Super-small thin films can superconduct

Scientists from Bar-Ilan University, Israel, and the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have worked together to create thin films that superconduct when cooled below 30 K (243C). Led by physicist Ivan Bozovic, the Brookhaven team used molecular beam epitaxy to build a material with alternating layers of copper-oxide, lanthanum, and strontium, a technique previously used to produce thin films that retain superconductivity within a single copper-oxide layer.

A fragment of superconducting thin film, patterned with nanoloops that measure 150 nm on a side (small) and 500 nm on a side (large), where the nanowires making up each loop have a diameter of 25 nm.

The team at Bar-Ilan then used electron-beam lithography to etch a pattern of thousands of loops into the surface of the material, forming “nanowires” on the sides of these loops that measured 25 nm in diameter and 150 to 500 nm in length. When measured, electrical resistance of the patterned arrays showed that they were indeed superconducting when cooled below about 30 K. The scientists also noted that when an external magnetic field was applied perpendicular to the loops, the loop resistance did not continue to increase steadily, but rather changed up and down in an oscillatory manner. “These oscillations in resistance have a large amplitude, and their frequency corresponds to discrete units (quanta) of magnetic flux — the measure of the strength of the magnetic field piercing the loops,” Bozovic said. “A material with such a discrete, switchable form of magneto-resistance — especially from the superconducting to the non-superconducting state — could be extremely useful for engineering new devices.”

The frequency of the oscillations in resistance may also hold some insight as to why copper-oxide materials become superconductors in the first place, and potentially lead to further designs for new materials.

The research appears in the June 13, 2010, online issue of “Nature Nanotechnology” ( www.nature.com/nnano/journal/vaop/ncurrent/abs/nnano.2010.111.html), and is supported by the DOE’s Office of Science, by the German Research Foundation through a German-Israeli cooperative agreement, and by a scholarship granted by the Israel Ministry of Science.

Christina D’Airo