Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×10¹⁹cm^?2) on the superconducting transition temperature T _c and the upper critical field H _c2 of polycrystalline MgB₂ samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T _c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH _c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T _c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.     

Doping dependence of correlation effects in K1 − x Fe2 − y Se2 superconductors: LDA’ + DMFT investigation

We present a detailed LDA’ + DMFT investigation of the doping dependence of correlation effects in the novel K_{1 ? x} Fe_{2 ? y} Se₂ superconductor. Calculations are performed at four different hole doping levels, starting from a hypothetical stoichiometric composition with the total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons toward the case of 26.52, which corresponds to the chemical composition K_0.76Fe_1.72Se₂ studied in recent ARPES experiments. In the general case, the increase in hole doping leads to quasiparticle bands in a wide energy window ±2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation-induced compression of Fe-3d LDA’ bandwidths stays almost the same, of the order of 1.3 for all hole concentrations. However, close to the Fermi level, the situation is more complicated. In the energy interval from ?1.0 eV to 0.4 eV, the bare Fe-3d LDA’ bands are compressed by significantly larger renormalization factors up to 5 with increased hole doping, while the value of Coulomb interaction remains the same. This fact manifests the increase in correlation effects with hole doping in the K_{1 ? x} Fe_{2 ? y} Se₂ system. Moreover, in contrast to typical pnictides, K_{1 ? x} Fe_{2 ? y} Se₂ does not have well-defined quasiparticle bands on the Fermi levels, but has a “pseudogap”-like dark region instead. We also find that with the growth of hole doping, Fe-3d orbitals of various symmetries are affected by correlations differently in different parts of the Brillouin zone. To illustrate this, we determine the quasiparticle mass renormalization factors and energy shifts that transform the bare Fe-3d LDA’ bands of various symmetries into LDA’ + DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy-dependent self-energy effects and can be used to analyze the available ARPES data.     

The Distributed-Suction Effect on the Development of Disturbances at the Nonlinear Stage of Their Evolution in the Boundary Layer

Doklady Physics - The results of experimental investigations into the effect of distributed suction through a finely perforated permeable section on the spatial development of disturbances at the...     

The donor-acceptor properties of exciplexes. Comparison of fluorescence quenching of excited aromatic molecules and their exciplexes

Exciplex reactivity in electron transfer reactions is investigated. Methods are proposed for the calculation of exciplex ionization potentials and electron affinities. Exciplex quenching rate constants have been measured. It is shown that exciplex quenching follows the same laws as that of excitcd molecules. Conditions of specific exciplex quenching are considered.