50,067 research outputs found
Investigation into the inadequacy of cRPA in reproducing screening in strongly correlated systems
The accuracy of the constrained random phase approximation(cRPA) method is
examined in multi-orbital Hubbard models containing all possible on-site
density-density interactions. Using DMFT, we show that the effective model
constructed using cRPA fails to reproduce the spectral properties of the
original full model in a wide parameter range. By comparing quantities such as
the density of states and quasiparticle residues of the full and the effective
models, we show that cRPA systematically overestimates the screening of Hubbard
U for DMFT impurity solvers. We instead propose a new method to investigate the
screening mechanism in the system using the local polarization, which is highly
successful in reproducing spectra and also shows that the true screening is far
less than that predicted by RPA. Furthermore, we compare the fully screened
interaction given by RPA and our new method and show that the RPA is
also overscreened and misses the signatures of local screening, which are
clearly present in our new method.Comment: 9 figures, 9 page
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
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
Hopping conductivity and insulator-metal transition in films of touching semiconductor nanocrystals
This paper is focused on the the variable-range hopping of electrons in
semiconductor nanocrystal (NC) films below the critical doping concentration
at which it becomes metallic. The hopping conductivity is described by
the Efros-Shklovskii law which depends on the localization length of electrons.
We study how the localization length grows with the doping concentration in
the film of touching NCs. For that we calculate the electron transfer matrix
element between neighboring NCs for two models when NCs touch by small
facets or just one point. We study two sources of disorder: variations of NC
diameters and random Coulomb potentials originating from random numbers of
donors in NCs. We use the ratio of to the disorder-induced NC level
dispersion to find the localization length of electrons due to the multi-step
elastic co-tunneling process. We found three different phases at
depending on the strength of disorder, the material, sizes of NCs and their
facets: 1) "insulator" where the localization length of electrons increases
monotonically with and 2) "oscillating insulator" when the localization
length (and the conductivity) oscillates with from the insulator base and
3) "blinking metal" where the localization length periodically diverges. The
first two phases were seen experimentally and we discuss how one can see the
more exotic third one. In all three the localization length diverges at
. This allows us to find
Essentiality landscape of metabolic networks
Local perturbations of individual metabolic reactions may result in different
levels of lethality, depending on their roles in metabolism and the size of
subsequent cascades induced by their failure. Moreover, essentiality of
individual metabolic reactions may show large variations within and across
species. Here we quantify their essentialities in hundreds of species by
computing the growth rate after removal of individual and pairs of reactions by
flux balance analysis. We find that about 10% of reactions are essential, i.e.,
growth stops without them, and most of the remaining reactions are redundant in
the metabolic network of each species. This large-scale and cross-species study
allows us to determine ad hoc ages of each reaction and species. We find that
when a reaction is older and contained in younger species, the reaction is more
likely to be essential. Such correlations of essentiality with the ages of
reactions and species may be attributable to the evolution of cellular
metabolism, in which alternative pathways are recruited to ensure the stability
of important reactions to various degrees across species
Solutions of Conformal Turbulence on a Half Plane
Exact solutions of conformal turbulence restricted on a upper half plane are
obtained. We show that the inertial range of homogeneous and isotropic
turbulence with constant enstrophy flux develops in a distant region from the
boundary. Thus in the presence of an anisotropic boundary, these exact
solutions of turbulence generalize Kolmogorov's solution consistently and
differ from the Polyakov's bulk case which requires a fine tunning of
coefficients. The simplest solution in our case is given by the minimal model
of and moreover we find a fixed point of solutions when
become large.Comment: 10pages, KHTP-93-07, SNUCTP-93-3
An Effective Action for Quasi-elastic Scatterings in QCD
A new effective action for the high energy quark-quark scatterings is
obtained by applying a scaling approximation to the QCD action. The propagators
are shown to factorize into the transverse and the longitudinal parts so that
the scattering amplitudes are given in terms of the products of two dimensional
-matrices. we show that our action provides a natural effective field theory
for the Lipatov's theory of quark scatterings with quasi-elastic unitarity. The
amplitude with quasi-elastic unitarity obtained from this action shows `Regge'
behavior and is eikonalized.Comment: 8 pages, uses Feynman.te
Spontaneous Breaking of Parity in 2+1-Dimensional Thirring Model
A new aspect of the vacuum structure of 2+1-dimensional Thirring model is
presented. Using the Fierz identity, we split the current-current four-Fermi
interaction in terms of a matrix valued auxiliary scalar field and compute its
effective potential. Energy consideration shows that contrary to earlier
expectations, parity in general is spontaneously broken at any finite order of
N, where N is the number of the two component spinors. In the large N limit,
there does not exist a stable vacuum of the theory thereby making the
application of the large N limit to Thirring model dangerous. A detailed
analysis for parity breaking solutions in N=2,3 cases is given.Comment: 6 pages, KHTP-94-05 /SNUCTP 94-3
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