Quasi-static Response of a Timoshenko Beam Loaded by an Elastically Supported Moving Rigid Beam

The present paper is concerned with the quasi-static response of an elastic beam, loaded by a rigid beam, which is slowly transported along the elastic beam. The elastic beam is modelled as a Timoshenko beam. The present paper provides a limiting case of the model with constant distributed load that is often considered in the study of transported masses. The rigid beam is connected to the Timoshenko beam by means of an interface modelled as a Winkler foundation. We present a non-dimensional study on the influence of the interface stiffness upon the displacement, bending moment and shear force of the Timoshenko beam, when the rigid beam is assumed to suffer a prescribed transverse displacement. Special emphasis is laid on the distribution of pressure transmitted by the interface between the Timoshenko beam and the rigid beam. Considerable pressure concentrations are found to take place and the locations of the maximum bending moments in the Timoshenko beam move towards the ends of the rigid beam

Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)

The phase diagram of non-hydrated Na(x)CoO(2) has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2) to a `Curie-Weiss metal' (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.Comment: 4 pages with 3 figures, changed conten

Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2

The determination by powder neutron diffraction of the ambient temperature crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is reported. The structures consist of triangular CoO2 layers with Na ions distributed in intervening charge reservoir layers. The shapes of the CoO6 octahedra that make up the CoO2 layers are found to be critically dependent on the electron count and on the distribution of the Na ions in the intervening layers, where two types of Na sites are available. Correlation of the shapes of cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram in NaxCoO2 is made, showing how structural and electronic degrees of freedom can be coupled in electrically conducting triangular lattice systems.Comment: 15 pages, 1 tables, 6 figures Submitted to Physical Review

NMR studies of Successive Phase Transitions in Na0.5CoO2 and K0.5CoO2

59Co- and 23Na-NMR measurements have been carried out on polycrystalline and c-axis aligned samples of Na0.5CoO2, which exhibits successive transitions at temperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also been carried out on c-axis aligned crystallites of K0.5CoO2 with similar successive transitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMR lines of 23Na nuclei explained by considering the quadrupolar frequencies nuQ ~1.32 and 1.40 MHz have been observed above Tc1, as is expected from the crystalline structure. Rather complicated but characteristic variation of the 23Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below Tc1 and the 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in a comparative way.Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jpn, correction is made in right colum of p6 (35th line) as K0.5CoO2-->Na0.5CoO

Charge order and superconductivity in a two-dimensional triangular lattice at n=2/3

To investigate the possibility of charge order and superconductivity in a doped two-dimensional triangular lattice, we study the extended Hubbard model with variational Monte Carlo method. At n=2/3, a commensurate filling for a triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V induces honeycomb-type charge order and antiferromagnetic spin order at U>10t. We also discuss the possibility of superconductivity induced by charge fluctuation and the relation to the superconductivity in Na_{0.35}CoO_{2}1.3H_{2}O and theta-type organic condoctors.Comment: 4 pages, 5 figure

A novel route to phase formation of cobalt oxyhydrates using KMnO4 as an oxidizing agent

We have first succeefully synthesized the sodium cobalt oxyhydrate superconductors using KMnO4 as a de-intercalating and oxidizing agent. It is a novel route to form the superconductive phase of NaxCoO2.yH2O without resorting to the commonly used Br2/CH3CN solution. The role of the KMnO4 is to de-intercalate the Na+ from the parent compound Na0.7CoO2 and oxidize the Co ion as a result. The higher molar ratio of KMnO4 relative to the sodium content tends to remove more Na+ from the parent compound and results in a slight expansion of the c-axis in the unit cell. The superconducting transition temperature is 4.6-3.8 K for samples treated by the aqueous KMnO4 solution with the molar ratio of KMnO4 relative to the sodium content in the range of 0.3 and 2.29.Comment: 10 pages, 3 figure