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Hui Hu

Publications and source records attributed to Hui Hu.

At least 163 records · Page 9Linked to original sources
arXiv2012

Critical temperature of a Rashba spin-orbit coupled Bose gas in harmonic traps

We investigate theoretically Bose-Einstein condensation of an ideal, trapped Bose gas in the presence of Rashba spin-orbit coupling. Analytic results for the critical temperature and condensate fraction are derived, based on a semi-classical approximation to the single-particle energy spectrum and density of states, and are compared with exact results obtained by explicitly summing discrete energy levels for small number of particles. We find a significant decrease of the critical temperature and of the condensate fraction due to a finite spin-orbit coupling. For large coupling strength and finite number of particles $N$, the critical temperature scales as $N^{2/5}$ and $N^{2/3}$ in three and two dimensions, respectively, contrasted to the predictions of $N^{1/3}$ and $N^{1/2}$ in the absence of spin-orbit coupling. Finite size corrections in three dimensions are also discussed.

cond-mat.quant-gas↗
arXiv2012

Half-quantum vortex state in a spin-orbit coupled Bose-Einstein condensate

We investigate theoretically the condensate state and collective excitations of a two-component Bose gas in two-dimensional harmonic traps subject to isotropic Rashba spin-orbit coupling. In the weakly interacting regime when the inter-species interaction is larger than the intra-species interaction ($g_{\uparrow\downarrow}>g$), we find that the condensate ground state has a half-quantum-angular-momentum vortex configuration with spatial rotational symmetry and skyrmion-type spin texture. Upon increasing the interatomic interaction beyond a threshold $g_{c}$, the ground state starts to involve higher-order angular momentum components and thus breaks the rotational symmetry. In the case of $g_{\uparrow\downarrow} g_{c}$ and $g_{\uparrow\downarrow}<g$) can be determined by solving the Bogoliubov equations for collective density oscillations of the half-quantum vortex state, and by analyzing the softening of mode frequencies. We present the phase diagram as functions of the interatomic interactions and the spin-orbit coupling. In addition, we directly simulate the time-dependent Gross-Pitaevskii equation to examine the dynamical properties of the system. Finally, we investigate the stability of the half-quantum vortex state against both the trap anisotropy and anisotropy in the spin-orbit coupling term.

cond-mat.quant-gas↗
arXiv2011

Density distribution of a trapped two-dimensional strongly interacting Fermi gas

We calculate and measure the density distribution and cloud size of a trapped two-dimensional $^{6}$Li Fermi gas near a Feshbach resonance at low temperatures. Density distributions and cloud sizes are calculated for a wide range of interaction parameters using a local density approximation (LDA) and a zero-temperature equation of state obtained from quantum Monte Carlo simulations reported by G. Bertaina and S. Giorgini, Phys. Rev. Lett. \textbf{106}, 110403 (2011). We find that LDA predictions agree well with experimental measurements across a Feshbach resonance. Theoretical results for Tan's contact parameter in a trapped gas are reported along with predictions for static structure factor at large momentum which could be measured in future Bragg spectroscopy experiments on two-dimensional Fermi gases.

cond-mat.quant-gas↗
arXiv2011

Confinement induced resonances in anharmonic waveguides

We develop the theory of anharmonic confinement-induced resonances (ACIR). These are caused by anharmonic excitation of the transverse motion of the center of mass (COM) of two bound atoms in a waveguide. As the transverse confinement becomes anisotropic, we find that the COM resonant solutions split for a quasi-1D system, in agreement with recent experiments. This is not found in harmonic confinement theories. A new resonance appears for repulsive couplings ($a_{3D}>0$) for a quasi-2D system, which is also not seen with harmonic confinement. After inclusion of anharmonic energy corrections within perturbation theory, we find that these ACIR resonances agree extremely well with anomalous 1D and 2D confinement induced resonance positions observed in recent experiments. Multiple even and odd order transverse ACIR resonances are identified in experimental data, including up to N=4 transverse COM quantum numbers.

cond-mat.quant-gas↗
arXiv2011

Rashba spin-orbit coupled atomic Fermi gases

We investigate theoretically BEC-BCS crossover physics in the presence of a Rashba spin-orbit coupling in a system of two-component Fermi gas with and without a Zeeman field that breaks the population balance between the two components. A new bound state (Rashba pair) emerges because of the spin-orbit interaction. We study the properties of Rashba pairs using a standard pair fluctuation theory. At zero temperature, the Rashba pairs condense into a macroscopic mixed spin state. We discuss in detail the experimental signatures for observing the condensation of Rashba pairs by calculating various physical observables which characterize the properties of the system and can be measured in experiment.

cond-mat.quant-gas↗
arXiv2011

Single Impurity In Ultracold Fermi Superfluids

The role of impurities as experimental probes in the detection of quantum material properties is well appreciated. Here we study the effect of a single classical magnetic impurity in trapped ultracold Fermi superfluids. Depending on its shape and strength, a magnetic impurity can induce single or multiple mid-gap bound states in a superfluid Fermi gas. The multiple mid-gap states could coincide with the development of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase within the superfluid. As an analog of the Scanning Tunneling Microscope, we propose a modified RF spectroscopic method to measure the local density of states which can be employed to detect these states and other quantum phases of cold atoms. A key result of our self consistent Bogoliubov-de Gennes calculations is that a magnetic impurity can controllably induce an FFLO state at currently accessible experimental parameters.

cond-mat.quant-gas↗
arXiv2011

Probing anisotropic superfluidity of rashbons in atomic Fermi gases

Motivated by the prospect of realizing a Fermi gas of $^{40}$K atoms with a synthetic non-Abelian gauge field, we investigate theoretically a strongly interacting Fermi gas in the presence of a Rashba spin-orbit coupling. As the two-fold spin degeneracy is lifted by spin-orbit interaction, bound pairs with mixed singlet and triplet pairings (referred to as rashbons) emerge, leading to an anisotropic superfluid. We show that this anisotropic superfluidity can be probed via measuring the momentum distribution and single-particle spectral function in a trapped atomic $^{40}$K cloud near a Feshbach resonance.

cond-mat.quant-gas↗
arXiv2011

Universal contact of strongly interacting fermions at finite temperatures

The recently discovered universal thermodynamic behaviour of dilute, strongly interacting Fermi gases also implies a universal structure in the many-body pair-correlation function at short distances, as quantified by the contact ${\cal I}$. This quantity is an excellent indicator of the presence of strong correlations in these systems, which provide a highly accessible physical model for other strongly correlated quantum fluids. Here we theoretically calculate the temperature dependence of this universal contact for a Fermi gas in free space and in a harmonic trap. At high temperatures above the Fermi degeneracy temperature, $T\gtrsim T_{F}$, we obtain a reliable non-perturbative quantum virial expansion up to third order. At low temperatures we compare different approximate strong coupling theories. These make different predictions, which need to be tested either by future experiments or advanced quantum Monte Carlo simulations. We conjecture that in the universal unitarity limit, the contact or correlation decreases monotonically with increasing temperature, unless the temperature is significantly lower than the critical temperature, $T\ll T_{c}\sim0.2T_{F}$. We also discuss briefly how to measure the universal contact either in homogeneous or harmonically trapped Fermi gases.

cond-mat.quant-gas↗
arXiv2011

Josephson effect in an atomic Fulde-Ferrell-Larkin-Ovchinnikov superfluid

We study theoretically two spatially separate quasi-one-dimensional atomic Fermi gases in a double-well trap. By tuning independently their spin polarizations, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid or a Bardeen-Cooper-Schrieffer (BCS) superfluid may be formed in each well. We seek the possibility of creating a spatially tunable atomic Josephson junction between two superfluids, which is supposed to be realizable via building a weak link at given positions of the double-well barrier. We show that within mean-field theory the maximum Josephson current is proportional to the order parameter in two wells. Thus, the spatial inhomogeneity of the FFLO order parameter in one well may be directly revealed through the current measurement with the position-tunable link. We anticipate that this type of Josephson measurements can provide a useful evidence for the existence of exotic FFLO superfluids. Possible experimental realizations of the Josephson measurements in atomic Fermi gases are discussed.

cond-mat.supr-con↗
arXiv2011

Universal dynamic structure factor of a strongly correlated Fermi gas

Universality of strongly interacting fermions is a topic of great interest in diverse fields. Here we investigate theoretically the universal dynamic density response of resonantly interacting ultracold Fermi atoms in the limit of either high temperature or large frequency: (1) At high temperature, we use quantum virial expansion to derive universal, non-perturbative expansion functions of dynamic structure factor; (2) At large momentum, we identify that the second-order expansion function gives the Wilson coefficient used in the operator expansion product method. The dynamic structure factor is therefore determined by its second-order expansion function with an overall normalization factor given by Tan's contact parameter. We show that the spin parallel and antiparallel dynamic structure factors have respectively a tail of the form $\sim \pm ω^{-5/2}$ for $ω\rightarrow\infty$, decaying slower than the total dynamic structure factor found previously ($\sim ω^{-7/2}$). Our predictions for dynamic structure factor at high temperature or large frequency are testable using Bragg spectroscopy for ultracold atomic Fermi gases.

cond-mat.quant-gas↗
arXiv2011

Quantitative comparison between theoretical predictions and experimental results for Bragg spectroscopy of a strongly interacting Fermi superfluid

Theoretical predictions for the dynamic structure factor of a harmonically trapped Fermi superfluid near the BEC-BCS crossover are compared with recent Bragg spectroscopy measurements at large transferred momenta. The calculations are based on a random-phase (or time-dependent Hartree-Fock-Gorkov) approximation generalized to the strongly interacting regime. Excellent agreement with experimental spectra at low temperatures is obtained, with no free parameters. Theoretical predictions for zero-temperature static structure factor are also found to agree well with the experimental results and independent theoretical calculations based on the exact Tan relations. The temperature dependence of the structure factors at unitarity is predicted.

cond-mat.quant-gas↗
arXiv2010

Confinement-induced resonance in quasi-one-dimensional systems under transversely anisotropic confinement

We theoretically investigate the confinement-induced resonance for quasi-one-dimensional quan- tum systems under transversely anisotropic confinement, using a two-body s-wave scattering model in the zero-energy collision limit. We predict a single resonance for any transverse anisotropy, whose position shows a slight downshift with increasing anisotropy. We compare our prediction with the recent experimental result by Haller et al. [Phys. Rev. Lett. 104, 153203 (2010)], in which two resonances are observed in the presence of transverse anisotropy. The discrepancy between theory and experiment remains to be resolved.

cond-mat.quant-gas↗
arXiv2010

Virial expansion for a strongly correlated Fermi gas with imbalanced spin populations

Quantum virial expansion provides an ideal tool to investigate the high-temperature properties of a strongly correlated Fermi gas. Here, we construct the virial expansion in the presence of spin population imbalance. Up to the third order, we calculate the high-temperature free energy of a unitary Fermi gas as a function of spin imbalance, with infinitely large, attractive or repulsive interactions. In the latter repulsive case, we show that there is no itinerant ferromagnetism when quantum virial expansion is applicable. We therefore estimate an upper bound for the ferromagnetic transition temperature $T_{c}$. For a harmonically trapped Fermi gas at unitarity, we find that $(T_{c})_{uppper}<T_{F}$, where $T_{F}$ is the Fermi temperature at the center of the trap. Our result for the high-temperature equations of state may confront future high-precision thermodynamic measurements.

cond-mat.quant-gas↗
arXiv2010

Universal thermodynamics of a strongly interacting Fermi gas: theory versus experiment

Strongly interacting, dilute Fermi gases exhibit a scale-invariant, universal thermodynamic behaviour. This is notoriously difficult to understand theoretically because of the absence of a small interaction parameter. Here we present a systematic comparison of theoretical predictions from different quantum many-body theories with recent experimental data of Nascimbene et. al. (arXiv:0911.0747v1). Our comparisons have no adjustable parameters, either theoretically or experimentally. A simple Gaussian pair fluctuation theory is shown to give the best quantitative agreement in the superfluid state below threshold. In the normal state, we also calculate the equation of state by using a quantum cluster expansion theory and explore in detail its applicability to low temperatures. Using the accurate experimental result for the thermodynamic function $S(T)$, we determine the temperature $T$ of a trapped Fermi gas at unitarity as a function of a non-interacting temperature $T_{i}$ which can be obtained by an adiabatic sweep to the free gas limit. By analyzing the recent experimental data, we find a normal-superfluid transition temperature $(T/T_{F})_{c}=0.19\pm0.02$ or $(T_{i}/T_{F})_{c}=0.16\pm0.02$ in a harmonic trap, where $T_{F}$ is the Fermi temperature for a trapped ideal, non-interacting Fermi gas.

cond-mat.quant-gas↗
arXiv2010

Dynamic response of strongly correlated Fermi gases in the quantum virial expansion

By developing a quantum virial expansion theory, we quantitatively calculate the dynamic density response function of a trapped strongly interacting Fermi gas at high temperatures near unitarity. A clear transition from atomic to molecular responses is identified in the spectra when crossing from the BCS to BEC regimes, in qualitative agreement with recent Bragg spectroscopy observations. Our virial expansion method provides a promising way to solve the challenging strong-coupling problems and is applicable to other dynamical properties of strongly correlated Fermi gases.

cond-mat.quant-gas↗
arXiv2010

Mean-field study of itinerant ferromagnetism in trapped ultracold Fermi gases: Beyond the local density approximation

We theoretically investigate the itinerant ferromagnetic transition of a spherically trapped ultracold Fermi gas with spin imbalance under strongly repulsive interatomic interactions. Our study is based on a self-consistent solution of the Hartree-Fock mean-field equations beyond the widely used local density approximation. We demonstrate that, while the local density approximation holds in the paramagnetic phase, after the ferromagnetic transition it leads to a quantitative discrepancy in various thermodynamic quantities even with large atom numbers. We determine the position of the phase transition by monitoring the shape change of the free energy curve with increasing the polarization at various interaction strengths.

cond-mat.quant-gas↗
arXiv2010

Pseudo-gap pairing in ultracold Fermi atoms

The crossover from a BEC (Bose-Einstein condensation) to a BCS (Bardeen-Cooper-Schrieffer) superfluid in dilute gases of ultracold Fermi atoms creates an ideal environment to enrich our knowledge of strongly correlated many-body systems. These experiments are relevant to a wide range of fields from condensed matter to astrophysics. The nature of pairing in strongly interacting Fermi gases can be readily studied, thus aiding our understanding of related problems in high-T_{c} superconductors, whose mechanism is still under debate. These are not well-understood due to the large interaction parameter. Here, we calculate the dynamical properties of a normal, trapped, and strongly correlated Fermi gas, by developing a quantum cluster expansion. In ultra-cold atomic physics one can measure the elementary excitations, using rf or Bragg spectroscopy. Our calculations for the single-particle spectral function agree with the recent measurements, and clearly demonstrate pseudogap pairing in the strongly interacting regime.

cond-mat.quant-gas↗