Abstract

Phenomenological theories take some experimental results, make a simple assumption and calculate the consequences. If they agree with experiment, the phenomenological theory is considered a success. With this definition all theories in physics, including BCS, are phenomenological theories, but conventionally the term is used in superconductivity to describe the London theory [1] and Ginzburg–Landau theory (G–L) [2] in which the assumptions do not involve the interactions between electrons. An early phenomenological theory is the ‘two-fluid model’ [3], which splits the electrons into two separate populations: the normal electrons and the superconducting electrons. Like many other concepts in superconductivity, this model originated in work on liquid helium and, in spite of being contrary to the spirit of the quantum mechanical picture of electrons as identical particles, remains an important part of the language of superconductivity.