Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z2: Condensed-Phase Dynamics, Solvation, and Statistical Mechanics
Sponsoring Units: DCPChair: Gilbert Nathanson, University of Wisconsin-Madison
Room: 102
Friday, March 7, 2014 11:15AM - 11:27AM |
Z2.00001: Solvated Hydroxide and Hydronium in Water Studied by ab initio Molecular Dynamics Based on PBE0 Hybrid Functional with van der Waals Interation Lixin Zheng, Charles Swartz, Xifan Wu The nature of the solvation structures of hydroxide (OH$^{-}$) and hydronium (H$_{3}$O$^{+}$) aqueous solutions is of fundamental interest. It is the prerequisite to understanding the mechanism of proton transfer (PT) through the autoprotolysis process in water. For a long time, ab initio calculations based on local or semi-local approximations have been used to study the solvated OH$^{-}$ and H$_{3}$O$^{+}$. Although successfully giving a qualitative description of the proton mechanism, the semi-local functional suffers from the delocalization error. In addition, the description of dispersion force is missing. To overcome the above errors, we perform ab initio molecular dynamics calculations in both solvated OH$^{-}$ and H$_{3}$O$^{+}$ based on PBE0 hybrid density functional and include the vander Waals interactions. It is found that, compared to the semi-local functional, the proton transfer rate in solvated OH$^{-}$ reduces due to the changes in the solvation structure. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z2.00002: Computational Modeling of Actinide Ions in Aqueous Solution Raymond Atta-Fynn Unraveling the chemical behavior of actinide species is difficult owing to the complex electronic structure of these species, the fact that many of these species can occur in multiple oxidation states, and the difficulties encountered in their experimental studies. First principles dynamical modeling, although computationally costly, allows us to gain rich insights into the behavior of actinide species. In this talk, we present results of the hydration shell structure and x-ray absorption spectra of aqueous actinides in different oxidation states including U(VI), U(V), U(IV), and Cm(III) using relativistic~\textit{ab initio }molecular dynamics at 300 K. We also probed the thermodynamics of hydrolysis by calculating the first acidity constant for uranium in all three oxidation states (IV, V, and VI). We predicted, for the first time, that UO$_{2}^{+}$ is a weak acid in solution with a pKa value of 8.5. This result is particularly important since no thermodynamic data are available for hydrolyzed species of U(V). In our most recent work on concentrated Cm(III) solutions, we showed that counter-ions can strengthen or weaken the solvent structure itself rather than just the water coordination number. These new results are better explained in terms of the hydrogen bond lifetimes of the solvents. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z2.00003: A new method for studying nucleation in aqueous environment Raffaela Cabriolu, Tianshu Li Probing nucleation of crystals in aqueous environment at the molecular level poses a major challenge in molecular simulations. The challenges are primarily attributed to two types of slow dynamics ubiquitously present in the nucleation process in aqueous environment: the rare event nature of nucleation and the sluggish dynamics of hydrogen-bond network. While advanced sampling method, such as transition path sampling or forward flux sampling, allows surpassing free energy barrier efficiently, it is unlikely to alleviate the second type of slow dynamics. Here we propose a new approach that combines Monte Carlo and molecular dynamics methods, to overcome the slow natural dynamics of tetrahedral bond network. The new approach exploits the flexibility of Monte Carlo trial moves and the collective motion of molecular dynamics. Recognizing the structural similarities between silicon and water (both belonging to ``tetrahedral materials''), we incorporate the bond-switching Monte Carlo move that was developed for generating random-network models for amorphous silicon, into our approach. The approach will allow modeling nucleation of ice or hydrates based on full atomistic water model. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z2.00004: On the Consequences of Clausius-Duhem Inequality for Electrolyte Solutions Martina Reis, Adalberto Bono Maurizio Sacchi Bassi Based on the fundamentals of thermo-statics, non-equilibrium thermodynamics theories frequently employ an entropy inequality, where the entropy flux is collinear to the heat flux, and the entropy supply is proportional to the energy supply. Although this assumption is suitable for many material bodies, e.g. heat-conducting viscous fluids, there is a class of materials for which these assumptions are not valid. By assuming that the entropy flux and the entropy supply are constitutive quantities, in this work it is demonstrated that the entropy flux for a reacting ionic mixture of non-volatile solutes presents a non-collinear term due to the diffusive fluxes. The consequences of the collinearity between the entropy flux and the heat flux, as well as the proportionality of the entropy supply and the energy supply on the stability of chemical systems are also investigated. Furthermore, by considering an electrolyte solution of non-volatile solutes in phase equilibrium with water vapor, and the constitutive nature of the entropy flux, the stability of a vapor-electrolyte solution interface is studied. Despite this work only deals with electrolyte solutions, the results presented can be easily extended to more complex chemical reacting systems. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z2.00005: Putting water on a lattice: The importance of long wavelength density fluctuations in theories of hydrophobic and interfacial phenomena. Suriyanarayanan Vaikuntanathan, Phillip Geissler The physics of air-water interfaces plays a central role in modern theories of the hydrophobic effect such as the Lum-Chandler-Weeks (LCW) theory. Implementing these theories, however, has been hampered by the difficulty of addressing fluctuations in the shape of such soft interfaces. We show that this challenge is a fundamental consequence of mapping long wavelength density variations onto discrete degrees of freedom. Specifically, through an analysis of the lattice gas model and related approximations, we identify a narrow parameter regime in which the lattice gas model can optimally be used to describe long wavelength liquid density fluctuations such as the capillary modes at a liquid-vapor interface. Coupling fluctuations in the lattice model to fluctuations on finer molecular scales through the least complicated realization of the LCW perspective, we obtain an effective Hamiltonian for lattice occupation variables in the presence of a hydrophobic solute. We show that this Hamiltonian - with no unknown parameters - in fact suffices to describe quantitatively the the solvation of hydrophobic objects with various shapes and sizes. This model is uniquely well suited for exploring hydrophobic and interfacial phenomena that involve disparate length scales. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z2.00006: Effetively trapping air or lqiud water for anti-icing applications Jianjun Wang Icing on solid surfaces leads to operational difficulties and high maintenance efforts for power networks, aircrafts, ships, ground transportation vehicles and house-hold refrigerators, to name but a few. In extreme cases, icing on surfaces causes disastrous events such as crash of aircrafts and collapse of power networks, which result in severe economic impact and large loss of life. This talk is focused on the fundamentals of the ice formation and adhesion of ice with solid substrates aiming for fighting against icing on solid surfaces. When the supercooling is low, it would be possible to remove supercooled liquid water from the solid surfaces before freezing occurs. To achieve this, we design and constructed surfaces that can trap the air at the subfreezing temperature thus condensed water microdroplets could be spontaneously removed after the coalescence. When the supercooling is high, icing on surfaces occurs spontaniously. In this case, we constructed coatings on which aqueous lubricating layer could be trapped, thus the ice adhesion on the coating is so low that the ice formed atop could be removed by a wind action or its own gravity. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z2.00007: Stretched-water equation of state at low temperatures Michael Santiago, Vinay Pagay, David Sessoms, Abraham Stroock Liquids can exist at negative pressure, a metastable state analogous to stretched rubber. This stretched state is ubiquitous. For instance, stretched water exists naturally in many plants, semi dry soils and porous materials, even food. Yet measurements of stretched water's thermodynamic properties are lacking because of experimental challenges. Such measurements could elucidate water's behavior in these settings, unfold the origins of water's anomalies, and perhaps provide conclusive evidence on whether a liquid-liquid critical point exists in highly supercooled water. Here we present the first isothermal measurements of the equation of state (EoS) of stretched water below room temperature, comparing them with predictions of existing theoretical models. For these measurements, we developed a microfabricated sensor that directly measures the pressure in a macroscopic volume of stretched water at known chemical potential and temperature. This sensor uses the metastable vapor liquid equilibrium technique to stretch the water, and is able to reach pressures down to -33 Mpa. Our results agree with the IAPWS EoS at 20 $^{\circ}$C and 15 $^{\circ}$C. We are currently taking further measurements at lower temperatures. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z2.00008: Charge-Controlled Colloids on Liquid-Liquid Interfaces Daniel A. Kunz, Bernd Reck, Vinothan N. Manoharan The tendency of colloidal particles to stabilize interfaces has been exploited for many years to generate Pickering emulsions with a variety of industrial applications. However, the exact stabilization mechanism and its dependence on the surface properties of the colloidal particles are not yet fully understood. We provide new interfacial studies on the nonequilibrium dynamics of a colloidal system with tunable surface charge density. We push individual sub-micron colloidal particles towards an oil-water interface and track their motion in three-dimensions using holographic microscopy to examine the influence of zeta potential on the dynamics of the system. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z2.00009: Toward an Accurate Model for the Line Shape Analysis of 2D Correlation Spectra Mohammadhasan Dinpajooh, Dmitry Matyushov Bilinear (linear plus quadratic) coupling of a dipolar polarizable chromophore to a Gaussian thermal bath (Q-model\footnote{D. V. Matyushov and G. A. Voth, J. Chem. Phys., \textbf{113}, 5413 (2000)}) is applied to study the effect of non-Gaussian statistics of the transition frequency on time-resolved linear and nonlinear correlation (2D) spectra. Exact lineshape functions of time-resolved fluorescence spectra and broadening functions of 2D correlation spectra (2DCS) are derived based on the summation of an infinite cumulant series of the transition frequency, in contrast to the two-cumulant approximation of the Gaussian models. The formal theory is supported by molecular dynamics simulations of a dipolar polarizable chromophore dissolved in a modified TIP4P water. We show that the Q-model, unlike the Gaussian model, can capture the asymmetry of 2DCS and bending of the center line in 2DCS reported experimentally. The theory provides a consistent formalism of the line shape analysis in cases when Gaussian models do not apply. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z2.00010: Structure and Dynamics of Octamethyl-POSS Nanoparticles Michael Crawford, Kerwin Dobbs, Robert Smalley, William Guise, Niina Jalarvo, Olivier Gourdon, Georg Ehlers, Anibal Ramirez-Cuesta, Christoph Wildgruber, Madhusudan Tiyagi, Sanat Kumar Polyoligosilsesquioxanes (POSS) are a large family of Si-O cage molecules that can be viewed as 1-2 nm diameter, monodisperse silica nanoparticles. Here we report the results of a study of the crystal structure and ligand dynamics of one of the simplest POSS nanoparticles, octamethyl-POSS or Si$_{\mathrm{8}}$O$_{\mathrm{12}}$(CH$_{\mathrm{3}})_{\mathrm{8}}$, where the central Si$_{\mathrm{8}}$O$_{\mathrm{12}}$ cage is surrounded by eight methyl ligands. Neutron powder diffraction, inelastic neutron scattering, Raman spectroscopy, and quasielastic neutron scattering were used to characterize the structural, vibrational and dynamical properties of the polycrystalline form of this material. The structural data clearly show the presence of strongly temperature dependent methyl group torsional vibrations. The torsional vibration energy, the magnitude of the torsional energy barrier, and the activation energy for methyl rotations over the barrier were determined from the neutron measurements. These results provide a detailed picture of the structure and ligand dynamics of this POSS molecule. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z2.00011: IR Spectroscopy of Gasses Evolved During Roasting Coffee Beans Alexander Clain, Xavier Capaldi, Samuel Amanuel We measured the IR spectra of the gasses that evolve during roasting of coffee beans. The spectra recorded at different temperature revealed that the intensity of certain IR bands increase as the temperature increases. For instance, the intensity of the CO$_{2}$ band increased by a factor of four and reached a plateau as the roasting temperature approached 200$^{\circ}$C. The intensity further increased as the temperature increased above 200$^{\circ}$C, however, in two steps. Similarly the intensity of the OH bands monotonically increased until 200$^{\circ}$C and then increased further in two rapid steps above 200$^{\circ}$C. The temperature ranges where IR intensities change in two steps coincides with the temperature ranges where typically commercial roasting is done and where the first and second ``cracks'' are heard during roasting. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z2.00012: A First-Principles Polarized Raman Method for Determining Whether a Sample is Crystalline or Isotropic Andrew Weisman, Kateri DuBay, Katherine Willets, Richard Friesner We have discovered a simple way to apply basic vibrational Raman spectroscopy to unambiguously determine whether a region of a sample is crystalline or isotropic. Unlike previous methods for determining crystallinity, ours is completely general and rigorously founded in the theory of Raman scattering; it applies independently to any mode of any material and requires no previously established relationships between peak parameters and degree of crystallinity. By applying this technique while scanning an unknown, heterogeneous sample, the borders of the crystalline and isotropic regions can be delineated clearly, after which more developed methods can be applied to determine the orientations of the crystalline regions, thereby completely characterizing the molecular structure of the sample. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z2.00013: Rotation-induced grain growth and stagnation in phase-field crystal models Jens Tarp, Mathias Bjerre, Luiza Angheluta, Joachim Mathiesen Polycrystalline microstructures are typically formed by thermal processes such as quenching or annealing of melts, through the nucleation and growth of grains of different crystallographic orientation. Since these microstructures have a controlling role on the large scale material properties, it is crucial to understand their formation and late stage evolution. Here we consider the grain growth and stagnation in polycrystalline microstructures using a phase-field crystal model. We identify a transition from a grain growth stagnation upon deep quenching below the melting temperature $T_{m}$ to a continuous coarsening at shallower quenching near $T_{m}$. We find that the grain evolution is mediated by local grain rotations. In the deep quenching regime, the grain assembly typically reaches a metastable state where the kinetic barrier for recrystallization across boundaries is too large and grain rotation with subsequent coalescence or boundary motion is infeasible. For quenching near $T_{m}$ the grain growth depends on the average rate of grain rotation, and follows a power-law behavior with time, with a scaling exponent that depends on the quenching depth. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z2.00014: Nested Sampling as a blind search method in condensed matter systems Robert Baldock, Gabor Csanyi, Livia Bartok-Partay, Albert Bartok-Partay Nested Sampling is a Bayesian blind search method for calculating the density of states. Nested Sampling samples almost entirely in the region of first order phase transitions, thereby efficiently delivering entire phase diagrams without specific system knowledge. Here we present the latest methods for applying nested sampling to condensed matter systems. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z2.00015: Pulling at the fabric of the exotic phase diagram for a simple 2D model Ahmad Almudallal, Ivan Saika-Voivod, Sergey Buldyrev We use computer simulation to study a simple, two-dimensional off-lattice model that was originally devised to understand the anomalous properties of water. The model comprises core-softened disks interacting through a repulsive square shoulder located inside a longer attractive square well. In calculating the phase diagram for the model we discover that the system exhibits the truly remarkable phenomenon of inverse melting, for which the system crystallizes upon isobaric heating, over a small range in pressure. Despite occurring in two dimensions, the melting transition is first order and to a liquid, rather than to a hexatic or quasicrystal phase. We find that by increasing the extent of the shoulder, we increase the pressure range over which inverse melting occurs. But as this range increases, the stability fields of other crystal phases must bend to accommodate the changing inverse melting line. This continues until the phase diagram breaks, with a triple point disappearing, new phases appearing, and a channel of liquid stability to low temperatures forming. [Preview Abstract] |
Session Z4: Focus Session: Magnetization Plateaus and Quantum Phase Transitions
Sponsoring Units: GMAGChair: Vivien Zapf, National High Magnetic Field Lab
Room: 112/110
Friday, March 7, 2014 11:15AM - 11:51AM |
Z4.00001: Magnetic Texture {\&} Frustration in Quantum Magnets via Strain Measurements to 100 Tesla Invited Speaker: Marcelo Jaime Strong geometrical frustration in magnets leads to exotic states, such as spin liquids, spin supersolids, magnetic solitons, and complex magnetic textures. SrCu$_{2}$(BO$_{3})_{2}$, a spin-1/2 Heisenberg antiferromagnet in the archetypical Shastry-Sutherland lattice, exhibits a rich spectrum of magnetization plateaus and stripe-like magnetic textures in applied fields. We observed new magnetic textures via optical FBG magnetostriction and magnetocaloric measurements in fields up to 100.75 Tesla at 73.6 T and at 82 T [1] which we attribute, using a controlled density matrix renormalization group approach, to a 2/5 plateau and to the long-predicted 1/2-saturation plateau. The plateau predicted at 2/5 saturation is particularly interesting since strain appears to be the only experimental probe with enough sensitivity to reveal it as magnetization probes see a much more gradual change in the same field range [2,3]. BiCu$_{2}$PO$_{6}$ is a frustrated two-leg spin ladder compound with a spin gap that can be closed with a magnetic field of approximately 20T to induce a soliton lattice [4,5]. Time permitting, I will also discuss magnetization, magnetostriction and specific heat vs magnetic fields to 65 T used to obtain the anisotropic (H,T) phase diagram in BiCu$_{2}$PO$_{6}$ single crystal samples. Work at the NHMFL was supported by the National Science Foundation, the US Department of Energy Office of Basic Energy Science through the project ``Science at 100 Tesla,'' and the State of Florida.\\[4pt] [1] M. Jaime, et al. \textit{Proc. Natl. Acad. Sci.} \textbf{109}, 12407 (2012).\\[0pt] [2] S.E. Sebastian, et al., \textit{Proc. Natl. Acad.} Sci. \textbf{105}, 20157 (2009).\\[0pt] [3] Y.H. Matsuda, et al., \textit{Phys. Rev. Lett.} \textbf{111}, 137204 (2013).\\[0pt] [4] Y. Kohama, et al., Phys. Rev. Lett. \textbf{109}, 167204 (2012). \\[0pt] [5] F. Cassola, et al., Phys. Rev. Lett. \textbf{110}, 187201 (2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z4.00002: Novel Spin Flop Transition of the S=1/2 Square-Kagome-Lattice Antiferromagnet Toru Sakai, Hiroki Nakano By means of the numerical diagonalization method, we study the S=1/2 Heisenberg antiferromagnet on the square-kagome lattice, similar to the kagome lattice. We examine the ground-state properties and the magnetization process of the model. It is found that a magnetization jump appears at the higher-field-side edge of the manetization plateau at the one-third height of the saturation. A spin-flop phenomenon is clearly observed at the jump even when the system is isotropic in the spin space. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z4.00003: Magnetic Field Induced Quantum Phase Transition in Multiferroic Vanadium Spinels E.-D. Mun, G.-W. Chern, V. Pardo, F. Rivadulla, R. Sinclair, H.D. Zhou, V.S. Zapf, C.D. Batista Vanadium spinels with the formula AV2O4 (A $=$ Cd, Mg, Zn, etc) show strong magnetic frustration due to a structure of corner-sharing tetrahedra. A tetragonal structural distortion and an ``up up down down'' magnetic ordering along diagonal chains result at low temperatures, which breaks spatial-inversion symmetry. CdV2O4 is insulating enough that this magnetic order produces ferroelectricity. Here we present data on CdV2O4 and MgV2O4, showing a field-induced quantum phase transition near 40 Tesla, which is a very small energy scale compared to the dominant magnetic exchange interactions. This transition suppresses ferroelectricity and produces a magnetization jump. We show that this transition can be explained by a model that includes spin-orbit coupling effects, and also a trigonal structural distortion at zero and applied magnetic fields. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z4.00004: Magnetization of rare earth kagome systems in pulsed fields Michael Hoch, Eun Mun, Neil Harrison, Haidong Zhou The rare earth kagome systems $R_{3}Ga_{5}SiO_{14}$ (R = Nd or Pr) exhibit cooperative paramagnetism at low temperatures. Evidence for correlated spin clusters in these weakly frustrated systems has previously been obtained in neutron scattering experiments. The present pulsed field (0 - 60 T) low temperature magnetization measurements on single crystals of $Nd_{3}Ga_{5}SiO_{14}$ (NGS) and $Pr_{3}Ga_{5}SiO_{14}$ (PGS) have revealed striking differences in the magnetic responses of these two materials. At 1.6 K NGS shows a low field plateau, saturation of the magnetization for $\mu_{0}H$ $>$ 10 T and significant hysteresis while the PGS magnetization does not saturate in fields up to 60 T and shows no hysteresis or plateaus. While $Nd^{3+}$ (J = 9/2) is a Kramers ion $Pr^{3+}$ (J = 4) is not. The exchange couplings J $\sim$ 1 K are similar for PGS and NGS but the crystal field splittings and anisotropies are quite different. The marked contrast in the behavior of the two kagome systems is attributed to differences in the spin cluster structures and dynamics. The pulsed field approach has great potential for investigating kagome cluster dynamics at low temperatures. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z4.00005: Pressure-induced quantum phase transitions in a frustrated spin liquid Alexandra Mannig, Matthias Thede, Dan H\"{u}vonen, Martin M{\aa}nsson, Andrey Zheludev, Rustem Khasanov, Elvezio Morenzoni With the frustrated gapped spin-1/2 quantum antiferromagnet $\mathrm{(C_4H_{12}N_2)Cu_2Cl_6}$ piperazinium hexachlorodicuprate (PHCC) we present an example of a pressure-induced quantum phase transition from the quantum spin liquid state to a magnetically ordered phase [1]. PHCC was investigated at hydrostatic pressures of up to 23.6 kbar with $\mu$SR techniques. The evaluation of the obtained data provided local field dependencies on temperature as well as pressure and allowed the mapping of a detailed $P$-$T$ phase diagram of PHCC. Thus, the quantum critical point that separates the non-ordered phase and a magnetically ordered phase at low pressures was found to lie between 4.2 and 4.4 kbar, which disagrees with recent suggestions of inelastic neutron scattering studies. The oscillations of this magnetically ordered phase indicate an incommensurate structure. In addition, an unexpected second magnetically ordered phase that exhibits a different oscillation behavior and considerably higher saturation fields occured at around 14 kbar.\\[4pt] [1] arXiv:1310.7807 [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z4.00006: Low temperature specific heat of frustrated antiferromagnet HoInCu$_{4}$ Franziska Weickert, Veronika Fritsch, Ryan Bambaugh, John Sarrao, Joe D. Thompson, Roman Movshovich We present low temperature specific heat measurements of single crystal HoInCu$_{4}$, down to 35 mK and in magnetic field up to 12 Tesla. Ho atoms are arranged in an FCC lattice of the edge-sharing tetrahedra, and undergo an antiferromagnetic ordering at $T_{N}=$0.76 K, with the frustration parameter $f =-\Theta_{\mathrm{CW}}$/T$_{\mathrm{N}}$ of 14.3 [1]. Magnetic AF order is suppressed in field $H_{0} \approx $ 4 T. The low temperature Schottky anomaly due to Ho evolves smoothly as a function of field through $H_{0}$ and $T_{N}$. The peak value of the anomaly remains roughly constant from 0 T to 12 T. The temperature of the anomaly's peak remains constant at $T_{Sch} \approx $ 170 mK for \textit{H\textless H}$_{0}$, and gradually increases above $H_{0}$ up to 300 mK at 12 T. This indicates a complete ordering of Ho spins in zero field as well as an increasing moment on Ho once the AF order is suppressed. The measured entropy of HoInCu$_{\mathrm{4}}$ at 12 T and 2K is 17.32 J/mol-K $\approx $ Rln8 expected for an I$=$7/2 nuclear spin of Ho. \\[4pt] [1] V. Fritsch, J. D. Thompson, and J. L. Sarrao, ``Spin and orbital frustration in $R$InCu4 ($R=$Gd, Dy, Ho, and Er)'', PRB \textbf{71}, 132401 (2005). [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z4.00007: Critical properties of the phase transition between a Bose glass and a magnetic Bose-Einstein condensate in a spin-1 Heisenberg model Stephan Haas, Rong Yu, Tommaso Roscilde Motivated by recent experiments on the disordered quantum magnets Ni(Cl$^{\mathrm{1-x}}$Br$^{\mathrm{x}})^{\mathrm{2}}$4SC(NH$^{\mathrm{2}})^{\mathrm{2}}$ (Br-doped DTN), we study the critical properties of the magnetic field induced phase transition between a Bose glass and a magnetic Bose-Einstein condensate in a spin-1 Heisenberg model for this system. We determine the location of the critical field of the transition and the order parameter critical exponent via quantum scaling from quantum Monte Carlo simulations. We find that the extracted value of the order parameter critical exponent is sensitive to both the location of the critical field and the field regime of the scaling. We have also calculated the spin excitations across the transition for this system with a SU(3) slave-boson mean-field method, and discuss the connections of our results to the recent neutron measurements. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z4.00008: Critical Exponents of the Superfluid-Bose Glass Transition in Three-Dimensions Zhiyuan Yao, Mikhail Kiselev, Karine da Costa, Nikolay Prokof'ev Disordered Bose-Hubbard model is key to understanding a number of strongly interacting systems from magnets and disordered superconductors to ultra-cold atoms in optical lattice. Although the emergence of the Bose glass phase is widely accepted, some basic features of the superfluid-to-Bose glass transition remain controversial. Specifically, recent experimental and numerical studies find that the values of the correlation length exponent $\nu \approx 0.7$ and the critical temperature exponent $\phi \approx 1.1$, are in strong violation of the key quantum critical relation $\phi=\nu z$ with $z=d=3$, where $z$ is the dynamic exponent. We present results of a Monte-Carlo (for the disordered Bose-Hubbard model and its classic J-current counterpart) that clearly demonstrate that previous investigations were done away from the quantum critical region and were severely influenced by strong density dependence on the chemical potential. When the quantum critical point is reached by increasing the disorder strength, the fluctuation region is broad and we find that $\phi \approx 3.0(3)$. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z4.00009: Magnetic structure and excitations in modified pyrochlore fluoride CsCr$_{2}$F$_{6}$ Sachith Dissanayake, Y. Qiu, M. Matsuda, H. Ueda, A. Hoser, S.-H. Lee In the newly synthesized fluorides, RbCr$_{2}$F$_{6}$ and CsCr$_{2}$F$_{6}$, the magnetic Cr ions have mixed ionic value of 3$+$ (Cr1) and 2$+$ (Cr2), and the two Cr1 and two Cr2 ions form a network of corner sharing tetrahedra. CsCr$_{2}$F$_{6}$ is an antiferromagnet with T$_{\mathrm{cw}}=$-40 K which long range orders below 18 K and undergoes a field induced transition around 4 T. Using elastic and inelastic neutron scattering measurements with and without application of an external magnetic field H, we examined the magnetic structure and excitations of CsCr$_{2}$F$_{6}$. Our results show that Cr2 spins are antiparallel along the c-axis while Cr1 spins are also antiparallel almost along c-axis but canted towards a-axis by 20$^{\circ}$. Upon application of H field, around 4 T, Cr1 moments spin-flop and start canting which corresponds to the jump in magnetization data. Linear spinwave calculations were also performed to shed light in understanding an effective spin hamiltonian for this system that explains our inelastic neutron scattering data with prominent excitation modes centered at 2.2 meV, 3.1 meV and 4.2 meV. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z4.00010: Quantum and Classical Criticality in a Dimerized Quantum Antiferromagnet Christian Rueegg, Philip Merchant, Desmond F. McMorrow, Karl. W. Kraemer, Martin Boehm, Bruce Normand We perform high-resolution neutron spectroscopy experiments to probe the spin excitations of the quantum antiferromagnet TlCuCl$_3$ throughout the phase diagram by controlling the pressure and temperature. Because this material has a pressure-induced quantum critical point (QCP) at $p_c = 1.07$ kb and a thermal ordering transition at $T_N(p)$ for $p > p_c$ [1], we demonstrate a number of remarkable properties arising at the interface between quantum and classical physics. Quantum and thermal fluctuations have very similar effects in melting the magnetically ordered phase and in opening excitation gaps, but they operate quite independently close to the QCP. In the QC regime there is robust $\omega/T$ scaling of the energies and $\Gamma/T$ scaling of the widths of the critically damped excitations. This scaling crosses over to a classical critical form in a narrow region around $T_N(p)$. The critically damped longitudinal, or Higgs, mode of the ordered phase [2] is exquisitely sensitive to thermal fluctuations and becomes overdamped in the classical regime. \\[4pt] [1] Ch. R\"uegg {\it et al.}, PRL {\bf 93}, 257201 (2004). \\[0pt] [2] Ch. R\"uegg {\it et al.}, PRL {\bf 100}, 205701 (2008). [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z4.00011: Constructing Two Distinct Spin Liquid States in a Layered Cubic Lattice Jin Xu, Kevin Beach We construct a family of short-range resonating-valence-bond wave functions on a layered cubic lattice, allowing for a tunable anisotropy in the amplitudes assigned to nearest-neighbour valence bonds along one axis. Monte Carlo simulations reveal that four phases are stabilized over the full range of the anisotropy parameter. They are separated from one another by a sequence of continuous quantum phase transitions. An antiferromagnetic phase, centered on the perfect isotropy point, intervenes between two {\it distinct} quantum spin liquid states. One of them is continuously deformable to the two-dimensional U(1) spin liquid, which is known to exhibit critical bond correlations. The other has both spin and bond correlations that decay exponentially. The existence of this second phase is proof that, contrary to expectations, neither a bipartite lattice structure nor a conventional Marshall sign rule is an impediment to realizing a fully gapped quantum spin liquid. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z4.00012: $Z_2$ Spin Liquid and Valence Bond Solid Quantum Phase Transition Kevin Slagle, Cenke Xu We propose a theory to describe the quantum phase transition between a $Z_2$ spin liquid and a valence bond solid (VBS) on a triangular lattice. This phase transition can not be described using the standard Landau-Ginzburg-Wilson (LGW) theory of spontaneous symmetry breaking because a $Z_2$ spin liquid has topological order, which can't be described by an order parameter. We develop a duality formalism which treats the spin liquid's topological vison excitation as a local particle, whose condensation will drive the $Z_2$ topological order into a VBS state. We have considered both isotropic and anisotropic triangular lattices, and second order phase transitions are found in both cases. At these transitions, the VBS order parameters are expected to have an enormous anomalous dimension. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z4.00013: Plasticity-Induced Magnetism in Frustrated Amorphous Solids George Hentschel, Itamar Procaccia, Bhaskar Sen Gupta Amorphous magnetic solids, like metallic glasses, with no macroscopic magnetic order due to random locally favoured orientations for individual spins exhibit a novel effect: the emergent growth of a macroscopic magnetic Order in the presence of an imposed mechanical strain in athermal conditions in the presence of a imposed magnetic field. The magnetic moment increases in steps whenever there is a plastic event. Thus plasticity induces the magnetic moment, acting as the effective noise driving the system towards equilibrium. We present results of atomistic simulations of this effect in a model of a magnetic amorphous solid subjected to pure shear and a magnetic field. While to elucidate the dependence on external strain and magnetic field we offer a mean-field theory that provides an adequate qualitative understanding of the observed phenomenon. [Preview Abstract] |
Session Z6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Manganite Thin Films
Sponsoring Units: DMP GMAGChair: H. Jeffrey Gardner, University of Nebraska-Lincoln
Room: 108
Friday, March 7, 2014 11:15AM - 11:27AM |
Z6.00001: Emergent Phenomena in La5/8Ca3/8MnO3 /Pr5/8Ca3/8MnO3 Superlattices Zhu Yinyan, Du Kai, Zhang Kai, Yin Lifeng, Shen Jian In this work, we study the magnetic and transport properties of La5/8Ca3/8MnO3 (LCMO)/Pr5/8Ca3/8MnO3(PCMO) superlattices. For comparison, La1$-$x$-$yPryCaxMnO3 (LPCMO) thin films with the same nominal doping concentration have also been grown and characterized. The spatial rearrangement of the doped cations and the presence of interfaces appear to have dramatic effect on the physical properties of the superlattices. Specifically, the transport and magnetic properties of the [LCMO2n/PCMOn]t show strong dependence on n, which differ greatly from those of the La10/24Pr5/24Ca3/8MnO3 thin films. It is especially striking that the n$=$1 superlattice has a metal-insulator transition temperature that nearly 100 K higher than that of the La10/24Pr5/24Ca3/8MnO3 thin film. These emergent phenomena are caused by the interplay between interfacial effect and artificial chemical phase separation between two contrasting ground states along the film stacking direction. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z6.00002: Anisotropic magnetoresistance in colossal magnetoresistive oxide La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ thin films Le Zhang, Vijay Singh, Anil Rajapitamahuni, Xia Hong We present our studies of the anisotropic magnetoresitance (AMR) in colossal magnetoresistive oxide La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ (LSMO, x $=$ 0.3, 0.5) thin films as a function of temperature, magnetic field and film thickness. LSMO thin films with thickness below 10 nm are grown on SrTiO$_{3}$ (001) and NdGaO$_{3}$(110) substrates via off-axis magnetron sputtering. X-ray diffraction and atomic force microscopy studies reveal high crystallinity and atomically smooth surfaces of these films. As the thickness of the films decreases, the metal insulator transition temperature (T$_{\mathrm{MI}})$ shifts to below the bulk value. Films thinner than 3 nm become totally insulating. We extract the AMR from the resistance change as a function of the orientation between current and magnetic field. AMR reaches the maximum value in the vicinity of T$_{\mathrm{MI}}$. At low magnetic field ($\sim$ 100 Oe), the angular dependence of AMR deviates from a sinusoidal shape, which is attributed to the effect of magnetocrystalline anisotropy. We discuss the effects of the carrier density, film thickness, and substrate strain on the AMR. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z6.00003: Ferromagnetic Metallic Edge State in Manganites Stripes Kai Du, Kai Zhang, Shuai Dong, Jian Shao, Jiebin Niu, Jinjie Chen, Yinyan Zhu, Lifeng Yin, Jian Shen Recently, spin-orbital interaction induced edge states in systems such as topological insulators and graphene ribbon have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we experimentally demonstrate that edge states do exist in strongly correlated oxides. Our observation is made by employing magnetic force microscope (MFM) and transport measurement techniques to study manganites stripes as a function of temperature and magnetic field. Distinct appearance of ferromagnetic metallic phase was observed along the edge of manganites stripes. The edge states have strong influence on the transport properties of the stripes, leading to an increase of metal-insulator transition (MIT) temperature with decreasing width of the stripes. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z6.00004: Telegraph Noise in LSMO Nanowires John J. Damasco, Nicholas T. Bronn, Xiaoqian M. Chen, Anoop R. Damodaran, Karthik Jambunathan, Lane W. Martin, Peter Abbamonte, Nadya Mason Lanthanum strontium manganites (La$_{1-x}$Sr$_{x}$MnO$_{3}$) are oxide materials that exhibit colossal magnetoresistance and have potential in spintronics applications. Their resistivity is affected by the competition between a double exchange mechanism and a Jahn-Teller distortion. We present magnetic-field dependent transport measurements on PLD-grown La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ nanowires. First, we show signatures of bimodal random telegraph noise that occur at temperatures below 100 K. We also discuss the temperature dependence of the resistance, which is consistent with a meta-stability in the nanowire. Both behaviors can be explained by a competition between ferromagnetic metallic and charge-ordered insulating domains, caused by double exchange and the Jahn-Teller distortion, respectively. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z6.00005: Low Temperature Magnetic Force Microscopy of La$_{\mathrm{2-2x}}$Sr$_{\mathrm{1+2x}}$Mn$_{2}$O$_{7}$ Neliza Leon Brito, J.-S. Zhou, J.B. Goodenough, Alex de Lozanne, Jeehoon Kim, Roman Movshovich We present micromagnetic studies of La$_{\mathrm{2-2x}}$Sr$_{\mathrm{1+2x}}$Mn$_{2}$O$_{7}$ (x $=$ 0.32, 0.36, 0.40) taken with a magnetic force microscope at 4 K in magnetic fields up to 5 T. The x $=$ 0.32 sample shows branching domains with magnetization in/out of the surface that evolve into stripes and bubbles as a function of increasing field until saturation is reached at $\sim$ 0.29 T. The rms average of the magnetic images show an unexpected non-monotonic dependence on field that is not observed in SQUID data of a sister sample. We speculate that this difference is caused by the micro vs. macro nature of the two measurements. The magnetic microstructure for the doping level of x $=$ 0.36 agrees with the expected in-plane magnetization. The sample reaches saturation by magnetic domain reorientation at \textbar H\textbar \textgreater 0.18 T. The x $=$ 0.40 sample also shows in-plane magnetization, but the increasing magnetic field does not appear to change noticeably the magnetic domain structure and only seems to change the magnitude of the contrast. From the drastic change in contrast, it would appear the sample reaches saturation around 0.4 T. Since this is the only sample that required polishing, it may be that this caused strong pinning of the domains at the surface. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z6.00006: Nanostructure engineering of epitaxial colossal magnetoresistive oxide thin films Anil Rajapitamahuni, Vijay Raj Singh, Le Zhang, Xia Hong We have fabricated nanostructured colossal magnetoresistive (CMR) oxide thin films to study the effect of phase separation. Using off-axis radio frequency magnetron sputtering, we have grown epitaxial single crystalline 4-6 nm La0.5Sr0.5MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO) films on (110) NdGaO3 and (001) SrTiO3 substrates, respectively. X-ray diffraction and atomic force microscopy characterizations show the films have high crystalinity and RMS roughness of 2-3 {\AA}. Films close to the electrical dead layer thickness (3-5 nm) are patterned into periodic depth modulation by e-beam lithography and fluorine based reactive ion etching. We fabricated nanostructures with periodic thickness variations of 1-2 nm and 100-200 nm periodicities. The etched patterns retain the atomic smoothness of the as grown films. We have the control of the etching depth with sub-nanometer precision on both LSMO and LCMO films. We discuss the effect of the periodicity and depth modulation of the nanostructures on the I-V characteristics and magnetotransport properties of these CMR thin films within the phase separation model. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z6.00007: Probing magnetic inhomogeneity in La0. 67Ca0.33MnO3 by optical spin wave resonances Yuhang Ren, Haibin Zhao, Yu Gong, David Scienzo, Yonatan Abranyos, Gunter Luepke, Qi Li We report on our recent study of spin wave resonances in ferromagnetic La0.67Ca0.33MnO3 (LCMO) thin films by ultrafast Kerr-rotation experiments. Coherent magnetization precessions were generated and detected by subpicosecond laser pulses. Confined higher order spin wave modes are identified in addition to fundamental modes at 10 K. We determine spin stiffness, magnetic and surface anisotropy parameters for both 60-nm and 100-nm LCMO samples. Significant changes in anisotropy parameters are explained by the formation and extension of magnetic inhomogeneity in the 100-nm LCMO film. Our results show the new picosecond time-resolved magneto-optical method is a powerful tool for detecting magnetic inhomogeneity. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z6.00008: Intrinsic Insulating Ferromagnetism in Manganese Oxide Thin Films Yusheng Hou, Hongjun Xiang, Xingao Gong Recently, LaMnO$_{\mathrm{3}}$ thin films attract considerable attentions not only because LaMnO$_{\mathrm{3}}$ is the most common magnetic component in all fabricated oxide superlattices/interfaces, but also because experiment observed exotic insulating ferromagnetism in LaMnO$_{\mathrm{3}}$ thin film grown on SrTiO$_{\mathrm{3}}$. However, there is no any model or theory/calculation to explain such striking insulating ferromagnetism. In this work, by means of genetic algorithm optimization, first-principles calculations and the orbital-degenerate double-exchange model studies, we successfully find the insulating ferromagnetic phase of the epitaxially strained LaMnO$_{\mathrm{3}}$ film grown on the cubic SrTiO$_{\mathrm{3}}$ substrate. The unexpected insulating ferromagnetism, which was observed experimentally but not fully understood, originates from the G-type orbital order $d_{{3z}^{2}-r^{2}} \mathord{\left/ {\vphantom {d_{{3z}^{2}-r^{2}} d_{x^{2}-y}}} \right. \kern-\nulldelimiterspace} d_{x^{2}-y}$ and the insulating gap opens as a result of both the orbital ordering and the strong electron-phonon coupling. Our work provides new insight into how a prototypical antiferromagnetic Mott insulator transforms into the ferromagnetic insulator. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z6.00009: Signatures of electronic phase separation in the Hall effect of anisotropically-strained La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ films with a magnetically driven percolative phase transition Liuqi Yu, Xiaohang Zhang, S. von Moln\'ar, P. Xiong, Lingfei Wang, W.B. Wu Hall measurements have been performed on La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) films with varying degrees of anisotropic strain. The strain is induced by epitaxial growth on NdGaO$_{3}$ substrate and controlled via post annealing. An antiferromagnetic insulating (AFI) state emerges upon annealing at low temperatures.\footnote{Z. Huang et al., JAP 105, 113919(2009)} The Hall effect (HE) data exhibit many unusual features that are indicative of a magnetically driven percolation. In the paramagnetic phase, it is found the Hall resistivity shows a distinct slope change at a constant critical magnetization over the temperature range, which is interpreted as a critical point of a magnetic field driven percolative phase transition. At lower temperatures near the metal-insulator transition, a negative Hall resistivity peak emerges. This Hall resistivity dip correlates with the emergence and strengthening of the AFI state, and is suppressed with the melting of the AFI state by an in-plane field. The Hall resistivity dips in LCMO resemble the giant HE in granular metal films near the composition-driven percolation,\footnote{X. X. Zhang et al., PRL 86, 5562 (2001)} which is interpreted as a result of the enhancement of Hall coefficient beyond the percolation point. The observations reveal an important manifestation of the magnetic field driven percolative phase transition in the HE of LCMO with an insulating background. Work supported in part by NSF DMR-0908625 and DMR-1308613. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z6.00010: Transport and magnetic properties of CMR manganites with antidot arrays Kai Zhang, Kai Du, Jiebin Niu, Wengang Wei, Jinjie Chen, Lifeng Yin, Jian Shen We fabricated and characterized a series of manganites thin film samples with different densities of antidots. With increasing antidot density, the samples show higher MIT temperature and lower resistivity under zero and low magnetic fields. These differences become smaller and finally vanished when the magnetic field is large enough to melt the charge ordered phase in the system, which is expected in our theoretical explanations. We believe that emerging edge states at the ring of antidotes play a significant role for observed metal-insulator transition and electrical transport properties, which are of great importance of real storage and sensor device design. Magnetic property measurements and theoretical simulation also support the conclusion. These results open up new ways to control and tune the strongly correlated oxides without introduce any new material or field. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z6.00011: Effects of preparation conditions on the physical properties of nickel ferrite thin films Hillary Belliveau, Manuel Bonilla, Patrick McArdle, Casey Miller The effects of the deposition temperature, pressure, and oxygen partial pressure were investigated on the structural, electrical, and magnetic properties of nickel ferrite (NiFe$_{\mathrm{2}}$O$_{\mathrm{4}})$ thin films grown by magnetron sputtering. The samples were grown on Si (100), Si (100) with 500nm of amorphous SiO$_{\mathrm{2}}$, and on MgO (100) substrates. Increasing the deposition temperature allowed the NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (004) XRD peak to appear at higher oxygen partial pressures. Between films of the same oxygen pressure, increasing the deposition temperature reduced the FWHM and increased the intensity of the NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (004) XRD peak. The films were 800 to 1000nm thick as measured by profilometry. Increasing the oxygen partial pressure reduces the intensity of the (004) NiFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ XRD peak, and allows us to tune the material from a conducting material (rho $=$ 114 microOhm*cm at T$=$220K) to an insulating one (rho$=$ 2475 Ohm*cm at T$=$220K). The resistivity at 220K decreased as the deposition temperature was increased for films grown at the same oxygen partial pressure. The magnetic properties were investigated with magneto-optical Kerr effect and vibrating sample magnetometry. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z6.00012: Magnetic properties of epitaxial hexagonal HoFeO$_{3}$ thin films Xiao Wang, Zhuyun Xiao, Xiaoshan Xu, Wenbin Wang, David Keavney, Yaohua Liu, X.M. Cheng Multiferroic materials exhibit multiple ferroic orders simultaneously and thus have great potential applications in information technology, sensing and actuation. Epitaxial hexagonal HoFeO$_{3}$ (h-HFO) films are very promising candidates as multiferroic materials with room temperature ferromagnetism, because magnetic Ho$^{3+}$ ions are expected to have stronger exchange interactions with Fe$^{3+}$ ions than the well-studied h-LuFeO$_{3}$ films. We report study of magnetic properties of epitaxial h-HFO thin films deposited using laser molecular beam epitaxy on Yttria-stabilized zirconia (YSZ) substrates. X-ray diffraction measurements confirmed the epitaxial registry and six-fold symmetry of the film. Temperature dependence of magnetization of the film measured by a Quantum Design SQUID magnetometer shows dominating paramagnetic characteristic. Element specific x-ray magnetic circular dichroism measurements performed at beamline 4-ID-C of the Advanced Photon Source show a ferromagnetic ordering of Fe and an exchange coupling between Ho$^{3+}$ and Fe$^{3+}$ ions. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z6.00013: Nonvolatile Three-Step Ferroelectric Switching in Tensile Strained BiFeO$_{3}$ Thin Films Jin Hong Lee, Kanghyun Chu, Kwang-Eun Kim, Chan-Ho Yang Misfit strain has been one of key control parameters to improve the magnetoelectric coupling between ferroelectricity and magnetism in multiferroic epitaxial thin films. Lately, it was discovered that a bulk-like phase of multiferroic bismuth ferrite (BiFeO$_{3})$, through compressive or tensile misfit strain, can be transformed into a highly-elongated tetragonal-like phase [1-3] or an orthorhombic phase [4], respectively, thereby offering new chances into magnetoelectric applications. Although the heteroepitaxial misfit strains via (001) interfaces have been intensively studied, strain effects arising from the other directional interfaces such as (110) have not been studied much. In this talk, we present the uniaxial-tensile-strain effects on the (110) oriented BiFeO$_{3}$ thin films. Our detailed piezoresponse force microscopy analysis, X-ray reciprocal space mapping, and Landau free energy modeling give strong evidences of electrically switchable, non-volatile, three out-of-plane polarization states. These findings provide useful implications for a new type of magnetoelectric devices based on phase competition.\\[4pt] [1] H. Bea \textit{et al}., Phys. Rev. Lett. \textbf{102}, 217603 (2009).\\[0pt] [2] R. J. Zeches \textit{et al}., Science \textbf{326}, 977 (2009).\\[0pt] [3] K. T. Ko \textit{et al}., Nat. Commun. \textbf{2}, 567 (2011).\\[0pt] [4] J. C. Yang \textit{et al}., Phys. Rev. Lett. \textbf{109}, 247606 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z6.00014: Photo-induced dynamics of charge ordering in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ probed by ultrafast hard x-ray diffraction Yi Zhu, Jason Hoffman, Clare Rowland, Donald Walko, John Freeland, Philip Ryan, Richard Schaller, Anand Bhattacharya, Haidan Wen La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ thin films exhibit strong charge ordering due to charge disproportionation of Fe$^{3+}$/Fe$^{5+}$ ions along the [111] direction below T$_{c}$ $\sim$ 200K. In this study, a La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ thin film was excited by ultrafast 355nm laser pulses, and the response of the charge ordering and the lattice was directly observed via ultrafast hard x-ray diffraction. We identified the threshold of the excitation laser fluence to melt the charge ordering diffraction peak. We also found charge ordering recovers in a few nanoseconds, faster than the lattice recovery. These findings indicate the non-thermal nature of the photo-induced dynamics of charge disordering in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$. [Preview Abstract] |
Session Z7: Focus Session: Magnetostructural Properties of Materials
Sponsoring Units: GMAGChair: Luqiao Liu, IBM Research
Room: 106
Friday, March 7, 2014 11:15AM - 11:51AM |
Z7.00001: Pathways for tailoring the magnetostructural behavior of FeRh-based systems Invited Speaker: Radhika Barua The prediction of phase transition temperatures in functional materials provides dual benefits of supplying insight into fundamental drivers underlying the phase transition, as well as enabling new and improved technological applications that employ the material. In this work, studies focused on understanding the magnetostructural phase transition of FeRh as a function of elemental substitution, provides guidance for tailoring phase transitions in this compound, with possible extensions to other intermetallic-based magnetostructural compounds. Clear trends in the magnetostructural temperatures ($T_{t})$ of alloys of composition Fe(Rh$_{1-x}$M$_{x})$ or (Fe$_{1-x}$M$_{x})$Rh (M $=$ 3$d$, 4$d$~or 5$d$~transition metals), as reported in literature since 1961, were identified and confirmed as a function of the valence band electron concentration (($s+d)$ electrons/atom) of the system. It is observed that substitution of 3$d $or 4$d $elements ($x\le $ 6.5 at{\%}) into B2-ordered FeRh compounds causes $T_{t} $to increase to a maximum around a critical valence band electron concentration ($e_{v}*) $of 8.50 electrons/atom and then decrease. Substitution of 5$d $elements echoes this trend but with an overall increase in $T_{t} $and a shift in $e_{v}* $to 8.52 electrons/atom. For $e_{v} $\textgreater 8.65 electrons/atom, FeRh-based alloys cease to adopt the B2-ordered crystallographic structure in favor of the chemically disordered A1-type structure or the ordered L1$_{0}$-type structure. This phenomenological model has been confirmed through synthesis and characterization of FeRh alloys with Cu, Ni and Au additions. The success of this model in confirming existing data trends in chemically-substituted FeRh and predicting new composition-transition temperature correlations emphasizes the strong interplay between the electronic spin configuration, the electronic band structure, and crystal lattice of this system. Further these results provide pathways for tailoring the magnetostructural behavior and the associated functional response of FeRh-based systems for potential technological applications. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z7.00002: Hysteretic properties of Nd2Fe14B-based permanent magnets: First principles and micromagnetic modeling Aleksander Wysocki, Denis Kukusta, Liqin Ke, Vladimir Antropov We combine ab initio electronic structure calculations with micromagnetic simulations to investigate permanent magnet properties of Nd2Fe14B-based systems. First, magnetic moments, anisotropy constants and exchange interactions of bulk Nd2Fe14B are calculated from first principles. These parameters are then used to construct a micromagnetic model for realistic samples and evaluate hysteresis loop at finite temperatures using Monte Carlo method. Several generic microstructures are considered including randomly oriented grains, hard/soft multilayers, and core/shell geometries. We find optimal grain sizes and hard phase/soft phase volume ratio which maximize maximum energy products of the systems. Further, we discuss the nature of the thermal spin reorientation effect in the bulk material and how it affects the finite temperature hysteretic properties. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z7.00003: Mechanochemical synthesis of submicron sized Nd-Fe-B particles using planetary ball mill Ozlem Koylu-Alkan, George C. Hadjipanayis, David J. Sellmyer Mechanochemical synthesis of Nd$_{2}$Fe$_{14}$B particles with size below 0.5 $\mu$m is done via planetary ball mill, followed by annealing of rare-earth oxides, iron oxide and boron oxide in the presence of a reducing agent (Ca) and a dispersant material (CaO). Compared to high energy ball mill, planetary mill gives a range in milling energy. Our purpose is to control the particle size and size distribution by changing the milling energy. In preliminary work, annealed particles in the CaO dispersant with coercivity 4.7 kOe were produced. After washing off the dispersant due to the interstitial modification of 2:14:1 phase with hydrogen, coercivity of the particles was decreased to 1.2 kOe. Electron micrographs of the samples showed that rectangular Nd$_{2}$Fe$_{14}$B particles are present with size distribution in the submicron range. The aim of this study is to obtain nanoparticles with a size below 500 nm and study the effect of size and surface on their magnetic properties. Work supported by DOE DE-FG02-04ERU612. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z7.00004: Influence of grain size on precipitation hardening in melt-spun Sm(Co, Fe, Cu, Zr)$_{\mathrm{z}}$ alloys Ozlem Koylu Alkan, Weiqiang Liu, Xiaocao Hu, George C. Hadjipanayis In this work, we have investigated the influence of grain size on precipitation hardening that takes place in 2:17 Sm-Co magnets. An alloy with a nominal composition of Sm(Co$_{0.72}$Fe$_{0.12}$Cu$_{0.13}$Zr$_{0.03})_{7.6}$ was prepared by arc-melting and subsequently melt-spinning. The grain size was controlled by varying the wheel speed from 5 m/s to 50 m/s. The melt-spun ribbons were subsequently isothermally aged at 850 $^{\circ}$C for 3 h followed by slow cooling at 0.7 K/min to 400 $^{\circ}$C. A single 1:7 phase was detected in the as-spun ribbons and the grain size of ribbons estimated by Scherrer's formula was found to decrease gradually with the increasing of the wheel speed from 5 m/s to 50 m/s. After aging, the 1:7 phase was decomposed into Sm$_{2}$(Co,Fe)$_{17}$ and Sm(Co,Cu)$_{5}$ phase. For the 5 m/s ribbon, the coercivity increased drastically form 0.8 kOe to 11.2 kOe. On the other hand, the coercivity of the 35 m/s ribbon showed a little increase from 4.7 kOe to 6.3 kOe. TEM studies are currently under way to study the microstructure as a function of grain size and the results will be reported. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z7.00005: Principle of Magnetodynamics for Composite Magnetic Pole Alexander Animalu It is shown in this paper that geometry provides the key to the new \textit{magnetodynamics} principle of operation of the machine (invented by Dr. Ezekiel Izuogu) which has an unexpected feature of driving a motor with static magnetic field. Essentially, because an array of like magnetic poles of the machine is arranged in a half circular array of a cylindrical geometry, the array creates a non-pointlike magnet pole that may be represented by a ``magnetic current loop'' at the position of the pivot of the movable arm. As a result, in three-dimensional space, it is possible to characterize the symmetry of the stator magnetic field \textbf{B} and the magnetic current loop \textbf{J} as a cube-hexagon system by a 6-vector (\textbf{J},\textbf{B}) (with \textbf{J}.\textbf{B}$\ne $\textbf{0)} comprising a 4x4 antisymmetric tensor analogous to the conventional electric and magnetic 6-vector \textbf{(E,B}) (with \textbf{E}.\textbf{B}$\ne $\textbf{0)} comprising the 4x4 antisymmetric tensor of classical electrodynamics The implications are discussed. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z7.00006: Magnetic domain formation in field-cooled metals Dirk Wulferding, Doohee Cho, Il Kyu Yang, Yoon Hee Jeong, Ji Ho Sung, Moon-Ho Jo, Han Woong Yeom, Jeehoon Kim We explore the effect of field-cooling on the formation of magnetic domains in films of the magnetic metals Ni and Fe using magnetic force microscopy. In particular, we study the dependence of the domain pattern on both film thickness and external magnetic field strength and compare the results to the domain structures of zero-field cooled specimens. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z7.00007: Thermodynamic Stability of ThMn$_{12}$-type CeFe$_{8}$M$_{4}$ Magnetic Compounds J.F. Herbst, Chen Zhou, F.E. Pinkerton Rare earth (R) elements such as Nd and Dy are critical constituents of high-performance Nd$_{2}$Fe$_{14}$B-type permanent magnets. Ongoing economic uncertainties have stimulated great interest in magnets that use alternative R materials, Ce in particular since it is the most abundant R element. While the intrinsic magnetic properties of known Ce-based compounds are inferior to those of their Nd-based cognates, they nevertheless offer the prospect of developing magnets with technical characteristics intermediate between those of Nd-Fe-B and ferrites. Moreover, there is ample opportunity to identify novel Ce systems. As a means of guiding the synthesis of new CeFe$_{\mathrm{12-x}}$M$_{\mathrm{x}}$ phases we have assessed the thermodynamic stability of ThMn$_{12}$-type CeFe$_{8}$M$_{4}$ compounds with 26 different elements M via density functional calculations. Compounds of this class are attractive since they can have larger Fe:R ratios than R$_{2}$Fe$_{14}$B, and in some cases additional processing such as nitriding, hydriding, or carbiding can substantially improve the magnetic properties. We critically compare the theoretical results with experiment. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z7.00008: Magnetic Field Dependent Phase Boundaries in Al-Cu alloys up to 35 Tesla Jason Cooley, Seth Imhoff, Martha Katz We report on the magnetic field dependence of the liquid-solid phase boundary in the Al-Cu alloy system between 0 and 17 at. {\%} Cu at fields up to 35 Tesla. Melting/freezing point measurements were performed using a purpose built Differential Thermal Analysis instrument capable of operating in the 32 mm bore of a 35T Bitter magnet at the National High Magnetic Field Laboratory DC Field facility in Tallahassee Florida. In general the application of field increases the melting point by approximately 10 degrees Celsius. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z7.00009: Effect of Co doping on the structural, magnetic and electron transport properties of Mn$_{2}$PtSn Heusler alloy Parashu Kharel, Yung Huh, Austin Nelson, Valloppilly Shah, Ralph Skomski, David Sellmyer Materials with high magnetic anisotropy and Curie temperature well above room temperature have huge potential for a range of applications including permanent magnet, high density recording and spintronic devices. Tetragonal Mn$_{2}$PtSn is one such Heuslar compounds which has been predicted to have very high magnetic anisotropy but its low Curie temperature (T$_{\mathrm{c}}=$ 374 K) is a drawback [1]. Our experimental investigation of the rapidly quenched nanostructured ribbons shows that a single phase Mn$_{2}$PtSn in the tetragonal structure cannot be easily prepared without the substitution of an external element. We have found that a partial replacement of Pt with Co in Mn$_{2}$PtSn stabilizes the tetragonal structure and also improves the magnetic properties. The experimentally observed values of the room-temperature saturation magnetization (M$_{\mathrm{s}}$) and Curie temperature (T$_{\mathrm{c}}$) are respectively 35 emu/g and 385 K for Mn$_{2}$PtSn and 43 emu/g and 516 K for Mn$_{2}$Pt$_{0.3}$Co$_{0.7}$Sn. The effect of cobalt on the magnetic anisotropy and electron transport properties of this material will be discussed.\\[4pt] [1] J. Winterlik et al., Adv. Mater. \textbf{24}, 6283 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z7.00010: Magnetic, Electrical and Structural study of Mn-Co-Sn Heusler Nanomaterials Yung Huh, P. Kharel, A. Nelson, V. Shah, R. Skomski, D. Sellmyer The nano-structured Mn$_{3-x}$Co$_{x}$Sn (x = 0, 0.3, 0.5, 0.7, 1.0) alloys were prepared using arc-melting, melt-spinning and thermal annealing. Mn$_{3}$Sn is stable in the hexagonal structure and it shows an antiferromagnetic spin order at room temperature. Mn$_{3-x}$Co$_{x}$Sn alloys maintained a hexagonal structure upon substituting Mn with Co up to x = 0.7, and then it transformed to cubic phases at x = 1.0. At room temperature Mn$_{3-x}$Co$_{x}$Sn (x = 0.5, 0.7, 1.0) exhibited ferromagnetic spin order. Mn$_{2.3}$Co$_{0.7}$Sn sample showed Curie temperature of 640 K. However, the transition temperatures are suppressed to 600 K for Mn$_{2.5}$Co$_{0.5}$Sn and Mn$_{2.0}$Co$_{1.0}$Sn. The room temperature saturation magnetization measured at 7.0 T increases with increasing amount of Co substitution, varying from 13 emu/g (x = 0.5), 25 emu/g (x = 0.7), and 50 emu/g (x = 1.0), respectively. The electrical resistivity of all the Co-substituted samples depends on temperature and decreases with increasing temperature from 5 K to room temperature. Interestingly, there observed a rapid upturn in the resistivity at 250 K for Mn$_{2.5}$Co$_{0.5}$Sn. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z7.00011: Magnetic properties of the off-stoichiometric Heusler alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$ using $^{55}$Mn NMR as a local probe Shaojie Yuan, Philip Kuhns, Michael Hoch, James Brooks, Arneil Reyes, Vijay Srivastava, Daniel Phelan, Richard James, Chris Leighton The off-stoichiometric Heusler-type alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$ have interesting properties and a rich phase diagram, stemming from the interplay between magnetic order, martensitic transformations, and ferroelasticity. Previous magnetization and small angle neutron scattering (SANS) measurements suggest that at low temperatures ferromagnetic (F) nanoscale clusters and antiferromagnetic (AF) regions coexist. As the temperature is raised above 50-100 K the F regions undergo superparamagnetic blocking while the AF matrix is thought to persist to higher temperatures. We have applied zero and low field nuclear magnetic resonance as a local probe to determine the temperature and field dependent behavior of the F and AF components for samples with x = 0, 7 and 15. For x = 7 evidence is obtained for two distinct Mn electronic environments which are characterized by different hyperfine fields. In addition, detailed information has been obtained on the evolution with temperature of the F and AF components. Results obtained for the x = 0 and x = 15 samples help to determine the nature of the ground state in these systems. A model which can account for the magnetic properties of the material will be presented, together with a modified phase diagram. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z7.00012: ABSTRACT WITHDRAWN |
Session Z8: Focus Session: Spin-Dynamics: Theory and Experiment
Sponsoring Units: GMAGChair: Lifa Zhang, University of Texas at Austin
Room: 104
Friday, March 7, 2014 11:15AM - 11:27AM |
Z8.00001: Angular Momentum of Phonons and Einstein-de Haas Effect Lifa Zhang, Qian Niu We study angular momentum of phonons in a magnetic crystal. In the presence of a spin-phonon interaction, we obtain a nonzero angular momentum of phonons, which is an odd function of magnetization. At zero temperature, phonon has a zero-point angular momentum besides a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches to zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein-de Haas effect. To obtain the change of angular momentum of electrons, the change of phonon angular momentum needs to be subtracted from the opposite change of lattice angular momentum. Furthermore, the finding of phonon angular momentum gives a potential method to study the spin-phonon interaction. Possible experiments on phonon angular momentum are also discussed. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z8.00002: Angular and Linear Momentum of Excited Ferromagnets Peng Yan, Akashdeep Kamra, Yunshan Cao, Gerrit Bauer The angular momentum vector of a Heisenberg ferromagnet with isotropic exchange interaction is conserved, while under uniaxial crystalline anisotropy the projection of the total spin along the easy axis is a constant of motion. Using Noether's theorem, we prove that these conservation laws persist in the presence of dipole-dipole interactions. However, spin and orbital angular momentum are not conserved separately anymore. We also define the linear momentum of ferromagnetic textures. We illustrate the general principles with special reference to spin transfer torques and identify the emergence of a non-adiabatic effective field acting on domain walls in ferromagnetic insulators [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z8.00003: Dynamic Magnetoelectric Effect in Ferromagnet|Superconductor Tunnel Junctions Mircea Trif, Yaroslav Tserkovnyak We study the magnetization dynamics in a ferromagnet$\mid$insulator$\mid$superconductor tunnel junction and the associated buildup of the electrical polarization. We show that for an open circuit, the induced voltage varies strongly and nonmonotonically with the precessional frequency, and can be enhanced significantly by the superconducting correlations. For frequencies much smaller or much larger than the superconducting gap, the voltage drops to zero, while when these two energy scales are comparable, the voltage is peaked at a value determined by the driving frequency. We comment on the potential utilization of the effect for the low-temperature spatially-resolved spectroscopy of magnetic dynamics. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z8.00004: Dependence of the demagnetization time on the exchange interaction and spin moment Guoping Zhang, Thomas F. George, Mingsu Si In femtomagnetism, the demagnetization time is at the center of laser-induced ultrafast demagnetization. It depends on various intrinsic and extrinsic parameters, but the experimental results are controversial, and in some cases the opposite effects are reported. In this presentation, we directly address how the exchange interaction and magnetic spin moment affect the demagnetization time. We employ a simple model that includes the exchange interaction and spin-orbit coupling. Then we derive an equation of motion for the spin moment change, from which a master equation is found. This equation explicitly shows how the demagnetization time is related to the exchange interaction and spin moment. This result can be directly compared with the latest experimental results. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z8.00005: Theoretical and experimental investigations of the electronic structure configuration during ultrafast demagnetization of Co Emrah Turgut, Patrik Grychtol, Dmitry Zusin, Henry C. Kapteyn, Margaret M. Murnane, Dominik Legut, Karel Carva, Peter M. Oppeneer, Stefan Mathias, Martin Aeschlimann, Claus M. Schneider, Justin Shaw, Ronny Knut, Hans Nembach, Thomas J. Silva We report on theoretical and experimental studies of the electronic structure configuration during the ultrafast demagnetization in Co thin films. After an ultrafast optical laser excitation of a ferromagnetic material, the magnetization of the material decreases rapidly in less than a picosecond. This ultrafast behavior has attracted a significant amount of attention for more than two decades; however, the underlying driving mechanism is still unclear. In this work, we use an extreme ultraviolet, broad-bandwidth, tabletop, ultrafast, and element-selective magnetization probe that employs the transverse magneto-optical Kerr effect to extract the energy- and time-resolved dynamics of the off-diagonal dielectric tensor element that is proportional to the magnetization. We compare our data with theoretical optical predictions based upon \textit{ab-initio} calculations of the electronic structure, with the ultimate goal of determining how the occupation of majority and minority states vs. energy evolves after ultrafast optical pumping. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z8.00006: Fast reversal of magnetic vortex chirality by electric current Weng-Lee Lim, RongHua Liu, Tolek Tyliszczak, Dmitry Berkov, Sergei Urazhdin We demonstrate reversal of magnetic vortex in a microscopic Pt/Permalloy bilayer disk by a nonuniform electric current in the plane of the disk. The switching is detected electronically by measuring the response to a small ac magnetic field, and confirmed by direct imaging with x-ray magnetic dichroism microscopy (XMCD). The magnetic contrasts obtained from time-resolved x-ray imaging indicate a fast and robust switching of magnetic vortex driven by electric current. The time-resolved XMCD measurements show that the characteristic switching time is less than 3 ns. Analysis from micromagnetic simulation shows that the reversal of the magnetic vortex is driven by a combination of the Oersted field due to the charge current and the spin transfer due to spin current generated by the spin Hall effect in Pt. The simulation reveals that the magnetization switching process of the magnetic vortex involves two distinct stages. The switching first proceeds with a fast dynamics and then evolves at a slower dynamics before reaching the final magnetic vortex state with opposite chirality, in agreement with the experimental result. The simulation also shows that the spin transfer torque (STT) accelerates the reversal of magnetic vortex in comparison to the case without STT. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z8.00007: Photo-induced Spin Angular Momentum Transfer into Antiferromagnetic Insulator Fan Fang, Yichun Fan, Xin Ma, J. Zhu, Q. Li, T.P. Ma, Y.Z. Wu, Z.H. Chen, H.B. Zhao, Gunter Luepke Spin angular momentum transfer into antiferromagnetic(AFM) insulator is observed in single crystalline Fe/CoO/MgO(001) heterostructure by time-resolved magneto-optical Kerr effect (TR-MOKE). The transfer process is mediated by the Heisenberg exchange coupling between Fe and CoO spins. Below the Neel temperature(TN) of CoO, the fact that effective Gilbert damping parameter $\alpha$ is independent of external magnetic field and it is enhanced with respect to the intrinsic damping in Fe/MgO, indicates that the damping process involves both the intrinsic spin relaxation and the transfer of Fe spin angular momentum to CoO spins via FM-AFM exchange coupling and then into the lattice by spin-orbit coupling. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z8.00008: Combined molecular and spin dynamics study of collective excitations in BCC iron Dilina Perera, David P. Landau, Don Nicholson, G. Malcolm Stocks Spin dynamics simulations of classical spin systems have revealed a substantial amount of information regarding the collective excitations in magnetic materials. However, much of the previous work has been restricted to lattice-based spin models that completely disregard the effect of lattice vibrations. Combining an empirical many body potential with a spin Hamiltonian parameterized by first principles calculations, we present a compressible magnetic model for BCC iron, which treats the dynamics of translational degrees of freedom on an equal footing with the magnetic (spin) degrees of freedom. This model provides us with a unified framework for performing combined molecular and spin dynamics simulations and make simultaneous quantitative measurements of the spin wave and vibrational spectrum. Results from our simulations reveal that the presence of lattice vibrations leads to softening and damping of spin waves, as well as evidence for a novel form of longitudinal spin wave excitation coupled with the longitudinal phonon mode of the same frequency. Furthermore, we will also discuss the influence of lattice vibrations at different temperatures and the implications of using different atomistic potentials. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z8.00009: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z8.00010: Laser Demagnetization Dynamics in Gadolinium from Time Resolved Photoemission John Bowlan, Bj\"orn Frietsch, Martin Teichmann, Robert Carley, Martin Weinelt The field of ultrafast magnetization dynamics has seen rapid progress in recent years and has the potential to enable magnetic data storage systems orders of magnitude faster than those based on conventional read/write heads. The dynamics of laser demagnetization in ferromagnetic Gadolinium depend on the transfer of energy and angular momentum between the metallic valence electrons and the core-like $4f$ electrons. Angle-Resolved Photoemission (ARPES) with femtosecond XUV laser pulses produced by high harmonic generation enables the direct measurement of the electronic band structure on a sub-picosecond time scale in a ``tabletop'' setup. Photoemission allows the magnetization dynamics of the valence and $4f$ bands to be tracked independently of one another. Thus, time-resolved photoemission is an alternative to experimental methods such as surface magnetic second harmonic generation (MSHG), the magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD). We applied this technique to study Gd(0001) films grown epitaxially on a W crystal. We find that the valence electrons demagnetize on a fs time scale, while the 4f electrons respond more slowly. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z8.00011: Pump-probe measurement of short and long-range exchange interactions in a rare-earth magnet using resonant x-ray diffraction Matthew Langner, Sujoy Roy, Yi-De Chuang, Rolf Versteeg, Yi Zhu, Marcus Hertlein, Thornton Glover, Karine Dumesnil, Robert Schoenlein The combined effects of spin-orbit interactions, magnetostriction, and long-range exchange coupling lead to a wide variety of magnetic phases in the rare earth magnets. In dysprosium, core level spins develop a spiral phase as a result of competition between short and long-range RKKY exchange interactions mediated by the conducting electrons. We use time-resolved resonant x-ray diffraction to directly probe the spiral order parameter of the core level magnetism in response to optical pumping of the conduction electrons that mediate the exchange interaction. The dynamics of the diffraction intensity and spiral turn angle occur on different time scales, and through free-energy analysis, we associate these dynamics with changes in the short and long-range exchange coupling. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z8.00012: Numerical Renormalization-Group computation of nuclear magnetic relaxation rates Krissia Zawadzki, Luiz N. Oliveira, Jos\'{e} Wilson M. Pinto We report an essentially exact numerical renormalization-group (NRG) computation of the temperature-dependent NMR rate $1/T_1$ of a probe at a distance $R$ from a magnetic impurity in a metallic host. We split the metallic states into two subsets, A and B. The former comprises electrons $a_k$ in $s$-wave states about the magnetic-impurity site. The coupling between the $a_k$ band and the impurity is described by the Anderson Hamiltonian, diagonalizable by the NRG procedure. Each state $b_k$ in the B subset is a linear combination of an $s$-wave state about the probe site with the degenerate $a_k$, constructed to be orthogonal to all the $a_k$'s. The $b_k$ band hence decouples from the impurity and is analytically treatable. We show that the relaxation rate has three components: (i) a constant associated with the $b_k$'s; (ii) a $T$-dependent term associated with the $a_k$'s, which decays in proportion to $1/(k_FR)^2$, where $k_F$ is the Fermi momentum; and (iii) another $T$-dependent term due to the interference between the $a_k$'s and the $b_k$'s. The interference term shows Friedel oscillations whose amplitude, proportional to $1/k_FR$, can be mapped onto the universal function of $T/T_K$ describing the Kondo resistivity. We compare our findings with results in the literature. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z8.00013: ABSTRACT MOVED TO A54.00014 |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z8.00014: Dynamic Phase Diagram of the DC-Pumped Magnon Condensates Scott Bender, Rembert Duine, Arne Brataas, Yaroslav Tserkovnyak We investigate the effects of nonlinear dynamics and damping by phonons on a system of electronically pumped Bose-Einstein condensed or normal phase magnons in a ferromagnet. The nonlinear effects are crucial to understanding the phenomenon of ``swasing." Meanwhile damping was heretofore neglected, since the pumped magnon condensates previously considered are quasi-equilibrium and considered only a much shorter timescale. We analyze the magnetic phase behavior in the presence of these two new effects, demonstrating the possibility of stable condensate and hysteresis. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z8.00015: Dynamics of a two-dimensional quantum spin liquid: signatures of emergent Majorana fermions and fluxes Johannes Knolle, Dimitry Kovrizhin, John Chalker, Roderich Moessner Topological states of matter present a wide variety of striking new phenomena. Prominent among these is the fractionalisation of electrons into unusual particles: Majorana fermions, Laughlin quasiparticles or magnetic monopoles. Their detection, however, is fundamentally complicated by the lack of any local order, such as, for example, the magnetisation in a ferromagnet. While there are now several instances of candidate topological spin liquids, their identification remains challenging. Here, we provide a complete and exact theoretical study of the dynamical structure factor of a two-dimensional quantum spin liquid in gapless and gapped (abelian and non-abelian) phases. We show that there are direct signatures--qualitative and quantitative--of the Majorana fermions and gauge fluxes emerging in Kitaev's honeycomb model. These include counterintuitive manifestations of quantum number fractionalisation, such as a neutron scattering response with a gap even in the presence of gapless excitations, and a sharp component despite the fractionalisation of electron spin. Our analysis identifies new varieties of the venerable X-ray edge problem and explores connections to the physics of quantum quenches. [Preview Abstract] |
Session Z12: Invited Session: Application of Synchronization in the Micro and Macro World
Sponsoring Units: GSNP DBIOChair: Juan Restrepo, University of Colorado, Boulder
Room: 205
Friday, March 7, 2014 11:15AM - 11:51AM |
Z12.00001: Quantum synchronization and the no-photon laser Invited Speaker: Murray Holland This talk will present a new approach to lasers that is based on the quantum synchronization of many atoms. Such lasers are predicted to produce light of unprecedented spectral purity and coherence, some two orders of magnitude better than any system available today. The idea is based on superradiant emission, where an ensemble of atoms with an extremely narrow atomic transition can phase-lock and form a macroscopic dipole that radiates light collectively. This is quite unlike a typical laser where atoms essentially act independently. The resulting light source is expected to have a spectral linewidth of just a few millihertz and could lead to more accurate and stable atomic clocks. Atomic clocks based on optical transitions have improved tremendously in recent years, giving clocks that tick $10^{15}$ times per second, and can have a fractional stability exceeding one part in $10^{16}$. This new sharper light source aims to push the frontier even further, so that fundamental tests of physics, such as the time variation of constants and tests of gravity, might even be possible. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z12.00002: Theory of injection locking and mutual synchronization of non-isochronous auto-oscillators in the presence of noise Invited Speaker: Andrei Slavin A theory of injection locking and mutual synchronization of non-isochronous (having oscillation frequency dependent on the oscillation amplitude) auto-oscillators (AO) in the presence of thermal noise is developed and illustrated on the examples of spin-torque and spin-Hall nano-oscillators. It is demonstrated that all the characteristics of the injection locking and mutual synchronization of AO (such as phase-locking and synchronization frequency bands and transition times to a phase-locked state after application of a driving signal) are determined by the non-isochronous parameters of an AO : damping rate of amplitude fluctuations $\Gamma_{p} $ and dimensionless nonlinearity coefficient $\nu $ (see Eqs. (27b) and (33) in [1]). It is also shown that the influence of the thermal noise leads to the appearance of an apparent threshold amplitude of the driving signal in the process of injection locking. The developed theory is used to quantitatively explain the phenomena of fractional [2] and parametric [3] synchronization observed in strongly non-isochronous spin-torque [2, 3] and spin-Hall [4] microwave nano-oscillators based on magnetic nano-structures and driven by either spin-polarized charge current or pure spin current. \\[4pt] [1] A. Slavin and V. Tiberkevich, IEEE Trans. Magn. \textbf{45}, 1875 (2009). \\[0pt] [2] S. Urazhdin et al., Phys. Rev.Lett. \textbf{105, }104101 (2010). \\[0pt] [3] S. Urazhdin et al., Phys. Rev. Lett. \textbf{105}, 237204 (2010). \\[0pt] [4] V. E. Demidov et al., Nature Materials \textbf{11}, 1028, doi:10.1038/nmat3459 (2012). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 1:03PM |
Z12.00003: Synchronization of Degrade-and-Fire Oscillations in Synthetic Gene Circuits Invited Speaker: Lev Tsimring This talk reviews our recent work on the synchronization of synthetic gene oscillators operating in the degrade-and-fire regime. Computational modeling and theoretical analysis show that the key mechanism of oscillations is a small delay in the negative feedback loop. In a strongly nonlinear regime, this time delay can lead to long-period oscillations in gene expression that can be characterized by ``degrade and fire'' dynamics. I will present experimental, analytical, and computational results for the intra-cellular as well as population-wide synchronization when oscillators are coupled either by common protease enzymes or by coupling the oscillators to a quorum sensing machinery that produces chemical inducers freely diffusing through cell membranes. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:39PM |
Z12.00004: Unusual synchronization behaviors during the electropolishing of silicon wafers: experiments and theory Invited Speaker: Katharina Krischer The electrodissolution of silicon wafers exhibits a variety of oscillatory spatio-temporal patterns. Most typically, rather peculiar cluster patterns emerge, which always exhibit a pronounced oscillation of the uniform mode. In addition, the spontaneous formation of synchronously and incoherently oscillating domains, i.e., a chimera-type state, is observed. I will first give an overview of the experimentally observed oscillatory states with emphasis on their synchronization behavior and then discuss the current theoretical understanding of their emergence. In particular, I will show that the patterns can be well reproduced with a modified complex Ginzburg-Landau equation with nonlinear global (amplitude) coupling. To elucidate the role of the nonlinear global coupling, the dynamics of an ensemble of identical nonlinearly coupled Stuart-Landau oscillators is also discussed. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 2:15PM |
Z12.00005: Equivalence of phase oscillator models Invited Speaker: Antonio Politi Various types of phase-oscillator models are discussed to identify mutual analogies and differences. In particular, an ensemble of leaky-integrate-and-fire neurons is compared with both Winfree- and Kuramoto-type models, showing that, contrary to the common belief, relevant differences are maintained even in the weak coupling limit. The comparison is mostly made by studying the linear stability of the splay state and its possible bifurcations. [Preview Abstract] |
Session Z13: Focus Session: Fe-based Superconductors-Nematicity
Sponsoring Units: DMPChair: Chris Homes, Brookhaven National Laboratory
Room: 207
Friday, March 7, 2014 11:15AM - 11:27AM |
Z13.00001: Nematic charge fluctuations and electron-phonon coupling in EuFe$_{2}$As$_{2}$ and SrFe$_{2}$As$_{2}$ Wei-Lu Zhang, Verner Thorsmolle, Hsiang-Hsi Kung, Philip Lubik, Alexander Lee, Girsh Blumberg, Pierre Richard, Hong Ding, Athena S. Sefat, Jack Gillett, Suchitra Sebastian We study phononic and electronic Raman scattering in the 122 iron pnictide parent compounds EuFe$_{2}$As$_{2}$ and SrFe$_{2}$As$_{2}$. Using polarized Raman spectra we identify all four Raman active phonon modes through the temperature range of 5-300 K from \textit{ab} plane and \textit{ac} plane. Strong interference of the phonon and the electronic continuum is observed in the A$_{g}$ channel below the tetragonal (D$_{4h}$) to orthorhombic (D$_{2h}$) (T-O) phase transition. Quasi-elastic Raman scattering induced by nematic fluctuations observed in the B$_{2g}$ channel in a wide temperature range above the T-O transition. The temperature dependence of the static Raman susceptibility derived from the Raman response $\chi'(0)= \frac{1}{\pi}P\int^{\infty}_{-\infty}\frac{\chi''(\omega)}{\omega}d\omega$ follows a Curie-Weiss like law A(T-T$_{0}$)$^{-1}$, where T$_{0}$ is about 50K below the T-O transition temperature. WLZ acknowledges ICAM support (NSF-IMI grant DMR-0844115), GB, VT and HHK acknowledge support by NSF DMR-1104884 and by U.S. DOE, BES, Award DE-SC0005463, AS acknowledges the support by US DOE, BES, Materials Sciences and Engineering Division. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z13.00002: Spin excitations in the A-type antiferromagnet CaCo$_{2}$As$_{2}$ R.J. McQueeney, B.G. Ueland, A. Sapkota, Abhishek Pandey, G.S. Tucker, D.L. Abernathy, J.L. Niedziela, A. Kreyssig, D.C. Johnston, A.I. Goldman CaCo$_{2}$As$_{2}$ is an A-type antiferromagnetic (AFM) metal with net ferromagnetic interactions between Co ions within the square-lattice layer and AFM interactions between layers. The material is isostructural to AFe$_{2}$As$_{2}$ (A=Ca, Sr, Ba) based high-temperature superconductors, although the stripe AFM magnetism is manifestly different from the nearly ferromagnetic CaCo$_{2}$As$_{2}$ compound. Surprisingly, AFM stripe spin correlations similar to those found in the iron arsenides are observed in the closely related paramagnetic SrCo$_{2}$As$_{2}$ system. We have studied spin excitations in CaCo$_{2}$As$_{2}$ using inelastic neutron scattering. While we find that low-energy Goldstone modes appear at the A-type AFM wavevector, broad and quasi-one-dimensional spin excitations spanning both A-type and stripe AFM wavevectors persist up to very high energies ($>$ 150 meV) and share many similarities to SrCo$_{2}$As$_{2}$. Combined with the small ordered moment ($<$ 0.5 $\mu_{B}$/Co) of CaCo$_{2}$As$_{2}$, the results suggest that the cobalt arsenides are itinerant magnetic systems where the net magnetic exchange interactions place them close to a quantum critical point separating stripe AFM and ferromagnetic ground states. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z13.00003: Anomalous behaviour of critical fields near a superconducting quantum critical point in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ C. Putzke, A. Carrington, P. Walmsley, L. Malone, J.D. Fletcher, P. See, D. Vignolles, C. Proust, S. Badoux, S. Kasahara, Y. Mazukami, T. Shibauchi, Y. Matsuda BaFe$_2$(As$_{1-x}$P$_x$)$_2$ presents one of the cleanest and clearest systems in which to study the influence of quantum critical fluctuations on high temperature superconductivity. In this material a sharp maximum in the magnetic penetration depth has been found at the quantum critical point (QCP $x=0.3$) where $T_c$ is maximal$^1$. Specific heat and de Haas-van Alphen effect measurements$^2$ show that this peak is driven by a corresponding increase in the quasiparticle effective mass. Based on these previous results a simple one-band theory would suggest that at the QCP we should expect a large increase in $H_{c2}$ and a corresponding dip in $H_{c1}$ . Actual measurements of these critical fields, which we present here, shows quite different behavior which we suggest is caused by an anomalous enhancement in the vortex core energy close to the QCP. \\ $^1$ K.Hashimoto \emph{et.al.}, Science \textbf{336}, 1554 (2012)\\ $^2$ P.Walmsley, C.Putzke \emph{et.al.}, Phys. Rev. Lett. \textbf{110}, 257002 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z13.00004: Electronic nematicity and its relation to magnetism in EuFe$_{2}$As$_{2}$ based superconductors Invited Speaker: Philipp Gegenwart The interplay of electronic, magnetic and structural degrees of freedom leads to the competition of various phases in iron pnictude superconductors. We have recently established the thermoelectric power under uniaxial pressure as sensitive novel probe of electronic nematicity in these materials, which is able to distinguish the influence of anisotropic magnetic fluctuations and orbital polarization [1]. We focus our attention on EuFe$_{2}$As$_{2}$ based systems, where the presence of local 4f moments leads to intriguing behavior: their magnetic ordering below 20 K, mediated by RKKY interaction, sensitively depends on the electronic structure, which can be tuned by chemical or hydrostatic pressure [2,3]. Such tuning results in different magnetic phases which coexist or compete with superconductivity [4]. Application of magnetic field below 20 K, acts differently along the orthorhombic a and b axis before the polarization of 4f moments is reached at 1.5 T. A giant magnetostriction of order 10$^{-3}$ is found, related to field induced de-twinning, which (partially) remains up to 200 K. Application of this effect to the study of the in-plane anisotropy in various properties is demonstrated. Work in collaboration with H.S. Jeevan, Y. Tokiwa, J. Maiwald, N. Bach, C. Stingl, S. Jiang, S. Zapf and M. Dressel.\\[4pt] [1] S. Jiang, H.S. Jeevan, J. Dong, P. Gegenwart, Phys. Rev. Lett. 110, 067001 (2013).\\[0pt] [2] H.S. Jeevan, D. Kasinathan, H. Rosner, P. Gegenwart. Phys. Rev. B 83, 054511 (2011).\\[0pt] [3] Y. Tokiwa, H.-S. H\"{u}bner, O. Beck, H.S. Jeevan, P. Gegenwart, Phys. Rev. B 86, 220505(R) (2012).\\[0pt] [4] S. Zapf, H.S. Jeevan, T. Ivek, F. Pfister, F. Klingert, S. Jiang, D. Wu, P. Gegenwart, R.K. Kremer, M. Dressel, Phys. Rev. Lett. 110, 237002 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z13.00005: The gap structure of BaFe$_2$(As$_{x}$P$_{1-x}$)$_2$ determined from specific heat measurements L. Malone, Y. Mizukami, P. Walmsley, C. Putzke, S. Kasahara, T. Terashima, T. Shibauchi, Y. Matsuda, A. Carrington The structure of the superconducting gap of the pncitide superconductors is an unresolved but crucial issue to understanding their mechanism of superconductivity. While some experiments and theories support a fully gapped s+/s- state, several experiments have revealed evidence for nodes in some families of pnictides. Detailed knowledge of the superconducting gap structure and how it varies as a function of material properties can be useful in helping to decide between microscopic theories. BaFe$_2$(As$_x$P$_{1-x}$)$_2$ is a pnictide family with a nodal gap structure and evidence for quantum critical behavior [1]. We have measured the specific heat of several samples of BaFe$_2$(As$_{x}$P$_{1-x}$)$_2$ in a range of $x$ values. We examine the temperature, field and field angle dependence of the specific heat to deduce the changes in the superconducting gap structure as the material is tuned to the quantum critical point. [1] K. Hashimoto et al, Science, 336, [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z13.00006: Doping Dependence of Resonant Spin Excitations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Raymond Osborn, John-Paul Castellan, Stephan Rosenkranz, Keith Taddei, Jared Allred, Omar Chmaissem, Sevda Avci, Duck-Young Chung, Helmut Claus, Mercouri Kanatzidis, Doug Abernathy, Matthew Stone The first spectroscopic evidence of unconventional s$_\pm$-symmetry in the iron-based superconductors was provided by inelastic neutron scattering on Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ with the observation of a resonant spin excitation at the wavevector, Q, that connected the hole and electron Fermi surfaces, centered at the zone center and zone boundary, respectively. Subsequent measurements as a function of hole doping showed that the resonant excitations split into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes and the resonant enhancement below T$_c$ falls to zero as the magnetic interactions weaken, in good agreement with RPA theory. We have now extended these measurements to investigate the doping dependence of BaFe$_2$(As$_{1-x}$P$_x$)$_2$, whose nominal isovalence has been the subject of debate, from $x=0.2$ to 0.6. We find no evidence of a splitting of the resonance with increasing $x$, but see a monotonic decrease in the resonant enhancement across the superconducting dome falling to zero close to $x=0.6$. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z13.00007: Magnetoresistance Near the Quantum Critical Point of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Ian Hayes, Arkady Shekhter, Ross McDonald, Nicholas Breznay, James Analytis We report on the magnetoresistance of the Iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ up to 65 Tesla. In addition to showing unconventional superconductivity, this compound has a quantum critical point associated with the suppression of the anti-ferromagnetic transition to zero temperature at a x = 0.33. We propose a simple model for the magnetoresistance of a quantum critical system; at the quantum critical point of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ this model captures the data quite well, while significant deviations are observed for samples far from the critical point. I will discuss this analysis in detail, as well as extensions to other systems. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z13.00008: Effect of Electron Irradiation on isovalenly substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$ C.P. Strehlow, Jason Murphy, Makariy Tanatar, Ruslan Prozorov, M. Konczykowski, N. Salovich, R.W. Giannetta, T. Kobayashi, S. Miyasaka, S. Tajima The effects of electron irradiation on the termperature-dependent London penetration depth, $\lambda$(T), have been investigated in annealed optimally doped single crystals of isovalently substituted SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=0.35$, using the tunnel diode resonator technique. The low temperature behavior of $\lambda$(T) changes under electron irradiation from almost $T-$linear to practically exponentially saturated behavior, similar to the observations in another isovalently substituted 122 pnictide, BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Furthermore, aluminum - coating technique was used to measure the absolute values of the London penetration depth, $\lambda$(0), which allowed calculations of the superfluid density, $\rho_s$. We conclude that, similar to BaFe$_2$(As$_{1-x}$P$_x$)$_2$, the superconducting properties of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ are compatible with $s_{\pm}$ pairing with accidental line nodes that are lifted by pair-breaking disorder. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z13.00009: Quantum Phase Transitions Inside the Superconducting Dome of the iron-based Superconductors Saurabh Maiti, Rafael Fernandes, Peter Woelfle, Andrey Chubukov In several iron-based superconductors, a nematic transition from the tetragonal to the orthorhombic phase precedes the onset of long-range magnetic order. As doping increases, both the nematic and magnetic ordered states are suppressed, and the two transitions lines cross separately the superconducting dome. In this talk, we discuss the fate of these two instabilities inside the superconducting dome. Using a microscopic electronic model in which nematicity arises from magnetic fluctuations, we show that both ordered states are able to coexist with superconductivity for a wide range of parameters. As the temperature is lowered below Tc, the two transitions merge, giving rise to a single simultaneous first-order nematic-magnetic transition at T=0. The changes in the magnetic spectrum caused by the coexistence with superconductivity makes this quantum phase transition weakly first-order, allowing strong fluctuations to existence in its vicinity. Our results are consistent with experimental findings in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, which indicate the existence of a single quantum phase transition inside the superconducting dome. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z13.00010: Linear magnetoconductivity in multiband spin-density-wave metals with nonideal nesting Alexei Koshelev In several parent iron-pnictide compounds the resistivity has an extended range of linear magnetic field dependence. We argue that there is a simple and natural explanation of this behavior. Spin density wave transition leads to Fermi-surface reconstruction corresponding to strong modification of the electronic spectrum near the nesting points. It is difficult for quasiparticles to pass through these points during their orbital motion in magnetic field, because they must turn sharply. As the area of the Fermi surface affected by the nesting points increases proportionally to magnetic field, this mechanism leads to the linear magnetoresistance. The crossover between the quadratic and linear regimes takes place at the field scale set by the SDW gap and scattering rate. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z13.00011: $T$-linear scattering rate in optimally doped K- and P-Ba122 iron-pnictides Y.M. Dai, C.C. Homes, R.P.S.M. Lobo, B. Xu, B. Shen, H. Xiao, X.G. Qiu, H.H. Wen The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reveal two groups of carriers with different scattering rates ($1/\tau$), described by two Drude components in the optical conductivity. A ``broad'' Drude component results in an incoherent background with a $T$-independent $1/\tau_b$, while a ``narrow'' Drude component reveals a $T$-linear scattering rate $1/\tau_n$ resulting in a resistivity $\rho_n \equiv 1/\sigma_{1n}(\omega \rightarrow 0)$ also linear in temperature. This fact explains the $T$-linear $\rho$ at low temperatures and the tendency to saturation at room temperature observed by transport measurements in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. The low frequency spectral weight increases with decreasing $T$, following an arctan($T$) dependence, which is also strong evidence for a $T$-linear scattering rate. A comparison to other materials with similar behavior suggests that the $T$-linear $1/\tau_n$ and $\rho_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ may arise out of scattering from spin fluctuations due to the proximity to a quantum critical point (QCP). Similar behaviors are found in the optimally doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_{2}$. [Preview Abstract] |
Session Z16: General Statistical and Nonlinear Physics II
Room: 401
Friday, March 7, 2014 11:15AM - 11:27AM |
Z16.00001: Non-adiabatic effect on quantum pumping Chikako Uchiyama We study quantum pumping for an anharmonic junction model which interacts with two kinds of bosonic environments. We provide an expression for the quantum pumping under a piecewise modulation of environmental temperatures with including non-adiabatic effect under Markovian approximation. The obtained formula is an extension of the one expressed with the geometrical phase(Phys. Rev. Lett. {\bf 104},170601 (2010)). This extension shows that the quantum pumping depends on the initial condition of the anharmonic junction just before the modulation, as well as the characteristic environmental parameters such as interaction strength and cut-off frequencies of spectral density other than the conditions of modulation. We clarify that the pumping current including non-adiabatic effect can be larger than that under the adiabatic condition. This means that we can find the optimal condition of the current by adjusting these parameters. (The article has been submitted as http://arxiv.org/submit/848201 and will be appeared soon.) [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z16.00002: Robustness of quantum multifractality Bertrand Georgeot, R\'emy Dubertrand, Ignacio Garcia-Mata, Olivier Giraud, Gabriel Lemari\'e, John Martin Several models where quantum wave functions display multifractal properties have been recently identified. In the quantum chaos field, they correspond to pseudointegrable systems, with properties intermediate between integrability and chaos. In condensed matter, they include electrons in a disordered potential at the Anderson metal-insulator transition. These multifractality properties lead to particular transport properties and appear in conjunction with specific types of spectral statistics. In parallel, progress in experimental techniques allow to observe finer and finer properties of the wavefunctions of quantum or wave systems, as well as to perform experiments with unprecedented control on the dynamics of the systems studied. In this context, this talk will discuss the robustness of multifractality in presence of small perturbations. We identify two distinct processes of multifractality destruction according to the type of perturbation, and specify a range of parameters where multifractality could indeed be observed in physical systems in presence of imperfections. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z16.00003: The ``Quantized Intrinsically Localized Modes" of a three-dimensional Lattice Derya Kanbur, Peter Riseborough The low-energy Intrinsically Localized Modes (ILMs) of a cubic lattice with nearest-neighbor interactions and quartic anharmonicity are examined using the Ladder Approximation.Due to the symmetry of the lattice and the isotropic nature of the anharmonic interaction,the ILMs are characterized by an intrinsic spin corresponding to either $S=0$ or $S=2$ as well as by their spatial symmetries.The lowest energy ILMs form preferentially for center of mass momenta at which several van-Hove singularities coalesce at the upper edge of the (non-interacting) two-phonon continuum.For $T=0$ and interactions larger than the critical value,the ILMs form above the top of the two-phonon continuum near the preferred values of $\underline{q}$,but fall into the continuum as $\underline{q}$ is shifted further away from the optimal value of $\underline{q}$.The critical value of the anharmonic interaction is found to be reduced for non-zero temperatures.The results are compared with experimental results on NaI. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z16.00004: Elastic wave propagation in the presence of linear and nonlinear dispersive mechanisms Romik Khajehtourian, Mahmoud Hussein The introduction of nonlinear and dispersive effects alters the dispersion of elastic waves in a solid medium. In this work, we derive an exact dispersion relation for longitudinal elastic wave propagation in a one-dimensional homogeneous thin rod in the presence of both linear and nonlinear dispersive mechanisms. Our amplitude- and radius-dependent exact dispersion relation contains the effects of finite strain, specifically Green-Lagrange strain, as well as lateral inertia. In general, the nonlinearity tends to steepen the waveform since large-amplitude waves are able to catch up with slower low-amplitude waves while the dispersion widens the waveform since large-wavelength waves cannot catch up with faster small-wavelength waves. The dispersion relation presented in this work provides information on both these mechanisms which may be used to elucidate the interplay between the waveform narrowing and widening effects due to nonlinearity and dispersion, respectively. A discussion is provided on the implications of the present analysis on elucidating the properties of shock and solitary waves. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z16.00005: Utilizing nonlinearity of transistors for reconfigurable chaos computation William Ditto, Behnam Kia A VLSI circuit design for chaos computing is presented that exploits the intrinsic nonlinearity of transistors to implement a novel approach for conventional and chaotic computing circuit design. In conventional digital circuit design and implementation, transistors are simply switched on or off. We argue that by using the full range of nonlinear dynamics of transistors, we can design and build more efficient computational elements and logic blocks. Furthermore, the nonlinearity of these transistor circuits can be used to program the logic block to implement different types of computational elements that can be reconfigured. Because the intrinsic nonlinear dynamics of the transistors are utilized the resulting circuits typically require fewer transistors compared to conventional digital circuits as we exploit the intrinsic nonlinearity of the transistors to realize computations. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z16.00006: Optical Asymmetry Induced by PT-symmetric Nonlinear Fano Resonances Nicholas Bender, Fakroddin Nazari, Hamidreza Ramezani, Mohammad Moravvej-Farshi, Demetrios Christodoulides, Tsampikos Kottos We introduce a new type of Fano resonances, realized in a photonic circuit which consists of two nonlinear PT-symmetric micro-resonators side-coupled to a waveguide, which have line-shape and resonance position that depends on the direction of the incident light. We utilize these features in order to induce asymmetric transport up to 47 dBs in the optical C-window. Our set-up requires low input power and does not compromise the power and frequency characteristics of the output signal. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z16.00007: Time Reversal Experiments in Chaotic Cavities Bo Xiao, Jen-Hao Yeh, Thomas Antonsen, Edward Ott, Steven Anlage Wave focusing through a strongly scattering medium has been an intriguing topic in the fields of optics, acoustics and electromagnetics. By introducing the time reversal technique, prior knowledge about each transmission channel is no longer needed since the step of sending waves through the medium measures this information. Many approaches have been explored to achieve better focusing quality, which is influenced by several factors, such as the propagation loss. We present a method to focus electromagnetic wave at an arbitrary location in ray-chaotic billiards or cavities using the time reversal technique. First, a ray-tracing algorithm calculates orbit information from knowledge of the cavity geometry. We use this information to generate a synthetic signal, which is then sent into the cavity as if it's the time reversed signal in the traditional time-reversal scheme. This method tries to obtain channel information numerically but has limited accuracy due to the loss, the coupling, the mode density, and the existence of chaotic. We discuss the effects of these factors by presenting experimental results on a low-loss superconducting cavity, changing the ports(to modify coupling) and frequency range(to vary the mode density), and modifying the cavity to obtain smaller Lyapunov exponents and thus longer Ehrenfest times to vary the time over which the semi-classical approximation is valid. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z16.00008: Matched Bipartite Digraph Representation of Generalized Dynamical System Formed by One-way Barriers John Li, John Mahoney, Kevin Mitchell We studied a dynamical system with stable and unstable manifolds that behave as one-way barriers, instead of separatrices in traditional dynamical system that are two-way barriers. This asymmetry gives rise to a richer dynamical behavior such as the overlapping of basins of attraction. The recently developed \emph{Burning Invariant Manifold} (BIM) theory took a dynamical system approach to understand front propagation in Advection-Reaction-Diffusion systems, which have BIMs as the one-way barriers. Through numerical simulations under BIM theory, we found that although both unstable and stable BIMs are one-way barriers, unstable BIMs are the ones that we can experimentally observe the fronts converging onto, and the stable BIMs act as the basin boundaries. We further hypothesized a duality relation between the stable and unstable BIMs. Under the duality hypothesis, we developed a mechanism of the behavior of the system by reducing it back to a traditional system based on topology, and we found a simplification of the system by to summarize the topological information into a Matched Bipartite directed graph (MB digraph). [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z16.00009: The Elusive Present: Hidden Past and Future Correlation and Why We Build Models Pooneh Mohammadiara Markov models assume that the present encodes all of a process's history. This is almost never the case if one randomly samples structured processes. So, how does this failure come about? How do measurements encode the past? And, how many are needed to capture correlations between the past and future? Here, we show how much information can be extracted from the past without having any information about the present. We show how to quantify this and then draw out the consequences. The most important of which is that when present hides past-future correlation we must build models. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z16.00010: The Role of Partial Enthalpy in Thermal Conductivity Calculations for Nanofluids Matthew Edwards, John Shelton Over the past decade, reports of significantly enhanced thermal conductivity in solutions of nanoscale particles (nanofluids) have elicited a great deal of interest due to the large number of applications for efficient heat transfer fluids. A common method for calculating the thermal conductivity of a nanofluid uses the autocorrelation of the microscopic heat flux (Green-Kubo formalism), which contains a correction for the net transport of enthalpy due to species diffusion. The partial enthalpy component of the correction term cannot be found from microscopic quantities and is often approximated by the partitioned enthalpy. Using NPT molecular dynamics simulations over a wide range of interaction energies, we show that this approximation leads to spurious enhancements with magnitudes similar to those reported in the literature. The discrepancy arises because the partitioned enthalpy neglects the change in fluid-fluid interaction enthalpy which occurs around solid particles; in systems with strong fluid-solid interactions this can be a substantial portion of the total enthalpy. This work suggests that the standard method for calculating thermal conductivity in nanofluids may be invalid and that actual conductivity enhancements are comparable to those predicted by Maxwell's theory. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z16.00011: Kramers-Wannier duality applied to the boolean satifiability problem Joe Mitchell, Benjamin Hsu, Victor Galitski Kramers-Wannier duality, first considered in 1941, is an exact technique used in statistical mechanics to relate two models together through an order-disorder transformation, and thereby study their structure and critical phenomena. The boolean satisfiability problem is one of the most important problems in computer science, specifically complexity theory; it is the first proven NP-complete problem. Using a mapping to a multi-spin Ising model in the limit of zero temperature, we present an application of Kramers-Wannier duality to this problem. This results in a novel relationship between solving the boolean satisfiability counting problem and a different computational problem: listing the non-negative solutions to a particular system of linear integer equations. This mapping relates the complexity of the two problems. We discuss the generality of Kramers-Wannier duality and its possible application to other computational problems. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z16.00012: Full counting statistics and the Edgeworth series for matrix product states Yifei Shi We consider full counting statistics of spin in matrix product states. In particular, we study the approach to gaussian distribution for magnetization. We derive the asymptotic corrections to the central limit theorem for magnetization distribution for finite but large blocks in analogy to the Edgeworth series. We also show how central limit theorem like behavior is modified for certain states with topological characteristics such as the AKLT state. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z16.00013: Emergent Newtonian dynamics and the geometric origin of mass Luca D'Alessio, Anatoli Polkovnikov We consider an arbitrary many-body system with possibly infinitely many degrees of freedom interacting with few macroscopic parameters which are allowed to slowly change in time. These degrees of freedom can represent positions of objects in space, their angles, shape distortions, magnetization, currents and so on. By extending the Kubo linear response theory to such setups we derive the dynamics of the macroscopic d.o.f. which takes the form of the emergent Newton's second law (force is equal to the mass times acceleration) with an extra dissipative term. We find the microscopic expression for the mass tensor relating it to the non-equal time correlation functions in equilibrium. In the classical (high-temperature) limit the mass tensor is given by the product of the inverse temperature and the Fubini-Study metric tensor determining the natural distance between the eigenstates of the Hamiltonian. For free particles this result reduces to the conventional definition of mass. This finding shows that any mass, at least in the classical limit, emerges from the distortions of the Hilbert space highlighting deep connections between any motion and geometry. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z16.00014: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z16.00015: News and views in discontinuous phase transitions Jan Nagler Recent progress in the theory of discontinuous percolation allow us to better understand the the sudden emergence of large-scale connectedness both in networked systems and on the lattice. We analytically study mechanisms for the amplification of critical fluctuations at the phase transition point, non-self-averaging and power law fluctuations. A single event analysis allow to establish criteria for discontinuous percolation transitions, even on the high-dimensional lattice. Some applications such as salad bowl percolation, and inverse fragmentation are discussed. [Preview Abstract] |
Friday, March 7, 2014 2:15PM - 2:27PM |
Z16.00016: Topological Properties of Combinational Logic Functions for Very Large Scale Integrated Circuits Elizabeth Hiteshue, Kelsey Irvin, Mary Lanzerotti, Graziano Vernizzi, Joseph Kujawski, Allan Weatherwax This talk presents topological properties of combinational logic functions implemented with basic logic gates. Combinational logic can be implemented in very large scale integrated circuits, including high-performance microprocessors. Prior work has produced an historically-equivalent (HE) interpretation of Mr. E. F. Rent's 1960 memos for today's complex circuitry, an application to modern microprocessors [1-5], and topological constraints for electronic circuits [6]. This talk will examine combinational logic blocks which may exhibit different connectivity and will evaluate their topological properties.\\[4pt] References: [1] B. Landman and R.Russo, IEEE Trans. Comput., vol. C-20, pp. 1469-1478, 1971; [2] M. Lanzerotti, G. Fiorenza, R. Rand, IBM Jnl. Res. and Develop., vol. 49, pp. 777-803, 2005; [3] M. Lanzerotti, G. Fiorenza, and R. Rand, IEEE Trans. VLSI Syst., vol. 12, pp. 1330-1347, 2004; [4] D. Stroobandt, IEEE Solid-State Circuits Mag., vol. 2, issue 1, pp. 21-27, 2010; [5] M. Lanzerotti, G. Fiorenza, R. Rand, 2011 Proc. APS March Mtg, Dallas, TX, 2011; [6] G. Vernizzi, M. Lanzerotti, J. Kujawski, A. Weatherwax, ``Topological Constraints for E. F. Rent's Work on Microminiature Packaging and Circuitry,'' IBM Jnl. Res. and Dev., in press. [Preview Abstract] |
Session Z17: Complex Networks and their Applications II
Sponsoring Units: GSNPRoom: 402
Friday, March 7, 2014 11:15AM - 11:27AM |
Z17.00001: Task-Based Cohesive Evolution of Dynamic Brain Networks Elizabeth Davison Applications of graph theory to neuroscience have resulted in significant progress towards a mechanistic understanding of the brain. Functional network representation of the brain has linked efficient network structure to psychometric intelligence and altered configurations with disease. Dynamic graphs provide us with tools to further study integral properties of the brain; specifically, the mathematical convention of hyperedges has allowed us to study the brain's cross-linked structure. Hyperedges capture the changes in network structure by identifying groups of brain regions with correlation patterns that change cohesively through time. We performed a hyperedge analysis on functional MRI data from 86 subjects and explored the cohesive evolution properties of their functional brain networks as they performed a series of tasks. Our results establish the hypergraph as a useful measure in understanding functional brain dynamics over tasks and reveal characteristic differences in the co-evolution structure of task-specific networks. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z17.00002: Evaluation of the satellite power system survivability with the selfish algorithm. Svetlana V. Poroseva, Jacob Lowe, Bryan E. Kaiser Engineering networks (electric power, gas, water, transportation systems, etc.) are traditionally designed for normal operating conditions. Reliability analysis provides tools for describing the network's performance under such conditions. In the modern society, the likelihood of adverse conditions has dramatically increased along with the scale and cost of the network's failure. For some networks such as, for example, those in spacecrafts or in military applications, adverse conditions are normal. Due to differences in the mathematical formulation, reliability analysis is not applicable to networks under adverse conditions. Instead, survivability analysis should be applied. Survivability analysis due to the network's topology is the emerging discipline. In Poroseva et al, IEEE ESTS, 2005, a network survivability analysis based on a probabilistic approach was proposed for networks with heterogeneous nodes. Later, the selfish algorithm was developed to evaluate the topological survivability of networks. The approach is applicable to networks with multiple sources and sinks. The application of this algorithm to a satellite electric power subsystem will be demonstrated at the conference. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z17.00003: A Simulation of Cooperation and Competition in Insurgent Networks Michael Gabbay Insurgencies are often characterized by multiple groups who share a common foe in the national government but have independent organizations which may differ with respect to social identities, ideologies, strategies, and their use of violence. These groups may cooperate in various ways such as conducting joint attacks, pooling resources, and establishing formal alliances or mergers. However, they may also compete with each other over popular support, recruitment of fighters, funding, allies, and ultimately military dominance. A network coevolution model of insurgent factional dynamics is presented which accounts for factors driving cooperation and competition. The model is formulated as a system of coupled ODEs which evolves network ties between insurgent groups along with group policies concerning the targets of violence. Simulation results are presented showing sharp transitions in network structure as model parameters are varied. Connections are drawn between the model results and empirical data from the Iraqi insurgency. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z17.00004: Scientific impact: the story of your big hit Roberta Sinatra, Dashun Wang, Pierre Deville, Chaoming Song, Albert-Laszlo Barabasi A gradual increase in performance through learning and practice characterize most trades, from sport to music or engineering, and common sense suggests this to be true in science as well. This prompts us to ask: what are the precise patterns that lead to scientific excellence? Does performance indeed improve throughout a scientific career? Are there quantifiable signs of an impending scientific hit? Using citation-based measures as a proxy of impact, we show that (i) major discoveries are not preceded by works of increasing impact, nor are followed by work of higher impact, (ii) the precise time ranking of the highest impact work in a scientist's career is uniformly random, with the higher probability to have a major discovery in the middle of scientific careers being due only to changes in productivity, (iii) there is a strong correlation between the highest impact work and average impact of a scientist's work. These findings suggest that the impact of a paper is drawn randomly from an impact distribution that is unique for each scientist. We present a model which allows to reconstruct the individual impact distribution, making possible to create synthetic careers that exhibit the same properties of the real data and to define a ranking based on the overall impact of a scientist. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z17.00005: Global Analysis of Food and Nutrition: What the Human Body Wants Seunghyeon Kim, Mathias Foo, Jaeyun Sung, Yong-Su Jin, Pan-Jun Kim There is currently an abundance of quantitative information regarding foods we consume, such as their total nutrient composition and daily nutritional requirements. In this study, we systematically analyzed such large-scale data of foods to better understand how the composition of foods affects their overall nutritional value. Herein, we constructed two types of networks that reflect nutritional data from about 700 food products: 1) The Food-food Network, in which each edge connects a pair of foods having similar nutritional contents; and 2) the Nutrient-nutrient Network, which is based on co-occurrence patterns of different nutrients across foods. By adopting the insight we obtained from the topological properties of these networks, we present a novel measure to quantify the overall nutritional value of a food, which we call the Nutritional Fitness (NF). Some nutrients can hinder foods from having high NF, acting as ``nutritional bottlenecks.'' Interestingly, a food's NF is not only affected by individual nutrients, but also pairs of nutrients. To this effect, foods with very high NFs tend to have unique nutrient pairs not observed from the majority of foods. To summarize, our study provides insight into how NF and nutrients are intricately related. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z17.00006: Distinguishing fiction from non-fiction with complex networks David M. Larue, Lincoln D. Carr, Linnea K. Jones, Joe T. Stevanak Complex Network Measures are applied to networks constructed from texts in English to demonstrate an initial viability in textual analysis. Texts from novels and short stories obtained from Project Gutenberg and news stories obtained from NPR are selected. Unique word stems in a text are used as nodes in an associated unweighted undirected network, with edges connecting words occurring within a certain number of words somewhere in the text. Various combinations of complex network measures are computed for each text's network. Fisher's Linear Discriminant analysis is used to build a parameter optimizing the ability to separate the texts according to their genre. Success rates in the 70\% range for correctly distinguishing fiction from non-fiction were obtained using edges defined as within four words, using 400 word samples from 400 texts from each of the two genres with some combinations of measures such as the power-law exponents of degree distributions and clustering coefficients. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z17.00007: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z17.00008: Community Structure of a Bank-Firm Credit Network in Japan Hiroshi Iyetomi, Yuki Matsuura We study temporal change of community structure in a Japanese credit network formed by banks and listed firms through their financial relations over the last 30 years. The credit connectedness is regarded as a potenital source of systemic risk. Our network is a bipartite graph consisting of two species of nodes connected with bidirectional links. The direction of links is identified with that of risk flows and their weights are relative credit/loan with respect to the targets. In a partial credit network obtained only with the links pointing from firms toward banks, the city banks forms one major community in most of the time period to share risk when firms go wrong. On the other hand, a partial network only with the links from banks toward firms is decomposed into communities of similar size each of which has its own city bank, reflecting the main-bank system in Japan. Finally we take overlapping parts of the two community sets to find cores of the risk concentration in the credit network. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z17.00009: Complex dynamics and scale invariance of one-dimensional memristive networks Yuriy Pershin, Valeriy Slipko, Massimiliano Di Ventra There is currently a great interest in resistive systems with memory, also called as ``memristive systems'', and their potential applications [1,2]. In this talk we show that even the simplest one-dimensional network formed by the most common memristive elements with voltage threshold bears nontrivial physical properties [3]. In particular, by taking into account the single element variability we find (1) dynamical acceleration and slowing down of the total resistance in adiabatic processes, (2) dependence of the final state on the history of the input signal with same initial conditions, (3) existence of switching avalanches in memristive ladders, and (4) independence of the dynamics voltage threshold with respect to the number of memristive elements in the network (scale invariance). An important criterion for this scale invariance is the presence of memristive systems with very small threshold voltages in the ensemble. [1] Y. V. Pershin and M. Di Ventra, Advances in Physics 60, 145-227 (2011). [2] M. Di Ventra and Y. V. Pershin, Nature Physics 9, 200 (2013). [3] Y. V. Pershin, V. A. Slipko, and M. Di Ventra, Phys. Rev. E 87, 022116 (2013). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z17.00010: Scaling of Various Dominating Sets in Scale-Free and Empirical Networks N. Derzsy, F. Molnar Jr., E. Czabarka, L. Szekely, B. Szymanski, G. Korniss We develop ensemble-based graph theoretical methods to approximate the size of minimum dominating sets (MDS) in scale-free networks. The MDS is found by a sequential greedy algorithm and the scale-free network samples are generated using the configuration model. Depending on the considered maximum degree cutoff, we analyze two subtypes of scale-free networks. We study the upper bound of random dominating sets and find that the numerical bound is lower than the analytical one. We propose a degree-based probabilistic selection that for a limiting case provides the smallest probabilistic dominating set. The method relies on a degree cutoff parameter and nodes having degrees above this cutoff are added to the dominating set (CDS). Our results reveal that with an optimal degree cutoff the CDS size is very close to the MDS. We provide analytical estimates for the uncorrelated version of scale-free networks to support our conjecture. We propose an efficient method to control the assortativity (measured by Spearman's rho) in networks. Applying this technique for the construction of our scale-free network ensembles, we provide a comprehensive analysis on the behavior of the probabilistic in comparison with the greedily selected dominating sets with respect to multiple network features. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z17.00011: Geometrical structure of Neural Networks: Geodesics, Jeffrey's Prior and Hyper-ribbons Lorien Hayden, Alex Alemi, James Sethna Neural networks are learning algorithms which are employed in a host of Machine Learning problems including speech recognition, object classification and data mining. In practice, neural networks learn a low dimensional representation of high dimensional data and define a model manifold which is an embedding of this low dimensional structure in the higher dimensional space. In this work, we explore the geometrical structure of a neural network model manifold. A Stacked Denoising Autoencoder and a Deep Belief Network are trained on handwritten digits from the MNIST database. Construction of geodesics along the surface and of slices taken from the high dimensional manifolds reveal a hierarchy of widths corresponding to a hyper-ribbon structure. This property indicates that neural networks fall into the class of sloppy models, in which certain parameter combinations dominate the behavior. Employing this information could prove valuable in designing both neural network architectures and training algorithms. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z17.00012: Modeling criticality in networks of neurons Shane Squires, Andrew Pomerance, Edward Ott, Michelle Girvan A recent series of experiments have suggested that networks of biological neurons operate near a critical point, separating two phases in which firing activity either decays or grows exponentially. In this talk, we propose and analyze a simple model of this behavior. Neurons may be connected via arbitrary networks of activating and inhibiting links, and they fire when their membrane voltage exceeds a threshold value. The main advantage of our model is that we can analyze the effects of network structure on the criticality of the system, while preserving realistic features of neurons, such as threshold-based firing behavior. At the critical point, we reproduce the empirically measured critical exponents for firing avalanches, and we discuss how changes to the network affect the tuning parameter for the phase transition. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z17.00013: Sampling networks with prescribed degree correlations Charo Del Genio, Kevin Bassler, P\'eter Erd\H{o}s, Istv\'an Miklos, Zolt\'an Toroczkai A feature of a network known to affect its structural and dynamical properties is the presence of correlations amongst the node degrees. Degree correlations are a measure of how much the connectivity of a node influences the connectivity of its neighbours, and they are fundamental in the study of processes such as the spreading of information or epidemics, the cascading failures of damaged systems and the evolution of social relations. We introduce a method, based on novel mathematical results, that allows the exact sampling of networks where the number of connections between nodes of any given connectivity is specified. Our algorithm provides a weight associated to each sample, thereby allowing network observables to be measured according to any desired distribution, and it is guaranteed to always terminate successfully in polynomial time. Thus, our new approach provides a preferred tool for scientists to model complex systems of current relevance, and enables researchers to precisely study correlated networks with broad societal importance. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z17.00014: Percolations on hypergraphs Bruno Coelho Coutinho, Y.-Y. Liu, H.-J. Zhou We analytically study the emergence of the giant component, two-core and core in uniform and non-uniform hypergraphs. We show that depending on the leaf definition and in the hypergraph rank distribution the 2-core can emerge as a hybrid phase transition our as a continuous phase transition and we provide a analytical condition for the existence of the hybrid phase transition. We found that in hyperpgrahs there are two meaningful versions of the greedy leaf removal (GLR), associated with two different leaves and intimately related with the vertex and edge cover problem. We study the emergence of the core for both cases, and we show that both of the cores emerge as a continuous phase transition for the considered distribution. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z17.00015: Quantum Phase Transitions: A Network Approach David L. Vargas, David M. Larue, Lincoln D. Carr Understanding the network structure of complex systems has opened up new avenues of research in sociology, biology, technology, and physics. In this talk we present evidence that complex network measures are able to identify the phases in two well known models. We distinguish the ferromagnetic and paramagnetic phases of the transverse Ising Hamiltonian. We also identify the Mott-insulator to superfluid transition of the Bose-Hubbard Hamiltonian. The network approach to the analysis of quantum phase transitions provides us with a new set of tools to explore the many body physics of quantum phase transitions. [Preview Abstract] |
Session Z18: Non-spherical Colloids and Complex Fluids
Chair: Ye Xu, University of PennsylvaniaRoom: 403
Friday, March 7, 2014 11:15AM - 11:27AM |
Z18.00001: Colloidal samaras Daniela Kraft, Raphael Wittkowski, Oliver Dlugosch, Hartmut L\"owen, David Pine We present a combined experimental and computational study of the sedimentation of colloidal particles composed of materials of different densities. We follow the sedimentation experimentally by confocal microscopy and compare the observed trajectories to simulation results. We find that particular mass distributions and shapes lead to a helical motion during sedimentation, similar to that of samaras. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z18.00002: Does mass play a role in partition functions even in low Reynolds number systems? Rebecca W. Perry, Nica Franklin, Vinothan N. Manoharan Classical statistical mechanics predicts that heavy components of a reconfigurable object will preferentially occupy positions at the edges of the object while lighter components will most often reside near the object's center of mass. This predicted influence of mass comes in through the rotational component of the partition function, which favors configurations with larger moments of inertia. It is tempting to apply these findings of statistical mechanics directly to colloidal systems, but is this appropriate when colloidal systems are immersed in liquid rather than surrounded by vaccuum? Does mass have a place in the partition function of colloidal clusters at low Reynolds numbers where we are accustomed to ignoring inertia? Here, we measure how silica microspheres distribute themselves when mixed with identically-sized polystyrene microspheres to form weakly-bound clusters of up to ten spheres. Using an array of microwells, we observe thousands of two-dimensional clusters to answer these fundamental questions. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z18.00003: Dynamics of non-spherical colloidal particles near and at oil-water interfaces Anna Wang, Thomas G. Dimiduk, Jerome Fung, Kundan Chaudhary, Jennifer A. Lewis, Sepideh Razavi, Ilona Kretzschmar, Vinothan N. Manoharan Whereas much is known about how spherical colloidal particles interact with and at oil-water interfaces, not much is known about their non-spherical counterparts. The rotation of non-spherically symmetric particles adds extra degrees of freedom to how such particles interact with each other and the interface, so to study their three-dimensional dynamics we must first be able to image the rotation which has so far only been possible in viscous fluids or for particles with large aspect ratios. Here we track both the three-dimensional translation and the rotation of non-spherical colloidal particles at high speeds using the discrete dipole approximation in conjunction with digital holographic microscopy. We study the dynamics of such particles at an oil-water interface to determine interactions and dynamics prior to or after attachment. We aim to connect these measurements to the formation and stability of Pickering emulsions. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z18.00004: Interaction of a colloidal sphere near a flat boundary Bhaskar Jyoti Krishnatreya, David G. Grier A colloidal sphere's diffusion is hindered near a surface due to hydrodynamic interactions . We study the hindered diffusion of a colloidal sphere near a glass surface using Digital Holographic Microscopy (DHM). Analysis of in-line holographic images of a diffusing colloidal sphere provides its three dimensional positions with nanometer resolution. We propose a general technique to determine the forces acting on the colloidal sphere near a flat boundary using Kernel Density Estimates (KDE), as a function of distance from the boundary. The results will help in understanding interactions between micron-sized colloidal particles near a boundary. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z18.00005: Brownian Motion of Boomerang Colloidal Particles Qi-Huo Wei, Andrew KOnya, Feng Wang, Jonathan V. Selinger, Kai Sun, Ayan Chakrabarty We present experimental and theoretical studies on the Brownian motion of boomerang colloidal particles confined between two glass plates. Our experimental observations show that the mean displacements are biased towards the center of hydrodynamic stress (CoH), and that the mean-square displacements exhibit a crossover from short-time faster to long-time slower diffusion with the short-time diffusion coefficients dependent on the points used for tracking. A model based on Langevin theory elucidates that these behaviors are ascribed to the superposition of two diffusive modes: the ellipsoidal motion of the CoH and the rotational motion of the tracking point with respect to the CoH. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z18.00006: Brownian Motion of Asymmetric Boomerang Colloidal Particles Ayan Chakrabarty, Andrew Konya, Feng Wang, Jonathan Selinger, Kai Sun, Qi-Huo Wei We used video microscopy and single particle tracking to study the diffusion and local behaviors of asymmetric boomerang particles in a quasi-two dimensional geometry. The motion is biased towards the center of hydrodynamic stress (CoH) and the mean square displacements of the particles are linear at short and long times with different diffusion coefficients and in the crossover regime it is sub-diffusive. Our model based on Langevin theory shows that these behaviors arise from the non-coincidence of the CoH with the center of the body. Since asymmetric boomerangs represent a class of rigid bodies of more generals shape, therefore our findings are generic and true for any non-skewed particle in two dimensions. Both experimental and theoretical results will be discussed. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z18.00007: A theory for depletion-induced colloidal membranes Louis Kang, Tom C. Lubensky Depletion-induced formation of colloidal membranes has been recently observed in suspensions of hard rods [E. Barry and Z. Dogic, Proc. Natl. Acad. Sci. U.S.A. {\bf 107}, 10348 (2010); T. Gibaud {\it et al.}, Nature {\bf 481}, 348 (2012)]. These membranes exhibit a variety of rich behaviors that must ultimately be driven by entropy alone. We propose an entropic model that can capture certain features of these membranes, including their curved edge shape and the presence of twist even with achiral rods. We calculate phenomenological parameters, such as the Frank twist constant and the Helfrich bending modulus, from physical quantities. Finally, we describe novel behaviors predicted by our model. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z18.00008: Improving the accuracy of DLVO theory for dense systems of macroions Niels Boon, Guillermo Ivan Guerrero, Rene van Roij, Monica Olvera de la Cruz The widely used DLVO pair potential was originally derived for a pair of interacting macroions in a dilute colloidal suspension. Here, we present a modified effective pair potential that is also accurate for non-dilute systems. Our new theory significantly deviates from the classical DLVO theory in dense systems. In essence, we propose a modification of Alexander's prescription for the charge renormalization procedure that is used to derive effective charges from highly charged macroions. By comparing pair correlation functions and pressures from computationally expensive primitive-model simulations, our new method demonstrates an improved accuracy w.r.t. Alexander's DLVO-based approach. We show that our method is also suitable to describe salt-free suspensions. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z18.00009: A model for restricted diffusion in complex fluids John de Bruyn, Jonathan Wylie We use a model originally due to Tanner [1] to study the diffusion of tracer particles in complex fluids both analytically and through Monte-Carlo simulations. The model consists of regions through which the particles diffuse freely, separated by membranes with a specified low permeability. The mean squared displacement of the particles calculated from the model agrees well with experimental data on the diffusion of particles in a concentrated colloidal suspension [2] when the membrane permeability is used as an adjustable parameter. Data on a micro-phase-separated polymer system [3] can be well modeled by considering two populations of particles constrained by membranes with different permeabilites. \\[4pt] [1] J. E. Tanner, J. Chem. Phys. 69, 1748 (1978).\\[0pt] [2] E. R. Weeks and D. A. Weitz, Chem. Phys. 284, 361 (2002).\\[0pt] [3] N. Yang, J. L. Hutter, and J. R. de Bruyn, J. Rheol. 56, 797 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z18.00010: Theory of the dynamics of evaporation-driven colloidal patterning C. Nadir Kaplan, Ning Wu, Shreyas Mandre, Joanna Aizenberg, L. Mahadevan In the suspensions of colloidal particles in a volatile liquid film, deposits of the solute form near the contact line due to the flow generated by evaporation. An enticingly simple and experimentally realizable model system of this mechanism is the drying of a spilled drop of coffee on the countertop. Similarly, patterns of periodic bands or continuous solid films are commonly observed on a substrate suspended vertically in a container of the colloidal solution. In order to characterize these patterns, we develop a multiphase model that couples both the liquid and solid flows, local variation of the particle concentration, the propagation dynamics of the solid front, and the liquid-air interface deformation. For vertical liquid films, we further determine the nature of the filming-banding transition and the phase boundary in terms of the volume fraction of the colloids. The results of our theory are in good agreement with direct observations of these patterns. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z18.00011: Towards a computational modeling of structure formation in colloidal drying Alexander Wagner We present lattice Boltzmann models at different scales for the simulation of colloidal drying in the presence of polymers and structure formation in resulting phase-separation fronts. When a drop of colloid polymer mixture is exposed to an environment in which the solvent in which these particles are suspended evaporates an accumulation of non-volotile material at the rim of the drop is observed (coffee ring effect). When the solvent concentration is reduced bejond a certain threshold, the colloid polymer mixture undergoes phase separation. The structures formed by this phase-separation is observed to depend on the processing conditions. In this presentation we will briefly present the experimental observations and our numerical approach to address the observed phenomena. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z18.00012: Rediscovering Red: Full-Spectrum Structural Color in Colloidal Glasses Sofia Magkiriadou, Jin-Gyu Park, Young-Seok Kim, Gi-Ra Yi, Vinothan N. Manoharan We use colloidal glasses to develop pigments with structural color: color that arises from interference rather than absorption. This pigmentation mechanism is common in blue birds, whose feather barbs often contain glassy microstructures. When a glass is illuminated, the spatial correlations between neighboring particles can give rise to constructive interference for a small range of wavelengths. Unlike the colors arising from Bragg diffraction in crystals, the colors of these ``photonic glasses'' are independent of angle due to the disordered, isotropic structure. However, there are no known examples of photonic glasses with pure structural red color, either in nature or in the lab. We present both experimental evidence and a model showing that the absence of red is due to the wavelength-dependence of the single-particle scattering cross-section. We show that this problem can be solved in ``inverse glasses,'' namely glasses composed of particles with refractive index lower than that of their medium. Although these systems are similar to those in birds, no known species uses this mechanism to create red. We use inverse glasses to make full-spectrum, angle-independent structural colors. This will enable the use of colloidal glasses as a new type of long-lasting, non-bleaching pigment. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z18.00013: Gravitational Drainage of Superspreader Films Stabilized by Disjoining Pressure Soumyadip Sett, Rakesh Sahu, Suman Sinha-Ray, Alexander Yarin Gravitational drainage from plane vertical films of two superspreaders SILWET L-77 and BREAK-THRU S278 and their respective ``cousin'' non-superspreaders SILWET L-7607 and BREAK-THRU S233 is studied experimentally and theoretically. The non-superspreader films showed ordered interferometric color bands, similar to those of ordinary surfactants and the film thickness decreased linearly in time. Their counterpart superspreaders showed complicated dynamic turbulent-like interferometric patterns and had an order of magnitude longer life time before bursting compared to that of the ``cousin'' non-superspreaders. The stabilization of the superspreader films and the nonlinear decrease of the film thickness with time are attributed to significant disjoining pressure associated with the van der Waals repulsion of the fluffy surfaces of the film formed by long superspreader bilayers hanging from the free surfaces. The non-superspreaders do not possess any significant disjoining pressure even in the film with thicknesses in the 30-50 nm range. The results show that gravitational drainage of vertical films is a useful simple tool for measuring disjoining pressure. [Preview Abstract] |
Session Z19: Supercooled Polymer Liquids and Glasses
Sponsoring Units: DPOLYChair: Zahra Fakhraai, University of Pennsylvania
Room: 404
Friday, March 7, 2014 11:15AM - 11:27AM |
Z19.00001: Nano-composites obtained by phase separation of polymer blends close to and below Tg Gregoire Julien We propose a model for describing the dynamics in polymer blends close to and below Tg. The model is solved on a 2D lattice corresponding to spatial scales a few 100 nm and a resolution corresponding to the scale of dynamical heterogeneities. In the course of spinodal decomposition at low temperatures we observe slow structures building, which coexist with faster ones. Simultaneously, the distribution of relaxation times evolves toward longer times, corresponding to an ageing process. Domains are found to grow like the logarithm of the time. By tuning the physical parameters of the systems (polymer mass, polymer Tg's and volume fraction) it should help making nano-composites with controlled morphologies on a scale typically of order 10 nm. We study also the melting dynamics of these systems, after the temperature is increased again in the totally miscible range. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z19.00002: Effect of Counteranion on Caged-like Dynamics of 1-alkyl-3-methylimidazolium-based Ionic Liquids Jenny Kim, Cheol Jeong, Madhu Tyagi, Christopher Soles Understanding physicochemical properties of ionic liquids (ILs) is essential to realize task-specific ILs. To better understand the structural and dynamic heterogeneity in ILs, we conducted quasielastic neutron scattering (QENS) measurements that cover time ranges from picosecond to nanosecond. Series of 1-alkyl-3-methylimidazolium-based ILs is chosen to explore the relationship between local dynamics and long-range translational dynamics. Two distinct dynamical processes have been examined: caged-like dynamics and jump-diffusion processes. Size and shape of a dynamic cage can be obtained by fitting elastic incoherent structure factor (EISF) with spherical and cylindrical Bessel functions. The cage geometry turns out to be strongly dependent on the counteranion. Residence time of ions or molecules attained from jump-diffusion model increased by several factors for the ILs with smaller and more isotropic cage which will lead to slower diffusion. Mean square displacements coupled with viscosity and conductivity by generalized localization model also confirms the relevance of caging to the ion transport and structural relaxation of ILs. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z19.00003: Dynamical Heterogeneities in Glasses from Fluctuating Mobility Generation and Transport: Two Equilibration Mechanisms in Glasses Apiwat Wisitsorasak, Peter G. Wolynes In the random first order transition theory, fluctuating mobility generation and transport are explored from an extended mode coupling theory of the glass transition that includes activated events. We numerically solve the continuum equations of mobility and temperature fields and find that the fluctuations which account for spatiotemporal structure in aging and rejuvenating glasses lead for dynamical heterogeneity in glass. Non-Gaussian distribution of free energy, stretch exponent $\beta$, and growing characteristic length are presented along with the four-point correlation function. Our results demonstrate that two equilibration mechanisms that have been observed in aged polymer glasses are the results of the heterogeneity and out-of-equilibrium behavior of glasses below the glass transition temperature. Numerical results of distribution of relaxation time agree with experimental observations. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z19.00004: Plastic Flow of Polymer Chains below $T_{\mathrm{g}}$ Induced by Jamming Transition Chao Teng, Gi Xue Polymer chains begin to flow when they are heated above $T_{\mathrm{g}}$. Other glassy systems, such as colloidal suspensions and granular materials, begin to flow when subjected to sufficiently large stresses. The equivalence of these two routes to flow is a basic tenet of jamming theory. However, a full understanding of jamming transition for polymer chains remains elusive. In this work, we adjust the polymer chain packing density by spry-drying and some other methods, and then apply shear stress at temperature far below its $T_{\mathrm{g}}$. The resulting pellet shows very similar features as the hot processed or solution casting samples, which strongly indicates that the plastic flow of polymer chains ever happened below $T_{\mathrm{g}}$. We found that the packing density and shear stress play important roles during the plastic flow process at low temperature, which is according with the jamming theory. This kind of plastic flow at low temperature shows its advantages when processing materials with bioactivity, which will be deactivated at high temperature. Furthermore, these findings also suggested an approach to understand the high mobility of surface layer of polymer thin film and nanoparticles. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z19.00005: Dynamical heterogeneity and structural relaxation in periodically deformed polymer glasses Nikolai Priezjev The dynamics of structural relaxation in a model polymer glass subject to spatially-homogeneous, time-periodic shear deformation is investigated using molecular dynamics simulations. We consider a coarse-grained bead-spring model of short polymer chains below the glass transition temperature. It is found that at small strain amplitudes, the segmental dynamics is nearly reversible over about 10,000 cycles, while at strain amplitudes above a few percent, polymer chains become fully relaxed after a hundred cycles. At the critical strain amplitude, the transition from slow to fast relaxation dynamics is associated with the largest number of dynamically correlated monomers as indicated by the peak value of the dynamical susceptibility. The analysis of individual monomer trajectories indicated that mobile monomers tend to assist their neighbors to become mobile and aggregate into relatively compact transient clusters. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z19.00006: Dramatic alteration of Tg of polystyrene confined in cylindrical nanopores Gi Xue, Chao Teng, Jie Xu, Linling Li Vitrification of polystyrene melt infiltrated in AAO template was greatly altered by geometrical confinement. DSC detected two distinguish Tgs for PS slowly cooled from the melt in nanopores. One was at a lower temperature than the bulk Tg and the other was at a higher temperature. The deviation between the two Tgs could reach as big as 60 $^{\circ}$C and increased with decreasing pore size. Surprisingly, Tg for the PS nanorodes recovered back to the bulk value when the template was removed, indicating the importance of the interfacial interaction. The major factor which induced such a strong confinement is the cooling rate. The coefficient of thermal expansion (CTE) of AAO template is about one order lower than that for PS. The mismatch of CTEs causes a strain induced stress during cooling: the PS chains tend to shrink and to de-wet from the walls; meanwhile the interfacial interaction tends to hold the chains back. The chains are subjected to a high residual stress by a fast cooling and would peel off from the wall. When the cooling rate is sufficiently slow, and the stress was relaxed during cooling. Then the chains were still stick on the wall, resulting in a strong interfacial constraints for chains. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z19.00007: Structural and Dynamical Heterogeneities in Thin FIlms of a Generic Glass-forming Liquid Amir Haji-Akbari, Pablo G. Debenedetti Recent discovery of stable glasses by Ediger and coworkers [1] has spurred an interest in structural and dynamical properties of atomic and molecular thin films. Here, we use molecular dynamics simulation to study thin films of a model glass-forming liquid, the Kob-Andersen binary Lennard Jones system [2], and compute profiles of structural properties such as densities, potential energies, stresses and lateral radial distribution functions, as well as dynamical properties such as relaxation times across the film. We observe the liquid to be stratified in the vicinity of the wall, but this stratification is not always accompanied by long-range order. We also observe two distinct dynamical regimes close to the liquid/solid interface. For weakly-interacting walls, a highly mobile region emerges with relaxation times smaller than the bulk, while for strongly-interacting walls, relaxation times can be several orders of magnitude larger in the same region than in the bulk. We are able to establish correlations between density modulations and normal stress modulations, and between relaxation time modulations and lateral stress modulations in the disordered regions of the film. \\[4pt] [1] Swallen SF, et al., Science 315: 353 (2007).\\[0pt] [2] Kob W, Andersen HC, Phys. Rev. E 51: 4626 (1995). [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z19.00008: Study of physical vapor deposited glasses of tris-naphthyl benzene based organic molecules Yue Zhang, Tianyi Liu, Ethan Glor, Guoyu Yang, Yi-Chih Lin, Zahra Fakhraai Stable glasses can be prepared by physical vapor deposition method (PVD), and these stable glasses will show greatly different properties compared to ordinary glasses prepared upon cooling a liquid, including higher density, higher thermal stability, increased charge transport ability and so on. Different organic molecule structures are also responsible for different glass structures and related glass properties. These properties strongly depend on the deposition temperature. We work with small organic molecules tris-naphthyl benzene (TNB) and molecules based on TNB that have slightly different substituents. These molecules will have different molecular weight and chemical structure such as $\pi $ stacking, which will have an influence on the structure of stable glasses, their glass transition temperature, Tg and density. In our study, we can vary both the physical and chemical properties of these organic molecules in a systematical way to have a better understanding of the relationship between molecule structure, glass structure and related properties. These studies allow us to probe whether the stability of these glasses correlate with their glass transition temperature, Tg or their chemical structure. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z19.00009: Glasses and Liquids Low on the Energy Landscape Prepared by Physical Vapor Deposition Shakeel Dalal, Zahra Fakhraai, Mark Ediger The lower portions of the potential energy landscape for glass-forming materials such as polymers and small molecules were historically inaccessible by experiments. Physical vapor deposition is uniquely able to prepare materials in this portion of the energy landscape, with the properties of the deposited material primarily modulated by the substrate temperature. Here we report on high-throughput experiments which utilize a temperature gradient stage to enable rapid screening of vapor-deposited organic glasses. Using ellipsometry, we characterize a 100 K range of substrate temperatures in a single experiment, allowing us to rapidly determine the density, kinetic stability, fictive temperature and molecular orientation of these glasses. Their properties fall into three temperature regimes. At substrate temperatures as low as 0.97T$_g$, we prepare materials which are equivalent to the supercooled liquid produced by cooling the melt. Below 0.9T$_g$ (1.16T$_K$) the properties of materials are kinetically controlled and highly tunable. At intermediate substrate temperatures we are able to produce materials whose bulk properties match those expected for the equilibrium supercooled liquid, down to 1.16T$_K$, but are structurally anisotropic. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z19.00010: Resolving the Puzzle of Two Glass Transitions in Miscible Polymer Blends Jacek Dudowicz, Jack Douglas, Karl Freed The existence of two glass transitions has widely been observed in experiments for miscible polymer blends. Qualitative explanations postulate models of local concentration enhancements (depletions) of component 1 (2) in the neighborhood of a chain of species 1 (2). The occurrence of two glass transition temperatures is analyzed and explained by using a merger of three statistical mechanical theories: the generalized entropy theory for glass-formation in binary homopolymer blends, the lattice cluster theory for the thermodynamics of binary polymer blends, and Kirkwood-Buff theory for concentration fluctuations in binary mixtures. Specific computations of glass transition temperatures are provided for blends of semi-flexible linear chains with varying stiffness. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z19.00011: Unified Theory of Activated Relaxation in Cold Liquids over 14 Decades in Time Kenneth Schweizer, Stephen Mirigian We formulate a predictive theory at the level of forces of activated relaxation in thermal liquids that covers in a unified manner the apparent Arrhenius, crossover and deeply supercooled regimes (J.Phys.Chem.Lett.4,3648(2013)). The alpha relaxation event involves coupled cage-scale hopping and a long range cooperative elastic distortion of the surrounding liquid, which results in two inter-related, but distinct, barriers. The strongly temperature and density dependent collective barrier is associated with a growing length scale, the shear modulus and density fluctuations. Thermal liquids are mapped to an effective hard sphere fluid based on matching long wavelength density fluctuation amplitudes. The theory is devoid of fit parameters, has no divergences at finite temperature nor below jamming, and captures the key features of the alpha relaxation time in molecular liquids from picoseconds to hundreds of seconds. The approach is extended to polymer liquids based on the Kuhn length as the key variable. The influence of chain length and backbone stiffness on the glass transition temperature and fragility have been studied where degree of polymerization enters via corrections to asymptotic conformational statistics. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z19.00012: Effects of hydrophobic aggregation on the charge transport mechanism of quaternary ammonium ionic liquids Philip Griffin, Adam Holt, Yangyang Wang, Vladimir Novikov, Joshua Sangoro, Alexei Sokolov Aprotic quaternary ammonium ionic liquids (ILs) are an important class of ILs due to their large electrochemical window and hydrophobicity. However, many of these ILs suffer from relatively low conductivity at room temperature which limits their use in electrochemical applications. In order to understand the nature of this low conductivity and its relation to the chemical structure of the alkyl ammonium cation, we have measured the charge transport properties and structural dynamics of the room temperature ionic liquid methyltrioctylammonium bistriflimide [m3oa][ntf2] over a broad temperature range using dielectric spectroscopy, dynamic light scattering, rheology, and pulsed field gradient nuclear magnetic resonance. We demonstrate that the low values of dc conductivity are due to the combined effects of significantly reduced ion mobility as well as reduced free ion concentration relative to other types of ILs. Secondly we find evidence for a mesoscopic scale structural relaxation process that we attribute to the reorientational motion of nanometer sized alkyl nanodomains. These two findings indicate that hydrophobic aggregation plays an important role in the charge transport mechanism of aprotic ammonium ionic liquids with long aliphatic side chains. [Preview Abstract] |
Session Z20: Focus Session: Organic Electronics and Photonics - Photonic and Electronic Properties
Sponsoring Units: DMP DPOLYChair: Bryan Boudouris, Purdue University
Room: 405
Friday, March 7, 2014 11:15AM - 11:27AM |
Z20.00001: Properties of guided modes in plasmonic aluminum quinoline waveguides Niranjala Wickremasinghe, Jonathan Thompson, Xiaosheng Wang, Heidrun Schmitzer, Hans Peter Wagner We investigate the mode properties of aluminum-quinoline (Alq$_{3})$ waveguides with embedded thin (approximately 10 nm thick) Mg$_{0.9}$Ag$_{0.1}$ metal layers at a wavelength of 633 nm using the m-line technique. The plasmonic waveguides were fabricated on a Pyrex substrate by organic molecular beam deposition. Our experiments show that TM$_{0}$ modes in an Alq$_{3}$ waveguides with a single centered metal layer and TM$_{0}$, TM$_{1}$ and TM$_{2}$ modes in a waveguide with three metal layers have higher effective refractive indices as compared to a pure Alq$_{3}$ reference waveguide. These modes are attributed to plasmon-like modes in agreement with model calculations considering a complex dielectric constant for the metal layer. TM modes which possess a node at the location of the metal layer essentially behave like dielectric modes. TE modes are more affected by the embedded metal layer(s). The number of TE modes is reduced and the mode coupling angles are significantly shifted. Only one TE mode is observed in the waveguide containing three metal layers which is in agreement with model calculations. The results show that strategically placed metal layers can be used to selectively excite plasmonic and dielectric TM modes and to shift and suppress TE modes. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z20.00002: Bragg Reflectors Based on Block Copolymer/ Polyhedral Oligomeric Silsesquioxanes (POSS) and TiO$_{2}$ Hybrid Nanocomposites Cheng Li, Nicholas Colella, James Watkins Maleamic acid functionalized polyhedral oligomeric silsesquioxanes (POSS) can interact with the poly (ethylene oxide) (PEO) block in Pluronics F108 block copolymer via hydrogen bonding to form well-ordered block copolymer nanocomposites. In this study, the block copolymer composites are spin coated into thin films and maleamic acid groups are thermal crosslinked to stabilize the nanocomposite structure. High temperature calcination of the stabilized nanocomposite yields a robust mesoporous silica thin film. By adjusting the loading of POSS into the block copolymer prior to calcination, the refractive index (RI) of mesoporous silica films can be tuned between 1.13 and 1.18. We show these low RI films can be sequentially layered with hybrid TiO$_{2}$ nanocomposite films that exhibit a RI of approximately 2.0 to yield efficient Bragg reflectors. The TiO2 films are prepared by the calcinations of polymer/anatase TiO2 nanoparticle composites with NP loadings as high as 90wt{\%}. Due to the porosity existing in each layer, the wavelength of the reflected light is sensitive to the adsorption of solvent vapors such as toluene, isopropanol, and tetrahydrofuran, or analytes, which suggest applications in sensors. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z20.00003: Gain enhancement in photorefractive polymers Carl Liebig, Steven Buller, Partha Banerjee, Sergey Basun, Pierre Blanche, Jayan Thomas, Corey Christenson, Nasser Peyghambarian, Dean Evans Photorefractive (PR) polymer materials have shown that they can be successfully used in display applications due to a diffraction efficiency that is close to unity [1]. The polymers rely on the cooperation between several components in order to generate the charge carriers, space-charge field, and the refractive index change as required for both diffractive and beam-coupling applications. The multi-component approach has several unforeseen consequences, such as multiple PR gratings (hole and electron) and sub-optimal phase shifts which decrease the potential (PR) gain [2]. We show that by applying electric fields close to the breakdown potential to PR polymers, the decreased beam coupling and diffraction efficiency can be overcome by reducing the grating competition (hole vs. electron) leading to an enhanced PR gain/efficiency [3]. \\[4pt] [1] P. A. Blanche, et al., Nature \textbf{468}, 80-83 (2010).\\[0pt] [2] P. P. Banerjee, et al., J. Appl. Phys. \textbf{111}, 013108 (2012).\\[0pt] [3] C. M. Liebig, et al., Opt. Exp. (In press 2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z20.00004: Large Area Printing of 3D Photonic Crystals James J. Watkins, Michael R. Beaulieu, Nicholas R. Hendricks, Rohit Kothari We have developed a readily scalable print, lift, and stack approach for producing large area, 3D photonic crystal (PC) structures. UV-assisted nanoimprint lithography (UV-NIL) was used to pattern grating structures comprised of highly filled nanoparticle polymer composite resists with tune-able refractive indices (RI). The gratings were robust and upon release from a support substrate were oriented and stacked to yield 3D PCs. The RI of the composite resists was tuned between 1.58 and 1.92 at 800 nm while maintaining excellent optical transparency. The grating structure dimensions, line width, depth, and pitch, were easily varied by simply changing the imprint mold. For example, a 6 layer log-pile stack was prepared using a composite resist a RI of 1.72 yielding 72 {\%} reflection at 900 nm. The process is scalable for roll-to-roll (R2R) production. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z20.00005: Tunable photonic multilayer sensors from photo-crosslinkable polymers Maria Chiappelli, Ryan Hayward The fabrication of tunable photonic multilayer sensors from stimuli-responsive, photo-crosslinkable polymers will be described. Benzophenone is covalently incorporated as a pendent photo-crosslinker, allowing for facile preparation of multilayer films by sequential spin-coating and crosslinking processes. Copolymer chemistries and layer thicknesses are selected to provide robust multilayer sensors which can show color changes across nearly the full visible spectrum due to the specific stimulus-responsive nature of the hydrated film stack. We will describe how this approach is extended to alternative sensor designs by tailoring the thickness and chemistry of each layer independently, allowing for the preparation of sensors which depend not only on the shift in wavelength of a reflectance peak, but also on the transition between Bragg mirrors and filters. Device design is optimized by photo-patterning sensor arrays on a single substrate, providing more efficient fabrication time as well as multi-functional sensors. Finally, radiation-sensitive multilayers, designed by choosing polymers which will preferentially degrade or crosslink under ionizing radiation, will also be described. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z20.00006: Positional Isomer Effects on Photomechanical Response of Azobenzene Functionalized Polyimides Jeong Jae Wie, David Wang, Kyung Min Lee, Loon-Seng Tan, Timothy White Azobenzene-functionalized polyimides (Azo-PIs) are materials capable of a large magnitude photomechanical responses. Recent work from our group has reported that photomechanical effects in these materials are strongly influenced by free volume and crystallinity. In this presentation, we will discuss a recently completed study in which a series of amorphous polyimides were prepared. The connectivity of the backbone of the imide units was intentionally varied to introduce rotational freedom, which is apparent as a $\beta $-transition. Comparatively, materials exhibiting a $\beta $-transition exhibit larger magnitude photomechanical effects. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z20.00007: Enhanced dielectric properties of electrically poled poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) multilayer films via interfacial polarization Jung-Kai Tseng, Matthew Mackey, Zheng Zhou, Joel Carr, Donald E. Schuele, Eric Baer, Lei Zhu Electrically poled poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) multilayer films can be considered as a polymer electret, which stores quasi-permanent charges (i.e., ions) at PVDF/PC interfaces. In this study, the corresponding dielectric properties of electrically poled PVDF/PC multilayer films are investigated experimentally. First, the bipolar hysteresis loop becomes narrower for the poled PVDF/PC multilayer films upon increasing the poling time, because the impurity ions in PVDF are locked at the PVDF/PC interfaces. Second, asymmetric DC conductivity in poled PVDF/PC multilayer films is observed because of the pre-existing electric field in the electret layers. When the pre-existing field is in the same direction of the applied external field, enhanced DC conductivity is observed in the leakage current measurement. In contrast, if the pre-existing field is opposite to the applied external field, decreased DC conductivity is seen. More experimental evidence of polarized charge at the PVDF/PC interfaces in poled PVDF/PC multilayer films is also manifested by thermally stimulated depolarization current (TSDC) experiments. [Preview Abstract] |
Session Z21: Polymer Composites: Nanocomposites
Sponsoring Units: DPOLYChair: Laura Clarke, North Carolina State University
Room: 406
Friday, March 7, 2014 11:15AM - 11:27AM |
Z21.00001: Molecular Simulation and Microstructure Characterization of Poly(p-phenylene/m-phenylene) Copolymers Robert Bubeck, Steven Keinath Molecular simulation and characterization of the molecular structure and microstructure of poly(p-phenylene/m-phenylene) copolymers were carried out. Tensile modulus, yield stress, and entanglement molecular weight were modeled as amorphous polymers as a function of m-phenylene content. Significant biphasic character, however, was observed for two copolymers in the melt (environs of 300$^{\circ}$C) using variable temperature synchrotron-based WAXS. Precise experimental determinations of entanglement molecular weights were frustrated by the occurrence of significant amounts of nematic mesophasic order in the rubbery and melt regimes of two commercial poly(p-phenylene/m-phenylene) examples. Nonetheless, entanglement molecular weights obtained by molecular modeling are useful for future experimental guidance because the level of order in the glassy phase near ambient temperature was found to be low (approx. 5{\%}) regardless of melt processing history. The biphasic nature of the melt may also be a contributor to more difficult melt processing. Based on both the modeling and WAXS measurements, it is believed that increasing m-phenylene content might improve toughness and processibility relative to the interrogated samples. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z21.00002: Synergy, Effect and Mechanism on Different Scale of organoclay Filled an Elastomer Blends by Rheological Method Xia Dong, Xianggui Liu, Charles C. Han, Dujin Wang The influence of organoclay on the phase separation temperature and phase separation kinetics of solution polymerized styrene butadiene rubber (SSBR)/low vinyl content polyisoprene (LPI) blends was investigated by rheological methods. The WAXD and the dynamic rheological test showed that the two components had similar interaction with the organoclay and could partially penetrate into the silicate layers. The intercalated clay could restrict the movement of polymer chains and act as a compatibilizer to reduce the phase separation temperature. The intercalated clays localized at the interface could act as an interface agent to reduce the interfacial tension, and also could act as a physical barrier to restrict the coarsening of the domains. Those two effects could slow down the phase separation kinetics and reduce the phase domain size. This effect became more obvious with increasing organoclay concentration. In addition, the addition of 3wt{\%} clay could form a rigid three-dimensional clay network which could dominant the rheological properties of the blend samples and stabilize the phase morphology permanently. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z21.00003: Polymer-polymer and hybrid clay-polymer complexes at liquid-liquid interfaces Yuhao Wang, Svetlana Sukhishvili We report on polymer-polymer and hybrid clay-polymer complex formation at oil-water interfaces. The complexes were composed of poly(methacrylic acid) (PMAA) and poly(N-isopropylacrylamide) (PNIPAM) or PNIPAM modified Laponite (L@PN). Interfacial surface tension, confocal laser scanning microscopy (CLSM) and cryogenic scanning electron microscopy (cryo-SEM) measurements were performed at various ratios of complex components and as a function of solution pH. The results reveal that interfacial PNIPAM/PMAA and L@PN/PMAA complexes are significantly more stable across the pH scale than their solution counterparts, probably because of the suppressed ionization of PMAA at the oil-water interface. In addition, we will discuss the effect of interfacial complex formation on PMAA chain dynamics, as measured by fluorescence-correlation spectroscopy (FCS), and demonstrate the use of these systems to control emulsion stability via changes in solution pH or temperature. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z21.00004: Controlled assembly of nanorods in block copolymer thin films Huikuan Chao, Dongliang Wang, Russell Composto, Robert Riggleman Block copolymers can be used as a template to order nanoparticles to obtain functional polymer nanocomposites. While progress has been made in understanding the distribution of spherical particles in the block copolymer, significantly less work has focused on the distribution of nanorods in block copolymers. Nanocomposites containing anisotropic particles could have enhanced mechanic, electrical and optical properties and become candidates for numerous applications such as conductive membranes or coatings with controlled optical properties. Understanding the thermodynamic origins that regulate the distribution of nanorods in block copolymers is of central importance in obtaining desired structures, and molecular modeling could be a powerful tool to guide experiments. In the talk, I will first introduce our extension of polymer field theory that enables the study of the equilibrium properties of block copolymer thin films containing nanorods. Then I will present how the geometry of the nanorod, including its aspect ratio and size, affects the distribution of nanorods in thin films. Finally, we will examine the role of surface wetting on the distribution of nanorods. We find the rods segregate to defects in the block copolymer structure, which agrees well with ongoing experiment. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z21.00005: Block copolymer templated growth of ZnO nanorod arrays with controlled size and spatial density Candice Pelligra, Chinedum Osuji The ability to control the diameter and spatial density of substrate-supported ZnO nanorods is critical for leveraging these nanomaterials in emerging applications. Specifically, in polymer-based photovoltaics (PV), there is a need to fabricate ZnO arrays with control over nanorod diameter and spacing to optimize device active area and to maximize exciton harvesting by matching the nanorod spacing to the exciton diffusion length in the polymer matrix. Self-assembly of block copolymers (BCPs) is well-suited to ordering nanoscopic domains over macroscopic areas with high levels of control in a low-cost, scalable manner. We present here a simple and robust method for templating the growth of vertically oriented ZnO nanorods with controlled diameter and spatial density based on the self-assembly of close-packed BCP micelles on pre-seeded substrates. Templating is accomplished using a wide range of BCP molecular weights and compositions to provide control of the ultimate nanowire diameter and areal density, respectively. Because the method relies on selective infiltration of nanorod precursor species through a hydrophobic micelle corona, it is readily extensible to a wide variety of block copolymers and nanomaterials. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z21.00006: Hybrid Thin Films Based Upon Polyoxometalates-Polymer Assembly Na Qi, Benxin Jing, Yingxi Zhu Block copolymers (BCPs) and polyoxometalates (POMs) have been used individually as building blocks for design and synthesis of novel functional materials. POM nanoclusters, the assemblies of transition metal oxides with well-defined atomic coordination structure, have been recently explored as novel nanomaterials... for catalysis, semiconductors, and even anti-cancer treatment due to their unique chemical, optical and electrical characteristics. We have explored the blending of inorganic POM nanocluster with BCPs into hierarchaically structured inorganic-organic hybrid nanocomposites. Using polystyrene-b-poly(ethylene oxide) (PS-b-PEO) thin films as the template, we have observed that the spatial organization of BCP thin films is modified by molybdenum based POM nanocluster to form 2D in-plane hexagonal ordered or 3D ordered network of POM-BCP assemblies, depending on the concentration ratio of POM to PS-b-PEO. The dielectric properties of such hybrid thin films can be enhanced by embedded POMs but show a strong dependence on the supramolecular structures of POM-polymer complexes. The assembly of nanoclusters in BCP-templated thin films could pave a new path to design new hybrid nanocomposites with uniquely combined functionality and material properties. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z21.00007: Two-dimensional networks of nanowires with large-scale continuity and connectivity patterned by the self-assembly of block copolymers Mark Stoykovich, Ian Campbell Self-assembled block copolymers in thin films have advantages for nanolithography including tunable and scalable feature sizes below 50 nm, and parallel patterning over large areas. Here we characterize the interconnectedness of two-dimensional networks self-assembled by a lamellar-forming diblock copolymer of polystyrene and poly(methyl methacrylate) in thin films. The topology of the network (its connectivity and large-scale continuity) was explored as a function of the copolymer composition and processing (i.e., solvent versus thermal annealing, film thickness, annealing temperature, annealing time). The block copolymer templates have then been used to fabricate metal and Si nanowires (less than 25 nm diameters) in the structure of the two-dimensional networks. The electrical and optical properties of the networks were measured over macroscopic areas, and were comparable to theoretical calculations based on the characteristic dimensions and network structure. Furthermore these two-dimensional nanowire networks have exciting mechanical properties; they can be stretched, compressed, twisted or folded with no significant changes in their optoelectronic characteristics, making such materials attractive for application in flexible or stretchable electronics. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z21.00008: Preparation of Low Band Gap Fibrillar Structures by Solvent Induced Crystallization Hsin-Wei Wang, Emily Pentzer, Todd Emerick, Thomas Russell Solution-induced crystallization of the low band gap polymer poly[$N$-9''-heptadecanyl-2,7-carbazole-\textit{alt}-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) was shown to give fibril-like structures of 40-60 nm width and $\sim$ 0.5 $\mu$m length. These structures, formed by heating and cooling PCDTBT in a marginal solvent, were characterized by AFM, TEM, GI-WAXS, and steady state absorption and emission spectroscopy. The width of the PCDTBT structures suggests that the polymer chains are oriented perpendicular to the fiber axis, while the observed undulated structures, as revealed by AFM, suggest that the nanostructures may be composed of smaller crystalline units, suggesting a crystal face-specific assembly. Surprisingly, no spectroscopic signatures in either absorption or emission were observed upon crystallization of PCDTBT, in sharp contrast to the well-known conjugated polymer poly(3-hexyl thiophene) (P3HT). The solution-based crystallization of PCDTBT offers insight into the self-assembly of conjugated polymers and a better understanding of their role in photovoltaic devices [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z21.00009: Controlling the thermomechanical behavior of nanoparticle/polymer films Dan Zhao, Dirk Schneider, George Fytas, Sanat K. Kumar Using Brillouin light scattering, we show that the thermomechanical properties of polymer nanocomposite films consisting of silica nanoparticles (NPs) and poly (2-vinylpyridine) (P2VP) critically depend on the casting solvent and thermal annealing. The composite films are solvent cast in either methylethylketone (MEK) or pyridine. In the MEK as-cast films, no remarkable dependence of elastic modulus on particle loading has been found, which suggests poor adhesion between the bound polymer chains and those in the bulk. Further, when the particle loading exceeds 20 wt{\%}, we observe another independently propagating phonon due to local bridging of silica NPs by P2VP chains. In contrast, when pyridine is used, the sound velocity vs. silica contents can be described by effective medium predictions. This implies that, at a mesoscopic scale, silica NPs are homogeneously dispersed in the P2VP matrices, preserving their potential attractions with P2VP. However, after thermal annealing, the solvent effect disappears. Additionally, we study the effect of the presence of NPs on the glass transition temperature (T$_{\mathrm{g}})$ of the resulting nanocomposites and find only a slight increase (5 K) in T$_{\mathrm{g}}$ for composites with a loading of 45 wt{\%}. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z21.00010: Synthesis of Biomimetic Branched Polymer Architectures Amanda Marciel, Danielle Mai, Charles Schroeder Development of sequence-defined or structurally-precise polymers as high-performance materials is a major challenge in materials science. In this work, we report a facile synthesis platform to produce monodisperse and stereochemically precise nucleotidomimetic polymers. Based on a top-down approach, we are able to precisely incorporate a wide-variety of functional group modifications in a simple two-step process. First, we utilize the natural ability of DNA polymerase to enzymatically incorporate chemically-modified monomers (e.g., C5-dibenzocyclooctyl dUTP) in a template-directed fashion. Second, we employ copper-free click chemistry to integrate the desired chemical functionality at precise locations along the polymer chain. In this way, we produced a variety of branched DNA homopolymer architectures including 3-arm star, symmetric H, and block-brush. Overall, this synthetic strategy allows for the systematic variation of oligomer length, stoichiometry, concentration, and environmental conditions to rapidly explore nucleotidomimetic polymer phase behavior for materials discovery. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z21.00011: Nanoscale phase separation in ultra-tough hydrogels Ryan Nixon, Jan Bart ten Hove, Adrian Orozco, W. Gregory Sawyer, Thomas Angelini Soft, wettable, water permeable materials that resist protein adsorption are essential to countless biomaterials, adaptive optics and microfluidics technologies. Hydrogels would be ideal for these applications, but are notoriously brittle and weak. For example, hydrogel coatings of synovial joint prosthetics exhibit irrecoverable damage after a single cycle of wear. The development of elastomer-like hydrogels that are tough, soft, and mechanically resilient would improve their versatility and create opportunities for a wide range of new applications. Here we present studies of an ultra-tough hydrogel, synthesized by the co-polymerization of two monomer species that polymerize at different rates and have strongly differing degrees of solvation. The resulting blended hydrogel network forms with both covalent and labile adhesive bonds, greatly improving recoverable energy dissipation and reducing fatigue relative to networks made from either constituent alone. We have studied the structural origins of the strengthening behavior using small angle x-ray scattering (SAXS) and found that the constituent polymers phase separate into nanoscale domains, which may prevent crack nucleation and propagation. [Preview Abstract] |
Session Z24: Neutron, Light, and X-ray Optics and Sources
Sponsoring Units: GIMSChair: Ao Teng, University of Tennessee
Room: 504
Friday, March 7, 2014 11:15AM - 11:27AM |
Z24.00001: Synchrotron X-ray Optics Testing at Beamline 1-BM at the Advanced Photon Source Albert Macrander, Naresh Kujala Beamline 1-BM at the Advanced Photon Source has been reconfigured, in part for testing of synchrotron optics with both monochromatic and white beams. Monochromatic energies between 6 and 30 keV are available. Primary agendas include both white beam and monochromatic beam topography, Talbot grating interferometry for measurement of coherence lengths and wavefronts, and micro-focusing. Recent examples will include topography of sapphire , tests of Kirkpatrick-Baez mirrors, and tests of multilayer Laue lenses. Analyzers for Inelastic X-ray Scattering has also been characterized by two user groups. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z24.00002: A diffractive-optic based nonlinear optical generation spectrometer for measurement of crystallographic and magnetic point group symmetries David Hsieh, Darius Torchinsky Nonlinear optical generation from a crystalline material can provide information about both its crystallographic and magnetic point group symmetries and may therefore be exploited as a complementary technique to diffraction based scattering probes. However, this is challenging to put into practice because the experiment should ideally be sensitive to all elements of the nonlinear optical susceptibility tensor. This involves being able to measure the intensity of nonlinear optical generation either transmitted or reflected from a crystal as a function of oblique light incidence direction, polarization and frequency. These requirements are even more difficult to achieve under extreme sample environments such as ultrahigh vacuum, low temperatures, high magnetic fields or high pressures. Here we present a novel experimental setup using a diffractive optic to realize such measurements. We demonstrate the efficacy of our scheme by showing results for low temperature lattice and magnetic symmetries of selected samples using optical second harmonic generation. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z24.00003: Lensless imaging of atomic surface structures using ptychography in reflection mode Chenhui Zhu, Ross Harder, Ana Diaz, Vladir Komanicky, Andi Barbour, Ruqing Xu, Xiaojing Huang, Yaohua Liu, Michael Pierce, Hoydoo You We propose that atomic structures on single crystal surfaces can be imaged using a variation of coherent x-ray diffractive imaging. This is a lensless ptychographic technique applied along the crystal truncation rod in reciprocal space. Simulations show that the highest sensitivity on the monolayer surface structure is obtained at the anti-Bragg condition. We demonstrate the feasibility of ptychographic reconstruction from experimental data collected in reflection mode by reconstruction of atomic steps on a crystal surface. This work and use of the Advanced Photon Source and the Electron Microscopy Center for Materials Research were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The work at Safarik University is supported by Slovak grant VEGA 1/0782/12 and ERDF EU grant under contract No. ITMS 26220120005 [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z24.00004: Hanbury Brown and Twiss interferometry at a free-electron laser Andrej Singer Measurements of second- and higher-order intensity correlation functions (so-called Hanbury Brown-Twiss experiment) performed at the free-electron laser (FEL) FLASH in the non-linear regime of its operation are presented. We demonstrate the high transverse coherence properties of the FEL beam with a degree of transverse coherence of about 80{\%} in the vertical direction and a degeneracy parameter on the order of 10$^{\mathrm{9}}$. Intensity correlation measurements in spatial and frequency domain yield an estimate of the FEL average pulse duration of 50 fs. These characteristics make the FEL similar to optical laser sources. Our measurements of the higher-order correlation functions indicate that FEL radiation obeys Gaussian statistics, which is characteristic to chaotic sources. A. Singer, U. Lorenz, F. Sorgenfrei, N. Gerasimova, J. Gulden, O. M. Yefanov, R. P. Kurta, A. Shabalin, R. Dronyak, R. Treusch, V. Kocharyan, E. Weckert, W. Wurth, and I. A. Vartanyants. Phys. Rev. Lett. \textbf{111}, 034802 (2013) [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z24.00005: An ultra-high energy resolution and wide energy range soft X-ray beamline in SSRF Y.-B. Shi, R. Ruben, L. Xue, Y. Zou, Y. Wang, R.-Z. Tai, H. Ding A new ultra-high energy resolution and wide energy range soft X-ray beamline is designed and is being constructed in Shanghai Synchrotron Radiation Facility (SSRF), which has two experimental stations: angle resolved photoemission spectroscopy (ARPES) and photoelectron emission microscopy (PEEM). The source is a pair of EPUs covering the evergy ranges 20 to 200 eV and 200 to 2000 eV with arbitrary polarized light. The beamline, based on a plane grating monochromator(PGM) and four variable line spacing gratings, will deliver flux higher than 1.0$\times10^{12}$ photons/s/0.01\%BW. A grating dedicated for high energies and very high resolution will provide more than 5$\times10^{10}$ photons/s at 1 keV with a resolution of 14 meV. The refocusing for both endstations is based on KB pairs with which the spot size will be 15$\times4.7 \mu m^{2}$ (hor.$\times$ver. FWHM) at ARPES and 9.9$\times4.8 \mu m^{2}$ (hor.$\times$ver. FWHM) at PEEM, respectively, with a 10 $\mu m$ exit slit. Owing to high thermal radiation from the source, a new method is used to eliminate the influence of the thermal deformation on mirrors in order to achieve ultra-high energy resolution. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z24.00006: kHz tabletop coherent soft X-ray source enabled by single-stage 10 mJ Ti:sapphire amplifier Chengyuan Ding, Wei Xiong, Tingting Fan, Daniel Hickstein, Henry Kapteyn, Margaret Murnane We present a tabletop source of bright, broadband, soft X-rays with photon energies up to 300 eV. By driving the harmonic generation process using 1.3 $\mu $m light in a high-pressure, phase matched geometry, we significantly enhance the soft X-ray flux and stability. We achieve a photon flux \textgreater 10$^{8}$~photons/s/1{\%} bandwidth, which emerges as a soft x-ray supercontinuum. Utilizing this broad bandwidth, we can implement high-quality x-ray absorption spectroscopy of multiple elements and transitions in a single spectrum. Near edge fine structure is obtained from molecular samples (CS$_{2}$ and SF$_{6})$. Furthermore, we demonstrate the applicability of this source to transient absorption studies by measuring the transient soft X-ray absorption of xenon plasma with high spectral sensitivity and femtosecond temporal resolution. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z24.00007: High average power, high repetition rate table-top soft x-ray lasers for applications in nanoscience and nanotechnology Brendan Reagan, Keith Wernsing, Cory Baumgarten, Leon Durivage, Mark Berrill, Alden Curtis, Federico Furch, Brad Luther, Mark Woolston, Dinesh Patel, Carmen Menoni, Vyacheslav Shlyaptsev, Jorge Rocca There is great interest in table-top sources of bright coherent soft x-ray radiation for nanoscale applications. We report the demonstration of a compact, high repetition rate soft x-ray laser operating at wavelengths between 10.9nm to 18.9nm, including the generation of 0.15mW average power at $\lambda =$18.9nm and 0.1mW average power at $\lambda =$13.9nm. These short wavelength lasers were driven by an all diode pumped, chirped pulse amplification laser based on cryogenically-cooled Yb:YAG amplifiers that produces 1 Joule, picosecond duration pulses at 100 Hz repetition rate. Irradiation of solid targets results in the production of plasmas with large transient population inversions on the 4d$^{\mathrm{1}}$S$_{\mathrm{0}}\to $4p$^{\mathrm{1}}$P$_{\mathrm{1}}$ transition of Ni-like ions. Optimization of this high repetition rate laser combined with the development of high shot capacity, rotating targets has allowed the uninterrupted operation of this soft x-ray laser for hundreds of thousands of consecutive shots, making it suitable for a number of applications requiring high photon flux at short wavelengths. Work was supported by the NSF ERC for Extreme Ultraviolet Science and Technology using equipment developed under NSF Award MRI-ARRA 09-561, and by the AMOS program of the Office of Basic Energy Sciences, US Department of Energy. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z24.00008: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z24.00009: Determining Source Directionality from a Scattering Based Neutron Detector System Stephan Young, Cory Hoshor, James Currie, Tom Oakes, Joseph Crow, Paul Scott, William Miller, Anthony Caruso Three-dimensional directional resolve of neutrons over a wide energy range has proven to be a grand challenge. Various attempts to solve the problem, including neutron cameras and proton recoil based detectors have been met with limited success, with costs in efficiency as well as dimensional and field of view restrictions. To achieve true three dimensional directional resolution, empirical data and Monte Carlo N-Particle Transport Code (MCNP) modeling were used to explore response of a volumetrically sensitive scattering based system to a Cf source of varying angular orientation, then compared through three dimensional cross correlation analysis. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z24.00010: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z24.00011: Elimination of Optical Artifacts from Transmission FTIR Spectra for Quantitative Bond Density Analysis Milan Milosevic, Sean King Fourier transform infrared (FTIR) spectroscopy is a powerful technique for characterizing the chemical structure and the short range order in bulk solids and thin films. However, transmission FTIR spectra of thin films deposited on optically thick substrates are complicated by the presence of numerous ``optical effects'' that arise from reflections at the two surfaces and the interface of the film and substrate. The convolution of these ``optical effects'' with the true absorption spectrum of the film can cause quantitative analyses of FTIR spectra of the films on substrate to yield errors as large as 90{\%} in integrated absorbance and other quantitative parameters. In this report, we describe and demonstrate a method that enables such ``optical effects'' to be removed with complete rigor allowing the true absorption coefficient spectrum of the film to be obtained. Unlike prior methods that address the problem as a combination of coherent and incoherent reflections in the film and substrate respectively, we use the exact theoretical expression for the transmittance of a thin film on an optically thick substrate. We show that this allows us to naturally separate the undesired ``optical effects'' and desired thin film absorption spectrum into two separate terms. Using both simulated and experimental data we demonstrate that the term containing ``optical effects'' can be cleanly subtracted from the experimental FTIR spectra to yield the true absorption coefficient spectrum of the thin film allowing for rigorous quantitative bond density and, potentially, elemental composition of the films to be calculated from the thus corrected spectra. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z24.00012: Low frequency all-optical thermoreflectance measurements of thin film thermal conductivities Kirby Myers, Hans D. Robinson We present an all-optical method for measuring the in-plane and cross-plane thermal conductivities in $\mu $m-thick films on substrates with dissimilar thermal conductivities. This method relies in thermooptically induced changes in reflectivity caused by a heating beam that is modulated with frequencies of up to 100 kHz. Because the method does not rely on ultrafast lasers, it is economical and can be incorporated in an experimental setup with a small footprint. Results from bulk samples and films with thermal conductivities ranging between about 1 Wm$^{\mathrm{-1}}$K$^{\mathrm{-1}}$and 100 Wm$^{\mathrm{-1}}$K$^{\mathrm{-1}}$will be presented. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z24.00013: Bio-inspired Artificial Apposition Compound Eye Jianliang Xiao, Youngmin Song, Yizhu Xie, Viktor Malyarchuk, Yonggang Huang, John Rogers In arthropods, evolution has created a remarkably sophisticated class of imaging system, with wide angle field of view, low aberrations, high acuity to motion and infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. We present ideas in materials, mechanics and integration schemes that enable scalable pathways to working, arthropod-inspired cameras in nearly full hemispherical shapes with surfaces densely populated by imaging elements (i.e. artificial ommatidia). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors, in co-integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated, to hemispherical shapes for integration into apposition cameras. Experimental and theoretical studies reveal key aspects of the materials science and physics of these systems. Imaging results and quantitative ray-tracing based modeling illustrate essential features of their operation. [Preview Abstract] |
Session Z25: Focus Session: Thermoelectrics - Phonons and Heat Conduction III
Sponsoring Units: DMP GERA FIAPChair: Austin Minnich
Room: 503
Friday, March 7, 2014 11:15AM - 11:51AM |
Z25.00001: Phonon Mean Free Path Spectra Measured by Broadband Frequency Domain Thermoreflectance Invited Speaker: Jonathan Malen Nonmetallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events$\relbar $their mean free paths (MFPs). Due to the breadth of the phonon MFP spectrum, nanostructuring of materials and devices can reduce thermal conductivity from bulk by scattering long MFP phonons, while short MFP phonons are unaffected. We have developed a novel approach called Broadband Frequency Domain Thermoreflectance (BB-FDTR) that uses high-frequency laser heating to generate non-Fourier heat conduction that can sort phonons based on their MFPs. BB-FDTR outputs thermal conductivity as a function of heating frequency. Through non-equilibrium Boltzmann Transport Equation models this data can be converted to thermal conductivity accumulation, which describes how thermal conductivity is summed from phonons with different MFPs. Relative to alternative approaches, BB-FDTR yields order-of-magnitude improvements in the resolution and breadth of the thermal conductivity accumulation function. We will present data for GaAs, GaN, AlN, Si, and SiC that show interesting commonalities near their respective Debye temperatures and suggest that there may be a universal phonon MFP spectrum for small unit cell non-metals in the high temperature limit. At the time of this abstract submission we are also working on measurements of semiconductor alloys and select metals that will be presented if completed by the conference. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z25.00002: Measurement of surface acoustic wave velocity using phase shift mask and application on thin film of thermoelectric material Dongyao Li, Peng Zhao, Noel Gunning, David Johnson, Ji-Cheng Zhao, David Cahill We describe a convenient approach for measuring the velocity $v_{SAW}$ of surface acoustic waves (SAWs) of the near-surface layer of a material through optical pump-probe measurements and apply this method, in combination with conventional picosecond acoustics, to determine a subset of the elastic constants of thin films of semiconducting misfit layered compounds. SAWs with a wavelength of 700 nm are generated and detected using an elastomeric polydimethylsiloxane (PDMS) phase-shift mask which is fabricated using a commercially-available Si grating as a mold. The velocity of SAWs of [(SnSe)$_{1.04}$]$_{m}$[MoSe$_{2}$]$_{n}$ synthesized by elemental reactants show subtle variations in their elastic constants as a function of m and n. Precise measurements of elastic constants will enable a better understanding of interfacial stiffness in nanoscale multilayers and the effects of phonon focusing on thermal conductivity. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z25.00003: Coherent phonon transport in Epitaxial Oxide Heterostructures Ajay Yadav, Aaron Swarz, Ramez Cheaito, Jayakanth Ravichandran, Patrick Hopkins, Arun Majumdar, Joel Moore, Ramamoorthy Ramesh Coherent transport of phonons was unambiguously observed [1] in superlattices of complex oxides, (SrTiO$_{3})_{\mathrm{m}}$/(CaTiO$_{3})_{\mathrm{n}}$ and (SrTiO$_{3})_{\mathrm{m}}$/(BaTiO$_{3})_{\mathrm{n}}$ manifested by a minimum in thermal conductivity as a function of superlattice interface density. To gain further insights into coherent regime of phonon transport, we systematically changed acoustic impedance mismatch between superlattice constituents and studied its effect on the transition from incoherent to coherent regime of heat transport. Further, in an attempt to manipulate transport of a broad range of phonons in the coherent regime, controllable disorder is introduced to attain both short-range and long-range phonon scattering. Quasi-periodic, controllable disorder is introduced by randomly stacking alternating layers of SrTiO$_{3}$ and CaTiO$_{3}$, for a given average interface density and volume fraction. In conclusion, our studies elucidate the effect of periodicity and impedance mismatch on both coherent and incoherent phonon scattering in epitaxial oxide heterostructures.\\[4pt] [1] J. Ravichandran, A. Yadav, R. Cheaito et al., accepted in \textit{Nature Materials }(2013). [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z25.00004: Phonon transmission and reflection antiresonances at the interface between solids with impurities as interference phenomena in atomic-scale phononic metamaterials Yury Kosevich, Haoxue Han, Sebastian Volz We study theoretically phonon transmission through the interface between two solid crystals, which contains heavy isotopic impurities and/or soft-force-constant defects. We perform analytical calculations of plane wave transmission and numerical molecular dynamics simulation of wave packet transmission, which give consistent with each other results. If the impurities do not fill completely the interface plane, longitudinal and transverse phonons have two passes to cross such interface, through the host and through the impurity atoms bonds. Destructive interference between these passes can result in total resonance reflection of the phonon. The phonon transmission antiresonance is followed by phonon reflection antiresonance at higher frequency. The random distribution of the defects at the interface and nonlinearity of atomic bonds do not deteriorate the reflection and transmission antiresonances. Such Fano-like phonon interference antiresonances can affect heat transport through interfaces and contacts between nanostructures with impurities. The antiresonances are realized in phonon transmission through a planar defect in Si crystal with segregated Ge atoms. The phonon antiresonances can be considered as interference phenomena in atomic-scale phononic metamaterials. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z25.00005: Directed phonon engineering in nanostructured Mn-Ge superlattices: Towards a description of heat transport in device-like structures Claudia Mangold, Joerg Behler, Davide Donadio Poor performance of thermoelectric materials severely limits the application of Peltier devices. Our work aims at the improvement of the efficiency of such devices by replacing standard p-n junctions with a membrane structure with nanofeatures. The low dimensionality of the membranes and the nanofeatures will ensure a reduction of the phononic thermal conductivity $\kappa$, thus enhancing the thermoelectric figure of merit, ZT=S$^2\sigma$T/$\kappa$. Mn-Ge compounds turned out to be excellent candidates for nanostructuring due to the broad structural variety[1]. We performed first-principles electronic structure calculations, in particular density functional theory, to characterize various Mn-Ge bulk species as well as Mn-Ge superlattices. To reach larger length scales we have constructed a transferable neural network potential[2] for Mn-Ge compounds to characterize nanostructured membranes up to device-like size and determine their thermal transport properties. This multiscale modeling approach is a powerful tool to design materials and devices with specifically engineered phonon properties and enhanced thermoelectric performances. [1]Jamet et al Nature Mat.5,653(2006);Jain et al J.Appl.Phys. 109,013911(2011) [2]Behler et al PRL 98,146401(2007);Phys.Stat.Sol.B 245,261(2008) [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z25.00006: First principles study of thermal conductance across the MgO/TiN interface Derek Stewart, Saikat Mukhopadhyay MgO and TiN are well lattice-matched crystals and their interface has one of the lowest thermal resistances currently measured. As such, it represents a key test for atomistic models for thermal interfacial resistance. In this work, we examine the phonon contribution to thermal transport across the epitaxial MgO/TiN (001) and (111) interfaces using an atomistic Green's function approach that incorporates interatomic force constants calculated using density functional theory. Since TiN is a metal, this approach will allow us to isolate the direct phonon contribution to thermal conductance across the interface. Calculated phonon dispersions for bulk MgO and TiN show good agreement with experiment. We will discuss how the predicted thermal interface resistance compares with values calculated using standard acoustic mismatch and diffusive mismatch models. We will also examine the impact of TiN nitrogen vacancies on both the bulk phonon dispersion and MgO/TiN thermal conductance. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z25.00007: Phonon impedance matching: minimizing interfacial thermal resistance of thin films Carlos Polanco, Jingjie Zhang, Avik Ghosh The challenge to minimize interfacial thermal resistance is to allow a broad band spectrum of phonons, with non-linear dispersion and well defined translational and rotational symmetries, to cross the interface. We explain how to minimize this resistance using a frequency dependent broadening matrix that generalizes the notion of acoustic impedance to the whole phonon spectrum including symmetries. We show how to ``match'' two given materials by joining them with a single atomic layer, with a multilayer material and with a graded superlattice. Atomic layer ``matching'' requires a layer with a mass close to the arithmetic mean (or spring constant close to the harmonic mean) to favor high frequency phonon transmission. For multilayer ``matching,'' we want a material with a broadening close to the geometric mean to maximize transmission peaks. For graded superlattices, a continuous sequence of geometric means translates to an exponentially varying broadening that generates a wide-band antireflection coating for both the coherent and incoherent limits. Our results are supported by ``first principles'' calculations of thermal conductance for $GaAs/Ga_xAl_{1-x}As/AlAs$ thin films using the Non-Equilibrium Greens Function formalism coupled with Density Functional Perturbation Theory. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z25.00008: Nanoscale thermal transport measurements: Bridging ultrafast and steady-state Brian Green, Mark Siemens Macroscale thermal transport is explained by classical thermal diffusion, but as nanostructure length scales are reduced towards the order of the phonon mean free path, transport of thermal energy takes on a fundamentally different character. Nanoscale effects emerge such as sensitivity to the presence of surfaces and the onset of ballistic transport. We investigate nanoscale thermal physics by comparing results from two different transport measurement techniques applied to systems of highly confined thermal transport: metallic thin films deposited on a suspended bridge structure. One technique uses the transient thermoreflectance (TTR) method to measure picosecond cooling dynamics following ultrafast laser heating in a micron-sized region of the metallic film deposited on the bridge; the second is a DC technique that measures transport driven by an Ohmically-generated thermal gradient across the bridge through the full volume of the film. We find that these very different methods give similar results of significantly reduced thermal conductivity relative to macroscale values. We compare TTR and DC results between differing film thicknesses, and evaluate conductance uniformity across film surfaces. In combination, the TTR and DC methods are powerful tools for investigating and understanding thermal transport at the nanoscale. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z25.00009: Nanocrystalline silicon thin films for thermoelectric applications Daniel Queen, Battogtokh Jugdersuren, Jim Culberston, Qi Wang, William Nemeth, Tom Metcalf, Xiao Liu Recent advances in thermoelectric materials have come from reductions in thermal conductivity by manipulating both chemical composition and nanostructure to limit the phonon mean free path. However, wide spread applications for some of these materials may be limited due to high raw material and integration costs. In this talk we will discuss our recent results on nanocrystalline silicon thin films deposited by both hot-wire and plasma enhanced chemical vapor deposition where the nanocrystal size and crystalline volume fraction are varied by dilution of the silane precursor gas with hydrogen. Nanocyrstalline silicon is an established material technology used in multijunction amorphous silicon solar cells and has the potential to be a low cost and scalable material for use in thermoelectric devices. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z25.00010: Phonon Thermal Transport in SiGe-based Nanocomposites for Thermoelectric Applications Zlatan Aksamija Silicon-germanium (SiGe) and Si/Si$_{1-x}$Ge$_{x}$ superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk Si$_{1-x}$Ge$_{x}$ alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocompoosites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we employ a \textit{momentum-dependent} specularity parameter p(\textbf{q})$=$exp(-4$\pi^{2}\Delta ^{2}$q$^{2}$cos$^{2}\theta )$. Our results show highly anisotropic thermal transport in SiGe nanocomposites below their bulk alloy counterparts. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z25.00011: Phonon Drag in Thin Films, Cases of Bi$_{2}$Te$_{3}$ and ZnTe Hang Chi, Ctirad Uher At low temperatures, in (semi-)conductors subjected to a thermal gradient, charge carriers (electrons and holes) are swept (dragged) by out-of-equilibrium phonons due to strong electron-phonon interaction, giving rise to a large contribution to the Seebeck coefficient called the phonon-drag effect. Such phenomenon was surprisingly observed in our recent transport study of highly mismatched alloys as potential thermoelectric materials: a significant phonon-drag thermopower reaching 1.5--2.5 mV/K was recorded for the first time in nitrogen-doped ZnTe epitaxial layers on GaAs (100). In thin films of Bi$_{2}$Te$_{3}$, we demonstrate a spectacular influence of substrate phonons on charge carriers. We show that one can control and tune the position and magnitude of the phonon-drag peak over a wide range of temperatures by depositing thin films on substrates with vastly different Debye temperatures. Our experiments also provide a way to study the nature of the phonon spectrum in thin films, which is rarely probed but clearly important for a complete understanding of thin film properties and the interplay of the substrate and films. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z25.00012: High Temperature Thermoelectric Properties of Gd doped InGaAs Thin Films Rachel Koltun, Ryan Need, Ashton Meginnis, Brian Schultz, Chris Palmstrom, John Bowers Doping III-As thin films with rare earths has been shown to increase the thermoelectric figure of merit (ZT) at high temperatures. Above the solubility limit, rare earth - arsenide nanoparticles precipitate out of molecular beam epitaxy grown films. These particles scatter phonons to reduce the thermal conductivity and act as a source of thermally activated carriers at high temperature. In this study, we compare the thermoelectric properties of Gd doped InGaAs to traditional doping methods (Si). Gd doped samples were grown to explore the doping effects below and above the solubility limit in InGaAs. This range also captures the peak ZT for these structures. Electrical conductivity and Seebeck coefficient were measured as a function of temperature. Gd doped InGaAs shows a higher doping efficiency than Er doped InGaAs, leading to better thermoelectric performance. However, Si has a much higher doping efficiency than any of the rare earths, leading to overall peak room temperature thermoelectric performance of Si doped InGaAs. Temperature dependent hall suggests that there may be a crossover point where enough carriers are thermally generated from nanoparticles to surpass the thermoelectric performance of Si doped InGaAs. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z25.00013: Nanophononic metamaterial: Thermal conductivity reduction by local resonance Bruce Davis, Mahmoud Hussein Engineered manipulation of phonons can yield beneficial thermal properties in semiconducting materials. One pivotal application relates to thermoelectric materials, or the concept of converting energy in the form of heat into electricity and vice-versa. The ability to use nanostructuring to reduce the thermal conductivity without negatively impacting the power factor provides a promising avenue for achieving high values of the thermoelectric energy conversion figure-of-merit, ZT. In this work, we propose a novel nanostructured material configuration that seeks to achieve this goal. Termed nanophononic metamaterial, the configuration is based on a silicon thin-film with a periodic array of pillars erected on one or two of the free surfaces. The pillars qualitatively alter the base thin-film phonon spectrum due to a hybridization mechanism between their local resonances and the underlying atomic lattice dispersion. Using an experimentally-fitted lattice-dynamics-based model, we conservatively predict a drop in the thermal conductivity to as low as 50{\%} of the corresponding uniform thin-film value despite the fact that the pillars add more phonon modes to the spectrum. [Preview Abstract] |
Session Z26: Novel Technologies and Algorithms
Sponsoring Units: DCOMPChair: Jack Wells, Oak Ridge National Laboratory
Room: 502
Friday, March 7, 2014 11:15AM - 11:27AM |
Z26.00001: A neural network representation of the potential energy surface in Si- and Si-Li systems Brad Malone, Ekin Cubuk, Efthimios Kaxiras A recently developed technique for extending calculations to longer length and time scales is based upon the training of the biologically-inspired neural network machine learning architecture to reproduce the potential energy surface. Trained with ab-initio density functional theory information, such neural networks can reproduce DFT-level accuracy in the study of processes traditionally limited to empirical potentials. We describe progress in constructing accurate neural network potentials for both elemental Si systems as well as Si systems which incorporate Li, the latter being of great current interest because of the promise of Si as an anode material in Li-ion batteries. These potentials allow for the study of interesting phase transformation behavior that occurs in these systems inaccessible by traditional approaches. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z26.00002: Machine Learning for Dynamical Mean Field Theory Louis-Francois Arsenault, Alejandro Lopez-Bezanilla, O. Anatole von Lilienfeld, P.B. Littlewood, Andy Millis Machine Learning (ML), an approach that infers new results from accumulated knowledge, is in use for a variety of tasks ranging from face and voice recognition to internet searching and has recently been gaining increasing importance in chemistry and physics [1]. In this talk, we investigate the possibility of using ML to solve the equations of dynamical mean field theory which otherwise requires the (numerically very expensive) solution of a quantum impurity model. Our ML scheme requires the relation between two functions: the hybridization function describing the bare (local) electronic structure of a material and the self-energy describing the many body physics. We discuss the parameterization of the two functions for the exact diagonalization solver and present examples, beginning with the Anderson Impurity model with a fixed bath density of states, demonstrating the advantages and the pitfalls of the method.\\[4pt] [1] J. Chem. Theory Comput., 9 3404 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z26.00003: Modeling quantum physics with machine learning Alejandro Lopez-Bezanilla, Louis-Francois Arsenault, Andrew Millis, Peter Littlewood, Anatole von Lilienfeld Machine Learning (ML) is a systematic way of inferring new results from sparse information. It directly allows for the resolution of computationally expensive sets of equations by making sense of accumulated knowledge and it is therefore an attractive method for providing computationally inexpensive 'solvers' for some of the important systems of condensed matter physics. In this talk a non-linear regression statistical model is introduced to demonstrate the utility of ML methods in solving quantum physics related problem, and is applied to the calculation of electronic transport in 1D channels. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z26.00004: Finite Temperature Quantum Effects on Confined Charges Jeffrey Wrighton, Sandipan Dutta, James Dufty The equilibrium density profile for charges confined in a harmonic trap is described for a wide range of temperatures and densities, including the strong coupling classical limit of dusty ion plasmas and low temperature limit of electrons in warm, dense matter. The theoretical description is based on a classical density functional theory (liquid state HNC approximation [1]) using effective quantum charge-charge and confining potentials [2]. Attention is focused on the role of quantum effects in shell formation. These quantum effects range from quantitative modifications of structure due to classical Coulomb correlations to qualitatively different quantum origins of shell structure due to exchange at temperatures below the Fermi temperature. \\[4pt] [1] J. Wrighton, J. W. Dufty, H. K\"{a}hlert, and M. Bonitz, Phys. Rev. E 80, 066405 (2009).\\[0pt] [2] Sandipan Dutta and James Dufty, Phys. Rev. E 87, 032102 (2013); EPL 102 67005 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z26.00005: Plasmon Excitations in a Triad of Coulomb-coupled spherical shells Bo Gao, Godfrey Gumbs, Antonios Balassis, Andrii Iurov, Danhong Huang Plasmon modes for a bundle of three spherical of two-dimensional electron gases (S2DEG's) have been obtained within the random-phase approximation (RPA). The inter-sphere Coulomb interaction matrix elements and their symmetry properties were also investigated in detail. The case of a bundle gives an adequate picture of the way in which the Coulomb interaction depends on orbital angular momentum quantum number $L$ and its projection $M$. We concluded that the interaction between the S2DEG's aligned at an angle of $45^0$ with the axis of quantization is negligible compared to the interaction along and perpendicular to the quantization axis. Consequently, the plasmon excitation frequencies reveal an interesting orientational anisotropic coupling to an external EM field probing the charge density oscillations. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z26.00006: Automated, ab initio calculations of X-ray spectra including many-body excitations and vibrational damping Kevin Jorissen, Shauna Story, John Rehr Accurate calculations of x-ray absorption spectra (XAS) often require linking several materials science codes [1]. To reduce the complexity and support the hardware requirements of such calculations, we have virtualized XAS modeling workflows using a Cloud-based approach, with interfacing and configuration of codes handled by developers, and virtual HPC resources allocated on demand [2]. When coupled to user-friendly GUIs this puts powerful multi-code simulations in the hands of general users. For instance, FEFF users can improve XAS interpretation and analysis using accurate ab initio Debye-Waller factors and self energy from the ABINIT DFT code, rather than semi-empirical models. Additionally, such workflows allow robust automation of large-scale calculation sets such as the Materials Project [3] where our approach could enable a theoretical spectroscopy database of many thousands of structures for systematic study of materials.\\[4pt] [1] Rehr et al., C.R. Phys. 10, 548 (2009).\\[0pt] [2] Jorissen et al., Comp. Phys. Comm. 183, 1922 (2012).\\[0pt] [3] www.materialsproject.org [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z26.00007: Quantitative simulation and density reconstruction in high-energy X-ray radiograph Li Tang, Haibo Xu Numerical radiograph using Monte Carlo method is used to study fidelity of density reconstruction in high-energy X-ray radiography. A density reconstruction method for a polyenergetic X-ray source and an object composed of different materials is proposed. The method includes energy spectrum, angular spectrum and spot size of photon source. And it includes mass absorption coefficients explicitly in density reconstruction as well. A constrained conjugate gradient algorithm and variation regularization are applied to determine material edges and density reconstruction of a French test object. It shows that the method is valid for density reconstruction and energy spectrum of imaging photons is important in obtaining accurate material densities in high-energy X-ray radiograph. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z26.00008: Compressed modes for variational problems in mathematical physics and compactly supported multiresolution basis for the Laplace operator Vidvuds Ozolins, Rongjie Lai, Russel Caflisch, Stanley Osher We will describe a general formalism for obtaining spatially localized (``sparse'') solutions to a class of problems in mathematical physics, which can be recast as variational optimization problems, such as the important case of Schr\"odinger's equation in quantum mechanics. Sparsity is achieved by adding an $L_1$ regularization term to the variational principle, which is shown to yield solutions with compact support (``compressed modes''). Linear combinations of these modes approximate the eigenvalue spectrum and eigenfunctions in a systematically improvable manner, and the localization properties of compressed modes make them an attractive choice for use with efficient numerical algorithms that scale linearly with the problem size. In addition, we introduce an $L_1$ regularized variational framework for developing a spatially localized basis, compressed plane waves (CPWs), that spans the eigenspace of a differential operator, for instance, the Laplace operator. Our approach generalizes the concept of plane waves to an orthogonal real-space basis with multiresolution capabilities. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z26.00009: Phase behavior of the 38-atom Lennard-Jones cluster Ray Sehgal, David Ford, Dimitrios Maroudas We have developed a coarse-grained description of the phase behavior of the isolated 38-atom Lennard-Jones cluster (LJ$_{\mathrm{38}})$. The model captures both the solid-solid polymorphic transitions that the cluster undergoes at low temperatures and the complex cluster breakup and melting transitions at higher temperatures. For this coarse model development, we employ the manifold learning technique of diffusion mapping. The outcome of the diffusion mapping analysis over a broad temperature range indicates that two order parameters are sufficient to describe the cluster's phase behavior; we have chosen two such appropriate order parameters that are metrics of condensation and overall crystallinity. In this well-justified coarse-variable space, we calculate the cluster's free energy landscape (FEL) as a function of temperature, employing Monte Carlo umbrella sampling. These FELs are used to quantify the phase behavior and onsets of phase transitions of the LJ$_{\mathrm{38}}$ cluster. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z26.00010: Solving Hydrogen Systems in a Gaussian-Sinc Mixed Basis Jonathan Jerke, C.J. Tymczak, Young Lee We introduce a Gaussian-Sinc electronic structure mixed basis calculation scheme. We solve the Schr\"{o}dinger's wave equation with a uniform magnetic field for a single electron in Hydrogen Systems in three dimensions. The scheme is inherently unbiased since most of the field is digital under the Sinc basis and the Gaussian are only meant to capture the cusp of atomic states. The entire scheme is translational invariant and the potentials are calculated properly and are necessarily off diagonal. In the absence of magnetic fields the scheme is variational bound. We generally find under arbitrary configurations of protons that we can achieve four significant digits after the decimal. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z26.00011: Kinetic barriers for Cd and Te adatoms on Cd and Te terminated CdTe (111) surface using {\it ab~ initio} simulations Ebadollah Naderi, Sachin P. Nanavati, Chiranjib Majumder, S.V. Ghaisas In the present work we have calculated using density functional theory (DFT), diffusion barrier potentials on both the CdTe (111) surfaces, Cd terminated (A-type) \& Te terminated (B-type). We employ nudge elastic band method (NEB) for obtaining the barrier potentials. The barrier is computed for Cd and for Te adatoms on both A-type and B-type surfaces. We report two energetically favourable positions along the normal to the surface, one above and other below the surface. The one above the surface has binding energy slightly more the one below. According to the results of this work, binding energy (in all cases) for adatoms are reasonable and close to experimental data. The barrier potential for hopping adatoms (Cd and Te) on both the surfaces is less than 0.35 eV. Apart from these most probable sites, there are other at least two sites on both the types of surfaces which are meta stable. We have also computed barriers for hopping to and from these meta stable positions. The present results can shade light on the defect formation mechanism in CdTe thin films during growth. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z26.00012: The CONV-3D code for DNS CFD calculation Vladimir Chudanov The CONV-3D code for DNS CFD calculation of thermal and hydrodynamics on Fast Reactor with use of supercomputers is developed. This code is highly effective in a scalability at the high performance computers such as ``Chebyshev'', ``Lomonosov'' (Moscow State University, Russia), Blue Gene/Q(ALCF MIRA, ANL). The scalability is reached up to 10$^{6}$ processors [1]. The code was validated on a series of the well known tests in a wide range of Rayleigh (10$^{6}$-10$^{16}$) and Reynolds (10$^{3}$-10$^{5}$. Such code was validated on the blind tests OECD/NEA of the turbulent intermixing in horizontal subchannels of the fuel assembly at normal pressure and temperature (Matis-H), of the flows in T-junction and the report IBRAE/ANL was published [2]. The good coincidence of numerical predictions with experimental data was reached, that specifies applicability of the developed approach for a prediction of thermal and hydrodynamics in a boundary layer at small Prandtl that is characteristic of the liquid metal reactors.\\[4pt] [1] V.V. Chudanov et al. The national supercomputer forum (NSCF-2013). Pereslavl-Zalesskii, 26-29 November, 2013.\\[0pt] [2] A.V. Obabko, P.F. Fischer, et al. ISBN 978-953-51-0987-7, Published: February 13, 2013 under CC BY 3.0 license. DOI: 10.5772/53143. 2013. [Preview Abstract] |
Session Z27: Focus Session: Friction and Wear at the Nano- and Micro-Scales
Sponsoring Units: DCOMPChair: Michael Chandross, Sandia National Laboratories
Room: 501
Friday, March 7, 2014 11:15AM - 11:51AM |
Z27.00001: Friction and Adhesion Behavior of Graphene and other Two-Dimensional Materials Invited Speaker: Robert Carpick Two-dimensional materials provide a rich playground for exploring new and unexpected physical phenomena, including tribological behavior such as friction and wear. This talk will focus on friction and adhesion behavior of nanoscale contacts with such materials. For contacts to graphene, MoS$_{2}$, NbS$_{2}$, and BN, we find that the friction force exhibits a significant dependence on the number of 2-D layers [1] which we attribute to an out-of-plane ``puckering'' deformation that occurs when the 2-D material is weakly bound to its substrate. However, adhesive behavior does not follow this dependence. Instead, we find that sliding can induce an increased adhesive force due to local delamination of the topmost layer of graphene [2]. Finally, we observe a large, order-of-magnitude increase in friction that occurs when graphene is fluorinated. This result is interpreted in the context of the Prandtl-Tomlinson model of stick-slip friction. \\[4pt] [1] \textit{Frictional characteristics of atomically thin sheets}. C. Lee, Q. Li, W. Kalb, X.-Z. Liu, H. Berger, R. W. Carpick, J. Hone. \textbf{Science}, 328, 76-80 (2010). \\[0pt] [2] \textit{Nanoscale adhesive properties of graphene: The effect of sliding history}. X.-Z. Liu, Q. Li, P. Egberts, and R.W. Carpick, \textbf{Adv. Mat. Interf.}, accepted (2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z27.00002: Ab-initio modelling of energy dissipation in nanotribological systems. A DFT study of fcc Cu(111) Michael Wolloch, Gregor Feldbauer, Peter Mohn, Josef Redinger, Andras Vernes Accurate modelling of the energy dissipation in sliding friction with \textit{ab-initio} methods in nanotribological systems poses a fundamental challenge in modern tribology. Here we present a quasi-static model to obtain the nanofrictional response of dry, wearless systems based on quantum mechanical all electron calculations. We propose a mechanism for energy dissipation, which relies on the atomic relaxations during sliding. We define two different ways of calculating the mean nanofriction force, both leading to an exponential friction versus load behaviour for all sliding directions. Since our approach does not impose any limits on lengths and directions of the sliding paths, we investigate arbitrary sliding directions for an fcc Cu(111) interface and detect two periodic paths which form the upper and lower bound of nanofriction. For long aperiodic paths the friction force convergences to a value in between these limits. For low loads we retrieve the Derjaguin generalization of Amontons-Coulomb kinetic friction law which appears to be valid all the way down to the nanoscale. We observe a non-vanishing Derjaguin-offset even for atomically flat surfaces in dry contact. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z27.00003: \textit{Ab-initio} simulations on adhesion and material transfer between contacting Al and TiN surfaces Gregor Feldbauer, Michael Wolloch, Peter Mohn, Josef Redinger, Andras Vernes Contacts of surfaces at the atomic scale are crucial in many modern applications from analytical techniques like indentation or AFM experiments to technologies such as nano- and micro-electro-mechanical-systems (N-/M-EMS). Furthermore, detailed insights into such contacts are fundamental for a better understanding of tribological processes like wear. A series of simulations is performed within the framework of Density Functional Theory (DFT) to investigate the approaching, contact and subsequent separation of two atomically flat surfaces consisting of different materials. Aluminum (Al) and titanium-nitride (TiN) slabs have been chosen as a model system representing the interaction between a soft and a hard material. The approaching and separation is simulated by moving one slab in discrete steps and allowing for electronic and ionic relaxations after each one. The simulations reveal the influences of different surface orientations ((001), (011), (111)) and alignments of the surfaces with respect to each other on the adhesion, equilibrium distance, charge distribution and material transfer between the surfaces. Material transfer is observed for configurations where the interface is stronger than the softer material. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:51PM |
Z27.00004: Friction at Interfaces of Metals and Alloys Invited Speaker: Shengfeng Cheng Pure metals such as gold that are frequently used in electrical contacts usually exhibit high adhesion and friction. However, nanocrystalline gold alloyed with minute amounts of Ni or Co can have low friction while still possessing low contact resistance. We used large-scale molecular dynamics simulations with validated EAM potentials to study the atomistic origin of friction reduction in metallic alloys. Three systems will be focused on in this talk: pure Ag, Ag-Cu alloy, and Ag-Au alloy. Our results reveal that different friction coefficients of metals and alloys are due to different sliding mechanisms. Dislocation-mediated plasticity dominates in pure metals or lattice-matched alloys and leads to high friction, while grain-boundary sliding mainly occurs in lattice-mismatched alloys that leads to low friction. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z27.00005: Failure of brittle heterogeneous materials: Intermittency or continuum regime Jonathan Bar\'es, Daniel Bonamy, Luc Barbier The problem of the solid fracture has occupied scientists and engineers for centuries. This phenomenon is classically addressed within the framework of continuum mechanics. Still, stress enhancement at crack tips makes the failure behavior observed at the continuum-level scale extremely dependent on the presence of microstructure inhomogeneities down to very small scales. This yields statistical aspects which, by essence, cannot be addressed using the conventional engineering continuum approaches. We addressed the problem numerically. The simulations invoke a recent statistical model mapping heterogeneous fracture with the depinning transition of an elastic manifold in a random potential. The numerical exploration of the parameter space allowed us to unravel when (i.e. which loading conditions, microstructure material parameters, material constants...) regular dynamics compatible with continuum approaches are expected to be observed, and when crackling dynamics calling for statistical approaches are observed. In this latter case, we have characterized quantitatively the dynamics statistic and its variations as a function of the input parameters. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z27.00006: Homogeneous Dislocation Nucleation - role of geometrical parameters and interatomic potentials Akanksha Garg, Asad Hasan, Craig Maloney We perform atomistic simulations of dislocation nucleation in defect free crystals in 2D and 3D during indentation with circular (2D) or spherical (3D) indenters of radius R. We study realistic interatomic potentials such as embedded atom method (EAM) potentials for Al in addition to simple pair-wise interactions such as linear springs. The dislocation embryo is localized along a line (or plane in 3D) of atoms with a lateral extent, $\xi$, at some depth, D, below the surface. For all potentials, in 2D, the scaled critical - load, $F_c/R$, and contact length, $C_c/R$, decrease to R independent values in the limit of large R. However, despite the R independence of $F_c/R$ and $C_c/R$, $\xi/R$ and $D/R$ display non-trivial scaling with R. Although both the interaction potential and the orientation of lattice affect the \emph{prefactors} in the scaling relations (e.g. crystal with springs is much harder than EAM Aluminum), all the \emph{scaling laws} are robust. Furthermore, we show that, despite the excellent prediction for the relation between F and C, Hertzian contact theory fails to correctly predict the strain underneath the indenter. This observation gives us hope that local nucleation criteria based on appropriate local strain may capture the nontrivial scaling laws. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z27.00007: Molecular Dynamics Simulation of the Phonon Conductivity in Cu-Ni Binary Alloy Yusuke Konishi, Tetsuya Fukushima, Kazunori Sato, Yoshihiro Asai, Hiroshi Katayama-Yoshida In 2010, a giant Peltier effect was observed in a Cu-Ni/Au junction [1]. It is considered that this giant Peltier effect is caused by nano-scale phase separation formed in the sputtering process. The giant Peltier coefficient in the Cu-Ni/Au junction indicates the great Seebeck coefficient in Cu-Ni alloy. Although this alloy is a prospective thermoelectric material because of its great Seebeck coefficient, the low phonon thermal conductivity is also necessary for a large thermoelectric coefficient ZT. In order to find conditions for the low phonon conductivity, we calculate the thermal conductivity in Cu-Ni Alloy in various shapes with or without nanostructures by using nonequilibrium molecular dynamics simulation. In this simulation, we use a semi-empirical potential [2] and the reverse nonequilibrium molecular dynamics [3] method. [1] A. Sugihara et al., Appl. Phys. Exp. 3, 065204 (2010). [2] J. Cai and Y. Y. Ye Phys. Rev. B 54, 8398 (1996). [3] F. M\"{u}ller-Plathe J. Chem. Phys. 106, 6082 (1997). [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z27.00008: Micrometer-scale molecular dynamics simulations on the lattice thermal conductivities of graphene and silicon Minkyu Park, Sun-Chul Lee, Yong-Sung Kim We calculated lattice thermal conductivity of graphene and silicon by using large-scale molecular dynamics simulations. In the molecular dynamics simulations, whether the non-equilibrium systems reach the steady states is rigorously investigated, and the times to reach the steady states are found to drastically increase with the lengths of the system. From the ballistic to the diffusive regime, the lattice thermal conductivities are explicitly calculated and found to keep increasing in a wide range of lengths with finally showing a converging behavior at 16 micrometer for graphene and 8 micrometer for silicon. That obtained macroscopic values of the lattice thermal conductivity of graphene and silicon are 3200 and 210 W/mK respectively. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z27.00009: Using Markov State Models to Study Self-Assembly Matthew Perkett, Michael Hagan In recent years, a number of algorithms have been developed to study rare events, which has resulted in paradigm shift from running a few long trajectories to gathering statistics from many shorter trajectories. Running many simulations in parallel to build a Markov State Model (MSM) is one such technique, which has been applied to protein folding with great success. We present an adaptation to the MSM framework that enables its application to a wide range of systems undergoing self-assembly. The feasibility of this approach is demonstrated on two different coarse-grained models for virus self-assembly. We find good agreement between the MSM calculations and brute force long simulations, with up to several orders of magnitude reduction in simulation time. [Preview Abstract] |
Session Z35: The Bose-Hubbard Model
Sponsoring Units: DAMOPChair: Ana Maria Rey, JILA
Room: 702
Friday, March 7, 2014 11:15AM - 11:27AM |
Z35.00001: Replica Symmetry Breaking in the Bose Glass Steven Thomson, Frank Kruger We investigate the nature of the Bose glass phase of the disordered Bose-Hubbard model and demonstrate the existence of a glass-like replica symmetry breaking (RSB) order parameter in terms of particle number fluctuations. Starting from a strong-coupling expansion around the atomic limit, we study the instability of the Mott insulator towards the formation of a Bose glass. We add some infinitesimal RSB, following the Parisi hierarchical approach in the most general form, and observe its flow under the momentum-shell renormalization group scheme. We find a new fixed point with one-step RSB, corresponding to the transition between the Mott insulator and a Bose glass phase with hitherto unseen RSB. This finding is consistent with the expectation of glassy behavior and previous results showing the breakdown of self-averaging. We discuss the possibility of measuring the glass-like order parameter in optical lattice experiments. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z35.00002: Hard-core lattice bosons: new insights from algebraic graph theory Randall W. Squires, David L. Feder Determining the characteristics of hard-core lattice bosons is a problem of long-standing interest in condensed matter physics. While in one-dimensional systems the ground state can be formally obtained via a mapping to free fermions, various properties (such as correlation functions) are often difficult to calculate. In this work we discuss the application of techniques from algebraic graph theory to hard-core lattice bosons in one dimension. Graphs are natural representations of many-body Hamiltonians, with vertices representing Fock basis states and edges representing matrix elements. ~We prove that the graphs for hard-core bosons and non-interacting bosons have identical connectivity; the only difference is the existence of edge weights. ~A formal mapping between the two is therefore possible by manipulating the graph incidence matrices. We explore the implications of these insights, in particular the intriguing possibility that ground-state properties of hard-core bosons can be calculated directly from those of non-interacting bosons. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z35.00003: Excitations and quantum phase transitions of Supersolid and Haldane Insulator phases in the extended one-dimensional bosonic Hubbard model George Batrouni, Valery Rousseau, Richard Scalettar, Beno\^It Gr\'emaud The Haldane Insulator is a gapped phase characterized by an exotic non-local order parameter. It appears in reduced dimensionality models such as spin chains and the one-dimensional bosonic Hubbard model (BHM) with contact and near neighbor repulsive interactions. The parameter regimes at which it might exist, and how it competes with alternate types of order, such as supersolidity, are studied using the Stochastic Green Function quantum Monte Carlo and the Density Matrix Renormalization Group. We show that, depending on the ratio of the near neighbor to contact interactions, this model exhibits charge density waves (CDW), superfluid (SF), supersolid (SS) and the recently identified Haldane insulating (HI) phases. We show that the HI exists only at the tip of the unit filling CDW lobe and that there is a stable SS phase over a very wide range of parameters. We also present results for the excitation spectra in the various quantum phases. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z35.00004: Ferromagnetic Phase Separated Region in the Polarized Two-species Bose Hubbard Model Kalani Hettiarachchilage, Valery G. Rousseau, Ka-Ming Tam, Juana Moreno, Mark Jarrell We study a doped two-dimensional bosonic Hubbard model with two hard-core species using quantum Monte Carlo simulations [Phys. Rev. B 88, 161101(R) (2013)] . Upon doping away from half-filling, we find several distinct phases including a phase separated ferromagnet with Mott behavior for the heavy species and both Mott and superfluid behaviors for the light species. Introducing an imbalance in the population between two species, we find a perfect phase separated ferromagnet. This phase exists for a broad range of temperatures and polarizations. By using finite size scaling of the susceptibilities, we find the critical exponent of the correlation length, $\nu = \displaystyle \frac{7}{4}$ which is the critical exponent for a two-dimensional Ising ferromagnet. Importantly, since the global entropy of this phase is relatively high, experimental observations in cold atoms may be achievable. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z35.00005: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z35.00006: Renormalization of Bose-Hubbard Parameters from Few Body Correlations of Cold Atoms Nikhil Monga, John Shumway, Kaden Hazzard, Erich Mueller, Steven Desch Cold atoms in an optical lattices can be a nearly ideal physical realization of a Bose-Hubbard model. However, the effective lattice-model parameters $t$ and $J$ are significantly renormalized by quantum correlations of the atoms, and this renormalization can depend on the site occupations. This occurs because interactions populate low energy states not explicitly present in the Bose-Hubbard model. We use path integral quantum Monte Carlo (PI-QMC) to calculate the effective matrix element, $t_{mn}$, for a bosonic atom to hop from a site with $m$ atoms to a neighbor site with $n$ atoms. We consider systems of up to five bosonic atoms on two sites. For the simple case of two atoms on two sites, the imaginary-time exchange frequency and double-occupation probability uniquely determine $t$ and $U$. For more particles, we extend our analysis to dynamics correlations of the site-occupation numbers, $\langle P_{mn}(\tau) P_{m'n'}(0)\rangle$, which are calculated by PI-QMC and compared with Bose-Hubbard correlations to infer the renormalized parameters. Unlike the present state-of-art determinations of these renormalizations, which use exact diagonalization for two-particles, our Monte Carlo approach is efficient for renormalizations arising from much larger numbers of atoms. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z35.00007: Two leg bosonic ladder in an external magnetic field at unit filling Antoine Sterdyniak, Marcello Dalmonte, Salvatore Manmana, Peter Zoller, Andreas L\"auchli Motivated by the recent experimental realizations of artificial gauge field on optical lattices, we study the two-leg bosonic ladder in an external magnetic field, both analytically using bozonization techniques and numerically using finite size density matrix renormalization group algorithm. At unit filling, interacting bosons can exhibit a rich variety of phases on ladders. At large interaction, they form a Mott insulator phase while, at smaller interaction, they exhibit a Meissner phase and, more intriguing floating and staggered vortex phases. A weak chiral Mott insulator phase is also found for intermediate interaction strength. We determine the phase diagram and Luttinger parameters from correlations functions and entanglement spectrum. While usually thought to be second order, some of the phase transitions appear to be first order. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z35.00008: Spectral functions in the 1D and 2D Bose Hubbard model Robert Ivancic, Eric Duchon, Nandini Trivedi We use state of the art numerical techniques including quantum Monte Carlo and maximum entropy methods to obtain the low energy excitation spectra in the superfluid and Mott-insulator phases of the Bose Hubbard model. These results are checked in 1D against Bethe Ansatz and tDMRG results and extended to 2D where such approaches are impossible. In the superfluid, we find linearly dispersing Bogoliubov sound modes as well as additional gapped modes broadened by interaction effects. In the Mott insulator, we find evidence for a finite gap and well defined quasiparticle excitations. We examine properties such as the excitation lifetime, density of states, and speed of sound as the system is tuned across the quantum phase transition that separates the superfluid and Mott states. These results provide an important theoretical framework for upcoming ultracold atom experiments in one and two dimensions. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z35.00009: Chiral Bosonic Mott Insulator on the Frustrated Triangular Lattice Siddharth Parameswaran, Michael Zaletel, Andreas R\"uegg, Ehud Altman We study the superfluid and insulating phases of interacting bosons on the triangular lattice with an inverted dispersion, corresponding to frustrated hopping between sites. The resulting single-particle dispersion has multiple minima at nonzero wavevectors in momentum space, in contrast to the unique zero-wavevector minimum of the unfrustrated problem. As a consequence, the superfluid phase is unstable against developing additional chiral order that breaks time reversal (T) and parity (P) symmetries by forming a condensate at nonzero wavevector. We demonstrate that the loss of superfluidity can lead to an even more exotic phase, the chiral Mott insulator, with nontrivial current order that breaks T, P. These results are obtained via variational estimates, as well as a combination of bosonization and DMRG of triangular ladders, which taken together permit a fairly complete characterization of the phase diagram. We discuss the relevance of these phases to optical lattice experiments, as well as signatures of chiral symmetry breaking in time-of-flight images. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z35.00010: Universal thermodynamics of the superfluid to Mott insulator transition Nicolas Dupuis, Adam Rancon The superfluid--Mott-insulator transition of a Bose gas in an optical lattice, when it occurs at constant density, belongs to the universality class of the quantum XY model. We discuss the thermodynamics of the two-dimensional quantum O(N) model for $N\geq 2$ in the vicinity of its zero-temperature quantum critical point, and in particular the universal scaling function ${\cal F}_N$ which determines the pressure $P(T)$. We show that the large-$N$ approach is unable to predict the (non-monotonuous) shape of ${\cal F}_N$ for $N < 10$, but ${\cal F}_N$ can be computed from a renormalization-group approach. Finally, we discuss the experimental determination of the scaling function ${\cal F}_2$ from the pressure of a Bose gas in an optical lattice near the superfluid--Mott-insulator transition. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z35.00011: Novel Quantum States of Bosons in Moat Bands Tigran Sedrakyan, Leonid Glazman, Alex Kamenev We study hard-core bosons on a class of frustrated lattices with the lowest Bloch band having a degenerate minimum along a closed contour in the reciprocal space -- the Moat. We suggest that the ground state of the system is given by non-condensed state, which may be viewed as a state of fermions subject to Chern-Simons gauge field. At fixed density of bosons, such a state exhibits domains of incompressible liquids. Their fixed densities are given by fractions of the reciprocal area enclosed by the minimal energy contour. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z35.00012: Self-localization in bosonic optical lattices: beyond mean-field theory Tadeusz Pudlik, Holger Hennig, Dirk Witthaut, David Campbell The combination of nonlinearity and discreteness allows cold bosonic atoms in optical lattices to support stable excitations, known as discrete breathers or intrinsic localized modes. Prior mean-field theory studies suggest such structures form spontaneously in the presence of dissipation, as long as the nonlinearity is strong enough. But how many atoms must be present in the lattice before these effects are observed? In our work, we address this question using numerically exact treatments of the dissipative Bose-Hubbard model. Our results suggest interesting phenomena may be seen in near-term experiments with just a few atoms per lattice site. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z35.00013: Quantum tricriticality at the superfluid-insulator transition of binary Bose mixtures: A quantum Monte-Carlo study Yasuyuki Kato, Daisuke Yamamoto, Ippei Danshita Criticality emerging near a tricritical point (TCP) is referred to as tricriticality. We study quantum tricriticality in the two-component Bose-Hubbard model on square lattices that describes Bose-Bose mixtures confined in optical lattices. We confirm the existence of quantum TCPs on a boundary of superfluid-insulator transition by means of the unbiased quantum Monte-Carlo method. Moreover, we analyze an effective field theory to derive the quantum tricritical behaviors analytically. We show that the quantum tricritical behaviors are pronounced in the chemical potential dependence of the superfluid transition temperature and the density fluctuation. We suggest that the quantum tricriticality may be observed in existing experimental setups of Bose-Bose mixtures in optical lattice.~ [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z35.00014: Bose-Hubbard model with ferromagnetic-like occupation-parity couplings and its realization in imbalanced fermionic superfluids in tubular optical lattices Kuei Sun, Carlos J. Bolech We study a Bose-Hubbard model with a nearest-neighbor occupation-parity coupling that can be considered as energy cost for a domain-wall link between two adjacent sites if their occupation parity is different (one even and the other odd). Our analysis shows that the parity coupling has non-trivial interplay with the tunneling and onsite repulsion, resulting in several exotic quantum phases. For example, a uniform system with zero tunneling can exhibit a pair-liquid phase or phase separation of two Mott insulators, while a trapped system with finite tunneling shows a wedding-cake structure of only even-filling Mott insulators or a structure of central regular superfluid and outer pair superfluid. In addition, we find similar physics in a recent experimental system of imbalanced Fermi gases in optical lattices producing a 2D array of 1D tubes, with the presence of an oscillatory superfluid order parameter (the Fulde-Ferrell-Larkin-Ovchinnikov or FFLO state). We show that the unpaired majority fermions on each tube have a bosonic behavior with cross-tube tunneling, on-tube repulsion, and interplay with the spatial parity of the FFLO order that contributes to the occupation-parity coupling. Therefore, such system provides a realization of our model in two dimensions. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z35.00015: Fermionic sound in Bose-Fermi mixtures Andrey Gromov, Barry Bradlyn Sound waves emerge as a result of spontaneously broken symmetry- translational in the case of solids and normal fluids and U(1) phase symmetry in the case of superfluids. Collective modes like these, which result from the breaking of conventional symmetries, usually have bosonic statistics. We explore the consequences of a subtle fermionic symmetry that appears in Bose-Fermi mixtures when both species have equal mass. In particular, we predict the existence of a novel fermionic collective excitation and comment on its properties. We show that this mode persists in the presence of a trapping potential and contact interaction. We describe the fate of these excitations when there is a small mass difference between the two particle species. Lastly, we discuss the possibility of observing this mode in experiments, for example in trapped $^{174}Yb-^{173}Yb$ Bose-Fermi mixtures. [Preview Abstract] |
Session Z36: Technological Applications in AMO Physics
Sponsoring Units: DAMOPChair: Andreas Nunnenkamp, University of Basel
Room: 703
Friday, March 7, 2014 11:15AM - 11:27AM |
Z36.00001: Imaging atoms from resonance fluorescence spectrum beyond the diffraction limit Zeyang Liao, Mohammad Al-Amri, M. Suhail Zubairy We calculate the resonance fluorescence spectrum of a linear chain of two-level atoms driven by a gradient coherent laser field. The result shows that we can determine the positions of atoms from the spectrum even when the atoms locate within subwavelength range and the dipole-dipole interaction is significant. This far-field resonance fluorescence localization microscopy method does not require point-by-point scanning and it may be more time-efficient. We also give a possible scheme to extract the position information in an extended region without requiring more peak power of laser. We also briefly discuss how to do a 2D imaging based on our scheme. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z36.00002: Compton Scattering with a Vortex Light Beam Mazen Nairat, David Voelz The Compton effect is applied to a vortex light beam. A photon in a vortex beam possesses spin angular momentum associated with the polarization and orbital angular momentum that consists of two orthogonal components: azimuthal and axial. The azimuthal part is directly proportional to the axial part. This study considers inelastic collision of a photon possessing angular momentum with a free electron. The conservation of angular momentum as well as total energy is applied to the photon-electron system to generalize the Compton scattering model. We describe the momentum exchange and characterize the Compton effect beyond the well-known photon wavelength shift to include other parameters such as the radius of gyration. Our analysis suggests that upon an exchange of angular momentum with an electron, it is possible for the scattered photon to have no wavelength to shift. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z36.00003: Diffraction in time: An exactly solvable model Arseni Goussev In optics, diffraction is typically portrayed as deflection of light incident upon an obstacle with sharp boundaries, that can not be accounted for by reflection or refraction. Interestingly, quantum mechanics allows for an additional, intrinsically time-dependent manifestation of the phenomenon: Owing to the dispersive nature of quantum matter waves, sudden changes in boundary conditions may cause the particle wave function to develop interference fringes akin to those in stationary (optical) diffraction problems. This phenomenon, pioneered in 1952 by Moshinsky [{\it Phys. Rev.} {\bf 88}, 625 (1952)] and presently referred to as ``diffraction in time,'' is at the heart of a vibrant area of experimental and theoretical research concerned with quantum transients. In my talk, I will introduce a new versatile exactly-solvable model of diffraction in time. The model describes dynamics of a quantum particle in the presence of an absorbing time-dependent barrier, and enables a quantitative description of diffraction and interference patterns in a large variety of setups.\\[4pt] [1] A. Goussev, {\it Phys. Rev. A} {\bf 87}, 053621 (2013).\\[0pt] [2] A. Goussev, {\it Phys. Rev. A} {\bf 85}, 013626 (2012). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z36.00004: Projection on tilted screens using a phase only spatial light modulator Javad R. Gatabi, Bill Mckenna, Kumar Pandey, Dan Tamir, Wilhelmus J. Geerts We are developing a new laser lithography exposure tool for use on non-flat substrates. Such a tool does currently not exists as commercial equipment used in the electronic industry uses high numerical aperture (NA) lenses to create patterns with critical dimensions down to 22 nm on very flat substrates ($+$/- 100 nm). The ability to pattern thin films on top of curved substrates with large topography differences allows for the development of new products and devices. We investigated the use of a phase only spatial modulator to project images on inclined and curved surfaces in such exposure equipment. Starting from the Raleigh-Sommerfeld diffraction equation, expressions for the diffraction between titled surfaces were derived. These expressions were used in an iterative algorithm to determine the modulator phase pattern required for a proper projection of an image on a curved surface. An approach similar to that of Gerchberg and Saxton [1] was followed. The algorithm was implemented using a Holoeye LCD phase only modulator. It was shown to be stable and converging for simple binary test patterns. A similar approach may be used for projection from a tilted surface to a curved surface. \\[4pt] [1] R. W. Gerchberg, W. O. Saxton, OPTIK, Vol. 35 (No.2) 237-246 (1972). [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z36.00005: Compressive sensing for spatial and spectral flame diagnostics David Starling, Joseph Ranalli Compressive sensing has been a valuable resource for use in quantum imaging, low light level depth mapping of natural scenes, object tracking and even for the improvement of miniature spectrometers via post processing. Experimentally, many optical compressive sensing techniques utilize a single pixel camera composed of a digital micromirror device or spatial light modulator coupled to one shot-noise limited detector. This method has the advantages of fast acquisition time and high signal to noise ratio. One currently unexplored area of study is the use of these techniques in the context of flame diagnostics. Optical diagnostics are employed for a variety of purposes in flames, including imaging of the heat release region (via chemiluminescence) and spatially resolved species and temperature measurement (via spontaneous Raman scattering). Compressive sensing has a dual role in this field, where the signals of interest are generally sparse and the mean photon flux is very low at the appropriate wavelengths. We show here that compressive sensing is beneficial in particular for the study of laminar, steady flames using Raman spectroscopy and flame chemiluminescence imaging, without the use of intensified CCDs, commercial spectrometers or high intensity pulse lasers. We present results from a theoretical study with experimental data to follow. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z36.00006: Reflective Optical and Microwave Limiters based on Non-Linear Localized Modes Eleana Makri, Hamidreza Ramezani, Tsampikos Kottos, Ilya Vitebskiy A limiter is a structure that controls signal transport by allowing the transmission of low intensity signals while blocking signals with excessively high intensity. Existing designs lead to the absorption of excessive high intensity which can cause their overheating and eventually their (self-)destruction. We introduce the concept of {\it reflective limiter} which is based on resonance transmission via a non-linear localized mode. Such a limiter does not absorb the high level radiation, but rather reflects it back to space. Importantly, the total reflection occurs within a broad frequency range and for any direction of incidence. The proposed concept can be applied to optical and microwave frequencies. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z36.00007: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z36.00008: ABSTRACT MOVED TO A34.00013 |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z36.00009: Regularization of spectral singularities in a PT-symmetric system with saturable nonlinearities Xuele Liu, Subhasish Dutta Gupta, Girish Agarwal Spectral response of a linear PT-symmetric system is known to lead to singularities leading to infinite transmission and reflection coefficients. Near such spectral singularities (because of the infinite growth of the amplitudes) the assumption of linearity of the system breaks down, and it is necessary to incorporate a nonlinear mechanism, which could saturate the growth of the scattering amplitudes. We show that an all-order nonlinearity, such as a saturable nonlinearity, can indeed limit the infinities associated with the linear PT-symmetric systems. In the example of a quasi-one dimensional wave-guide with equal loss and gain segments, we demonstrate this regularisation. Our numerical simulation is based on the exact Helmholtz equation with the saturable gain and loss. We further present explicit numerical results to demonstrate optical diode action, whereby, light is allowed to pass only in one preferential direction, and it is blocked for the other direction. The switching and near-perfect isolation of the PT-symmetric device can find many applications in optical signal processing and chip-level integration. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z36.00010: A Clifford Algebra Description of Polarization Optics David Yevick, George Soliman The polarization changes induced by optical components are represented as Clifford algebra transformations. This yields a unified formalism for polarized and partially polarized light and for the frequency dependence of polarization in the presence of polarization mode dispersion and polarization dependent loss. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z36.00011: CO$_{2}$ TEA Laser-Enhanced Laser Ablation Molecular Isotopic Spectrometry (TELLAMIS) Staci R. Brown, Charlemagne A. Akpovo, Alan Ford, Kenley Herbert, Lewis Johnson Recently, it has been shown that the relative abundance of isotopes in enriched materials can be determined via laser-induced breakdown spectroscopy (LIBS) in a technique known as laser-ablation molecular isotopic spectroscopy (LAMIS). The original LAMIS work has focused on single-pulse (SP) LIBS for the excitation. However, dual-pulse (DP) LIBS reduces shot-to-shot variation and can lower detection limits of an element by about an order of magnitude or more. It also has the potential to improve the accuracy of the determination of the relative abundances of isotopes in LAMIS by minimizing the signal-to-noise ratio. In this work, a DP-LIBS technique for improving LAMIS relative-abundance information from a sample is presented. The new technique, called (TEA) Transverse-Excited breakdown in Atmosphere Laser-Enhanced Laser Ablation Molecular Isotopic Spectrometry (TELLAMIS), uses a carbon dioxide (CO$_{2})$ laser to increase the breakdown emission from LIBS in the LAMIS method. This technique is demonstrated on a collection of relative abundance isotopes of boron- 10 and boron-11 in varying concentrations in boric acid. Least-squares fitting to theoretical models are used to deduce plasma parameters and understand reproducibility of results. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z36.00012: Ultracold Cesium source for FIB below 1nm: Milling, deposition, and lithography Ross Martin-Wells We discuss the applications, models, and apparatus for a high-brightness, low-emittance Focused Ion Beam(FIB) source from ultracold cesium atoms. We propose a system where the atoms are cooled in two Magneto-Optic Traps(MOT) and degenerate sideband cooled in an optical lattice, decreasing the temperature to 100-300nK. Temperatures in the nanokelvin regime mean that by adapting current ion lens techniques from an Liquid Metal Ion Source (LMIS) column, our cesium ion beam can be focused to $<$1 nm. The ionization process at these ultracold temperatures is studied using a Monte Carlo simulation, determining the velocity distribution of the ions. After ionization via a two-frequency excitation, the cloud is accelerated by electric fields in the same way as LMIS FIB systems. We also discuss: (1)production of a much higher current continuous wave ultracold atom source for nanofabrication and (2) the use of interferometrically stabilized optical interference masks as a system to print structures of linewidth $<$1nm by the deposition of atoms from a variety of ultracold sources. These tools could make major contributions in the fields of lithography and microscopy in nanofabrication, conducting milling operations as well as deposition and microscopy far below the photon-diffraction limit [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z36.00013: Superpositions of Free Electron Vortices and Measurement of Matter Wave Gouy Phase Benjamin McMorran, Tyler Harvey, Jordan Pierce, Martin Linck We demonstrate superpositions of free electron matter wave orbital states using nanofabricated diffraction holograms. The orbital superposition is comprised of an electron beam that is a coherent mixture of two overlapped, co-propagating vortex beam modes with different topological charge. Whereas a pure mode electron vortex beam forms an annular spot when projected onto an imaging detector, the superposition has an intensity profile that is broken into azimuthal lobes. The number of lobes is given by the absolute difference in topological charge between the two orbital components. We created superpositions of vortices with various topological charges, from $m_{\ell}$ = 0 to 15. We use these superposition states to measure the Gouy phase measurement for matter waves. We discuss the possibility of using these beams to measure magnetic fields. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z36.00014: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z36.00015: Ion Production in the Presence of Isotopes for Various Gasses Benjamin Graber, Rongjia Tao, Dong Ho Wu Gamma radiation creates ions in gasses. Since each gas has unique ionization energy, the positive and negative ion production rate will be dependent on the radiation energy. Each radioactive isotope has a characteristic gamma ray spectra. Combining these ideas, by using a few chambers containing different gasses with ion counters, one can construct a nuclear material detector that can identify an isotope. This experiment was carried out using Ar, CO2, O2, N2, regular air and humid air environments. These environments were exposed to Am, Ba, Co, Cs and Na while the positive and negative ions were counted over time. This procedure confirmed our expected result. This detection method is currently patent pending. [Preview Abstract] |
Session Z37: Carbon Nanofoams and Composites
Sponsoring Units: DCMPChair: David Tomanek, Michigan State University
Room: 705/707
Friday, March 7, 2014 11:15AM - 11:27AM |
Z37.00001: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z37.00002: Dielectric characterization of multi-walled carbon nanotube nanocomposites as a function of ultraviolet exposure at microwave frequency Nathan Orloff, Christian J. Long, Kevin Twedt, Thomas Lam, Jabez McClelland, Jan Obrzut, J. Alexander Liddle We investigate multi-walled carbon nanotube epoxy composites as a function of ultraviolet exposure. As the epoxy is etched away from the composite, we found that the multi-walled carbon nanotubes form a thin conducting layer on the surface. We then characterize the multi-walled carbon nanotube composites by atomic force microscopy, lithium ion microscopy, and microwave cavity perturbation at each value of ultraviolet exposure. We perform our measurements on a set of neat samples made from a stoichiometric mixture bisphenol A epoxy resin and another set that contains a mass fraction of 3.5{\%} multi-walled carbon nanotubes. The samples were then exposed to ultraviolet radiation to etch the surface for different durations of time. At the 7.31 GHz, we measured the permittivity and loss tangent of the unexposed epoxy to be $\varepsilon \quad =$ 2.93$+$/-0.11 and tan$\delta =$ 0.029$+$/-0.002, respectively. The unexposed epoxy with a mass fraction of 3.5{\%} multi-walled carbon nanotubes had a permittivity of $\varepsilon $ $=$ 8.01$+$/-0.48 and loss tangent of tan$\delta =$0.144$+$/-0.011. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z37.00003: Electric and Magnetic Polarizability Tensors of Carbon Nanotubes and Graphene Sheets with Different Morphologies: A Numerical Study Fernando Vargas-Lara, Jack F. Douglas, Ahmed Hassan, Edward Garboczi The addition of Carbon Nanotubes (CNTs) and Graphene Sheets (GSs) in polymeric melts affects the electromagnetic response of the resulting composites. This effect strongly depends on the shape of the CNTs and GSs. In this study, we explore how the morphology of individual CNTs and GSs immerse on a dielectric material is related to their electromagnetic signature. In the microwave region, the wavelength is much larger than the size of the CNTs or GSs. Hence, their electromagnetic response in the far field is determined by the electric polarizability tensor, the magnetic polarizability tensor, and the dielectric properties of the CNTs. To determine these properties, we first generate CNTs and GSs with different morphology via molecular dynamic simulations of coarse-grained models for CNTs and GSs, whose mechanical properties mimic the ones predicted by atomistic simulations and experiments. We, next compute the electric polarizability tensors of the aforementioned objects using the path integrator ZENO. Considering this information as a reference, we calculated the magnetic polarizability tensor using finite element calculations (low frequency 3D COMSOL simulations). We finally report these properties as well as their connection with other shape descriptors. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z37.00004: Local curvature and relative stability of graphitic carbon nanostructures Jie Guan, Zhongqi Jin, David Tomanek We propose a way to estimate the relative stability of graphitic nanostructures including fullerenes, nanotubes and schwarzites using continuum elasticity theory. The key quantity is the local deformation energy with respect to graphene, which we determine by estimating the two principal radii of curvature at each lattice site using the Bertrand-Diquet-Puiseux theorem. We find an impressive level of agreement between strain energies based on local curvature and {\em ab initio} density functional calculations. We demonstrate that our approach correctly determines strain energy differences between nanotubes with different chiral indices (n,m) and zero Gaussian curvature, C$_n$ fullerenes with $20{\le}n{\le}72$ atoms and positive Gaussian curvature, and selected schwarzites with negative Gaussian curvature. In contrast to other methods, our approach correctly determines even the energy differences between different isomers of fullerenes such as C$_{28}$, C$_{36}$ and C$_{38}$. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z37.00005: Static and dynamic response of bucky sponges Theyaraman Ramathasan, Mehmet Karakaya, Ramakrishna Podila, Chiara Daraio, Apparao Rao Here we present the static and dynamic mechanical behavior of a three dimensional, interconnected, carbon nanotube (CNT) based, spongy material termed the bucky sponge. We adopted a facile top-down synthesis approach by judiciously mixing carbon micro-fibers with CNTs to create bucky sponges with controlled porosity and density. Static and dynamic tests were performed using a customized setup based on geometric Moir\'{e} interferometry and high-speed microscopic imaging. In both quasi-static and dynamic experiments, the bucky sponges exhibited highly nonlinear foam-like stress-strain response with hysteretic dissipation. The energy dissipated at 80{\%} compressive strain is in the order of 500 kJ/m$^{\mathrm{3}}$, which is nearly 25 times more than the energy dissipated by commercial foams with similar densities. Dynamic unloading modulus of bucky sponges varies between 25-250 MPa depending on the maximum strain attained and they show exceptional resilience to impact by recovering more than 70{\%} of the deformation. Bucky sponges with tailored microstructure and mechanical properties have the potential to be used in applications requiring impact mitigation, vibration damping, and separating oil from water. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z37.00006: Impact response of helically coiled carbon nanotube foams Chiara Daraio, Mehmet Karakaya, Ramakrishna Podila, Thevaraman Ramathasan, Apparao Rao We examined the dynamic response of helically coiled carbon nanotube (HCCNT) foams (K. Yang et al., \textit{Advanced Materials}~\textbf{20}, 179 (2008)) in an impact testing set up developed in our laboratory, which is based on geometric Moir\'{e} interferometry and high-speed microscopic imaging. Dynamic force and displacement histories were measured during the impact, from which the dynamic constitutive response was obtained. HCCNT foams exhibit nonlinear foam-like dynamic stress-strain response with an exceptional ability to mitigate impact forces and dissipate energy through hysteresis. The time-resolved image sequences obtained using high-speed microscopic imaging showed a progressive deformation in a preferred direction along the thickness of the foam during the impact. We attribute this finding to the inherent density gradient introduced in the foams during the chemical vapor deposition process. Due to their energy dissipative and cushioning characteristics, HCCNT foams can find potential use in impact mitigation, packaging and vibration damping applications. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z37.00007: Energetics of Boron Doping of Carbon Pores Carlos Wexler, Alexander St. John, Matthew Connolly Carbon-based materials show promise, given their light weight, large surface areas and low cost for storage of hydrogen and other gases, e.g., for energy applications. Alas, the interaction of H2 and carbon, 4-5kJ/mol, is insufficient for room-temperature operation. Boron doping of carbon materials could raise the binding energy of H2 to 12-15kJ/mol. The nature of the incorporation of boron into a carbon structure has not been studied so far. In this talk we will address the energetics of boron incorporation into a carbon matrix via adsorption and decomposition of decaborane by first principles calculations. These demonstrate: (a) A strong adsorption of decaborane to carbon (70-80kJ/mol) resulting in easy incorporation of decaborane, sufficient for up to 10-20\% B:C at low decaborane vapour pressures. (b) Identification that boron acts as an electron acceptor when incorporated substitutionally into a graphene-like material, as expected due to its valence. (c) The electrostatic field near the molecule is responsible for ca. 2/3 of the enhancement of the H2-adsorbent interaction in aromatic compounds such as pyrene, coronene and ovalene. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z37.00008: {\em Ab initio} study of novel carbon nanofoam structure as an anode material for Li secondary battery Hanjin Park, Sora Park, Seoung-Hun Kang, Young-Kyun Kwon Using ab inito density functional theory, we investigate the adsorption and diffusion properties of Li atoms on a new carbon nanostructure, which may be used as an anode of Li secondary battery. We focus on a special carbon nanofoam structure consisting of Schwarzite structures with negative Gaussian curvature as core parts, which are interconnected through (4,4) CNT segments. Considering the symmetry of the nanofoam structure, we find various Li adsorption sites exhibiting relatively large binding energies (${\agt}2.00$~eV). Based on these adsorption sites, we identify several diffusion paths on the outside or inside surface of the nanofoam structure and examine the diffusion barriers along the paths. Our results show that Li atom can diffuse almost freely due to its low energy barriers on both outside and inside surfaces. Finally, we also evaluate the energy gain tendency and the volume expansion as well as the average binding energy while adding Li atoms to estimate the Li-capacity and recyclability of the system, which are important characterisitics for anode materials. We conclude that the carbon nanofoam structure would be better as an anode material than graphite in Li capacity and volume expansion. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z37.00009: Unusual conduction mechanism at graphitic carbon foam surfaces: An \textit{ab initio} study David Tomanek, Zhen Zhu, Zacharias G. Fthenakis, Jie Guan Using {\em ab initio} electronic structure and quantum conductance calculations, we identify an unusual conduction mechanism at the surface of a previously described graphitic carbon foam structure. The emergence of a new, topologically protected conduction band in this semiconducting system is intimately linked to the topology of the foam. In contrast to conduction bands of graphitic structures, which are related to nearest-neighbor interactions between $p_\perp$ orbitals normal to the surface, the new band responsible for metallic behavior derives from interactions between $p_\|$ orbitals lying in the surface plane. The conducting surface state occurs on bare and hydrogen-terminated surfaces and is thus unrelated to dangling bonds. We find that the conductance behavior can be further significantly modified by surface patterning. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z37.00010: Thermal conductivity of one-, two- and three-dimensional sp2 carbon Luiz Felipe Pereira, Ivana Savic, Davide Donadio Carbon atoms can form structures in one, two and three dimensions due to their unique chemical versatility. In terms of thermal conductivity, carbon polymorphs cover a wide range from very low values with amorphous carbon to very high values with diamond, carbon nanotubes and graphene. Schwarzites are a class of three-dimensional fully covalent sp2-bonded carbon polymorphs, with the same local chemical environment as graphene and carbon nanotubes, but negative Gaussian curvature. We calculate the thermal conductivity of a (10,0) carbon nanotube, graphene and two schwarzites with different curvature, by molecular dynamics simulations based on the Tersoff empirical potential. We find that schwarzites present a thermal conductivity two orders of magnitude smaller than nanotubes and graphene. The reason for such large difference is explained by anharmonic lattice dynamics calculations, which show that phonon group velocities and mean free paths are much smaller in schwarzites than in nanotubes and graphene. Their reduced thermal conductivity, in addition to tunable electronic properties, indicate that schwarzites could pave the way towards all-carbon thermoelectric technology with high conversion efficiency. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z37.00011: Adsorption-induced breathing in nanoporous carbon Matthew Connolly, Carlos Wexler In most adsorption studies it is assumed that the adsorbent conformation is not changed by the adsorption; this assumption underlies most conceptual and theoretical framework used for characterization of porous materials and of adsorption in general. Recently, the behavior of the solid has come under review: when a gas enters pores with sizes comparable with the range of the van der Walls forces an excess pressure or tension exists. Here we present a theoretical, computational and experimental demonstration of breathing (expansion) of graphene-like adsorbents (graphene oxide frameworks, GOFs): Molecular dynamics simulations show the potential for supercritical hydrogen to open new pores in carbons. Grand Canonical Monte Carlo perturbative calculations demonstrate a reduction of the free energy of strip-shaped pores with gas loading upon a conformational change that increases the net size of micropores. Experimentally, reversible pore expansion during adsorption was measured by x-ray scattering for GOFs. These breathing modes have significant consequences for medium- to high-pressure adsorption, with modified adsorption isotherms that may require re-interpretation of standard models. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z37.00012: Strain effect on electronic properties of low-dimensional $\gamma$-graphyne : first principles study Hyeonsu Lee, Seoung-Hun Kang, Sora Park, Chang-Sun Lee, Young-Kyun Kwon Using first-principles calculations, we study the interplay between structural and electronic properties of $\gamma$-graphyne nanotubes ($\gamma$GNTs) consisting of hexagonal carbon rings and acetylenic linkages. We first identify the equilibrium structures of various $\gamma$GNTs classified in terms of chirality: $(n,0)$ denotes an armchair-type tube, whereas $(n,n)$ does a zigzag-type, in contrast with CNTs. Then their Young's moduli are calcuated to be a few hundreds in GPa, which are smaller than those of CNTs. We verify that all $\gamma$GNTs are intrinsic semicondutors with energy gap ($\alt 1.22$ eV) decreasing with tube diameter. It is, however, found that axial strain can significantly modifies the electronic structures of semiconducting $\gamma$GNTs. Very intriguingly, even semiconductor-metal transition occurs under compressive strain: all armchair $\gamma$GNTs, exept for (3,0) $\gamma$GNT with small diameter, become metallic, while only some types of zigzag $\gamma$GNTs metallic under compression. To explain the origin of such electronic structure modifications, we examine the effect of structural change on the band structures of two-dimensional $\gamma$-graphyne sheet under strains and match them with the band structure of $\gamma$GNTs using the zone-folding scheme. [Preview Abstract] |
Session Z39: Invited Session: Strongly Correlated Electron Systems, Transition Metal Oxides, Vanadates
Sponsoring Units: DCMPChair: Dimitri Basov, University of California, San Diego
Room: Mile High Ballroom 2A-3A
Friday, March 7, 2014 11:15AM - 11:51AM |
Z39.00001: Giant reversible structural and electronic changes in liquid gated epitaxial films of VO2 Invited Speaker: Stuart S Parkin |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z39.00002: Bad Metallic Behavior in Model Hamiltonian Studies and in Transition Metal Oxides Invited Speaker: Gabriel Kotliar We investigate the transport properties of a correlated metal within dynamical mean-field theory. Canonical Fermi liquid behavior emerges only below a very low temperature scale $T_{\mathrm{FL}}$. Surprisingly the quasiparticle scattering rate follows a quadratic temperature dependence up to much higher temperatures and crosses over to saturated behavior around a temperature scale $T_{\mathrm{sat}}$ indicating the existence of ``hidden'' Fermi liquid behavior. The non-Fermi-liquid transport above $T_{\mathrm{FL}}$, in particular the linear-in-$T$ resistivity, is shown to be a result of a strongly temperature dependent band dispersion. We derive simple expressions for the resistivity, Hall angle, thermoelectric power and Nernst coefficient in terms of a temperature dependent renormalized band structure and the quasiparticle scattering rate. We discuss the implications of the results for numerous transition metal oxides and other correlated materials connecting the non Fermi liquid transport with anomalous transfer of spectral weight. \\[4pt] References:\\[0pt] W Xu, K Haule, G Kotliar PRL 11 , 036401 (2013).\\[0pt] X Deng, J Mravlje, M Ferrero, G Kotliar, A Georges PRL 110, 086401 (2013).\\[0pt] B Lazarovits, K Kim, K Haule, G Kotliar, PRB 81, 115115(2010). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 1:03PM |
Z39.00003: Vanadium Dioxide: a reconfigurable disordered metamaterial Invited Speaker: Federico Capasso In VO$_{2}$ thin films, the Insulator-to-Metal transition occurs gradually with increasing temperature: Nanoscale inclusions of the metallic phase emerge in the surrounding insulating-phase VO$_{2}$, which grow and these metallic inclusions are much smaller than the scale of the wavelength at infrared frequencies, and thus VO$_{2}$ can be viewed as a natural, reconfigurable,disordered metamaterial with variable effective optical properties across the phase transition. connect in a percolation process, eventually leading to a fully metallic state at the end of the transition. In Ref. [1], this unique temperature-dependent dispersion of the effective medium was used to demonstrate that a film of VO$_{2}$, with thickness ( $\cong $ 150 nm) much smaller than the wavelength, deposited on sapphire can operate as a temperature tunable absorber; in particular, nearly perfect absorption was achieved at a particular temperature for a narrow range of infrared wavelengths. The reflectivity of such a device varies dramatically and non-monotonically across the phase transition, with the strong absorption feature appearing during an intermediate state of VO$_{2}$ as a result of coupling to an ``ultra-thin-film resonance'' [2]. Since the emissivity of an object is equal to its frequency-dependent absorptivity (Kirchoff's law) such a thin-film VO$_{2}$-sapphire structure is expected to have an emissivity that also depends strongly and non-monotonically on temperature. This structure displays ``perfect'' blackbody-like thermal emissivity over a narrow wavelength range (approximately 40 cm$^{-1})$, surpassing the emissivity of our black-soot reference [3]. We observed large broadband negative differential thermal emittance over a \textgreater 10 C range: Upon heating, the VO$_{\mathrm{2}}$-sapphire structure emits less thermal radiation and appears colder on an infrared camera [3]. Our experimental approach allows for a direct measurement and extraction of the wavelength- and temperature-dependent thermal emittance. Collaborations with M. A. Kats, S. Ramanathan, D. Sharma, R. Blanchard, P. Genevet, J. Lin, S. Zhang, C. Ko, Z. Yang, M. M. Qazilbash, D. N. Basov are gratefully acknowledged.\\[4pt] [1] M. A. Kats et al. Appl. Phys. Lett. 101,221101 (2012).\\[0pt] [2] M. A. Kats, R. Blanchard, P. Genevet, and F. Capasso, Nat. Mater. 12, 20 (2012).\\[0pt] [3] M. A. Kats et al. PRX 3, 041004 (2013). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:39PM |
Z39.00004: The metal-insulator triple point in vanadium dioxide Invited Speaker: David Cobden The metal-insulator transition (MIT) in vanadium dioxide is a candidate for optical and electrical switching applications. However, being a first-order solid-state phase transition makes it challenging to study reproducibly in any detail. The combination of the change in unit cell shape, symmetry reduction, long range of elastic distortion, and latent heat leads to domain structure, hysteresis, and cracking of even the highest quality samples. At the MIT two stable insulating phases (M1 and M2) occur in addition to the metallic phase (R), but their phase stability diagram was poorly known. To establish it precisely we studied single-crystal nanobeams of VO$_{\mathrm{2}}$ in a purpose-built nanomechanical strain apparatus. We were able to measure the transition temperature accurately to be 65.0 $+$- 0.1 $^{\mathrm{o}}$C, to determine the phase boundary slopes, and to detect the intermediate metastable triclinic (T) phase where it is metastable towards M2. We were surprised to find that the transition occurs precisely at the solid-state triple point of the metallic and two insulating phases, a fact that is not explained by existing theories. See J.H. Park et al, Nature 500, 431-4 (August 2013), doi:10.1038/nature12425. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 2:15PM |
Z39.00005: Ultrafast and Ultrasmall Spectroscopy of Phase Transition in VO$_{2}$ Invited Speaker: Mengkun Liu Recent advances in optical spectroscopy facilitate the probing of vibrational and electronic properties of materials with unprecedented spatial and time resolution (down to $\sim$ 10 nanometers and $\sim$ 10-s femtoseconds, respectively). In this talk, we report on ultrafast and ultrasmall aspects of the insulator-to-metal transition (IMT) in a canonical correlated electron material, vanadium dioxide (VO$_{2})$. Using scattering-type scanning near-field optical microscopy (s-SNOM) and spectroscopy (nano-FTIR), we revealed unidirectional conducting stripes in strained VO$_{2}$ films at sub-micrometer scale over a wide temperature range (320K-380K). Investigating the formation of this microscopic stripe state, we resolved the enigma of the macroscopic electronic anisotropy and disentangled three distinct stages of the VO$_{2}$ phase transition [Phys. Rev. Lett. 111 (9), 096602 (2013) and follow-up studies]. Furthermore, with newly developed terahertz (THz) pump THz probe spectroscopy, we demonstrated the first THz-field-induced insulator-to-metal switching experiments. We show that high-field THz pulses can effectively reduce the Coulomb-induced potential barrier for carrier transport and lead to subsequent rapid lattice heating. The fundamental electric-field-switching time of VO$_{2}$ can be in the order of a few picoseconds, with which the direct current measurements are incapable to measure due to instrumental limitations [Nature, 487, 345--348 (2012)]. With these comprehensive studies we offer unique insights into the electron and phonon evolution at fundamental time, energy and length scales. These novel spectroscopic techniques also provide universal methodologies for studying many other classes of transition metal oxides and phase transition materials. [Preview Abstract] |
Session Z40: Invited Session: Unconvential Superconducting Pairing in Heavy Fermion Materials
Sponsoring Units: DCMPChair: Qimiao Si, Rice University
Room: Mile High Ballroom 2B-3B
Friday, March 7, 2014 11:15AM - 11:51AM |
Z40.00001: Visualizing nodal superconductivity and heavy fermion formation in CeCoIn$_{5}$ Invited Speaker: Brian Zhou In solids containing elements with $f-$orbitals, the interaction between $f$-electron spins and those of itinerant electrons leads to the development of low-energy excitations with heavy effective mass. Previously, we used the scanning tunneling microscope (STM) to visualize the scattering of quasiparticles and detect their mass enhancement with the lowering of temperature in the prototypical Ce-115 heavy fermion family. Tunneling into different surface terminations revealed the composite nature of these heavy excitations, arising from the entanglement of conduction and $f$ electrons [1]. Here, by extending our techniques to milli-Kelvin temperature and high magnetic field, we first observe a spectroscopic pseudogap in the tunneling density of states of the heavy quasiparticles both prior to superconductivity and also above the critical field, indicating the development of further correlations from which the unconventional superconducting state arises. Quasiparticle interference (QPI) measurements in the superconducting and normal states demonstrate the onset of strong particle-hole asymmetry in the superconducting state, dissimilar from previous STM QPI studies of gap symmetry. Nevertheless, we can directly pinpoint the d-wave nature of Cooper pairing through visualizing the spatial symmetry of quasi-particle bound states in the vicinity of atomic scale defects [2]. Work done in the collaboration with R. E. Baumbach, J. D. Thompson, E. D. Bauer, and A. Yazdani. Primary financial support from DOE-BES.\\[4pt] [1] P. Aynajian, E. da Silva Neto, et al., Nature, 486, 201 (2012).\\[0pt] [2] B. B. Zhou, S. Misra, et al. Nature Phys., 9, 474 (2013). [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:27PM |
Z40.00002: f-electron mediated Cooper Pairing in CeCoIn$_5$ Invited Speaker: Dirk Morr Recent experimental breakthroughs in scanning tunneling spectroscopy have made it possible to probe how the complex electronic structure of the heavy fermion compound CeCoIn$_5$ evolves with decreasing temperature, eventually leading to the emergence of an unconventional superconducting state [1,2]. Using a recently developed theoretical model for quasi-particle interference (QPI) spectroscopy in heavy fermion materials [3,4], we demonstrate that the experimental QPI data are consistent with a superconducting order parameter of $d_{x^2 y^2}$-symmetry, possessing a complex, multi-band momentum space structure [1]. Furthermore, we show that the unprecedented insight into the complex electronic structure of CeCoIn$_5$ above $T_c$ opens a new path for identifying quantitatively the superconducting pairing potential, arising from the strong antiferromagnetic correlations in the heavy $f$-band [5]. Using this pairing potential to solve the multi-band superconducting gap equations provides us with a series of quantitative predictions for the critical temperature, the momentum space structure of the superconducting gaps, the phase sensitive QPI signature of the $d_{x^2 y^2}$ pairing symmetry, the spin-lattice relaxation rate, and the form of the magnetic ``spinresonance.'' The quantitative agreement between these predictions and the measured properties of superconducting CeCoIn$_5$ provides strong evidence for Cooper pairing being mediated by $f$-electron magnetism. \\[4pt] [1] M. P. Allan, F. Massee, D. K. Morr, J. van Dyke, A.W. Rost, A. P. Mackenzie, C. Petrovic and J. C. Davis, Nature Physics 9, 468 (2013).\\[0pt] [2] B.B. Zhou, S. Misra, E.H. da Silva Neto, P. Aynajian, R.E. Baumbach, J.D. Thompson, E.D. Bauer, and A. Yazdani, Nature Physics, 9, 474 (2013)\\[0pt] [3] F. Parisen~Toldin, J. Figgins, S. Kirchner, and D.K. Morr, Phys. Rev. B 88, 081101(R) (2013) \\[0pt] [4] T. Yuan, J.Figgins, and D.K. Morr, Phys. Rev. B 86, 035129 (2012). \\[0pt] [5] J. van Dyke F. Massee, M. P. Allan, J. C. Davis, C. Petrovic and D. K. Morr, submitted (2013). [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 1:03PM |
Z40.00003: Slicing a Kondo lattice: the quest for exotic superconductivity in artificially engineered Ce-based superlattices Invited Speaker: Yuji Matsuda Condensed matter systems that are both low-dimensional and strongly interacting often exhibit unusual electronic properties, with the high-$T_c$ superconductivity in cuprates and iron pnictides as the most prominent example. A metallic state with the strongest electron correlation is realized in heavy fermion compounds, whose electronic structure is essentially 3D. Recently, by fabricating epitaxial superlattices built of alternating layers of Ce-based heavy-fermion and La- or Yb-based conventional nonmagnetic metals, we have succeeded in confining heavy fermions to two dimensions, resulting in slices of 2D Kondo lattice. In CeIn$_3$/LaIn$_3$ superlattices, 2D heavy fermions display striking deviations from the standard Fermi liquid properties, and these are associated with the dimensional tuning of quantum criticality [1]. Moreover, superconductivity is observed in CeCoIn$_5$/YbCoIn$_5$ superlatttices even in the superlattice with only one-unit-cell-thick CeCoIn$_5$ layers [2]. These superconducting superlattices with atomic layer thickness exhibit highly unusual behaviors, including striking enhancement and highly unusual angular dependence of $H_{c2}$ [3]. We discuss these phenomena in terms of extremely strong coupling superconducting nature as a result of two-dimensionalization, and the entanglement of Pauli paramagnetism and Rashba interaction associated with the local inversion symmetry breaking at the heavy fermion interface. The heavy fermion superlattices offer a new playground for exploring exotic superconducting phases.\\[4pt] In collaboration with M. Shimozawa, S.K. Goh, H. Shishido, Y. Mizukami, T. Watashige, R. Endo, R. Kobayashi, T. Shibauchi and T. Terashima (Kyoto).\\[4pt] [1] H. Shishido {\it et al}. Science {\bf 327}, 980 (2010).\\[0pt] [2] Y. Mizukami {\it et al}. Nature Physics {\bf 7}, 849 (2011).\\[0pt] [3] S. K. Goh {\it et al}. Phys. Rev. Lett. {\bf 109}, 157006 (2012). [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:39PM |
Z40.00004: Nodal quasiparticle dynamics in the heavy fermion superconductor CeCoIn$_5$ revealed by precision microwave spectroscopy Invited Speaker: David Broun CeCoIn$_5$ is a heavy fermion superconductor with strong similarities to the high-$T_c$ cuprates, including quasi-two-dimensionality, proximity to antiferromagnetism and probable $d$-wave pairing arising from a non-Fermi-liquid normal state. Experiments allowing detailed comparisons of the electronic properties of these two types of superconductor are of particular interest, but in most cases are difficult to realize, due to their very different transition temperatures. Here we use low-temperature microwave spectroscopy to study the charge dynamics of the CeCoIn$_5$ superconducting state. The similarities to cuprates, in particular to ultra-clean YBa$_2$Cu$_3$O$_y$, are striking: the frequency and temperature dependence of the quasiparticle conductivity are instantly recognizable, a consequence of rapid suppression of quasiparticle scattering below $T_c$; and penetration-depth data, when properly treated, reveal a clean, linear temperature dependence of the quasiparticle contribution to superfluid density. The measurements also expose key differences, including prominent multiband effects and a temperature-dependent renormalization of the quasiparticle mass. C.J.S. Truncik et al., Nat. Comm. 4, 2477 (2013). [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 2:15PM |
Z40.00005: Charge Aspects of Composite Pair Superconductivity Invited Speaker: Rebecca Flint Conventional Cooper pairs form from well-defined electronic quasiparticles, making the internal structure of the pair irrelevant. However, in the 115 family of superconductors [1-3], the heavy electrons are forming as they pair and the internal pair structure becomes as important as the pairing mechanism. Conventional spin fluctuation mediated pairing cannot capture the direct transition from incoherent local moments to heavy fermion superconductivity, but the formation of composite pairs favored by the two channel Kondo effect can [4]. These composite pairs are local d-wave pairs formed by two conduction electrons in orthogonal Kondo channels screening the same local moment. Composite pairing shares the same symmetries as magnetically mediated pairing, however, only composite pairing necessarily involves a redistribution of charge within the unit cell originating from the internal pair structure, both as a monopole (valence change) and a quadrupole effect [5]. This redistribution will onset sharply at the superconducting transition temperature. A smoking gun test for composite pairing is therefore a sharp signature at Tc - for example, a cusp in the Mossbauer isomer shift in NpPd$_5$Al$_2$ or in the NQR shift in (Ce,Pu)CoIn$_5$. \\[4pt] [1] J. L.Sarrao and J.D. Thompson, JPSJ 76, 051013(2007).\\[0pt] [2] E. D. Bauer et al. J. Phys Cond. Mat. 24, 052206 (2012).\\[0pt] [3] D. Aoki et al, JPSJ 76, 063701 (2008).\\[0pt] [4] R. Flint, M. Dzero and P. Coleman, Nat. Phys. 4, 643 (2008).\\[0pt] [5] R. Flint, A. Nevidomskyy and P. Coleman, PRB 84, 064514 (2011). [Preview Abstract] |
Session Z46: Theory of Majorana States in Superconductors
Chair: Greg Boyd, Georgetown UniversityRoom: Mile High Ballroom 4E
Friday, March 7, 2014 11:15AM - 11:27AM |
Z46.00001: Majorana surface and vortex states in three dimensional nodal noncentrosymmetric superconductors Po-Yao Chang, Shunji Matsuura, Andreas Schnyder, Shinsei Ryu We investigate Majorana surface and vortex states in three dimensional noncentrosymmetric superconductors (NCSs) that have antisymmetric spin-orbit coupling and exhibit an admixture of spin singlet and triplet superconducting pairings. By exact diagonalization of Bogoliubov-de Gennes Hamiltonians, we show different scenarios of Majorana surface and vortex states, which coexist with the surface flat bands originated from the nodal rings in the bulk: (i) there are no additional surface and vortex states; (ii) there are a Fermi arc on the surface and a flat band localized at the core of a vortex line; (iii) there are a Majorna cone state protected by a ${Z}_2$ topological invariant on the surface and a helical state localized at the core of a vortex line. By turning off the singlet superconducting pairing, these three different scenarios adiabatically connect to a trivial gapped superconductor, a nodal superconductor with two nodal points (a superconducting and time-reversal symmetric analogue of Weyl semimetal), and a fully gapped topological superconductor, respectively. The latter indicates NCSs can share interesting topological properties with fully gapped topological superconductors and widens the possibility of searching physics of topological superconductors in NCSs. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z46.00002: Spin texture of topological superconductor surface states Philip Brydon, Andreas Schnyder, Carsten Timm Noncentrosymmetric superconductors (NCS), where singlet and triplet pairing coexist, are examples of topologically non-trivial gapless systems, and hence display nondegenerate flat-band surface states at certain surfaces [1]. Using quasiclassical methods, we construct the edge-state wavefunctions and calculate the edge-state spectra [2]. We show that the edge states, in particular the flat bands, generically show strong spin polarization which is odd in the surface momentum [3]. The spin polarization is mostly fixed by the spin-orbit coupling, but also depends upon the relative strength of singlet to triplet pairing. Not only does the spin polarization suppress the effect of impurity scattering, but it also is responsible for strong edge currents at interfaces with ferromagnets [3]. [1] A. P. Schnyder and S. Ryu, Phys. Rev. B {\bf 84}, 060504(R) (2011). [2] P. M. R. Brydon, A. P. Schnyder, and C. Timm, Phys. Rev. B {\bf 84}, 020501(R) (2011); A. P. Schnyder, P. M. R. Brydon, and C. Timm, Phys. Rev. B {\bf 85}, 024522 (2012). [3] A. P. Schnyder, C. Timm, and P. M. R. Brydon, Phys. Rev. Lett. {\bf 111}, 077001 (2013). [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z46.00003: Magnetic Field Response and Chiral Symmetry of Time Reversal Invariant Topological Superconductors Eugen Dumitrescu, Jay D. Sau, Sumtanta Tewari We study the magnetic ?eld response of the Majorana Kramers pairs of a one-dimensional time-reversal invariant (TRI) superconductors (class DIII) with or without a coexisting chirality symmetry. For unbroken TR and chirality invariance the parameter regimes for nontrivial values of the ($Z_2$) DIII-invariant and the ($Z$) BDI chiral invariant coincide. However, broken TR may or may not be accompanied by broken chirality, and if chiral symmetry is unbroken the pair of Majorana fermions (MFs) at a given end survives the loss of TR symmetry in an entire plane perpendicular to the spin-orbit coupling field. Conversely, we show that broken chirality may or may not be accompanied by broken TR, and if TR is unbroken, the pair of MFs survives the loss of broken chirality. In addition to explaining the anomalous magnetic field response of all the DIII class TS systems proposed in the literature, we provide a realistic route to engineer a ``true'' TR-invariant TS, whose pair of MFs at each end is split by an applied Zeeman field in arbitrary direction. We also prove that, quite generally, the splitting of the MFs by TR-breaking fields in TRI superconductors is highly anisotropic in spin space, even in the absence of the topological chiral symmetry. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z46.00004: Planar tunneling spectroscopy of topological insulators and superconductors Wan Kyu Park, C. Jones, L. Sun, M. Worek, R. Tapping, L.H. Greene, J. Schneeloch, R.D. Zhong, Z.J. Xu, G. Gu Tunneling spectroscopy has been widely adopted for the study of electronic density of states. Using this technique, we investigate topological surface states and superconducting proximity effect in topological insulators and superconductors. Planar tunnel junctions are prepared via sputter deposition of Nb and/or AlO$_{\mathrm{x}}$ tunnel barrier onto cleaved or polished ({\&} ion-beam cleaned) surfaces of these materials. Interplay between superconducting pair potential in Nb and spin-momentum locking in the surface states of (Bi,Sb)$_{2}$Se$_{3}$, a confirmed topological insulator, is studied as a function of the Nb layer thickness. Our results have shown that tunneling conductance does not reveal any new features down to 150 {\AA} of Nb. Further investigations are under way to clarify the influence of interface cleanliness as well as the location of the chemical potential. Measurements of tunneling conductance into the surface states of some known and candidate topological insulators will also be discussed. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z46.00005: BDI Class Topological Superconductors and Generating Correlated Spin Currents in Quantum Anomalous Hall insulators James He, Jiansheng Wu, Ting-Pong Choy, Xiong-Jun Liu, Y. Tanaka, K.T. Law In this work, we show that a one dimensional AIII class topological insulator, which supports fermionic end states, can be turned into a BDI class topological superconductor (TS) through proximity effect. The resulting BDI TS has two topological phases with one or two Majorana end states at each end of the wire respectively. Interestingly, in the phase with two Majorana end states, the BDI TS causes zero-bias conductance dips in tunneling spectroscopy experiments due to destructive interference of Andreev reflection amplitudes caused by the two Majorana end states. More importantly, this BDI TS can induces resonant crossed Andreev reflections in a normal lead/BDI TS/ normal lead junction, in which an electron from one lead is reflected as a hole in the other lead with probability of unity. Moreover, we show that the currents in the two normal leads are perfectly correlated and spin polarized with opposite spin-polarization. Therefore, BDI TS can be used to generate correlated spin currents. We demonstrate that a quantum Anomalous Hall insulator in proximity to a superconductor can be used to realize the proposed BDI TS. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z46.00006: Majorana Fermion induced Selective Equal Spin Andreev Reflections Kam Tuen Law, James He, Tai Kai Ng, Patrick Lee It is known that a Majorana fermion end state of a topological superconductor can induce resonant Andreev reflections at a normal lead/topological superconductor junction. However, the details of the Andreev reflection processes have not been studied before. Surprisingly, in this work, we show that Majorana fermions induce a special type of Andreev reflections we call \textit{selective equal spin Andreev reflections }(SESARs). In SESAR processes, incoming electrons with certain spin polarization in the normal lead are reflected as counter-propagating holes with the same spin. More importantly, the spin polarization direction of the electrons, which can undergo Andreev reflections, is selected by the Majorana fermion end state. On the contrary, electrons with opposite spin polarization are always reflected as electrons with unchanged spin and they cannot undergo Andreev reflections. Due to SESARs, the current in the normal lead is spin-polarized. Therefore, a topological superconductor, which supports Majorana fermions, can be used as a novel device to create fully spin-polarized currents in paramagnetic leads. We point out that SESARs can also be used to detect Majorana fermions in topological superconductors. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z46.00007: Realizing Majorana zero mode by Proximity Effect between Topological Insulator and d-wave Superconductor Zixiang Li, Cheung Chan, Hong Yao We study the proximity effect between a topological insulator (TI) and a d-wave superconductor systematically. We find that because of the difference of lattice structures between the topological insulator and the d-wave superconductor, a finite s-wave component, coexisting with d-wave component, in superconducting pairing emerges in the surface states of topological insulators. Moreover, we show that disorder has significant effects on suppressing d-wave pairing and enhancing s-wave pairing in topological insulators. This result qualitatively explains the recent experimental work, which reports the nearly isotropic pairing gap on the TI's surface states induced by proximity with a d-wave superconductor [Nature Physics 9, 621-625 (2013)]. Finally, we consider the system of TI and d-wave superconductor in presence of vortices and find evidences of Majorana zero mode in the vortex core, which may be detectable in future experiments. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z46.00008: Zero-bias peak and soft superconducting gap in differential conductance calculations of semiconductor-based Majorana nanostructures John Stenger, Tudor Stanescu Recent experiments on semiconductor wire-superconductor hybrid structures aiming to realize and detect zero-energy Majorana bound states have revealed the presence of a substantial sub-gap conductance, in addition to the predicted zero-bias anomaly that appears, as expected, above a certain critical magnetic field. The origin of this soft gap is controversial and remains highly problematic. It has recently been suggested [1] that the coupling of the semiconductor nanowire to a large normal-metal lead is, in fact, the reason for the soft gap. Here, we confirm this mechanism by explicitly calculating the differential conductance of a normal metal -- semiconductor wire -- superconductor hybrid structure and we discuss the dependence of the soft gap on the relevant system parameters. \\[4pt] [1] Tudor D. Stanescu, Roman M. Lutchyn, and S. Das Sarma, arXiv:1311.2075 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z46.00009: Local Adiabatic Mixing of Majorana Kramers pairs in DIII wires Konrad W\"{o}lms, Ady Stern, Karsten Flensberg We consider coherence of localized Kramers pairs of Majorana fermions in a DIII topological superconductors and show that they get mixed by adiabatic processes, even if the instantaneous Hamiltonian stays in the DIII class at each instant of time. In particular, we compute the associated Berry curvature analytically for an illustrative toy model as well as numerically for a simple DIII wire model and discuss the general conditions for the mixing to be finite. The mixing occurs for a wide range of perturbations, for example even by electrical noise alone. Our calculation thus shows that quantum information stored in time-reversal symmetric Majorana Kramers pairs are not topologically protected. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z46.00010: Experimental search for Majorana fermions in chains of magnetic atoms on a superconductor Stevan Nadj-Perge, Ilya Drozdov, Sangjun Jeon, Jungpil Seo, Andrei Bernevig, Ali Yazdani The ongoing search for Majorana fermions (MF) is currently hindered by various disorder effects which can mimic signatures of MF modes. To overcome this problem, cleaner systems are needed in which MF modes can be readily distinguished from disorder induced effects. In this talk I will present novel experimental approach to realize MF modes in chains of magnetic atoms on the surface of an s-wave superconductor. Our experimental efforts are motivated by model calculations which show that such chains can support topological superconductivity with MF end modes [1]. Surprisingly, even short chains consisting of tens of atoms can host well resolved Majorana modes under suitable conditions depending on the relative spin orientations of adjacent atoms. We realize magnetic chains using self-assembled growth technique and probe their electronic structure using scanning tunneling microscopy. Results from spatially resolved spectroscopic mapping reveal zero energy modes at the chain ends, consistent with the existence of MF modes in this system. [1] S. Nadj-Perge, I. K. Drozdov, B. A. Bernevig., Ali Yazdani, Phys. Rev. B 88, 020407(R) (2013). [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z46.00011: Majorana bound states in two-channel time-reversal-symmetric nanowire systems Erikas Gaidamauskas, Jens Paaske, Karsten Flensberg In this work consider time-reversal-symmetric two-channel semiconducting nanowires proximity coupled to the s-wave superconductor. We made an analysis for the conditions for a topologicaly non-trivial phase, and find that necessary requirements are 1) the determinant of the pairing matrix in channel space must be negative, 2) spatial inversion symmetry must be broken, and 3) the two channels must have different spin-orbit couplings. The first condition can be realized in semiconducting nanowire systems with different tunnel couplings between the channels and superconductor, while the parity can be broken by tuning the chemical potentials of the channels. For the case of parallel spin-orbit directions, we derive the expression for the topological invariant applying the block diagonalization of the Hamiltonian into the two chiral symmetric blocks. Making the projection to the low-energy sector we solve for the bound states explicitly and investigate the influence of the magnetic field. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z46.00012: Tunneling spectroscopy of a spiral Luttinger liquid in contact with superconductors Dong E. Liu, Alex Levchenko One-dimensional wires with Rashba spin-orbit coupling, magnetic field, and strong electron-electron interactions are described by a spiral Luttinger liquid model. We develop a theory to investigate the tunneling density of states into a spiral Luttinger liquid in contact with superconductors at its two ends. This approach provides a way to disentangle the delicate interplay between superconducting correlations and strong electron interactions. If the wire-superconductor boundary is dominated by Andreev reflection, we find that in the vicinity of the interface the zero-bias tunneling anomaly reveals a power law enhancement with the unusual exponent. This zero-bias due to Andreev reflections may coexist and thus mask possible peak due to Majorana bound states. Far away from the interface strong correlations inherent to the Luttinger liquid prevail and restore conventional suppression of the tunneling density of states at the Fermi level, which acquires a Friedel-like oscillatory envelope with the period renormalized by the strength of the interaction. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z46.00013: Competitive pairing effects on interacting fermions: superconductivity, topology, and entanglement entropy Jiansheng Wu, Ching-Kai Chiu, Kuei Sun, Hsiang-Hsuan Hung We study spin-half fermions in one-dimensional chain lattices in which we identify three triplet and one singlet channels independently tunable for the Cooper pairing. We conduct a comprehensive analysis on the model, including a mean-field treatment on a large-size case and an exact-diagonalization method on a finite-size case. We find that the competitive pairing effects can lead to singlet, triplet and mixed superconducting states in both cases. The mean-field Hamiltonian has $Z_2$ topological invariant for symmetry class DIII determined by these pairings. The tunability of our model makes it particularly interesting for studying the interaction effects on one-dimensional topological superconductors in search for Majorana fermions. Furthermore, we consider a two-dimensional interacting model, the mean-field Hamiltonian of which corresponds to a topological superconductors. Its topological order can be confirmed by length-independent entanglement entropy. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z46.00014: Majorana states in helical Shiba chains and ladders Teemu Ojanen, Kim Poyhonen, Alex Westsrom, Joel Rontynen Motivated by recent proposals to realize Majorana bound states in chains and arrays of magnetic atoms deposited on top of a superconductor, we study the topological properties of various chain structures, ladders and two-dimensional arrangements exhibiting magnetic helices. We show that magnetic domain walls where the chirality of a magnetic helix is inverted support two protected Majorana states giving rise to a tunneling conductance peak twice the height of a single Majorana state. Multiple overlapping Majorana states are protected by chiral symmetry which is present in systems exhibiting planar magnetic textures. Thus the topological properties of coupled chains exhibit nontrivial behaviour as a function of the number of chains beyond the even-odd dichotomy expected from ${Z}_2$ classification. In addition, it is possible that a ladder of two or more coupled chains exhibit Majorana edge states even when decoupled chains are trivial. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z46.00015: Superconductor with intrinsic topological order induced by Coloumb repulsion Evelyn Tang, Xiao-Gang Wen We study a lattice system which at commensurate fillings supports fractional quantum Hall states; here we explore what happens at incommensurate fillings. As excitations are believed to be anyons, we assume that doping the system creates a finite density of anyon excitations. The presence of a lattice allows access to a new regime in which the anyon kinetic energy dominates. This leads to a gas of anyons which can condense to form a charged superfluid, driven by repulsive interactions and time-reversal symmetry breaking. We find ground states including those with intrinsic topological order, i.e. containing fractionalized quasiparticles. The relative stability of these states are compared using different flux-attachment approaches; lastly we discuss their physical properties and methods for experimental detection. [Preview Abstract] |
Session Z47: Superconductivity: Pseudogap, Fluctuations, Anisotropy
Chair: Werner Hanke, University of WuerzburgRoom: Mile High Ballroom 4F
Friday, March 7, 2014 11:15AM - 11:27AM |
Z47.00001: Superconductivity in Sr$_2$RuO$_4$: Two-Dimensional versus One-Dimensional Origin of Pairing Werner Hanke, Christian Platt, Ronny Thomale There is growing experimental evidence that Sr$_2$RuO$_4$ displays an unconventional superconducting (SC) state with a chiral p-wave, i.e. ``p+ip'' symmetry. The continuing strong interest in this superconductor has recently been fuelled by speculations of the possible realization of topologically protected edge modes. However, there exists also a variety of challenges for the chiral p-wave state, such as a power-law behavior in the specific heat, no clear observation of edge currents and conflicting pictures for the pairing mechanism (2D versus 1D). In this contribution, we discuss our recent theory of SC in a 3-orbital model of Sr$_2$RuO$_4$ [1], and extensions thereof, in light of these challenges, in particular, of recent tunneling spectroscopy data, which were interpreted as evidence for a quasi-1D origin for SC [2].\\[4pt] [1] Q. H. Wang, C. Platt, Y. Yang, C. Honerkamp, F. C. Zhang, W. Hanke, T. M. Rice, R. Thomale, Eur. Phys. Lett. 104, 17013 (2013).\\[0pt] [2] I.A. Firmo, S. Lederer, C. Lupien, A. P. Mackenzie, J. C. Davis, and S. A. Kivelson, Phys. Rev. B 88, 134521 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z47.00002: RG Approach to Superconductivity in Sr$_2$RuO$_4$ : Multi-Band and Spin-Orbit Effects Thomas Scaffidi, Jesper Romers, Steven Simon We study the superconductivity pairing mechanism in Sr$_2$RuO$_4$ in the limit of small interaction by extending a renormalization group (RG) calculation developed by Raghu et al (Phys Rev B.81.224505) to include multiband and spin-orbit coupling (SOC) effects. We show these effects to be crucial to discriminate between the possible order parameters. In contrast to the usual theory of an ``active'' gamma band with a large superconducting gap and ``passive'' alpha and beta bands with smaller gaps induced by a proximity effect, we obtain pseudo-spin triplet gaps of the same order of magnitude on all three bands for a large range of interaction parameters. The inclusion of SOC in the microscopic model allows us to study ab initio the breaking of degeneracy between the different d vector orientations. Implications for experiments will be discussed. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z47.00003: Fluctuation exchange analysis of quasiparticle properties of strontium ruthenate John Deisz, Tim Kidd We utilize a weak-coupling approximation, FLEX, to analyze quasiparticle properties of strontium ruthenate. Utilizing first-principles-derived parameters for the band structure, spin-orbit interaction and Coulomb and exchange interactions, we quantitatively reproduce the band-structure renormalization and linewidths observed in photoemission. However, the electronic specific heat coefficient is underestimated by a factor of two and superconducting $T_c$ is overestimated by 30 K. Analysis of of the band and momentum-resolved self-energy reveals an essentially momentum-independent, Fermi-liquid-like self-energy with the exception of the $\gamma$ band along the $\Gamma$-M cut for which non-Fermi-liquid behavior is observed. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z47.00004: Manipulating superconducting tendency in ruthenates through Fermi surface engineering Yi-Ting Hsu, Jian-Huang She, Bulat Burganov, Carolina Adamo, Darrell Schlom, Kyle Shen, Eun-Ah Kim $\rm{Sr_2RuO_4}$ is the leading material candidate for topological triplet superconductivity yet its low transition temperature ($T_c$) limits experimental investigation of the system. One of the leading proposals for the mechanism of the observed superconductivity is the one dimensional band driven superconductivity mediated by antiferromagnetic fluctuations. Within this proposal a perturbative RG approach on a microscopic model with purely repulsive interactions yielded dominant triplet pairing tendency. In this approach the fermiology plays a key role in tilting the balance among different pairing possibilities and the superconducting $T_c$. This implies one can manipulate superconductivity through Fermi surface engineering. Motivated by the recent experimental advance in the growth of $\rm{Ba_2RuO_4}$ films where an isovalent substitution of $\rm{Sr^{2+}}$ by $\rm{Ba^{2+}}$ produces negative chemical pressure, we investigate how the resulting changes in Fermi surface affect superconducting instability using the perturbative RG approach. We take the band structure fitted to Fermi surface measured using angle resolved photoemission spectroscopy as our input. We then compare the results to known effects of hydrostatic pressure. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z47.00005: The collective mode spectrum and electromagnetic response of spin-triplet models for the superconducting Sr$_2$RuO$_4$ Hao Wu, Suk-Bum Chung, James Sauls Collective modes in unconventional superconductors provide spectroscopic signatures of the broken symmetries of the superconducting ground state. We consider the effects of Fermi surface and pairing anisotropy on the mode energies and selection rules for the coupling of the order parameter collective modes to an electromagnetic field for spin-triplet pairing models of the superconducting phase of Sr$_2$RuO$_4$. We show that the well known doubly degenerate clapping modes with frequency $\sqrt{2} \Delta$ are split by anisotropy. We report calculations of the mode frequencies and their damping by pairbreaking based on our microscopic pairing model for Sr$_2$RuO$_4$. We report results for the current response to in plane electromagnetic field from both the single particle and collective mode excitations. The clapping modes give strong absorption peaks at frequencies below the maximal pair-breaking energy of $2\Delta$. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z47.00006: Evidence from tunneling spectroscopy for a quasi-one-dimensional origin of superconductivity in Sr$_2$RuO$_4$ S. Lederer, I.A. Firmo, C. Lupien, A.P. Mackenzie, J.C. Davis, S.A. Kivelson To establish the mechanism of unconventional superconductivity in Sr$_2$RuO$_4$, a prerequisite is direct information concering the momentum-space structure of the energy gaps $\Delta_i(k)$, and in particular whether the pairing strength is stronger (``dominant'') on the quasi-one-dimensional ($\alpha$ and $\beta$) or on the quasi-two-dimensional ($\gamma$) Fermi surfaces. We present scanning tunneling spectroscopy measurements of the density of states spectra in the superconducting state of Sr$_2$RuO$_4$ for $0.1 T_c |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z47.00007: Nodal Behavior of the Superconducting State in Sr${_2}$RuO${_4}$ S.J. Kuhn, M.R. Eskildsen, C. Rastovski, C.D. Dewhurst, J. Gavilano, Y. Maeno Multiple experimental and theoretical studies provide compelling support for triplet pairing of electrons and an odd, $p$-wave order parameter symmetry in superconducting Sr$_2$RuO$_4$. However, seemingly contradictory experimental results have left important questions concerning the detailed structure and coupling of the orbital and spin parts of the order parameter in this compound unresolved. The nodal behavior constrains the possibilities for the order parameter. We have used small-angle neutron scattering (SANS) to study the scattering intensity of the vortex lattice (VL) in Sr$_2$RuO$_4$, with $H$ along the [110] crystalline direction. Because the VL anisotropy in Sr$_2$RuO$_4$ is large [C. Rastovski $et$ $al.$, Phys. Rev. Lett. {\bf 111}, 087003 (2013)], this measurement effectively determines the current along the field direction, and hence the penetration depth along [1$\bar{1}$0]. The intensity vs. temperature curve is linear for $T/T{_c}$ of 0.25-0.75, but flattens out as $T$ approaches 0. This is consistent with a gap mode in the [110] direction, coupled with non-local effects. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z47.00008: Competition Between Antiferromagnetism and Ferromagnetism in Sr$_{2}$RuO$_{4}$ Probed by Mn and Co Doping Zhiqiang Mao, John Ortmann, Jinyu Liu, Jin Hu, M. Zhu, Jin Peng, M. Matsuda, Xianglin Ke Spin-triplet superconductivity in Sr$_{2}$RuO$_{4}$ has attracted enormous interest. Like other unconventional superconductors, superconductivity in Sr$_{2}$RuO$_{4}$ is in close proximity to magnetic instability. Undoped Sr$_{2}$RuO$_{4}$ exhibits incommensurate antiferromagnetic (AFM) fluctuations, which can evolve into static, short-range AFM order via Ti doping. Moreover, weak ferromagnetic (FM) coupling in Sr$_{2}$RuO$_{4}$ has also been suggested by NMR/neutron scattering experiments and studies on Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ and Sr$_{2-y}$La$_{y}$RuO$_{4}$. In this talk, we will report bulk static, short-range FM order in Sr$_{2}$RuO$_{4}$ triggered by \textless 2{\%} Co doping, showing superconductivity in Sr$_{2}$RuO$_{4}$ is much closer to FM instability than previously reported in Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. We also find Mn doping can effectively establish incommensurate AFM order, with $T_{N}$ $\sim$ 50 K for 3{\%} Mn doping. These new results highlight the important role of competing magnetic fluctuations in determining superconducting properties of Sr$_{2}$RuO$_{4}$[1]. \\[4pt] [1] Ortmann et al., Scientific Report 3, 2950 (2013). [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z47.00009: Anomalous Knight Shift properties in a variety of Superconductors Bianca Hall, Richard Klemm Anomalous Knight shift properties have been found in a variety of superconductors below the superconducting transition temperature. In most instances this anomaly is an observed Knight shift along one axis but no observed Knight shift perpendicular to this axis (usually the c-axis). Materials that show this anomaly include LiFeAs, UPt3, CeTIn5 (T$=$Co and Ir), Tl2Ba2CuO6$+$y, and YBa2Cu3O7-$\delta $. Another anomaly occurs in the material Na0.35CoO2$\cdot$1.3H2O, where NMR data for the Knight shift done on 59Co and 23Na do not agree. Additionally, NMR data of FeSe show no shift, which is contrary to the material's s-wave pairing superconductivity. Finally, Sr2RuO4 has a constant Knight shift, which is contrary to upper critical field measurements and superconducting gap measurements. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z47.00010: Observation of unconventional Little-Parks resistance oscillations in mesoscopic rings of Sr$_2$RuO$_4$ Xinxin Cai, Yiqun Ying, David Fobes, Tijiang Liu, Zhiqiang Mao, Ying Liu Spin-triplet superconductor Sr$_2$RuO$_4$ may host half-flux-quantum (h/4e) states, which carry Majorana modes useful for topological quantum computing. Recent cantilever magnetometry measurements on micron-size samples of Sr$_2$RuO$_4$ in the presence of an in-plane magnetic field revealed unusual magnetization steps suggesting the presence of both integer and half-integer flux quanta. To observe h/4e resistance oscillations that will not only provide independent confirmation of the existence of the half-flux-quantum states but also provide insights into the physical origin of this novel phenomenon, we fabricated mesoscopic superconducting rings of Sr$_2$RuO$_4$ and carried out Little-Parks resistance oscillation measurements. Without the application of an in-plane field, resistance oscillations with a full-flux period (h/2e) and a large amplitude were observed. A pronounced second set of resistance peaks was found in one sample when the in-plane field and the measurement current were sufficiently large. This sample featured a large critical current density of $\sim 10^5\mathrm{A/cm^2}$, which we believe is of significance. Preparation and measurements of more samples are currently underway. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z47.00011: Unconventional Magnetoresistance Oscillations in Sr$_2$RuO$_4$ Kevin Roberts, Victor Vakaryuk Unconventional quantum oscillations have been detected in Sr$_2$RuO$_4$ which cannot be explained by the traditional Little-Parks effect [1]. To gain insight into the problem we use the theory of oscillatory magnetoresistance induced by thermally excited vortex transitions [2]. We numerically obtain energy barriers for vortex entry, calculate the resulting magnetoresistance, and compare our results with experimental data.$\\ \\$ [1] X. Cai, Y.A. Ying, N.E. Staley, Y. Xin, D. Fobes, T.J. Liu, Z.Q. Mao, and Y. Liu, Phys. Rev. B, $\textbf{87}$, 081104(R) (2013)$\\$ [2] I. Sochnikov, A. Shaulov, Y. Yeshurun, G. Logvenov, and I. Bozovic, Phys. Rev. B, $\textbf{82}$, 094513 (2010) [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z47.00012: Non-topological nature of the edge current in a chiral p-wave superconductor Wen Huang, Edward Taylor, Samuel Lederer, Srinivas Raghu, Catherine Kallin Influential work by Volovik and others (see e.g., [1] and [2]) has tried to establish an analog of the quantum Hall effect in chiral superconductors subject to a spatially varying chemical potential. It is further argued that this quantized static nonlocal Hall conductivity (taking the frequency to zero before the momentum) leads to a topological edge current insofar as the edge can be thought of as a spatially varying chemical potential. Using Bogoliubov-de Gennes calculations as well as diagrammatic calculations of the Hall conductivity using exact Ward identities, we find this analogy breaks down in two respects. First, we show that for a rapidly varying chemical potential, such as would arise at the edge, there can be significant deviations from what is expected from a linear response formulation of the problem. Second, even for a slowly varying chemical potential, we show that the quantization of the Hall current is violated for non-Galilean invariant systems. In contrast to the quantum Hall effect in topological insulators, U(1) symmetry breaking in topological superconductors eliminates the possibility of a topological current response. [1] G. E. Volovik, Sov. Phys. JETP 67, 1804 (1988). [2] J. Goryo and K. Ishikawa, Phys. Lett. A 246, 549 (1998). [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z47.00013: Parametric suppression of edge currents in Sr$_2$RuO$_4$ due to surface roughness Srinivas Raghu, Samuel Lederer, Wen Huang, Edward Taylor, Catherine Kallin The unconventional superconductor Sr$_2$RuO$_4$ is widely believed to have chiral p-wave symmetry, which requires spontaneous charge currents (of some magnitude) at sample edges. However, scanning magnetometry experiments[1], have set an upper bound on currents at least two orders of magnitude smaller than original theoretical predictions[2]. We propose that the currents are suppressed due to surface roughness, which quenches superconductivity within approximately a coherence length of the edge. The system can then be modeled as a normal metal in contact with a chiral p-wave superconductor. We justify this model with spectroscopic evidence from tunneling conductance[3], and calculate the edge current in the model. The current is suppressed by a factor on the order of $\Delta_0/E_F$, which suffices to resolve the seeming contradiction between chiral p-wave pairing and undetectable edge currents. \\ \\ 1. C. W. Hicks, et al. Phys. Rev. B 81, 214501 (2010). \\ 2. M. Matsumoto and M. Sigrist, J. Phys. Soc. Jpn. 68, 994 (1999). \\ 3. S. Kashiwaya et al. Phys. Rev. Lett. 107, 077003 (2011). [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z47.00014: Strongly correlated Dirac electrons and f-wave superconductivity in Ga- herbertsmithite Igor Mazin, Harald Jeschke, Frank Lechermann, Hunpyo Lee, Mario Fink, Ronny Thomale, Roser Valent\'I Herbertsmithite ZnCu$_3$(OH)$_6$C$_{12}$ is essentially the only real-world realization of the ideal single-orbital Kagome model. Being half-doping, it is a Mott insulator. In the nn $p-d$ TB model, it maps exactly onto a single $s$-orbital Kagome Hamiltonian, in particular, exhibits topologically protected Dirac points (DP) at the 4/3 doping. We propose to achieve this doping by substituting Ga for Zn. Such Ga-herbertsmithite (GHS) would be a rare example of a material with strongly correlated Dirac electrons at symmetry-protected locations in the Brillouin zone. We have investigated GHS by means of DFT, TB-DCA and the Slave Bosons approaches and searched for Mott and/or charge order instabilities, and found that it remains metallic and uniform, retaining the DPs. Such a metal with strongly correlated DP electrons would have rather unique topological, magnetic and transport properties. In particular, we show analytically and using fRG that when back-doped with Zn, GHS would harbor unconventional spin-fluctuation driven superconductivity which by symmetry must be $f$-wave of the $+-+-+-$ type. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z47.00015: Physical Properties of Modified Compositions of Strontium Ruthenates Armen Gulian, Vahan Nikoghosyan We performed systematic research on ceramic materials Sr2RuO4 with Sulfur, Selenium or Tellurium added, in combination with other dopants such as: Au, Pt, Al, Zn, Mn, Ba, Na, Ca, Os, Co, Ni, Fe, and Ir. Data on resistive, magnetic, structural, compositional, morphological and other physical properties are obtained, and the most interesting results are presented, as well as corresponding synthesis conditions. [Preview Abstract] |
Session Z48: Graphene Transport: Role of Defects and Interfaces
Sponsoring Units: DCMPChair: Masahiro Ishigami, University of Central Florida
Room: Mile High Ballroom 1A-1B
Friday, March 7, 2014 11:15AM - 11:27AM |
Z48.00001: Negative correlation between charge carrier density and mobility fluctuations in graphene Jie Pan, Jianming Lu, Ping Sheng By carrying out simultaneous longitudinal and Hall measurements in graphene, we find that the 1/f noise for the charge carrier density is negatively correlated to that of mobility, with a governing behavior that differs significantly from the relation between their mean values. The correlation in the noise data can be quantitatively explained by a single parameter theory whose underlying physics is the trapping and de-trapping of the fluctuating charge carriers by the oppositely charged Coulomb scattering centers. This can alter the effective density of long-range scattering centers in a transient manner, with the consequent fluctuating effect on the mobility. The longitudinal noise turns out to be dominated by the remaining component of the mobility fluctuations, and display no correlation to the Hall noise. Due to the negative correlation between charge carrier density and mobility fluctuations, the normalized PSD is smaller than that of the Hall noise. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z48.00002: Current densities due to electron-hole puddles in graphene flakes at the charge neutrality point Leandro Lima, Caio Lewenkopf Graphene flakes show a typical conductivity minimum of about $e^2/h$, almost independent of sample mobility, at the charge neutrality point. This is at odds with the notion that as the mobility increases, and graphene becomes more ballistic, its density of states (DOS) and conductivity at the charge neutrality point should vanish. The observed conductivity minimum is often attributed to the presence of electron-hole charge puddles, that give rise to an effective local-dependent chemical potential. In this way, the local chemical potential fluctuates creating p and n-doped regions and the electronic transport is facilitated by Klein tunneling through the p and n-doped domains. Although very attractive, there is little quantitative support for this this picture. We revisit this problem and analyze the transport properties using a self-consistent recursive Green's functions technique with spin resolution that includes the electronic interaction modeled by a mean field Hubbard term. We calculate electronic current densities between neighboring carbon sites near the p-n interface and relate the electronic propagation to the puddles charge, size and shapes. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z48.00003: Ballistic interference in ultraclean suspended monolayer graphene Christian Schonenberger, Peter Rickhaus, Romain Maurand, Peter Makk, Samuel Hess, Endre Tovari, Markus Weiss, Ming-Hao Liu, Klaus Richter We have developed a versatile technology that allows to suspend graphene and complement it with arbitrary bottom and top-gate structures. Using current annealing we demonstrate exceptional high mobililties in monolayer graphene approaching $100$ m$^2$/Vs. These suspended devices are ballistic over micrometer length scales and display intriguing interference patterns in the electrical con-ductance when different gate potentials are applied. Specifically we will discuss different types of Fabry-Perot resonances that appear in different gate voltage regimes of ballistic pn devices [1]. We will go beyond our recent publication [1] and also show electric transport measurements in magnetic field, where intriguing features appear in the intermediate field range in between the low-field Klein-tunneling regime and the quantum Hall regime. We observe a large number of non-dispersing states which might be due to so-called snake states confined to the pn interface. We will also discuss first results on electron guiding in ultraclean monolayer graphene.\\[4pt] [1] P. Rickhaus et al., Nature Communications 4, 2342 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z48.00004: Multiple Scattering of Dirac Fermions in Two Dimensions Mahmoud M. Asmar, Sergio E. Ulloa The low energy dispersion of electrons in graphene-as well as surface states of three dimensional topological insulators- are characterized by a linear dispersion, leading to interesting dynamical properties. The presence of potential scattering centers, such as impurities in real samples or artificially created gated regions, also reflect the ``massless'' nature of electrons in these materials. The study of Dirac fermion scattering from single potential obstacles is made possible through partial wave methods. In the case of closely-spaced potential obstacles (high defect concentration), one should consider multiple scattering effects. Using separation of variables, Graf's addition rules, and far field matching, one can generalize the partial wave method to the case of many scatterers, and obtain physical observables for such problem. We present our study of the scattering problem of Dirac fermions from multiple potential obstacles, with focus on the two-center problem. We discuss the dependence of the differential cross section on the separation, and different potential shifts caused by these obstacles, and compare these results with the differential cross section for a single scattering center. We also study the minimal conditions that allow the observation of Klein tunneling. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z48.00005: Temperature-dependent Transport Properties of Graphene Bochen Zhong, Amol Singh, Ahsan Uddin, Goutam Koley, Richard Webb Temperature-dependent transport properties of graphene synthesized by chemical vapor deposition (CVD) on a Cu thin sheet have been investigated. Raman spectra of our samples show good quality of the CVD graphene. We have measured the temperature dependence of conductivity, charge-carrier density and Hall mobility of graphene by patterning them into micrometer-sized Hall bars. Quantum Hall effect has been observed when the temperature is about 60 Kelvin, which is the evidence for single-layer graphene. Furthermore, the results of temperature dependence of Hall mobility indicate that impurity and defect scattering is the primary scattering mechanism at low temperature, while substrate surface polar phonon scattering is dominant at high temperature. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z48.00006: The Low Frequency Noise Spectrum in Gated Epitaxial Graphene Field Effect Transistors D. Kurt Gaskill, H.K. Chan, V.D. Wheeler, V.K. Nagareddy, L.O. Nyakiti, A. Nath, R.L. Myers-Ward, Z.R. Robinson, N.Y. Garces, M.V. Rao, J.P. Goss, N.G. Wright, C.R. Eddy, Jr., A.B. Horsfall The low frequency noise (LFN) spectrum characteristics in ungated and gated \textit{ca.} 1 ML graphene field effect transistor structures are presented. Synthesis was via the Ar ambient method in a commercial reactor on semi-insulating on-axis 6H(0001)SiC. Samples were processed using photolithography before dielectric deposition; Ti/Au stack was used for ohmic and gate contacts. High-$\kappa $ dielectric deposition used F-functionalization followed by atomic layer deposition of 15 nm Al$_{2}$O$_{3}$ or HfO$_{2}$. The LFN data was averaged over 5 different samples on the same substrate for each oxide case. The LFN spectrum, proportional to 1/f, was similar in magnitude for both bare and dielectric covered graphene, implying the F-functionalization process does not appreciably add noise generation-recombination centers. Both gate oxides showed noise hysteresis ($\sim$ 15{\%}) although it was more pronounced for the HfO$_{2}$ devices. The LFN increased with increasing carrier concentration but decreased with increasing mobility implying that the empirical Hooge model cannot explain the origin of the noise and points to carrier scattering mechanisms as the noise source. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z48.00007: Monitoring the electrical property of graphene transistor by the oxygen vacancy generation of top oxide layer Tae Kwang Kim, Hye Won Du, So Myeong Shin, Jong-Hyuk Yoon, Eun-Kyu Lee, Seungmin Cho, Sunae Seo Fermi level tuning in graphene is crucial for the applications such as conducting electrode or semiconducting electronic device. It is generally achieved by both non-covalent and covalent molecular doping. Former is related with weak Vander Waals interaction which keeps electronic band structure of graphene intact. However, the molecular doping is sensitive to the air exposure so that the degradation of electrical property induce reliability issue without passivation. Here, we suggest self-passivated and well-controlled graphene doping by changing the resistivity of interfacial oxide. The oxygen in transition metal oxide is released at the high temperature under vacuum due to the concentration gradient at the interface leaving electrons which is probable to be conduction electrons by thermal activation. This indicates oxygen vacancy generates impurity level close to conduction band minimum and forms insulating oxide into N-type semiconductor. We monitored the annealing time dependent electron doping concentration of graphene under vacuum. Along with the change of charge neutrality point, to understand the conduction mechanism of graphene at room temperature with Fermi level increase, we investigated the mobility variation of electron and hole carrier versus doping concentration. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z48.00008: The exploration of bandgap opening in graphene oxides by electrical measurements Wen-Bin Jian, Sheng-Tsung Wang, Yen-Fu Lin, Pei-Ching Yeh, Baruch Rosenstein, A. Torgeman, Lain-Jong Li, Xufeng Zhou, Zhaoping Liu Chemical or electrochemical exfoliation with post reduction is a vital, mass-production method to make few-layer graphene or reduced graphene oxides (rGOs). It was argued that several structures, including C-O-C and C-OH groups, are formed at graphene surface. The rGO flakes are reduced by hydrazine or thermal annealing whereas a small ratio of residue graphene oxides remains on surface. Unlike the metallic graphene, graphene oxides and rGOs show a bandgap with low conductivity. Several atomic models of rGOs were proposed but lacking experimental corroborations. We prepared rGOs with different ratios of remaining oxides. Two ohmic contacts and one tunneling junction were fabricated on each flake. The oxygen coverage, decided by the resistivity of rGO flakes, is in the range from 8 to 23\%. Electron transport was studied from measurements of $R$-$T$ data and fitted to two-dimensional Mott's variable range hopping. The tunneling junction and differential conductance measurements disclosed the density of states (DOS) in rGO flakes. Both transport and DOS measurements indicate an electronic phase transition at an oxygen coverage of ~15\%. The DOS variation and bandgap opening are reproduced from theoretical calculation. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z48.00009: Kinetic and chemical stability of graphene oxide layers Si Zhou, Angelo Bongiorno Chemical functionalization of graphene holds great promise to open new applications of graphene in technology. Here we combine density functional theory (DFT) and Monte Carlo calculations to study both the stability and structure of graphene layers functionalized with epoxide and hydroxyl species. Our calculations show that sparse functionalizations of graphene are unstable in air at room temperature. However, oxygen groups diffuse and are prone to form dense agglomerates. To investigate these phenomena, we use DFT calculations to first map the interaction of functionalities on graphene, and then to device a simple energy scheme to both compute the Gibbs free energy of formation of arbitrary functionalizations of graphene and predict the structure resulting from diffusion and agglomeration processes. We find that the stability of graphene oxide increases for increasing both the O:C ratio and ageing time. The structure of the aged layers consists of a non-homogeneous phase of highly oxidized regions surrounded by areas of pristine graphene. Within the oxidized domains, formation of energetically stable motifs reduces the likelihood of occurrence of decomposition reactions, thereby enhancing the kinetic stability of the oxidized layer. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z48.00010: Vertical Graphene-base transistor on GaN substrate Ahmad Zubair, Omair Saadat, Yi Song, Jing Kong, Mildred Dresselhaus, Tomas Palacios The high carrier mobility, saturation velocity and thermal conductivity make graphene an attractive candidate for RF electronics. In addition to conventional lateral transistors, several alternative vertical device structures like hot electron transistors have been demonstrated to be promising for RF applications. The unique combination of sub-nanometer thickness and high conductivity makes graphene an excellent base material for hot electron transistors by lowering the base transit time in these vertical devices. The demonstrated graphene-base hot electron transistor performance is limited by low current density and low common-base current gain. In this work, we fabricated a graphene-base transistor on GaN/AlGaN heterostructure. We studied the tunneling from GaN/AlGaN heterojunction to graphene and compared with other demonstrated vertical graphene-base devices. We also investigated the effect of AlGaN thickness and different filtering barriers on both room temperature and low temperature transport characteristics of the fabricated devices. With careful design and optimization of the structure, graphene-base transistors on GaN substrate can be a potential candidate for future graphene RF electronics. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z48.00011: Graphene junction field-effect transistor Tzu-Min Ou, Tomoko Borsa, Bart Van Zeghbroeck We have demonstrated for the first time a novel graphene transistor gated by a graphene/semiconductor junction rather than an insulating gate. The transistor operates much like a semiconductor junction Field Effect Transistor (jFET) where the depletion layer charge in the semiconductor modulates the mobile charge in the channel. The channel in our case is the graphene rather than another semiconductor layer. An increased reverse bias of the graphene/n-silicon junction increases the positive charge in the depletion region and thereby reduces the total charge in the graphene. We fabricated individual graphene/silicon junctions as well as graphene jFETs (GjFETs) on n-type (4.5x10$^{\mathrm{15}}$ cm$^{\mathrm{-3}})$ silicon with Cr/Au electrodes and 3$\mu $m gate length. As a control device, we also fabricated back-gated graphene MOSFETs using a 90nm SiO$_{\mathrm{2}}$ on a p-type silicon substrate (10$^{\mathrm{19}}$ cm$^{\mathrm{-3}})$. The graphene was grown by APCVD on copper foil and transferred with PMMA onto the silicon substrate. The GjFET exhibited an on-off ratio of 3.75, an intrinsic graphene doping of 1.75x10$^{\mathrm{12}}$ cm$^{\mathrm{-2}}$, compared to 1.17x10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ in the MOSFET, and reached the Dirac point at 13.5V. Characteristics of the junctions and transistors were measured as a function of temperature and in response to light. Experimental data and a comparison with simulations will be presented. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z48.00012: Electrical transport in graphene-carbon nanotube hybrid junctions Jhao-Wun Huang, Cheng Pan, Hang Zhang, Fenglin Wang, Son Tran, Lei Jing, Marc Bockrath, Jeanie Lau We performed transport experiments in 1D-2D hybrid systems consisting of graphene and single-walled carbon nanotube junctions. We fabricated suspended graphene-carbon nanotube junctions by transferring monolayer graphene sheets onto single-walled carbon nanotubes and etching the SiO2/Si substrates in hydrofluoric acid. We measured the transport properties as a function of magnetic field and gate voltage and electric field. Coulomb blockade feature at 260mK was observed. More of our latest data will be presented with theoretical models. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z48.00013: Fabrication of Ultra-low-contact-resistance Graphene Devices Wei Sun Leong, John T.L. Thong Reactive graphene surface is important to facilitate carriers transport from graphene to other contacting material or vice versa. In this work, we present a technique that is both simple and complementary metal-oxide-semiconductor (CMOS) compatible to generate a significant amount of nano-sized pores in the graphene surface. Edge termination of the created pores in graphene was verified to be of pure zigzag configuration. A number of graphene field-effect transistors were fabricated such that the graphene channel remained intact and the graphene under metal contacts were made porous. These graphene devices exhibit very low contact resistance which is about 60{\%} better than that required for the silicon-based technology at 22 nm node with much higher mobility. In addition, approaches to control the size of pores created in graphene down to sub-nanometer regime and their density will be discussed. In short, the findings suggest that the creation of well-defined nanopores in graphene could be a promising method to enhance interaction of graphene with the contacting metal and thus optimizing the performance of graphene devices. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z48.00014: Reducing Carrier Density Pinning at Graphene/Metal Interfaces Using Interfacial Multilayer Graphene Akinobu Kanda, Kenta Katakura, Yu Ito, Shintaro Nihei, Rineka Hiraide, Hirokazu Tanaka, Youiti Ootuka, Hikari Tomori In graphene field effect transistors, as the channel length becomes shorter, the apparent field effect mobility gets smaller. One of the origins of the mobility reduction is carrier injection from metal electrodes (source and drain) to graphene mainly due to the difference in the work function, which makes the carrier density at the interface insensitive to the gate voltage. This carrier density pinning at the interface is unfavorable not only for graphene applications to field effect devices but for the observation of some kinds of Dirac Fermionic behaviors of electrons in graphene, such as specular Andreev reflection at graphene/superconductor interfaces. One of the possible solution for lifting the carrier density pinning is to insert an interfacial layer between the graphene film and electrode metals. Here, we report our attempt to form multilayer graphene at the interface and the observed modulation of the transport property by the interfacial layer. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z48.00015: ABSTRACT WITHDRAWN |
Session Z49: Focus Session: Ferroelectric, Multiferroic and Polar Oxide Heterostructures
Sponsoring Units: DMPChair: Qi Li, Pennsylvania State University
Room: Mile High Ballroom 1C
Friday, March 7, 2014 11:15AM - 11:51AM |
Z49.00001: Interface enhanced functionalities in BaTiO$_3$/CaTiO$_3$ superlattices Invited Speaker: Xifan Wu Interface engineering of oxide thin films has led to the development of many intriguing physical properties and new functionalities, in which the oxygen rotation and tilting take an crucial role. The oxygen octahedral tilt has been considered to be a coherent motion in the oxide thin-films, based on which the tilt is often neglected in the modeling of ABO$_3$ superlattices. However, combined with state-of-art experimental high-resolution electron microscopic image, our first-principles results clearly show that oxygen octahedral tilt should be more appropriately defined by the tilting angles of two individual pyramids. Each pyramid will tilt rather independently as a function of its local chemical environment. Considering the oxygen octahedral rotation at the same time, the new picture of oxygen octahedral tilting will induce a novel interface effect, in which an unstable structure in bulk CaTiO$_3$ will be stabilized at the interface in BaTiO$_3$/CaTiO$_3$ superlattice. This novel interface effect induces large polarizations both in-plane and out-of-plane with a corresponding enhanced piezoelectricity. The above scenario successfully explains the recent experimental discoveries in BaTiO$_3$/CaTiO$_3$ superlattices by H. Lee's and P. Evan's groups respectively. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z49.00002: First-principles modeling of piezoelectric response of perovskite superlattices: the case of BaTiO$_3$/CaTiO$_3$ Qibin Zhou, Karin Rabe In multicomponent ABO$_3$ superlattices, instabilities belonging to individual bulk constituents strongly interact with each other through the interfaces. Such interactions in superlattices lead to rich behavior beyond that of simple perovskites, and in particular can lead to enhanced piezoelectric response.* In this work, we studied short-period BaTiO$_3$/CaTiO$_3$ superlattices with varying layer thicknesses and overall composition. Our first-principles calculations reveal a phase transition between ferroelectric and dielectric phases at a BaTiO$_3$ fraction close to 50\% and enhanced piezoelectricity in the ferroelectric phase. A first-principles-based model, extending a previous analysis for PbTiO$_3$/BaTiO$_3$ superlattices, is constructed to predict the phase transition, the polarization and tetragonality, and the enhanced piezoelectricity. The further extension of this modeling approach to a wider range of perovskite superlattices will be discussed. \\[0.3in] *V. R. Cooper and K. M. Rabe, Phys. Rev. B 79, 180101 (2009). [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z49.00003: Temperature-composition phase diagram of PbTiO$_{3}$/CaTiO$_{3}$ superlattices Matthew Dawber, Benjamin Bein, John Sinsheimer, Sara J. Callori, Hsiang-Chun Hsing, Mohammed Humed Yusuf, Huma Yusuf An experimental enhancement of the piezoelectric response and dielectric constant can be achieved in artificially layered epitaxial PbTiO$_{3}$/CaTiO$_{3}$ superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice are changed from sample to sample, evidence for polarization rotation is found in multiple x-ray diffraction measurements and associated measurements of functional properties. Here we report on synchrotron x-ray diffraction measurements performed at X22C at the National Synchrotron Light Source at Brookhaven National Laboratory and the MS SD beam line at the Swiss Light Source at the Paul Scherrer Institute. Through these measurements we studied the rotation of the ferroelectric polarization direction as a function of both composition and temperature. This work provides significant insight into the polarization rotation mechanism in general, and illuminates routes for exploiting it in artificially layered structures to produce enhanced piezoelectric materials. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z49.00004: In-situ x-ray diffraction studies of the epitaxial growth of BaTiO$_{3}$/SrTiO$_{3}$ superlattices Benjamin Bein, John Sinsheimer, Sara Callori, Hsiang-Chun Hsing, Mohammed Humed Yusuf, Priya Chinta, Randall Headrick, Matthew Dawber The growth of BaTiO$_{3}$/SrTiO$_{3}$ superlattices on SrTiO$_{3}$ substrates using off axis RF magnetron sputtering was monitored by in-situ x-ray diffraction at X21 at the National Synchrotron Light Source at Brookhaven National Laboratory. The (00$\frac{1}{2}$) surface reflection was used to measure the growth rates of BaTiO$_{3}$ and SrTiO$_{3}$. By rocking the sample in front of an area detector, reciprocal space maps around the (001) and (101) peaks can be rapidly acquired during the growth of the superlattice. This allows the evolution of the materials lattice parameters and the superlattice structure to be continually monitored during the growth of these structures. An interesting observation is that despite the elevated deposition temperature, ferroelectric stripe domains appear, and the evolution of these with superlattice thickness was also monitored during the growth process. The eventual relaxation of the superlattices above a critical thickness was also monitored and can be compared to post-deposition atomic force microscopy measurements of film morphology. These studies provide insight into the evolution of ferroelectric properties during the growth of highly strained epitaxial ferroelectric heterostructures. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z49.00005: Direct Observation of Film Polarization and Oxygen Vacancies at the BaTiO$_{3}$/SrTiO$_{3}$/GaAs Interface Qiao Qiao, Rocio Contreras-Guerrero, Ravi Droopad, Stephen Pennycook, Sokrates Pantelides, Serdar Ogut, Robert Klie We report successful growth of BaTiO$_{3}$ thin films on GaAs (001) with a SrTiO$_{3}$ buffer layer using oxide molecular beam epitaxy (MBE), and investigate the oxide/semiconductor interface using atomic-resolution imaging, electron energy loss spectroscopy (EELS) and first principles density functional theory (DFT). Atomic-resolution Z-contrast and annular bright field (ABF) images of BaTiO$_{3}$/SrTiO$_{3}$/GaAs reveal atomically sharp interfaces and show no sign of interfacial diffusion or extensive sensitivity to the electron beam. ABF images also show that the first SrO monolayer in contact with the GaAs substrate is highly oxygen deficient, and the SrTiO$_{3}$ buffer layer has an out of plane polarization due to the presence of oxygen vacancies, which can be directly observed by the displacement between the Ti and O columns. The Ti $L_{2,3}$ and O $K$ edge spectra from the SrTiO$_{3}$/GaAs interfacial Ti columns indicate the presence of oxygen vacancies and a distortion of the TiO$_{6}$ octahedra. DFT calculations show that O vacancies form preferentially at the SrTiO$_{3}$/GaAs interface, where they polarize the SrTiO$_{3}$, and, in turn, inhibit the ferroelectric switching in the BaTiO$_{3}$. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z49.00006: Structural characterization of PbTiO$_{3}$/SrTiO$_{3}$ superlattices under an applied external field Stephanie Fernandez-Pena, Pavlo Zubko, Celine Lichtensteiger, Jean-Marc Triscone Understanding ferroelectricity in ultrathin films is important both from the fundamental as well as the technological points of view. At these thicknesses, electrostatics plays a key role and often leads to the formation of 180 degree domains that form in order to minimize the depolarization field and subsequently dominate the functional properties of ultrathin ferroelectrics. Superlattice structures combining ferroelectrics and dielectrics, where such domains form a regular pattern that can be probed using X-ray diffraction, are an ideal system for studying ferroelectric nanodomains and their response to applied electric fields (P.Zubko, et al., PRL104, 2010) . Reciprocal space maps reveal domain satellites up to third order around the main superlattice peak and their evolution under applied fields is followed from RT to 30 K. Our detailed study maps the domain evolution as well as the piezoelectric response of superlattices with different PbTiO$_{3}$/SrTiO$_{3}$ periodicities. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z49.00007: Spatial Variation and Temporal Fluctuation of Domains at Equilibrium in a PbTiO3/SrTiO3 Superlattice Qingteng Zhang, Pice Chen, Margaret Cosgriff, Mohammed Yusuf, Zhonghou Cai, Ross Harder, Sara Callori, Matthew Dawber, Paul Evans The spontaneous polarization of ferroelectric thin films often forms a periodic domain pattern in order to minimize the electrostatic energy. The coherent x-ray scattering pattern of serpentine striped domains in a PbTiO3/SrTiO3 superlattice exhibits a series of intensity speckles arising from the nanoscale disorder of the domain pattern. The detailed variation of the domain pattern in space and time can be measured by observing the decorrelation of the speckles in a series of measurements at varying positions and times. We show here that the serpentine domains do not show repetition of spatial patterns on the order of approximately 1000 domain periods. The temporal fluctuation of the domains is fit by a model that describes similar slow dynamics in jammed soft matter systems. Change of domain structures is observed after repeated excitation by short-duration electric-field pulses. The decorrelation is incomplete following electric pulses that are large enough induce a transition to a uniform polarization state, indicating that the domain pattern regenerated after each pulse is at least partially determined by heterogeneity in the superlattice structure. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z49.00008: Dielectric Properties of Artificially Layered Ferroelectrics Pavlo Zubko, Stephanie Fernandez-Pena, Celine Lichtensteiger, Jean-Marc Triscone Over the past decade, superlattices composed of ferroelectric and paraelectric oxides have received a great deal of attention due to the rich physics arising from the complex electrostatic interactions in these materials, as well as the discovery of novel interface phenomena. The strong depolarizing fields induced by the presence of the paraelectric layers, lead to the formation of a stable structure of regular nanoscale domains, which dominate the functional properties of these artificially layered materials. The dielectric properties of PbTiO3-SrTiO3 superlattices were studied using impedance spectroscopy over a broad range of temperatures. A giant enhancement of the dielectric permittivity due to reversible motion of nanoscale domains was observed and domain-wall relaxation dynamics were studied as a function of ferroelectric and paraelectric layer thickness, temperature, and magnitude and frequency of the applied field. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z49.00009: Studies of local polarization in complex oxide multiferroic interfaces by aberration corrected STEM-EELS Gabriel Sanchez-Santolino, Javier Tornos, Carlos Leon, Mar\'Ia Varela, Stephen J. Pennycook, Jacobo Santamar\'Ia Interfaces in complex oxide heterostructures are responsible for exciting new physics, which is directly related to the chemical, structural and electronic properties at the atomic scale. Here, we study artificial multiferroic heterostructures combining ferromagnetic \textit{La}$_{0.7}$\textit{Sr}$_{0.3}$\textit{MnO}$_{3}$ with ferroelectric \textit{BaTiO}$_{3}$ by atomic resolution aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy. Measurements of the atomic positions in the STEM images permit calculating relative displacements and hence, local polarization. Polarization gradients can be observed in annular bright field images which seem to be correlated to strain gradients associated with the large lattice mismatch between barriers and electrodes. Spectroscopic measurements suggest the presence of O vacancies through the ferroelectric layers. Understanding the effect of the charge carriers associated with the oxygen vacancies may be the key to control the dynamics of domain walls in these heterostructures. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z49.00010: Charge Control of Interface Magnetization at Oxide Heterointerface G. Luepke, X. Ma, H. Zhai, F. Fang, A. Kumar, R.S. Katiyar, S. Dussan, H.B. Zhao, J.F. Scott The complex oxide heterointerface is key to the development of emerging multiferroic and spintronic technologies with new functionality. Even so, direct characterization of the interfacial spin state is missing, which prevents further interpretation of the coupling between spin and other ordering parameters at such oxide heterointerfaces, and impedes the development of future interface-based devices. Here we use the interface-specific Magnetization-induced Second-Harmonic Generation (MSHG) technique to investigate the interfacial magnetic state of the multiferroic (MF) heterostructure PbZr0.52Ti0.48O3 / La0.67Sr0.33MnO3 (PZT/LSMO) and its dependence on the charge state. We observe a gradual transition from ferromagnetic (FM) to canted anti-ferromagnetic (AFM) phase in the first unit cell layer at the heterointerface with increasing hole doping. Moreover, the exchange coupling between interface and bulk is weak, independently of the carrier filling. Our results provide new insight into the interface spin system of MF heterostructures, and have implications for developing electric field control of spin switches and magnetic tunneling junctions. [Preview Abstract] |
Session Z50: Plasmonics and Metamaterials
Sponsoring Units: DCMPChair: Christos Argyropoulos, Duke University
Room: Mile High Ballroom 1D
Friday, March 7, 2014 11:15AM - 11:27AM |
Z50.00001: Germanium-Based Plasmonic Nanojunctions Kenneth Evans, Pavlo Zolotavin, Douglas Natelson The fabrication of robust optoelectronic devices which function on length scales well below the wavelength of light is an important step in the development of light-based electronics. We present a method for the production of reliable near-IR light detection in germanium films on plasmonically-active gold nanojunctions. We show polarization measurements consistent with the existence of plasmon-enhanced absorption in these structures, making possible the careful study of the effect of highly local plasmons in the gold leads on the photogenerated carriers in the germanium. We discuss the photoconductive mechanism in these structures and the potential for high-efficiency, scalable photodevices through changes to the device geometry. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z50.00002: Dirac-Like Plasmons in Honeycomb Lattice of Metallic Nanoparticles Claire Woollacott, Guillaume Weick, William L. Barnes, Ortwin Hess, Eros Mariani We consider a two-dimensional (2D) honeycomb array of metallic nanoparticles, each supporting a localized surface plasmon, and study the quantum properties of the collective plasmonic modes resulting from the near-field dipole interaction between nanoparticles. We analytically investigate the dispersion, effective Hamiltonian and eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. For polarization pointing normal to the plane, the spectrum presents Dirac cones similar to those present in the electronic band structure of graphene. The effective Dirac Hamiltonian and corresponding spinor eigenstates represent Dirac-like massless bosonic excitation, presenting similar effects to electrons in graphene, for example, non-trivial Berry phase and the absence of backscattering off smooth inhomogeneities. However, by tilting the polarisation, the Dirac points can be manipulated and a gap can be controllably opened in the spectrum. Therefore the properties of this metamaterial can be manipulated by the incident light polarization, paving the way for a fully tunable plasmonic analogue of graphene. - G. Weick, C. Woollacott, W. Barnes, O. Hess and E. Mariani, Phys. Rev. Lett. 110, 106801 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z50.00003: Tailoring the plasmonic modes of metal nanoparticle arrays with lattice anisotropy King Chun Lai, Sze Fung Lee, Kin Wah Yu We have studied the plasmonic band structure of three-dimensional lattices of nanoparticles under external electromagnetic waves. The long-range dipolar forces among polarized nanoparticles lead to the collective motion of the dipole moments to form plasmon. The resulting plasmonic dispersion thus depends on the polarizability of individual particle and the lattice structure of the whole system. We tailor sets of desirable plasmonic modes through varying the polarizability of nanoparticles or lattice anisotropy which can be tuned by incident GHz ultrasonic waves. Similar work of one-dimensional particle chain was contributed by Maier (2003), but we further extend the system into three-dimensional cases. In order to deal with the long-range interactions, we adopt the Ewald method to develop a viable means for calculating the plasmonic dispersion relation. Furthermore, we consider the formalism for diatomic basis of nanoshell. The plasmonic modes of each particle may couple and form hybridized plasmonic band attributed to level repulsion effect. This method provides a flexible way to manipulate plasmonic wave in a lattice by tuning the characteristic parameters of particle shape or lattice structure. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z50.00004: Selective reflection by deteriorated phase accumulation in Fabry-Perot cavity with aperiodic metallic nanomesh entry windows Tianyi Sun, Chuanfei Guo, Krzysztof Kempa, Zhifeng Ren A Fabry-Perot reflection filter, consisting of semi-transparent metal and dielectric layers on opaque metals, is featured by selective absorption determined by the phase difference of waves from the two interfaces. In such systems, semi-transparency is usually realized by layers of reflective metals thinner than the penetration depth of the light. Here we present a filter cavity with entry windows not made of traditional thin layers, but of aperiodic metallic random nanomeshes thicker than the penetration depth, fabricated by grain boundary lithography. It is shown that due to the deteriorated phase caused by the interface between the random nanomesh and the dielectric layer, the width and location of the resonances can be tuned by metallic coverage. Further experiments show that this phenomenon can be used in designing aperiodic plasmonic metamaterial structures for visible and infrared applications. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z50.00005: Symmetry-broken metamaterial absorbers as reflectionless directional couplers for surface plasmon polaritons in the visible range Fan Ye, Michael J. Burns, Michael J. Naughton Recently, gradient-index meta-surfaces have been shown to have the ability to manipulate wavefronts at will in a reflectionless manner in the GHz range, including the extreme example of converting freely propagating waves into surface waves with high-efficiency. Upon approaching the visible regime, the gradient-index concept encounters difficulties due to fabrication limitations. Here, we demonstrate theoretically and experimentally that asymmetric, periodic, two-element metal-insulator-metal structures can serve as reflectionless directional convertors between freely propagating visible photons and surface plasmon polaritons (SPP). Coupling between propagating modes caused by the broken symmetry and localized modes generated by individual elements is shown to be the main mechanism of this high-efficiency process. Direct experimental evidence is obtained for reflectionless ($<$ 8\% measured reflectance) directional SPP coupling in the visible range, with a directionality of 99.7\%. A novel measurement scheme is developed for the characterization of absolute reflectance of samples with micron-sized area under normal incidence in the visible range. Our results are meaningful for integrated nanoplasmonics, plasmonic logic, and plasmonic light harvesting, among others. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z50.00006: Edge state and photon emission rate in plasmon chains Chi Wai Ling, Kin Hung Fung, Siu Fung Yu Topology band theory has explained many important electronic phenomena in condensed matter physics, like quantum hall effect and topological insulators. We consider realistic plasmonic nanoparticles as strongly coupled ``atoms'' and study the topological properties in the plasmon bands. Zak phase and edge state frequency in chains of plasmonic nanoparticles are studied analytically. Photon emission rate enhanced by such an edge state is also analyzed by solving the Maxwell's equations using FDTD method. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z50.00007: Electromagnetic properties of gain-doped epsilon-near-zero metamaterials Ka Ki Ng, Naijing Deng, Kin Wah Yu Sun and Yu designed broadband gain-doped epsilon-near-zero (GENZ) metamaterials in 2012. The large loss of epsilon-near-zero metamaterials is compensated by gain to get a great advantage in practical applications. Previous works have demonstrated that the electromagnetic properties of gain media differ from that of lossy media. For example, the original Kramers-Kronig relations are no longer suitable for certain gain media. Therefore, we have studied the electromagnetic properties of GENZ and determined whether it is possible to develop such relations for GENZ over a broad frequency range. In order to investigate the validity of Kramers-Kronig relations for GENZ, we have studied the electromagnetic properties of a GENZ slab which is considered as a homogeneous layer with frequency dependent permittivity. The associated Fresnel equations for GENZ is studied using Laplace transform analysis. The changes caused by the two pumped sources at lower and higher frequencies is demonstrated. Finally, the associated Kramers-Kronig relation is validated after a contact with the causality and electromagnetic properties of active media. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z50.00008: Focusing electromagnetic waves in subwavelength structures by lossy anisotropic media Hui Kin Kwok, Sze Fung Lee, King Chun Lai, Kin Wah Yu Different from subwavelength focusing by negative refractive index slab and microsphere array, we have proposed a new mechanism of subwavelength focusing by lossy radial anisotropy which helps to beat the diffraction limit in near field microscopy. We consider the propagation of a plane polarized electromagnetic wave incident on a nanosphere with anisotropic complex permittivity. For a sphere with complex radial permittivity implying a loss effect on radial component of the electric field, only the tangential component of the field survives, and thus the Poynting vectors can be made to concentrate at the centre of the sphere by increasing the complex permittivity to enhance the radial loss. Extending the study of electromagnetic scattering by coated spheres with lossless radial anisotropy by Gao et al.(2008), we consider a lossy medium and calculate the distribution of electromagnetic field by solving Maxwell's equations with expressing the fields in terms of Debye potentials. Considering the sphere of subwavelength scale and thus the incident fields are nearly quasistatic, the calculation of Poynting vectors inside reveals a focusing effect as expected. This nanosphere also offers a possible way to convert light into a nanospot which leads to applications in computing. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z50.00009: Broadband Dipole Bonding with Epsilon-near-zero Material Naijing Deng, Kin Wah Yu Epsilon-near-zero dielectrics (ENZ) is known to be responsible for static dipole levitation. Force equilibrium, or dipole bonding appears when the wavelength is comparable to the separation distance. For oscillating dipoles, the bonding position would be dependent on the dispersive dielectric responses of the ENZ plane. We propose a ENZ designing scheme, allowing dipoles with broad frequency range to be bonded simultaneously at one specific position. In the meantime, the Kramer-Kronig relation of ENZ dielectrics is not violated. The dipole mass would linearly shift the bonding position and may bring viable applications in particle filtration. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z50.00010: A Multi-Mode Hybrid Plasmonic Waveguide with Enhanced Confinement and Propagation John Colanduoni, Daniel Nikolov, Huizhong Xu Waveguides capable of achieving high confinement with low loss are a key goal in the developing field of plasmonics. A hybrid waveguide, which consists of a dielectric wire above a dielectric-metal interface, has been previously proposed with such desirable properties. By exciting this geometry with an aperture in the metal that takes advantage of the extraordinary transmission through sub-wavelength apertures, it is possible to strongly couple to multiple modes. The real part of the fundamental mode is in fact capable of exceeding the index of refraction of all the materials used while maintaining a small imaginary part, as a result of appropriate choice of materials for the dielectric wire and the metal. As the amplitude of the most confined mode is significantly larger than the amplitude of the other modes with poor confinement, this geometry can enable enhanced confinement and propagation in light guiding applications. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z50.00011: Making structured metals transparency for broadband and wide-incidence-angle electromagnetic waves Renhao Fan, Ruwen Peng, Xianrong Huang, Mu Wang Very recently, we have demonstrated that one-dimensional metallic gratings can become transparent and completely antireflective for extremely broadband electromagnetic (EM) waves under oblique incidence. However, the oblique-incidence geometry, is inconvenient for the technological applications. To overcome this drawback, here we instead use oblique metal gratings with optimal tilt angles to achieve normal-incidence broadband transparence for EM waves. Further we use two-dimensional periodic metallic cuboids to achieve broadband and broad-angle high transmission and antireflection. By introducing such metallic cuboids arrays into silicon solar cells, we find that high performance of light trapping in the cells can be obtained with a significant enhancement of the ultimate quantum efficiency. The structured metals, which achieve broadband and broad-angle high transmission for EM waves, may have many other potential applications, such as transparent conducting panels, white-beam polarizers, and stealth objects. References: R. H. Fan, R. W. Peng, X. R. Huang et al., Adv. Mater. 24, 1980 (2012); R. H. Fan, J. Li, R. W. Peng et al., Appl. Phys. Lett. 102, 171904 (2013); and R. H. Fan, L. H. Zhu, R. W. Peng et al., Physical Review B, 87, 195444 (2013). [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z50.00012: Dark Acoustic Metamaterials Jun Mei, Guancong Ma, Min Yang, Zhiyu Yang, Weijia Wen, Ping Sheng The attenuation of low-frequency sound has been a challenging task because the dissipation of materials in this regime is inherently weak. Here we show that by using thin elastic membranes decorated with asymmetric rigid platelets, the resulting acoustic metamaterials can reach almost unity absorption at frequencies where the relevant sound wavelength in air can be three orders of magnitude larger than the membrane thickness. At resonances, the measured displacement profiles show slope discontinuities around the platelet perimeters, implying significantly enhanced elastic curvature energy is concentrated in these small volumes. This thereby gives rise to strong absorption similar to a cavity system, even though the system is geometrically open. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z50.00013: Asymmetric Invisibility Cloaking Theory Based on the Concept of Effective Electromagnetic Fields for Photons Tomo Amemiya, Masato Taki, Toru Kanazawa, Shigehisa Arai The asymmetric invisibility cloak is a special cloak with unidirectional transparency; that is, a person in the cloak should not be seen from the outside but should be able to see the outside. Existing theories of designing invisibility cloaks cannot be used for asymmetric cloaking because they are based on the transformation optics that uses Riemannian metric tensor independent of direction. To overcome this problem, we propose introducing directionality into invisibility cloaking. Our theory is based on ``the theory of effective magnetic field for photons'' proposed by Stanford University.\footnote{K. Fang \textit{et al.}, Nature Photon. \textbf{6}, 782 (2012).} To realize asymmetric cloaking, we have extended the Stanford's theory to add the concept of ``effective electric field for photons.'' The effective electric and the magnetic field can be generated using a photonc resonator lattice, which is a kind of metamaterial. The Hamiltonian for photons in these fields has a similar form to that of the Hamiltonian for a charged particle in an electromagnetic field. An incident photon therefore experiences a ``Lorentz-like'' and a ``Coulomb-like'' force and shows asymmetric movement depending of its travelling direction.We show the procedure of designing actual invisibility cloaks using the photonc resonator lattice and confirm their operation with the aid of computer simulation. [Preview Abstract] |
Session Z52: Superconductivity: Tunneling Phenomena
Sponsoring Units: DCMPChair: Michael Osofsky, Navel Research Laboratory
Room: Mile High Ballroom 1F
Friday, March 7, 2014 11:15AM - 11:27AM |
Z52.00001: Non-equilibrium theory of tunneling into localized states in superconductors Ivar Martin, Dima Mozyrsky A single static magnetic impurity in a fully-gapped superconductor leads to formation of an intragap quasiparticle bound state. At temperatures much lower than the superconducting transition temperature, the energy relaxation and spin dephasing of the state are expected to be exponentially suppressed. The presence of such a state can be detected in electron tunneling experiments. Here we show, that even for an arbitrarily weak tunneling strength, the differential tunneling conductance is symmetric with respect to the sign of applied bias. This is in contrast to the standard expectation that the conductance is proportional to the local density of states, which may be particle-hole asymmetric. The standard result can be recovered is one assumes either a finite density of impurity states, or that impurities are coupled to another, non-superconducting equilibrium bath. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z52.00002: Conductance Measurements of Magnesium Diboride-based Josephson Junctions Below 1 Kelvin: Beyond the 2-Gap Model Steven Carabello, Joseph Lambert, Jerome Mlack, Wenqing Dai, Qi Li, Ke Chen, Daniel Cunnane, C.G. Zhuang, X.X. Xi, Roberto Ramos Theoretical and experimental studies have probed the nature of magnesium diboride's two superconducting energy gaps $\Delta_\pi$ and $\Delta_\sigma$. Several theoretical analyses have predicted fine structures within each energy gap, with recent experiments revealing similar structures. We have performed high-resolution tunneling measurements of low-transparency Josephson junctions using ``terraced,'' ``columnar,'' and c-axis MgB$_2$ films separated by its native oxide from either lead (Pb) or tin (Sn) counter-electrodes. Using high-resolution I-V data at $T$ as low as 23mK, we observe sub-structures within both energy gaps. We also observe sharp peaks in the subgap that identify, to high precision, the energy gap values of the junction counter-electrodes (Pb and Sn). These lead us to conclude that the substructures seen in the gaps are due to MgB$_2$. We then fit the data using simplified two-gap and four-gap models with variable weights and broadening factors. By demonstrating the inadequacy of a simple two-gap model in fitting the data, we illustrate that some distinctions between theoretical models of energy gap substructures are experimentally observable. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z52.00003: Current oscillations in ultra-small superconducting Nb-Nb junctions formed by STM at mK temperatures Michael Dreyer, Anita Roychowdhury, Rami Dana Using etched Nb STM tips we formed ultra-small tunnel junctions on a Nb crystal at an effective temperature of $\sim$ 200 mK using an Oxford dilution refrigerator. The Nb crystal was prepared in UHV and then transferred into the mK STM. The resulting superconductor-insulator-superconductor (SIS) junction displayed several sub-gap features from multiple Andreev reflections to a zero bias conductance peak. The latter showed features of a Josephson junction in the phase diffusion limit [2] with side structures due to the electrical environment [3]. Upon microwave irradiation the peak split into multiple peaks in accordance with theory [4], verifying Josephson tunneling. In addition we observed bias dependent oscillations of the tunneling current. The oscillations where recorded at a rate of 10 kS/s while acquiring conventional dI/dV or I(V) spectroscopic curves. Histograms of the current for each bias voltage step then reveal the nature of the oscillation. It ranges from multiple states in certain bias regions through pure oscillations to supercurrent-normal switching. Fourier transform of the current show in some cases a bias dependence of the main frequencies. Possible causes will be discussed. \\[4pt] [1] A. Roychowdhury, et al., arXiv:1311.1855 (2013).\\[0pt] [2] M. Ivanchenko and L.A Zil'berman, Sov. Phys. JETP, \textbf{28}, 1272 (1969).\\[0pt] [3] G. Ingold and H. Grabert, Phys. Rev. B., \textbf{50}, 395 (1994).\\[0pt] [4] G. Falci, V. Bubunja and G. Schon, Z. Phys. B., \textbf{85}, 451 (1991) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z52.00004: Point Contact Spectroscopy Study of the New Superconductor Nb2PdxSe5 Yeping Jiang, Xiaohang Zhang, Ichiro Takeuchi, Richard Greene, Seunghyun Khim, Bumsung Lee, Kee-Hoon Kim We have systematically investigated the temperature dependence of the energy gap structure for the new quasi-one-dimensional superconductor Nb2PdxSe5 by point contact spectroscopy (PCS). Our studies were performed on highly transparent Andreev reflection junctions evidenced by sharp and dramatic conductance enhancements at low temperatures. By applying the BTK model, we find that the energy gap scales with the transition temperature (Tc$=$5.5 K) in a BCS-like manner. Details of this and a few anomalous features of the PCS will be presented. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z52.00005: STM Spectroscopy Probe of Field Depairing and Vortex Lattice Transition in a Multiband Superconductor I. Fridman, V. Lukic, C. Kloc, C. Petrovic, J.Y.T. Wei The Cooper pairing in a variety of superconductors involves carriers from multiple bands, which can optimize the pairing phase space and provide novel pairing interactions. We have developed a novel technique to probe multiband pairing, using a directional diamagnetic supercurrent to perturb the quasiparticle density-of-states spectrum, and measuring the spectral evolution due to pair breaking by finite superfluid momentum. This technique is demonstrated on the layered superconductor 2H-NbSe2, using a scanning tunneling microscope (STM) at 300 mK with an in-plane magnetic field up to 9 T [1]. The STM spectroscopy measurements revealed unambiguous evidence for multiband pairing [2], as well as a novel reorientation transition of the in-plane vortex lattice [3]. We will discuss the first-order and quantum-critical characteristics of this transition, in terms of the geometric frustration of a distorted hexagonal vortex lattice, and show that this transition is intimately related to the multiband pairing. \\[4pt] [1] I. Fridman et al., Applied Physics Letters 99, 192505 (2011).\\[0pt] [2] I. Fridman et al., arXiv:1110.6490.\\[0pt] [3] I. Fridman et al., arXiv:1303.3559. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z52.00006: Low-Bias Anomaly and Tunnel-Fluctuoscopy Andreas Glatz, Andrey Varlamov, Valerii Vinokur Electron tunneling spectroscopy pioneered by Esaki and Giaever offers a powerful tool for studying electronic spectra in superconductors. The phenomenological theory by Giaever and Megerle related the tunneling current to the electronic densities of states and to the difference of their equilibrium distribution functions in electrodes. This led to impressive discoveries having revealed, in particular, of the wide, $eV_{pg}\sim\Delta_{BCS}$, pseudogap in the tunneling spectrum of superconductors above their critical temperatures. However, it turns that this standard approach is insufficient to reveal the nontrivial, related to Andreev reflection of the tunneling electrons from superconducting fluctuation domains in the biased electrode, zero-bias anomaly carrying important information about the scattering, interactions, and decoherence. Here, operating in frameworks of the microscopic theory of tunneling, we report the existence of a such low-energy singularity in a tunneling conductivity of N-I-N(S) junction directly indicating on the presence of fluctuating Cooper pairs. Our findings mark a radical departure from the conventional picture of the ZBA and open new horizons for quantitative analysis of electronic spectra of superconductors in fluctuation regime. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z52.00007: Tunneling Spectroscopy of MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N Thin Films Grown by Atomic Layer Deposition Chaoyue Cao, Nickolas Groll, Jeffrey Klug, Nicholas Becker, Serdar Altin, Thomas Proslier, John Zasadzinski Tunneling I(V) and dI/dV vs. V are reported on superconducting thin films of MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N using a point contact method with a Au tip. The films are grown by the chemical process of atomic layer deposition (ALD) onto various substrates (Si, quartz, sapphire) held at 450 C. Resistively measured superconducting Tc values up to 12K and 13K are found for the MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N respectively. Artificial tunnel barriers (1-3 nm thick) of Al$_{\mathrm{2}}$O$_{\mathrm{3}}$, also grown by ALD, are shown to provide much improved tunneling characteristics compared to the native oxides. Relatively high quality gap features are observed with zero-bias conductance values as low as $\sim$ 10{\%} of the high bias values. Gap parameters $\Delta$ $\sim$ 2.0meV are found for the MoN and $\Delta$ $\sim$ 2.0-2.4 meV for the Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N which follow the BCS temperature dependence and close near the measured film Tc indicating bulk superconductivity at the surface. The suitability of such conformal ALD grown films for potential superconducting devices is discussed. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z52.00008: Observing tunnel magnetoresistance in junctions comprising of superconductors with Zeeman-split energy bands Bin Li, Guo-Xing Miao, Jagadeesh S. Moodera The spin-splitting of the quasiparticle density of states (DOS) in a superconductor due to Zeeman energy can lead to a highly field responsive spintronic device. We present our magnetotunneling studies in superconductor/insulator/ferromagnet tunnel junctions in which the superconducting quasiparticle DOS is energy split by an internal exchange field at the interface from an adjacent ferromagnetic insulator EuS layer. A tunnel magnetoresistance (TMR) as large as 36\% is observed, and that only occurs in the superconducting state. Tunnel conductance simulation suggests that the TMR originates from the conductance variation resulting from spin selective quasiparticle tunneling. Our results show that in addition to the naturally existent spin imbalance at Fermi level in ferromagnets that gives rise to conventional TMR in standard magnetic junction (MTJs), we can manipulate tunnel conductance by tailoring spin dependent density of states with interfacial exchange fields. Furthermore, a similar TMR is also observed even with a tunnel junction with both superconducting electrodes that have exchange split DOS. [B. Li, G.-X. Miao, and J. S. Moodera, \textit{Phys. Rev. B} \textbf{88}, 161105(R) (2013)] [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z52.00009: Macroscopic Refrigeration Using Superconducting Tunnel Junctions Peter Lowell, Galen O'Neil, Jason Underwood, Xiaohang Zhang, Joel Ullom Sub-kelvin temperatures are often a prerequisite for modern scientific experiments, such as quantum information processing, astrophysical missions looking for dark energy signatures and tabletop time resolved x-ray spectroscopy. Existing methods of reaching these temperatures, such as dilution refrigerators, are bulky and costly. In order to increase the accessibility of sub-Kelvin temperatures, we have developed a new method of refrigeration using normal-metal/insulator/superconductor (NIS) tunnel junctions. NIS junctions cool the electrons in the normal metal since the hottest electrons selectively tunnel from the normal metal into the superconductor. By extending the normal metal onto a thermally isolated membrane, the cold electrons can cool the phonons through the electron-phonon coupling. When these junctions are combined with a pumped $^{3}$He system, they provide a potentially inexpensive method of reaching these temperatures. Using only three devices, each with a junction area of approximately 3,500 $\mu$m$^{2}$, we have cooled a 2 cm$^{3}$ Cu plate from 290 mK to 256 mK. We will present these experimental results along with recent modeling predictions that strongly suggest that further refinements will allow cooling from 300 mK to 120 mK. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z52.00010: Superconductivity of lanthanum revisited Peter Loeptien, Lihui Zhou, Jens Wiebe, Alexander Ako Khajetoorians, Roland Wiesendanger The thickness dependence of the superconductivity in clean hexagonal lanthanum films grown on tungsten (110) is studied by means of scanning tunneling microscopy (STM) and spectroscopy (STS). Fitting of the measured spectra to BCS theory yields the superconducting energy gaps from which the critical temperatures are determined. For the case of thick, bulk-like films, the bulk energy gap and critical temperature of dhcp lanthanum turn out to be considerably higher as compared to values from the literature measured by other techniques. In thin films the superconductivity is quenched by the boundary condition for the superconducting wavefunction imposed by the substrate and surface, leading to a linear decrease of the superconducting transition temperature as a function of the inverse film thickness. This opens up the possibility to grow lanthanum films with defined superconducting properties. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z52.00011: Ultra-high vacuum fabrication of metal/insulator/superconductor junctions for spin polarization measurements Zachary Barcikowski, Joshua Pomeroy Using a unique ultra-high vacuum (UHV) deposition chamber equipped with electron gun deposition sources, sputter deposition and plasma oxidation, we are depositing shadow mask defined tunnel junctions. These unique capabilities allow us to assess the importance of creating high quality tunneling materials in the ultra-thin regime where abrupt chemical interfaces and near-ideal stoichiometries are important. In this talk, I will present experimental details about this unique system and discuss devices being fabricated, including spectroscopy measurement techniques using the superconductor quasi-particle DOS as an analyzer. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z52.00012: The Effect of Inelastic Scattering of Charge Carriers on the Reliability of the Value of the Spin Polarization as Determined from Superconductor/Ferromagnet Point Contact Conductance Data Paul J. Dolan, Jr., Charles W. Smith An extended BTK model for charge transport in a superconductor/ferromagnet point contact can be used to determine the value of the spin polarization of the ferromagnet. We estimate the effect of inelastic scattering of charge carriers in the active region of the contact on the reliability of the value of the polarization as determined from conductance data. The effect can be substantial and depends upon contact transparency. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z52.00013: Cooper pair splitting in isolated NSN island Pochen Tai, Chia-Heng Sun, Jheng-An Jiang, Cen-Shawn Wu, Jeng-Chung Chen, Yung-Fu Chen Cooper pair is in a maximally entangled two-particle state, and may have applications on solid state version of quantum teleportation. We investigate Cooper pair splitting, a charge-transfer process by crossed Andreev reflection (CAR), in a three-island system. The system consists of one superconducting island (S) and two normal-metal islands (N), while S connects to two N via two tunnel junctions. When the charging energy of the system is the dominant energy factor, this system is suitable to study charge transfers down to single-electron regime. Two single-electron transistors as charge sensors are capacitively coupled to two N, respectively, to observe charge tunneling events. Several competing charge-transfer processes as long with CAR also occur in two S/N interfaces, such as quasi-particle tunneling, cotunneling, and ordinary Andreev reflection. Correlation measurements of charge fluctuation at two S/N interfaces should help to tell CAR process apart from other competing processes. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z52.00014: Temperature Dependence of Quasiparticle Spectral Weight and Coherence in High $T_c$ Superconductors Yang He, Jessie Zhang, Jennifer Hoffman Superconductivity arises from the Cooper pairing of quasiparticles on the Fermi surface. Understanding the formation of Cooper pairs is an essential step towards unveiling the mechanism of high $T_c$ superconductivity. We compare scanning tunneling microscope investigations of the temperature dependence of quasiparticle spectral weight and quasiparticle interference in several families of high $T_c$ materials. We calculate the coherent spectral weight related to superconductivity, despite the coexistence of competing orders. The relation between pairing temperature and coherent spectral weight is discussed. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z52.00015: Coulomb Blockade of Shot Noise Denis Vion, Carles Altimiras, Olivier Parlavecchio, Philippe Joyez, Patrice Roche, Daniel Esteve, Fabien Portier We observe the suppression of the finite frequency shot-noise produced by a voltage biased tunnel junction due to its interaction with a single electromagnetic mode of high impedance. The junction is embedded in a superconducting lambda/4 resonator containing a dense SQUID array yielding a resonator characteristic impedance in the kOhm range and a resonant frequency tunable in the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb blockade on the tunnel junction (about 30{\%} reduction in the differential conductance) and allows an efficient measurement of the spectral density of the current fluctuations at the resonator frequency. The observed blockade of shot-noise is found in agreement with an extension of the dynamical Coulomb blockade theory. [Preview Abstract] |
Session Z53: Surfaces, Interfaces, and Thin Films: Growth, Processing, and Characterization
Sponsoring Units: DMPRoom: Mile High Ballroom 2C
Friday, March 7, 2014 11:15AM - 11:27AM |
Z53.00001: First-principles calculation of oxidation process of SiC substrate Tomoya Ono, Shoichiro Saito The atomic structure of 4H-SiC(0001)/SiO$_2$ interface is still under discussion since the conclusion derived by HRTEM is different from that by SIMS and XPS. The oxidation processes as well as the CO desorption of a SiC substrate are investigated by first-principles calculations. We employ SiC surface and SiC/SiO$_2$ interface models to imitate initial and middle stages of oxidation, respectively. O atoms are inserted between Si-C bond sequentially and the energy gain of the CO desorption is calculated by removing a C atom as a form of CO molecule. We found that the CO desorption becomes preferable when the number of inserted O atoms is three because the perfect SiO$_2$ network remains after the CO desorption and the stress is relaxed by removing the CO molecule from the SiC/SiO$_2$ interface. When C atoms are not removed as CO molecules at the interface, the unoxized Si-C bond remains in SiO$_2$. We also investigate the CO desorption from SiO$_2$. The energy gain of the CO desorption indicates that the CO desorption is unfavorable because the rearrangement of Si-O bonds costs more energy than the CO desorption. Thus, C atoms are immediately emitted from the interface as CO molecules and not kicked out from SiO$_2$. Our results support the conclusion obtained by SIMS and XPS. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z53.00002: Properties of ZrB$_{2}$ Thin Films Grown by E-Beam Evaporation Robert Lad, David Stewart, Julia Sell, George Bernhardt, David Frankel Zirconium diboride (ZrB$_{2}$) is a candidate material for many high temperature applications because it has a high melting point, high hardness, thermal shock resistance, and metallic conductivity. However, very little work has been reported concerning growth of ZrB$_{2}$ thin films and high temperature oxidation behavior. In this study, ZrB$_{2}$ films with nominal thickness of 200 nm have been deposited using electron-beam evaporation of either ZrB$_{2}$ pellets or elemental B and Zr sources. The ZrB$_{2}$ source yields a film that has a 1:1 Zr:B average composition as measured by X-ray photoelectron spectroscopy, consisting of ZrB$_{2}$ precipitates within an amorphous Zr matrix as determined by X-ray diffraction. Use of elemental B and Zr sources allows precise control of film growth over a range of stoichiometries and yields ZrB$_{2}$ films with much lower oxygen contamination. After annealing ZrB$_{2}$ films to 1200$^{\circ}$C in air, oxidation leads to a loss of B and formation of a textured monoclinic ZrO$_{2}$ phase. Several strategies, including deposition of a thin Al$_{2}$O$_{3}$ capping layer over the ZrB$_{2}$ film are being pursued in an attempt to stabilize the electrically conductive ZrB$_{2}$ phase at high temperature, where it can be used for high temperature electronic devices in harsh environments. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z53.00003: Understanding nanoscale mechanical properties of materials using ultrafast EUV photoacoustics K. Hoogeboom-Pot, E. Turgut, J. Shaw, J. Hernandez-Charpak, M. Murnane, H. Kapteyn, D. Nardi How do the elastic properties of materials evolve as a nanostructure builds up layer by layer? A host of questions in nanoscience, nanotechnology, quantum dot systems and more rely on an answer to this issue; but our ability to probe mechanical properties is severely constrained at dimensions below 100 nm. With tabletop high harmonic generation (HHG), we overcome these limitations by extending non-destructive visible photoacoustics to extreme ultraviolet (EUV) wavelengths. The short wavelength of EUV light, combined with the coherence and ultrashort pulses of HHG creates a unique and powerful probe of nanostructured materials on their intrinsic length and time scales. We study a series of ultrathin bilayer (10-nm Ni/0-6-nm Ta) nanostructures on SiO$_{2}$ substrates. A femtosecond infrared pulse excites longitudinal acoustic waves (LAWs) within the nanostructures and surface acoustic waves (SAWs) in the substrate. Diffraction of a time-delayed EUV probe pulse monitors the dynamics. LAW resonances are directly related to the bilayer thickness and effective speed of sound; their dependence on Ta-layer thickness reveals that the LAW velocities of both Ni and Ta differ from bulk values. The changing mass of Ta also affects the SAW frequency, allowing us to extract nanoscale densities. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z53.00004: Heteroepitaxy of group IV-VI nitrides by atomic layer deposition Jeffrey Klug, Nicholas Becker, Carlos Alvarez, Nickolas Groll, Chaoyue Cao, Matthew Weimer, Michael Pellin, John Zasadzinski, Thomas Proslier Heteroepitaxial growth of selected group IV-VI nitrides on various orientations of $\alpha$-Al$_2$O$_3$ and MgO is demonstrated using atomic layer deposition. High quality, epitaxial films are produced at significantly lower temperatures than required by conventional deposition methods. The influence of substrate orientation on film structure and morphology as well as film resistivity and superconductivity are discussed. Transport measurements reveal a reduced room temperature resistivity and increased residual resistance ratio (RRR) for films deposited on lattice-matched substrates compared to polycrystalline samples deposited concurrently on native-oxide Si(001) and fused quartz substrates. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z53.00005: Transpassive Dissolution of Copper and Rapid Formation of Brilliant Colored Copper Oxide Films Narjes Fredj, T. David Burleigh This investigation describes an electrochemical technique for growing adhesive copper oxide films on copper with attractive colors ranging from gold-brown to pearl with intermediate colors from red violet to gold green. The technique consists of anodically dissolving copper at transpassive potentials in hot sodium hydroxide, and then depositing brilliant color films of Cu$_{\mathrm{2}}$O onto the surface of copper after the anodic potential has been turned off. The color of the copper oxide film depends on the temperature, the anodic potential, the time t$_{\mathrm{1}}$ of polarization, and the time t$_{\mathrm{2}}$, which is the time of immersion after potential has been turned off. The brilliant colored films were characterized using glancing angle x-ray diffraction, and the film was found to be primarily Cu$_{\mathrm{2}}$O. Cyclic voltammetry, chronopotentiometry, scanning electron microscopy, and x-ray photoelectron spectroscopy were also used to characterize these films. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z53.00006: Annealing Temperature Tuned Structural and Optical Properties of ZnO Sputtered Thin Films Arshad Bhatti, Madiha Sabeen, Malik Abdul-Rehman, Muhammad Bhopal, Awais Ali, Faisal Nasim We report the role of annealing in oxygen environment on the structure and optical properties of zinc oxide (ZnO) thin films grown in oxygen deficient environment. Thin films of two different thicknesses (300 nm and 500 nm) were sputter deposited and annealed from 400 to 800 $^{\circ}$C. X-ray diffraction showed better crystallinity of 500 nm thick films than 300 nm on annealing; however 300 nm films had grown in preferred orientation along the c-axis. The grain sizes increased from 0.1 to 0.45 $\mu $m from the as -- grown to the annealed at 800 $^{\circ}$C, respectively. Raman spectra showed blue shift in E$_{2}$High and 3E$_{2}$LO modes with the increase in the annealing temperature attributed to the enhancement of oxygen vacancies. The A$_{1}$ (TA$+$TO) mode showed red shift due to reduction in structural and surface defects. Room temperature PL showed two bands corresponding to UV and visible were due to band edge emission and defect assisted luminescence and showed distinctively different behavior in the annealed films. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z53.00007: Formation of a TiO(001) epitaxial thin film on TiO$_2$(110) induced by ion bombardment Oscar Rodriguez, Beatriz Martinez, Irene Palacio, Arantzazu Mascaraque, Jesus Lopez, German Castro, Juan Rubio, Pilar Ferrer, Matteo Monti, Jose Marco, Juan Beltran, Carmen Munoz TiO$_2$ is one of the most studied oxides. This is mainly due to its outstanding photochemical properties. More recently, a growing interest in this oxide has arisen in relationship with resistive switching mechanisms and its possible use in memristors. For all applications low energy ion bombardment has been frequently used to modify or enhace its properties. In this work we show that high doses of ion bombardment tranforms the upper layers of TiO$_2$(110) into epitaxial TiO(001). Surface diffraction and chemical analysis shows the initial transformation of the surface into a disordered suboxide which gives rise to an ordered layer of the monoxide after long irradiation times. This is achieved thanks to the oxygen preferential sputtering, the good registry between the TiO$_2$(110) (rutile) and the TiO(001) (rocksalt) structures (as salso shown by DFT calculations) and the diffusion induced by ion bombardment. To the best of our knowledge, this is the first time that low energy ion bombardment is used to transform the surface of an oxide into another material with a different crystalline structure, while keeping the single crystalline character of the surface. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z53.00008: Enhanced electrical properties by post thermal nitridation in atomic-layer-deposited HfO$_{2}$ on InP Yu-Seon Kang, Dae-Kyoung Kim, Hang-Kyu Kang, Kwang-Sik Jeong, Mann-Ho Cho, Dae-Hong Ko, Hyoungsub Kim, Jung-Hye Seo, Dong-Chan Kim We investigated the effects of post-nitridation in HfO$_{2}$ thin films grown on InP by atomic layer deposition on the structural, chemical, and electrical properties of the resultant film as well as its thermal stability compared to samples that were only thermally-annealed by comprehensive physical, electrical, and theoretical analyses. By post-deposition annealing under NH$_{3}$ vapor at 600${^\circ}$, an InN layer formed at the HfO$_{2}$/InP interface and ionized NH$_{x}$ was incorporated in the HfO$_{2}$ film. Accordingly, interfacial reactions were effectively suppressed in nitrided HfO$_{2}$/InP by controlling out-diffusion of In or P atoms from the substrate. Nitridation of HfO$_{2}$/InP modulated energy band parameters at the HfO$_{2}$/InP interface, thereby decreasing leakage current. Moreover, the nitridation process significantly suppressed the generation of D$_{it}$ due to controlled diffusion of In and P. DFT calculations showed that In$_{i}$ and P$_{i}$ in HfO$_{2}$ are closely related, with defect states within the band gap of InP. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z53.00009: Pulsed Laser Deposition Growth of Delafossite (CuFeO$_{2})$ thin films and multilayers Toyanath Joshi, Piero Ferrari, Pavel Borisov, Alejandro Cabrera, David Lederman Owing to its narrow band gap (\textless 2 eV) and p-type conductivity delafossite CuFeO$_{2}$ is attractive for applications in the field of solar energy conversion. Obtaining pure phase CuFeO$_{2}$ thin films, however, is relatively difficult. It is necessary to maintain the lowest possible Cu valency ($+$1) in order to avoid forming the comparably stable spinel compound CuFe$_{2}$O$_{4}$. We present a systematic study of the pulsed laser deposition (PLD) growth conditions for epitaxial (00.1) oriented CuFeO$_{2}$ thin films on Al$_{2}$O$_{3}$ (00.1) substrates. The secondary impurity phase, CuFe$_{2}$O$_{4}$, was removed completely by optimizing the growth conditions. RHEED, XRD and TEM showed that the pure phase delafossite films are highly epitaxial to the substrate. The chemical purity was verified by Raman and XPS. The indirect bandgap of 1.15 eV was measured using infrared reflectivity, and is in agreement with the CuFeO$_{2}$ bulk value. Finally, we discuss the growth and structural characterization of delafossite multilayers, CuFeO$_{2}$/CuGaO$_{2}$. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z53.00010: Fabrication and Characterization of Metal-Patterned SrCo$_{0.9}$Nb$_{0.1}$O$_{3-\delta}$ Thin Film Cathodes with Well-defined Geometry Iwnetim Abate, WooChul Jung, Sossina Haile A major obstacle to the study of fundamental properties of candidate cathode materials is the morphological complexity of the electrode-electrolyte interface in fuel cells. This complexity prevents a true determination of the catalytic mechanisms. To address this challenge, photolithography patterning technique has been used to make considerably simplified and well-defined electrode geometries. However, the time required for such fabrication is extreme. In this work, we employ a simple shadow-mask-patterning method to fabricate a perovskite oxide-metal composite structure. First, a dense thin film of SrCo$_{0.9}$Nb$_{0.1}$O$_{3}$(SCN) is grown on a Y$_{0.16}$Zr$_{0.84}$O$_{1.92}$ (YSZ) single crystal substrate by pulsed laser deposition. Patterned metal layers are subsequently deposited by DC sputtering with a shadow mask. Thermal stability and electrochemical properties of the fabricated composite cathodes are investigated by optical microscopy, scanning electron microscopy and AC impedance spectroscopy (ACIS). [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z53.00011: Resistive Switching Behaviour of SrCoO$_{x}$ thin films Chang Uk Jung, Octolia Togibasa Tambunan, Bo Wha Lee, Bae Ho Park, Ji-Yong Park, Myung Rae Cho, Yun Daniel Park, Seung Jin Kang, Miyoung Kim, Cheol Seong Hwang Resistance random access memory using metal-oxide insulator-metal structure is attracting considerable attention due to their potential high scalability and low switching current. The resistance switching behavior in many oxides is suggested to associate with local oxygen migration. An insulating brownmillerite SrCoO$_{\mathrm{2.5}}$ has been found to transform topotactically to conducting perovskite SrCoO$_{\mathrm{3}}$, due to the easy oxygen migration even at room temperature. Therefore, the SrCoO$_{x}$ offers a great opportunity to study the switching mechanism based on local oxygen migration. In this report, we succeed to fabricate TE/SrCoO$_{x}$/BE/SrTiO$_{\mathrm{3}}$ devices. The fabrication process covered the 100 nm SCO on 50 nm patterened bottom electrode using pulsed laser deposition. Furthermore, the 80 nm top electrodes by litography patterning was deposited using e-beam evaporator metal deposition. From the TE/SCO/BE memory cell we observed resistance switching with some evidence of conducting filament. We discuss the switching mechanism through the analysis of composition, structure, and dimension of the filaments. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z53.00012: Sulfide and Oxide-Sulfide Combinatorial Libraries by Co-Sputtering with an Atomic Sulfur Source Joshua Ford, Adam Welch, Christopher Caskey, Bart Van Zeghbroeck, Philip Parilla, David Ginley, Andriy Zakutayev, John Perkins We report a deposition method with improved control over the sulfur content in thin-films through the addition of a radio frequency (RF) solids atom source (cracker) to a multiple-source sputtering system. Co-sputtering from one or two targets provides a compositional gradient across a 2'' substrate. In addition, a temperature gradient orthogonal to the composition gradient is induced. An RF solids cracker is used to provide controllable amounts of activated sulfur during the deposition. The composition gradient, temperature gradient and activated sulfur can be used concurrently to control the composition and phase of the thin films. In proof-of-principle experiments, Cu$_{\mathrm{2}}$S films have been grown from both Cu and Cu$_{\mathrm{2}}$O targets as well as Bi$_{\mathrm{x}}$O$_{\mathrm{y}}$S$_{\mathrm{z}}$ films with tunable oxygen to sulfur ratios from a Bi$_{\mathrm{2}}$O$_{\mathrm{3}}$ target. Further, the independent tuning of anion and cation ratios is demonstrated by the growth of BiCuOS. The successful growth of both sulfide and oxide-sulfide compounds demonstrates the viability of this hybrid approach. Finally, this hybrid deposition approach is likely extendable to phosphides and oxide-phosphides. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z53.00013: Silicon LEDs fabricated using high fluence carbon ion implantation into heated silicon targets Sarah Purdy, John R. McLeod, Himanshu Rai, Andrew Knights, Michael Bradley Fabrication of silicon compatible light emitting diodes (LEDs) is a major goal for modern integrated circuit designers. Ion implantation is a materials processing technique that can be used to engineer materials for this application, by making small to large changes in the stoichiometry at the surface or subsurface of a material. In this study carbon ions were implanted at 20 keV and fluences of 3, 5, 7 and 10 x10$^{16}$ ions/cm$^{2}$ into silicon wafers which were maintained at 400 $^{\circ}$C during the implantation process. The implanted wafers were then annealed at 1000 $^{\circ}$C in flowing nitrogen for 1h. FTIR was used to confirm the formation of SiC. The electronic structure of the implanted and annealed wafers was probed using X-Ray absorption spectroscopy at the silicon L2,3-edge. After treatment, physical vapor deposition was used to apply metal contacts onto the wafers: a semi-transparent Schottky contact (25nm Au) on the implanted surface, and an Ohmic contact (150nm Al) on the back of the wafer. The current-voltage curves and light emission spectra of the resulting Schottky LEDs were collected using a water-cooled electroluminescence testing system. The fabricated LEDs showed turn-on voltages of $\sim$ 2-3V, and the emission spectra showed a broad luminescence band in the orange to infrared (550-900nm) region. The devices are bright, broadband emitters, easily visible to the naked eye and represent one prototype silicon photonics device architecture. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z53.00014: Thin Film Substrates from the Raman spectroscopy point of view Lev Gasparov, Theo Jegorel, Lars Loetgering, Srimanta Middey, Jak Chakhalian We have investigated ten standard single crystal substrates of complex oxides on the account of their applicability in the Raman spectroscopy based thin film research. In this study we suggest a spectra normalization procedure that utilises a comparison of the substrate's Raman spectra to those of well-established Raman reference materials. We demonstrate that MgO, LaGaO$_{3}$, (LaAlO$_{3}$)$_{0.3}$(Sr$_{2}$AlTaO$_{6}$)$_{0.7}$ (LSAT), DyScO$_{3}$, YAlO$_{3}$, and LaAlO$_{3}$ can be of potential use for a Raman based thin film research. At the same time TiO$_{2}$ (rutile), NdGaO$_{3}$, SrLaAlO$_{4}$, and SrTiO$_{3}$ single crystals exhibit multiple phonon modes accompanied by strong Raman background that substantially hinder the Raman based thin film experiments. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z53.00015: ABSTRACT WITHDRAWN |
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