60 research outputs found
Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition
The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and
one In/InOx film have been measured in zero field using a two-coil mutual
inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to
have three contributions: the inductive superfluid, renormalized by nonvortex
phase fluctuations; conventional vortex-antivortex pairs, whose contribution
turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii
unbinding temperature; and an anomalous contribution. The latter is
predominantly resistive, persists well below the KTB temperature, and is weakly
dependent on frequency down to remarkably low frequencies, at least 100 Hz. It
increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about
half the energy to create a vortex-antivortex pair, indicating that the
frequency dependence is that of individual excitations, rather than critical
behavior.Comment: 10 pages, 10 figs; subm PR
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde
Transverse phase-locking in fully frustrated Josephson junction arrays: a new type of fractional giant steps
We study, analytically and numerically, phase locking of driven vortex
lattices in fully-frustrated Josephson junction arrays at zero temperature. We
consider the case when an ac current is applied {\it perpendicular} to a dc
current. We observe phase locking, steps in the current-voltage
characteristics, with a dependence on external ac-drive amplitude and frequency
qualitatively different from the Shapiro steps, observed when the ac and dc
currents are applied in parallel. Further, the critical current increases with
increasing transverse ac-drive amplitude, while it decreases for longitudinal
ac-drive. The critical current and the phase-locked current step width,
increase quadratically with (small) amplitudes of the ac-drive. For larger
amplitudes of the transverse ac-signal, we find windows where the critical
current is hysteretic, and windows where phase locking is suppressed due to
dynamical instabilities. We characterize the dynamical states around the
phase-locking interference condition in the curve with voltage noise,
Lyapunov exponents and Poincar\'e sections. We find that zero temperature
phase-locking behavior in large fully frustrated arrays is well described by an
effective four plaquette model.Comment: 12 pages, 11 figure
Critical Exponents of the Fully Frustrated 2-D Xy Model
We present a detailed study of the critical properties of the 2-D XY model
with maximal frustration in a square lattice. We use extensive Monte Carlo
simulations to study the thermodynamics of the spin and chiral degrees of
freedom, concentrating on their correlation functions. The gauge invariant
spin-spin correlation functions are calculated close to the critical point for
lattice sizes up to ; the chiral correlation functions are
studied on lattices up to . We find that the critical exponents of
the spin phase transition are , and , which are to be
compared with the unfrustrated XY model exponents and . We
also find that the critical exponents of the chiral transition are
, , , and
, which are different from the expected 2-D Ising
critical exponents. The spin-phase transition occurs at which
is about 7\% above the estimated chiral critical temperature . However, because of the size of the statistical errors, it is
difficult to decide with certainty whether the transitions occur at the same or
at slightly different temperatures. Finally, the jump in the helicity modulus
in the fully frustrated system is found to be about 23\% below the unfrustrated
universal value. The most important consequence of these results is that the
fully frustrated XY model appears to be in a novel universality class. Recent
successful comparisons of some of these results with experimental data are also
briefly discussed. (TO APPEAR IN PRB)Comment: 47 pages (PHYZZX
Quantum Interference in Superconducting Wire Networks and Josephson Junction Arrays: Analytical Approach based on Multiple-Loop Aharonov-Bohm Feynman Path-Integrals
We investigate analytically and numerically the mean-field
superconducting-normal phase boundaries of two-dimensional superconducting wire
networks and Josephson junction arrays immersed in a transverse magnetic field.
The geometries we consider include square, honeycomb, triangular, and kagome'
lattices. Our approach is based on an analytical study of multiple-loop
Aharonov-Bohm effects: the quantum interference between different electron
closed paths where each one of them encloses a net magnetic flux. Specifically,
we compute exactly the sums of magnetic phase factors, i.e., the lattice path
integrals, on all closed lattice paths of different lengths. A very large
number, e.g., up to for the square lattice, exact lattice path
integrals are obtained. Analytic results of these lattice path integrals then
enable us to obtain the resistive transition temperature as a continuous
function of the field. In particular, we can analyze measurable effects on the
superconducting transition temperature, , as a function of the magnetic
filed , originating from electron trajectories over loops of various
lengths. In addition to systematically deriving previously observed features,
and understanding the physical origin of the dips in as a result of
multiple-loop quantum interference effects, we also find novel results. In
particular, we explicitly derive the self-similarity in the phase diagram of
square networks. Our approach allows us to analyze the complex structure
present in the phase boundaries from the viewpoint of quantum interference
effects due to the electron motion on the underlying lattices.Comment: 18 PRB-type pages, plus 8 large figure
MMP-13 stimulates osteoclast differentiation and activation in tumour breast bone metastases
INTRODUCTION:
The increased bone degradation in osteolytic metastases depends on stimulation of mature osteoclasts and on continuous differentiation of new pre-osteoclasts. Metalloproteinases (MMP)-13 is expressed in a broad range of primary malignant tumours and it is emerging as a novel biomarker. Recent data suggest a direct role of MMP-13 in dissolving bone matrix complementing the activity of MMP-9 and other enzymes. Tumour-microenvironment interactions alter gene expression in malignant breast tumour cells promoting osteolytic bone metastasis. Gene expression profiles revealed that MMP-13 was among the up-regulated genes in tumour-bone interface and its abrogation reduced bone erosion. The precise mechanism remained not fully understood. Our purpose was to further investigate the mechanistic role of MMP-13 in bone osteolytic lesions.
METHODS:
MDA-MB-231 breast cancer cells that express MMP-13 were used as a model for in vitro and in vivo experiments. Conditioned media from MDA-MB-231 cells were added to peripheral blood mononuclear cultures to monitor pre-osteoclast differentiation and activation. Bone erosion was evaluated after injection of MMP-13-silenced MDA-MB-231 cells into nude mice femurs.
RESULTS:
MMP-13 was co-expressed by human breast tumour bone metastases with its activator MT1-MMP. MMP-13 was up-regulated in breast cancer cells after in vitro stimulation with IL-8 and was responsible for increased bone resorption and osteoclastogenesis, both of which were reduced by MMP inhibitors. We hypothesized that MMP-13 might be directly involved in the loop promoting pre-osteoclast differentiation and activity. We obtained further evidence for a direct role of MMP-13 in bone metastasis by a silencing approach: conditioned media from MDA-MB-231 after MMP-13 abrogation or co-cultivation of silenced cells with pre-osteoclast were unable to increase pre-osteoclast differentiation and resorption activity. MMP-13 activated pre-MMP-9 and promoted the cleavage of galectin-3, a suppressor of osteoclastogenesis, thus contributing to pre-osteoclast differentiation. Accordingly, MMP-13 abrogation in tumour cells injected into the femurs of nude mice reduced the differentiation of TRAP positive cells in bone marrow and within the tumour mass as well as bone erosion.
CONCLUSIONS:
These results indicate that within the inflammatory bone microenvironment MMP-13 production was up-regulated in breast tumour cells leading to increased pre-osteoclast differentiation and their subsequent activation
EcoTILLING by sequencing reveals polymorphisms in genes encoding starch synthases that are associated with low glycemic response in rice
Polarized Fusion: An Idea More Than Thirty Years Old! What Are We Waiting for?
The present status of the fusion research is strictly connected to government
investments on the corresponding research projects like ITER, or the proposed
IGNITOR and DEMO reactors. The production of energy by nuclear fusion is a perfect
option that could give “breath” to the planet. Recent agreements on limiting the
global climate change and plans for our future life on the planet require the reduction
of energy production by carbon-based resources. But even the use of nuclear
resources by fission implicates a non negligible risk for our civilization, either by
disasters like in Chernobyl or in Fukushima, or by the release of the fission products
into environment. CO2 emissions into the atmosphere and the growing and developing
population urgently require to put more effort into fusion programs worldwide.
An additional option for any fusion program could be the use of polarized fuel. It
still requires intense effort on the development of the necessary technologies, but it
is a realistic option to increase the energy output of different types of fusion reactors
and to increase the cost efficiency. First of all we would like to give an overview on
the current situation in energy production and recent climate development. Secondly,
we would like to provide an introduction to the contents of this volume, devoted to
nuclear fusion using polarized fuels
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