Nai Phuan Ong

Low temperature phase transitions and crystal structure of Na0.5CoO2

Specific heat measurements on Na0.5CoO2 show that the transitions observed at 87 and 53xa0K in the resistivity and magnetic susceptibility are accompanied by changes in entropy, whereas the one near 20xa0K is not. Electron diffraction studies suggest that the 87xa0K transition has a structural component. The crystal structure of Na0.5CoO2, determined by powder neutron diffraction, consists of layers of edge-shared CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The Na ions form one-dimensional zigzag chains. Two types of Co ions are also found in chains.

Chemical instability of the cobalt oxyhydrate superconductor under ambient conditions

Abstract The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2·1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of the series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 A implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9xa0A at nominally constant chemical doping.

Vorticity and the Nernst effect in cuprate superconductors

We present a review of the vortex-Nernst effect in the 3 cuprate families La2-xSrxCuO4, Bi2Sr2CaCu2O8, and YBa2Cu3Oy, and discuss the scenario that the superconducting transition in the hole-doped cuprates corresponds to the destruction of long-range phase coherence rather than the vanishing of the order-parameter amplitude.

Field tuning of the electron and hole populations in the ruthenate Bi3Ru3O11

Experiments on the Hall coefficient RH and heat capacity C reveal an unusual, compensated electronic ground state in the ruthenate Bi3Ru3O11. At low temperature T, RH decreases linearly with magnetic field |H| for fields larger than the field scale set by the Zeeman energy. The results suggest that the electron and hole populations are tuned by H in opposite directions via coupling of the spins to the field. As T is decreased below 5 K, the curve C(T)/T vs. T2 shows an anomalous flattening consistent with a rapidly growing Sommerfeld parameter γ(T). We discuss shifts of the electron and hole chemical potentials by H to interpret the observed behavior of RH.

Phase coherence and the Nernst effect at magic angles in organic conductors

A giant Nernst signal was recently observed for fields near crystallographic directions in (TMTSF)2PF6. Such large Nernst signals are most naturally associated with the motion of pancake vortices. We propose a model in which phase coherence is destroyed throughout the sample except in planes closely aligned with the applied field H. A small tilt above or below the plane changes the direction and density of the penetrating vortices and leads to a Nernst signal that varies with the tilt angle of H as observed. The resistance notches at magic angles are understood in terms of flux-flow dissipation from field-induced vortices.

Quasi-particle mean-free-path and thermal hall conductivity in YBa2Cu3O7

Abstract We have measured the Hall component of the thermal conductivity κ xy e in the mixed state of YBCO. κ xy e is observed to be unusually large. From its strong field dependence we deduce the quasi-particle mean free path l 0 . Between 90 K and 15 K, l 0 increases rapidly from 80 A to 2500 A.

Strongly nonlinear magnetization above Tc in

Using high-resolution magnetometry we have investigated in detail the magnetization M above the critical temperature Tc in Bi2Sr2CaCu2O8. In a broad range of temperature T above Tc, we find that M(T,H) is strongly non-linear in the field H. We show that as T → Tc, the susceptibility χ(T,H) diverges to very large values (χ → − 1) if measured in weak H. In addition, M(H) displays an anomalous non-analytic form M ~ H1/δ in weak fields with a strongly T-dependent exponent δ(T). These features strongly support the proposal that, above Tc, the pair condensate survives to support significant London rigidity.