Frustrated and Allowed Structural Transitions at the Fermi Surface: Chemical Pressures and Principal Orbital Components in the Emergence of Charge Density Waves

Charge Density Waves (CDW) often compete or coexist with other emergent quantum phenomena, such as superconducting, topological, and magnetic phases. In developing design principles for quantum materials, an understanding of the origins of CDWs is then essential. Attempts to meet this challenge have ranged from focusing on the geometry of a material's Fermi surface and its potential for nesting to computing electron–phonon coupling matrix elements; an examination of chemical factors, such as atomic packing and crystal orbital shape, offer new opportunities. In this Article, we explore how our understanding of CDW formation can be enhanced by employing a dual evaluation of their atomic packing and electronic context. Using BaFe2Al9, ScV6Sn6, and RbV3Sb5 as test cases, we demonstrate how Density Functional Theory-Chemical Pressure (DFT-CP) schemes can reveal networks of positive CPs that scaffold paths of soft atomic motion needed for the geometrical distortions that accompany CDW transitions. To assess the electronic aspects of the CDW transitions, we develop a principal components-style analysis of the crystal orbitals near the Fermi surface, entitled principal orbital components analysis. In this approach we construct a basis of periodic orbitals ranked by the degree of their contributions to the states near the Fermi energy. A comparison of the principal orbital components with the CP schemes of the compounds offers a powerful criterion for the favorability of CDW emergence: in all three test cases the paths of soft atomic motion templated by the CP scheme involve atoms that are heavily featured in at least one of the highly ranked principal orbital components, such that the displacements strongly influence the energies of many crystal orbitals at the Fermi energy. In addition, an analysis of the k-space dependence of the principal orbital component contributions on the Fermi surface reveals that specific q-vectors are available to realize these effects, which may be distinct from the nesting vectors obtained from an analysis of the Fermi surface geometry alone. Based on these results, we propose an approach that aims to identify CDW candidates among the vast diversity of intermetallic phases.