Photogalvanic effect in Weyl semimetals from first principles

Abstract

Using first-principles calculations, we investigate the photogalvanic effect in the Weyl semimetal material TaAs. We find colossal photocurrents caused by the Weyl points in the band structure in a wide range of laser frequency. Our calculations reveal that the photocurrent is predominantly contributed by the three-band transition from the occupied Weyl band to the empty Weyl band via an intermediate band away from the Weyl cone, for excitations both by linearly and circularly polarized lights. Therefore, it is essential to sum over all three-band transitions by considering a full set of Bloch bands (both Weyl bands and trivial bands) in the first-principles band structure while it does not suffice to only consider the two-band direct transition within a Weyl cone. The calculated photoconductivities are well consistent with recent experiment measurements. Our work provides the first first-principles calculation on nonlinear optical phenomena of Weyl semimetals and serves as a deep understanding of the photogalvanic effects in complexed materials. Introduction. Weyl fermions correspond to the massless solutions of Dirac equation [1]and have been observed in solids as quasiparticles recently [2][3][4][5][6]. Related materials are called Weyl semimetals (WSM) [7][8][9][10][11][12][13]. A WSM gives rise to linearly band-crossing points called Weyl points (WPs) in the momentum space. WPs are monopoles of the Berry curvature [14,15] with finite chirality and are related to the chiral anomaly in the context of high-energy physics [16][17][18][19] and unique surface Fermi arcs [2].

Chiral terahertz wave emission from the Weyl semimetal TaAs

“…This is because the excitation from the Weyl band to the high-lying band is accompanied by a large and rapid change in the effective velocity of the charged quasiparticles. The microscopic mechanism is quite different from previous findings in the WSMs upon light excitations with small photon energies [22,23,27], where the Weyl band tilting plays the pivotal role.…”

Ab initio calculation of the shift photocurrent by Wannier interpolation

“…This phenomenom was intensively studied in the 60s and 70s, particularly in ferroelectric oxides such as BaTiO 3 [4]. In recent years it has attracted renewed interest in view of potential applications in novel solar-cell designs [5][6][7], and in connection with topological insulators [8][9][10] and Weyl semimetals [11][12][13].…”

Nonlinear Absorption and Photocurrent in Weyl Semimetals

“…The specific feature of Weyl semimetals is the circular photo‐galvanic effect (CPGE)—generation of the photocurrent which inverses its direction at inversion of the helicity of the absorbed light. As the universal CPGE current is determined by the Weyl cone chirality, those two values have topological nature and define Weyl cone topological properties . The specifics is that the CPGE current density has a “quantized” generation rate which, except for light intensity, is given by a combination of fundamental constants independent of both details of the real band structure of the system and the light frequency .…”