82 research outputs found
Columnar defects and vortex fluctuations in layered superconductors
We investigate fluctuations of Josephson-coupled pancake vortices in layered
superconductors in the presence of columnar defects. We study the
thermodynamics of a single pancake stack pinned by columnar defects and obtain
the temperature dependence of localization length, pinning energy and critical
current. We study the creep regime and compute the crossover current between
line-like creep and pancake-like creep motion. We find that columnar defects
effectively increase interlayer Josephson coupling by suppressing thermal
fluctuations of pancakes. This leads to an upward shift in the decoupling line
most pronounced around the matching field.Comment: 5 pages, REVTeX, no figure
Vortex Plastic Flow, , Deep in the Bose Glass and Mott-Insulator Regimes
We present simulations of flux-gradient-driven superconducting vortices
interacting with strong columnar pinning defects as an external field is
quasi-statically swept from zero through a matching field . We
analyze several measurable quantities, including the local flux density , magnetization , critical current , and the
individual vortex flow paths. We find a significant change in the behavior of
these quantities as the local flux density crosses , and quantify it
for many microscopic pinning parameters. Further, we find that for a given pin
density can be enhanced by maximizing the distance between the pins
for .Comment: 4 pages, 4 PostScript Figure
Intrinsic Tunneling in Cuprates and Manganites
The most anisotropic high temperature superconductors like Bi2Sr2CaCu2O8, as
well as the recently discovered layered manganite La1.4Sr1.6Mn2O7 are layered
metallic systems where the interlayer current transport occurs via sequential
tunneling of charge carriers. As a consequence, in Bi2Sr2CaCu2O8 adjacent CuO2
double layers form an intrinsic Josephson tunnel junction while in in
La1.4Sr1.6Mn2O7 tunneling of spin polarized charge carriers between adjacent
MnO2 layers leads to an intrinsic spin valve effect. We present and discuss
interlayer transport experiments for both systems. To perform the experiments
small sized mesa structures were patterned on top of single crystals of the
above materials defining stacks of a small number of intrinsic Josephson
junctions and intrinsic spin valves, respectively.Comment: 6 pages, 8 figure
Quantum depinning of a pancake-vortex from a columnar defect
We consider the problem of the depinning of a weakly driven ()
pancake vortex from a columnar defect in a Josephson-coupled superconductor,
where denotes the force acting on the vortex ( is the critical
force).
The dynamics of the vortex is supposed to be of the Hall type. The Euclidean
action is calculated in the entire temperature range; the result
is universal and does not depend on the detailed form of the pinning potential.
We show that the transition from quantum to classical behavior is second-order
like with the temperature of the transition scaling like
Special attention is paid to the regime of applicability of our results, in
particular, the influence of the large vortex mass appearing in the superclean
limit is discussed.Comment: 11 pages, RevTeX, 4 figures inserte
Thermally activated Hall creep of flux lines from a columnar defect
We analyse the thermally activated depinning of an elastic string (line
tension ) governed by Hall dynamics from a columnar defect modelled
as a cylindrical potential well of depth for the case of a small
external force An effective 1D field Hamiltonian is derived in order to
describe the 2D string motion. At high temperatures the decay rate is
proportional to with a constant of order of the
critical force and U(F) \sim{\left ({\epsilon V_{0}})}^{{1}/{2}}{V_{0}/{F}}
the activation energy. The results are applied to vortices pinned by columnar
defects in superclean superconductors.Comment: 12 pages, RevTeX, 2 figures inserte
Vortex wandering in a forest of splayed columnar defects
We investigate the scaling properties of single flux lines in a random
pinning landscape consisting of splayed columnar defects. Such correlated
defects can be injected into Type II superconductors by inducing nuclear
fission or via direct heavy ion irradiation. The result is often very efficient
pinning of the vortices which gives, e.g., a strongly enhanced critical
current. The wandering exponent \zeta and the free energy exponent \omega of a
single flux line in such a disordered environment are obtained analytically
from scaling arguments combined with extreme-value statistics. In contrast to
the case of point disorder, where these exponents are universal, we find a
dependence of the exponents on details in the probability distribution of the
low lying energies of the columnar defects. The analytical results show
excellent agreement with numerical transfer matrix calculations in two and
three dimensions.Comment: 11 pages, 9 figure
Vortex Pinning and the Non-Hermitian Mott Transition
The boson Hubbard model has been extensively studied as a model of the zero
temperature superfluid/insulator transition in Helium-4 on periodic substrates.
It can also serve as a model for vortex lines in superconductors with a
magnetic field parallel to a periodic array of columnar pins, due to a formal
analogy between the vortex lines and the statistical mechanics of quantum
bosons. When the magnetic field has a component perpendicular to the pins, this
analogy yields a non-Hermitian boson Hubbard model. At integer filling, we find
that for small transverse fields, the insulating phase is preserved, and the
transverse field is exponentially screened away from the boundaries of the
superconductor. At larger transverse fields, a ``superfluid'' phase of tilted,
entangled vortices appears. The universality class of the transition is found
to be that of vortex lines entering the Meissner phase at H_{c1}, with the
additional feature that the direction of the tilted vortices at the transition
bears a non-trivial relationship to the direction of the applied magnetic
field. The properties of the Mott Insulator and flux liquid phases with tilt
are also discussed.Comment: 20 pages, 12 figures included in text; to appear in Physical Review
Interactions, Distribution of Pinning Energies, and Transport in the Bose Glass Phase of Vortices in Superconductors
We study the ground state and low energy excitations of vortices pinned to
columnar defects in superconductors, taking into account the long--range
interaction between the fluxons. We consider the ``underfilled'' situation in
the Bose glass phase, where each flux line is attached to one of the defects,
while some pins remain unoccupied. By exploiting an analogy with disordered
semiconductors, we calculate the spatial configurations in the ground state, as
well as the distribution of pinning energies, using a zero--temperature Monte
Carlo algorithm minimizing the total energy with respect to all possible
one--vortex transfers. Intervortex repulsion leads to strong correlations
whenever the London penetration depth exceeds the fluxon spacing. A pronounced
peak appears in the static structure factor for low filling fractions . Interactions lead to a broad Coulomb gap in the distribution of
pinning energies near the chemical potential , separating
the occupied and empty pins. The vanishing of at leads to a
considerable reduction of variable--range hopping vortex transport by
correlated flux line pinning.Comment: 16 pages (twocolumn), revtex, 16 figures not appended, please contact
[email protected]
Bioactive Hydroperoxyl Cembranoids from the Red Sea Soft Coral Sarcophyton glaucum
A chemical investigation of an ethyl acetate extract of the Red Sea soft coral Sarcophyton glaucum has led to the isolation of two peroxide diterpenes, 11(S) hydroperoxylsarcoph-12(20)-ene (1), and 12(S)-hydroperoxylsarcoph-10-ene (2), as well as 8-epi-sarcophinone (3). In addition to these three new compounds, two known structures were identified including: ent-sarcophine (4) and sarcophine (5). Structures were elucidated by spectroscopic analysis, with the relative configuration of 1 and 2 confirmed by X-ray diffraction. Isolated compounds were found to be inhibitors of cytochrome P450 1A activity as well as inducers of glutathione S-transferases (GST), quinone reductase (QR), and epoxide hydrolase (mEH) establishing chemo-preventive and tumor anti-initiating activity for these characterized metabolites
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