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Register a new userElectrostatic modification of infrared response in gated structures based on VO2
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We investigate the changes in the infrared response due to charge carriers introduced by electrostatic doping of the correlated insulator vanadium dioxide (VO2) integrated in the architecture of the field effect transistor. Accumulation of holes at the VO2 interface with the gate dielectric leads to an increase in infrared absorption. This phenomenon is observed only in the insulator-to-metal transition regime of VO2 with coexisting metallic and insulating regions. We postulate that doped holes lead to the growth of the metallic islands thereby promoting percolation, an effect that persists upon removal of the applied gate voltage.
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Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide (VO2) in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nano-scale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasiparticle mass in the metallic puddles. The experimental approach employed here sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.
Electrical control of magnetism is a long-standing goal in physics and technology, recently developed electrolyte gating techniques providing a promising route to realization. Validating a recent theoretical prediction, here we demonstrate large enhancement of electrostatic modulation of ferromagnetic order in ion-gel-gated ultrathin La$_{0.5}$Sr$_{0.5}$CoO$_{3-delta}$ by thickness-tuning to the brink of a magnetic percolation transition. Application of only 3-4 V then drives a transition from a short-range-ordered insulator to a robust long-range ferromagnetic metal, realizing giant electrostatic Curie temperature modulation over a 150 K window. In operando polarized neutron reflectometry confirms gate-controlled ferromagnetism, also demonstrating unusually deep penetration of induced magnetization, in further agreement with theory.
We have measured photoemission spectra of two kinds of TiO$_2$-capped VO$_2$ thin films, namely, that with rutile-type TiO$_2$ (r-TiO$_2$/VO$_2$) and that with amorphous TiO$_2$ (a-TiO$_2$/VO$_2$) capping layers. Below the Metal-insulator transition temperature of the VO$_2$ thin films, $sim 300$ K, metallic states were not observed for the interfaces with TiO$_2$, in contrast with the interfaces between the band insulator SrTiO$_3$ and the Mott insulator LaTiO$_3$ in spite of the fact that both TiO$_2$ and SrTiO$_3$ are band insulators with $d^0$ electronic configurations and both VO$_2$ and LaTiO$_3$ are Mott insulators with $d^1$ electronic configurations. We discuss possible origins of this difference and suggest the importance of the polarity discontinuity of the interfaces. Stronger incoherent part was observed in r-TiO$_2$/VO$_2$ than in a-TiO$_2$/VO$_2$, suggesting Ti-V atomic diffusion due to the higher deposition temperature for r-TiO$_2$/VO$_2$.
We report on the observation of stable electrical oscillation in Pt/VO2 bilayer strips, in which the Pt overlayer serves the dual purposes of heating up the VO2 and weakening the electric field in the VO2 layer. Systematic measurements in an ultrahigh vacuum nanoprobe system show that the oscillation frequency increases with the bias current and/or with decreasing device dimension. In contrast to most VO2-based oscillators reported to date, which are electrically triggered, current-induced Joule heating in the Pt overlayer is found to play a dominant role in the generation of oscillation in Pt/VO2 bilayers. A simple model involving thermally triggered transition of VO2 on a heat sink is able to account for the experimental observations. The results in this work provide an alternative view of the triggering mechanism in VO2-based oscillators.
The optical/infrared properties of films of vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) have been investigated via ellipsometry and near-normal incidence reflectance measurements from far infrared to ultraviolet frequencies. Significant changes occur in the optical conductivity of both VO2 and V2O3 across the metal-insulator transitions at least up to (and possibly beyond) 6 eV. We argue that such changes in optical conductivity and electronic spectral weight over a broad frequency range is evidence of the important role of electronic correlations to the metal-insulator transitions in both of these vanadium oxides. We observe a sharp optical transition with possible final state (exciton) effects in the insulating phase of VO2. This sharp optical transition occurs between narrow a1g bands that arise from the quasi-one-dimensional chains of vanadium dimers. Electronic correlations in the metallic phases of both VO2 and V2O3 lead to reduction of the kinetic energy of the charge carriers compared to band theory values, with paramagnetic metallic V2O3 showing evidence of stronger correlations compared to rutile metallic VO2.
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