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The experimental determination of the scaling of the superconducting critical temperature (T-c) vs the Fermi temperature (T-f) of the holes in the boron sigma subband is presented. The Fermi level has been tuned near the "shape resonance," i.e., the two- to three-dimensional crossover of the Fermi surface of the boron sigma subband by changing the Al/Mg content in Al1-xMgxB2. The product k(f)xi(0) of the Fermi wave vector (k(f)) times the superconducting Pippard coherence length (xi(0)), that is a measure of the pairing strength, remains constant, k(f)xi(0)=90 for x>0.66. This high-T-c phase occurs in the boron superlattice under a tensile microstrain in the range 3%

The single-crystal X-ray diffractometer installed at the PWA Laboratory in Frascati is described. The Huber four-circle goniometer is controlled by an IBM-compatible personal computer by means of a plug-in board interface based on the LSI chip Am9513. The program CS running under MS-DOS performs a wide variety of operations from random search to data collection. The angular resolution of the diffractometer is 0.0025 °, while the instrumental resolution at 8 keV, with an Si(lll) monochromator, is 0.0026 °. After preliminary tests, 2Zn insulin crystals were mounted on the diffractometer and reflections were measured using synchrotron radiation. The rocking curves show full widths at half-maxima (FWHM) in the range 0.013-0.031 ° mainly due to the mosaicity.

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