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June 14, 2010

Brookhaven National Lab Creates Nano-Patterned Superconducting Thin Films

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A fragment of a superconducting thin film patterned with nano-loops measuring 150 nanometers on a side (small) and 500 nanometers on a side (large), where the nano wires making up each loop have a diameter of 25 nanometers.
Brookhaven National Laboratory has fabricated thin films patterned with large arrays of nanowires and loops that are superconducting — able to carry electric current with no resistance — when cooled below about 30 kelvin (-243 degrees Celsius). Even more interesting, the scientists showed they could change the material’s electrical resistance in an unexpected way by placing the material in an external magnetic field. Superconducting nanowires and nano-loops might eventually be useful for new electronic devices

Nature Nanotechnology - Large oscillations of the magnetoresistance in nanopatterned high-temperature superconducting films



It has been a long-standing dream to fabricate superconducting nano-scale wires for faster, more powerful electronics. However, this has turned out to be very difficult if not impossible with conventional superconductors because the minimal size for the sample to be superconducting — known as the coherence length — is large. For example, in the case of niobium, the most widely used superconductor, it is about 40 nanometers. Very thin nano-wires made of such materials wouldn’t act as superconductors.

In layered copper-oxide superconductors, the coherence length is much smaller — about one or two nanometers within the copper-oxide plane, and as small as a tenth of a nanometer out-of-plane. The fact that these materials operate at warmer temperatures, reducing the need for costly cooling, makes them even more attractive for real-world applications.


Abstract

Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometres. Here, we report the results of measurements on loops of La1.84Sr0.16CuO4, a high-temperature superconductor that loses its resistance to electric currents when cooled below ~38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little–Parks effect. Moreover, unlike Little–Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry or with a period of h/4e, as predicted for superconductors that exhibit stripes.

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