How doping controls the structure and properties of vanadium dioxide: A direct comparison of nonequilibrium photo-doping with static W-doping.

VO₂ is a widely studied, prototypical strongly correlated material that exhibits an insulator-to-metal transition (IMT) coupled to a crystallographic phase transition from a low-symmetry monoclinic (M1, insulating) phase to a high-symmetry metallic rutile (R, metallic) phase at a critical temperature Tc ≈ 68 °C. Previous work has also studied laser-induced transitions in VO₂, revealing the existence of a metastable phase (M*), accessible via ultrafast laser excitation of M1, that retains the M1 crystal symmetry but exhibits 1D anti-ferroelectric order and metallic-phase conductivity [1].  The M* phase does not appear when cycling temperature in pure VO₂.

Tungsten (substitutional) doping has been shown to be an effective path to lower Tc (~ 20 °C per atomic percent of W) [2]. W-doping also has the effect of decreasing the resistivity contrast of the IMT and increasing the hysteresis width.  Here, through the comparison of temperature, photo-doping, W-doping and time-dependent (ultrafast) electron diffraction measurements on VO₂ we show that M* is stable at equilibrium in W-doped samples.  M* is conspicuously present in the region around Tc in thermally cycled W-doped VO₂ but only emerges via photo-doping in pure VO₂.  Our study demonstrates the effective equivalence of nonequilibrium photodoping and equilibrium (static) W-doping in the VO₂ system and the potential to better understand the nonequilibrium phases of VO₂ through the effects of static equilibrium W-doping, a much easier computational task.