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.
