Professor Stephen Blundell

Abstract:

The physics of matter in the condensed state is concerned with problems in which the number of constituent particles is vastly greater than can be easily comprehended. The inherent physical limitations of the human mind are fundamental and restrict the way in which we can interact with and learn about the universe. This presents challenges for developing scientific explanations that are met by emergent narratives, concepts and arguments that have a non-trivial relationship to the underlying microphysics. By examining examples within condensed matter physics, and also from cellular automata, I show how such emergent narratives efficiently describe elements of reality.

Abstract:

© 2017 Elsevier Inc. Polycrystalline samples of La 3 Ni 2 NbO 9 and La 3 Ni 2 TaO 9 have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La 3 Ni 2 SbO 9 . On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B′ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La 3 Ni 2 B′O 9 (B′ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La 3 Ni 2 NbO 9 behaves as a spin glass below 29 K but significant short-range interactions are present in La 3 Ni 2 TaO 9 below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.

Abstract:

Polycrystalline samples of La₃Ni₂NbO₉ and La₃Ni₂TaO₉ have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La₃Ni₂SbO₉. On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B’ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La₃Ni₂B’O₉ (B’ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La₃Ni₂NbO₉ behaves as a spin glass below 29 K but significant short-range interactions are present in La₃Ni₂TaO₉ below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.