arXiv subjects

L. Q. Lai

Publications and source records attributed to L. Q. Lai.

10 recordsLinked to original sources

arXiv2026

Dynamical control of particle jets from a driven condensate in a one-dimensional lattice with double-well potential

We investigate the nonlinear dynamics of a Bose-Einstein condensate trapped in a double-well potential of a one-dimensional lattice, where the interatomic interactions are periodically modulated in time. In the typical case of a symmetric double-well, we observe collective particle emission under resonant driving, where the excitation regimes are explicitly constrained by the interplay between the drive strength and the hopping amplitude. By introducing a depth asymmetry between the wells, we find that moderate bias specifically enhances the emission rate, while large asymmetry suppresses it. The particle jets can be further controlled by modulating the hopping amplitudes, where the emission is weakened for finite hopping imbalances. These results outline the roles of asymmetry and external driving in precisely manipulating quantum many-body transport, and may offer insights into the design of atomtronic devices.

cond-mat.quant-gas↗

arXiv2026

Dynamics of interacting bosons in a two-leg ring ladder with artificial magnetic flux and ac-driven modulations

We investigate the nonequilibrium dynamics of interacting bosons in a two-leg ring ladder pierced by an artificial magnetic flux, where the particles are initially localized in the central sites of both rings, and the ac-driven local energy shifts are applied to the remaining lattice sites. Within the mean-field approximation, we demonstrate the emergence of nonlinear self-trapping for strong interparticle interactions, and characterize the distinct excitation regimes in the absence of the inter-ring tunneling. The artificial magnetic flux typically introduces Peierls phase factors, which induces complex-valued hopping amplitudes and leads to directed net particle currents along the chains. By further incorporating the finite inter-ring coupling and biased intra-ring hopping, we reveal that the tuning of the drive frequency and Peierls phase allows the precise control over both the intensity and direction of particle currents, which facilitates the transition between chiral and antichiral dynamics. These findings offer insights into the coherent manipulation of matter-wave transports in closed-loop lattice configurations and the exploration of nonequilibrium synthetic quantum systems in related fields.

cond-mat.quant-gas↗

arXiv2025

Generally covariant geometric momentum and geometric potential for a Dirac fermion on a two-dimensional hypersurface

Geometric momentum is the appropriate momentum for a particle constrained to move on a curved surface, which depends on the extrinsic curvature and leads to observable effects, and curvature-induced quantum potentials appear for a nonrelativistic free particle on the surface. In the context of multi-component quantum states, the geometric momentum should be rewritten as a generally covariant geometric momentum, which contains an additional term defined as the gauge potential. For a Dirac fermion constrained on a two-dimensional hypersurface, we derive the generally covariant geometric momentum, and demonstrate that no curvature-induced geometric potentials arise on a pseudosphere or a helical surface. The dynamical quantization conditions are verified to be effective in dealing with constrained systems on hypersurfaces, enabling the derivation of both the generally convariant geometric momentum and the geometric potential for a spin particle constrained on parametrically defined surfaces.

quant-ph↗

arXiv2025

Instability and particle current control of a parametrically driven Bose-Einstein condensate in a ring-shaped lattice

We investigate the dynamics of a Bose-Einstein condensate in a one-dimensional ring-shaped lattice with the Peierls phase and site-dependent modulations, where the condensate is confined in a single deep trap and the interparticle interaction strength is modulated by a time-periodic driving field. The system has a finite spectrum, which limits the excitation regimes, and the Peierls phase typically induces imbalanced complex hopping amplitudes in each direction, leading to nonzero net particle currents along the lattice chain, which can hold nearly persistent even when the driving field is turned off after half of the period. The configuration provides a specific way for the coherent control of particle currents in many-body quantum systems with the help of an external driving field, and promotes the possible applications in future closed-loop atom circuits.

cond-mat.quant-gas↗

arXiv2024

Interference-induced suppression of particle emission from a Bose-Einstein condensate in lattice with time-periodic modulations

Emission of matter-wave jets from a parametrically driven condensate has attracted significant experimental and theoretical attention due to the appealing visual effects and potential metrological applications. In this work, we investigate the collective particle emission from a Bose-Einstein condensate confined in a one-dimensional lattice with periodically modulated interparticle interactions. We give the regimes for discrete modes, and find that the emission can be distinctly suppressed. The configuration induces a broad band, but few particles are ejected due to the interference of the matter waves. We further qualitatively model the emission process, and demonstrate the short-time behaviors. This engineering provides a way for manipulating the propagation of particles and the corresponding dynamics of condensates in lattices, and may find use in the dynamical excitation control of other nonequilibrium problems with time-periodic driving.

cond-mat.quant-gas↗

arXiv2024

Intermittent emission of particles from a Bose-Einstein condensate in a one-dimensional lattice

We investigate particle emission from a Bose-Einstein condensate with periodically modulated interactions in a one-dimensional lattice. Within perturbative analysis, which leads to instabilities for discrete modes, we obtain the main regimes where the system can emit a large particle jet, and find that the emission is distinctly intermittent rather than continuous. The time evolution of the trapped particles exhibits a stair-like decay, and a larger drive induces a more significant intermittency. We further shed light on the dynamics of the stimulating process, and demonstrate that instead of a real suspension, the intermittency represents a build-up stage of the system. The theoretical framework might be generalized to the explorations on multiple-site systems with analogous configurations and couplings, and offer new insights into other fundamental nonequilibrium problems.

cond-mat.quant-gas↗

arXiv2022

Resonant enhancement of particle emission from a parametrically driven condensate in a one-dimensional lattice

Motivated by recent experiments, we investigate particle emission from a Bose-Einstein condensate in a one-dimensional lattice, where the interaction strength is periodically modulated. The modulated interactions parametrically excite a collective mode, leading to density oscillations. These collective oscillations in turn drive particle emission. This multistep process amplifies the drive, producing larger particle jets. We find that the amplitude dependence of the emission rate has a characteristic threshold behavior, as seen in experiments.

cond-mat.quant-gas↗

arXiv2021

Emission of particles from a parametrically driven condensate in a one-dimensional lattice

Motivated by recent experiments, we calculate particle emission from a Bose-Einstein condensate trapped in a single deep well of a one-dimensional lattice when the interaction strength is modulated. In addition to pair emission, which has been widely studied, we observe single-particle emission. Within linear response, we are able to write closed-form expressions for the single-particle emission rates and reduce the pair emission rates to one-dimensional integrals. The full nonlinear theory of single-particle emission is reduced to a single variable integrodifferential equation, which we numerically solve.

cond-mat.quant-gas↗

arXiv2021

The curvature-induced gauge potential and the geometric momentum for a particle on a hypersphere

A particle that is constrained to freely move on a hyperspherical surface in an $N\left( \geq 2\right) $ dimensional flat space experiences a curvature-induced gauge potential, whose form was given long ago (J. Math. Phys. \textbf{34}(1993)2827). We demonstrate that the momentum for the particle on the hypersphere is the geometric one including the gauge potential and its components obey the commutation relations $\left[ p_{i},p_{j}\right] =-i\hbar J_{ij}/r^{2}$, in which $\hbar $ is the Planck's constant, and $p_{i}$ ($i,j=1,2,3,...N$) denotes the $i-$th component of the geometric momentum, and $J_{ij}$ specifies the $ij-$th component of the generalized\textit{\ angular momentum} containing both the orbital part and the coupling of the generators of continuous rotational symmetry group $% SO(N-1)$ and curvature, and $r$ denotes the radius of the $N-1$ dimensional hypersphere.

hep-th↗

arXiv2019

Charge nonconservation of molecular devices in the presence of a nonlocal potential

In the presence of a nonlocal potential in molecular device systems, generally the charge conservation cannot be satisfied, and in literatures the modifications of the conventional definition of current were given to solve this problem. We demonstrate that, however, the nonconservation is not due to the invalidation of the conventional definition of current, but originates respectively from the improper approximations to electron-electron interactions and the inappropriate definition of current using pseudo wave functions in pseudopotential implementations. In this work, we propose a nonlocal-potential formulation of the interactions to fulfill the charge conservation and also give a discussion about the calculation of current when the pseudopotential is involved. As an example of application of our formulation, we further present the calculated results of a double-barrier model.

cond-mat.mes-hall↗