Structural origin of enhanced critical temperature in ultrafine multilayers of cuprate superconducting films

Abstract

The interface layers of ${\mathrm{La}}_{2\ensuremath{-}x}$${\mathrm{Sr}}_{x}$${\mathrm{CuO}}_{4}$ (LSCO) thin films epitaxially grown on ${\mathrm{LaSrAlO}}_{4}$ substrates by molecular beam epitaxy were investigated using transmission electron microscopy (TEM). In single-phase LSCO film, we observed an irregular layering sequence near the interface between the film and the substrate, as well as an abundance of oxygen vacancies in ${\mathrm{CuO}}_{2}$ layers. A multilayer LSCO film with a high critical temperature (${T}_{c}$ = 44.5 K) showed perfect interfaces between the sublayers. Furthermore, by combining scanning TEM, electron energy-loss spectroscopy, and electron holography, we show that there is little or no interdiffusion between the sublayers. The interfacial defects and oxygen vacancies reduce ${T}_{c}$, while the compressive strain in high-quality multilayers enhances ${T}_{c}$.