54 research outputs found
A two-coil mutual inductance technique to study matching effect in disordered NbN thin films
Although matching effects in superconducting anti-dot arrays have been
studied extensively through magneto-resistance oscillations, these
investigations have been restricted to a very narrow temperature window close
to the superconducting transition. Here we report a "two coil" mutual
inductance technique, which allows the study of this phenomenon deep in the
superconducting state, through a direct measurement of the magnetic field
variation of the shielding response. We demonstrate how this technique can be
used to resolve outstanding issues on the origin of matching effects in
superconducting thin films with periodic array of holes grown on anodized
alumina membranes
Origin of Matching Effect in Anti-dot Array of Superconducting NbN Thin Films
We investigate the origin of matching effect observed in disordered
superconducting NbN thin films with periodic array of holes. In addition to the
periodic variation in the electrical resistance just above the superconducting
transition temperature, Tc0, we find pronounced periodic variations with
magnetic field in all dynamical quantities which can be influenced by flux-line
motion under an external drive such as the magnetic shielding response and the
critical current which survive in some samples down to temperatures as low as
0.09Tc0. In contrast, the superconducting energy gap, D which is a true
thermodynamic quantity does not show any periodic variation with magnetic
fields for the same films. Our results show that commensurate pinning of the
flux line lattice driven by vortex-vortex interaction is the dominant mechanism
for the observed matching effects in these superconducting anti-dot films
rather than Little-Parks like quantum interference effect.Comment: 18 pages, 6 figure
Frequency dependent superfluid stiffness in the pseudogap regime in strongly disordered NbN thin films
We measure the frequency dependence of the complex ac conductivity of NbN
films with different levels of disorder in frequency range 0.4-20 GHz. Films
with low disorder exhibit a narrow dynamic fluctuation regime above T_c as
expected for a conventional superconductor. However, for strongly disordered
samples, the fluctuation regime extends well above T_c, with a strongly
frequency-dependent superfluid stiffness which disappears only at a temperature
T* close to the pseudogap temperature obtained from scanning tunneling
measurements. Such a finite-frequency response is associated to a marked
slowing down of the superconducting fluctuations already below T*. The
corresponding large length-scale fluctuations suggest a scenario of thermal
phase fluctuations between superconducting domains in a strongly disordered
s-wave superconductor.Comment: pdf file: 18 pages including figure
Spin-Polarisation measurement using NbN-Insulator-Ferromagnet Tunnel Junction with oxidized barrier
We report a two-step process for the fabrication of
superconductor-insulator-normal metal tunnel junctions using NbN as the
superconducting electrode and its surface oxide as the insulating tunnel
barrier, and investigate its efficacy in measuring spin-polarisation of
ferromagnets using the Meservey-Tedrow technique. We observe that for NbN film
thickness below 10 nm, under the application of parallel magnetic field, the
superconducting density of states show clear "Zeeman" splitting into spin-up
and spin-down sub-bands. Tunnelling measurements on devices where ferromagnetic
Co is used as the normal electrode shows that these devices can be used to
reliably measure spin polarisation of a ferromagnet at temperatures up to 1.6
K. The simplicity of our fabrication process, and the ability to perform
spin-polarisation measurements at 4He temperatures makes NbN a very attractive
candidate for spin polarisation measurements
Probing long-range correlations in the Berezinskii-Kosterlitz-Thouless fluctuation regime of ultra-thin NbN superconducting films using transport noise measurements
We probe the presence of long-range correlations in phase fluctuations by
analyzing the higher-order spectrum of resistance fluctuations in ultra-thin
NbN superconducting films. The non-Gaussian component of resistance
fluctuations is found to be sensitive to film thickness close to the
transition, which allows us to distinguish between mean field and
Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transitions. The
extent of non-Gaussianity was found to be bounded by the BKT and mean field
transition temperatures and depend strongly on the roughness and structural
inhomogeneity of the superconducting films. Our experiment outlines a novel
fluctuation-based kinetic probe in detecting the nature of superconductivity in
disordered low-dimensional materials.Comment: submitted to PR
Vortex matching effect in engineered thin films of NbN
We report robust vortex matching effects in antidot arrays fabricated on thin
films of NbN. The near absence of hysteresis between field sweep directions
indicates a negligible residual pinning in the host thin film. Owing to the
very small coherence length of NbN thin films (), the observations
suggests the possibility of probing physics of vortices at true nanometer
length scales in suitably fabricated structures.Comment: Submitted to Appl. Phys. Let
- …