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Advances in Mathematical Physics
Volume 2017 (2017), Article ID 6265427, 9 pages
Research Article

Quasi-Particles, Thermodynamic Consistency, and the Gap Equation

Dipartimento di Fisica “E. Fermi”, Università di Pisa and INFN Sezione di Pisa, Largo B. Pontecorvo 2, 56127 Pisa, Italy

Correspondence should be addressed to Enore Guadagnini

Received 18 April 2017; Revised 29 May 2017; Accepted 6 June 2017; Published 10 July 2017

Academic Editor: Xavier Leoncini

Copyright © 2017 Enore Guadagnini. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The thermodynamic potentials of superconducting electrons are derived by means of the Bogoliubov-Valatin formalism. The thermodynamic potentials can be obtained by computing the free energy of a gas of quasi-particles, whose energy spectrum is conditional on the gap function. However, the nontrivial dependence of the gap on the temperature jeopardises the validity of the standard thermodynamic relations. In this article, it is shown how the thermodynamic consistency (i.e., the validity of the Maxwell relations) is recovered, and the correction terms to the quasi-particles potentials are computed. It is shown that the Bogoliubov-Valatin transformation avoids the problem of the thermodynamic consistency of the quasi-particle approach; in fact, the correct identification of the variables, which are associated with the quasi-particles, leads to a precise calculation of the quasi-particles vacuum energy and of the dependence of the chemical potential on the electron density. The stationarity condition for the grand potential coincides with the gap equation, which guarantees the thermodynamic consistency. The expressions of various thermodynamic potentials, as functions of the variables, are produced in the low temperature limit; as a final check, a rederivation of the condensation energy is presented.