Research Highlights
Plume attachment thresholds for fire spread in canyon-like geometries
Research into bushfire related accidents around the world has consistently identified a link between very intense and rapid fire propagation (known as fire eruption or fire blow-up) and steep, confined topography. In particular, a number of accidents that have resulted in the grievous injury or deaths of fire-fighters have occurred in canyons or on steep slopes. In many of these instances, surviving witnesses describe the sudden onset of an ‘upslope wind’, which immediately precedes the escalation in fire behaviour. This sudden upslope wind is typically assumed to be the main contributing factor driving the fire eruption, even in cases where weather conditions prior to the eruption were relatively benign or even windless.
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Resonant x-ray diffraction reveals hexadecapole order
The rare earth borocarbides exhibit a wide variety of ordered phases. These include superconductivity and different magnetic orders ranging from those with very long wavelength sinusoidal modulations to triple-k antiferromagnetism. Perhaps most interesting are the rare earth borocarbides with the three lanthanides that exhibit an ordering of the 4f orbitals (that is, Tb, Dy, and Ho). In TbB2C2, the (ferro-) orbital ordering is coincident with the magnetism, whilst in DyB2C2 the (antiferro-) orbital ordering occurs before a combined magnetic/orbital phase. In HoB2C2 there is an incommensurate magnetic phase before a reorientation transition occurs to a commensurate antiferromagnet accompanied by the onset of orbital order.
We study the Ho 4f multipoles in the combined magnetic and orbitally ordered phase of HoB2C2 combining soft x-ray resonant diffraction, non resonant x-ray diffraction and neutron diffraction. We show that the magnetic and orbital orders have distinct temperature dependences and that the low temperature phase is dominated by hexadecapole order.
Hexadecapole order dominates
the Ho 4f electronic shell
Andrew Princep, Annemieke Mulders, Wayne Hutchison et.al., J. Phys.: Condens. Matter 24 075602, featured in IOP LabTalk http://iopscience.iop.org/0953-8984/labtalk-article/48531.
The elusive crystal at the bottom of the well
Strontium titanate (SrTiO3) turns into an electrically polarised system when normal oxygen-16 is substituted with its heavier isotope oxygen-18. It is normally accepted that this substitution should have no effect and experiment has not been able to reveal how the atoms are arranged in the polarised system. We solved this problem using a computer cluster for simulations of atomic motion. Normally at absolute zero temperature atoms reside in the deepest hole that they can find in the energy landscape. Nevertheless, the atoms move around very slightly in these holes even at absolute zero (as the quantum-mechanical uncertainty principle requires). In strontium titanate there is a slight distortion to the shape of the holes so that even zero-point atomic motion prevents the atoms from reaching a final stable configuration. The trick is that because oxygen-18 atoms are slightly heavier than oxygen-16 their zero-point atomic motion is correspondingly less so that in this case the atoms find the final stable configuration. But what is this configuration? For a number of reasons this is difficult to determine experimentally so we analysed the atomic vibrations to find the direction in which the atoms would move as the zero-point atomic motion is quenched. We obtained atomic positions that are not only stable, but also give a structure of strontium titanate that displays the same properties as those so far observed experimentally in strontium titanate containing oxygen-18.
Maciej Bartkowiak, Mohana Yethiraj, Don Kearley, Annemieke Mulders. Phys. Rev. B 83, 064102 (2011).
Perturbed Angular Correlation Spectrometer
One of the two new Perturbed Angular Correlation Spectrometers at
UNSW@ADFA produces first results.
Jake Warner (PhD
student),
Dr Wolf-Dietrich Zeitz (Visitor to PEMS), Dr Laura Gladkis, Assoc. Prof. Heiko
Timmers and Dr William Kemp debate how to improve the detection sensitivity of the new perturbed angular correlation spectrometer. In 2010 the spectrometer produced data leading to the discovery of a dopant-defect complex in germanium. The identification of such defect complexes improves new crystallisation techniques for germanium materials that are developed for the next generation of semiconductor devices.