Dr Dharmalingam Prabhakaran

Abstract:

Strongly correlated electron systems give an opportunity to manipulate charge, orbital, magnetic and structural phases of matter. Here we show that the insulating phase where charges are localized can be delocalized through photo-excitation which, in turn changes the structure locally, inducing an orthorhombic to rhombohedral phase transition. The I-M transition was witnessed for La(1-x)Sr(x)MnO(3) compounds in Raman spectra and photo-induced conduction simultaneously. A simple continuous argon ion laser source was used for optical excitation. The photon energy was 2.53 eV and the power can be chosen anywhere between 5 and 45 mW. Our studies clearly bring out the role of local disorder in the form of Jahn-Teller distortion in the localization of electrons.

Abstract:

We investigated the effect of uniaxial and hydrostatic pressure on resistivity and ac-magnetic susceptibility of two-dimensional layered manganite, La1.25Sr1.75Mn2O7 (LSMO125) to investigate the lattice effect on magnetic and electronic properties. Asymmetric role of uniaxial pressure, || and ⊥ to c-axis on the spin flop and charge transport has been revealed while comparing hydrostatic pressure. Uniaxial pressure along c-axis increases metal-insulator transition temperature (TMI) and ferromagnetic ordering temperature (TC), whereas it decreases the resistivity along ab-plane (ρab). In contrast to pressure along c-axis, TMI and TC decrease, whereas the resistivity along c-axis (ρc) increases with pressure || to ab-plane. ρc/ρab is quite large, increasing with pressure and shows a peak at around TMI. Uniaxial pressure behaviour is strongly related to the Mn-O-Mn linkage between MnO2 layers and the spin reorientation from the apical axis to the basal plane and vice versa with pressure. Both ρab and ρc decrease whereas TMI and TC increases under hydrostatic pressure. Influence of spin and charge on magnetic and electrical properties under hydrostatic pressure are explained by pressure-induced cant between the MnO2 bilayers and variation in bond lengths. The different pressure driving rates of TMI while measuring ρab and ρc confirms that there is a strong competition between the in and out plane components under hydrostatic pressure. © 2008 Elsevier B.V. All rights reserved.

Abstract:

We present resonant x-ray scattering experimental data from YVO3. By scattering at the vanadium K edge we are able to observe diffraction from the anisotropic tensor of susceptibility at the Bragg forbidden (010). The resonant energy spectra from these reflections are unusually complex, giving an indication of the crystal field distortions around the vanadium site.

Abstract:

The magnetism of charge-stripe-ordered La1.55Sr 0.45NiO4 was studied by a combination of neutron diffraction, muon-spin relaxation (μSR), and bulk susceptibility. Long-range magnetic ordering was observed at a lower temperature by μSR than by neutron diffraction, consistent with a glassy transition to the ordered phase. A second magnetic transition is detected by all techniques and is consistent with a spin reorientation. On cooling below TSR=42 K the spins reorientate from lying 32.9±0.6° away from the stripe direction at 70 K to 56.8±0.4° at 10 K. The magnetic order was observed by neutron diffraction to have both three-dimensional and two-dimensional (without any correlation along the c axis) characters. μSR measurements confirmed this and are consistent with a single magnetically ordered spin-stripe phase. The effects of checkerboard charge order on the ordered phase and the characteristics of the phase diagram of the spin reorientation in charge-ordered La2-xSrxNiO4 are commented on. © 2008 The American Physical Society.