Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y3: Novel Physics and Quantum Interference in Weakly Disordered Graphene Devices
Sponsoring Units: DCMPChair: Alberto Morpurgo, Technical Univerisity of Delft
Room: Morial Convention Center RO2 - RO3
Friday, March 14, 2008 11:15AM - 11:51AM |
Y3.00001: Theoretical studies of quantum interference effects in graphene Invited Speaker: We review the recently-developed theory of weak localization in monolayer and bilayer graphene [Phys. Rev. Lett. 97, 146805 (2006); Phys. Rev. Lett. 98, 176806 (2007)]. Owing to the chiral nature of electrons in a monolayer of graphite (graphene) one can expect weak antilocalization and a positive weak-field magnetoresistance in it. For high-density monolayer graphene and for any-density bilayers, the dominant factor affecting weak localization properties is trigonal warping of graphene bands, which reflects asymmetry of the carrier dispersion with respect to the center of the corresponding valley. The suppression of weak antilocalization by trigonal warping is accompanied by a similar effect caused by random- bond disorder (due to bending of a graphene sheet) and by dislocation/antidislocation pairs. As a result, weak localization in graphene can be observed only in samples with sufficiently strong inter-valley scattering due to atomically sharp scatterers or by edges in a narrow wire, reflected by a characteristic form of conventional negative magnetoresistance. We show this by evaluating the dependence of the magnetoresistance of graphene on relaxation rates associated with various possible ways of breaking a ``hidden'' valley symmetry of the system. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y3.00002: Quantum interference in Epitaxial Graphene: Evidence for Chiral Electrons Invited Speaker: The extraordinary transport properties of graphene originate from its unique band structure. Electrons in the band have chirality, which correlates the directions of the momentum and the psuedospin. This chirality prevents electrons from being back-scattered, hence causing a particular quantum phase coherent phenomenon, called weak-antilocalization. Recent theoretical work suggested that multilayer epitaxial graphene (EG) grown on SiC possesses essentially same band structure as single layer graphene because of the rotational stacking order between layers. We have investigated the magnetoresistance of EG. We found that weak anti-localization manifests itself as a broad cusp-like depression in the longitudinal resistance for magnetic fields 10 mT $B$ 5 T. An extremely sharp weak-localization resistance peak at $B=0$ is also observed. These features quantitatively agree with graphene weak-(anti)localization theory implying the chiral electronic character of the samples. Scattering contributions from the trapped char ges in the substrate and from trigonal warping due to the graphite layer on top are tentatively identified. The phase coherence length was found about 1 $\mu$m at 4.2 K and the main phase-breaking mechanism was $e-e$ scattering. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y3.00003: Quantum Interference in Single and Bilayer Graphene Invited Speaker: It is known that interference of charge carriers scattered by impurities results in a quantum correction to the conductivity of a two-dimensional (2D) system. This phenomenon of weak localisation (WL) is usually seen as magnetoconductivity, as magnetic field changes the phase of interfering waves. Here we show that quantum interference in graphene -- a single layer of carbon atoms [1] -- is very different from that in conventional 2D systems. Due to the \textit{chiral} nature of carriers, it becomes sensitive to different \textit{elastic} scattering mechanisms. By changing the geometry and quality of samples we show that quantum interference in graphene can take a variety of forms, and that WL is a sensitive tool to detect defects in graphene crystals [2]. We perform a comparative study of WL in single-layers and bilayers. Although the two systems are different in their spectrum (massless and massive fermions, respectively), the carriers in both are chiral. As a result WL in a bilayer is also affected by elastic scattering [3]. Analysis of the magnetoresistance using theories [4] allows us to determine the phase-breaking time as well as times of inter- and intra-valley scattering, which together control WL. They are found at different carrier densities, including the electro-neutrality point where the nominal carrier density is zero. We show that in all cases WL is not suppressed, and that the reason for this is strong inter-valley scattering. The study of WL is complemented by AFM imaging of the surface which provides information about the nature of the defects responsible for the different manifestations of WL in graphene systems. In addition to the studies of WL, we perform analysis of universal conductance fluctuations (UCF) in both systems. They have the same physical origin as WL -- quantum interference -- and are controlled by the same characteristic times. We compare the times found from analysis of WL and UCF. \newline [1] K.S. Novoselov \textit{et al}., Science \textbf{306}, 666 (2004) \newline [2] F.V.Tikhonenko \textit{et al.}, arXiv: 0708.1700 (to be published in Phys. Rev. Lett.) \newline [3] R.V. Gorbachev \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 176805 (2007). \newline [4] E. McCann et al., Phys. Rev. Lett. \textbf{97}, 146805 (2006); K. Kechedzhi \textit{et al}., Phys. Rev. Lett. \textbf{98}, 176806 (2007). [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y3.00004: Experimental studies of conductance fluctuation and tunneling spectroscopy of weakly disordered graphene devices Invited Speaker: We measured the conductance fluctuation and tunneling spectra of single-, bi- and trilayer graphene prepared by mechanical exfoliation. Reproducible fluctuations in conductance as a function of applied gate voltage or magnetic field were found in the electric transport measurements. As the Fermi energy was tuned to near the charge neutral point, the amplitude of the conductance fluctuation was suppressed quickly from a value consistent with universal conductance fluctuations, even though the devices were still well within the weakly disordered regime. The physical origin of the observed suppression may be related to the presence of edge states, or puddles of electrons or holes in graphene devices. The tunneling spectra of planar tunnel junctions of micron size were found to exhibit interesting features that evolve with the backgate voltage, temperature, and applied magnetic field. The implications of these observations will be discussed. This work is done in collaboration with Neal Staley, Conor Puls, and Haohua Wang. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y3.00005: Excitations from Filled Landau Levels in Graphene Invited Speaker: Recent experimental progress has allowed the fabrication of graphene, a two-dimensional honeycomb lattice of carbon atoms that forms the basic planar structure in graphite. Graphene exhibits a host of interesting properties that can be understood in terms of a non-interacting system whose single-particle excitations are described by the Dirac equation. What, then, is the role of the Coulomb interactions between electrons in graphene? In this talk, I will present a study of the collective excitations of graphene in the quantum Hall regime. These excitations open a window into the nature of Coulomb interaction effects and may be observable by optical spectroscopy. Such excitations are well-understood in the case of the standard two-dimensional electron gas (2DEG), in which the low-lying collective mode spectrum may be interpreted in terms of a single particle-hole pair bound into a stable exciton by Coulomb forces. Using a similar analysis for graphene, we find that, in spite of the linear electronic dispersion near the Dirac points, the exciton spectrum is qualitatively quite similar to that of the 2DEG. On the other hand, the additional pseudospin degree of freedom strongly enhances many-body corrections relative to the 2DEG case. We also find that the presence or absence of certain branches of the exciton spectrum is sensitive to the number of filled spin and pseudospin sublevels. Finally, I will discuss these results in relation to infrared spectroscopy measurements and comment on the screening of the Coulomb interaction in graphene. [Preview Abstract] |
Session Y4: New Symmetries and Excitations in Multiferroics
Sponsoring Units: DCMPChair: Dennis Drew, University of Maryland
Room: Morial Convention Center 206
Friday, March 14, 2008 11:15AM - 11:51AM |
Y4.00001: Observation of ferrotoroidic order in LiCoPO$_4$ Invited Speaker: Domains are an essential property of any ferroic material. Three forms of ferroic order (ferromagnetism, ferroelectricity, ferroelasticity) are widely known. It is currently debated whether to include an ordered arrangement of magnetic vortices as fourth form of ferroic order termed ferrotoroidicity [1]. Although there are reasons to do this from the point of view of thermodynamics a crucial hallmark of the ferroic state, i.e., a ferrotoroidic domain structure, has not been observed before. Here ferrotoroidic domains are spatially resolved by optical second harmonic generation in LiCoPO$_4$ where they coexist with independent antiferromagnetic domains [2]. The origin of ferrotoroidicity in LiCoPO$_4$ is discussed and the general relation between ferrotoroidicity and antiferromagnetism or spin-spiral magnetism will be highlighted. Their space- and time asymmetric nature relates ferrotoroidics to multiferroics with magnetoelectric phase control and other systems in which space and time asymmetry leads to exciting possibilities for future application.\newline $[1]$ C. Ederer, N.A. Spaldin, arXiv:0706.1974v1 [cond-mat.str-el], Phys.\ Rev.\ B, in press (2007) \newline $[2]$ B.B. Van Aken, J.P. Rivera, H. Schmid, M. Fiebig, Nature {\bf 449}, 702 (2007) [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y4.00002: Symmetry in Multiferroics Invited Speaker: Symmetries govern Nature ubiquitously from the beauty of human faces to the local gauge invariance of quantum field theory. Magnetic order in frustrated magnets can occur without space inversion symmetry. When it relaxes to the magnetically-ordered configuration through exchange-striction, lattice can also loose inversion symmetry, leading to the presence of ferroelectric polarization. In these magnetically-driven ferroelectrics, dielectric properties turn out to be highly susceptible to applied magnetic fields. Both symmetric and antisymmetric exchange coupling can be involved in the exchange-striction. One form of symmetry often broken in Nature is the symmetry between left- and right-handedness. For example, the manner in which light propagates naturally selects one handedness, and is customarily described by a right-handed rule, depicting the relationship among the oscillating electric field, magnetic field and propagation vector of light. Chiral molecules also have a definite handedness, and given the preponderance of chiral molecules, it is not surprising that most complex proteins as well as their constituent amino acids are chiral. What is remarkable however, is that most of naturally occurring amino acids share the same chirality; only left-handedness. Such handedness, or chirality, appears to be a characteristic signature of life. In the multiferroic spinel CoCr$_{2}$O$_{4}$, conical magnetic order accompanies ferroelectric polarization as well as ferromagnetic moment. The relevant handedness and chirality in the multiferroic state will be also discussed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y4.00003: Towards a microscopic theory of toroidal moments in periodic crystals Invited Speaker: The recent resurgence of interest in magnetoelectric multiferroics has prompted discussion of the relevance of the concept of magnetic toroidal moments in such systems. In particular, the toroidal moment has the same symmetry as the antisymmetric part of the linear magnetoelectric tensor, suggesting a role in mediating coupling between magnetization and electric polarization in multiferroics. In addition, materials in which the toroidal moments are aligned cooperatively -- so-called ferrotoroidics -- have been proposed to complete the group of primary ferroics\footnote{B.B. Van Aken, J.P. Rivera, H. Schmid and M. Fiebig, Nature 449, 702 (2007).}. Here we review the basic microscopic and macroscopic definitions of toroidal moments and illustrate the difficulties in evaluating the toroidal moment of an infinite periodic system. We show that periodic boundary conditions give rise to a multivaluedness of the toroidal moment per unit cell, in close analogy to the case of the electric polarization in bulk periodic crystals. We then evaluate the toroidal moments of several multiferroic and magnetoelectric materials (BaNiF$_4$, LiCoPO$_4$, GaFeO$_3$ and BiFeO$_3$) in the ``localized dipole limit'', where the toroidal moment is caused by a time- and space-reversal symmetry-breaking arrangement of localized magnetic moments\footnote{C. Ederer and N.A. Spaldin, arXiv:0706.1974v1 [cond-mat.str.el], Phys. Rev. B in press.}. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y4.00004: Role of Spin current in multiferroic behavior Invited Speaker: |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y4.00005: Electromagnons in multiferroics Invited Speaker: Multiferroic materials with simultaneous magnetic and ferroelectric order exhibit strong cross coupling between electric and magnetic phenomena. One important new effect is the strong coupling between the low lying magnetic and lattice excitations to produce spin waves that interact strongly with light by acquiring electric dipole activity from the phonons. As a result, these excitations, which are called electromagnons, produce contributions to the static dielectric constant which appear in the ordered phases and that can be manipulated with an applied magnetic field. This appears to be the origin of the giant magneto-capacitance effect observed in these multiferroics. Predicted more than three decades ago, electromagnons were reliably observed only recently. In my talk, I will discuss electromagnons in two classes of multiferroic materials: RMnO3 and RMn2O5 (R = Y, Rare Earth) in which the multiferroicity derives from different mechanisms. Correspondingly the electromagnons in these two materials systems have characteristically different spectra and selection rules. The electromagnon H-T phase diagrams for Eu0.75Y0.25MnO3, TbMnO3, TbMn2O5 will be presented. I will also discuss the outstanding problems in understanding these novel excitations and the prospects for electromagnons in other materials. [Preview Abstract] |
Session Y5: Charged and Ion-Containing Polymers II
Sponsoring Units: DPOLYChair: Ron Hedden, Pennsylvania State University
Room: Morial Convention Center RO1
Friday, March 14, 2008 11:15AM - 11:51AM |
Y5.00001: Manipulating Assembly, Disassembly and Exchange in Responsive Polyelectrolyte Multilayers Invited Speaker: Polyelectrolyte multilayer assembly is based on the alternating adsorption of multilvalent positively and negatively charged species to create ionically crosslinked thin films with nanoscale control of film composition and function. We have utilized this method of assembly to manipulate ion transport, molecular transport, and electrochemical transport in these films, enabling the generation of a range of organic and organic-inorganic devices. Biological materials applications are also derived from such films, enabling their use as drug delivery devices. In each of these applications, it is desired to control interdiffusion and exchange within the multilayer systems to maintain desired function and generate isolated regions of composition and function within the z-direction of the film. Here we address these applications and means of controlling this phenomenon. Furthermore, it is desirable to induce controlled means of disassembly of these multilayer thin films. We will address a number of approaches for achieving this, including hydrolytic degradation, hydrogen bond dissociation, and controlled deconstruction on electrochemical impulse. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y5.00002: Using Folding Pathways to Predict Protein Structure Invited Speaker: Since the demonstration that the amino acid sequence of a protein encodes its structure, the prediction of structure from sequence remains an outstanding problem that impacts numerous scientific disciplines. By iteratively fixing secondary structure assignments of residues during Monte Carlo simulations of folding, a coarse grained model without homology information or explicit side chains outperforms current homology-based secondary structure prediction methods. The computationally rapid algorithm also generates tertiary structures with backbone conformations of comparable accuracy to existing all-atom methods for many small proteins, particularly for low homology sequences. Given appropriate search strategies and scoring functions, reduced representations can accurately predict secondary structure as well as three-dimensional structures, thereby increasing the size of proteins approachable by \textit{ab initio} methods and the accuracy of template-based methods, in particular for sequences with low homology. In addition, we will discuss recent advances in understanding non-linear electrostatic contributions to transfer free energies in continuum electrostatic models. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y5.00003: Temporal and Spatial Distributions of Water in Ion-Containing Perfluorosulfonic Polymers Invited Speaker: The spatial distribution and the molecular dynamics of water in perfluorinated ionomer polymeric membranes (Nafion 11x in the acid form) were quantified at several hydration levels by Atomic Force Microscopy and Dielectric Relaxation Spectroscopy, respectively. A variety of concurrent AFM modes, including interleave and intermittent contact methods, is necessary to map the water-containing domains on the polymer surfaces, whereas at least two different dielectric relaxation setups are needed to record the range of water dynamics that develop in these systems as the hydration level changes. The competition between sulfonic-group/water attraction and water/water hydrogen-bonding, in addition to confinement effects, give rise to at least three ``states'' of water, manifested through distinct dynamical behaviors: The fastest process observed was identified as the cooperative picosecond relaxation of free/isotropic, bulk-like water, whereas the slowest process --with microsecond relaxation times-- corresponds to water molecules strongly bound to the charged sulfonic groups. An intermediate relaxation, in the picosecond range and about three times slower than those of bulk water, is shown to contain substantial dynamical heterogeneities and most probably corresponds to a variety of local environments that are cumulatively defined as ``loosely bound'' water. AFM studies, probing the same surfaces at various hydration levels, provides insights on the location and geometry of the water domains that contribute to the various dynamical ``states''. Both the spatial and temporal distributions of water are sensitive to the sample preparation conditions, especially with respect to the geometry and dynamics of the ``loosely bound'' water domains. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y5.00004: Microrheological studies of solvent-response dynamics of polyelectrolytes Invited Speaker: We have developed a dialysis cell for microrheology, which provides unique capabilities for studying microstructural dynamics of macromolecular solutions due to sudden changes in solvent composition (e.g., ionic strength, pH, solvent/cosolvent ratio). The device was used to determine the response of sodium sulphonated polystyrene (NaPSS) solutions of different molecular weights to changes in counterion concentration. In general, polyelectrolyte chains collapse upon addition of counter-ions, but recent numerical simulations by Hsiao and Luijten (PRL 97, 2006) predicted reexpansion at high concentrations of multivalent counterions. We tested and confirmed these predictions for trivalent chloride salts (Al, In, Tl), although the effect is subtle and strongly varies between the cations investigated. Another study employed the dialysis cell to characterize pH-induced swelling and deswelling of colloidal microgel particles of a poly(N-isopropylacrylamide)-co-(acrylic acid) copolymer. The acid copolymer caases pH-responsiveness, swelling the particles at high pH due to deprotonation. In dilute suspensions of these particles, we studied the swelling response for different AAc fractions as a function of pH. In the concentrated suspensions, pH-induced particle expansion can cause transitions between fluid, glassy and crystalline phases. Data will be presented on the dynamics of the observed phase behavior. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y5.00005: Ion- and pH-dependent volume transitions in biopolymer gels Invited Speaker: Swelling and collapse of polyelectrolyte gels are the result of a balance of different interactions that control the osmotic pressure and network elasticity. In biopolymer systems ions often play a central role in determining the phase behavior. For example, DNA condensation induced by multivalent cations is crucial for its packaging. It is known that biological processes, such as nerve excitation and muscle contraction, are mediated by divalent cations. In general, relatively little is known about the interaction between multivalent ions and charged biopolymers due to the lack of an appropriate theory and the absence of a sufficiently broad base of experimental data. Recent experimental observations made by anomalous small-angle X-ray scattering indicate that the spatial extent of the counterion cloud is significantly reduced in the case of divalent ions relative to the monovalent ions. An understanding of ion induced swelling/collapse transition in polyelectrolyte gels may shed light on the mechanism of important physiological processes. We compare the effects of pH, ionic strength and counterion valence on the structure and osmotic properties of biopolymer gels. Systematic studies made on DNA gels indicate that monovalent salts gradually reduce gel swelling but do not cause discontinuous volume transition. Introducing calcium ions into the gels produces a reversible volume change. Similarly, decreasing the pH in the surrounding environment leads to shrinkage of the swollen networks. Scattering observations reveal that cations mediate the equilibrium properties by modifying the local environment and the organization of the polymer chains. Osmotic pressure measurements detect significant differences between the effects of pH and ion valence. [Preview Abstract] |
Session Y6: Industrial Advanced Characterizations
Sponsoring Units: FIAPChair: Mark Disko, Exxon Mobile Research and Engineering Co.
Room: Morial Convention Center RO4
Friday, March 14, 2008 11:15AM - 11:51AM |
Y6.00001: Local strain analysis for CMOS technology by Raman and Nano-Raman spectroscopy Invited Speaker: In present and future high performance CMOS devices, the dedicated introduction of mechanical strain into active silicon regions is an important challenge to progress technology. For both basic understanding of the structure-strain relationship and for developing improved device structures, local measurement of the strain state has become essential. A promising technique that meets these requirements is Raman spectroscopy, enabling strain and composition determination in silicon structures on the $\mu $m-scale with high accuracy. Locally enhanced Raman intensities due to tip enhanced Raman spectroscopy (TERS) can be utilized to downscale the spatial resolution of Raman spectroscopy significantly below the optical diffraction limit. It will be shown for various strained silicon and silicon-germanium film structures, that optical near-field information can be obtained on a scale promising for CMOS device characterization. Local strain measurements are achieved utilizing TERS with metal coated AFM tips positioned in the region of interest. Experimental results of $\mu $Raman and of TERS scans for the strain distribution in island and line structures of thin films are discussed as well as possibilities and limitations for further improvement of the spatial Raman resolution. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y6.00002: Hydrogen storage by physisorption on Metal Organic Frameworks Invited Speaker: Cryo-adsorption systems based on materials with high specific surface areas have the main advantage that they can store and release hydrogen with fast kinetics and high reversibility over multiples cycles. Recently Metal Organic Frameworks (MOFs) have been proposed as promising adsorbents for hydrogen. These crystallographically well organized hybrid solids resulting from the three dimensional connection of inorganic clusters using organic linkers show the largest specific surface areas of all known crystalline solids. The determination of the relationships between physical properties (chemistry, structure, surface area {\ldots}) of the MOFs and their hydrogen storage behavior is a key step in the characterization of these materials, if they are to be designed for hydrogen storage applications. Excess hydrogen sorption measurements for different MOFs will be presented. We show that maximum hydrogen uptake at high pressure and 77K does not always scale with the specific surface area. A linear correlation trend only apply within a class of specific materials and breaks down when the surface area measurement does not represent the surface sites that are available to H2. The influence of pore size and shape will also be discussed by comparing several MOFs with different structure types. The hydrogen adsorption and binding energy at low pressure are strongly dependent on the metal ions and the pore size. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y6.00003: Characterization of the pore geometry of porous media and the saturating fluids using 2-dimensional diffusion - NMR relaxation measurements Invited Speaker: |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y6.00004: Non- linear Optical Spectroscopy of Interfaces Invited Speaker: The critical role played by interfaces can be extended to areas such as nanometer size sensors, electronic devices, fuel cells, composite materials, corrosion, lubrication, oil recovery, catalysis, biology, and aqueous environments. The development of superior technologies in these areas can be driven by rapid advances in interfacial science. Second-order nonlinear optical spectroscopy, particularly sum-frequency generation (SFG) spectroscopy, is well suited for advanced characterization of interfaces. It possesses long penetration depths and intrinsic interface specificity. This talk discusses the general principle of SFG spectroscopy as applied in a petrochemical industry with brief examples related to polymer surfaces, aqueous/oxide interfaces and adsorption of molecular additives from liquid onto solid surfaces. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y6.00005: Development of Silicide Contacts for CMOS devices: Advantages of using Synchrotron Radiation Invited Speaker: Silicide films have been used for close to two decades as the contact to the source, drain and gate of state-of-the-art complementary-metal-oxide-semiconductor (CMOS) devices. The desired properties for this contact layer have limited the choice of applicable silicides to low resistivity TiSi$_{2}$, CoSi$_{2}$ and NiSi. The stringent and evolving requirements in material properties imposed by continuous scaling have forced modifications to current materials or implementation of the following best candidate. The recent conversion from CoSi$_{2}$ for the 90 nm technology to NiSi in 65 nm technology and beyond represent a good example of the complexity associated with the integration of a new material. The requirements necessary to achieve performance in current devices are so stringent that even a material such as NiSi studied for more than 3 decades exhibited unexpected characteristics in very thin films and in small devices. The use of intense x-ray beams allows for the characterization of such materials in reduced dimensions and has brought to light multiple unknown behaviors. For example, the morphological stability of NiSi is much lower than originally expected, a result of the complexity in the phase sequence, of the strong anisotropy in properties related to the non-cubic crystal structure and of the very peculiar texture of these films. This early thermal degradation of the contacts has been controlled through process optimization and recently through the use of Ni alloys. In this presentation the impact of using intense x-ray sources for materials optimization will be discussed. The access to these powerful setups has allowed the rapid characterization and optimization of large parameter spaces necessary to develop the knowledge for implementation of new materials in state-of-the-art devices. [Preview Abstract] |
Session Y7: Control of Light with Bacteriorhodopsin
Sponsoring Units: DBP DCPChair: Gopal Rao, University of Massachusetts-Boston
Room: Morial Convention Center RO5
Friday, March 14, 2008 11:15AM - 11:51AM |
Y7.00001: Earle K. Plyler Prize Talk: Stark Realities Invited Speaker: Stark spectroscopy is the effect of an electric field on a spectrum. Measurements of the Stark effect give information on the change in dipole moment and polarizability for a spectroscopic transition. The great majority of Stark effect measurements have been and still are made in the gas phase where spectroscopic transitions are very narrow and a Stark splitting can be readily measured. There are many fewer examples of Stark spectroscopy measurements in condensed phases, largely because of the perceived difficulty of applying a large electric field. While this is the case for liquid samples, where molecular alignment and low breakdown voltages complicate the measurement, it is simple to immobilize the molecule of interest, either by embedding it in a thin polymer film or by freezing the solvent. The latter is completely general and any sample that forms a high quality optical glass, including protein samples, can be studied. In this talk I will present an overview of applications of Stark effects to diverse systems. We divide the phenomenon into two broad classes: classical Stark effects, where the applied field acts as a perturbation shifting a transition; and non-classical Stark effects, where the applied field affects the intrinsic absorption lineshape and/or populations of states. Classical Stark effects provide quantitative information on the dipolar nature of excited states for electronic or vibrational transitions. Once calibrated, the spectroscopic transition can be used to probe electric fields in organized complex systems such as proteins and changes in those fields accompanying mutations, catalysis, ligand binding and folding. Vibrational Stark effects are particularly useful in this context, and this has led to diverse strategies for introducing unique and sensitive probes for electrostatic fields in proteins. Non-classical Stark effects embrace the many effects that electric fields can have on reaction dynamics, particularly involving electron transfer, either photoinduced or in mixed valence systems. For such systems, the electric field can alter the absorption or emission lineshape substantially because the potential surface depends upon the field and the spectrum depends on the shape of the potential. Examples of each type will be presented. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y7.00002: Protein-Based Three-Dimensional Memories and Associative Processors Invited Speaker: The field of bioelectronics has benefited from the fact that nature has often solved problems of a similar nature to those which must be solved to create molecular electronic or photonic devices that operate with efficiency and reliability. Retinal proteins show great promise in bioelectronic devices because they operate with high efficiency ($\sim $0.65{\%}), high cyclicity ($>$10$^{7})$, operate over an extended wavelength range (360 -- 630 nm) and can convert light into changes in voltage, pH, absorption or refractive index. This talk will focus on a retinal protein called bacteriorhodopsin, the proton pump of the organism \textit{Halobacterium salinarum}. Two memories based on this protein will be described. The first is an optical three-dimensional memory. This memory stores information using volume elements (voxels), and provides as much as a thousand-fold improvement in effective capacity over current technology. A unique branching reaction of a variant of bacteriorhodopsin is used to turn each protein into an optically addressed latched AND gate. Although three working prototypes have been developed, a number of cost/performance and architectural issues must be resolved prior to commercialization. The major issue is that the native protein provides a very inefficient branching reaction. Genetic engineering has improved performance by nearly 500-fold, but a further order of magnitude improvement is needed. Protein-based holographic associative memories will also be discussed. The human brain stores and retrieves information via association, and human intelligence is intimately connected to the nature and enormous capacity of this associative search and retrieval process. To a first order approximation, creativity can be viewed as the association of two seemingly disparate concepts to form a totally new construct. Thus, artificial intelligence requires large scale associative memories. Current computer hardware does not provide an optimal environment for creating artificial intelligence due to the serial nature of random access memories. Software cannot provide a satisfactory work-around that does not introduce unacceptable latency. Holographic associative memories provide a useful approach to large scale associative recall. Bacteriorhodopsin has long been recognized for its outstanding holographic properties, and when utilized in the Paek and Psaltis design, provides a high-speed real-time associative memory with variable thresholding and feedback. What remains is to make an associative memory capable of high-speed association and long-term data storage. The use of directed evolution to create a protein with the necessary unique properties will be discussed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y7.00003: Optical Fourier and Holographic Techniques for Medical Image Processing with Bacteriorhodopsin Invited Speaker: The biological photochrome bacteriorhodopsin (bR) shows many intrinsic optical and physical properties. The active chromophore~in bR is a retinal group which absorbs light and goes through a photocycle. The unique feature of the system is its flexibility -- the photocycle can be~optically controllable since the process of photoisomerization can go in both directions depending on wavelength, intensity and polarization of the incident light, opening a variety of possibilities for manipulating amplitude, phase, polarization and index of refraction of the incident light. Over the years we studied the basic nonlinear optics and successfully exploited the unique properties for several optical spatial filtering techniques with applications in medical image processing. For nonlinear Fourier filtering, the photo-controlled light modulating characteristics of bR films are exploited. At the Fourier plane, the spatial frequency information carried by a blue probe beam at 442 nm is selectively manipulated in the bR film by changing the position and intensity of a yellow control beam at 568 nm. In transient Fourier holography, photoisomerizative gratings are recorded and reconstructed in bR films. Desired spatial frequencies are obtained by matching the reference beam intensity to that of the particular frequency band in object beam. A novel feature of the technique is the ability to transient display of selected spatial frequencies in the reconstructing process which enables radiologists to study the features of interest in time scale.~The results offer useful information to radiologists for early detection of breast cancer. Some of the highlights will be presented. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y7.00004: Light manipulation with Bacteriorhodopsin membrane self-assembled on high-Q photonic structures Invited Speaker: Resonant photonic structures such as ring resonators and photonic crystal nanocavities interact evanescently with biological material assembled on a reflecting interface. Quality (Q-) factors $\sim$10$^6$ and sub-wavelength modal (V-) volumes significantly enhance the interaction so that tuning of microcavity resonances by only few molecules is feasible. Since only few constituents are required, the molecular-photonic interface can be fashioned from self-organizing principles that govern interaction of organic and biological polymers. We demonstrate this bottom-up approach with photochromic Bacteriorhodopsin membrane which we self-assemble on various microcavities. The hybrid molecular-photonic architectures exhibit high Q/V-values and are sensitive to photoinduced molecular transitions and other non-linearities which we utilize for demonstrations of all-optical switching, routing and molecular analysis. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y7.00005: The Integration of Bacteriorhodopsin Proteins with Semiconductor Heterostructure Devices Invited Speaker: Bioelectronics has emerged as one of the most rapidly developing fields among the active frontiers of interdisciplinary research. A major thrust in this field is aimed at the coupling of the technologically-unmatched performance of biological systems, such as neural and sensing functions, with the well developed technology of microelectronics and optoelectronics. To this end we have studied the integration of a suitably engineered protein, \textit{bacteriorhodopsin} (BR), with semiconductor optoelectronic devices and circuits. Successful integration will potentially lead to ultrasensitive sensors with polarization selectivity and built-in preprocessing capabilities that will be useful for high speed tracking, motion and edge detection, biological detection, and artificial vision systems. In this presentation we will summarize our progresses in this area, which include fundamental studies on the transient dynamics of photo-induced charge shift in BR and the coupling mechanism at protein-semiconductor interface for effective immobilizing and selectively integrating light sensitive proteins with microelectronic devices and circuits, and the device engineering of BR-transistor-integrated optical sensors as well as their applications in phototransceiver circuits. \newline \newline Work done in collaboration with Pallab Bhattacharya, Jonghyun Shin, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI; Robert R. Birge, Department of Chemistry, University of Connecticut, Storrs, CT 06269; and Gy\"{o}rgy V\'{a}r\'{o}, Institute of Biophysics, Biological Research Center of the Hungarian Academy of Science, H-6701 Szeged, Hungary. [Preview Abstract] |
Friday, March 14, 2008 2:15PM - 2:51PM |
Y7.00006: All-Optical Switching in Bacteriorhodopsin Based on Excited-State Absorption Invited Speaker: Switching light with light is of tremendous importance for both fundamental and applied science. The advent of nano-bio-photonics has led to the design, synthesis and characterization of novel biomolecules that exhibit an efficient nonlinear optical response, which can be utilized for designing all-optical biomolecular switches. Bacteriorhodopsin (bR) protein found in the purple membrane of \textit{Halobacterium halobium }has been the focus of intense research due to its unique properties that can also be tailored by physical, chemical and genetic engineering techniques to suit desired applications. The talk would focus on our recent results on all-optical switching in bR and its mutants, based on excited-state absorption, using the pump-probe technique. We would discuss the all-optical control of various features of the switching characteristics such as switching contrast, switching time, switching pump intensity, switched probe profile and phase, and relative phase-shift. Optimized conditions for all-optical switching that include optimized values of the small-signal absorption coefficient (for cw case), the pump pulse width and concentration for maximum switching contrast (for pulsed case), would be presented. We would discuss the desired optimal spectral and kinetic properties for device applications. We would also discuss the application of all-optical switching to design low power all-optical computing devices, such as, spatial light modulators, logic gates and multiplexers and compare their performance with other natural photoreceptors such as pharaonis phoborhodopsin, proteorhodopsin, photoactive yellow protein and the blue light plant photoreceptor phototropin. [Preview Abstract] |
Session Y8: Hydrodynamics of Surfaces and Films
Sponsoring Units: DFDChair: Elizabeth Mann, Kent State University
Room: Morial Convention Center RO6
Friday, March 14, 2008 11:15AM - 11:27AM |
Y8.00001: Capillary ratchet: Hydrodynamics of capillary feeding in shorebirds Manu Prakash, David Quere, John Bush Bill morphologies are highly specialized to particular foraging strategies in birds, as is apparent from the large diversity of beak shapes observed in nature. Here we present an experimental and analytical study of capillary feeding in shorebirds. We highlight the critical role of contact angle hysteresis in capillary feeding. Our study provides a simple physical rationalization for the observation of multiple mandibular spreading cycles in feeding, necessary to overcome contact line resistance. We also find a unique geometrical optima in beak opening and closing angles for the most efficient drop transport. This capillary ratchet mechanism may also find applications in micro scale fluid transport, such as valveless pumping of fluid drops. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y8.00002: Rheology and Microrheology of Actin-Lipid Composites at the Air-Water Interface Robert Walder, Alex Levine, Christoph Schmidt, Michael Dennin We report on the mechanical properties of a composite material that is a combination of a Langmuir monolayer chemically linked to an actin filament network. This composite system is a 2 dimensional analogue of a cellular membrane and is also expected to have interesting nonlinear mechanical properties. To measure these mechanical properties, we employ traditional rheology and have developed unique microrheological capabilities based on an optical tweezer setup combined with a Couette surface rheometer. This combination of techniques will allow the study of both bulk and local mechanical responses of the composite material to external forces. Studying such materials allows us to simultaneously study a biomimetic material that should provide useful insights into the mechanical properties of biological cells, while also providing a 2 dimensional soft matter system to study the properties of semi-flexible polymer networks. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y8.00003: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y8.00004: Elasticity and capillarity: wet hairs and origami Jos\'e Bico, Lingguo Du, Benoit Roman, Jerome Guilet Capillary forces are responsible for a large range of everyday observations : the shape of rain droplets, the imbibition of a sponge, the clumping of wet hair into bundles. Although they are often negligible on macroscopic structures, surface capillary forces may overcome volume forces at small scales and deform compliant micro-structures. Capillary-induced sticking can prevent the actuation of mobile elements in MEMS, or even cause their collapse. Capillary forces also have important consequences in biology such as the buckling of the airway lumen induced by surface tension, which can eventually cause the lethal closure of lung airways. We will review a few experimental situations where capillary forces are able to deform two types of objects: rods, and thin sheets. For instance, the nanotubes of a ``carbon nanotube carpet'' self-assemble into conical ``teepee'' structures after the evaporation of a solvent and can produce intriguing cellular patterns. Similarly macroscopic wet hairs tend to assemble into bundles through a cascade of successive pairings. The comparison of the physical ingredients involved in these phenomena, attracting capillary forces acting against bending elasticity, leads to a characteristic length: a slander structure longer than this ``elasto-capillary'' length is considerably bent by capillary forces. The case of thin sheets is trickier because of geometrical constrains, which generally leads to singularities. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y8.00005: Flow fields in soap films: effects of surface viscosity and film thickness Vikram Prasad, Eric R. Weeks A soap film is a thin fluid layer (10 nm to 10 microns thick) separated from bulk air phases above and below it by two surfactant monolayers. We measure the flow field in these films by two-particle microrheology, which looks at the correlated Brownian motion of pairs of embedded tracer particles separated by a distance R. In thin soap films with the thickness comparable to the particle size, and with mobile surfactant interfacial layers, this flow field is long ranged. On the other hand, the flow field in a 3D fluid is known to decay as 1/R. We vary the thickness of the soap film, the mobility (surface viscosity) of the interface and the size of the polystyrene probe particles to quantify the transition of the hydrodynamics of the film from quasi-2D to 3D-like behavior. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y8.00006: Effective Viscosity of a Dilute Suspension of Membrane-bound Inclusions Mark L. Henle, Alex J. Levine In 1906, Einstein famously calculated the effective viscosity of a dilute solution of spheres suspended in a viscous solvent [\textit{Annalen der Physik} \textbf{19}, 289 (1906)]. In this talk, we consider the two-dimensional analogue of~this problem: that is, we calculate the effective viscosity of a dilute suspension of disks embedded in a two-dimensional fluid membrane. The rheological properties of particle-decorated membranes and fluid-fluid interfaces are important in a variety of soft matter systems. ~For example, the cell membrane contains a suspension of membrane-bound inclusions (e.g. transmembrane proteins, lipid rafts), which modifies the transport kinetics of the membrane. Also, the interfacial viscosity of liquid-liquid interfaces in colloid-stabilized emulsions plays a key role in preventing droplet coalescence. We include the dissipation caused by flows both within the membrane and in the surrounding bulk fluids. When the flows within the membrane dominate the dissipation, the particle suspension effectively shifts the membrane viscosity. Conversely, when flows induced in the bulk fluids dominate the dissipation, the suspension in the membrane shifts the bulk viscosity. In both limits, we obtain simple analytic expressions for the appropriate effective viscosity. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y8.00007: Contact line motions of drying solutions Francois Lequeux, Cecile Monteux, Astrid Tay, Emmanuelle Rio, Laurent Limat, Guillaume Berteloot, Adrien Daerr If most the studies on wetting deal with pure liquids in the absence of evaporation, in practical situations, the liquid is often a solution with an evaporating solvent. This is encountered both in coating and in surface cleaning. In that case, the contact line of a solution is the location of many divergent phenomena. The hydrodynamics dissipation diverges at the contact line: 1) the drying rate diverges at the contact line 2) the concentration diverges at the contact line. The coupling of these phenomena leads to complex effect for the contact line motion. We have observed that an advancing contact line of a colloidal suspension exhibit a stick-slip motion. Moreover, for similar reasons in the case, an advancing contact line of a polymer solution, the contact angle exhibit a minimum as a function of velocity -- at which the polymer accumulates on a length of typically 5 nm in the vicinity of the contact line. All these phenomena can explained quantitatively using simple scaling arguments that we will present. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y8.00008: Obtaining Reproducible Slip Measurements on Smooth Hydrophobic Surfaces Sean P. McBride, B.M. Law Over the past decade, the world market for microfluidic technologies and applications of such devices has soared. The slip length parameter at the liquid-surface interface of these devices describes how easily a fluid flows over the surface. As microfluidic devices decrease in size, slip becomes very important. Despite the undeniable success of these devices in recent years, the literature illustrates that numerous discrepancies exist for the slip magnitude measured using different experimental methods. As the need for smaller microfluidic devices approaches a consistent experimental method is needed to obtain reproducible slip results. The method employed to study slip, in this research, uses an Atomic Force Microscope (AFM) to obtain the hydrodynamic force exerted on a colloidal cantilever which is immersed in a homologous series of test liquids and driven toward a smooth hydrophobic surface. The surfaces are prepared using silicon wafers with 0.4nm RMS over a 5x5um area and coated with hexadecyltricholorsilane (HTS) via cold liquid deposition. This method provides reliable and reproducible slip measurements that are consistent with a constant slip length over a wide shear rate range. This research was supported by NSF grant DMR-0603144. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y8.00009: Fabrication of non-aging superhydrophobic surfaces by packing flower-like hematite particles Anmin Cao, Liangliang Cao, Di Gao We demonstrate the fabrication of non-aging superhydrophobic surfaces by packing flower-like micrometer-sized hematite particles. Although hematite is intrinsically hydrophilic, the nanometer-sized protrusions on the particles form textures with overhanging structures that prevent water from entering into the textures and induce a macroscopic superhydrophobic phenomenon. These superhydrophobic surfaces do not age even in extremely oxidative environments---they retain the superhydrophobicity after being stored in ambient laboratory air for 4 months, heated to 800 degree C in air for 10 hours, and exposed to ultraviolet ozone for 10 hours. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y8.00010: Liquid slip probed by second harmonic generation Dan Lis, Steve Granick, Bae Sung Chul Second harmonic generation has been used to probe how a solid surface responds to flow past it. The surface is quartz, the measurements are made in total internal reflection configuration, and comparison of responses to light with s and p incident polarisation allows us to determine the orientation of dye molecules physisorbed before the onset of the shear flow. By monitoring the orientation of the dye at different fluid viscosity and different shear rate, we deduce the surprising relation between shear rate and surface stress. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y8.00011: Determination of Inter-Phase Line Tension in Langmuir Films Elizabeth K. Mann, Lu Zou, Jacob R. Wintersmith, Andrew J. Bernoff, James C. Alexander, J. Adin Mann, Jr., Prem Basnet, Edgar E. Kooijman The hydrodynamic response of a thin fluid film, whether a Langmuir monolayer at the air/water interface or a cell membrane, is difficult to model, since it involves the coupling of both bulk and surfaces phases. However, such hydrodynamic response is not only intrinsically critical for transport within the layer, it also provides the major available means to evaluate an important parameter for phase-separated layers, the line tension. We have developed a line-integral formulation of the hydrodynamic response of phase-separated layers with short-ranged forces, and tested it by comparisons between numerical simulations based on this model and experiment. These experiments both validate the model and demonstrate that the line tension can be determined with unprecedented accuracy and precision. Two systems have been studied to date: a simple smectic liquid crystal multilayer and coexistence between phases in binary lipid/cholesterol mixed layers. For the latter case, long-range dipole-dipole interactions are introduced into the model. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y8.00012: ABSTRACT WITHDRAWN |
Session Y9: BEC/Matter Optics/Atom Interferometry
Sponsoring Units: DAMOPChair: Conjung Wu, University of California, San Diego
Room: Morial Convention Center RO7
Friday, March 14, 2008 11:15AM - 11:27AM |
Y9.00001: Phase diagram of a Bose gas near a wide Feshbach resonance Lan Yin The phase diagram of a homogeneous Bose gas with a repulsive interaction near a wide Feshbach resonance is studied at zero temperature. The Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the scattering length $a$ exceeds a critical value depending on the atom density $n$, $na^3>0.035$, the molecular excitation energy is imaginary and the atomic BEC state is dynamically unstable against molecule formation. The BEC state of diatomic molecules has lower energy, where the atomic excitation is gapped and the molecular excitation is gapless. However when the scattering length is above another critical value, $na^3>0.0164$, the molecular BEC state becomes a unstable coherent mixture of atoms and molecules. (http://arxiv.org/abs/0710.5318)(cond-mat/0710.5318) [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y9.00002: Initial States of BEC mixtures Produced by Cooling in the Presence of a Feshbach Resonance Laura Halmo, Mark Edwards We have studied the types of Bose--Einstein condensate (BEC) mixtures produced as a result of different cooling paths. These results are relevant to a recent experiment in which a mixture of $^{85}$Rb and $^{87}$Rb BECs was cooled in three stages: (1) optical pre--cooling, (2) evaporative cooling in a magnetic trap, and (3) evaporatively cooled in an optical trap. We assume that, upon transfer to the optical trap, the state of the mixture of thermal gases can be represented by the superposition of a small number of low--lying trap eigenstates each with high occupation. In this case, the bose field operator can be approximated as a $c$--number and its evolution will be governed by the nonlinear Schr\"odinger equation. We investigated the density profiles that resulted from different initial thermal distributions as well as non--thermal initial distributions. We also performed studies of the effect of varying the $^{85}$Rb--$^{85}$Rb scattering length via a Feshbach resonance. We found condensate states that differ markedly from the standard Thomas--Fermi ground states of the Gross--Piteavskii equation. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y9.00003: Interference between atomic Bardeen-Cooper-Schrieffer gases Tun Wang, Susanne Yelin We study the interference between two atomic Bardeen-Cooper- Schrieffer (BCS) gases using noise correlations. Fringes as seen in the interference between two Bose-Einstein Condensates (BECs) do not to exist due to the requirement that two BCS gases have to initially overlap to interfere. This requirement results from the fact that the spin up and spin down fermions in a Cooper pair have opposite momenta. Nevertheless, BCS gases still interfere with each other, and their interference patterns share many aspects with those of BECs. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y9.00004: Single and double reflection Michelson atom interferometers in a weakly-confining magnetic trap Rudra Kafle, James Stickney, Dana Anderson, Alex Zozulya We analyze the operation of a BEC based atom interferometer, where the atoms are held in a weakly-confining magnetic trap and manipulated with diffraction gratings produced by counter-propagating laser beams. A simple analytic model is developed to describe the dynamics of the interferometer. It is used to find the region of parameters corresponding to high values of the interference fringe contrast for both single and double reflection geometries. We demonstrate that for a double reflection interferometer the coherence time can be increased by shifting the recombination time. Finally, we compare the theory with recent experimental realizations of these interferometers and estimate when phase diffusion and finite temperature phase fluctuations become important. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y9.00005: Prospects for a Gradient Magnetometer Atom Interferometer Frank A. Narducci, Jon P. Davis Atom interferometers form the basis for state-of-the-art sensors, including gravimeters, gravity gradiometers, gyroscopes and atomic clocks. Notably absent from this list are magnetometers, which can have a wide range of applications ranging from military to medical applications. We propose a scheme to realize an atom interferometer {\em gradient} magnetometer. We begin by demonstrating a light-pulse magnetic beam-splitter. The analysis is based on a full multi-level 2-laser field Maxwell-Bloch model including state selection rules, polarization selectivity, laser detuning, and Doppler averaging. We then consider an ensemble of atoms subject to a $\pi/2-\pi-\pi/2$ pulse sequence. The phase of the interference pattern depends on the phase of the action along the classical path and on the phase of the combined laser fields imprinted on the atoms during the pulse sequence. From this analysis, we conclude that, to first order, the phase of the interferometer output is insensitive to the field across the interferometer, but is sensitive to the {\em gradient} of the field. Using realizable numbers from existing interferometers, we show that a gradient magnetometer of this type has can have a greater gradient sensitivity than many current magnetic sensors. We discuss the status of our current experiments using ultra-cold atoms. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y9.00006: Bose--Josephson Junction with Binary Mixture of Bosonic Atoms Mark Edwards, Jeffrey Heward, Indubala Satija, Radha Balakrishnan, Phillip Naudus We consider a bose--Josephson junction consisting of a binary mixture of two weakly coupled Bose--Einstein condensates confined in a symmetric double--well external potential. In a single condensate confined in a double--well potential, when the condensate wavefunction is approximated as a linear combination of the lowest two eigenmodes of the potential, the result is a dynamical system analogous to those that describe the current and phase across a Josephson junction. Josephson oscillations and nonlinear self--trapping are among the effects predicted by this dynamical system. Using the same two--mode approximation, the condensate mixture can be mapped to two coupled, non--rigid pendula. Although the system is found to exhibit periodic dynamics, the tunneling dynamics of the individual components can be periodic, quasiperiodic, as well as chaotic. We also investigate the experimental signatures of these effects and the goodness of the two--mode approximation by solving the coupled Gross--Pitaevskii equations that govern the behavior of the system. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y9.00007: Dynamics of phase separation in cold-atom boson-fermion mixtures Dmitry Solenov, Dmitry Mozyrsky We study the kinetics of the first order phase separation transition in boson-fermion cold atom mixtures. At low enough temperatures such a transition is driven by quantum fluctuations responsible for the formation of critical nuclei of a stable phase. Based on a microscopic description of interacting boson-fermion mixtures we derive an effective action for the critical droplet and obtain an asymptotic expression for the nucleation rate in the vicinity of the phase transition and near the spinodal instability of the mixed phase. We show that in the former case the transition rate is significantly modified by dissipation due to interaction with fermion excitations. The regimes where quantum nucleation can be experimentally observed in cold atom systems are identified. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y9.00008: Quantum phases of mixtures of atoms and molecules on optical lattices Valy Rousseau, Peter Denteneer We investigate the phase diagram of a two-species Bose-Hubbard model with an additional conversion term, where two particles from the first species can be converted into one particle of the second species, and vice-versa. The model can be related to ultra-cold atoms experiments where Feshbach resonance, used to tune the scattering length, produces long-lived bound states viewed as diatomic molecules. The model is solved exactly by means of Quantum Monte Carlo simulations. We find that the model exhibits an exotic incompressible ``Super-Mott'' phase where the particles from both species can flow with signs of superfluidity, but with anti-correlations such that there is no global supercurrent. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y9.00009: First order behaviour of Bose Fermi mixtures across a Feshbach resonance Charles Mathy, Francesca Marchetti, Meera Parish, David Huse We analyze the phase diagram of a mixture of bosonic and fermionic atoms, whose interaction is tuned by varying a magnetic field across a Feshbach resonance. To this end, we introduce a two-channel model and study it with a mean field approach. The phase diagram is found to contain both second order and first order phase transitions, which leads to a regime of densities where phase separation is predicted. We explain why our model is consistent with the experimental observation of collapse, which is usually captured by a single- channel model, and discuss in which systems one is most likely to encounter the physics we are describing. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y9.00010: Boson-Fermion mixture and superconducting phases on the honeycomb lattice Doron Bergman, Peter Orth, Karyn Le Hur We explore theoretically the different phases of a Boson-Fermion mixture in a honeycomb lattice model. With realistic band structure and interactions, we find that much like phonons in a solid, the bosonic atoms induce effective attractive interactions between the fermions. The attractive interactions can then lead to a number of superconducting phases, which we explore. Using a Bogoliubov and mean-field approach, as well as a full RG treatment, we derive the phase diagram. Possible phases of the system include s-wave as well as $p+i p$ superconducting states. We also analyze the nature of the vortices in the different superconducting states, as these are of great interest as a possible realization of emergent non-Abelian statistics. We explain how tuning between different superconducting phases can be achieved in a cold atomic gas realization of this system. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y9.00011: Supersymmetry and Goldstino-like Mode in Bose-Fermi Mixtures Yue Yu, Kun Yang Supersymmetry is assumed to be a basic symmetry of the world in many high energy theories, but none of the super partners of any known elementary particle has been observed yet. We argue that supersymmetry can also be realized and studied in ultracold atomic systems with a mixture of bosons and fermions, with properly tuned interactions and single particle dispersion. We further show that in such non-releativistic systems supersymmetry is either spontaneously broken, or explicitly broken by a chemical potential difference between the bosons and fermions. In both cases the system supports a sharp fermionic collective mode similar to the Goldstino mode in high-energy physics, due to supersymmetry. We also discuss possible ways to detect this mode experimentally. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y9.00012: Preemptive phase-transitions in multicomponent BECs Steinar Kragset, Eskil Kulset Dahl, Egor Babaev, Asle Sudbo We use analytical arguments and large-scale Monte-Carlo simulations to investigate phase transitions between various complex superfluid phases in a two-component Bose-Einstein condensate with varying non- dissipative drag between the two components. We focus on establishing the phase- diagram and investigate in detail the individual and composite superfluid densities that the system features, using a representation in terms of the phases of the superfluid ordering fields. In particular, we describe a novel preemptive scenario, whereby drag induces a first-order phase transition from the interplay between two phase-transitions that individually would have been in the $3DXY$-universality class. Our results may shed light on similar phenomena occuring in certain multicomponent superconductors and in scenarios of deconfined quantum criticality in certain quantum antiferromagnetic systems. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y9.00013: Chaos Threshold in Bose-Hubbard Model Amy Cassidy, Vanja Dunjko, Maxim Olshanii The goal of this work is to determine the criterion for chaos in the one-dimensional mean-field Bose-Hubbard model. We investigate the time evolution of this system with a few low-energy momentum modes excited initially. A threshold for chaos is identified from calculations of the largest Lyapunov exponent, which is compared with the predictions of the Chrikov criterion of overlapping resonances. Additionally, the results are compared with a closely related fully integrable model. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y9.00014: Quantum quench dynamics in analytically solvable one-dimensional models Anibal Iucci, Miguel A. Cazalilla, Thierry Giamarchi In connection with experiments in cold atomic systems, we consider the non-equilibrium dynamics of some analytically solvable one-dimensional systems which undergo a quantum quench. In this quench one or several of the parameters of the Hamiltonian of an interacting quantum system are changed over a very short time scale. In particular, we concentrate on the Luttinger model and the sine-Gordon model in the Luther-Emery point. For the latter, we show that the order parameter and the two-point correlation function relax in the long time limit to the values determined by a generalized Gibbs ensemble first discussed by J. T. Jaynes [Phys. Rev. \textbf{106}, 620 (1957); \textbf{108}, 171 (1957)], and recently conjectured by M. Rigol \emph{et.al.} [Phys. Rev. Lett. \textbf{98}, 050405 (2007)] to apply to the non-equilibrium dynamics of integrable systems. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y9.00015: Supercurrent survival under Rosen-Zener quench of hard core bosons Israel Klich, Courtney Lannert, Gil Refael We study the survival of super-currents in a system of impenetrable bosons subject to a quantum quench from its critical superfluid phase to an insulating phase. We show that the evolution of the current when the quench follows a Rosen-Zener profile is exactly solvable. This allows us to analyze a quench of arbitrary rate, from a sudden destruction of the superfluid to a slow opening of a gap. The decay and oscillations of the current are analytically derived, and studied numerically along with the momentum distribution after the quench. In the case of small supercurrent boosts $\nu$, we find that the current surviving at long times is proportional to $\nu^3$ [Preview Abstract] |
Session Y10: Superconducting Nanostructures II
Sponsoring Units: DCMPChair: Timir Datta, University of South Carolina
Room: Morial Convention Center RO8
Friday, March 14, 2008 11:15AM - 11:27AM |
Y10.00001: Studying pinning on the nanoscale by vortex dragging in a YBa$_2$Cu$_3$O$_{6.991}$ single crystal O. M. Auslaender, Lan Luan, E. Zeldov, K. A. Moler, D. A. Bonn, Ruixing Liang, W. N. Hardy We have used a magnetic force microscope to drag individual, well isolated vortices in a detwinned YBa$_2$Cu$_3$O$_{6.991}$ single crystal. At this slight degree of overdoping, a vortex can be described as a one-dimensional elastic string. We find an angle dependent dragging distance, implying that it is easier to drag a vortex along the Cu-O chains than across them. We understand this as a manifestation of single vortex weak collective pinning (WCP) by oxygen vacancies along the Cu-O chains, the dominant source of pinning in our sample. Single vortex WCP arises when individual pinning sites are too weak individually, but are able to compete with elasticity by cumulative effect. Using the usual single vortex WCP assumption of isotropic point pinning sites at uncorrelated positions we find that the anisotropy of superconductivity in YBa$_2$Cu$_3$O$_{6.991}$ only partially accounts for the angle dependence. Relaxing that assumption to account for the known tendency of the vacancies to cluster along the Cu-O chains, we find that single vortex WCP describes the dragging anisotropy quantitatively. This picture is further supported by the observation that vortex motion is more erratic along the chains than across them. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y10.00002: Fabrication and characterization of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Aharonov-Bohm rings and ultra-long nanowires PaiChia Kuo, Jessie Shiue, Patrick Morales, J.Y.T. Wei We report a novel technique to fabricate YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) nanodevices with characteristic length scale smaller than the YBCO penetration depth. The nanodevices presented here are Aharonov-Bohm rings $\sim $ 1.5 $\mu $m in diameter, and 200 nm nanowires $\sim $ 300 $\mu $m in length. These devices have Tc behaviors similar to that of unpatterned YBCO thin films. Fabrication of nanostructured complex oxide is a challenge even with advanced thin film growth techniques since either chemical or physical etching tends to compromise the film properties. The effective method is to epitaxially deposit thin film onto nano-patterned oxide substrate without any post-deposition treatment. Our novel technique takes advantage of the 3D micromachining capability of focused-ion-beam to nano-pattern the oxide substrate without the inherent surface damage and edge rounding problems caused by the energetic ion beam. This method is a reliable way to fabricate nanostructures of complex oxides and hence enables the studies of their properties. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y10.00003: Electrical Transport Properties of Nanostructured YBa$_2$Cu$_3$O$_{7-\delta}$ Rings and Wires P. Morales, J.Y.T. Wei, P.C. Kuo, J. Shiue, M.K. Wu The resistance and current-voltage characteristics of nanostructured high-T$_c$ superconducting YBa$_2$Cu$_3$O$_{7-\delta}$ rings and wires were studied as a function of temperature and applied magnetic field. The rings and wires were fabricated by pulsed laser deposition of YBa$_2$Cu$_3$O$_{7-\delta}$ on patterned SrTiO$_3$ substrates. The substrates were patterned using two different techniques. The first technique is based on selective epitaxial growth, and the second, using a method based on focused ion beam. Nanostructured superconducting rings were fabricated with a diameter of 1.5${\mu}$m with the width of the arms of the rings being 150nm. The low field magnetoresistance of the rings exhibit characteristics indicative of quantum interference effects. Nanostructured superconducting wires were fabricated with lengths up to 300${\mu}$m and widths as small as 200nm. The current-voltage characteristics of the wires exhibit discontinuities under current biasing and s-shaped non-linearities under voltage biasing characteristic of the formation of phase slip lines, the 2D analog of phase-slip centers. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y10.00004: Flux and bias driven superconducting to normal transition in an SNS proximity dc SQUID Jian Wei, Paul Cadden-Zimansky, Venkat Chandrasekhar We measure the magnetoresistance of a dc SNS SQUID in the form of a mesoscopic normal-metal loop in contact with two superconducting electrodes. Below the transition temperature of the superconducting leads, large $h/2e$ periodic magnetoresistance oscillations can be observed when the normal sections of the SNS junctions enter a proximity regime induced by the superconducting electrodes. As the temperature is lowered, the entire device becomes superconducting. In this regime, sharp switching from the zero-resistance state to a finite-resistance state is seen at half-integer flux quanta. With the application of a dc bias current at even lower temperatures, periodic switching from the superconducting state to the fully normal state can be produced with the external field. The observation of periodic flux-driven transitions in this device suggests that beyond the current SQUID theory for SIS junctions the development of SQUID theory for SNS junctions that incorporates the kinetic energy of the coherent electrons in the junctions is needed. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y10.00005: Charge and mass of Cooper pairs in small superconducting rings. Victor Vakaryuk It is well known that response of a neutral fermionic superfluid to rotation or a superconductor to magnetic field and/or rotation involves$\,$ such$\,$ characteristics of a Cooper pair as, stretching terms a little, its mass $2m$ and charge $2e$. Here $m$ and $e$ are essentially\footnote{ignoring tiny relativistic corrections} bare mass and charge of particles constituting the Cooper pair e.g.~electrons in case of superconductors. On a phenomenological level this is a consequence of the fact that expressions for currents are written for pairs of particles. We analyze this situation in BCS framework and show that for superfluids mesoscopically constrained in (at least) one spatial dimension the pair's mass and/or charge become smaller than their values for the bulk case (i.e.~$2m$ and/or $2e$). One of the implications of this result is the absence of $hc/2e$ harmonic in the response of small superconducting rings or tubes to external magnetic field. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y10.00006: Emergence of $h/e$-period oscillations in the critical temperature of small superconducting rings enclosing magnetic flux Tzu-Chieh Wei, Paul M. Goldbart The Little-Parks critical-temperature oscillations, with magnetic flux, of a large-radius hollow cylindrical superconductor have a period $h/2e$. This oscillation period reflects the binding of electrons into Cooper pairs. On the other hand, the single-electron Aharonov-Bohm oscillations in the resistance or persistent current in a clean metallic ring have period $h/e$. By using the Gor'kov approach to BCS theory, we investigate oscillations in the critical temperature of a superconducting ring, for radii that are comparable to the superconducting coherence length. In this regime, oscillations in the critical temperature of period $h/e$ emerge, in addition to the usual Little-Parks-period oscillations. We argue that in the clean limit there is a superconductor-normal phase transition at nonzero flux, as the ring radius becomes sufficiently small, and that this transition can be either second- or first-order, depending on the ring radius and the external flux. In the dirty limit, we argue that the transition is rendered second-order, which results in continuous quantum phase transitions tuned by flux and radius. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y10.00007: Current fluctuations in rough Josephson tunnel junctions Frank Wilhelm, Georg Heinrich The barrier material of superconducting tunnel junction has become the focus of interest as there is evidence that it limits the intrinsic quantum coherence of superconducting qubits. It is also potentially responsible for 1/f noise in SQUIDs. We study the model of a ``sieve'' junction of many opaque transport channels plus few pinholes, modeling a rough tunnel barrier. Even if the pinholes have a small effect on the subgap current, they completely dominate the shot noise at low voltages. Remarkably, even a fully open pinhole contributes shot noise because the size of the charge quantum it carries is uncertain. The full-counting statistics of charge transfer leads to a multimodal distribution. It is discussed, to what extent this distribution can be interpreted as the onset of telegraph or 1/f noise. This theoretical work is based on an extensive full counting statistics calculation using Keldysh Green's function. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y10.00008: Josephson effect through a resonant level coupled to a single oscillator mode Mikael Fogelstrom, Jonas Skoldberg, Tomas Lofwander Motivated by very recent experiments on superconducting transport through single-walled carbon nanotubes, we investigate the supercurrent through a one-level quantum dot connected to a single phonon mode. Calculations are done using non-equilibrium Green's function methods within a self-consistent Born approximation, i.e. assuming that the tunneling rate is much larger that the effective electron-phonon coupling. We calculate both the modified Andreev-bound state spectrum and the renormalization of the phonon density-of-states in situations when the phonon-mode is either in or out-off thermal equilibrium with the electron system. Based on our calculations we discuss possible Andreev-state spectroscopy. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y10.00009: Quantum Phase Slips in 1D Josephson Junction arrays Jack Lidmar One-dimensional arrays of Josephson junctions can undergo a zero temperature superconductor-insulator quantum phase transition by tuning the Josephson coupling. Quantum phase slips (QPS) play the key role in this transition: In the superconducting regime they are very rare, while in the insulating Coulomb blockade regime, they proliferate and destroy the phase coherence. We derive an expression for the QPS rate that is amenable to quantum Monte Carlo simulations and perform calculations in a realistic model of an array over a wide range of parameters including the transition region. In particular we can determine the scaling properties of the QPS rate at the transition. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y10.00010: Macroscopic Resonant Tunneling through Andreev Interferometers Jeff Weiss, Marlies Goorden, Philippe Jacquod We investigate the conductance through and the spectrum of ballistic {\it Andreev interferometers}, chaotic quantum dots attached to two $s$-wave superconductors, as a function of the phase difference $\phi$ between the two order parameters. A combination of analytical techniques -- random matrix theory, Nazarov's circuit theory and the trajectory-based semiclassical theory -- allows us to explore the quantum-to-classical crossover in detail. When the superconductors are not phase-biased, $\phi=0$, we recover known results that the spectrum of the quantum dot exhibits an excitation gap, while the conductance across two normal leads carrying $N_{\rm N}$ channels and connected to the dot via tunnel contacts of transparency $\Gamma_{\rm N}$ is $\propto \Gamma_{\rm N}^2 N_{\rm N}$. In contrast, when $\phi=\pi$, the excitation gap closes and the conductance becomes $G \propto \Gamma_{\rm N} N_{\rm N}$ in the universal regime. In the tunneling regime, $\Gamma_{\rm N} \ll 1$, resonant contributions induce an order-of-magnitude enhancement of the conductance towards $G \propto N_{\rm N}$ in the short-wavelength limit. We relate this to the emergence of a giant peak in the density of states at the Fermi level. Our predictions are corroborated by numerical simulations. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y10.00011: Measurement of the Magnetization of Mesoscopic Superconducting Rings with Cantilever Torsional Magnetometry Will Shanks, Ania Bleszynski-Jayich, Jack Harris We have measured the magnetization of micron-sized aluminum rings in the superconducting state as a function of magnetic flux threading the rings. The rings were fabricated on the ends of 400 $\mu $m long, 400 nm thick silicon cantilevers, which act as sensitive torque meters. By measuring the shift in resonant frequency of the cantilever as a function of applied magnetic field, we are able to determine the ring's magnetization. Our measurements are in qualitative agreement with previous studies of similar rings. The results are promising for other proposed measurements of closed mesoscopic electronic samples using cantilever torsional magnetometry. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y10.00012: Interplay of superconductivity and ferromagnetism in ultra-small metallic grains Sebastian Schmidt, Yoram Alhassid, Kris van Houcke We investigate the competition between superconductivity and ferromagnetism in ultra-small metallic grains in a regime where both phases can coexist. We use an effective Hamiltonian in the mesoscopic regime that combines a BCS-like pairing term and a ferromagnetic Stoner-like spin exchange term. The presence of spin jumps in the ground-state phase diagram of the grain is a unique feature of the coexistence of pairing and ferromagnetic correlations. We show that the coexistence regime can be made accessible to experiments by tuning an external Zeeman field [1]. We also study the transport properties of the grain in the Coulomb blockade regime and identify signatures of the competition between superconductivity and ferromagnetism in the mesoscopic fluctuations of the conductance [2]. \newline [1] S. Schmidt, Y. Alhassid and K. Van Houcke, Europhys. Lett. 80, 47004 (2007). \newline [2] S. Schmidt, Y. Alhassid, to be published (2007). [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y10.00013: The optimal inhomogeneity for superconductivity - finite size studies Wei-Feng Tsai, Hong Yao, Steven Kivelson, Andreas Lauchli We report the results of exact diagonalization studies of Hubbard models on
a $4\times 4$ square lattice with periodic boundary conditions and various
degrees and patterns of inhomogeneity. Inhomogeneities are represented by
different patterns of inequivalent hopping integrals ($t$ and ${t}')$, such
that for ${t}'=t$, the model is ``homogeneous'', while for ${t}'< |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y10.00014: Charge redistribution phenomena at the interfaces of the HTS, insulating and metallic oxides Vladimir Butko, Gennady Logvenov, David Reagor, Ivan Bozovic Interface and low dimensional phenomena are currently a focus of the active and broad scientific investigations. We have fabricated atomically sharp interface structures of the La2-xSrxCuO4 , SrRuO3 , SrTiO3 and LaSrAlO4 by using the atomic layer by layer molecular beam epitaxy MBE, Rf- sputtering and Ion Beam Preferential Etching (IBPE). Based on our structural, transport and inductance studies we discussed possible mechanisms of the surface metallic state in the IBPE SrTiO3 [1], and elevated superconducting temperature observed in the La2-xSrxCuO4 bi-layers [2]. [1] Reagor D.W., Butko V.Y. Highly conductive nanolayers on strontium titanate produced by preferential ion beam etching. Nature Materials 4 (8): 593-596 Aug., 2005. [2] Bozovic I, Logvenov G, Belca I, et al. Epitaxial strain and superconductivity in La2-xSrxCO4 thin films. Physical Review Letters 89 (10) Art. 107001, Sep.2, 2002. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y10.00015: Observation of phase transition from Tomonaga-Luttinger liquid states to superconductive phase in carbon nanotubes Junji Haruyama, Masaharu Matsudaira, Naoyoshi Murata, Yuko Yagi, Erick Einarson, Shigeo Maruyama, Toshiki Sugai, Hisanori Shinohara A carbon nanotube (CNT) is a one-dimensional (1D) ballistic conductor, which has Tomonaga-Luttinger liquid (TLL) state that arises from the repulsive Coulomb interaction between electrons. In contrast, the phonon-mediated attractive Coulomb interaction leads to BCS-type superconductivity (SC) in 2D and 3D conductors. Thus, interplay of SC with the TLL states in CNTs has attracted considerable attention [1]. The experimental report, however, was only in our multi-walled CNTs (MWNTs) [2]. Here, we report the detailed observation of change in 1D Coulomb interaction in the MWNTs. The results indicate occurrence of the phase transition from the TLL states to the SC phase, as energy decreases. The small number of shells with current flow in the partially end-bonded MWNTs makes the observation possible. \newline [1] e.g., D.Loss et al., Phys.Rev.B 50, 12160 (1994-II), E.Perfetto et al., Phys.Rev.B 74, 201403(R) (2006) \newline [2] I.Takesue, J.Haruyama., et al., Phys.Rev.Lett.96, 057001 (2006) [Preview Abstract] |
Session Y11: Oxide Spectroscopy
Sponsoring Units: DCMPChair: J. Steven Dodge, Simon Fraser University
Room: Morial Convention Center RO9
Friday, March 14, 2008 11:15AM - 11:27AM |
Y11.00001: Transient Grating Measurements of YBa$_{2}$Cu$_{3}$O$_{7-x}$ films C.L.S. Kantner, J.D. Koralek, I. Hetel, T.R. Lemberger, J. Orenstein Transient photoinduced changes in the transmission of YBa$_{2}$Cu$_{3}$O$_{7-x}$ thin films have been measured using the charge-grating technique. In this technique two non-collinear pulses are interfered to create a sinusoidal spatial variation in superfluid density, which leads to a grating in the transmission coefficient. Evolution of the amplitude and phase of this grating in the time-domain can be monitored by the coherent detection of a time-delayed diffracted probe pulse. Previous experiments using conventional pump-probe methods have revealed at least two components in the change in transmission. Resolving these components is inherently ambiguous as their relative phase is undetermined. We report on the use of the phase sensitivity of the transient grating experiment to separate the individual components of the two-component wave form. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y11.00002: Visible-pump, THz-probe measurement of the photocarrier mobility in an undoped cuprate Jesse Petersen, J. Steven Dodge, Ruixing Liang We present new experimental results on the mobility of photoexcited carriers in Sr$_2$CuCl$_2$O$_2$, an undoped cuprate. We use ultrafast laser pulses to excite photocarriers in the antiferromagnetic insulating lattice. We then probe the low-frequency dynamical conductivity of the resulting nonequilibrium state with time-domain terahertz spectroscopy. We observe the prompt onset of photoconductivity followed by a non-exponential decay on ps timescales. Assuming all photoexcited carriers are free we observe a peak mobility of $\sim$0.1 cm$^2$/Vs, much lower than the Hall mobility in chemically doped systems with similar carrier concentrations \footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)}. Such a low mobility suggests the formation of polarons or excitons after photoexcitation. We will also discuss the temperature dependence of the nonequilibrium state. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y11.00003: Time-resolved spectroscopy of the charge-transfer gap in Sr$_2$CuO$_2$Cl$_2$ J. Steven Dodge, Andreas Schumacher, Lance Miller, Daniel Chemla We present energy- and time-resolved pump-probe spectroscopy near the charge-transfer gap in the undoped cuprate compound Sr$_2$CuO$_2$Cl$_2$. Upon photoexcitation, an increase in absorption is observed for energies below 1.95~eV, whereas a decrease occurs above 1.95~eV. Overall, the spectral weight is not conserved over the probe range of 1.6-2.3 eV. No hole-burning is observable at the pump energy $E_{\mathrm{pump}} = 2.1$~eV. The transient spectral changes appear as one spectral unit instantaneously after the excitation, and they decay, again as one spectral unit, on a picosecond time scale. The photoinduced response relates simply to the thermal response, indicating a common boson-mediated origin. These results support a theoretical model that places the gap energy near 1.5 eV, well below the peak in the charge-transfer absorption spectrum.\footnote{K. M. Shen \textit{et al.}, Phys.~Rev.~B \textbf{75}, 075115 (2007).} In this model, the photoexcited state decays rapidly to the gap energy via phonon emission, and the presence of the additional phonons then has the same effect on the charge-transfer absorption as an increase in the equilibrium lattice temperature. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y11.00004: Unusual Spectral Signatures in Na-doped Oxychloride High Temperature Superconductors M. Brunner, K.-H. Kim, H.-G. Lee, S.-I. Lee, M.R. Peterson, B.S. Shastry, G.-H. Gweon Electron spectroscopy studies on Na-doped oxychloride samples (Na-CCOC; (NaCa)$_2$CuO$_2$Cl$_2$) have received much attention recently, due to the facts that Na-CCOC samples are easy to cleave, the crystal structure is relatively simple among the cuprates, and the spectroscopy reveals enhanced signatures of putative ``competing order.'' Here, we report the electronic structure of Na-CCOC investigated by angle resolved photoelectron spectroscopy (ARPES) as a function of momentum, energy and doping, with an emphasis on unusual nature of some signatures clearly observed in this family of the cuprates. We discuss those signatures in relation to the recently discovered high energy dispersion anomaly and the suggested charge order. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y11.00005: Competition scenario in Raman and ARPES experiments in electron-doped cuprates Belen Valenzuela, Elena Bascones Raman and ARPES experiments in the superconducting state of electron doped cuprates have shown deviations from the predictions of a monotonic d-wave superconducting gap. Two scenarios have been proposed to explain these experiments: a non-monotonic BCS gap on a large Fermi surface and a two band model due to the truncation of the Fermi surface by antiferromatic correlations. We calculate angle resolved photoemision spectrum and Raman signal in the superconducting phase using a two-band model where superconductivity and antiferromagnetism competes and compare our results with others theoretical scenarios. We also discuss the effect of the non-monotonicity of the gap on the tunneling experiments. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y11.00006: Appearance of Universal Metallic Dispersion in a Doped Mott Insulator Hsin Lin, S. Sahrakorpi, R.S. Markiewicz, M. Lindroos, X.J. Zhou, T. Yoshida, W.L. Yang, T. Kakeshita, H. Eisaki, S. Uchida, Seiki Komiya, Yoichi Ando, F. Zhou, Z.X. Zhao, T. Sasagawa, A. Fujimori, Z. Hussain, Z.-X. Shen, A. Bansil Under strong electronic correlations the parent compounds of all cuprate high-temperature superconductors assume the so-called Mott-Hubbard insulating state. By what routes these insulators accomplish the miraculous transformation into superconductors with the addition of electrons or holes is a question of intense current interest. In this study we consider the classic superconductor La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over the wide doping range of $x=0.03-0.30$, delineating how the electronic spectrum evolves with doping for binding energies extending to several hundred meV's. Our analysis indicates that this Mott insulator contains `nascent' or `preformed' metallic states, which develop finite spectral weight with doping, but otherwise undergo relatively little change in dispersion over a wide doping range. Our findings challenge existing theoretical scenarios for cuprates. Work supported in part by the USDOE. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y11.00007: Systematic ARPES study of Ln$_x$Bi$_2$Sr$_{2-x}$CuO$_{6+\delta}$ Zhihui Pan, Madhab Neupane, Yiming Xu, Ziqiang Wang, Huiqian Luo, Lei Fang, Haihu Wen, Hong Ding Ln$_{x}$Bi$_{2}$Sr$_{2-x}$CuO$_{6+\delta}$ is a good candidate to investigate the effects of charge doping and potential disorder to the properties of the high-Tc cuprates. The samples with different Ln elements exhibits very different property, and can used as a probe to study the superconductivity with different Ln substitutions. High-quality single crystals of Ln$_{x}$Bi$_{2}$Sr$_{2-x}$CuO$_{6+\delta}$(Bi and La) have been synthesized over a wide substitution range. We will report our high-resolution ARPES results on of Ln$_{x}$Bi$_{2}$Sr$_{2-x}$CuO$_{6+\delta}$. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y11.00008: Strong correlation effects in the electron momentum density distribution of La$_{2-x}$Sr$_x$CuO$_4$ B. Barbiellini, P.E. Mijnarends, S. Kaprzyk, R.S. Markiewicz , M. Itou, Y. Sakurai, K. Yamada, A. Bansil Compton scattering offers unique capabilities for measuring absolute spectral weights, which are not obtainable by other spectroscopies. Moreover, this technique is genuinely a bulk- sensitive probe. In this connection, we have carried out a series of high resolution Compton scattering measurements on oriented single crystals of La$_{2-x}$Sr$_x$CuO$_4$ at three different dopings x=0, 0.15 and 0.30, together with corresponding computations based on the LDA and on models for treating strong correlation effects. Theoretical predictions are compared and contrasted with the experimentally reconstructed two-dimensional electron momentum densities to identify strong correlation effects in the spectra, and to delineate how these effects evolve as the system undergoes the transition from the Mott insulator to the superconductor. Work supported in part by the USDOE. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y11.00009: New analysis of the electronic excitations in the Mott insulator Nd$_2$CuO$_4$ using momentum-dependent intensity maps Guillaume Chabot-Couture, Jason N. Hancock, Diego M. Casa, Patrick K. Mang, Thomas Gog, Martin Greven Resonant inelastic X-ray scattering (RIXS) is a rapidly advancing technique that allows the measurement of electronic excitations in correlated-electron systems. In order to extend this technique, we developed a new circle to perform azimuthal rotations of the crystal. This allowed us to perform precise measurements of the electronic excitations as a function of the momentum transfer in the copper-oxygen planes and the incident photon polarization. As a result, we discovered that Cu K edge RIXS is at most weakly polarization dependent when the polarization is kept in the copper-oxygen planes. We used this new circle to determine the intensity variations of different energy-loss excitations across many Brillouin zones. These intensity maps as well as inelastic spectra measured at high-symmetry points in the Brillouin zone allow an improved analysis of the electronic-excitation spectral weight in this material. We discuss our findings in the context of past measurements on other cuprate systems. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y11.00010: Valence Electronic Structure of YBCO Probed by X-ray Standing Waves Jorg Zegenhagen, Sebastian Thiess, Tien-Lin Lee, Carmela Aruta, Chengtian Lin, Federica Venturini, Nicholas Brookes, Bruce C.C. Cowie The photoelectron emission spectrum of the valence band of the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) in the X-ray regime is dominated by contributions from the copper sites (d states) because of the comparably large cross section. With the help of the x-ray standing wave (XSW) technique in combination with photoelectron spectroscopy, it is possible to separate the contributions to the valence band originating from different lattice sites, even if they are populated by the same element. In this way, by applying the XSW method with photoelectron spectroscopy, we discriminated the contributions to the YBCO valence band coming from the nonequivalent in-plane and chain copper sites. Within the resolution of our measurements, the contributions of Cu-I and Cu-II to the valence band were found to be identical. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y11.00011: Doping Dependence of Electronic Excitations in La$_{2-x}$Sr$_{x}$CuO$_{4}$ Observed by K-edge RIXS D.S. Ellis, Jungho Kim, J.P. Hill, S. Wakimoto, R.J. Birgeneau, T. Gog, D. Casa, Y.-J. Kim Resonant inelastic xray scattering (RIXS) spectra of the cuprate La$_{2-x}$Sr$_{x}$CuO$_{4 }$are measured for single crystal samples with progressively larger value of x, ranging from undoped to the overdoped regime. As x is increased in the underdoped region, the lowest energy excitation above the charge transfer gap is sharply suppressed, but many of the general spectral features and overall spectral weight distribution above the gap do not appreciably change, and the broad intensity around the same energy of the exciton remains constant. As the sample becomes overdoped, a much more pronounced change of the spectral weight above the charge transfer gap occurs. An in-gap state at $\sim $1.8 eV in the undoped case, which shows no dispersion with momentum, broadens and shifts down in energy as the doping is increased. The amount of the observed shift is to within an order of magnitude of the change calculated from simple crystal field model. . [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y11.00012: Charge superstructures in Zn-doped La$_{2-x}$Sr$_x$CuO$_4$ J. C. Lee, A. Rusydi, S. Smadici, S. Wang, P. Abbamonte, M. Enoki, M. Fujita, M. Ruebhausen, K. Yamada We have observed valence band charge order in both twinned and untwinned samples of La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05} $O$_{4}$ with resonant soft x-ray scattering. In the untwinned sample the order was observed to be mainly electronic and centered at the (0,0.084,2)$_o$ position in reciprocal space, indicating diagonal charge order with period $12 a_o$, where $a_o$ is the orthorhombic lattice parameter. This order has approximately half the wavelength of the magnetic order previsouly observed with neutron scattering* in this system, suggesting a stripe interpretation. Preliminary measurements on a twinned sample revealed four satellites at (0,$K$,2)$_o$, where $K$ takes on integer multiples of the value 0.011. Relationships between these effects and the crystal structure of La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$ will be discussed. *M. Matsuda, et. al., Phys. Rev. B {\bf 73}, 140503(R) (2006) [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y11.00013: The model high-T$_C$ superconductor HgBa$_2$CuO$_{4+\delta}$: a quantitative annealing, transport and magnetic susceptibility study Neven Barisic, Yuan Li, Guillaume Chabot-Couture, Yu Guichuan, Yongchan Cho, Xudong Zhao, Martin Greven The investigation of the physical properties of high-T$_C$ superconductors (HTSCs) is complicated by several materials-related obstacles. In order to obtain reliable experimental results, homogeneous single crystals are needed, which are difficult to obtain due to chemical and/or electronic disorder. HgBa$_2$CuO$_{4+\delta}$ (Hg1201) is a particularly interesting HTSC, since it possesses the highest superconducting transition temperature (T$_C$=97 K) among single Cu-O layer compounds, a simple crystal structure, and the property to confine disorder relatively far a way from the superconducting pivotal Cu-O layers. Recently, we reported a new recipe for the growth of unprecedentedly large, gram-sized monocrystals of Hg1201 [1]. Here, we demonstrate that it is possible to select samples of the highest quality, with very few vortex pinning centers, and to dope them uniformly over a wide range of hole concentration. Furthermore, we show that those crystals can be cleaved and contacted with high-quality electrical contacts. These results make Hg1201 a particularly interesting model system for extensive experimental investigation. [1] X. Zhao et al., Adv. Mater. 18, 3243 (2006) [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y11.00014: ABSTRACT WITHDRAWN |
Session Y12: General Theory
Sponsoring Units: DCOMPChair: Anne Chaka, National Institute of Standards and Technology
Room: Morial Convention Center 203
Friday, March 14, 2008 11:15AM - 11:27AM |
Y12.00001: A Study of Null and Time-Like Angular Geodesics in Kerr-De Sitter Space-Time Nabin Malakar, Udayaraj Khanal In this work, some aspects of the angular null and time-like geodesics in Kerr-de Sitter space-time are investigated. The trajectory, of massive and mass-less particles in various geometries, is described by the geodesics. In particular, trajectories of massive and mass-less test particles around a spherically symmetric gravitating-rotating body with asymptotically flat space-time is depicted by geodesic motion in Kerr space-time. A rotating black hole in asymptotically de Sitter space-time can be described by Kerr-de Sitter geometry. Such a space-time includes a finite cosmological constant ($\lambda$). Physically, $\lambda$ is related to the vacuum energy density and implies a pre-existing curvature. The trajectories, of particles around Kerr-de Sitter black hole in a circular orbit, are unstable and oscillate about the equatorial plane but do not reach the singularity. These geodesics are found to be confined within a maximum angle about the equatorial plane. This maximum angle is dependent on the cosmological constant. So it is inferred that the thickness of accretion disks should be related to the value of cosmological constant. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y12.00002: The relativistic infinite plane Preston Jones In general relativity there have been several proposals as to what constitutes a uniform field. We give the gravitational field due to an infinite plane with finite mass per unit area, and show that this is the closest general relativistic analog to the Newtonian uniform field. Although we work in 4D we show that the 5D generalization of this solution is the starting point for many current research papers in particle physics and cosmology dealing with infinite extra dimension theories known as brane world models. This physical picture of the brane world models as higher dimensional versions of the general relativistic plane allows one to understand many of the features of these models in simple terms. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y12.00003: Einstein's 1918 Position on the Role of Ether in Relativity Theory Tom Morton In his 1918 ``Dialog about Objections to the Theory of Relativity'' Einstein emphasized the potential applicability of the Ether concept in General Relativity. He noted that ``Lorentz brought a rigid substance embodying coordinate systems into play, but SRT denied the existence of all ether concepts. However, GRT is different, and empty space has physical qualities characterized as the components of gravitational potential. This situation can very well be interpreted by speaking of an ether whose state varies from point to point. However, one has to be careful not to attribute to this ether any matter-like properties such as a distinct velocity at each point.'' This presentation seeks to identify the qualities required of an ether. Concepts might include ether as a cause of gravity, and as the source medium for the production of electric charge. The ether might conceivably take the form of a diffuse mixture of positive and negative electric charge. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y12.00004: Absolute Planck Values: Moving Beyond the Arbitrary Assignment of Unity John Laubenstein Planck Values provide a valuable tool in efforts to understand basic universal relationships; however, they fall short of having any truly intrinsic value. Planck Values come with the assumption that unity can be assigned to up to five of the fundamental universal constants. While constraining these values to unity may be convenient, it by no means ensures that intelligent life anywhere in the universe would make the same assumptions. Further, the peculiar value of the inverse fine structure constant of 137 suggests that it is naive to assume that any of the physical constants are equal to unity or any other simplistic value. Through an analysis of gravitation and electrostatic force, the IWPD Research Center has derived a logical argument for a revised set of Planck Values that represent absolute values with true universal significance. Of greatest importance, is a recalculated Planck Mass that serves as a truly fundamental unit of mass at the quantum scale. This finding contrasts with the significantly large value associated with the current Planck Mass and provides new information that may be critical in the search to unify General Relativity with Quantum Mechanics. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y12.00005: Quantum Information, Entropy, ALPHA, Hubble Time, and Dark Energy, Linked? Shantilal Goradia The postulation of fundamental constants by Newton, Einstein and Planck gave us natural units at Planck scale. Additional postulates may explain coupling constants. About sixty orders of magnitude of Planck times equal Hubble time ($W)$. Substitution of $W$ in Boltzmann's entropy equation ($S=k$ \textit{ln} $W$; with Boltzmann constant $k $= 1 in natural units, and using the natural logarithm to probe nature) equates the statistical entropy ($S)$ of the universe to about 137, the reciprocal of the fine-structure constant (\textit{$\alpha $}). Thermodynamic entropy (\textit{dS} = $\delta Q$/$T)$, a consequence of statistical entropy, implies that the fine-structure constant generates heat out of vacuum energy or dark energy. We draw support from the insights of Maxwell's demon (1867), Gamow (1967) and Eddington (1949). In information theory, entropy is linked to a measure of uncertainty, indicating that the fine-structure constant is greater than or equal to the reciprocal of the natural logarithm of the age of the universe: $\alpha \ge 1 \mathord{\left/ {\vphantom {1 {lnW}}} \right. \kern-\nulldelimiterspace} {lnW}$. The postulation in [1] (a draft of a 2008 planned review paper) will address further issues. [1] S. Goradia, What is Fine Structure Constant? http://www.arxiv.org/abs/physics/0210040v3 (revised 1/6/2007) [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y12.00006: A Zero free Parameter and Zero Counter Term Requirement Found By Replacing the Gauge Derivative General Covariance with Metric Nontrivial General Covariance in The Dirac Equation David Maker We replace the general covariance in the gauge derivatives in the Standard Model (SM) with a metric general covariance. The result is a new Dirac equation pde ($\surd $\textbf{\textit{g}}$_{\mu \mu }$\textit{$\gamma $}$_{\mu }$\textit{$\partial \psi $/$\partial $x}$_{\mu }$\textit{+i$\omega \psi $=0 }(sum on $\mu )$ with~g$_{oo}$=1-e$^{2}$/rm$_{e}$c$^{2}\equiv $1-r$_{H}$/r) requiring \textbf{no} free parameters, instead of the 18 of the SM, where here r$_{H}$ just sets the spatial scale. To illustrate the power of this technique we note here that equivalence principle considerations allow only \textit{one} type of charge e. Also g$_{oo}$ =0 at r=r$_{H}$ with stability the result. Note also that near r$_{H}$ the 2P$_{3/2}$ state for this new Dirac equation gives a azimuthal trifolium, 3 lobe shape; so this \textbf{ONE} charge e (so don't need \textbf{ color} to guarantee this) spends \textbf{1/3} of its time in each lobe (\textbf{fractionally charged} lobes), the lobe structure is locked into the center of mass \textbf{(asymptotic freedom}), there are\textbf{ six} 2P states (corresponding to the 6 flavors);~ which are the~~\textbf{main properties of quarks}!~without invoking free parameters. The S matrix of this new pde gives the W and Z as resonances and does not require renormalization counterterms or free parameters thereby restoring sanity to theoretical physics. It is vital that the physics community adopt this method if it is to break out of the 30 years of stagnation created by the confusion caused by these free parameters and counterterms. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y12.00007: The Alternate Atomic Model Evan Ragland An alternate atomic model posits concentric electron and nucleon fields spinning together about an empty center. It is alternative to the generally accepted planetary system in which electron point particles orbit about a center clump of nucleon point particles. Introduced in 1992 as an alternative to the standard model of the nucleus it applies scientific space-time knowledge unknown when the standard model was conceived. Originally advanced in the spirit of alternative equivalence it evolved to model the entire atomic structure plus many features of space-time. Structural definitions assume space-time properties of: unidirectional expansion, special relativity, electrical field, magnetic field, spin field, gravity field, and space-time surface effect. Field effects are associated with Faraday lines of field force. Model properties feature symmetry and complementarily. Mass structures of the electron, proton,neutron, and protium atom plus the atomic and nuclear constituencies of all elements are developed. In addition the nuclear strong force is defined, the magnetic anomaly explained, etc. Model logic constructs the neutron as the complement of the hydrogen (protium) atom. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y12.00008: Proof of the Wave Nature of Plants Orvin Wagner I assume plants operate with a set of frequencies. These frequencies and the means of these frequencies are equal in all directions. We can then write (v$_{h}$/$\lambda )_{avh}$=(v$_{v}$/$\lambda )_{avv}$ where the subscripts h and v represent horizontal and vertical respectively and av is average,. or v$_{v}$/v$_{h}$=(1/$\lambda _{h})_{av}$/(1/$\lambda _{v})_{av}$. I use an internodal spacing as $\lambda $/2 or the the distance between adjacent branches, leaves, etc. The ratios, v$_{v}$/v$_{h}$, are ratios of small integers for sufficient samplings. For example, for Ponderosa pine the ratio is 3/1 or for delicious apple 4/3. Note that these ratios represent the shape of the tree or other plant and their interactions with gravity. These ratios are derivable by other means such as use the ratio of {\#} of horizontal needles per unit length from a horizontal sample to the {\#} of needles per unit length from a vertical sample from p-pine. Or measure the vertical and horizontal velocities. My literature provides many other proofs of the wave nature of plants. I suggest that the waves in and related waves outside of plants (outside 4.9 m/s) are a dark matter related since they travel at such low velocities. See my present web site at home.budget.net/$\sim $oedphd. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y12.00009: Post modern physic (a revolution in modern physics) Shahram Malekzadeh When you make a deep Study in Classic and modern physics you will find great and numerous weak points. On the other hand when you want to dismiss a wrong theory you should accept the other maybe incorrect theory so I think that we should write the physics from very beginning. Here I mentioned new theories on each field that can be a start for other scientists. Theories like ``cyclone theory'' which is about light travel or ``matrix theory'' which denotes that the materials are not made only atoms but an external capsule named matrix. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y12.00010: The mass, energy, space and time system theory-MEST Dayong Cao Things have their own system of mass, energy, space and time of themself. (The MEST for short thereinafter). Mass is density, energy is force, time is frequency, spac is amplitude square. There are the transmutation between space-time and mass-energy. When they get a balance, they get the inertia and eigenvalue system. The inertia, eigenvalue and the transmutative balance system equation are being put forward. In quantum mechanics, the quality of radiate wave mainly is quantum space-time while it's mass-energy is ancillary; The nature of the macrosubstance is mainly mass-energy while space-time are ancillary. Mass-energy are like the earth center while space-time are like the heaven around. An Nuclear of an atom is like the earth center while the charge cloud of an atom(radiate wave) is like the heaven around. Macro is like the earth center while micro is like the heaven around. Physics system of The MEST of heaven and earth, which unites both macrophysics and microphysics. New space-time equation and photoelectric conversion equation are put forward, and get new mass-energy equation which equal the mass-energy relation of Einstein and the equation of electrodynamics. Black hole is space-time center while the dark matter-energy is the radiate mass-energy wave around Black hole. The dark matter-energy is from the Black hole. Black hole and Dark matter-energy can reduce the frequence of the light of star. [Preview Abstract] |
Session Y13: Classical Molecular Dynamics and Fluids
Sponsoring Units: DCOMPChair: Brian Good, NASA GRC
Room: Morial Convention Center 204
Friday, March 14, 2008 11:15AM - 11:27AM |
Y13.00001: Canonical sampling through velocity rescaling Giovanni Bussi, Davide Donadio, Michele Parrinello We present [1] a new molecular dynamics algorithm for sampling the canonical distribution. In this approach the velocities of all the particles are rescaled by a properly chosen random factor. The algorithm is formally justified and it is shown that, in spite of its stochastic nature, a quantity can still be defined that remains constant during the evolution. In numerical applications this quantity can be used to measure the accuracy of the sampling. We illustrate the properties of this new method on Lennard-Jones and TIP4P water models in the solid and liquid phases. Its performance is excellent and largely independent on the thermostat parameter also with regard to the dynamic properties. [1] G. Bussi, D. Donadio and M. Parrinello, J. Chem. Phys. 126, 014101 (2007) [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y13.00002: Well-tempered metadynamics: a smoothly-converging and tunable free-energy method Alessandro Barducci, Giovanni Bussi, Michele Parrinello We present [1] a method for determining the free energy dependence on a selected number of order parameters using an adaptive bias. The formalism provides a unified description which has metadynamics and canonical sampling as limiting cases. Convergence and errors can be rigorously and easily controlled. The parameters of the simulation can be tuned so as to focus the computational effort only on the physically relevantregions of the order parameter space. The algorithm is tested on the reconstruction of alanine dipeptide free energy landscape. [1] A. Barducci, G. Bussi and M. Parrinello, Phys. Rev. Lett., accepted (2007). [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y13.00003: Hard-sphere variational CPMD approach Gerald Faussurier, Christophe Blancard, Pier Luigi Silvestrelli We present a variational method to determine the total free energy of the electron and ion system using the Gibbs-Bogolyubov inequality and a hard-sphere reference system applied to the quantum molecular dynamics code CPMD. Numerical results and comparisons with quantum molecular dynamics simulations and experiments are presented and discussed for dense and expanded aluminum. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y13.00004: Classical Simulation {\it versus} Perturbation Theory of Anharmonicity in 2D Lennard-Jones Triangular Lattice Xiao Shen, Tao Sun, Julie Stern, Philip B. Allen Classical molecular dynamics simulation and perturbation theory are two methods that can treat the anharmonicity in solids. Classical molecular dynamics simulation can treat anharmonic effects to high order. Perturbation theory beyond lowest order is difficult, and has convergence issues. However, perturbation theory easily treats the thermodynamic limit, while simulation is necessarily done on a finite system. This raises interesting questions such as whether molecular dynamic simulation will give the correct decay rate when the phonon mean free path is larger than the simulation cell. We try to answer such questions, and explore the limits of both methods, by comparing their results. The system we studied is a two dimensional triangular lattice model with a Lennard-Jones potential. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y13.00005: Dissipation in an electric field-driven synthetic rotary caltrop-based molecular motor Corina Barbu, Vincent Crespi A molecular caltrop has a three-legged base for attachment to a substrate and a vertical molecular shaft functionalized with a dipole-carrying molecular rotor at the upper end. The desired rotational motion of the rotor can generate dissipation when the motor is driven at frequencies which are close to the natural frequencies of soft vibrational modes in the structure or librational of the rotator about field direction. Classical molecular dynamics simulations elucidate the role of these resonances and investigate motor performance under external drive. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y13.00006: Convergence Rates of a Dynamic Monte Carlo Rejection-Free Method for Interacting particles Marta Guerra, Mark Novotny, Hiroshi Watanabe, Nobuyasu Ito We calculated the efficiency of a Rejection-Free Monte Carlo method\footnote{H. Watanawe, S. Yukawa, M.A. Novotny and N. Ito, \textit{Efficiency of Rejection-free dynamic Monte Carlo methods for homogeneous spin models, hard disk systems, and hard sphere system}, Phys. Rew. E, \textbf{74}, 026707 (2006)} in the limit of low temperatures and/or high densities for $d$-dimensional particles interacting through a repulsive power-law $r^p$ as well as Lennard-Jones Interactions. Theoretically we find the algorithmic efficiency is proportional to $\rho^{\frac{p+2}{2}}T^{-\frac{d}{2}}$ where $\rho$ is the particle density and T the temperature. For different powers ($p$) in 1, 2 and 3 dimensions as a function of $T$ and $\rho$, we report results in agreement with our theoretical predictions [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y13.00007: Introducing the Reduced Monte Carlo Scheme (RMCS) with Application for Hard Sphere Equation of State and First Order Phase Transition Uduzei Edgal, David Huber This is the first demonstration of a novel approach, the ``Reduced Monte Carlo Scheme'' (RMCS), developed for the investigation of the statistical thermodynamic properties of multi-scale material systems (classical and quantum) over the entire temperature and density range, with arbitrary inter-particle interactions. RMCS employs a new ensemble, the ``nearest neighbor ensemble'' involving the PDF for ``n'' nearest neighbor configurations. RMCS results for the equation of state of the hard sphere system from low densities to densities in the neighborhood of closest packing will be discussed. The power of RMCS as a materials investigative tool will be tested within the region of first order phase transition as well as for correct asymptotic behavior of the solid phase equation of state. Early results show that RMCS provides accurate results in the fluid phase employing ``small'' n values, thus suggesting that RMCS may provide a highly efficient computational scheme in contrast to traditional Monte Carlo methods which normally require ``large'' systems for computation. Also to be discussed briefly, is the outlook for the development of the quantum version of RMCS for quantum systems. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y13.00008: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y13.00009: Angular momentum form of Verlet algorithm for rigid molecules Miyabi Hiyama, Tomoyuki Kinjo, Shiaki Hyodo We seek to make an algorithm based on the Verlet method which could be applied to non-Hamiltonian and explicit time dependent Hamiltonian systems for rigid molecules. For the first step of this aim, we will propose an algorithm based on the Verlet method for rigid molecules and investigate the characteristics of this algorithm for simple system. In our algorithm, the equations of motion for rigid molecules are integrated by Verlet framework in the angular momentum form. This simple algorithm is named `the angular momentum Verlet algorithm'. We will show the results of MD simulations for 125 carbon tetrachloride molecules using the angular momentum Verlet algorithm. The relative total energy fluctuations are compared with those using the standard leap-frog and the Gear predictor-corrector algorithms. The energy drift using the angular momentum Verlet algorithm is smaller than that using the leap-frog or the Gear predictor-corrector algorithms, especially in the case of MD simulation with the large time interval. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y13.00010: Absorbing boundary conditions for molecular dynamics and multiscale modeling S. Namilae, D.M. Nicholson, P.K.V.V. Nukala, C.Y. Gao, Y.N. Osetsky, D.J. Keffer We present an application of differential equation based local absorbing boundary conditions to molecular dynamics. The absorbing boundary conditions result in the absorbtion of the majority of waves incident perpendicular to the bounding surface. We demonstrate that boundary conditions developed for the wave equation can be applied to molecular dynamics. Comparisons with damping material boundary conditions are discussed. The concept is extended to the formulation of an atomistic-continuum multiscale scheme with handshaking between the regions based on absorbing boundary conditions. The multiscale model is effective in minimizing spurious reflections at the interface. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y13.00011: Coupling atomistic molecular dynamics and fluctuating hydrodynamics: shear and sound Rafael Delgado-Buscalioni, Gianni de Fabritiis Bridging spatio-temporal scales is the main objective of multiscale modeling and one of the hot-topics in the simulation community. However, compared to gas and solid phase, hybrid schemes based on molecular-continuum domain decomposition of the liquid phase are relatively less developed. The present hybrid model (see PRL 97, 134501 and PRE 76, 036709) is the first to include several decisive features: the molecular domain is described with atomistic accuracy (chemical specificity), and it is embedded within a continuum fluid description based on the Landau-Lifshitz fluctuating hydrodynamics equations. The hybrid scheme is thermodynamically consistent (e.g. the MD domain is an open subsystem in agreement with the grand canonical ensemble) and fluctuations of mass, momentum and stress are seamlessly connected across the molecular-continuum interface. As the scheme is based on mass and momentum conservation, it enables to solve shear and sound waves traveling across both domains. Due to its relevance we consider water as working solvent. As a test case, we have studied the reflection of sound waves by a lipid monolayer (DMPC) immersed in aqueous solvent. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y13.00012: Concurrent triple-scale simulation of molecular liquids Matej Praprotnik, Kurt Kremer, Rafael Delgado-Buscalioni We present a triple-scale simulation of a molecular liquid, in which the atomistic, coarse-grained and continuum descriptions of the liquid are concurrently coupled. The presented approach successfully sorts out the problem of large molecules insertion in the hybrid particle-continuum simulations and thus opens up the possibility to perform efficient grand-canonical molecular dynamics simulations of open molecular liquid systems. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y13.00013: Averaged Equations for Species Interactions in Binary Particulate Systems Duan Zhang, Jin Liu Averaged equations for disperse two-phase flows are relatively well-studied compared to averaged equations for binary particulate systems. Disperse two-phase flows can be viewed as a limit of binary particulate system, in which the continuous phase consists of large amount of small particles, such as molecules, and the disperse phase consists of smaller number of large particles. Therefore averaged equations for disperse two-phase flows provide a guidance for the derivation of averaged equations for binary particulate systems. A correct system of averaged equations for binary particulate systems has to recover the averaged equations for disperse two-phase flows in this limit. In this talk it is shown that this can only be done by introducing an interspecies stress in a binary particulate system. Although the framework of deriving the averaged equations is applicable to general particulate systems, numerical simulations are performed for a granular system to study the behaviors of the species exchange force, intraspecies stresses and interspecies stress. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y13.00014: The Scaling of Atomistic Fluid Dynamics Simulations John Barber, Kai Kadau, Timothy Germann, Berni Alder A series of large-scale atomistic simulations of the Rayleigh-Taylor instability was performed using up to 5.7 billion particles. The results of these simulations, which included a wide range of time and length scales, suggest that atomistic fluid dynamics simulations exhibit a scaling similar to that predicted for Navier-Stokes solvers. Furthermore, quantitative comparison to a macroscopic Rayleigh-Taylor experiment further suggests that the results of atomistic simulations - even for complex non-stationary flows - can be scaled up to describe larger systems. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y13.00015: A numerical method for miscible two-fluid flow at a large density ratio Siina Haapanen A computational algorithm for direct numerical simulation of a binary system of miscible fluids at a large density ratio is described. The flow is three-dimensional, with two of the three spatial dimensions periodic. A pseudo-spectral discretization is used in the periodic directions, and an eigth order compact finite difference scheme is utilized in the non-periodic direction. The Mach number of the flow is small, and the equations of motion are integrated forward in time using a fractional step method. A constant coefficient elliptic equation (Poisson equation) is solved to determine the pressure. The method is applied to test problems including the Rayleigh-Taylor instability and a miscible two-fluid shear flow. The accuracy of the method, and its stability at a large density ratio of the fluids are discussed. [Preview Abstract] |
Session Y15: Focus Session: Quantum Metrology and Control: Fundamental Limits and Applications
Sponsoring Units: GQIChair: Lorenza Viola, Dartmouth College
Room: Morial Convention Center 207
Friday, March 14, 2008 11:15AM - 11:51AM |
Y15.00001: Quantum Enhanced Sensing, Measurement, and Control Invited Speaker: This talk investigates how quantum-mechanical effects such as squeezing and entanglement can be used to enhance the precision and sensitivity of imaging, measurement, and control. Entanglement can give a substantial enhancement to sensitivity even in the presence of high levels of noise and loss. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y15.00002: Environmental Constraints in Practical Photonic Quantum Sensing. Yaakov Weinstein, Gerald Gilbert, Michael Hamrick We report on research directed to problems associated with the propagation of photonic signal states in quantum sensing. Attention is devoted to constraints associated to realistic propagation environments for practical applications of quantum sensing. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y15.00003: Quantum efficiency of binary-outcome solid-state detectors Alexander N. Korotkov We discuss the definitions of the quantum efficiency for binary-output detectors of solid-state qubits, focusing on the subclass of quantum non-demolition detectors. Similar to the previously considered case of linear detectors, the definitions of the quantum efficiency are based on the relation between the ensemble decoherence and the information acquired from the measurement (this information determines the lower bound for the ensemble decoherence). Quantum efficiency is analyzed for several models of binary-outcome detectors, including indirect projective measurement, linear detector in a binary-output regime, detector for a phase qubit, and detector based on tunneling into continuum. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y15.00004: Quantum Metrology with Product States Animesh Datta, Sergio Boixo, Steven Flammia, Anil Shaji, Carlton Caves, Emilio Bagan We study the performance of initial product states of $n$-body systems in generalized quantum metrology protocols that involve estimating an unknown coupling constant in a nonlinear k-body ($k \ll n$) Hamiltonian. We obtain the theoretical lower bound on the uncertainty in the estimate of the parameter. For arbitrary initial states, the lower bound scales as $1/n^k$, and for initial product states, it scales as $1/n^{k-1/2}$. We show that the latter scaling can be achieved using simple, separable measurements. We formulate a simple model, based on the evolution of angular-momentum coherent states, which explains the $O(n^{-3/2})$ scaling for $k = 2$, implementable with Bose-Einstein condensates; the model shows that the entanglement generated by the quadratic Hamiltonian does not play a role in the enhanced sensitivity scaling. We show that phase decoherence does not affect the $O(n^{-3/2})$ sensitivity scaling for initial product states. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y15.00005: Efficient feedback controllers for continuous-time quantum error correction Andrew Landahl, Brad Chase, J.M. Geremia We present an efficient approach to continuous-time quantum error correction that extends the low-dimensional quantum filtering methodology developed by van Handel and Mabuchi [quant-ph/0511221 (2005)] to include error recovery operations in the form of real-time quantum feedback. We expect this paradigm to be useful for systems in which error recovery operations cannot be applied instantaneously. While we could not find an exact low-dimensional filter that combined both continuous syndrome measurement and a feedback Hamiltonian appropriate for error recovery, we developed an approximate reduced-dimensional model to do so. Simulations of the five-qubit code subjected to the symmetric depolarizing channel suggests that error correction based on our approximate filter performs essentially identically to correction based on an exact quantum dynamical model. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y15.00006: Passive Cooling of a Micromechanical Oscillator with a Resonant Electric Circuit K.R. Brown, J. Britton, R.J. Epstein, J. Chiaverini, D. Leibfried, D.J. Wineland Currently there is considerable interest in the cooling of macroscopic mechanical oscillators, as strong cooling may allow one to reach the quantum regime of such oscillators. Recent advances in fabrication and cooling techniques have brought this regime much closer. Here we present theoretical and experimental results for cooling of the fundamental mode of a miniature cantilever by capacitively coupling it to a driven rf resonant circuit. Cooling results from the rf capacitive force, which is phase shifted relative to the cantilever motion due to the finite decay time of the resonant circuit. If this force varies with an appropriate phase shift relative to the motion of the cantilever, it can oppose the velocity of the cantilever, leading to cooling. We demonstrate this technique by cooling a 7 kHz cantilever from room temperature to 45 K, obtaining reasonable agreement with a model for the cooling, damping, and frequency shift. Extending the method to higher frequencies in a cryogenic system could enable ground state cooling and may prove simpler than related optical experiments in a low temperature apparatus. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y15.00007: Sensitive orthogonal optical monitoring of a micromechanical oscillator Ako Chijioke, John Lawall Optical sensing of oscillations of a mechanical microresonator is of crucial interest for a number of purposes including observation of quantum behavior of macroscopic objects and force microscopy. The majority of optical sensing schemes use light aligned with the axis of mechanical oscillation. We present sensitive monitoring of the oscillations of a micromechanical resonator orthogonal to the field in an optical cavity, and its particular advantages. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y15.00008: Sideband Resolved Cooling of a Nanomechanical Resonator Parametrically Coupled to a Microwave Resonator Jared Hertzberg, Tristan Rocheleau, Tchefor Ndukum, Keith Schwab We have created a nanostructure formed by a radio-frequency nanomechanica (NEMS) resonator capacitively coupled to a 5 GHz superconducting, co-planar waveguide (CPW) resonator. Recently, we have shown that it is possible to passively cool a NEMS resonator to within a few tens of quanta of its ground state, N = 25 [1]. By driving this coupled system at a frequency $\omega_{pump} = \omega_{CPW} - \omega_{NEMS}$, we expect to produce an active cooling process in the sideband resolved limit which in principle [2] should be capable of preparing the ground state of motion,with occupation factors N << 1. In future work, we expect to be able to demonstrate backaction evading position detection and ultimately squeezed quantum states of the mechanical device by using more advanced pumping schemes, such as double sideband pumping. [1] A. Naik et al, Nature 443, 193 - 196 (2006) [2] F. Marquardt et al, Phys. Rev. Lett. 99, 093902 (2007) [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y15.00009: Parametric Amplification of Quantum Signals with a Josephson Ring Modulator Nicolas Bergeal, Flavius Schakert, Michael Metcalfe, Vladimir Manucharyan, Rajamani Vijayaraghavan, Markus Brink, Michel Devoret Quantum Mechanics puts a limit on how small the degradation of information passing through a phase preserving amplifier can be. It is known theoretically that the minimum noise added by the amplifier to the signal amounts at least to half a photon at the signal frequency.~ Is it possible~to construct a~practical amplifier working at microwave frequencies that would reach this quantum limit? We have developed a new device aiming at answering this question, which is of practical importance for the readout of solid state qubits, and more generally, for the measurement of very weak signals in various areas of science. The device is based on a ring of four Josephson junctions which connects two microwave resonators corresponding to the signal and idler modes. It can be operated both as an amplifier and a frequency converter. Theoretical aspects and experimental results will be presented. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y15.00010: Microwave characterization of Josephson junction arrays in superconducting regime for Coulomb blockade and quantum metrology Vladimir Manucharyan, Michael Metcalfe, Jens Koch, Luigi Frunzio, Markus Brink, Nicolas Bergeal, Leonid Glazman, Michel Devoret Although the phenomenon of Bloch Oscillations could in principle lead to a primary standard of electrical current, it requires in practice the embedding of a Josephson junction in an electrodynamic environment with microwave impedance much greater than resistance quantum for Cooper pairs. A promising candidate for such environment is an array of Josephson tunnel junctions in the superconducting (non-insulating) regime. We have developed a new technique to dispersively probe the electromagnetic properties of such arrays. We access the RF impedance of the array by placing it as a ``mirror'' in a high-Q planar superconducting microwave resonator, whose phase and magnitude response are measured. The advantage of this configuration is that, while measuring the RF property of the array, we can pass DC current through it. This serves three purposes: i) emulating the situation of a current standard experiment with arrays, ii) providing a knob for control experiments on the RF dissipation in arrays, iii) exploring a novel out-of-equilibrium non-linear collective system. Our experiment also contributes to the physics of superconducting qubits and nano-wires. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y15.00011: Impact of Atomic Gap Size on Sensitivity and Backaction of APC Displacement Detectors N.E. Flowers-Jacobs, K.W. Lehnert Recently our group created a mesoscopic displacement detector formed by coupling an atomic point contact (APC) to a nanomechanical beam and demonstrated a displacement imprecision limited by the fundamental shot-noise in the number of electrons that tunnel across the APC [1]. We continue this work by using a cryogenic apparatus that flexes the device substrate to mechanically adjust the size of the APC atomic gap {\it in situ}. The resulting changes in the APC displacement detector's intrinsic noise properties are measured by observing the 1 K random thermal motion of the nanomechanical beam at resonance frequencies up to 200 MHz. The goal of this work is to explore the effect of atomic gap size and shape on displacement sensitivity, understand the origin of the observed measurement backaction, and measure the recoil force of tunneling electrons. \newline \newline [1] N. E. Flowers-Jacobs, D. R. Schmidt, and K. W. Lehnert, {\it Phys. Rev. Lett.} {\bf 98}, 096804 (2007) [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y15.00012: Quantum Zeno Effect in Detection of Itinerant Microwave Photons Ferdinand Helmer, Matteo Mariantoni, Enrique Solano, Florian Marquardt We analyze detection of itinerant photons using a QND measurement. We show that the backaction due to the continuous measurement poses a fundamental limit for the fidelity of detection in such a scheme. We illustrate this using a setup where signal photons have to enter a cavity in order to be detected dispersively. The measurement signal in this approach is the phase shift imparted to an intense beam passing through a second cavity mode. The restrictions on the fidelity are a consequence of the Quantum Zeno effect, and we discuss both analytical results and quantum trajectory simulations of the measurement process. Finally, we briefly mention a possible experimental realisation in the context of superconducting circuit QED. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y15.00013: Rapid Purification protocols for Optical Homodyning Aravind Chiruvelli Recently Jacobs (PRA 67 030301 (2003)) and Wiseman and Ralph (New J. Phys 8, 90(2006)) have discovered rapid purification protocols for a qubit using quantum feedback. We present these protocols in optical setting. These could also be very useful in quantum state preparation for various uses in metrology and control. [Preview Abstract] |
Session Y18: Dynamics of Thin Polymer Films
Sponsoring Units: DPOLYChair: Peter Green, University of Michigan
Room: Morial Convention Center 210
Friday, March 14, 2008 11:15AM - 11:27AM |
Y18.00001: The effect of confinement on the structure of polystyrene melt films Mrinmay K. Mukhopadhyay, Sunil K. Sinha, Laurence B. Lurio, Curt DeCaro, Zhang Jiang, Michael Sprung The structure factor of thin, Si supported, polystyrene films has been measured using grazing incidence wide angle diffuse x-ray scattering. Measurements were made as function of thickness and molecular weight from bulk-like films down to films of thickness of the polymer radius of gyration. A standing wave technique was employed to isolate the scattering component from the film interior. We observe a diffuse background and a liquid scattering ring whose intensity, for thick films, depends only on the magnitude of the scattering vector. In thinner films the intensity in the scattering ring is strongly concentrated along the surface normal direction. We interpret this peak as due to the chain-chain correlations and the concentration of scattering along the surface normal is indicating preferential stacking of the polymer chains parallel to the surface. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y18.00002: Tuning the Glass Transition Temperature over 100 K using Polymer-Polymer Interfaces Connie B. Roth, Rodney D. Priestley, Soyoung Kim, John M. Torkelson During the past decade considerable research has focused on the impact of the free surface and substrate interactions on the glass transition temperature (Tg) in nanoconfined geometries. For example, the large (up to 80 K) Tg reductions that have been observed in free-standing films indicate that we still have much to learn about the nature of the glass transition. Here we focus on an entirely different kind of interface, a polymer-polymer interface, which we show can have an even stronger impact on the Tg dynamics than a free surface. We demonstrate that the interactions across a narrow polymer-polymer interface are sufficient to tune the Tg of a single polymer material by over a 100 K simply by changing the type of polymer in the adjoining layer. The cooperative segmental dynamics of the two immiscible polymers are strongly coupled over length scales of several tens of nanometers. These results have significant impact on our understanding of the glass transition in multilayer films and nanostructured polymer blends with large amounts of polymer-polymer interface. These findings also suggest new methods for controlling polymer properties in nanoconfined geometries. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y18.00003: The viscoelastic properties of ultrathin polymer films as measured with a novel nanobubble inflation technique. Paul OConnell, Gregory McKenna Using a nano-bubble inflation technique developed within our laboratory, we have measured the absolute biaxial compliance of polymer films as thin as 11.3 nm. Previous results have shown that the degree of reduction in Tg with film thickness is not universal viz., PVAc shows no reduction even for the thinnest films while the PS shows a significant reduction at a thickness below approximately 80nm. In addition the rubbery plateau region for both materials shows dramatic stiffening as the thickness is reduced ($>$300 times) and scales as approximately the square of film thickness. We have extended the analysis of the data to directly determine the creep compliance function from the measured data rather than the minimization routine used previously. Creep compliance master curves constructed from data at varying thicknesses show that time-temperature superposition is valid even at the thinnest film thickness. The time-temperature shift factors are consistent with a WLF-type dependence and indicate a reduction in Tg for PS at 11.3nm of 53K while no significant reduction ($<$ 3K) is seen for PVAc. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y18.00004: Fabricating Nanoscale Gratings with Gradient Pattern Height by Annealing Imprinted Polymer Patterns Yifu Ding, Hyunwook Ro, Jirun Sun, Jing Zhou, Sheng Lin-Gibson, Christopher Soles The evolution of nanoimprinted polymer patterns during isothermal annealing is driven by the interplay of the pattern features, material properties of the polymer, and the polymer/substrate interactions. With proper control of these factors, a range of hierarchical nanostructures can be fabricated through thermal annealing of the imprinted polymer patterns. Here we demonstrate an example of creating polystyrene (PS) gratings with gradient pattern height. This is achieved by annealing the imprinted PS gratings under a temperature gradient. In the simplest case, the pattern decay rate is determined by the viscosity and surface tension of the PS. Consequently, the degree of the gradient pattern height can be well controlled through the ``fragility'' of the PS, i.e. its temperature dependence of the viscosity. Such a gradient grating is extremely useful in the combinatorial studies of the effect of the surface topology on the cell behaviors and controlled wettability. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y18.00005: Substrate and chain size dependence of near surface dynamics of glassy polymers Dongping Qi, Zahra Fakhraai, James Forrest We report on the application of nanohole relaxation technique to study the surface relaxation of i-PMMA thin films. This allows us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface. By studying the film thickness dependence of the near-free surface relaxation for films on different substrates we are able to determine the range over which the substrate directly affects the free surface relaxation. This also allows us to determine a limiting thickness where the free surface relaxation is not affected by the substrate. For such films we determine the Mw dependence of the near free surface relaxation time and find a surprising linear Mw dependence. The Mw dependence is discussed in terms of possible motions as well as polymer configurations near the free surface. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y18.00006: Molecular Simulation of Confined Polymer Films : Structure, Dynamics and the Glass Transition Vikram Kuppa, Gregory Rutledge Molecular Dynamics simulations are used to probe the structure and dynamics of polymers in extreme confinements. The simulations mimic intercalated nanocomposites in which polymer chains are trapped in nanometer sized slit pores between layered inorganic surfaces : our system consists of thin films of bead-spring chains spatially restricted in one dimension by surfaces comprised of monomer beads arranged in an FCC configuration. The responses of the system are studied as a function of slit spacing, polymer-wall interaction strength and temperature. The glass transition temperatures, as well as the fragility of the confined films are seen to increase with increasing confinement and with increasing attraction of the polymer with the confining wall. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y18.00007: Probing Relaxation in Glassy Freestanding Diblock Copolymer Films Adam N. Raegen, Andrew B. Croll, Kari Dalnoki-Veress We employ an axi-symmetric deformation and modulus test (ADAM) to measure the response of a thin freestanding diblock copolymer film to an external load. The method measures the deformation of a spincast film when an axi-symmetric load is applied by a flat circular punch. The flat punch minimises uncertainties in the experiment, while the use of spincast films provides a very smooth contact surface. The use of diblock copolymers allows us to change the internal structure of the film from disordered to ordered (lamellar) and surface topography (flat if there are an integer number of lamellae, and islands, bicontinuous or holes for non-integer) by changing the annealing history and thickness of the sample. We discuss our results in terms of the elastic modulus and creep compliance of the films. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y18.00008: Ellipsometric Investigation of the Surface Dynamics of a Polymer Film near the Glass Transition Temperature Ashis Mukhopadhyay, Christopher Grabowski We have investigated the surface dynamics of poly (butyl methacrylate) films by using a phase-modulated ellipsometer, which can measure thickness at angstrom-level resolutions. Experiments were performed for a range of temperatures, both above and below the glass transition temperature (Tg) of this system. Thickness-thickness correlation functions were calculated at each temperature using ellipsometry data collected at 200 Hz frequency. Our results indicate that, above Tg, the relaxation time stays relatively constant ($\sim $1 sec) and the correlation functions obey a simple exponential decay. As Tg is approached, a dramatic increase in the relaxation time is observed and the correlation functions are best fitted with a stretched exponential Kohlrausch-Williams-Watts (KWW) relation. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y18.00009: Confinement effects on the dynamics of polymers Hugues Bodiguel, Guang Yin Jing, Christian Fretigny Dewetting experiments of thin polystyrene films on a liquid substrate are performed in the vicinity of the glass transition. It is shown that this technique reveals the extensional creep compliance function of the polymer. The viscosity in the flow regime is very much reduced when the film thickness becomes comparable to the gyration radius of the polymer. This long time behaviour may be associated with the large length scales involved in the viscous flow which should be modified by confinement. On the contrary, the rubbery plateau remains unaffected by the confinement up to a fraction of the coil size. This can be related to the short length scales involved in the rubber elasticity. In the viscoelastic region, physical aging of the sample is clearly evidenced: Structural recovery complicates the short times dewetting response. Preliminary results are presented of the confinement effects on aging properties of ultra thin films as revealed in this original way. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y18.00010: Suppression of the Tg-Confinement Effect in Thin Polymer Films by the Presence of an Anti-Plasticizer Soyoung Kim, Manish Mundra, Connie Roth, John Torkelson The effect of film thickness on the glass transition temperature (Tg) of poly(vinyl acetate) supported on silica was studied via ellipsometry for films that were in the bone-dry state and also with several weight percent water sorbed into the film. The presence of water resulted in a decrease of the Tg of bulk poly(vinyl acetate) films but an increase in the density of the films. This combination of effects indicates that water acts as an anti-plasticizer in poly(vinyl acetate). We demonstrate that the bone-dry poly(vinyl acetate) films exhibit a significant reduction in Tg with decreasing film thickness below about 50 nm. In contrast, the poly(vinyl acetate films) containing several weight percent water exhibited no reduction in Tg relative to bulk Tg with decreasing film thickness down to a thickness of about 10 nm. These results are in accord with recent theoretical analysis (Riggelman et al., PRL 97, 045502 (2006)) indicating that the presence of anti-plasticizers leads to a suppression of confinement effects on the behavior of polymer films and indicate that caution should be employed when studying confinement effects in polymer systems that may sorb significant levels of water from the atmosphere. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y18.00011: New Measurements of the Effects of Confinement on the Glass Transition Temperature of Freely Standing Polymer Films John Torkelson, Soyoung Kim, Connie Roth Pioneering work by the Dutcher group (PRL 77, 2002 (1996)) a dozen years ago led to the first measurement of the Tg-confinement effect in freely standing polymer films. Their studies were especially intriguing because of the observations of very large Tg reductions relative to bulk Tg and a strong molecular weight dependence of the Tg-confinement effect. Such a molecular weight dependence is absent in the Tg-confinement effect of supported polymer films. Because of experimental difficulties associated with freely standing films, especially when the films are less than 100 nm thick, only a few related experiments have been reported by other research groups. Here we describe new results involving the measurement of Tg via the temperature dependence of fluorescence intensity of dyes labeled at trace levels to the polymer chains. Present measurements on freely standing films of poly(methyl methacrylate) (PMMA) have demonstrated reductions in Tg relative to bulk values of 20 to 25 K in films of 25-40 nm thickness. Tg reductions of at least 5 K are observed when PMMA films are 80 nm thick. Studies are also underway with polystyrene films and with polymers of different molecular weight. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y18.00012: Glass transition in ultra thin polymeric films measured by differential AC-Chip calorimetry H. Huth, A. Minakov, C. Schick The film thickness dependency of glass transition in polymer films is still controversially discussed. For different experimental probes different dependencies are observed and a generally accepted link to molecular mobility is not yet established. Calorimetry has proven to provide useful information about glass transition, because it establishes a direct link to energetic characterization. In several cases a direct comparison with results from other dynamic methods like dielectric spectroscopy is possible giving further insights. For thin films in the $\mu $m{\ldots}nm range standard calorimetric methods are mostly not applicable. In the recent years there are new developments in the field of calorimetry which overcome these limitations. We set up a differential AC-chip calorimeter capable to measure the glass transition in nanometer thin films with pJ/K sensitivity. Changes in heat capacity can be measured for sample masses below one nanogram even above room temperature as needed for the study of the glass transition in nanometer thin polymeric films. The glass transition in thin films was determined at well defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits - neither at constant frequency nor for the traces in the activation diagrams. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y18.00013: Relaxation Kinetics of Nanostructures on Polymer Surface: Effect of Orientation, Spatial Confinement, and Chain Mobility H.G. Peng, Y.P. Kong, A.F. Yee Nanostructures provide an opportunity for studying relaxation and chain dynamics of polymers when the radius of gyration is not small compared with the dimension of the structure. PS (PDI=1.03-1.05, Mw=6.4 to 1571 kg/mol) gratings of varying line-widths (600 nm, 270 nm, and 30 nm) were fabricated by nanoimprint lithography. When annealed at T $\sim $ bulk Tg, the grating height monitored with an AFM relaxes as surface tension and other driving forces overcome the polymer viscosity. The temperature for rapid relaxation decreases as the feature size diminishes for all molecular weights (MWs), but a simple explanation based on surface enhanced mobility fails to explain the results. The residual molecular orientation effect is identified as the main relaxation driving force for gratings of MWs much larger than the entanglement MW. Comparison between the various nanostructure sizes allows to observe the spatial confinement effect and to determine whether a thin mobile surface layer exists. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y18.00014: Direct Measurements of Heterogeneous Viscosity Distributions in Ultrathin Polymer Films Tadanori Koga, C. Li, J. Koo, J. Jiang, M. Rafailovich, S. Narayanan, D. Lee, L. Lurio, S. Sinha We will present direct measurements of depth dependence of the polymer chain dynamics in single ultrathin films via the diffusive motion of gold nanoparticles dispersed in the films. The technique used was x-ray photon correlation spectroscopy (XPCS) which serves a fingerprint of the Brownian motion of the gold nanoparticles. By intensifying the probing electrical field in the region of interest, we could measure the viscosity at the surface and at the position close to the center of the film separately. The XPCS results showed that most chains had at least some direct contacts with an immobile layer (absorbed with a substrate) up to 3$R_{g}$ thick from the substrate, where R$_{g}$ is the radius of polymer gyration, and hence the lateral dynamics of the polymer chains was significantly reduced. [Preview Abstract] |
Session Y22: Interfaces and Adhesion II
Sponsoring Units: DPOLYChair: Alexei Sokolov, University of Akron
Room: Morial Convention Center 214
Friday, March 14, 2008 11:15AM - 11:27AM |
Y22.00001: Polymer monolayer -- substrate adhesion strength Moshe Gottlieb, Haim Dvir Polymer monolayers have been deposited on several chemically different solid substrates. The substrates ranged from hydrophobic to hydrophilic and from chemically inert to highly reactive. In addition few of the surfaces were also exposed to ionizing irradiation. The extent of surface coverage and surface topology were experimentally determined for the different surfaces and polymers. The adsorbed layer thickness was determined optically. The strength of polymer interaction with the substrate was investigated using contact-mode Atomic Force Microscopy. Typically, for each polymer a characteristic layer thickness was measured irrespective of the nature of the surface or strength of adhesion. Adhesion strength was attributed mainly to van der Waals interactions with no indications of large scale covalent bonding between the polymer and the surface even for highly reactive surfaces. Hydrophobic interactions, surface topology, and initial conditions existing during film deposition seem to dominate the interaction between the polymer and the substrate. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y22.00002: Role of Interfacially Active Diblock Copolymers toward Controlling the Glass Transition of Thin Polymer Films Hyunjoon Oh, Peter Green We show that small concentrations of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) diblock copolymers significantly alter the thickness, h, dependence of the glass transition of polystyrene (PS) films supported by silicon substrates. The T$_{g}$ can be induced to increase, or decrease, with decreasing h or to be independent of h. T$_{g}$-shifts of as much of 35\r{ }C are obtained for films h $<$ 30 nm. The copolymers form micelles and the critical micelle concentration ($\phi _{cmc})$ of the copolymer in the thin films is considerably larger than for the bulk, larger than 15{\%}. In fact micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains required to saturate the interfaces. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y22.00003: Weak interfaces for UV cure nanoimprint lithography Frances Houle, Ann Fornof, Eva Simonyi, Dolores Miller, Hoa Truong Nanoimprint lithography using a photocurable organic resist provides a means of patterning substrates with a spatial resolution in the few nm range. The usefulness of the technique is limited by defect generation during template removal, which involves fracture at the interface between the template and the newly cured polymer. Although it is critical to have the lowest possible interfacial fracture toughness (Gc less than 0.1 Jm-2) to avoid cohesive failure in the polymer, there is little understanding on how to achieve this using reacting low viscosity resist fluids. Studies of debonding of a series of free-radical cured polyhedral silsesquioxane crosslinker formulations containing selected reactive diluents from fluorosilane-coated quartz template materials will be described. At constant diluent fraction the storage modulus of cured resists follows trends in initial reaction rate, not diluent Tg. Adhesion is uncorrelated with both Tg and storage modulus. XPS studies of near-interface compositions indicate that component segregation within the resist fluid on contact with the template, prior to cure, plays a significant role in controlling the fracture process. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y22.00004: Spincoating of ultrathin chitosan films Chris Murray, John Dutcher We have studied the spincoating of ultrathin chitosan films onto silicon wafer substrates from dilute solutions of chitosan dissolved in acetic acid solutions. This particular example of spincoating presents unique difficulties due to the non-volatility of the solvent, but also provides unique information since the spincoating process is slow enough to allow detailed measurements of the drying of the film. The resulting film thickness, as measured using ellipsometry, is a strong function of the relative humidity (RH) in the surrounding atmosphere, which can be easily controlled. By using a simple model for the dependence of film thickness on spin speed and RH, we obtain a measure of water uptake in chitosan films that can be compared with that estimated from sorption isotherms measured using ellipsometry. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y22.00005: Protein Diffusion at the Interface of Responsive Polymer Thin Films Shengqin Wang, Yingxi Elaine Zhu Protein adhesion at polymer interfaces has been much explored, yet the interfacial friction is not. We employ fluorescence correlation spectroscopy (FCS) and single-molecule imaging to examine the translational dynamic processes of protein at the responsive polymer interfaces, whose surface hydrophobicity and interfacial viscoelasticity are tunable experimentally. We focus on the dynamics of human serum albumin (HSA) and lubrin, a nutritious protein in synovial fluids, at the interface of responsive poly (N-isopropylacrylamide) (PNIPAM) brush layers. The effects of PNIPAM brush thickness, grafting density, and surface hydrophobicity on protein interfacial diffusivity are investigated. We observe the coupling of the local protein dynamics at the protein-PNIPAM interfaces with the interfacial viscoelasicity of PNIPAM brush thin films. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y22.00006: Structure and dynamics of molecules undergoing lubricated sliding Kumar Nanjundiah, Anish Kurian, Ping Hsu, Ali Dhinojwala The presence of a thin fluid layer is crucial in reducing wear and energy dissipation and is important in many areas such as tribology, adhesion, micro-fluidics, study of earthquakes and biolubrication. It has been shown using force measurements that the molecules undergo abrupt liquid-to-solid transition upon confinement and shear melting on sliding. Experiments and simulations have provided important clues but no definite answers. We have designed a friction cell that allows us to simultaneously probe the structure of the confined molecules using infrared-visible sum frequency generation spectroscopy in conjunction with friction and adhesion. Changes in the structure of liquid molecules upon confinement and during sliding will be presented. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y22.00007: Measurement advances to follow polymer thin film reaction-diffusion processes. Vivek Prabhu, Shuhui Kang, Kristopher Lavery, Kwang-Woo Choi, Wen-li Wu, Eric K. Lin Polymer thin films are used as imaging layers for photolithography to define high spatial resolution features for the semiconductor industry. These chemically amplified photoresist materials, however, may be reaching their intrinsic limits as desired feature sizes approach macromolecular dimensions. A photoacid-catalyzed reaction defines a chemical image which is subsequently resolved by dissolution in an aqueous base solution. A method to characterize the reaction-diffusion process was developed using infrared spectroscopy and tested by neutron reflectivity. We determine the thin film reaction kinetics, photoacid trapping behavior, and photoacid diffusivity by measuring the reaction kinetics. The temperature-dependence and mechanism for observed pinning of the reaction-diffusion front will be discussed. These results permit an analysis of the latent image formation which is a crucial for photolithography resolution and fidelity. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y22.00008: Methanol Diffusion into Thin Ionomer Films: An \textit{in situ} Study Using Neutron Reflectometry . Lilin He THUSITHA, N. ETAMPAWALA DVORA, PERAHIA $^{ }$Department of Chemistry, Clemson University, Clemson, SC 29634 JAROSLAW MAJEWSKI, Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545 CHRISTOPHER J. CORNELIUS$^{ }$Sandia National Laboratories, MS 0886, Albuquerque, New Mexico 87185-0886 The penetration of solvent into a polymer that consists of incompatable groups is determined by the specific interactions with the guest molecule, where interfacial structure and dynamics of the polymer affect the onset of the process. The current work presents a neutron reflectometry study of the penetration of methanol into sulfonated polyphenlylene thin films. The ionomer films were exposed to saturated deuterated methanol vapor and reflectometry patterns were recorded until equilibrium was reached. The process incorporates two stages where the vapors first wet the surface and then penetrate into the film. Significant swelling takes place as soon as the film is exposed to the vapors. Similar to previous studied in water, the onset diffusion is Fickian followed by an anomalous diffusion process. The entire process however is faster than that observed for water. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y22.00009: Pattern Formation in Dewetting Nanoparticle/Polymer Bilayers Alan Esker, Rituparna Paul, Ufuk Karabiyik, Michael Swift, John Hottle Comprised of inorganic cores and flexible organic coronae with 1 -- 2 nm diameter monodisperse sizes, polyhedral oligomeric silsesquioxanes (POSS) are ideal model nanofillers. Our discovery that one POSS derivative, trisilanolphenyl-POSS (TPP), can form Langmuir-Blodgett (LB) films on hydrophobic substrates, allows us to create thin film bilayers of precisely controlled thickness and architecture. Work with poly(t-butylacrylate) (PtBA)/TPP bilayers reveals a two-step dewetting mechanism in which the upper TPP layer dewets first, followed by the formation of isolated holes with intricate, fractal, nanofiller aggregates. Like the PtBA/TPP bilayers, polystyrene (PS)/TPP bilayers also undergo a two-step dewetting mechanism. However, the upper TPP layer initially forms cracks that may arise from mismatches in thermal expansion coefficients. These cracks then serve as nucleation sites for complete dewetting of the entire bilayer. Understanding the rich diversity of surface patterns that can be formed from relatively simple processes is a key feature of this work. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y22.00010: Case II diffusion and solvent-polymer films drying: a meso-scale model Didier Long, Mireille Souche This model is based on the fact that dynamics in liquids close to the glass transition is spatially heterogeneous [Ediger2000,Souche2007], with characteristic size 3 to 4nm in van der Waals liquids. Before considering large scale diffusion experiments, we consider first the evolution of the dynamics of a layer of thickness 3 to 4nm, submitted to an arbitrary time varying activity a(t). This procedure allows in principle to calculate a constitutive relation for the dynamics of solvent- polymer mixtures, that can then be used for calculating the evolution of macroscopic samples in contact with a reservoir of solvent. We show how these constitutive relations allow for explaining case-II diffusion in glassy polymers and provide a physical interpretation for the parameters of the Thomas-Windle model. Regarding the process of film drying, we show that films up to 1 micrometer thick can be almost completely dried in an accessible experimental time, even at temperatures well below the polymer glass transition temperature. This is a consequence: 1- of the presence of the fast path 2- of the film being out equilibrium, and in a dynamical state which is must faster than the one it would have at equilibrium. When drying a thicker film, we show that a glassy crust may appear on the free surface, as has been shown experimentally. Ediger M.D., Annu. Rev. Chem., 51 (2000) 99; Souche M. and Long D., Europhys. Lett, 77 (2007) 48002 [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y22.00011: Nonsolvent-induced dewetting of thin polymer films Tong-Fei Shi, Lin Xu, Li-Jia An The dewetting of thin liquid films is important to various technological processes. Most of the studies on dewetting are through thermal dewetting, whereas solvent-induced dewetting has received very little attention. Generally the main difference between thermal dewetting and solvent-induced dewetting is that the cause of instability is the long-range force of van der Waals interactions in the thermal dewetting whereas it is the short-range force of polar interactions in the solvent-induced dewetting. In these reports on solvent-induced dewetting, nearly all solvents, which are chosen, can dissolve the polymers. However, few reports focus on the dewetting induced by nonsolvent, which cannot dissolve polymer. In this work, the process of nonsolvent-induced dewetting of thin polystyrene (PS) films on hydrophilic surfaces at room temperature has been studied by using water as a nonsolvent. It is observed that the process of nonsolvent-induced dewetting is greatly different from other previous dewetting processes. The PS film is in non-viscous state. A mechanism of nonsolvent-induced dewetting, different from other previous dewetting mechanisms, is deduced: penetration, replacement and coalescent. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y22.00012: Mechanical Properties of Thin Polymer Films Studied by Atomic Force Microscopy Blandine Jerome, Christian Vialleton, Laurent Chazeau, Elisabeth Charlaix Introducing inorganic nanoparticles into a polymer is known to modify the macroscopic mechanical properties of the material. This is often interpreted by assuming the presence of a polymer layer with different properties at the interface with the particles. There is however little direct information available on the mechanical properties of such an interfacial layer. We have used an Atomic Force Microscope (AFM) as a nano-indenter to probe the mechanical response of thin poly(styrene butadiene) random copolymers deposited on oxidized silicon wafers (model silica surface). Indentations were performed at different approach and retraction speeds at room temperature (polymer in the rubbery state) on films with thicknesses ranging from 40nm to 500nm. Approach and retraction curves obtained at high speeds are characteristic of the indentation of an elastic material with an adhesive tip/polymer contact. At low speeds, the adhesion forces dominate for low applied forces, while the elasticity of the polymer dominates the behaviour at high applied load. This allows us to separate the mechanical response of the polymer film from the tip-polymer adhesion that involves some dissipation taking place close to the contact line between the polymer free surface and the tip. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y22.00013: Molecular Dynamics Simulations of Adhesion at Epoxy Interfaces Sarah-Jane Frankland, Thomas Clancy, Thomas Gates With composite materials becoming more prevalent as metal parts are being replaced on aircraft, adhesives are being developed for composite bonds which are suitable for the various thermal, mechanical and environmental changes that take place over the lifetime of the aircraft. The key molecular structure-property relationships that enable the chemical compatibility of the adhesive with the adherend can be identified with molecular dynamics simulation (MD). MD can assess the role of different chemical moieties in the adhesive, and their behavior in the presence or absence of solvents under different thermo-mechanical conditions. In the present work, MD simulations are used to calculate factors that affect the work of adhesion with and without solvent present. MD simulations are carried out at the interface between components of epoxy-based adhesives and composite adherends. The simulations utilize molecular models of networks which are representative of specific chemistries of the epoxy system. The simulations include both bulk and interface models of the components. The paper will present the simulation methodology, and the results for the work of adhesion.. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y22.00014: Interfacial Properties of Polydimethylsiloxane-Water Systems Ahmed E. Ismail, Gary S. Grest, Mark J. Stevens, Mesfin Tsige, David R. Heine Polydimethylsiloxane (PDMS) is a main constituent of silicone adhesives, which have a wide use as adhesives. Often these adhesives are used as sealants. The interaction between water and PDMS is of fundamental importance. To improve our understanding at the molecular level, we have performed molecular dynamics (MD) simulations of PDMS in the presence of water, with the long-term goal of studying how water molecules effect debonding at the surface. Knowledge of the basic interfacial properties of a multicomponent system, such as the surface tension, contact angle, and diffusion constant, are essential to obtain the proper dynamic behavior in a molecular simulation of adhesion and wetting processes. Explicit-atom simulations of 10$^5$ or more atoms were used to determine liquid-vapor surface tension and the contact angle for water on the surface of PDMS. We present results for the dependence of the surface tension on chain length and end-group functionality. [Preview Abstract] |
Session Y23: Focus Session: Electronic Structure of Complex Oxides
Sponsoring Units: DMP GMAGChair: Bill Butler, University of Alabama
Room: Morial Convention Center 215
Friday, March 14, 2008 11:15AM - 11:51AM |
Y23.00001: Magnetic Moment Collapse-Driven Mott Transition in MnO Invited Speaker: Metal-insulator transition in strongly correlated electron systems has been one of the central themes of condensed matter physics for a few decades. In the simplest model system, the single band Hubbard model, the transition, which is still not completely understood, is driven by the ratio of the on-site repulsion to the bare bandwidth. Real materials with multiple bands offer possibility of alternative scenarios of the metal-insulator transition. In this talk we will present a numerical study of MnO under high pressure using combination of the standard bandstructure theory and modern many-body methods (dynamical mean-field theory). Our results reveal a close relationship between the high-spin to low-spin transition and metallization, which can be interpreted as the moment collapse driving the metallization. We find an isostructural volume collapse of about 13\% accompaning the transition. While the moment collapse, which is essentially an atomic effect, is obtained by most electronic structure methods, the metal-insulator transtion can be described reliably only when dynamical correlations are taken into account. We find our results to compare very well to the available experimental data. In order to demonstrate the capability of the computational method PES and ARPES spectra obtained on the related NiO with and without hole doping will also be presented. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y23.00002: Origin of magnetism in the Fe$_2$O$_3$-FeTiO$_3$ system from correlated band theory Rossitza Pentcheva, Hasan Sadat Nabi The high remanent magnetization measured in exsolutions of the canted antiferromagnet hematite (Fe$_2$O$_3$) and room-temperature paramagnet ilmenite (FeTiO$_3$) has recently received considerable attention not only in the geoscience community [1] but also for possible spintronics applications. To resolve the microscopic origin of magnetism in this system, we have performed density functional theory calculations, varying systematically the concentration, distribution, and charge state of Ti (Fe) in a hematite (ilmenite) host. We find that including electronic correlation within the LDA+U approach is decisive to obtain the correct ground state of the end members, $\alpha$-Fe$^{3+}$$_2$O$_3$ and Fe$^{2+}$Ti$^{4+}$O$_3$. In a single Ti layer in the hematite host, Ti is not inert as commonly assumed but plays an active role in compensating the charge mismatch at the interface and the emergence of magnetism and the preferred charge state is Ti$^{3+}$, Fe$^{3+}$. As soon as a thicker ilmenite-like block forms, the most favorable compensation mechanism is through Ti$^{4+}$ and a disproportionation in the Fe contact layer in Fe$^{2+}$, Fe$^{3+}$ giving theoretical evidence for the {\sl lamellar magnetism hypothesis} [1]. The substitution of Ti (or Fe) in Fe$_2$O$_3$ (FeTiO$_3$) leads to impurity levels in the band gap and in some cases to half-metallic behavior. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y23.00003: Electronic structure of Mn and Fe oxides Walter Harrison We present a clear, simple tight-binding representation of the electronic structure and cohesive energy (energy of atomization) of MnO, Mn$_{2}$O$_{3}$, and MnO$_{2}$, in which the formal charge states Mn$^{2+}$, Mn$^{3+}$, and Mn$^{4+}$, respectively, occur. It is based upon localized cluster orbitals for each Mn and its six oxygen neighbors. This approach is fundamentally different from local-density theory (or LDA+U), and perhaps diametrically opposite to Dynamical Mean Field Theory. Electronic states were calculated self-consistently using existing parameters [1], but it is found that the charge \textit{density} is quite insensitive to charge \textit{state}, so that the starting parameters are adequate. The cohesive energy per Mn is dominated by the transfer of two $s$ electrons to oxygen $p$ states, the same for all three compounds. The differing transfer of majority $d$ electrons to oxygen $p$ states, and the coupling between them, accounts for the observed variation in cohesion in the series. The same description applies to the perovskites, such as La$_{x}$Sr$_{1-x}$MnO$_{3}$, and can be used for FeO, Fe$_{2}$O$_{3}$ (and FeO$_{2})$, Because the formulation is local, it is equally applicable to impurities, defects and surfaces. \newline [1] Walter A. Harrison, \textit{Elementary Electronic Structure,} World Scientific (Singapore, 1999), revised edition (2004). [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y23.00004: Configurational Electronic Entropy and the Phase Diagram of Mixed-Valence Oxides: The Cases of Li$_x$FePO$_4$ and Fe$_3$O_4$ Fei Zhou, Thomax Maxisch, Gerbrand Ceder We demonstrate that configurational electronic entropy, previously neglected, in {\it ab initio} thermodynamics of materials can qualitatively modify the finite-temperature phase stability of mixed-valence oxides, in our case Li$_x$FePO$_4$. First-principles LDA+U calculations were performed on 245 Li$_x$FePO$_4$ structures with different lithium/vacancy and electron/hole distributions, and Monte Carlo simulations were used to determine the phase diagram based on a coupled cluster-expansion model. While transformations from low-T ordered or immiscible states are almost always driven by configurational disorder (i.e.\ random occupation of lattice sites by multiple species), in FePO$_4$--LiFePO$_4$ the formation of a solid solution is almost entirely driven by electronic, rather than ionic configurational entropy. We argue that such an electronic entropic mechanism, rather than an ionic one, may be relevant to most other mixed-valence systems. Details in Phys. Rev. Lett. {\bf 97}, 155704 (2006). Recently we have studied the Verwey transition in magnetite Fe$_3$O$_4$. The configurational entropy of the $t_{2g}$ electrons on the iron B sub-lattice is found to lead to a first-order phase transition, although the the mechanism is substantially more complicated than that Verwey originally proposed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y23.00005: Can pristine semiconducting oxides be ferromagnetic? Hoa Hong Nguyen, A. Barla, J. Sakai, Que Huong Nguyen The recent finding of FM in HfO$_{2}$ thin films of Coey's group has urged us to re-judge the role of TM doping in introducing FM into semiconducting oxides. Our observation of FM in undoped TiO$_{2}$, HfO$_{2}$, In$_{2}$O$_{3}$, ZnO, and SnO$_{2}$ confirmed that magnetism is possible in pristine oxide thin films, and FM is likely due to oxygen vacancies. This assumption is confirmed by our XMCD measurement on TiO$_{2}$ films: The FM in TiO$_{2}$ films is indeed intrinsic, and stems from both O-2$p$ and Ti-3$d$ electrons. In semiconducting oxides, the origin of magnetism is not due to the doping, but oxygen vacancies/defects. A big issue is how to find a more appropriate model to explain better the mechanism. We propose a model based on an electronic structure calculation using the tight binding method in the confinement configuration. Vacancy sites in TiO$_{2}$, HfO$_{2}$, In$_{2}$O$_{3}$ films could create spin splitting and high spin state, so that the exchange interaction between the electrons surrounding the oxygen vacancy with the local field of symmetry could lead to a FM ground state. Calculations give the results of 3.18 $\mu _{B}$/vac for TiO$_{2}$, 3.05 $\mu _{B}$ /vac for HfO$_{2}$ and 0.16 $\mu _{B}$ /vac for In$_{2}$O$_{3}$. This model suggests that confinement effects play an important role in shaping up magnetic properties of low dimension systems. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y23.00006: Charge regulation via a negative feedback: transition metal atoms in semiconductors and insulators Hannes Raebiger, Stephan Lany, Alex Zunger Transition metal (TM) atoms in semiconductors and insulators produce energy levels in the band gap, whose occupation can be altered by shifting the Fermi level e.g. via doping. Changes in level occupation correspond to changes in the formal oxidation state. Such changes are associated with inward/outward lattice relaxations recorded as ``ionic radii'', different magnetic moments, and a core shift in x-ray photoemission. We show, via density-functional calculations within the plane-wave supercell method for TM atoms including Cr, Mn, Fe, and Co in the semiconductor hosts GaAs and Cu$_2$O, as well as in the ionic insulator host MgO, that changes in gap-level occupation result in only very small changes of charge on the TM atom itself. We show that this is due to an inherent negative feedback that regulates the TM charge via a TM--ligand rehybridization. Further, the inward/outward lattice relaxations and XPS core shifts, often associated with a change of the TM charge, in fact follow from the TM-ligand rehybridization, as the TM charge is kept unchanged via the inherent negative feedback. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y23.00007: Electronic Structure of Cubic Copper Monoxide P.M. Grant, W. Siemons, G. Koster, R.H. Hammond, T.H. Geballe We report the calculation of the band structure and optical properties of the rocksalt form of copper monoxide. Although this particular crystal structure does not exist in bulk form for CuO, at least two groups, including ourselves, have succeeded in growing by ``forced epitaxy,'' several atomic layers of cubic CuO on rocksalt proxy substrates such as MgO and STO. For our computation, we employed the DFT/LDA+U method known to give valid results for rocksalt NiO and FeO. Our results show cubic CuO, like these two materials, to be an antiferromagnetic Mott-Hubbard insulator whose band gap is primarily determined by charge transfer between filled O 2p bands and empty Cu 4s states, with localization of the Cu 3d hole by on-site coulomb repulsion frustrating what otherwise would be metallic behavior. We compare our results with NiO, FeO and the natural form of CuO found in the monoclinic mineral tenorite. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y23.00008: Instabilities of coupled Cu$_2$O$_5$ ladders Florian Schuetz, Brad Marston The spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ has a complex phase diagram including charge-density-wave order as well as unconventional superconductivity under high pressure. Due to its quasi-one-dimensional nature\footnote{S. Lee, J. B. Marston, J. O. Fjaerestad, Phys. Rev. B {\bf 72}, 075126.} fundamental questions about the high-T$_c$ cuprates might be more easily addressed in this context. However, due to the spatial proximity of neighboring ladders inter-ladder Coulomb repulsion as well as hopping between ladders might still be important. Using the functional renormalization group\footnote{M. Salmhofer and C. Honerkamp, Prog. Theor. Physics {\bf{105}}, 1 (2001).} and an analysis of generalized susceptibilities \footnote{D. Zanchi and H. J. Schulz, Phys. Rev. B {\bf 61}, 13609 (2000); C. J. Halboth and W. Metzner, Phys. Rev. Lett. {\bf 85}, 5162 (2000).}, we study a model of coupled Cu$_2$O$_5$ ladders \footnote{K. Wohlfeld, A. M. Oles, and G. A. Sawatzky, Phys. Rev. B {\bf 75}, 180501(R) (2007).}. We investigate instabilities towards charge, spin, and pairing order as a function of hole doping, inter-ladder hopping, and interaction strength starting from experimentally relevant hopping parameters\footnote{T. F. A. M\"u{}ller, {\it et al.}, Phys. Rev. B {\bf 57}, R12655 (1998).}. [Preview Abstract] |
Session Y24: Nanotube Devices and Applications
Sponsoring Units: DCMPChair: Gary Pennington, Army Research Laboratory
Room: Morial Convention Center 216
Friday, March 14, 2008 11:15AM - 11:27AM |
Y24.00001: Self-Assembled, Self-Aligned Carbon Nanotube Thin Film Transistors Michael Engel, Joshua Small, Yu-Ming Lin, Alex Green, Mark Hersam, Phaedon Avouris Carbon nanotube field effect transistors possess superb device characteristics for electronic applications. However, the non-selective nature of nanotube synthesis, difficulty in accurate nanotube placement, and the high device impedance of single tube devices pose major challenges in the integration of carbon nanotubes in large-scale electronic devices. Here we present a novel approach to address these issues. Carbon nanotubes used in this study have been purified and separated by their electronic structure, where the semiconducting tube percentage is as high as 99{\%}, confirmed by both transport measurements on individual nanotubes and by optical absorption spectra. Through a simple self-assembly technique, we have produced aligned nanotube arrays. Thin film transistors based on these aligned nanotube arrays are fabricated with both back- and top-gate layouts, showing good switching performance and a high drive current. It is found that top-gated and back-gated devices exhibit distinct switching behaviors due to screening effects. Results on device channel length dependence will also be presented. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y24.00002: Temperature Measurement of Carbon Nanotube FETs by Raman Spectroscopy Hootan Farhat, Hyungbin Son, Ying Feng, Mildred Dresselhaus, Jing Kong Heat dissipation is an important concern for nanoscale electronic devices. Freely suspended carbon nanotubes experience self heating during electron transport due to a lack dissipation channels for acoustic phonons[1]. Nanotubes lying on a SiO2 substrate, however, are often assumed to be in good thermal contact with the underlying substrate [2]. In this work we show that there is substantial self-heating in nanotubes lying on a SiO2 substrate. We use Raman spectroscopy to monitor the temperature of carbon nanotube field effect transistors (FETs) as a function of the applied bias voltage. The temperature is determined from the shift in frequency and the broadening of the high energy Raman modes. Our results suggest that nanotubes FETs on a substrate can reach temperatures upwards of 700K before saturation. [1] Pop et al., PRL 95, 155505 (2005) [2] Lazzeri et al., PRB 73, 165419 (2006) [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y24.00003: Gate-all-around carbon nanotube field-effect transistor Zhihong Chen, Damon Farmer, Sheng Xu, Roy Gordon, Phaedon Avouris, Joerg Appenzeller The ultra-thin body of carbon nanotubes allows for aggressive channel length scaling while maintaining excellent gate control. In general, a gate-all-around (GAA) structure is expected to be the ideal geometry that maximizes electrostatic gate control in FETs. Combining the ultra-thin body of a carbon nanotube with a GAA device geometry is a natural choice for ultimate device design. In this talk, we demonstrate a gate-all-around single wall carbon nanotube field-effect transistor. This is the first successful experimental implementation of an off-chip gate and gate-dielectric assembly with subsequent deposition on a suitable substrate. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y24.00004: MnxGe1-x nanowires field effect transistor for spintronics applications Xinhai Han, Masaaki Ogawa, Mingsheng Wang, Kang L. Wang, Justin D. Holmes Group IV Dilute Magnetic Semiconductors (DMS) materials attract much attention not only because of the potential for integration of DMSs with current COMS technology, but also the enhanced spin lifetime and coherent length due to small spin-orbit coupling and lattice inversion symmetry. On the other hand, nanowires are the versatile building blocks for the assembly of functional devices to do fundamental studies in nanoscale. Here we presents Mn$_{x}$Ge$_{1-x}$ (Mn $\sim $ 0.5-1{\%}) nanowires in which there are no detectable secondary phases and the Curie temperature (Tc) is higher than 400 K. Single Mn$_{x}$Ge$_{1-x}$ nanowire back gated field effect transistors (FETs) were fabricated and studied, and $p-$type depletion mode was observed with an on/off ratio of 10$^{4}$, threshold voltage of $\sim $ 0.53 V, maximum transconductance of 0.2 \textit{$\mu $}S, and subthreshold swing (SS) of 210 mV/decade. The mobility was estimated to be around 340 cm$^{2}$/Vs. These results show the high performance of our Mn$_{x}$Ge$_{1-x}$ nanowire FET, which indicates the Mn$_{x}$Ge$_{1-x}$ nanowires could be the promising building blocks for both electrical and spintronics devices. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y24.00005: Transient Random Telegraph Signal in carbon nanotube field effect transistors Tsz-Wah Chan, Brian Burke, Kenny Evans, Keith Williams We have studied transient \textit{Random Telegraph Signal} (RTS) induced in carbon nanotube-channel field effect transistors (FETs) by operating them at high bias. RTS arises from the population and depopulation of charge traps at specific energies that are scanned by sweeping the gate in a FET. At high bias, surface adsorbates/dopants interact with the SWNT and produce transient charge traps, which are manifest in the RTS signature. Transient RTS has been seen at temperatures from 200K up to room temperature. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y24.00006: Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices Gunnar Gunnarsson, Jelena Trbovic, Christian Schoenenberger Spin field-effect transistor has been recently realized in single wall carbon nanotube (SWCNT) devices contacted with NiPd alloy [1]. In order to separate charge related effects from that of pure spin transport we measure a non-local voltage in SWCNTs by using a four-terminal structure. The four contacts divide the tube into three quantum dots (QD) which we control by the back-gate voltage V$_{g}$. We inject the current through the first QD by using excitation voltage of 200 $\mu $V and measure the non-local signal V$_{nl}$ across the third QD. We measure large \textit{oscillating} non-local voltage as a function of V$_{g}$ with amplitude of V$_{nl}\sim $ 2$\mu $V [2]. While the classical resistor model can account for the negative sign of the non-local voltage its large amplitude needs deeper understanding. We discuss the origin of this large non-local signal and its effect on the non-local spin transport measurements in this type of devices. [1] S. Sahoo, et al. Nature Phys. \textbf{1}, 99 (2005). [2] G. Gunnarsson et al., arXiv:0710.0365v1. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y24.00007: Probing the electrostatics of a metal-carbon nanotube Schottky diode using capacitive measurements Yu-Chih Tseng, Jeffrey Bokor Capacitance-voltage measurement is a technique widely used to characterize metal-semiconductor contacts. We apply this technique to measure the capacitance-voltage across a p-type Schottky contact formed by titanium and a semiconducting carbon nanotube. Ohmic and Schottky contacts are made on the nanotube using palladium and titanium, respectively. The results agree qualitatively with simulations done using a Poisson-Schroedinger solver, considering only the electrostatics. We found additional frequency-dependent effects in the capacitance measurement that indicate the presence of electronic states arising from adsorbates or defects on the length of the nanotube. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y24.00008: Hypergolic fuel detection using Single Walled Nanotube Networks Sharvil Desai, Kapila Hewaparakrama, Gamini Sumanasekera Reliable and accurate detection of hypergolic fuels is vital to U. S. Missile Defense Agency. In this research a simple and highly sensitive SWNT network sensor was developed for real time monitoring of hydrazine leaks to ppm level concentrations. Upon exposure to hydrazine vapor, the resistance of n-type (after degassing) nanotubes is observed to decrease rapidly. The response time exhibits a linear dependence on the concentrations of the vapor. It was also found that the resistance of the sample can be recovered by pumping on the sample and exposing to UV light. The experimental results support chemical adsorption of hydrazine on SWNTs. Theoretical results of hydrazine-SWNT interaction [1] are compared with the experimental observations. Results of similar study on ammonia, dimethyl hydrazine, and naphthalene will also be presented. [1] Min Yu, C. S. Jayanthi, Shi-Yu Wu, APS 2008 [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y24.00009: Electrically Tunable Spin Polarization in a Carbon-Nanotube Spin Diode Chris Merchant, Nina Markovic We have studied the current through a carbon nanotube quantum dot with one ferromagnetic and one normal-metal lead. For the values of gate voltage at which the normal lead is resonant with the single available non-degenerate energy level on the dot, we observe a pronounced decrease in the current for one bias direction. We show that this rectification is spin-dependent, and that it stems from the interplay between the spin accumulation and the Coulomb blockade on the quantum dot. Our results imply that the current is spin-polarized for one direction of the bias, and that the degree of spin polarization is fully and precisely tunable using the gate and bias voltages. As the operation of this spin diode does not require high magnetic fields or optics, it could be used as a building block for electrically controlled spintronic devices. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y24.00010: Detection of adsorbed gas atoms on suspended single-walled carbon nanotube micro-balances Zenghui Wang, Jiang Wei, Oscar Vilches, David Cobden Monolayers of gas atoms or molecules adsorbed on suspended single-walled carbon nanotubes offer the opportunity to study the phases and phase transitions of a unique low dimensional system.~ They are expected to resemble the well studied 2D monolayers on planar graphite, but with tight cylindrical boundary conditions imposed.~ The adsorbed density can be measured by using the nanotube itself as a vibrating microbalance, whose vibration amplitude is detected through the induced modulation of the conductance.~ We are initially studying the noble gases Ne, Kr and Xe, which are attractive for their simplicity and which show discontinuous phase transitions on 2D graphite that from basic considerations should be altered or suppressed as the dimensionality is reduced.~ We will also study oxygen monolayers, because oxygen has more complex 2D ordering on graphite, being magnetic and nonspherical, and because of the surprisingly large doping effect reported for oxygen on nanotubes which remains to be fully understood.~ We will survey the resonant behavior of a number of nanotube microbalances we have made, including examples with quality factor greater than 1000, and the damping effect of a gaseous environment.~ We will then report on our progress in detecting adsorbed layers and phase transitions in them. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y24.00011: Ab initio design of reallistic nanotube sensors Adalberto Fazzio, Alexandre Rocha, Mariana Rossi, Antonio J.R. da Silva The understanding of the electronic transport properties of nanoscopic devices present tantalizing possibilities. In particular it has been demonstrated that carbon nanotubes can be used as sensors for hazardous gases. Large scale computer simulations have an important role to play in predicting the transport properties of such systems. In order to do so one must take into account devices which are a few hundred nanometers in length and present defects randomly distributed along the structure. These defects act as binding sites for the molecules one wishes to detect. In this work we initially use density functional theory (DFT) to determine the most likely defects in highly nitrogen-doped carbon nanotubes, and to calculate the dissociation path of ammonia and hydrogen sulphide molecules onto these defects. Finally we use a combination of DFT and recursive Green's functions techniques to first assemble and then calculte the electronic transport properties of nanotubes up to 200 nm in length and with defects randomly distributed along the structure. We demonstrate that these nanotubes present relatively large resistance changes even at low coverages which leads to highly sensitive devices. The result is a new paradigm in computer-aided sensor design, where one can simulate realistic sensors. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y24.00012: Utilizing Carbon Nanotubes for 1-D Mass Transport Gavi Begtrup, A. Zettl Precision control of the size and placement of materials on the nanoscale creates many opportunities for customizable materials. Recent reports have shown that carbon nanotubes act as efficient one-dimensional mass transport platforms. We have designed nanotube devices on custom fabricated electron transparent substrates compatible with transmission electron microscopy in order to study mass transport mechanisms and applications in situ. Here we report the results of these studies. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y24.00013: Measurement of metal/carbon nanotube contact resistance by shortening contact length Chun Lan, Dmitri Zakharov, Ronald Reifenberger Estimating the contact resistance to achieve a minimum contact length of a nanotube interconnect to a nanoscale electronic device is a major challenge. In this study, we describe a novel experiment using a focused ion beam to sequentially shorten the contact length between a nanotube and an evaporated metallic film. We develop a theoretical model that relates the measured resistance change as a function of contact length to the intrinsic linear resistivity of the nanotube as well as the specific contact resistivity between the nanotube and the deposited metallic film. In this way, we arrive at an estimate for the optimal contact length of the metal film to the carbon nanotube. The results for Au and Ag contacts to multi-wall carbon nanotubes will be summarized. Our method is quite general and can be used to accurately determine the contact resistance of any metallic film to a wide variety of different nanotubes and nanowires. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y24.00014: Bolometric Response of a Single-Wall Carbon Nanotube Daniel Santavicca, Joel Chudow, Yan Yin, Markus Brink, Anthony Annunziata, Luigi Frunzio, Daniel Prober, Alan True, Charles Schmuttenmaer, Meninder Purewal, Yuri Zuev, Philip Kim We report on the low temperature bolometric (thermal) rf response of individual metallic single-wall carbon nanotubes. This response is used to determine the thermal conductance of the nanotube. Previous work has demonstrated heterodyne mixing in individual carbon nanotubes using either an electrical I-V nonlinearity or a gate-modulated conductance. We distinguish between bolometric mixing and the response due to non-thermal electrical nonlinearities. These experiments are a precursor to proposed terahertz measurements of the frequency-dependent bolometric response of an individual single-wall nanotube. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y24.00015: I-V transport measurements of a single unsupported MWCNT under various bending deformations. Suenne Kim, Jeehoon Kim, Morgann Berg, Alex de Lozanne Using a home-made low-temperature high-vacuum probe setup we have obtained more details about the transport characteristics of multiwall carbon nanotubes (MWCNTs). We report our experimental studies on the improvement of the nanowelding between the CNTs and a metallic (W) probe tip in our SEM, which gives a clean and firm contact that satisfies for both electrical and mechanical requirements. We observe hysteresis of the I-V curves between bending and un-bending cycles, effective and efficient fabrication of junctions in the MWCNTs and their respective I-V characteristics, and the deformation-dependent saturation behaviors in the I-V curves of the MWCNTs. All these observations may be qualitatively understood using a simple phenomenological model for localization effects in the deformed hexagonal lattice of graphene. [Preview Abstract] |
Session Y25: Theory and Simulations III
Sponsoring Units: DPOLYChair: Hank Ashbaugh, Tulane University
Room: Morial Convention Center 217
Friday, March 14, 2008 11:15AM - 11:27AM |
Y25.00001: Early Stage Crystallization in Isotactic Polypropylene: Influence of Nanofillers Rahmi Ozisik, Xiaofeng Chen, Sanat Kumar, Phillip Choi Formation of helices in isotactic polypropylene was studied using on-lattice, coarse-grained, Metropolis Monte Carlo simulations. Influence of polymer-particle interaction and particle size on polymer crystallization was studied by inserting isotropic particles into neat iPP melt. Results indicated that the surface of isotropic particle exerts a strong orientation effect on helices and their ordering. In addition, isotropic particle shows a length scale effect on the formation of long helical structures at low temperatures, i.e., below melting temperature. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y25.00002: Early Stage Crystallization in Isotactic Polypropylene: Influence of Substrate-Polymer Interaction and Confinement Xiaofeng Chen, Rahmi Ozisik, Sanat Kumar, Phillip Choi Formation of helices in isotactic polypropylene was studied using on-lattice, coarse-grained, Metropolis Monte Carlo simulations. Influence of polymer-substrate interaction on polymer crystallization was studied by placing iPP chains on a flat surface. Results indicated that attractive interaction between polymer and particle plays a dominant role in the formation of helical structures. Repulsive interaction excludes polymer chains from the neighborhood of the surface and triggers crystallization transition earlier (at higher temperatures). Irrespective of the energy potential used, flat surface always influences the orientation of the helices to be parallel to the surface. Confinement effect was also investigated by changing the gallery spacing between two flat surfaces. Confinement significantly prohibits the growth of long helical structures but has no effect on the overall helicity as well as the ordering of helices. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y25.00003: Growth, non-coalescence and assembly of water drops that form ordered arrays over evaporating polymer solutions Vivek Sharma, Mohan Srinivasarao Breath figures are patterns formed, when cold solid or liquid substrates contact humid air. Typically, the condensed water drops exhibit a range of sizes, and their self-similar growth is marked by coalescence in late stages. But in the breath figures formed on evaporating polymer solutions exposed to the blast of humid air, non-coalescent drops grow and self-assemble into close packed arrays of nearly monodisperse drops. These drops evaporate away leaving an ordered array of air bubbles in polymer film. In this study, we elucidate the physics that drives nucleation, growth, non-coalescence and assembly of drops. We compute the growth kinetics of a droplet population under the mass and heat transport of water vapor that are intimately coupled with the corresponding fluxes of the evaporating solvent. We elucidate the role of solvent, polymer and air flow conditions and determine why the drops are non-sticky and why drops and pores are monodisperse. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y25.00004: Theory of competitive counterion adsorption on flexible polyelectrolytes: Divalent salts Arindam Kundagrami, M. Muthukumar Counterion distribution around an isolated flexible polyelectrolyte in the presence of a divalent salt is evaluated using the adsorption model [M. Muthukumar, J. Chem. Phys. {\bf 120}, 9343 (2004)] that considers temperature, salt concentration, and local dielectric heterogeneity as physical variables in the system. Self consistent calculations of effective charge and size of polymer show that divalent counterions replace condensed monovalent counterions in competitive adsorption. The theory further predicts that at modest physical conditions, polymer charge is compensated and reversed with increasing divalent salt. Consequently, the polyelectrolyte collapses and reswells, respectively. Lower temperatures and higher degrees of dielectric heterogeneity enhance condensation of all species of ions. Complete diagram of states for the effective charge is calculated as functions of temperature and salt concentration. A simple theory of ion-bridging is also presented which predicts a first-order collapse of polyelectrolytes. The theoretical predictions are in agreement with generic results from experiments and simulations. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y25.00005: Confinement free energy of flexible polyelectrolytes in spherical cavities Rajeev Kumar, M. Muthukumar A single flexible polyelectrolyte chain in a spherical cavity is analyzed using self-consistent field theory (SCFT) in the presence of solvent molecules and salt ions. It is found that the confinement of the chain leads to creation of a charge density wave along with the development of a potential difference across the centre of cavity and the surface. We have computed different energetic and entropic contributions to the free energy of the system. In particular, the role of wall-segment repulsive interactions and concentration fluctuations (at one loop level) in free energy has been explored. Results for the finite size corrections to free energy and osmotic pressure will be presented. Predictions about the effects of salt concentration, chain length, radius of the cavity, electrostatic interaction strength, degree of ionization and solvent quality will also be presented. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y25.00006: Simulation study of proton transport in ionomers Philip Taylor, Elshad Allahyarov Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion-like ionomer by the imposition of a strong electric field. We observe that proton transport through this polymer electrolyte membrane is accompanied by morphological changes that include the formation of structures aligned along the direction of the applied field. The polar head groups of the ionomer side chains assemble into clusters, which then form rod-like formations, and these cylindrical structures then assemble into a hexagonally ordered array aligned with the direction of current flow. For dry ionomers, at current densities in excess of 1 A/cm$^2$ these rod-like clusters undergo an inner micro-phase separation, in which distinct wire-like lines of sulfonate head groups are accompanied by similar wire-like alignments of bound protons. The clusters appear to be of two types. If there are two, four, or five lines of sulfonates then there is an equal number of lines of protons, but if there are three lines of sulfonates then they are accompanied by four lines of protons. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexagonal array of rod-like structures remains, but the microphase separation disappears below the threshold current of 1 A/cm$^2$. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y25.00007: Polymer capture by electro-osmotic flow of oppositely charged nanopores Chiu Tai Andrew Wong, M. Muthukumar We have addressed theoretically the hydrodynamic effect on the translocation of DNA through nanopores. We consider the cases of nanopore surface charge being opposite to the charge of the translocating polymer. We show that, because of the high electric field across the nanopore in DNA translocation experiments, electro-osmotic flow is able to create an absorbing region comparable to the size of the polymer around the nanopore. Within this capturing region, the velocity gradient of the fluid flow is high enough for the polymer to undergo coil-stretch transition. The stretched conformation reduces the entropic barrier of translocation. The diffusion limited translocation rate is found to be proportional to the applied voltage. In our theory, many experimental variables (electric field, surface potential, pore radius, dielectric constant, temperature, and salt concentration) appear through a single universal parameter. We have made quantitative predictions on the size of the adsorption region near the pore for the polymer and on the rate of translocation. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y25.00008: Depletion interaction and effect of polydispersity in non-adsorbing polymer solutions Dadong Yan, Shuang Yang, C.C. Han, An-Chang Shi The depletion effect between two spherical colloidal particles in non-adsorbing polymer solutions is investigated using the self-consistent field theory. The density distributions of polymer segments, the depleted amount and depletion potential are calculated numerically in bi-spherical coordinates. The effects of chain length, bulk concentration, and solvency are also investigated for the dilute regime, semidilute regime and high concentration. Also, the effect of polymer polydispersity on the depletion interaction between two plates immersed in a non-adsorbing polymer solution with Schulz molecular weight distribution is studied within self-consistent-field theory. For the case of two large spheres the Derjaguin approximation is used to study the effect of polydispersity. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y25.00009: Unimolecular spreading of a molecular brush on adsorbing surface. Ekaterina Zhulina, Sergey Panyukov, Michael Rubinstein Using scaling concepts and the analytical self-consistent field theory we explore different conformations of a molecular brush on a planar substrate in nonsolvent environment (air-solid interface). The relationship between architecture and stress in adsorbed macromolecule is determined in terms of spreading parameter, grafting density and degree of polymerization of the side chains. A novel tentlike shape of molecular cross-section as well as rectangular and combined (tentlike + rectangular) conformations are predicted and examined. We demonstrate that strong adsorption of densely branched macromolecules on a planar substrate can lead to stress in molecular backbone sufficient to break covalent bonds. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y25.00010: Effect of chain stiffness on structural and thermodynamic properties of polymer melts Jutta Luettmer-Strathmann Static and dynamic properties of polymers are affected by the stiffness of the chains. In this work, we investigate structural and thermodynamic properties of a lattice model for semiflexible polymer chains. The model is an extension of Shaffer's bond- fluctuation model [1] and includes attractive interactions between monomers and an adjustable bending penalty that determines the Kuhn segment length. For isolated chains, a competition between monomer-monomer interactions and bending penalties determines the chain conformations at low temperatures. For dense melts, packing effects play an important role in the structure and thermodynamics of the polymeric liquid. In order to investigate static properties as a function of temperature and chain stiffness, we perform Wang-Landau type simulations and construct densities of states over the two- dimensional state space of monomer-monomer and bending contributions to the internal energy. In addition, we present first results from an algorithm for equation-of-state effects in lattice models. \newline [1] J. S. Shaffer, J. Chem. Phys. 101, 4205 (1994). [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y25.00011: Ameba-like diffusion in two-dimensional polymer melts: how critical exponents determine the structural relaxation Torsten Kreer, Hendrik Meyer, Joerg Baschnagel By means of numerical investigations we demonstrate that the structural relaxation of linear polymers in two dimensional (space-filling) melts is characterized by ameba-like diffusion, where the chains relax via frictional dissipation at their interfacial contact lines. The perimeter length of the contact line determines a new length scale, which does not exist in three dimensions. We show how this length scale follows from the critical exponents, which hence characterize not only the static but also the dynamic properties of the melt. Our data is in agreement with recent theoretical predictions, concerning the time-dependence of single-monomer mean-square displacements and the scaling of concomitant relaxation times with the degree of polymerization. For the latter we demonstrate a density crossover-scaling as an additional test for ameba-like relaxation. We compare our results to the conceptually different Rouse model, which predicts numerically close exponents. Our data can clearly rule out the classical picture as the relevant relaxation mechanism in two-dimensional polymer melts. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y25.00012: Connections between static and dynamic properties of athermal polymer melts: a Monte Carlo simulation study Nenad Stojilovic, Jutta Luettmer-Strathmann The motion of individual chains in polymer melts and blends is governed by local friction and entanglement effects. In simulations and experiments it can be difficult to separate these effects since both local friction coefficients and entanglement lengths depend on the thermodynamic state and the chain structure and since many systems display neither ideal Rouse nor fully entangled dynamics. In this work, we investigate local and chain dynamics of athermal polymer melts with Monte Carlo simulations of two versions of Shaffer's bond- fluctuation model [J. S. Shaffer, J. Chem. Phys. 101, 4205 (1994)]. In the first version, bonds are allowed to cross each other with the result that the chains do not entangle; in the second, bond crossings are prohibited and entanglement effects become apparent. Since both versions of the model have very similar static properties, local friction and entanglement effects can be separated. With simulations for a range of densities and chain lengths, we investigate connections between static and dynamic properties, in particular, scaling with the packing length and the size of the moving segment responsible for local friction. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y25.00013: Thermodynamic modeling of melt deformation. J.P. Ibar In current tube models, the chain is the focus of interest; it is \textit{the} statistical system. For entangled chains, part of the chain (of molecular weight Me) becomes the system. In the statistical model of this paper, systems are sets of conformers coherently interactive, where interactive coupling is defined with respect to 2 types of interaction between conformers, covalent or intermolecular. The duality is described by a new statistics, a crossed-statistics, which calculates the conformational state of all conformers, not just whether they are cis, gauche or trans, but also whether they are either covalently or inter-molecularly bonded. Entanglements manifestation result from a disturbance of the crossed-statistics by the increase of the number of covalently bonded conformers resulting in thermodynamically stable dual phases. The deformation of a statistical system results from a change of the conformation population between the flexed and trans conformations in the direction of the imposed macroscopic field vector. Shear or elongational flow mechanisms differ for the amount played by diffusion in feeding the deformed systems with undeformed (or relaxed) conformers to minimize the total energy required to accommodate the macroscopic deformation. Strain, strain rate and temperature determine how many systems are deformed and to what extent. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y25.00014: Can a material clock based model describe highly non-linear creep? Grigori Medvedev, James Caruthers Most constitutive models developed to date assume that the non-linear behavior observed in glassy polymers it is due to a deformation dependent material clock, where the challenge is to find the correct functional form of how the rate of relaxation depends upon structural variables. A two order-of-magnitude change in mobility upon deformation has been observed experimentally [1], confirming that a material clock is a necessary component of any constitutive description. A non-linear viscoelastic constitutive theory [2], where the clock is configurational energy based, captures with a single parameter set a wide variety of phenomena, including yield, stress/ volume/ enthalpy relaxation, and physical aging. The model is also successful in describing linear creep and recovery; however, it fails dramatically in case of highly non-linear creep in the glass transition region. No change in the functional form of the material clock can significantly improve the prediction of the model for non-linear creep. We postulate that spatial dynamic heterogeneity of glassy materials, which is well established experimentally, must be incorporated into the constitutive model in order to describe non-linear creep. [1] Ediger, et al., APS Meeting - Denver, CO March, 2007. [2] Caruthers, et al., Polymer, 45, 4577-4597, 2004. [Preview Abstract] |
Session Y27: Focus Session: Kagome Magnets
Sponsoring Units: GMAGChair: Ying Chen, National Institute of Standards and Technology
Room: Morial Convention Center 219
Friday, March 14, 2008 11:15AM - 11:27AM |
Y27.00001: Magnetic Diffuse Scattering in the Frustrated Kagome Antiferromagnet YBaCo$_{4}$O$_{7}$ Pascal Manuel, Laurent Chapon, Paolo Radaelli, John Mitchell, Hong Zheng Cobalt oxides of composition RBaCo$_{4}$O$_{7}$ (R=Y, Tb-Lu) crystallize with a lattice structure topologically related to that of the pyrochlore. Considering only the magnetic transition metal sublattice, R-114 appears as Kagome sheets linked by triangular layers and is therefore expected to provide a new materials class for exploring geometric magnetic frustration. We have recently shown that stoichiometric R-114 compound orders antiferromagnetically into a long-range ordered (LRO) structure with features common to the $\sqrt 3 \ast \sqrt 3$ negative chirality spin arrangements often found in Kagome net systems. In contrast, small excesses of O added to the system, as little as 0.1, destroys this LRO state. To explore the nature of the frustrated magnetism in this novel system, we have measured magnetic diffuse scattering on YBaCo$_{4}$O$_{7}$ and YBaCo$_{4}$O$_{7.1}$ single crystals at the ISIS facility. Large maps of reciprocal space in several planes have been recorded showing a very structured diffuse scattering. The data compared to models obtained by the Monte-Carlo method using the metropolis algorithm, reveal the exact nature of the disordered ground state in this new class of frustrated magnets. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y27.00002: $^{63}$Cu, $^{35}$Cl, and $^{1}$H NMR in the S=1/2 Kagom\'{e} Lattice ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ Takashi Imai, E.A. Nytkoc, B.M. Bartlett, M.P. Shores, D.G. Nocera ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ (S=1/2) is a promising new candidate for an ideal Kagom\'{e} Heisenberg antiferromagnet, because there is no magnetic phase transition down to $\sim $50 mK. We investigated its local magnetic and lattice environments with NMR techniques (ArXiv:cond-mat/0703141). From $^{35}$Cl Knight shift data, we demonstrate that the intrinsic spin susceptibility follows a Curie-Weiss law down to $\sim $0.2J, then decreases toward T = 0. Comparison of $^{1}$H and $^{35}$Cl spin-lattice relaxation rate 1/T$_{1}$ evidences for slow freezing of the lattice near $\sim $50 K, presumably associated with OH bonds. Spin dynamics near T = 0 obey a power-law behavior in the presence of high magnetic fields. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y27.00003: Theory of Collinear ordering on the Kagome Lattice of Zn-paratacamite Lars Fritz, Michael Lawler, Yong-Baek Kim, Subir Sachdev We present a theory of the collinearly ordered phase discovered in a recent neutron scattering experiment [1] on Zn-paratacamite, Zn$_x$Cu$_ {4-x}$(OH)$_6$Cl$_2$, at small $x$. Zn-paratacamite has been considered as an excellent model system for the kagome lattice of spin-1/2 moments with antiferromagnetic exchange interactions. We studied both the classical and quantum Heisenberg models on the distorted kagome lattice appropriate for Zn-paratacamite with small x. Our theory naturally explains the emergence of a N\'eel phase. A theory for the Valence-Bond Solid phase also discovered in this experiment is presented in a related contributed talk.\newline \newline [1] S.-H. Lee et al., Nature Materials, 6, 853 [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y27.00004: Theory of the Valence-Bond-Solid Phase on the Kagome Lattice of Zn-paratacamite Michael Lawler, Lars Fritz, Yong Baek Kim, Subir Sachdev We present a theory of the valence-bond-solid phase discovered in a recent neutron scattering experiment [1] on Zn-paratacamite, Zn$_x$Cu$_{4-x}$(OH)$_6$Cl$_2$, at small $x$. A theory of the Neel phase also discovered in this experiment will be presented in a related contributed talk. Zn-paratacamite has been considered as an excellent model system for the kagome lattice of spin-1/2 moments with antiferromagnetic exchange interactions. We study both the classical and quantum Heisenberg models on the distorted kagome lattice appropriate for Zn-paratacamite with small x. Our theory naturally explains the emergence of the valence bond solid phase. We suggest future inelastic neutron and elastic X-ray scattering experiments that can test our predictions. [1] S.-H. Lee et al., Nature Materials, 6, 853 [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y27.00005: New natural spin-1/2 kagom\'{e} systems --- kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$ and haydeeite Cu$_3$Mg(OH)$_6$Cl$_2$ Oleg Janson, Helge Rosner New natural spin-1/2 systems with kagom\'{e} layers --- kapellasite Cu$_3$Zn(OH)$_6$Cl$_2$ and haydeeite Cu$_3$Mg(OH)$_6$Cl$_2$ --- are studied by full potential density functional calculations using the fplo6.00-24 code. The band structure, obtained by a paramagnetic calculation, was used to solve a tight-binding model. The transfer integrals were mapped subsequently to a Hubbard model and to a Heisenberg model, giving an estimate for the antiferromagnetic (AF) exchange. The total exchange, containing AF and ferromagnetic (FM) parts, was derived from LSDA + \textsl{U} supercell calculations. As the main result, we find that in both compounds only two exchange integrals are relevant: the nearest neighbour exchange \textsl{J}$_1$ and the interaction \textsl{J}$_{\mathrm{d}}$ along the diagonals of the Cu$^{2+}$ hexagons. Surprisingly, the size of these integrals depends strongly on the O---H bond length which was therefor optimized with respect to the total energy, resulting in about 1 \AA for both compounds. Using the optimized O---H bond length, we find \textsl{J}$_{1}>\sl{J}_{\mathrm{d}}$ in kapellasite and \textsl{J}$_{1}\sim\textsl{J}_{\mathrm{d}}$ in haydeeite. According to our results, kapellasite can be described as a modified kagom\'{e} lattice, while interpenetrating chains should be considered for haydeeite. Our results should encourage new experimental studies of these interesting materials. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y27.00006: Classical antiferromagnet on a hyper-kagome lattice John Hopkinson, Sergei Isakov, Hae-Young Kee, Yong Baek Kim Motivated by recent experiments on Na$_4$Ir$_3$O$_8$ [Y. Okamoto {\it{et al.}}, Phys. Rev. Lett. 99, 167402 (2007)], we study the classical antiferromagnet on a frustrated three-dimensional lattice obtained by selectively removing one of four sites in each tetrahedron of the pyrochlore lattice. This ``hyper-kagome'' lattice consists of corner-sharing triangles. We present (J. Hopkinson {\it{et al.}}, Phys. Rev. Lett. 99, 037201 (2007)) the results of large-$N$ mean field theory and Monte Carlo computations on $O(N)$ classical spin models. We find the classical ground states to be highly degenerate. Nonetheless, at low temperatures, nematic order emerges via ``order by disorder'' in the Heisenberg model ($N$=3), representing the dominance of coplanar spin configurations. Above this transition, the spin-spin correlations show a dipolar form which can be understood to arise from a generalized ``Gauss' law'' constraint. The relevance of these results to ongoing neutron scattering measurements will be discussed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y27.00007: Tuning of the spin liquid ground state in the kagome system Pr$_3$Ga$_5$XO$_{14}$ C. R. Wiebe, H. D. Zhou, Y.-J. Jo, M. A. Castellano, L. Balicas, M. J. Case, Y. Qiu, J. R. D. Copley, V. Ramachandran, N. S. Dalal, J. S. Gardner We report on the single crystal growth of the series of kagome oxides Pr$_3$Ga$_5$XO$_{14}$ (X = Si, Ti, Ge, and Sn). The material Pr$_3$Ga$_5$SiO$_{14}$ has near neighbor antiferromagnetic interactions between the Pr spins ($\theta$ = -2.3 K), but there is no long range order down to 0.035 mK (f $\sim$ 66). The presence of 2D low energy spin excitations results in a strong T$^2$ component to the specific heat typical of other kagome systems such as SCGO. By tuning the size of the lattice through substitution on the Si site, one can adjust the exchange between the spins in a regular fashion. Our data shows a systematic decrease in the amplitude of the T$^2$ component of the specific heat as the magnetic exchange becomes weaker through Ti and Ge substitution. In the case of Sn doping, the system orders as the dipolar interactions dominate over the weak antiferromagnetic exchange. To our knowledge, this is the first example of a tunable spin liquid kagome system. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y27.00008: Thermodynamics of Ising Spins on the Triangular Kagome Lattice Yen Lee Loh, Daoxin Yao, Erica W. Carlson In the compounds $\mbox{Cu}_{9}\mbox{X}_2(\mbox{cpa})_{6}\cdot x\mbox{H}_2\mbox{O}$ (cpa=2-carboxypentonic acid; X=F,Cl,Br), the Cu spins form a fascinating and unique pattern called a triangular kagome lattice (TKL). We present a detailed study of Ising spins on such a lattice using exact methods and Monte Carlo simulation. We calculate the free energy, internal energy, specific heat, entropy, sublattice magnetizations, and susceptibility, and we find a rich phase diagram as a function of coupling constants, temperature, and applied magnetic field. In the frustrated regime at $T=0$, the system effectively decouples into independent degrees of freedom, giving residual entropy $s_0=\frac{1}{9} \ln 72$ per spin and correlation length $\xi=0$ -- an interesting contrast with the triangular and kagome lattice Ising models. Applying a field induces a critical phase (related to the honeycomb lattice dimer model) that has irrational entropy $0.0359$ per spin and $1/r^2$ correlations that should be detectable by neutron scattering. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y27.00009: Spin Triplet Excitations of the Heisenberg Antiferromagnet on the Kagome Lattice Kwon Park, Bohm-Jung Yang, Yong Baek Kim, Jaejun Yu The Kagome lattice Heisenberg antiferromagnet is one of the most frustrated spin systems in two dimension, which has generated various theoretical proposals for the ground state. While recent experiments strongly suggest that the ground state is not magnetically ordered, identification of the true ground state remains highly controversial. Possible candidate phases include various spin liquids and a valence bond solid, particularly with a 36-site unit cell. It is therefore important to theoretically explore decisive properties of the candidate ground states which can be directly compared with experiments. To this end, we investigate the low-energy spin triplet excitations of the valence bond solid state with a 36-site unit cell, which are gapped in contrast to spin singlet excitations. Implications to future experiments are discussed. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y27.00010: Multiple magnetic phases in the frustrated spin-dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ E. C. Samulon, Y.-J. Jo, P. Sengupta, G. M. Schmiedeshoff, C. D. Batista, M. Jaime, L. Balicas, I. R. Fisher Ba$_{3}$Mn$_{2}$O$_{8}$ is a spin-dimer compound based on S=1 3d$^{2}$ Mn$^{5+}$ ions on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material. Specific heat, magnetocaloric effect, torque magnetometry and magnetostriction measurements were performed in magnetic fields up to 32T and temperatures down to 20 mK. These measurements reveal the presence of multiple ordered states for fields above H$_{c1}\sim $8.7T. Both single-ion anisotropy and geometric frustration play crucial roles in determining the phase diagram. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y27.00011: $^{135,137}$Ba NMR study of Ba$_{3}$Mn$_{2}$O$_{8}$ Steve Suh, W.G. Clark, Guoqing Wu, S.E. Brown, E.C. Samulon, I.R. Fisher, C.D. Batista, A.P. Reyes, P. Kuhns, L.L. Lumata We report results from $^{135,137}$Ba NMR spectroscopy and relaxation rate (1/$T_{1})$ measurements in single crystal Ba$_{3}$Mn$_{2}$O$_{8}$, an $S$ =1 dimer system with a singlet ground state. Thermodynamic measurements have shown it has multiple field-induced phase transitions for fields exceeding a critical field $H_{c1}\simeq $90 kOe and varying with field orientation. We have evaluated the hyperfine couplings and electric field gradients in the normal phase for one of the two inequivalent Ba sites, and find a significant anisotropic component to the hyperfine coupling. Measurements of 1/$T_{1}$ made at fixed fields down to temperatures $T<$0.4 K are consistent with critical behavior in the vicinity of $H_{c1}$. However, lower temperatures are needed to clarify the universality class. Goals for upcoming experiments include a determination of the spectrum in the low-symmetry phases and an evaluation of 1/$T_{1}$ for $T<$0.4 K. This work is supported at UCLA by NSF Grants 0520552 (SEB), DMR-00334869 (WGC), Stanford by DMR-0134613 (IRF), and NHMFL by 0084173 and the State of Florida. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y27.00012: Ground State Phase Diagram of the Heisenberg Model on Anisotropic Triangular Lattice. Tommaso Pardini, Rajiv R.P. Singh We study the spin-half and spin-one Heisenberg models on the anisotropic triangular lattice with interactions $J_1$ and $J_2$. The model interpolates between the limits of square lattice ($J_1=0$), triangular lattice ($J_1=J_2$) and decoupled one dimensional linear chains ($J_2=0$). Results are obtained by means of linked-cluster series expansions around the colinear antiferromagnetic phase (CAF) and the non colinear antiferromagnetic phase (NCAF), also known as the spiral phase. For the spin-half model, both phases can be stabilized within our calculations for small $J_2$. However, the NCAF phase always appears to have a lower energy. The pitch of the spiral is substantially renormalized from the classical values. For the spin-one model, we find a transition from the Haldane gap phase to the NCAF phase as a function of $J_1/J_2$. Interchain coupling required for this transition is more than a factor of $30$ larger than when the chains are coupled in an unfrustrated square-lattice geometry. The CAF phase does not appeared to be stabilized for any value of $J_1/J_2$ for the spin-one model. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y27.00013: Spin order and excitations of a model triangular antiferromagnet Wei Bao, Yiming Qiu, Yingxia Wang, Kuo Li, Jianghua Lin, Ross Erwin The triangular antiferromagnet is a model system situating close to the boundary between a three-sublattice order and a quantum-liquid state, due to delicate balance among magnetic interaction, quantum fluctuations and geometrical frustration. The unique topology of the non-collinear three-sublattice order has profound consequences in finite-temperature phase-transitions and spin excitations, which are not yet fully understood. Experimental investigation on such issues has been impeded by imperfect materials which fail to represent the theoretical model at low temperatures. Here we show by neutron scattering that the three-sublattice order in the exceptional new material La$_2$Ca$_2$MnO$_7$ remains two-dimensional down to 40 mK. The order parameter and critical spin fluctuations suggest a phase transition at 3.8 K, but the in-plane correlation length becomes resolution-limited only below 1.8 K. While the spin-wave cone at low energy and the softening of high-energy modes in current theories are supported by our observations, measured spectral intensity above the upper energy limit of spin-waves distribution and a pseudogap developing below 1.8 K are not anticipated. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y27.00014: Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co$_{3}$V$_{2}$O$_{8}$ Mehmet Ramazanoglu, J. Pat Clancy, A. John Berlinsky, Bruce D. Gaulin, Carl Adams, Zahra Yamani, Ritta Szymczak, Henryk Szymczak, Jan Fink-Finowicki We have performed inelastic neutron scattering measurements on a single crystal sample of cobalt vanadate, Co$_{3}$V$_{2}$O$_{8}$ (CVO). The magnetic Co ions in CVO decorate a stacked, Kagome staircase crystal structure, and the system displays a complex sequence of ordered magnetic phases, culminating in a ferromagnetic phase at low $T$ [1,2]. We studied the spin wave dispersion relations and intensities within the Kagome planes and in the low temperature ferromagnetic phase which the system enters below $T_{c}\sim $6 K. Linear spin wave theory can model the qualitative features and much quantitative detail of these inelastic neutron measurements. These results show strongly anisotropic exchange interactions within this plane, and interesting finite lifetimes to the spin waves well below $T_{c}$. \newline [1] R. Szymczak et. al. PRB 73, 094425, (2006) \newline [2] Y. Chen et. al. PRB 74,014430, (2006) [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y27.00015: Oxygen Sublattice Tuning of Magnetic Order in the Kagom\'{e} Antiferromagnet YBaCo$_{4}$O$_{7+\delta}$ J.F. Mitchell, O. Chmaissem, H. Zheng, A. Huq, P. Stephens YBaCo$_{4}$O$_{7+\delta}$ (Y-114) is a relatively new compound that contains the Kagom\'{e} net motif and is structurally related to the pyrochlore lattice, differing only in the stacking of the triangular layers that link successive Kagom\'{e} planes. We have previously shown that the stoichiometric compound orders antiferromagnetically (AFM) at T$_{N}$ = 108 K into a structure that compromises a collinear arrangement along the $c$-axis and a 120$^{o}$ structure (akin to the well-known $\sqrt{3} \mbox{x} \sqrt{3}$ supercell) in the Kagom\'{e} planes. This ordered state is considered to result from a symmetry-breaking structural distortion that lifts the geometric frustration. Here we show from neutron diffraction the effect of added oxygen on the structure and magnetism of the parent compound. By controlling this parameter, we can tune the system from the AFM ordered ground state into a disordered state. We discuss two possible mechanisms for this evolution: (1) suppressed structural distortion leading to geometric frustration, and (2) the formation of S=0 Co$^{3+}$ centers that can break magnetic exchange pathways. [Preview Abstract] |
Session Y28: Superlattices and Nanostructures: Electronic Properties III
Sponsoring Units: DCMPChair: Xuedong Hu, University at Buffalo SUNY
Room: Morial Convention Center 220
Friday, March 14, 2008 11:15AM - 11:27AM |
Y28.00001: Layer interdependence of transport in an undoped electron-hole bilayer Christian Morath, John Seamons, John Reno, Mike Lilly Recently interest in the layer interdependence of a bilayer's transport has emerged. To examine this dependence the layer transport properties in an undoped electron-hole bilayer (uEHBL) device were measured as a function of density, inter-layer electric field and temperature. The uEHBL device consisted of a tunable, independently-contacted two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) induced in distinct GaAs quantum wells separated by a 30 nm Al$_{.9}$Ga$_{.1}$ As barrier. At T = 0.3 K, the 2DHG mobility increased with increasing 2DEG density, while the opposite effect was not observed. Decreasing the inter-layer electric field also increased 2DHG mobility without affecting the 2DEG mobility. This also decreased 2DHG Coulomb drag suggesting the inter-layer separation was increased. Distinct temperature dependencies were also measured for each layer's density and resistivity. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y28.00002: Electronic doping in heterostructures of strongly correlated materials Ivan Gonzalez, Roger G. Melko, Elbio Dagotto Heterostructures of strongly correlated materials have attracted much attention recently. One of the main points of interest is the possibility of the stabilization of new phases at the interface between two different strongly correlated materials. In this talk, we present a study of the electronic properties of a heterostructure made of strongly correlated materials. The heterostructure is built up by alternating several layers of two different materials. The layers are thin enough (about 10 units cells) so the charges can be transferred all throughout the heterostructure. Calculations are performed using the Density Matrix Renormalization Group algorithm together with a Poisson equation formalism to account for the charge redistribution produced by the interfaces. We show that for realistic values of the parameters of the model the properties of the heterostructure are greatly determined by the behaviour at the interfaces. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y28.00003: Exciton Formation in Coulomb Drag Measurements of Electron-Hole Bilayers J.A. Seamons, C.P. Morath, J.L. Reno, M.P. Lilly Since it was predicted over two decades ago, there has been intense interest in exciton condensation in coupled-well bilayer systems. While exciton condensation effects have been evident in optically-generated indirect excitons and quantum Hall bilayers, transport experiments in electron-hole bilayers in the regime of exciton condensation have proven to be extremely difficult. Results of Coulomb drag ($\rho _{DRAG})$ measurements at zero magnetic field on new undoped electron-hole bilayer devices formed in GaAs/Al$_{0.9}$Ga$_{0.1}$As double quantum well heterostructures are presented. For devices with 30 nm barriers $\rho _{DRAG}$ demonstrates T$^{2}$ behavior consistent with two Fermi liquids. In 20 nm barrier devices a dramatic upturn in the 2DHG Coulomb drag voltage occurs below T=1K. This upturn signals an increase in inter-layer coupling consistent with exciton formation. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y28.00004: Electronic states in magnetic quantum dots and quantum-dot molecules: Coulomb interaction effects and spontaneous symmetry breaking Alexander Govorov, Wei Zhang We investigate theoretically few-electron states in semi-magnetic quantum dots and quantum-dot molecules [1,2]. A double quantum-dot system made of diluted magnetic semiconductor behaves unlike the usual molecules. In a semiconductor double quantum dot or in a diatomic molecule, the ground state of a single carrier is described by a symmetric orbital. In a magnetic material molecule, new ground states with broken symmetry can appear due the competition between the tunneling and magnetic polaron energy. With decreasing temperature, the ground state changes from the normal symmetric state to a state with spontaneously broken symmetry. Interestingly, the symmetry of a magnetic molecule is recovered at very low temperatures. A magnetic double quantum dot with broken-symmetry phases can be used as a voltage-controlled nanoscale memory cell. [1] A. O. Govorov, Phys. Rev. B 72, 075359 (2005). [2] W. Zhang, T. Dong, and A. O. Govorov, Phys. Rev. B 76, 075319 (2007). [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y28.00005: The Kondo Effect and Rashba Spin-Orbit Coupling Justin Malecki We present the results of a study of the influence that weak Rashba spin-orbit coupling has on the Kondo effect induced by a magnetic impurity in a two dimensional electronic system. It is shown that the Kondo effect is robust against such coupling of momentum and spin, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature $T_K$ in such systems. Applications to semiconductor quantum dots and magnetic atoms on metallic surfaces are discussed. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y28.00006: Fano-Kondo effect in a two-level system with triple quantum dots Tetsufumi Tanamoto, Yoshifumi Nishi, Shinobu Fujita Quantum dot (QD) systems have been providing opportunities to probe a wide variety of many-body effects in microelectronic structures. Recently, the Fano effect, which appears as a result of quantum interference between a discrete single energy level and a major electronic system, has attracted the interests of many researches [1]. Here, we theoretically study the Fano-Kondo effect and Fano effect in a triple QD system, where two QDs constitute a two- level system and the other QD works in a detector with electrodes. When two QDs are coupled, bonding and anti-bonding states are formed. It is expected that the detector current reflects these electronic states. Indeed, we found that the Fano dip is modulated by strongly coupled QDs with a slow detector. We also compare noise properties of Fano-Kondo effect with those of Fano effect, and we found that, depending on the coupling strength among the QDs, noise and the Fano factor are greatly modulated for a slow detector. These suggest a new method of reading out qubit states [2].-- [1] A. W. Rushforth et al., Phys. Rev. B 73, 081305 (2006). [2] T. Tanamoto et al., Phys. Rev. B 76, 155319 (2007); arXiv:0710.0912. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y28.00007: Kondo physics with ac driving in the single electron transistor subjected to finite bias Ali Goker We employ the time-dependent non-crossing approximation to study the time averaged conductance for a single electron transistor in the Kondo regime when the dot level is sinusoidally driven from its equilibrium position by means of a gate voltage in finite bias. We find that the average conductance exhibits considerable deviation from the monotonous reduction when the applied bias is equal to the driving frequency of the dot level. We attribute this behaviour to the overlap of the satellite Kondo peaks with the split Kondo resonances formed at each lead's Fermi level. We display the spectral function to put our interpretation into more rigorous footing. We also investigate the effect of the temperature and the driving frequency on the observed enhancement. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y28.00008: Interplay of Interactions and Phase Coherence in Open Quantum Dots Ileana Rau, Michael Grobis, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon The effect of Coulomb interactions on the electronic properties of a confined quantum system greatly weakens when electrons are allowed to rapidly enter and exit the system. For electron transport through a quantum dot, increasing the coupling of the dot to nearby leads causes a transition from the Coulomb blockade regime to a regime dominated by interference phenomena. We have investigated this transition in large, micron-sized quantum dots and have found that Coulomb blockade effects persist in a regime where they had generally been assumed absent: when a dot is coupled by one fully transmitting mode to each of two leads. We discuss the interplay of these residual Coulomb interactions with phase coherent transport through a dot. We also examine how the subtle suppression of conductance by these Coulomb interactions affects the electron dephasing rate at low temperatures in open quantum dots. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y28.00009: Numerical estimate of correlation energy for two electrons confined in 2D quantum dot Takuma Okunishi, Yuki Negishi, Masakazu Muraguchi, Kyozaburo Takeda Local density approximation (LDA) is now widely accepted to design the novel materials as well as to predict new phenomena. In practical use of LDA, estimate of correlation energy is crucial. Tanatar et al. have given the well-known comprehensive expression of LDA correlation energy for 2D electron gas [1]. However, a straightforward use of this correlation energy to two electrons confined in 2D quantum dot (QD) leads such an inconsistent result that the correlation energy can overcome the Coulomb one because their correlation expression includes only 2D confinement. Specific correlation energy should be requested in accordance with the degree of the QD confinement. So we have obtained the correlation energy for two electrons confined in 2D QD by employing multi-reference configuration interaction (MRCI) technique. Now we are trying to improve the LDA correlation expression by refining their parameters and study time dependent phenomena of electrons in 2D QD by applying the improved LDA. [1] B. Tanatar and D. M. Ceperley, Phys. Rev. B \textbf{39}, 5005 (1989). [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y28.00010: Electron Tunneling Counting Statistics of a GaAs Quantum Dot at Thermal Equilibrium Xinchang Zhang, Ming Xiao, Eli Yablonovitch, Hongwen Jiang Full counting statistics (FCS) is an innovative way to investigate current fluctuations of mesoscopic conductors which can provide additional information beyond the conventional average current measurement [1]. Suppression of the 2nd moment and the 3rd moment were observed in a many-electron quantum dot(QD)under nonequilibrium conditions [1]. Here we studied the FCS of single electron tunneling of a GaAs QD in the few electron regime at thermal equilibrium in the in-plane magnetic fields. The device consists of a multiple-surface-gates defined GaAs QD integrated with a very sensitive, high bandwidth field effect transitor (FET) channel for the QD charge state read-out. Monitoring the FET current revealed two sequences of random telegraph signals which represent the electron tunneling onto and off the QD in real time. When the QD level is aligned with Fermi level of the reservoir, the statistics shows a maximum value of both mean ($<$n$>)$ and standard deviation ({\$}$\backslash $sigma{\$}), but a minimum skewness in its distribution function. It was also found that an in-plane magnetic field suppresses both $<$n$>$ and {\$}$\backslash $sigma{\$}, but enhances the skewness. [1], S. Gustavsson et. al, PRL 96, 76695(2006). . [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y28.00011: Non-Markovian Dynamics of Charge Carriers in Quantum Dots at High Bias Eduardo Vaz, Jordan Kyriakidis We have investigated the dynamics of bound particles in multi-level current-carrying quantum dots. We look specifically in the regime of resonant tunneling transport, where several channels are available for transport. Through the non-Markovian Born-Redfield formalism, we investigate the real-time evolution of the confined particles including transport-induced decoherence and relaxation. In the case of a coherent superposition between states with different particle number, we find that coherence may be preserved even in the presence of tunneling into and out of the dot. Real-time results are presented for various asymmetries of tunnel barriers and tunneling rates into different orbitals. [Preview Abstract] |
Session Y29: Focus Session: Carbon Nanotubes and Related Materials XVI: Mechanical and Thermal Properties
Sponsoring Units: DMPChair: David Tomanek, Michigan State University
Room: Morial Convention Center 221
Friday, March 14, 2008 11:15AM - 11:27AM |
Y29.00001: Anisotropic Casimir interactions between a one-dimensional object (nanotube) and a polar substrate Slava V. Rotkin, Alexey G. Petrov, John A. Rogers The energy of Casimir interaction of a polarizable one-dimensional object (1DO), e.g. a nanotube, and a polar substrate was estimated. Within our model the energy of the dipole moment induced in the 1DO by the external electric field of the fluctuations of the quantized surface optical phonon modes is evaluated. Such polariton modes are known to exist in polar insulators and have the electric field with an exponentially decreasing wing in vacuum. If the polarization tensor of the 1DO is not isotropic, an orientation dependent Casimir force may arise. To the best of our knowledge, such anisotropic Casimir interaction has not been considered before and may lead to an orientation of long flexible objects, like nanotubes, at polar substrates. The interaction energy is derived analytically for the case of a single-wall nanotube on the ST-cut quartz. Besides a material dependent energy constant, it is proportional to the ratio of the volume of interacting segment of the nanotube and cube of the distance to the substrate. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y29.00002: Dynamics and energy dissipation of nano-graphite mechanical devices. Zhiping Xu Controllable mechanical motion of nano-structures holds great interests because of their applications in the nano-electromechanical systems (NEMS). One of the novel models proposed was the nano-graphitic materials based devices, where planar or cylindrical graphene layers act as moving parts and the motion is managed by the van der Waals force between them. Comparing with the multi-walled carbon nanotubes, nano-graphite flakes have an accessible scale for current techniques. Recent experiments using nano-mechanical manipulator have shown self-retraction motion of micrometer graphite layers after mechanical extrusion (Zheng et al. submitted to PRL). However, persistent oscillation as expected was not observed. The short lifetime implies severe energy dissipation. Analysis based on MD simulation show that the coupling with rotation and lattice vibration contribute significantly. Furthermore we have discussed the effects of edge instabilities, surface contamination and non-planar deformation, which also introduce complexities into the dynamics as approved by the experimental observation. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y29.00003: Large Negative Thermal Expansion of an Individual Carbon Nanotube J. Zhang, L. Ji, J. Zuo It is of fundamental value to understand the thermo-mechanical properties of individual carbon nanotubes (CNTs). The coefficient of thermal expansion (CTE) of CNTs has been a subject of considerable debate in the literature with more recent works predicting thermal contraction. Because of the small size, experimental measurement of individual CNTs is very difficult; So far only limited data was reported by X-ray diffraction that measured the average CTE of many tubes. Here, we use nanoarea electron diffraction to measure the CTE of an individual Multi-walled carbon nanotube (MWCNT) and correlate the CTE with the tube atomic structure. All the 4 walls of this individual MWCNT show apparent radial diameter thermal contraction from 297 to 827k, and thermal expansion from 827 to 1027k. The radial CTE has strong diameter dependence between 297 and 827k; It changes from (-6.48$\pm$0.46) E-5 (1/K) for the wall with the theoretical diameter 16.4 A to (-2.37$\pm$0.77) E-5 (1/K) for the wall with the theoretical diameter 37.3 A, which means smaller diameter wall contracts more. On the other hand, all the 4 walls of this individual MWCNT show apparent axial thermal contraction from 297 to 1073k. The axial CTE is independent of the diameter, and the average axial CTE for different walls is (-1.30$\pm$0.07) E-5 (1/K). [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y29.00004: Optical Measurement of Thermal Contact Resistance in Suspended Carbon Nanotubes I-Kai Hsu, Rajay Kumar, Adam Bushmaker, Michael T. Pettes, Li Shi, Todd Brintlinger, Michael S. Fuhrer, John Cumings, Stephen B. Cronin We observe the local temperature increase profile $\Delta T(x)$ along suspended carbon nanotubes (CNTs) by converting the shifts in the $G$-band Raman mode to temperature. By deconvolving the temperature profile using the Fourier heat transport equation, we determine the thermal contact resistance ($R_{c})$ relative to the intrinsic thermal resistance of the nanotube itself ($R_{NT})$. The curvature of the temperature profile is found to be dominated by the ratio of $R_{NT }$to $R_{c}$. Moreover, the difference between the left and right thermal contact resistances can also be differentiated via the offset of the temperature increase at the ends of the suspended CNT. The results show the ratio of the contact thermal resistance to the nanotube thermal resistance to range from 0.02 to 17. The measurement is also able to distinguish between ballistic and diffusive thermal transport. We find diffusive thermal transport to dominate the heat transport in all nanotubes measured in this study. The authors would like to acknowledge support from DOE Award Nos. DE-FG02-07ER46376 and DE-FG02-07ER46377. I.K. Hsu \textit{et al.}, Applied Physics Letters (in press). [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y29.00005: Thermal Conductance Measurement of Metal-CNT Composites using Micro-Sized Suspended Structure Ki Sung Suh, Jung Hoon Bak, Byung Yang Lee, Seunghun Hong, Yun Daniel Park As CNTs have a unique structure and remarkable physical properties, CNT composites have attracted much attention from many researchers. Especially the thermal properties of CNTs and their composite materials have been studied intensively, because CNT has very good thermal transport properties [1-5]. For example, thermal conductivity of CNT is known to be much larger than that of metals such as Ag, Au, Cu and Al. To study the thermal conductance of metal-CNT composites, we have fabricated the micro-sized suspended structures. By using e-beam lithography and metallization, two thermometers have been patterned on the GaAs substrates. Thermal links made of metal or metal-CNT composite also have been patterned between the two thermometers. Then GaAs substrate has been under-etched to form suspended structures. We will show the fabrication methods and measurement scheme using these microstructures. $^{*}$ parkyd@phya.snu.ac.kr [1] J.A. Eastman \textit{et al.}, Appl. Phys. Lett. \textbf{78}, 718 (2001). [2] S.U.S. Choi \textit{et al.}, Appl. Phys. Lett. \textbf{79}, 2252 (2001). [3] M.J. Biercuk \textit{et al.}, Appl. Phys. Lett. \textbf{80}, 2767 (2002). [4] R. Ramasubramaniam \textit{et al.}, Appl. Phys. Lett. \textbf{80}, 4647 (2003). [5] H.Q. Xia \textit{et al.}, Appl. Phys. Lett. \textbf{94}, 4967 (2003). [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y29.00006: Thermal conductivity of isolated and interacting carbon nanotubes Davide Donadio, Giulia Galli We have investigated the thermal conductivity of single wall carbon nanotubes (SWCNT) either isolated or in contact with external media, by using equilibrium molecular dynamics and the Boltzmann transport equation\footnote{Chantrenne {\sl et al.} J. Appl. Phys. \textbf{97}, 104318 (2005).}. We show that, contrary to existing controversies, both methods yield a finite value of the thermal conductivity for infinitely long tubes, as opposed to the case of 1D momentum conserving systems\footnote{O. Narayan and S. Ramaswamy, Phys. Rev. Lett. \textbf{89}, 200601 (2002).}. Acoustic and flexure modes with mean free paths of the order of a few micron, as observed also in experiments\footnote{C.~Yu, {\sl et al.} Nano Lett. \textbf{5}, 1842 (2005).}, are identified as major contributors to the high value of SWCNT conductivity. We also find that the interaction with an external medium may substantially decrease the lifetime of the low frequency vibrations, reducing the thermal conductivity by up to two orders of magnitude. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y29.00007: Determination of carbon nanotube wall thickness and elasticity by atomic force micrsocopy. Tristan DeBorde, Caleb Joiner, Matthew Leyden, Ethan Minot To understand the operation of carbon nanotube (CNT) devices it is important to determine whether nanotubes are single-walled or multi-walled. Transmission electron microscopy of CNTs has previously been the only tool available to count the number of graphene sheets forming the wall of a CNT. We show that atomic force microscopy can measure CNT wall thickness by squeezing individual nanotubes between a tip and a hard surface. Full compression of single-walled and double-walled CNTs can be achieved either by a static force or by ac-mode imaging, allowing clear determination of wall number. Direct measurements of compression forces are used to determine the elastic properties of the wall, yielding the bending modulus of graphene. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y29.00008: Pressure Induced Changes in the Atomic and Electronic Structure of Carbon Nanotubes Sumit Saxena, Trevor A. Tyson We present first principle density functional calculations on small diameter single walled carbon nanotubes to explore the changes in their electronic structure and atomic arrangement under hydrostatic compression. Simulations on zigzag (n, 0) SWCNT 6$\le $n$\le $9 using the full potential projector augmented wave and ultra-soft pseudo potentials were conducted. Large structure-related changes are found in the density of states at the Fermi energy. The cross sections of small tubes exhibits deformations not predicted by classical models. The structural cross sections of large diameter tubes$^{1}$ (10, 0) calculated under moderate pressure are consistent with the reported results. The details of calculations and other results will be presented. This work is supported in part by NSF DMR-0512196. \begin{enumerate} \item Paul Tangney, Rodrigo B. Capaz, Catalin D. Spataru, Marvin L. Cohen and Steven G. Louie, Nano Lett. 5, 2268 (2005). \end{enumerate} [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y29.00009: Nanomechanical energy transfer in carbon nanotubes: fundamental insights from molecular dynamics simulations Giovanna Lani, P. Alex Greaney, Giancarlo Cicero, Jeffrey C. Grossman Single wall carbon nanotubes have been employed as oscillating elements in nanoeletromechanical resonators (NEMS), attaining very high frequencies but disappointingly low quality factors. Despite the amount of work regarding internal friction, intrinsic dissipation within such nanoscale systems is still poorly understood. In this work we employ molecular dynamics simulations to gain insight into how energy is dissipated in a plucked CNT. It is found that dissipation exhibits two regimes depending on the background temperature. At high temperature, the energy decay is exponential, resembling the behavior of a classical damped oscillator, while at low temperatures an initial transient region is observed during which there is little damping. Increasing the duration of this transient region could be a route for engineering higher Q factors NEMS resonators. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y29.00010: A Failure Criterion for Single-Walled Carbon Nanotubes Based on Molecular Mechanics Antonio Avila, Guilherme Lacerda Single-walled carbon nanotubes (SWNT) are the natural choice for high performance materials. The problem, however, rises when the experimental data are compared against each other. The large variability of experimental data lead to development of a new set of numerical simulations called molecular mechanics, which is a ``symbiotic'' association of molecular dynamics and solid mechanics. This papers deals with a molecular mechanics simulations of single-walled carbon nanotubes. Three SWNT configurations and its combinations were simulated, i.e. armchair, zigzag and chiral. The failure criterion introduced is based on modified Morse's potential with dissociation energy of 124 Kcal/mol and an inflection point considered is around 13{\%} of strain. The numerical data are in good agreement with data from Belytschko et al. (2002) where the failure occurred at 10.6{\%} strain at 65.2 GPa of stress. To be able to identify the highest stress concentration region, one end of the SWNT all degrees-of-freedom were fixed and a prescribed axial displacement was applied at the opposite end. The Sadoc (chiral-chiral) configuration had the highest stress at the smallest chiral SWNT. For the Dunlap configuration (chiral-zigzag) the highest stress occurred at chiral part close to the pentagon location. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y29.00011: Excitons in Single-Walled Carbon Nanotubes in Different Local Environments: Effects of Strain and Disorder on Magnetic Brightening T.A. Searles, D.J. Hilton, J. Shaver, W.D. Rice, Y.-D. Jho, S.A. McGill, J.A. Fagan, E.K. Hobbie, J. Kono Recent experiments on single-walled carbon nanotubes (SWNTs) have shown that in the presence of a high magnetic field the two lowest-energy spin-singlet exciton states become bright [1]. Furthermore, this ``magnetic brightening'', or increase in photoluminescence (PL) intensity as a function of magnetic flux through each SWNT, increases as the temperature decreases. Here, we report results of temperature-dependent magneto-PL from 2 to 200 K and up to 45 T on SWNTs of the same stock solution suspended in four different local environments. We compared both the brightening and temperature dependence of tubes stretch aligned and unaligned in poly-acrylic acid matrices. As expected, the tubes aligned at high magnetic field exhibited more brightening than those unaligned. We also investigated the behavior of SWNTs in two other matrices, iota-Carrageenan and gelatin. Along with the expected peak shifting and broadening from the effects of strain, we found that the temperature dependence changes with local environment. [1] S. Zaric \textit{et al}., PRL \textbf{96}, 016406 (2006); J. Shaver \textit{et al}., Nano Lett. \textbf{7}, 1851 (2007); I. B. Mortimer and R. J. Nicholas, PRL \textbf{98}, 027404 (2007). [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y29.00012: Carbon nanotube nanomechanical mass sensors Hsin-Ying Chiu, Peter Hung, Henk Postma, Marc Bockrath Single-walled carbon nanotubes are arguably the lightest and smallest wires in the world, and have recently been shown to act as nanomechanical resonators [1]. As a result, single-wall carbon nanotubes are excellent candidates for highly sensitive mass sensing [2]. We observed the down shift of the resonant frequency of a suspended double-clamped carbon nanotube resonator at cryogenic temperatures upon helium mass loading. Using a straightforward estimate of the nanotube mass, the observed frequency shift corresponds to the mass of $\sim $1000 helium atoms, which is the zeptogram range. This is considerably smaller than found previously with nanotube resonators, and comparable to that found using nanowire resonators [3]. Our noise floor is currently $\sim $1 Xenon atom per root Hz, which may enable single-atom detection in future experiments. [1] Vera Sazonova, et al., Nature \textbf{431}, 284 (2004). [2] H. B. Peng, et al. Phys. Rev. Lett. \textbf{97}, 087203 (2006) [3] Y. T. Yang, et al. Nano Lett. \textbf{6}, 583 (2006). [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y29.00013: Anharmonic phonon lifetimes in carbon nanotubes, graphene and graphite Nicola Bonini, Michele Lazzeri, Francesco Mauri, Nicola Marzari In this work we present a first-principles study of the anharmonic phonon lifetimes of the key vibrational modes that most strongly interact with electrons in carbon nanotubes, graphene and graphite. The calculations of both harmonic and anharmonic properties are performed using density-functional theory and density-functional perturbation theory. Our results---in excellent agreement with the available experimental data---provide a microscopic characterization of the energy relaxation mechanisms and of the relative importance of the individual decay channels. We will discuss the relevance of these results to elucidate the role of non-equilibrium phonon populations in high-field electronic transport. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y29.00014: Measurement of mechanical properties of graphene using nanoindentation Changgu Lee, Xiaoding Wei, Jeffrey Kysar, James Hone Mechanical properties of graphene have been measured using AFM nanoindentation. Mono-, bi-, and tri-layer graphene sheets are suspended over micron-sized circular hole arrays. Force-displacement curves obtained by AFM nanoindentation allow the extraction of mechanical properties such as Young's modulus and fracture strength using equations for thin circular membranes. In order to verify the validity of the equations, the experimental and analytical results were compared with finite element simulation. The analytical equations fitted to the measurements show that Young's modulus is 0.9-1.2 TPa and the fracture strength is 90-150 GPa for up to 3 layers. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y29.00015: Electrical conductivity of individual, thermally reduced graphene oxide sheets Inhwa Jung, Dmitriy Dikin, Richard Piner, Rod Ruoff Electrical properties of individual graphene oxide sheets were investigated. Graphene oxide itself is insulating, but its conductivity is finite and measureable following heat treatment in vacuum. The dependence on temperature and time for reduction of graphene oxide were fit to a standard chemical kinetics rate law and from this an activation energy of 30 kcal/mole was found. I-V curves, obtained at several stages of the chemical reduction achieved by heating, are non-linear and slightly asymmetric. The effect of applying an electric field via a back gate and the resulting change in resistance was measured at different temperatures and at different stages of reduction. The maximum conductivity by thermal annealing graphene oxide sheets was 85 S/m at room temperature and zero gate potential. This value was determined based on 4-probe pseudo Van der Pauw measurements and numerical modeling and using 1.0 nm as the sheet thickness. [Preview Abstract] |
Session Y30: Low Dimensional Systems
Sponsoring Units: DCMPChair: Michael Mehl, Naval Research Laboratory
Room: Morial Convention Center 222
Friday, March 14, 2008 11:15AM - 11:27AM |
Y30.00001: Inelastic neutron scattering from confined molecular oxygen. Paul Sokol, Duncan Kilburn We report results from experiments measuring the generalized density of states in confined solid molecular oxygen. It is known from previous experiments that fundamental properties of liquids and solids, such as phase transition temperatures and intermolecular structure can be altered by confining them in porous media (pores typically in the angstrom to nanometer range). It is reasonable therefore to ask the question: what is the effect of confinement on collective excitations in the material, and can these changes be exploited in a technological setting? Using inelastic neutron scattering we find that both the structure and generalized density of states of solid molecular oxygen are altered by confining it in a templated porous glass with a mean pore diameter of 100 Angstroms. The structure, in the Q-range which we were able to measure, resembles that of an amorphous material and the density of states are shifted to lower energy excitations. One possible application for such a material is as moderator material in a very cold neutron source. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y30.00002: Imaging One-Electron Quantum Dots in InAs/InP Nanowires Halvar Trodahl, Erin Boyd, R.M. Westervelt, Linus Froberg, Lars Samuelson Quantum dots formed in InAs/InP heterostructure nanowires are attractive candidates for nanoelectronics, spintronics, and quantum information processing. It has been shown that with the use of a liquid helium cooled scanning probe microscope (SPM) the electronic charge state of a single quantum dot can be chosen by merely moving the tip of the SPM with respect to the dot [1]. Simulations show that this technique can also be implemented for double quantum dots with interdot separation less than 30nm. By applying a magnetic field the electronic states of a quantum dot can be spin split. It is possible to image this splitting with the local, movable electrostatic gating of the SPM. A magnetic field applied to a double dot system can be used to create a spin filter; a simple movement of the SPM tip with respect to the dot structure can determine which spin passes through the filter. \newline [1] A. Bleszynski et al., 28th Int. Conf. Physics of Semiconductors, 2006. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y30.00003: Imaging the Wavefunction of a One-Electron Quantum Dot Erin E. Boyd, Halvar J. Trodahl, Parisa Fallahi, R.M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots grown in InAs/InP nanowires are promising contenders for nanoelectronics. A fundamental understanding of the quantum behavior of the electron is important for the design of quantum devices. We have developed an imaging technique to image the electron wavefunction of a quantum state inside a long InAs dot (length$>$diameter) formed by InP barriers, using a liquid He-4 cooled scanning probe microscope [1]. The electrostatic potential of the tip dents the wavefunction and changes the energy of the quantum state by an amount proportional to the electron probability density at the tip position. Using Coulomb blockade conductance images of the dot, the energy change vs. tip position can be found. By deconvolving the measured energy shift with the tip potential, one can extract the electron probability density, using first-order perturbation theory. [1] P. Fallahi, PhD Thesis, Harvard Univ (2006). [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y30.00004: Theoretical study of spin relaxation in a carbon nanotube quantum dot Brian Bezanson, Xuedong Hu Carbon nanotubes offer an attractive environment for coherent spin manipulation due to the small population of nuclear spins and weak spin-orbit interaction. While a couple of specific spin relaxation mechanisms have been investigated theoretically[1][2], there is still no comprehensive study of spin lifetimes in carbon nanotubes. In the present study we calculate the spin decay rate for electrons in gate-defined quantum dots on carbon nanotubes due to the spin-orbit and electron-phonon interactions. More specifically, we explore effects of magnetic field strength and orientation, tube diameter and chirality, and confinement. \newline [1] Y. G. Semenov, K. W. Kim, G. J. Iafrate, Phys. Rev. B 75, 045429 (2007) \newline [2] K. M. Borysenko, Y. G. Semenov, K. W. Kim, J. M. Zavada, arXiv 0710.3382 (2007) [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y30.00005: Analysis of unusual splitting of Kondo peak in the differential conductance of a carbon nanotube quantum dot Jeffrey Stephens, Jerome Licini, A.T. Charlie Johnson, Doug Strachan, Danvers Johnston, Sam Khamis Carbon nanotubes grown by chemical vapor deposition on an oxidized silicon substrate were contacted to form a gated sample of parallel tubes. Testing was done at low temperature and high magnetic field using a dilution refrigerator and superconducting magnet. The current versus voltage graph shows asymmetry with respect to zero volts. The differential conductance (dI/dV) is computed and yields some intriguing behavior. The previous asymmetry is more apparent as is a sharp increase in conductivity near zero voltage. Temperature data further suggests a conductance peak at near zero voltage consistent with the Kondo effect. High magnetic fields, 0 to 11 Tesla in 0.5 Tesla increments, are used to probe the conductance behavior. The magnetic field tests yield unusual shapes and splitting at two critical fields. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y30.00006: Probing edge-localized states of graphene quantum dots on Co(0001) Daejin Eom, Kwang Rim, Hui Zhou, Michael Lefenfeld, Li Liu, Shengxiong Xiao, Colin Nuckolls, George Flynn, Tony Heinz Two-dimensional graphene sheets of finite lateral extent are expected to show characteristic edge states at their boundaries. In particular, for zigzag edges, highly degenerate localized states have been predicted theoretically (Ref. 1) and probed by STM (Ref. 2). Such boundary effects are expected to be particularly prominent for nanometer-scale graphene quantum dots, structures for which the proportion of edge atoms is significant. In this paper we present investigations of graphene quantum dots that we have prepared by annealing carbon- bearing precursor molecules on a Co(0001) surface. Using scanning tunneling microscopy as a local probe of the physical and electronic structure, we report results on the nature of edge states for quantum dots of differing geometrical shape. We observed prominent edge-localized states for triangular quantum dots, whereas these features are suppressed for quantum dots of hexagonal shape. These observations are consistent with numerical simulations of the expected electronic structure. 1. M. Fujita et. al., J. Phys. Soc. Jpn. 65, 1920 (1996) 2. Y. Niimi et. al., Phys. Rev. B 73, 085421 (2006) [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y30.00007: Surface Functionalized Carbogenic Quantum Dots A.B. Bourlinos, A. Stassinopoulos, A. Anglos, S.H. Anastasiadis, R. Zboril, M. Karakassides, E.P. Giannelis Surface functionalized carbon-based quantum dots (C-QDs) are formed in-situ in a single-step process via thermal carbonization of suitable molecular precursors based on ammonium citrate salts. The as-synthesized nanoparticles have near spherical morphology and size around 7nm. Using different surface modifiers, we can form hydrophobic or hydrophilic capped C-QDs, which can be dispersed in organic or aqueous solvents, respectively. These C-QDs fluoresce strongly upon optical excitation. We believe that the fine size of the C-QDs combined with their disorder structure favor a high concentration of defect sites at the surface of the nanoparticles that, upon stabilization by the attached organic groups, give rise to the observed emissions. It is further noted that the emission band shifts to shorter wavelengths as the excitation is blue-shifted. In a series of studies, the emission quantum yield of C-QDs was found to be around 4\%. These types of materials are promising as fluorescent tags for biological application. Sponsored by the ULF-FORTH (Laserlab- Europe) and by the Greek GSRT. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y30.00008: Scanning Probe Microscopy Characterization of Electrical Properties of Bimetallic Core Shell Nanostructures Regina Ragan, Sangyeob Lee, Aniketa Shinde, Satoru Emori Metallic nanoparticles have shown enhanced catalytic activity compared to their bulk counterparts potentially due to changes in electronic properties at the nanoscale. Challenges in nanoscale catalysis studies include the fabrication of monodisperse nanostructures as well as a fundamental knowledge of the electronic properties at the nanometer length scale. Our group addresses these issues by fabricating dense ordered arrays of bimetallic core-shell nanostructures and characterizing structures with scanning tunneling spectroscopy and Kelvin probe force microscopy. Self assembled rare earth disilicide nanowires are used as templates for Pt and Au nanostructures on Si(001). We will present electronic characterization of these structures with nanometer scale resolution using STS and KPFM. STS measurements of RESi2 nanowires will be presented that show enhanced tunneling as compared to thin films as well as size dependant rectification ratios when comparing islands and wires of various width. KPFM is used to measure the work function of various sizes of RESi2 nanostructures providing a fundamental basis for understanding catalytic behavior in terms chemical activity of the nanostructures. KPFM data reveals a higher CPD for DySi2 nanowires than islands with $\Delta \Phi$ nanoisland-nanowire found to be 230meV. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y30.00009: Controlled fabrication and electrical properties of long quasi-one-dimensional superconducting nanowire arrays Ke Xu, James Heath Quasi-one-dimensional superconducting nanowires are an interesting and ideal system to examine fundamental superconductivity physics and size effects on superconductivity. We report a general method for reliably fabricating quasi-one-dimensional superconducting nanowire arrays, with good control over nanowire cross section and length, and with full compatibility with device processing methods. We investigate Nb nanowires with individual nanowire cross sectional areas that range from bulk-like to 10 $\times $ 11 nm, and with lengths from 1 to 100 micrometers. Nanowire size effects are systematically studied. In particular, a comprehensive investigation of influence of nanowire length on superconductivity is reported for the first time. All results are interpreted within the context of phase-slip models. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y30.00010: Optical Interferometry of Gas Pressure Damped Silicon Nanobridges and Nanocantilevers O. Svitelskiy, N. Liu, V. Sauer, J. Losby, M. Belov, E. Finley, K.M. Cheng, M. Freeman, W. Hiebert The growing interest in NEMS, in particular in nanobridges and nanocantilevers, is determined by the prospective of their usage as hypersensitive sensors of various physical factors: mass, tension, pressure, viscosity, etc. In order to investigate their properties under damping, a series of NEMS with different sizes was prepared from standard SOI wafers by the chemical etching after electron beam lithography. The surfaces were coated by layers of Al, Au and/or Cr in different combinations. The quality of the fabricated NEMS was evaluated by SEM imaging. The resonant frequencies of the NEMS varied in the range of 10-1000 MHz. The damping was introduced by means of pressurized gas in specially built optical pressure chamber capable to hold up to 5 atmospheres with glass window and not less than 160 atmospheres if equipped with sapphire window. We demonstrate that the NEMS Q- factor, the amplitude and the frequency of their resonances show considerable dependence on the value of the pressure in the chamber. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y30.00011: Periodic and Quasiperiodic Nanostructures: Accessing Complex Architectures Through Designer Phase Masks Cheong Yang Koh, Edwin Thomas In this work, we show how one may design phase mask architectures in order to achieve complex 3-dimensional periodic and quasiperiodic nanostructures through considerations of the symmetries of the phase masks. By making use of the fact that phase mask interference lithography is essentially a case of light propagation through the non-modulated direction of a finite photonic crystal slab, we show that the diffracted beams obey the symmetry restrictions of the corresponding phase mask, which allow us to determine and design the polarizations and directions of the exiting beams which interfere coherently in the substrate, subsequently leading to the formation of 3-dimensional nanostructures which are periodic or quasiperiodic. The extension of this approach towards quasiperiodic structures is straightforward when working in Fourier space, which identifies the propagating eigen-modes within the phase mask, or photonic crystal slab. This allows us to rationally design structures with targeted properties, utilizing this Fourier space approach. We show several examples of this approach in achieving this method of fabrication for both periodic and quasiperiodic nanostructures. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y30.00012: Field Emission Enhancement and the Field-Screening Effect Reduction using Carbon Nanopipettes as Cold Cathodes Abdelilah Safir, David Mudd, Mehdi Yazdanpanah, Vladimir Dobrokhotov, Gamini Sumanasekera, Robert Cohn In this work, we report a recent experimental study of high emission current densities exceeding 10mA/cm$^{2}$ and breakdown electric field lower than 5Volts/$\mu $m from novel cold cathodes such as conical shaped carbon nanopipettes (CNP). CNP were grown by CVD on Pt wire and have apex as sharp as 10nm with length between 3-6$\mu $m. The emission experiments were conducted under vacuum in a scanning electron microscope for individual CNP and in a dedicated chamber for bulk samples. CNP's conical bases and low density contribute significantly to the reduction of the screening effect and to the field emission enhancement. The experimental value for the field enhancement factor, $\gamma$, was about 867. Comparing emission results taken from CNP and aligned multiwall carbon nanotubes (MWNT) show that the ratio between $\gamma_{CNP}$ and $\gamma_{MWNT}$ is $\sim $1.6 which contributes to the reduction of screening effect. The emission from multilayers of graphene was also studied. High emission current (20$\mu $A) demonstrates promising emission properties of graphene. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y30.00013: Dicke effect in a multi-ripple electron waveguide. Hoshik Lee, Linda Reichl We compute the electron transmission through a bi-ripple electron waveguide. We numerically observe a resonance splitting, in this {\it open quantum system}, which is analogous to the Dicke effect in quantum optics. We also plot S- matrix poles in the complex energy plane, and find that two symmetry related poles contribute to the resonance splitting. We find that the symmetric resonant states are easily coupled to the leads, but the anti-symmetric states are not. We show the resonance splitting is due to a indirect interaction between wavefunctions in each cavity using a simple model. We also show that one of S-matrix poles withdraws from the real axis as a ripple is added. It turns out that the width of the resonance for $N$-ripple waveguide is $N$ times larger than the resonance width of a mono-ripple waveguide. It agrees with the result of the Dicke model. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y30.00014: What is Quantum in Quantum Pumping: The role of the Phase and Asymmetries Kunal Das, Tomas Opatrny By studying several examples, we show that quantum pumping does not always necessitate a strictly quantum description, neither is phase always a necessary concept. The same quantum mechanical picture of pumping encompass a variety of distinct mechanisms, some can be simulated by classical mechanisms while others can be explained only in a quantum picture; the role of the phase of the wavefunction is the crucial differentiator. We also show that most pumping processes have a previously unconsidered antisymmetric component which contributes significantly to the instantaneous current at each terminal but causes no net charge transfer . We have also computed the exact pumped current for arbitrary rates of time variation for certain potentials, not just in the adiabatic regime as has been previously studied. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y30.00015: Evanescent-wave current through nanometer-scale conductor with generalized channel decomposition in non-equillibrium Green's function theory Hiroshi Shinaoka, Takeo Hoshi, Takeo Fujiwara In optics, evanescent wave is known as a decay mode without dissipation, which appears at total reflecting surfaces and surfaces of nanoparticles. Even though such decay mode can be found, in principle, also in electronic current in nanometer-scale conductors, the evanescent wave effect has not yet been investigated systematically, in materials with electronic structure. We present a novel eigen-channel decomposition method in non-equilibrium Green's function formalism. By applying this method to nanometer-scale d-band metal wires, we found decaying behavior of electron density and backward current flows near electrodes, which is evidence of evanescent waves. We also found that the evanescent waves cause conductor-length dependence of the transmission, which is detectable in experiments. Dependence of the evanescent waves on materials and structures are also discussed from a point of view of band structures and their connectivity at electrodes. [Preview Abstract] |
Session Y31: Focus Session: Sodium Cobaltites
Sponsoring Units: DMP GMAGChair: David Singh, Oak Ridge National Laboratory
Room: Morial Convention Center 223
Friday, March 14, 2008 11:15AM - 11:51AM |
Y31.00001: Charge order and anomalous magnetism in the Na cobaltates Invited Speaker: The layered Na cobaltates have some analogies with the cuprates~as 2D conductivity occurs in the CoO$_{2}$ planes and doping can be modified by changing the Na content. Also ordered magnetic phases have been evidenced, but unexpectedly for large values of $x$ for which one would expect a hole doping of the band insulator NaCoO$_{2}$. Indeed, in the high crystal field on the Co sites in these compounds, an ionic picture for the Co states would correspond to low spin configurations Co$^{3+}$, S=0 or Co$^{4+}$, S=1/2. We shall present SQUID and $^{23}$Na and $^{59}$Co NMR data [1] taken on samples synthetized and characterized by X ray cristallography in LLB, Saclay. We evidence that the Co charge is uniform for $x$=0.35 as in the hydrated superconducting phase. For high Na contents the samples are found to display ordered Na structures or mixtures of those, with different $x$ values. In pure phases isolated for specific $x$ values, we evidence a charge disproportionation into non magnetic Co$^{3+}$ and more magnetic Co sites with an average charge of about Co$^{3.5+}$, except for $x$=0.5 [2]. This hole delocalization and charge order occur both for paramagnetic and AF phases [3]. NMR investigations of the dynamic susceptibilities allow us to characterize the nature of the in plane electronic correlations in most parts of the phase diagram. Contrary to the case of most cuprates for which dopant disorder is quite influential, the hole doping achieved in cobaltate samples is associated with the insertion of well ordered Na planar structures. They have to be taken into account to explain theoretically the metallicity, the magnetic properties and their evolution with doping. \newline [1] \textit{I. Mukhamedchine, H. Alloul, G. Collin et N. Blanchard, Phys. Rev. Letters, }\textbf{\textit{ 94}}\textit{, 247602 (2005). } \newline [2] http://arxiv.org/find/cond-mat/1/au:+Bobroff\_J/0/1/0/all/0/1, J. Bobroff; http://arxiv.org/find/cond-mat/1/au:+Lang\_G/0/1/0/all/0/1, G. Lang; http://arxiv.org/find/cond-mat/1/au:+Alloul\_H/0/1/0/all/0/1, H. Alloul; http://arxiv.org/find/cond-mat/1/au:+Blanchard\_N/0/1/0/all/0/1, N. Blanchard and http://arxiv.org/find/cond-mat/1/au:+Collin\_G/0/1/0/all/0/1, G. Collin, \textit{Phys. Rev. Letters \textbf{\textit{96}}, 107201 (2006) .} \newline [3] \textit{I. Mukhamedchine, H. Alloul, G. Collin et N. Blanchard, condmat /0703561. } [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y31.00002: Sodium Ordering Phase Transition in Sodium Cobaltate D.J.P. Morris, M. Roger, D.A. Tennant, M.J. Gutmann, J.P. Goff, D. Prabhakaran, J.-U. Hoffmann, R. Feyerherm, E. Dudzik, K. Kiefer Na$_{x}$CoO$_{2}$ has emerged as a system of fundamental scientific interest because of its highly unusual electrical and magnetic properties. Using neutron and x-ray diffraction we have detected long-range 3D ordering of Na$^{+}$ ions in single crystals, and demonstrate a kaleidoscope of Na$^{+}$ ion patterns as a function of concentration and temperature [1]. Large scale numerical simulations reveal the ordering principle for this system, the formation of tri-vacancy charged droplets that then order long range, and the structure factors from these defect clusters are in good agreement with the observed neutron diffraction intensities. Superstructure transitions are observed in the diffraction data which are explained by a change from a stripe structure of tri-vacancies. The results readily explain many of the observed electrical and magnetic properties, including the formation of ferromagnetic sheets in the CoO$_{2}$ layers over this composition range, and the 3D nature of the magnetic excitations. [1] M. Roger et al. Nature 445, 631 (2007) [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y31.00003: New magnetic states in Na$_{x}$CoO$_{2}$ induced by controlled Na order. T. F. Schulze, P. S. Haefliger, Ch. Niedermayer, K. Mattenberger, S. Bubenhofer, B. Batlogg We prove the direct link between low temperature magnetism and high-temperature Na$^{+}$ ordering in Na$_{x}$CoO$_{2}$ using the example of a so far unreported magnetic transition at 8K . The new magnetic state carries a weak ferromagnetic moment parallel to the CoO$_{2}$ layers. The 8K feature has been characterized in detail and its dependence on a diffusive Na+ rearrangement around 200K is demonstrated. The diffusive process is found to slow down around 200K, and the characteristic time scale reaches several hours at 195K. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered Na+ Coulomb potential on the itinerant electrons in the CoO$_{2}$ layers is evident, and new ways to experimentally revisit the Na$_{x}$CoO$_{2}$ phase diagram are discussed. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y31.00004: Precise control of the Na nonstoichiometry in Na$_{x}$CoO$_{2}$ Yoshihiko Okamoto, Atsushi Nishio, Yoko Kiuchi, Zenji Hiroi Na$_{x}$CoO$_{2}$ has been attracting great interests in terms of a correlation between its electronic properties and Na nonstoichiometry. Many groups have reported that a Na rich phase ($x \quad \sim $ 0.7) is a Curie-Weiss metal and a poor phase ($x \quad \sim $ 0.3) is a Pauli paramagnetic metal. The origin of this difference, however, has not been confirmed yet, mainly because of difficulty in controlling precisely the Na nonstoichiometry. We succeeded in synthesizing a series of polycrystalline samples of Na$_{x}$CoO$_{2}$ with well-controlled Na content by the solid-state reaction instead of the solution reaction previously used. We prepared polycrystalline Na$_{x}$CoO$_{2}$ (0.58 $\le \quad x \quad \le $ 0.63) by a solid-state reaction of Na$_{0.71}$CoO$_{2}$ and Na$_{0.5}$CoO$_{2}$ at 200$^{\circ}$C. Furthermore, fine tuning of Na content in the range of 0.62 $< \quad x \quad <$ 0.63 was carried out by a solid-state reaction of Na$_{0.62}$CoO$_{2}$ and Na$_{0.63}$CoO$_{2}$. Magnetic susceptibility of Na$_{x}$CoO$_{2}$ exhibited Curie-Weiss behavior at $x \quad \ge $ 0.621 while nearly temperature independent paramagnetism at $x \quad \le $ 0.620. Such a drastic change of magnetism clearly indicates that the magnetic phase boundary is located in an extremely narrow range around $x$ = 0.62. We think that the difference originates from a change of the Fermi surface topology with electron filling probably associated with the dip in the a$_{1g}$ band near the $\Gamma $ point. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y31.00005: Systematic ARPES study on Na-rich Na$_{x}$CoO$_{2}$ (0.75$<$x$<$1) Y.-M Xu, P. Richard, M. Neupane, F.-C. Chou, C.-T. Lin, M. Gao, Z. Wang, H. Ding The phase diagram of the cobaltite Na$_{x}$CoO$_{2}$, with varying Na concentration x, is very rich and complicated. At the high-doping regime (x$>$0.75), the system was found to be more correlated, with a spin-density-wave state emerging at low temperatures. A stable phase was found with $\sqrt {13} \times \sqrt {13} $ symmetrical superstructure at Na-rich doping cobaltite. We will report our recent ARPES results of the Na-rich Na$_{x}$CoO$_{2}$ (0.75$<$x$<$1) samples. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y31.00006: Fermi surface pockets in Na$_x$CoO$_2$ for $ x = 0.71$ and $0.84$: Localization effects and emergence of a quantum spin-liquid ground state Luis Balicas, Younjung Jo, Fangcheng Chou, Patrick Lee Here we report the observation of Fermi surface (FS) pockets via the Shubnikov de Haas effect in Na$_x$CoO$_2$ for $x = 0.71$ and $0.84$, respectively. Our observations indicate that the FS of each compound can intersect their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, only if a precise number of holes per unit cell is \emph{localized}. For $0.71 \leq x < 0.75$ the coexistence of itinerant carriers and localized $S =1/2$ spins on a paramagnetic triangular lattice leads, at low temperatures, to the observation of non Fermi-liquid behavior in the electrical transport and heat capacity properties. Namely, an anomalous exponent in the temperature dependence of the resistivity and a logarithmic divergence of the heat capacity divided by temperature as the temperature is lowered. These observations suggest the possibility of a unique quantum spin-liquid ground state resulting from the interplay between itinerant carriers and fluctuating $S=1/2$ spins on a frustrated lattice. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y31.00007: ARPES investigation of the electronic properties of PdCoO$_{2}$ I.M. Vishik, W.S. Lee, H. Takatsu, D.H. Lu, R.G. Moore, Y. Maeno, Z.-X. Shen The triangular-lattice layered cobaltates have emerged as an exciting new correlated electron system.~ Although tremendous progress has been made with Sodium Cobaltate (Na$_{x}$CoO$_{2})$ there still remain disputes, most notably, the precise Fermi surface topology.~ In order to gain another perspective, we have studied a related compound, Palladium Cobaltate (PdCoO$_{2})$. This is a chemically stable metallic oxide that can be prepared with very high purity, allowing us to study a clean system that lacks the complications introduced by doping in Na$_{x}$CoO$_{2}$. We present the first ARPES measurements on PdCoO$_{2}$, focusing on determining the Fermi surface topology, comparing observed band structure to theory, and making connections with bulk measurements. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y31.00008: a$_1g$-e$_g$' splitting and the small Fermi surface pockets in Na$_x$CoO$_2$ Michelle Johannes, Devina Pillay, Igor Mazin, Ole Andersen Because DFT calculations and ARPES experiments disagree on the existence of six small Fermi surface pockets in NaxCoO2, it has been suggested that correlation effects neglected by the LDA may be responsible for suppressing the eg-derived pockets. Recent DMFT work has shown that such suppression is only possible if the position of the eg' band is lower than that of the a1g band, prior to correlation effects. Here we show that the energy difference between band positions, $\Delta$=$\epsilon_{eg'}$-$\epsilon_{a1g}$ strongly depends on Na content, Na positions, and on whether bands stem from the surface or bulk. We show that the Coulomb field of the Na ions is enough to shift the a1g band beneath the eg band, even though simple crystal field arguments would suggest the opposite. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y31.00009: Resonant Ultrasound Studies of Quasi-2D Na$_{x}$CoO$_{2}$ (0.7 $\le $ x $\le $ 0.77) Timothy Cagle, Veerle Keppens, Rongying Jin Cobalt-based oxides have been shown to represent a new paradigm for a good thermoelectric material. These materials violate all of the traditional guidelines to help identify a potentially good thermoelectric material. In order to obtain a better understanding of its physical properties, we have initiated the synthesis of layered Na$_{x}$CoO$_{2}$, and started a study of the elastic properties of these materials using Resonant Ultrasound Spectroscopy (RUS). In this work, we discuss our results for sodium cobaltate compounds with 0.7 $\le $ x $\le $ 0.77. Single crystals were successfully grown in a floating-zone furnace, and the elastic constants have been measured as a function of temperature (5-300K) and in magnetic fields up to 5 Tesla. The resulting plots of elasticity vs. temperature clearly reflect a transition below 50 K, which is believed to be associated with rearrangement of the Na-atoms between the CoO$_{2}$ planes. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y31.00010: Thermo-magnetic effect of cobalt oxides Wataru Koshibae, Sadamichi Maekawa The cobalt oxide, Na$_x$CoO$_2$, shows not only a large thermoelectric response but also an anomalous high-temperature Hall effect: The Hall coefficient increases linearly as a function of temperature and the magnitude comes to no fewer than 8 times as large as the expected Drude value. On the electron system with the large thermopower and the large Hall coefficient, an interesting behavior is expected in the response to a magnetic field upon a temperature gradient. We have studied the electronic state of the cobalt oxide and found that the electronic structure reflects the nature of the kagom\'e lattice hidden in the CoO$_2$ layer. We will show the importance of the hidden kagom\'e lattice structure in the emergence of the anomalous Hall effect and the close relationship between the Hall and Nernst coefficients. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y31.00011: Nodal $d+id$ pairing and topological phases on the triangular lattice: unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O Sen Zhou, Ziqiang Wang We show that the finite angular momentum pairing on the triangular lattice has point nodes in the complex gap function. A topological quantum phase transition takes place through a gapless critical state at a specific carrier density $x_c$ where the normal state Fermi surface crosses these isolated nodes. For spin singlet pairing, we show that the second nearest neighbor $d+id$ pairing is the dominate superconducting channel. The gapless critical state appears at $x_c\simeq0.25$ for the sodium cobaltates. It has six Dirac points and is topologically nontrivial with a $T^3$ spin relaxation rate below $T_c$. This theory provides a consistent explanation for the unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y31.00012: Unusual Muon Spin Relaxation in Hydrated Cobaltite Superconductors Scott L. Stubbs, Jess H. Brewer, Jun Sugiyama, Yutaka Y. Ikedo, Peter L. Russo, Eduardo J. Ansaldo, Kim H. Chow, Hiroto Ohta, Kazuyoshi Yoshimura Muon spin relaxation ($\mu^+$SR) was studied in zero (ZF) and weak transverse magnetic field (TF) in Na$_x$(H$_3$O)$_z$CoO$_2\cdot y$H$_2$O and its deuterated analog. In ZF, the muon relaxes (surprisingly) faster for the deuterated sample. Detectable effects of superconductivity are also surprisingly subtle in both cases. [Preview Abstract] |
Session Y32: Focus Session: Theory and Simulations of Magnetism II
Sponsoring Units: DCOMP DMP GMAGChair: Peter Dederichs, IFF Research Center Juelich
Room: Morial Convention Center 225
Friday, March 14, 2008 11:15AM - 11:51AM |
Y32.00001: Homochiral magnetic structures at surfaces Invited Speaker: Electrons propagating in the vicinity of inversion asymmetric environments such as surfaces, interfaces, ultrathin films or nanostructures can give rise to an important antisymmetric exchange interaction, known as Dzyaloshinskii-Moriya (DM) interaction. Although this interaction, favoring spatially rotating spin structures, is in principle known for about 50 years, its consequences for the magnetic structure in low-dimensional magnets remained nearly unexplored and has been basically overlooked the past 20 years. Theoretical models considering isotropic exchange, magnetic anisotropy and the DM interaction display a rich phase diagram of complex magnetic phases on different length scales depending on the strength of the different contributions. Today, it is unknown how large is the strength of the DM interaction. Is this just a small perturbation to the collinear uniaxial ferro- or antiferromagnetic state, determined by exchange and magnetic anisotropy or is it strong enough to create new phases which had been overlooked? Surprisingly little first-principles calculations deal with the DM interaction. There might be several reasons for this: The investigation requires the treatment of non-collinear magnetism together with spin-orbit interactions of large magnetic structures in low-symmetry situations. We developed a perturbative strategy implemented into the FLAPW code {\tt FLEUR} which can cope with this challenge. We show by first- principles calculations based on the vector-spin density formulation of the density- functional theory (DFT) that that there are circumstances whre the DM interaction is indeed sufficiently strong to compete with the interactions that favor collinear spin alignment causing magnetic phases of unique handedness e.g.homochiral magnetic phases such as a left rotation cycloidal spiral in Mn on W(110) [M.~Bode {\em et al.}, Nature {\bf 447}, 190 (2007)] or favoring magnetic domain-walls with unique turning sense. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y32.00002: First-principles Calculation of Magnetic Anisotropy of a Single Atom on a Surface Chiung-Yuan Lin, Barbara Jones Recent progress on scanning tunneling microscopy has made it possible to position (in atomic-scale accuracy) and probe single magnetic atom on material surfaces. Targeting the fabrication of a single-atom data storage bit, we perform first-principles calculations of single magnetic atoms (Fe and Mn) on a surface. Structure relaxation determines the binding structures of the magnetic atoms to its surroundings. Charge analysis indicates that the magnetic atoms form polar covalent bonds with the surface. Spin density is found to spread up to 4{\AA} from the magnetic atom, which is qualitatively similar to that reported in DFT calculations of molecular magnets. Total energies with spin-orbit interaction included are calculated in different spin orientations, and are compared to the anisotropy axes measured in the experiments. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y32.00003: First-principles study on the surface half-metallicity of CaC in the zinc-blende structure Kailun Yao, Guoying Gao, Zuli Liu We investigate the electronic structure and the surface half-metallicity of CaC in the zinc-blende structure by using the first-principles full-potential linearized augmented plane-wave (FPLAPW) method. It is found that the (1 1 0) surface preserves the half-metallic character of the bulk, while in the case of the (0 0 1) surfaces including the Ca-terminated and C-terminated surfaces the surface states destroy the half-metallicity. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y32.00004: Finite Hubbard clusters with large spin polarization Erik Nielsen, R.N. Bhatt A generalized Hubbard model can be used to characterize hydrogenic impurities in semiconductors. It has been shown that the ground state spin of such impurity clusters is very sensitive to a cluster's electron number and geometry [1]. An understanding of how these factors affect cluster magnetization is particularly relevant in light of the current ability to position phosphorus donors with nanometer accuracy within bulk silicon [2]. We present numerical results for two-dimensional clusters showing the effect of geometry and electron-hole asymmetry present in real systems of hydrogenic donors. We also consider the robustness of high-spin cluster ground states to perturbations of site position, and discuss the general features of clusters found to possess a high-spin ground state, in particular the fully spin-polarized state.\newline [1] Erik Nielsen and R. N. Bhatt, Phys. Rev. B 76, 161202 (2007). \newline [2] J. L. O'Brien et al., Phys. Rev. B 64, 161401 (2001). [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y32.00005: Magnetic ground state of small nanoparticles: Cr trimers on Au(111) Invited Speaker: The development of scanning tunneling microscopy (STM) and the ability to build clusters with well-controlled structures permit the study of various effects induced by local interactions within magnetic nanoclusters. However, for a clear interpretation of experimental results, first principles studies are often required, even when they are sometimes too demanding. In this talk we show a new way to map the energy of supported magnetic nanoparticles obtained from first principles calculations onto a classical spin Hamiltonian. The half-filled valence configuration of Cr yields a large magnetic moment and strong antiferromagnetic inter-atomic bonding leads in turn to magnetic frustration and complex spin phenomena. The simplest system exhibiting such properties is a trimer. The electronic structure of the Cr trimers are calculated by means of a fully relativistic Green's function embedding method. The relativistic treatment of the electronic structure leads to a proper account of spin-orbit coupling giving rise to tensorial exchange interactions and magnetic anisotropies influencing the formation of non-collinear ground states. In additon, we show that the inclusion of fourth-order terms into the spin-model largely enhance the accuracy of the mapping. The magnetic ground-state of the trimers are found as the solution of the Landau-Lifshitz-Gilbert equations. In case of an equilateral Cr trimer we show that the Dzaloshinsky-Moriya interactions lift the degeneracy of the 120$^\circ$ N\'eel states with different chirality. For the linear and the isosceles Cr trimers we obtain collinear antiferromagnetic ground states. We also address the issue of choosing the reference state inherent to methods based on the magnetic force theorem in the context to the equilateral Cr trimer. This freedom of the method might cause an ambiguity in determining the magnetic ground state of systems exhibiting metastable states close to the ground state. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y32.00006: A Numerical Method to Study Highly Correlated Nanostructures: The Logarithmic Discretization Embedded Cluster Approximation E. Anda, G. Chiappe, C. Busser, M. Davidovich, G. Martins, F. H-Meisner, E. Dagotto A numerical algorithm to study transport properties of highly correlated local structures is proposed. The method, dubbed the Logarithmic Discretization Embedded Cluster Approximation (LDECA), consists of diagonalizing a finite cluster containing the many-body terms of the Hamiltonian and embedding it into the rest of the system, combined with Wilson's ideas of a logarithmic discretization of the representation of the Hamiltonian. LDECA's rapid convergence eliminates finite-size effects commonly present in the embedding cluster approximation (ECA) method. The physics associated with both one embedded dot and a string of two dots side-coupled to leads is discussed. In the former case, our results accurately agree with Bethe ansatz (BA) data, while in the latter, the results are framed in the conceptual background of a two-stage Kondo problem. A diagrammatic expansion provides the theoretical foundation for the method. It is argued that LDECA allows for the study of complex problems that are beyond the reach of currently available numerical methods. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y32.00007: Coulomb and spin-orbit effects in quantum dot molecules under harmonic fields Lilia Meza-Montes, Arezky H. Rodriguez, Sergio E. Ulloa The time evolution of a two-electron quantum dot molecule under strong harmonic electric fields is studied. The wave function is determined in terms of the single-electron orbitals using the Floquet approach. We pay particular attention to the evolution of the spin states of the system, as the surface inversion asymmetry (Rashba-type) and bulk inversion asymmetry (Dresselhaus-type) spin-orbit effects are known to introduce spin mixing. We also study the role of a perpendicular magnetic field, which is shown to have dramatic effects on the dynamics. We present an analysis of the physical behavior of the system in terms of the quasi-energy spectrum, and study the time evolution of the occupation probabilities of the dots. Conditions for singlet-triplet mixing, similar to the spin-flips observed in the single-electron case, are analyzed. These results are relevant for applications in spin-controlled devices. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y32.00008: Electron Transport in Nanogranular Ferromagnets Igor Beloborodov, Andreas Glatz, Valerii Vinokur I will discuss electronic transport properties of ferromagnetic nanoparticle arrays and nanodomain materials near the Curie temperature in the limit of weak coupling between the grains. The conductivity is calculated in the Ohmic and non-Ohmic regimes and the magnetoresistance jump in the resistivity at the transition temperature is estimated. The results are applicable for many emerging materials, including artificially self-assembled nanoparticle arrays and a certain class of manganites, where localization effects within the clusters can be neglected. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y32.00009: Inverse band structure optimization of (InAs)/(GaAs) (001) nanostructures for thermophotovoltaics Paulo Piquini, Peter Graf, Alex Zunger Thermophotovoltaic materials converting black-body thermal radiation to electricity often require conversion efficiency for materials with direct band gaps of 0.6 eV. Random In$_{0.53}$Ga$_{0.47}$As alloy lattice $matched$ to InP have a gap around 0.76 eV, too big for this application. Therefore, difficult to grow lattice-$mismatched$ In-rich InGaAs alloys have been attempted in the past. Here we suggest to use (InAs)$_n$/(GaAs)$_m$ {\it ordered superlattices} (rather than random), lattice $matched$ to InP substrates. Using empirical pseudopotential calculations and genetic algorithm methods we look for the sequence of InAs and GaAs {\it pure} layers that simultaneously lead to a target band gap of 0.6 eV and has a minimum in-plane stress (strain balance condition). Further, since for (InAs)$_n$ layers with n$>$5 the two-dimensional growth is unstable and SK quantum dots are seen to form, we restrict the value for the period of the InAs layers to be always lower than 5. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y32.00010: Spin-blockade in a Hubbard chain with spin-dependent impurities Chang-Qin Wu, Yao Yao We investigate the spin/charge transport in a one-dimensional strongly-correlated system using adaptive time-dependent density matrix renormalization group method. The model we consider is a non-half-filled Hubbard chain with spin-dependent impurities, which is found to display the blockade of spin current while little influence on charge current. We have considered (1) the spread of a wave packet of both spin and charge in the Hubbard chain and (2) the spin and charge currents induced by a spin-dependent voltage bias that is applied to the ideal leads attached at the ends of this Hubbard chain. It is found that the spin-charge separation plays a key role in the spin-blockade and a large on-site repulsion U is required for more effective utilization of this phenomenon in some spin-related devices, like spin memory. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y32.00011: Signature of hyperfine interaction through current properties in quantum dots. Ernesto Cota, Fernando Rojas, Sergio E. Ulloa Several experiments have been carried out to observe and control spin properties of electrons in quantum dots subject to the hyperfine interaction due to nuclear spins. In this work, we study the manifestation of the hyperfine interaction through current calculations in one and two quantum dots. We use the density matrix master equation approach in the stationary regime taking into account an external magnetic field and the nuclear magnetic field in the quasistatic approximation characterized by the statistical fluctuations of the components of the nuclear field. As a first step, we study the case of a single quantum dot with one orbital. We study in detail the effects on the current of the hyperfine interaction, temperature and external magnetic field and we find that it is possible to obtain information on the hyperfine interaction directly from the differential conductance. We extend the model to study a double quantum dot with one and two electrons, including tunneling and exchange interactions, where the signature of the hyperfine interaction is more involved. [Preview Abstract] |
Session Y33: Focus Session: Diluted Magnetic Oxides
Sponsoring Units: GMAG FIAP DMPChair: Stuf Wolf, University of Virginia
Room: Morial Convention Center 224
Friday, March 14, 2008 11:15AM - 11:51AM |
Y33.00001: Room-Temperature Electron Spin Dynamics in Free-Standing ZnO Quantum Dots Invited Speaker: |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y33.00002: Insulator-ferromagnetic metal transition in anatase Fe:TiO$_2$ Enju Sakai, Yasushi Hirose, Taro Hitosugi, Toshihiro Shimada, Tetsuya Hasegawa Local Fe valence states in Fe-doped anatase TiO$_2$ (Fe:TiO$_2 $) were investigated in relation with transport and magnetic properties. Anatase Fe:TiO$_2$ films were deposited on LaAlO$_3 $ (100) substrates by pulsed laser deposition technique. Amounts of oxygen vacancies in the films were controlled by varying partial oxygen pressure during deposition ($P_{O_2}$). Magnetic and transport properties of the synthesized Fe:TiO$_2$ films measured by SQUID and conventional fourt probe measurements. An insulator-ferromagnetic metal transition was clearly observed between $P_{O_2}$= 1$\times 10^{-6}$ and $3 \times 10^{-6}$ Torr. X-ray photoemission spectroscopy (XPS) measurements have revealed that the local Fe valence state changes from 3+ to 2+, accompanied with the insulator- ferromagnetic metal transition. These results strongly suggest that carriers bound to Fe-oxygen vacancy pairs form magnetic polarons, and that mutual overlap of magnetic polarons triggers the insulator to ferromagnetic metal transition in Fe:TiO$_2$. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y33.00003: Insights into magnetically doped semiconductors from soft x-rays D.J. Keavney, D.B. Buchholz, Q. Ma, R. Chang, T.C. Droubay, T.C. Kaspar, S. Chambers Soft x-ray absorption and dichroism provide a crucial test of the intrinsic nature of magnetism in doped oxide semiconductors. Experiments on Mn, Co, and Cu- doped ZnO reveal that the magnetic dopants have primarily paramagnetic field and temperature dependence regardless of whether the bulk behavior is ferromagnetic. In PLD-grown ferromagnetic Cu:ZnO, no zero-field dichroism is detected at the Cu L, O K, or Zn L edges. In MOVCD-grown Co:ZnO, we find a small remanent signal that is consistent with bulk magnetization measurements of $\sim $0.04$\mu _{B}$/Co ion, however the signal is insufficient to rule out metallic Co as its origin. These results suggest that the origins of ferromagnetism in doped oxides may be unrelated to the presence of magnetic dopants. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y33.00004: Structural, Magnetic and Transport Properties of a New Class of Ferromagnetic Semiconductors/Metals: (Ba, Sr)M$_{2\pm x}$Ru$_{4-x}$O$_{11}$ (M = Fe, Co) Larysa Shlyk, Lance De Long, Sergiy Kryukov, Barbara Sch\"upp-Niewa, Rainer Niewa Single crystals (mm size) of (Ba, Sr)Fe$_{2+x}$Ru$_{4-x}$O$_{11}$ and (Ba, Sr)Co$_{2+x}$Ru$_{4-x}$O$_{11}$ were grown for the first time. X-ray refinements confirmed a hexagonal space group (P6$_{3}$/mmc, No. 194) with two crystallographic sites having mixed Ru and Fe/Co occupation, and one site occupied exclusively by the 3d species. Structural parameters and charge balance suggest oxidation states Co$^{2+}$ and mixed Ru$^{3+}$/Ru$^{5+}$ in the Co compound, and mixed Fe$^{2+}$/Fe$^{3+}$ and Ru$^{3+}$/Ru$^{5+}$ in the Fe compound. The physical properties of these single crystals are sensitive to site disorder among the transition metal ions. Magnetic and transport measurements show the Co-bearing barium ferrite is a ferromagnetic metal below 105 K. In contrast, Fe-bearing barium and strontium compounds exhibit long-range ferromagnetic order at temperatures above 400 K, and narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. These characteristics make the Fe-bearing materials promising new candidates for spintronic applications. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y33.00005: Spin coherence of electrons in singly-charged quantum dots Invited Speaker: We present experimental and theoretical study on electron spin coherence in ensemble of n-type doped InGaAs/GaAs quantum dots containing one electron per dot. A pump-probe time-resolved Faraday rotation technique is exploited. Long-lived spin precession of resident electrons in external magnetic fields is observed with the dephasing time of spin ensemble exceeding 6 ns. Rabi oscillations of the Faraday rotation amplitude has been detected confirming the suggested model of generation electron coherent via excitation of coherent superposition of the trion state and the resident electron [1]. We show that the ensemble dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins. This mode-locking leads to constructive interference of contributions to Faraday rotation, and presents potential applications based on robust quantum coherency within an ensemble of dots [2, 3]. Under these experimental conditions spins of the dots nuclei are aligned in a way that all dots in the ensemble contribute to the coherent signal with a potential to focus the electron Larmor frequencies in the ensemble to a single mode [4]. The used optical technique allows to recover the coherence time of a single quantum dot. The measured spin coherence time is 3 microseconds, which is four orders of magnitude longer than the ensemble dephasing time of 400 picoseconds. This work is done in collaboration with A. Greilich, I. A. Yugova, R. Oulton, M. Bayer, A. Shabaev, Al. L. Efros, D. Reuter and A. D. Wieck. \newline \newline [1] A. Greilich, et al., Phys. Rev. Lett. 96, 227401 (2006). \newline [2] A. Greilich, et al., Science 313, 331 (2006). \newline [3] A. Greilich, et al., Phys. Rev. B 75, 233301 (2007). \newline [4] A. Greilich, et al., Science 317, 1896 (2007). [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y33.00006: Impurity-Ion pair induced high-temperature ferromagnetism in Co-doped ZnO Stefano Sanvito, Chaitanya Das Pemmaraju, Ruairi Hanafin, Thomas Archer, Hans Benjamin Braun Magnetic 3d-ions doped into wide-gap oxides show signature of room temperature ferromagnetism, although their concentration is two orders of magnitude smaller than that of conventional magnets. The prototype of these exceptional materials is Co-doped ZnO, for which an explanation of the room temperature ferromagnetism is still elusive. Here we demonstrate that magnetism originates from Co2+ oxygen-vacancy pairs with a partially filled level close to the ZnO conduction band minimum. The magnetic interaction between these pairs is sufficiently long-ranged to cause percolation at moderate concentrations. However, magnetically correlated clusters large enough to show hysteresis at room temperature already form below the percolation threshold and explain the current experimental findings. Our work demonstrates that the magnetism in ZnO:Co is entirely governed by intrinsic defects and a phase diagram is presented. This suggests a recipe for tailoring the magnetic properties of spintronics materials by controlling their intrinsic defects. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y33.00007: Magnetic 3$d$ interactions in ZnO and In$_{2}$O$_{3}$ in a band-gap corrected approach Stephan Lany, H. Raebiger, A. Zunger The electronic and magnetic configuration of 3$d$ transition metal (TM) impurities in wide-gap oxides like ZnO and In$_{2}$O$_{3}$ is misrepresented in the standard LDA and GGA approximations to density functional theory: Because the conduction band minimum lies energetically much too low, the spin-polarized impurity states wrongly occur as resonances inside the conduction band rather than as gap states. Due to spurious ``charge spilling'' from the TM impurity state into the host conduction band, the magnetic moment and the occupancy of the TM impurity state is incorrect, and the TM state becomes partially occupied, which is prone to cause overestimated ferromagnetic interactions. These errors are not corrected by the LDA+U or GGA+U methods often applied to TM-$d$ states. In our band-gap corrected approach, we augment the GGA+U functional by empirical non-local external potentials (NLEP) for the $s$- and $p$-states of Oxygen and the cations. In this approach the correct spin and orbital configuration of the TM impurity-states is recovered. In the absence of additional doping, we find generally short-ranged magnetic interactions, and pronounced Jahn-Teller effects in case of partially occupied gap states. Additional electron-doping can lead to more long-range ferromagnetic interactions for those TM-dopants that have unoccupied $d$-states which hybridize strongly with the conduction band. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y33.00008: Study of the anomalous Hall effect, magnetoresistance, and magnetic anisotropy in ZnO:Co and ZnO:Mn thin films Z. Yang, Z. Zuo, Y. Pu, M. Biasini, W. Beyermann, J. Shi, J. Liu ZnO-based diluted magnetic semiconductor (DMS) materials have attracted much attention in these years due to the theoretical prediction of above room-temperature ferromagnetism. So far, most of the experiments were focused to the study of the magnetization of the ZnO DMS materials. However, the magnetization study, by only providing a global information on the moment of the material, cannot distinguish the intrinsic magnetic properties from the extrinsic contributions. The ZnO thin films were grown on sapphire substrates by molecular-beam epitaxy. The Co- and Mn-implantations were performed on the as-grown ZnO samples with different free carrier concentrations. Anomalous Hall effect (AHE) and magnetoresistance measurements were performed on the ZnO:Co and ZnO:Mn thin films. AHE hysteresis loops were observed in both ZnO:Co and ZnO:Mn thin films, which confirm the intrinsic ferromagnetism in both films. However, the AHE hysteresis loops are distinctly weaker than the magnetization hysteresis loops ($M-H)$ measured by SQUID. Therefore we conclude that both intrinsic and extrinsic ferromagnetism co-exist in the samples. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y33.00009: Influence of oxygen partial pressure on structural, transport and magnetic properties of Co doped TiO2 films Bakhtyar Ali, Abdul Rumaiz, Arif Ozbay, S. Ismat Shah, Edmund Nowak Crystal structure, transport and magnetic properties of Co dopedTiO2 laser ablated thin films are investigated and are found to have a strong dependence on the oxygen partial pressure. X-ray diffraction reveals the presence of mixed phase material containing anatase and rutile. However, these phases intertransform with the change in the oxygen partial pressure in the chamber during the growth of the films under the same temperature and other growth conditions. Electrical conductivity enhances as more oxygen vacancies are created. Concomitantly, the magnetization increases with increased vacancy concentration. The electrical transport data is suggesting that the conduction is dominated by polarons. The activation energies obtained are in the range from 100 to 150meV, typical for semiconducting oxides. APS Membership: Pending [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y33.00010: Spin-polarized tunneling study of the room temperature spin filter CoFe2O4 Ana Ramos, Jean-Baptiste Moussy, Richard Mattana, Frederic Petroff, Tiffany Santos, Guo-Xing Miao, Jagadeesh Moodera The spin filter effect has the potential of generating highly spin-polarized electron currents by the spin selective transport of electrons across a ferromagnetic tunnel barrier. In this work, we investigate the spin-polarized tunneling characteristics of cobalt ferrite (CoFe2O4), which we show is a room temperature spin filter. Tunnel junctions containing epitaxial CoFe2O4(111) tunnel barriers have been grown by oxygen plasma-assisted molecular beam epitaxy. Their structural, chemical and magnetic properties having previously been optimized by a number of in situ and ex situ methods, we focus on the spin-polarized tunneling in the CoFe2O4-based systems using different measurement techniques. Following the demonstration of spin filtering by TMR measurements, both at low temperature and at room temperature, we further investigate the spin filter characteristics of CoFe2O4 in detail. In particular, we pay special attention to the influence of defects on the spin polarization, as well as the role of different spin-detecting electrodes. [Preview Abstract] |
Session Y36: Focus Session: Artificial Neurons
Sponsoring Units: FIAPChair: Unil Perera, Georgia State University
Room: Morial Convention Center 228
Friday, March 14, 2008 11:15AM - 11:51AM |
Y36.00001: Understanding the dynamical control of animal movement Invited Speaker: Over the last 50 years, neurophysiologists have described many neural circuits that transform sensory input into motor commands, while biomechanicians and behavioral biologists have described many patterns of animal movement that occur in response to sensory input. Attempts to link these two have been frustrated by our technical inability to record from the necessary neurons in a freely behaving animal. As a result, we don't know how these neural circuits function in the closed loop context of free behavior, where the sensory and motor context changes on a millisecond time-scale. To address this problem, we have developed a software package, AnimatLab (www.AnimatLab.com), that enables users to reconstruct an animal's body and its relevant neural circuits, to link them at the sensory and motor ends, and through simulation, to test their ability to reproduce appropriate patterns of the animal's movements in a simulated Newtonian world. A Windows-based program, AnimatLab consists of a neural editor, a body editor, a world editor, stimulus and recording facilities, neural and physics engines, and an interactive 3-D graphical display. We have used AnimatLab to study three patterns of behavior: the grasshopper jump, crayfish escape, and crayfish leg movements used in postural control, walking, reaching and grasping. In each instance, the simulation helped identify constraints on both nervous function and biomechanical performance that have provided the basis for new experiments. Colleagues elsewhere have begun to use AnimatLab to study control of paw movements in cats and postural control in humans. We have also used AnimatLab simulations to guide the development of an autonomous hexapod robot in which the neural control circuitry is downloaded to the robot from the test computer. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y36.00002: Biomechanical Analysis of Locust Jumping in a Physically Realistic Virtual Environment David Cofer, Gennady Cymbalyuk, William Heitler, Donald Edwards The biomechanical and neural components that underlie locust jumping have been extensively studied. Previous research suggested that jump energy is stored primarily in the extensor apodeme, and in a band of cuticle called the semi-lunar process (SLP). As it has thus far proven impossible to experimentally alter the SLP without rendering a locust unable to jump, it has not been possible to test whether the energy stored in the SLP has a significant impact on the jump. To address problems such as this we have developed a software toolkit, AnimatLab, which allows researchers to build and test virtual organisms. We used this software to build a virtual locust, and then asked how the SLP is utilized during jumping. The results show that without the SLP the jump distance was reduced by almost half. Further, the simulations were also able to show that loss of the SLP had a significant impact on the final phase of the jump. We are currently working on postural control mechanisms for targeted jumping in locust. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y36.00003: Design of real-time locomotion generator with map-based neuronal models Nikolai Rulkov, Joseph Ayers, Mark Hunt We are developing an electronic nervous system for a biomimetic robot based on an established neurobiological model system, the Sea Lamprey. Undulatory locomotion of the lamprey is coordinated by a concatenated network of over 100 segmental central pattern generators (CPGs). To achieve real time operation in a DSP chip, we are using simple phenomenological models of neurons and synapses based on the dynamics of nonlinear maps. CPG networks based on known neuronal circuitry have replicated main properties of the dynamical behavior of the animal model. The results of numerical studies of the neuronal activity coordinating various swimming patterns in the reduced model of the CPG are considered. Both ascending and descending connections between segmental CPGs can mediate both forward and backward propagating flexion waves based on anterior or posterior bias by descending premotor commands. Bilaterally asymmetric biases of descending commands can mediate turning. The CPG outputs control 5 shape memory alloy actuators on each side to generate coordinated undulations. Two dorsal and ventral pitch actuators control the angle between the hull and undulator to control dive and climb. Descending commands are modulated by an analog compass, inclinometers, accelerometers and a short baseline sonar array to mediate homing by the vehicle on a sonar beacon. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y36.00004: Genesis and synchronization properties of fast neural oscillations Maxim Bazhenov, Nikolai Rulkov Fast neural network oscillations in gamma (30-80 Hz) range are associated with attentiveness and sensory perception and have strong relation to both cognitive processing and temporal binding of sensory stimuli. These oscillations are found in different brain systems including cerebral cortex, hippocampus and olfactory bulb. Cortical gamma oscillations may become synchronized within 1-2 msec over distances up to a few millimeters. In this study we used computational network models to analyze basic synaptic mechanisms and synchronization properties of fast neural oscillations. Using the network models of synaptically coupled pyramidal neurons (up to 500,000 cells) and fast spiking interneurons (up to 125,000 cells) we found that the strength of feedback inhibition determined the network synchronization state: either global network oscillations with near zero phase lag between remote sites or waves of gamma activity propagating through the network. Long-range excitatory connections between pyramidal cells were not required for long-range synchronization. The model predicts that local inhibitory circuits can mediate global network synchronization with phase delays being much smaller than activity propagation time between remote network sites. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y36.00005: Periodic vs. Transient Estimation of Phase Response Curves Jianxia Cui, Srisairam Achuthan, Carmen Canavier, Robert Butera Phase response curves (PRCs) for a single neuron are often used to predict the synchrony of mutually coupled neurons.~ Previous theoretical work on pulse coupled oscillators used single pulse perturbations. We propose an alternate method in which functional PRCs (FPRCs) are generated using a train of pulses applied at a fixed delay after a spike. Experimental FPRCs in \textit{Aplysia} pacemaker neurons were different from single pulse PRCs because of adaptation. Adaptation was incorporated by plotting the effective period, observed just after the pulse train is terminated, as a function of the entrained period during the pulse train. The effective intrinsic period was used iteratively in the prediction method instead of the unperturbed intrinsic period. Incorporating adaptation improved the accuracy of prediction of phase-locked modes in a model network of adapting oscillators characterized by both single pulse and multiple pulse PRCs compared to those characterized by single pulse PRCs alone. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y36.00006: Homoclinic Spike adding in a neuronal model in the presence of noise Ibiyinka Fuwape, Alexander Neiman, Andrey Shilnikov We study the influence of noise on a spike adding transitions within the bursting activity in a Hodgkin-Huxley-type model of the leech heart interneuron. Spike adding in this model occur via homoclinic bifurcation of a saddle periodic orbit. Although narrow chaotic regions are observed near bifurcation transition, overall bursting dynamics is regular and is characterized by a constant number of spikes per burst. Experimental studies, however, show variability of bursting patterns whereby number of spikes per burst varies randomly. Thus, introduction of external synaptic noise is a necessary step to account for variability of burst durations observed experimentally. We show that near every such transition the neuron is highly sensitive to random perturbations that lead to and enhance broadly the regions of chaotic dynamics of the cell. For each spike adding transition there is a critical noise level beyond which the dynamics of the neuron becomes chaotic throughout the entire region of the given transition. Noise-induced chaotic dynamics is characterized in terms of the Lyapunov exponents and the Shannon entropy and reflects variability of firing patterns with various numbers of spikes per burst, traversing wide range of the neuron's parameters [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:27PM |
Y36.00007: Adaptive Neurotechnology for Making Neural Circuits Functional . Invited Speaker: Two of the most important trends in recent technological developments are that technology is increasingly integrated with biological systems and that it is increasingly adaptive in its capabilities. Neuroprosthetic systems that provide lost sensorimotor function after a neural disability offer a platform to investigate this interplay between biological and engineered systems. Adaptive neurotechnology (hardware and software) could be designed to be biomimetic, guided by the physical and programmatic constraints observed in biological systems, and allow for real-time learning, stability, and error correction. An example will present biomimetic neural-network hardware that can be interfaced with the isolated spinal cord of a lower vertebrate to allow phase-locked real-time neural control. Another will present adaptive neural network control algorithms for functional electrical stimulation of the peripheral nervous system to provide desired movements of paralyzed limbs in rodents or people. Ultimately, the frontier lies in being able to utilize the adaptive neurotechnology to promote neuroplasticity in the living system on a long-time scale under co-adaptive conditions. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y36.00008: Investigating the Dynamics of Functional Brain Networks with MRI Shella Keilholz, Waqas Majeed Functional Magnetic Resonance Imaging (fMRI) is sensitive to changes in blood oxygenation levels. While fMRI has traditionally mapped changes in these levels that localize to brain areas activated by an external stimulus, recent work has focused on detecting correlated, non-stimulus-related fluctuations in the fMRI signal throughout the brain. These fluctuations are believed to arise from spontaneous variations in local neural activity, and so correlated fluctuations from different brain areas may indicate coordinated activity. Maps of ``functional connectivity'' based upon these fluctuations show reproducible patterns of correlated signals. To date, research has focused on steady-state networks that persist over the entire imaging session (minutes). We are exploring the possibility of detecting changes in network activity on much shorter time scales (seconds). Preliminary analysis shows that power in the frequency band used to map functional connectivity varies over time, and that power differences correspond to changes in correlation between areas. We also detected phase differences in fluctuations that are consistent with propagating waves. These results indicate that time-varying analysis of fMRI data may provide insight into the dynamics of functional networks in the brain. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y36.00009: Estimating Granger causality from Fourier and wavelet transforms of time series data Mukesh Dhamala Experiments in many fields of science and engineering yield data in the form of time series. The Fourier and wavelet transform-based nonparametric methods are used widely to study the spectral characteristics of these time series data. We have recently extended the framework of nonparametric spectral methods to include the estimation of Granger causality spectra for assessing directionalinfluences. We illustrate the utility of the proposed methods using artificial data and real brain data . [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y36.00010: Anomalous Effect of Surface Diffusion on NMR Signal: Tracing the Fiber Geometry Vadym Apalkov, Neranjan Edirisinghe, Gennady Cymbalyuk We show the strong qualitative effect of the surface diffusion channel on the echo attenuation of the NMR signal from restricted geometry, e.g. fiber system. In some range of parameters of the system the residual echo signal, which is obtained by subtracting the background value, can have anomalous behavior, which means that the echo signal has a maximum value at some finite value of the magnitude of the gradient pulses. This fact can be used to enhance the accuracy of the measurements by studying the echo signal around the maximum value. Effect described here could be also used for tuning the MRI measurements to trace fibers with particular characteristic diameters or for timely detection of changes in the diffusion coefficients and fiber diameters. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y36.00011: Application of real time systems to the analysis of neuronal dynamics Gennady Cymbalyuk, Andrey Shilnikov Neurons exhibit various activity regimes and transitions in between. The central pattern generator controlling the leech's heartbeat contains identified pairs of mutually inhibitory neurons (Calabrese et al. 1995). We describe real time systems approaches to the analysis of their activity. The hybrid system consists of a living neuron and a model neuron (or an artificial silicon neuron) interacting in the real time. Dynamic clamp is used to implement artificial ionic currents and synapses in the system (Sharp et al. 1993). Our study determines the mechanisms underlying and regulating bursting activity, based on intrinsic membrane dynamics and network interactions. The complexity of endogenous dynamics originates from the diversity of ionic currents operating on different time scales. Hybrid system analysis and slow-fast dynamical systems analysis have been combined in our studies of bursting, its origin and transformations in heart interneurons both as single cells and in the mutually inhibitory configuration. [Preview Abstract] |
Session Y37: Thermodynamic and Transport Properties of Semiconductors
Sponsoring Units: FIAPChair: Terry Alford, Arizona State University
Room: Morial Convention Center 229
Friday, March 14, 2008 11:15AM - 11:27AM |
Y37.00001: Impact ionization rate calculation based on $GW$ approximation Takao Kotani, Mark van Schilfgaarde Impact ionization (IMI) means the electron-hole pair production due to high energy electron (or hole). It can control the performance of devices related to the high-field transport process. For solar cell, it was proposed to use IMI for efficient energy conversion. The IMI rate can be identified as the lifetime of an electron (imaginary part of the self-energy). Since the electron-hole pair spectrum is included in the screened Coulomb interaction $W$, it should be important to use full $W$ for the calculation of the lifetime. However, to our knowledge, no such calculation has been presented for semiconductors until now. For example, Ref.[1] uses a model $W$. In addition, the conventional formalism of Ref.[1] contains some double-counting problem. We will show our calculation for the IMI rate based on our recently-developed quasi-particle self-consistent $GW$ method for semiconductors. Our calculation predicts a rather smaller IMI rate. [1] A. Kuligk, N. Fitzer, and R. Redmer, PRB71, 085201(2005) [2] T.Kotani, M. van Schilfgaarde, and S.V. Faleev, PRB76, 165106 (2007) [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y37.00002: First-principles calculations of mobilities in ultra-thin double-gate MOSFETs Oscar D. Restrepo, Kalman Varga, Blair Tuttle, Sokrates T. Pantelides Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO$_{2}$. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005). [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y37.00003: Composition Dependence of the Hole Mobility in Dilute GaSb$_{x}$As$_{1-x}$ Kirstin Alberi, O.D. Dubon, K.M. Yu, W. Walukiewicz, J.A. Gupta, J.-M. Baribeau Highly p-type doped GaSb$_{x}$As$_{1-x}$, long considered a promising component for III-V-based double heterojunction bipolar transistors, exhibits an unusually abrupt reduction in the hole mobility in the dilute Sb alloy composition range (x $<$ 0.2), which cannot be completely explained by conventional carrier scattering models. We show that this behavior is due to the reconfiguration of the alloy's valence band structure through an anticrossing interaction between the localized and extended $p$-states of the Sb impurities and GaAs host as described by a valence band anticrossing model [1]. Our model suggests that the drop is due to the significant upward movement of the valence band edge as well as an increase in the heavy hole effective mass that enhance the scattering processes in this composition range. K. Alberi, \textit{et al.}, \textit{Phys. Rev. B}, \textbf{75}, 045203 (2007). [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y37.00004: Enhanced Hall coefficient in InAs/AlSb $\mu $-Hall bars induced by ballistic electron scattering from interfacial impurities Goran Mihajlovic, John E. Pearson, Axel Hoffmann, Samuel D. Bader, Mark Field, Gerard J. Sullivan We fabricated micrometer-sized Hall bar channels of variable width $w$ from an InAs/AlSb quantum-well, two-dimensional electron system and studied their electrical response in perpendicular magnetic fields. For the narrowest channels ($w \quad \sim $ 1 $\mu $m) at low fields ($<$0.5 T) and 5 K, we observed that the Hall coefficient increases above its classical value. This increase persists up to temperatures of order 100 K, but its magnitude decreases with increasing channel width and disappears for $w \quad \sim $ 4 $\mu $m. At the same time, the longitudinal resistance decreases with increasing magnetic field. The strong negative magnetoresistance is present even for the widest channels, suggesting that boundary scattering is only partially responsible for its observation. We show that both results can be explained by a mechanism of large-angle scattering of ballistic electrons from non-ionized impurities residing at the InAs/AlSb interfaces. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y37.00005: Superradiance And Electron Transport Through Nanosystems Luca Celardo, Lev Kaplan Electron transport through a sequence of potential wells is investigated. Using the effective non hermitian Hamiltonian approach to open systems, the transition to a superradiance regime is shown to occur. The consequences of the superradiance transition on the conductance, including negative differential conductance, are investigated. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y37.00006: Thermal conductivity of amorphous silicon films Joseph Feldman, Xiao Liu, R. Crandall, N. Bernstein, M. Mehl, D. Papaconstantopoulos We measured the thermal conductivity of an 80 $\mu $m thick amorphous silicon film from 80K to room temperature. The amorphous silicon sample was prepared by hot-wire chemical-vapor deposition with 1 at. {\%} hydrogen, which was found previously to contain almost no atomic tunneling states that is common in amorphous solids. The value of the thermal conductivity is about a factor of two larger than previous results. To explain this unusually large thermal conductivity, we report on a Kubo theory that makes use of a tight binding electronic structure of a 1000 atom model. We include the low frequency modes that the Kubo model does not take into account because of its finite size. By considering Rayleigh and boundary scattering, and scattering of tunneling states, our theory can explain not only our result but also previous ones as well. We conclude that the large thermal conductivity of our film is attributed to the lack of scattering of the low frequency modes by the tunneling states. Therefore, low frequency modes can make significant contribution to heat transport even at near room temperature. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y37.00007: Thermoelectric properties of nanoporous Si Joo-Hyoung Lee, Jeffrey C. Grossman, John Reed, Giulia Galli Improvements in thermoelectric (TE) materials could lead to efficient solid-state energy-conversion for environmentally benign power generation and refrigeration. This realization would require a large increase to $\sim 3$ in the thermoelectric figure of merit $ZT$ at room temperature. Recent experiments have shown promise for practical applications of TE materials such as Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices and PbSeTe/PbTe quantum dot superlattices, yielding $ZT$ of $2.4$ and $1.3-1.6$, respectively. In addition, there have been recent attempts to use Si for TE pplications due to its structural simplicity and the possibility of utilizing existing Si-based manufacturing processes. In the present work, we report theoretical studies on thermoelectric properties of Si with periodically arranged nanometer-sized pores ({\it nanoporous Si}). Specifically, we calculate the electrical conductivity, Seebeck coefficient and figure of merit of nanoporous Si for a range of configurations using a combined {\it ab inito} electronic structure calculation and Boltzmann transport approach at room temperature. The results show a substantial increase in $ZT$ compared with that of bulk Si, similar to recent findings for $ZT$ in Si nanowires. Approaches for increasing $ZT$ further in this porous material will also be discussed. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y37.00008: ac conductivity in low dimensional structures: acoustic study A. Suslov, I. Drichko, I. Smirnov, A. Dyakonov, Yu. Galperin, V. Vinokur Surface acoustic waves (SAWs) were used for contactless measurements of ac conductivity in low dimensional structures. The value of complex ac conductivity was extracted from simultaneous measurements of the sound attenuation and velocity. The measurements were performed in the frequency range 17-300MHz, at temperatures down to 0.3K and in the magnetic fields up to 18T. Such measurements allowed to study, for example, mechanisms of conductivity in a dense array of SiGe quantum dots and localization of the 2D carries in GaAs/AlGaAs and Si/SiGe heterostructures in the ultraquantum limit. An extended list of coauthors will be presented during the presentation. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y37.00009: Effects of capture, escape and confinement on SAW-dragged photocurrents in a single QW Godfrey Gumbs, Danhong Huang, Michael Pepper A dual-plasma model is developed for studying the steady-state transport of SAW-dragged photocurrents of 1D confined-state carriers. This model includes the effects of the quantum confinement and the tunneling escape of SAW-dragged 1D carriers, as well as the effects of the inelastic capture of 2D continuous-state carriers and the space-charge field. The numerical results uncover a high photocurrent gain due to suppressed recombination of 1D carriers in a crossover region of the sample between an absorption strip and a surface gate. Based on a discrete model, responsivities for the SAW-dragged photocurrents in a quantum well are calculated as functions of the gate voltage, photon flux, SAW power and frequency and temperature, respectively. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y37.00010: Evidence of Real-Space Transfer in Buried-Channel Ge$_{x}$C$_{1-x}$ Devices En-Shao Liu, David Kelly, Joseph Donnelly, Emanuel Tutuc, Sanjay Banerjee We present experimental evidence of real-space transfer (RST) in buried-channel Ge$_{x}$C$_{1-x}$ p-type metal-oxide-semiconductor field effect transistors (MOSFET) containing a Si cap layer. The output characteristics of these devices reveal a negative differential resistance (NDR) below 150K, at the onset of the saturation regime. This observation indicates a charge transfer from Ge$_{x}$C$_{1-x}$ layer into the Si cap at sufficiently large drain bias values. The lower hole mobility of the Si cap with respect to the Ge$_{x}$C$_{1-x}$, translates into a drain current reduction, hence the observed NDR. Our low-field, temperature-dependent mobility measurements indeed reveal a higher effective carrier mobility in the buried-channel Ge$_{x}$C$_{1-x}$ layer with respect to a Si-reference sample, which suggests that the observed NDR is caused by RST of holes from the Ge$_{x}$C$_{1-x}$ into the Si layer. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y37.00011: Tunable Channel Interference in an Aharonov-Bohm Ring Yiping Lin, Pei-Jung Wu, Kuan Ting Lin, J. C. Chen, T. Ueda, S. Komiyama We have investigated the Aharonov-Bohm effect in a quasi one-dimensional ring on a GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure, which is defined by two metallic arc gates coupled to each branch of the ring. Each gate can be separately biased to uniformly squeeze the channel width of electrons, thereby externally tuning the transverse modes in the interference paths. The oscillatory magnetoconductance of the device is systematically studied by varying the number of channels in each path. We have observed the evidence of phase shifts in the magnetoconductance oscillations due to the suppression of the mode numbers on the ring path. Though the periodicity is not well resolved, qualitatively our data support the random phase shifts between the successive modes. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y37.00012: Energy Relaxation in the Bloch-Gruneisen Regime Probed by Weak Antilocalization (WAL) Measurements in GaN Heterostructures Hailing Cheng, Cagliyan Kurdak, Necmi Biyikli, Jinqiao Xie, Hadis Morkoc Electron-phonon (e-p) interaction was investigated in wurtzite Al$_{0.15}$Ga$_{0.85}$N/AlN/GaN and Al$_{0.83}$In$_{0.17}$N/AlN/GaN heterostructures with polarization induced two dimensional electron gases in the Bloch-Gruneisen regime. WAL and Shubnikov-de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. With gate voltage, we cover a carrier density range from 3.41$\times $10$^{12}$cm$^{-2}$ to 4.92$\times $10$^{12}$cm$^{-2}$. Moreover, we used the WAL as a thermometer to measure the electron temperature T$_{e}$ as a function of the bias current. We find the power dissipated per electron P$_{e}$ is proportional to T$_{e}^{4 }$ due to piezoelectric acoustic phonon emission by hot electrons. We calculated P$_{e}$ as a function of T$_{e}$ without using any adjustable parameters for both static and dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment the static screening case is expected to be applicable; however, our data are in better agreement with the dynamic screening case. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y37.00013: Bloch Oscillations and Saturation to a Steady-State Current of an Electron Gas in a Modulated Quantum-Wire Superlattice in a High Electric Field S.K. Lyo, D. Huang, W. Pan We present rigorous theoretical results for the time-dependent and steady-state nonlinear DC current of an electron gas in a periodically modulated one-dimensional semiconductor quantum wire in a high electric field. The theoretical model considers electron-phonon and impurity scattering microscopically in the degenerate and the nondegenerate regime in a tight-binding model. The time-dependent oscillatory and saturation (i.e., steady-state) currents are studied as a function of the field, the radius of the wire, the elastic scattering rate, the lattice period, and the temperature. The radius controls the inelastic scattering rate. The distinctive roles of elastic and inelastic scattering for the current are contrasted and examined. Finally, we compare the results with those from an exact analytic formalism based on a relaxation-time model. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y37.00014: Shot Noise in SU(N) Quantum Dot Kondo Effects Pavel Vitushinskiy, Karyn Le Hur, Aashish Clerk We study effect of interactions on transport properties of mesoscopic conductors whose low-temperature behavior corresponds to SU(N) Kondo model where cases of N=2 and N=4 describe spin-1/2 and carbon nanotube quantum dots and thus are experimentally relevant. Unlike previous studies, we find that there exist two distinct physical mechanisms via which two-particle interactions modify shot noise: scattering process with N-dependent effective charge and enhancement of coherent partition noise. We also account for possible deviation from perfect models such as asymmetry of couplings to source and drain as well as presence of residual potential scattering at low temperatures. The method we propose is not specific just to Kondo effect quantum dots and can be applied to a wide variety of different mesoscopic systems. [Preview Abstract] |
Session Y39: Other Topics in Statistical Physics
Sponsoring Units: GSNPChair: Harvey Gould, Clark University
Room: Morial Convention Center 231
Friday, March 14, 2008 11:15AM - 11:27AM |
Y39.00001: Topology and the Transition to Spatiotemporal Chaos Nicholas Ouellette, J.P. Gollub Locations in a vector field where the field magnitude vanishes have special topological significance. For the case of the velocity field of a two-dimensional incompressible fluid, these points come in two types: elliptic (in vortex cores) and hyperbolic (non-rotating stagnation points). Here, we show a novel method for identifying these special points in experimental data sets by considering the local curvature of fluid element trajectories in a thin layer of conducting fluid driven by electromagnetic forcing. By constructing a curvature field, we show that regions of locally high curvature indicate the presence of hyperbolic and elliptic points. By then tracking the motion of these points in time, we show that their dynamics shed light on the transition of the flow to a spatiotemporally chaotic state. When the driving is weak, the hyperbolic and elliptic points are pinned to locations determined by the forcing geometry; when the driving is strong, they wander over the flow domain and interact pairwise. By comparing the behavior of several base flows, we show that our methods are robust even for complex flow situations. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y39.00002: ABSTRACT HAS BEEN MOVED TO SESSION D9 |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y39.00003: The dependence of vortex shedding on the aeroelastic response of a bluff body Marcel Ilie Airflow over a vertical flat plate is investigated as a function of Reynolds number, using Large Eddy Simulation. It is generally known that the structure of the wake, behind a bluff body, exhibits very complex turbulent flow patterns. In many practical applications the bluff bodies are flexible structures and this characteristic enables them to respond to the aerodynamic loads. The fluid-structure interaction phenomenon is of critical importance due to the inheriting danger associated with the vortex induced vibrations. The periodic shedding of vortices may result in significant fluctuating loading on the body. When the shedding frequency is close to one of the characteristic frequencies of the body, the resonant oscillations of the body can be excited, causing damaging instabilities. In the present analysis, the dependence of vortex shedding on the aeroelastic response of a vertical flat plate in cross-flow is investigated. A CFD based algorithm, using Large Eddy Simulation, is developed for the investigation of a strong (two-way) aeroelastic coupling between a subsonic flow and a flexible flat plate. The results of the present analysis indicate that there is a strong fluid-structure coupling. It was observed that the aeroelastic response of the flat plate is a function of Reynolds number. Also, the aeroelastic response of the flat plate influences the vortex shedding. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y39.00004: VKS: a turbulent homogeneous dynamo with liquid sodium Michael Berhanu The magnetic field of the earth and of most astrophysical objects result from turbulent flows of electrically conducting fluids: the kinetic energy of the flow is converted into magnetic energy by dynamo effect. In September 2006 we observed this effect for the first time in a closed homogeneous turbulent flow of liquid sodium at very high Reynolds number in the Von-Karman Sodium (VKS). Despite the strong level of the fluctuations of the flow,we observed the growth and saturation of a stationary global mode of the magnetic field at the experiment's characteristic length. Does turbulence act as noise or does it participate in the magnetic generation process? If we modify the global properties of the flow, we observe transitions between different magnetic field modes, going from stationary to oscillatory, and, near the frontiers between these modes, interesting dynamical behaviours occur, such as bursts and relaxations cycles. In particular we found a state with reversals of the magnetic field similar to those of the Earth recorded on geological time scale. These evolutions present some features of low dimensional chaos, compatible with the interaction between few modes. Finally we observe for the first time bistability from a stationary dynamo to an oscillatory one. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y39.00005: Dynamical Phase Transitions and Scaling Laws in the Response of a Rhythmically Perturbed Neuron Jan Engelbrecht, Renato Mirollo In order to explore how a local rhythm influences the timing of a neuron's spikes, we study the dynamics of an integrate-and-fire model neuron with an oscillatory stimulus. The frustration due to the competition between the neuron's natural firing period and that of the oscillatory rhythm leads to a rich structure of asymptotic phase locking patterns and ordering dynamics. The phase transitions between these states can be classified as either tangent or discontinuous bifurcations, each with its own characteristic scaling laws. The discontinuous bifurcations exhibit a new kind of phase transition that may be viewed as intermediate between continuous and first order, while tangent bifurcations behave like continuous transitions with a diverging coherence scale. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y39.00006: Correlations, fluctuations and stability of a finite-size network of coupled oscillators Michael Buice, Carson Chow The incoherent state of the Kuramoto model of coupled oscillators exhibits marginal modes in mean field theory. We demonstrate that corrections due to finite size effects render these modes stable in the subcritical case, i.e. when the population is not synchronous. This demonstration is facilitated by the construction of a non-equilibrium statistical field theoretic formulation of a generic model of coupled oscillators. This theory is consistent with previous results. In the all-to-all case, the fluctuations in this theory are due completely to finite size corrections, which can be calculated in an expansion in 1/N, where N is the number of oscillators. The N $\rightarrow$ infinity limit of this theory is what is traditionally called mean field theory for the Kuramoto model. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y39.00007: Time-Shifts and Correlations in Synchronized Chaos Jonathan Blakely, Ned Corron We introduce a new method for predicting characteristics of the synchronized state achieved by a wide class of uni-directional coupling schemes. Specifically, we derive a transfer function from the coupling model that provides estimates of the correlation between the drive and response waveforms, and of the time shift (i.e., lag or anticipation) of the synchronized state. Notably, this approach does not require modeling or simulation of the full coupled system. To demonstrate the method, we compare its predictions to simulations of a variety of different coupled oscillator systems as well as to an experimental system of coupled chaotic electronic circuits. Finally, we show that the transfer function can be exploited to design novel coupling schemes that significantly improve the correlation and increase the maximum achievable time shift. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y39.00008: A linear reformulation of the Kuramoto model of self-synchronizing coupled oscillators David Roberts This talk will present a linear reformulation of the Kuramoto model describing a self-synchronizing phase transition in a system of globally coupled oscillators that in general have different intrinsic frequencies. The reformulated model provides an alternative coherent framework through which one can analytically tackle synchronization problems that are not amenable to the original Kuramoto analysis. It allows one to 1) find an analytic solution for a new class of continuum systems and 2) solve explicitly for the synchronization order parameter and the critical point of the phase-locking transition for a system with a finite number of oscillators (unlike the original Kuramoto model, which is solvable implicitly only in the mean-field limit). It also makes it possible to probe the system's dynamics as it moves towards a steady state. While this talk will cover only systems with global coupling, the new formalism introduced by the linear reformulation also lends itself to solving systems that exhibit local or asymmetric coupling. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y39.00009: Constructing almost invariant sets for multi-stable systems Lora Billings, Ira Schwartz We consider the problem of noise driven dynamical systems possessing deterministic multiple stable invariant sets. Noise typically creates a single attractor by mixing the underlying deterministic basins of attraction. We show how to approximate the distributions of the almost invariant stochastic attractors probabilistically. We employ the tools from stochastic Markov operator theory to describe the dynamical evolution. Given a stochastic kernel with a known distribution, we approximate the almost invariant sets by translating the problem into a spectral problem. We illustrate the method on a model from epidemiology in a large population. This method distinguishes two almost invariant sets, having large and small outbreaks. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y39.00010: The Critical Properties of Two-dimensional Oscillator Arrays Gabriele Migliorini We present a renormalization group study of two dimensional arrays of oscillators, with dissipative, short range interactions. We consider the case of non-identical oscillators, with distributed intrinsic frequencies within the array and study the steady-state properties of the system. In two dimensions no macroscopic mutual entrainment is found but, for identical oscillators, critical behavior of the Berezinskii-Kosterlitz-Thouless type is shown to be present. We then discuss the stability of (BKT) order in the physical case of distributed quenched random frequencies. In order to do that, we show how the steady-state dynamical properties of the two dimensional array of non-identical oscillators are related to the equilibrium properties of the XY model with quenched randomness, that has been already studied in the past. We propose a novel set of recursion relations to study this system within the Migdal Kadanoff renormalization group scheme, by mean of the discrete clock-state formulation. We compute the phase diagram in the presence of random dissipative coupling, at finite values of the clock state parameter. Possible experimental applications in two dimensional arrays of microelectromechanical oscillators are briefly suggested. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y39.00011: Correlations of Coupled Logistic Maps John Harrison, Richard Taylor, Gus Hart Many systems in the world are non-linear and therefore often chaotic. Moreover, many systems influence or are influenced by other physical systems. That is, systems are often coupled to other systems. In an effort to uncover the fundamental issues of coupled systems, we have studied a system of coupled logistic maps. The logistic map, arguably the simplest chaotic system, shows unusual correlations when coupled to a second logistic map. We use a master--slave coupling, where the first map influences the second, but not the other way around. At low coupling strengths the correlations are complex but the two maps do not completely synchronize. At higher coupling strengths, the two maps ``lock'', becoming synchronized. The value of coupling that causes the two maps to lock can be determined analytically. Intriguingly, at intermediate couplings strengths, periodic forcing by the master can result in chaotic behavior in the slave. [Preview Abstract] |
Session Y40: Bionanotechnology
Sponsoring Units: DBP FIAPChair: David Nolte, Purdue University
Room: Morial Convention Center 232
Friday, March 14, 2008 11:15AM - 11:27AM |
Y40.00001: Probing Single Molecule Capture By a Solid State Nanopore Marc Gershow, Jene Golvochenko We investigate the capture of a single molecule of DNA from solution by a solid state nanopore. We model the motion of DNA near the pore as due to a combination of electrophoretic and thermal forces. We test this model by reversing the driving voltage and recapturing individual molecules soon after they translocate through the nanopore. We find that DNA molecules are drawn to the pore by electric forces over micron scale distances, that the probability of capture decreases linearly with distance from the pore over this distance, and that, in our experimental conditions (120 mV applied voltage, $\sim $5 nm dia pore), molecules enter the pore immediately upon arrival. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y40.00002: Characterization of DNA molecules through solid-state nanopores Shanshan Wu, Venkat Balagurusamy, Xinsheng Ling We will report the latest results of our ongoing experiment using solid-state nanopores to characterize DNA molecules assisted by optical tweezers. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y40.00003: Computer simulations of polymer reversal inside a pore Lei Huang, Dmitrii E. Makarov Translocation of biopolymers through pores is implicated in many biological phenomena. Confinement within pores often breaks ergodicity on biological time scales by creating large entropic barriers to rearrangements of the chain. We study one example of such hindered rearrangement, in which the chain reverses its direction inside a long pore. Our goal is two-fold. First, we study the dependence of the polymer reversal timescale on the pore size and on the polymer length. Second, we examine the ability of simple theories, such as transition state theory (TST) and Kramers' theory to quantitatively describe a transition in a system with a complex energy landscape. We find that one-dimensional TST using the polymer extension along the pore axis as the reaction coordinate adequately accounts for the exponentially strong dependence of the reversal rate on the pore radius $r $and the polymer length $N$, while the transmission factor, i.e., the ratio of the exact rate and the TST one, has a much weaker, power law $r$ and $N$ dependence. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y40.00004: Refoldable Peptide Barrel -- Carbon Nanotube Junctions Alexey Titov, Boyang Wang, Petr Kral We design hybrid bio-nano-junctions formed by cylindrical peptide structures covalently attached to carbon nanotubes. The cylinders are composed of 5 pairs of antiparallel peptide strands that are ``one-to-one'' matched and covalently bonded through ester and amide bonds to the terminal C atoms in two (20,0) carbon nanotubes. The remaining terminal carbons in the CNTs are replaced by nitrogens, forming embedded quinoline-like structures. The used peptide strands are composed of charged amino acids that form cylindrical patterns with preferred stable configurations. By applying a torque to the nanotubes, we can reversibly fold and control the overall structure of the peptide barrels. The junctions might allow the collection and delivery of drugs and activation of biological molecules attached to them. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y40.00005: In vitro optical measurements of the interaction between human lung cells and single-wall carbon nanotubes M. L. Becker, J. A. Fagan, J. Chun, B. J. Bauer, E. K. Hobbie The intrinsic band gap fluorescence of individual semiconducting single-wall carbon nanotubes (SWNTs) stabilized with single-stranded DNA and deoxycholate surfactant is exploited to optically measure the interaction between human lung cells and length-fractionated SWNTs. Using near-infrared (NIR) fluorescence microscopy in microfluidic flow platforms, live human lung fibroblasts (IMR-90) are exposed to controlled quantities of length-sorted single wall nanotubes, and the cellular interaction and uptake of the SWNTs is optically monitored in real space-time. Cell mortality is shown to result from the uptake of shorter nanotubes and is correlated with both SWNT length and concentration. The NIR optical measurements are used to identify potential uptake mechanisms and quantify the kinetics of the interaction. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y40.00006: AFM and XPS Studies of Oligodeoxyribonucleotides associated with Single Walled Carbon Nanotubes Roya R. Lahiji, B. D. Dolash, D. Zemlyanov, D. E. Bergstrom, R. Reifenberger Oligodeoxyribonucleotides (ODN) disperse single wall carbon nanotubes (SCWNTs) in aqueous solution \textit{via} sonication.$^{...................[1]}$ By developing procedures that produce ODN:SWCNT hybrids uniformly dispersed in an aqueous solution, new biological applications will emerge. We have studied ODN T30:SWCNT hybrids that form after different preparation techniques. Deposition of the resulting ODN T30:SWCNT hybrids onto both insulating and conducting substrates have been studied. AFM under ambient conditions reveals localized features decorating individual SWCNTs having an approximate height consistent with the dimensions of single stranded T30 ODN. XPS confirmed the decorative features are ODN.$^{...........[2]}$ Taking advantage of the ODN negative charge, we studied the deposition of ODN:SWCNT hybrids on Au substrates using electrodeposition techniques. Electrodeposition has advantages since it does not require fuctionalization of ODN or the substrate prior to deposition. Applying a positive potential to the Au substrate can produce a uniform deposit of T30 ODN:SWCNT hybrids. The electrodeposited ODN:SWCNTs were further studied using AFM and XPS. .[1] M. Zheng et al, \textit{Nat Mater} \textbf{2003}, 2, 338. [2] R. R. Lahiji et al, \textit{Small} \textbf{2007}, 3, 1912. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y40.00007: Atomic Scale Force Spectroscopy of Protein Active Sites J.T. Sage, B.M. Leu, W. Sturhahn, E.E. Alp Isotope-specific nuclear resonances allow X-ray photons to monitor local forces at the atomic scale. We use nuclear resonance vibrational spectroscopy (NRVS) to investigate $^{57}$Fe embedded in protein matrices. Access to the complete spectrum of Fe vibrations allows experimental determination of effective local force constants. The\textit{ stiffness }reflects the force required to displace the probe nucleus with respect to its nearest neighbors and provides a direct probe of local structure. In contrast, the \textit{resilience} describes the force required to displace the probe atom with neighboring atoms free to respond, and determines the thermal fluctuations of the Fe. We find that additional covalent links to the protein increase the resilience of the Fe site in cytochrome c as compared to myoglobin [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y40.00008: The Directed Assembly of Conducting Polymer Nanowires Bret Flanders, Prem Thapa, Ray Baughman, Norman Barisci The Directed Electrochemical Nanowire Assembly (DENA) technique is a single-step approach to fabricating metallic nanowires and interconnecting them with external circuitry or other objects. Here we expand this technique to include the growth of non-metallic wires. From aqueous pyrrole solutions, individual wires were grown from the tip of one electrode across a 30 $\mu $m electrode gap and into contact with the tip of the other electrode. Energy dispersive spectroscopy is used to show that the wires are composed of doped polypyrrole. The conductance of these nanowires will be discussed, as well as the interfacing of DENA nanowires with biological cells for cell stimulation studies. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y40.00009: Directed Electrochemical Nanowire Assembly Prem Thapa, Bret Flanders \textit{Directed Electrochemical Nanowire Assembly} (DENA) technique is a single-step approach to fabricating metallic nanowires and interconnecting them with external circuitry. We have previously shown that these are near single crystalline metallic nanowires and that they interconnect with on-chip electrodes with very small contact resistances. Here we discuss the user-directed growth of these nanowires up to inter-electrode targets, such as biological cells. Recent results on the delivery of small voltages to these targets will be discussed, as well. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y40.00010: Electrochemical measurement of DNA in a nanofluidic channel Chih-kuan Tung, Robert Riehn, Robert H. Austin The elongation of genomic length DNA in confining nanochannels is not only a fascinating exercise in polymer dynamics, but also is of great interest in biotechnology because the elongation of the confined molecule is directly proportional to the actual length of the molecule in basepairs. We will present a way to construct nanochannels using sacrificial PMMA ebeam lithography and to measure non-immobilized DNA molecules inside such a channel electrochemically. This kind of measurements can lead us to fast and precise electronic length measurement, which will open the door to a number of important areas in genomics such as gene exchange and evolution dynamics of single cells. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y40.00011: A nanofluidic lateral Coulter counter Yong Sun, Junhan Pan, Robert Riehn Optical detection has been the mainstay of detection in many micro-and nanofluidic systems. However, the need for labeling and alignment of detectors or registration of images has slowed transition to application and increased cost. On the other hand, when traditional Coulter counters that detect along the axis of a fluidic channel are integrated with nanofluidics on a chip, leakage and limited resolution often present problems. For that reason, we are developing an electronic detection mechanism that is based on the impedance change of an electrode pair that is laterally integrated with a nanochannel. We will present detection of 50 nm polystyrene beads, and will discuss possible applications in the detection of biological molecules such as DNA. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y40.00012: On the virus capsid assembly and encapsulation of foreign materials Chao Chen, Bogdan Dragnea, Cheng Kao, Adam Zlotnick Icosahedral virus capsids are one of the simplest biological structures, yet poorly understood from a molecular physics point of view, e.g., the paradox between its stability and flexibility, its interaction with the virus genome, and its assembly thermodynamics and kinetics. In the hope of elucidating these problems, we have created a study platform based on virus-like particles (VLPs) -- inorganic nanoparticles encapsulated inside icosahedral virus capsids in place of their genomic cores. These nanoparticles were successfully incorporated when coated with carboxylate-terminated poly-ethyleneglycols, implying a dominating electro-static interaction between the virus capsid and the core. The nano-particle size determines the T number of the icosahedral cage and the efficiency of encapsulation. The current work seeks to understand the mechanism of capsid assembly process using VLP as a model system. Empty capsid assembly kinetics has been also studied for comparison. A simplified kinetically limiting model based on a previously reported master equation model is proposed. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y40.00013: Assembly and Characterization of NanoComplexes: Quantum Dot Encapsulated Liposomes Angela Hight Walker, Emren Esenturk, Peter Yim, Jeeseong Hwang Liposome complexes have received significant attention for a variety of biochemical and biomedical applications including drug targeting and drug delivery and tumor imaging and diagnostics. Semiconductor nano-crystals, also known as quantum dots, are now beginning to be used in similar biochemical experiments. Like fluorescent dyes, these quantum dots have the ability to reliably fluoresce at pre-engineered wavelengths. However, these nano-crystals have lifetimes significantly longer comparable to fluorescence dye counterparts. We have successfully encapsulated approximately 10nm CdSe nano-crystals inside approximately 100nm liposomes and studied the resulting complex using fluorescence resonance energy transfer (FRET) microscopy. Further studies were performed using transmission electron microscopy (TEM) showing the details of the encapsulation, and Raman spectroscopy to examine their structural details. Our nano-manufactured quantum dot liposome complexes do not bleach over periods of hours and are general enough to allow the addition of drugs targeted for the vectored cells thus offering the ability to both image and medicate simultaneously over a long period of time. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y40.00014: Selective Cell Growth on Fibronectin-Carbon Nanotube Hybrid Nanostructures Seon Namgung, Sung Young Park, Byung Yang Lee, Minbaek Lee, Jwa-Min Nam, Seunghun Hong Carbon nanotubes (CNT) have been considered a promising material for biological applications including biosensors, therapeutic application, and nano-structured scaffolds. However, there are still controversies associated with toxicity and biocompatibility of CNTs on live cells. Here, we report general strategy to functionalize CNTs with cell adhesion molecules (fibronectins) for selective and stable adhesion of cells on CNTs. Interestingly, more fibronectins were adsorbed and activated on CNTs rather than on hydrophobic self assembled monolayers (SAMs) or bare substrates (SiO$_{2})$. We demonstrate the functionality of fibronectins on CNTs with immunofluorescence and molecule-level force measurement study using atomic force microscopy (AFM). These fibronectin-CNT hybrid nanostructures were successfully applied to attract cells selectively onto predefined regions on the substrate. Our strategy was generally available on various cell types including mesenchymal stem cells, KB cells, and NIH3T3 fibroblast cells (\textit{Advanced Materials} \textbf{19,} 2530-2534 (2007)). We will also discuss about its impacts on cell biology combined with CNTs. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y40.00015: Large-Scale `Linker-Free Assembly' of swCNT-Based Biosensor Arrays Donghee Sohn, Byung Yang Lee, Seunghun Hong Biosensors based on single-walled carbon nanotubes (swCNTs) have received a great deal of attention due to their potential applications such as genotyping, disease diagnosis, food analysis, etc. However, a lack of reliable mass-production method for such swCNT-based biosensor has been holding back their practical applications. One promising mass-production method for swCNT-based biosensor arrays can be \textit{`linker-free assembly'} process (Nature Nanotechnology 1, 66 (2006)), where non-polar patterns guide the \textit{`selective assembly'} and \textit{`precision alignment'} of carbon nanotubes on bare substrates without using any external forces such as liquid flow, etc. We used this method to fabricate large-scale assembly of swCNT-based integrated devices on virtually general substrates including SiO$_{2}$, Si, Al, Au, etc. To utilize swCNT devices for biosensors, we functionalized swCNT devices on SiO$_{2}$ with receptor biomolecules such as enzyme L-glutamate oxidase or biotin. And then, we could detect the target biomolecules (L-glutamate or streptavidin, respectively) with high sensitivity and selectivity by monitoring the conductance change of swCNT junctions in aqueous environment. These studies provide biological implications on neurotransmitters and proteins onto swCNT patterned surface. [Preview Abstract] |
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