Properties of ultra-thin vanadium layers in V/Ru superlattices

The properties of ultra-thin vanadium layers in V/Ru(0001) superlattices grown by molecular beam epitaxy were studied. The atomic structure of V was investigated by various methods including reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD) and polarized x-ray-absorption fine structure (PXAFS). It appears that, for up to three atomic layers, V adopts a slightly distorted hexagonal-close-packed (hcp) structure induced by pseudomorphic growth on Ru(0001). By increasing the V thickness to four atomic layers, this structure almost completely relaxes towards the body-centered-cubic (bcc) bulk structure. This sharp structural transition is also manifested in the electronic properties. A reduced superconducting transition temperature between 0.6 and 1.05 K was found in the bcc V/hcp Ru superlattice, while superconductivity is quenched in the superlattice with hexagonal V. This behavior might be linked to the existence of a ferromagnetic ground state of the metastable V induced by the hybridization of the d-bands at the hcp V/Ru interface, as predicted from first-principles density-functional theory.