边缘费米玻璃的THz二维相干光谱观测
Observation of a marginal Fermi glass using THz 2D coherent spectroscopy
摘要：
凝聚态物理中一个长期存在的问题是，一个强无序的相互作用绝缘体是否可以映射到一个有效的非相互作用局域激发系统。我们利用太赫兹二维相干光谱新技术研究了掺磷硅中3D金属-绝缘体转变（MIT）的绝缘侧。尽管这些材料本质上是无序的，但我们观察到了相干激发和强光子回波，这为我们研究它们的衰变过程提供了一种强有力的方法。我们提取的第一个测量的能量弛豫（$T_1$）和退相干（$T_2$）的时间接近MIT在这个经典的系统。我们观察到，（i）两个弛豫速率在激发频率中是线性的，斜率接近1，（ii）能量弛豫时间尺度$T_1$随着温度的升高而反直觉地增加，（iii）相干弛豫时间尺度$T_2$在5 K和25 K之间几乎没有温度依赖性，但是随着材料向MIT掺杂而反直觉地增加。我们认为，这些功能意味着（a）系统的行为作为一个很好的孤立的电子系统的时间尺度上的利益，和（b）弛豫是由电子-电子相互作用控制。我们讨论了可能解释这种行为的潜在弛豫通道。我们的观察构成了一个定性的新现象，由强无序和强电子-电子相互作用的相互作用驱动，我们称之为边缘费米玻璃。
英文摘要：
A longstanding open problem in condensed matter physics is whether or not a strongly disordered interacting insulator can be mapped to a system of effectively non-interacting localized excitations. We investigate this issue on the insulating side of the 3D metal-insulator transition (MIT) in phosphorus doped silicon using the new technique of terahertz two dimensional coherent spectroscopy. Despite the intrinsically disordered nature of these materials, we observe coherent excitations and strong photon echoes that provide us with a powerful method for the study of their decay processes. We extract the first measurements of energy relaxation ($T_1$) and decoherence ($T_2$) times close to the MIT in this classic system. We observe that (i) both relaxation rates are linear in excitation frequency with a slope close to unity, (ii) the energy relaxation timescale $T_1$ counterintuitively increases with increasing temperature and (iii) the coherence relaxation timescale $T_2$ has little temperature dependence between 5 K and 25 K, but counterintuitively increases as the material is doped towards the MIT. We argue that these features imply that (a) the system behaves as a well isolated electronic system on the timescales of interest, and (b) relaxation is controlled by electron-electron interactions. We discuss the potential relaxation channels that may explain the behavior. Our observations constitute a qualitatively new phenomenology, driven by the interplay of strong disorder and strong electron-electron interactions, which we dub the marginal Fermi glass.
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