Straining to observe the M2 phase in epitaxial VO$_2$ films*

It has been more than a decade since it was shown that the transition temperature T$_{MIT}$ of VO$_2$ in epitaxial thin films can be tuned by compressive and tensile strain along the rutile c-axis. Since this discovery, uniaxial strain studies of VO$_2$ nanobeams have demonstrated that compressive strain indeed lowers T$_{MIT}$, thus stabilizing the metallic rutile phase. However, even minor tensile strain induces an intermediate insulating monoclinic M2 phase. Whether this phase can be stabilized in thin films remains contentious owing to the constraints of sample and/or interface quality. Here, we present hard x-ray photoelectron spectroscopy and temperature-dependent soft x-ray absorption spectroscopy of high quality ultrathin epitaxial VO$_2$ films on TiO$_2$ (001) and (100) substrates. The VO$_2$/TiO$_2$(001) are absent of intermediate phases and maintain a MIT similar to unstrained VO$_2$, while the VO$_2$/TiO$_2$(100) films display a stable M2 phase between the M1 and rutile endpoint phases. We discuss our findings in terms of differences between uniaxial and biaxial strain.