Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z12: Phase Transitions at Surfaces |
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Sponsoring Units: DCMP DMP Chair: Robert Bartynski, Rutgers University Room: 308 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z12.00001: A martensitic-like transition in a normal alkane Jeffrey Hutter, Shailesh Nene, Eric Karhu, Roberta Flemming The normal alkanes, C$_{n}$H$_{2n+2}$, with a structure consisting of a single chain, are the simplest hydrocarbons. These are an interesting class of material, both in terms of their intrinsic properties and the fact that many biological molecules contain hydrocarbon domains. Normal alkanes exhibit an unusual phase diagram with several solid phases, some of which---the ``rotator phases''---are characterized by positional order without long-range orientational order. We have found a striking pattern of twinned, striped domains that occurs in thin layers the monoclinic rotator RV phase of tricosane (C$_{23}$H$_{48}$). We have studied this structure, and its transitions to other phases, by X-ray diffraction, as well as by optical and atomic-force microscopy. Intriguingly, transitions between the RV phase and the RI orthorhombic phase lying at higher temperatures appear to be diffusionless, and preserve molecular-scale features even after multiple transitions between the phases. These properties are reminiscent of martensitic transformations, which are better-known in metal alloys, but occur here at convenient temperatures and with slow kinetics. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z12.00002: Monolayer solids of short (perfluoro)alkanes on graphite L.W. Bruch Calculations are reported for the relative stability of monolayer solid latices on graphite for C$_2$H$_6$, C$_3$H$_8$, C$_2$F$_6$, and C$_3$F$_8$. Triangular, centered rectangular and two-sublattice herringbone lattices are treated. The calculations use all-atom (AA) models and are based on non-bonding interactions formulated for three dimensional dense phases of alkanes and perfluoroalkanes. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z12.00003: Simulation of methane on Al(111) Majid Karimi, Carl LeBlond, Wahyu Setyawan, Stefano Curtarolo, Renee Diehl Classical many-body interatomic potentials for methane molecules interacting with Al(111) are developed using the embedded-atom method (EAM). The optimized EAM parameters for methane-Al(111) are obtained by fitting to the data generated from the first principles calculations. Adsorption of methane on the Al(111) substrate is studied using the Grand Canonical Monte-Carlo method. Adsorption isotherms are calculated at several temperatures near the triple point temperature of methane Tt = 90K, and structural parameters obtained from the adsorbed density profiles. The structures and the thermodynamics of film growth are compared to methane adsorption on other metal surfaces, graphite and the d-Al-Co-Ni quasicrystal. This research is supported by NSF and ACS-PRF. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z12.00004: Conformation-controlled networking of H-bonded assemblies on surfaces M. Enache, M. Matena, M. Stoehr, T.A. Jung, A. Llanes-Pallas, D. Bonifazi In order to prepare networks that could undergo phase transitions through a thermally-induced inversion of the molecular conformation leading to a variation of the intermolecular interactions, directional intermolecular forces can be regarded as promising candidates. In particular, H-bonding interactions will be exploited since their intermolecular interaction strength and geometry can be controlled by the number and arrangement of available H-bonding donor or acceptor moieties. We studied the 2D self-organization of a conjugated molecule bearing terminal 2,6-di(acylamino)pyridine moieties [1], which are well-known to form H-bonds, on a Ag(111) surface by STM. The hexagonal porous network, which is formed for room temperature deposition, is transformed into a close-packed rhombic pattern by a thermally induced trans-cis inversion of the terminal groups. This transformation can be explained by the fact that the system wants to minimize its energy: at the same time the free surface energy is minimized while the number of H-bonds per terminal group is doubled from two to four. [1]~A. Llanes-Pallas et al., Angew. Chem. Int. Ed. 2008, 47, 7726 [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z12.00005: IR enhancement and the surface potential of n-alkanethiol SAMs on GaAs(001) Gregory M. Marshall, Gregory P. Lopinski, Farid Bensebaa, Jan J. Dubowski n-Alkanethiol self-assembled monolayers (SAMs) were prepared on the GaAs(001) surface according to [1]. FTIR modal analysis of the CH$_{2}$ stretching mode region (2800-3000 cm$^{-1})$ verified SAM structural coherence, revealed evidence of an assembly threshold and allowed the absorption coefficient of the SAM phase to be directly measured. A 6x enhancement factor was observed relative to coefficients derived from the liquid and polycrystalline phases. This effect is reviewed in terms of the molecular order and is largely attributed to the chemical properties of the interface [2]. Confirmation is provided by Kelvin Probe measurement of the sheet dipole potential, interpreted in terms of the Cooperative Molecular Field Effect [3]. [1] McGuiness et al., J. Am. Chem. Soc. 128, 5231 (2006). [2] Marshall et al., submitted. [3] Cahen et al., Adv. Funct. Mater. 15, 1571 (2005). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z12.00006: Faceting of Ru($11\bar {2}0)$ Surface: A Model System for Catalysis Quantong Shen, Wenhua Chen, Hao Wang, Robert Bartynski We have studied NO$_{2}$-induced faceting of a Ru($11\bar {2}0)$ surface by means of low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES). By annealing the sample at $>$ 600 K in NO$_{2}$ (10$^{-8}$ Torr), the surface becomes fully faceted as revealed by LEED although it is rather smooth, with only two layers exposed. The faceted surface remains the same at NO$_{2}$ exposure ranging from 20 L to 12000 L and is stable for substrate temperature $T \quad <$ 850 K.The STM results confirmed the LEED observations and showed that the faceted surface consists of sawtooth ridges along the [0001] direction with typical dimensions of $\sim $5 nm in width and $>$ 100nm in length. We have found that the faceted O/Ru surface is very active for NH$_{3}$ decomposition to produce H$_{2}$ with high selectivity to N$_{2}$ at room temperature. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z12.00007: Oxygen-induced nano-faceting of the Pd(112) surface Alina Vlad, Andreas Stierle, Rasmus Westerstroem, Edvin Lundgren, Helmut Dosch Extensive efforts are currently made to understand the elementary steps in heterogeneous catalytic reactions, with the ultimate goal of designing more efficient catalysts. The structure of the catalytically active particles, as well as the temperature and the gas phase pressure, play a decisive role in the behavior of these systems. We studied the interaction of oxygen with the Pd(112) surface from ultrahigh vacuum to atmospheric oxygen pressures by means of {\em in-situ\/} surface x-ray diffraction, high resolution core level spectroscopy and scanning tunneling microscopy. The rearrangement of the stepped (112) surface into different type of facets was observed and is strongly dependent on the oxygen pressure. The transition from different adsorbed-oxygen phases to the bulk oxide was also followed as a function of the oxygen pressure. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z12.00008: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z12.00009: Phase-field theory for phase transition dynamics of reconstructed semiconductor surfaces Bang-Gui Liu We propose a natural two-speed model for the phase dynamics of Si(111) 7$\times $7 phase transition to unreconstructed phase. Our simulated results show that a 7$\times $7 island decays with its shape kept unchanged, and its area decay rate is a constant increasing with its initial area. LEEM experiments concerned are explained, which confirms that the dimer chains and corner holes are broken first, and then the stacking fault is remedied slowly. We also propose a phase-field-crystal model for the (2$\times $1)-(1$\times $1) phase transitions of Si(001) and Ge(001) surfaces. Simulated periodic arrays of dimerization is consistent with STM images. Calculated temperature dependence of the dimerization indicates that normal dimers and broken ones coexist between T$_{L}$ and T$_{H}$, and a first-order phase transition takes place in between. This phase-field method is a reliable approach to phase dynamics of surface phase transitions. Phys. Rev. Lett. 100, 056103 (2008). [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z12.00010: Investigation of Structural Phase Transitions on Wurtzite Gallium Nitride Surfaces Tianjiao Chen, Abhijit Chinchore, Yinghao Liu, Kangkang Wang, Wenzhi Lin, Arthur Smith Surface structures of wurtzite gallium nitride (w-GaN) have been investigated previously,[1][2] and it is well known that above 300K there exist order-disorder phase transitions. For N-polar w-GaN (000-1) at 300K, a family of surface reconstructions occurs, including 1$\times $1, 3$\times $3, 6$\times $6, and c(6$\times $12). Not much is known, however, about what happens to these structures as they are cooled below 300K. We have recently developed a new epitaxy/analysis system, including a sample stage which can be both heated and cooled. The N-polar w-GaN surfaces are prepared using rf N-plasma-assisted molecular beam epitaxy, and monitored in-situ using reflection high energy electron diffraction (RHEED). The approach is to monitor the [11-20] and [10-10] RHEED diffractions during cryogenic cooling, starting with the 1$\times $1 or 3$\times $3 structures. A critical issue to explore is the interrelationship between surface gallium concentration and structural deformation. This study may provide the missing link to new reconstructions of w-GaN recently observed using LT scanning tunneling microscopy.[3] This work is supported by NSF (Grant No. 0730257). [1] A. R. Smith et al., Phys. Rev. Lett. \textbf{79}, 3934 (1997). [2] A. R. Smith et al., Surface Science \textbf{423}, 70 (1999). [3] D. Acharya, S.-W. Hla et al., unpublished. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z12.00011: Magnetism of bulk Cr$_{2}$O$_{3 }$and its (0001) surface: An ab initio study Siqi Shi, Aleksander Wysocki, Kirill Belashchenko Magnetic properties of bulk Cr$_{2}$O$_{3}$ and its (0001) surface are studied using the LSDA+U method. Magnetic energies are well fitted by the Heisenberg model and the Neel temperature is calculated using the quantum pair-cluster approximation. Very good agreement with experiment is found for the equilibrium volume, spectral density, local magnetic moment, band gap, and the Neel temperature. The stable (0001) surface is known to be terminated by a Cr semilayer [1], but its detailed structure is unknown. We identify two competing surface sites. The configurational surface Hamiltonian is constructed from supercells and the structural thermodynamics is studied. We find that in a wide range of temperatures about 1/3 of Cr atoms are below the oxygen layer and that there is a ordering phase transition from $\sqrt 3 \times \sqrt 3 $ to $1\times 1$ structure. Further, we find that (0001) surface has a unique feature of having an uncompensated magnetic moment that is not destroyed by surface roughness. This phenomenon makes Cr$_{2}$O$_{3}$ a promising exchange bias application. [1] M. Bender, \textit{et. al. }J. Phys.: Condens. Matter \textbf{7}, 5289 (1995). . [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z12.00012: Fingerprints of surface magnetism in Cr$_{2}$O$_{3}$ based exchange bias heterostructures Xi He, Yi Wang, Ch. Binek Magnetoelectric materials experienced a recent revival as promising components of novel spintronic devices [1, 2, 3]. Since the magnetoelectric (ME) effect is relativistically small in traditional antiferromagnetic (AF) compounds like Cr$_{2}$O$_{3}$ (max. \textit{$\alpha $}\textit{zz }\textit{$\approx $}4ps/m) and also cross-coupling between ferroic order parameters is typically small in the modern multiferroics, it is a challenge to electrically induce sufficient magnetization required for the envisioned device applications. In exchange bias systems the bias field depends critically on the AF interface magnetization. Hence, a strong relation between the latter and the surface magnetization of the free Cr$_{2}$O$_{3 }$pinning layer can be expected. Our recent research indicates that there are surface magnetic phase transitions in free Cr$_{2}$O$_{3}$ (111) films accompanying surface structural phase transitions. Well defined AF interface magnetization is initialized through ME annealing to T=20K. Subsequently, the interface magnetization is thermally driven through phase transitions at T=120 and 210K. Their effects on the exchange bias are studied in Cr$_{2}$O$_{3}$ (111)/CoPt films with the help of polar Kerr and SQUID magnetometry. [1] P. Borisov et al. Phys. Rev. Lett. 94, 117203 (2005). [2] Ch. Binek, B.Doudin, J. Phys. Condens. Matter 17, L39 (2005). [3] R. Ramesh et al. 2007 \textit{Nature Materials }6 21. Financial support by NSF through Career DMR-0547887, MRSEC DMR-0820521 and the NRI. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z12.00013: Magnetic Susceptibility $\chi$ of O$_2$ in Confined Geometries Timothy Prisk, Paul Sokol Bulk solid oxygen exhibits three distinct crystallographic and paramagnetic phases, $\alpha$, $\beta$, $\gamma$ (in order of ascending temperature). The thermodynamic behavior of some systems, including their possible phases and phase transition temperatures, are known to change in interesting and nontrivial ways when these systems are subjected to confined geometries. Recent work by Kilburn and Sokol on the phonon density of states for confined solid oxygen indicates that the first of these crystallographic phases, $\alpha $, is completely suppressed. The magnetic susceptibility $\chi$ of solid oxygen confined within various porous materials will be presented. The effect of confinement on the magnitude of the susceptibility $\chi$ and the transition temperatures will be discussed. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z12.00014: Size dependent transport of amorphous Indium Oxide films. Swati Soman, Dan Shahar Superconductivity in presence of disorder is a topic of interest among experimentalists as well as theoreticians for past several decades. Experiments performed on disordered films of various materials, elemental as well as mixture, have demonstrated Superconductor to Insulator transition (SIT) with increase in disorder or externally applied magnetic field. Disorder is difficult to quantify. However, in an experiment it is controlled by tuning film thickness and/or composition. We present experimental evidence of SIT, in disordered, amorphous Indium Oxide (a:InO$_{x})$ films, which is tuned by films' lateral dimensions. By fabricating films of same thickness and composition in Hall bar geometry and changing only the sizes of Hall bars, we observe that the sheet resistance per square, R changes with the size of the square, contrary to its definition. The systematic dependence of R on square size is observed to occur only for a critical disorder, similar to percolation model. The observations suggest an inhomogeneous nature of transport near SIT in our samples which are found to be structurally homogeneous. We postulate that such size dependent transport properties are possible to observe in disordered films of other materials that exhibit SIT. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z12.00015: Impact of Boron Additions on the A1 to L1$_{0 }$Phase Transformation in FePt Alloy Films B. Wang, K. Barmak The combination of high magnetocrystalline anisotropy energy density and good corrosion resistance has resulted in significant interest in L1$_{0}$ ordered alloys such as FePt for ultrahigh density, heat assisted magnetic recording (HAMR) media, with areal storage densities of $\ge $1Tb/in$^{2}$. When deposited at room temperature, these FePt forms in the chemically-disordered A1 state, requiring a post-deposition anneal to form the ordered L1$_{0}$ phase. Previous work has shown that the composition of FePt films has a significant impact on the kinetics and thermodynamics of the A1 to L1$_{0}$ phase transformation. In this paper, we report on the impact of ternary additions of B to FePt on the thermodynamic and kinetic parameters of the transformation. We also compare and contrast the impact of B additions with those of ternary additions of Cu and Ni. It is shown that the introduction of B into Pt-rich FePt films lowers the activation energy and the kinetic ordering temperature for the A1 to L1$_{0}$ phase transformation. However, this impact becomes negligible when B is introduced into Fe-rich FePt films. [Preview Abstract] |
Friday, March 20, 2009 2:15PM - 2:27PM |
Z12.00016: ABSTRACT WITHDRAWN |
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