22 research outputs found
Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T in MgB
Results of FLMTO-GGA (full-potential linear muffin-tin orbital -- generalized
gradient approximation) calculations of the band structure and boron electric
field gradients (EFG) for the new medium-T superconductor (MTSC), MgB,
and related diborides MB, M=Be, Al, Sc, Ti, V, Cr, Mo and Ta are reported.
The boron EFG variations are found to be related to specific features of their
band structure and particularly to the M-B hybridization. The strong charge
anisotropy at the B site in MgB is completely defined by the valence
electrons - a property which sets MgB apart from other diborides. The boron
EFG in MgB is weakly dependent of applied pressure: the B p electron
anisotropy increases with pressure, but it is partly compensated by the
increase of core charge assymetry. The concentration of holes in bonding
bands is found to decrease slightly from 0.067 to 0.062 holes/B under
a pressure of 10 GPa. Despite a small decrease of N(E), the Hopfield
parameter increases with pressure and we believe that the main reason for the
reduction under pressure of the superconducting transition temperature, T,
is the strong pressure dependence of phonon frequencies, which is sufficient to
compensate the electronic effects.Comment: 12 pages, 3 figure
Fermi Surfaces of Diborides: MgB2 and ZrB2
We provide a comparison of accurate full potential band calculations of the
Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen
date of Yelland et al. and Tanaka et al., respectively. The discrepancies in
areas in MgB2 can be removed by a shift of sigma-bands downward with respect to
pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged
electron-phonon coupling constants lambda(sigma)=1.3 (both orbits),
lambda(pi)=0.5. The required band shifts, which we interpret as an exchange
attraction for sigma states beyond local density band theory, reduces the
number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of
superconductivity in MgB2 a somewhat closer call than previously recognized,
and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure
Band structure of ZrB2, VB2, NbB2, and TaB2 hexagonal diborides: Comparison with superconducting MgB2
The complex nature of superconductivity in MgB2 as revealed by the reduced total isotope effect
The MRCAT insertion device beamline at the advanced photon source
© 2000 American Institute of Physics Inc.. All rights reserved. The Materials Research Collaborative Access Team (MRCAT) beamline 10-ID at the Advanced Photon Source has been in Commisioning since January 1998. This Beamline has been designed as a flexible spectroscopy, scattering, and diffraction facility for the study of materials in situ. Over 70 users have performed experiments at the beamline during this period. Details about the beamline layout, components, performance and configuration for typical experiments will be presented