Abstract
Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO2, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO2 from 1.86 to 2.44 on the band structure and insulator–metal transitions are presented for the first time. 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 pi^{*} 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.
Highlights
Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO2, whose properties have been an issue for accurate prediction by common theoretical probes
Their presence introduces additional structural distortion due to lattice s train[30,31], and gives rise to overlapping chemical and magnetic signatures, complicating the study of phase transition behavior. These issues can be avoided if methods exist for the controlled introduction of native defects (V and O) that would enable the study of resulting band structure changes and its corresponding influence on insulator-to-metal transition (IMT)
The combined results of electrical resistivity and photoemission measurements show that the doping-induced IMT process is controlled by the lattice V3+/V5+ ratio
Summary
Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO2, whose properties have been an issue for accurate prediction by common theoretical probes. 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. Our results shed a new light on the IMT process They indicate that stoichiometric V O2 in the metallic state is multi-valent with co-existing V3+ and V 5+ ions that are a result of charge fluctuation of lattice V 4+ cations. Localization of charge carriers at the disproportionated cationic sites, likely due to strong correlation effects, together with lattice distortions, lead to the nucleation of an insulating phase. Results show that CrO2 is a half-metallic ferromagnet due to its mixed valent lattice composed of C r3+ and C r4+ ions[23,24,25]
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