1,445 research outputs found
Application of Finite Strain Landau Theory To High Pressure Phase Transitions
In this paper we explain how to set up what is in fact the only possible
consistent construction scheme for a Landau theory of high pressure phase
transitions that systematically allows to take into account elastic
nonlinearities. We also show how to incorporate available information on the
pressure dependence of elastic constants taken from experiment or simulation.
We apply our new theory to the example of the high pressure cubic-tetragonal
phase transition in Strontium Titanate, a model perovskite that has played a
central role in the development of the theory of structural phase transitions.
Armed with pressure dependent elastic constants calculated by density
functional theory, we give a both qualitatively as well as quantitatively
satisfying description of recent high precision experimental data. Our
nonlinear theory also allows to predict a number of additional elastic
transition anomalies that are accessible to experiment.Comment: submitted to Phys. Rev. Let
Ab initio prediction of magnetically dead layers in freestanding -Ce(111)
It is well known that the surface of nonmagnetic -Ce is magnetically
ordered, i.e., -like. One then might conjecture, in agreement with
previous theoretical predictions, that the -Ce may also exhibit at its
surfaces even more strongly enhanced -like magnetic ordering.
Nonetheless, our result shows that the (111)-surfaces of magnetic -Ce
are neither spin nor orbitally polarized, i.e., -like. Therefore, we
predict, in contrast to the nonmagnetic -phase which tends to produce
magnetically ordered -like thin layers at its free surfaces, the
magnetic -phase has a tendency to form -like dead layers. This
study, which explains the suppressed (promoted) surface magnetic moments of
-Ce (-Ce), shows that how nanoscale can reverse physical
properties by going from bulk to the surface in isostructural - and
-phases of cerium. We predict using our freestanding surface results
that a typical unreactive and non-diffusive substrate can dramatically
influence the magnetic surface of cerium thin films in contrast to most of the
uncorrelated thin films and strongly correlated transition metals. Our result
implies that magnetic surface moments of -Ce(111) can be suddenly
disappeared by increasing lattice mismatch at the interface of a typical
unreactive and non-diffusive substrate with cerium overlayers.Comment: 6 pages, 3 figures, 1 tabl
Charge order in Magnetite. An LDA+ study
The electronic structure of the monoclinic structure of FeO is
studied using both the local density approximation (LDA) and the LDA+. The
LDA gives only a small charge disproportionation, thus excluding that the
structural distortion should be sufficient to give a charge order. The LDA+
results in a charge disproportion along the c-axis in good agreement with the
experiment. We also show how the effective can be calculated within the
augmented plane wave methods
Electronic structure of copper intercalated transition metal dichalcogenides: First-principles calculations
We report first principles calculations, within density functional theory, of
copper intercalated titanium diselenides, CuxTiSe2, for values of x ranging
from 0 to 0.11. The effect of intercalation on the energy bands and densities
of states of the host material is studied in order to better understand the
cause of the superconductivity that was recently observed in these structures.
We find that charge transfer from the copper atoms to the metal dichalcogenide
host layers causes a gradual reduction in the number of holes in the otherwise
semi-metallic pristine TiSe2, thus suppressing the charge density wave
transition at low temperatures, and a corresponding increase in the density of
states at the Fermi level. These effects are probably what drive the
superconducting transition in the intercalated systems.Comment: 8 pages, 6 figure
Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients
In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which
are divalent. This was experimentally determined in 1977 by perturbed angular
correlation measurements of the electric-field gradient on a 111Cd impurity. At
that time, the data were interpreted using a point charge model, which is now
known to be unphysical and unreliable. This makes the valency determination
potentially questionable. We revisit these data, and analyze them using ab
initio calculations of the electric-field gradient. From these calculations,
the physical mechanism that is responsible for the influence of the valency on
the electric-field gradient is derived. A generally applicable scheme to
interpret electric-field gradients is used, which in a transparent way
correlates the size of the field gradient with chemical properties of the
system.Comment: 10 page
Implementation of screened hybrid functionals based on the Yukawa potential within the LAPW basis set
The implementation of screened hybrid functionals into the WIEN2k code, which
is based on the LAPW basis set, is reported. The Hartree-Fock exchange energy
and potential are screened by means of the Yukawa potential as proposed by
Bylander and Kleinman [Phys. Rev. B 41, 7868 (1990)] for the calculation of the
electronic structure of solids with the screened-exchange local density
approximation. Details of the formalism, which is based on the method of
Massidda, Posternak, and Baldereschi [Phys. Rev. B 48, 5058 (1993)] for the
unscreened Hartree-Fock exchange are given. The results for the
transition-energy and structural properties of several test cases are
presented. Results of calculations of the Cu electric-field gradient in Cu2O
are also presented, and it is shown that the hybrid functionals are much more
accurate than the standard local-density or generalized gradient
approximations
Laser-induced spin protection and switching in a specially designed magnetic dot: A theoretical investigation
Most laser-induced femtosecond magnetism investigations are done in magnetic
thin films. Nanostructured magnetic dots, with their reduced dimensionality,
present new opportunities for spin manipulation. Here we predict that if a
magnetic dot has a dipole-forbidden transition between the lowest occupied
molecular orbital (LUMO) and the highest unoccupied molecular orbital (HOMO),
but a dipole-allowed transition between LUMO+1 and HOMO, electromagnetically
inducedtransparency can be used to prevent ultrafast laser-induced spin
momentum reduction, or spin protection. This is realized through a strong dump
pulse to funnel the population into LUMO+1. If the time delay between the pump
and dump pulses is longer than 60 fs, a population inversion starts and spin
switching is achieved. Thesepredictions are detectable experimentally.Comment: 6 pages, three figur
Stabilization of the high-spin state of Co in LaCoRhO
The rhodium doping in the LaCoRhO perovskite series
() has been studied by X-ray diffraction, electric transport and
magnetization measurements, complemented by electronic structure GGA+U
calculations in supercell for different concentration regimes. No charge
transfer between Co and Rh is evidenced. The diamagnetic ground
state of LaCoO, based on Co in low-spin (LS) state, is disturbed
even by a small doping of Rh. The driving force is the elastic energy connected
with incorporation of a large Rh cation into the matrix of small LS
Co cations, which is relaxed by formation of large Co in
high-spin (HS) state in the next-nearest sites to the inserted Rh atom. With
increasing temperature, the population of Co in HS state increases
through thermal excitation, and a saturated phase is obtained close to room
temperature, consisting of a nearest-neighbor correlation of small (LS
Co) and large (HS Co and LS Rh) cations in a kind of
double perovskite structure. The stabilizing role of elastic and electronic
energy contributions is demonstrated in supercell calculations for dilute Rh
concentration compared to other dopants with various trivalent ionic radius.Comment: 8 pages, 8 figure
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