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CMP
Credit: Jack Hobhouse

Dr Dharmalingam Prabhakaran

Researcher

Research theme

  • Quantum materials

Sub department

  • Condensed Matter Physics

Research groups

  • Synthesis and crystal growth
dharmalingam.prabhakaran@physics.ox.ac.uk
Telephone: 01865 (2)72270,01865 (2)72351,01865 (2)72341
Clarendon Laboratory, room 177,377,373
  • About
  • Publications

Investigation of the spin state of Co in LaCo O3 at room temperature: Ab initio calculations and high-resolution photoemission spectroscopy of single crystals

Physical Review B - Condensed Matter and Materials Physics 77:4 (2008)

Authors:

SK Pandey, A Kumar, S Patil, VRR Medicherla, RS Singh, K Maiti, D Prabhakaran, AT Boothroyd, AV Pimpale

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.
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Terahertz-frequency conductivity of charge stripes in the antiferromagnet La5/3Sr1/3NiO4

IRMMW-THz2007 - Conference Digest of the Joint 32nd International Conference on Infrared and Millimetre Waves, and 15th International Conference on Terahertz Electronics (2007) 869-870

Authors:

J Lloyd-Hughes, D Prabhakaran, E Castro-Camus, AT Boothroyd, MB Johnston

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.
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X-ray scattering study of the order parameters in multiferroic TbMn O3

Physical Review B - Condensed Matter and Materials Physics 76:18 (2007)

Authors:

D Mannix, DF McMorrow, RA Ewings, AT Boothroyd, D Prabhakaran, Y Joly, B Janousova, C Mazzoli, L Paolasini, SB Wilkins

Abstract:

We report on an extensive investigation of the multiferroic compound TbMn O3 using x-ray scattering techniques. Nonresonant x-ray magnetic scattering (NRXMS) was used to characterize the domain population of the single crystal used in our experiments. This revealed that the dominant domain is overwhelmingly A type. The temperature dependence of the intensity and wave vector associated with the incommensurate magnetic order was found to be in good agreement with neutron scattering data. X-ray resonant scattering experiments were performed in the vicinity of the Mn K and Tb L3 edges in the high-temperature collinear phase, the intermediate temperature cycloidal and ferroelectric phase, and the low-temperature phase. In the collinear phase, where according to neutron diffraction only the Mn sublattice is ordered, resonant E1-E1 satellites were found at the Mn K edge associated with A -type but also F -type peaks. Detailed measurements of the azimuthal dependence of the F -type satellites (and their absence in the NRXMS experiments) leads us to conclude that they are most likely nonmagnetic in origin. We suggest instead that they may be associated with an induced charge multipole. At the Tb L3 edge, resonant A - and F -type satellites were observed in the collinear phase again associated with E1-E1 events. We attribute these to a polarization of the Tb 5d states by the ordering of the Mn sublattice. On cooling into the cycloidal and ferroelectric phase, a new set of resonant satellites appear corresponding to C -type order. These appear at the Tb L3 edge only. In addition to a dominant E1-E1 component in the σ- π′ channel, a weaker component is found in the preedge with σ- σ′ polarization and displaced by -7 eV with respect to the E1-E1 component. Comprehensive calculations of the x-ray scattering cross section were performed using the FDMNES code. These calculations show that the unrotated σ- σ′ component of the Tb L3 C -type peaks appearing in the ferroelectric phase contains a contribution from a multipole that is odd with respect to both space and time, known in various contexts as the anapole. Our experiments thus provide tentative evidence for the existence of a type of anapolar order parameter in the rare-earth manganite class of mulitferroic compounds. © 2007 The American Physical Society.
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Magnetism in geometrically frustrated YMnO3 under hydrostatic pressure studied with muon spin relaxation.

Phys Rev Lett 98:19 (2007) 197203

Authors:

T Lancaster, SJ Blundell, D Andreica, M Janoschek, B Roessli, SN Gvasaliya, K Conder, E Pomjakushina, ML Brooks, PJ Baker, D Prabhakaran, W Hayes, FL Pratt

Abstract:

The ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T(N) approximately equal to 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature T(N) increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.
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Sodium ordering and the control of magnetism in sodium cobaltate

Journal of Magnetism and Magnetic Materials 310:2 SUPPL. PART 1 (2007) 810-812

Authors:

DJP Morris, M Roger, DA Tennant, JP Goff, MJ Gutmann, JU Hoffmann, D Prabhakaran, N Shannon, B Lake, PP Deen

Abstract:

The long-range three-dimensional ordering of Na+ ions was studied in a sample of composition Na0.75CoO2 using single-crystal neutron diffraction. Large-scale numerical simulations reveal the ordering principle for this system, the formation of multi-vacancy charged droplets then order long range, and the structure factors from these defect clusters are in good agreement with the observed neutron diffraction intensities. The electrostatic potential is found to be the dominant factor in determining the sodium ordering and its associated distortion field. The superstructures induce a periodic potential in the CoO2, giving potential wells that are larger than the single-particle hopping frequency and so able to localize holes. The results readily explain many of the observed electrical and magnetic properties, including the three dimensionality of the magnetic excitations. © 2006 Elsevier B.V. All rights reserved.
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