Itinerant ferromagnetism in half-metallic CoS_2

We have investigated electronic and magnetic properties of the pyrite-type CoS_2 using the linearized muffin-tin orbital (LMTO) band method. We have obtained the ferromagnetic ground state with nearly half-metallic nature. The half-metallic stability is studied by using the fixed spin moment method. The non-negligible orbital magnetic moment of Co 3d electrons is obtained as $\mu_L = 0.06 \mu_B$ in the local spin density approximation (LSDA). The calculated ratio of the orbital to spin angular momenta / = 0.15 is consistent with experiment. The effect of the Coulomb correlation between Co 3d electrons is also explored with the LSDA + U method. The Coulomb correlation at Co sites is not so large, $U \lesssim 1$ eV, and so CoS_2 is possibly categorized as an itinerant ferromagnet. It is found that the observed electronic and magnetic behaviors of CoS_2 can be described better by the LSDA than by the LSDA + U.Comment: 4 pages, 3 postscript figure

The energy spectrum of all-particle cosmic rays around the knee region observed with the Tibet-III air-shower array

We have already reported the first result on the all-particle spectrum around the knee region based on data from 2000 November to 2001 October observed by the Tibet-III air-shower array. In this paper, we present an updated result using data set collected in the period from 2000 November through 2004 October in a wide range over 3 decades between $10^{14}$ eV and $10^{17}$ eV, in which the position of the knee is clearly seen at around 4 PeV. The spectral index is -2.68 $\pm$ 0.02(stat.) below 1PeV, while it is -3.12 $\pm$ 0.01(stat.) above 4 PeV in the case of QGSJET+HD model, and various systematic errors are under study now.Comment: 12 pages, 7 figures, accepted by Advances in space researc

Moon Shadow by Cosmic Rays under the Influence of Geomagnetic Field and Search for Antiprotons at Multi-TeV Energies

We have observed the shadowing of galactic cosmic ray flux in the direction of the moon, the so-called moon shadow, using the Tibet-III air shower array operating at Yangbajing (4300 m a.s.l.) in Tibet since 1999. Almost all cosmic rays are positively charged; for that reason, they are bent by the geomagnetic field, thereby shifting the moon shadow westward. The cosmic rays will also produce an additional shadow in the eastward direction of the moon if cosmic rays contain negatively charged particles, such as antiprotons, with some fraction. We selected 1.5 x10^{10} air shower events with energy beyond about 3 TeV from the dataset observed by the Tibet-III air shower array and detected the moon shadow at $\sim 40 \sigma$ level. The center of the moon was detected in the direction away from the apparent center of the moon by 0.23$^\circ$ to the west. Based on these data and a full Monte Carlo simulation, we searched for the existence of the shadow produced by antiprotons at the multi-TeV energy region. No evidence of the existence of antiprotons was found in this energy region. We obtained the 90% confidence level upper limit of the flux ratio of antiprotons to protons as 7% at multi-TeV energies.Comment: 13pages,4figures; Accepted for publication in Astroparticle Physic