arXiv subjects

W. Mao

Publications and source records attributed to W. Mao.

6 recordsLinked to original sources

arXiv2018

Modification of structural disorder by hydrostatic-pressure in superconducting YBa$_{2}$Cu$_{3}$O$_{6.73}$ cuprate

Compelling efforts to improve the critical temperature ($T_{c}$) of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important, however, much remains unclear. Here we studied ortho-III YBa$_{2}$Cu$_{3}$O$_{6.73}$ (YBCO) using x-ray scattering under hydrostatic-pressure (HP) up to ~6.0 GPa. We found the reinforced oxygen order (OO) of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge transfer phenomenon between the CuO$_{2}$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge density wave (CDW) signature in CuO$_{2}$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the CuO$_{2}$ plane. Using these results, we appropriately explain why pressure-condition can achieve higher $T_{c}$ compared with the optimal $T_{c}$ under ambient pressure in YBa$_{2}$Cu$_{3}$O$_{6+x}$. As an implication of these results, finally, we have discussed that the change in disorder could make it easier for YBa$_{2}$Cu$_{3}$O$_{6+x}$ to undergo a transition to the nematic order under an external magnetic field.

cond-mat.supr-con↗

arXiv2003

Spin dynamics from Majorana fermions

Using the Majorana fermion representation of spin-1/2 local moments, we show how it is possible to directly read off the dynamic spin correlation and susceptibility from the one-particle propagator of the Majorana fermion. We illustrate our method by applying it to the spin dynamics of a non-equilibrium quantum dot, computing the voltage-dependent spin relaxation rate and showing that, at weak coupling, the fluctuation-dissipation relation for the spin of a quantum dot is voltage-dependent. We confirm the voltage-dependent Curie susceptibility recently found by Parcollet and Hooley [Phys. Rev. B {\bf 66}, 085315 (2002)].

cond-mat.str-el↗

arXiv2002

Quantum Reciprocity Conjecture for the Non-Equilibrium Steady State

By considering the lack of history dependence in the non-equilibrium steady state of a quantum system we are led to conjecture that in such a system, there is a set of quantum mechanical observables whose retarded response functions are insensitive to the arrow of time, and which consequently satisfy a quantum analog of the Onsager reciprocity relations. Systems which satisfy this conjecture can be described by an effective Free energy functional. We demonstrate that the conjecture holds in a resonant level model of a multi-lead quantum dot.

cond-mat.str-el↗

arXiv2002

Magnetization of a D.C. biased quantum dot

Using a quantum generalization of the Onsager principle of microscopic reversibility, the magnetization of a system in a non-equilibrium steady state quantum dot is formulated as a response of the interaction energy to an external field. This formulation permits a direct and compact computation of the steady-state magnetization of a non-equilibrium quantum dot as a differential of the interaction energy. Unlike the direct computation of the magnetization using perturbative Keldysh methods, this approach does not require the use of a point splitting procedure. Our results nevertheless support earlier calculations made in the limit of zero field, and they support the survival of strong coupling to arbitrarily large voltages, both at zero field, and under the conditions where the chemical potential difference $eV$ becomes equal to the spin-flip energy in a field $eV = g μ_{B}B$.

cond-mat.mes-hall↗

arXiv2000

Equilibrium and nonequilibrium properties associated with the chiral phase transition at finite density in the Gross-Neveu Model

We study the dynamics of the chiral phase transition at finite density in the Gross-Neveu (GN) model in the leading order in large-N approximation. The phase structure of the GN model in this approximation has the property that there is a tricritical point at a fixed temperature and chemical potential separating regions where the chiral transition is first order from that where it is second order. We consider evolutions starting in local thermal and chemical equilibrium in the massless unbroken phase for conditions pertaining to traversing a first or second order phase transition. We assume boost invariant kinematics and determine the evolution of the order parameter $σ$, the energy density and pressure as well as the effective temperature, chemical potential and interpolating number densities as a function of the proper time $τ$. We find that before the phase transition, the system behaves as if it were an ideal fluid in local thermal equilibrium with equation of state $p=ε$. After the phase transition, the system quickly reaches its true broken symmetry vacuum value for the fermion mass and for the energy density. The single particle distribution functions for Fermions and anti-Fermions go far out of equilibrium as soon as the plasma traverses the chiral phase transition. We have also determined the spatial dependence of the "pion" Green's function $<\barψ(x) γ_5 ψ(x) \barψ(0) γ_5 ψ(0)>$ as a function of the proper time.

hep-ph↗

arXiv1999

Finite Density of States in a Mixed State of d_x^2-y^2+id_xy Superconductor

We have calculated the density of states of quasiparticles in a d_x^2-y^2+id_xy superconductor, and show that in the mixed state the quasiparticle spectrum remains gapless because of the Doppler shift by superflow. It was found that if the d_{xy} order gap $Δ_1\propto \sqrt{H}$ as suggested by experiments, then thermal conductivity $κ\propto \sqrt{H}$ in accord with experimental data at lowest temperatures. This is an appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999). We now also discuss the disorder effects and analyze the H log H crossover at small fields. We argue that H log H regime is present and disorder effect is dominant as the field-induced seconary gap is small at small fields.

cond-mat.supr-con↗