Superconductivity of strongly correlated electrons in copper and ruthenium oxides within the t-J-I model

We propose a t-J-I model with direct ferromagnetic exchange I to explain the superconductivity of copper oxides and the ruthenate Sr₂RuO₄ on the basis of the analysis of the electronic structure of these substances. We analyze the possible p-and d-type superconducting solutions. Solutions of the s type with singlet pairings are impossible in the strong-electron-correlations regime, and p-type solutions correspond to triplet superconductivity and is formed near the ferromagnetic instability threshold in ruthenates. The solution with the symmetry near the antiferromagnetic instability threshold corresponds to copper oxides. We also discuss the reason for the high values of the superconducting transition temperature (T _c ∼100 K) in copper oxides and the low values (T _c ∼1 K) in ruthenates. Zh. éksp. Teor. Fiz. 116, 655–670 (August 1999)     

Electrochemical transient investigations on the diffusion of minority charge carriers in YSZ doped by transition metal oxides

The voltage relaxation of galvanic cells with zirconia based electrolytes polarised between an inert Pt electrode and a Pt/air electrode is analysed to obtain the diffusion coefficients of holes and electrons. The hole diffusion coefficient can be reduced by replacing zirconium with guest ions of different size, e.g. Nb⁵⁺ and Ti⁴⁺. The TZP phase with 3 mol% Y₂O₃ of dopant has a higher hole diffusion coefficient than the CYZ phase doped with 8 mol% Y₂O₃. 1 and 3 mol% p-type MnO_1.5 doping increases the conductivity of holes in CYZ to a large extend, but does not influence the diffusivity. This indicates that the doping increases the hole conductivity through an increased concentration of holes. In the case of 10 and 15 mol% MnO_1.5 doped Z3Y, the electronic conductivity is dominant. The chemical diffusion coefficients which are related to the oxygen vacancies were determined by GITT. The results show that the chemical diffusion coefficient of oxygen vacancies is much larger than that for holes in zirconia.     

Cluster theory of charge-transfer excitations in strongly correlated oxides

The cluster model of p-d and d-d charge-transfer transitions in strongly correlated 3d oxides is briefly reviewed. The application of this model is illustrated by an analysis of experimental optical spectra of orthorhombic manganites of the LaMnO₃ type.     

Skyrmion-approach study of the magnetic properties and spin kinetics of weakly doped cuprates

The evolution of the magnetic and kinetic properties of quasi-two-dimensional cuprates has been studied upon doping CuO₂ planes with electron holes. The notion of thermal skyrmions and hole-induced skyrmions was used to determine the spin-correlation length and nuclear relaxation rate as functions of temperature and hole concentration.     

Spinons in the strongly correlated copper oxide chains in SrCuO2

We have investigated the spin dynamics in the strongly correlated chain copper oxide SrCuO2 for energies up to greater, similar 0.6 eV using inelastic neutron scattering. We observe a gapless continuum of magnetic excitations, which is well described by the "Müller ansatz" for the two-spinon continuum in the S=1/2 antiferromagnetic Heisenberg spin chain. The lower boundary of the continuum extends up to approximately 360 meV, which corresponds to an exchange constant J=226(12) meV.     

The mobility of charge carriers in molecularly doped polymer films with various thicknesses

The dependence of the mobility of charge carriers in molecularly doped polycarbonate on the thickness of the polymer was studied at two dopant DEH concentrations (30 and 50 wt %). The mobility of holes in the more concentrated polymer did decrease as the film thickness increased in close agreement with the predictions of the theory of weakly nonequilibrium transport with a dispersion parameter of 0.75. For films with a lower dopant concentration, such a categorical statement cannot be made, because mobility can either decrease as the film thickness increases or remain almost unchanged depending on the method for data processing. For this reason, it is not recommended to use the dependence of the time of flight on film thickness as a criterion for selecting the mechanism of the transport of charge carriers in molecularly doped polymers.     

Quantum transport of slow charge carriers in quasicrystals and correlated systems

We show that the semiclassical model of conduction breaks down if the mean free path of charge carriers is smaller than a typical extension of their wave function. This situation is realized for sufficiently slow charge carriers and leads to a transition from a metalliclike to an insulatinglike regime when scattering by defects increases. This explains the unconventional conduction properties of quasicrystals and related alloys. The conduction properties of some heavy fermions or polaronic systems, where charge carriers are also slow, present a deep analogy.     

Isotope effect in the model of strongly correlated electrons with the magnetic and phonon superconducting pairing mechanisms

Peculiarities of the temperature isotope effect in a BCS-type theory describing the exchange and phonon mechanisms of Cooper pairing in a system of strongly correlated electrons are considered. The electron-phonon interaction constant is determined from the fitting of the calculated value of the isotope-effect index to the observed value with the parameters of La_{2 − x} Sr _x CuO₄ obtained from ab initio calculations. The value of this constant indicates that the contribution from the traditional pairing mechanism to the superconducting state is of the same order of magnitude as the contribution from the magnetic mechanism.     

Interrelation of magnetic and Jahn-Teller orderings in weakly doped lanthanium-strontium

The mutual influence of ferromagnetic and Jahn-Teller ordering in weakly doped manganites with the content La_{1 ? x} Sr _x MnO₃ (0.17 < x < 0.175) is considered. A phenomenological explanation of colossal magnetic resistance based on suppression of the local Jahn-Teller structure of distorted MnO₆ octahedrons by means of an applied magnetic field is suggested.     

The ground state of a strongly correlated fermion system in a magnetic field

A new variational method is used to investigate the ground state of the Hubbard model with a half-filled band for a one-dimensional chain, a planar square lattice, and a simple cubic lattice. A metamagnetic transition is found to occur in a one-dimensional chain and a simple square lattice. A simple cubic lattice does not undergo the metamagnetic transition.     

Metal-ligand interplay in strongly correlated oxides: a parametrized phase diagram for pressure-induced spin transitions

We investigate the magnetic properties of archetypal transition-metal oxides MnO, FeO, CoO, and NiO under very high pressure by x-ray emission spectroscopy at the Kbeta line. We observe a strong modification of the magnetism in the megabar range in all the samples except NiO. The results are analyzed within a multiplet approach including charge-transfer effects. The spectral changes are well accounted for by changes of the ligand field acting on the d electrons and allows us to extract the d-hybridization strength, O-2p bandwidth and ionic crystal field across the magnetic transition. This approach allows first-hand insight into the mechanism of the pressure-induced spin transition.     

A doubly degenerate Hubbard model in the strongly correlated limit

Using a doubly degenerate Hubbard model we derive an effective Hamiltonian in the limiting case where the Coulomb repulsion is much greater than the hopping constant. A brief discussion of this Hamiltonian is given.     

The effective Hamiltonian of the singlet-triplet model for copper oxides

The effective Hamiltonian for a realistic multiband p-d model is developed. In the case of electron doping, the Hamiltonian coincides with that for the standard t-J model. For hole doping, the singlet-triplet t-J model takes place.     

Relaxation and dynamics of charge carriers in organic polymer semiconductors: Polyacetylene (review)

The results of investigations of cis-polyacetylene and trans-polyacetylene by EPR spectroscopy in the 2-mm wave range are systematized and generalized. The nature and dynamic properties of nonlinear soliton-type carriers are discussed, along with the charge transfer mechanism in neutral polyacetylene. Fiz. Tverd. Tela (St. Petersburg) 39, 3–17 (January 1997)