arXiv subjects

Xin-Yuan Gao

Publications and source records attributed to Xin-Yuan Gao.

14 recordsLinked to original sources

arXiv2026

Microscopic Origin of Emergent Elliptic Flow and Molecule Formation in Strongly Interacting Quasi-Two-Dimensional Few-Body Systems

Recent experiments simulating two-dimensional few-fermion systems have observed emergent hydrodynamic behavior, i.e., interaction-driven elliptic flow by adding fermions two at a time [S.~Brandstetter et al., Nat. Phys. (2025)]. Due to the curse of dimensionality and strong correlations, capturing such phenomena beyond two particles remains challenging. Here, we use the ab initio time-dependent explicitly correlated Gaussian (TDECG) method to quantitatively reproduce these experimental observations. With only a moderate number of correlated Gaussian basis functions, our approach obtains converged dynamical observables for systems up to six particles. Furthermore, real-time access to the many-body wavefunction and two-point correlation functions enables us to visualize the transformation from a strongly interacting gas to a stream of paired molecules, i.e., a dynamical BCS-BEC crossover.

cond-mat.quant-gas↗

arXiv2026

In-situ Observation of Magnetostriction Crossover in a Strongly Dipolar Two-Dimensional Bose Gas

Magnetostriction, the anisotropic spatial deformation, is a hallmark of dipolar gases with strong long-range interactions, yet it poses a challenge for in-situ characterization. Here, we observe a magnetostriction crossover from the strongly anisotropic superfluid phase to the nearly isotropic normal phase using in-situ imaging of quasi-two-dimensional 166Er gases. Then, we develop a quasi-2D Hartree-Fock-mean-field framework that provides a robust tool for interaction-aware thermometry, enabling the determination of temperature and chemical potential across all dipole orientations from a single fit. We further demonstrate that the low-density wings effectively obey a local-density equation of state. Finally, we reveals the crossover from the isotropic thermal wings to the anisotropic coherent core in a single in-situ image, providing a pathway for future accurate studies of strongly dipolar superfluidity and thermodynamics in 2D.

cond-mat.quant-gas↗

arXiv2025

Penetration of surface effects on structural relaxation and particle hops in glassy films

A free surface induces enhanced dynamics in glass formers. We study the dynamical enhancement of glassy films with a distinguishable-particle lattice model of glass free of elastic effects. We demonstrate that the thickness of the surface mobile layer depends on temperature differently under different definitions, although all are based on local structure relaxation rate. The rate can be fitted to a double exponential form with an exponential-of-power-law tail. Our approach and results exclude elasticity as the unique mechanism for the tail. Layer-resolved particle hopping rate, potentially a key measure for activated hopping, is also studied but it exhibits much shallower surface effects.

cond-mat.soft↗

arXiv2025

Dynamical Phase Transition of Dissipative Fermionic Superfluids

Driven-dissipative open quantum many-body systems exhibit rich phases that are characterized by the steady states in the long-time dynamics. However, lossy open systems inevitably decay to the vacuum, making their transient evolution the primary focus. Assuming the Hartree-Fock-Bogoliubov ansatz, we derive a generalized time-dependent Hartree-Fock-Bogoliubov equation based on the least action principle for open quantum systems. By solving the quench dynamics after abruptly introducing inelastic scattering or one-body loss in the Bardeen-Cooper-Schrieffer limit, we reveal a generic dynamical phase transition: the superfluid order parameter vanishes non-analytically while the superfluid fraction's first-order time derivative undergoes a discontinuous change at a finite critical time. This marks a new paradigm of dynamical phase transitions, distinct from those in closed systems, where the initial state must be finely tuned.

cond-mat.quant-gas↗

arXiv2024

Fate of thermalization of ultracold fermions with two-body dissipation

Two-body dissipation due to chemical reactions occurs in both ultracold fermionic and bosonic molecular gases. Despite recent advances in achieving quantum degeneracy, the loss dynamics are typically described phenomenologically using rate equations, often assuming thermalization during chemical reactions. From the first principles, we analyze particle loss, temperature evolution, and momentum distributions in single-component Fermi gases using the inelastic quantum Boltzmann equation. Our results prove that the conventional two-body loss model is valid for trapped systems, though it fails to describe the dynamics in homogeneous systems accurately. Interestingly, we find that systems prepared near or above quantum degeneracy can thermalize spontaneously, even in the absence of elastic collisions, while systems initialized deep in degeneracy display non-equilibrium behavior. Our calculations are in good agreement with recent experimental data from trapped systems and could be further tested in atomic systems with induced two-body loss in box potentials.

cond-mat.quant-gas↗

arXiv2024

Exact Thermodynamics For Weakly Interacting Normal-Phase Quantum Gases: Equations of State For All Partial Waves

While the thermodynamics for bosonic systems with weak $s$-wave interactions has been known for decades, a general and systematic extension to higher partial waves has not yet been reported. We provide closed-form expressions for the equations of state for weakly interacting systems with arbitrary partial waves in the normal phase. Thermodynamics, including contact, loss rate, and compressibility, are derived over the entire temperature regime. Our results offer an improved thermometer for ultracold atoms and molecules with weak high-partial wave interactions.

cond-mat.quant-gas↗

arXiv2024

Thermodynamics of Spin-Imbalanced Fermi Gases with SU(N) Symmetric Interaction

Thermodynamics of degenerate Fermi gases has been extensively studied through various aspects such as Pauli blocking effects, collective modes, BCS superfluidity, and more. Despite this, multi-component fermions with imbalanced spin configurations remain largely unexplored, particularly beyond the two-component scenario. In this work, we generalize the thermodynamic study of SU($N$) fermions to spin-imbalanced configurations based on density fluctuations. Theoretically, we provide closed-form expressions of density fluctuation across all temperature ranges for general spin population setups. Experimentally, after calibrating the measurements with deeply degenerate $^{173}$Yb Fermi gases under spin-balanced configurations ($N\leq$~6), we examine the density fluctuations in spin-imbalanced systems. Specifically, we investigate two-species and four-species configurations to validate our theoretical predictions. Our analysis indicates that interaction enhancement effects can be significant even in highly spin-imbalanced systems. Finally, as an application, we use this approach to examine the decoherence process. Our study provides a deeper understanding of the thermodynamic features of spin-imbalanced multi-component Fermi gases and opens new avenues for exploring complex quantum many-body systems.

cond-mat.quant-gas↗

arXiv2024

Surface mobility gradient and emergent facilitation in glassy films

Confining glassy polymer into films can substantially modify their local and film-averaged properties. We present a lattice model of film geometry with void-mediated facilitation behaviors but free from any elasticity effect. We analyze the spatially varying viscosity to delineate the transport property of glassy films. The film mobility measurements reported by [Yang et. al., Science, 2010, 328, 1676] are successfully reproduced. The flow exhibits a crossover from simple viscous flow to a surface-dominated regime as temperature decreases. The propagation of a highly mobile front induced by the free surface is visualized in real space. Our approach provides a microscopic treatment of the observed glassy phenomena.

cond-mat.soft↗

arXiv2024

The distinguishable-particle lattice model of glasses in three dimensions

The nature of glassy states in realistic finite dimensions is still under fierce debate. Lattice models can offer valuable insights and facilitate deeper theoretical understanding. Recently, a disordered-interacting lattice model with distinguishable particles in two dimensions (2D) has been shown to produce a wide range of dynamical properties of structural glasses, including the slow and heterogeneous characteristics of the glassy dynamics, various fragility behaviors of glasses, and so on. These findings support the usefulness of this model for modeling structural glasses. An important question is whether such properties still hold in the more realistic three dimensions. In this study, we aim to extend the distinguishable-particle lattice model (DPLM) to three dimensions (3D) and explore the corresponding glassy dynamics. Through extensive kinetic Monte Carlo simulations, we found that the 3D DPLM exhibits many typical glassy behaviors, such as plateaus in the mean square displacement of particles and the self-intermediate scattering function, dynamic heterogeneity, variability of glass fragilities, and so on, validating the effectiveness of the DPLM in a broader realistic setting. The observed glassy behaviors of the 3D DPLM appear similar to those of its 2D counterpart, in accordance with recent findings in molecular models of glasses. We further investigate the role of void-induced motions in dynamical relaxations and discuss their relation to dynamic facilitation. As lattice models tend to keep the minimal but important modeling elements, they are typically much more amenable to analysis. Therefore, we envisage that the DPLM will benefit future theoretical developments, such as the configuration tree theory, towards a more comprehensive understanding of structural glasses.

cond-mat.soft↗

arXiv2023

Temperature-Dependent Contact of Weakly Interacting Single-Component Fermi Gases and Loss Rate of Degenerate Polar Molecules

Motivated by the experimental realization of single-component degenerate Fermi gases of polar ground state KRb molecules with intrinsic two-body losses [L. De Marco, G. Valtolina, K. Matsuda, W. G. Tobias, J. P. Covey, and J. Ye, A degenerate Fermi gas of polar molecules, Science 363, 853 (2019)], this work studies the finite-temperature loss rate of single-component Fermi gases with weak interactions. First, we establish a relationship between the two-body loss rate and the $p$-wave contact. Second, we evaluate the contact of the homogeneous system in the low-temperature regime using $p$-wave Fermi liquid theory and in the high-temperature regime using the second-order virial expansion. Third, conjecturing that there are no phase transitions between the two temperature regimes, we smoothly interpolate the results to intermediate temperatures. It is found that the contact is constant at temperatures close to zero and increases first quadratically with increasing temperature and finally -- in agreement with the Bethe-Wigner threshold law -- linearly at high temperatures. Fourth, applying the local-density approximation, we obtain the loss-rate coefficient for the harmonically trapped system, reproducing the experimental KRb loss measurements within a unified theoretical framework over a wide temperature regime without fitting parameters. Our results for the contact are not only applicable to molecular $p$-wave gases but also to atomic single-component Fermi gases, such as 40K and 6Li.

cond-mat.quant-gas↗

arXiv2023

Kauzmann Paradox: a crossover due to diminishing local excitations

The configurational entropy of supercooled liquids extrapolates to zero at the Kauzmann temperature, causing a crisis called the Kauzmann paradox. Here, using a class of multicomponent lattice glass models, we study a resolution of the paradox characterized by a sudden but smooth turn in the entropy as temperature goes sufficiently low. A scalar variant of the models reproduces the Kauzmann paradox with thermodynamic properties at very low temperatures dominated by correlations. An exactly solvable vector variant without correlation illustrates that a sudden entropy turn occurs when discrete local excitations are largely suppressed. Despite being disordered and infinitely degenerate, the ground states have zero entropy per particle.

cond-mat.stat-mech↗

arXiv2023

Towards Relating Fragile-To-Strong Transition to Fragile Glass

Glass formers are in general classified as strong or fragile depending on whether their relaxation rates follow Arrhenius or super-Arrhenius temperature dependence. There are however notable exceptions such as water, which exhibit a fragile-to-strong (FTS) transition and behave as fragile and strong respectively at high and low temperatures. In this work, the FTS transition is studied using a distinguishable-particle lattice model previously demonstrated to be capable of simulating both strong and fragile glasses [Phys. Rev. Lett. 125, 265703 (2020)]. Starting with a bimodal pair-interaction distribution appropriate for fragile glasses, we show that by narrowing down the energy dispersion in the low-energy component of the distribution, a FTS transition is observed. The transition occurs at a temperature at which the stretching exponent of the relaxation is minimized, in agreement with previous molecular dynamics simulations.

cond-mat.soft↗

arXiv2022

Diffusion coefficient power laws and defect-driven glassy dynamics in swap acceleration

Particle swaps can drastically accelerate dynamics in glass. The mechanism is expected to be vital for a fundamental understanding of glassy dynamics. To extract defining features, we propose a partial swappability with a fraction {ϕ_s} of swap-initiating particles, which can only swap locally with each other or with regular particles. We focus on the swap-dominating regime. At all temperatures studied, particle diffusion coefficients scale with {ϕ_s} in unexpected power laws with temperature-dependent exponents, consistent with the kinetic picture of glass transition. At small {ϕ_s}, swap-initiators, becoming defect particles, induce remarkably typical glassy dynamics of regular particles. This supports defect models of glass.

cond-mat.dis-nn↗

arXiv2022

Emergence of two-level systems in glass formers: a kinetic Monte Carlo study

Using a distinguishable-particle lattice model based on void-induced dynamics, we successfully reproduce the well-known linear relation between heat capacity and temperature at very low temperatures. The heat capacity is dominated by two-level systems formed due to the strong localization of voids to two neighboring sites, and can be exactly calculated in the limit of ultrastable glasses. Similar but weaker localization at higher temperatures accounts for the glass transition. The result supports the conventional two-level tunneling picture by revealing how two-level systems emerge from random particle interactions, which also cause the glass transition. Our approach provides a unified framework for relating microscopic dynamics of glasses at room and cryogenic temperatures.

cond-mat.soft↗