arXiv subjects

T. Giamarchi

Publications and source records attributed to T. Giamarchi.

At least 91 records · Page 5Linked to original sources

arXiv2006

Chemical pressure and hidden one-dimensional behavior in rare earth tri-telluride charge density wave compounds

We report on the first optical measurements of the rare-earth tri-telluride charge-density-wave systems. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density-wave gap excitation, respectively. The data analysis displays a diminishing impact of the charge-density-wave condensate on the electronic properties with decreasing lattice constant across the rare-earth series. We propose a possible mechanism describing this behavior and we suggest the presence of a one-dimensional character in these two-dimensional compounds. We also envisage that interactions and umklapp processes might play a relevant role in the formation of the charge-density-wave state in these compounds.

cond-mat.str-el↗

arXiv2006

Strong correlations in low dimensional systems

I describe in these notes the physical properties of one dimensional interacting quantum particles. In one dimension the combined effects of interactions and quantum fluctuations lead to a radically new physics quite different from the one existing in the higher dimensional world. Although the general physics and concepts are presented, I focuss in these notes on the properties of interacting bosons, with a special emphasis on cold atomic physics in optical lattices. The method of bosonization used to tackle such problems is presented. It is then used to solve two fundamental problems. The first one is the action of a periodic potential, leading to a superfluid to (Mott)-Insulator transition. The second is the action of a random potential that transforms the superfluid in phase localized by disorder, the Bose glass. Some discussion of other interesting extensions of these studies is given.

cond-mat.stat-mech↗

arXiv2006

Interacting Bose gases in quasi-one dimensional optical lattices

We study a two-dimensional array of coupled one-dimensional (1D) tubes of interacting bosons. Such systems can be produced by loading ultra-cold atoms in anisotropic optical lattices. We investigate the effects of coupling the tubes via hopping of the bosons (i.e. Josephson coupling). In the absence of a periodic potential along the tubes, or when such potential is incommensurate with the boson density, the system undergoes a transition from an array of incoherent Tomonaga-Luttinger liquids at high temperature to an anisotropic Bose-Einstein condensate (BEC), at low temperature. We determine the transition temperature and long wave-length excitations of the BEC. In addition to the usual gapless (Goldstone) mode found in standard superfluids, we also find a gapped mode associated with fluctuations of the amplitude of the order parameter. When a commensurate periodic potential is applied along the tubes, they can become 1D Mott insulators. Intertube hopping leads to a deconfinement quantum phase transition between the 1D Mott insulators and the anisotropic BEC. We also take into account the finite size of the gas tubes as realized in actual experiments. We map out the phase diagram of the quasi-1D lattice and compare our results with the existing experiments on such systems.

cond-mat.mes-hall↗

arXiv2006

Deconfinement and cold atoms in optical lattices

Despite the fact that by now one dimensional and three dimensional systems of interacting particles are reasonably well understood, very little is known on how to go from the one dimensional physics to the three dimensional one. This is in particular true in a quasi-one dimensional geometry where the hopping of particles between one dimensional chains or tubes can lead to a dimensional crossover between a Luttinger liquid and more conventional high dimensional states. Such a situation is relevant to many physical systems. Recently cold atoms in optical traps have provided a unique and controllable system in which to investigate this physics. We thus analyze a system made of coupled one dimensional tubes of interacting fermions. We explore the observable consequences, such as the phase diagram for isolated tubes, and the possibility to realize unusual superfluid phases in coupled tubes systems.

cond-mat.str-el↗

arXiv2006

Breakup of the Fermi surface near the Mott transition in low-dimensional systems

We investigate the Mott transition in weakly-coupled one-dimensional (1d) fermionic chains. Using a generalization of Dynamic Mean Field Theory, we show that the Mott gap is suppressed at some critical hopping $t_{\perp}^{c2}$. 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.

cond-mat.str-el↗

arXiv2005

Disorder Effects in Fluctuating One-Dimensional Interacting Systems

The zero temperature localization of interacting electrons coupled to a two-dimensional quenched random potential, and constrained to move on a fluctuating one-dimensional string embedded in the disordered plane, is studied using a perturbative renormalization group approach. In the reference frame of the electrons the impurities are dynamical and their localizing effect is expected to decrease. We consider several models for the string dynamics and find that while the extent of the delocalized regime indeed grows with the degree of string fluctuations, the critical interaction strength, which determines the localization-delocalization transition for infinitesimal disorder,does not change unless the fluctuations are softer than those of a simple elastic string.

cond-mat.str-el↗

arXiv2005

Dynamic Compressibility and aging in Wigner crystals and quantum glasses

We study the non-equilibrium linear response of quantum elastic systems pinned by quenched disorder with Schwinger-Keldysh real-time techniques complemented by a mean-field variational approach. We find (i) a quasi-equilibrium regime in which the analytic continuation from the imaginary-time replica results holds provided the marginality condition is enforced; (ii) an aging regime. The conductivity and compressibility are computed. The latter is found to cross over from its dynamic to static value on a scale set by the waiting time after a quench, an effect which can be probed in experiments in {\it e.g.} Wigner glasses.

cond-mat.dis-nn↗

arXiv2005

Tunnelling in organic superconductors

We discuss the possibility of deciding on the symmetry of the superconducting phase in the organic superconductors (Bechgaard salts), using tunnelling experiments. We first briefly review the properties of organic superconductors, and the possibility to have unconventional (triplet) superconductivity in these systems. We then present a simple scheme for computing the full current-voltage characteristics for tunnelling experiments within the framework of the non-equilibrium Keldysh Green function formalism. This formalism is flexible enough to address different pairing symmetries combined with magnetic fields and finite temperatures at arbitrary bias voltages. We then discuss extensively how to apply these results to probe for the symmetry of the superconducting order parameter.

cond-mat.str-el↗

arXiv2005

Coulombian Disorder in Periodic Systems

We study the effect of unscreened charged impurities on periodic systems. We show that the long wavelength component of the disorder becomes long ranged and dominates static correlation functions. On the other hand, because of the statistical tilt symmetry, dynamical properties such as pinning remain unaffected. As a concrete example, we focus on the effect of Coulombian disorder generated by charged impurities, on 3D charge density waves with non local elasticity. We calculate the x-ray intensity and find that it is identical to the one produced by thermal fluctuations in a disorder-free smectic-A. We discuss the consequences of these results for experiments.

cond-mat.dis-nn↗

arXiv2005

Adiabatic-antiadiabatic crossover in a spin-Peierls chain

We consider an XXZ spin-1/2 chain coupled to optical phonons with non-zero frequency $ω_0$. In the adiabatic limit (small $ω_0$), the chain is expected to spontaneously dimerize and open a spin gap, while the phonons become static. In the antiadiabatic limit (large $ω_0$), phonons are expected to give rise to frustration, so that dimerization and formation of spin-gap are obtained only when the spin-phonon interaction is large enough. We study this crossover using bosonization technique. The effective action is solved both by the Self Consistent Harmonic Approximation (SCHA)and by Renormalization Group (RG) approach starting from a bosonized description. The SCHA allows to analyze the lowfrequency regime and determine the coupling constant associated with the spin-Peierls transition. However, it fails to describe the SU(2) invariant limit. This limit is tackled by the RG. Three regimes are found. For $ω_0\llΔ_s$, where $Δ_s$ is the gap in the static limit $ω_0\to 0$, the system is in the adiabatic regime, and the gap remains of order $Δ_s$. For $ω_0>Δ_s$, the system enters the antiadiabatic regime, and the gap decreases rapidly as $ω_0$ increases. Finally, for $ω_0>ω_{BKT}$, where $ω_{BKT}$ is an increasing function of the spin phonon coupling, the spin gap vanishes via a Berezinskii-Kosterlitz-Thouless transition. Our results are discussed in relation with numerical and experimental studies of spin-Peierls systems.

cond-mat.str-el↗

arXiv2005

Dynamics of disordered elastic systems

We review in these notes some dynamical properties of interfaces in random media submitted to an external force. We focuss in particular to the response to a very small force (so called creep motion) and discuss various theoretical aspects of this problem. We consider in details in particular the case of a one dimensional interface (domain wall).

cond-mat.dis-nn↗

arXiv2005

Keldysh study of point-contact tunneling between superconductors

We revisit the problem of point-contact tunnel junctions involving one-dimensional superconductors and present a simple scheme for computing the full current-voltage characteristics within the framework of the non-equilibrium Keldysh Green function formalism. We address the effects of different pairing symmetries combined with magnetic fields and finite temperatures at arbitrary bias voltages. We discuss extensively the importance of these results for present-day experiments. In particular, we propose ways of measuring the effects found when the two sides of the junction have dissimilar superconducting gaps and when the symmetry of the superconducting states is not the one of spin-singlet pairing. This last point is of relevance for the study of the superconducting state of certain organic materials like the Bechgaard salts and, to some extent, for ruthenium compounds.

cond-mat.supr-con↗

arXiv2005

Spin-1/2 fermions on spin-dependent optical lattices

We study the phase diagram of one dimensional spin one-half fermionic cold atoms. The two ``spin'' species can have different hopping or mass. The phase diagram at equal densities of the species is found to be very rich, Mott insulators as well as superfluids. We also briefly discuss coupling 1D systems together, and some experimental signatures of these phases. In particular, we compute the spin structure factor in the small momentum limit, which should allow the spin gap to be detected.

cond-mat.soft↗

arXiv2004

Domain wall roughness in epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films

The static configuration of ferroelectric domain walls was investigated using atomic force microscopy on epitaxial PbZr0.2Ti0.8O3 thin films. Measurements of domain wall roughness reveal a power law growth of the correlation function of relative displacements B(L) ~ L^(2zeta) with zeta ~ 0.26 at short length scales L, followed by an apparent saturation at large L. In the same films, the dynamic exponent mu was found to be ~ 0.6 from independent measurements of domain wall creep. These results give an effective domain wall dimensionality of d=2.5, in good agreement with theoretical calculations for a two-dimensional elastic interface in the presence of random-bond disorder and long range dipolar interactions.

cond-mat.dis-nn↗

arXiv2004

Nanoscale studies of domain wall motion in epitaxial ferroelectric thin films

Atomic force microscopy was used to investigate ferroelectric switching and nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements of the writing time dependence of domain size reveal a two-step process in which nucleation is followed by radial domain growth. During this growth, the domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric field, characteristic of a creep process. The domain wall motion was analyzed both in the context of stochastic nucleation in a periodic potential as well as the canonical creep motion of an elastic manifold in a disorder potential. The dimensionality of the films suggests that disorder is at the origin of the observed domain wall creep. To investigate the effects of changing the disorder in the films, defects were introduced during crystal growth (a-axis inclusions) or by heavy ion irradiation, producing films with planar and columnar defects, respectively. The presence of these defects was found to significantly decrease the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films and 0.19 - 0.31 in the films containing a-axis inclusions.

cond-mat.dis-nn↗

arXiv2004

Scaling Behavior of the Longitudinal and Transverse Transport in Quasi One-Dimensional Organic Conductors

We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{\prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below T=70 K, the dc resistivity follows a power-law with $ρ_a$ and $ρ_{b^{\prime}}$ proportional to $T^2$ while $ρ_{c^*}\propto T$. Above $T = 100$ K the exponents extracted from the data for the $a$ and $c^*$ axes are consiste1nt with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about 100 K. The $b^\prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{\prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $ρ_a/ρ_{b^{\prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200 K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature.

cond-mat.str-el↗

arXiv2004

Zero field Wigner crystal

A candidate for the insulating phase of the 2D electron gas, seen in high mobility 2D MOSFETS and heterojunctions, is a Wigner crystal pinned by the incipient disorder. With this in view, we study the effect of collective pinning on the physical properties of the crystal formed in zero external magnetic field. We use an elastic theory to describe to long wavelength modes of the crystal. The disorder is treated using the standard Gaussian variational method. We calculate various physical properties of the system with particular emphasis on their density dependence. We revisit the problem of compressibility in this system and present results for the compressibility obtained via effective capacitance measurements in experiments using bilayers. We present results for the dynamical conductivity, surface acoustic wave anomalies and the power radiated by the crystal through phonon emission at finite temperatures.

cond-mat.str-el↗

arXiv2004

Deconfinement in a 2D optical lattice of coupled 1D boson systems

We show that a two-dimensional (2D) array of 1D interacting boson tubes has a deconfinement transition between a 1D Mott insulator and a 3D superfluid for commensurate fillings and a dimensional crossover for the incommensurate case. We determine the phase diagram and excitations of this system and discuss the consequences for Bose condensates loaded in 2D optical lattices.

cond-mat.soft↗