Dr Dharmalingam Prabhakaran

Abstract:

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat (cp), thermal conductivity (K), and thermal diffusivity (D) around the antiferromagnetic to paramagnetic phase transition in CoO and NiO single crystals. Up to now, no agreement on the critical behavior of both oxides has been obtained. The results for seven samples grown in different laboratories have been compared. We have found that, irrespective of the origin of the samples, the critical exponent and amplitude ratio of cp, D, and K in NiO agree with the predictions of the three-dimensional Heisenberg model for isotropic antiferromagnets, which contradicts previous results. On the other hand, the sharpness of the peak of specific heat and of the dip of thermal diffusivity, together with the value of the critical exponent α in CoO, highly increase as crystal quality improves. For the best sample, a value α=0.81 has been obtained, which is not only higher than previously reported results, but far away from any universality class. © 2008 The American Physical Society.

Abstract:

By means of neutron scattering we have determined new branches of magnetic excitations in orbitally active CoO (TN=290 K) up to 15 THz and for temperatures from 6 to 450 K. Data were taken in the (1 1 1) direction in six single-crystal zones. From the dependence on temperature and Q we have identified several branches of magnetic excitation. We describe a model for the coupled orbital and spin states of Co2+ subject to a crystal field and tetragonal distortion. © 2007.

Abstract:

Polarized- and unpolarized-neutron scattering was used to study the temperature evolution of the magnetic excitations of charge ordered La5/3Sr1/3NiO4. We studied two features in detail: (i) the resonance-like scattering at 27 meV in the quasi-two-dimensional magnetic excitations from the ordered Ni2+ (S = 1) spins, and (ii) the diffuse scattering associated with quasi-one-dimensional antiferromagnetic correlations along the stripes. Although both these features persist up to at least similar to 125 K, their temperature dependences are found to be quite different. We argue from the results that the resonance- like feature observed in La2-xSrxNiO4 (x similar to 1/3) is not caused by a coupling between the spin dynamics of the two magnetic sub-systems.

Abstract:

We investigate the spin state of LaCo O3 using state-of-the-art photoemission spectroscopy and ab initio band structure calculations. The GGA+U calculations provide a good description of the ground state for the experimentally estimated value of electron correlation strength U. In addition to the correlation effect, spin-orbit interaction is observed to play a significant role in the case of intermediate spin and high spin configurations. The comparison of the calculated Co 3d and O 2p partial density of states with the experimental valence band spectra indicates that at room temperature, Co has dominant intermediate spin state configuration and that the contribution from high spin configuration may not be significant at this temperature. The line shape of the La 5p and O 2s core level spectra could be reproduced well within these ab initio calculations. © 2008 The American Physical Society.

Abstract:

We report the complex refractive index of La5/3Sr 1/3NiO4 over the terahertz frequency range, obtained using time-domain spectroscopy. Negligible change in the complex refractive index with magnetic flux densities up to 6 T was seen, while changes were observed as the lattice temperature was increased from 1.5K to the charge-ordering temperature at 220 K. The terahertz frequency response therefore originates from the dielectric function rather than the magnetic permeability.