High temperature superconductivity

      Superconductivity is a remarkable discovery in the 20th century. Ever since Kamerlingh Onnes discovered the superconductivity of mercury (1911) at 4.2 K, superconducting materials have been attracting more and more interest of research in condensed matter physics. High-temperature superconductors have much higher transition temperatures than mercury (or liquid helium) and is beyond the traditional Bardeen-Cooper-Schrieffer (BCS) theory. The theory of high-temperature superconductivity is still an active area of research.

      Currently there are many types of high-temperature superconductors, including copper-based, iron-based, and organic superconductors. Copper-based superconductors are formed due to the strong correlation of interacting electrons on copper oxide surfaces. Iron-based superconductors are composed of iron arsenic, iron phosphorus or iron selenium and other elements. Like copper oxide surface, iron arsenic, iron phosphorus or iron selenium can also increase the effective electron-electron interaction to give rise to high-temperature superconductivity. Organic superconductivity is even more special. Recently, studies have found that pig brains can be superconducting at high temperatures, but a theoretical explanation is still yet inconclusive. In addition to finding new materials, scientists have also used high pressure and high electron filling density to further increase the superconducting transition temperature.

      There are many aspects of applications for superconductors. First, since superconductors are perfectly diamagnetic, when large hadron colliders (LHC) accelerate particles, they use superconductors to increase the magnetic field strength to reduce the cyclotron radius of the accelerating particles. Second, the anti-gravity effect caused by diamagnetism is also applied to the orbit of the maglev train, in order to reduce the friction and reduce energy consumption. Third, if the superconducting zero resistance can be applied to power transmission, it can also reduce the waste of energy. However, since normal temperature and atmospheric superconductivity have not been found yet, the concept of superconducting cable has not yet been realized.

       Our group works on topological superconductivity, which can occur if a superconductor is connected to a topological material, or if the superconductor itself has a topologically non-planar state. The Majorana zero-energy mode can appear in the boundary state of the topological superconductor. The Majorana zero-energy mode no longer obeys Fermi statistics, but obeys non-Abelian statistics, which means that the order of transformation changes the state of the system. This transformation order changes the characteristics of the system state and is promising for making topological quantum computers.