66 research outputs found
Barkhausen Noise and Critical Scaling in the Demagnetization Curve
The demagnetization curve, or initial magnetization curve, is studied by
examining the embedded Barkhausen noise using the non-equilibrium, zero
temperature random-field Ising model. The demagnetization curve is found to
reflect the critical point seen as the system's disorder is changed. Critical
scaling is found for avalanche sizes and the size and number of spanning
avalanches. The critical exponents are derived from those related to the
saturation loop and subloops. Finally, the behavior in the presence of long
range demagnetizing fields is discussed. Results are presented for simulations
of up to one million spins.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Average shape of fluctuations for subdiffusive walks
We study the average shape of fluctuations for subdiffusive processes, i.e.,
processes with uncorrelated increments but where the waiting time distribution
has a broad power-law tail. This shape is obtained analytically by means of a
fractional diffusion approach. We find that, in contrast with processes where
the waiting time between increments has finite variance, the fluctuation shape
is no longer a semicircle: it tends to adopt a table-like form as the
subdiffusive character of the process increases. The theoretical predictions
are compared with numerical simulation results.Comment: 4 pages, 6 figures. Accepted for publication Phys. Rev. E (Replaced
for the latest version, in press.) Section II rewritte
Time-dependent Gutzwiller theory of magnetic excitations in the Hubbard model
We use a spin-rotational invariant Gutzwiller energy functional to compute
random-phase-approximation-like (RPA) fluctuations on top of the Gutzwiller
approximation (GA). The method can be viewed as an extension of the previously
developed GA+RPA approach for the charge sector [G. Seibold and J. Lorenzana,
Phys. Rev. Lett. {\bf 86}, 2605 (2001)] with respect to the inclusion of the
magnetic excitations. Unlike the charge case, no assumptions about the time
evolution of the double occupancy are needed in this case. Interestingly, in a
spin-rotational invariant system, we find the correct degeneracy between
triplet excitations, showing the consistency of both computations. Since no
restrictions are imposed on the symmetry of the underlying saddle-point
solution, our approach is suitable for the evaluation of the magnetic
susceptibility and dynamical structure factor in strongly correlated
inhomogeneous systems. We present a detailed study of the quality of our
approach by comparing with exact diagonalization results and show its much
higher accuracy compared to the conventional Hartree-Fock+RPA theory. In
infinite dimensions, where the GA becomes exact for the Gutzwiller variational
energy, we evaluate ferromagnetic and antiferromagnetic instabilities from the
transverse magnetic susceptibility. The resulting phase diagram is in complete
agreement with previous variational computations.Comment: 12 pages, 8 figure
Evolution of avalanche conducting states in electrorheological liquids
Charge transport in electrorheological fluids is studied experimentally under
strongly nonequlibrium conditions. By injecting an electrical current into a
suspension of conducting nanoparticles we are able to initiate a process of
self-organization which leads, in certain cases, to formation of a stable
pattern which consists of continuous conducting chains of particles. The
evolution of the dissipative state in such system is a complex process. It
starts as an avalanche process characterized by nucleation, growth, and thermal
destruction of such dissipative elements as continuous conducting chains of
particles as well as electroconvective vortices. A power-law distribution of
avalanche sizes and durations, observed at this stage of the evolution,
indicates that the system is in a self-organized critical state. A sharp
transition into an avalanche-free state with a stable pattern of conducting
chains is observed when the power dissipated in the fluid reaches its maximum.
We propose a simple evolution model which obeys the maximum power condition and
also shows a power-law distribution of the avalanche sizes.Comment: 15 pages, 6 figure
Frozen spatial chaos induced by boundaries
We show that rather simple but non-trivial boundary conditions could induce
the appearance of spatial chaos (that is stationary, stable, but spatially
disordered configurations) in extended dynamical systems with very simple
dynamics. We exemplify the phenomenon with a nonlinear reaction-diffusion
equation in a two-dimensional undulated domain. Concepts from the theory of
dynamical systems, and a transverse-single-mode approximation are used to
describe the spatially chaotic structures.Comment: 9 pages, 6 figures, submitted for publication; for related work visit
http://www.imedea.uib.es/~victo
Simplifying superstring and D-brane actions in AdS(4) x CP(3) superbackground
By making an appropriate choice for gauge fixing kappa-symmetry we obtain a
relatively simple form of the actions for a D=11 superparticle in AdS(4) x
S(7)/Z_k, and for a D0-brane, fundamental string and D2-branes in the AdS(4) x
CP(3) superbackground. They can be used to study various problems of string
theory and the AdS4/CFT3 correspondence, especially in regions of the theory
which are not reachable by the OSp(6|4)/U(3) x SO(1,3) supercoset sigma-model.
In particular, we present a simple form of the gauge-fixed superstring action
in AdS(4) x CP(3) and briefly discuss issues of its T-dualization.Comment: 1+36 pages, v2,v3 clarifications and references adde
Semiclassical strings in AdS(3) X S^2
In this paper, we investigate the semiclassical strings in AdS(3)XS^2, in
which the string configuration of AdS(3) is classified to three cases depending
on the parameters. Each of these has a different anomalous dimension
proportional to logS, S^(1/3) and S, where S is a angular momentum on AdS(3).
Further we generalize the dispersion relations for various string configuration
on AdS(3)XS^2.Comment: 15 pages, added reference
Review of AdS/CFT Integrability, Chapter IV.3: N=6 Chern-Simons and Strings on AdS4xCP3
We review the duality and integrability of N=6 superconformal Chern-Simons
theory in three dimensions and IIA superstring theory on the background
AdS4xCP3. We introduce both of these models and describe how their degrees of
freedom are mapped to excitations of a long-range integrable spin-chain.
Finally, we discuss the properties of the Bethe equations, the S-matrix and the
algebraic curve that are special to this correspondence and differ from the
case of N=4 SYM theory and strings on AdS5xS5.Comment: 22 pages, see also overview article arXiv:1012.3982, v2: references
to other chapters updated, v3: references added, v4: brief discussion of
giant magnons added, further minor changes, published version, v5: union of
v3 and v4 because changes made in v3 were accidentally lost in v
Z_3 Quantum Criticality in a spin-1/2 chain model
The stability of the magnetization plateau phase of the XXZ spin-1/2
Heisenberg chain with competing interactions is investigated upon switching on
a staggered transverse magnetic field. Within a bosonization approach, it is
shown that the low-energy properties of the model are described by an effective
two-dimensional XY model in a three-fold symmetry-breaking field. A phase
transition in the three-state Potts universality class is expected separating
the plateau phase to a phase where the spins are polarized along the
staggered magnetic field. The Z critical properties of the transition are
determined within the bosonization approach.Comment: 5 pages, revised versio
Worldsheet spectrum in AdS(4)/CFT(3) correspondence
The AdS(4)/CFT(3) duality is a new example of an integrable and exactly
solvable AdS/CFT system. There is, however, a puzzling mismatch between the
number of degrees of freedom used in the exact solution (4B+4F scattering
states) and 8B+8F transverse oscillation modes of critical superstring theory.
We offer a resolution of this puzzle by arguing that half of the string modes
dissolve in the continuum of two-particle states once alpha' corrections are
taken into account. We also check that the conjectured exact S-matrix of
AdS(4)/CFT(3) agrees with the tree-level worldsheet calculation.Comment: 19 pages, 2 figures; v2: misprints in (2.3), (2.11) and (A.1)
corrected, a reference added; v3: misprints in (B.6), (B.9) and (B.12)
corrected; v4: footnote 4 adde
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