On structural instabilities in itinerant magnets and superconductors

The influence of a structural distortion on the itinerant antiferromagnetic (IAF) phase and on the superconducting (S) phase is investigated using a free electron band structure for the electron and hole pockets. For equal concentrations of electrons and holes a metastable phase is found in which the lattice distortion coexists with the IAF or S phase. For unequal concentrations of electrons and holes the critical temperature and the value of the order parameter for the IAF or S phase (in the coexistence region) will always be enhanced by the onset of the structural distortion. The theoretical predictions are compared in the IAF case with the occurrence of a spin flip transition in chromium accompanied by a tetragonal distortion, and in the S case with experimental results on A-15 compounds exhibiting a martensitic phase transition.