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In 1987 a major breakthrough occurred in materials science. A new family of materials was discovered that became superconducting above the temperature at which nitrogen gas liquifies, namely, 77 K or –196°C. Within months of the discovery, a wide variety of experimental techniques were brought to bear in order to measure the properties of these materials and to gain an understanding of why they superconduct at such high temperatures. Among the techniques used were electromagnetic absorption in both the normal and the superconducting states. The measurements enabled the determination of a wide variety of properties, and in some instances led to the observation of new effects not seen by other measu- ments, such as the existence of weak-link microwave absorption at low dc magnetic fields. The number of different properties and the degree of detail that can be obtained from magnetic field- and temperature-dependent studies of electromagnetic abso- tion are not widely appreciated. For example, these measurements can provide information on the band gap, critical fields, the H–T irreversibility line, the amount of trapped flux, and even information about the symmetry of the wave function of the Cooper pairs. It is possible to use low dc magnetic field-induced absorption of microwaves with derivative detection to verify the presence of superconductivity in a matter of minutes, and the measurements are often more straightforward than others. For example, they do not require the physical contact with the sample that is necessary when using four-probe resistivity to detect superconductivity.
The microwave absorption at 9.2 kmc in films of gold and copper plated onto bulk tin has been measured as a function of the temperature in the range 1.4 to 4.2 K. In the limit of zero temperature the results indicate the presence of an energy gap of 0.61 x 10 to the -23rd power J at the surface of a 190 angstrom film of gold and at the surface of a 500 angstrom film of copper on tin. The normal bulk resistivity of the gold was estimated from D.C. measurements as well as from the microwave data. The theory of the proximity effect, that is the influence of adjoining normal and superconducting metals, as given by DeGennes and Werthamer was applied to the measurements and yielded an estimate for N(O)V for gold of 0.009. Assuming that the sign of N(O)V is positive this leads to a very low transition temperature. (Author).