A. S. Ghosh

Polar octahedral rotations, cation displacement and ferroelectricity in multiferroic SmCrO3

Our thorough synchrotron diffraction studies provide a clue on the origin of ferroelectricity in SmCrO3. Careful observation demonstrates that polar order develops in the paramagnetic state. Rietveld refinement of the diffraction data confirms that emergence of polar order is correlated with the structural transformation from centrosymmetric Pbnm to non-centrosymmetric Pna21 space group of the distorted orthorhombic structure. Rotations of polar CrO6 octahedra and Sm displacement are proposed to be correlated with the emergence of polar order, which is extended over a wide temperature range and increases gradually with decreasing temperature. This is consistent with the relaxor behavior as evident from the frequency-dependent dielectric response satisfying the Vogel-Fulcher law. A non-collinear to collinear spin transformation is suggested well below the spin reorientation transition. Appearance of ferroelectricity without any correlation to the antiferromagnetic order in SmCrO3 suggests a new class of ferroelectricity. All-electron full-potential first-principles calculation demonstrates significant Sm-Cr hybridization near the Fermi level, which substantiates the experimental findings.

Atypical multiferroicity of HoCrO3 in bulk and film geometry

We report ferroelectricity in antiferromagnetic HoCrO3 with a reasonably large value of spontaneous electric polarization (∼0.32 μC cm−2 at 10 K) from the measurement of pyroelectric current. The appearance of ferroelectricity is further confirmed in the film geometry. Intriguingly, the onset of polar order is observed at a significantly higher temperature (∼240 K) than the Neel temperature (TN = 142 K). The survival of remanent polarization is confirmed from the electric polarization hysteresis loops above and below TN for polycrystalline and film specimens. Structural analysis suggests that polar oxygen octahedral rotations and Ho displacements in the non-centrosymmetric Pna21 space group engineer ferroelectricity. The results indicate that occurrence of polar order in HoCrO3 resembles neither the typical case of proper ferroelectrics nor the improper ferroelectrics.

PS-H SCATTERING USING A PROJECTILE ELASTIC CLOSE-COUPLING APPROXIMATION

A projectile elastic close-coupling approximation with electron exchange is employed to study Ps-H scattering using the basis set [Ps(1s) + H(1s,2s,2p)] at low and medium energies. The s-, p- and d-wave phase shifts are reported below 5.1 eV along with the integrated cross sections for elastic (also quenching) and target inelastic transitions. The present prediction is compared with other theoretical predictions and found to be consistent. The conversion ratio approaches the value 0.25 with the increase of incident Ps energy.

Ps - He scattering using a static-exchange model

The static-exchange model has been employed to investigate Ps - He atom scattering at low and intermediate energies using Hylleraas and Hartree - Fock wavefunctions for the He atom. The total elastic cross sections and s-, p- and d-wave phase shifts are reported and our present phase shifts are compared with those existing predictions. The effect of electron exchange is found to be significant at low and medium energies and the scattering parameters are found to be sensitive to the choice of wavefunction.

Low-energy positronium-hydrogen elastic scattering using the six-state close coupling approximation

Positronium-hydrogen elastic scattering is investigated using the six-state full close coupling approximation employing the basis set Ps(1s,2s,2p) + H(1s,2s,2p). S-wave singlet and triplet scattering and also the corresponding scattering length and effective range are compared with the corresponding predictions of Drachman and Houston and the Belfast group. We also report phase shifts up to l = 2 and the integrated elastic cross sections. Present results indicate that the effect of target excitation as well as the effect of excitations of both the atoms are significant in the energy region considered. We also report the angle integrated Ps(1s) + H(1s) → Ps(2p) + H(1s) and Ps(1s) + H(1s) → Ps(1s) + H(2p) excitation cross sections.

The effect of positronium formation in e+-Li and e+-Na scattering

Abstract We studied e + Li and e + Na scattering using the close-coupling approximation in the static and coupled static expansion schemes. The effect of the positronium formation on the elastic channel is found to be strong in both cases. In the case of the lithium atom the effect is dramatic; the inclusion of the positronium formation channel transforms the purely repulsive effective e + Li S wave (static) potential to a predominantly attractive (coupled static) potential. In this case, in the static model δ(0)−δ(∞)=0, whereas in the coupled static model δ(0)−δ(∞)=π. According to Levinsons theorem this suggests the presence of a S wave bound or continuum bound state in the e + Li system.

Study of positronium formation in positron - hydrogen molecule scattering

We investigate ground-state positronium formation cross sections in e C -H2 scattering using simplified second-order perturbative Born series. The second-order angle- integrated capture cross sections are also found to be in very good agreement with the experimental results of Fromme et al and Diana et al in the energy region E> 65 eV. Present second-order differential cross sections show a qualitative difference over the first-order results and agree well with the recent measurement of Tang and Surko at 80 and 100 eV. In the laboratory, the hydrogen molecule, along with a few heavier atoms . 64 Cu; Ar/ ,i s widely used as a target for sources of the positronium (Ps) beam. Positronium, the particle- antiparticle bound state and the isotope of the hydrogen atom, interacts more intimately with matter and is now extensively used as a vital probe in the surface study of solids and ionic crystals (Brookhaven National Laboratory), in the study of transport properties of positrons in tokamak fusion plasma (San Diego group) and in other different branches of physics and chemistry. Angular distribution of Ps formation is an essential parameter in tuning a Ps beam as a probe. A large number of elaborate theoretical calculations have been performed in the study of the capture process in positron-atom scattering vis-` a-vis a large number of experimental studies that have also been performed on positron-atom and positron-molecule scattering (Fornari et al 1983, Griffith 1984, Fromme et al 1988, Diana et al 1986). But the theoretical study of positron-molecule scattering is rare and is restricted to only first-order calculations (Sural and Mukherjee 1970, Ray et al 1980, Bussard et al 1979, Biswas et al 1991a, b). As the capture process is regarded as a multivertex process, at least a second-order theory is needed to have a reliable prediction for the capture parameters in a molecular target. Considering all these facts we investigate Ps formation in e C -H2 scattering using a simplified second-order Born series. Multicentredness of the molecular target is the main stumbling block in performing any quantal calculation on it. In the elastic scattering amplitude for e- diatomic molecule collisions a two-centred integral appears in the amplitude. The inclusion of Ps formation adds one more centre to the problem. Moreover, due to the multicentredness of the molecule, one is required to perform an averaging over the molecular orientations, i.e. integration over two more dimensions. These are the probable reasons for the scarcity of calculations even with the simplest molecule, H2. To the best of our knowledge, this paper will be the first estimate with a second-order term for Ps formation in a molecule. In other words we demonstrate a possible way to incorporate the second-order term for the capture cross sections in positron-diatomic molecule scattering.

Correlation of macroscopic ion dynamics with microscopic length scale and modification of network structure in ion conducting mixed network former glasses

We have studied silver ion dynamics in mixed network former molybdophosphate glasses by varying the glass network formers ratio. We have shown that the macroscopic ion transport is correlated to the characteristic microscopic length scale and the modification of the phosphate network structure due to the introduction of molybdate network former. The time-temperature superposition of the conductivity spectra onto a single master curve is independent of the mixing of the network formers.

Mechanistic Insight into the Molecular TiO2-mediated Gas Phase Detoxication of DMMP: A Theoretical Approach

The detoxication of DMMP (dimethyl methylphosphonate) mediated by molecular TiO2 has been investigated computationally using density functional theory (DFT). From our previous studies, it is evident that the unimolecular detoxication of OPCs (organophosphorus compounds) is kinetically unfeasible at room temperature due to the significantly high activation barrier. Thus, the aim of our work is to find out whether molecular TiO2 can make any significant impact on the kinetic feasibility of the detoxication processes or not. Here, we have identified a total of three detoxication pathways, where in the first step the detoxication occurs through H-abstraction with the assistance of TiO2, and in the second step, the titanium complex is separated from the respective phospho-titanium complexes. The outcomes reveal that the TiO2-mediated detoxication pathways are at least 20.0 kcal/mol more favorable than their respective unimolecular pathways and that among them, the α-H-mediated isomerization is found to be the most feasible pathway. When the separation of a titanium complex is under consideration, the double H2O-assisted mechanism is found to be the favored pathway. Overall, the entire work provides a widespread idea about the efficiency of molecular TiO2-assisted detoxication of DMMP, which can be well applicable to other OPCs also.

The effect of core electrons in -Na scattering

The scattering of positrons off sodium targets has been investigated using the coupled static model. The sodium atom is represented by the one-active-electron model in which all the electrons of the target have been considered explicitly and the loosely bound valence electron is only involved in transitions. The scattering parameters are presented at low and medium energies. Appreciable differences are noticed between the present results and those obtained by the one-electron model with and without the core potential.