E. Sigmund

Percolative phase separation in La2CuO4+δ and La2−xSrxCuO4

We report on the formation of conducting phases in slightly doped La2CuO4 samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.

UNRESTRICTED SLAVE-BOSON MEAN-FIELD APPROXIMATION FOR THE TWO-DIMENSIONAL HUBBARD MODEL

The Kotliar-Ruckenstein slave-boson scheme is used to allow for an unrestricted variation of the bosonic and fermionic fields on the saddle-point level. Various inhomogeneous solutions, such as spin polarons and domain walls, are discussed within the two-dimensional Hubbard model and compared with results of unrestricted Hartree-Fock (HF) calculations. We find that the present approach drastically reduces the polarization of these states and leads to increased delocalized wave functions as compared to the HF model. The interaction between two spin polarons turns out to be attractive over a wide range of the on-site repulsion

On the existence of percolative phase separation in high-Tc cuprates

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g-tensor analysis of S− and Se− centers in cubic crystals

. In addition we obtain the crossover from vertical to diagonal domain walls at a higher value of

On the Role of Electronic and Chemical Phase Separation: Susceptibility and Conductivity Experiments on La2−xCuO4+δ

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High-Tc superconductivity: A nonadiabatic theory of interacting solitons

than predicted by HF.

On the Vibronic Laser Theory

The phase separation behavior of superconducting cuprate systems is studied. Using experimental data obtained from electrical resistivity and susceptibility measurements on La2CuO4+δ we demonstrate that the phase separation is of electronic and percolative nature. In addition, the experiments clearly prove the coexistence of bulk superconductivity and long-range antiferromagnetic (afm) order.

The exciton localization problem: a numerically exact approach to the 2-site model

Experimental results for the electron spin resonance of the S− and Se− centers in alkali halide crystals are analysed and a theoretical model for their description is established. Most of the experimental results can be explained by assuming a strong T-t Jahn-Teller effect in the ground-state system combined with a spin-orbit coupling of similar strength. In particular, the principal g-tensor values, which are of importance for comparison with experiment are derived. They show good agreement with the measured data.

Influence of quantum fluctuations on solitary-wave acoustic polaron motion

The mechanisms contributing to the metalization and the formation of the conducting and at low temperature superconducting phase in high-T c super–conductors are discussed. Related experimental investigations on La2−xCuO4+δ are reviewed with special emphasis on our recent magnetic susceptibility and conductivity experiments on low excess oxygen doped ceramic and single crystal samples of La2−xCuO4+δ . Particular attention was paid to the thermal history of the samples following our early observation that the superconducting phase can be significantly reduced by rapidly quenching the samples to temperatures below about 220 K. The reviewed experiments give very strong evidence that the formation of the conducting and at low temperature superconducting phase in high-T c superconductors follows the concept of percolative electronic phase separation.

Field induced optical transition probabilities between vibrational states: Exact rabi solution for a vibronic system

Abstract A possible explanation of high- T c superconductivity in the layered perovskites is the formation of solitons by charge carriers (holes) and the lattice vibrations. This formation is induced by a resonant energy exchange process between electronic transitions and high-frequency (asymmetric) optical phonons. The strong coupling of the holes to a 1g breathing modes leads to an attractive interaction between the solitons and in this way to a pair creation and to a BCS like superconductivity. The transition temperature T c strongly depends on the spatial extension of the solitons.