Two-gap superconductivity in MgB2_{2}: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport

Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate

We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate (Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The large number of observed vibrational modes is related to the low symmetry of the molecule, and they are grouped together in clusters. Analysis of the mode character based on molecular dynamics simulations and model compound studies shows that all vibrations are complex; motion from a majority of atoms in the molecule contribute to most modes. Three features involving intramolecular vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes with applied magnetic field. The structure near 284 cm$^{-1}$ displays the largest deviation with field and is mainly intensity related. A comparison between the temperature dependent absorption difference spectra, the gradual low-temperature cluster framework distortion as assessed by neutron diffraction data, and field dependent absorption difference spectra suggests that this mode may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte

Solar flare-related eruptions followed by long-lasting occultation of the emission in the He II 304 A line and in microwaves

Plasma with a temperature close to the chromospheric one is ejected in solar eruptions. Such plasma can occult some part of emission of compact sources in active regions as well as quiet solar areas. Absorption phenomena can be observed in the microwave range as the so-called 'negative bursts' and also in the He II 304 A line. The paper considers three eruptive events associated with rather powerful flares. Parameters of absorbing material of an eruption are estimated from multi-frequency records of a 'negative burst' in one event. 'Destruction' of an eruptive filament and its dispersion like a cloud over a huge area observed as a giant depression of the 304 A line emission has been revealed in a few events. One such event out of three ones known to us is considered in this paper. Another event is a possibility.Comment: 23 pages, 8 figures, submitted for publication in Astronomy Report

The Tc amplification by quantum interference effects in diborides

The model of two (s and p) channel superconductivity known to be necessary to explain the superconductivity in MgB2 has been applied to the Al1-xMgxB2 diborides by tuning x from MgB2 to AlMgB4. The evolution of the interband coupling parameter (probing the strength of the interchannel pairing due to quantum interference effects) and the two gaps in the s and p channel as a function of x have been calculated. While in MgB2 the quantum interference effects give an amplification of Tc by a factor of 1.5 in comparison with the dominant intra s band single channel pairing, in AlMgB4 the amplification is about 100, in comparison with the dominant intra p band single channel pairing.Comment: 7 pages, 6 figure

Infrared optical properties of Pr2CuO4

The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a wide frequency range at a variety of temperatures, and the optical properties determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency conductivity increases quickly with temperature; the resistivity follows the form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical gap of ~ 1.2 eV. Transport measurements show that at low temperature the resistivity deviates from activated behavior and follows the form e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to variable-range hopping between localized states in the gap. The four infrared-active Eu modes dominate the infrared optical properties. Below ~ 200 K, a striking new feature appears near the low-frequency Eu mode, and there is additional new fine structure at high frequency. A normal coordinate analysis has been performed and the detailed nature of the zone-center vibrations determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction. Several possible mechanisms related to the antiferromagnetism in this material are proposed to explain the sudden appearance of this and other new spectral features at low temperature.Comment: 11 pages, 7 embedded EPS figures, REVTeX