Coexistence and interplay of superconductivity and ferromagnetism in URhGe

As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe2, URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was first observed in 2001. Recently, we discovered a second pocket of superconductivity. This new pocket of superconductivity appears at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at HR = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. The possibility that magnetic fluctuations emerging from a quantum critical point provide the pairing mechanism for superconductivity is discussed.

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