Enhancement of magnetic ordering temperature in iron substituted ytterbium manganate (YbMn1−xFexO3)

Oxides of the type YbMn_1−xFe_xO₃; x≤0.3 showing multiferroic behavior have been synthesized by the solid state route. These oxides crystallize in the hexagonal structure known for the parent YbMnO₃ with the c/a ratio increasing with Fe substitution. The distortion of the MnO₅ polyhedra (tbp) decreases and the Mn-O-Mn bonds in the a-b plane become shorter with Fe-substitution. Magnetic ordering is observed from the low temperature neutron diffraction study. The compounds were found to be antiferromagnetic and the ordering temperature T_N increased from 82 K for pure YbMnO₃ to 95 K for YbMn_0.7Fe_0.3O₃. Variable temperature dielectric measurements (15-110 K) show an anomaly in the dielectric constant at temperatures close to the antiferromagnetic ordering temperature for all the compositions, showing a unique correlation between the magnetic and electric field. The increase in the ordering temperature in YbMn_1−xFe_xO₃ is explained on the basis of increase in covalence of Mn/Fe-O-Mn/Fe bonds (shorter) with iron substitution.     

Highly Uniform Nano and Mesostructures of Silica Obtained by Reverse Micellar and Hydrothermal Methods

Silica nanoparticles with controlled size and morphology and a high degree of monodispersity have been synthesized using single and double microemulsion systems employing a cationic surfactant (CTAB) and a non-ionic surfactant (Tergitol). Depending on the type of surfactant aggregate acting as templates, very different morphologies were formed. Nanospindles of silica of ~200–300 nm in length and nanofibres could be obtained by suitably controlling the reverse micellar methodology. The hydrothermal method resulted in highly porous and uniform spheres of diameter ~300 nm which appears to be formed from aggregates of small silica nanoparticles of size ~10 nm. The surface area of the nanoparticles (119 m²/g) was found to be much higher than the corresponding bulk SiO₂ (500–600 nm) which had a surface area of 22 m²/g. The hydrothermally obtained spheres of size ~300 nm show a surface area of 35 m²/g. In honor of Prof. C.N.R. Rao, FRS, on his 75th birth anniversary.     

Pressure effects on the superconducting transition of ytterbium doped Ce0.6Yb0.4FeAsO0.9F0.1

The Effect of pressure on the superconducting transition temperature of Yb doped Ce_0.6Yb_0.4FeAsO_0.9F_0.1 has been investigated for the first time using resistivity and magnetization studies. Increase in chemical pressure by substitution of smaller Yb³⁺ ions in place of Ce³⁺ ions results in a significant enhancement of T_c from 38 K (Yb free) to 47 K (40% Yb). Enhancement in T_c with external pressure has been observed for this compound up to a maximum value of T_c = 48.7 K at 1 GPa, beyond which T_c starts decreasing monotonously. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.     

Novel 1122 thallium cuprates showing high Tc superconductivity: TlCa1-xY,Ba2Cu2O y,T,1-xPb x CaSr2Cu2O y and TlCa0.5Ln0.5Sr2Cu2O y

Substitution of Ca by Y in TlCaBa₂Cu₂O _y does not favour superconductivity, but substitution of Tl by Pb or of Ca by Ln (Ln = Y or rare earth) in TlCaSr₂Cu₂O _y results in high T_c superconductivity (T_c π 60–90 K). TlCa_1-xLn _x Sr₂Cu₂O _y is a new series of high T_c superconductors, but the x = 0.0 composition does not exhibit bulk superconductivity.     

Regge phenomenology of inclusive reactions

This project is devoted to updating the Regge phenomenology of inclusive reaction and correlation, with the help of the low p_T data acquired over the last 5 years or so. The analyses over the fragmentation region are largely based on the Mueller-Regge formalism. For the central cross-section and correlation data, however, the Mueller-Regge formalism is seen to be inadequate; and the analyses are largely based on the multiperipheral cluster model and the Gribov calculus. The effort has been to make the individual sections self-contained, at the cost of some duplication. The important gaps remaining in the data, as also in the theoretical interpretation of the available data, are brought out at the end of the relevant sections.     

InclusiveK *'s and∑ *'s production in 360 GeV/cpp interactions using the European hybrid spectrometer

We have studied the inclusive production of strange meson and baryon resonances in 360 GeV/cpp interactions. Cross sections forK ^*+(892) andK ^*?(892) are estimated to be 4.42±0.62 mb and 2.54±0.47 mb respectively and those for∑ ⁺(1385) and∑ ^?(1385) are 0.67±0.11 mb and 0.26±0.07 mb respectively. Longitudinal and transverse momentum distributions forK ^*(892) and Σ(1385) are presented, as well as for their decay productsK ⁰ and Λ. An estimation is given on theK ^*(1430) and∑ ^*(1915) productions.     

Spectroscopic Study of Cerium-Doped Silica Gel Monoliths and Their Densified Derivatives

Cerium is exploited as a probe cation for elucidating the structure of an alkoxide-derived silica gel and its progressive evolution to a glass network as a function of heat-treatment up to 1000°C. At intermediate temperatures, the host structure exhibits inhomogeneity due to insufficient formation of siloxane bonds, which is reflected by at least two different sites and co-ordination spheres (termed “high” and “low” water ligation) for cerium. This is proved by the response of the gels heated up to 700°C to rehydration. Further formation of Si−O−Si network (900°C) leads to the destruction of the “high water” sites of cerium and progression towards a glassy structure. It is, however, only after heat-treatment at 1000°C that a dense silica glass network, not responding to rehydration, is finally obtained with cerium ions embedded in it.