For semiconductors and insulators, it is commonly believed that in-gap transitions into nonlocalized states are smoothly suppressed in the clean limit; i.e., at zero temperature, their contribution vanishes due to the unavailability of states. We present a novel type of subgap response which shows that this intuition does not generalize beyond linear response. Namely, we find that the dc current due to the bulk photovoltaic effect can be finite and mostly temperature independent in an allowed window of subgap transitions. We expect that a moderate range of excitation energies lies between the bulk energy gap and the mobility edge where this effect is observable. Using a simplified relaxation time model for the band broadening, we find the subgap dc current to be temperature independent for noninteracting systems but temperature dependent for strongly interacting systems. Thus, the subgap response may be used to distinguish whether a state is single-particle localized or many-body localized.

中文翻译：

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消失的亚带隙光电流作为寿命效应的探针

对于半导体和绝缘体，通常认为在清洁极限内能平稳地抑制间隙内过渡到非局部状态。也就是说，在零温度下，由于状态的不可用，其贡献消失了。我们提出了一种新型的子间隙响应，它表明这种直觉不会超越线性响应。即，我们发现，由于整体光伏效应而产生的直流电流可以是有限的，并且在允许的子间隙跃迁窗口内，温度与温度无关。我们期望在大的能隙和迁移率边缘之间存在适度范围的激发能，在该处可以观察到这种效应。使用简化的弛豫时间模型进行频带扩展，我们发现，非间隙系统的子间隙直流电流与温度无关，而对于强相互作用的系统，则与温度无关。因此，subgap响应可以用于区分状态是单粒子定位还是多体定位。