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An abrupt first-order metal-insulator transition (MIT) as a current jump has been observed by applying a dc electric field to Mott insulator VO2-based two-terminal devices. The size of the jumps was measured to be asymmetrical depending on the direction of the applied voltage due to heating effects. The structure of VO2 is investigated by micro-Raman scattering experiments. An analysis of the Raman-active Ag modes at 195 and 222cm−1, explained by pairing and tilting of V cations, and 622cm−1, shows that the modes below a low compliance (restricted) current do not change when the MIT occurs, whereas a structural phase transition above the low compliance current is found to occur secondarily, due to heating effects in the device induced by the MIT. The MIT has applications in the development of high-speed and high-gain switching devices.

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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...