Xiaoling Cui

Universal Trimers Induced by Spin-Orbit Coupling in Ultracold Fermi Gases

In this Letter we address the issue of how synthetic spin-orbit (SO) coupling can strongly affect three-body physics in ultracold atomic gases. We consider a system which consists of three fermionic atoms, including two spinless heavy atoms and one spin-1/2 light atom subjected to an isotropic SO coupling. We find that SO coupling can induce universal three-body bound states with a negative s-wave scattering length at a smaller mass ratio, where no trimer bound state can exist if in the absence of SO coupling. The energies of these trimers are independent of the high-energy cutoff, and therefore they are universal ones. Moreover, the resulting atom-dimer resonance can be effectively controlled by SO coupling strength. Our results can be applied to systems like a 6Li and 40K mixture.

Phase separation in mixtures of repulsive Fermi gases driven by mass difference.

We show that phase separation must occur in a mixture of fermions with repulsive interaction if their mass difference is sufficiently large. This phenomenon is highly dimension dependent. Consequently, the density profiles of phase separated 3D mixtures are very different from those in 1D. Noting that the ferromagnetic transition of a spin-1/2 repulsive Fermi gas is the equal mass limit of the phase separation in mixtures, we show from the Bethe ansatz solution that a ferromagnetic transition will take place in the scattering states when the interaction passes through the strongly repulsive regime and becomes attractive.

Visualizing the Efimov Correlation in Bose Polarons

The Bose polaron is a quasiparticle of an impurity dressed by surrounding bosons. In few-body physics, it is known that two identical bosons and a third distinguishable particle can form a sequence of Efimov bound states in the vicinity of interspecies scattering resonance. On the other hand, in the Bose polaron system with an impurity atom embedded in many bosons, no signature of Efimov physics has been reported in the existing spectroscopy measurements to date. In this Letter, we propose that a large mass imbalance between a light impurity and heavy bosons can help produce visible signatures of Efimov physics in such a spectroscopy measurement. Using the diagrammatic approach in the virial expansion to include three-body effects from pair-wise interactions, we determine the impurity self-energy and its spectral function. Taking the ^{6}Li-^{133}Cs system as a concrete example, we find two visible Efimov branches in the polaron spectrum, as well as their hybridizations with the attractive polaron branch. We also discuss the general scenarios for observing the signature of Efimov physics in polaron systems. This work paves the way for experimentally exploring intriguing few-body correlations in a many-body system in the near future.

Enhancement of condensate depletion due to spin-orbit coupling

We show that spin-orbit coupling (SOC) significantly enhances the depletion of a homogeneous Bose-Einstein condensate in three dimensions. With decreasing anisotropy of the SOC, both the quantum and thermal depletion increase. In particular, different types of SOC give rise to qualitatively different dependences of the condensate depletion on microscopic variables including the scattering length, the strength of the SOC, and the temperature, which can be directly observed once these types of SOC are realized in experiments. Moreover, we point out that thermal depletion in three dimensions becomes logarithmically divergent at any given finite temperature when both the SOC and the interaction approach the isotropic limit.

Resonance Scattering in Optical Lattices and Molecules: Interband versus Intraband Effects

We study the low-energy two-body scattering in optical lattices with higher-band effects included in an effective potential, using a renormalization group approach. The approach captures the most dominating higher-band effects as well as all multiple scattering processes in the lowest band. For an arbitrary negative free space scattering length (a(s)), a resonance of low-energy scattering occurs as the lattice potential depths reach a critical value v(c); these resonances, with continuously tunable positions v(c) and widths W, can be mainly driven either by intraband or both intra- and interband effects depending on the magnitude of a(s). We have also studied the scattering amplitudes and the formation of molecules when interband effects are dominating, and discussed an intimate relation between molecules for negative a(s) and repulsively bound states pioneered by Winkler et al. [Nature (London) 441, 853 (2006)].

Polarons in Ultracold Fermi Superfluids

We study a new type of Fermi polaron induced by an impurity interacting with an ultracold Fermi superfluid. Due to the three-component nature of the system, the polaron can become trimer-like with a non-universal energy spectrum. We identify multiple avoided crossings between impurity- and trimer-like solutions in both the attractive and the repulsive polaron spectra. In particular, the widths of avoided crossings gradually increase as the Fermi superfluid undergoes a crossover from the BCS side towards the BEC side, which suggests instabilities towards three-body losses. Such losses can be reduced for interaction potentials with small effective ranges. We also demonstrate, using the second-order perturbation theory, that the mean-field evaluation of the fermion-impurity interaction energy is inadequate even for small fermion-impurity scattering lengths, due to the essential effects of Fermi superfluid and short-range physics in such a system. Our results are practically useful for cold atom experiments on mixtures.

Universal one-dimensional atomic gases near odd-wave resonance

We show the renormalization of the contact interaction for odd-wave scattering in one-dimension (1D). Based on the renormalized interaction, we exactly solve the two-body problem in a harmonic trap and further explore the universal properties of spin-polarized fermions near odd-wave resonance by using the operator-product-expansion method. It is found that the high-momentum distribution behaves as

Universal Borromean Binding in Spin-Orbit-Coupled Ultracold Fermi Gases

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Three-component Ultracold Fermi Gases with Spin-Orbit Coupling

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Partial entropy in finite-temperature phase transitions

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