In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of VO2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68 degrees C. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted hole excitation, which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate nonequilibrium state.

The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO2-based two-terminal device. When a constant voltage is applied to the device, an abrupt current jump is observed with temperature. With increasing applied voltages, the transition temperature of the MIT current jump decreases. We find a monoclinic and electronically correlated metal (MCM) phase between the abrupt current jump and the structural phase transition (SPT). After the transition from insulator to metal, a linear increase in current (or conductivity) is shown with temperature until the current becomes a constant maximum value above TSP T ≈68• C. The SPT is confirmed by micro-Raman spectroscopy measurements. Optical microscopy analysis reveals the absence of the local current path in micro scale in the VO2 device. The current uniformly flows throughout the surface of the VO2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.PACS numbers: 71.27. +a, 71.30.+hThe first-order Mott discontinuous metal-insulator transition (MIT) has been studied as a function of temperature in numerous materials such as Ti 2 O 3 , V 2 O 3 , and VO 2 etc [1]. Almost all have a transition temperature, T MIT , below room temperature except VO 2 which has T MIT ≈68• C. In particular, VO 2 thin films were used for fabrication of two-and three-terminal devices controlled by an electric field [2]. A high-speed Mott switching device using an abrupt current jump as observed in I-V measurements was predicted for manufacturing in the nano-level transistor regime [3,4].Moreover, Raman experiments [5] for a VO 2 film have showed monoclinic-insulator peaks after the film had undergone an electric-field-induced transition from an insulator to a metal. Furthermore, tetragonal-metal peaks have been associated with the structural phase transition (SPT) above 68• C. Also no evidence of phonon softening near the transition temperature has been found by the temperature dependence of Raman spectra measured with a VO 2 single crystal and a thin film [6]. These results support the electron correlation model of the MIT. However, some reports argue that the electric field-induced MIT is due to Joule heating by current and is accompanied by SPT, and that, furthermore, the local current path or current filament formed by the dielectric breakdown [7] can also cause the jump (MIT). The dielectric breakdown was described by depinning and the collective transport of charge carriers above a threshold voltage. Here, we try to elucidate this ambiguity through the analysis of our present research.Another interesting aspect in VO 2 is that the T MIT can be modified by doping [8,9] and stress [10]. VO 2 thin films deposited on (001) and (110) TiO 2 substrates showed a modified T MIT of 27 and 96• C, respectively, where the c-axis length was stressed by a lattice mismatch between the film and the substrate [10]. The modification of the T MIT by doping and stress is restricted to within a fixed temperature, whereas the T MIT indu...

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We systematically investigate the characteristics of an electrical oscillation observed in two-terminal vanadium dioxide ͑VO 2 ͒ devices. These oscillations are observed at room temperature in a simple electrical circuit without inductive components. The circuit is composed only of a dc voltage source, the VO 2 device, and a standard resistor connected in series with the device. We explain why the observed oscillations are a result of the percolative metal-to-insulator transition ͑MIT͒ of VO 2 and the coexistence of the metal and insulating phases. Specifically, oscillations are attributed to the construction and destruction of capacitive regions composed of regions of the semiconducting phase, ͑as dielectric material͒ and metallic phase electron carriers, induced by the MIT ͑as capacitor electrodes͒. Since the coexistence of these phases-and thus the capacitive regions-is destroyed by elevated temperature, the MIT oscillation is not explained in terms of significant heat input but rather in terms of a voltage-triggered effect. It is also discussed whether the current jump at the onset of the oscillations is driven by Mott physics or by Peierls physics relying on a structural phase transition. Furthermore, the electrical parameter space surrounding these oscillations is explored, and a generation window is identified. Within this generation window, the oscillation frequency can be continuously tuned by adjusting applied voltage or by an external circuit component, such as resistor or added capacitor. The frequency of oscillations can be increased up to Ͼ1 MHz.

In this letter, we report an observation of room temperature electrical oscillation in vanadium dioxide (VO2), a representative strongly correlated material showing a metal-insulator transition. An electric circuit for the oscillation is simply composed of a voltage source and two-terminal VO2 thin film device serially connected with a standard resistor. The systematic procedures where the oscillation occurred were explained based on the electrical relationship between the VO2 device and resistor, and the generation window of the oscillation was determined. In particular, the oscillation frequency could be controlled by adjusting an external voltage and increased up to >0.5MHz.