The Tokyo Metropolitan University's researchers have discovered that crystals of a recently found superconducting material, a layered bismuth chalcogenide with a four-fold symmetric structure, reveals only two-fold symmetry in its superconductivity. The researchers are yet to understand the origin of superconductivity in these structures and this discovery indicates a connection with an enigmatic class of materials identified as nematic superconductors and the extraordinary mechanisms by which superconductivity can emerge at easier-to-reach temperatures.
As materials, superconductors have remarkably low electrical resistance. These materials have already seen several applications to powerful electromagnets, especially in medical magnetic resonance imaging (MRI) units, where scientists use them to generate the strong magnetic fields necessary for high-resolution non-invasive imaging.
Nonetheless, considerable barriers exist that prevents more widespread usage such as for power transmission over long distances. Significantly, conventional superconductivity only arises at notably low temperatures. The earliest superconductors with "high-temperature" were discovered in the latter half of the 1980s, and scientists still debate the mechanisms behind how they work.
The Tokyo Metropolitan University's Prof. Yoshikazu Mizuguchi, in 2012, was able to succeed in engineering layered bismuth chalcogenide material with alternating superconducting and insulating layers for the first time.
Materials containing elements from group 16 of the periodic table are the chalcogenides. Currently, the same group have measured single crystals of the materials and found that the rotational symmetry characteristics of the crystalline structure are not replicated in how the superconductivity changes with orientation.
The team studied the material that incorporated layers of superconductivity formed of sulfur, bismuth, and selenium, and insulating layers made of lanthanum, fluorine, and oxygen. Essentially, segments of the chalcogenide had four-fold rotational (or tetragonal) symmetry specifically the same when rotated by 180 degrees.
Further analysis at different temperatures did not propose any changes to the structure. The conclusion was that this breakage of symmetry must originate from the arrangement of the electrons in the layer.
The nematic phases' concept arises from liquid crystals, where disordered, amorphous arrays of rod-like particles can point in the same direction, breaking rotational symmetry while remaining distributed over space.
In recent time, it has been hypothesized that electronic nematicity, something similar in the electronic structure of materials, may be behind the emergence of superconductivity in high-temperature superconductors.
It was explicit in this discovery that connects this highly customizable system to high-temperature superconductors like copper and iron-based materials. It is the hope of this group that further investigation will reveal critical insights into how otherwise widely different materials give rise to the same behavior, and how they function.
