Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B15: Metals: Lattice, Low D, Phonons |
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Sponsoring Units: DCMP Chair: Larry Boyer, Naval Research Laboratory Room: Baltimore Convention Center 311 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B15.00001: Anomalous equilibrium volume change of magnetic Fe-Al crystals Martin Fri\'{a}k, J\"{o}rg Neugebauer Iron aluminides represent a very promising class of intermetallic materials with great potential for substituting stainless steels at elevated and high temperatures. Experiments observed an anomalous equilibrium volume behaviour as a function of concentration in Fe-rich compounds [1]. This effect has been tentatively assigned to be due to an order-disorder transition. We have studied the role of magnetism in Fe-Al crystals employing density functional theory (DFT) within the generalized gradient approximation (GGA). The excess energies, equilibrium lattice parameters and magnetic states have been determined for a dense set of different iron concentrations and a large variety of atomic configurations. Both external and internal relaxations were allowed. The spin-polarized calculations for \emph{ordered} ferromagnetic Fe-rich compounds nicely reproduce the anomalous volume behaviour, i.e. the effect is \emph{not} related to an order-disorder transition. Analyzing different magnetic states we identified the change in magnetism to be the driving force. In fact, performing the same calculations but switching off magnetism removed the anomalous volume dependence and showed a clear linear dependence. Based on these results the importance of order-disorder transition in Fe-Al systems is revisited. [1] R. A. Buckley and S. Kaviani, Mat. Sci. Eng. A258, 173 (1998). [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B15.00002: Internal stress generation during electrochemically deposited Volmer-Weber thin films. Tianzhi Luo, Robert Cammarata The real-time \textit{in situ} stress generation during electrochemical deposition of metallic thin films deposited by an island (Volmer-Weber) growth process will be presented. The stresses were measured using a sensitive substrate curvature system optimized for use in an electrochemical cell. Films were deposited on crystalline and amorphous substrates. In many cases, the films displayed an early stage compressive stress, followed by a large tensile jump, and finally ending with a compressive stress at large thickness. Also, interruption of growth led to significant relaxation of the stress. These behaviors were qualitatively similar to those found for evaporated films, and suggest that similar stress generation mechanisms operate for both types of depositions. In other systems, the stress generation behavior was much different, generally displaying a small compressive stress throughout. Correlation of the stress behaviors with microstructural evolution will be presented, and possible mechanisms for the generation of the stresses will be discussed. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B15.00003: Anomalous Magnetic Particle Distribution in Annealed Granular CuCo Thin Films Jian-Qing Wang, Nam H. Kim, Jardon Peck A SQUID magnetometer was used to measure ZFC and FC c-T data to study the microstructure of annealed granular Cu80Co20 thin films. The films of thicknesses ranging from 40 nm to 1 mm with 20\% vol. Co were magnetron sputtered at base pressure $<$ 1.5 10- 7 torr. The annealing was done at various temperatures up to 400$^{\circ}$C in a tube furnace with a reducing gas flow. As-deposits showed the standard single-peak c-T curve with an average size of 3.0 nm in dia. for the Co nanoparticles as calculated by Curie-Weiss analysis. Langevin fitting to magnetization at elevated temperature of 300 K also provided the same particle size. As annealing temperature increased, the first peak occurring around 40K decreased in height while the second, occurring around 300 K, grew. These anomalous features, observed over a wide range of thickness (40-200 nm), appeared to result from microstructural evolution, namely an increase in large particle population at the expense of smaller particles. It is likely that the film contains a mixture of small particles and larger particles that are induced by annealing, which gives rise to the double peak structure. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B15.00004: Metal Diffusion in Granular Bismuth Thin Films John Sadleir Bismuth's exotic electro-thermal properties makes it attractive for many device applications. Despite such incentives, fabrication of high quality bismuth films has proven difficult, and measured properties of such films are highly variable in the literature. Implementing a bismuth deposition process in device fabrication presents additional challenges—particularly at interfaces due to the inherent granularity and surface roughness of its films, its low melting point, and its tendency to diffuse and form undesired intermetallic phases. We report on the properties of the granular bismuth films grown at NASA Goddard and studies of solid state diffusion of Au and Cu in these films. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B15.00005: Temperature effect on structure of copper phthalocyanine thin films Kedar Manandhar, Kenneth Park, Zhen Song, Tanhong Cai, Jan Hrbek In situ deposited heteroepitaxial thin films of copper phthalocyanine (CuPc) on Ag(111) surfaces have been investigated by scanning tunneling microscopy. The molecules lay flat on the substrate forming a well ordered closed pack square lattice. A lattice vector of ordered film makes 4$^{0 }\pm $ 0.5$^{0 }$with [0-11] direction of Ag (111) lattice. The lattice vectors of the overlayer have been measured 14{\AA}x14{\AA} corresponding to the surface molecular density of 5.10 x 10$^{13 }$molecules/cm$^{2}$. The close packed square lattice remains highly ordered when annealed up to 500$^{0}$K. However further annealing of film to $\sim $ 700$^{0}$K desorbs 75 {\%} of molecules leaving submonolayers coverage of molecules. The remaining molecules aggregate to form dendrite like form, which appears to result from dimerization and polymerization. Molecular orientation in well ordered film, structural change of molecules in dendrite like form, and further discussion of temperature effects on CuPc thin films will be presented. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B15.00006: Lattice defects and electronic properties of graphene Francisco Guinea, Nuno M.R. Peres, Antonio H. Castro Neto We analyze the electronic properties of graphene. Extended lattice defects, and deviations from electron-hole symmetry, lead to self doping, and to a finite Fermi surface. Point lattice defects give rise to a finite inverse elastic lifetime at low energies, and to a universal value of the dc conductivity at low frequencies or temperatures. Transport properties at optical frequencies are also investigated. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B15.00007: Electron-phonon coupling at the Be(0001) Surface Timur Kim, Philip Hofmann, TeYu Chien, Hong Liu, Ward Plummer The temperature dependent angle-resolved photoemission spectrum (ARPES) of the surface state centered at $\Gamma $ point of Be(0001) surface along the $\Gamma $-K and the $\Gamma $-M directions have been measured as a function of binding energy and momentum. The surface state centered at $\Gamma $ point is highly symmetric in reciprocal space. The temperature dependent ARPES data along these two directions were analyzed by means of energy distribution curves (EDCs) and momentum distribution curves (MDCs) with the help of the spectral function. From the analysis, the information yields the magnitude of the imaginary part of the self energy as a function of E and \textbf{k}. Attempts to extract the electron-phonon coupling constant from this data are so dependent upon the model for the phonons and the bare band structure that it is impossible to extract any useful information. But the E and \textbf{k }dependence of the imaginary part of the self-energy can be compared directly to first-principles calculations. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B15.00008: Wannier-Stark Ladders in Torsional Waves Guillermo Monsivais, Rafael M\'{e}ndez-S\'{a}nchez, Alfredo Diaz-de-Anda, Jorge Flores, Luis Guti\'{e}rrez, Alejandro Morales We study the normal modes of torsional waves in an elastic rod consisting of a set of $n $circular cylinders of varying length determined by a parameter $\gamma $. We present experimental, theoretical, and numerical results. It is shown that some analogies to the Wannier-Stark ladders, originally introduced by Wannier (1960), are exhibited by this classical system. The ladders consist of a series of equidistant energy levels for the electrons in a crystal in the presence of a static external electric field, the nearest-neighbor spacing being proportional to the intensity of the external field. For the case of torsional waves in the rod, we have observed a similar behavior: the vibrations of the rod show resonances of equidistant frequencies, the nearest neighbor spacing being proportional to $\gamma $, associated with the geometry of the rod. One should point out, however, that the analogy is not perfect. \newline \newline References: \newline Wannier G. H. (1960) Wave Functions and Effective Hamiltonian for Bloch Electrons in an Electric Field, Phys. Rev. \textbf{117}, 432-439; Wannier G. H. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B15.00009: Resonant frequency shifts of Al micro-resonator by electric currents and nonlinear oscillation Seong Soo Kim, Jung Ho Kim, Kookrin Char Mechanical microbridge resonators are fabricated from polycrystallin aluminum thin film using micromachining processes. A movement of the microbridge smaller than a nm can be measured by optical method. The microstructures are self-oscillating or can be actuated by an external electrostatic field, and oscillations with quality factors up to 2400 in vacuum were measured. The observed resonant frequencies reveal the important role of the internal stress in thin metal film. A substantial resonance frequency shift induced by electric current was observed, which we attribute to thermal expansion caused by Joule heating. Nonlinear oscillation was observed readily due to the large length to thickness aspect ratio ($\sim $1000). Amplitude-dependent resonant frequency shifts show qualitative changes of the resonator response. We will try to correlate the motions of grain boundaries and/or dislocations with the resonator response. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B15.00010: Collective Excitations in a Molten Early transition Metal Ayman Said, Harald Sinn, Ahmet Alatas, Clement Burns, David Price, Marry Saboungi, Walter Schirmacher We report inelastic x-ray scattering measurements of the atomic collective excitations of an early transition metal, namely liquid titanium at T=2023K. The data show well defined sound excitations with a low damping constant comparable to the damping observed in liquid alkaline metals. The data agree very well with the predictions of mode-coupling theory. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B15.00011: Modified Debye spectra in disordered solids J.C. Lashley, D.J. Safarik, M.F. Hundley, R.B. Schwarz, A. Planes Low-energy modes, as manifested by an excess phonon heat capacity, are ubiquitous in glasses and is often thought to be a signature of a glassy or disordered state. However, this feature is also observed in negative thermal expansion crystals, thermoelectrics, and shape-memory alloys. We compare the peaks in metallic glasses and shape-memory alloys as a function of disorder and crystallinity. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 2:03PM |
B15.00012: Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses Invited Speaker: We consider the effect of the internal nuclear quadrupole interaction on quantum tunneling of complex multi-atomic two-level systems. Two distinct regimes of strong and weak interactions are found. They depend on the relationship between the characteristic energy of the internal interaction $\lambda_{\ast}$, which is directly proportional to the number of tunneling atoms per tunneling system, and a bare tunneling coupling strength $\Delta_{0}$. When $\Delta_{0}>\lambda_{\ast},$ the internal interaction is negligible and tunneling remains coherent, being defined by the strength of $\Delta_{0}$. When $\Delta_{0}<\lambda_{\ast}$, coherent tunneling breaks down and the effective tunneling amplitude decreases by an exponentially small overlap factor $\eta^{\ast}\ll1$ between the internal ground states of the left and right well, affecting thermal and kinetic properties of tunneling systems. \par The theory is applied to interpret the anomalous behavior of the resonant dielectric susceptibility in amorphous solids for $T\leq5mK$ in terms of the nuclear quadrupole interaction. This interaction breaks coherent tunneling for $T\leq5mK$, where the characteristic tunneling amplitude becomes comparable with the interaction strength. We suggest clarifying experiments using external magnetic fields to test the theories predictions and to shed some light on the internal structure of tunneling systems in amorphous solids. [Preview Abstract] |
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