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

The interface layers of La2-xSrxCuO4 (LSCO) thin films epitaxially grown on LaSrAlO4 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 CuO2 layers. A multilayer LSCO film with a high critical temperature (Tc = 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 Tc, while the compressive strain in high-quality multilayers enhances Tc. [ABSTRACT FROM AUTHOR]