Phase transitions in vortex lattices of hexagonal exotic superconductors

In this paper the author suggest a theoretical description of magnetic phase transitions in hexagonal exotic superconductors with an order parameter having two complex components. For different values of magnetic field H and the Ginzburg-Landau functional coefficients the author has qualitatively considered the problem of the energetically most favored structure of the vortex lattice near Hc2 when H is along the sixfold symmetry axis. Near Hc2 a single-quantum lattice (SQL) could be regular triangular or rectangular. When the field decreases, a transition from a SQL to a two-quantum vortex lattice (TQL) is realized in the widest range of the functional coefficients. For further decrease in H a reverse TQL-SQL transition should occur in superconductors with a Ginzburg-Landau constant x much-gt 1. A coefficient range in which singular vortices are converted into nonsingular vortices have also been found. The values of the magnetic fields corresponding to these phase transitions are calculated as functions of the functional coefficients