31 research outputs found
Superconducting properties and Hall Effect of epitaxial NbN thin films
We have measured the magnetotransport properties and Hall effect of a series
of epitaxial NbN films grown on (100) oriented single crystalline MgO substrate
under different conditions using reactive magnetron sputtering. Hall effect
measurements reveal that the carrier density in NbN thin films is sensitive to
the growth condition. The carrier density increases by a factor of 3 between
the film with highest normal state resistivity (rho_n~3.83micro-Ohm-m) and
lowest transition temperature (Tc~9.99K) and the film with lowest normal state
resistivity (rho_n~0.94micro-Ohm-m) and highest transition temperature
(Tc~16.11K) while the mobility of carriers does not change significantly except
for the most resistive films. Our results show that the Tc of NbN is governed
primarily by the carrier density rather than disorder scattering. By varying
the carrier concentration during growth we can vary the effective disorder
(kF_l) from the moderately clean limit to the dirty limit which makes this
system ideal to study the interplay of carrier density and disorder on the
superconducting properties of an s-wave superconductor.Comment: ps files with figures (accepted for publication in Phys. Rev. B
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
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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