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arXiv 1202.4705cond-mat.mes-hallphysics.chem-ph
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精确平衡约化密度矩阵公式I:噪声、无序和温度对激子系统局域化的影响

An exact equilibrium reduced density matrix formulation I: The influence of noise, disorder, and temperature on localization in excitonic systems

Jeremy M. Moix, Yang Zhao, Jianshu Cao

摘要:

本文提出了一种计算具有体系浴哈密顿量的体系的整体平衡约化密度矩阵的精确方法。该方法是基于随机解开的影响功能,出现在虚时间路径积分形式主义的量子统计力学。然后,这种方法被应用到研究的热噪声,静态无序,激子系统的相干长度和温度的影响。作为有偏和无偏系统的代表性例子,注意力分别集中在FMO和LH 2的良好表征的捕光复合物上。由于偏置,FMO是完全本地化的网站基础在低温下,而LH2是完全离域。在后者中,静态无序的存在导致低温下相干长度的平台,随着无序强度的增加,该平台变得越来越明显。然而,噪声的引入排除了这种影响。在有偏系统中,环境可以增加系统的相干长度,但只会减少无偏系统的相干长度。最后强调,对于典型的光捕获系统中的环境参数值,系统和浴纠缠在单激发流形的平衡。也就是说,即使在室温下,密度矩阵也不能被描述为通常假设的产物状态。简化的LH2的密度矩阵被证明是精确的协议与以前的精确量子动力学计算的稳态极限。

英文摘要:

An exact method to compute the entire equilibrium reduced density matrix for systems characterized by a system-bath Hamiltonian is presented. The approach is based upon a stochastic unraveling of the influence functional that appears in the imaginary time path integral formalism of quantum statistical mechanics. This method is then applied to study the effects of thermal noise, static disorder, and temperature on the coherence length in excitonic systems. As representative examples of biased and unbiased systems, attention is focused on the well-characterized light harvesting complexes of FMO and LH2, respectively. Due to the bias, FMO is completely localized in the site basis at low temperatures, whereas LH2 is completely delocalized. In the latter, the presence of static disorder leads to a plateau in the coherence length at low temperature that becomes increasingly pronounced with increasing strength of the disorder. The introduction of noise, however, precludes this effect. In biased systems, it is shown that the environment may increase the coherence length, but only decrease that of unbiased systems. Finally it is emphasized that for typical values of the environmental parameters in light harvesting systems, the system and bath are entangled at equilibrium in the single excitation manifold. That is, the density matrix cannot be described as a product state as is often assumed, even at room temperature. The reduced density matrix of LH2 is shown to be in precise agreement with the steady state limit of previous exact quantum dynamics calculations.

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