Probing the Energy Gap

Abstract

The most direct tool for the determination of the excitation gap of superconductors is spectroscopy. Spectroscopy probes the basic excitation of a system by absorption and emission of a well known amount of energy. In general, different kind of probes can be used such as photons, neutrons and electrons, as long as the initial and final states with respect to energy and momentum can be determined. In the following sections tunnelling spectroscopy and the related methods of scanning tunnelling spectroscopy and point-contact spectroscopy will be discussed, threemethods which have been frequently used for the investigation of conventional and unconventional superconductors. Of course, there are further methods, e.g. the photoemission spectroscopy which has played an important role for the investigation of the high-T_c material. However, most of the superconducting compounds discussed in this book have T_c’s of the order of 1K and therefore, an excitation gap below 1meV, where these methods have serious drawbacks.