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Quantum criticality and Fermi surface instabilities in the ferromagnetic superconductor UCoGe

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UCoGe shows microscopic coexistence of ferromagnetism and superconductivity with a Curie temperature TC=2.7K and a superconducting transition temperature Tsc=0.6K. The Curie temperature decreases with pressure and vanishes at a critical pressure: pc~1GPa. We measured resistivity and magnetoresistance for field along c axis in UCoGe up to 10.5GPa. Superconductivity was observed both below pc and above pc up to 4 GPa with a maximum of Tsc just below the critical pressure. Non Fermi liquid behavior can be observed on a broad pressure range around the critical pressure. The temperature and pressure dependence of the upper critical field shows the strong influence of magnetic field on the pairing interaction. We report also here magnetoresistance Hall effect and thermopower measurements on UCoGe under high magnetic field applied along the easy magnetization axis c. Quantum oscillations were detected both in resistivity and Seebeck effect. A cascade of Fermi surface reconstructions without magnetic phase transitions were observed in high magnetic fields up to 34 T and they were discussed in terms of topological Lifshitz transitions. Shubnikov-de Haas oscillations were also measured under pressure up to 2.5GPa and the evolution of Fermi surface through the critical pressure is relatively small.

This talk is part of the Quantum Matter Seminar series.

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