Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

Abstract

In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, the ^[59]Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to the spontaneous magnetization axis ^[59]K^[a,b]. ^[59]K^[a,b] shows to be almost constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev [Phys. Rev. B 81 180504 (2010)].