28 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
Electronic structure induced reconstruction and magnetic ordering at the LaAlOSrTiO interface
Using local density approximation (LDA) calculations we predict
GdFeO-like rotation of TiO octahedra at the -type interface between
LaAlO and SrTiO. The narrowing of the Ti bandwidth which results
means that for very modest values of , LDA calculations predict charge
and spin ordering at the interface. Recent experimental evidence for magnetic
interface ordering may be understood in terms of the close proximity of an
antiferromagnetic insulating ground state to a ferromagnetic metallic excited
state
Magnetoelastic coupling in iron
Exchange interactions in {\alpha}- and {\gamma}-Fe are investigated within an
ab-initio spin spiral approach. We have performed total energy calculations for
different magnetic structures as a function of lattice distortions, related
with various cell volumes and the Bain tetragonal deformations. The effective
exchange parameters in {\gamma}-Fe are very sensitive to the lattice
distortions, leading to the ferromagnetic ground state for the tetragonal
deformation or increase of the volume cell. At the same time, the
magnetic-structure-independent part of the total energy changes very slowly
with the tetragonal deformations. The computational results demonstrate a
strong mutual dependence of crystal and magnetic structures in Fe and explain
the observable "anti-Invar" behavior of thermal expansion coefficient in
{\gamma}-Fe.Comment: Submitted to Phys. Rev.
Effect of magnetic state on the transition in iron: First-principle calculations of the Bain transformation path
Energetics of the fcc () - bcc () lattice transformation by
the Bain tetragonal deformation is calculated for both magnetically ordered and
paramagnetic (disordered local moment) states of iron. The first-principle
computational results manifest a relevance of the magnetic order in a scenario
of the - transition and reveal a special role of the Curie
temperature of -Fe, , where a character of the transformation is
changed. At a cooling down to the temperatures one can expect that
the transformation is developed as a lattice instability whereas for
it follows a standard mechanism of creation and growth of an embryo of the new
phase. It explains a closeness of to the temperature of start of the
martensitic transformation, .Comment: 4 pages, 3 figures, submitted in Phys. Rev. Letter
Superexchange Interactions in Orthorhombically Distorted Titanates RTiO3 (R= Y, Gd, Sm, and La)
Starting from the multiorbital Hubbard model for the t2g-bands of RTiO3 (R=
Y, Gd, Sm, and La), where all parameters have been derived from the
first-principles calculations, we construct an effective superexchange (SE)
spin model, by treating transfer integrals as a perturbation. We consider four
approximations for the SE interactions: (i) the canonical crystal-field (CF)
theory, where the form of the the occupied t2g-orbitals is dictated by the CF
splitting, and three extensions, namely (ii) the relativistic one, where
occupied orbitals are confined within the lowest Kramers doublet obtained from
the diagonalization of the crystal field and relativistic spin-orbit (SO)
interactions; (iii) the finite-temperature extension, which consider the effect
of thermal orbital fluctuations near the CF configuration; (iv) the
many-electron extension, which is based on the diagonalization of the full
Hamiltonian constructed in the basis of two-electron states separately for each
bond of the system. The main results are summarized as follows. (i) Thermal
fluctuations of the orbital degrees of freedom can substantially reduce the
value of the magnetic transition temperature. (ii) The anisotropic and
antisymmetric Dzyaloshinsky-Moriya interactions are rigorously derived and
their implications to the magnetic properties are discussed. (iii) The CF
theory, although applicable for YTiO3 and high-temperature structures of GdTiO3
and SmTiO3, breaks down in LaTiO3. Instead, the combination of the
many-electron effects and SO interaction can be responsible for the AFM
character of interatomic correlations in LaTiO3. (iv) The SE interactions in
YTiO3 strongly depend on the details of the crystal structure. Distortions in
the low-temperature structure tend to weaken the ferromagnetic interactions.Comment: 23 pages, 9 tables, 4 figure