Time reversal and lattice symmetry breaking in Nd₂Ir₂O₇ observed by Raman scattering spectroscopy

Many intriguing phenomena such as anomalous Hall effect, metal-insulator transition, and quadratic band touching, has been discovered in pyrochlore iridate family thanks to the interplay of spin orbit coupling, electron correlations, and magnetic frustration. Among them, Nd₂Ir₂O₇ was suggested to be a promising candidate to realize Weyl semimetal state upon breaking of the time-reversal symmetry (TRS) due to all-in-all-out order of Ir moments. In this work, we present a comprehensive low-temperature Raman scattering study of Nd₂Ir₂O₇ single crystals. Our polarized Raman spectra reveals an abruptly opening of a partial gap of 24 meV in Raman susceptibility below T_N = 33 K, possibly due to the splitting of the quadratic band touching. TRS breaking was evidenced by the appearance of the one magnon excitations at 15 meV below T_N. This magnon excitation hardens and broadens upon further cooling down to 10 K, when the Nd moments show strong magnetic correlations. The occurrence of a new magnetic peak at 32 meV below 7 K indicates the accomplishment of the Nd long-range magnetic order, while subtle changes in phonons point on a lattice distortion. These results suggest a proximity of Nd₂Ir₂O₇ to the Weyl semimetal state between 33 and 10 K.