2,712 research outputs found
Calculations of Hubbard U from first-principles
The Hubbard \emph{U} of the \emph{3d} transition metal series as well as
SrVO, YTiO, Ce and Gd has been estimated using a recently proposed
scheme based on the random-phase approximation. The values obtained are
generally in good accord with the values often used in model calculations but
for some cases the estimated values are somewhat smaller than those used in the
literature. We have also calculated the frequency-dependent \emph{U} for some
of the materials. The strong frequency dependence of \emph{U} in some of the
cases considered in this paper suggests that the static value of \emph{U} may
not be the most appropriate one to use in model calculations. We have also made
comparison with the constrained LDA method and found some discrepancies in a
number of cases. We emphasize that our scheme and the constrained LDA method
theoretically ought to give similar results and the discrepancies may be
attributed to technical difficulties in performing calculations based on
currently implemented constrained LDA schemes.Comment: 24 pages, 13 figures; Submitted to Phys. Rev.
Screened Interaction and Self-Energy in an Infinitesimally Polarized Electron Gas via the Kukkonen-Overhauser Method
The screened electron-electron interaction and the
electron self-energy in an infinitesimally polarized electron gas are derived
by extending the approach of Kukkonen and Overhauser. Various quantities in the
expression for are identified in terms of the relevant
response functions of the electron gas. The self-energy is obtained from
by making use of the GW method which in this case
represents a consistent approximation. Contact with previous calculations is
made.Comment: 7 page
Neutral winds derived from IRI parameters and from the HWM87 wind model for the sundial campaign of September, 1986
Meridional neutral winds derived from the height of the maximum ionization of the F2 layer are compared with values from results of the HWM87 empirical neutral wind model. The time period considered is the SUNDIAL-2 campaign, 21 Sept. through 5 Oct. 1986. Winds were derived from measurements by a global network of ionosondes, as well as from similar quantities generated by the International Reference Ionosphere. Global wind patterns from the three sources are similar. Differences tend to be the result of local or transient phenomena that are either too rapid to be described by the order of harmonics of the empirical models, or are the result of temporal changes not reproduced by models based on average conditions
A dynamo theory prediction for solar cycle 22: Sunspot number, radio flux, exospheric temperature, and total density at 400 km
Using the dynamo theory method to predict solar activity, a value for the smoothed sunspot number of 109 + or - 20 is obtained for solar cycle 22. The predicted cycle is expected to peak near December, 1990 + or - 1 year. Concommitantly, F(10.7) radio flux is expected to reach a smoothed value of 158 + or - 18 flux units. Global mean exospheric temperature is expected to reach 1060 + or - 50 K and global total average total thermospheric density at 400 km is expected to reach 4.3 x 10 to the -15th gm/cu cm + or - 25 percent
GW approximation with self-screening correction
The \emph{GW} approximation takes into account electrostatic self-interaction
contained in the Hartree potential through the exchange potential. However, it
has been known for a long time that the approximation contains self-screening
error as evident in the case of the hydrogen atom. When applied to the hydrogen
atom, the \emph{GW} approximation does not yield the exact result for the
electron removal spectra because of the presence of self-screening: the hole
left behind is erroneously screened by the only electron in the system which is
no longer present. We present a scheme to take into account self-screening and
show that the removal of self-screening is equivalent to including exchange
diagrams, as far as self-screening is concerned. The scheme is tested on a
model hydrogen dimer and it is shown that the scheme yields the exact result to
second order in where and are respectively
the onsite and offsite Hubbard interaction parameters and the hopping
parameter.Comment: 9 pages, 2 figures; Submitted to Phys. Rev.
Activation gaps for the fractional quantum Hall effect: realistic treatment of transverse thickness
The activation gaps for fractional quantum Hall states at filling fractions
are computed for heterojunction, square quantum well, as well as
parabolic quantum well geometries, using an interaction potential calculated
from a self-consistent electronic structure calculation in the local density
approximation. The finite thickness is estimated to make 30% correction
to the gap in the heterojunction geometry for typical parameters, which
accounts for roughly half of the discrepancy between the experiment and
theoretical gaps computed for a pure two dimensional system. Certain model
interactions are also considered. It is found that the activation energies
behave qualitatively differently depending on whether the interaction is of
longer or shorter range than the Coulomb interaction; there are indications
that fractional Hall states close to the Fermi sea are destabilized for the
latter.Comment: 32 pages, 13 figure
Electronic structure and magnetism in doped semiconducting half-Heusler compounds
We have studied in details the electronic structure and magnetism in M (Mn
and Cr) doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn
(XMYZ) in a wide concentration range using local-spin density
functional method in the framework of tight-binding linearized muffin tin
orbital method(TB-LMTO) and supercell approach. Our calculations indicate that
some of these compounds are not only ferromagnetic but also half-metallic and
may be useful for spintronics applications. The electronic structure of the
doped systems is analyzed with the aid of a simple model where we have
considered the interaction between the dopant transition metal (M) and the
valence band X-Z hybrid. We have shown that the strong X-d - M-d interaction
places the M-d states close to the Fermi level with the M-t states lying
higher in energy in comparison to the M-e states. Depending on the number
of available d-electrons, ferromagnetism is realized provided the d-manifold is
partially occupied. The tendencies toward ferromagnetic(FM) or
antiferromagnetic(AFM) behavior are discussed within Anderson-Hasegawa models
of super-exchange and double-exchange. In our calculations for Mn doped NiTiSn,
the strong preference for FM over AFM ordering suggests a possible high Curie
temperature for these systems.Comment: 14 pages, 6 figure
Empirical wind model for the middle and lower atmosphere. Part 1: Local time average
The HWM90 thermospheric wind model was revised in the lower thermosphere and extended into the mesosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, and longitude (stationary wave 1). The model represents a smoothed compromise between the data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Root mean square differences between data and model are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal wind, and 10 m/s and 4 m/s, respectively, for meridional wind
Band structure analysis of the conduction-band mass anisotropy in 6H and 4H SiC
The band structures of 6H and 4H SiC calculated by means of the FP-LMTO
method are used to determine the effective mass tensors for their
conduction-band minima. The results are shown to be consistent with recent
optically detected cyclotron resonance measurements and predict an unusual band
filling dependence for 6H-SiC.Comment: 5 pages including 4 postscript figures incorporated with epsfig figs.
available as part 2: sicfig.uu self-extracting file to appear in Phys. Rev.
B: Aug. 15 (Rapid Communications
Collective fields in the functional renormalization group for fermions, Ward identities, and the exact solution of the Tomonaga-Luttinger model
We develop a new formulation of the functional renormalization group (RG) for
interacting fermions. Our approach unifies the purely fermionic formulation
based on the Grassmannian functional integral, which has been used in recent
years by many authors, with the traditional Wilsonian RG approach to quantum
systems pioneered by Hertz [Phys. Rev. B 14, 1165 (1976)], which attempts to
describe the infrared behavior of the system in terms of an effective bosonic
theory associated with the soft modes of the underlying fermionic problem. In
our approach, we decouple the interaction by means of a suitable
Hubbard-Stratonovich transformation (following the Hertz-approach), but do not
eliminate the fermions; instead, we derive an exact hierarchy of RG flow
equations for the irreducible vertices of the resulting coupled field theory
involving both fermionic and bosonic fields. The freedom of choosing a momentum
transfer cutoff for the bosonic soft modes in addition to the usual band cutoff
for the fermions opens the possibility of new RG schemes. In particular, we
show how the exact solution of the Tomonaga-Luttinger model emerges from the
functional RG if one works with a momentum transfer cutoff. Then the Ward
identities associated with the local particle conservation at each Fermi point
are valid at every stage of the RG flow and provide a solution of an infinite
hierarchy of flow equations for the irreducible vertices. The RG flow equation
for the irreducible single-particle self-energy can then be closed and can be
reduced to a linear integro-differential equation, the solution of which yields
the result familiar from bosonization. We suggest new truncation schemes of the
exact hierarchy of flow equations, which might be useful even outside the weak
coupling regime.Comment: 27 pages, 15 figures; published version, some typos correcte
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