非中心对称体系中位移电流的局域光激发
Local photo-excitation of shift current in noncentrosymmetric systems
摘要：
固体中的光电流是一种重要的物理现象，在太阳能电池等领域有着广泛的应用。在传统的光电导体中，通过光照射产生的电子和空穴被外部电场分离，导致电流流入电极。非中心对称系统中的位移电流与传统的光电流不同，因为它不需要外部电场，而且更值得注意的是，位移电流是由布洛赫波函数所固有的贝里相位驱动的。它类似于基态的极化电流，但在光的泵浦下，它是由非平衡稳态连续支持的直流电流。在这里，我们从理论上表明，通过采用Keldysh-Floquet形式主义适用于一个简单的一维模型，本地光激发可以诱导的移位电流，这是独立的位置和宽度的激发区域，也是系统的长度。这一特征与传统的光电流形成鲜明对比，当样品在中间局部激发并朝向电极增加时，传统的光电流被抑制。这一发现揭示了转移电流的非常规性质，并将为设计固体中的高效光伏效应铺平道路。
英文摘要：
Photocurrent in solids is an important phenomenon with many applications including the solar cells. In conventional photoconductors, the electrons and holes created by light irradiation are separated by the external electric field, resulting in a current flowing into electrodes. Shift current in noncentrosymmetric systems is distinct from this conventional photocurrent in the sense that no external electric field is needed, and, more remarkably, is driven by the Berry phase inherent to the Bloch wavefunction. It is analogous to the polarization current in the ground state but is a d.c. current continuously supported by the nonequilibrium steady state under the pumping by light. Here we show theoretically, by employing Keldysh-Floquet formalism applied to a simple one-dimensional model, that the local photo excitation can induce the shift current which is independent of the position and width of the excited region and also the length of the system. This feature is in stark contrast to the conventional photocurrent, which is suppressed when the sample is excited locally at the middle and increases towards the electrodes. This finding reveals the unconventional nature of shift current and will pave a way to design a highly efficient photovoltaic effect in solids.
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