Nakamura Kiyoto, Tanimura Yoshitaka
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyoku, Kyoto 606-8502, Japan.
J Chem Phys. 2021 Dec 28;155(24):244109. doi: 10.1063/5.0074047.
Complex environments, such as molecular matrices and biological material, play a fundamental role in many important dynamic processes in condensed phases. Because it is extremely difficult to conduct full quantum dynamics simulations on such environments due to their many degrees of freedom, here, we treat in detail the environment only around the main system of interest (the subenvironment), while the other degrees of freedom needed to maintain the equilibrium temperature are described by a simple harmonic bath, which we call a quantum thermostat. The noise generated by the subenvironment is spatially non-local and non-Gaussian and cannot be characterized by the fluctuation-dissipation theorem. We describe this model by simulating the dynamics of a two-level system (TLS) that interacts with a subenvironment consisting of a one-dimensional XXZ spin chain. The hierarchical Schrödinger equations of motion are employed to describe the quantum thermostat, allowing for time-irreversible simulations of the dynamics at arbitrary temperature. To see the effects of a quantum phase transition of the subenvironment, we investigate the decoherence and relaxation processes of the TLS at zero and finite temperatures for various values of the spin anisotropy. We observed the decoherence of the TLS at finite temperature even when the anisotropy of the XXZ model is enormous. We also found that the population-relaxation dynamics of the TLS changed in a complex manner with the change in the anisotropy and the ferromagnetic or antiferromagnetic orders of spins.
复杂环境，如分子矩阵和生物材料，在凝聚相的许多重要动态过程中起着基础性作用。由于此类环境具有多个自由度，对其进行完整的量子动力学模拟极为困难，因此，在这里，我们仅详细处理感兴趣的主要系统（子环境）周围的环境，而维持平衡温度所需的其他自由度则由一个简单的谐振子浴来描述，我们称之为量子恒温器。子环境产生的噪声在空间上是非局部且非高斯的，无法用涨落耗散定理来表征。我们通过模拟一个与由一维XXZ自旋链组成的子环境相互作用的两能级系统（TLS）的动力学来描述这个模型。采用分层薛定谔运动方程来描述量子恒温器，从而能够在任意温度下对动力学进行时间不可逆模拟。为了观察子环境的量子相变的影响，我们研究了在零温和有限温度下，对于各种自旋各向异性值，TLS的退相干和弛豫过程。我们观察到，即使XXZ模型的各向异性极大，TLS在有限温度下仍会发生退相干。我们还发现，TLS的布居弛豫动力学随着各向异性以及自旋的铁磁或反铁磁序的变化而以复杂的方式改变。
