Unveiling the electronic structure of pseudo-tetragonal WO$_3$ thin films

WO$_3$ is a binary 5d compound which has attracted remarkable attention due
to the vast array of structural transitions that it undergoes in its bulk form.
In the bulk, a wide range of electronic properties has been demonstrated,
including metal-insulator transitions and superconductivity upon doping. In
this context, the synthesis of WO$_3$ thin films holds considerable promise for
stabilizing targeted electronic phase diagrams and embedding them in
technological applications. However, to date, the electronic structure of
WO$_3$ thin films is experimentally unexplored, and only characterized by
numerical calculations. Underpinning such properties experimentally would be
important to understand not only the collective behavior of electrons in this
transition metal oxide, but also to explain and engineer both the observed
optical responses to carriers' concentration and its prized catalytic activity.
Here, by means of tensile strain, we stabilize WO$_3$ thin films into a stable
phase, which we call pseudo-tetragonal, and we unveil its electronic structure
by combining photoelectron spectroscopy and density functional theory
calculations. This study constitutes the experimental demonstration of the
electronic structure of WO$_3$ thin-films and allows us to pin down the first
experimental benchmarks of the fermiology of this system.