30 research outputs found
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
Emergence of nanoscale inhomogeneity in the superconducting state of a homogeneously disordered conventional superconductor, NbN
The notion of spontaneous formation of an inhomogeneous superconducting state
is at the heart of most theories attempting to understand the superconducting
state in the presence of strong disorder. Using scanning tunneling spectroscopy
and high resolution scanning transmission electron microscopy, we
experimentally demonstrate that under the competing effects of strong
homogeneous disorder and superconducting correlations, the superconducting
state of a conventional superconductor, NbN, spontaneously segregates into
domains. Tracking these domains as a function of temperature we observe that
the superconducting domains persist across the bulk superconducting transition,
Tc, and disappear close to the pseudogap temperature, T*, where signatures of
superconducting correlations disappear from the tunneling spectrum and the
superfluid response of the system
Phase diagram and upper critical field of homogenously disordered epitaxial 3-dimensional NbN films
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 kFl-->1 the superconducting transition temperature (Tc) and
minimum conductivity (sigma_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 at
kFl~1.Comment: To appear in Journal of Superconductivity and Novel Magnetism
(ICSM2010 proceedings
Phase diagram of a strongly disordered s-wave superconductor, NbN, close to the metal-insulator transition
We present a phase diagram as a function of disorder in three-dimensional NbN
thin films, as the system enters the critical disorder for the destruction of
the superconducting state. The superconducting state is investigated using a
combination of magnetotransport and tunneling spectroscopy measurements. Our
studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4),
the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the
superconducting energy gap vanishes at the temperature where electrical
resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state
emerges where a gap in the electronic spectrum persists up to temperatures much
higher than Tc, suggesting that Cooper pairs continue to exist in the system
even after the zero resistance state is destroyed. Finally, very strongly
disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at
low temperatures, suggesting that localized Cooper pairs continue to survive in
the system even after the global superconducting ground state is completely
destroyed.Comment: pdf file with figures (Modified Version
Penetration depth and tunneling studies in very thin epitaxial NbN films
We investigate evolution of the magnetic penetration depth and
superconducting energy gap in epitaxial NbN films using a low frequency mutual
inductance technique and tunneling spectroscopy using a low temperature
scanning tunneling microscope (STM). The superconducting transition temperature
(Tc) for films grown under optimal growth conditions decreases monotonically
from 15.87K to 9.16K as the film thickness is decreased from 50nm to 3nm. With
decrease in film thickness delta(0) monotonically decreases, whereas lambda(0)
monotonically increases. We observe that Tc, lambda(o) and delta(0) are well
described by Bardeen-Cooper-Schrieffer (BCS) theory in all films other than the
two thinnest ones where we see evidence of the Kosterlitz-Thouless-Berezinski
(KTB) transition close to Tc.Comment: Modified version with extensive analysis of the KTB transition. One
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