127 research outputs found
Elevated temperature clears chytrid fungus infections from tadpoles of the midwife toad, Alytes obstetricans
The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) is sensitive to high temperature. Hence, exposing amphibians to high temperature may be a method to clear Bd infection. However, the effect of exposure to elevated
temperature has never been tested in larval stages or temperate species.We experimentally exposed tadpoles of the toad Alytes obstetricans to low, medium and high temperatures and found that most, but not all, tadpoles lost the infection when exposed to temperatures higher than 26◦C for 5 days. Thus, exposure to elevated temperatures can be used to treat tadpoles against Bd infection
Exponential Decay of Correlations in a Model for Strongly Disordered 2D Nematic Elastomers
Lattice Monte-Carlo simulations were performed to study the equilibrium
ordering in a two-dimensional nematic system with quenched random disorder.
When the disordering field, which competes against the aligning effect of the
Frank elasticity, is sufficiently strong, the long-range correlation of the
director orientation is found to decay as a simple exponential, Exp[-r/x]. The
correlation length {x} itself also decays exponentially with increasing
strength of the disordering field. This result represents a new type of
behavior, distinct from the Gaussian and power-law decays predicted by some
theories.Comment: Latex file (4 pages) + 2 EPS figure
Plasticity and memory effects in the vortex solid phase of twinned YBa2Cu3O7 single crystals
We report on marked memory effects in the vortex system of twinned YBa2Cu3O7
single crystals observed in ac susceptibility measurements. We show that the
vortex system can be trapped in different metastable states with variable
degree of order arising in response to different system histories. The pressure
exerted by the oscillating ac field assists the vortex system in ordering,
locally reducing the critical current density in the penetrated outer zone of
the sample. The robustness of the ordered and disordered states together with
the spatial profile of the critical current density lead to the observed memory
effects
Peak effect, vortex-lattice melting-line and order - disorder transition in conventional and high-T superconductors
We investigate the order - disorder transition line from a Bragg glass to an
amorphous vortex glass in the H-T phase diagram of three-dimensional type-II
superconductors with account of both pinning-caused and thermal fluctuations of
the vortex lattice. Our approach is based on the Lindemann criterion and on
results of the collective pinning theory and generalizes previous work of other
authors. It is shown that the shapes of the order - disorder transition line
and the vortex lattice melting curve are determined only by the Ginzburg
number, which characterizes thermal fluctuations, and by a parameter which
describes the strength of the quenched disorder in the flux-line lattice. In
the framework of this unified approach we obtain the H-T phase diagrams for
both conventional and high-Tc superconductors. Several well-known experimental
results concerning the fishtail effect and the phase diagram of high-Tc
superconductors are naturally explained by assuming that a peak effect in the
critical current density versus H signalizes the order - disorder transition
line in superconductors with point defects.Comment: 15 pages including 11 figure
Slow stress relaxation in randomly disordered nematic elastomers and gels
Randomly disordered (polydomain) liquid crystalline elastomers align under
stress. We study the dynamics of stress relaxation before, during and after the
Polydomain-Monodomain transition. The results for different materials show the
universal ultra-slow logarithmic behaviour, especially pronounced in the region
of the transition. The data is approximated very well by an equation Sigma(t) ~
Sigma_{eq} + A/(1+ Alpha Log[t]). We propose a theoretical model based on the
concept of cooperative mechanical resistance for the re-orientation of each
domain, attempting to follow the soft-deformation pathway. The exact model
solution can be approximated by compact analytical expressions valid at short
and at long times of relaxation, with two model parameters determined from the
data.Comment: 4 pages (two-column), 5 EPS figures (included via epsfig
History effects and pinning regimes in solid vortex matter
We propose a phenomenological model that accounts for the history effects
observed in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys.
Rev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the
model is the assumption that the penetrating ac magnetic field modifies the
vortex lattice mobility, trapping different robust dynamical states in
different regions of the sample. We discuss in detail on the response of the
superconductor to an ac magnetic field when the vortex lattice mobility is not
uniform inside the sample. We begin with an analytical description for a simple
geometry (slab) and then we perform numerical calculations for a strip in a
transverse magnetic field which include relaxation effects. In calculations,
the vortex system is assumed to coexist in different pinning regimes. The
vortex behavior in the regions where the induced current density j has been
always below a given threshold (j_c^>) is described by an elastic Campbell-like
regime (or a critical state regime with local high critical current density,
j_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the
critical current density is modified to j_c^) at
regions where vortices have been forced to oscillate by a current density
larger than j_c^>. Experimentally, an initial state with high critical current
density (j_c^>) can be obtained by zero field cooling, field cooling (with no
applied ac field) or by shaking the vortex lattice with an asymmetrical (e.g.
sawtooth) field. We compare our calculations with experimental ac
susceptibility results in YBa2Cu3O7 single crystals.Comment: 11 pages, 7 figures. To be published in PR
Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals
We report on transport and ac susceptibility studies below the peak effect in
twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak
effect can be partially inhibited by forcing vortices to move with an ac
driving current. The vortex system can be additionally ordered below a
well-defined temperature where elastic interactions between vortices overcome
pinning-generated stress and a plastic to elastic crossover seems to occur. The
combined effect of these two processes results in vortex structures with
different mobilities that give place to history effects.Comment: 4 pages, 4 figures. Published in PRB Rapid Comm., February 1, 200
Low field vortex dynamics over seven time decades in a Bi_2Sr_2CaCu_2O_{8+\delta} single crystal for temperatures 13 K < T < 83 K
Using a custom made dc-SQUID magnetometer, we have measured the time
relaxation of the remanent magnetization M_rem of a Bi_2Sr_2CaCu_2O_{8+\delta}
single crystal from the fully critical state for temperatures 13 K < T < 83 K.
The measurements cover a time window of seven decades 10^{-2} s < t < 10^5 s,
so that the current density j can be studied from values very close to j_c down
to values considerably smaller than j_c. From the data we have obtained: (i)
the flux creep activation barriers U as a function of current density j, (ii)
the current-voltage characteristics E(j) in a typical range of 10^{-7} V/cm to
10^{-15} V/cm, and (iii) the critical current density j_c(0) at T = 0. Three
different regimes of vortex dynamics are observed: For temperatures T < 20 K
the activation barrier U(j) is logarithmic, no unique functional dependence
U(j) could be found for the intermediate temperature interval 20 K < T < 40 K,
and finally for T > 40 K the activation barrier U(j) follows a power-law
behavior with an exponent mu = 0.6. From the analysis of the data within the
weak collective pinning theory for strongly layered superconductors, it is
argued that for temperatures T < 20 K pancake-vortices are pinned individually,
while for temperatures T > 40 K pinning involves large collectively pinned
vortex bundles. A description of the vortex dynamics in the intermediate
temperature interval 20 K < T < 40 K is given on the basis of a qualitative low
field phase diagram of the vortex state in Bi_2Sr_2CaCu_2O_{8+\delta}. Within
this description a second peak in the magnetization loop should occur for
temperatures between 20 K and 40 K, as it has been observed in several
magnetization measurements in the literature.Comment: 12 pages, 10 figure
Soft and non-soft structural transitions in disordered nematic networks
Properties of disordered nematic elastomers and gels are theoretically
investigated with emphasis on the roles of non-local elastic interactions and
crosslinking conditions. Networks originally crosslinked in the isotropic phase
lose their long-range orientational order by the action of quenched random
stresses, which we incorporate into the affine-deformation model of nematic
rubber elasticity. We present a detailed picture of mechanical quasi-Goldstone
modes, which accounts for an almost completely soft polydomain-monodomain (P-M)
transition under strain as well as a ``four-leaf clover'' pattern in
depolarized light scattering intensity. Dynamical relaxation of the domain
structure is studied using a simple model. The peak wavenumber of the structure
factor obeys a power-law-type slow kinetics and goes to zero in true mechanical
equilibrium. The effect of quenched disorder on director fluctuation in the
monodomain state is analyzed. The random frozen contribution to the fluctuation
amplitude dominates the thermal one, at long wavelengths and near the P-M
transition threshold. We also study networks obtained by crosslinking
polydomain nematic polymer melts. The memory of initial director configuration
acts as correlated and strong quenched disorder, which renders the P-M
transition non-soft. The spatial distribution of the elastic free energy is
strongly dehomogenized by external strain, in contrast to the case of
isotropically crosslinked networks.Comment: 19 pages, 15 EPS figure
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