Elemental chalcogens as a minimal model for charge and orbital order

Helices of increased electron density emerging spontaneously in materials containing multiple, interacting density waves, are an example of how orbital and charge degrees of freedom may combine to form a single ordered phase. Although a macroscopic order parameter theory describing this behaviour has been proposed and experimentally tested, a microscopic understanding of such simultaneous orbital and electronic order in specific materials is still lacking. Here we present the elemental chalcogens selenium and tellurium as model materials for the development of combined charge and orbital order. We formulate minimal models capturing the formation of spiral structures consisting of ordered occupied orbitals and increased charge density, both in terms of a macroscopic Landau theory and a microscopic Hamiltonian. Both reproduce the known chiral crystal structure and are consistent with its observed thermal evolution and behaviour under pressure. The combination of microscopic and macroscopic frameworks allows us to distil the essential ingredients in the emergence of combined orbital and charge order, and may serve as a general guide to predicting and understanding spontaneous chirality as well as other, more general, types of combined charge and orbital order in other materials.