Niobium-Tantalum Superlattices Grown by Molecular Beam Epitaxy: Structure, Transport, and Superconducting Properties

Abstract

Single crystal Nb-Ta superlattices with wavelengths from 20 to 800 (ANGSTROM) were fabricated in this work by Molecular Beam Epitaxy (MBE) techniques. For the first time, electronic mean free paths and crystal domain sizes were much larger than the imposed wavelength in metallic superlattices. X-ray structure factor determinations of superlattices grown in four different crystal orientations found that interfacial alloying was strongly direction dependent. The extent of intermixing was larger than predicted by simple diffusion, indicating mixing due to the kinetics of the deposition process.

The superconducting transition temperatures of superlattices in the short wavelength limit were significantly larger than the Cooper theory predictions, suggesting that even for the shortest wavelengths the superconducting gap function of a one dimensional superlattice differs from that of a three dimensional random alloy.

Superconducting tunnelling measurements of pure Nb films produced electron-coupled phonon spectra, (alpha)('2)((omega))F((omega)), comparable to published results on bulk foils, but showed distinct differences for {110} and {111} crystals. MBE interfaces may be of sufficient quality to permit tunnelling measurements sensitive to crystal direction effects.

Raman scattering studies of MBE-grown crystals of Nb, V, and Ta revealed electron-coupled phonon densities of states, which are compared to results from tunnelling and neutron diffraction. A search for acoustic phonons folded into optical branches by the artificially-imposed periodic potential in Nb-Ta superlattices was unsuccessful.