Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A1: Spectroscopy of Two-Dimensional Electronic Systems
Sponsoring Units: DCMPChair: Aron Pinczuk, Columbia University
Room: Morial Convention Center LaLouisiane AB
Monday, March 10, 2008 8:00AM - 8:36AM |
A1.00001: Fractionally Charged Excitations in Optical Emission Spectroscopy Invited Speaker: We discuss recent experiments and theory of the signatures of fractionally charged excitations in optical emission spectroscopy of two dimensional electrons subjected to a high magnetic field [1]. We show that the two flux quanta in a composite fermion interacting with an exciton lead to filling factor dependent features in optical emission spectrum symmetric around filling factor 1/2 while fractionally charged excitations lead to fractionally charged exciton. In the vicinity of the incompressible filling factor 1/3 state we observe a doublet structure in the emission line, corresponding to excitations of the incompressible fluid. At filling factors lower then 1/3 , corresponding to the transition to a compressible, metallic state, a new emission line appears which is attributed to the fractionally charged quasi-exciton. These observations are supported by extensive numerical calculations of the emission spectrum of finite number of electrons and holes on a Haldane sphere. \newline [1] M. Byszewski, B. Chwalisz, D.K. Maude, M.L. Sadowski, M. Potemski, T. Saku, and Y. Hirayama, S. Studenikin, D. G. Austing, A.S. Sachrajda, and P. Hawrylak, Nature Physics 2, 239 (2006). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A1.00002: Soft spin waves and magnetic instability in Skyrmion systems Invited Speaker: In this work the highly correlated ground states of spin in 2D electron layers (2DES) near filling factor $\nu $=1 are probed by inelastic light scattering [1]. In this filling factor range the ground state of the 2DES is affected by the proliferation of spin-charge textures known as Skyrmions. Recent experiments [2] have suggested the possibility of observing a long range Skyrme crystal phase with non-collinear magnetic order at low temperatures. The magnetic properties of the 2DES close to $\nu $=1 are studied by the direct measurement of the low-lying spin wave excitations by inelastic light scattering between 2.5K and 40mK. We discovered a very low energy spin wave that emerges on both sides of $\nu $=1. The spin wave is well below the Zeeman energy and exhibit surprising soft behavior with temperature changes: its energy increases with temperature and reaches the Zeeman energy for temperatures above 2K. These results suggest an instability of the 2DES towards magnetic order at low temperatures and filling factors close to $\nu $=1. The spin excitation spectra are consistent with the ordering of the in-plane components of spin in a square Skyrme crystal phase proposed in theoretical evaluations [3], but never fully confirmed by experiments. Our experiments create venues for the determination of Skyrme crystal phases from measurements of low-lying spin excitations by inelastic light scattering. \newline [1] Y. Gallais et al, arXiv:0709.3240 \newline [2] G. Gervais et al., Phys.Rev.Lett. 94, 196803 (2005) \newline [3] L. Brey et al., Phys.Rev.Lett. 75, 2562 (1995) [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A1.00003: High Resolution Spectroscopy of the Quantum Hall Liquid Invited Speaker: We present precise and unprecedentedly high resolution spectra of the tunneling density of states (TDOS) of a cold two dimensional electron system (2DES) in GaAs over an energy range from 15 meV above to 15 meV below the Fermi surface. The results provide the first direct measurements of the width of the single-particle exchange gap and lifetimes in the quantum Hall system. At higher energies, we show the first observations of exchange-induced spin-splittings in fully filled or unfilled Landau levels not at the Fermi energy. The results demonstrate a counter-intuitive fact: the high energy spectrum reflects correlations that only appear at very low temperatures. For instance, upon raising the temperature from 100 mK (0.01 meV) to 1 K (0.1 meV) changes are seen in the spectrum at 10 meV away from the Fermi energy. Along with measurements of exchange splittings and lifetimes, we observe an unpredicted new structure appearing only at high magnetic fields and low temperatures that appears to be a long lived quasi-particle. The results are made possible by a novel technique, time domain capacitance spectroscopy. It allows us to measure the TDOS of a 2DES with resolution only limited by temperature, even at large tunneling energies. In TDCS, sharp voltage pulses disequilibrate a 2DES from a nearby metallic contact inducing a tunnel current perpendicular to the plane of the 2DES. We detect this current by monitoring the image charge of the tunneling electrons on a distant electrode. No ohmic contact to the 2DES is required. The technique works even when the 2DES is empty or has vanishing in-plane conductivity, as frequently occurs in the quantum Hall effect. Importantly, we can eliminate the effects of ohmic heating in the experiment by using short duty cycle pulses, with currents flowing only 0.01\% of the time. The obtained spectra reveal the beautiful and difficult to reach structure present far from the Fermi surface in the quantum Hall system. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A1.00004: Magnetotunneling spectroscopy: Imaging electron wavefunctions and measuring electron dispersion curves in GaMnAs- and GaAsN-based heterostructures Invited Speaker: Magnetotunnelling spectroscopy is a powerful tool for imaging the wavefunctions of electrons in quantum wires [1] and dots [2] and for measuring the energy-wavevector dispersion curves of holes [3] and electrons [4] in novel quantum well structures. It uses the effect of the Lorentz force to tune the in-plane momentum of a tunneling electron when it enters a quantum-confined structure [4]. This talk will describe recent work to spatially image the ground and excited state wavefunction of electrons confined in quantum dots in ferromagnetic GaMnAs tunnel diodes. These dots are formed by the electrostatic potential arising from clusters of charged Mn interstitial donors. It will also be shown how the fragmented conduction electron dispersion curves of GaAsN give rise to highly non-linear electron dynamics and a new type of negative differential conductivity effect. \newline [1] Beton et al, Phys Rev Lett \textbf{75}, 1996 (1995); [2] Vdovin et al, Science 290, 122 (2000) and Patane et al, Phys Rev B \textbf{65}, 165308 (2002); [3] Hayden et al, Phys Rev Lett \textbf{66}, 1749 (1991); [4] Endicott et al, Phys Rev Lett \textbf{91} 126802 (2003) [Preview Abstract] |
Session A2: New Developments in HTSC I
Sponsoring Units: DCMPChair: Stephen Kivelson, Stanford University
Room: Morial Convention Center LaLouisiane C
Monday, March 10, 2008 8:00AM - 8:36AM |
A2.00001: Microscopic Theory of the Phenomena in Cuprates Invited Speaker: This work is based on two principles, (1) that the microscopic model for the Cuprates must reflect their unique properties, and (2) that there exists a quantum critical point in the superconducting region of the phase diagram of the Cuprates which marks the end of an unusual ordered phase and whose quantum fluctuations determine the ``strange metal'' or marginal fermi-liquid properties. A mean-field theory of such a microscopic model predicts the time-reversal breaking order parameter in the underdoped region which has now been observed experimentally in four different families of Cuprates. The quantum critical fluctuations of this order parameter are governed by topological excitations and are derived to have the $\omega/T$ scaling and spatial locality suggested long ago for the marginal fermi-liquid phase. The coupling of the topological excitations to the fermions is shown to be $\propto {\bf \nabla}\times{\bf j}$, where ${\bf j}$ is the fermion current operator. Such a coupling produces an attractive pairing interaction in the $d$-wave channel. Experimental evidence for the applicability of the ideas and calculations to properties of cuprates and several predictions are provided. \newline \newline Work in the past three years on this problem was done in collaboration with Vivek Aji, Arcadi Shehter and Lijun Zhu. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A2.00002: Dichroism in the pseudogap phase observed through high precision Sagnac interferometry Invited Speaker: Polar Kerr effect in the high-$T_c$ superconductor $\mathrm{YBa_2Cu_3O}_{6+x}$\footnote{Jing Xia, Elizabeth Schemm, G. Deutscher, S. A. Kivelson, D. A. Bonn, W. N. Hardy, R. Liang, W. Siemons, G. Koster, M. M. Fejer, and A. Kapitulnik, arXiv:0711.2494 (2007).} was measured at zero magnetic field with high precision using a cryogenic Sagnac fiber interferometer. We observed non-zero Kerr rotations on the order of $\sim$ 1 $\mu$rad appearing near the pseudogap temperature $T^*$, and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature. We will show results from single crystals and oriented films. Preliminary results on other high-temperature superconductors will also be discussed. \par Work done in collaboration with Jing Xia and Aharon Kapitulnik. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A2.00003: Orbital-Current phases in one- and two-dimensional strongly correlated systems Invited Speaker: Although we now know that strongly correlated systems can have several type of conventional order ranging from charge or spin order to superconductivity, the possibility that they present also more exotic phases remains an elusive but very challenging question. In particular whether such systems can have orbital current order for realistic interactions has been strongly debated. Recently this question has come to attention again due to theoretical proposals and subsequent neutron scattering experiments suggesting that this could be the case in the pseudogap phase of High Tc. Tackling this issue directly for the two dimensional case is difficult, since no uncontrolled method can be used beyond exact diagonalization for very small clusters. However this question can be looked at on the one dimensional (ladder) version of this problem, where such orbital current phases can be studied by well controlled methods such as bosonization. I will present the results we obtained on these systems and discuss in particular the comparison between the case of a simple Hubbard ladder [1] versus a three band model (Cu-O ladder) [2]. In order to tackle these issues for the two-dimensional case, we have performed a variational Monte Carlo analysis for a three band Cu-O model. This technique although depending on the quality of the variational wave function has the advantage of being essentially free of numerical problems. I will discuss the phases obtained by this approach as well as the possible experimental consequences. \\ These works are a collaboration with E. Orignac; P. Chudzinski and M. Gabay; C. Weber, A. La\"uchli and F. Mila.\\{} [1] E. Orignac and T. Giamarchi, PRB 56 7167 (1997); \\{} [2] P. Chudzinskii, M. Gabay and T. Giamarchi, PRB 76, 161101 (R) (2007); [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A2.00004: Inhomogeneous Superconductivity in YBa$_2$Cu$_3$O$_y$ and La$_{2-x}$Sr$_x$CuO$_4$ Above $T_c$ Invited Speaker: An exciting development in the immense research effort focused on resolving the origin of high-$T_c$ superconductivity, is the growing experimental evidence for signatures of superconductivity in cuprate materials at temperatures far above $T_c$. Recent STM experiments on Bi$_2 $Sr$_2$CaCu$_2$O$_{8+\delta}$ have provided new insight into the precise nature of these pairing correlations, by revealing the occurrence of nanometre-sized pairing regions above $T_c$. Whether nanoscale inhomogeneous superconductivity is universal to the cuprates, and whether $T_c$ is driven by Kosterlitz- Thouless physics or Josephson coupling between nanometre-sized superconducting regions are matters of current debate. Very recently we have used $\mu$SR to probe the local response in the bulk of YBa$_2$Cu$_3$O$_y$ and La$_{2-x}$Sr$_x$CuO$_4$ single crystals to a large applied magnetic field (H = 7 T). At temperatures above $T_c$, we detect a spatially inhomogeneous magnetic field that tracks the hole-doping dependences of both $T_c$ and the superfluid density at $T = 0$ K. Our experiments are inconsistent with the field inhomogeneity above $T_c$ being caused by electronic magnetic moments or a vortex liquid. Instead they are explained by the existence of nanometre-size superconducting regions with a local $T_c$ that exceeds the bulk $T_c$. In YBa$_2$Cu$_3$O$_y$, we detect a spatially inhomogeneous response to field that persists beyond $T = 200$ K, indicating that the basic ingredients for superconductivity near room temperature already exist in spatially localized regions of this material. A lingering question is the origin of the weak magnetism detected earlier in YBa$_2$Cu$_3$O$_y$ by zero-field $\mu$SR, at temperatures below the pseudogap temperature $T^*$. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A2.00005: Observation of a Sharp Magnetic Transition at the Pseudogap Temperature in YBa$_{2}$Cu$_{3}$O$_{6.6}$ Invited Speaker: Polarized neutron diffraction has been used to demonstrate magnetic order in a crystal of YBa$_{2}$Cu$_{3}$O$_{6.6}$ that displays an exceptionally sharp superconducting transition. Earlier experiments showed a gradual increase of intensity of magnetically scattered neutrons from underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$ crystals for reflections that do not break translational symmetry. The present experiment confirms this but displays a much sharper transition demonstrating that the width of the magnetic transition depends on the quality of the superconducting transition. This relationship is strengthened by the fact that other magnetic properties sensitive to the superconductivity, such the resonance and excitation spectra are also exceptionally well defined when measured with the same sample. The magnetic order is observed at the temperature of the pseudogap transition and the close relationship between the quality of the magnetic properties and the superconductivity suggests that the pseudogap is directly connected to the magnetic order. [Preview Abstract] |
Session A3: Frontiers in Computational Materials
Sponsoring Units: DCOMP TMSChair: Giulia Galli, University of California, Davis
Room: Morial Convention Center RO2 - RO3
Monday, March 10, 2008 8:00AM - 8:36AM |
A3.00001: Multi-Scale Modeling from First-Principles Invited Speaker: Electronic structure theory (the nature of the chemical bond) is the base and the finest scale for multi-scale modeling of the function of materials. Frequently it is assumed that details at this base do not matter when length and time scales approach meso- or macroscopic proportions (e.g. {$\mu$}m and minutes). In this talk I will show for various examples that details matter indeed. When accuracy is lacking at the base, there is little hope for predictive results at any level of modeling that follows. I will also emphasize the importance of reversible mapping and error control between the different levels of multi-scale modeling when moving up the chain of methods to successively increasing spatial and temporal dimensions. -- In this context I will also address the sometimes problematic accuracy of present day density functional theory methods and show how it can be determined and errors corrected. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A3.00002: Computational Approaches for Strongly Correlated Materials: an Electronic Structure Theory Perspective. Invited Speaker: Density functional theory known to work well for weakly correlated materials fails to attack real strongly correlated phenomena, and recent progress in understanding those using many-body model-hamiltonian-based dynamical mean-field theory has triggered developments of new approaches for computational material science in searching for alternatives to DFT. In this talk one of such new techniques, a spectral density functional theory, which considers total free energy as a functional of a local electronic Green function, will be discussed. Applications of the method to compute energetics, spectroscopy, lattice dynamics and exchange interactions of classes of materials such as heavy fermion and high temperature superconductors as well as actinide systems will be given. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A3.00003: First-principles studies of electrical transport in nanoscale molecular junctions Invited Speaker: Understanding the conductance of individual molecular junctions is a forefront topic in theoretical nanoscience. The development of a general, efficient atomistic approach for treating an open system out of equilibrium with good accuracy, and then using it to inform experiment, is a significant open challenge in the field. Here I will describe studies where first-principles techniques, based on density functional theory (DFT) and beyond, are used to investigate some of the fundamental issues associated with single-molecule transport measurements. After a brief summary of previous work, a DFT-based scattering-state approach is presented and applied to H$_2$ and amine-Au linked molecular junctions [1], two systems for which there exist reliable data [2]. Similar to most ab initio studies, we rely on a Landauer approach within DFT for junction conductance. Using this framework, which has proven relatively accurate for metallic point contacts, good agreement with experiment is obtained for the H$_2$ conductance. For amine-Au linked junctions, however, the computed conductance is significantly larger than that measured,although structural trends are reproduced by the calculations. To explore this further, we draw on GW calculations of a prototypical metal-molecule contact, benzene on graphite, where interfacial polarization effects are found to drastically modify frontier orbital energies [3]. A physically motivated model self-energy correction is developed from our GW calculations,applied to the amine case, and shown to quantitatively explain the discrepancy with experiment. The importance of many-electron corrections beyond DFT for accurately computing molecular conductance and understanding experiments is thoroughly discussed. [1] S. Y. Quek {\it et al.}, Nano Lett {\bf 7}, 3482 (2007); K. H. Khoo {\it et al.}, submitted (2007). [2] R. Smit {\it et al.}, Nature {\bf 419}, 906 (2002); L. Venkataraman {\it et al.}, Nature {\bf 442} ,904 (2006). [3] J. B. Neaton {\it et al.}, Phys. Rev. Lett. {\bf 97}, 216405 (2006). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A3.00004: First-Principles Thermodynamics and Kinetics of Advanced Hydrogen Storage Materials Invited Speaker: Hydrogen-fueled vehicles require a cost-effective, light-weight material that binds hydrogen strongly enough to be stable at ambient pressures and temperatures but weakly enough to liberate H2 with minimal heat input. Since none of the simple metal hydrides satisfy all the requirements for a practical H2 storage system, recent research efforts have turned to complex hydrides and advanced multicomponent material compositions. We will show that first-principles density-functional theory (DFT) calculations have become a valuable tool for understanding and predicting novel hydrogen storage materials. Recent studies in our group have used DFT calculations to (i) predict crystal structures of new solid-state hydrides, (ii) determine phase diagrams and thermodynamically favored reaction pathways in multinary hydrides, and (iii) study microscopic kinetics of diffusion, phase transformations, and hydrogen release. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A3.00005: Liquid Metal Embrittlement: new understanding for an old problem Invited Speaker: When liquid metals are brought into contact with other polycrystalline metals, deep liquid-filled grooves often form at the intersections of grain boundaries and the solid-liquid interface. In some systems, e.g., Al-Ga, Cu-Bi and Ni-Bi, the liquid film quickly penetrates deep into the solid along the grain boundaries and leads to brittle, intergranular fracture under the influence of modest stresses. This is a form of liquid metal embrittlement (LME).~ This phenomenon is ubiquitous in material processing and is particularly important in nuclear reactor scenarios in which liquid metals are used as coolants and as spallation targets. The penetration of a liquid phase along the grain boundary is a complex phenomenon, involving several different types of simultaneous processes. The tendency for and rate of LME are also sensitive to externally controllable factors such as temperature and applied stress. Because of the interplay between the underlying phenomena that occur in LME, it has been difficult to perform experiments that can be interpreted to understand which processes control LME and which are simply parasitic. We study LME by performing molecular dynamics simulations of an Al bicrystal in contact with liquid Ga and investigate how Ga penetrates along the grain boundaries during the early stages of the wetting process. We use the simulation results to propose a new mechanism for LME and compare it with general trends gleaned from a series of LME experimental studies. [Preview Abstract] |
Session A4: Polymers at Surfaces: Adhesion, Tribology and Patterning
Sponsoring Units: DPOLYChair: Steve Granick, University of Illinois at Urbana-Champaign
Room: Morial Convention Center 206
Monday, March 10, 2008 8:00AM - 8:36AM |
A4.00001: Interfacial engineering using heteropolymers with adjustable monomer sequences (HAMS) Invited Speaker: Heteropolymers with adjustable monomer sequences (HAMS) represent a new type of functional random copolymers that could play an important role in emerging areas pertaining to interfacial science and polymer assembly. Due to their disordered but tailorable co-monomer sequence distribution HAMS are capable of adsorbing selectively at interfaces and recognizing patterns on chemical targets (i.e., chemically patterned substrates). HAMS are synthesized in a laboratory by `coloring' the segments of a collapsed homopolymer, A, with a functionalizing agent, B, and then unraveling the resultant polymer to yield a random sequence of A and B blocks, which `remembers' its original collapsed conformation and hence prefers some conformations over others. In the presentation, we will provide details pertaining to the experimental formation of HAMS and studying their physico-chemical characteristics. We will also provide examples of a few case studies that unravel the tailorable interfacial and self-assembly character of HAMS made of poly(styrene-co-4-bromostyrene) and its derivatives. In addition, we present results of computer simulation studies providing molecular insight into forming HAMS. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A4.00002: Carbon Nanotube-Based Synthetic Gecko Tapes Invited Speaker: Wall-climbing geckos have unique ability to attach to different surfaces without the use of any viscoelastic glues. On coming in contact with any surface, the micron-size gecko foot-hairs deform, enabling molecular contact over large areas, thus translating weak van der Waals (vdW) interactions into enormous shear forces. We will present our recent results on the development of synthetic gecko tape using aligned carbon nanotubes to mimic the keratin hairs found on gecko feet. The patterned carbon nanotube-based gecko tape can support a shear stress (36 N/cm$^{2})$ nearly four times higher than the gecko foot and sticks to a variety of surfaces, including Teflon. Both the micron-size setae (replicated by nanotube bundles) and nanometer-size spatulas (individual nanotubes) are necessary to achieve macroscopic shear adhesion and to translate the weak vdW interactions into high shear forces. The carbon nanotube based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics and space applications. The mechanism behind these large shear forces and self-cleaning properties of these carbon nanotube based synthetic gecko tapes will be discussed. This work was performed in collaboration with graduate students Liehui Ge, and Sunny Sethi, and collaborators from RPI; Lijie Ci and Professor Pulickel Ajayan. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A4.00003: Polymer adhesion at surfaces: biological adhesive proteins and their synthetic mimics Invited Speaker: Mussels are famous for their ability to permanently adhere to a wide variety of wet surfaces, such as rocks, metal and polymer ship hulls, and wood structures. They accomplish this through specialized proteins collectively referred to as mussel adhesive proteins (MAPs). The biophysical aspects of MAP adhesion is being revealed through the use of single molecule force measurements. The results provide insight into the adhesive roles of key amino acids found in these proteins, including the magnitude of adhesive forces, cooperative effects, and their self-healing properties. This molecular-level information is being incorporated into designs of biomimetic polymer coatings for a variety of applications. Our biomimetic approach to polymer design will be illustrated by a few examples where adhesive constituents found in MAPs are exploited to make wet-adhesive polymer coatings. In addition, small molecule analogs of MAPs can be used to apply thin functional films onto virtually any material surface using a facile approach. These coatings have a variety of potential uses in microelectronics, water treatment, prevention of environmental biofouling, and for control of biointerfacial phenomena at the surfaces of medical/diagnostic devices. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A4.00004: Patterning inorganic nanoparticles in Polymer Films Invited Speaker: |
Monday, March 10, 2008 10:24AM - 11:00AM |
A4.00005: Wrinkling, Crumpling and Snapping for Surface Property Control Invited Speaker: Upon the development of a critical stress, many materials and geometries experience a mechanical instability, which produces significant changes in geometry with very small changes in stress. In nature, mechanical instabilities are ubiquitous with the definition of shape, morphology, and function. Examples range from wrinkles on human skin to the snapping of Venus Flytrap leaflets. Inspired by these examples and others, we use elastic instabilities to control the morphology and function of soft polymer surfaces. We present three strategies. The first is a novel approach for the development of surface wrinkles on a top-constrained elastomer surface. We demonstrate and understand the control of kinetically-trapped and equilibrium wrinkle morphologies associated with changes in the materials properties and geometric constraint. These structures are stabilized to create surfaces with enhanced adhesion and advantageous optical properties. A second strategy is based on the controlled buckling of surface attached sheets. This method allows the fabrication of responsive surface structures that are prone to snap-through instabilities and the fabrication of pattern features that are difficult, if not impossible, to achieve with any other method. The third strategy brings the bio-inspired surface control full circle with the use of mechanical instabilities to control and characterize monolayer sheets of biological cells. [Preview Abstract] |
Session A5: Supersolid 4He: A New State of Matter
Sponsoring Units: DCMPChair: John Beamish, University of Alberta
Room: Morial Convention Center RO1
Monday, March 10, 2008 8:00AM - 8:36AM |
A5.00001: Probable heat capacity signature of the supersolid transition Invited Speaker: We report our heat capacity measurements [1] of solid $^{4}$He down to 40mK, well into the apparent supersolid region of the phase diagram. We observed a broad peak in the specific heat centered near 75mK in $^{4}$He samples containing 1ppb, 0.3ppm, and 10ppm $^{3}$He impurities. In addition, our measurements of samples containing 10ppm and 30ppm of $^{3}$He have revealed a temperature-independent contribution to the heat capacity that scales with the number of isotopic impurities. New measurements with higher resolution are in progress. \newline [1] Nature (London) 449, 1025 (2007). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A5.00002: Probing the upper limit of the nonclassical rotational inertia Invited Speaker: Recently, we have used torsional oscillators to study the dependence of the nonclassical rotational inertia on sample confinement, expressed as surface to volume ratio S/V [1]. When we decreased the width of annular helium sample we observed an increase of the supersolid fraction by three orders of magnitude to 20 \% in a 150 $\mu$m wide annulus. As an extension of those measurements, we have built torsional oscillators with even smaller gaps down to 25 microns. We will give a brief description of the experimental setup and present the results of those measurements. \newline [1] A.S.C. Rittner and J.D. Reppy, Phys. Rev. Lett. \textbf{98}, 175302(2007) [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A5.00003: Frequency dependence and Hysteretic behavior in Non-Classical Rotational Inertia of Solid $^{4}$He Invited Speaker: We have constructed a compound torsional oscillator having two resonance frequencies for studying non-classical rotational inertia (NCRI) of solid $^{4}$He. The oscillator allows us to study NCRI and supersolid effects of the \textit{identical} solid $^{4}$He sample grown in a cylindrical container at 496 and 1173 Hz. We have grown and studied solid samples with final solid pressures between 27 and 42 bar. The observed features are qualitatively similar in all solid samples\textbf{.} NCRI fractions at sufficiently low oscillation drive and at the lowest temperature are only about 0.1 {\%} and consistent with cylindrical cells in other laboratories. NCRI fraction below 35 mK does not depend on frequency nor temperature. At T $>$ 35 mK, NCRI fraction observed in the lower mode is smaller than that in the higher mode. ``Transition'' into supersolid state occurs at a higher temperature in the higher mode than the lower one. The peak in extra dissipation due to solid $^{4}$He is greater in the lower mode by a factor 1.7 than in the higher mode. The frequency dependence of the magnitude of NCRI will be compared with existing theoretical predictions. In addition to the frequency dependent effects at low oscillation drive, we have observed hysteretic behavior in NCRI fraction depending on the history of oscillation drive and temperature from the normal state above 300 mK to low temperatures. We find that the supersolid state below 40 mK can have different NCRI fractions depending on the particular sequence of oscillation amplitude. Above about 50 mK, however, NCRI fraction does not depend on the history of oscillation amplitude changes. We also observe a time dependent overshoot in the dissipation of solid when the NCRI fraction is increased by decreasing the oscillation drive. The general behavior of this relaxation phenomenon is rather complex depending on temperature, history of oscillation amplitude and memory effects. Some of the observations share common features with vortex motion and glassy behavior. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A5.00004: Superfluid defects in solid Helium-4: grain boundaries, dislocations, superglass Invited Speaker: First principle quantum Monte Carlo simulations and recent experiments all point out that ideal, defect free, crystals of Helium-4 are not supersolid. Contrary to naive expectation that disorder inhibits superfluidity and suppresses superfluid response due to localization effects, disorder in quantum solids works in the opposite direction: if particles in the ideal crystal are already localized then defects can help to make the sample superfluid. The theory of superfluidity in lower dimensional defective structures embedded in a higher dimensional bulk has its own twists since such phenomena as superfluidity, roughening and defect mobility may be strongly linked. An unusual behavior is expected in the 3D network of 1D liquid channels when normal state at temperatures orders of magnitude above $T_c$ is indistinguishable from that of a superfluid. The possibilities for ``designing'' crystallographic defects are countless, and in the strongly correlated system each case (superfluid or not) has to be considered separately. We find that generic grain boundaries and the screw dislocation along the z-axis are superfluid, while special types of boundaries and edge dislocations are insulating [1,2]. We also find that Helium-4 can form a metastable superfluid glass [3]. Whether these findings are relevant for the explanation of supersolid and other experiments remains an open question. \begin{enumerate} \item L. Pollet, M. Boninsegni, A.B. Kuklov, N.V. Prokof'ev, B.V. Svistunov, and M. Troyer, Phys. Rev. Lett. {\bf 98}, 135301 (2007). \item M. Boninsegni, A.B. Kuklov, L. Pollet, N.V. Prokof'ev, B.V. Svistunov, and M. Troyer, Phys. Rev. Lett. {\bf 99}, 035301 (2007). \item M. Boninsegni, N. Prokof'ev, and B. Svistunov, Phys. Rev. Lett. {\bf 96}, 105301 (2006). \end{enumerate} [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A5.00005: Vortex liquid Invited Speaker: |
Session A6: Novel Channel Materials for CMOS Technology
Sponsoring Units: FIAPChair: Matthew Copel, IBM Thomas J. Watson Research Center
Room: Morial Convention Center RO4
Monday, March 10, 2008 8:00AM - 8:36AM |
A6.00001: III-V MOSFETs: From Materials {\&} Physics to Devices Invited Speaker: Gallium-Arsenide metal-oxide-semiconductor field-effect-transistors (MOSFET) have finally been demonstrated with performance metrics matching the predictions of semiconductor device models. Recent discoveries and inventions in many areas including materials and fabrication, semiconductor physics, interface chemistry, semiconductor interface analysis, and semiconductor device design and process have contributed to this success. In my invited talk, I will review some select areas including the unique properties of interfaces formed between Ga$_{2}$O molecules and a GaAs surface, a high permittivity ($\kappa \quad \cong $ 20) GdGaO/Ga$_{2}$O$_{3}$ dielectric stack providing both a device quality interface and band-offsets on GaAs required for MOSFET operation, a semiconductor heterostructure for mitigation of high band-edge interface-state density, and device design criteria for high electron channel mobility and MOSFET drive current. Performance metrics of present metal-gate GaAs enhancement-mode MOSFETs such as electron channel mobility, drive current, transconductance, and threshold voltage will be discussed. GaAs MOSFETs with In$_{0.3}$Ga$_{0.7}$As channel layers exhibit typical electron peak mobilities exceeding 5,000 cm$^{2}$/Vs, an improvement of a factor of 20 over silicon based high-$\kappa $ metal-gate inversion-mode MOSFETs. Even higher electron mobilities surpassing 12,000 cm$^{2}$/Vs have been measured in In$_{0.75}$Ga$_{0.25}$As channel layers. Beside the use of channel materials such as In$_{x}$Ga$_{1-x}$As with high bulk electron mobility, the physics of device operation is distinctively different from silicon inversion-mode MOSFETs. III-V MOSFET are now considered an option for CMOS based circuitry beyond the 22 nm node of the International Technology Roadmap for Semiconductors. High channel mobilities and the first successful implantation of III-V MOSFETs seem to justify such contemplation, however, many obstacles remain. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A6.00002: Stability of Metal Oxide/Ge and Metal Oxide/III-V Interfaces and Implications for Low Defect Density MOS Devices Invited Speaker: The need to achieve high performance in MOS transistors as they scale to their ultimate size limits prompts interest in channel materials, such as Ge and III-V compound semiconductors, which exhibit larger intrinsic carrier mobilities than Si. Given the need to reduce gate leakage current density while maintaining electrostatic control of the devices, it is necessary to deposit high-k gate dielectrics onto these novel channel materials. Unlike silicon, high mobility channel materials do not form a highly-stable and stoichiometric native oxide; therefore, control of the state of oxidation at the metal oxide dielectric/channel interface during and after gate dielectric deposition is essential. This presentation will summarize findings reported to date on 1) chemical stability of Ge and III-V surfaces in the presence of oxygen and 2) oxide/channel defect formation and passivation. New results on pre-high-k chemical surface preparation, structural modification during metal oxide deposition and the resulting effects on MOS capacitor and transistor characteristics will also be presented, with emphasis on Al$_{2}$O$_{3}$ and HfO$_{2}$ gate insulators grown by atomic layer deposition onto Ge and InGaAs channels. \textit{In situ} and \textit{ex situ} monitoring of chemical bonding at the gate insulator/channel interface by photoelectron spectroscopy will be correlated with the D$_{it}$, fixed charge and charge trapping behavior of MOS devices. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A6.00003: Dangling-bond defects and hydrogen passivation in germanium Invited Speaker: The application of germanium in complementary metal-oxide semiconductor (CMOS) technology is hampered by high interface-state densities, the microscopic origin of which has remained elusive. Using first-principles calculations, we have investigated the atomic and electronic structure of prototype germanium dangling-bond defects [1]. The computational approach is based on density functional theory, and in order to overcome band-gap problems we have also performed quasiparticle calculations based on the GW approach. Surprisingly, the germanium dangling bonds give rise to electronic levels below the valence-band maximum. They therefore occur exclusively in the negative charge state, explaining why they have eluded observation with electron spin resonance. The associated fixed charge is likely responsible for threshold-voltage shifts and poor performance of n-channel transistors. At silicon/silicon dioxide interfaces, hydrogen is successfully used to passivate dangling-bond defects. We have therefore also investigated the interaction of hydrogen with germanium. In contrast to silicon and other semiconductors in which hydrogen behaves as an amphoteric impurity, interstitial hydrogen in germanium is stable only in the negative charge state, i.e., it behaves exclusively as an acceptor. Passivation of dangling bonds by hydrogen will therefore be ineffective, again explaining experimental observations. Other cases where unusual interfacial defects and problems with hydrogen passivation may occur will be discussed. \\ \\ Work performed in collaboration with A. Janotti, P. Rinke, and C. G. Van de Walle, and supported by the Semiconductor Research Corporation. \\ \\ 1. J. R. Weber, A. Janotti, P. Rinke, and C. G. Van de Walle, Appl. Phys. Lett. 91, 142101 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A6.00004: Scanning-tunneling microscopy and spectroscopy of oxide deposition on III-V semiconductor surfaces Invited Speaker: The correlation between the atomic bonding structure and the electronic structure at oxide-semiconductor interfaces is critical to understanding how atomic scale changes in electronic structure can cause localization of electrons or holes at these interfaces. All logic devices function by having an electric field perturb the electronic structure of a semiconductor to change its resistance thereby activating the device. The key material in this process is the interface between the gate oxide and the semiconductor. Any fixed charge or defects which trap electrons or holes destroy the device operation because the electric field will be terminated by interface charges instead of being transmitted into the semiconductor where the electrons or holes are conducted. We have used atomically resolved scanning tunneling microscopy (STM) images and scanning tunneling spectra (STS) to determine the atomic and electronic structure at the gate-oxide semiconductor interface. Our research focuses upon the group III-V semiconductors (GaAs, InGaAs, InAs) since they offer electron speeds up to 30x greater than silicon as well as germanium since it offers 3x higher hole speeds than silicon. In general, electronically passive interfaces are formed when oxide deposition does not disrupt the semiconductor lattice but instead restores the semiconductor surface atoms back to more bulk-like electronic structure. Even in the absence of a lattice disruption, oxide deposition can create new states in the bandgap thereby pinning the Fermi level by two mechanisms: direct (the adsorbate induced states in the bandgap region) and/or indirect (generation of undimerized surface atoms). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A6.00005: Monte Carlo simulations of carrier flow in novel gate materials Invited Speaker: |
Session A7: Oscillations Without Transcription in Vivo and in Vitro
Sponsoring Units: DBP GSNPChair: Michal Zochowski, University of Michigan
Room: Morial Convention Center RO5
Monday, March 10, 2008 8:00AM - 8:36AM |
A7.00001: Molecular synchronization, the Kai system, and biological oscillators Invited Speaker: In most textbook examples, oscillations in cell biology are driven by the periodic creation and destruction of one or more chemical species. The past few years, however, have seen growing interest in a different sort of oscillator. In these systems, the total concentrations of the major protein components are constant, but the molecules move sequentially through a cycle of different states (e.g. covalent modifications). Macroscopic oscillations appear when the progress of the many individual molecules becomes \textit{synchronized}. The recently-characterized cyanobacterial circadian clock provides a particularly elegant example of such molecular synchronization. Remarkably, with only the 3 proteins KaiA, KaiB, and KaiC, a $\sim$24 hour oscillation in KaiC phosphorylation can be reconstituted \textit{in vitro}. We can thus dissect this biochemical circuit in almost unprecedented detail. Here, we give an overview of the Kai system and its relationship to other oscillators. We begin with a review of the major experimental facts about the Kai system, emphasizing possible mechanisms to synchronize KaiC phosphorylation. We then examine in more detail models in which this synchronization occurs through sequestration of KaiA via \textit{differential affinity}: KaiA, which stimulates KaiC phosphorylation, has a higher affinity for KaiC during certain stages of the phosphorylation cycle; as long as some KaiC molecules at these stages are present in the reaction mixture, they bind all the available KaiA, thereby preventing the other KaiC's from being phosphorylated and proceeding through the cycle. We also discuss the implications of this mechanism for phenomena such as temperature compensation. Finally, we suggest that, in light of lessons learned from the Kai system, a number of other biological oscillators can fruitfully be viewed as examples of molecular synchronization. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A7.00002: Monomer exchange and the hourglass model of protein-based oscillators Invited Speaker: Circadian rhythms in photosynthetic cyanobacteria are under the control of a three protein biochemical network that generates oscillations in the phosphorylation level of one of the proteins. This oscillatory signal has a period of roughly 24 hours and regulates many biological processes in the bacteria to the day and night cycle. The molecular view of the phosphorylation process is that one of the proteins forms a hexameric complex whose phosphorylation levels rise and fall based on the activity of the other two proteins. Each hexameric complex thus functions as an independent molecular clock. However the bacteria contains many such clocks and so how do they interact to generate a coherent oscillating signal? In this talk I will discuss a model that suggests that monomer exchange between hexamers helps to synchronize the population of clocks during the daylight portion of the oscillation. Other synchronizing mechanisms will be highlighted for the other portion of the cycle. Recent experiments will be discussed in light of the exchange model. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A7.00003: Perfect Robust Network Architecture of a Bacterial Circadian Clock Invited Speaker: The circadian core clock of cyanobacteria consists of only three proteins, KaiA, KaiB, and KaiC. The readout of the clock status is given by the phosphorylation level of KaiC hexamers that oszillates with 23h period even under {\it in vitro} conditions. We present a circadian clock model based on mass action kinetics that shows almost perfect agreement with the experimentally found phosphorylation dynamics. The model consists of two feedback loops, with the main oscillatory mechanism realised by a negative feedback via sequestration of the enhancer of KaiC autophosphorylation, that is KaiA. These feedbacks are the reason for the observed outstanding robustness of the KaiABC clock that keeps phase, frequency and amplitude even under concerted serveral fold changes of the Kai protein concentrations. \newline [1] Clodong S. et al, Molecular Systems Biology, (2007) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A7.00004: Stability and Noise in the Cyanobacterial Circadian Clock Invited Speaker: Accuracy in cellular function has to be achieved despite random fluctuations (noise) in the concentrations of different molecular constituents inside and outside the cell. Single cell in vivo monitoring reveals that individual cells generate autonomous circadian rhythms in protein abundance. In multi-cellular organisms, the individual cell rhythms appear to be noisy with drifting phases and frequencies. However, the whole organism is significantly more accurate, the temporal precision being achieved most probably via intercellular coupling of the individual noisy oscillators. In cyanobacteria, we have shown that single cell oscillators are impressively stable and a first estimation rules out strong intercellular coupling. Interestingly, these prokaryotes also have the simplest molecular mechanism at the heart of their circadian clock. In the absence of transcriptional activity in vivo, as well alone in vitro, the three clock proteins KaiA, KaiB and KaiC generate a self-sustained circadian oscillation of autophosphorylation and dephosphorylation. Recent chemical kinetics models provide a possible understanding of the three-protein oscillator, but the measured in vivo stability remains yet unexplained. Is the clock stability a built-in property for each bacterium or does a weak intercellular coupling, make them appear like that? To address this question we first theoretically designed our experiment to be able to distinguish coupling, even weak, from phase diffusion. As the precision of our evaluation increases with the length of the experiments, we continuously monitor, for a couple of weeks, mixtures of cell populations with different initial phases. The inherent experimental noise contribution, initially dominant, is reduced by enhanced statistics. In addition, in situ entrainment experiments confirm our ability to detect a coupling of the circadian oscillator to an external force and to describe explicitly the dynamic change of the mean phase. We report a value of the coupling constant that is small compared to the diffusion constant. These results therefore confirm that the cyanobacterial clock stability is a built-in property: the cyanobacterian clock mechanism is not only the simplest but also the most robust. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A7.00005: Chromosome oscillations in mitosis Invited Speaker: Successful cell division necessitates a tight regulation of chromosome movement via the activity of molecular motors. Many of the key players at the origin of the forces generating the motion have been identified, but their spatial and temporal organization remains elusive. In animal cells, chromosomes periodically switch between phases of movement towards and away from the pole. This characteristic oscillatory behaviour cannot be explained by the current models of chromosome positioning and congression. We perform a self-contained theoretical analysis in which the motion of mono-oriented chromosomes results from the competition between the activity of the kinetochore and chromokinesin motors on the chromosome arms. Our analysis, consistent with the available experimental data, proposes that the interplay between the aster-like morphology of the spindle and the collective kinetics of molecular motors is at the origin of chromosome oscillations, positioning and congression. It provides a natural explanation for the so-called chromosome directional instability and for the mechanism by which chromosomes sense their position in space. In addition, we estimate the in vivo velocity of chromokinesins at vanishing load and propose new experiments to assess the mechanism at the origin of chromosome movement in cell division. [Preview Abstract] |
Session A8: Colloidal Self-Assembly I
Sponsoring Units: DFDChair: Hans Wyss, Harvard University
Room: Morial Convention Center RO6
Monday, March 10, 2008 8:00AM - 8:12AM |
A8.00001: Directed Self-Assembly of Spherical Particles Natalie Arkus, Guangnan Meng, Vinothan Manoharan, Michael Brenner \par We examine the kinetics and energetics of self-assembly in systems containing a small number of spherical colloidal nanoparticles using a combination of theory, simulation, and experiment. We then explore how the addition of spherically symmetric binding specificity can be used to direct the self-assembly of a given structure. \par Using graph theoretic, numerical, and algebraic geometric techniques, we denumerate all possible packings for a system of $n$ particles. We map out the energy landscape of these packings, which is determined not only by the value of the potential energy at these minima, but also by the vibrational normal modes of the structures. Experiments for a 6 particle system show that the likelihood of a given packing follows this expected equilibrium distribution. \par To explore the kinetics of packing formation, we simulate the self-assembly of these systems in the irreversible binding limit. For the 6 particle system, this reveals that the kinetics required to form one of the packings is highly unlikely, resulting in the other packing forming with 100\% probability. With the addition of binding specificity however, we can cause the unlikely packing to form with 100\% probability. We show how the addition of binding specificity effects the energetic landscape of these systems, and that it alone is sufficient to direct self-assembly. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A8.00002: Controlling assembly of micro- and nano-particle systems with DNA. Dmytro Nykypanchuk, Mathew Maye, Daniel van der Lelie, Oleg Gang Addressable biological interactions provide attractive platform for rational self-assembly, however the strength of such interactions are often difficult to control. Here we present an approach where DNA molecules are used to balance attractive and repulsive interactions during particles assembly while preserving the interaction addressability. We show, that by changing the composition and structure of DNA shall of micro- (2 um) or nanoparticles (10 nm), assembly kinetics, aggregate sizes, and the systems melting properties can be tuned. At constant environmental conditions, this strategy allows for rational control of interaction energy landscape for nano- and micro-systems in a wide dynamic range. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A8.00003: Evaporation-Driven Assembly of Microspheres with Polymer in Emulsion Droplets Keng-hui Lin, Liang-jie Lai, Chih-Chung Chang, Hui Chen We study the packing of colloidal microspheres mixed with polymer in oil-in-water emulsion droplets through evaporation. The addition of polymer produce non-unique configurations of final clusters when the number of particles $N$ inside the droplet is larger than 4. The cluster configurations are classified into three categories based on the symmetry. Stablized colloidal clusters of spherical packings are observed. Observation on packing process shed light to the mechanisms which cause different and non-unique structures. The osmotic pressure and interparticle interaction due to polymer play important roles in packing. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A8.00004: Entropy-driven self-assembly of dimers Issei Nakamura, An-Chang Shi Supramolecular self-assembly is an important phenomenon with applications ranging from chemical synthesis to biological systems. Although the driving force of assembly is the weak non-covalent intermolecular interaction such as hydrogen bonding and dispersion force, the self-assembly is a result from balancing the enthalpic and entropic contributions. In general, the disassembled/disordered phase is expected as temperature is raised because of the entropic gain from the components of the aggregate. However, it has been observed that the self-assembled/ordered phase can be promoted with increasing temperature. This implies that the self-assembly is driven by entropy. In order to provide a better understanding of this entropy-driven transition, we have studied a statistical mechanical model for the aggregation of macromolecular dimers immersed in solvents. The model demonstrates that solvent molecules absorbed on the surface of the solute are released with increasing temperature, leading to an increase of the total entropy of the system. Consequently, the cooperative stability of the dimeric state is induced. The thermodynamic features of this transition are analyzed. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A8.00005: Hydrodynamic interactions effects on the dipole-induced self-assembly of $\beta$-peptides and Brownian-induced polymer pore translocation Juan Hernandez-Ortiz, Michael Graham, Juan de Pablo A novel method that scales linearly with the number of particles is used to study Brownian-systems considering fluctuating hydrodynamic interactions. The method is demostrated in the concept of two applications: the dipole-induced self-assembly of $\beta$-peptides and the Brownian-motion-induced translocation of a polymer thought a rectangular pore. The method includes the long-range interactions by the Green's function formalism. It allows the consideration of peptides at intermediate concentrations and the inclusion of the non-periodic domain of the translocation. The hydrodynamics interactions affect the dynamics of the peptides agglomeration and the mean-squared-displacement indicates significant changes in the long-time diffusion coefficient. The polymer translocation is study using a transition path sampling based methodology. In particular it is used to calculate the translocation rate constant. Even for a single bead there are differences once hydrodynamics are included. These differences are due to the changes of mobility near walls and the change in polymer chain diffusion coefficient. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A8.00006: Self-assembly of complex shaped colloidal particles Adeline Perro, Vinothan N. Manoharan We have developed a new method to produce hybrid particles with polyhedral shapes in very high yield (liter quantities at up to 75{\%} purity) using a combination of emulsion polymerization and inorganic surface chemistry. The optical properties of these particles are tailored for studying their dynamics and self-assembly. For example, we produce systems that consist of index-matched sphere doublets with a small strongly scattering inorganic core between the two spheres, allowing us to track the center of mass of each doublet. We have generalized the preparation procedure to create even more complex geometries, including hybrid tetrahedra and octahedra. We present some preliminary studies on the self-assembly of these systems based on various optical experiments, including confocal microscopy, light scattering, and digital holographic microscopy. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A8.00007: Progress on systems of DNA modified colloidal particles for self-replication Paul Chaikin, Mirjam Leunissen, Remi Dreyfus, Roujie Sha, Nadrian Seeman, David Grier, David Pine Our goal is to create new materials that can self-replicate and self-assemble. For this, we modify the interactions between micrometer-sized colloids by coating them with single-stranded DNA `sticky ends', which specifically recognize complementary sequences on other colloids. We find that the aggregation-dissociation behavior is fully reversible for at least tens of temperature cycles. Using magnetic beads or optical tweezers, we form a chain-like `seed' structure, which acts as a template to assemble copies of itself from a soup of singlets. To determine what are the preferred binding sites, we studied the interactions between the singlets and their complementary particles in the seed. Important in our replication scheme is that each particle has two different types of sticky ends: one for `longitudinal' bonding along the chain and another for `transverse' bonding between seed and daughter chains. Contrary to the transverse linkers, the longitudinal linkers form AT/TA bonds, which can be crosslinked with an intercalator and UV irradiation. In this way, we permanently fix the seed and its copies. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A8.00008: Binary Colloidal Assembly by Dielectrophoresis Peter Hoffman, Yingxi Elaine Zhu Dielectrophoresis (DEP)-driven colloidal assembly has been recently explored as a new route to manipulate colloids and rapidly form nanostructured materials. In this talk, we demonstrate that colloidal particles of varied sizes can be assembled with controllable packing configurations in the presence of AC-electrical fields. We investigate binary latex particles of varied size ratios from 0.25 to 0.8 and directly monitor the dynamic assembly process with final structural characterization by using high-speed confocal microscopy. We observe rich phase behaviors of binary colloidal assembly with a strong dependence of applied AC-field frequency and medium conductivity. The obtained structural phase diagram can be well predicted by the DEP mobility and the Peclet number. We also present a mechanism that underlies the colloidal charge polarization due to charge segregation and entrainment within the double layer at several distinct frequencies, which cannot be explained by the classical Maxwell-Wagner theory. We recently also employ the same mechanism to form binary colloidal crystals. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A8.00009: Unusual aggregation behavior of colloids coated with palindromic DNA Mirjam Leunissen, Remi Dreyfus, David Pine, Paul Chaikin Coating particles with single-stranded DNA `sticky ends' gives excellent control over the specificity, strength and range of their interactions. Usually, a pair of complementary `Watson-Crick' sequences is used to obtain thermoreversible binding of different colloids. However, for certain purposes one could also use self-complementary `palindrome' sequences. Using light microscopy, we studied the aggregation behavior of micrometer-sized palindrome-coated colloids. Unlike Watson-Crick sticky ends, we found that it is of great importance whether the palindrome sticky end is attached to a flexible single strand or a rigid double-stranded `rod'. While the latter system displayed normal dissociation at elevated temperature, the former system showed enhanced aggregation with increasing temperature and no aggregation during fast temperature quenches. We explain these unusual observations by a competition between intra- and interparticle bonds. This provides us with an additional level of control over the interparticle bonding, besides the sequence of the sticky ends, the salt concentration and the DNA density on the beads. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A8.00010: Functionalized Au nanoparticles in solution Gary S. Grest, J. Matthew D. Lane The properties of functionalized Au nanoparticles in decane and water were studied by large-scale explicit atom, molecular dynamics simulations. Gold nanoparticles functionalized with S-(CH$_2$)$_9$-X alkanethiols chains (X = COOH or CH$_3$) were studied at the liquid-vapor interface and in the bulk. The structure of the functional groups on the nanoparticle was found to depend strongly on the end group and solvent. At the interface methyl terminated nanoparticles repel the water and move toward the vapor while in decane, the decane molecules engulf the nanoparticle. In the bulk, results for the nanoparticle/nanoparticle pair correlation function and nanoparticle diffusion will be presented as a function of nanoparticle concentration. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04- 94AL85000. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A8.00011: DNA-Grafted Janus Particles Ching Hsueh, Keng-hui Lin, Wen-Tau Juan Recently there have been advances in generating Janus microspheres whose two hemispheres have different chemical compositions [1-4]. The new types of particles open up possibilities for assembly of complex structures. Here we attach DNA molecules onto one side of Janus microspheres. The new type of colloidal particles resembles surfactant molecules and may give us interesting new structures.\\ \textbf{Reference:} [1] Y. Lu, H. Xiong, X. Jiang, Y.Xia, M. Prentiss and G. M.Whitesides, J. Am. Chem. Soc. \textbf{125}, 12724 (2003) [2] O.Cayre, V. N.Paunov and O. D. Velev, J. Mater. Chem. \textbf{13}, 2445 (2003) [3] R. F. Shepherd, J. C. Conrad, S. K. Rhodes, D. R. Link, M. Marquez, D. A. Weitz and J. A. Lewis, Langmuir \textbf{22}, 8618 (2006) [4] L. Hong, S. Jiang and S. Granick, Langmuir \textbf{22}, 9495 (2006) [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A8.00012: The importance of repulsion in the aggregation-dissociation behavior of DNA coated colloids Remi Dreyfus, Mirjam Leunissen, Roujie Sha, Nadrian Seeman, David Grier, David Pine, Paul Chaikin Coating particles with DNA gives excellent control over the specificity, strength and range of their interactions. In our experiments, a pair of complementary `Watson-Crick' sequences is used to obtain thermoreversible binding of different colloids. Colloids coated with such complementary `sticky' DNA aggregate when they are mixed together, the aggregates dissolve again when they are heated. We investigate the melting behavior of colloids coated with both sticky and non-sticky DNA. By changing the ratio between the sticky and non-sticky DNA, the obtained melting curves have very different melting temperatures, but almost the same sharpness. We show that the sharpness does not change because a high number of bonds bridges the particles, and that the strong shift in melting temperature is due to a repulsive interaction induced by the confinement of the non-sticky DNA when the particles come close together. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A8.00013: Self Assembly of Colloidal Particles at Small $N$ Guangnan Meng, Natalie Arkus, Ryan McGorty, Michael Brenner, Vinothan Manoharan We confine a small number ($N\approx10$) of micron-sized colloidal particles within micro-wells, and we use this finite system to study the process of self-assembly. The reversible aggregation of colloidal particles is controlled by a short-range depletion attraction, which is induced by poly($N$-isopropylacrylamide) nano particles. We use digital holographic microscopy to monitor the structural and kinetic properties of self-assembled colloidal clusters, and we use micro-wells to collect ensemble statistics. We compare our experimental results with theory and simulations, which probe how energetics and kinetics affect the packing structures. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A8.00014: 2D crystals of Janus amphiphilic colloidal spheres Shan Jiang, Stephen Anthony, Angelo Cacciuto, Erik Luijten, Steve Granick Colloidal spheres with one side hydrophilic and the other side hydrophobic assemble into 2D crystals with hexagonal translational order complemented by a high degree of orientational organization. Factors that determine the crystal structure are investigated, especially the dependence on ionic strength and on Janus balance. Depending on these variables, patches of the orientational order can be altered: from doublets to extended lines containing dozens of particles. Janus particles with different geometry (Janus balance) self-assemble into different cluster structures. Collective motion is evident from time-resolved optical microscopy. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A8.00015: Harnessing Elastic Instability for the self-assembly of complex patterns Elisabetta Matsumoto, Ying Zhang, Anna Peter, Pei-Chun Lin, Randall Kamien, Shu Yang Directed pattern formation through the self-assembly of complex polymer systems promises to be a powerful approach in the pursuit of novel, transformative technologies. Current approaches to create desired motifs at the nanoscale utilize flow, shear, fields, and other externally imposed, top-down forces. Nature, on the other hand, provides us with a plethora of examples of intrinsic, bottom-up effects: from the phyllotactic growth of plants to animal stripes to fingerprints, instabilities, packing constraints, and simple geometries can drive the formation of delicate, detailed, and beautiful patterns. By harnessing the elastic instability in flexible poly(dimethylsiloxane) (PDMS) membranes with a square lattice of circular pores exposed to a solvent, we distort the pores into a pattern featuring long-range orientational order. Within linear elasticity theory, we find the groundstate configuration of a lattice of interacting deformation elements, or ``dislocation dipoles'' to be in complete agreement with the observed pattern. Our theory allows us a means to design the patterns formed by such elastic frustration. [Preview Abstract] |
Session A9: Focus Session: Fluid Dynamics of Animal Motion
Sponsoring Units: DFD DBPChair: Kenny Breuer, Brown University
Room: Morial Convention Center RO7
Monday, March 10, 2008 8:00AM - 8:12AM |
A9.00001: Comparing flight strategies in species of fruit flies Itai Cohen, Leif Ristroph, Gordon Berman, Z. Jane Wang Observing different species of fruit flies offers an opportunity to compare flight strategies for insects of varying size but of nearly identical body and wing architecture. Using automated three-dimensional high-speed videography, we have captured many beautiful flight sequences of untethered fruit flies. From this data we have extracted the complete body and wing kinematics and determined the fluid forces acting on the wings using custom-written tracking and analysis software. We find that, in addition to lift, drag plays an important role in providing the vertical force needed for these insects to stay aloft. Moreover, our data base in combination with various numerical analysis techniques is allowing us to resolve whether these insects are flapping in the most efficient manner possible. Answers to this line of questions are important for determining what role if any evolution has played in determining how these insect fly. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A9.00002: Flow-induced attraction of swimming microorganisms by surfaces Eric Lauga, Allison Berke, Linda Turner, Howard Berg In this talk, we present an experimental and theoretical investigation of the accumulation of swimming cells by nearby surfaces. First, we present results of an experiment aiming at measuring the distribution of smooth-swimming E. coli when moving in a density-matched fluid and between two glass plates; the distribution for the bacteria concentration is found to peak near the glass plates. We then present a physical model for the observed attraction, based on the hydrodynamics interactions between the swimming cells and the walls. We show that such interactions result in a reorientation of the cells in the direction parallel to the surfaces, and an attraction of these (parallel) cells by the nearest wall. Our results are exploited to obtain an estimate of the propulsive force of smooth-swimming E. coli. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A9.00003: Effects of hydrodynamic interactions in bacterial swimming. Suddhashil Chattopadhyay, Xiao Lun Wu The lack of precise experimental data has prevented the investigation of the effects of long range hydrodynamic interactions in bacterial swimming. We perform measurements on various strains of bacteria with the aid of optical tweezers to shed light on this aspect of bacterial motility. Geometrical parameters recorded by fluorescence microscopy are used with theories which model flagella propulsion (Resistive force theory \& Lighthill's formulation which includes long range interactions). Comparison of the predictions of these theories with experimental data, observed directly from swimming bacterium, led to the conclusion that while long range inetractions were important for single polar flagellated strains (Vibrio Alginolyticus \& Caulobacter Crescentus), local force theory was adequate to describe the swimming of multi-flagellated Esherichia Coli. We performed additional measurements on E. Coli minicells (miniature cells with single polar flagellum) to try and determine the cause of this apparent effect of shielding of long range interactions in multiple flagellated bacteria. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A9.00004: Lagrangian studies of animal swimming and aquatic predator-prey interactions Invited Speaker: Experimental studies of animal swimming have been traditionally based on an Eulerian perspective in which the time-dependent flow field surrounding the animal is measured at fixed locations in space. The measured velocity field and its derivatives (e.g. vorticity) can, in principle, be used to deduce the forces, energetics, and fluid transport associated with locomotion in real fluids. However, achieving a connection between measurements of these Eulerian fields and the dynamics of locomotion has proven difficult in practice. We present the application of Lagrangian methods of flow analysis in which the time-dependent trajectories of individual tracer particles in the flow are measured experimentally and subsequently interrogated using dynamical systems tools in order to quantitatively resolve the dynamics of animal swimming. The Lagrangian methods are shown to be readily extended to time-dependent measurements in three spatial dimensions and to in situ field measurements using a recently developed self-contained underwater velocimetry apparatus (SCUVA). Case studies of jellyfish and other aquatic animals observed in the laboratory and in marine environments are used to illustrate the proposed approach. We also show that predator-prey interactions during jellyfish swimming can be addressed using the aforementioned Lagrangian methods in combination with the Maxey-Riley equations for inertial particles in fluid flow. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A9.00005: Shape transformations and propulsion due to an elastic filament rotating in a viscous fluid Bian Qian, Thomas Powers, Kenneth Breuer The deformation of an elastic filament in a viscous liquid is central to the mechanics of motility in cells ranging from \textit{E. coli} to sperm. We use experiments and theory to study the shape transition of a flexible rod rotating in a viscous fluid and set at an angle to the axis of rotation. In the experiments, two modes of operation are studied: constant torque and constant speed, and the shape of the filament is measured using stereoscopic imaging. At low applied torque, the rod bends gently, while at high torque, the rod adopts a helical shape with the tip close to the axis of rotation. At constant torque, the transition from the splayed form to the helical form is abrupt, accompanied by a sharp increase in the rotational speed. As the torque is decreased, the shape change exhibits hysteresis, transitioning back to the splayed form at a lower torque. At constant speed, the shape transition is continuous characterized by a region of decreasing torque that persists until the transition to the helical form is complete. Calculations based on slender body and resistive force theory predict the torque-speed relationship and the filament shape throughout the entire operating range, and show excellent agreement with the experiments. The propulsive force is predicted to increase sharply after the shape transformation, at which point the efficiency is also predicted to reach a maximum. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A9.00006: Propulsion by directional adhesion John Bush, Manu Prakash The rough, hairy integument of water-walking arthropods is well known to be responsible for their water-repellency; we here consider its additional propulsive role. We demonstrate that the tilted flexible leg hairs of water-walking arthropods render the leg cuticle directionally anisotropic: contact lines advance most readily towards the leg tips. The dynamical role of the resulting unidirectional adhesion is explored, and yields new insight into the manner in which water-walking arthropods generate thrust, glide and leap from the free surface. We thus provide new rationale for the fundamental topological difference in the roughness on plants and insects, and suggest novel directions for biomimetic design of smart, hydrophobic surfaces. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A9.00007: The role of the ventral pedal waves in the locomotion of terrestrial gastropods Janice Lai, Robert D. Shepherd, Juan C. del Alamo, Javier Rodriguez-Rodriguez, Juan C. Lasheras The locomotion of terrestrial gastropods exhibits unique characteristics which allow these animals to crawl on steep surfaces. Gastropods move by gliding over a ventral foot lubricated by mucus. They generate trains of pedal waves through periodic muscle contractions in the central portion of the ventral foot, producing a forward traction, while the rim of the foot adheres to the substrate and generates suction forces. We analyzed the kinematics and dynamics of locomotion by conducting two sets of experiments. In the first set, we used digital image processing to correlate the frequency and wavelength of the pedal waves to the migration velocity. In the second set, we computed the traction and adhesion forces produced by these animals from measurements of the deformation of an elastic substrate of known properties. We found that the strain energy exerted by the animal on the substrate is quasi-periodic, and explored a possible correlation between the mean speed of migration and the period of this energy fluctuation. In addition, we found that the pedal waves accelerate as they move forward along the ventral foot producing the symmetry break necessary for the generation of a net traction force. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A9.00008: Flying, swimming and fluttering in 3D: potential flow around a rectangular deformable plate Christophe Eloy, Lionel Schouveiler The interaction between a flexible rectangular plate and the flow around it can serve as a model for several phenomena. This situation arises in many problems of animal locomotion as well as industrial ones such as airfoil flutter. So far, most models have assumed a 2D problem for the sake of simplicity. We show here how to extend these models to include the finite plate aspect ratio in the analysis. We consider a rectangular deformable plate moving in a uniform flow at small amplitude such that the plate and its wake remain in the same plane at first order. The potential flow around the plate is calculated in the Fourier space and then averaged along the span. The result is a new integral equation for the vorticity distribution both inside the solid plate and in its wake. It means that the 3D effects can be taken into account by simply modifying the potential of a point-vortex (or equivalently the Green function of the Laplace's equation). [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A9.00009: Symmetry breaking in gastropod locomotion through acceleration or deceleration of the pedal waves Juan C. del Alamo, Javier Rodriguez-Rodriguez, Janice Lai, Robert D. Shepherd, Juan C. Lasheras Marine and terrestrial gastropods move by gliding over a ventral foot that is lubricated by secreted mucus (terrestrial) or simply by water (marine). The rim of the ventral foot generates suction forces that keep the animal adhered to the substrate. The central part of the foot produces a forward traction force by generating trains of pedal waves through periodic muscle contractions. Recent experiments show that, in some gastropods, these pedal waves become faster and longer as they move forward, suggesting a mechanism for breaking the symmetry in the flow between the pedal waves and the substrate. To investigate this mechanism, we have analyzed theoretically a two-dimensional lubrication layer between a train of waves of slowly varying length and speed, and a flat, rigid, impermeable surface. The inhomogeneity of the pedal waves has been modeled through multiple-scale asymptotics. We have considered a Newtonian fluid to separate the effect of this inhomogeneity from the viscoelastic symmetry breaking reported in previous works. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A9.00010: The unsteady flow over a bat wing in mid-downstroke. Florian Muijres, Christoffer Johansson, Ryan Barfield, Marta Wolf, Geoffrey Spedding, Anders Hedenstrom Birds, bats and insects have provided inspiration for human-designed small-scale flying machines, and while insects have long been known to rely on unsteady separated flows for their above-average aerodynamic performance at small-scale, the details of air flows over bird and bat wings have been harder to elucidate, mainly because of the extra complexity and precautions required in live experiments. Here we report on the first experiments of the airflow around a bat wing in free (but trained) flight in a low-turbulence wind tunnel. The aerodynamics of fixed wings at these Reynolds numbers are notoriously sensitive to small disturbances of the initially laminar, attached boundary layer, but these flight experiments show that the instantaneous flow fields around the flapping wing bear almost no resemblance to an equivalent fixed-wing experiment. The circulation increment due to the presence of a strong leading-edge vortex is estimated to provide a significant fraction of the total lift. Implications for the design and control of micro-air vehicles are considered. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A9.00011: Mechanics of Mammalian Swimming Timothy Wei, Paul Legac, Frank Fish, Terrie Williams, Russell Mark, Sean Hutchison Propulsion of large mammals ($i.e.$ dolphins and humans) has been of great interest for both technological and athletic reasons. The foundational question is how fast can a mammal swim? Digital Particle Image Velocimetry (DPIV) has been modified to be safely used on swimmers and dolphins. Experiments of dolphins performing various swimming behaviors were performed at the Long Marine Laboratory, University of California, Santa Cruz. Vortices generated by the dolphins' tail motions were used to estimate thrust production. Also, a two-dimensional dynamic force balance was constructed to study and improve the mechanics of elite swimmers. Paired with an underwater video camera, the forces seen could be directly related to the motion of the swimmer. These force measurements could be correlated to time resolved DPIV measurements of flow around the swimmers. Measurements made with swimmers, Megan Jendrick (2000 Olympic gold medalist) and Ariana Kukors (4x US National Champion), as well as data from trials with two dolphins will be presented. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A9.00012: Synchronization and hydrodynamic interactions Thomas Powers, Bian Qian, Kenneth Breuer Cilia and flagella commonly beat in a coordinated manner. Examples include the flagella that Volvox colonies use to move, the cilia that sweep foreign particles up out of the human airway, and the nodal cilia that set up the flow that determines the left-right axis in developing vertebrate embryos. In this talk we present an experimental study of how hydrodynamic interactions can lead to coordination in a simple idealized system: two nearby paddles driven with fixed torques in a highly viscous fluid. The paddles attain a synchronized state in which they rotate together with a phase difference of 90 degrees. We discuss how synchronization depends on system parameters and present numerical calculations using the method of regularized stokeslets. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A9.00013: An experimental and numerical study of fluid flow generated by a single nodal cilium Xingzhou Yang, Lisa Fauci, Arshad Kudrolli A rotating nodal cilia is said to generate fluid flow in the node of a developing embryo by posterior tilt leading to the left-right asymmetry of the mammalian body. In order to develop a physical understanding of the flow generated and the effect of the enclosing chamber, we perform scaled-up fluid-mechanics experiments and numerical simulations using the method of Regularized Stokeslets for zero Reynolds number. Important mechanical parameters, such as the geometry of the rods, dimensions of the tank, and the ratio of viscous to elastic stresses can be scaled to match typical cilia and cell. Digital imaging and tracer particle tracking techniques are used to measure the location and shape of the rods and the fluid flow. We will discuss the nature of the hydrodynamic velocity fields which are found to be more complex than anticipated by previous studies. [Preview Abstract] |
Session A10: Tunneling on Cuprate Superconductors
Sponsoring Units: DMPChair: Igor Zutic, University at Buffalo
Room: Morial Convention Center RO8
Monday, March 10, 2008 8:00AM - 8:12AM |
A10.00001: Local tunneling probe of low-energy Andreev states on (110) Y$_{0.95}$Ca$_{0.05}$Ba$_2$Cu$_3$O$_{7-\delta}$ thin films in an applied magnetic field J.H. Ngai, R. Beck, G. Leibovitch, G. Deutscher, J.Y.T. Wei Cryomagnetic scanning tunneling spectroscopy (STS) was performed on (110)-oriented Y$_{0.95}$Ca$_{0.05}$Ba$_2$Cu$_3$O$_ {7-\delta}$ thin films, in order to reveal coherence-length scale information on the symmetry of the high-$T_c$ order parameter (OP) in a magnetic field. In zero-field at 4.2K, both spontaneously split and unsplit zero-bias conductance peaks (ZBCP) are seen in the STS spectrum. The two types of peak spectra exhibit increasing splitting in a field applied along the $c$-axis of the film. Both spontaneous and field-induced ZBCP splitting indicate a lifting in the degeneracy of the low- energy Andreev states, consistent with time-reversal symmetry breaking. These results are discussed within the context of the Doppler effect as well as intrinsic \emph{vs.} field-induced complex components in the high-$T_c$ OP. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A10.00002: Magnetic-field effect on the quasiparticle excitation observed by the tunneling spectroscopy in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystal Yi Xuan, H.J. Tao, Z.Z. Li, B.R. Zhao, Z.X. Zhao In the $d$-wave superconductor, due to the existence of nodes of the pairing gap, the field-dependence of the quasiparticle density of state has been predicted to be essentially different from the behavior in the $s$-wave case [1]. Here we report the planar junction tunneling spectra at different magnetic fields in the high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ to study how the field modifies the quasiparticle excitation. [1] G. E. Volovik, JETP Lett. \textbf{58}, 469 (1993). [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A10.00003: Role of competing orders (COs) in the low-energy pseudogap (PG) phenomena and quasiparticle (QP) excitations of hole- and electron-type cuprate superconductors A.D. Beyer, M.S. Grinolds, M.L. Teague, N.-C. Yeh, S.-I. Lee Our cryogenic scanning tunneling spectroscopic studies of spatially resolved QP density of states (DOS) in hole-type YBa$_{2}$Cu$_{3}$O$_{x}$ and electron-type La$_{0.1}$Sr$_{0.9}$CuO$_{2}$ cuprate superconductors (SC) reveal that the existence of COs in the cuprates can account for many seeming non-universal phenomena. Namely, we analyze the low-energy QP excitation spectra by using a microscopic model of coexisting SC/CO, with density-wave type COs, and find that various spectral characteristics are uniquely determined by the parameters $\Delta _{SC}$, V$_{CO}$, Q, $\eta $, and $\Gamma $ ($\Delta _{SC}$: SC gap, V$_{CO}$: CO gap, Q: CO wave-vector, $\eta $: strength of quantum fluctuations, $\Gamma $: line-width of QP spectral peak). For instance, V$_{CO}>\Delta _{SC}$ (V$_{CO}$\underline {$<$}$\Delta _{SC})$ in hole- (electron-) type cuprates can account for the presence (absence) of the low-energy PG. Anomalous momentum-dependent QP properties such as the Fermi arcs and antiferromagnetic hot spots can also be explained. In finite magnetic fields, the QP DOS inside the vortex core of both types of cuprates reveal unconventional PG-like features at energies comparable to the V$_{CO}$ values derived by our analysis. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A10.00004: Spatially Resolved Quasiparticle (QP) Spectra in the Vortex State of Electron-Type Cuprate Superconductor (SC) La$_{0.1}$Sr$_{0.9}$CuO$_{2}$(La-112) M.L. Teague, A.D. Beyer, M. Grinolds, N.-C. Yeh, S.I. Lee The low-energy excitations of cuprate superconductors are unconventional and are susceptible to changes in temperature, doping and magnetic field. Our recent experiments and microscopic theoretical analysis suggest that these phenomena may be attributed to the presence of competing orders (COs) and strong quantum fluctuations. Here we present our scanning tunneling spectroscopic studies that support coexisting SC/CO in La-112, an optimally-doped electron-type cuprate with $T_{c}$=43 K. In zero-field, the histogram of the QP spectra at 9 K over an (100$\times $100) nm$^{2}$ area reveals a single set of spatially homogeneous peaks at $\Delta _{eff}$ = 11.8$\pm $1.5 meV. Our analysis of the data (with two energy gaps of SC and CO, $\Delta _{SC}$ and V$_{CO})$ suggests $\Delta _{eff}$ = [($\Delta _{SC})^{2}$+(V$_{CO})^{2}$]$^{1/2}$, $\Delta _{SC}>$V$_{CO}$, and the presence of quantum fluctuations. With increasing magnetic field to 6 Tesla, $\Delta _{eff}$ shifts downward to 10.0 meV due to the overall suppression of $\Delta _{SC}$, and the spatial variation of the spectra are consistent with vortex periodicity. Moreover, pseudogap-like spectral features occur at $\sim \pm $V$_{CO}$ inside the vortex cores while the spectra outside the vortex cores retain a gap value at $\sim \pm \Delta _{eff}$. This work was supported by NSF Grant DMR-0405088. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A10.00005: Importance of matrix element effects in the scanning tunneling spectra of $Bi_2Sr_2CaCu_2O_{8+\delta}$ Arun Bansil, Jouko Nieminen, Robert Markiewicz, Hsin Lin Scanning tunneling microscopy/spectroscopy (STM/STS) techniques have entered the realm of high-Tc's impressively by offering atomic scale real space resolution and meV resolution in bias voltages. STM/STS spectra, however, represent a complex mapping of electronic states of interest related to the CuO2 planes, since the tunneling current must reach the tip after being filtered through the overlayers (e.g. SrO and BiO in Bi2212). We have developed a Green function approach for realistic modeling of STM/STS spectra of the cuprates, where the tunneling current is evaluated directly including the effect of overlayers, with all orbitals within a few eV's of the Fermi energy $E_F$ accounted for. Our computations show the presence of strong matrix element effects, which lead to significant differences between the dI/dV spectra and the local density of states (LDOS) of CuO2 planes. For instance, the $d_{x^2-y^2}$ signal is found to be dominated by non-vertical hopping between the CuO$_2$ and BiO layers. A substantial electron-hole anisotropy of the tunneling spectrum, which is in accord with experiments, is naturally explained by the contribution from $d_ {z^2}$ and other orbitals below $E_F$. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A10.00006: Temperature dependent Scanning Tunneling Spectroscopy of impurities in Bi2Sr2CuO6+x through the transition temperature Kamalesh Chatterjee, Douglus Wise, Michael Boyer, Takeshi Kondo, Tsunehiro Takeuchi, Hiroshi Ikuta, Yayu Wang, Eric Hudson Scanning Tunneling Microscopy has been used to study detailed electronic spectrum in atomic scale defects in high temperature superconductors. We present Scanning Tunneling Spectroscopy of impurities in Bi$_{2}$Sr$_{2}$CuO$_{6+x }$(Bi-2201) over a wide range of temperatures. Surprisingly, native impurity resonances, similar to the ones previously observed in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212), spatially coexist with the superconducting gap at low temperatures and survive almost unchanged through the superconducting transition temperature T$_{c}$. We shall discuss the implications of these findings on the relationship between superconducting gap and pseudogap in these materials. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A10.00007: Visualizing Two Gaps in the High Temperature Superconductor Bi-2201 Michael Boyer, W.D. Wise, Kamalesh Chatterjee, Ming Yi, T. Kondo, T. Takeuchi, H. Ikuta, E.W. Hudson The relationship between the superconducting and pseudogap states in the cuprates has been a subject of much interest as well as debate in the HTS community. At the forefront of this debate is whether the pseudogap exists below Tc, and if it does, in what capacity. We present scanning tunneling microscopy measurements which provide evidence for two distinct but simultaneously coexisting gaps in the density of states of Bi2Sr2CuO6+x (Bi-2201) below Tc; one identified as the superconducting gap and the other, the pseudogap. In addition, we discuss our preliminary doping dependence measurements showing that the small (superconducting) gap scales with Tc while the larger (pseudogap) gap scales with T* indicating a consistency with their identification. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A10.00008: Temperature and doping dependent FT-STS studies of Bi-2201 William Wise, Kamalesh Chatterjee, Michael Boyer, Yayu Wang, Takeshi Kondo, Tsunehiro Takeuchi, Hiroshi Ikuta, Eric Hudson We present Fourier transform scanning tunneling spectroscopy (FT-STS) results in superconducting Bi-2201. The data presented are drawn from samples at a variety of dopings, and from experiments both below and above the superconducting transition temperature T$_{c}$. A number of unexpected trends are revealed by this systematic study. We interpret the results in light of several theories and make comparisons to results from other crystal systems. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A10.00009: Bogoliubov angle and visualization of particle-hole mixture in superconductors. Kazuhiro Fujita, Ilya Grigorenko, Jinho Lee, Miao Wang, Jian Xin Zhu, J.C. Davis, Hiroshi Eisaki, Shin-ichi Uchida, Alexander V. Balatsky We propose a new technique to visualize particle-hole mixture in high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, using the Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM). Depending on the polarity of bias voltage, SI-STM can sample the particle and hole content of a superconducting excitation, Bogoliubov quasiparticle. `Bogoliubov angle' (BA) is a measure of the relative weight of particle and hole amplitude in the Bogoliubov quasiparticle, which is determined by taking ratio of the differential conductance at positive and negative biases. BA allows one to measure directly the energy and position dependent particle-hole admixture and therefore visualize robustness of superconducting state locally. We will demonstrate the power of this new technique and discuss the momentum space (q-space) electronic structure in the talk. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A10.00010: Phase-sensitive scattering of Bogoliubov quasi-particles in (Ca,Na)$_2$CuO$_2$Cl$_2$ under magnetic field T. Hanaguri, Y. Kohsaka, M. Ono, M. Maltseva, P. Coleman, I. Yamada, M. Azuma, M. Takano, K. Ohishi, H. Takagi Magnetic-field effect on the quasi-particle interference (QPI) in Ca$_{1.86}$Na$_{0.14}$CuO$_2$Cl$_2$ ($T_{\rm c} \sim 28$~K) has been studied using Fourier-transform spectroscopic-imaging scanning tunneling microscopy. In the absence of magnetic field, all scattering vectors expected from the octet model for QPI [1] were detected in the Fourier-transformed conductance-ratio maps [2]. We have found that magnetic field enhances (suppresses) the amplitude of the standing waves due to QPI if the signs of the $d$-wave superconducting order parameter in momentum space are the same (different) between initial and final states of the scatterings. Such sensitivity to the phase of the order parameter indicates that coherence factors play an important role. \newline [1] K. McElroy {\it et al.}, Nature {\bf 422}, 592 (2003). \newline [2] T. Hanaguri {\it et al.}, Nature Phys. published online/nphys753. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A10.00011: Evolution of the electronic excitation spectrum with strongly diminishing hole-density in superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ J.W. Alldredge, Jinho Lee, K. McElroy, M. Wang, K. Fujita, Y. Kohsaka, C. Taylor, H. Eisaki, S. Uchida, P.J. Hirschfeld, J.C. Davis We use tunneling spectroscopy to measure the T$>$0 spectrum of electronic excitations N(E) over a wide range of hole-density in superconducting Bi2212. We introduce a parameterization for N(E) based an anisotropic d-wave energy-gap plus an effective scattering rate which varies linearly with energy. We demonstrate that this form of N(E) allows successful fitting of differential tunnelling conductance spectra throughout much of the Bi2212 phase diagram. A single, particle-hole symmetric, anisotropic energy-gap, in combination with a strongly energy dependent effective scattering rate, can describe the excitations without recourse to an additional energy gap of another ordered state. However we also observe two distinct and diverging energy scales in the system: the energy-gap maximum $\Delta _{1}$ and a lower energy scale $\Delta _{0}$ separating the spatially homogeneous and heterogeneous electronic structures. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A10.00012: Temperature Evolution of Local Pairing {\&} Electron-Boson Coupling in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Kenjiro K. Gomes, Abhay Pasupathy, Aakash Pushp, Colin Parker, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani Recently, we have shown using variable temperature scanning tunneling microscopy measurements that the pairing in high-Tc superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ persists in nanoscale regions at temperatures above Tc. [1] Using the ability to track the same atomic location with temperature, we have examined the evolution of the electronic states from well below Tc to above the temperature at which the pairs first form locally. We will present these results for an overdoped sample. Using these measurements we extract the evolution of the pairing gap with temperature and show that pairing gaps at different atomic sites close at different temperatures. Our technique also allows us to quantitatively analyze the local electron-boson coupling for different atomic sites with different pairing strengths. Our results show that there is no connection between the variation of the gap magnitude and the bosonic mode associated with the ``dip-hump'' feature in the spectra. [1] Gomes et al. Nature 447, 569--572 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A10.00013: Electronic Origin of the Nanoscale Variation of Pairing Gaps in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Abhay Pasupathy, Kenjiro K. Gomes, Aakash Pushp, Colin Parker, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani The magnitude of the low-temperature superconducting gap measured in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ shows large nanoscale spatial variations. On raising the temperature, these superconducting gaps disappear locally at temperatures above the superconducting transition temperature Tc. We present the first atomically resolved measurements of the spectrum of overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ at temperatures where all gaps have closed. We show that the shape of this ``normal'' state spectrum is inhomogeneous over a length scale similar to that of the gap variation at low temperature. We then track the same lattice sites down to low temperature and observe the opening of gaps in the spectrum. We will discuss the relationship between the shape of the spectrum measured in the ``normal'' state at high temperature and the size of the low-temperature superconducting gap at the same atomic site. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A10.00014: Temperature Evolution of the Electronic States {\&} Multiple Gap Features in Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$. Aakash Pushp, Abhay Pasupathy, Kenjiro K. Gomes, Colin Parker, Shimpei Ono, Yoichi Ando, Ali Yazdani Like Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$, Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6 }$samples show inhomogeneous gaps in the DOS at low temperature. We present atomic resolution STM spectroscopy measurements of the evolution of the DOS with temperature for optimal (y=0.4) and overdoped (y$<$0.4) samples and compare these measurements to Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. In Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$, the low temperature (T$<$Tc) spectra of overdoped samples are all characterized by a single d-wave gap with sharp coherence peaks. In contrast, Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6 }$shows a variety of spectra at low temperature ranging from those without gaps to ones with multiple gap features [1]. By measuring the temperature evolution of these gap features at given lattice sites for various doping levels, we estimate the local temperature at which pairing develops in these samples. [1] Boyer et al., Nat Phys 3, 802 (2007) [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A10.00015: Local Signatures and Spectral Inversion of Bosonic Mode Coupling in a High-Temperature Superconductor Laila S. Mattos, Christopher R. Moon, Brian K. Foster, Gabriel Zeltzer, Martin Greven, Hari C. Manoharan High-resolution spectroscopy and mapping of the high-temperature superconductor Bi$_{2}$Sr$_{2}$Ca$_{0.93}$Y$_{0.07}$Cu$_{2}$O$_{8+d}$ was performed with a custom-built low-temperature scanning tunneling microscope. At optimal doping ($T_{c} \quad \approx $ 96 K), these materials exhibit~unusually strong spectral sidebands characteristic of electron-boson coupling. The magnitude of these Holstein-like $d$-wave density of states replicas facilitates a spectral inversion procedure, from which the critical parameters of the bosonic coupling can be directly extracted. This results in electron-boson coupling $\lambda \quad \approx $ 0.2-0.4 and boson energy $\Omega \quad \approx $ 70-90 meV.~Origins and implications of these excitations will be discussed. [Preview Abstract] |
Session A11: Focus Session: MgB2-like: Exotic Behavior in MgB2-like Materials
Sponsoring Units: DMPChair: Xiaoxing Xi, Pennsylvania State University
Room: Morial Convention Center RO9
Monday, March 10, 2008 8:00AM - 8:12AM |
A11.00001: High quality MgB$_{2}$ thin films in clean and dirty limit realized via HPCVD Chenggang Zhuang, Sheng Meng, Qingrong Feng, Zizhao Gan, Huan Yang, Ying Jia, Haihu Wen, Xiaoxing Xi Pure and carbon-doped MgB$_{2}$ thin films are fabricated using hybrid physical-chemical vapor deposition. Excellent properties are obtained in the pure samples, including T$_{c}\sim $41.4K, \textit{$\rho $}${\rm g}$(42K)$\sim $0.3$\mu \Omega$ cm, which indicates that films are in the clean limit. Ultrahigh critical current density, up to 1x10$^{8}$A/cm$^{2}$ at 5K and zero fields, approaching the theoretical deparing current of MgB$_{2}$, was observed in a 150nm-wide microbridge, in agreement with the results derived using a Bean Model from the magnetization measurements. Large normal state magnetoresistance before the transition, about 40{\%} at 7T, and absence of flux jump at low temperature and low field regions in the magnetization curves strongly support that the film is very clean. To enhance upper critical field and flux pinning, carbon doping is employed and realized in a dual-heater HPCVD setup from the thermal decomposition of CH$_{4}$. $\mu _{0}$H$_{c2}$(0)$\sim $54T, J$_{c}$(5K,6T)$\sim $3x10$^{5}$A/cm$^{2}$, as well as T$_{c}$(0)$\sim $34K and \textit{$\rho $}${\rm g}$(42K)$\sim $47$\mu \Omega $cm are achieved in the carbon-doped films. By controlling the carbon concentration, MgB$_{2}$ films in dirty limit can be prepared. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A11.00002: Superconducting properties of MoN microfibers and thin films Amar Karki, Edem Okudzeto, Phil Adams, David Young We present the superconducting transport properties measurements on polycrystalline MoN synthesized in the form of thin films and microfibers. The samples were prepared by heating Mo films and Mo-coated carbon fibers to temperatures between 850 $^{o}$C and 1000 $^{o}$C for different time periods in a flowing stream of ammonia (NH$_{3})$ gas under atmospheric pressure. The microfibers and thin films had a transition temperature Tc $\sim $ 12.2 K, which is substantially higher than that reported for MoN films grown by other methods. We also present critical current measurements on microfibers which consisted of a 50-nm thick layer of polycrystalline MoN synthesized directly onto 5 micron-diameter carbon fibers. The microfibers supported current densities in excess of 10$^{7}$ A/cm$^{2}$ well below Tc. Near Tc, Jc was well described by the power law [1-(T/Tc)$^{2}$]$^{3/2}$. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A11.00003: Theory of Raman Scattering from Leggett's Collective Mode in a Multiple Band Superconductor: Application to MgB$_{2}$ Miles Klein Using an extension of BCS theory to a two-band superconductor, Leggett showed that if the relevant parameters obeyed certain conditions a collective mode would exist corresponding to the counter flow of the two condensates.$^{1}$ I have extended earlier work on electronic Raman in superconductors$^{2}$ to the multiple band case in order to incorporate Leggett's theory. The following effects have been included: (a) Vertex correction in the particle/hole channel where the Raman vertex acts. (b) Realistic parameters that apply to MgB$_{2}$ yielding a counter flow mode that decays into the pair-breaking continuum associated with the lower gap $\pi $ band. (c) Large finite wave-vector effects due to the relatively large Fermi velocity of the $\pi $ band. (d) Integration over the wave-vector in part (c) necessitated by the exponential decay of the photon fields traveling into and out of the metallic sample. A comparison to the results of Blumberg$^{3}$ will be given. $^{1}$A.J. Leggett, Progr. Theor. Phys. \textbf{36}, 901 (1966). $^{2}$M.V. Klein and S.B. Dierker, Phys. Rev. B\textbf{29}, 4976 (1984). $^{3}$G. Blumberg et al., Phys. Rev. Lett. \textbf{99}, (2007); arXiv:0710.2803. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A11.00004: MgB$_{2}$: Novel properties due to multibands Invited Speaker: About 40 years ago A.J. Leggett proposed a new collective mode arising from cross-tunneling of Cooper pairs residing on different Fermi surfaces of a multiband superconductor: Leggett's collective mode is caused by a counter flow of the interacting superfluids leading to small fluctuations of the relative phase of the condensates while the total electron density is locally conserved.\footnote{A.J. Leggett, Progr. Theor. Phys. \textbf{36}, 901 (1966).} Here we present direct spectroscopic observation of the Leggett's excitation in the MgB$_{2}$ superconductor containig two pairs of Fermi surfaces resulting from $\pi$- and $\sigma$-bands. Electronic Raman scattering studies have revealed three distinct superconducting (SC) features: (i) a clean threshold of Raman intensity at 4.6 meV consistent with the $\pi$-band SC gap; (ii) the SC pair breaking coherence peak at 13.5 meV consistent with excitations above the $\sigma$-band gap; and (iii) the SC collective mode at 9.4 meV which we assign to an excitation first discussed by Leggett.\footnote{G. Blumberg et al., Phys. Rev. Lett. \textbf{99} (2007); \urllink{arXiv:0710.2803}{http://arxiv.org/pdf/0710.2803}.} Our calculation of the Raman response function for MgB$_{2}$ superconductor based on multiband interaction matrices by first principle computations show good agreement with spectroscopic observations. The temperature and field dependencies for all three features (i) -- (iii) have been established;\footnote{G. Blumberg et al., Physica (Amsterdam) \textbf{456C}, 75 (2007).} the effects of magnetic field on the pair cross-tunneling in multiband system will be discussed. In addition, anharmonicity and superconductivity-induced self-energy effects for the E$_{2g}$ boron stretching phonon have been studied.\footnote{A. Mialitsin et al., Phys. Rev. B \textbf{75}, 020509(R) (2007).} We show that anharmonic two-phonon decay is mainly responsible for the unusually large linewidth of the E$_{2g}$ mode. We observe 2.5\% hardening of the E$_{2g}$ phonon frequency upon cooling into the SC state and estimate the electron-phonon coupling strength associated with this renormalization. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A11.00005: Microwave properties of epitaxial MgB2 films and micro-bridges M.V. Costache, G.X. Miao, J.S. Moodera The high transition temperature and simple AlB$_{2}$ structure make the MgB$_{2}$ superconductor a promising new material for application in superconducting electronic devices. In order to reach this goal, in addition to the development of MgB2 Josephson Junctions (JJ), we have explored the superconducting properties of lithographically patterned micron sized bridges in high quality epitaxial MgB2 films. These thin film micron size wire bridges show JJ characteristics. The current-voltage (I-V) characteristics show the expected JJ behavior as a function of temperature, magnetic field and microwave radiation including hysteresis. Large change in the hysteresis behavior is observed when the microwave power is varied. The large hysteretic I-V can have potential as a memory element. The details will be discussed. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A11.00006: Force-Detected NMR Study of Single-Crystal MgB$_2$ using Ultrasensitive Oscillators* Han-Jong Chia, Mark Monti, Samaresh Guchhait, John Markert, Jae-Hyuk Choi, Sung-Ik Lee MgB$_2$ is a unique superconductor with a relatively high $T_c$ and two nearly independent electronic bands. An NMR study of $^{11}$B in MgB$_2$ using polycrystalline samples [1] did not observe any two-band effects, nor a Hebel-Slichter coherence peak, possibly due to large $H_{c2}$ anisotropy (and thus a distribution of $T_c$'s). Anisotropic NMR studies of MgB$_2$ have proven difficult due to the small size ($\sim 10\ \mu$m) of high-quality crystals. A large-single-crystal conventional NMR study [2] could not probe the superconducting state due to line broadening. We have set out to use the exquisite sensitivity of Nuclear Magnetic Resonance Force Microscopy (NMRFM) to probe the behavior of $^{11}$B in single crystal MgB$_2$. We have fabricated ultrasensitive mechanical oscillators using e-beam lithography to facilitate detection of the weak $^{11}$B resonance; these have resonance frequencies of 1--10 kHz, spring constants of $\sim 10^{-4}$ N/m, and quality factors $>$3000 at 77 K. We report our initial detection of the $^{11}$B nuclear resonance and our plans to study relaxation rates in single crystal MgB$_2$. [1] H. Kotegawa {\it et al.}, {\it Phys.~Rev.~Lett.} {\bf 87}, 127001 (2001). [2] S.~Str\"assle {\it et al.}, {\it Physica C} {\bf 466}, 168 (2007). *Supported by NSF DMR-0605828 and DGE-0549417. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A11.00007: High pressure synthesis of single crystalline MgB$_{2}$ M. Tillman, G. Lapertot, R. Prozorov, C. Martin, S.L. Bud'ko, P.C. Canfield We report the results of single crystal growth of MgB$_{2}$.~ A high pressure furnace, using a 19 mm truncation edge length, cubic anvil capable of reaching 3.3 GPa and 2000 C was used to grow crystals of MgB$_{2}$ out of the Mg-B-N ternary.~ Design, setup, and calibration will be discussed as well as correlations between pressure and temperature profiles and crystal size.~ Results of measurements of penetration depth and H$_{c2}$(T) on single crystals will be shown as well as the results of initial doping studies. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A11.00008: Electric-field-effect studies of atomically thin flakes of superconducting NbSe$_2$ prepared by mechanical exfoliation Neal Staley, Jain Wu, Peter Eklund, Ying Liu, Linjun Li, Zhuan Xu Recent years have yielded many studies on electric field modulated superconductivity. In order to achieve the high carrier density changes needed to modulate superconductivity, two main approaches have been tried, ultra thin films grown by in situ quench deposition or few-layer single crystalline films of superconductors grown by pulsed laser deposition or molecular beam epitaxy. However in both cases, difficulties have been encountered, ultra thin films are subject to large amounts of disorder, and it is difficult if not impossible to grow single layered superconductors. Using a simple micromechanical exfoliation technique, we are able to create single crystal single layered graphite, featuring a linear density of states, allowing its physical properties to be tuned by gate voltage. Using this mechanical exfoliation procedure we fabricated ultra thin single crystalline NbSe$_2$ flakes ranging from double layered to many layered as estimated using an optical technique correlated to AFM and Raman spectroscopy measurements. Using a lithography-free, ``all dry'' process we fabricated devices showing modulated T$_c$ with applied gate voltage as well as a superconductor insulator transition tuned by the number of layers. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A11.00009: Low Energy Electronic Structure of an Excitonic CDW Melted Novel Superconductor Dong Qian, David Hsieh, Lewis Wray, Yuqi Xia, E. Morosan, R.J. Cava, M.Z. Hasan A superconducting (SC) state has very recently been observed upon successful doping of the charge-density wave (CDW) ordered triangular lattice TiSe$_2$ with copper. Using angle-resolved photoemission spectroscopy (ARPES) we studied the doping evolution of the electronic structure of Cu$_x$TiSe$_2$. The momentum space locations of the doped electrons that form the Fermi sea of the parent superconductor is identified. With increasing electron doping, we observe a significant rise of chemical potential which is found to destabilize the long range CDW order. At the same time the emergence of a large density of states in the form of a narrow electron pocket near the L- points of the Brillouin zone favors the onset of superconductivity within the BCS-Eliashberg scenario. With doping, we find that the kinematic nesting volume increases whereas the coherence of the CDW order sharply drops. The k- space electron distributions highlight the unconventional interplay of CDW to SC cross-over achieved through non-magnetic copper doping. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A11.00010: Sandwich-type MgB$_{2}$ tunnel junctions Ke Chen, Yi Cui, Qi Li, Xiaoxing Xi Properties of superconductors, such as band gap, density of states (DOS) of quasiparticles, and effective phonon spectra, can be studied by electrical tunneling with a high energy resolution of several $k_{B}T$. Sandwich-type MgB$_{2}$ tunnel junctions with Pb or Ag as the counter-electrodes were made on Hybrid Physical-Chemical Vapor Deposition MgB$_{2}$ films with thermal oxide tunnel barrier. The tunnel junctions exhibit very small subgap current at 4.2 K. Fit to the BCS DOS relation, the $\pi $-band and $\sigma $-band gaps of MgB$_{2}$ are found to be 2.3 and 7.4 meV at 4.2 K respectively. Josephson tunneling was observed with $I_{c}R_{n}$ product following the BCS temperature dependence relation, and the critical current showing a Fraunhofer pattern modulated by the applied magnetic field. Effective phonon modes are revealed at 42, 61, and 74 meV. These superb tunnel junctions give us deep insights into the properties of MgB$_{2}$. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A11.00011: Hybridized Abrikosov Flux-lines and Pancake Vortices in Two-band Superconductors with Mixed Dimensionality K. Tanaka, M. Eschrig We study electronic structure and thermodynamic properties of a two-band superconductor, in which one band is ballistic and quasi-two dimensional (2D), and the other is diffusive and three dimensional (3D). We assume that superconductivity in the 3D diffusive band is ``weak'', i.e., mostly induced, as is the case in MgB$_2$. Hybridization with the ``weak'' 3D diffusive band has significant and intriguing influence on the electronic properties of the ``strong'' 2D ballistic band. In particular, the effects of Coulomb interactions in the diffusive band and unusual Kramer-Pesch effect are examined. Furthermore, based on a circular-cell approximation within the quasiclassical theory of superconductivity, we explore the effects of magnetic field on vortex structure in such a two-band superconductor. We discuss hybridization of Abrikosov flux lines in the 3D diffusive band with pancake vortices in the 2D ballistic band. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A11.00012: Low-temperature transport in highly disordered films of superconducting magnesium diboride Nicholas Breznay, Aharon Kapitulnik Many features make magnesium diboride an interesting model system for understanding the behavior of two dimensional BCS superconductors, including its sensitivity to disorder and low spin-orbit coupling. We study the transport behavior of highly disordered magnesium diboride films, and will review preliminary low-temperature magnetoresistance measurements in the regime of high magnetic fields. We connect these results to recent work on multilayered films prepared using a similar process and also to similar work on other model systems, and review prospects for future study. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A11.00013: T$_{c}$ vs Isotopic Mass and vs Residual Resistivity Investigation in MgB$_{2}$ Marina Putti, Matteo Tropeano, Paolo Brotto, Carlo Ferdeghini, Enrico Galleani , Pietro Manfrinetti, Andrea Palenzona Almost five years after the discovery of superconductivity in MgB$_{2}$ the isotope effect on T$_{c}$ is not yet understood (M. Calandra et al, Physica C456, 38 (2007) and references therein). The isotope effect is mainly due to the B atoms reflecting the important role of B vibrations in determining T$_{c}$. Detailed two bands calculation leads to $\alpha $(B) of the order of 0.4--0.45, in disagreement with experiments which evaluated $\alpha $(B) = 0.30. Anharmonicity was proposed as a possible explanation for the reduced B isotope coefficient, but recently it was emphasized that such an explanation needs to be reconsidered. On the other hand, recent investigations on the effect of disorder on T$_{c}$ pointed out that samples with residual resistivity ($\rho _{0})$ of few $\mu \Omega $cm present T$_{c}$ variations comparable with the intrinsic variations due to isotopic effect. This calls for new investigations of isotopic effect in samples with controlled amount of disorder. Ultra clean Mg$^{10}$B$_{2}$ and Mg$^{11}$B$_{2}$ samples ($\rho _{0}\sim $0.5 $\mu \Omega $cm) were damaged respectively with annealing and neutron irradiation and T$_{c}$ and resistivity were measured. T$_{c}$ vs $\rho _{0}$ plot shows in both cases a linear relationship allowing us to extrapolate T$_{c}$ ($\rho $=0)${\rm g}$ for both the sample series. $\alpha $(B) evaluated by these intrinsic T$_{c}$ values confirms results of previous report and the crucial role of disorder in determining T$_{c}$ has been proved. [Preview Abstract] |
Session A12: Strongly Correlated 1D Systems
Sponsoring Units: DCMPChair: Gregory Fiete, California Institute of Technology
Room: Morial Convention Center 203
Monday, March 10, 2008 8:00AM - 8:12AM |
A12.00001: Local density of states in Luttinger liquids with a dynamically-generated spin gap Dirk Schuricht, Akbar Jaefari, Fabian Essler, Eduardo Fradkin We present a theory of STM spectroscopy in semi-infinite 1D strongly correlated electron systems with a spin-gap. We calculate the local density of states of semi-infinite Luttinger liquids with open boundary conditions and a dynamically generated spin gap at zero temperature. In order to perform this calculation, we use the boundary state formalism [1] together with a form factor expansion in the infinite system. We find explicit expressions for the local density of states as a function of the gap size and the distance from the boundary. We show how a local spectroscopic probe of this type can be used a)to detect the spectrum of fractionalized massive solitons and their dynamics, b)the existence of boundary states and c)charge order induced by the boundary. We will discuss the relevance of this work to STM experiments in 1D systems and in quasi-1D systems used to model stripe phases in strongly correlated systems [2]. [1] S. Ghoshal and A. Zamolodchikov, Int. J. Mod. Phys. 9, 3841(1994), ibid. 9, E4353(1994). [2] S. A. Kivelson, I. P. Bindloss, E. Fradkin, V. Oganesyan, J. Tranquada, A. Kapitulnik and C. Howald, Rev. Mod. Phys. 75, 1201(2003). [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A12.00002: Spin-Incoherent Luttinger Liquid-Superconductor Hybrid Systems Dagim Tilahun, Gregory Fiete Strongly interacting one-dimensional systems exhibit the exotic property of {\em spin-charge separation} where a repulsive interaction suppresses the velocity of the spin degree of freedom while enhancing that of the charge. Spin-incoherent Luttinger liquids exist in the regime where the temperature is much higher than the characteristic energy of the spin sector (thermalized, random spins) but much less than that of the charge. We study a hybrid system consisting of a spin-incoherent Luttinger liquid adjoined at one or both ends to a superconductor, and find robust features that can be used as clear experimental signatures of spin-incoherence. We find the tunneling density of states diverges at low energies and exhibits a universal frequency dependence independent of the strength of the interactions in the system. We also find that in spite of exponentially decaying pair correlations with distance into the spin-incoherent Luttinger liquid, the Josephson current remains robust. Compared to the zero temperature Luttinger liquid case there is a factor of 2 reduction in the critical current and a halving of the period in the phase difference between the superconductors. Our results open the way for a new class of experiments in the spin-incoherent regime of one dimensional systems. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A12.00003: Temperature dependence of the anomalous exponent in Li(0.9)Mo(6)O(17) that reveals Luttinger Liquid behavior Rene Matzdorf, Tatjana Novgorodov, Bernard Nansseu, Michael Waelsch, Jian He, Rongying Jin, David Mandrus Scanning tunneling spectroscopy (STS) has been used to study the Luttinger-liquid behavior of the purple bronze Li(0.9)Mo(6)O(17) in the temperature range 5K < T < 300K. In the entire temperature range the suppression of density of states at the Fermi-energy could be fitted very good by a model describing the tunneling into a Luttinger liquid at ambient temperature. The power-law exponent extracted from these fits reveals a significant increase above 200K. It changes from $\alpha$=0.6 at low temperature to $\alpha$=1.0 at room temperature. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A12.00004: Specific heat of a one-dimensional interacting Fermi system Andrey Chubukov, Dmitrr Maslov, Ronojoy Saha We re-visit the issue of the temperature dependence of the specific heat $% C(T)$ for interacting fermions in 1D. The charge component $C_c (T)$ scales linearly with $T$, but the spin component $C_s (T)$ displays a more complex behavior with $T$ as it depends on the backscattering amplitude, $g_1$, which scales down under RG transformation and eventually behaves as $g_1 (T) \sim 1/\log T$. We show, however, by direct perturbative calculations that $C_s (T)$ is strictly linear in $T$ to order $g^2_1$ as it contains the renormalized backscattering amplitude not on the scale of $T$, but at the cutoff scale set by the momentum dependence of the interaction around $2k_F$. The running amplitude $g_1 (T)$ appears only at third order and gives rise to an extra $T/\log^3 T$ term in $C_s (T)$. This agrees with the results obtained by a variety of bosonization techniques. We also show how to obtain the same expansion in $g_1$ within the sine-Gordon model. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A12.00005: Entanglement and quantum phase transition in the ground state of one-dimensional asymmetric Hubbard model Wenling Chan, Shi-Jian Gu We study the quantum phase transition by means of entanglement in the ground state of the one-dimensional asymmetric Hubbard model. Both the half-filling and away from half-filling cases are investigated. The local entanglement between the middle two sites with the rest of the system, and the block entanglement between the left and right portions of the system, are calculated by the DMRG method. We find that the entanglements show interesting scaling and singular behavior around the phase transition line. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A12.00006: Is the Drude weight a thermodynamic quantity? Marcos Rigol, B. Sriram Shastry Transport properties distinguish metals from insulators, and superconductors from ideal metals. In one dimension, they can help differentiate integrable from nonintegrable systems. The Drude weight (or charge stiffness) is found to be nonzero in integrable metals, even at very large temperature, whereas it vanishes for generic (nonintegrable) systems. In systems with periodic boundary conditions, the Drude weight can be identified as the coefficient accompanying the zero-frequency delta peak in the real part of the electrical conductivity. Paradoxically on the other hand, for systems with open boundary conditions, it can be shown that this coefficient is identically zero for any finite system, regardless of its integrability. For the Drude weight to be a thermodynamically meaningful quantity, both kinds of boundary conditions should produce the same answer in the thermodynamic limit. We resolve this paradox using analytical and numerical methods. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A12.00007: Phase diagram of hole doped two-leg $Cu$-$O$ ladders Piotr Chudzinski, Marc Gabay, Thierry Giamarchi In the weak coupling limit, we establish the phase diagram of a two-leg ladder with a unit cell containing both $Cu$ and $O$ atoms, as a function of doping. We use bosonization and design a specific RG procedure to handle the additional degrees of freedom. Significant differences are found with the single orbital case; for purely repulsive interactions, a completely massless quantum critical region is obtained at intermediate carrier concentrations(well inside the bands). For some finite value of direct hopping between oxygen atoms the ground state consists of an incommensurate pattern of orbital currents plus a spin or charge density wave (DW) structure. The experimental relevance of these findings is also discussed. We have calculated the NMR properties like Knight shift and relaxation rate at each atom inside the elementary cell. We make a prediction that different temperature dependance indicates the phase of the measured system. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A12.00008: Groundstate fidelity and the spin one chain Ian McCulloch it has been recognized quite recently that the groundstate fidelity, that is, the overlap of the groundstate wavefunctions as a function of interaction strength, can be used to obtain phase boundaries and exponents \emph{without} a-priori knowledge of the order parameter. This procedure is easy to apply in a wide class of numerical algorithms based on matrix product states, of which the Density Matrix Renormalization Group (DMRG) is the most famous. I will give a brief overview of the technique, and demonstrate that the fidelity reveals \emph{all} features of the bilinear-biquadratic spin one chain, while almost certainly ruling out the appearance of a (gapped or critical) nematic phase in the vicinity of $\theta = -3\pi / 4$. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A12.00009: Magnetism of one-dimensional Wigner lattices and its impact on charge order Maria Daghofer, Reinhard Noack, Peter Horsch We report the phase diagram of a quarter-filled Wigner lattice described by the 1D Hubbard-Wigner model with nearest and next-nearest neighbor hopping $t_1$ and $t_2$. In the $t_1$-$t_2$ plane, we find a region at negative $t_2$ with fully saturated ferromagnetic ground states due to kinetic exchange interactions, while the remaining phase diagram is controlled by antiferromagnetic exchange. We also observe a strong influence of magnetism on the charge structure factor, in contrast to the expectation that charge ordering in the Wigner lattice is well described by spinless fermions. Our results, obtained using the density-matrix renormalization group and exact diagonalization, can be transparently explained within the framework of an effective low-energy Hamiltonian. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A12.00010: Critical lines and massive phases in quantum spin ladders with dimerization Javier Almeida, Miguel Angel Matin-Delgado, German Sierra We study here various types of S=1/2 spin ladders with odd and even number of legs and intrinsically dimerized patterns. The low temperature physics of these systems is very rich and in fact their ground state at T=0 may undergo a quantum phase transition as we vary the microscopic parameters of the lattices. To study these many-body systems we will use the density matrix renormalization group (DMRG) algorithm, and will present accurate estimations of the critical lines found in these models as well as different measurements to characterize the nature of the ground state. We will see that the valence bond solid picture seems to be a proper description of the massive phases on each side of the phase diagram and will show evidences of this nature by means of a particular order parameter denoted generalized string order parameter. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A12.00011: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 10:12AM - 10:24AM |
A12.00012: Magnetostriction and thermal expansion on 1D chain compound Azurite Frederik W. Fabris, Vivien Zapf, Sonia Francoual, Marcelo Jaime, Alex Lacerda Azurite is a natural mineral with a chemical structure Cu$_{3}$(OH$_{2})$(CO$_{3})_{2}$. This compound is a frustrated triangular quantum magnet consisting of Cu S=1/2 atoms arranged alternately to form infinite chains along the b axis. The magnetic behavior of this compound reflects the existence of both monomers and dimers of S = 1/2 Cu. A magnetization plateau at 1/3 of the saturization magnetization is observed in M vs H measurements between 11 and 30 T due to saturization of the monomers. For fields above the plateau, the magnetic field energy exceeds the dimer bonding and thus the dimers cant and then align with the field. The magnetic structure and the detailed phase diagram in temperature and field are largely unknown or controversial. A recent report [1] in the specific heat behavior suggests a more complicated structure than previously thought. In addition, recent ultrasound measurements [2] indicate significant magnetoelastic coupling must be taken into account. We have acquired interesting results on magnetic torque, magnetostriction and thermal expansion. We have demonstrated that significant, anisotropic magnetostriction occurs in azurite, giving us an indication of the magnetically induced structural distortions. [1] Yasu Takano, personal communication. [2] Lang et al, J. Phys.: Conf. Series 51, 1, (2006). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A12.00013: Quantum Monte Carlo study of a spin-Peierls model in a magnetic field Jeongpil Song, R.T. Clay We present results of a quantum Monte Carlo study of a quasi one-dimensional XY spin model coupled to quantum phonons. We compare different updating techniques for the Stochastic Series Expansion method and present autocorrelation time data. We are able to reduce autocorrelation times by using loop update techniques for both spin and phonon degrees of freedom. We determine the critical phonon coupling for the spin-Peierls state, and discuss the dependence on the phonon frequency, magnetic field, and inter-chain coupling. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A12.00014: Quantum solvation in optical lattice You Wenlong, Gu Shijian, Lin Haiqing We have studied the ground state phase diagram of the one dimensional asymmetric t-J-Jz model at various filling by different methods. The reduced dimensionality and asymmetric hopping could lead to interesting dynamics which can be conjectured to demonstrate quantum solvation process with fermion character of the quantum solvent. The system potentially can be demonstrated in optical lattice. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A12.00015: Effective model parameters for the spin-Peierls system TiOCl from first principles Yuzhong Zhang, Roser Valenti, Harald Jeschke The inorganic spin-Peierls system TiOCl is studied in the frame of Density Functional Theory (DFT) by the projector augmented wave (PAW) and linearized augmented plane wave (LAPW) methods. A two-dimensional frustrated spin Peierls model is proposed to describe the system. The model parameters, such as spin exchange couplings in a, b, and c directions, are estimated by the LAPW method. With the help of the eigenvectors of the dynamical matrix, the spin-phonon couplings and the elastic constant are determined by the PAW method. The reliability of these model parameters is demonstrated by an exact diagonalization and a mean-field calculation as well as by comparison to available experiments. [Preview Abstract] |
Session A13: Focus Session: Simulations of Matter at Extreme Conditions I: Hydrogen Helium, and Planetary Materials
Sponsoring Units: DCOMP GSCCMChair: Burkhard Militzer, University of California, Berkeley
Room: Morial Convention Center 204
Monday, March 10, 2008 8:00AM - 8:36AM |
A13.00001: Quantum Monte Carlo Simulations of Warm Dense Hydrogen Invited Speaker: Quantum Monte Carlo methods are the most accurate and general methods for computing total electronic energies. However, in general, they have been limited to high temperatures or to zero temperature. In recent years, we and others have been working on methods [1] that utilize the Born Oppenheimer approximation to allow simulations coupling the correlated quantum systems and a system of ions. Using quantum Monte Carlo, one estimates the Born-Oppenheimer energy change for a movement of the ions which is then used in a Monte Carlo simulation of the ionic degrees of freedom. The quantum effects of the ionic degrees of freedom and the boundary conditions on the phase of the wavefunction can be integrated over. We have performed simulations of dense hydrogen down to temperatures of 300K. We have used this method to determine the equation of state of warm dense hydrogen, to study the cross-over from the molecular liquid to the atomic liquid [2] and for the melting temperature of solid atomic hydrogen [3]. \newline [1] C. Pierleoni and D. M. Ceperley, ChemPhysChem 6, 1 (2005); physics/0501013. \newline [2] K. Delaney, C. Pierleoni and D. M. Ceperley, Phys. Rev. Letts. 97, 235702 (2006). \newline [3] C. Pierleoni, D. M. Ceperley and M. Holzmann, Phys Rev. Letts. 93, 146402 (2004). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A13.00002: Electrical conductivity of liquid Hydrogen Fei Lin, Kris Delaney, Miguel Morales, Carlo Pierleoni, Richard Martin, David Ceperley DC electrical conductivity of liquid Hydrogen under high pressure has been measured by shock-wave experiments a long time ago [Phys. Rev. Lett. 76, 1860 (1996)], however, an accurate theoretical calculation of electrical conductivity is still unavailable. Ab-initio DFT calculations seem to overestimate the DC conductivity value by about 6 times. On the other hand, coupled electron-ion Monte Carlo (CEIMC) simulation [Phys. Rev. Lett. 97, 235702 (2006)] has predicted different high-pressure Hydrogen molecular-atomic transition than the DFT calculation. In this talk I will report our preliminary electrical conductivity results from CEIMC simulations using the Kubo formula with energies and current-current matrix elements computed with correlated quantum Monte Carlo methods. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A13.00003: Properties of Hydrogen-Helium Mixtures at High Pressure and Temperature Saad Khairallah, Jan Vorberger, Burkhard Militzer Most of the over 200 recently discovered extrasolar planets are giant gas planets that consist primarily of dense, hot hydrogen and helium. Using density functional molecular dynamics (DFT-MD) simulations, we study these fluids at the extreme conditions found in planet interiors. We characterize the interaction of hydrogen and helium, analyze the electronic properties, and report on structural changes in the fluid as a function of density and temperature. We further study the influence of helium concentrations on the stability of hydrogen molecules. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A13.00004: Ultra-High-Pressure Water Martin French, Ronald Redmer, Thomas R. Mattsson We present the first all-electron QMD simulations of water in the ultra-high-pressure regime up to conditions typical for the deep interior of Jupiter and Saturn. We calculate the equation of state and the Hugoniot curve and study the structural properties via pair correlation functions and self-diffusion coefficients. In the ultra-dense superionic phase, we find a continuous transition in the protonic structure. Water at conditions of Jupiter's core (i.e. 20000 K, 50 Mbar, 11 g/cm$^3$) forms a fluid dense plasma. Supported by the DFG within SFB 652. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A13.00005: Ab initio Molecular Dynamics Simulations of Water Under Shock Compression: Chemistry Behind Shock Fronts Laurence Fried, Nir Goldman, Alessandro Curioni, Christopher Mundy, I.-F.W. Kuo, Evan Reed We report herein first principles simulations of water under shock loading of velocities from 5 - 11 km/s. Accurate description of the plateau in the ionic conductivity at high pressures and temperatures is of particular importance to models of the planetary dynamo mechanism in Neptune and Uranus. We attribute this plateau to the exceedingly short- lived molecular and ionic states that occur in water under these extreme conditions. In particular, at the intersection of the shock hugoniot and Neptune isentrope we observe transient metallization that we attribute to the formation of short-lived negatively charged species that contribute electronic states at or around the band gap. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A13.00006: Molecular structures of CO$_{2}$ and N$_{2}$O under pressure Stanimir A. Bonev, Brendan Osberg Carbon dioxide and nitrous oxide are investigated at pressures up to 50 GPa and 1000 K using \textit{ab initio} methods. In this pressure-temperature range, both materials have a number of stable molecular structures. We demonstrate that the constituent molecules in these structures do not undergo significant changes and that the proposed phases are consistent with experimental data. The differences found between the CO$_{2}$ and N$_{2}$O phase diagrams can be understood in terms of the polarity of the N$_{2}$O molecules. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A13.00007: Quantum Monte Carlo Computations for Equations of State, Phase Transitions, and Elasticity of Silica R.E. Cohen, B. Militzer, Z. Wu, K. Driver, P.L. Rios, M. Towler, R. Needs We have performed Quantum Monte Carlo (QMC) computations for silica in the quartz, stishovite, and $\alpha$-PbO$_2$ structures as functions of compression. In spite of the great success of DFT there is still need for improvement. The local density approximation (LDA) gives excellent results for individual silica phases, but LDA predicts stishovite to be the stable ground state rather than quartz. The Generalized Gradient Approximation (GGA) does give correct energy differences, but other properties, such as the bulk moduli, are worse with the GGA than the LDA. We included thermal contributions using density functional perturbation theory with the code ABINIT. We have also computed the shear elastic constant c$_{11}$-c$_{12}$ in stishovite, which is associated with the phase transition to the CaCl$_2$ structure, with QMC. We find excellent agreement with experiments. We find that the main differences between QMC and DFT are crystalline phase dependent energy and pressure shifts. This work is supported by NSF grants EAR-0530282, EAR-0310139, and by DOE contract DE-FG02-99ER45795 to John Wilkins. Computations were performed on blueice at NCAR under a BTS grant, and on Tungsten and Abe at NCSA, and at the Carnegie Institution of Washington. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A13.00008: The effect of temperature and anisotropic pressure on the phase transitions in $\alpha$-cristobalite Roman Martonak, Davide Donadio, Paolo Raiteri, Michele Parrinello The role of temperature and anisotropy of the pressure tensor in the pressure--induced transformations of $\alpha$-cristobalite is investigated by means of first principle molecular dynamics combined with an improved version of the metadynamics algorithm for the study of solid-solid phase transitions\footnote{R.~Marto\v{n}\'{a}k, D. Donadio, A. R. Oganov and M. Parrinello, Nat. Mat. \textbf{5}, 623 (2006).} We reproduce for the first time the transition to $\alpha-$PbO$_2$ as found in experiments\footnote{L.~S. Dubrovinsky {\it et al.} Chem. Phys. Lett. \textbf{333}, 264 (2001).} and we observe that the transition paths are qualitatively different and yield different products whether the applied pressure is hydrostatic or not. While in hydrostatic conditions $\alpha-$PbO$_2$ or stishovite is obtained depending on the temperature and initial conditions, more complicated pathways with several metastable structures are followed upon non-hydrostatic compression and post-stishovite phases are obtained. Based of our simulations, we predict the metastability of a new class of high pressure polymorphs of silica obtained by non-hydrostatic compression. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A13.00009: The Melting Line of Molecular Hydrogen at High Pressure Shanti Deemyad, Isaac Silvera We have measured the melting line of molecular hydrogen to pressure P=82 GPa in a diamond anvil cell (DAC) using pulsed laser heating and found a peak at P=64.5$\pm$4 GPa and T=1055$\pm$20 K. Previous attempts to measure the melting temperature of hydrogen in a DAC by CW ohmic heating methods were limited by the diffusion of hydrogen at elevated temperatures. We have developed a technique of temperature determination in a pulsed laser heated DAC and succeeded to achieve much higher pressures and temperatures. In the pulsed laser method hydrogen diffusion is suppressed allowing access to high temperatures at elevated pressures. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A13.00010: Post-perovskite MgCO$_3$ phase at pressures up to 800 GPa Jones Tsz-Kai Wan The high-pressure phases of magnesite (MgCO$_3$) are investigated by variable cell first-principles molecular dynamics simulations. At pressures compatible to lower mantle conditions ($\sim$120 GPa), the carbon atoms are surrounded by 4 oxygen atoms, which is consistent with the work of Skorodumova et al. (2005). Perovskite phase is observed at pressures greater than 300 GPa, but its stability is still subject to further studies. Stable post-perovskite structure is observed at pressures up to 800 GPa, and is found to be more stable than the perovskite phase. The results may bring important implications to interior models of giant planets, which may lead to a better understanding in giant planets physics. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A13.00011: First-Principles Studies of Anisotropic Constitutive Relationships in Nitromethane and RDX Michael Conroy, Ivan Oleynik, Sergey Zybin, Carter White One of the goals of energetic materials (EMs) research is to obtain accurate equations of state (EOS). These EOS establish fundamental relationships between thermodynamic variables and provide necessary input for the description of materials at the mesoscopic and continuum levels. Ultimately, these EOS are governed by interactions at the atomic scale, and the investigation of these relationships provides an opportunity to connect the shock sensitivity of EMs with underlying atomic-scale structure. In order to investigate this fundamental structure-property relationship, we performed first-principles density functional theory studies of hydrostatic and uniaxial compressions in several crystallographic directions of nitromethane and RDX. Equilibrium properties, including lattice parameters and elastic constants, as well as the hydrostatic equation of state obtained from our calculations are compared with experiment. The shear stresses upon uniaxial compression will be examined, and the possibility of a correlation of their behavior with shock sensitivity will be discussed. [Preview Abstract] |
Session A14: Focus Session: Quantum Simulation of Condensed Matter Systems With Ultracold Atoms
Sponsoring Units: GQI DAMOPChair: Ivan Deutsch, University of New Mexico
Room: Morial Convention Center 205
Monday, March 10, 2008 8:00AM - 8:12AM |
A14.00001: Simulating Charged Particles in a Magnetic Field with Ultra-cold Atoms Using Light-induced Effective Gauge Fields Yu-Ju Lin, William Phillips, James Porto, Ian Spielman We experimentally study light-induced gauge potentials in a $^{87}$Rb Bose-Einstein condensate. Instead of rotating the trap, we prepare the atoms in a spatially-varying optically dressed state. The atomic spin state is dressed by a spatially varying two-photon Raman coupling between the three $F=1$ hyperfine ground states. The resulting effective magnetic field is equivalent to rotating the condensate (and transforming to the rotating frame), and thus generates vortices. The inter-vortex distance is given by $\sqrt{2\pi} l_B$. Using the technique, the minimum possible $l_B \approx\sqrt{R_{\rm TF} \lambda/8 \pi}$ is the magnetic length for a uniform field, $R_{\rm TF}$ is the condensate diameter, and $\lambda\approx805\ {\rm nm}$ is the optical wavelength. We prepare the condensate in the dressed state, whose projection onto internal states of various state-dependent Bragg momenta are well understood. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A14.00002: Two-Dimensional Electron Gas with Cold Atoms in Non-Abelian Gauge Potentials Indubala Satija, Daniel Dakin, J. Y. Vaishnav, Charles W. Clark Motivated by the possibility of creating non-Abelian fields using cold atoms in optical lattices, we study two-dimensional electron gases in a lattice, subjected to such fields. In the continuum limit, the system characterized by a two-component ``magnetic flux" describes a harmonic oscillator existing in two different charge states (mimicking a particle-hole pair). A key feature of the non-Abelian system is a splitting of the Landau energy levels, which broaden into bands. These Landau bands result in a coarse-grained Hofstadter ``moth." Furthermore, the bands overlap, leading to effective relativistic effects. Similar features also characterize the corresponding 2D lattice problem when at least one of the components of the magnetic flux is an irrational number. Some unique aspects of the transport properties of the non-Abelian system are the possibility of inducing localization by varying the quasimomentum, and the absence of localization of certain zero-energy states exhibiting a linear dispersion relation. Furthermore, non-Abelian systems provide an interesting localization scenario where the localization transition is accompanied by a transition from relativistic to non-relativistic theory. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A14.00003: Observing {\it Zitterbewegung} in Ultracold Atoms J.Y. Vaishnav, Charles W. Clark We propose an optical lattice scheme which would permit the experimental observation of {\it Zitterbewegung} (ZB) with ultracold, neutral atoms. A four-level ``tripod" variant of the usual setup for stimulated Raman adiabatic passage (STIRAP) has been proposed for generating non-Abelian gauge fields. [1] Dirac-like Hamiltonians, which exhibit ZB, are simple examples of such non-Abelian gauge fields; we show how a variety of them can arise, and how ZB can be observed, in a tripod system. We predict that the ZB should occur at experimentally accessible frequencies and amplitudes. \newline [1] J. Ruseckas, G. Juzeli{\=u}nas, P. \"{O}hberg, M. Fleischhauer, {\it Physical Review Letters} {\bf 95}, 010404 (2005). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A14.00004: Detecting the Bose glass in optical lattices Tommaso Roscilde We theoretically propose a method for the unambiguous experimental detection of Bose-glass behavior in the central region of a system of bosons trapped in an optical lattice system, and to discriminate the Bose glass from more conventional Mott and band insulators. The method is based on probing the compressibility of the system in the trap center by gradually increasing the trap frequency. Straightforward measurements of the average particle density in the center of the trap and of the momentum distribution allow to detect the migration of particles from the wings into \emph{localized} states in the center of the system under trap squeezing. We discuss the application of the method to simple optical lattices, and to commensurate and incommensurate optical superlattices; we moreover discuss the potential of trap squeezing techniques to probe the density of states of exotic phases realized in optical lattices. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A14.00005: Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate$^1$ Yong P. Chen$^2$, J. Hitchcock, D. Dries, M. Junker, C. Welford, R. G. Hulet We have studied both the transport and phase coherence properties of a Bose-Einstein condensate (BEC) of $^7$Li atoms subject to a disordered potential with tunable strength ($V_D$). The BEC is created in an elongated optical trap, while the disordered potential is produced by laser speckle. We probe transport of the disordered BEC by either slowly or abruptly offsetting the trap relative to the disordered potential. At high $V_D$, we observe pinning of the disordered BEC and suppression of its dipole excitation, consistent with the transition to an insulator. We use \textit{in-situ} imaging to detect density modulation, while time-of-flight (TOF) imaging is used to probe phase coherence. At moderate $V_D$, we observe small density fluctuations in the \textit{in-situ} images, and random but \textit{reproducible} interference patterns in the TOF images. This interference reflects phase coherence in the disordered BEC and is interpreted as \textit{speckle for matter waves}. At higher $V_D$, the TOF interference contrast diminishes while the \textit{in-situ} density fluctuations increase, signifying a fragmented ``granular" condensate with little phase coherence. $^1$Supported by NSF, ONR, NASA, Welch and Keck Foundations. $^2$Now at Purdue University. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A14.00006: Effects of disorder on the interacting Fermi gases in one-dimensional optical lattices B. Tanatar, Gao Xianlong, M. Polini, M. P. Tosi Interacting two-component Fermi gases loaded in a one-dimensional (1D) lattice and subjected to an harmonic trapping potential exhibit interesting compound phases in which fluid regions coexist with local Mott-insulator and/or band-insulator regions. Motivated by experiments on cold atoms inside disordered optical lattices, we present a theoretical study of the effects of a random potential on these ground-state phases. We employ density-functional theory within the local-density approximation to determine the density distribution of fermions in these phases. The exchange-correlation potential is obtained from the Lieb-Wu exact solution of Fermi-Hubbard model. On-site disorder and harmonic trap are treated as external potentials. We find that disorder has two main effects: it destroys the local insulating regions if it is sufficiently strong compared with the on-site atom-atom repulsion, and it induces an anomaly in the compressibility at low density from quenching of percolation. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A14.00007: Probing non local order parameters in highly correlated Bose insulators Invited Speaker: Ground states of integer spin chains are known since the late 80's to sustain highly non local order described by infinite string operators of the spins. Such states defy the usual Landau theory description and can be considered simple prototypes of topological order. Recently we showed that spinless Bose insulators with nearest neighbor or longer range repulsion in one dimension can exhibit similar string order in terms of the boson density [1]. The tunability of cold atomic systems would allow much more flexibility in probing the non local order than spin systems do. For example the bosons can be tuned across a quantum phase transition between the exotic insulator, which we term Haldane insulator, and the usual Mott insulator. Investigating how the transition responds to external perturbations lends direct access to properties of the string order parameter. I will demonstrate this with several new results obtained from a field theoretic description of the phases and confirmed by numerical calculations using DMRG. Particularly revealing of the unusual character of the string order is the prediction that any external perturbation, which breaks the lattice inversion symmetry, would eliminate the distinction between the Haldane and Mott phases and allow a fully gapped adiabatic connection between them. This is remarkable given that neither phase involves spontaneous breaking of lattice inversion symmetry. We also predict that inter-chain tunneling destroys the direct phase transition between the two insulators by establishing an intermediate superfluid phase. Finally I will discuss how the new phases and phase transitions may be realized and probed in actual experiments with ultra cold atoms or polar molecules. \newline [1] E. G. Dalla Torre, E. Berg and E. Altman, Phys. Rev. Lett. 97, 260401 (2006) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A14.00008: A Hexagonal Lattice Ion Trap for Quantum Simulation of Spin Models Ziliang Lin, Robert Clark, Yufei Ge, Isaac Chuang Quantum simulations of one dimensional spin systems are being implemented by ions in linear Paul traps; however, a natural extension to two dimensional quantum spin simulations cannot be realized in the linear Paul trap geometry. Planar lattice traps not only offer the possibility for two dimensional simulations, but also hold two advantages over linear traps: first, neighboring ions in lattice traps have well defined and uniform spacings; second, quantum simulations with planar traps can be scaled up more easily than with linear traps. We develop a hexagonal lattice trap that allows vibrational coupling between ions due to their Coulomb repulsion, which is essential for effective spin-spin interaction. We present fabrication details, preliminary testing results, and a proposal for simulating geometrical spin frustration with three ions in a triangular configuration. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A14.00009: Trapped-Ion Quantum Simulations of Spin Systems: From Two Qubits to Thousands Warren Lybarger, Dana Berkeland, John Chiaverini Due to the exponential growth of a quantum system's state-space with its size, the current technological limit for simulating the evolution of many-quantum-spin systems with classical computers (CC) is 36 spin-$\frac{1}{2}$ particles. While CC's cannot be scaled to meet the exponentially increased demand in computational resources, mapping the Hamiltonians of such problems onto that of a quantum simulator (QS) completely avoids this exponential scaling problem, allowing for efficient simulations of much larger systems. QS may be the first attainable application of quantum information processing, enabling exploration of parts of the phase space not accessible in the original system and possibly providing an exponential speedup of computations for even just a few tens of interacting qubits when compared to CC methods. Following the work of Porras and Cirac [Phys. Rev. Lett. 92, 207901-1 (2004)] we discuss the status of an experiment at Los Alamos for demonstrating a proof of principle QS of an Ising-like spin-spin interaction in a transverse magnetic field. We also discuss a novel architecture for microfabricated ion trap arrays geared toward enabling large scale QS and one-way quantum computing with potentially thousands of ions [arXiv:0711.0233]. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A14.00010: Quantitative determination of the Hubbard model phase diagram from optical lattice experiments: overcoming the singular nature of the thermodynamic limit Vivaldo Campo, Jr., Klaus Capelle, Jorge Quintanilla, Chris Hooley We propose an experiment to obtain the phase diagram of the fermionic Hubbard model, for any dimensionality, using cold atoms in optical lattices. It is based on measuring the total energy for a sequence of trap profiles. It combines finite-size scaling with an additional `finite-curvature scaling' necessary to reach the homogeneous limit. We illustrate its viability in the 1D case, simulating experimental data in the Bethe-Ansatz local density approximation. Including experimental errors, the filling corresponding to the Mott transition can be determined with better than 3 per cent accuracy. The main obstacle that our method overcomes is the singular nature of the thermodynamic limit of atom traps. We discuss this surprising phenomenon and describe a simpler experiment that could be used to demonstrate it. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A14.00011: The p$_{x,y}$-orbital counterpart of graphene -- cold fermions in the honeycomb optical lattices Congjun Wu, Doron Bergman, Leon Balents, Sankar Das Sarma We study the ground states of cold atoms in the tight-binding bands built from p-orbitals on a two dimensional honeycomb optical lattice. The band structure includes two completely flat bands. Exact many-body ground states with on-site repulsion can be found at low particle densities, for both fermions and bosons. We find crystalline order at n=1/6 with a $\sqrt{3} \times \sqrt{3}$ structure breaking a number of discrete lattice symmetries. In fermionic systems, if the repulsion is strong enough, we find the bonding strength becomes \emph{dimerized} at n=1/2. Experimental signatures of crystalline order can be detected through the noise correlations in time of flight experiments. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A14.00012: DMRG Studies for Strongly-Correlated Fermions on a Triangular Optical Lattice Masahiko Okumura, Susumu Yamada, Masahiko Machida Strongly-interacting fermions in a triangular lattice attract much attention because not only the interaction but also the geometrical frustration is expected to cause non-trivial behaviors. In solids, most of materials parameters, e.g., interaction strength between electrons (fermions), fermion density, and crystalline potential are almost fixed depending on the sample fabrication and our research area are restricted. In contrast, cold atom systems enable to study it systematically because some crucial parameters are precisely controllable. Thus, we expect that the cold Fermi atoms on a triangular lattice bring us information on both strongly-correlated and frustrated systems. In this study, we examine a system described by the triangular Hubbard model by using the parallel density-matrix renormalization group (DMRG). In addition, we also investigate effects of random potential made by speckle laser in the system. We evaluate the effects of frustration, strong interaction, and randomness simultaneously. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A14.00013: Order parameter statistics at a quantum phase transition Austen Lamacraft, Paul Fendley Universality implies that at a second order phase transition the probability distribution of the order parameter takes a universal scaling form. This distribution is a natural way to characterize the quantum critical properties of ultracold atomic gases, since its histogram may be readily obtained by repeated `single-shot' measurements. In this work we obtain the exact scaling probability distribution for the simplest quantum critical point: that of the transverse field Ising model in 1D. Using a novel identity for the Ising model correlation functions, we map the problem to a particular case of the anisotropic Kondo model. [Preview Abstract] |
Session A15: Focus Session: Theory of Magnetization Dynamics
Sponsoring Units: DCOMP DMP GMAGChair: Allan MacDonald, University Of Texas, Austin
Room: Morial Convention Center 207
Monday, March 10, 2008 8:00AM - 8:12AM |
A15.00001: Bias dependence of magnetic exchange coupling. Paul Haney, Mark Stiles, Christian Heiliger, Allan MacDonald An applied electrical bias can change the interlayer coupling in magnetic multilayers and magnetic tunnel junctions. The bias dependence of these changes is controversial; it is not clear whether the changes depend linearly or quadratically on the applied voltage. Motivated by this controversy, as well as proposals to exploit bias-dependent exchange coupling to accomplish current induced magnetic switching, we compute the bias-dependence of interlayer exchange coupling in magnetic multilayers and tunnel junctions. For simple tight-binding models, we derive expressions for this dependence, describe the special cases in which this dependence is particularly large, and derive the extent to which zero-bias expressions for interlayer coupling remain valid for biased systems. We also examine the related question of the bias-dependence of intralayer exchange interactions in a single ferromagnetic layer, and discuss experimental consequences of bias-modulated exchange stiffness, including induced changes in the Curie temperature and spin wave dispersion. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A15.00002: Spin current and rectification in quantum wires Feifei Li, Bernd Braunecker, Dima Feldman We study the spin and charge currents in a one-channel quantum wire with strong electron interactions in a uniform static magnetic field. We show that a dc-spin current can be generated by an ac voltage in the presence of an asymmetric potential barrier, e.g., two point scatterers of unequal strength. In an interval of voltages, the spin current increases with the decrease of the voltage bias as a negative power of the voltage. We find that the spin dc-current in units of $\hbar/2$ per second can greatly exceed the charge current in units of electron charge per second. Neither spin-polarized particle injection nor time-dependent magnetic fields are required for the generation of the spin current. \newline \newline [1] D. E. Feldman, S. Scheidl, and V. M. Vinokur, Phys. Rev. Lett. 94, 186809 (2005). \newline [2] Bernd Braunecker, D. E. Feldman, and Feifei Li, Phys. Rev. B 76, 085119 (2007). [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A15.00003: Vortices and Antivortices as Harmonic Oscillators B. Krueger, M. Bolte, A. Drews, U. Merkt, G. Meier, D. Pfannkuche In experiments the distinction between current-induced dynamics and the dynamics induced by the Oersted field of the current is still an open problem. Here we investigate the gyroscopic motion of current- and field-driven magnetic vortices/antivortices in micro- or nanostructured thin-film elements by analytical calculations and by numerical simulations. Starting from the micromagnetic equation of motion extended by the spin torque introduced by Zhang and Li, we derive an analytical expression for the current- and field-driven trajectory of the vortex/antivortex. The gyroscopic motion is well described by modeling the stray-field energy as a harmonic-oscillator potential. For small harmonic excitations the vortex/antivortex cores perform an elliptical rotation around their equilibrium positions. Our analytical model allows to calculate the amplitude and phase of the gyration. The phase of the rotation and the ratio between semi-axes are determined by the frequency and amplitudes of Oersted field and spin torque. The analytical result is compared to micromagnetic simulations with good accordance.[1] Even though the influence of weak magnetic fields on the vortex/antivortex trajectories is small, the phase of the rotation is significantly changed. Thus, the model can estimate the Oersted field's contribution in spin-torque experiments.[1] B. Krueger et al., Phys. Rev. B 76, December (2007). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A15.00004: Current-induced torques in magnetic textures and in antiferromagnets Invited Speaker: Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. Spin pumping is the opposite of spin transfer, namely the generation of spin currents by a time-dependent magnetization. In this talk I will discuss recent theoretical work aimed at understanding current-induced torques and spin pumping in situations that spin is not fully conserved, due to e.g., spin-orbit interactions, or when conservation of spin can not be used to infer order-parameter dynamics, as is the case in antiferromagnets. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A15.00005: Spin transfer and the role of spin-motive-forces for spin valves and domain walls Stewart Barnes The interaction of magnetic domains with electrical currents has potentially far reaching applications for spintronics. The requirements of energy conservation are reflected by spin- (smf) and electro-motive-forces (emf) [1] . For spin-valves and domain walls this smf redistributes the currents between the different possible conduction channels in a manner that significantly modifies the dynamics and introduces magnetic relaxation. Our Berry phase approach to domain walls [1,2] has been extended to spin-valves. The results are consistent with the requirements of angular momentum and energy conservation but differ in a number of important ways from those obtained when the Sloncewski torque transfer term is added to the Landau-Liftshitz equations with either Gilbert or Landau-Liftshitz relaxation [3].\break [1] S. E. Barnes and S. Maekawa: Phys. Rev. Lett. {\bf 98}, 246601 (2007)\break [2] S. E. Barnes and S. Maekawa: Phys. Rev. Lett. {\bf 95}, 107204 (2005).\break [3] See e.g., Concepts in Spin Electronics, ed. by S. Maekawa (Oxford Press, 2006). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A15.00006: Dynamic and temperature effects in spin-transfer switching Dorin Cimpoesu, Huy Pham, Alexandru Stancu, Leonard Spinu Recently, the current-induced spin-transfer torque has been proposed as a convenient writing process in high density magnetic random access memory. With increasing demand on the access time, the current pulse shape become important. Also, with memory area density increasing and the memory cell size further shrinking the study of thermal fluctuations in these magnetic structures becomes of extreme importance for their recording thermal stability. In this paper we have studied the dynamic switching in a spin-transfer memory and its dependence on thermal effects. The magnetic layers are assumed to be in the shape of ellipsoids, and each magnetic layer is assumed to be a single domain. The model is based on stochastic Landau-Lifshitz-Gilbert equation, which is numerically integrated, and the switching diagrams, as a function of current pulse amplitude and duration, are presented. Instead of a clear border between switching and non-switching areas we have a transition region, with a layer-like structure with switching/non-switching areas, where the final state is sensitive to current pulse amplitude and duration, to damping constant, and to thermal fluctuations. The extent of the instability region is increasing with the applied current sweep rate. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A15.00007: Frequency degeneracy in spin-torque induced precession Shuxia Wang, Pieter Visscher Magnetic precession in nanometer elements, first studied by Stoner and Wohlfarth in 1948, is central to the understanding of fast switching in magnetic information storage devices. Periodic orbits have recently gained more attention because they can be stabilized (and their frequencies measured) by spin torque techniques. We have observed a surprising and so far largely unexplained degeneracy in this system: when (as is often the case) there are two orbits with the same energy, even if their shapes and sizes are very different, their frequencies turn out to be the same. Although this is easy to show in the highest-symmetry (uniaxial) case, we find it is true far more generally -- for any quadratic energy function with arbitrary anisotropy tensor and arbitrary external magnetic field. We have calculated the frequencies for a random selection of anisotropy tensors and magnetic fields, will show examples of asymmetrical orbits, whose frequencies are equal within numerical accuracy ($\approx 10^{-6}$). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A15.00008: Evaluation of Gilbert damping in half metals Claudia K.A. Mewes, Chunsheng Liu, Mairbek Chshiev, Tim Mewes, William H. Butler According to Kambersk\'{y}'s spin torque correlation model of Gilbert damping [1,2], precessional damping in magnetic systems occurs through a combination of spin-flip exciations and orbital excitations. In half-metallic systems, Gilbert damping is expected to be reduced because of the absence of spin-flip scattering. This makes half-metals interesting potential candidates for information storage technologies especially for use in CPP/GMR read head devices and spin-torque MRAM. Using a combination of first principle calculations to predict the band structure for the half-metal of interest and an extended H\"{u}ckel tight binding model we calculate and discuss the Gilbert damping within the spin torque correlation model for different half-metallic structures, including the Heusler alloys Co$_{2}$MnSi, Co$_{2}$MnGe. [1] V. Kambersk\'{y}, Czech. J. Phys. B \textbf{26}, 1366 (1976). [2] B. Heinrich, D. Fraitov\'{a} and V. Kambersk\'{y}, Phys. Stat. Sol. \textbf{23}, 501 (1967). [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A15.00009: Evaluation of Gilbert damping in transition metals using tight binding schemes Chunsheng Liu, Claudia K.A. Mewes, Mairbek Chshiev, Tim Mewes, William H. Butler Recently first principle calculations of the damping in transition metals have reproduced the unusual temperature dependence observed experimentally [1, 2]. Here we present an alternative method to calculate the Gilbert damping within Kambersk\'{y}'s spin torque correlation model using a combination of first principle calculations and an extended H\"{u}ckel tight binding model. In our scheme we use ab initio calculations (VASP) including spin orbit coupling to obtain the band structure of the transition metal of interest. With the knowledge of the band structure we use a fitting procedure to construct an extended H\"{u}ckel tight binding model which then allows the evaluation of the Gilbert damping parameter. Because of the simplicity of our Hamiltonian, we can converge the integral over the Brillouin of the spin-orbit torque without extraordinary computational effort. We show that our results are in good agreement with the results obtained from previous calculations. [1] K. Gilmore, Y.U. Idzerda and M.D. Stiles, Phys. Rev. Let. \textbf{99}, 027204 (2007). [2] V. Kambersk\'{y}, Phys. Rev. B \textbf{76}, 134416 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A15.00010: Saturation of spin-polarized current in nanometer scale aluminum grains Chris Malec, Yaguang Wei, Dragomir Davidovic \\\\We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples ($\sim G\Omega$), spin-polarized current is carried only via the ground state and the low-lying excited states, leading to a saturation in spin polarized current with bias voltage. Both a qualitative argument based on relaxation rates, and a non-equilibrium transport model are developed and compared. In two samples, the spin-relaxation rate $T_1^{-1}$ for some of the low-lying excited states is comparable to the electron tunnelling rate: $T_1^{-1}\approx 1.5\cdot 10^6 s^{-1}$ and $10^7s^{-1}$, meaning the spin of an electron confined in a metallic grain is highly stable. The ratio of $T_1^{-1}$ to the electron-phonon relaxation rate is in quantitative agreement with the Elliot-Yafet scaling, an evidence that spin-relaxation in Al grains is driven by the spin-orbit interaction. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A15.00011: Kondo Resonance in the Presence of Spin-Polarized Currents Yunong Qi, Jian-Xin Zhu, Shufeng Zhang, Chin-Shen Ting We propose an improved method of the equation of motion approach to study the Kondo problem in spin-dependent non-equilibrium conditions. We found that the previously introduced additional renormalization for non-equilibrium Kondo effects is not required when we use a proper decoupling scheme. Our improved formulation is then applied to address the spin-split Kondo peaks when a spin current injects into a Kondo system. We believe that this work significantly advances our understanding of the non-equilibrium Kondo physics, and our predictions of the Kondo resonance are timely for the application of non-equilibrium spin-related phenomena. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A15.00012: Resolution-dependent mechanisms for bimodal switching time distributions in simulated Fe nanopillars S.H. Thompson, G. Brown, P.A. Rikvold, M.A. Novotny Numerical simulations of magnetization reversals of iron nanopillars in off-axis applied fields at different lattice resolutions reveal bimodal distributions in the switching times (first-passage times through $0$ of the longitudinal magnetization, $M_{\mathrm{z}}$). We show that the mechanisms responsible for these distributions are resolution-dependent. The highest-resolution model, in which the computational cell is smaller than the exchange length, is three-dimensional. Here, the bimodal distribution results from a reversal process in which the pillar sometimes avoids a metastable free-energy well. At medium resolution, the pillar is modeled as a $1$-D stack of spins. The bimodal distribution then reflects whether the reversal starts from one or both ends. Finally, for a low-resolution model in the form of a single spin with an anisotropic potential, the bimodal distribution is an artifact of the definition of a switching event: the result of the spin precessing close to $M_{\mathrm{z}}$~$=0$. While the zero- and one-dimensional models display bimodal switching-time distributions, the mechanisms are different than for the three-dimensional model. Only the latter captures the mechanism that is most interesting from an experimental and device-application point of view. [Preview Abstract] |
Session A16: Dynamics of Nucleic Acids
Sponsoring Units: DBP DPOLYChair: Igor Aronson, Argonne National Laboratory
Room: Morial Convention Center 208
Monday, March 10, 2008 8:00AM - 8:12AM |
A16.00001: Biochemistry on a leash: A mechanism for ligand recruitment via tethered binding sites Daniel Reeves, Keith Cheveralls, Jane Kondev The diffusion limited reaction rate for ligand-receptor systems is typically estimated as the rate at which the ligand stumbles upon the receptor site by three-dimensional Brownian motion. We consider a mechanism that improves upon this limiting rate by placing a binding site on a flexible polymer. The tethered binding site explores the vicinity of the receptor site via polymer diffusion. After binding the ligand, the tether directly transfers it to the receptor site. This is in contrast with existing model mechanisms that involve non-specific binding that confines the ligand to lower-dimensional diffusion. The proposed mechanism may be relevant to the biological actin capping protein formin, which increases actin polymerization rates when bound to the growing tip of actin filaments. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A16.00002: Backtracking and error correction in DNA transcription Margaritis Voliotis, Netta Cohen, Carmen Molina-Paris, Tanniemola Liverpool Genetic information is encoded in the nucleotide sequence of the DNA. This sequence contains the instruction code of the cell - determining protein structure and function, and hence cell function and fate. The viability and endurance of organisms crucially depend on the fidelity with which genetic information is transcribed/translated (during mRNA and protein production) and replicated (during DNA replication). However, thermodynamics introduces significant fluctuations which would incur massive error rates if efficient proofreading mechanisms were not in place. Here, we examine a putative mechanism for error correction during DNA transcription, which relies on backtracking of the RNA polymerase (RNAP). We develop an error correction model that incorporates RNAP translocation, backtracking pauses and mRNA cleavage. We calculate the error rate as a function of the relevant rates (translocation, cleavage, backtracking and polymerization) and show that the its theoretical limit is equivalent to that accomplished by a multiple-step kinetic proofreading mechanism. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A16.00003: Effects of crosslinks on motor-mediated filament organization Igor Aranson, Falko Ziebert, Lev Tsimring Crosslinks and molecular motors play an important role in the organization of cytoskeletal filament networks. Here we incorporate the effect of crosslinks into our model of polar motor-filament organization, through suppressing the relative sliding of filaments in the course of motor-mediated alignment. We show that this modification leads to a nontrivial macroscopic behavior, namely the oriented state exhibits a transverse instability in contrast to the isotropic instability that occurs without crosslinks. This transverse instability leads to the formation of dense extended bundles of oriented filaments, similar to recently observed structures in actomyosin. This model also can be applied to situations with two oppositely directed motor species or motors with different processing speeds. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A16.00004: Collective alignment of polar filaments by molecular motors Falko Ziebert, Igor Aranson We study the alignment of polar biofilaments, such as microtubules and actin, subject to the action of multiple molecular motors attached simultaneously to more than one filament. Focusing on a paradigm micromechanical model of only two filaments interacting with multiple motors, we were able to investigate in detail the dynamics of the filaments' alignment. While almost no alignment occurs in the case of a single motor, we show that the filaments become perfectly aligned due to the collective action of the motors working together. Our studies revealed that the the alignment time is governed by the magnitude of the fluctuations in the motor force. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A16.00005: Detecting cooperative sequences in the binding of RNA Polymerase-II Kimberly Glass, Julian Rozenberg, Michelle Girvan, Wolfgang Losert, Ed Ott, Charles Vinson Regulation of the expression level of genes is a key biological process controlled largely by the 1000 base pair (bp) sequence preceding each gene (the promoter region). Within that region transcription factor binding sites (TFBS), 5-10 bp long sequences, act individually or cooperate together in the recruitment of, and therefore subsequent gene transcription by, RNA Polymerase-II (RNAP). We have measured the binding of RNAP to promoters on a genome-wide basis using Chromatin Immunoprecipitation (ChIP-on-Chip) microarray assays. Using all 8-base pair long sequences as a test set, we have identified the DNA sequences that are enriched in promoters with high RNAP binding values. We are able to demonstrate that virtually all sequences enriched in such promoters contain a CpG dinucleotide, indicating that TFBS that contain the CpG dinucleotide are involved in RNAP binding to promoters. Further analysis shows that the presence of pairs of CpG containing sequences cooperate to enhance the binding of RNAP to the promoter. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A16.00006: Dynamic self-assembly of nanocomposite ring structures through the interaction of thermodynamic and energy-dissipating processes Haiqing Liu, Erik Spoerke, Marlene Bachand, Steven Koch, Bruce Bunker, George Bachand Self-assembly of nanostructured materials occurs in thermodynamic and energy-dissipating systems. We've described a unique self-assembly scheme in which non-equilibrium nanocomposites are formed by the interaction of energy dissipation and thermodynamics. Three distinct composite structures (mobile linear, rotating circular and immobile aggregated composites) are formed when streptavidin-coated quantum dots are introduced to biotinylated microtubules that are being transported by kinesin. The circular nanocomposites occur only in a delicately balanced regime when thermodynamic and energy-dissipating components interact cooperatively. Linear translation and axial rotation of microtubules drive the formation of mechanical strain within the composites, which ultimately defines the structural shape and rotational direction. Disassembly of these composites occurs spontaneously, as well as induced by the addition of free biotin. Exploitation of dynamic self-assembly promises nanostructured materials with revolutionary behaviors that are unattainable through conventional self-assembly. \newline *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, or the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A16.00007: How Large are Cooperative Effects in Hydrogen Bonded Molecular Chains? Martin Fuchs, Matthias Scheffler, Joel Ireta Intermolecular hydrogen bonds play an eminent role in a wide range of materials. In particular, they are critical for the secondary structure stabilization of biopolymers like proteins and nucleic acids. Arrays of hydrogen bonds (hbs), such as in chains or helices, often display a cooperative strengthening of the individual hbs. This cooperativity is crucial for understanding the stability and properties of hydrogen bonded materials. Here we investigate the hb cooperativity in model chains of HCl, HF, HCN, formamide, and 4-pyridone, i.e. molecules forming weak to strong hbs. We calculate the hb strength of infinitely long chains using density-functional theory (DFT) with the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA). We show that for large intermolecular separations, the hbs in the infinite chain strengthen by 20\% over the respective molecular dimers, consistent with dipolar electrostatics [1]. At the equilibrium separation, the hbs strengthen significantly further (up to 260\% for HF), with additional stabilization from induced dipolar interactions. Comparing with results from higher-level calculations (MP2 and quantum Monte Carlo) we find that DFT faithfully describes the cooperativity in these systems in which the hbs are close to linear. [1] P.B. Allen, J. Chem. Phys. {\bf 120}, 2951 (2004) [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A16.00008: Single stranded DNA hairpin loop kinetics: A Brownian dynamics study Martin Kenward, Kevin Dorfman The dynamics of single stranded DNA (ssDNA) molecules play a role in a number of biological functions and have a found uses in several microfluidic applications. In particular, ssDNA having complementary sequences at their ends can form hairpin loops in which the complementary sections bind to one another. The appearance of these loops and fluctuations between {\it open} and {\it closed} states depend on a number of variables including: the degree of complementarity of the end sequences, temperature and strength of hydrogen bonding and base stacking. In this study, we present a Brownian dynamics model which is used to examine the kinetics of the hairpin loop formation. We present results for the melting behavior of hairpins as a function of temperature and other system parameters. We also present results for the kinetic rate constants $k_{-}$ and $k_{+}$ corresponding to the open-closed and closed-open transitions respectively. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A16.00009: Statistical Analysis of the Chemotactic Motility Cycle of Amoeboid Cells. Baldomero Alonso-Latorre, Juan C. del Alamo, Ruedi Meili, Richard A. Firtel, Juan C. Lasheras Amoeboid motility results from the repetition of stereotypic steps that produce quasi-periodic oscillations of cell length and speed. We characterize the steps of the motility cycle of \textit{Dictyostelium }cells crawling on elastic substrates by analyzing their traction forces. Using a high-resolution force cytometry method for wild type cells and mutants with contractility and adhesion defects, we find that the time evolution of the traction forces is quasi-periodic, with a period (T) that correlates strongly with the cell speed (V) according to a simple law VT=L. The constant L is the distance traveled per cycle. The magnitude of the traction forces exerted by the cells does not correlate with the cell speed, suggesting that the speed of migration is determined by the ability of the cell to rapidly repeat the phases of the motility cycle. Phase average statistics allow us to combine time sequences of force maps derived from different cells to obtain a spatio-temporal representation of a canonical motility cycle divided into four steps: protrusion, contraction, retraction and relaxation. We find that myosin II-dependent contraction is present in all the steps of the wild-type motility cycle,including protrusion. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A16.00010: Force generated by polymerization of actin filaments Coraline Brangbour, Olivia du Roure, Emmanuele Helfer, Marc Fermigier, Marie-France Carlier, Jerome Bibette, Jean Baudry Actin polymerization drives protrusions at the cell surface and leads to cell motility. Using magnetic colloids, we measure how the chemical reaction of polymerization generates mechanical forces. We detail in particular the force-velocity relation of growing actin filaments; and discuss how the stalling force is affected by the mean number of filaments between two beads. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A16.00011: Probing Brownian Motion of an Ellipsoid with an External Force Shao-Qing Zhang, Wu-Pei Su Brownian motion has translational and rotational degrees of freedom. Anisotropy in the shape of a Brownian particle leads to dissipative coupling between transitional and rotational motion. However, the coupling effects, which depend on the initial orientation of an ellipsoid, cannot be detected by most typical experimental techniques. To surmount the hurdle between theoretical predictions and experimental measurements, we present a theoretical scheme for uncovering the translation- rotation coupling by applying a constant external force to an ellipsoid in a two-dimensional suspension. The geometry of the ellipsoid can be determined using the first two cumulants. An anisotropy-isotropy alternation is found in the cumulant series. We also discuss the probability distribution function (PDF) of lab-frame displacements to gain insight into the significance of anisotropy of a Brownian particle in diverse environments. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A16.00012: Probing Protein Conformations at the Oil-water Interface Using Single-Molecule Force Spectroscopy Ahmed Touhami, Marcela Alexander, Milena Corredig , John Dutcher The present work aims at a deeper understanding of the conformational changes in Beta-lactoglobulin (BLG) protein adsorbed onto the oil-in-water emulsion interfaces due to variations in pH. Mechanical unfolding of BLG using AFM-single-molecule force spectroscopy (AFM-SMFS) was performed on single oil droplets that were mechanically trapped in a polycarbonate filter. The changes in the contour length upon each unfolding event were determined by fitting the WLC model of polymer elasticity to each of the BLG peaks. Our results show clearly that at pH 2.5 BLG exists as a dimer in which each monomer is similar to two Immunoglobulin domains. At pH 6.8 BLG on the oil droplets adopts a conformation consisting of domains with a contour length of 11 nm. Furthermore, at pH 9 the interactions between the AFM tip and the BLG layer on the oil droplet surface are dominated by a huge repulsion due to the highly negatively charged BLG layer. This study demonstrates a novel application of AFM-SMFS to investigate the underlying mechanisms by which proteins can be used to stabilize food products. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A16.00013: Structural motifs of biomolecules Hoang Trinh, Jayanth Banavar, Amos Maritan, Chiara Poletto, Antonio Trovato, John Maddocks, Andrzej Stasiak Biomolecular structures are assemblies of emergent anisotropic building modules such as uniaxial helices or biaxial strands. We provide an approach to understanding a marginally compact phase of matter that is occupied by proteins and DNA. This phase, which is in some respects analogous to the liquid crystal phase for chain molecules, stabilizes a range of shapes that can be obtained by sequence-independent interactions occurring intra- and intermolecularly between polymeric molecules. We present a singularity free self-interaction for a tube in the continuum limit and show that this results in the tube being positioned in the marginally compact phase. Our work provides a unified framework for understanding the building blocks of biomolecules. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A16.00014: Instabilities of ordered chiral active suspensions Tapan Adhyapak, Davide Marenduzzo, Sriram Ramaswamy Suspensions of actively contractile or tensile filaments with simple orientational order are hydrodynamically unstable. Here we study, analytically as well as numerically, the manner in which the presence of cholesteric order competes with this instability. [Preview Abstract] |
Session A17: Cold Fusion
Sponsoring Units: DCMPChair: Scott Chubb, Naval Research Laboratory
Room: Morial Convention Center 209
Monday, March 10, 2008 8:00AM - 8:12AM |
A17.00001: The Significance of Replication Michael C.H. McKrubre, Francis L. Tanzella, Vittorio Violante Much has been made of an apparent lack of reproducibility in so called ``cold fusion'' experiments. In this paper we will demonstrate that this failure, while real, was the result of inability to meet critical threshold criteria: a thermodynamic loading, dynamic flux and disequilibrium trigger. Recent experiments, performed independently at SRI and ENEA, have successfully replicated powerful excess heat results obtained initially by Energetics in Israel. This success and high levels of experiment reproducibility are attributed to two critical factors that allow these threshold barriers to be surpassed: i)achievement and maintenance of a high level of control of the metallurgy of the bulk palladium metal host and the cathode surface morphology, guided by initial studies at ENEA and the University of Rome,ii) use of a novel non steady-state cathode current stimulus, proposed and developed by Energetics. With simultaneous high deuterium loading and high flux, excess heat effects were measured in both Isoperibolic and Mass Flow calorimeters at factors several times greater than the electrical input power and several orders of magnitude larger than the sum of all conceivable chemical reactions. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A17.00002: Public-Interest and Level-of-Evidence Considerations in Cold Fusion Public Policy Thomas Grinshaw Cold fusion (CF) protagonists and antagonists would no doubt agree that scientific processes have been challenged in the CF case. The public interest in CF turns on two questions: What are the potential benefits? What is the probability that CF is ``real''? Potential benefits have been agreed on since CF announcement in 1989. The probability of CF reality may be assessed based on level of evidence (LoE): preponderance of evidence (PoE); clear and convincing evidence (CCE); and beyond a reasonable doubt (BRD). PoE, from civil law, indicates a probability of 50\% or higher. BRD, from criminal law, has a probability approaching 90\%. CCE, in between, thus has a 70-75\% probability. CF experimental evidence, based on: 1) initial affirmations, 2) the large number of corroborations since marginalization, and 3) particularly demonstrative experiments, reasonably indicates at least a PoE level of evidence for excess heat. A case can also be made for a CCE (but probably not for a BRD) LoE. In either the PoE or CCE scenario a clear need is demonstrated for change in policy toward CR, given its potential benefits to humanity. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A17.00003: Anomalous Radiation Produced by Glow Discharge in Deuterium Containing Oxygen Edmund Storms, Brian Scanlan ElectroMagnetic Radiation (EMR) and anomalous radiation (potentially produced by nuclear reactions, involving high energy particles), in a low-voltage discharge in a gas containing deuterium was measured using a Geiger counter located within the apparatus. This radiation is found to consist of energetic particles that are produced only when the voltage is above a critical value. In addition, the emission is very sensitive to the presence of oxygen in the gas. The intensity of the reaction producing the radiation could be fit by a power function when compared to the applied voltage. The effect of EMR and other sources of noise that might be attributed to the anomalous radiation are discussed. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A17.00004: Low Energy Nuclear Reaction Products at Surfaces David J. Nagel This paper examines the evidence for LENR occurring on or very near to the surface of materials. Several types of experimental indications for LENR surface reactions have been reported and will be reviewed. LENR result in two types of products, energy and the appearance of new elements. The level of instantaneous power production can be written as the product of four factors: (1) the total area of the surface on which the reactions can occur, (2) the fraction of the area that is active at any time, (3) the reaction rate, that is, the number of reactions per unit active area per second, and (4) the energy produced per reaction. Each of these factors, and their limits, are reviewed. A graphical means of relating these four factors over their wide variations has been devised. The instantaneous generation of atoms of new elements can also be written as the product of the first three factors and the new elemental mass produced per reaction. Again, a graphical means of presenting the factors and their results over many orders of magnitude has been developed. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A17.00005: Evidence and Theory for Cluster Reactions in LENRs George H. Miley, Heinz Hora, Andrei Lipson, Prajakti Joshi Shrestha A distinctive array reaction products attributed to nuclear reactions was observed earlier in the ``Patterson'' flowing packed-bed type electrolytic cell experiments using multi-layer thin films of metals on mm-size plastic beads. The swimming electron layer and a new magic number theory were proposed to explain this. More recently these theories have been expanded into a ``D-Pd-D cluster'' model to explain a wider range of transmutation experiments. This cluster model is consistent with certain measurements of energetic charged-particle emission during thin film electrolysis, with observations suggesting localized reactions and also with x-ray production during plasma bombardment experiments. The cluster reaction concept and supporting experimental data will be discussed in this presentation. In addition to explaining , if understood and optimized, cluster reactions could lead to an important new power source based on Low Energy Nuclear Reactions (LENRs). A conceptual power cell based on a novel electrode design that promotes cluster reactions is presented. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A17.00006: Compound Nucleus Reactions in LENR, Analogy to Uranium Fission Heinrich Hora, George Miley, Karl Philberth The discovery of nuclear fission by Hahn and Strassmann was based on a very rare microanalytical result that could not initially indicate the very complicated details of this most important process. A similarity is discussed for the low energy nuclear reactions (LENRs) with analogies to the yield structure found in measurements of uranium fission. The LENR product distribution measured earlier in a reproducible way in experiments with thin film electrodes and a high density deuteron concentration in palladium has several striking similarities with the uranium fission fragment yield curve.\footnote{G.H. Miley and J.A. Patterson, J. New Energy 1, 11 (1996); G.H. Miley et al, Proc ICCF6, p. 629 (1997).}This comparison is specifically focussed to the Maruhn-Greiner local maximum of the distribution within the large-scale minimum when the fission nuclei are excited. Implications for uranium fission are discussed in comparison with LENR relative to the identification of fission a hypothetical compound nuclear reaction via a element $^{306}$X$_{126}$ with double magic numbers. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A17.00007: New Mechanism for Explaing LENR and Certain forms of Technological and Natural Catastrophes Fangil Gareev We proposed\footnote{ F.A. Gareev and I.E. Zhidkova, Proc. of the 12th international conference on cold fusion, Yokohama, Japan 27 November -2 December 2005.} a new mechanism for low energy nuclear reactions (LENR): cooperative resonance processes involving the whole the system - nuclei + atoms + condensed matter can occur at a smaller threshold energies than the corresponding ones on free constituents. The cooperative processes can be induced and enhanced by low energy external fields. The excess heat is the emission of internal energy and transmutations at LENR are the result of a redistribution of internal energy of the whole system. The lack of financial support and ignorance by mainstream physicists has resulted in the LENR field not being accepted. We postulate that LENR can lead to catastrophes, potentially including, the runaway evcnt involving the reactor at the Chernobyl Nuclear Power Plant, the explosion of the twin towers during the 11 September 2001 World Trade Center collapse, in New York, the explosion of transformers in Moscow, catastrophes of submarines, and other phenomena associated with a cooperative resonance synchronization mechanism. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A17.00008: Predictability of Theory, and Collaboration with Experimentalists in CMNS Xing Zhong Li Condensed Matter Nuclear Science has confirmed 2 outstanding experimental results: 144.5W of continuous ``Excess Heat'' in 10 minutes(ICCF-3,1992) and the nuclear transmutation induced by deuterium flux on the Pd surface (ICCF-8, 2000). Theory predicted neutron emission based on the previous beam-target experiments. It was a wrong guidance, because there was no ``commensurate neutron'' detected. The collaboration with experimentalists helped theorist to modify their prediction in the past 19 years. Theorists might imagine that ``high loading ratio'' was necessary; then, the experiments said ``deuterium flux was more important.'' Resonant tunneling theory imagined again ``any resonance in inelastic scattering (nuclear reaction) had to be accompanied by a resonance in elastic scattering (diffusion); hence, a peak in excess heat should be correlated with a peak in deuterium flux.'' The experiments seem to confirm this imagination\footnote{J. Phys. D: Appl. Phys.36 3095(2003)}. The next 2 predictions are: (1) Adjusting the loading rate to form a steady state for resonant tunneling;(2)Neutrino detection from this steady state to confirm its nuclear nature. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A17.00009: Comparison of SPAWAR Co-deposition Experimental Data and Competing Condensed Matter Nuclear Science Theories Lawrence P.G. Forsley, Pamela Mosier-Boss The SPAWAR PdD co-deposition protocol has been replicated in several laboratories and shown to produce apparent nuclear tracks in solid-state CR-39 detectors. Additional spectroscopic gamma ray measurements have been carried out using either high resolution, cryogenically cooled germanium or lower resolution sodium iodide detectors. These results are at odds with many of the competing theories in this field, suggesting the need to acquire additional temporally and spectrally resolved nuclear data. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A17.00010: Multiple Etching of CR-39 Nuclear Track Detectors used in SPAWAR Co-Dep Experiment Pamela Mosier-Boss, Lawrence P.G. Forsley Previously published results involving the use of the solid state track detectors, CR -39, have brought into question whether or not purported tracks are of nuclear origin. One method of determining this is to serially etch and scan these track detectors so as to determine the approximate depth of the tracks. This method, coupled with a computer code incorporating bulk and track etching rates in CR-39 for alpha particles, gives good agreement with tracks seen in SPAWAR co-deposition experiments as compared to known alpha sources. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A17.00011: Spatial and Temporal Resolution of Three Sites Characterizing Lattice-Assisted Nuclear Reactions (LANR) Mitchell Swartz We present developing evidence that three different sites (physical locations in the solid state) are involved in lattice-assisted nuclear reactions (LANR). By expanding the equation first developed by Prof. David Nagel at ICCF-13\footnote{Nagel, D., ``Rates for LENRs at Surfaces'', ICCF-13}, we correlate observations of excess heat and de novo helium-4 production to three different physical locations and to the optimal operating points (OOPs) which are now known to characterize LANR systems\footnote{Swartz. M., G. Verner, ``Excess Heat from Low Electrical Conductivity Heavy Water Spiral-Wound Pd/D2O/Pt and Pd/D2O-PdCl2/Pt Devices'', ICCF-10 (Camb. MA), Proceedings of ICCF-10, (2003).}. This observation will be shown to be consistent with our previous reports of distinct time constants which characterize the tardive thermal power regime\footnote{Swartz. M., G. Verner, ``Dual Ohmic Controls Improve Understanding of `Heat after Death' '', Transactions American Nuclear Society, vol. 93, ISSN:0003-018X, 891-892 (2005)} (`heat after death'), which results after all input electrical power is terminated to an active LANR device. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A17.00012: Effects of Applied Magnetic Fields on Aqueous Electrolysis John Dash, Craig Cousins, Jon Anderton, Jian Tian Evidence that nuclear reactions occur during Pd/D co-deposition has been reported \footnote{Pamela A. Mosier-Boss, Stanislaw Szpak, and Frank E. Gordon, Abstract Submitted for the March 07 Meeting of The American Physical Society.}. These reactions were found to be enhanced in the presence of either an external electric or magnetic field. We have studied the interaction of applied magnetic fields with aqueous electrolysis. The electrolysis cell was placed between the pole pieces of an electromagnet, with the magnetic field normal to the electric field. Appreciable turbulence was observed with electrolysis current density of 0.05 A/cm2 and applied magnetic field of 0.8 tesla. Turbulence increased with increase in current density, up to 0.2 A/cm2, and/or increased magnetic field strength, up to 0.8 tesla. These effects are documented on a video tape. Results of studies of electrolysis of heavy water with a palladium cathode in an applied magnetic field, including heat measurements, surface topography, and surface composition, will be compared with results obtained from an identical cell without an applied magnetic field. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A17.00013: D2 Fusion in Ionic Solid + Nanometal Composites Talbot Chubb Interfaces between ionic solids and nanometals seem to provide an environment that promotes Bloch deuterium with 2-dimensional lattice symmetry. Electrolysis-loaded powdered ZrO$_2$ + nanoPd composite produced 10-W excess heat for 400 hr (1.4 x 10$^7$ J).\footnote{Y. Arata and Y-C Zhang, Proc. Jaspan Acad. 78B, 57 (2002).}. This compares with best plasma fusion runs of 16 MW of fusion heat for $\le$ 1s ($\le$ 1.6 x 10$^7$ J). The fusion heat was less than the input energy).\footnote{C. Cookson, Financial Times, Energy Section, 14 (9 Nov. 2007).} In 2004, Arata and Zhang pressure-loaded ZrO$_2$ + nanoPd with D$_2$ at 140 $^o$C and produced an estimated steady 0.6 W of fusion heat.\footnote{Y. Arata and Y-C Zhang, Proc. ICCF12, 44 (2006).}$^,$\footnote{T.A. Chubb, Proc. ICCF13, (submitted 2007).} The ionic oxide + nanometal composites absorb abnormal amounts of hydrogen gas.\footnote{S-i. Yamaura et al., J. Mater. Res. 17, 1329 (2002).} [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A17.00014: Resonant Electromagnetic Interaction in Low Energy Nuclear Reactions Scott Chubb Basic ideas about how resonant electromagnetic interaction (EMI) can take place in finite solids are reviewed. These ideas not only provide a basis for conventional, electron energy band theory (which explains charge and heat transport in solids), but they also explain how through finite size effects, it is possible to create many of the kinds of effects envisioned by Giuliano Preparata. The underlying formalism predicts that the orientation of the external fields in the SPAWAR protocol\footnote{Krivit, Steven B., New Energy Times, 2007, issue 21, item 10. http://newenergytimes.com/news/2007/NET21.htm}$^,$\footnote{Szpak, S.; Mosier-Boss, P.A.; Gordon, F.E. Further evidence of nuclear reactions in the Pd lattice: emission of charged particles. Naturwissenschaften 94,511(2007).}.has direct bearing on the emission of high-energy particles. Resonant EMI also implies that nano-scale solids, of a particular size, provide an optimal environment for initiating Low Energy Nuclear Reactions (LENR) in the PdD system. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A17.00015: Connection of Preparata QED Theory and D-Pd-D Cluster Theory for Cold Fusion Reactions Linchon Wu, George Miley G. Preparata earlier proposed a radical new QED theory, and had just begun application to cold fusion prior to his untimely death.\footnote{G. Preparata, ``QED Coherence in Matter'', World Scientific Press, Singapore, 1997.} We have since used a variation of his theory to explain D-Pd-D cluster reactions in certain cold fusion experiments.\footnote{George H. Miley, et al., ``Evidence and Theory for Cluster Reactions In LENRs'', these APS Proceedings.} An ensemble of D atoms loaded into Pd can assume two different configurations coupled with the intrinsic EM field. A coherent state forms above critical density and temperature thresholds. This new state has lower energy than the Pd lattice where D-Pd-D atoms oscillate between the two configurations in tune with an EM field arising from vacuum fluctuations and co-resonating atoms. This form ``coherence domains'' (CDs) of micron size for PdD. CDs hold two mesoscopic components, a coherent fraction of D-Pd-D and a non- coherent fraction of interstitial Ds.. Large ``cage'' of CD's forms with a definite phase and zero entropy, trapping a ``gas'' of non-coherent Ds. In our D-Pd-D cluster theory this results in intense nuclear reactions. [Preview Abstract] |
Session A18: Focus Session: Multiscale Modeling: Polymers, Nanocomposites, and Biomacromolecules
Sponsoring Units: DPOLY DCOMP DBPChair: John Curro, University of New Mexico
Room: Morial Convention Center 210
Monday, March 10, 2008 8:00AM - 8:36AM |
A18.00001: Aneesur Rahman Prize Talk: Dynamics of Entangled Polymer Melts: Perceptive from Molecular Dynamics Simulations Invited Speaker: Twenty years ago at the APS March Meeting, Kurt Kremer and I presented the first numerical evidence from computer simulations that the reptation model of Edwards and de Gennes correctly describes the dynamics of entangled linear polymer melts. For chains longer than the entanglement length $N_e$, the monomers of a chain move predominantly along their own contour. The distinctive signature of reptation dynamics, which we observed, was that on intermediate time scales, the mean squared displacement of a monomer increases with time as $t^ {1/4}$. Though these early simulations were limited to chains of a few $N_e$, they demonstrated the potential of computer simulations to contribute to our understanding of polymer dynamics. Here I will review the progress over the past twenty years and present an outlook for the future in modeling entangled polymer melts and networks. With present day computers coupled with efficient parallel molecular dynamics codes, it is now possible to follow the equilibrium dynamics of chains of length $10-20N_e$ from the early Rouse regime to the long time diffusive regime. Result of these simulations support the earlier results obtained on chains of only a few $N_e$. Further evidence for the tube models of polymer dynamics has been obtained by identifying the primitive path mesh that characterizes the microscopic topological state of the computer- generated conformations of the chains. In particular, the plateau moduli derived on the basis of this analysis quantitatively reproduce experimental data for a wide spectrum of entangled polymer liquids including semi-dilute theta solutions of synthetic polymers, the corresponding dense melts, and solutions of semi-flexible (bio)polymers such as f-actin or suspensions of rodlike viruses. We also find that in agreement with the reptation model, the stress, end-to-end distance and entanglement length of an entangled melt subjected to uniaxial elongation, all relax on the same time scale. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A18.00002: Microrheology of Nanospheres in Rod Suspensions Victor Pryamitsyn, Venkat Ganesan Many biological processes and applications involve the motion of small (nanoscale) spherical particles through a dense (typically) semiflexible polymer matrix. While much theoretical work has characterized the motion of such particles in the limit of its size being much larger than the mesh size of the matrix, very limited understanding exists of the (more biologically relevant) crossover regime to the case where the particle size becomes comparable to the mesh size. Recently we have developed a new computer simulation method to simulate the dynamical and rheological properties of colloid suspensions of in a variety of complex fluids. We first present the results of its generalization to the dynamics and linear rheological properties of dilute, semidilute and concentrated rods suspensions in a simple fluid. Subsequently, we use this method to characterize the mobility and diffusive dynamics of nanoscale spheres in rod matrices, while paying special attention to the length scales of the fluid which characterize the hydrodynamic screening and overall viscous motion. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A18.00003: Peptide binding to sheet silicate and metal nanoparticles: Insight from atomistic simulation Hendrik Heinz, Ras B. Pandey, Lawrence Drummy, Richard A. Vaia, Rajesh R. Naik, Barry L. Farmer Short peptides (8 to 12 amino acids, excluding Cys) bind selectively to nanoparticles composed of Au, Pd, and montmorillonite depending on the sequence of amino acids, as evidenced by laboratory screening of several billion peptides. The molecular reasons for binding versus non-binding and the specificity toward a certain surface are analyzed by molecular dynamics simulation, using recent force field extensions for fcc metals and sheet silicates to reproduce surface and interface energies with $<$10{\%} deviation compared to experiment. Polarization on even metal surfaces ranges from 3 to 5 kcal/mol and non-covalent binding energies from 0 and 80 kcal/mol per dodecapeptide. Adsorption energies, changes in chain conformation, Ramachandran plots, and orientational parameters, are analyzed in conjunction with NMR, TEM, and other experimental data. On montmorillonite, an ion exchange reaction of Lys side groups against alkali ions as well as interactions between alkali cations and polar groups in the peptide are explained. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A18.00004: Adsorption-desorption of peptide chains on Au surface by a coarse-grained Monte Carlo simulation Ras B. Pandey, Hendrik Heinz, Lawrence R. Drummy, Richard A. Vaia, Rajesh R. Naik, Barry L. Farmer Using a coarse grained description, we study stability of the structure and dynamics of several peptide chains (A3, Flg, Pro10, Gly10, Pd2, Pd4) at gold surfaces on a cubic lattice. Although the structural details within the amino acid groups are ignored, the specificity of their interactions is incorporated in our computer simulation modeling of these peptide chains on a cubic lattice. Appropriate coarse-grained interactions (Lennard-Jones) among the amino acid nodes, solvent, and the gold surface with different strength are guided by the atomistic simulations and X-ray crystallographic data; the molecular weight of each amino acid groups is also considered. Peptide chains execute their stochastic motion and their proximity to the generic gold surface is monitored. Mobility of each amino acid (node), its energy, and correlations to their neighboring constituents are analyzed. Some of these results are consistent with the atomistic simulation. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A18.00005: Characterization of the translocation of polymers driven through nanopores using molecular dynamics simulations Hendrick de Haan, Gary W. Slater The passage of a polymer through a narrow pore (translocation) is a fundamental process with a wide range of biological applications.~ In particular, threading DNA through nanopores promises to have important implications for the next generation of DNA sequencing techniques.~ In this work, simulations of the translocation of polymers being driven through a narrow, short nanopore are conducted via the Espresso Molecular Dynamics simulation package using the Lattice-Boltzmann algorithm to include hydrodynamics.~ In this talk, results from simulations in which an external field is applied within the pore or to one end of the polymer are presented and compared.~ Characterization of the scaling of the translocation time with the number of monomers as well as details of the anomalous diffusion exhibited by the translocation coordinate will be given. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A18.00006: Coupling of atomistic and mesoscopic scales: visualizing the translocation of biopolymers through nanopores Maria Fyta, Simone Melchionna, Massimo Bernaschi, Efthimios Kaxiras, Sauro Succi We investigate the process of biopolymer translocation through a narrow pore using a multiscale approach, which combines Langevin Molecular-Dynamics with a mesoscopic Lattice-Boltzmann method for the solvent dynamics. We analyze the statistical features of the translocation process through extensive simulations over various polymer conformations and lengths. The translocation time obeys a power law dependence to the polymer length with an exponent $1.28 \pm 0.01$ in very good agreement with experiments of DNA translocation through solid state pores. We focus on the morphological aspects of the translocation dynamics, the folding behavior of the translocating molecule and the associated cooperation of the surrounding solvent, and report the first computational evidence of quantized current blockades. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A18.00007: Coarse-Grained Kinetic Modeling of Polymer Networks with Non-Affine Slip-Tube Behavior and Heterogeneous Microstructure Brian Pasquini, Fernando Escobedo, Yong Lak Joo Elements of existing entanglement network models have been extended to better account for non-affice slip-tube behavior and to incorporate the effect of heterogeneous spatial domains. Starting with the Density Cloud Model (DCM) framework from Terzis et. al., an entanglement bond potential acting at each entanglement point is introduced. This potential mimics the non-affine tethering from network theories, and in combination with slippage accurately reproduces Non-Affine Slip-Tube behavior. This framework can easily be extended to study the effect of polymer architecture on the mechanical response of the resulting networks. Secondly, the temporary bond from the model of Termonia and Smith is combined with the DCM framework to simulate rigid domains within a matrix of soft polymer network. The modulus of the additional bonds sets the elastic properties of the rigid domain, while DCM entanglement relaxation assures that local deformation remains consistent with the bulk polymer density. The effect of rigid domain size on initial modulus is reported. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A18.00008: Simulating thermal transport in high contrast composite media Harshadewa S. Gunawardana, Kieran Mullen, Dimitrios V. Papavassiliou In dealing with transport in composites systems, high contrast materials pose a special problem for numerical simulation: the time scale or step size in the high thermal conductivity material must be much smaller than in the low conductivity material. High conductivity inclusions can be treated as having an infinite conductivity, removing the need to model transport within the high conductivity inclusions. We develop a random walk algorithm to model thermal transport in composites with high conductivity. We observed the standard random walk algorithm leads to non uniform temperature distribution at the vicinity of the high conductivity inclusion violating the second law of thermodynamics. We show how a standard random walk algorithm can be altered to improve speed while still preserving the second law of thermodynamics. We demonstrate the algorithm in 1D and 3D systems. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A18.00009: Predictive Morphology Models for Crystalline Polymers Jacob Harvey, Zhicheng Xiao, Yvonne Akpalu Modeling of small-angle scattering data provides information on heterogeneities on sizes on the order of 10 Angstroms and larger. The typical size, shape and arrangement of the heterogeneity can be determined by applying models to the scattering intensity $I(q)$. When there is a distribution in the size of structures present and when a system is densely packed, it is likely that models that can be used for analysis may not provide a unique description of the structure. With the recent interest developing predictive models for molecular level control over the properties of polymers, it is desirable to determine all unique structural and morphological contributions to a scattering curve without assuming a model. However, by using a multi-scale approach (i.e. light and X-ray scattering spanning multiple size scales), it may be possible to build unique models for crystalline polymers. We will show that hybrids of statistical methods can be used to decouple scattering data into unique structural components. We will show how our approach can be used to discover analytical models and to develop a set of descriptors that can be used to predict scattering curves for several other polymers that share a similar structure or crystallization condition. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A18.00010: Coarse-grained Molecular Dynamics Simulations and Analysis of Poly(L-lactic Acid) (PLLA) Melt Gaurav Manik, Hemant Nanavati, Upendra Natarajan We present coarse-grained (CG) MD simulations of the melt structure of PLLA, a very useful biodegradable polymer. Our CGMD simulations consider entire repeat unit as a one bead and use IBI scheme. The CG potentials and forces are obtained after performing atomistic MD of 52 PLLA tetramer molecules and employing the probability distributions for the corresponding CG lengths, angles, dihedrals, and the radial distribution function. The initial energy-minimized samples are equilibrated for density in NPT ensemble, followed by structural equilibration. The simulated characteristic ratio (2.13) and density (1.123g/cc) at 450K are in excellent agreement with the expt. values$^{1}$ of 2.2 and 1.152g/cc at 413K. The equilibrated structures were analyzed for primitive path properties, tube diameter, $a_{pp}$, entanglement length, $N_{e}$, etc., using Kroger's Z-code$^{2}$. This yields $N_{e}$=61.8 and $a_{pp}$=55.7A$^{o}$ for longest chains (N=1000) and compare favorably with expt.$^{ }$values.$^{1}$, 55 and 47.7A$^{o}$ (413K). \newline \newline [1] Dorgan, J R., Janzen, J and Clayton, M.,\textit{J. Rheology},49,607,\textbf{2005} \newline (a) Kroger, M., \textit{Comput. Phys. Commun}., 168, 209, \textbf{2005}. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A18.00011: Modeling the Thermodynamics of the Interaction of Nanoparticles with Cell Membranes Valeriy Ginzburg, Sudhakar Balijepalli Interactions between nanoparticles and cell membranes may play a crucial role in determining the cytotoxicity of nanoparticles as well as their potential application as drug delivery vehicles or therapeutic agents. It has been shown that such interactions are often determined not by biochemical but by physico-chemical factors (e.g., nanoparticle size, hydrophobicity, and surface charge density). Here, we propose a mesoscale thermodynamic model describing the transitions in membrane morphology observed after exposure to various types of nanoparticles. Our simulations demonstrate under which conditions (determined by particle size and hydrophilic/hydrophobic interactions) the particles can adsorb into the membrane or compromise the membrane integrity to result in the formation of nano-sized holes. The model could be refined to include a more accurate description of various phospholipid membranes, and its results could be applied in the design of specific nanoparticles for various biomedical applications. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A18.00012: Structure and dynamics of a model polymer nanocomposites Monojoy Goswami, Bobby Sumpter We investigate the structure and the dynamics of a model polymer nanocomposite (PNC) through molecular dynamics (MD) simulations in the canonical ensemble (NVT). Several computer experiments have been carried out at different temperatures for different Lennard-Jones well-depth and filler volume fraction. We studied the real space pair correlation functions and collective scattering structure factors of the PNC melt. This structural analysis has been compared with the previous theoretical and experimental works. The reinforcement of the nanocomposite have been investigated using stress-stress autocorrelation ($\sigma_{xy}(t)$) function for different temperatures. At lower temperatures, $\sigma_{xy}(t)$ shows strong reinforcement of the nanocomposite while at higher temperatures it relaxes quite fast. Diffusion of nanoparticles in the composite has been investigated and compared with earlier works. The effect of sizes and shapes of the nanoparticles has also been investigated in this work. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A18.00013: Strategies for design of polymeric nanoparticles Jiwu Liu, Michael Mackay, Phillip Duxbury Recently polymer nanoparticles have been synthesized using single chains as macromolecular precursors, providing unprecedented control of nanoparticle size and function. We present the results of molecular dynamics simulations which provide detailed insight into the formation kinetics of specific polymeric nanoparticles and which also predict design strategies for formation of interesting new targets. Nanoparticles are formed through chemical crosslinking which is possible when reactive species on the chain backbone are in close proximity. Since the chemical crosslinking is highly irreversible, nanoparticles formed in this way do not unfold on heating, in contrast to the familiar case of thermal denaturing of proteins. Synthesis of precursors with an alphabet of orthogonal crosslinkers provides a rich phase space for design of polymeric nanoparticles. For example, our simulations indicate that an alphabet of three orthogonal crosslinkers enables self-assembly of two-faced or Janus nanoparticles and a variety of other morphologies. [Preview Abstract] |
Session A19: Magnetic and Superconducting Properties
Sponsoring Units: DCMPChair: Wenguang Zhu, University of Tennessee
Room: Morial Convention Center 211
Monday, March 10, 2008 8:00AM - 8:12AM |
A19.00001: Transport in ultrathin gold films decorated with magnetic Gd atoms Micol Alemani, Erik Helgren, Addison Hugel, Frances Hellman We have performed four-probe transport measurements of ultrathin Au films decorated with Gd ad-atoms. The samples were prepared by quench condensation, i.e., sequential evaporation on a cryogenically cooled substrate under UHV conditions while monitoring the film thickness and resistance. Electrically continuous Au films at thickness of about 2 mono-layers of material are grown on an amorphous Ge wetting layer. The quench condensation method provides a sensitive control on the sample growth process, allowing us to tune the morphological and electrical configuration of the system. The ultrathin gold films develop from an insulating to a metallic state as a function of film thickness. The temperature dependence of the Au conductivity for different thickness is studied. It evolves from hopping transport for the insulating films, to a ln T dependence for thicker films. For gold films in the insulating regime we found a decreasing resistance by adding Gd. This is in agreement with a decreasing tunneling barrier height between metallic atoms. The Gd magnetic moments are randomly oriented for isolated atoms. This magnetic disorder leads to scattering of the charge carriers and a reduced conductivity compared to nonmagnetic materials. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A19.00002: From Order to Disorder and Back: Co on Narrow Stepped Cu Nader Zaki, Denis Potapenko, Richard Osgood, Jr., Peter Johnson Bimetallic surface systems allow a ready template to explore the compositional dependence of surface phases. When these systems involve a vicinal substrate, the surface also becomes a template for nanoscale-phase formations. In this regard, we examine the bimetallic system of Co on Cu(775), due the wide-spread interest in the magnetic phenomena of the related Co/Cu(111). We present an STM imaging study of this surface to show that it is possible to observe self-assembly of reduced-dimension quantum structures. These observations show a rich set of bimetallic phase transitions as a function of coverage - moving from wires at low coverage to step-induced ordered islands at high coverage. At coverage of less than 0.1ML, we observe growth of sharp, straight 2-atom-wide Co wires; topographic measurements suggest an interesting interpretation of recent DFT computations on such a system. Increasing coverage causes a marked change of the step spacing and causes the surface to be covered with an ordered array of 2-D islands beyond a critical deposition amount. Thus, as coverage increases, the Cu step structure evolves from straight ordered step edges to concave-shaped edges and then from disordered to ordered islands. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A19.00003: Electron charge and spin pairing (pseudo)gaps and Nagaoka instabilities in nanoclusters Armen Kocharian, Gayanath Fernando, Tun Wang, Kalum Palandage, Jim Davenport The electron pairings, phase separation and magnetism in various frustrated Hubbard clusters are studied exactly with emphasis on tetrahedron and octahedron under doping, magnetic field and temperature. Small clusters yield intriguing insight into charge spin separation and invoked thermal condensation of electron charge and spin in more than one bosonic mode. The spin saturated phase in so called Nagaoka state is found equivalent to ferromagnetic Mott-Hubbard like insulator with (negative) spin pairing gap, while non maximum spin ground state is of BCS-like metallic origin with equal charge (negative) and spin (positive) pairing gaps. The calculated phase diagrams resemble a number of inhomogeneous coherent and incoherent paired phases in high T$_c$ cuprates, fullerene molecules, Co and Nb nanoparticles. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A19.00004: Superconductivity of Ultra-thin Pb films on Semiconductor Substrates: A Scanning Tunneling Microscopy/Spectroscopy Study Shengyong Qin, Jungdae Kim, Alexander Ako Khajetoorians, Chih-Kang Shih Ultra-thin Pb films on semiconductor substrates have exhibited many intriguing phenomena manifested by the quantum confinement of electronic states. Quantum stability has been a topic of interest for many years. Recently, it was shown that quantum confinements also play an interesting role on superconductivity. Oscillations of superconductivity gap and Tc as a function of film thickness have been observed in Pb/Si(111) and Pb/Ge(111) systems. Moreover, it is found that the superconductivity remains very robust even for films as thin as 5 ML. An interesting question arises as to what extent the robustness of superconductivity remains in even thinner regime. By using a different surface template, namely $\surd $3x$\surd $3-$\alpha $ Pb/Si(111) surface, we have grown uniform Pb films down to 2 ML. The film shows preferred thicknesses of 2ML and 4ML, presumably a manifestation of the quantum stability. We find that the superconducting gap remains robust down to 4ML and shows BCS-like temperature dependence. For 2ML films, we find a much smaller gap at 4.2K. Whether or not it corresponds to superconducting gap is under investigation. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A19.00005: Magnetic field dependence of Interface Superconductivity in LSCO/LCO bilayers Scott Riggs, Fedor Balakirev, Albert Migliori, Greg Boebinger, Gena Logvenov, Anthony Bollinger, Adrian Gozar, Ivan Bozovic Interface superconductivity (IS) with a high T$_{c}$ has been discovered recently in bi-layer films consisting of a thin layer of La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ (overdoped and metallic but not superconducting) covered with a thin layer of La$_{2}$CuO$_{4}$ (undoped, insulating, and antiferromagnetic) grown by molecular beam epitaxy (MBE)$^{1}$. Here we report on a study of magneto-transport properties in such IS systems. By measuring the magnetic-field dependence of in-plane longitudinal and Hall resistivities we find the temperature dependence of the upper critical field (H$_{c2})$. Other findings and inferences on the nature of IS and T$_{c}$ enhancement will be discussed as well. $^{1}$A. Gozar, G. Logvenov, A. T. Bollinger and I. Bozovic, ``Interface superconductivity between a metal and a Mott insulator'', submitted for publication. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A19.00006: Separation of the strain and finite size effect on the ferromagnetic properties of La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films Changkun Xie, Joseph Budnick, Barrett Wells, Joseph Woicik The ferromagnetic properties of epitaxial La$_{0.5}$Sr$_{0.5} $CoO$_3$ thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thickness and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of T$_C$ versus in-plane strain suggests that the ferromagnetic transition temperature is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should be applicable to any ordering phase transition in thin films. A leading theory for the ferromagnetism in La$_{0.5}$Sr$_{0.5} $CoO$_3$ is the double exchange mechanism. This model relies upon Co-O-Co electron hopping so that a strong dependence on bond length is expected. Our recent EXAFS results will examine whether the double exchange mechanism quantitatively predicts the strain dependence we have measured. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A19.00007: Giant Magnetoelectric Coupling in Asymmetric Ferromagnet-Ferroelectric-Metal Trilayers: Toward Electric Control of Magnetization Tianyi Cai, Sheng Ju, Junren Shi, Enge Wang, Zhenya Li, Qian Niu By examining electron screening at the surface of ferromagnetic metals and semiconductors, we propose an asymmetric ferromagnet-ferroelectric-metal trilayer to realize giant magnetoelectric effect. The origin is the strengthened accumulation of spin-polarized screening electrons at the ferromagnet-ferroelectric interface. Including the electrostatic energy from such spin-polarized charges in the Laudau-Devibshire free energy for ferroelectics, a magnetoelectric coupling term $\chi P^2M^2$ is derived, and consequently an electric control of magnetization hysteries is observed. The dependence of $\chi$ as well as the ME effect on the choice of materials is discussed for some real ferromagnets. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A19.00008: Room temperature ferromagnetism in as-deposited and post-annealed Co-doped ZnO films Xiao-Hong Xu, Xiao-Li Li, G.A. Gehring The Co-doped ZnO thin films were prepared on $c$-cut sapphire substrates by magnetron co-sputtering, and then annealed at various temperatures in vacuum. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the magnetization of the annealed Zn$_{0.88}$Co$_{0.12}$O films is increased about one order of magnitude in comparison with the corresponding as-deposited one. Both X-ray diffraction and TEM results show that there are not any Co and Co oxides secondary phases. Optical spectrometry indicates that Co$^{2+}$enters the tetrahedral sites of the wurtzite structure of ZnO host and substitutes for Zn$^{2+}$. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A19.00009: Ferromagnetic $Mn_{3-\delta } Ga$ On Wurtzite GaN: Initial Stages Of Growth By Molecular Beam Epitaxy Kangkang Wang, Abhjit Chinchore, Erdong Lu, Wenzhi Lin, Jeongihm Pak, Arthur R. Smith Ferromagnetic (FM) metal/wide band-gap bilayers are of great interest due to their potential for novel spintronics applications, such as blue and ultra-violet spin light-emitting diodes$^{[1]}$. It has been reported$^{[2]}$ that $Mn_{3-\delta } Ga$, a promising FM alloy, can be grown epitaxially on top of w-GaN(0001) with controllable magnetism via controlling of the Mn:Ga flux ratio. Here we report studies on the initial stages of growth of MnGa on w-GaN. Growth experiments were carried out in a UHV chamber using molecular beam epitaxy with rf (N$_{2})$-plasma, on both N- and Ga-polar substrates. Reflection high-energy electron diffraction (RHEED) data suggest that at the initial stages of growth, the surface structures depend on the substrate polarity. This may be due to the structural differences between the N-polar and the Ga-polar GaN surfaces. Stoichometry dependence of initial stages of growth is also being investigated. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No.0304314). Equipment support from ONR is also acknowledged. [1] S.A.Wolf \textit{et al}, Science \textbf{294}, 1488 (2001) [2] E.Lu \textit{et al}, Phys.Rev.Lett. \textbf{97}, 146101 (2006) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A19.00010: Molecular Beam Epitaxial Growth of Iron Nitrides on Zinc-Blende Gallium Nitride(001) Jeongihm Pak, Wenzhi Lin, Abhijit Chinchore, Kangkang Wang, Arthur R. Smith Iron nitrides are attractive materials for their high magnetic moments, corrosion, and oxidation resistance. We present the successful epitaxial growth of iron nitride on zinc-blende gallium nitride (c-GaN) in order to develop a novel magnetic transition metal nitride/semiconductor system. First, GaN is grown on magnesium oxide (MgO) substrates having (001) orientation using rf N$_{2}$-plasma molecular beam epitaxy. Then we grow FeN at substrate temperature of $\sim $ 210 $^{\circ}$C up to a thickness of $\sim $ 10.5 nm. \textit{In-situ} reflection high-energy electron diffraction (RHEED) is used to monitor the surface during growth. Initial results suggest that the epitaxial relationship is FeN[001] $\vert \vert $ GaN[001] and FeN[100] $\vert \vert $ GaN[100]. Work in progress is to investigate the surface using \textit{in-situ} scanning tunneling microscopy (STM) to reveal the surface structure at atomic scale, as well as to explore more Fe-rich magnetic phases. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A19.00011: Synthesis of Co$_{1-x}$Fe$_{2+x}$O$_{4}$: Towards Spin Polarized Ferrites Jarrett Moyer, Hui-Qiong Wang, Carlos Vaz, Eric Altman, Victor Henrich Ferrites are promising materials for spintronic devices, since they are predicted to exhibit high spin polarizations [1]. Thin-film cobalt ferrite (CoFe$_{2}$O$_{4})$ has a large saturation magnetization and magnetic coercivity, but is insulating [2]. In this work, epitaxial Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films are grown by MBE on Fe$_{3}$O$_{4}$(001) and MgO(001), where a fraction of the Co$^{2+}$ ions are replaced with Fe$^{2+}$. LEED, RHEED and XRD confirm the crystal structure. Stoichiometry and cation valence states are ascertained by XPS, and the electronic structure near the Fermi level is determined by UPS. We show that, by varying the stoichiometry of Co$_{1-x}$Fe$_{2+x}$O$_{4}$, we can tailor its electronic properties, which may lead to a conductive, spin polarized ferrite. [1] J. Cibert, et al., C.R. Physique \textbf{6} (2005) 977. [2] W. Huang, et al., J. Crystal Growth \textbf{300} (2007) 426. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A19.00012: A surface-driven route to novel magnetic structures: Manganese on Si(100)(2x1) Petra Reinke, Hui Liu, Christopher Nolph The combination of Silicon with an element with a large magnetic moment such as Manganese is highly desirable for the development of novel spintronics devices. We present a study on the surface-driven synthesis of Mn-nanostructures on the Si(100) (2x1) surface using STM and photoelectron spectroscopy. The Si-surface functions as a template and monoatomic Mn-nanowires are formed, which always run perpendicular to the Si-dimer rows. Their length and spatial distribution is used to derive a model for the wire formation. The bonding of Mn to the Si, which is decisive for the resultant magnetic properties, are presented. The transition to a silicide is kinetically hindered and controlled by the Si- mobility. In the next step a Ge-overlayer is deposited, and analyzed with voltage dependent STM. In the low-adatom-mobility regime the Ge-growth is unperturbed by the presence of Mn, and the Mn-nanostructure is preserved and embedded. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A19.00013: High-resolution ARPES studies of atomically uniform Pb films on Si(111). Shaolong He, Masashi Arita, Masahiro Sawada, Shan Qiao, Hirofumi Namatame, Masaki Taniguchi Investigations on the fundamental physics evolved in the low-dimensional systems are of great interests both in basic research and in potential applications. Atomically uniform metal thin films on semiconductor substrate are the simplest quasi-2D electron systems, which demonstrate quantum confinement and form the quantum well states (QWS). Recently, atomically uniform Pb films on Si(111) have demonstrated novel properties induced by the quantum size effects. We have manufactured atomically uniform Pb films on Si(111)-7x7 surface. The dependence of the Pb films band structures on thickness has been studied by high-resolution angle-resolved photoemission spectroscopy (ARPES). In addition, we have investigated the superconductivity properties in such a quasi-2D electron systems by measuring the ARPES below superconducting transition temperature. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A19.00014: Unusual Hall effect due to carrier delocalization. Xiaohang Zhang, S. von Molnar, P. Xiong, Z. Fisk Recently, an unusual Hall effect (HE) in antiferromagnetic YbRh$_{2}$Si$_{2}$ was reported.$^{1}$ Here, we describe the observation of a similar HE in ferromagnetic EuB$_{6}$. The unusual HE is characterized by two distinct slopes in the Hall resistivity as a function of applied magnetic field: a small slope in the ferromagnetic state and a large slope at high temperatures. In the paramagnetic state just above the Curie-Weiss temperature $\theta $ (intermediate temperatures), the Hall resistivity switches from the large slope at low fields to the small slope at high fields. The phenomenon cannot be attributed to the anomalous HE since the change in the Hall slope is not accompanied by saturation of magnetization. Moreover, the switching field was found to depend linearly on temperature and vanish right at $\theta $. We show that the switching occurs at a certain magnetization at which carriers are delocalized due to the overlapping of magnetic polarons. A quantitative fit to the HE data has been obtained based on this model of carrier delocalization. The model and analysis were successfully applied to the published HE data on YbRh$_{2}$Si$_{2}$, which suggests a possible relation between carrier delocalization and quantum criticality. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. $^{1}$S. Paschen et al., Nature \textbf{432}, 881 (2004). [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A19.00015: Spin-dependent band structure of the ferromagnetic semimetal EuB$_{6}$ Peng Xiong, X. Zhang, S. von Molnar, Z. Fisk The spin polarization of EuB$_{6}$ crystals has been measured using Andreev reflection spectroscopy. The conductance spectra of the EuB$_{6}$/Pb junctions are well-described by the spin-polarized BTK model, which yields a spin polarization of about 56{\%}. The results demonstrate that ferromagnetic EuB$_{6}$ is not half-metallic. Further analyses of the Hall effect and magnetoresistivity indicate a semi-metallic band structure with complete spin polarization for the hole band only. The values and the spread of the measured spin polarization are \textit{quantitatively} consistent with Fermi surface determined by quantum oscillation measurements$^{1}$ and carrier densities obtained from standard two-band model fits to the low temperature magnetoresistivity and Hall resistivity. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. $^{1}$R. Goodrich et al., PRB \textbf{58}, 14896 (1998); M. Aronson et al., PRB \textbf{59}, 4720 (1999). [Preview Abstract] |
Session A20: Focus Session: Quantum Dots and Semiconductor Surface Nanostructures
Sponsoring Units: DMPChair: Kristen Fichthorn, Pennsylvania State University
Room: Morial Convention Center 212
Monday, March 10, 2008 8:00AM - 8:12AM |
A20.00001: Phase-field modeling of solute precipitation and dissolution at solid-fluid interface Zhijie Xu, Paul Meakin Phase phase-field methods have been developed to simulate a variety of processes in which interface dynamics play a critical role. The mathematical formulation of a phase field approach to the dynamics of liquid solid interfaces that evolve due to precipitation and/or dissolution will be presented. For the purpose of illustration, and comparison with other methods, phase field simulations have been carried out assuming first order reaction dissolution/precipitation) kinetics. In contrast to solidification processes controlled by a temperature field that is continuous at the solid/liquid interface, with a discontinuous temperature gradient, precipitation/dissolution is controlled by a solute concentration field that is discontinuous at the solid/liquid interface. The sharp-interface asymptotic analysis of the phase-field equations for solidification by Karma and Rappel [Phys. Rev. \textbf{E57} (1998) 4342] have been extended to demonstrate that the phase-field equations converge to the proper sharp-interface limit for the precipitation/dissolution problem. The mathematical model has been validated for a one-dimensional precipitation/dissolution problem by comparison with the analytical solutions of the free-boundary problem. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A20.00002: Island Size Scaling and Evolution during Strained Film Epitaxy Zhi-Feng Huang, Ken Elder Strained film growth often gives rise to the self assembly of nanostructures such as quantum dots or islands. While there has been and continues to be much interest in such behavior the fundamental mechanisms that determine the precise morphologies remain unclear. In this work the phase field crystal model, which incorporates the atomic length and diffusive time scales, and the corresponding amplitude equations are used to examine this phenomenon. Direct numerical simulations of the model and a linear stability analysis of the amplitude equations are presented. The results predict that the average island size is inversely proportional to the strain. This result is consistent with recent experiments on SiGe, but inconsistent with the predictions of classic continuum elasticity theory (or the Asaro-Tiller-Grinfeld instability). Basic mechanisms identified in our study, which are associated with the crystalline nature but absent in all continuum approaches, are discussed. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A20.00003: Optimal Capping Layer Thickness for Stacked Quantum Dots Xiaobin Niu, Christian Ratsch, Young-Ju Lee, Russel Caflisch We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A20.00004: Modeling the complex evolution of self-assembled quantum dots Invited Speaker: In heteroepitaxy, misfit strain often leads to spontaneous formation of islands. Such islands have attracted great interest as ``self-assembled quantum dots". The growth of these quantum dots is remarkably rich, exhibiting alloy intermixing, island coarsening, trench formation, and even spontaneous {\it lateral} motion of islands. Islands also interact with topographic features on the substrate, providing a means for controlling the position of quantum dots. The diverse experimental observations provide an ideal opportunity to test and extend our theoretical understanding of growth at the nanoscale. We find that much of the complexity arises because there is a strong thermodynamic driving force for intermixing (to increase entropy and reduce strain energy) as well as for morphological evolution (to reduce strain energy); but this evolution must occur under the kinetic constraint of diffusion occurring only at the surface. Simulations of such constrained evolution directly reproduce many of the observed phenomena [Y. Tu and J. Tersoff, Phys. Phys. Lett. 98, 096103 (2007)]. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A20.00005: Effect of Elastic Inhomogeneity and Anisotropy on the Order of Epitaxial Self-Assembled Quantum Dots Chandan Kumar, Lawrence Friedman Growth of epitaxial self-assembled semiconductor quantum dots (SAQDs) is of particular interest in the development of quantum dot based devices such as quantum computing architectures, laser diodes, and other optoelectronic devices. The ordering of these SAQDs is critical for the development of these devices. Understanding what factors the order of these SAQDs depend on, is important for guiding both experiments and simulations. Most theoretical and numerical models approximate the film substrate system as a semi-infinite solid. Although models based on such an approximation have been able to predict some general behaviour in confirmation with the experimental results, predictions about a quantitative measure would be less approximate if the models could incorporate elastic inhomogeneity. The presented linear stochastic model for SAQD growth incorporates elastic inhomogeneity and anisotropy along with stochastic surface diffusion to produce a more refined quantitative model for SAQD order estimation. For the Ge/Si film-substrate system it is found that at the critical film height such an approximation could lead to an error of $\sim12$\% in the estimation of average spacing between SAQDs and an error of $\sim24$\% in the estimation of number of correlated dots for small height fluctuations. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A20.00006: Composition Maps in Strained Alloy Quantum Dots Nikhil Medhekar, Vishwanath Hegadekatte, Vivek Shenoy Knowledge of composition profiles within self-assembled SiGe and InGaAs quantum dots is critical for applications in optoelectronic and memory devices as variations in composition at the nanoscale can substantially influence their electronic properties. Obtaining the quantitative description of composition profiles in the quantum dot is a challenging task due to the coupling between composition variations, shape of the quantum dots and long-range elastic interactions. In this talk, we present an efficient scheme that combines the finite element analysis with an optimization scheme based on a quadratic programming method to determine equilibrium profiles in strained quantum dots. Composition profiles are found to strongly depend on the shape of the quantum dots, as strain relaxation in dots with steeper sidewalls allows for segregation of the larger alloy component in the regions near the apex. Based on these observations, we have developed a phase diagram that shows the degree of segregation of the alloy components in the phase space spanned by the temperature (which governs chemical mixing) and the shape of the dot. Further, we find that the segregation of the alloy components can substantially reduce the critical dot size for the transition between the shapes with different facets. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A20.00007: An Accelerated Molecular Dynamics Study of the GaAs (001) $\beta $2(2x4) Reconstruction Maria Mignogna, Kristen Fichthorn The GaAs (001) $\beta $2(2x4) reconstruction is the most commonly used substrate for growth in GaAs homoepitaxy by molecular beam epitaxy. While the atomic positions of the $\beta $2(2x4) unit cell have been determined, reflection high energy electron diffraction and scanning tunneling microscopy images show long range disorder on this surface[1]. It is hypothesized that domains of anti-phase $\beta $2(2x4) unit cells can be created by vacancies or As dimer shifts. Accelerated molecular dynamics (MD) allows us to examine atomic scale processes that can lead to this disorder. We have developed an adaptive accelerated MD scheme based on the bond boost method of Miron and Fichthorn[2]. The adaptive method is suitable for the rough energy landscape presented by GaAs (001). In the adaptive method, both the length thresholds for determining transition states and the magnitude of the boost are calculated on the fly. We are able to extend the physical timescale of the simulation by several orders of magnitude. We see events that lead to small domains of As dimers shifting. By simulating RHEED images of the surface, we link the disorder to experiment. [1] D.W. Pashley, J.H. Neave, B.A. Joyce, Surf. Sci., \textbf{582}, 189 (2005) [2] R.A Miron, K.A. Fichthorn, J. Chem. Phys., \textbf{119}, 6210 (2003) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A20.00008: Strain and Piezoelectric Effects on the Electronic Structure of Coupled In$_{x}$Ga$_{1-x}$As/GaAs Self-Assembled Quantum Dots Usman Muhammad, Shaikh Ahmed, Gerhard Klimeck In$_{x}$Ga$_{1-x}$As/GaAs coupled quantum dot systems have gained much attention for optical and quantum computing applications. Due to strain, originating from the assembly of lattice- mismatched semiconductors, the quantum dot arrays tend to grow in the vertical direction. These vertically stacked quantum dots are strongly coupled through the strain field, which is atomistically inhomogeneous and penetrates deep into the GaAs buffer layer surrounding the dots. Crystal symmetry and atomistic details of interfaces are extremely important in such systems. Also piezoelectric fields must be taken into account to properly model the experimentally observed symmetry breaking and the introduction of a global shift in the energy spectra of the system. In this work, we present a detailed description of strain and piezoelectric potential effects on the electronic structure of closely coupled identical and non-identical quantum dot systems using sp$^{3}$d$^{5}$s* nearest neighbor empirical tight binding model. We show that strain causes strong mixing of s- and p- electron energy levels in strongly coupled quantum dot, splits heavy hole and light hole bands and even reverses their order within dots. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A20.00009: Structure competition in growth of In island on Si{111} from first-principles calculations Cai-Zhuang Wang, Min Ji, J. Chen, M. Hupalo, M.C. Tringides, K.M. Ho We have carried out first principles calculations to understand the growth of indium island on Si{111} substrate which have been observed to have an interesting FCC and BCT structure competition. Our calculations show that quantum size effect (QSE) plays an important role in different island structure formation. Furthermore, the interface energy between In and Si substrate also controls the relative stabilily of different island structures. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A20.00010: Thermodynamic potentials in closed and open nanocrystalline systems: Si-Ge islands on Si(001). Marina S. Leite, Angelo Malachias, Stefan W. Kycia, Ted I. Kamins, R. Stanley Williams, Gilberto Medeiros-Ribeiro The driving forces for alloying in Si-Ge epitaxial nanocrystalline islands were quantified experimentally. Closed and open systems were emulated by controlling surface diffusion kinetics [1]. Grazing Incidence X-Ray Diffraction (GIXRD) experiments were performed to map the composition and the strain distribution within the Si-Ge:Si(001) islands, permitting the evaluation of the relevant thermodynamic potentials for alloying. For the closed system the elastic strain energy increased, which was more than compensated by the increase in the local mixing entropy [2]. In contrast, for the open system, the elastic energy decreased and the mixing entropy increased, driven by the intermixing originated from the inflow of Si from the reservoir. For both systems, the evolution of the composition leads to a lowering of the Gibbs free energy. The results were in full agreement with a theoretical prediction of the optimum concentration for epitaxial islands. [1] M. S. Leite \textit{et al}, Phys. Rev. Lett. \textbf{98}, 165901 (2007). [2] G. Medeiros-Ribeiro \textit{et al}, Nano Lett. \textbf{7}, 223 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A20.00011: Characterization of MBE grown PbSe Quantum Dots Nathaniel Becker, Dustin Klein, Tim Kidd Lead selenide (PbSe) has been shown to be an excellent candidate for solar cell research due to its ability to allow the possibility for multiple electronic carrier production by absorption of a single photon. These quantum dots (QDs) were created using molecular beam epitaxy (MBE) to evaporate PbSe onto clean and modified silicon and germanium substrates. Control of lattice strain was achieved by the deposition of buffer layers onto clean Si(111)in ultra-high vacuum. The MBE technique allows for structural control at the atomic level. We have investigated the samples using Auger spectroscopy, scanning probe and scanning electron microscopy to determine their suitability for solar cell applications. Specifically, we investigated structural properties such as size, distribution, and uniformity to correlate such features with the electronic properties of the sample. Our initial results indicate the structural properties can be controlled with careful tuning of the substrate surface properties. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A20.00012: Self Assembling Quantum Dot Aggregates in Liquid Crystal Matrices Christopher Ferri, M. Gallardo, Y. Verma, D. Kelley, S. Ghosh A system of colloidal quantum dots (QDs) embedded in a matrix of highly directional and ordered liquid crystal (LC) molecules at room temperature offers the novel potential of promoting controllable aggregation of the QDs. Photoluminescence (PL) of GaSe QDs embedded in a LC matrix, studied using a confocal-microscopy setup, shows considerable red-shift in the emission spectrum of the QD-LC composite. While bare QDs in solution emit at 485 nm, mixing with LC molecules results in an emission centered at 500 nm, system suggesting their aggregation into longer structures in the matrix. A high resolution two-dimensional spatial map of the PL on the sample provided evidence of the organization of QDs into these ordered domains. Application of in-plane electric fields further enhances the aggregation effect of the QDs and emission spectrum is red-shifted to around 525 nm. Furthermore, as the aligning electric field increases the degree of ordering of the liquid crystal molecules, the polarization (P) of the emission of the aggregated QDs rotates in step with that of the LCs' directionality. Unlike the disc-shaped GaSe QDs, investigations on LC and CdSe QD system failed to show such dramatic aggregation effects. [Preview Abstract] |
Session A21: Focus Session: Reversibly Associating Polymers: Theory and Experiments
Sponsoring Units: DPOLY DBPChair: Ken Shull, Northwestern University
Room: Morial Convention Center 213
Monday, March 10, 2008 8:00AM - 8:12AM |
A21.00001: Self Assembly of Mixed-Valence Ionic Amphiphiles into Faceted Vesicles Megan Greenfield, Graziano Vernizzi, Liam Palmer, Samuel Stupp, Monica Olvera de la Cruz We show that anionic and cationic amphiphiles of unequal charge can co-assemble into small faceted vesicles and we propose a theoretical model to explain the faceting behavior. The strong electrostatic interaction between the +3 and -1 head groups increases the Columbic cohesion energy of the amphiphiles and should favor the formation of a two-dimensional, flat ionic surface. The vesicle surface can form edges by breaking the ionic lattice, which can be visualized as faceted shapes. Our results demonstrate that a large charge imbalance between the cationic and anionic head groups of amphiphiles enables their coassembly into facetted vesicles. We anticipate this work to be a starting point for rationally designing new self-assembled supramolecular structures. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A21.00002: Electrospining Solutions of Associating Polymers -- the Case of Stereocomplex PMMA Matija Crne, Jung Park, Mohan Srinivasarao A mixture of isotactic and syndiotactic PMMA polymers (also called a ``stereocomplex PMMA'') forms a supramolecular helical structure, which is held together by non-covalent bonding. This association is thermoreversible and solvent-sensitive. If the concentration of stereocomplex PMMA is high enough, the solutions form thermoreversible physical gels. We have examined the influence this associating behavior has on the process of electrospinning. In our work, we have used solutions of stereocomplex PMMA to study the effect of physical gelation interaction on the fiber morphology and compared it to the solutions of atactic PMMA of similar molecular weight. We have found that the stereocomplex PMMA solutions do not follow the same empirical rules that are applicable for linear polymers in solution. Instead, the concentration necessary for the production of smooth, continous fibers is much lower. We ascribe this extraordinary behavior to the ability of these polymers to associate and form a network during the electrospinning process. The elasticity of the fluid jet thus rises and results in a more stable jet. The resulting fibers are interesting, as they have greater temperature stability than regular atactic PMMA. Therefore they have a bigger processing window for making composites with better mechanical properties. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A21.00003: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 8:36AM - 9:12AM |
A21.00004: Unique Properties of Reversibly Associating Polymer Networks Invited Speaker: Reversibly associating functional groups offer the polymer physicist with a new tool to develop stimuli-responsive polymers. Our focus has been to attach reversibly associating groups onto rubbery network polymers. Free radical copolymerization was used to synthesize a series of crosslinked poly(n-butylacrylate)s containing quadruple H-bonding ureidopyrimidinone (UPy) side-groups. Resulting elastomeric networks contain both covalent and dynamic non-covalent crosslinks, and this unique architecture is shown to affect viscoelastic behavior and mass-transport properties. Shape-memory effects are studied quantitatively using thermomechanical techniques. Experiments show how reversible interactions, such as hydrogen bonding, are capable of stabilizing mechanically strained states. Unlike conventional shape-memory polymers, these dynamic networks lack a well-defined shape recovery temperature. Instead, their shape recovery rate depends on temperature. To further study the dynamics and temperature dependence of mechanical relaxation, isothermal creep experiments and dynamic mechanical analysis were performed. Creep data, acquired at several different temperatures, are fit to a simple viscoelastic model. Fit viscosities exhibit Arrhenius-like temperature dependence with activation energies of $\sim $90 kJ/mol, which is in rough agreement with H-bond dissociation barriers. Molecular transport through dynamic networks is studied using gravimetric sorption and dye-diffusion techniques. Diffusion depends on temperature, network architecture, solute size, and the interaction between the solute and the network. Membranes with high temperature-sensitive diffusion properties may be useful in applications such as transdermal drug delivery, microfluidics, or liquid chemical separation processes. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A21.00005: Computer Simulations of Semi-flexible Polymer Chains Venkat Padmanabhan, Sanat K. Kumar, Arun Yethiraj Monte Carlo Simulations are performed to obtain the isotropic-nematic (IN) transition in systems with semi-flexible polymer chains of different lengths. The chains are modeled as spherical beads that interact via a square-well potential. Bonded beads are connected by strings chosen so that bond length varies between 1.01$\sigma $ and 1.05$\sigma $ (where $\sigma $ is the hard sphere diameter). The stiffness of the molecules is controlled via a potential between beads separated by two bonds; this potential restricts the distance between these beads to be between 2.02$\sigma $ and 2.1$\sigma $. The vapor-liquid coexistence and isotropic-nematic (IN) coexistence curves are obtained using computer simulations. An IN transition is found for N$_{b} \quad \ge $ 10. The density, at which the IN transition occurs, moves to higher values as N$_{b}$ is increased and then drops on further increase. This is analogous to the initial increase in the critical density for pure alkanes as the chain length is increased. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A21.00006: Reversible Networks by Hydrogen Bonding of ABA Triblock Copolymers in an Ionic Liquid Timothy Lodge, Atsushi Noro, Yushu Matsushita Ion gels, comprising a polymeric network solvated by an ionic liquid, are of great interest as, e.g., gate dielectrics in plastic electronics, polymer electrolytes with high ionic conductivity, actuators and artificial muscles, gas separation media, and sensors. We have explored the thermoreversible gelation of a model system containing poly(2-vinyl pyridine-b-ethyl acrylate-b-2-vinyl pyridine) ) (VEAV) triblocks dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) [emi][tfsi], where the EA blocks are under good solvent conditions. Addition of poly(vinyl phenol) (PVPh) creates hydrogen bonds with the V end blocks, leading to gelation. As each end block and each PVPh ``crosslinker'' has about 50 segments, up to 50 hydrogen bonds may be formed per block. Consequently, over the temperature range 30 -- 160 C the longest relaxation time of the gel, related to the ``sticker'' lifetime, varies by about 12 orders of magnitude. The thermal stability of the ionic liquid thus provides an unprecedented opportunity to study the dynamic properties of reversible networks over a wide range of timescale. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A21.00007: Examination of the Structure of Molten Hydrogen-Bonded Supramolecular Diblock Copolymers Kathleen Feldman, Matthew Kade, Craig Hawker, Edward Kramer A new strategy for synthesizing low polydispersity polymers containing multiple hydrogen bonding (MHB) groups at one chain end capable of heterodimerization in both solution and the melt has been developed. Two well-known MHB systems were chosen for initial studies---2-ureido-4[1H]-pyrimidinone (UPy) and 1,8-diamidonaphthyridine (Napy), and ATRP initiators containing either UPy or Napy were synthesized. These initiators were shown to produce well-defined (meth)acrylic polymers with the desired MHB functionality present at the chain end. To characterize the effectiveness of the MHB interaction in the melt, blends were cast into films, annealed at various temperatures above T$_{g}$, then quenched and their structure analyzed by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was shown that the nature of the hydrogen bonding group(s) present in the blend has a significant effect on structure and thermal behavior, and in particular blends of UPy- and Napy-functional chains contain a significant fraction of ``supramolecular diblock copolymers.'' [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A21.00008: Thermoreversible bond formation in end-linking, difunctional polymer blends Richard Elliott, Glenn Fredrickson We investigate theoretically thermoreversible bond formation and phase transitions in a system composed of linear, end-linking, difunctional polymers. In this melt, two distinct species of equal-length links assemble into chains of varying sequences and polymerization depending on the affinities of binding between the blocks. Several limits, such as that of purely heterogeneous bonding which forms chains of alternating block sequences, are explored. Interactions between dissimilar segments are described with the Flory-Huggins contact potential and a mean-field approach is employed to investigate the effects of prevalent bonding and the onset of meso-scale ordering. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A21.00009: Inter- and Intra-Molecular Interactions of Associative Polymers in Solution M. Wei, R. David, Julie Kornfield Model polymers with matched backbone length are used to examine the effects of degree of functionalization and type of interaction (self-associating or donor-acceptor) on shear and extensional rheology of associative polymer solutions. Series of polymers were prepared by functionalization of 500 and 1300 kg/mol polybutadiene chains with carboxylic acid side groups (A stickers, self-associating) and tertiary amine side groups (N stickers, forming strong hydrogen donor-acceptor interactions with A stickers). We found that stickers drive phase separation at extents of functionalization as low at 0.5 mol{\%}. Intramolecular associations dominated the behavior of A-functionalized chains even at semi-dilute concentrations, leading to chain collapse and reduced shear and extentional viscosities. Finally, we found that intermolecular interactions were much more favorable for dilute mixtures of A-functionalized and N-functionalized chains (as evidenced by increased zero-shear viscosity and by the formation of large aggregates), but that associations still reduced solution elasticity and extentional viscosity in elongational flow. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A21.00010: Molecular Dynamics Simulation of Polyelectrolyte Brushes: From Hemispherical Micelles to Maze-like Aggregates. Jan-Michael Carrillo, Andrey Dobrynin We present results of the molecular dynamics simulations of the effects of solvent quality, strength of the electrostatic interactions, chain degree of polymerization and grafting density on the conformations of planar polyelectrolyte brushes in salt-free solutions. The polyelectrolyte brush could form: (1) hemispherical micelle aggregates, (2) vertically oriented cylindrical micelles, (3) maze-like aggregate structures, or (4) thin polymeric layer uniformly covering the substrate. These different brush structures appear as a result of the fine interplay between electrostatic and monomer-monomer attractive interactions. The brush thickness depends nonmonotonically on the value of the Bjerrum length. This nonmonotonic dependence is due to counterion condensing inside the brush. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A21.00011: Effect of Hydrogen-Bonding Junctions on Microphase Separation in Block Copolymers Greg Stone, Jim Hedrick, Fredrik Nederberg, Nitash Balsara The morphology of poly(styrene-block- trimethylene carbonate) (PS-PTMC) copolymers with and without thiourea groups at the junction between the blocks was studied by a combination of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The thiourea groups are known to exhibit inter-molecular hydrogen bonding. We demonstrate that the presence of thiourea groups results in increased segregation between PS and PTMC blocks. We focus on symmetric systems with total molecular weights in the 5 kg/mol range. In conventional block copolymers without hydrogen bonding groups it is difficult to obtain strong segregation in low molecular weight systems because the product chi*N controls segregation (chi is the Flory-Huggins interaction parameter and N is the number of monomers per chain). The incorporation of hydrogen bonding groups may provide a route for the generation of patterns with small, sharply defined features using block copolymers. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A21.00012: Relating Chain Structure to Physical Properties of Branched Polymers Ramnath Ramachandran, Gregory Beaucage, Amit S. Kulkarni, Vassilios Galiatsatos, Douglas C. McFaddin We investigated linear and branched polyethylene (PE) using small-angle neutron scattering (SANS). The experiments were conducted on dilute solutions of PE in deuterated p-xylene. A variety of structural information$^{\dag }$ such as fractal dimension ($d_{f})$, connectivity dimension ($c)$, minimum path dimension ($d_{min})$, long chain branch fraction ($\phi _{br})$, radius of gyration ($R_{g})$ and persistence length ($l_{p})$ were obtained. Such information presents a qualitative and quantitative assessment of branching in polymers. Theoretical models such as `binary contacts per pervaded volume' model$^{\ast }$ were employed to correlate the structural information of the polymer to its entanglement molecular weight ($M_{e})$. $M_{e}$ was used to predict physical properties such as plateau modulus ($G_N^0 )$ and zero-shear viscosity (\textit{$\eta $}$_{0})$. We relate physical properties of branched polymers to their structural properties.$^{ \dag }$Beaucage G. \textit{Physical Review E} \textbf{70},031401 (2004) $^{\ast }$Colby \textit{et al}.\textit{ Macromolecules }\textbf{25,} p.996 (1992) [Preview Abstract] |
Session A22: Focus Session: Hybrid Organic-Inorganic Nanomaterials I: Patterning and Self Assembly
Sponsoring Units: DPOLYChair: R Kannan, Wayne State
Room: Morial Convention Center 214
Monday, March 10, 2008 8:00AM - 8:12AM |
A22.00001: Connecting quantum dots and bionanoparticles in hybrid nanoscale ultra-thin films Ravisubhash Tangirala, Yunxia Hu, Qingling Zhang, Jinbo He, Thomas Russell, Todd Emrick Aldehyde-functionalized CdSe quantum dots and nanorods, and horse spleen ferritin bionanoparticles, were co-assembled at an oil-water interface. Reaction of the aldehydes with the surface-available amines on the ferritin particles enabled cross-linking at the interface, converting the assembled nanoparticles into robust ultra-thin films. The cross-linked capsules and sheets thus made by aldehyde-amine conjugation could be disrupted by addition of acid. Reductive amination chemistry could be performed to convert these degradable capsules and sheets into structures with irreversible cross-linking. Fluorescence confocal microscopy, scanning force microscopy and pendant drop tensiometry were used to characterize these hybrid nanoparticle-based materials, and transmission electron microscopy (TEM) confirmed the presence of both the synthetic and naturally derived nanoparticles. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A22.00002: Janus Au Nanoparticle Patterning via Polymer Single Crystals Christopher Li, Bing Li Gold nanoparticles (AuNPs) possess remarkable electrical and optical properties coupled with good chemical stability. Thus, they have wide potential applications in fabrication of advanced optical and electronic devices. In order to efficiently transfer their outstanding properties from nano- to micro-scale, patterned AuNPs are desired. Here, we report a novel method using polymer single crystals as substrates to pattern AuNPs. By using thiol-terminated polyethylene oxide (HS-PEO) as the substrate, we were able to assemble AuNPs into different patterns on the PEO single crystals. Furthermore, these AuNPs are asymmetrically functionalized (Janus NPs). A variety of shapes including chains and sheets were formed, and the size and areal density of the AuNP assemblies can be controlled through the AuNP adding time and the ratio between AuNPs and polymer single crystals, respectively. Because of the asymmetrical nature of the AuNPs, the resultant chains and sheets are both asymmetric (Janus chains and Janus sheets), which could lead to a variety of interesting optic properties. We anticipate that our approach would provide a facile means to pattern AuNPs, and it could be extended to other metal and semiconductor NPs. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A22.00003: Organic-Inorganic Nanocomposites via Directly Grafting Conjugated Polymers onto Quantum Dots Zhiqun Lin, Jun Xu, Jun Wang, Mike Mitchell, Prasun Mukherjee, Malika Jeffries-El, Jacob W. Petrich Nanocomposites of poly(3-hexylthiophene)-cadmium selenide (P3HT-CdSe) were synthesized by \textit{directly} grafting vinyl terminated P3HT onto [(4-bromophenyl)methyl]dioctylphosphine oxide (DOPO-Br)-functionalized CdSe quantum dot (QD) surfaces via a mild palladium-catalyzed Heck coupling, \textit{thereby dispensing with the need for ligand exchange chemistry}. \textit{The resulting P3HT}-\textit{CdSe nanocomposites possess a well-defined interface, thus significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between these two components}. The photophysical properties of nanocomposites were found to differ from the conventional composites in which P3HT and CdSe QDs were physically mixed. Solid-state emission spectra of nanocomposites suggested the charge transfer from P3HT to CdSe QDs, while the energy transfer from 3.5-nm CdSe QD to P3HT was implicated in the P3HT/CdSe composites. A faster decay in lifetime further confirmed the occurrence of charge transfer in P3HT-CdSe nanocomposites. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A22.00004: Amplification of Self-Assembled Nanopatterns: Bilayer Approach to High Aspect-Ratio Cylindrical Nanopore Arrays and Their Use for Templating Functional Materials Ho-Cheol Kim, Oun-Ho Park, Joy Cheng, Mark Hart, Robert Miller, Hiroshi Ito Thin films containing well-defined nanoscopic cylindrical pores oriented perpendicular to the surface are highly desirable for a variety of applications. Of great interest is to use the porous structures for templating a variety of functional materials into 3D nanostructures. Previous reports employ the porous oxide of anodized aluminum or thin films of block copolymers for generating high aspect-ratio nanoporous films. It is, however, still desirable to develop a generalized, simple and reliable method to fabricate nanoporous templates with controlled pore dimensions, high aspect-ratio, substrate non-selectivity and uniformity over large areas. We report a relatively simple and reproducible method for generating nanoporous templates. The approach we used in this study provides nanoporous films with controlled pore diameters (8nm - 25nm in this study) and high aspect-ratio over large areas of a variety of substrates. We also report the application of the templates for generation of nanostructured copper sulfide and titania and their functional properties. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A22.00005: Nanoparticle Arrays via Self Assembled Peptide Templates Nikhil Sharma, Matthew Lamm, Darrin Pochan The bottom up approach towards nano-scale patterning presents the possibility of creating hierarchical architectures through simple self-assembly strategies. Herein, we demonstrate the use of a peptidic template for the construction of parallel, linear arrays of inorganic nanoparticles. A 20 amino acid peptide, consisting of alternating hydrophilic (lysine) and hydrophobic (valine) residues flanking a central diproline turn sequence (VKVKVKVKVPPTKVKVKVKV-NH$_{2})$ was employed as a nano-scale template for the organization of 2nm gold particles. This peptide self assembles into a laminated fibrillar morphology in solution and has a periodic nanostructure consisting of alternating hydrophobic and hydrophilic layers with a lateral periodicity of 2.5 nm. Negatively charged gold nanoparticles are templated into the positively charged lysine layer through electrostatic interaction and are aligned within the template that itself swells to a periodic spacing of 4.0 nm in order to accommodate the particles. These 1D nanoparticle arrays have potential applications in fields like nano-electronics, and we are currently attempting to create arrays of quantum dots and hetero-structures of metal and semiconductor particles. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A22.00006: Study of Alkanethiol Self-Assembled Structure Grown on Silver Liang Hu, Zishu Zhang, Mikhail Yu. Efremov, Eric A. Olson, Ming Zhang, Lito de la Rama, Leslie H. Allen Alkanethiol self-assembled structures grown on surface-supported Ag clusters (3D) are studied by measuring heat capacity with nanocalorimeter at high scanning rate (40,000 K/s), and compared with the self-assembled monolayer (SAMs) grown on planar Ag surface (2D). There is more amount of alkanethiol assembled on 3D Ag clusters compared with 2D SAMs, and the melting transition occurs at a higher temperature with sharper melting peak. This is due to the formation of layered Ag-alkanethiolate structure demonstrated by X-ray diffraction. IR spectrum indicates this self-assembled structure has a high degree of conformational order. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:48AM |
A22.00007: Ordered Inorganic/Organic Composites via Novel Templates and Techniques Invited Speaker: The fabrication of well ordered nanocomposite materials offers new opportunities for device applications. Recently we reported that nanostructured templates with sub-10 nm domains can be easily obtained through the blending of disordered polymer surfactants containing poly(ethylene oxide) as the hydrophilic block with homopolymers including poly (acrylic acid), poly (4-vinyl phenol) and poly (styrene sulfonate) that selectively associate with the PEO block through hydrogen bonding. These inexpensive blends are strongly segregated, yielding well ordered domains. Moreover, the functionalities imparted by the homopolymers provide convenient handles for binding active materials such as nanoparticles and for promoting in situ, phase selective reactions to produce hierarchical metal oxide polymer composites. The behavior of the template systems and their use for the fabrication of well ordered polymer/nanoparticle, metal/oxide polymer, and metal oxide/polymer/nanoparticle composites using aqueous routes or via the 3-D replication of the template structure in supercritical fluids will be discussed. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A22.00008: Two-Dimensional Confinement of Nanorods in Block Copolymer Domains Yu Liu, Ranjan Deshmukh, Russell Composto To control their unique electrical and optical properties, one-dimensional metallic/semiconductor nanoparticles need to be aligned and assembled within a host material. In our research, we investigated the assembly of gold nanorods (NRs) in films of a symmetric diblock copolymer, poly(styrene-b-methyl methacrylate), PS-b-PMMA (211kg/mol). The NR length and diameter was 42nm and 13nm, respectively. The NRs were grafted with a short PEG-brush (5kg/mol). During solvent annealing, the NRs become oriented as the PS-b-PMMA chains assemble into a parallel lamellar morphology. The NRs are selectively sequestered and confined in the PMMA domains, which are narrower than the NR length, due to the favorable interaction between the PEG brush and PMMA block. This confinement orients 71{\%} of NRs within $\pm$ 5$^{\circ}$ of the lamella plane. This route to produce alternating layers containing conducting NRs separated by dielectric domains has the potential for fabricating self-assembled nanodevices. The thermal stability of the NRs in PS-b-PMMA and homo PMMA films will also be discussed. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A22.00009: Surfactant Directed Assembly of ZnS Nanocrystals Youli Li, Cyrus Safinya, Jacob Israelachvili, Nataly Belman, Yuval Golan Post-synthesis assembly of nanocrystals into ordered two- and three-dimensional super-structures is an important step in many technological applications. A method was developed to use chain-crystalline surfactants to link nanocrystals into 2D and 3D assemblies with precise control of inter-particle distances as well as crystallographic orientation of the nanocrystals. X-ray diffraction and transmission electron microscopy data revealed that ZnS nanocrystals synthesized in octadecylamine (ODA) self-assemble into a highly ordered structure composed of stacked sheets with in-plane 2D-crystalline order. The unit cell of the nanocrystal superlattice (including type and dimensions) is directly related to the crystal structure of the pure surfactant, and thus can be tuned predictably by varying the surfactant chain length. This method can be used to assemble a broad range of nanocrystals and particles (Work supported by ONR N00014-05-1-0540, DOE DE-FG02-06ER46314, NSF DMR-0619171 and BSF 2002059). [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A22.00010: End-functionalized triblock copolymers as a robust template for assembly of nanoparticles Rastko Sknepnek, Joshua Anderson, Monica Lamm, Joerg Schmalian, Alex Travesset Polymers with attached end groups with specific affinity for nanoparticles, i.e., functionalized polymers, are shown to provide a robust templating strategy, where the aggregated nanoparticles follow the mesoscopic order imposed by the polymeric matrix. In this talk, we will present a detailed investigation of triblock polymer templating as a function of both nanoparticle size and concentration . Next to phases such as hexagons or gyroids, regularly observed in copolymer solutions and melts, we find a novel square columnar phase of two interpenetrating line-lattices of micellar cylinders and aligned nanoparticles. We argue that this phase is a realization of the packing problem of binary mixtures of disks. Our study suggests that nanoparticle templating with functionalized block copolymers can provide a simple and efficient tool for assembling novel materials with nanometer scale resolution. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A22.00011: Nanoparticle Ordering in Diblock Copolymer-based Supramolecular Systems Thomas Schilling, Ting Xu, Shih-Huang Tung, Yue Wu A well-controlled nanoparticle dispersion within a polymer matrix can have a significant impact on a wide range of material properties. A simple, efficient strategy to direct nanoparticle assembly within nanoscopic domains has been developed. Cadmium selenide (CdSe) nanoparticles were directly blended with diblock copolymer-based supramolecules wherein small molecules were attached to the side chain of one block via hydrogen-bonding. Co-assembly of nanoparticle and supramolecule were examined using SAXS and TEM, confirming fine ordering of the nanoparticles. The influence of nanoparticle concentration on supramolecule microphase separation and nanoparticle ordering was studied. Various functionalities should be readily incorporable into these systems by simply varying the nanoparticle type. The hierarchical assembly of supramolecule and nanoparticles presents many new opportunities for the fabrication of functional nanodevices. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A22.00012: The metal insulator transition in self-assembled gold nanoparticle wires M. E. Reeves, Jianwei Sun, J. A. Hoffmann, Jasper Nijdam, Guebre Tessema We report the temperature dependence of wires assembled from spherical nanoparticles by a relatively new technique, vertical colloidal deposition. This is a simple, one-step method for assembling spherical nanoparticles into wires without the need for lithographic templating. It is effective for a variety of conducting and nonconducting nanoparticles and substrates, and the only material requirement is that the nanoparticles be placed in a colloidal suspension that is wettable on the desired substrate. The shape of the meniscus defines the wire's geometry, and we report the synthesis and physical properties of wires several millimeters long by a few micrometers wide. When wires are formed from 12 nm gold nanoparticles, they exhibit a weak metallic temperature dependence. Those assembled from 6 nm nanoparticles show activated behavior. Post-processing also effects the conductivity of the nanoparticle wires, such as when they are intercalated with proteins or other short organic molecules. Evidence for the metal insulator transition in these materials will be presented and discussed. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A22.00013: Effects of Electric-Magnetic Fields on Hybrid Excitons in a Semiconductor Quantum Dot Coated by an Organic Shell Que Huong Nguyen The effect of electric and magnetic fields on the hybrid exciton of an organic-coated semiconductor spherical quantum dot is considered. Due to dipole-dipole interaction at the semiconductor dot- organic shell interface, a strong mixing of the Wannier-Mott exciton in the quantum dot and the Frenkel exciton of the organic medium occurs, resulting in a new hybrid exciton having priorities of both kinds of excitons as large exciton radius and large oscillator strength. The hybrid excitons are as sensitive to external perturbation as Wannier-Mott excitons. We investigate the effect of mutual presence of electric and magnetic fields on the hybrid exciton of the isolated semiconductor quantum dot such as CdSe (core) clothed by an organic or glass coating for two configuration of fields when the electric and magnetic fields are parallel and orthogonal. The fields affect the eigenfunctions (by a Stark -effect distortion and by the magnetic field-induced distortion) and the Frenkel-Wannier coupling term. Upon the application of the magnetic and electric fields the coupling term between the two kinds of excitons increases. The most important feature of this system is by adjusting the magnetic field and electric field, one can tune the resonance between the two kinds of excitons to get different regions of mixing to obtain the expected high non-linearity. [Preview Abstract] |
Session A23: Focus Session: Manganites I
Sponsoring Units: DMP GMAGChair: J. J. Neumeier, Montana State University
Room: Morial Convention Center 215
Monday, March 10, 2008 8:00AM - 8:36AM |
A23.00001: Sliding charge density wave in manganites Invited Speaker: Stripe and checkerboard phases appear in many metal oxide compounds, and are thought to be linked to exotic behaviour such as high temperature superconductivity and colossal magnetoresistance. It is therefore extremely important to understand the fundamental nature of such phases. The so-called stripe phase of the manganites has long been interpreted as the localisation of charge at atomic sites. Here, we present resistance measurements on La$_{0.50}$Ca$_{0.50}$MnO$_3$ which strongly suggest that this state is in fact a prototypical charge density wave (CDW) which undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. The CDW-type behaviour of the manganite superstructure suggests that unusual transport and structural properties do not require exotic physics, but could emerge when a well-understood phase (the CDW) coexists with disorder. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A23.00002: Polaron dynamics in La$_{0.7}$Ba$_{0.3}$MnO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Y. Chen, B. G. Ueland, J. W. Lynn, S. Barilo, Y. Mukovski, R. Privezentsev In the pervoskite manganites La$_{1-x}$A$_{x}$MnO$_{3}$ (Ca, Sr and Ba), the spin, lattice, charge and orbital degrees of freedom are intimately coupled , leading to a rich phase diagram. La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) is a metallic ferromagnet at temperatures below T$_{c} \quad \sim $ 257 K and colossal magnetoresistivity is observed and associated with the formation of nanoscale polarons that develop at elevated temperatures with an ordering wave vector of $\sim $ (1/4,1/4,0). As the paramagnetic-insulating state is entered, a purely elastic component to the structural polaron scattering signals the development of the correlated polaron glass phase in LCMO. We investigated the structure and dynamics of these polarons through and above the ferromagnetic-metallic to paramagnetic-insulating transition by neutron scattering on single crystals of La$_{0.7}$Ba$_{0.3}$MnO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$. In contrast to LCMO, no static polaron correlations are observed, however, we find that dynamic polaron correlations exist. We have measured the energy and temperature dependence of the dynamic polaron correlations. The formation of the dynamic polarons is not directly related to T$_{c}$. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A23.00003: Composition Dependence of Polaron Formation and Dynamics in La$_{1-x}$Ca$_{x}$MnO$_{3}$ B.G. Ueland, Y. Chen, J.W. Lynn, Y.M. Mukovskii, R. Privezentsev The colossal magnetoresistive perovskites La$_{1-x}$Ca$_{x}$MnO$_{3}$ have generated much interest due to the intimate correlation between their magnetic and transport properties. It is well known that for the doping range 0.22$<$x$<$0.47 a transition from a paramagnetic insulating to a ferromagnetic metallic state occurs upon cooling below T$_{C}\sim $150-250 K, and experiments have shown that the transition between these states is closely tied to the formation of lattice polarons. Recent neutron scattering experiments have directly observed that these polarons freeze into a glassy state at T=T$_{c}$ for materials with x=0.33, which is responsible for the observed first order nature of the ferromagnetic transition at this level of doping. While it has been proposed that this glass state occurs as x$\sim $0, its dependence on doping and temperature has yet to be fully mapped out. Here we present results from elastic and inelastic neutron scattering experiments performed on single crystal samples of these materials for x=0.15, 0.2, and 0.25, which have been undertaken to determine the direction in which the boundary for the glass phase evolves with x. Preliminary results indicate that in the insulating state (x$<$0.22), no static polaron scattering is observed (in zero field), in contrast to the results for the metallic regime. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A23.00004: Current-induced persistent ferromagnetic metallic state in an Fe-doped manganite Hideaki Sakai, Yoshinori Tokura A persistent ferromagnetic metallic state has been observed to be induced by excitation of an electric current for a single crystal of 7\% Fe-doped (La$_{0.7}$Pr$_{0.3}$)$_{0.65}$Ca$_ {0.35}$MnO$_3$, a typical ``relaxor ferromagnet"[1] with coexisting clusters of the ferromagnetic metal and charge- orbital-ordered insulator. According to the simultaneous measurement of the resistance and magnetization, the induced ferromagnetic state is likely to form a filamentary pathway, where the increase in magnetization is estimated to be $\sim\!0.4$ $\mu_{\rm B}$ per Mn. Distinguishing the current heating effect, which tends to conversely decrease the ferromagnetic fraction, from the intrinsic effect, we have successfully demonstrated the reproducible switching of both the resistance and magnetization by changing the magnitudes of the applied voltages. \newline [1] H. Sakai {\it et al.}, Phys. Rev. B {\bf 76}, 155112 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A23.00005: Terahertz time domain spectroscopy of ordered and disordered half-doped manganites K.R. Mavani, M. Nagai, H. Yada, K. Tanaka, D.S. Rana, I. Kawayama, M. Tonouchi Terahertz (THz) time domain spectroscopic studies were carried on Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and Nd$_{0.5}$Ca$_{0.48}$Ba$_{0.02}$MnO$_{3}$ (NCBMO) charge-ordered (CO) manganite thin films. Temperature dependent complex optical conductivity ($\tilde {\sigma }=\sigma _1 +i\sigma _2 )$ was studied to probe the cation disorder effects on low energy charge dynamics. The frequency dependent $\sigma _{1}$ suggests formation of charge-density-waves in these manganites. A doping induced cation disorder, as in the case of NCBMO, weakens the CO state.$^{1}$ However, at THz frequency, there are subtle effects of disorder on $\tilde {\sigma }$of NCBMO in a definite low temperature range. As temperature increases to 150 K (N\'{e}el temperature), changes are observed on the trends of temperature dependent dielectric constant ($\varepsilon _{1})$ and $\sigma _{1}$. The Ba doping effects are dominant above the charge-ordering transition temperature. The \textit{$\varepsilon $}$_{1}$ and \textit{$\sigma $}$_{1}$, both increase with temperature and show a scaling relationship, \textit{$\varepsilon $}$_{1~\propto }\sigma _{1}$, that is consistent with similar result observed for (PrCaSr)MnO$_{3}$ manganite.$^{2}$ $^{1}$Mavani \textit{et al.}, Europhys. Lett. (in print). $^{2}$Pimenove \textit{et al.}, Phys. Rev. B 73,220407(R). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A23.00006: High Magnetic Field Time-resolved Optical Study of Manganites. Paula Sahanggamu, Sanhita Ghosh, Stephen McGill, Haidong Zhou, Ben Conner, Christopher Wiebe We study the effects of applied electric fields and large magnetic fields on the optical properties of La$_{(1-x)}$Ca$_{x}$MnO$_{3}$ (x$\sim $0.18) (LCMO) and Pr$_{(1-x)}$Ca$_{x}$MnO$_{3}$ (x$\sim $0.5) (PCMO) using time-resolved techniques. Our measurements are performed from 4 K to room temperature and in dc magnetic fields up to 31 T. The conductivity of the low-temperature ferromagnetic state in LCMO is altered by the application of an electric field and these electrically-induced changes are further modified in the presence of an applied magnetic field. We demonstrate that time-resolved optical methods are capable of capturing these mixed electronic and magnetic effects. In addition, we performed time-resolved Kerr effect measurements in PCMO in an attempt to gain further insight into the loss of strong charge/orbital ordering in the presence of large applied magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A23.00007: Properties of Doped Manganese Oxides and the Origin of Colossal Magnetoresistance I. Felner, I. Nowik, D. Orgad, M. I. Tsindlekht, A. Frydman, D. I. Golosov, N. Ossi, Y. M. Mukovskii We report a series of magnetic and transport measurments on high-quality single crystal manganate samples in the colossal magnetoresistive (CMR) regime. A small iron doping allows also for a M\"{o}ssbauer spectroscopy study, showing both an unusual line broadening within the ferromagnetic phase and a coexistence of ferro- and paramagnetic contributions in the critical region. The resistivity peak (which gives rise to the CMR) occurs at a somewhat higher temperature, suggesting that it {\it does not} correspond to a threshhold of percolation between metallic (ferromagnetic) and insulating (paramagnetic) phases. In addition, our magnetic and transport data appear to imply that the paramagnetic-ferromagnetic (Curie) transition and the metal-insulator transition (the resistivity maximum) are two distinct phenomena on their own right, with distinct (although probably interrelated) physical origins. We speculate that these results can be understood phenomenologically within the framework of a two-fluid model. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A23.00008: Monte Carlo study of the one-orbital model for manganites in the CMR regime Rong Yu, Cengiz Sen, Gonzalo Alvarez, Elbio Dagotto The thermal phase transition in the one-orbital model for manganites with cooperative phonons and superexchange coupling is studied using Monte Carlo simulations. In the absence of on-site disorder, the model has a second order phase transition from a paramagnetic insulating phase to a ferromagnetic metallic phase in the CMR regime. The CMR effect is found to be associated with short-range charge and spin correlations in the insulating phase. This clearly shows that the CMR effect arises from the competition between the ferromagnetic and a charge-ordered competing states. Motivated by a recent STM experiment, we also studied the local density of states in this one-orbital model. We show how the experimental results can be understood within our model. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A23.00009: A New Correlated Model of Colossal Magnetoresistive Manganese Oxides D.I. Golosov A new minimal model is proposed for the doped manganese oxides which exhibit colossal magnetoresistance (CMR), involving (in addition to the ionic spins) a broad spin-majority conduction band as well as nearly localised spin-minority electron states. We outline the reasons to introduce this model, and discuss some of its properties. In particular, a mean field analysis yields an interaction-induced enhancement of the interband hybridisation and implies an emergence of a small energy scale, linked to the longitudinal spin dynamics, charge fluctuations, and presumably also to the carrier transport. We suggest that these conclusions merit further theoretical and experimental investigation in the context of the CMR manganates. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A23.00010: Combined Effect of Bond- and Potential-Disorder in Half-Doped Manganites Sanjeev Kumar, Arno Kampf We analyze the effects of both bond- and potential-disorder in the vicinity of a first-order metal insulator transition in a two-band model for manganites using a real-space Monte Carlo method. Our results reveal a novel charge-ordered state coexisting with spin-glass behavior. We provide the basis for understanding the phase diagrams of half-doped manganites, and contrast the effects of bond- and potential-disorder and the combination of both. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A23.00011: Nanoscale ferromagnetism and doping effect in manganites S. Mori, K. Yoshidome, Y. Horibe, T. Asaka, Y. Matsui, K. Takenaka We investigated temperature variation of ferromagnetic (FM) microstructures in La$_{1-x}$Sr$_{x}$MnO$_{3}$ (x=0.175) by low-temperature Lorentz microscopy. Also, changes of the FM domain structures by substituting non-magnetic Al$^{3+}$ ion for Mn one were examined. We succeeded in observing changes of magnetic domain structures from large FM domains with the average width of 1$\mu $m to stripe-shaped FM ones with the 100nm width in the metallic phase. On the other hand, the insulator phase was found in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.02). We found that the insulating phase exhibits a characteristic bi-modal distribution of the FM domains, which is characterized as the coexisting state of distinct types of the FM domains; one is the stripe-shaped FM domains and the other is the plate-shaped one. This mixture of distinct types of the FM domains breaks the coherence of the conductivity and results in the insulating character in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.02). In addition, we found the nanoscale FM domains around the rhombohedral-to-orthorhombic structural transition temperature in La$_{1-x}$Sr$_{x}$Mn$_{1-y}$Al$_{y}$O$_{3}$ (x=0.175, y=0.04). [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A23.00012: Multiple Metal-Insulator Transitions in LPCMO Wires T.Z. Ward, S.H. Liang, K. Fuchigami, L.F. Yin, E. Daggotto, E.W. Plummer, J. Shen The two hottest areas of research in condensed matter physics are complexity and nanoscale physics. Interestingly, these two areas have little overlap as most of the nanophysics research work is conducted using ``simple'' materials of metals or semiconductors instead of complex materials such as transition metal oxides (TMOs). However, due to the strong electronic correlation, it is exactly the transition metal oxides that will most likely lead to observations of striking new phenomena under spatial confinement. In this work, spatially confined La$_{.325}$Pr$_{.3}$Ca$_{.375}$MnO$_{3}$ (LPCMO) is shown to exhibit never before seen transport properties which reveal a double metal-insulator transition. These findings shine new light on the processes at play in LPCMO, as we use this novel technique to probe the material. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A23.00013: Ferromagnetic tendencies at the surface of AF/CO bulk manganites Shuai Dong, Rong Yu, Seiji Yunoki, Elbio Dagotto Previous investigations have shown that the surface of a ferromagnetic (FM) manganite has antiferromagnetic (AFM) tendencies. However, the development of ferromagnetism has been observed experimentally in some AFM charge-ordered (CO) manganites by reducing the grain size down to the nanoscale. Similar phenomenon was also found in other nanosized CO oxides, such as La$_{1/3}$Sr$_{2/3}$FeO$_{3}$. To clarify these puzzling observations, using Monte Carlo techniques we studied the FM Kondo model, using open boundary conditions to simulate a surface. For some values of the couplings, the previously known tendency to an AFM outer shell was found for the case of a bulk FM state. But for other couplings, the opposite effect was observed: the existence of a FM layer at the surface of a AFM/CO material. Details will be provided in this presentation. [Preview Abstract] |
Session A24: Focus Session: Transport in Nanostructures I: STM and Atomic Control
Sponsoring Units: DMPChair: Nathan Guisinger, National Institute of Standards and Technology
Room: Morial Convention Center 216
Monday, March 10, 2008 8:00AM - 8:12AM |
A24.00001: Single electron tunneling measurements of Titanium Silicide islands on Si(100) J.L. Tedesco, J.E. Rowe, R.J. Nemanich Titanium silicide (TiSi$_{2})$ islands have been formed by the ultrahigh vacuum (UHV) deposition of thin films of titanium ($<$ 1 nm) on clean Si(100) surfaces followed by annealing to $\sim $800$^{o}$C. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have been performed on these islands to demonstrate single electron tunneling (SET). Evidence of Coulomb staircase peaks corresponding to SET has been identified in current-voltage (I-V) curves recorded from islands at room temperature. Predictions of the orthodox model were found to agree with our data, except for slight discrepancies of the shape of the I-V curves at current steps. Many islands that were expected to exhibit SET did not do so. Potential reasons for the absence of SET include Schottky barrier lowering due to Fermi level pinning, and interfacial faceting which was identified as the most likely reason for the absence of observable SET. The positive SET results establish that a Schottky barrier can be used as an effective tunnel junction in a future double barrier tunnel junction (DBTJ) device. Possible approaches to improve the reliability based on control and engineering of surface and interface electronic bands will be discussed. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A24.00002: Tunneling Spectroscopy of Ultrathin Insulating Films: CuN on Cu(100). Charles D. Ruggiero, Taeyoung Choi, Jay A. Gupta Insulating films comprising only a few atomic layers offer insight into the evolution of electronic structure at the nanoscale and are useful for controlling electronic coupling of adsorbates. We have studied the electronic structure of one monolayer thick CuN islands grown on Cu(100) with a low temperature (5K), ultrahigh vacuum scanning tunneling microscope. We find that CuN acts as an insulator, with a band gap that exceeds 4~eV. Measurements of the tunneling barrier height and image potential states indicate that the CuN work function is $\sim $0.9~eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory. We find no significant dependence of these results on CuN coverage, from small islands ($\sim $ 10~nm$^{2})$ to complete films. This suggests that collective electronic structure is already established in the smallest islands. We use the CuN films to decouple metal nanoclusters from the Cu surface electron density. Tunneling spectra of few-atom Nb clusters on CuN reveal an atomic resonance that is not observed for clusters on Cu. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A24.00003: STM studies of transport through single azobenzene molecules Taeyoung Choi, Jay A. Gupta A microscopic understanding of electrical contact to molecules is needed to improve the performance and reproducibility of devices based on organic materials. We use a low-temperature scanning tunneling microscope to study current flow through single molecules where the contacts and local environment are characterized with atomic resolution. Azobenzene was adsorbed on a Cu(100) surface partially covered with one-monolayer thick islands of CuN, an insulator with a bandgap exceeding 4~eV. Peaks in tunneling spectra attributed to molecular orbitals are observed for molecules on CuN, but not on Cu. This is consistent with the decrease in hybridization expected for adsorbates on insulating films. Current flow through azobenzene molecules which bridge Cu and CuN regions is highly asymmetric with bias voltage, suggestive of rectifying behavior. Atomically precise contacts can be made to the molecules by using the STM tip to manipulate nearby metal atoms. Spectroscopic imaging is used to monitor changes in molecular orbitals due to charge transfer between molecule and contacts. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A24.00004: An atomic switch of electron propagation on Ge (001) by tunneling carrier injection Invited Speaker: Reversible switching of electronic conduction through atom manipulation is one of the important subjects of nanoscience. However, different conducting pathways were not clearly observed with atomic resolution. We have demonstrated the correlation between the change of surface atomic position by tunneling carrier injection and that of the reflection of one-dimensional (1D) surface-state electrons on the Ge (001) surface with a low density of heterogeneous Sn-Ge dimers. [1] On the clean Ge(001) surface, two adjacent atoms form a buckled dimer, and the buckling orientation of the Ge dimer can be locally and reversibly controlled by carrier injection to the surface from the STM tip. [2] The unoccupied surface $\pi^{*}$-electron behaves like a 1D free electron along the Ge dimer row. When Sn atoms are deposited on the clean Ge(001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge dimer position in the surface. [3] An atomic switch is realized for the $\pi^{*}$ electrons in the Ge dimer- row direction by injection carriers to reversibly flip the buckling orientation of a single Sn-Ge dimer in the dimer row. When the Sn atom of the heterogeneous dimer is at the lower position, the 1D electrons are reflected and a standing wave of this state is observed. Whereas, when it is at the upper position, the 1D electrons pass through the heterogeneous dimer, and no standing wave is observed. In this state, the lower atom of the dimer is Ge, and the $\pi^{*}$ state at the dimer is little different from that of the Ge-Ge dimers. \newline \newline [1] K. Tomatsu, K. Nakatsuji, T. Iimori, Y. Takagi, H. Kusuhara, A. Ishii, F. Komori; Science \textbf{315}, 1696, 2007. \newline [2] Y. Takagi, Y. Yoshimoto, K. Nakatsuji, F. Komori; Surf. Sci. \textbf{559}, 1, 2004. \newline [3] K. Tomatsu, K. Nakatsuji, T. Iimori, F. Komori; Surf. Sci. \textbf{601}, 1736, 2007. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A24.00005: Quantum Channels and Conductance Oscillations in TiOx Nano-switches Feng Miao, J. Joshua Yang, Duncan R. Stewart, R. Stanley Williams, Chun Ning Lau We investigate conductance switching in Pt/TiOx/Pt devices by pressure-modulated conductance microscopy. For devices with conductance G$>>$G$_{Q}$ and G$<<$ G$_{Q}$, where G$_{Q}$ is the conductance quantum, localized pressure-induced conductance peaks are observed, indicating formation of nanoscale conductance pathways on the electrodes. We postulate that these nano-conducting channels are related to the drift of oxygen vacancies under electrical field. For devices with G$\sim $ 1- 2 G$_{Q}$, in addition of conductance peaks, we also observed conductance dips and oscillations in response to localized pressure. These results suggest formation of quantum conductance channels in our devices, and can be modeled by considering interfering electron waves between two partially transmitting electrodes. Our findings suggest the possible use of these devices as atomic-scale switches. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A24.00006: Electronic switching in nanoscale titanium oxide devices Duncan Stewart, J. Joshua Yang, Julien Borghetti, Douglas Ohlberg, Matthew Pickett, Feng Miao, R. Stanley Williams Titanium metal is widely used as a top metal contact for nanoscale molecular electronic devices, where it has been assumed to form a few-atom-thick Ti carbide overlayer. Using a vacuum delamination technique we expose and analyze chemically pristine buried titanium/organic monolayer interfaces from devices that have displayed `molecular electronic switching'. We establish that under many conditions the titanium instead forms a \textit{few-nanometer-thick Ti oxide} overlayer. Both TiO2 and reduced TiOx species exist -- this mixed stoichiometry Ti oxide is responsible for the electronic switching. In the separate field of `conventional' nano-electronics, oxide based electrical-resistance switches are pursued for next generation nonvolatile random access memories (R-RAMs). However, the metal/oxide/metal switching mechanism is poorly understood. We demonstrate in Pt/TiOx/Pt nanocrosspoint devices that the switching is channeling (on) and recovering (off) the Schottky barrier at the Pt/TiO2 interface due to the creation and drift of positively charged oxygen vacancies under electric field. Engineered oxygen vacancy profiles predictively control the switching polarity and conductance to support a general physics model of switching in these devices. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A24.00007: Electron transport simulations through organic adlayers on metal surfaces Manuel Cobian, Nicolas Lorente, Pablo Ordejon Molecular entities at the interface with an inorganic surface are the basis for new hybrid functional materials for microelectronics. In most cases, strong bonding of molecules to metal surfaces perturbs the discrete molecular energy levels leading to a broadening of the molecular density of states. Deposition of $C_{60}$ on a Au(111) surface previously exposed to tetraphenyladamantane give rises to a nanostructured organic layer where the electronic coupling between the $C_{60}$ and the Au(111) surface is significantly reduced compared to $C_{60}$ on a clean Au(111) surface. Calculations based on Density Functional Theory reveal that intermolecular interactions lock $C_{60}$ into a particular orientation in agreement with Scanning Tunneling Microscopy experiments. This system exhibits the presence of negative differential resistance which can be understood by simulations of the transport properties at the ab-initio level using TRANSIESTA. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A24.00008: Scanning tunneling spectroscopy (STS) of DNA and G4-DNA molecules Danny Porath, Errez Shapir, Hezy Cohen, Alexander Kotlyar, Rosa Di Felice Attempts to resolve the energy level structure of single DNA molecules by STS span over the last two decades, thanks to this technique ability to probe the local density of states of objects deposited on a surface. Success was hindered by extreme technical difficulties in stable deposition and reproducibility. By using STS at 78 \r{ }K, for the first time we disclose the energy spectrum of poly(G)-poly(C) DNA molecules deposited on gold. The tunneling current-voltage characteristics and their derivative curves exhibit a clear gap and a peak structure around the gap. By means of ab initio Density Functional Theory calculations the character of the observed peaks is assigned to orbitals originating from the different molecular components. Limited fluctuations in the I-V curves are observed and statistically characterized. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A24.00009: Microscopic Characterization of Organic/Metal Interfaces: a Combined DFT and Many-Body Perturbation Theory Study Yan Li, Deyu Lu, Giulia Galli Aromatic molecules and molecular assemblies have received widespread attention as possible components of molecular electronic devices. An essential prerequisite to understand their stability and transport properties is the microscopic characterization of the interface formed with metallic leads. We present a comprehensive, first-principles study of the interface of Au(111) and a representative aromatic isocyanide molecular SAM (phenylenediisocyanide). We provide predictions about the binding geometries, coverage and stability properties, which are in good agreement with experimental measurements. We also discuss the electronic properties of the organic/metal interface by including self-energy corrections through many-body perturbation theory (GWA), and surface polarization effects. Our results indicate that electronic structure calculations beyond DFT are required to make an accurate assessment of energy level alignments between SAMs and the metallic leads. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A24.00010: Scanning tunneling spectroscopy of mass selected Ag clusters on C$_{60}$ functionalized surfaces Heinz H\"{o}vel, Stefanie Duffe, Lukas Patryarcha, Chunrong Yin, Bernd von Issendorff, Michael Moseler Scanning tunneling spectroscopy (STS), which can be used to study the electronic properties of individual clusters on surfaces [1] is combined with the deposition of mass selected Ag clusters (from Ag$^+_{55}$ to Ag$^+_{561\pm 5}$). A functionalization of the substrate with C$_{60}$ layers proved to be very useful to ensure the soft landing of the clusters and to bind them to fixed positions on the surface [2]. The knowledge of the exact cluster size shows that one has to interpret the STM derived cluster height carefully, considering details of the STM imaging process. For the soft landed clusters we measured identical spectral features for individual clusters with the same selected size using STS at 5\,K. \par \noindent [1] H. H\"{o}vel, I. Barke, Prog. Surf. Sci. 81 (2006) 53. \par \noindent [2] S. Duffe et al., Eur. Phys. J. D (2007), published online. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A24.00011: Rectification in Porphyrin/Fullerene Dyads on Au(111)* Dominic Britti, Ray Phaneuf, Francesca Matino, Valentina Arima, Manuel Piacenza, Fabio Della Sala, Giuseppe Maruccio, Roberta Del Sole, Giuseppe Mele, Giuseppe Vasapollo, Roberto Cingolani, Ross Rinaldi We present an ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy study of \textit{ex-situ} self assembled supramolecular dyads, consisting of fulleropyrrolidines (PyC$_{2}$C$_{60})$ axially ligated to zinc(II) tetraphenylporphyrin (ZnTPP), self organized by axial ligation to a 4-aminothiophenol (4-ATP), self assembled monolayer on gold (111). By highly diluting the PyC$_{2}$C$_{60}$ solution, and subsequently annealing in vacuum, isolated dyads are obtained; these show both bias polarity-dependent apparent height in STM images, and highly rectifying behaviour in tunneling spectroscopy. First principles density functional theory calculations clarify the conformational and the electronic properties of the 4-ATP/ZnTPP/PyC$_{2}$C$_{60}$ system. The rectifying behavior is explained using a model based on the Aviram-Ratner mechanism. * Work supported by the CNR-INFM, by a NSF US-Italy Cooperative Research Program {\#}OISE-0242579, by the SpiDME European project and by MIUR FIRB 2003 `SYNERGY' grant. Lab for Physical Sciences and in part by a NSF-MRSEC, DMR{\#} 0520471. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A24.00012: Electron transport study of surface-supported nanostructures with a quadraprobe scanning tunneling microscope Tae-Hwan Kim, John Wendelken, An-Ping Li We report the study of the electron transport and structural properties of nanostructured materials with a low temperature quadraprobe scanning tunneling microscope (STM) system. Self-assembled nanostructures including epitaxially grown nanowires and atomic chains have been fabricated by doping with small amounts of metal atoms (Au, Gd, Y, Ag) on a Si surface. The local electronic states and transport properties have been characterized in situ. This research took the advantage of our recent development of the quadraprobe STM system. As a ``nano'' version of a four-probe station, the quadraprobe STM system provides an integrated research platform with a low temperature four-probe STM, a molecular-beam epitaxy growth chamber, a high resolution scanning electron microscope, and a scanning Auger microscope. The four STM probes can be driven independently with sub-nanometer precision, enabling conventional STM imaging and four-point electrical transport study of surface electronic systems and nanostructured materials at temperatures down to 10 K. *Email: apli@ornl.gov [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A24.00013: Metallic Transport and Anderson Localization on In Atomic Layers on Silicon Shiro Yamazaki, Yoshikazu Hosomura, Iwao Matsuda, Rei Hobara, Shuji Hasegawa Metallic temperature dependence of electrical resistance have not been observed except extremely limited number of examples[1] below 100K in atomic-scale low-dimensional metal systems due to Anderson localization. Si(111)-$\sqrt 7 \times \sqrt 3 $-In surface reconstruction consist of 1.2 ML In atoms. According to ARPES study, the surface is 2D metal with the large Fermi wave number (k$_{F}$=14nm$^{-1})$ and the large electron density (4.6$\times $10$^{14}$eV$^{-1}$cm$^{-2})$, leading to a low resistance [2]. By using variable-temperature micro-four-point probe method [3], low resistance and metallic transport was found down to 10 K. It is quantitatively explained by the ARPES result by using Boltzmann equation $R_{2D} =\frac{4\pi ^2\lambda m^\ast }{e^2\hbar k_F^2 }k_B T$. By introducing defect, it shows semiconducting temperature dependence of variable range hopping due to Anderson localization. [1]K. Lee, \textit{et al}. , Nature \textbf{441}, 65 (2006). [2]E. Rotenberg, \textit{et al}. , Phys. Rev. Lett. \textbf{91}, 246404 (2003). [3]T. Tanikawa,\textit{ et al}. , e-J. Surf. Sci. Nanotech. \textbf{1}, 50 (2003) [Preview Abstract] |
Session A25: Block Copolymer Thin Films
Sponsoring Units: DPOLYChair: Alamgir Karim, National Institute of Standards and Technology
Room: Morial Convention Center 217
Monday, March 10, 2008 8:00AM - 8:12AM |
A25.00001: Control of P(S-b-PMMA) Orientation on Organosilicate Substrates by Thermal Treatment Kookheon Char, Hyoseon Suh One of most important factors to control the orientation of microdomains in block copolymer films is the wetting behavior of a block copolymer at an interface. From this perspective, we studied the wetting behavior of P(S-b-MMA) block copolymer (BCP) thin films on silsesquioxane-based organosilicate (OS) substrates, which have potential applications as interlayer dielectrics (ILDs) for next generation semiconductor devices. We controlled the surface energy of OS substrates by changing the treatment temperature. As the treatment temperature was increased, the wetting behavior of a P(S-b-MMA) film on OS substrates changed from the asymmetric to the symmetric wetting, which allowed us to find the optimum treatment temperature for the neutral wetting behavior. Changes in the orientation of BCP microdomains on OS substrates were analyzed by AFM, FE-SEM, and GI-SAXS. These results show the potential that the OS ILDs prepared in this study do not require any additional surface modifications such as random copolymer brushes or SAMs for BCP microdomains to orient perpendicular to the substrate, which could ultimately be applied to BCP lithography. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A25.00002: Functionalization of PEO Nanocylinder Array Structure in Block Copolymer Thin Film. Kaori Kamata, Tomokazu Iyoda Phase-separated nanostructure of block copolymer has been intensively studied as one of the most promising candidates for many important nanotechnological applications, because the feature size of domain structure generated by block copolymers is ranging to a level of several nanometers. We have studied the nanostructures of block copolymers, which are based on the use of amphiphilic block copolymer, PEO-b-PMA(Az), to establish the long range orderness onto the substrate. Here, we describe the nanotemplating process with domain selective doping of PEO cylindrical domains, using various kinds of metal cation sources, such as Fe or Pb ions. Furthermore, the ionic liquid also hybridized into the PEO nanocylinders to form liquid nanochannels perpendicular to the substrate. A wide variety of cations were successfully incorporated through the coordination bonding with the oxygen in PEO chain. The resulting metal cation hybrid films could offer the metallic nanopillar arrays, with the nanostructure-specific electrochemical, electric or magnetic properties. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A25.00003: Block copolymer mask with cylindrical nanochannels for wet nanopatterning on silicon wafer Ryoko Watanabe, Kaori Kamata, Tomokazu Iyoda Microphase-separated structure of block copolymer (BC) thin film has been considered as a mass-productive nanomask, in which the periodicity of the pattern can be tuned by molecular weight. We have developed a series of amphiphilic BC, PEO$_{m}b$-PMA(Az)$_{n}$, consisting of hydrophilic, ion-conductive poly(ethylene oxide) (PEO) and hydrophobic poly(methacrylate) with azobenzene in its side chain. This BC forms hexagonally arranged PEO cylinders which span across the thickness of the film after thermal annealing. In this study, PEO cylinders were blended with poly(ethylene glycol) monomethylether to provide effective nanochannels that can perpendicularly diffuse etching species for wet etching of silicon wafer As a quick and total wet nanopatterning process, a silicon wafer covered by the BC mask was annealed at 140 degrees C for 1 h and immersed into 33 wt{\%} NH$_{4}$F aqueous solution for 3 min. AFM observation of the resulting silicon wafer surface revealed a hexagonally arranged nanohole array with 24 nm of center-to-center distance and 10 nm of diameter was fabricated. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A25.00004: Is a short fluorinated segment sufficient to induce interfacial rearrangements in diblock co-polymers? Umesh Shrestha, Dvora Perahia, Stephan Clarson The interfacial structure of thin diblock co-polymer films is a result of internal segregation between the blocks and their affinity to the interfaces. Introducing a fluorinated group affects significantly the segregation of the fluorinated block to the interface. The interfacial structure and dynamics of thin layers of a diblock co-polymer Polytrifluoro propyl methyl siloxane-polystyrene (PTEPMS-PS) with the fraction of the fluorinated block ranging from 0.03 to 0.5was studied by neutron reflectivity. The uniqueness of this diblock lies in the presence siloxane group and a fluorinated group intercalated into one of the blocks where the siloxane offers flexibility and the fluorine exhibits low interfacial energies. While the air interface is dominated by fluorinated segments for all volume fractions, layering is observed even for the shortest fluorinated segments. The rate of rearrangement upon annealing varies with the size of the fluorinated block. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A25.00005: Multi-block copolymers in thin films. Panagiotis Maniadis, Edward Kober, Turab Lookman We study the behavior of an $\{AB\}_n$ multi-block copolymer confined to a thin film, using self consistent field theory (SCFT) methods. Due to the breaking of symmetry in the direction of confinement, the propagators do not obey the usual diffusion equation. We derive the diffusion equation which correctly describes the confined polymer system and find that it differs from the original in an area which is approximately 3 times the Kuhn length of the polymer, close to the surface of the film. We use the modified diffusion equation to study the structure of the confined polymer. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A25.00006: Two-Dimensional Instabilities in Patterned Diblock Copolymer Films Joseph Parete, Andrew B. Croll, John S. Preston, Kari Dalnoki-Veress We have developed a novel surface tension-driven laser lithography technique that enables the effective generation of microstructures in polymer systems. Combining this approach with naturally occurring instabilities in symmetric diblock copolymer films allows us to produce structures far smaller then those imposed by the resolution of the laser patterning procedure. An example of this is the Plateau-Rayleigh instability, which results in the breaking up of an extended cylinder into an arrangement of smaller, evenly spaced droplets. Since our diblock system can be considered an ideal two-dimensional system, the creation of various structures using laser lithography provides an excellent probe for studying instabilities in two dimensions. Optical and atomic force microscope measurements are presented and discussed in terms of a linear stability analysis. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A25.00007: Structure and dynamics of block copolymer films by XPCS Hyunjung Kim, Heeju Lee, Young Joo Lee, Sanghoon Song, Zhang Jiang, Sunil K. Sinha, A. Ruehm We have measured the structure and the dynamics of block copolymer films in the melt using X-ray Photon Correlation Spectroscopy. Block-copolymers films used in this study have an internal structure of spherical micelles. This ought to have a strong influence on the physical properties of the thin films. The results from the surface dynamics are compared with the theory of overdamped thermal capillary waves on thin films. By changing the incident angle, the surface dynamics and the micelle dynamics were selectively measured. The obtained viscosity will be compared with the value from the mechanical measurement of the bulk material. The surface tension obtained from static grazing incidence scattering data shows that a PDMS layer segregates to the free surface of the film. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A25.00008: Sphere-Forming and Cylinder-Forming Block Copolymer Thin Films Aligned Under Double Shear Andrew Marencic, Richard Register, Paul Chaikin Studies have shown that stress transmitted through a viscous layer to a sphere- or cylinder-forming block copolymer thin film can orient the microdomains in the direction of the imposed shear. For the creation of complex patterns, reorientation of these unidirectionally oriented films is necessary. Here we demonstrate the ability of shear to realign the microdomains along a second direction, as imaged by atomic-force microscopy. The results are in qualitative agreement with our previously proposed phenomenological model; however, the stress required for realignment is larger (factor of 2.7 for sphere-formers and factor of 1.2 for cylinder-formers) than the stress required to orient a film from the polygrain state. We also observed grain boundary generation within the transition region between alignment with the first shear direction and alignment with the second shear direction. No noticeable change in the dislocation density or its angular distribution was observed in the sphere-forming block copolymer thin films following the second shear. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A25.00009: Rapid Directed Assembly of Block Copolymer Films on chemically patterned surfaces at Elevated Temperatures Adam Welander, Paul Nealey We report on the rapid directed assembly of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer thin films at elevated temperatures well above the glass transition temperature (Tg) on chemically patterned surfaces. The time needed for defect free assembly, where the chemical pattern (L$_{S})$ closely matches the natural length of the block copolymer (L$_{0})$, is strongly dependant on the annealing temperature. Annealing times range from 150 minutes at 180 \r{ }C to 3 minutes at 230 \r{ }C. This system behavior is well described as a simple thermally activated process with an apparent activation energy (\textit{$\Delta $E}$_{a})$ of 182 kJ/mol and a polymer diffusion coefficient of 7.5E-15 cm$^{2}$s$^{-1}$ at 190 \r{ }C. Modeling this behavior predicts annealing times of 13.5 seconds at 250 \r{ }C and 1.9 seconds at 280 \r{ }C. While these times are difficult to investigate experimentally, a one minute anneal at these elevated temperatures not only shows perfect assembly where L$_{S}$ = L$_{0}$, but also where L$_{S} <$ L$_{0}$. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A25.00010: Precise Control of 3-dimensional Block Copolymer Assembly using 2-dimensional Chemical Templates Sangcheol Kim, Hae-Jeong Lee, Ronald L. Jones, Alamgir Karim, R.M. Briber, Ho-Cheol Kim Chemically heterogeneous substrates, where the periodicities of the chemical pattern are close to those of lamellar microdomains of block copolymers, have been used as an effective route to align the microdomains with a low number density of defects. Macroscopic chemical heterogeneity is, however, still valuable for providing insights on ordering behavior of block copolymers. Using a series of chemical patterns with micrometers length scales, we studied the wetting and self-assembly behavior of poly(styrene-b-methyl methacrylate) (PS-b-PMMA). We found that the variation in morphology and orientation of microdomains are governed by the degree of chemical contrast and the size of the underlying chemical patterns. The two-dimensional chemical template is shown to precisely control the three dimensional assembly of the block copolymer film. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A25.00011: Novel Complex Nanostructures from Directed Assembly of Block Copolymers on Incommensurate Surface Patterns. Sang Ouk Kim, Bong Hoon Kim, Harun H. Solak, Dong Meng, Qiang Wang Self-assembled nanostructures of block copolymer thin films have attracted enormous attention as useful templates for nanofabrication. We present that a chemically nanopatterned surface prepared by photolithography is able to direct the assembly of a block copolymer thin film to form a novel complex nanostructure. When a cylindrical block copolymer was assembled on a stripe pattern, whose pattern period was twice as large as a natural lattice size of the bulk nanostructure, a new structure was produced, where cylinders were alternately oriented parallel and perpendicularly to the surface. Self-consistent field calculations supported the emergence of the new structure, providing insight into the detailed structure and formation mechanism. Our work suggests that the combining top-down and bottom-up approaches may provide a versatile pathway for fabricating well-registered complex nanostructure, potentially useful in diverse advanced applications. References; S. O. Kim, et al. Nature 424, 411 (2003); M. P. Stoykovich, et al. Science 308, 1442 (2005); S. O. Kim, et al. Macromolecules 39, 5466 (2006); S. O. Kim, et al. Advanced Materials 19, 3271 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A25.00012: Templated Self-Assembly of Asymmetric Ternary Blends of Block Copolymers and Homopolymers Karl Stuen, Francois Detcheverry, Carla Thomas, Richard Farrell, Michael Morris, Juan de Pablo, Paul Nealey Templated assembly of ternary blends of cylinder-forming PS-\textit{block}-PMMA and homopolymers of PS and PMMA was investigated experimentally and with Monte Carlo simulations of a coarse-grained model. The blends were deposited into trench features coated with a neutral brush to induce cylinder orientation perpendicular to the substrate. The ternary blends were used to systematically control the commensurability between the blend and trenches of constant width. Important patterning parameters such as the degree of perfection, the domain spacing perpendicular and parallel to the confining template, the domain uniformity, and shape of the unit cell were quantified as a function of blend composition. The cylindrical nanostructures appear to have improved uniformity across the trench width compared with spherical systems. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A25.00013: Directed Assembly of Asymmetric Ternary Block Copolymer-Homopolymer Blends Thin Films on Checkerboard Trimming Chemical Pattern Huiman Kang, Paul F. Nealey Typical 2-dimensional projections of ordered block copolymer morphologies in thin films include periodic lines or hexagonal arrays of spots and therefore may not be suitable for patterning even strictly periodic device-oriented arrays, such as checkerboard trimming patterns (alternating lines and dashed lines) used in the fabrication of dynamic random access memory (DRAM). Here we direct the assembly of asymmetric ternary polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA)/homopolymer PS/homopolymer PMMA blends on checkerboard trimming chemical patterns. The degree of perfection and domain uniformity of the assembled films was quantified as a function of overall composition of PS or PMMA and total homopolymer fraction in the blends. By matching polymer volume fractions and pattern area fractions, blends composed of lamellae-forming block copolymer and substantially asymmetric fractions of the two homopolymers could be assembled into equilibrated line and dash morphologies. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A25.00014: Influence of added copolymers on thin film polymer blends studied by atomic force microscopy: surface morphology and dewetting Dean Waldow, Jenifer Hoffert, Kris Peterson Atomic force microscopy (AFM) was used to study thin film polymer blends of polystyrene (PS) and polybutadiene (PB). These blends were studied with increasing concentration of block, random, and graft copolymers. Thin films were prepared by spin coating from a good solvent, which resulted in film thicknesses of approximately 65 nm. The AFM data were collected in AC mode and included both topographic and phase data. The topography data were analyzed using lateral Fourier Transform analysis as well as real space techniques both lateral and normal to the surface. The results suggest that lateral phase separated domain size decreases with increasing copolymer concentration for each of additives studied and for all concentrations. The evolution of the phase separated morphologies and dewetting was also studied by an anneal / quench technique as well as real time monitoring at temperatures above the PS glass transition temperature. Results for the domain evolution experiments demonstrate significant suppression of lateral domain growth and some suppression of large encapsulated domains seen in the binary blend. The graft copolymer demonstrated the most significant effects with the diblock and random copolymers following in significance respectively. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A25.00015: Controlled Dimensions of Nanostructures in Asymmetric Ternary Blends of Block Copolymers and Homopolymers in Thin Films Carla Thomas, Karl Stuen, Nicola Ferrier, Paul Nealey Ternary blends of cylinder-forming PS-\textit{block}-PMMA and homopolymers of PS and PMMA were investigated in thin films on a neutral brush with the domains oriented perpendicular to the substrate. Five blends with homopolymers having different degrees of polymerization (N) were studied to quantify the dimensional scaling of the domains and suitability for patterning. In the dry brush regime, the best pattern uniformity and fewest defects were found when N of the homopolymers matched N of the corresponding block of the block copolymer. The lattice spacing of the cylindrical array, $D$, and cylinder diameter uniformity changed with $\phi _{H}$, the homopolymer volume fraction in the film, and N of the hompolymers$_{.}$ With the ternary blends, $D$ could be increased up to 200 percent relative to the neat block copolymer; the ternary blends allow $\phi _{H }$greater than 0.4 without inducing a phase change. [Preview Abstract] |
Session A26: Focus Session: Photophysics of Cold Molecules I
Sponsoring Units: DCPChair: Andrey Vilesov, University of Southern California
Room: Morial Convention Center 218
Monday, March 10, 2008 8:00AM - 8:36AM |
A26.00001: Isolation of Molecules in Helium Nanodroplets: Spectroscopy and Dynamics at Ultra-cold Temperatures Invited Speaker: The isolation of atoms, molecules, clusters or nano-sized complexes in helium nanodroplets allows detailed spectroscopic studies at temperatures in the millikelvin range. Moreover, femtosecond real-time spectroscopy has been introduced to study dynamical processes in the ultracold helium environment. On the one hand, wave packet propagation opens a window to dynamical processes, allowing even a view to superfluid properties at the nanoscale. This is exemplified at decoherence effects in the wave packet propargation of small molecules attached to the droplets. On the other hand, high-resolution mass spectra using both femtosecond photoionisation (PI) as well as electron impact ionization enable us to characterize reactive processes at temperatures in the millikelvin range. As an example, alkali cluster -- water complexes are formed in helium droplets. By recording multi-photon PI spectra we can distinguish between reactive processes of the neutral clusters and ionic reactions occurring after ionisation of the alkali cluster component. These studies pave the way to time-resolved reaction dynamics at very low temperatures. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A26.00002: Nonequilibrium magnesium complexes formed in helium nanodroplets Josef Tiggesb\"aumker, Andreas Przystawik, Sebastian G\"ode, Karl-Heinz Meiwes-Broer Doping helium droplets with alkaline earth atoms is an interesting tool to investigate the interaction with the superfluid helium. Magnesium is a corner case regarding the degree of solvation in helium [1,2] which may enable the detection of quantized vortices in helium droplets. In this contribution we add another facet to the discussion. The absorption of helium droplets doped with magnesium atoms is measured with resonant two-photon ionization at different combinations of droplet size and the number of doped Mg atoms. This enables the unambiguous identification of the absorption of an isolated atom inside the droplet centered around 279\,nm. When increasing the Mg content of the droplet we find evidence for the formation of metastable, weakly bound Mg complexes. After excitation of such a complex it collapses to a Mg cluster on a timescale of 20\,ps. \newline [1] J. Reho \emph{et al}., J. Chem. Phys. {\bf 112}, 8409 (2000) \newline [2] Y. Ren and V.V. Kresin, Phys. Rev. A {\bf 76}, 043204 (2007) [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A26.00003: Superfluid $^4$He density functional theory in 2-D cylindrical coordinates Jussi Eloranta, Sean French, Steven Fiedler Bosonic density functional theory describing superfluid $^4$He is formulated in 2-D cylindrical coordinates and a numerical implementation of the model using a regular spatial grid basis is presented. The 2-D formulation has many important applications as the 1-D treatment cannot, for example, describe translational motion of atoms and molecules solvated in the liquid and the 3-D theory is usually computationally too expensive, especially when describing dynamics in bulk superfluid $^4$He. The theory is implemented in both real and imaginary time forms for allowing solution of both time-dependent and time-independent problems. Two test cases for the developed method are presented and the results are compared against the previously published results. Finally, the method is applied to describe solvation of single wall carbon nanotubes in superfluid $^4$He at 0 K and the implications of the results to dynamic liquid response are discussed. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A26.00004: Photo-induced isomerization and chemical reaction dynamics in superfluid helium droplets Jeremy Merritt, Gary Douberly, Roger Miller Near threshold photo-induced isomerization and photo-induced chemical reactions have long been sough after as sensitive probes of the underlying potential energy surface. One of the most important questions asked is how the initially bright quantum state couples to the reaction coordinate, and thus relates to energy transfer in general. Helium droplets have now allowed us to stabilize entrance channel clusters behind very small reaction barriers such that vibrational excitation may result in reaction. Through two examples, namely the isomerization of the 2 binary complexes of HF-HCN {\{}Douberly et al. PCCP 2005, 7,463{\}}, and the induced reaction of the gallium-HCN complex {\{}Merritt et al. JPCA 2007, DOI:10.1021/jp074981e{\}} we will show how the branching ratios for reaction and predissociation can determined and the influence of the superfluid He solvent. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A26.00005: Rydberg States of Na-doped Helium Nanodroplets Marcel Drabbels The dynamics of excited states of Na atoms deposited on the surface of helium nanodroplets has been investigated with velocity map ion imaging, photoelectron spectroscopy and time-of-flight mass-spectroscopy. For the first time, the excitation spectra of Na-doped helium nanodroplets corresponding to Rydberg states of Na atoms have been measured from the lowest excited 3p state up to the ionization threshold. All lines in the excitation spectra are shifted and broadened with respect to the corresponding atomic lines. In addition to bare Na* atoms also Na*He$_{N}$ ($N$ = 1-6) exciplexes are detected upon excitation. Photoelectron spectroscopy reveals the desorption of Na* not only in the initially excited states but also in lower lying states, indicating that relaxation plays an important role. The recorded velocity distributions show interesting characteristics: for the lowest states the mean kinetic energy of Na* increases linearly with excitation energy. The velocity distributions of Na*He$_{N}$ exciplexes do not manifest such remarkable properties. The observations can be largely explained by assuming that the interaction of Na* with the helium nanodroplet can be described by the sum of Na*-He pair potentials. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A26.00006: Photoelectron imaging of doped helium nanodroplets Daniel Neumark Photoelectron images of helium nanodroplets doped with Kr and Ne atoms are reported. The images and resulting photoelectron spectra were obtained using tunable synchrotron radiation to ionize the droplets. Droplets were excited at 21.6 eV, corresponding to a strong droplet electronic excitation. The rare gas dopant is then ionized via a Penning excitation transfer process. The electron kinetic energy distributions reflect complex ionization and electron escape dynamics. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A26.00007: High Resolution Fluorescence Excitation and Dispersed Emission Spectra of Organic Molecules in Superfluid Helium Nanodroplets Alkwin Slenczka, Ricarda Riechers, Dominik Pentlehner, Alexander Vdovin Superfluid helium droplets serve as a very gentle cryogenic matrix for molecular spectroscopy. The low temperature and high thermal conductivity of superfluid helium droplets are of great advantage for the investigation of dispersed emission spectra of molecules. As a complement to the fluorescence excitation spectrum the emission spectra provide important details on dynamic processes of intramolecular as well as intermolecular nature. This will be demonstrated for various examples such as intramolecular proton tunnelling, isomeric van der Waals complexes, tautomerization and microsolvation. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A26.00008: Coherent boson dynamics in strongly localized potentials - helium excitations at planar aromatic molecules and trapped cold atoms Invited Speaker: Planar aromatic molecules provide strongly localizing potentials for helium that considerably modify the local superfluid properties of a solvating helium environment. I shall describe some of the effects of these interactions on the solvation structure and spectroscopy of tetracene and phthalocyanine in helium droplets, comparing results of zero and finite temperature quantum Monte Carlo simulations with experimental data. The helium atoms closest to the molecule are seen to show similarities to trapped cold atoms in multi-well potentials. Studies of cold bosons with attractive and repulsive interactions in double well potentials will also be presented, showing formation of squeezed and quantum superposition states of cold atoms. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A26.00009: Fragmentation dynamics inside helium nanodroplets: new theoretical results Nadine Halberstadt, David Bonhommeau, Marius Lewerenz We present a theoretical study on the effect of a helium nanodroplet environment on the fragmentation dynamics of embedded rare gas cluster ions. The helium atoms are treated explicitly, with zero-point effects taken into account through an effective helium-helium interaction potential. All the nonadiabatic effects between electronic states of the ionized rare gas cluster are taken into account. Our results reveal new mechanisms for the cooling by helium, and show that the dopant can be ejected from the helium droplet. These results will be presented and discussed. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A26.00010: Suppressing the fragmentation of fragile molecules in helium nanodroplets by co-embedding with water: Possible role of the electric dipole moment Yanfei Ren, Vitaly Kresin When fragile molecules are embedded in liquid helium nanodroplets, electron-impact ionization usually leads to fragmentation which is as extensive as for isolated gas-phase molecules. This occurs because of the energy release accompanying charge transfer from the impurity to the He$^{+}$ hole created by electron bombardment. However, in experiments with glycine, polyglycine peptide chains, and alkane hydrocarbon chains we found that if a few molecules of water are co-embedded with these molecules, the fragmentation of the latter is drastically reduced or completely eliminated. On the other hand, the fragmentation of alkanethiols remains unaffected. On the basis of these observations, it is proposed that the fragmentation ``buffering'' effect may correlate with the magnitude of the impurity's electric dipole moment, which steers the migration of the ionizing He$^{+ }$hole through the droplet. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A26.00011: Path integral investigation of the electronic spectra of He-tetracene clusters Heather D. Whitley, K. Birgitta Whaley Planar aromatic molecules (PAMs) are nanoscale precursors to bulk graphite. Their electronic spectra have been extensively studied in $^{4}$He nanodroplets and show a number of unusual spectroscopic features. We have conducted many-body quantum simulations of tetracene in He nanodroplets to probe the 1.1 cm$^{-1}$ spectral splitting of the electronic origin seen for this PAM. We calculate spectral shifts and He density profiles via path integral quantum Monte Carlo simulations. The spectral splitting is examined using a path integral correlation function approach to determine the lowest-lying vibrational excitation frequencies for small He$_{N}$-tetracene clusters. Simulations in the S$_{1}$ state of tetracene utilize a semi-empirical perturbative interaction potential for a He atom with a PAM. Results for the splitting of the electronic origin and the spectral shifts are in good agreement with experiment. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Session A27: Focus Session: Pyrochlores
Sponsoring Units: GMAGChair: Jason Gardner, National Institute of Standards and Technology
Room: Morial Convention Center 219
Monday, March 10, 2008 8:00AM - 8:36AM |
A27.00001: Low temperature spin dynamics and high pressure effects in frustrated pyrochlores Invited Speaker: Frustrated pyrochlores R$_{2}$M$_{2}$O$_{7}$, where R$^{3+}$ is a rare earth and M$^{4+}$ a transition or sp metal ion, show a large variety of exotic magnetic states due to the geometrical frustration of the pyrochlore lattice, consisting of corner sharing tetrahedra for both R and M ions. Neutron scattering allows one to measure their magnetic ground state as well as the spin fluctuations, in a microscopic way. An applied pressure may change the subtle energy balance between magnetic interactions, inducing new magnetic states. In this talk, I will review recent neutron results on Terbium pyrochlores, investigated by high pressure neutron diffraction and inelastic neutron scattering. Tb$_{2}$M$_{2}$O$_{7}$ pyrochlores show respectively a spin liquid state for M=Ti [1], an ordered spin ice state for M= Sn [2], and a spin glass state with chemical order for M=Mo [3]. In Tb$_{2}$Ti$_{2}$O$_{7}$ spin liquid, where only Tb$^{3+}$ ions are magnetic, an applied pressure induces long range antiferromagnetic order due to a small distortion of the lattice and magneto elastic coupling [4]. In Tb$_{2}$Sn$_{2}$O$_{7}$, the substitution of Ti$^{4+}$ by the bigger Sn$^{4+}$ ion expands the lattice, inducing a long range ordered \textit{ferromagnetic }state, with the local structure of a spin ice [2] and unconventional spin fluctuations [2,5]. The local ground state and excited crystal field states of the Tb$^{3+}$ ion were recently investigated by inelastic neutron scattering in both compounds [6]. Tb$_{2}$Mo$_{2}$O$_{7}$, where Mo$^{4+}$ ions are also magnetic, shows an even more rich behaviour, due to the complex interaction between frustrated Tb and Mo lattices, having respectively localized and itinerant magnetism. In Tb$_{2}$Mo$_{2}$O$_{7}$ spin glass, the lattice expansion induced by Tb/La substitution yields an ordered ferromagnetic state, which transforms back to spin glass under applied pressure [7]. New data about the spin fluctuations in these compounds, as measured by inelastic neutron scattering, will be presented. The talk will be dedicated to the memory of Igor Goncharenko, a renowned high pressure and neutron physicist, who died accidentally on Nov. 4$^{th}$, 2007. \newline [1] J. S. Gardner \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 1012, (1999). \newline [2] I. Mirebeau \textit{et al.,} Phys. Rev. Lett. \textbf{94}, 246402, (2005). \newline [3] B. D. Gaulin \textit{et al.,} Phys. Rev. Lett. \textbf{69}, 3244, (1992). \newline [4] I. Mirebeau \textit{et al.} Nature \textbf{420}, 54 (2002); Phys. Rev. Lett. 93, 187204, (2004). \newline [5] F. Bert \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 117203, (2006)~; P. Dalmas de R\'{e}otier \textit{et al.}, Phys. Rev. Lett.\textbf{ 96}, 127202, (2006). \newline [6] I. Mirebeau, P. Bonville, M. Hennion, Phys. Rev. \textbf{76}, 184436, (2007). \newline [7] A. Apetrei et al, Phys. Rev. Lett. \textbf{97}, 206401, (2006). [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A27.00002: Spin Structure of Three Dimensional Frustrated Magnetic System of Tb$_{2}$Mo$_{2}$O$_{7}$ Deepak Singh, Joel Helton, Jeffrey Lynn, Young Lee The pyrochlore compound Tb$_{2}$Mo$_{2}$O$_{7}$ has been of interest since it exhibits spin-glass behavior despite the apparent lack of chemical disorder. This compound crystallizes in a cubic space group in which both the Tb and Mo atoms form three-dimensional networks of corner-sharing tetrahedra. Thus, each magnetic ion resides on a highly frustrated pyrochlore lattice. To clarify the nature of the spin correlations in the spin-glass state of Tb$_{2}$Mo$_{2}$O$_{7}$, we have succeeded in growing single crystal samples. Magnetic susceptibility (zero field cooled and field cooled) measurements confirm the spin-glass behavior, similar to that observed in previous powder samples. Recently we performed elastic neutron scattering measurements on a single crystal sample and identified the \textbf{Q-}vectors associated with the short-range diffuse scattering. The spin structure for this compound, consistent with elastic scattering, will be discussed. In addition, recent inelastic neutron scattering measurements will be discussed. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A27.00003: Field driven phases in the geometrically frustrated dipolar Heisenberg pyrochlore antiferromagnet Gd$_2$Ti$_2$O$_7$ Matthew Enjalran, Adrian Del Maestro, Michel J.P. Gingras The rare-earth pyrochlore gadolinium titanate, Gd$_2$Ti$_2$O$_7$, represents an excellent experimental realization of a Heisenberg antiferromagnet (AFM) in a frustrated geometry with weak long-range dipole-dipole interactions (approximately 20$\%$ of nearest neighbor AFM exchange). Experiments on Gd$_2$Ti$_2$O$_7$ in a magnetic field reveal a complex phase diagram associated with the breaking of spatial symmetries of the pyrochlore lattice as the field is applied along select symmetry directions. We study a model of classical Heisenberg spins (O(3) symmetry) on a pyrochlore lattice with exchange and dipolar interactions within mean-field theory. Using parameters relevant to the material system, we develop phase diagrams in finite magnetic fields. Our results our compared to experiments on Gd$_2$Ti$_2$O$_7$ (and Gd$_2$Sn$_2$O$_7$). [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A27.00004: Induced Magnetic Order in Yb$_2$Ti$_2$O$_7$ Carl Adams, K.A. Ross, J.P. Ruff, B.D. Gaulin, H. Dabkowska, Y. Qiu, J.R.D. Copley, J.S. Gardner Yb$_2$Ti$_2$O$_7$ is part of family of pyrochlore materials with a magnetic rare-earth and a non-magnetic transition metal. In similar compounds containing holmium, erbium, or terbium the geometric frustration results in several exotic ground states including spin-ice, spin-liquid, field-induced ordering, and low-dimensional behavior. The ytterbium-based compound has a strong anomaly in the specific heat at 240~mK [Hodges {\em et al.}, J Phys Cond Mat {\bf 13}, 9301 (2001)] but without accompanying long range order. We have made measurements of the elastic and inelastic neutron scattering on a large single crystal of Yb$_2$Ti$_2$O$_7$ over a broad range of the $(hhl)$ scattering plane using the DCS spectrometer at the NIST Center for Neutron Research. Our studies have confirmed the presence of diffuse ``rods'' of scattering along the (111) directions that persist below 100~mK. However, when the sample is field-cooled in fields as low as 0.5~T the rods of scattering are replaced by well-defined magnetic excitations. We will present the results of our studies at a variety of temperatures and fields. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A27.00005: Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er$_2$Ti$_2$O$_7$ Jacob Ruff, Patrick Clancy, Bruce Gaulin, Hanna Dabkowska, Mehmet Ramazanoglu, Jason Gardner, Alex Bourque, Mary Anne White, Yiming Qiu, John Copley We report recent single crystal neutron scattering measurements of the XY pyrochlore antiferromaget Er$_2$Ti$_2$O$_7$, in zero and non-zero magnetic field applied along the (110) direction. Previous results show a magnetically-ordered ground state which is suggested to be stabilized by an unusual order-by-disorder mechanism driven by quantum fluctuations [1]. We present measurements of the low-lying spin excitations of this system, revealing Goldstone modes previously thought to be absent [1]. Also, we explore the response of the ground state to applied magnetic fields, which are shown to induce a quantum phase transition separating the low-field antiferromagnetically ordered phase from the high-field polarized phase. \newline [1] J.D.M. Champion et al. ``Er$_2$Ti$_2$O$_7$: Evidence of quantum order by disorder in a frustrated antiferromagnet'' PRB 68 (2003) [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A27.00006: Lattice distortion in the spin liquid phase of Tb$_2$Ti$_2$O$_7$ Stephanie Curnoe The Tb ions in the pyrochlore antiferromagnet Tb$_2$Ti$_2$O$_7$ form a corner-shared tetrahedral network. It has been shown that the quantum anti-ferromagnetic ground state of a single tetrahedron accounts for short wavelength correlations observed in inelastic neutron scattering experiments. A bulk $k=0$ lattice distortion, specifically an $A_{2u}$ displacement of the Tb ions, can account for the isolation of single tetrahedra in the tetrahedral network. Such a distortion results in a space group reduction $Fd\bar{3}m \rightarrow F\bar{4}3m$ which is consistent with experimental observations. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A27.00007: Non-monotonic zero point entropy in diluted spin ice X. Ke, R. Freitas, B. Ueland, G. Lau, M. Dahlberg, R. Cava, R. Moessner, P. Schiffer Water ice and spin ice are important model systems in which theory can directly account for ``zero point'' entropy associated with quenched configurational disorder. Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with non-magnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90{\%}. Investigations of both Ho and Dy spin ices reveal that the zero point entropy depends non-monotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice. \newline X. Ke, et al, Phys. Rev. Lett. \textbf{99}, 137203 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A27.00008: Thermodynamic and magnetic studies of novel spin ice materials M.L. Dahlberg, X. Ke, B.G. Ueland, P. Schiffer, D.V. West, R.J. Cava We report systematic low temperature measurements of the dc magnetization, ac susceptibility, and heat capacity of $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ (x=0, 0.25, and 0.5), and $Dy_{2}NbScO_{7}$. We find evidence for Ising-like single ion ground states in the $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ materials, and these materials possess nearly the same zero-point entropy as the canonical spin ices $Ho_{2}Ti_{2}O_{7}$ and $Dy_{2}Ti_{2}O_{7}$. These results strongly suggest that the $Dy_{2}Sn_{2-x}Sb_{x}O_{7+x/2}$ materials studied have spin-ice states at low temperatures despite the cation disorder on the B sites of the pyrochlore lattice We also observe a somewhat reduced zero-point entropy in $Dy_{2}NbScO_{7}$, which is possibly associated with a higher level of cation disorder [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A27.00009: Direct observation of the ice rule in demagnetized artificial kagome spin ice Yi Qi, Todd Brintlinger, John Cumings, Paula Mellado, Oleg Tchernyshyov Artificially designed magnetic structures have proven to be good analogs for the study of spin ice.~ We studied an artificial kagome spin ice, which is lithographically patterned using permalloy elements $\sim $23nm thick, $\sim $100nm wide, and $\sim $500nm long.~ Lorentz TEM is used to unambiguously determine each local pseudospin.~Even with a rigorous randomizing demagnetization process, we directly observe an exclusive ice rule and ice-like short range correlations. This is the first time that the ice rule has been confirmed by direct counting of the nearest-neighbors in any ice system, real or artificial. The further-neighbor correlations we observe are larger in magnitude than predicted for a random ice-rule ensemble, and we attribute this to the effects of dipolar interactions. Further, we investigate the entropy associated with artificial kagome spin ice based on the general concept of Shannon entropy [1].~ These results will be presented and compared with relevant theoretical studies. [1] C.E. Shannon, Bell System Technical Journal, 27, 379 (1948) [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A27.00010: Magnetization reversal in artificial kagome ice. Paula Mellado, Oleg Tchernyshyov, Yi Qi, Todd Brintlinger, John Cumings We study magnetization dynamics in an artificial kagome spin ice realized as a honeycomb network of connected ferromagnetic nanowires [1]. Our model is focused on magnetic charges defined as the flux of magnetization into a network site. In this system, the allowed values of magnetic charge are $\pm 1$ and $\pm 3$, while in the original square ice [2] they are 0, $\pm 2$, and $\pm 4$. The ice rule is equivalent to the minimization of the absolute value of magnetic charge. In our model, magnetization reversal in a given link is triggered when the total magnetic field at one of its sites reaches a critical value and the site emits a domain wall with charge $\pm 2$, which propagates the entire length of the link reversing the magnetization. The resulting redistribution of magnetic charges provides a positive feedback by increasing the local values of the magnetic field at neighboring sites. A sufficiently strong feedback triggers an avalanche-like reversal observed experimentally. That and an inherently dissipative character of the reversal process suggest interesting parallels with granular materials and sheds light on the physics of rotational demagnetization of spin ice [3]. Supported by NSF Grant DMR-0348679. [1] M. Tanaka \textit{et al.}, Phys. Rev. B \textbf{73}, 052411 (2006). [2] R. F. Wang \textit{et al.}, Nature \textbf{439}, 303 (2006). [3] C. Nisoli \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 217203 (2007). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A27.00011: Neutron Scattering from Magnetically Frustrated Ruthenium Pyrochlores Invited Speaker: Spin-1 ruthenium pyrochlores feature strong exchange interactions and deeply suppressed N\'{e}el ordering. In addition a doping induced metallic phase has been demonstrated. I discuss neutron scattering experiments that explore the strongly frustrated quantum magnetism of Y$_{2}$Ru$_{2}$O$_{7}$ [1] and Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ [2]. In Y$_{2}$Ru$_{2}$O$_{7}$ ($\Theta _{CW}$= --1100 K, T$_{N}$=77 K) much of the magnetic spectral weight is concentrated in a 20 meV spectral peak above an 11 meV low temperature gap in the excitation spectrum. In Pr$_{2-x}$Bi$_{x}$Ru$_{2}$O$_{7}$ magnetic order which occurs for T$<$T$_{N}$=160 K when $x$=0, is suppressed when the material becomes a metal for $x\approx $1. I discuss the anomalous low energy spin dynamics associated with praseodymium for $x$=0 and $x$=1 and relate the results to other rare-earth pyrochlores. \newline \newline [1] J. van Duijn, N. Hur, J. W. Taylor, Y. Qiu, Q. Z. Huang, S.-W. Cheong, C. Broholm, and T. G. Perring, Submitted to Phys. Rev. B Rapid Communications, cond-mat 0707.2678v1 (2007). \newline [2] J. van Duijn, K. H. Kim, N. Hur, D. Adroja, M. A. Adams, Q. Z. Huang, M. Jaime, S.-W. Cheong, C. Broholm, and T. G. Perring, Phys. Rev. Lett.\textbf{ 94}, 177201 (2005). [Preview Abstract] |
Session A28: Focus Session: Optical Properties of Nanostructures I: Carbon Nanotubes
Sponsoring Units: DMPChair: Riichiro Saito, Tohoku University
Room: Morial Convention Center 220
Monday, March 10, 2008 8:00AM - 8:36AM |
A28.00001: Optical Spectroscopy of Individual Single-Walled Carbon Nanotubes Invited Speaker: Single-walled carbon nanotubes (SWCNTs) constitute a fascinating class of 1-dimensional materials. While the electrical, thermal, and mechanical properties of SWCNTs have been studied in depth for some time, only recently have their optical properties emerged as a major topic of research. This interest reflects several factors: The importance of nanotube optical properties for analysis of their structure, quality, and growth; the potential of nanotubes for diverse optoelectronic applications; and the unique physics of the excited states and dynamical processes in these 1-D structures. In this paper we will present an overview of our current understanding of excited electronic states in nanotubes and of the methods and applications of single-molecule optical spectroscopy of carbon nanotubes. We will present recent results on optical spectroscopy of individual carbon nanotubes as perturbed by their external environment, including by the application of strain and deposition of adsorbates. These studies were carried out in collaboration with the research groups of Profs. Louis Brus, Jim Hone, and Stephen O'Brien at Columbia and Prof. Feng Wang at UC Berkeley. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A28.00002: Photophysics of Nanostructures: Tubes, Sheets, and Ribbons Invited Speaker: The restricted geometry of nanostructures often gives rise to novel, unexpected properties and phenomena. In particular, symmetry and many-electron effects can play a vital role in determining the behaviors of these systems. In this talk, I discuss some recent work on using first-principles theory and computation (employing the GW-BSE approach) to understand and predict the electronic structure and optical response of carbon nanostructures including nanotubes, graphene and graphene nanoribbons. We show that, owing to reduced dimensionality, many-electron interaction (self-energy and excitonic) effects change qualitatively the nature of the photo-excited states in both semiconducting and metallic carbon nanotubes. Exciton states with extraordinarily large binding energies and an unusual spectrum (arising from an ``anti-screening'' phenomenon) are predicted for the semiconducting tubes. Moreover, unlike bulk systems, we discover that excitons also exist in the metallic carbon nanotubes. Similar studies show that excitonic effects are equally dominant in the optical spectra of the graphene nanoribbons, although the characters of the exciton states are quite different. Other quasi-1D systems, such as the boron-nitride nanotubes and semiconductor nanowires, exhibit similar behavior. We discuss the physics behind these phenomena and present comparison of our theoretical predictions to recent measurements. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A28.00003: Effective One-Dimensional Electron-Hole Interaction in Single-Walled Carbon Nanotubes Jack Deslippe, Mario Dipoppa, David Prendergast, Rodrigo Capaz, Steven Louie Using the results of ab initio GW-Bethe-Salpeter-Equation (GW-BSE) calculations on the excitonic effects in single-walled carbon nanotubes (SWCNTs), we derive a 1D quantum model for the electron-hole interaction in both semiconducting and metallic SWCNTs. The model includes the important effects of spatial dependent screening and reproduces the exciton binding energies and envelope wave functions of the complete GW-BSE solution of the electron-hole excited states. The inclusion of the spatial dependence in the dielectric function is essential to capture the positioning of the higher exciton states in the spectrum whose calculated energies differ dramatically from those obtained using previous models based on constant dielectric screening. The present effective interaction can be used to calculate the binding energies of exciton states in a range of SWCNTs, which would be impractical by ab initio study. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A28.00004: Environmental effect for exciton transition energy of single carbon nanotubes Riichiro Saito, Kentaro Sato, Park JinSung, Yuhei Miyauchi, Shigeo Maruyama, Mildred Dresselhaus, Gene Dresselhaus The exiton transition energies of single wall carbon nanotubes which are observed in resonance Raman, photo-absorption, and photoluminescense spectroscopies, depend on the surrouding materials (environmental effect). The environmental effect can be explained by screeining of the excitonic states by the dielectric materials. We calculate the transition energies for many different (n,m) carbon nanotubes up to 4eV and to 3nm in diameter. The calculated results are compare with many experimental data with different conditions for samples. The energy shift for the exciton transition energies can be explained by a fitting parameter of static dielectric constants of surrouding materials. However we will show that the effective dielectric constant has a unique, type, metallicity, diameter, and energy dependence of the dielectric constants in order to reproduce the exciton energies for the wide range of diameter and excitation energies. By analyzing the data, we will give a simple formula for the dielectric constants for carbon nanotubes themselves and the surrounding materials as a function of chirality and diameter of single wall carbon nanotubes. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A28.00005: Surface Exciton-Plasmons in Carbon Nanotubes Igor Bondarev, Kevin Tatur, Lilia Woods We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We use our previously developed Green's function formalism to quantize an electromagnetic field in the presence of quasi-1D absorbing bodies [1]. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength due to the presence of weakly-dispersive low-energy ($\sim $0.5-2eV) interband surface plasmon modes [2] and large exciton excitation energies $\sim $1eV in small-diameter nanotubes [3]. We estimate the exciton-plasmon Rabi splitting to be $\sim $0.01-0.1eV which is close to that observed in organic semiconductors [4] and much larger than that reported for hybrid semiconductor-metal nanoparticle molecules [5]. We calculate the exciton absorption lineshape and demonstrate a clear line splitting effect as the exciton energy is tuned to the closest interband surface plasmon resonance. \newline [1] I.V.Bondarev and Ph.Lambin, Phys. Rev. B72, 035451 (2005). [2] T.Pichler, et al., Phys. Rev. Lett. 80, 4729 (1998). [3] D.Spataru, et al., Phys. Rev. Lett. 95, 247402 (2005). [4] J.Belessa, et al., Phys. Rev. Lett. 93, 036404 (2004). [5] W.Zhang, A.O.Govorov, G.W.Bryant, Phys. Rev. Lett. 97, 146804 (2006). [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:24AM |
A28.00006: Cross-polarized optical absorption of single-walled carbon nanotubes probed by photoluminescence excitation spectroscopy, UV-Vis-IR and polarized Raman Scatterings Invited Speaker: Because of the depolarization effect, or so-called antenna effect, optical absorption of single-walled carbon nanotubes (SWNTs) is weak when excited by light polarized perpendicular to the nanotube axis. However, in photoluminescence (PL) excitation spectra of isolated SWNTs, PL peaks due to cross-polarized excitation can be clearly identified. By decomposing the cross-polarized component, the optical transition energy of E12 or E21 can be? measured, and the smaller exciton binding energy for perpendicular excitations is concluded [1]. Cross-polarized absorption is dominant in the absorption of a vertically aligned film of SWNTs [2] when excited from the top of the film. In our previous study, a pi-plasmon absorption at 5.25 eV was revealed in contrast to 4.5 eV for parallel excitation [3]. Resonant Raman scattering from such a film is also influenced by the cross-polarized excitation [4]. Even though a Kataura plot for the E33 and E44 range has been proposed by using such a vertically aligned film [5], polarized Raman scattering spectra reveal more complicated features in the system because of the small bundle size, typically 5-8 nanotubes [6]. \newline References: \newline [1] Y. Miyauchi, M. Oba, S. Maruyama, Phys. Rev. B 74 (2006) 205440. \newline [2] Y. Murakami, S. Chiashi, Y. Miyauchi, M. Hu, M. Ogura, T. Okubo, S. Maruyama, Chem. Phys. Lett. 385 (2004) 298. \newline [3] Y. Murakami, E. Einarsson, T. Edamura, S. Maruyama, Phys. Rev. Lett. 94 (2005) 087402. \newline [4] Y. Murakami, S. Chiashi, E. Einarsson, S. Maruyama, Phys. Rev. B 71 (2005) 085403. \newline [5] P. T. Araujo, S. K. Doorn, S. Kilina, S. Tretiak, E. Einarsson, S. Maruyama, H. Chacham, M. A. Pimenta, A. Jorio, Phys. Rev. Lett. 88 (2007) 067401. \newline [6] E. Einarsson, H. Shiozawa, C. Kramberger, M. H. Ruemmeli, A. Gruneis, T. Pichler, S. Maruyama, J. Phys. Chem. C (2007) published on Web. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 11:00AM |
A28.00007: Carbon nanotube excited states: the role of the environment Invited Speaker: The nature of the excited states and the radiative and non-radiative decay of isolated CNTs will be briefly reviewed. I will then focus on the role of the environment, external fields and photon confinement on these properties. I will discuss the use of photovoltage microscopy as a means of imaging environment-induced potential fluctuations in CNTs, presence and size of Schottky barriers at contacts and the extent of band-bending. Scanning resonant micro-Raman scattering will be used to evaluate the CNT excited state shifts induced by the substrate, and, also the phonon frequency and linewidth changes due to the fields of trapped substrate charges, and charge-transfer effects. Variations in local charge density will be determined by making use of the dependence of electron-phonon coupling on the local charge density. The non-radiative decay of free and localized excitons will then be examined and it will be shown that a phonon-assisted electronic decay mechanism made possible by interaction with the environment can dominate the lifetime of excited CNTs. Finally, results on the modification of the radiative properties of CNTs by changing the photon field mode density for CNTs enclosed in micro-cavities will be presented. [Preview Abstract] |
Session A29: Focus Session: Carbon Nanotubes and Related Materials I: Graphene Transport
Sponsoring Units: DMPChair: Stephen Cronin, University of Southern California
Room: Morial Convention Center 221
Monday, March 10, 2008 8:00AM - 8:12AM |
A29.00001: Thermopower of Few-Layer Graphene Devices Peng Wei, Wenzhong Bao, Yong Pu, Chun Ning Lau, Jing Shi We report thermopower measurements of few-layer graphene (FLG) devices at low temperatures. FLG flakes are separated by mechanical exfoliation and then connected to form devices using electron beam lithography. FLG devices are fabricated on a 300 nm-thick SiO2 layer which separates from the heavily-doped silicon substrate used as a gate. As the gate voltage is swept from -55 V to 55 V, the electrical conductivity of FLG devices undergoes a minimum. In the meantime, the Seebeck coefficient changes the sign near the conductivity minimum, marking the transition from p- to n-type in graphene. By directly comparing the derivative of the logarithmic conductivity with the measured Seebeck coefficient, we have experimentally validated the Mott relation. We have also measured the Seebeck coefficient and electrical conductivity under magnetic fields up to 8T. Detailed analysis of the experimental data will be presented. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A29.00002: Measurement of the Thermal Conductance at the Graphene-Quartz Interface by Optical Pump-Probe Spectroscopy Chun Hung Lui, Kin Fai Mak, Yang Wu, Tony Heinz We have determined the interfacial thermal conductance of single and multi-layer graphene samples prepared on quartz substrates by mechanical exfoliation of graphite. The measurements were performed by suddenly heating the graphene sample with a femtosecond optical laser pulse and then monitoring the sample's subsequent temperature evolution through the slight changes in reflectivity experienced by a time-delayed optical probe pulse. For the study of thermal transport, the transient response occurring on a time scale of tens of picoseconds was relevant. A faster transient related to non-equilibrium electronic excitation was also observed at early delay times. By studying the dependence of the slow decay component on the number of graphene layers in the sample, we could identify interfacial heat flow as the relaxation mechanism. An interfacial conductance in excess of 5,000W/cm$^{2}$K was deduced for the graphene-quartz system which is in the same order of magnitude compared to similar measurements on carbon nanotube suspensions [Huxtable et al. Nature Materials 2003]. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A29.00003: A graphene-based atomic-scale switch Brian Standley, Marc Bockrath Graphene's remarkable mechanical and electrical properties combined with its compatibility with existing planar CMOS technology make it an attractive material for novel computing devices. Thus far work has focused primarily on realizing transistor functionality. To complement this effort, we have developed a graphene-based switch that realizes a non-volatile memory element. Our devices have demonstrated tens of thousands of writing cycles and long retention times. Additionally, the devices' atomic-scale dimensions correspond to bit densities far greater than present-day memory technologies. We will present the fabrication process, switching behavior, and further performance characterization. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A29.00004: Electronic properties of graphene Invited Speaker: Graphene is a first two-dimensional atomic crystal. In my talk I'll overview our latest results on the electronic properties of this material. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A29.00005: Charged Impurity Scattering in Graphene Masa Ishigami, Jianhao Chen, C. Jang, E.D. Williams, M.S. Fuhrer We have measured the impact of charged impurity scattering on the transport properties of graphene sheets [1]. We vary the density of adsorbed potassium atoms in our experiment up to $5 \times 10^{12} K/cm^2$ on the surface of graphene based-devices which are otherwise devoid of any surface adsorbates [2] in ultra high vacuum environment. Adsorbed potassium decreases the charge carrier mobility, renders the gate-dependent conductivity linear, shifts the minimum conductivity point in gate voltage, broadens the width of minimum conductivity region, and lowers the minimum conductivity. Our results are in qualitative agreement with a recent Boltzmann transport calculation [3]. New features, such as asymmetric response of electron-hole mobility and the observation of a ``residual'' conductivity (the extrapolation of the linear gate-voltage dependent conductivity to the minimum conductivity point) near 2 $e^2/h$, indicate transport properties beyond the simple Boltzmann picture. [1] J.H.Chen et al., http://xxx.lanl.gov/abs/0708.2408. [2] M.Ishigami et al., Nano Letters, 7, 1643 (2007). [3] S. Adam et al., PNAS 104, 18392 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A29.00006: Conductivity and Fano factor in disordered graphene Eduardo R. Mucciolo, Caio H. Lewenkopf, Antonio H. Castro Neto Using the recursive Green's function method, we study the problem of electron transport in a disordered single-layer graphene sheet. The conductivity is of order $e^2/h$ and its dependence on the carrier density has a scaling form that is controlled solely by the disorder strength and the ratio between the sample size and the correlation length of the disorder potential. The shot noise Fano factor is shown to have a narrow dip near the neutrality point for weak disorder and to develop a nearly doping independent behavior at strong disorder. Our results are in good qualitative and quantitative agreement with experiments and provide a way for extracting microscopic information about the magnitude of extrinsic disorder in graphene. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A29.00007: The conductivity of pure graphene Subir Sachdev, Lars Fritz, Joerg Schmalian Pure graphene, in the absence of impurities or bias voltage, is described by a theory of Dirac fermions with Coulomb interactions. We argue that this theory has a finite conductivity, $\sigma$, and show that at frequencies $\omega \ll k_B T/\hbar$ (where $T$ is absolute temperature) $\sigma = \Xi (e^2/h) (\ln (W/T))^2 $, where $W$ is the bandwidth, and $\Xi$ is a {\em universal} number. We compute $\Xi$ by the solution of a quantum Boltzmann equation. The influence of a dilute concentration of impurities and finite bias voltage is also discussed. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A29.00008: Theoretical study of graphene transport regimes Shaffique Adam, S. Das Sarma In recent work [Adam et al. Proc. Natl. Acad. Sci. 104, 18392 (2007); Y.-W. Tan et al. arXiv:0707.1807, Phys. Rev. Lett., in press (2007)], we argued that the transport properties of currently available experimental graphene samples are dominated by diffusive carriers scattering off Coulomb impurity centers typically located in the substrate. In the current paper we study graphene monolayers, bilayers and nanoribbons and show theoretically that by tuning external parameters, one can access several different transport regimes ranging from the aforementioned diffusive Boltzmann transport to phase-coherent ballistic transport to classical percolation through puddles of electrons and holes. This work is supported by U.S. ONR and NRI-NSF. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A29.00009: Impact of physisorbed species on transport properties of graphene Chaun Jang, Jianhao Chen, Shudong Xiao, Masa Ishigami, Ellen Williams, Michael Fuhrer We have measured the impact of physisorbed species, including Argon, Krypton, Nitrogen, water and Benzene, on the transport properties of mechanically-exfoliated graphene sheets on SiO$_{2}$/Si in an ultra-high vacuum environment at temperatures near 30 K. We controlled the gas dosage down to the sub-monolayer level and found species-specific effects on the field-effect mobility of graphene. We observed the influence of different molecular sizes, molecular dipole moment, and intermolecular interactions. We will discuss our results in the context of recent theoretical calculations within the Boltzmann transport framework. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A29.00010: Stress-induced Controlled Fabrication of Graphene Nano Ribbons and Carbon Nanotubes via Electrostatic force and electrical transport properties of freely suspended graphene monolayers and bi-layers. Anton Sidorov, David Mudd, Vladimir Dobrokhotov, C.S. Jayanthi, Shi-Yu Wu, Gamini Sumanasekera A simple electrostatic technique to transfer loosely bound graphene sheets from a freshly cleaved highly oriented pyrolitic graphite (HOPG) to a desired substrate has been recently reported (Sidorov \textit{et al}., Nanotechnology, 2007). Here we demonstrate that this technique can be further extended to roll/scroll graphene sheets in a controllable manner by changing the environment during this electrostatic deposition. Deposition under high vacuum (10$^{-7}$ Torr) is observed to deposit extremely flat graphene monolayers on a substrate. In contrast, high density of completely scrolled graphene layers are observed in hydrogen atmosphere and in the presence of an electrostatic field. No scrolling was seen in He atmosphere; but partial scrolling is seen in nitrogen atmosphere under the influence of an electrostatic force. It is believed that in addition to the stress induced due to the adsorption of hydrogen, an additional electrostatic field is necessary to scroll the graphene layers loosely bound to HOPG. Also electrical transport properties of monolayers and bi-layers of graphene layers freely suspended between two electrodes and deposited between trenches on a substrate will be presented and compared. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A29.00011: Interaction of Si atoms and Si-based radicals with carbon nanotubes and graphene monolayers Kiseok Chang, Savas Berber, David Tom\'anek We use {\em ab initio} density functional calculations to study the interaction of Si atoms and Si-based radicals, such as SiH$_3$, with single-wall carbon nanotubes and graphene monolayers. We find that both Si atoms and radicals form a strong chemisorption bond, accompanied by a small relaxation and a locally increased sp$^3$ bond character of the graphitic nanostructure. We identify the optimum adsorption geometries at different adsorbate coverages and adsorbate-related changes in the electronic structure and vibration spectra of the systems. We propose that successful functionalization of carbon nanotubes or graphene by Si atoms or Si-based radicals can be verified by studying changes in the radial breathing mode of nanotubes and the G-band of graphitic nanocarbons using Raman spectroscopy. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A29.00012: First-principles Studies of Metal Adsorption on Graphene Kevin T. Chan, J. B. Neaton, Marvin L. Cohen Quantitative first-principles theory can aid in understanding many experimental phenomena involving metal adsorption on graphene and carbon nanotubes, including adatom mass transport, modification of electronic, mechanical, and magnetic properties, and adhesion and efficacy of electrical contacts. In this work, the binding energy and geometry, charge transfer, work function, and electronic structure of adatom-graphene systems are calculated using first-principles density functional theory for a variety of metal elements. Trends in these calculated data are analyzed, and their implications for graphene-based devices are discussed. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A29.00013: Carbon-based ion and molecular channels Kyaw Sint, Boyang Wang, Petr Kral We design ion and molecular channels based on layered carboneous materials, with chemically-functionalized pore entrances. Our molecular dynamics simulations demonstrate that these ultra-narrow pores, with diameters around 1 nm, are highly selective to the charges and sizes of the passing (Na$^{+}$ and Cl$^{-})$ ions and short alkanes. We demonstrate that the molecular flows through these pores can be easily controlled by electrical and mechanical means. These artificial pores could be integrated in fluidic nanodevices and lab-on-a-chip techniques with numerous potential applications. [1] Kyaw Sint, Boyang Wang and Petr Kral, submitted. [2] Boyang Wang and Petr Kral, JACS 128, 15984 (2006). [Preview Abstract] |
Session A30: Electronic Properties of Graphene and Related Structures I
Sponsoring Units: DCMPChair: Nicola Bonini, Massachusetts Institute of Technology
Room: Morial Convention Center 222
Monday, March 10, 2008 8:00AM - 8:12AM |
A30.00001: First-principles calculations of zigzag-edge graphene nanostrips with different edge species John W. Mintmire, Daniel Gunlycke, Junwen Li, Carter T. White First-principles calculations have suggested that zigzag-edge graphene nanostrips terminated with hydrogen atoms have edge states which exhibit magnetic behavior. However, it is not clear that zigzag-edge graphene nanostrips terminated with other atoms or functional groups also show similar magnetic behavior. Our local-spin-density calculations suggest that some zigzag-edge nanostrips, including oxygen-terminated nanostrips, have no magnetic edges. One reason could be that there is charge transfer at the edges which effectively dopes the pi-orbital network, causing the spin polarization to collapse. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A30.00002: Hydrogenation of graphitic nanocarbons Savas Berber, David Tom\'anek, Eunja Kim, Philippe F. Weck, Glen P. Miller We apply {\em ab initio} density functional calculations to study the hydrogenation of graphitic nanocarbons including fullerenes, onions and nanotubes using diethylenetriamine (DETA) as hydrogenation reagent. Our results indicate that transfer of atomic hydrogen from the amine end-group of chemisorbed DETA molecules to nanocarbons is an exothermic reaction. We explore the optimum pathway for the hydrogenation reaction and find the activation energy associated with sigmatropic rearrangement of chemisorbed hydrogen atoms to lie near 1 eV, thus facilitating formation of energetically favorable adsorbate structures by surface diffusion. Chemisorbed hydrogen assists in a local sp$^2$ to sp$^3$ bonding conversion of the graphitic nanocarbons, causing large-scale structural changes ranging from local relaxations in nanotubes to shell opening in multi-wall onions. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A30.00003: Electronic Structure Study of Edge Saturated Graphene Nanoribbons Yiming Zhang, Philip Shemella, P.M. Ajayan, Saroj Nayak Using density functional theory and GW method, we have studied how the electronic structures of graphene nanoribbons responds to the edge saturation. The energy gaps and effective mass of the nanoribbons are highly sensitive to the edge states, as well as the nanoribbon width. This suggests a new approach to modify the electronic structure of graphene nanoribbons by tweaking the edge saturation. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A30.00004: Imaging massless Dirac fermion flow in graphene nanoribbons Liviu P. Z\^arbo, Branislav K. Nikoli\'c Since its recent experimental discovery, graphene has been the focus of intense theoretical and experimental research. Its unconventional electronic structure characterized by the linear momentum dispersion of electrons which behave as massless Dirac particles at half-filling makes graphene an ideal candidate not only for investigating fundamental physics questions, but also for constructing new nanoscale devices. Despite its importance for device applications, there are very few studies of local features of electronic transport in graphene nanoribbons (GNRs). Moreover, the application of recently advanced scanning probe techniques to imaging electronic flow in graphene is expected to lead to many interesting discoveries. Therefore, we investigate the local features of charge transport through GNRs, by employing our bond current formalism which expresses the local current fluxes flowing between neighboring sites of the hexagonal lattice in terms of nonequilibrium Green functions. We show that, while the charge density profiles for clean zigzag graphene nanoribbons (ZGNRs) close to Dirac point peak at the edges due to the zero-energy edge states, the current densities concentrate towards the nanoribbon center. The analysis of local charge current flow explains unusual transport properties of ZGNRs such as low sensitivity of current flow to edge vacancies or long-range impurities. Journal Ref: EPL {\bf 80}, 47001 (2007). [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A30.00005: First principles study of graphene nanoribbons and nanorectangles Philip Shemella, Li Chen, Yu Zhou, Yiming Zhang, Sreekala Subbulakshmi, Pulickel Ajayan, Saroj Nayak We have studied the finite size effect on the electronic structure of graphene nanoribbons (GNRs) using first principles density functional techniques. In particular, we have computed the energy gap dependence on the width and length for zero-dimensional nanorectangles for both the armchair and zigzag ribbons; and compared to the one-dimensional (infinite length) cases. One-dimensional armchair ribbons are expected to be metallic if the number of carbon atoms across the ribbon is N = 3M-1, and non-metallic N $\ne $ 3M-1, where M is an integer. In addition to quantum confinement along the width of the ribbon for metallic widths, an additional finite size effect emerges along the length of ribbons only for non-metallic armchair ribbons. The origin of additional quantum confinement in these structures is explained based on the energy states near the Fermi energy. The differences between zero- and one-dimensional electronic structure properties are considered with the addition of passivating groups and their effect on the electronic properties of graphenes and their impact on nanoelectronics devices are discussed. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A30.00006: Band gaps in armchair-edge graphene nanostrips Carter T. White, Daniel Gunlycke, John W. Mintmire First-principles calculations have shown that all graphene nanostrips terminated with hydrogen atoms exhibit band gaps at the Fermi level. In the case of armchair-edge nanostrips, the calculations contradict a first-nearest-neighbor tight-binding prediction that one third of these nanostrips should be metallic. At the one-electron level, at least two independent causes for the band gaps in these nanostrips have been suggested, namely lattice distortion and long-range interactions. In this presentation, we present theoretical calculations of arbitrary armchair-edge nanostrips. The model, which includes distortion of edge atoms and third-nearest-neighbor interactions, leads to band gaps and band structures which are in good agreement with those obtained from our first-principles calculations. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A30.00007: Study of partial oxidation of zigzag graphene 1-d ribbons S. Sreekala, Y. Zhang, P.M. Ajayan, S.K. Nayak We study the effect of partial oxidation of graphene 1-d ribbons using the first principle- density functional theory. We have considered zigzag graphene, n=8 with four dangling carbon on each edge. The zigzag graphene 1-d ribbon is a zero bandgap material, when it is functionalized completely with hydrogen atoms. However, when two of these hydrogen atoms are replaced by oxygen, the band gap opens. This is due to the fact that the oxygen forms double bond with the carbon and hence disrupts the delocalization of the $\pi $ and $\pi ^{\ast }$ bond. This functionalization does not induce magnetization. On further increase of oxygen, the band decreases. When oxidation is more than 75{\%} on one side of the graphene ribbon or on either sides, the lone pair of electrons of the oxygen induces magnetization to some of the carbon atoms. Also some configurations of partially oxidized graphene show that antiferromagnetic order is the stable ordering in these systems. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A30.00008: Graphene Nanoribbon(GNR) based Nanoelectronics for Interconnect Applications and Logic Devices using First Principles Calculations Yu Zhou, Subbalakshmi Sreekala, Yiming Zhang, Philip Shemella, Pulickel Ajayan, Saroj Nayak We have studied electronic structures of graphene ribbon based nanoelectronics using first principles density functional techniques for interconnect applications as well as for logic devices. The conductance behaviors of them are computed based on Non-equilibrium Green's Function. For example, we have calculated the energy gap and I-V curve of Schottyky diode built by connecting two zig-zag GNRs with different passivations. All new configurations will show nonlinear I-V behavior and explicit step feature is observed in the I-V plot as well. There is also a small charge transfer at the junction area for this new configuration which is more like a traditional diode, which leads to different phenomena during the negative biasing. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A30.00009: Zigzag graphene ribbons with intrinsic spin-orbit and electron-electron interactions. Mahdi Zarea, Carlos B\"usser, Nancy Sandler The effects of intrinsic spin-orbit (I-SO) and Coulomb interactions on low-energy properties of finite width graphene zigzag ribbons are studied by means of tight-binding Hamiltonian. We derive analytic expressions for eigenstates and energies in the presence of the I-SO interaction in the hard-wall boundary limit. A detailed study of the spatial dependence of spin-filtered edge states [1] shows different edge localizations as the Dirac point is reached. Tight-binding numerical calculations reproduce exactly the analytic expression obtained for the band-structure. Coulomb interactions are included and treated within the bosonization approximation. We find that small momentum transfer scattering terms open a charge-gap in neutral ribbons, while keeping the spin sector gapless. Our numerical results suggest an exponentially vanishing gap in terms of the ribbon width for large values of the I-SO coupling constant, in clear contrast with results found for armchair ribbons [2]. [1] C.L. Kane and E.J. Mele, Phys. Rev. Lett. {\bf 95}, 226801 (2005). [2] M. Zarea and N. Sandler, Phys. Rev. Lett. Dec (2007) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A30.00010: Atomic-scale studies of nanometer-sized graphene on semiconducting surfaces. Justin Koepke, Kyle Ritter, Kevin He, Joseph Lyding We have performed atomic level studies of graphene on semiconducting surfaces using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) [1]. By mechanically exfoliating graphite and using an in-situ dry contact transfer technique [2], we observe predominantly single and double layers of atomically clean graphene with lateral dimensions of 2-20 nm. Room temperature scanning tunneling spectroscopy measurements of the 2-10 nm monolayer pieces display a size-dependent energy gap ranging from 0.1-1 eV, while monolayers with lateral dimensions of 20 nm exhibit a finite density of states at the Fermi level. [1] K.A. Ritter and J.W. Lyding, Nanotechnology, in press (http://arxiv.org/pdf/0711.0050). [2] P.M. Albrecht and J.W. Lyding, APL 83, 5029 (2003). [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A30.00011: Ab-initio calculation of bonding, charge redistribution and transfer of graphene on amorphous silica Ning Shen, Jorge Sofo We study the effects of an amorphous silica substrate on the electronic structure and electron density of graphene using Density Functional Theory. We observe that the silica substrate transfers charge to the graphene layer and the workfunction of the combined system is lower than that of an isolated graphene sheet. The inhomogeneous charge distribution of the substrate induces an inhomogeneous charge redistribution on the graphene layer, which is experimentally observed as electron and hole puddles. The binding energy between one graphene layer and the substrate is weak and reveals no sign of chemical bonding. This can also be inferred from a rigid band shift observed in the system. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A30.00012: Electron states of mono- and bilayer graphene on SiC probed by STM Fran\c{c}ois Varchon, Pierre Mallet, C\'ecile Naud, Claire Berger, Laurence Magaud, Jean-Yves Veuillen We present a scanning-tunneling microscopy (STM) study of a gently graphitized 6H-SiC(0001) surface in ultrahigh vaccum [1]. From an analysis of atomic scale images, we identify two different kinds of atomic scale contrasts, which we attribute to mono- and bilayer (or trilayer) graphene capping a C-rich interface. At any temperature, both terraces show quantum interferences generated by point defects. Such interferences are a fingerprint of pi-like states close to the Fermi level. We conclude that the metallic states of the first graphene layer are almost unperturbed by the underlying interface, in agreement with recent ab initio studies [2] and photoemission experiments [3]. However, a significant density of interface states is detected close to the Fermi level in the C-rich interface. [1] P. Mallet et al., Phys. Rev. B 76, 041403(R) (2007) [2] F. Varchon et al., Phys. Rev. Lett. 99, 126805 (2007) [3] A. Bostwick et al., Nat. Phys. 3, 36 (2007) [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A30.00013: Magnetism in graphene nanoislands and nanovoids Juan Jose Palacios, Joaquin Fernandez-Rossier, Luis Brey The rules to predict the magnetic state of both graphene nanoislands and nanovoids in otherwise perfect graphene systems are presented. We discuss how the shape of the island or void, the associated imbalance in the number of atoms belonging to the two graphene sublattices, the existence of zero-energy states, and the total and local magnetic moment are intimately related. We consider electronic interactions both in a mean-field approximation of the one-orbital Hubbard model and with density functional calculations. The magnetic properties of nanometer-sized graphene structures with triangular and hexagonal shapes terminated by zigzag edges happen to be drastically different[1]. In the case of voids in semiconductor ribbons, we study the magnetism associated to a single void and the magnetic interactions developed between them[2]. [1] J. Fern\'andez-Rossier and J. J. Palacios, Phys. Rev. Lett. {\bf 99}, 177204 (2007). [2] J. J. Palacios,J. Fern\'andez-Rossier, L. Brey, in preparation. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A30.00014: The electronic structure of graphene layers on SiO$_{2}$ substrate Yong-Ju Kang, Joongoo Kang, Kee Joo Chang Graphene is a single layer of carbon atoms packed in a honeycomb lattice, and its quasiparticles behave like massless Dirac fermions. Since graphene is usually supported and deposited on dielectric materials such as SiO$_{2}$ and SiC, interactions between graphene and substrate atoms can modify the electronic structure of graphene. In this work we study the structural and electronic properties of a few graphene layers on SiO$_{2}$ surfaces through first-principles calculations within the local-density-functional approximation. We examine interactions between graphene layers and Si- and O- terminated surfaces of $\alpha $-quartz and the substrate-induced doping effect. For a single graphene layer, we find that graphene strongly interacts with the O-terminated surface. A charge transfer occurs from the graphene to the surface O atoms, leading to the p-type doping. For a bilayer graphene in AB stacking, the charge transfer mostly occurs for the graphene layer right on the substrate, resulting in an asymmetric distribution of electron charges between two graphene layers and thus a gap opening at the Dirac point. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A30.00015: Pair condensation in electron-hole graphene bi-layer Rafi Bistritzer, Hongki Min, Jung-Jung Su, Allan H. MacDonald Using self consistent Hartree Fock calculations we show that an electron-hole graphene bi-layer exhibits spontaneous interlayer coherence when the distance between layers is sufficiently small. We describe the unique transport properties of the emergent dipolar superfluid and relate them to the nature of the underlying massless Dirac particles. [Preview Abstract] |
Session A31: Focus Session: Oxide Interfaces I
Sponsoring Units: DMP GMAGChair: D. Schlom, Pennsylvania State University
Room: Morial Convention Center 223
Monday, March 10, 2008 8:00AM - 8:36AM |
A31.00001: Electronic and magnetic effects at complex oxide interfaces Invited Speaker: Remarkable electronic and magnetic behavior results from electronic reconstruction at interfaces in complex oxides. In recent years this has been strikingly shown especially for interfaces to SrTiO$_{3}$, for example with LaTiO$_{3}$ and LaAlO$_{3}$. Depending on the atomic arrangement of these interfaces, they can become conducting, remain insulating, show magnetic activity and even become superconducting. An important aspect concerns the role of oxygen vacancies, it is not completely clear yet to what extent they are decisive for some of these properties. In this talk I will discuss recent experimental and theoretical studies on such interfaces. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A31.00002: Magnetic anisotropy modulation in Fe$_3$O$_4$/BaTiO$_3$(100) epitaxial structures C.A.F. Vaz, J. Hoffman, A. Posadas, C.H. Ahn Renewed interest in `classical' ferroic materials has been accompanied by the study of a new class of multiferroic composite materials, based on magnetic and dielectric multilayer structures. One motivation is the search for materials that allow independent control of both the magnetic and electric properties. In this context, we investigate the modulation of the magnetic anisotropy of a 10 nm magnetite (Fe$_3$O$_4$) film grown epitaxially by off-axis magnetron sputtering on (001)BaTiO$_3$ (BTO). SQUID and magnetoresistance measurements as a function of temperature show a series of discontinuities that are attributed to changes in the strain of the magnetite film via elastic coupling with the substrate, as the latter undergoes a series of structural phase transitions. Magnetic hysteresis loops carried out at temperatures above and below each transformation in the BTO elucidate the variation of the effective anisotropy of the Fe$_3$O$_4$ film. The possibility of using the piezoelectric response of BTO to modulate the magnetic anisotropy of magnetite films is discussed. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A31.00003: Epitaxial strain-mediated spin-state transitions: can we switch off magnetism? James Rondinelli, Nicola Spaldin We use first-principles density functional theory calculations to explore spin-state transitions in epitaxially strained LaCoO$_3$. While high-spin to low-spin state transitions in minerals are common in geophysics, where pressures can reach over 200~GPa, we explore whether heteroepitaxial strain can achieve similar transitions with moderate strain in thin films. LaCoO$_3$ is known to undergo a low-spin ($S$=0, t$_{2g}^6e_g^0$) to intermediate-spin ($S$=1, t$_{2g}^5e_g^1$) or high-spin ($S$=2, t$_{2g}^4e_g^2$) state transition with increasing temperature, and thus makes it a promising candidate material for strain-mediated spin transitions. Here we discuss the physics of the low-spin transition and changes in the electronic structure of LaCoO$_3$, most notably, the metal-insulator transition that accompanies the spin-state transitions with epitaxial strain. As thin film growth techniques continue to reach atomic-level precision, we suggest this is another approach for controlling magnetism in complex oxide heterostructures. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A31.00004: Density functional study of ferromagnetic ferroelectric LaMnO$_{3}$/BaTiO$_{3}$ superlattice Leonard Kleinman, Bhagawan Sahu, Adrian Ciucivara Using the GGA + $U$ density functional, we have calculated the lattice constants, atomic positions, magnetization, and ferroelectric polarization of a (LaMnO$_{3})_{4.5}$(BaTiO$_{3})_{4.5}$ superlattice containing five LaO and TiO$_{2}$ planes and four MnO$_{2}$ and BaO planes. Although LaMnO$_{3}$ is antiferromagnetic, it is ferromagnetic in the superlattice. An approximation to the ferroelectric polarization, obtained from a comparison of superlattice and bulk crystal atomic displacements and unit cell volumes, is found to agree reasonably well with the polarization obtained from a Berry phase calculation. The electric polarization points along the longer in-plane lattice vector, while the atomic spins, after the spin-orbit interaction and spin noncollinearity are included, all point in directions close to that lattice vector. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A31.00005: Electron Localization and Interface Magnetism in CaRuO$_3$/CaMnO$_3$ Superlattices J.W. Freeland, J. Chakhalian, J.J. Kavich, B. Keimer, H.N. Lee We present a study of interface physics in superlattices composed of a G type anti-ferromagnetic insulator (CaMnO$_3$) and a paramagnetic metal (CaRuO$_3$). Using laser MBE, ultra-thin superlattices were grown with the structure [CaRuO$_3$(N u.c)/CaMnO$_3$(10 u.c.)]x6 on LaAlO$_3$(100) substrates. Polarized x-ray probes clearly show a large Mn magnetic moment, which is attributed to a canted anti-ferromagnetic state at the interface. The electronic state of the system was probed by current-in-plane transport measurements. Due to the large resistivity of CMO, magneto-transport measurements probe primarily the behavior of the carriers in the CRO layer, which show a non-bulk like insulating behavior that is attributed to disorder induced localization from interface scattering. Field dependence shows a large negative magneto-resistance (MR), which results from strong scattering of the carriers by Mn spins at the interface since the MR drops with increasing CRO layer thickness. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. JC is funded by U.S. DOD-ARO under Contract No. 0402-17291. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A31.00006: Exploring artificial layered heterostructures of LaM'O3/LaM''O3 (M'M''= NiCr, FeCr and NiV). J. Liu, M. Kareev, J.W. Freeland, A. Kareev, H.N. Lee, J. Chakhalian Digital synthesis of atomically sharp interfaces between strongly correlated electron systems can provide a template to build completely new materials. Here we present our results on magnetism and electronic structure in LaM'O3/LaM''O3 (M'M''= NiCr, FeCr and NiV) superlattices by using polarized X-ray spectroscopies. Using laser MBE, the (111) and (100) oriented ultra-thin superlattices were grown with alternating layer thicknesses of 1 unit cell. In the bulk, LaMO3 (M=Cr,Fe,V) are antiferromagnetic insulators while LaNiO3 is a paramagnetic metal. The evolution of element specific magnetism and charge at the interface of LFO/LCO, LNO/LVO and LNO/LCO superlattices with temperature and an applied magnetic field will be discussed in detail. The superlattice results will be contrasted to the bulk magnetic properties of the constituent layers. The work has been supported by U.S. DOD-ARO under Contract No. 0402-17291. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A31.00007: Strain effect on Magnetism at the Manganite Interfaces: SrMnO$_3$/LaMnO$_3$ Birabar Nanda, Sashi Satpathy Recently it has been shown that new magnetic and electronic phases can be produced by varying the strain condition at the manganite interfaces[1]. From the density-functional studies of the electronic structure at the interface of SrMnO$_3$ and LaMnO$_3$ we show that the epitaxial strain, which enforces a tetragonal distortion, splits the itinerant interface Mn-e$_g$ states to x$^2$-y$^2$ and 3z$^2$-1 states. If the strain is tensile in the plane the x$^2$-y$^2$ orbital becomes more occupied, enhancing thereby the ferromagnetic double exchange which overcomes the antiferromagnetic super exchange between the core t$_{2g}$ states to produce a net in-plane ferromagnetic interaction. Due to the lower occupancy of the 3z$^2$-1 orbitals, the super exchange supercedes the double exchange to produce out-of-plane antiferromagnetic ordering. For in-plane compressive strain higher occupancy of 3z$^2$-1 orbital results in the out-of-plane ferromagnetic ordering while in-plane ordering remains antiferromagnetic. Without any epitaxial strain, the itinerant x$^2$-y$^2$ and 3z$^2$-1 states are more or less equally occupied and ferromagnetic ordering prevails both in-plane and out-of-plane. While for the tensile strain we find the heterostrutcure to be metallic, for the compressive strain an insulating phase is obtained if the strain is sufficiently large. This work was supported by DOE-DE-FG02-00ER45818. \\ 1. H. Yamada et al. Appl. Phys. Lett. {\bf 89}, 052506 (2006) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A31.00008: Asymmetric and modulated magnetic profiles in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices S.J. May, S.G.E. te Velthuis, M.R. Fitzsimmons, A.B. Shah, J.M. Zuo, X. Zhai, J.N. Eckstein, S.D. Bader, A. Bhattacharya We have determined the magnetic depth profile of MBE-grown ferromagnetic (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices, where $n$ is nominally equal to 3 and 5. Polarized neutron reflectivity measurements reveal the existence of a modulated magnetic structure that repeats with the superlattice period in both samples. For $n$=5, a moment of $\sim$2.6 $\mu_B$/Mn is measured in the LaMnO$_3$ (LMO) layer, while the moment in the middle of the SrMnO$_3$ (SMO) layer is negligible. The magnetization at the interfaces is found to be asymmetric with an enhanced moment residing at the LMO/SMO interfaces but not at the SMO/LMO interfaces. The origin of the magnetic asymmetry at the interfaces is elucidated from comparison with the structural properties determined by x-ray reflectivity and transmission electron microscopy. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A31.00009: Magnetic depth profiles of complex oxide F/S/F trilayers. C. Visani, Z. Sefrioui, C. Leon, J. Santamaria, S.G.E. te Velthuis, A. Hoffmann, Norbert M. Nemes, M. Garcia-Hernandez, M.R. Fitzsimmons, B.J. Kirby The origin of the large magnetoresitance in epitaxial F/S/F trilayers composed of highly spin polarized ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and high-T$_{c}$ superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) is investigated by characterizing the magnetic structure. Polarized neutron reflectometry experiments have determined the detailed magnetization depth profiles in trilayers with varying YBCO layer thicknesses. In addition to inhomogeneous magnetization profiles, rotation of the magnetization during the magnetization reversal for the films with thick ($\ge $ 17.7 nm) YBCO layers has been observed. The results are consistent with the presence of an (in plane) easy-axis tilted away from the (100) direction. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A31.00010: Origins of Anomalous Ferromagnetism in F/AF LCMO Multilayers B. J. Kirby, S. M. Watson, M. Kareev, J. Chakhalian Unexpected behavior can emerge from magneto-electronic interactions at the interface between two different strongly correlated electron systems. Exchange bias - giving a ferromagnet (F) a preferred direction via coupling with an antiferromagnet (AF) - is a phenomenon of great fundamental and applied research interest. Both topics are pertinent in the case of the interface between F and AF La[1-x]Ca[x]MnO3 (LCMO) layers. Depending on x, LCMO can be F (x = 1/3 Ca) or AF (x = 2/3 Ca), and exchange bias has been reported in superlattices consisting of such layers. Surprisingly SQUID magnetometry has shown that the saturation moment of such a structure increases as the nominally AF layer thickness is increased [1]. This has been attributed to electronic effects that cause F order to extend into the nominally AF layer. However, the location of the extra moment cannot be determined with bulk magnetometry techniques. Thus, we have used polarized neutron and x-ray reflectometry to measure the magnetic and structural depth profiles in an exchange biased x=1/3 LCMO / x=2/3 LCMO bilayer. Our results suggest that the magnetic profile extends beyond the x = 1/3 layer, implying that some F order indeed exists in the nominally AF x = 2/3 layer. [1] G. Campillo, et al., J. Appl. Phys. 97, 10K104 (2005). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A31.00011: Ferroelectric field effect modulation of the magnetic properties of colossal magnetoresistive La$_{1-x}$Sr$_x$MnO$_3$ Jason Hoffman, Hajo Molegraaf, Xia Hong, Jean-Marc Triscone, C.H. Ahn In this work, we have tuned the magnetic properties of the colossal magnetoresistive oxide La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) using a ferroelectric field effect approach. Epitaxial LSMO thin films and ferroelectric Pb(Zr,Ti)O$_3$ (PZT) / LSMO heterostructures were fabricated using off-axis RF magnetron sputtering. LSMO films with high crystalline quality and atomically smooth surfaces have been achieved. Using the magneto-optical Kerr effect (MOKE), we measured a shift in the Curie temperature (T$_{\mathrm{c}}$) of LSMO upon switching the polarization direction of PZT, in agreement with the modulation of T$_{\mathrm{c}}$ found from magnetotransport measurements. Unlike the traditional chemical doping approach, the ferroelectric field effect approach allows one to control the magnetism in the system reversibly, without introducing additional lattice disorder/distortion. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A31.00012: Structure and properties of CaMnO$_3$/SrMnO$_3$/BaMnO$_3$ superlattices from first principles Shen Li, Seongshik Oh, Karin Rabe Previous theoretical and experimental studies have shown that three-component, or ``tri-color'' superlattices can exhibit intrinsic electric polarization due to inversion-symmetry breaking in the layer sequence. In ferromagnetic inversion-symmetry-breaking superlattices, controlled symmetry lowering is similarly expected to lead to interesting new and tunable properties. Here, we present results of first-principles density-functional-theory calculations for short-period CaMnO$_3$/SrMnO$_3$/BaMnO$_3$ superlattices, using VASP. The ground state structure, magnetic ordering, polarization and dielectric response will be presented. The role of epitaxial strain in the individual layers and the role of layer sequence will be explored. Connections to experimental studies and prospects for future work will be discussed. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A31.00013: Phase separation at the La$_{1-x}$Sr$_{x}$CoO$_{3}$ / SrTiO$_{3}$ (001) interface from thickness and doping dependent magnetotransport M.A. Torija, M. Sharma, C. Leighton Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) has received considerable attention with regard to nanoscale magnetic phase separation. Fabrication of epitaxial films provides a means to study this phase separation under dimensional confinement and at interfaces. Moreover, the characteristic intercluster ``GMR'' effect observed in the phase-separated state of this material provides a simple means to probe phase separation even in very thin films. We have found that even at x = 0.5 (a homogeneous ferromagnetic metal in bulk), sufficiently thin films ($<$ 60 {\AA}) grown on SrTiO$_{3}$ (001) show a crossover to a reduced moment insulating phase, with the characteristic intercluster MR effect, i.e. phase separation occurs. By measuring the thickness dependence of the magnetotransport as a function of doping we have assembled a three-dimensional phase diagram in temperature-doping-thickness space. The interfacial phase-separated region increases dramatically as x is decreased (to over 250 {\AA} at x = 0.2) and the non-ferromagnetic phase boundary is approached. The increased prominence of phase separation as proximity to the non-ferromagnetic phase is increased provides a strong hint to the origin of this interfacial phase separation. [Preview Abstract] |
Session A32: Disordered Magnetic Materials
Sponsoring Units: GMAGChair: Leonard Spinu, University of New Orleans
Room: Morial Convention Center 225
Monday, March 10, 2008 8:00AM - 8:12AM |
A32.00001: Transport in strongly disordered classical spin chains Vadim Oganesyan, Arijeet Pal, David Huse We present a numerical study of diffusion of energy at high temperature through strongly disordered arrays of interacting classicals spins with Hamiltonian dynamics. We find that quenched randomness strongly supresses transport, with diffusion constant apparently becoming smaller than any power of spin-spin interaction rescaled by randomness. We have looked for but not found signs of a classical many-body localization transition at any finite strength of disorder. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A32.00002: Ferromagnetism in Melt-spun Gd$_{0.946}$Fe$_{0.054}$ Paul Shand, Nicholas Jensen, Justin Bohnet, Jared Goertzen, Jeffrey Shield, David Schmitter, Geoffrey Shelburne, Diandra Leslie-Pelecky The ac susceptibility and dc magnetization at various temperatures have been measured for a melt-spun Gd$_{0.946}$ Fe$_{0.054}$ alloy. The grain size was $\approx $100 nm. A sharp paramagnetic-to-ferromagnetic transition was observed at a temperature close to that of pure Gd. Effective critical exponents and the critical temperature $T_{C}$ were extracted by using modified Arrott plots and Kouvel-Fisher analysis. The values obtained were \textit{$\beta $}$_{eff}$ = 0.398 $\pm $ 0.004, \textit{$\gamma $}$_{ eff}$ = 1.24 $\pm $ 0.02, \textit{$\delta $}$_{ eff}$ = 3.83 $\pm $ 0.05, and $T_{C}$ = 290.25 $\pm $ 0.17 K. These exponent values do not satisfy the Widom scaling relation \textit{$\beta \delta $ }= (\textit{$\beta $}+\textit{$\gamma $}). The \textit{$\beta $}$_{eff}$ and \textit{$\gamma $}$_{eff}$ values for ms-Gd$_{0.946}$ Fe$_{0.054}$ are similar to those obtained for pure Gd in the same temperature interval around $T_{C}$. This is in consonance with x-ray microanalysis measurements indicating that the grains are nearly pure Gd. The lower-than-expected value of \textit{$\delta $}$_{eff}$\textit{${\rm g}$ }may be due to the effect of increased anisotropy due to the presence of Fe in the grain-boundary regions. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A32.00003: Observation of spin-wave mediated Altshuler-Aronov and weak localization corrections to the conductivity in thin films of gadolinium Rajiv Misra, Arthur F. Hebard, Khandker Muttalib, Peter Woelfle We present a study of quantum corrections to the conductivity tensor of thin ferromagnetic gadolinium films. Using the sheet resistance as a measure of disorder, \textit{in situ} magnetotransport studies were performed on a series of gadolinium films deposited onto sapphire substrates having sheet resistance $R_{0 }\equiv $~$R_{xx}$ (5K) varying over the range 428~$\Omega $ ($\sim $135{\AA}) to 4011 $\Omega $ ($\sim $35 {\AA}). For temperatures $T$~$<$~30 K and $R_{0} \quad <$ 4011 $\Omega $, we observe the simultaneous presence of two types of quantum correction to the Drude conductivity, $\sigma =\sigma _{Drude} +\Delta \sigma _{SpinWaveMediated} +\Delta \sigma _{WL} $. The characteristic feature of the first correction is an approximately linear increase with temperature of conductivity, and we attribute this as a spin-wave mediated Altshuler-Aronov correction to conductivity. The second correction to the Drude conductivity comes from weak localization, with a characteristic logarithmic temperature dependence of conductivity with a prefactor ${e^2} \mathord{\left/ {\vphantom {{e^2} {2\pi ^2\hbar }}} \right. \kern-\nulldelimiterspace} {2\pi ^2\hbar }$ in 2D. We observe a breakdown of this behavior at a sheet resistance $R_{0}$ =4011 $\Omega $, which is very close to the quantum of resistance, $\hbar \mathord{\left/ {\vphantom {\hbar {e^2\approx 4100\Omega }}} \right. \kern-\nulldelimiterspace} {e^2\approx 4100\Omega }$. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A32.00004: Monte Carlo study of the three-dimensional Coulomb glass Brigitte Surer, Helmut G. Katzgraber, Gergely T. Zimanyi, Brandon A. Allgood The memory and hysteresis effects found in strongly-disordered electron systems can be explained by the existence of a glassy phase, the Coulomb glass. Efros and Shklovskii have predicted the emergence of a soft Coulomb gap, resulting from the long-range interactions between the localized electrons. However, the relationship between the soft Coulomb gap in the density of states and the electron's glassy behavior has been a long-standing unresolved question. Only recently has it been surmised within the framework of a mean field theory that the disordered electron system undergoes a replica symmetry breaking transition at a finite temperature, similar to the Sherrington-Kirkpatrick model of spin glasses. Because it is not clear, however, whether the transition persists beyond the mean-field approximation, we study in detail the critical behavior and the shape of the Coulomb gap in three space dimensions using Monte Carlo methods. Furthermore, we compare our results for the (random-energy) Coulomb glass model to previous results on a random lattice version of the model. Since these models possess different symmetries, the equivalence of the phase diagrams is far from obvious, contrary to previous claims. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A32.00005: Reentrant spin-glass behavior and enhanced Curie temperature in epitaxial MnSi Eric Karhu, Samer Kahwaji, Ted Monchesky, Krista Raffel, Michael Robertson, Christian Maunders We grew single crystal MnSi(111) thin films on Si(111) substrates by molecular beam epitaxy. The 3{\%} lattice mismatch created an in-plane tensile strain of $\varepsilon _{\vert \vert }$ = 0.005 $\pm $ 0.001, as measured by transmission electron microscopy, and induced an out-of-plane compressive strain $\varepsilon _{\bot }$= -0.0033 $\pm $ 0.0001, as determined by x-ray diffraction. The MnSi(111) films displayed two magnetic phases. The first transition from a paramagnetic phase to a phase with long range magnetic order occurs with an enhanced Curie temperature T$_{C}$ = 40 K as compared to bulk MnSi, which develops helical magnetic structure below T$_{C}$ = 29.5~K. This increase in T$_{C}$ can be explained by an in-plane strain. A second phase transition to spin glass, below T$_{f}$ = 35 K, maybe due to geometric frustration created by the compressive out-of-plain strain. We propose a relationship between the reentrant spin-glass behavior and the partial magnetic order reported for bulk MnSi under pressure. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A32.00006: Dynamical behavior of spin clusters in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Robert Smith, Michael Hoch, William Moulton, Phillip Kuhns, Gregory Boebinger, Arneil Reyes, John Mitchell Previous work has provided evidence for magnetic glassy behavior in the hole-doped cobaltite system La$_{1-x}$Sr$_{x}$CoO$_{3 }$(LSCO). Models proposed to describe the interesting and unusual magneto-transport properties of LSCO involve hole-rich clusters in a hole-poor matrix. The glassy properties, which are not well understood, have variously been interpreted in terms of spin glass and cluster glass components. The present $^{139}$La NMR spectral lineshape measurements on single crystal LSCO, that map the hyperfine field distribution in the x$-T$ plane, confirm the presence of magnetic clusters, identified as spin polarons, and provide a phase inhomogeneity diagram. NMR relaxation rates have been used to probe the dynamical behavior of the system at the nanoscale level in macroscopically insulating and metallic samples as a function of temperature in the range 4-280 K. For x less than the metal-insulator critical concentration x$_{C }$= 0.17 evidence has been obtained for two classes of glassy component with different characteristic correlation time distributions and freezing temperatures. The magnetic glass properties persist above x$_{C}$. A spin polaron model is used to explain the results. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A32.00007: Local field distributions in spin glasses David Sherrington, Helmut G. Katzgraber, Stefan Boettcher Numerical results for the local field distributions of a family of Ising spin-glass models are presented. In particular, the Edwards-Anderson model in dimensions two, three, and four is considered, as well as spin glasses with long-range power-law-modulated interactions that interpolate between a nearest-neighbor Edwards-Anderson system in one dimension and the infinite-range Sherrington-Kirkpatrick model. Remarkably, the local field distributions only depend weakly on the range of the interactions and the dimensionality and show strong similarities except for near zero local field. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A32.00008: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 9:36AM - 9:48AM |
A32.00009: The Antiferromagnetic SO(3) Heisenberg Quantum Spin-Glass with Short Range Interaction Eduardo Marino, Carlos Conceicao We study the quenched disordered magnetic system which is obtained from the 2D SO(3) quantum Heisenberg model, with nearest neighbors interaction, by taking the random values of the exchange couplings as given by a Gaussian probability distribution centered in a value of the coupling that corresponds to anti-ferromagnetic order. Using coherent states, we map this system onto a generalization of the SO(3) nonlinear sigma model, containing different flavors, which correspond to the replicas and a quartic interaction. We then integrate over the transverse components and perform a mean-field calculation of the free energy density in the limit of zero replicas. The phase diagram of the system is then obtained and shows a critical curve, starting at a quantum critical point at T=0 separating a paramagnetic from a spin-glass phase. The stability of the phases is demonstrated by an analysis of the Hessian matrix of the free energy. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A32.00010: On the ordering of Ising spin glasses in a field Helmut G. Katzgraber, Thomas Jorg, Florent Krzakala We study the existence of a spin-glass phase in a magnetic field in three space dimensions using a novel approach where the Monte Carlo simulations are performed along a nontrivial path in the magnetic field--temperature plane which must cross any putative de Almeida-Thouless line. The method is first tested on the mean-field version of the Edwards-Anderson Ising spin glass on a Bethe lattice where we compute analytically the instability line that separates the spin glass from the paramagnetic state. While the de Almeida-Thouless line is clearly reproduced by our simulations on the mean-field Bethe lattice, no such instability line can be found numerically for the short-range three-dimensional model. We thus conclude that there is no such instability line for three-dimensional short-range Ising spin glasses. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A32.00011: Spin glass of a diluted Ising dipolar system Ka-Ming Tam, Michel Gingras The diluted dipolar Ising system has been regarded as a standard example which exhibits spin glass properties. Recent studies have challenged the existence of spin glass phase transition in one of the materials in this category, $LiHo_{x}Y_{1-x}F_{4}$. Using Monte Carlo simulations, we calculate various quantities to address the current controversy of a possible spin glass phase transition in this material. Beside the conventional method to locate the spin glass transition by observing the crossing of Binder ratios of magnetization moments, another crucial probe for the nature of spin glass, order parameter fluctuations, is studied via the so-called fluctuation sensitive parameters. Crossing is observed in the Binder ratio of overlap order parameter, and non-trivial structures of overlap order parameter are obtained at low temperature. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A32.00012: Nonlinear and ac Susceptibility of the Dilute Ising Magnet LiHo$_x$Y$_{1-x}$F$_4$ Jeffrey Quilliam, Shuchao Meng, Chas Mugford, Jan Kycia Recent work has called into question the existence of a spin glass transition in the dilute dipolar Ising magnet LiHo$_x$Y$_{1-x}$F$_4$ [1]. Other work has suggested that there is an exotic spin liquid phase found at a Ho concentration of $x = 0.045$ [2]. In order to carefully study the dynamics of this system, we have put together a SQUID magnetometer which allows for measurements of ac susceptibility and nonlinear susceptibility over a large frequency range. We present results from measurements on single crystals of LiHo$_x$Y$_{1-x}$F$_4$, particularly on an $x = 0.045$ sample, in an attempt to either reproduce the exotic ``anti-glass'' physics that was previously observed or to detect a spin glass transition. [1] P. E. Jonnson et al. PRL 98, 256403 (2007) [2] S. Ghosh et al. Science 296, 2195 (2002) [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A32.00013: Low-temperature properties of the dilute dipolar magnet LiHo$_x$Y$_{1-x}$F$_4$ Anders Biltmo, Patrik Henelius The phase diagram of the rare-earth compound LiHo$_x$Y$_{1-x}$F$_4$ is considered as a function of dilution. At low temperatures the material is a good realization of a dipolar Ising magnet. The net magnetization vanishes at high dilution and the glassy behavior that ensues has several interesting features, including a proposed anti-glass phase and anomalous peaks in the specific heat. In this talk we will show results obtained with Monte Carlo techniques and compare them with recent experimental data. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A32.00014: Quantum and Classical Glass Transitions in $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$ D. M. Silevitch, C. Ancona-Torres, G. Aeppli, T. F. Rosenbaum When performed in the proper low field, low frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in transverse field, $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$, prove the existence of a spin glass transition in both the classical and quantum limits for x = 0.167 and 0.198. The classical behavior tracks for the two concentrations, but the quantum glasses differ because of the competing effects of entangled spins and local random fields. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A32.00015: Study of the dipolar coupled Ising system $\mathrm{LiHo_xY_{1-x}F_4}$ using muon spin relaxation. Jose Rodriguez, A.A. Aczel, S.R. Dunsiger, G.J. MacDougall, G.M. Luke, P.L. Russo, A.T. Savici, Y.J. Uemura, C.R. Wiebe In $\mathrm{LiHo_xY_{1-x}F_4}$ magnetic Ho ions have an Ising character and interact mainly through the magnetic dipolar interaction. For $x=1$ the ground state of the system is ferromagnetic and as $x$ decreases a disordered ground state with glassy properties arises. If $x$ is decreased further, the system enters a phase sometimes referred to as ``antiglass". Both the non-canonical glassy state and the not yet understood ``antiglass", have never been systematically studied using a microscopic probe. We performed muon spin relaxation measurements in five samples (x=0.018, 0.045, 0.08, 0.12 and 0.25) which span these disordered phases. In this talk we will show from the microscopic point of view, how does the glassy state manifests itself as well as how does the evolution from the glass to the ``antiglass" occurs. [Preview Abstract] |
Session A33: Focus Session: Spin Dependent Phenomena in Semiconductors: I
Sponsoring Units: DMP GMAG FIAPChair: Nitin Samarth, Pennsylvania State University
Room: Morial Convention Center 224
Monday, March 10, 2008 8:00AM - 8:12AM |
A33.00001: Cross-sectional scanning tunneling microscopy of Ga$_{1-x}$Mn$_{x}$As/GaAs Heterostructures Pedram Roushan, Anthony Richardella, Shawn Mack, David Awschalom, Ali Yazdani We have used a cryogenic scanning tunneling microscope (STM) to perform cross-sectional imaging studies of GaMnAs heterostructures. The heterostructures, consisting of a p-type buffer followed by a 3{\%} Mn doped layer, were grown on a n-type GaAs substrate by molecular-beam epitaxy and cleaved in situ for STM measurements. The topographic measurements on the GaMnAs layer showed a variety of long range electronic structure modulations on the order of a few nm due to high level of disorder and compensation. Combining bias-dependent imaging and spectroscopy, we have used the STM to identify electronic features due to Mn dopants and other defects. In particular, we find that Mn dopants on the top most layer act as deep acceptors and exhibits similar topographic and spectroscopic features as our previous work [1] on Mn adatoms substituted into GaAs using STM manipulation techniques. [1] D. Kitchen, A. Richardella, J-M. Tang, M. Flatte, A. Yazdani, Nature 442, 436--439 (2006) [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A33.00002: Spectroscopic Mapping of Electronic States near Fermi Energy in GaMnAs Anthony Richardella, Pedram Roushan, Shawn Mack, David Awschalom, Ali Yazdani We have used atomically resolved spectroscopic mapping with the scanning tunneling microscope (STM) to probe the spatial structure of electronic states in GaMnAs/GaAs heterostructures. Mapping the electronic states over a wide range of energies across the heterostructure, we find significant differences from the p-type buffer layer to the Mn-doped layer. The features include gap narrowing and in gap resonances, some of which are due to the bulk electronic structure of GaMnAs. We focus on these bulk states, in particular a broad state that is observed near the valance band above the Fermi energy. We analyze the spatial distribution of these states in conjunction with our observations of disorder in the sample and variations in the electrostatic potential landscape due to compensation. Finally, we discuss the implications of our results to the questions of localization, band tails and whether highly doped GaMnAs has an impurity band separated from or merged with the host valance band. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A33.00003: STM studies of spin-spin interactions between Mn acceptors in p-type GaAs Donghun Lee, David R. Daughton, Jay A. Gupta We use a custom STM operating in a cryogenic, ultrahigh vacuum environment to study spin-spin interactions in semiconductors at the single-impurity level. By applying a voltage pulse with the STM tip, single magnetic impurities (e.g. Mn) can be substituted for Ga atoms in the first layer of the GaAs(110) surface. It was previously found that pairs of Mn acceptors exhibit an exchange splitting which depends on their separation and crystal orientation [2]. We are developing a capability for spin-polarized STM to better study long range magnetic ordering between pairs and larger clusters of Mn acceptors. To characterize the magnetic orientation of our STM tips, we have prepared samples such as Co/Cu(111) which exhibits out-of-plane magnetization, and Cr(001), which is an antiferromagnet with in-plane magnetic contrast between alternating terraces. http://www.physics.ohio-state.edu/$\sim $jgupta [2] D. Kitchen et al., Nature 442, 436-439 (2006) [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A33.00004: Magnetic Memory Effects in Coherent Magnetization Dynamics of GaMnAs: From Non-equilibrium to Thermal Regime Ingrid Cotoros, Jigang Wang, Xinyu Liu, Jacek Furdyna, Jaroslav Chovan, Ilias Perakis, Daniel Chemla III-Mn-V heterostructures exhibit rich spin memory effects and their magnetic properties show strong responses to external stimuli (light, electrical gate and current) via carrier density tuning. Here we report on the coherent magnetization dynamics due to laser excitation of transient carriers in GaMnAs, distinctly depending on the initial magnetic state the system is prepared in. We identify two distinct temporal regimes that reveal a complex scenario of spin reorientation, marked by the transition from a highly non-equilibrium, non-thermal, transient carrier-mediated regime ($<$ 300 femtosecond), to a thermal, lattice-heating regime (on the picosecond time scale). The ultrafast, sub-picosecond response can be used as footing for ultrafast optical detection of magnetic memory states. This observed femtosecond cooperative magnetic phenomenon may represent an as-yet-undiscovered universal principle in all carrier-mediated ferromagnetic materials, offering potential perspectives for terahertz (10$^{12}$ Hz) speed ``spintronic'' devices and functional systems. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A33.00005: Epitaxial engineering of ferromagnetism in (GaAs)/(Ga,Mn)As digital superlattices. M.J. Wilson, G. Xiang, B.L. Sheu, P. Schiffer, N. Samarth Recent theory predicts novel pathways for the epitaxial engineering of ferromagnetism in GaAs/MnAs digital superlattices grown along different crystalline directions [Franceschetti et al, PRL 97, 047202 (2006)]. This has motivated us to systematically study GaAs/(Ga,Mn)As digital superlattices grown along the [001], [110], [201] and [311] directions. We have characterized these samples using SQUID magnetometry, magnetotransport,TEM and SIMS. We observe clear trends in the Curie temperature that are correlated with the epitaxy direction and the GaAs spacer layer thickness. Our results suggest that -- for a given Mn concentration -- the Curie temperature of (Ga,Mn)As may depend on the geometrical arrangement of the Mn ions. We also explore alternative explanations, such as an orientation dependence of the Mn incorporation rate and defect formation energies. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A33.00006: Stoichiometric growth of high Curie temperature heavily-alloyed GaMnAs S. Mack, R.C. Myers, J.T. Heron, A.C. Gossard, D.D. Awschalom Previous work indicates that single-phase, high-Mn incorporation ($\ge $9{\%}) in GaAs can be obtained via MBE at very low substrate temperatures with film thicknesses on the order of a few nanometers. Here we present the properties of smooth, single-crystalline GaMnAs samples with Mn densities between 9 and 18{\%} and continuously tuned arsenic stoichiometry using a combinatorial, non-rotated growth method.[1] Systematic, reproducible thick films (100 nm) display optimal magnetic, electronic, and structural properties in a narrow band of As:Ga flux ratios at the stoichiometric condition, where the Curie temperature is maximum. Post-growth annealing increases the Curie temperature while lowering the lattice constant, indicating that Mn interstitials are the dominant compensating defect in high-Mn containing GaMnAs. Curie temperatures from many samples grown with varying conditions all reach a maximum near the previously reported maximum ($\sim$165K). \newline [1] R. C. Myers\textit{ et al.}, Physical Review B 74, 9 (2006). [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A33.00007: Ultrafast Coercivity Dynamics in GaMnAs Kimberley Hall, Jeremy Zahn, Samuel March, Xinyu Liu, Jacek Furdyna The hole-mediated ferromagnetism in III-Mn-V diluted magnetic semiconductors opens up a whole host of possibilities for future multifunctional devices. Control over the ferromagnetic properties in these materials through hole density modulation has been demonstrated using electrical gates [1] and CW optical excitation [2], and more recently using femtosecond optical excitation [3,4]. Using time-resolved magneto-optical Kerr Effect spectroscopy, we have measured the magnetization and coercivity dynamics in GaMnAs. Our experiments reveal a subpicosecond ferromagnetic to paramagnetic phase transition followed by coercivity enhancement on longer time scales. Our findings are promising for possible applications in ultrafast, nonthermal magneto-optical recording using diluted magnetic semiconductors. [1] H. Ohno et al., Nature 408, 944 (2000). [2] S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997). [3] J. Wang et al., Phys. Rev. Lett. 95, 167401 (2005). [4] J. Wang et al., Phys. Rev. Lett. 98, 217401 (2007). [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A33.00008: Magnetic domain structure study of a ferromagnetic semiconductor using a home-made low temperature scanning Hall probe microscope Seongsoo Kweon, Alex De Lozanne, Nitin Samarth GaMnAs is a ferromagnetic semiconductor actively studied for basic research and for the possibility of application to spintronic devices. To study the local magnetic properties of this material the magnetic force microscope (MFM) is too invasive (by affecting the domains in the sample) or not sensitive enough (due to the weak magnetization of the GaMnAs). We have therefore built a scanning Hall probe microscope (SHPM) to complement our MFM studies. We use a lock-in amplifier to supply a bias current of 1-10$\mu $A and to measure the Hall voltage. We calibrated this home-made SHPM with a computer hard disk sample. Comparing images of this sample obtained with MFM and SHPM we show that our home-made SHPM is operating well. We observed the domain structure of 30-nm thick Ga$_{0.94}$Mn$_{0.06}$As epilayer grown on a 700nm-thick In$_{0.13}$Ga$_{0.87}$As buffer covering a GaAs substrate. We will study the magnetic domain structure as a function of temperature with varying external magnetic fields. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A33.00009: Si co-doping of GaMnAs: a solution for removing As antisites Y.J. Cho, X. Liu, J.K. Furdyna The primary defects that degrade the ferromagnetism of GaMnAs are believed to be Mn interstitials and As antisites. The former can be reduced by low temperature annealing after growth. However, so far no solution for removing As antisites in GaMnAs has been developed. In this connection we report the effect of Si co-doping on Ga$_{1-x}$Mn$_{x}$As films. For $x <$ 0.05, Si co-doping decreases the Curie temperature compared to undoped GaMnAs. However, at higher Mn concentration Si co-doping has the desirable effect of improving both ferromagnetic and structural properties of GaMnAs. To achieve a high Mn concentration in GaMnAs the growth temperature has to be sufficiently low to prevent the formation of MnAs clusters, but this introduces a high density of As antisites in GaMnAs, thus degrading its structural and ferromagnetic properties. However, when such growth of GaMnAs is accompanied by Si co-doping, x ray diffraction results show that such co-doping removes almost all As antisites in GaMnAs with high Mn concentration. Furthermore, magneto-transport and magnetization measurements show that the ferromagnetic properties of thick Si co-doped Ga$_{1-x}$Mn$_{x}$As (x $>$ 0.1) films are greatly improved compared to samples without Si. This suggests that Si co-doping provides an effective solution for removing As antisites in GaMnAs at high Mn concentrations. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A33.00010: Ferromagnetism in epitaxial InMnSb films Nidhi Parashar, Bruce Wessels The structure and ferromagnetic properties of epitaxial In$_{1-x}$Mn$_{x}$Sb semiconductor films deposited using metal-organic vapor phase epitaxy were investigated. Films were single phase as determined by x-ray diffraction for x = 0.01 to 0.05. A rocking curve width of 0.3 degrees was measured in $\theta $-2$\theta $ x-ray scans. XRD $\phi $-scans indicated that the films were epitaxial. Films are ferromagnetic at room temperature as indicated by hysteretic behavior. For an In$_{0.965}$Mn$_{0.035}$Sb epitaxial film a saturation magnetization (Ms) and coercive field (H$_{C})$ of 20 emu/cm3 and 240 G respectively were measured at 295 K. The field cooled and zero field cooled magnetization curves exhibit reversible behavior confirming the absence of any impurity phase. The temperature dependent magnetization was well-described by a Brillouin function. The Curie temperature (T$_{C})$ was above 400 K as determined from the field cooled magnetization temperature dependence. High temperature SQUID measurements will also be presented. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:36AM |
A33.00011: Self-organized magnetic GeMn nanocolumns in germanium Invited Speaker: Spintronics requires spin injectors compatible with silicon technology and operating at room temperature. Potential candidates are group-IV ferromagnetic semiconductors like Mn-doped silicon or germanium. In this presentation, we show that co-evaporating Ge and Mn on Ge(001) and GaAs(001) substrates using low-temperature MBE leads to the formation of self-assembled Mn-rich nanocolumns. These nanocolumns are observed in a wide range of growth temperatures (80\r{ }C to 180\r{ }C) and Mn concentrations (1{\%} to 11{\%}). However the deposition rate is kept very low in order to favor the 2D spinodal decomposition which promotes the growth of nanocolumns. In this talk, we first present a complete phase diagram of nanocolumns as a function of growth temperature and Mn concentration focusing on their size, density, crystalline structure and magnetic properties. In particular, we could demonstrate that at low growth temperature it is possible to tune the columns density and at higher growth temperatures their size distribution. Moreover vertical self-organization of nanocolumns in (GeMn/Ge) superlattices could be achieved. At low growth temperatures, nanocolumns exhibit the diamond structure of Ge and contain up to 30 {\%} of Mn. By combining ab-initio calculations and EXAFS measurements, we could suggest a realistic building block of the nanocolumns. In parallel we have studied the crystalline structure of nanocolumns using grazing incidence x-ray diffraction on synchrotron radiation facilities. We then correlated the magnetic properties like magnetic anisotropy of nanocolumns to their structure by combining SQUID and EPR measurements in a three-dimensional geometry. Finally magneto-transport measurements were performed to evidence the coupling between carriers and the magnetic nanocolumns. CIP measurements mostly give information on the Ge matrix electronic properties and CPP measurements on the nanocolumns. We show the first CPP measurements on a single nanocolumn using nanocontacts. [Preview Abstract] |
Session A35: Focus Session: Negative Index Materials I
Sponsoring Units: FIAPChair: Shih-Yuan Wang, Hewlett Packard
Room: Morial Convention Center 227
Monday, March 10, 2008 8:00AM - 8:12AM |
A35.00001: Discrete Breathers in magnetic metamaterials in one and two dimensions. Giorgos Tsironis, Maria Eleftheriou, Nikos Lazarides We study the formation, stability as well as mobility of discrete breathers (DBs) in magnetic metamaterials in one and two dimensions. Magnetic metamaterials consisted by split ring resonators (SRRs) exhibit large magnetic response at Terahertz and optical frequencies. We use nonlinear arrays of SRRs where DBs arise as a result of the nonlinearity and discreteness. We consider different geometries of SRRs in both dimensions and find several different types of Hamiltonian nonlinear excitations such as dark, single-site and multibreathers. We also consider the dissipative version of the problem where DBs are formed as well. In the latter case, DBs locally alter the paramagnetic character of MMs to a diamagnetic. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A35.00002: Negative response and intrinsic localization in rf SQUID metamaterials. George Tsironis, Nikolaos Lazarides, Maria Eleftheriou A periodic array of rf SQUIDs in an alternating magnetic field acts as an inherently nonlinear magnetic metamaterial, due to the nonlinearity of the Josephson element and the resonant properties of the SQUIDs themselves. Neighboring SQUIDs are weakly coupled due to magnetic dipole-dipole interaction through their mutual inductances, allowing an effective medium description if the wavelength of the applied field is much larger than the array period. We found that SQUID arrays can provide negative magnetic response, and thus negative permeability, above the resonance frequency of the individual SQUIDs. Moreover, that response can be tuned by the applied flux. Dissipative SQUID arrays, modeled as discrete networks, support nonlinear excitations in the form of intrinsic localized modes (discrete breathers). We found that dissipative discrete breathers exist in both one- and two-dimensional SQUID arrays. Furthermore, those breathers alter locally the magnetic response of an array from paramagnetic to diamagnetic or vice versa. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A35.00003: Hybrid metamaterials for dynamic tuning Tom Driscoll, Sabarni Palit, Mumtaz Qazilbash, Marcus Brehm, F. Keilmann, B. Chae, H. Kim, Dimitri Basov, Nan Marie-Jokerst, David Smith Advances in the field of metamaterials have created many new and exciting devices, but the performance and applicability of these devices to date have been hindered by the reliance on a dispersive resonance. In this talk we present a metamaterial device with the ability to dynamically tune the center frequency of its far-infrared resonance in real-time, alleviating many of the limitations of dispersion. Our device combines the familiar Split Ring Resonator (SRR) element with a thin film of rare earth oxide possessing a metal-to-insulator phase transition that occurs just above room temperature. During this phase transition, the electromagnetic responses of the oxide-film and SRR become intertwined, creating a sort of hybrid metamaterial. This interaction allows us to manipulate the resonance of the SRR via the oxide-film material properties. The device exhibits a dynamically tunable resonance -- shifting center frequency of the magnetically active SRR mode by as much as 20{\%} within 2 degrees Kelvin of temperature control. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A35.00004: Tunable plasmonic nanostructures and nanolenses in optical domain Invited Speaker: We have designed and studied various periodic metamaterials with the resonant response in the IR and optical range. In particular, the stacks of metallic films with periodic hole arrays separated by dielectric layers (fishnet, FN) have been demonstrated to have a negative index at IR frequencies. We were able to control the index of refraction of a fishnet with an amorphous semiconductor spacer layer in a pump-probe experiment [1]. This opens up ways for modulating the light at the nanoscale. We have also observed strong second- and third- harmonic generation with this metamaterial. We discuss various uses of a gain material to compensate the losses. Arrays of metallic nanoparticles or holes support individual and collective plasmonic excitations that contribute to surface enhanced Raman scattering (SERS) with thousand-fold field enhancement factor that can be used for single-molecule detection and other applications of the ``light at the nanoscale.'' \newline \newline [1] E. Kim, et al., Appl. Phys. Lett. 91, 173105 (2007) \newline [2] E.V. Ponizovskaya, A.M.Bratkovsky, Appl.Phys. A 87, 161 (2007) [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A35.00005: Negative Refraction Index in Magnetic Semiconductors Adil-Gerai Kussow, Alkim Akyurtlu A novel principally homogeneous, non-composite magnetic semiconductor, or Chromium doped Indium Oxide, with the Curie temperature well above room temperature with natural negative refraction index in the THz range will be presented. The negative refraction index arises due to the overlapping of the negative permittivity in the plasmon subsystem and the negative permeability in the spin wave (magnon) subsystem within the same frequency domain. Since the losses in the magnetic mode are almost negligible, and the additional scattering losses due to the inhomogeneities are not present in our homogeneous medium, the total losses are exclusively due to the plasmon decay. Consequently, the negative refraction index wave has losses approximately 5 times smaller than losses in any of the currently known inhomogeneous designs. The parameters of both plasmon and magnon subsystems are calculated from the extended Band Theory, and first principles, respectively, and validated with available experimental data. Analytical expressions which describe the negative refraction index band are also presented. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A35.00006: Negative refraction and an optical analogue of a directional spin valve in multiferroic materials Kei Sawada, Shuichi Murakami, Naoto Nagaosa We present a new mechanism for negative refraction caused by a magnetoelectric (ME) effect. The ME effect appears in multiferroic materials in which spatial inversion and time-reversal symmetries are simultaneously broken. Such symmetry breakings allow us to control the spontaneous electric polarization by a magnetic field. We study polaritonic states in multiferroics and show that an asymmetric dispersion relation due to the ME effect gives rise to an optical analogue of a directional spin valve and a one-way waveguide as well as the negative refraction. We also estimate a realistic size of the effect. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A35.00007: Infrared Properties of NiO-SrTiO$_{3}$ Composites I.D. Vugmeyster, K. Kastella, C. Knill, R. Merlin, J.F. Whitaker, J.A. Azurdia, S.N. Karlsdottir, V. Tomeckova, C. Torres-Garibay, J.W. Halloran, G.O. Andreev, D.N. Basov Magnetic-dielectric composites with overlapping magnetic- and electric-dipole resonances are promising candidates in the search for artificial systems with negative refractive index [1]. Here we report on the fabrication and infrared characterization of NiO-SrTiO$_{3 }$~ceramics. Transmission and reflection data were obtained in the 10-700 cm$^{-1}$ range using both a THz time-domain and a FTIR spectrometer. The spectra show features associated with the bulk antiferromagnetic resonance of NiO and the soft mode of SrTiO$_{3}$, as well as new collective modes of the aggregate. The results are in qualitative agreement with effective medium theories. \newline [1] S. D. Kirby, M. Lee, R. B. van Dover, J. Phys. D \textbf{40,} 1161 (2007) [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A35.00008: Measurements of the electric susceptibilities of Au nanorods at optical frequencies Jake Fontana, Peter Palffy-Muhoray, Nicholas Kotov, Ashish Agarwal Accurate knowledge of the electric susceptibilities of nanoparticles is of key importance in the design of optical metamaterials. We have determined the principal values of the susceptibility tensor of Au nanorods by measuring the real and imaginary phase shift of light transmitted by Au nanorod suspensions in organic solvents. The nanorods were aligned by an externally applied low frequency electric field. The real and imaginary parts of the phase shift were determined using a conoscopic Mach-Zehnder interferometer with a dye laser and a spectrophotometer, respectively. We discuss our procedure of extracting the principal values of the susceptibility tensor as function of wavelength from the experimental data. We consider the implications of our results for the construction of optical negative index metamaterials. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A35.00009: Tuned permeability in terahertz split-ring resonators for devices and sensors Greg Andreev, Tom Driscoll, Dimitri Basov, Massimiliano Diventra, Sabarni Palit, Sang Yeon Cho, Nan Marie Jokerst, David Smith A process is demonstrated for tuning the magnetic resonance frequency of a fixed split-ring resonator array, by way of adding material near the split-ring elements. Applying drops of a silicon-nanospheres/ethanol solution to the surface of the sample decreases the magnetic resonance frequency of the split-ring array in incremental steps of 0.03 THz. This fine tuning is done post fabrication and is demonstrated to be reversible. The exhibited sensitivity of the split-ring resonance frequency to the presence of silicon nanospheres also suggests further application possibilities as a sensor device. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A35.00010: Double Negative Index of Refraction Observed in a Single Layer of Closed Ring Resonators Zhao Hao, Michael Martin, Bruce Harteneck, Stefano Cabrini, Erik Anderson We report the results of a spectroscopic study of a single layer of nanoscale metallic single closed ring resonators on a free- standing thin membrane at near-normal and grazing angles of incidence$^{[1]}$. When the magnetic component of the light is perpendicular to the ring plane, we observe a so-called ``double'' negative index of refraction at near-infrared frequencies attributed to a strong magnetic dipolar resonance and a broad electric resonance in this metamaterial. We experimentally identify the different resonance modes and the spectral region of negative refractive index on a series of samples with different feature and lattice sizes, using multi- oscillator fits and comparing to electromagnetic simulations. \newline [1] Z. Hao, M. C. Martin, B. Harteneck, S. Cabrini, E. H. Anderson, Appl. Phys. Lett., in press (2008). [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A35.00011: Z-scan measurement of oriented Au nanoparticle suspensions Piotr Lesiak, Michele Moreira, Peter Palffy-Muhoray, Nickolas Kotov, Ashish Agarwal The Z-scan technique, developed by the CREOL group$^{1}$, is a simple and effective method for measuring intensity dependent optical nonlinearities of materials. We have carried out Z-scan measurements of gold nanorods suspended in organic solvents using a CW laser. A low frequency external electric field was used to orient the nanoparticles$^{2}$. We present our experimental results for the real and imaginary parts of the nonlinear phase shift as function of the applied aligning electric field. We consider a variety of possible contributing physical mechanisms, and compare their expected contributions with experimental observations. [1] M. Sheik, A.A. Said, and E.W. Van Stryland, \textit{Opt. Lett.} \textbf{14}, 955 (1989). [2] J. Fontana, and P. Palffy-Muhoray, APS March meeting 2008, New Orleans, LA (2008). [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A35.00012: Numerical simulation of the non-local optical response of nanoparticles Jeremy Neal, Peter Palffy-Muhoray The interaction of nanoparticles with light is a primary focus of research in negative index materials. When the wavelength of light is comparable to the particle size, significant non-local effects are expected in the electric and magnetic response of the nanoparticles. It has been suggested that the spatially non-local response may be taken into account via the bianisotropic formalism for the constitutive equations. We have carried out computer simulations of the optical response of nanoparticles using both the discrete dipole approximation and the finite integration technique to determine the effectiveness of these bianisotropic constitutive equations. We present our results, which indicate that the approach of Agranovich et al.[1] provides a better description of the non-local optical response than the bianisotropic formalism. \newline [1] V.M. Agranovich and V.L. Ginzburg, ``Spatial dispersion in crystal optics and the theory of excitons'', (Interscience, London, 1966). [Preview Abstract] |
Session A36: Focus Session: X-ray and Electron Optics and Microscopy
Sponsoring Units: GIMSChair: Timothy Graber, University of Chicago
Room: Morial Convention Center 228
Monday, March 10, 2008 8:00AM - 8:36AM |
A36.00001: Invited Speaker: |
Monday, March 10, 2008 8:36AM - 8:48AM |
A36.00002: Optimal conditions for combining a transmission x-ray microscope with a grating interferometer Xianghui Xiao, Qun Shen Transmission x-ray microscope (TXM) is a powerful imaging tool that can provide resolution down to 15$\sim $20 nm. Grating interferometer (GI) is a recently established imaging technique with which both phase and attenuation information of an arbitrary specimen can be extracted in a straightforward way. The achievable resolution of a GI is limited by either the grating analyzer period or the detector pixel size, which is currently at about micron level. It is natural to imagine that combing a TXM with a GI (TXMGI) will give ability to image a weak-absorbing specimen with high resolution. However, it is not trivial to obtain reliable structure information from a TXMGI. In this presentation, we will discuss the dependence of the interferogram on three key parameters in a TXMGI, i.e. the coherence of the illumination beam, the numerical aperture of the TXM, and the grating period. Based on that result, the optimal conditions and the limits on the achievable resolution are obtained. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A36.00003: Consideration in the Reconstruction of 3D atomic Structure from X-Ray Yuhao Wang, J. Bai, T.A. Tyson, P. Siddons, G. De Geronimo X-Ray Holography is a promising technique for recovery of the three dimensional structure of materials. With the advent of high flux sources and fast x-ray detectors this method is under serious consideration as main stream technique. Simulations based on spherical atomic scattering factors method are performed to estimate the effects of wave front curvature by application to specific systems. An assessment is made of the distortions which arise if artifacts such as errors in the position of the origin exists. Simulations are performed to determine the influence of counting rates on the image reconstruction. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A36.00004: Synchrotron topographic studies of stacking faults in type-IIa diamond crystals XianRong Huang, Albert Macrander, Jozef Maj High-quality diamond is an ideal material for various synchrotron x-ray optical applications. However, diamond crystals generally contain various defects, among which stacking faults (SFs) are more detrimental since they are planar defects extended up to square centimeters. In this presentation, we will introduce monochromatic synchrotron topographic studies of SFs (as well as other defects) in type-IIa diamond crystals. SFs show strong contrast at the tails of the rocking curve, thus broadening the rocking curve width. The outcrops of SFs on the crystal surface still show sharp white-line contrast at the Bragg peak, indicating strong strains or lattice misorientations near the outcrops. From the variation of SF contrast with the rocking angle, we obtain a detailed picture how the extended SFs influence the diffraction performance of diamond crystals. A straightforward diffraction contrast mechanism of SFs will also be presented in addition to the dynamical theory description. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A36.00005: Design, nanofabrication and testing of silicon and diamond hard X-ray optics A.F. Isakovic, A. Stein, J.B. Warren, K. Evans-Lutterodt, S. Narayanan, M. Sprung, A. Sandy We have designed, fabricated and tested silicon (Si) and diamond based X-ray kinoform lenses. In design and nanofabrication of such X-ray optics elements, surface roughness and wall verticality are among the tasks of critical importance for achieving as-designed performance. Our cyclic cryogenic RIE method [1], developed to deal with such stringent nanofabrication requirements, is comparable in performance to licensed Bosch process, as established in measurements of the surface roughness, and the etch rate ($>$2 microns/min for Si, $\sim $100 nm/min for diamond) and verticality ($<$1deg over 100 microns) parameters. We compare nanofabrication procedures for Si and diamond lenses, and discuss relative merits of Si and diamond as materials for X-ray lenses. We also show sub-100 nm spot size tests of kinoform Si-based X-ray focusing optics, determined via knife edge measurements at APS 8-ID, and a preliminary results of tests of diamond-based kinoform lens at NSLS, performed at X13B and APS 8-ID. [1] A. F. Isakovic et al., submitted for publication. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A36.00006: X-ray Reflectivity and Power Spectral Density of Smoothly Polished Silicon Lahsen Assoufid, Albert Macrander, Suresh Narayanan, Ruben Khachatryan, Sunil Sinha Silicon polished by means of chemical-mechanical-polishing has been studied. A finely crevaced top surface was seen in the AFM data. The power-spectral-density was measured by means of interferometry and by atomic force microscopy, and a roughness value of 0.21- 0.23 nm rms was found by integration. X-ray reflectivity data for 10 keV x-rays were obtained at the Advanced Photon Source, and a roughness of 0.22 - 0.30 nm was found to be roughly consistent with these data. A surface layer with a slightly higher density than that of crystalline silicon was needed to model the x-ray reflectivity. Crevaces 3.6 nm deep and resulting in land areas having 85{\%} coverage were invoked for the modeling. A total layer thickness of 7.4 nm was invoked for the modeling . That is, the crevaces penetrated roughly half way through the total layer thickness. Due to the overall agreement between the two very different techniques for measuring roughness, namely, PSD and x-ray reflectivity data, we consider these results to accurately quantify roughnesses for a silicon surface that is near the state-of-art for smoothness. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A36.00007: Wide-angle incidence x-ray waveguides prepared by micro-/nano-technology using crystal surface diffraction Sung-Yu Chen, Yu-Chi Shen, Mau-Sen Chiu, Chia-Hung Chu, Yuriy P. Stetsko, Bo-Yuan Shew, Shih-Lin Chang Grazing incidence x-ray waveguides have been most studied because of its simple geometry and its applicability for all photon energies. However, wide-angle incidence waveguides are also essential for modern x-ray optics, as far as coupling/guiding x-ray beams into given directions are concerned. To investigate this possibility we have prepared waveguides on silicon wafers by x-ray lithographic technique. The waveguides are 100$\mu $m high and 1cm long with different widths and the distance between the adjacent waveguides is 2.5 mm. Both the top and bottom surface of a waveguide are plated with gold. With this type of waveguides we have actually observed the effects of guiding x-rays in both lateral and vertical directions using (113) surface diffraction in Au/Si waveguide systems. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A36.00008: Fluctuation electron microscopy studies of complex structured materials Gongpu Zhao, Annick Roug\'ee, Peter Buseck, Michael Treacy Fluctuation electron microscopy (FEM) is a hybrid imaging-diffraction technique. This technique is particularly sensitive to paracrystalline structures of dimension 0.5-2 nm, which are difficult to detect by either imaging or diffraction techniques alone. It has been successfully deployed to study paracrystalline structures in amorphous silicon, germanium thin film. This technique has also been used to study metallic glasses and oxide glasses. Until now, FEM has not been used to study disordered geological materials. In this talk we present our FEM studies of shungite, a naturally occurring disordered carbonaceous material, reveal that trace quantities of tightly curved graphene structures such as C60, or fragments of C60, is present in shungite. We also present results from our study of metamict zircon, whose crystal structure is destroyed by self-radiation during naturally occurring $\alpha $ decay events. Work is in progress to study the structural evolution during the metamictization process. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A36.00009: Holography with Low Energy Electrons - a New Tool for Structural Biology Tatiana Latychevskaia, Hans-Werner Fink Holography is widely used for three-dimensional imaging of macroscopic objects using visible light. The same principle can also be applied for imaging of individual molecules like DNA or larger objects, for instance viruses. The holograms are recorded with coherent low energy electrons with wavelength, and thus potential resolution, in the sub-nanometer regime. The experimental setup together with holograms of individual biological molecules and their numerical reconstructions shall be presented. Current experimental and theoretical challenges of holography with low energy electrons will also be addressed. Strong forward scattering of electron waves has been taken into account for the reconstruction process. Since most biological molecules exhibit phase shifting rather than absorbing properties, the retrieval of the phase parallel to the absorbing properties of an object has been realized. On the experimental side, a method towards a significantly improved signal to noise ratio in holograms has been established by acquiring several hundred short pulsed holograms followed by a cross correlation alignment. Finally, the solution to the twin image problem in holography will be presented and reconstructed twin-image free experimental holograms will be shown. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A36.00010: Theory of ultrafast electron diffraction: the role of the pulse properties. John Sipe, Ania Michalik, Eugene Sherman We present a general formalism for scattering of electron bunches used in ultrafast electron diffraction (UED) experiments that incorporates characteristic parameters of the incident electron bunch. To perform the scattering calculation, we associate the classical distribution function, which describes the electron bunch just before scattering, with the asymptotic-in Wigner distribution. Using single-scattering and far-field approximations appropriate for typical UED experimental conditions, and considering the effects of the bunch parameters on the scattered signal, we derive two diffraction expressions. We derive a Fraunhofer type expression suitable for scattering from small samples, such as molecules, and a Fresnel type expression appropriate for scattering from large targets such as thin films. In our analysis we also identify the coherence length of an electron bunch. We present sample numerical calculation for scattering by nanosize particles based on our model, and discuss the effects of bunch and scattering target parameters on the diffraction signal. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A36.00011: Determination of Adsorbed C$_{60}$ Nanostructures by Low-Energy Electron Diffraction M.A. Van Hove, G.M. Gavaza, Z.X. Yu, G. Xu, S.Y. Tong, W.W. Pai, C.H. Lin We have recently extended to nanostructures the basic theoretical capabilities of surface structure determination by Low Energy Electron Diffraction (LEED), by adopting a non-periodic cluster approach and substantially accelerating the computation time for complex structures. In this contribution, we describe two further theoretical enhancements and their application to experimental data for buckyballs adsorbed on a Cu(111) surface. One enhancement addresses occasional situations where strong multiple scattering causes poor convergence: this is solved by treating all scattering within subclusters of a few atoms with accurate matrix inversion. Secondly, for the structure determination of complex nanostructures, an efficient search method is essential: for that purpose a modified version of tensor LEED is adapted to nanostructures, called NanoTensorLEED. We exhibit the resulting ability to analyze detailed nanostructures with the case of buckyballs adsorbed on a Cu(111) surface. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A36.00012: RHEED-TRAXS as a tool for in-situ stoichiometry control. Sandeep Chandril, Cameron Keenan, Thomas Myers, David Lederman RHEED-total reflection x-ray spectroscopy (-TRAXS) is an in-situ chemical and structural characterization technique which is highly surface sensitive. This consists of a grazing-angle electron beam from which characteristic x-rays from the sample are measured also at grazing angles. We have demonstrated that monolayer sensitivity in Y and Mn films on GaN can be achieved. We have also developed a theoretical model for the angular dependence of the x-ray K\textit{$\alpha $} peaks for the thin films, based on Parratt's formalism for x-ray reflectivity and the electron trajectory simulation software CASINO, to correct for grazing angle electron beam as a source for x-rays. As the angular dependence is highly dependent upon the film thickness and the smoothness of the film, it can be used to determine the deposition rate of individual elements as well as the interface chemical roughness [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A36.00013: First background free measurement of the inelastic tail of the Auger electron spectrum down to 0 eV A. H. Weiss, S. Mukherjee, M.P. Nadesalingam, N. G. Fazleev Background free measurements of the inelastic tail of the Auger electron energy spectrum were performed by using very low energy positrons to excite Auger transitions in Au and Cu via positron-electron annihilation. The kinetic energy of the incident positrons (1.5eV) was set below the energy threshold required to excite secondary electrons resulting in a Auger spectra that was completely free of collision induced secondary electrons. The measured spectra contain contributions solely from either annihilation induced Auger electrons or annihilation induced Auger electrons that have lost energy on the way out. By using the time of flight technique it was possible to measure the inelastic tail of the Auger electron energy spectrum down to 0 eV. [Preview Abstract] |
Session A37: Semiconductors I: Growth and Processing
Sponsoring Units: FIAPChair: Max. G. Lagally, University of Wisconsin-Madison
Room: Morial Convention Center 229
Monday, March 10, 2008 8:00AM - 8:12AM |
A37.00001: Single phase In$_{x}$Ga$_{1-x}$N (0.25 $\le $ x $\le $ 0.63) alloys synthesized by MOCVD. Bed Pantha, Li Jing, Jingyu Lin, Hongxing Jiang In$_{x}$Ga$_{1-x}$N alloys have received much attention due to their applications in optoelectronic devices operating in the near infrared region to the near UV region as the band gap of In$_{x}$Ga$_{1-x}$N alloys can be continuously tuned from $\sim $ 0.7 eV (InN) to 3.4 eV (GaN). Recently, it has been suggested that high quality In-rich InGaN alloys offer great potential applications in many important areas as follows; (1) high efficiency multijunction solar cell, (2) high efficiency photoelectronchemical (PEC) cell, and (3) thermoelectric (TE) devices. Our recent experimental results show that In-rich InGaN alloys could be as good as SiGe alloys in terms of figure of merit (ZT) for TE applications. However many experimental techniques have proven that growth of In-rich InGaN alloys is extremely challenging due to the solid phase miscibility gap between InN and GaN. Here we present the growth of single phase InGaN alloys with high In-contents by metal organic chemical vapor deposition on AlN/Al$_{2}$O$_{3}$ and/or GaN/Al$_{2}$O$_{3}$ templates. X-ray diffraction was employed to determine indium content. Single peak of wide range theta-2theta scan of (002) plane confirmed that there is no evidence of phase separation. Optical and electrical properties and surface morphology were also studied by photoluminescence, Hall-effect and atomic force microscopy measurements, respectively. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A37.00002: High Mobility InN epilayers grown on AlN templates by MOCVD Neelam Khan, Ashok Sedhain, Jing Li, Jingyu Lin, Hongxing Jiang Among III-nitrides, InN has the smallest electron effective mass, the largest mobility and smallest direct band gap. These distinguished properties make InN an interesting material for the applications in high speed electronic devices and full color displays. However, obtaining InN and In-rich InGaN epilayers with controllable conductivity is still a challenging task. We report here on the growth and transport property studies of InN epilayers on AlN templates, as compared to GaN templates. Significant improvements in the electrical and optical properties of InN epilayers were observed on AlN templates. A hall mobility of 1390 cm$^{2}$/Vs with a carrier concentration of 1x10$^{19}$cm$^{-3}$ at room temperature was observed, which is highest value reported for MOCVD grown InN epilayers. The photoluminescence emission spectra revealed band to band emission peak at $\sim $0.75 eV. The effects of post growth annealing on transport, structural and optical properties of undoped and Mg-doped InN epilayers will also be presented. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A37.00003: Growth of the BN -- nano - structured materials using borazine decomposition by Laser Chemical Vapor Synthesis Arturo Hidalgo, Vladimir Makarov, Dachun Huang, Gerardo Morell, Brad Weiner We describe BN nanostructured materials growth by Laser Chemical Vapor Synthesis (LCVS) using the precursor borazine B$_{3}$N$_{3}$H$_{6}$. As result due to laser induced creation of the active chemical intermediates in the bulk volume, and further development of the ``dark'' chain processes in the borazine vapor with formation of the BN- nano-tubes and hydrogen gas. The phenomenological model qualitatively describing the observed phenomenon was developed and applied to explanation of the studied effects. The variation of radiation density (J/cm$^{2}$) for both harmonics and pressure is used for optimized the amount obtained. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A37.00004: Anomalous Coherent Bragg Rod Analysis Studies of GaAs/InGaAs Divine Kumah, Naji Husseni, Codrin Cionca, Alex Riposan, Joanna Mirecki Millunchick, Phil Willmott, Roy Clarke, Yizhak Yacoby A considerable amount of work has been carried out recently in correlating growth conditions with electronically observed properties in Group III-V systems using a variety of characterization techniques. Ambiguity in interpretation of most characterization techniques arises due to difficulties in separating roughness effects from segregation and inter-diffusion of atomic species. We apply x-ray resonant techniques to the Coherent Bragg Rod Analysis (COBRA) phase retrieval procedure to produce high resolution electron density maps from bragg rod scans to determine with a high degree of accuracy, the relative concentrations of In and Ga in a system comprising of 1ML of GaAs on InGaAs. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A37.00005: Growth and Characterization of Non-Polar, A-Plane ZnO Thin Films Priya V. Chinta, O. Lozano, P. Wadekar, L.H. Chu, Q.Y. Chen*, W.K. Chu, H.W. Seo, L.W. Tu, N.J. Ho The growth and characterizations of non-polar a-axis-oriented epitaxial films of ZnO deposited by magnetron sputtering were investigated in comparison with the polar c-axis-oriented counterparts. The single-phase a-axis-oriented films were obtained at 700-degree C substrate temperature, whereas both a- and c-axis orientations were observed when deposited at lower temperatures ranging from 400 degree C to 600 degree C. The structural, morphological and temperature effects on photoluminescence (PL) behaviors have been studied. X-ray rocking curves show that the FWHM of (11-20) peak increases with temperature up to 600 degree C but start to decreases beyond 700C. Atomic force microscopy reveals significant changes in surface morphology as the growth temperature varies. Optical properties of the differently oriented films will be presented. PL emission in UV range was observed for the a-plane ZnO films which has been attributed to neutral donor bound exciton. The effect of Al-doping will also be discussed. *Also with NSYSU. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A37.00006: Pinpoint Growth Mechanism Of ZnO Nanoprisms Da-Jun Shu, Xiang Xiong, Mu Wang We investigate the growth mechanism of ZnO nanoprisms synthesized by thermal evaporation method. Temperature is tuned to control the growth driving force while other conditions are fixed. Classical nucleation theory and growth dynamics are used to analyze the competition between growth in lateral and vertical directions. Interfacial diffusion properties, step edge diffusion barrier and several other factors affecting the growth of nanostructures are taken into account. Based on these considerations we have established a model which suggests a quantitative relation between temperature and the size of nanoprisms. Programmed cooling processes are introduced into the thermal evaporation to verify the theoretical expectations. It is also demonstrated that a morphology-controllable hierarchical prisms, which is expected from our theoretical model, can be easily achieved by tuning the temperature. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A37.00007: Titanium oxide nano-clusters prepared via amorphous solid water: Improved overlap with solar spectrum Micha Asscher The need to develop reliable and cost effective alternative energy sources is rapidly growing. Solar light is among the most important and promising, therefore new materials are searched for to effectively overlap and harvest the solar spectrum. Here we describe the preparation of titanium oxide nano-clusters utilizing amorphous solid water (ASW) as a reactive buffer layer to assist the growth of titanium oxide clusters. These clusters were grown in-vacuum on top of ASW layers at 120K. Upon evaporation of the water layer, seed Ti(OH)$_{n}$ clusters polymerize via a solid state ``Sol-Gel-like'' mechanism. Self termination process, dictated by the evaporated water vapor, results in hemispherical clusters 5-7 nm in diameter. The clusters were analyzed by XPS at 300K. The clusters grown this way were studied by employing 4K scanning tunneling spectroscopy, revealing a typical diode-like I-V profile. An apparent band gap of 2.3$\pm $0.5 eV was obtained, significantly narrower than the bulk value of TiO$_{2}$ crystal (3.2 eV). Thermal stability of these defect rich clusters need to be studied, since this may prove important for photo-catalysis and photovoltaic applications. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A37.00008: On the logarithmic-normal distribution in nucleation and growth processes Andreas Bill, Anthony Teran, Ralf B. Bergmann The logarithmic-normal (lognormal) distribution is one of the most frequently observed distributions in nature and describes a large number of physical, biological and even sociological phenomena. However, a derivation of this distribution from first principles is lacking. We propose a differential equation governing the time development of grain size distribution in random nucleation and growth processes. The solution of this equation provides an analytical derivation of size distributions that has a form of the lognormal type. The resulting expression is used to discuss the grain size distribution of solid phase crystallized Si-films. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A37.00009: Si nanomembranes with mixed crystal orientations Shelley Scott, Deborah Cottrill, Donald Savage, Max Lagally Higher-carrier-mobility CMOS devices enhance processor speed. Carrier mobility can be optimized by fabricating mixed regions of Si(110) (high hole mobility) and Si(001) (high electron mobility) on a single substrate, so-called hybrid-orientation technology (HOT). We fabricate a mixed-crystal-orientation material using Si nanomembrane (SiNM) transfer and overgrowth. The top Si layer of SOI(110) is patterned with an array of holes and removed from its handle substrate, creating a Si(110)NM, which is then bonded to Si(001). We deposit Si over the structure with CVD, which is much faster on Si(001) than on Si(110), allowing planarization of the surface (i.e., hole filling), to produce a flat mesh of Si(001) and Si(110) regions. We characterize the mesh with XRD, SEM, and AFM. We can fabricate HOT membranes for transfer to various (including flexible) substrates, and can incorporate strain to tune mobilities. With strained Si(110)NMs, we expect a hole mobility enhancement of $\sim $70{\%} over Si(001) while maintaining the high electron mobility of Si(001), thereby dramatically reducing the mobility imbalance between n and p-type devices. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A37.00010: Strain Relaxation in Elastically Strain-Sharing Silicon (110) Nanomembranes Deborah Cottrill, Shelley Scott, Donald Savage, Max Lagally Tensilely strained Si(110) has potential for advanced CMOS and p-MOS devices because, depending on strain and current direction, hole mobility can be increased to 150{\%} of that of unstrained Si(001) and an electron mobility nearly equal. Previous efforts to strain Si(110) tensilely have relied on the formation of partial dislocations for strain relaxation, resulting in large asymmetric components of the in-plane strain and high threading dislocation densities, which alter device performance. We use thin tri-layer heterostructures of Si/SiGe/Si(110) grown with MBE on Si(110)-on-insulator that elastically strain share when released from the handling substrate. Besides their very high flexibility, these nanomembranes (NMs) are virtually dislocation free and exhibit a lower degree of asymmetric in-plane strain relaxation than achieved with threading dislocation relaxation; the NMs rely on elastic strain transfer rather than partial dislocation propagation. We use AFM, XRD, and Raman to characterize growth, strain transfer, and strain anisotropy. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A37.00011: A Novel Route for the Synthesis of Graphene by Microwave Plasma Enhanced Chemical Vapor Deposition Alexander Malesevic, Manish Pal Chowdhury, Liang Zhang, Annick Vanhulsel, Chris Van Haesendonck A novel route for the synthesis of graphene by means of microwave plasma enhanced chemical vapor deposition is presented. This technique outclasses its competitors in many ways since it is less elaborate and better reproducible than micromechanical cleavage of graphite and less expensive than thermal decomposition of silicon carbide wafers. Methane diluted with hydrogen is decomposed in a high power microwave plasma and the resulting carbon radicals recombine on the surface of any substrate that withstands temperatures up to 700\r{ }C. A broad range of substrates were successfully tested including silicon, quartz, stainless steel and many metals. The resulting carbon nanostructures are freestanding graphene flakes, only four to six atomic layers thick but up to several micrometers wide and high. The flakes are perpendicular aligned to the substrate surface. Thorough qualitative analysis lead to the conclusion that the flakes are highly crystalline sp2 carbon nanostructures with few defects or impurities. A possible growth scheme is proposed and field emission measurements of as grown flakes reveal a low turn on voltage of only 3V/$\mu $m which is a promising value for possible future applications. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A37.00012: Structure of Iron Silicon Germanide and Osmium Silicide Epitaxial Films Measured by X-Ray Absorption Spectroscopy Nader Elmarhoumi, R. Cottier, F. Amir, G. Merchan, A. Roy, H. Geisler, C. A. Ventrice Jr., T. D. Golding Some of the iron- and osmium-based metal silicide and germinide phases have been predicted to be direct band gap semiconductors. Therefore, they show promise for use as optoelectronic materials. We have used synchrotron-based x-ray absorption spectroscopy to study the structure of iron silicon germanide and osmium silicide films grown by molecular beam epitaxy. Osmium silicide films which are primarily in the Os$_{2}$Si$_{3}$ phase and a series of Fe(Si$_{1-x}$Ge$_{x})_{2}$ films with a nominal Ge concentration of up to x = 0.04 have been grown. X-ray absorption near edge structure (XANES) measurements on both the iron silicon germinide and osmium silicide films has been performed. An absorption edge shift of 0.9 eV is observed for the osmium silicide films; however, no shift was observed for the iron silicon germinide films. Extended x-ray absorption fine structure (EXAFS) measurements have also been performed on the iron silicon germinide films. The nearest neighbor coordination corresponding to the $\beta $-FeSi phase of iron silicide provides the best fit with the EXAFS data. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A37.00013: Studies of the Ge(100) Surface Using a Low Energy Positron Beam: The Effects of Surface Reconstructions on Positron Trapping and Annihilation Characteristics N. G. Fazleev, A. H. Weiss Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(100) surface. The PAES spectrum from the Ge(100) surface displays several strong Auger peaks corresponding to M4,5N1N2,3 , M2,3M4,5M4,5 , M2,3M4,5V, and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level electrons. The experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons with relevant Ge core-level electrons for the reconstructed Ge(100)-p(2x1), Ge(100)-p(2x2), and Ge(100)-c(4x2) surfaces. Estimates of positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction of the topmost layers of clean Ge(100). These results are compared to the ones obtained for the reconstructed Si(100)-(2x1) and Si(100)-p(2x2) surfaces. A comparison with PAES data reveals an agreement with theoretical core annihilation probabilities for the Auger transitions considered. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A37.00014: X-ray Photoelectron Spectroscopic Investigation of Oxidation of Hafnium John Hickman, Steven McDonough, R.L. Miller, M.A. Seabolt, G.A. Nixon, A.R. Chourasia X-ray photoelectron spectroscopy has been employed to investigate the oxidation of hafnium. Thin films (20 {\AA}) of elemental hafnium were deposited on silicon substrates using e-beam technique. Two types of samples were investigated. In one type, the substrate was annealed at the desired temperature after the deposition. In the other type, the substrate was kept at the desired temperature during the deposition. The substrate temperatures were kept at 100, 200, 300, 400, 500, and 600$^{\circ}$C. Hafnium is observed to get deposited mostly as hafnium dioxide with some suboxide. The amount of the suboxide is found to vary with the processing conditions. The concentration of the dioxide and the suboxide were determined by curve fitting the spectra. The fitting was performed using the parameters determined from fitting pure elemental hafnium spectrum and pure hafnium dioxide spectrum. [Preview Abstract] |
Session A38: Focus Session: Biocompatibility
Sponsoring Units: DBP DMPChair: Eric Cochran, Iowa State University
Room: Morial Convention Center 230
Monday, March 10, 2008 8:00AM - 8:36AM |
A38.00001: The quantification of biocompatibility: toward a new definition Invited Speaker: Implantable medical devices, and the biomaterials that comprise them, form a {\$}100B business worldwide. Medical devices save lives and/or improve the quality of life for millions. Tissue engineering also makes extensive use of biomaterials -- biomaterials are an enabling technology for tissue engineering. A central word to understanding the effectiveness of such materials and devices is biocompatibility. The word ``biocompatible'' is widely used in reference to biomaterials and medical devices and most everyone has some value understanding of its meaning. Many formal definitions have been proposed for this word, but it is still largely used in an imprecise manner. Four descriptions or definitions of biocompatibility will be reviewed: a widely adopted definition from a consensus conference, a surgeon's perspective on this word, the regulatory agency view and the factors that clearly influence biocompatibility. In this talk, the classical definition of biocompatibility will be contrasted to a newer definition embracing molecular concepts and the understanding of normal wound healing. The biological data on the \textit{in vivo} healing responses of mammals to implants will be described. A strategy to improve the healing of biomaterials will be presented. It is based upon surface molecular engineering. First, non-specific protein adsorption must be inhibited. Strategies to achieve this design parameter will be presented. Then methods to deliver the specific protein signals will be addressed. Matricellular proteins such as osteopontin, thrombospondin 2 and SPARC will be introduced with an emphasis on exploiting the special reactivity of such proteins. A discussion of the influence of surface textures and porosities will also be presented. Finally a new scheme based upon macrophage phenotypic pathways will be proposed that may allow a quantitative measure of extent of biocompatibility. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A38.00002: Biocompatibility of implantable biomedical devices Invited Speaker: Biomedical devices have been broadly used to treat human disease, especially chronic diseases where pharmaceuticals are less effective. Heart valve and artificial joint are examples. Biomedical devices perform by delivering therapies such as electric stimulations, mechanical supports and biological actions. While the uses of biomedical devices are highly successful they can trigger adverse biological reactions as well. The property that medical devices perform with intended functions but not causing unacceptable adverse effects was called biocompatibility in the early time. As our understanding of biomaterial-biological interactions getting broader, biocompatibility has more meanings. In this talk, I will present some adverse biological reactions observed with implantable biomedical devices. Among them are surface fouling of implantable sensors, calcification with vascular devices, restenosis with stents, foreign particle migration and mechanical fractures of devices due to inflammation reactions. While these effects are repeatable, there are very few quantitative data and theories to define them. The purpose of this presentation is to introduce this biocompatibility concept to biophysicists to stimulate research interests at different angles. An open question is how to quantitatively understand the biocompatibility that, like many other biological processes, has not been quantified experimentally. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A38.00003: A Gaussian theory of the response of heart frequency to ventilator Yan Lu, Michael Deem, Anton Burykin, Timothy Buchman Extensive studies suggest that there exists a coupling between the human heart and respiration. We constructed a simple Gaussian Markovian propagation model to describe the influence of ventilator on patient's heart frequency. We show that for sedated patients, the theory captures the essential correlations between heart rate and induced ventilation during patient's spontaneous breathing and by so doing successfully predicts the response of heart rate to application of the ventilator. We also discuss the cases in which the theory fails, all of which were none sedated patients. We believe sedation disconnects high brain activities from cardiac and respiratory functions leaving just the primitive response. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A38.00004: Synchronization of Cardio-Respiratory Dynamics in Critically Ill Patients. Anton Burykin, Timothy Buchman We studied changes in cardio-respiratory synchronization and dynamics of cardiovascular system during transition from mechanical ventilation to spontaneous respiration in critically ill patients. This observational study exploits a standard clinical practice---the spontaneous breathing trial (SBT). The SBT consists of a period of mechanical ventilation, followed by a period of spontaneous breathing, followed by resumption of mechanical ventilation. We collected continuous respiratory, cardiac (EKG), and blood pressure signals of mechanically ventilated patients before, during and after SBT. The data were analyzed by means of spectral analysis, phase dynamics, and entropy measures. Mechanical ventilation appears to affect not only the lungs but also the cardiac and vascular systems. Spontaneous cardiovascular rhythms are entrained by the mechanical ventilator and are drawn into synchrony. Sudden interruption of mechanical ventilation causes gross desynchronization, which is restored by reinstitution of mechanical ventilation. The data suggest (1) therapies intended to support one organ system may propagate unanticipated effects to other organ systems and (2) sustained therapies may adversely affect recovery of normal organ system interactions. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A38.00005: Crystal Structure Properties of Human Teeth as a Function of Age Th. Leventouri, A. Kyriacou, R. Venturelli, A. Antonakos, E. Liarokapis, V. Perdikatsis We report on crystal structure studies of human teeth as a function of age in the range of 5-87 years. The crystallinity of the hydroxyapatite, which is the main dental mineral phase in teeth, decreases with age in a systematic way starting at $\sim $ 40 years old teeth. The average crystallite size decreases from $\sim $40 nm to $\sim $12 nm in the age range 30 to 60 years old and then it remains practically constant. The a-lattice constant decreases in a similar systematic way and it is associated with the carbonate content of the tooth. Development of the secondary phases with the tooth-age questions the crystallographic structure of the dental apatite. FTIR spectroscopy reveals both types of carbonate substitution, but B-type substitution is greater by a factor of 4 than the A-type. An increase of the carbonate content with the tooth age is also deduced from the ratio of the v$_{2}$ CO$_{3}$ to the v$_{1}$ PO$_{4}$ IR modes from 17 to 70 years of age. TGA measurements confirm the results of both experimental methods. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A38.00006: Procedure to Measure Effect of Excess Body Mass on Musculoskeleture: II. Implementation Saami J. Shaibani There are a number of ways in which the musculoskeletal system can be affected by excess body mass. One representative quantity for these is the torque exerted on the spinal column about a horizontal lateral axis; hence, its use as an illustrative mechanical indicator in the research reported here. Values of the torque are determined for all subjects in an exceptionally broad adult population that was developed during a companion study. Increases in body mass index caused nearly uniform increases in torque for all height percentiles in both sexes. Overweight individuals had torques that were 35 and 30 percent greater (females and males, respectively) than those for healthy individuals of the same height. Corresponding increases for obese individuals occurred at the much higher levels of 75 and 66 percent. Any resulting musculosketal damage from this is in addition to other problems arising from obesity, such as heart disease, diabetes, and high blood pressure. However, whereas the latter can be treated or managed with medication, some facets of the former might be irreversible and/or irremediable. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A38.00007: Extremely high paw accelerations during paw shake in the cat: A mechanism revealed by computer simulations Alexander Klishko, David Cofer, Donald Edwards, Boris Prilutsky Paw shake response is a reflex aimed at removing an irritating stimulus from the paw by imparting to it high periodic accelerations (\underline {$>$}10 g). These values seem too high to be produced by distal muscles exclusively. According to Prilutsky et al. (2005), resultant hip moments during paw shake are much greater than distal joint moments, whereas distal joint velocities and accelerations exceed those of the proximal joints. The goal of this study was to examine how proximal hip muscles could contribute to high paw accelerations. Using software AnimatLab, we developed a 2D model of the cat hindlimb consisting of 5 rigid segments with 4 hinge joints and 11 muscles spanning all joints. The muscles were assumed passive except for those crossing the hip. When in simulations the hip muscles were reciprocally activated to periodically flex and extend the hip joint with a typical paw shake frequency of 10 Hz, the hindlimb segments demonstrated motion resembling experimental observations: linear and angular velocities and accelerations of the distal segments exceeded several fold the values of the proximal segments. Simulated paw shake revealed features of a whip-like motion. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A38.00008: Is movement organization in cat paw shake response optimal? Boris Prilutsky, Alexander Klishko Animal musculoskeletal systems are highly redundant: they have more kinematic degrees of freedom and muscles than strictly necessary to execute a given motor task. Such redundancy gives the animal many choices in selecting kinematic and muscle activity patterns to achieve movement goal. Given a stereotypic execution of cat paw shake response (very fast periodic oscillations of the paw) among deferent cats despite the motor redundancy, we hypothesized that the movement strategy in this reflex is optimal. The goal of this study was to test several physiologically plausible cost functions, optimizations of which could explain the functional significance of chosen movement strategy in paw shake. A 2D, 5 degrees-of-freedom forward dynamics cat hindlimb model was developed. The model has 5 body segments, 4 frictionless hinge joints, 11 muscles with realistic muscle and activation dynamics. Muscle activations were computed using a simulated annealing optimization algorithm and several cost functions. The best match between simulated and experimentally recorded muscle activity patterns was obtained when peak paw acceleration was maximized. [Preview Abstract] |
Session A39: Focus Session: Elasticity and Geometry of Thin Objects
Sponsoring Units: GSNPChair: Pedro Reis, Massachusetts Institute of Technology
Room: Morial Convention Center 231
Monday, March 10, 2008 8:00AM - 8:12AM |
A39.00001: Delamination of thin elastic sheets from soft, sticky substrates Dominic Vella, Pedro Reis, Denis Bartolo, Jose Bico, Arezki Boudaoud, Benoit Roman We study the compression of a soft elastic substrate with a thin sheet adhered to its surface. In this situation, it is energetically expensive for the thin sheet to alter its length. Instead, it accommodates its excess length by delamination from the substrate, allowing it to bend out of the plane. Rather than forming a single `blister', however, we observe the formation of several blisters with a characteristic size. Here, we investigate the dependence of this characteristic blister size on the material properties of the system using a combination of experimental and theoretical analyses. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A39.00002: The frustrating tearing of adhesive tape Benoit Roman, Eugenio Hamm, Pedro M. Reis, M. LeBlanc, Enrique Cerda When trying to remove adhesive tape, one often only manages to peel off a useless pointy strip: the fracture tips on both sides of the pulled strip seem to attract each-other, and merge in a finite distance. Why don't they repel each other and lead to a continually increasing width of the strip, as one would like to? We will present an experimental and theoretical study of this pinch-off phenomenon in the rupture of peeled adhesive sheets. The cut shapes are very reproducibles, and we will show that the geometry of the peeling fold, where elastic energy is concentrated, plays a major role here. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A39.00003: Instability of an elastic knot under twist Basile Audoly, Nicolas Clauvelin, Sebastien Neukirch In a recent paper, we derived a solution to the Kirchhoff equations representing a knotted elastic rod held by a tensile force applied at its ends. This problem has been formulated as the minimization of a curvature energy in the presence of a topological constraint. We extend this analysis to the case of a knot subjected to both a tensile force and a twisting moment. We unveil a striking instability that can be easily reproduced with a piece of computer cord: a simple knot, initially comprising a large loop merging with a localized braid, can be unfolded under applied twist into a symmetric shape resembling the figure of eight. Doing so, it becomes much easier to untie. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 9:12AM |
A39.00004: On the statistical physics of folding and crumpling Invited Speaker: Unfolding a ball of crumpled paper reveals numerous ridges with a wide distribution of sizes. How can we describe the statistics of sizes and energies? Can we understand this system using the tools of statistical physics? During my talk, I will review the various experimental and theoretical approaches that we used to tackle these questions, which are typical of glasses and granular media. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A39.00005: The Shape of the Optimal Javelin Yossi Farjoun, John Neu To find the shape of a javelin whose vibrations dampen the fastest, we seek to maximize the eigenvalue of the first eigen-mode of a vibrating rod. The problem is related to (and is inspired by) the classical problems of finding the tallest and strongest columns solved by J. B. Keller [1], and J. B. Keller and F. I. Niordson [2]. A 4${}^{\rm th}$ order ODE for the maximizing eigen-mode is readily found, however it is ill-conditioned at the boundaries, and standard numerical methods fails. Using a similarity solution, we ``peel away'' the singularity, and solve the remaining part ``backwards''. The resulting shape has a frequency of vibration 5 times larger than that of the uniform-diameter rod. The method of solution is applicable to other similar problems. For example, we confirm the shape of the tallest column with it. \newline \newline {[1]} The Strongest Column / J. B. Keller ; Arch. Rat. Mech. Anal. 1960 {\bf(5)}, pp. 275--285 \newline {[2]} The Tallest Column / J. B. Keller and F. I. Niordson ; J. Math. Mech. 1966 {\bf (16)}, pp. 433--446 [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A39.00006: Impacts on thin elastic sheets Romain Vermorel, Nicolas Vandenberghe, Emmanuel Villermaux The radial cracks developing from the impact point of a projectile on a windshield are of common experience. We investigate the origin of this phenomenon using thin elastic sheets as an experimental model. A projectile launched at controlled speed impacts a free membrane at rest. A tensile front sets out from the point of impact and propagates radially at the speed of sound. Flexural waves can propagate in the extended area. Specifically, the interaction between the rigid body and the elastic sheet gives birth to a conical flexural shape whose base expands radially at a well defined velocity. During the propagation of both the tensile and flexural fronts, the radial tensile stress field results in a compressive stress in the azimuthal direction, which triggers a buckling instability. That instability is responsible for the formation of radial folds, with a well defined azimuthal wave number. Based on detailed experimental observations and measurements, we propose a model to understand the wave motion and stress field consecutive to the impact; in addition, we provide a prediction for the number of folds selected during the buckling instability as a function of the relevant parameters, including impact velocity. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A39.00007: Spiraling Cracks in Thin Sheets Victor Romero, Benoit Roman, Enrique Cerda A wide kind of everyday-life industrial products come in a thin package that needs to be torn open by the user, and the opening is not always easy. We built a simple setup to study crack propagation in thin sheets coupled with large out-of-plane displacement : A cylindrical tool is inserted in a straight incision in a thin sheet, and is pushed against the sheet perpendicularly to that incision, eventually propagating a crack. When the blunt tool is continually pushed against the lip, we found that the crack follows a very robust spiraling path. Experiments may be interpreted in terms of ``Spira Mirabilis'' (logarithmic spiral). Starting with crack theory argument, we will show that the early behavior of the cut path follows a portion of a logathmic spiral, and that the path tends to another spiral with a different pitch as the crack adds more turns. Our crack experiment illustrates the fact that thin sheets mechanics is deeply connected to geometry, and finally spirals characteristics allow us to measure material crack properties of the thin layer used. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A39.00008: Interaction Between Two Localized Wrinkle Patterns Jiangshui Huang, Wim H. de Jeu, Narayanan Menon, Thomas P. Russell A drop of water placed on the surface of a freely floating ultrathin polymer film produces a radial wrinkling pattern due to the capillary force it exerts on the film. ~We have previously characterized [1] the number N and length L of the wrinkles. We now study the interaction between two such localized wrinkling patterns each induced by one drop of water. The patterns distort, and radial symmetry about each drop is lost, with the wrinkles extending further along the line between the drops. When the drops are brought closer, a single long wrinkle forms along this axis. We use the distance at which this connecting wrinkle appears to quantify the range of the interaction between the wrinkles. We will present data for this interaction length as a function of other length scales in the experiment. \newline 1. Full reference here. Science 317, 650(2007) [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A39.00009: Pattern transformation triggered by deformation. Tom Mullin Periodic elastomeric cellular solids are subjected to uniaxial compression and novel transformations of the patterned structures are found upon reaching a critical value of applied load. The results of a numerical investigation reveal that the pattern switch is triggered by a reversible elastic instability. Excellent quantitative agreement between numerical and experimental results is found and the transformations are found to be remarkably uniform across the samples. Moreover the phenomenon is found to be robust for a range of soft solids including rubber and jelly. *Joint work with M.C. Boyce, K. Bertoldi and S. Deschanel, MIT. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A39.00010: Granular Silo collapse: an experimental study Eric Clement, Gustavo Gutierriez, Philippe Boltenhagen, Jose Lanuza We present an experimental work that develop some basic insight into the pre-buckling behavior and the buckling transition toward plastic collapse of a granular silo. We study different patterns of deformation generated on thin paper cylindrical shells during granular discharge. We study the collapse threshold for different bed height, flow rates and grain sizes. We compare the patterns that appear during the discharge of spherical beads, with those obtained in the axially compressed cylindrical shells. When the height of the granular column is close to the collapse threshold, we describe a ladder like pattern that rises around the cylinder surface in a spiral path of diamond shaped localizations, and develops into a plastic collapsing fold that grows around the collapsing silo. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A39.00011: Sudden ridge collapse in the stress relaxation of thin crumpled polymer films Ingo Dierking, Paul Archer Uniform compression of thin crumpled sheets subjected to a constant weight has been shown to exhibit a remarkably wide range of scaling behaviour, covering up to five orders of magnitude [1], i.e. time scales from seconds to weeks. We demonstrate that this scaling behaviour is not smooth, but rather interrupted by sudden changes in height of the uniformly compressed crumple, which we attribute to sudden ridge collapses. The height of the discontinuous jumps due to sudden ridge collapse during the compression process increases with increasing thickness of the polymer film. This is attributed to the fact that thick films exhibit a smaller defect density, but increased defect length. Interestingly, when plotting the time laps between successive ridge collapses as a function of time, the data collapses to a single line for all film thicknesses, with a slope of d$\Delta $t/dt=1 over a scaling regime of four orders of magnitude. Possible explanations will be discussed. [1] K. Matan, R.B. Williams, T.A. Witten, S.R. Nagel, Phys. Rev. Lett., 88, (2002), 076101. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A39.00012: Geometry, mechanics and statistical physics in crumpled structures Laurent Bou\'e, Arezki Boudaoud, Mokhtar Adda-Bedia, St\'ephanie Deboeuf, Eytan Katzav There's been a recent surge of interest in the study of low-dimensional packed elastic manifolds. In fact, the simple act of crumpling a piece of paper does require the simultaneous interaction of many fascinating mechanisms. These include energy condensation from large length scales to small singular structures, topological self-avoidance and complex phase space landscapes reminiscent of frustration in the context of glassy systems. We will present a numerical experiment modeling the folding of an elastic rod (1D) restricted to a shrinking 2D space. The confinement is obtained by preparing an initially disordered elastic line embedded in a quadratic potential. Varying the strength of this confining potential shows that many metastable states can be observed. We are interested in a statistical analysis of the emerging folded patterns. We will discuss the relevance of our results with recent theoretical models (inspired by the free-volume theory of Edwards in the context of granular matter) and recent experiments of crumpled paper. \newline Some references: L. Bou\'e {\it et al}, PRL {\bf 97} (2006) 166104, L. Bou\'e and E. Katzav EPL {\bf 80} (2007) 54002, E. Katzav, M. Adda-Bedia and A. Boudaoud PNAS {\bf 103} (2006) 18900-18904. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A39.00013: Inside a Ball of Crumpled Aluminum Foil Anne Dominique Cambou, Narayanan Menon We have studied the three-dimensional geometry of a crumpled sheet via x-ray CT scans. We crumple circular sheets of aluminum with thicknesses of 30--50$\mu m$ and diameter 100000$\mu m$ into spherical balls of diameter 15000$\mu m$ to 20000$\mu m$. We then perform CT scans with a resolution of 6$\mu m^3$/voxel. This allows us to fully resolve the conformation of the sheet. We use the reconstructed CT images to determine the mass distribution inside the crumpled ball. We also report on a box-counting analysis to assess the fractal nature of the mass distribution. [Preview Abstract] |
Session A40: Phase Stability and Phase Transitions
Sponsoring Units: DCMPChair: Stefano Curtarolo, Duke University
Room: Morial Convention Center 232
Monday, March 10, 2008 8:00AM - 8:12AM |
A40.00001: First-principles solution to the problem of Mo lattice stability Igor Abrikosov, Christian Asker, Arkady Mikhaylushkin, Anatoly Belonoshko The energy differences between the ground state body-centred structure (bcc) and closed-packed face-centred structure (fcc) structures for transition metals in the middle of the series show unusually large disagreements when they are obtained by the thermochemical approach based on the analysis of experimental data or by first-principles electronic structure calculations. Considering a typical example, the lattice stability of Mo, we present a solution to this long-standing problem. In contrast to conventional total energy calculations within Density Functional Theory framework, we carry out \textit{ab initio }molecular dynamics simulations for the two phases at high temperature. We show that at these conditions both bcc and fcc structures of Mo are dynamically stable, and the difference in their configurational energies decreases dramatically as compared to the zero temperature result, approaching the value derived by means of the thermochmical approach. We show that the main contribution to the effect comes from the modification of the canonical band structure for bcc and fcc phases due to lattice vibrations at high temperature, and discuss consequences of our finding for future first-principles simulations of phase stability. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A40.00002: Link between structural and mechanical stability of fcc- and bcc-based ordered Mg-Li alloys. Maje Phasha, Phuti Ngoepe, Hasani Chauke, Duc Nguyen-Manh, David Pettifor The first principles pseudopotential calculations based on the Perdew-Burke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) within density functional theory (DFT) have been used to successfully investigate the electronic and elastic properties of cubic-based Mg-Li alloys. The heats of formation, Jones-type analysis and mechanical elasticity were utilized in predicting structural stability profile, and their results consistent with each other. An interesting correlation between shear modulus (C$\prime )$ and the predicted energy differences of corresponding bcc and fcc ordered compounds relative to hcp Mg and Li lattices is observed. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A40.00003: Density-Functional and CALPHAD Studies of U-Zr Alloys Alexander Landa, Per Soderlind, Patrice Turchi, Levente Vitos, Andrei Ruban The U-Zr alloy alloys have been recognized as a fuel for liquid-metal fast breeder reactors. First-principles methods are employed to study ground-state properties of U-Zr alloys for the most important phases observed experimentally, namely \textit{$\gamma $} (bcc) and \textit{$\delta $} (C32). Effective interatomic interactions obtained from the screened GPM, incorporating KKR-ASA-CPA, have been applied in MC simulations to derive the \textit{$\gamma $}-phase miscibility gap. EMTO-CPA method has been applied to study properties of the open \textit{$\delta $}-phase. Results of \textit{ab initi}o calculations are compared with experimental data and CALPHAD assessment. Then, the CALPHAD assessed U-Zr phase diagram is contrasted with the one predicted with the input from \textit{ab initio}. This work shows that an overall validity of a combined \textit{ab initio}--CALPHAD approach to thermodynamic properties exists, and that the knowledge and quantitative output gained from quantum mechanics on phase stability and its relation to $f$-bonding, can be used to explore other actinide-based systems, for which experimental data are sparse or lacking. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A40.00004: Phase Stability Studies of Ni3Al and Pt3Al Structures Hasani Chauke, Ralf Drautz, Benoit Minisini, Phuti Ngoepe, David Pettifor The structural instability of cubic L12 against the non-cubic DO'C and tP16 Pt3Al have been investigated in direct comparison with Ni3Al, using plane-wave pseudopotential methods within the local density approximation. We predict that the deleterious DO'C is more stable than cubic ductile L12 Pt3Al, in agreement with the experimental observation of the LT DO'C phase and HT L12 phase. In contrast to the Pt-Al, the L12 Ni3Al is ground state and is consistent with the experimental phase diagram. The transformation from L12 into DO'C has been investigated more explicitly, where the transformation path along DO'C leads to a stable phase at displacement parameter u=0.041?. However the Ni3Al phase, is stable at u=0.0 corresponding to the cubic L12 phase, as expected. The phonon dispersion spectra have been used to confirm the relative structural trend where a soft mode was detected for L12 and DO'C which is not found in tP16 Pt3Al. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A40.00005: Where are Nature's missing structures Gus L.W. Hart Our society's environmental and economic progress depends on the development of high-performance materials such as lightweight alloys, high-energy-density battery materials, recyclable motor vehicle and building components, and energy-efficient lighting. Meeting these needs requires us to understand the central role of crystal structure in a material's properties. Despite more than 50 years of progress in first- principles calculations, it is still impossible in most materials to infer ground-state properties purely from a knowledge of their atomic components--a situation described as `scandalous' in the well-known essay by Maddox. Many methods attempt to predict crystal structures and compound stability, but here I take a different tack--to infer the existence of structures on the basis of combinatorics and geometric simplicity. The method identifies `least random' structures, for which the energy is an extremum (maximum or minimum). Although the key to the generic nature of the approach is energy minimization, the extrema are found in a chemistry-independent way. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A40.00006: Fully {\it ab initio} melting curve of aluminium up to 350 GPa Johann Bouchet, Francois Bottin, Gerald Jomard We performed {\it ab initio} molecular dynamics simulations to compute the melting curve of aluminium. Both the two-phase (TP) and heat-until melt (HUM) methods have been considered. The former describes an heterogeneous mechanism with a well-defined melting temperature T$_{\rm m}$. On the other hand, in the HUM the crystal melts homogeneously and can be overheated above T$_{\rm m}$. The limit of superheating is the critical temperature T$_{\rm LS}$. Calculations are carried out in the NVT ensemble up to 350 GPa with supercells of different sizes. For each method, we present the convergence of the melting curves as a function of the number of atoms, and compare our results with previous calculations and experiments. We also discuss the evolution of the degree of overheating with respect to the pressure. At last, by means of NPT simulations we also compute volume change on melting, which is in good agreement with previous free energy calculations. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A40.00007: Free energies from ab initio calculations for liquid Mg Raquel Lizarraga, Carl Greeff We performed free energy calculations for liquid metals from {\it ab initio} potential surfaces by means of Monte Carlo methods that involve sampling on the potential surface defined by a reference system. This allows for large gains in efficiency because the random walk is carried out on the much faster reference potential, and the {\it ab initio} potential is only evaluated on a small subset of uncorrelated configurations. This is highly desirable since direct free energy calculations for liquid metals from ab initio potential surfaces are very computationally intensive. Our calculations on liquid magnesium shows that we can obtain free energies accurate at the meV/atom level with only 100 evaluations of the {\it ab initio} total energies. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A40.00008: Ultrafast melting and solidification of Ag studied by time-resolved third harmonic generation Wai Lun Chan, Virginia McCreary, Alexie Lagoutchev, Yinon Ashkenazy, Kwangu Kang, David Cahill, Robert Averback We study the transformation of Ag between the solid and liquid phases using pump-probe femtosecond laser experiments. A pump with fluences of 100-500 mJ cm$^{-2}$ is used to heat the sample while third harmonic generation of light from a time-delayed probe is used to determine the structure as a function of time. For the melting experiment, we find that the solid begins to melt before the phonons are thermalized by the electrons, according to the two-temperature model. In addition, we find by using Ag thin films with different thicknesses, that the depth of heat deposition is less than 100nm. Both observations suggest that the established two-temperature model is insufficient to explain the melting kinetics in Ag by the femto-second laser. For the solidification experiment, we are able to determine the solidification velocity as a function of undercooling down to half of the melting temperature. The results are compared to predictions form molecular dynamics simulations. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A40.00009: Revisiting the segregation driving forces rule: the Coupled Three Effects Model J\'{e}r\^{o}me Creuze, Isabelle Braems, Fabienne Berthier, Christine Mottet, Guy Tr\'{e}glia, Bernard Legrand Separating the surface segregation enthalpy into three elementary contributions (cohesive, alloy and size) has been proposed by many authors, but rarely tested quantitatively. Such a separation rule, derived from a tight-binding Hamiltonian years ago, has yielded very satisfying results for various environments (surfaces, grain boundaries and clusters) for the Cu-Ag system and for many other alloys, but recently stumbled over the Co-Pt system. We propose a new approach based on a systematic study of the permutation enthalpies, both in the bulk and in the surface, as a function of the mixed interaction involved in the $N$-body interatomic potentials derived from the electronic structure. We then show that both the disagreement observed for Co-Pt and the agreement mentioned for Cu-Ag can be explained by the variation of the effective pair interactions in the surface and by the existence of coupling coefficients between the three effects. Finally, we introduce a new decomposition, the Coupled Three Effects Model (CTEM), that is valid for systems with both size and cohesive effects. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A40.00010: Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. {\%} Ga. Jason R. Jeffries, K.J.M. Blobaum, M.A. Wall, A.J. Schwartz The gallium-stabilized Pu-2.0 at. {\%} Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable delta phase to the monoclinic alpha-prime phase near -120 $^{\circ}$C. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation diagram. While the underlying mechanisms responsible for the double-C behavior are currently unresolved, recent experiments suggest that a conditioning treatment influences the upper-C. As such, the effects of the conditioning treatment can provide valuable insight into the mechanisms dominating the phase transition. A differential scanning calorimeter (DSC) is used to investigate the effects of conditioning temperature and time upon the delta/alpha-prime transition. The results will be discussed as they pertain to radiation damage, nucleation, embryo formation, or phase-field stability. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A40.00011: New Transformation Path of Shape Memory NiTi N. Hatcher, O. Yu. Kontsevoi, A.J. Freeman The detailed mechanism of structural evolution during the martensitic transformation in NiTi is not fully understood. To fully characterize the transformation path and to precisely determine the structure of its martensitic phase, we employ the highly-precise all-electron full-potential linearized augmented plane wave (FLAPW) method\footnote{Wimmer,\,Krakauer,\,Weinert,\,and\,Freeman,\,Phys.\,Rev.\,B,\,{\bf 24},\,864\,(1981)}, and identify the governing processes of the martensitic transformations between the B2, B19, R, B19$'$ and recently proposed B33 and BCO phases. Several precursor phenomena for displacive phase transformations in the B2, B19, and R phases are identified, i.e. soft C$_{44}$ and C$'$ elastic constants, regions of Fermi surface nesting, and instabilities towards electronic topological transitions. By means of generalized stacking fault calculations, we find that the B2 phase has a low resistance to $\langle100\rangle\{110\}$ shear. A new, barrrierless transformation path from B2 to B19$'$ is established by calculating a bilayer $\langle100\rangle\{110\}$ shear with full structural optimization which leads to a monoclinic intermediate phase at $1/2$a displacement; a relaxation of this structure's monoclinic angle results in the B19$'$ phase. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A40.00012: Magneto-Resistance at the Pre-Martensite Transition in Ni$_{2}$MnGa C. P. Opeil, J. C. Lashley, J. L. Smith A magneto-resistance and angle resolved photoemission (ARPES) study of the pre-martensite phase of the ferromagnetic shape memory alloy single crystal Ni$_{2}$MnGa, reveals a temperature (235 \underline {$>$} T \underline {$>$} 190 K) and field dependent (0 -- 1 T) positive/negative magneto-resistance slope. Previous inelastic neutron scattering experiments (Zheludev et al., PRB \textbf{51}, 1995) on this Heusler alloy indicate a phonon branch [110]-TA$_{2}$ softening in the pre-martensite phase along \textbf{q} = (1/3, 1/3, 0).~ This phonon softening combine with our ARPES data show that significant depletion of states (pseudo gap) occur at the premartensitic transition temperature. Recent results (Shapiro et al., EPL \textbf{77}, 2007) reveal phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting.~ Our experimental results will be discussed in light of electron-phonon coupling. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A40.00013: Optical properties of ErH$_{2+x}$: First principles calculations and experimental measurements. Clark Snow, Thomas Mattsson Rare earth and transition metal hydrides exhibit many interesting physical phenomena, from metal-semiconductor transitions to transparency changes at RT as a function of hydrogen content. Electrical resistivity measurements by P. Vajda (1) indicate that ErH$_{2+x}$ undergoes a metal-semiconductor transition between 240-290K and an antiferromagnetic transition between 1.75-2.3K depending on hydrogen content. These same transitions should also cause profound changes in the optical properties. This work will present first principles calculations of the optical properties of ErH$_{2+x}$ from 300-600K where x ranges from -0.2 to 0.2. The calculations will be compared to experimental results on thin films of ErH$_{2+x}$ which were obtained as a function of temperature from 10-600K and hydrogen content. \newline [1] P. Vajda and J.N. Daou, Phys. Rev. B., Vol. 49, p. 3275 (1994). [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A40.00014: Role of defects and impurities on the thermal stability of transition-metal nitrides and carbides L. Tsetseris, N. Kalfagiannis, S. Logothetidis, S. T. Pantelides Transition-metal nitrides and carbides are used in a variety of applications because of their renowned hardness and stability. Here, we present the results of first-principles calculations on point defects and impurities in the prototype systems of TiN and TiC, and in HfN and ZrN. We find features which are common to all systems, while we unravel also key differences. In cases, for example N interstitials in TiN, the interaction between defects is attractive and it favors the formation of defect complexes. Moreover, we show that the atomic-scale mechanisms of interaction and migration of point defects and their complexes can account for various changes of transition metal nitrides and carbides after annealing at widely different temperatures. Finally, we discuss the fundamentals of trapping and release of the most common impurities in TiN. The work was supported in part by the McMinn Endowment at Vanderbilt University, AFOSR MURI Grant FA9550-05-1-0306, and GSRT-PENED- 03ED613. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A40.00015: Atom Transport in Random Close Packed Metal Alloys under Thermal Forcing Yong W. Kim Alloy making entails disparate pyro-metallurgical pathways, contributing to variability in elemental composition profiles. Surface segregation by constituent elements is a long-standing example. Thermal cycling and exposures to intense fluxes of energetic particles and photons, as in fission and fusion reactors, force the movement of composition profiles. Transport properties thus become dependent of materials' history. We note that a non-crystalline alloy specimen is a randomly close packed assembly of atoms, and, as such, contains a distribution of residual nano-crystallites. The primary effect of varied forcing mechanisms is to convert atoms bound in nano-crystallites into those of glassy configurations. Spatial fluctuation is increased, specific to elemental species. In this paper we present a modeling of thermal conversion of crystallite atoms into a glassy state. The distribution function of nano-crystallites by size at room temperature is modeled by the distribution of nano-clusters formed from an atomic vapor plume.[Kim, Lee, Belony, Rev. Sci. Instr. \textbf{17}, 10F115 (2006)] At a given temperature, equilibrium dissociation of a nano-crystallite into `glassy' atoms is treated by the law of mass action. The equation of state is fashioned after the thermal expansion of the specimen volume with respect to a reference. A large set of simultaneous dissociation equations is solved iteratively. Work supported in part by NSF-DMR(Metals). [Preview Abstract] |
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