Phase Transitions in Correlated Oxides Modulated through Electrochemical Gating

Abstract

Strongly correlated oxides have many unusual properties, such as high-temperature superconductivity, colossal magnetoresistance, spin polarization, and metal-insulator phase transitions (MIT) that can be induced by various types of external stimuli, such as temperature, strain, pressure, electron and chemical doping, and light. VO2 undergoes an electronic and structural phase transition at 67°C from a non-magnetic monoclinic (M1) phase to a paramagnetic rutile (R) phase. Modulation of the transition temperature through oxygen over-stoichiometry is difficult to achieve in VO2 through annealing due to nucleation of the V2O5 phase within the VO2 lattice. However, electrochemical doping at room temperature using suitable electrolyte allows for systematic modulation of oxygen stoichiometry in VO2 through the regulation of charge carrier injection. Using this method, the oxygen stoichiometry in VO2 was modulated from 1.86 to 2.44 and its effects on the band structure and MIT was studied through detailed electrical and photoemission studies. In oxygen over-stoichiometric VO2 the TIMT was shifted from 67°C to 118°C, as shown in Figure 1.The results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO2 results in the formation of e− and h+ pairs that lead to delocalized polaronic V3+ and V5+ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V3+ electrons at V5+ sites, which results in the formation of V4+–V4+ dimers, and removal of π* screening electrons. It is shown that the nature of phase transitions is linked to the lattice V3+/V5+ concentrations of stoichiometric VO2 and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition. Figure 1