Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H37: Focus Session: Nanoscale Materials Physics of Phase Transitions II |
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Sponsoring Units: DMP Chair: Eric Mazur, Harvard University Room: Baltimore Convention Center 340 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H37.00001: Electronic Transport in Molecularly-Linked Gold Nanoparticle Films Near the Metal-Insulator Transition. Al-Amin Dhirani, Jeffrey Dunford, Yoshinori Suganuma We have investigated the temperature ($T)$ dependence of the conductance ($g)$ of 1,4-butane dithiol linked Au nanoparticle films. Over a wide range of temperatures (20 K to 300 K), the conductance behaves as $g=g_{0} $ exp(-($T_{0}$/$T)^{1/2}))$. Such a temperature dependence is predicted by an Efros-Shklovskii variable-range hopping model based on a competition between Coulombic and inter-cluster tunneling processes. However, we find that hopping distances are too large (62 nm to 720 nm at 100 K) to be consistent with tunneling between clusters, and tend to scale with cluster size. We propose a modified ``quasi-localized hopping'' model based on competition between single-electron cluster charging and intra-cluster electron backscattering to explain this temperature dependence. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H37.00002: Plasticity and energy absorption at the nanoscale: kinetics and chemical control Matteo Cococcioni, Gerbrand Ceder, Nicola Marzari Shock-induced phase transitions of group-IV nanoparticles are investigated using first-principles molecular dynamics in the electronic-enthalpy formalism [1]. In some of these systems a shock compression can induce plastic deformations, leading to to the absorption of the mechanical energy coming from the impact. Kinetic factors greatly affect phase transformations, and so only some of the thermodynamically stable structures are explored. Still, transitions are always nucleated in the core of the nanoparticles, and are accompanied by the formation of small close-packed clusters of characteristic shape. Also, as the particle's size grows, amorphization remains confined in the core region. Finally, we highlight how chemical doping can be used to affect nucleation, and to tune pressure thresholds; this could be relevant e.g. for target applications such as the design of novel impact-absorbing nanostructured materials. \newline \newline [1] M. Cococcioni, F. Mauri, G. Ceder and N. Marzari, PRL (2005). [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H37.00003: Magnetic response and enhanced Tc in the 1D to 2D crossover regime of mesoscopic superconducting aluminum rings. Nicholas C. Koshnick, Hendrik Bluhm, Martin E. Huber, Kathryn A. Moler We have studied more than 40 mesoscopic Aluminum rings by positioning a scannable SQUID susceptometer over each ring individually, and have found a striking non-monotonic dependence of Tc on linewidth. By measuring the magnetic response, or current in the ring, as a function of applied flux and temperature, it is possible to extract $\lambda(T)$ and $\xi(T)$. The rings vary in width (40-350nm) and diameter (1-4um) with a nominal thickness of 35nm. For rings with linewidths above 150nm ($\approx 2 \xi(T = 0)$), the extracted superfluid density shows a high-temperature tail up to temperatures as high as 15\% above the 1D and bulk thin film critical temperatures. In this regime, we also find evidence for the saturation of $\xi(T)$ on the order of the ring's linewidth. We speculate that these results indicate the particular importance of Tc disorder in the crossover from quasi-1D to 2D superconductivity. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H37.00004: Fluctuation diamagnetism in mesoscopic Aluminum rings Hendrik Bluhm, Nicholas C. Koshnick, Martin E. Huber, Kathryn A. Moler We have measured the magnetic response of mesoscopic, superconducting Al rings using a high sensitivity scanning SQUID microscope. We find that for sufficiently small, quasi one- dimensional rings, the rounding of the superconducting transition due to thermal fluctuations is in good agreement with theoretical preditctions. The main effects are a fluctuation tail in the amplitude of the response near $T_c$ and a suppression of the apparent $T_c$ as determined from the temperature dependence of the superfluid density below the fluctuation tail compared to the extracted mean field $T_c$ by several mK. In larger rings, the tail is less pronounced due to the lack of phase coherence. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H37.00005: Magnon characterization in magnetic quantum dots Mohammad-Reza Mozaffari, Keivan Esfarjani The spin structure in a magnetic dot is studied as a function of exchange coupling strength or dot size, within the semiclassical approximation on a discrete lattice. In addition to the exchange term, the dipole interaction has also been included in our calculations. As the exchange coupling is decreased or the size is increased, the ground state undergoes a phase change from a single domain ferromagnet to a spin vortex. The line separating these two phases has been calculated numerically for small system sizes. Magnon frequencies in such a dot have also been calculated in both phases by the linearized equation of motion method. These calculations have also been checked by comparing them to the spin autocorrelation function spectrum. Furthermore, the modes have been characterized for both the ferromagnetic and the vortex phase. The modes responsible for the instability of the vortex and the ferromagntic phases have been identified, and can provide the mechanism for spin reorientation in these phases. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:51PM |
H37.00006: Magnetic multilayers on nanospheres. Invited Speaker: In modern magnetic recording materials the `superparamagnetic effect' [1] has become increasingly important as new magnetic hard disk drive products are designed for higher storage densities. In this regard, nanoparticle media [2], where two-dimensional arrays of monodisperse nanoparticles with high magnetic anisotropy are used, is assumed to be the ideal future magnetic recording material. In this presentation a novel magnetic gradient nanomaterial, which has been created by depositing Co/Pd multilayers onto two-dimensional arrays of self-assembled nanoparticles [3] will be introduced. The magnetic nanostructures formed on top of the particles are in a magnetically exchange-isolated quasi-single-domain state. This nanoscale system is quite distinct from the classical geometries: Neither extrinsic properties nor the intrinsic properties are uniform in space. The film is extended over a wide region of the sphere and thus shows substantial curvature. The film thickness varies and so do the intrinsic magnetic properties most notable the magneto-crystalline anisotropy, which is a key factor affecting the fundamental nature of the reversal process. The specific magnetic characteristics of such a gradient nanomaterial and in particular its impact on the reversal mechanism will be discussed. Angle-dependent reversal studies were performed for different particle sizes and the experimental results were interpreted using micromagnetic simulations. The simulations account for the angular dependence of the hysteresis loops and provide a detailed picture of how the local magnetization evolves during reversal, offering new opportunities in the functionalization of magnetic nanostructures for storage applications. [1] A. Moser et al., J. Phys. D: Appl. Phys. 35, (2002) R157. [2] S. Sun et al., Science 87, (2000) 1989. [3] M. Albrecht et al., Nature Mater. 4, (2005) 203. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H37.00007: Coulomb effects and hopping transport in granular metals Andrei Lopatin, Igor Beloborodov, Valerii Vinokur We investigate effects of Coulomb interaction and hopping transport in the insulator phase of granular metals and quantum dot arrays considering both spatially periodic as well as irregular grain/dot arraignments. We study the Mott transition between the insulating and metallic phases in a strictly periodic system and find the dependence of the Mott gap on the intergranular coupling. In this case the conductivity of the insulating state has the activation form with the Mott gap entering the exponent. In the irregular arrays the electrostatic disorder induces the finite density of states near the Fermi level giving rise to the variable range hopping conductivity. We derive the transport properties of the irregular array in the dielectric, low coupling limit and show that the conductivity follows the Efros-Shklovskii law. We develop a theory of tunneling through a chain of grains and discuss in detail both elastic and inelastic cotunneling mechanisms; the former dominates at very low temperatures and very low applied electric fields, while the inelastic mechanism controls tunneling at high temperature/fields. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H37.00008: Distinguishing Magnetic Behavior on the Slater-Pauling Curve applied to nanoscale thin films. K.R. Podolak, M.G. Birke, N. Janke-Gilman, R.F. Willis The Slater-Pauling curve is a plot of the stoichiometric mean saturation magnetization per atom as a function of the electron filling of the d-bands in the transition metals and their alloys. When the elemental magnetic moments are aligned, the volcano-shape curve plots the variation of the mean magnetic moment. It has been predicted that a narrowing of the d-bands in nanostructured magnetic materials will influence both the magnitude of the magnetic moments as well as the overall magnetic alignment through increased spin-orbit coupling [1]. In this paper, we report dichroism in the core-level x-ray photoemission spectra of the elements which reveals and distinguishes the changing magnitudes of the magnetic moments and the changing alignment of these moments with changing composition. The 3p core-level spectral densities of multiplet spin-ordered states show a width W proportional to the local Zeeman splitting of levels, and a magnetic dichroism amplitude A sensitive to the non-local magnetic order (anisotropy). In nanoscale epitaxial layers, small deviations from the bulk Slater-Pauling curve are observed due to enhanced moments due to d-band narrowing [2]. It is predicted that a plot of the spectral parameters W, A will show a different dependence of temperature approaching Tc, the critical ordering temperature. [1] S. Handschuh {\&} S. Bl\"{u}gel, Solid State Communications. \underline {105}, 633 (1998). [2] R.F. Willis {\&} N. Janke-Gilman, Europhysics Letters. \underline {69}, 411 (2005). [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H37.00009: Localization of coupled plasmon modes in graded plasmonic chains Jun Jun Xiao, K. Yakubo, K. W. Yu Electromagnetic energy propagation and localization in plasmonics offer the potential for integrated nanophotonics. Plasmonic waves occur naturally inside the subwavelength scale with lateral confinement below the diffraction limit. However, controlling their localization/delocalization behavior in the propagating (longitudinal) direction is also desirable. We present results of longitudinal localization-delocalization tuning of coupled plasmon modes in graded chains of metallic nanodots. Two graded models will be studied: incremental spacing between the nanoparticles and graded index of refraction in the host medium. The coupled plasmons in these systems exhibit strong localization when detuned from the Mie plasmon frequency, showing a tunable passband in finite size system. To understand the localization mechanism, we map the problem onto an equivalent system of one-dimensional chain of coupled harmonic oscillators, whose coupling strength is gradually varied from one end to the other, with additional on-site potentials. The results can be applied to analogous graded systems. Confining and transmitting electromagnetic energy in these structures may pave new way for many fruitful applications in plasmonics. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H37.00010: A study of Co thin films grown on Si(111) with Al buffer layer Zi Q. Qiu, J. Wu, J. Choi, J. Li, A. Scholl, A. Doran Growth and magnetic properties of Co/Si(111) and Co/Al/Si(111) were investigated using Scanning Tunneling Microscopy (STM), Surface Magneto-Optic Kerr Effect (SMOKE), and Photoemission Electron Spectroscopy (PEEM). For Co film grown directly on Si(111), silicide is formed at the interface which leads to a loss of magnetization for ultrathin Co film. By growing an Al buffer layer at the Co/Si interface, we found that the Al buffer layer effectively reduces the silicide formation and SMOKE signal was detected at a much thinner Co thickness as compared with Co film directly deposited on Si(111). Magnetic domain imaging using PEEM confirms the SMOKE measurement. STM measurements were also performed to study morphology change after inserting the Al buffer layer. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H37.00011: Experimental Evidence for a Dynamic Phase Transition in Ultra-thin Magnetic Films D.T. Robb, Y.H. Xu, O. Hellwig, A. Berger, M.A. Novotny, P.A. Rikvold We present the first convincing evidence for an experimental
observation of a Dynamic Phase Transition (DPT) in a magnetic
system: an ultra-thin [Co(0.4nm)/Pt(0.7nm)]$_3$ multilayer,
which is well modeled by a two-dimensional kinetic Ising
system. This DPT, as a function of the period $P$ of an applied
alternating magnetic field, has been observed previously in
simulations of magnetic systems [1]. For several values of $P$
and bias field $H_b$, the magnetization was measured for 50
cycles of the field [2]. The order parameter, which was
identified in simulations as the magnetization averaged over
the {\it i}th cycle, $Q_i$, was obtained from the experimental
data as a time series. Kinetic Monte Carlo simulations produced
close agreement with the experimental data for the order
parameter averaged over the final 30 cycles, $ |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H37.00012: Structural and Magnetic Phase Diagrams of Co and Mn Doped Ge (100) Epitaxial Films Brian Collins, Liang He, Frank Tsui, Yuncheng Zhong, Stefan Vogt, Yong Chu We present studies of structural and magnetic properties of epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ magnetic semiconductors, grown by combinatorial MBE on Ge (100) substrates. Structural effects were examined \textit{in-situ} by reflection high energy electron diffraction and \textit{ex-situ} by microbeam x-ray diffraction. A ternary epitaxial phase diagram has been determined for doping concentrations (x and y for Co and Mn respectively) up to 30 at {\%}, where regions of coherent epitaxy and associated strain states, and regions of rough disordered growth and the nature of the disorders have been examined and identified. Specifically, within a narrow range of compositions around the atomic ratio between Co and Mn (x/y) of 3, the observed epitaxial strain is low ($<$0.1{\%}) and coherent epitaxial growth is most stable, where the thin film lattice constants obey Vegard's law and they match those of the Ge at x/y =3. Within this region of compositions, high quality epitaxial films of p-type magnetic semiconductors can be stabilized for total doping concentration of transition metal dopants as high as 13 at {\%} and with T$_{C}$ as high as 300 K. A ternary magnetic phase diagram has been measured using magnetooptic Kerr effect. The interplays between structural effects and magnetism have been investigated. [Preview Abstract] |
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