Optical absorption driven by dynamical symmetry breaking in indium oxide

Indium oxide is a wide band gap semiconductor that provides the platform for most n-type transparent conductors. Optical absorption is dominated by a strong edge starting at the optical gap around 3.7 eV, but the material also exhibits a prominent absorption tail starting around 2.7 eV. We use first principles methods to show that this tail arises from interband transitions that are dipole forbidden at the static lattice level, but become dipole allowed via a dynamical symmetry breaking induced by nuclear motion. We also report the temperature dependence of the absorption onset, which exhibits a redshift with heating, driven by a combination of electron-phonon coupling and thermal expansion. We argue that the role of dynamical symmetry breaking in optical absorption is a general feature of semiconductors and that the computational design of novel materials for optical applications, ranging from transparent conductors to solar cells, should incorporate the lattice dynamics of the crystal.