Unconventional Superconductivity in Uranium Ditelluride

Superconductivity remains one of the most important topics in condensed matter physics, arising from the pairing of two fermions, in either a singlet or triplet spin configurations. Spin-triplet superconductivity has attracted a lot of interest due to its intriguing physics and potential application in recent years. While superfluid helium-3 has been rigorously established to possess triplet character, no triplet superconducting analog has yet been conclusively identified. This dissertation investigates the heavy fermion compound uranium ditelluride (UTe2), one of the most promising candidates for spin-triplet superconductivity. UTe2 exhibits extremely high upper critical fields of up to 70 Tesla, minimal change in the local spin susceptibility upon the crossing superconducting transition temperature, and a remarkable phase diagram comprising multiple distinct superconducting phases intertwined with complex magnetic fluctuations, metamagnetic transitions, and ordered states. Nevertheless, conflicting results between numerous experimental studies have impeded progress in understanding the nature of the superconductivity in UTe2. Initial studies on low- quality UTe2 samples suggested time-reversal symmetry breaking and a multi-component superconducting order parameter, based on polar Kerr effect and specific heat measurements. Subsequent work using cleaner samples failed to reproduce these results, attributing the discrepancies to inhomogeneities in early-generation crystals. One part of this dissertation focuses on providing a detailed and systematic experimental study of the superconducting and normal state properties of UTe2 in the ultraclean limit to firmly elucidate – in the absence of impurities or inhomogeneities – the intrinsic properties of this material. Remarkably, the field-reinforced superconducting phase shows acute sensitivity to the impurity level in the sample and sheds light on its origin from Cooper pairs mediated by metamagnetic fluctuations. The pressure-dependent magnetic susceptibility study of UTe2 single crystals shows a superconducting transition within another superconducting state, highlighting the difference between these superconducting states. Another remarkable result in this dissertation is the discovery of a quantum critical line in the Ha-Hb-Hc Cartesian field space. UTe2 undergoes a metamagnetic transition in which a large field along the b-axis is applied. Application of transverse field components along the a-axis and c-axis was found to suppress the critical endpoint of the metamagnetic transition towards zero temperature. Two-axis rotational studies discover that the critical endpoint is suppressed into a series of quantum critical endpoints. These quantum critical endpoints connect into a quantum critical line in the three-dimensional field space and bound the metamagnetic transition surface. The quantum fluctuations accompanying this quantum critical line likely mediate the pairing of the field-induced superconducting phase, resulting in its exotic toroidal shape and enhanced critical temperature. In combination, this dissertation establishes a comprehensive framework for the discus- sion of the intrinsic properties of UTe2. The careful mapping of the phase diagram reveals a new type of enhanced-dimensional quantum critical phase boundary and provides new insights into the mechanisms underlying spin-triplet superconductivity.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Wu, Zheyu

Advisor dc:contributor.advisor

• Grosche, Friedrich

• Superconductivity
• Quantum Criticality

Licence

• https://www.repository.cam.ac.uk/bitstreams/6016a778-7616-4a5f-bcd6-6a5e4190629f/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0009-0009-3839-5611

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/393247