arXiv subjects

T. Yamagami

Publications and source records attributed to T. Yamagami.

18 recordsLinked to original sources

arXiv2018

Room-temperature Low-field Colossal Magneto-resistance in Double-perovskite Manganite

The gigantic decrease of resistance by an applied magnetic field, which is often referred to as colossal magnetoresistance (CMR), has been an attracting phenomenon in strongly correlated electron systems. The discovery of CMR in manganese oxide compounds has developed the science of strong coupling among charge, orbital, and spin degrees of freedom. CMR is also attracting scientists from the viewpoint of possible applications to sensors, memories, and so on. However, no application using CMR effect has been achieved so far, partly because the CMR materials which satisfy all of the required conditions for the application, namely, high operating temperature, low operating magnetic field, and sharp resistive change, have not been discovered. Here we report a resistance change of more than two-orders of magnitude at a magnetic field lower than 2 T near 300 K in an A-site ordered NdBaMn_2_O_6_ crystal. When temperature and a magnetic field sweep from insulating (metallic) phase to metallic (insulating) phase, the insulating (metallic) conduction changes to the metallic (insulating) conduction within 1 K and 0.5 T, respectively. The CMR is ascribed to the melting of the charge ordering. The entropy change which is estimated from the B-T phase diagram is smaller than what is expected for the charge and orbital ordering. The suppression of the entropy change is attributable to the loss of the short range ferromagnetic fluctuation of Mn spin moments, which an important key of the high temperature and low magnetic field CMR effect.

cond-mat.str-el↗

arXiv2016

Measurements of cosmic-ray proton and helium spectra from the BESS-Polar long-duration balloon flights over Antarctica

The BESS-Polar Collaboration measured the energy spectra of cosmic-ray protons and helium during two long-duration balloon flights over Antarctica in December 2004 and December 2007, at substantially different levels of solar modulation. Proton and helium spectra probe the origin and propagation history of cosmic rays in the galaxy, and are essential to calculations of the expected spectra of cosmic-ray antiprotons, positrons, and electrons from interactions of primary cosmic-ray nuclei with the interstellar gas, and to calculations of atmospheric muons and neutrinos. We report absolute spectra at the top of the atmosphere for cosmic-ray protons in the kinetic energy range 0.2-160 GeV and helium nuclei 0.15-80 GeV/nucleon. The corresponding magnetic rigidity ranges are 0.6-160 GV for protons and 1.1-160 GV for helium. These spectra are compared to measurements from previous BESS flights and from ATIC-2, PAMELA, and AMS-02. We also report the ratio of the proton and helium fluxes from 1.1 GV to 160 GV and compare to ratios from PAMELA and AMS-02.

astro-ph.HE↗

arXiv2012

Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the BESS-Polar collaboration has searched for antihelium in the cosmic radiation with higher sensitivity than any reported investigation. BESS- Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among 8.4\times 10^6 |Z| = 2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0\times 10^7 |Z| = 2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit of 6.9 \times 10^-8 was determined by combining all the BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies from 1.6 to 14 GV, an upper limit of 1.0 \times 10^-7 was determined for the combined BESS-Polar data. These are the most stringent limits obtained to date.

astro-ph.CO↗

arXiv2011

Measurement of the cosmic-ray antiproton spectrum at solar minimum with a long-duration balloon flight over Antarctica

The energy spectrum of cosmic-ray antiprotons from 0.17 to 3.5 GeV has been measured using 7886 antiprotons detected by BESS-Polar II during a long-duration flight over Antarctica near solar minimum in December 2007 and January 2008. This shows good consistency with secondary antiproton calculations. Cosmologically primary antiprotons have been investigated by comparing measured and calculated antiproton spectra. BESS-Polar II data show no evidence of primary antiprotons from evaporation of primordial black holes.

astro-ph.HE↗

arXiv2008

Measurement of cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1-4.2 GeV. In this paper, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiprotons to probe the effect of charge sign dependent drift in the solar modulation.

astro-ph↗

arXiv2008

High-energy electron observations by PPB-BETS flight in Antarctica

We have observed cosmic-ray electrons from 10 GeV to 800 GeV by a long duration balloon flight using Polar Patrol Balloon (PPB) in Antarctica. The observation was carried out for 13 days at an average altitude of 35 km in January 2004. The detector is an imaging calorimeter composed of scintillating-fiber belts and plastic scintillators inserted between lead plates with 9 radiation lengths. The performance of the detector has been confirmed by the CERN-SPS beam test and also investigated by Monte-Carlo simulations. New telemetry system using a commercial satellite of Iridium, power supply by solar batteries, and automatic level control using CPU have successfully been developed and operated during the flight. From the long duration balloon observations, we derived the energy spectrum of cosmic-ray electrons in the energy range from 100 GeV to 800 GeV. In addition, for the first time we derived the electron arrival directions above 100 GeV, which is consistent with the isotropic distribution.

astro-ph↗

arXiv2007

Measurements of 0.2 to 20 GeV/n cosmic-ray proton and helium spectra from 1997 through 2002 with the BESS spectrometer

We measured low energy cosmic-ray proton and helium spectra in the kinetic energy range 0.215 - 21.5 GeV/n at different solar activities during a period from 1997 to 2002. The observations were carried out with the BESS spectrometer launched on a balloon at Lynn Lake, Canada. A calculation for the correction of secondary particle backgrounds from the overlying atmosphere was improved by using the measured spectra at small atmospheric depths ranging from 5 through 37 g/cm^2. The uncertainties including statistical and systematic errors of the obtained spectra at the top of atmosphere are 5-7 % for protons and 6-9 % for helium nuclei in the energy range 0.5 - 5 GeV/n.

astro-ph↗

arXiv2005

Measurements of Atmospheric Antiprotons

We measured atmospheric antiproton spectra in the energy range 0.2 to 3.4 GeV, at sea level and at balloon altitude in the atmospheric depth range 4.5 to 26 g/cm^2. The observed energy spectra, including our previous measurements at mountain altitude, were compared with estimated spectra calculated on various assumptions regarding the energy distribution of antiprotons that interacted with air nuclei.

astro-ph↗

arXiv2005

Search for Cosmic-Ray Antideuterons

We performed a search for cosmic-ray antideuterons using data collected during four BESS balloon flights from 1997 to 2000. No candidate was found. We derived, for the first time, an upper limit of 1.9E-4 (m^2 s sr GeV/nucleon)^(-1) for the differential flux of cosmic-ray antideuterons, at the 95% confidence level, between 0.17 and 1.15 GeV/nucleon at the top of the atmosphere.

astro-ph↗

arXiv2004

Measurements of Primary and Atmospheric Cosmic-Ray Spectra with the BESS-TeV Spectrometer

Primary and atmospheric cosmic-ray spectra were precisely measured with the BESS-TeV spectrometer. The spectrometer was upgraded from BESS-98 to achieve seven times higher resolution in momentum measurement. We report absolute fluxes of primary protons and helium nuclei in the energy ranges, 1-540 GeV and 1-250 GeV/n, respectively, and absolute flux of atmospheric muons in the momentum range 0.6-400 GeV/c.

astro-ph↗

arXiv2003

Measurements of Proton, Helium and Muon Spectra at Small Atmospheric Depths with the BESS Spectrometer

The cosmic-ray proton, helium, and muon spectra at small atmospheric depths of 4.5 -- 28 g/cm^2 were precisely measured during the slow descending period of the BESS-2001 balloon flight. The variation of atmospheric secondary particle fluxes as a function of atmospheric depth provides fundamental information to study hadronic interactions of the primary cosmic rays with the atmosphere.

astro-ph↗

arXiv2002

Atmospheric gamma-ray observation with the BETS detectorfor calibrating atmospheric neutrino flux calculations

We observed atmospheric gamma-rays around 10 GeV at balloon altitudes (15~25 km) and at a mountain (2770 m a.s.l). The observed results were compared with Monte Carlo calculations to find that an interaction model (Lund Fritiof1.6) used in an old neutrino flux calculation was not good enough for describing the observed values. In stead, we found that two other nuclear interaction models, Lund Fritiof7.02 and dpmjet3.03, gave much better agreement with the observations. Our data will serve for examining nuclear interaction models and for deriving a reliable absolute atmospheric neutrino flux in the GeV region.

hep-ex↗

arXiv2002

Precise Measurements of Atmospheric Muon Fluxes with the BESS Spectrometer

The vertical absolute fluxes of atmospheric muons and muon charge ratio have been measured precisely at different geomagnetic locations by using the BESS spectrometer. The observations had been performed at sea level (30 m above sea level) in Tsukuba, Japan, and at 360 m above sea level in Lynn Lake, Canada. The vertical cutoff rigidities in Tsukuba (36.2 N, 140.1 E) and in Lynn Lake (56.5 N, 101.0 W) are 11.4 GV and 0.4 GV, respectively. We have obtained vertical fluxes of positive and negative muons in a momentum range from 0.6 to 20 GeV/c with systematic errors less than 3 % in both measurements. By comparing the data collected at two different geomagnetic latitudes, we have seen an effect of cutoff rigidity. The dependence on the atmospheric pressure and temperature, and the solar modulation effect have been also clearly observed. We also clearly observed the decrease of charge ratio of muons at low momentum side with at higher cutoff rigidity region.

astro-ph↗

arXiv2001

ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036

ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals coherent X-ray pulsation from this source for the first time. The source was first detected in the serendipitous Einstein observation and later identified as a radio pulsar. Possible pulsation in the gamma-ray band has been detected from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA band is characterized by a hard power-law type emission with a photon index of about 2.3, when fitted with a single power-law function modified with absorption. An additional blackbody component of about 0.14 keV increases the quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR B0633+17 (Geminga), and PSR B0656+14.

astro-ph↗

arXiv2000

Precise Measurement of Cosmic-Ray Proton and Helium Spectra with the BESS Spectrometer

We report cosmic-ray proton and helium spectra in energy ranges of 1 to 120 GeV and 1 to 54 GeV/nucleon, respectively, measured by a balloon flight of the BESS spectrometer in 1998. The magnetic-rigidity of the cosmic-rays was reliably determined by highly precise measurement of the circular track in a uniform solenoidal magnetic field of 1 Tesla. Those spectra were determined within overall uncertainties of +-5 % for protons and +- 10 % for helium nuclei including statistical and systematic errors.

astro-ph↗

arXiv1999

Precision Measurement of Cosmic-Ray Antiproton Spectrum

The energy spectrum of cosmic-ray antiprotons has been measured in the range 0.18 to 3.56 GeV, based on 458 antiprotons collected by BESS in recent solar-minimum period. We have detected for the first time a distinctive peak at 2 GeV of antiprotons originating from cosmic-ray interactions with the interstellar gas. The peak spectrum is reproduced by theoretical calculations, implying that the propagation models are basically correct and that different cosmic-ray species undergo a universal propagation. Future BESS flights toward the solar maximum will help us to study the solar modulation and the propagation in detail and to search for primary antiproton components.

astro-ph↗

arXiv1998

Measurement of Low-Energy Cosmic-Ray Antiprotons at Solar Minimum

The absolute fluxes of the cosmic-ray antiprotons at solar minimum are measured in the energy range 0.18 to 1.4 GeV, based on 43 events unambiguously detected in BESS '95 data. The resultant energy spectrum appears to be flat below 1 GeV, compatible with a possible admixture of primary antiproton component with a soft energy spectrum, while the possibility of secondary antiprotons alone explaining the data cannot be excluded with the present accuracy. Further improvement of statistical accuracy and extension of the energy range are planned in future BESS flights.

astro-ph↗

arXiv1997

A New Limit on the Flux of Cosmic Antihelium

A very sensitive search for cosmic-ray antihelium was performed using data obtained from three scientific flights of BESS magnetic rigidity spectrometer. We have not observed any antihelium; this places a model-independent upper limit (95 % C.L.) on the antihelium flux of 6*10**(-4) m**(-2)sr**(-1)s**(-1) at the top of the atmosphere in the rigidity region 1 to 16 GV, after correcting for the estimated interaction loss of antihelium in the air and in the instrument. The corresponding upper limit on the Hebar/He flux ratio is 3.1*10**(-6), 30 times more stringent than the limits obtained in similar rigidity regions with magnetic spectrometers previous to BESS.

astro-ph↗