Brent A. Richert

A tight-binding description of high-temperature superconductors

A single tight-binding model, with transferable parameters, provides a good description of the electronic structures of all currently known high-temperature superconductors, in the sense that results in the local density approximation are satisfactorily reproduced. Using this model, one can study the effects of atomic substitutions, vacancies, and structural modifications.

LaO/BaO/SrO oxygen(p)-metal(d) excitons as the mechanism of high-temperature superconductivity

Abstract The discovery that cubic Ba 0.6 K 0.4 BiO 3 is a high-temperature superconductor, with T c ≈ 34 K, poses a challenge to theories based on CuO 2 planes. On the other hand, this material, like all other currently-known high- T c superconductors, contains metallic planes adjacent to insulating metal-oxide layers. In every case, these layers can support virtual oxygen p to metal d excitations.

Effect of Oxygen Vacancies on the Electronic Structure of La2-xMxCuO4-y

The band structure of La2-xMxCuO4 has been calculated using a semiempirical tight-binding Hamiltonian and the virtual crystal approximation. The results are similar for Ba, Sr, and Ca (as the metal M), and for all small values of x. When oxygen vacancies are introduced on the 01 site (short bond), there is a dramatic effect on the electronic structure for \veck along the ΓΔ line of the Brillouin zone. Vacancies on the 02 site (long bond) are found to have relatively little effect on the electronic structure.

Tight‐binding description of high‐temperature superconductors

A single tight‐binding model, with fully transferable parameters, provides a good description of the electronic structures of all currently‐known high‐temperature superconductors, including the copper oxides La2−xSrxCuO4, YBa2Cu3O7, Bi2CaSr2Cu2O8, Tl2Ba2CuO6, Tl2CaBa2Cu2O8, and Tl2Ca2Ba2Cu3O10 and the bismuth oxide Ba1−xKxBiO3 (within the one‐electron approximation). The energy bands, local densities of states, and atomic valences have been calculated for each material.

Atomic substitution in YBa2Cu3O7: Modification of the electronic structure

We have performed semiempirical tight‐binding calculations of the electronic structure of YBa2Cu3O7, with d and s orbitals included for all the metal atoms and p and s orbitals for the oxygen. Here we report studies of Fe, Co, Ni, and Zn replacing Cu; of Tl, Pb, and Bi replacing Y; and of F and N replacing O in this material. In each case, the modification of the local densities of states, the atomic valences, and the Fermi energy was calculated.

Vacancies as dopants in high-temperature superconductors

The shift in the Fermi energy EF and the modification of the density of states rho (E) have been calculated for oxygen vacancies in La1.85Sr0.15CuO4-y and YBa2Cu3O7-y and for lanthanum vacancies in La2-xCuO4. Oxygen vacancies are found to act as donors in every case, and La vacancies as acceptors. These conclusions are in accord with the well known observations that oxygen vacancies decrease Tc, and with the suggestion that the superconductivity of nominally pure La2CuO4 could be due to La vacancies.

Pairing interaction V(q↘) for sandwich excitons

We consider an excitonic model in which the electron is localized in a d orbital on a metal site, and the hole is in a linear combination of p orbitals on the surrounding oxygen sites, with both electron and hole in an insulating LaO, Bao, etc. layer adjacent to a metallic CuO2 or BiO2 plane. This mechanism appears to provide a natural and consistent interpretation of high‐temperature superconductivity in both the cuprates and bismuthates.

Chemical and structural effects on the electronic states of (Hg, Tl, Pb, Bi)-based cuprate superconductors

The electronic energy bands have been calculated for the new single-layer and triple-layer Hg-based superconductors, HgBa2CuO4 and HgBa2Ca2Cu3Oy, for the analogous Tl-and Bi-based materials, and for hypothetical Pb-based materials. As one moves across the last row of the periodic table, from Hg to Bi,s- andp-derived bands pass below the Fermi energy, to influence hole doping of the copper oxide planes and transport in the other layers of the material. The dispersion of these bands is significantly affected by the crystal structure. The calculations were performed with a simple chemical model.

The little mechanism: A consistent interpretation of high-temperature superconductivity

Abstract We consider an excitonic model in which the electron is localized in a d orbital on a metal site, and the hole is in a linear combination of p orbitals on the surrounding oxygen on a metal with both electron and hole in an insulating LaO, BaO, etc. layer adjacent to a metallic CuO 2 or BiO 2 plane. Even for rather large effective dielectric constants, some of the excitons are found to be tightly bound and consequently to have sizable transition densities α e * ( x → ) β h ( x → ) . We calculate the attractive carrier-carrier interaction V ( q → ) , and use it to estimate the superconducting T c as a function of the parameters in the model. This excitonic mechanism appears to provide a natural and consistent interpretation of high-temperature superconductivity in both the cuprates and bismuthates.