动态外场作用下Ising自旋体系的非平衡动态相变

系统地考察了Ising自旋体系的动力学方程对三种不同性质的驱动外场（正弦波 、方波和锯齿波）的动态响应及其相应的非平衡动态相变特征.在正弦波和方波的驱动场 作用下，体系存在分别对应于低温对称破缺的铁磁有序态和高温对称顺磁无序态的动态非平衡转变，相应的动态转变相界上存在区分连续转变和非连续转变的三临界点；而锯齿波驱动 场情形下体系始终维持对称性破缺的有序态.体系动态转变表现出的上述差异与作用外场的驱动特征有关.确定了表征相应动态相变相界的临界驱动外场振幅h0C和频率 ωc、体系的温度tc, 并给予了分析讨论 关键词： Ising自旋体系 非平衡动态相变 对称性 平均场

Nonequilibrium dynamic phase transition of an Ising spin system driven by various oscillating field

We have studied both the dynamic response and the relevant nonequilibrium dynamical phase transition of an Ising spin system subject to three sorts of oscillating field i.e. sinusoidal, square and sawtooth waves. The above three sorts of external field drive dynamically the Ising spin system in either simply gradual (abrupt) way or their combination respectively. In the case of both sinusoidal and square ways, it was observed that the Ising spin system displays a low_temperature symmetry-breaking ordered phase and a high_temperature symmetric disordered phase as well as the dynamic transition between two dynamical phases above. We also detected the tri_critical point separating high_temperature continuous dynamic transition and low_temperature discontinuous one on the boundary of dynamic transition. The trend of dynamic transition boundary and the dependence of tri_critical point upon the system temperature, the frequency and amplitude of the driving field were revealed as well. In contrast, no dynamic transition occurs any longer and the system always stays in symmetry-breaking ordered state if the external field takes the form of sawtooth wave. The preceding discrepancy in dynamic response and transition is attributed to the perturbative characteristic of different oscillating fields.