306 research outputs found
First-principles Theory of Nonlocal Screening in Graphene
Using the quasiparticle self-consistent GW (QSGW) and local-density (LD)
approximations, we calculate the q-dependent static dielectric function, and
derive an effective 2D dielectric function corresponding to screening of point
charges. In the q-to-0 limit, the 2D function is found to scale approximately
as the square root of the macroscopic dielectric function. Its value is ~4, a
factor approximately 1.5 larger than predictions of Dirac model. Both kinds of
dielectric functions depend strongly on q, in contrast with the Dirac model.
The QSGW approximation is shown to describe QP levels very well, with small
systematic errors analogous to bulk sp semiconductors. Local-field effects are
rather more important in graphene than in bulk semiconductors.Comment: 9 pages, 2 figure
Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN
We present calculations of magnetic exchange interactions and critical
temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density
approximation is combined with a linear-response technique to map the magnetic
energy onto a Heisenberg hamiltonion, but no significant further approximations
are made. Special quasi-random structures in large unit cells are used to
accurately model the disorder. T_c is computed using both a spin-dynamics
approach and the cluster variation method developed for the classical
Heisenberg model.
We show the following: (i) configurational disorder results in large
dispersions in the pairwise exchange interactions; (ii) the disorder strongly
reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is
predicted from total-energy considerations, further reduces T_c. Additionally
the exchange interactions J(R) are found to decay exponentially with distance
R^3 on average; and the mean-field approximation is found to be a very poor
predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of
spin-orbit coupling on T_c is considered. With all these factors taken into
account, T_c is reasonably predicted by the local spin-density approximation in
MnGaAs without the need to invoke compensation by donor impurities.Comment: 10 pages, 3 figure
Calculations of spin-disorder resistivity from first principles
Spin-disorder resistivity of Fe and Ni is studied using the noncollinear
density functional theory. The Landauer conductance is averaged over random
disorder configurations and fitted to Ohm's law. The distribution function is
approximated by the mean-field theory. The dependence of spin-disorder
resistivity on magnetization in Fe is found to be in excellent agreement with
the results for the isotropic s-d model. In the fully disordered state,
spin-disorder resistivity for Fe is close to experiment, while for fcc Ni it
exceeds the experimental value by a factor of 2.3. This result indicates strong
magnetic short-range order in Ni at the Curie temperature.Comment: 3 pages, 3 figure
Direct Method for Calculating Temperature-Dependent Transport Properties
We show how temperature-induced disorder can be combined in a direct way with
first-principles scattering theory to study diffusive transport in real
materials. Excellent (good) agreement with experiment is found for the
resistivity of Cu, Pd, Pt (and Fe) when lattice (and spin) disorder are
calculated from first principles. For Fe, the agreement with experiment is
limited by how well the magnetization (of itinerant ferromagnets) can be
calculated as a function of temperature. By introducing a simple Debye-like
model of spin disorder parameterized to reproduce the experimental
magnetization, the temperature dependence of the average resistivity, the
anisotropic magnetoresistance and the spin polarization of a NiFe
alloy are calculated and found to be in good agreement with existing data.
Extension of the method to complex, inhomogeneous materials as well as to the
calculation of other finite-temperature physical properties within the
adiabatic approximation is straightforward.Comment: Accepted as a Rapid Communication in Physical Review
The effects of interface morphology on Schottky barrier heights: a case study on Al/GaAs(001)
The problem of Fermi-level pinning at semiconductor-metal contacts is
readdressed starting from first-principles calculations for Al/GaAs. We give
quantitative evidence that the Schottky barrier height is very little affected
by any structural distortions on the metal side---including elongations of the
metal-semiconductor bond (i.e. interface strain)---whereas it strongly depends
on the interface structure on the semiconductor side. A rationale for these
findings is given in terms of the interface dipole generated by the ionic
effective charges.Comment: 5 pages, latex file, 2 postscript figures automatically include
Self-energies in itinerant magnets: A focus on Fe and Ni
We present a detailed study of local and non-local correlations in the
electronic structure of elemental transition metals carried out by means of the
Quasiparticle Self-consistent GW (QSGW ) and Dynamical Mean Field Theory
(DMFT). Recent high resolution ARPES and Haas-van Alphen data of two typical
transition metal systems (Fe and Ni) are used as case study. (i) We find that
the properties of Fe are very well described by QSGW. Agreement with cyclotron
and very clean ARPES measurements is excellent, provided that final-state
scattering is taken into account. This establishes the exceptional reliability
of QSGW also in metallic systems. (ii) Nonetheless QSGW alone is not able to
provide an adequate description of the Ni ARPES data due to strong local spin
fluctuations. We surmount this deficiency by combining nonlocal charge
fluctuations in QSGW with local spin fluctuations in DMFT (QSGW + 'Magnetic
DMFT'). (iii) Finally we show that the dynamics of the local fluctuations are
actually not crucial. The addition of an external static field can lead to
similarly good results if non-local correlations are included through QSGW
Gradual Absorption of Intraperitoneal Insulin Results in Impaired Glucose Tolerance in Recipients of Microencapsulated Islet Grafts
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