ABINITIO ELECTRONIC-STRUCTURE CALCULATIONS ON THE NB TA MULTILAYER SYSTEM

Ab initio electronic-structure calculations are performed for Nb/Ta multilayers with small modulation wavelengths in the [100], [110] and [111] BCC modulation directions. The [110] modulated Nb/Ta multilayer with the smallest modulation wavelength was also calculated with an 8% increased modulation wavelength on the one hand, with an undisturbed structure but containing 50% interstitial hydrogen on the other hand, and with a combination of both. The ease with which Nb/Ta multilayers can be grown in different modulation directions is understood from total energy calculations which show energy differences of less than 1 mRyd between the various directions. Further, it is found that interstitial hydrogen lowers the density of states at the Fermi level and that, but to a lesser extent, the lattice expansion on hydrogenation raises the density of states at the Fermi level. The net effect of a lowering of the density of states at the Fermi level is in agreement with the measured reduction of the superconducting critical temperature on hydrogenation.