65 research outputs found
Fractal structures in systems made of small magnetic particles
We have found that in a system consisting of small magnetic particles a phenomenon related to the formation of fractal structures may arise. The fractal features may arise not only in the distribution of magnetic moments but also in their energy spectrum. The magnetization and the susceptibility of the system also display fractal characteristics. The multiple structures are associated with exponentially many locally stable minima in a highly complex energy landscape. The signature of these fractal structures can be experimentally detected by various methods
Critical fields and devil's staircase in superconducting ladders
We have determined the ground state for both a ladder array of Josephson
junctions and a ladder of thin superconducting wires. We find that the
repulsive interaction between vortices falls off exponentially with separation.
The fact that the interaction is short-range leads to novel phenomena. The
ground state vortex density exhibits a complete devil's staircase as the
applied magnetic field is increased, each step producing a pair of
metal-insulator transitions. The critical fields in the staircase are all
calculated analytically and depend only on the connectivity of the ladder and
the area of the elementary plaquette. In particular the normal square ladder
contains no vortices at all until the flux per plaquette reaches 0.5/sqrt{3}
flux quanta.Comment: 4 pages (Revtex), 3 postscript figure
Coherence of the lattice polarization in large-polaron motion
Main problems of the large polaron theory are considered. We demonstrate that the problem of searching the ground stationary state of a system of coupled fields with translation-invariant Hamiltonian can have a solution of the form f(r−vt), v→0, i.e., the solution with the spontaneously broken translational symmetry. Such a state can be a ground state of a large polaron in case of strong electron-phonon coupling when the spontaneous break of the translational symmetry results from the phonon vacuum deformation by the electric field of the charge carrier. The correctness of the classical representation of the polarization field in the theory of a strongly coupled large polaron is proved on the base of the theory of the quantum-coherent states of the phonon field. The use of this representation has enabled us to show that extremely high losses of the electron energy in dielectric parts of cold cathodes occurring when the carrier velocity is lower than the threshold for the single-phonon radiation are due to coherent phonon radiation by polarons like Cherenkov effect. It is this radiation that results in the predicted Thornber and Feynman dependence of the carrier steady-state velocity on the applied electric field strength. The coherent phonon radiation generated by polaron current can be detected in experiments on the neutrons scattering. The primary directions of the neutrons scattering depend on the polarons steady-state velocity and, hence, on the applied field strength. The coherent phonon radiation stemming from supersonic thermal motion of polarons causes a giant increase of the resistance in a corresponding temperature interval
Numerical simulation of antiferromagnetically coupled nanomagnets
We study the dynamical behaviour of a system that consists of three identical elongated nanomagnets. The magnets are coupled antiferromagnetically and subjected to periodically changing external magnetic field. The numerical simulation of the system reveals the qualitatively different kinds of hysteresis loops
Scaling Properties of the Two-Chain Model
Scaling properties of a self-dual field-theoretical model, describing two
weakl$spinless Luttinger chains, are studied. A crossover to a sine-Gordon
massive phase, with strongly developed two-particleinterchain correlations, is
described. It is argued that, in a wide range of the in-chain interaction,
renormalization of the interchain hopping amplitude is determined by the
Luttinger liquid effects.Comment: 14 pages Latex, accepted Physics Letters
Planar isotropic two-phase systemsin perpendicular magnetic field: effective conductivity
Three explicit approximate expressions for the effective conductivity sigma_e
of various planar isotropic two-phase systems in a magnetic field are obtained
using the dual linear fractional transformation, connecting sigma_e of these
systems with and without magnetic field. The obtained results are applicable
for two-phase systems (regular and nonregular as well as random), satisfying
the symmetry and self-duality conditions, and allow to describe sigma_e of
various two-dimensional and layered inhomogeneous media at arbitrary phase
concentrations and magnetic fields. All these results admit a direct
experimental checking.Comment: 10 pages, Latex2e, 3 figure
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Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study
Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors
Duality and exact results for conductivity of 2D isotropic heterophase systems in magnetic field
Using a fact that the effective conductivity sigma_{e} of 2D random
heterophase systems in the orthogonal magnetic field is transformed under some
subgroup of the linear fractional group, connected with a group of linear
transformations of two conserved currents, the exact values for sigma_{e} of
isotropic heterophase systems are found. As known, for binary (N=2) systems a
determination of exact values of both conductivities (diagonal sigma_{ed} and
transverse Hall sigma_{et}) is possible only at equal phase concentrations and
arbitrary values of partial conductivities. For heterophase (N > 2) systems
this method gives exact values of effective conductivities, when their partial
conductivities belong to some hypersurfaces in the space of these partial
conductivities and the phase concentrations are pairwise equal. In all these
cases sigma_e does not depend on phase concentrations. The complete,
3-parametric, explicit transformation, connecting sigma_e in binary systems
with a magnetic field and without it, is constructedComment: 15 pages, 3 figures, Latex2
Large linear magnetoresistivity in strongly inhomogeneous planar and layered systems
Explicit expressions for magnetoresistance of planar and layered strongly
inhomogeneous two-phase systems are obtained, using exact dual transformation,
connecting effective conductivities of in-plane isotropic two-phase systems
with and without magnetic field. These expressions allow to describe the
magnetoresistance of various inhomogeneous media at arbitrary concentrations
and magnetic fields . All expressions show large linear
magnetoresistance effect with different dependencies on the phase
concentrations. The corresponding plots of the - and -dependencies of
are represented for various values, respectively, of magnetic field
and concentrations at some values of inhomogeneity parameter. The obtained
results show a remarkable similarity with the existing experimental data on
linear magnetoresistance in silver chalcogenides A possible
physical explanation of this similarity is proposed. It is shown that the
random, stripe type, structures of inhomogeneities are the most suitable for a
fabrication of magnetic sensors and a storage of information at room
temperatures.Comment: 12 pages, 2 figures, Latex2
Second harmonics and compensation effect in ceramic superconductors
A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model the ceramic superconductors. The
nonlinear ac susceptibility and the compensation effect are studied by Monte
Carlo simulations in this model. The compensation effect is shown to be due to
the existence of the chiral glass phase. We demonstrate, in agreement with
experiments, that this effect may be present in the ceramic superconductors
which show the paramagnetic Meissner effect.Comment: 6 pages, latex, 4 figures, Phys. Rev. B (accepted
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