Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session CC: Materials I |
Hide Abstracts |
Chair: Rene Lopez, University of North Carolina Room: Holiday Inn Brownstone Lincoln |
Thursday, October 30, 2008 10:45AM - 10:57AM |
CC.00001: Dynamics of two dimensional alkylsiloxane self assembled monolayers Mary Scott, Derrick Stevens, Jason Bochinski, Laura Clarke Self assembled monolayers (SAMs) are commonly implemented as a method to easily and permanently modify surface properties. For many existing applications, SAMs are considered essentially static systems; however, motion within disordered SAMs is of interest to the thin film community, and such systems could be studied as simplified examples of glassy materials, where density is explicitly controllable and molecule-molecule interactions can be tuned. In this study, sensitive, temperature dependent dielectric spectroscopy has been used to study molecular motion within two dimensional alkylsiloxane SAMs. Highly disordered SAMs of varying density were grown, with the intention of maximizing the motion within the films. A cooperative relaxation is observed in films with an alkyl chain length greater than three carbons long. This interacting motion has similar dynamics to a previously reported polyethylene-like glass transition occurring in phase segregated alkyl side chains of polymers with various backbones. At high film densities, an additional, local relaxation occurs. This relaxation, which has been previously observed in three dimensional SAMs, is attributed to sub-chain rotation. [Preview Abstract] |
Thursday, October 30, 2008 10:57AM - 11:09AM |
CC.00002: Shockley-type Surface State Modification for Enantiopure vs. Racemic Tartaric Acid on Ag(111) Nancy Santagata, Pengshun Luo, Bryce Davis, Thomas Pearl Shockley-type surface states exist on several metal surfaces and are characterized by electron confinement by the vacuum barrier on one side and a band gap in the bulk on the other. We will report the observation of Shockley-type surface state energy shifts as a signature of adsorbate structure for tartaric acid (C4H6O6) on Ag(111). The adsorption of both enantiopure and racemic tartaric acid in the submonolayer regime was studied with low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) and modeled with density functional theory (DFT). We find that the surface state, which is occupied on the clean Ag(111) surface (67 meV below the Fermi level), experiences a positive energy shift and becomes unoccupied after the adsorption of both forms of tartaric acid. The magnitude of the shift differs, however, for films composed of either enantiopure or racemic domains, and we attribute these relative shifts to unique adsorbate units. DFT is used to confirm the molecular-level adsorbate arrangements that lead to the experimentally observed behavior. [Preview Abstract] |
Thursday, October 30, 2008 11:09AM - 11:21AM |
CC.00003: Magnetic and magnetotransport properties of organic trilayers of alkanethiol self-assembled monolayers sandwiched between ferromagnetic thin films William Rice, Paul Hoertz, Jeremy Niskala, Jeff Haller, Mike Feng, Wei You, Frank Tsui Magnetic and magnetotransport properties of organic spin valve structures have been studied. The organic trilayer spin valve structure consists of a self-assembled monolayer (SAM) of alkanethiol and a conducting polymer layer sandwiched between two ferromagnetic metal contacts, a Ni film as the bottom contact and a Co film as the top contact. Each trilayer is confined in a square well-like structure (approximately 40 microns across) surrounded by 500nm thick photoresist to provide both electrical isolation and mechanical support for 4-terminal vertical transport measurements. The SAM was formed on the Ni surface on the bottom of the well. The conducting polymer layer was spin coated on top of the SAM prior to the deposition of the Co film. Magnetooptic Kerr Effect (MOKE) measurements show independent switching of the ferromagnetic layers at approximately 50 and 100 Gauss. Electrical transport measurements were carried out as a function of bias voltage, temperature and field, in order to explore spin-dependent transport through the organic interlayer. [Preview Abstract] |
Thursday, October 30, 2008 11:21AM - 11:33AM |
CC.00004: Advanced spectroscopy and imaging studies of multiferroicity in YMnO$_{3}$ and BiFeO$_{3}$ Relja Vasic, Marc Ulrich, Jack Rowe, Jerry Lucovsky, Joseph Fontcuberta, Xavier Marti, Jurek Sadowski, Haidong Zhou, James Brooks, Chris Wiebe There has been recent research interest in a number of magnetic ferroelectrics, including YMnO$_{3}$, a hexagonal perovskite that is antiferromagnetic [Ne\'{e}l temperature (T$_{N})$ between 70 and 130 K] and ferroelectric [Curie temperature (T$_{C})$ between 570 and 990 K] in the ground state and perovskite BiFeO$_{3}$ which is ferroelectric (T$_{C}$ $\sim $ 1103 K) and antiferromagnetic (T$_{N} \quad \sim $ 643 K), exhibiting weak magnetism at room temperature due to a residual moment from a canted spin structure. These systems can be understood by competition between local interactions on several ion sites. We report synchrotron based spectroscopy and low-energy electron microscopy (LEEM) imaging studies of sample surface and bulk for magnetoelectric coupling and spin ordering in multiferroic heterostructures and single crystals. Preliminary results indicate the importance of oxygen vacancies in ferroic properties in thin films of YMnO$_{3}$. Photoemission and x-ray absorption spectroscopy of electronic structure indicate relations between strain and crystallographic structure of epitaxial thin films grown on different substrates. The goal of this study is better understanding the interface effects and spin ordering in multiferroic heterostructures vs single crystals. [Preview Abstract] |
Thursday, October 30, 2008 11:33AM - 11:45AM |
CC.00005: Magnetism of \{Cr$_{10}$Cu$_2$\}: A story of the interplay between experiment and theory Larry Engelhardt It is a common view that theoretical results are irrelevant without experimentation and experimental results cannot be understood without theories. However, it is often the case that research in physics falls strictly under the heading of ``theory'' or ``experiment'' with limited communication between the two. Our recent analysis of a particular magnetic molecule, \{Cr$_{10}$Cu$_2$\}, provides a striking example to the contrary: X-ray diffraction measurements allowed us to formulate a general theoretical model; magnetic susceptibility measurements were used to refine the model; the model provided low temperature predictions; these predictions were verified experimentally; certain features of these measurements provided additional new insights about the theory; in turn, the theory gave a better understanding of the experimental technique itself; and this led to additional predictions for future experiments. (Background information about each of the quantities that have been measured and/or calculated will be provided.) [Preview Abstract] |
Thursday, October 30, 2008 11:45AM - 11:57AM |
CC.00006: Onset of ferrielectricity and the hidden nature of nanoscale polarization in ferroelectric thin films Matias Nunez, Marco Buongiorno Nardelli Using calculations from first principles we have elucidated the nanoscale organization and local polarization in ferroelectric thin films between metallic contacts. We discovered a ferrielectric pattern of polarization in what was originally thought to be a simple ferroelectric domain. Applying the layer polarization concept (Wu et al., Phys. Rev. Lett. 97, 107602 (2006)), we analyzed the polarization profile for different film thicknesses. The results (M. Nunez and M. B. Nardelli, Phys. Rev. Letters, to be published) unveil a peculiar spatial pattern where individual atomic layers acquire uncompensated opposing dipoles in what was originally thought to be a simple ferroelectric domain. This ferrielectric behavior arises as consequence of the complex energetic competition between the interface effects, the depolarization field, and the mutual interaction of the layer dipoles. Morover, as the thickness of the film is varied, we show that the system undergoes a ferroelectric-to-ferrielectric phase transition at a critical thickness. These results are interpreted using a simple classical model where the interface effects are explicitly taken into account. We propose a method in order to carefully tune the spatial polarization pattern of the film (M. Nunez and M. B. Nardelli, Appl. Phys. Lett. 92, 1 (2008)). [Preview Abstract] |
Thursday, October 30, 2008 11:57AM - 12:09PM |
CC.00007: ABSTRACT WITHDRAWN |
Thursday, October 30, 2008 12:09PM - 12:21PM |
CC.00008: Elasticity and Electron Fractionalization in Graphene William Shively, Dmitri Khveshchenko Much of the recent excitement over graphene comes from the fact that, at half-filling, the energy spectrum may be effectively described by a pair of Dirac fermions, giving rise to a host of effective (2+1)-d chiral gauge theoretic phenomena. In the presence of lattice distortions, hopping electrons bind to topological defects in the honeycomb lattice structure, which can lead to electron fractionalization. Recent work [c.f. Hou et al, PRL \textbf{98} (2007); Jackiw \& Pi, PRL \textbf{98} (2007)] has shown that for Peierls distortions - which in the case of graphene would be described by a Kekul\'{e} lattice dimerization pattern - such fractionalization may occur when electrons interact with topologically-induced vortices. Approaching the problem differently, here we develop a general theory of elasticity for honeycomb lattice structures with various non-trivial dimerization patterns, and explore concomitant possibilities of electron fractionalization. [Preview Abstract] |
Thursday, October 30, 2008 12:21PM - 12:33PM |
CC.00009: Epitaxial graphene growth on several SiC surface orientations at high and ultrahigh vacuum pressure Andreas Sandin, Zhengang Wang, Xianhua Kong, J.E. (Jack) Rowe We use Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) measurements on several SiC surface orientations and have measured an Auger Electron Spectroscopy (AES) signature confirming the formation of graphene at high ($\sim $10$^{-8}$ torr) and ultrahigh ($\sim $10$^{-10}$ torr) vacuum pressure. In addition, low energy electron diffraction (LEED) and ex situ AFM have been used \textit{in situ} to characterize the graphene layers. Sample preparation and characterization both \textit{in situ} and \textit{ex situ} is an important prerequisite for a final uniform graphene film. A chemical mechanical polish of the initial SiC gives atomically flat surfaces with SiC bilayer-step heights. \textit{In situ} direct current annealing at $\sim $900 C is performed to remove the native oxide and further prolonged annealing activates step flow growth of graphene on SiC. The decomposition of Si on bare SiC is shown to be more rapid at higher pressures possibly due to residual gases such as CO and H$_{2}$O while having much less effect on the 6sq root3 surface reconstruction due to its inert properties. AES is combined with STM to characterize the surface morphology combined with LEED to image surface graphene layer transformations. The half-cell stacking-fault symmetry appears to be preferred on the Si polar face as reported by others and gives the most uniform graphene growth. [Preview Abstract] |
Thursday, October 30, 2008 12:33PM - 12:45PM |
CC.00010: Epitaxial and stoichiometric effects on structural and chemical ordering in Heusler alloys Brian A. Collins, Liang He, Frank Tsui, Yuncheng Zhong, Yong S. Chu The Heusler alloys of Co$_{2}$MnGe and Co$_{2}$MnSi have been predicted to be half-metallic, where the minority spin density of states shows a gap at the Fermi Level. However, half-metallicity has not yet been realized owing to its expected sensitivity to atomic disorders associated with off-stoichiometry and epitaxial constraints. Combinatorial epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{z}$ and Co$_{x}$Mn$_{y}$Si$_{z}$ have been grown on Ge (111) substrates in and around the Heusler stoichiometry using molecular beam epitaxy. The structural and chemical ordering of the films has been examined using synchrotron x-ray microbeam techniques, including x-ray diffraction and energy dependent anomalous diffraction. A comprehensive model has been developed for anomalous diffraction, allowing for detection and quantification of various disorders even at small amounts, including site-dependent vacancies and elemental site swapping. The x-ray experiments reveal that the ordering is very sensitive to the Co:Mn atomic ratio and that epitaxial strain can cause a shift in the composition of highest structural ordering away from the Heusler stoichiometry, accompanied by increased chemical disorders. These findings have made it possible to explore spin dependent states as a function of structural and chemical ordering. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700