Quantum criticality in the ferromagnetic superconductor UCoGe under pressure and magnetic field

The pressure-temperature phase diagram of the orthorhombic ferromagnetic superconductor UCoGe was determined by resistivity measurements up to 10.5GPa. The Curie temperature TC is suppressed with pressure and vanishes at the critical pressure pc≈1GPa. Superconductivity is observed in both the ferromagnetic state at low pressure, and in the paramagnetic state above pc up to about 4GPa. Non-Fermi-liquid behavior appears in a large pressure range. The resistivity varies linearly with temperature around pc and evolves continuously with pressure to a T2 Fermi-liquid behavior for p-5 GPa. The residual resistivity as a function of pressure shows a maximum far above pc at p-7.2GPa and the amplitude of the inelastic scattering term of the resistivity decreases by more than one order in magnitude at p, which appears to mark the entrance into a weakly correlated regime. The pressure dependence of the upper critical field for magnetic field applied along the b and c axis illustrates the drastic difference in the field dependence of the ferromagnetic superconducting pairing. While for Hb axis Hc2(T) is driven by the suppression of the ferromagnetic order, it is dominated by the strong initial suppression of the ferromagnetic fluctuations for a field applied in the easy magnetization axis c.