Phase Diagram and Upper Critical Field of Homogeneously Disordered Epitaxial 3-Dimensional NbN Films

Abstract

We report the evolution of superconducting properties with disorder, in 3 dimensional homogeneously disordered epitaxial NbN thin films. The effective disorder in NbN is controlled from moderately clean limit down to Anderson metal-insulator transition by changing the deposition conditions. We propose a phase diagram for NbN in temperature-disorder plane. With increasing disorder we observe that as k F l→1 the superconducting transition temperature (T c ) and normal state conductivity in the limit T 0 (σ 0 ) go to zero. The phase diagram shows that in homogeneously disordered 3-D NbN films, the metal-insulator transition and the superconductor-insulator transition occur at a single quantum critical point, k F l~1.

Highly oriented, free-standing, superconducting NbN films growth on chemical vapor deposited graphene

“…This value of the upper critical field for NbN grown on CVD graphene is considerably larger than the 20 T reported on NbN films grown of MgO with similar transition temperature 24 and suggests that the NbN/graphene films can be useful for high magnetic field applications. Figure 4 shows the resistance vs. temperature curves for films deposited for different times which correspond to different film thicknesses ranging from 10 nm to 250 nm.…”

Enhancement of the finite-frequency superfluid response in the pseudogap regime of strongly disordered superconducting films

“…T c , defined as the temperature at which dc resistance goes below our measurable limit varies in the range T c ≈ 15.71 – 3.14 K. The effective disorder, characterized using the product of the Fermi wave vector, k F , and electronic mean free path, l , is in the range k F l ~ 9.5 – 1.8. The thickness ( t ) of all films was ~50 nm which is much larger than the dirty limit coherence length20, ξ 0 ~ 4−8 nm. The phase diagram of disordered NbN established earlier from STS and transport measurements on similar samples21 reveal that samples with T c < 6 K show a pronounced PG phase with T* ~ 6–7 K.…”

“…We convert σ''(ω) into J (from eqn. 1) using the experimental values of ξ 0 obtained from upper critical field measurements20. For T ≪ T c , J is frequency independent, showing that the phase rigidity is established over all time scales.…”

Amplitude, density, and current correlations of strongly disordered superconductors

“…In particular when the superfluid stiffness D s is the lowest energy scale in the problem one would expect that T c ∝ D s , so that also the upper critical field will scale with D s , as suggested for example by a recent analysis of the microwave conductivity at finite magnetic field in disordered InO x . 44 In this sense, even though at intermediate disorder the decrease of H c2 measured experimentally 45 can be interpreted as an increase of ξ 0 due to the weakening of the SC order parameter, as the SIT is approached one should not attribute the vanishing of H c2 ∝ T c to a divergence of ξ A 0 discussed above.…”