Massive parallel calculations for phase transition driven by cooperative mechanisms with different time constants

在本研究中，主要关注的是有序-无序转换。作为初步计算，我们通过明确区分声子振动过程和原子扩散过程的时间常数，在fcc系统的无序区进行了短程有序扩散强度计算。结果表明，扩散强度的温度依赖性表现为偏离平衡轨迹的动力学效应。通过纯数学分析，发现动力学效应的出现与一个函数的变换和反变换有关。通过对Ni3Pt金属间化合物一阶跃迁的电阻率测量，观察到一种伪临界慢化现象。为了揭示这一机制，我们尝试在聚类变化的四面体近似和路径概率方法的框架下进行数值计算。在传统的路径概率方法中，路径概率函数中的原子扩散项没有考虑明显的温度依赖性。在本研究中，另一方面，介绍了热活化过程。所得到的弛豫时间再现了除接近转变温度外的普通温区的正常温度依赖性。然后，进一步引入了活化能的轻微温度依赖性，并证实了在转变温度附近弛豫时间突然增加的实验结果得到了满意的再现。当考虑路径概率函数中涉及的激活过程的物理意义时，人们意识到这个附加的激活过程对应于导致不同时间常数的两个不同的松弛过程。

In the present study, the main focus is placed on order-disorder transition.As a preliminary calculation, we performed short range order diffuse intensity calculation in the disordered region of fcc system by explicitly distinguishing the time constant of phonon vibration process and atomic diffusion process. It is revealed that kinetic effects appear in the temperature dependency of diffuse intensity as a deviation from the equilibrium locus.A purely mathematical analysis was followed and it was found that the appearance of kinetic effects corresponds to transformation and inverse transformation of a function.A pseudo-critical slowing down phenomenon has been observed by electrical resistivity measurements for Ni3Pt intermetallic compound which undergoes first order transition. In order to reveal the mechanism, we attempted numerical calculation within the frameworks of the Tetrahedron Approximation of the Cluster Variation and Path Probability Methods. In the conventional Path Probability Method, an explicit temperature dependency is not considered in the atomic diffusion term in the path probability function. In the present study, on the other hand, a thermal activation process is introduced. The resultant relaxation time reproduces the normal temperature dependency for an ordinary temperature region except near transition temperature. Then, a slight temperature dependency is further introduced in the activation energy, and it was confirmed that the experimental result of abrupt increase of relaxation time in the vicinity of transition temperature is satisfactorily reproduced. When one considers the physical meanings of activation processes involved in the path probability function, one realizes that this additional activation process corresponds to the two distinguished relaxation processes resulting in the different time constants.