N. M. Plakida

High temperature superconductivity

This book contains papers presented at the international seminar on High Temperature Superconductivity. Included are the following papers: Oxygen disorder, superstructure and the dependence of T{sub c} on structure in oxide superconductors, structural instability and high temperature superconductivity, Theoretical aspects of radiation effects on high-T{sub c} superconductors.

High-Temperature Cuprate Superconductors

When Bednorz and Müller discovered the superconductivity in a compound La-Ba-Cu-O in 1986, it was considered as a breakthrough in the research of the superconductivity. This leads to the discovery of the other cuprate superconductors, and immediately the transition temperature of the synthesizedmaterials reached to the liquid nitrogen temperature. Today the maximum transition temperature of the cuprate superconductors changes from 35K for La2−xSrxCuO4 to 138K for Hg1−xTlxBa2Ca2Cu3Oy (the highest record under normal pressure, which extends to ∼160K at high pressure) [1]. 2.1 General Properties of Cuprates 2.1.1 Crystal Structure As a common property, all cuprate superconductors have a layered perovskite structure that consists of alternating conducting CuO2 layers and insulating blocking layers that provide charge carriers to the CuO2 layers. Even though this is the general scheme, the numbers of the CuO2 layers are different in different cuprate families. In Fig. 2.1, as an example to 1-, 2-, and 3layer cuprate families had been shown. In this thesis, we worked with double layer YBa2Cu3Oy system, because the additional structures, namely the CuO-chains in this system give some advantage in our measurements by increasing the conductivity along the c-axis. YBa2Cu3Oy is a system, where the charge doping can be adjusted with oxygen annealing. And this material belongs to the hole doped case, where we will show the electronic phase diagram in the next section. In cuprates, it is also possible to create hole doping in the system with substituting positive ions by ions of smaller valence, such as Sr2+ to La3+ sites in La2−xSrxCuO4. Zn-substitution that been used in this study, on the other hand does not alter the doping level.

Electron spectrum in high-temperature cuprate superconductors

A microscopic theory for the electron spectrum of the CuO2 plane within an effective p-d Hubbard model is proposed. The Dyson equation for the single-electron Green’s function in terms of the Hubbard operators is derived and solved self-consistently for the self-energy evaluated in the noncrossing approximation. Electron scattering on spin fluctuations induced by the kinematic interaction is described by a dynamical spin susceptibility with a continuous spectrum. The doping and temperature dependence of electron dispersions, spectral functions, the Fermi surface, and the coupling constant λ are studied in the hole-doped case. At low doping, an arc-type Fermi surface and a pseudogap in the spectral function close to the Brillouin zone boundary are observed.

Dynamic spin susceptibility in the t-J model

The dynamic spin susceptibility is calculated for the t-J model in the paramagnetic phase using the memory function method in terms of the Hubbard operators. A self-consistent system of equations for the memory function is obtained in the mode-coupling approximation. Both itinerant hole excitations and localized spin fluctuations contribute to the memory function. The spin dynamics have a diffusive character in the hydrodynamic limit; spin-wave-like excitations are regained in the high-frequency region.

Charge- and magnetic-ordering in a two-orbital double-exchange model for manganites

Phase diagram of half-doped perovskite manganites is studied within the extended double-exchange model. To demonstrate the role of orbital degrees of freedom both one- and two-orbital models are examined. A rich phase diagram is obtained in the mean-field theory at zero temperature as a function of J (antiferromagnetic (AFM) superexchange interaction) and V (intersite Coulomb repulsion). For the one-orbital model a charge-ordered (CO) state appears at any value of V with different types of magnetic order which changes with increasing J from ferromagnetic (F) to AFM ones of the types A, C and G. The orbital degeneracy results in appearance of a new CE-type spin order that is favorable due to opening of thedimerizationgap at the Fermi surface. In addition, the CO state appears only for V>V_c for F and CE states while C-type AFM state disappears and A-type AFM state is observed only at small values of V as a charge disordered one. The relevance of our results to the experimental data are dicussed.

Dynamic spin susceptibility of superconducting cuprates: A microscopic theory of the magnetic resonance mode

A microscopic theory of the dynamic spin susceptibility (DSS) in the superconducting state within the t-J model is presented. It is based on an exact representation for the DSS obtained by applying the Mori-type projection technique for the relaxation function in terms of Hubbard operators. The static spin susceptibility is evaluated by a sum-rule-conserving generalized mean-field approximation, while the self-energy is calculated in the mode-coupling approximation. The spectrum of spin excitations is studied in the underdoped and optimally doped regions. The DSS reveals a resonance mode (RM) at the antiferromagnetic wave vector Q = pi(1,1) at low temperatures due to a strong suppression of the damping of spin excitations. This is explained by an involvement of spin excitations in the decay process besides the particle-hole continuum usually considered in random-phase-type approximations. The spin gap in the spin-excitation spectrum at Q plays a dominant role in limiting the decay in comparison with the superconducting gap which results in the observation of the RM even above

On the theory of superconductivity in the extended Hubbard model

T_c

Optical and dc conductivities of cuprates: Spin fluctuation scattering in the t-J model

in the underdoped region. A good agreement with inelastic neutron-scattering experiments on the RM in YBCO compounds is found.

Spin excitations and thermodynamics of the antiferromagnetic Heisenberg model on the layered honeycomb lattice

A microscopic theory of superconductivity in the extended Hubbard model which takes into account the intersite Coulomb repulsion and electron-phonon interaction is developed in the limit of strong correlations. The Dyson equation for normal and pair Green functions expressed in terms of the Hubbard operators is derived. The self-energy is obtained in the noncrossing approximation. In the normal state, antiferromagnetic short-range correlations result in the electronic spectrum with a narrow bandwidth. We calculate superconducting Tc by taking into account the pairing mediated by charge and spin fluctuations and phonons. We found the d-wave pairing with high-Tc mediated by spin fluctuations induced by the strong kinematic interaction for the Hubbard operators. Contributions to the d-wave pairing coming from the intersite Coulomb repulsion and phonons turned out to be small.

Theory of spin-wave excitations of metallic A-type antiferromagnetic manganites.

A microscopic theory of the electrical conductivity