arXiv subjects

Antoine Georges

Publications and source records attributed to Antoine Georges.

At least 127 records · Page 7Linked to original sources

arXiv2014

Theory of nonlinear phononics for coherent light-control of solids

We present a microscopic theory for ultrafast control of solids with high-intensity terahertz frequency optical pulses. When resonant with selected infrared-active vibrations, these pulses transiently modify the crystal structure and lead to new collective electronic properties. The theory predicts the dynamical path taken by the crystal lattice using first-principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. Two classes of dynamics are identified. In the perturbative regime, displacements along the normal mode coordinate of symmetry-preserving Raman active modes can be achieved by cubic anharmonicities. This explains the light-induced insulator-to-metal transition reported experimentally in manganites. We predict a regime in which ultrafast instabilities that break crystal symmetry can be induced. This nonperturbative effect involves a quartic anharmonic coupling and occurs above a critical threshold, below which the nonlinear dynamics of the driven mode displays softening and dynamical stabilization.

cond-mat.str-el↗

arXiv2014

Ground States of a Bose-Hubbard Ladder in an Artificial Magnetic Field: Field-Theoretical Approach

We consider a Bose-Hubbard ladder subject to an artificial magnetic flux and discuss its different ground states, their physical properties, and the quantum phase transitions between them. A low-energy effective field theory is derived, in the two distinct regimes of a small and large magnetic flux, using a bosonization technique starting from the weak-coupling limit. Based on this effective field theory, the ground-state phase diagram at a filling of one particle per site is investigated for a small flux and for a flux equal to $π$ per plaquette. For $π$-flux, this analysis reveals a tricritical point which has been overlooked in previous studies. In addition, the Mott insulating state at a small magnetic flux is found to display Meissner currents.

cond-mat.quant-gas↗

arXiv2014

Peltier cooling of fermionic quantum gases

We propose a cooling scheme for fermionic quantum gases, based on the principles of the Peltier thermoelectric effect and energy filtering. The system to be cooled is connected to another harmonically trapped gas acting as a reservoir. The cooling is achieved by two simultaneous processes: (i) the system is evaporatively cooled and (ii) cold fermions from deep below the Fermi surface of the reservoir are injected below the Fermi level of the system, in order to fill the 'holes' in the energy distribution. This is achieved by a suitable energy dependence of the transmission coefficient connecting the system to the reservoir. The two processes can be viewed as simultaneous evaporative cooling of particles and holes. We show that both a significantly lower entropy per particle and faster cooling rate can be achieved than by using only evaporative cooling.

cond-mat.quant-gas↗

arXiv2013

Dynamical Mean Field Approximation Applied to Quantum Field Theory

We apply the Dynamical Mean Field (DMFT) approximation to the real, scalar phi^4 quantum field theory. By comparing to lattice Monte Carlo calculations, perturbation theory and standard mean field theory, we test the quality of the approximation in two, three, four and five dimensions. The quantities considered in these tests are the critical coupling for the transition to the ordered phase and the associated critical exponents nu and beta. We also map out the phase diagram in four dimensions. In two and three dimensions, DMFT incorrectly predicts a first order phase transition for all bare quartic couplings, which is problematic, because the second order nature of the phase transition of lattice phi^4-theory is crucial for taking the continuum limit. Nevertheless, by extrapolating the behaviour away from the phase transition, one can obtain critical couplings and critical exponents. They differ from those of mean field theory and are much closer to the correct values. In four dimensions the transition is second order for small quartic couplings and turns weakly first order as the coupling increases beyond a tricritical value. In dimensions five and higher, DMFT gives qualitatively correct results, predicts reasonable values for the critical exponents and considerably more accurate critical couplings than standard mean field theory. The approximation works best for small values of the quartic coupling. We investigate the change from first to second order transition in the local limit of DMFT which is computationally much less intensive. We also discuss technical issues related to the convergence of the non-linear self-consistency equation solver and the solution of the effective single-site model using Fourier-space Monte Carlo updates in the presence of a phi^4-interaction.

hep-lat↗

arXiv2013

Emergence of glass-like dynamics for dissipative and strongly interacting bosons

We study the dynamics of a strongly interacting bosonic quantum gas in an optical lattice potential under the effect of a dissipative environment. We show that the interplay between the dissipative process and the Hamiltonian evolution leads to an unconventional dynamical behavior of local number fluctuations. In particular we show, both analytically and numerically, the emergence of an anomalous diffusive evolution in configuration space at short times and, at long times, an unconventional dynamics dominated by rare events. Such rare events, common in disordered and frustrated systems, are due here to strong interactions. This complex two-stage dynamics reveals information on the level structure of the strongly interacting gas.

cond-mat.quant-gas↗

arXiv2013

Probing thermoelectric transport with cold atoms

We propose experimental protocols to reveal thermoelectric and thermal effects in the transport properties of ultracold fermionic atoms, using the two-terminal setup recently realized at ETH. We show in particular that, for two reservoirs having equal particle numbers but different temperatures initially, the observation of a transient particle number imbalance during equilibration is a direct evidence of thermoelectric (off-diagonal) transport coefficients. This is a time-dependent analogue of the Seebeck effect, and a corresponding analogue of the Peltier effect can be proposed. We reveal that in addition to the thermoelectric coupling of the constriction a thermoelectric coupling also arises due to the finite dilatation coefficient of the reservoirs. We present a theoretical analysis of the protocols, and assess their feasibility by estimating the corresponding temperature and particle number imbalances in realistic current experimental conditions.

cond-mat.quant-gas↗

arXiv2013

Strongly correlated bosons and fermions in optical lattices

These lectures are an introduction to the physics of strongly correlated fermions and bosons. They are specially targeted for the experimental realizations that have been provided by cold atomic gases in optical lattices.

cond-mat.quant-gas↗

arXiv2013

A thermoelectric heat engine with ultracold atoms

Thermoelectric effects, such as the generation of a particle current by a temperature gradient, have their origin in a reversible coupling between heat and particle flows. These effects are fundamental probes for materials and have applications to cooling and power generation. Here we demonstrate thermoelectricity in a fermionic cold atoms channel, ballistic or diffusive, connected to two reservoirs. We show that the magnitude of the effect and the efficiency of energy conversion can be optimized by controlling the geometry or disorder strength. Our observations are in quantitative agreement with a theoretical model based on the Landauer-Bu ttiker formalism. Our device provides a controllable model-system to explore mechanisms of energy conversion and realizes a cold atom based heat engine.

cond-mat.quant-gas↗

arXiv2013

Theoretical prediction and spectroscopic fingerprints of an orbital transition in CeCu2Si2

We show that the heavy-fermion compound CeCu2Si2 undergoes a transition between two regimes dominated by different crystal-field states. At low pressure P and low temperature T the Ce 4f electron resides in the atomic crystal-field ground state, while at high P or T the electron occupancy and spectral weight is transferred to an excited crystal-field level that hybridizes more strongly with itinerant states. These findings result from first-principles dynamical-mean-field-theory calculations. We predict experimental signatures of this orbital transition in X-ray spectroscopy. The corresponding fluctuations may be responsible for the second high-pressure superconducting dome observed in this and similar materials.

cond-mat.str-el↗

arXiv2013

Non-Drude universal scaling laws for the optical response of local Fermi liquids

We 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 hbar omega ~ kB T. 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.

cond-mat.str-el↗

arXiv2013

How bad metals turn good: spectroscopic signatures of resilient quasiparticles

We investigate transport in strongly correlated metals. Within dynamical mean-field theory, we calculate the resistivity, thermopower, optical conductivity and thermodynamic properties of a hole-doped Mott insulator. Two well-separated temperature scales are identified: T_FL below which Landau Fermi liquid behavior applies, and T_MIR above which the resistivity exceeds the Mott-Ioffe-Regel value and `bad-metal' behavior is found. We show that quasiparticle excitations remain well-defined above T_FL and dominate transport throughout the intermediate regime T_FL < T_MIR. The lifetime of these `resilient quasiparticles' is longer for electron-like excitations, and this pronounced particle-hole asymmetry has important consequences for the thermopower. The crossover into the bad-metal regime corresponds to the disappearance of these excitations, and has clear signatures in optical spectroscopy.

cond-mat.str-el↗

arXiv2012

Quantum oscillations in ultracold Fermi gases : realizations with rotating gases or artificial gauge fields

We consider the angular momentum of a harmonically trapped, noninteracting Fermi gas subject to either rotation or to an artificial gauge field. The angular momentum of the gas is shown to display oscillations as a function of the particle number or chemical potential. This phenomenon is analogous to the de Haas - van Alphen oscillations of the magnetization in the solid-state context. However, key differences exist between the solid-state and ultracold atomic gases that we point out and analyze. We explore the dependence of the visibility of these oscillations on the physical parameters and propose two experimental protocols for their observation. Due to the very strong dependence of the amplitude of the oscillations on temperature, we propose their use as a sensitive thermometer for Fermi gases in the low temperature regime.

cond-mat.quant-gas↗

arXiv2012

Dissipative quantum systems: from two to many atoms

We study the dynamics of bosonic atoms in a double well potential under the influence of dissipation. The main effect of dissipation is to destroy quantum coherence and to drive the system towards a unique steady state. We study how the atom-atom interaction affects the decoherence process. We use a systematic approach considering different atomic densities. We show that, for two atoms, the interaction already strongly suppresses decoherence: a phenomenon we refer to as "interaction impeded decoherence". For many atoms, thanks to the increased complexity of the system, the nature of the decoherence process is dramatically altered giving rise to an algebraic instead of exponential decay.

cond-mat.quant-gas↗

arXiv2012

Interaction-induced impeding of decoherence and anomalous diffusion

We study how the interplay of dissipation and interactions affects the dynamics of a bosonic many-body quantum system. In the presence of both dissipation and strongly repulsive interactions, observables such as the coherence and the compressibility display three dynamical regimes: an initial exponential variation followed by a power-law regime and finally a slow exponential convergence to their asymptotic values corresponding to the infinite temperature state. These very long-time scales arise as dissipation forces the population of states disfavored by interactions. The long-time, strong coupling dynamics are understood by performing a mapping onto a classical diffusion process displaying non-Brownian behavior. While both dissipation and strong interactions tend to suppress coherence when acting separately, we find that strong interaction impedes the decoherence process generated by the dissipation.

quant-ph↗

arXiv2012

Strong electronic correlations from Hund's coupling

Strong electronic correlations are often associated with the proximity of a Mott insulating state. In recent years however, it has become increasingly clear that the Hund's rule coupling (intra-atomic exchange) is responsible for strong correlations in multi-orbital metallic materials which are not close to a Mott insulator. Hund's coupling has two effects: it influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi-liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. The theoretical understanding and physical properties of these `Hund's metals' are reviewed, together with the relevance of this concept to transition-metal oxides of the 3d, and especially 4d series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).

cond-mat.str-el↗

arXiv2011

Thinking locally: reflections on Dynamical Mean-Field Theory from a high-temperature/high energy perspective

When spatial correlations are short-range, the physics of strongly correlated systems is controlled by local quantum fluctuations. In those regimes, Dynamical Mean-Field Theory can be viewed as a `compass' which provides guidance on the relevant degrees of freedom and their effective dynamics over intermediate energy scales. These intermediate energy scales and associated crossovers play a crucial role in the physics of strongly correlated materials.

cond-mat.str-el↗

arXiv2011

Strong correlations enhanced by charge-ordering in highly doped cobaltates

We present an explanation for the puzzling spectral and transport properties of layered cobaltates close to the band-insulator limit, which relies on the key effect of charge ordering. Blocking a significant fraction of the lattice sites deeply modifies the electronic structure in a way that is shown to be quantitatively consistent with photoemission experiments. It also makes the system highly sensitive to interactions (especially to intersite ones), hence accounting for the strong correlations effects observed in this regime, such as the high effective mass and quasiparticle scattering rate. These conclusions are supported by a theoretical study of an extended Hubbard model with a realistic band structure on an effective kagomè lattice.

cond-mat.str-el↗

arXiv2011

Importance of electronic correlations for structural and magnetic properties of the iron pnictide superconductor LaFeAsO

We present calculations of structural and magnetic properties of the iron-pnictide superconductor LaFeAsO including electron-electron correlations. For this purpose we apply a fully charge self-consistent combination of Density-Functional Theory with the Dynamical Mean-Field theory, allowing for the calculation of total energies. We find that the inclusion of correlation effects gives a good agreement of the Arsenic z position with experimental data even in the paramagnetic (high-temperature) phase. Going to low temperatures, we study the formation of the ordered moment in the striped spin-density-wave phase, yielding an ordered moment of about 0.60, again in good agreement with experiments. This shows that the inclusion of correlation effects improves both structural and magnetic properties of LaFeAsO at the same time.

cond-mat.str-el↗