Bulk Photovoltaic Effect
Bulk Photovoltaic Effect (BPVE) in noncentrosymmetric materials is completely different from "pn junction" solar cell mechanism. Shift current is an interesting model to describe BPVE. This is a second order process with electron wavefunctioins excited twice by the light. Based on perturbation theory, it is possible to use firstprinciple method to calculate the transition intensity and shift vector in order to obtain shortcircuit current. 
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Shift current theory displays quite good agreement with the experiments in BaTiO_{3}. It successfully predicted magnitude, sign and whole spectral profile of BaTiO_{3}, including the notable dependence of current direction on the polarization. 

BiFeO_{3} is a widely studied material with both ferromagnetic and ferroelectric properties in room temperature. We can use firstprinciple method to calculate all the tensor elements of its shift current response. 
Pure spin current means that "up" spin and "down" spin move in opposite directions with same magnitude and the total charge current will be zero. Bulk Photovoltaic pure spin current in antiferromagnetics originates from mirror symmetry of two sublattices with different spins. Without strongly spinorbital coupling or circular light, it can generate pure spin current. 
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