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 with forming a spatially modulated order parameter, was predicted decades ago, but only handful evidence has been reported in anisotropic two-dimensional layered superconductors restricted to the extreme requirements on the materials and experimental conditions, exceptionally not found in three-dimensional isotropic superconductors yet. This thesis presents experimental evidence for the possible FFLO state in two isotropic superconductors, Ti4Ir2O and KOs2O6 , using thermodynamic bulk probes.
High-resolution magnetization and specific heat measurements on Ti4Ir2O reveal an upper critical field Ti4Ir2O 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 Hc2.
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 theory exploration of the FFLO state.