The question of how thin cuprate layers can be while still retaining high-temperature
superconductivity (HTS) has been challenging to address, in part because experimental studies
require the synthesis of near-perfect ultrathin HTS layers and ways to profile the superconducting
properties such as the critical temperature and the superfluid density across interfaces with atomic
resolution. We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal
(La1.65Sr0.45CuO4) and a cuprate insulator (La2CuO4) in which each layer is just three unit cells
thick. We selectively doped layers with isovalent Zn atoms, which suppress superconductivity and
act as markers, to show that this interface HTS occurs within a single CuO2 plane. This approach
may also be useful in fabricating HTS devices.
We use resonant soft x-ray scattering (RSXS) to quantify the hole distribution in a superlattice of
insulating La2CuO4 (LCO) and overdoped La2-xSrxCuO4 (LSCO). 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. The LCO layers were found to be optimally doped, suggesting
they are the main drivers of superconductivity. Our results demonstrate the utility of RSXS for
separating electronic from structural effects at oxide interfaces.
The realization of high- transition- temperature (high-Tc) superconductivity confined
to nanometre-sized interfaces has been a long- standing goal because of potential
applications(1,2) and the opportunity to study quantum phenomena in reduced dimensions(3,4).
This has been, however, a challenging target: in conventional metals, the high electron
density restricts interface effects (such as carrier depletion or accumulation) to a
region much narrower than the coherence length, which is the scale necessary for superconductivity
to occur. By contrast, in copper oxides the carrier density is low whereas Tc is high and the
coherence length very short, which provides an opportunity - but at a price: the interface must
be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator
(La2CuO4) and a metal (La1.55Sr0.45CuO4), neither of which is superconducting in isolation.
In these bilayers, Tc is either similar to 15 K or similar to 30 K, depending on the layering
sequence. This highly robust phenomenon is confined within 2 - 3nm of the interface. If such a
bilayer is exposed to ozone, Tc exceeds 50 K, and this enhanced superconductivity is also shown
to originate from an interface layer about 1 - 2 unit cells thick. Enhancement of Tc in bilayer
systems was observed previously(5) but the essential role of the interface was not recognized at
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