Archive ouverte UNIGE | last documents for author 'Christophe Berthod'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Christophe Berthod'engBand Filling and Cross Quantum Capacitance in Ion-Gated Semiconducting Transition Metal Dichalcogenide Monolayershttps://archive-ouverte.unige.ch/unige:127193https://archive-ouverte.unige.ch/unige:127193Ionic liquid gated field-effect transistors (FETs) based on semiconducting transition metal dichalcogenides (TMDs) are used to study a rich variety of extremely interesting physical phenomena, but important aspects of how charge carriers are accumulated in these systems are not understood. We address these issues by means of a systematic experimental study of transport in monolayer MoSe2 and WSe2 as a function of magnetic field and gate voltage, exploring accumulated densities of carriers ranging from approximately 1014 cm–2 holes in the valence band to 4 × 1014 cm–2 electrons in the conduction band. We identify the conditions when the chemical potential enters different valleys in the monolayer band structure (the K and Q valley in the conduction band and the two spin-split K-valleys in the valence band) and find that an independent electron picture describes the occupation of states well. Unexpectedly, however, the experiments show very large changes in the device capacitance when multiple valleys are occupied that are not at all compatible with the commonly expected quantum capacitance contribution of these systems, CQ = e2/ (dμ/dn). A theoretical analysis of all terms responsible for the total capacitance shows that under general conditions a term is present besides the usual quantum capacitance, which becomes important for very small distances between the capacitor plates. This term, which we call cross quantum capacitance, originates from screening of the electric field generated by charges on one plate from charges sitting on the other plate. The effect is negligible in normal capacitors but large in ionic liquid FETs because of the atomic proximity between the ions in the gate and the accumulated charges on the TMD, and it accounts for all our experimental observations. Our findings therefore reveal an important contribution to the capacitance of physical systems that had been virtually entirely neglected until now.Mon, 02 Dec 2019 11:21:55 +0100High-Resolution Photoemission on Sr2RuO4 Reveals Correlation-Enhanced Effective Spin-Orbit Coupling and Dominantly Local Self-Energieshttps://archive-ouverte.unige.ch/unige:123134https://archive-ouverte.unige.ch/unige:123134We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2, due to electronic correlations. The self-energies for the β and γ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion of Sr2RuO4.Fri, 13 Sep 2019 15:08:21 +0200Impurity coupled to a lattice with disorderhttps://archive-ouverte.unige.ch/unige:110714https://archive-ouverte.unige.ch/unige:110714We study the time-dependent occupation of an impurity state hybridized with a continuum of extended or localized states. Of particular interest is the return probability, which gives the long-time limit of the average impurity occupation. In the extended case, the return probability is zero unless there are bound states of the impurity and continuum. We present exact expressions for the return probability of an impurity state coupled to a lattice, and show that the existence of bound states depends on the dimension of the lattice. In a disordered lattice with localized eigenstates, the finite extent of the eigenstates results in a nonzero return probability. We investigate different parameter regimes numerically by exact diagonalization, and show that the return probability can serve as a measure of the localization length in the regime of weak hybridization and disorder. Possible experimental realizations with ultracold atoms are discussed.Tue, 13 Nov 2018 12:09:19 +0100Signatures of nodeless multiband superconductivity and particle-hole crossover in the vortex cores of FeTe0.55Se0.45https://archive-ouverte.unige.ch/unige:109947https://archive-ouverte.unige.ch/unige:109947Scanning tunneling experiments on single crystals of superconducting FeTe0.55Se0.45 have recently provided evidence for discrete energy levels inside vortices. Although predicted long ago, such levels are seldom resolved due to extrinsic (temperature, instrumentation) and intrinsic (quasiparticle scattering) limitations. We study a microscopic multiband model with parameters appropriate for FeTe0.55Se0.45. We confirm the existence of well-separated bound states and show that the chemical disorder due to random occupation of the chalcogen site does not affect significantly the vortex-core electronic structure. We further analyze the vortex bound states by projecting the local density of states on angular-momentum eigenstates. A rather complex pattern of bound states emerges from the multiband and mixed electron-hole nature of the normal-state carriers. The character of the vortex states changes from hole-like with negative angular momentum at low energy to electron-like with positive angular momentum at higher energy within the superconducting gap. We show that disorder in the arrangement of vortices most likely explains the differences found experimentally when comparing different vortices.Fri, 26 Oct 2018 14:12:41 +0200Tilted vortex cores and superconducting gap anisotropy in 2H-NbSe2https://archive-ouverte.unige.ch/unige:106037https://archive-ouverte.unige.ch/unige:106037The superconductor 2H-NbSe2 features vortices with a sixfold star shape when the magnetic field is applied perpendicular to the plane of the hexagonal crystal structure. This is due to the anisotropy in the quasi-two-dimensional Fermi surface tubes oriented along the c axis. But the properties of another, three-dimensional, pocket are unknown, in spite that it has a dominant contribution in many experiments. Here we measure vortices in tilted magnetic fields using a scanning tunneling microscope. We find patterns of stripes due to in-plane vortices and show how these change with the in-plane direction of the magnetic field. The shape of outgoing vortices is successfully compared with detailed calculations of vortex cores. Our data show that vortices exit at an angle with the surface and that the sixfold gap anisotropy is present over the whole Fermi surface.Mon, 02 Jul 2018 15:50:02 +0200Observation 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 +0100Modulation of the superconducting critical temperature due to quantum confinement at the LaAlO3/SrTiO3 interfacehttps://archive-ouverte.unige.ch/unige:97170https://archive-ouverte.unige.ch/unige:97170Superconductivity develops in bulk doped SrTiO3 and at the LaAlO3/SrTiO3 interface with a dome-shaped density dependence of the critical temperature Tc , despite different dimensionalities and geometries. We propose that the Tc dome of LaAlO3/SrTiO3 is a shape resonance due to quantum confinement of superconducting bulk SrTiO3. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for Tc in both geometries. For bulk SrTiO3, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate Tc in the confined configuration. We compare the calculated Tc to transport experiments and provide an explanation as to why the optimal Tc's are so close to each other in two-dimensional interfaces and the three-dimensional bulk material.Mon, 02 Oct 2017 09:19:18 +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 +0200Interplay of the pseudogap and the BCS gap for heteropairs in 40K-6Li mixturehttps://archive-ouverte.unige.ch/unige:90704https://archive-ouverte.unige.ch/unige:90704The description of heteropairs like 40K-6Li near and in the superconducting state requires a fully self-consistent theory (see Hanai R. and Ohashi Y., Phys. Rev. A, 90 (2014) 043622). We derive analytic pseudogap Green's functions for the "normal" and superconducting states from the Luttinger-Ward theory with the T-matrix in the static separable approximation. The self-consistency in the closing loop of self-energy has two pronounced effects on the single-particle spectrum. First, the single-particle excitations decay before the asymptotic quasiparticle propagation is established, therefore the normal state is not a Fermi liquid. Second, the pseudogap has a V shape even for s-wave pairing. The V-shaped pseudogap and the U-shaped BCS gap interfere resulting in slope breaks of the gap walls and the in-gap states in the density of states. Various consequences of an incomplete self-consistency are demonstrated.Wed, 21 Dec 2016 15:21:31 +0100Vortex spectroscopy in the vortex glass: A real-space numerical approachhttps://archive-ouverte.unige.ch/unige:89667https://archive-ouverte.unige.ch/unige:89667A method is presented to solve the Bogoliubov-de Gennes equations with arbitrary distributions of vortices. The real-space Green's function approach based on Chebyshev polynomials is complemented by a gauge transformation which allows one to treat finite as well as infinite, ordered as well as disordered vortex configurations. This tool gives unprecedented access to vortex lattices at very low magnetic fields and glassy phases. After describing in detail the method and its implementation, we use it to address a series of problems related to d-wave superconductivity on the square lattice. We first study the continuity of the vortex-core energy spectrum and its evolution from the quantum regime to the semiclassical limit; we investigate the effect of the band structure on the vortex by following the self-consistent solution through a Lifshitz transition; we then study the evolution from the vortex lattice to the isolated-vortex limit with decreasing field and show that a new emerging length scale controls this transition; finally, we perform a statistical study of the vortex-core local density of states in the presence of positional disorder in the vortex lattice. The calculations reveal a number of qualitative differences between the properties of vortices in the quantum and semiclassical regimes.Wed, 30 Nov 2016 17:11:50 +0100Rise and fall of shape resonances in thin films of BCS superconductorshttps://archive-ouverte.unige.ch/unige:87802https://archive-ouverte.unige.ch/unige:87802The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature Tc. Previous calculations have reported large discontinuities of Tc when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that Tc is a continuous function of the film thickness. We also find that Tc is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement Tc increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass.Tue, 27 Sep 2016 12:58:39 +0200BCS superconductivity near the band edge: Exact results for one and several bandshttps://archive-ouverte.unige.ch/unige:85393https://archive-ouverte.unige.ch/unige:85393We revisit the problem of a BCS superconductor in the regime where the Fermi energy is smaller than the Debye energy. This regime is relevant for low-density superconductors such as SrTiO3 that are not in the BEC limit, as well as in the problem of “shape resonances” associated with the confinement of a three-dimensional superconductor. While the problem is not new, exact results were lacking in the low-density limit. In two dimensions, we find that the initial rise of the pairing temperature Tc at low density n is nonanalytic and faster than any power of n. In three dimensions, we also find that Tc is nonanalytic, but starts with zero slope at weak coupling and infinite slope at strong coupling. Self-consistent treatment of the chemical potential and energy dependence of the density of states are crucial ingredients to obtain these results. We also present exact results for multiband systems and confirm our analytical expressions by numerical simulations.Wed, 20 Jul 2016 16:01:59 +0200Revisiting the vortex-core tunnelling spectroscopy in YBa2Cu3O7−δhttps://archive-ouverte.unige.ch/unige:82277https://archive-ouverte.unige.ch/unige:82277The observation by scanning tunnelling spectroscopy (STS) of Abrikosov vortex cores in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-delta}$ (Y123) has revealed a robust pair of electron-hole symmetric states at finite subgap energy. Their interpretation remains an open question because theory predicts a different signature in the vortex cores, characterised by a strong zero-bias conductance peak. We present STS data on very homogeneous Y123 at 0.4~K revealing that the subgap features do not belong to vortices: they are actually observed everywhere along the surface with high spatial and energy reproducibility, even in the absence of magnetic field. Detailed analysis and modelling show that these states remain unpaired in the superconducting phase and belong to an incoherent channel which contributes to the tunnelling signal in parallel with the superconducting density of states.Mon, 04 Apr 2016 09:51:10 +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 +0100Bogoliubov quasiparticles coupled to the antiferromagnetic spin mode in a vortex corehttps://archive-ouverte.unige.ch/unige:78647https://archive-ouverte.unige.ch/unige:78647In copper- and iron-based unconventional superconductors, the Bogoliubov quasiparticles interact with a spin resonance at momentum $(pi,pi)$. This interaction is revealed by specific signatures in the quasiparticle spectroscopies, like kinks in photoemission and dips in tunneling. We study these signatures, as they appear inside and around a vortex core in the local density of states (LDOS), a property accessible experimentally by scanning tunneling spectroscopy. Our model retains the whole nonlocal structure of the self-energy in space and time and is therefore not amenable to a Hamiltonian treatment using Bogoliubov--de Gennes equations. The interaction with the spin resonance does not suppress the zero-bias peak at the vortex center, although it reduces its spectral weight; neither does it smear out the vortex LDOS, but rather it adds structure to it. Some of the signatures we find may have been already measured in FeSe, but remained unnoticed. We compare the LDOS as a function of both energy and position with and without coupling to the spin resonance and observe, in particular, that the quasiparticle interference patterns around the vortex are strongly damped by the coupling. We study in detail the transfer of spectral weight induced both locally and globally by the interaction and also by the formation of the vortex. Finally, we introduce a new way of imaging the quasiparticles in real space, which combines locality and momentum-space sensitivity. This approach allows one to access quasiparticle properties that are not contained in the LDOS.Tue, 15 Dec 2015 13:38:32 +0100Second-order response theory of radio-frequency spectroscopy for cold atomshttps://archive-ouverte.unige.ch/unige:74294https://archive-ouverte.unige.ch/unige:74294We present a theoretical description of the radio-frequency (rf) spectroscopy of fermionic atomic gases, based on the second-order response theory at finite temperature. This approach takes into account the energy resolution due to the envelope of the rf pulse. For a noninteracting final state, the momentum- and energy-resolved rf intensity depends on the fermion spectral function and pulse envelope. The contributions due to interactions in the final state can be classified by means of diagrams. Using this formalism, as well as the local density approximation in two and three dimensions, we study the interplay of inhomogeneities and Hartree energy in forming the line shape of the rf signal. We show that the effects of inhomogeneities can be minimized by taking advantage of interactions in the final state, and we discuss the most relevant final-state effects at low temperature and density, in particular the effect of a finite lifetime.Thu, 23 Jul 2015 09:54:52 +0200Large modulation of the Shubnikov–de Haas oscillations by the Rashba interaction at the LaAlO3/SrTiO3 interfacehttps://archive-ouverte.unige.ch/unige:42124https://archive-ouverte.unige.ch/unige:42124We investigate the two-dimensional Fermi surface of high-mobility LaAlO3/ SrTiO3 interfaces using Shubnikov-de Haas oscillations. Our analysis of the oscillation pattern underscores the key role played by the Rashba spin–orbit interaction brought about by the breaking of inversion symmetry, as well as the dominant contribution of the heavy dxz/dyz orbitals on electrical transport. We furthermore bring into light the complex evolution of the oscillations with the carrier density, which is tuned by the field effect.Mon, 24 Nov 2014 12:30:11 +0100Optical Response of Sr2RuO4 Reveals Universal Fermi-Liquid Scaling and Quasiparticles Beyond Landau Theoryhttps://archive-ouverte.unige.ch/unige:39703https://archive-ouverte.unige.ch/unige:39703We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr2RuO4 obeys scaling relations for its frequency (ω) and temperature (T) dependence in accordancewithFermi-liquidtheory.Inthethermalrelaxationregime,1/τ~(ħω)^2+(pπkT)^2 withp=2, and ω/T scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling, and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing “resilient” quasiparticle excitations above the Fermi energy.Mon, 25 Aug 2014 13:25:03 +0200A theory of the strain-dependent critical field in Nb3Sn, based on anharmonic phonon generationhttps://archive-ouverte.unige.ch/unige:33264https://archive-ouverte.unige.ch/unige:33264We propose a theory to explain the strain dependence of the critical properties in A15 superconductors. Starting from the strong-coupling formula for the critical temperature, and assuming that the strain sensitivity stems mostly from the electron-phonon alpha^2F function, we link the strain dependence of the critical properties to a widening of alpha^2F. This widening is attributed to the nonlinear generation of phonons, which takes place in the anharmonic deformation potential induced by the strain. Based on the theory of sum- and difference-frequency wave generation in nonlinear media, we obtain an explicit connection between the widening of alpha^2F and the anharmonic energy. The resulting model is fit to experimental datasets for Nb3Sn, and the anharmonic energy extracted from the fits is compared with first-principles calculations.Wed, 15 Jan 2014 14:53:12 +0100Quasiparticle spectra of Abrikosov vortices in a uniform supercurrent flowhttps://archive-ouverte.unige.ch/unige:30754https://archive-ouverte.unige.ch/unige:30754abstract not availableMon, 28 Oct 2013 14:50:24 +0100Strong-coupling analysis of scanning tunneling spectra in Bi2Sr2Ca2Cu3O10+δhttps://archive-ouverte.unige.ch/unige:29036https://archive-ouverte.unige.ch/unige:29036We study a series of spectra measured in the superconducting state of optimally-doped Bi-2223 by scanning tunneling spectroscopy. Each spectrum, as well as the average of spectra presenting the same gap, is fitted using a strong-coupling model taking into account the band structure, the BCS gap, and the interaction of electrons with the spin resonance. After describing our measurements and the main characteristics of the strong-coupling model, we report the whole set of parameters determined from the fits, and we discuss trends as a function of the gap magnitude. We also simulate angle-resolved photoemission spectra, and compare with recent experimental results.Mon, 05 Aug 2013 10:28:39 +0200Spectroscopic evidence for Fermi liquid-like energy and temperature dependence of the relaxation rate in the pseudogap phase of the cuprateshttps://archive-ouverte.unige.ch/unige:27147https://archive-ouverte.unige.ch/unige:27147Cuprate high-Tc superconductors exhibit enigmatic behavior in the nonsuperconducting state. For carrier concentrations near “optimal doping” (with respect to the highest Tcs) the transport and spectroscopic properties are unlike those of a Landau–Fermi liquid. On the Mott-insulating side of the optimal carrier concentration, which corresponds to underdoping, a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface and causes a breakup of the Fermi surface into disconnected nodal and antinodal sectors. Here, we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. The lifetime τ(ω, T) of a quasi-particle depends on its energy ω as well as on the temperature T. For a Fermi liquid, 1/τ(ω, T) is expected to collapse on a universal function proportional to (ℏω)2 + (pπkBT)2. Magneto-transport experiments, which probe the properties in the limit ω = 0, have provided indications for the presence of a T2 dependence of the dc (ω = 0) resistivity of different cuprate materials. However, Fermi liquid behavior is very much about the energy dependence of the lifetime, and this can only be addressed by spectroscopic techniques. Our optical experiments confirm the aforementioned universal ω- and T dependence of 1/τ(ω, T), with p ∼ 1.5. Our data thus provide a piece of evidence in favor of a Fermi liquid-like scenario of the pseudogap phase of the cuprates.Wed, 03 Apr 2013 08:27:17 +0200Modeling scanning tunneling spectra of Bi2Sr2CaCu2O8+δhttps://archive-ouverte.unige.ch/unige:26791https://archive-ouverte.unige.ch/unige:26791Recent angle-resolved photoemission spectroscopy (ARPES) and neutron scattering data have provided ingredients for the interpretation of scanning tunneling spectra on Bi2Sr2CaCu2O8+δ. We analyze the low-temperature tunneling spectra, from oxygen overdoped to underdoped samples, including details about the bilayer splitting and the neutron resonance peak. Two van Hove singularities are identified: the first is integrated in the coherence peaks, the second is heavily broadened at higher binding energy. The shape of the tunneling spectra suggests a strong coupling of the quasiparticles with a collective mode, and a comparison with photoemission shows that the scattering rate in tunneling is an order of magnitude smaller than in ARPES. Finally, the theoretical spectra calculated with an isotropic tunneling matrix element are in better agreement with the experimental data than those obtained with anisotropic matrix elements.Wed, 13 Mar 2013 15:01:23 +0100Vorticity and vortex-core states in type-II superconductorshttps://archive-ouverte.unige.ch/unige:26790https://archive-ouverte.unige.ch/unige:26790The origin of the vortex-core states in s-wave and dx2−y2-wave superconductors is investigated by means of some selected numerical experiments. By relaxing the self-consistency condition in the Bogoliubov–de Gennes equations and tuning the order parameter in the core region, it is shown that the suppression of the superfluid density in the core is not a necessary condition for the core states to form. This excludes “potential well” types of interpretations for the core states. The topological defect in the phase of the order parameter, however, plays a crucial role. This fact is explained by considering the effect of the vortex supercurrent on the Bogoliubov quasiparticles and illustrated by comparing conventional vortices to multiply quantized vortices and vortex-antivortex pairs. The core states are also found to be extremely robust against random disorder of the phase.Wed, 13 Mar 2013 14:58:56 +0100Node-like excitations in superconducting PbMo6S8 probed by scanning tunneling spectroscopyhttps://archive-ouverte.unige.ch/unige:26789https://archive-ouverte.unige.ch/unige:26789We present a scanning tunneling spectroscopy study on the Chevrel phase PbMo6S8, an extreme type-II superconductor with a coherence length only slightly larger than in high-Tc cuprates. Tunneling spectra measured on atomically flat terraces are spatially homogeneous and show well-defined coherence peaks. The low-energy spectral weight, the zero bias conductance, and the temperature dependence of the gap are incompatible with a conventional isotropic s-wave interpretation, revealing the presence of low-energy excitations in the superconducting state. We show that our data are consistent with the presence of nodes in the superconducting gap.Wed, 13 Mar 2013 14:55:09 +0100Hall effect in strongly correlated low-dimensional systemshttps://archive-ouverte.unige.ch/unige:26788https://archive-ouverte.unige.ch/unige:26788We investigate the Hall effect in a quasi-one-dimensional system made of weakly coupled Luttinger liquids at half filling. Using a memory function approach, we compute the Hall coefficient as a function of temperature and frequency in the presence of umklapp scattering. We find a power-law correction to the free-fermion value (band value), with an exponent depending on the Luttinger parameter Kρ. At sufficiently high temperature or frequency, the Hall coefficient approaches the band value.Wed, 13 Mar 2013 14:51:15 +0100Heterovalent interlayers and interface states: An ab initio study of GaAs/Si/GaAs (110) and (100) heterostructureshttps://archive-ouverte.unige.ch/unige:26787https://archive-ouverte.unige.ch/unige:26787We have investigated ab initio the existence of localized states and resonances in abrupt GaAs∕Si∕GaAs (110)- and (100)-oriented heterostructures incorporating one or two monolayers (MLs) of Si, as well as in the fully developed Si∕GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si∕GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band, the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bonding properties and atomic characters, we are able to follow the evolution of the localized states and resonances from the fully developed Si∕GaAs binary junction to the ternary GaAs∕Si∕GaAs (110) systems incorporating two or one ML(s) of Si. This approach also allows us to show the link between the interface states of the (110) and (100) systems. Finally, the conditions for the existence of localized states at the Si∕GaAs (110) interface are discussed based on a Koster-Slater model developed for the interface-state problem.Wed, 13 Mar 2013 14:48:34 +0100Gorkov equations for a pseudogapped high-temperature superconductorhttps://archive-ouverte.unige.ch/unige:26786https://archive-ouverte.unige.ch/unige:26786A phenomenological theory of superconductivity based on the two-body Cooperon propagator is presented. This theory takes the form of a modified Gorkov equation for the Green's function and allows one to model the effect of local superconducting correlations and long-range phase fluctuations on the spectral properties of high-temperature superconductors, both above and below Tc. A model is proposed for the Cooperon propagator, which provides a simple physical picture of the pseudogap phenomenon, as well as insights into the doping dependence of the spectral properties. Numerical calculations of the density of states and spectral functions based on this model are also presented, and compared with the experimental tunneling (STM) and photoemission (ARPES) data. It is found, in particular, that the sharpness of the peaks in the density of states is related to the strength and the range of the superconducting correlations and that the apparent pseudogap in STM and ARPES can be different, although the underlying model is the same.Wed, 13 Mar 2013 14:42:49 +0100Cooperon propagator description of high-temperature superconductivityhttps://archive-ouverte.unige.ch/unige:26785https://archive-ouverte.unige.ch/unige:26785A phenomenological description of the high-Tc superconductors based on the Cooperon propagator is presented. This model allows one to study the effects of local pairing correlations and long-range phase fluctuations on the same footing, both above and below Tc. Based on numerical calculations, it is shown that the two types of correlations contribute to the gap/pseudogap in the single-particle excitation spectra. The concourse of these two effects can induce low energy states, which should be observable in underdoped materials at very low temperature.Wed, 13 Mar 2013 14:41:13 +0100Breakup of the Fermi Surface Near the Mott Transition in Low-Dimensional Systemshttps://archive-ouverte.unige.ch/unige:26784https://archive-ouverte.unige.ch/unige:26784We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping t⊥c2. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like “arcs” in photoemission experiments.Wed, 13 Mar 2013 14:38:48 +0100Schottky barrier heights at polar metal/semiconductor interfaceshttps://archive-ouverte.unige.ch/unige:26783https://archive-ouverte.unige.ch/unige:26783Using a first-principle pseudopotential approach, we have investigated the Schottky barrier heights of abrupt Al/Ge, Al/GaAs, Al/AlAs, and Al/ZnSe (100) junctions, and their dependence on the semiconductor chemical composition and surface termination. A model based on linear-response theory is developed, which provides a simple, yet accurate description of the barrier-height variations with the chemical composition of the semiconductor. The larger barrier values found for the anion-terminated surface than for the cation-terminated surface are explained in terms of the screened charge of the polar semiconductor surface and its image charge at the metal surface. Atomic-scale computations show how the classical image charge concept, valid for charges placed at large distances from the metal, extends to distances shorter than the decay length of the metal-induced-gap states.Wed, 13 Mar 2013 14:37:42 +0100Density of States in High- Tc Superconductor Vorticeshttps://archive-ouverte.unige.ch/unige:26782https://archive-ouverte.unige.ch/unige:26782We calculated the electronic structure of a vortex in a pseudogapped superconductor within a model featuring strong correlations. With increasing strength of the correlations, the BCS core states are suppressed and the spectra inside and outside the core become similar. If the correlations are short range, we find new core states in agreement with the observations in YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ. Our results point to a common phenomenology for these two systems and indicate that normal-state correlations survive below Tc without taking part in the overall phase coherence.Wed, 13 Mar 2013 14:36:21 +0100Scanning tunneling spectroscopy of high-temperature superconductorshttps://archive-ouverte.unige.ch/unige:26781https://archive-ouverte.unige.ch/unige:26781Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.Wed, 13 Mar 2013 14:34:50 +0100Non-Drude universal scaling laws for the optical response of local Fermi liquidshttps://archive-ouverte.unige.ch/unige:26780https://archive-ouverte.unige.ch/unige:26780We investigate the frequency and temperature dependence of the low-energy electron dynamics in a Landau Fermi liquid with a local self-energy. We show that the frequency and temperature dependencies of the optical conductivity obey universal scaling forms, for which explicit analytical expressions are obtained. For the optical conductivity and the associated memory function, we obtain a number of surprising features that differ qualitatively from the Drude model and are universal characteristics of a Fermi liquid. Different physical regimes of scaling are identified, with marked non-Drude features in the regime where ℏω∼kBT. These analytical results for the optical conductivity are compared to numerical calculations for the doped Hubbard model within dynamical mean-field theory. For the “universal” low-energy electrodynamics, we obtain perfect agreement between numerical calculations and analytical scaling laws. Both results show that the optical conductivity displays a non-Drude “foot,” which could be easily mistaken as a signature of breakdown of the Fermi liquid, while it actually is a striking signature of its applicability. The aforementioned scaling laws provide a quantitative tool for the experimental identification and analysis of the Fermi-liquid state using optical spectroscopy, and a powerful method for the identification of alternative states of matter, when applicable.Wed, 13 Mar 2013 14:32:40 +0100Two-Dimensional Fermi Liquid with Attractive Interactionshttps://archive-ouverte.unige.ch/unige:23853https://archive-ouverte.unige.ch/unige:23853We realize and study an attractively interacting two-dimensional Fermi liquid. Using momentum-resolved photoemission spectroscopy, we measure the self-energy, determine the contact parameter of the short-range interaction potential, and find their dependence on the interaction strength. We successfully compare the measurements to a theoretical analysis, properly taking into account the finite temperature, harmonic trap, and the averaging over several two-dimensional gases with different peak densities.Wed, 07 Nov 2012 12:24:27 +0100Tunneling conductance and local density of states in tight-binding junctionshttps://archive-ouverte.unige.ch/unige:23852https://archive-ouverte.unige.ch/unige:23852We study the relationship between the differential conductance and the local density of states in tight-binding tunnel junctions where the junction geometry can be varied between the point-contact and the planar-contact limits. The conductances are found to differ significantly in these two limiting cases. We also examine how the matrix element influences the tunneling characteristics and produces contrast in a simple model of scanning tunneling microscope (STM). Some implications regarding the interpretation of STM spectroscopic data in the cuprates are discussed. The calculations are carried out within the real-space Keldysh formalism.Wed, 07 Nov 2012 12:23:03 +0100First direct observation of the Van Hove singularity in the tunnelling spectra of cuprateshttps://archive-ouverte.unige.ch/unige:23851https://archive-ouverte.unige.ch/unige:23851In two-dimensional (2D) lattices, the electronic levels are unevenly spaced, and the density of states (DOS) displays a logarithmic divergence known as the Van Hove singularity (VHS). This is the case in particular for the layered cuprate superconductors. The scanning tunnelling microscope (STM) probes the DOS, and is therefore the ideal tool to observe the VHS. No STM study of cuprate superconductors has reported such an observation so far giving rise to a debate about the possibility of observing directly the normal state DOS in the tunnelling spectra. In this study, we show for the first time that the VHS is unambiguously observed in STM measurements performed on the cuprate Bi2Sr2CuO6+δ (Bi-2201). Beside closing the debate, our analysis proves the presence of the pseudogap in the overdoped side of the phase diagram of Bi-2201 and discredits the scenario of the pseudogap phase crossing the superconducting dome.Wed, 07 Nov 2012 12:21:37 +0100Scanning tunneling spectroscopy of high Tc cuprateshttps://archive-ouverte.unige.ch/unige:23850https://archive-ouverte.unige.ch/unige:23850Tunneling spectroscopy played a central role in the experimental verification of the microscopic theory of superconductivity in the classical superconductors. In the case of high-temperature superconductors (HTS), the initial attempts to adopt the same approach were hampered by various problems related to the complexity of these materials. The progresses made in synthesizing high quality samples, and the use of scanning tunneling microscopy/spectroscopy (STM/STS) on these compounds allowed to overcome the main difficulties. In this review we present some of the experimental highlights obtained with STM/STS techniques over the last decade. Most of the results confirm the fact that this new class of materials differ noticeably from the conventional BCS superconductors, and provide convincing arguments towards the understanding of the microscopic mechanisms at the origin of high-temperature superconductivityWed, 07 Nov 2012 12:05:29 +0100Multiband Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8https://archive-ouverte.unige.ch/unige:23846https://archive-ouverte.unige.ch/unige:23846Sub-Kelvin scanning tunneling spectroscopy in the Chevrel phases SnMo6S8 and PbMo6S8 reveals two distinct superconducting gaps with Δ1=3 meV, Δ2∼1.0 meV and Δ1=3.1 meV, Δ2∼1.4 meV, respectively. The gap distribution is strongly anisotropic, with Δ2 predominantly seen when scanning across unit-cell steps on the (001) sample surface. The spectra are well fitted by an anisotropic two-band BCS s-wave gap function. Our spectroscopic data are confirmed by electronic heat capacity measurements, which also provide evidence for a twin-gap scenario.Wed, 07 Nov 2012 11:00:12 +0100Levy de Castro et al. Replyhttps://archive-ouverte.unige.ch/unige:23845https://archive-ouverte.unige.ch/unige:23845A Reply to the Comment by Flora Onufrieva and Pierre Pfeuty.Wed, 07 Nov 2012 10:52:48 +0100Tunneling spectra of strongly coupled superconductors: Role of dimensionalityhttps://archive-ouverte.unige.ch/unige:23844https://archive-ouverte.unige.ch/unige:23844We investigate numerically the signatures of collective modes in the tunneling spectra of superconductors. The larger strength of the signatures observed in the high-Tc superconductors, as compared to classical low-Tc materials, is explained by the low dimensionality of these layered compounds. We also show that the strong-coupling structures are dips (zeros in the d2I/dV2 spectrum) in d-wave superconductors, rather than the steps (peaks in d2I/dV2) observed in classical s-wave superconductors. Finally we question the usefulness of effective density of states models for the analysis of tunneling data in d-wave superconductors.Wed, 07 Nov 2012 10:48:43 +0100Imaging the Essential Role of Spin Fluctuations in High-Tc Superconductivityhttps://archive-ouverte.unige.ch/unige:23843https://archive-ouverte.unige.ch/unige:23843We have used scanning tunneling spectroscopy to investigate short-length electronic correlations in three-layer Bi2Sr2Ca2Cu3O10+δ (Bi-2223). We show that the superconducting gap and the energy Ωdip, defined as the difference between the dip minimum and the gap, are both modulated in space following the lattice superstructure and are locally anticorrelated. Based on fits of our data to a microscopic strong-coupling model, we show that Ωdip is an accurate measure of the collective-mode energy in Bi-2223. We conclude that the collective mode responsible for the dip is a local excitation with a doping dependent energy and is most likely the (π, π) spin resonance.Wed, 07 Nov 2012 10:47:09 +0100Hall effect on the triangular latticehttps://archive-ouverte.unige.ch/unige:23842https://archive-ouverte.unige.ch/unige:23842We investigate the high-frequency Hall effect on a two-dimensional triangular lattice with nearest-neighbor hopping and a local Hubbard interaction. The complete temperature and doping dependencies of the high-frequency Hall coefficient RH are evaluated analytically and numerically for small, intermediate, and strong interactions using various approximation schemes. We find that RH follows the semiclassical 1/qn∗ law near T=0 but exhibits a striking T-linear behavior with an interaction- and doping-dependent slope at high temperature. We compare our results with previous theories as well as Hall measurements performed in cobaltates.Wed, 07 Nov 2012 10:45:16 +0100Scanning Tunneling Spectroscopy in the Superconducting State and Vortex Cores of the β-Pyrochlore KOs2O6https://archive-ouverte.unige.ch/unige:23841https://archive-ouverte.unige.ch/unige:23841We performed the first scanning tunneling spectroscopy measurements on the pyrochlore superconductor KOs2O6 (Tc=9.6 K) in both zero magnetic field and the vortex state at several temperatures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31–40 Å, in agreement with the value derived from the upper critical field Hc2. We observe a reduction in size of the vortex cores (and hence the coherence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported.Wed, 07 Nov 2012 10:36:58 +0100Preeminent Role of the Van Hove Singularity in the Strong-Coupling Analysis of Scanning Tunneling Spectroscopy for Two-Dimensional Cuprate Superconductorshttps://archive-ouverte.unige.ch/unige:23840https://archive-ouverte.unige.ch/unige:23840In two dimensions the noninteracting density of states displays a van Hove singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent in three dimensions. We show that due to this VHS the strong-coupling analysis of tunneling spectra in high-Tc superconductors must be reconsidered. Based on a microscopic model which reproduces the experimental data with excellent accuracy, we elucidate the peculiar role played by the VHS in shaping the tunneling spectra, and show that more conventional analysis of strong-coupling effects can lead to severe errors.Wed, 07 Nov 2012 10:31:38 +0100