Jul 4, 2007
Return of the nanostripes
A research team in Germany has discovered nanoscale stripe-like structures in high-temperature superconductors made from samarium, barium, copper and oxygen. Many physicists believe that such structures are responsible for the ability of these superconductors to carry currents without resistance, but others think they aren't. The new result could not only open up this debate again, but also shed more light on the origins of high-temperature superconductivity, which has been one of the biggest mysteries in physics for the last two decades.
All high-temperature superconductors consist of parallel planes of copper oxide, with other elements sandwiched in between these layers. The copper atoms lie on a square lattice and the charge is carried by "holes" sitting on oxygen sites. Previous X-ray scattering measurements on yttrium barium copper oxide superconductors revealed spectra containing diffuse features that were attributed to the formation of stripes in the copper oxide planes. Many physicists believe that these stripes serve as channels along which the super-current can flow.
In 2004, however, researchers in Germany found that these features had their origins in oxygen defects instead. Meanwhile, an independent team in the US observed "nanodomains", suggesting the same superstructures seen by the German group. These results meant that the stripes of charge might not be responsible for the ability of high-temperature superconductors to carry current without resistance after all.
Now, Michael Koblischka and colleagues at Saarland University in Saarbruecken have observed nanoscale stripe-like structures in samarium barium copper oxide (SmBCO). The stripes are sometimes parallel over several microns and sometimes wavy. The researchers say that the structures may act as effective "pinning centres" thanks to their small-scale periodicity, which is typically 10–60 nm. This is the ideal pinning-centre size for these materials to achieve high critical current densities even at elevated temperatures of around 77 K.
Koblischka and co-workers saw the stripes in single crystals of SmBCO grown by the so-called top-speed pulling technique and in melt-textured samples. Detailed atomic-force and scanning-tunnelling-microscopy measurements revealed that the nanostripes are formed by chains of individual nanoclusters from unit cells of the samarium-rich phase, Sm1+xBa2–xCu3Oy.
"The higher transition temperature, Tc (of 93.5 K) and the larger critical current densities, Jc (of around 38,000 A/cm2 at T = 77 K and 2 T applied field) make these SmBCO materials interesting for bulk applications, such as levitation," Koblischka told nanotechweb.org. "Although the reason for their improved Jc is not yet clear, the appearance of the nanostripes may be the key."
The researchers reckon that controlling these pinning structures, which run through the whole sample volume, could help improve the Jc further – especially at high external magnetic fields.
The work was reported in Supercond. Sci. Technol..
About the author
Belle Dumé is contributing editor at nanotechweb.org