arXiv subjects

X. -J. Liu

Publications and source records attributed to X. -J. Liu.

9 recordsLinked to original sources

arXiv2025

Evidence of mini-jet emission in a large emission zone from a magnetically-dominated gamma-ray burst jet

The second brightest GRB in history, GRB230307A, provides an ideal laboratory to study the mechanism of GRB prompt emission thanks to its extraordinarily high photon statistics and its single episode activity. Here we demonstrate that the rapidly variable components of its prompt emission compose an overall broad single pulse-like profile. Although these individual rapid components are aligned in time across all energy bands, this overall profile conspires to show a well-defined energy-dependent behavior which is typically seen in single GRB pulses. Such a feature demonstrates that the prompt emission of this burst is from many individual emitting units that are casually linked in a emission site at a large distance from the central engine. Such a scenario is in natural consistency with the internal-collision-induced magnetic reconnection and turbulence framework, which invokes many mini-jets due to local magnetic reconnection that constantly appear and disappear in a global magnetically-dominated jet.

astro-ph.HE↗

arXiv2016

Criteria for 2D kinematics in an interacting Fermi gas

Ultracold Fermi gases subject to tight transverse confinement offer a highly controllable setting to study the two-dimensional (2D) BCS to Berezinskii-Kosterlitz-Thouless superfluid crossover. Achieving the 2D regime requires confining particles to their transverse ground state which presents challenges in interacting systems. Here, we establish the conditions for an interacting Fermi gas to behave kinematically 2D. Transverse excitations are detected by measuring the transverse expansion rate which displays a sudden increase when the atom number exceeds a critical value $N_{2D}$ signifying a density driven departure from 2D kinematics. For weak interactions $N_{2D}$ is set by the aspect ratio of the trap. Close to a Feshbach resonance, however, the stronger interactions reduce $N_{2D}$ and excitations appear at lower density.

cond-mat.quant-gas↗

arXiv2012

Deep inner-shell multiphoton ionization by intense x-ray free-electron laser pulses

We have investigated multiphoton multiple ionization dynamics of argon and xenon atoms using a new x-ray free electron laser (XFEL) facility, SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, and identified that highly charged Xe ions with the charge state up to +26 are produced predominantly via four-photon absorption as well as highly charged Ar ions with the charge state up to +10 are produced via two-photon absorption at a photon energy of 5.5 keV. The absolute fluence of the XFEL pulse, needed for comparison between theory and experiment, has been determined using two-photon processes in the argon atom with the help of benchmark ab initio calculations. Our experimental results, in combination with a newly developed theoretical model for heavy atoms, demonstrate the occurrence of multiphoton absorption involving deep inner shells.

physics.atom-ph↗

arXiv2010

Universal behavior of pair correlations in a strongly interacting Fermi gas

We show that short-range pair correlations in a strongly interacting Fermi gas follow a simple universal law described by Tan's relations. This is achieved through measurements of the static structure factor which displays a universal scaling proportional to the ratio of Tan's contact to the momentum $C/q$. Bragg spectroscopy of ultracold $^6$Li atoms from a periodic optical potential is used to measure the structure factor for a wide range of momenta and interaction strengths, providing broad confirmation of this universal law. We calibrate our Bragg spectra using the $f$-sum rule, which is found to improve the accuracy of the structure factor measurement.

cond-mat.quant-gas↗

arXiv2010

Second sound and the density response function in uniform superfluid atomic gases

Recently there has been renewed interest in second sound in superfluid Bose and Fermi gases. By using two-fluid hydrodynamic theory, we review the density response $χ_{nn}(\bq,ω)$ of these systems as a tool to identify second sound in experiments based on density probes. Our work generalizes the well-known studies of the dynamic structure factor $S(\bq,ω)$ in superfluid $^4$He in the critical region. We show that, in the unitary limit of uniform superfluid Fermi gases, the relative weight of second vs. first sound in the compressibility sum rule is given by the Landau--Placzek ratio $\lp\equiv (\bar{c}_p-\bar{c}_v)/\bar{c}_v$ for all temperatures below $T_c$. In contrast to superfluid $^4$He, $\lp$ is much larger in strongly interacting Fermi gases, being already of order unity for $T \sim 0.8 T_c$, thereby providing promising opportunities to excite second sound with density probes. The relative weights of first and second sound are quite different in $S(\bq,ω)$ (measured in pulse propagation studies) as compared to $\mathrm{Im}χ_{nn}(\bq,ω)$ (measured in two-photon Bragg scattering). We show that first and second sound in $S(\bq,ω)$ in a strongly interacting Bose-condensed gas are similar to those in a Fermi gas at unitarity. However, in a weakly interacting Bose gas, first and second sound are mainly uncoupled oscillations of the thermal cloud and condensate, respectively, and second sound has most of the spectral weight in $S(\bq,ω)$. We also discuss the behaviour of the superfluid and normal fluid velocity fields involved in first and second sound.

cond-mat.quant-gas↗

arXiv2009

First and second sound in a strongly interacting Fermi gas

Using a variational approach, we solve the equations of two-fluid hydrodynamics for a uniform and trapped Fermi gas at unitarity. In the uniform case, we find that the first and second sound modes are remarkably similar to those in superfluid Helium, a consequence of strong interactions. In the presence of harmonic trapping, first and second sound become degenerate at certain temperatures. At these points, second sound hybridizes with first sound and is strongly coupled with density fluctuations, giving a promising way of observing second sound. We also discuss the possibility of exciting second sound by generating local heat perturbations.

cond-mat.quant-gas↗

arXiv2007

Non-Abelian gauge field effects and its relevance to spinning particle dynamics in the technology of spintronics

We describe formally the precession of spin vector about the k-space effective magnetic field in condensed matter system with spin orbital effects as constituting a local transformation of the electron wavefunction which necessarily invokes the SU(2) transformation rule to ensure covariance. We showed a "no-precession" condition as pre-requisite for the spin gauge field to exert its influence on spin particle motion. The effects of the spin gauge field on spin particle motion were shown to be consistent in both classical and quantum pictures, which hence should underpin theoretical explanations for important effects in anomalous Hall, spin Hall, spin torque, optical Magnus, geometric quantum computation.

quant-ph↗

arXiv2005

Ultra-cold fermions in optical lattices

We summarize recent theoretical results for the signatures of strongly correlated ultra-cold fermions in optical lattices. In particular, we focus on: collective mode calculations, where a sharp decrease in collective mode frequency is predicted at the onset of the Mott metal-insulator transition; and correlation functions at finite temperature, where we employ a new exact technique that applies the stochastic gauge technique with a Gaussian operator basis.

cond-mat.other↗

arXiv2004

Correlated Quantum Memory: Manipulating Atomic Entanglement via Electromagnetically Induced Transparency

We propose a feasible scheme of quantum state storage and manipulation via electromagnetically induced transparency (EIT) in flexibly $united$ multi-ensembles of three-level atoms. For different atomic array configurations, one can properly steer the signal and the control lights to generate different forms of atomic entanglement within the framework of linear optics. These results shed new light on designing the versatile quantum memory devices by using, e.g., an atomic grid.

quant-ph↗