Dr Dharmalingam Prabhakaran

Abstract:

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity of the perovskite manganite family La1-xSr xMnO3 on a set of single crystals with doping range 0≤x≤0.35. Taking into account that the inverse of the thermal diffusivity has the same critical behavior as the specific heat, the critical exponent a of the magnetic transitions has been obtained. The results point to short-range interaction models for pure magnetic transitions. In the pure and lightly doped samples (x<0.10), where the transition is antiferromagnetic-paramagnetic, the critical exponent is consistent with the Heisenberg model (α=-0.11). For the highly doped samples (x>0.28), where there is a pure ferromagnetic- paramagnetic transition, the critical exponent is exactly that of an Ising behavior (α=+0.11). For 0.10≤x<0.28 no universality class was found and this behavior has been discussed taking into account the complexity of the phase diagram in this concentration range.

Abstract:

Muonium centres in Cu2O, Ag2O and CdO show hyperfine parameters spanning four orders of magnitude. They exemplify the three different categories of hydrogen defect centre in semiconducting and dielectric solids, with very different electronic structure and electrical activity, namely quasi-atomic (possibly deep acceptor), deep donor and shallow donor. © Springer 2005.

Abstract:

We have investigated the optimum conditions for the growth of Na xCoO2 single crystals by the optical floating-zone technique. A significant reduction in Na loss, was achieved by using a high-pressure growth atmosphere. A high-pressure oxygen-rich atmosphere during crystal growth was found to reduce the presence of CO3O4 impurity phase in the grown crystal. The magnetic properties of powder and single crystal samples were measured by superconducting quantum interference device (SQUID) magnetometry and found to vary according to the preparation conditions. © 2004 Elsevier B.V. All rights reserved.

Abstract:

In this paper we report studies of the charge stripe ordering using high resolution x-ray scattering in the nickelate system La2-xSr xNiO4 with doping levels of x=0.33, x=0.30, and x=0.275. The charge stripes for all doping levels were found to be two dimensional in nature with a high degree of correlation in the a-b plane. The in-plane inverse correlation length in the lower doped systems was greater than that in the x=0.33 system and is greatest for x=0.275, consistent with the stripes becoming less correlated as the doping level is decreased from x=0.33. However, the charge ordering in the x=0.33 system was observed to be more two dimensional in nature with a greater inverse correlation length between planes. The interaction between the lattice and charge order was observed to stabilize the charge ordering, even in the absence of spin ordering. In the x=0.30 and x=0.275 systems no long-range charge order could exist without the presence of the magnetic order. In both systems the charge stripes were incommensurate and on heating the incommensurability increased towards the stable ε=0.33 value as the stripes gained thermal energy to overcome the Coulombic repulsion. In all these systems the integrated intensity of the stripe reflections showed a clear difference in behavior from previous neutron studies with an initial increase in the integrated intensity as the temperature was increased, which was not found in the neutron studies.

Abstract:

Magnetic order has been investigated in stripe-ordered La 2-xSrxNiO4 (x=0.275,0.37,0.4) by dc magnetization and by polarized- and unpolarized-neutron diffraction. In the magnetically ordered phase, all three compositions exhibit a magnetic transition consistent with a spin reorientation in the ab plane. For x=0.37, the spin axes rotate from an angle of 37.7°±0.3° to the stripe direction at 71 K, to 52.3°±0.20 at 2 K. The x=0.275 and 0.4 compounds were found to undergo a similar spin reorientation. A spin reorientation has now been observed to occur for five different doping levels in the range of 0.275≤x≤0.5, suggesting that this spin transition is an intrinsic property of the stripe phase.