Apr 15 2019
Scientists at Tokyo Metropolitan University have discovered that crystals of a recently identified superconducting material, a layered bismuth chalcogenide with a four-fold symmetric structure, exhibits only two-fold symmetry in its superconductivity.
The source of superconductivity in these structures is still not well understood; this discovery proposes a link with an enigmatic class of materials called nematic superconductors and the exceptional mechanisms by which superconductivity can be exhibited at easier-to-reach temperatures.
Superconductors are materials that have very low electrical resistance. They have already been used in several applications to powerful electromagnets, specifically in medical magnetic resonance imaging (MRI) units, where they are employed to produce the strong magnetic fields needed for high-resolution non-invasive imaging.
However, considerable barriers are present which hinder more extensive usage, for example, power transmission over long distances. The most remarkable is that traditional superconductivity only arises at very low temperatures. The first “high-temperature” superconductors were only discovered in the second half of the 1980s, and the underlying mechanisms of how they function are still intensely argued.
Prof. Yoshikazu Mizuguchi of Tokyo Metropolitan University, for the first time, successfully engineered layered bismuth chalcogenide materials (chalcogenides are materials comprising of elements from group 16 of the periodic table) with alternating layers of insulating and superconducting materials, in 2012. Currently, the same group has carried out measurements on single crystals of the material and discovered that the rotational symmetry characteristics of the crystalline structure are not replicated in how the superconductivity varies with orientation.
The material investigated by the team comprised of superconducting layers composed of bismuth, selenium, and sulfur, and insulating layers composed of fluorine, lanthanum, and oxygen. Notably, the chalcogenide layers had four-fold rotational (or tetragonal) symmetry meaning it has the same symmetry even when rotated by 90°.
However, when the group determined the magnetoresistance of the material at various orientations, they only observed two-fold symmetry, i.e., the same when rotated by 180°. Additional analyses at various temperatures did not imply any variations to the structure; they deduced that this breakage of symmetry must be due to the arrangement of the electrons in the layer.
The idea of nematic phases originates from liquid crystals, where disordered, amorphous series of rod-like particles can point in the same direction, violating rotational symmetry while remaining randomly distributed across space. Very lately, it has been assumed that something similar in the electronic structure of materials, electronic nematicity, may be the reason for the appearance of superconductivity in high-temperature superconductors.
This discovery apparently associates this highly modifiable system with high-temperature superconductors such as copper and iron-based materials. The group expects that further study will offer important insights into how otherwise broadly different materials lead to similar behavior, and how they function.
This research was supported in part by Collaborative Research with IMR, Tohoku Univ. (proposal number: 17H0074) and Grants-in-Aid for Scientific Research (Nos. 15H05886, 16H04493, 17K19058).