Showing papers in "Journal of the Korean Physical Society in 2009"

Blandford-Znajek mechanism versus Penrose process

Abstract: During the three decades since its theoretical discovery the Blandford-Znajek process of extracting the rotational energy of black holes has become one of the foundation stones in the building of modern relativistic astrophysics. However, it is also true that for a long time its physics was not well understood, as evidenced by the controversy that surrounded it since 1990s. Thanks to the efforts of many theorists during the last decade the state of affairs is gradually improving. In this lecture I attempt to explain the key ingredients of this process in more or less systematic, rigorous, and at the same time relatively simple fashion. A particular attention is paid to the similarities and differences between the Blandford-Znajek and Penrose processes. To this purpose I formulate the notion of energy counter flow. The concept of horizon membrane is replaced with the concept of vacuum as an electromagnetically active medium. The effect of negative phase velocity of electromagnetic waves in the black hole ergosphere is also discussed.

D_sD^* Molecule as an Axial Meson

Abstract: We use QCD sum rules to study the possible existence of a $D_s\bar{D}^*+ D_s^*\bar{D}$ molecule with the quantum number $J^P=1^+$. We consider the contributions of condensates up to dimension eight and work at leading order in $\alpha_s$. We obtain $m_{D_sD^*}=(3.96\pm0.10) \GeV$ around 100 MeV above the mass of the meson X(3872). The proposed state is a natural generalized state to the strangeness sector of the X(3872), which was also found to be consistent with a multiquark state from a previous QCD sum rule analysis.

Pinhole Camera Measurements of Prompt Gamma-rays for Detection of Beam Range Variation in Proton Therapy

Abstract: We have designed and constructed a pinhole camera system to monitor variations in the beam range during the course of proton beam therapy. The camera consists of a shielding enclosure made of lead plates with a pinhole aperture and a CsI(Tl) scintillation detector. The design of the camera was performed using a Monte Carlo program to optimize the shielding structure. The proton energy was assumed to be 50 MeV for empirical verification by using a proton beam and a water phantom at the proton-beam facility of the Korea Cancer Center Hospital. The enclosure thickness was determined so as to reduce the background radiation well below the prompt gamma distribution through the pinhole aperture. The aperture was shaped and located to view the endpoint of the proton range. Experimental results showed that the gamma distribution was sensitive to variations in the proton beam range of the order of 1 mm in water.

Synthesis of Zinc Ferrite and Its Photocatalytic Application under Visible Light

Abstract: ZnFe2O4 with a spinel crystal structure was synthesized by using the solid-state reaction method. A single-phase ZnFe2O4 was obtained in the temperature range of 900 { 1200 C. The as-synthesized particles were agglomerates of crystals with an average size of 51.9 nm, as estimated from X-ray di raction analysis. The band gap of the n-type semiconducting metal oxide, as determined by using UV-vis di use re ectance spectroscopy (UV-DRS) was found to be 1.90 eV (657 nm). The photocatalytic activity of ZnFe2O4 was investigated by using the photo-decomposition of isopropyl alcohol (IPA) under visible light ( 420 nm) and was found to be much higher than that of the well-known TiO2 xNx photocatalyst.

Is Dark Matter a BEC or Scalar Field

Abstract: This is a brief review on the history of the Bose-Einstein condensate (BEC) or boson star model of galactic dark matter halos, where ultra-light scalar dark matter particles condense in a single BEC quantum state. The halos can be described as a self-gravitating, possibly self-interacting, coherent scalar field. On a scale larger than galaxies, dark matter behaves like cold dark matter while below that scale the quantum mechanical nature suppresses the dark matter structure formation due to the minimum length scale determined by the mass m > ∼ 10eV and the self-interaction of the particles. This property could alleviate the cusp problem and missing satellite problems of the ΛCDM model. Furthermore, this model well reproduces the observed rotation curves of spiral and dwarf galaxies, which makes the model promising.

First-Principles Study of the Surface of Wurtzite ZnO and ZnS - Implications for Nanostructure Formation

Abstract: The microscopic properties of several ZnO and ZnS surfaces of low Miller index are investigated through rst-principles calculation of the surface energy, crystal structure and electronic structure. Generally, the non-polar surfaces such as (10 10) and (11 20) are found to be more stable than the polar (0001)-Zn surface. The (11 20) surface is found to be the most stable for both ZnO and ZnS semiconductors and the (10 10) surface is found to have a slightly larger surface energy. These are consistent with the abundant observations of ZnO nanostructures having (11 20) and (10 10) surfaces. The near-surface structures and electronic structures are discussed.

Passivation of Bottom-Gate IGZO Thin Film Transistors

Abstract: We have studied transparent bottom-gate TFTs (thin film transistors) using amorphous IGZO (In-Ga-Zn-O) as an active channel material. The TFT devices had inverse co-planar structures. Source/drain and gate electrodes were constituted by ITO sputtered with a DC-RF magnetron sputter system, and an alkaline-free glass was used as a substrate. The gate insulator was Al2O3 formed by using an atomic layer deposition (ALD) method at 150 ◦C. An active layer was formed by off-axis RF magnetron sputtering and post-annealing was performed with a hot plate or a vacuum oven. The field effect mobilities and the sub-threshold swings of the IGZO TFTs were 12 ∼ 18 cm/Vs and 0.2 ∼ 0.6 V/dec, respectively. However, the hysteresis on I-V characteristics was relatively large without passivation. Thus, we passivated the TFT devices with inorganic and organic materials. After the organic passivation and post-heat treatments, the hysteresis was remarkably reduced without deterioration of the electrical characteristics.

Optimization for Axial Resolution, Depth Range, and Sensitivity of Spectral Domain Optical Coherence Tomography at 1.3 µm.

Abstract: We have developed high-speed and high-resolution spectral domain optical coherence tomography at 1.3 µm using an InGaAs line-scan camera and a broadband light source with the bandwidth of 170 nm that produces a theoretical axial resolution of 4.4 µm in air. We compared axial resolutions from point spread functions (PSFs) and depth ranges while changing the full spectral bandwidth detected by the camera and describing the optimization process for the axial resolution, the depth range, and the sensitivity for SD-OCT system. We found that SD-OCT at 1.3 µm cannot satisfy the conditions both below the axial resolution of 5 µm and above the depth range of 2 mm because of the restricted pixel number of the line-scan camera. To scan a large depth range, the axial resolution has to be sacrificed. In addition, the sensitivity rolls off slowly as a function of the depth if a large depth range is scanned. On the other hand, if the axial resolution needs to be close to the theoretical one, the depth range becomes limited and the sensitivity decays quickly. Since we have to maintain a reasonable depth range of 2.0 mm, we chose the spectrum full bandwidth of 214 nm captured by the detector to balance the axial resolution of 8.2 µm. In this setting, the sensitivity of our OCT system was measured at 107.1 dB. Theoretical and experimental results are compared and presented in this paper.

Frequency and Temperature Dependence of the Dielectric Properties of a PCB Substrate for Advanced Packaging Applications

Abstract: The frequencyand temperature-dependent dielectric properties for printed circuit board (PCB) applications are investigated in the frequency range from 1 kHz to 1 MHz and the temperature range from 300 K to 600 K. The applicable range of frequency for the capacitance-voltage (C-V) technique is discussed. An analytical model based on the experimental results and the theoretical analysis is proposed to give rise to a comprehensive understanding of the correlations between the operation temperature and the dielectric properties, where the temperature-dependent dipole property is investigated.

Properties of AZO thin films for solar cells deposited on Polycarbonate Substrate

Abstract: We prepared Al-doped ZnO (AZO) thin films on polycarbonate (PC) substrates at room temperature by using a facing target sputtering (FTS) method The PC substrate has advantages of light weight and easy deformation Therefore, a study of deposition on a flexible substrate is necessary In this study, we used a PC substrate with a thickness of 200 µm, and the transmittance was above 90 % in the visible range The AZO thin films were prepared at various sputtering powers and various PO2 (O2 gas /Ar gas +O2 gas) The properties of the AZO thin films were measured by using X-ray diractometer, an UV/VIS spectrometer, a Hall measurement system, and a field-emission scanning electron microscope (SEM) From the results, the lowest resistivity of the AZO thin films was 879 ◊ 10 4 [-cm], and the transmittance of several AZO thin films was over 90 % in visible range

Secondary Electron Generation in Electron-beam-irradiated Solids: Resolution Limits to Nanolithography

Abstract: We have investigated the secondary electron generation in electron-beam-irradiated solids by means of a Monte Carlo simulation. The slow secondary electron energy was found to be independent of the position and the incident energy of the electron beam, and the electron beam broadening in thin films due to secondary electrons was found to be at least 5 – 10 nm, setting limits to the nanolithographic resolution.

Quantum Interference Effects in InAs Semiconductor Nanowires

Abstract: We report quantum interference effects in InAs semiconductor nanowires strongly coupled to superconducting electrodes. In the normal state, universal conductance fluctuations are investigated as a function of the magnetic field, the temperature, the bias and the gate voltage. The results are found to be in good agreement with theoretical predictions for weakly disordered one-dimensional conductors. In the superconducting state, the fluctuation amplitude is enhanced by a factor up to $\sim$1.6, which is attributed to a doubling of the charge transport via Andreev reflection. At a temperature of 4.2 K, well above the Thouless temperature, conductance fluctuations are almost entirely suppressed and the nanowire conductance exhibits anomalous quantization in steps of $e^2/h$.

Preparation of ITO and IZO thin films by using Facing Target Sputtering (FTS) method

Abstract: In study, for transparent conductive oxides (TCOs) we investigated the electrical and the optical properties of thin films of indium oxide (In2O3) doped with 10 wt.% tin oxide (SnO2) and of indium oxide (In2O3) doped with 10 wt.% zinc oxide thin films in facing targets sputtering (FTS) system. All thin films were prepared on the glass at room temperature and at various oxygen contents in the sputter gas. All as-deposited thin films were annealed at various temperatures in an air atmosphere. The electrical and the optical properties of the as-deposited thin films were investigated by using a four point probe, an UV/VIS spectrometer, an X-ray diffractometer (XRD), and a Hall Effect measurement. As a result, with increasing oxygen content in the sputter gas, the optical transmittance in the visible range of all thin films increased (>80 %). ITO and IZO thin films had resistivities of 6.7 × 10−4 Ω·cm and 4.5 × 10−4 Ω·cm respectively at the optimum oxygen contents. After post-annealing, the structure of ITO thin film was polycrystalline, but the IZO thin films had amorphous.