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Coupled structural and electronic phase transitions underlie themultifunctional properties of strongly-correlated materials. For example,colossal magnetoresistance1,2 in manganites involves phase transition fromparamagnetic insulator to ferromagnetic metal linked to a structuralJahn-Teller distortion3. Vanadium dioxide (VO2) likewise exhibits aninsulator-to-metal transition (IMT) at ~67oC with abrupt changes in transportand optical properties and coupled to a structural phase transition (SPT) frommonoclinic to tetragonal4. The IMT and SPT hystereses are signatures offirst-order phase transition tracking the nucleation to stabilization of a newphase. Here we have for the first time measured independently the IMT and SPThystereses in epitaxial VO2 films, and shown that the hystereses are notcongruent. From the measured volume fractions of the two phases in the regionof strong correlation, we have computed the evolving dielectric function underan effective-medium approximation. But the computed dielectric functions couldnot reproduce the measured IMT, implying that there is a strongly correlatedmetallic phase that is not in the stable rutile structure, consistent withQazilbash et al5. Search for a corresponding macroscopic structuralintermediate also yielded negative result.

Non-congruence of thermally driven structural and electronic transition in VO2