Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3

We report first principles density functional perturbation theory calculations and inelastic neutron scattering measurements of the phonon density of states, dispersion relations and electromechanical response of PbTiO3, BaTiO3 and SrTiO3. The phonon density-of-states of the quantum paraelectric SrTiO3 is found to be fundamentally distinct from that of ferroelectric PbTiO3 and BaTiO3 with a large 70-90 meV phonon band-gap. The phonon dispersion and electromechanical response of PbTiO3 reveal giant anisotropies. The interplay of covalent bonding and ferroelectricity, strongly modulates the electromechanical response and give rise to spectacular signatures in the phonon spectra. The computed charge densities have been used to study the bonding in these perovskites. Distinct bonding characteristics in the ferroelectric and paraelectric phases give rise to spectacular vibrational signatures. While a large phonon band-gap in ATiO3 perovskites seems a characteristic of quantum paraelectrics, anisotropy of the phonon spectra correlates well with ferroelectric strength. These correlations between the phonon spectra and ferroelectricity, can guide future efforts at custom designing still more effective piezoelectrics for applications. These results suggest that vibrational spectroscopy can help design novel materials.Comment: 11 pages, 4 color figures and 2 Table

The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?

We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the far-infrared region using Migdal-Eliashberg theory and found it inconsistent with standard electron-phonon coupling: Whereas the superconducting state data could be explained using moderate coupling, \lambda=0.7, the normal state properties indicate \lambda \le 0.2. We have found that such behaviour could be understood using a simple model consisting of weak standard electron-phonon coupling plus weak coupling to an unspecified high energy excitation near 0.4 eV. This model is found to be in general agreement with the reflectivity data, except for the predicted superconducting gap size. The additional high energy excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys. Rev.

Phase diagram of the one-dimensional extended attractive Hubbard model for large nearest-neighbor repulsion

We consider the extended Hubbard model with attractive on-site interaction U and nearest-neighbor repulsions V. We construct an effective Hamiltonian H_{eff} for hopping t<<V and arbitrary U<0. Retaining the most important terms, H_{eff} can be mapped onto two XXZ models, solved by the Bethe ansatz. The quantum phase diagram shows two Luttinger liquid phases and a region of phase separation between them. For density n<0.422 and U<-4, singlet superconducting correlations dominate at large distances. For some parameters, the results are in qualitative agreement with experiments in BaKBiO.Comment: 6 pages, 3 figures, submitted to Phys. Rev.

4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)

We have performed inelastic magnetic neutron scattering experiments on La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low energies. In all samples we find at high temperatures a quasielastic line (Lorentzian) with a line width which decreases on lowering the temperature. The temperature dependence of the quasielastic line width Gamma/2(T) can be explained with an Orbach-process, i.e. a relaxation via the coupling between crystal field excitations and phonons. At low temperatures the Nd-4f magnetic response S(Q,omega) correlates with the electronic properties of the CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line vanishes below 80 K and an inelastic excitation occurs. This directly indicates the splitting of the Nd3+ ground state Kramers doublet due to the static antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18 superconductivity is strongly suppressed. In these compounds we observe a temperature independent broad quasielastic line of Gaussian shape below T about 30 K. This suggests a distribution of various internal fields on different Nd sites and is interpreted in the frame of the stripe model. In La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.

The Hydration Structure at Yttria-Stabilized Cubic Zirconia (110)-Water Interface with Sub-Angstrom Resolution

The interfacial hydration structure of yttria-stabilized cubic zirconia (110) surface in contact with water was determined with ~0.5&amp;#8201;&amp;Aring; resolution by high-resolution X-ray reflectivity measurement. The terminal layer shows a reduced electron density compared to the following substrate lattice layers, which indicates there are additional defects generated by metal depletion as well as intrinsic oxygen vacancies, both of which are apparently filled by water species. Above this top surface layer, two additional adsorbed layers are observed forming a characteristic interfacial hydration structure. The first adsorbed layer shows abnormally high density as pure water and likely includes metal species, whereas the second layer consists of pure water. The observed interfacial hydration structure seems responsible for local equilibration of the defective surface in water and eventually regulating the long-term degradation processes. The multitude of water interactions with the zirconia surface results in the complex but highly ordered interfacial structure constituting the reaction front.ope

Defects of Tensioned Membrane Structures (TMS) In Tropics

The wide use of tensioned membrane structures (TMS) becomes prominent in many designs because of its aesthetic, ergonomic, and economical nature. Recently, TMS has been applied in the tropics with success, yet defects specific to this region have received little attention. Through a questionnaire survey of 890 uses and technicians of TMS in three different areas in Malaysia, this study identified the most frequently occurring TMS defects in the tropics, such as "deterioration of roof coatings", "corrosion or fatigue in fixings", "fungal decay, mould growth, and dirt in membrane", "corrosion in anchor cables", and "degradation of fabrics". These defects are quite different from those in other climatic zones. The top five causes of TMS defects in the tropics are weather, aging, design, construction/installation, and material selection. However, only "corrosion or fatigue in fixings" is a significant predictor for "deterioration of roof coatings". While this is not a causal effect, a practical implication is that TMS maintenance workers do not have to climb up to the roof to check the deterioration in coating but only need to predict the deterioration through the corrosion levels of the fixings. Further, seven countermeasures for TMS in tropic are recommended. This study is the first comprehensive study examining tensioned membrane structure defects in the tropics

Neutron scattering study of Kondo lattice antiferromagnet YbNiSi3

The Kondo lattice antiferromagnet YbNiSi3 was investigated by neutron scattering. The magnetic structure of YbNiSi3 was determined by neutron diffraction on a single-crystalline sample. Inelastic scattering experiments were also performed on a pulverized sample to study the crystalline electric field (CEF) excitations. Two broad CEF excitations were observed, from which the CEF parameters were determined. The temperature dependence of the magnetic susceptibility chi and the magnetic specific heat Cmag were calculated using the determined CEF model, and compared with previous results.Comment: 10 pages, 7 figures,submitted to JPS

Diagnostic performance of the Minimal Eating Observation and Nutrition Form - Version II (MEONF-II) and Nutritional Risk Screening 2002 (NRS 2002) among hospital inpatients - a cross-sectional study

<p>Abstract</p> <p>Background</p> <p>The usefulness of the nutritional screening tool Minimal Eating Observation and Nutrition Form - Version II (MEONF-II) relative to Nutritional Risk Screening 2002 (NRS 2002) remains untested. Here we attempted to fill this gap by testing the diagnostic performance and user-friendliness of the MEONF-II and the NRS 2002 in relation to the Mini Nutritional Assessment (MNA) among hospital inpatients.</p> <p>Methods</p> <p>Eighty seven hospital inpatients were assessed for nutritional status with the 18-item MNA (considered as the gold standard), and screened with the NRS 2002 and the MEONF-II.</p> <p>Results</p> <p>The MEONF-II sensitivity (0.61), specificity (0.79), and accuracy (0.68) were acceptable. The corresponding figures for NRS 2002 were 0.37, 0.82 and 0.55, respectively. MEONF-II and NRS 2002 took five minutes each to complete. Assessors considered MEONF-II instructions and items to be easy to understand and complete (96-99%), and the items to be relevant (87%). For NRS 2002, the corresponding figures were 75-93% and 79%, respectively.</p> <p>Conclusions</p> <p>The MEONF-II is an easy to use, relatively quick and sensitive screening tool to assess risk of undernutrition among hospital inpatients. With respect to user-friendliness and sensitivity the MEONF-II seems to perform better than the NRS 2002, although larger studies are needed for firm conclusions. The different scoring systems for undernutrition appear to identify overlapping but not identical patient groups. A potential limitation with the study is that the MNA was used as gold standard among patients younger than 65 years.</p