Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors

We address the question of whether anisotropic superconductivity is compatible with the evidently weak sensitivity of the critical temperature Tc to sample quality in the high-Tc copper oxides. We examine this issue quantitatively by solving the strong-coupling Eliashberg equations numerically as well as analytically for s-wave impurity scattering within the second Born approximation. For pairing interactions with a characteristically low energy scale, we find an approximately universal dependence of the d-wave superconducting transition temperature on the planar residual resistivity which is independent of the details of the microscopic pairing. These results, in conjunction with future systematic experiments, should help elucidate the symmetry of the order parameter in the cuprates.Comment: 13 pages, 4 figures upon request, revtex version

The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO

In the framework of the model assuming the formation of NUC on the pairs of Cu ions in CuO$_{2}$ plane the mechanism of hole carrier generation is considered and the interpretation of pseudogap and 60 K-phases in $YBa_{2}Cu_{3}O_{6+\delta}$. is offered. The calculated dependences of hole concentration in $YBa_{2}Cu_{3}O_{6+\delta}$ on doping $\delta$ and temperature are found to be in a perfect quantitative agreement with experimental data. As follows from the model the pseudogap has superconducting nature and arises at temperature $T^{*}>T_{c\infty}>T_{c}$ in small clusters uniting a number of NUC's due to large fluctuations of NUC occupation. Here $T_{c\infty}$ and $T_{c}$ are the superconducting transition temperatures of infinite and finite clusters of NUC's, correspondingly. The calculated $T^{*}(\delta)$ and $T_{n}(\delta)$ dependences are in accordance with experiment. The area between $T^{*}(\delta)$ and $T_{n}(\delta)$ corresponds to the area of fluctuations where small clusters fluctuate between superconducting and normal states owing to fluctuations of NUC occupation. The results may serve as important arguments in favor of the proposed model of HTSC.Comment: 12 pages, 7 figure

Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion

A multiband CuO Hubbard model is studied which incorporates long-range (LR) repulsive Coulomb interactions. In the atomic limit, it is shown that a charge-transfer from copper to oxygen ions occurs as the strength of the LR interaction is increased. The regime of phase separation becomes unstable, and is replaced by a uniform state with doubly occupied oxygens. As the holes become mobile a superfluid condensate is formed, as suggested by a numerical analysis of pairing correlation functions and flux quantization. Although most of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy, preprint ORNL-CCIP/93/1

High-energy magnon dispersion in the half-filled Hubbard model: A comparison with La2_2CuO4_4

We use quantum Monte Carlo methods and single-mode approximation to study the magnon dispersion in the 2D half-filled Hubbard and phonon-coupled Heisenberg models. We find that in the Hubbard model with $U/t< 8$, high-energy magnon dispersion is similar to those observed in inelastic neutron scattering experiments in ${La}_2{CuO}_4$. On the other hand, our studies of a 2D Heisenberg model coupled to dynamic optical bond phonons, fails to reproduce the experimental dispersion. These results can be interpreted as evidence for intermediate $U/t$ and charge fluctuations in the cuprate materials

Effective three-band model for double perovskites

We start from a six-band model describing the transition-metal t2g orbitals of half-metallic double perovskite systems, such as Sr2FeMoO6, in which only one of the transition metal ions (Fe) contains important intratomic repulsion Ufe. By eliminating the Mo orbitals using a low-energy reduction similar to that used in the cuprates, we construct a Hamiltonian which contains only effective t2g Fe orbitals. This allows to treat exactly Ufe, and most of the Fe-Mo hopping. As an application, we treat the effective Hamiltonian in the slave-boson mean-field approximation and calculate the position of the metal-insulator transition and other quantities as a function of pressure or on-site energy difference.Comment: 8 pages, 3 figure

Electron-lattice interaction and its impact on high Tc superconductivity

In this Colloquium, the main features of the electron-lattice interaction are discussed and high values of the critical temperature up to room temperature could be provided. While the issue of the mechanism of superconductivity in the high Tc cuprates continues to be controversial, one can state that there have been many experimental results demonstrating that the lattice makes a strong impact on the pairing of electrons. The polaronic nature of the carriers is also a manifestation of strong electron-lattice interaction. One can propose an experiment that allows an unambiguous determination of the intermediate boson (phonon, magnon, exciton, etc.) which provides the pairing. The electron-lattice interaction increases for nanosystems, and this is due to an effective increase in the density of states

Polarons as Nucleation Droplets in Non-Degenerate Polymers

We present a study of the nucleation mechanism that allows the decay of the metastable phase (trans-cisoid) to the stable phase (cis-transoid) in quasi one-dimensional non-degenerate polymers within the continuum electron-phonon model. The electron-phonon configurations that lead to the decay, i.e. the critical droplets (or transition state), are identified as polarons of the metastable phase. We obtain an estimate for the decay rate via thermal activation within a range of parameters consistent with experimental values for the gap of the cis-configuration. It is pointed out that, upon doping, the activation barriers of the excited states are quite smaller and the decay rate is greatly enhanced. Typical activation energies for electron or hole polarons are $\approx 0.1$ eV and the typical size for a critical droplet (polaron) is about $20 \AA$. Decay via quantum nucleation is also studied and it is found that the crossover temperature between quantum nucleation and thermal activation is of order $T_c \leq 40 ^oK$. Metastable configurations of non-degenerate polymers may provide examples for mesoscopic quantum tunneling.Comment: REVTEX 3.0, 28 PAGES, 3 FIGURES AVAILABLE UPON REQUEST, PITT 94-0

Range of the t--J model parameters for CuO2_{2} plane: experimental data constraints

The t-J model effective hopping integral is determined from the three-band Hubbard model for the charge carriers in CuO$_{2}$ plane. For this purpose the values of the superexchange constant $J$ and the charge-transfer gap $E_{gap}$ are calculated in the framework of the three-band model. Fitting values of $J$ and $E_{gap}$ to the experimental data allows to narrow the uncertainty region of the three-band model parameters. As a result, the $t/J$ ratio of the t-J model is fixed in the range $2.4 \div 2.7$ for holes and $2.5 \div 3.0$ for electrons. Formation of the Frenkel exciton is justified and the main features of the charge-transfer spectrum are correctly described in the framework of this approach.Comment: 20pp., REVTEX 3.0, (11 figures), report 66