Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B9: Phase Transitions, Dynamics, and Excitations |
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Sponsoring Units: DCMP Chair: Alberto Pimpinelli, Université Blaise Pascal Room: Baltimore Convention Center 301 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B9.00001: Competing periodicities in fractionally filled quasi-1D bands P.C. Snijders, S. Rogge, H.H. Weitering We present a variable temperature Scanning Tunneling Microscopy and Spectroscopy (STM and STS) study of the Si(553)-Au atomic chain reconstruction. This quasi one-dimensional (1D) system undergoes at least two charge density wave (CDW) transitions at low temperature, which can be attributed to electronic instabilities in the fractionally-filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the imaged chains. This Peierls state is ultimately overcome by a competing tripleperiod CDW, which in turn is accompanied by a $\times $2 periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly half-filled and quarter-filled 1D bands. The mobility of atomic scale dislocations in the $\times $3 CDW state indicates the possibility of manipulating phase solitons carrying a (spin,charge) of (1/2,$\pm $e/3) or (0,$\pm $2e/3). [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B9.00002: Order-disorder phase transitions: a DFT - (Wang-Landau) MC study Mira Todorova, Mikael Borg, Catherine Stampfl, Matthias Scheffler Using a hybrid statistical mechanics method comprising the use of a Lattice-gas Hamiltonian (LGH) determined from ensity-functional theory and subsequent Monte Carlo (MC) calculations, we obtain a phase diagram for Na-Al surface alloys from first-principles and compare it to experimental results [1]. A safe approach towards parametrizing a LGH from the self-consistent evaluation of the electronic structure starts with an analytic form for the long-range pair-interactions. This expression is corrected in the short and medium range using DFT derived data, also including nearsighted many-body terms. An ensuing cross-validation is of utmost importance to ensure that the corrections (to the long-range part) are assessed with an optimum accuracy. The thus extracted LGH is used to perform MC calculations, also using the new Wang-Landau MC algorithm [2], which allows us to reliably determine the transition temperature.\\ $[1]$ M. Borg {\it et al.}, Chem. Phys.Chem. {\bf 6}, 1923 (2005).\\ $[2]$ F. Wang and D.P. Landau, Phys. Rev. Lett. {\bf 86}, 2050 (2001). [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B9.00003: Bose Einstein condensation of magnons in mesoscopic ferromagnets. L.H. Bennett, E. Della Torre, R.E. Watson Bulk magnetic materials are comprised of magnetic domains. As the size of the sample is reduced, it forms a single domain state. Further reduction of particle size yields a superparamagnetic state. There is an apparent phase transition\footnote{J.J. Becker, ``Precipitation and magnetic annealing in a Cu-Co alloy,'' \textit{Trans Met SocAIME,} \textbf{212} 138-144 (1958).} between the single domain state and the superparamagnetic state at some critical size in the single-domain mesoscopic region. We had found\footnote{E. Della Torre, L.H. Bennett, and R.E. Watson, ``Extension of the Bloch T$^{3/2}$ law to magnetic nanostructures: Bose-Einstein condensation'', \textit{Phys. Rev. Lett. }\textbf{94, }147210 (2005).}$^{,}$\footnote{S. Rao, E. Della Torre, L. H. Bennett, H. M. Seyoum, and R.E. Watson, ``Temperature variation of the fluctuation field in Co/Pt'', \textit{J. Appl. Phys.} \textbf{97}, 10N113 (2005).} a Bose-Einstein condensation in a number of mesoscopic ferromagnets (30 - 60 nm diam). We hypothesize that the superparamagnetic-to-single domain ferromagnetic transition involves the same type of Bose-Einstein condensation. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B9.00004: Bose-Einstein Condensation of Ni spin degrees of freedom observed from susceptibility measurements at high magnetic field in NiCl$_{2}$-4SC(NH$_2$)$_2$ Catalin Martin, Kenneth Purcell, Timothy Murphy, Eric Palm, Stan Tozer, Vivien Zapf, Alex Lacerda, Armando Paduan-Filho The organic compound NiCl$_{2}$-4SC(NH$_2$)$_2$ is a new candidate for Bose-Einstein condensation (BEC) of spins system. Involving a technique based on a self-resonant LC-oscillator, we measured the change in magnetic susceptibility of NiCl$_{2}$- 4SC(NH$_2$)$_2$ in magnetic field up to 18 Tesla, and temperatures down to 60 mK. We have found the existence of two phase boundaries. One appears at H$_{c1}\sim$2 T, where the gap between the $S_{z}$= 0 ground state and the $S_{z}$=-1 excited state is overcome by the Zeeman effect and the spin condensation starts. The other boundary is at H$_{c2}\sim$12 T, where the system saturates. Our results on temperature and angular dependence of critical fields will be discussed in connection with previous experimental reports and theoretical predictions for BEC. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B9.00005: Spin Density Wave Suppression in Thin Films of Cr. Daniel Queen, Zoe Boekelheide, David Cooke, Frances Hellman The magnetic and electronic properties of thin films of Cr are critical to their use in GMR multilayers and are not the same as bulk Cr. For example, itinerant spin density waves have been shown to be suppressed in thin films of Cr. This suppression has been attributed to disorder in the microstructure of the films. For bulk Cr the electronic contribution to the heat capacity $\gamma $=3.5 J/mol$\cdot $K$^{2}$ for non-magnetic Cr and $\gamma $=1.4 J/mol$\cdot $K$^{2}$ for magnetic Cr. Recent heat capacity measurements on Cr thin films have found $\gamma $=3.2 J/mol$\cdot $K$^{2}$ in agreement with the nonmagnetic bulk value. These Cr films also have a reduced $\theta _{D}$ with respect to the bulk and indicate a softening of phonon modes as seen in nanocrystalline materials. We present magnetization and heat capacity results for Cr thin films. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B9.00006: Phase transitions in metal-oxide particulate-surface interactions: insights through ab initio study Daniel Freedman, Tomas Arias We present results for {\em ab initio} molecular dynamics calculations of collisions between metal-oxide particulates and metal-oxide surfaces. Detailed examinations of the resulting trajectories suggest particulate internal kinetic energy as a key parameter in obtaining smooth laminar deposition of thin films. Based on {\em ab initio} calculations and more extensive supporting classical molecular dynamics modelling, we propose a phase-diagram which maps both translational and internal kinetic energy to modes of crystal growth. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B9.00007: Optical Measurements of Grain Boundary Melting Erik Thomson, John Wettlaufer, Larry Wilen Bulk properties of polycrystalline solids are strongly influenced by effects of grain boundary melting. While numerical simulations and theory support the idea of disorder along grain boundaries, direct experimental access to the interface of two crystals in thermodynamic equilibrium remains difficult. Polycrystalline ice, however, possesses unique properties which lend it to experimental probing. In addition to its transparency and birefringency the melting temperature of ice, which is relatively close to ambient temperatures, make it an ideal solid within which to experimentally explore grain boundary melting. Here using light scattering by a 4mW Helium-Neon laser we directly explore the boundary in an ice bicrystal, prepared within a thin ice growth cell. Reflected light intensity is measured as a function of the thermodynamic variables, temperature and impurity concentration. C-axis orientation of individual crystals can be determined by systematically measuring the difference between incident and transmitted polarized beams. Assuming the index of refraction for bulk water, for any melted layer, we anticipate a greater than 10{\%} change in reflected signal strength for a 15 angstrom melt layer. Experimental results are compared with a recent theoretical study of impurity driven grain boundary melting. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B9.00008: Photoelectron Emission Microscopy at the Manganese L-edge of Thin La$_{1-x}$ Sr$_{x}$ Mn O$_{3}$ Films through the Phase Transition M.A. DeLeon, T. Tyson, C. Dubourdieu, A. Scholl, A. Doran Surface magnetization of La$_{1-x}$ Sr$_{x}$ Mn O$_{3}$ films has been observed directly by x-ray photoelectron emission microscopy (XPEEM) at the manganese L-edge. Two sets of films of similar thickness range have been grown via metal-organic chemical vapor deposition (MOCVD) on LaAlO$_{3}$ and SrTiO$_{3}$ substrates. The data provides direct observation of magnetic domain melting and growth through the transition temperature. A comparison of domain geometry between the two sets of films will be presented. The effect of the coupling of the strain (tensile vs. compressive) with the magnetization will be discussed. This research is supported by NSF DMR-0209243 and DMR-0512196. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B9.00009: Thermodynamic Investigation of n-Hexane and Cyclohexane on MgO (100) Surfaces Peter N. Yaron, John Z. Larese Thermodynamic properties of thin films of n-hexane and cyclohexane adsorbed on MgO (100) surfaces were investigated and compared using high-resolution volumetric isotherm techniques. A series of high-resolution adsorption isotherm measurements were recorded for n-hexane and cyclohexane between 197K to 255K and 230K to 280K respectively using an automated volumetric isotherm apparatus. Adsorption data was used to determine thermodynamic quantities e.g. the two-dimensional isothermal compressibility and isosteric heats of adsorption and to identify regions where phase transitions might occur. Evidence is found for the presence of two layering transitions in both systems. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B9.00010: Complex Conductance Measurements of Ultra-thin MoGe films near the Superconductor-Insulator Transition Lukas Urban, Michael Callahan, Ali Yazdani The application of a magnetic field drives a two-dimensional superconductor through an unexpected conducting state into an insulating state. While this transition has been studied using electrical transport techniques, we present a different way to study the magnetic field-tuned transition. Using a two coil mutual inductance probe inside a top-loading dilution refrigerator, we measure the complex conductance of Mo$_{43}$Ge$_{57}$ thin films as we vary the temperature or apply a magnetic field. From the complex conductance we determine the superconducting electron density around this field-tuned transition. This work was supported by NSF grants DMR-98-75565 and DMR-03-1529632, U.S. Department of Energy grant DEFG-02-91ER4539 through the Frederick Seitz Materials Research Laboratory and Office of Naval Research grant N000140110071. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B9.00011: Crystallization behaviors of $n$-nonadecane in confined space: observation of metastable phase induced by surface freezing Dujin Wang, Baoquan Xie, Haifeng Shi, Xia Dong, Ying Zhao, Charles C. Han, Duanfu Xu Crystallization and phase transition behaviors of $n$-nonadecane in microcapsules was studied with the combination of differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (XRD). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase (R$_{II})$, which has not been reported anywhere else. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of R$_{II}$ phase and turns the transient R$_{II}$ phase to a `long-lived' metastable phase. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B9.00012: Ordering and Dynamics of CN$^{-}$/Cu(001) Surfaces Erkan Ciftlikli, Ian Shuttleworth, Alexei Ermakov, Jane Hinch C$_{2}$N$_{2}$ adsorption on Cu(001) is largely dissociative, yielding adsorbed CN$^{-}$ species. Exposure dependent angular-resolved Helium Atom Scattering (HAS) measurements show the initial development of a diffuse backscattered intensity, followed by the onset of c(10x6) superstructure domain growth only in a limited exposure temperature range. Diffraction from an ordered phase is not observed for exposures above 323K, nor below 223K. Yet, even at optimal deposition temperatures, a diffuse scattering contribution remains at saturation and persists beyond 473K. Energy and angular resolved measurements show that the diffuse He intensity is strongly inelastic, and multiphonon-like; i.e. not showing features with resolvable discrete energies. The surface temperature and momentum exchange dependencies of this intensity will elaborate the nature of the dynamics of CN$^{-}$ on Cu(001) surfaces. The CN$^{-}$ coverage dependence of the inelastic intensity also illustrates the influence of intermolecular interactions in CN$^{-}$ motion. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B9.00013: Theoretical treatment of non-vibrationally-relaxed electron transfer in organic solar cells Kuo Kan Liang, Chih-Kai Lin, Huan-Cheng Chang, Michitoshi Hayashi, Sheng Hsien Lin In this work we shall show how to calculate the single vibronic- level electron transfer rate constant, which will be compared with the thermal averaged one. To apply the theoretical results to the dye- sensitized nano-crystalline semiconductor (Gr\"{a}tzel type) solar cells, we use a simple model to describe how we model the final state of the eletron- transfer process (oxidized dye and reduced semiconductor). Numerical calculations of the single-level electron transfer rate constants and the simulation of the quantum beat in the photo-excited state and the product state will be performed to demonstrate the theoretical results. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B9.00014: Investigating chemicurrent production from hyperthermal-energy ion impacts S.A. Moody, M.P. Ray, C.E. Sosolik The production of chemicurrents by thermal-energy molecular beam scattering from thin-film Schottky diode surfaces has generated considerable interest in the field of gas-surface physics. These experiments promise to give insight into energy transfer processes at surfaces. However, few experiments have investigated chemicurrent production at higher impact energies. We propose experiments to study beam impact events on Schottky diode/thin film surfaces using hyperthermal ion beams, with the added capability of studying the mechanism of charge transfer. These experiments will utilize our lab's highly versatile beamline. The beamline is ideal for exploring chemicurrent phenomena at higher energies because of the capability it gives us to produce well-characterized ion beams at hyperthermal- and low-energies with a wide variety of gas and solid-state sources. This talk will focus on optimization of our beamline for this experiment. Specific attention will be given to the addition of a new load lock/sample transfer system and the electronic interfacing required for simultaneous scattering/chemicurrent data acquisition. Preliminary results from hyperthermal impact events on thin film Schottky diodes are also presented. [Preview Abstract] |
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