Critical behavior of tetragonal systems with coupled order parameters. Application toUXTeandU2N2Zcompounds

By representation analysis of $\mathrm{U}X\mathrm{Te}$ and ${\mathrm{U}}_2{\mathrm{N}}_{2}Z$ compounds a Landau-Ginzburg-Wilson Hamiltonian with two coupled order parameters is constructed. Using the exact renormalization-group technique in $d=4-\epsilon$ dimensions the competition between the order parameters is analyzed in detail. It is shown that the isotropic bicritical behavior is not affected by the anisotropic coupling within the vector order parameter and that the critical behavior for ${\mathrm{U}}_2$ ${\mathrm{N}}_2$Te should be of Heisenberg-like type, whereas for $\mathrm{U}X\mathrm{Te}$ compounds it should be of Ising-like type.