Understanding the metal-insulator transition in VO2 from quantum Monte Carlo, DMFT, and experiment

Vanadium dioxide displays the quintessential example of metal-insulator transition (MIT) physics in a strongly correlated material. Despite numerous studies, the nature of the MIT is still controversial and new perspectives are needed. Recent experiments view rutile VO2 as an unconventional metal due to its anomalously low electronic thermal conductivity. Due to strong correlations in VO2, beyond DFT approaches are required and here we study pristine and non-stoichiometric VO2 with quantum Monte Carlo and DMFT. New perspective is provided by the momentum distribution, which contains no discontinuity in the metallic phase, indicating a non-Fermi liquid metal consistent with experimental findings. Quasi-1D back-scattering along the rutile c-axis is reminiscent of a Tomanaga-Luttinger liquid, where the scattering is induced by impurities. In non-stoichiometric VO2 the calculated spectral function indicates a competition between a_1g and e^π_g orbitals which have a role in the formation of the insulating state. DMFT-VCA calculations show that the a_1g/e^π_g orbital dichroism falls below its pristine value at a doping concentration of δ=0.07, in near agreement with the experimentally determined critical doping threshold for the suppression of the insulating state.