368 research outputs found
Dynamic melting of confined vortex matter
We study {\em dynamic} melting of confined vortex matter moving in
disordered, mesoscopic channels by mode-locking experiments. The dynamic
melting transition, characterized by a collapse of the mode-locking effect,
strongly depends on the frequency, i.e. on the average velocity of the
vortices. The associated dynamic ordering velocity diverges upon approaching
the equilibrium melting line as . The
data provide the first direct evidence for velocity dependent melting and show
that the phenomenon also takes place in a system under disordered confinement.
\pacs{74.25.Qt,83.50.Ha,64.70.Dv,64.60.Ht}Comment: Some small changes have been made. 4 pages, 4 figures included.
Accepted for publication in Phys. Rev. Let
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
Vortex dynamics in superconducting channels with periodic constrictions
Vortices confined to superconducting easy flow channels with periodic
constrictions exhibit reversible oscillations in the critical current at which
vortices begin moving as the external magnetic field is varied. This
commensurability scales with the channel shape and arrangement, although
screening effects play an important role. For large magnetic fields, some of
the vortices become pinned outside of the channels, leading to magnetic
hysteresis in the critical current. Some channel configurations also exhibit a
dynamical hysteresis in the flux-flow regime near the matching fields
Evidence for a dynamic phase transition in [Co/Pt]_3 magnetic multilayers
A dynamic phase transition (DPT) with respect to the period P of an applied
alternating magnetic field has been observed previously in numerical
simulations of magnetic systems. However, experimental evidence for this DPT
has thus far been limited to qualitative observations of hysteresis loop
collapse in studies of hysteresis loop area scaling. Here, we present
significantly stronger evidence for the experimental observation of this DPT,
in a [Co(4 A)/Pt(7 A)]_3-multilayer system with strong perpendicular
anisotropy. We applied an out-of-plane, time-varying (sawtooth) field to the
[Co/Pt]_3 multilayer, in the presence of a small additional constant field,
H_b. We then measured the resulting out-of-plane magnetization time series to
produce nonequilibrium phase diagrams (NEPDs) of the cycle-averaged
magnetization, Q, and its variance, Var(Q), as functions of P and H_b. The
experimental NEPDs are found to strongly resemble those calculated from
simulations of a kinetic Ising model under analagous conditions. The similarity
of the experimental and simulated NEPDs, in particular the presence of a
localized peak in the variance Var(Q) in the experimental results, constitutes
strong evidence for the presence of this DPT in our magnetic multilayer
samples. Technical challenges related to the hysteretic nature and response
time of the electromagnet used to generate the time-varying applied field
precluded us from extracting meaningful critical scaling exponents from the
current data. However, based on our results, we propose refinements to the
experimental procedure which could potentially enable the determination of
critical exponents in the future.Comment: substantial revision; 26 pages, 9 figures; to appear in Phys. Rev.
Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study
We report measurements of the magnetic penetration depth \lambda_m(T) in the
presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals.
Warming, after magnetic field is applied to a zero-field cooled sample, results
in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained
upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the
possible relation of our results to the vortex decoupling, unbinding, and
dimensional crossover.Comment: M2S-HTSC-V
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
Vortex liquid correlations induced by in-plane field in underdoped Bi2Sr2CaCu2O8+d
By measuring the Josephson Plasma Resonance, we have probed the influence of
an in-plane magnetic field on the pancake vortex correlations along the c-axis
in heavily underdoped Bi2Sr2CaCu2O8+d (Tc = 72.4 +/- 0.6 K) single crystals
both in the vortex liquid and in the vortex solid phase. Whereas the in-plane
field enhances the interlayer phase coherence in the liquid state close to the
melting line, it slightly depresses it in the solid state. This is interpreted
as the result of an attractive force between pancake vortices and Josephson
vortices, apparently also present in the vortex liquid state. The results
unveil a boundary between a correlated vortex liquid in which pancakes adapt to
Josephson vortices, and the usual homogeneous liquid.Comment: 2 pages, submitted to the Proceedings of M2S HTSC VIII Dresde
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