6,904 research outputs found
On the Magnetism of the Normal State in MgB2
The experimentally observed ferromagnetism in MgB2 in the normal state is
attributed to micro phase separated inclusions of iron. This magnetic character
is also observed when the iron content of the samples is reduced below 20
micro-g/g, however in these samples the diamagnetism of MgB2 is apparent and is
measured. It is found experimentally that the diamagnetic susceptibility at
room temperature of B element, MgB2 and MgB4 is close to the ratio 1:2:4,
suggesting that the diamagnetism in these borides is confined to the boron
atoms. This observation supports a picture in which the two electrons of Mg are
donated to B in MgB2Comment: 6 pages, 4 figures; references corrected; PDF-file introduce
Dielectric Constant and Charging Energy in Array of Touching Nanocrystals
We calculate the effective macroscopic dielectric constant of
a periodic array of spherical nanocrystals (NCs) with dielectric constant
immersed in the medium with dielectric constant . For an array of NCs with the diameter and the distance
between their centers, which are separated by the small distance or touch each other by small facets with radius what is
equivalent to , we derive two analytical asymptotics of the
function in the limit .
Using the scaling hypothesis we interpolate between them near to obtain
new approximated function for . It agrees with existing numerical calculations for
, while the standard mean-field Maxwell-Garnett
and Bruggeman approximations fail to describe percolation-like behavior of
near . We also show that in this case the charging
energy of a single NC in an array of touching NCs has a non-trivial
relationship to , namely ,
where varies from 1.59 to 1.95 depending on the studied
three-dimensional lattices. Our approximation for can be
used instead of mean field Maxwell-Garnett and Bruggeman approximations to
describe percolation like transitions near for other material
characteristics of NC arrays, such as conductivity
Weak, Strong and Linear Convergence of a Double-Layer Fixed Point Algorithm
In this article we consider a consistent convex feasibility problem in a real
Hilbert space defined by a finite family of sets . We are interested, in
particular, in the case where for each , , is a cutter and
is a proximity function. Moreover,
we make the following assumption: the computation of is at most as
difficult as the evaluation of and this is at most as difficult as
projecting onto . We study a double-layer fixed point algorithm which
applies two types of controls in every iteration step. The first one -- the
outer control -- is assumed to be almost cyclic. The second one -- the inner
control -- determines the most important sets from those offered by the first
one. The selection is made in terms of proximity functions. The convergence
results presented in this manuscript depend on the conditions which first, bind
together the sets, the operators and the proximity functions and second,
connect the inner and outer controls. In particular, weak regularity
(demi-closedness principle), bounded regularity and bounded linear regularity
imply weak, strong and linear convergence of our algorithm, respectively. The
framework presented in this paper covers many known (subgradient) projection
algorithms already existing in the literature; for example, those applied with
(almost) cyclic, remotest-set, maximum displacement, most-violated constraint
and simultaneous controls. In addition, we provide several new examples, where
the double-layer approach indeed accelerates the convergence speed as we
demonstrate numerically.Comment: accepted for publication in SIAM Journal on Optimization (SIOPT
Finitely Convergent Deterministic and Stochastic Iterative Methods for Solving Convex Feasibility Problems
We propose finitely convergent methods for solving convex feasibility
problems defined over a possibly infinite pool of constraints. Following other
works in this area, we assume that the interior of the solution set is nonempty
and that certain overrelaxation parameters form a divergent series. We combine
our methods with a very general class of deterministic control sequences where,
roughly speaking, we require that sooner or later we encounter a violated
constraint if one exists. This requirement is satisfied, in particular, by the
cyclic, repetitive and remotest set controls. Moreover, it is almost surely
satisfied for random controls
Surface roughness scattering in multisubband accumulation layers
Accumulation layers with very large concentrations of electrons where many
subbands are filled became recently available due to ionic liquid and other new
methods of gating. The low temperature mobility in such layers is limited by
the surface roughness scattering. However theories of roughness scattering so
far dealt only with the small-density single subband two-dimensional electron
gas (2DEG). Here we develop a theory of roughness-scattering limited mobility
for the multisubband large concentration case. We show that with growing 2D
electron concentration the surface dimensionless conductivity
first decreases as and then saturates as
, where and are the characteristic
length and height of the surface roughness, is the effective Bohr radius.
This means that in spite of the shrinkage of the 2DEG width and the related
increase of the scattering rate, the 2DEG remains a good metal. Thus, there is
no re-entrant metal-insulator transition at high concentrations conjectured by
Das Sarma and Hwang [PRB 89, 121413 (2014)].Comment: A few corrections to the version published in PRB are included here
in this versio
Accumulation, inversion, and depletion layers in SrTiO
We study potential and electron density depth profiles in accumulation,
inversion and depletion layers in crystals with large and nonlinear dielectric
response such as . We describe the lattice dielectric
response using the Landau-Ginzburg free energy expansion. In accumulation and
inversion layers we arrive at new nonlinear dependencies of the width of
the electron gas on applied electric field . Particularly important is the
predicted electron density profile of accumulation layers (including the
interface) , where . We compare this profile with available data and find
satifactory agreement. For a depletion layer we find an unconventional
nonlinear dependence of capacitance on voltage. We also evaluate the role of
spatial dispersion in the dielectric response by adding a gradient term to the
Landau-Ginzburg free energy
Collapse of electrons to a donor cluster in SrTiO
It is known that a nucleus with charge where creates
electron-positron pairs from the vacuum. These electrons collapse onto the
nucleus resulting in a net charge while the positrons are emitted. This
effect is due to the relativistic dispersion law. The same reason leads to the
collapse of electrons to the charged impurity with a large charge number in
narrow-band gap semiconductors and Weyl semimetals as well as graphene. In this
paper, a similar effect of electron collapse and charge renormalization is
found for donor clusters in SrTiO (STO), but with a very different origin.
At low temperatures, STO has an enormously large dielectric constant. Because
of this, the nonlinear dielectric response becomes dominant when the electric
field is not too small. We show that this leads to the collapse of surrounding
electrons into a charged spherical donor cluster with radius when its total
charge number exceeds a critical value where is the
lattice constant. Using the Thomas-Fermi approach, we find that the net charge
grows with until exceeds another value .
After this point, remains . We extend our results to the case
of long cylindrical clusters. Our predictions can be tested by creating discs
and stripes of charge on the STO surface
Electron gas induced in SrTiO
This mini-review is dedicated to the 85th birthday of Prof. L. V. Keldysh,
from whom we have learned so much. In this paper we study the potential and
electron density depth profiles in surface accumulation layers in crystals with
a large and nonlinear dielectric response such as SrTiO (STO) in the cases
of planar, spherical and cylindrical geometries. The electron gas can be
created by applying an induction to the STO surface. We describe the
lattice dielectric response of STO using the Landau-Ginzburg free energy
expansion and employ the Thomas-Fermi (TF) approximation for the electron gas.
For the planar geometry we arrive at the electron density profile , where . We extend our results to
overlapping electron gases in GTO/STO/GTO multi-heterojunctions and electron
gases created by spill-out from NSTO (heavily -type doped STO) layers into
STO. Generalization of our approach to a spherical donor cluster creating a big
TF atom with electrons in STO brings us to the problem of supercharged nuclei.
It is known that for an atom with nuclear charge , where ,
electrons collapse onto the nucleus resulting in a net charge . Here,
instead of relativistic physics, the collapse is caused by the nonlinear
dielectric response. Electrons collapse into the charged spherical donor
cluster with radius when its total charge number exceeds the critical
value , where is the lattice constant. The net charge
grows with until exceeds . After this
point, the charge number of the compact core remains , with
the rest electrons forming a sparse Thomas-Fermi electron atmosphere
around it. We extend our results to the case of long cylindrical clusters as
well.Comment: mini-review dedicated to the 85th birthday of Prof. L. V. Keldys
Anomalous conductivity, Hall factor, magnetoresistance, and thermopower of accumulation layer in
We study the low temperature conductivity of the electron accumulation layer
induced by the very strong electric field at the surface of
sample. Due to the strongly nonlinear lattice dielectric response, the
three-dimensional density of electrons in such a layer decays with the
distance from the surface very slowly as . We show
that when the mobility is limited by the surface scattering the contribution of
such a tail to the conductivity diverges at large because of growing time
electrons need to reach the surface. We explore truncation of this divergence
by the finite sample width, by the bulk scattering rate, or by the crossover to
the bulk linear dielectric response with the dielectric constant . As a
result we arrive at the anomalously large mobility, which depends not only on
the rate of the surface scattering, but also on the physics of truncation.
Similar anomalous behavior is found for the Hall factor, the magnetoresistance,
and the thermopower
A whole-genome approach to identifying protein binding sites: promoters in Methanocaldococcus (Methanococcus) jannaschii
We have adapted an electrophoretic mobility shift assay (EMSA) to isolate genomic DNA fragments that bind the archaeal transcription initiation factors TATA-binding protein (TBP) and transcription factor B (TFB) to perform a genome-wide search for promoters. Mobility-shifted fragments were cloned, tested for their ability to compete with known promoter-containing fragments for a limited concentration of transcription factors, and sequenced. We applied the method to search for promoters in the genome of Methanocaldococcus jannaschii. Selection was most efficient for promoters of tRNA genes and genes for several presumed small non-coding RNAs (ncRNA). Protein-coding gene promoters were dramatically underrepresented relative to their frequency in the genome. The repeated isolation of these genomic regions was partially rectified by including a hybridization-based screening. Sequence alignment of the affinity-selected promoters revealed previously identified TATA box, BRE, and the putative initiator element. In addition, the conserved bases immediately upstream and downstream of the BRE and TATA box suggest that the composition and structure of archaeal natural promoters are more complicated
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