International Journal of Modern Physics B

The last decade has seen the emergence of new circular and uni-axial, or forward backward, birefringence and dichroism due to magnetic, nonlinear electromagnetic, and under the correct conditions, alternating electric fields. In this review, we explain some of our contributions to this new area of chemical physics, with special reference to fundamental and applied phenomena such as parity violation and nonlinear optical resonance effects set up by various conjugate products of the Maxwellian electromagnetic field.

We review and analyze the data on the thermal conductivity of both ceramic and single crystal samples of high temperature superconductors. A universal pattern can be extracted and interpreted in the following way: phonons are the main heat carriers in these materials, and in the high temperature range the thermal conductivity κ is almost constant due to phonon scattering against disorder; below the superconducting transition temperature κ increases as phonon scattering against carriers condensing into the superconducting state decreases and at still lower temperatures there is a region in which a T² law is obeyed that most probably is due to resonant phonon scattering against low energy excitations, i.e. tunneling systems similar to those found in disordered materials. The origin of the relevant disorder is discussed.

This review describes a Fermi liquid picture of high-T_c superconductivity. A density-of-states (dos) peak associated with the CuO₂-plane van Hove singularity causes a peak in T_c as a function of hole doping. Strong correlation effects drive a Mott transition at half filling. For intermediate doping, the electronic system is unstable against phase separation, with one phase near the insulating state, the other near the T_c peak.

The large dos leads to competition between superconductivity and structural instability, in analogy with the A15 compounds. The superconductivity appears to be driven by strong electron-phonon coupling, enhanced by fluctuation effects.

In this paper we show that the existence of microscopic defects (both point and linear) in the crystal lattice changes the elastic moduli of the solid. Owing to the change of the shear modulus induced by equilibrium defects, the relative shear modulus becomes temperature dependent and this dependence is close to the empirical value. For a wide class of metals, the values of the melting temperature obtained from the requirement that the shear modulus turn to zero are in good agreement with experiment.

The impurity-anharmonicity interactions, along with anharmonicities, mass and force constant changes have been taken into account to investigate the expressions for microwave attenuation in anharmonic crystals. A remarkable change in the phonon frequency shifts and widths has been reported due to the defect anharmonicity interactions. The usual method of central and non-central force constant parameters has been ignored here and parameter free expressions have been reported.

We consider the extended Hubbard model including repulsion U_pd between nearest-neighbor Cu and O holes. In the limit of strong intra-atomic correlations and weak hopping, we derive an effective Hamiltonian for the O holes. This Hamiltonian contains interactions between O holes with a common nearest-neighbor Cu ion. The effective interaction between two holes is repulsive for small U_pd and attractive otherwise. Bound states between two holes exist only if the effective interaction between them is sufficiently large and negative.

We consider the Ising model on the triangular lattice with anisotropic pair interactions, staggered triplet interactions, and an external magnetic field. Using the Yang-Lee zero consideration and a star-triangle transformation which eliminates the triplet interactions, we obtain regimes in the parameter space which are free of phase transitions.