Large bulk photovoltaic effect and spontaneous polarization of   single-layer monochalcogenides
  We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current ($\sim$100 $\mu$A/V$^2$), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective three-dimensional electric polarization ($\sim$1.9 C/m$^2$). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials 
Tonatiuh Rangel and Benjamin M. Fregoso and Bernardo S. Mendoza and   Takahiro Morimoto and Joel E. Moore and Jeffrey B. Neaton
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.str-el