Brief discussion of the experimental background
Superconductivity is one of the most studied areas of condensed matter physics. The discovery by H. Kamerlingh-Onnes in 1911 demonstrated that, at 4.3 K, Hg had zero resistance and remained in this state at temperatures below the transition temperature Tc (Fig. 14.1). It appeared that the solid had made a transition to a new state, and this visualization of the superconducting transition as a change of state remained. The transition was found to be very sharp with ΔT ~ 10−2 K, and no crystallographic changes were noted.
Further tests of various materials yielded new superconductors, and it became clear that non-metals were not superconducting, while metals with higher room-temperature resistivities tended to be more likely candidates for superconductivity. Another feature of superconductors was that at finite frequencies ω, the resistance ρ(ω) remained zero up to a critical value ω = ωc , where ħωc became known as the optical superconducting energy gap (Fig. 14.2). The relation ħω c ≈ 3.5k B Tc was established experimentally and later derived theoretically for a large class of superconductors.
The maximum Tc for materials increased at an average rate of 1 K every three years until the mid 1980s when it was around 25 K. In 1986, Bednorz and Müller found superconductivity in a copper oxide material in the 50 K range, and, soon afterward, Chu and collaborators achieved Tc 's above liquid nitrogen temperature and, in 1996, achieved Tc 's around 160 K. At this time, this is the highest Tc achieved in this class of materials.
An important early discovery was the existence of persistent currents, where supercurrents, once generated, persist in a superconducting ring for as long as the ring is kept below Tc . A magnetic field is induced in the hole of the ring and the system is very stable. Theoretical estimates of the time for the decay of the currents at T ≪ Tc are enormously long, perhaps of the order of the age of the universe. Another important magnetic field effect is the destruction of superconductivity in the presence of a magnetic field above a value called the critical field Hc (Fig. 14.3). The critical field is temperature dependent, rising from Hc (Tc ) = 0 to a maximum Hc (T = 0), which for superconductors like Hg is only around several hundred G.