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We present recent observation results of Sgr A* at millimeter obtained with VLBI arrays in Korea and Japan.
7 mm monitoring of Sgr A* is part of our AGN large project. The results at 7 epochs during 2013-2014, including high resolution maps, flux density and two-dimensional size measurements are presented. The source shows no significant variation in flux and structure related to the G2 encounter in 2014. According to recent MHD simulations by kawashima et al., flux and magnetic field energy can be expected to increase several years after the encounter; We will keep our monitoring in order to test this prediction.
Astrometric observations of Sgr A* were performed in 2015 at 7 and 3.5 millimeter simultaneously. Source-frequency phase referencing was applied and a combined ”core-shift” of Sgr A* and a nearby calibrator was measured. Future observations and analysis are necessary to determine the core-shift in each source.
Much attempts of statistical approach have been made to study the origin of heavy elements, distribution of dark matter and evolution of clusters of galaxies. Henry et al.(1991) reported a power-law relation; LX ∝ kTγ, γ ~ 2.7. Edge and Stwert(1991) found significant scatter in the correlation using 45 clusters. David at al.(1993) reported γ ~ 3.4 using 104 clusters.
NGC 3079 has very luminous water megamaser from the nucleus, the peak of the spectrum being blueshifted by 180 km s−1 from the systemic velocity of the galaxy (Vsys = 1131 km s−1) (Henkel et al. 1984, Haschick & Baan 1985). Core-jet like continuum structure is also found in the nuclear region (Irwin & Seaquist 1988). No velocity drift for main features of water maser (VLSR = 941–975 km s−1) has been shown (Nakai et al. 1995). However, the drift was recently detected for the maser of 1190 km s−1 (Nakai 1997). HI and OH absorptions are detected in the nucleus (Haschick & Baan 1985, Irwin & Seaquist 1991). Thus this galaxy is very unique object to investigate water masers, continuum structure and absorption features all together with VLBI.
We conducted an observation of weak HI and OH absorption features in the parsec-scale nuclear region of NGC 3079 using a global VLBI network; the VLBA, the VLA and the Green Bank 43-m telescope of NRAO. One mas corresponds to 0.076 pc in NGC 3079.
We present multi-frequency observations towards the nucleus of NGC 3079 using both Japan VLBI Network (J-Net) and the VLBA including phased VLA and Effelsberg. We detected two continuum components at 1.4 and 8.4 GHz, and determined the spectral indices for each component. Further, we found several H ɪ absorption features towards the continuum components whose velocity gradient is opposite to the galaxy rotation. Assuming rotation, the central mass is estimated to be 3 × 106M⊙. The water maser distribution extends in North-South direction along the galactic disk, while no clear indication of a Keplerian rotating disk was found.
NGC 3079 has very luminous water megamaser from the nucleus, the peak of the spectrum being blueshifted by 180 km s-1 from the systemic velocity of the galaxy (Vsys = 1131 km s-1) (Henkel et al. 1984, Haschick & Baan 1985). Core-jet like continuum structure is also found in the nuclear region (Irwin & Seaquist 1988). No velocity drift for main features of water maser (VLSR — 941-975 km s1) has been shown (Nakai et al. 1995). However, the drift was recently detected for the maser of 1190 km s-1 (Nakai 1997). HI and OH absorptions were detected in the nucleus (Haschick & Baan 1985, Irwin & Seaquist 1991). Thus this galaxy is very unique object to investigate water masers, continuum structure and absorption features all together with VLBI.
The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between γ-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the γ-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at >120 pc from the nucleus are proposed as candidate site(s) of γ-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV γ-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.
We present the internal proper motion of 6.7-GHz methanol masers in S269, an Ultra Compact HII region. The maser distribution in S269 consists of several maser groups, and the spatial structure of the main groups A and B are consistent with the past VLBI image. The remarkable result of comparing the two VLBI maps is that 6.7-GHz methanol maser distribution and velocity range within each group have been kept for eight years. Angular separation between the two groups A and B increases by 3.6 mas, which corresponds to a velocity of 11.5 km s−1.
We present VLBI maps of the 6.7 GHz methanol maser emission in 32 sources obtained using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). All of the observed sources provide new VLBI maps, and the spatial morphologies have been classified into five categories similar to the results obtained from European VLBI Network observations (Bartkiewicz et al. 2009). The 32 methanol sources are being monitored to measure the relative proper motions of the methanol maser spots.
We report an essentially complete characterization of energies and relaxation processes of the lowest seven electronic states of the N-V (nitrogen-vacancy) center in diamond using several different nonlinear laser spectroscopie techniques. We have also applied ultrafast optical techniques to measure dielectric properties of CVD and bulk diamond in the 0.3–1.6 THz range for the first time.
The capability of discharge-pumped vacuum ultraviolet F2 molecular laser for laser annealing of heavily ion implanted semiconductor was demonstrated for the first time using Se+ heavily ion implanted GaAs. Cr-doped semi-insulationg GaAs wafers were used as the substrates, and the Se+ implantation energy and dose were controlled to 100 keV and 1× 1015 cm-2, respectively. Samples were annealed using a F2 molecular laser ( wavelength = 157 nm ) with a single pulse ( width ~ 20 ns ) in the energy density range from 200 to 800 mJ/cm2 in a nitrogen atmosphere. In addition, furnace annealing was done on separate samples at 850 ºC for 20 minutes in a purified hydrogen atmosphere for comparison. Characterization of the samples was carried out using Raman scattering and ellipsometry. The laser annealed samples exhibited intense Raman scattering LO phonon peaks whose intensity increased with increasing laser power density, whereas the furnace annealed samples exhibited a very weak LO phonon peak. It was demonstrated for the first time that VUV photons can be very effective in annealing ion implantation damage as compared with conventional furnace annealing. The behavior of Raman scattering spectra as a function of laser energy density was explained quantitatively by a “spatial correlation” model. The model made it possible to estimate the average size of the recovered crystal regions in samples for any given laser energy density.
The charge collection efficiency of a diode with a retrograde well was estimated using focused ion beam irradiation at 400 keV and 2 MeV. The retrograde well was found to effectively suppress a collection of charge carriers created by energetic particles. The charge collection efficiency of the diode with the retrograde well was ~ 25 % lower than that with the conventional well when 400 keV ~ 2 MeV protons were irradiated normal to diodes. This result was in good agreement with device simulation.
Epitaxial growth of CeO2 layers on silicon (100) substrates is studied using electron-beam evaporation under substrate bias application in an ultrahigh vacuum. Both negative and positive biases are proved to be effective for lowering the epitaxial temperature. Sample current characteristics are measured as a function of the bias voltage. Under negative bias conditions, as the bias voltage increases, the sample current varies from negative to positive with a transition point at -42 V and then reaches a saturation value of ∼ +4 μA above -60 V. Use of a negative bias of -60 V leads to epitaxial growth temperature lowering of ∼, 40°C. Under a positive bias, the sample current is negative and its absolute value increases with the bias voltage, where the sample current components are to be anions and electrons (46°) as determined by mass separation with a magnetic field application. It is experimentally clarified that the degree of enhancement of epitaxial growth is greater than that in the negative bias experiment (750°C at +60 V bias, i. e., epitaxial growth temperature lowering of 70°C) and the enhancement is attributed to the electron component. It is found that a negative current of ∼ -0.15 mA flows at zero bias, indicating that even in conventional evaporation, electrons somewhat promote epitaxial growth.
Double ion implanted 4H-SiC bipolar junction transistors (BJTs) are fabricated by Al and N ion implantation to the base and emitter. The current gain of 3 is obtained at the base Al concentration of 1 × ∼ 1017 /cm3. The collector current as a function of the base Gummel number suggests that double ion implanted 4H-SiC BJT operates in the intrinsic region below the emitter in the low injection level. The high base resistance restricts the base current at VBE as low as 3 V.
In this paper, we demonstrate that high temperature and short time EBAS annealing is effective to obtain low sheet resistance without surface roughening in heavily Al-implanted 4H-SiC (0001) samples (Al concentration: 1.0 × 1021 /cm3, thickness: 0.3 microns, total dose: 2.6 × 1016 /cm2). The sheet resistance and rms surface roughness of the sample annealed at 1800 °C for 0.5 min is estimated to be 4.8k ohm/sq. and 0.82 nm, respectively. Also, we discuss the advantage of EBAS annealing for the suppression of surface roughening during annealing.
The impact of the conduction band offset (CBO) between window/Cu(In,Ga)Se2 (CIGS) layers on the light soaking (LS) effect in CIGS solar cells has been studied with continuous CBO control using a (Zn,Mg)O (ZMO) window layer. Two types of CIGS solar cells with different window/buffer/absorber layers configurations were fabricated, i.e., ZMO/CIGS (without buffer layer) and ZMO/CdS/CIGS structures. The CBO values between ZMO and CIGS layers were controlled to -0.15~0.25 eV. Plus and minus signs of CBO indicate the conduction band minimums of ZMO above and below that of CIGS, respectively. Current-voltage (J-V) characteristics of the solar cells with different LS durations revealed that a positive CBO value higher than 0.16 eV induces J-V curve distortion, i.e., LS effect, and all the J-V characteristics stabilized in 30 min. The degrees of the LS effect were dominated by the CBO value between ZMO and CIGS layers in the both structure regardless of the existence of CdS buffer layers. These results indicate that the LS effect is dominated by the highest barrier for photo-generated electrons in the conduction band diagram, i.e., the CBO between ZMO and CIGS layers, and quantitatively the LS effect emerges the CBO value higher than 0.16 eV.
We have demonstrated enhancement-mode n-channel gallium nitride (GaN) MOSFETs on Si (111) substrates with high-temperature operation up to 300 °C. The GaN MOSFETs have good normally-off operation with the threshold voltages of +2.7 V. The MOSFET exhibits good output characteristics from room temperature to 300 °C. The leakage current at 300°C is less than 100 pA/mm at the drain-to-source voltage of 0.1 V. The on-state resistance of MOSFET at 300°C is about 1.5 times as high as that at room temperature. These results indicate that GaN MOSFET is suitable for high-temperature operation compared with AlGaN/GaN HFET.
We investigated triple ion implanted 4H-SiC BJT with etched extrinsic base regions. To remove the defects induced by ion implantation between emitter and base regions, the characteristics of triple ion implanted 4H-SiC BJT were significantly improved. Maximum common current gain was improved from 1.7 to 7.5.
We demonstrated electrical characteristics of operational amplifier (OPAMP) circuits fabricated by GaN/AlGaN/GaN HEMTs operating over 100 oC. GaN/AlGaN/GaN HEMTs, with the extremely low source resistance were fabricated by multiple ion implantation, precisely controlled ion-implanted (I/I) resistors and Schottky barrier diodes were integrated on the silicon substrate. The GaN cap layer on the AlGaN was grown to decrease the gate leakage current and current collapse for AlGaN/GaN HEMTs.