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Title: Experimental studies of electronic correlations in quantum materials
Author: Davies, Nathaniel
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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In this thesis I present experimental studies of three quantum materials, resolving an important unanswered question about the novel microscopic physics in each case. I first present inelastic neutron scattering measurements on a powder sample of the unconventional, high-temperature superconductor lithium iron selenide hydroxide, Li1-xFexODFe1-ySe (x = 0.16; y = 0.02, Tc = 41 K). The spectrum shows an enhanced intensity below Tc at energy transfers below the superconducting pair breaking energy, with broad maxima at two different wavevectors. The behaviour of this feature is consistent with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular intercalated iron selenide, Li0.6(ND2)0.2(ND3)0.8Fe2Se2. The signal can be described with a characteristic two-dimensional wave vector which is consistent with the nesting vector between electron Fermi sheets. These results place a strong constraint on the pairing symmetry in this unconventional superconductor and may have relevance to the high temperature superconductor monolayer FeSe due to structural and electronic similarities between the two systems. In Na2Ti2Pn2O (Pn = As, Sb), two important members of the titanium oxypnictide family of superconductors, I then present single crystal x-ray diffraction data which reveal a charge superstructure that appears below density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements I am able to completely determine the symmetry and all atomic positions in the distorted phase. I also analyse angle resolved photoemission spectroscopy (ARPES) data, which show band folding and back bending consistent with a density wave with the same symmetry as the lattice distortion as well as evidence for Fermi surface nesting which may help drive the density wave transition. The results provide direct evidence for phonon-assisted charge density wave order in Na2Ti2Pn2O. This represents the first complete characterisation of the density wave phase in a titanium oxypnictide, information which is crucial in microscopic modelling of the superconductivity in this family. Finally I present a detailed study of the pyrochlore osmate Y2Os2O7 via a wide variety of experimental techniques. I observe a small, non-zero paramagnetic moment and spin freezing at temperature Tf = 5 K, consistent with previous results, and show based on high-field magnetisation measurements that the paramagnetic moment is very likely to be due to large effective moments located on a small fraction ~ 2% of Os sites. Comparison of single-ion energy level calculations with resonant inelastic x-ray scattering (RIXS) data yields a non-magnetic Jeff = 0 ground state on the majority Os4+ sites with the spin-orbit interaction, Hund's coupling and trigonal distortion of OsO6 octahedra all important in modelling the experimentally observed spectra. These results fully explain the unexpected magnetism in Y2Os2O7, and the single-ion spectrum may prove useful to inform the search for novel excitonic magnetism in other, related 5d materials.
Supervisor: Boothroyd, Andrew Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available