Description
- Abstract:
- The combined effect of electronic correlation and strong spin-orbit-coupling(SOC) can give rise to a variety of exotic quantum phases. Double perovskites provide a simple structure to study the spin-orbit-lattice entangled states. In this thesis, focusing on the 5d osmate double perovskite system, we conduct a combination of work including theoretical model simulation, first-principle calculation, and nuclear magnetic resonance experiments to understand the fundamental physical properties of this material system. For the 5d Mott insulator Ba2NaOsO6, by conducting spin-spin relaxation measurements and applying quadrupolar noise spectroscopy, we addressed a long-standing missing entropy problem. We found that quadrupolar noise with a Lorentzian distribution persists up to a high temperature above its structural transition, indicating quadrupolar domains which account for the missing entropy in the system. Also by carrying out a classical Monte Carlo simulation using 4 sites per unit cell for a 5d1 double perovskite model with strong SOC, we found that the non-zero quadrupolar moment Q(3z^2-r^2) arises due to additional symmetry breaking that was not captured in earlier mean-field treatment with 2 sites per unit cell, showing consistency with the two non-zero quadrupolar moments proposed in the quadrupolar phase of Ba2MgReO6. Furthermore, we improved the earlier point charge approximation calculation on the electric field gradient of Ba2NaOsO6 and we identified the local orthorhombic structural distortion for Na-O octahedra in Ba2NaOsO6 of around 0.01 angstrom. Our first principle calculation on Ba2NaOsO6 also found the existence of a staggered orbital ordering pattern accompanying its canted ferromagnetic order, which is characterized by the different selective occupation of d orbitals on the two-sublattice Os ions. Besides Ba2NaOsO6, we have also conducted comprehensive NMR measurements on its isostructural, isovalent compound Ba2LiOsO6. We found that the metamagnetic transition at 5.75T is possibly a spin-flop transition and the ground magnetic state is more likely to be a 3D antiferromagnet. Electron doping effect on 5d1 Ba2NaOsO6 has also been studied on powder compounds Ba2NaxCa(1-x)OsO6 (0 < x < 1). We found that all the doped samples remain as magnetic insulators despite the added electrons. Powder spectrum simulation indicates that similar to the Ba2NaOsO6 case, a "broken local point symmetry" phase with orthorhombic symmetry occurs above magnetic transition for all doped samples. Under the collinear two sublattices canted AFM model, the ground magnetic states evolve from canted FM state to collinear AFM state with the increase of doping electrons. Whether there is multipolar ordering in these samples needs further study using complementary techniques in the future.
- Notes:
- Thesis (Ph. D.)--Brown University, 2022
Citation
Cong, Rong,
"Magnetic and Structural Properties of 5d Osmate Double Perovskites Probed by Nuclear Magnetic Resonance"
(2022).
Physics Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.26300/7tx8-gq83
