Bulletin of the American Physical Society
2005 New England Sections of the APS and AAPT Joint Fall Meeting
Friday–Saturday, October 14–15, 2005; Burlington, VT
Session B: Condensed Matter Physics |
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Chair: Dennis Clougherty and Shaheen Malghani Room: University of Vermont Angell B106 |
Saturday, October 15, 2005 8:00AM - 8:12AM |
B.00001: The Effects of Parallel Tempering on the Autocorrelation Time in Simulations of the Two-Dimensional Ising Spin Glass Jennifer E. Houle, Susan R. McKay The two-dimensional Ising spin glass is a notoriously difficult system to simulate, since the autocorrelation time becomes extremely long as the system approaches zero temperature. [1] Thus, this system is an ideal testing ground for algorithms designed to circumvent the difficulties inherent in simulating a frustrated system with an intricate free energy landscape. In this study, we have applied parallel tempering [2,3] to the bimodal spin glass on a square lattice in two dimensions. Our results show that the autocorrelation time can be substantially shortened through this approach, so the simulations yield realistic system properties in substantially fewer Monte Carlo steps. The algorithmic impact on calculated system properties varies. For example, internal energies computed using conventional and parallel tempering show excellent agreement even at low temperatures, whereas the Edwards-Anderson order parameter is more accurately obtained with parallel tempering. 1. W.L. McMillan, Phys. Rev. B, \textbf{28} 5216, 1983. 2. K. Hukushima and K. Nemoto, J. Phys. Soc. Japan, \textbf{65} 1604, 1996. 3. H.G. Katzgraber and L.W. Lee, Phys. Rev. B, \textbf{71 }134404, 2005. [Preview Abstract] |
Saturday, October 15, 2005 8:12AM - 8:24AM |
B.00002: Phase diagram of a geometrically frustrated dipolar Heisenberg pyrochlore antiferromagnet Matthew Enjalran Rare-earth pyrochlore oxide materials of general chemical formula A$_2$B$_2$O$_7$ (A$^{3+}$= rare-earth, B$^{4+}$= transition metal) have attracted a great deal of attention because the combination of chemical composition and lattice geometry produces a plethora of interesting physical phenomena in these materials. In insulating systems, rare-earth ions with large magnetic moments reside on the corners of a geometrically frustrated network of corner sharing tetrahedra and interact via exchange and long-range dipole-dipole interactions. Experiments have observed spin liquid, spin glass, spin ice, and magnetically ordered phases. In the gadolinium titanate pyrochlore, Gd$_2$Ti$_2$O$_7$, an excellent realization of a Heisenberg antiferromagnet, experiments reveal a two step transition sequence to a partially ordered magnetic phase at 50 mK in zero applied field. In finite magnetic fields, a rich phase diagram of multiple field driven transitions is observed for magnetic fields applied along the high symmetry directions of the lattice. We study a model of classical Heisenberg spins (O(3) symmetry) on a pyrochlore lattice with exchange and dipolar interactions within mean-field theory. Using parameters relevant to the material system, we develop phase diagrams in zero and finite magnetic fields. Our results our compared to recent experiments on Gd$_2$Ti$_2$O$_7$ and Gd$_2$Sn$_2$O$_7$. [Preview Abstract] |
Saturday, October 15, 2005 8:24AM - 8:36AM |
B.00003: Lateral Organization in Biomembranes from Simple Non-Equilibrium Models with Nearest Neighbor Interactions Andrew P. Paradis, Susan R. McKay, Samuel T. Hess Biological function relies strongly on the regulated segregation of biomembrane constituents, yet understanding of the mechanisms for such organization remains limited. This study focuses on the distribution of saturated and unsaturated lipids interacting and moving on a two-dimensional triangular lattice, simulated using a Metropolis algorithm. Interaction energies between the two species are adjustable within the model, as are the rate and size of simulated endo- and exocytosis events. These events keep the system substantially out of equilibrium and yield a striped phase comparable to those seen experimentally. [1] This phase is robust, occurring over a wide range of interactions. By adding additional species such as cholesterol, this simple model shows promise for quantitatively predicting specific details of biomembrane phase behavior. \newline \newline [1] Baumgart T, Hess ST, and Webb WW. Nature. 2003 Oct 23;425(6960):821-4. [Preview Abstract] |
Saturday, October 15, 2005 8:36AM - 8:48AM |
B.00004: Soluble and thermally stable molecules for Organic Light-Emitting Transistors Fabio Cicoira, Clara Santato, Manuela Melucci, Laura Favaretto, Massimo Gazzano, Michele Muccini, Barbarella Giovanna We report on the design, synthesis, thin film growth and optoelectronic properties of a newly synthesized dithienothiophene derivative. Atiomic Force Microscopy and Laser Scanning Confocal Microscopy show that both vacuum sublimed and drop cast DTT7Me films have good silicon dioxide surface coverage. X-ray diffraction reveals that the films have a highly ordered structure. Very interestingly for future technological applications, Organic Light Emitting Transistors (OLETs) were successfully fabricated not only employing vacuum sublimation but also drop casting. Our results introduce several novelties in organic optoelectronics both from fundamental and practical point of view: the demonstration of a novel multifunctional material able to conjugate good charge transport and electroluminescence in FET configuration and the first solution processed OLET based on non polymeric systems. This last result paves the way towards all plastic, low temperature processed, large area organic optoelectronics. [Preview Abstract] |
Saturday, October 15, 2005 8:48AM - 9:38AM |
B.00005: Dripping, Jetting, Drops and Wetting: The Magic of Microfluidics. David Weitz Microfluidic techniques enable easy visualization of many fluid structures, permitting the study of hydrodynamic instabilities, allowing them to be constructively harnessed.~ This talk will explore droplet formation, and will show how hydrodynamic instabilities can be exploited to create fascinating new structures, and how the drops can be used as nanoreactors. [Preview Abstract] |
Saturday, October 15, 2005 9:38AM - 10:28AM |
B.00006: Molecular Fluidics Sergie S. Sheiko |
Saturday, October 15, 2005 10:28AM - 11:18AM |
B.00007: Melting of Two-Dimensional Colloidal Crystals A.D. Dinsmore, J.R. Savage, D. Blair, A.J. Levine, J. Machta The process by which crystals melt into a fluid is an interesting topic that, despite being very common, has proved difficult to study experimentally because of the high speed and small size of atoms. Using colloidal spheres as model `atoms,' however, we have monitored the kinetics of the melting of crystallites formed in the presence of a short-ranged depletion attraction, whose strength was adjusted by means of the temperature. We followed the motions of hundreds of individual particles with high resolution. We found that large crystallites melted at a steady rate that was limited by bond-breaking. The melting kinetics, however, drastically sped up when the size reached 20-30, below which the melting was approximately diffusion-limited. The results are robust and are observed at different temperatures above the melting point. Computer simulations of a similar system show similar behavior while providing additional insight into the effective elastic constants inside the small crystallites. Implications for melting in a broader range of examples will be discussed. [Preview Abstract] |
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