Aoyama Gakuin University

About: Aoyama Gakuin University is a education organization based out in Tokyo, Japan. It is known for research contribution in the topics: Superconductivity & Thin film. The organization has 3494 authors who have published 6419 publications receiving 115648 citations. The organization is also known as: Aoyama gakuin daigaku.

Evidence for strong-coupling s-wave superconductivity in MgB2: 11B-NMR study of MgB2 and the related materials

Abstract: We report the results of nuclear spin–lattice relaxation time, T 1 of 11 B in MgB 2 , Al-doped MgB 2 and NbB 2 . A T 1 T =constant behavior was observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state, 1/ T 1 shows a tiny coherence peak just below T c and decreases exponentially, demonstrating an s-wave superconductivity. The magnitude of SC gap is estimated as 2 Δ / k B T c ∼5, which is quite larger than the weak-coupling value in the BCS theory, 2 Δ / k B T c =3.5. This rules out a possibility for the multiple SC gaps that was suggested in many literatures. The anisotropy in the upper critical field was also corroborated by the T 1 measurement on a bulk polycrystalline sample. In NbB 2 with a low value of T c =5 K, the T 1 measurement revealed the distinct coherence peak just below T c , followed by an exponential decrease with a magnitude of SC gap 2 Δ / k B T c =3.1. These data are consistent with the weak-coupling BCS theory. As Al 3+ is substituted for Mg 2+ , 1/ T 1 T , which is proportional to the square of the density of states at the Fermi level N ( E F ), decreases. The variation in T c against the relative changes in N ( E F ) deduced from 1/ T 1 T is well fitted in terms of the McMillan equation by assuming a characteristic phonon frequency ω ∼ 700 K and an electron–phonon coupling constant λ ∼ 0.87. Thus obtained values are in good agreement with the values suggested by the theoretical works. The high- T c superconductivity in MgB 2 is shown to occur through the strong coupling with high frequency phonons.

Output feedback control for fuzzy systems with immeasurable premise variables

Abstract: This paper is concerned with the output feedback control for fuzzy systems with immeasurable premise variables. When we consider a fuzzy system, the selection of the premise variable is important. If it is the state of the system, a fuzzy system describes a wide class of nonlinear systems. However, the state is not measurable in the output feedback control problem. Hence, the premise variable is not unknown. In this case, the separation principle, in general, does not hold. This causes a difficulty in control design. In order to overcome this difficulty, a new approach to the output feedback control is introduced. Our approach converts the output feedback stabilization into the H ∞ control problem where the terms related to the premise variable is considered an unknown signal. The method does not only stabilizes the system but also takes care of the control performance. A numerical example is given to show the effectiveness of our output feedback control.

Optical follow-up observations of PTF10qts, a luminous broad-lined Type Ic supernova found by the Palomar Transient Factory

Abstract: We present optical photometry and spectroscopy of the broad-lined Type Ic supernova (SN Ic-BL) PTF10qts, which was discovered as part of the Palomar Transient Factory. The SN was located in a dwarf galaxy of magnitude r = 21.1 at a redshift z = 0.0907. We find that the R-band light curve is a poor proxy for bolometric data and use photometric and spectroscopic data to construct and constrain the bolometric light curve. The derived bolometric magnitude at maximum light is M_bol = −18.51 ± 0.2 mag, comparable to that of SN 1998bw (M_bol = −18.7 mag) which was associated with a gamma-ray burst (GRB). PTF10qts is one of the most luminous SNe Ic-BL observed without an accompanying GRB. We estimate the physical parameters of the explosion using data from our programme of follow-up observations, finding that it produced a larger mass of radioactive nickel compared to other SNe Ic-BL with similar inferred ejecta masses and kinetic energies. The progenitor of the event was likely an ∼20 M_⊙ star.

Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

Abstract: 23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57) cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10^(34-39) ergs. Notably, the peak X-ray luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau and L_X, respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

JEM-EUSO: Meteor and nuclearite observations

Abstract: Meteor and fireball observations are key to the derivation of both the inventory and physical characterization of small solar system bodies orbiting in the vicinity of the Earth. For several decades, observation of these phenomena has only been possible via ground-based instruments. The proposed JEM-EUSO mission has the potential to become the first operational space-based platform to share this capability. In comparison to the observation of extremely energetic cosmic ray events, which is the primary objective of JEM-EUSO, meteor phenomena are very slow, since their typical speeds are of the order of a few tens of km/sec (whereas cosmic rays travel at light speed). The observing strategy developed to detect meteors may also be applied to the detection of nuclearites, which have higher velocities, a wider range of possible trajectories, but move well below the speed of light and can therefore be considered as slow events for JEM-EUSO. The possible detection of nuclearites greatly enhances the scientific rationale behind the JEM-EUSO mission.