Engineering

Publication Search Results

Now showing 1 - 7 of 7
  • (1997) Han, Shaowei; Zhao, Yong; Zhang, Hua; Gu, Genda; Russell, Graeme; Koshizuka, N
    Journal Article
    A resistivity anomaly near the superconducting transition temperature is observed in the ab-plane of as-grown Bi2Sr2CaCu2O8 (Bi-2 2 1 2) single crystals. The resistivity at the peak is much larger than the normal state resistivity around 100 K. The peak vanished gradually with increasing the applied magnetic field. I---V curves of the Bi-2 2 1 2 samples also exhibit a peak close to the transition steps, vanishing gradually with increasing intrinsic granularity of high-Tc superconductors.

  • (1997) Zhao, Yong; Zhang, Hua; Gu, Genda; Han, Shaowei; Russell, Graeme; Koshizuka, N
    Journal Article
    Voltage noise of the Bi2Sr2CaCu2O3+x single crystal below Tc is by an asymptotic power spectrum method. The low-frequency noise, power, Sa(T), shows a peak at Y = Tp. This peak is found to be closely related to the vortex lattice melting temperature. Near the peak T = Tp the power spectrum, Sa(f), exhibits a power-law fall-off, while at much lower temperatures Sa(f) dramaticaily deviates from the power-law behaviour, showing some broad peaks which are superposed upon a background with an exponential-law characteristic. The estimated correlation dimension of the noise data reveals that the vortex motion is highly correlatas at the early stage of melting, then develops into an extended dissipative dynamic system with self-organized criticality, showing a 1/fa behaviour. The correlated motion disappears when the lattice completely melts.

  • (1997) Russell, Graeme; Gu, Genda; Han, Shaowei; Zhao, Yong; Zhang, Hua; Koshizuka, N
    Journal Article
    A resistivity anomaly near the superconducting transition temperature is observed in the ab-plane of as-grown Bi2Sr2CaCu2O8 (Bi-2 2 1 2) single crystals. The resistivity at the peak is much larger than the normal state resistivity around 100 K. The peak vanished gradually with increasing the applied magnetic field. I---V curves of the Bi-2 2 1 2 samples also exhibit a peak close to the transition steps, vanishing gradually with increasing intrinsic granularity of high-Tc superconductors.

  • (1998) Han, Shaowei; Zhao, Yong; Yang, C; Cheng, Chun-Chung; Zhang, Hua; Gu, Genda; Russell, Graeme; Koshizu, N
    Journal Article

  • (1997) Zhao, Yong; Zhang, Hua; Gu, Genda; Han, Shaowei; Russell, Graeme; Koshizuka, N
    Journal Article

  • (1998) Han, Shaowei; Zhao, Yong; Yang, C; Cheng, Chun-Chung; Zhang, Hua; Gu, Genda; Russell, Graeme; Koshizuka, N
    Journal Article
    The vortex-glass-liquid transition in Bi---Sr---Ca---Cu---O (BSCCO) intergrowth single crystals in which some CuO2 trilayers are inserted into the Bi2Sr2CaCu2O8 matrix has been studied. The curvature of the log V-logI curves for CuO2 trilayers is found to change from negative to positive with increasing magnetic field and a scaling behaviour for the vortex-glass state is observed. The magnetic field dependence of the vortex-glass transition temperature, TG, qualitatively obeys the relation H(TG) (Tc − Tg)2v0, with v0 = 1.15 ± 0.2, which is larger than the 2/3 predicted theoretically for an isotropic 3D system. Our results reveal that the vortex-glass transition exists in a system constructed with CuO2 trilayers.

  • (2022) Nguyen, Minh Triet
    Thesis
    Singlet fission is a photo-physical process that generates two triplet excitons from one singlet exciton and can potentially enhance efficiency in photovoltaic systems. The combination of photovoltaics and singlet fission is a novel field for solar energy conversion when there is much interest in renewable, non-destructive, and continuously available energy sources. Singlet fission can also overcome thermalization losses in photovoltaics, which happens in traditional cells when the incident photon energy is higher than the silicon bandgap energy, using a carrier multiplication mechanism. This thesis will design, construct, and characterize photovoltaic devices incorporating singlet fission materials to study singlet fission in practical application. The research focuses on materials characterization, spin dynamics, and electron transfers between acene and the semiconductor layer in Au/TiO2 ballistic cells, and the incorporation of singlet fission layers on silicon-based cell structures. In detail, a set of investigations was developed and summarized by implementing singlet fission materials into a state-of-the-art ballistic photovoltaic device and silicon-based solar cell. The studies demonstrate proof of concept and rationally explain the process. The first part of the thesis investigates thin films of pentacene, TIPS-pentacene, and tetracene via crystallinity, morphology, absorption, and thickness characterization. Additionally, Au and TiO2 layers in Schottky device structures were optimized to achieve the best performance for energy transfer from an applied dye layer (merbromin). The drop-casted dye layer influences the device performance by increasing short-circuit current and open-circuit voltage, demonstrating the ability of charge transfer between the device and the applied film. This device structure provides a test bed for studying charge and energy transfer from singlet fission films. The latter part of the thesis describes several investigations to understand singlet fission in a thin film using this architecture. Magneto-photoconductivity measurements were primarily used to observe the spin dynamics via photoconductivity under an external magnetic field. Control experiments with bare Au/TiO2 devices showed no observable magneto-photoconductivity signal. In contrast, devices with pentacene and tetracene singlet fission layers showed a strong magnetoconductivity effect caused by ballistic electron transfer from the singlet fission layer into the TiO2 n-type semiconductor through an ultra-thin gold layer inserted between the layers. A qualitatively different behavior is seen between the pentacene and tetracene, which reveals that the energy alignment plays a crucial part in the charge transfer between the singlet fission layer and the device. The last section investigates the application of pentacene and tetracene evaporated thin-films as sensitizer layers to a silicon-based solar cell. The optimized Si cell structure with the annealing treatment improved the cell's performance by increasing short-circuit current and open-circuit voltage. The deposition of pentacene and tetracene as sensitizer layers into the device showed some results but posed several challenges that need to be addressed. As the current-voltage and external quantum efficiency measurements were taken, it was observed that material interfaces need to be designed to fully achieve the singlet fission of the acene layer into the Si devices.