Dr Dharmalingam Prabhakaran

Abstract:

The origin of colossal magnetoresistance (CMR) in manganese oxides is among the most challenging problems in condensed-matter physics today. The true nature of the low-temperature electronic phase of these materials is heavily debated. By combining photoemission and tunnelling data, we show that in the archetypal bilayer system La 2-2x Sr 1+2x Mn 2 O 7 , polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state of bilayer manganites supports a vanishing coherent quasi-particle spectral weight at the Fermi level throughout k-space. The incoherence of the charge carriers, resulting from strong electron-lattice interactions and the accompanying orbital physics, offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopies. The stacking number N is the key factor for true metallic behaviour, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in this system.

Abstract:

Magneto-electric multiferroics exemplified by TbMnO 3 possess both magnetic and ferroelectric long-range order. The magnetic order is mostly understood, whereas the nature of the ferroelectricity has remained more elusive. Competing models proposed to explain the ferroelectricity are associated respectively with charge transfer and ionic displacements. Exploiting the magneto-electric coupling, we used an electric field to produce a single magnetic domain state, and a magnetic field to induce ionic displacements. Under these conditions, interference between charge and magnetic x-ray scattering arose, encoding the amplitude and phase of the displacements. When combined with a theoretical analysis, our data allow us to resolve the ionic displacements at the femtoscale, and show that such displacements make a substantial contribution to the zero-field ferroelectric moment.

Abstract:

Fluctuation dynamics in magnetization plateaus is a relatively poorly explored area in frustrated magnetism. Here we use muon spin relaxation to determine the fluctuation timescale and associated field distribution width in the partial magnetization plateau of Ca3Co2O6. The muon spin relaxation rate has a simple and characteristic field dependence which we model and, by fitting to the data at 15K, we extract a fluctuation timescale τ = 880(30)ps and a field distribution width Δ = 40.6(3)mT. Comparison with previous results on Ca3Co2O6 suggests that this fluctuation timescale can be associated with short-range, slowly fluctuating magnetic order. © 2011 IOP Publishing Ltd.

Abstract:

Temperature and magnetic field dependence of the vortex penetration into a superconductor and the resulting trapped vortex field (the vortex remanent state) were investigated for Bi2Sr2CaCu2O 8+x (BSCCO) and YBa2Cu3O6+x (YBCO) single crystals and BSCCO thin films. The experiments revealed changes in the pinning regime (the magnitude and magnetic relaxation) of the trapped vortex field with an increasing temperature. The trapped vortex field, obtained by applying a constant magnetic field, exhibits a maximum at a certain temperature, that separates the partial vortex penetration regime at low temperatures from the complete vortex penetration state at higher temperatures. The corresponding vortex remanent states in these two regimes are characterized by two distinctly different relaxations, the logarithmic and the nonlogarithmic ones at temperatures below and above the maximum, respectively, for both BSCCO and YBCO. At temperatures close to Tc surface/geometric barrier affect the relaxation rates. © 2011 American Physical Society.