Observation of a symmetry change in the gap function in heavy-fermion superconductor PrOs4Sb12

Abstract The superconducting gap structure of heavy-fermion superconductor PrOs 4 Sb 12 was investigated by using magneto-thermal conductivity measurements. We demonstrate that a symmetry change in the gap function takes place deep inside the superconducting phase, giving a clear evidence of the presence of two distinct superconducting phases with twofold and fourfold symmetries.