Magnetic Dipole Transitions in the Second Harmonic Generation Response of the LnAlSi (Ln = La, Ce, Pr, Nd) Series of Weyl Semimetals

Optical transitions are usually considered in the electric dipole (ED) approximation since magnetic dipole (MD) and higher order multipole transitions are typically orders of magnitude weaker. Although some compounds can host MD transitions whose strength rivals that of the ED, teasing them apart can be difficult at linear order because the form of the linear suscepibility tensors for the two responses χ_ij is typically the same. In contrast, separating ED and MD responses is simplified in second harmonic generation (SHG) because the higher rank nature of the second order optical susceptibility tensor χ_ijk can result in quite different placement of their respective nonzero components, allowing each response to be selectively probed. To date, SHG MD emission has only been observed in the presence of magnetic order. In this talk, we present evidence of strong MD transitions that occur in the non-magnetically ordered states of the LnAlSi (Ln = La, Ce, Pr, Nd) series of Weyl semimetals and we discuss this observation in the context of the physics of these materials.