ABSTRACT
The key thermodynamic quantity distinguishing superconducting state from any other known state of matter is condensation energy. The fact that at thermodynamic equilibrium the free energy of any system takes a minimum of its possible values implies that the condensation energy in superconductors is consumed with the maximum possible effectiveness or with maximum possible saving. Condensation energy is an internal resource allowing the specimen to stay superconducting in magnetic field. In diamagnetics the change of free energy at constant temperature equals the change of internal energy. The Peierls-London model provides a clear and simple picture of thermodynamic properties of the intermediate state and serves as a general guide for understanding of magnetic properties of type-I superconductors. Magnetic properties of pinning-free type-II superconductors of the transverse geometry represent equilibrium properties of pure vortex matter in view of the absence of effects associated with the side surface and pinning.
