The intermetallic compounds AAl 2-x Si x , where A = Ca, Sr or Ba, crystallize in the C32 structure, same as the recently discovered MgB 2 with a high superconducting transition temperature of 39 K. For x = 1, superconductivity has been observed in AAlSi with A = Ca and Sr, but not with A = Ba. The transition temperatures are 7.8 and 5.1 K, respectively for CaAlSi and SrAlSi. The CaAl 2-x Si x compound system display a T c -peak at x = 1, a possible x-induced electronic transition at x ~ 0.75 and a possible miscibility gap near x ~ 1.1 which results in a very broad superconducting transition. The Seebeck coefficients of AAlSi indicate that their carriers are predominantly electrons in nature, in contrast to the holes in MgB 2 .

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The small intergrain effect of MgB 2 on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature is only 40 K. However, band-structure calculations predict that a higher than that of MgB 2 is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise , but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa 2 , AGa 2 Si , and AAl 2 Si , where = Ca, Sr, or Ba, which are isostructural to MgB 2 and have greater lattice parameters than MgB 2 . In spite of the greater lattice parameters, AGa 2 are not superconducting. However, all pseudobinary compounds AGa 2 Si and AAl 2 Si for 0 6 1 5 are superconducting. Superconductivity is reported for the first time in SrAlSi, with of 5.1 K. Although varies with in a similar fashion for all members of the series, no specific correlation between and lattice parameters or ionic mass is observed. The maximum of these compound series with different 's varies between 5.5 and 7.8 K, much lower than that of MgB 2 . The results strongly suggest the unique role of B in the superconductivity of C32 intermetallic compounds. They also demonstrate that factors additional to the lattice parameters and densities of states must play an important role, and that the rigid-band model is not sufficient to account for the observations. Index Terms-Intermetallic compounds, lattice parameters, superconducting materials, valence electron density.T HE discovery of the 40-K superconductor MgB with a simple hexagonal C32 structure [1] has generated great interest for both practical applications and fundamental research. In particular, the small intergrain effect of MgB on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature is only 40 K. However, band-structure calculations predict an enhancement of within the C32 structure if the lattice can be expanded. This prediction is consistent with the negative pressure effect on its , observed recently [2], [3] which has been understood semiquantitatively in terms of a rigid-band model by a pressure-induced decrease of the density of states at the Fermi energy and an increase of the characteristic phonon frequency [4], both resulting in a decrease of the electron-phonon coupling constant. The large isotope effect detected [5] demonstrates that electron-phonon interaction should play a major role in the occurrence of superconductivity at such a high in MgB and the conventional BCS theory has been used to explain t...

The nanostructures self-organized via lateral composition modulation in 140 period (InAs) 13 /(GaSb) 13 superlattices grown by molecular beam epitaxy have been studied by highresolution x-ray diffraction and infrared absorption. Three samples were analyzed in this study; two with lateral composition modulation and one without. X-ray reciprocal space map scans were taken to determine the average morphology of the modulated structures. Both vertical and lateral satellite peaks were observed for the samples with composition modulation, indicating the formation of two-dimensional nanowire arrays. The vertical wavelength measured for the two samples was twice the period intended by the growers. This is due to the face-centered cubic type stacking of the nanowires. Infrared absorption spectra of these two samples were compared to the spectra of the sample with no lateral composition modulation. Transitions involving the heavy-and light-hole bands in the GaSb hole quantum well and the electron subbands of the InAs electron quantum well were not evident for the samples with lateral composition modulation, indicating that the nanostructure of the lateral composition modulation affects the optical response of the sample, which is important for optoelectronic device applications.