SEARCH · Searcharxiv

Results for “cond-mat.quant-gas”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16Linked to original sources

arXiv2024

Universality of Efimov states in highly mass-imbalanced cold-atom mixtures with van der Waals and dipole interactions

We study three-body systems in a mass-imbalanced two-component cold-atom mixture, and we investigate the three-body parameter of their Efimov states for both bosonic and fermionic systems, with a major focus on the Er-Er-Li Efimov states. For a system interacting solely via van der Waals interactions, the van der Waals universality of the three-body parameter is analytically derived using the quantum defect theory. With the addition of a perturbative dipole interaction between the heavy atoms, the three-body parameters of the bosonic and fermionic Efimov states are found to behave differently. When the dipole interaction is as strong as the van der Waals interaction, corresponding to realistic Er-Er-Li Efimov states, we show that the van der Waals universality persists once the effects of the non-perturbative dipole interaction are renormalized into the s-wave and p-wave scattering parameters between the heavy atoms. For a dipole interaction much stronger than the van der Waals interaction, we find that the universality of the Efimov states can be alternatively characterized by a quasi-one-dimensional scattering parameter due to a strong anisotropic deformation of the Efimov wavefunctions. Our work thus clarifies the interplay of isotropic and anisotropic forces in the universality of the Efimov states. Based on the renormalized van der Waals universality, the three-body parameter is estimated for specific isotopes of Er-Li cold-atom mixtures.

cond-mat.quant-gas↗

arXiv2011

Spontaneous symmetry breaking in Schrödinger lattices with two nonlinear sites

We introduce discrete systems in the form of straight (infinite) and ring-shaped chains, with two symmetrically placed nonlinear sites. The systems can be implemented in nonlinear optics (as waveguiding arrays) and BEC (by means of an optical lattice). A full set of exact analytical solutions for symmetric, asymmetric, and antisymmetric localized modes is found, and their stability is investigated in a numerical form. The symmetry-breaking bifurcation (SBB), through which the asymmetric modes emerge from the symmetric ones, is found to be of the subcritical type. It is transformed into a supercritical bifurcation if the nonlinearity is localized in relatively broad domains around two central sites, and also in the ring of a small size, i.e., in effectively nonlocal settings. The family of antisymmetric modes does not undergo bifurcations, and features both stable and unstable portions. The evolution of unstable localized modes is investigated by means of direct simulations. In particular, unstable asymmetric states, which exist in the case of the subcritical bifurcation, give rise to breathers oscillating between the nonlinear sites, thus restoring an effective dynamical symmetry between them.

nlin.PS↗

arXiv2011

Stability of nonstationary states of spin-1 Bose-Einstein condensates

The stability of nonstationary states of homogeneous spin-1 Bose-Einstein condensates is studied by performing Bogoliubov analysis in a frame of reference where the state is stationary. In particular, the effect of an external magnetic field is examined. It is found that a nonzero magnetic field introduces instability in a $^{23}$Na condensate. The wavelengths of this instability can be controlled by tuning the strength of the magnetic field. In a $^{87}$Rb condensate this instability is present already at zero magnetic field. Furthermore, an analytical bound for the size of a stable condensate is found, and a condition for the validity of the single-mode approximation is presented. Realization of the system in a toroidal trap is discussed and the full time development is simulated.

cond-mat.quant-gas↗

arXiv2012

Merging and alignment of Dirac points in a shaken honeycomb optical lattice

Inspired by the recent creation of the honeycomb optical lattice and the realization of the Mott insulating state in a square lattice by shaking, we study here the shaken honeycomb optical lattice. For a periodic shaking of the lattice, a Floquet theory may be applied to derive a time-independent Hamiltonian. In this effective description, the hopping parameters are renormalized by a Bessel function, which depends on the shaking direction, amplitude and frequency. Consequently, the hopping parameters can vanish and even change sign, in an anisotropic manner, thus yielding different band structures. Here, we study the merging and the alignment of Dirac points and dimensional crossovers from the two dimensional system to one dimensional chains and zero dimensional dimers. We also consider next-nearest-neighbor hopping, which breaks the particle-hole symmetry and leads to a metallic phase when it becomes dominant over the nearest-neighbor hopping. Furthermore, we include weak repulsive on-site interactions and find the density profiles for different values of the hopping parameters and interactions, both in a homogeneous system and in the presence of a trapping potential. Our results may be experimentally observed by using momentum-resolved Raman spectroscopy.

cond-mat.quant-gas↗

arXiv2012

Quantum dynamics and entanglement of a 1D Fermi gas released from a trap

We investigate the entanglement properties of the nonequilibrium dynamics of one-dimensional noninteracting Fermi gases released from a trap. The gas of N particles is initially in the ground state within hard-wall or harmonic traps, then it expands after dropping the trap. We compute the time dependence of the von Neumann and Renyi entanglement entropies and the particle fluctuations of spatial intervals around the original trap, in the limit of a large number N of particles. The results for these observables apply to one-dimensional gases of impenetrable bosons as well. We identify different dynamical regimes at small and large times, depending also on the initial condition, whether it is that of a hard-wall or harmonic trap. In particular, we analytically show that the expansion from hard-wall traps is characterized by the asymptotic small-time behavior $S \approx (1/3)\ln(1/t)$ of the von Neumann entanglement entropy, and the relation $S\approx π^2 V/3$ where V is the particle variance, which are analogous to the equilibrium behaviors whose leading logarithms are essentially determined by the corresponding conformal field theory with central charge $c=1$. The time dependence of the entanglement entropy of extended regions during the expansion from harmonic traps shows the remarkable property that it can be expressed as a global time-dependent rescaling of the space dependence of the initial equilibrium entanglement entropy.

cond-mat.stat-mech↗

arXiv2011

Quantum Walk of Two Interacting Bosons

We study the effect of interactions on the bosonic two-particle quantum walk and its corresponding spatial correlations. The combined effect of interactions and Hanbury-Brown Twiss interference results in unique spatial correlations which depend on the strength of the interaction, but not on its sign. The results are explained in light of the two-particle spectrum and the physics of attractively and repulsively bound pairs. We experimentally measure the weak interaction limit of these effects in nonlinear photonic lattices. Finally, we discuss an experimental approach to observe the strong interaction limit using single atoms in optical lattices.

quant-ph↗

arXiv2012

Numerical Computation of Dynamically Important Excited States of Many-Body Systems

We present an extension of the time-dependent Density Matrix Renormalization Group (t-DMRG), also known as Time Evolving Block Decimation algorithm (TEBD), allowing for the computation of dynamically important excited states of one-dimensional many-body systems. We show its practical use for analyzing the dynamical properties and excitations of the Bose-Hubbard model describing ultracold atoms loaded in an optical lattice from a Bose-Einstein condensate. This allows for a deeper understanding of nonadiabaticity in experimental realizations of insulating phases.

cond-mat.quant-gas↗

arXiv2012

Energy level statistics of interacting trapped bosons

It is an well established fact that statistical properties of energy level spectra are the most efficient tool to characterize nonintegrable quantum systems. The study of statistical properties and spectral fluctuation in the interacting many boson systems have developed a new interest in this direction. Specially we are interested in the weakly interacting trapped bosons in the context of Bose-Einstein condensation (BEC) as the energy spectrum shows a transition from the collective to single particle nature with the increase in the number of levels. However this has received less attention as it is believed that the system may exhibit Poisson like fluctuations due to the existence of external harmonic trap. Here we compute numerically the energy levels of the zero-temperature many-boson systems which are weakly interacting through the van der Waals potential and are in the 3D confined harmonic potential. We study the nearest neighbour spacing distribution and the spectral rigidity by unfolding the spectrum. It is found that increase in number of energy levels for repulsive BEC induces a transition from a Wigner like form displaying level repulsion to Poisson distribution for P(s). It does not follow the GOE prediction. For repulsive interaction, the lower levels are correlated and manifest level repulsion. For intermediate levels P (s) shows mixed statistic which clearly signifies the existence of two energy scales: external trap and interatomic interaction. Whereas for very high levels the trapping potential dominates, genarating Poisson distribution. Comparison with mean-field results for lower levels are also presented. For attractive BEC near the critical point we observe the Shrielman like peak near s=0 which signifies the presence of large number of quasi-degenerate states.

cond-mat.quant-gas↗

arXiv2012

Quantum Bose and Fermi gases with large negative scattering length in the 2-body S-matrix approximation

We study both Bose and Fermi gases at finite temperature and density in an approximation that sums an infinite number of many body processes that are reducible to 2-body scatterings. This is done for arbitrary negative scattering length, which interpolates between the ideal and unitary gas limits. In the unitary limit, we compute the first four virial coefficients within our approximation. The second virial coefficient is exact, and we extend the previously known result for fermions to bosons, and also for both bosons and fermions for the upper branch on the other side of unitarity (infinitely large positive scattering length). Assuming bosons can exist in a meta-stable state before undergoing mechanical collapse, we map out the critical temperatures for strongly coupled Bose-Einstein condensation as a function of scattering length.

cond-mat.quant-gas↗

arXiv2012

Zero density limit extrapolation of the superfluid transition temperature in a unitary atomic Fermi gas on a lattice

The superfluid transition temperature $T_c$ of a unitary Fermi gas on a three-dimensional isotropic lattice with an attractive on-site interaction is investigated as a function of density $n$, from half filling down to $5.0\times 10^{-7}$ per unit cell, using a pairing fluctuation theory. We show that except at very low densities ($n^{1/3} <0.2$), where $T_c/E_F$ is linear in $n^{1/3}$, $T_c/E_F$ exhibits significant higher order nonlinear dependence on $n^{1/3}$. Therefore, linear extrapolation using results at intermediate densities such as in typical quantum Monte Carlo simulations leads to a significant underestimate of the zero density limit of $T_c/E_F$. Our result, $T_c/E_F=0.256$, at $n=0$ is subject to reduction from particle-hole fluctuations and incoherent single particle self energy corrections.

cond-mat.quant-gas↗

arXiv2012

Symmetry breaking and singularity structure in Bose-Einstein condensates

We determine the trajectories of vortex singularities that arise after a single vortex is broken by a discretely symmetric impulse in the context of Bose-Einstein condensates in a harmonic trap. The dynamics of these singularities are analyzed to determine the form of the imprinted motion. We find that the symmetry-breaking process introduces two effective forces: a repulsive harmonic force that causes the daughter trajectories to be ejected from the parent singularity, and a Magnus force that introduces a torque about the axis of symmetry. For the analytical non-interacting case we find that the parent singularity is reconstructed from the daughter singularities after one period of the trapping frequency. The interactions between singularities in the weakly interacting system do not allow the parent vortex to be reconstructed. Analytic trajectories were compared to the actual minima of the wavefunction, showing less 0.5% error for impulse strength of (v=0.00005). We show that these solutions are valid within the impulse regime for various impulse strengths using numerical integration of the Gross-Pitaevskii equation. We also show that the actual duration of the symmetry breaking potential does not significantly change the dynamics of the system as long as the strength is below (v=0.0005).

cond-mat.quant-gas↗

arXiv2012

Atom-dimer scattering amplitude for fermionic mixtures with different masses: s-wave and p-wave contributions

We study near a Feshbach resonance, as a function of the mass ratio, the fermion-dimer scattering amplitude in fermionic mixtures of two fermion species. When masses are equal the physical situation is known to be quite simple. We show that, when the mass ratio is increased, the situation becomes much more complex. For the s-wave contribution we obtain an analytical solution in the asymptotic limit of very large mass ratio. In this regime the s-wave scattering amplitude displays a large number of zeros, essentially linked to the known large value of the fermion-dimer scattering length in this regime. We find by an exact numerical calculation that a zero is still present for a mass ratio of 15. For the p-wave contribution we make our study below the mass ratio of 8.17, where a fermion-dimer bound state appears. We find that a strong p-wave resonance is present at low energy, due to a virtual bound state, in the fermion-dimer system, which is a forerunner of the real bound state. This resonance becomes prominent in the mass ratio range around the one corresponding to the $^{40}$K - $^6$Li mixtures, much studied experimentally. This resonance should affect a number of physical properties. These include the equation of state of unbalanced mixtures at very low temperature but also the equation of state of balanced mixtures at moderate or high temperature. The frequency and the damping of collective modes should also provide a convenient way to evidence this resonance. Finally it should be possible to modify the effective mass of one of the fermionic species by making use of an optical lattice. This would allow to study the strong dependence of the resonance as a function of the mass ratio of the two fermionic elements. In particular one could check if the virtual bound state is relevant for the instabilities of these mixtures.

cond-mat.quant-gas↗

arXiv2013

Loop Structure Stability of a Double-Well-Lattice BEC

In this work, we consider excited many-body mean-field states of bosons in a double-well optical lattice by investigating stationary Bloch solutions to the non-linear equations of motion. We show that, for any positive interaction strength, a loop structure emerges at the edge of the band structure whose existence is entirely due to interactions. This can be contrasted to the case of a conventional optical (Bravais) lattice where a loop appears only above a critical repulsive interaction strength. Motivated by the possibility of realizing such non-linear Bloch states experimentally, we analyze the collective excitations about these non-linear stationary states and thereby establish conditions for the system's energetic and dynamical stability. We find that there are regimes that are dynamically stable and thus apt to be realized experimentally.

cond-mat.quant-gas↗

arXiv2012

Observation of interspecies Li-Cs Feshbach resonances

We report on the observation of nineteen interspecies Feshbach resonances in an optically trapped ultracold Bose-Fermi mixture of ^{133}Cs and ^{6}Li in the two energetically lowest spin states. We assign the resonances to s- and p-wave molecular channels by a coupled-channels calculation, resulting in an accurate determination of LiCs ground state potentials. Fits of the resonance position based on the undressed Asymptotic Bound State model do not provide the same level of accuracy as the coupled-channels calculation. Several broad s-wave resonances provide prospects to create fermionic LiCs molecules with a large dipole moment via Feshbach association followed by stimulated Raman passage. Two of the s-wave resonances overlap with a zero crossing of the Cs scattering length which offers prospects for the investigation of polarons in an ultracold Li-Cs mixture.

cond-mat.quant-gas↗

arXiv2012

Non-equilibrium dynamics of Ising models with decoherence: an exact solution

The interplay between interactions and decoherence in many-body systems is of fundamental importance in quantum physics: Decoherence can degrade correlations, but can also give rise to a variety of rich dynamical and steady-state behaviors. We obtain an exact analytic solution for the non-equilibrium dynamics of Ising models with arbitrary interactions and subject to the most general form of local Markovian decoherence. Our solution shows that decoherence affects the relaxation of observables more than predicted by single-particle considerations. It also reveals a dynamical phase transition, specifically a Hopf bifurcation, which is absent at the single-particle level. These calculations are applicable to ongoing quantum information and emulation efforts using a variety of atomic, molecular, optical, and solid-state systems.

quant-ph↗

arXiv2013

Self-trapping of Fermi and Bose gases under spatially modulated repulsive nonlinearity and transverse confinement

We show that self-localized ground states can be created in the spin-balanced gas of fermions with repulsion between the spin components, whose strength grows from the center to periphery, in combination with the harmonic-oscillator (HO) trapping potential acting in one or two transverse directions. We also consider the ground state in the non-interacting Fermi gas under the action of the spatially growing tightness of the one- or two-dimensional (1D or 2D) HO confinement. These settings are considered in the framework of the Thomas-Fermi-von Weizsacker (TF-vW) density functional. It is found that the vW correction to the simple TF approximation (the gradient term) is nearly negligible in all situations. The properties of the ground state under the action of the 2D and 1D HO confinement with the tightness growing in the transverse directions is investigated too for the Bose-Einstein condensate (BEC) with the self-repulsive nonlinearity.

cond-mat.quant-gas↗

arXiv2013

Inhomogeneous Fulde-Ferrell superfluidity in spin-orbit coupled atomic Fermi gases

Inhomogeneous superfluidity lies at the heart of many intriguing phenomena in quantum physics. It is believed to play a central role in unconventional organic or heavy-fermion superconductors, chiral quark matter, and neutron star glitches. However, so far even the simplest form of inhomogeneous superfluidity, the Fulde-Ferrell (FF) pairing state with a single centre-of-mass momentum, is not conclusively observed due to the intrinsic complexibility of any realistic Fermi systems in nature. Here we theoretically predict that the controlled setting of ultracold fermionic atoms with synthetic spin-orbit coupling induced by a two-photon Raman process, demonstrated recently in cold-atom laboratories, provides a promising route to realize the long-sought FF superfluidity. At experimentally accessible low temperatures (i.e., $0.05T_{F}$, where $T_{F}$ is the Fermi temperature), the FF superfluid state dominates the phase diagram, in sharp contrast to the conventional case without spin-orbit coupling. We show that the finite centre-of-mass momentum carried by Cooper pairs is directly measurable via momentum-resolved radio-frequency spectroscopy. Our work opens the way to direct observation and characterization of inhomogeneous superfluidity.

cond-mat.quant-gas↗

arXiv2013

High resolution photoassociation spectroscopy of the $^{6}$Li$_2$ $1^{3}Σ_{g}^{+}$ state

We present experimental observations of seven vibrational levels $v'=20-26$ of the $1^{3}Σ_{g}^{+}$ excited state of Li$_2$ molecules by the photoassociation (PA) of a degenerate Fermi gas of $^6$Li atoms. For each vibrational level, we resolve the rotational structure using a Feshbach resonance to enhance the PA rates from p-wave collisions. We also, for the first time, determine the spin-spin and spin-rotation interaction constants for this state. The absolute uncertainty of our measurements is $\pm 0.00002$ cm$^{-1}$ ($\pm 600$ kHz). We use these new data to further refine an analytic potential for this state.

physics.atom-ph↗