Atomic Interaction Strength Research Articles

Dynamics of bubble-shaped Bose–Einstein condensates on two-dimensional cross-section in micro-gravity environment

We investigated the dynamic evolution and interference phenomena of bubble-shaped Bose–Einstein condensates achievable in a micro-gravity environment. Using numerical solutions of the Gross–Pitaevskii equation describing the dynamic evolution of the bubble-shaped Bose–Einstein condensates, we plotted the evolution of the wave function density distribution on its two-dimensional (2D) cross-section and analysed the resulting patterns. We found that changes in the strength of atomic interactions and initial momentum can affect the dynamic evolution of the bubble-shaped Bose–Einstein condensates and their interference fringes. Notably, we have observed that when the initial momentum is sufficiently high, the thickness of the bubble-shaped Bose–Einstein condensate undergoes a counterintuitive thinning, which is a counterintuitive result that requires further investigation. Our findings are poised to advance our comprehension of the physical essence of bubble-shaped Bose–Einstein condensates and to facilitate the development of relevant experiments in micro-gravity environments.

Investigation on the influence mechanism of wall-fluid interaction on flow around nanoscale circular cylinder

The influence of atomic interaction strength between solid wall and fluid on the flow around a nanoscale circular cylinder was numerically probed with the molecular dynamics method. The results show that when the flow velocity is 2.2 and Reynolds number is 23, vortices are periodically formed behind the circular cylinder at the nanoscale, and go through the development and shedding. With the weakening of wall-fluid interaction between cylinder surface and fluid atoms, the vortex shedding period decreases, the Strouhal number increases, the weak wall-fluid interaction is beneficial for increasing vortex shedding frequency. It reveals that the length of time-averaged vortex behind the nanoscale cylinder decreases slightly with the decrease of wall-fluid interaction. Furthermore, the fluid density and velocity are symmetrically distributed along the central axis of nanoscale cylinder. As the wall-fluid interaction decreases, the binding force of cylinder surface to the fluid atoms decreases, which results in the decrease of the minimum value of fluid density in the upper right and lower right regions near the cylinder. And, the argon fluid is more prone to occur velocity slip on the cylinder surface, the corresponding velocity near the cylinder surface increases significantly.

Stability and superfluidity of the Bose-Einstein condensate in a two-leg ladder with magnetic field.

The stability and superfluidity of the Bose-Einstein condensate in two-leg ladder with magnetic field are studied. The dispersion relation and the phase diagram of the system are obtained. Three phases are revealed: the Meissner phase, the biased ladder (BL) phase, and the vortex phase. The dispersion relation and phase transition of the system strongly depend on the magnitude of atomic interaction strength, the rung-to-leg coupling ratio and the magnetic flux. Particularly, the change of the energy band structure in the phase transition region is modified significantly by the atomic interaction strength. Furthermore, based on the Bogoliubov theory, the energetic and dynamical stability of the system are invested. The stability phase diagram in the full parameter space is presented, and the dependence of superfluidity on the dispersion relation is illustrated explicitly. The atomic interaction strength can produce dynamical instability in the energetic unstable region and can expand the superfluid region. The results show that the stability of the system can be controlled by the atomic interaction strength, the rung-to-leg coupling ratio and the magnetic flux. In addition, the excitation spectrums in the Meissner phase, BL phase and vortex phase are further studied. The modulation of the excitation spectrum and the energetic stability of the system by the atomic interaction strength, the rung-to-leg coupling ratio and magnetic flux is discussed. Finally, through the numerical simulation, the dynamical instability of the system is verified by the time evolution of the Bloch wave and rung current. This provides a theoretical basis for controlling the superfluidity of the system.

Polaron and molecular states of a spin-orbit coupled impurity in a spinless Fermi sea* *Project supported by the National Natural Science Foundation of China (Grant No. 11875195) and the Foundation of Beijing Education Committees (Grant Nos. CIT&TCD201804074 and KZ201810028043).

We investigate the polaron and molecular states of a fermionic atom with one-dimensional spin–orbit coupling (SOC) coupled to a three-dimensional spinless Fermi sea. Because of the interplay among the SOC, Raman coupling and spin-selected interatomic interactions, the polaron state induced by the spin–orbit coupled impurity exhibits quite unique features. We find that the energy dispersion of the polaron generally has a double-minimum structure, which results in a finite center-of-mass (c.m.) momentum in the ground state, different from the zero-momentum polarons where SOC are introduced into the majority atoms. By further tuning the parameters such as the atomic interaction strength, a discontinuous transition between the polarons with different c.m. momenta may occur, signaled by the singular behavior of the quasiparticle residue and effective mass of the polaron. Meanwhile, the molecular state as well as the polaron-to-molecule transition is also strongly affected by the Raman coupling and the effective Zeeman field, which are introduced by the lasers generating SOC on the impurity atom. We also discuss the effects of a more general spin-dependent interaction and mass ratio. These results would be beneficial for the study of impurity physics brought by SOC.

Cyclotron dynamics of a Bose—Einstein condensate in a quadruple-well potential with synthetic gauge fields

We investigate the cyclotron dynamics of Bose-Einstein condensate (BEC) in a quadruple-well potential with synthetic gauge fields. We use laser-assisted tunneling to generate large tunable effective magnetic fields for BEC. The mean position of BEC follows an orbit that simulated the cyclotron orbits of charged particles in a magnetic field. In the absence of atomic interaction, atom dynamics may exhibit periodic or quasi-periodic cyclotron orbits. In the presence of atomic interaction, the system may exhibit self-trapping, which depends on synthetic gauge fields and atomic interaction strength. In particular, the competition between synthetic gauge fields and atomic interaction leads to the generation of several discontinuous parameter windows for the transition to self-trapping, which is obviously different from that without synthetic gauge fields.

Periodic quenches across the Berezinskii-Kosterlitz-Thouless phase transition

The quenched dynamics of an ultracold homogeneous atomic two-dimensional Bose gas subjected to periodic quenches across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition are discussed. Specifically, we address the effect of periodic cycling of the effective atomic interaction strength between a thermal disordered state above, and a highly ordered state below the critical BKT interaction strength, by means of numerical simulations of the stochastic projected Gross-Pitaevskii equation. Probing the emerging dynamics as a function of the frequency of sinusoidal driving from low to high frequencies reveals diverse dynamical features, including phase-lagged quasi adiabatic reversible condensate formation, resonant excitation consistent with an intrinsic system relaxation timescale, and gradual establishment of dynamically-recurring or time-averaged non-equilibrium states with enhanced coherence which are neither condensed, nor thermal. Our study paves the way for experimental observation of such driven non-equilibrium ultracold superfluid states.

Interaction induced localization of a spin–orbit-coupled Bose-Einstein condensate in a double-well potential

We investigate atomic interaction induced localization of a spin–orbit coupled Bose-Einstein condensate loaded in a double-well potential. We confirm the existence of the localization in the presence of the spin–orbit coupling for differences between the spin-dependent nonlinear atomic interaction strength between intraspin and interspin. We find a quantum phase transition from spin balance to spin localization for the different strengths of nonlinear atomic interaction strength. We also obtain the parametric dependence of the transitions to spin localization. The results play a constructive role to control spin localization of spin–orbit coupled Bose-Einstein condensate in a double-well potential.

Quantum transport in ultracold atoms

Ultracold atoms confined by engineered magnetic or optical potentials are ideal systems for studying phenomena otherwise difficult to realize or probe in the solid state because their atomic interaction strength, number of species, density, and geometry can be independently controlled. This review focuses on quantum transport phenomena in atomic gases that mirror and oftentimes either better elucidate or show fundamental differences with those observed in mesoscopic and nanoscopic systems. We discuss significant progress in performing transport experiments in atomic gases, contrast similarities and differences between transport in cold atoms and in condensed matter systems, and survey inspiring theoretical predictions that are difficult to verify in conventional setups. These results further demonstrate the versatility offered by atomic systems in the study of nonequilibrium phenomena and their promise for novel applications.

Orbital hyperfine interaction and qubit dephasing in carbon nanotube quantum dots

Hyperfine interaction (HF) is of key importance for the functionality of solid-state quantum information processing, as it affects qubit coherence and enables nuclear-spin quantum memories. In this work, we complete the theory of the basic HF mechanisms (Fermi contact, dipolar, orbital) in carbon nanotube quantum dots by providing a theoretical description of the orbital HF. We find that orbital HF induces an interaction between the nuclear spins of the nanotube lattice and the valley degree of freedom of the electrons confined in the quantum dot. We show that the resulting nuclear-spin--electron-valley interaction (i) is approximately of Ising type; (ii) is essentially local, in the sense that a radius- and dot-length-independent atomic interaction strength can be defined; and (iii) has an atomic interaction strength that is comparable to the combined strength of the Fermi contact and dipolar interactions. We argue that orbital HF provides a new decoherence mechanism for single-electron valley qubits and spin-valley qubits in a range of multivalley materials. We explicitly evaluate the corresponding inhomogeneous dephasing time ${T}_{2}^{*}$ for a nanotube-based valley qubit.

Three-dimensional ring vortex solitons and their stabilities in Bose-Einstein condensates under magnetic confinement

A three-dimensional study of the ring vortex solitons is conducted for both attractive and repulsive Bose-Einstein condensates subject to harmonic potential confinement. A family of stationary ring vortex solitons, which is defined by the radial excitation number and the winding number of the intrinsic vorticity, are obtained numerically for a given atomic interaction strength. We find that stabilities of the ground and radially excited states of the ring vortex soliton are dependent on the winding number differently. The ground state of the ring vortex soliton with the large winding number is unstable dynamically against random perturbation. The radially excited state of the ring vortex soliton with large winding number corresponds to the increased collapse threshold and therefore can be made stable for sufficiently small atomic interaction strengths. The ground and radially excited states also demonstrate different dynamic evolutions under large atomic interaction strengths. The former exhibits simultaneous symmetric splitting in the transverse plane, while the latter displays periodic expand-merge cycles in the longitudinal direction.

PERIODIC MODULATION ON THE BOSE–JOSEPHSON JUNCTION IN AN OPTICAL CAVITY

The periodic modulation on dynamic behavior of Bose–Josephson junction dispersively coupled to a single mode high-finesse optical cavity is investigated. The eigenenergies and the phase diagram of the system are obtained using the numerical methods under the mean field approximation. It is shown that the large atomic interaction strength will lead the eigenenergies to show a bistable and a loop structure. It is also found that the periodic modulation can change the period and frequency of the motion of the atoms and the intra cavity photons drastically. Furthermore, the strong condensate-field interplay will enhance the oscillation of the atoms and make some trapped atoms start oscillating. For certain parameters of the cavity field, the relative population oscillation exhibits a process from order to chaos.

Parametric and geometric resonances of collective oscillation modes in Bose–Einstein condensates

We analytically and numerically study nonlinear dynamics in Bose–Einstein condensates (BECs) induced either by a harmonic modulation of the interaction or by the geometry of the trapping potential. To analytically describe BEC dynamics, we use a perturbative expansion based on the Poincaré–Lindstedt analysis of a Gaussian variational ansatz, whereas in the numerical approach we use numerical solutions of both a variational system of equations and the full time-dependent Gross–Pitaevskii equation. The harmonic modulation of the atomic s-wave scattering length of a BEC of 7Li was achieved recently via Feshbach resonance, and such a modulation leads to a number of nonlinear effects, which we describe within our approach: mode coupling, higher harmonics generation and significant shifts in the frequencies of collective modes. In addition to the strength of atomic interactions, the geometry of the trapping potential is another key factor for the dynamics of the condensate, as well as for its collective modes. The asymmetry of the confining potential leads to important nonlinear effects, including resonances in the frequencies of collective modes of the condensate. We study in detail such geometric resonances and derive explicit analytic results for frequency shifts for the case of an axially symmetric condensate with two- and three-body interactions. Analytically obtained results are verified by extensive numerical simulations.

Observation of a one-dimensional Tonks-Girardeau gas.

We report the observation of a one-dimensional (1D) Tonks-Girardeau (TG) gas of bosons moving freely in 1D. Although TG gas bosons are strongly interacting, they behave very much like noninteracting fermions. We enter the TG regime with cold rubidium-87 atoms by trapping them with a combination of two light traps. By changing the trap intensities, and hence the atomic interaction strength, the atoms can be made to act either like a Bose-Einstein condensate or like a TG gas. We measure the total 1D energy and the length of the gas. With no free parameters and over a wide range of coupling strengths, our data fit the exact solution for the ground state of a 1D Bose gas.

The competition between surface segregation and compositional ordering in alloys: theory and experimental observations of segregation versus temperature peaked curves

Abstract The tendency to form ordered structures in metallic alloys can be manifested in prominent deviations from the equilibrium surface segregation behavior characteristic to random solid solutions. Depending on the relative strength of atomic interactions, the occurrence of long-range order (LRO) or short-range order (SRO) can suppress the segregation tendency of an alloy constituent. Since this suppressing effect is weakened with increasing temperature, segregation levels can increase concurrently with entropy-driven compositional disordering. In ordered alloys, such endothermic segregation process should be followed by the well-known monotonously decreasing McLean–Langmuir temperature dependence starting near the order–disorder transition temperature. The resultant peaked segregation versus temperature curve, expected for systems with relatively strong ordering tendencies, has been relatively little explored theoretically or experimentally. Some basic theoretical considerations regarding segregation/ordering competition as well as pertinent experimental results are briefly discussed, including surface segregation affected by LRO, especially in Pt3Sn(111) alloy and SRO effects (Al in Ni–9%Al solid solution).