Magnesium diboride (MgB₂) is an ionic compound with a hexagonal crystal structure. It is an intercalation type compound with alternating layers of magnesium and boron.
Researchers discovered in 2001 that a seemingly inconspicuous compound, magnesium diboride, turns into a superconductor at a temperature slightly close to the absolute temperature of 40K (ie -233°C). Its transition temperature is almost twice that of other superconductors of the same type, and its actual working temperature is 20~30K. To reach this temperature, liquid neon, liquid hydrogen or closed-cycle refrigerators can be used to complete the cooling. These methods are simpler and cheaper than the industrial cooling of niobium alloys (4K) with liquid helium. Once doped with carbon or other impurities, magnesium boride can maintain superconductivity as good as niobium alloy, or even better, in the presence of a magnetic field or current passing through. Its potential applications include superconducting magnets, power transmission lines, and sensitive magnetic field detectors.
Among metal materials, multi-band and multi-Fermi noodles are a common feature. When the material enters the superconducting state, the superconducting energy gap is generally opened on the Fermi surface, so multiple energy bands will lead to the appearance of multiple energy gaps. In many superconducting materials, the multi-band effect will be greatly weakened due to the extremely strong inter-band scattering. However, in some superconducting materials with quasi-two-dimensional characteristics, multi-band and multi-gap effects will appear due to the orthogonality of the electron motion wave functions above different energy bands. The recently discovered iron-based superconductors also exhibit a certain multiband effect. This is currently an important direction of superconducting materials and physics research.
Magnesium diboride is a typical multi-band superconductor. It has two hole-type σ bands, one hole-type π band and one electron-type π band. Due to the special configuration of the Fermi surface (the π band is three-dimensional and the σ band is quasi-two-dimensional), its The wave vectors of electrons in different energy bands are in an orthogonal state, so that the inter-band scattering is not very strong, which makes the superconductor's multi-band characteristics outstanding. Magnetoresistance is a powerful means to detect changes in the scattering rate of electrons in cyclotron motion, and Hall effect can detect information such as the number of carriers. Therefore, combining magnetoresistance and Hall effect can deduce the scattering rate of electrons in different energy bands.
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