Coexistence of superconductivity and other electronic correlations in U2Ti and UTe2

Abstract

The coexistence of different electronic orders in condensed matter systems is an actively explored area of experimental and theoretical research. This is both to advance understanding and, since different phases, like superconductivity or magnetism, can be utilised on the microscopic and macroscopic scale, for device applications. Exploration of the complex phase diagrams which emerge in many actinide materials requires careful synthesis of samples, as the microstructure and lattice defects of samples can affect thermodynamic and transport properties.

Characterisation with microstructure probes and crystallographic diffraction is important for an informed interpretation of the resulting material properties. This thesis investigates the synthesis and characterisation of two uranium binary compounds, U2Ti and UTe2, with coexisting electronic correlations, paying particular attention to the effect of synthesis conditions on the microstructure, inclusion of other phases, and lattice disorder.

A computational study of U2Ti predicted it to undergo a transition to charge density wave (CDW) order on the basis of density functional theory calculations of the phonon spectrum. A previously reported experimental investigation of the material identified it as a bulk superconductor (SC) at 0.38 K, but did not identify a CDW transition. Coexistence of SC and CDW order is especially interesting as both compete for states at the Fermi level and gap the Fermi surface, with the former leading to a zero resistance state and the latter giving rise to an insulating state (or decrease in conductivity) in many systems. The motivation for studying U2Ti was thus to investigate the predicted CDW order and the possible coexistence with superconductivity. Synthesis of U2Ti single crystals is challenging due to a disordered to ordered solid transition. Therefore, we explored the synthesis conditions and their impact on the properties of samples produced. Through a combination of thermodynamic, electrical transport and xray diffraction (XRD) measurements (conducted at the ESRF, Grenoble, France) we confirm the emergence of incommensurate CDW order at 71K followed by a commensuratation transition at 46 K. The measurements also suggest that at ambient pressure, U2Ti is not a bulk SC and previous identification of SC order in the system was due to inclusions. Energy dispersive x-ray (EDX) analysis is used to identify the inclusion of an α-U phase and, in combination with XRD measurements, we are able to identify the alignment of this inclusion with the bulk U2Ti lattice.

UTe2 is an unconventional SC actively being studied to determine its order parameter and pairing mechanism. Due to the persistence of SC order in magnetic fields exceeding 40 T, the material is considered a candidate for spin triplet pairing, mediated by ferromagnetic fluctuations evidenced by the divergence of the dc magnetic susceptibility at low temperature. However, under pressures exceeding 1.7GPa antiferromagnetic order emerges upon the destruction of SC order. In order to make conclusive statements about the SC order pure samples of UTe2 are required, and the variation of thermodynamic and transport properties reported in the literature needed to be related to the growth conditions of samples.

Therefore, the investigation presented in this thesis focused on departing from the standard growth conditions used widely in other studies to investigate the impact on sample properties. It is shown that varying the stoichiometry of the reactants in the chemical vapour transport growth of the compound can produce samples of higher superconducting transition temperature and lower residual heat capacity.

This also impacts the magnetic susceptibility below 15K suggesting some impact on magnetic fluctuations. From magnetic susceptibility measurements on samples from different syntheses we are able to identify ferromagnetic impurities while, EDX measurements suggest how these impurities are distributed. We discuss the difficulty in identifying the differences in microstructural and crystallographic properties of samples with different SC transition temperatures, which remains an outstanding issue in the literature.

The investigation of U2Ti has identified a new uranium based CDW compound and revealed that SC order previously identified is likely associated with inclusions and/or filamentary SC. The results motivate the further synthesis of bulk single crystal samples to explore the phase diagram of this material. The work on UTe2 has moved forward the understanding of the synthesis conditions on the superconducting and magnetic properties, as well as identifying inclusions. The results suggested that an early explanation of the superconducting order of UTe2 which involved pairing of only one spin channel of electrons, with a remaining unpaired electron liquid contributing to a residual Sommerfeld coefficient of heat capacity, is unlikely. The precise form of disorder in UTe2 that drives variations in the superconducting and magnetic properties remains an outstanding issue.

Though it is determined, by XRD, that non-SC samples are compositionally distinct from SC samples due to the presence of uranium vacancies in the crystal structure. More broadly, the combination of these investigations contributes to the exploration of the coexistence of superconductivity and other electronic correlations (and order) in uranium containing compounds and motivates new areas of exploration.