533 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
Surface quality studies of high T superconductors of the Hg , Tl and HgTl-families: RBS and resonant C and O backscattering studies
The composition, crystallinity, uniformity, purity, and thermal stability of cuprate superconductors have been studied by Rutherford backscattering and channeling spectrometry, and 3.045 MeV He oxygen non-Rutherford resonant scattering. Further experiments have been performed with 1.75 MeV Hcarbon non-Rutherford resonant scattering. Three sets of samples were studied : HgBaCuO (Hg1201), HgT1BaCaCuO (Hg,T1-1223) and T1BaCuO/LaA10 (T1-2201), either in bulk or as an epitaxial thin film. It was observed that the superconductors exhibit a metal deficiency near the surface, which is largely compensated by excess oxygen. Moreover, the samples are significantly contaminated with carbon within the probing region of the H beam. The thermal stability and surface degradation were studied in both oxidizing ambient and vacuum. As a general trend, the heavy metal deficiency — and consequently the compensating oxygen excess — is enhanced as the temperature increases
Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure
We demonstrate the emergence of superconductivity in pseudo-binary silicide
SrNixSi2-x. The compound exhibits a structural phase transition from the cubic
SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon
substituting Ni for Si at approximately x = 0.1. The hexagonal structure is
stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in
the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant,
as confirmed by the Pauli paramagnetic behavior.Comment: 12 pages, 5 figure
Theoretical evidences for enhanced superconducting transition temperature of CaSi in a high-pressure AlB phase
By means of first-principles calculations, we studied stable lattice
structures and estimated superconducting transition temperature of CaSi at
high pressure. Our simulation showed stability of the AlB structure in a
pressure range above 17 GPa. In this structure, doubly degenerated optical
phonon modes, in which the neighboring silicon atoms oscillate alternately in a
silicon plane, show prominently strong interaction with the conduction
electrons. In addition there exists a softened optical mode (out-of-plan motion
of silicon atoms), whose strength of the electron-phonon interaction is nearly
the same as the above mode. The density of states at the Fermi level in the
AlB structure is higher than that in the trigonal structure. These findings
and the estimation of the transition temperature strongly suggest that higher
is expected in the AlB structure than the trigonal structures
which are known so far.Comment: 6 pages and 11 figure
Molecular Control of the Catalytic Properties of Rhodium Nanoparticles in Supported Ionic Liquid Phase (SILP) Systems
Rhodium nanoparticles (NPs) immobilized on imidazolium-based supported ionic liquid phases (Rh@SILP) act as effective catalysts for the hydrogenation of biomass-derived furfuralacetone. The structure of ionic liquid-type (IL) molecular modifiers was systematically varied regarding spacer, side chain, and anion to assess the influence on the NP synthesis and their catalytic properties. Well-dispersed Rh NPs with diameters in the range of 0.6–2.0 nm were formed on all SILP materials, whereby the actual size was dependent significantly on the IL structure. The resulting variations in catalytic activity for hydrogenation of the C═O moiety in furfuralacetone allowed control of the product selectivity to obtain either the saturated alcohol or the ketone in high yield. Experiments conducted under batch and continuous flow conditions demonstrated that Rh NPs immobilized on SILPs with suitable IL structures are more active and much more stable than Rh@SiO2 catalyst synthesized on unmodified silica
Estimates of electronic interaction parameters for LaO compounds (=Ti-Ni) from ab-initio approaches
We have analyzed the ab-initio local density approximation band structure
calculations for the family of perovskite oxides, LaO with =Ti-Ni
within a parametrized nearest neighbor tight-binding model and extracted
various interaction strengths. We study the systematics in these interaction
parameters across the transition metal series and discuss the relevance of
these in a many-body description of these oxides. The results obtained here
compare well with estimates of these parameters obtained via analysis of
electron spectroscopic results in conjunction with the Anderson impurity model.
The dependence of the hopping interaction strength, t, is found to be
approximately .Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct
15,1996
Geology of the Dhaulagiri-Annapurna-Manaslu Himalaya, Western Region, Nepal. 1:200,000
Geological mapping of mountains belts is fundamental to understanding their structure and evolution. Here, a 1:200,000 scale geological map of the central Himalaya of Western Region, Nepal is presented. This map represents a compilation of previously published maps, integrated with new geological field data. The wide spatial coverage of the map and the accompanying cross sections reveal the detailed structure of the Dhaulagiri-Annapurna-Manaslu Himalaya. The addition of modern topographic and infrastructure data makes this map suitable for navigation through the region
Pressure Effects and Large Polarons in Layered MgB_2 Superconductor
We consider the dependence of the MgB_2 superconducting critical temperature
on the pressure. Our model exploits the influence of the large polarons on the
band structure of the layered MgB_2 superconductor. Namely, the hole
Pekar-Froehlich polarons form quasi two-dimensional potential wells in the
boron plane which shift the positions of the sigma- and pi-bands. This energy
shift depends on the pressure and the Cooper pairing of the correlated
sigma-electrons happens inside polaron wells. The results obtained are as
follows: dT_c/dp = -\alpha (5.2 \pm 0.9) K/GPa or dT_c/dp = -\alpha (6.9\pm
1.1) K/GPa for a different choice of the Grueneisen parameter. Being compared
with known experimental data they give us a resonable interval for the value of
the Froehlich electron-phonon coupling constant: \alpha = 0.15 - 0.45.Comment: 6 pages, 1 fig, LaTeX, subm. to Phys. Rev.
Sleep deprivation and Modafinil affect cortical sources of resting state electroencephalographic rhythms in healthy young adults
Objective: It has been reported that sleep deprivation affects the neurophysiological mechanisms underpinning the vigilance. Here, we tested the following hypotheses in the PharmaCog project (www.pharmacog.org): (i) sleep deprivation may alter posterior cortical delta and alpha sources of resting state eyes-closed electroencephalographic (rsEEG) rhythms in healthy young adults; (ii) after the sleep deprivation, a vigilance enhancer may recover those rsEEG source markers. Methods: rsEEG data were recorded in 36 healthy young adults before (Pre-sleep deprivation) and after (Post-sleep deprivation) one night of sleep deprivation. In the Post-sleep deprivation, these data were collected after a single dose of PLACEBO or MODAFINIL. rsEEG cortical sources were estimated by eLORETA freeware. Results: In the PLACEBO condition, the sleep deprivation induced an increase and a decrease in posterior delta (2–4 Hz) and alpha (8–13 Hz) source activities, respectively. In the MODAFINIL condition, the vigilance enhancer partially recovered those source activities. Conclusions: The present results suggest that posterior delta and alpha source activities may be both related to the regulation of human brain arousal and vigilance in quiet wakefulness. Significance: Future research in healthy young adults may use this methodology to preselect new symptomatic drug candidates designed to normalize brain arousal and vigilance in seniors with dementia
C-axis lattice dynamics in Bi-based cuprate superconductors
We present results of a systematic study of the c axis lattice dynamics in
single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer
Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both
experimental data obtained by spectral ellipsometry on single crystals and
theoretical calculations. The calculations are carried out within the framework
of a classical shell model, which includes long-range Coulomb interactions and
short-range interactions of the Buckingham form in a system of polarizable
ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and
Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes
of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve
good agreement between the calculated A2u eigenfrequencies and the experimental
values of the c axis TO phonon frequencies which allows us to make a reliable
phonon mode assignment for all three Bi-based cuprate superconductors. We also
present the results of our shell model calculations for the Gamma-point A1g
symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is
based on the published experimental Raman spectra. The
superconductivity-induced phonon anomalies recently observed in the c axis
infrared and resonant Raman scattering spectra in trilayer Bi2223 are
consistently explained with the suggested assignment.Comment: 29 pages, 13 figure
- …