Dr Dharmalingam Prabhakaran

Abstract:

Low-temperature transmission electron microscopy and x-ray diffraction (XRD) studies have been carried out on pellet and powder samples of Pr0.5 Ca0.5 MnO3. These studies have revealed appearance of a different type of charge-orbital ordered (COO) phase, resulting due to flipping of eg orbitals from d3 x2 - r2 / d3 y2 - r2 to d3 x2 - r2 / d3 z2 - r2 configuration. This orbital flip results in a changeover of the COO superlattice-ordering vector from (1/2,0,0) to (1/4,1/2,1/4) in the Pnma phase. This COO phase coexists with the conventional COO phase. Low-temperature XRD studies show that the COO phase appears only in pellet sample and not in the corresponding powder sample. The powder sample shows only conventional COO phase. Volume fractions of conventional and the other type COO phases in pellet sample of Pr0.5 Ca0.5 MnO3 is estimated to be ∼55% and 45%, respectively. The occurrence of orbital flip has been attributed to local strain building up in the pellet sample. The strain builds up during cooling because manganite has anisotropic thermal expansion coefficients. © 2008 The American Physical Society.

Abstract:

An ultrafast electronic phase transition, associated with melting of orbital order, is driven in La1/2Sr3/2MnO4 by selectively exciting the Mn-O stretching mode with femtosecond pulses at 16 microm wavelength. The energy coupled into this vibration is less than 1% of that necessary to induce the transition thermally. Nonthermal melting of this electronic phase originates from coherent lattice displacements comparable to the static Jahn-Teller distortion.

Abstract:

Soft x-ray resonant scattering (XRS) has been used to observe directly, for the first time, the ordering of localized electronic states on both the Mn and the Tb sites in multiferroic TbMnO3. Large resonant enhancements of the x-ray scattering cross-section were observed when the incident photon energy was tuned to either the Mn L or Tb M edges which provide information on the Mn 3d and Tb 4f electronic states, respectively. The temperature dependence of the XRS signal establishes, in a model independent way, that in the high-temperature phase (28 K≤T≤42 K) the Mn 3d sublattice displays long-range order. The Tb 4f sublattices are found to order only on entering the combined ferroelectric/magnetic state below 28 K. Our results are discussed with respect to recent hard XRS experiments (sensitive to spatially extended orbitals) and neutron scattering. © 2008 IOP Publishing Ltd.

Abstract:

Ultra-small superparamagnetic iron oxide nanoparticles (SPIOs) were synthesized by co-precipitation of iron chloride salts with ammonia and then encapsulated with thin (~2nm) layers of silica. The particles have been characterized for size, diffraction pattern, surface charge, and magnetic properties. This rapid and economical synthesis has a number of industrial applications; however, the silica-coated particles have been optimized for use in medical applications as MR contrast agents, biosensors, DNA capturing, bioseparation and enzyme immobilization.

Abstract:

We have investigated charge dynamics in the static stripe ordered phase of La2-x Srx Ni O4+δ at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies (1-10 meV). The thermally activated dynamic response of the charge stripes is found to be characteristic of a collective mode such as a pinned charge density wave. At incommensurate doping levels, the pinning energy is reduced, owing to the presence of real-space defects in the stripe order, and a pronounced increase in the oscillator strength is seen. The results provide compelling evidence for the existence of low-energy collective modes of the charge stripes. © 2008 The American Physical Society.