Raman signatures of quadratic band touching state and strong spin-phonon coupling in pyrochlore iridates (Sm1−xBix)2

In this work, we report low-temperature Raman spectroscopic measurement to investigate the temperature dependence of phonons and magnetic as well as electronic excitations in ${({\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x})}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ for $x=0$, 0.02, 0.035, 0.05, and 0.10. A strong spin-phonon coupling induced phonon renormalization is observed below magnetic transition temperature ${T}_{N}$ for samples with $x\ensuremath{\le}0.035$. An underlying broad continuum attributed to pronounced spin-spin correlations is observed below ${T}_{N}$ for pure ${\mathrm{Sm}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. Significant phonon anomalies at lower temperatures, much below ${T}_{N}$, for $x=0$ and 0.02, are observed and are attributed to a possible crossover into the Weyl semimetal state. Most interestingly, we observe clear signatures of quadratic band touching in $x=0.05$ and 0.10 systems in terms of a broad electronic continuum at low temperatures analyzed using the Raman susceptibility for a non-Fermi liquid, corroborating the inference from recent transport measurements.