Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X6: Frontiers in Computational Materials |
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Sponsoring Units: DCOMP Chair: Mei-Yin Chou, Georgia Tech Room: LACC 502A |
Friday, March 25, 2005 8:00AM - 8:36AM |
X6.00001: Accuracy of high throughput ab-initio methods in predicting crystal structures of metals: review of 80 binary alloys Invited Speaker: Predicting and characterizing the crystal structure of new alloys is a key problem in materials research and development. It is typically addressed with either accurate {\it ab initio} calculations on a small set of candidate structures or with empirical rules that have been extracted from a large body of experimental information, but have uncertain predictive power. One inherent limitation of most {\it ab initio} approaches is that they do not make explicit use of results of previous calculations when studying a new system. In heuristic models, a large set of experimental observations is used to extract rules that rationalize crystal structure with a few simple physical parameters (e.g. atomic radii, electronegativities, etc.). An innovative and powerful tool to tackle the prediction problem is the “high-throughput” {\it ab initio} method, which makes use of robust automated techniques to perform many thousands of calculations. By creating a huge database of over 14,000 {\it ab initio} structural optimizations on a set of 80 intermetallic binary alloys, and by implementing a new data mining technique, we are able to drastically reduce the time necessary to obtain accurate results on novel systems [1]. Furthermore, the huge amount of {\it ab initio} information contained in the database provides a unique opportunity for comparison with experimental results [2]. The accuracy of state of the art density functional theory pseudopotential methods is addressed in a large scale study of intermetallic systems [2]. [1] S. Curtarolo, D. Morgan, K. Persson, J. Rodgers, and G. Ceder, {\it Predicting Crystal Structures with Data Mining of Quantum Calculations}, Phys. Rev. Lett. {\bf 91}, 135503 (2003). [2] S. Curtarolo, D. Morgan, and G. Ceder, {\it Accuracy of ab-initio methods in predicting the crystal structures of metals: review of 80 binary alloys}, Calphad (2005) [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 9:12AM |
X6.00002: Ab-initio simulations of systems of biochemical and
biological interests Invited Speaker: |
Friday, March 25, 2005 9:12AM - 9:48AM |
X6.00003: Emergence of Complex States in CMR Manganites and High-Tc Cuprates Invited Speaker: Recent developments in the context of theory and experiments for manganites and cuprates will be discussed. It will be argued that the presence of nanoscale phase separation is at the heart of the colossal magnetoresistance phenomenon [1]. Simulation results support this view, as well as experimental data. These effects are not limited to manganites, but they may appear in other compounds as well, such as the high-Tc cuprates. New results will be presented in this area, on the phenomenological competition between antiferromagnetism and d-wave superconductivity, suggesting the possibility of ``colossal'' effects in this context [2]. This is compatible with the recent discovery of ``giant proximity effects'' in Cu-oxides [3]. All this suggests that clustered or mixed-phase states could form a new paradigm for the understanding of compounds in condensed matter physics. Work in collaboration with G. Alvarez, M. Mayr, A. Moreo, C. Sen, and I. Sergienko, supported by NSF DMR. [1] A. Moreo et al., Science 283, 2034 (1999); E.D., T. Hotta and A. Moreo, Physics Reports 344,1 (2001); E.D., ``Nanoscale Phase Separation and Colossal Magnetoresistance'', Springer-Verlag, 2002. [2] G. Alvarez et al., cond-mat/0401474, PRB to appear. [3] I. Bozovic et al., Phys. Rev. Lett. 93, 157002 (2004) [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:24AM |
X6.00004: Unusuall magnetic anisotropies in artificial surface nanostructures: nano-wires, corals and particles on Pt and Cu surfaces Invited Speaker: Magnetic nanostructures are often viewed as they may replace nanocrystalline materials in the quest for ever increasing density of magnetic data storage. Computational studies of these systems based on first-principles methods are nowadays possible with high accuracy on realistic system sizes. Consequently, quantities which are very difficult or currently not possible to measure can be calculated and analyzed. In this talk i present the basic building blocks of a theory and respective computational tools to calculate the magnetic properties of surface nanostructures based on a relativistic multiple scattering theory, including the description of the host surface and the embedding procedure. The theory will be illustrated by calculating various magnetic properties including Magnetic Anisotropy Energies of nanostructures ranging from single impurities to quantum corals. Since many of such systems exhibit ground states which are canted and/or non-collinear, a zero temperature spin-dynamics technique is described as an efficient method to find them. This is illustrated on Co and Fe chains on Pt(111) surface where comparison with experiment is possible. At the end i will show how general symmetry arguments can be used to extend the results of calculations. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 11:00AM |
X6.00005: Predicition and Discovery of High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Crystalline Barriers Invited Speaker: Tunneling magnetoresistance in excess of 200{\%} has recently been observed in magnetic tunnel junctions using bcc Fe or bcc CoFe electrodes with crystalline MgO tunnel barriers[1,2]. These results demonstrate that tunneling magnetoresistance depends on more than the ``electrode polarization''. This talk will describe the calculations that predicted high TMR in these and other systems[3,4,5]. These calculations helped us to understand certain principles that may lead to high TMR through coherent electron tunneling. They can be briefly summarized as follows: (1) If the symmetry of a Bloch state can be preserved as electrons cross the interfaces between the electrode and the tunnel barrier, this be used to advantage for spin filtering. (2) Evanescent states of different symmetries decay at different rates in the barrier. (3) Interfacial bonding can be very important in determining the probability that an electron can traverse the interface. (4) Electrons of disallowed symmetry cannot propagate in an electrode. Once these simple principles are understood, simple band codes can be used to screen and to develop heterostructures with the proper symmetries to obtain high TMR. [1] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant AND S.-H. Yang, ``Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers,'' Nature Materials, Advance Online Publication [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, ``Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions,'' Nature Materials, Advance Online Publication [3] W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, ``Spin-dependent tunneling conductance of Fe $\vert $ MgO $\vert $ Fe sandwiches'' Phys. Rev. B \textbf{63}, 054416 (2001) [4] J. Mathon, A. Umerski, ``Theory of tunneling magnetoresistance of an epitaxial Fe/MgO/Fe(001) junction,'' Phys. Rev. B \textbf{63}, 220403(R) (2001). [5] X.-G. Zhang, and W. H. Butler, ``Large magnetoresistance in bcc Co/MgO/Co and FeCo/MgO/FeCo tunnel junctions,'' Phys. Rev. B \textbf{70}, 172407 (2004) [Preview Abstract] |
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