The optical contrast in a phase change material is concomitant with its structural transition. We connect these two by first recognizing that Friedel oscillations couple electrons propagating in opposite directions and supply an additional Coulomb energy. As the crystal switches phase, this energy acquires time dependence and the Landau-Zener mechanism operates, steering population transfer from the valence to the conduction band and vice versa. Spectroscopy suggests that the oscillator energy dominates the optical properties and a calculation involving the crystalline field and spin-orbit interaction yields good estimates for of both structural phases. Further analysis relates the optical gap with the crystalline-field energy as well as activation energy for electrical conduction. This last property characterizes the amorphous phase, thereby furnishing a link between the crystalline field and the activation energy and ultimately with the resistance drift exponent. Providing optical means to quantify resistance drift in PCMs could circumvent the need for fabricating expensive devices and performing time consuming measurements.
Negative differential resistance and characteristic nonlinear electromagnetic response of a Topological Insulator
表现出负微分电阻的材料在涉及微波产生的技术中具有重要的应用，其范围从运动传感到射电天文学。尽管它们很有用，但很少有物理机制产生具有这种特性的材料，即用于耿氏二极管的GaAs。在这项工作中，我们表明，负微分电阻也普遍出现在狄拉克环系统，其中的一个例子已被实验观察到的表面态的拓扑绝缘体。这种新的实现负微分电阻是基于一个完全不同的物理机制，从耿氏效应，依赖于响应曲线的特征非单调性，在非零温度，化学势，质量间隙和杂质散射的存在下保持稳健。因此，它为工程应用开辟了新的可能性，例如高度寻求太赫兹信号生成的频率上转换器件。我们的结果可以测试薄膜的Bi 2 Se 3拓扑绝缘体，并预计定性地举行，即使在没有严格的线性狄拉克色散，将是在更一般的样品的Bi 2 Se 3和其他材料的情况下，拓扑非平凡的费米海区域。
