The structure of magnesium diboride (MgB2) is a hexagonal system with a layer of magnesium atoms between the two layers of boron atoms. Studies have shown that the superconductivity of magnesium diboride is derived from the boron atomic layer. Scientists used a “doping” approach to find superconductors with higher critical temperatures based on MgB2. There are usually two methods of doping: one is electron doping, MgB2 -yXy, X = Be, C, N, O, that is, partially replacing the B element in MgB2 with elements such as Be, C, N, O, etc. So far, it has not been found that electron doping has the effect of increasing the critical temperature; the other is hole doping, Mgl -xMxB2 , M = Al , Be , Ca , Cu , Li , Na , Zn , ie Al , Be , Ca , Cu, Li, Na, Zn and other elements partially replace the Mg element. Chinese scientists have found that magnesium diboride (Mg0.8Cu0.2B2) doped with 20% copper has superconductivity, its superconducting transition starting temperature is 49K, and the zero-resistance temperature is 45.6K, which is the new type of magnesium diboride. The highest critical temperature in the superconductor. Mg0 .8Cu0 .2B2 is mainly a mixture of MgB2 and Cu2Mg, and its crystal structure is still a hexagonal system. Compared with MgB2, the hexagonal lattice is slightly shortened in the a-axis and c-axis directions. For conventional alloy superconductors, non-magnetic impurities often have the effect of increasing the critical temperature. The findings of Chinese scientists show that the non-magnetic impurity Cu2Mg plays a significant role in increasing the critical temperature of the magnesium diboride superconductor. On the other hand, American scientists have estimated the critical temperature of the new superconductor of magnesium diboride to 70K according to the BCS theory. The measurement of thermodynamic parameters shows that magnesium diboride is a typical type II superconductor with a lower critical magnetic field of 300 Oe and an upper critical magnetic field of 12. 5 × 104 Oe. The type II superconductor is filled with a quantized magnetic flux, and unless the flux is pinned in some way, energy loss occurs when the superconducting current flows. The high pressure experiments of different research groups show that the critical temperature of magnesium diboride decreases with increasing pressure. Theoretical calculations also prove this. National research groups used various advanced methods, such as scanning tunneling spectroscopy, high-resolution photoelectron spectroscopy, etc., to measure the energy gap values ??at different temperatures, and the results were roughly consistent with the BCS theory.
A new hot spot in superconductivity research – magnesium diboride