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Hui Zhai

Publications and source records attributed to Hui Zhai.

162 records · Page 9Linked to original sources
arXiv2007

Effects of a Fluctuating Interface between a Superfluid and a Polarized Fermi Gas

Motivated by recent experiments in trapped Fermi gas with spin population imbalance, we discuss the effects of the quantum and thermal fluctuations of the interface between a fully paired superfluid core and a fully polarized Fermi gas. We demonstrate that even if there is no true partially polarized thermodynamic phase in bulk, the interface fluctuation can give rise to a partially polarized transition regime in trap. Our theory yields a definite prediction for the functional forms of the spatial profile of spin polarization and pairing gap, and we show that the spin-resolved density profiles measured by both the MIT and Rice groups obey this function form. We also show that sufficient large fluctuation will lead to a visibly unequal density even at the center of the cloud. We hope this picture can shed lights on the controversial discrepancies in recent experiments.

cond-mat.supr-con↗
arXiv2007

Criterion for bosonic superfluidity in an optical lattice

We show that the current method of determining superfluidity in optical lattices based on a visibly sharp bosonic momentum distribution $n({\bf k})$ can be misleading, for even a normal Bose gas can have a similarly sharp $n({\bf k})$. We show that superfluidity in a homogeneous system can be detected from the so-called visibility $(v)$ of $n({\bf k})$ $-$ that $v$ must be 1 within $O(N^{-2/3})$, where $N$ is the number of bosons. We also show that the T=0 visibility of trapped lattice bosons is far higher than what is obtained in some current experiments, suggesting strong temperature effects and that these states can be normal. These normal states allow one to explore the physics in the quantum critical region.

cond-mat.other↗
arXiv2007

Superfluid-Insulator Transition of Strongly Interacting Fermi Gases in Optical Lattices

We study a quantum phase transition between fermion superfluid (SF) and band insulator (BI) of fermions in optical lattices. The destruction of the band insulator is driven by the energy gain in promoting fermions from valance band to various conducting bands to form Cooper pairs. We show that the transition must take place in rather shallow lattice height, $V_{o}/E_{R}$ between 2.23 and 4.14. The latter is the prediction of mean field theory while the former is the value for opening a band gap. As one moves across resonance to the molecule side, the SF-BI transition evolves into the SF-Mott insulator transition of bosonic molecules. We shall also present the global phase diagram for SF-Insulator transition for the BCS-BEC family.

cond-mat.supr-con↗
arXiv2006

Critical Rotational Frequency for Superfluid Fermionic Gases across a Feshbach Resonance

We present a method to determine the critical rotational frequencies for superfluidity of both uniform and trapped Fermi gases across wide Feshbach resonance. It is found that as one approaches the resonance from the BCS side, beyond a critical scattering length, pairing is so robust that superfluidity cannot be destroyed by rotation. Moreover, the critical frequency has a sequence of jumps revealing the appearance of Landau levels, which are particularly prominent for systems up to a few thousand particles. For rotational frequency below an "ultimate" critical frequency, defined to be the lowest frequency at which the center of the cloud goes normal, a trapped gas has a superfluid core surrounded by a normal gas, as seen in recent experiments.

cond-mat.supr-con↗
arXiv2006

Superfluidity in Three-species Mixture of Fermi Gases across Feshbach Resonances

In this letter a generalization of the BEC-BCS crossover theory to a multicomponent superfluid is presented by studying a three-species mixture of Fermi gas across two Feshbach resonances. At the BEC side of resonances, two kinds of molecules are stable which gives rise to a two-component Bose condensate. This two-component superfluid state can be experimentally identified from the radio-frequency spectroscopy, density profile and short noise measurements. As approaching the BCS side of resonances, the superfluidity will break down at some point and yield a first-order quantum phase transition to normal state, due to the mismatch of three Fermi surfaces. Phase separation instability will occur around the critical regime.

cond-mat.supr-con↗
arXiv2006

Homogeneous Fermion Superfluid with Unequal Spin Populations

For decades, the conventional view is that an s-wave BCS superfluid can not support uniform spin polarization due to a gap $Δ$ in the quasiparticle excitation spectrum. We show that this is an artifact of the dismissal of quasiparticle interactions $V_{qp}^{}$ in the conventional approach at the outset. Such interactions can cause triplet fluctuations in the ground state and hence non-zero spin polarization at "magnetic field" $h<Δ$. The resulting ground state is a pairing state of quasiparticles on the ``BCS vacuum". For sufficiently large $V_{qp}$, the spin polarization of at unitarity has the simple form $m\propto μ^{1/2}$. Our study is motivated by the recent experiments at Rice which found evidence of a homogenous superfluid state with uniform spin polarization.

cond-mat.supr-con↗
arXiv2005

Vortex Lattice in the Planar Bose-Einstein Condensates with Dipolar Interactions

In this letter we investigate the effects of dipole-dipole interactions on the vortex lattices in fast rotating Bose-Einstein condensates. For single planar condensate, we show that the triangular lattice structure will be unfavorable when the s-wave interaction is attractive and exceeds a critical value. It will first change to a square lattice, and then become more and more flat with the increase of s-wave attraction, until the collapse of the condensate. For an array of coupled planar condensates, we discuss how the dipole-dipole interactions between neighboring condensates compete with the quantum tunneling processes, which affects the relative displacement of two neighboring vortex lattices and leads to the loss of phase coherence between different condensates.

cond-mat.other↗
arXiv2005

Pairing between Atoms and Molecules in a Boson-Fermion Resonant Mixture

We consider a mixture of fermionic and bosonic atoms nearby interspecies Feshbach resonances, which have been observed recently in $^6$Li-$^{23}$Na mixture by MIT group, and in $^{40}$K-$^{87}$Rb mixture by JILA group. We point out that the fermion-boson bound state, namely the heteronuclear molecules, will coexist with the fermionic atoms in a wide parameter region, and the attraction between fermionic atoms and molecules will lead to the formation of atom-molecule pairs. The pairing structure is studied in detail, and, in particular, we highlight the possible realization of the Fulde-Ferrel-Larkin-Ovchinnikov state in this system.

cond-mat.supr-con↗
arXiv2005

A Wave Function Describing Superfluidity in a Perfect Crystal

We propose a many-body wave function that exhibits both diagonal and off-diagonal long-range order. Incorporating short-range correlations due to interatomic repulsion, this wave function is shown to allow condensation of zero-point lattice vibrations and phase rigidity. In the presence of an external velocity field, such a perfect crystal will develop non-classical rotational inertia, exhibiting the supersolid behavior. In a sample calculation we show that the superfluid fraction in this state can be as large as of order 0.01 in a reasonable range of microscopic parameters. The relevance to the recent experimental evidence of a supersolid state by Chan and Kim is discussed.

cond-mat.stat-mech↗
arXiv2005

Magnetically Stabilized Order. II: Critical states and algebraically ordered nematic spin liquids in one-dimensional optical lattices

We investigate the Zeeman-field-driven quantum phase transitions between singlet spin liquids and algebraically ordered O(2) nematic spin liquids of spin-one bosons in one-dimensional optical lattices. We find that the critical behavior is characterized by condensation of hardcore bosons instead of ideal magnons in high dimensional lattices. Critical exponents are strongly renormalized by hardcore interactions and critical states are equivalent to the free Fermion model up to the Friedal oscillations. We also find that the algebraically ordered nematic spin liquids close to critical points are fully characterized by the Luttinger liquid dynamics with Luttinger liquid parameters magnetically tunable. The Bethe Ansatz solution has been applied to determine the critical magnetization and nematic correlations.

cond-mat.stat-mech↗
arXiv2005

Understanding of the Vortex Lattice Behavior Induced by Selective Removal of Atoms from a Bose-Einstein Condensate

It is known that the quantized vortices in a superfluid can be described by a dual electromagnetic model through the duality transformation. Recently a new technique, which can selectively remove atoms from a Bose-Einstein condensate, was applied to the vortex lattice state of the rapid rotating Boson gases. The increase of vortex number, dynamical formation of giant vortex, the oscillation and damping of vortex motion were observed in these experiments. In this paper we will discuss these observations in the framework of dual description and show how to understood these observations naturally.

cond-mat.mes-hall↗
arXiv2005

Two Generalizations of $η$ Pairing in Extended Hubbard Models

A rigorous result about Hubbard model obtained by C.N.Yang [Phys.Rev.Lett.63,2144(1989)] is generalized to two kinds of extended Hubbard models. One is the Hubbard model in two-dimensional triangular lattice, and the other is the Hubbard model for high spins fermions. We obtain the conditions under which a modified $η$-pairing operator is an eigen-operator, thus these models have a series of eigenstates possessing off-diagonal long-range order.

cond-mat.supr-con↗
arXiv2004

Vortex Dipole in a Trapped Atomic Bose-Einstein Condensate

We calculate the angular momentum and energy of a vortex dipole in a trapped atomic Bose-Einstein condensate. Fully analytic expressions are obtained. We apply the results to understand a novel phenomenon in the MIT group experiment, an excellent agreement is achieved, and further experimental investigation is proposed to confirm this vortex dipole mechanism. We then suggest an effective generation and detection of vortex dipole for experimental realization. Application of the sum rule to calculate collective mode frequency splitting is also discussed.

cond-mat.supr-con↗
arXiv2004

Double-Layer Bose-Einstein Condensates with Large Number of Vortices

In this paper we systematically study the double layer vortex lattice model, which is proposed to illustrate the interplay between the physics of a fast rotating Bose-Einstein condensate and the macroscopic quantum tunnelling. The phase diagram of the system is obtained. We find that under certain conditions the system will exhibit one novel phase transition, which is consequence of competition between inter-layer coherent hopping and inter-layer density-density interaction. In one phase the vortices in one layer coincide with those in the other layer. And in another phase two sets of vortex lattices are staggered, and as a result the quantum tunnelling between two layers is suppressed. To obtain the phase diagram we use two kinds of mean field theories which are quantum Hall mean field and Thomas-Fermi mean field. Two different criteria for the transition taking place are obtained respectively, which reveals some fundamental differences between these two mean field states. The sliding mode excitation is also discussed.

cond-mat.mes-hall↗
arXiv2003

Skyrmion Excitation in Two-Dimensional Spinor Bose-Einstein Condensate

We study the properties of coreless vortices(skyrmion) in spinor Bose-Einstein condensate. We find that this excitation is always energetically unstable, it always decays to an uniform spin texture. We obtain the skyrmion energy as a function of its size and position, a key quantity in understanding the decay process. We also point out that the decay rate of a skyrmion with high winding number will be slower. The interaction between skyrmions and other excitation modes are also discussed.

cond-mat.stat-mech↗
arXiv2003

Quantum coherence of double-well BEC: a SU(2)-coherent-state path-integral approach

Macroscopic quantum coherence of Bose gas in a double-well potential is studied based on SU(2)-coherent-state path-integral. The ground state and fluctuations around it can be obtained by this method. In this picture, one can obtain macroscopic quantum superposition states for attractive Bose gas. The coherent gap of degenerate ground states is obtained with the instanton technique. The phenomenon of macroscopic quantum self-trapping is also discussed.

cond-mat↗
arXiv2002

Entanglement Properties of Some Fractional Quantum Hall Liquids

We study the entanglement properties of some fractional quantum Hall liquids. We calculate the entanglement of the Laughlin wave function and the wave functions that are generated by the K-matrix using the modified entanglement measure of indistinguishable fermions that is first proposed by Paškauskas and You[1].

quant-ph↗