Iron-based superconductors: A new addition to the family

A new iron-based superconductor with critical temperature higher than 40 K has been synthesized.

Fig. 1: Structure of the new superconductor (Fe₂As₂)(Ca₄(Mg,Ti)₃O_y).

High-temperature superconductivity has puzzled and intrigued physicists and material scientists for over two decades. In classical, low-temperature superconductors, the disappearance of resistivity below the ‘critical’ temperature (T_c) and the expulsion of magnetic flux by the Meissner effect are generally understood in terms of interactions between electrons with the host lattice, which can be described by the Bardeen-Cooper and Schieffer (BCS) theory. This theory, however, has been unable to describe the type of superconductivity observed in the more recently discovered high-temperature superconductors, and the origin of superconductivity in these materials remains elusive. The discovery of superconductivity with a T_c of up to 56 K in a completely new class of materials based on iron just over two years ago gave rise to a considerable revitalization of the field, with the promise of new, crucial insights that could unveil the mechanism responsible for superconductivity in general. The iron-based ‘pnictide’ superconductors usually have a rather simple layered structure. However, some compounds have been discovered in which the pnictide layers, in which the superconductivity is believed to occur, are separated by large blocking layers of oxides with the ‘perovskite‘ structure. The interesting aspects of these compounds are that their structures and compositions can be easily varied by inserting additional elements, thus allowing the synthesis of new superconductors with possibly higher T_c. This is the strategy that Hiraku Ogino and colleagues from the University of Tokyo in Japan¹ followed to synthesize the new superconductor (Fe₂As₂)(Ca₄(Mg,Ti)₃O_y), which has a T_c higher than 40 K. The team had previously synthesized other compounds with perovskite-type blocking layers. An interesting observation is that the T_c of the new compounds is not dependent on the spacing between pnictide layers but on their in-plane lengths. This suggests that the local structure of the pnictide layers plays a major role in determining the critical temperature, and that the two-dimensionality of the crystal structure, on the other hand, has little influence. “The discovery of this compound indicates that there is still considerable room for development of new iron-based superconductors. Exploration of iron-based superconductors with new structural types may lead to new breakthroughs in T_c,” says Ogino.