79 research outputs found
Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells
We present a mean field theory of ferromagnetism in diluted magnetic
semiconductor quantum wells. When subband mixing due to exchange interactions
between quantum well free carriers and magnetic impurities is neglected,
analytic result can be obtained for the dependence of the critical temperature
and the spontaneous magnetization on the distribution of magnetic impurities
and the quantum well width. The validity of this approximate theory has been
tested by comparing its predictions with those from numerical self-consistent
field calculations. Interactions among free carriers, accounted for using the
local-spin-density approximation, substantially enhance the critical
temperature. We demonstrate that an external bias potential can tune the
critical temperature through a wide range.Comment: 4 pages, 3 figures, submitted to Phys. Rev.
Structural and Magnetic Properties of Trigonal Iron
First principles calculations of the electronic structure of trigonal iron
were performed using density function theory. The results are used to predict
lattice spacings, magnetic moments and elastic properties; these are in good
agreement with experiment for both the bcc and fcc structures. We find however,
that in extracting these quantities great care must be taken in interpreting
numerical fits to the calculated total energies. In addition, the results for
bulk iron give insight into the properties of thin iron films. Thin films grown
on substrates with mismatched lattice constants often have non-cubic symmetry.
If they are thicker than a few monolayers their electronic structure is similar
to a bulk material with an appropriately distorted geometry, as in our trigonal
calculations. We recast our bulk results in terms of an iron film grown on the
(111) surface of an fcc substrate, and find the predicted strain energies and
moments accurately reflect the trends for iron growth on a variety of
substrates.Comment: 11 pages, RevTeX,4 tar'd,compressed, uuencoded Postscript figure
Density Functional Study of Cubic to Rhombohedral Transition in -AlF
Under heating, -AlF undergoes a structural phase transition from
rhombohedral to cubic at temperature around 730 K. The density functional
method is used to examine the =0 energy surface in the structural parameter
space, and finds the minimum in good agreement with the observed rhombohedral
structure. The energy surface and electronic wave-functions at the minimum are
then used to calculate properties including density of states, -point
phonon modes, and the dielectric function. The dipole formed at each fluorine
ion in the low temperature phase is also calculated, and is used in a classical
electrostatic picture to examine possible antiferroelectric aspects of this
phase transition.Comment: A 6-page manuscript with 4 figures and 4 table
All-electron GW calculation based on the LAPW method: application to wurtzite ZnO
We present a new, all-electron implementation of the GW approximation and
apply it to wurtzite ZnO. Eigenfunctions computed in the local-density
approximation (LDA) by the full-potential linearized augmented-plane-wave
(LAPW) or the linearized muffin-tin-orbital (LMTO) method supply the input for
generating the Green function G and the screened Coulomb interaction W. A mixed
basis is used for the expansion of W, consisting of plane waves in the
interstitial region and augmented-wavefunction products in the
augmentation-sphere regions. The frequency-dependence of the dielectric
function is computed within the random-phase approximation (RPA), without a
plasmon-pole approximation. The Zn 3d orbitals are treated as valence states
within the LDA; both core and valence states are included in the self-energy
calculation. The calculated bandgap is smaller than experiment by about 1eV, in
contrast to previously reported GW results. Self-energy corrections are
orbital-dependent, and push down the deep O 2s and Zn 3d levels by about 1eV
relative to the LDA. The d level shifts closer to experiment but the size of
shift is underestimated, suggesting that the RPA overscreens localized states.Comment: 10 pages, 3 figures, submitted to Phys. Rev.
Surface Half-Metallicity of CrAs in the Zinc-Blende Structure
The development of new techniques such as the molecular beam epitaxy have
enabled the growth of thin films of materials presenting novel properties.
Recently it was made possible to grow a CrAs thin-film in the zinc-blende
structure. In this contribution, the full-potential screened KKR method is used
to study the electronic and magnetic properties of bulk CrAs in this novel
phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to
be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice
constants with a total spin magnetic moment of 3 . The Cr-terminated
surface retains the half-ferromagnetic character of the bulk, while in the case
of the As-termination the surface states destroy the gap in the minority-spin
band.Comment: 4 pages, 2 figures, new text, new titl
Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field
Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs
buffer of a standard heterostructure were investigated by magnetotransport
measurements. In magnetic fields oriented parallel to the electron layers, the
magnetoresistance exhibits an oscillation associated with the depopulation of
the higher occupied subband and the field-induced transition into a decoupled
bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields
allow to reconstruct the evolution of the electron concentration in the
individual subbands as a function of the in-plane magnetic field. The
characteristics of the system derived experimentally are in quantitative
agreement with numerical self-consistent-field calculations of the electronic
structure.Comment: 6 pages, 5 figure
Large Orbital Magnetic Moment and Coulomb Correlation effects in FeBr2
We have performed an all-electron fully relativistic density functional
calculation to study the magnetic properties of FeBr2. We show for the first
time that the correlation effect enhances the contribution from orbital degrees
of freedom of electrons to the total magnetic moment on Fe as
opposed to common notion of nearly total quenching of the orbital moment on
Fe site. The insulating nature of the system is correctly predicted when
the Hubbard parameter U is included. Energy bands around the gap are very
narrow in width and originate from the localized Fe-3 orbitals, which
indicates that FeBr2 is a typical example of the Mott insulator.Comment: 4 pages, 3 figures, revtex4, PRB accepte
Orbital effect of in-plane magnetic field on quantum transport in chaotic lateral dots
We show how the in-plane magnetic field, which breaks time-reversal and
rotational symmetries of the orbital motion of electrons in a heterostructure
due to the momentum-dependent inter-subband mixing, affects weak localisation
correction to conductance of a large-area chaotic lateral quantum dot and
parameteric dependences of universal conductance fluctuations in it.Comment: 4 pages with a figur
Influence of Carbon Concentration on the Superconductivity in MgCxNi3
The influence of carbon concentration on the superconductivity (SC) in
MgCNi has been investigated by measuring the low temperature specific
heat combined with first principles electronic structure calculation. It is
found that the specific heat coefficient of the
superconducting sample () in normal state is twice that of the
non-superconducting one (). The comparison of measured
and the calculated electronic density of states (DOS) shows that the
effective mass renormalization changes remarkably as the carbon concentration
changes. The large mass renormalization for the superconducting sample and the
low (7K) indicate that more than one kind of boson mediated
electron-electron interactions exist in MgCNi.Comment: 4 pages, 4 figure
Lorenz function of BiTe/SbTe superlattices
Combining first principles density functional theory and semi-classical
Boltzmann transport, the anisotropic Lorenz function was studied for
thermoelectric BiTe/SbTe superlattices and their bulk
constituents. It was found that already for the bulk materials BiTe
and SbTe, the Lorenz function is not a pellucid function on charge
carrier concentration and temperature. For electron-doped
BiTe/SbTe superlattices large oscillatory deviations
for the Lorenz function from the metallic limit were found even at high charge
carrier concentrations. The latter can be referred to quantum well effects,
which occur at distinct superlattice periods
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