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Correlation-driven insulator-metal transition in near-ideal vanadium dioxide films

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO2. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO2 films are preceded by the purely-electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (+/- 0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically-promising material.