Fig. 1: Electron coupling to TO1 phonon.
From: Theory of superconductivity in doped quantum paraelectrics
Two types of symmetry allowed couplings between t2g electrons and the TO phonons. In a two phonon process (a), a pair of TO phonons, each denoted by a wavy line oriented along the displacement axis, couples to the electron density. b Single-phonon processes can occur provided the tunneling occur between two distinct t2g orbitals on the nearest neighbors. Along the crystalline x axis, a TO displacement along the z-direction couples the dxy and dyz orbitals. The wavy line denotes the TO phonon displacement axis.
