Phase transition between commensurate and incommensurate phases in a lattice and their formation

A number of anomalies have been detected in the behavior of the critical current of superconducting films placed in a parallel magnetic field. The dependence of the critical current Ic on the field is oscillatory in nature, and in a certain range of fields and temperatures Ic does not depend on the temperature. It is shown that the anomalous behavior is observed only in films with smooth, mutually parallel surfaces. The laws discovered are explained on the basis of concepts concerning a phase transition between commensurate and incommensurate phase in a lattice of vortices and their images. The commensurate phase corresponds to the formation of a regular periodic lattice, in which case the fixed period of the lattice of images that is determined by the thickness of the film is equal to or a multiple of the period of the vortex lattice in the film, which is determined by the value of the external magnetic field.

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