Theoretical analysis of high-temperature superconductivity in oxides.

ABSTRACTThe availability of large amounts of Buckminsterfullerene has allowed a plethora of experimental investigations on fullerenes. The chemical and physical studies have focussed on synthesis, isomerism, magnetism, spectroscopy and high temperature superconductivity in doped materials. The chemical reactivities of fullerenes have been defined and most of the studies are dominated by C60 isomers. Some of the observed activities of fullerenes parlled those of alkeies. In our previous studies, the reactivity of the 6-6' bond with respect to eco- addition was described. Current studies have exploited the olefinic nature of the 6-6' bond and analyzed the effect of the addition on cluster stability and frontier character. In this work, we describe the mechanisms of simple substitution and analyze stability and orbital effects for the addition chemistry of C60 with multiple species. Evolving changes in orbital frontier character are analyzed with respect to site directed chemistry exhibited by C60.

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REMARK ON THE EXISTENCE OF LONG-RANGE ORDER IN QUASI-TWO-DIMENSIONAL HUBBARD MODEL

Modern Physics Letters B ◽

10.1142/s0217984996000389 ◽

1996 ◽

Vol 10 (08) ◽

pp. 341-346 ◽

Cited By ~ 1

Author(s):

A. BELKASRI ◽

J.L. RICHARD

Keyword(s):

Thin Film ◽

High Temperature ◽

Theoretical Analysis ◽

Hubbard Model ◽

Long Range ◽

High Temperature Superconductivity ◽

Type Model ◽

Long Range Order ◽

Two Dimensional ◽

2D System

Recently in many works on the mechanism of high temperature superconductivity (see for example Refs. 1–6), quasi-averages like <ck↑c−k↓> were considered even in the case of a dimension less or equal two. But it is well known from the old work of Hohenberg7 that these quasi-averages are zero at T≠0 in case of 1 and 2 dimensions. In this communication we generalize the Hohenberg’s result to any kind of Hubbard type model on lattice and prove that in the case of quasi-two-dimension, the theorem of Hohenberg is not in contradiction with having <ck↑c−k↓>≠0 (at T≠0). In practice this makes sense to compare the data for a thin film (which can be considered as quasi-2D system) to the theoretical analysis based on quasi-two-dimensional models, but not for strictly two-dimensional case.

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High-Temperature Superconductivity in Graphite-Sulfur Composites: Theoretical Analysis

MRS Proceedings ◽

10.1557/proc-689-e5.2 ◽

2001 ◽

Vol 689 ◽

Author(s):

D. S. Galvão ◽

B. Laks ◽

R. R. da Silva ◽

J. H. S. Torres ◽

Y. Kopelevich

Keyword(s):

Electronic Structure ◽

High Temperature ◽

Theoretical Analysis ◽

Density Of States ◽

High Temperature Superconductivity ◽

Tight Binding ◽

Two Dimensional ◽

Negative Factor ◽

Structure Changes ◽

Geometric Curvature

ABSTRACTRecently superconductivity in graphite-sulfur composites was experimentally observed. In this work we have analyzed the electronic structure changes associated with the presence of sulfur atoms in one and two dimensional graphite layers. We have considered ordered and disordered sulfur atoms distributions in many configurations. The density of states (DOS) of these structures were obtained using the negative factor counting (NFC) technique coupled to a tight-binding hamiltonian (Hückel type). Our results indicate that the incorporation of sulfur atoms at edge graphite layers (changing their global geometric curvature and increasing the DOS at the Fermi level) might be in the origin of the graphite superconductivity.

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