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