Role of spin-phonon and electron-phonon interactions in the phonon renormalization of (Eu1−xBix)2Ir2O7 across the metal-insulator phase transition: Temperature-dependent Raman and x-ray studies

Abstract

We report temperature-dependent Raman scattering and x-ray diffraction studies of pyrochlore iridates ( Eu 1 − x Bi x)2Ir2O7, for x=0, 0.02, 0.035, 0.05, and 0.1. The temperature variation in Raman experiments spans from 4 to 300 K, covering the metal-insulator phase transition accompanied by paramagnetic–to–all-in/all-out (AIAO) spin ordering (TN). These systems also show a Weyl semimetal (WSM) phase at low temperatures (<∼50K). The Ir-O-Ir bond bending mode A1g (510 cm−1) shows anomalous softening (for x=0.0, 0.02, 0.035, and 0.05) in the magnetically ordered AIAO state, arising primarily from the spin-phonon interaction due to the phonon modulation of the Dzyaloshinskii-Moriya spin-exchange interaction. The two stretching modes T 12g (307 cm−1) and T 22g (382 cm−1) harden significantly in the magnetic insulating phase. The T 2g phonons (for x=0.0, 0.02, 0.035, and 0.05) also show anomalous temperature dependence of their mode frequencies above TN due to strong electron-phonon coupling. The signatures of the WSM state are observed clearly in phonon renormalization <50 K(in x=0.02) due to strong electron-phonon interaction. Our experimental results establish strong magneto-elastic coupling below TN and significant electron-phonon interactions in the metallic phase above TN as well as in the low-temperature WSM state.