Bulletin of the American Physical Society
Inaugural Fall 2009 Meeting of the Prairie Section of the APS
Volume 54, Number 17
Thursday–Saturday, November 12–14, 2009; Iowa City, Iowa
Session N2: Condensed Matter Physics III |
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Chair: Craig Pryor, University of Iowa Room: IMU 243 (Ballroom) |
Saturday, November 14, 2009 8:30AM - 8:42AM |
N2.00001: Influence of magnetism on phonons in CaFe$_{2}$As$_{2}$ as seen via inelastic x-ray scattering Steven Hahn, Yongbin Lee, Ni Ni, Paul Canfield, Alan Goldman, Robert McQueeney, Bruce Harmon, Ahmet Alatas, Bogdan Leu, Ercan Alp, Duck Young Chung, Iliya Todorov, Mercouri Kanatzidis In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe$_{2}$As$_{2}$ using inelastic x-ray scattering on single-crystal samples. These measurements were compared to\textit{ ab initio }calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature tetragonal phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. [Preview Abstract] |
Saturday, November 14, 2009 8:42AM - 8:54AM |
N2.00002: Analyzing Magnetic Molecules Using TDR Steven Yeninas, Ruslan Prozorov, Marshall Luban Since the early nineties, much interest has grown in the field of magnetic molecules due to the fact that at suitably low temperatures, intermolecular interactions can be ignored. As a result, studying crystalline samples can be reduced to analyzing the discrete spectrum of magnetic energy levels within an individual molecule. As the size and complexity of magnetic molecules continues to grow, we see that low temperature DC magnetization measurements are restricted to regions of ground state level crossings, demanding a more detailed experimental technique. However, using a tunnel diode resonator (TDR) to measure the dynamic magnetic susceptibility in the millikelvin range, we can probe the magnetic spectrum in both the ground state and low-lying excited states. The TDR technique has recently been used to investigate the magnetic molecules Cr$_{12}$Cu$_{2}$ and Cr$_{10}$Cu$_{2}$. When compared with theoretical quantum Monte Carlo (QMC) simulations, we find the TDR results to be in excellent agreement with the predicted energy spectrum. This demonstrates that the QMC model can be a valuable quantitative tool for predicting properties of magnetic molecules; as well, the TDR technique is demonstrated to be a unique and powerful tool for analyzing the magnetic spectrum. [Preview Abstract] |
Saturday, November 14, 2009 8:54AM - 9:06AM |
N2.00003: Exchange-correlation energy functionals for electrons in two dimensions Stefano Pittalis, E. R\"as\"anen, C. Proetto, M. Marques, E.K.U. Gross Two-dimensional (2D) electronic systems have attracted vast interest since the beginning of semiconductor technology. The investigation of electronic properties of these 2D structures form a significant part of condensed matter and materials physics research. Among the available theoretical and computational methods to deal with many-electron systems is the density-functional theory (DFT). The fundamental quantity in DFT is the exchange-correlation (xc) energy functional, which embodies all the effect of the electron-electron interactions. In practice, this functional needs to be approximated. Many approximations have been developed for three-dimensional (3D) systems, where considerable advances beyond the commonly used local spin-density approximation (LSDA) were achieved. Unfortunately, most of the popular 3D approximations are inadequate for 2D systems. Hence, there is a clear need for new approximations specifically developed for 2D systems. Following this important need, efficient and practical expressions for the xc-energy of electrons in 2D are presented. Numerical results for finite systems show that the proposed functionals outperform the standard 2D LSDA. [Preview Abstract] |
Saturday, November 14, 2009 9:06AM - 9:18AM |
N2.00004: Spin torque and charge resistance of ferromagnetic semiconductor $2\pi$ and $\pi$ domain walls E.A. Golovatski, M.E. Flatt\'e Charge resistance and spin torque are generated by coherent carrier transport through ferromagnetic $2\pi$ domain walls, but with qualitatively different trends than for $\pi$ walls. We calculate charge and spin transport and torque for $\pi$ and $2\pi$ domain walls in a ferromagnetic semiconductor. Under coherent transport conditions, analytic solutions for spin-dependent transmission and reflection coefficients are possible [1,2]. The $2\pi$ wall resistance has a maximum at an intermediate wall width; the $\pi$ wall resistance monotonically decreases with width. The spin torque on a $\pi$ wall is highly nonlinear and insensitive to width, except for very thin walls. In $2\pi$ walls, large nonlinear spin torque is generated over a range of intermediate wall widths, but vanishes for very thin and very thick walls. We find the peak domain wall velocity is larger for a $2\pi$ wall than a $\pi$ wall, suggesting unexpected nonlinearities in magnetoelectronic devices incorporating domain wall motion.\\[4pt] [1] P. Levy and S. Zhang, PRL 79, 5110 (1997)\\[0pt] [2] G. Vignale and M. E. Flatt\'e, PRL 89, 098302 (2002) [Preview Abstract] |
Saturday, November 14, 2009 9:18AM - 9:54AM |
N2.00005: Coulomb drag and spin Hall Drag Invited Speaker: Double-layer structures consisting of two parallel quantum wells separated by a potential barrier are an important class of nanoscale electronic devices. Each layer hosts a quasi-two dimensional electron gas and electrons interact across the barrier via the Coulomb interaction. When an electric current is driven in one of the layers the Coulomb interaction causes a charge accumulation in the other layer. This phenomenon, known as {\em Coulomb drag}, is of fundamental interest as a probe of electron correlations. Another effect of great interest is the {\em Spin Hall Effect}, i.e. the generation of spin accumulation by an electric current. This is due to spin-orbit interactions and has recently received great attention not only because of its theoretical subtlety but also for its usefulness as a source of spin-polarized currents. In this talk I describe a new effect, which arises from the combination of spin Hall effect and Coulomb drag. I call it {\em Spin Hall Drag}. The effect consists in the generation of transversal spin accumulation in one layer by an electric current in the other layer. Microscopic calculations indicate that the induced spin accumulation, although considerably smaller than the one observed in the ordinary spin Hall effect, is large enough to be detected in optical rotation experiments.\\[4pt] In collaboration with Samvel Badalyan, University of Regensburg. [Preview Abstract] |
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