380 research outputs found
Depinning of a vortex chain in a disordered flow channel
We study depinning of vortex chains in channels formed by static, disordered
vortex arrays. Depinning is governed either by the barrier for defect
nucleation or for defect motion, depending on whether the chain periodicity is
commensurate or incommensurate with the surrounding arrays. We analyze the
reduction of the gap between these barriers as function of disorder. At large
disorder, commensurability becomes irrelevant and the pinning force is reduced
to a small fraction of the ideal shear strength of ordered channels.
Implications for experiments on channel devices are discussed.Comment: 5 pages, 4 figures. Accepted for publication in Europhysics Letter
Depinning and dynamics of vortices confined in mesoscopic flow channels
We study the behavior of vortex matter in artificial flow channels confined
by pinned vortices in the channel edges (CE's). The critical current is
governed by the interaction with static vortices in the CE's. We study
structural changes associated with (in)commensurability between the channel
width and the natural row spacing , and their effect on . The
behavior depends crucially on the presence of disorder in the CE arrays. For
ordered CE's, maxima in occur at matching ( integer), while
for defects along the CE's cause a vanishing . For weak CE
disorder, the sharp peaks in at become smeared via nucleation
and pinning of defects. The corresponding quasi-1D row configurations can
be described by a (disordered)sine-Gordon model. For larger disorder and
, levels at of the ideal lattice strength
. Around 'half filling' (), disorder causes new
features, namely {\it misaligned} defects and coexistence of and
rows in the channel. This causes a {\it maximum} in around mismatch,
while smoothly decreases towards matching due to annealing of the
misaligned regions. We study the evolution of static and dynamic structures on
changing , the relation between modulations of and transverse
fluctuations and dynamic ordering of the arrays. The numerical results at
strong disorder show good qualitative agreement with recent mode-locking
experiments.Comment: 29 pages, 32 figure
Optical Properties of TiN Thin Films close to the Superconductor-Insulator Transition
We present the intrinsic optical properties over a broad spectral range of
TiN thin films deposited on a Si/SiO substrate. We analyze the measured
reflectivity spectra of the film-substrate multilayer structure within a
well-establish procedure based on the Fresnel equation and extract the real
part of the optical conductivity of TiN. We identify the metallic contribution
as well as the finite energy excitations and disentangle the spectral weight
distribution among them. The absorption spectrum of TiN bears some similarities
with the electrodynamic response observed in the normal state of the
high-temperature superconductors. Particularly, a mid-infrared feature in the
optical conductivity is quite reminiscent of a pseudogap-like excitation
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