arXiv subjects

Wei Yi

Publications and source records attributed to Wei Yi.

At least 199 records · Page 11Linked to original sources

arXiv2014

High mobility back-gated InAs/GaSb double quantum well grown on GaSb substrate

We report a backgated InAs/GaSb double quantum well device grown on GaSb substrate. The use of the native substrate allows for high materials quality with electron mobility in excess of 500,000 cm2/Vs at sheet charge density of 8x1011 cm-2 and approaching 100,000 cm2/Vs near the charge neutrality point (CNP). Lattice matching between the quantum well structure and the substrate eliminates the need for a thick buffer, enabling large back gate capacitance and efficient coupling with the conduction channels in the quantum wells. As a result, quantum Hall effects are observed in both electron and hole regimes across the hybridization gap.

cond-mat.mes-hall↗

arXiv2014

Pairing Superfluidity in Spin-Orbit Coupled Ultracold Fermi Gases

We review some recent progresses on the study of ultracold Fermi gases with synthetic spin-orbit coupling. In particular, we focus on the pairing superfluidity in these systems at zero temperature. Recent studies have shown that different forms of spin-orbit coupling in various spatial dimensions can lead to a wealth of novel pairing superfluidity. A common theme of these variations is the emergence of new pairing mechanisms which are direct results of spin-orbit-coupling-modified single-particle dispersion spectra. As different configurations can give rise to single-particle dispersion spectra with drastic differences in symmetry, spin dependence and low-energy density of states, spin-orbit coupling is potentially a powerful tool of quantum control, which, when combined with other available control schemes in ultracold atomic gases, will enable us to engineer novel states of matter.

cond-mat.quant-gas↗

arXiv2014

Signatures of Bose-Einstein condensation in an optical lattice

We discuss typical experimental signatures for the Bose-Einstein condensation (BEC) of an ultracold Bose gas in an inhomogeneous optical lattice at finite temperature. Applying the Hartree-Fock-Bogoliubov-Popov formalism, we calculate quantities such as the momentum-space density distribution, visibility and peak width as the system is tuned through the superfluid to normal phase transition. Different from previous studies, we consider systems with fixed total particle number, which is of direct experimental relevance. We show that the onset of BEC is accompanied by sharp features in all these signatures, which can be probed via typical time-of-flight imaging techniques. In particular, we find a two-platform structure in the peak width across the phase transition. We show that the onset of condensation is related to the emergence of the higher platform, which can be used as an effective experimental signature.

cond-mat.quant-gas↗

arXiv2014

Universal Borromean Binding in Spin-Orbit Coupled Ultracold Fermi Gases

Borromean rings and Borromean binding, a class of intriguing phenomena as three objects are linked (bound) together while any two of them are unlinked (unbound), widely exist in nature and have been found in systems of biology, chemistry and physics. Previous studies have suggested that the occurrence of such a binding in physical systems typically relies on the microscopic details of pairwise interaction potentials at short-range, and is therefore non-universal. Here, we report a new type of Borromean binding in ultracold Fermi gases with Rashba spin-orbit coupling, which is {\it universal} against short-range interaction details, with its binding energy only dependent on the s-wave scattering length and the spin-orbit coupling strength. We show that the occurrence of this universal Borromean binding is facilitated by the symmetry of the single-particle dispersion under spin-orbit coupling, and is therefore {\it symmetry-selective} rather than interaction-selective. The state is robust over a wide range of mass ratio between composing fermions, which are accessible by Li-Li, K-K and K-Li mixtures in cold atoms experiments. Our results reveal the importance of symmetry factor in few-body physics, and shed light on the emergence of new quantum phases in a many-body system with exotic few-body correlations.

cond-mat.quant-gas↗

arXiv2014

BCS-BEC crossover and quantum phase transition in an ultracold Fermi gas under spin-orbit coupling

In this work, we study the BCS-BEC crossover and quantum phase transition in a Fermi gas under Rashba spin-orbit coupling close to a Feshbach resonance. By adopting a two-channel model, we take into account of the closed channel molecules, and show that combined with spin-orbit coupling, a finite background scattering in the open channel can lead to two branches of solution for both the two-body and the many-body ground states. The branching of the two-body bound state solution originates from the avoided crossing between bound states in the open and the closed channels, respectively. For the many-body states, we identify a quantum phase transition in the upper branch regardless of the sign of the background scattering length, which is in clear contrast to the case without spin-orbit coupling. For systems with negative background scattering length in particular, we show that the bound state in the open channel, and hence the quantum phase transition in the upper branch, are induced by spin-orbit coupling. We then characterize the critical detuning of the quantum phase transition for both positive and negative background scattering lengths, and demonstrate the optimal parameters for the critical point to be probed experimentally.

cond-mat.quant-gas↗

arXiv2014

Three-component Ultracold Fermi Gases with Spin-Orbit Coupling

We investigate the pairing physics in a three-component Fermi-Fermi mixture, where a few impurities are immersed in a non-interacting spin-$\frac{1}{2}$ Fermi gas with synthetic spin-orbit coupling (SOC), and interact attractively with one spin species in the Fermi gas. Due to the interplay of SOC and spin-selective interaction, the molecular state intrinsically acquires a non-zero center-of-mass momentum, which results in a new type of Fulde-Ferrell (FF) pairing in spin-orbit coupled Fermi systems. The existence of the Fermi sea can also lead to the competition between FF-like molecular states with different center-of-mass momenta, which corresponds to a first-order transition between FF phases in the thermodynamic limit. As the interaction strength is tuned, a polaron-molecule transition occurs in the highly imbalanced system, where the boundary varies non-monotonically with SOC parameters and gives rise to the reentrance of polaron states. The rich physics in this system can be probed using existing experimental techniques.

cond-mat.quant-gas↗

arXiv2014

Topological phase transition in the quench dynamics of a one-dimensional Fermi gas

We study the quench dynamics of a one-dimensional ultracold Fermi gas in an optical lattice potential with synthetic spin-orbit coupling. At equilibrium, the ground state of the system can undergo a topological phase transition and become a topological superfluid with Majorana edge states. As the interaction is quenched near the topological phase boundary, we identify an interesting dynamical phase transition of the quenched state in the long-time limit, characterized by an abrupt change of the pairing gap at a critical quenched interaction strength. We further demonstrate the topological nature of this dynamical phase transition from edge-state analysis of the quenched states. Our findings provide interesting clues for the understanding of topological phase transitions in dynamical processes, and can be useful for the dynamical detection of Majorana edge states in corresponding systems.

cond-mat.quant-gas↗

arXiv2013

Unconventional Fulde-Ferrell-Larkin-Ovchinnikov pairing states in a Fermi gas with spin-orbit coupling

We study the phase diagram in a two-dimensional Fermi gas with the synthetic spin-orbit coupling that has recently been realized experimentally. In particular, we characterize in detail the properties and the stability region of the unconventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in such a system, which are induced by spin-orbit coupling and Fermi surface asymmetry. We identify several distinct nodal FFLO states by studying the topology of their respective gapless contours in momentum space. We then examine the phase structure and the number density distributions in a typical harmonic trapping potential under the local density approximation. Our studies provide detailed information on the FFLO pairing states with spin-orbit coupling and Fermi surface asymmetry, and will facilitate experimental detection of these interesting pairing states in the future.

cond-mat.quant-gas↗

arXiv2013

Topological Fulde-Ferrel-Larkin-Ovchinnikov states in Spin-orbit Coupled Fermi Gases

Pairing in an attractively interacting two-component Fermi gas in the absence of the inversion symmetry and/or the time-reversal symmetry may give rise to exotic superfluid states. Notable examples range from the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state with a finite center-of-mass momentum in a polarized Fermi gas, to the topological superfluid state in a two-dimensional Fermi gas under Rashba spin-orbit coupling and an out-of-plane Zeeman field. Here, we show that a topological FFLO state can be stabilized in a two-dimensional Fermi gas with Rashba spin-orbit coupling and both in-plane and out-of-plane Zeeman fields. We characterize the topological FFLO state by a non-trivial Berry phase, and demonstrate the stability region of the state on the zero-temperature phase diagram. Given its unique properties in both the quasi-particle dispersion spectra and the momentum distribution, signatures of the topological FFLO state can be detected using existing experimental techniques.

cond-mat.quant-gas↗

arXiv2013

Exotic pairing states in a Fermi gas with three-dimensional spin-orbit coupling

We investigate properties of exotic pairing states in a three-dimensional Fermi gas with three-dimensional spin-orbit coupling and an effective Zeeman field. The interplay of spin-orbit coupling, effective Zeeman field and pairing can lead to first-order phase transitions between different phases, and to interesting nodal superfluid states with gapless surfaces in the momentum space. We then demonstrate that pairing states with zero center-of-mass momentum are unstable against finite center-of-mass momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, with the center-of-mass momentum of the pairs opposite to the direction of the effective Zeeman field. Unlike conventional FFLO states, these FFLO states are induced by the coexistence of spin-orbit coupling and Fermi surface deformation, and have intriguing features like first-order transitions between different FFLO states, nodal FFLO states with gapless surfaces in momentum space, and exotic fully gapped FFLO states. With the recent theoretical proposals for realizing three-dimensional spin-orbit coupling in ultracold atom gases, our work is helpful for the future experimental studies, and provides valuable information for the general understanding of pairing physics in spin-orbit coupled fermionic systems.

cond-mat.quant-gas↗

arXiv2013

Significance of dressed molecules in a quasi-two-dimensional polarized Fermi gas

We investigate the properties of a spin-orbit coupled quasi-two-dimensional Fermi gas with tunable s-wave interaction between the two spin species. By analyzing the two-body bound state, we find that the population of the excited states in the tightly-confined axial direction can be significant when the two-body binding energy becomes comparable or exceeds the axial confinement. Since the Rashba spin-orbit coupling that we study here tends to enhance the two-body binding energy, this effect can become prominent at unitarity or even on the BCS side of the Feshbach resonance. To study the impact of these excited modes along the third dimension, we adopt an effective two-dimensional Hamiltonian in the form of a two-channel model, where the dressed molecules in the closed channel consist of the conventional Feshbach molecules as well as the excited states occupation in the axial direction. With properly renormalized interactions between atoms and dressed molecules, we find that both the density distribution and the phase structure in the trap can be significantly modified near a wide Feshbach resonance. In particular, the stability region of the topological superfluid phase is increased. Our findings are helpful for the experimental search for the topological superfluid phase in ultra-cold Fermi gases, and have interesting implications for quasi-low-dimensional polarized Fermi gases in general.

cond-mat.quant-gas↗

arXiv2013

Unconventional superfluid in a two-dimensional Fermi gas with anisotropic spin-orbit coupling and Zeeman fields

We study the phase diagram of a two-dimensional ultracold Fermi gas with the synthetic spin-orbit coupling (SOC) that has recently been realized at NIST. Due to the coexistence of anisotropic SOC and effective Zeeman fields in the NIST scheme, the system shows rich structure of phase separation involving exotic gapless superfluid and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states with different center-of-mass momentum. In particular, we characterize the stability region of FFLO states and demonstrate their unique features under SOC. We then show that the effective transverse Zeeman field in the NIST scheme can qualitatively change the landscape of the thermodynamic potential which leads to intriguing effects such as the disappearance of pairing instability, the competition between different FFLO states, and the stabilization of a fully gapped FFLO state. These interesting features may be probed for example by measuring the in-situ density profiles or by the momentum-resolved radio-frequency spectroscopy.

cond-mat.quant-gas↗

arXiv2012

Symmetry Protected Topological Phases in Spin Ladders with Two-body Interactions

Spin-1/2 two-legged ladders respecting inter-leg exchange symmetry and D2 spin rotation symmetry have new symmetry protected topological (SPT) phases which are different from the Haldane phase. Three of the new SPT phases are tx,ty,tz, which all have symmetry protected two-fold degenerate edge states on each end of the open boundaries. However, the edge states in different phases have different response to magnetic field. For example, the edge states in the tz phase will be split by the magnetic field along the z-direction, but not by the fields in the x- and y-directions. We give the Hamiltonian that realizes each SPT phase and demonstrate a proof-of-principle quantum simulation scheme for Hamiltonians of the t0 and tz phases based on coupled-QED-cavity ladder.

cond-mat.str-el↗

arXiv2012

Molecule and polaron in a highly polarized two-dimensional Fermi gas with spin-orbit coupling

We show that spin-orbit coupling (SOC) gives rise to pairing instability in a highly polarized two-dimensional Fermi gas for arbitrary interaction strength. The pairing instability can lead to a Fulde-Ferrell-Larkin-Ovchinnikov-like molecular state, which undergoes a first-order transition into a pairing state with zero center-of-mass momentum as the parameters are tuned. These pairing states are metastable against a polaron state dressed by particle-hole fluctuations for small SOC. At large SOC, a polaron-molecule transition exists, which suggests a phase transition between the topological superfluid state and the normal state for a highly polarized Fermi gas in the thermodynamic limit. As polarization in a Fermi gas with SOC is induced by the effective Zeeman field, we also discuss the influences of the effective Zeeman field on the ground state of the system. Our findings may be tested directly in future experiments.

cond-mat.quant-gas↗

arXiv2012

Superconductivity suppression of Ba0.5K0.5Fe2-2xM2xAs2 single crystals by substitution of transition-metal (M = Mn, Ru, Co, Ni, Cu, and Zn)

We investigated the doping effects of magnetic and nonmagnetic impurities on the single-crystalline p-type Ba0.5K0.5Fe2-2xM2xAs2 (M = Mn, Ru, Co, Ni, Cu and Zn) superconductors. The superconductivity indicates robustly against impurity of Ru, while weakly against the impurities of Mn, Co, Ni, Cu, and Zn. However, the present Tc suppression rate of both magnetic and nonmagnetic impurities remains much lower than what was expected for the s\pm-wave model. The temperature dependence of resistivity data is observed an obvious low-T upturn for the crystals doped with high-level impurity, which is due to the occurrence of localization. Thus, the relatively weak Tc suppression effect from Mn, Co, Ni, Cu, and Zn are considered as a result of localization rather than pair-breaking effect in s\pm-wave model.

cond-mat.supr-con↗

arXiv2012

Thermal and Electrical Properties of Multiwall Carbon Nanotubes

In this dissertation, thermal and electrical properties of aligned multiwall carbon nanotubes (MWNTs) prepared by thermal decomposition of hydrocarbons have been experimentally studied. The thesis consists of six chapters. Ch1 is an introduction. In Ch2, sample preparation and characterizations are described. In Ch3, by using a self-heating 3-Omega method, the specific heat, thermal diffusivity and thermal conductivity of MWNTs are measured. MWNTs of 20-40 nm diameter show a linear specific heat over a temperature range of 10-300 K, suggesting that inter-wall coupling in MWNTs is rather weak compared with that of graphite. The thermal conductivity shows a crossover from linear temperature dependence to a square law at ~120K, with a rather low room-temperature amplitude which may have resulted from structural defects. In Ch4, four-wire tunneling spectroscopy of junctions between MWNTs and a normal metal is measured. The Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of tunneling density of states that can be fitted numerically with the environmental quantum-fluctuation theory. At low temperatures, an asymmetric conductance anomaly near zero bias is observed, which is interpreted as Fano resonance in the strong tunneling regime. In Ch5, the thermoelectric power (TEP) and longitudinal magnetoresistance (MR) of MWNTs are measured. A moderate positive TEP with metallic-like linear temperature dependence is found, suggesting that the electron-hole symmetry in metallic MWNTs is broken. Periodic oscillations in MR are observed at 20 mK when a longitudinal magnetic field is applied. The period of oscillation agrees well with the period h/2e of Altshuler-Aronov-Spivak (AAS) effect if only the outermost graphene wall contributes to conductance, clearly indicating quantum-interference effects at low temperatures. Ch6 presents the main conclusions.

cond-mat.mes-hall↗

arXiv2010

Phase Diagram of Pressure-induced Superconductivity and its Relation to Hall Coefficient in Bi2Te3 Single Crystal

Pressure-induced superconductivity and its relation to corresponding Hall coefficient (RH) have been reported for Bi2Te3, one of known topological insulators. A full phase diagram is presented which shows a complex dependence of the superconducting transition temperature as a function of pressure over an extensive range. High-pressure RH measurements reveal a close relation of these complex behaviors, particularly, a dramatic change of dRH/dP before structural phase transition and a pressure-induced crossover on RH in the high pressure phase were observed.

cond-mat.supr-con↗

arXiv2010

Evidence for Competition between Superconductivity and Kondo Effect in CeFeAsO0.7F0.3 under High Pressure

We discover a pressure induced quantum phase transition from the superconducting state to the non-superconducting Kondo screened phase associated with a 2% volume collapse in CeFeAsO0.7F0.3 through measurements of high-pressure resistance, synchrotron x-ray diffraction, and x-ray absorption spectroscopy (XAS) in a diamond anvil cell. Our XAS data of Ce-L3 in CeFeAsO0.7F0.3 clearly show a spectral weight transfer from the main line to the satellite line after the transition, demonstrating the formation of the Kondo singlets under pressure in CeFeAsO1-xFx. Our results have revealed a physical picture of a pressure-induce competition between Kondo singlets and BCS singlets in the Ce-pnictide superconductors.

cond-mat.supr-con↗