350 research outputs found
De Haas-van Alphen effect in two- and quasi two-dimensional metals and superconductors
An analytical form of the quantum magnetization oscillations (de Haas-van
Alphen effect) is derived for two- and quasi two-dimensional metals in normal
and superconducting mixed states. The theory is developed under condition that
the chemical potential is much greater than the cyclotron frequency, which is
proved to be valid for using grand canonical ensemble in the systems of low
dimensionality. Effects of impurity, temperature, spin-splitting and vortex
lattice - in the case of superconductors of type II -, are taken into account.
Contrary to the three dimensional case, the oscillations in sufficiently pure
systems of low dimensionality and at sufficiently low temperatures are
characterized by a saw-tooth wave form, which smoothened with temperature and
concentration of impurities growth. In the normal quasi two-dimensional
systems, the expression for the magnetization oscillations includes an extra
factor expressed through the transfer integral between the layers. The
additional damping effect due to the vortex lattice is found. The criterion of
proximity to the upper critical field for the observation of de Haas-van Alphen
effect in the superconducting mixed state is established.Comment: 18 pages, Latex, revised versio
Quantum oscillations in graphene in the presence of disorder and interactions
Quantum oscillations in graphene is discussed. The effect of interactions are
addressed by Kohn's theorem regarding de Haas-van Alphen oscillations, which
states that electron-electron interactions cannot affect the oscillation
frequencies as long as disorder is neglected and the system is sufficiently
screened, which should be valid for chemical potentials not very close to the
Dirac point. We determine the positions of Landau levels in the presence of
potential disorder from exact transfer matrix and finite size diagonalization
calculations. The positions are shown to be unshifted even for moderate
disorder; stronger disorder, can, however, lead to shifts, but this also
appears minimal even for disorder width as large as one-half of the bare
hopping matrix element on the graphene lattice. Shubnikov-de Haas oscillations
of the conductivity are calculated analytically within a self-consistent Born
approximation of impurity scattering. The oscillatory part of the conductivity
follows the widely invoked Lifshitz-Kosevich form when certain mass and
frequency parameters are properly interpreted.Comment: Appendix A was removed, as the content of it is already contained in
Ref. 17. Thanks to M. A. H. Vozmedian
Intrinsic optical bistability of thin films of linear molecular aggregates: The one-exciton approximation
We perform a theoretical study of the nonlinear optical response of an
ultrathin film consisting of oriented linear aggregates. A single aggregate is
described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder.
The exciton wave functions and energies are found exactly by numerically
diagonalizing the Hamiltonian. The principal restriction we impose is that only
the optical transitions between the ground state and optically dominant states
of the one-exciton manifold are considered, whereas transitions to other
states, including those of higher exciton manifolds, are neglected. The optical
dynamics of the system is treated within the framework of truncated optical
Maxwell-Bloch equations in which the electric polarization is calculated by
using a joint distribution of the transition frequency and the transition
dipole moment of the optically dominant states. This function contains all the
statistical information about these two quantities that govern the optical
response, and is obtained numerically by sampling many disorder realizations.
We derive a steady-state equation that establishes a relationship between the
output and input intensities of the electric field and show that within a
certain range of the parameter space this equation exhibits a three-valued
solution for the output field. A time-domain analysis is employed to
investigate the stability of different branches of the three-valued solutions
and to get insight into switching times. We discuss the possibility to
experimentally verify the bistable behavior.Comment: 13 two-column pages, 8 figures, accepted to the Journal of Chemical
Physic
Observation of the March Maximum in the Daemon Flux from Neos in the Year 2005: New Efforts and New Effects
The experiments of 2005 aimed at detection of low-velocity (~10-15 km s-1)
daemons falling on to the Earth's surface from Near-Earth, Almost Circular
Heliocentric Orbits (NEACHOs) have corroborated once more the existence of the
March maximum in their flux by raising its confidence level to 99.99%. In
addition, these experiments permitted us to identify several FEU-167-1-type PM
tubes, with a few times thicker inner Al coating, which appear to be capable to
detect, without any scintillator, the crossing of negatively charged daemons.
As a result, detection efficiency increases tens of times, thus raising the
measured level of the March daemon flux to f > 0.5E-7 cm-2s-1.Comment: 14 page
Spectrum of an open disordered quasi-two-dimensional electron system: strong orbital effect of the weak in-plane magnetic field
The effect of an in-plane magnetic field upon open quasi-two-dimensional
electron and hole systems is investigated in terms of the carrier ground-state
spectrum. The magnetic field, classified as weak from the viewpoint of
correlation between size parameters of classical electron motion and the gate
potential spatial profile is shown to efficiently cut off extended modes from
the spectrum and to change singularly the mode density of states (MDOS). The
reduction in the number of current-carrying modes, right up to zero in magnetic
fields of moderate strength, can be viewed as the cause of
magnetic-field-driven metal-to-insulator transition widely observed in
two-dimensional systems. Both the mode number reduction and the MDOS
singularity appear to be most pronounced in the mode states dephasing
associated with their scattering by quenched-disorder potential. This sort of
dephasing is proven to dominate the dephasing which involves solely the
magnetic field whatever level of the disorder.Comment: RevTeX-4 class, 12 pages, 5 eps figure
Relativistic diffusion
We discuss a relativistic diffusion in the proper time in an approach of
Schay and Dudley. We derive (Langevin) stochastic differential equations in
various coordinates.We show that in some coordinates the stochastic
differential equations become linear. We obtain momentum probability
distribution in an explicit form.We discuss a relativistic particle diffusing
in an external electromagnetic field. We solve the Langevin equations in the
case of parallel electric and magnetic fields. We derive a kinetic equation for
the evolution of the probability distribution.We discuss drag terms leading to
an equilibrium distribution.The relativistic analog of the Ornstein-Uhlenbeck
process is not unique. We show that if the drag comes from a diffusion
approximation to the master equation then its form is strongly restricted. The
drag leading to the Tsallis equilibrium distribution satisfies this restriction
whereas the one of the Juettner distribution does not. We show that any
function of the relativistic energy can be the equilibrium distribution for a
particle in a static electric field. A preliminary study of the time evolution
with friction is presented. It is shown that the problem is equivalent to
quantum mechanics of a particle moving on a hyperboloid with a potential
determined by the drag. A relation to diffusions appearing in heavy ion
collisions is briefly discussed.Comment: 9 pages,some numerical factors correcte
Instability of Magnons in Two-dimensional Antiferromagnet at High Magnetic Fields
Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is
studied by a combination of bulk measurements, neutron diffraction, and
inelastic neutron scattering techniques. Easy plane type antiferromagnetic
order is identified at K. The exchange interactions are estimated
as = 27.8(3)eV and = 1.0(1) eV, and the saturation
field is 9.75 T. Magnetic excitation measurements with high
experimental resolution setup by triple axis neutron spectrometer reveals the
instability of one magnon excitation in the field range of .Comment: 5 pgase, 5 figuers, to be published in PRB R
A Theory of Magnets with Competing Double Exchange and Superexchange Interactions
We study the competition between ferromagnetic double exchange (DE) and
nearest-neighbour antiferromagnetic exchange in CMR materials. Towards this
end, a single site mean field theory is proposed which emphasizes the
hopping-mediated nature of the DE contribution. We find that the competition
between these two exchange interactions leads to ferro- or antiferromagnetic
order with incomplete saturation of the (sub)lattice magnetization. This
conclusion is in contrast to previous results in the literature which find a
canted spin arrangement under similar circumstances. We attribute this
difference to the highly anisotropic exchange interactions used elsewhere. The
associated experimental implications are discussed.Comment: 4 pages, Latex-Revtex, 3 PostScript figures. Please see report
cond-mat/980523
1D quantum models with correlated disorder vs. classical oscillators with coloured noise
We perform an analytical study of the correspondence between a classical
oscillator with frequency perturbed by a coloured noise and the one-dimensional
Anderson-type model with correlated diagonal disorder. It is rigorously shown
that localisation of electronic states in the quantum model corresponds to
exponential divergence of nearby trajectories of the classical random
oscillator. We discuss the relation between the localisation length for the
quantum model and the rate of energy growth for the stochastic oscillator.
Finally, we examine the problem of electron transmission through a finite
disordered barrier by considering the evolution of the classical oscillator.Comment: 23 pages, LaTeX fil
Multifractals Competing with Solitons on Fibonacci Optical Lattice
We study the stationary states for the nonlinear Schr\"odinger equation on
the Fibonacci lattice which is expected to be realized by Bose-Einstein
condensates loaded into an optical lattice. When the model does not have a
nonlinear term, the wavefunctions and the spectrum are known to show fractal
structures. Such wavefunctions are called critical. We present a phase diagram
of the energy spectrum for varying the nonlinearity. It consists of three
portions, a forbidden region, the spectrum of critical states, and the spectrum
of stationary solitons. We show that the energy spectrum of critical states
remains intact irrespective of the nonlinearity in the sea of a large number of
stationary solitons.Comment: 5 pages, 4 figures, major revision, references adde
- …