Novel Photogalvanic Effects in Weyl Semimetals

Photovoltaic effect is attracting growing interest from the viewpoint of the geometrical quantal phase of Bloch wavefunctions, i.e., Berry phase. The interband transitions between bands causes the shift of the electrons due to the difference in the intra-cell coordinates encoded as the Berry phase connection, resulting in the shift current without the external bias voltage. On the other hand, it has been known that the Weyl fermion is the source or sink of the Berry curvature, i.e., monopole or anti-monopole in momentum space, and leads to the enhanced Berry phase phenomena. Therefore, it is natural to expect the novel photovoltaic effect there. In this talk, I will describe the physical mechanism of shift current, and its features such as I-V characteristics, dependence on the mean free time, and noise, and discuss its realizations in Weyl semimetals such as the surface state of topological insulator, and TaAs. This talk is based on the works with collaborators T. Morimoto, K.W. Kim, H.Ishizuka, Y.Zhang, J.v. d.Brink, C. Felser, B. Yan, N. Ogawa, T. Hayata, M. Ueda, M. Sotome, M. Nakamura, J. Fujioka, M. Ogino, Y. Kaneko, M. Kawasaki, and Y.Tokura.