Dynamical simulations on the two-dimensional XY model with random-phase shift

Using resistively-shunted-junction dynamics, we numerically investigate the two-dimensional XY model with random phase shift. The critical temperatures and critical exponents are determined by dynamic scaling analysis. For weak disorder strengths, the system undergoes a Kosterlitz-Thouless (KT). A non-KT type phase transition is also observed for strong disorders. A genuine continuous depinning transition at zero temperature and creep motion at low temperature are also studied for various disorder strengths. The relevant critical currents and critical exponents are evaluated, and a non-Arrhenius creep motion is observed in the low temperature phases.     

Breakdown of scaling in the nonequilibrium critical dynamics of the two-dimensional XY model

The approach to equilibrium, from a nonequilibrium initial state, in a system at its critical point is usually described by a scaling theory with a single growing length scale, xi(t) approximately t(1/z), where z is the dynamic exponent that governs the equilibrium dynamics. We show that, for the 2D XY model, the rate of approach to equilibrium depends on the initial condition. In particular, xi(t) approximately t(1/2) if no free vortices are present in the initial state, while xi(t) approximately (t/lnt)(1/2) if free vortices are present.     

Correlation properties of anisotropic XY model with a sudden quench

Starting from a general Hamiltonian which may undergo a quantum phase transition (QPT) with the change of a controllable parameter, we obtain a general conclusion that in a sudden quench system, when the final Hamiltonian is fixed, the behavior of the time-averaged expectation of any observable has close relationship with the gapless excitation of the initial Hamiltonian. To clarify our conclusion, we investigate the two-spin correlation of a XY chain in a transverse field under a sudden quench at zero temperature. The critical property of the derivative of quench two-spin correlation and the long-range correlation of the quench system are analyzed.     

Deconfinement in the two-dimensional XY model

The unbinding of vortex-antivortex pairs for the classical two-dimensional XY model in a magnetic field is studied. A single such pair is connected by a string of overturned spins, leading to linear confinement. We show that this system supports two phase transitions, one in which closed strings proliferate, and a second in which vortices unbind. The transitions are shown to be dual to one another, and are remarkably continuous. Possible consequences for a variety of systems are discussed.     

Dynamics of the anisotropic two-dimensional XY model

In this paper we study the dynamics of the two-dimensional XY model with single-ion anisotropy, and spin S = 1, in the large D phase, and low temperatures, using the bond operator formalism. The in-plane structure factor is a delta function. The out of plane shows a three peak structure, which merges in a single peak at the Brillouin zone boundary. We analyze also spin currents generated by a magnetic field gradient. The spin conductivity is calculated, at finite temperature, using the Kubo formula. The model shows unconventional ballistic spin transport at finite temperature. The computed spin conductivity exhibits a nonzero Drude weight at finite temperature. For ω< 2m, where m is the energy gap, the spin conductivity is described solely by the Drude weight. There is a regular contribution to the spin conductivity for ω> 2m, which persist in the zero temperature limit. The conductivity at the critical point, and for small frequencies, is (gμ_B)²/ħ times a universal scaling function of ħω/k_B T.     

Autocorrelation exponent of conserved spin systems in the scaling regime following a critical quench

We study the autocorrelation function of a conserved spin system following a quench at the critical temperature. Defining the correlation length L(t) approximately t(1/z), we find that for times t' and t satisfying L(t')infinity limit, we show that lambda(')(c)=d+2 and phi=z/2. We give a heuristic argument suggesting that this result is, in fact, valid for any dimension d and spin vector dimension n. We present numerical simulations for the conserved Ising model in d=1 and d=2, which are fully consistent with the present theory.     

Zero-temperature ordering in the two-dimensional quantum XY model

We have calculated the probability distribution for the staggered magnetization at T=0 for the 2D antiferromagnetic quantum XY model on finite lattices. For the ground state, the distribution shows evidence of isotropic magnetization ordering on the xy-plane. Based on data on seven lattices up to 26 sites, the extrapolated value of the staggered magnetization is 0.448±0.003 in the thermodynamic limit.     

Phase diagram for the two-dimensional quantum anisotropic XY model

A self-consistent harmonic approximation is used to study the Kosterlitz–Thouless phase transition and the quantum phase transition at T=0 K in the two-dimensional anisotropic quantum XY model.     

Exact results for quench dynamics and defect production in a two-dimensional model

We show that for a d-dimensional model in which a quench with a rate tau(-1) takes the system across a (d-m)-dimensional critical surface, the defect density scales as n approximately 1/tau(mnu/(znu+1)), where nu and z are the correlation length and dynamical critical exponents characterizing the critical surface. We explicitly demonstrate that the Kitaev model provides an example of such a scaling with d = 2 and m = nu = z = 1. We also provide the first example of an exact calculation of some multispin correlation functions for a two-dimensional model that can be used to determine the correlation between the defects. We suggest possible experiments to test our theory.     

Dynamical phase transitions in the two-dimensional ANNNI model

We study the phase diagram of the two-dimensional anisotropic next-nearest neighbor Ising (ANNNI) model by comparing the time evolution of two distinct spin configurations submitted to the same thermal noise. We clearly see several dynamical transitions between ferromagnetic, paramagnetic, antiphase, and floating phases. These dynamical transitions seem to occur rather close to the transition lines determined previously in the literature.