Some examples of recent research carried out using MuSR are shown below:
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
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.
Evidence for Time-Reversal Symmetry Breaking in LaNiC2
AD Hillier and J Quintanilla (ISIS), R Cywinski (Huddersfield)
Contact: Dr Adrian Hillier
Further reading: Phys. Rev. Lett. 102 (2009) 117007
Muon spin relaxation experiments were carried out on the noncentrosymmetric intermetallic superconductor LaNiC2. The onset of superconductivity was found to coincide with the appearance of spontaneous magnetic fields, implying that in the superconducting state time-reversal symmetry is broken.
Crystal structure and magnetic field distribution in LaNiC2
Localization of the superfluid density in the superconducting planes of high-Tc superconductors
PJ Baker et al. (Oxford), FL Pratt (ISIS), SJ Kwon (Samsung)
Contact: Dr Peter Baker
Further reading: Phys. Rev. Lett. 102 (2009) 087002
Muon spin rotation measurements were made on two series of Bi-based high temperature superconductors where organic molecules allowed the spacing of the copper oxide planes to be increased by up to a factor of three. The results showed that while the increased layer spacing had no effect on the critical temperature, the density of the superconducting pairs of electrons decreased geometrically with the increasing layer spacing. This contradicts the first Uemura relation stating that the critical temperature is proportional to the superfluid density, but is consistent with its conventional interpretation: the density of pairs within the superconducting layers is a primary factor in determining the critical temperature of cuprate superconductors.
Intercalated Bi2212 showing the position of the organic molecules that increase the layer spacing.