114 research outputs found
A dynamic localization of 2D electrons at mesoscopic length scales
We have investigated the local magneto-transport in high-quality 2D electron
systems at low carrier densities. The positive magneto-resistance in
perpendicular magnetic field in the strongly insulating regime has been
measured to evaluate the spatial concentration of localized states within a
mesoscopic region of the samples. An independent measurement of the electron
density within the same region shows an unexpected correspondence between the
density of electrons in the metallic regime and that of the localized states in
the insulating phase. We have argued that this correspondence manifests a rigid
distribution of electrons at low densities.Comment: 8 pages (incl 4 figures), double colum
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Negative hopping magnetoresistance of two-dimensional electron gas in a smooth random potential
We show that the tunnel coupling between semiclassical states localized in
different minima of a smooth random potential increases when magnetic field is
applied. This increase originates from the difference in gauge factors which
electron wave functions belonging to different electron ``lakes'' acquire in
the presence of the field. We illustrate the increase of coupling by a model
calculation of tunneling through a saddle point separating two adjacent lakes.
In the common case, when the barrier between two lakes is much narrower than
their size, the characteristic magnetic field is determined by the area of the
lakes, and thus may be quite small. The effect of the field on coupling
constants leads to a negative magnetoresistance in low-temperature conduction.Comment: 9 pages RevTe
Coulomb Glasses: A Comparison Between Mean Field and Monte Carlo Results
Recently a local mean field theory for both eqilibrium and transport
properties of the Coulomb glass was proposed [A. Amir et al., Phys. Rev. B 77,
165207 (2008); 80, 245214 (2009)]. We compare the predictions of this theory to
the results of dynamic Monte Carlo simulations. In a thermal equilibrium state
we compare the density of states and the occupation probabilities. We also
study the transition rates between different states and find that the mean
field rates underestimate a certain class of important transitions. We propose
modified rates to be used in the mean field approach which take into account
correlations at the minimal level in the sense that transitions are only to
take place from an occupied to an empty site. We show that this modification
accounts for most of the difference between the mean field and Monte Carlo
rates. The linear response conductance is shown to exhibit the Efros-Shklovskii
behaviour in both the mean field and Monte Carlo approaches, but the mean field
method strongly underestimates the current at low temperatures. When using the
modified rates better agreement is achieved
Variable range cotunneling and conductivity of a granular metal
The Efros-Shklovskii law for the conductivity of granular metals is
interpreted as a result of a variable range cotunneling process. The
cotunneling between distant resonant grains is predominantly elastic at low T
<< T_c, while it is inelastic (i.e., accompanied by creation of electron-hole
pairs on a string of intermediate non-resonant grains) at T > T_c. The
corresponding E-S temperature T_ES in the latter case is slightly
(logarithmically) T-dependent. The magnetoresistance in the two cases is
different: it may be relatively strong and negative at T much below T_c, while
at T>T_c it is suppressed due to inelastic processes which destroy the
interference.Comment: Submitted to JETP Letter
Uniformity transition for ray intensities in random media
This paper analyses a model for the intensity of distribution for rays propagating without absorption in a random medium. The random medium is modelled as a dynamical map. After N iterations, the intensity is modelled as a sum S of N contributions from different trajectories, each of which is a product of N independent identically distributed random variables xk, representing successive focussing or de-focussing events. The number of ray trajectories reaching a given point is assumed to proliferate exponentially: N=ΛN, for some Λ>1. We investigate the probability distribution of S. We find a phase transition as parameters of the model are varied. There is a phase where the fluctuations of S are suppressed as N → ∞, and a phase where the S has large fluctuations, for which we provide a large deviation analysis
Differential scanning calorimetry as a method for the control of vegetable oils
Differential scanning calorimetry (DSC) was used to study the thermophysical properties of oils of amaranth, corn, flax, sunflower, rapeseed, milk thistle, camelina, and pumpkin seed, liquid at room temperature. The characteristic thermal effects of these oils (temperatures of the maxima of endothermic peaks and their areas in the DSC thermograms) were determined. Endothermic peaks of different intensities on the melting curves of liquid vegetable oils in the ranges from -40 to -15°C, from-25 to -8°C, from -19 to +6°C, and from -10 to +4 °C as identification factors are discusse
Population of isomers in decay of the giant dipole resonance
The value of an isomeric ratio (IR) in N=81 isotones (Ba, Ce,
Nd and Sm) is studied by means of the ( reaction.
This quantity measures a probability to populate the isomeric state in respect
to the ground state population. In ( reactions, the giant dipole
resonance (GDR) is excited and after its decay by a neutron emission, the
nucleus has an excitation energy of a few MeV. The forthcoming decay
by direct or cascade transitions deexcites the nucleus into an isomeric or
ground state. It has been observed experimentally that the IR for Ba
and Ce equals about 0.13 while in two heavier isotones it is even less
than half the size. To explain this effect, the structure of the excited states
in the energy region up to 6.5 MeV has been calculated within the Quasiparticle
Phonon Model. Many states are found connected to the ground and isomeric states
by , and transitions. The single-particle component of the wave
function is responsible for the large values of the transitions. The calculated
value of the isomeric ratio is in very good agreement with the experimental
data for all isotones. A slightly different value of maximum energy with which
the nuclei rest after neutron decay of the GDR is responsible for the reported
effect of the A-dependence of the IR.Comment: 16 pages, 4 Fig
A Numerical Study of Coulomb Interaction Effects on 2D Hopping Transport
We have extended our supercomputer-enabled Monte Carlo simulations of hopping
transport in completely disordered 2D conductors to the case of substantial
electron-electron Coulomb interaction. Such interaction may not only suppress
the average value of hopping current, but also affect its fluctuations rather
substantially. In particular, the spectral density of current
fluctuations exhibits, at sufficiently low frequencies, a -like increase
which approximately follows the Hooge scaling, even at vanishing temperature.
At higher , there is a crossover to a broad range of frequencies in which
is nearly constant, hence allowing characterization of the current
noise by the effective Fano factor F\equiv S_I(f)/2e \left. For
sufficiently large conductor samples and low temperatures, the Fano factor is
suppressed below the Schottky value (F=1), scaling with the length of the
conductor as . The exponent is significantly
affected by the Coulomb interaction effects, changing from when such effects are negligible to virtually unity when they are
substantial. The scaling parameter , interpreted as the average
percolation cluster length along the electric field direction, scales as when Coulomb interaction effects are negligible
and when such effects are substantial, in
good agreement with estimates based on the theory of directed percolation.Comment: 19 pages, 7 figures. Fixed minor typos and updated reference
A Numerical Study of Transport and Shot Noise at 2D Hopping
We have used modern supercomputer facilities to carry out extensive Monte
Carlo simulations of 2D hopping (at negligible Coulomb interaction) in
conductors with the completely random distribution of localized sites in both
space and energy, within a broad range of the applied electric field and
temperature , both within and beyond the variable-range hopping region. The
calculated properties include not only dc current and statistics of localized
site occupation and hop lengths, but also the current fluctuation spectrum.
Within the calculation accuracy, the model does not exhibit noise, so
that the low-frequency noise at low temperatures may be characterized by the
Fano factor . For sufficiently large samples, scales with conductor
length as , where , and
parameter is interpreted as the average percolation cluster length. At
relatively low , the electric field dependence of parameter is
compatible with the law which follows from directed
percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference
- …