Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials III |
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Sponsoring Units: DCP Chair: Roberto Rodrigues, Virginia Commonwealth University Room: Morial Convention Center 213 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H21.00001: Probing electronic and magnetic properties of atomic and molecular clusters with sharp tips Invited Speaker: Probing magnetic and transport properties on a local basis with the tip of a scanning tunneling microscope (STM) allows establishing close links with the exciting field of magnetic read and write processes.~ Some examples of applications of this approach to magnetic nanostructures will be shown. First of all, the fundamental properties of Co nanoclusters, on metal surfaces will be presented. These clusters have been probed by low temperature dI/dV spectroscopy (STS). It is found that occupied electronic surface states of the Co clusters are sensitive to the crystallographic stacking and furthermore exhibit a downward energy shift as the cluster size decreases. \textit{Ab initio} calculations confirm that the observed shift is due to the size dependent mesoscopic relaxation in the clusters. When a magnetic tip is used in low temperature spin polarized (SP)-STM experiment, it is possible to reveal spin polarized feature in the local density of states. For example, one is able to identify two magnetization states of the nanometer Co clusters, corresponding to the parallel or antiparallel configuration with respect to the tip polarization. Progress in the emerging field of spintronics strongly relies on the fundamental understanding of electron/spin transport and magnetic phenomena in reduced dimensions, down to the extreme limit of individual molecules, or even single atoms where sizeable quantum effects are expected. Electronic and magnetic properties of Co atoms and metal-based molecular magnets adsorbed on magnetic nano-islands or on non magnetic surfaces will be presented. On the example of Co-phtalocyanines prepared in UHV at 4.6 K, it will be shown that dI/dV characteristics are representative of both, the nature of the molecule and also its interaction with the substrate. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H21.00002: First Principles Theory of Supported Clusters with Complex Magnetic Order. Invited Speaker: It is demonstrated that the magnetic interactions can be drastically different for nano-sized systems compared to those of bulk or surfaces. In a real-space formalism we have developed a technique to calculate non-collinear magnetization structures and hence exchange interactions. Our results for magnetic Cr, Mn and Fe clusters supported on a Cu(111) surface show that the magnetic ordering as a rule is non-collinear and can not always be described using a simple Heisenberg Hamiltonian. We argue that the use of \textit{ab initio} calculations allowing for non-collinear coupling between atomic spins constitute an efficient and reliable way of analyzing nano-sized magnets. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H21.00003: Magnetic properties and stability of metalloinorganic clusters Roberto Robles, Shiv Khanna Theoretical studies on the structure, stability, electronic structure and magnetic properties of binary clusters Si$_n$TM$_m$ (n=1-8, TM=Cr,Mn) have been carried out within a density functional formalism using the generalized gradient approximation. The stability of the clusters as a function size is analyzed in terms of several criteria, like the progression in bonding energy and HOMO-LUMO gap, with the ultimate objective of identifying the simple rules that can guide the search of stable species. The magnetic properties of the clusters are investigated by considering different ferromagnetic and antiferromagnetic arrangements of the local spin moments and optimizing the geometry and the spin state to determine the ground state including possible isomers. The possible use of these clusters as building blocks of cluster assemblies is discussed, and finally, the interest of these assemblies in the design of materials which could be used in the field of spintronics is briefly considered. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H21.00004: Intermediate valence, local antiferromagnetic coupling and the Kondo effect in ytterbium organometallic molecules C.H. Booth, W.W. Lukens, M.D. Walter, D. Kazhdan, R.A. Andersen, E.D. Bauer, L. Maron, O. Eisenstein Studying magnetic ions coupling to aromatic rings in organometallic molecules provides an analogous route to studying the Kondo effect in nanoscale systems. We extend the number of molecules displaying such effects in their magnetism and x-ray absorption spectroscopy from cerocene [Ce(C$_8$H$_8$)$_2$] and Cp*$_2$Yb(bipy) [Cp*=pentamethylcyclopentadienyl, bipy=bipyridine) to a collection of Cp*$_2$Yb($L$) molecules, where $L$ is one of various bipyridyl or diazadiene ligands. Clear trends are observed in both the magnetic susceptibility and the Yb valence that indicate changes in the fundamental temperature scale. CASSCF calculations indicate the intermediate valence is primarily due to a configuration interaction between the open-shell $f^{13}\pi^{*1}$ and the closed-shell $f^{14}\pi^{*0}$ spin-singlet states, in direct analogy to the Kondo effect in intermetallic systems. These studies increase the range of molecular species where such properties are observed, and point toward understanding the ubiquity of such effects and their involvement in fundamental bonding and magnetism in organometallic molecules. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H21.00005: The structure, stability, and magnetic properties of Au(111)/NiO(111) interface: density functional theory study K.L. Yao, Y.L. Li, Z.L. Liu We studied the electronic structure of Au(111)/NiO(111) interface in accordance with the two models of NiO(111) surface. The work of adhesion, the spin magnetic moment, the stability and the electronic properties of the Au(111)/NiO(111) interface were calculated by density functional theory (DFT). The calculated results of Au(111)/NiO(111) interface were then compared with non-polar Au(100)/NiO(100) interface. At the same time, the total density of states (DOS) of Au(111)/NiO(111) interface corresponding to the two models were also calculated. The calculations reveal that the Ni-terminated and the oxidized interfaces have antiferromagnetic properties, while the O-terminated interface exhibits ferromagnetic properties. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:24AM |
H21.00006: Investigating the Molecular Level Details of Catalytic Oxidation Reactions Invited Speaker: Gas-phase cluster reactivity studies are providing significant insight into the molecular level mechanisms of oxidation reactions occurring on catalytic surfaces. Our experimental approach, employing tandem mass spectrometry, uses mass selected metal oxide clusters to model specific catalytic active sites. This technique enables investigation of the influence of factors such as size, stoichiometry, charge state, and elemental composition on the reactivity of catalytic materials. Particular emphasis is on identifying species with enhanced activity for the selective oxidation of simple hydrocarbons and atmospheric pollutants. Recent findings pertain to the kinetics of ethylene oxidation in the presence of vanadium oxides and the oxidation of carbon monoxide in the presence of gold and iron. Through a combination of experiments and theoretical calculations we establish structure-reactivity relationships and propose general reaction mechanisms for these catalytic processes. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H21.00007: Effect of ligand on the geometric and electronic structure of Au$_{13}$ cluster Ghazal Shafai, Sampyo Hong, Talat Rahman, Massimo Bertino We have carried out calculations based on the density functional theory in the projector augmented wave scheme (PAW) and the pseudopotential approach, to examine the effect of the ligand on the geometric and electronic structure of Au$_{13}$ cluster. We find \textit{bare} Au$_{13}$ to form a flat flake, in agreement with previous theoretical calculations. This structure is lower in energy by 2.60 eV in comparison with the well ordered icosahedron geometry. Our results show, however, that the \textit{Au}$_{13}$\textit{ cluster covered with ligands of phosphine (PH}$_{3}$\textit{) forms a stable spherical structure} \textit{(icosahedron) }in agreement with the experiment [1] which is by 0.08 eV lower in energy when compared to the flat-flake complex. If the phosphine is replaced by H, the spherical structure is no longer stable, but it still maintains a 3 dimensional form, signifying the effect of the ligand in stabilization of the structure. We observe a narrow d-band for flat-flake gold atoms in the complex, while in the icosahedron structure the d-band is wider. We also find a stronger overlap between the p orbitals of the P atom with d orbitals of gold atoms in the icosahedron complex. [1] M. F. Bertino et. al. Phys. Chem. B Lett. 110, 21416 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H21.00008: Superheating, Melting and Precursors to Melting in Metal Nanoparticles Dmitri Schebarchov, Shaun Hendy We have investigated precursors to melting in metal nanoparticles using molecular dynamics in the microcanonical ensemble. At the onset of solid-liquid phase coexistence, we find first-order transitions in clusters with non-melting facets (i.e. facets that are not wet by the melt such as Pb (111)), and continuous transitions otherwise. In sufficiently small clusters however, we find that static solid-liquid coexistence is unstable. Further, the size at which the instability arises, and even the melting temperature, depends on the ability of the melt to wet the solid facets of the cluster. In particles with non-melting facets we show that the melting temperature can exceed that of the bulk material. Finally, we also discuss a range of solid-solid transitions that have been observed to occur during solid-liquid phase coexistence, some of which are again driven by the preference of the melt to wet certain crystal facets. [Preview Abstract] |
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