Novel spin, charge, and lattice dynamics in pyrochlore iridates : Raman and X-ray diffraction studies at low temperatures and high pressure

Abstract

This thesis investigates phonon dynamics, electronic, and spin excitations along with structural instabilities and phase transitions in rare earth pyrochlore iridates, A2Ir2O7 (A = Pr, Gd, Dy, and Er) and Bi-doped Sm2Ir2O7 (Sm1−xBix)2Ir2O7 using Raman scattering and synchrotron-based X-ray diffraction studies under extreme conditions such as low-temperature (down to 4 K) and high-pressure (up to 25 GPa). The temperature dependence Raman studies on (Sm1-xBi)2Ir2O7 show strong spin-phonon coupling (SPC) for x ≤ 0.035. Additionally, for x = 0 and 0.02, phonon dynamics suggests a possible transition to a Weyl semimetal state. Higher Bi concentrations (x = 0.05 and 0.10) show clear signatures of a Quadratic Band Touching (QBT) inferred from electronic Raman scattering (ERS) associated background, suggesting non-Fermi liquid (NFL) behavior. The temperature-dependent studies on Pr2Ir2O7 also show ERS, inferring NFL state as the characteristic of the "QBT- Luttinger semimetal" behavior below a critical temperature TQ. We observe strong electron-phonon coupling (EPC) within the QBT phase in terms of Fano asymmetry of an Ir-O-Ir vibration and anomalous behaviors of phonon frequencies and linewidths. Through analysis of magnon energies in A2Ir2O7, we have estimated the isotropic exchange (J) and Dzyaloshinskii-Moriya interaction (DMI) parameter (D), resulting in D/J ratios of ∼0.15 for Gd2Ir2O7 and Dy2Ir2O7. The Raman data for Gd, Dy, and Er iridates also confirms the presence of strong SPC within the AIAO-ordered state by the phonon modulation of the DM spin-exchange interaction.

High-pressure X-ray diffraction and Raman studies are carried out to unveil the structural evolution of the doped and undoped iridates. We observe pressure-induced iso-structural phase transitions associated with the rearrangement of IrO6 octahedra within the pyrochlore lattice at ambient temperature. The transition pressure Pc at ∼11.2 GPa in Sm2Ir2O7 decreases to ∼10.2 and 9 GPa for samples with Bi-substitutions of x = 0.02 and 0.10, respectively. The critical pressure (Pc) correlates inversely with the A-site cation radius; Pc decreases from ∼10.8 GPa (Er, smallest cation) to ∼7.5 GPa (Pr, largest cation) in A2Ir2O7. The anomalous decrease in the linewidth of three phonons related to Ir-O-Ir (A1g and Eg) and Ir-O (T2g4) vibrations across all samples up to the critical pressure (Pc) has been attributed to reduced electron-phonon interaction due to enhanced electronic bandwidth under pressure.