A TWO-BAND APPROACH TO MgB_2 SUPERCONDUCTIVITY

摘要

The discovery of MgB_2 superconductivity with T_c ～ 39K has enriched the family of high-temperature superconductors. Recent wide research activity incorporates investigations of various properties of MgB_2 and related compounds, calculations of electron- and phonon spectra and elaboration of models describing the superconductivity. For the pairing mechanism in MgB_2 experimentally established boron isotope effect, the s-wave nature of the order parameter(s) and the hole nature of the conductivity are of guiding significance. Analogously to cuprates the transition temperature (T-c) of MgB_2 is essentially influenced by doping. Both electron- (Mg_(1-x)Al_x B_2) and (additional)hole doping reduce T_c. On the doping scale the parent MgB_2 for a whole class of compounds seems to be positioned in the optimal doping region of the phase diagram. First MgB_2 superconductivity descriptions used the BCS-scheme referring to high phonon frequencies connected with the small boron mass and strong electron-phonon interaction present. However, the classical mechanism has been probably exploited on the maximal limit at this. Further experimental investigations have established manifestation of two superconductivity gaps in responses to various probes . Gap-to-T_c ratios of non-BCS universality have been also found. Therefore the applications of two-band type models have been natural. Such models are known for a long time and numerous versions using various physical basis elements have been elaborated for cuprates in last decade, e.g. for review.