Study And Exploration On New Superconductor MgB₂

MgB2 is discovered to be a superconductor at a remarkably high critical temperature -38.5K, the highest in the similar metallic compounds. Its higher Tc , simple lattice structure and lower cost have made it the most potential superconductor. The measuring of low-temperature specific heat is an important and effective method to study the structure of electronic states, the atomic vibration of lattice, phase transition and structure of grain boundary. We have measured the low-temperature specific heat of MgB2, and analyzed thoroughly its lattice and electronic contribution respectively. And the resistances of Cu-doped MgB2 samples are measured and the reason of discovered linear resistance is analyzed. a) The analyse of normal state specific heat of MgB2 shows : there is a strong Einstain mode at about 310K. Therefore its lattice specific heat composes of Debye and Einstain contribution. After extended the formula of normal state specific heat to the superconducting state, we obtain its electronic specific heat of superconducting state. At the temperature far below Tc, the electronic specific heat deviates the BCS-like pattern largely. This indicates the existence of the second energy gap. So, we fit the specific heat of its superconducting state by two-gap model, and get the magnitude and relative proportion of the two gaps. b) We have compared the specific heats of ambient and high pressure sintered MgB2. It shows that the leap of electronic specific heat â–³Ces of high pressure prepared sample at Tc is smaller than that of ambient pressure prepared sample. The reason is perhaps that the method of high-pressure sinter changes the shape of the Fermi faces, and reduces the contribution of one pair of Fermi faces and increases the contribution of the other. The mechanism is possibly related to the Proximity interacting of the Fermi faces. c) In the study of the doped samples of MgB2, we notice that when Cu concentration x=0.2, it shows entirely linear temperature dependence of resistance. By contract with the resistance of Cu2Mg, the reason of the abnormality is not due to the portion of MgB2, but other components in our sample.