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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...

Using pre-designed trains of femtosecond optical pulses, we have selectively excited coherent phonons of the radial breathing mode of specific-chirality single-walled carbon nanotubes within an ensemble sample. By analyzing the initial phase of the phonon oscillations, we prove that the tube diameter initially increases in response to ultrafast photoexcitation. Furthermore, from excitation profiles, we demonstrate that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when the tube diameter undergoes radial breathing mode oscillations.PACS numbers: 78.67. Ch,71.35.Ji, Single-walled carbon nanotubes (SWNTs), hollow onedimensional nanostructures with unique electronic, mechanical, and optical properties, come in a variety of species, or chiralities. Some of them are metallic and others semiconducting, depending on their chiral indices (n,m) [1,2,3]. This diversity, while making them such unusual nanomaterials, often makes it challenging to extract reliable parameters on chirality-dependent properties from experimental results on ensemble samples. Currently, there are world-wide efforts on SWNT purification, separation, and enrichment, producing promising results [4,5,6,7,8]. However, a standard for fabrication of these samples has yet to be established.Here, we present a novel method that allows us to study single-chirality nanotubes even though the sample contains nanotubes of many different chiralities. Specifically, we have utilized the techniques of femtosecond pulse shaping [9,10,11] in ultrafast pump-probe spectroscopy of SWNTs to selectively excite the coherent lattice vibrations [12,13] of the radial breathing mode (RBM) of specific chiralities. The excitation spectra of such coherent phonons (CPs) provide chirality-specific information on the processes of light absorption, phonon generation, and phonon-induced band structure modulations in unprecedented detail. In particular, the excitation-energy-dependence of the phase of the CP oscillations provides direct, time-domain evidence that band gap oscillations follow the diameter oscillations in the RBM coherent phonon mode.The sample studied was a micelle-suspended SWNT solution, where the SWNTs (HiPco batch HPR 104) were suspended as individuals with sodium cholate [14]. The optical setup was that of standard degenerate pumpprobe spectroscopy, but chirality selectivity was achieved by using multiple pulse trains, with a pulse-to-pulse interval corresponding to the period of a specific RBM mode. Among different species of nanotubes, those having RBM frequencies that are matched to the repetition rate of multiple pulse trains will generate large amplitude coherent oscillations with increasing oscillatory response to each pulse, while others will have diminished coherent responses [15,16,17]. The tailoring of multiple pulse trains from femtosecond pulses was achieved using the pulse-shaping technique described elsewhere [10]. Pulse trains are incident on an ensemble of nanotubes as a pump beam, and coherent RBM osc...

Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one can generate and detect coherent phonons in chirality-specific semiconducting single-walled carbon nanotubes. The signals are resonantly enhanced when the pump photon energy coincides with an interband exciton resonance, and analysis of such data provides a wealth of information on the chirality-dependence of light absorption, phonon generation, and phonon-induced band structure modulations. To explain our experimental results, we have developed a microscopic theory for the generation and detection of coherent phonons in single-walled carbon nanotubes using a tight-binding model for the electronic states and a valence force field model for the phonons. We find that the coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We compared our theoretical results with experimental results on mod 2 nanotubes and found that our model provides satisfactory overall trends in the relative strengths of the coherent phonon signal both within and between different mod 2 families. We also find that the coherent phonon intensities are considerably weaker in mod 1 nanotubes in comparison with mod 2 nanotubes, which is also in excellent agreement with experiment.

The aim of this study was to compare the pre- and post-impact three-dimensional kinematics of the ball and racquet during first and second serves performed by elite tennis players. Data were collected from four male and four female right-handed professional players during competition using two high-speed cameras (200 Hz). For each player, one first serve and one second serve from the 'deuce' or right service court that landed within the specified target area were analysed. To test for significant differences between the first and second serves, Wilcoxon tests (P < or = 0.05) were performed on selected parameters. The results indicate that the ball travelled forward and to the left during the flight phase of the toss in all but one trial. The average pre-impact ball forward location for the first serve was significantly more in front and had a higher associated forward ball velocity than the corresponding values for the second serve. On average, the decrease in post-impact ball speed from the first to the second serve was 24.1%. No significant differences between the first and second serves were found in the pre-impact racquet head speed and orientation, which was represented as a unit vector perpendicular to the racquet face. The major adjustments made by the players when going from the first to second serve were a decrease in pre-impact ball forward location (P < or = 0.01) and an increase in the pre-impact racquet vertical and lateral velocities (both P < or = 0.05). This implies that the players tossed the ball closer to the body and imparted topspin and sidespin on the ball by changing the racquet vertical and lateral velocities when going from the first to the second serve.