Superconductivity in layered iron germanide

New materials are the lifeblood of condensed matter research. The discovery of high temperature superconductivity in certain iron-pnictide compounds has since 2008 motivated a wide-ranging search for new superconductors. Superconductivity has since been found in numerous iron compounds, in contrast with its more limited occurrence among the cuprates. Iron-based superconductors can be categorised into diverse structure families, such as the ’11-11′ (e.g. SmFeAsO), ‘1-2-2’ (BaFe₂As₂), ‘1-1-1’ (LiFeP) and ‘1-1’ (FeSe) systems. All these materials have in common that they combine iron with a group-V (pnictogen) or group-VI (chalcogen) element in a layered structure.

This restriction to pnictides or chalcogenides is now being challenged. A team of scientists at the University of Cambridge and at University College London have widened the search to group-IV element Ge and found evidence for superconductivity, albeit at a relatively low temperature. In the ‘1-2-2’ compound YFe₂Ge₂, they report a superconducting transition below about 1.8 K. They also observed an unusual power-law temperature dependence of the electrical resistivity above 1.8 K, suggesting that the superconductivity develops out of an anomalous normal state. Moreover, the heat capacity in the normal state is unusually high; it is among the highest found in transition metal compounds, and about an order of magnitude higher than what would be expected from numerical calculations, suggesting strong electronic correlations.

Although the nominal Fe valence in YFe₂Ge₂ is the same as in the isostructural iron-arsenides KFe₂As₂, RbFe₂As₂ and CsFe₂As₂, which have similar superconducting transition temperatures and a similarly enhanced heat capacity as YFe₂Ge₂, numerical calculations suggest that the electronic structure of YFe₂Ge₂ is very different to that of these iron-arsenide superconductors. The origin of superconductivity in YFe₂Ge₂ remains a wide open question, which will require more detailed experimental and theoretical studies and may itself point towards further superconductors, possibly with higher transition temperatures.