Andrew Boothroyd

Abstract:

By combining bulk properties, neutron diffraction, and nonresonant x-ray diffraction measurements, we demonstrate that the new multiferroic Cu 3Nb2O8 becomes polar simultaneously with the appearance of generalized helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins-an observation that cannot be reconciled with the conventional theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically induced structural chirality. © 2011 American Physical Society.

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.