arXiv subjects

Zhigang Wu

Publications and source records attributed to Zhigang Wu.

At least 73 records · Page 4Linked to original sources

arXiv2017

Emergent Symmetry at Superradiance Transition of a Bose Condensate in Two Crossed Beam Cavities

Recently the ETH group has reported an experiment on superradiant transition of a Bose condensate in two crossed beam cavities. The surprise is that they find that across the superradiant transition, the cavity light can be emitted in any superposition of these two cavity modes. This indicates an additional $U(1)$ symmetry that does not exist in the full Hamiltonian. In this letter we show that this symmetry is an emergent symmetry in the vicinity of the phase transition. We identify all the necessary conditions that are required for this emergent $U(1)$ symmetry and show that the ETH experiment is a special case that satisfies these conditions. We further show that the superradiant transition in this system can also be driven to a first order one when the system is tuned away from the point having the emergent symmetry.

cond-mat.quant-gas↗

arXiv2017

A mixed dimensional Bose polaron

A new generation of cold atom experiments trapping atomic mixtures in species selective optical potentials opens up the intriguing possibility to create systems in which different atoms live in different spatial dimensions. Inspired by this, we investigate a mixed dimensional Bose polaron consisting of an impurity particle moving in a two-dimensional (2D) layer immersed in a 3D Bose-Einstein condensate (BEC), using a theory that includes the mixed dimensional vacuum scattering between the impurity and the bosons exactly. We show that similarly to the pure 3D case, this system exhibits a well-defined polaron state for attractive boson-impurity interaction that evolves smoothly into a mixed-dimensional dimer for strong attraction, as well as a well-defined polaron state for weak repulsive interaction, which becomes over-damped for strong interaction. We furthermore find that the properties of the polaron depend only weakly on the gas parameter of the BEC as long as the Bogoliubov theory remains a valid description for the BEC. This indicates that higher order correlations between the impurity and the bosons are suppressed by the mixed dimensional geometry in comparison to a pure 3D system, and that the mixed dimensional polaron has universal properties in the unitarity limit of the impurity-boson interaction.

cond-mat.quant-gas↗

arXiv2017

Long range mediated interactions in a mixed dimensional system

We present a mixed-dimensional atomic gas system to unambiguously detect and systematically probe mediated interactions. In our scheme, fermionic atoms are confined in two parallel planes and interact via exchange of elementary excitations in a three-dimensional background gas. This interaction gives rise to a frequency shift of the out-of-phase dipole oscillations of the two clouds, which we calculate using a strong coupling theory taking the two-body mixed-dimensional scattering into account exactly. The shift is shown to be easily measurable for strong interactions and can be used as a probe for mediated interactions.

cond-mat.quant-gas↗

arXiv2017

Topological superfluidity of lattice fermions inside a Bose-Einstein condensate

We calculate the phase diagram of identical fermions in a 2-dimensional (2D) lattice immersed in a 3D Bose-Einstein condensate (BEC). The fermions exchange density fluctuations in the BEC, which gives rise to an attractive induced interaction. The resulting zero temperature phase diagram exhibits topological $p_x+ip_y$ superfluid phases as well as a phase separation region. We show how to use the flexibility of the Bose-Fermi mixture to tune the induced interaction, so that it maximises the pairing between nearest neighbour sites, whereas phase separation originating from long range interactions is suppressed. Finally, we calculate the Berezinskii-Kosterlitz-Thouless (BKT) critical temperature of the topological superfluid in the lattice and discuss experimental realisations.

cond-mat.quant-gas↗

arXiv2016

Topological superfluid in a Fermi-Bose mixture with a high critical temperature

We show that a two-dimensional (2D) spin-polarised Fermi gas immersed in a 3D Bose-Einstein condensate (BEC) constitutes a very promising system to realise a $p_x+ip_y$ superfluid. The fermions attract each other via an induced interaction mediated by the bosons, and the resulting pairing is analysed with retardation effects fully taken into account. This is further combined with Berezinskii-Kosterlitz-Thouless (BKT) theory to obtain reliable results for the superfluid critical temperature. We show that both the strength and the range of the induced interaction can be tuned experimentally, which can be used to make the critical temperature approach the maximum value allowed by general BKT theory. Moreover, this is achieved while keeping the Fermi-Bose interaction weak so that three-body losses are small. Our results show that realising a topological superfluid with atomic Fermi-Bose mixtures is within experimental reach.

cond-mat.quant-gas↗

arXiv2016

Pointwise estimates for bipolar compressible Navier-Stokes-Poisson system in dimension three

The Cauchy problem of the bipolar Navier-Stokes-Poisson system (1.1) in dimension three is considered. We obtain the pointwise estimates of the time-asymptotic shape of the solution, which exhibit generalized Huygens' principle as the Navier-Stokes system. This phenomenon is the the most important difference from the unipolar Navier-Stokes-Poisson system. Due to non-conservative structure of the system (1.1) and interplay of two carriers which counteracts the influence of electric field (a nonlocal term), some new observations are essential for the proof. We make full use of the conservative structure of the system for the total density and total momentum, and the mechanism of the linearized unipolar Navier-Stokes-Poisson system together with the special form of the nonlinear terms in the system for the difference of densities and the difference of momentums. Lastly, as a byproduct, we extend the usual $L^2(\mathbb{R}^3)$-decay rate to $L^p(\mathbb{R}^3)$-decay rate with $p>1$ and also improve former decay rates in part.

math.AP↗

arXiv2015

Liquid crystal phases of two-dimensional dipolar gases and Berezinskii-Kosterlitz-Thouless melting

Liquid crystals are phases of matter intermediate between crystals and liquids. Whereas classical liquid crystals have been known for a long time and are used in electro-optical displays, much less is known about their quantum counterparts. There is growing evidence that quantum liquid crystals play a central role in many electron systems including high temperature superconductors, but a quantitative understanding is lacking due to disorder and other complications. Here, we analyse the quantum phase diagram of a two-dimensional dipolar gas, which exhibits stripe, nematic and supersolid phases. We calculate the stiffness constants determining the stability of the nematic and stripe phases, and the melting of the stripes set by the proliferation of topological defects is analysed microscopically. Our results for the critical temperatures of these phases demonstrate that a controlled study of the interplay between quantum liquid and superfluid phases is within experimental reach for the first time, using dipolar gases.

cond-mat.quant-gas↗

arXiv2015

A study on the shielding mechanisms of SOI pixel detector

In order to tackle the charge injection issue that had perplexed the counting type SOI pixel for years, two successive chips CPIXTEG3 and CPIXTEG3b were developed utilizing two shielding mechanisms, Nested-well and Double-SOI, in the LAPIS process. A TCAD simulation showed the shielding effectiveness influenced by the high sheet resistance of shielding layers. Test structures specially designed to measure the crosstalk associated to charge injection were implemented in CPIXTEG3/3b. Measurement results proved that using shielding layer is indispensable for counting type pixel and Double-SOI is superior to Nested-well in terms of shielding effectiveness and design flexibility.

physics.ins-det↗

arXiv2015

Probing the optical conductivity of trapped charge-neutral quantum gases

We study a harmonically confined atomic gas which is subjected to an additional external potential such as an optical lattice. Using a linear response formulation, we determine the response of the gas to a small, time-dependent displacement of the harmonic trap and derive a simple exact relation showing that the centre-of-mass position of the atomic cloud is directly related to the global optical conductivity of the system. We demonstrate the usefulness of this approach by calculating the optical conductivity of bosonic atoms in an optical lattice. In the Mott insulating phase, there is clear evidence of an optical Mott gap, providing a proof-of-principle demonstration that the global optical conductivity gives high-quality information about the exci- tations of strongly-correlated quantum gases.

cond-mat.quant-gas↗

arXiv2014

Coexistence of density wave and superfluid order in a dipolar Fermi gas

We analyse the coexistence of superfluid and density wave (stripe) order in a quasi-two-dimensional gas of dipolar fermions aligned by an external field. Remarkably, the anisotropic nature of the dipolar interaction allows for such a coexistence in a large region of the zero temperature phase diagram. In this region, the repulsive part of the interaction drives the stripe formation and the attractive part induces the pairing, resulting in a supersolid with $p$-wave Cooper pairs aligned along the stripes. From a momentum space perspective, the stability of the supersolid phase is due to the fact that the stripe order renders the Fermi surface only partially gapped, leaving gapless regions that are most important for $p$-wave pairing. We finally demonstrate how this supersolid phase can be detected in time-of-flight experiments.

cond-mat.quant-gas↗

arXiv2013

Mean-field yrast spectrum of a two-component Bose gas in ring geometry: persistent currents at higher angular momentum

We use analytic soliton solutions of a two-component Bose gas in ring geometry to analyze the mean-field yrast spectrum of the system. We find that the spectrum exhibits a surprisingly rich structure as a result of the interplay of interparticle interactions and population imbalance. We discuss the implication of these results in regard to the possibility of persistent currents at higher angular momenta.

cond-mat.quant-gas↗

arXiv2013

Optimal decay rate of the bipolar Euler-Poisson system with damping in $\mathbb{R}^3$

By rewriting a bipolar Euler-Poisson equations with damping into an Euler equation with damping coupled with an Euler-Poisson equation with damping, and using a new spectral analysis, we obtain the optimal decay results of the solutions in $L^2$-norm, which improve theose in \cite{Li3, Wu3}. More precisely, the velocities $u_1,u_2$ decay at the $L^2-$rate $(1+t)^{-{5}{4}}$, which is faster than the normal $L^2-$rate $(1+t)^{-{3}{4}}$ for the Heat equation and the Navier-Stokes equations. In addition, the disparity of two densities $ρ_1-ρ_2$ and the disparity of two velocities $u_1-u_2$ decay at the $L^2$-rate $(1+t)^{-2}$.

math.AP↗

arXiv2012

Decay of the solution to the bipolar Euler-Poisson system with damping in $\mathbb{R}^3$

We construct the global solution to the Cauchy's problem of the bipolar Euler-Poisson equations with damping in $\mathbb{R}^3$ when $H^3$ norm of the initial data is small. If further, the $\dot{H}^{-s}$ norm ($0\leq s<3/2)$ or $\dot{B}_{2,\infty}^{-s}$ norm ($0<s\leq3/2$) of the initial data is bounded, we give the optimal decay rates of the solution. As a byproduct, the decay results of the $L^p-L^2$ ($1\leq p\leq2$) type hold without the smallness of the $L^p$ norm of the initial data. In particular, we deduce that $\|\nabla^k(ρ_1-ρ_2)\|_{L^2} \sim(1+t)^{-5/4-\frac{k}{2}}$ and $\|\nabla^k(ρ_i-\barρ,u_i,\nablaϕ)\|_{L^2} \sim(1+t)^{-3/4-\frac{k}{2}}$. We improve the decay results in Li and Yang \cite{Li3}(\emph{J.Differential Equations} 252(2012), 768-791), where they showed the decay rates as $\|\nabla^k(ρ_i-\barρ)\|_{L^2} \sim(1+t)^{-3/4-\frac{k}{2}}$ and $\|\nabla^k(u_i,\nablaϕ)\|_{L^2} \sim(1+t)^{-1/4-\frac{k}{2}}$, when the $H^3\cap L^1$ norm of the initial data is small. Our analysis is motivated by the technique developed recently in Guo and Wang \cite{Guo}(\emph{Comm. Partial Differential Equations} 37(2012), 2165-2208) with some modifications.

math.AP↗

arXiv2012

Decay of a model system of radiating gas

This paper is concerned with optimal time-decay estimates of solutions of the Cauchy problem to a model system of the radiating gas in $\mathbb{R}^n$. Compared to Liu and Kawashima (2011) \cite{Liu1} and Wang and Wang (2009) \cite{Wang}, without smallness assumption of initial perturbation in $L^1$-norm, we study large time behavior of small amplitude classical solutions to the Cauchy problem. The optimal $H^N$-norm time-decay rates of the solutions in $\mathbb{R}^n$ with $1\leq n\leq4$ are obtained by applying the Fourier splitting method introduced in Schonbek (1980) \cite{Schonbek1} with a slight modification and an energy method. Furthermore, basing on a refined pure energy method introduced in Guo and Wang \cite{Guo} (2011), we give optimal $L^p$-$L^2(\mathbb{R}^3)$ decay estimates of the derivatives of solutions when initial perturbation is bounded in $L^p$-norm with some $p\in(1,2]$.

math.AP↗

arXiv2012

Tuning polarization and elasticity properties by uniaxial stress in BiFeO3

The changes of polarizational, elastic and structural properties of mutiferroic BiFeO3 under [111] direction uniaxial stress are calculated using density functional theory within the Perdew-Burke- Ernzerhof revised for solids (PBEsol) + U approach, and compared with available measurements or predictions. A large ferroelectric polarization of 87.5 ?C=cm2 is found, agreeing with other theoretical and experimental values, and the polarization changes near-linearly within the uniaxial stress range of 8 GPa to -8 GPa. This property can be used to indirectly control the radiative recombination of luminous organ and the electrical properties of transistor structure. In addition, we have also investigated the elastic stiffness coefficients and the elastic compliance coefficients in the same uniaxial stress range, which provide helpful guidance for the applications of BiFeO3.

cond-mat.mtrl-sci↗

arXiv2010

Quantum Monte Carlo for minerals at high pressure: Phase stability, equations of state, and elasticity of silica

Silica is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. As the simplest silicates, understanding pure silica is a prerequisite to understanding the rocky part of the Earth, its majority. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals, but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrodinger's equation. Using ground state QMC plus phonons within the quasiharmonic approximation from density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth's core-mantle boundary. Our results provide the most well-constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the most dense alpha-PbO2 structure above the core-insulating D" layer, suggesting the absence of significant free silica in the bulk lower mantle, which has been assumed but never proven. We also find an accurate shear elastic constant and its geophysically important softening with pressure.

cond-mat.mtrl-sci↗

arXiv2007

Melting of the vortex lattice in layered superconductors

The structure function of the vortex lattice of layered superconductor is calculated to one-loop order. Based on a phenomenological melting criterion concerning the Debye-Waller factor, we calculate the melting line of the vortex lattice, and compare our results to Monte Carlo simulation and experiment. We find that our results are quantitatively in good agreement with the Monte Carlo results. Moreover, our analytic calculation of the melting line of BSCCO fits the experiment reasonably well in a temperature range not far from Tc.

cond-mat.supr-con↗

arXiv2006

A More Accurate Generalized Gradient Approximation for Solids

We present a new nonempirical density functional generalized gradient approximation (GGA) that gives significant improvements for lattice constants, crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. The new functional is based on a diffuse radial cutoff for the exchange-hole in real space, and the analytic gradient expansion of the exchange energy for small gradients. There are no adjustable parameters, the constraining conditions of PBE are maintained, and the functional is easily implemented in existing codes.

cond-mat.mtrl-sci↗