On the nature of superconductivity in magnesium diboride MgB2. Quantitative analysis on the basis of indirect-exchange pairing

In this paper, we undertake a quantitative analysis of superconductivity recently observed in MgB2, with a high critical temperature of 39 K. The analysis assumes a direct analogy with high-Tc cuprates; it is based on indirect-exchange Cooper pairing of electrons (quasiparticles) via diamagnetic units B1− replacing O2− in the cuprates, the latter applied in many previous high-Tc systems. Following these analogies, we consider MgB2 to be a self-doped superconductor, with a ratio between B1− and neutral boron determined in the first instance by the electronegativity of boron. The relevant equations in the formalism are solved imposing experimental constraints on their solution (critical temperature, pressure coefficient of Tc at low pressure, equilibrium condition). It is found that the indirect-exchange formalism provides a unique set of parameters which, in particular, reproduce the dependence of Tc on pressure P including the decrease in slope dTc/dP at pressures >10 GPa. We comment on isotope coefficients for this system (10B replaced by 11B) and draw further essential support for the adopted indirect-exchange basis of high-Tc superconductivity from recent results by Pan et al. (Nature 413 (2001) 282), pertaining to microscopic electronic phenomena observed in a Bi-cuprate by scanning tunnelling methods