Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is broken1,2,3,4. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention5,6,7. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons8,9,10. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, χ(2), leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic χ(2) in the TMMPs TaAs, TaP and NbAs. With the fundamental and second-harmonic fields oriented parallel to the polar axis, the value of χ(2) is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs11 and ZnTe12, and in fact larger than reported in any crystal to date.
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We thank B. M. Fregoso, T. R. Gordillo, J. Neaton and Y. R. Shen for helpful discussions and B. Xu for sharing refractive index data of TaAs. Measurements and modelling were performed at the Lawrence Berkeley National Laboratory in the Quantum Materials program supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under Contract No. DE-AC02-05CH11231. J.O., L.W. and A.L. received support for performing and analysing optical measurements from the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4537 to J.O. at UC Berkeley. Sample growth was supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative Grant GBMF4374 to J.A. at UC Berkeley. T.M. is supported by the Gordon and Betty Moore Foundation’s EPiQS Initiative Theory Center Grant GBMF4307 to UC Berkeley. J.E.M. received support for travel from the Simons Foundation. The authors would like to thank Nobumichi Tamura for his help in performing crystal diffraction and orientation on beamline 12.3.2 at the Advanced Light Source. N. Tamura and the ALS are supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231. J. A. and N. N. acknowledge support by the Office of Naval Research under the Electrical Sensors and Network Research Division, Award No. N00014-15-1-2674.
About this article
Nature Photonics (2018)