Superconducting Gap Measurements at Extreme Conditions

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Despite the additional experimental difficulty required for high pressure measurements, the ability to tune materials with applied pressure opens up a remarkable phase space of novel structures, stoichiometries and exploration through the rich phase diagrams of condensed matter systems. Exploring the interplay of these ordered phases is vital in understanding the underlying superconductivity present in many of these such materials. For the field of high temperature superconductivity, the hydride family of superconductors have the highest experimentally verified critical temperatures, but are only currently stable and accessible with experiments in the megabar pressure range. As the current best candidates for room-temperature superconductivity, research is vital in characterising these hydride compounds at extreme conditions. This thesis covers a variety of compounds and experimental techniques, but grouped under the topic of superconductors under pressure. In particular, these measurements focus on characterising the pairing gap, a key parameter in understanding the nature of superconductivity in all superconductors.

Firstly, hydrostatic pressure up to 8 GPa has been used to suppress the charge-density wave phase in the transition metal dichalchogenide NbSe2, where measurements of the magnetic susceptibility track the superconducting critical temperature with applied pressure. Here, the onset temperature of the charge density wave (CDW) and superconducting phases (SC) show a clear anticorrelation, revealing a competition for electronic density of states between these two phases. Magnetisation measurements also show a sharp increase in the lower critical field with applied pressure that cannot be solely explained by a SC-CDW competition, but instead indicates unconventional superconducting behaviour in NbSe2. Fits to the temperature-dependent superfluid density are used to measure a superconducting gap magnitude as a function of pressure.

Secondly, measurements of two elemental superconductors are presented under extreme conditions. Raman measurements on elemental sulphur observe a clear asymmetric excitation between 80 and 150 GPa that can be attributed to the CDW amplitude mode. This mode is observed to undergo a weakly first-order phase transition at the SIV-SV structural phase boundary, with additional phonon modes visible below 150 GPa used to map the series of structural transitions that occur above ambient pressure. Electrical measurements on the same sample reveal a sharp increase in the superconducting critical temperature Tc at high pressure that, contrary to previous interpretations, is shown to not be related to a structural transition. Instead, fits to the temperature-dependent electrical resistivity provide evidence for a pressure-induced electronic transition from a single to multiband superconductor. Likewise, tunneling spectroscopy measurements on elemental lanthanum are used to probe the superconducting gap structure at 90 GPa, where nonlinear IV features indicate that Fmmm La is a strong-coupled multigap superconductor.

Lanthanum hydride samples with critical temperatures of 70 - 80K have been synthesised from evaporated lanthanum films and ammonia borane as a hydrogen donor. These samples shed light on high-Tc metastable phases in this system, and measurements of the upper critical field Hc2 are presented on stable phases down to low temperature. An unusual Hc2(T) is observed, which can be well described by an extremely strong electron-phonon coupling. X-ray diffraction measurements on a similar Tc synthesised lanthanum hydride sample are analysed, which shows that the formation of stable low symmetry LaH10 structures are responsible for the observed critical temperatures. Preliminary tunneling spectroscopy measurements on LaHx are also introduced, observing clear signatures of superconductivity with evidence for multiple superconducting gaps.

Finally, the synthesis of H3S is shown via elemental sulphur and the hydrogen donor ammonia borane, where a sharp superconducting transition at 197 K is observed at 155 GPa. Low temperature Raman spectroscopy measurements observe a novel excitation at 80 - 90 meV that is associated with the quasiparticle peak at the superconducting pairing gap energy. Comprehensive spatial and temperature dependencies of this excitation link this peak to the synthesised superconducting H3S. In sharp contrast to conventional BCS superconductivity, the temperature dependence of this excitation shows evidence for preformed electron pairs above Tc, with H3S in the BCS-BEC crossover regime.
Date of Award22 Mar 2022
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorSven Friedemann (Supervisor) & Antony Carrington (Supervisor)

Keywords

  • Superconductivity
  • High Pressure
  • Charge order

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