ON THE TEMPERATURE-DEPENDENCE OF THE LONDON PENETRATION DEPTH IN A SUPERCONDUCTOR

In the framework of the electron-phonon theory of superconductivity [1], it is shown that the magnetic field penetration depth delta-L increases like a certain power of temperature at T << T(c) due to the low-lying excitations in the phonon spectrum. For the acoustic phonons with the density of states approximately omega-2 the penetration depth increases approximately T5. The origin of such a high power of T is the same as that in the case of resistivity of the normal metal: the phonon corrections to the electromagnetic vertex should be taken into account, and major terms (approximately T3) cancel, the surviving ones having a higher power of T. The possibility of linear and quadratic terms in delta-L(T) is discussed in a model of electrons interacting with two-level centers [2].