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Hui Zhai

Publications and source records attributed to Hui Zhai.

At least 145 records · Page 8Linked to original sources
arXiv2011

Spin-Orbit Coupled Fermi Gases across a Feshbach Resonance

In this letter we study both ground state properties and the superfluid transition temperature of a spin-1/2 Fermi gas across a Feshbach resonance with a synthetic spin-orbit coupling, using mean-field theory and exact solution of two-body problem. We show that a strong spin-orbit coupling can significantly enhance the pairing gap for 1/(k_F a_s)<=0 due to increased density-of-state. Strong spin-orbit coupling also significantly enhances the superfluid transition temperature when 1/(k_F a_s)<=0, while suppresses it slightly when 1/(k_F a_s)>0. The universal interaction energy and pair size at resonance are also discussed.

cond-mat.quant-gas↗
arXiv2011

Stability Condition of a Strongly Interacting Boson-Fermion Mixture across an Inter-Species Feshbach Resonance

We study the properties of dilute bosons immersed in a single component Fermi sea across a broad boson-fermion Feshbach resonance. The stability of the mixture requires that the bare interaction between bosons exceeds a critical value, which is a universal function of the boson-fermion scattering length, and exhibits a maximum in the unitary region. We calculate the quantum depletion, momentum distribution and the boson contact parameter across the resonance. The transition from condensate to molecular Fermi gas is also discussed.

cond-mat.quant-gas↗
arXiv2011

The electronic instabilities of the Iron-based superconductors: a variational Monte-Carlo study

We report the first variational Monte Carlo (VMC) study of the iron-based superconductors. We use realistic band structures, and the ordering instabilities/variational ansatzs are suggested by previous functional renormalization group calculations. We examine antiferromagnetism, superconducting pairing, normal state fermi surface distortion, and orbital order in the antiferromagnetic state.

cond-mat.str-el↗
arXiv2011

Spatially Modulated Interaction Induced Bound States and Scattering Resonances

We study the two-body problem with a spatially modulated interaction potential using a two-channel model, in which the inter-channel coupling is provided by an optical standing wave and its strength modulates periodically in space. As the modulation amplitudes increases, there will appear a sequence of bound states. Part of them will cause divergence of the effective scattering length, defined through the phase shift in the asymptotic behavior of scattering states. We also discuss how the local scattering length, defined through short-range behavior of scattering states, modulates spatially in different regimes. These results provide a theoretical guideline for new control technique in cold atom toolbox, in particular, for alkali-earth-(like) atoms where the inelastic loss is small.

cond-mat.quant-gas↗
arXiv2010

Degeneracy of Many-body Quantum States in an Optical Lattice with a Uniform Magnetic Field

We prove a theorem that shows the degeneracy of many-body states depends on total particle number and flux filling ratio, for particles in a periodic lattice and under a uniform magnetic field. Non-interacting fermions and weakly interacting bosons are given as two examples. For the later case, this phenomena can also be understood in terms of destructive quantum interferences of multiple symmetry related tunneling paths between classical energy minima, which is reminiscent of the spin-parity effect discovered in magnetic molecular cluster. We also show that the quantum ground state of a mesoscopic number of bosons in this system is not a simple mean-field state but a fragmented state even for very weak interactions.

cond-mat.quant-gas↗
arXiv2010

Spin-Orbit Coupled Spinor Bose-Einstein Condensates

An effective spin-orbit coupling can be generated in cold atom system by engineering atom-light interactions. In this letter we study spin-1/2 and spin-1 Bose-Einstein condensates with Rashba spin-orbit coupling, and find that the condensate wave function will develop non-trivial structures. From numerical simulation we have identified two different phases. In one phase the ground state is a single plane wave, and often we find the system splits into domains and an array of vortices plays the role as domain wall. In this phase, time-reversal symmetry is broken. In the other phase the condensate wave function is a standing wave and it forms spin stripe. The transition between them is driven by interactions between bosons. We also provide an analytical understanding of these results and determines the transition point between the two phases.

cond-mat.quant-gas↗
arXiv2010

Nodes in the Gap Function of LaFePO, the Gap Function of the Fe(Se,Te) Systems, and the STM Signature of the s$_{\pm}$ Pairing

We reiterate, in more details, our previous proposal of using quasi-particle interference to determine the pairing form factor in iron-based superconductors. We also present our functional renormalization group(FRG) results on LaFePO and Fe(Se,Te) superconductors. In particular we found that the leading pairing channel in LaFePO is nodal s$_{\pm}$, with nodes on electron Fermi surfaces. For Fe(Se,Te) system we found fully gapped s$_{\pm}$ pairing, with substantial gap anisotropy on electron Fermi surfaces, and large gap is concentrated in regions with dominant $xy$ orbital character. We further fit the form factor obtained by FRG to real space orbital basis pairing picture, which shows more clearly the differences between different iron-based superconductors.

cond-mat.supr-con↗
arXiv2010

Stability of a Fully Magnetized Ferromagnetic state in Repulsively Interacting Ultracold Fermi Gases

We construct a variational wave function to study whether a fully polarized Fermi sea is energetically stable against a single spin flip. Our variational wave function contains sufficient short-range correlation at least to the same level as Gutzwiller's projected wave function. For Hubbard lattice model and continuum model with pure repulsive interaction, we show a fully polarized Fermi sea is generally unstable even when the repulsive strength becomes infinite. While for a resonance model, ferromagnetic state is possible if the s-wave scattering length is positive and sufficiently large, and the system is prepared in scattering state orthogonal to molecular bound state. However, we can not rule out the possibility that more exotic correlation can destabilize the ferromagnetic state.

cond-mat.quant-gas↗
arXiv2010

Pairing and Vortex Lattices for Interacting Fermions in Optical Lattices with a Large Magnetic Field

We study the structure of pairing order parameter for spin-1/2 fermions with attractive interactions in a square lattice under a uniform magnetic field. Because the magnetic translation symmetry gives a unique degeneracy in the single-particle spectrum, the wave function has both zero and finite momentum components co-existing, and their relative phases are determined by a self-consistent mean-field theory. We present a microscopic calculation that can determine the vortex lattice structure in the superfluid phase for different flux densities. Phase transition from a Hofstadter insulator to a superfluid phase is also discussed.

cond-mat.quant-gas↗
arXiv2010

Spin Helix of Magnetic Impurities in Two-dimensional Helical Metal

We analyze the Ruderman-Kettel-Kasuya-Yosida(RKKY) interaction between magnetic impurities embedded in the helical metal on the surface of three-dimensional topological insulators. Apart from the conventional RKKY terms, the spin-momentum locking of conduction electrons also leads to a significant Dzyaloshinskii-Moriya (DM) interaction between impurity spins. For a chain of magnetic impurities, the DM term can result in single-handed spin helix on the surface. The handedness of spin helix is locked with the sign of Fermi velocity of the emergent Dirac fermions on the surface. We also show the polarization of impurity spins can be controlled via electric voltage for dilute magnetic impurity concentration.

cond-mat.mtrl-sci↗
arXiv2009

Correlated versus Ferromagnetic State in Repulsively Interacting Two-Component Fermi Gases

Whether a spin-1/2 Fermi gas will become ferromagnetic as the strength of repulsive interaction increases is a long-standing controversial issue. Recently this problem is studied experimentally by Jo et al, Science, 325, 1521 (2009) in which the authors claim a ferromagnetic transition is observed. This work is to point out the results of this experiment can not distinguish whether the system is in a ferromagnetic state or in a non-magnetic but strongly short-range correlated state. A conclusive experimental demonstration of ferromagnetism relies on the observation of ferromagnetic domains.

cond-mat.quant-gas↗
arXiv2009

Antiferromagnetically Driven Electronic Correlation in Iron Pnictides and Cuprates

The iron pnictides and the cuprates represent two families of materials, where strong antiferromagnetic correlation drives three other distinct ordering tendencies: (1) superconducting pairing, (2) Fermi surface distortion, and (3) orbital current order. We propose that (1)-(3) and the antiferromagnetic correlation are the hallmarks of a class of strongly correlated materials to which the cuprates and pnictides belong. In this paper we present the results of the functional renormalization group studies to support the above claim. In addition, we show that as a function of the interlayer hopping parameter, the double layer Hubbard model nicely interpolates between the cuprate and the iron pnictide physics. Finally, as a check, we will present the renormalization group study of a ladder version of the iron pnictide, and compare the results to those of the two-dimensional model.

cond-mat.supr-con↗
arXiv2009

Nodal Spin Density Wave and band topology of the FeAs based materials

The recently discovered FeAs-based materials exhibit a $(π,0)$ Spin Density Wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two band model and a more realistic 5-band model. The positions of the nodes are different in the two models and can be used to detected the validity of each model.

cond-mat.supr-con↗
arXiv2008

A Numerical Renormalization Group Study of the Superconducting and Spin Density Wave Instabilities in MFeAsO$_{1-x}$F$_x$ Compounds

We apply the fermion renormalization group method, implemented numerically by Honerkamp et.al., to a two-band model of FeAs-based materials. At half filling we find the $(π,0)$ or $(0,π)$ spin density wave order and a sub-dominant superconducting pairing tendency. Due to a topological reason, the spin density wave gap has nodes on the fermi surfaces. Away from half filling we find an unconventional s-wave and a sub-dominant $d_{x^2-y^2}$ pairing instability. The former has $s$ symmetry around the hole fermi surface but exhibits $s+d_{x^2-y^2}$ symmetry around the electron pockets where the 90 degree rotation is broken. The pairing mechanism is inter-pocket pair hopping. Interestingly, the same interaction also drives the antiferromagnetism.

cond-mat.supr-con↗
arXiv2008

Quantum Antiferromagnetism of Fermions in Optical Lattices with Half-filled p-band

We study Fermi gases in a three-dimensional optical lattice with five fermions per site, i.e. the s-band is completely filled and the p-band with three-fold degeneracy is half filled. We show that, for repulsive interaction between fermions, the system will exhibit spin-3/2 antiferromagnetic order at low temperature. This conclusion is obtained in strong interaction regime by strong coupling expansion which yields an isotropic spin-3/2 Heisenberg model, and also in weak interaction regime by Hatree-Fock mean-field theory and analysis of Fermi surface nesting. We show that the critical temperature for this antiferromagnetism of a p-band Mott insulator is about two orders of magnitudes higher than that of an $s$-band Mott insulator, which is close to the lowest temperature attainable nowadays.

cond-mat.str-el↗
arXiv2008

Quantum Hall Transition near a Fermion Feshbach Resonance in a Rotating Trap

We consider two-species of fermions in a rotating trap that interact via an s-wave Feshbach resonance, at total Landau level filling factor two (or one for each species). We show that the system undergoes a quantum phase transition from a fermion integer quantum Hall state to a boson fractional quantum Hall state as the pairing interaction strength increases, with the transition occurring near the resonance. The effective field theory for the transition is shown to be that of a (emergent) massless relativistic bosonic field coupled to a Chern-Simons gauge field, with the coupling giving rise to semionic statistics to the emergent particles.

cond-mat.mes-hall↗
arXiv2007

Trapped Fermi Gases in Rotating Optical Lattices: Realization and Detection of the Topological Hofstadter Insulator

We consider a gas of non-interacting spinless fermions in a rotating optical lattice and calculate the density profile of the gas in an external confinement potential. The density profile exhibits distinct plateaus, which correspond to gaps in the single particle spectrum known as the Hofstadter butterfly. The plateaus result from insulating behavior whenever the Fermi energy lies within a gap. We discuss the necessary conditions to realize the Hofstadter insulator in a cold atom setup and show how the quantized Hall conductance can be measured from density profiles using the Středa formula.

cond-mat.mes-hall↗