Might muons and a microwave make marvellous materials?
Muons make the most of waste heat
ISIS: Making muons for a quarter of a century
Muonium and hydrogen centres in sulphur
Muons explore a new plateau
An outsider’s view: a novel muon study of frustration
SR Giblin, JDM Champion (ISIS), HD Zhou and CR Wiebe (Florida State University, USA), JS Gardener (NIST, USA), I Terry (Durham University), S Calder, T Fennell, ST Bramwell (University College London)
Contact: Dr Sean Giblin, email@example.com
Further reading: SR Giblin et al., Phys. Rev. Lett. 101 (2008) 237201
Frustration occurs when it is not possible to satisfy all interactions. For example, a magnetic atom might want its spin direction to be misaligned with that of a neighbouring atom (if the interactions are antiferromagnetic). But, for some arrangements of atoms, it’s possible to find that misalignment with one neighbour prevents misalignment with another – producing frustration. Frustration plays an important role in a diverse range of physics, from magnetism to protein folding. Pyrochlores – magnetic materials with atoms arranged in a particular way that leads to frustration – are fascinating as by changing one atom the frustration behaviour changes, culminating in properties such as a ‘spin liquid’, ‘spin glass’ or ‘spin ice’.
The frustration in pyrochlore Tb2Sn2O7 has previously led some to believe it exhibits a novel state of magnetism in which the magnetisation direction reverses multiple times a second. This is not how a permanent magnet normally behaves. We tested the behaviour using muons implanted into silver in front of the sample (rather than into the sample itself). If the sample behaved like a permanent magnet field lines would penetrate the silver and be detectable by the muons. This is indeed what is revealed – so that Tb2Sn2O7 does indeed behave like a permanent magnet below its transition temperature of 0.87K.
An oscillatory signal in the muon data is a clear indication of static internal magnetic fields in Tb2Sn2O7. The inset shows the temperature dependence of the internal field below the transition.
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Muon studies of organic semiconductors used in display screens
AJ Drew et al (University of Fribourg, Switzerland), FL Pratt (ISIS), NA Morley (University of Sheffield), P Desai (Queen Mary, University of London), R Scheuermann (PSI, Switzerland)
Contact: Dr AJ Drew, firstname.lastname@example.org, Dr FL Pratt email@example.com
Further reading: A.J. Drew, F.L. Pratt et al, Phys. Rev. Lett. 100 (2008) 116601
Electronic devices based on organic semiconductors such as Alq3 (tris[8-hydroxy-quinoline] aluminum) are revolutionising electroluminescent displays and large-area electronics, as these organics are economically favourable, can be easily processed in large areas, have tunable electronic properties, and are simple to grow into high quality thin films.
Even though the mechanisms of charge transport in such organic conductors are fundamental to their operation, many aspects of organic charge transport are still only poorly understood. Progress in this area may be pivotal to utilizing these materials to their fullest extent.
Implanted muons provide a powerful local probe technique for studying the dynamics of mobile spins. Muon spin relaxation studies at ISIS have been used to probe the charge carrier motion in Alq3 as a function of temperature. The charge mobilities obtained in this way are significantly larger than those obtained from direct transport measurements in polycrystalline films and thus provide an estimate for the intrinsic upper limit for the mobility that might be achievable in high quality bulk material.
The propellor-shaped molecules of Alq3,which are used as an active constituent of organic light emitting diode (OLED) displays.
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