Archive ouverte UNIGE | last documents for author 'Corinna Kollath'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Corinna Kollath'engBound 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 +0100Temperature dependence of the NMR spin-lattice relaxation rate for spin-1/2 chainshttps://archive-ouverte.unige.ch/unige:92623https://archive-ouverte.unige.ch/unige:92623We use recent developments in the framework of a time-dependent matrix product state method to compute the nuclear magnetic resonance relaxation rate 1/T1 for spin-1/2 chains under magnetic field and for different Hamiltonians (XXX, XXZ, isotropically dimerized). We compute numerically the temperature dependence of the 1/T1. We consider both gapped and gapless phases, and also the proximity of quantum critical points. At temperatures much lower than the typical exchange energy scale, our results are in excellent agreement with analytical results, such as the ones derived from the Tomonaga-Luttinger liquid (TLL) theory and bosonization, which are valid in this regime. We also cover the regime for which the temperature T is comparable to the exchange coupling. In this case analytical theories are not appropriate, but this regime is relevant for various new compounds with exchange couplings in the range of tens of Kelvin. For the gapped phases, either the fully polarized phase for spin chains or the low-magnetic-field phase for the dimerized systems, we find an exponential decrease in /(kB T ) of the relaxation time and can compute the gap . Close to the quantum critical point our results are in good agreement with the scaling behavior based on the existence of free excitations.Wed, 15 Mar 2017 14:52:45 +0100Numerical study of thermal effects in low dimensional quantum spin systemshttps://archive-ouverte.unige.ch/unige:90530https://archive-ouverte.unige.ch/unige:90530This thesis is composed by three projects in which we investigate thermal effects in low dimensional quantum spin systems. Thanks to recent developments in DMRG/MPS techniques, it is possible to simulate for such systems the dynamics at finite temperature. This allows us in the first two parts to study the temperature dependence of the NMR spin-lattice relaxation rate and of the INS spectra. Numerical calculations are performed for a wide range of temperatures. This represents a bridge between analytical results available for very low and very high temperatures. Interesting deviations from bosonization results valid at low energy, are seen at temperatures comparable to the energy scale of the system. In the third part we study the temperature and magnetic field dependence of the Grüneisen parameter, a valuable quantity for the detection and investigation of quantum phase transitions. Using an effective theory valid close to criticality, we show deviations from the critical behavior.Mon, 19 Dec 2016 14:02:40 +0100Magnetic entropy landscape and Grüneisen parameter of a quantum spin ladderhttps://archive-ouverte.unige.ch/unige:87815https://archive-ouverte.unige.ch/unige:87815We present measurements of the magnetic entropy landscape and Grüneisen parameter of the Cu2+ complex (C5 H12 N)2 CuCl4 in a magnetic field. Our thermodynamic measurements are in very good agreement with a theoretical description by a S = 1/2 Heisenberg ladder model. Due to its excellent experimental access, the compound crosses two quantum critical points in the applied range of the magnetic field, first from a gapped unpolarized state to a critical phase and then to a gapped fully polarized state. This behavior is reflected directly in the magnetic entropy map. Due to the remarkable properties of the magnetic Gru ̈neisen parameter, we are able to discuss the validity of critical behavior with respect to temperature and magnetic field for this model quasi-one-dimensional system.Tue, 27 Sep 2016 13:36:03 +0200Modulation spectroscopy with ultracold fermions in an optical latticehttps://archive-ouverte.unige.ch/unige:36080https://archive-ouverte.unige.ch/unige:36080We propose an experimental setup of ultracold fermions in an optical lattice to determine the pairing gap in a superfluid state and the spin ordering in a Mott-insulating state. The idea is to apply a periodic modulation of the lattice potential and to use the thereby induced double occupancy to probe the system. We show by full time-dependent calculation using the adaptive time-dependent density-matrix renormalization-group method that the position of the peak in the spectrum of the induced double occupancy gives the pairing energy in a superfluid and the interaction energy in a Mott insulator, respectively. In the Mott insulator we relate the spectral weight of the peak to the spin ordering at finite temperature using perturbative calculations.Mon, 28 Apr 2014 16:55:03 +0200Spectroscopy of Ultracold Atoms by Periodic Lattice Modulationshttps://archive-ouverte.unige.ch/unige:36069https://archive-ouverte.unige.ch/unige:36069We present a nonperturbative analysis of a new experimental technique for probing ultracold bosons in an optical lattice by periodic lattice depth modulations. This is done using the time-dependent density-matrix renormalization group method. We find that sharp energy absorption peaks are not unique to the Mott insulating phase at commensurate filling but also exist for superfluids at incommensurate filling. For strong interactions, the peak structure provides an experimental measure of the interaction strength. Moreover, the peak height of the peaks at ℏω≳2U can be employed as a measure of the incommensurability of the system.Mon, 28 Apr 2014 16:52:45 +0200Spin-charge separation in two-component Bose gaseshttps://archive-ouverte.unige.ch/unige:36059https://archive-ouverte.unige.ch/unige:36059We show that one of the key characteristics of interacting one-dimensional electronic quantum systems, the separation of spin and charge, can be observed in a two-component system of bosonic ultracold atoms even close to a competing phase separation regime. To this purpose we determine the real-time evolution of a single particle excitation and the single particle spectral function using density-matrix renormalization group techniques. Due to efficient bosonic cooling and good tunability this setup exhibits very good conditions for observing this strong correlation effect. In anticipation of experimental realizations we calculate the velocities for spin and charge perturbations for a wide range of parameters.Mon, 28 Apr 2014 15:55:40 +0200Scanning tunneling microscopy for ultracold atomshttps://archive-ouverte.unige.ch/unige:36058https://archive-ouverte.unige.ch/unige:36058We propose a versatile experimental probe for cold atomic gases analogous to the scanning tunneling microscope (STM) in condensed matter. This probe uses the coherent coupling of a single particle to the system. Depending on the measurement sequence, our probe allows us to obtain either the local density and spatial density correlations, with a resolution on the nanometer scale, or the single particle correlation function in real time. We discuss applications of this scheme to the various possible phases for a two dimensional Hubbard system of fermions in an optical lattice.Mon, 28 Apr 2014 15:55:08 +0200Field-controlled magnetic order in the quantum spin-ladder system (Hpip)2CuBr4https://archive-ouverte.unige.ch/unige:36045https://archive-ouverte.unige.ch/unige:36045Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This “classical” phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction.Mon, 28 Apr 2014 14:26:05 +0200Thermodynamics of the Spin Luttinger Liquid in a Model Ladder Materialhttps://archive-ouverte.unige.ch/unige:36044https://archive-ouverte.unige.ch/unige:36044The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modeling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the entire phase diagram.Mon, 28 Apr 2014 14:25:10 +0200Controlling Luttinger Liquid Physics in Spin Ladders under a Magnetic Fieldhttps://archive-ouverte.unige.ch/unige:36043https://archive-ouverte.unige.ch/unige:36043We present a 14N nuclear magnetic resonance study of a single crystal of CuBr4(C5H12N)2 (BPCB) consisting of weakly coupled spin-1/2 Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate T−11 at 250 mK and for (ii) the phase transition to a 3D ordered phase occurring below 110 mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and the Luttinger liquid model tested in detail over the whole fermion band.Mon, 28 Apr 2014 14:24:31 +0200Quasiperiodic Bose-Hubbard model and localization in one-dimensional cold atomic gaseshttps://archive-ouverte.unige.ch/unige:36039https://archive-ouverte.unige.ch/unige:36039We compute the phase diagram of the one-dimensional Bose-Hubbard model with a quasiperiodic potential by means of the density-matrix renormalization group technique. This model describes the physics of cold atoms loaded in an optical lattice in the presence of a superlattice potential whose wavelength is incommensurate with the main lattice wavelength. After discussing the conditions under which the model can be realized experimentally, the study of the density vs the chemical potential curves for a nontrapped system unveils the existence of gapped phases at incommensurate densities interpreted as incommensurate charge-density-wave phases. Furthermore, a localization transition is known to occur above a critical value of the potential depth V2 in the case of free and hard-core bosons. We extend these results to soft-core bosons for which the phase diagrams at fixed densities display new features compared with the phase diagrams known for random box distribution disorder. In particular, a direct transition from the superfluid phase to the Mott-insulating phase is found at finite V2. Evidence for reentrances of the superfluid phase upon increasing interactions is presented. We finally comment on different ways to probe the emergent quantum phases and most importantly, the existence of a critical value for the localization transition. The latter feature can be investigated by looking at the expansion of the cloud after releasing the trap.Mon, 28 Apr 2014 14:19:15 +0200Excitations in two-component Bose gaseshttps://archive-ouverte.unige.ch/unige:36037https://archive-ouverte.unige.ch/unige:36037In this paper, we study a strongly correlated quantum system that has become amenable to experiment by the advent of ultracold bosonic atoms in optical lattices, a chain of two different bosonic constituents. Excitations in this system are first considered within the framework of bosonization and the Luttinger liquid theory which are applicable if the Luttinger liquid parameters are determined numerically. The occurrence of a bosonic counterpart of fermionic spin–charge separation is signalled by a characteristic two-peak structure in the spectral functions found by dynamical density-matrix renormalization group (DMRG) in good agreement with analytical predictions. Experimentally, single-particle excitations as probed by spectral functions are currently not accessible in cold atoms. Therefore we consider the modifications needed for current experiments, namely the investigation of the real-time evolution of density perturbations instead of single-particle excitations, a slight inequivalence between the two intraspecies interactions in actual experiments, and the presence of a confining trap potential. Using time-dependent DMRG, we show that only quantitative modifications occur. With an eye to the simulation of strongly correlated quantum systems far from equilibrium, we detect a strong dependence of the time-evolution of entanglement entropy on the initial perturbation.Mon, 28 Apr 2014 14:18:01 +0200Dipolar Bosons in a Planar Array of One-Dimensional Tubeshttps://archive-ouverte.unige.ch/unige:36035https://archive-ouverte.unige.ch/unige:36035We investigate bosonic atoms or molecules interacting via dipolar interactions in a planar array of one-dimensional tubes. We consider the situation in which the dipoles are oriented perpendicular to the tubes by an external field. We find various quantum phases reaching from a “sliding Luttinger liquid” phase to a two-dimensional charge density wave ordered phase. Two different kinds of charge density wave order occur: a stripe phase in which the bosons in different tubes are aligned and a checkerboard phase. We further point out how to distinguish the occurring phases experimentally.Mon, 28 Apr 2014 14:16:44 +0200Statics and dynamics of weakly coupled antiferromagnetic spin-1/2 ladders in a magnetic fieldhttps://archive-ouverte.unige.ch/unige:35992https://archive-ouverte.unige.ch/unige:35992We investigate weakly coupled spin-1/2 ladders in a magnetic field. The work is motivated by recent experiments on the compound (5CH12N2)CuBr4 (BPCB). We use a combination of numerical and analytical methods, in particular, the density-matrix renormalization group (DMRG) technique, to explore the phase diagram and the excitation spectra of such a system. We give detailed results on the temperature dependence of the magnetization and the specific heat, and the magnetic-field dependence of the nuclear-magnetic-resonance relaxation rate of single ladders. For coupled ladders, treating the weak interladder coupling within a mean-field or quantum Monte Carlo approach, we compute the transition temperature of triplet condensation and its corresponding antiferromagnetic order parameter. Existing experimental measurements are discussed and compared to our theoretical results. Furthermore, we compute, using time-dependent DMRG, the dynamical correlations of a single spin ladder. Our results allow to describe directly the inelastic neutron scattering cross section up to high energies. We focus on the evolution of the spectra with the magnetic field and compare their behavior for different couplings. The characteristic features of the spectra are interpreted using different analytical approaches such as the mapping onto a spin chain, a Luttinger liquid or onto a t-J model. For values of parameters for which such measurements exist, we compare our results to inelastic neutron scattering experiments on the compound BPCB and find excellent agreement. We make additional predictions for the high-energy part of the spectrum that are potentially testable in future experimentsMon, 28 Apr 2014 10:29:04 +0200Spectral and Thermodynamic Properties of a Strong-Leg Quantum Spin Ladderhttps://archive-ouverte.unige.ch/unige:35936https://archive-ouverte.unige.ch/unige:35936The strong-leg S=1/2 Heisenberg spin ladder system (C7H10N)2CuBr4 is investigated using density matrix renormalization group calculations, inelastic neutron scattering, and bulk magnetothermodynamic measurements. Measurements showed qualitative differences compared to the strong-rung case. A long-lived two-triplon bound state is confirmed to persist across most of the Brillouin zone in a zero field. In applied fields, in the Tomonaga-Luttinger spin-liquid phase, elementary excitations are attractive, rather than repulsive. In the presence of weak interladder interactions, the strong-leg system is considerably more prone to three-dimensional orderingTue, 22 Apr 2014 15:25:30 +0200Electron Spin Resonance Shift in Spin Ladder Compoundshttps://archive-ouverte.unige.ch/unige:35933https://archive-ouverte.unige.ch/unige:35933We analyze the effects of different coupling anisotropies in a spin-1/2 ladder on the electron spin resonance (ESR) shift. Combining a perturbative expression in the anisotropies with density matrix renormalization group computation of the short range correlations at finite temperature, we provide the full temperature and magnetic field evolution of the ESR paramagnetic shift. We show that for well chosen parameters the ESR shift can be in principle used to extract quantitatively the anisotropies and, as an example, discuss the material BPCB.Tue, 22 Apr 2014 15:23:16 +0200Spectrum of a Magnetized Strong-Leg Quantum Spin Ladderhttps://archive-ouverte.unige.ch/unige:35451https://archive-ouverte.unige.ch/unige:35451Inelastic neutron scattering is used to measure the spin excitation spectrum of the Heisenberg S = 1/2 ladder material (C7H10N)2CuBr4 in its entirety, both in the gapped spin liquid and the magnetic field- induced Tomonaga-Luttinger spin liquid regimes. A fundamental change of the spin dynamics is observed between these two regimes. Density matrix renormalization group calculations quantitatively reproduce and help understand the observed commensurate and incommensurate excitations. The results validate long-standing quantum field-theoretical predictions but also test the limits of that approach.Tue, 08 Apr 2014 13:34:45 +0200Symmetric and asymmetric excitations of a strong-leg quantum spin ladderhttps://archive-ouverte.unige.ch/unige:35449https://archive-ouverte.unige.ch/unige:35449The zero-field excitation spectrum of the strong-leg spin ladder (C7H10N)2CuBr4 is studied with a neutron time-of-flight technique. The spectrum is decomposed into its symmetric and asymmetric parts with respect to the rung momentum and compared with theoretical results obtained by the density matrix renormalization group method. Additionally, the calculated dynamical correlations are shown for a wide range of rung and leg coupling ratios in order to point out the evolution of arising excitations, as, e.g., of the two-magnon bound state from the strong to the weak coupling limit.Tue, 08 Apr 2014 13:27:56 +0200Spin ladders and quantum simulators for Tomonaga–Luttinger liquidshttps://archive-ouverte.unige.ch/unige:35448https://archive-ouverte.unige.ch/unige:35448Magnetic insulators have proven to be usable as quantum simulators for itinerant interacting quantum systems. In particular the compound (C5H12N)2CuBr4 (for short: (Hpip)2CuBr4) was shown to be a remarkable realization of a Tomonaga–Luttinger liquid (TLL) and allowed us to quantitatively test the TLL theory. Substitution weakly disorders this class of compounds and thus allows us to use them to tackle questions pertaining to the effect of disorder in TLL as well, such as that of the formation of the Bose glass. In this paper we present, as a first step in this direction, a study of the properties of the related (Hpip)2CuCl4 compound. We determine the exchange couplings and compute the temperature and magnetic field dependence of the specific heat, using a finite temperature density matrix renormalization group procedure. Comparison with the measured specific heat at zero magnetic field confirms the exchange parameters and Hamiltonian for the (Hpip)2CuCl4 compound, giving the basis needed to begin studying the disorder effects.Tue, 08 Apr 2014 13:16:46 +0200Competition of spin and charge excitations in the one-dimensional Hubbard modelhttps://archive-ouverte.unige.ch/unige:35410https://archive-ouverte.unige.ch/unige:35410Motivated by recent experiments with ultracold fermionic atoms in optical lattices, we study finite temperature magnetic correlations, as singlet and triplet correlations, and the double occupancy in the one-dimensional Hubbard model. We point out that for intermediate interaction strengths the double occupancy has an intriguing doubly nonmonotonic temperature dependence due to the competition between spin and charge modes, related to the Pomeranchuk effect. Furthermore, we determine properties of magnetic correlations in the temperature regimes relevant for current cold atom experiments and discuss effects of the trap on spatially integrated observables. We estimate the entropy and the temperature reached in the experiment by Greif, Uehlinger, Jotzu, Tarruell, and Esslinger [Science 340, 1307 (2013)].Mon, 07 Apr 2014 14:53:59 +0200Statics and dynamics of weakly coupled antiferromagnetic spin-1/2 ladders in a magnetic fieldhttps://archive-ouverte.unige.ch/unige:17334https://archive-ouverte.unige.ch/unige:17334We investigate weakly coupled spin-1/2 ladders in a magnetic field. The work is motivated by recent experiments on the compound (C5H12N)2CuBr4 (BPCB). We use a combination of numerical and analytical methods, in particular the density matrix renormalization group (DMRG) technique, to explore the phase diagram and the excitation spectra of such a system. We give detailed results on the temperature dependence of the magnetization and the specific heat, and the magnetic field dependence of the nuclear magnetic resonance (NMR) relaxation rate of single ladders. For coupled ladders, treating the weak interladder coupling within a mean field approach, we compute the transition temperature of triplet condensation and its corresponding antiferromagnetic order parameter. Existing xperimental measurements are discussed and compared to our theoretical results. Furthermore we compute, using time dependent DMRG, the dynamical correlations of a single spin ladder. Our results allow to directly describe the inelastic neutron scattering cross section up to high energies. We focus on the evolution of the spectra with the magnetic field and compare their behavior for different couplings. The characteristic features of the spectra are interpreted using different analytical approaches such as the mapping onto a spin chain, a Luttinger liquid (LL) or onto a t-J model. For values of parameters for which such measurements exist, we compare our results to nelastic neutron scattering experiments on the compound BPCB and find xcellent agreement. We make additional predictions for the high energy part of the spectrum that are potentially testable in future experiments.Mon, 07 Nov 2011 08:59:01 +0100