Recently discovered superconductor has unexpected properties

Researchers from Tokyo Metropolitan University have found that crystals of a recently discovered superconducting material, a layered bismuth chalcogenide with a four-fold symmetric structure, shows only two-fold symmetry in its superconductivity.

While research into superconductivity structures is not particularly well understood, this research suggests a connection with an enigmatic class of materials known as nematic superconductors and a range of mechanisms that could promote superconductivity at much easier-to-reach temperatures. Superconductors are materials with extremely low electrical resistance, but significant barriers exist which prevent more widespread usage.

In 2012, Prof Yoshikazu Mizuguchi of Tokyo Metropolitan University succeeded in engineering layered bismuth chalcogenide materials with alternating insulating and superconducting layers for the first time. The same team have now taken measurements on single crystals of the material and found that the rotational symmetry characteristics of the crystalline structure are not replicated in how the superconductivity changes with orientation.

The material the group studied consisted of superconducting layers made of bismuth, sulfur and selenium, and insulating layers made of lanthanum, fluorine and oxygen. Importantly, the chalcogenide layers had four-fold rotational (or tetragonal) symmetry i.e. the same when rotated by 90 degrees.

When the team measured the magnetoresistance of the material at different orientations they only found two-fold symmetry i.e. the same when rotated by 180 degrees. Further analyses at different temperatures did not suggest any changes to the structure; they concluded that this breakage of symmetry must arise from the arrangement of the electrons in the layer.

The concept of nematic phases comes from liquid crystals, where disordered, amorphous arrays of rod-like particles can point in the same direction, breaking rotational symmetry while remaining randomly distributed over space.

Recently it has been hypothesised that something similar in the electronic structure of materials, electronic nematicity, may be behind the emergence of superconductivity in high temperature superconductors.

This finding clearly links this highly customisable system to high temperature superconductors like copper and iron-based materials.

The team hope that further investigation will reveal critical insights into how otherwise widely different materials give rise to similar behaviour, and how they work.

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