title: Enhancement of the superconducting transition temperature of MgB2 by a strain-induced bond-stretching mode softening.
abstract: We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.
authors.affiliation: Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
authors.forename: A V
authors.initials: AV
authors.lastname: Pogrebnyakov
authors.forename: J M
authors.initials: JM
authors.lastname: Redwing
authors.forename: S
authors.initials: S
authors.lastname: Raghavan
authors.forename: V
authors.initials: V
authors.lastname: Vaithyanathan
authors.forename: D G
authors.initials: DG
authors.lastname: Schlom
authors.forename: S Y
authors.initials: SY
authors.lastname: Xu
authors.forename: Qi
authors.initials: Q
authors.lastname: Li
authors.forename: D A
authors.initials: DA
authors.lastname: Tenne
authors.forename: A
authors.initials: A
authors.lastname: Soukiassian
authors.forename: X X
authors.initials: XX
authors.lastname: Xi
authors.forename: M D
authors.initials: MD
authors.lastname: Johannes
authors.forename: D
authors.initials: D
authors.lastname: Kasinathan
authors.forename: W E
authors.initials: WE
authors.lastname: Pickett
authors.forename: J S
authors.initials: JS
authors.lastname: Wu
authors.forename: J C H
authors.initials: JC
authors.lastname: Spence