Young Scientist Award, Abstracts

Evidence for a Dynamic Jahn-Teller Effect in PrO2

C.H. Gardiner1 A.T. Boothroyd1 S.J.S. Lister1 P. Santini1 B.D. Rainford2 L.D. Noailles3 D.B. Currie4 R.S. Eccleston5 R.I. Bewley5
1Department of Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom
2Department of Physics, University of Southampton, Southampton, SO17 1BJ, United Kingdom
3Inorganic Chemistry Laboratory, University of Oxford, Oxford, OX1 3QR, United Kingdom
4Department of Chemistry, University of Southampton, Southampton, SO17 1BJ, United Kingdom
5ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom
Recent neutron scattering research into the properties of PrO2 has produced unusual results which are suggestive of a strong magneto-elastic coupling between phonon modes and crystal field states [1,2]. This leads to a dynamic Jahn-Teller effect in the ground state and a bound state between a phonon and a crystal field excitation. Measurements of the magnetic excitation spectrum of PrO2 over the energy range 0-1200 meV show broad features as well as crystal field excitations characteristic of a Pr4+ ion. The ordered magnetic moment of the Pr4+ ion has been measured in a single crystal, and found to be anomalously small, in agreement with measurements made on polycrystalline samples [2]. The reduction of the ordered moment from that expected in a cubic crystal field and the broad features in the excitation spectrum can be reproduced qualitatively by a model based on magneto-elastic coupling.
[1] A.T. Boothroyd et al Phys. Rev. Lett. 86, 2082 (2001)
[2] S. Kern et al, Solid State Communications 49, 295 (1984)

Spin dynamics in the high-field phase of quantum critical S=1/2 TlCuCl3

Ch. Rüegg1 N. Cavadini1 A. Furrer1 K. Krämer2 H.U. Güdel2 P. Vorderwisch3 H. Mutka4
1Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen, Switzerland
2Departement für Chemie und Biochemie, Universität Bern, CH-3000 Bern 9, Switzerland
3Hahn-Meitner-Institut, BENSC, D-14109 Berlin (Wannsee), Germany
4Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9, France
An external magnetic field suppresses the spin energy gap in singlet ground state S=1/2 TlCuCl3. The system becomes quantum critical at Hc~ 5.5 T, where the energy gap between the lowest Zeeman split triplet level and the nonmagnetic ground state vanishes. Antiferromagnetic ordering is reported above Hc, which underlines the collective nature of the observed quantum phase transition. The intrinsic parameters of the title compound allow to access the critical region microscopically by neutron scattering. A substantial study of the spin dynamics in the high-field phase of TlCuCl3 at T=2 K up to H=12 T was performed for the first time. The results indicate two dynamical regimes, which can be understood within characteristically renormalized triplet modes and a low-lying dynamics of possibly collective origin.

The Equilibria in the AlN-Al2O3-Y2O3 Ternary System - Thermodynamic and Neutron Diffraction

M. Medraj1 R. Hammond3 J. Root3 W. T. Thompson2 R. A. L. Drew1
1McGill University, Montreal, Quebec, Canada
2Royal Military College, Kingston, Ontario, Canada
3Neutron Program for Materials Research, National Research Centre, Chalk River, Ontario, Canada
Aluminum nitride (AlN) is currently used in electronic packaging and engineering ceramics. It offers both higher thermal conductivity and superior electrical insulating properties compared with Al2O3. Yttria (Y2O3) is the best additive for AlN sintering, and it has been shown that AlN densifies by a liquid phase mechanism, where the surface oxide, Al2O3, reacts with the oxide additive, Y2O3, to form a Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is becoming essential for further development of the AlN. Binary diagrams of Al2O3-Y2O3, AlN-Al2O3, and AlN-Y2O3 were thermodynamically modeled. The obtained Gibbs free energies of components, stoichiometric phases and solution parameters were used for the calculation of isothermal sections and liquidus surface of AlN-Al2O3-Y2O3 system. The predicted ternary phase diagram was verified experimentally using in situ high temperature neutron diffractometry. The ternary phase diagram AlN-Al2O3-Y2O3 has been constructed for the first time in this work.

Film thickness dependence of structure formation in ultra-thin polymer blend films

J. S. Gutmann1,3 P. Müller-Buschbaum2 M. Stamm3
1Department of Materials Science and Engineering, Cornell University, Bard Hall, Ithaca, NY, 14853, USA
2Physik Department, LS E13, James-Franck-Straße 1, TU-München, 85747 Garching, Germany
3IPF Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany
We investigated the film thickness dependence of structure formation in ultra-thin polymer blend films prepared from mixtures. As a model system we used binary mixtures of poly-styrene-stat-para-bromostyrene copolymers dissolved in toluene as a common solvent. The ultra thin films were prepared via spin coating of the polymer solutions onto silicon (100) substrates. In total three different samples series differing in the miscibility of the blended polymers and varying film thickness were prepared. The resulting morphologies were investigated with scanning force microscopy, reflectometry and grazing incidence scattering techniques in order to obtain a picture of the sample at and below the sample surface.

Rietveld refinement of neutron, synchrotron and combined powder diffraction data of cement clinker

V. Peterson1 B. Hunter2 L. Aldridge2 A. Ray1
1University of Technology, Sydney
2Australian Nuclear Science and Technology Organisation
An ordinary Portland cement clinker and a NIST standard reference material clinker were characterised using neutron and synchrotron powder diffraction. Rietveld analysis was performed using the program LHPM, with individual as well as combined data sets. Comparisons were made between phase quantifications from these different data sets, and with published results from other methods. Synchrotron refinements gave results consistent with the literature phase quantifications, while the lower resolution of the neutron data resulted in refinements with poor literature correlation for tricalcium and dicalcium silicates. Upon refinement of the atomic positions of triclinic tricalcium and dicalcium silicates, relatively minor adjustments to the structures resulted in significant improvement in the fit of the phases. This suggested that the published structures are not identical to those found in clinker materials.