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N. J. Zhou

Publications and source records attributed to N. J. Zhou.

14 recordsLinked to original sources
arXiv2019

Spin-reorientation critical dynamics in the two-dimensional XY model with a domain wall

In recent years, static and dynamic properties of non-$180^\circ$ domain walls in magnetic materials have attracted a great deal of interest. In this paper, spin-reorientation critical dynamics in the two-dimensional XY model is investigated with Monte Carlo simulations and theoretical analyses based on the Langevin equation. At the Kosterlitz-Thouless phase transition, dynamic scaling behaviors of the magnetization and the two-time correlation function are carefully analyzed, and critical exponents are accurately determined. When the initial value of the angle between adjacent domains is slightly lower than $π$, a critical exponent is introduced to characterize the abnormal power-law increase of the magnetization in the horizontal direction inside the domain interface, which is measured to be $ψ=0.0568(8)$. Besides, the relation $ψ=η/2z$ is analytically deduced from the Langevin dynamics in the long-wavelength approximation, well consistent with numerical results.

cond-mat.stat-mech↗
arXiv2014

Corrections to scaling in the dynamic approach to the phase transition with quenched disorder

With dynamic Monte Carlo simulations, we investigate the continuous phase transition in the three-dimensional three-state random-bond Potts model. We propose a useful technique to deal with the strong corrections to the dynamic scaling form. The critical point, static exponents $β$ and $ν$, and dynamic exponent $z$ are accurately determined. Particularly, the results support that the exponent $ν$ satisfies the lower bound $ν\geqslant 2/d$.

cond-mat.stat-mech↗
arXiv2014

Nonsteady dynamic properties of a domain wall for the creep state under an alternating driving field

With Monte Carlo simulations, the nonsteady dynamics properties of a domain wall have been systematically investigated for the thermally activated creep state under an alternating driving field. Taking the driven random-field Ising model in two dimensions as an example, two distinct growth stages of the domain interface are identified with both the correlation length and roughness function. One stage belongs to the universality class of the random depositions, and the other to that of the quenched Edwards-Wilkinson equation. In the latter case, due to the dynamic effect of overhangs, the domain interface may exhibit an intrinsic anomalous scaling behavior, different from that of the quenched Edwards-Wilkinson equation.

cond-mat.stat-mech↗
arXiv2013

Dynamic approach to finite-temperature magnetic phase transitions in the extended J1- J2 model with vacancy order

The recently discovered iron-based superconductors A$_{y}$Fe$_{2-x}$Se$_{2}$ ($A$=K, Rb, Cs, Tl) show a long-range antiferromagnetic order with an unexpected high transition temperature $T_N \sim 550$ K and a unique $\sqrt{5} \times \sqrt{5}$ vacancy order. Taking the extended $J_1$-$J_2$ model as a minimal model, we investigate the finite-temperature magnetic phase transitions in a square lattice with a $\sqrt{5} \times \sqrt{5}$ vacancy superstructure by using large-scale Monte Carlo simulations. By the parallel tempering technique, the block spin checkerboard and stripe antiferromagnetic states are detected to be the groundstates for three representative sets of model parameters. The short-time dynamic approach is applied to accurately determine the critical temperature as well as the static and dynamic exponents. Our results indicate that the dramatic enhancement of the critical temperature as observed in experiments should be mainly due to a combination effect of the vacancy order and the block lattice contraction.

cond-mat.supr-con↗
arXiv2012

Depinning phase transition in two-dimensional clock model with quenched randomness

With Monte Carlo simulations, we systematically investigate the depinning phase transition in the two-dimensional driven random-field clock model. Based on the short-time dynamic approach, we determine the transition field and critical exponents. The results show that the critical exponents vary with the form of the random-field distribution and the strength of the random fields, and the roughening dynamics of the domain interface belongs to the new subclass with $ζ\neq ζ_{loc} \neq ζ_s$ and $ζ_{loc} \neq 1$. More importantly, we find that the transition field and critical exponents change with the initial orientations of the magnetization of the two ordered domains.

cond-mat.dis-nn↗
arXiv2012

Creep motion of a domain wall in the two-dimensional random-field Ising model with a driving field

With Monte Carlo simulations, we study the creep motion of a domain wall in the two-dimensional random-field Ising model with a driving field. We observe the nonlinear fieldvelocity relation, and determine the creep exponent μ. To further investigate the universality class of the creep motion, we also measure the roughness exponent ζ and energy barrier exponent ψ from the zero-field relaxation process. We find that all the exponents depend on the strength of disorder.

cond-mat.dis-nn↗
arXiv2012

Hamiltonian equation of motion and depinning phase transition in two-dimensional magnets

Based on the Hamiltonian equation of motion of the $ϕ^4$ theory with quenched disorder, we investigate the depinning phase transition of the domain-wall motion in two-dimensional magnets. With the short-time dynamic approach, we numerically determine the transition field, and the static and dynamic critical exponents. The results show that the fundamental Hamiltonian equation of motion belongs to a universality class very different from those effective equations of motion.

cond-mat.dis-nn↗
arXiv2012

Critical domain-wall dynamics of model B

With Monte Carlo methods, we simulate the critical domain-wall dynamics of model B, taking the two-dimensional Ising model as an example. In the macroscopic short-time regime, a dynamic scaling form is revealed. Due to the existence of the quasi-random walkers, the magnetization shows intrinsic dependence on the lattice size $L$. A new exponent which governs the $L$-dependence of the magnetization is measured to be $σ=0.243(8)$.

cond-mat.stat-mech↗
arXiv2012

Universality class of the depinning transition in the two-dimensional Ising model with quenched disorder

With Monte Carlo methods, we investigate the universality class of the depinning transition in the two-dimensional Ising model with quenched random fields. Based on the short-time dynamic approach, we accurately determine the depinning transition field and both static and dynamic critical exponents. The critical exponents vary significantly with the form and strength of the random fields, but exhibit independence on the updating schemes of the Monte Carlo algorithm. From the roughness exponents $ζ, ζ_{loc}$ and $ζ_s$, one may judge that the depinning transition of the random-field Ising model belongs to the new dynamic universality class with $ζ\neq ζ_{loc}\neq ζ_s$ and $ζ_{loc} \neq 1$. The crossover from the second-order phase transition to the first-order one is observed for the uniform distribution of the random fields, but it is not present for the Gaussian distribution.

physics.comp-ph↗
arXiv2012

Short-time domain-wall dynamics in the random-field Ising model with a driving field

With Monte Carlo methods, we investigate the relaxation dynamics of a domain wall in the two-dimensional random-field Ising model with a driving field. The short-time dynamic behavior at the depinning transition is carefully examined, and the roughening process of the domain wall is observed. Based on the short-time dynamic scaling form, we accurately determine the transition field, static and dynamic exponents, and local and global roughness exponents. In contrast to the usual assumption, the results indicate that the domain interface does not belong to the universality class of the Edwards-Wilkinson equation. In particular, due to the dynamic effect of overhangs, the domain interface exhibits intrinsic anomalous scaling and spatial multiscaling behaviors, compatible with the experiments

cond-mat.stat-mech↗
arXiv2012

Nonequilibrium critical dynamics with domain wall and surface

With Monte Carlo simulations, we investigate the relaxation dynamics with a domain wall for magnetic systems at the critical temperature. The dynamic scaling behavior is carefully analyzed, and a dynamic roughening process is observed. For comparison, similar analysis is applied to the relaxation dynamics with a free or disordered surface.

cond-mat.stat-mech↗
arXiv2012

Dynamic effect of overhangs and islands at the depinning transition in two-dimensional magnets

With the Monte Carlo methods, we systematically investigate the short-time dynamics of domain-wall motion in the two-dimensional random-field Ising model with a driving field ?DRFIM?. We accurately determine the depinning transition field and critical exponents. Through two different definitions of the domain interface, we examine the dynamics of overhangs and islands. At the depinning transition, the dynamic effect of overhangs and islands reaches maximum. We argue that this should be an important mechanism leading the DRFIM model to a different universality class from the Edwards-Wilkinson equation with quenched disorder

cond-mat.stat-mech↗
arXiv2012

Dynamic relaxation of topological defect at Kosterlitz-Thouless phase transition

With Monte Carlo methods we study the dynamic relaxation of a vortex state at the Kosterlitz-Thouless phase transition of the two-dimensional XY model. A local pseudo-magnetization is introduced to characterize the symmetric structure of the dynamic systems. The dynamic scaling behavior of the pseudo-magnetization and Binder cumulant is carefully analyzed, and the critical exponents are determined. To illustrate the dynamic effect of the topological defect, similar analysis for the the dynamic relaxation with a spin-wave initial state is also performed for comparison. We demonstrate that a limited amount of quenched disorder in the core of the vortex state may alter the dynamic universality class. Further, theoretical calculations based on the long-wave approximation are presented.

physics.comp-ph↗