May 07, 2008

Iron and arsenic superconductors could be path to room temperature superconductors

Previously nextbigfuture has discussed the new family of superconducting materials based on iron and arsenic compounds

The Christian Science Monitor indicates some experts say they've seen hints suggesting that these new materials should post impressive magnetic-field numbers soon. The iron in these new superconductors could allow some applications such as wires and electronics and more efficient engines to be developed more easily if they can carry higher current, have stronger magnetic fields or if the material is tougher and more robust than the cuprate superconductors.

New experiments at Cornell have verified a theory that variations in the distance between atoms in cuprate superconductors account for differences in the temperature at which the material begins to superconduct. A better understanding of the process could lead to superconductors that work at higher temperatures. Within most cuprate crystals, the copper and oxygen atoms are arranged in pyramids, with an oxygen atom at the apex. Theorists have proposed that superconductivity can be modified when dopants alter the crystal structure and push this apex-atom down or sideways, changing the way its electrons interact with those in the atoms in the pyramid base. The researchers also verified that electron pairing is more likely in the vicinity of dopant atoms, at completely random locations in the crystal. Both effects are taking place at the same time, Davis said, and both result from the squeezing of the copper-oxide pyramid.

Cuprate crystals consist of layers of copper oxide interleaved with layers of other atoms. Copper and oxygen atoms usually form a pyramid with the oxygen atom at the apex located in an adjacent layer. Cornell research now shows that other atoms pushing that oxygen out of position creates superconductivity.

The biggest spurt in work on the new superconductors has come from China. Among other things, it has the laboratory labor force that can systematically look at ingredients with properties similar to those in the original recipe and try them out. China and Japan both place high priority on such work because they realize that new materials tend to translate into new technologies, says George Crabtree, a researcher at the Argonne National Laboratory outside Chicago.

A research paper: Multiband magnetism and superconductivity in Fe-based compounds by
Vladimir Cvetkovic and Zlatko Tesanovic, Department of Physics & Astronomy, The Johns Hopkins University, Baltimore
have constructed a simplified tightbinding
model which they believe qualitatively describes the physics of FeAs layers in Fe-based superconductors. The researchers indicate that the recent discovery of high Tc superconductivity in Fe-based compounds has reignited interest in different pathways to the room temperature superconductivity. The iron arsenic superconductors could be showing exciton-assisted superconductivity, long-anticipated but never unambiguously observed.

They evaluate analytically the elementary particle-hole response in charge, spin and multiband channels and use the results to discuss various features of the SDW/AF
order and superconductivity. They stress the importance of puckering of As atoms in promoting d-electron itinerancy and argue that high Tc of Fe-based superconductors
might be essentially tied to the multiband character of their Fermi surface. It is tempting to speculate that different Tc’s obtained for different rare-earth substitutions might be related to the different degree of puckering in FeAs layers.