Magnetotransport studies of strong correlations and superconductivity in strange and quantum critical metals

Abstract

Ever since the concept of a Fermi gas was invented, detailed knowledge of the electronic ground state has formed the basis for our understanding of metals. In strange and quantum critical metals, understandingthe electronic ground state is difficult as a result of strong interactions between the electrons, interactions that are intricately connected to unconventional superconductivity. It is my hope that the magnetotransport studies presented in this thesis help improve this understanding.

The strongest interactions of all manifest at quantum critical points (QCPs) and in the cuprate strange metal. In many of these compounds a striking, non-saturating, H-linear magnetoresistance (MR) has been observed. Impeded orbital motion is presented as a new, simple phenomenological principle which may account for this phenomenon. In this scenario, the H-linear MR is explained by the k-selectivity of strong correlation effects (independent of their nature), which form regions on the Fermi surface that impede cyclotron motion. I argue that strongly correlated electron systems satisfy Boltzmann transport theory, but escape the relaxation time approximation. Implications are discussed and NbSe2 is proposed as a prototypical candidate for impeded orbital motion in a Fermi liquid regime.

Experiments and modelling have been undertaken since to test impeded orbital motion specifically in NbSe2 and good qualitative and quantitative agreement was found. These results are the first bulk evidence of depletion of spectral weight at the Fermi level (rather than Fermi surface reconstruction) and the origin of density wave order in the short coherence length limit. Further studies are encouraged to see whether this is a general phenomenon in density wave systems with short coherence lengths. A second study into the electronic ground state of a correlated metal are reported, namely measurements of c-axis angle-dependent magnetoresistance (ADMR) and quantum oscillations in FeSe alongside advances in the computational technique of ADMR modelling itself. The data obtained thus far point to a strongly barrel shaped hole pocket at Γ alongside a single electron pocket.

A key area of interest is the connection between strongly correlated ground states and the emergence of superconductivity. New measurements of the superconducting dome as well as theoretical calculations on the electronic ground state are presented for prototypical charge density wave (CDW) compound TiSe2 under hydrostatic pressure. The data show a superconducting dome centred at the charge density wave endpoint at 5 GPa. At 2 GPa, a Lifshitz transition is identified in which electrons and holes arise at the Fermi level well before the end of the CDW order. This pressure coincides with the onset of superconductivity and implies superconductivity is enabled by the Lifshitz transition, similar to BaFe2As2 except via charge rather than spin degrees of freedom. Strong evidence is presented of a new kind of unconventional superconductivity, namely driven by an interband optical phonon mode, which is also responsible for CDW order. If upheld, TiSe2 may represent the first quantum critical superconductor driven by charge degrees of freedom.

Date of Award	9 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nigel E Hussey (Supervisor) & Sven Friedemann (Supervisor)