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 $10.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. The Curie temperature ${T}_{C}$ is suppressed with pressure and vanishes at the critical pressure ${p}_{c}\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Superconductivity is observed in both the ferromagnetic state at low pressure, and in the paramagnetic state above ${p}_{c}$ up to about $4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Non-Fermi-liquid behavior appears in a large pressure range. The resistivity varies linearly with temperature around ${p}_{c}$ and evolves continuously with pressure to a ${T}^{2}$ Fermi-liquid behavior for $p⪆5$ GPa. The residual resistivity as a function of pressure shows a maximum far above ${p}_{c}$ at ${p}^{\ensuremath{\star}}=7.2\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ and the amplitude of the inelastic scattering term of the resistivity decreases by more than one order in magnitude at ${p}^{\ensuremath{\star}}$, 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 $H\ensuremath{\parallel}b$ axis ${H}_{c2}(T)$ 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 $c$.