PHASE-TRANSITIONS IN THE FLUX LINE LATTICES OF HEXAGONAL UNCONVENTIONAL SUPERCONDUCTORS

The theoretical description is given of the magnetic phase transitions in hexagonal exotic superconductors with the two-component order parameter. For various values of the magnetic filed H and Ginzburg - Landau functional parameters, the question about the most energetically favorable structure of the flux line lattice (FLL) near H(c2) for H in the direction of the hexagonal symmetry-axis of a crystal is considered quantitatively. In the close vicinity of H(c2) the one-quantum lattice may be regularly triangular or rectangular. With decreasing the field in the widest range of the functional parameters the one-to-two quantum lattice transition occurs. For superconductors with the Ginzburg - Landau constant k >> 1, with the further decrease of H the reverse transition from two-quantum to one-quantum lattices should take place. The parameter region where the singular-to-nonsingular FLL transition takes place is found as well. The values of the magnetic fields corresponding to the phase transitions concerned are calculated as a function of the functional parameters.