arXiv subjects

N. Ishikawa

Publications and source records attributed to N. Ishikawa.

5 recordsLinked to original sources

arXiv2011

Observational Search for PeV-EeV Tau Neutrino from GRB081203A

We report the first observational search for tau neutrinos from gamma ray bursts (GRBs) using one of the Ashra light collectors. The earth-skimming tau-neutrino technique of imaging Cherenkov tau showers was applied as a detection method. We set stringent upper limits on the tau-neutrino fluence in PeV-EeV region for 3780 s (between 2.83 and 1.78 hours before) and another 3780 s (between 21.2 and 22.2 hours after) surrounding GRB081203A triggered by the Swift satellite. This first search for PeV-EeV tau neutrino complements other experiments in energy range and methodology, and suggests the prologue of "multi-particle astronomy" with a precise determination of time and location.

astro-ph.HE↗

arXiv2006

Swift and Suzaku Observations of the X-Ray Afterglow from the GRB 060105

Results are presented of early X-ray afterglow observations of GRB 060105 by Swift and Suzaku. The bright, long gamma-ray burst GRB 060105 triggered the Swift Burst Alert Telescope (BAT) at 06:49:28 on 5 January 2006. The Suzaku team commenced a pre-planned target of opportunity observation at 19 ks (5.3 hr) after the Swift trigger. Following the prompt emission and successive very steep decay, a shallow decay was observed from T_0+187 s to T_0+1287 s. After an observation gap during T_0 +(1.5-3) ks, an extremely early steep decay was observed in T_0+(4-30) ks. The lightcurve flattened again at T_0+30 ks, and another steep decay followed from T_0+50 ks to the end of observations. Both steep decays exhibited decay indices of 2.3 - 2.4. This very early break, if it is a jet break, is the earliest case among X-ray afterglow observations, suggesting a very narrow jet whose opening angle is well below 1 degree. The unique Suzaku/XIS data allow us to set very tight upper limits on line emission or absorption in this GRB. For the reported pseudo-redshift of z=4.0+/-1.3 the upper limit on the iron line equivalent width is 50 eV.

astro-ph↗

arXiv2005

Quantum tunneling of magnetization in lanthanide single-molecule magnets, bis(phthalocyaninato)terbium and bis(phthalocyaninato)-dysprosium anions

Magnetization versus field measurements were performed on single crystals of [(Pc)2TbIII0.02YIII0.98]^TBA+ and [(Pc)2DyIII0.02YIII0.98]^TBA+ (Pc: phthalocyaninato, TBA: tetrabutylammonium) at 0.04 K. The [(Pc)2TbIII] complex, the first lanthanide single-molecule magnet, exhibited clear staircase-like structures, which are assigned to resonant quantum tunneling between entangled states of the electron and nuclear spin systems.

cond-mat.mes-hall↗

arXiv2005

Nuclear Spin Driven Quantum Tunneling of Magnetization in a New Lanthanide Single-Molecule Magnet: Bis(phthalocyaninato)holmium anion

The first measurements of magnetization hysteresis loops on diluted single crystal of [(Pc)2Ho]-TBA+ (Pc: phthalocyaninato, TBA: tetrabutylammonium) in the subkelvin temperature range are reported. Characteristic staircase-like structure was observed, indicating the occurrence of the quantum tunneling of magnetization (QTM), which is a characteristic feature of SMMs. The quantum process in the new lanthanide SMMs is due to resonant quantum tunneling between entangled states of the electronic and nuclear spin systems, which is an essentially different mechanism from those of the known transition-metal-cluster SMMs. Evidence of the two-body quantum process was also observed for the first time in lanthanide complex systems.

cond-mat.mes-hall↗

arXiv2004

Vortex phase diagram in BSCCO with damage tracks created by 30 MeV fullerene irradiation

Using 30 MeV C60 fullerene irradiation, we have produced latent tracks of diameter 20 nm and length 200 nm, near the surface of single crystalline BSCCO. A preliminary transmission electron microscopy study shows evidence for a very high density of deposited energy, and the ejection of material from the track core in very thin specimens. The latent tracks reveal themselves to be exceptionally strong pinning centers for vortices in the superconducting mixed state. Both the critical current density and magnetic irreversibility line are significantly enhanced. The irradiated crystals present salient features of the (B,T) phase diagram of vortex matter both of pristine crystals, such as the first order vortex phase transition, and the exponential Bose-glass line characteristic of heavy ion-irradiated crystals. We show that the latter is manifestly independent of the pinning potential.

cond-mat.supr-con↗