Using a combination of unique tools to build and analyze films just nanometers thick, a group of researchers found subtle structural changes that are known to alter superconducting properties. Their findings stress the importance of studying both the superficial and deep features of superconducting materials, which transport electricity with zero resistance and are envisioned for use in applications like ultrafast, power-saving electronics or more efficient electricity lines.
The study was spurred by a surprising discovery made in 2008 by a Brookhaven-led research team: an ultrathin film — just a few nanometers, or billionths of a meter, thick — made of two non-superconducting cuprates (a metal and an insulator) actually exhibits superconductivity at the interface between the layers. This finding has triggered a debate about the super-property’s origin and the possibility to turn the material’s transition temperature thermostat higher.
But in order to reveal more information about this unique ultrathin film — which is made from a cuprate containing lanthanum, strontium, copper, and oxygen and appropriately called LSCO — scientists need a new set of tools that characterize the full film, not just the surface.
The researchers grew a variety of states of LSCO using a unique molecular beam epitaxy system at Brookhaven that allows materials to be deposited atom by atom, giving precise control of each layer's thickness. By varying the concentration of strontium in the cuprate, the scientists can create LSCO that behaves in three different ways: like a metal, an insulator, or a superconductor.
They then used x-ray diffraction at Argonne National Laboratory’s Advanced Photon Source to determine the positions of the atoms inside the film. This was only possible by combining a synchrotron technique called coherent Bragg rod analysis (COBRA) with Difference Mapping, a slower but more accurate procedure. The result is a precise technique that’s two orders of magnitude quicker than Difference Mapping alone.
This diffraction data was used to map the distance of LSCO’s out-of-plane (apical) oxygen atoms from the copper-oxygen plane, a characteristic known to affect the transition temperature of high-temperature superconductors.
The group’s results showed an unexpected discrepancy. The apical distance stays the same in metallic and superconducting LSCO films but increases in the metallic-insulating LSCO bilayer films previously studied by the researchers.
These results show that the crystal structure of the few layers nearest to the surface can be very different from that inside the bulk — and in an unexpected ways.
H. Zhou, Y. Yacoby, V.Y. Butko, G. Logvenov, I. Božović, R. Pindak, “Anomalous Expansion of the Copper-Apical-Oxygen Distance in Superconducting Cuprate Bilayers,” PNAS, 107(18), 8103 (2010).
Left: Ron Pindak (left) and Ivan Božović at NSLS beamline X21,
which could be upgraded to perform COBRA measurements as a jump-start program for
Right: The measured electron density in two atomic planes of the interfacial superconducting bilayer LSCO film showing the positions of the atoms in two metallic (M) and three insulating (I) unit cells Left: in the (100) plane, the white lines highlight the projected shapes of the CuO6 octahedra, in particular the elongation near the surface; Right: in (110) plane, the white lines highlight the projected profiles of the La-apical O planes, in particular the enhanced corrugation near the surface.