196 research outputs found
Structural distortions and orbital ordering in LaTiO3 and YTiO3
Theoretical investigations of the electronic, magnetic and structural
properties of LaTiO3 and YTiO3 have been made. In the framework of GGA and
GGA+U scheme we analyzed the effect of the local Coulomb interaction (U) value
on the atomic forces acting in the experimental structure. The optimal
parameters of the electron-electron on-site interactions as well as the orbital
configurations and magnetic properties are determined.Comment: 6 pages 6 figures, better quality pictures are avelable via e-mail,
Submitted to Europhysics Letter
Non-local Coulomb interactions and metal-insulator transition in TiO: a cluster LDA+DMFT approach
We present an ab initio quantum theory of the metal-insulator transition in
TiO. The recently developed cluster LDA+DMFT scheme is applied to
describe the many-body features of this compound. The conventional single site
DMFT cannot reproduce a low temperature insulating phase for any reasonable
values of the Coulomb interaction. We show that the non-local Coulomb
interactions and the strong chemical bonding within Ti-Ti pair is the origin of
the small gap insulating ground state of TiO
Dynamical singlets and correlation-assisted Peierls transition in VO2
A theory of the metal-insulator transition in vanadium dioxide from the
high-temperature rutile to the low- temperature monoclinic phase is proposed on
the basis of cluster dynamical mean field theory, in conjunction with the
density functional scheme. The interplay of strong electronic Coulomb
interactions and structural distortions, in particular the dimerization of
vanadium atoms in the low temperature phase, plays a crucial role. We find that
VO2 is not a conventional Mott insulator, but that the formation of dynamical
V-V singlet pairs due to strong Coulomb correlations is necessary to trigger
the opening of a Peierls gap.Comment: 5 page
Magnetic fluctuations and effective magnetic moments in \gamma-iron due to electronic structure peculiarities
Applying the local density and dynamical mean field approximations to
paramagnetic \gamma-iron we revisit the problem of theoretical description of
magnetic properties in a wide temperature range. We show that contrary to
\alpha-iron, the frequency dependence of the electronic self-energy has a
quasiparticle form for both, t_{2g} and e_g states. In the temperature range
T=1200-1500 K, where \gamma-iron exist in nature, this substance can be
nevertheless characterized by temperature-dependent effective local moments,
which yield relatively narrow peaks in the real part of the local magnetic
susceptibility. At the same time, at low temperatures \gamma-iron (which is
realized in precipitates) is better described in terms of itinerant picture. In
particular, the nesting features of the Fermi surfaces yield maximum of the
static magnetic susceptibility at the incommensurate wave vector q_{max}
belonging the direction q_X-q_W (q_X=(2\pi/a)(1,0,0),q_W=(2\pi/a)(1,1/2,0), a
is a lattice parameter) in agreement with the experimental data. This state is
found however to compete closely with the states characterized by magnetic wave
vectors along the directions q_X-q_L-q_K, where q_L=(2\pi/a)(1/2,1/2,1/2),
q_K=(2\pi/a)(3/4,3/4,0). From the analysis of the uniform magnetic
susceptibility we find that contrary to \alpha-iron, the Curie-Weiss law is not
fulfilled in a broad temperature range, although the inverse susceptibility is
nearly linear in the moderate-temperature region (1200-1500 K). The
non-linearity of the inverse uniform magnetic susceptibility in a broader
temperature range is due to the density of states peak located close to the
Fermi level. The effective exchange integrals in the paramagnetic phase are
estimated on the base of momentum dependent susceptibility.Comment: 10 pages, 11 figure
Mott transition and suppression of orbital fluctuations in orthorhombic 3 perovskites
Using Wannier-functions, a low-energy Hamiltonian is derived for
orthorhombic transition-metal oxides. Electronic correlations are
treated with a new implementation of dynamical mean-field theory for non-cubic
systems. Good agreement with photoemission data is obtained. The interplay of
correlation effects and cation covalency (GdFeO-type distortions) is
found to suppress orbital fluctuations in LaTiO and even more in
YTiO, and to favor the transition to the insulating state.Comment: 4 pages, 3 figures; revised manuscrip
Targeting the Wnt pathways for therapies
The Wnt/β-catenin signaling pathway is crucial in animal development from sponges to humans. Its activity in the adulthood is less general, with exceptions having huge medical importance. Namely, improper activation of this pathway is carcinogenic in many tissues, most notably in the colon, liver and the breast. On the other hand, the Wnt/β-catenin signaling must be re-activated in cases of tissue damage, and insufficient activation results in regeneration failure and degeneration. These both medically important implications are unified by the emerging importance of this signaling pathway in the control of proliferation of various types of stem cells, crucial for tissue regeneration and, in case of cancer stem cells - cancer progression and relapse. This article aims at briefly reviewing the current state of knowledge in the field of Wnt signaling, followed by a detailed discussion of current medical developments targeting distinct branches of the Wnt pathway for anti-cancer and pro-regeneration therapies
Double Counting in LDA+DMFT - The Example of NiO
An intrinsic issue of the LDA+DMFT approach is the so called double counting
of interaction terms. How to choose the double-counting potential in a manner
that is both physically sound and consistent is unknown. We have conducted an
extensive study of the charge transfer system NiO in the LDA+DMFT framework
using quantum Monte Carlo and exact diagonalization as impurity solvers. By
explicitly treating the double-counting correction as an adjustable parameter
we systematically investigated the effects of different choices for the double
counting on the spectral function. Different methods for fixing the double
counting can drive the result from Mott insulating to almost metallic. We
propose a reasonable scheme for the determination of double-counting
corrections for insulating systems.Comment: 7 pages, 6 figure
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