Double critical regimes at the superconductor-metal transition in ultrathin niobium films

The upper critical field ($H_{c2}$) and the magnetic field induced superconductor-metal transition (SMT) are studied in ultrathin Nb films of thickness $d$ ranging from 1.2 nm to 20 nm, which undergo a transition from polycrystalline to amorphous structure at the thickness $d \simeq 3.3$ nm. The $H_{c2}$ is found to be orbitally limited in polycrystalline films, and paramagnetically limited in the amorphous films. The SMT is observed in all films with $d < 11.3$ nm, with the critical field $B_c$ approximately constant in polycrystalline films, and decreasing as a power-law with the film conductance $G$ in the amorphous films. The scaling analysis identifies two different critical regimes in the amorphous films, with the critical exponents consistent with (2+1)D $XY$ model for a 2D superconductor in the clean (disordered) limit at high (low) temperatures, respectively; in addition, at the lowest $T$ quantum metal phase is observed. These results suggest inhomogeneous nature of the amorphous films, in the form of superconducting islands immersed in the metallic background. Some unusual features, not observed in other systems, include the suppression of SC correlations on cooling, and the exponent of the power law in the dependence of $B_c$ on $G$, which differs from theoretical predictions. These features may be caused by paramagnetic pair breaking, or by the proximity of quantum metal phase.