S. M. Rao

Simultaneous measurement of pressure evolution of crystal structure and superconductivity in FeSe0.92 using designer diamonds

Simultaneous high-pressure X-ray diffraction and electrical resistance measurements have been carried out on a PbO-type ?-FeSe0.92 compound to a pressure of 44?GPa and temperatures down to 4?K using designer diamond anvils at synchrotron source. At ambient temperature, a structural phase transition from a tetragonal (P4/nmm) phase to an orthorhombic (Pbnm) phase is observed at 11?GPa and the Pbnm phase persists up to 74?GPa. The superconducting transition temperature (TC) increases rapidly with pressure reaching a maximum of ?28?K at ?6?GPa and decreases at higher pressures, disappearing completely at 14.6?GPa. Simultaneous pressure-dependent X-ray diffraction and resistance measurements at low temperatures show superconductivity only in a low-pressure orthorhombic (Cmma) phase of the ?-FeSe0.92. Upon increasing pressure at 10?K near TC, crystalline phases change from a mixture of orthorhombic (Cmma) and hexagonal (P63/mmc) phases to a high-pressure orthorhombic (Pbnm) phase near 6.4?GPa where TC is maximum.

Investigation of the valence states of Fe and Co in Fe1−xCoxOy (0<x<1) thin films by x-ray absorption spectroscopy

The unusual magnetic properties of thin films of Fe1−xCoxOy {0≤x≤1} ferrite solid solutions have been explained through an investigation of their electronic structure. The x-ray absorption near-edge structure (XANES) spectra of the Fe L3-edge and K-edges show that Co replaces Fe3+ in the tetrahedral (Td) sites up to x = 0.34 and octahedral (Oh) sites from 0.39<x<0.89. The Co L3-edge and O K-edge results indicate mixed valence states of Co as it replaces Fe at dissimilar sites. From the results, it is concluded that the magnetic properties of the thin films of the spinel Fe1−xCoxOy arise from the Fe3+ at the Td sites. The replacement of Fe3+ and Fe2+ by Co at the Oh sites results in a crossover to the rocksalt structure and a loss of the magnetic properties.

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...

Investigation of the unoccupied states in Sr2YRuO6 single crystals doped with Cu

The electronic structure single crystals of the new double perovskite Sr2YRu1−xCuxO6 (x = 0–0.05) has been investigated by x-ray absorption near-edge structure (XANES) spectroscopy. The Cu K-edge spectra show that Cu is incorporated as Cu1+ and Cu2+ in the crystals grown in air and in an oxygen atmosphere and Ru5+ is seen in the Ru K-edge spectra. The O K-edge and the Ru L3-edge spectra indicate an increase in the unoccupied states (hole concentration) as x is increased, accompanied by a lattice distortion from an increased O–Ru–O bond angle. These changes observed even with a small Cu addition are interpreted to result from the Cu 3d–O 2p hybridization as Cu replaces Ru in the octahedral sites.

Magnetic and superconducting properties of single crystals of Sr2HoRu1−xCuxO6 grown from high temperature solutions

Single crystals of Sr2HoRu1−xCuxO6 (with x=0–0.2), measuring 2–3mm across have been grown from PbO–PbF2 based solutions in the temperature range of 1250–1150°C. The crystals exhibit octahedral morphology and belong to the monoclinic space group P21∕n. While Sr2HoRuO6 is found to be antiferromagnetic with weak ferromagnetism below 30K, the solid solutions containing Cu exhibit a diamagnetic transition at 31K which increases in magnitude and temperature with increasing Cu. Through a correlation of magnetic and calorimetric properties, these crystals are concluded to be spin-glass superconductors.

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.