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 (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.