arXiv subjects

Qiang Gu

Publications and source records attributed to Qiang Gu.

At least 73 records · Page 4Linked to original sources

arXiv2014

First-order chiral to non-chiral transition in the angular dependence of the upper critical induction of the Scharnberg-Klemm $p$-wave pair state

We calculate the temperature $T$ and angular $(θ,ϕ)$ dependence of the upper critical induction $B_{c2}(θ,ϕ,T)$ for parallel-spin superconductors with an axially symmetric $p$-wave pairing interaction pinned to the lattice and a dominant ellipsoidal Fermi surface (FS). For all FS anisotropies, the chiral Scharnberg-Klemm state $B_{c2}(θ,ϕ,T)$ exceeds that of the chiral Anderson-Brinkman-Morel state, and exhibits a kink at $θ=θ^{*}(T,ϕ)$, indicative of a first-order transition from its chiral, nodal-direction behavior to its non-chiral, antinodal-direction behavior. Applicability to Sr$_2$RuO$_4$, UCoGe, and topological superconductors such as Cu$_x$Bi$_2$Se$_3$ is discussed.

cond-mat.supr-con↗

arXiv2013

Magnetic properties of two-dimensional charged spin-1 Bose gases

Within the mean-field theory, we investigate the magnetic properties of a charged spin-1 Bose gas in two dimension. In this system the diamagnetism competes with paramagnetism, where Lande-factor $g$ is introduced to describe the strength of the paramagnetic effect. The system presents a crossover from diamagnetism to paramagnetism with the increasing of Lande-factor. The critical value of the Lande-factor, $g_{c}$, is discussed as a function of the temperature and magnetic field. We get the same value of $g_{c}$ both in the low temperature and strong magnetic field limit. Our results also show that in very weak magnetic field no condensation happens in the two dimensional charged spin-1 Bose gas.

cond-mat.stat-mech↗

arXiv2013

Plasma effect in the longitudinal space charge induced microbunching instability for low energy electron beams

The microbunching instability usually exists in the LINAC of a free electron laser (FEL) facility. In many cases, the longitudinal space charge (LSC) is a dominant factor that generates the instability. For the highly bright electron beams, the plasma effect is found to be non-trivial in the development of the instability. In this paper, starting from the Vlasov and Poisson equations in the multiple-dimensional phase space, we perform the straightforward analysis of the microbunching instability based on the explicit formula of the longitudinal electric field introduced by the density perturbation in the longitudinal direction, in such a way to be highly comparable to the well-developed method for higher energy beams. This method generally applies in both the cases with and without acceleration and independent of lattice components. The results show that for a electron beam with small transverse emittance at low energies, which is always the case in the injector of a free electron laser device, the plasma effect results in the oscillation of the longitudinal electric field in the modified plasma frequency that depends on the transverse size of the beam, and the Landau damping effect in the longitudinal electric field due to the uncorrelated longitudinal velocity spread during the beam transportation. These two effects both play important roles in the development of the instability. As the result, the energy modulation driven by the LSC impedance differs from the regular value significantly and the discrepancy leads to the noticeable change of the final gain of the instability.

physics.acc-ph↗

arXiv2013

Radio frequency spectrum of fermions near a narrow Feshbach resonance

We calculate the radio frequency (RF) spectrum of fermionic atoms near a narrow Feshbach resonance, explaining observations made in ultracold samples of $^6\rm{Li}$ [E. L. Hazlett {\it et al.}, Phys. Rev. Lett. {\bf 108}, 045304 (2012)]. We use a two channel resonance model to show that the RF spectrum contains two peaks. In the wide-resonance limit, nearly all spectral weight lies in one of these peaks, and typically the second peak is very broad. We find strong temperature dependence, which can be traced to the energy dependence of the two-particle scattering. In addition to microscopic calculations, we use sum rule arguments to find generic features of the spectrum which are model independent.

cond-mat.quant-gas↗

arXiv2013

Dynamics of Bose-Einstein condensates in a one-dimensional optical lattice with double-well potential

We study dynamical behaviors of the weakly interacting Bose-Einstein condensate in the one-dimensional optical lattice with an overall double-well potential by solving the time-dependent Gross-Pitaevskii equation. It is observed that the double-well potential dominates the dynamics of such a system even if the lattice depth is several times larger than the height of the double-well potential. This result suggests that the condensate flows without resistance in the periodic lattice just like the case of a single particle moving in periodic potentials. Nevertheless, the effective mass of atoms is increased, which can be experimentally verified since it is connected to the Josephson oscillation frequency. Moreover, the periodic lattice enhances the nonlinearity of the double-well condensate, making the condensate more "self-trapped" in the $π$-mode self-trapping regime.

cond-mat.quant-gas↗

arXiv2013

Simulation studies on laser pulse stability for Dalian Coherent Light Source

Dalian Coherent Light Source will use a 300MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150-50nm by high gain harmonic generation free electron laser (FEL) scheme. To generate stable FEL pulses, stringent tolerance budget is required for the LINAC output parameters, such as the mean beam energy stability, electron bunch arrival time jitter, peak current variation and the transverse beam position offset. In order to provide guidance for the design of the Dalian Coherent Light Source, in this paper, the sensitivity of FEL pulse energy fluctuation to various error sources of the electron bunch was performed using intensive start-to-end FEL simulations.

physics.acc-ph↗

arXiv2013

Anomalous angular dependence of the upper critical induction of orthorhombic ferromagnetic superconductors with completely broken p-wave symmetry

We employ the Klemm-Clem transformations to map the equations of motion for the Green functions of a clean superconductor with a general ellipsoidal Fermi surface (FS) characterized by the effective masses $m_1, m_2$, and $m_3$ in the presence of an arbitrarily directed magnetic induction ${\bm B}=B(\sinθ\cosϕ,\sinθ\sinϕ,\cosθ)$ onto those of a spherical FS. We then obtain the transformed gap equation for a transformed pairing interaction $\tilde{V}(\hat{\tilde{\bm k}},\hat{\tilde{\bm k}}')$ appropriate for any orbital order parameter symmetry. We use these results to calculate the upper critical induction $B_{c2}(θ,ϕ)$ for an orthorhombic ferromagnetic superconductor with transition temperatures $T_{\rm Curie}>T_c$. We assume the FS is split by strong spin-orbit coupling, with a single parallel-spin ($\uparrow\uparrow$) pairing interaction of the \textit{p}-wave polar state form locked onto the $\hat{\bm e}_3$ crystal axis normal to the spontaneous magnetization ${\bm M}_0\perp\hat{\bm e}_3$ due to the ferromagnetism. The orbital harmonic oscillator eigenvalues are modified according to $B\rightarrow Bα$, where $α(θ,ϕ)=\sqrt{m_3/m}\sqrt{\cos^2θ+γ^{-2}(ϕ)\sin^2θ}$, $γ^2(ϕ)=m_3/(m_1\cos^2ϕ+m_2\sin^2ϕ)$ and $m=(m_1m_2m_3)^{1/3}$. At fixed $ϕ$, the order parameter anisotropy causes $B_{c2}$ to exhibit a novel $θ$-dependence, which for $γ^2(ϕ)>3$ becomes a double peak at $0^{\circ}<θ^{*}<90^{\circ}$ and at $180^{\circ}-θ^{*}$, providing a sensitive bulk test of the order parameter orbital symmetry in both phases of URhGe and in similar compounds still to be discovered.

cond-mat.supr-con↗

arXiv2013

Study of microbunching instability in the linac of a soft X-ray FEL Facility

The development of the microbunching instability is studied for the linac of the proposed Shanghai Soft X-ray Free-Electron-Laser facility (SXFEL) by analytic formulae as well as numerical simulations with the aid of two different codes. The process is investigated in detail and the growth rates (gains) of the instability under various conditions are compared. The results indicate that the limitations from numerical computations in the present simulation model must be taken into account. Moreover, the appearance of higher-order mode excitations in the simulations suggests that further improvement of the current theory is necessary. A mechanism of introducing shot noise into beam profile as the beam passes through a chicane is proposed. The key issues that drive the instability are analyzed.

physics.acc-ph↗

arXiv2013

Coexistence of p-wave Cooper pairing and ferromagnetism

A two-band model for coexistence of p-wave superconductivity with localized ferromagnetism is studied using the equation of motion approach. It shows that ferromagnetic and superconducting states enhance each other but in a different way from that of the one-band model. The Curie temperature is not only determined by the exchange interactions between localized spins, but also can be increased with the coupling between electrons and spins, and with the p-wave Cooper-pairing interaction. These results are complementary to those of the one-band model, which suggest that the Curie temperature is unlikely to ever be below the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2013

Dynamics of double-well Bose-Einstein Condensates subject to external Gaussian white noise

Dynamical properties of the Bose-Einstein condensate in double-well potential subject to Gaussian white noise are investigated by numerically solving the time-dependent Gross-Pitaevskii equation. The Gaussian white noise is used to describe influence of the random environmental disturbance on the double-well condensate. Dynamical evolutions from three different initial states, the Josephson oscillation state, the running phase and $π$-mode macroscopic quantum self-trapping states are considered. It is shown that the system is rather robust with respect to the weak noise whose strength is small and change rate is high. If the evolution time is sufficiently long, the weak noise will finally drive the system to evolve from high energy states to low energy states, but in a manner rather different from the energy-dissipation effect. In presence of strong noise with either large strength or slow change rate, the double-well condensate may exhibit very irregular dynamical behaviors.

cond-mat.quant-gas↗

arXiv2012

Dissipation effect in the double-well Bose-Einstein Condensate

Dynamics of the double-well Bose-Einstein condensate subject to energy dissipation is studied by solving a reduced one-dimensional time-dependent Gross-Pitaevskii equation numerically. We first reproduce the phase space diagram of the system without dissipation systematically, and then calculate evolutionary trajectories of dissipated systems. It is clearly shown that the dissipation can drive the system to evolve gradually from the $π$-mode quantum macroscopic self-trapping state, a state with relatively higher energy, to the lowest energy stationary state in which particles distribute equally in the two wells. The average phase and phase distribution in each well are discussed as well. We show that the phase distribution varies slowly in each well but may exhibit abrupt changes near the barrier. This sudden change occurs at the minimum position in particle density profile. We also note that the average phase in each well varies much faster with time than the phase difference between two wells.

cond-mat.quant-gas↗

arXiv2012

Magnetic properties of charged spin-1 Bose gases with ferromagnetic coupling

Magnetic properties of a charged spin-1 Bose gas with ferromagnetic interactions is investigated within mean-field theory. It is shown that a competition between paramagnetism, diamagnetism and ferromagnetism exists in this system. It is shown that diamagnetism, being concerned with spontaneous magnetization, cannot exceed ferromagnetism in very weak magnetic field. The critical value of reduced ferromagnetic coupling of paramagnetic phase to ferromagnetic phase transition $\bar I_{c}$ increases with increasing temperature. The Lande-factor $g$ is introduced to describe the strength of paramagnetic effect which comes from the spin degree of freedom. The magnetization density $\bar M$ increases monotonically with $g$ for fixed reduced ferromagnetic coupling $\bar I$ as $\bar I>\bar I_{c}$. In a weak magnetic field, ferromagnetism makes immense contribution to the magnetization density. While at a high magnetic field, the diamagnetism inclines to saturate. Evidence for condensation can be seen in the magnetization density at weak magnetic field.

cond-mat.stat-mech↗

arXiv2011

Berezinskii-Kosterlitz-Thouless transition of two-dimensional Bose gases in a synthetic magnetic field

We study the Berezinskii-Kosterlitz-Thouless transition of two-dimensional Bose gases in a synthetic magnetic field using the standard Metropolis Monte Carlo method. The system is described by the frustrated XY model and the critical temperature is calculated though the absence of central peak of the wave function in momentum space, which can be directly measured by the time-of-flight absorbing imaging in cold atoms experiments. The results of our work show agreement with former studies on superconducting Josephson arrays.

cond-mat.quant-gas↗

arXiv2011

Upper critical field of p-wave superconductors with orthorhombic symmetry

We extend the Scharnberg-klemm theory of $H_{c2}$ in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong applied magnetic fields. For some partially broken symmetry cases, $H_{c2}$ can mimic upward curvature in all three crystal axis directions, and reasonably good fits to some of the the low-field UCoGe data are obtained.

cond-mat.supr-con↗

arXiv2011

The QE numerical simulation of PEA semiconductor photocathode

Several kinds of models have already been proposed for explaining the photoemission process. The exact photoemission theory of semiconductor photocathode was not well established after decades of research. In this paper an integral equation of quantum efficiency (QE) is constructed to describe the photoemission of positive electron affinity (PEA) semiconductor photocathode based on three-step photoemission model. The influences of forbidden gap, electron affinity, photon energy, incident angle, degree of polarization, refractive index, extinction coefficient, initial/final electron energy, relaxation time and external electric field on the QE of PEA semiconductor photocathode are taken into account. In addition, a computer code is also programmed to calculate the QE of K2CsSb photocathode theoretically at 532nm wavelength, the result is in line with the experimental value by and large. What are the reasons caused to the distinction between the experimental measuring and theoretical QE are discussed.

physics.acc-ph↗

arXiv2011

Upper critical field of p-wave ferromagnetic superconductors with orthorhombic symmetry

We extended the Scharnberg-Klemm theory of $H_{c2}(T)$ in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong magnetic fields. For some partially broken symmetry cases, $H_{c2}(T)$ can mimic upward curvature in all three crystal axis directions, and reasonably good fits to some of the UCoGe data are obtained.

cond-mat.supr-con↗

arXiv2011

Spontaneous separation of two-component Fermi gases in a double-well trap

The two-component Fermi gas in a double-well trap is studied using the density functional theory and the density profile of each component is calculated within the Thomas-Fermi approximation. We show that the two components are spatially separate in the two wells once the repulsive interaction exceeds the Stoner point, signaling the occurrence of the ferromagnetic transition. Therefore, the double-well trap helps to explore itinerant ferromagnetism in atomic Fermi gases, since the spontaneous separation can be examined by measuring component populations in one well.

cond-mat.quant-gas↗

arXiv2011

Dynamics of Two-Component Bose-Einstein Condensates Coupled with Environment

We investigate the dynamics of an open Bose-Einstein condensate system consisting of two hyperfine states of the same atomic species which are coupled by tunable Raman laser. It is already suggested that the detuning between the laser frequency and transition frequency affect significantly on the dynamics of the pure condensate. Here we show that the detuning effect is suppressed by noise and dissipation caused by the environment. The increase of coherence and purity are also displayed for specific parameters. As a verification to the lowest-order approximation we derive the hierarchy of motion equations in the second-order approximation. It turns out that the former one can describe the dynamical evolution qualitatively for weak noise and dissipation and quantitatively for strong noise and dissipation.

cond-mat.quant-gas↗