A systematic increase in TC with biaxial tensile strain, to well beyond the bulk value, was reported. The tensile strain increased with film thickness, caused primarily by the coalescence of initially nucleated discrete islands (Volmer-Weber growth mode). The TC increase was observed in epitaxial films on SiC and sapphire substrates, although the TC values were different for the 2 substrates due to different lattice parameters and thermal expansion coefficients. First-principles calculations identified the underlying mechanism for the TC increase as being softening of the bond-stretching E2g phonon mode. This conclusion was confirmed by Raman scattering measurements. The result suggested that E2g phonon softening was a possible avenue to achieving even higher TC in MgB2-related material systems.

Enhancement of the Superconducting Transition Temperature of MgB2 by a Strain-Induced Bond-Stretching Mode Softening. A.V.Pogrebnyakov, J.M.Redwing, S.Raghavan, V.Vaithyanathan, D.G.Schlom, S.Y.Xu, Q.Li, D.A.Tenne, A.Soukiassian, X.X.Xi, M.D.Johannes, D.Kasinathan, W.E.Pickett, J.S.Wu, J.C.Spence: Physical Review Letters, 2004, 93[14], 147006 (3pp)