484 research outputs found
Temperature dependence of the energy dissipation in dynamic force microscopy
The dissipation of energy in dynamic force microscopy is usually described in
terms of an adhesion hysteresis mechanism. This mechanism should become less
efficient with increasing temperature. To verify this prediction we have
measured topography and dissipation data with dynamic force microscopy in the
temperature range from 100 K up to 300 K. We used
3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) grown on KBr(001), both
materials exhibiting a strong dissipation signal at large frequency shifts. At
room temperature, the energy dissipated into the sample (or tip) is 1.9
eV/cycle for PTCDA and 2.7 eV/cycle for KBr, respectively, and is in good
agreement with an adhesion hysteresis mechanism. The energy dissipation over
the PTCDA surface decreases with increasing temperature yielding a negative
temperature coefficient. For the KBr substrate, we find the opposite behaviour:
an increase of dissipated energy with increasing temperature. While the
negative temperature coefficient in case of PTCDA agrees rather well with the
adhesion hysteresis model, the positive slope found for KBr points to a
hitherto unknown dissipation mechanism
Simulation Studies on the Stability of the Vortex-Glass Order
The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J.
Phys. Soc. Jpn. Vol.69 No.1 (2000
Irreversibility, Mechanical Entanglement and Thermal Melting in Superconducting Vortex Crystals with Point Impurities
We discuss the onset of irreversibility and entanglement of vortex lines in
high Tc superconductors due to point disorder and thermal fluctuations using a
simplified cage model. A combination of Flory arguments, known results from
directed polymers in random media, and a Lindemann criterion are used to
estimate the field and temperature dependence of irreversibility, mechanical
entanglement and thermal melting. The qualitative features of this dependence,
including its nonmonotonicity when disorder is sufficiently strong, are in good
agreement with recent experiments.Comment: 7 pages, uses RevTeX, multicol.sty and epsf.sty, 5 EPS figures
include
Melting and Dimensionality of the Vortex Lattice in Underdoped YBa2Cu3O6.60
Muon spin rotation measurements of the magnetic field distribution in the
vortex state of the oxygen deficient high-Tc superconductor YBa{2}Cu{3}O{6.60}
reveal a vortex-lattice melting transition at much lower temperature than that
in the fully oxygenated material. The transition is best described by a model
in which adjacent layers of ``pancake'' vortices decouple in the liquid phase.
Evidence is also found for a pinning-induced crossover from a solid 3D to
quasi-2D vortex lattice, similar to that observed in the highly anisotropic
superconductor Bi{2+x}Sr{2-x}CaCu{2}O{8+y}.Comment: 8 pages, 4 figures, 5 postscript file
Longitudinal Current Dissipation in Bose-glass Superconductors
A scaling theory of vortex motion in Bose glass superconductors with currents
parallel to the common direction of the magnetic field and columnar defects is
presented. Above the Bose-glass transition the longitudinal DC resistivity
vanishes much faster than the
corresponding transverse resistivity , thus {\it reversing} the usual anisotropy of electrical transport in
the normal state of layered superconductors. In the presence of a current at an angle with the common field and columnar defect axis, the
electric field angle approaches as .
Scaling also predicts the behavior of penetration depths for the AC currents as
, and implies a {\it jump discontinuity} at in
the superfluid density describing transport parallel to the columns.Comment: 5 pages, revte
Critical Fluctuations and Disorder at the Vortex Liquid to Crystal Transition in Type-II Superconductors
We present a functional renormalization group (FRG) analysis of a
Landau-Ginzburg model of type-II superconductors (generalized to complex
fields) in a magnetic field, both for a pure system, and in the presence of
quenched random impurities. Our analysis is based on a previous FRG treatment
of the pure case [E.Br\'ezin et. al., Phys. Rev. B, {\bf 31}, 7124 (1985)]
which is an expansion in . If the coupling functions are
restricted to the space of functions with non-zero support only at reciprocal
lattice vectors corresponding to the Abrikosov lattice, we find a stable FRG
fixed point in the presence of disorder for , identical to that of the
disordered model in dimensions. The pure system has a stable fixed
point only for and so the physical case () is likely to have a
first order transition. We speculate that the recent experimental findings that
disorder removes the apparent first order transition are consistent with these
calculations.Comment: 4 pages, no figures, typeset using revtex (v3.0
First-Order Melting of a Moving Vortex Lattice: Effects of Disorder
We study the melting of a moving vortex lattice through numerical simulations
with the current driven 3D XY model with disorder. We find that there is a
first-order phase transition even for large disorder when the corresponding
equilibrium transition is continuous. The low temperature phase is an
anisotropic moving glass.Comment: Important changes from original version. Finite size analysis of
results has been added. Figure 2 has been changed. There is a new additional
Figure. To be published in Physical Review Letter
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