Kyoto University, Japan, announced on October 14, 2016 that a phenomenon has been discovered in superconductors of copper-doped selenium compounds: a pair of electrons that induces a superconducting state will spontaneously align, enabling the formation of electron pairs. The intensity is weakened in a specific direction. Since this phenomenon is similar to the nematic state of liquid crystals (liquid crystal molecules align in a certain direction, a special direction will occur, resulting in the breaking of the rotational symmetry), it is called "nematic superconductivity."
“Symmetric spontaneous breakage†is an extremely important concept in the entire field of physics. The superconductivity of the current-carrying electron composition that leads to the disappearance of resistance is also closely related to the breakage of symmetry. For example, "p-wave" superconductors and "d-wave" superconductors have been discovered previously that have "magnetoid-like time-reversal symmetry breaking" superconductors, phase characterized by superconducting pairs, and special direction-changing properties. Wait. However, no “rotational symmetry breaking superconductivity†has been found in which the strength of the superconducting pair changes with the direction.
With regard to "rotational symmetry breaking superconductivity," its more rigorous definition is "no matter what the structural equivalence direction is, the strength of the superconductor pair is not the same". The researchers used a single-crystal sample of CuxBi2Se3, a substance that incorporates copper ions in the crystal of Bi2Se3, a selenium compound, and measured the specific heat related to the bonding strength of the superconductor pair under the control of the direction of the magnetic field. The substance is formed by stacking triangular structures of atoms. When the magnetic field rotates in parallel with this layer, the specific heat shows a period of 180 degrees with respect to the magnetic field angle.
There are three equivalent directions in the structure of the material. It was originally envisaged that when the magnetic field is oriented in these equivalent directions, the specific heat is the same, and a vibration occurs at a period of 60 degrees (sometimes 120 degrees). . However, it was found that the period observed was longer than expected, and that the maximum magnetic field (upper critical magnetic field) that could maintain the superconducting state would also occur in the direction of the magnetic field with a period of 180 degrees, and therefore in three equivalents. In the direction, the bonding strength of the superconducting pair weakens in one of the directions, thereby achieving nematic superconductivity.
As it is known theoretically, the nematic superconducting state in this material is a superconducting superconducting state that has received much attention in recent years. Therefore, this result is used to measure the overall properties of the sample when measuring specific heat. The topological superconducting state is shown. Furthermore, nematic superconductivity is a nematic state in which superconductivity forms for this quantum mechanical state, and therefore this state is very novel compared to known nematic states.
In the future, the nature of nematic superconductors also needs to be ascertained. For example, in the nematic liquid crystal state, the direction of molecular arrangement can be controlled by an external voltage, and if the direction of the nematic superconductor can also be controlled by an external method, it is expected to open up new applications for superconducting devices. In addition, the results of this study also proved that CuxBi2Se3 is a topological superconductor by thermodynamics, which is a big step towards the goal of quantum computing.
The research was conducted jointly by Kyoto University and the University of Cologne in Germany, Kyoto University of Technology, and the Japan Institute for Atomic Energy Research and Development. Relevant results were published on the online version of Nature Physics, a scientific journal issued by the Nature Publishing Group, on October 11, 2016. Moreover, the journal also publishes relevant reports on this achievement in the "News and Views" of the focus paper. (Special Contributor: Kudosuke)
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