International Journal of Modern Physics B

In the present paper we review our findings on ethylene adsorption on clean and oxygen covered Ag(001) surfaces investigated by dosing the gas with a Supersonic Molecular Beam and analysing the adsorption state either by High Resolution Electron Energy Loss Spectroscopy or by High Resolution X Rays Photoemission Spectroscopy. The final adsorption state depends on the translational and on the internal energy of the gas-phase molecules and on the presence of defects. At low translational energy ethylene either physisorbs or very weakly chemisorbs at flat terrace sites. The physisorption probability is thereby hindered by rotational excitation. A more strongly bound, π bonded, state forms at higher translational energy, the activation barrier being related to the energy needed to form the relevant defect at which chemisorption takes place. A further even more strongly bound state forms only when dosing vibrationally excited molecules from the gas phase.

An attempt is made to understand the isotope effects measured by Zhao et al. in LaSrCuO in terms of a previous theory for high T_c superconductors, a theory based on pair coupling by soft lattice modes. In the theory, the lattice elastic energy originating with the ion-electron correlation limits the gap. Here we note that this elastic energy does not depend directly on the ionic masses. However, the mechanical lattice parameters in the neighborhood of a structural phase transition are so sensitive that the free energy of oscillation and, hence, the masses are able to have an influence. Minimization of the free energy of the solid with respect to the lattice deformation leads to an equation of state, as basis for finding the effects of the ionic mass changes. With the measured isotope effects at several values of doping as input to the calculation, we obtain, as output, very plausible results for the Fermi energy, the lattice softening, and the electron-phonon coupling constant.

We have investigated the effect of the partial substitution of Sb for Ru in Ru_{1 - x}Sb_xSr₂(Sm_0.7Ce_0.3)₂Cu₂O_{10 - δ}, (Ru:1222) prepared by conventional solid state reaction method, using X-ray diffraction analysis, electrical resistivity and DC-magnetic susceptibility measurements. Sb doping reduces the conductivity of the system and the onset superconducting transition temperature decreases from 42 K for x = 0 to 21 K for x = 0.06. This maybe due to a distortion of RuO₆ octahedral, which is responsible of the increase in hole localization. The corresponding magnetic moment in the investigated samples is about 0.1 μ_B for Ru. Furthermore we discuss the influence of phase composition on superconducting and magnetic order.

The main equations of the theory of superconductivity in non-adiabatic systems with ferromagnetic arrangement of spins of impurity atoms are derived. The influence of non-adiabaticity on the temperature of superconducting transition T_C as well as on the critical concentration of impurity is also studied. The behavior of the temperature of magnetic arrangement of spins T_k as function of concentration of impurity c in the superconducting state for the non-adiabatic systems is investigated as well. The phase diagram (T, c) is built and it is proved that the effects of non-adiabaticity favor a raise in the field with coexistence of superconductivity and ferromagnetism.

We study the electrical conductivity problem of DNA, applying the Holstein's polaron model to a double strand of DNA. We show that the Holstein's polarons act as donors and acceptors in DNA and that the existence of such donors and acceptors provides us the extrinsic semiconductor character of DNA. This character of DNA may explain with a unified point of view almost all the known results recently obtained by experiments in the study of conductivity problems of DNA, which seem so controversial so far.

We examine the effect of the phase of the external light fields on the dynamics of spin fluctuations in a coupled two-component Bose–Einstein condensates. We discover that the phase of the coherent coupling between the two components to be an easily tunable experimental parameter that can conveniently manipulate the spin squeezing in our given system.

We study the superconductivity in MgB₂ using a tight binding model to investigate the doping dependence of the order parameter and the critical temperature. We consider both the anisotropic and the isotropic s-wave pairing to study the coexistence of the two gaps. A good agreement between the existing experiments and our theoretical curves is observed. The temperature dependence of order parameter, specific heat and penetration depth at half filling are also demonstrated and are found to be in accord with the available experimental predictions.

Based on Feynman's explanation that a Cooper pair is "a bound pair act as a Bose particle", we propose a bosonic phase operator formalism and a bosonic Hamiltonian model for Josephson junction. The Cooper pair number — phase uncertainty relation is thus established. The corresponding minimum uncertainty state is derived which turns out to be a phase state.

We report here on the structural and superconducting properties of RE_{1 - 2x}Pr_xCa_xSr_0.8Ba_1.2Cu₃O_{7 - δ} (RE = Er and Gd, 0 ≤ x ≤ 0.2) high temperature superconductors. The transition temperature (T_c) of these compounds decreases when the Pr and Ca concentrations are increased. At constant Pr and Ca concentrations, the T_c of Er_{1 - 2x}Pr_xCa_xSr_0.8Ba_1.2Cu₃O_{7 - δ} superconductor system is higher than that observed for the Gd_{1 - 2x}Pr_xCa_xSr_0.8Ba_1.2Cu₃O_{7 - δ} system. For the Gd_0.8Pr_0.1Ca_0.1Sr_0.8Ba_1.2Cu₃O_{7 - δ} sample, after heating in argon followed by oxygen annealing, the orthorhombicity increases accompanied by 10 K increase in T_c. These results show that though our data indicate an ion-size effect, the T_c suppression rate dT_c/dx depends very much on the heat treatment adopted to prepare the samples.

A crossover distribution specified by two parameters (ω, η) is introduced to describe the transition of spectral statistics from Poissonian to the Gaussian unitary ensemble. During the transition, appropriate values of (ω, η) will move along a route from (0,0) to (1,1) in the (ω, η) plane. This difference between the route of a nerve-cell-like fractal-based complex and that of a nebula-like fractal-based complex elicits different spectral behaviors during the Anderson transition in these fractal-based complexes with random magnetic fields.

The structural and magnetic properties of a mixed oxide-fluoride vitreous matrix 3B₂O₃·CaF₂ containing iron ions were investigated. Information concerning the structural units involving iron ions, their valence states and the type of interactions between them were obtained using EPR and magnetic susceptibility measurements.