H. Kang

Silicon nitride formation by low energy N+ and N+2 ion beams

Reactions of N+ and N+2 ions with Si(100) surface are examined as a function of both ion kinetic energy and dose using a low energy ion beam instrument. The Si surface is exposed to low energy (1–300 eV) ion beams in an ultrahigh vacuum environment and the resulting surface species are characterized by Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The absolute reaction probability Pr is measured for nitridation processes. Pr(N+) has a value of ∼0.25 and stays constant in the energy range of 1–25 eV. Pr(N+2) increases from zero to ∼0.25 in the same range. Continued exposure of the ion beams to a dose ≳5×1015 ions/cm2 leads to a saturation and formation of a dense and stable silicon nitride layer. Variation of Pr with energy and dose is explained in terms of elementary reaction steps such as charge neutralization of the projectile ion, collisional dissociation of N+2, nitridation reaction, and chemically induced desorption of surface nitrogen species. A mechanism is proposed to exp...

Discovery of CO2 precursor in the reaction of CO and O on Pt(111)

Using the technique of Cs+ reactive ion scattering (Cs+ RIS), we show that reaction of CO and O on a Pt(111) surface produces a CO2 precursor species that remains stable on the surface up to 300 K. The CO2 species desorbs with an activation energy of 46±3 kJ mol−1, whereas the direct CO2 gas formation via Langmuir–Hinshelwood (LH) reaction between CO and O occurs with an activation energy of 75±4 kJ mol−1. The observation reveals a new pathway for CO oxidation reaction, which is different from the direct LH pathway.

High pressure synthesis and superconductivity of Ba1-xKxBiO3 (0.35 < x < 1)

The whole region of superconductivity in the Ba1−xKxBiO3 (BKBO) system (0.35<x<1) has been investigated by synthesizing the samples with the high pressure and high temperature technique. As the K doping ratio (x) increases, the superconducting transition temperature (Tc) of the BKBO system decreases from 30.4 K for x=0.37 to very low diamagnetic signal below 3 K for x=0.74. The synthesis of the samples without including any bariums in the starting composition (x=1) resulted in different Tc and lattice parameters from sample to sample with partial substitutions of Bi at the K site in final products. With detailed synthesis information of the BKBO samples which varies with x, the evolution of Tc by doping in the cubic BKBO system is compared with other superconducting bismuthates and discussed on a basis of its electronic band structure.

Kinetic energy dependence of N+ and N2+ reactions with Si(100)

Abstract Reactions of N + and N 2 + ions with a Si(100) surface are investigated in the kinetic energy range 1–25 eV. The Si surface is exposed to low energy beams of these ions in an ultrahigh vacuum ion-surface collision chamber, and the reacted surface is analyzed using Auger electron spectroscopy and ultraviolet photoelectron spectroscopy. The N + reaction leads to efficient silicon nitride formation, its reaction probability P r (N + ) being ∼ 0.25 at all energies. The N 2 + reaction efficiency changes with e P r (N 2 + ) increasing from zero to ∼ 0.25 in this energy range. The energy dependence of P r is explained in terms of eleme reaction steps such as ion neutralization, collisional dissociation of N 2 + , and Si-N bond formation.

SIMULTANEOUS OBSERVATION OF WATER AND CLASS I METHANOL MASERS TOWARD CLASS II METHANOL MASER SOURCES

We present a simultaneous single-dish survey of 22 GHz water maser and 44 GHz and 95 GHz class I methanol masers toward 77 6.7 GHz class II methanol maser sources, which were selected from the Arecibo methanol maser Galactic plane survey (AMGPS) catalog.Water maser emission is detected in 39 (51%) sources, of which 15 are new detections. Methanol maser emission at 44 GHz and 95 GHz is found in 25 (32%) and 19 (25%) sources, of which 21 and 13 sources are newly detected, respectively. We find 4 high-velocity (> 30 km/s) water maser sources, including 3 dominant blue- or redshifted outflows.The 95 GHz masers always appear with the 44 GHz maser emission. They are strongly correlated with 44 GHz masers in velocity, flux density, and luminosity, while they are not correlated with either water or 6.7 GHz class II methanol masers. The average peak flux density ratio of 95 GHz to 44 GHz masers is close to unity, which is two times higher than previous estimates. The flux densities of class I methanol masers are more closely correlated with the associated BGPS core mass than those of water or class II methanol masers. Using the large velocity gradient (LVG) model and assuming unsaturated class I methanol maser emission, we derive the fractional abundance of methanol to be in a range of 4.2*10^-8 to 2.3*10^-6, with a median value of 3.3\pm2.7*10^-7.

Systems of pancreatic beta-cells and glucose regulation.

We present mathematical models for systems of beta-cells in pancreatic islets. The first topic begins with the effects of noise and coupling strength on bursting action potentials of beta-cells. From the discussion, the regular bursts are produced by a proper amount of noise and coupling strength. Furthermore, the bursting duration and period depend on the cluster size of beta-cells. We also observe the real size of islets mostly consisting of beta-cells and obtain the size distribution of islets. In addition, we derive either log-normal or Weibull distributions of the islet sizes based on recent observation on islet growth. Islets of Langerhans are composed of several endocrine cells which interact with each other. Considering asymmetric and inhibitory interactions of these endocrine cells, we introduce a simple islet model consisting of alpha-, beta-, and delta-cells. Finally, a whole feedback model for glucose regulation is constructed, connecting the microscopic bursting mechanism and the macroscopic blood glucose regulation of the body. We analyzed these models via numerical simulations based on in vivo and in vitro experimental data.

Magnetoresistance and thermoelectric power of La-chalcogenides (La2.989S4, La2.985Se4, Ce3S4)

Abstract The electrical transport properties of La-chalcogenides (La 2.989 S 4 , La 2.985 Se 4 , Ce 3 S 4 ) are investigated by measuring the magnetoresistance and thermoelectric power (TEP). The superconducting La 2.989 S 4 with T c =8.2 K shows a clear phase transition at around T ∼90 K, which is consistent with the structural phase transition temperature. La 2.985 Se 4 is also superconducting with T c =5.7 K. In the case of Ce 3 S 4 , the ferromagnetic transition is observed at T m =7.2 K and there is no superconducting or structural phase transition. The temperature dependence of TEP of the above three La-chalcogenides shows metallic characteristics with magnitudes of −10 μ V/K at room temperature. The broad peak at low temperature in TEP of Ce 3 S 4 is discussed with the role of the f-electron in Ce 3 S 4 comparing with superconducting La 2.989 S 4 and La 2.985 Se 4 .

Anomalous reentrant resistance phenomena in superconducting Sr1-xKxBiO3: Recovery of superconductivity with electric or magnetic field

The temperature-dependent resistivity of Sr1 − xKxBiO3, with x = 0.4–0.6, has been measured as a function of the magnetic field (or electrical current). Although X-ray diffraction results are more or less identical to single phase for the measured 10 samples, the EDS results indicate that the potassium content, x, varies from 0.4 to 0.6 and the electrical resistance varies quite sensitively from sample to sample. For the samples of resistivity less than 4 mΩ cm at room temperature with ρ(15 K)/ρ(273 K) < 0.9, superconductivity is observed with Tc ∼ 12 K. Other samples exhibit a reentrant resistance below the superconducting transition temperature. The reentrant resistance, however, decreases as the external magnetic field (or electrical current) is applied, and some samples show the recovery of superconductivity upon the application of a magnetic field (or electrical current). Disorderness in the junction area between superconducting grains seems to be vital for the observed anomalous reentrant resistance. Further investigations are on to understand this intriguing phenomena.

Chemical manipulation of edge-contact and encapsulated graphene by dissociated hydrogen adsorption

Due to its important role in the mass production of electronics, chemical functionalization is commonly utilized during the development of graphene-based electronics. Dramatically improved graphene devices with encapsulated edge-contact configurations represent an additional application of a suitable functionalization process. In this study, we report the partial functionalization of a novel device by dissociative hydrogen adsorption. We investigate the H2 adsorption process, which results in a change in the electrical properties using an encapsulated edge-contact device. When using chemical functionalization, we found that hydrogenation allows the realization of high-quality two-dimensional material-based electronics.

DEVELOPMENT OF 230 GHZ RADIO RECEIVER SYSTEM FOR SRAO

We develop a radio receiver system operating at � � 1.3 mm for the 6 m telescope of Seoul Radio Astronomy Observatory. It consists of a dual polarization receiver, a couple of IF processing units, two FFT spectrometers, and associated software. By adopting sideband-separating superconductor mixers with image band terminated to waveguide load at 4.2 K, we achieve TRX � 100 K and Tsys less than 150 K at best weather condition over 210-250 GHz frequency range. The intermediate frequency signal of 3.5-4.5 GHz is down converted to 0-1 GHz and fed into the FFT spectrometers. The spectrometer covers 1 GHz bandwidth with a spectral resolution of 61 KHz. Test observations are conducted toward several radio sources to evaluate the performance of the system. Aperture and beam efficiencies measured by observing planets are found to be typically 44 � 59% and 47 � 61%, respectively over the RF band, which are consistent with those measured at 3 mm band previously.