Mixed state inA−15-structure superconductors: Finite temperature

The martensitic transformation in the $A-15$-structure compounds causes a drastic change of the electronic structure in the vicinity of the Fermi level, and as a result the mixed state of the compounds becomes highly anisotropic. In a previous paper, it was shown that the domains elongated along an applied field in ${\mathrm{V}}_3$Si become unstable when the field strength exceeds a critical value and this instability may explain the anomaly observed in the ultrasonic and specific-heat measurements in the mixed state of ${\mathrm{V}}_3$Si. The analysis in the previous paper is restricted to the case of absolute zero temperature. In the present paper, we study the case of finite temperature; we calculate the temperature dependence of the Gibbs free energy and the lower critical field which depend on the direction of the tetragonal axis relative to an external magnetic field. The results are compared with experimental data.