Low magnetic field phase diagram of UCoGe

Abstract

We propose an alternate scenario for the relation between ferromagnetism (FM) and superconductivity (SC) in UCoGe at ambient pressure. On the basis of neutron polarimetry measurement performed in strictly zero field on a single-crystal at ambient pressure, we claim that the SC phase does exist in spite of lack of long-range FM order. Moreover, the SC regime dominates to the paramagnetic (PM) state in small external magnetic fields. Above the critical SC temperature, TSC = 0.64 K, the zero-field state is characterized by a regime with strong FM spin fluctuations, suggesting the proximity of the FM quantum critical point. We present an ambient-pressure phase diagram of UCoGe based on our resistivity, magnetization and neutron polarimetry results.