Spin Gap of Two-Dimensional Antiferromagnet Representing CaV4_4O9_9

We examined a two-dimensional Heisenberg model with two kinds of exchange energies, $J_e$ and $J_c$. This model describes localized spins at vanadium ions in a layer of CaV$_4$O$_9$, for which a spin gap is found by a recent experiment. Comparing the high temperature expansion of the magnetic susceptibility to experimental data, we determined the exchange energies as $J_e \simeq$ 610 K and $J_c \simeq$ 150 K. By the numerical diagonalization we estimated the spin gap as $\Delta \sim 0.2J_e \simeq$ 120 K, which consists with the experimental value 107 K. Frustration by finite $J_c$ enhances the spin gap.Comment: 12 pages of LaTex, 4 figures availavule upon reques

The effects of small ice crystals on the infrared radiative properties of cirrus clouds

To be successful in the development of satellite retrieval methodologies for the determination of cirrus cloud properties, we must have fundamental scattering and absorption data on nonspherical ice crystals that are found in cirrus clouds. Recent aircraft observations (Platt et al. 1989) reveal that there is a large amount of small ice particles, on the order of 10 micron, in cirrus clouds. Thus it is important to explore the potential differences in the scattering and absorption properties of ice crystals with respect to their sizes and shapes. In this study the effects of nonspherical small ice crystals on the infrared radiative properties of cirrus clouds are investigated using light scattering properties of spheroidal particles. In Section 2, using the anomalous diffraction theory for spheres and results from the exact spheroid scattering program, efficient parameterization equations are developed for calculations of the scattering and absorption properties for small ice crystals. Parameterization formulas are also developed for large ice crystals using results computed from the geometric ray-tracing technique and the Fraunhofer diffraction theory for spheroids and hexagonal crystals. This is presented in Section 3. Finally, applications to the satellite remote sensing are described in Section 4

The read-out system of spatial distribution of thermoluminescence in meteorites

The thermoluminescence (TL) technique used for dating the terrestrial age of meteorites is based on the TL fading of interior samples. The depth dependence of the TL for Antarctic meteorites with fusion crust is measured. Usually, meteorites are powdered and their TL measured under a photomultiplier. In this case, a TL spatial distribution of a cross section of antarctic meteorites is measured using a read out system of spatial distribution of TL, since a meteorite is made up of inhomogeneous material. Antarctic meteorites MET-78028(L6) and ALH-77278(L13) are used

Two-Staged Magnetoresistance Driven by Ising-like Spin Sublattice in SrCo6O11

A two-staged, uniaxial magnetoresistive effect has been discovered in SrCo6O11 having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field-dependence of magnetization. The stepwise field-dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.Comment: 14 pages, 4 figures, accepted for publication in Phys. Rev. Let

Single crystal MgB2 with anisotropic superconducting properties

The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the research of its superconducting mechanism and its applications.Comment: 7 pages pdf fil

Superconducting Properties of MgB2 Bulk Materials Prepared by High Pressure Sintering

High-density bulk materials of a newly discovered 40K intermetallic MgB2 superconductor were prepared by high pressure sintering. Superconducting transition with the onset temperature of 39K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M-H) curve shows the behavior of a typical Type II superconductor and the lower critical field Hc1(0) estimated from M-H curve is 0.032T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 2x104A/cm2 at 20K and 1T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18T.Comment: 3Pages PD

Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet Observed by the Specific Heat

Specific heat experiments on single crystals of the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C_mag) is proportional to temperature (T) above a critical field H_c, at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C_mag/T increases as the magnetic field approaches H_c from above. The data are in good quantitative agreement with the prediction of the c=1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing a conclusive evidence for a Tomonaga-Luttinger liquid.Comment: 4 pages, 4 postscript figure

Macroscopic quantum tunneling and phase diffusion in a La2−x_{2-x}Srx_xCuO4_4 intrinsic Josephson junction stack

We performed measurements of switching current distribution in a submicron La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) intrinsic Josephson junction (IJJ) stack in a wide temperature range. The escape rate saturates below approximately 2\,K, indicating that the escape event is dominated by a macroscopic quantum tunneling (MQT) process with a crossover temperature $T^{*}\approx2\,$K. We applied the theory of MQT for IJJ stacks, taking into account dissipation and the phase re-trapping effect in the LSCO IJJ stack. The theory is in good agreement with the experiment both in the MQT and in the thermal activation regimes.Comment: 9 pages, 7 figure

Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

We have studied local magnetic moment and electronic phase separation in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption spectroscopy. Detailed temperature dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T$_c$) and the coexisting phases exchange spectral weights with the low spin state gaining intensity at the expense of the higher spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at T$_c$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low spin and high spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of electronically inhomogeneous K$_{x}$Fe$_{2-y}$Se$_2$ system.Comment: 6 pages, 5 figure