Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2
Siddharth Joshi, Nicholas Smieszek, Vidhya Chakrapani
Abstract
Abstract Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO 2 , whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO 2 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 VO 2 results in the formation of e − and h + pairs that lead to delocalized polaronic V 3+ and V 5+ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V 3+ electrons at V 5+ sites, which results in the formation of V 4+ –V 4+ dimers, and removal of $$\pi^{*}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>π</mml:mi> <mml:mrow> <mml:mrow/> <mml:mo>∗</mml:mo> </mml:mrow> </mml:msup> </mml:math> screening electrons. It is shown that the nature of phase transitions is linked to the lattice V 3+ /V 5+ concentrations of stoichiometric VO 2 and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition.