1,780 research outputs found
Self-organized circular flow of classical point particles
We consider newtonian dynamics of charged particles on the circle with
nearest neigbour interaction with Coulomb repulsive potential . Also
there is an external accelerating force which is nonzero only on a small part
of the circle. We construct homogeneous solutions where the velocities of all
particles are approximately equal and their density is approximately uniform.
This gives a qualitative mathematical model for some features of the direct
electric current (DC), in agreement with a suggestion by R. Feynman
Temperature dependent correlations in covalent insulators
Motivated by the peculiar behavior of FeSi and FeSb2 we study the effect of
local electronic correlations on magnetic, transport and optical properties in
a specific type of band insulator, namely a covalent insulator. Investigating a
minimum model of covalent insulator within a single-site dynamical mean-field
approximation we are able to obtain the crossover from low temperature
non-magnetic insulator to high-temperature paramagnetic metal with parameters
realistic for FeSi and FeSb2 systems. Our results show that the behavior of
FeSi does not imply microscopic description in terms of Kondo insulator
(periodic Anderson model) as can be often found in the literature, but in fact
reflects generic properties of a broader class of materials.Comment: 4 pages, 4 figure
Theoretical Analysis of Electronic and Magnetic Properties of NaVO: Crucial Role of the Orbital Degrees of Freedom
Using realistic low-energy model with parameters derived from the
first-principles electronic structure calculation, we address the origin of the
quasi-one-dimensional behavior in orthorhombic NaVO, consisting of the
double chains of edge-sharing VO octahedra. We argue that the geometrical
aspect alone does not explain the experimentally observed anisotropy of
electronic and magnetic properties of NaVO. Instead, we attribute the
unique behavior of NaVO to one particular type of the orbital ordering,
which respects the orthorhombic symmetry. This orbital ordering acts to
divide all states into two types: the `localized' ones, which are
antisymmetric with respect to the mirror reflection , and
the symmetric `delocalized' ones. Thus, NaVO can be classified as the
double exchange system. The directional orientation of symmetric orbitals,
which form the metallic band, appears to be sufficient to explain both
quasi-one-dimensional character of interatomic magnetic interactions and the
anisotropy of electrical resistivity.Comment: 16 pages, 4 figure
Orbital Magnetization of Quantum Spin Hall Insulator Nanoparticles
Both spin and orbital degrees of freedom contribute to the magnetic moment of
isolated atoms. However, when inserted in crystals, atomic orbital moments are
quenched because of the lack of rotational symmetry that protects them when
isolated. Thus, the dominant contribution to the magnetization of magnetic
materials comes from electronic spin. Here we show that nanoislands of quantum
spin Hall insulators can host robust orbital edge magnetism whenever their
highest occupied Kramers doublet is singly occupied, upgrading the spin edge
current into a charge current. The resulting orbital magnetization scales
linearly with size, outweighing the spin contribution for islands of a few nm
in size. This linear scaling is specific of the Dirac edge states and very
different from Schrodinger electrons in quantum rings. Modelling Bi(111)
flakes, whose edge states have been recently observed, we show that orbital
magnetization is robust with respect to disorder, thermal agitation, shape of
the island and crystallographic direction of the edges, reflecting its
topological protection.Comment: 7 pages, 5 figures, + Supporting Informatio
Bi(111) thin film with insulating interior but metallic surfaces
The electrical conductance of molecular beam epitaxial Bi on BaF2(111) was
measured as a function of both film thickness (4-540 nm) and temperature (5-300
K). Unlike bulk Bi as a prototype semimetal, the Bi thin films up to 90 nm are
found to be insulating in the interiors but metallic on the surfaces. This
result has not only resolved unambiguously the long controversy about the
existence of semimetal-semiconductor transition in Bi thin film but also
provided a straightforward interpretation for the long-puzzled temperature
dependence of the resistivity of Bi thin films, which in turn might suggest
some potential applications in spintronics
Electrostatic tuning of magnetism at the conducting (111) (LaSr)(AlTa)/SrTiO interface
We present measurements of the low temperature electrical transport
properties of the two dimensional carrier gas that forms at the interface of
(LaSr)(AlTa)/SrTiO (LSAT/STO) as
a function of applied back gate voltage, . As is found in (111)
LaAlO/SrTiO interfaces, the low-field Hall coefficient is
electron-like, but shows a sharp reduction in magnitude below 20 V,
indicating the presence of hole-like carriers in the system. This same value of
correlates approximately with the gate voltage below which the
magnetoresistance evolves from nonhysteretic to hysteretic behavior at
millikelvin temperatures, signaling the onset of magnetic order in the system.
We believe our results can provide insight into the mechanism of magnetism in
SrTiO based systems.Comment: 5 pages, 3 figure
Relativistic Klein-Gordon-Maxwell multistream model for quantum plasmas
A multistream model for spinless electrons in a relativistic quantum plasma
is introduced by means of a suitable fluid-like version of the
Klein-Gordon-Maxwell system. The one and two-stream cases are treated in
detail. A new linear instability condition for two-stream quantum plasmas is
obtained, generalizing the previously known non-relativistic results. In both
the one and two-stream cases, steady-state solutions reduce the model to a set
of coupled nonlinear ordinary differential equations, which can be numerically
solved, yielding a manifold of nonlinear periodic and soliton structures. The
validity conditions for the applicability of the model are addressed
Theory of Nonlinear Matter Waves in Optical Lattices
We consider several effects of the matter wave dynamics which can be observed
in Bose-Einstein condensates embedded into optical lattices. For low-density
condensates we derive approximate evolution equations, the form of which
depends on relation among the main spatial scales of the system. Reduction of
the Gross-Pitaevskii equation to a lattice model (the tight-binding
approximation) is also presented. Within the framework of the obtained models
we consider modulational instability of the condensate, solitary and periodic
matter waves, paying special attention to different limits of the solutions,
i.e. to smooth movable gap solitons and to strongly localized discrete modes.
We also discuss how the Feshbach resonance, a linear force, and lattice defects
affect the nonlinear matter waves.Comment: Modern Physics Letters B (invited brief review), 25 pages, 9 figure
Localized basis sets for unbound electrons in nanoelectronics
It is shown how unbound electron wave functions can be expanded in a suitably
chosen localized basis sets for any desired range of energies. In particular,
we focus on the use of gaussian basis sets, commonly used in first-principles
codes. The possible usefulness of these basis sets in a first-principles
description of field emission or scanning tunneling microscopy at large bias is
illustrated by studying a simpler related phenomenon: The lifetime of an
electron in a H atom subjected to a strong electric field.Comment: 6 pages, 5 figures, accepted by J. Chem. Phys. (http://jcp.aip.org/
Dynamical structure factor of a nonlinear Klein-Gordon lattice
The quantum modes of a nonlinear Klein-Gordon lattice have been computed
numerically [L. Proville, Phys. Rev. B 71, 104306 (2005)]. The on-site
nonlinearity has been found to lead to phonon bound states. In the present
paper, we compute numerically the dynamical structure factor so as to simulate
the coherent scattering cross section at low temperature. The inelastic
contribution is studied as a function of the on-site anharmonicity.
Interestingly, our numerical method is not limited to the weak anharmonicity
and permits one to study thoroughly the spectra of nonlinear phonons
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