Soft-modes and electronic topological transitions in p- and d-electron materials

The Fermi surface of a metal can be altered through the application of isotropic pressure, symmetry-breaking anisotropic strains, or chemical substitution. Changes in Fermi surface topology, such as the opening of a gap, or the creation or disappearance of a pocket, are known as Lifshitz transitions, and can dramatically influence the electronic, magnetic and mechanical properties of a material. This talk will focus on two examples of such transitions: the Burgers distortion that transforms body centered cubic metals to hexagonal close packed is driven by a Jahn-Teller-Peierls transition associated with a pseudogap in d_xy orbitals created by symmetry breaking; the collapse transitions in tetragonal AFe₂As₂ pnictides (A = Ca, Sr, Ba) are driven by the disappearance of a p_z electron pocket upon the application of pressure. These collapse transitions are contrasted with the discontinuous tetragonal to orthorhombic transition in CaFe₂As₂, which is thermodynamic rather than mechanical in character. Both the Burgers distortion and the collapse transition are connected with soft mode behaviors.