Role of spin-phonon and electron-phonon interactions in the phonon renormalization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:msub><mml:mi>Eu</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Bi</mml:mi><mml:mi>x</mml:mi></mml:msub></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Ir</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>7</mml:mn></mml:msub></mml:mrow></mml:math> across the metal-insulator phase transition: Temperature-dependent Raman and x-ray studies
Anoop Thomas, Prachi Telang, Kshiti Mishra, Martin Cesnek, Jozef Bednarčík, D. V. S. Muthu, Surjeet Singh, A. K. Sood
Abstract
We report temperature-dependent Raman scattering and x-ray diffraction studies of pyrochlore iridates ${({\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x})}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, 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 (${T}_{\mathrm{N}}$). These systems also show a Weyl semimetal (WSM) phase at low temperatures ($<\ensuremath{\sim}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). The Ir-O-Ir bond bending mode ${A}_{1g}$ (510 $\mathrm{c}{\mathrm{m}}^{\text{\ensuremath{-}}1}$) 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 ${T}_{2g}^{1}$ (307 $\mathrm{c}{\mathrm{m}}^{\text{\ensuremath{-}}1}$) and ${T}_{2g}^{2}$ (382 $\mathrm{c}{\mathrm{m}}^{\text{\ensuremath{-}}1}$) 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 ${T}_{\mathrm{N}}$ due to strong electron-phonon coupling. The signatures of the WSM state are observed clearly in phonon renormalization $<50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ (in $x=0.02$) due to strong electron-phonon interaction. Our experimental results establish strong magneto-elastic coupling below ${T}_{\mathrm{N}}$ and significant electron-phonon interactions in the metallic phase above ${T}_{\mathrm{N}}$ as well as in the low-temperature WSM state.