arXiv subjects

Hui Zhai

Publications and source records attributed to Hui Zhai.

At least 127 records · Page 7Linked to original sources

arXiv2014

Floquet Topological States in Shaking Optical Lattices

In this letter we propose realistic schemes to realize topologically nontrivial Floquet states by shaking optical lattices, using both one-dimension lattice and two-dimensional honeycomb lattice as examples. The topological phase in the two-dimensional model exhibits quantum anomalous Hall effect. The transition between topological trivial and nontrivial states can be easily controlled by shaking frequency and amplitude. Our schemes have two major advantages. First, both the static Hamiltonian and the shaking scheme are sufficiently simple to implement. Secondly, it requires relatively small shaking amplitude and therefore heating can be minimized. These two advantages make our scheme much more practical.

cond-mat.quant-gas↗

arXiv2013

Superradiance of Degenerate Fermi Gases in a Cavity

In this letter we consider spinless Fermi gases placed inside a cavity and study the critical strength of pumping field for driving a superradiance transition. We emphasize that Fermi surface nesting effect strongly enhances the superradiance tendency. Around certain fillings, when the Fermi surface is nearly nested with a relevant nesting momentum, the susceptibility of the system toward a checkboard density-wave ordered state is strongly enhanced, because of which a much smaller (sometime even vanishingly small) critical pumping field strength can lead to superradiance. This leads to interesting reentrance behavior and topologically distinct structure in the phase diagram. Away from these fillings, the Pauli exclusion principle brings about the dominant effect for which the critical pumping strength is lowered in the low-density regime and increased in the high-density regime, in comparison to a Bose gas with same density. These results open the prospect of studying rich phenomena of degenerate Fermi gases in cavity.

cond-mat.quant-gas↗

arXiv2013

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

In this letter we address the issue 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 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 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 ${}^6$Li and ${}^{40}$K mixture.

cond-mat.quant-gas↗

arXiv2013

Quasi-One-Dimensional Dipolar Quantum Gases

In this letter we consider dipolar quantum gases in a quasi-one-dimensional tube with dipole moment perpendicular to the tube direction. We deduce the effective one-dimensional interaction potential and show that this potential is not purely repulsive, but rather has an attractive part due to high-order scattering processes through transverse excited states. The attractive part can induce bound state and cause scattering resonances. This represents the dipole induced resonance in low-dimension. We work out an unconventional behavior of low-energy phase shift for this effective potential and show how it evolves across a resonance. Based on the phase shift, the interaction energy of spinless bosons is obtained using asymptotic Bethe ansatz. Despite of long-range nature of dipolar interaction, we find that a behavior similar as short-range Lieb-Linger gas emerges at the resonance regime.

cond-mat.quant-gas↗

arXiv2013

Spin-Orbit Coupling Induced Coherent Production of Feshbach Molecules in a Degenerate Fermi Gas

In this work we demonstrate a dynamic process in which SO coupling can coherently produce s-wave Feshbach molecules from a fully polarized Fermi gas, and can induce a coherent oscillation between Feshbach molecules and spin polarized gas. For comparison, we also show that such phenomena are absent if the inter-component coupling is momentum-independent. This demonstrates experimentally that SO coupling does provide finite matrix element between a singlet state and a triplet state, and therefore, implies the bound pairs of a system with SO coupling have triplet p-wave component, which can become topological superfluid by further cooling these pairs to condensation and confining them to lower dimension.

cond-mat.quant-gas↗

arXiv2013

Synthetic Gauge Field with Highly Magnetic Lanthanide Atoms

We present a scheme for generating a synthetic magnetic field and spin-orbit coupling via Raman coupling in highly magnetic lanthanide atoms such as dysprosium. Employing these atoms offer several advantages for realizing strongly correlated states and exotic spinor phases. The large spin and narrow optical transitions of these atoms allow the generation of synthetic magnetic fields an order of magnitude larger than those in the alkalis, but with considerable reduction of the heating rate for equal Raman coupling. The effective hamiltonian of these systems differs from that of the alkalis' by an additional nematic coupling term, which leads to a phase transition in the dressed states as detuning varies. For \text{high-spin} condensates, spin-orbit coupling leads to a spatially periodic structure, which is described in Majorana representation by a set of points moving periodically on a unit sphere. We name this a "Majorana spinor helix" in analogy to the persistent spin-1/2 helix observed in electronic systems.

cond-mat.quant-gas↗

arXiv2013

Stability of Excited Dressed States with Spin-Orbit Coupling

We study the decay behaviors of ultracold atoms in metastable states with spin-orbit coupling (SOC), and demonstrate that there are two SOC-induced decay mechanisms. One arises from the trapping potential and the other is due to interatomic collision. We present general schemes for calculating decay rates from these two mechanisms, and illustrate how the decay rates can be controlled by experimental parameters.We experimentally measure the decay rates over a broad parameter region, and the results agree well with theoretical calculations. This work provides an insight for both quantum simulation involving metastable dressed states and studies on few-body problems with SO coupling.

cond-mat.quant-gas↗

arXiv2012

Properties of Bose Gases with Raman-Induced Spin-Orbit Coupling

In this paper we investigate the properties of Bose gases with Raman-induced spin-orbit(SO) coupling. It is found that the SO coupling can greatly modify the single particle density-of-state, and thus lead to non-monotonic behavior of the condensate depletion, the Lee-Huang-Yang correction of ground-state energy and the transition temperature of a non-interacting Bose-Einstein condensate. The presence of the SO coupling also breaks the Galilean invariance, and this gives two different critical velocities, corresponding to the movement of the condensate and the impurity respectively. Finally, we show that with SO coupling, the interactions modify the BEC transition temperature even at Hartree-Fock level, in contrast to the ordinary Bose gas without SO coupling. All results presented here can be directly verified in the current cold atom experiments using Raman laser-induced gauge field.

cond-mat.quant-gas↗

arXiv2012

Superfluidity of Bosons in Kagome Lattices with Frustration

In this letter we consider spinless bosons in a Kagome lattice with nearest-neighbor hopping and on-site interaction, and the sign of hopping is inverted by insetting a π flux in each triangle of Kagome lattice so that the lowest single particle band is perfectly flat. We show that in the high density limit, despite of the infinite degeneracy of the single particle ground states, interaction will select out the Bloch state at the K point of Brillouin zone for boson condensation at the lowest temperature. As temperature increases, the single boson superfluid order can be easily destroyed, while an exotic triple-boson paired superfluid order will remain. We establish that this trion superfluid exists in a broad temperature regime until the temperature is increased to the same order of hopping and then the system turns into normal phases. Finally we show that time of flight measurement of momentum distribution and its noise correlation can be used to distinguish these three phases.

cond-mat.quant-gas↗

arXiv2012

Fermion Pairing across a Dipolar Interaction Induced Resonance

It is known from the solution of the two-body problem that an anisotropic dipolar interaction can give rise to s-wave scattering resonances, which are named as dipolar interaction induced resonaces (DIIR). In this letter, we study zero-temperature many-body physics of a two-component Fermi gas across a DIIR. In the low-density regime, it is very striking that the resulting pairing order parameter is a nearly isotropic singlet pairing and the physics can be well described by an s-wave resonant interaction potential with finite range corrections, despite of the anisotropic nature of dipolar interaction. The pairing energy is as strong as a unitary Fermi gas nearby a magnetic Feshbach resonance. In the high density regime, the anisotropic effect plays an important role. We find phase transitions from singlet pairing to a state with mixed singlet and triplet pairing, and then from mixed pairing to pure triplet pairing. The state with mixed pairing spontaneously breaks the time-reversal symmetry.

cond-mat.quant-gas↗

arXiv2012

Collective Dipole Oscillation of a Spin-Orbit Coupled Bose-Einstein Condensate

We present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. Dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region, as a function of spin-orbit coupling parameters as well as oscillation amplitude. Anharmonic properties beyond effective-mass approximation are revealed, such as amplitude-dependent frequency and finite oscillation frequency at place with divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility--a static physical quantity--can be measured via dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates magnetic nonzero-momentum condensate from nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a bench mark for recently developed theoretical approaches.

cond-mat.quant-gas↗

arXiv2012

Interaction Induced Hall Response in a Spin-Orbit Coupled Bose-Einstein Condensate

In this letter we consider the dynamic behaviors of spin-orbit coupled Bose condensates realized in recent experiments. We show that there exists an interaction induced ac Hall response which is absent in a non-interacting system. This condensate has two distinct equilibrium phases known as the plane wave phase and the stripe phase. In the plane wave phase, we show that an ac longitudinal current will induce an ac radial current in the transverse direction, and vice versa, as a cooperation effect of spin-velocity locking and spin-dependent interaction. In the stripe phase, we show that the dominant longitudinal response to a transverse radial current is sliding of the density stripe, because it is the low-lying excitation mode originated from spontaneous spatial translational symmetry breaking in this phase.

cond-mat.quant-gas↗

arXiv2012

Highly Polarized Fermi Gases across a Narrow Feshbach Resonance

We address the phase of a highly polarized Fermi gas across a narrow Feshbach resonance starting from the problem of a single down spin fermion immersed in a Fermi sea of up spins. Both polaron and pairing states are considered using the variational wave function approach, and we find that the polaron to pairing transition will take place at the BCS side of the resonance, strongly in contrast to a wide resonance where the transition is located at the BEC side. For pairing phase, we find out the critical strength of repulsive interaction between pairs above which the mixture of pairs and fermions will not phase separate. Therefore, nearby a narrow resonance, it is quite likely that magnetism can coexist with s-wave BCS superfluidity at large Zeeman field, which is a remarkable property absent in conventional BCS superconductors (or fermion pair superfluids).

cond-mat.quant-gas↗

arXiv2012

Spin-Orbit Coupled Degenerate Fermi Gases

Spin-orbit coupling plays an increasingly important role in the modern condensed matter physics. For instance, it gives birth to topological insulators and topological superconductors. Quantum simulation of spin-orbit coupling using ultracold Fermi gases will offer opportunities to study these new phenomena in a more controllable setting. Here we report the first experimental study of a spin-orbit coupled Fermi gas. We observe spin dephasing in spin dynamics and momentum distribution asymmetry in the equilibrium state as hallmarks of spin-orbit coupling. We also observe evidences of Lifshitz transition where the topology of Fermi surfaces change. This serves as an important first step toward finding Majorana fermions in this system.

cond-mat.quant-gas↗

arXiv2012

s-Wave Scattering Resonances Induced by Dipolar Interactions of Polar Molecules

We show that s-wave scattering resonances induced by dipolar interactions in a polar molecular gas have a universal large and positive effective range, which is very different from Feshbach resonances realized in cold atoms before, where the effective range is either negligible or negative. Such a difference has important consequence in many-body physics. At high temperature regime, a positive effective range gives rise to stronger repulsive interaction energy for positive scattering length, and weaker attractive interaction energy for negative scattering length. While at low-temperatures, we study polaron problem formed by single impurity molecule, and we find that the polaron binding energy increases at the BEC side and decreases at the BCS side. All these effects are in opposite to narrow Feshbach resonances where the effective range is negative.

cond-mat.quant-gas↗

arXiv2012

Searching for Non-Abelian Phases in the Bose-Einstein Condensate of Dysprosium

The recently realized Bose-Einstein condensate of Dysprosium will become a spin-8 spinor condensate at ultralow magnetic fields. In such a high spin condensate, many phases with different symmetries can exist. Among them the most interesting ones are those with non-abelian point group symmetry. In this letter we discuss the variety of symmetry phases in a spin-8 condensate resulting from numerical solutions of the Hamiltonian. We show that these symmetries can be determined uniquely from the measurements of density population on each spin component in an ultralow magnetic field, together with the measurements of the collective modes in the zero-field limit. This method can also be applied to Bose-Einstein condensate of other magnetic atoms, such as Cr and Er.

cond-mat.quant-gas↗

arXiv2011

Spin-Orbit Coupled Quantum Gases

In this review we will discuss the experimental and theoretical progresses in studying spin-orbit coupled degenerate atomic gases during the last two years. We shall first review a series of pioneering experiments in generating synthetic gauge potentials and spin-orbit coupling in atomic gases by engineering atom-light interaction. Realization of spin-orbit coupled quantum gases opens a new avenue in cold atom physics, and also brings out a lot of new physical problems. In particular, the interplay between spin-orbit coupling and inter-atomic interaction leads to many intriguing phenomena. Here, by reviewing recent theoretical studies of both interacting bosons and fermions with isotropic Rashba spin-orbit coupling, the key message delivered here is that spin-orbit coupling can enhance the interaction effects, and make the interaction effects much more dramatic even in the weakly interacting regime.

cond-mat.quant-gas↗

arXiv2011

Paired Superfluidity and Fractionalized Vortices in Spin-orbit Coupled Bosons

In this letter we study finite temperature properties of spin-1/2 interacting bosons with spin-orbit coupling in two dimensions. When the ground state has stripe order, we show that thermal fluctuations will first melt the stripe order and lead to a superfluid of boson pairs if the spin-orbit coupling is isotropic or nearly isotropic. Such a phase supports fractionalized quantum vortices. The Kosterlize-Thouless transition from superfluid to normal state is driven by proliferation of half vortices. When the ground state is a plane wave state, the transition to normal state is driven by conventional Kosterlize-Thouless transition. However, the critical temperature will drop to zero for isotropic spin-orbit coupling.

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