Recent progress of probing correlated electron states by point contact spectroscopy
Recent progress of probing correlated electron states by point contact spectroscopy
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点接触光谱探测相关电子态的最新进展
DOI：
10.1088/0034-4885/79/9/094502
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发表时间：
2015
影响因子：
18.1
通讯作者：
L. Greene
中科院分区：
文献类型：
--
作者：
Wei‐Cheng Lee;L. Greene
本文综述了点接触光谱学（PCS）从相关电子材料中提取光谱信息的最新进展，重点介绍了非超导态电子材料。PCS已被用于检测普通金属中的玻色子激发，其中特征（例如声子）通常小于测量电导的1%。在超导状态下，点接触安德烈夫反射（PCAR）被广泛用于研究各种超导体中超导隙的性质。众所周知，相应的电导可以用Blonder-Tinkham-Klapwijk （BTK）理论精确拟合，其中发生在点接触结附近的AR由三个参数模拟；超导间隙，准粒子散射率，以及描述结处势垒强度的无量纲参数Z。AR可以达到背景电导的100%，并且只在超导体的情况下出现。近十年来，越来越多的实验结果表明，点接触电导率可以揭示非超导状态下不寻常的单电子动力学的新特征，为探索相关材料中竞争相的性质提供了新的思路。为了正确地解释这些新特征，重新检查点接触结的建模，用于描述单电子动力学的形式，特别是在点接触光谱中，以及应该计算以理解电导的物理量是至关重要的。我们将总结从不同方法发展的点接触光谱理论，并强调区分点接触光谱与基于隧道的探针的概念差异。此外，我们将展示Schwinger-Kadanoff-Baym-Keldysh （SKBK）形式以及随机分布在结上的纳米尺度点接触的适当建模如何得出点接触电导与有效态密度成正比的结论，如果电子自能已知，则可以计算出有效态密度。铁基超导体和重费米子化合物的实验数据将在此框架下进行分析。这些最新的发展将点接触光谱的适用性扩展到相关材料，这将有助于我们更深入地了解强相关系统中的单电子动力学。
We review recent progress in point contact spectroscopy (PCS) to extract spectroscopic information out of correlated electron materials, with the emphasis on non-superconducting states. PCS has been used to detect bosonic excitations in normal metals, where signatures (e.g. phonons) are usually less than 1% of the measured conductance. In the superconducting state, point contact Andreev reflection (PCAR) has been widely used to study properties of the superconducting gap in various superconductors. It has been well-recognized that the corresponding conductance can be accurately fitted by the Blonder–Tinkham–Klapwijk (BTK) theory in which the AR occurring near the point contact junction is modeled by three parameters; the superconducting gap, the quasiparticle scattering rate, and a dimensionless parameter, Z, describing the strength of the potential barrier at the junction. AR can be as large as 100% of the background conductance, and only arises in the case of superconductors. In the last decade, there have been more and more experimental results suggesting that the point contact conductance could reveal new features associated with the unusual single electron dynamics in non-superconducting states, shedding a new light on exploring the nature of the competing phases in correlated materials. To correctly interpret these new features, it is crucial to re-examine the modeling of the point contact junctions, the formalism used to describe the single electron dynamics particularly in point contact spectroscopy, and the physical quantity that should be computed to understand the conductance. We will summarize the theories for point contact spectroscopy developed from different approaches and highlight these conceptual differences distinguishing point contact spectroscopy from tunneling-based probes. Moreover, we will show how the Schwinger–Kadanoff–Baym–Keldysh (SKBK) formalism together with the appropriate modeling of the nano-scale point contacts randomly distributed across the junction leads to the conclusion that the point contact conductance is proportional to the effective density of states, a physical quantity that can be computed if the electron self energy is known. The experimental data on iron based superconductors and heavy fermion compounds will be analyzed in this framework. These recent developments have extended the applicability of point contact spectroscopy to correlated materials, which will help us achieve a deeper understanding of the single electron dynamics in strongly correlated systems.