Theoretical investigation of twin boundaries in WO$_3$: Structure, properties and implications for superconductivity

We present a theoretical study of the structure and functionality of ferroelastic domain walls in tungsten trioxide, WO$_3$. WO$_3$ has a rich structural phase diagram, with the stability and properties of the various structural phases strongly affected both by temperature and by electron doping. The existence of superconductivity is of particular interest, with the underlying mechanism as of now not well understood. In addition, reports of enhanced superconductivity at structural domain walls are particularly intriguing. Focusing specifically on the orthorhombic $\beta$ phase, we calculate the structure and properties of the domain walls both with and without electron doping. We use two theoretical approaches: Landau-Ginzburg theory, with free energies constructed from symmetry considerations and parameters extracted from our first-principles density functional calculations, and direct calculation using large-scale, GPU-enabled density functional theory. We find that the structure of the $\beta$-phase domain walls resembles that of the bulk tetragonal $\alpha_1$ phase, and that the electronic charge tends to accumulate at the walls. Motivated by this finding, we perform ab initio computations of electron-phonon coupling in the bulk $\alpha_1$ structure and extract the superconducting critical temperatures , $T_c$, within Bardeen-Cooper-Schrieffer theory. Our results provide insight into the experimentally observed unusual trend of decreasing Tc with increasing electronic charge carrier concentration.