Role of defects in determining the magnetic ground state of ytterbium titanate.

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.

Diffusion mechanism in the sodium-ion battery material sodium cobaltate.

High performance batteries based on the movement of Li ions in Li x CoO2 have made possible a revolution in mobile electronic technology, from laptops to mobile phones. However, the scarcity of Li and the demand for energy storage for renewables has led to intense interest in Na-ion batteries, including structurally-related Na x CoO2. Here we have determined the diffusion mechanism for Na0.8CoO2 using diffuse x-ray scattering, quasi-elastic neutron scattering and ab-initio molecular dynamics simulations, and we find that the sodium ordering provides diffusion pathways and governs the diffusion rate. Above T ~ 290 K the so-called partially disordered stripe superstructure provides channels for quasi-1D diffusion, and melting of the sodium ordering leads to 2D superionic diffusion above T ~ 370 K. We obtain quantitative agreement between our microscopic study of the hopping mechanism and bulk self-diffusion measurements. Our approach can be applied widely to other Na- or Li-ion battery materials.

Self-interaction free local exchange potentials applied to metallic systems.

We extend the formalism of local exchange methods to calculate and investigate the electronic structure of metals. It is well-known that the Hartree-Fock method when applied to metals shows unphysical behaviour, however the accurate treatment of exchange via DFT's exact exchange method and using our local Fock exchange method can be used to describe metallic band structures accurately.

Hopping Time Scales and the Phonon-Liquid Electron-Crystal Picture in Thermoelectric Copper Selenide.

The suppression of transverse phonons by liquidlike diffusion in superionic conductors has been proposed as a means to dramatically reduce thermal conductivity in thermoelectric materials [H. Lui et al. Nat. Mater. 11, 422 (2012)NMAACR1476-112210.1038/nmat3273]. We have measured the ion transport and lattice dynamics in the original phonon-liquid electron-crystal Cu_{2}Se using neutron spectroscopy. We show that hopping time scales are too slow to significantly affect lattice vibrations and that the transverse phonons persist at all temperatures. Substantial changes to the phonon spectrum occur well below the transition to the superionic phase, and the ultralow thermal conductivity is instead attributed to anharmonicity.

Multiband One-Dimensional Electronic Structure and Spectroscopic Signature of Tomonaga-Luttinger Liquid Behavior in K_{2}Cr_{3}As_{3}.

We present angle-resolved photoemission spectroscopy measurements of the quasi-one-dimensional superconductor K_{2}Cr_{3}As_{3}. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ∼200 meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superconductivity in this family of compounds.

A local Fock-exchange potential in Kohn-Sham equations.

We derive and employ a local potential to represent the Fock exchange operator in electronic single-particle equations. This local Fock-exchange (LFX) potential is very similar to the exact exchange (EXX) potential in density functional theory (DFT). The practical software implementation of the two potentials (LFX and EXX) yields robust and accurate results for a variety of systems (semiconductors, transition metal oxides) where Hartree-Fock and popular approximations of DFT typically fail. This includes examples traditionally considered qualitatively inaccessible to calculations that omit correlation.

Role of disorder in the thermodynamics and atomic dynamics of glasses.

We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO2. The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higher-density crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate.

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Room temperature single-crystal diffuse scattering and ab initio lattice dynamics in CaTiSiO5.

Single-crystal diffuse scattering data have been collected at room temperature on synthetic titanite using both neutrons and high-energy x-rays. A simple ball-and-springs model reproduces the observed diffuse scattering well, confirming its origin to be primarily due to thermal motion of the atoms. Ab initio phonons are calculated using density-functional perturbation theory and are shown to reproduce the experimental diffuse scattering. The observed diffuse x-ray and neutron scattering patterns are consistent with a summation of mode frequencies and displacement eigenvectors associated with the entire phonon spectrum, rather than with a simple, short-range static displacement. A band gap is observed between 600 and 700 cm(-1) with only two modes crossing this region, both associated with antiferroelectric Ti-O motion along a. One of these modes (of Bu symmetry), displays a large LO-TO mode-splitting (562-701.4 cm(-1)) and has a dominant component coming from Ti-O bond-stretching and, thus, the mode-splitting is related to the polarizability of the Ti-O bonds along the chain direction. Similar mode-splitting is observed in piezo- and ferroelectric materials. The calculated phonon dispersion model may be of use to others in future to understand the phase transition at higher temperatures, as well as in the interpretation of measured phonon dispersion curves.

Emergence of crystal-like atomic dynamics in glasses at the nanometer scale.

The vibrational dynamics of a permanently densified silica glass is compared to the one of an α-quartz polycrystal, the silica polymorph of the same density and local structure. The combined use of inelastic x-ray scattering experiments and ab initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic length ξ of a few nanometers, corresponding to about 20 interatomic distances. In the microscopic regime the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the elastic modulus of the glass presents spatial heterogeneities of an average size a ~ ξ/2 π.

Influence of deuteration on lithium acetate dihydrate studied by inelastic X-ray scattering, density functional theory, thermal expansion, elastic and thermodynamic measurements.

The influence of deuteration on the properties of lithium acetate dihydrate has been investigated by thermal expansion measurements, ultrasound spectroscopy and calorimetry. Inelastic X-ray scattering has been employed to investigate if the low temperature structural phase transition can be detected by a change in the vibrational spectrum. Density functional theory, DFT, calculations have been employed to complement the experimental investigations. The thermal expansion coefficients and the specific heat of the deuterated compound differ significantly from the protonated form. The differences in the elastic stiffness coefficients are just above the detection limit of the technique employed here. Temperature dependent inelastic X-ray spectroscopic measurements show no significant change of the vibrational spectrum when crossing the transition temperature. The DFT calculations show that the methyl group dynamics are best described in the framework of coupled rotators of opposing methyl groups. One of the coupled rotational modes corresponds to a hindered rotator with a barrier of 15 meV, while the other is a free rotator.

3D imaging of the Fermi surface by thermal diffuse scattering.

We use thermal diffuse scattering of x rays to visualize the lens-shaped portions of the Fermi surface in metallic zinc. Our interpretation of the nature of the observed scattered intensity anomalies is supported by the incorporation of inelastic x-ray scattering measurements as well as ab initio calculations of the electronic structure and lattice dynamics. Our work demonstrates that thermal diffuse scattering complements well-established techniques and is a powerful tool in its own right for studying the shape of the Fermi surface through the associated electron-phonon coupling.

Spin singlet formation in MgTi2O4: evidence of a helical dimerization pattern.

The transition-metal spinel MgTi2O4 undergoes a metal-insulator (M-I) transition on cooling below T(M-I)=260 K. A sharp reduction of the magnetic susceptibility below T(M-I) suggests the onset of a magnetic singlet state. Using high-resolution synchrotron and neutron powder diffraction, we have solved the low-temperature crystal structure of MgTi2O4, which is found to contain dimers with short Ti-Ti distances (the locations of the spin singlets) alternating with long bonds to form helices. Band structure calculations based on hybrid exchange density functional theory show that, at low temperatures, MgTi2O4 is an orbitally ordered band insulator.

Residual visual functions of guide dog owners in the UK.

Numbers of guide dog owners (GDOs) in the United Kingdom reached 4700 by the end of 1998. Despite this growing trend, little is known about the nature of their visual loss. This paper reports the results of a national three-centre investigation into the residual visual functions and ophthalmic conditions of guide dog owners. Random samples of GDOs (Scotland n = 82, England n = 77, and Northern Ireland n = 87) underwent a detailed visual analysis and interview. GDOs had an overall median age of 53 years. Nationally, they make up just 2.4% of the registered blind population. All GDOs were found to have profound loss of visual acuity, contrast sensitivity or visual fields, but only 43% were totally blind. GDOs in Scotland retained higher levels of residual visual function than those in the other two regions. The main causes of visual loss were congenital and early onset degenerative eye disease (retinitis pigmentosa 18%, optic atrophy 10%). Results taken in conjunction with epidemiological registration trends suggest that the past growth in numbers of GDOs is unlikely to be sustainable. Implications for mobility service providers are discussed. It is suggested that increased optometric input and a multidisciplinary approach could assist present and potential guide dog owners.

Correctable visual impairment in stroke rehabilitation patients.

BACKGROUND: after stroke, visual impairment may exacerbate the impact of other impairments on overall disability and negatively influence rehabilitation. OBJECTIVE: to examine the visual status of patients after stroke and determine whether this can be improved by simple interventions. DESIGN: prospective study. SETTINGS: stroke rehabilitation unit in a Belfast teaching hospital. SUBJECTS: 77 consecutive patients admitted for rehabilitation after acute stroke. METHODS: full optometric and ophthalmic assessment within 2 weeks of admission. RESULTS: of 70 patients with glasses, 19 did not have their glasses in hospital before prompting and 18 had glasses in unacceptable condition. Twenty patients had impaired visual acuity (6/12 or worse) with existing glasses (if helpful); 11 of these improved to better than 6/12 with refractive correction. CONCLUSIONS: stroke professionals need to enquire about patients' spectacles and assess their condition. Patients with reduced visual acuity in the absence of significant non-refractive disease should be referred to an optometrist: in this series 14% of patients had visual impairment which benefited from refractive correction.

Ophthalmic and visual profile of guide dog owners in Scotland.

BACKGROUND/AIMS: Out of an estimated 90,000 visually impaired people in Scotland, 509 make use of a guide dog. Initial research in Northern Ireland suggests that the ophthalmic profile of guide dog owners (GDOs) is highly specific. The aim of this study was to compare the ophthalmic and visual characteristics of Scottish GDOs with other groups of visually impaired people. METHODS: A random sample of GDOs from central and northern Scotland (n = 82) underwent a detailed assessment of residual vision (distance and near acuity, visual fields, contrast, and glare sensitivity). Comparative data were obtained from two populations of visually impaired non-GDOs-one group attending hospital ophthalmic and low vision clinics (n = 50) the other social services rehabilitation clients (n = 35). All participants completed a questionnaire to elicit ophthalmic history, age, and registration details. RESULTS: GDOs were found to be significantly younger and more profoundly visually impaired than non-GDOs. The main causes of visual impairment were retinitis pigmentosa (23%), optic atrophy (15%), and retinopathy of prematurity (7%). Ninety nine per cent of GDOs were registered blind and had been visually impaired for an average of 39 years. Only 31% were totally blind. CONCLUSION: GDOs represent a unique minority of the visually impaired population. Epidemiological registration trends would suggest that the numbers of young profoundly visually impaired people are unlikely to increase relative to their elderly counterparts. This has implications on the future demand for guide dog ownership.

Varifocal contact lenses for the correction of presbyopia.

A Nissel +1.75 D PS45 lens' label power seems to be made up of about +0.75 D distance and +0.75 D near addition. There are individual variations which will depend on pupil size, corneal shape, lens position and ocular aberrations. When a +0.75 Bausch & Lomb U3 lens was used as a control, there was also a significant increase in 'accommodation' measured (+0.75 D), but this could not be explained. When two other lenses with an aspheric surface were studied, they also showed a near addition effect. In this supplementary study the changes were explained by changes in spectacle accommodation. A lens with two spherical surfaces did not show the expected increase in spectacle accommodation. Astigmatism, as a general rule, was not corrected by the PS45 lens, although the very occasional case did show some reduction. Visual acuity and contrast sensitivity at 8 cpd and 3 cpd were not significantly affected when compared to the control lens.