Showing papers by "Shoji Yamamoto published in 2012"

Alfvén eigenmode properties and dynamics in the TJ-II stellarator

Abstract: Alfven eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density ne, electric potential φ and poloidal magnetic field Bpol with frequencies 50 kHz < fAE < 300 kHz. The LP, MP and HIBP data showed a high level of coherency for specific branches of AEs. Poloidal mode wave-vectors kθ, mode numbers m(m < 8) and propagation velocities Vθ ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfven law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector kθ and mode number m remained constant, (ii) the cross-phases between the oscillations in Bpol, ne and electric potential remained constant, having an individual value for each AE branch, (iii) Vθ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in Epol and ne, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

Magnetic Properties of the S=2 Heisenberg Antiferromagnetic Chain Compound MnCl3(bpy)

Abstract: We report the results of magnetic susceptibilities at temperatures between 2 and 300 K, and magnetization in magnetic fields of up to 52 T on polycrystalline samples of MnCl3(bpy) (bpy=2, 2'-bipyridine) and the comparison with numerical calculations. This compound is one of the rare examples of the spin 2 quasi-one-dimensional Heisenberg antiferromagnet, and the magnetic properties of tiny single crystal samples were reported previously. The temperature dependence of magnetic susceptibility and the magnetization curve after subtracting the contribution of magnetic impurity are well fitted to those calculated by a quantum Monte Carlo method with the intrachain exchange constant J/kB =31.2 K and the g-value g=2.02 which are comparable to reported values (J/kB =34.8±1.6 K and g=2.04±0.04).

Theoretical Scenario for the Proton NMR Observation of Field-Induced Level Crossings in a Single Crystal of [Cr8F8(piv)16]

Abstract: Motivated by the stimulative NMR observations of field-induced level crossings in molecular antiferromagnetic rings, we construct a microscopic theory of the spin dynamics in single-crystalline Cr 8 F 8 pivalates. We reproduce the low-temperature 1 H NMR relaxation rate 1/ T 1 as a function of magnetic field, which is single-peaked as usual but double-peaked intriguingly around the first and second level crossings, respectively. It is not poor crystallization such as mosaicity and gemination, but combined efforts of weak intramolecular antisymmetric interactions and nonnegligible intermolecular couplings that should split 1/ T 1 peaks substantially.

Phase competition and photomagnetism in supramolecular assemblies based on octacyanomolybdates of square antiprism configuration

Abstract: We construct a microscopic theory on the photomagnetism in the cyano-bridged bimetallic assemblies Cu2[Mo(CN)8]·𝓍H2O. Going through a grouptheoretical analysis on the ground-state properties, we simulate photoirradiation and the resultant magnetization dynamics with a time-dependent Hartree-Fock method. It is not c-axis-polarized photons but those polarized in the ab plane that induce a macroscopic magnetization. Absorption of the photon energy is double-stepped and the occurrence of a significant magnetization is simultaneous with the second stage (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)