223 research outputs found
Orbital occupation, local spin and exchange interactions in V2O3
We present the results of an LDA and LDA+U band structure study of the
monoclinic and the corundum phases of V2O3 and argue that the most prominent
(spin 1/2) models used to describe the semiconductor metal transition are not
valid. Contrary to the generally accepted assumptions we find that the large on
site Coulomb and exchange interactions result in a total local spin of 1 rather
than 1/2 and especially an orbital occupation which removes the orbital
degeneracies and the freedom for orbital ordering. The calculated exchange
interaction parameters lead to a magnetic structure consistent with experiment
again without the need of orbital ordering. While the low-temperature
monoclinic distortion of the corundum crystal structure produces a very small
effect on electronic structure of v2o3, the change of magnetic order leads to
drastic differences in band widths and band gaps. The low temperature
monoclinic phase clearly favors the experimentally observed magnetic structure,
but calculations for corundum crystal structure gave two consistent sets of
exchange interaction parameters with nearly degenerate total energies
suggesting a kind of frustration in the paramagnetic phase. These results
strongly suggest that the phase transitions in V2O3 which is so often quoted as
the example of a S=1/2 Mott Hubbard system have a different origin. So back to
the drawing board
The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3
We present first principles LDA+U calculations of electronic structure and
magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state
transition was found in our calculations using experimental crystallographic
data for both materials with a much higher transition temperature for HoCoO3,
which agrees well with the experimental estimations. Low Spin state t6e0
(non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+)
ions happens due to the competition between crystal field t_2g-e_g splitting
and effective exchange interaction between 3 spin-orbitals. We show that the
difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the
smaller ionic radius of Ho ion comparing with La ion results in stronger
crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3)
and hence tip the balance between the Low Spin and Intermediate Spin states to
the non-magnetic solution in HoCoO3.Comment: 13 pages, 6 figure
Time-dependent scattering theory for Schr\"odinger operators on scattering manifolds
We construct a time-dependent scattering theory for Schr\"odinger operators
on a manifold with asymptotically conic structure. We use the two-space
scattering theory formalism, and a reference operator on a space of the form
, where is the boundary of at infinity. We
prove the existence and the completeness of the wave operators, and show that
our scattering matrix is equivalent to the absolute scattering matrix, which is
defined in terms of the asymptotic expansion of generalized eigenfunctions. Our
method is functional analytic, and we use no microlocal analysis in this paper.Comment: 24 page
Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3
The combination of bandstructure theory in the local density approximation
with dynamical mean field theory was recently successfully applied to
VO -- a material which undergoes the f amous Mott-Hubbard
metal-insulator transition upon Cr doping. The aim of this sh ort paper is to
emphasize two aspects of our recent results: (i) the filling of the
Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the
Mott-Hubbard transition in this system which is not characterized by a diver
gence of the effective mass for the -orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding
The study of the canonical Watson-Crick DNA base pairs by Moller-Plesset perturbation method: the nature of their stability
Gas-phase gradient optimization was carried out on the canonical Watson-Crick DNA base pairs using the second-order Moller-Plesset (MP2) perturbation method at the 6-31G* and 6-31G*(0.25) basis sets. It is detected that full geometry optimization at the MP2 level leads to an intrinsically nonplanar propeller-twisted and buckled geometry of G-C and A-T base pairs. Morokuma-Kitaura (MK) and reduced variational space (RVS) methods of the decomposition for molecular Hartree-Fock interaction energies were used for the investigation of the hydrogen bonding in the Watson-Crick base pairs in question. It is shown that the stability of the hydrogen-bonded DNA base pairs originates mainly from electrostatic interactions. At the same time the polarization, charge transfer and dispersion interactions also make considerable contribution to the attraction energy of bases.Використовуючи теорію збурень Моллера-Плессета у другому порядку, проведено градієнтну оптимізацію канонічних уотсон-криківських пар основ у базисах 6-31G* і 6-31G*(0,25). Знайдено, що повна оптимізація геометрії на рівні МР2 спричинює внутрішньо неплоску пропелер-повернуту і вигнуту геометрію пар основ G-C і А-Т. Для дослідження водневих зв'язків в уотсон-криківських парах використано методи Мороку ми-Кітаури та зменшеного варіаційного простору для розкладу хартрі-фоківської енергії молекулярної взаємодії. Показано, що стабільність водневих зв'язків пар основ переважно з'являється за рахунок електростатичної взаємодії. В той же час поляризаційна і исперсійна компоненти та взаємодія з перенесенням заряду також роблять значний внесок в енергію притяжіння основ.Используя теорию возмущений Моллера-Плессета во втором порядке, проведена градиентная оптимизация канонических уотсон-криковских пар оснований в базисах 6-31G* и 6-31G* (0,25). Обнаружено, что полная оптимизация геометрии на уровне МР2 приводит к внутренне неплоской пропеллер-повернутой и изогнутой геометрии пар оснований G-C и А-Т. Для исследования водородных связей в уотсон-криковских парах оснований использованы методы Морокумы-Китауры и уменьшенного вариационного пространства для разложения хартри-фоковской энергии молекулярного взаимодействия. Показано, что стабильность водородно-связанных пар оснований возникает, как правило, благодаря электростатическим взаимодействиям. В то же время поляризационная и дисперсионная компоненты, а также взаимодействие с переносом заряда в свою очередь вносят значительный вклад в энергию притяжения оснований
Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory
We use first principles density functional theory to calculate electronic and
magnetic properties of TiF3 using the full potential linearized augmented plane
wave method. The LDA approximation predicts a fully saturated ferromagnetic
metal and finds degenerate energy minima for high and low symmetry structures.
The experimentally observed Jahn-Teller phase transition at Tc=370K can not be
driven by the electron-phonon interaction alone, which is usually described
accurately by LDA.
Electron correlations beyond LDA are essential to lift the degeneracy of the
singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are
important, the direction of the t2g-level splitting is determined by the
dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic
insulator with an orbitally ordered ground state. The input parameters U=8.1 eV
and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on
the TiF ion using the molecular NRLMOL code. We estimate the
Heisenberg exchange constant for spin-1/2 on a cubic lattice to be
approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per
TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
Orbital Ordering in Paramagnetic LaMnO3 and KCuF3
{\it Ab-initio} studies of the stability of orbital ordering, its coupling to
magnetic structure and its possible origins (electron-phonon and/or
electron-electron interactions) are reported for two perovskite systems,
LaMnO and KCuF. We present a new Average Spin State (ASS) calculational
scheme that allowed us to treat a paramagnetic state. Using this scheme, we
succesfully described the experimental magnetic/orbital phase diagram of both
LaMnO and KCuF in crystal structures when the Jahn-Teller distortions
are neglected. Hence, we conclude that the orbital ordering in both compounds
is purely electronic in origin.Comment: 10 pages, 5 figure
On-site Coulomb interaction and the magnetism of (GaMn)N and (GaMn)As
We use the local density approximation (LDA) and LDA+U schemes to study the
magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying
number of holes. We show that for both systems and both calculational schemes
the presence of holes is crucial for establishing ferromagnetism. For both
systems, the introduction of increases delocalization of the holes and,
simultaneously, decreases the p-d interaction. Since these two trends exert
opposite influences on the Mn-Mn exchange interaction the character of the
variation of the Curie temperature (T) cannot be predicted without direct
calculation. We show that the variation of T is different for two systems.
For low Mn concentrations we obtain the tendency to increasing T in the
case of (GaMn)N whereas an opposite tendency to decreasing T is obtained
for (GaMn)As. We reveal the origin of this difference by inspecting the
properties of the densities of states and holes for both systems. The main body
of calculations is performed within a supercell approach. The Curie
temperatures calculated within the coherent potential approximation to atomic
disorder are reported for comparison. Both approaches give similar qualitative
behavior. The results of calculations are related to the experimental data.Comment: to appear in Physical Review
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
The Vainshtein mechanism in the Decoupling Limit of massive gravity
We investigate static spherically symmetric solutions of nonlinear massive
gravities. We first identify, in an ansatz appropriate to the study of those
solutions, the analog of the decoupling limit (DL) that has been used in the
Goldstone picture description. We show that the system of equations left over
in the DL has regular solutions featuring a Vainshtein-like recovery of
solutions of General Relativity (GR). Hence, the singularities found to arise
integrating the full nonlinear system of equations are not present in the DL,
despite the fact those singularities are usually thought to be due to a
negative energy mode also seen in this limit. Moreover, we show that the
scaling conjectured by Vainshtein at small radius is only a limiting case in an
infinite family of non singular solutions each showing a Vainshtein recovery of
GR solutions below the Vainshtein radius but a different common scaling at
small distances. This new scaling is shown to be associated with a zero mode of
the nonlinearities left over in the DL. We also show that, in the DL, this
scaling allows for a recovery of GR solutions even for potentials where the
original Vainshtein mechanism is not working. Our results imply either that the
DL misses some important features of nonlinear massive gravities or that
important features of the solutions of the full nonlinear theory have been
overlooked. They could also have interesting outcomes for the DGP model and
related proposals.Comment: 52 pages, 7 figures; v3: minor typos corrected, discussion of the
validity of the Decoupling Limit extended; accepted for publication in JHE
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