HYDROSTATIC PRESSURE DEPENDENCE OF THE SUPERCONDUCTING AND STRUCTURAL PROPERTIES OF MgB2

The past ¿fteen years have witnessed some of the more memorable discoveries in the ninety-year-old ¿eld of superconductivity, including the high-AS oxides in 1986 [1], the alkali-doped fullerenes in 1991 [2], the charge-injected fullerenes in 2000 [3], and the binary compound MgB2 in January 2001 [4]. The discovery of superconductivity at such high temperatures (40 K) in the simple rc R-metal compound MgB2 was quite unexpected. The absence [5] of the problematic weak-link behavior of the high-AS oxides and the relative ease of synthesis in various forms [6] has raised hopes that MgB2 may be suitable for numerous technological applications. To aid in the search for related compounds with even better superconducting properties and to help identify the pairing mechanism, a great deal of current research is dedicated to fully characterizing MgB2 in both its normal and superconducting states. A wide range of experiments, including isotope effect [7, 8], heat capacity [9, 10], inelastic neutron scattering [11,12], NMR [13], and photoemission spectroscopy [14], support the picture that MgB2 is a phonon-mediated BCS superconductor in the moderate coupling regime. The fact that the B isotope effect is ¿fteen times that for Mg [8] is clear evidence that the superconducting pairing originates within the graphite-like B2-layers, consistent with electronic structure calculations [12,15–18] whereby MgB2 is a quasi-2D material with strong covalent bonding within the boron layers. The anisotropy in the superconducting properties is appreciable, the upper critical ¿eld ratio M S2*M S S2 reportedly being  . [19] or 2 . [20], but far less than that observed in the high-AS oxides [21]. A full characterization of all anisotropic properties