1,162 research outputs found
Measurement of the vortex-core radius by scanning tunneling microscopy
Using a scanning tunneling microscope operated in a spectroscopic mode we imaged flux-line lattices in niobium diselenide at various external magnetic fields. From the evaluation of a large number of tunneling-current profiles taken across the individual vortices we deduced the dependence of the vortex-code radius on the applied magnetic field. It was found that the core radius shows a pronounced decrease with increasing field, even for H/Hc2<<1. This behavior is qualitatively well characterized by self-consistent solutions of the Usadel equations
Low-energy excitations in the three-dimensional random-field Ising model
The random-field Ising model (RFIM), one of the basic models for quenched
disorder, can be studied numerically with the help of efficient ground-state
algorithms. In this study, we extend these algorithm by various methods in
order to analyze low-energy excitations for the three-dimensional RFIM with
Gaussian distributed disorder that appear in the form of clusters of connected
spins. We analyze several properties of these clusters. Our results support the
validity of the droplet-model description for the RFIM.Comment: 10 pages, 9 figure
Generating droplets in two-dimensional Ising spin glasses by using matching algorithms
We study the behavior of droplets for two dimensional Ising spin glasses with
Gaussian interactions. We use an exact matching algorithm which enables study
of systems with linear dimension L up to 240, which is larger than is possible
with other approaches. But the method only allows certain classes of droplets
to be generated. We study single-bond, cross and a category of fixed volume
droplets as well as first excitations. By comparison with similar or equivalent
droplets generated in previous works, the advantages but also the limitations
of this approach are revealed. In particular we have studied the scaling
behavior of the droplet energies and droplet sizes. In most cases, a crossover
of the data can be observed such that for large sizes the behavior is
compatible with the one-exponent scenario of the droplet theory. Only for the
case of first excitations, no clear conclusion can be reached, probably because
even with the matching approach the accessible system sizes are still too
small.Comment: 11 pages, 16 figures, revte
Perturbation theories for the S=1/2 spin ladder with four-spin ring exchange
The isotropic S=1/2 antiferromagnetic spin ladder with additional four-spin
ring exchange is studied perturbatively in the strong coupling regime with the
help of cluster expansion technique, and by means of bosonization in the weak
coupling limit. It is found that a sufficiently large strength of ring exchange
leads to a second-order phase transition, and the shape of the boundary in the
vicinity of the known exact transition point is obtained. The critical exponent
for the gap is found to be , in agreement both with exact results
available for the dimer line and with the bosonization analysis. The phase
emerging for high values of the ring exchange is argued to be gapped and
spontaneously dimerized. The results for the transition line from strong
coupling and from weak coupling match with each other naturally.Comment: 8 pages, 4 figures, some minor changes in text and reference
Electron self-trapping on a nano-circle
We study the self-trapping of quasiparticles (electrons, holes, excitons,
etc) in a molecular chain with the structure of a ring, taking into account the
electron-phonon interaction and the radial and tangential deformations of the
chain. A discrete system of equations is obtained and solved numerically. The
analytical solutions for the wave function of a quasiparticle and for the
molecule displacements that determine the distortion of the ring, are also
obtained and solved in the continuum approximation. The numerical solutions of
the system of discrete nonlinear equations reveals several regimes of
quasiparticle localisation in the chain which depend on the values of the
parameters of the system. It is shown that the transversal deformation of the
chain favours the formation of a soliton.Comment: 43 pages 9 figure
T=0 Phase Diagram of the Double-Exchange Model
We present the T=0 phase diagram of the double-exchange model (ferromagnetic
Kondo lattice model) for all values of the carrier concentration and Hund's
couplng , within dynamical mean field theory. We find that depending on the
values of and , the ground state is either a ferromagnet, a commensurate
antiferromagnet or some other incommensurate phase with intermediate wave
vectors . The antiferromagnetic phase is separated by first order phase
boundaries and wide regimes of phase separation. The transition from the
ferromagnetic phase to an incommensurate phase is second order.Comment: 4 pages, 5 figures. The analysis now includes incommensurate phases
with arbitrary wave vectors. Correspondingly, the figures have been change
Percolation in three-dimensional random field Ising magnets
The structure of the three-dimensional random field Ising magnet is studied
by ground state calculations. We investigate the percolation of the minority
spin orientation in the paramagnetic phase above the bulk phase transition,
located at [Delta/J]_c ~= 2.27, where Delta is the standard deviation of the
Gaussian random fields (J=1). With an external field H there is a disorder
strength dependent critical field +/- H_c(Delta) for the down (or up) spin
spanning. The percolation transition is in the standard percolation
universality class. H_c ~ (Delta - Delta_p)^{delta}, where Delta_p = 2.43 +/-
0.01 and delta = 1.31 +/- 0.03, implying a critical line for Delta_c < Delta <=
Delta_p. When, with zero external field, Delta is decreased from a large value
there is a transition from the simultaneous up and down spin spanning, with
probability Pi_{uparrow downarrow} = 1.00 to Pi_{uparrow downarrow} = 0. This
is located at Delta = 2.32 +/- 0.01, i.e., above Delta_c. The spanning cluster
has the fractal dimension of standard percolation D_f = 2.53 at H = H_c(Delta).
We provide evidence that this is asymptotically true even at H=0 for Delta_c <
Delta <= Delta_p beyond a crossover scale that diverges as Delta_c is
approached from above. Percolation implies extra finite size effects in the
ground states of the 3D RFIM.Comment: replaced with version to appear in Physical Review
Parasitic nematodes exert antimicrobial activity and benefit from microbiota-driven support for host immune regulation
Intestinal parasitic nematodes live in intimate contact with the host microbiota. Changes in the microbiome composition during nematode infection affect immune control of the parasites and shifts in the abundance of bacterial groups have been linked to the immunoregulatory potential of nematodes. Here we asked if the small intestinal parasite Heligmosomoides polygyrus produces factors with antimicrobial activity, senses its microbial environment and if the anti-nematode immune and regulatory responses are altered in mice devoid of gut microbes. We found that H. polygyrus excretory/secretory products exhibited antimicrobial activity against gram(+/-) bacteria. Parasites from germ-free mice displayed alterations in gene expression, comprising factors with putative antimicrobial functions such as chitinase and lysozyme. Infected germ-free mice developed increased small intestinal Th2 responses coinciding with a reduction in local Foxp3(+)RORγt(+) regulatory T cells and decreased parasite fecundity. Our data suggest that nematodes sense their microbial surrounding and have evolved factors that limit the outgrowth of certain microbes. Moreover, the parasites benefit from microbiota-driven immune regulatory circuits, as an increased ratio of intestinal Th2 effector to regulatory T cells coincides with reduced parasite fitness in germ-free mice
Linewidth of single photon transitions in Mn-acetate
We use time-domain terahertz spectroscopy to measure the position and
linewidth of single photon transitions in Mn-acetate. This linewidth is
compared to the linewidth measured in tunneling experiments. We conclude that
local magnetic fields (due to dipole or hyperfine interactions) cannot be
responsible for the observed linewidth, and suggest that the linewidth is due
to variations in the anisotropy constants for different clusters. We also
calculate a lower limit on the dipole field distribution that would be expected
due to random orientations of clusters and find that collective effects must
narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review
Vortex Interactions and Thermally Induced Crossover from Type-I to Type-II Superconductivity
We have computed the effective interaction between vortices in the
Ginzburg-Landau model from large-scale Monte-Carlo simulations, taking thermal
fluctuations of matter fields and gauge fields fully into account close to the
critical temperature. We find a change, in the form of a crossover, from
attractive to repulsive effective vortex interactions in an intermediate range
of Ginzburg-Landau parameters upon increasing
the temperature in the superconducting state. This corresponds to a thermally
induced crossover from \typeI to \typeII superconductivity around a temperature
, which we map out in the vicinity of the
metal-to-superconductor transition. In order to see this crossover, it is
essential to include amplitude fluctuations of the matter field, in addition to
phase-fluctuations and gauge-field fluctuations. We present a simple physical
picture of the crossover, and relate it to observations in \metal{Ta} and
\metal{Nb} elemental superconductors which have low-temperature values of
in the relevant range.Comment: 9 pages, 6 figures. Accepted for publication in Physical Review
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