arXiv subjects

Benoit Gremaud

Publications and source records attributed to Benoit Gremaud.

16 recordsLinked to original sources

arXiv2023

Momentum-space signatures of the Anderson transition in a symplectic, two-dimensional, disordered ultracold gas

We study Anderson Localization in two dimensional (2D) disordered spin-orbit systems described by the Gaussian symplectic ensemble using momentum-space signatures such as the coherent backscattering (CBS) anti-peak, and the coherent forward scattering (CFS) peak. Significantly, these momentum-space features are readily accessible in ultracold atom experiments through absorption imaging after time-of-flight expansion. The critical exponent and mobility edge of the metal-insulator transition are successfully obtained in this model through a finite-time analysis of the CBS width. An anomalous residual diffusion, unique to 2D, is identified at the transition point where the system changes from a metal to an insulator. A spin localization phenomenon is also observed in the deep localized regime.

cond-mat.dis-nn↗

arXiv2020

Magnetoelastic interaction in the two-dimensional magnetic material MnPS$_3$ studied by first principles calculations and Raman experiments

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS$_3$. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm$^{-1}$, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

cond-mat.mes-hall↗

arXiv2010

The superfluid fountain effect in a Bose-Einstein condensate

We consider a simple experimental setup, based on a harmonic confinement, where a Bose-Einstein condensate and a thermal cloud of weakly interacting alkali atoms are trapped in two different vessels connected by a narrow channel. Using the classical field approximation, as described in J. Phys. B 40, R1 (2007) and optimized in Phys. Rev. A 81, 013629 (2010) for an arbitrary trapping potential, we theoretically investigate the analog of the celebrated superfluid helium fountain effect. We show that this thermo-mechanical effect might indeed be observed in this system. By analyzing the dynamics of the system, we are able to identify the superfluid and normal components of the flow as well as to distinguish the condensate fraction from the superfluid component. We show that the superfluid component can easily flow from the colder vessel to the hotter one while the normal component is practically blocked in the latter.

cond-mat.quant-gas↗

arXiv2010

Emergent Spin Liquids in the Hubbard Model on the Anisotropic Honeycomb Lattice

We study the repulsive Hubbard model on an anisotropic honeycomb lattice within a mean-field and a slave-rotor treatment. In addition to the known semi-metallic and band-insulating phases, obtained for very weak interactions, and the anti-ferromagnetic phase at large couplings, various insulating spin-liquid phases develop at intermediate couplings. Whereas some of these spin liquids have gapless spinon excitations, a gapped one occupies a large region of the phase diagram and becomes the predominant phase for large hopping anisotropies. This phase can be understood in terms of weakly-coupled strongly dimerized states.

cond-mat.quant-gas↗

arXiv2010

Topological quantum phase transitions of attractive spinless fermions in a honeycomb lattice

We investigate a spinless Fermi gas trapped in a honeycomb optical lattice with attractive nearest-neighbor interactions. At zero temperature, mean-field theory predicts three quantum phase transitions, two being topological. At low interactions, the system is semi-metallic. Increasing the interaction further, the semi-metal destabilizes into a fully gapped superfluid. At larger interactions, a topological transition occurs and this superfluid phase becomes gapless, with Dirac-like dispersion relations. Finally, increasing again the interaction, a second topological transition occurs and the gapless superfluid is replaced by a different fully gapped superfluid phase. We analyze these different quantum phases as the temperature and the lattice filling are varied.

cond-mat.quant-gas↗

arXiv2009

Ultracold Fermions in a Graphene-Type Optical Lattice

Some important features of the graphene physics can be reproduced by loading ultracold fermionic atoms in a two-dimensional optical lattice with honeycomb symmetry and we address here its experimental feasibility. We analyze in great details the optical lattice generated by the coherent superposition of three coplanar running laser waves with respective angles $2π/3$. The corresponding band structure displays Dirac cones located at the corners of the Brillouin zone and close to half-filling this system is well described by massless Dirac fermions. We characterize their properties by accurately deriving the nearest-neighbor hopping parameter $t_0$ as a function of the optical lattice parameters. Our semi-classical instanton method proves in excellent agreement with an exact numerical diagonalization of the full Hamilton operator in the tight-binding regime. We conclude that the temperature range needed to access the Dirac fermions regime is within experimental reach. We also analyze imperfections in the laser configuration as they lead to optical lattice distortions which affect the Dirac fermions. We show that the Dirac cones do survive up to some critical intensity or angle mismatches which are easily controlled in actual experiments. In the tight-binding regime, we predict, and numerically confirm, that these critical mismatches are inversely proportional to the square-root of the optical potential strength. We also briefly discuss the interesting possibility of fine-tuning the mass of the Dirac fermions by controlling the laser phase in an optical lattice generated by the incoherent superposition of three coplanar independent standing waves with respective angles $2π/3$.

quant-ph↗

arXiv2009

Coherent propagation of waves in random media with weak nonlinearity

We develop a diagrammatic theory for transport of waves in disordered media with weak nonlinearity. We first represent the solution of the nonlinear wave equation as a nonlinear Born series. From this, we construct nonlinear ladder and crossed diagrams for the average wave intensity. Then, we sum up the diagrammatic series completely, i.e. nonperturbatively in the strength of the nonlinearity, and thereby obtain integral equations describing both nonlinear diffusive transport and coherent backscattering of the average intensity. As main result, we find that the nonlinearity significantly influences the magnitude of the coherent backscattering effect. Depending on the type of nonlinearity, coherent backscattering is either enhanced or suppressed, as compared to the linear case.

cond-mat.mes-hall↗

arXiv2009

Enhancement of radiation trapping for quasi-resonant scatterers at low temperature

We present a transport equation for the incoherent propagation of radiation inside a quasi-resonant atomic gas at low temperature. The derivation is based on a generalized Bethe-Salpeter equation taking into account the motion of the atoms. The obtained equation is similar to the radiative transfer equation. It is solved numerically by an original Monte Carlo approach in the case of a slab geometry. The partial frequency redistribution caused by the small velocity of the scatterers make the emitted flux outside the system and the energy density inside the medium to behave differently than in the case of complete frequency redistribution. In particular, the long time dependence of the specific intensity (escape factor) is slightly different from the Holstein prediction.

cond-mat.dis-nn↗

arXiv2008

Speckle instability: coherent effects in nonlinear disordered media

We numerically investigate the properties of speckle patterns formed by nonlinear point scatterers. We show that, in the weak localization regime, dynamical instability appears, eventually leading to chaotic behavior of the system. Analysing the statistical properties of the instability thresholds for different values of the system size and disorder strength, a scaling law is emphasized. The later is also found to govern the smallest decay rate of the linear system, putting thus forward the crucial importance of interference effects. This is also underlined by the fact that coherent backscattering is still observed even in the chaotic regime.

cond-mat.mes-hall↗

arXiv2007

Nonlinear coherent transport of waves in disordered media

We present a diagrammatic theory for coherent backscattering from disordered dilute media in the nonlinear regime. The approach is non-perturbative in the strength of the nonlinearity. We show that the coherent backscattering enhancement factor is strongly affected by the nonlinearity, and corroborate these results by numerical simulations. Our theory can be applied to several physical scenarios like scattering of light in nonlinear Kerr media, or propagation of matter waves in disordered potentials.

cond-mat.mes-hall↗

arXiv2005

Coherent Backscattering of Light with Nonlinear Atomic Scatterers

We study coherent backscattering of a monochromatic laser by a dilute gas of cold two-level atoms in the weakly nonlinear regime. The nonlinear response of the atoms results in a modification of both the average field propagation (nonlinear refractive index) and the scattering events. Using a perturbative approach, the nonlinear effects arise from inelastic two-photon scattering processes. We present a detailed diagrammatic derivation of the elastic and inelastic components of the backscattering signal both for scalar and vectorial photons. Especially, we show that the coherent backscattering phenomenon originates in some cases from the interference between three different scattering amplitudes. This is in marked contrast with the linear regime where it is due to the interference between two different scattering amplitudes. In particular we show that, if elastically scattered photons are filtered out from the photo-detection signal, the nonlinear backscattering enhancement factor exceeds the linear barrier two, consistently with a three-amplitude interference effect.

quant-ph↗

arXiv2004

Bloch oscillations of ultracold atoms: a tool for a metrological determination of $h/m_{Rb}$

We use Bloch oscillations in a horizontal moving standing wave to transfer a large number of photon recoils to atoms with a high efficiency (99.5% per cycle). By measuring the photon recoil of $^{87}Rb$, using velocity selective Raman transitions to select a subrecoil velocity class and to measure the final accelerated velocity class, we have determined $h/m_{Rb}$ with a relative precision of 0.4 ppm. To exploit the high momentum transfer efficiency of our method, we are developing a vertical standing wave set-up. This will allow us to measure $h/m_{Rb}$ better than $10^{-8}$ and hence the fine structure constant $α$ with an uncertainty close to the most accurate value coming from the ($g-2$) determination.

physics.atom-ph↗

arXiv2002

$\hbar$ corrections in semi-classical formula for smooth chaotic dynamics

The validity of semiclassical expansions in the power of $\hbar$ for the quantum Green's function have been extensively tested for billiards systems, but in the case of chaotic dynamics with smooth potential, even if formula are existing, a quantitative comparison is still missing. In this paper, extending the theory developed by Gaspard et al., Adv. Chem. Phys. XC 105 (1995), based on the classical Green's functions, we present an efficient method allowing the calculation of $\hbar$ corrections for the propagator, the quantum Green's function, and their traces. Especially, we show that the previously published expressions for $\hbar$ corrections to the traces are incomplete.

nlin.CD↗

arXiv2001

Quantum Expression of Classical Chaos

Even as we understand for long that the world is quantal and buried in it is classical dynamics which is chaotic, finding eigenfunctions analytically from the the Schroedinger equation has turned out to be a near-impossibility. Here, we discover a class of chaotic quantum systems for which we obtain some analytically exact eigenfunctions in closed form. This paves way to an exact classical and quantum mechanical treatment of chaotic systems. Furthermore, we bring out connections, underlying the discovery, between different areas of physics and mathematics related to universality in fluctuations observed in a wide variety of complex quantum systems.

nlin.CD↗

arXiv1999

Ghost orbits in the diamagnetic hydrogen spectrum using harmonic inversion

The harmonic inversion method is applied in the case of the hydrogen atom in a magnetic field to extract classical information from the quantum photo-ionization cross-section. The study is made close to a saddle-node bifurcation for which the usual semi-classical formulas give diverging contributions. All quantities (actions, stabilities and Maslov indices) for real orbits above the bifurcation and for complex ghost orbits below the bifurcation, are found to be in excellent agreement with the modified semi-classical predictions based on a normal form approach.

chao-dyn↗

arXiv1998

Photo-ionization of the helium atom close to the double ionization threshold: towards the Ericson regime

We calculate the photo-ionization cross-section from the ground state of the helium atom, using the complex rotation method and diagonalization of sparse matrices. This produces directly the positions and widths of the doubly excited 1Po resonances together with the photo-ionization cross-section. Our calculations up to the N=9 threshold are in perfect agreement with recent experimental data and show the transition from a regular structure at low energy to a chaotic one at high energy, where various resonances strongly overlap.

chao-dyn↗