Chung-Ting Ke

Anisotropic dielectric and ferroelectric response of multiferroic LiCu2O2 in magnetic field

LiCu2O2 is the first multiferroic cuprate to be reported and its ferroelectricity is induced by complex magnetic ordering in ground state, which is still in controversy today. Herein, we have grown nearly untwinned LiCu2O2 single crystals of high quality and systematically investigated their dielectric and ferroelectric behaviours in external magnetic fields. The highly anisotropic response observed in different magnetic fields apparently contradicts the prevalent bc- or ab-plane cycloidal spin model. Our observations give strong evidence supporting a new helimagnetic picture in which the normal of the spin helix plane is along the diagonal of CuO4 squares which form the quasi-1D spin chains by edge-sharing. Further analysis suggests that the spin helix in the ground state is elliptical and in the intermediate state the present c-axis collinear SDW model is applicable with some appropriate modifications. In addition, our studies show that the dielectric and ferroelectric measurements could be used as probes for the characterization of the complex spin structures in multiferroic materials due to the close tie between their magnetic and electric orderings.

Convective solution transport - An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent

An improved technique of convective solution transport using KCl as solvent at 840–790 °C (where mass transport takes place across a vertical temperature gradient) is described for the growth of crystals of the recently discovered superconductor α-FeSex (x = 1−0.8). The crystals were annealed in situ at 400–350 °C for 20–30 h to improve the superconducting properties. Hexagonal plate like crystals measuring 5–6 mm across and 0.25–0.5 mm thick were obtained. High resolution transmission electron microscopy (HRTEM) measurements show good crystallinity and the energy dispersive x-ray analysis (EDX) gives a composition very close to the starting powders. The zero resistance temperature of the crystals was found to increase from 6.5 to 9 K as the composition is decreased from x = 0.95 to 0.9 and decrease thereafter. Similar behavior was also observed in the powder x-ray diffraction (XRD) patterns and Raman spectra with the main peak shifting to higher value until 0.9 and decrease thereafter. In addition the XR...

Nonstoichiometry of LixCu2O2+δ single crystal and its relation to magnetic ordering

We report the first systematic study on the nonstoichiometry of Li/Cu ionic ratios x and oxygen defect concentrations δ in LixCu2O2+δ solid solution. Nonstoichiometric crystals with intentional Li-deficient (x=0.97) or Li-excessive (x=1.06) concentrations were prepared by slow-cooling and floating-zone methods. By applying different oxygen pressures during growth, different magnitudes of oxygen defects were obtained, revealing varied magnetic and electrical properties. Our results strongly suggest that Li/Cu and oxygen nonstoichiometry in these crystals are responsible for the varied temperatures of helimagnetic ordering.

Publisher’s Note: “Convective solution transport—An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent” [J. Appl. Phys. 110, 113919 (2011)]

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