Understanding of metal-insulator transition in VO₂ based on experimental and theoretical investigations of magnetic features

The metal-insulator transition temperature T_c in VO₂ is experimentally shown to be almost the same as a magnetic transition temperature T_m characterized by an abrupt decrease in susceptibility, suggesting the evidence of the same underlying origin for both transitions. The measurement of susceptibility shows that it weakly increases on cooling for temperature range of T > T_m, sharply decreases near T_m and then unusually increases on further cooling. A theoretical approach for such unusual observations in susceptibility near T_m or below is performed by modeling electrons from each two adjacent V⁴⁺ ions distributed along V-chains as a two-electron system, which indicates that the spin exchange between electrons could cause a level splitting into a singlet (S = 0) level of lower energy and a triplet (S = 1) level of higher energy. The observed abrupt decrease in susceptibility near T_m is explained to be due to that the sample enters the singlet state in which two electrons from adjacent V⁴⁺ ions are paired into dimers in spin antiparallel. By considering paramagnetic contribution of unpaired electrons created by the thermal activation from singlet to triplet levels, an expression for susceptibility is proposed to quantitatively explain the unusual temperature-dependent susceptibility observed at low temperatures. Based on the approach to magnetic features, the observed metal-insulator transition is explained to be due to a transition from high-temperature Pauli paramagnetic metallic state of V⁴⁺ions to low-temperature dimerized state of strong electronic localization.