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Renyuan Liao

Publications and source records attributed to Renyuan Liao.

At least 19 recordsLinked to original sources
arXiv2026

Tuning Density and Spin Ordering of Degenerate Fermi Gases in an Optical Cavity

We investigate a spin-degenerate Fermi gas coupled to a high-finesse optical cavity, where the competition between scalar and vectorial couplings is controlled by the relative polarization angle of the pump and cavity fields. We find that the phase transition threshold is synergistically determined by the scalar-vectorial coupling weight and Pauli blocking, with the latter dictating the critical pump lattice depth required for the onset of superradiance. For a two-component Fermi gas with opposite spins, the population ratio drives two distinct types of phase transitions corresponding to real-space phase separation: continuous and discontinuous. Nevertheless, the boundary of the phase transition remains fundamentally governed by the scalar-vectorial coupling competition. We clarify the impact of the relative polarization angle on phase transitions of the system; these results also apply to bosonic systems. Our results provide valuable theoretical insights for future experimental realizations.

cond-mat.quant-gas↗
arXiv2026

Phase transition, phase separation and mode softening of a two-component Bose-Einstein condensate in an optical cavity

We investigate the superradiant phase transition in a two-component Bose-Einstein condensate with distinct atomic detunings, confined in an optical cavity and driven by a transverse pump laser. By combining perturbation theory and numerical simulations, we demonstrate that the phase transition is dominated by the red-detuned component, resulting in a phase diagram completely different from that of a single-component case under blue-detuned condition. The system exhibits spontaneous phase separation between the two components, manifested as alternating stripe patterns in the normal phase and distinct Bragg gratings in the superradiant phase. Furthermore, the Bogoliubov excitation spectrum reveals roton-type mode softening, indicating that the phase transition also corresponds to the superfluid-to-lattice supersolid transition. Our findings provide insights into the interplay between atomic detunings and collective quantum many-body phenomena, offering potential applications in quantum simulation and optical switching technologies.

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arXiv2025

Non-Equilibrium Probing of Topological Supersolids in Spin-Orbit-Coupled Dipolar Condensates

A chiral supersolid is a quantum phase that simultaneously exhibits crystalline order, superfluidity, and topological spin texture, with spontaneously broken translational, U(1) gauge, and chiral symmetries. Here, we demonstrate a chiral supersolid with tunable non-equilibrium dynamics in a spin-orbit coupled dipolar Bose-Einstein condensate. By adjusting dipolar interaction and spin-orbit coupling, we uncover two distinct quantum phase transitions: (i) a first-order transition from a single skyrmion superfluid to a triangular meron supersolid, and (ii) a second-order transition from this superfluid to a square skyrmion supersolid. These phases are characterized by their lattice symmetries, nonclassical rotational inertia, and spin textures. Under parity-time symmetric dissipation, we predict phase-dependent damping of the current oscillations, directly linked to the superfluid fraction. The predicted chiral supersolid phase can be experimentally observed in ultracold magnetic atoms with spin-orbit coupling. Our results establish dipolar quantum gases as a platform for designing topological matter with spintronic functionality.

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arXiv2025

Unified Field-integral Thermodynamics of Bose Mixtures: Stability and Critical Behavior

We establish a unified thermodynamic framework for Bose mixtures at finite temperatures based on the functional field integral, within which the decision on whether to discard the anomalous densities, when determining the density configuration and stability matrix, yields distinct theories. Beyond the existing Hartree-Fock approximation and Ota-Giorgini-Stringari theory, retaining the anomalous densities throughout will provide a completely self-consistent thermodynamic description, requiring the combination of the Hartree-Fock-Bogoliubov approximation and the representative statistical ensemble. Comparing three approaches for predicting magnetic susceptibility, we highlight the role of anomalous densities in stabilizing superfluid mixtures. We further unveil that Feshbach coupling can either expand the regime of atomic and molecular superfluids, or induce a phase transition to a pure molecular superfluid, depending on their density ratio. Importantly, we show that thermal fluctuations will trigger a phase transition from stable to unstable mixtures, where anomalous densities can serve as distinct signatures for experimental observation.

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arXiv2025

Asymptotic Freedom of Two Heavy Impurities in a Bose-Einstein Condensate

We consider two heavy impurities immersed in a Bose-Einstein condensate, and calculate the self-energy using the Wilsonian renormalization. The polaron energy, quasiparticle residue and damping rate are extracted from the self-energy. We demonstrate that various effective potentials emerge from the polaron energy under the specific conditions. In the limit of large separation between the impurities, the polaron spectrum converges to the results for a single impurity, exhibiting an attractive-repulsive crossover across the Feshbach resonance. The boundary of this crossover is identified through the analysis of the damping rate. We highlight that repulsive-dominant polarons can exist as long as the impurities are sufficiently close, even when the impurity-boson interactions are attractive. Additionally, we observe that the two impurities become asymptotically free in the repulsive polaron regime. These results are verifiable and offer a fresh perspective on the interaction dynamics between two polarons.

cond-mat.quant-gas↗
arXiv2025

Theoretical Exploration of Phase Transitions in a Cavity-BEC System with Two Crossed Optical Pumps

We consider a Bose-Einstein condensation (BEC) inside an optical cavity and two crossed coherent pump fields. We determine the phase boundary separating the normal superfluid phase and the superradiance phase, perturbatively. In the regime of negative cavity detuning, we map out the phase diagrams both for an attractive and a repulsive optical lattice. It turns out that the situation is quite different in two cases. Specifically, in the case of attractive lattice, if a system is in the superradiant phase with one pump laser, adding another pump does not drive the system out of the superradiance phase. While for the repulsive lattice, increasing another pump potential have suppressive effects on the superradiance. We also find that, in the case of attractive lattice, equally increasing two pump lattice potentials can induce a transition from the normal phase to the superradiance phase. In stark contrast, for the repulsive lattice, the system will remain in the normal phase as the pump depths are tuned within a wide range, independent of the cavity detuning and the decay rate.

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arXiv2025

Mediated Interactions and Damping Effects in Superfluid Mixtures of Bose and Fermi Gases

We investigate the homogeneous superfluid mixtures of Bardeen-Cooper-Schrieffer~(BCS) superfluid originating from pairing two-species fermionic atoms and superfluidity stemming from condensation of bosonic atoms. By integrating out the freedoms associated with the BCS superfluid, we derive the fermion-mediated interactions between bosons, which is attractive and can be tuned from long range in the BCS region to short range in the region of Bose-Einstein condensation (BEC) of molecular dimers. By analyzing the Bogoliubov spectrum and the damping rate of bosonic superfluid, we map out the phase diagram spanned by the boson-fermion mass ratio and the boson-fermion coupling strength, which consists of a phase separation region and two phase mixing regions with and without Landau damping. The three different phases can coexist at a tricritical point, which moves toward low boson-fermion mass ratio and high boson-fermion scattering length as the fermion-fermion interaction strength is tuned up on the BCS side.

cond-mat.quant-gas↗
arXiv2024

Quantum Triticality of Bosonic Atomic-Molecular Mixtures with Feshbach Coupling

We develop a functional integral formulation for a homogeneous bosonic atomic-molecular mixture with Feshbach coupling in three-spatial dimensions. Taking phase stability into account, we establish a rich ground-state phase diagram, which features three regions: molecular superfluid (MSF), atomic-molecular superfluid (AMSF), and phase separation (PS). The system can accommodate up to two tricritical points where the three regions meet, with one tricritical point being intrinsic and the other being conditional. Strikingly, we show that the sound velocity vanishes as the AMSF phase touches on the border of phase separation lines. We find that quantum fluctuations correction to the ground-state energy and quantum depletion of the condensates vary nonmonotonically with Feshbach coupling strength as well as molecular percentage. Correlation functions such as pairing amplitudes, density structure factor and spin density structure factor show characteristic behaviors when the system crosses phase transitions. Our work paves the way for future advancement toward understanding salient physics of atom-molecular mixtures.

cond-mat.quant-gas↗
arXiv2020

Ultracold Bose Mixtures with Spin-Dependent Fermion-Mediated Interactions

We develop a functional integral formulation for binary Bose-Einstein condensates coupled to polarized fermions. We find that spin-dependent fermion-mediated interactions have dramatic effects on the properties of the binary condensates. The quasiparticle spectrum features two branches. The upper branch, which is of density nature, gets modified by the induced interactions, while the lower branch, which is of spin nature, is left intact. The ground-state phase diagram consists of stable region of miscible phases and unstable region toward phase separation. In the stable region, it is further classified by the damping of excitations of the upper branch. We show that it is possible to find region of well-defined, long-lived quasiparticle excitations by tuning relevant parameters, such as boson-fermion mass ratio, boson-fermion number density ratio, and interspecies interactions between bosons as well. We explore the effects of quantum fluctuation due to the effective potential on the binary condensates. It turns out that both the density structure factor and spin density structure factor fulfill the Feynman relation, except that the latter is immune to the fermion-mediated interactions.

cond-mat.quant-gas↗
arXiv2020

Polarizing the Medium: Fermion-Mediated Interactions between Bosons

We consider a homogeneous mixture of bosons and polarized fermions. We find that long-range and attractive fermion-mediated interactions between bosons have dramatic effects on the properties of the bosons. We construct the phase diagram spanned by boson-fermion mass ratio and boson-fermion scattering parameter. It consists of stable region of mixing and unstable region toward phase separation. In stable mixing phase, the collective long-wavelength excitations can either be well-behaved with infinite lifetime or be finite in lifetime suffered from the Landau damping. We examine the effects of the induced interaction on the properties of weakly interacting bosons. It turns out that the induced interaction not only enhances the repulsion between the bosons against collapse but also enhances the stability of the superfluid state by suppressing quantum depletion.

cond-mat.quant-gas↗
arXiv2019

Tuning Dissipation and Excitations in Superfluid Fermi Gases with a Moving Impurity

We develop a method to extract the dissipation for a heavy moving impurity immersed in superfluid Fermi gases. The drag force is derived analytically. As a reward, we are able to extract the dynamical structure factor, from which density excitations of the system is carefully examined. We show that dissipations through drag force is associated with two types of excitations, one being single-particle and the other being collective. We map out the critical velocity for dissipation across the BEC-BCS crossover, consistent with existing experiments. For a magnetic impurity, we show that the dissipation is immune to collective excitations. Our study clearly manifests that dissipation and associated excitations can be controlled by coupling superfluid Fermi gases with a moving impurity, and paves the way for further exploring intriguing realm of nonequilibrium phenomena and dissipation dynamics.

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arXiv2018

Searching for Supersolidity in Ultracold Atomic Bose Condensates with Rashba Spin-Orbit Coupling

We developed functional integral formulation for the stripe phase of a spinor Bose-Einstein condensates with Rashba spin-orbit coupling. The excitation spectrum is found to exhibit double gapless band structures, identified to be two Goldstone modes resulting from spontaneously broken internal gauge symmetry and translational invariance symmetry. The sound velocities display anisotropic behaviors with the lower branch vanishes in the direction perpendicular to the stripe in the x-y plane. At the transition point between the plane wave phase and the stripe phase, physical quantities such as fluctuation correction to the ground state energy and quantum depletion of the condensates exhibit discontinuity, characteristic of the first order phase transition. Despite strong quantum fluctuations induced by Rashba spin-orbit coupling, we show that the supersolid phase is stable against quantum depletion. Finally we extend our formulation to finite temperatures to account for interactions between excitations.

cond-mat.quant-gas↗
arXiv2018

Theoretical exploration of competing phases of lattice Bose gases in a cavity

We consider bosonic atoms loaded into optical lattices with cavity-mediated infinite-range interactions. Competing short- and global-range interactions cultivates a rich phase diagram. With a systematic field-theoretical perspective, we present an $\emph{analytical}$ construction of global ground-state phase diagram. We find that the infinite-range interaction enhances the fluctuation of the number density. In the strong coupling regime, we find four branches of elementary excitations with two being "partilce-like" and two being "hole-like", and that the excitation gap becomes soft at the phase boundary between compressible phases and incompressible phases. We derive an effective theory describing compressible superfluid and supersolid states. To complement this perturbative study, we construct a self-consistent mean-field theory and find numerical results consistent with our theoretical analysis. We map out the phase diagram and find that a charge density wave may undergo a structure phase transition to a different charge density wave before it finally enters into the supersolid phase driven by increasing the hopping amplitude.

cond-mat.quant-gas↗
arXiv2017

Hidden long-range order in a spin-orbit coupled two-dimensional Bose gas

A spin-orbit coupled two-dimensional (2D) Bose gas is shown to simultaneously possess quasi and true long-range order in the total and relative phase sectors, respectively. The total phase undergoes a Berenzinskii- Kosterlitz-Thouless transition to a low temperature phase with quasi long-range order, as expected for a two- dimensional quantum gas. Additionally, the relative phase undergoes an Ising-type transition building up true long-range order, which is induced by the anisotropic spin-orbit coupling. Based on the Bogoliubov approach, expressions for the total- and relative-phase fluctuations are derived analytically for the low temperature regime. Numerical simulations of the stochastic projected Gross-Pitaevskii equation (SPGPE) give a good agreement with the analytical predictions.

cond-mat.quant-gas↗
arXiv2017

Bose-Einstein condensates in the presence of Weyl spin-orbit coupling

We consider two-component Bose-Einstein condensates subject to Weyl spin-orbit coupling. We obtain mean-field ground state phase diagram by variational method. In the regime where interspecies coupling is larger than intraspecies coupling, the system is found to be fully polarized and condensed at a finite momentum lying along the quantization axis. We characterize this phase by studying the excitation spectrum, the sound velocity, the quantum depletion of condensates, the shift of ground state energy, and the static structure factor. We find that spin-orbit coupling and interspecies coupling generally leads to competing effects.

cond-mat.quant-gas↗
arXiv2015

Motion of an Impurity in a Bose-Einstein Condensate with Weyl Spin-Orbit Coupling: Non-collinear Drag Force and Anisotropic Critical Velocity

We consider the motion of a point-like impurity through a three-dimensional two-component Bose-Einstein condensate subject to Weyl spin-orbit coupling. Using linear-response theory, we calculate the drag force felt by the impurity and the associated anisotropic critical velocity from the spectrum of elementary excitations. The drag force is shown to be generally not collinear with the velocity of the impurity. This unusual behavior is a consequence of condensation into a finite-momentum state due to the spin-orbit coupling.

cond-mat.quant-gas↗
arXiv2015

Multicriticality, Metastability, and Roton Feature in Bose-Einstein Condensates with Three-Dimensional Spin-Orbit Coupling

We theoretically study homogeneously trapped atomic Bose-Einstein condensates where all three momentum components couple to a pseudo-spin-$1/2$ degree of freedom. Tuning the anisotropies of spin-orbit coupling and the spin-dependent interactions is shown to provide access to a rich phase diagram with a tetracritical point, first-order phase transitions, and multiple metastable phases of stripe and plane-wave character. The elementary excitation spectrum of the axial plane-wave phase features an anisotropic roton feature and can be used to probe the phase diagram. In addition to providing a versatile laboratory for studying fundamental concepts in statistical physics, the emergence of metastable phases creates new opportunities for observing false-vacuum decay and bubble nucleation in ultra-cold-atom experiments.

cond-mat.quant-gas↗
arXiv2014

Spin-Orbit Coupled Bose Gases at Finite Temperatures

Spin-orbit coupling is predicted to have dramatic effects on thermal properties of a two-component atomic Bose gas. We show that in three spatial dimensions it lowers the critical temperature of condensation and enhances thermal depletion of the condensate fraction. In two dimensions we show that spin-orbit coupling destroys superfluidity at any finite temperature, modifying dramatically the cerebrated Berezinskii-Kosterlitz-Thouless scenario. We explain this by the increase of the number of low energy states induced by spin-orbit coupling, enhancing the role of quantum fluctuations.

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