High Temperature Superconductors

Ceramic materials are expected to be insulators -- certainly not superconductors, but that is just what Georg Bednorz and Alex Muller found when they studied the conductivity of a lanthanum-barium-copper oxide ceramic in 1986. Its critical temperature of 30 K was the highest which had been measured to date, but their discovery started a surge of activity which discovered superconducting behavior as high as 125 K.

Cuprate Superconductor Phases

Illustrative of the complexity of the high-temperature superconductor materials is this phase diagram which applies to the cuprate materials. At very low doping, they show the long range order of an antiferromagnet.

Doping breaks up the antiferromagnetic order and they become insulators. Only with doping fraction between about 0.1 and 0.2 do they become superconductors.

Energy Gap in Superconductors as a Function of Temperature

The effective energy gap in superconductors can be measured in microwave absorption experiments. The data at left offer general confirmation of the BCS theory of superconductivity.

Vanadium Heat Capacity

The heat capacity of superconducting vanadium is very different from that of vanadium which is kept in the normal state by imposing a magnetic field on the sample. The exponential increase in heat capacity near the critical temperature suggests an energy bandgap for the superconducting material. This evidence for a bandgap is one of the pieces of experimental evidence which supports the BCS theory of superconductivity.

Exponential Heat Capacity

As it is warmed toward its critical temperature, the heat capacity of vanadium increases 100-fold in just 4 K. This exponential increase suggests an energy gap which must be bridged by thermal energy. This energy gap evidence was part of the experimental motivation for the BCS theory of superconductivity. Compare heat capacity of normal and superconducting vanadium.