38 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
Quantum phase transition in few-layer NbSe probed through quantized conductance fluctuations
We present the first observation of dynamically modulated quantum phase
transition (QPT) between two distinct charge density wave (CDW) phases in
2-dimensional 2H-NbSe. There is recent spectroscopic evidence for the
presence of these two quantum phases, but its evidence in bulk measurements
remained elusive. We studied suspended, ultra-thin \nbse devices fabricated on
piezoelectric substrates - with tunable flakes thickness, disorder level and
strain. We find a surprising evolution of the conductance fluctuation spectra
across the CDW temperature: the conductance fluctuates between two precise
values, separated by a quantum of conductance. These quantized fluctuations
disappear for disordered and on-substrate devices. With the help of mean-field
calculations, these observations can be explained as to arise from dynamical
phase transition between the two CDW states. To affirm this idea, we vary the
lateral strain across the device via piezoelectric medium and map out the phase
diagram near the quantum critical point (QCP). The results resolve a
long-standing mystery of the anomalously large spectroscopic gap in NbSe
Structure and dynamics of a pinned vortex liquid in superconducting a-Re_xZr (x ~ 6) thin film
We report the formation of a pinned vortex liquid spanning a very large
region of the magnetic field-temperature parameter space in a 5 nm thick
amorphous superconducting Re_xZr (x~6) (a-ReZr) thin film, using a combination
of low-temperature scanning tunnelling spectroscopic (STS) imaging and
magnetotransport measurements. The nature of the vortex liquid differs
significantly from a regular liquid. Analysing series of STS images captured as
a function of time, we observe that the interplay of pinning and intervortex
interactions produces a very inhomogeneous state, where some vortices remain
static, whereas others move forming a percolating network along which vortices
are mobile. With increase in temperature or magnetic field this network becomes
denser eventually encompassing all vortices. Our results provide key insight on
the nature of a pinned vortex liquid and some of the peculiarities in the
transport properties of ultrathin superconducting films.Comment: Main text with Supplementar