Role of polar nanoregions with weak random fields in Pb-based perovskite ferroelectrics

In relaxor ferroelectrics, the role of randomly orientated polar nanoregions (PNRs) with weak random fields (RFs) is one of the most puzzling issues of materials science. The relaxation time of polarization fluctuations of PNRs, which manifests themselves as a central peak (CP) in inelastic light scattering, is the important physical quantity to understand the dynamics of PNRs. Here, the angular and temperature dependences of depolarized and polarized CPs in 0.44Pb(Mg1/3Nb2/3)O3-0.56PbTiO3 single crystals with weak RFs have been studied by Raman and Brillouin scattering, respectively. The CPs observed in Raman scattering show the very clear angular dependence which is consistent with the local tetragonal symmetry. It is different from the well-known local rhombohedral symmetry with strong RFs for Pb(Mg1/3Nb2/3)O3. In Brillouin scattering, depolarized and polarized CPs show two relaxation processes corresponding to transverse and longitudinal fluctuations of PNRs. The remarkable slowing down towards the Curie temperature was observed for transverse fluctuations in local tetragonal symmetry

High pressure Ca-VI phase between 158-180 GPa: Stability, electronic structure and superconductivity

We have performed ab initio calculations for new high-pressure phase of Ca-VI between 158-180 GPa. The study includes elastic parameters of mono- and poly-crystalline aggregates, electronic band structure, lattice dynamics and superconductivity. The calculations show that the orthorhombic Pnma structure is mechanically and dynamically stable in the pressure range studied. The structure is superconducting in the entire pressure range and the calculated Tc (~25K) is maximum at ~172 GPa, where the transfer of charges from 4s to 3d may be thought to be completed.Comment: 8 pages, 4 figures; PACS number(s): 74.70.Ad, 62.20.de, 71.20.-b, 74.20.Pq, 74.25.Kc, 74.62.Fj; Keywords: Calcium; High pressure; Electronic band structure; Phonon spectrum; Elastic constants; Superconducto

Electric field and aging effects of uniaxial ferroelectrics Sr x Ba1-x Nb2O6 probed by Brillouin scattering

This study was supported in part by the Marubun Research Promotion Foundation and JSPS KAKENHI Grant Number JP17K05030.Static and dynamic heterogeneity of disordered system is one of the current topics in materials science. In disordered ferroelectric materials with random fields, dynamic polar nanoregions (PNRs) appear at Burns temperature and freeze into nanodomain state below Curie temperature (T C). This state is very sensitive to external electric field and aging by which it gradually switches into macrodomain state. However, the role of PNRs in such states below T C is still a puzzling issue of materials science. Electric field and aging effects of uniaxial ferroelectric Sr x Ba1-x Nb2O6 (x = 0.40, SBN40) single crystals were studied using Brillouin scattering to clarify the critical nature of PNRs in domain states below T C. On field heating, a broad anomaly in longitudinal acoustic (LA) velocity at low temperature region was due to an incomplete alignment of nanodomains caused by the interaction between PNRs. A sharp anomaly near T C was attributed to the complete switching of nanodomain to macrodomain state owing to the lack of interaction among PNRs. After isothermal aging below T C, the noticeable increase of LA velocity was observed. It was unaffected by cyclic temperature measurements up to T C, and recovered to initial state outside of a narrow temperature range above and below aging temperature.Japan Society for the Promotion of Scienc

New MAX phase superconductor Ti2GeC: A first-principles study

This is the first DFT-based first-principles prediction of the detailed optical and thermodynamic properties, including Vickers hardness and Fermi surface of 211 MAX phase Ti2GeC for which superconductivity (Tc~ 9.5 K) was reported very recently. The calculated structural properties are in excellent agreement with experiments. Our results on elastic parameters indicate a slight elastic anisotropy and brittleness of the compound. The chemical bonding is seen to be a combination of covalent, ionic and metallic nature. The rather stronger covalent bonding is responsible for its high Vickers hardness of 11.6 GPa. The investigated Fermi surface is formed mainly by the low-dispersive bands, which should be responsible for the presence of superconductivity in Ti2GeC. All the optical properties are evaluated and analyzed for two different polarization directions of incident photon. The temperature and pressure dependence of primitive cell volume, thermal expansion coefficient, specific heats, bulk modulus, and Debye temperature of Ti2GeC are derived from the quasi-harmonic Debye model with phononic effect and the various implications are discussed in details

New Superconducting RbFe2As2: A First-principles Investigation

RbFe2As2 has recently been reported to be a bulk superconductor with Tc = 2.6 K in the undoped state, in contrast to undoped BaFe2As2 with a magnetic ground state. We present here the results of the first-principles calculations of the structural, elastic and electronic properties for this newest superconductor and discuss its behaviour in relation to other related systems. Keywords: RbFe2As2, Electronic structure; Elastic constant; Superconductivity. PACS: 74.70.Dd, 74.10.+v, 74.20.Pq, 75.25.LdComment: 6 pages, 2 figures, 3 table

Electric field and aging effects of uniaxial ferroelectrics Sr x Ba1−x Nb2O6 probed by Brillouin scattering

Static and dynamic heterogeneity of disordered system is one of the current topics in materials science. In disordered ferroelectric materials with random fields, dynamic polar nanoregions (PNRs) appear at Burns temperature and freeze into nanodomain state below Curie temperature (TC). This state is very sensitive to external electric field and aging by which it gradually switches into macrodomain state. However, the role of PNRs in such states below TC is still a puzzling issue of materials science. Electric field and aging effects of uniaxial ferroelectric Sr x Ba1−x Nb2O6 (x = 0.40, SBN40) single crystals were studied using Brillouin scattering to clarify the critical nature of PNRs in domain states below TC. On field heating, a broad anomaly in longitudinal acoustic (LA) velocity at low temperature region was due to an incomplete alignment of nanodomains caused by the interaction between PNRs. A sharp anomaly near TC was attributed to the complete switching of nanodomain to macrodomain state owing to the lack of interaction among PNRs. After isothermal aging below TC, the noticeable increase of LA velocity was observed. It was unaffected by cyclic temperature measurements up to TC, and recovered to initial state outside of a narrow temperature range above and below aging temperature

Pilot study on the distribution of caseous lymphadenitis in Korean native goats and the relationship between sex and age in disease occurrence

Caseous lymphadenitis (CLA) caused by Corynebacterium pseudotuberculosis is a chronic disease that affects goats. The Korean native goat (KNG) is the most popular goat breed raised in Korea. In this study, the distribution of CLA in the KNG population and the relevance of sex and age in disease development were determined. From March 2020 to February 2021, 1,177 KNGs from 110 farms were assessed. The distribution of CLA in animals was determined by a veterinary inspector who performed gross examinations of goat carcasses and confirmed diagnoses via polymerase chain reaction and bacteriological examination. The CLA detection rate in the KNG population was 19.80%, and more than half of the farms were affected by the disease (56.36%). A statistically significant difference was observed among the sex groups, with castrated males (13.98%) having the lowest detection rate, followed by intact males (22.48%) and females (24.09%), suggesting that castration has a positive effect on reducing the risk of CLA (p < 0.05). In terms of age groups, an increased detection rate of 28.16% was observed in the late adult (>2 years) group. Phylogenetic analysis indicated that the RNA polymerase beta subunit-encoding gene can effectively differentiate biovar ovis from biovar equi and can be used for further epidemiological studies of C. pseudotuberculosis. This is the first nationwide surveillance study of CLA distribution to confirm the continuous occurrence of CLA in Korean goat farms. Future studies should include risk factor analyses of CLA based on herd levels to prevent and control this disease in Korea

New superconductor (Na0.25K0.45) Ba3Bi4O12: A first-principles study

(Na_0.25K_0.45)Ba₃Bi₄O₁₂, having an <em>A</em>-site-ordered double perovskite structure, with a maximum <em>T_<em>c</em></em> ∼ 27 K has very recently been discovered through hydrothermal synthesis at 593 K. The structural, elastic, electronic, and thermal properties of the new synthesized compound have been investigated theoretically. Here we have employed the pseudo-potential plane-wave (PP-PW) approach based on the density functional (DFT) theory, within the generalized gradient approximation (GGA). The elastic constants (<em>C_ij</em>), Pugh‘s ratio, Cauchy‘s pressure and other elastic parameters are derived first time. We have discussed the bonding nature in the light of the electronic valence charge density. Both electron and hole-like Fermi surfaces are present in the compound under study which indicate the multiple-band nature of (Na_0.25K_0.45)Ba₃Bi₄O₁₂. The compound is indicated to be a strongly coupled superconductor which is based on the estimated e-ph coupling constant. The thermodynamic properties at elevated temperature and pressure are calculated and analyzed for the first time by using quasi-harmonic model which includes vibrational term