Pnictides go critical

The two most important families of so-called high-temperature superconductors, those based on copper (the cuprates) and those based on iron (the pnictides), turn out to have a fundamental difference. In the work published in Nature Physics on 19 January 2014, high magnetic fields were used to strip away the superconductivity of pnictides. This reveals more about the nature of the metallic (or resistive) state from which the superconductivity (the zero-resistive state) emerges. The researchers compared the results with earlier research on cuprates, which was published in Science in 2009.

Whatever scatters the electrons in the metallic state, thus causing it to become resistive, may also be the same interaction that causes the electrons to form the superconducting state. Thus, it is believed that studying the nature of the underlying metallic state can provide important clues as to the origin of the superconductivity - always the holy grail in this area of research.

Many researchers in the field think that superconductivity in the two families of high-temperature superconductors share a common origin - the vibrations (or fluctuations) of the spins on the copper or iron atoms within the crystalline lattice. Theory argues that superconductivity is optimised when these fluctuations become ‘critical’, i.e. when the ordering of these spins occurs at zero Kelvin - the absolute zero of our temperature scale.

‘This article lends strong support to this idea for the iron-based pnictides’, Nigel Hussey, director of the HFML at Radboud University, explains. ‘The nature of the underlying resistive state in the copper-based superconductors was revealed by our group in an earlier paper published in Science back in 2009. The striking differences now revealed between the two families casts doubt on whether the same theoretical arguments can be applied to the copper-based superconductors, whose superconducting transition temperatures are about twice as high. For them, a fundamentally new theory may be required.’

Reference
Transport near a quantum critical point in BaFe₂(As_1-xP_x)₂
James G. Analytis, H-H. Kuo, Ross D. McDonald, Mark Wartenbe, P. M. C. Rourke, N. E. Hussey and I. R. Fisher DOI: 10.1038/NPHYS2869 Nature Physics January 19 2014

Left: Temperature-tuning parameter phase diagram for a conventional quantum critical system described in terms of the power-law exponent of the temperature-dependence of the electrical resistance. Right: Comparative phase diagram for high temperature superconductors. The white dashed line indicates the superconducting transition temperature in zero magnetic field. In the presence of a high magnetic field, the veil of superconductivity is stripped away to reveal the metallic ground state underneath and the persistence of the linear-in-temperature resistance across the phase diagram, an observation that is difficult to explain within conventional quantum critical scenarios. (R. A. Cooper et al., Science 323, 603 (2009)).

Colour plot of the power-law exponent of the temperature dependence of the electrical resistance in the iron-based pnictide superconductors revealed by this work (J. G. Analytis et al. Nature Physics, in press (2014)). It resembles to a large extent the right hand side of the left hand panel above, i.e. the behaviour expected for a conventional quantum critical system, and contrasts markedly with what has been observed previously in the copper-oxide high temperature superconductors.