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Type of Resource (primo): dissertations
Name: Personal
Name: Personal
Name: Personal
Name: Personal
Name: Corporate
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Note: thesis: Thesis (Ph. D.)--Brown University, 2023
Genre (aat): theses
Abstract: Spin, orbit, and lattice dynamics are coupled in strong spin-orbit coupling systems. The Lacunar spinel, GaTa$_{4}$Se$_{8}$, is a unique Mott insulator that hosts molecular spin-orbit J$_{eff}$-3/2 states as correlation units. However, little is known about the spin-orbital ground state and dynamics in this system. In this thesis, I discuss a series of comprehensive x-ray and neutron total scattering and inelastic neutron scattering measurements that uncover a fluctuating spin-orbital dimer phase in this cluster Mott insulator. GaTa$_{4}$Se$_{8}$ has concurrent structural and magnetic transitions at 50 K, forming a spin-orbital singlet valence bond solid ground state. As a result of strong spin-orbit coupling and orbital degeneracy, the magnetic ground state couples with lattice distortion. Our total scattering measurements show that the lattice is locally distorted up to 300 K, indicating that the spin-orbital singlet transition is an order-disorder type. Our inelastic neutron measurements directly reveal the lattice fluctuations preceding the transition and provide direct evidence of spin-orbital fluctuation. To investigate the interplay between spin-orbit coupling, the Jahn-Teller effect, and exchange coupling. We carried out a series of inelastic neutron scattering measurements on lacunar spinels GaM$_{4}$X$_{8}$ (M=V, Nb, Ta; X=S, Se). Our results reveal the phonon anomaly in stronger spin-orbit coupling compounds (M=Nb, Ta), while this feature is missing in the vanadates with dominating Jahn-Teller effect. Combining our pair distribution function, synchrotron crystallography, and density functional theory, we demonstrate the spin-orbital ordering and fluctuations in lacunar spinels. Under high pressure, GaTa$_{4}$Se$_{8}$ undergoes insulator-metal and metal-superconducting phase transitions at 5 GPa and 10 GPa, respectively. The persistence of the J$_{eff}$-3/2 ground state in both phases suggests that the superconducting phase could be unconventional and topological. To gain a better understanding of the phase transitions, we performed high-pressure x-ray diffraction to identify the structural phase transition that takes place in the superconducting phase.
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