arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 2007.11343cond-mat.str-el
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电子-声子系统的温度-密度矩阵重整化群方法:热力学和Holstein极化子谱函数

Finite-temperature density-matrix renormalization group method for electron-phonon systems: Thermodynamics and Holstein-polaron spectral functions

David Jansen, Janez Bonča, Fabian Heidrich-Meisner

摘要:

利用密度矩阵重整化群方法研究了Holstein极化子的热力学性质和有限温度谱函数。我们的方法结合了纯化和局部基优化(LBO)作为一个有效的治疗声子模式。LBO是一种通过对角化局部约简密度矩阵来寻找最优局部基的方法。通过将状态变换为这个基,可以截断局部希尔伯特空间,对于很宽的参数范围,精度损失可以忽略不计。在这项工作中,我们专注于Holstein模型的大,小极化子之间的交叉制度。在这里,没有解析解存在,我们表明,在低温下的热期望值是独立的声子希尔伯特空间截断的基础上选择足够大。然后,我们证明,我们可以提取的电子光谱函数,并建立从有限温度Lanczos方法的结果一致。我们还计算了电子发射谱和声子谱函数,并表明所有的计算都显着简化的局部基优化。我们观察到,随着温度的升高,电子发射谱的谱重向低频和大动量方向移动。声子谱函数经历了一个大的加宽和极化子峰在大动量得到显着平坦,几乎完全合并到自由声子峰。

英文摘要:

We investigate the thermodynamics and finite-temperature spectral functions of the Holstein polaron using a density-matrix renormalization group method. Our method combines purification and local basis optimization (LBO) as an efficient treatment of phonon modes. LBO is a scheme which relies on finding the optimal local basis by diagonalizing the local reduced density matrix. By transforming the state into this basis, one can truncate the local Hilbert space with a negligible loss of accuracy for a wide range of parameters. In this work, we focus on the crossover regime between large and small polarons of the Holstein model. Here, no analytical solution exists and we show that the thermal expectation values at low temperatures are independent of the phonon Hilbert space truncation provided the basis is chosen large enough. We then demonstrate that we can extract the electron spectral function and establish consistency with results from a finite-temperature Lanczos method. We additionally calculate the electron emission spectrum and the phonon spectral function and show that all the computations are significantly simplified by the local basis optimization. We observe that the electron emission spectrum shifts spectral weight to both lower frequencies and larger momenta as the temperature is increased. The phonon spectral function experiences a large broadening and the polaron peak at large momenta gets significantly flattened and merges almost completely into the free-phonon peak.

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