Soft x-ray spectroscopy experiments on the near K-edge of B in MB2 (M=Mg, Al, Ta, and Nb)

Soft X-ray absorption and emission measurements are performed for the K- edge of B in MB$_2$ (M=Mg, Al, Ta and Nb). Unique feature of MgB$_2$ with a high density of B 2$p_{xy}(\sigma)$-state below and above the Fermi edge, which extends to 1 eV above the edge, is confirmed. In contrast, the B 2$p$ density of states in AlB$_2$ and TaB$_2$, both of occupied and unoccupied states, decreased linearly towards the Fermi energy and showed a dip at the Fermi energy. Furthermore, there is a broadening of the peaks with $p\sigma$-character in XES and XAS of AlB$_2$, which is due to the increase of three dimensionality in the $p\sigma$-band in AlB$_2$. The DOS of NbB$_2$ has a dip just below the Fermi energy. The present results indicate that the large DOS of B-2$p\sigma$ states near the Fermi energy are crucial for the superconductivity of MgB$_2$.Comment: 3 pages text and 4 pages figures. accepted for publication to Phys. Rev.

SOFT X-RAY EMISSION SPECTRA AND THE BONDING OF ALUMINUM

Les spectres L2,3 de rayons x-mous (SXE) de l'aluminium sont présentés pour 1'aluminium métallique, les alliages Al-Mn, les alliages dilues Al-Mg, les composés intermetalliques LiAl et Ni3Al, l'alliage semiconducteur (Al-Ga)As et l'isolant Al2O3. Les spectres fournissent une mesure de la s densité d'états locale partielle (PDOS) localisée aux atomes d'aluminium et montrent des structures remarquables qui peuvent être associées à chacum des types de liaisons dans les solides - métallique, covalent et ionique. Les spectres de l'aluminium métallique et des alliages Al-Mn ont des densités d'états paraboliques caractéristiques d'une PDOS d'électrons quasi libres. L'aluminium dans ses alliages avec le Mg a une PDOS métallique et le spectre présente une forte augmentation dans la région des basses énergies, associée à l'écrantage de la charge supplémentaire du coeur de l'ion aluminum. Dans LiAl, l'Al a une symétrie locale tétragonale, le spectre est semblable à celui du silicium et la liaison est une combinaison des liaisons covalente et métallique. Dans les alliages AlxGa1-xAs l'aluminium a une symétrie locale tétragonale mais le spectre caractéristique de forme covalente est fortement modifié par la présence d'une forte composante ionique dans la liaison. Enfin, le spectre de l'Al2O3 présente des structures caractéristiques d'orbitales électroniques localisées et une liaison ionique.The L2,3 soft x-ray emission (SXE) spectra of Al in metallic Al, Al-Mn alloys, dilute Al-Mg alloys, the intermetallic compounds LiAl and Ni3Al, the semiconducting alloy (Al-Ga)As and insulating Al2O3 are presented. The spectra provide a measure of the s-like partial density of states (PDOS) localized at the Al atoms and show prominant qualitative features that may be identified with each of the major types of bonding in solids, ie. metallic, covalent and ionic. The spectra of metallic Al and of the Al-Mn alloys have parabolic densities of states characteristic of nearly free electron PDOS's. Al in Mg alloys have metallic PDOS's with a strong enhancement of the low energy region of the spectrum which is associated with screening of the extra charge on the Al ion core. In LiAl, the Al has tetragonal local symmetry, the spectrum is similar to that of silicon and the bonding is a combination of covalent and metallic. In AlxGa1-x As alloys, the Al has tetragonal local symmety, but the spectrum is strongly modified from the characteristic covalent form by the presence of a strong ionic component in the bonding. Finally, the spectrum of Al2O3 shows features characteristic of localized electronic orbitals and ionic bonding

The structural homogeneity of boron carbide thin films fabricated using plasma-enhanced chemical vapor deposition from B\u3csub\u3e5\u3c/sub\u3eH\u3csub\u3e9\u3csup\u3e+\u3c/sup\u3e\u3c/sub\u3eCH\u3csub\u3e4\u3c/sub\u3e

Boron carbide thin films of several B/C ratios have been deposited on Si(111) using plasma-enhanced chemical vapor deposition from nido-pentaborane(9) (B5H9) and methane (CH4). X-ray diffraction studies of boron carbide thin films on Si(111) exhibited characteristic microcrystalline diffraction lines. Soft x-ray emission spectroscopy was used to verify that the local electronic structure and composition of each sample corresponded to a homogeneous solid solution boron carbide phase