arXiv subjects

H. Kobayashi

Publications and source records attributed to H. Kobayashi.

At least 55 records · Page 3Linked to original sources

arXiv2008

Microstructure and kinematics of H2O masers in the massive star forming region IRAS 06061+2151

We have made multi-epoch VLBI observations of H2O maser emission in the massive star forming region IRAS 06061+2151 with the Japanese VLBI network (JVN) from 2005 May to 2007 October. The detected maser features are distributed within an 1\arcsec$\times$1\arcsec (2000 au$\times$2000 au at the source position) around the ultra-compact H {\small\bf II} region seen in radio continuum emission. Their bipolar morphology and expanding motion traced through their relative proper motions indicate that they are excited by an energetic bipolar outflow. Our three-dimensional model fitting has shown that the maser kinematical structure in IRAS 06061+2151 is able to be explained by a biconical outflow with a large opening angle ($>$ 50\degr). The position angle of the flow major axis coincides very well with that of the large scale jet seen in 2.1$\:μ\rmn{m}$ hydrogen emission. This maser geometry indicates the existence of dual structures composed of a collimated jet and a less collimated massive molecular flow. We have also detected a large velocity gradient in the southern maser group. This can be explained by a very small (on a scale of several tens of au) and clumpy (the density contrast by an order of magnitude or more) structure of the parental cloud. Such a structure may be formed by strong instability of shock front or splitting of high density core.

astro-ph↗

arXiv2008

NMR Evidence for Antiferromagnetic Transition in the Single-Component Molecular Conductor, [Au(tmdt)_{2}] at 110 K

We present the results of a ^{1}H NMR study of the single-component molecular conductor, [Au(tmdt)_{2}]. A steep increase in the NMR line width and a peak formation of the nuclear spin-lattice relaxation rate, 1/T_{1}, were observed at around 110 K. This behavior provides clear and microscopic evidences for a magnetic phase transition at considerably high temperature among organic conductors. The observed variation in 1/T_{1} with respect to temperature indicates the highly correlated nature of the metallic phase.

cond-mat.str-el↗

arXiv2008

Optical-phonon mediated exciton energy relaxation with highly preserved spin states Optical-phonon mediated exciton energy relaxation with highly preserved spin states in a single quantum dot

High degree of preservation of spin states during energy relaxation processes mediated by optical phonons is demonstrated in a single quantum dot. Optical-phonon resonance and relevant suppression of spin relaxation are clearly identified as dip structures in photoluminescence excitation spectra probed by the positive trion emission. The absence of continuum states makes this observation possible under the cross-circularly polarized detection with respect to a circularly polarized pumping. Consequently, distinguishably high degree of circular polarization up to ~0.85 is achieved without applying external magnetic field at the optical-phonon resonance. Rate equation analysis reveals that the spin-flip probability during energy relaxation is restricted to less than 7.5%. It is also indicated that the spin flip time of the positive trion ground state is extended by more than 3 times compared with that of neutral exciton ground state. This corresponds to the spin flip time longer than 11 ns for the positive trion ground state. The influence of nuclear polarization to the present measurements is also discussed.

cond-mat.mtrl-sci↗

arXiv2007

The 2006 Radio Outburst of a Microquasar Cyg X-3: Observation and Data

We present the results of the multi-frequency observations of radio outburst of the microquasar Cyg X-3 in February and March 2006 with the Nobeyama 45-m telescope, the Nobeyama Millimeter Array, and the Yamaguchi 32-m telescope. Since the prediction of a flare by RATAN-600, the source has been monitored from Jan 27 (UT) with these radio telescopes. At the eighteenth day after the quench of the activity, successive flares exceeding 1 Jy were observed successfully. The time scale of the variability in the active phase is presumably shorter in higher frequency bands. We also present the result of a follow-up VLBI observation at 8.4 GHz with the Japanese VLBI Network (JVN) 2.6 days after the first rise. The VLBI image exhibits a single core with a size of <8 mas (80 AU). The observed image was almost stable, although the core showed rapid variation in flux density. No jet structure was seen at a sensitivity of $T_b = 7.5\times 10^5$ K.

astro-ph↗

arXiv2007

$^{77}$Se NMR evidence for the Jaccarino-Peter mechanism in the field induced superconductor, $λ$(BETS)$_2$FeCl$_4$}

We have performed $^{77}$Se NMR on a single crystal sample of the field induced superconductor $λ$-(BETS)$_{2}$FeCl$_{4}$. Our results obtained in the paramagnetic state provide a microscopic insight on the exchange interaction $J$ between the spins \textbf{s} of the BETS $π$ conduction electrons and the Fe localized $d$ spins \textbf{S}. The absolute value of the Knight shift \textbf{K} decreases when the polarization of the Fe spins increases. This reflects the ``negative'' spin polarization of the $π$ electrons through the exchange interaction $J$. The value of $J$ has been estimated from the temperature and the magnetic field dependence of \textbf{K} and found in good agreement with that deduced from transport measurements (L. Balicas \textit{et al}. Phys. Rev. Lett. \textbf{87}, 067002 (2001)). This provides a direct microscopic evidence that the field induced superconductivity is due to the compensation effect predicted by Jaccarino and Peter (Phys. Rev. Lett. \textbf{9}, 290 (1962)). Furthermore, an anomalous broadening of the NMR line has been observed at low temperature, which suggests the existence of charge disproportionation in the metallic state neighboring the superconducting phase.

cond-mat.str-el↗

arXiv2007

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from the mid- to far-infrared. The instruments also have the capability for imaging and spectroscopy in the wavelength range 2 - 180 micron in the pointed observation mode, occasionally inserted into the continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90 percent of the whole sky with higher spatial resolution and wider wavelength coverage than that of the previous IRAS all-sky survey. Point source catalogues of the All-Sky Survey will be released to the astronomical community. The pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

astro-ph↗

arXiv2007

$^{77}$Se NMR measurements of the $π-d$ exchange field in the organic conductor $λ-$(BETS)$_{2}$FeCl$_{4}$

$^{77}$Se-NMR spectrum and frequency shift measurements in the paramagnetic metal (PM) and antiferromagnetic insulating (AFI) phases are reported for a small single crystal of the organic conductor $λ-$(BETS)$_{2}$FeCl$_{4}$ as a function of temperature ($T$) and field alignment for an applied magnetic field $B_{0}$ = 9 T. The results show that in the low $T$ limit, where the localized Fe$^{3+}$ spins ($S_{d}$ = 5/2) are almost fully polarized, the conduction electrons (Se $π$-electrons, spin $s_π$ = 1/2) in the BETS molecules experience an exchange field ($\bf{B}$$_{πd}$) from the Fe$^{3+}$ spins with a value of $-$ 32.7 $\pm$ 1.5 T at 5 K and 9 T aligned opposite to $\bf{B}$$_{0}$. This large negative value of $\bf{B}$$_{πd}$ is consistent with that predicted by the resistivity measurements and supports the Jaccarino-Peter internal field-compensation mechanism being responsible for the origin of field-induced superconductivity.

cond-mat.str-el↗

arXiv2006

VERA 22 GHz fringe search survey

This paper presents results of a survey search for bright compact radio sources at 22 GHz with the VERA radio-interferometer. Each source from a list of 2494 objects was observed in one scan for 2 minutes. The purpose of this survey was to find compact extragalactic sources bright enough at 22 GHz to be useful as phase calibrators. Observed sources were either a) within 6 degrees of the Galactic plane, or b) within 11 degrees from the Galactic center; or c) within 2 degrees from known water masers. Among the observed sources, 549 were detected, including 180 extragalactic objects which were not previously observed with the very long baseline interferometry technique. Estimates of the correlated flux densities of the detected sources are presented.

astro-ph↗

arXiv2006

Compensation of Effective Field in the Field-Induced Superconductor k-(BETS)2FeBr4 Observed by 77Se NMR

We report results of 77Se NMR frequency shift in the normal state of the organic charge-transfer-salt k-(BETS)2FeBr4 which shows magnetic field-induced superconductivity (FISC). From a simple mean field analysis, we determined the field and the temperature dependences of the magnetization m_{pi} of the πconduction electrons on BETS molecules. We found that the Fe spins are antiferromagnetically coupled to the pi electrons and determined the exchange field to be J = -2.3T/mu_B. The exchange field from the fully saturated Fe moments (5 mu_B) is compensated by an external field of 12T. This is close to the central field of the FISC phase, consistent with the Jaccarino-Peter local field-compensation mechanism for FISC (Phys. Rev. Lett. 9, 290 (1962)).

cond-mat.supr-con↗

arXiv2005

EMR Measurements of Field-Induced Superconductor lamda-(BETS)2FexGa1-xCl4

We have performed high-field Electron Magnetic Resonance (EMR) measurements of lamda-(BETS)2FexGa1-xCl4 to clarify the role of Pi- and d-electrons in the paramagnetic (PM) and field-induced superconducting (FISC) phases. We found that this system has a non-negligible Pi-d interaction that is very sensitive to the sample cooling rate. The Pi-d interaction is weaker when the sample is rapidly cooled which might affect its FISC phase. The results of the organic alloy lamda-(BETS)2FexGa1-xCl4 (x=0.6) will be presented in this paper.

cond-mat.supr-con↗

arXiv2005

Multi-epoch VERA Observations of H2O masers in OH 43.8-0.1

We report on the multi-epoch observations of H2O maser emission in star forming region OH 43.8-0.1 carried out with VLBI Exploration of Radio Astrometry (VERA). The large-scale maser distributions obtained by single-beam VLBI mapping reveal new maser spots scattered in area of 0.7 x 1.0 arcsec, in addition to a `shell-like' structure with a scale of 0.3 x 0.5 arcsec which was previously mapped by Downes et al.(1979). Proper motions are also obtained for 43 spots based on 5-epoch monitoring with a time span of 281 days. The distributions of proper motions show a systematic outflow in the north-south direction with an expansion velocity of ~8 km/s, and overall distributions of maser spots as well as proper motions are better represented by a bipolar flow plus a central maser cluster with a complex structure, rather than a shell with a uniform expansion such as those found in Cep A R5 and W75N VLA2. The distance to OH 43.8-0.1 is also estimated based on the statistical parallax, yielding D = 2.8 +/- 0.5 kpc. This distance is consistent with a near kinematic distance and rules out a far kinematics distance (~9 kpc), and the LSR velocity of OH 43.8-0.1 combined with the distance provides a constraint on the flatness of the galactic rotation curve, that there is no systematic difference in rotation speeds at the Sun and at the position of OH 43.8-0.1, which is located at the galacto-centric radius of ~6.3 kpc.

astro-ph↗

arXiv2005

A low-cost time-hopping impulse radio system for high data rate transmission

We present an efficient, low-cost implementation of time-hopping impulse radio that fulfills the spectral mask mandated by the FCC and is suitable for high-data-rate, short-range communications. Key features are: (i) all-baseband implementation that obviates the need for passband components, (ii) symbol-rate (not chip rate) sampling, A/D conversion, and digital signal processing, (iii) fast acquisition due to novel search algorithms, (iv) spectral shaping that can be adapted to accommodate different spectrum regulations and interference environments. Computer simulations show that this system can provide 110Mbit/s at 7-10m distance, as well as higher data rates at shorter distances under FCC emissions limits. Due to the spreading concept of time-hopping impulse radio, the system can sustain multiple simultaneous users, and can suppress narrowband interference effectively.

cs.IT↗

arXiv2004

VERA Observation of the W49N H2O Maser Outburst in 2003 October

We report on a strong outburst of the W49N H2O maser observed with VERA. Single-dish monitoring with VERA 20 m telescopes detected a strong outburst of the maser feature at V_LSR = -30.7 km/s in 2003 October. The outburst had a duration of ~100 days and a peak intensity of 7.9 x 10^4 Jy, being one of the strongest outbursts in W49N observed so far. VLBI observations with the VERA array were also carried out near to the maximum phase of the outburst, and the outburst spot was identified in the VLBI map. While the map was in good agreement with previous studies, showing three major concentrations of maser spots, we found a newly formed arc-like structure in the central maser concentration, which may be a shock front powered by a forming star or a star cluster. The outburst spot was found to be located on the arc-like structure, indicating a possible connection of the present outburst to a shock phenomenon.

astro-ph↗

arXiv2004

High Field ESR Study of the pi-d Correlated Organic Conductor lambda-(BETS)2Fe0.6Ga0.4Cl4

Submillimeter and millimeter wave electron spin resonance (ESR) measurements of the pi-d correlated organic conductor lambda-(BETS)2Fe0.6Ga0.4Cl4 have been performed. Antiferromagnetic resonance (AFMR) has been observed in the insulating antiferromagnetic phase, and its frequency-field dependence can be reproduced by the biaxial anisotropic AFMR theory. We find that in this alloy system, the easy-axis is near the b-axis, unlike previous results for the pure lambda-(BETS)2FeCl4 salts where it is closer to the c*-axis. We have also observed electron paramagnetic resonance (EPR) in the metallic phase at higher fields where the g-value is shown to be temperature and frequency dependent for field applied along the c*-axis. This behavior indicates the existence of strong pi-d interaction. Our measurements further show the magnetic anisotropy associated with the anions (the D term in the spin Hamiltonian) is |D|~0.11 cm-1.

cond-mat.supr-con↗

arXiv2003

Mapping the Fermi velocity in the quasi-2D organic conductor k-(BEDT-TTF)2I3

We demonstrate a new method for determining the Fermi velocity in quasi-two-dimensional (Q2D) organic conductors. Application of a magnetic field parallel to the conducting layers results in periodic open orbit quasiparticle trajectories along the Q2D Fermi surface. Averaging of this motion over the Fermi surface leads to a resonance in the interlayer microwave conductivity. The resonance frequency is simply related to the extremal value of the Fermi velocity perpendicular to the applied field. Thus, angle dependent microwave studies enable a complete mapping of the Fermi velocity. We illustrate the applicability of this method for the highly-2D organic conductor k-(BEDT-TTF)2I3.

cond-mat.str-el↗

arXiv2003

First Fringe Detection with VERA's Dual-Beam System and its Phase Referencing Capability

We present the results of first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The first dual-beam observations of a pair of H2O maser sources W49N and OH43.8-0.1 have been carried out on 2002 May 29 and July 23, and fringes of H2O maser lines at 22 GHz have been successfully detected. While the residual fringe phases of both sources show rapid variations over 360 degree due to the atmospheric fluctuation, the differential phase between the two sources remains constant for 1 hour with r.m.s. of 8 degree, demonstrating that the atmospheric phase fluctuation is removed effectively by the dual-beam phase referencing. The analysis based on Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for arbitrarily long time. These results demonstrate VERA's high capability of phase referencing, indicating that VERA is a promising tool for phase referencing VLBI astrometry at 10 microarcsec level accuracy.

astro-ph↗

arXiv2002

Optical Alignment System for the PHENIX Muon Tracking Chambers

A micron-precision optical alignment system (OASys) for the PHENIX muon tracking chambers is developed. To ensure the required mass resolution of vector meson detection, the relative alignment between three tracking station chambers must be monitored with a precision of 25$μ$m. The OASys is a straightness monitoring system comprised of a light source, lens and CCD camera, used for determining the initial placement as well as for monitoring the time dependent movement of the chambers on a micron scale.

nucl-ex↗

arXiv2002

Magnetic field - temperature phase diagram of quasi-two-dimensional organic superconductor lambda-(BETS)_2 GaCl_4 studied via thermal conductivity

The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H applied parallel to the Q2D plane. The phase diagram determined from this bulk measurement shows notable dependence on the sample quality. In dirty samples the upper critical field H_{c2} is consistent with the Pauli paramagnetic limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In contrast in clean samples H_{c2}(T) shows no saturation towards low temperatures and the feature in kappa(H) is replaced by two slope changes reminiscent of second-order transitions. The peculiarity was observed below ~ 0.33T_c and disappeared on field inclination to the plane when the orbital suppression of superconductivity became dominant. This behavior is consistent with the formation of a superconducting state with spatially modulated order parameter in clean samples.

cond-mat.supr-con↗