1PO1-50 Constraints on the Application of the Holstein model to High T_c Superconductors

Marco Zoli, Istituto Nazionale Fisica della Materia, Dipartimento di Fisica, Universita' di Camerino, Italy

Presenting Author: M. Zoli

I study the conditions under which strong coupling perturbation theory can be applied to the Molecular Crystal Model, a fundamental tool for the analysis of the polaron properties. The polaron effective mass has been computed in different regimes ranging from the fully antiadiabatic to the fully adiabatic one. The Polaron mass becomes essentially dimension independent for sufficiently strong intermolecular coupling strengths and converge to much lower values than those traditionally obtained in small polaron theory.

I find evidence for a self-trapping transition in a moderately adiabatic regime, peculiar of the high T_c systems, at an electron-phonon coupling of about 3. The model is applied to calculate the c-axis electrical resisitivity in some high T_c compounds.

1PO1-51 Temperature Dependence of the Strength of the Interlayer Coupling of anisotropic HTS in Normal State

Galina Sergeeva, Kharkov Institute of Physics and Technology, Akademicheskaya Street 1, 310108 Kharkov, Ukraine

Presenting Author: G. Sergeeva

The nature of strong temperature dependence of the strength of interlayer coupling t^ (T) for anisotropic HTS in normal state is studied. It is shown that with decreasing T up to the temperature of superconducting transition T_c the decreasing of t^ (T) is due by 2D superconducting fluctuations in CuO₂ plane. At T£ T_c0, where T_c0 is the temperature of superconducting transition in CuO₂ plane, these fluctuations lead to the different temperature dependencies of in-plane correlation length x _ab(T)» (1-T/T_c0)^-1/2 and of out-of-plane correlation length x _c(T)» const, that leads to the dependence t^ (T)» ( x _c(T)/ x _ab(T))² » (1-T/T_c0).

At T_c p T p T_c0 the difference of the phases of the order parameter on fluctuation 2D-vortices and anti-vortices leads to additional mechanism of t^ (T) decreasing. In superconducting state at T£ T_c the temperature dependencies of in-plane and of out-of-plane correlation lengths are equal, and the strength of interlayer coupling t^ (T) does not depend on the temperature.

1PO1-52 Model for square planar superconductors which includes correlation and vibronic interaction

Sven Larsson and Agris Klimkans, Physical Chemistry Dept., Chalmers University of Technology, S-41296 Gothenburg, Sweden

Presenting Author: S. Larsson

4-site 4-electron models which include electron correlation and Jahn-Teller type distortions have been developed for different types of chemical systems (for example cyclobutadiene). Such models may be extended to any square-planar system and show charge density wave, spin density wave, or pair conducting phases. Electron correlation and lattice coupling are included.

The model explains a number of experimental facts for layered, square-planar superconductors such as d-wave symmetry in cuprates. We hope to show that the currents are identical to the superconducting currents of the Meissner effect, and similar to the order parameter. Pairing attraction is due to resonance between different oxidation states (chemistry), alternatively (physics) between CDW and SDW states. This interaction is directly coupled to nuclear motions and leads to the superconducting gap. Calculation of gap is shown.

The model further explains why cuprate perovskites are superconducting but not, for example, corresponding coboltates. It further explains superconductivity in the beginning of d-series and possibly in some 'unconventional' systems, such as alkali-ammonia.

1PO1-53 Brief Report on the Mechanism of Superconductivity in Cuprates

Ara M. Kechiantz, Institute for Physical Researches, National Academy of Sciences of Armenia. 40-8 Charentz Street, Yerevan, 375025, Armenia

Presenting Author: A.M. Kechiantz

The fair ‘reverse engineering’ and analyzing of superconducting cuprates’ properties are made with emphasis on the distribution of carriers within crystal, over sites that are favourable for carriers moving.

The favourable sites are arranged into regular sub-lattice of the ionic lattice. The self-consistent repulsion energy that causes the cohesion between carriers, the characteristic features that the repulsion imposes on carriers and the basic equations that determine regular and random distributions of carriers over the sub-lattices of favourable sites are obtained and discussed.

The numerical computation of the basic equations reveals the strong trend towards ordering of carriers into isolated clusters of regular electronic lattice. Second order phase transition that isolated clusters merge into the infinite cluster of regular electronic lattice emerges from the computation. The results that the maximum of the T_c dependence on the doping occurs at the 0.2 carriers per site in the square lattice of favourable sites, and that isolated mobile clusters result in the linear dependence of resistivity on temperature just above the T_c support the conclusion that the ordering of carriers into the mobile electronic lattice causes unconventional superconductivity in cuprates.

1PO1-54 Coulomb coupling of like charges due to negative reduced effective mass

Vladimir I. Belyavskii, Vladimir V. Kapaev, and Yurii V. Kopaev, P.N. Lebedev Physical Institute of Russian Academy of Sciences, 117924 Moscow, Russia

Presenting Author: Y.V. Kopaev

We have shown previously the possibility of the formation of quasistationary states (QSS’s) of electron or hole pairs by repulsive interaction in the cases corresponding to different signs of the reduced effective masses. Such situation can take place in high-temperature superconductors.

Provided that QSS life time is large enough one may assume that such pairs are specific quasiparticles subordinate to a statistics other than simple Fermi one. If the energy of a QSS decreases due to the negative sign of one of the effective masses it may be a decrease of the full energy of the electron system with respect to the energy corresponding to any one-electron approximation. As a result one may expect a formation of a new state of the electron system having superconductive properties. Such possibility depends on Fermi surface disposition with respect to Van Hove singularities.

Replacing short-range screened Coulomb electron-electron interaction by a strongly localized potential, we have calculated the scattering amplitude corresponding to relative motion of the components of the pair. This allowed us to evaluate the pair energy and the life time and also to determine a condition which gave rise to a long-living QSS.

The estimation of the concentration of pairs corresponding to long-living QSS’s gives a value in the range of 10¹⁷-10¹⁹ cm^-3.

1PO1-55 The Nature of a Correct Theory of High Temperature Superconductivity

John D. Dow, Department of Physics, Arizona State University, Tempe, AZ 85187-1504, USA and Howard A. Blackstead, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA.

Presenting Author: J.D. Dow

Analyses of data for many materials reveal that (i) in all known cases, high-temperature superconductors are p-type, and none are n-type; (ii) the oxygen ions carry the holes that superconduct; (iii) these hole-carrying oxygen ions are primarily in the charge-reservoirs, not in the cuprate-planes; (iv) cuprate-planes are unnecessary for high-temperature superconductivity; (v) the Cooper-pairing is conventional, namely phononic and excitonic, not magnetic (e.g., not by spin-fluctuations); (vi) materials with the same charge-reservoir layers have almost the same critical temperatures; but (vii) materials with cuprate planes in similar environments (e.g., bounded by rare-earth layers and SrO layers) do not have even similar critical temperatures.

A correct theory of the superconductivity is necessarily BCS-like, with novel hole-pairing. Even in superconductors as common as NdBa₂Cu₃O₇, the cuprate planes are clearly not the main superconducting layers.

1PO1-56 Four Predicted High Temperature Superconductors

John D. Dow, Department of Physics, Arizona State University,Tempe, AZ 85187-1504, USA and Howard A. Blackstead, Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA.

Presenting Author: J.D. Dow

Granular superconductivity has been observed, as predicted, in the materials Pr_1.5Ce_0.5Sr₂Cu₂NbO₁₀ [1] and Eu_1.5Ce_0.5Sr₂Cu₂TiO₁₀ [2]. These are the third and fourth high-temperature superconductors we predicted that subsequently did superconduct --- the first two were PrBa₂Cu₃O₇ and Gd_2-zCe_zSr₂Cu₂TiO₁₀. The predictions were based on the idea that the charge-reservoirs, not the cuprate-planes, are the primary superconductors.

These results are consistent with the observations that the superconductivity of PrBa₂Cu₃O₇, NdBa₂Cu₃O₇, and Eu_2-zCe_zSr₂Cu₂RuO₁₀ cannot originate in the cuprate-planes.

[1] H.A. Blackstead, J.D. Dow, I. Felner, H. Luo, D.B. Pulling, and W.B. Yelon, Int. J. Mod. Phys. B 12, 3074 (1998).

[2] H.A. Blackstead and J.D. Dow, I. Felner, and D.B. Pulling, to be published.

1PO1-57 The arrow of time and the character of pairing interactions

Matias Moreno, Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000, México, D.F., México, and Rosa M. Mendez-Moreno, Departamento de Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, Apartado Postal 21-092, 04021 México, D.F., México

Presenting Author: M. Moreno

Using a recently found generalization of the gap equation, we study the minimal conditions of the pairing interactions which could develop time reversal violation. Explicit solutions valid for weakly coupled systems in which the fermions generate a discrete quantum number are presented. The relevance of this study for high T_c systems (HTS) is evaluated; in this context we find that the BCS formalism extended to multicomponent systems supports high T_c superconductor systems even in the weak coupling regime. These solutions break the traditional, one component, R relation between gap size to T_c value.

1PO1-58 Parametrization of Tl-based high-T_c superconductors in the generalized supersymmetric theory.

Rosa M. Mendez-Moreno, Departamento de Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, Apartado Postal 21-092, 04021 México, D.F., México, and Matias Moreno, Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364. 01000, México, D.F., México

Presenting Author: R.M. Mendez-Moreno

Using a recently proposed formalism based in a generalization of supersymmetry, we can write the gap equation for multicomponent systems. Within this framework we present a parametrization of Tl-based superconductor cuprates. These materials, which present critical temperatures above 100 K, can be described within the proposed formalism even in the weak coupling BCS framework.

1PO1-59 A New Understanding of the BCS Hamiltonian

J.D. Fan and Y.M. Malozovsky, Department of Physics, Southern University and A&M College, P.O. Box 9767, Baton Rouge, LA 70813, USA

Presenting Author: J.D. Fan

It is shown that the BCS Hamiltonian can easily be derived from the second quantization formalism with the condition of k + k' = 0 (also, s + s ' = 0). This Hamiltonian may be understood in either particle-particle channel or particle-hole channel. The former is the well-known BCS point based on the idea of Cooper's pairing. In contrast to the conventional interpretation of this condition and the interaction v(k, k') in the BCS Hamiltonian, it is proposed that the above condition is a statistical grouping condition of sorting electrons pair by pair, and that v(k, k') is naturally attractive because it represents the Coulomb interaction between an electron and a hole in k-space. However, the interaction in the particle-particle channel, i.e. between two electrons, is still repulsive as given by the Coulomb law. This interpretation is particularly for fermions only based on the Pauli exclusion principle.

Further, it is indicated that the ground state energy with the electron-hole pairs is lower than that with the electron-electron Cooper's pairs. Therefore, the ground state in the particle-hole channel is preferable for a many-electron system. In this sense, the concept of Cooper's pairing is unnecessary for superconductivity and the BCS theory remains valid without it. In addition, many controversies encountered in high-temperature superconductivity will disappear if the concept of Cooper's pairing is discarded. Low- and high-temperature superconductivity can thus be unified in a unique theory from the repulsive Coulomb interaction between electrons.