Instability of isolated triplet excitations on the Shastry-Sutherland lattice (SSL)

Configurations of singlets and triplets on the SSL have been proposed in the literature as variational ground states of the Shastry-Sutherland model at fixed magnetization M. We prove, that isolated triplet excitations on the SSL are unstable if the coupling alpha falls below a critical value alpha_c=2.0 (approx.). The instability should be visible in the compound SrCu_2(BO_3)_2 where a coupling alpha^*=1.48 is realized.Comment: 4 pages, 4 figures, RevTe

Magnetization Plateaus in the Shastry-Sutherland Model for SrCu_2(BO_3)_2: Results of Fourth-Order Perturbation Expansion with a Low-Density Approximation

Magnetization plateaus in the Shastry-Sutherland model for ${\rm SrCu_2(BO_3)_2}$ are studied by the perturbation expansion method. The fourth-order effective Hamiltonian which describes the dynamics of triplet dimers (TD's) with $S^{\rm tot}_z=1$ in the singlet sea is derived and then partially diagonalized for the space that consists of the TD configurations with the lowest second-order energy. The fourth-order terms are treated within a low-density approximation. Our procedure makes clear how TD interactions are responsible for the formation of magnetization plateaus. Particularly, the 1/4-plateau is obtained by the fourth-neighbor TD repulsion in the fourth-order perturbation, and a diagonal stripe arrangement of TD's appears at this plateau.Comment: 8 pages, 12 figure

Two-Triplet-Dimer Excitation Spectra in the Shastry-Sutherland Model for SrCu_2(BO_3)_2

By using the perturbation expansion up to the fifth order, we study the two-triplet-dimer excitation spectra in the Shastry-Sutherland model, where the localized nature of a triplet-dimer, the propagation of a triplet-dimer pair by the correlated hopping and the long-range interactions between triplet-dimers play an essential role. It is found that the dispersion relations for first-neighbor triplet-dimer pair excitations with S=1 and p-type symmetry qualitatively explain the second-lowest branch observed in the neutron inelastic scattering experiment. It is also predicted that the second-lowest branch consists of two components, p_x- and p_y-states, with slightly different excitation energies. The origin of the singlet mode at 3.7meV observed in the Raman scattering experiment is also discussed.Comment: 5 pages, 3 figure

Relationship Between Gas Depth Profiles in Compost Heap and Gas Emission

Rosana G. Moreira, Editor-in-Chief; Texas A&M UniversityThis is an Invited Paper from International Commission of Agricultural Engineering (CIGR, Commission Internationale du Genie Rural) E-Journal Volume 5 (2003): Y. Fukumoto, H. Rom, and P. Dahl. Relationship Between Gas Depth Profiles in Compost Heap and Gas Emission. Vol. V. May 2003

Effect of Forest Grazing in Summer on Grazing Behaviour, Heart Beat and Heat Production of Beef Cows

In Japan summer is very hot and humid, particularly in daytime, therefore the supply of shade to animals is important. At the same time, a lot of forest and partial forest area is under-utilised in most of the mountain areas. Introduction of animals to forest areas might result in not only less labour demanding animal management, but also in the efficient weeding in the forest (Sugimoto et al., 1999). We compared the grazing behaviour, heart beat and heat production of beef cows in partial forest with those in the normal grazing place outside the forest

Investigation of over-horizon VHF radio signals associated with earthquakes

International audienceThis paper presents the preliminary results on the possible reception of over-horizon VHF radio signals from a FM (frequency modulation) transmitter during abnormal situations (probably in close association with earthquakes). The transmitter is FM Sendai (frequency = 77.1 MHz) and the receiving station is our university (UEC) at Chofu, Tokyo, with the transmitter-receiver distance being about 310 km. We first show a typical example of the reception of over-horizon VHF signals in which we present the amplitude behavior, azimuthal and incident angle estimation, etc. Then, we study the correlation between a signal anomaly and an earthquake and it seems that the over-horizon VHF signals observed are probably associated with earthquakes, observed about 7 to 0 days before an earthquake. The direction finding measurements have indicated that such signals are due to favorable tropospheric (but not ionospheric) conditions attributed to the effects of earthquakes

Evolution of Multiphase Hot Interstellar Medium in Elliptical Galaxies

We present the results of a variety of simulations concerning the evolution of multiphase (inhomogeneous) hot interstellar medium (ISM) in elliptical galaxies. We assume the gases ejected from stars do not mix globally with the circumferential gas. The ejected gas components evolve separately according to their birth time, position, and origin. We consider cases where supernova remnants (SNRs) mix with local ISM. The components with high metal abundance and/or high density cool and drop out of the hot ISM gas faster than the other components because of their high metal abundance and/or density. This makes the average metal abundance of the hot ISM low. Furthermore, since the metal abundance of mass-loss gas decreases with radius, gas inflow from outer region makes the average metal abundance of the hot ISM smaller than that of mass-loss gas in the inner region. As gas ejection rate of stellar system decreases, mass fraction of mass-loss gas ejected at outer region increases in a galaxy. If the mixing of SNRs is ineffective, our model predicts that observed [Si/Fe] and [Mg/Fe] should decrease towards the galactic center because of strong iron emission by SNRs. In the outer region, where the cooling of time of the ISM is long, the selective cooling is ineffective and most of gas components remain hot. Thus, the metal abundance of the ISM in this region directly reflects that of the gas ejected from stars. Our model shows that supernovae are not effective heating sources in the inner region of elliptical galaxies, because most of the energy released by them radiates. Therefore, cooling flow is established even if the supernova rate is high. Mixing of SNRs with ambient ISM makes the energy transfer between supernova explosion and ambient ISM more effective.Comment: 21 pages (AASTeX), 14 figures, accepted for publication in The Astrophysical Journa