Synthesis and crystal growth

Abstract:

The long-range three-dimensional ordering of Na+ ions was studied in a sample of composition Na0.75CoO2 using single-crystal neutron diffraction. Large-scale numerical simulations reveal the ordering principle for this system, the formation of multi-vacancy charged droplets then order long range, and the structure factors from these defect clusters are in good agreement with the observed neutron diffraction intensities. The electrostatic potential is found to be the dominant factor in determining the sodium ordering and its associated distortion field. The superstructures induce a periodic potential in the CoO2, giving potential wells that are larger than the single-particle hopping frequency and so able to localize holes. The results readily explain many of the observed electrical and magnetic properties, including the three dimensionality of the magnetic excitations. © 2006 Elsevier B.V. All rights reserved.

Abstract:

The incommensurate magnetic excitations of spin-charge ordered La2NiO4.11 were studied by inelastic neutron scattering. With increasing energy up to similar to 20meV the maximum intensity of the spin excitations is observed to shift slightly towards the 2D antiferromagnetic wave vector (1/2, 1/2). This asymmetry in the magnon dispersion about the incommensurate wave vector is a similar effect, though less marked, to what has been observed in the layered cuprate superconductors. (c) 2006 Elsevier B.V. All rights reserved.

Abstract:

We present the results of muon-spin relaxation (μ+ SR) measurements on the kagome staircase compounds Ni3 V2 O8 and Co3 V2 O8. The magnetic behavior of these materials may be described in terms of two inequivalent magnetic ion sites, known as spine sites and cross-tie sites. Our μ+ SR results allow us to probe each of these sites individually, revealing the distribution of the local magnetic fields near these positions. We are able not only to confirm the magnetic structures of the various phases proposed on the basis of bulk measurements but also to give an insight into the temperature evolution of the local field distribution in each phase. © 2007 The American Physical Society.

Abstract:

Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

Abstract:

We report the complex refractive index of La5/3Sr1/3NiO4 over the terahertz frequency range, obtained using time-domain spectroscopy. Negligible change in the complex refractive index with magnetic flux densities up to 6T was seen, while changes were observed as the lattice temperature was increased from 1.5 K to the charge-ordering temperature at 220 K. The terahertz frequency response therefore originates from the dielectric function rather than the magnetic permeability.