Electrical Transport Studies of Superconductors at Extreme Conditions

  • Sam Cross

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Advances in diamond anvil cell capabilities have pushed experimentally attainable pressures far into the megabar regime. In combination with low temperatures and high–magnetic fields, high–pressure experiments greatly expand the potential phase space for exploration of matter at the extremes of
experimental conditions. This thesis presents electrical transport studies of the transition metal dichalcogenide 2H−NbSe2 as well as lanthanum and lutetium hydrides, under the extreme conditions of high–pressure, low–temperature and high–magnetic fields.

To begin, a high-pressure magnetotransport study of NbSe2 is presented, a prototypical system for the coexistence of charge-density wave (CDW) order and superconductivity. By suppressing the CDW order with hydrostatic pressure, measurements up to 35 T reveal close links between the anomalous linear magnetoresistance and strong scattering associated with the CDW. Above the CDW critical pressure, PCDW = 4.4 GPa, the magnetoresistance tends toward saturation at 5.8 GPa. However, non-saturating linear magnetoresistance re–emerges at 9.4 GPa that follows the empirical quadrature form, for which various possible mechanisms are discussed. Additionally, the observation of Shubnikov-de Haas (SdH) oscillations in NbSe2 is presented. At high–pressure in the absence of CDW order, high frequencies corresponding to a large Fermi surface of NbSe2 are observed for the first time, with DFT calculations providing some insight into their origin. Low frequency oscillations within the CDW phase are also observed, with the frequency and quasiparticle effective mass tracked on approach to the CDW end point. One of the first ambient pressure SdH studies of the small selenium–derived pancake Fermi surface is also presented.

The remaining experimental chapters focus on superconductivity in hydrides. Firstly, the synthesis and characterisation of a novel high-Tc lanthanum hydride film is presented. High-pressure synchrotron x-ray diffraction reveals the cubic A15 lanthanum sublattice indicative of a type-I clathrate hydride, a novel structure-type that has very recently emerged in multiple hydride systems. The stoichiometry is estimated to be close to La4H23, whereby the hydrogen atoms adopt a Weaire–Phelan arrangement comprised of two distinct hydrogen cage-like networks surrounding the lanthanum atoms. Resistance measurements confirm superconductivity at 95 GPa with Tc ∼ 90 K, with the suppression of Tc observed in magnetic field. The results highlight the stabilisation of a high-symmetry phase at much reduced pressures relative to the current record–Tc hydride LaH10, with La4H23 so–far exhibiting the highest Tc amongst its structure–type.

Finally, the synthesis and characterisation of lutetium hydride films is presented. Motivated by the recent report of near-ambient superconductivity in nitrogen-doped lutetium hydride, a novel synthesis technique is detailed, with successful stabilisation of both trigonal and cubic lutetium hydride films at pressures close to 2 GPa. In synthesising the cubic phase, nitrogen impurities were introduced
in an attempt to reproduce the near-ambient superconductivity. Both phases were found to be insulating. The work joins the growing experimental effort that refutes such claims of near-ambient superconductivity, though reveal a possible novel electronic phase of cubic lutetium hydride. Further, a high–pressure study demonstrates the stabilisation of a cubic lutetium hydride phase at 140 GPa,
attributed to a stoichiometry close to LuH3. Superconductivity is evidenced in resistance measurements with Tc ∼ 8 K, lower than that of elemental lutetium at the same pressure in contradiction with previous work. This work further demonstrates the inherent difficulty to stabilise high-Tc clathrate
lutetium hydrides relative to other members of the rare-earth hydride family.
Date of Award1 Oct 2024
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorSven Friedemann (Supervisor) & Antony Carrington (Supervisor)

Cite this

'