National Synchrotron Light Source Lunch Time Seminar

Several groups have recently reported that the onset temperature of superconductivity in certain copper oxides may be enhanced - perhaps even exceeding the bulk maximum - at interfaces in artificial structures fabricated by atomic layer-by-layer deposition techniques [1,2]. A key question is whether this effect is purely structural, i.e. arising from cation interdiffusion between the layers, or arises from something more fundamental like the formation of a carrier accumulation region, such as occurs at a metal-on-semiconductor interface. To address this issue we used resonant soft x-ray scattering (RSXS) to quantify the hole distribution in a superlattice of insulating La2CuO4 (LCO, x=0) and overdoped, nonsuperconducting La2-xSrxCuO4 (LSCO, x=0.36). Despite its nonsuperconducting constituents, this structure is superconducting with Tc = 38 K. We found that the conducting holes redistribute electronically from LSCO to the LCO layers, indicating the existence of an electronic, charge accumulation region. The LCO layers were found to be optimally doped, suggesting they are the main drivers of superconductivity. Our results demonstrate that nontrivial electronic effects take place in oxide heterostructures, and may be observed with RSXS.