B. H. Mok

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...

Study of the magnetic interactions in Ba2PrRu1−xCuxO6 using neutron powder diffraction

High-resolution neutron powder diffraction has been used to determine the crystal and magnetic structures of Ba2PrRuO6 and Ba2PrRu0.90Cu0.10O6. Both materials are 2116 distorted double perovskites and the magnetic structure, which develops below ∼110 K, consists of two interpenetrating Type I antiferromagnetic sublattices of the Ru5+ and Pr3+ ions. When saturated, the moments are ∼1.9 μB for Ru5+ and ∼1.1 μB for Pr3+ in the c-direction, though copper doping introduces an extra ab-component of ∼0.65 μB. From the study of variable temperature neutron diffraction data the principal magnetic interactions in the materials have been determined. Although an overall antiferromagnetic structure is adopted, the Ru–O–Pr ferromagnetic interaction is a factor of ∼4 times stronger than the Ru–O–O–Ru antiferromagnetic interaction.

Elastic stiffness of single-crystalline FeSe measured by picosecond ultrasonics

We report investigations on the elasticity of superconducting FeSe using picosecond ultrasonic technique. The tetragonal (001) FeSe thin film, deposited on a processed SrTiO3 substrate by the pulsed laser deposition, exhibits distinct c-axis preferred orientation and single-crystalline features as a result of the x-ray diffraction. The high-quality crystallinity thus enables quantitative examinations of anisotropic stiffness coefficients (C33) of FeSe, correlating to the interatomic interaction in the simplest iron-based superconductor. Our experiment indicates a room-temperature C33 of 40.9 ± 0.4 GPa and material stiffening of 4.3% at low temperatures, which can be explained by the weakening of anharmonic phonon–phonon interactions.

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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Magnetic Ordering in the Mixed Ruthenium-Copper Oxide Ba2Pr(Ru1−xCux)O6 System

We have synthesized a series of mixed ruthenium-copper oxides of general formulae Ba2PrRu1−xCuxO6 with x=0, 0.05, 0.1, 0.15 by high temperature solid-state reaction process. From the SQUID measurements, we observed a strong diamagnetic response on zero-field-cooling (ZFC), meanwhile a ferromagnetic-like transition under field cooling, both at about 11 K. In addition, we also observed a broad transition at about 125 K by SQUID measurements. Electrical resistivity measurements also show a sharp resistive drop at about 150 K, suggesting the presence of a magnetic transition. High intensity and high resolution neutron diffraction studies further confirm the magnetic phase transition at ∼100 K into a Type I antiferromagnetic structure.