Abstract
Recent studies suggest that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Here we investigate the nature of SC state in ultrathin Nb films, of thickness d ranging from 1.2 to 20 nm, which undergo a transition from amorphous to polycrystalline structure at the thickness d simeq 3.3 nm. We show that the properties of SC state are very different in polycrystalline and amorphous films. The upper critical field (H_{c2}) is orbitally limited in the first case, and paramagnetically limited in the latter. The magnetic field induced superconductor-metal transition is observed, with the critical field approximately constant or decreasing as a power-law with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity exists above H_{c2}, particularly pronounced in amorphous films, signaling the presence of fluctuating Cooper pairs. These observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximity-coupled SC islands. An usual evolution of SC correlations on cooling is observed in amorphous films, likely related to the effect of quantum fluctuations on the proximity-induced phase coherence.
Highlights
Recent studies suggest that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous
SC correlations is observed in the amorphous films on lowering of temperature, which we propose to explain by the influence of quantum fluctuations on the proximity-induced phase coherence
The Hc2 and the B-induced superconductor-metal transition (SMT) have been examined in ultrathin Nb films, which undergo a transition from polycrystalline to amorphous structure on the decrease of thickness
Summary
Recent studies suggest that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Fluctuating Cooper pairs (FCP) surviving across the SIT/SMT phase boundary, well into the normal state, may be responsible for many unusual transport phenomena, such as, for example, large positive magnetoresistance (MR) peak, large Nernst effect, negative MR at high magnetic fields, or magnetic field induced reentrant behaviors[5,6,7,8,9,10,11,12] Some of these phenomena are well described by theories of the FCP, developed over last two decades[13,14,15]. The most important finding in the present study is the strong indication of the development of proximitycoupled SC islands in the amorphous films, what results in SC state with properties distinctly different from those observed in polycrystalline films These distinct properties include upper critical field ( Hc2 ), the scaling properties in the vicinity of the SMT, and the negative contribution of SC fluctuations to conductivity above Hc2 , large in amorphous films, consistent with the theories of the FCP13–15. SC correlations is observed in the amorphous films on lowering of temperature, which we propose to explain by the influence of quantum fluctuations on the proximity-induced phase coherence
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