Archive ouverte UNIGE | last documentshttps://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGEengStructural and Electronic Properties of Liquid and Amorphous SiO2: An Ab Initio Molecular Dynamics Studyhttps://archive-ouverte.unige.ch/unige:114786https://archive-ouverte.unige.ch/unige:114786We performed a first-principles molecular dynamics study of liquid SiO2 at a temperature of 3500 K, followed by a rapid quench to 300 K obtaining a perfectly chemically ordered amorphous network. Structural and electronic properties of our amorphous sample are in good agreement with neutron diffraction, x-ray photoemission, and optical experiments. On the basis of the partial structure factors, we investigated the origin of the first sharp diffraction peak. Disorder affects differently the localization properties of valence and conduction band states, as suggested by experimental mobilities of electrons and holes.Tue, 05 Mar 2019 11:41:09 +0100Ab Initio Molecular Dynamics Study of First-Order Phase Transitions: Melting of Siliconhttps://archive-ouverte.unige.ch/unige:114639https://archive-ouverte.unige.ch/unige:114639We present a scheme to compute the thermodynamic properties and the phase stability of materials based on parameter-free microscopic quantum theory. Taking silicon as an example we show that properties like the specific entropy, the specific volume, or the heat capacity of a solid and a liquid can be calculated accurately. In particular, we can locate the solid-liquid phase boundary and compute how thermodynamic properties change upon melting. This greatly extends the range of first-principles predictions of materials properties.Thu, 28 Feb 2019 14:46:41 +0100Evidence for long localization length along b Axis PrBa2Cu3O7 in a Axis YBa2Cu3O7https://archive-ouverte.unige.ch/unige:114638https://archive-ouverte.unige.ch/unige:114638We have investigated resistive transitions of a axis YBa₂Cu₃O₇/a,b axis PrBa₂Cu₃O₇ superlattices in a magnetic field to study the correlation length of vortices along the a axis direction of YBa₂Cu₃O₇ across an a,b-axis oriented PrBa₂Cu₃O₇ barrier. The correlation of vortices across up to 480 Å of PrBa₂Cu₃O₇ is evidence consistent with long proximity coupling coherence lengths in PrBa₂Cu₃O₇ found in sandwich type YBa₂Cu₃O₇/PrBa₂Cu₃O₇/YBa₂Cu₃O₇ Josephson junctions. Investigation of a-b plane and b axis transport of undoped and cobalt doped PrBa₂Cu₃O₇ films suggests that a long localization length along the b axis of PrBa₂Cu₃O₇ is the origin of this long range proximity coupling.Thu, 28 Feb 2019 14:45:39 +0100Direct Vortex Lattice Imaging and Tunneling Spectroscopy of Flux Lines on YBa2Cu3O7-δhttps://archive-ouverte.unige.ch/unige:114299https://archive-ouverte.unige.ch/unige:114299We report the observation of the flux line lattice in YBa2Cu3O7−δ by scanning tunneling microscopy. The measurements were carried out at 4.2 K and in a magnetic field of 6 T applied along the c axis. The vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77°. We also report local tunneling spectroscopy into a vortex core which reveals two peaks separated by about 11 meV. The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure.Thu, 14 Feb 2019 16:26:25 +0100Electrostatic Tuning of the Hole Density in NdBa2Cu3O7-δ Films and its Effect on the Hall Responsehttps://archive-ouverte.unige.ch/unige:114263https://archive-ouverte.unige.ch/unige:114263We have used the ferroelectric field effect in heterostructures based on superconducting NdBa2Cu3O7-delta and ferroelectric Pb(Zr0.2Ti0.8)O-3 to electrostatically modulate in a reversible and nonvolatile fashion the hole carrier density of the superconducting layer. Reversing the ferroelectric polarization induces a constant relative change in the resistivity and Hall constant of 9% and 6%, respectively, at all temperatures above the superconducting transition. The cotangent of the Hall angle displays a T-2 dependence with a slope that increases as the carrier density is reduced.Wed, 13 Feb 2019 13:37:45 +0100Ab initio Studies on High Pressure Phases of Icehttps://archive-ouverte.unige.ch/unige:113915https://archive-ouverte.unige.ch/unige:113915The pressure-induced transition of H₂O into the ice X phase, characterized by symmetric hydrogen bonding, is studied using ab initio molecular dynamics combined with ultrasoft pseudopotentials. A good description of the hydrogen bond is obtained only after gradient corrections to the local-density approximation are included. The transition into ice X is predicted at 49 GPa, in good agreement with experiment, when proton quantum fluctuations are treated within mean-field theory. Molecular-dynamics simulations show that a mode-softening description of the transition is appropriate.Wed, 06 Feb 2019 10:25:28 +0100Measurement of the Dynamical Structure Factor of a 1D Interacting Fermi Gashttps://archive-ouverte.unige.ch/unige:113836https://archive-ouverte.unige.ch/unige:113836We present measurements of the dynamical structure factor Sðq; ωÞ of an interacting one-dimensional Fermi gas for small excitation energies. We use the two lowest hyperfine levels of the 6Li atom to form a pseudospin-1=2 system whose s-wave interactions are tunable via a Feshbach resonance. The atoms are confined to one dimension by a two-dimensional optical lattice. Bragg spectroscopy is used to measure a response of the gas to density (“charge”) mode excitations at a momentum q and frequency ω, as a function of the interaction strength. The spectrum is obtained by varying ω, while the angle between two laser beams determines q, which is fixed to be less than the Fermi momentum kF. The measurements agree well with Tomonaga-Luttinger theory.Mon, 04 Feb 2019 14:21:28 +0100Transconducting Transition for a Dynamic Boundary Coupled to Several Luttinger Liquidshttps://archive-ouverte.unige.ch/unige:113834https://archive-ouverte.unige.ch/unige:113834We study a dynamic boundary, e.g., a mobile impurity, coupled to N independent Tomonaga-Luttinger liquids (TLLs) each with interaction parameter K. We demonstrate that for N ≥ 2 there is a quantum phase transition at K ≥ 1=2, where the TLL phases lock together at the particle position, resulting in a nonzero transconductance equal to e2=Nh. The transition line terminates for strong coupling at K 1⁄4 1 − ð1=NÞ, consistent with results at large N. Another type of a dynamic boundary is a superconducting (or a Bose- Einstein condensate) grain coupled to N ≥ 2 TLLs; here the transition signals also the onset of a relevant Josephson coupling.Mon, 04 Feb 2019 14:14:32 +0100Towards Understanding the Origin of Cosmic-Ray Positronshttps://archive-ouverte.unige.ch/unige:113787https://archive-ouverte.unige.ch/unige:113787Precision measurements of cosmic ray positrons are presented up to 1 TeV based on 1.9 million positrons collected by the Alpha Magnetic Spectrometer on the International Space Station. The positron flux exhibits complex energy dependence. Its distinctive properties are (a) a significant excess starting from 25.2±1.8 GeV compared to the lower-energy, power-law trend, (b) a sharp dropoff above 284+91−64 GeV, (c) in the entire energy range the positron flux is well described by the sum of a term associated with the positrons produced in the collision of cosmic rays, which dominates at low energies, and a new source term of positrons, which dominates at high energies, and (d) a finite energy cutoff of the source term of Es=810+310−180 GeV is established with a significance of more than 4σ. These experimental data on cosmic ray positrons show that, at high energies, they predominantly originate either from dark matter annihilation or from other astrophysical sources.Fri, 01 Feb 2019 15:12:59 +0100The μτ and eτ Decays of Top Quark Pairs Produced in p¯p Collisions at √s=1.8TeVhttps://archive-ouverte.unige.ch/unige:113064https://archive-ouverte.unige.ch/unige:113064We present a search for dilepton events from t¯t production with one electron or muon and one hadronically decaying τ lepton from the decay t¯t→(ℓνℓ)(τντ)b¯b, (ℓ=e,μ), using the Collider Detector at Fermilab. In a 109pb⁻¹ data sample of p¯p collisions at √s=1.8TeV we expect ∼1 signal event and a total background of ∼2 events; we observe 4 candidate events (2eτ and 2μτ). Three of these events have jets identified as b candidates, compared to an estimated background of 0.28±0.02 events.Fri, 18 Jan 2019 14:53:30 +0100Search for New Particles Decaying into b¯b and Produced in Association with W Bosons Decaying into eν or μν at the Fermilab Tevatronhttps://archive-ouverte.unige.ch/unige:113063https://archive-ouverte.unige.ch/unige:113063We search for new particles that decay into b¯b and are produced with W bosons in p¯p collisions at √s=1.8TeV. The search uses 109±7pb⁻¹ accumulated by the CDF experiment at Fermilab. We select events with an eν or μν, and two jets, one of them b tagged. The number of events and the two-jet mass distribution are consistent with expectations. Using W+Higgs production as a model for the acceptance, we set an upper limit on the production cross section times branching ratio for the new particle ranging from 14 to 19 pb (95% C.L.) as the particle mass varies from 70 to 120GeV/c².Fri, 18 Jan 2019 14:49:39 +0100Search for New Gauge Bosons Decaying into Dileptons in ¯pp Collisions at √s = 1.8 TeVhttps://archive-ouverte.unige.ch/unige:113062https://archive-ouverte.unige.ch/unige:113062We have searched for heavy neutral gauge bosons (Z′) in dielectron and dimuon decay modes using 110pb−1 of ¯pp collisions at √s=1.8TeV collected with the Collider Detector at Fermilab. We present a limit on the production cross section times branching ratio of a Z′ boson decaying into dileptons as a function of Z′ mass. For mass MZ′>600GeV/c2, the upper limit is 40 fb at 95% confidence level. We set the lower mass limits of 690, 590, 620, 595, 565, 630, and 600GeV/c2 for Z′SM, Zψ, Zη, Zχ, ZI, ZLR, and ZALRM, respectively.Fri, 18 Jan 2019 14:46:47 +0100Properties of Jets in W Boson Events from 1.8 TeV ¯pp Collisionshttps://archive-ouverte.unige.ch/unige:112628https://archive-ouverte.unige.ch/unige:112628We present a study of events with W bosons and hadronic jets produced in ¯pp collisions at a center of mass energy of 1.8 TeV. The data consist of 51 400 W±→e±ν decay candidates from 108pb−1 of integrated luminosity collected using the CDF detector at the Fermilab Tevatron Collider. Cross sections and jet production properties have been measured for W+≥1 to ≥4 jet events. The data are compared to predictions of leading-order QCD matrix element calculations with added gluon radiation and simulated parton fragmentation.Wed, 09 Jan 2019 09:03:37 +0100Limits on Quark-Lepton Compositeness Scales from Dileptons Produced in 1.8 TeV p¯p Collisionshttps://archive-ouverte.unige.ch/unige:112627https://archive-ouverte.unige.ch/unige:112627The dilepton mass spectrum in p¯p→l+l−+X interactions is studied using dielectrons (ee) and dimuons (μμ) in 110pb−1 of data collected with the Collider Detector at Fermilab. The data are consistent with standard model predictions. The mass spectrum, being a probe for new physics, is examined for new interactions of quarks and leptons from a common composite structure. Assuming a contact interaction with the conventional coupling g20/4π=1, limits on chiral quark-electron and quark-muon compositeness scales in the range of 2.5 to 4.2 TeV are obtained.Wed, 09 Jan 2019 09:02:15 +0100Measurement of Diffractive Dijet Production at the Fermilab Tevatronhttps://archive-ouverte.unige.ch/unige:112626https://archive-ouverte.unige.ch/unige:112626We report the observation and measurement of the rate of diffractive dijet production at the Fermilab Tevatron ¯pp collider at √s=1.8TeV. In events with two jets of ET>20GeV, 1.8<|η|<3.5, and η1η2>0, we find that the diffractive to nondiffractive production ratio is RJJ=[0.75±0.05(stat)±0.09(syst)]%. By comparing this result, in combination with our measured rate for diffractive W boson production reported previously, with predictions based on a hard partonic pomeron structure, we determine the pomeron gluon fraction to be fg=0.7±0.2.Wed, 09 Jan 2019 09:01:01 +0100Search for First Generation Leptoquark Pair Production in p¯p Collisions at √s = 1.8 TeVhttps://archive-ouverte.unige.ch/unige:112620https://archive-ouverte.unige.ch/unige:112620We present the results of a search for first generation scalar leptoquarks pairs (S1¯S1) in 110±7pb−1 of data collected by the Collider Detector at Fermilab. We assume both leptoquarks decay to an electron and a quark. Three candidate events, with masses below 140GeV/c2 and consistent with background expectations, are observed. We obtain a 95% confidence level upper limit on the production cross section as a function of the leptoquark mass. Using a next-to-leading order calculation of S1¯S1 production, we exclude scalar leptoquarks with mass less than 213GeV/c2 at 95% C.L. for a branching ratio into eq equal to 1.Wed, 09 Jan 2019 08:47:07 +0100Secure Quantum Key Distribution over 421 km of Optical Fiberhttps://archive-ouverte.unige.ch/unige:112310https://archive-ouverte.unige.ch/unige:112310We present a quantum key distribution system with a 2.5 GHz repetition rate using a three-state timebin protocol combined with a one-decoy approach. Taking advantage of superconducting single-photon detectors optimized for quantum key distribution and ultralow-loss fiber, we can distribute secret keys at a maximum distance of 421 km and obtain secret key rates of 6.5 bps over 405 km.Fri, 14 Dec 2018 14:41:59 +0100Violation of Bell Inequalities by Photons More Than 10 km Aparthttps://archive-ouverte.unige.ch/unige:112219https://archive-ouverte.unige.ch/unige:112219A Franson-type test of Bell inequalities by photons 10.9 km apart is presented. Energy-time entangled photon pairs are measured using two-channel analyzers, leading to a violation of the inequalities by 16 standard deviations without subtracting accidental coincidences. Subtracting them, a two-photon interference visibility of 95.5% is observed, demonstrating that distances up to 10 km have no significant effect on entanglement. This sets quantum cryptography with photon pairs as a practical competitor to the schemes based on weak pulses.Wed, 12 Dec 2018 12:16:15 +0100Nonequilibrium Superconductivity and Quasiparticle Dynamics Studied by Photoinduced Activation of mm-Wave Absorptionhttps://archive-ouverte.unige.ch/unige:112117https://archive-ouverte.unige.ch/unige:112117We present a study of nonequilibrium superconductivity in DyBa2Cu3O7−δ using photoinduced activation of mm-wave absorption. We monitor the time evolution of the thin film transmissivity at 5cm−1 subject to pulsed infrared radiation. In addition to a positive bolometric signal we observe a second, faster, decay with a sign opposite to the bolometric signal for T>40K. We attribute this to the unusual properties of quasiparticles residing near the nodes of an unconventional superconductor, resulting in a strong enhancement of the recombination time.Fri, 07 Dec 2018 12:23:50 +0100Search for Resonant and Nonresonant Higgs Boson Pair Production in the bb¯τ+τ− Decay Channel in pp Collisions at √s = 13 TeV with the ATLAS Detectorhttps://archive-ouverte.unige.ch/unige:111803https://archive-ouverte.unige.ch/unige:111803A search for resonant and nonresonant pair production of Higgs bosons in the bb¯τ+τ− final state is presented. The search uses 36.1 fb−1 of pp collision data with s=13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. Decays of the τ-lepton pairs with at least one τ lepton decaying to final states with hadrons and a neutrino are considered. No significant excess above the expected background is observed in the data. The cross-section times branching ratio for nonresonant Higgs boson pair production is constrained to be less than 30.9 fb, 12.7 times the standard model expectation, at 95% confidence level. The data are also analyzed to probe resonant Higgs boson pair production, constraining a model with an extended Higgs sector based on two doublets and a Randall-Sundrum bulk graviton model. Upper limits are placed on the resonant Higgs boson pair production cross-section times branching ratio, excluding resonances X in the mass range 305 GeV<mX<402 GeV in the simplified hMSSM minimal supersymmetric model for tanβ=2 and excluding bulk Randall-Sundrum gravitons GKK in the mass range 325 GeV<mGKK<885 GeV for k/M¯Pl=1.Tue, 04 Dec 2018 10:35:06 +0100Combination of the Searches for Pair-Produced Vectorlike Partners of the Third-Generation Quarks at √s = 13 TeV with the ATLAS Detectorhttps://archive-ouverte.unige.ch/unige:111794https://archive-ouverte.unige.ch/unige:111794A combination of the searches for pair-produced vectorlike partners of the top and bottom quarks in various decay channels (T→Zt/Wb/Ht, B→Zb/Wt/Hb) is performed using 36.1 fb−1 of pp collision data at s=13 TeV with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the standard model background prediction in all individual searches. Therefore, combined 95% confidence-level upper limits are set on the production cross section for a range of vectorlike quark scenarios, significantly improving upon the reach of the individual searches. Model-independent limits are set assuming the vectorlike quarks decay to standard model particles. A singlet T is excluded for masses below 1.31 TeV and a singlet B is excluded for masses below 1.22 TeV. Assuming a weak isospin (T,B) doublet and |VTb|≪|VtB|, T and B masses below 1.37 TeV are excluded.Tue, 04 Dec 2018 10:34:53 +0100Probing the Quantum Interference between Singly and Doubly Resonant Top-Quark Production in pp Collisions at √s = 13 TeV with the ATLAS Detectorhttps://archive-ouverte.unige.ch/unige:110369https://archive-ouverte.unige.ch/unige:110369This Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a W boson and a b-quark are significant. Events with exactly two leptons (ee, μμ, or eμ) and two b-tagged jets that satisfy a multiparticle invariant mass requirement are selected from 36.1 fb−1 of proton-proton collision data taken at s=13 TeV with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within 2σ of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.Mon, 05 Nov 2018 09:34:03 +0100Search for CP Violation in Neutrino and Antineutrino Oscillations by the T2K Experiment with 2.2×1021 Protons on Targethttps://archive-ouverte.unige.ch/unige:110363https://archive-ouverte.unige.ch/unige:110363The T2K experiment measures muon neutrino disappearance and electron neutrino appearance in accelerator-produced neutrino and antineutrino beams. With an exposure of 14.7(7.6)×1020 protons on target in the neutrino (antineutrino) mode, 89 νe candidates and seven anti-νe candidates are observed, while 67.5 and 9.0 are expected for δCP=0 and normal mass ordering. The obtained 2σ confidence interval for the CP-violating phase, δCP, does not include the CP-conserving cases (δCP=0, π). The best-fit values of other parameters are sin2θ23=0.526−0.036+0.032 and Δm322=2.463−0.070+0.071×10−3 eV2/c4.Mon, 05 Nov 2018 09:34:01 +0100Observation of Out-of-Plane Spin Texture in a SrTiO3(111) Two-Dimensional Electron Gashttps://archive-ouverte.unige.ch/unige:109117https://archive-ouverte.unige.ch/unige:109117We explore the second order bilinear magnetoelectric resistance (BMER) effect in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO3 (111) surface. We find evidence of a spin-split band structure with the archetypal spin-momentum locking of the Rashba effect for the in-plane component. Under an out-of-plane magnetic field, we find a BMER signal that breaks the sixfold symmetry of the electronic dispersion, which is a fingerprint for the presence of a momentum-dependent out-of-plane spin component. Relativistic electronic structure calculations reproduce this spin texture and indicate that the out-of-plane component is a ubiquitous property of oxide 2DEGs arising from strong crystal field effects. We further show that the BMER response of the SrTiO3 (111) 2DEG is tunable and unexpectedly large.Fri, 12 Oct 2018 12:44:26 +0200Hall Anomaly and Vortex-Lattice Melting in Superconducting Single Crystal YBa2Cu3O7-δhttps://archive-ouverte.unige.ch/unige:107903https://archive-ouverte.unige.ch/unige:107903Subnanovolt resolution longitudinal and Hall voltages are measured in an ultrapure YBa2Cu3O7−δ single crystal. The Hall anomaly and the first-order vortex-lattice melting transition are observed simultaneously. Changes in the dynamic behavior of the vortex solid and liquid are correlated with features of the Hall conductivity σxy. With the magnetic field oriented at an angle from the twin boundaries, the Hall conductivity sharply decreases toward large negative values at the vortex-lattice melting transition.Mon, 17 Sep 2018 15:55:33 +0200Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnethttps://archive-ouverte.unige.ch/unige:107697https://archive-ouverte.unige.ch/unige:107697We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF2(H2O)2]2pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.Wed, 12 Sep 2018 17:27:42 +0200Search for Low-Mass Dijet Resonances Using Trigger-Level Jets with the ATLAS Detector in pp Collisions at √s = 13 TeVhttps://archive-ouverte.unige.ch/unige:107312https://archive-ouverte.unige.ch/unige:107312Searches for dijet resonances with sub-TeV masses using the ATLAS detector at the Large Hadron Collider can be statistically limited by the bandwidth available to inclusive single-jet triggers, whose data-collection rates at low transverse momentum are much lower than the rate from standard model multijet production. This Letter describes a new search for dijet resonances where this limitation is overcome by recording only the event information calculated by the jet trigger algorithms, thereby allowing much higher event rates with reduced storage needs. The search targets low-mass dijet resonances in the range 450–1800 GeV. The analyzed data set has an integrated luminosity of up to 29.3 fb−1 and was recorded at a center-of-mass energy of 13 TeV. No excesses are found; limits are set on Gaussian-shaped contributions to the dijet mass distribution from new particles and on a model of dark-matter particles with axial-vector couplings to quarks.Mon, 03 Sep 2018 09:00:46 +0200Measurement of the Soft-Drop Jet Mass in pp Collisions at √s = 13 TeV with the ATLAS Detectorhttps://archive-ouverte.unige.ch/unige:107309https://archive-ouverte.unige.ch/unige:107309Jet substructure observables have significantly extended the search program for physics beyond the standard model at the Large Hadron Collider. The state-of-the-art tools have been motivated by theoretical calculations, but there has never been a direct comparison between data and calculations of jet substructure observables that are accurate beyond leading-logarithm approximation. Such observables are significant not only for probing the collinear regime of QCD that is largely unexplored at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. This Letter documents a measurement of the first jet substructure quantity at a hadron collider to be calculated at next-to-next-to-leading-logarithm accuracy. The normalized, differential cross section is measured as a function of log10ρ2, where ρ is the ratio of the soft-drop mass to the ungroomed jet transverse momentum. This quantity is measured in dijet events from 32.9 fb−1 of s=13 TeV proton-proton collisions recorded by the ATLAS detector. The data are unfolded to correct for detector effects and compared to precise QCD calculations and leading-logarithm particle-level Monte Carlo simulations.Mon, 03 Sep 2018 09:00:45 +0200Precision Measurement of Cosmic-Ray Nitrogen and its Primary and Secondary Components with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:106849https://archive-ouverte.unige.ch/unige:106849A precision measurement of the nitrogen flux with rigidity (momentum per unit charge) from 2.2 GV to 3.3 TV based on 2.2×10^6 events is presented. The detailed rigidity dependence of the nitrogen flux spectral index is presented for the first time. The spectral index rapidly hardens at high rigidities and becomes identical to the spectral indices of primary He, C, and O cosmic rays above ∼700 GV. We observed that the nitrogen flux ΦN can be presented as the sum of its primary component ΦPN and secondary component ΦSN, ΦN=ΦPN+ΦSN, and we found ΦN is well described by the weighted sum of the oxygen flux ΦO (primary cosmic rays) and the boron flux ΦB (secondary cosmic rays), with ΦPN=(0.090±0.002)×ΦO and ΦSN=(0.62±0.02)×ΦB over the entire rigidity range. This corresponds to a change of the contribution of the secondary cosmic ray component in the nitrogen flux from 70% at a few GV to <30% above 1 TV.Mon, 06 Aug 2018 14:55:38 +0200Observation of Complex Time Structures in the Cosmic-Ray Electron and Positron Fluxes with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:106844https://archive-ouverte.unige.ch/unige:106844We present high-statistics, precision measurements of the detailed time and energy dependence of the primary cosmic-ray electron flux and positron flux over 79 Bartels rotations from May 2011 to May 2017 in the energy range from 1 to 50 GeV. For the first time, the charge-sign dependent modulation during solar maximum has been investigated in detail by leptons alone. Based on 23.5×10^6 events, we report the observation of short-term structures on the timescale of months coincident in both the electron flux and the positron flux. These structures are not visible in the e+/e− flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over 830±30 days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by 260±30 days from 1 to 6 GeV.Mon, 06 Aug 2018 14:51:34 +0200Observation of Fine Time Structures in the Cosmic Proton and Helium Fluxes with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:106841https://archive-ouverte.unige.ch/unige:106841We present the precision measurement from May 2011 to May 2017 (79 Bartels rotations) of the proton fluxes at rigidities from 1 to 60 GV and the helium fluxes from 1.9 to 60 GV based on a total of 1×10^9 events collected with the Alpha Magnetic Spectrometer aboard the International Space Station. This measurement is in solar cycle 24, which has the solar maximum in April 2014. We observed that, below 40 GV, the proton flux and the helium flux show nearly identical fine structures in both time and relative amplitude. The amplitudes of the flux structures decrease with increasing rigidity and vanish above 40 GV. The amplitudes of the structures are reduced during the time period, which started one year after solar maximum, when the proton and helium fluxes steadily increase. Above ∼3 GV the p/He flux ratio is time independent. We observed that below ∼3 GV the ratio has a long-term decrease coinciding with the period during which the fluxes start to rise.Mon, 06 Aug 2018 14:40:22 +0200Measurement of Final-State Correlations in Neutrino Muon-Proton Mesonless Production on Hydrocarbon at ⟨Eν⟩=3 GeVhttps://archive-ouverte.unige.ch/unige:106819https://archive-ouverte.unige.ch/unige:106819Final-state kinematic imbalances are measured in mesonless production of νμ+A→μ−+p+X in the MINERvA tracker. Initial- and final-state nuclear effects are probed using the direction of the μ−−p transverse momentum imbalance and the initial-state momentum of the struck neutron. Differential cross sections are compared to predictions based on current approaches to medium modeling. These models underpredict the cross section at intermediate intranuclear momentum transfers that generally exceed the Fermi momenta. As neutrino interaction models need to correctly incorporate the effect of the nucleus in order to predict neutrino energy resolution in oscillation experiments, this result points to a region of phase space where additional cross section strength is needed in current models, and demonstrates a new technique that would be suitable for use in fine-grained liquid argon detectors where the effect of the nucleus may be even larger.Mon, 06 Aug 2018 08:23:39 +0200Antineutrino Charged-Current Reactions on Hydrocarbon with Low Momentum Transferhttps://archive-ouverte.unige.ch/unige:106248https://archive-ouverte.unige.ch/unige:106248We report on multinucleon effects in low momentum transfer (<0.8 GeV/c) antineutrino interactions on plastic (CH) scintillator. These data are from the 2010–2011 antineutrino phase of the MINERvA experiment at Fermilab. The hadronic energy spectrum of this inclusive sample is well described when a screening effect at a low energy transfer and a two-nucleon knockout process are added to a relativistic Fermi gas model of quasielastic, Δ resonance, and higher resonance processes. In this analysis, model elements introduced to describe previously published neutrino results have quantitatively similar benefits for this antineutrino sample. We present the results as a double-differential cross section to accelerate the investigation of alternate models for antineutrino scattering off nuclei.Thu, 05 Jul 2018 12:09:43 +0200Hydrogenation-Induced Insulating State in the Intermetallic Compound LaMg2Nihttps://archive-ouverte.unige.ch/unige:104971https://archive-ouverte.unige.ch/unige:104971Hydrogenation-induced metal-semiconductor transitions usually occur in simple systems based on rare earths and/or magnesium, accompanied by major reconstructions of the metal host (atom shifts >2 Å). We report on the first such transition in a quaternary system based on a transition element. Metallic LaMg2Ni absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 which has a nearly unchanged metal host structure (atom shifts <0.7 Å). The transition is induced by a charge transfer of conduction electrons into tetrahedral [NiH4]4− complexes having closed-shell electron configurations.Tue, 05 Jun 2018 10:50:57 +0200Search for a Structure in the Bs0π± Invariant Mass Spectrum with the ATLAS Experimenthttps://archive-ouverte.unige.ch/unige:104829https://archive-ouverte.unige.ch/unige:104829A search for the narrow structure, X(5568), reported by the D0 Collaboration in the decay sequence X→Bs0π±, Bs0→J/ψϕ, is presented. The analysis is based on a data sample recorded with the ATLAS detector at the LHC corresponding to 4.9 fb−1 of pp collisions at 7 TeV and 19.5 fb−1 at 8 TeV. No significant signal was found. Upper limits on the number of signal events, with properties corresponding to those reported by D0, and on the X production rate relative to Bs0 mesons, ρX, were determined at 95% confidence level. The results are N(X)<382 and ρX<0.015 for Bs0 mesons with transverse momenta above 10 GeV, and N(X)<356 and ρX<0.016 for transverse momenta above 15 GeV. Limits are also set for potential Bs0π± resonances in the mass range 5550 to 5700 MeV.Fri, 01 Jun 2018 10:36:02 +0200Search for the Exotic Meson X(5568) with the Collider Detector at Fermilabhttps://archive-ouverte.unige.ch/unige:104828https://archive-ouverte.unige.ch/unige:104828A search for the exotic meson X(5568) decaying into the Bs0π± final state is performed using data corresponding to 9.6 fb−1 from pp¯ collisions at s=1960 GeV recorded by the Collider Detector at Fermilab. No evidence for this state is found and an upper limit of 6.7% at the 95% confidence level is set on the fraction of Bs0 produced through the X(5568)→Bs0π± process.Fri, 01 Jun 2018 10:36:02 +0200Search for the Decay of the Higgs Boson to Charm Quarks with the ATLAS Experimenthttps://archive-ouverte.unige.ch/unige:104825https://archive-ouverte.unige.ch/unige:104825A direct search for the standard model Higgs boson decaying to a pair of charm quarks is presented. Associated production of the Higgs and Z bosons, in the decay mode ZH→ℓ+ℓ−cc¯ is studied. A data set with an integrated luminosity of 36.1 fb−1 of pp collisions at s=13TeV recorded by the ATLAS experiment at the LHC is used. The H→cc¯ signature is identified using charm-tagging algorithms. The observed (expected) upper limit on σ(pp→ZH)×B(H→cc¯) is 2.7 (3.9−1.1+2.1) pb at the 95% confidence level for a Higgs boson mass of 125 GeV, while the standard model value is 26 fb.Fri, 01 Jun 2018 10:36:01 +0200Beyond Positivity Bounds and the Fate of Massive Gravityhttps://archive-ouverte.unige.ch/unige:104094https://archive-ouverte.unige.ch/unige:104094We constrain effective field theories by going beyond the familiar positivity bounds that follow from unitarity, analyticity, and crossing symmetry of the scattering amplitudes. As interesting examples, we discuss the implications of the bounds for the Galileon and ghost-free massive gravity. The combination of our theoretical bounds with the experimental constraints on the graviton mass implies that the latter is either ruled out or unable to describe gravitational phenomena, let alone to consistently implement the Vainshtein mechanism, down to the relevant scales of fifth-force experiments, where general relativity has been successfully tested. We also show that the Galileon theory must contain symmetry-breaking terms that are at most one-loop suppressed compared to the symmetry-preserving ones. We comment as well on other interesting applications of our bounds.Mon, 07 May 2018 08:24:00 +0200Search for High-Mass Resonances Decaying to τν in pp Collisions at √s = 13 TeV with the ATLAS Detectorhttps://archive-ouverte.unige.ch/unige:104090https://archive-ouverte.unige.ch/unige:104090A search for high-mass resonances decaying to τν using proton-proton collisions at s=13 TeV produced by the Large Hadron Collider is presented. Only τ-lepton decays with hadrons in the final state are considered. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb−1. No statistically significant excess above the standard model expectation is observed; model-independent upper limits are set on the visible τν production cross section. Heavy W′ bosons with masses less than 3.7 TeV in the sequential standard model and masses less than 2.2–3.8 TeV depending on the coupling in the nonuniversal G(221) model are excluded at the 95% credibility level.Mon, 07 May 2018 08:23:59 +0200Cosmological Signature of the Standard Model Higgs Vacuum Instability: Primordial Black Holes as Dark Matterhttps://archive-ouverte.unige.ch/unige:103325https://archive-ouverte.unige.ch/unige:103325For the current central values of the Higgs boson and top quark masses, the standard model Higgs potential develops an instability at a scale of the order of 1011 GeV. We show that a cosmological signature of such instability could be dark matter in the form of primordial black holes seeded by Higgs fluctuations during inflation. The existence of dark matter might not require physics beyond the standard model.Thu, 05 Apr 2018 10:50:44 +0200Interplay between Depletion and Double-Layer Forces Acting between Charged Particles in Solutions of Like-Charged Polyelectrolyteshttps://archive-ouverte.unige.ch/unige:103253https://archive-ouverte.unige.ch/unige:103253Direct force measurements between negatively charged silica particles in the presence of a like-charged strong polyelectrolyte were carried out with an atomic force microscope. The force profiles can be quantitatively interpreted as a superposition of depletion and double-layer forces. The depletion forces are modeled with a damped oscillatory profile, while the double-layer forces with the mean-field Poisson- Boltzmann theory for a strongly asymmetric electrolyte, whereby an effective valence must be assigned to the polyelectrolyte. This effective valence is substantially smaller than the bare valence due to ion condensation effects. The unusual aspect of the electrical double layer in these systems is the exclusion of the like-charged polyelectrolyte from the vicinity of the surface, leading to a strongly nonexponential diffuse ionic layer that is dominated by counterions and has a well-defined thickness. As the oscillatory depletion force sets in right after this layer, this condition can be used to predict the phase of the oscillatory depletion force.Wed, 28 Mar 2018 15:21:05 +0200Measurement of Atmospheric Neutrino Oscillations at 6–56 GeV with IceCube DeepCorehttps://archive-ouverte.unige.ch/unige:102647https://archive-ouverte.unige.ch/unige:102647We present a measurement of the atmospheric neutrino oscillation parameters using three years of data from the IceCube Neutrino Observatory. The DeepCore infill array in the center of IceCube enables the detection and reconstruction of neutrinos produced by the interaction of cosmic rays in Earth’s atmosphere at energies as low as <math><mo>∼</mo><mn>5</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>. That energy threshold permits measurements of muon neutrino disappearance, over a range of baselines up to the diameter of the Earth, probing the same range of <math><mrow><mi>L</mi><mo>/</mo><msub><mrow><mi>E</mi></mrow><mrow><mi>ν</mi></mrow></msub></mrow></math> as long-baseline experiments but with substantially higher-energy neutrinos. This analysis uses neutrinos from the full sky with reconstructed energies from 5.6 to 56 GeV. We measure <math><mrow><mi>Δ</mi><msubsup><mrow><mi>m</mi></mrow><mrow><mn>32</mn></mrow><mrow><mn>2</mn></mrow></msubsup><mo>=</mo><msubsup><mrow><mn>2.31</mn></mrow><mrow><mo>−</mo><mn>0.13</mn></mrow><mrow><mo>+</mo><mn>0.11</mn></mrow></msubsup><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>eV</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math> and <math><msup><mi>sin</mi><mn>2</mn></msup><msub><mi>θ</mi><mn>23</mn></msub><mo>=</mo><mn>0.5</mn><msubsup><mn>1</mn><mrow><mo>−</mo><mn>0.09</mn></mrow><mrow><mo>+</mo><mn>0.07</mn></mrow></msubsup></math>, assuming normal neutrino mass ordering. These results are consistent with, and of similar precision to, those from accelerator- and reactor-based experiments.Fri, 02 Mar 2018 14:32:15 +0100Combined Forward-Backward Asymmetry Measurements in Top-Antitop Quark Production at the Tevatronhttps://archive-ouverte.unige.ch/unige:102495https://archive-ouverte.unige.ch/unige:102495The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of s√=1.96TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is Att¯FB=0.128±0.025. The combined inclusive and differential asymmetries are consistent with recent standard model predictions.Thu, 22 Feb 2018 16:01:01 +0100Observation of New Properties of Secondary Cosmic Rays Lithium, Beryllium, and Boron by the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:101349https://archive-ouverte.unige.ch/unige:101349We report on the observation of new properties of secondary cosmic rays Li, Be, and B measured in the rigidity (momentum per unit charge) range 1.9 GV to 3.3 TV with a total of 5.4×10^6 nuclei collected by AMS during the first five years of operation aboard the International Space Station. The Li and B fluxes have an identical rigidity dependence above 7 GV and all three fluxes have an identical rigidity dependence above 30 GV with the Li/Be flux ratio of 2.0±0.1. The three fluxes deviate from a single power law above 200 GV in an identical way. This behavior of secondary cosmic rays has also been observed in the AMS measurement of primary cosmic rays He, C, and O but the rigidity dependences of primary cosmic rays and of secondary cosmic rays are distinctly different. In particular, above 200 GV, the secondary cosmic rays harden more than the primary cosmic rays.Mon, 15 Jan 2018 09:34:51 +0100Spatiotemporal patterns in ultraslow domain wall creep dynamicshttps://archive-ouverte.unige.ch/unige:101047https://archive-ouverte.unige.ch/unige:101047In the presence of impurities, ferromagnetic and ferroelectric domain walls slide only above a finite external field. Close to this depinning threshold, they proceed by large and abrupt jumps called avalanches, while, at much smaller fields, these interfaces creep by thermal activation. In this Letter, we develop a novel numerical technique that captures the ultraslow creep regime over huge time scales. We point out the existence of activated events that involve collective reorganizations similar to avalanches, but, at variance with them, display correlated spatiotemporal patterns that resemble the complex sequence of aftershocks observed after a large earthquake. Remarkably, we show that events assemble in independent clusters that display at large scales the same statistics as critical depinning avalanches. We foresee these correlated dynamics being experimentally accessible by magnetooptical imaging of ferromagnetic films.Tue, 02 Jan 2018 13:53:58 +0100Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladderhttps://archive-ouverte.unige.ch/unige:101046https://archive-ouverte.unige.ch/unige:101046The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.Tue, 02 Jan 2018 13:53:08 +0100Observation of the Identical Rigidity Dependence of He, C, and O Cosmic Rays at High Rigidities by the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:100533https://archive-ouverte.unige.ch/unige:100533We report the observation of new properties of primary cosmic rays He, C, and O measured in the rigidity (momentum/charge) range 2 GV to 3 TV with 90×10^6 helium, 8.4×10^6 carbon, and 7.0×10^6 oxygen nuclei collected by the Alpha Magnetic Spectrometer (AMS) during the first five years of operation. Above 60 GV, these three spectra have identical rigidity dependence. They all deviate from a single power law above 200 GV and harden in an identical way.Tue, 19 Dec 2017 15:05:48 +0100Multimode and Long-Lived Quantum Correlations Between Photons and Spins in a Crystalhttps://archive-ouverte.unige.ch/unige:100453https://archive-ouverte.unige.ch/unige:100453The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on the entanglement of remote matter systems, which in turn requires the creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the Duan-Lukin-Cirac-Zoller (DLCZ) scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a 151Eu3+-doped Y2SiO5 crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy-Schwarz inequality. Our results show that solid-state rare-earth-ion-doped crystals could be used to generate remote multimode entanglement, an important resource for future quantum networks.Mon, 18 Dec 2017 11:06:10 +0100Observation of Caroli–de Gennes–Matricon Vortex States in YBa2Cu3O7-δhttps://archive-ouverte.unige.ch/unige:99962https://archive-ouverte.unige.ch/unige:99962The copper oxides present the highest superconducting temperature and properties at odds with other compounds, suggestive of a fundamentally different superconductivity. In particular, the Abrikosov vortices fail to exhibit localized states expected and observed in all clean superconductors. We have explored the possibility that the elusive vortex-core signatures are actually present but weak. Combining local tunneling measurements with large-scale theoretical modeling, we positively identify the vortex states in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub>. We explain their spectrum and the observed variations thereof from one vortex to the next by considering the effects of nearby vortices and disorder in the vortex lattice. We argue that the superconductivity of copper oxides is conventional, but the spectroscopic signature does not look so because the superconducting carriers are a minority.Tue, 05 Dec 2017 11:18:15 +0100Reconstruction of the diamond (111) surfacehttps://archive-ouverte.unige.ch/unige:99311https://archive-ouverte.unige.ch/unige:99311The (2×1) reconstruction of the (111) surface of diamond has been characterized via a series of first principles molecular dynamics calculations. The reconstructed clean surface is found to consist of dimerized π-bonded chains with no buckling and to be stable against thermal fluctuations up to about 2500 K. Our results for the atomic and electronic structure agree well with the experimental information available.Tue, 21 Nov 2017 11:45:32 +0100Precision Measurement of the Helium Flux in Primary Cosmic Rays of Rigidities 1.9 GV to 3 TV with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:98561https://archive-ouverte.unige.ch/unige:98561Knowledge of the precise rigidity dependence of the helium flux is important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a single power law.Mon, 06 Nov 2017 09:57:08 +0100Antiproton Flux, Antiproton-to-Proton Flux Ratio, and Properties of Elementary Particle Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:98560https://archive-ouverte.unige.ch/unige:98560A precision measurement by AMS of the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV is presented based on 3.49×105 antiproton events and 2.42×109 proton events. The fluxes and flux ratios of charged elementary particles in cosmic rays are also presented. In the absolute rigidity range ∼60 to ∼500 GV, the antiproton ¯p, proton p, and positron e+ fluxes are found to have nearly identical rigidity dependence and the electron e− flux exhibits a different rigidity dependence. Below 60 GV, the (¯p/p), (¯p/e+), and (p/e+) flux ratios each reaches a maximum. From ∼60 to ∼500 GV, the (¯p/p), (¯p/e+), and (p/e+) flux ratios show no rigidity dependence. These are new observations of the properties of elementary particles in the cosmos.Mon, 06 Nov 2017 09:54:06 +0100Precision Measurement of the Boron to Carbon Flux Ratio in Cosmic Rays from 1.9 GV to 2.6 TV with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:98559https://archive-ouverte.unige.ch/unige:98559Knowledge of the rigidity dependence of the boron to carbon flux ratio (B/C) is important in understanding the propagation of cosmic rays. The precise measurement of the B/C ratio from 1.9 GV to 2.6 TV, based on 2.3 million boron and 8.3 million carbon nuclei collected by AMS during the first 5 years of operation, is presented. The detailed variation with rigidity of the B/C spectral index is reported for the first time. The B/C ratio does not show any significant structures in contrast to many cosmic ray models that require such structures at high rigidities. Remarkably, above 65 GV, the B/C ratio is well described by a single power law RΔ with index Δ=−0.333±0.014(fit)±0.005(syst), in good agreement with the Kolmogorov theory of turbulence which predicts Δ=−1/3 asymptotically.Mon, 06 Nov 2017 09:51:04 +0100Precision Measurement of the (e+ + e-) Flux in Primary Cosmic Rays from 0.5 GeV to 1 TeV with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:98257https://archive-ouverte.unige.ch/unige:98257We present a measurement of the cosmic ray (e+ + e−) flux in the range 0.5 GeV to 1 TeV based on the analysis of 10.6 million (e+ + e−) events collected by AMS. The statistics and the resolution of AMS provide a precision measurement of the flux. The flux is smooth and reveals new and distinct information. Above 30.2 GeV, the flux can be described by a single power law with a spectral index gamma= -3.170 +- 0.008(stat + sys) +- 0.008(energy scale).Tue, 31 Oct 2017 08:51:41 +0100Precision Measurement of the Proton Flux in Primary Cosmic Rays from Rigidity 1 GV to 1.8 TV with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:98256https://archive-ouverte.unige.ch/unige:98256A precise measurement of the proton flux in primary cosmic rays with rigidity (momentum/charge) from 1 GV to 1.8 TV is presented based on 300 million events. Knowledge of the rigidity dependence of the proton flux is important in understanding the origin, acceleration, and propagation of cosmic rays. We present the detailed variation with rigidity of the flux spectral index for the first time. The spectral index progressively hardens at high rigidities.Tue, 31 Oct 2017 08:47:57 +0100High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:97662https://archive-ouverte.unige.ch/unige:97662A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500 GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above ~200 GeV the positron fraction no longer exhibits an increase in energy.Mon, 16 Oct 2017 14:27:18 +0200Electron and Positron Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Stationhttps://archive-ouverte.unige.ch/unige:97660https://archive-ouverte.unige.ch/unige:97660Precision measurements by the Alpha Magnetic Spectrometer on the International Space Station of the primary cosmic-ray electron flux in the range 0.5 to 700 GeV and the positron flux in the range 0.5 to 500 GeV are presented. The electron flux and the positron flux each require a description beyond a single power-law spectrum. Both the electron flux and the positron flux change their behavior at ~30 GeV but the fluxes are significantly different in their magnitude and energy dependence. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The determination of the differing behavior of the spectral indices versus energy is a new observation and provides important information on the origins of cosmic-ray electrons and positrons.Mon, 16 Oct 2017 14:18:09 +0200Demonstration of Einstein-Podolsky-Rosen Steering Using Single-Photon Path Entanglement and Displacement-Based Detectionhttps://archive-ouverte.unige.ch/unige:96472https://archive-ouverte.unige.ch/unige:96472We demonstrate the violation of an EPR steering inequality developed for single photon path entanglement with displacement-based detection. We use a high-rate source of heralded single-photon path-entangled states, combined with high-efficiency superconducting-based detectors, in a scheme that is free of any post-selection and thus immune to the detection loophole. This result conclusively demonstrates single-photon entanglement in a one-sided device-independent scenario, and opens the way towards implementations of device-independent quantum technologies within the paradigm of path entanglement.Mon, 04 Sep 2017 12:00:52 +0200Dimensional Crossover in a Charge Density Wave Material Probed by Angle-Resolved Photoemission Spectroscopyhttps://archive-ouverte.unige.ch/unige:94365https://archive-ouverte.unige.ch/unige:94365High-resolution angle-resolved photoemission spectroscopy data reveal evidence of a crossover from one-dimensional (1D) to three-dimensional (3D) behavior in the prototypical charge density wave (CDW) material NbSe3. In the low-temperature 3D regime, gaps in the electronic structure are observed due to two incommensurate CDWs, in agreement with x-ray diffraction and electronic-structure calculations. At higher temperatures we observe a spectral weight depletion that approaches the power-law behavior expected in one dimension. From the warping of the quasi-1D Fermi surface at low temperatures, we extract the energy scale of the dimensional crossover. This is corroborated by a detailed analysis of the density of states, which reveals a change in dimensional behavior dependent on binding energy. Our results offer an important insight into the dimensionality of excitations in quasi-1D materials.Mon, 22 May 2017 09:25:02 +0200Gas-solid phase transition in laser multiple filamentationhttps://archive-ouverte.unige.ch/unige:94058https://archive-ouverte.unige.ch/unige:94058While propagating in transparent media, near-infrared multi-terawatt (TW) laser beams break up in a multitude of filaments of typically 100-200 um diameter with peak intensities as high as 10 to 100~TW/cm$^{2}$. We observe a phase transition at incident beam intensities of 0.4~TW/cm$^2$, where the interaction between filaments induce solid-like 2-dimensional crystals with a 2.7 mm lattice constant, independent of the initial beam diameter. Below 0.4~TW/cm$^2$, we evidence a mixed phase state in which some filaments are closely packed in localized clusters, nucleated on inhomogeneities (seeds) in the transverse intensity profile of the beam, and other are sparse with almost no interaction with their neighbors, similar to a gas. This analogy with a thermodynamic gas-solid phase transition is confirmed by calculating the interaction Hamiltonian between neighboring filaments, which takes into account the effect of diffraction, Kerr self-focusing and plasma generation. The shape of the effective potential is close to a Morse potential with an equilibrium bond length close to the observed value.Tue, 09 May 2017 10:41:53 +0200Ab initio study of positron trapping at a vacancy in GaAshttps://archive-ouverte.unige.ch/unige:93806https://archive-ouverte.unige.ch/unige:93806We present a first-principles study of positron trapping at a negatively charged As vacancy in GaAs. Lattice relaxations induced both by the presence of the defect and of the positron have been included in a self-consistent way. In the presence of a positron, the volume of the vacancy increases and its symmetry is lowered. The positron wave function is well localized in the defect. Calculated positron lifetime and angular correlations of annihilation photons are in good agreement with recent experiments.Mon, 24 Apr 2017 11:15:49 +0200Quantifying Photonic High-Dimensional Entanglementhttps://archive-ouverte.unige.ch/unige:93007https://archive-ouverte.unige.ch/unige:93007High-dimensional entanglement offers promising perspectives in quantum information science. In practice, however, the main challenge is to devise efficient methods to characterize high-dimensional entanglement, based on the available experimental data which is usually rather limited. Here we report the characterization and certification of high-dimensional entanglement in photon pairs, encoded in temporal modes. Building upon recently developed theoretical methods, we certify an entanglement of formation of 2.09(7) ebits in a time-bin implementation, and 4.1(1) ebits in an energy-time implementation. These results are based on very limited sets of local measurements, which illustrates the practical relevance of these methods.Mon, 03 Apr 2017 11:15:07 +0200Laser-Driven Multiferroics and Ultrafast Spin Current Generationhttps://archive-ouverte.unige.ch/unige:92626https://archive-ouverte.unige.ch/unige:92626We propose an ultrafast way to generate spin chirality and spin current in a class of multiferroic magnets using a terahertz circularly polarized laser. Using the Floquet formalism for periodically driven systems, we show that it is possible to dynamically control the Dzyaloshinskii-Moriya interaction in materials with magnetoelectric coupling. This is supported by numerical calculations, by which additional resonant phenomena are found. Specifically, when a static magnetic field is applied in addition to the circularly polarized laser, a large resonant enhancement of spin chirality is observed resembling the electron spin resonance. Spin current is generated when the laser is spatially modulated by chiral plasmonic structures and could be detected using optospintronic devices.Wed, 15 Mar 2017 14:54:29 +0100New Analytic Approach to Multivelocity Annihilation in the Kinetic Theory of Reactionshttps://archive-ouverte.unige.ch/unige:92188https://archive-ouverte.unige.ch/unige:92188A new, exact, analytic approach to multivelocity, one-species, ballistic annihilation in one dimension is proposed. For an arbitrary one-particle initial velocity distribution, the problem can be solved rigorously in terms of the two-particle conditional probability, which obeys a closed nonlinear integro-differential equation. We present a method for solving this equation for an arbitrary discrete velocity distribution. This method is applied to the three-velocity case. The outcome of numerical simulations compares well with our exact results.Tue, 28 Feb 2017 11:14:49 +0100Integrability and Ideal Conductance at Finite Temperatureshttps://archive-ouverte.unige.ch/unige:91491https://archive-ouverte.unige.ch/unige:91491We analyse the finite temperature charge stiffness D(T>0), by a generalization of Kohn's method, for the problem of a particle interacting with a fermionic bath in one dimension. We present analytical evidence, using the Bethe ansatz method, that D(T>0) is finite in the integrable case where the mass of the particle equals the mass of the fermions and numerical evidence that it vanishes in the nonintegrable one of unequal masses. We conjecture that a finite D(T>0) is a generic property of integrable systems.Tue, 31 Jan 2017 11:33:19 +0100Comment on "Scaling Anomalies in Reaction Front Dynamics of Confined Systems"https://archive-ouverte.unige.ch/unige:91475https://archive-ouverte.unige.ch/unige:91475abstract not availableMon, 30 Jan 2017 14:20:45 +0100Full Elasticity Tensor from Thermal Diffuse Scatteringhttps://archive-ouverte.unige.ch/unige:91447https://archive-ouverte.unige.ch/unige:91447We present a method for the precise determination of the full elasticity tensor from a single crystal diffraction experiment using monochromatic x rays. For the two benchmark systems calcite and magnesium oxide, we show that the measurement of thermal diffuse scattering in the proximity of Bragg reflections provides accurate values of the complete set of elastic constants. This approach allows for a reliable and model-free determination of the elastic properties and can be performed together with crystal structure investigation in the same experiment.Mon, 30 Jan 2017 08:59:34 +0100Reaction-diffusion cellular automata model for the formation of Leisegang patternshttps://archive-ouverte.unige.ch/unige:91286https://archive-ouverte.unige.ch/unige:91286Cellular automata models for the formation of Liesegang structures are proposed. This novel approach, which takes into account the fluctuations for the first time, describes the problem at a microscopic scale, in terms of reaction, diffusion, nucleation, and aggregation processes. We present large scale numerical simulations which provide clear verifications of the time and spacing laws and predict a novel behavior for the widths of the patterns. We show that two different microscopic reaction schemes are possible for producing Liesegang structures and we propose a phase diagram showing the different types of possible patterns.Tue, 24 Jan 2017 12:10:36 +0100Stripe and Short Range Order in the Charge Density Wave of 1T-CuxTiSe2https://archive-ouverte.unige.ch/unige:90894https://archive-ouverte.unige.ch/unige:90894We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-CuxTiSe2 by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x < 0.02) and a breaking up of the commensurate order into 2 × 2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.Mon, 09 Jan 2017 09:44:51 +0100Algorithmic Construction of Local Hidden Variable Models for Entangled Quantum Stateshttps://archive-ouverte.unige.ch/unige:90701https://archive-ouverte.unige.ch/unige:90701Constructing local hidden variable (LHV) models for entangled quantum states is a fundamental problem, with implications for the foundations of quantum theory and for quantum information processing. It is, however, a challenging problem, as the model should reproduce quantum predictions for all possible local measurements. Here we present a simple method for building LHV models, applicable to any entangled state and considering continuous sets of measurements. This leads to a sequence of tests which, in the limit, fully captures the set of quantum states admitting a LHV model. Similar methods are developed for local hidden state models. We illustrate the practical relevance of these methods with several examples.Wed, 21 Dec 2016 15:16:37 +0100High-Precision Measurement of the Dzyaloshinsky-Moriya Interaction between Two Rare-Earth Ions in a Solidhttps://archive-ouverte.unige.ch/unige:90520https://archive-ouverte.unige.ch/unige:90520We report on a direct measurement of the pairwise antisymmetric exchange interaction, known as the Dzyaloshinsky-Moriya interaction (DMI), in a Nd3+-doped YVO4 crystal. To this end, we introduce a broadband electron spin resonance technique coupled with an optical detection scheme which selectively detects only one Nd3+−Nd3+ pair. Using this technique we can fully measure the spin-spin coupling tensor, allowing us to experimentally determine both the strength and direction of the DMI vector. We believe that this ability to fully determine the interaction Hamiltonian is of interest for studying the numerous magnetic phenomena where the DMI interaction is of fundamental importance, including multiferroics. We also detect a singlet-triplet transition within the pair, with a highly suppressed magnetic-field dependence, which suggests that such systems could form singlet-triplet qubits with long coherence times for quantum information applications.Mon, 19 Dec 2016 11:56:01 +0100Coherent Spin Control at the Quantum Level in an Ensemble-Based Optical Memoryhttps://archive-ouverte.unige.ch/unige:90518https://archive-ouverte.unige.ch/unige:90518Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.Mon, 19 Dec 2016 11:51:42 +0100Temporal Multimode Storage of Entangled Photon Pairshttps://archive-ouverte.unige.ch/unige:90247https://archive-ouverte.unige.ch/unige:90247Multiplexed quantum memories capable of storing and processing entangled photons are essential for the development of quantum networks. In this context, we demonstrate and certify the simultaneous storage and retrieval of two entangled photons inside a solid-state quantum memory and measure a temporal multimode capacity of ten modes. This is achieved by producing two polarization-entangled pairs from parametric down-conversion and mapping one photon of each pair onto a rare-earth-ion-doped (REID) crystal using the atomic frequency comb (AFC) protocol. We develop a concept of indirect entanglement witnesses, which can be used as Schmidt number witnesses, and we use it to experimentally certify the presence of more than one entangled pair retrieved from the quantum memory. Our work puts forward REID-AFC as a platform compatible with temporal multiplexing of several entangled photon pairs along with a new entanglement certification method, useful for the characterization of multiplexed quantum memories.Mon, 12 Dec 2016 12:03:46 +0100Demonstration of Light-Matter Micro-Macro Quantum Correlationshttps://archive-ouverte.unige.ch/unige:89853https://archive-ouverte.unige.ch/unige:89853Quantum mechanics predicts microscopic phenomena with undeniable success. Nevertheless, current theoretical and experimental efforts still do not yield conclusive evidence that there is or is not a fundamental limitation on the possibility to observe quantum phenomena at the macroscopic scale. This question prompted several experimental efforts producing quantum superpositions of large quantum states in light or matter.We report on the observation of quantum correlations, revealed using an entanglement witness, between a single photon and an atomic ensemble of billions of ions frozen in a crystal. The matter part of the state involves the superposition of two macroscopically distinguishable solid-state components composed of several tens of atomic excitations. Assuming the insignificance of the time ordering our experiment indirectly shows lightmatter micro-macro entanglement. Our approach leverages from quantum memory techniques and could be used in other systems to expand the size of quantum superpositions in matter.Mon, 05 Dec 2016 11:36:19 +010024-Hour Relativistic Bit Commitmenthttps://archive-ouverte.unige.ch/unige:88082https://archive-ouverte.unige.ch/unige:88082Bit commitment is a fundamental cryptographic primitive in which a party wishes to commit a secret bit to another party. Perfect security between mistrustful parties is unfortunately impossible to achieve through the asynchronous exchange of classical and quantum messages. Perfect security can nonetheless be achieved if each party splits into two agents exchanging classical information at times and locations satisfying strict relativistic constraints. A relativistic multiround protocol to achieve this was previously proposed and used to implement a 2-millisecond commitment time. Much longer durations were initially thought to be insecure, but recent theoretical progress showed that this is not so. In this Letter, we report on the implementation of a 24-hour bit commitment solely based on timed high-speed optical communication and fast data processing, with all agents located within the city of Geneva. This duration is more than 6 orders of magnitude longer than before, and we argue that it could be extended to one year and allow much more flexibility on the locations of the agents. Our implementation offers a practical and viable solution for use in applications such as digital signatures, secure voting and honesty-preserving auctions.Mon, 10 Oct 2016 09:34:03 +0200Pinning-Dependent Field-Driven Domain Wall Dynamics and Thermal Scaling in an Ultrathin Pt/Co/Pt Magnetic Filmhttps://archive-ouverte.unige.ch/unige:87813https://archive-ouverte.unige.ch/unige:87813Magnetic-field-driven domain wall motion in an ultrathin Pt/Co(0.45 nm)/Pt ferromagnetic film with perpendicular anisotropy is studied over a wide temperature range. Three different pinning dependent dynamical regimes are clearly identified: the creep, the thermally assisted flux flow, and the depinning, as well as their corresponding crossovers. The wall elastic energy and microscopic parameters characterizing the pinning are determined. Both the extracted thermal rounding exponent at the depinning transition, ψ=0.15, and the Larkin length crossover exponent, φ=0.24, fit well with the numerical predictions.Tue, 27 Sep 2016 13:33:38 +0200Competing Regimes of Motion of 1D Mobile Impuritieshttps://archive-ouverte.unige.ch/unige:87812https://archive-ouverte.unige.ch/unige:87812We show that a distinguishable mobile impurity inside a one-dimensional many-body state at zero temperature generally does not behave like a quasiparticle. Instead, both the impurity dynamics as well as the ground state of the bath are fundamentally transformed by a diverging number of zero-energy excitations being generated, leading to what we call infrared-dominated (ID) dynamics. Combining analytics and density matrix renormalization group numerics, we provide a general formula for the power law governing ID dynamics at zero momentum, discuss a threshold beyond which quasiparticle dynamics may occur again, and study the competition between the ID and quasiparticle universality classes at larger impurity momenta.Tue, 27 Sep 2016 13:32:29 +0200Observation of a Disordered Bosonic Insulator from Weak to Strong Interactionshttps://archive-ouverte.unige.ch/unige:87811https://archive-ouverte.unige.ch/unige:87811We employ ultracold atoms with controllable disorder and interaction to study the paradigmatic problem of disordered bosons in the full disorder-interaction plane. Combining measurements of coherence, transport and excitation spectra, we get evidence of an insulating regime extending from weak to strong interaction and surrounding a superfluidlike regime, in general agreement with the theory. For strong interaction, we reveal the presence of a strongly correlated Bose glass coexisting with a Mott insulator.Tue, 27 Sep 2016 13:30:20 +0200Lattice-Assisted Spectroscopy: A Generalized Scanning Tunneling Microscope for Ultracold Atomshttps://archive-ouverte.unige.ch/unige:87806https://archive-ouverte.unige.ch/unige:87806We propose a scheme to measure the frequency-resolved local particle and hole spectra of any optical lattice-confined system of correlated ultracold atoms that offers single-site addressing and imaging, which is now an experimental reality. Combining perturbation theory and time-dependent density matrix renormalization group simulations, we quantitatively test and validate this approach of lattice-assisted spectroscopy on several one-dimensional example systems, such as the superfluid and Mott insulator, with and without a parabolic trap, and finally on edge states of the bosonic Su-Schrieffer-Heeger model. We highlight extensions of our basic scheme to obtain an even wider variety of interesting and important frequency resolved spectra.Tue, 27 Sep 2016 13:19:29 +0200Joint Measurability, Einstein-Podolsky-Rosen Steering, and Bell Nonlocalityhttps://archive-ouverte.unige.ch/unige:84468https://archive-ouverte.unige.ch/unige:84468We investigate the relation between the incompatibility of quantum measurements and quantum nonlocality. We show that a set of measurements is not jointly measurable (i.e., incompatible) if and only if it can be used for demonstrating Einstein-Podolsky-Rosen steering, a form of quantum nonlocality. Moreover, we discuss the connection between Bell nonlocality and joint measurability, and give evidence that both notions are inequivalent. Specifically, we exhibit a set of incompatible quantum measurements and show that it does not violate a large class of Bell inequalities. This suggests the existence of incompatible quantum measurements which are Bell local, similarly to certain entangled states which admit a local hidden variable model.Mon, 13 Jun 2016 11:19:04 +0200Device-Independent Tests of Entropyhttps://archive-ouverte.unige.ch/unige:84463https://archive-ouverte.unige.ch/unige:84463We show that the entropy of a message can be tested in a device-independent way. Specifically, we consider a prepare-and-measure scenario with classical or quantum communication, and develop two different methods for placing lower bounds on the communication entropy, given observable data. The first method is based on the framework of causal inference networks. The second technique, based on convex optimization, shows that quantum communication provides an advantage over classical, in the sense of requiring a lower entropy to reproduce given data. These ideas may serve as a basis for novel applications in device-independent quantum information processing.Mon, 13 Jun 2016 11:12:01 +0200Local hidden variable models for entangled quantum states using finite shared randomnesshttps://archive-ouverte.unige.ch/unige:84461https://archive-ouverte.unige.ch/unige:84461The statistics of local measurements performed on certain entangled states can be reproduced using a local hidden variable (LHV) model. While all known models make use of an infinite amount of shared randomness---the physical relevance of which is questionable---we show that essentially all entangled states admitting a LHV model can be simulated with finite shared randomness. Our most economical model simulates noisy two-qubit Werner states using only 3.58 bits of shared randomness. We also discuss the case of POVMs, and the simulation of nonlocal states with finite shared randomness and finite communication. Our work represents a first step towards quantifying the cost of LHV models for entangled quantum states.Mon, 13 Jun 2016 11:10:49 +0200Practical Relativistic Bit Commitmenthttps://archive-ouverte.unige.ch/unige:84119https://archive-ouverte.unige.ch/unige:84119Bit commitment is a fundamental cryptographic primitive in which Alice wishes to commit a secret bit to Bob. Perfectly secure bit commitment between two mistrustful parties is impossible through an asynchronous exchange of quantum information. Perfect security is, however, possible when Alice and Bob each split into several agents exchanging classical information at times and locations suitably chosen to satisfy specific relativistic constraints. In this Letter we first revisit a previously proposed scheme [C. Crépeau et al., Lect. Notes Comput. Sci. 7073, 407 (2011)] that realizes bit commitment using only classical communication. We prove that the protocol is secure against quantum adversaries for a duration limited by the light-speed communication time between the locations of the agents. We then propose a novel multiround scheme based on finite-field arithmetic that extends the commitment time beyond this limit, and we prove its security against classical attacks. Finally, we present an implementation of these protocols using dedicated hardware and we demonstrate a 2 ms-long bit commitment over a distance of 131 km. By positioning the agents on antipodal points on the surface of Earth, the commitment time could possibly be extended to 212 ms.Wed, 01 Jun 2016 13:44:18 +0200Self-Testing Quantum Random Number Generatorhttps://archive-ouverte.unige.ch/unige:83961https://archive-ouverte.unige.ch/unige:83961A central issue in randomness generation is to estimate the entropy of the output data generated by a given device. Here we present a protocol for self-testing quantum random number generation, in which the entropy of the raw data can be monitored in real-time. In turn, this allows the user to adapt the randomness extraction procedure, in order to continuously generate high quality random bits. Using a fully optical implementation, we demonstrate that our protocol is practical and efficient, and illustrate its self-testing capacity.Mon, 30 May 2016 11:16:43 +0200Resource-Efficient Measurement-Device-Independent Entanglement Witnesshttps://archive-ouverte.unige.ch/unige:83960https://archive-ouverte.unige.ch/unige:83960Imperfections in experimental measurement schemes can lead to falsely identifying, or over estimating, entanglement in a quantum system. A recent solution to this is to define schemes that are robust to measurement imperfections - measurement device independent entanglement witness (MDI-EW). Here we introduce a novel approach for MDI-EW, which significantly reduces the experimental complexity and is applicable to a wide range of physical systems. The scheme requires no prior description of the state, is detection loop-hole free, robust to classical communication, and works for all entangled qubit states. We focus on photonic entanglement, experimentally generating and testing bipartite Werner states, varying the entanglement from the maximally entangled Bell state, past the bound for nonlocal states and down to the separable bound of 1/3. We witness entanglement down to an entangled state fraction close to 0.4. These results could be of particular interest for device independent quantum random number generation.Mon, 30 May 2016 11:15:37 +0200Quantum Nonlocality with Arbitrary Limited Detection Efficiencyhttps://archive-ouverte.unige.ch/unige:83949https://archive-ouverte.unige.ch/unige:83949The demonstration and use of nonlocality, as defined by Bell's theorem, rely strongly on dealing with non-detection events due to losses and detectors' inefficiencies. Otherwise, the so-called detection loophole could be exploited. The only way to avoid this is to have detection efficiencies that are above a certain threshold. We introduce the intermediate assumption of limited detection efficiency, that is, in each run of the experiment, the overall detection efficiency is lower bounded by $eta_{min} > 0$. Hence, in an adversarial scenario, the adversaries have arbitrary large but not full control over the inefficiencies. We analyse the set of possible correlations that fulfill Limited Detection Locality (LDL) and show that they necessarily satisfy some linear Bell-like inequalities. We prove that quantum theory predicts the violation of one of these inequalities for all $eta_{min} > 0$. Hence, nonlocality can be demonstrated with arbitrarily small limited detection efficiencies. We validate this assumption experimentally via a twin-photon implementation in which two users are provided with one photon each out of a partially entangled pair. We exploit on each side a passive switch followed by two measurement devices with fixed settings. Assuming the switches are not fully controlled by an adversary, nor by hypothetical local variables, we reveal the nonlocality of the established correlations despite a low overall detection efficiency.Mon, 30 May 2016 10:03:59 +0200Revealing Genuine Optical-Path Entanglementhttps://archive-ouverte.unige.ch/unige:83944https://archive-ouverte.unige.ch/unige:83944How can one detect entanglement between multiple optical paths sharing a single photon? We address this question by proposing a scalable protocol, which only uses local measurements where single photon detection is combined with small displacement operations. The resulting entanglement witness does not require postselection, nor assumptions about the photon number in each path. Furthermore, it guarantees that entanglement lies in a subspace with at most one photon per optical path and reveals genuinely multipartite entanglement. We demonstrate its scalability and resistance to loss by performing various experiments with two and three optical paths. We anticipate applications of our results for quantum network certification.Mon, 30 May 2016 09:53:17 +0200Demonstration of Quantum Nonlocality in the Presence of Measurement Dependencehttps://archive-ouverte.unige.ch/unige:83942https://archive-ouverte.unige.ch/unige:83942Quantum nonlocality stands as a resource for device independent quantum information processing (DIQIP), such as, for instance, device independent quantum key distribution. We investigate, exper- imentally, the assumption of limited measurement dependence, i.e., that the measurement settings used in Bell inequality tests or DIQIP are partially influenced by the source of entangled particle and/or by an adversary. Using a recently derived Bell-like inequality [G. Pütz, Phys. Rev. Lett. 113, 190402 (2014)] and a 99% fidelity source of partially entangled polarization photonic qubits, we obtain a clear violation of the inequality, excluding a much larger range of measurement dependent local models than would be possible with an adapted Clauser-Horne-Shimony-Holt (CHSH) inequality. It is therefore shown that the measurement independence assumption can be widely relaxed while still demonstrating quantum nonlocality.Mon, 30 May 2016 09:50:10 +0200Nonlinear Interaction between Single Photonshttps://archive-ouverte.unige.ch/unige:83938https://archive-ouverte.unige.ch/unige:83938Harnessing nonlinearities strong enough to allow single photons to interact with one another is not only a fascinating challenge but also central to numerous advanced applications in quantum information science. Here we report the nonlinear interaction between two single photons. Each photon is generated in independent parametric down-conversion sources. They are subsequently combined in a nonlinear waveguide where they are converted into a single photon of higher energy by the process of sum- frequency generation. Our approach results in the direct generation of photon triplets. More generally, it highlights the potential for quantum nonlinear optics with integrated devices and, as the photons are at telecom wavelengths, it opens the way towards novel applications in quantum communication such as device-independent quantum key distribution.Mon, 30 May 2016 09:44:07 +0200Genuinely Multipartite Entangled Quantum States with Fully Local Hidden Variable Models and Hidden Multipartite Nonlocalityhttps://archive-ouverte.unige.ch/unige:83549https://archive-ouverte.unige.ch/unige:83549The relation between entanglement and nonlocality is discussed in the case of multipartite quantum systems. We show that, for any number of parties, there exist genuinely multipartite entangled states that admit a fully local hidden variable model, i.e., where all parties are separated. Hence, although these states exhibit the strongest form of multipartite entanglement, they cannot lead to Bell inequality violation considering general nonsequential local measurements. Then, we show that the nonlocality of these states can nevertheless be activated using sequences of local measurements, thus revealing genuine multipartite hidden nonlocality.Mon, 09 May 2016 09:37:58 +0200Collapse of the Mott Gap and Emergence of a Nodal Liquid in Lightly Doped Sr2IrO4https://archive-ouverte.unige.ch/unige:80004https://archive-ouverte.unige.ch/unige:80004We report angle resolved photoemission experiments on the electron doped Heisenberg antiferromagnet Sr1−xLax2IrO4. For a doping level of x 1⁄4 0.05, we find an unusual metallic state with coherent nodal excitations and an antinodal pseudogap bearing strong similarities with underdoped cuprates. This state emerges from a rapid collapse of the Mott gap with doping resulting in a large underlying Fermi surface that is backfolded by a (π,π) reciprocal lattice vector which we attribute to the intrinsic structural distortion of Sr2IrO4.Mon, 25 Jan 2016 12:17:26 +0100Structure of Self-Assembled Mn Atom Chains on Si(001)https://archive-ouverte.unige.ch/unige:78890https://archive-ouverte.unige.ch/unige:78890Mn has been found to self-assemble into atomic chains running perpendicular to the surface dimer reconstruction on Si(001). They differ from other atomic chains by a striking asymmetric appearance in filled state scanning tunneling microscopy (STM) images. This has prompted complicated structural models involving up to three Mn atoms per chain unit. Combining STM, atomic force microscopy, and density functional theory we find that a simple necklacelike chain of single Mn atoms reproduces all their prominent features, including their asymmetry not captured by current models. The upshot is a remarkably simpler structure for modeling the electronic and magnetic properties of Mn atom chains on Si(001).Tue, 22 Dec 2015 11:55:06 +0100Spin-Glass Model Governs Laser Multiple Filamentationhttps://archive-ouverte.unige.ch/unige:74168https://archive-ouverte.unige.ch/unige:74168We show that multiple filamentation patterns in high-power laser beams, can be described by means of two statistical physics concepts, namely self-similarity of the patterns over two nested scales, and nearest-neighbor interactions of classical rotators. The resulting lattice spin model perfectly reproduces the evolution of intense laser pulses as simulated by the Non-Linear Schr"odinger Equation, shedding a new light on multiple filamentation. As a side benefit, this approach drastically reduces the computing time by two orders of magnitude as compared to the standard simulation methods of laser filamentation.Wed, 15 Jul 2015 15:59:30 +0200Laser Filamentation as a New Phase Transition Universality Classhttps://archive-ouverte.unige.ch/unige:54894https://archive-ouverte.unige.ch/unige:54894We show that the onset of laser multiple filamentation can be described as a critical phenomenon that we characterize both experimentally and numerically by measuring a set of seven critical exponents. This phase transition deviates from any existing universality class and offers a unique perspective of conducting two-dimensional experiments of statistical physics at a human scale.Mon, 30 Mar 2015 10:54:04 +0200High-Frequency Subsurface and Bulk Dynamics of Liquid Indiumhttps://archive-ouverte.unige.ch/unige:48341https://archive-ouverte.unige.ch/unige:48341We have performed bulk and surface-sensitive inelastic x-ray scattering experiments on liquid indium with 3 meV energy resolution. The experimental data are well reproduced within a generalized hydrodynamic model including structural and microscopic relaxation processes. We find a longitudinal viscosity of 22 mPa s in the near-surface region compared to 7.4 mPa s in the bulk. The origin of the increase is associated with a slowing down of the collective dynamics in a subsurface region of 4.6 nm.Wed, 18 Mar 2015 14:59:15 +0100Emergence of Crystal-like Atomic Dynamics in Glasses at the Nanometer Scalehttps://archive-ouverte.unige.ch/unige:48339https://archive-ouverte.unige.ch/unige:48339The vibrational dynamics of a permanently densified silica glass is compared to the one of an α-quartz polycrystal, the silica polymorph of the same density and local structure. The combined use of inelastic x-ray scattering experiments and ab initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic length ξ of a few nanometers, corresponding to about 20 interatomic distances. In the microscopic regime the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the elastic modulus of the glass presents spatial heterogeneities of an average size a∼ξ/2π.Wed, 18 Mar 2015 14:55:52 +0100Role of Disorder in the Thermodynamics and Atomic Dynamics of Glasseshttps://archive-ouverte.unige.ch/unige:48337https://archive-ouverte.unige.ch/unige:48337We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO2. The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higher-density crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.Wed, 18 Mar 2015 14:52:21 +0100Experimental Methods for Detecting Entanglementhttps://archive-ouverte.unige.ch/unige:47356https://archive-ouverte.unige.ch/unige:47356Here we present experimental realizations of two new entanglement detection methods: a three-measurement Bell inequality inequivalent to the Clauser-Horne-Shimony-Holt inequality and a nonlinear Bell-type inequality based on the negativity measure. In addition, we provide an experimental and theoretical comparison between these new methods and several techniques already in use: the traditional Clauser-Horne-Shimony-Holt inequality, the entanglement witness, and complete state tomography.Fri, 27 Feb 2015 14:37:41 +0100Device-Independent Entanglement Quantification and Related Applicationshttps://archive-ouverte.unige.ch/unige:47341https://archive-ouverte.unige.ch/unige:47341We present a general method to quantify both bipartite and multipartite entanglement in a device-independent manner, meaning that we put a lower bound on the amount of entanglement present in a system based on the observed data only but independent of any quantum description of the employed devices. Some of the bounds we obtain, such as for the Clauser-Horne-Shimony-Holt Bell inequality or the Svetlichny inequality, are shown to be tight. Besides, device-independent entanglement quantification can serve as a basis for numerous tasks. We show in particular that our method provides a rigorous way to construct dimension witnesses, gives new insights into the question whether bound entangled states can violate a Bell inequality, and can be used to construct device-independent entanglement witnesses involving an arbitrary number of parties.Fri, 27 Feb 2015 14:20:55 +0100Origin of Interface Magnetism in BiMnO3/SrTiO3 and LaAlO3/SrTiO3 Heterostructureshttps://archive-ouverte.unige.ch/unige:47227https://archive-ouverte.unige.ch/unige:47227Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.Tue, 24 Feb 2015 09:32:30 +0100