258 research outputs found
Electron energy spectrum and magnetic interactions in high-T(sub c) superconductors
The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T sub N1 and T sub N2. The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T sub N2 reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T sub N1 and T sub N2 depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J sub ij parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors
Crystal chemical and quantum chemical studies of Ba(Sr)-Nb oxide compounds
The information available on the BaO(SrO)-NbO-NbO2 system with the niobium atom in the lower oxidation degree is very limited. Very few compounds have been found previously in this system. They are BaNbO3, SrxNbO3(0,7=x=1), Ba2Nb2O9, SrNb8O14; and some suggestions on the BaNb8O14 existence have been made also. At the same time Nb-based oxide compounds could be quite interesting in the search of new noncopper high T(sub c) superconductors Researchers studied Ba(Sr) NbxO2x-2 and Ba2(Sr2)-NbxO2x-1 compositions in the phase diagram of BaO(SrO)-NbO-NbO2 system. The synthesis of the materials was carried out in vacuum at the temperatures of 1000 to 1500 C. Barium carbonate and niobium pentoxide were used as initial components. X-ray analysis was carried out
Implementation of the LDA+U method using the full potential linearized augmented plane wave basis
We provide a straightforward and efficient procedure to combine LDA+U total
energy functional with the full potential linearized augmented plane wave
method. A detailed derivation of the LDA+U Kohn-Sham type equations is
presented for the augmented plane wave basis set, and a simple
``second-variation'' based procedure for self-consistent LDA+U calculations is
given. The method is applied to calculate electronic structure and magnetic
properties of NiO and Gd. The magnetic moments and band eigenvalues obtained
are in very good quantitative agreement with previous full potential LMTO
calculations. We point out that LDA+U reduces the total d charge on Ni by 0.1
in NiO
Ginzburg-Landau Expansion and the Slope of the Upper Critical Field in Disordered Superconductors with Anisotropic Pairing
It is demonstrated that the slope of the upper critical field
in superconductors with -wave pairing drops rather
fast with concentration of normal impurities, while in superconductors with
anisotropic -wave pairing grows, and in the limit of
strong disorder is described by the known dependences of the theory of
``dirty'' superconductors. This allows to use the measurements of in
disordered superconductors to discriminate between these different types of
pairing in high-temperature and heavy-fermion superconductors.Comment: 7 pages, 5 figures, RevTeX 3.0, 4 Postscript figures attached;
Submitted to JETP Letter
Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation
Starting from exact expression for the dynamical spin susceptibility in the
time-dependent density functional theory a controversial issue about exchange
interaction parameters and spin-wave excitation spectra of itinerant electron
ferromagnets is reconsidered. It is shown that the original expressions for
exchange integrals based on the magnetic force theorem (J. Phys. F14 L125
(1984)) are optimal for the calculations of the magnon spectrum whereas static
response function is better described by the ``renormalized'' magnetic force
theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is
confirmed by the {\it ab initio} calculations for Fe and Ni.Comment: 12 pages, 2 figures, submitted to JPC
Electron energy spectrum and magnetic interactions in high-Tc superconductors
The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T(sub N1) and T(sub N2). The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T(sub N2) reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T(sub N1) and T(sub N2) which depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg Hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J(sub ij) parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors
Interplay of p-d and d-d charge transfer transitions in rare-earth perovskite manganites
We have performed both theoretical and experimental study of optical response
of parent perovskite manganites RMnO_3 with a main goal to elucidate nature of
clearly visible optical features. Starting with a simple cluster model approach
we addressed the both one-center (p-d) and two-center (d-d) charge transfer
(CT) transitions, their polarization properties, the role played by structural
parameters, orbital mixing, and spin degree of freedom. Optical complex
dielectric function of single crystalline samples of RMnO_3 (R=La, Pr, Nd, Sm,
Eu) was measured by ellipsometric technique at room temperature in the spectral
range from 1.0 to 5.0 eV for two light polarizations: E \parallel c and E \perp
c. The comparative analysis of the spectral behavior of \varepsilon _1 and
\varepsilon _2 is believed to provide a more reliable assignment of spectral
features. We have found an overall agreement between experimental spectra and
theoretical predictions based on the theory of one-center p-d CT transitions
and inter-site d-d CT transitions. Our experimental data and theoretical
analysis evidence a dual nature of the dielectric gap in nominally
stoichiometric matrix of perovskite manganites RMnO_3, it is formed by a
superposition of forbidden or weak dipole allowed p-d CT transitions and
inter-site d-d CT transitions. In fact, the parent perovskite manganites RMnO_3
should rather be sorted neither into the CT insulator nor the Mott-Hubbard
insulator in the Zaanen, Sawatzky, Allen scheme.Comment: 20 pages, 6 figure
First- principle calculations of magnetic interactions in correlated systems
We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.Comment: 17 pages, 3 Postscript figure
Theory of Coherent -Axis Josephson Tunneling between Layered Superconductors
We calculate exactly the Josephson current for -axis coherent tunneling
between two layered superconductors, each with internal coherent tight-binding
intra- and interlayer quasiparticle dispersions. Our results also apply when
one or both of the superconductors is a bulk material, and include the usually
neglected effects of surface states. For weak tunneling, our results reduce to
our previous results derived using the tunneling Hamiltonian. Our results are
also correct for strong tunneling. However, the -axis tunneling results of
Tanaka and Kashiwaya are shown to be incorrect in any limit. In addition, we
consider the -axis coherent critical current between two identical layered
superconductors twisted an angle about the -axis with respect to
each other. Regardless of the order parameter symmetry, our coherent tunneling
results using a tight-binding intralayer quasiparticle dispersion are
inconsistent with the recent -axis twist bicrystal
BiSrCaCuO twist junction experiments of Li {\it et
al.}Comment: 11 pages, 13 figures, submitted to Physical Review
Temperature dependence of the collective mode and its influence on the band splitting in bilayer cuprates
The recently observed bilayer splitting in high-T cuprates is analyzed
within a model where the charge carriers are coupled to a phenomenological
bosonic spectrum which interpolates between the marginal Fermi liquid structure
and collective mode type behavior as a function of temperature. We argue that
the origin of the collective mode is probably associated with dynamic
incommensurate charge density waves. Moreover it is shown that the resulting
temperature dependence of the self-energy is in good agreement with
as extracted from angle-resolved photoemission data.Comment: 6 pages, 4 figures, accepted for PR
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