Kinson C. Kam

Electrical properties of inorganic nanowire–polymer composites

Composites of nanowires of ZnO, RuO2 and Ag with polyaniline (PANI) as well as polypyrrole (PPy) have been prepared, for the first time, by an in-situ process, in order to investigate their electrical properties. Characterization by electron microscopy and IR spectroscopy indicates that there is considerable interaction between the oxide nanowires and the polymer. The room-temperature resistivity of the composites prepared in-situ varies in the 0.01–400 Ω cm range depending on the composition. While the resistivities of the PANI–ZnONW and PPy–ZnONW composites prepared by the in-situ process are generally higher than that of PANI/PPy, those of PANI–RuO2NW and PANI–AgNW are lower. Composites of ZnONW with polyaniline prepared by an ex-situ process exhibit a resistivity close to that of polyaniline.

Pressure-induced structural transitions in Tb-pyrochlore oxides

The structure of Tb2Ti2O7, Tb2Sn2O7, and Tb2TiSnO7 were studied at high pressures (P<1Mbar) using synchrotron radiation and angular dispersive x-ray diffraction. The cubic lattice was shown to distort into a monoclinic phase before exhibiting total structural disorder (amorphization) in all three samples. Our experiments demonstrate that partial replacement of Sn with Ti significantly enhances the bulk modulus and increases the structural stability (crystalline state) to much higher pressures. Furthermore, pressure induces anion before cation disorder during the order-disorder transition. These results are compared to radiation damage studies that show order-disorder transitions.

Frustrated spin correlations in diluted spin ice Ho2−xLaxTi2O7

We have studied the evolution of the structural properties as well as the static and dynamic spin correlations of spin ice Ho(2)Ti(2)O(7), where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samples Ho(2-x)La(x)Ti(2)O(7) was characterized by x-ray and neutron diffraction and by Ho L(III)-edge and Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements. It is found that the pyrochlore structure remains intact until about x = 0.3, but a systematic increase in local disorder with increasing La concentration is observed in the EXAFS data, especially from the Ti K edge. Quasi-elastic neutron scattering and ac susceptibility measurements show that, in x≤0.4 samples at temperatures above macroscopic freezing, the spin-spin correlations are short ranged and dynamic in nature. The main difference with pure spin ice in the dynamics is the appearance of a second, faster, relaxation process.