Abstract
Competing phenomena in low dimensional systems can generate exotic electronic phases, either through symmetry breaking or a non-trivial topology. In two-dimensional (2D) systems, the interplay between superfluidity, disorder and repulsive interactions is especially fruitful in this respect although both the exact nature of the phases and the microscopic processes at play are still open questions. In particular, in 2D, once superconductivity is destroyed by disorder, an insulating ground state is expected to emerge, as a result of a direct superconductor-to-insulator quantum phase transition. In such systems, no metallic state is theoretically expected to survive to the slightest disorder. Here we map out the phase diagram of amorphous NbSi thin films as functions of disorder and film thickness, with two metallic phases in between the superconducting and insulating ones. These two dissipative states, defined by a resistance which extrapolates to a finite value in the zero temperature limit, each bear a specific dependence on disorder. We argue that they originate from an inhomogeneous destruction of superconductivity, even if the system is morphologically homogeneous. Our results suggest that superconducting fluctuations can favor metallic states that would not otherwise exist.
Highlights
In 2D systems, disorder induces quantum interferences between electronic wave functions, eventually leading to their localization
We present the phase diagram of a-NbSi thin films across the disorder-induced Superconductor-to-Insulator Transition (SIT), along with the characterization of corresponding phases
We have considered 40 samples with compositions x from 8.5 to 18.5%, thicknesses d from 4 to 50 nm, and submitted to heat treatments at temperatures θht from 70 to 250 °C
Summary
In 2D systems, disorder induces quantum interferences between electronic wave functions, eventually leading to their localization. We will here examine the transport features of the films as a function of σN, the normal state conductivity at T = 500 mK, taken as a measure of disorder.
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