M410_15c 345..348

NATURE | VOL 410 | 15 MARCH 2001 | www.nature.com 345 The onsets of the transitions are all in the vicinity of 36 K, the transition temperature for x ˆ 0:10. There is no bulk superconducting transition for x ˆ 0:3 and x ˆ 0:4. The data are interpreted as indicating that bulk superconductivity in single-phase material is suppressed for x greater than 0.10. The superconducting transitions observed in the two-phase region are due to both the decreasing amount of the bulk superconducting phase with increasing Al content, and also to the distribution of Al contents in the powders. The structural and magnetic data taken together therefore indicate that the disappearance of bulk superconductivity in Mg1-xAlxB2 occurs at the same Al concentrations at which a structural transition occursÐthis transition results in the partial collapse of the separation between boron planes. The similarity of the calculated electronic density of states (DOS) for MgB2 and AlB2 (ref. 6) indicates that the effect of substituting Al for Mg can be considered primarily as a simple ®lling of available electronic states, with one electron donated per Al, within a `rigid band' picture. The calculations show that there is a sharp drop in the DOS of MgB2 at only slightly higher electron concentrations. The gradual decrease of Tc from 38 to 36 K with increasing Al content in the single-phase region below x ˆ 0:1 may therefore be due to the expected decrease of the DOS at EF with increasing electron count, consistent with a conventional origin for the superconductivity. High-resolution structural study of uniform powders, if they prove possible to synthesize, would help to determine how the superconductivity and the structural transition are linked on an electronic level. The proximity of MgB2 to a structural instability is consistent with a general picture for conventional high-temperature superconductors as being near chemical phase instability owing to strong electron±lattice coupling. However, our data for Mg1-xAlxB2 do not suggest that the structural instability in this case is driven by competition between superconductivity and a structural distortion that would decrease the density of electronic states at the Fermi energy; this is because the Al substitution both decreases Tc and decreases the DOS at EF before the transition occurs. It is at ®rst sight surprising that the observed structural instability in MgB2 involves a partial collapse of the separation of the boron layers, not a change in the boron±boron in-plane distance. It will be of interest to determine whether this collapse is associated with special characteristics of the electronic band ®lling near 0.1 excess electrons per cell. M