arXiv subjects

Chuanwei Zhang

Publications and source records attributed to Chuanwei Zhang.

At least 91 records · Page 5Linked to original sources

arXiv2016

Phase tunable Josephson junction and spontaneous mass current in a spin-orbit coupled Fermi superfluid

Atomtronics has the potential for engineering new types of functional devices, such as Josephson junctions (JJs). Previous studies have mainly focused on JJs whose ground states have 0 or $π$ superconducting phase difference across the junctions, while arbitrarily tunable phase JJs may have important applications in superconducting electronics and quantum computation. Here we show that a phase tunable JJ can be implemented in a spin-orbit coupled cold atomic gas with the magnetic tunneling barrier generated by a spin-dependent focused laser beam. We consider the JJ confined in either a linear harmonic trap or a circular ring trap. In the ring trap, the magnetic barrier induces a spontaneous mass current for the ground state of the JJ, demonstrating the magnetoelectric effects of cold atoms.

cond-mat.quant-gas↗

arXiv2016

Structured Weyl Points in Spin-Orbit Coupled Fermionic Superfluids

We demonstrate that a Weyl point, widely examined in 3D Weyl semimetals and superfluids, can develop a pair of non-degenerate gapless spheres. Such a bouquet of two spheres is characterized by three distinct topological invariants of manifolds with full energy gaps, i.e., the Chern number of a 0D point inside one developed sphere, the winding number of a 1D loop around the original Weyl point, and the Chern number of a 2D surface enclosing the whole bouquet. We show that such structured Weyl points can be realized in the superfluid quasiparticle spectrum of a 3D degenerate Fermi gas subject to spin-orbit couplings and Zeeman fields, which supports Fulde-Ferrell superfluids as the ground state.

cond-mat.quant-gas↗

arXiv2016

Fulde-Ferrell Superfluids without Spin Imbalance in Driven Optical Lattices

Spin-imbalanced ultracold Fermi gases have been widely studied recently as a platform for exploring the long-sought Fulde-Ferrell-Larkin-Ovchinnikov superfluid phases, but so far conclusive evidence has not been found. Here we propose to realize an Fulde-Ferrell (FF) superfluid without spin imbalance in a three-dimensional fermionic cold atom optical lattice, where $s$- and $p$-orbital bands of the lattice are coupled by another weak moving optical lattice. Such coupling leads to a spin-independent asymmetric Fermi surface, which, together with the $s$-wave scattering interaction between two spins, yields an FF type of superfluid pairing. Unlike traditional schemes, our proposal does not rely on the spin imbalance (or an equivalent Zeeman field) to induce the Fermi surface mismatch and provides a completely new route for realizing FF superfluids.

cond-mat.quant-gas↗

arXiv2016

Interacting spin-orbit-coupled spin-1 Bose-Einstein condensates

The recent experimental realization of spin-orbit (SO) coupling for spin-1 ultracold atoms opens an interesting avenue for exploring SO-coupling-related physics in large-spin systems, which is generally unattainable in electronic materials. In this paper, we study the effects of interactions between atoms on the ground states and collective excitations of SO-coupled spin-1 Bose-Einstein condensates (BECs) in the presence of a spin-tensor potential. We find that ferromagnetic interaction between atoms can induce a stripe phase exhibiting in-phase or out-of-phase modulating patterns between spin-tensor and zero-spin-component density waves. We characterize the phase transitions between different phases using the spin-tensor density as well as the collective dipole motion of the BEC. We show that there exists a double maxon-roton structure in the Bogoliubov-excitation spectrum, attributed to the three band minima of the SO-coupled spin-1 BEC.

cond-mat.quant-gas↗

arXiv2015

Dirac and Weyl Rings in Three Dimensional Cold Atom Optical Lattices

Recently three dimensional topological quantum materials with gapless energy spectra have attracted considerable interests in many branches of physics. Besides the celebrated example, Dirac and Weyl points which possess gapless point structures in the underlying energy dispersion, the topologically protected gapless spectrum can also occur along a ring, named Dirac and Weyl nodal rings. Ultra-cold atomic gases provide an ideal platform for exploring new topological materials with designed symmetries. However, whether Dirac and Weyl rings can exist in the single-particle spectrum of cold atoms remains elusive. Here we propose a realistic model for realizing Dirac and Weyl rings in the single-particle band dispersion of a cold atom optical lattice. Our scheme is based on previously experimentally already implemented Raman coupling setup for realizing spin-orbit coupling. Without the Zeeman field, the model preserves both pseudo-time-reversal and inversion symmetries, allowing Dirac rings. The Dirac rings split into Weyl rings with a Zeeman field that breaks the pseudo-time-reversal symmetry. We examine the superfluidity of attractive Fermi gases in this model and also find Dirac and Weyl rings in the quasiparticle spectrum.

cond-mat.quant-gas↗

arXiv2015

Topological superfluids with time-reversal symmetry from $s$-wave interaction in a bilayer system

Topological superconducting phases with time-reversal (TR) symmetry have been widely explored in recent years. However the involved unconventional pairings are generally implausible in realistic materials. Here we demonstrate via detailed self-consistent calculation that these topological phases with TR symmetry in DIII and BDI classes can be realized in a spin-orbit coupled bilayer system with only $s$-wave interaction. The bilayer freedom enables the definition of TR symmetry between the layers even in the presence of local Zeeman fields, which we propose to be realized using four laser beams. The gapped phase in DIII class is characterized by $\mathbb{Z}_2$, while all the gapless phases in these two classes are characterized by nontrivial winding numbers and are also manifested from the Majorana flat bands. We also unveil the intimate relation between TR symmetry and mirror symmetry due to phase locking effect between the two layers, which harbors the mirror symmetry protected topological phases. We finally demonstrate that these phases will not be spoiled by interlayer pairings.

cond-mat.quant-gas↗

arXiv2015

Dynamical spin-density waves in a spin-orbit-coupled Bose-Einstein condensate

Synthetic spin-orbit (SO) coupling, an important ingredient for quantum simulation of many exotic condensed matter physics, has recently attracted considerable attention. The static and dynamic properties of a SO coupled Bose-Einstein condensate (BEC) have been extensively studied in both theory and experiment. Here we numerically investigate the generation and propagation of a \textit{dynamical} spin-density wave (SDW) in a SO coupled BEC using a fast moving Gaussian-shaped barrier. We find that the SDW wavelength is sensitive to the barrier's velocity while varies slightly with the barrier's peak potential or width. We qualitatively explain the generation of SDW by considering a rectangular barrier in a one dimensional system. Our results may motivate future experimental and theoretical investigations of rich dynamics in the SO coupled BEC induced by a moving barrier.

cond-mat.quant-gas↗

arXiv2015

Spin - orbital-angular-momentum coupling in Bose-Einstein condensates

Spin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultracold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultracold atoms using higher-order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground-state phase diagrams of SOAM-coupled Bose-Einstein condensates on a ring trap and explore their applications in gravitational force detection. Our results may provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling, such as collective excitations.

cond-mat.quant-gas↗

arXiv2015

Spin-momentum coupled Bose-Einstein condensates with lattice band pseudospins

The quantum emulation of spin-momentum coupling (SMC), a crucial ingredient for the emergence of topological phases, is currently drawing considerable interest. In previous quantum gas experiments, typically two atomic hyperfine states were chosen as pseudospins. Here, we report the observation of a new kind of SMC achieved by loading a Bose-Einstein condensate (BEC) into periodically driven optical lattices. The s- and p-bands of a static lattice, which act as pseudospins, are coupled through an additional moving lattice which induces a momentum dependent coupling between the two pseudospins, resulting in s-p hybrid Floquet-Bloch bands. We investigate the band structures by measuring the quasimomentum of the BEC for different velocities and strengths of the moving lattice and compare our measurements to theoretical predictions. The realization of SMC with lattice bands as pseudospins paves the way for engineering novel quantum matter using hybrid orbital bands.

cond-mat.quant-gas↗

arXiv2015

Quantum phases of Bose-Einstein condensates with synthetic spin - orbital-angular-momentum coupling

The experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves the way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for an experimental investigation of SOAM-coupled BECs.

cond-mat.quant-gas↗

arXiv2015

Dzyaloshinskii-Moriya Interaction and Spiral Order in Spin-orbit Coupled Optical Lattices

We show that the recent experimental realization of spin-orbit coupling in ultracold atomic gases can be used to study different types of spin spiral order and resulting multiferroic effects. Spin-orbit coupling in optical lattices can give rise to the Dzyaloshinskii-Moriya (DM) spin interaction which is essential for spin spiral order. By taking into account spin-orbit coupling and an external Zeeman field, we derive an effective spin model in the Mott insulator regime at half filling and demonstrate that the DM interaction in optical lattices can be made extremely strong with realistic experimental parameters. The rich finite temperature phase diagrams of the effective spin models for fermions and bosons are obtained via classical Monte Carlo simulations.

cond-mat.quant-gas↗

arXiv2015

Bright solitons in a 2D spin-orbit-coupled dipolar Bose-Einstein condensate

We study a two-dimensional spin-orbit-coupled dipolar Bose-Einstein condensate with repulsive contact interactions by both the variational method and the imaginary time evolution of the Gross-Pitaevskii equation. The dipoles are completely polarized along one direction in the 2D plane so as to provide an effective attractive dipole-dipole interaction. We find two types of solitons as the ground states arising from such attractive interactions: a plane wave soliton with a spatially varying phase and a stripe soliton with a spatially oscillating density for each component. Both types of solitons possess smaller size and higher anisotropy than the soliton without spin-orbit coupling. Finally, we discuss the properties of moving solitons, which are nontrivial because of the violation of Galilean invariance.

cond-mat.quant-gas↗

arXiv2015

Berezinskii-Kosterlitz-Thouless Phase Transition in 2D Spin-Orbit Coupled Fulde-Ferrell Superfluids

The experimental observation of traditional Zeeman-field induced Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids has been hindered by various challenges, in particular, the requirement of low dimensional systems. In 2D, finite temperature phase fluctuations lead to extremely small Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for FFLO superfluids, raising serious concerns regarding their experimental observability. Recently, it was shown that FFLO superfluids can be realized using a Rashba spin-orbit coupled Fermi gas subject to Zeeman fields, which may also support topological excitations such as Majorana fermions in 2D. Here we address the finite temperature BKT transition issue in this system, which may exhibit gapped, gapless, topological, and gapless topological FF phases. We find a large BKT transition temperature due to large effective superfluid densities, making it possible to observe 2D FF superfluids at finite temperature. In addition, we show that gapless FF superfluids can be stable due to their positive superfluid densities. These findings pave the way for the experimental observation of 2D gapped and gapless FF superfluids and their associated topological excitations at finite temperature.

cond-mat.quant-gas↗

arXiv2015

1D topological chains with Majorana fermions in 2D non-topological optical lattices

The recent experimental realization of 1D equal Rashba-Dresselhaus spin-orbit coupling (ERD-SOC) for cold atoms provide a disorder-free platform for the implementation and observation of Majorana fermions (MFs), similar to the well studied solid state nanowire/superconductor heterostructures. However, the corresponding 1D chains of cold atoms possess strong quantum fluctuation, which may destroy the superfluids and MFs. In this Letter, we show that such 1D topological chains with MFs may be on demand generated in a 2D non-topological optical lattice with 1D ERD-SOC by modifying local potentials on target locations using experimentally already implemented atomic gas microscopes or patterned (e.g., double or triple well) optical lattices. All ingredients in our scheme have been experimentally realized and the combination of them may pave the way for the experimental observation of MFs in a clean system.

cond-mat.quant-gas↗

arXiv2015

Exciton-polaritons in transition-metal dichalcogenides and their direct excitation via energy transfer

Excitons, composite electron-hole quasiparticles, are known to play an important role in optoelectronic phenomena in many semiconducting materials. Recent experiments and theory indicate that the band-gap optics of the newly discovered monolayer transition-metal dichalcogenides (TMDs) is dominated by tightly bound valley excitons. The strong interaction of excitons with long-range electromagnetic fields in these 2D systems can significantly affect their intrinsic properties. Here, we develop a semi-classical framework for intrinsic exciton-polaritons in monolayer TMDs that treats their dispersion and radiative decay on the same footing and can incorporate effects of the dielectric environment. It is demonstrated how both inter- and intra-valley long-range interactions influence the dispersion and decay of the polaritonic eigenstates. We also show that exciton-polaritons can be efficiently excited via resonance energy transfer from quantum emitters such as quantum dots, which may be useful for various applications.

cond-mat.mes-hall↗

arXiv2014

Topological Fulde-Ferrell Superfluids of a Spin-Orbit Coupled Fermi Gas

Topological Fermi superfluids have played the central role in various fields of physics. However, all previous studies focus on the cases where Cooper pairs have zero center-of-mass momenta (i.e. normal superfluids). The topology of Fulde-Ferrell superfluids with nonzero momentum pairings have never been explored until recent findings that Fulde-Ferrell superfluids in a spin-orbit coupled Fermi gas can accommodate Majorana fermions in real space in low dimensions and Weyl fermions in momentum space in three dimension. In this review, we first discuss the mechanism of pairings in spin-orbit coupled Fermi gases in optical lattices subject to Zeeman fields, showing that spin-orbit coupling as well as Zeeman fields enhance Fulde-Ferrell states while suppress Larkin-Ovchinnikov states. We then present the low temperature phase diagram including both FF superfluids and topological FF superfluids phases in both two dimension and three dimension. In one dimension, Majorana fermions as well as phase dependent order parameter are visualized. In three dimension, we show the properties of Weyl fermions in momentum space such as anisotropic linear dispersion, Fermi arch, and gaplessness away from $k_{\perp}=0$. Finally, we discuss some possible methods to probe FF superfluids and topological FF superfluids in cold atom systems.

cond-mat.quant-gas↗

arXiv2014

Dark Solitons with Majorana Fermions in Spin-Orbit-Coupled Fermi Gases

We show that a single dark soliton can exist in a spin-orbit-coupled Fermi gas with a high spin imbalance, where spin-orbit coupling favors uniform superfluids over non-uniform Fulde-Ferrell-Larkin-Ovchinnikov states, leading to dark soliton excitations in highly imbalanced gases. Above a critical spin imbalance, two topological Majorana fermions (MFs) without interactions can coexist inside a dark soliton, paving a way for manipulating MFs through controlling solitons. At the topological transition point, the atom density contrast across the soliton suddenly vanishes, suggesting a signature for identifying topological solitons.

cond-mat.quant-gas↗

arXiv2014

Floquet FFLO superfluids and Majorana fermions in a shaken fermionic optical lattice

Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids, Cooper pairings with finite momentum, and Majorana fermions (MFs), quasiparticles with non-Abelian exchange statistics, are two topics under intensive investigation in the past several decades, but unambiguous experimental evidences for them have not been found yet in any physical system. Here we show that the recent experimentally realized cold atom shaken optical lattice provides a new pathway to realize FFLO superfluids and MFs. By tuning shaken lattice parameters (shaking frequency and amplitude), various coupling between the s- and p-orbitals of the lattice (denoted as the pseudo-spins) can be generated. We show that the combination of the inverted s- and p-band dispersions, the engineered pseudo-spin coupling, and the attractive on-site atom interaction, naturally allows the observation of FFLO superfluids as well as MFs in different parameter regions. While without interaction the system is a topological insulator (TI) with edge states, the MFs in the superfluid may be found to be in the conduction or valence band, distinguished from previous TI-based schemes that utilize edge states inside the band gap.

cond-mat.quant-gas↗