1,542 research outputs found
On the magnetization of two-dimensional superconductors
We calculate the magnetization of a two-dimensional superconductor in a
perpendicular magnetic field near its Kosterlitz-Thouless transition and at
lower temperatures. We find that the critical behavior is more complex than
assumed in the literature and that, in particular, the critical magnetization
is {\it not} field independent as naive scaling predicts. In the low
temperature phase we find a substantial fluctuation renormalization of the
mean-field result. We compare our analysis with the data on the cuprates.Comment: 8 pages, 3 figure
Finite Size Effects in Vortex Localization
The equilibrium properties of flux lines pinned by columnar disorder are
studied, using the analogy with the time evolution of a diffusing scalar
density in a randomly amplifying medium. Near H_{c1}, the physical features of
the vortices in the localized phase are shown to be determined by the density
of states near the band edge. As a result, H_{c1} is inversely proportional to
the logarithm of the sample size, and the screening length of the perpendicular
magnetic field decreases with temperature. For large tilt the extended ground
state turns out to wander in the plane perpendicular to the defects with
exponents corresponding to a directed polymer in a random medium, and the
energy difference between two competing metastable states in this case is
extensive. The divergence of the effective potential associated with strong
pinning centers as the tilt approaches its critical value is discussed as well.Comment: 10 pages, 2 figure
Nernst effect in the vortex-liquid regime of a type-II superconductor
We measure the transverse thermoelectric coefficient in
simulations of type-II superconductors in the vortex liquid regime, using the
time-dependent Ginzburg-Landau (TDGL) equation with thermal noise. Our results
are in reasonably good quantitative agreement with experimental data on cuprate
samples, suggesting that this simple model of superconducting fluctuations
contains much of the physics behind the large Nernst effect observed in these
materials.Comment: 6 pages. Expanded version of text. New Fig.
Zero Temperature Dynamics of the Weakly Disordered Ising Model
The Glauber dynamics of the pure and weakly disordered random-bond 2d Ising
model is studied at zero-temperature. A single characteristic length scale,
, is extracted from the equal time correlation function. In the pure
case, the persistence probability decreases algebraically with the coarsening
length scale. In the disordered case, three distinct regimes are identified: a
short time regime where the behaviour is pure-like; an intermediate regime
where the persistence probability decays non-algebraically with time; and a
long time regime where the domains freeze and there is a cessation of growth.
In the intermediate regime, we find that , where
. The value of is consistent with that
found for the pure 2d Ising model at zero-temperature. Our results in the
intermediate regime are consistent with a logarithmic decay of the persistence
probability with time, , where .Comment: references updated, very minor amendment to abstract and the
labelling of figures. To be published in Phys Rev E (Rapid Communications), 1
March 199
Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice
We study the ground state of a spin-half Heisenberg antiferromagnet on the
stacked kagome lattice by using a spin-rotation-invariant Green's-function
method. Since the pure two-dimensional kagome antiferromagnet is most likely a
magnetically disordered quantum spin liquid, we investigate the question
whether the coupling of kagome layers in a stacked three-dimensional system may
lead to a magnetically ordered ground state. We present spin-spin correlation
functions and correlation lengths. For comparison we apply also linear spin
wave theory. Our results provide strong evidence that the system remains
short-range ordered independent of the sign and the strength of the interlayer
coupling
The diurnal vertical dynamics of cape hake and their potential prey
The Cape hakes Merluccius capensis and M. paradoxus are dominant predators over the Namibian shelf. They are found in a water column that includes myctophids and other mesopelagic fish, euphausiids andcephalopods. Together with their cohabitant potential prey, hake are known to undertake diurnal vertical migrations, aggregating near the bottom during daylight, but migrating off the bottom at night. An attempt to determine the underlying mechanisms of this diurnal migration by means of underwater acoustics and trawling was made at a single location on the central Namibian shelf at a depth of 350 m during four consecutive days in April 1996. Large M. capensis, 50–75 cm total length, dominated just over the sea bed, whereas 30–40 cm M. paradoxus were most abundant 5–50 m off the bottom, suggesting that the smaller M. paradoxus had to remain higher in the water column to avoid being eaten by the larger M. capensis. Large hake of both species preyed preferentially on fish, whereas the smaller hake preferred euphausiids, although there was some evidence of euphausiid consumption by most hake. There was no distinct daily feeding rhythm in either species of hake, although there was some evidence of evening predation dominating. This may indicate a feeding strategy where vision is not important
U(1) spin liquids and valence bond solids in a large-N three-dimensional Heisenberg model
We study possible quantum ground states of the Sp(N) generalized Heisenberg
model on a cubic lattice with nearest-neighbor and next-nearest-neighbor
exchange interactions. The phase diagram is obtained in the large-N limit and
fluctuation effects are considered via appropriate gauge theories. In
particular, we find three U(1) spin liquid phases with different short-range
magnetic correlations. These phases are characterized by deconfined gapped
spinons, gapped monopoles, and gapless ``photons''. As N becomes smaller, a
confinement transition from these phases to valence bond solids (VBS) may
occur. This transition is studied by using duality and analyzing the resulting
theory of monopoles coupled to a non-compact dual gauge field; the condensation
of the monopoles leads to VBS phases. We determine the resulting VBS phases
emerging from two of the three spin liquid states. On the other hand, the spin
liquid state near J_1 \approx J_2 appears to be more stable against monopole
condensation and could be a promising candidate for a spin liquid state in real
systems.Comment: revtex file 12 pages, 17 figure
TGF-β-induced growth inhibition in B-cell lymphoma correlates with Smad1/5 signalling and constitutively active p38 MAPK
<p>Abstract</p> <p>Background</p> <p>Cytokines of the transforming growth factor β (TGF-β) superfamily exert effects on proliferation, apoptosis and differentiation in various cell types. Cancer cells frequently acquire resistance to the anti-proliferative signals of TGF-β, which can be due to mutations in proteins of the signalling cascade. We compared the TGF-β-related signalling properties in B-cell lymphoma cell lines that were sensitive or resistant to TGF-β-induced anti-proliferative effects.</p> <p>Results</p> <p>TGF-β sensitive cell lines expressed higher cell surface levels of the activin receptor-like kinase 5 (Alk-5), a TGF-β receptor type 1. The expression levels of the other TGF-β and bone morphogenetic protein receptors were comparable in the different cell lines. TGF-β-induced phosphorylation of Smad2 was similar in TGF-β sensitive and resistant cell lines. In contrast, activation of Smad1/5 was restricted to cells that were sensitive to growth inhibition by TGF-β. Moreover, with activin A we detected limited anti-proliferative effects, strong phosphorylation of Smad2, but no Smad1/5 phosphorylation. Up-regulation of the TGF-β target genes Id1 and Pai-1 was identified in the TGF-β sensitive cell lines. Constitutive phosphorylation of MAPK p38 was restricted to the TGF-β sensitive cell lines. Inhibition of p38 MAPK led to reduced sensitivity to TGF-β.</p> <p>Conclusions</p> <p>We suggest that phosphorylation of Smad1/5 is important for the anti-proliferative effects of TGF-β in B-cell lymphoma. Alk-5 was highly expressed in the sensitive cell lines, and might be important for signalling through Smad1/5. Our results indicate a role for p38 MAPK in the regulation of TGF-β-induced anti-proliferative effects.</p
Quantum fluctuations and glassy behavior: The case of a quantum particle in a random potential
In this paper we expand our previous investigation of a quantum particle
subject to the action of a random potential plus a fixed harmonic potential at
a finite temperature T. In the classical limit the system reduces to a
well-known ``toy'' model for an interface in a random medium. It also applies
to a single quantum particle like an an electron subject to random
interactions, where the harmonic potential can be tuned to mimic the effect of
a finite box. Using the variational approximation, or alternatively, the limit
of large spatial dimensions, together with the use the replica method, and are
able to solve the model and obtain its phase diagram in the
plane, where is the particle's mass. The phase diagram is similar to that
of a quantum spin-glass in a transverse field, where the variable
plays the role of the transverse field. The glassy phase is characterized by
replica-symmetry-breaking. The quantum transition at zero temperature is also
discussed.Comment: revised version, 23 pages, revtex, 5 postscript figures in a separate
file figures.u
Orientational order on curved surfaces - the high temperature region
We study orientational order, subject to thermal fluctuations, on a fixed
curved surface. We derive, in particular, the average density of zeros of
Gaussian distributed vector fields on a closed Riemannian manifold. Results are
compared with the density of disclination charges obtained from a Coulomb gas
model. Our model describes the disordered state of two dimensional objects with
orientational degrees of freedom, such as vector ordering in Langmuir
monolayers and lipid bilayers above the hexatic to fluid transition.Comment: final version, 13 Pages, 2 figures, uses iopart.cl
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