Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L25: Organic Spintronic Materials and Nano-Spintronic Materials |
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Sponsoring Units: DPOLY DMP Room: Colorado Convention Center 203 |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L25.00001: BREAK
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Tuesday, March 6, 2007 3:06PM - 3:42PM |
L25.00002: Understanding electronic properties at organic/silicon interfaces from first principles Invited Speaker: Organic/inorganic interfaces often possess properties that are significantly different from those of the organic molecules and the inorganic substrate that comprise them, due to both inter- molecular and molecule-substrate interactions. In this talk, I show how we explore such electronic effects using first principles calculations of prototypical silicon/organic interfaces. I focus on dipole depolarization effects, demonstrated for benzene derivatives on Si(111), and on interface-induced gap states, demonstrated on alkyl chains on Si (111). By comparing the results to both experiment and phenomenological models, we rationalize these effects and predict their manifestation in a range of organic electronic structures and devices. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L25.00003: Ferromagnetism in a Porphyrin-based Organic Semiconductor J. Moreno, M.A. Majidi, W.A. Schwalm, R.S. Fishman Current efforts in growing supramolecular quasi two-dimensional magnetic organic semiconductors, such as porphyrin-based or bimetallic oxalates materials, have not been followed by close theoretical studies of their magnetic properties. Interplay between experimental and theoretical approaches is needed to increase their ferromagnetic transition temperatures, which are still quite low. Our aim is to contribute to the theoretical effort by studying a simplified model of a two-dimensional array of magnetic ions embedded in a porphyrin matrix. Since the distance between the local moments is very large their magnetic couplings are mediated by the metal-like extended pi-orbitals. Therefore, our approach is based on a Double-Exchange Hamiltonian with effective hopping between magnetic ions derived from Huckel model. We solve this model using the Dynamical Mean Field Approximation (DMFA) including several ions on the local site. In order to predict the optimal magnetic properties, we calculate the ferromagnetic transition temperature, magnetization and susceptibility for a range of parameters. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L25.00004: Self-Assembly of Magnetic Molecules on GaN(000\underline 1) Saw W. Hla, Danda P. Acharya, Violeta Iancu, Erdong Lu, Arthur R. Smith Self-assembled clusters of TBrPP-Co molecules are formed on a freshly grown nitrogen polar GaN (000\underline {1}) surface. The structural and electronic properties of the molecular clusters are then studied by using a scanning tunneling microscopy and spectroscopy at low-temperature (4.6 K) under an ultra-high-vacuum condition. The TBrPP-Co molecule has a spin-active cobalt atom caged at the center of porphyrin unit and four bromo-phenyl groups are attached to its four corners. On GaN(000\underline {1}), the molecules bind the surface through the bromo-phenyl units and form a saddle conformation, in which the central part of the molecule is bent by lifting the two pyrrole units of the porphyrin macrocycle. Within the self-assembled molecular clusters on this surface, the molecules are aligned either parallel or 90 degree rotated to each other. This molecule-substrate system may be useful for spintronic applications. This work is supported by NSF-NIRT grant number DMR 0304314. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L25.00005: Atomic and Electronic Structure of a Novel Two-Dimensional Molecular Magnet System Anthony Caruso, Trevor Tyson, Douglas Schulz, Wolfgang Caliebe Molecular magnet systems show much promise to replace standard metals and metal oxide systems in a broad range of application due to the comparative simplicity of processing. Understanding the coupling of the spins in these systems is important to determine their full range of applicability. We have studied the local atomic and electronic structure of a recently synthesized Mn carboxylate system which forms two-dimensional interconnected rings. To understand the spin interactions, the local atomic and electronic structure about the Mn sites was investigated by x-ray absorption spectroscopy and high resolution x-ray emission spectroscopy. The valence and spin configuration are described. Comparisons are made between the coupling of Mn sites via the oxygen atom with standard magnetic oxide systems such as the manganties. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L25.00006: Growth and electronic structure of tetracyanoethylene on noble metals studied by scanning tunneling microscopy Daniel Wegner, Ryan Yamachika, Yayu Wang, Bart Bartlett, Jeff Long, Mike Crommie Tetracyanoethylene (TCNE, C$_{2}$(CN)$_{4})$ is a $\pi $-electron acceptor with a very strong electron affinity that easily forms charge-transfer complexes with other organic molecules and metals. We have performed STM and STS of isolated TCNE molecules and ordered sub-monolayer coverages on noble-metal surfaces in order to study the competition between intermolecule and molecule-substrate interactions, and the impact this might have on film-growth and electronic structure. HOMO and LUMO peaks were observed for single TCNE molecules on Ag and Au substrates using STS, but not for Cu substrates which react more strongly with TCNE. The spatial distribution of the TCNE HOMO, as observed in dI/dV maps, fits well with DFT calculations and shows that TCNE is in a negatively charged state on these metal substrates. dI/dV maps of ordered TCNE arrays indicate that neighboring TCNE molecules interact strongly with each other in some cases. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L25.00007: Strong electron-phonon interaction in e-e correlated molecular systems Yuri Dahnovsky Molecular systems (molecules) with strong electron-phonon interaction are described in terms of the Green functions. In the case of the strong e-ph interaction a general scheme that includes of the Dyson equations for the electron and phonon Green functions, is not productive. Hence, the different methodology is developed where the unitary transformation (that included both electron and phonon subsystems) is used. In this case the Dyson equation for the electron Green function is not valid any longer. Different approximations are proposed. The developed approach is extremely important for electron transfer reactions without single electron transfer assumption. It can be also used in the transport in molecular junction devices. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L25.00008: Electron Spin Relaxation in Hole Polaron States of Conjugated Porphyrin Arrays Paul J. Angiolillo, Paul R. Frail, Nora Graneto, Devlin Murdock, Michael J. Therien It has been previously demonstrated that stable hole-polaron states may be produced in a family of highly $\pi $-conjugated (porphinato)Zn(II) in which the monomeric units are bridged by ethyne linkages. Furthermore, EPR results verify that hole delocalization or incoherent hopping occur over substantial distances ($\sim $ 7.5 nm) along a single conjugated backbone. The electron spin relaxation times in traditional conducting materials are on the order of picoseconds. Preliminary data gleaned from progressive microwave saturation will show that electron spin relaxation times in these materials are on the order of 1$\mu $s at 298 K in both solution and in film architectures and moreover are relatively insensitive to oligomer length with distances spanning 1.4 to 7.5 nm. Since hopping rates have been observed to be on the order of 10$^{+7}$ Hz, it is possible then for spin memory of the hole polaron to be retained during its migratory process. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L25.00009: Spin states and their relaxation in transition-metallorganic self-assembled molecules Zhi-Gang Yu The coexistence of spins localized on transition-metal ions and mobile charges on the $\pi $-conjugated ligands in transition-metallorganic self-assembled molecules (TMSAMs) makes these molecules attractive for molecular spintronic devices and quantum computing. We present our theoretical results on the spin states localized on the transition-metal ion in a TMSAM using both the first-principles approaches and the ligand-field theory. Then we construct a spin Hamiltonian to calculate spin lifetimes and identify the dominant spin relaxation mechanisms in the molecule. We also discuss the relation between the spin states on the transition-metal ion and the charge transport along the $\pi $-conjugated ligand in the molecule. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L25.00010: Low Temperature STM Investigation of Molecular Kondo Effect Gayani Perera, Violeta Iancu, Saw-Wai Hla We investigate site-dependent Kondo effect on TBrPP-Co molecules on a Cu(111) surface at 4.6 K using scanning tunneling microscopy and spectroscopy [1]. The TBrPP-Co molecule has a spin-active cobalt atom caged at the center of porphyrin unit and four bromo-phenyl groups are attached to its four corners. On Cu(111), the molecules can anchor on the surface with either planar or saddle conformation [2]. For the current study, we choose only planar type molecules, in which the porphyrin unit is lying parallel to the surface and the molecule binds through the surface via four bromo-phenyl units as well as central porphyrin unit. The Kondo temperature of 170 K is measured above the cobalt atom location, i.e. at the center of the molecule. The observed Kondo effect is caused by spin-electron coupling between the cobalt atom of the molecule and the free electrons from the surface [2,3]. We find that the Kondo effect observed here is contributed from both surface and bulk states of Cu(111). This work is supported by the US Department of Energy Basic Energy Sciences grant no. DE-FG02-02ER46012. [1] G. Perera, V. Iancu, Luis G.G.V. Dias da Silva, S.E. Ulloa, and S.-W. Hla. Submitted. [2] V. Iancu, A. Deshpande, and S.-W. Hla, Nano Lett. 6 (2006) 820-823. [3] V. Iancu, A. Deshpande, and S.-W. Hla, Phys. Rev. Lett. (2006) in press. [Preview Abstract] |
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