ABSTRACT
A hybrid growth process was developed in order to epitaxially integrate nano-layers of the multi-functional perovskite BaTiO3 onto Si(001) substrates. In particular, we combined molecular beam epitaxy (MBE) with radio-frequency sputtering. Due to its strong influence on the functional properties, the crystalline structure of the layers was thoroughly investigated throughout our study. MBE-grown seed layers are tetragonal and c-axis oriented up to a thickness of 20 nm. A transition into a-axis films is visible for thicker layers. When the seed layer thickness exceeds 6 nm, subsequently sputtered BaTiO3 films are epitaxial. However, their crystalline structure, their orientation with respect to the substrate, and their morphology are strongly dependent on the deposition and post-deposition thermal budget. Consistently with their crystalline symmetry, thin MBE BaTiO3 films are piezo- and ferroelectric with a spontaneous polarization perpendicular to the surface. Also for thick films, the functional response, as determined via piezo-force microscopy, is in good agreement with the structural properties.
ABSTRACT
By capacitively charging an underdoped ultrathin La2-xSrxCuO4 film with an electric field applied across a gate insulator with a high dielectric constant, relative changes of the areal superfluid density ns of unprecedented strength were observed in measurements of the film kinetic inductance. Although ns appears to be substantially reduced by disorder, the data provide, for the first time on the same sample, direct compelling evidence for the Uemura relation Tc proportional to ns(T=0) in the underdoped regime of copper-oxide superconductors.
