3,566 research outputs found
Lattice Distortion and Magnetism of 3d- Perovskite Oxides
Several puzzling aspects of interplay of the experimental lattice distortion
and the the magnetic properties of four narrow -band perovskite oxides
(YTiO, LaTiO, YVO, and LaVO) are clarified using results of
first-principles electronic structure calculations. First, we derive parameters
of the effective Hubbard-type Hamiltonian for the isolated bands using
newly developed downfolding method for the kinetic-energy part and a hybrid
approach, based on the combination of the random-phase approximation and the
constraint local-density approximation, for the screened Coulomb interaction
part. Then, we solve the obtained Hamiltonian using a number of techniques,
including the mean-field Hartree-Fock (HF) approximation, the second-order
perturbation theory for the correlation energy, and a variational superexchange
theory. Even though the crystal-field splitting is not particularly large to
quench the orbital degrees of freedom, the crystal distortion imposes a severe
constraint on the form of the possible orbital states, which favor the
formation of the experimentally observed magnetic structures in YTiO,
YVO_, and LaVO even at the HF level. Beyond the HF approximation, the
correlations effects systematically improve the agreement with the experimental
data. Using the same type of approximations we could not reproduce the correct
magnetic ground state of LaTiO. However, we expect that the situation may
change by systematically improving the level of approximations for dealing with
the correlation effects.Comment: 30 pages, 17 figures, 8 tables, high-quality figures are available
via e-mai
Theoretical study of the accuracy limits for the optical resonance frequency measurements
The principal limits for the accuracy of the resonance frequency measurements
set by the asymmetry of the natural resonance line shape are studied and
applied to the recent accurate frequency measurements in the two-photon 1s-2s
resonance and in the one-photon 1s-2p resonance in hydrogen atom. This limit
for 1s-2s resonance is found to be Hz compared to the accuracy
achieved in experiment Hz. In case of deuterium atom the limit is
essentially larger: Hz. For 1s-2p resonance the accuracy limit is
0.17 MHz while the uncertainty of the recent frequency measurement is about 6
MHz.Comment: to be published in Physical Review Letter
Fingerprints of Spin-Orbital Physics in Crystalline O
The alkali hyperoxide KO is a molecular analog of strongly-correlated
systems, comprising of orbitally degenerate magnetic O ions. Using
first-principles electronic structure calculations, we set up an effective
spin-orbital model for the low-energy \textit{molecular} orbitals and argue
that many anomalous properties of KO replicate the status of its orbital
system in various temperature regimes.Comment: 4 pages, 2 figures, 1 tabl
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
Ground State Properties and Optical Conductivity of the Transition Metal Oxide
Combining first-principles calculations with a technique for many-body
problems, we investigate properties of the transition metal oxide from the microscopic point of view. By using the local density
approximation (LDA), the high-energy band structure is obtained, while screened
Coulomb interactions are derived from the constrained LDA and the GW method.
The renormalization of the kinetic energy is determined from the GW method. By
these downfolding procedures, an effective Hamiltonian at low energies is
derived. Applying the path integral renormalization group method to this
Hamiltonian, we obtain ground state properties such as the magnetic and orbital
orders. Obtained results are consistent with experiments within available data.
We find that is close to the metal-insulator transition.
Furthermore, because of the coexistence and competition of ferromagnetic and
antiferromgnetic exchange interactions in this system, an antiferromagnetic and
orbital-ordered state with a nontrivial and large unit cell structure is
predicted in the ground state. The calculated optical conductivity shows
characteristic shoulder structure in agreement with the experimental results.
This suggests an orbital selective reduction of the Mott gap.Comment: 38pages, 22figure
Optimized Effective Potential for Extended Hubbard Model
Antiferromagnetic and charge ordered Hartree-Fock solutions of the one-band
Hubbard model with on-site and nearest-neighbor Coulomb repulsions are exactly
mapped onto an auxiliary local Kohn-Sham (KS) problem within a
density-functional theory. The mapping provides a new insight into the
interpretation of the KS equations. (i) With an appropriate choice of the basic
variable, there is a universal form of the KS potential, which is applicable
both for the antiferromagnetic and the charge ordered solutions. (ii) The
Kohn-Sham and Hartree-Fock eigenvalues are interconnected by a scaling
transformation. (iii) The band-gap problem is attributed to the derivative
discontinuity of the basic variable as the function of the electron number,
rather than a finite discontinuity of the KS potential. (iv) It is argued that
the conductivity gap and the energies of spin-wave excitations can be entirely
defined by the parameters of the ground state and the KS eigenvalues.Comment: 21 page, 3 figure
QED Calculation of E1M1 and E1E2 Transition Probabilities in One-Electron Ions with Arbitrary Nuclear Charge
The quantum electrodynamical theory of the two-photon transitions in
hydrogenlike ions is presented. The emission probability for 2s1/2 -> 2E1+1s1/2
transitions is calculated and compared to the results of the previous
calculations. The emission probabilities 2p12 -> E1E2+1s1/2 and 2p1/2 ->
E1M1+1s1/2 are also calculated for the nuclear charge Z values 1-100. This is
the first calculation of the two latter probabilities. The results are given in
two different gauges.Comment: 14 pages, 4 tables, 1 figur
Extension of the sum rule for the transition rates between multiplets to the multiphoton case
The sum rule for the transition rates between the components of two
multiplets, known for the one-photon transitions, is extended to the
multiphoton transitions in hydrogen and hydrogen-like ions. As an example the
transitions 3p-2p, 4p-3p and 4d-3d are considered. The numerical results are
compared with previous calculations.Comment: 10 pages, 4 table
Asymmetry of the natural line profile for the hydrogen atom
The asymmetry of the natural line profile for transitions in hydrogen-like
atoms is evaluated within a QED framework. For the Lyman-alpha
absorption transition in neutral hydrogen this asymmetry results in an
additional energy shift of 2.929856 Hz. For the transition
it amounts to -1.512674 Hz. As a new feature this correction turns out to be
process dependent. The quoted numbers refer to the Compton-scattering process.Comment: RevTex, 7 Latex pages, 1 figur
Novel miR390-Dependent Transacting siRNA Precursors in Plants Revealed by a PCR-Based Experimental Approach and Database Analysis
TAS loci in plant genomes encode transacting small interfering RNAs (ta-siRNAs) that regulate expression of a number of genes. The function of TAS3 precursor in Arabidopsis thaliana is controlled by two miR390 target sites flanking two ta-siARF sequences targeting mRNAs of ARF transcription factors. Cleavage of the 3â˛-miR390-site initiates ta-siRNAs biogenesis. Here we describe the new method for identification of plant ta-siRNA precursors based on PCR with oligodeoxyribonucleotide primers mimicking miR390. The method was found to be efficient for dicotiledonous plants, cycads, and mosses. Based on sequences of amplified loci and a database analysis, a novel type of miR390-dependent TAS sequences was identified in dicots. These TAS loci are characterized by a smaller distance between miR390 sites compared to TAS3, a single copy of ta-siARF, and a sequence conservation pattern pointing to the possibility that processing of novel TAS-like locus is initiated by cleavage of the 5â˛-terminal miR390 target site
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