B A Dasannacharya

Texture formation and enhanced critical currents in YBa2Cu3O7

We report an increase of a factor up to 5 in JC on mixing varying amounts of Ag2O with YBa2Cu3O7. The increase correlates with an increase in texture as seen by x‐ray diffraction intensities. Microstructure indicates the formation of platelets with c axis normal to the platelets.

Observation of librational modes of water molecules in single crystal hydrates by neutron scattering

Abstract The librational motions of water molecules in single crystals of Ba(ClO3)2.H2O and K2C2O4.H2O are studied by the technique of neutron scattering. The assignments of the observed peaks in the two cases are discussed with reference to features affecting the intensity, namely, the orientation of the crystal and the polarisation vector of the librational motions. Unambiguous assignments have been made for the rocking modes of vibration.

Small angle neutron scattering from micellar solutions of triton X-100

Micellar solutions of non-ionic surfactant triton X-100 (8% by weight) show phase separation at cloud pointTcp ∼ 335 K. This paper reports results of small angle neutron scattering (SANS) experiments from this solution as a function of temperature between 298 and 332 K. The range of wave-vector transferQ, covered in these experiments is from 0.02 to 0.15 Å−1. It is seen that as one approachesTcp, the neutron scattering cross section diverges in the region of lowQ (<0.06 Å−1) while it is independent of temperature in region of largeQ(>0.06 Å−1). We believe that the divergence of scattering at lowQ with an increase in temperature is because of changes in the structure factorS(Q) of the solution. The measured distributions have been analyzed using four different models for inter-micellar potential. The models used to calculate the structure factorS(Q) are (1) mean spherical approximation (MSA) with Yukawa tail for attractive potential, (2) MSA with an attractive square well potential, (3) random phase approximation (RPA) with an attractive square-well potential and (4) Sticky hard sphere model (attractive square-well potential with Percus-Yevick approximation). The strengths of the attractive potential required to fit the SANS data are (−6.6 to − 14.4)/kt for model (1), (− 6.6 to − 15.0)/kt for model (2), (− 3.8 to − 7.3)/kB T for model (3) and (−2 to −2.7)/kt for model (4). On the basis of reasonableness of the derived strength of the potential near the phase separation temperature and its relative temperature dependence, it is concluded that present data favour the Sticky hard sphere model.

Study of molecular reorientation in solids by slow-neutron scattering

Abstract Molecular reorientation in (NH4)2SO4 has been studied using slow-neutron scattering. The results are compared with Skolds model and NMR measurements, and used to demonstrate the capabilities of the neutron-scattering method for probing reorientations in solids.

Neutron inelastic scattering from mixed salts [(NH4)xK1−x]2SO4

Abstract Neutron inelastic scattering measurements have been performed on [(NH4)xK1−x]2SO4. It is shown that NH4+ preferentially occupies one of the two available inequivalent sites at smaller concentrations. NH4+ at the other site seems to be necessary for the occurance of ferroelectricity in these compounds.

Dynamics of liquid ammonia from cold neutron scattering

The scattering of 4.1 Å neutrons by liquid ammonia has been measured at 218 K in the angular range of 30–90° using the Trombay rotating crystal spectrometer. The experimental data, after correcting for multiple scattering, have been compared with model calculations, and it is shown that it is possible to get detailed information about the rotational correlation function on the basis of neutron experiments alone. The model assumes Langevin diffusion for translational motions. Rotational motions are described by means of an orientational correlation function which has a gaseous behaviour for times up toτ0 and then changes over to a diffusive character with a rotational diffusion constantDr. Within the framework of the model the correlation function can be described withDr=0.28×1013sec−1 andτ0=0.57×10−18sec. Corrections for multiple scattering and their dependence on model parameters are discussed.

Lattice dynamics of beryllium

Phonon frequencies in beryllium along the principal symmetry directions have been determined by means of the slow neutron inelastic scattering technique. The data are analysed in terms of a six-neighbour force constant model and the force constants are evaluated. It is concluded that strong tensor forces are present in beryllium and the importance of this finding to basic theories of lattice dynamics is pointed out.

Critical current measurements on extruded YBa2Cu3O y wires

The successful extrusion of short lengths of brittle wires of YBa2Cu3Oy that are superconducting above 77 K is reported. Detailed critical current measurements are presented on wires having undergone different physical treatments. The highest critical current obtained from V-I data is ⋍ 90 A/cm2, but this particular wire shows clear evidence of flux-pinning forces corresponding to ⋍ 130 A/cm2.

Normalisation and assessment of neutron diffraction data from liquids

AbstractA couple of normalisation criteria have been proposed to assess liquid structure data over the past decade. In this paper, a critical examination of these and other plausible criteria is made. Neutron diffraction data from liquid krypton measured by Clayton and Heaton is used to study these criteria. It is shown that if the structure factorS (Q)=1+γ (Q) satisfies the Krogh-Moe relation, namelyn