Bulletin of the American Physical Society
2005 TSAPS/AAPT/SPS Joint Fall Meeting
Thursday–Saturday, October 20–22, 2005; Houston, TX
Session C2: Condensed Matter III |
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Room: Waldorf Astoria A 210A |
Friday, October 21, 2005 3:00PM - 3:12PM |
C2.00001: Anisotropic lattice anomalies and the pressure effect on the ferroelectricity in multiferroic HoMn$_{2}$O$_{5}$ Clarina dela Cruz, Fei Yen, Bernd Lorenz, Ching-Wu Chu, Marin Gospodinov, William Ratcliff, Jeffrey Lynn Distinctive anomalies in the linear thermal expansivities along all principal axes were observed in multiferroic HoMn$_{2}$O$_{5}$ at the magnetic (T$_{N1}$ and T$_{N2})$ and ferroelectric (T$_{C1}$ and T$_{C2})$ transitions with a notable negative thermal expansivity along the $c$-axis for T$<$25K. These measurements provide a clear evidence of significant coupling between the magnetic and lattice orders in this material. Two main anomalies were observed, one at the onset of ferroelectricity (T$_{C1})$ and the other at T$_{C2}$ where another sharp change in the dielectric order was seen. The lattice anomalies were highly anisotropic where the $a $and $ b$-axes expand as the $c$-axis shrinks upon cooling through T$_{C1}$ and T$_{C2}$. The incommensurate AFM ordering of the Mn$^{3+}$ spins has been associated with T$_{N1}$. This is characterized by a high degree of frustration that subsequently drives the ferroelectric displacements at the lock-in transition to a commensurate magnetic structure. The second magnetic anomaly at T$_{N2}$ was shown via neutron diffraction to be due to a spin-reorientation phase transition. Dielectric measurements under isotropic pressure revealed that the two ferroelectric transitions are correlated and that the pressure stabilizes the ferroelectricity below T$_{C2.}$ [Preview Abstract] |
Friday, October 21, 2005 3:12PM - 3:24PM |
C2.00002: Extraordinarily Large Diffraction Efficiencies by Permanent Gratings in Carbon Nanotube and Methyl Red Doped Liquid Crystal Gene Carlisle, Saunab Ghosh, Yousef Suleiman We present a detailed description on the preparation of nematic liquid crystal (E7) cells doped with only methyl red as well as doped with single-wall carbon nanotubes and methyl red. Permanent grating were written by use of either 532-nm or 488-nm pump beams and probed with a 670-nm beam. The cells doped with carbon nanotubes and methyl red produced extremely high diffraction efficiencies of 60 percent. Cells doped only with methyl red generated diffraction efficiencies around 10 percent. The angular dependence of polarized absorption spectra and diffraction are in agreement with a photochromic mechanism for the phase modulation. The permanent gratings are quite stable when stored at ambient conditions for over one year. [Preview Abstract] |
Friday, October 21, 2005 3:24PM - 3:36PM |
C2.00003: The Spin Polarization at the Metal-Insulator Transition R.V.A. Srivastava, W. Teizer, F. Hellman, R.C. Dynes We have extracted the spin-polarized (SP) density of states (DOS) of 3-dimensional amorphous (a-) Gd$_{x}$Si$_{1-x}$ in the quantum critical regime (QCR) of a magnetic field tunable metal-insulator transition (MIT) by measuring the SP tunneling conductance of an Al/Al$_{2}$O$_{3}$/a-Gd$_{x}$Si$_{1-x}$ planar tunnel junction at T=25mK and H$\le $3.0T. We have applied SP Abrikosov-Gorkov DOS to fit the data, leading to a significant improvement over prior attempts to use a SP Bardeen-Cooper-Schrieffer DOS. We find a large polarization near the MIT of a-Gd$_{x}$Si$_{1-x}$ (x=0.14). We have determined polarization values at different applied magnetic fields allowing for a study of the polarization dependence in the QCR. [Preview Abstract] |
Friday, October 21, 2005 3:36PM - 3:48PM |
C2.00004: Morphology and structure of copper-phthalocyanine and iron-phthalocyanine thin films on Ag(111) Kedar Manandhar, Kenneth Park, Zhen Song, Tanhong Cai, Shuguo Ma, Jan Hrbek Copper-phthalocyanine and iron-phthalocyanine have been in situ deposited onto Ag(111) surface at room temperature by vacuum sublimation. Thin heteroepitaxial films formed thereof have been investigated using scanning tunneling microscopy. When annealed up to $\sim $ 500\r{ }K, both molecules form well-ordered square lattices on the hexagonal substrate lattice but result in different molecular orientations within the surface unit cell. For CuPc, benzo units of the molecules are rotated by approximately 30\r{ } with respect to the surface lattice vectors forming a high-density packing whereas for FePc the benzo units are rotated by about 45\r{ }. Consequently, the square lattice constant for CuPc measured at $\sim $14 {\AA} is significantly smaller than that of $\sim $16 {\AA} for FePc. Annealing to $\sim $700\r{ }K results desorption of as much as 70{\%} of surface monolayer of phthalocyanine molecules. The aggregates are observed in dendrite-like form, and further discussion of temperature effects on CuPc and FePc thin films will be presented. [Preview Abstract] |
Friday, October 21, 2005 3:48PM - 4:00PM |
C2.00005: Response of Biological Molecules to Ultrafast Laser Pulses Petra Sauer, Roland Allen Our earlier work on organic molecules containing only carbon and hydrogen has been extended to biological molecules which also contain nitrogen and oxygen. We will present preliminary results, with animations, for the response of dipicolinic acid and retinal to femtosecond-scale laser pulses. We will also discuss how a realistic model was developed which (1) provides a reasonably accurate representation of chemical bonding in complex molecules and (2) is computationally efficient enough to permit simulations of the coupled electronic and nuclear dynamics with a time step of order 10 attoseconds. One starts with a diatomic molecule, fitting the Hamiltonian to the principal electronic energy gap, and an effective repulsive potential to the bond length and vibrational frequency. However, the model must work more globally, in a variety of different chemical environments, and to achieve this broader goal requires further refinements and testing. There are various subtleties. For example, with a minimal basis set, the highest energy state is unphysically senstive to changes in bond length, because it is not pushed down by higher-lying states. We will discuss preliminary work on quantum control of molecules via changes in the duration, intensity, polarization, and other laser pulse parameters. [Preview Abstract] |
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