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Events on Thursday, March 31st, 2011

R. G. Herb Condensed Matter Seminar
Quantum Phases of the Cairo Pentagonal Lattice
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Ioannis Rousochatzakis, Max-Planck Insitute for the Physics of Complex Systems, Dresden
Abstract: We present an analytical and numerical study of the spin S=1/2 antiferromagnetic Heisenberg model on the Cairo pentagonal lattice. This is the dual of the Shastry-Sutherland lattice and has been discussed as a possible new candidate for having a spin liquid ground state [1]. More recently a S=5/2 version of this model has been realized in the Bi2Fe4O9 system [2]. Here we use a model with two different types of exchange couplings and investigate the nature of the ground state as a function of their ratio. This strategy allows us to understand the nature of a number of phases and derive effective models for their description with and without a magnetic field. Of particular interest is a surprising interplay between a collinear and a four-sublattice orthogonal phase due to an underlying order-by-disorder mechanism. Furthermore we address the issue of possible nonmagnetic ground states such as singlet and spin nematic phases.

[1] K. S. Raman, R. Moessner, and S. L. Sondhi, PRB 72, 064413 (2005)
[2] E. Ressouche, V. Simonet, B. Canals, M. Gospodinov, and V. Skumryev, PRL 103, 267204 (2009)
Host: Natalia Perkins
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Astronomy Colloquium
"The HERMES project - reconstructing the ancient Galaxy"
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Joss Bland-Hawthorn, Federation Fellow, University of Sydney
Abstract: The first building blocks of our Galaxy were likely to have been laid down in the first 300 million years after the Big Bang. The first stars that came into being in those early structures have long since disappeared but they may have left behind unique chemical signatures in today's stellar populations. Over the next 13 billion years, the Galaxy grew through a series of mergers and acquisitions into the corporate giant that surrounds us today - 100 billion stars and counting. This was undoubtedly a very complex process that we are far from understanding even with the most sophisticated computer simulations. All stars show evidence of chemical signatures that say something about their past. In this talk, I will describe the basic motivation of the HERMES survey which will attempt to reconstruct the early Galaxy history through the technique of chemical tagging. The HERMES instrument is a major new $10M facility at the AAT that will see first light in 2013.
Host: Professor Snezana Stanimirovic
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