Exploring the unusual phase diagram of elemental uranium

: structural and electronic characterisation of thin film U metal

Abstract

The bulk phase diagram of uranium metal contains three allotropes - orthorhombic α-U, tetragonal β-U and cubic γ-U. It is well-known that the orthorhombic ground state structure hosts a complex series of three-dimensional charge density waves (α₁, α₂, α₃) below 43 K as well as an ambient pressure superconducting state whose onset temperature varies with sample quality (T_c ≈ 0 − 2 K). Diffraction studies have shown that epitaxial strain engineering can be used to manipulate the CDW in thin films of α-U, but there are still no published low temperature electronic transport or band structure measurements of these systems. It has also been shown that a fourth allotrope of uranium can be stabilised only as a thin film, though little is known about the elusive hexagonal close-packed structure and its link to the three bulk phases. To further explore the unusual structural and electronic properties of this fascinating actinide, careful selection and optimisation of substrates, buffer layers and deposition conditions has been performed here in order to stabilise of uranium thin films in various structures and orientations via d.c. magnetron sputtering.

Four new epitaxial systems of α-U have now been synthesised in addition to the two previously known configurations. The crystallographic domain structures and optimal deposition temperatures required for the stabilisation of these systems have been determined with X-ray diffraction and reflectivity techniques. Strain-induced changes to the CDW and SC states in layers of α-U with a-axes that are nominally strained (a > a_bulk) and unstrained (a ≈ a_bulk) have been studied for the first time via low temperature magnetotransport and spectroscopic techniques, with surface sensitive measurements showing possible hexagonal surface reconstructions in both α-U(001) and α-U(110) thin films. These studies provide some new insight into the interplay between the low temperature ground states of α-U and offer a possible explanation for the unpredictability of the superconducting transition temperatures historically seen in bulk crystals.

A novel, repeatable technique has also been developed for the growth of textured, metastable thin films of hexagonal close-packed-like uranium. Thickness studies were used to offer some insight into the wide variation of lattice parameters seen in the literature and the time-dependent transitions of the hexagonal layers into various orientations of α-U have been tracked for the first time. Several symmetry-based mechanisms describing transitions between the metastable hexagonal structure and the orthorhombic ground state have been suggested. Summaries of the ideal deposition conditions for all known α-U and hcp-U configurations are provided here for future use.

Date of Award	20 Jun 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Chris Bell (Supervisor) & Ross S Springell (Supervisor)