Superconductors

Abstract

Superconductors superconductor are characterized by an anomalous temperature dependence of the electrical resistivity. Below a critical temperature Tc, their resistivity drops by more than a factor of 1010. In superconductors the magnetic flux density B = μrBa induced by an externally applied field Ha is zero, like in ideal diamagnets with μr = 0 (Meißner–Ochsenfeld effect). If Ha exceeds a critical value Hc the superconductor becomes normal conducting. But the magnetic induction B decreases from Ba at the free surface to B = 0 in the interior through a layer of finite thickness characterized by the Landau penetration depth λ. The critical field varies with temperature as (formula presented) Hc(T)=Hc(0)[1-(T/Tc)2],whereHc(0)=Hc(T=0K). According to the isothermal field dependence of the magnetization I(Ha) = - μ0Ha, two types of superconductors may be differentiated, as shown in Fig. 21.1: Type I superconductors such as Pb with a sudden drop of −I, at Hc; all pure metallic elements and their dilute solid solutions belong to this group.Type II superconductors such as Pb-In15 which are characterized by a lower critical field Hc1 at which the drop of −I sets in and an upper critical field Hc2 at which −I reaches 0.