174 research outputs found

    Temperature dependence of Mott transition in VO_2 and programmable critical temperature sensor

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    The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO_2-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 T_{SPT}=68^oC. 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 VO_2 device. The current uniformly flows throughout the surface of the VO_2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.Comment: 4 pages, 3 figure

    Nanostructured Iron Sulfide/N, S Dual-Doped Carbon Nanotube-Graphene Composites as Efficient Electrocatalysts for Oxygen Reduction Reaction

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    Nanostructured FeS dispersed onto N, S dual-doped carbon nanotube-graphene composite support (FeS/N,S:CNT-GR) was prepared by a simple synthetic method. Annealing an ethanol slurry of Fe precursor, thiourea, carbon nanotube, and graphene oxide at 973 K under N-2 atmosphere and subsequent acid treatment produced FeS nanoparticles distributed onto the N, S-doped carbon nanotube-graphene support. The synthesized FeS/N,S:CNT-GR catalyst exhibited significantly enhanced electrochemical performance in the oxygen reduction reaction (ORR) compared with bare FeS, FeS/N,S:GR, and FeS/N,S:CNT with a small half-wave potential (0.827 V) in an alkaline electrolyte. The improved ORR performance, comparable to that of commercial Pt/C, could be attributed to synergy between the small FeS nanoparticles with a high activity and the N, S-doped carbon nanotube-graphene composite support providing high electrical conductivity, large surface area, and additional active sites

    Observation of First-Order Metal-Insulator Transition without Structural Phase Transition in VO_2

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    An abrupt first-order metal-insulator transition (MIT) without structural phase transition is first observed by current-voltage measurements and micro-Raman scattering experiments, when a DC electric field is applied to a Mott insulator VO_2 based two-terminal device. An abrupt current jump is measured at a critical electric field. The Raman-shift frequency and the bandwidth of the most predominant Raman-active A_g mode, excited by the electric field, do not change through the abrupt MIT, while, they, excited by temperature, pronouncedly soften and damp (structural MIT), respectively. This structural MIT is found to occur secondarily.Comment: 4 pages, 4 figure

    Thermally Activated Delayed Fluorescence Materials Based on Homoconjugation Effect of Donor–Acceptor Triptycenes

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    Donor–acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2, were synthesized and their emissive properties were studied. They exhibited a blue-green fluorescence with emission lifetimes on the order of a microsecond in cyclohexane at room temperature. The long lifetime emission is quenched by O[subscript 2] and is attributed to thermally activated delayed florescence (TADF). Unimolecular TADF is made possible by the separation and weak coupling due to homoconjugation of the HOMO and LUMO on different arms of the three-dimensional donor–acceptor triptycene. Organic light emitting devices (OLEDs) were fabricated using TPA-QNX(CN)2 and TPA-PRZ(CN)2 as emitters which displayed electroluminescence with efficiencies as high as 9.4% EQE.Samsung (Firm)Japan Society for the Promotion of Scienc

    Monoclinic and Correlated Metal Phase in VO_2 as Evidence of the Mott Transition: Coherent Phonon Analysis

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    In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 THz and 6.0 THz indicating the rutile metal phase of VO_2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68^oC. The monoclinic and correlated metal(MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photo-assisted 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 non-equilibrium state.Comment: 4 pages, 2 figure

    Visceral fat thickness and its associations with pubertal and metabolic parameters among girls with precocious puberty

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    Purpose This study aimed to investigate associations of central obesity with sexual maturation and metabolic parameters in Korean girls with precocious puberty. Methods This retrospective study evaluated data from 72 girls under 8 years of age with a chief complaint of early breast development. The patients were categorized as central precocious puberty (CPP) subjects or non-CPP subjects based on their gonadotropin-releasing hormone stimulation test results. Visceral fat thickness (VFT) was measured using ultrasonography and defined as the distance from the linea alba to the aorta. Patient anthropometric, metabolic, and hormonal parameters were also evaluated. Results Increased VFT was correlated with an earlier onset of thelarche among all study subjects (r=-0.307, P=0.034). Overweight CPP subjects showed higher insulin resistance than normal weight CPP subjects. Insulin resistance was not significantly different between overweight and normal weight non-CPP subjects. VFT was not significantly different between CPP and non-CPP subjects (2.22±0.79 cm vs. 2.74±1.47 cm, P=0.169). However, overweight and obese CPP subjects (body mass index percentile>85%) had lower VFT than non-CPP obese subjects. Conclusions Central obesity, defined using ultrasonography-measured VFT, might be associated with early pubertal development in Korean girls. However, VFT was not higher in CPP than non-CPP patients and was not significantly correlated with insulin resistance. Further longitudinal studies with a larger cohort are needed

    Efficacy and Long-Term Follow Up of Combination Therapy with Interferon Alpha and Ribavirin for Chronic Hepatitis C in Korea

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    Combination therapy with interferon alpha (IFN-α) and ribavirin for 24 or 48 weeks according to HCV genotype has improved the overall sustained virological response (SVR) rates to approximately 40%. The aim of this study was to investigate the long-term efficacy of combination therapy with IFN-α and ribavirin for chronic hepatitis C in Koreans. One hundred thirty-eight patients with chronic hepatitis C who received this combination therapy between 1995 and 2003 were analyzed retrospectively. All patients were treated with IFN-α 3-6 million units three times weekly in combination with 900-1200 mg/day of ribavirin for 24 weeks. The overall SVR rate was 41.3%. Patients were followed up for a median of 41 months (range, 12-105 months) after completion of therapy. In all of the SVR patients (57 patients), SVR was conserved during the follow-up period. None of the patients progressed to decompensated liver disease or hepatocellular carcinoma (HCC). However, 5 of the 81 non-SVR patients (6.2%) progressed to decompensated liver disease or HCC. In conclusion, combination therapy with IFN-α and ribavirin shows good long-term efficacy in patients with chronic hepatitis C in Korea, one of the highest endemic areas of hepatitis B virus (HBV) infection
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