Shedding light on nematic superconducting states

Solid-state systems harbour numerous intriguing behaviours — and probably as many mysteries. An outstanding example is superconductivity. In a superconductor, electrons flow without any resistance once the material is cooled below a critical temperature. This is of obvious technological relevance. Yet, for many known superconductors the underlying physical mechanisms remain not fully understood. These gaps in understanding in turn hinder, for example, systematic searches for novel superconductors with critical temperatures closer to room temperature.

The difficult to grasp the microscopic mechanisms that are at play in these materials comes not least from the complex interplay of factors that contribute to the observed macroscopic behaviour. These factors typically are related to magnetic, electronic and/or structural properties of the material, and disentangling them is notoriously difficult. Therefore the interest in experimental studies such as the one presented earlier this month by Manuel Chinotti, a PhD student of Prof. Leonardo Degiorgi in the Laboratory for Solid State Physics, and colleagues in his group, in the Ames Laboratory (US) and at the Karlsruhe Institute of Technology (Germany). Writing in Physical Review B, they report how from one single experiment evidence can be found of all the ingredients determining defining properties of one specific superconductor they studied.

The team looked at a prominent member of the family of iron-based superconductors. The first superconductor containing iron was reported in 2008. That discovery came as quite a surprise. On the one hand, elemental iron is strongly magnetic, and typically magnetism tends to destroy superconductivity. On the other hand, the temperature at which the material becomes superconducting was unexpectedly high. But understanding how these ‘exotic’ superconductors work remains an outstanding challenge.

Fresh experimental insight comes now from the study of Chinotti and co-workers. They studied crystals of iron selenide (FeSe), an iron-based superconductor that has garnered particular interest in recent years. FeSe exhibits at 90 K a transition from a tetragonal to an orthorhombic crystal structure. This symmetry-breaking process occurs in other iron-based superconductors as well, and there is evidence that the transition is driven by electronic degrees of freedom (rather than by lattice vibrations). In particular, in the ‘nematic’ phase — that is, below the tetragonal-to-orthorhombic transition — several electronic properties of these materials have been found to be anisotropic, indicating that electrons have a role in the transition. However, in most iron-based superconductors, the nematic phase transition is accompanied by changes in magnetic properties, making it difficult to separate electronic, magnetic and structural effects. Not so in FeSe. Throughout the entire temperature range from 90 K down to the transition to superconductivity at 8 K and below, there is no magnetic ordering in this material. This means that FeSe offers a platform for studying the nematic phase in relative isolation.