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 145 records · Page 8Linked to original sources

arXiv2014

On the theory of inhomogeneous Bose-Einstein condensation of magnons in yttrium garnet

The Bose-Einstein condensation (BEC) of magnons created by a strong pumping in ferromagnetic thin films of yttrium iron garnet used as systems of finite size is considered analytically. Such a peculiarity, typical for this magnetic material, as the presence of a minimum in the spectrum of spin waves at a finite value of the wave vector is taken into account. The definition of hightemperature BEC is introduced and its characteristics are discussed. A role of boundary conditions for spin variables is analyzed, and it is shown that in the case of free spins on the boundary the magnon lattice can form in the system. The factors responsible for its appearance are discussed.

cond-mat.quant-gas↗

arXiv2014

Optimizing a spin-flip Zeeman slower

We present a design of a spin-flip Zeeman slower controlled by a fast feedback circuit for a sodium Bose-Einstein condensate apparatus. We also demonstrate how the efficiency of the slower strongly depends on its intrinsic parameters, and compare these observations with a few theoretical models. Our findings lead to a simple three-step procedure of designing an optimal Zeeman slower for neutral atoms, especially for those atomic species with high initial velocities, such as sodium and lithium atoms.

cond-mat.quant-gas↗

arXiv2014

Possible critical regions for the ground state of a Bose gas in a spherical trap

With the help of perturbation theory, we study the ground state of a Bose gas in a spherical trap, using the solution in the Thomas--Fermi approximation as the zero approximation. We have found within a certain approximation that, in some very narrow intervals of values of the magnetic field of a trap, the solution deviates strongly from that in the Thomas--Fermi approximation. If the magnetic field is equal to one of such critical values, the size (or even the shape) of the condensate cloud should significantly differ from the Thomas--Fermi one.

cond-mat.quant-gas↗

arXiv2014

Collective behaviour of large number of vortices in the plane

We investigate the dynamics of $N$ point vortices in the plane, in the limit of large $N$. We consider {\em relative equilibria}, which are rigidly rotating lattice-like configurations of vortices. These configurations were observed in several recent experiments [Durkin and Fajans, Phys. Fluids (2000) 12, 289-293; Grzybowski {\em et.al} PRE (2001)64, 011603]. We show that these solutions and their stability are fully characterized via a related {\em aggregation model} which was recently investigated in the context of biological swarms [Fetecau {\em et.al.}, Nonlinearity (2011) 2681; Bertozzi {\em et.al.}, M3AS (2011)]. By utilizing this connection, we give explicit analytic formulae for many of the configurations that have been observed experimentally. These include configurations of vortices of equal strength; the $N+1$ configurations of $N$ vortices of equal strength and one vortex of much higher strength; and more generally, $N+K$ configurations. We also give examples of configurations that have not been studied experimentally, including $N+2$ configurations where $N$ vortices aggregate inside an ellipse. Finally, we introduce an artificial ``damping'' to the vortex dynamics, in an attempt to explain the phenomenon of crystalization that is often observed in real experiments. The diffusion breaks the conservative structure of vortex dynamics so that any initial conditions converge to the lattice-like relative equilibrium.

cond-mat.quant-gas↗

arXiv2014

Quantum Walks of Two Interacting Particles in One Dimension

We investigate continuous-time quantum walks of two indistinguishable particles (bosons, fermions or hard-core bosons) in one-dimensional lattices with nearest-neighbour interactions. The two interacting particles can undergo independent- and/or co-walking dependent on both quantum statistics and interaction strength. We find that two strongly interacting particles may form a bound state and then co-walk like a single composite particle with statistics-dependent propagation speed. Such an effective single-particle picture of co-walking is analytically derived in the context of degenerate perturbation and the analytical results are well consistent with direct numerical simulation. In addition to implementing universal quantum computation and observing bound states, two-particle quantum walks offer a novel route to detecting quantum statistics. Our theoretical results can be examined in experiments of light propagations in two-dimensional waveguide arrays or spin-impurity dynamics of ultracold atoms in one-dimensional optical lattices.

quant-ph↗

arXiv2014

An analytical approach to quantum phase transitions of ultracold Bose systems in bipartite optical lattices: Along the avenue of Green's function

In this paper, we present a generalized Green's function method which can be used to investigate the quantum phase transitions analytically in a systematic way for ultracold Bose systems in bipartite optical lattices. As an example, to the lowest order, we calculate the quantum phase boundaries of the localized states (Mott insulator or charge density wave) of an ultracold Bose system in a $d$-dimensional hypercubic optical lattice with nearest-neighbor repulsive interactions. Due to the inhomogeneity of the system, in the generalized Green's function method, cumuants on different sublattices are calculated separately, together with re-summed Green's function technique, the analytical expression of the phase boundaries of the localized phases in the system is presented.

cond-mat.stat-mech↗

arXiv2014

Ultracold Quantum Gases in Optical Lattice with Topological Defects: Its Physics and Experimental Proposal

In this paper we give a proposal to realize optical lattices with manipulated dislocations and study the physics of ultracold quantum gas on a two-dimensional (2D) optical square lattice with dislocations. In particular, the dislocations may induce fractional topological flux on 2D Peierls optical lattice. These results pave new approach to study the quantum many-body systems on an optical lattice with controllable topological lattice-defects, including the dislocations, topological fluxes.

cond-mat.quant-gas↗

arXiv2014

High-fidelity contact pseudopotentials and p-wave superconductivity

We develop ultratransferable pseudopotentials for the contact interaction that are 100 times more accurate than contemporary approximations. The pseudopotential offers scattering properties very similar to the contact potential, has a smooth profile to accelerate numerics by a factor of up to 4,000, and, for positive scattering lengths, does not support an unwanted bound state. We demonstrate these advantages in a Diffusion Monte Carlo study of fermions with repulsive interactions, delivering the first numerical evidence for the formation of a p-wave superconducting state.

cond-mat.quant-gas↗

arXiv2014

Bose-Einstein condensates in multiple well potentials from a variational path integral

We apply a path integral variational approach to obtain analytical expressions for condensate wave functions of an ultracold, interacting trapped Bose gases. As in many recent experiments, the particles are confined in a 1D or 3D harmonic oscillator trap which is superimposed by a periodic potential in one direction. Based on the first order cumulant expansion with respect to a harmonic trial action, and emplyoing a mean-field approximation, optimal variational parameters are obtained by minimizing an analytical expression for the ground state energy. Our largely analytical results for energy and condensate wave function are in good agreement with fully numerical calculations based on the Gross-Pitaevskii equation.

cond-mat.quant-gas↗

arXiv2014

Phases, transitions, and patterns in the one-dimensional Extended Bose-Hubbard model

We carry out an extensive study of the phase diagram of the extended Bose Hubbard model, with a mean filling of one boson per site, in one dimension by using the density matrix renormalization group and show that it can have Superfluid (SF), Mott-insulator (MI), density-wave (DW) and Haldane-insulator (HI) phases depending on the precise value of filling and how edge states are handled. We show that the critical exponents and central charge for the HI-DW, MI-HI and SF-MI transitions are consistent with those for models in the two-dimensional Ising, Gaussian, and Berezinskii-Kosterlitz-Thouless (BKT) universality classes, respectively; and we suggest that the SF-HI transition may be more exotic than a simple BKT transition.

cond-mat.quant-gas↗

arXiv2014

On the BCS gap equation for superfluid fermionic gases

We present a rigorous derivation of the BCS gap equation for superfluid fermionic gases with point interactions. Our starting point is the BCS energy functional, whose minimizer we investigate in the limit when the range of the interaction potential goes to zero.

math-ph↗

arXiv2014

Lefschetz thimble Monte Carlo for many body theories: application to the repulsive Hubbard model away from half filling

Recently, a new method, based on stochastic integration on the surfaces of steepest descent of the action, was introduced to tackle the sign problem in quantum field theories. We show how this method can be used in many body theories to perform fully non-perturbative calculations of quantum corrections about mean field solutions. We discuss an explicit algorithm for implementing our method, and present results for the repulsive Hubbard model away from half-filling at intermediate temperatures. Our results are consistent with those from the recent state of the art cluster dynamical mean field theory calculations.

cond-mat.str-el↗

arXiv2014

Entropy is in Flux

The science of thermodynamics was put together in the Nineteenth Century to describe large systems in equilibrium. One part of thermodynamics defines entropy for equilibrium systems and demands an ever-increasing entropy for non-equilibrium ones. However, starting with the work of Ludwig Boltzmann in 1872, and continuing to the present day, various models of non-equilibrium behavior have been put together with the specific aim of generalizing the concept of entropy to non-equilibrium situations. This kind of entropy has been termed {\em kinetic entropy} to distinguish it from the thermodynamic variety. Knowledge of kinetic entropy started from Boltzmann's insight about his equation for the time dependence of gaseous systems. In this paper, his result is stated as a definition of kinetic entropy in terms of a local equation for the entropy density. This definition is then applied to Landau's theory of the Fermi liquid thereby giving the kinetic entropy within that theory. Entropy has been defined and used for a wide variety of situations in which a condensed matter system has been allowed to relax for a sufficient period so that the very most rapid fluctuations have been ironed out. One of the broadest applications of non-equilibrium analysis considers quantum degenerate systems using Martin-Schwinger Green's functions\cite{MS} as generalized of Wigner functions, $g^<$ and $g^>$. This paper describes once again these how the quantum kinetic equations for these functions give locally defined conservation laws for mass momentum and energy. In local thermodynamic equilibrium, this kinetic theory enables a reasonable local definition of entropy density. However, when the system is outside of local equilibrium, this definition fails. It is speculated that quantum entanglement is the source of this failure.

cond-mat.stat-mech↗

arXiv2014

Two dispersion curves for a one-dimensional interacting Bose gas under zero boundary conditions

The influence of boundaries and non-point character of interatomic interaction on the dispersion law has been studied for a uniform Bose gas in a one-dimensional vessel. The non-point character of interaction was taken into account using the Gross equation, which is more general than the Gross-Pitaevskii one. In the framework of this approach, the well-known Bogolyubov dispersion mode \hbarω(k)=[(\hbar^{2}k^{2}/2m) ^{2}+qnν(k)\hbar^{2}k^{2}/m]^{1/2} (q=1) was obtained, as well as a new one, which is described by the same formula, but with q= 1/2. The new mode emerges owing to the account of boundaries and the non-point character of interaction: this mode is absent when either the Gross equation for a cyclic system or the Gross-Pitaevskii equation for a cyclic system or a system with boundaries is solved. Capabilities for the new mode to be observed are discussed.

cond-mat.quant-gas↗

arXiv2014

Control of spin current in a Bose gas by bang-bang pulses

We generate spin currents in an $^{87}$Rb spin-2 Bose-Einstein condensate by application of a magnetic field gradient. The spin current destroys the spin polarization, leading to a sudden onset of two-body collisions. In addition, the spin coherence, as measured by the fringe contrast using Ramsey interferometry, is reduced drastically but experiences a weak revival due to in-trap oscillations. The spin current can be controlled using periodic $π$ pulses (bang-bang control), producing longer spin coherence times. Our results show that spin coherence can be maintained even in the presence of spin currents, with applications to quantum sensing in noisy environments.

cond-mat.quant-gas↗

arXiv2017

Fractional excitations in one-dimensional fermionic superfluids

We study the soliton modes carrying fractional quantum numbers in one-dimensional superfluids. In the $s$-wave pairing superfluid with the phase of the order parameter twisted by opposite angles $\pm φ/2$ at the two ends there is an emergent complex $Z_2$ soliton mode carrying fractional spin number $φ/(2π)$ if there is only one pairing branch. We demonstrate that in finite systems of length $L$, the spin density for one pairing branch in the presence of a single soliton mode consists of two terms, a localized spin density profile carrying fractional quantum number $φ/(2π)$, and a uniform background $-φ/(2πL)$. The latter one vanishes in the thermodynamic limit leaving a single soliton mode carrying fractional excitation, however it is essential to keep the total quantum number conserved in finite systems. This analysis is also applicable to other systems with fractional quantum numbers, thus provides a mechanism to understand the compatibility of the emergence of fractional charges with the integral quantization of charges in a finite system. For the $p$-wave pairing superfluid with the chemical potential interpolating between the strong and weak pairing phases, the $Z_2$ soliton is associated with a Majorana zero mode. By introducing the dimension density, we argue that the Majorana zero mode may be understood as an object with 1/2 dimension of the single particle Hilbert space. We conjecture a connection of the dimension density of one-dimensional solitons with the quantum dimension of topological excitations.

cond-mat.str-el↗

arXiv2017

Criticality and hidden criticality in multi-species Bose-Einstein condensates

A general approach is proposed to solve the coupled Gross-Pitaevskii equations (CGP) for K-species Bose-Einstein condensates (BEC). Analytical solutions have been obtained under the Thomas-Fermi approximation. We aim at finding out the common features of the K-species BEC. In particular, two types of phase-transitions, full-state-transition and partial-state transition, are found. In the former all species are involved in the transition, while in the latter only a few specified species are essentially involved. This leads to the criticality and the hidden criticality (previously found in multi-band superconductivity). We further found that the former originates from the singularity of the whole matrix of the CGP, while the latter originates from the singularity of a specified sub-matrix (which is contributed by only a few specified species). It is emphasized that the singularity is not a by-product of the TFA, but is inherent in the CGP.

cond-mat.quant-gas↗

arXiv2018

Chaos and $\mathcal{PT}$-symmetry breaking transitions in a driven, nonlinear dimer with balanced gain and loss

Dynamics of a simple system, such as a two-state (dimer) model, are dramatically changed in the presence of interactions and external driving, and the resultant unitary dynamics show both regular and chaotic regions. We investigate the non-unitary dynamics of such a dimer in the presence of balanced gain and loss for the two states, i.e. a $\mathcal{PT}$ symmetric dimer. We find that at low and high driving frequencies, the $\mathcal{PT}$-symmetric dimer motion continues to be regular, and the system is in the $\mathcal{PT}$-symmetric state. On that other hand, for intermediate driving frequency, the system shows chaotic motion, and is usually in the $\mathcal{PT}$-symmetry broken state. Our results elucidate the interplay between the $\mathcal{PT}$-symmetry breaking transitions and regular-chaotic transitions in an experimentally accessible toy model.

nlin.CD↗