Fulde-Ferrell-Larkin-Ovchinnikov States in Isotropic Superconductors

Abstract

The investigation of the unconventional superconductivity and the mechanism behind it has never stopped. The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, which is established near the Pauli limit and can make the superconductor persist with a much higher magnetic field than its Pauli limit by forming a spatially modulated order parameter, was predicted decades ago. However, only a handful of evidence has been reported in anisotropic two-dimensional layered superconductors restricted to the extreme requirements on the materials and experimental conditions, particularly not found in three-dimensional isotropic superconductors yet. This thesis presents the first experimental evidence for the possible formation of an FFLO state in two fully isotropic superconductors, Ti₄Ir₂O and KOs₂O₆, using thermodynamic bulk probes.

High-resolution magnetization and specific heat measurements on Ti₄Ir₂O reveal an upper critical field 𝐻_c2 exceeding the Pauli limit. Anomalies in the specific heat data at the Pauli limit indicate a phase transition from the conventional superconducting state to an FFLO state. The 𝐻−𝑇 phase diagram exhibits a deviation from the WHH model near the Pauli limit and a slight characteristic upturn in 𝐻_c2.

We then conducted resistance and specific heat measurements under high magnetic fields on KOs₂O₆. The jump-like anomaly around the Pauli limit in the specific heat data indicates the formation of an FFLO state, which is further confirmed by the field and temperature dependent specific heat data and the 𝐻−𝑇 phase diagram.

The results in this thesis provide an explanation for the violation of the Pauli limit in these two superconductors, and add two fully isotropic superconductors to the list of potential FFLO superconductors for the first time, which advances the understanding of unconventional high-field superconductivity and provides new platforms for the theoretical exploration of the FFLO state.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Rolf Walter LORTZ (Supervisor)