Charge Dynamics in a Doped Valence-Bond Solid System

Abstract

Many complex transition-metal oxides show the insulator-metal transition, on which the vanishing conductivity is driven either by the divergent carrier mass or by the decrease of the carrier density. The electron-correlation driven insulator-metal transition mostly shows the former type, while the insulator-metal transition in high-temperature superconducting cuprates is known to be of the latter type. In this chapter, we present another example of the carrier-density driven insulator-metal transition in the d-electron system Ti$_{2}$O$_{3}$. We systematically study the charge dynamics in thermally and doping induced insulator-metal transitions of (Ti$_{1-\mathrm{{x}}}$V$_{\mathrm{{x}}}$)$_{2}$O$_{3}$. The origin of the observed doped-insulator-like characteristics, such as small Drude weight proportional to the doping level x, is proved to stem from the robust singlet formation on the Ti-Ti dimer. In addition, we closely investigate the dynamics of the doped holes in (Ti$_{1-\mathrm{{x}}}$V$_{\mathrm{{x}}}$)$_{2}$O$_{3}$ and have found that the doped holes show the ferromagnetic correlation and large negative magnetoresistance, while showing an extraordinarily large effective mass. We have ascribed such strong mass renormalization to a polaron effect originating from the strong electron-phonon coupling on the dimer sites.