Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals

摘要

Although Weyl fermions have proven elusive in high-energy physics, theirexistence as emergent quasiparticles has been predicted in certain crystallinesolids in which either inversion or time-reversal symmetry isbroken\cite{WanPRB2011,BurkovPRL2011, WengPRX2015,HuangNatComm2015}. Recentlythey have been observed in transition metal monopnictides (TMMPs) such as TaAs,a class of noncentrosymmetric materials that heretofore received only limitedattention \cite{XuScience2015, LvPRX2015, YangNatPhys2015}. The question thatarises now is whether these materials will exhibit novel, enhanced, ortechnologically applicable electronic properties. The TMMPs are polar metals, arare subset of inversion-breaking crystals that would allow spontaneouspolarization, were it not screened by conduction electrons\cite{anderson1965symmetry,shi2013ferroelectric,kim2016polar}. Despite theabsence of spontaneous polarization, polar metals can exhibit other signaturesof inversion-symmetry breaking, most notably second-order nonlinear opticalpolarizability, $\chi^{(2)}$, leading to phenomena such as opticalrectification and second-harmonic generation (SHG). Here we report measurementsof SHG that reveal a giant, anisotropic $\chi^{(2)}$ in the TMMPs TaAs, TaP,and NbAs. With the fundamental and second harmonic fields oriented parallel tothe polar axis, the value of $\chi^{(2)}$ is larger by almost one order ofmagnitude than its value in the archetypal electro-optic materials GaAs\cite{bergfeld2003second} and ZnTe \cite{wagner1998dispersion}, and in factlarger than reported in any crystal to date.