Thickness induced superconductor-insulator transition in epitaxial La-Sr-Cu-O

For many years, the superconductor-to-insulator transition (SIT) is studied by controlling external magnetic field, gating voltage, and hydrostatic pressure. In particular, for thin film systems, film thickness is also a candidate parameter by changing the dimensionality of system. We investigate SIT for the thickness dependent epitaxial La$_{\mathrm{1.85}}$Sr$_{\mathrm{0.15}}$CuO$_{\mathrm{4\thinspace }}$thin films on LaAlO$_{\mathrm{3\thinspace }}$with various perspectives. Electronic transport measurement shows thickness dependent $T_{\mathrm{c\thinspace }}$and SIT occurs at a critical thickness of \textasciitilde 15 nm. By using transmission electronic microscopy imaging, it directly supports high quality of the epitaxial films with minimizing dislocations in the atomic resolution. X-ray diffraction and reciprocal space map represent that $c$-axis and in-plane lattice parameters exhibit no significant change and fully strained on substrate for all thicknesses. In addition, x-ray photoemission spectroscopy for O 1s and Cu 2p core level spectra also reveals a similar electronic structure irrelevant to the thickness. We will discuss possible mechanisms for the observed SIT.