Anomalous Expansion of the Cu-Apical O Distance in Superconducting Cuprate Oxide Bilayers

Interfaces between complex oxides have received considerable attention due to the observation of fascinating quasi two- dimensional phenomena such as a high-mobility electron gas, interfacial ferromagnetism, and, recently, the observation of interfacial high-temperature superconductivity in epitaxially grown bilayers of metallic La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ and insulating La$_{2}$CuO$_{4}$ on LaSrAlO$_{4}$ substrates$^{1}$. To help understand the mechanism underlying the observed interfacial superconductivity, we directly measured the 3D electron density of this epitaxial bilayer system with sub- atomic resolution using the Coherent Bragg Rod Analysis phase retrieval method$^{2}$. The Cu to apical O distance, which is believed to be a critical parameter controlling T$_{C}$, was found to expand dramatically from the substrate/film interface towards the surface. The correlation between structural features and interface transport properties will be discussed. $^{1}$A. Gozar et al., Nature, 455, 782(2008). $^{2}$Y. Yacoby et al., Phys. Rev. B, 77, 195426(2008).