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[en] We consider a synchronous spacetime with pure trace extrinsic curvature slices, and first prove that spatial metric is Einstein if and only if the electric part of the spacetime Weyl tensor vanishes, and then that spatial metric has a constant curvature if and only if the spacetime Weyl tensor vanishes. We also consider the cases when the spacetime has a harmonic Weyl tensor and is vacuum with and without a cosmological constant. In the last case, the spacetime reduces to Minkowski, de Sitter and anti-de Sitter spacetimes.
[en] The full potential linearized augmented plane-wave (FP-LAPW) scheme of computation is used to explore the electronic and magnetic properties of Fe doped into YN. Band structure calculations show that YN is a semicon ductor with a narrow indirect band gap of 0.08 eV along Γ-X direction. Optical properties such as reflectivity, absorption coefficient are reported and are discussed on the basis of corresponding electronic structure. Spin polarized results indicate that the ground state of Y_1_-_xFe_xN (x=0.06, 0.12, 0.25) is ferromagnetic with a high moment on Fe-atom and zero moment on Y and N atoms, except in the case of 25 % doping. A discussion of the transport properties of YN and Y_1_-_xFe_xN is given in order to get insights of the Fe substitution effects.
[en] The optical response of ZnAl2X4 (X = S, Se, Te) compounds to photon energy as well as to temperature variations has been studied by using self consistent full potential linearized augmented plane wave (FP-LAPW) method with Wu-Cohen generalized gradient approximation (WC-GGA) and mBJ potential. From the energy band structures, a direct bandgap is observed in all the zinc aluminides compounds, which reduces in the order S>Se>Te. The mBJ potential brings the calculated energy band gap close to the experimental one. From the density of states, it is observed that there is a strong hybridization of Zn-d, Al-p and X-p states. The optical nature of the compounds is described in terms of the absorption coefficient, reflectivity etc. The peaks observed in the photocurrent spectra of ZnAl2Se4 corresponds to the various structures at 3.52, 3.61 and 3.717 eV observed in the calculated conductivity and absorbance spectra
[en] Sodium cooled fast breeder reactors (FBRs) may experience accidental leakage of hot liquid sodium in the inert equipment cells and reactor cavity. The leaked sodium at temperature ranging from 120degC to 550degC can come in contact with the sacrificial layer of limestone concrete. In order to study the thermal and chemical impact of sodium on the limestone concrete, five experimental runs were carried out under different test conditions simulating accident scenarios as realistically as possible. In each experimental run, a given mass of liquid sodium preheated to a specified temperature was dumped on the surface of concrete specimen housed in a test vessel with argon atmosphere. The sodium pool formed on the concrete was heated with an immersion heater to maintain the pool temperature at pre-selected level. The temperatures at various strategic locations were continuously monitored throughout the test run. Online measurement of pressure, hydrogen gas and oxygen gas in argon atmosphere was conducted. The solid samples of sodium debris were retrieved from the posttest concrete specimen by manual core drilling device for chemical analysis of reacted and un-reacted sodium. After cleaning the sodium debris, a power-drilling machine was employed to collect powder samples at regular depth interval from the concrete block floor to determine residual free and bound water. This paper presents some of the dominant thermal and chemical features related to structural safety of the concrete. Among the thermal parameters, on-set time and residence period for Energetic Thermal Transients (ETT) along with peak and average heat generation rates are evaluated. Chemical parameters such as rate and extent of water release from concrete, sodium consumption, sodium hydroxide production and sodium emission into argon atmosphere are also elucidated. Physicochemical characteristics of post-test sodium and concrete debris were investigated. Moreover spatial distribution of sodium, free and bound water in the post-test concrete blocks was studied. Preliminary analysis of test results revealed that when hot sodium at 500degC was discharged on cold concrete block, it monotonically cooled with low degree of interaction. But supply of external heat to sodium pool has triggered considerable reaction with or without occurrence of ETT phase under the given test conditions. Critical analysis of published data on large scale tests has indicated that ETT phase has emerged without in situ heating of sodium pool on limestone concrete even at initial sodium temperature of 420degC. (author)
[en] We present results of ab initio study of ordered vacancy compounds of mercury. The electronic structure, charge density, optical and transport properties of the semiconductor family HgGa_2X_4 (X=S, Se, Te) are calculated using the full potential linearized augmented plane wave method which is based on the density functional theory. A direct bandgap is observed in these compounds, which reduces in the order S>Se>Te. From the density of states it is observed that there is strong hybridization of Hg-d, Ga-d and X-p states. The optical properties show a red shift with increasing size and atomic no. of the chalcogenide atoms. We have also reported the transport properties of mercury thiogallates for the first time. The selenide compound exhibits n-type nature whereas HgGa_2S_4 and HgGa_2Te_4 show p-type behavior. The power factor and ZT for the HGS increases at low temperatures, the figure of merit is highest for HgGa_2Se_4 (1.17) at 19 K.
[en] The structural, electronic, thermal, and optical properties of cubic non magnetic CeX_3(X=Pd, Rh) compounds which crystallize in the Au_3Cu structure have been studied using the projected augmented wave (PAW) method within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange correlation potential. In this paper we have calculated the band structure which are interpreted using the density of states. The optical properties such as extinction coefficients clearly illustrate the changes in CeX_3 due to intercalation of boron. Lattice instability is observed in CePd_3B from the calculated dynamical properties.
[en] The structural, electronic, thermal, and optical properties of borides of cubic non-magnetic YX_3 (X=Rh, Pd) compounds and their borides which crystallize in the AuCu_3 structure have been studied using the density functional theory (DFT). The flat bands in the vicinity of E_F which are associated with superconductivity appear in YPd_3 and YRh_3 band structures. However, the B s-states enhance the flat band only in YRh_3B. The optical properties clearly show that boron insertion modifies the absorption and transmittance. The YX_3 alloys and their borides exhibit valuable changes in the thermopower and ZT. It is observed that the properties of the Y-X intermetallics change significantly for the Y-Rh and Y-Pd alloys and the presence of single boron atom modifies the properties to a great extent.
[en] We have studied the electronic, transport and vibrational properties of low temperature superconductor β-PdBi_2. The band manifold clearly demonstrates the 2D-layered structure with multiple gaps. The intersection of bands at E_F in the Γ-P, Γ-N directions gives rise to complicated Fermi surface topology, which contains quite complicated multiple connected sheets, as well as hole and electron-like pockets. From the low temperature specific heat, we have estimated the electron-phonon coupling constant λ_e_l_-_p_h which has a very high value of 3.66. The vibrational properties clearly illustrates that the strong coupling makes the lattice unstable. The calculated properties confirm that β-PdBi_2 is an intermediate coupling superconductor
[en] Amongst the family of anti-perovskites, the materials with general formula RM3X (R and M are metals, X=B, C, N) are isostructural with superconducting MgCNi3. From a survey of literature it was found that the Y-Pd-C-B system has one of the highest superconducting transition temperatures (∼ 23 K) amongst the intermetallic compounds. Since YPd3 with AuCu3 structure has good hydrogen storage properties, boron and carbon can also be easily incorporated at its interstitials sites. The electronic properties of borides and carbides of Y-Pd have therefore been investigated by density functional theory based full potential linearized augmented plane wave (FP-LAPW) and projector augmented wave (PAW) methods. The energy bands, Fermi surfaces and density of states as well as the transport and vibrational properties were calculated to study the stability and superconducting properties of these alloys
[en] Highlights: • Both YIn3 and YSn3 exhibit type I superconductivity (λel-ph < 1). • Smaller N(EF) and λel-ph: instability in phonon mode due to stronger Y−In/In−In bonds lowers Tc in YIn3. • Insertion of Sn-atom in YIn3 increases N(EF) and γ, with lowering of vibrational frequency. • Tc is related to the valence electron concentration, ternary YIn2Sn may exhibit intermediate superconducting Tc. - Abstract: Non-magnetic YIn3, LaIn3 and LuIn3 with a superconducting transition temperature Tc of 0.78, 0.71 and 0.24 K were investigated for superconductivity. Similarly, rare-earth compound LaSn3 has been reported to exhibit superconductivity around 6.25 K, whereas the non-magnetic YSn3 is a superconductor with Tc of 7 K. The substitution of 13th group In-atoms by 14th group Sn-atoms is seen to enhance Tc by nearly one order, although the lattice parameters increase by ∼1.0% in YSn3 compared to YIn3 compound. It is observed from the ground state properties that the slight difference in the energy band structures of YIn3, YIn2Sn and YSn3 gives rise to various complex Fermi surfaces which are multiply connected and exhibit vast differences. The Fermi level lies on a sharp peak in YSn3 which has a higher density of states N(EF), whereas Fermi level lies on the shoulder of a sharp peak in YIn3. The electron localization function (ELF) and difference charge density maps clearly illustrate the difference in the nature of bonding; the Y−Sn bonds are clearly more ionic (due to larger bond length) than Y−In bonds. These results are consistent with the Bader charges which show loss of charges from Y-atoms and a gain of charges by In/Sn atoms. The dynamical properties also clearly illustrate the difference in the nature of bonds in YX3 intermetallics. A softening of the lowermost acoustic modes is observed in YIn3, whereas all the modes in YSn3 are observed to have positive frequencies which imply its greater stability. Since λel-ph < 1, both YIn3 and YSn3 compounds exhibit type I superconductivity according to BCS theory. However, the smaller N(EF) obtained from the density of states (DOS); the electron-phonon coupling constant λel-ph obtained from the temperature dependent specific heat as well as the instability in phonon modes due to stronger Y−In and In−In bonds in YIn3 may be the cause of lower Tc and filamentary nature of superconductivity. Insertion of Sn-atom in the YIn3 lattice further consolidates the superconducting nature due to increase in N(EF) and γ (electronic component of specific heat), along with lowering of the frequency of imaginary modes from 5.6 THz to 1.5–0.6 THz. Thus Tc is directly related to the valence electron concentration and ternary YIn2Sn may exhibit intermediate superconducting transition temperature.