Electrical Transport Studies In Superconductors At Extreme Pressures

Abstract

In this thesis, electrical transport measurements were performed on superconducting, charge density wave, transition metal dichalcogenide compounds and hydride superconductors under extreme pressures. Firstly, Shubnikov-de Haas quantum oscillations and the masses of electrical carriers were measured in TiSe₂. Hydrostatic pressure was used to suppress the charge density wave, which resulted in several clear frequencies emerging and no indications of quantum critical mass enhancements were observed. 
Secondly, the interaction between the charge density wave and superconductivity in NbSe₂
was investigated. By suppressing the charge density wave with hydrostatic pressure, it was shown that there is no quantum critical enhancement in superconductivity around the critical point because the transition becomes 1st-order. Comprehensive magnetotransport measurements analysed within a four-band model suggest the presence of a pseudogap state coexisting with and outside of the charge density wave, which is further supported by Shubnikov-de Haas oscillations observing Fermi arcs generated by the pseudogap. This pseudogap state is also suppressed with pressure alongside the charge density wave and both states are destroyed at 4.4GPa.
Measurements of the high temperature superconductors sulfur, yttrium, and lanthanum
hydrides are presented alongside measurements on their precursor elements. Sulfur trihydride was synthesised from ammonia borane and observed to have similar superconducting properties to previous work, whilst Raman measurements on elemental sulfur observed a charge density wave amplitudon coexisting with superconductivity. Novel thin-film deposition techniques were used to synthesise lanthanum and yttrium hydrides, which promise to aid future syntheses of predicted room-temperature superconducting ternary hydrides. Superconductivity in yttrium thin-films was measured to 164GPa and agree with predictions for an Fddd crystal structure, which is believed to be universal for rare-earth elements. Finally, a low symmetry lanthanum superhydride was synthesised with a drastically lower superconducting temperature than the high-symmetry cubic phase observed previously.

Date of Award	11 May 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Sven Friedemann (Supervisor) & Antony Carrington (Supervisor)