Multipole expansion for magnetic structures: A generation scheme for symmetry-adapted orthonormal basis set in crystallographic point group

We propose a systematic method to generate a complete orthonormal basis set of multipole expansion for magnetic structures in arbitrary crystal structure. The key idea is the introduction of a virtual atomic cluster of a target crystal, on which we can clearly define the magnetic configurations corresponding to symmetry-adapted multipole moments. The magnetic configurations are then mapped onto the crystal so as to preserve the magnetic point group of the multipole moments, leading to the magnetic structures classified according to the irreducible representations of crystallographic point group. We apply the present scheme to pyrhochlore and hexagonal ABO3 crystal structures, and demonstrate that the multipole expansion is useful to investigate the macroscopic responses of antiferromagnets

TeV Gamma Ray Emission from Southern Sky Objects and CANGAROO Project

We report recent results of the CANGAROO Collaboration on very high energy gamma ray emission from pulsars, their nebulae, SNR and AGN in the southern sky. Observations are made in South Australia using the imaging technique of detecting atmospheric Cerenkov light from gamma rays higher than about 1 TeV. The detected gamma rays are most likely produced by the inverse Compton process by electrons which also radiate synchrotron X-rays. Together with information from longer wavelengths, our results can be used to infer the strength of magnetic field in the emission region of gamma rays as well as the energy of the progenitor electrons. A description of the CANGAROO project is also given, as well as details of the new telescope of 7 m diameter which is scheduled to be in operation within two years.Comment: 5 pages, 1 figure, LaTeX 2.09 with aipproc.sty & epsfig.sty, to appear in proceedings of the 4th Compton Symposium, Williamsburg, 199

trans-Bis(N,N-diethyl­ethylenediamine)­nickel(II) dibromide

The structure of the title compound, [Ni(C6H16N2)2]Br2 or [Ni(Et2en)2]Br2 (Et2en is asymmetric N,N-diethyl­ethylene­diamine), containing an NiII atom (site symmetry ) in square-planar NiN4 coordination, is described and contrasted with related structures containing NiII in octa­hedral coordination with axial X − ligands (X − = variable anions). The dialkyl­ated N atom has an appreciably longer bond length to the NiII atom [1.9666 (13) Å] than does the unsubstituted N atom [1.9202 (14) Å]. The Ni—N bond lengths in [Ni(Et2en)2]Br2 are significantly shorter than corresponding values in tetra­gonally distorted [Ni(Et2en)2 X 2] compounds (X = −O2CCF3, OH2, or −NCS), which have a triplet ground state. The electronic configuration in these axially ligated [Ni(Et2en)2 X 2] compounds populates the metal-based d x 2 -y 2 orbital, which is Ni—N anti­bonding in character. Each Et2en ligand in each [Ni(Et2en)2]2+ cation forms a pair of N—H⋯Br hydrogen bonds to the Br− anions, one above and below the NiN4 square plane. Thus, a ribbon of alternating Br− pairs and [Ni(Et2en)2]2+ cations that are canted at 65° relative to one another is formed by hydrogen bonds

Detection of Gamma Rays of Up to 50 TeV From the Crab Nebula

Gamma rays with energies greater than 7 TeV from the Crab pulsar/nebula have been observed at large zenith angles, using the Imaging Atmospheric Technique from Woomera, South Australia. CANGAROO data taken in 1992, 1993 and 1995 indicate that the energy spectrum extends up to at least 50 TeV, without a change of the index of the power law spectrum. The observed differential spectrum is \noindent $(2.01\pm 0.36)\times 10^{-13}(E/{7 TeV})^{-2.53 \pm 0.18} TeV^{-1}cm^{-2}s^{-1}$ between 7 TeV and 50 TeV. There is no apparent cut-off. The spectrum for photon energies above $\sim$10 TeV allows the maximum particle acceleration energy to be inferred, and implies that this unpulsed emission does not originate near the light cylinder of the pulsar, but in the nebula where the magnetic field is not strong enough to allow pair creation from the TeV photons. The hard gamma-ray energy spectrum above 10 TeV also provides information about the varying role of seed photons for the inverse Compton process at these high energies, as well as a possible contribution of $\pi ^{\circ}$-gamma rays from proton collisions.Comment: 19 pages, 4 figures, LaTeX2.09 with AASTeX 4.0 maros, to appear in Astrophys. J. Let

Partial Disorder and Metal-Insulator Transition in the Periodic Anderson Model on a Triangular Lattice

Ground state of the periodic Anderson model on a triangular lattice is systematically investigated by the mean-field approximation. We found that the model exhibits two different types of partially disordered states: one is at half filling and the other is at other commensurate fillings. In the latter case, the kinetic energy is lowered by forming an extensive network involving both magnetic and nonmagnetic sites, in sharp contrast to the former case in which the nonmagnetic sites are rather isolated. This spatially extended nature of nonmagnetic sites yields a metallic partially-disordered state by hole doping. We discuss the mechanism of the metal-insulator transition by the change of electronic structure.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp