Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y1: Superconductivity and Quantum Transport in Nanowires
Sponsoring Units: DCMPChair: Alexei Berzryadi, University of Illinois at Urbana-Champaign
Room: Spirit of Pittsburgh Ballroom A
Friday, March 20, 2009 8:00AM - 8:36AM |
Y1.00001: Mapping out the Superconductor-Insulator Phase Diagram for Nanowires. Invited Speaker: We establish the superconductor-insulator phase diagram for quasi-one-dimensional wires by measuring about 100 MoGe nanowires with length in a range 30-500 nm. All wires can be clearly separated into two groups: superconducting ones with the wire resistance dropping rapidly with cooling, roughly following the Arrhenius activation law, and insulating wires, which exhibit a weak Coulomb blockade behavior. The phase boundary between superconducting and insulating wires is consistent with the Chakravarty-Schmid-Bulgadaev criteria, namely with the critical resistance of a wire being equal to quantum resistance, i.e. 6.5 kOhms. We argue that small deviations from this phase boundary in short thin wires are caused by magnetic moments that forms on a wire surface. The evidence for the presence of the moments comes from an anomalous enhancement of the critical current by magnetic field detected at low temperatures. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y1.00002: Statistics of superconductive-resistive switching in nanowires: An effective probe for resolving phase-slip events Invited Speaker: Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states and impart a non-zero resistance to superconducting nanowires. They play a fundamental role in determining the fate of superconductivity in nanowires. Conversely, superconducting nanowires provide an ideal setting for accessing non-trivial fluctuations driven by thermal activation and---at low temperatures---by quantum tunneling of a one-dimensional field. However, this potential has not been fully realized because resistance measurements, on the one hand, are capable of capturing only the averaged phase-slip behavior, and on the other hand, are incapable of pinning down the low temperature phase-slip behavior, as the measured resistance values drop below the noise floor. On going beyond the linear-response regime, the I-V characteristics show a hysteretic behavior. As the current is ramped up repeatedly, the state switches from a superconductive to a resistive one, doing so at somewhat random current values below the depairing critical current. The distribution of these switching currents was studied recently [1]. In this talk, I will report on the rather counter-intuitive temperature dependence of the distribution and its theoretical understanding via a stochastic model developed in Ref [2]. I will show that although, in general, several phase-slip events are necessary to induce switching, there is an experimentally accessible temperature- and current-range for which a single phase-slip event is sufficient to switch the wire to the normal (resistive) state. I will conclude by arguing that switching-current statistics provide an effective probe to resolve individual phase-slip events and in addition offer unprecedented access to quantum phase-slip tunneling events. \\[4pt] [1] M. Sahu, M.-H. Bae, A. Rogachev, D. Pekker, T.-C. Wei, N. Shah, P. M. Goldbart, and A. Bezryadin, arXiv:0804.2251\\[0pt] [2] N. Shah, D. Pekker, and P. M. Goldbart, Phys. Rev. Lett. 101, 207001 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y1.00003: Theory of the pairbreaking superconductor-metal transition in nanowires Invited Speaker: We present a detailed description of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism. The dissipative critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex components of the superconducting order parameter (one in the physical case). The fluctuation corrections to the electrical ($\sigma$) and thermal ($\kappa$) conductivities are determined, and we find that $\sigma$ has a non-monotonic temperature dependence in the metallic phase which may be consistent with recent experimental results on ultra-narrow wires. In the quantum critical regime, the ratio of the thermal to electrical conductivity displays a linear temperature dependence and thus the Wiedemann-Franz law is obeyed, with a new universal experimentally verifiable Lorenz number. We also examined the influence of quenched disorder on the superconductor-metal transition. The self-consistent pairing eigenmodes of a quasi-one dimensional wire were determined numerically. Our results support the proposal by Hoyos {\em et al.\/} (Phys. Rev. Lett. {\bf 99}, 230601 (2007)) that the transition is described by the same strong disorder fixed point describing the onset of ferromagnetism in the quantum Ising model in a transverse field. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y1.00004: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y1.00005: Thermally activated phase slips in superconducting nanowires Invited Speaker: We reanalyze the problem of thermally activated phase slips which can dominate the behavior of sufficiently thin superconducting wires at temperatures close to T$_c$. With the aid of an effective action approach we evaluate the TAPS rate which turns out to exceed the rate found by McCumber and Halperin, Phys. Rev. B 1, 1054 (1970) within the time-dependent Ginzburg-Landau analysis by the factor 1/(1-T/TC). Additional differences in the results of these two approaches arise at bias currents close to the Ginzburg-Landau critical current where the TAPS rate becomes bigger. We also derive a simple formula for the voltage noise across the superconducting wire in terms of the TAPS rate. Our results can be verified in modern experiments with superconducting nanowires. [Preview Abstract] |
Session Y2: Advances in Optical Microscopy
Sponsoring Units: DCMPChair: David Weitz, Harvard University
Room: Spirit of Pittsburgh Ballroom BC
Friday, March 20, 2009 8:00AM - 8:36AM |
Y2.00001: Structured illumination microscopy (SIM) Invited Speaker: |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y2.00002: Differential Dynamic Microscopy: a simple means to measure dynamics with a microscope Invited Speaker: Optical microscopy is an excellent tool to investigate the structure and dynamics of soft and biological materials. In this contribution we present a novel scheme to measure the dynamics of a system using an ordinary microscope [1]. This scheme is based on the spatial Fourier analysis of a time series of microscopy images, which enables us to study the relaxation of the intensity Fourier components at different spatial frequencies. This quantifies the dynamic activity of the system at different wave-vectors, giving access to information similar to the one obtained in dynamic light scattering experiments. Our technique termed Differential Dynamic Microscopy (DDM) is capable of monitoring the dynamics of both objects that are larger and smaller than the wavelength of light. The remarkable simplicity of DDM makes it suitable for the use in any laboratory that is equipped with an ordinary microscope.\\[4pt] [1] Roberto Cerbino and Veronique Trappe, ``Differential Dynamic Microscopy: Probing Wave Vector Dependent Dynamics with a Microscope,'' Phys. Rev. Lett. 100, 188102 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y2.00003: Holographic video microscopy Invited Speaker: |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y2.00004: New techniques for fluorescence background rejection in microscopy and endoscopy Invited Speaker: Confocal microscopy is a popular technique in the bioimaging community, mainly because it provides optical sectioning. However, its standard implementation requires 3-dimensional scanning of focused illumination throughout the sample. Efficient non-scanning alternatives have been implemented, among which the simple and well-established incoherent structured illumination microscopy (SIM) [1]. We recently proposed a similar technique, called Dynamic Speckle Illumination (DSI) microscopy, wherein the incoherent grid illumination pattern is replaced with a coherent speckle illumination pattern from a laser, taking advantage of the fact that speckle contrast is highly maintained in a scattering media, making the technique well adapted to tissue imaging [2]. DSI microscopy relies on the illumination of a sample with a sequence of dynamic speckle patterns and an image processing algorithm based only on an a priori knowledge of speckle statistics. The choice of this post-processing algorithm is crucial to obtain a good sectioning strength: in particular, we developed a novel post-processing algorithm based one wavelet pre-filtering of the raw images and obtained near-confocal fluorescence sectioning in a mouse brain labeled with GFP, with a good image quality maintained throughout a depth of $\sim $100 $\mu $m [3]. In the purpose of imaging fluorescent tissue at higher depth, we recently applied structured illumination to endoscopy. We used a similar set-up wherein the illumination pattern (a one-dimensional grid) is transported to the sample with an imaging fiber bundle with miniaturized objective and the fluorescence image is collected through the same bundle. Using a post-processing algorithm similar to the one previously described [3], we obtained high-quality images of a fluorescein-labeled rat colonic mucosa [4], establishing the potential of our endomicroscope for bioimaging applications. \\[4pt] \underline {Ref:} \\[0pt] [1] M. A. A. Neil \textit{et al}, Opt. Lett. 22, 1905 (1997) \\[0pt] [2] C. Ventalon \textit{et al}, Opt. Lett. \textbf{30}, 3350 (2005) \\[0pt] [3] C. Ventalon \textit{et al}, Opt. Lett. \textbf{32,} 1417 (2007) \\[0pt] [4] N. Bozinovic \textit{et al}, Opt. Express \textbf{16}, 8016 (2008) [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y2.00005: Photoactivated localization microscopy combined with single particle tracking Invited Speaker: |
Session Y3: Multi-valley Electron Systems in the Quantum Limit
Sponsoring Units: DCMPChair: Wei Pan, Sandia National Laboratories
Room: 301/302
Friday, March 20, 2009 8:00AM - 8:36AM |
Y3.00001: Phase Transitions of Dirac Electrons in Bismuth Invited Speaker: The Fermi Surface (FS) in elemental bismuth consists of 3 electron ellipsoids and one hole ellipsoid [1]. The accidental coincidence of the hole and electron caliper areas when the field $\bf H$ is aligned with the trigonal axis $\bf Z$ has long stymied analyses of the quantum oscillations. Because of current strong interest in how electrons with Dirac dispersion behave in intense fields, we have renewed attack on this problem [2] using high-resolution torque magnetometry in fields up to 31 T and at temperatures $T$ down to 300 mK. When $\bf H$ is tilted with respect to $\bf Z$ by a slight angle $\theta$, the torque $\vec{\tau}$ on the sample derived from the 3 electron ellipsoids dominates the torque from the hole FS, allowing the Landau Level crossings of the Dirac electron to be resolved. By measuring the curves of $\vec{\tau}$ vs. $H$ at 19 values of $\theta$ straddling the trigonal axes, we completely resolve the Landau Levels of the Dirac electrons. A new result is the detection of jumps in the transverse magnetization when $H$ exceeds the quantum limit of the electron pockets. By tracing the jumps in the plane of $H$ vs. $\theta$, we uncover a region in which the Dirac electrons enter a new ground state. Within this cone-shaped region, Landau Level anomalies are severely suppressed. We interpret the state as one in which the 3-fold valley degeneracy of the Dirac gas is lifted to form a many-body state. The unusual nature of the magnetization within this region will be described. \\[4pt] [1] M. H. Cohen and E. I. Blount, Phil. Mag. {\bf 5}, 115 (1960). \\[0pt] [2] Lu Li {\it et al.}, Science {\bf 321}, 547 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y3.00002: Hall Plateaus at magic angles in ultraquantum Bismuth Invited Speaker: The behaviour of a three-dimensional electron gas in the presence of a magnetic field strong enough to put all carriers in the first Landau level (i.e. beyond the quantum limit) is a longstanding question of theoretical condensed matter physics [1]. This issue has been recently explored by two high-field experiments on elemental semi-metal Bismuth. In a first study of transport coefficients (which are dominated by hole-like carriers), the Nernst coefficient presented three unexpected maxima that are concomitant with quasi-plateaux in the Hall coefficient [2]. In a second series of experiments, torque magnetometry (which mainly probes the three Dirac valley electron pockets) detected a field-induced phase transition [3]. The full understanding of the electron and hole behaviours above the quantum limit of pure Bi is therefore still under debate. In this talk, we will present our measurement of the Hall resistivity and torque magnetometry with magnetic field up to 31 T and rotating in the trigonal-bisectrix plane [4]. The Hall response is dominated by the hole pockets according to its sign as well as the period and the angular dependence of its quantum oscillations. In the vicinity of the quantum limit, it presents additional anomalies which are the fingerprints of the electron pockets. We found that for particular orientations of the magnetic field (namely ``magic angles''), the Hall response becomes field-independent within the experimental resolution around 20T. This drastic dependence of the plateaux on the field orientation provides strong constraints for theoretical scenarios. \\[4pt] [1] Bertrand I. Halperin, \emph{Japanese Journal of Applied Physics}, {\bf 26}, Supplement 26-3 (1987).\\[0pt] [2] Kamran Behnia, Luis Balicas, Yakov Kopelevich, \emph{Science}, {\bf 317}, 1729 (2008).\\[0pt] [3] Lu Li, J. G. Checkelsky, Y. S. Hor, C. Uher, A. F. Hebard, R. J. Cava, and N. P. Ong , \emph{Science}, {\bf 321}, 5888 (2008).\\[0pt] [4] Beno\^it Fauqu\'e, Luis Balicas, Ilya Sheikin, Jean Paul Issi and Kamran Behnia, to be published [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y3.00003: Bismuth and graphite in the ultraquantum limit: signatures of fractional quantum Hall effect Invited Speaker: Bismuth and graphite are semimetals that possess both conventional massive and Dirac-like quasiparticle spectra. High quality graphite is a multi-layer system with nearly decoupled two-dimensional (2D) graphene planes, in which the integer quantum Hall effect has already been found [1]. On the other hand, the fractional quantum Hall effect (FQHE) has been observed for 3D bismuth in the ultraquantum limit (UCL), i. e. above the field that pulls all carriers into the lowest Landau level [2]. Recent measurements performed on quasi-2D graphite in magnetic field up to B = 57 T revealed well defined plateaus in the Hall resistance for B $>$ 10 T, suggesting also the FQHE occurrence in graphite in the UCL [3]. A striking similarity of the obtained results with the FQHE measured for 2D electron system in a GaAs/AlGaAs quantum well [4] is found. Our present results indicate the interplay between FQHE and charge density wave states in graphite. We discuss the FQHE occurrence in bismuth and graphite within the framework of available theoretical models. \\[4pt] [1] Y. Kopelevich, J. H. S. Torres, R. R. da Silva et al., Phys. Rev. Lett. 90,156402 (2003). \\[0pt] [2] [K. Behnia, L. Balicas, and Y. Kopelevich, Science 317, 1729 (2007). \\[0pt] [3] Y. Kopelevich, B. Raquet, M. Goiran et al. (unpublished). \\[0pt] [4] W. Pan, H. L. Stormer, D. C. Tsui et al., Phys. Rev. Lett. 88, 176802 (2002). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y3.00004: AlAs 2D Electrons at High Magnetic Field: The Role of Spin and Valley Degree of Freedom Invited Speaker: Two-dimensional (2D) electrons in AlAs quantum wells occupy multiple conduction-band minima (or valleys) at the X point of the Brillouin zone. These valleys have large effective mass (m*) and g-factor compared to the standard GaAs electrons, and are also highly anisotropic. The system is rather unique in that, with proper choice of well width and by applying in situ symmetry-breaking strain in the plane, one can control the occupation of different valleys, thus rendering a system with tuneable m*, g-factor, Fermi contour anisotropy, and with single, double, or triple valley degeneracy. By adding a magnetic field, we obtain a system which allows us to explore very rich physics ensuing from the valley and spin degrees of freedom in a strongly interacting system. In this presentation, I will highlight some of our latest results on 2D electrons confined to wide AlAs quantum wells where the electrons reside in two in-plane valleys whose occupation can be controlled via the application of strain. I will present the results of our m* measurements, via analyzing the temperature dependence of the Shubnikov-de Haas oscillations. The measured m* shows a strong dependence on the occupation of valley and spin subbands, reflecting the electron- electron interaction in this system. Most remarkably, m* is suppressed with respect the band value when the 2D electrons are fully spin- and valley-polarized. I will also discuss the relation of m* suppression to the 2D metal-insulator transition problem. Our studies also include measurements of the valley susceptibility (dependence of valley population on applied strain) and the valley polarization of the fractional quantum Hall effect composite fermions. While part of our observations can be explained well by a simple Landau level fan diagram for composite fermions with a valley degree of freedom, there are some surprises. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y3.00005: High Mobility Sixfold Valley Degenerate Electrons on Silicon [111] Surfaces Invited Speaker: The 111 surface of silicon is predicted to retain the sixfold valley degeneracy of the ideal bulk crystal. We have developed a method for fabricating field effect transistors using vacuum as a dielectric in order to study electron transport on the bare hydrogen-terminated surface, free from the complications created by intrinsic disorder at Si-Si$O_2$ interfaces. The resulting devices display very high mobilities (up to 110,000 $cm^2/Vs$ at 70mK, more than twice as large as the best silicon MOSFETs), enabling us to probe valley-dependent transport dynamics to a much greater degree than previously possible. Measurements made on a recent device over a density range of $n_s=0.7-7\times10^{11}/cm^2$ reveal considerable information about the nature of this degeneracy and its role in 2D transport. In particular, we find (at $n_s$=6.7) that 1) low field Shubnikov-de Haas oscillations reveal a clearly sixfold degenerate system and allow us to establish an upper bound on the valley splitting of 0.2K 2) longitudinal resistivity at B=0 displays a strong temperature dependence, consistent with predictions that large valley degeneracy should enhance screening[1] and 3) the Hall coefficient near B=0 is modified by the presence of multiple valleys, and we can use this correction to measure the intervalley Coulomb drag and its temperature dependence. [1] E. H. Hwang and S. Das Sarma. PRB 75, 073301 (2007) [Preview Abstract] |
Session Y4: New insight into Exchange Bias from Advanced Scattering Techniques
Sponsoring Units: GMAGChair: Julie Borchers, National Institute of Standards and Technology
Room: 306/307
Friday, March 20, 2009 8:00AM - 8:36AM |
Y4.00001: Neutron and X-Ray Scattering Studies of the Exchange Bias Problem Invited Speaker: Exchange Bias, i.e. the shift of the hysteresis loop of a ferromagnet in juxtaposition to an antiferromagnet, is a phenomenon that has been known for fifty years, and has already been put to wide commercial use in devices such as magnetic read-heads and other devices. Nevertheless a detailed understanding of the effect has proved to be fairly controversial, notwithstanding much research on this problem over the years, and the development of several alternative theoretical models. This is partly due to the necessity of understanding the details of the interactions and the magnetic structure across and in the vicinity of the interface between the ferromagnet and the antiferromagnet. The details of how interface roughness and other defects affect exchange bias and the details of how magnetic domains are established on both sides of the interface are still not well understood. Non-destructive probing of such buried interfaces is conveniently accomplished with neutron scattering or synchrotron X-ray techniques such as X-Ray Magnetic Circular Dichroism, Photoemission imaging or Resonant Magnetic X-ray Scattering, and these types of experiments have been increasingly employed over the last decade. We shall attempt to discuss what has been learned from such experiments and what crucial issues remain unresolved, with particular emphasis on recent studies of the Co/FeF$_2$ and Permalloy/CoO exchange bias bilayer systems. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y4.00002: Measuring interfacial magnetic configurations with Polarized Neutron Reflectometry Invited Speaker: Polarized neutron reflectivity (PNR) is ideally suited for imaging both vertical structural and magnetic variations in the complex magnetic multilayers [1]. During the talk will be described particularly how this technique allows obtaining the magnetic depth-profile of exchange-coupled bilayer. For instance, Gd40Fe60/ Tb12Fe88 is a model system to study exchange-bias phenomena origin in anti-ferromagnetically coupled AF/FM system, like FeF2/Fe. In these systems, unusual properties are observed such as a transition from positive to negative exchange bias field HE as the cooling field Hcf is swept from small to large positive value [2]. Combining complementary techniques that are macroscopic magnetization measurements and PNR, we have demonstrated that the above properties, e.g. the cooling field dependence of HE, come from an interfacial domain wall (iDW) frozen in the TbFe as the sample is cooled down under a field [3, 4]. Moreover, PNR measurements have recently revealed that the frozen iDW is metastable and that the exchange bias training effect in TbFe/GdFe results from the thermally assisted relaxation of the iDW, with field cycling [4, 5]. Overall, PNR studies concerning the TbFe/GdFe have brought strong insights into the exchange bias mechanisms in exchange coupled hard/soft systems with in-plane anisotropy. However we have demonstrated as well that this powerful technique can be applied to systems with perpendicular magnetic anisotropy (PMA). Although, in that case, the perpendicular moments are parallel to the scattering vector and do not give rise to scattering via the neutron selection rules, we have used a unconventional geometry to obtain a depth-dependent magnetic profile of a PMA exchange-coupled system. Specifically, we have characterized antiferromagnetically-coupled, TbFeCo/[Co/Pd] multilayers [6]. \\[4pt] [1] K.V. O'Donovan et al., Phys. Rev. Lett. 88, 067201 (2002). \\[0pt] [2] J. Nogues and al. Phys. Rev. Lett. 76, 4624 (1996) \\[0pt] [3] Y. Henry et al., Phys. Rev. B 73, 134420 (2006) \\[0pt] [4] T. Hauet et al., Phys. Rev. Lett. 96, 067207 (2006) \\[0pt] [5] T. Hauet et al., Appl. Phys. Lett. 91, 022505 (2007) \\[0pt] [6] S. Watson et al., Appl. Phys. Lett. 92, 202507 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y4.00003: Inelastic neutron scattering studies of exchange biased core-shell nanoparticles Invited Speaker: Inelastic neutron scattering (INS) measurements of nanoparticle systems are very few, and we report here the first investigation of exchange biased core-shell nanoparticles. We present a study of spin dynamics in core-shell Co/CoO nanoparticles, which display an exchange bias field of 0.6T. We have used INS measurements to determine how the magnetic dynamics are affected, both by the onset of antiferromagnetic (AF) order at 250K and the subsequent onset of the exchange bias effect at 200K. At the highest temperatures, the scattering consists of two quasielastic peaks. The narrower peak is ascribed to superparamagnetic reorientations of the Co core. The broader peak originates with moments in the CoO shell. Surprisingly, their dynamics speed up with decreased temperature, suggesting that the CoO shell absorbs some of the magnetic energy of the core as exchange blocking is approached. Below 200K, the scattering is dominated by an inelastic peak at $\sim $3meV. The integrated spin wave intensity grows when the temperature is reduced below 200K, reaches a maximum near 150K, and nearly vanishes at the low temperatures. We attribute this peak to AF spin waves in the CoO shell, and their lack of dispersion and overall energy scale are consistent with predictions for low energy spin waves in bulk CoO [3,4]. It is remarkable that bulk-like spin wave behavior is observed in the CoO shell, which is only 4 nm thick. \\[3pt] References \\[0pt] [1] M. Feygenson et al., in preparation \\[0pt] [2] Inderhees et al., Phys. Rev. Lett., 101, 117202 (2008) \\[0pt] [3] Daniel et al., Phys. Rev. 177, 932 (1969) \\[0pt] [4] Sakurai et al., Phys. Rev. 167, 510 (1968) [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y4.00004: Exchange bias in core/shell magnetic nanoparticles: experimental results and numerical simulations Invited Speaker: In this talk, we will review some of the main experimental observations related to the occurrence of exchange bias in magnetic systems, focusing the attention on the phenomenology appearing in nanoparticles with core/shell structure as compared to thin film bilayers [1]. The main open questions posed by the experimental observations will be discussed and contrasted to existing theories and models for exchange bias [1]. We will also present some recent numerical simulations [2-4] based on a simple model of a core/shell nanoparticle, showing evidence that the magnetic order of interfacial spins accounts for most of the experimental observations. Finally, we will discuss the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in amorphous zirconia matrix [5]. The deposition method allows controlling the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber. The nature of the nanoparticles embedded in the nonmagnetic matrix may be monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoO$_{x}$, with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. This is a model system to study some of the features of exchange bias in nanoparticles, such as particle size dependence, induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and blocking of the AFM clusters due to proximity to the FM phase. \\[4pt] [1] For a recent review see, for example, ``Exchange bias phenomenology and models of core/shell nanoparticles''; Iglesias, O.; Labarta, A.; and Batlle, X. Journal of Nanoscience and Nanotechnology \textbf{8}, 2761 (2008). \\[0pt] [2] ``Microscopic origin of exchange bias in core/shell nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A.; Physical Review B\textbf{ 72}, 212401 (2005). \\[0pt] [3] ``Modelling exchange bias in core/shell nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A., Journal of Physics-Condensed Matter \textbf{19}, 406232 (2007). \\[0pt] [4] ``Particle size and cooling field dependence of exchange bias in core/shell magnetic nanoparticles''; Iglesias, O.; Batlle, X.; Labarta, A.; Journal of Physics D: Applied Physics \textbf{41}, 134010 (2008). \\[0pt] [5] ``Exchange coupling in Co-CoO$_{x}$ nanoparticles in zirconia matrix''; M. Kovylina, M. Garcia del Muro, Z. Konstantinovic, O. Iglesias, M. Varela, A. Labarta and X. Batlle (submitted). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y4.00005: Small-Angle Neutron Scattering Studies of Magnetic Correlation Lengths in Nanoparticle Assemblies Invited Speaker: Small-angle neutron scattering (SANS) measurements of ordered arrays of surfactant-coated magnetic nanoparticle reveal characteristic length scales associated with interparticle and intraparticle magnetic ordering. The high degree of uniformity in the monodisperse nanoparticle size and spacing leads to a pronounced diffraction peak and allows for a straightforward determination of these length scales [1]. There are notable differences in these length scales depending on the particle moment, which depends on the material (Fe, Co, Fe3O4) and diameter, and also on whether the metal particle core is surrounded by an oxide shell. For 8.5 nm particles containing an Fe core and thick Fe3O4 shell, evidence of a spin flop phase is seen in the magnetite shell when a field is applied , but not when the shell thickness is $\sim $0.5 nm [2]. 8.0 nm particles with an e-Co core and 0.75 nm CoO shell show no exchange bias effects while similar particles with a 2 nm thick shell so significant training effects below 90 K. Polarized SANS studied of 7 nm Fe3O4 nanoparticle assemblies show the ability to resolve the magnetization components in 3D. \\[4pt] [1] M. Sachan, C. Bonnoit, S. A. Majetich, Y. Ijiri, P. O. Mensah-Bonsu, J. A. Borchers, and J. J. Rhyne, \textit{Appl. Phys. Lett. }\textbf{92}, 152503 (2008). \\[0pt] [2] Yumi Ijiri, Christopher V. Kelly, Julie A. Borchers, James J. Rhyne, Dorothy F. Farrell, Sara A. Majetich, \textit{Appl. Phys. Lett.} \textbf{86}, 243102-243104 (2005). \\[0pt] [3] K. L. Krycka, R. Booth, J. A. Borchers, W. C. Chen, C. Conlon, T. Gentile, C. Hogg, Y. Ijiri, M. Laver, B. B. Maranville, S. A. Majetich, J. Rhyne, and S. M. Watson, Physica B (submitted). [Preview Abstract] |
Session Y5: Self-Organization in Biological Cells and Tissues II
Sponsoring Units: DBPChair: Ray Goldstein, University of Cambridge
Room: 401/402
Friday, March 20, 2009 8:00AM - 8:36AM |
Y5.00001: Cells anticipate periodic events Invited Speaker: We show that an amoeboid organism can anticipate the timing of periodic events. The plasmodium of the true slime mold \textit{Physarum polycephalum} moves rapidly under favourable conditions, but stops moving when transferred to less-favourable conditions. Plasmodia exposed to unfavourable conditions, presented in three consecutive pulses at constant intervals, reduced their locomotive speed in response to each episode. When subsequently subjected to favourable conditions, the plasmodia spontaneously reduced their locomotive speed at the time point when the next unfavourable episode would have occurred. This implied anticipation of impending environmental change. After this behaviour had been evoked several times, the locomotion of the plasmodia returned to normal; however, the anticipatory response could subsequently be induced by a single unfavourable pulse, implying recall of the memorized periodicity. We explored the mechanisms underlying these behaviours from a dynamical systems perspective. Our results hint at the cellular origins of primitive intelligence and imply that simple dynamics might be sufficient to explain its emergence. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y5.00002: The Physics of Cardiac Fibrillation: Strings that kill Invited Speaker: Fibrillation is a state of spatio-temporal chaos in a 3d-biological excitable medium, namely the heart muscle. The building blocks are wave-emitting three-dimensional topological singularities in the electric excitation field of the tissue. These string like singularities send out a rotating wave fields with very fast frequencies (up to 10 times normal heart rate) and thus dominate over the pacemaker. The incoherent electrical excitation of the spatio-temporal chaotic dynamics leads to an unsynchronized contraction of the cardiac muscle and to the loss of the pumping action, and if untreated to death. Due to the topological nature of the spatio-temporal chaotic state it is very difficult to control. Current defibrillation technologies use strong electric field pulses (1 kV, 30 A, 12 ms) to reset the whole muscle. Here we report that natural muscle heterogeneities act as wave emitting sites when a weak electric field pulse is applied across the tissue. We report theoretical predictions on the physics and support the findings by results from experiment. This work was conducted in collaboration with Stefan Luther (MPIDS), Falvio Fenton ( Cornell), Amgad Squires (Cornell), Robert Gilmour (Cornell), Valentin Krinsky (MPIDS), Alain Pumir (NIce). [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y5.00003: Spatiotemporal patterns of voltage and calcium signaling in heart cells and tissue Invited Speaker: This talk will describe recent progress made in understanding oscillatory patterns of voltage and calcium signals that precede the onset of electromechanical wave turbulence in the main chambers of the heart. Results will illustrate how both physiologically detailed and abstract models have proven useful to cope with the bewildering molecular complexity of cardiac biology and to bridge phenomena on cellular and tissue scales. A main conclusion is that those oscillatory patterns can be self-organized, resulting from symmetry-breaking linear instabilities, or/and a manifestation of underling tissue heterogeneities. Thus studying the evolution of those patterns provides a valuable indirect probe of complex physiological processes that render the heart susceptible to the sudden onset of lethal heart rhythm disorders. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y5.00004: Synchronization of Eukaryotic Flagella and the Evolution of Multicellularity Invited Speaker: Flagella, among the most highly conserved structures in eukaryotes, are responsible for such tasks as fluid transport, motility and phototaxis, establishment of embryonic left-right asymmetry, and intercellular communication, and are thought to have played a key role in the development of multicellularity. These tasks are usually performed by the coordinated action of groups of flagella (from pairs to thousands), which display various types of spatio-temporal organization. The origin and quantitative characterization of flagellar synchronization has remained an important open problem, involving interplay between intracellular biochemistry and interflagellar mechanical/hydrodynamic coupling. The Volvocine green algae serve as useful model organisms for the study of these phenomena, as they form a lineage spanning from unicellular {\it Chlamydomonas} to germ-soma differentiated {\it Volvox}, having as many as 50,000 biflagellated surface somatic cells. In this talk I will describe extensive studies [1], using micromanipulation and high-speed imaging, of the flagellar synchronization of two key species - {\it Chlamydomonas reinhardtii} and {\it Volvox carteri} - over tens of thousands of cycles. With {\it Chlamydomonas} we find that the flagellar dynamics moves back and forth between a stochastic synchronized state consistent with a simple model of hydrodynamically coupled noisy oscillators, and a deterministic one driven by a large interflagellar frequency difference. These results reconcile previously contradictory studies, based on short observations, showing only one or the other of these two states, and, more importantly, show that the flagellar beat frequencies themselves are regulated by the cell. Moreover, high-resolution three-dimensional tracking of swimming cells provides strong evidence that these dynamical states are related to reorientation events in the trajectories, yielding a eukaryotic equivalent of the ``run and tumble'' motion of peritrichously flagellated bacteria. The degree of synchronization is found to depend upon the presence of external fluid flow, an important aspect of the dynamics in the context of evolutionary transitions to multicellularity. Comparison is made with dynamics of somatic cells of {\it Volvox}, which we have found can display metachronal waves, not previously reported in this organism. Implications of these findings for phototactic steering are also discussed. \vskip 0.2cm [1] M.Polin, I. Tuval, K. Drescher, J.P. Gollub, and R.E. Goldstein, submitted (2009). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y5.00005: Single cell motility and trail formation in populations of microglia Invited Speaker: Microglia are a special type of glia cell in brain that has immune responses. They constitute about 20 \% of the total glia population within the brain. Compared to other glia cells, microglia are very motile, constantly moving to destroy pathogens and to remove dead neurons. While doing so, they exhibit interesting body shapes, have cell-to-cell communications, and have chemotatic responses to each other. Interestingly, our recent in vitro studies show that their unusual motile behaviors can self-organize to form trails, similar to those in populations of ants. We have studied the changes in the physical properties of these trails by varying the cell population density and by changing the degree of spatial inhomogeneities (``pathogens''). Our experimental observations can be quite faithfully reproduced by a simple mathematical model involving many motile cells whose mechanical motion are driven by actin polymerization and depolymerization process within the individual cell body and by external chemical gradients. [Preview Abstract] |
Session Y6: Novel Orbital Quantum Phases in Cold Atom Optical Lattices
Sponsoring Units: DCMP DAMOPChair: Steve Girvin, Yale University
Room: 406
Friday, March 20, 2009 8:00AM - 8:36AM |
Y6.00001: Dynamics of ultracold atoms in higher lattice orbitals Invited Speaker: Ultracold quantum gases in far detuned optical lattices have enabled many intriguing experiments studying a new regime of strongly correlated quantum systems. So far, such experiments have mostly concentrated on atoms in the vibrational ground state of the lattice band structure. Here, we report on the realization of a multiorbital system with ultracold atoms in the excited bands of a 3D optical lattice by selectively controlling the band population along a given lattice direction. The lifetime of the atoms in the excited band is found to be considerably longer (10-100 times) than the characteristic time scale for intersite tunneling, thus opening the path for orbital selective many-body physics with ultracold atoms in optical lattices. Upon exciting the atoms from an initial lowest band Mott-insulating state to higher lying bands, we observe the dynamical emergence of long-range coherence in 1D (and also 2D) at nonzero quasimomentum, providing a possible route for Bose-Einstein condensation to nonzero momentum. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y6.00002: Control of Bosons in a 2D optical lattice with checkerboard staggered field. Invited Speaker: I will describe experiments to control the vibrational and spin degrees of freedom of ultra-cold Rb atoms in a novel 2D optical lattice. This unit cell of the lattice can be dynamically transformed between a single-site and two-site configuration, allowing us to manipulate the vibrational degree of freedom of atoms in the unit cell, and merge separated atoms into the same site. In addition, the vector light shift of the optical lattice acts as a Zeeman field for the atoms on every other site of the lattice, providing a checkerboard staggered field. We use this field to control the spins on the two sub-lattices separately. I will also discuss the possibility of using this field to prepare low entropy anti-ferromagnetic states. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y6.00003: Non-zero momentum Bose-Einstein condensation of orbital atoms Invited Speaker: Bose-Einstein condensation (BEC) is often associated with zero momentum to which a macroscopic fraction of bosons condense. Here we propose a new class of meta-stable quantum states where bosons condense at non-zero momenta, defying the paradigm. This becomes possible when bosonic atoms are confined in the $p$-orbital Bloch bands of an optical lattice rather than the usual $s$-orbital. A recent experiment at Mainz confirmed the discovery of such an exotic BEC with alkali-metal atoms in a 3D cubic lattice with anisotropic optical potentials. Non-zero momentum suggests crystalline order. Our theoretical studies further found that such non-zero momentum BECs are also naturally orbital ordered superfluids due to the fascinating, less studied center-of-mass $p$-wave symmetry (e.g., a vortex-like $p_x+ip_y$ condensate). Varying with the geometry from standard optical lattices to double-well lattices, the interesting orderings include staggered orbital currents, stripes of angular momenta, and modulated super-current density wave. Different than a phase of coexisting orders such as supersolidity, this new class of states is characterized by a single order parameter. Work done in collaboration with J. Moore, S. Das Sarma, V. M. Stojanovic, C. Wu, and E. Zhao. \\[4pt] References:\\[0pt] [1] W. V. Liu, C. Wu, Phys. Rev. A {\bf 74}, 013607 (2006). \\[0pt] [2] C. Wu, W. V. Liu, J. Moore, S. Das Sarma, Phys. Rev. Lett. {\bf 97}, 190406 (2006). \\[0pt] [3] V. M. Stojanovic, C. Wu, W. V. Liu, S. Das Sarma, Phys. Rev. Lett. {\bf 101}, 125301 (2008). \\[0pt] [4] E. Zhao and W. V. Liu, Phys. Rev. Lett. {\bf 100}, 160403 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y6.00004: Novel orbital physics with fermions in optical lattices Invited Speaker: Orbitals, a degree of freedom characterized by orbital degeneracy and spatial anisotropy and independent of change and spin, play important roles in magnetism and superconductivity in transition metal oxides. In this talk, we will show that the rapid progress of cold atom physics has opened up an opportunity to study novel features of orbital physics, which do not appear usual solid state systems. In particular, the p$_{x;y}$-orbital system of the honeycomb lattice exhibits amazingly rich and fundamentally different behavior from that in the $p_z$-orbital system of graphene. Its flat band structure dramatically amplifies interaction effects, providing a natural way to study non-perturbative strong correlation phenomena such as Wigner crystallization, and ferromagnetism which is an important field in condensed matter physics but has not attracted much attention in the cold atom community. Furthermore, in the Mott-insulating states, the orbital degree of freedom enables superexchange interactions as spin does. We will show how spatial anisotropy generates frustration in such systems, which leads to a promising way to the exciting orbital liquid states. At last, we will present that a topological insulating phase occurs in the presence of the lattice rotation, as an orbital analogy of the quantum anomalous Hall effect of electron systems. \\[3pt] References:\\[0pt] 1. Congjun Wu, ``Orbital analogue of quantum anomalous Hall effect in $p$-band systems,'' Phys. Rev. Lett. 101, 186807 (2008). \\[0pt] 2. Shizhong Zhang, Congjun Wu, ``Proposed realization of itinerant ferromagnetism in optical lattices,'' arXiv:0805.3031. \\[0pt] 3. Congjun Wu, ``Orbital orderings and frustrations of p-band systems in optical lattices,'' Phys. Rev. Lett. 100, 200406 (2008).\\[0pt] 4. Congjun Wu , and S. Das Sarma, ``The $p_{x,y}$-orbital counterpart of graphene: cold atoms in the honeycomb optical lattice,'' Phys. Rev. B 77, 235107 (2008).\\[0pt] 5. Congjun Wu, Doron Bergman, Leon Balents, and S. Das Sarma, ``Flat bands and Wigner crystallization in the honeycomb optical attice,'' Phys. Rev. Lett. 99, 70401 (2007). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y6.00005: Realization of Extended Hubbard Models with Tailored Orbitals in Optical Lattices Invited Speaker: Optical lattices containing cold atoms represent nearly ideal manifestations of Hubbard models free from disorder, defects, impurities and lattice phonons. Experiments with bosonic alkali atoms confined to the lowest optical lattice band demonstrate strongly correlated phases including the superfluid and Mott insulator that arise from a real space contact interaction between atoms. Can other quantum condensed phases of matter be observed in these systems? We show that promoting bosons to higher bands effectively extends the range of the contact interaction. Quasi-localized orbitals in higher bands overlap with nearest neighbors. They can be modeled with extended Bose-Hubbard models that harbor density wave and supersolid phases. Bosons promoted to higher bands can decay but the purity of optical lattice systems limits possible decay mechanisms (e.g. phonons). We propose that long-lived metastable states of bosons promoted to higher bands of optical lattices may therefore provide a route to a novel class of extended Hubbard models. [Preview Abstract] |
Session Y7: The Statistical Properties of Forces in Jammed Granular Matter
Sponsoring Units: GSNP DCMPChair: Wim van Saarloos, Leiden University
Room: 407
Friday, March 20, 2009 8:00AM - 8:36AM |
Y7.00001: Statistical Mechanics of Jammed Matter Invited Speaker: Jammed systems consist of large numbers of macroscopic particles. As such, they are inherently statistical in nature. However, in general, key assumptions of ordinary statistical mechanics need not apply. For instance, energy does not flow in a meaningful way from a thermal bath to such systems. And energy need not be conserved. However, experiments and simulations have shown that there are well defined distributions for such important properties as forces, contact numbers, etc. And new theoretical constructions have been proposed, starting with Edwards et al. The present symposium highlights recent developments for the statistics of jammed matter. This talk reviews the overall field, and highlights recent work in granular systems[1]. Brian Tighe[2] will describe new results from a force ensemble approach proposed recently by Snoeijer et al. Silke Henkes will describe a different force-based ensemble approach that yields a generalized partition function[3]. Eric Corwin will describe state-of-the-art experiments on dense emulsions[4]. And Matthias Schr\"{o}ter will present novel experiments on fluidized suspensions that address the issue of jamming and glassy behavior[5]. So, do we have a complete description of jammed matter? Not yet, but these talks, as well as other exciting developments in the field, show that there has been enormous progress, towards that end. \\[4pt] [1] T. S. Majmudar et al., Nature {\bf 435}, 1079 (2005); Phys. Rev. Lett. {\bf 98} 058001 (2007). \\[0pt] [2] B. P. Tighe, A. R. T. van Eerd, and T. J. H. Vlugt , Phys. Rev. Lett. 100, 238001 (2008). \\[0pt] [3] S. Henkes, C. O'Hern and B. Chakrabory, Phys. Rev. Lett. 99, 038002 (2007). \\[0pt] [4] J. Bruji\'{c} et al., Phys. Rev. Lett. 98, 248001 (2007). \\[0pt] [5] M. Schr\"{o}oter, D. I. Goldman, and H. L. Swinney, Phys. Rev. E 71, 030301(R) (2005). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y7.00002: A statistical mechanics framework for static granular matter Invited Speaker: It is still an open question if the formalism of equilibrium statistical mechanics can be extended to athermal granular media. A number of authors have used a maximum entropy approach with a flat measure in configuration space to derive the equivalent of the canonical ensemble for the total volume or for the boundary forces as conserved quantities. We have generalized the force-based ensemble to the full force-moment tensor, and allow for the effects of a measure that is not flat. At the isostatic point, this formalism allows us to compute the force distribution exactly, and we obtain an exponential if we choose a flat measure. We use this result as a baseline to investigate the effects of the measure, and we also study the link to a recently proposed ensemble which incorporates an additional stress-based conserved quantity. At a coarse-grained level, the jamming transition can be studied within this framework by postulating a field-theoretical model for the density of states. We construct a minimal model based on symmetry arguments and a positivity constraint for the pressure, which incorporates force and torque balance through the Airy stress function. Unlike in continuum elasticity, the material constants crucially depend on the imposed boundary stresses. The model predicts that the jamming transition is characterized by a vanishing phase space available to the system as the pressure goes to zero. We are able to calculate correlation functions for the components of the stress tensor and compare them to simulation results. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y7.00003: Confocal measurements of emulsions leading to a statistical model for frictionless, polydisperse packings Invited Speaker: We use confocal microscopy to image a frictionless, polydisperse emulsion in 3D. Using a deconvolution technique we determine the position and radius of every droplet. This information allows us to calculate the network of nearest neighbors and the local packing fraction around each droplet. Additionally, we exploit an enhanced fluorescence at the points of droplet contacts to determine the contact network. Based on our observations we build a simple statistical model in which the complexity of the global packing is distilled into a local stochastic process. We show that, locally, the packing problem may be reduced to the random assembly of nearest neighbors, followed by a random choice of contacts among them. Our model is based on only two parameters, the available solid angle around each particle and the ratio of contacts to neighbors, which are both directly obtained from experiments. We find that this ``granocentric'' view captures the essential properties of the polydisperse emulsion packing, ranging from the microscopic distributions of nearest neighbours and contacts to local density fluctuations and all the way to the global packing density. This model suggests a general principle of organization for random packing. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y7.00004: Statistics of force networks Invited Speaker: We study the statistics of contact forces in the force network ensemble, a minimal model of jammed granular media that emphasizes the role of vector force balance. We show that the force probability distribution can be calculated analytically by way of an analogy to equilibrium ensemble methods. In two dimensions the large force tail decays asymptotically as a Gaussian, distinct from earlier predictions, due to the existence of a conserved quantity related to the presence of local vector force balance. We confirm our predictions with highly accurate statistical sampling -- we sample the force distribution over more than 40 decades -- permitting unambiguous confrontation of theory with numerics. We show how the conserved quantity arises naturally within the context of any constant stress ensemble. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y7.00005: Measuring the configurational temperature of granular media Invited Speaker: Twenty years ago Edwards and Oakeshott suggested developing a statistical mechanics of static granular media based on the idea that the logarithm of the number of mechanically stable states of a specific sample constitutes the relevant entropy [1]. From this entropy then, a configurational temperature, named compactivity, could be derived. However, in the absence of an appropriate thermometer to measure compactivity, the question if it is indeed a relevant state variable remained untested. Only recently it was shown that the steady state volume fluctuations of a periodically driven sample can be used to measure the compactivity of a granular sample including its dependence on volume fraction and surface friction of the particles [2]. This opens up the possibility of studying questions like the existence of a zeroth law of granular thermodynamics or the relationship between compactivity and other forms of granular temperature. \newline [1] Edwards and Oakeshott, Physica A {\bf 157}, 1080 (1989). \newline [2] M. Schr\"{o}ter, D. Goldman, and H. L. Swinney Phys. Rev. E {\bf 71}, 030301(R) (2005) [Preview Abstract] |
Session Y8: Pushing the Limits of Mechanical Detection
Sponsoring Units: GIMSChair: Dan Rugar, IBM Almaden
Room: 414/415
Friday, March 20, 2009 8:00AM - 8:36AM |
Y8.00001: Quantum Optical Control of Micromechanics Invited Speaker: Massive mechanical resonators are now approaching the quantum regime. This opens up not only a spectrum of new applications but also a previously inaccessible parameter range for macroscopic quantum experiments on systems consisting of up to $10^{20}$ atoms. Quantum optics provides a rich toolbox to prepare and detect quantum states of mechanical systems, in particular by combining nano- and micromechanical resonators with high-finesse cavities. I will report on our recent experiments in Vienna on laser cooling micromechanical systems towards the quantum ground state by using radiation pressure. I will also discuss the prospects and experimental challenges of generating and detecting optomechanical entanglement, which is at the heart of Schr\"{o}dinger's cat paradox. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y8.00002: Measurement of Dispersive Coupling Between a Nanoresonator and a Superconducting Qubit Invited Speaker: Incorporating superconducting qubit technology into nanoelectromechanical systems (NEMS) should enable the observation of quantum behavior in NEMS. Ultimately, it is expected that coupled qubit-NEMS systems could serve as a test bed for studying fundamental issues of quantum mechanics including the quantum limits of measurement and the quantum-classical divide. Proposals in the literature posit the qubits as veritable toolboxes for preparing, manipulating and measuring quantum states of a nanomechanical resonator (or `nanoresonator'), and range from the nondestructive read-out of quantized-energy states (or `Fock states') to the generation of Schrodinger-cat states. In an initial step toward implementing these advanced strategies, we have performed the first measurements of a nanoresonator coupled to a superconducting qubit, the Cooper-pair box (CPB). We find that the coupling produces a CPB-state-dependent shift in the frequency of the nanoresonator that is analogous to the single-atom phase shifts experienced by superconducting resonators in the dispersive limit of cavity quantum electrodynamics (CQED). In my talk, I will report on our latest measurements of the dispersive interaction between the CPB and nanoresonator, including how we utilize it to read-out quantum interference effects in the CPB. In the end, I will discuss how the interaction could soon be utilized for exploring the quantum limit of NEMS. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y8.00003: Nanoscale Magnetic Resonance Imaging Based on Ultrasensitive Force Detection Invited Speaker: Magnetic Resonance Force Microscopy (MRFM) seeks to dramatically improve the sensitivity and resolution of magnetic resonance imaging (MRI), perhaps ultimately down to the molecular scale. It uses force detection to circumvent the sensitivity limits inherent in conventional inductively-detected MRI. By using an ultrasensitive, single crystal silicon cantilever cooled to 300 mK, we can detect forces smaller than 1 aN, allowing us to sense the magnetism from small ensembles of nuclear spins. We have used tobacco mosaic virus as a test object, detecting the hydrogen signal. Using three-dimensional scans and mathematical deconvolution algorithms, we have made 3D reconstructions of the viruses with resolution down to roughly 4 nm. This represents a 10$^{8}\times $ improvement in minimum detectable volume compared to the best conventional MRI. Advancing the technique further will require reducing the force noise, increasing the achieved magnetic field gradients, and making use of the inherent chemical sensitivity of magnetic resonance. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y8.00004: A Cantilever-based apparatus for detecting micron-scale deviations from Newtonian gravity Invited Speaker: To test new theories of physics beyond the Standard Model, we have built a low temperature probe to measure forces as small as 10$^{-18}$ N between masses separated by distances on the order of 20 microns. Our experiment is fundamentally a Cavendish-type experiment in the sense that its purpose is to directly measure the force between two masses [1]. A cryogenic helium gas bearing is used to rotate a disc containing a drive mass pattern of alternating density under a small test mass mounted on a micromachined cantilever. Any mass-dependent force between the two will produce a time-varying force on the test mass, and consequently a time-varying displacement of the cantilever. This displacement is read out with a laser interferometer, and the position of the drive mass is simultaneously recorded using an optical encoder. The displacement is then averaged over many cycles and converted to a force using measured properties of the cantilever. This AC ``lock-in'' type measurement enables significant noise rejection and allows us to operate on resonance to take advantage of the cantilever's high quality factor. A novel feature of the apparatus is the utilization of feedback regulation of the response of the microcantilever using the radiation pressure of a laser. Our approach does not require a high-finesse cavity, and the feedback force is due solely to the momentum of the photons in the second laser. \\[4pt] [1] D.M. Weld, J. Xia, B. Cabrera, and A. Kapitulnik, Phys. Rev. D 77, 062006 (2008). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 11:00AM |
Y8.00005: The quantum limit and beyond in gravitational wave detection Invited Speaker: The sensitivity of current and next generation interferometric gravitational wave detectors is limited by quantum mechanics. We will explore this quantum limit, one aspect of which arises from the radiation pressure that laser light exerts on the movable mirrors of the interferometer. I will describe experiments in which the light force dominates the mechanical forces to such an extent that the mirror oscillators are optically trapped and cooled. Laser cooling of macroscopic mechanical oscillators has applications in high precision force and position measurements, gravitational wave detection, and exploration of the classical-quantum boundary. I will discuss the status of a variety of experimental efforts worldwide are working to approach the quantum regime, with the goal of observing non-classical effects such as quantum back-action, squeezing and entanglement of the light and mirror states, and conclude with an outlook on prospects for observation of quantum effects in macroscopic human-scale objects. [Preview Abstract] |
Session Y9: Stochastic Processes in Biological Systems II
Sponsoring Units: GSNPChair: Leah Shaw, The College of William and Mary
Room: 303
Friday, March 20, 2009 8:00AM - 8:12AM |
Y9.00001: A Stochastic Single-Molecule Event Triggers Phenotype Switching of a Bacterial Cell Sunney Xie, Paul Choi, Long Cai By monitoring fluorescently labeled lactose permease with single-molecule sensitivity, we investigated the molecular mechanism of how an Escherichia coli cell with the lac operon switches from one phenotype to another. At intermediate inducer concentrations, a population of genetically identical cells exhibits two phenotypes: induced cells with highly fluorescent membranes and uninduced cells with a small number of membrane-bound permeases. We found that this basal-level expression results from partial dissociation of the tetrameric lactose repressor from one of its operators on looped DNA. In contrast, infrequent events of complete dissociation of the repressor from DNA result in large bursts of permease expression that trigger induction of the lac operon. Hence, a stochastic single-molecule event determines a cell's phenotype. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y9.00002: The role of time scales in extrinsic noise propagation Srividya Iyer-Biswas, Juan Manuel Pedraza, C. Jayaprakash Cell-to cell variability in the number of proteins has been studied extensively experimentally. There are many sources of this stochastic variability or noise that can be classified as intrinsic, due to the stochasticity of chemical reactions and extrinsic, due to environmental differences. The different stages in the production of proteins in response to a stimulus, the signaling cascade before transcription, transcription, and translation are characterized by different time scales. We analyze how these time scales determine the effect of the reactions at each stage on different sources of noise. For example, even if intrinsic noise dominates the fluctuations in mRNA number, for typical degradation rates, extrinsic noise can dominate corresponding protein number fluctuations. Such results are important in determining the importance of intrinsic noise at earlier stages of a genetic network on the products of subsequent stages. We examine cases in which the dynamics of the extrinsic noise can lead to differences from cases in which extrinsic noise arises from static (in time) cell-to-cell variations. We will interpret the experiments of Pedraza et al*. in the light of these results. *J. M. Pedraza et al, Science 25 March 2005:Vol. 307. no. 5717, pp. 1965 - 1969. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y9.00003: Determination of the equilibrium free energy for pulled single molecules from nonequilibrium work measurements Liao Chen The Jarzynski equality (JE) is widely accepted for extracting equilibrium free energy from non-equilibrium work measured in single-molecule pulling experiments, even though questions remain on its validity and applicability. In this talk, I will show that the JE is actually inapplicable outside the near-equilibrium regime. I will also present a new fluctuation-dissipation theorem (FDT) that is derived within the context of Brownian dynamics. The new FDT agrees with the JE in the near equilibrium regime but it is valid in far nonequilibrium regime where the JE does not stand. \textit{In silico} experiments of unfolding polypeptides show that the new FDT is indeed accurate for far non-equilibrium processes. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y9.00004: Efficient stochastic sampling of first passage times for multi-scale simulations Navodit Misra, Russell Schwartz Monte Carlo methods have become increasingly popular for simulating stochastic dynamics in biological systems. However, the standard Stochastic Simulation Algorithm (SSA) can become highly inefficient for multi-timescale problems, where important events occur in parallel and at a much slower rate than other relatively unimportant events. We present two new algorithms based on the spectral analysis of Continuous Time Markov Model (CTMM) graphs to accelerate sampling of rare events in SSA models. These methods are well suited for simulating a broad class of ``stiff'' reaction networks such as models of bond networks and nucleation-limited self-assembly in biological systems. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y9.00005: Dynamics of segregation of polymers in a confined geometry Ya Liu, Bulbul Chakraborty Chromosomes are enormous DNA molecules living in the crowded, confined environment of a cell. They carry important genetic information and are stably propagated to new generations through replication. During the replication, two identical DNA molecules are generated and segregate rapidly into opposite pole of the cell. We have used numerical simulation to investigate the effects of confinement on the segregation of two identical self-avoiding chains. Simulation shows the existence of a transition from a mixing state to a demixing state with changes in the confining geometry. Using the blob picture, we construct a free energy function that depends on the distance between the two chains. We describe the dynamics of segregation as a stochastic process driven by this energy function. We will present comparisons of our theoretical results with numerical simulations. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y9.00006: Modeling DNA unhooking from a single post as a translocation process Nabil Laachi, Jaeseol Cho, Kevin Dorfman We will present theoretical results on the stochastic unhooking of a long DNA chain from an isolated, stationary micropillar obtained by mapping the unhooking process to the translocation of a long chain through a nanopore. We show how stochastic methods, developed for DNA translocation, can thus be utilized to study chain-post unhooking. In particular, implementing such methods leads to the full probability distribution of the unhooking time and the ensuing moments in a fast and efficient manner for a wide range of chain and field parameters. The results thus obtained compare favorably to more realistic (and computationally intense) Brownian Dynamics simulation data, indicating that the finite size of the insulating micropillar and the elasticity of the DNA make at most a small contribution to the dynamics. We will also address the relevant electric fields and time scales for this process, making a connection between the theoretical data obtained here and experimental separations. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y9.00007: Evolutionary advantage of a mixed strategy for the competence phenotype in bacteria Christopher Wylie, Herbert Levine, David Kessler Under certain stressful conditions, bacterial species such as \textit{B. subtilis} undergo a differentiation process in which a \textit{finite subpopulation} transiently and stochastically enters the ``competent'' state. This state is defined by the ability to import and homologously incorporate extracellular DNA fragments into the genome. This ability is accompanied by a reduced growth rate that tends to slow adaptive evolution. On the other hand, the increased genetic diversity generated by recombination tends to speed evolution. Using stochastic simulation and analytic methods, we show that this tradeoff implies that a ``mixed strategy'' optimizes the rate at which populations acquire beneficial mutations. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y9.00008: Physical limits on computation by assemblies of allosteric proteins John Robinson Assemblies of allosteric proteins are the principle information processing devices in biology. Using the Ca$^{2+}$-sensitive cardiac regulatory assembly as a paradigm for Brownian computation, we examine how system complexity and system resetting impose physical limits on computation. Nearest-neighbor-limited interactions among assembly components constrains the topology of the system's macrostate free energy landscape and produces degenerate transition probabilities. As a result, signaling fidelity and deactivation kinetics can not be simultaneously optimized. This imposes an upper limit on the rate of information processing by assemblies of allosteric proteins that couple to a single ligand type. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y9.00009: Calcium waves in the the maturing oocyte Aman Ullah, Ghanim Ullah, Peter Jung, Khaled Machaca Calcium waves in oocytes are sustained by release of Ca2+ from the endoplasmic reticulum (ER) through clustered release channels. As the oocytes matures, a) the calcium waves slow down by about a factor of two, b) the overall duration of Ca2+ elevation grows substantially, and c) the cell is more susceptible to wave initiation. At the same time, the kinetics of release of Ca2+ from a single cluster is changed only insignificantly. Based on a computational model that accurately reproduces elemental Ca2+ release kinetics from channel clusters, we propose that the changing spatial organization of signaling effectors is a common underlying cause for all the above described observations as the Ca2+ signaling machinery matures. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y9.00010: Calcium puffs: From microdomain to a channel. Divya Swaminathan, Peter Jung Calcium puffs describe the release of calcium ($Ca^{2+}$) ions from internal stores into the cytosol through clusters of up to tens of ion channels. It is believed that during the release process, when the channels open, steep $Ca^{2+}$ concentration gradients are established around the cluster. These large local concentrations are consequential as they determine the opening and closing rates of the ion channel and therefore control receptor kinetics. We present a computational study, wherein we simulate the release and diffusion of $Ca^{2+}$ and its interaction with buffers and indicator dyes around one channel cluster. Our goal is to relate local steep $Ca^{2+}$ gradients with experimentally observable microdomain-averaged $Ca^{2+}$ concentrations thereby putting the high concentration hypothesis to test. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y9.00011: Dynamical Phase Transitions In Periodically Driven Model Neurons Jan R. Engelbrecht$^1$, Renato Mirollo$^2$ Transitions between dynamical states in integrate-and-fire (IF) neuron models with periodic stimuli result from tangent or discontinuous bifurcations of a return map. We study their characteristic scaling laws and show that discontinuous bifurcations exhibit a new kind of phase transition intermediate between continuous and first order. We then consider a much more complicated Hodgkin-Huxley type of model and show that in the presence of periodic stimuli an attracting 2D invariant subspace develops in the 7D state space. A Poincare section on this subspace yields a 1D return map, remarkably similar to the IF case. This reduction to 1D map dynamics should extend to real neurons in a periodic current clamp setting. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y9.00012: Stochastic and Deterministic Flagellar Dynamics Provide a Mechanism for Eukaryotic Swimming Reorientation Marco Polin, Idan Tuval, Knut Drescher, Raymond Goldstein The biflagellated alga \textit{Chlamydomonas reinhardtii} is a good model organism to study the origin of flagellar synchronization. Here we employ high-speed imaging to study the beating of the two flagella of \textit{Chlamydomonas}, and show that a single cell can alternate between two distinct dynamical regimes: asynchronous and synchronous. The asynchronous state is characterized by a large interflagellar frequency difference. In the synchronous state, the flagella beat in phase for lengthy periods, interrupted episodically by an extra beat of either flagellum. The statistics of these events are consistent with a model of hydrodynamically coupled noisy oscillators. Previous observations have suggested that the two flagella have well separated intrinsic beat frequencies, and are synchronized by their mutual coupling. Our analysis shows instead that the synchronized state is incompatible with coupling-induced synchronization of flagella with those intrinsic frequencies. This suggests that the beat frequencies themselves are under the control of the cell. Moreover, high-resolution three-dimensional tracking of swimming cells provides strong evidence that these dynamical states are related to non-phototactic reorientation events in the trajectories, yielding a eukaryotic equivalent of the ``run and tumble'' motion of peritrichously flagellated bacteria. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y9.00013: Active stochastic oscillations and amplification of mechanical stimuli in a hair cell model Lijuan Han, Alexander Neiman We study signal transduction in spontaneously oscillating hair bundles of an auditory hair cell using a computational model. The effects of intrinsic noise from the Brownian motion of hair bundles and from stochastic fluctuations of transduction ion channels as well as periodic fluctuations of the receptor potential are taken into account. The model shows the explosion of a canard trajectory near the Hopf bifurcation. We have found that the system's gain of weak mechanical stimuli can be greatly enhanced when the system operates slightly beyond the Hopf bifurcation, i.e. in the canard region. The gain can also be optimized by tuning the noise intensity. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y9.00014: The co-existence of spirals with multiple rates of rotation Joseph Tranquillo Two findings in homogeneous reaction-diffusion media are that a single spiral may break into multiple spirals and that rapidly rotating spirals push slowly rotating spirals to domain boundaries. These two findings together fail to explain how cardiac tissue can support multiple stable spirals with different periods of rotation. Numerical simulations are presented in which a thin inhomogeneous region forms a functionally protective barrier between spirals rotating at different rates. The only requirement of the insulating region is that it partially block alternating wavefronts from the fast spiral. Parameters of both reaction and diffusion can result in functional insulation and multiple insulating regions can result in the broad frequency spectrum characteristic of cardiac fibrillation. The results suggest that the healthy ventricle, although containing intrinsic inhomogeneities, is functionally connected, while disease may create functionally disconnected regions. This simple mechanism may shed light on why defibrillation and pacing are not always successful, and why some patients are more susceptible to a transition from tachycardia to fibrillation. [Preview Abstract] |
Session Y10: Focus Session: Optical Properties of Nanostructures VI: Artificially Structured Materials
Sponsoring Units: DMPChair: Stefano Cabrini, Molecular Foundry, Lawrence Berkeley National Laboratory
Room: 304
Friday, March 20, 2009 8:00AM - 8:36AM |
Y10.00001: Green's Functions in Nanoscience Invited Speaker: Theoretical nanoscience is a fast growing area in physics. It gains momentum with the recent advances in related areas such as nanodevice design, synthesis and characterization of novel nanostructures, nanoscale imaging and spectroscopy. Several techniques common to quantum chemistry and condensed matter physics have been applied successfully to the modeling of nanoscale structures. In particular, methods based on many-body Green's functions (MBGF) are becoming more and more popular. One of the reasons for this success is that these methods have predicted several phenomena at nanoscale, for example the peculiar dimensional confinement of excitons in nanostructures. Further advances in quantum transport and exciton dynamics can be foreseen. Moreover, algorithms tailored to confined systems have made calculations of Green's functions in nanostructures much more manageable [1]. With those algorithms, we were able to investigate the properties of correlated excitations in clusters of semiconductors (CdSe and silicon)[1,2]. We were also able to explain the properties of electronic excitations in fullerenes and other organic compounds [1,3]. This talk will present an overview of the current stage of MBGF techniques, discuss the various approximations that have been proposed, and review recent advances. \\[4pt] References: \\[0pt] [1] M. L. Tiago and J. R. Chelikowsky, Phys. Rev. B 73, 205334 (2006) \\[0pt] [2] M. L. Del Puerto, M. L. Tiago and J. R. Chelikowsky, Phys. Rev. Lett.97, 096401 (2006). \\[0pt] [3] M. L. Tiago, P. R. C. Kent, R. Q. Hood, and F. A. Reboredo, J. Chem. Phys. 129, 084311 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y10.00002: First Principles Absorption Spectra of Cu$_n$ ($n=1-10$) Clusters Kopinjol Baishya, Juan C. Idrobo, Serdar Ogut, Mingli Yang, Koblar A. Jackson, Julius Jellinek First principles optical absorption spectra, obtained within time-dependent density functional theory, for the ground state and low-energy isomers of Cu$_n$ ($n = 1- 10$) are presented. Overall our theoretical results exhibit good agreement with available experimental data. We analyze the orbital character of the optical excitations as a function of size and energy. Compared to noble metal clusters of Ag and Au in the same size range,\footnote {J. C. Idrobo {\em et al.} Phys. Rev. B {\bf 76}, 205422 (2007); K. Baishya {\em et al.} {\em ibid.} {\bf 78}, 075439 (2008).} we find that Cu clusters have much higher d-electron contribution to low-energy optical excitations. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y10.00003: ABSTRACT HAS BEEN MOVED TO S1.00259 |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y10.00004: Calculations of second harmonic generation by periodically-structured surfaces William Schaich We have done finite-difference time-domain (fdtd) calculations of second harmonic generation (shg) from various surface structures. The periodic structures include pairs of metal strips, split-ring resonators, and fishnets. The nonlinear response is described by a set of parameters that multiply products of linear field components. These fields and the resultant second harmonic radiated fields are found in separate fdtd calculations. The shg spectra do not always correlate well with that seen in linear response. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y10.00005: Optical properties of the Folic Acid/APTMS/TiO$_{2}$ nanosystems Volodymyr Turkowski, Michael Leuenberger, Talat Rahman, Duy Le, Sudipta Seal, Sanku Mallik, Andre Gesquiere Our photoluminescence experiments on folic acid (FA) conjugated nanoparticles of ${\rm TiO_{2}}$, ${\rm CeO_{2}}$ and ${\rm SiO_{2}}$ show great promise for a variety of optoelectronic applications for these materials, in particular in the field of modern molecular photoelectronic devices, since they demonstrate a dramatic increase of the photoemission intensity at wavelengths between 500 to 700 nm when the nanoparticles are coated with the 3-aminopropyltrimethoxylane (APTMS) linker/spacer molecule. We report here results of accompanying time-dependent density- functional theory studies of the FA/APTMS/${\rm TiO_{2}}$ nanosystems by using the B3LYP exchange-correlation potential. We demonstrate that the large increase of the photoemission is due to enhanced optical transitions which involve the intermediate energy levels related to the APTMS states. We present details of the geometric and electronic structure and excited states of our nanosystems and their dependence on the characteristics of the nanoparticle. We discuss possible optoelectronic applications for this effect. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y10.00006: Optical Properties and Aging of Gasochromic WO$_{3}$ Rudresh Ghosh, Matthew B. Baker, Rene Lopez WO$_{3}$ as a possible optical gas sensor has gained increasing importance with H$_{2}$ becoming a major fuel of the future. This has led to efforts to understand the theoretical and practical aspects of the gasochromic behavior of WO$_{3}$. WO$_{3}$ films were fabricated using pulsed laser deposition (PLD). Morphological and stoichiometric ratios of films obtained were observed as functions of deposition parameters. We present the optical constants induced by 2{\%} H$_{2}$:Ar in WO$_{3}$ films. This allows us to obtain the limits of the gasochromic change in comparison to ion injection. It was found using Langmuir's adsorption equation that at low H$_{2}$ concentrations a high sensitivity is predicted but the coloration could saturate at 57.9 {\%} of the material's maximum ion adsorption. Poisoning of the films was also addressed by coating with a permeable polydimethylsiloxane layer. It is shown that gasochromic degradation is prevented thus eliminating common atmospheric gases as possible contaminants. Our studies suggest WO$_{3}$ thin films as highly sensitive and stable optical hydrogen sensors. . [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y10.00007: {\it Ab initio} Bethe-Salpeter Equation approach for aperiodic materials and core-excitations H.M. Lawler, J. Vinson, J.J. Rehr, E.L. Shirley We have recently developed an interface dubbed AI2NBSE between the Bethe-Salpeter optical spectroscopy code NBSE developed at NIST and the {\it ab initio} electronic structure code ABINIT [1]. This interface facilitates first-principles calculations of valence-band dielectric response including excitonic effects in insulating crystals. Here we report on the extension of this interface for calculations of 1) dielectric response in complex nano-scale, disordered, and molecular systems, and 2) core-level UV and x-ray response. For the treatment of complex systems, we discuss issues of cell selection, basis size, and the treatment of the screened electron- hole interaction. For the core level response, we address various strategies including explicit treatments of core and semi-core states with plane waves and the PAW representation. *Supported by DOE Grant DE-FG03-97ER45623. [1] H. M. Lawler, J. J. Rehr, F. Vila, S. D. Dalosto, E. L. Shirley and Z. H. Levine, Phys. Rev. B {\bf 78}, 205108 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y10.00008: Franz-Keldysh effect in the interband optical absorption of quantum wires Harold Spector, Congxin Xia We present a theoretical calculation of the effect of an electric field applied either parallel or perpendicular to the axis of a rectangular quantum wire on the interband optical absorption. We find that the application of the electric field decreases the optical absorption coefficient for both the parallel and perpendicular to the axis electric field configurations. The absorption is greater when the electric field is along the direction of carrier confinement than when it is along the axis of the wire. This difference is due to the effect of the field on the overlap of the electron and hole wave functions. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y10.00009: Surface Enhanced Raman Spectroscopy (SERS) of pyridine on Pt Qingzhen Hao, Lasse Jensen, Peter Eklund SERS studies were carried out on vertically oriented Pt cylinders patterned on quartz substrates via e-beam lithography. Optical absorption indicates that Localized Surface Plasmon Resonance (LSPR) of the Pt cylinders in the UV region, around 300nm. Discrete Dipole Approximation (DDA) simulation was performed to confirm the position of the substrate's LSPR and also to map the electric field distribution inside and at the surface of the Pt, which allows us to estimate the Electromagnetic enhancement factor (EF). Experimentally, we demonstrate that the total SERS EF is about 5$\times $10$^{4}$ using 514.5nm excitation (far away from the LSPR resonance). Using time-dependent density functional theory we have calculated the off-resonance chemical SERS enhancements of pyridine interacting with small Pt clusters. Our results show that the enhancements are much larger than results obtained for small Ag clusters. We are currently exploring the enhancements for different Pt cluster sizes as well as the importance of charge-transfer excitations. These results will provide detailed insights into the mechanism responsible for the chemical enhancement in SERS. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y10.00010: The Effect of Roughened Metallic Films on Colloidal Quantum Dot Energy Transfer Christopher Ferri, Somnath Ghosh, Brent Rich, Michelle Khine, Sayantani Ghosh We investigate self-organized, roughened metallic surfaces as a platform for enhanced energy transfer between colloidal Cadmium Selenide (CdSe) quantum dots (QD). Pre-stressed thermoplastic substrates are sputter coated with gold palladium (AuPd) to create thin films. When heated, due to differing coefficients of thermal expansion of the plastic and metal, the AuPd film buckles to form micro- to nano-meter sized structures. QDs deposited on these self-organized metallic structures exhibit changes in their static and dynamic optical characteristics, which include spectral red-shift and multiple recombination decay rates. These observations can be attributed to a combination of enhanced electronic coupling between close-packed QDs and plasmonic coupling between the QD and metallic structures. We then leverage these properties to fabricate controlled, directional structures using this self-organized method which can be utilized as biochemical sensors. [Preview Abstract] |
Session Y11: Superlattices and Nanostructures - Electronic Properties
Sponsoring Units: DCMPChair: Stefan Badescu, Naval Research Laboratory
Room: 305
Friday, March 20, 2009 8:00AM - 8:12AM |
Y11.00001: Finding structures with specific properties in complex configurational spaces using multi-target inverse band structure approach Paulo Piquini, Alex Zunger The conventional strategy to look for materials with desired properties is to use physical intuition to select some candidates among an enormous number of possibilities.Apart the very special cases, the solutions to these search problems are far from obvious. The inverse band structure (IBS) approach, on the other hand, search for the desired electronic structures (instead of atomic configurations) from the beginning. Here we illustrate the power of this inverse approach by applying it to the simultaneous engineering of multi-target problems, which encompass huge configurational spaces: (i) the search of a specific band gap in the quaternary (In,Ga)(As,Sb) semiconductors(a) lattice-matched to InP and, (ii) the stacking sequence of (In,Ga)As/InP superlattices leading to band gaps and strains within the range suitable for thermophotovoltaic applications(b). \\[3pt] (a) P. Piquini, P.A. Graf, and A. Zunger, Phys. Rev. Lett. {\bf 100}, 186403 (2008); \\[0pt] (b) P. Piquini and A. Zunger, Phys. Rev. B {\bf 78}, 161302 (2008) [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y11.00002: First-principles studies of structure and electronic level alignment at nanoscale CdSe/CdTe heterojunctions Shenyuan Yang, David Prendergast, Jeffrey Neaton Group II-VI semiconductor nanostructured heterojunctions with type-II interfacial band offsets have many potential applications in nanoscale optoelectronics and photovoltaics. A key open question is how the electronic level alignment at nanoscale heterojunctions differs from that at their bulk counterparts, and whether bulk intuition can be used to understand their electronic behavior. We use density functional theory and beyond to study the structure and electronic properties of CdSe/CdTe interfaces in bulk-planar and nanowire form. Both periodic ``superlattice'' geometries and slabs finite along the surface normal are compared. We compute interface atomic and electronic structure, and for small-diameter nanowires, we discuss the impact of quantum confinement, intrinsic strain, and organic ligand passivation on the electronic properties of the interface. Our results are discussed in the context of recent experiments. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y11.00003: \textit{P}-type InSb and In$_{x}$Ga$_{1-x}$As quantum wells remotely doped with Be Chomani Gaspe, Madhavie Edirisooriya, Tetsuya Mishima, Michael Santos CMOS circuits require $p-$type transistors with high hole mobility, in addition to \textit{n{\-}}type transistors with high electron mobility. We have observed room-temperature hole mobilities of 100 and 600 cm$^{2}$/Vs in In$_{x}$Ga$_{1-x}$As and InSb quantum wells, respectively. The In$_{x}$Ga$_{1-x}$As wells are remotely doped with Be in In$_{x}$Al$_{1-x}$As barrier layers, and grown on InP substrates. The InSb wells are remotely doped with Be in Al$_{x}$In$_{1-x}$Sb barrier layers, and grown on GaAs substrates. We will discuss the effects of strain, structural parameters, and defect density on hole mobility in InSb and In$_{x}$Ga$_{1-x}$As quantum wells. In zinc-blende semiconductors, a narrower band gap leads to smaller effective masses for electrons and holes. Strain and confinement increase the energy splitting between holes with light in-plane mass and those with heavy in-plane mass. This work was supported by the NSF Grants DMR-0808086 and DMR-0520550. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y11.00004: Predicted Band Structures of Wurtzite III-V Semiconductors Based on Empirical Pseudopotentials Amrit De, Craig Pryor The electronic properties of III-V semiconductor nanowhiskers present a problem since the nanowhiskers often crystallize in wurtzite (WZ) form while the corresponding bulk materials are zincblende. Using empirical pseudopotentials, including spin-orbit coupling, we have computed the bulk band structures for non-nitride III-V semiconductors in WZ form, which may be used for electronic structure calculations of nanowhiskers. The calculations make use of transferable model pseudopotentials fit to the zincblende form. We find that due to the stronger breaking of inversion symmetry in the WZ form, there are larger zero field spin splittings than in the corresponding zincblende materials, making WZ nanowhiskers good candidates for novel spin based devices. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y11.00005: Fundamental properties of TiO$_{2}$ nanostructures: the effects of the size confinement and the surface coverage Giovanni Cantele, Amilcare Iacomino, Fabio Trani, Domenico Ninno, Ivan Marri, Stefano Ossicini The titanium dioxide (TiO$_{2}$) complexes are widely investigated for their multipurpose capabilities. We discuss here a characterization of anatase TiO$% _{2}$ 0D clusters (NCs) and 1D nanowires (NWs) in the framework of ab initio DFT calculations. Based on both theoretical and experimental evidences, we defined an anatase TiO$_{2}$ NC by modifying a perfect bipyramidal morphology and then used this NC as a chain repetition unit in the anatase NW. We studied the size confinement and analyzed the effect of surface coverage by functionalization with simple water-derived adsorbates. We found that the structural reconstruction fit the available experimental data, that the band gap shift depends on the crystallinity and that the hydration is important in stabilizing the nanostructures. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y11.00006: Ion scattering from Au nanoclusters formed by buffer layer assisted growth Snjezana Balaz, Jory Yarmoff Ion scattering is used to probe the atomic and electronic structure of Au nanocrystals grown by Buffer Layer Assister Growth (BLAG). Amorphous solid water (ASW) was adsorbed as a buffer layer onto SiO$_2$/Si(111) at liquid nitrogen temperature. Au was then evaporated onto the buffer layer to form nanoclusters. The samples were subsequently annealed to room temperature, causing the water to desorb and the clusters to deposit directly onto the substrate. Time-of-flight (TOF) spectroscopy was used to measure 2 keV $^7$Li$^+$ and $^{39}$K$^+$ ions scattered from Au atoms both at low temperature when the clusters reside atop the buffer layer, and after desorbing the water. Small Au depositions yielded a sharp single scattering peak that indicates single layer structures. Following larger depositions, multiple scattering features were present indicating the formation of multilayer nanoclusters. The neutral fraction of scattered K$^{+}$, which provides an indication of the filled quantum states, starts at $\sim $50{\%} for small Au coverages and decreases with further deposition, indicating changes in the quantum state occupancy with cluster size. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y11.00007: Cyclotron Resonance of Two-Dimensional Hole Systems in InSb Quantum Wells James Coker, R. Doezema, M. Edirisooriya, T. Mishima, Mike Santos, X. Pan, G. Sanders, C. Stanton, L. Tung, Y.-J. Wang In order to realize high-performance InSb CMOS circuits, p-type InSb QW transistors with a high room-temperature mobility are necessary. We report on an experimental study of cyclotron resonance in InSb QWs with Al$_{0.20}$In$_{0.80}$Sb barriers doped with Be. Magnetic fields up to 17.5T were applied perpendicular to the QWs, at a temperature of 4.2K. At fields less than 4T, we deduce a hole effective mass of 0.05$\sim $0.1m$_{o}$ for densities of 2$\sim $5$\times $10$^{11}$cm$^{-2}$, which suggests that high hole mobilities are possible. At higher fields, we observe separate features for different spin-conserving transitions between neighboring Landau levels. The energies of the features depend on the levels' spin index and Landau level indices. The energies and intensities are explained by a modified Pidgeon-Brown model that explicitly incorporates pseudomorphic mechanical strain. This work was supported by the NSF under Grants DMR-0808086 and DMR-0520550. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y11.00008: Structural and electronic properties of crystals of thiolate-capped Au nanoparticles doped with donor and acceptor molecules: first-principles calculations Ronaldo Batista, Jonathan Martins, H\'elio Chacham Structural and electronic properties of crystals of etiolate- capped Au nanoparticles doped with donor and acceptor molecules: first-principles calculations We perform first-principles calculations for crystals composed of periodic assemblies of Au$_{38}$ nanoparticles that are capped with methylthiol molecules. We consider fcc structures, consistent with recent experimental results [1]. We also consider that the nanoparticle crystals can be doped with either donor (tetrabutylammonium) or acceptor (hexafluorophosphate) molecules, also consistent with recent experiments [2]. We find that the most stable positions of the dopant molecules are located near the nanoparticle surfaces, and not near interstitial positions. We also find that the electron chemical potential changes linearly with the impurity concentration, up to a critical concentration, consistent with recent experiments [2]. Above that critical concentration, a new regime is observed due to a partial charge transfer between dopants and nanoparticles. These features are reproduced by a simple capacitive model for the crystal. [1] Ab\'ecassis et al. Phys. Rev. Lett. 100, 115504 (2008) [2] Boettcher S. W. et al. Nature Mater. 6, 592–596 (2007). [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y11.00009: Interaction effects in conductance of quasi-1D channels formed from AlGaAs/GaAs 2DEG: Crossover from weakly-disordered Fermi liquid to Luttinger liquid Matthew Bell, Andrei Sergeev, Jonathan Bird, Vladimir Mitin, Aleksandr Verevkin We investigated the conductance of a long and narrow high-mobility channel and observed crossover from weakly-disordered multi-channel Fermi liquid [1] to Luttinger liquid with decreasing channel width to $\sim$100nm. Quasi-one-dimensional channels were formed from an AlGaAs/GaAs heterostructure using the split-gate technique. The lengths of the channels were 100 $\mu$m. The width of the channels were varied from a lithographic width of 500 nm to $\sim$50 nm by applying negative bias to the split-gate. The effective electron concentration and the channel widths were evaluated from magnetoresistance measurements. In the range of channel widths 500 - 100 nm, at temperatures 1-10 K we clearly observe the logarithmic temperature dependences of the conductance. These dependences are adequately explained by effects of electron-electron interaction in weakly-disordered quasi-one dimensional (with respect to the interaction) Fermi liquid [1]. When the width further decreases, the logarithmic dependences change to power-law dependences, which are typical for Luttinger liquid. This crossover takes place when the channel width corresponds to 2-3 one-dimensional subbands. [1] Sergeev et.al., Phys. Rev. B. 69, 075310 (2004). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y11.00010: Giant Orbital Paramagnetism in Nanometer Scale 2DEG Strips Michael Harrison An elementary calculation shows that Landau diamagnetism becomes significantly altered and very large paramagnetic effects emerge at low tempersatures in nanoscale 2DEG strips penetrated by a perpendicular applied magnetic field and bounded by a parabolic potential, such as may arise from negative voltage applied to a split gate. These novel results are described by an expression which manifests the total system magnetization as a difference between evolved orbital paramagnetism and altered diamagnetism. These predicted effects correspond to drift motion of electrons parallel to the strip length arising from Landau eigenstates that are non-degenerate in the combined presence of a perpendicular applied magnetic field and electric fields associated with a confining parabolic potential. A new heterostructured magnetic material based on orbital electronic motion in 2DEG strips is proposed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y11.00011: Excitonic condensation with different pairing symmetries in double quantum wells Christopher Jamell Double quantum wells with one containing electrons and the other containing holes as carriers are a promising candidate for condensation of dipolar excitons with lifetime much larger than lifetime of excitons in bulk semiconductors. When the inter-well distance is comparable to the interparticle distance within a single well, $d \leq r_s a_B$, inter-well coherence is expected to lead to an excitonic condensation. We explore the ground state of a balanced system as a function of inter-well distance $d$ and the carrier density $n_{2D}$. We present Hartree-Fock mean-field results for the quasiparticle and order parameter dispersion with different pairing symmetries. We obtain the quasiparticle density of states in each case. These results lay the ground work for mean-field study of excitonic condensate states with spontaneously broken translational symmetry. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y11.00012: Coulomb Blockade in a Field Emitting Freely Suspended Island Chulki Kim, Hyun S. Kim, Hua Qin, Robert H. Blick We observe staircase current-voltage characteristics from an isolated nanomechanical island. The island is fixed by CF$_{2}$ connections, which makes the structure suspended 1µm above SiO$_{2}$. The noteworthy difference to the ``orthodox'' single electron transistors is the fact that we observe Coulomb blockade in conjunction with field emission. We can trace and reproduce the transition from staircase in the high bias regime. A theoretical model based on field emission current reproduces the experimental data. The full profile of the current-voltage measurement shows the transition from Coulomb staircase current to island field emission current. [Preview Abstract] |
Session Y12: Electronic and Lattice Properties, Including Quantum Size Effects
Sponsoring Units: DCMP DMPChair: Tai Chiang, University of Illlinois, Champaign-Urbana
Room: 308
Friday, March 20, 2009 8:00AM - 8:12AM |
Y12.00001: Bi(114): A quasi one-dimensional metal with strong spin-orbit splitting Ph. Hofmann, J.W. Wells, H. Dil, F. Meier, J. Lobo-Checa, V.N. Petrov, J. Osterwalder, M.M. Ugeda, I. Fernandez-Torrente, J.I. Pascual, E. Rienks, M.F. Jensen The (114) vicinal surface of the semimetal Bi is found to support a quasi one-dimensional, metallic surface state. As required by symmetry, the state is degenerate along the $\bar{\Gamma}-\bar{Y}$ line of the surface Brillouin zone with a binding energy of $\approx$100 meV. In the $\bar{\Gamma}-\bar{X}$ direction the degeneracy is lifted by the strong spin-orbit interaction, as directly shown by spin-resolved photoemission. This results in a Fermi surface consisting of two closely separated, paralell lines of opposite spin direction. We discuss these findings in the light of the recently discovered topological stability of surface states on BiSb topological insulators. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y12.00002: Plasmon Resonances in Ultrathin Magnesium Films Mustafa M. Ozer, Eun Ju Moon, Adolfo G. Eguiluz, Hanno H. Weitering Low temperature growth of Mg on Si(111) results in the formation of atomically smooth thin films with precisely controlled film thickness. We employed x-ray photoelectron spectroscopy to monitor the evolution of a sharp shake-up satellite in the Mg 1s core level as a function of the film thickness. For films with thicknesses between five and twenty five atomic layers the energy position of this peak is inversely proportional to the square of the film thickness. These results are consistent with the existence of quantized plasmons, which we interpret on the basis of theoretical (hydrodynamics and RPA) descriptions of the density-response function. We demonstrate that the observed loss feature corresponds to the n = 1 antisymmetric normal mode of the thin film, consistent with the fact that in the ultrathin film limit the symmetric plasmons have vanishing spectral weight - a striking manifestation of the role of size quantization on plasmon resonances in precisely controlled nanostructures. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y12.00003: Structural analysis of the surface of bilayered ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ Biao Hu, D. Mandrus, A.P. Li, M.H. Pan, Shuheng Pan, V.B. Nascimento, E.W. Plummer, R. Jin The bilayered ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits interesting structural properties. For example, in the bulk the in-plane lattice parameter $a$ expands while the out of plane lattice parameter $c$ contracts as temperature increases, accompanied by a rotation of RuO$_{6}$ octahedra. Remarkably, the broken translational symmetry at the surface causes a tilt of RuO$_{6}$ octahedra. The Low Energy Electron Diffraction (LEED) $I-V$ analysis shows that the tilt angle of RuO$_{6}$ is temperature dependent and hysteretic. The correlation between RuO$_{6}$ tilt at the surface, the surface thermal expansion, and the electrical properties were investigated using Scanning Tunneling Spectroscopy, (LEED)-$I-V, $and Scanning Electron Microscopy. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y12.00004: Interface-Induced Complex Electronic Interference Structures in Ag Films on Ge(111) Tai Chiang, Yang Liu, Nathan Speer, Shu Tang, Tom Miller We have mapped the electronic structure of atomically-uniform films of Ag grown on Ge(111) by angle-resolved photoemission. Cuts in momentum space at constant energies near the Fermi surface reveal intricate patterns resembling interfering waves emanating from multiple centers. The measured dispersion relations exhibit zigzag patterns with multiple energy gaps. These features are attributed to the mixing of electronic standing waves by the Ag-Ge interface potential, as confirmed by the observed pattern symmetry and by an experimentally deduced interaction strength that scales as the inverse film thickness. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y12.00005: Influence of quantum well states on the formation of surface Au/Pb alloy in Pb/Si(111) quantum thin films. Jungdae Kim, Shengyong Qin, Alexander Khajetoorians, Wenguang Zhu, Zhenyu Zhang, Chih-Kang Shih The thickness dependence of Au/Pb alloy formation on thin Pb quantum films is studied using \textit{in-situ} low temperature STM/S. Sub-monolayer Au was deposited on to thin Pb films on Si(111) substrates. When deposition is carried out at a substrate temperature close to room temperature, it is found that local Au/Pb surface alloys are formed in the form of nano-islands, with preferential formation probability at certain thicknesses. STS data shows this is directly related to quantum well states (QWS) of underlying Pb mesas. When the growth is carried out at low temperature ($\sim $80K), the alloy formation probability doesn't show strong thickness preference, but QWS has still strong influence on the nature of the Au/Pb alloy. Two types of Au/Pb alloy nano-islands are formed with quite different electronic properties. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y12.00006: Coherent quantum-well electronic structure in bimetallic Pb/Ag films prepared on Si(111). Matthew Brinkley, Yang Liu, Nathan Speer, Thomas Miller, Tai-Chang Chiang Angle-resolved photoemission is employed to investigate the electronic structure of Pb films of various thicknesses grown on atomically uniform Ag(111) films. The Ag films, which were deposited on Si(111) substrates, host fully confined electrons at energies within the absolute gap of Si and partially confined electrons outside. The question is: What is the electronic structure of the Pb films prepared over the Ag films? Our results reveal that the quantized electronic structure of the Ag films can be detected for Pb overlayers with thicknesses much larger than the photoemission escape depth. Comprehensive simulations have been performed and are in agreement with the experimental results. This study reveals a strong coherent coupling of the Ag and Pb electronic structures despite the incommensurate Ag/Pb interface. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y12.00007: Pseudogap Mediated by Quantum-Size Effects in Lead Islands Kedong Wang, Xieqiu Zhang, M.M.T. Loy, T.-C. Chiang, Xudong Xiao Quantum confinement effects in both metallic and semiconducting materials are subjects of intense prevailing interest. For thin films and islands of Pb grown on semiconductor surface, quantum well states have been clearly identified and their consequences in growth, thermal stability, and superconductivity have been well studied. In this talk, we will present scanning tunneling spectroscopy measurement results of Pb islands on Si(111) at high energy resolution that reveal a novel pseudogap, or a pseudopeak in special cases, around the Fermi level in addition to the usual quantum well states. These gap or peak features persist to temperatures as high as $\sim $80 K and are uniquely related to the quantum well nanostructure of the Pb islands. A systematic analysis indicates that electron-phonon scattering is responsible for the observed electronic structure. The behavior of the pseudogap has a strong resemblance to that of the pseudogap in high temperature superconductors and certain connections may be speculated. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y12.00008: Quantized Electronic Structure and Growth of Pb Films on Highly Oriented Pyrolytic Graphite Yang Liu, Jens Paggel, Mary Upton, Tom Miller, Tai Chiang We have measured the electronic structure of thin Pb films grown on highly oriented pyrolytic graphite (HOPG) by angle-resolved photoemission spectroscopy. Quantum well states (QWS) corresponding to confined Pb valence electrons are observed. Although the films are rough, the thickness distribution is sufficiently narrow to allow a unique assignment for each QWS peak in terms of a quantum number and the exact film thickness in atomic layers. The even film thicknesses are found to be much more prevalent than the odd film thicknesses. These results are consistent with an available first-principles calculation of the surface energies of freestanding films; an implication is that the interaction between the Pb film and the HOPG substrate is weak. The effective masses of QWS at the surface zone center agree well with the results calculated from the bulk Pb band structure, in sharp contrast to the strongly enhanced or anomalous effective masses in Pb films grown on Si(111) as reported previously. This finding indicates that the anomalous effective masses in Pb/Si(111) are not caused by increased electron correlation effects in a confined geometry, but rather attributable to a strong interfacial interaction between the QWS and the substrate electronic structure. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y12.00009: Quantum stability and superconductive properties of atomically smooth ultrathin alloy films of thermodynamically immiscible metal elements Eun Ju Moon, Mustafa M. Ozer, James R. Thompson, Hanno H. Weitering Pb and Ga are immiscible in bulk form. However, atomically smooth ultrathin films of Pb $_{1-x}$Ga $_{x}$ alloy (x=0.06) can be stabilized on a Si(111)7x7 substrate through the quantum size effect. The quantum stability and superconductive properties of these films were investigated using STM, XPS, and SQUID magnetometry measurements. Quantum stabilized growth defects, consisting of deep holes extending to the film-substrate interface, act as pinning centers for vortices in the superconducting state. The pinning centers support an extraordinarily robust critical state with critical current densities in excess of 3 MA/cm2 in 10 monolayer thick films. Anomalies in the dc magnetization and ac magnetic response below 2.5-3.5 K indicate a reduction of the flux pinning below these temperatures, which we attribute to the nature of the holes (deep holes as opposed to blind holes in pure Pb films). The present study highlights the possibility of growing new alloys beyond the solid solubility limit and controlling critical state properties in the quantum regime. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y12.00010: First-principles study of SiON/SiC(0001) surface insulating layer Yasunobu Ando, Kazuto Akagi, Shinji Tsuneyuki, Tetsuroh Shirasawa, Hiroshi Tochihara Silicon oxynitride (SiON) formed on a 6H-SiC (0001) surface could be an ideal insulating layer in the application of SiC for future semiconductor devices: it is not amorphous but crystalline with only three-atomic-layer (5A) thickness, while STS-IV measurements show that the surface has a band gap as large as crystalline SiO2 (Shirasawa et al., Phys. Rev. Lett. 98, 136105(2007)). In this study, we precisely calculated band gap profile of the system in the surface normal direction with a first-principles method based on density functional theory to find reasonable agreement with that obtained by the element-specific XAS and XES measurements. In view of its application to a metal-oxide-semiconductor (MOS) structure, we also investigated the effect of aluminum atoms deposited on the insulating layer at low coverage. Starting from several candidate structures, we optimized the structure and investigated the band gap profile. We found that adsorbate-originated states appeared within the large band gap at the surface, though the states are confined within a very thin layer apparently preserving the surface insulating layer. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y12.00011: Electronic structures of intrinsic \textit{n}-type SrTiO$_{3}$-LaAlO$_{3}$ interface: density and spatial distribution of free carriers Won-joon Son, Eunae Cho, Seungwu Han The seminal paper by Ohtomo and Hwang reporting the unexpected conductivity when two perovskite insulators, SrTiO$_{3}$ and LaAlO$_{3}$, formed an atomically abrupt interface along [001] direction, has aroused an immense interest on the origin of the conductivity. While it is widely accepted that the intrinsic $n$-type interface is conducting, the carrier density and its spatial distribution are not fully understood. In presentation, first-principles study for establishing the fundamental property of the charge carrier in the intrinsic $n$-type LaAlO$_{3}$/SrTiO$_{3}$ interface is introduced. To this end, large model systems including up to 20 layers of SrTiO$_{3}$ and 1--10 layers of LaAlO$_{3}$ are employed. The charge carrier is observed when LaAlO$_{3}$ is larger than 3 unit cells and it converges to 0.5 electrons per unit cell. It is also found that the charge carriers transferred from LaAlO$_{3}$ surface are mostly localized within a few layers from the interface. Furthermore, the electronic states are quantized at the interface with different localization widths, which is similar to electrons in a wedge potential. The carrier density contributed by the interface-localized state shows a good agreement with the experiment. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y12.00012: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y12.00013: Positron states and annihilation characteristics at the reconstructed (100) and (111) surfaces of Si with adsorbed hydrogen and oxygen. N.G. Fazleev, W.B. Maddox, A.H. Weiss We present results of theoretical studies of positron surface states, work functions and annihilation characteristics at the reconstructed (100) and (111) surfaces of Si with adsorbed hydrogen and oxygen. Calculations are performed taking into account geometrically optimized surface structures using a modified superimposed-atom method, and employing the corrugated-mirror model in a full three-dimensional geometry. The effects of adsorption of hydrogen and oxygen on the positron binding energy at the reconstructed (100) and (111) surfaces of Si and the positron work function are explored. The positron surface-state annihilation characteristics are computed for different hydrogen and oxygen coverages of both reconstructed surfaces of Si. The obtained results predict attenuation of the Si positron-annihilation-induced Auger electron signal intensity with the increase of the hydrogen and oxygen coverage consistent with experimental data. These studies confirm that positron-annihilation-induced Auger electron spectroscopy can be used to study changes in the properties of semiconductor surfaces due to the presence of adsorbed hydrogen and oxygen. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y12.00014: Fabrication and transport measurement of periodic wavy structures of Si/SiGe nanomembranes Minrui Yu, Robert Blick, Arnold Kiefer, Don Savage, Max Lagally We demonstrate fabrication of completely under-etched Hall-bars made from Si/SiGe nanomembranes with a highly doped SiGe layer. The sample material is epitaxially grown by chemical vapor deposition (CVD) on silicon-on-insulator (SOI) wafers. Hall-bars are defined by optical photolithography and etched by reactive ion etching (RIE). They are then completely released from the substrate through hydrogen fluoride (HF) vapor etching. The lattice mismatch between silicon (Si) and germanium (Ge) generates an initial strain inside the material, which tends to recover once the sacrificial oxide layer is removed. This combined with carefully designed geometric constraints causes the structures to buckle and generate periodic wavy patterns after releasing and rebonding to the wafer surface. We study the magneto-transport at both room and low temperatures, with and without light illumination. Our results show the effect of strain on band structure and electron mobility. This will further the understanding of mechanically modulated electron transport. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y12.00015: Electron accumulation on bare and hydrogenated indium nitride surfaces Brian Thoms, Rudra Bhatta, Ananta Acharya, Mustafa Alevli, Nikolaus Dietz Electron accumulation layers which affect device and contact properties have been reported on several semiconductor surfaces such as InAs, InN, and CdO. Adsorbates have been shown to affect the electron density on InAs surfaces, however, surface termination effects for InN have not been determined. In this work surface-sensitive electron spectroscopic techniques are used to study both the electron accumulation layer and the surface structure on N-polar InN. High resolution electron energy loss spectroscopy (HREELS) has been used to characterize the surface electron accumulation by observing changes in the energy of the conduction band plasmon loss with variations in incident electron energy. In addition, HREELS along with low energy electron diffraction and Auger electron spectroscopy allow characterization of the surface structure and bonding. By this method it is shown that both hydrogen-terminated and bare N-polar InN surfaces exhibit electron accumulation. These results indicate that surface electron accumulation on InN is not due to indium-indium bonding and is not substantially affected by the presence or absence of surface hydrogen, but may instead be intrinsic to the N-polar InN surface. The effects of other adsorbates will also be discussed. [Preview Abstract] |
Session Y13: General Theoretical and Computational Methods
Sponsoring Units: DCOMPChair: Christine Wu, Lawrence Livermore National Laboratory
Room: 309
Friday, March 20, 2009 8:00AM - 8:12AM |
Y13.00001: A global EMC/FDTD simulation tool for modeling THz wave interaction with conductive media K. J. Willis, S. C. Hagness, I. Knezevic We present a computational tool for modeling the interaction between THz electromagnetic waves and conductive media. By coupling the ensemble Monte Carlo (EMC) simulator of carrier dynamics and the finite-difference time-domain (FDTD) solver of Maxwell's equations, we have developed a robust and versatile global simulator that interactively tracks the field-particle dynamics. The global simulator enables accurate numerical examination of wave propagation in conductive media under circumstances in which macroscopic materials properties are not well established, such as under THz-frequency wave irradiation of highly doped semiconductors. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y13.00002: GPU Based Acceleration of First Principles Calculations Hidekazu Tomono, Toshiaki Iitaka, Kazuo Tsumuraya The saturation of the acceleration using the silicon devices has required the parallel computing using multiple CPU's (central processing units). The parallel computing has been widely used in the field of the high-performance computing. On the other hand, graphics processing units (GPU's) were designed to accelerate graphic applications in 1978. NVIDIA Co.\ began to provide CUDA for C-language users to manipulate the GPU's in 2007. They applied it to computational fluid dynamics, medical real time simulation and astronomical N-body problem among others. This is the GPGPU (general-purpose computation on GPU's), which is faster in operation than CPU in the fields of linear algebras, FFT, and others. We have experienced that one- dimensional CUFFT ver1.1 (GPU-FFT) is eight times faster than FFTW for single-precision case. We implement the GPU-FFT into our in-house first principles planewave code, in which the hot spot is the FFT routine. We will present the performance of the implementation. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y13.00003: Saturation effects in dispersion interactions Bo E. Sernelius The interest in Casimir interactions has been very strong during the last decade. This increase in interest was spurred by the torsion pendulum experiment by Lamoreaux which produced results with good enough accuracy for the comparison between theory and experiment to be feasible. Theory and experiment agree quite well for low temperatures. However at room temperature, where most experiments are performed there are serious deviations. Each new experiment has lead to new puzzling discrepancies between theory and experiment. Theorists have been forced to resort to phenomenological approaches to the problems, with new prescriptions for each new experiment. Here we address three experiments: Casimir pressure between a gold sphere and a gold plate; Casimir force between a gold sphere and a laser excited silicon membrane; Casimir force between a Rb atom and a fused silica wall. In all these different experiments we show that inclusion of saturation effects makes the discrepancies go away. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y13.00004: Symmetries of non-relativistic quantum-mechanical Hamiltonian Bojan Tunguz In non-relativistic quantum mechanics the most fundamental invariance group of the Hamiltonian is the Galilean group of transformations: the group spatial and temporal translations and rotations. The quantum-mechanical wave functions in that view belong to an infinite-dimensional representation of the Galilean group, and the generators are represented with first-order differential operators. In this work we look at all the higher order differential operators that commute with the Hamiltonian and construct the most general group that leaves the Hamiltonian invariant. We show how the Galilean group fits within this group, and we show how the interaction terms break the symmetry of the free-particle Hamiltonian. We argue that the interpretation of the Hamiltonian in terms of individual interacting particles is the consequence of this broken symmetry. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y13.00005: Planck's High Temperature Catastrophe in Observational Astronomy:- (NASA proves Planck wrong) Clarence A. Gall Planck's black body radiation law $\left( I_{P}=\frac{c_{1}} {\lambda ^{5}}\frac{1}{e^{\frac{c_{2}}{\lambda T}}~~-1}\right) $predicts that a hotter body (higher T) should always emit more intensely than a colder body (lower T) throughout the entire EMR spectrum. However, space age infrared astronomy contradicts this prediction! It is now known that as observation moves from the visible to the near-, mid- and far infrared; increasingly cold celestial objects come into view while hotter ones fade and disappear (http://coolcosmos.ipac.caltech.edu/cosmic\_classroom/ir\_tutori al/irregions.html). Were Planck's law valid, the hottest stars would never disappear; and colder objects would not be detected. This can only be described as a high temperature catastrophe (BAPS, April Meeting 2008, H12.3, St Louis, MO) for Planck's law! On the other hand, Gall's black body radiation law $\left( I_{G}=\sigma \frac{T^{6}}{b^{2}}\lambda e^{-\frac {\lambda T}{b}}\right) $ (http://sites.google.com/site/purefieldphysics) predicts that as wavelength increases, there is a crossover point above which a colder object will emit more intensely than a hotter one. Hence colder objects will appear and hotter ones will eventually disappear from view. The crossover point for black bodies at 6000K and 100K is 12.066 microns. These calculations with Gall's law are in overall agreement with observational infrared astronomy. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y13.00006: Expressing n dimensions as n-1 John Laubenstein The IWPD Scale Metrics team has explored a different conceptualization of multiple dimensions through a model expressing n dimensions as n -- 1. This is achieved by aligning time and a spatial dimension along the same orientation. We have shown that time and distance along the same axis in combination with a scalar is equivalent to two orthogonal dimensions. Scale Metrics is simply a different conceptualization of multi-dimensions; however, it requires a change in the modeling of gravitation since time is no longer considered to be orthogonal to the three spatial dimensions. A model for Scale Metrics gravity has been developed and in the process a quantum theory of gravitation emerges. Why entertain IWPD Scale Metrics? Because the Standard Model has not been successful in the unification of GR with QT. A new model of gravitation built on a foundation of quantum concepts (as opposed to a quantum fix to an inherently classical geometric theory) may be of benefit in the ultimate search for the unification of gravitation with quantum theory. Further, since Scale Metrics provides nothing more than a different way to conceptualize multiple dimensions in a manner that is equivalent to 4 vectors -- it replaces nothing, but rather serves only to complement past and current achievements while providing a new view of quantum gravitation. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y13.00007: The mass, energy, space and time system theory-MEST- the theory of relativity and the quantum mechanics Dayong Cao Things have their own physical system of mass energy, space and time of themselves. (The MEST for short thereinafter) So we can use it to unite both the physical system of any thing. There are the transmutation between space-time and mass-energy. There are the conservation of space-time and mass-energy.The uf force like the gravitation is from space-time, the down force like repulsion is from mass-energy. So there are the positive and negative curvature. We need to develop the theory of relativity and get the new equation. By using the mass-energy wave equation, deduce the new uncertainty principle, uncertainty and probability can not be divided. New wave web equation are being put forward. The quantum mechanics is the mass and energy theory of relativity; the theory of relativity is the space and time theory of relativity. MEST can unite both them. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y13.00008: Is Sound a Property of Space or Vacuum? Venkata Chaganti Absolutist regards SPACE as an entity in its own right with properties of its own. Further more, relationists, scientists and laymen attribute many different sorts of properties to SPACE. In fact, we are able to distinguish every material from the other by its properties. We all know that SPACE is attributed by some of the properties listed below. 1. Electromagnetic Properties like permittivity and permeability. 2. Empty SPACE is a poor conductor. 3. Empty SPACE is transparent. 4. SPACE is penetrable by any particle / material. 5. SPACE is incapable of action. 6. SPACE is immovable. 7. SPACE is infinite. 8. SPACE is isotropic. 9. All bodies are place in SPACE. 10. SPACE is Isotropic. And many more properties can be made quite precise without any reference to an embedding space surrounding the SPACE of interest. SPACE as a physical object only makes sense if it can be detected, or if it can exert physical influences. Does the motion of an object through SPACE lead to detectable effects? Can SPACE act on a moving body in the way that the sea acts on a moving fish? In this paper an attempt is made to show that SPACE is indeed like any other medium / material and also SOUND is its property. Statistical Mechanics is used as a tool. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y13.00009: New Generally Covariant Generalization of the Dirac Equation Not Requiring Gauges David Maker We introduce a new pde ($\Sigma _{\mu }\surd \kappa _{\mu \mu }$\textit{$\gamma $}$_{\mu }$\textit{$\partial \psi $/$\partial $x}$_{\mu }$\textit{-$\omega \psi $=0}) with spherically symmetric diagonalized $\kappa _{00}$ = 1-r$_{H}$ =1/$\kappa _{rr}$ giving it general covariance. If r$_{H}$ =2e$^{2}$/m$_{e}$c$^{2}$ this new pde reduces to the standard Dirac equation as r$\to \infty $. Next we solve this equation directly using separation of variables (e.g., 2P, 2S, 1S terms). Note metric time component $\kappa _{oo}$=0 at r=r$_{H}$ and so clocks slow down with \textit{baryon 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 \textit{color} to guarantee this) spends $1/3$ of its time in each lobe (\textit{fractionally charged} lobes), the lobe structure is locked into the center of mass \textbf{(}\textit{asymptotic freedom}), there are \textit{six }2P states (corresponding to the 6 flavors);~the P wave scattering gives the \textit{jets}\textbf{,} all these properties together constituting the~\textit{main properties of quarks!}~without invoking the many free parameters, gauge conditions of QCD. Also the 2S$_{1/2}$ is the\textbf{ }\textit{tauon} and the 1S$_{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$is the \textit{muon} here. The S matrix of this new pde gives the \textit{W and Z as resonances and does not require renormalization counterterms or free parameters. }Thus we get nuclear, weak and E{\&}M phenomenology as\textit{ one} step solutions of this new pde, not requiring the standard method's pathology of adhoc assumptions such as gauges and counterterms, 19 free parameters (you can vary any way you want) that have confused, blocked the progress of theoretical physics for the past 30 years. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y13.00010: The darkness of mere being: DM and DE explained James Beichler In previous APS meetings, I have presented a geometrical explanation of Dark Matter and Dark Energy that makes testable predictions and is thus completely falsifiable. The theory is based on a macroscopically extended fourth dimension of space that yields a five-dimensional space-time structure. In this structure, the four-dimensional space-time of relativity is extrinsically curved in the higher spatial dimension. Dark Matter is curvature in the higher dimension that is not directly associated with local matter, but instead results from an interaction between local matter or curvature and the global curvature due to all matter in the universe. Criticisms that the theory was not mathematical have now been overcome and a simple algebraic formula that corresponds to the geometry of the four-dimensional structure of space has been derived. The algebraic formula appears to be Newtonian, but it implies a five-dimensional unified field structure such as that developed by Kaluza in 1921 and extended by Einstein and his colleagues in the late 1930s. The new equation also shows how gravity can be quantized on the basis of relativity without hypothesizing the discrete nature of matter, i.e., the existence of specific ``particles'' of gravity, inherent in quantum mechanics, the Standard Model and other quantum models. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y13.00011: Charged Particles are Preventing from Moving Faster than the Speed of Light by Light Itself Randy Wayne Many problems in classical mechanics are solved by assuming that friction is negligible. At velocities close to the speed of light, however, friction is never negligible as a consequence of the dilatant optical molasses that results from the temperature-dependent blackbody distribution of photons. A body moving at relativistic velocities experiences the blackbody radiation as being Doppler shifted. This adds a nonlinear velocity-dependent component of friction. By accounting for this thermodynamic friction, I have obtained an equation of motion that is applicable for modeling the movement of particles at relativistic velocities. While the predictions of the opto-mechanical model are qualitatively consistent with the predictions of the Theory of Special Relativity in terms of the nonlinear relationship between force and acceleration, there are quantitative and testable differences. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y13.00012: Why So Many More Americans Die in Fires Lawrence Cranberg ``Why So Many More Americans Die in Fires'' is the headline on Page 3 of The New York Times' full-page story on December 22, l991, by D. G. McNeil, Jr. This is a partial report based on personal experience with domestic fire making for thermal comfort since l975 (1) and a published claim (2) of unique safety benefits.The McNeil report attributes the problem to ``A Case of Bad Attitude'' and ``A Reliance on Technology.'' That implies a ``bad attitude'' on the part of technologists - a conclusion consistent with this technologist's thirty-five years of experience with fellow technologists, who has found ``buck-passing'' the favorite recourse of technologists in the highest places in government even though, as McNeil has written, ``Many children never wake up. Smoke or toxic gases overcome them as they sleep. When fire fighters lift them, their imprints remain.'' Regrettably, in this author's experience, the courts have also displayed a ``bad attitude'' where ``life and death issues'' have been pleaded. 1. L. Cranberg, Slot Flame Stablity with Hohlraum Radiation Pattern, BAPS, Series II, Vol. 20, No. 9, Sept., l978. 2. L. Cranberg, Fireplace Firesafety, Fire Journal, Letter, May/June,l987 [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y13.00013: A new Virtual Crystal Approximation approach Roberta Poloni, Jorge I\~{n}iguez, Alberto Garcia, Enric Canadell It is well known that the virtual crystal approximation (VCA) provides an efficient method for studying disordered alloys and solid solutions by first-principles. Although several studies have reported VCA results regarding stability issues by using energetic considerations, here we propose a new approach based on alternative structural indicators. The reason for this is that it is still not clear whether energy comparisons for different virtual compositions are trustworthy. Our non-fully-predictive scheme makes use of some experimental information in order address structural problems like atomic ordering and/or partial occupation at some site. We look at different figures of merit (energy derivatives), depending on the amount of experimental information taken into account, and we minimize them with respect to different possible structural configurations. By applying our approach to a wide number of well known systems (oxinitrides, borocarbides, perovskites, etc.) we have been able to reproduce the experimental structure in all cases. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y13.00014: Solvent-Shift Monte Carlo: A cluster algorithm for solvated atomistic and coarse-grained systems David Earl, Christopher Hixson, James Benigni We present a cluster algorithm for the efficient simulation of solvated molecules that we term solvent-shift Monte Carlo (SSMC). The algorithm involves a conformational change in a solvated solute molecule of interest, followed by a geometrical rotation of solvent particles. The method satisfies detailed balance and can be applied to existing schemes to sample conformational space, where an axis or plane of rotation can be defined. We demonstrate that the algorithm significantly enhances the sampling of phase space in solvated systems, and may be easily combined with other advanced sampling techniques such as parallel tempering. [Preview Abstract] |
Session Y14: Nonequilibrium and Templated Assembly
Sponsoring Units: DFDChair: Jack Douglas, National Institute of Standards and Technology
Room: 315
Friday, March 20, 2009 8:00AM - 8:12AM |
Y14.00001: Speeding up the understanding of Vertical Deposition of Diluted Colloids Wenceslao Gonz\'alez-Vi\~nas, Maximiliano Giuliani, Moorthi Pichumani We measured the speed of contact line in vertical deposition of diluted micron sized polymeric colloids. We correlated these results with the obtained morphologies for the deposits. We show that low velocities correspond to the formation of monolayer and high velocities to multilayer. These new results are explained in terms of the local concentration of particles in the suspension near the contact line and the porosity of the pre-deposited structure. The effect of an applied electric field to the system is also reported. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y14.00002: Kinetics of formation and disintegration of ionic and non-ionic spherical micelles Gunjan Mohan, Dmitry Kopelevich Dynamics of self-assembly and structural transitions in amphiphilic systems play an important role in various industrial and biological processes. Main challenge in computational modeling of these dynamics is a complex interplay between various length- and time-scales. In this talk, we discuss development of a multi-scale model for formation and disintegration of non-ionic and ionic spherical micelles. This study is performed under the assumption that the dominant mechanism of micelle formation (disintegration) is a stepwise addition (removal) of individual surfactant monomers to (from) a surfactant aggregate. A series of molecular dynamics simulations is used to develop reduced stochastic models for these elementary processes. It is demonstrated that these processes involve complex interactions of the translational degree of freedom (i.e., distance between centers of mass of the aggregate and the monomer) with degrees of freedom corresponding to the monomer orientation and the micellar shape and microstructure. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y14.00003: Using fluid flow to control the structure of soluble surfactants deposited through receding contact lines. Benjamin Beppler, Kalyani Varanasi, Stephen Garoff, Kristina Woods, Guennadi Evmenenko Moving contact lines are often used to deposit soluble organic molecules in applications such as spin coating and dip coating. In this study, we demonstrate that altering the flow field near such a contact line fundamentally changes the deposited surfactant structure. At slow contact line speeds, the substrate emerges dry. The rolling fluid motion near the contact line deposits a densely packed, tilted monolayer of surfactant along the emerging solid-vapor interface. Above a critical contact line speed, an evaporating thin film is entrained on the emerging substrate. Surfactant concentration constantly increases in this confined environment due to solvent evaporation. Monodisperse crystalline islands nucleate and grow on the surface with sizes and shapes controlled by varying the deposition conditions. These results contrast with disordered deposits that result from evaporation at a pinned contact line. Our results suggest that dip-coating with control of dipping speed and evaporation rate produces unique assembled structures and may provide better control of deposition through moving contact lines. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y14.00004: Rapid Convective Deposition For Fabrication of Microlens Arrays James Gilchrist, Pisist Kumnorkaew, Nelson Tansu, Yik-Khoon Ee Micron-sized microspheres were deposited into thin films via rapid convective deposition, similar to the `coffee ring effect' using a similar method to that studied by Prevo and Velev, Langmuir, 2003. By varying deposition rate and blade angle, the optimal operating ranges in which 2D close-packed arrays of microspheres existed were obtained. Previous models do not consider the effect of blade angle and blade surface energy on the deposition rate. Using a confocal laser scanning microscope, dynamic self-assembly of colloidal particles under capillary force during solvent evaporation was revealed. The resulting microstructure controlled by varying the macroscale parameters and interaction between substrate and colloidal particles played an important role in formation of ordered crystalline arrays. These interactions were explored through a model comparing the residence time of a particle in the thin film and the characteristic time of capillary-driven crystallization to describe the morphology and microstructure of deposited particles. Fabricated microlens arrays assembled on LEDs using this process were demonstrated to enhance performance by 300{\%}. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y14.00005: Permanent Flow-Induced Phase Transitions in Wormlike Surfactant Micelle Solutions Mukund Vasudevan, Eric Buse, Amy Shen, Bamin Khomami, Radhakrishna Sureshkumar It is well known that certain wormlike micelle solutions form flow-induced structures under shear flow. This structure transition is typically accompanied by an enhancement in the shear viscosity and the emergence of a new gel phase. However, such transitions are generally believed to be reversible, i.e., upon flow stoppage, the structure relaxes to equilibrium. In this work, we show that by subjecting translucent wormlike micelle solutions to high flow deformation over a rapid time scale, permanent flow-induced structures can be formed. We will discuss the phenomenology and plausible physical mechanisms underlying this discovery. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y14.00006: Understanding the structure of porous materials created by freeze casting Stephen Barr, Erik Luijten When a suspension of colloidal particles in water freezes, dendrites of ice with high aspect ratios are formed which can either engulf or reject the particles based on their size and the velocity of the advancing ice front. As the particles are pushed between the dendrites, concentrated regions of colloidal particles are formed. Recent experiments have shown that this can be exploited to create strong, lightweight, porous materials. We investigate this process using molecular dynamics simulations, focusing on the effect of the ice front velocity on the structure of the resulting material. We develop a simulation model which accounts for particle engulfment or rejection by the dendrites. We study both columnar and lamellar geometries. Our main finding is that variation of the front velocity not only affects the particle concentration in the interdendritic regions, but also the degree of order of the resulting solid. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y14.00007: End-Functionalized Triblock Copolymers as a Guide for Nanoparticle Ordering Rastko Sknepnek, Joshua Anderson, Monica Lamm, Joerg Schmalian, Alex Travesset Using molecular dynamics simulations we show that triblock copolymers, designed to have specific affinity for nanoparticles at the chain ends, can successfully mediate assembly of nanoparticle/copolymer composites. In this talk, we will present a detailed investigation of the phase diagram of these nanocomposites as a function of both nanoparticle size and concentration. We find a rich phase diagram with two striking features. The first is the existence of an unconventional square columnar phase of two interpenetrating line-lattices of micellar cylinders and aligned nanoparticles, and the second is a drastically enhanced stability of the gyroid phase. We interpret the origin of the square columnar phase by making an analogy to the packing of binary mixtures of disks. Based on the analysis of stretching of copolymers we argue that nanoparticles help stabilize gyroid order and drastically widen the region of its stability. Our study suggests that combining nanoparticles with functionalized block copolymers can provide a simple and efficient tool for assembling novel materials with nanometer scale resolution. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y14.00008: Unexpectedly wide distributions in the stochastic synthesis of functionalized nanoparticles. Jack Waddell, Douglas Mullen, Bradford Orr, Mark Banaszak Holl, Leonard Sander Functionalized nanoparticles are promising devices with a variety of applications, such as the targeted delivery of chemotherapy drugs to cancer cells. Their properties depend on the specifics of the distribution of functional groups on the nanoparticle. Stochastic ligand conjugation is an efficient strategy for synthesizing large quantities of functionalized nanoparticles. We developed a kinetic model for the study of ligand distribution on a generation 5 poly(amidoamine) dendrimer, as measured by HPLC and SPR. We found a cooperative effect in single species ligation, leading to a broader-than-Poisson distribution of ligands on nanoparticles, and suggesting a high spatial correlation of functional groups. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y14.00009: Self assembly of silica nanoparticles in a surfactant mesophase K. Sharma, K. Guruswamy, O. Mondain-Monval, I. Ly We examine the organization of silica particles in a hexagonal mesophase of a non-ionic surfactant, C$_{12}$E$_{9}$ in water. The mesophase has a characteristic length scale (cylinder center-to-center distance, a $\sim $ 5.7 nm). We vary the size of the silica particles from $\sim $ 2 nm ($<$ a) to $\sim $500 nm ($>>$ a), to examine the effect of particle size, and use a combination of SAXS, freeze fracture TEM and optical microscopy to characterize our materials. We show that particles $<$ a behave like a solvent and template the mesophase. Particles with a size $\sim $ a are partitioned into a dispersed phase and into strand-like aggregates. Particles $>$ a phase separate from the mesophase and form strand-like aggregates that organize into a network. The formation of this network is reversible and heating into the high temperature isotropic phase leads to dispersion of the particles. Unusually an \textit{increase} in hexagonal-isotropic transition temperature is seen for the mesophase-particle composites. We show that the network forms by expulsion of the particles from growing hexagonal phase domains -- as these domains grow, the particles are concentrated in the isotropic regions until they jam to form the network. We show that we are able to tune the mesh size of the particulate networks by changing the cooling rate. [Preview Abstract] |
Session Y15: Liquid Crystals III
Sponsoring Units: DFDChair: Luz Martinez-Miranda, University of Maryland
Room: 316
Friday, March 20, 2009 8:00AM - 8:12AM |
Y15.00001: Magnetic-field induced isotropic to nematic liquid crystal phase transition J.T. Gleeson, T.B. Ostapenko, D. B. Wiant, S.N. Sprunt, A. Jakli We report on measurements of magnetic field induced nematic order in the bent-core liquid crystal 4-chlororesorcinol bis[4- (4-n-dodecyloxybenzoyloxy) benzoate]. Using the 31 Tesla solenoid at the National High Magnetic Field Laboratory, we have observed, at temperatures less than one degree above the clearing point, a first-order transition to the nematic phase. The critical magnetic field at which this occurs increases with temperature. We discuss these results within the context of both Maier-Saupe and Landau-deGennes mean field models for the nematic-isotropic transition. The implications of possible tetrahedratic order are also discussed. To our knowledge, this is the first observation of such a magnetic field-induced transition in a thermotropic liquid crystal; the reasons for which this behavior is now attainable are discussed. \\ \\ This work was supported by the NSF (DMR-0606160) and Kent State University. Work performed at NHMFL supported by NSF cooperative agreement DMR- 0084173, the State of Florida and the DOE. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y15.00002: High Magnetic Field-Induced Birefringence in Lyotropic Chromonic Liquid Crystals T. Ostapenko, Yu. Nastishin, J.T. Gleeson, S.N. Sprunt, O.D. Lavrentovich, P.J. Collings We studied the effect of magnetic-field induced birefringence of a 14{\%} solution of disodium cromoglycate (DSCG) in water at temperatures above the nematic-isotropic coexistence region. According to Landau-deGennes mean field theory, we expect to find a linear relationship between the inverse of the induced birefringence, $\Delta $n, and the quantity (T-T*), where T* is the stability limit of the isotropic phase. Using the 31 T resistive magnet at the National High Magnetic Field Laboratory, we observed that, as we increase the temperature above the coexistence region, we deviate from this linear dependence. Our data shows that $\Delta $n goes to zero, whereas Landau-deGennes predicts that $\Delta $n should decrease asymptotically. This may be due to the lack of isodesmic aggregate formation at a finite temperature above the coexistence region.\\ \\ Supported by NSF (DMR-0710544 and DMR-0606160). Work performed at NHMFL, supported by NSF cooperative agreements DMR-0084173, the State of Florida and the DOE. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y15.00003: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y15.00004: Chromonic liquid crystalline properties of dyes Xuxia Yao, Jung Park, Mohan Srinivasarao As a new class of lyotropic liquid crystals, chromonic liquid crystals (CLCs) can self-assemble into an ordered complex fluid, potentially useful for organic solar cells. Different from common amphiphilic lyotropic mesophases, CLCs have no optimum aggregation size, which implies the order parameter increases with concentration. We used capillary flow and magnetic field to induce alignment in chromonic dyes and studied the aggregation behavior by Vis-spectroscopy, the phase behavior by POM and DSC, and the order distribution by Raman Scattering. We also investigated how the molecular structures influence the structures of mesogens and the morphology in the dried film which will further influence the charge mobility in the solar cells. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y15.00005: Liquid Crystalline Phase Transition of Colloidal Platelets with Identical Thickness Dazhi Sun, Zhengdong Cheng, Hung-Jue Sue The disorder -- order transition in discotic colloids has been an active research area since the observation of the nematic phase in clay suspensions by I. Langmuir in 1938. In the past decade, synthetic platelets have been used extensively to investigate the discotic liquid crystal phase transitions. Here, we report the phase behavior of a new model platelet system -- alpha-zirconium phosphate (ZrP). After exfoliation, the monolayer ZrP platelets possess uniform thickness, but have a high polydispersity in diameter. We observed an isotropic -- nematic transition in our system upon increasing the platelet volume fraction, followed by the formation of the discotic smectic phase, an elusive phase that has been rarely seen in discotic liquid crystals. The discotic smectic phase (domain) is characterized by X-ray diffraction, high-resolution transmission electron microscopy, and optical microscopy. The equation of state (EOS) of our system is also discussed. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y15.00006: Study of the Isotropic-Nematic and the Nematic-Smectic-A Phase Transitions in Carbon Nanotubes and Liquid Crystal Composites Krishna Sigdel, Germano Iannacchione A high-resolution ac-calorimetric study of the isotropic to nematic (\emph{I-N}) and the nematic to smectic-A (\emph{N-SmA}) phase transitions of carbon nanotubes (CNTs) and liquid crystal octyl-cyanobiphenyl (8CB) composites (8CB+CNTs) as a function of CNTs concentration is reported. Scans were performed on heating and cooling for all samples (0.5-6 wt\% of CNTs) over a wide temperature range well above and below the two transitions in pure 8CB. Both the \emph{I-N} and the \emph{N-SmA} transitions evolve in character and have their transition temperatures shift lower as the wt\% of CNTs increases. For intermediate wt\% values, new transitions features are observed, which suggest new phase ordering of the CNTs within the liquid crystal host. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y15.00007: Recovery and stabilization of a reversed phase sequence in a ternary liquid crystal mixture Ronald Pindak, Shun Wang, LiDong Pan, B.K. McCoy, Suntao Wang, H.T. Nguyen, Cheng-Cher Huang The nOHFBBB1M7 (n =10) compound, 10OHF, shows a reversed SmC*$_{FI2}$ - SmC* phase sequence, unique among all known antiferroelectric liquid crystals. This reversed phase sequence is stabilized upon doping with 9OTBBB1M7(C9) or 11OTBBB1M7(C11). In contrast, doping with homologous compounds (n = 9, 11, or 12) eliminates the SmC*$_{FI2}$ phase. One 10OHF/11OHF mixture without the SmC*$_{FI2}$ phase was selected for further studies. Adding C9 into this mixture revives the reversed phase sequence. Unexpectedly, even though 11OHF destabilizes the SmC*$_{FI2}$ phase in binary mixtures with 10OHF, it significantly increases the SmC*$_{FI2}$ temperature range in 10OHF/11OHF/C9 ternary mixtures. The extended temperature range is important for device applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y15.00008: Room-Temperature Liquid Crystal Blue Phases Stefanie Taushanoff, Khoa Van Le, Robert Twieg, Antal Jakli The ``blue phases'' of a highly chiral liquid crystal are defect-studded structures of double-twist cylinders that are laced together. The three phases, BPI*, BPII* and BPIII* differ only in the packing of the double-twist cylinders. Until recently, blue phases were of limited practical use because they appeared for only a very narrow temperature range. Mixtures that show BPI* and BPII* phases for wide temperature ranges at or around room temperature are now available [1]. Relatively wide temperature BPIII (the blue fog) phase so far was available only at very high temperatures [2]. Here we present mixtures with room-temperature wide range BPIII* phase and compare the ability of chiral dopants to form the different blue phases in a base nematic mixture. PDLC films cast with blue-phase material are also examined.\\[3pt] [1] H. Coles and M. Pivnenko, Nature 2005 436-18 997-1000 \\[0pt] [2] C. V. Yelamaggad, I. S. Shashikala, G. Liao, D.S. Shankar Rao, S. K. Prasad , Q. Li A. Jakli, Chem. Mater Comm, 2006, 18, 6100-6102 [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y15.00009: Hybrid shells of nematic liquid crystal Alberto Fernandez-Nieves, Teresa Lopez-Leon We investigate the consequences of changing the boundary conditions for the nematic director at the outer surface of a spherical shell from planar to homeotropic. We find there are different routes to the final equilibrium configuration, depending on the initial shell structure. For bipolar shells, which are shells having two pairs of s=+1 boojums on either surface, a disclination ring forms, shrinks and disappears in a process that is highly reminiscent of that seen in bipolar drops. By contrast, shells with four s=+1/2 defects develop open disclination lines in the inner surface; these lines form between the original s=+1/2 defects and force their approach and coalescence. These results highlight the fascinating range of behaviors that are driven by the interplay between topological constraints and the nematic order of liquid crystals. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y15.00010: Optical waveguiding in bent core liquid crystal filaments Antal Jakli, Jake Fontana, Chris Bailey, Wolfgang Weissflog, Istvan Janossy, Peter Palffy-Muhoray We demonstrate optical waveguiding in recently discovered free-standing bent core liquid crystal filaments. The bent core liquid crystal molecules self-assemble into a novel cylindrical geometry that is ``solid-like'' in the radial direction of the filament and liquid in the axial direction of the filament. Waveguiding properties of filaments of millimeter lengths were characterized. The transmitted power density through the filament was independent of temperature from 180oC to near room temperature. Initial defect of newly pulled filaments were found to self anneal, thus leaving perfectly defect free fibers, where light scattering was found to be insignificant. The absorbance was found to be strongly wavelength dependent in the visible regime and very small in the infrared range. A self-assembled optical waveguide with self-annealing fluid properties may have promising applications in optical communications. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y15.00011: Dielectric properties of bent-core nematic materials Peter Salamon, Nandor Eber, Samuel Sprunt, James Gleeson, Antal Jakli We report of dielectric spectroscopy measurements on bent-core nematic liquid crystals. The components of the relative dielectric permittivity and the dielectric loss have been measured as functions of frequency and temperature in the case of various bent-core mesogens in their nematic and isotropic phases. The results show that these liquid crystals have extraordinary dielectric behaviors if we compare them to the traditional calamitic materials, such as -- they show low frequency ( $<$ 10 kHz ) relaxations. Distortion elastic constants measurements reveal that K$_{1}$ / K$_{3} \quad \sim$ 1 in contrast to typical calamitics. The reasons for these anomalous behaviors will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y15.00012: Second Harmonic Generation in a Bent-core Nematic Liquid Crystal. Seung Ho Hong, Antal Jakli, James Gleeson, Samuel Sprunt, Brettt Ellman We studied second harmonic generation (SHG) as a function of optical polarization in a magnetically-aligned bent-core nematic liquid crystal (BCN). At the isotropic to nematic transition we detect the onset of a weak SH signal, which stays approximately constant through the nematic phase. Our results for polarization selectivity and for cells of different thickness indicate that the signal from the BCN cannot be explained by quadrupoles, defects in director orientation, fluctuations or cell-surface polarization. We discuss models for a noncentrosymmetric component of the BCN structure that can explain our data. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y15.00013: Viscoelastic parameters and flexoelectric effect in a bent-core nematic liquid crystal studied by dynamic light scattering Madhabi Majumdar, K. Neupane, James T. Gleeson, Antal Jakli, Samuel Sprunt Recent measurements of the flexure-induced electric polarization in certain bent-core nematics (BCNs) have demonstrated a giant flexoelectric effect [1]. We present a study of nematic elasticities and viscosities in one of these compounds, together with an attempt to characterize the flexoelectricity by its effect on director fluctuations. Our results combined with a reanalysis of earlier data indicate that the flexoelectricity is a phenomenon distinct from the ordinary director modes; additionally we observe unusual, very slow fluctuations in polarized scattering which suggest the BCN has a heterogeneous, ``glassy'' character. Dilution in a miscible calamitic indicates a dramatic development of the slow dynamics between 30 wt {\%} and 60 wt {\%} BCN. We suggest a model to account for both our present results and the giant flexoelectricity discovered in [1]. Reference: [1] J. Harden \textit{et al}., \textit{Phys. Rev. Lett., 97,} 157802 (2006). Acknowledgement: NSF DMR-0606160. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y15.00014: Large Flow-Birefringence of Nematogenic Bent-Core Liquid Crystals Christopher Bailey, Katalin Fodor-Csorba, Rafael Verduzco, James Gleeson, Samuel Sprunt, Antal Jakli We have found that bent-core liquid crystalline materials show exceptionally large flow birefringence in their isotropic liquid phase. The flow birefringence is over two orders of magnitude larger than usual for low molecular weight liquid crystals. Comparing the flow birefringence per unit viscosity, the observed values are an order of magnitude larger than low molecular weight and side-chain polymeric calamitic liquid crystals. This large flow birefringence is attributed to the nanostructure of these materials that contain temporary smectic clusters of a few smectic layers, which exist even in their isotropic phase. These smectic clusters appear to shear align resulting in the observed flow birefringence behavior. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y15.00015: Activated Kinetics of Nematic and Smectic Phase Transitions in an Aligned Matrix of Nano-colloidal Liquid Crystalline Gel Dipti Sharma This study investigates an interesting thermal behavior of an aligned aerosil nano-colloidal system in the aligned matrix of octyl-cyanobiphenyl liquid crystal. This system was prepared by solvent dispersion method (SDM) where different densities of aerosil nanoparticles were added into octyl-cyanobiphenyl liquid crystal. Then samples were cycled into magnetic field during SmA-I transition to get an aligned matrix of nanocolloids. Heating scans were performed at various heating rates from 20 to 1 K min$^{-1}$ using DSC. Aligned samples follow Arrhenius behavior and showed a decrease in transitions temperature for SmA--N and N--I transitions when compared with the unaligned samples. The activation energy of the aligned system increases and its respective enthalpy decreases for the lowest density of aerosils 0.05 g cm$^{-3}$, then for the further increase of aerosil density, the activation energy decreases and its respective enthalpy increases. The second order transition SmA--N shows a higher activated kinetics than the weak first order N--I transition. This can be explained in terms of molecular interaction in between aerosil nanoparticles and aligned liquid crystal molecules, and developed strain in the matrix of the aligned system. [Preview Abstract] |
Session Y16: Elastomers and Gels II
Sponsoring Units: DPOLYChair: Ryan Hayward, University of Massachusetts
Room: 317
Friday, March 20, 2009 8:00AM - 8:12AM |
Y16.00001: Deuterium NMR studies of segment orientation in PDMS unimodal and bimodal endlinked networks. Claude Cohen, Geoffrey Genesky, T. Michael Duncan Polymer segment orientation in elastomers is revealed by solid state deuterium NMR spectra: earlier work has focused on the frequency split between the peaks of the spectra from stretched elastomers rather than the details of the lineshape. The split has classically been interpreted as a measure of polymer segment order parameter S caused by excluded volume interactions between neighboring segments. We synthesized deuterated PDMS chains of about 5000 g/mol and 80,000 g/mol to probe the segmental orientation of each component separately in bimodal networks. Even in the unstretched state, the spectra for the labeled short chain networks show an evolving lineshape with varying short chain content. We compute the average absolute value of the frequency shift of the entire spectrum to better account for the highly aligned segments. This method allows us to probe the chain segment alignment with increasing strain in both unimodal and bimodal networks and confirms Monte Carlo simulation results. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y16.00002: Volume-phase Transitions and Confined Water in Surface-tethered Poly(N-isopropylacrylamide) Networks Ajay Vidyasagar, Ryan Toomey We present a simple approach for studying volume-phase transitions in thin, mechanically anchored responsive polymer networks. The approach is based on the photo- cross-linking of copolymers synthesized from N- isopropylacrylamide NIPAAm) and methacroyloxybenzophenone(MaBP). We monitored the swelling of poly(NIPAAm-co-MaBP) networks in contact with aqueous solution as a function of cross-link density with both neutron reflection and ATR-FTIR. Neutron reflection reveals that the volume-phase transition of tethered poly(NIPAAm) networks coincides with the miscibility gap of linear poly(NIPAAm) solutions. Whether or not the poly(NIPAAm) network undergoes a continuous or discontinuous collapse depends on its degree of cross-linking. At cross-link densities above 5 mole$\%$, the network remains in the single-phase region of the linear solution phase diagram and shows only a continuous transition between the swollen and collapsed states. Moreover, in the collapsed state, 30-35$\%$ by volume of water remains in the network, which is independent of cross-link density. The relative position of the FTIR absorption peaks during the collapse follows very closely the amount of water in the layer and can be related to the local dielectric environment within the network. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y16.00003: Strain induced alignment of particles in an elastomer host Ben Spott, Jeremy Neal, Peter Palffy-Muhoray The properties of composite materials consisting of anisometric nanoparticles dispersed in a host depend on the orientation of the nanoparticles. It is interesting to ask how mechanical strain of the composite affects their orientation. We have carried out experiments stretching both two- and three-dimensional rubber samples containing rigid rod-like particles and measured their orientational order parameter as a function strain. We discuss these results, and make connections with theoretical models. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y16.00004: Dynamics of uniaxially oriented elastomers using dielectric spectroscopy HyungKi Lee, Daniel Fragiadakis, Darren Martin, James Runt We summarize our initial dielectric spectroscopy investigation of the dynamics of oriented segmented polyurethanes and crosslinked polyisoprene elastomers. A specially designed uniaxial stretching rig is used to control the draw ratio, and the electric field is applied normal to the draw direction. For the segmented PUs, we observe a dramatic reduction in relaxation strength of the soft phase segmental process with increasing extension ratio, accompanied by a modest decrease in relaxation frequency. Crosslinking of the polyisoprene was accomplished with dicumyl peroxide and the dynamics of uncrosslinked and crosslinked versions are investigated in the undrawn state and at different extension ratios. Complimentary analysis of the crosslinked PI is conducted with wide angle X- ray diffraction to examine possible strain-induced crystallization, DSC, and swelling experiments. Quantitative analysis of relaxation strengths and shapes as a function of draw ratio will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y16.00005: Shape-Memory Polymers Based on Fatty Acid-Filled Elastomeric Ionomers Elise Izzo, Robert Weiss Shape memory polymers (SMPs) have applications as medical devices, actuators, sensors, artificial muscles, switches, smart textiles, and self-deployable structures. All previous design of SMPs has involved synthesizing new polymers or modifying existing polymers. This paper describes a new type of SMP based on blends of an elastomeric ionomer and low molar mass fatty acids or their salts (FAS). Shape memory elastomers were prepared from mixtures of a sulfonated EPDM ionomer and various amounts of a FAS (e.g., zinc stearate, zinc oleate, and various aliphalic acids). Nanophase separation of the metal sulfonate groups provided the ``permanent'' crosslinks, while sub-microscopic crystals of the low molecular weight FAS provided a physical crosslink needed for the temporary shape. The material was deformed above the melting point of the FAS and the new shape was fixed by cooling the material while under stress to below the melting point of the FAS. Polar interactions between the ionomer and the FAS stabilized the dispersion of the FAS in the polymer and provided the continuity between the phases that allowed the crystals of the FAS to provide a second network of physical crosslinks. The temporary shape was erased and the material returned to the primary shape by heating above the melting point of the FAS. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y16.00006: Dynamics of Segmented Polyurethane Elastomers Using Dielectric Spectroscopy James Runt, Daniel Fragiadakis, Alicia Castagna, Taeyi Choi This investigation focuses on the molecular dynamics of segmented polyurethane copolymers with different hard segment contents (30 to 52 wt percent) and soft segment chemistries. Methylene bis(p-phenyl isocyanate) and 1,4-butanediol constitute the hard segments in all materials under investigation, while soft segments include poly(tetramethylene oxide) and a 80-20 mixture of poly(dimethylsiloxane) and poly(hexamethylene oxide). The dynamics of these materials were explored over a wide temperature and frequency range using dielectric spectroscopy. In addition to investigating the details of segmental and local processes, three dielectric relaxations above Tg were observed for the first time in segmented polyurethanes, and their origin discussed in the presentation. For example, the highest temperature process is assigned to Maxwell-Wagner-Sillars interfacial polarization. The strength of the MWS process is a sensitive indicator of the change in microphase-separated character. It disappears at a temperature similar to that at which the small-angle X-ray scattering maximum disappears, indicating the transformation to the single phase state. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y16.00007: Polydomain to monodomain transition in nematic liquid crystal elastomers Badel Mbanga, Fangfu Ye, Jonathan Selinger, Robin Selinger Liquid crystal elastomers are crosslinked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. Deformation of an initially polydomain nematic elastomer film induces a transition to the monodomain configuration. We model this phenomenon using a recently developed finite element elastodynamics simulation method. We study the rate-dependent material response upon uniaxial extension and resolve the textures that form along with the associated stress-strain behavior. Our simulations yield qualitative agreement with experimental observations. This model allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y16.00008: Liquid crystal elastomers: Bent core flexo-electricity Martin Chambers, Rafael Verduzco, Samuel Sprunt, James T. Gleeson, Antal Jakli We report on the swelling of calamitic liquid crystal elastomers (LCE) with bent-core mesogens (BCM); this swelling took place at a temperature where both materials were in their isotropic phase. The BCM used varied in the degree of saturation of their hydrocarbon tails, which affects both viscosity and phase behaviour. We determined both swelling magnitude and dynamics. The host LCE systems homogeneously imbibe BCM up to 30-40 {\%} mol. Based on differential scanning calorimetry, shape change anisotropy, and optical birefringence measurements, the swollen elastomers are all found to exhibit nematic phases, with some possessing a lower temperature smectic phase. Bent-core liquid crystal elastomers and swollen calamitic LCE in BCM were investigated for the flexoelectric properties by inducing a mechanical deformation. The value of the bend flexoelectric constant, e$_{3 }$of the swollen BCM containing LCE systems is comparable of that of the neat bent-core liquid crystal. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y16.00009: Photoinduced distortions of polydomain liquid crystal elastomer samples Wilder Iglesias, Peter Palffy-Muhoray We have studied the effects of illumination of a dye doped polydomain nematic elastomer. The sample was on a glass substrate, and its free surface was illuminated by polarized light from an Ar+ laser. The intensity of the reflected light in the far field was studied to probe photoinduced deformations. Above a threshold intensity, a target pattern appeared in the far field, indicating sample deformation. We discuss the experimental observations, and relate these to the processes such as photoinduced surface relief. Experimental results are compared with the predictions of simple theory. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y16.00010: Dynamic Aspect of Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers Kenji Urayama, Atsushi Fukunaga, Toshikazu Takigawa, Antonio DeSimone, Luciano Teresi The nematic elastomers swollen by low molecular mass liquid crystals exhibit a macroscopic deformation with a significant change in birefringence in fast response to electric fields. We experimentally characterize the dynamics of this ``electro-opto-mechanical effect.'' The optical and mechanical rise times (in response to field-on) decrease in nearly proportion to the square of field strength, while the corresponding decay times (in response to field-off) are almost independent of field strength. The optical rise and decay times are about one order of magnitude smaller than the mechanical ones. We also propose a minimal model to describe the main features of both static and dynamic characteristics of this phenomenon observed experimentally. 1) Urayama, K., Honda, S., Takigawa, T., \textit{Macromolecules}, \textbf{2006}, $39$, 1943. 2) Fukunaga, A., Urayama, K., Takigawa, T., DeSimone, A., Teresi, L., \textit{Macromolecules}, in press. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y16.00011: Analysis of Diffusion through Dynamic Network Polymers using Multi-photon Fluorescence Recovery after Photobleaching Jiahui Li, Kelley Sullivan, Edward Brown, Mitchell Anthamatten Multi-Photon Fluorescence Recovery after Photobleaching (FRAP) techniques are utilized to study small molecule transport through polymer networks containing multiple hydrogen-bonding functional groups. Experiments involve uniformly dispersing small dye molecules (fluorophores) into functionalized polymers and networks. Polymer samples are then locally bleached, and the recovery of fluorescence is studied using 2-photon fluorescence microscopy. By curve-fitting fluorescence recovery curves to a model, diffusion coefficients are obtained. We have investigated series of polymer networks containing H-bonding interactions with different compositions at different temperatures. The diffusion coefficient through these polymer networks shows an expected Arrhenius-like temperature dependence. The influence of hydrogen bonding and network architecture on transport activation energies will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y16.00012: Gelling Mechanism of Aluminum Di-Soaps in Oils Xiaorong Wang, Mindaugas Rackaitis This work demonstrates that aluminum di-soaps form nano-sized spherical micelles in the oils and that the aggregation of these micelles forms a network that gives rise to a gel formation -- thereby refuting a long-held belief that the gel formation was the result of linear polymeric chains of aluminum association. The discovery of such aluminum nano-particles could expand application of these materials to new technologies because these materials are chemically inert, odorless and nontoxic, and have been widely used in greases, paints, gels, cosmetics, drugs and foods. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y16.00013: Surface Plasmon Resonance Effects in Responsive Polyelectrolyte/Gold Nanoparticles Hydrogel Thin Films Ihor Tokarev, Iryna Tokareva, Venkateshwarlu Gopishetty, Sergiy Minko In this study, we explored localized surface plasmon resonance excited in gold nanoparticles coupled with continuous and macroporous stimuli-responsive hydrogel thin films. The 100-nm-thick porous hydrogel film with vertically aligned cylindrical pores decorated with spherical Au nanoparticles (synthesized in the hydrogel) and placed on Au islands (prepared on a glass substrate) enabled the highly-sensitive optical detection of changes in the swelling degree of hydrogel induced by an external stimulus (pH). The strong optical response of our sensing platform is attributed to the electromagnetic coupling between the nanoparticles and islands that is highly sensitive to the inter-particle spacing. The transformation of a chemical signal into the optical effect can be used for analytical applications. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y16.00014: Gelation, Dynamics and Mechanics of Associating Polymers: Exploring Parameter Space Robert S. Hoy, Glenn H. Fredrickson We present simulation studies of associating polymer melts, where a fraction of the monomers are ``sticky.'' A coarse grained hybrid MD/MC model is used to accurately model polymer dynamics and qualitatively capture chemical kinetics. The effects of varying sticky bond strength, sticky monomer concentration and placement along chains, chain length N, temperature T, and chemical kinetics are examined. All have independent and interesting effects on the dynamics and mechanics. We focus on the ``reversible'' gel regime where almost all chains are instantaneously connected to the network, yet chains are delocalized and the system displays complex time and strain- dependent properties. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y16.00015: Step Cycle Deformation Processing of Elastomers and Gels Based on Semicrystalline Polyolefin-based Block Copolymers Fanny Deplace, Zhigang Wang, Nathaniel A. Lynd, Atsushi Hotta, Glenn H. Fredrickson, Edward J. Kramer, Jeffrey M. Rose, Anna E. Cherian, Geoffrey W. Coates, Hisashi Ohtaki, K. Hirokane, F. Yamada, Yong-Woo Shin, Fumihiko Shimizu Recent catalysts have enabled the synthesis of block copolymers with semicrystalline syndiotactic and isotactic polypropylene endblocks and amorphous ethylene-r-propylene midblocks. In these copolymers, the crystals play the role of physical crosslinks which can deform plastically under stretching. Neat elastomers, gels in mineral oil and gels from which mineral oil has been extracted have been subjected to step cycle tensile tests. The incremental plastic deformation of the crystals has dramatic effects on the true stress versus extension ratio curves. Moreover, small and wide angle X-ray scattering experiments during step cyclic tests revealed the evolution of the microstructure of the crystalline blocks: crystal fibrils and crystals in the fibrils are oriented parallel to the tensile direction at large strains and once the stress has decreased to zero, the fibrils return to being randomly oriented again. [Preview Abstract] |
Session Y17: Focus Session: Quantum Metrology and Nanomechanics
Sponsoring Units: GQIChair: David DiVincenzo, IBM
Room: 318
Friday, March 20, 2009 8:00AM - 8:36AM |
Y17.00001: High-sensitivity diamond magnetometer with nanoscale resolution Invited Speaker: We will discuss our recent work on using isolated electronic spins in the solid-state as sensitive magnetic sensors [1,2]. This novel approach to magnetometry is enabled by the good coherence properties of electronic qubits, such as the spins associated with Nitrogen-Vacancy (NV) centers in diamond, as well as by advanced techniques for their coherent control. The key feature of this solid-state magnetometer is the possibility to confine the sensing spins into a crystal of nanometer size that can be brought extremely close to the magnetic field source, thus achieving high spatial resolution. Our first experiments demonstrate that the resulting magnetic sensor provides an unprecedented combination of ultra-high sensitivity and spatial resolution. The ultimate sensitivity limit is set by the interaction of the spin sensor with its environment and in particular the nuclear and electronic spin bath. As an outlook, we will discuss how engineering, controlling or harnessing the environment can lead to better sensitivity, even beyond the standard quantum limit. Finally, we will outline several exciting applications of the novel magnetic sensors in areas ranging from bio- and materials science to fundamental physics and single electronic and nuclear spin detection. \\[4pt] [1] J. M. Taylor, et al. Nature Phys. 4, 810-816 (2008). \\[0pt] [2] J. R. Maze, et al. Nature 455, 644 - 647 (2008) [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y17.00002: Coupling Nitrogen Vacancy Centers in a Diamond Nanopillar to a Silica Microsphere Khodadad Dinyari, Mats Larsson, Hailin Wang Nitrogen vacancy (NV) centers in a diamond nanopillar have been coupled to the whispering gallery modes (WGMs) of a silica microsphere. The NV centers were coupled to the WGMs by positioning a nanopillar near the equator of the microsphere with nanometer precision. For cavity QED studies, WGMs with $l=m$ are of interest due to their small mode volumes. It was observed that when a 200 nm diameter nanopillar was optimally coupled to this particular mode in a 50 $\mu m$ diameter microsphere, the quality factor (Q) was only reduced to 2x10$^{6}$ from an initial Q of 6x10$^{6}$. The nanopillars were fabricated from a bulk single crystal diamond by means of reactive ion etching, resulting in nanopillars with diameters as small as 200 nm with a height of approximately 1 $\mu m$. We estimate that a 140 nm pillar would allow a cavity linewidth of order 20 MHz, comparable to the zero phonon linewidth of a NV center. Producing a nanopillar with a 140 nm diameter is well within our fabrication technique making this composite system a suitable candidate for strong-coupling cavity QED. This nanopillar-microsphere system circumvents the poor controllability of nanocrystal based microresonator systems and utilizes the exceptional properties of both NV centers in bulk crystals and the ultra-high Q of silica microspheres. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y17.00003: An Ensemble of NV-centers in Diamond Coupled to a Flux Qubit David Marcos, Martijn Wubs, R. Aguado, M.D. Lukin, Anders Sorensen In the last years, the interface between quantum optics and solid state physics has been explored, and various models for hybrid qubits have been proposed. We consider a superconducting flux qubit coupled to an ensemble of NV-centers in diamond. An effective model is derived, showing how coherent transfer between both two level systems can be achieved, and the possible advantages of the system as the building block of a quantum computer. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y17.00004: Getting Information on Independently Prepared Quantum States -- When are Individual Measurements as Powerful as Joint Measurements? Chi-Hang Fred Fung, H. F. Chau Given a composite quantum system in which the states of the subsystems are independently (but not necessarily identically) prepared, we construct separate measurements on the subsystems from any given joint measurement such that the former always give at least as large information as the latter. This construction offers new insights into the understanding of measurements on this type of composite systems. Moreover, this construction essentially proves the intuition that separate measurements on the subsystems are sufficient to extract the maximal information about the separately prepared subsystems, thus making a joint measurement unnecessary. Furthermore, our result implies that individual attacks are as powerful as collective attacks in obtaining information on the raw key in quantum key distribution. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y17.00005: Quantum noise interference as a route to ground state cooling in cavity electromechanics Aashish Clerk, Florian Elste, Steve Girvin We present a theoretical analysis of a novel cavity electromechanical (or optomechanical) system where a mechanical resonator directly modulates the damping rate $\kappa$ of a driven microwave (or optical) cavity. We show that due to a destructive interference of quantum noise, the driven cavity can effectively act like a zero-temperature bath {\it irrespective} of the ratio $\kappa / \omega_M$, where $\omega_M$ is the mechanical frequency. This scheme thus allows one to cool the mechanical resonator to its ground state without requiring the cavity to be in the so-called good cavity limit $\kappa \ll \omega_M$. This behavior is in sharp contrast to the more common setup with a parametric coupling (where the mechanics modulates the frequency of the cavity); there, ground state cooling is only possible in the good cavity limit [1,2]. We also show that this system can be used to perform quantum-limited position measurements. The system described here could be implemented directly using setups similar to those used in recent experiments in cavity electromechanics [3]. \\[4pt] [1]~F. Marquardt \textit{et al.}, Phys.\ Rev.Lett.\ \textbf{99}, 093902 (2007).\\[0pt] [2]~I. Wilson-Rae \textit{et al.}, Phys.Rev.\ Lett.\ \textbf{99}, 093901 (2007).\\[0pt] [3]~J. D. Teufel \textit{et al.}, Phys.\ Rev.Lett.\ \textbf{101}, 197203 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y17.00006: Force sensitivity of a nanomechanical oscillator in a microwave cavity Jennifer Harlow, John Teufel, Tobias Donner, Konrad Lehnert We describe our efforts to realize ultrasensitive force detection based on sensing the motion of nanomechanical oscillators embedded in superconducting resonant microwave cavities. Such a force sensor requires a readout mechanism quiet enough that the sensitivity is limited by thermal noise of the oscillator, as we recently demonstrated [1]. Force sensitivity is optimized by low mass, high-Q mechanical oscillators which have been cooled to dilution refrigeration temperatures. With this goal in mind, we fabricate high-Q ($Q>10^5$), picogram mechanical beams with MHz resonance frequencies. We report measurements with sub-$aN/\sqrt{Hz}$ force sensitivity and discuss prospects for further progress. [1] C. A. Regal, J. D. Teufel, and K. W. Lehnert, Nature Physics 4, 555 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y17.00007: Resolved-Sideband Cooling of Nanomechanical Motion within a Microwave Cavity John Teufel, Jennifer Harlow, Tobias Donner, Michael Demoret, Konrad Lehnert We present recent experiments in which we couple the motion of a high-Q nanomechanical oscillator to the microwave fields in a superconducting resonant circuit [1]. This microwave optomechanical system is operated in the resolved-sideband regime in which the mechanical resonance frequency exceeds the cavity bandwidth. In this regime, the dynamical backaction of the microwave radiation further cools the mechanical motion from dilution refrigerator temperatures to even lower thermal occupancy. Recent improvements increase both the optomechanical coupling strength and the power handling capability of the cavity. We report progress toward cooling to the mechanical ground state with this system. [1] J. D. Teufel, J. W. Harlow, C. A. Regal and K. W. Lehnert, Phys. Rev. Lett., 101, 197203 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y17.00008: Photon-Mediated Magnetic Cooling of~a Micromechanical Oscillator Joonho Jang, Raffi Budakian In recent years, a number of techniques have been developed to cool a mode of a micromechanical oscillator to the ground state. In this talk, I will present a new scheme for cooling a micromechanical oscillator involving the interaction of a micron-size superconductor, attached to the cantilever, with an external magnetic field. When the cantilever is placed inside an optical cavity, the absorption of photons by the superconductor gives rise to a retarded force that modifies the damping of the oscillator. Initial measurements using NbSe$_{2}$, show approximately a factor of 25 reduction in the mode temperature from 5 K to 200 mK. By optimizing the cavity finesse, the magnetic field configuration, and the superconductor quasiparticle lifetime, a further reduction of 10$^{3}${\-}10$^{4}$ in the cantilever mode temperature could be realized. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y17.00009: Parametric Amplification and Detection of Nanomechanical Motion Jared Hertzberg, Tristan Rocheleau, Tchefor Ndukum, Keith Schwab We have performed experiments with a 5.57 MHz nanomechanical resonator (NR) capacitively coupled to a 5 GHz superconducting microwave resonator and cooled to a temperature of 142mK. When driving with two microwave tones, a configuration appropriate for back-action evading measurements of a single motional quadrature, we find that a parametric instability appears at high drive powers. Due to the interference of the microwave tones, the capacitive frequency shift of the NR is periodically modulated at twice the mechanical frequency, resulting in a degenerate parametric amplification of the mechanical motion. In this regime, we demonstrate mechanical gains of up to 11.6dB and parametrically reduced linewidths of 2.1 Hz, resulting in a position resolution near the standard quantum limit. Although this effect is expected to limit the back-action evasion dynamics, it is useful for mechanical preamplification and noise squeezing, subjects of future work. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y17.00010: Back action evading quantum limited measurements of a nanomechanical resonator Tchefor Ndukum, Tristan Rocheleau, Jared Hertzberg, Keith Schwab By driving a 5GHz superconducting, co-planar waveguide (CPW) resonator coupled to a radio-frequency nanomechanical resonator with both red- and blue-detuned, phase coherent microwave signals, we demonstrate amplifier noise back action evading(BAE) detection of one quadrature of nanomechanical motion. With this method we show precise measurements of a single motional quadrature with additive measurement noise of 4 times the zero point amplitude, and a reduction in sensitivity to injected measurement noise of a factor of 43 in comparison to a single tone, non-BAE measurement. We have also found a parametric instability which limits the coupling strength possible in our device, which will be described elsewhere. With straightforward improvements to the microwave resonator, we expect to be able to demonstrate sensitivity to one quadrature with additive measurement noise below the zero-point level, a necessary ingredient to produce and measure squeezed states of motion. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y17.00011: Sideband Resolved Cooling of a Nanomechanical Resonator Parametrically Coupled to a Microwave Resonator Tristan Rocheleau, Tchefor Ndukum, Jared Hertzberg, Keith Schwab We have fabricated a nanostructure formed by a radio-frequency nanomechanical (NEMS) resonator capacitively coupled to an aluminum 5 GHz superconducting, co-planar waveguide (CPW) resonator with 50 $\Omega$ characteristic impedance.By driving this coupled system at a frequency $\omega_{pump} =\omega_{CPW} - \omega_{NEMS}$, we demonstrate back action cooling effects of a single NEMS mode achieving cooling from temperatures of 100mK to $<$10mK, with the lowest occupation factor of N$<$30. We have recently demonstrated a Nb, 130 $\Omega$ 5 GHz, Q=15,000 microwave resonator which we expect to be capable of cooling the NEMS close to ground state. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y17.00012: Ground state cooling of nanomechanical resonator via linear coupling in a superconducting circuit Lin Tian In recent experiments, it has been demonstrated that radiation pressure-like coupling between a nanomechanical resonator and a superconducting resonator can be explored for the cooling of the nanomechanical mode. In this work, We present a ground state cooling scheme for a nanomechanical resonator linearly coupled with a superconducting LC oscillator. The linear coupling, when periodically modulated at red detuning, up-converts the low-frequency nanomechanical mode to the high- frequency LC oscillator mode and generates backaction force that can cool the nanomechanical mode to its ground state in the resolved-sideband regime. Compared with schemes using radiation pressure-like coupling, the LC oscillator mode doesn't need to be driven to high photon occupation number in our scheme. We calculate the cooling rate and the stationary occupation number of the nanomechanical mode and show that ground state can be reached with practical device parameters. A detailed study of our model shows that the quantum backaction noise that limits the cooling process is due to the counter rotating terms in the linear coupling. The scheme can be compared with laser cooling for the atomic systems as well. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y17.00013: Quantum Measurements of Coupled Systems L. Fedichkin, M. Shapiro, M. I. Dykman Quantum measurements are often performed on coupled systems. Such measurements are of interest for various proposed realizations of a quantum computer where the qubit-qubit coupling may not be completely turned off. Because of the coupling, the stationary state wave functions are not fully localized on individual qubits even where the energies of neighboring qubits are tuned away from each other. As a result, an instantaneous projective single-qubit measurement gives the state population with an error. We show that the error may be significantly reduced. This is accomplished by tuning the detector close to resonance with the measured qubit. The qubit- detector coupling should be small compared to the decay width $\gamma$ of the excited level of the detector. For such tuning, there is a broad time interval where the probability of an error in detecting an excitation on the resonant qubit and distinguishing it from other excitations is smaller than that for a projective measurement by a factor $\sim (\gamma/\Delta E)^2$, where $\Delta E$ is the difference in the qubit energies. The results bear on the scalability of quantum computers with permanently coupled qubits. [Preview Abstract] |
Session Y18: Focus Session: Surface Instabilities and Adsorbed or Grafted Layers
Sponsoring Units: DPOLYRoom: 319
Friday, March 20, 2009 8:00AM - 8:36AM |
Y18.00001: Drying-Mediated Self-Assembly of Highly Ordered Complex Structures: From Polymers to Nanoparticles Invited Speaker: Drying of a sessile drop containing nonvolatile solutes readily self-assembles into a number of concentric ``coffee rings'' by repetitive ``stick-slip'' motion of the three-phase contact line. However, due mainly to lack of control over the evaporation process of the drop, the challenge remains to use evaporative self-assembly to rationally ``synthesize'' ``coffee rings'' of different shapes and sizes of high regularity and fidelity. Here, we report a facile, robust, and one-step evaporation method for producing in a precisely controllable manner versatile microstructures possessing high regularity, dispensing with the need for lithographic techniques and externally applied fields. Polymer or nanocrystal solutions are confined in a simple geometry comprised of a curved surface placed upon a flat substrate. By changing the shape of the upper surface of the imposed geometry, the controlled, evaporative self-assembly of polymer or nanocrystal solutions yields a variety of complex, intriguing, and well-ordered structures over large areas. As such, this method represents a significant advance in creating regularly organized, complex structures with potential applications in microelectronics, optoelectronics, and biotechnology, among other areas. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y18.00002: Morphology of Osmotically-Driven Surface Buckles Derek Breid, Alfred Crosby The ordering of osmotically-driven surface buckles on an elastically-supported stiff plate depends strongly upon the dominant stresses acting on the surface during formation. For example, conditions which lead to buckle initiation at the center of the plate yield hexagonal dimple arrays characteristic of an isotropic surface stress, while buckles initiated at the edges align radially, indicating a theta-dominated state of compressive stress. Here, we present experimentally-observed buckling morphologies resulting from a variety of surface geometries, as well as Finite Element Modeling results which provide insight into the specific evolution of stresses which led to the formation of these morphologies. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y18.00003: Lateral Instability of Nanoimprinted Polymer Patterns during Thermal Annealing Yifu Ding, Kyle Alvine, Hyunwook Ro, Manish Kulkarni, Jack Douglas, Christopher Soles Nanoimprint Lithography (NIL) is a promising candidate for next generation lithography. Dense polymer patterns with relatively high aspect ratios can be created using NIL. Upon thermal annealing, the as-imprinted polymer patterns will smooth out due to the surface energy and sometimes residual stress within the patterns. Both effects will predominately drive the pattern decay vertically. Here, we present that under certain patterning conditions, lateral instability of the surface patterns, driven by surface fluctuations, can also become an effective mechanism for reducing the total surface area. The characteristics of the lateral instability were successfully carried out by annealing the patterns under a temperature gradient. We demonstrate that the lateral instability can be controlled by the imprinting conditions, and can be greatly enhanced by the additions of surfactants. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y18.00004: Mechanical instabilities in periodic porous elasto-plastic solids. Srikanth Singamaneni, Katia Bertoldi, Sehoon Chang, Ji-Hyun Jang, Seth Young, Edwin Thomas, Mary Boyce, Vladimir Tsukruk We describe the transformation of the periodic microporous structures fabricated by interference lithography followed by their freezing below glass transition. Periodic porous microstructures subjected to internal compressive stresses can undergo sudden structural transformation at a critical strain. The pattern transformation of collapsed pores is caused by the stresses originated during the polymerization of acrylic acid (rubbery component) inside of cylindrical pores and the subsequent solvent evaporation in the organized microporous structure. The results of a non-linear numerical investigation confirm the critical role of the bifurcation of the periodic solid under compressive stresses. In striking contrast to the earlier observations of elastic instabilities in porous elastomeric solids, the elastic-plastic nature of the crosslinked periodic microstructure studied here provides for the ability to lock in the transformed pattern with complete relaxation of the internal stresses. By confining the polymerization of acrylic acid to localized porous areas complex microscopic periodic structures are obtained. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y18.00005: Peculiarities in liquid phase of Styrene Butadiene rubber surface induced by Atomic Force Microscopy-assisted electrostatic nanolithography Mindaugas Rackaitis, Sergei Lyuksyutov, Dmytro Kashyn, Pavel Paramonov, Robert Mallik Nanoscale surface changes are reported for styrene butadiene rubber (SBR) films (10-100 nm) using protocol derived from Atomic Force Microscopy (AFM) electrostatic nanolithography. Under appropriate tip bias conditions, the electric field magnitude induced in SBR films is of the order of 10$^{8}$-10$^{9}$ V m$^{-1}$, which is sufficiently large to initiate cross-linking in the rubber. Peaklike surface features, surrounded by a circular trough and a raised ring, are observed after completing AFMEN-based protocol. The nanostructure dimensions vary from 0.5-20-nm high and 50-200-nm in diameter. The topology of the nanostructures is attributed to the interplay between film thickness (10 nm and thinner) and the radial component electrostatic pressure. Modeling of the electric field based on the numerical solution of Laplace equation for cylindrical geometry suggests that non-uniformity of electric field plays an important role in nanostructure formation. The stability of the features which remain stable for days suggests cross-linking between macromolecules at the nanoscale. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y18.00006: Diffusion-Controlled, Self-Organized Growth of Symmetric Wrinkling Patterns Christopher M. Stafford, Jun Young Chung, Adam J. Nolte The formation of self-organized wrinkling patterns is a potential route for generating such tunable ordered patterns on surfaces across many length scales. Here, we demonstrate that surface wrinkling of ultraviolet/ozone (UVO) treated polymer films through osmotically driven swelling by solvent vapor sorption leads to unique and intriguing patterns, some of which have not been previously reported. The type of pattern and speed of its growth is coupled to the degree of UVO crosslinking and the rate of solvent diffusion into the film from a localized defect. This simple yet novel approach could serve as a test-bed for studying topography-driven phenomena such as wettability and adhesion and diffusion related processes, as well as facilitate a better understanding of dynamic self-assembly. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y18.00007: Transition from Non-interacting to Interacting Regime of Tethered Polymer Chains Ryan Van Horn, Joseph X. Zheng, Ming-Siao Hsiao, Bernard Lotz, Edwin L. Thomas, Jutta Luettmer-Strathmann, Stephen Z.D. Cheng Tethered polymer chains have become an important area of research over the last few decades. Their unique properties make them appealing for various applications. The tethering density of the chains determines the state of the chains as well as the resulting properties, as shown by various theoretical and experimental work. Our group uses single crystals of crystalline-amorphous diblock copolymers to study tethered polymer chains. This system provides better control of tethering density and molecular weight as compared to previous methods. Previous work on PS-b-PEO and PS-b-PLLA shows, for the first time, the reduced tethering density value of the interacting regime transition regardless of molecular weight or solvent quality. Other factors that were not addressed include adsorption and chain stiffness effects. The work presented here will look at the role of chain adsorption using miscible polymers, namely PMMA-b-PLLA, and chain stiffness with PEO-b-PCL where PEO is the amorphous block. In addition to experimental results, Monte Carlo simulations were used to estimate the chain conformation of adsorbed PMMA. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y18.00008: Internal Structure of Diblock Copolymer Brushes. Bulent Akgun, Charles F. Majkrzak, Sushil Satija, Gokce Ugur, William J. Brittain, Mark D. Foster, Xuefa Li, Jin Wang Although diblock copolymer brushes (DCBs) have been studied due to their stimuli-responsive behavior, their internal structure has been unknown. We have resolved the internal structure of DCBs using neutron reflectivity and grazing incidence small-angle X-ray scattering (GISAXS). The internal brush structure of DCBs of polystyrene-$b$-poly(methyl acrylate) (PS-b-PMA) and PMA-$b$-PS depends strongly on the block sequence and the value of $\chi $N. For the thinnest films a model of two layers with an interfacial region of finite width provides a good description of the data. For dPS-b-PMA films that are thicker and of sufficiently asymmetric composition, a third layer must be included. The necessity of including a third layer is consistent with lateral ordering of some type in the center of the brush, as evidenced by correlation peaks in GISAXS data. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y18.00009: Steric effects on the phase behavior of end-tethered temperature-responsive polymers Charles Hogshead, Evangelos Manias End-tethering polymers from a planar surface at high grafting density results in an apparent spatial confinement originating from interchain steric repulsion. This effective confinement can alter the phase behavior of temperature-responsive polymers in solution, relative to their bulk solution behavior. Here we report on experiments where we synthesized tethered polymer layers with a gradient in grafting density. The systems studied were temperature-responsive alternating copolymers in aqueous solutions. Under-water AFM was used to directly observe the tethered chain collapse upon heating through the bulk LCST, and the resulting reversible adhesion switch. The transition of the tethered layers occurs at temperatures that are similar to the binodal points of the respective solution, but over a much broader temperature range. This behavior is consistent with the collapse being a cooperative conformational transition, reflecting the effects of chain confinement, rather than a first-order thermodynamic LCST transition (as these same polymers exhibit in bulk aqueous solutions). The onset-temperature of the transition, of both the tethered layers and the LCST, was tailored by varying the copolymer composition, i.e., by tuning the hydrophilic/hydrophobic balance within the chain. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y18.00010: Self-Assembling Pi-conjugated Monolayer on Silicon J.-C. Lin, J. Kellar, J.-H. Kim, S. Nguyen, M. Hersam, M. Bedzyk, K. Bevan Pi-conjugated molecules play an important role in molecular electronic applications. For conductivity, the intrinsic ordering of the structure strongly influences its efficiency but is difficult to characterize by conventional scanning probe and IR spectroscopy techniques. In the present study, we combine a compliment of techniques, including XSW(X-ray standing waves), AFM, XPS, XRR(X-ray reflectivity), XRF(X-ray fluorescence), and DFT(Density functional theory) to determine the atomic scale molecular configurations and packing densities of two self-assembled aromatic monolayers (SAMs) grown on H-passivated silicon. P-4-bromophenyl-ethynyl-phenyl-acetylene, which has two phenyl rings, is directly compared with p-4-bromophenyl-acetylene, which has only one phenyl ring. The results show a local dense packing in spite of the overall coverage being somewhat less than 0.5 monolayers. This packing of the p-4-bromophenyl-ethynyl-phenyl-acetylene SAM suggests the average spacing between molecules is within the pi-pi interaction range, which will contribute to the charge transport. The detailed atomic structure of SAMs are also constructed using our characterization package. The result suggests the possibility of the application of self-assembling method on the growth of molecular electronics. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y18.00011: The Dynamic Reinforcement of Polyvinyl Alcohol (PVA) as a Result of Non-equilibrium State of Polymer Supermolecular Structures and their Confinement in Nanofibers Eyal Zussman, Emil Shaked, Arkadi Arinstein The results of mechanical testing of PVA -based electrospun nanofibers and bulk in static and dynamic modes are presented. An increase in the elastic moduli resulting from sample deformation was observed in both the bulk and as-spun fibers. This increase occurs when the deformation rate exceeds a critical value and can be attributed to the non-equilibrium dynamics of the supermolecular structures of the polymer matrix. That is, the evolution of these supermolecular structures results in an observably extended relaxation time. It is noted that the rate of the modulus increase of the nanofibers is nearly double that of the bulk fibers' rate. This difference can be explained by confinement influence on the polymer matrix of the nanofibers. In addition, the tests revealed that the, \textit{Tg}, of the nanofiber is noticeably higher than that of bulk specimen. Reinforcing the nanofibrs by cellulose whiskers showing that the dependence of the effective modulus on the whisker concentration has an initial increase that changes to a decrease when the whisker concentration exceeds 2 {\%}. Such behavior can be explained in the framework of an aggregation concept -- when the cluster size reaches that of the fiber diameter (cluster confinement), the whisker distribution becomes inhomogeneous and results in a measurable weakening of the composite. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y18.00012: Phenomenology of Polymer Thin Film Dewetting during Vapor Deposition Polymerization Mitchell Anthamatten, Xichong Chen Initiated chemical vapor deposition (iCVD) is a solventless technique to grow polymer thin films directly from gas phase feeds. The free radical technique involves the dissociation of gaseous initiator followed by adsorption onto a surface and subsequent polymerization with monomer to produce linear or crosslinked polymer films. We have designed and built an axisymmetric hot-zone iCVD vacuum reactor. Using this reactor, smooth poly (methyl methacrylate) films are grown from methyl methacrylate and t-butyl peroxide gas feeds. When solvent vapors are added to the process, we observe dewetting of vapor deposited polymer films. The objective of the current study is to understand the phenomenology of the observed solvent-induced dewetting. White light interferometery was used to investigate the surface topography of dewetted structures. The observed length scale depends on several process parameters including the deposition rate, the type of solvent used, and the surface free energy. Higher deposition rates suppress dewetting, and higher solvent content leads to dewetted structures with larger length scales. A dynamic model is applied to explain how droplet size and droplet aerial density depend on time and process parameters. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y18.00013: Nano-adsorbents control surface properties of polyurethane SuPing Lyu, Darrel Untereker, Jim Schley, Tom Grailer, Anna Belu, Chris Hobot, Terri Bartlett, Randy Sparer Additives are minor but critical components that polymers need for processing and applications. However, these additives may also have adverse effects, e.g. for polymeric biomaterials, leaching additives can change surface properties, and may lead to poor biocompatibility. How to use additives yet keep them from detrimental behaviors is challenging. Diffusion barriers may be used to slow down the additive migration but it is difficult to stop it. In this paper, we introduced the concept of ``nano-adsorbents'' in polymers. These nano-adsorbents confined the additives within the polymers via thermodynamically interacting with them. While the additives are still present in polymers to provide intended functions, they are thermodynamically constrained from freely migrating to the surface. Nano sized-fillers were selected due to their high surface-to-volume ratio. This new use of nano-fillers for polymers was demonstrated with a biomedical polyurethane and a surface coated nano-clay that thermodynamically attracts the additive in the polyurethane. [Preview Abstract] |
Session Y19: Surfaces and Adsorption I
Sponsoring Units: DPOLYChair: Zhiqun Lin, Iowa State University
Room: 320
Friday, March 20, 2009 8:00AM - 8:12AM |
Y19.00001: Sequence-directed organization in self-assembled monolayers of beta-peptides on solid surfaces: A Monte Carlo simulation study Jagannath Mondal, Bong June Sung, Arun Yethiraj The sequence-directed organization of self-assembled monolayers (SAM) of amphiphilic $\beta$-peptides adsorbed on gold surfaces is studied using Monte Carlo simulations. A phenomenological model is considered where each (helical) molecule is represented by a rigid nano-rod with the side groups at appropriate locations. This model effectively distinguishes between two, namely globally amphiphilic (GA) and non-globally amphiphilic(non-GA), sequence-isomers of an amphiphilic $\beta$ peptide Y-(ACHC-ACHC-K)$_3$. The simulations show that the GA isomers have a high degree of orientational order that is not exhibited by the non-GA isomers, consistent with experiment. The simulations quantify a subtle balance between electrostatic, hydrophilic, and hydrophobic interactions on the self-assembly of $\beta$-peptides on surfaces. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y19.00002: Thermal Response of PNIPAM Brushes Studied by Numerical Self-Consistent Field Calculations Dong Meng, Qiang WANG Unlike most polymers, poly(N-isopropyl acrylamide) (PNIPAM) exhibits a lower critical solution temperature in water, i.e., PNIPAM chains are soluble (expand) at low temperatures and insoluble (collapse) at high temperatures. This property has been used to create ``smart'' surfaces of PNIAPM brushes that can switch wettability, porosity and cell-adhesion properties as temperature is changed. Such thermal response of PNIPAM brushes are strongly affected by both chain length and grafting density. In this study we use numerical self-consistent field calculations with a composition-dependent Flory-Huggins parameter obtained from experiments to study the thermal response of PNIPAM brushes. The effects of chain length and grafting density are systematically investigated to provide guidance to experimental design of PNIPAM brushes for targeted applications. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y19.00003: Time evolution of the structure of fluorinated alkanes near solid surfaces Mesfin Tsige The study of thin films of physisorbed molecules has attracted much experimental and theoretical interest in the past few decades. This is mainly due to numerous reports of anomalous behavior in the structural and dynamical properties of polymer thin films at interfaces and the expected consequences in their wide range of practical applications. Although great advances have been made in our understanding of the physics and chemistry of polymer melt/solid and polymer melt/vapor interfaces, there remain a number of fundamental as well as practical issues that need to be addressed regarding the influence of interfaces on a polymer film. One of the issues, which I think no systematic study has been yet conducted on, is the stability or time-evolution of the molecular layering in a film next to a solid surface. The main focus of my presentation will be on our recent computer simulation study result that shows that the extent of layering in an alkane film next to a solid substrate may oscillate in time with an amplitude and period that strongly depends on temperature. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y19.00004: Activation of CO$_{2}$ on transition metal surfaces and oxide supported metal thin films Sujata Paul, Marco Buongiorno Nardelli Using first principles simulations based on Density Functional Theory, we have investigated the adsorption and activation properties of CO$_{2}$ on a variety of transition metal surfaces and oxide supported metal thin films. We intend to focus on the chemical conversion of CO$_{2}$ through heterogeneous catalysis using surfaces and interfaces where there is nanoscale control over charge density at the reactive sites. The activation of CO$_{2}$ on clean metal surfaces is possible at very high temperatures and the situations changes drastically when reaction happens on oxide supported metal thin film. The chemical reactivity of the molecule on the surface depends on the charge rearrangement at the metal-alkaline earth oxide interface. We want to understand the possible catalytic systems and characterize the relevant geometrical and electronic parameters related to the reaction mechanisms, rates and yield. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y19.00005: Dynamics of an Adsorbed Polymer Chain Joshua Kalb, Sanat Kumar Because of the current precision in fluorescent labeling, it is possible to label single polymers such as DNA or PEG and track their dynamical and equilibrium properties in the bulk as well as near attractive surfaces [Maier et. al., Macro. 2000][Sukhishvili et. al., Macro. 2002]. Recent evidence from these experiments and related simulations has shown that the dynamics of a single polymer near an attractive surface appear diffusive, however further evidence coming from the 'diffusion coefficient' implies a different process other than diffusion is at work such as reptation, `hover crafting', or `hopping' [Sukhishvili et. al., Macro. 2002]. In general, these possible dynamical behaviors are determined by the length of the polymer itself as well as the microscopic details of the attractive surface which include the density, strength, and distribution of attractive surface sites[Desai et. al., PRL 2007][Qian et. al., PRL 2007]. In this presentation, we investigate the effects of microscopic surface sites on single polymer dynamics through DMD simulations and compare these results to the properties of the chain in the bulk and near a flat attractive surface. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y19.00006: Investigation of Carboxymethyl Cellulose Adsorption onto Regenerated Cellulose Surfaces via Quartz Crystal Microbalance with Dissipation Monitoring and Surface Plasmon Resonance Spectroscopy Zelin Liu, Paul Gatenholm, Alan Esker The adsorption of anionic polyeletrolytes, sodium salts of carboxymethyl celluloses (CMC), with different degrees of substitution (DS = 0.9 and 1.2) from aqueous electrolyte solutions onto regenerated cellulose surface was studied via quartz microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR). The influence of both calcium chloride (CaCl$_{2})$ and sodium chloride (NaCl) was examined. Both QCM-D and SPR results indicate that CMC adsorption onto regenerated cellulose surfaces increases with increasing electrolyte concentration and CaCl$_{2}$ (divalent cation) showed a significant effect on CMC adsorption compared to NaCl (monovalent cation) at the same ionic strength. Voigt-based viscoelastic modeling of the QCM-D data and analysis of the SPR data are consistent with the existence of a swollen CMC layer on the cellulose surface with a viscosity of $\sim $1.3×10$^{-3}$ kg m$^{-1}$ s$^{-1}$ and an elastic shear modulus of $\sim $10$^{5}$ kg m$^{-1}$ s$^{-2}$. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y19.00007: Theory of Polymer Adsorption Onto Selected Chemically Patterned Substrates. Alexander Chervanyov, Gert Heinrich We theoretically studied the reversible adsorption of polymers onto selected rigid and soft chemically non-uniform substrates with an emphasis on the polymer adsorption onto the selective binary mixed brushes. In the course of our study, we developed two independent theoretical methods, the self-consistent perturbation expansion and the transfer operator formalism, which made it possible to thoroughly investigate the density structure of polymers adsorbed onto chemically non-uniform substrates. As successive stages of our research, we applied the above theoretical methods to the study of the polymer adsorption onto the selected substrates, as follows: (i) periodically patterned rigid surface; (ii) randomly patterned rigid surface; (iii) surface that bears an array of periodically distributed adsorption centers; (iv) ripple, random and dimple morphologies of the binary mixed brush. By comparing the results obtained for the above listed systems we derived the qualitative trends that are common for the polymer adsorption onto the investigated chemically non-uniform substrates. In this talk, we discuss what are the main factors that influence the polymer adsorption onto the patterned substrates and how to reduce/enhance the polymer adsorption by way of manipulating these competitive factors. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y19.00008: Cationic xylan adsorption onto self-assembled monolayers and model cellulose surfaces Alan Esker, Abdulaziz Kaya, Daniel Drazenovich, Wolfgang Glasser, Katrin Schwikal, Thomas Heinze Self-assembly of cationic hydroxypropyltrimethylammonium xylans (HPMAs) with different degrees of substitution (DS) onto self-assembled monolayers (SAMs) and model cellulose surfaces has been investigated by surface plasmon resonance (SPR). Maximal adsorption of HPMAs onto COOH-terminated SAMs occurs at an intermediate DS=0.10. Ionic strength effects on adsorbed amount follow different trends at low and high DS values which qualitatively agree with predictions of scaling theory for polyelectrolyte adsorption. For adsorption onto model cellulose surfaces and OH-terminated SAMs, surface excess values are relatively low compared to COOH-terminated SAMs. For adsorption onto CH$_{3}$-terminated SAMs, solubility of the HPMAs plays an important role as HPMA adsorption decreases with increasing DS values. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y19.00009: Introduction of a novel surface plasmon resonance imaging method for use in the study of quantitative kinetic surface interactions Scott Allen, Oleh Tanchak, John Dutcher The surface plasmon resonance (SPR) phenomenon is widely used as a surface sensitive probe of biomolecular surface interactions. SPR imaging (SPRi) is an experimental mode that takes advantage of the SPR phenomenon to directly visualize nanoscopic changes to surfaces using simple optics and a CCD camera. SPRi, performed at a fixed angle of incidence, enables one to study association, dissociation and degradation processes in a multi-arrayed format without the need for fluorescent tags. A discussion of specific challenges associated with performing kinetic measurements at a fixed angle of incidence will be presented. A novel SPRi method that enables one to perform reproducible quantitative kinetic measurements will be introduced, along with experimental examples of the use of this technique. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y19.00010: Pentacene on Cu(911): A Density Functional Theory study Abdelkader Kara I use density functional theory to calculate the adsorption energies, geometric and electronic structures of pentacene (C$_{22}$H$_{14})$ on the Cu(911) vicinal surface. The pentacene molecule is found to adsorb nearly flat (slightly arched at the edges) on top of the terrace but close to the step with an adsorption energy of 1.3 eV. The adsorption geometry on Cu(911) is found to differ from that of on Cu(110); but there is a strong bonding between the center carbon atoms of the molecule and Cu atoms near the step. In accord with experimental observations, the arched geometry induces brightness at the edges of the molecule in the STM images. The alignments of the energy levels at the interface induce a change in the work function of about -0.4 eV, in qualitative agreement with the observed -0.9 eV, which is an indication of charge transfer from the molecule to the surface. The details of the electronic structure at the interface will be presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y19.00011: Dynamics within alkylsiloxane SAMs studied by sensitive dielectric spectroscopy Mary Scott, Derrick Stevens, Jason Bochinski, Laura Clarke Self assembled monolayers (SAMs) are a ubiquitous tool in modern research and their static structure has been extensively studied. Fewer investigations have addressed dynamics within these systems; however, such motions within SAMs will affect surface properties such as friction and blocking ability (permeability). In this study, sensitive, dielectric spectroscopy over a broad temperature range (4-400 K) has been employed to study relaxations within planar alkylsiloxane SAMs[1] . Highly disordered SAMs of varying density were grown by vapor deposition. Two dielectric relaxations were observed. The first, a polyethylene-like relaxation similar to that previously reported in phase-segregated alkyl side-chain polymers, is observed for all films with alkyl chains containing four or more carbons. This is an interacting or glassy relaxation. The second motion, which is observable only at high film densities, is a local mode, which follows an Arrhenius dependence on temperature, and has been previously assigned to a sub-chain rotation. [1] M.C. Scott, D.R. Stevens, J.R. Bochinski, L.I. Clarke, ACS Nano. DOI: 10.1021/nn800543j. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y19.00012: Control of The Morphology of Super-Hydrophobic Surfaces Robert Weiss, Andrey Dobrynin, Xueyuan Wang Textured surfaces consisting of nanometer to micrometer-sized polymer particles were prepared by rapid evaporation of the solvent for a dilute polymer solution. The size and supermolecular structure of the particles and particle aggregates was controlled by the rate of the solvent evaporation. The process is believed to be a spinodal decomposition of the polymer film during casting and the morphology is dictated by pinning the spinodal decomposition process. The surfaces exhibit high water contact angles greater than 150 degrees, making them candidates for applications requiring super-hydrophobicity. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y19.00013: Biomimetic and responsive artificial surfaces that quantitatively reproduce the water repellency of a Lotus leaf Spiros H. Anastasiadis, Vassilia Zorba, Emmanuel Stratakis, Marios Barberoglou, Emmanuel Spanakis, Panagiotis Tzanetakis, Costas Fotakis, Anca Mateescu, Maria Vamvakaki We report an efficient method for preparing superhydrophobic and highly water repellent surfaces by irradiating silicon wafers with femtosecond laser pulses and subsequently coating them with alkylsilanes or polymer brushes. Such surfaces exhibit controlled dual-scale roughness at the micro- and the nano-scale and water contact angle properties very similar to those of the Lotus leaf. The water repellency of the artificial surfaces is quantified by studying the restitution coefficient of water droplets bouncing off the surfaces as a function of the droplet impact velocity; this is the first time such a direct comparison of performance is made and it clearly demonstrates the possibility of designing highly efficient biomimetic water repellent surfaces. When a polymer brush is ``grafted from'' these surfaces based on a pH-sensitive polymer, these artificially structured surfaces can alter their behavior from super-hydrophilic (after immersion in a low pH buffer) to super-hydrophobic and water-repellent (following immersion to a high pH buffer). Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y19.00014: Haloform Adsorption on Crystalline Copolymers of Vinylidene Fluoride with Trifluoroethylene Carolina C. Ilie, Jie Xiao, Peter A. Dowben Bromoform absorption on crystalline polyvinylidene fluoride with trifluoroethylene, P(VDF-TrFE 70:30) was investigated by photoemission and inverse photoemission and found to be associative and reversible. Molecular bromoform adsorption appears to be an activated process at 120 K with enhanced adsorption following the initial adsorption of bromoform. Strong intermolecular interactions are also implicated in the presence of a weak shake off or screened photoemission final state, whose intensity scales with the unscreened photoemission final state. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y19.00015: Shear-controlled Micro-nano Scaled Super-Hydrophobic Surfaces with Tunable Sliding Angles from isotactic-Polypropylene / Polypropylene Chlorid Blend Xia Dong, Charles C. Han, Song Hong, Yonghua Yao With proper selection of shear and thermal conditions, super- hydrophobic polymeric surfaces (Contact angle higher than 150$^{\circ}$) with tunable sliding angles (From less than 1$^{\circ}$ to higher than 90$^{\circ}$) can be prepared from isotactic polypropylene (iPP)/ polypropylene chloride (PPC) blend under ambient atmosphere. No further modification with low-surface-energy component is needed. The formed surfaces have good thermal property, chemical and moisture resistance and potentially low manufacturing cost. [Preview Abstract] |
Session Y20: Biological-Synthetic Hybrid Materials
Sponsoring Units: DPOLYChair: Ali Dhinojwala, University of Akron
Room: 321
Friday, March 20, 2009 8:00AM - 8:36AM |
Y20.00001: Peptide Folding and Consequent Self-assembly for Shear Thinning Hydrogels with Immediate Recovery Invited Speaker: The local nano- and overall network structure, and resultant viscoelastic properties, of hydrogels that are formed via beta-hairpin self-assembly will be presented. The 20 amino acid peptides have been shown to intramolecularly fold and intermolecularly self-assemble into a rigid hydrogel based on environmental cues such as pH, salt, and temperature including physiological conditions. The hydrogel is composed of a network of fibrils that are 3 nm wide that physically crosslink (i.e. entangle and branch) with no covalent crosslinking required. Slight design variations of the peptide sequence allow for tunability of the self-assembly/hydrogelation kinetics. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network. This physical assembly process allows the encapsulation of desired payloads into the gel network such as large macromolecules or living cells. Importantly, once formed into a solid, the self-supporting gel network can be disrupted by the introduction of a shear stress. The system can shear thin but immediately reheal to a stiff solid on the cessation of the shear stress. This shear thinning and recovery behavior makes them interesting candidates for injectable delivery in vivo. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y20.00002: Designing `smart' hydrogels using peptide-responsive conjugates Alberto Saiani, Jean-Baptiste Guibaud, Aline Miller Polymers displaying phase transitions in aqueous solution from a hydrophilic to hydrophobic state are of widespread interest as `smart' biomedical materials. Hydrogels formed from self-assembling peptides have also attracted considerable attention in the past decade. These hydrogels are based on the self-assembly of short peptides into supra-molecular fibres that entangle or associate to form three dimensional networks and, ultimately, self-supporting hydrogels. In order to create a new generation of hydrogel based on self-assembling peptides but possessing an internal transition that can be used as a trigger to release a drug molecule or a specific biological signal we have synthesized a new family of peptide-polymer conjugates using free radical polymerization. We were able to create a new generation of `smart' hydrogel in which gelation is driven by the peptide while the polymer phase transition can be used to release in a controlled fashion a specific signal or drug molecules. These materials are though to be of prime interest for tissue culture applications where they can be used to deliver specific signals stimulating a specific cell response. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y20.00003: ``Backpack'' Functionalized Living Immune Cells Albert Swiston, Soong Ho Um, Darrell Irvine, Robert Cohen, Michael Rubner We demonstrate that functional polymeric ``backpacks'' built from polyelectrolyte multilayers (PEMs) can be attached to a fraction of the surface area of living, individual lymphocytes. Backpacks containing fluorescent polymers, superparamagnetic nanoparticles, and commercially available quantum dots have been attached to B and T-cells, which may be spatially manipulated using a magnetic field. Since the backpack does not occlude the entire cellular surface from the environment, this technique allows functional synthetic payloads to be attached to a cell that is free to perform its native functions, thereby synergistically utilizing both biological and synthetic functionalities. For instance, we have shown that backpack-modified T-cells are able to migrate on surfaces for several hours following backpack attachment. Possible payloads within the PEM backpack include drugs, vaccine antigens, thermally responsive polymers, nanoparticles, and imaging agents. We will discuss how this approach has broad potential for applications in bioimaging, single-cell functionalization, immune system and tissue engineering, and cell-based therapeutics where cell-environment interactions are critical. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y20.00004: Reversible Structural Transition of a DNA Lipid Film. Matthew Tirrell, Surekha Gajria, Thorsten Neumann, Luc Jaeger Polyanions such as nucleic acids (RNA and DNA) can self-assemble with cationic lipids via electrostatic complexation, driven thermodynamically by the release of counterions. The structures of these complexes in water have been studied extensively and are recognized as potentially useful in the field of gene delivery. The structure of films in water is dominated by the nature of the lipid. Within these lamellar complexes in aqueous solution the lipid assumes a bilayer formation and the DNA is a double helix. It is possible to obtain dry nucleic acid-lipid films when the dissolved cationic lipid complex of DDAB and nucleic acid is cast on a solid. These self-standing films have been characterized by tensile properties and nucleic acid intercalation experiments. The tensile properties of these films are adjustable by mixing different molecular weights. It was expected that these films would have the same characteristic structure as these complexes in water. However, our work shows that the film undergoes a transition from double stranded helical DNA complexed with a bilayer of DDAB in the wet state, while in the dry state we observed a repeat unit of single stranded DNA complexed with a monolayer of DDAB. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y20.00005: Gecko-Inspired Carbon Nanotube-Based Adhesives Liehui Ge, Sunny Sethi, Anubha Goyal, Lijie Ci, Pulickel Ajayan, Ali Dhinojwala Nature has developed hierarchical hairy structure on the wall-climbing gecko's foot, consisting of microscopic hairs called setae, which further split into hundreds of smaller structures called spatulas. In the last five years, numerous attempts to mimic gecko foot-hair using polymer soft molding and photolithography methods have been reported. However, most of these polymer-based synthetic gecko hairs fall short of the clinging ability of geckos. Vertically aligned carbon nanotubes (CNT) have shown strong adhesion at nanometer scale. Here, we present our work on developing CNT-based macroscopic flexible tape mimicking the hierarchical structure found on gecko's foot. The synthetic gecko tape is made by transferring aligned CNT array onto flexible polymer tape. The unpatterned CNT-gecko tape can support a shear force stress similar to gecko foot (10 N/cm$^{2})$. The supported shear stress increase by a factor of four, when we use micro-patterned CNT patches (50 to 500 $\mu $m). We find that both setae (replicated by CNT bundles) and spatulas (individual CNT) are necessary to achieve large macroscopic shear adhesion. The carbon nanotube-based tape offers an excellent synthetic option as a dry conductive reversible adhesive in microelectronics, robotics, and space applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y20.00006: Sensing Structures Inspired by Blind Cave Fish Michael E. McConney, Nannan Chen, David Lu, Kyle D. Anderson, Huan Hu, Chang Liu, Vladimir V. Tsukruk Blind cave fish, with degenerated non-functioning eyes, have evolved to ``see'' their hydrodynamic environment by using the flow receptors of the lateral line system. The hair-cell receptors are encapsulated in a hydrogel-like material, called a cupula, which increases the sensitivity of the hair-cell receptors by coupling their motion to the surrounding flowing media. We characterized the viscoelastic properties and of blind cave fish cupulae by using colloidal-probe spectroscopy in fluid. A photo-patternable hydrogel with similar properties was developed to mimic the fish receptor coupling structure. Flow-based measurements indicated that the hydrogels enhance drag through increased surface area, but also inherent material properties. These bio-inspired structures endowed micro-fabricated flow sensors with sensitivities rivaling that of fish. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y20.00007: Giant super-helix formation from aqueous bioinspired block copolymers. H.K. Murnen, A.M. Rosales, R.N. Zuckermann, R.A. Segalman Polypeptoids are a class of bioinspired polymers based on N-substituted glycines with the side group bonded to the backbone nitrogen rather than the alpha carbon as in natural polypeptides. Due to the lack of backbone hydrogen bonding and the sequence specific synthesis of these materials, side chain interactions can be designed to induce the formation of macromolecular structures in aqueous solution. An amphiphilic block copolypeptoid consisting of a hydrophobic block, poly[N-(2-phenylethyl)glycine] and a hydrophilic block, poly[N-(2-carboxyethyl)glycine] is found to form giant superhelices in aqueous solution by scanning and transmission electron microscopy and atomic force microscopy. With a diameter greater than 600nm, these helices are much larger than the fully extended length of the molecules (7 nm). Furthermore, while the molecules are completely achiral, the helices are all left handed and remarkably regular (pitch 670nm, length greater than 2 microns). We will discuss possible preferential chain conformations that may provide the driving force for the superstructure. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y20.00008: The effects of supramolecular network topology on hapten-receptor avidity Jason Benkoski, Andrew Mason, Jill La Favors, Joshua Wolfe Antibodies produced in the early stages of the immune response have much lower affinities for a given antigen than those produced later on. Nature compensates for the initial weakness of these associative bonds by synthesizing multivalent antibodies. The total binding strength, represented by the avidity constant, is equal to the product of the affinity constants for the individual hapten/receptor sites. However, under realistic conditions the individual binding sites do not act independently. Factors such as steric hindrance, intramolecular stresses, and competitive binding can significantly alter the relationship between affinity and avidity. We investigate the influence of these factors on a model system consisting of synthetic multifunctional nanoparticles and polymers. Each polymer or nanoparticle is decorated with either multiple antigens (thromboxane B2) or multiple antibodies. We then measure the association and dissociation in real time using Surface Plasmon Resonance Spectrometry (SPR). By using synthetic polymers and nanoparticles, we are able to systematically control the degree of functionality, flexibility, and distance between receptor and hapten sites. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y20.00009: Dynamic DNA Interactions with Functionalized Colloids Lu Zhang, Yingxi Elaine Zhu Many biomedical processes, such as protein adsorption, DNA hybridization and enzyme reactivity, are intimately related to their interactions with surfaces and complex ionic environments, yet the details of biomacromolecular interaction remain insufficiently understood. In this work, we use confocal laser scanning microscopy to examine the interaction between DNA molecules and functionalized colloidal particles in aqueous suspension. We observe an intriguing attractive interaction between DNAs and carboxyl-functionalized silica particles of varied sizes from 50 nm to 3 um, resulting in complex DNA-colloid aggregation with a strong dependence on DNA/colloid size ration and ionic strength. As colloidal size becomes larger than DNA dimensions, colloidal doublets and triplets with adsorbed DNAs are observed at high DNA concentration and ionic strength. The intriguing DNA-colloid complex structures are further confirmed by SEM and appear stable for at least 2 weeks. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y20.00010: Growth of well-defined metal and oxide nanoparticles on biological surfaces Vladimir Tsukruk We present a brief overview of our recent studies in the field of bio-enabled surface-mediated growth of inorganic nanoparticles at room temperature and ambient conditions. We demonstrate that all titania, gold, and silver nanoparticles can be grown with relatively monodisperse diameter within 4-6 nm surrounded by biological shells of 1-2 nm thick. As biological templates we utilized ultrathin, molecular uniform and micropatterned surface layers of two different proteins: silk fibroin (for growth of gold and silver nanoparticles) and silaffin (for growth of titania nanoparticles). To identify the grown nanophases and chemical composition/secondary structure of biological templates we applied combined AFM, SEM, TEM, XPS, SERS, UV-vis, and ATR-FTIR techniques. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y20.00011: Fluorescent DNA-bound Ag nanoclusters Patrick O'Neill, Lourdes Velazquez, Kim Weirich, Deborah Fygenson Few-atom fluorescent Ag nanoclusters self-assemble on short, synthetic DNA strands, and exhibit sequence and structure dependent fluorescence ranging from the blue to the near infrared. Here we report UV excitation as a ubiquitous feature of these emitters. Each emitter thus has two excitation peaks: a visible peak which is cluster-dependent, and a UV peak which has the same wavelength for all DNA-bound Ag clusters. This UV peak corresponds to resonant absorbance by the DNA bases, and produces the same emission spectra as visible excitation, suggesting energy transfer from the DNA bases to the Ag cluster. We make use of this UV excitation to image the emitters in unstained polyacrylamide gels, and show that electrophoresis can be used to create a pure solution of green DNA:Ag11 clusters from an inhomogeneous red solution of DNA:Ag$>$12 clusters. [Preview Abstract] |
Session Y22: Focus Session: Spin Resonance in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Stephen Lyon, Princeton University
Room: 324
Friday, March 20, 2009 8:00AM - 8:36AM |
Y22.00001: Dynamic Nuclear Polarization in Silicon Invited Speaker: Silicon is a promising material for spintronics and spin-based quantum information processing. However, the highly mixed state of the nuclear spins can be a significant limitation, whether the nuclear spins are used as qubits or act as an environment for the electronic spins. We report the results of recent experiments to hyperpolarize the $^{29}$Si spins in silicon. We used microwave-induced dynamic nuclear polarization to achieve 5\% polarization of the $^{29}$Si in micro-crystalline silicon powder [1], and 5--8\% polarization in antimony- and phosphorus-doped silicon wafers. Since silicon has long T$_1$ relaxation times, polarized silicon micro- and nanoparticles could be of use in magnetic resonance imaging. In the powders the $^{29}$ Si nuclei in the amorphous region (containing unpaired electrons) are polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. In the wafers the DNP is driven by an Overhauser mechanism within exchange-coupled clusters of donors. \\[4pt] [1] A. Dementyev, D. G. Cory, C. Ramanathan, {\em Phys. Rev. Lett.}, {\bf 100}, Article 127601 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y22.00002: Electrical detection of dynamic nuclear polarization and nuclear magnetic resonance in ferromagnet-semiconductor heterostructures Mun Chan, J. Zhang, Q. Hu, T. Kondo, E. Garlid, C.J. Palmstrom, P.A. Crowell We report all-electrical measurements of dynamic nuclear polarization (DNP) and nuclear magnetic resonance (NMR) in ferromagnet-semiconductor heterostructures. Nuclei in GaAs are polarized by electron spins injected through an epitaxial Fe/GaAs Schottky tunnel barrier. In an oblique Hanle geometry, electron spin depolarization due to the hyperfine field is detected by measuring the change in spin-dependent electrochemical potential with a Fe contact.We also report electrical measurements of both magnetic and current driven NMR. A transverse magnetic field or the injection current is modulated at the NMR frequency to suppress DNP, resulting in changes of up to 80{\%} in the spin-dependent voltage. We measured resonant frequencies of different isotopes in the GaAs channel. Harmonics of the fundamental transitions are observed, reflecting the existence of either quadrupolar coupling or dipole-dipole interactions. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y22.00003: Electric-field control of a hydrogenic donor's spin in a semiconductor Amrit De, Craig E. Pryor, Michael E. Flatt\'e The orbital wave function of an electron bound to a single donor in a semiconductor can be modulated by an applied AC electric field, which affects the electron spin dynamics via the spin-orbit interaction. Numerical calculations of the spin dynamics of a single hydrogenic donor (Si) using a real-space multi-band $k\cdot p$ formalism show that in addition to breaking the high symmetry of the hydrogenic donor state, the g-tensor has a strong nonlinear dependence on the applied fields. By explicitly integrating the time dependent Schr\"odinger equation it is seen that Rabi oscillations can be obtained for electric fields modulated at sub-harmonics of the Larmor frequency. The Rabi frequencies obtained from sub-harmonic modulation depend on the magnitudes of the AC and DC components of the electric field. For a purely AC field, the highest Rabi frequency is obtained when E is driven at the 2nd sub-harmonic of the Larmor frequency. Apart from suggesting ways to measure g-tensor anisotropies and nonlinearities, these results also suggest the possibility of direct frequency domain measurements of Rabi frequencies. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y22.00004: Electronic spin polarization and spin-dependent band structure in GaAs probed by optically-pumped NMR (OPNMR) Sophia Hayes, Kannan Ramaswamy, Stacy Mui, Scott Crooker, Xingyuan Pan, Gary Sanders, Christopher Stanton Traditionally, magnetic fields have played an important role in determining the band structure of a material (de Haas-van Alphen techniques for metals and cyclotron resonance or magneto-absorption for semiconductors). We report optically pumped NMR (OPNMR) spectra of 69Ga spins in bulk semi-insulating GaAs generated by a narrowband laser. OPNMR involves optical orientation of electrons in a semiconductor with NMR detection of the polarized nuclear spins to which they are coupled. These experiments resolve fine details of the spin-dependent electronic structure of the valence bands. By comparing the oscillations in the OPNMR signal intensity with theoretical calculations, we have mapped out the conduction band electronic spin polarization. We show that OPNMR experiments in combination with theoretical simulations have the potential to reveal information about spin polarization and electronic structure of bulk semiconductors with far greater sensitivity than conventional techniques such as magneto-absorption. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y22.00005: Theory of the Spin-Splitting of the Valence Band Landau Levels in GaAs X. Pan, G.D. Sanders, C.J. Stanton, K. Ramaswamy, S. Mui, S.E. Hayes, S.A. Crooker We present calculations of the spin-dependent band structure and magneto-optical absorption spectra in bulk GaAs. Our calculations are based on the 8-band Pidgeon-Brown model. Optical properties are calculated within the golden rule approximation and compared with experiments. Although GaAs has a small $g$-factor leading to nearly spin-degenerate conduction band Landau levels, the valence band Landau levels are spin-split and the spin splitting can be observed in the circularly polarized magneto-absorption spectra. By carefully analyzing the energy band structure and the absorption spectra together, we identify the origins of all the optical transitions. We also separate contributions to the absorption coefficient from spin-up electrons and spin-down electrons to get the conduction band electron spin polarization. This information is used to compute the optically-pumped NMR (OPNMR) signal. We demonstrate that OPNMR can provide unique insight into the spin-dependent valence band electronic states. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y22.00006: High bandwidth EDMR detection H. Huebl, L.H. Willems van Beveren, R.P. Starrett, D.R. McCamey, A.J. Ferguson Several proposals discuss the realization of quantum computation with the help of the spin degree of freedom in semiconductors. Electrically detected magnetic resonance (EDMR) provides a well established tool to investigate spin states in semiconductors which was recently extended to investigate the spin dynamics of phosphorus donors in silicon. Typically, the detection bandwidth of EDMR is limited by the characteristic RC time constant of the sample. In this contribution we show that by embedding the sample in a LRC resonant circuit, a so-called tank circuit, it is possible to overcome this limitations. Here, we investigate a silicon MOSFET where the microwave magnetic field to induce the spin transitions is generated on chip by a shorted coplanar stripline[1]. We monitor the MOSFET resistance with a current preamplifier and in-situ by the response of the LRC resonant circuit and observe a spin resonance signature in both cases. Investigating the detection bandwidth by using frequency modulation of the microwaves applied indicates that the spin signature observed with the tank circuit is limited at the high end currently by the experimental setup. This shows that this method has the expected high bandwidth opening the view to faster phenomena in EDMR in a more direct manner. [1] Willems van Beveren et al., APL {\bf{93}}, 072102 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y22.00007: Optically Detected Electron Spin Resonance of GaAs Spin-LEDs John Colton, Steve Brown, Benjamin Heaton, Daniel Jenson, Michael Johnson, Aaron Jones GaAs ``spin-LED'' samples give off circularly polarized light due to spin-polarized electrons being injected into a quantum well diode structure. The spin dynamics of these electrons have been studied through electron spin resonance (ESR), with the resonance being optically detected by a change in the circular polarization of the emitted light. Results of the ESR experiments will be presented. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y22.00008: Spin lifetime properties of a quantum well GaAs sample measured by optically detected magnetic resonance Benjamin Heaton, John Colton, Steve Brown, Daniel Jenson, Michael Johnson, Aaron Jones Optically detected Kerr rotation techniques were used to measure spin properties in GaAs. The samples studied were MBE-grown 14 nm n-type GaAs quantum wells. Magnetic resonance was observed with great sensitivity as the probe laser was tuned to the exciton resonance. The g-factor was measured to be $\vert $g$\vert $=0.35. The T$_{2}$* lifetime measured from the width of the ODMR peaks was 52 ns. Results from pulsed microwave Rabi oscillation and spin echo experiments (to measure the T$_{2}$ spin coherence lifetime) are presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y22.00009: Ballistic Spin Resonance Sergey Frolov, Silvia Luescher, Wing-Wa Yu, Yuan Ren, Joshua Folk, Werner Wegscheider We demonstrate spin resonance driven by ballistic motion of electrons and mediated by spin-orbit interaction in a micron-scale channel of GaAs/AlGaAs two-dimensional electron gas. The resonance is observed when the frequency of electron bouncing trajectories in the channel matches the spin precession frequency set by a large in-plane magnetic field. The resonance is manifested as a suppression of pure spin currents that are generated in the channel by injection through quantum point contacts. The resonant frequency (10-50 GHz) can be tuned by varying electron density or channel width, as well as by bending the electron trajectories with a small out-of-plane magnetic field. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y22.00010: Scaling behavior of spin-dependent scattering off Neutral Donors in Silicon Field-Effect Transistors C. C. Lo, J. Bokor, T. Schenkel, J. He, A. M. Tyryshkin, S.A. Lyon Spin-dependent scattering of conduction electrons by neutral impurities is a promising route towards donor nuclear spin-state readout for donor qubits in silicon. Using electron spin resonance techniques, the donor nuclear spin-state can be extracted from the position of the resonance signal. Contrary to readout schemes involving Coulomb/spin blockade or other single electron phenomenon, spin-dependent scattering can be observed and studied in the presence of an ensemble of donors. In our experiments we study neutral impurity scattering of two-dimensional conduction electrons by donor impurities in field-effect transistors [1]. In this talk, we will discuss the scaling behavior of donor resonance signals using electrically detected magnetic resonance techniques in devices with different sizes. [1] C. C. Lo et al, App. Phys. Lett., 91, 242106 (2007) [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y22.00011: Electron Spin Resonance in Si/SiGe Heterostructures at 350 mK Jianhua He, A.M. Tyryshkin, S.A. Lyon, D.E. Savage, M.A. Eriksson Si/SiGe heterostructures are one of the promising matrices for electron spins as qubits in a silicon-based quantum computer. Many electron spin resonance (ESR) measurements have been done to characterize 2D electron spins embedded in such structures at temperatures above 2 K. Here we report the first CW and pulsed ESR experiments in Si/SiGe heterostructures in a $^{3}$He system at 350 mK. Electron beam lithography was used to pattern a large area (16 mm$^{2})$ of a CVD grown modulation doped Si/SiGe quantum well (QW) into an array of $\sim $100 nm quantum dots (300 nm pitch) which has been wet etched about half-way through the doped layer. In the dark, only one signal is observed, which shows a Curie-like temperature dependence indicative of isolated spins. After brief illumination, two more signals appear: a line having the same g-factor as an unpatterned QW sample (g=2.0003) and another line which disappears upon annealing to 20 K. The first of these lines (g=2.0003) shows a Pauli temperature dependence consistent with many-electron quantum dots, and a T$_{2}$ relaxation time of about 150 ns at 350 mK. The origin of these ESR signals and their relaxation mechanisms will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y22.00012: Electron spin resonance in silicon MOS structures down to 0.36 K S. Shankar, A. M. Tyryshkin, S. A. Lyon While transport of 2-dimensional (2D) electrons has been routinely measured down to few mK, performing electron spin resonance (ESR) at low temperatures is challenging. We report measurements of the paramagnetic susceptibility of 2D electrons in a silicon metal-oxide-semiconductor (MOS) structure using ESR at 0.34 T for temperatures down to 0.36 K. When the MOS gate is biased below the threshold voltage, we measure electrons weakly confined below the conduction band edge and find that the susceptibility follows a Curie-like 1/T temperature dependence. The Curie susceptibility of confined electrons suggests that they are independent electrons confined by disorder at the Si-SiO$_2$ interface. At gate voltages above threshold the signal arises from 2D conduction electrons, whose susceptibility is expected to follow a simple Pauli temperature dependence, i.e., constant at low temperatures. Surprisingly, at an electron density of $2.8 \times 10^{11}$ cm$^{-2}$ (Fermi temperature = 20 K), as the temperature is reduced from 4.2 K to 0.36 K, the susceptibility actually drops by a factor of 2. Furthermore, this effect becomes more pronounced at higher 2D electron densities. The drop in susceptibility for 2D conduction electrons at low temperature is unexpected and remains to be explained. [Preview Abstract] |
Session Y24: Focus Session: Hydrogen Storage: Atomic and Molecular Motions
Sponsoring Units: DMPChair: Eric Majzoub, University of Missouri--St. Louis
Room: 326
Friday, March 20, 2009 8:00AM - 8:36AM |
Y24.00001: Atomic Motions in Ionic Hydrides: MgH$_{2}$, NaMg$_{3}$H$_{3}$, and LiBH$_{4}$ Invited Speaker: In hydrogen storage, rapid hydrogen diffusion is a key component for rapid reaction rates of dehydriding and rehydriding. In metallic systems, the light interstitial H atoms typically do display rapid diffusion. However, recent interest has focused on ionic and complex hydrides of light metal-atoms because of their high weight fractions of reversible hydrogen. These ionic complex hydrides generally reveal slow hydrogen diffusion and resultingly slow reaction kinetics. We report here studies of H diffusion using NMR in several such hydrides. In MgH$_{2}$, the rate $\omega _{H}$ of H hopping remains too slow to narrow the H NMR up to 400 $^{\circ}$C. T$_{1D}$ measurements, however, can detect the motion and find an activation energy of 1.72 eV, the first reported direct measurement of diffusion in MgH$_{2}$. In ball-milled (bm) material with Nb$_{2}$O$_{5}$ catalyst additive, a fraction of the resonance intensity is narrowed starting at 50 $^{\circ}$C, with the narrow fraction growing to 30{\%} by 400 $^{\circ}$C. A model for continuous growth of the narrow line, based on a wide distribution of motion rates, is presented. Ball-milling also greatly increases the laboratory-frame relaxation rates, T$_{1}^{-1}$, from paramagnetic defects created by the mechanical process. In bm NaMgH$_{3}$, an even larger fraction of the resonance is motionally-narrowed, growing to nearly 100{\%} by 300 $^{\circ}$C. Clearly, ball-milling has a much more profound effect on ionic hydrides than the simple reduction of grain sizes and diffusion distances. In coarse-grain LiBH$_{4}$ (with 13.8 weight{\%} reversible hydrogen), an orientationally disordered solid phase occurs above 110 $^{\circ}$C. Above the transition, the rate of Li ion diffusion increases remarkably. H diffusion starts to narrow the H NMR line around 170 $^{\circ}$C, continuing to narrow up to the melt near 280 $^{\circ}$C. To distinguish diffusion of (already rapidly rotating) BH$_{4}$ units from H exchange between neighboring BH$_{4}$, the $^{11}$B resonance was studied. The boron line central transition becomes much narrower (400 Hz) than the width (1500 Hz) expected from Van Vleck M$_{2}$ for the case of static boron spins (with rapid Li and H diffusion). Thus, intact BH$_{4}$ units are the diffusing species. Even in molten LiBH$_{4}$, the BH$_{4}$ lifetime is found to be at least 2 seconds from observations of the B-H J-coupling pattern, so it is probably much larger in the solid. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y24.00002: Understanding and Enhancing Hydrogen Diffusion in MgH$_{2}$ and NaMgH$_{3}$ David Sholl, Shiqiang Hao The transport properties of hydrogen in metal hydrides are crucial to the kinetics of H$_{2}$ storage in these materials. We use first-principles calculations to identify the defects that are relevant for H transport in MgH$_{2}$ and NaMgH$_{3}$. In both materials, the physically relevant defects are charged and H diffusion is dominated by mobility of negatively charged interstitial H. Interestingly, the diffusion of these species occurs via concerted mechanisms with low energy barriers. To improve the charged interstitial H diffusivity, a series of transition-metal additives are screened to lower the formation energy of mobile defects. Our results provide a practical way to examine and alter H diffusion in light metal hydrides. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y24.00003: Quasielastic Neutron Scattering of Hydrogen Adsorbed in KC$_{24}$ Justin Purewal, James Keith, Channing Ahn, Brent Fultz, Craig Brown Quasielastic neutron scattering (QENS) and volumetric techniques were used to study the adsorption of H$_{2}$ by the stage-2 potassium graphite intercalation compound KC$_{24}$. A zero-coverage sorption enthalpy of 8.5~kJ/mol was measured from H$_{2}$ isotherms recorded at 77~K and 87~K. The saturation H$_{2}$ adsorption amount at 77~K was 1.2~mass\%, corresponding to a stoichiometry of KC$_{24}$(H$_{2}$)$_{2.0}$. Quasielastic neutron scattering spectra for KC$_{24}$(H$_{2}$)$_{1.0}$ were collected at temperatures between 40~K and 80~K on a chopper spectrometer and a backscattering spectrometer. Two distinct H$_{2}$ diffusion processes were identified with characteristic times of approximately $\tau = 10$~ps and $\tau = 350$~ps at 60~K, respectively. By operating the backscattering spectrometer in fixed window mode, the total elastic scattering of KC$_{24}$(H$_{2}$)$_{1.0}$ was measured as a function of temperature. A sharp decrease in elastic intensity was observed at 35~K due to the onset of quasielastic scattering. This was interpreted as a melting transition of the H$_{2}$ adsorbate in KC$_{24}$. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y24.00004: Novel geometry for simultaneous resistive, Hall and optical measurement of MgH$_{x}$ thin films D.W. Koon, C. C. W. Griffin, J.R. Ares, F. Leardini, C. Sanchez We describe a novel specimen geometry we have used to simultaneously probe optical transmission, sheet resistance and sheet Hall resistance in 100nm Mg films during hydrogen absorption. A Mg-film cloverleaf overlaps four rectangular Pd pads at the corners of a glass slide, a variation on a two-pad geometry used by Ingason and Olafsson for resistive studies of Pd-capped MgH$_{x}$ films [J. Alloys and Compounds \textbf{404- 406} (2005), 469-72.]. Hydrogen diffuses laterally through the Pd pads before entering the magnesium layer from below. The sample holder also includes an LED-photodiode pair for measuring [monochromatic] optical transmission. We show that the simultaneous measurement of these three quantities during the metal-to-insulator transition in a hydriding MgH$_{x}$ film allows for a more complete understanding of the hydriding process in these films. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y24.00005: Proton Tunneling: A Decay Channel of the O-H Stretch Mode in $\mathrm{KTaO}_{3}$ Erik Spahr, Michael Stavola, Lanlin Wen, Lynn Boatner, Leonard Feldman, Norman Tolk, Gunter L\"{u}pke Proton vibrational dynamics play a key role in the important processes of hydrogen diffusion and transport. In particular, perovskite structured proton conductors are an important class of hydrogen transport materials with a wide range of potential applications. We have measured for the first time the vibrational lifetimes of the O-H and O-D stretch modes in the perovskite oxide, $\mathrm{KTaO}_{3}$, by pump-probe infrared spectroscopy. Both stretch modes are exceptionally long lived and exhibit a large ``reverse'' isotope effect, due to a phonon-assisted proton tunneling process, which involves the O-Ta-O bending motion. The excited-state tunneling rate is found to be seven orders of magnitude larger than from the ground state in the proton conducting oxide, $\mathrm{BaCeO}_{3}$ [1]. [1] I. Kuskovsky et al., Phys. Rev. B \textbf{60}, R3713 (1999). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y24.00006: Reversible Low Temperature Hydrogen Storage Using Ternary Borides Wen Li, John Vajo, Robert Cumberland, Ping Liu, Son-Jong Hwang, Chul Kim, Robert Bowman Among many materials for hydrogen storage, complex borohydride of light metals with high hydrogen capacity, have been studied extensively. However, the thermodynamic and kinetic properties of borohydrides limit their ability to cycle hydrogen reversibly at low temperature. For example, although LiBH$_{4}$ is thermodynamically quite stable, the formation of LiBH$_{4}$ from LiH + B requires elevated temperatures and pressures of up to 600 \r{ }C and 150 bar. Here, we report ternary borides with active boron species that can be hydrogenated forming [BH$_{4}$]$^{-}$ anions at temperatures as low as $\sim $280\r{ }C. These ternary borides were prepared through milling of precursors followed by thermal treatment under inert atmosphere. Samples were then milled with additional binary hydrides before hydrogenation. Analysis using FTIR and $^{11}$B MAS NMR indicated that the ternary borides were hydrogenated to [BH$_{4}$]$^{-}$ species with good kinetics. After hydrogenation, the mixture could be cycled with dehydrogenation occurring in two steps that begin at 280\r{ }C and 345\r{ }C, respectively. Characterization using FTIR, $^{11}$B MAS NMR, and XRD, indicates that the [BH$_{4}$]$^{-}$ anions are consumed in the first dehydrogenation step. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y24.00007: Thermodynamic and kinetic destabilization in LiBH$_{4}$-based hydride systems John Vajo, Wen Li, Ping Liu The LiBH$_{4}$/MgH$_{2}$ destabilized hydride system has attracted considerable attention recently because it is a reversible system, based on the [BH$_{4}$]$^{-}$ anion, with a high hydrogen capacity (11.6 wt {\%} gravimetric). However, hydrogen release during a temperature ramp occurs in two steps with dehydrogenation of MgH$_{2}$ to Mg + H$_{2}$ occurring first followed by reaction of Mg with LiBH$_{4}$ to form LiH + MgB$_{2}$ + H$_{2}$. These two steps occur despite there being a direct reaction that is thermodynamically allowed at lower temperatures. In this talk we describe a LiBH$_{4}$-based hydride system that is kinetically as well as thermodynamically destabilized. In this system, a direct (concerted) dehydrogenation reaction of LiBH$_{4}$ occurs at temperatures lower than the dehydrogenation temperature of any of the components separately. In addition, the system is reversible with an equilibrium hydrogen pressure (based on preliminary measurements) that is $\sim $20X higher than the pressure for the LiBH$_{4}$/MgH$_{2}$ system. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y24.00008: Structure and librational dynamics in borohydrides Monika Hartl, Michael Wolverton, Alice Acatrinei, Abhijit Bhattacharyya, Luke Daemen Borohydrides are candidates for reversible hydrogen storage. The attention accorded to this class of materials is supported by extensive hydrogenation/dehydrogenation thermodynamic measurements. However, the underlying chemical reaction mechanisms remain uncertain. We used neutron diffraction and inelastic neutron scattering, together with a computational approach, to examine the connection between structure and dynamics in several borohydrides and the possible role played by dynamics in the approach to the dehydrogenation transition state. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y24.00009: First-principles Study on the Vibration Modes and Electronic Structure of Alkali and Alkaline-earth Amides and Alanates Takao Tsumuraya, Tatsuya Shishidou, Tamio Oguchi Light alkaline and alkaline-earth metal hydrides such as amides $M$(NH$_2$)$_n$ and alanates $M$(AlH$_4$)$_n$ ($M$=K, Na, Li, Ca, and Mg) have attracted a growing interest as reversible hydrogen storage materials recently because of their innately high hydrogen contents. [1, 2] We study the electronic structure of the amides and alanates with different cations, focusing on the role of cation states from first-principles calculations based on the all-electron FLAPW method. Calculated breathing stretch vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing stretch vibration frequencies and the structural parameters of NH$_2$ vary in accordance with the ionization energy of cation, which may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH$_2$)$^-$. We elucidate the microscopic mechanism of correlations between the breathing stretch vibration frequencies of N-H and structural parameters by analyzing the calculated electronic structure from a view point of the molecular-orbitals. A similar tendency in the alanates is also discussed. [1] P. Chen, Z. Xiong, J. Luo, J. Lin and K.L. Tan, Nature \textbf{420}, 302 (2002). [2] B. Bogdanovi and M. Schwickardi, J. Alloys Compd. \textbf{253-254}, 1 (1997). [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y24.00010: Neutron spectroscopy of $\gamma $-AlH$_{3}$ Alexander Kolesnikov, Jason Graetz, Craig Jensen, Walker Langley, Vladimir Antonov The density of vibrational states, G(E), for $\gamma $-AlH$_{3}$ is measured by inelastic neutron scattering. The obtained spectrum noticeably differs from that of $\alpha $-AlH$_{3}$, because $\alpha $-AlH$_{3}$ has a structure built of corner-sharing AlH$_{6}$ octahedra, while the $\gamma $-AlH$_{3}$ is composed of both corner- and edge-sharing AlH$_{6}$ octahedra. The first acoustic phonon peak in the G(E) of $\gamma $-AlH$_{3}$ appears at a lower energy and the band of translational modes extends to higher energies (55 vs. 42 meV) than in the spectrum of $\alpha $-AlH$_{3}$. The bands of Al-H bending modes are observed at about the same energies of 60--140 meV in $\gamma $-AlH$_{3}$ and $\alpha $-AlH$_{3}$, only the structures of the bands are different. The Al-H stretching modes in the $\gamma $-phase show eight peaks in the range 145 to 265 meV, while in the $\alpha $-phase they exhibits only two peaks at 200 and 235 meV. The G(E) spectrum of $\gamma $-AlH$_{3}$ is in good agreement with recent simulations [Y. Wang \textit{et al., Phys. Rev. B} \textbf{77} (2008) 014101], which took into account the existence of a unique double-bridge bond between certain Al and H atoms. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y24.00011: Cubic Metallic Phase of Aluminum Hydride Showing Improved Hydrogen Desorption Ralph H. Scheicher, Duck Young Kim, Rajeev Ahuja AlH$_3$ is of great interest for hydrogen storage applications, with a particularly attractive feature being its large hydrogen capacity of 10 wt.\%. Here we report the results of our density functional theory study of the dehydrogenation properties in a cubic phase of AlH$_3$. The metallic nature of the electronic structure entails a more favorable hydrogen removal energy which is lowered by 75\% compared to the insulating hexagonal phase. This remarkable reduction in the Al--H bond strength might bear important consequences for feasible applications of AlH$_3$ as an on-board hydrogen storage material for mobile applications. Suggestions are made how the cubic phase could be prepared and stabilized at ambient pressure by off-board quenching. See also: R.\ H.\ Scheicher, D.\ Y.\ Kim, S.\ Leb\`egue, B.\ Arnaud, M.Alouani, and R.\ Ahuja, {\it Appl.\ Phys.\ Lett.} {\bf 92}, 201903 (2008) and D.\ Y.\ Kim, R.\ H.\ Scheicher, and R.\ Ahuja, {\it Phys.\ Rev.\ B} {\bf 78}, 100102(R) (2008). [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y24.00012: Regeneration of Aluminum Hydride studied with Raman Microscopy. David Lacina, Jason Graetz, J.J. Reilly We are interested in developing new methods to form aluminum hydride directly from aluminum powder and hydrogen. Due to the low free energy of formation, aluminum and hydrogen require extremely high pressures to react and form the hydride. It is possible to form alane directly at low pressure when it is catalyzed with a small amount of titanium (2 mol {\%}) and stabilized as an adduct. We have studied the formation of amine-alanes by direct hydrogenation of aluminum and have attempted to understand the mechanisms behind these reversible reactions and the role of the catalyst. We will present the results from our recent survey of possible reactions between aluminum, hydrogen and various amines. We will also present the results of a Raman spectroscopy study of the alane polymorphs at ambient and high pressure and alane amines. [Preview Abstract] |
Session Y25: Focus Session: Graphene XVIII: Functionalization and Growth II
Sponsoring Units: DMPChair: Yong-Jie Wang, NHMFL, Florida State University
Room: 327
Friday, March 20, 2009 8:00AM - 8:12AM |
Y25.00001: Edge states and nitrogen substitutional doping in carbon nanoribbons Jie Jiang, Wenchang Lu, Jerry Bernholc, Piotr Boguslawski The edge states in carbon nanoribbons and the electronic and magnetic properties in N-doped carbon nanoribbons are investigated within density functional theory. While the ground state of zigzag ribbons is spin polarized, defects at the edges destroy the polarization and lead to a non-magnetic ground state. Scanning tunneling spectroscopy will thus show different features depending on edge quality. Turning to substitutionally doped carbon nanoribbons, the impurity states are elongated along the ribbon width by edge and confinement effects, which also affect their ionization energies in armchair ribbons. Formation energy calculations reveal that N atoms preferentially occupy edge sites in carbon nanoribbons. The extra electron from the donor suppresses the spin-polarization and tailor the relative magnetization at the two edges in zigzag ribbons. The interplay of impurity and edge states in zigzag ribbons leads to rich electronic effects, resulting in semiconducting or metallic behavior depending on the dopant position. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y25.00002: Counting Graphene Layers on Glass by Optical Reflection Microscopy Helgi Skulason, Peter Gaskell, Chris Rodenchuk, Thomas Szkopek Using optical reflection microscopy we can locate and count graphene layers on a bulk glass substrate. This is a reliable and low cost technique for graphene flake metrology. Optical reflection measurements are in agreement with the universal optical conductance of graphene. We present measurements of the optical conductivity of graphitic flakes showing a transition from few layer graphene to bulk graphite. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y25.00003: Molecular Physisorption on Graphene. David Carey, Thomas Connolly Ab initio calculations using both LDA and GGA functionals have been used to examine the binding energy, optimum binding intermolecular separations, molecular orientation dependence for a range of graphene lattice sites with oxygen containing molecules such as CO and NO. For all sites investigated NO has a higher binding energy than CO. For example, we find that the most stable sites are for the intermolecular axis parallel to the plane of the graphene layer with a binding energy of 195 meV for NO and 131 meV for CO using LDA VWN functional. Using the GGA PW91 functional the corresponding binding energies are 45 meV and 28 meV. When the CO or NO molecular axis is perpendicular to the graphene layer, orientation with the O atom oriented away from the graphene layer are found to be favoured than those with the O atom closer to the graphene layer. Molecular physisorption on graphene is discussed. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y25.00004: Interface structure for growth of epitaxial graphene on SiC(0001) S.H. Rhim, G. Sun, L. Li, M. Weinert In spite of the enormous effort devoted to the study of the epitaxial growth of graphene on SiC, there is not yet a consensus regarding the structure of the interface between graphene and the substrate. There have been a long standing discrepancy between low energy electron diffraction (LEED) and STM patterns regarding the periodicity of graphene on SiC(0001); the theoretical studies of the of $6\sqrt{3}\times6\sqrt{3}$ \footnote{S. Kim, J. Ihm, H. J. Choi, and Y. W. Son, Phys. Rev. Lett. {\bf 100}, 176802 (2008).} or $\sqrt{3}\times\sqrt{3}$ \footnote{F. Varchon {\em et al}, Phys. Rev. Lett. {\bf 99}, 126805 (2007); A. Mattausch and O. Pankratov, Phys. Rev. Lett. {\bf 99}, 076802 (2007} periodicity, while describing some aspects, disagree in important details with scanning tunneling microscopy (STM) images. We present a combined theoretical and experimental study, employing density functional calculations and STM, to investigate this issue. We propose the formation of a defected graphene layer at the interface, and then subsequent growth of graphene. The calculated bias-dependent STM images are in good agreement with our STM images, and provide insight into the details of the interface structure. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y25.00005: In-situ IR studies of graphene oxide reduction. Muge Acik, Laurence Goux, Yves Chabal Thermal reduction of graphene oxide (GO) synthesized by Hummer's method is studied by \textit{in-situ} infrared absorption spectroscopy in a vacuum reactor. Initially, water and hydroxyl groups are removed (100$^{o}$C), with release of CO$_{2}$. Upon reduction of epoxides and carbonyl groups, the appearance of sp2-bonded carbon (C=C bonds) is evident with detection of in-plane and out of plane vibrations. However, oxygen remains in the structure in the form of COC bonds even after 700$^{o}$C anneal. Around 290$^{o}$C, a strong increase of the absorbance associated with structure changes of GO is observed. The increase of the refractive index is attributed to an increase of electrical conductivity after reduction of GO. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y25.00006: AFM local oxidation nanolithography of graphene Lishan Weng, Liyuan Zhang, Yong P. Chen, Leonid P. Rokhinson We demonstrate the local oxidation nanopatterning of graphene films by an atomic force microscope. The technique provides a method to form insulating trenches in graphene flakes and to fabricate nanodevices with sub-nanometer precision. By utilizing this technique, a 25-nm-wide nanoribbon and submicron size nanoring were fabricated from a graphene flake. In addition we found that either trenches or bumps can be written on the graphene surface depending on the lithography conditions. It is proposed that the trenches are created by defect-associated oxidation whereas the bumps are incorporation of oxygen into the graphene lattice. Some of the bumps disappear with time as quickly as in a few minutes or as slow as in a few days. We also further investigate the possibility to remove the bumps in a controllable manner by writing trenches on top, applying opposite voltage or change the environmental conditions. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y25.00007: Hydrogen Saturation of Graphene Nanoribbons: edge states suppression and gap behavior Thiago Martins, Antonio J. R. da Silva, Adalberto Fazzio Di-hydrogenated zigzag edges Graphene Nanoribbons (2H-ZZ-GNR) are more stable than the usually studied mono-hydrogenated [1] passivation (H-ZZ-GNR). Using density functional theory, we studied a variety of 2H-ZZ-GNR configurations. We investigated how the interaction between the CH$_{2}$ units depends on their separation at the same edge as well as on the width of the ribbon. We observe, in agreement with previous studies [1], that the 2H-ZZ-GNR passivation suppresses the presence of edge states, thus eliminating the magnetic instability of H-ZZ-GNR that is responsible for the gap opening in the anti-ferromagnetic ground state configuration. Moreover, there is a reduction of coupling between edge and bulk carbon atoms, resulting in a band structure whose gap is dominated by bulk bands and confinement effects. We also studied the behavior of the gap as a function of the ribbon's width, and we observed that it quickly closes as the width is increased. \\[3pt] [1] T. Wassmann, A. P. Seitsonen, A. M. Saitta, M. Lazzeri, and F. Mauri, Phys. Rev. Lett. {\bf 101}, 096402 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y25.00008: Anomalous magnetic susceptibility and Hall effect from valley degrees of freedom Tianyi Cai, Wang Yao, Junren Shi, Qian Niu With a staggered sublattice potential, sizable gaps can occur in epitaxial graphenen films. Magnetic and transport properties of this system are studied. We predict large signal of magnetic susceptibility and relate it to the intrinsic large magnetic moments of electrons. There is also an anomalous contribution to the ordinary Hall effect, which is due to the valley dependent Berry phase. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y25.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y25.00010: Si diffusion on and between graphene sheets Lede Xian, M.Y. Chou The growth of epitaxial graphene (EG) on the SiC substrate is accompanied by the evaporation of Si atoms during the growth process. The continuing loss of Si atoms takes place even after the surface graphene sheets have been formed. This atomic transport is believed to be a key element in establishing a growth mechanism to model and control the process. Using density functional theory (DFT) calculations, we have studied the diffusion of Si atoms on a single layer of graphene and between graphene sheets. The potential energy surfaces are explored. For single-layer graphene, the diffusion barrier for Si is relatively low. While for multilayers, some buckling of graphene sheets will appear and the stacking pattern also plays a role. The connection with the growth process will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y25.00011: Gauge field for the edge states in graphene Ken-ichi Sasaki, Shuichi Murakami, Riichiro Saito By considering a continuous model for graphene, we study a special gauge field for the edge state. The gauge field explains the properties of the edge state such as the existence only on the zigzag edge, the partial appearance in the k-space, and the energy position around the Fermi energy. The gauge field polarizes the pseudospin. The applications of the gauge field to the ferromagnetism of edge states and the electron-phonon interaction are reported on. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y25.00012: Collective properties of magnetobiexcitons in quantum wells' and graphene superlattices Oleg Berman, Roman Kezerashvili, Yurii Lozovik The Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field are reported. The two different Hamiltonians of a dilute gas of magnetoexcitons with a dipole-dipole repulsion in superlattices consisting of both QWs and graphene layers (GLs) in the limit of high magnetic field have been reduced to one effective Hamiltonian a dilute gas of two-dimensional excitons with the renormalized effective mass of the magnetoexciton, which depends on the magntic field. The instability of the ground state of the system of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The stable system of indirect quasi-two-dimensional magnetobiexcitons, consisting of pair of indirect excitons with opposite dipole moments is considered. The density of the superfluid component $% n_{s}(T)$ and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two- dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both the QW and graphene realizations. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y25.00013: The Electronic Structure of Few-Layer Graphene: Probing the Evolution from a 2-Dimensional Sheet to a 3-Dimensional Solid by Optical Spectroscopy Kin Fai Mak, Matthew Sfeir, James Misewich, Tony Heinz The evolution of the electronic structure of few-layer graphene, for $n$ = 1, 2, 3, {\ldots}, 8 atomic layers, was characterized by optical absorption spectroscopy. Each thickness of few-layer graphene exhibited well defined and distinct infrared absorption peaks associated with interband transitions. The positions of the peaks were found to obey a simple scaling relation with layer thickness. The principal features of the experimental spectra for all samples could be described consistently in terms of the electronic states of the parent graphite material through application of a specific zone-folding construct obtained when only nearest-layer interactions are considered. Both the experiment and analysis permit one to follow the convergence of the multilayer graphene response to that of graphite with increasing sample thickness. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y25.00014: Edge effects in Bilayer Graphene Nanoribbons Matheus P. Lima, Adalberto Fazzio, Antonio J. R. da Silva We investigate the geometrical and electronic structure of zigzag bilayer graphene nanoribbons (B-ZGNR), with widths that range from $w=0.6$ to $w=4.5$ $nm$. The layers are in the Bernal stacking, which means that there are two types of C atoms, those that are positioned above the center of the hexagons of the other layer, defining a B-sublattice, and those right on top of the C atoms of the other layer, forming an A-sublattice. When we cut the layer along the zigzag edge, there are two possible alignments, $\alpha$, where the outermost edge atoms belong to the A- sublattice, and $\beta$, where the outermost edge atoms belong to the B-sublattice. Thus, only the inter-layer edge interaction differs. We found that the $\alpha$ alignment is energetically favorable, with an inter-layer edges attraction, whereas for the $\beta$ there is an inter-layer edges repulsion. These edge-related forces cause a deviation from the exact Bernal stacking, resulting in a non-monotonic behavior of the energy gap with the width $w$ for the $\alpha$ B-ZGNR, with a maximum value at $w\approx 3.5nm$. This is a consequence of the competition between bulk and strongly attractive edge interactions. All results were obtained using density functional theory calculations with the inclusion of parametrized van der Waals interactions. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y25.00015: Measuring a Butterfly with Graphene Andres Concha The Hofstadter butterfly (HB) is a hierarchical structure that emerges as a consequence of the commensuration of two length scales, the magnetic length $l\sim \frac{1}{H}$ and the lattice spacing $a$ between atomic sites. We argue that by using a set of scalar potentials and an external magnetic field it is possible to measure and manipulate the HB in graphene. Our claim is based in the fact that in graphene, close to the Dirac point the theory becomes critical and as such no length scale is present in the low energy description. Thus the only relevant length scales are dictated by the magnetic length and the distance between potential barriers. It is shown that the Hall conductance in the minigaps should be directly measurable with current available technology. [Preview Abstract] |
Session Y26: Glassy and Amorphous Systems
Sponsoring Units: DCMPRoom: 328
Friday, March 20, 2009 8:00AM - 8:12AM |
Y26.00001: Long Term Aging of As$_{x}$Se$_{1-x }$glasses and the Intermediate Phase Ping Chen, Jacob Wachtman, P. Boolchand The reversibility window in As$_{x}$Se$_{1-x}$ glasses was reported\footnote{D.G. Georgiev et al. Phys. Rev. B \underline {62}, R9228(2000).} 8 years ago to reside in the 28{\%} $<$ x $<$ 37{\%} range. We have re-examined those samples in m-DSC and Raman scattering. Both the 8 year old hermetically sealed samples in Al holders (set A) and samples from the same batch preparation but stored in plastic vials (set B) at laboratory ambient environment were studied. The reversibility window in samples of set A, after 8 years of aging, is found to be intact. In set B, analysis of the T$_{g}$ endotherm becomes difficult because of a precursor exotherm that appears in the 32{\%} $<$ x $<$ 60{\%} range, and steadily increases with x. In addition, in both set of samples, one observes a sub T$_{g}$ endotherm upon aging in the 90 $^{\circ}$C $<$ T $<$ 120 $^{\circ}$C range. These m-DSC results supported by Raman scattering suggest that the exotherm is due to light induced nanocrystallization (nc) of As$_{4}$Se$_{4}$ fragments (an extrinsic effect), while the sub-T$_{g}$ feature is due to nc fragments of trigonal Se formed upon long term aging(an intrinsic effect). These findings will be compared to a recent report.\footnote{R. Golovchak et al. Phys. Rev. B \underline {78}, 014202(2008).} [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y26.00002: Molecular structure of virgin and T$_{g}$ cycled (Ag$_{2}$Se)$_{x}$ (AsSe)$_{1-x}$ bulk glasses Jacob Wachtman, Ping Chen, P. Boochand AsSe, the base glass (x = 0) in the titled ternary, is an interesting example of a chalcogenide that is partially de-mixed into As$_{4}$Se$_{4}$ molecules segregated from a connected AsSe network, with the latter determining glass network properties. Raman scattering reveals sharp modes of the Realgar molecules that are superimposed on broad modes coming from of the backbone. Upon T$_{g}$ cycling virgin samples (as quenched melts), the concentration of de-mixed As$_{4}$Se$_{4}$ molecules decreases, suggesting that thermally induced polymerization occurs; molecules break up to form part of the connective tissue. Modulated DSC experiments reveal a broad exotherm near 140 $^{\circ}$C in virgin samples, which becomes nearly extinct in T$_{g}$ cycled samples. The exotherm may represent Realgar molecules nano-crystallizing as the temperature approaches T$_{g}$. Compositional trends in thermal parameters such as T$_{g}$(x), $\Delta $C$_{p}$(x), and the $\Delta $H$_{nr}$(x) as a function of Ag$_{2}$Se content `x' of the glasses will be reported. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y26.00003: Atomistic Picture of the Intermediate Phase in GexSe1-x Glasses: A Joint Theoretical and Experimental Study Fakhar ul Inam, Gang Chen, Denyago Tafen, David Drabold Raman and calorimetric studies on~~GexSe1-x glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenide and other glasses. Here, Ab-initio molecular dynamics (MD) based models of these glasses are discussed, and an atomistic picture of the IP, based upon the models and available experiments, is presented. A thorough analysis of our models reveals that the IP in GexSe1-x glasses appears due to the competition between the percolating~GeSe2 phase~and the a-Se phase, which gives rise to the ``flattening'' of the observables in the IP window. We present X-Ray Absorption Near Edge Structure (XANES) measurements on germanium selenide glasses in the IP composition range, and detect an electronic signature of the IP in terms of the shift of the XANES white line (WL) and non-linear behavior o their intensities in the IP window. We show that these models appear to properly represent the XANES results. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y26.00004: Boson modes and Floppy modes in network glasses D. Novita, Ping Chen, P. Boolchand Network glasses differ from their crystalline counterparts in a significant way- the presence of an excess of low-frequency vibrations. Here we show that in a covalent\footnote{P. Chen et al. ArXiv 0810.3637} ( As$_{x}$S$_{1-x})$ and in a super-ionic\footnote{D. Novita et al. ArXiv 0808.1154} (AgI$_{(x)}$AgPO$_{3(1-x)})$ glass system, Raman Bose peak scattering strength (I$_{B})$ displays commonalities; in the flexible phase of these systems, I$_{B}$ is found to increase almost linearly as networks become more flexible or their connectivity decreases. Moreover, the rate at which dI$_{B}$/d\textbf{\textit{r}} changes is reminiscent of the variation df/d\textbf{\textit{r}} of the floppy mode count (f) with \textbf{\textit{r }}in rigidity theory. These results suggest that at least in the flexible phase, contributions to the boson peak must result in large part from floppy modes. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y26.00005: Characterization of dry B$_{2}$O$_{3}$ glass K. Vignarooban, D. Novita, Ping Chen, P. Boolchand A sample of Puratronic B$_{2}$O$_{3}$ (Aesar) was vacuum ( 10$^{-6 }$Torr) melted in a Pt crucible at 520\r{ }C for 3 days and slow cooled to room temperature to obtain a glass. All sample manipulations were performed in a N$_{2}$ gas purged glove box. T$_{g }$ of the sample from inflexion point of the reversing heat flow in an m-DSC experiment ,using a scan rate of 3\r{ }C/min, gave a value of T$_{g}$ (mDSC) = 308(1)\r{ }C. A traditional DSC experiment, using a scan rate of 10\r{ }C/min, gave a value of T$_{g}$(DSC) = 309(2)\r{ }C. Our T$_{g}$ (DSC) value is 12\r{ }C to 20\r{ }C higher than previous reports$^{2}$ using the same scan rate. Vibrational features in IR reflectance in the 1200-1600 cm$^{-1}$ range (LO and TO modes) , and in the 3200-3600 cm$^{-1}$ range ( free and bonded water) evolve as transparent platelets are exposed to laboratory environment, providing evidence for water reactivity of dry samples. Raman scattering$^{3}$ results complement IR reflectance ones. We confirm$^{2}$ density of dry samples (1.805(4)gms/cm$^{3})$ to be somewhat less than wet ones (1.815(4) gms/cm$^{3})$. $^{2}$ Ramos et al. JNCS \underline {221}, 170 (1997). $^{3~}$F. Galeener et al, PRB \underline {22}, 3983 (1980). [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y26.00006: Silicon under mechanical shear: molecular dynamics study Ali Kerrache, Normand Mousseau, Laurent J. Lewis Relaxation processes and defect behavior in amorphous silicon (a-Si) under shear are investigated by molecular dynamics simulations using the empirical Environment Dependent Inter-atomic Potential. Shear deformations allow us to reproduce a vast range of interesting dynamics in disordered materials. For example, it has been shown recently that high-energy ion irradiation deforms plastically a-Si samples, following a pattern similar to the application of a shear. While large shear reproduce high-irradiation effects, moderate one can accelerate defect diffusion. A controlled application of shear can therefore help us to understand better the nature of defect diffusion in a-Si, in addition to generating new defects that could be placed with the appropriate external field at critical positions or even lead, in some cases, to crystallization of the a-Si. The properties of a-Si under shear are analyzed as a function of imposed shear velocity and as a function of the distance from the wall. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y26.00007: Liquid-liquid transition in supercooled silicon determined by first-principles simulation P. Ganesh, M. Widom First principles molecular dynamics simulations reveal a liquid-liquid phase transition in supercooled elemental silicon. Two phases coexist below T$_{c} \quad \sim $ 1232K. The low density phase is nearly tetra-coordinated, with a pseudogap at the Fermi surface, while the high density phase is more highly coordinated and metallic in nature. The transition is observed through the formation of van der Waals loops in pressure-volume isotherms below T$_{c}$. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y26.00008: High thermal conductivity of a hydrogenated amorphous silicon film J.L. Feldman, Xiao Liu, D.G. Cahill, R.S. Crandall, Noam Bernstein, D.M. Photiadis, M.J. Mehl, D.A. Papaconstantopoulos, Ho-Soon Yang We measured the thermal conductivity $\kappa$ of an 80~$\mu$m thick hydrogenated amorphous silicon ({\em a}-Si:H) film from 80\,K to room temperature with the 3$\omega$ method and at room temperature with the time-domain thermoreflectance (TDTR) method. The {\em a}-Si:H sample with 1~at.\% hydrogen was prepared by hot-wire chemical-vapor deposition (HWCVD), a procedure which was found previously to produce superior material properties with a near absent atomic tunneling states that are ubiquitous in glasses. We find that $\kappa$ is higher than any of the previous temperature dependent measurements, and shows a strong phonon mean free path dependence. We also performed numerical calculations on three 1000 atom models using Kubo theory and a tight binding electronic structure method. Due to the restraints of the TDTR results on low frequency extrapolations of calculated phonon diffusivities, the Kubo thermal conductivityis seen to be too small to explain our experiments. We conclude that the HWCVD a-Si:H sample has superior structural ordering relative to any amorphous silicon previously studied. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y26.00009: Specific heat of amorphous materials outside of the universal regime: $a$-Si and $a$-Si:H Daniel Queen, Qi Wang, Richard Crandall, Frances Hellman We present specific heat measurements for $a$-Si and $a$-Si:H films that are known to lack the broad distribution of tunneling level systems (TLS) as measured by internal friction. Below 1K the TLS model describes the universal behavior seen in specific heat, thermal conductivity, and other measurements. However, this model does not address the universal high temperature features, namely, the plateau in k and peak in C/T$^{3}$ that occur around 10K. Tetrahedrally bonded systems, such as $a$-Si and $a$-Ge, have long been thought to lack TLS due to the over constrained nature of their bonding. These materials prove difficult to quench from a bulk melt but are routinely grown by vapor deposition. We use our MEMS nanocalorimeter for heat capacity measurements between 2-300K on a range of $a$-Si and $a-$Si:H films prepared by e-beam evaporation and Hot-wire CVD. Changes in the high temperature specific heat will be discussed for films that are known to have orders of magnitude lower densities of TLS as measured by internal friction. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y26.00010: Hydrogen microstructure and voids in amorphous silicon hydride: A first principles study Rajendra Timilsina , Parthapratim Biswas We study distribution of hydrogen and hydride configurations in several models of a-Si:H using first-principles density functional calculations. Motivated by recent experimental result via small angle x-ray scattering, which reveals the presence of large voids (of linear dimension up to 4 nm) in a-Si:H, we develop models for a range of concentration, and study the effect of voids on hydrogen distribution. In particular, we investigate the presence of voids in two different concentration regime: high (14 at. \% and above) and low (below 14 at. \%) following a recent experimental observation from infrared absorption spectroscopy . The bonding environment of H atoms, and the local electronic structure near the voids are also presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y26.00011: Atomistic modeling of amorphous silicon carbide: A first-principles study Parthapratim Biswas, Raymond Atta-Fynn Localized basis {\it ab initio} molecular dynamics techniques within density functional theoretic framework have been used to model a realistic atomistic configuration of amorphous silicon carbide ({\it a}-Si$_{0.5}$C$_{0.5}$) containing 1000 atoms. The structural, electronic and vibrational properties have been studied and compared to existing theoretical models and available experimental data. Our study clearly reveals that the short-range chemical order in this material is predominant due to presence of heteronuclear Si-C bonds with coordination defect concentration less than 5\% and the chemical disorder parameter was $\chi$ = 0.083. Our 1000-atom model shows the presence of a clean gap in the spectrum and we also study the nature of the localization of the electronic band tail states as well as the vibrational eigenmodes. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y26.00012: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y26.00013: Glass Formability of Aqueous Solutions, and the Critical Nucleation Radius of Cubic Ice Ryan Dunn, Matthew Warkentin, Robert Thorne We have determined critical cooling rates and critical warming rates for a range of concentrations of different solutes in aqueous solutions using high-speed video microscopy. Our results show that the glass formability of aqueous solutions is exponential in the concentration for all solutes tested, with a different characteristic concentration for each solute. The characteristic concentration correlates with molecular radius. A simple modification of critical droplet theory relates the characteristic concentration to the critical nucleation radius in pure water, and explains the relationship between molecular radius and the characteristic. This simple, general theory of glass formability in aqueous solutions is important at a fundamental level, and will also have broad consequences for the field of cryobiology. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y26.00014: Nanocalorimetry as a means to explore thin films of vapor-deposited organic glasses Kenneth L. Kearns, Heiko Huth, Mathias Ahrenberg, Christoph Schick, M. D. Ediger Vapor deposition was used to prepare nanometer thick films of small molecule organic glasses. Films of indomethacin (IMC) and 1,3,5-(tris)naphthylbenzene (TNB) with a range of stabilities and thicknesses were created and characterized using differential nanocalorimetry. The heat capacity-like calorimetric signal was lower for the stable vapor-deposited glass films at temperatures below the glass transition T$_{g}$ than for an ordinary glass prepared by cooling the liquid. A gradual increase in the calorimetric signal was observed during the isothermal transformation above T$_{g}$ from stable to ordinary glass with the fastest transformation taking place in about 200 $\tau _{\alpha }$. The time for this transformation was dependent on film thickness with 350 nm thick films transforming approximately 5 times faster than 40 $\mu $m thick films. Aging experiments on the ordinary glass also showed a thickness dependence with thinner films aging more rapidly. This thickness dependent behavior is consistent with a mechanism where the dynamics at the film interfaces are faster than those in the bulk. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y26.00015: Evidence for a new molecular packing at low temperatures in vapor-deposited indomethacin Kevin Dawson, Ken Kearns, Werner Steffen, Lian Yu, Mark Ediger Thin films of the low molecular weight organic glassformer indomethacin were prepared at different substrate temperatures using physical vapor deposition. When analyzed by wide angle x-ray scattering, samples prepared at T$_{g}$-50 K showed a broad high intensity peak that is not present in samples prepared at T$_{g}$. When such samples were annealed at T$_{g}$+4 K the extra peak eventually vanished but only after 24,000 s (more then 500 $\tau _{\alpha })$. At low deposition rates the WAXS signature of this new molecular packing is only observed for substrate temperatures below T$_{g}$-20 K. Based on WAXS data on supercooled indomethacin, the new WAXS peak is unexpected. These results suggest that ordinary indomethacin glasses aged to equilibrium 20 K below the conventional T$_{g}$ would undergo a first order transition to a new amorphous phase. [Preview Abstract] |
Session Y27: Mechanical Properties of Nanomaterials
Sponsoring Units: DCMPChair: John Sullivan, Sandia National Laboratories
Room: 329
Friday, March 20, 2009 8:00AM - 8:12AM |
Y27.00001: Carbon nanotube dispersed liquid crystal: A nano electromechanical system Rajratan Basu, Germano Iannacchione Electric field induced director orientation of a nematic liquid crystal (LC) + carbon nanotube (CNT) system reveals insights on switching behavior for this anisotropic composite. Once the field goes off, the LC+CNT system relaxes back to the original orientation through a mechanical rotation, revealing the intrinsic dynamics. LC molecules and CNTs cooperatively form local \textit{pseudonematic }domains in the isotropic phase due to strong LC-CNT interactions. These field-responsive anisotropic domains do not relax back to the original orientation on switching of the field off, which could find potential applications in memory devices. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y27.00002: Strong ``position squared'' optical readout of a micromechanical oscillator Jack Sankey, Andrew Jayich, Benjamin Zwickl, Cheng Yang, Jack Harris Optomechanical devices with a flexible SiN membrane inside an optical cavity allow for very high optical finesse and high mechanical quality factor in a single device. They also provide fundamentally new functionality: the cavity detuning can have a quadratic dependence on the membrane position. This enables a measurement of ``position squared'' ($x^2$) and in principle a QND phonon number readout of the membrane. However, the readout achieved using a single cavity mode is not sensitive enough to observe quantum jumps between phonon Fock states. Here we demonstrate an $x^2$-sensitivity that is orders of magnitude stronger using two nearly-degenerate transverse cavity modes, and that we can tune this sensitivity somewhat by tilting the membrane. We derive a perturbatve treatment that describes the interactions between the transverse optical modes and achieve good agreement with observation using realistic parameters. We also show that the $x^2$-coupling should be easily tunable over a wide range via mm-scale membrane displacements along the cavity axis. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y27.00003: \textit{In-situ} Transduction of the In-Plane and Out-of-Plane Modes of Nanowire-based Very-High Frequency Electromechanical Resonators Wayne Fung, Wei Lu Recent advances in nanoelectromechanical systems (NEMS) promise important applications such as mass and force sensing, rf signal generation and timing, and quantum measurement studies. Chemically synthesized nanowires appear especially attractive for NEMS because of their atomically smooth surfaces and large aspect ratios. Here we report the measurements of doubly-clamped beam mechanical resonators using SnO$_{2}$ nanowires with widths ranging from 30 to 80 nm. The devices are electrostatically actuated and detected on-chip using a dual-gate setup and an all-electronic transduction scheme. This setup also allows us to independently actuate and tune the resonant frequencies of both the in-plane and out-of-plane modes of vibration \textit{in situ}, potentially leading to NEMS-based practical applications. Our devices exhibit resonant frequencies ranging from 30 to 100 MHz, quality factors up to 2000, force sensitivities down to 10$^{-14}$ N/Hz$^{1/2}$, and mass sensitivities down to 5 x 10$^{-17}$ g. The frequency of the in-plane and out-of-plane modes can be tuned within $\pm $1 MHz of their nominal values at gate voltages of $\pm $5 V. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y27.00004: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y27.00005: Mechanical properties of metal-repaired defective carbon nanotubes Guangping Zheng Carbon nanotubes (CNs) are promising in the producing of strong and light structural materials because of their unique mechanical properties such as ultrahigh mechanical strength and large ultimate tensile strain. However, CNs are not defect free. Instead, several types of intrinsic defects exist, hence the mechanical strength and ductility of CN can be significantly lower than those of an ideal one. In this study, 3d transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) or clusters are filled into the defective sites of single-walled (SW) CN containing vacancy defects, resulting in stable repaired SWCN. The mechanical and electronic properties of the repaired SWCNs are investigated by spin-polarized density functional theory. The results indicate that the 3d transition-metal atoms acting as substitutional defects can substantially modify the electronic structure and magnetization of an un-deformed CN. Compared with defective SWCN, the metal-repaired CN shows significant enhancements in mechanical strength and ductility that are close to or even better than those of pristine CNs. The underlying physics of these behaviors are analyzed by the structural transformation, electronic structures and spin and charge distributions during the tensile tests. Strong magneto-mechanical coupling effect is found to be responsible for the enhanced mechanical behaviors of metal-CN hybrid structures. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y27.00006: Mechanics of Nanometric Water Wire Manhee Lee, Baekman Sung, Bongsu Kim, Jongwoo Kim, Wonho Jhe Water has been one of the perfect newtonian viscous liquids, which are exactly described by navier-stokes equation. However, it has been found that the effective shear viscosity of water confined between mica crystals at few nanometer thickness is very different from the one of 3-dimensional bulk water. While some researchers have measured very high viscoelasticity of the confined liquid [1], the other researchers reported the fluidic nature of water confined between mica surfaces at $<$3.5 nm interfacial separation like bulk-water viscosity [2]. These conflicting results concerning the mechanical properties of nanometric water have been continually reported for the past several years. None of them clearly clarified the mechanical properties of nanometric water, and the detailed behavior of the viscoelasticity within a tip-sample separation less than 1 nm has not been measured. Here, we investigate a nanometric water cluster formed between AFM tip and sample surface and present the nano-mechanical properties of it including viscoelasticity, dissipation energy, and phase transitions. [1] Y. Zhu and S. Granick, Phys. Rev. Lett. \textbf{87}, 096104 (2001). [2] U. Raviv, P. Laurat, and J. Klein, Nature (London) \textbf{413}, 51 (2001). [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y27.00007: Torsional Stick-Slip Behavior in WS$_{2}$ Nanotubes K.S. Nagapriya, Ohad Goldbart, Ifat Kaplan-Ashiri, Gotthard Seifert, Reshef Tenne, Ernesto Joselevich We experimentally observed atomic-scale torsional stick-slip behavior in individual nanotubes of tungsten disulfide (WS$_{2})$. When an external torque is applied to a WS$_{2}$ nanotube, all its walls initially stick and twist together, until a critical torsion angle, at which the outer wall slips and twists around the inner walls, further undergoing a series of stick-slip torque oscillations. This is contrary to what happens in a multi-wall carbon nanotube, where an external torque causes the outer wall to slip and twist smoothly around the inner walls. We present a theoretical model based on DFTB calculations, which explains the torsional stick-slip behavior of WS$_{2}$ nanotubes in terms of a competition between the effects of the in-plane shear stiffness of the WS$_{2}$ walls and the inter-wall friction arising from the atomic corrugation of the interaction between adjacent WS$_{2}$ walls. K. S. Nagapriya, Ohad Goldbart, Ifat Kaplan-Ashiri, Gotthard Seifert, Reshef Tenne, and Ernesto Joselevich, Phys. Rev. Lett. \textbf{101}, 195501 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y27.00008: Optical Requirements for Quantum Mechanics with Micromechanical Systems Dustin Kleckner, Brian Pepper, Evan Jeffrey, Petro Sonin, Dirk Bouwmeester Interest in micro-optomechanical systems is motivated by the desire to test quantum mechanics on relatively massive scales. Proposals for realizing these effects have extremely challenging technical requirements, particularly with regards to optical quality. We present the results of simulations designed to determine the effects of imperfections in real systems on the maximum achievable optical finesse. Additionally, we will discuss recent progress in our efforts to fabricate novel devices with the required properties. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y27.00009: A Servo-Controlled Atomic Break Junction: Closing the Gap between Experiment and Modeling Douglas Smith, Jon Pratt, Francesca Tavazza Measurement of force and displacement in atomic-scale deformation experiments enables direct measurement of atomic bond strength and stiffness, promises insight into phenomena like adhesion and friction, and promises to provide reliable experimental data for atomistic models. Such experiments require displacement accuracy well below atomic dimensions and force accuracy below predicted atomic-bond rupture forces of 1.5 nN. A break junction instrument has been realized that uses a fiber-optic laser interferometer to hold the position of a gold contact stable to better than 5 pm for tens of minutes in vacuum at 4 K. This has allowed a careful study of quantized electron transport through a gold junction as it is drawn down to a single Au-Au bond and broken. Density functional calculations show that the junction necks down to a single bond that breaks with a reproducible rupture force that is independent of the crystallographic orientation of the original junction. The work represents one of the first direct deformation studies of an experimental system small enough to be modeled in its entirety using density functional methods. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y27.00010: High-performance supercapacitors, actuators and elastomeric composites based on CNT assemblies Mikhail Kozlov, Jiyoung Oh, MinKyoon Shin, Raquel Robles, M\'arcio Lima, Ray Baughman A number of materials ranging from carbon nanotube (CNT) yarns, sheets and CNT-based composites to shape memory alloys (SMA) have been explored for the application in the area of energy conversion and storage. Highly porous sheets comprised of Single Walled Carbon Nanotubes and doped polypyrrole (SWNT-PPy) were found to possess remarkably high specific capacitance of about 131 F/g; CNT-elastomeric polymer composites exhibited electrical conductivity of about 0.5 S/cm and can be stretched by 1400{\%}. We found that if powered electrically, the isometric stress generated by the CNT-based actuators could be as large as 12 MPa. This approaches the stress generation capability of commercial ferroelectrics and is significantly larger than that of natural muscles. We also report several types of artificial muscles that convert the chemical energy of high--energy-density fuels to mechanical energy. Because of more than 30 times higher energy density obtainable for fuels like methanol, compared to that for the most advanced batteries, the major expected benefits are dramatic increase in energy conversion efficiency, work capacity, power performance. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y27.00011: Determination of Intrinsic Damping in a MWNT using the Harmonic Detection of Resonance Method Doyl Dickel, Gayatri Keskar, Malcolm Skove, Apparao Rao Harmonic Detection of Resonance (HDR) method has been shown to be an effective method of electrically determining the resonant frequency of cantilevered structures at both the micro- and nanometer scale. Previously, HDR has been used effectively to study nonlinear behavior in highly anharmonic systems, as a gas sensor, and to determine the resonant frequency of nanoscale structures such as a Multi-wall Carbon Nanotube (MWNT). In addition, HDR method has been used for determining material properties such as the Young's Modulus. Here, we provide a simple model describing the theory underlying the HDR method and a demonstration of its use to determine the resonant behavior of a MWNT. Finally, we report the effects of varying pressures on both the resonant frequency and quality factor of the MWNT. We also estimate the intrinsic damping inherent in the MWNT from these effects and show its correlation with defect density. The MWNT examined was found to have a resonant frequency for its primary mode of oscillation of 2.79 MHz with a quality factor of 10.15 at a pressure less than 1 Pa. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y27.00012: Eigenmodes and Vibration Spectra of Ag$_2$Ga Nanoneedles Measured Using Laser Doppler Vibrometry R. Reifenberger, L. Biedermann, R. Tung, A. Raman, M. Yazdanpanah, R. Cohn Applications for selectively grown Ag$_2$Ga nanoneedles include high-aspect ratio conductive AFM tips, mass sensors, force sensors, and high resonant frequency nano-cantilevers. To help enable these applications, reliable estimates for the elastic modulus of these nanoneedles and the quality factors of their oscillations are of interest. We have used Laser Doppler Vibrometry (LDV) to measure the vibration spectra and eigenmodes of individual Ag$_2$Ga nanoneedles. An advantage of this technique is that the vibration spectra between 0 and 20~MHz can be measured with high frequency resolution, allowing the eigenfrequencies and quality factors of each resonance to be accurately determined. Using Euler-Bernoulli beam theory, the elastic modulus and spring constant can be calculated from the nanoneedles' eigenfrequencies and the dimensions of the nanoneedles. The techniques developed can be used to measure the vibrational spectra of any suspended nanowire with high frequency resolution. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y27.00013: Digital Batteries Alfred Hubler The energy density in conventional capacitors is limited by sparking. We present nano-capacitor arrays, where - like in laser diodes and quantum wells [1] - quantization prevents dielectric breakthrough. We show that the energy density and the power/weight ratio are very high, possibly larger than in hydrogen [2]. Digital batteries are a potential clean energy source for cars, laptops, and mobile devices. The technology is related to flash drives. However, because of the high energy density, safety is a concern. Digital batteries can be easily and safely charged and discharged. In the discharged state they pose no danger. Even if a charged digital battery were to explode, it would produce no radioactive waste, no long-term radiation, and probably could be designed to produce no noxious chemicals. We discuss methodologies to prevent shorts and other measures to make digital batteries safe. [1] H. Higuraskh, A. Toriumi, F. Yamaguchi, K. Kawamura, A. Hubler, Correlation Tunnel Device, U. S. Patent No. 5,679,961 (1997) [2] Alfred Hubler, http://server10.how-why.com/blog/ [Preview Abstract] |
Session Y28: Nanotechnology I
Sponsoring Units: FIAPChair: Jason Benkoski, Applied Physics Laboratory, Johns Hopkins University
Room: 330
Friday, March 20, 2009 8:00AM - 8:12AM |
Y28.00001: Nanoscale Assembly of Actuating Cilia-Mimetic Lance Baird, Jennifer Breidenich, Bruce Land, Allen Hayes, Jason Benkoski, Pei Keng, Jeffrey Pyun The cilium is among the smallest mechanical actuators found in nature. We have taken inspiration from this design to create magnetic nanochains, measuring approximately 1-5 $\mu $m long and 25 nm in diameter. Fabricated from the self-assembly of cobalt nanoparticles, these flexible filaments actuate in an oscillating magnetic field. The cobalt nanoparticles were functionalized with a polystyrene/benzaldehyde surface coating, thus allowing the particles to form imine bonds with one another in the presence of a diamine terminated polyethylene glycol. These imine bonds effectively cross-linked the particles and held the nanochains together in the absence of a magnetic field. Using design of experiments (DOE) to efficiently screen the effects of cobalt nanoparticle concentration, crosslinker concentration, and surface chemistry, we determined that the morphology of the final structures could be explained primarily by physical interactions (i.e. magnetic forces) rather than chemistry. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y28.00002: Amplification by 1/f noise with stochastic resonance in silicon-based nanomechanical resonators. Diego N. Guerra, Tyler Dunn, Pritiraj Mohanty We report signal amplification by 1/f noise with stochastic resonance in a nanomechanical two-state system of a nonlinear silicon resonator. The addition of 1/f noise to a sub-threshold modulation signal enhances the likelihood of an electrostatically driven resonator switching between its two states in the hysteretic region. Considering the prevalence of 1/f noise in integrated circuits, signal enhancement demonstrated here, using a fully on-chip electronic actuation/detection scheme, suggests potentially beneficial use of the otherwise detrimental noise. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y28.00003: High Frequency Antennas for Wireless Transmissions of Audio and Video Signals Using Threads Spun From Long Multi-Wall Carbon Nanotubes. David Mast, Chaminda Jayasinghe, Mark Schulz, Vesselin Shanov We have used threads spun from long multiwall carbon nanotubes (MWCNT) to make antennas for audio and video broadcasts (transmission and reception) at GHz frequencies. The MWCNT used to make the threads have outer diameters from about 6 nm to 30 nm. These MWCNT's have been grown in lengths up to 18 mm. The diameter of the CNT threads used to fabricate the high frequency antennas was 25 microns. Initial measurements consist of 1) transmission and reception of a CW signals at f$_{ }$= 694 MHz and 1388MHz , 2) the transmission and detection of a CW signal plus sidebands at $\pm $ 100kHz, 3) the broadcast and reception of an AM modulated audio signal, 4) the broadcast and reception of composite video images, 5) the simultaneous broadcast and reception of audio signals from a single CNT antenna, and 6) the simultaneous transmission and/or reception at multiple frequencies from a single CNT thread antenna. The results of using the CNT thread antenna for these transmissions will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y28.00004: Noise color and asymmetry in stochastic resonance with silicon nanomechanical resonators Tyler Dunn, Diego N. Guerra, Pritiraj Mohanty Stochastic resonance (SR) with white noise has been well established as a potential signal amplification mechanism in nanomechanical two-state systems. While white noise represents the archetypal stimulus for SR, typical operating environments for nanomechanical devices often contain different classes of noise, particularly colored noise with a $1/f$ spectrum. As a result, improved understanding of the effects of noise color is necessary in maximizing device performance. Here, we report measurements of SR in a silicon nanomechanical resonator using $1/f$ noise and exponentially correlated Ornstein-Uhlenbeck noise. Power spectral densities and residence time distributions provide insight into asymmetry of the bistable amplitude states, and evidence suggests that $1/f^{\alpha }$ spectra with increasing noise exponent $\alpha $ may lead to increasing asymmetry in the system, reducing the achievable signal-to-noise ratio. Furthermore, we explore the effects of correlation time $\tau $ on SR with the use of exponentially correlated noise. We find monotonic suppression of the spectral amplification as the correlation time increases. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y28.00005: Room-Temperature Single-Electron Transistors fabricated using CMOS-compatible processes Vishva Ray, Ramkumar Subramanian, Pradeep Bhadrachalam, Seong Jin Koh A critical requirement for the fabrication of single-electron devices is that the device components (Coulomb island, source, drain, and gate electrodes) be arranged with nanometer scale precision. We present a new single-electron device structure which consists of vertically stacked source and drain electrodes separated by a thin dielectric film. Using this configuration, we were able to control the gap between the electrodes with nanometer scale precision over an entire wafer, thereby allowing the concurrent fabrication of many device units in parallel processing. Coulomb islands (10 nm Au nanoparticles) were positioned in the gap between the source and the drain electrodes. Individually addressable gate electrodes were then incorporated in these devices, also in complete parallel processing. These devices have yielded clear single-electron transport characteristics (Coulomb blockade/staircase and Coulomb oscillations) at room temperature as well as at low temperatures ($\sim $10 K). The experimental data is in excellent agreement with the orthodox theory of single-electron tunneling. This study suggests that the fabrication of chip-level integrated systems of single-electron devices may now be possible using current CMOS fabrication technology. (ONR (N00014-05-1-0030), NSF CAREER (ECS-0449958), and THECB ARP (003656-0014-2006)). [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y28.00006: Electronic-structure modulation transistor: A new switch with few kT supply voltage Hassan Raza, Tehseen Raza, Tuo-Hung Hou, Edwin Kan We present a novel electronic-structure modulation transistor (EMT) for post-CMOS logic applications. The device is based on the electronic structure modulation of the channel by an external gate voltage. Its functionality is theoretically analyzed using single-band tight-binding model and non-equilibrium Green's function formalism. We report that the EMT is expected to have very large ON/OFF current ratio with reasonable self gain using a few kT Vdd. We provide an experimental proof-of-concept device of the proposed mechanism in a double gated structure using a 20 nm long and 10 um wide channel consisting of Au nanocrystals (NCs) and nitride traps. Putting negative charge on the NCs is results in wavefunction extension over larger distance due to lifting of the energy levels, resulting in reduction of the effective barrier. In transfer characteristics, we find a nonlinear dependence of the drain current on gate voltage and charge stored in the channel, which we attribute to the wavefunction modulation of the Au NCs due to charging. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y28.00007: The electronic structure of diodes probed under bias Per-Anders Glans, Jinghua Guo, Jeong Park, Somorjai Gabor Chemists have known for decades that when metal nano-particles are affixed to a catalytically inactive oxide surface, the catalytic turnover rate of the array is more than 10 times that of a metal surface alone. However, the mechanism behind the effect is not clear. To understand the catalytic activity of the interface between the metal nano-structures and the oxide substrate, we have investigated the electronic structure of Pt and Pd doped diodes on a TiO$_2$ substrate. The devices were put under bias during the measurements in an attempt to reproduce the potential differences found over the diode when used as a catalyst. This is challenging for electron based measuring techniques, but using photon-in, photon-out techniques we have successfully probed the electronic structure of Pt and Pd doped diodes under bias. The results from soft x-ray absorption and emission will be presented. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y28.00008: Spatial Wavefunction Switched Field-Effect Transistors (SWS-FETs): A Novel Device with Multiple States and Functionality Faquir Jain, Evan Heller An asymmetric coupled quantum well transport channel FET is shown to confine carriers in either the lower of the two wells, both wells, or upper well (adjacent to the gate insulator) depending on the gate voltage. That is, as the gate voltage is increased above threshold in n-channel FET, the electron wavefunction is spatially switched, which in turn change the operating characteristics. A Spatial Wavefunction Switched (SWS) FET, having two coupled wells in the channel, provides four states 00, 01, 10, 11 corresponding to wavefunction location. No wavefunction being the OFF (00 state), electrons in well W2 (01 state), in well W1 (10 state), and both Wells W1-W2 (11 state). Simulation has verified the spatial switching in SiGe as well as InGaAs coupled well FET structures. The wavefunctions/carrier locations get more pronounced and result in additional states when the transport channel is configured as a quantum dot (QD) channel. Preliminary simulation of quantum dot gate 3-state structures [1], configured as SWS-QD channel FET, will also be presented. [1]. F.C. Jain, E. Heller, S. Karmakar, and J. Chandy, Device and Circuit Modeling using Novel 3-State Quantum Dot Gate FETs, ISDRS Proc 2007. *Supported in part by ONR Contract N00014-06-1-0016 and NSF ECS 0622068 grant. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y28.00009: Ultrathin germanium-on-insulator tunneling field effect transistors D. Kazazis, P. Jannaty, A. Zaslavsky, C. Le Royer, C. Tabone, L. Clavelier As the CMOS downscaling is approaching its limits, there is greater need for alternative and unconventional devices to continue enhancing the performance of electronics. We report on the fabrication and electrical characterization of a CMOS-compatible germanium-on-insulator (GeOI) tunneling field effect transistor (TFET) device that can in principle switch more sharply than a standard FET. The source-drain current in the TFET is based on interband tunneling between an inversion channel and a counterdoped drain electrode. Taking advantage of the narrower bandgap of germanium, the devices are fabricated in ultra-thin GeOI and consist of a heavily $p$-type doped, epitaxially grown drain, an $n$-type ion-implanted source and a standard high-\textit{$\kappa $} dielectric gate stack with channel lengths down to 400 \textit{nm}. The devices exhibit a reasonable on-off current ratio of more than 10$^{2}$ and improved on current compared to silicon-on-insulator TFETs. Current-voltage measurements at room and low temperatures will be presented to characterize the behavior of the fabricated transistors. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y28.00010: Full Recovery of PFET NBTI and NFET PBTI of high-k metal gate MOSFETs with high temperature bake Anastasios Katsetos, James Stathis, Fernando Guarin High-k metal gate MOSFET devices exhibit Bias Temperature Instability (BTI) degradation mechanisms. The p-channel Field Effect Transistor (PFET) has NBTI which results in threshold voltage (Vt) decrease and drive current (Ion) decrease when the gate is biased negatively with respect to the channel. The n-channel Field Effect Transistor (NFET) has PBTI which results in threshold voltage (Vt) increase and drive current (Ion) decrease when the gate is biased positively with respect to the channel. The amount of NBTI and PBTI is process dependent and depends on temperature, gate voltage, time and gate oxide thickness. PBTI has stronger dependence on voltage than NBTI and NBTI has stronger temperature dependence than PBTI. However, with a high temperature (370 $^{\circ}$C) bake, full recovery of both NBTI and PBTI is achieved and the devices behave like unstressed devices on repeated BTI stress. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y28.00011: Band Structure Engineering of PtSi Alex Slepko, Alexander A. Demkov PtSi is being considered as a contact material in field effect transistors. It has an additional advantage of having a low Shottky barrier to p-type Si. The relatively low conductivity of PtSi compared for example to pure Pt can be traced to the low density of states at the Fermi level. In this theoretical study we discuss a method to increase the conductivity of PtSi by manipulating the density of states through alloying. The scheme is based on substituting Pt atoms by Ti atoms to shift the Fermi level to a higher density of states region. We find identify a compound with the carrier concentration 2.7 times larger than that of bulk PtSi. We estimate the formation energies of the compounds and determine the solubility limit of Ti in PtSi at room temperature. We analyze the effect of doping with Ti on the work function for the (121) surface (the lowest energy surface orientation of PtSi). Moreover, we study possible schemes to lower the formation energies of the alloys by further doping with boron, carbon, gallium and aluminum. We identify a stable alloy in the case of aluminum doping. All calculations are done within the framework of density functional theory. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y28.00012: Ideal SiC Schottky Barrier Diodes Fabricated Using Refractory Metal Borides Tom Oder, Rani Kummari We present results of n-type 4H-SiC Schottky barrier diodes fabricated using several refractory metal boride Schottky contacts deposited on SiC held at various temperatures. From the electrical properties determined by current-voltage measurements, diodes with contacts deposited on SiC substrates held at 600 $^{o}$C had average ideality factors in the range 1.04 -- 1.09 and Schottky barrier heights of 1.02 eV -- 1.14 eV; and these values remained unchanged after the diodes were annealed at 600 $^{o}$C for 20 min. Diodes with contacts deposited on substrates held at 20 $^{o}$C had much higher ideality factors which decreased only slightly after the annealing. The Rutherford backscattering spectroscopy spectra of these contacts revealed a systematic decrease in oxygen with increase in the deposition temperature. The improved electrical properties and thermal stability are attributed to the removal of oxygen from the boride/SiC interface during high temperature deposition. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y28.00013: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y28.00014: Achieving a low interfacial density of states in atomic layer deposited Al$_{2}$O$_{3 }$on In$_{0.53}$Ga$_{0.47}$As Han-Chin Chiu, L.T. Tung, Y.H. Chang, Y.J. Lee, C.C. Chang, M. Hong, J. Kwo Atomic-layer-deposited (ALD) Al$_{2}$O$_{3}$ dielectrics on In$_{0.53}$Ga$_{0.47}$As with short air exposure between oxide and semiconductor deposition has been demonstrated nearly ideal capacitance-voltage ($C-V)$ characteristics with negligible frequency dispersion at flat-band and accumulation. A relationship of surface potential versus gate voltage derived by the excellent quasi-static $C-V$ curve shows high efficiency of 63{\%} for Fermi-level movement near the mid-gap. A low mean interfacial density of states ($\overline D it) \quad \sim $ 2.5x10$^{11}$ cm$^{-2}$eV$^{-1}$ was determined using the charge pumping method, which was also employed to probe the depth profile of bulk trap density ($N_{bt})$ and the energy dependence of $D_{it}$ measured at 50kHz: a low $N_{bt} \quad \sim $ 7x10$^{19}$ cm$^{-3}$ and $D_{it}$ of 2-4x10$^{11}$ cm$^{-2}$eV$^{-1}$ in the lower half of the band-gap and a higher $D_{it}$ of $\sim $10$^{12}$ cm$^{-2}$eV$^{-1}$ in the upper half of the band-gap. The employment of charge pumping method has given a more accurate determination of $D_{it}$, which is usually overestimated using other commonly methods such as Terman, conductance, and high-low frequencies, due to the influence of weak inversion at room temperature. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y28.00015: Palladium and Palladium-Carbon Nanotube Composite Nanomechanical Resonator SungWan Cho, YoungDuck Kim, JungHoon Bak, JuHyung Lee, SeungRan Lee, KookRin Char, SeungHun Hong, Yun Daniel Park For its bio-compatibility, conductivity and optical reflectivity, metallic thin films are an attractive choice to realize multifunctional micromechanical resonators. However, moderate elastic properties of metallic thin films are ill suited for high frequency applications. Meanwhile, Carbon nanotubes have shown great potential with superior electrical and mechanical properties. Combined Metal-CNT nanolaminates have increased strengths and are less susceptible to onset of mechanical nonlinearity compared to equivalent metal beams without CNT. With palladium's good affinity to CNT to further study the role of the metal-CNT interface, we realized doubly clamped beam and torsional resonators from Palladium and Palladium/CNT composite. Resonance frequencies were detected using optical modulation technique with different wavelength at room temperature under moderate vacuum. Comparing the dynamic flexural response of Pd and Pd/CNT doubly clamped beam and torsional resonators, we will also discuss the difference between Pd-CNT and Al-CNT resonators as well as actuating the resonators electrostatically and optically [Preview Abstract] |
Session Y29: Focus Session: Spin Liquids
Sponsoring Units: GMAGChair: Tsuyoshi Kimura, Osaka University
Room: 333
Friday, March 20, 2009 8:00AM - 8:12AM |
Y29.00001: Topological pairing of skyrmions and symmetry breaking in low-dimensional SU(N) antiferromagnets Alexei Kolezhuk I study what happens to a so-called SU(N) antiferromagnet when the high SU(N) symmetry getss explicitly broken. Physically, such SU(N) antiferromagnets can be realized in cold atom systems in optical lattices (particularly, the N=3 case corresponds to spin-1 bosons), and similar models might be possibly relevant for some magnetic materials like $\rm NiGa_2S_4$. I consider two perturbations breaking the SU(N) symmetry down to O(N) and SU(N-1), respectively, and study the phase diagram of the system. Breaking symmetry has a twofold effect: except favoring a certain type of order (spin-nematic or antiferromagnetic), it also affects the topological (Berry) phases. It is shown that the physically interesting case N=3 is very special: the effect of ``topological pairing'' of skyrmions leads to a change in the degeneracy of the disordered phase in case of SU(N) to O(N) perturbation, and the SU(N) to SU(N-1) perturbation brings the system into a critical phase. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y29.00002: Cascade of quantum phase transitions in the spin-1/2 triangular-lattice antiferromagnet Cs$_{2}$CuBr$_{4}$ Nathanael Fortune, Adrienne Wilson-Muenchow, Scott Hannahs , Yasu Takano, Yasuo Yoshida, Todd Sherline, Toshio Ono, Hidekazu Tanaka In classical magnetic spin systems, geometric frustration leads to a large number of states of identical energy. We report here calorimetric and magnetocaloric evidence that in Cs$_{2}$CuBr$_{4}$ --- a geometrically frustrated Heisenberg S= 1/2 triangular antiferromagnet --- quantum fluctuations stabilize a series of gapped collinear spin states bounded by first-order transitions at simple increasing fractions of the saturation magnetization for fields directed along the c axis. Only the first of these quantum phase transitions has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. The Dzyaloshinskii-Moriya interaction breaks the symmetry when the magnetic field is directed along the triangular layers, providing one possible explanation for the directional dependence and the 1st order nature of the transitions. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y29.00003: Dzyaloshinskii-Moriya interactions and non-magnetic impurities in the s=1/2 kagome antiferromagnet Ioannis Rousochatzakis, Salvatore Manmana, Andreas Laeuchli, Bruce Normand, Frederic Mila Motivated by recent NMR experiments[1] on ZnCu$_3$(OH)$_6$Cl$_2$, we present an exact diagonalization study of the combined effect of non-magnetic impurities and Dzyaloshinskii-Moriya (DM) interactions in the $s=1/2$ Kagom\'e antiferromagnet. The magnetization response and the correlation matrix data reveal that the dimer freezing which occurs around the vacancy for $D=0$ [2,3] ($D$ is the magnitude of the DM vectors) persists up to $D/J \simeq 0.07$, above which a phase transition to the $Q=0$ semiclassical 120$^{\circ}$ state[4] takes place. Surprisingly however, the dimers next to the vacancy remain strong up to $D/J \sim 2-3$, i.e. well above the critical point. Implications for ZnCu$_3$(OH)$_6$Cl$_2$ will be discussed. 1. A. Olariu, {\it et al.}, Phys. Rev. Lett. {\bf 100}, 087202 (2008). 2. S. Dommange, {\it et al.}, Phys. Rev. B {\bf 68}, 224416 (2003). 3. A. L\"auchli, {\it et al.}, Phys. Rev. B {\bf 76}, 144113 (2007). 4. O. C\'epas, {\it et al.}, Phys. Rev. B {\bf 78}, 140405 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y29.00004: Neel ordering and the stability of the spin 1/2 kagome lattice antiferromagnet in Zn-Paratacamite Michael J. Lawler, Erik S. Sorensen, Yong Baek Kim Zn-Paratacamite is a rare spin 1/2 antiferromagnetic insulator with an ideal kagome lattice structure in part of its phase diagram. As a function of Zn doping, this material undergoes a structural distortion which relieves the frustration and introduces magnetic order in the ground state, though the precise nature of the order is not clear at this point. In this talk, I will present strong evidence for Neel ordering in the distorted phase of Zn-Paratacamite through the application of quantum monte-carlo and exact diagonalization methods to the appropriate Heisenberg model. These numerical results strongly support a recent Schwinger-boson mean field theory of Zn-Paratacamite. Furthermore, our results indicate a large basin of stability of the ideal kagome lattice ground state in the presence of this type of distortion. This suggests that the ideal kagome ground state may be stable towards weak doping-induced distortion, though further studies of local effects may be necessary to make a firm conclusion. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y29.00005: Magnetization steps on the spin liquid ground state of the $S $= 1/2 kagome-like antiferromagnet Cu$_{3}$V$_{2}$O$_{7}$(OH)$_{2}$2H$_{2}$O Hiroyuki Yoshida, Yoshihiko Okamoto, Takashi Tayama, Toshiro Sakakibara, Masashi Tokunaga, Akira Matsuo, Yasuo Narumi, Koichi Kindo, Makoto Yoshida, Masashi Takigawa, Zenji Hiroi The ground state of the $S$ = 1/2 kagome antiferromagnet (KAFM) is expected to be a spin liquid with a finite spin gap \textit{$\Delta $ }$\sim \quad J$ / 20. Here, we report the magnetic properties of $S$ = 1/2 KAFM Cu$_{3}$V$_{2}$O$_{7}$(OH)$_{2}$$\cdot$2H$_{2}$O studied by magnetization, specific heat and V NMR measurements. Neither magnetic long- range order nor a spin gap has been detected down to 60 mK, in spite of a large antiferromagnetic interaction $J$ = 86 K, suggesting a gapless spin liquid. Surprisingly, we observed three step-like increases in magnetization at $H_{S1}$ = 4.3, $H_{S2}$ = 25.5, and $H_{S3}$ = 46 T, which implies that there exist at least four kinds of spin liquid or other quantum state under magnetic fields. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y29.00006: $^{69,71}$Ga NMR Probe of the Spin Dynamics in the Rare-Earth Kagom\'{e} Pr$_3$Ga$_5$SiO$_{14}$ Lloyd Lumata, K.-Y. Choi, T. Besara, M. J. R. Hoch, H. D. Zhou, J. S. Brooks, P. L. Kuhns, A. P. Reyes, N. S. Dalal, C. R. Wiebe We report $^{69,71}$Ga nuclear magnetic resonance investigation of the spin dynamics in the rare-earth kagom\'{e} system Pr$_3$Ga$_5$SiO$_{14}$. We find that the spin-lattice relaxation rate $^{69}1/T_1$ exhibits a peak around 30 K, below which the Pr$^{3+}$ spin correlation time $\tau$ shows novel field-dependent behavior consistent with a field-dependent gap in the excitation spectrum. The spin-spin relaxation rate $^{69}1/T_{2}$ exhibits a maximum at a lower temperature (10 K) below which field-dependent power-law behavior close to $T^{2}$ is observed. These results point to the interplay of single-ion anisotropy and field-induced formation of nanoscale magnetic clusters consistent with recent neutron scattering measurements. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y29.00007: Ba$_3$Cr$_2$O$_8$, a new non-Cu based quantum s=1/2 spin singlet system Invited Speaker: Field-induced condensation of magnons has been experimentally observed in several weakly coupled quantum ($s = 1/2$) dimer systems that are based on Cu$^{2+}$ ions, such as TlCuCl$_3$ and BaCuSi$_2$O$_6$, and it has been adequately described by the Bose-Einstein condensation (BEC) theory. However, the robustness of such descriptions can only be truly evaluated with investigation into complementary materials, in particular materials that are based on non-Cu$^{2+}$ ions. Recently, a new spin dimer system, Ba3Cr2O8 has been found, where Cr$^{5+}$ ($s=1/2$) ion with the unusual 5+ electronic valence forms quantum dimers along the $c$-axis and a frustrating triangular lattice in the $ab$-plane. Using elastic and inelastic neutron scattering measurements on single crystals and a powder sample, we have characterized the magnetic interactions to show that Ba$_3$Cr$_2$O$_8$ is indeed an excellent model system of weakly coupled quantum dimers [1]. We have also investigated the field-induced condensation of magnons in this compound, using specific heat, bulk magnetization, and elastic neutron scattering measurements under an external magnetic field. The experimental results and comparison to the theories will be discussed. *This work is in collaboration with J.-H. Kim, S. Ji, S.-H. Lee (University of Virginia), H. Ueda, Y. Ueda (ISSP, University of Tokyo), H. Nojiri (IMR, Tohoku University), B. Lake, K. Rule (Helmholtz Centre Berlin). \\[4pt] [1] M. Kofu {\it et al}., cond-mat/0809.5069 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y29.00008: BEC of triplon in the complex quantum spin liquid BaCuSi$_2$O$_6$ Raivo Stern, Steffen Kraemer, Mladen Horvatic, Ivo Heinmaa, Enno Joon, Claude Berthier, Tsuyoshi Kimura, Joel Mesot We present a $^{63,65}$Cu and $^{29}$Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi$_2$O$_6$ in the magnetic field range 13-26 T and at temperatures as low as 50~mK. NMR data and neutron scattering experiments in the gapped phase reveal that below 90~K different intra-dimer exchange couplings and different gaps ($\Delta_{\rm{B}}/\Delta_{\rm{A}}$ = 1.16) exist in every second plane along the $c$-axis, in addition to a planar incommensurate (IC) modulation. $^{29}$Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at $H_{\rm{c1}}$ = 23.35 T the average boson density $\overline{n}$ of the Bose-Einstein condensate (BEC) is strongly modulated along the $c$-axis with a density ratio for every second plane $\overline{n}_{\rm{A}}/\overline{n}_{\rm{B}} \simeq 5$. An IC modulation of the local density is also present in each plane. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y29.00009: High-Frequency EPR Studies of the Antiferromagnet Spin Dimer Compound Ba$_{3}$Mn$_{2}$O$_{8}$ Changhyun Koo, Stephen Hill, Eric C. Samulon, Ian R. Fisher Ba$_{3}$Mn$_{2}$O$_{8}$ is a triangular lattice antiferromagnetic spin-dimer system based on pairs of spin $S$ = 1 Mn$^{5+}$ ions. In zero-field, Ba$_{3}$Mn$_{2}$O$_{8}$ possesses a singlet ground state. Meanwhile, application of a large magnetic field induces several ordered phases associated with the closing of gaps to the excited triplet and quintet states. Field orientation dependent high frequency electron paramagnetic resonance (HFEPR) measurements for a single crystal of Ba$_{3}$Mn$_{2}$O$_{8}$ have been carried out in fields up to 45~T using the hybrid magnet at the National High Magnetic Field Laboratory. Broad low-frequency transitions are observed in the field ranges close to $\sim $9~T and $\sim $32~T, suggesting inter-spin multiplet excitations. Sharper resonances are observed at higher frequencies, which persist to 42~T. Meanwhile, low-field studies provide information on the magnetic anisotropy of the system, which is of the easy-plane type with $D$ = +0.375 K. We attempt to account for the observed HFEPR spectra in the context of a model recently developed to explain the magnetic properties of this compound [E. C. Samulon et al., Phys. Rev. B \textbf{77}, 214441 (2008)]. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y29.00010: $^{135,137}$Ba NMR study of condensed phase and critical behavior in \textit{S}=1 spin dimer system Ba$_{3}$Mn$_{2}$O$_{8}$ Steve Suh, W.G. Clark, S.E. Brown, E.C. Samulon, I.R. Fisher, C.D. Batista, A.P. Reyes, P. Kuhns, L.L. Lumata, J.S. Brooks Ba$_{3}$Mn$_{2}$O$_{8}$ is a trigonal $S=$1 spin dimer system, in which we have used $^{135,137}$Ba NMR spectroscopy and relaxation to study the static and dynamic properties of the field-induced phases and phase transitions to temperatures as low as 20mK, and with emphasis on magnetic fields up to 12T. Specifically, we report on the nature and variation of the order parameter in the condensed phases as well as the form of the spin lattice relaxation in the condensed phases, and compare to behavior anticipated for BEC and 3DXY universality classes. The temperature dependence of magnetization and the relaxation rate at the quantum critical point at H$_{c}\sim $9T is also reported. For the case of the magnetization, a 3D to 2D crossover is evident, and shown consistent with calculations based on the magnon dispersion relations (M.B. Stone et. al, PRL 100, 237201 (2008)). [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y29.00011: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y29.00012: The symmetry of the spin Hamiltonian in herbertsmithite, a spin-1/2 kagom\'e lattice Oren Ofer, Amit Keren We present magnetization measurements on oriented powder of ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ along and perpendicular to the orienting field. We find a dramatic difference in the magnetization between the two directions. It is biggest at low measurement fields $H$ or high temperatures. We show that the difference at high temperatures must emerge from Ising-like exchange anisotropy. This allows us to explain muon spin rotation data at $T\rightarrow 0$ in terms of an exotic ferromagnetic ground state. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y29.00013: Magnetic Excitations in the Stacked Quantum Magnets NaNiO$_2$ and LiNiO$_2$ J.P. Clancy, B.D. Gaulin, J.P.C. Ruff, K.A. Ross, G.J. Van Gastel, D.L. Abernathy, M.B. Stone NaNiO$_2$ and LiNiO$_2$ are isostructural stacked triangular lattice quantum magnets, in which magnetism is conventionally thought to arise due to spin 1/2 moments carried by Ni$^{3+}$ ions. Surprisingly, while NaNiO$_2$ undergoes a cooperative Jahn-Teller transition at 480K and magnetically orders below T$_N$ $\sim$ 23K, LiNiO$_2$ undergoes a glass transition at T$_g$ $\sim$ 9K and remains disordered down to the lowest measured temperatures. The absence of long-range magnetic order in LiNiO$_2$ has been attributed to either geometric frustration caused by mixing of the Li and Ni sublattices, or orbital degeneracy due to the absence of a coherent Jahn-Teller distortion. We have performed time of flight neutron scattering measurements on polycrystalline samples of NaNiO$_2$ and LiNiO$_2$ using the wide Angular-Range Chopper Spectrometer (ARCS) at the SNS. Our measurements reveal previously unobserved magnetic excitations at relatively high energy transfers, which we associate with ferromagnetic spin waves mediated by in-plane interactions. We also find evidence of critical scattering in NaNiO$_2$ near the magnetic phase transition at T$_N$. These results will be compared with previous measurements collected using the DCS at NIST. [Preview Abstract] |
Session Y30: Focus Session: LAO/STO Interfaces
Sponsoring Units: GMAG DMPChair: Anand Bhattacharya, Argonne National Laboratory
Room: 334
Friday, March 20, 2009 8:00AM - 8:12AM |
Y30.00001: Coulomb Catastrophe and the Origin of the Sheet Carrier Density at the n-type LaAlO$_3$/SrTiO$_3$ interface: What do band calculations tell us? Zoran Popovic, Sashi Satpathy, Richard Martin Transport measurements of the two-dimensional electron gas (2DEG) at the intrinsic n-type LaAlO$_3$/SrTiO$_3$ interface have found a density of carriers much lower than expected from the ``Coulomb catastrophe" arguments. From a detail density-functional study, we suggest how this discrepancy may be reconciled. We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties. Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y30.00002: Quantum nature of two-dimensional electron gas confinement at LaAlO$_{3}$/SrTiO$_{3}$ interfaces Karolina Janicka, Julian Velev, Evgeny Tsymbal Replace this text with your abstract body. The discovery of highly conducting interface between two insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$ has attracted significant interest due to possible applications in all-oxide electronic devices. The two-dimensional electron gas (2DEG) formed at LaAlO$_{3}$/SrTiO$_{3}$ interfaces exhibits extremely high mobility and high density of carriers. Stimulated by this discovery we perform density functional calculations to understand the mechanism controlling the confinement width of the two-dimensional electron gas (2DEG) at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. We find that the 2DEG confinement can be explained by the formation of metal induced gap states (MIGS) in the band gap of SrTiO$_{3}$. These states are formed as the result of quantum-mechanical tunneling of the charge created at the interface due to electronic reconstruction. The penetration depth of the MIGS into the insulator is controlled by the lowest-decay-rate evanescent states of SrTiO$_{3}$, as determined by its complex band structure. Our calculations predict that the 2DEG is confined in SrTiO$_{3}$ within about 1 nm at the interface. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y30.00003: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y30.00004: Effect of Stoichiometry on the Two-Dimensional Electron Gas at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Grown by MBE. Maitri Warusawithana, Charles Brooks, Darrell Schlom, Stefan Thiel, Jochen Mannhart, Nicolas Reyren, Andrea Caviglia, Stefano Gariglio, Jean-Marc Triscone The discovery of a quasi 2-dimensional electron gas (q2-DEG) at the interface between SrTiO$_{3}$ and LaAlO$_{3}$ has enabled a number of exciting developments. So far this q2-DEG has been observed only in films grown by pulsed-laser deposition, which raised a question as to whether this manifestation has a connection with defects that result from the dynamics of the growth scheme employed. We find that a q2-DEG can also be obtained using the more gentle growth technique, molecular-beam epitaxy, and that control of the stoichiometry of the LaAlO$_{3}$ layer is key to its existence. Small changes in the composition of the LaAlO$_{3}$ layer affect the conductivity at the heterointerface. With appropriate stoichiometry the electron gas transitions into a superconducting state below $\sim $200 mK. Interesting possibilities that stem from these findings of composition control on conductivity and the ability to obtain this q2-DEG under the framework of molecular-beam epitaxy will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y30.00005: Two Dimensional Electron Gas Formed at Inverted SrTiO3-LaAlO3 Interface Mao Zheng, Brian Mulcahy, Xiaofang Zhai, James Eckstein We have grown and tested inverted 2DEG structures consisting of SrTiO3 layers grown on top of thick LaAlO3 films. By engineering the layering, the interface can be made n-type. Contact to the 2DEG is relatively easy in this geometry since the current does not have to pass through the high band gap LAO layer. We have obtained a 2-D carrier density of 2x10\^{}14 electrons/cm\^{}2 at room temperature along with a mobility of 10 cm\^{}2/Vs. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y30.00006: Electrostatic doping in oxide heterostructures. Alexander A. Demkov, Jaekwang Lee, Na Sai Recent experiments on perovskite heterostructures grown by methods ranging from molecular beam epitaxy to pulsed laser deposition suggest the existence of two dimensional electron gas of high mobility at the oxide/oxide interface, and even a possibility of a superconducting state. Both p-type and n-type interfaces have been reported. However, the origin of charge in these insulating materials is still under debate. We report a first-principles study of several heterostructures where we employ the internal filed in a polar oxide LaAlO$_{3}$ to demonstrate the possibility of the electrostatic doping, an effect similar to a well known polar catastrophe in e.g., III-V semiconductors. $_{ }$We use density functional theory at the LDA+U level. We mainly focus on the electronic structure of the oxide/oxide junctions. The results of our calculations suggest that once the critical thickness of the aluminate layer is reached the internal electric field is sufficient to produce the electrostatic doping. We will discuss simple estimates for the temperature of the superconducting transition and the role of oxygen-related defects such as vacancies in the electronic structure and thermodynamic stability of these fascinating oxide structures. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:48AM |
Y30.00007: Nanoscale analysis of high-mobility electron gases at SrTiO$_{3}$ interfaces and surfaces Invited Speaker: Electronic reconstructions or defects localized next to an interface between two oxides may lead to dramatic modifications of their physical properties. One intriguing example of such phenomena is the formation of high-mobility two-dimensional electron gases (2DEG) at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ (STO), two insulating dielectric perovskite oxides. To characterize crucial aspects of these 2DEG such as the spatial extension as well as carrier density profiles, sophisticated techniques are required. Here I explain how we used the synergetic combination of different advanced characterization tools including depth-resolved positron annihilation spectroscopy, conductive-tip atomic force microscopy [1], electron energy loss spectroscopy [2] or low-temperature high-magnetic field measurements [3, 4], to characterize with nanometric space resolution high-mobility electron gases at STO interfaces and surfaces. Our results emphasize the relevance of using interface/surface characterization tools that resolve spatially the physical properties and also detect spatial tiny changes of stoichiometry at the interface/surface of complex oxide structures. \\[4pt] [1] M. Basletic et al., Nature Materials, 7, 621 - 625 (2008) \\[0pt] [2] J.-L. Maurice et al., Europhys. Lett. 82, 17003 (2008) \\[0pt] [3] G. Herranz et al., Phys. Rev. Lett. 98, 216803 (2007) \\[0pt] [4] G. Herranz et al., Phys. Rev. B 73, 064403 (2006) [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y30.00008: Band offsets between SrTiO$_3$ and LaAlO$_3$ Kristopher E. Andersen, C. Stephen Hellberg Although separately SrTiO$_3$ and LaAlO$_3$ are both band insulators, together a highly mobile, quasi-2D electron gas can form at their interface. Several mechanisms have been proposed to produce this electron gas, including the electrostatic divergence within LaAlO$_3$. A critical property in understanding this divergence is the valence band offset between SrTiO$_3$ and LaAlO$_3$. However, because the electrostatic potential can diverge, it is not clear \emph{where} the valence band offset should be defined; an issue that may affect experimental band offset measurements. In this talk, the band offsets between SrTiO$_3$ and LaAlO$_3$ are presented within the framework of density functional theory. Both the layer projected density of states and macroscopically averaged potential are used to find the valence band offset for thin films and multilayers. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y30.00009: Transport and band profiles of MBE grown LaAlO$_{3}$/ SrTiO$_{3}$ Yaron Segal, J.H. Ngai, J.W. Reiner, F.J. Walker, C.H. Ahn Previously reported properties of the LaAlO$_{3}$/SrTiO$_{3}$ system showed strong dependence on growth parameters. This makes it difficult to identify the key physical quantities. To isolate the role of oxygen content, we grew LaAlO$_{3}$ films on SrTiO$_ {3}$ using MBE. The thermal evaporation of MBE minimizes potential damage to the substrate and interface. The samples were then annealed at low temperature for a prolonged time, thus raising the oxygen content without damaging the structural integrity of the film. Transport and X-ray photoemission measurements were performed before and after annealing on films grown on both terminations of the substrate. Transport measurements show the in-plane conductivity decreases by several orders of magnitude upon annealing. A dependence on film thickness appears in certain oxygen content regimes, where it can be interpreted as being controlled by oxygen diffusion. Photoemission measurements reveal an intriguing band structure in the LaAlO$_{3}$ film. The termination of the SrTiO$_ {3}$ determines the direction of apparent band bending, for which we discuss possible models. Our results imply that while transport behavior of this system is dominated by oxygen diffusion, the atomic details of the interface have a substantial impact on band structure. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y30.00010: Resonant Anomalous Synchrotron X-Ray Studies of LaAlO$_{3}$ Films on SrTiO$_{3}$(001) Dillon Fong, Tim Fister, Marie-Ingrid Richard, Stephan Hruszkewycz, Jeffrey Eastman, Paul Fuoss, Sung Seok Seo, Ho Nyung Lee The high conductivity present at the interface between LaAlO$_{3}$ and TiO$_{2}$-terminated SrTiO$_{3}$(001) has been attributed to an electronic reconstruction [1] or atomic intermixing [2], both induced by the polar discontinuity. LaAlO$_{3}$ films with thicknesses equal to or thinner than a critical thickness (three unit cells [3]), however, can maintain the interface dipole, and no reconstruction (electronic or atomic) is expected. In this study, we employ resonant anomalous x-ray scattering at the Ti K-edge to investigate the structure and properties of the LaAlO$_{3}$ films both above and below the critical thickness. For films thicker than the critical thickness, an interfacial layer is observed to form. The structure and chemical properties of the interfacial layer as determined by both resonant scattering and x-ray spectroscopy will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y30.00011: Oxygen vacancy, charge doping, and polarization screening in LaAlO$_3$/SrTiO$_3$ interface Yun Li, Jaejun Yu A high mobility electron gas has been observed in the $n$-type (TiO$_2$)/(LaO) interface between two insulators: non-polar SrTiO$_{3}$ (STO) and polar LaAlO$_{3}$ (LAO) and the mechanism of conductivity and dimensionality of electron gas at the interface have been intensively investigated in various experiments. There are two mechanisms suggested for the observed conductivity at the interface: electronic reconstruction and oxygen vacancy. We carried out density-functional-theory calculations to investigate the distribution of electron carriers for the $n$-type LAO/STO interfaces with and without oxygen vacancy. When no oxygen vacancy is present, the critical thickness of LAO film for conducting interface was found to be consistent with experiments. The induced carrier density at the interface without oxygen vacancy turns out to be an order of magnitude smaller than the one expected from the electronic reconstruction. This implies that the lattice polarization takes a significant role in charge screening. On the other hand, when oxygen vacancies are present, the vacancy-induced states are found to affect the carrier doping as well as the screening of polar electric field of LAO film. From the results, we propose that the upper limit of carrier doping should be 0.375 electrons per unit cell. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y30.00012: The effect of strain on the SrTiO3/LaAlO3 heterointerface C. Tyler Diggans, Kristopher E. Andersen, C. Stephen Hellberg Recent experiments have shown that it is possible to form a highly mobile, quasi-2D electron gas at the interface between SrTiO$_{3}$ and LaAlO$_{3}$. Although the origin of this effect is still debated, there is growing consensus that under certain growth conditions (e.g. high oxygen pressures) it is caused by the diverging electric potential within LaAlO$_{3}$ -the so-called polar catastrophe. One aspect of this system that has not been previously considered is the electric polarization of SrTiO$_{3}$, which can be effectively tuned by strain. This polarization can partially or fully compensate the diverging LaAlO$_{3}$ potential, and must be considered if SrTiO$_{3}$/LaAlO$_{3}$ is to be grown on a substrate such as Si. In this talk, an electrostatic model is presented to show how the polar catastrophe is modified by polarization within strained SrTiO$_{3}$. This model is supported by first-principles calculations on strained SrTiO$_{3}$/LaAlO$_{3}$ multilayers of varying thickness. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y30.00013: Polarization effects and the source of electrons in two-dimensional electron gas at insulating oxide heterointerfaces H.W. Jang, D.A. Felker, C.M. Folkman, D.L. Profitt, S.H. Baek, M.S. Rzchowski, C.B. Eom, K. Janicka, Y. Wang, M.K. Niranjan, E.Y. Tsymbal The discovery of a two-dimensional electron gas (2DEG) at the heterointerface between insulating perovskite oxides LaAlO$_{3}$ and SrTiO$_{3}$ has stimulated intensive theoretical and experimental studies on the origin of the 2DEG. Empirically, the electron density has been found to be strongly dependent on the oxygen partial pressure during growth and the thickness of the polar LaAlO$_{3}$ layer. Understanding and controlling the source of the electrons in 2DEGs at oxide heterointerfaces is important for the optimization of their performance. In this talk, we will discuss electrical properties of heterointerfaces between various polar perovskites and SrTiO$_{3}$ fabricated by pulsed laser ablation. The importance of built-in electric fields due to polarization-induced charges, bandgap energy, and oxygen vacancies in the polar layers is highlighted using a simple analysis based on electrostatics. The experimental Hall data are then compared with the theoretical result from first-principles calculations. [Preview Abstract] |
Session Y31: Focus Session: Neutron Scattering
Sponsoring Units: GMAGChair: Jason Lashley, Los Alamos National Laboratory
Room: 335
Friday, March 20, 2009 8:00AM - 8:12AM |
Y31.00001: Anomalous spin-waves in triangular lattice antiferromagnets Mike Zhitomirsky, Sasha Chernyshev The distinct features of spin-wave excitations in the triangular-lattice antiferromagnet are (i) finite lifetime at zero temperature due to spontaneous two-magnon decays, (ii) strong renormalization of magnon energies $\varepsilon_{\bf k}$ with respect to the harmonic result, and (iii) logarithmic singularities in the decay rate $\Gamma_{\bf k}$. Detailed quantitative results are obtained for the magnon spectrum of the spin-1/2 model using both the on-shell and off-shell solutions of the Dyson's equation. In the low-energy limit magnons remain well-defined excitations but with anomalous decay rates. At high energies, magnons are heavily damped with decay rates reaching $(2\Gamma_{\bf k}/\varepsilon_{\bf k})\sim 0.3$. The on-shell solution shows logarithmic singularities in $\Gamma_{\bf k}$ with the concomitant jump-like discontinuities in Re[$\varepsilon_{\bf k}$] along certain contours in the momentum space. Such singularities are even more prominent in the magnon spectral function $A({\bf k},\omega)$. Although the off-shell solution removes such log-singularities, the decay rates remain strongly enhanced. We also discuss the role of higher-order corrections and show that such singularities may lead to complete disappearance of the spectrum in the vicinity of certain ${\bf k}$-points. We conclude that magnon decays and singularities must be prominent in a wide class of noncollinear antiferromagnets. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y31.00002: Two Dimensional Correlations and Field Induced Order in the Pyrochlore Ferromagnet Yb$_{2}$Ti$_{2}$O$_{7}$ Kate Ross, J.P.C. Ruff, B.D. Gaulin, C.P. Adams, J.S. Gardner, H.A. Dabkowska, Y. Qiu, J.R.D. Copley The rare earth pyrochlore magnets are of intense current interest due to their often unconventional magnetic behaviour. The pyrochlore lattice, which is occupied by the magnetic ions in such materials, is the 3D archetype for geometric frustration. We have studied the frustrated pyrochlore magnet, Yb$_{2}$Ti$_{2} $O$_{7}$, in single crystal form using time-of-flight neutron scattering. Our study confirms the presence of diffuse rods of scattering, which indicate unexpected 2D magnetic correlations in this cubic system. The diffuse scattering is measured both above and below a previously reported first order transition at T$_c$ = 240mK. Although rod- like scattering persists below T$_c$, three dimensional correlations develop, indicating a build-up of interplane correlations. Yet Yb$_{2}$Ti$_{2}$O$_{7}$ continues to resist long range order down to $\sim$50mK. We have discovered, however, that the application of a small magnetic field along the [110] direction readily induces a long range ordered magnetic state, as evidenced by well defined spin wave excitations. We will present a magnetic phase diagram for Yb$_ {2}$Ti$_{2}$O$_{7}$ based on the results of this study. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y31.00003: Beyond Simple Bilayers in the Triangular Lattice Dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ Matthew Stone, Mark Lumsden, Sung Chang, Eric Samulon, Cristian Batista, Kirrily Rule, Eric Ressouche, Beatrice Grenier, Ian Fisher We present single crystal inelastic neutron scattering measurements of the $S=1$ dimerized quasi-twodimensional antiferromagnet Ba$_{3}$Mn$_{2}$O$_{8}$. The singlet-triplet dispersion reveals nearest-neighbor and nextnearest- neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the $c$ axis and leads to a quasi-two-dimensional behavior. We also present magnetic field-dependent neutron scattering measurements examining long range order and excitations within the field-induced ordered phases in Ba$_{3}$Mn$_{2}$O$_{8}$. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 9:12AM |
Y31.00004: Spin waves and quantum criticality in the frustrated XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$ Invited Speaker: Geometrically frustrated magnetism in the three dimensional pyrochlore lattice often gives rise to unconventional magnetic states at low temperatures. The effective elimination of leading energetic terms via geometric cancellation allows weak perturbations to ultimately determine the behaviour of a magnetic material. In this talk, I will describe the manifold of low energy states in the antiferromagnetic XY pyrochlore Er$_2$Ti$_2$O$_7$, as measured by the time-of-flight neutron scattering technique. In zero applied magnetic field, the ground state is purported to be selected either by unusual anisotropic interactions, or via thermal and/or quantum fluctuations in an example of order-by-disorder [1,2]. Recently, we have shown that this state consists of coexisting short and long range orders[3], and that both orders can be destroyed by the application of modest magnetic fields [3]. This disordering of the magnetic system involves a seemingly continuous quantum critical point at $\mu_{0}H_c \sim$ 1.6 Tesla [3]. The properties of the induced quantum paramagnetic state, and the dependence of these properties on the direction of applied field will be discussed. I will also make comparisons with a recent theoretical treatment of Er$_2$Ti$_2$O$_7$ [2]. \\[4pt] [1] Champion et al. Phys. Rev. B. 68, 020401 (2003)\\[0pt] [2] McClarty et al. arXiv:0810.2483v2\\[0pt] [3] Ruff et al. Phys. Rev. Lett. 101, 147205 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y31.00005: Recent neutron scattering results from Gd-based pyrochlore oxides Jason Gardner In my presentation I will present recent results that have determined the spin-spin correlations in the geometrically frustrated magnets Gd$_{2}$Sn$_{2}$O$_{7}$ and Gd$_{2}$Ti$_{2}$O$_{7}$. This will include polarised neutron diffraction, inelastic neutron scattering and neutron spin echo data. One sample of particular interest is Gd$_{2}$Sn$_{2}$O$_{7}$ which is believed to be a good approximation to a Heisenberg antiferromagnet on a pyrochlore lattice with exchange and dipole-dipole interactions. Theoretically such a system is expected to enter long range ordered ground state known as the ``Palmer Chalker'' state [1]. We show conclusively, through neutron scattering data, that the system indeed enters an ordered state with the Palmer-Chalker spin configuration below T$_{c}$ = 1 K [2-3]. Within this state we have also observed long range collective spin dynamics, spin waves. This work has been performed in collaboration with many research groups including G. Ehlers (SNS), R. Stewart (ISIS). \\[0pt] [1] S. E. Palmer and J. T. Chalker, Phys. Rev. B 62, 488 (2000). \\[0pt] [2] J. R. Stewart, G. Ehlers, A. S. Wills, S. T. Bramwell, and J. S. Gardner, J. Phys.: Condens. Matter 16, L321 (2004). \\[0pt] [3] J R Stewart, J S Gardner, Y. Qiu and G Ehlers, Phys. Rev. B. 78, 132410 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y31.00006: Electric Control of Spin Chirality in Multiferroic Ni3V2O8 Ivelisse Cabrera, Michel Kenzelmann, Gavin Lawes, Oksana Zaharko, Collin Broholm We discuss electric control of spin chirality in the magnetically frustrated multiferroic Ni$_3$V$_2$O$_8$ through polarized magnetic neutron diffraction. Cooling to the cycloidal magneto-electric phase in an electric field $\mathbf{E}$ causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with $\mathbf{E}$. Comprehensive polarized neutron diffraction measurements establish the chiral nature of the long range ordered spin configuration and its response to $\mathbf{E}$. Concomitant evolution of chiral and ferroelectric domains is observed by comparing polarized neutron diffraction data to pyroelectric current measurements and hysteresis under different poling conditions [1]. \newline \newline [1] I. Cabrera, M. Kenzelmann, G. Lawes, Y. Chen, W. C. Chen, R. Erwin, T. R. Gentile, J. Leao, J. W. Lynn, N. Rogado, R. J. Cava, and C. Broholm, (unpublished), 2008. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y31.00007: CEF groundstate of the frustrated quantum magnets SrRE$_2$O$_4$ (RE= Dy, Ho) A. Desilets-Benoit, A. D. Bianchi, V. Pomjakushin, B. R. Hansen, M. Kenzelmann, R. J. Cava We have measured the crystalline electric field (CEF) niveaus of the magnetic ions in the frustrated quantum magnets SrRE$_2$O$_4$ with RE = Dy and Ho by inelastic neutron scattering. SrRE$_2$O$_4$ crystallizes in a $pnam$ structure, which as four in-equivalent rare earth sites, leading to a large degree of geometrical frustration. Fitting a CEF level scheme to the experimental data has allowed us to determine the CEF ground state of this system. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:24AM |
Y31.00008: Dynamics of excitations in a one-dimensional Bose liquid Invited Speaker: The dynamical structure factor $S(q,\omega)$ of interacting 1D liquid is studied. This quantity has become experimentally accessible in the recent experiments on cold atoms and neutron scattering off spin chains. We find power law non-analyticities $S(q,\omega) \propto \left(\omega - \epsilon_{1,2}(q) \right)^{-\mu(q)}$ at the kinematical thresholds characterized by the momentum dependent exponents evauated in a broad range of parameters. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y31.00009: Magnetic properties of LuFe$_{2}$O$_{4+\delta }$ Fan Wang, Jungho Kim, G. Xu, S.M. Shapiro, G.D. Gu, Y. Lee, Y.-J. Kim LuFe$_{2}$O$_{4+\delta }$ (LFO) has been drawing much attention as a potential multiferroic compound. We show that the oxygen stoichiometry plays an important role in determining the magnetic properties of LFO. The sample with excess oxygen shows two magnetic transitions at 236K and 228K. The transition at 236K is a paramagnetic to ferrimagnetic transition, and below 228 K the system enters a spin glass phase. The DC magnetization shows strong time and history dependence, while the AC susceptibility exhibits dynamic scaling behaviour similar to that of canonical spin glass systems. This spin glass behaviour seems to disappear in the more stoichiometric sample. In this second sample, long range ferrimagnetic ordering has been observed with neutron scattering experiments. Our x-ray scattering on single crystal and x-ray powder diffraction experiments both show there is a structural change around 170K, which seems to be responsible for an observed low field anomaly. When a small magnetic field is applied, magnetization is only turned on during heating the sample. With large applied field, a metamagnetic transition was observed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y31.00010: Phonon renormalization of the N\`{e}el transition in KCuF$_{3}$ J.C.T. Lee, S. Yuan, A. Rusydi, S. Smadici, S.L. Cooper, E. Fradkin, P. Abbamonte Critical magnetic fluctuations in the one-dimensional antiferromagnet KCuF$_3$, in the form of diffuse scattering around the magnetic (001) Bragg peak, have been studied with resonant soft x-ray scattering. Using x-rays near the Cu L$_{3}$ edge to exploit the 2\textit{p} $\to$ 3\textit{d} dipole transition, the (001) was directly observed at temperatures ranging from 23K to above the transition temperature (T$_{N}$ $\approx$ 43K). Notably, the phase transition exhibits hysteresis, with T$_{N}$ sensitive to whether the sample was cooled or heated prior to measurement. This suggests that the phase transition is weakly first order, as might be expected by a transition renormalized by phonons. The temperature dependence of the coherence length and the diffuse scattering, as well as the role played by Jahn-Teller phonons in the transition are discussed. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y31.00011: The influence of magnetic field on Cu(tn)Cl$_{2}$ - two-dimensional quantum magnet with N\'{e}el ground state. A. Orendacova, M. Orendac, L. Sedlakova, A. Feher, K. Siemensmeyer, A. Buchsteiner, J. S. Xia, L. Yin, M.W. Meisel AC susceptibility and neutron elastic scattering experiments were performed on Cu(tn)Cl$_{2}$ [1] to explore the origin of a field induced anomaly previously observed in specific heat below 1 K and in finite fields up to 7 T [2]. Isothermal AC susceptibility scans, made down to 40 mK and in fields up to 10 T, confirmed and extended the B vs. T phase diagram obtained from the specific heat data. Neutron spectra of a polycrystalline sample studied down to 0.5 K in B = 0 and 4 T did not reveal any magnetic peaks expected below 1 K and indicated a slight change of (200) nuclear peak intensity induced by the magnetic field. The possible reasons of the absence of a phase transition to ordered state will be discussed. [1] V. Zelenak \textit{et al.}, Inorg. Chem. \underline {45} (2006) 1774. [2] A. Orendacova \textit{et al.}, in preparation. [Preview Abstract] |
Session Y32: Focus Session: Current-Induced Magnetic Switching
Sponsoring Units: GMAG DMP FIAPChair: Yi Ji, University of Delaware
Room: 336
Friday, March 20, 2009 8:00AM - 8:12AM |
Y32.00001: Effect of spin diffusion in the polarizer on current-induced magnetic switching Scott Button, Sergei Urazhdin Current-induced magnetic switching of a nanomagnet occurs due to the spin transfer torque exerted by current spin-polarized by another ferromagnet. Efficient switching is generally achieved by enhancing the polarizing properties of the latter. However, calculations show that switching is affected not only by the polarizing properties of the polarizer, but also by the electron diffusion in this layer [1,2]. To test the effects of spin diffusion in the polarizer on current-induced switching, we performed measurements of magnetic multilayer nanopillars with three different structures of the polarizing magnetic layer: a thick Co layer, a thin Co layer, and a bilayer consisting of a thin Co layer and a strongly spin-flipping FeMn alloy. In the pillars with a thick Co polarizer, the switching currents dramatically increase below 130 K, while the magnetoresistance exhibits a nonmonotonic dependence on temperature with a peak at 130 K. In contrast, the samples with a thin Co polarizer exhibit weak monotonic dependencies of switching and magnetoresistance on temperature. We discuss the implications of our results for our understanding of spin-dependent diffusion in magnetic multilayers. [1] A.A. Kovalev, A. Brataas, and G.E.W. Bauer, Phys. Rev. B 66, 224424 (2002). [2] Zhang, P.M. Levy, and A. Fert, Phys. Rev. Lett. 88, 236601 (2002). [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y32.00002: Spin transfer torque switching of Co nanoparticles Han Zou, Xiaojun Wang, Yi Ji Spin transfer torque effect has potential application in Magnetic Random Access Memory (MRAM) devices as a way to address the memory elements. Most spin transfer studies are based on patterned multilayer thin films with 100 nm lateral dimension. In this work, we demonstrate the feasibility of the spin transfer switching of a few cobalt nanoparticles with a diameter of $<$ 5 nm at 4.2 K. The motivation arises from the prospect of device miniaturization and the capability to manipulate an individual magnetic nanoparticle. We use a multilayer thin film Cu(100nm)/Co(10nm)/Cu(3nm)/Co(0.5nm)/Au(2nm). The 0.5 nm Co layer is not continuous, and it consists of isolated Co particles formed due to surface tension. A mechanical point contact is formed on the multilayer film at 4.2K. By varying the size of the contact, the number of nanoparticles underneath a point contact can be controlled between $\sim $5 and $\sim $50. Hysteretic loops in \textit{dV/dI }-- $I$ measurements clearly indicates spin-transfer switching. The \textit{dV/dI-I} curves are qualitatively different between point contacts involving only few particles (5-10) and those involving many particles (40-50). [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y32.00003: Stochastic Resonance Driven by Spin Torque Xiao Cheng, Carl Boone, Jian Zhu, Ilya Krivorotov Application of a microwave ac current to a spin valve gives rise to a rectified voltage due to magnetization dynamics driven by ac spin torque. We study the effect of dc current bias on these dynamics in spin valves with superparamagnetic free layers. We observe large enhancement of the rectified voltage (up to two orders of magnitude) along a line in the dc current - magnetic field phase diagram of the system. This enhancement arises from large-amplitude nonlinear dynamics of magnetization of the free layer induced by the combined action of ac and dc spin torques. For small out-of-plane external magnetic field, the enhanced rectified signal is observed at low frequencies ($<$1GHz) of the ac drive. This signal enhancement arises from adiabatic stochastic resonance of magnetization of the free layer driven by ac spin torque. For large out-of-plane magnetic field, the rectified signal enhancement is found at the ac drive frequencies of several GHz. We interpret this new type of large-amplitude high-frequency dynamics as non-adiabatic stochastic resonance of magnetization. Temperature-dependent measurements of the rectified signal confirm the stochastic resonance nature of the observed phenomena. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y32.00004: Reduction of spin-torque switching currents by partially canceling the free layer demagnetization field Luqiao Liu, Takahiro Moriyama, Dan Ralph, Robert Buhrman A small switching or excitation current is crucial for the successful application of spin torque (ST) in magnetic memory and on-chip oscillator devices. The required ST current for an in-plane-polarized nanomagnet is proportional to its effective field, within which the out-of-plane component (H$_{z})$ dominates. This large H$_{z}$, however, does not contribute to the thermal stability of the free layer. So it will be of great advantage if we can reduce H$_{z}$. Co/Ni multilayer structures have been shown to exhibit perpendicular anisotropy and we have precisely controlled the thickness of these multilayer components so that this crystalline anisotropy can be used to cancel the demagnetization field, reducing H$_{z}$ to a value comparable to the in-plane geometry-dependent coercive field. In comparison to a control sample with a relatively higher H$_{z}$ and the same magnetic volume, we find that the low H$_{z}$ sample has much smaller ST reversal currents in both the quasi-state thermally activated and short pulse reversal regimes. The fact that the free layer magnetization lies in plane and the fixed layer(s) can be conventional magnetic material(s) makes it more tractable to deal with the dipole coupling between the free and reference layers, and should also facilitate the incorporation of this approach in high performance ST devices that utilize magnetic tunnel junctions. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y32.00005: Effects of rf current on critical field for magnetization reversal in spin torque devices Wenyu Chen, Sylvia Florez, Jordan Katine, Matthew Carey, Liesl Folks, Bruce Terris Current induced switching assisted by rf current has recently been observed in spin torque devices at low temperature [1, 2]. This effect allows control of spin transfer induced magnetization reversal through the frequency of an injected rf current. In this study, the effects of the rf current injection on critical field for magnetization reversal in spin valve junctions have been investigated. Measurements were conducted at room temperature, and the magnetic field was applied along the easy axis of the junction. An rf current was injected into the nanojunction at various frequencies ranging between 1 and 20 GHz. The dynamic resistance, dV/dI, was measured as a function of the rf frequency, power and the dc bias current while ramping the magnetic field. The rf current injection was observed to change the critical field for free layer magnetization reversal when the intrinsic spin-transfer-induced dynamics is frequency-locked with the injected rf. The results will be discussed in the context of macrospin models of spin transfer in metallic spin valve structures. [1] S. H. Florez et al. Phys. Rev. B 78, 184403 (2008) [2] Y.-T. Cui et al. Phys. Rev. B 77, 214440 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:36AM |
Y32.00006: A Three Terminal Approach to Spin-Torque Written MRAM Cells Invited Speaker: Magnetic random access memory (MRAM) is a potentially superior alternative to silicon-based memories due to a combination of properties including non-volatility, fast read/write times, and low power consumption. Future MRAM technologies have been considered which use the spin transfer effect as a mechanism for bit element writing. Here, a spin polarized current passing through a ferromagnetic element is used to reverse its moment via an exchange of angular momentum, as opposed to the magnetic fields from remote write lines used in more conventional toggle MRAM [1]. However, the large current densities required for spin transfer reversal create significant barrier wearout issues in the magnetic tunnel junctions (MTJs) used as bit elements. One possible solution is to develop a nanopillar structure where a third electrode can be made to any point within a thin-film multilayer stack, substantially enhancing the versatility of the device by providing the means of applying independent electrical biases to two separate parts of the device. Using experimental results and micromagnetic simulations, I will discuss a joint magnetic spin valve/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode [2]. A spatially nonuniform spin-polarized current flowing into the free layer via the low-resistance spin valve path can reverse the magnetic orientation of the free layer as a consequence of the spin torque effect, by nucleating a reversal domain at the spin injection site that propagates across the free layer. The free layer magnetic state can then be read out separately via the higher-resistance magnetic tunnel junction. This three-terminal structure provides a strategy for developing high performance spin-torque MRAM cells which avoids the need to apply a large voltage across a MTJ during the writing step, thereby enhancing device reliability, while retaining the benefits of a high-impedance MTJ for read-out. \\[4pt] [1] Slaughter J.M., Dave R.W., DeHerrera M., Durlam M., Engel B.N., Janesky J., Rizzo N.D., Tehrani S., Fundamentals of MRAM technology, \textit{Journal of} \textit{Superconductivity: Incorporating Novel Magnetism }15, 19 (2002). \\[0pt] [2] Braganca P.M., Katine J.A., Emley N.C., Mauri D., Childress J.R., Rice P.M., Delenia E., Ralph D.C., Buhrman R.A., \textit{IEEE Trans. Nanotechnol.}, In press (2008). [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y32.00007: Invariant Form of the Spin-Transfer Switching Condition Inti Sodemann, Yaroslaw Bazaliy Conventional spin-transfer (ST) device with one fixed and one free layer is considered in the macrospin approximation for the case of constant driving current. The expression for the critical current capable of pushing the free layer magnetization out of the local energy minimum is obtained in an invariant form. It is found that the relevant quantity is the divergence of the spin-transfer torque, and not the strength of the torque itself. This shows that there is no essential difference between current induced switching in collinear and non-collinear geometries. The result further provides a qualitative picture of the influence of ST torque angular dependence on the switching current and allows to understand when the Slonczewski spin polarization coefficient $g(\theta)$ can, or cannot be approximated by a constant. We discuss the implications of the derived formulas for the engineering of low current devices. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y32.00008: Exchange assisted spin transfer torque switching Xi Chen, Randall Victora The main challenge in the application of spin transfer torque switching is the high current required to reverse the magnetization. We propose a composite structure containing soft and hard magnetic layers that significantly lowered the switching current. The dynamic phase diagram of the structure is studied using a macrospin model, with Landau-Lifshitz-Gilbert equation including a spin torque term. It is shown that an optimal exchange coupling strength exists with a value around half the anisotropy of the hard layer. By using multiple soft layers with graded anisotropy, a further reduction can be achieved. We also show that the switching current grows linearly with the damping constant in the soft layer. This means that a low damping, soft material can facilitate the reversal of the hard layer and reduce the switching current by over an order of magnitude. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y32.00009: Structural Characterization of Spin-Torque Oscillators Sarah C. Parks, K. Li, A. Hauser, J. E. Thompson, J. Ciraldo, J. Emerick, J. Lucy, F. Y. Yang, E. Johnston-Halperin The discovery of current-induced magnetodynamics in giant magnetoresistive (GMR) trilayers promises a novel platform for microwave electronics. One of the keys to developing this potential has been the development of nanoscale fabrication techniques, typically resulting in either nanopillar or point-contact geometries. As a result, a considerable technical barrier to further progress is the fidelity of current nanoscale patterning techniques. In an effort to address this challenge, we present the results of development efforts aimed at fabricating prototype point-contact spin torque oscillator (PC-STO) structures with a focused ion beam (FIB). The flexibility of FIB-based nanofabrication allows \textit{in situ} cross sectional imaging of contact structure, and these results are correlated with DC magnetotransport. This fabrication approach enables the rapid generation of structures in arbitrary geometries, and in conjunction with cross-sectional imaging promises increased control of device to device variation and the development of novel PC-STO structures. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y32.00010: AC and DC voltage driven magnetization dynamics in magnetic nanostructures Oleg Tretiakov, Aditi Mitra We study a geometry involving a thin ferromagnetic (F) layer sandwiched between two normal metal (N) leads. The system is driven out of equilibrium by the simultaneous application of an external dc and ac voltage across the N-F-N structure. The Keldysh diagrammatic approach is used to study the system which reveals that one of the effects of the external drive is to produce noise with a non-trivial frequency dependence. We determine the effect of the noise on the magnetization dynamics, and also present results for how the current-voltage characteristics of the structure is affected by the nonequilibrium dynamics of the ferromagnetic layer. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y32.00011: All Spin Digital Circuits Behtash Behin-Aein, Deepanjan Datta, Sayeef Salahuddin, Supriyo Datta Switching of a magnetic free layer using spin polarized current has been demonstrated in Magnetic Tunnel Junction (MTJ) devices. Currently MTJ's are being studied for memory and microwave oscillator applications. The purpose of this talk is to explore a modified MTJ where a clock pulse via the fixed layer facilities the switching of the free layer in accordance with a weak bias provided by an input magnet in the form of a spin current. Based on the Landau-Lifshitz-Gilbert equation (LLG) augmented with spin torque functions, we show the switching energy and the switching time of the free layer which indicates the possibility of very low power digital logic applications. Ordinary digital circuits store information in the form of capacitor charges that communicate through electrical interconnects. The purpose of this paper is to show that modified MTJ's can be the basis for all spin digital circuits. Our primary objective is to stimulate proof of concept experiments that could usher in a whole new set of devices suitable for spintronic circuits. [Preview Abstract] |
Session Y33: Nanowires and Quantum Dots
Sponsoring Units: DCMPChair: Milind Kunchur, University of South Carolina
Room: 403
Friday, March 20, 2009 8:00AM - 8:12AM |
Y33.00001: Digital Wires Benny Brown, Alfred Hubler We study hardware implementations of cellular automata as reliable, adjustable, and secure commication lines. We discuss energy efficient digital wires on a nano-scale, all-optical digital wires, and digital wires as power lines and present performance data of a prototype digital wire, a six cells wide and ten cells long Boolean network. We show that digital wires have the following advantages: (i) Fixed pulse shape (pulses have a rectangular shape with a constant height and a constant width and produce no echos); (ii) Robust against electric smog. Digital wires based on semiconductor technology are effectively inert against electro-magnetic radiation, except for low-frequency radiation (heat) and high frequency radiation (X-rays). Digital wires based on plasma technology have in addition a very high tolerance for heat and X-rays. In digital wire the pulse speed can be rapidly adjusted. Signals on digital wires can be encrypted. Some digital wires can be used as general purpose computers. The data and the code are the input of the wire. Then both travel along the wire and `collide'. The collision is the computation. The result travels to the end of the wire, for further processing, as parallel input by a CPU, an actuator, or another digital wire. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y33.00002: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y33.00003: Renormalization of the dephasing by zero point fluctuations Swarnali Bandopadhyay, Doron Cohen One of the most fundamental properties of an quantum particle is to maintain its phase-coherence. When an quantum particle is coupled to a fluctuating environment its wave-function gets phase-randomised. During the last decade a controversy has emerged in the mesoscopic literature regarding the role of zero-point-fuctuations (ZPF) in low temperature dephasing. We propose an exactly solvable model for dephasing due to short range scattering with environmental modes in dephasing at low temperature. Unlike the Caldeira-Leggett model where the interaction is with an homogeneous fluctuating field of force, here we consider the environment consisting of infinitely many localized fluctuating modes with (say) Ohmic spectral function and the interaction is local as in ``s-scattering". We find that in low temperature ZPF can enhance the inelastic cross-section. Our study shows [Phys. Rev. B {\bf 77}, 155438 (2008)] we need finite temperature to see the effect. Thus indirectly ZPF might contribute to the dephasing at low temperature. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y33.00004: Helical [110] gold nanowires make longer linear atomic chains Edgard Amorim, Edison da Silva Experiments performed on nanowires (NWs) synthesized by electron beam irradiation technique have shown that gold NWs formed along the [110] direction become helical when the NWs are sufficiently thin [1]. Moreover, helical and other non-crystalline structures have been theoretically predicted to other few metals [2]. Our study using tight-binding molecular dynamics show that gold NWs formed along the [110] direction reconstruct upon stress to form helical NWs. We discuss this formation and our results seem to indicate that an intrinsic mechanism is responsible for the formation of the helical structure. These helical NWs evolve on stretching to form linear atomic chains (LACs) and because they do not form symmetrical tips, these NWs produce longer LACs than other NWs. We use \textit{ab initio} calculations to study the NW obtained from the tigth-binding simulations at stages close to rupture and compare LAC distances obtained with both methods. Furthermore, we investigate the electronic structure of the NW close to rupture [3]. [1] Y. Kondo, and K. Takayanagi, Science \textbf{289}, 606 (2000). [2] O. Gulseren, F. Ercolessi and E. Tosatti, Phys. Rev. Lett. \textbf{80}, 3775 (1998). [3] E.P.M. Amorim and E.Z. da Silva, Phys. Rev. Lett. \textbf{101}, 125502 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y33.00005: Hall effect detection of time-reversal symmetry breaking under AC driving Alexei Chepelianskii, Sophie Gueron, Frederic Pierre, Antonella Cavanna, Bernard Etienne, Helene Bouchiat In a four terminal sample microscopic time-reversibility leads to symmetry relations between resistance measurements where the role of current and voltage leads are exchanged. These reciprocity relations are a manifestation of general Onsager-Casimir symmetries in equilibrium systems. We investigate experimentally the validity of time reversal symmetry in a $GaAs/Ga_{1-x}Al_xAs$ Hall bar irradiated by an external AC field at zero magnetic fields. For inhomogeneous AC fields we find strong deviations from reciprocity relations and show that their origin can be understood from the the billiard model of a Hall junction. Under homogeneous irradiation the symmetry is more robust indicating that time-reversal symmetry is preserved. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y33.00006: On time-dependent counting statistics of mesoscopic electron transport Wolfgang Belzig Full counting statistics (FCS) has emerged as a key concept to understand quantum transport in mesoscopic systems like heterostructures, quantum wires, and quantum dots. The knowlegde of the FCS not only enables to predict all measurable zero-frequency quantities accessible via charge detection, but also allows to identify the elementary transport events and the correlations between them. We demonstrate this concept for a standard quantum point contact between normal and/or superconducting leads under dc- and ac-bias. [M. Vanevic, Yu. V. Nazarov, W. Belzig, Phys. Rev. Lett. 99, 076601 (2007)] Finally we address the question, how these concepts can be applied to time-resolved current measurements. [A. Bednorz and W. Belzig, Phys. Rev. Lett. 101, 206803 (2008)] [Preview Abstract] |
Session Y34: Superconducting Devices and Applications
Sponsoring Units: DCMPChair: Galen O'Neil and Robert Horansky, National Institute of Standards and Technology
Room: 404
Friday, March 20, 2009 8:00AM - 8:12AM |
Y34.00001: Designing electron refrigerators for improved cooling with an expanded thermal model Galen O'Neil, Erik Larson, Joel Ullom Normal-metal/insulator/superconductor (NIS) tunnel junctions can act as refrigerators below 1K. ~Biasing the junction such that only thermally excited electrons have energy higher than the superconducting gap causes selective tunneling which cools the normal metal electrode. ~Because of their small size, low mass, and absence of moving parts, NIS refrigerators are an attractive cooling technology for space and industrial applications. ~We have demonstrated temperature reductions of 100mK from bath temperatures near 300mK. ~For example, we operated a superconducting x-ray detector at 160mK with a cryostat bath temperature of 260mK by using NIS junctions for the additional cooling. ~We will show a more complete thermal model of a large area NIS refrigerator accounting for quasiparticle injection, diffusion, and imperfect trapping. Using this model to guide NIS refrigerator design we expect to achieve our goal of cooling from 300mK to 100mK. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y34.00002: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y34.00003: Nanomechanical parametric amplification and oscillation via electrostatic coupling to Cooper-pair box Junho Suh, Matt LaHaye, Pierre Echternach, Keith Schwab, Michael Roukes Nanomechanical resonator coupled to a Cooper-pair box exhibits frequency modulation as a function of the gate voltage when the qubit is adiabatically tuned. We utilize this effect to demonstrate mechanical degenerate parametric amplification and oscillation. Gain above 30dB and self-oscillation is observed. This technique would provide an efficient way to enhance the force sensitivity in the measurement of coupled dynamics of nanomechanical resonanator modes and a solid state qubit. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y34.00004: Back-action and self-oscillation in the Double Pump Josephson Parametric Amplifier (DPA) Archana Kamal, Adam Marblestone, Michel H. Devoret Josephson Parametric amplifiers are the most promising candidates for reaching the quantum limit of amplification at RF frequencies. The DPA employs 2 pumps. The dual pumps ensure separation between the signal and the pump frequencies, which is necessary to observe delicate effects, such as self-oscillations. We present the model of the DPA. Our calculations are based on Input-Output Theory, and can easily be generalized to any coupled system involving parametric interactions. We analyze the operation of the device, taking into account the feedback introduced by the reaction of the signal amplification on the pump power and we compute various response functions - signal/idler gain, internal gain, and steady state signal response. To account for this back-action between signal and pump, we adopt a mean-field approach and self-consistently explore the boundary between amplification and self-oscillation. The potential of the DPA for quantum-limited measurements and as a squeezer is also discussed. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y34.00005: Detection and Reset Dynamics of Superconducting Nanowire Single Photon Detectors Anthony Annunziata, Orlando Quaranta, Daniel Santavicca, Joel Chudow, Luigi Frunzio, Aviad Frydman, Michael Rooks, Daniel Prober We investigate the single photon detection and reset mechanisms in superconducting nanowires, which have received recent, widespread attention for use in applications requiring fast optical and near infrared photon counting. These devices are fabricated from ultra-thin Nb and NbN films and read out by measuring short ($\sim $1 ns) transient voltage spikes that result from the localized suppression of superconductivity by an absorbed photon. We find that intrinsic electro-thermal instabilities necessitate a low impedance ($<$50 ohms) readout circuit for stable resetting to the superconducting state after detecting a photon. The actual impedance needed depends on the detailed physical properties of the device. We also investigate the detection mechanism and report the dependence of detection efficiency on the temperature and dc bias current as well as on film disorder, from which we present a model of the detection mechanism. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y34.00006: Measurements of a YBCO superconducting quantum interference filter with planar ion-damaged Josephson junctions Steven M. Anton, Shane A. Cybart, Stephen M. Wu, John Clarke, R.C. Dynes We have fabricated a two dimensional series-parallel array of 565 $\times$28 \linebreak YBa$_2$Cu$_3$O$_{7-\delta}$ thin film ion damage Josephson junctions. The loop areas were varied incommensurately from 28.5 to 90~$\mu$m$^2$ so that the response of the current-biased array to an applied magnetic field is a prominent peak in voltage centered at zero field. We measured voltage versus applied magnetic field characteristics for several static bias currents at different temperatures. Additionally, we measured current-voltage ($I$-$V$) characteristics of the array with no applied magnetic field at several temperatures. To fit the $I$-$V$ characteristics, we assumed that each parallel section of the array could be modeled as a single resistively shunted junction with critical currents following a normal distribution. Fitting the summed voltage contributions of the parallel sections to measurements of the array, we computed the resistance and critical current mean and spread. The standard deviation of these critical currents was 15~percent at 74~K. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y34.00007: Cryogenic Broadband Impedance-Matched Absorptive Microwave Filters Daniel Slichter, Ofer Naaman, Irfan Siddiqi We report Johnson noise and S parameter measurements of a broadband impedance-matched low pass microwave filter consisting of a section of lossy stripline transmission line. The thermal noise power generated by the filter was measured in the frequency band of 1.2 GHz - 1.8 GHz at temperatures from 30 mK to 300 mK. The noise power was comparable to that of a 50$\Omega$ reference load held at the same temperature and measured with the same microwave measurement chain using a cryogenic mechanical switch. Transmission measurements on a filter with $f_{3dB}$=1.3 GHz show that the filter's cutoff characteristics remain essentially unchanged between room temperature and 20 mK. The filters are robust to thermal cycling and are simple to manufacture. We have used these filters to obtain low Cooper pair temperatures in high-bandwidth microwave measurements on superconducting devices. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y34.00008: The Electronic Properties of AlN Tunnel Barriers and the Effect of Oxygen Impurities Yun Li, John Read, Pinshane Huang, Hsin-wei Tseng, Robert Buhrman The use of ultra-thin aluminum nitride (AlN) barrier layers can result in Josephson Junctions (JJ's) with both very high critical current densities and low sub-gap leakage [1-4], demonstrating that AlN is a superior JJ tunnel barrier material in the ultra-thin barrier limit. We have utilized scanning tunneling spectroscopy (STS) and analytical scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS) to investigate thin AlN layers formed on Nb/Al bilayers by treating the Al surface with an atomic nitrogen beam. Under optimum nitridation conditions the resultant $\sim $1nm AlN barrier layers have small, $\sim $ 1 eV, but well defined band gaps and stable surfaces in UHV, with the absence of band-tail states extending close to the Fermi energy, which is in sharp contrast to the case for AlO$_{x }$layers formed by thermal oxidation [5]. The AlN barrier layers are however quite sensitive to even low levels of background oxygen (O) exposure, either during or after the nitridation process, which reacts O into the barrier layer and results in the formation of low energy band-tail states and an unstable surface. [1] Zijlstra et al., APL 91, 233102 (2007); [2] Wang et al., APL 64, 2034 (1994); [3] Kleinsasser et al., IEEE TAS 5, 2318 (1995); [4] Kaul et al., JMRS 20, 3047 (2005); [5] Mather et al., APL 86, 242504 (2005) [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y34.00009: Dispersive Microwave Readout of NanoSQUIDs R. Vijay, Aidin Fathalizadeh, Irfan Siddiqi, Michael Hatridge, John Clarke Superconducting quantum interference devices (SQUIDs) based on Josephson tunnel junctions have long been used as sensitive magnetic flux detectors. NanoSQUIDs, which use submicron weak link junctions for enhanced flux coupling,~are~attractive candidates for magnetic measurements of molecules.~ We present a novel~method~for nanoSQUID readout~which involves embedding the SQUID in a superconducting transmission line cavity operating at microwave frequency. The magnetic flux dependence of the total SQUID inductance modulates the cavity resonant frequency; these frequency changes are determined using microwave reflectometry. This dispersive microwave measurement allows detection of changes in magnetic flux at submicrosecond timescales without creating dissipation in the vicinity of the molecule. Moreover, we can exploit the Josephson nonlinearity of the nanoSQUID for bifurcation amplification to~enhance sensitivity. Optimization of the nanoSQUID design and cavity parameters for maximizing detector sensitivity and bandwidth is discussed. We also discuss the various sources of noise in this measurement scheme and how to minimize their impact. This work is supported by AFOSR and USDOE. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y34.00010: Dispersive microwave magnetometry of single molecule magnet crystals M. Hatridge, J. Clarke, R. Vijay, J.J. Lee, I. Siddiqi, G. Timco, R. Winpenny Dispersive microwave readout of SQUIDs is a promising technique for magnetometry of molecular magnets, as it offers good sensitivity, high bandwidth, and minimal back action. We have fabricated a sample consisting of a 1.5 GHz aluminum coplanar stripline resonator with a quality factor Q=1000, terminated with a Al-AlO$_{x}$-Al unshunted tunnel junction SQUID with loop area 9 $\mu $m$^{2}$. Flux signals coupled to the SQUID loop are detected as changes in the phase of the reflected microwave signal. We present data characterizing the effective flux noise of the detector as a function of microwave drive power and flux bias. By exploiting the enhanced sensitivity when the resonator is operated in the nonlinear regime we achieve a minimum effective flux noise of 0.5-1 $\mu \Phi _{0}$ / rt Hz. This effective flux noise in the nonhysteretic regime is presently limited by our system noise temperature of 10 K. Data showing low field T$_{1}$ dispersion measured on a small crystal of Cr$_{7}$Ni diluted in Cr$_{8}$ will be presented. We acknowledge support from AFOSR and the US DOE. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y34.00011: Single-artificial-atom lasing using a voltage-biased superconducting charge qubit Robert Johansson, Sahel Ashhab, Alexandre Zagoskin, Franco Nori We consider a system composed of a single artificial atom coupled to a cavity mode. The artificial atom is biased such that the most dominant relaxation process in the system takes the atom from its ground state to its excited state, thus ensuring population inversion. Even under this condition, lasing action can be suppressed if the `relaxation' rate, i.e. the pumping rate, is larger than a certain threshold value. Using simple transition-rate arguments and a semiclassical calculation, we derive analytic expressions for the lasing suppression condition and the state of the cavity in both the lasing and suppressed-lasing regimes. The results of numerical calculations agree very well with the analytically derived results. Our analysis and results are relevant to the recently realized superconducting artificial-atom laser. [arXiv:0803.1209] [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y34.00012: A scanning SQUID microscope for imaging high-frequency magnetic fields C. P. Vlahacos, F. C. Wellstood, J. Matthews We have developed a large-bandwidth scanning SQUID microscope in order to spatially image high frequency magnetic fields. By using a hysteretic Nb dc-SQUID and a pulsed sampling technique, rather than a non-hysteretic SQUID and a flux-locked loop, we have overcome the bandwidth limitations of existing scanning SQUID microscopes, which typically only image below about 1 MHz. The microscope allows for non-contact time-varying magnetic field images to be taken of room temperature samples with time steps down to 50 ps and spatial resolution ultimately limited by the size of the SQUID to about 10 $\mu $m. Towards this end, results will be presented on the design, development, and operation of a cryo-cooled 4.2 K scanning SQUID microscope with a bandwidth of dc to 3 GHz and a sensitivity of about 52.4 nT per sample. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y34.00013: The Josephson Microwave Photomultiplier Brendan Osberg, Jay Gambetta, Frank Wilhelm The current lack of single microwave photon counters -as opposed to microwave amplifiers- has become a problematic omission in the toolkit of available circuit QED devices. Hence, we propose a microwave photo-multiplier based on a modified phase qubit. Such a system, trapped in the metastable state, can be activated over its potential energy barrier by an incoming photon, creating an avalanche effect analogous to current photo-diodes. Linear coupling of the junction flux with the radiation field, in the weak damping regime, permits photodetection from an arbitrary quantum source in the GHz range. We model this device theoretically and investigate its sensitivity, bandwidth, efficiency, and dark-count rate using the Langevine stochastic differential equations and a path integral approach. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y34.00014: Transport Properties of a Hybrid SET-SQUID Device in Tunable Dissipative Environment. Shuchao Meng, Jeffrey Quilliam, Chas Mugford, Andy Sachrajda , Jan Kycia We will present measurements of transport properties of a new type of superconducting device, designed to allow a fully adjustable Hamiltonian with charge term, Josephson coupling term, and dissipation term. This device consists of a superconducting Single Electron Transistor (sSET) and two Superconducting Quantum Interference Devices (SQUIDs). A 2D electron gas embedded 90nm below the substrate surface provides a tunable dissipative environment. A small magnetic field can be applied to drive this hybrid device from the strong Josephson coupling regime to the SET regime. Dissipation and temperature dependence of the switching current out of the zero-voltage state show different characteristics for different settings of competition between Josephson coupling and charging energy. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y34.00015: Cooper Pair Wavefunction Approach to SNS Junctions Yong-Jihn Kim Recently we introduced a Cooper pair wavefunction approach to the Josephson effects [1]. The approach led to the discovery of threshold resistance in the SIS junctions and more accurate temperature dependence of the DC supercurrent. We apply the approach to the SNS junctions. The Cooper pair wavefunction in the superconductor penetrates into the normal metal, leading to proximity-effect-induced Josephson coupling. The resulting Josephson coupling energy and the supercurrents are calculated from the overlap of the Cooper pair wavefunction of the superconductor and the induced Cooper pair wavefunction of the normal metal near the interface. We calculate the magnitude of the superccurrent at 0K and the temperature dependence of the supercurrent. We compare our theoretical results with experiments. [1] Yong-Jihn Kim, J. Appl. Phys. Vol. 103, 103902 (2008). [Preview Abstract] |
Session Y35: Focus Session: Iron Pnictides and Other Novel Superconductors XVI: Hc2 and Vortex Dynamics
Sponsoring Units: DMPChair: Ruslan Prozorov, Ames Laboratory
Room: 405
Friday, March 20, 2009 8:00AM - 8:12AM |
Y35.00001: Fluorine Doping Effect On H$_{c2}$ and R$_{h}$ In LaFeAsO$_{1-x}$F$_{x}$ Y. Kohama, S. Riggs, F. Balakirev, M. Jaime, Y. Kamihara, T. Atake, M. Hirano, H. Hosono The iron arsenide superconductors discovered earlier this year have attracted much interest, and some families showing high-$T_{c}$ have been identified. \textit{Ln}FeAsO (\textit{Ln}; lanthanide) is the first copper-free family of compounds with $T_{c}$ exceeding 50 K. Here, we present the first systematic study of $H_{c2}$ and $R_{H}$ in the wide fluorine doping region (LaFeAsO$_{1-x}$F$_{x}$; $x$ = 0, 0.25, 0.05, 0.07, 0.11 and 0.14). We found that $H_{c2}$ increases monotonically with decreasing $x$, while the superconducting phase diagram ($T_{c}$, $x)$ displays the classic dome-shaped structure. Furthermore, the shape of $H_{c2}(T)$ depends strongly on $x$. This, according to Gurevich's model, suggests a multiband electronic structure. The Hall resistivity for non-superconducting samples $x$ = 0, 0.025 show a non-linear magnetic field dependence, which also supports a multiband electronic structure interpretation. In addition, the estimated low-field limit of the $R_{H}$ for $x$ = 0, 0.025 detect a gap opening at the structural transition and magnetic transition. The evidence for multiband electronic structure links these materials to famous multiband superconductor MgB$_{2}$ rather than high-$T_{c}$ cuprates. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y35.00002: Upper critical fields of NdFeAsO$_{0.7}$F$_{0.3}$ single crystal J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, Ivana Raicevic, A. Gurevich, D.C. Larbalestier, F.F. Balakirev, L. Fang, P. Cheng, Y. Jia, H.H. Wen We present measurements of the resistivity and the upper critical field $H_{c2}$ of NdFeAs O$_{0.7}$F$_{0.3}$ single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of $H_{c2}$ is comparable to $\sim100$~T of high $T_c$ cuprates. $H_{c2}(T)$ parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of $H_{c2}$ shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as $T$ decreases, from 9.2 at 44 K to 5 at 34 K. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of $H_{c2}$, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on $H_{c2}(T)$, which may significantly reduce $H_{c2}(0)$ as compared to $H_{c2}(0)\sim200-300$ T based on extrapolations of $H_{c2}(T)$ near $T_c$ down to low temperatures. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y35.00003: Determination of anisotropic $H_{c2}$ in (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single crystals C. H. Mielke, M. M. Altarawneh, K. Collar, N. Ni, S. L. Bud'ko, P. C. Canfield The radio frequency penetration depth was measured in the superconductor (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ under pulsed magnetic fields extending to 60 tesla and down to 14 K. Using these data we are able to infer a $H_{c2}(T)$, $H-T$ phase diagram, for applied fields parallel and perpendicular to the crystallographic $c$-axis. The upper critical field curvature is different for the respective orientations but they each remain positive down to 14 K. The upper critical field anisotropy is moderate, $\approx 3.5$ close to $T_c$, and drops with the decrease of temperature, reaching $\approx 1.2$ at $14 K$. These data and analysis indicate that (i) (Ba$_{0.55}$K$_{0.45}$)Fe$_{2}$As$_{2}$ anisotropy diminishes with temperature and has an unusual temperature dependence, (ii) $H_{c2} (T=0)$ for this compound may easily approach fields of 75 tesla. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y35.00004: Thermodynamic estimation of the upper critical field slope of doped SmFeAsO from fluctuation conductivity in the critical regime Marina Putti, Ilaria Pallecchi, Carlo Fanciulli, Matteo Tropeano, Maurizio Ferretti, Alberto Martinelli, Andrea Palenzona, Carlo Ferdeghini We measure magnetotransport in SmFeAs(O$_{1-x}$F$_{x})$ polycrystalline samples up to 28T and we extract the upper critical fields, using different criteria. Due to fluctuation effects, not negligible magnetoresistance and resistivity not saturating to a residual value at Tc, H$_{c2}$ values turn out to be strongly criterion-dependent. In order to circumvent this problem, we propose a thermodynamic estimation of the upper critical field slope dH$_{c2}$/dT based on the analysis of conductivity fluctuations in the critical regime at high fields. Indeed, in this regime we find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than $\mu _{0}$H$_{LLL}\sim $8T, which allows to extract a high field slope as large as -12T/K for the optimally doped sample SmFeAs(O$_{0.85}$F$_{0.15})$. A comparison of the fluctuation behavior with that of high-T$_{c}$ cuprates indicates that this H$_{LLL}$ value may be related to the H$_{c2}$ and $\kappa $ values higher than those of cuprates.. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y35.00005: Upper critical field in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ and FeSe$_{1-x}$ Te$_{x}$ compounds Chiara Tarantini, Jan Jaroszynski, Jianyi Jiang, Alex Gurevich, David C. Larbalestier, Rongyin Jin, Athena S. Sefat, Michael A. McGuire, Brian C. Sales, David G. Mandrus We report H$_{c2}$ measurements in high magnetic field up to 31 T on Ba(Fe$_{1-x}$Co$_{1-x})_{2}$As$_{2}$ and FeSe$_{1-x}$ Te$_{x}$\textbf{ }pnictide compounds for different doping levels. Both materials exhibit a very high upper critical fields and unconventional temperature dependencies of H$_{c2}$(T) with the extremely high slopes dH$_{c2}$/dT from 10 to 30 T/K near T$_{c}$ and a relatively low anisotropy: $\gamma ={H_{c2} ^{//}} \mathord{\left/ {\vphantom {{H_{c2} ^{//}} {H_{c2} ^\bot }}} \right. \kern-\nulldelimiterspace} {H_{c2} ^\bot }$ for the doped ternary compound and $\gamma \approx 1.1\div 1.2$ for the binary one. The observed temperature dependences of H$_{c2}$(T) and the high H$_{c2}$ values well above the BCS paramagnetic limit indicate the importance of the Zeeman pairbreaking effects in these compounds. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y35.00006: Thermodynamic Determination of the Upper Critical Field and Anisotropy of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ Single Crystals Wai -K. Kwok, Ulrich Welp, Ruobing Xie, Alexei Koshelev, John Schlueter, Jiong Hua, Hui-qian Luo, Zhao-sheng Wang, Gang Mu, Hai-hu Wen We present anisotropic heat capacity measurements of the upper critical field of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ single crystals in fields up to 8 Tesla. In zero-magnetic field a clear step in the heat capacity is observed at T$_{c} \sim $36K. Using an entropy conserving construction we determined the transition temperatures in applied fields and the upper critical field slopes dH$_{c2 \vert \vert c}$/dT = -6.5 T/K and dH$_{c2 \vert \vert ab}$/dT -17.4 T/K, the latter showing record high critical field slope near T$_{c}$. The temperature dependence of the specific heat of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ indicates strong coupling effects. Based on the experimental values of the upper critical field slopes, we determined the Ginzburg parameter, coherence and penetration lengths, anisotropy and thermodynamic critical fields. We also present magnetization measurements and discuss their implications on the nature of critical currents in this material. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y35.00007: Inter- and intra-granular current properties of iron pnictide superconductors Akiyasu Yamamoto, Anatolii Polyanskii, Jianyi Jiang, Fumitake Kametani, Marina Putti, Chiara Tarantini, Frank Hunte, Jan Jaroszynski, Eric Hellstrom, Alex Gurevich, David Larbalestier The iron pnictide superconductors have very high upper critical field $B_{c2}$(0) of possibly over 100 T for 1111 and 50-70 T for 122. We have recently shown [1,2] that polycrystalline 1111 samples exhibit electromagnetic granular behavior, perhaps in an analogous way to that seen now to be intrinsic to the HTS cuprates. Detailed investigation is proceeding in parallel with serious efforts to make more single phase samples, since it appears that all present polycrystalline oxypnictides are multi-phase. In particular we are using magneto-optical imaging to study the local variation of current density and then performing detailed microstructural analysis by SEM, TEM and orientation analysis to understand intergranular current flow. At the present time we see that samples are multi-phase, often with a grain boundary wetting phase, but even so the global $J_{c}$ attains 1000-4000 A/cm$^{2}$, some 10-40 times that seen in single phase YBCO randomly oriented polycrystalline. On the other hand, very high intra-grain critical current owing to the strong pinning reminiscent of Nb-Ti is observed in the Co doped Ba122 pnictide. We will report on our latest results on the inter- and intra-granular current properties in the high-$T_{c}$ pnictides. [1] A. Yamamoto et al., Appl. Phys. Lett. \textbf{92}, 252501 (2008). [2] A. Yamamoto et al., Supercond. Sci. Technol. \textbf{21}, 095008 (2008). [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y35.00008: Magneto-optical imaging of flux distribution in single crystals Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ of various doping level $x$. Erick C. Blomberg, P. Prommapan, M. A. Tanatar, V. G. Kogan, N. Ni, S. L. Bud'ko, P. C. Canfield, R. Prozorov Near optimal doping of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ crystals exhibit uniform superconductivity and vortex properties similar to high-T$_c$ cuprates \footnote{R. Prozorov \textit{et al.}, Phys. Rev. B in print (2008). arXiv:0810.1338}. In this contribution, single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with the measured doping level x=3.8\%, 4.7\%, 5.8\%, 7.4\%, 10\%, and 11.8\%, covering all regimes - from underdoped to overdoped were studied using real-time magneto-optical imaging. Inhomogeneity of the superconducting state as well as field and temperature dependencies of the magnetic induction distribution were analyzed. Superconductivity is homogeneous at all except for the highest doping level. The results are correlated with macroscopic transport and magnetic measurements. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y35.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y35.00010: Similarity Between Fe-based Pnictide Superconductors and High-T$_c$ Cuprates Revealed by the Irreversible Magnetic Behavior in the Vortex State Ruslan Prozorov, M.A. Tanatar, C. Martin, R.T. Gordon, E.C. Blomberg, P. Prommapan, V.G. Kogan, N. Ni, M.E. Tillman, S.L. Bud'ko, P.C. Canfield Static and dynamic measurements of DC magnetization and direct real-time magneto-optical imaging have revealed unconventional vortex behavior in NdFeAsO$_{0.9}$F$_{0.1}$\footnote{R. Prozorov \textit{et al.} arXiv:0805.2783 (2008)} and Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$\footnote{R. Prozorov \textit{et al.}, Phys. Rev. B in print (2008). arXiv:0810.1338} single crystals. In particular, the fishtail effect, giant magnetic relaxation and a crossover from elastic to plastic vortex creep, reminiscent of similar features found in pure single crystals of Y-Ba-Cu-O and Nd-Ce-Cu-O cuprates. Moreover, despite weak anisotropy of the London penetration depth and second critical field, vortex properties of the pnictides are shown to be very anisotropic. We discuss possible explanations of the observed behavior and conclude that pnictide superconductors bridge a gap between s-wave multiband MgB$_2$ and highly anisotropic d-wave cuprates. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y35.00011: Flux Pinning and Structural Inhomogeneity in Superconducting BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ Single Crystals Qiang Li, Juan Zhou, Jiufeng Tu, Yuhang Ren, Linjun Li, Yongkang Luo, Hang Chen, Guanghan Cao, Zhu'an Xu We report coordinated studies of flux pinning behavior and structural inhomogeneity in BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ single crystals with superconducting transition temperature at 24 K, in order to understand the flux pinning mechanism in the iron-based superconducting materials. Static and dynamic behavior of vortices are investigated by transport, bulk magnetization, and quantitative magneto-optical imaging techniques, while high resolution analytical TEM is used to investigate the structural inhomogeneity down to atomistic level. Frequent flux jump, and enhanced flux pinning at elevated magnetic field, corresponding to the ``fish tail'' in magnetization hysteresis, are often observed. We will discuss the relationship between the flux pinning behavior and detailed structural properties in this and other related superconducting materials. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y35.00012: Thermally-activated and temperature-independent magnetic relaxation in aligned grains of NdFeAsO(F) James R. Thompson, Y. L. Zuev, D. K. Christen, E. D. Specht, R. Jin, B.C. Sales, M. A. McGuire, A. Sefat, D. G. Mandrus We have studied flux creep in a magnetically-aligned powder of NdFeAsOF and found it to be strikingly similar to the situation in cuprates. The magnetic relaxation rate S=-dlnM$_{irr}$/dlnt is linear in temperature at low temperatures. There is an extrapolated~finite creep rate of about S=-0.02 at T=0, indicative of a quantum tunneling of vortices under energy barriers. This quantum creep rate is field-independent. From the temperature-dependent creep data we have obtained activation energy as a function of persistent current density, U(J). Comparison with existing creep theories will be made. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y35.00013: Magnetic imaging of vortices and inhomogeneity in Ba(Fe,Co)$_2$As$_2$ by magnetic force microscopy Weida Wu, S. Park, Linjun Li, Yongkang Li, Hang Chen, Guanghan Cao, Zhu'an Xu Single crystals of BaFe$_{2-x}$Co$_x$As$_2$ synthesized by FeAs flux method were studied by variable temperature magnetic force microscopy (VT-MFM). The nominal Co doping concentrations range from underdoped region (x$\sim$0.1) to optimum doped region (x$\sim$0.2) of the superconducting dome. Sharp superconducting transitions ($\sim$1K) indicate good sample quality. Individual Abrikosov vortices were visualized by VT-MFM below Tc at low magnetic field. The temperature dependence of vortex configuration indicates a strong pinning effect, which is supported by Bean-model behavior\footnote{C.P. Bean, PRL, 8, 250 (1962).} observed at high magnetic field. Results of magnetic inhomogeneity of underdoped samples will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y35.00014: Isotropic low-temperature upper critical field in (Ba,K)Fe$_2$As$_2$ Huiqiu Yuan, Scot Baily, John Singleton, Fedor Balakirev, G.F. Chen, J.L. Luo, N.L. Wang Furious activity has been generated by the discovery of superconductivity in the iron-arsenic-based compounds ReFeAs(O,F) (Re = lanthanide) and (A,K)Fe$_2$As$_2$ (A=Ba, Sr). Superconducting $T_{\rm c}$s as high as 55~K have been observed, provoking comparisons with the ``high $T_{\rm c}$'' cuprates. The layered crystal structure of the cuprates led to speculations that reduced dimensionality is necessary for ``high-temperature'' superconductivity; at first sight, the iron-arsenic compounds, which also possess layered structures, give additional credence to this idea. However, we report measurements in magnetic fields of up to 60~T, necessary to overcome the large upper critical fields, that demonstrate that the superconducting properties of single crystals of (Ba,K)Fe$_2$As$_2$ are in fact rather three dimensional. This is markedly different from the highly anisotropic properties of all previously-known layered superconductors (e.g. the cuprates and the crystalline organic metals); we suggest that it is attributable to the distinctive electronic structure of the iron-arsenide compounds. Our measurements indicate that in contrast to the assumptions based on the cuprates, reduced dimensionality is not a prerequisite for ``high-temperature'' superconductivity. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y35.00015: Unconventional magnetic field-temperature superconducting phase-diagram in Co doped BaFe$_2$As$_2$ Younjung Jo, R.H. Liu, H. Chen, X.H. Chen, L. Balicas Here, we report electrical transport and preliminary torque magnetometry measurements in underdoped Ba(Fe$_{1-x}$Co$_2$As$_2$ single crystals under high magnetic fields. For fields perpendicular to the conducting planes one observes an upward curvature for the temperature dependence of the upper critical field $H_{C2} (T)$ which contrasts markedly with one expects for conventional superconductors. While for fields along the conducting planes $H_{C2} (T)$ behaves nearly linearly in temperature down to low temperatures. For both configurations $H_{C2} (T)$ is found to surpass the weak coupling Pauli limiting field e.g., for a sample displaying a superconducting transition temperature $T_c \simeq 9.7$ K one observes $H_{C2} (T \rightarrow 0$ K $\geq 35$ T for fields along an in-plane direction. This value surpasses the expected Pauli limiting field by a factor of two. Thus, the resulting phase-diagram presents strong similarities with the one reported for poly-crystalline LaFeASO$_{1-x}$F$_x$ and which was recently claimed to be described by a model based on a two-gap strong coupled superconducting state. Our on-going torque magnetometry measurements are expected to check the validity of this interpretation. [Preview Abstract] |
Session Y36: Organic and Hybrid Surfaces and Interfaces
Sponsoring Units: DMP DPOLYChair: V. Podzorov, Rutgers University
Room: 408
Friday, March 20, 2009 8:00AM - 8:12AM |
Y36.00001: Diffusivity control in molecule-on-metal systems using electric fields Y.Y. Zhang, N. Jiang, S.X. Du, H.-J. Gao, Matthew J. Beck, Sokrates T. Pantelides Electronic devices constructed from molecule-on-metal systems are actively being explored for applications in logic and memory devices, sensors, fuel cells, and solar cells. The implementation of practical molecular electronic devices requires molecule-on-metal systems in which the arrangement of active molecules is fixed or can be controlled and the contact-molecule-contact system exhibits desirable electronic properties. Fe (II) Phthalocyanine (FePc) on Au (111) exhibits a number of promising electronic properties, but diffuses rapidly at room temperature. Using scanning tunneling microscopy and density functional theory calculations we show that applied electric fields can be employed to enhance or retard the diffusivity of FePc molecules on Au (111) independent of temperature. These results demonstrate the possibility of dynamic field-guided patterning of molecule-on-metal systems. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y36.00002: Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance Wolfgang Kalb, Kurt Mattenberger, Bertram Batlogg We studied the influence of oxygen on the electronic trap states in a pentacene thin film kept under highly controlled conditions. This was done by temperature-dependent gated four-terminal measurements on pentacene thin-film transistors prior to and after controlled oxygen exposure. We developed and used a scheme that allows for the calculation of the essential transport parameters, such as the trap DOS, in an unambiguous way. The results are free from parasitic contact artifacts. Oxidation of pentacene in light leads to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of 10$^{18}$ cm$^{-3}$. The measurements reveal how these traps affect the key device parameters, i.e. subthreshold performance and field-effect mobility. The study supports the assumption of a mobility edge for charge transport, and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y36.00003: Grazing-incidence X-ray Diffraction of Tetracene Thin Films on Hydrogenated Si(001) Substrate De-Tong Jiang, Andrew Tersigni, Chang-Yong Kim, Jun Shi, Robert Gordon, Ning Chen, Xiaorong Qin Ex situ grazing-incidence X-ray diffraction (GIXD) and wide angle Bragg diffraction have been performed on UHV epitaxially grown tetracene thin films on H/Si(001)-2x1 substrates. The in-plane lattices of the crystalline films were characterized by 2D reciprocal space imaging of the in-plane (11), (12) and (20) GIXD diffraction spots and the out-place lattices were characterized by the wide angle Bragg diffraction. The thickness of the tetracene films ranged from 1.2 monolayer (ML) to 15 ML. H/Si(001)-2x1 substrates with different surface roughness were used. The results indicate that the film structure characteristics are strongly influenced by the substrate conditions and under favorable conditions the homogeneous thin-film phase could dominate the growth up to about 8 ML. The implications of the results to the growth mechanisms and to thin film electronics applications will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y36.00004: Nanoscale conductivity measurements on organic thin films and interfaces Justin Wells, Fei Song, Philip Hofmann Despite the importance of conductance measurements to bulk solid state physics, there is poor understanding of surface and nano-scale conductance - despite the relevance to smaller devices and the development of novel concepts for electronics. Of particular interest in this respect are self-organized organic nano-structures, which offer a virtually unlimited design freedom. Electron delocalization and transport in such systems is of great current interest. In this work, we utilise a recently developed nanoscale multi-contact mono-cantilever probe with a minimum spacing of 250 nm. We measure the conductivity of the Si(111)($\sqrt{3}\times\sqrt{3}$)Ag surface, and the corresponding changes in the conductivity which occur when organic absorbates are present. By increasing the coverage from sub-monolayer to multilayer, it is possible to see doping of the underlying Ag layer, as well as conduction through the organic film. These measurements are supported by PES and NEXAFS studies, and thus can be interpreted in terms of charge transfer and geometric structure. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y36.00005: Density Functional Theory of Transition Metal Phthalocyanines Noa Marom, Leeor Kronik Metal phthalocyanines (MPc's) are a family of highly stable molecules that, as a molecular solid, form organic semiconductors. They have been used in a broad range of applications, e.g., light emitting diodes, solar cells, gas sensors, thin film transistors, and even as single molecule devices. Here, we present a systematic density functional theory (DFT) study of the electronic structure of selected transition metal Pc's: CuPc, NiPc, CoPc, MnPc, and FePc. We critically assess the performance of several semi-local and hybrid exchange-correlation functionals for these systems, and compare the results to experimental photoemission data. For the low-spin systems CuPc, NiPc, and CoPc, we show that semi-local functionals fail qualitatively, primarily because of under- binding of localized orbitals due to self-interaction errors. For the intermediate-spin systems, MnPc and FePc, we show that DFT calculations are extremely sensitive to the choice of functional and basis set with respect to the obtained electronic configuration and to symmetry breaking. However, interestingly, all simulated spectra are in good agreement with experiment despite the differences in the underlying electronic configurations. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y36.00006: Cascade and Accumulation of Spin at CuPc/GaAs (100) interface Huanjun Ding, Irfan Irfan, Yongli Gao, Mirko Cinchetti, Marina Sanchez-Albaneda, Jan-Peter Wustenberg, Oleksiy Andreyev, Michael Bauer, Martin Aeschlimann We have investigated the spin dynamics in organic semiconductor, copper phthalocyanine (CuPc), with spin and time resolved two photon photoemission spectroscopy (STR-2PPE). Spin polarized electrons are generated optically from GaAs substrate, and injected into the unoccupied states of CuPc film. The apparent spin relaxation time is observed to have strong energy dependence. The spin polarization at high energy levels decreases much faster than that of the low energy levels. The experimental results are then explained by a cascade model. The calculation suggests that the spin information of the hot electrons can be well preserved during the energy relaxation process. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y36.00007: Electronic structure of MoO3 insertion layer at the interface between organic semiconductor and indium tin oxide (ITO). Huanjun Ding, Irfan Irfan, Yongli Gao, Frank So We have investigated the electronic structure of the interfaces formed by inserting thin layer of MoO$_{3}$ in between indium tin oxide (ITO) and different organic semiconductors, such as aluminium phthalocyanine chloride (AlPcCl) and copper phthalocyanine (CuPc), with photoemission and inverse photoemission spectroscopy (PES and IPES). The presents of MoO$_{3}$ layer at the interface increases the workfunction dramatically. As a result, the organic HOMO is almost aligned with the Fermi level (E$_{f})$ at the AlPc-Cl/MoO$_{3}$ interface. For thicker AlPc-Cl layers, gradual band bending is observed. However, the recovery of the HOMO is incomplete for AlPc-Cl thickness of 200 {\AA}, leading to a great reduction of the hole injection barrier compare to the case without MoO$_{3}$. Similar situation is found in case of CuPc/MoO$_{3}$, although the energy levels are almost fully recovered for CuPc film thicker than 200 {\AA}. The energy level alignment of these interfaces will be discussed to explain the improvement induced by MoO$_{3}$ layer in device performance. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y36.00008: HPLC and Semi-Prep Scale Fractionations of Poly(3-alkyl thiophenes) Scott LeFevre, Heungyeol Choi, Taihyun Chang, Chang Ryu Molecular weight and polydispersity play a crucial role in crystal formation for thin-films of conducting polymers and subsequently their charge mobility. In order to obtain well defined conducting polymer samples, high performance liquid chromatography (HPLC) analysis and separation of poly(3-alkyl thiophene) (P3AT) systems has been explored. In particular a precipitation-redissolution technique has been employed for the fractionation of P3ATs. Both solvent composition and temperature have been manipulated to tune the solvent quality on both hydrophobic and hydrophilic type stationary phases in order to obtain more well-defined samples of these electrically conducting polymers. In addition to HPLC separations on an analytical scale, semi-prep scale LC separations in the regime of 10mg to 100mg has also been carried out as proof of concept. Finally the interplay between solvent quality and the surface energy has been investigated using HPLC stationary phases consisting of either bare silica or C18 bonded silica. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y36.00009: Tailored Assembly of Organic Molecular Nanofibers into Advanced Donor-Acceptor Architectures Volodimyr Duzhko, Michael J. Kelley, Kenneth D. Singer Non-covalent self-assembly of organic molecules in organic solvents provides a multi-functional approach toward producing organic semiconducting nanostructures having versatile, well-ordered, architectures\footnote{ V. Duzhko and K.D. Singer, J. Phys. Chem. C 2007, 111, 27.} that are potentially integrable into useful electronic, optoelectronic and photonic device architectures. Aiming at molecular-scale tailoring of electron donor-acceptor blend architectures and rational engineering of their functionality for photovoltaic applications, we discuss our approach of solvent-based, electric-field-assisted\footnote{ V. Duzhko, J. Du, C.A. Zorman, and K.D. Singer, J. Phys. Chem. C 2008, 112, 12081.} integration of self-assembled donor (phthalocyanine) nanofibers into an acceptor (perylene diimide) matrix (or vice versa). We present results of our systematic spectroscopic, X-ray diffraction and scanning electron microscopy studies revealing the structure and morphology of neat fibers and fiber blends in various phases. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y36.00010: The substitution effect on the reorganization energy of metal free phthalocyanine Choongkeun Lee, Karl Sohlberg Many discotic (disk like) materials such as phthalocyanine are of interest for use in organic electronic devices because of their high charge mobility. The mobility of various discotic materials has been studied using the Marcus formalism. In the Marcus formalism, charge mobility is depends on two parameters, reorganization energy and coupling matrix constant. Of these two parameters the reorganization energy has more influence on the charge hopping rate. A small change in reorganization energy leads to a large change of charge mobility. We have employed electronic structure methods to describe substitution effects on the reorganization energy of phthalocyanine. The substitutions on the external phenyl rings have almost no influence on reorganization energy, but the substitutions on the internal nitrogen in phthalocyanine have strong influence on reorganization energy. The detailed relation between reorganization energy and substitution will be presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y36.00011: Rational design of the shape and periphery of discotics: a synthetic way towards high charge carrier mobilities Denis Andrienko, Xinliang Feng, Valentina Marcon, Wojciech Pisula, James Kirkpatrick, Ferdinand Grozema, Kurt Kremer, Klaus M{\"u}llen Conjugated materials offer the revolutionary prospect of producing semiconductor devices at low cost. The best to date discotics are built around the coronene unit and possess six fold symmetry. In the discotic phase six fold symmetric molecules stack with an average azimuthal twist of 30 deg, whereas the angle which would lead to the greatest electronic coupling and hence highest charge mobility is 60 deg. Here, a molecule with three fold symmetry and alternating hydrophilic/hydrophobic side chains is synthesized and X-ray scattering is used to prove the formation of the desired helical microstructure. Pulse radiolysis time resolved microwave conductivity measurements show that the material has indeed a very high mobility in the plastic crystalline phase, in the range of $0.1-0.2\, \rm cm^2 / Vs$. The physical structure of the assemblies of molecules are simulated using molecular dynamics. This, together with quantum chemical techniques, allows the computation of charge mobilities without fitting parameters. The calculations prove that mobility is still limited by structural defects and that a defect free assembly would lead to mobilities in excess of $10\, \rm cm^2 / Vs$. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y36.00012: Anisotropic phases in ferromagnetic ultrathin films from multipolar interactions Daniel Barci, Daniel Stariolo We present a model to describe complex phases observed at mesoscopic scales in ultrathin magnetic films with perpendicular anisotropy. The model is based on the interaction between magnetic dipolar as well as quadrupolar moments. This model has a very rich phase diagram. In the special case of films with strong perpendicular anisotropy, a nematic phase, characterized by orientational (stripe-like) but not translational order, is predicted. The isotropic-nematic transition belongs to the Kosterliz-Thouless type in the thermodynamic limit. However, we find that in actual experimental scales the fluctuations of the nematic order parameter are regularized by the sample size, and real orientational order, as predicted by mean field, should be observable. The transition may be characterized experimentally from measurements of the magnetic structure factor, from which the nematic order parameter is derived. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y36.00013: ABSTRACT WITHDRAWN |
Session Y37: Mesoscopic Systems, Clusters, and Nanoscale Systems I
Sponsoring Units: DCPChair: Julius Jellinek, Argonne National Laboratory
Room: 409
Friday, March 20, 2009 8:00AM - 8:12AM |
Y37.00001: Single-Molecule Interfacial Electron Transfer Dynamics at Dye-Sensitized TiO$_{2}$ Nanoparticles H. Peter Lu, Yuanmin Wang, Yufan He Interfacial electron transfer dynamics is important for environmental and catalytic reactions. Extensive ensemble-averaged studies have indicated inhomogeneous and complex dynamics of interfacial ET reaction. To characterize the inhomogeniety and the complex mechanism, we have applied single-molecule spectroscopy and correlated AFM/STM imaging to study the Interfacial ET dynamics of dye molecules adsorbed at the surface of TiO$_{2}$ nanoparticles. The interfacial ET activity of individual dye molecules showed fluctuations and intermittency at time scale of milliseconds to seconds. The fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the dynamics. Furthermore, we have applied site-specific AFM-Raman spectroscopy on analyzing ET associated mode-specific vibrational reorganization energy barriers. Our experiments revealed site-to-site variations in the vibrational reorganization energy barriers in the interfacial ET systems. Our recent experiments on single-molecule metal-to-ligand electron transfer (3) and single-molecule STM manipulation will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y37.00002: Phase Diagram of a Model of Nanoparticles in Electrolyte Solutions Xiaofei Li, Steven Lettieri, Nathaniel Wentzel, James Gunton We obtain accurate fluid-fluid coexistence curves for a recent simple model of interacting nanoparticles that includes the effects of ion-dispersion forces. It has been proposed that these ion dispersion forces provide at least a partial explanation for the Hofmeister effect [Phys. Rev. Lett., 87:168103, 2001]. We study a model of aluminum oxide nanoparticle [Colloids and Surfaces A, 319:98-102, 2008] for three different electrolyte solutions with added salt type being sodium chloride, sodium iodide and a non-polarizable salt. We observe that the fluid-fluid coexistence curves depend substantially on the identity of added salt; this provides an efficient way of tuning the phase behavior of nanoparticles. The methods we employ include finite-size scaling (FSS), multicanonical histogram reweighting and Gibbs ensemble methods. We show that, as expected, all three cases belong to the universality class. The scaling fields and critical point parameters are obtained in the thermodynamic limit of infinite system size by extrapolation of our FSS results. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y37.00003: Prediction of Zeolite Framework Types by a Machine Learning Approach Shujiang Yang, Mohammed Lach-hab, Iosif Vaisman, Estela Blaisten-Barojas Zeolites are microporous crystalline materials with highly regular framework structures consisting of molecular-sized pores and channels. Characteristic framework types of zeolites are traditionally determined by the combined information of coordination sequences and vertex symbols. Here we present a machine learning model for classifying zeolite crystals according to their framework types. An eighteen-dimensional feature vector is defined including topological descriptors and physical/chemical properties of zeolite crystals [Microporous and Mesoporous Materials 117, 339 (2009)]. Trained with crystallographic data of known zeolites, the new model can predict the framework types of unknown zeolite crystals with up to 98 \% accuracy. Compared with conventional methods, the machine learning model is more robust handling crystal disorder and/or crystal defects in a more effective manner. This model can be adapted for classifying and clustering other crystalline species. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y37.00004: Site-specific polarizabilities as predictors of optimal binding sites of H$_{2}$O on Na$_{n}$ clusters Li Ma, Koblar Jackson, Julius Jellinek We have used density functional theory (DFT), in the generalized gradient approximation to study the adsorption of water molecules on sodium clusters, Na$_{n}$, for n = 7, 12, 18, and 25. These clusters span a range of sizes and surface topographies. In each case, we conduct an extensive search to identify the optimal binding site of the ad-molecule on the cluster. We analyze the results within the framework of cluster polarizabilities, making use of a new methodology for partitioning the cluster polarizability into atomic components (Jackson et al., J. Chem. Phys. \textbf{129}, 144309 (2008)). We show that the most favorable adsorption sites are at surface atoms that have the largest atomic polarizabilities. We will compare and contrast these results with corresponding findings for the adsorption of O$_{2}$ and NH$_{3}$ molecules on Na$_{n}$ clusters. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y37.00005: Effect of adsorbates on the isomer stability of Ir$_4$ clusters Vladan Stevanovic, Zeljko Sljivancanin, Alfonso Baldereschi The relative stability of Ir$_4$ isomers, both in the gas phase and on MgO(100) substrate, is studied using density functional theory. The square Ir$_4$ is the most stable in both cases. The metastable tetrahedral isomer, which experimental data suggest as the most stable form of Ir$_4$ on MgO(100), is highly distorted by the strong Ir--O interactions. The relative stability of Ir$_4$ isomers is strongly altered by adsorption of a single C atom since the binding energy of the C adatom to tetrahedral and butterfly Ir$_4$ is much larger ($\sim$ 1.7 eV) than that to the square one, both in the gas-phase and on MgO(100). After carbon adsorption, the most stable structure of Ir$_4$ is the butterfly geometry for free clusters and the ``tetrahedral'' one for Ir$_4$/MgO(100). The C adatom binds in a bridge configuration in all cases and reduces the distortions produced by the MgO substrate. Energetics and equilibrium atomic geometries will also be discussed for the adsorption ({\it i}) one H or O atom and ({\it ii}) one CO molecule. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y37.00006: Control and manipulation of Au nanocatalysis: effects of metal oxide support thickness and composition Bokwon Yoon, Chris Harding, Vahideh Habibpour, Sebastian Kunz, Adrian Nam-Su Farnbacher, Ueli Heiz, Uzi Landman Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses, are explored through temperature programmed reaction measurements on mass selected 20-atom gold clusters and via first-principles density-functional theory calculations. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films, and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low and high temperature CO oxidation reactions are calculated and analyzed. Along with the oxidation reactions on stoichiometric ultra thin MgO films we also study reactions catalyzed by Au$_{20}$ nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo, and on defect-rich thick MgO surfaces containing oxygen vacancy defects. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y37.00007: Role of anharmonic contributions for the elasticity of ice Mira Todorova, Lars Ismer, J\"org Neugebauer Water, one of the simplest molecules in chemistry, forms a liquid and solid phase with features essential to live and environment. Many of these can be attributed to hydrogen bonding, but that does not mean that they are fully understood. Ice should be an easier material to understand, because its molecules are arranged on a regular lattice. Yet even the determination of such basic properties as the bulk modulus and the elastic constants proves to be a challenge. Using first principles calculations we investigate the bulk properties of hexagonal ice. Our initial density-functional theory calculations (GGA-PBE level) yield values, which are much too high when compared to experiment. Even though the consideration of thermal effects within the quasi-harmonic approximation leads to a qualitative agreement between measured and calculated quantities, such as the linear expansion coefficient, ice remains much too hard. The large overestimation of the ice' softness demonstrates the importance of anharmonic contributions, which will be shown to be crucial and lead to a dramatic reduction of the bulk modulus and the elastic constants. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y37.00008: An Assessment of Hubbard U Corrections on Manganese Oxide Clusters Elise Y. Li, Davide Ceresoli, Nicola Marzari Conventional density-functional approaches often fail in offering an accurate description of the spin-resolved energetics in transition metal complexes, due to spurious self-interaction errors (SIE). Previous studies have shown that a self-consistent DFT + U approach [1] can accurately correct SIE in TM complexes, providing excellent agreement with high-level quantum chemistry calculations. In this work we report a systematic evaluation of DFT + U in a series of small manganese oxide clusters (MnO$_{x}$, x=1-4) and their anions, focusing on structural, electronic and magnetic properties. It is found that DFT + U succeeds in systems where the valence electrons are essentially localized on the TM ion, but progressively worsens when the coordination number increases and more covalency comes into play between the TM center and neighboring atoms, pointing to the importance of choosing the appropriate correlated manifold when Hubbard corrections are added. [1] H. J. Kulik, M. Cococcioni, D. A. Scherlis, and N. Marzari, Phys. Rev. Lett., 2006, 97, 103001 . [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y37.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y37.00010: Advances in a Joint Density-Functional Theory for Electronic Systems in Contact with Liquid Water: A New Form of Density Functional for Water Johannes Lischner, Tomas Arias We present a framework for studying complex electronic systems, such as biological molecules or electrochemical interfaces, that are dissolved in liquid water. The key ingredient, that renders calculations possible, is the usage of an approximate, yet accurate ``classical'' density-functional theory of water, while the electronic system is described by traditional Kohn-Sham theory. The electronic system (e.g. biosolute) is then coupled to the aqueous environment via molecular pseudopotentials. Here, we present a new form of ``classical'' density functional for water which is the first to properly account for the structure of the molecule in an exact way. We accomplish this by writing the free energy in terms of three effective potentials, one for the oxygen and one for each hydrogen atom, in which fictitious non-interacting water molecules move. In this talk, we will show that the resulting functional correctly reproduces the following properties of water: the linear and {\em nonlinear} dielectric response, the site-site correlation functions, the surface tension, the bulk modulus of the liquid and its variation with pressure, the density of the liquid and the vapor phases, and their coexistance. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y37.00011: The AM05 density functional applied to the water molecule, dimer, and bulk liquid Ann E. Mattsson, Thomas R. Mattsson We show that the AM05 exchange-correlation density functional (Armiento and Mattsson, Phys.~Rev.~B~{\bf 72}, 085108 (2005)) yields a H$_2$O dimer binding energy of 4.9 kcal/mol. The result is thus within 0.15 kcal/mol of CCSD(T) level theory ($5.02 \pm 0.05$ kcal/mol). We compare the AM05 results with those of five other functionals: LDA, PBE, PBEsol, RPBE, and BLYP. For liquid water, AM05 yields an O-O pair correlation function that is more structured than the ones of PBE and BLYP, which, in turn, are more structured than the one of RPBE. However, LDA and PBEsol yields more structured water than AM05. We confirm that accuracy in the water dimer binding energy is not a strong indicator for the fidelity of the resulting structure of liquid water. We will also report on the performance of AM05 for other systems and discuss the sub-system functional scheme used in the construction of AM05. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y37.00012: Heat capacities of aluminum clusters Anne Starace, Baopeng Cao, Oscar Judd, Martin Jarrold Clusters of certain elements are known to undergo phase transitions from solid-like to liquid-like states. Aluminum clusters have emerged as a model system for metal cluster phase transitions [1]. We report here the measurement of heat capacities of cationic clusters containing 84 to 127 Al atoms using a multi-collision induced dissociation mass spectrometry method [2]. We find two major changes in the heat capacities with increasing cluster size: (1) the fluctuations in the temperature of the phase transition vary more smoothly and (2) the peaks in heat capacity become sharper. Furthermore, we have found a range of cluster sizes (115-117 atoms) that contain two distinct peaks, separated by baseline, in their heat capacities. The origin of the extra peaks in the heat capacity, which is suspected to be due either to a pre-melting transition or to a solid-to-solid transition prior to the melting transition, will be further investigated by means of annealing experiments. The current work extends prior work on singly charged Al cluster cations having 16-83 atoms [2, 3]. [1] Breaux, G. A.; Neal, C. M.; Cao, B.; Jarrold, M. F. Physical Review Letters 2005, 94. [2] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Journal of the American Society for Mass Spectrometry 2007, 18, 74-81. [3] Neal, C. M.; Starace, A. K.; Jarrold, M. F. Physical Review B 2007, 76. [4] This work is supported by NSF. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y37.00013: From Atomic Clusters towards Nano-Materials with Controlled properties S.N. Khanna, M.C. Qian, A.C. Reber, J.U. Reveles, R. Robles, P.A. Clayborne, S.V. Ong, K. Casalenuovo, A.W. Castleman Jr., A. Sen, P.W. Weiss, H. Saavedra, A. Ugrinov, N. Chaki One pathway towards the synthesis of nanomaterials with controllable properties is to assemble solids using chosen clusters as the building blocks. The talk will outline a new protocol that enables synthesis of nanomaterials from clusters and highlight how the character of the cluster emerges in the assembled material. Through studies on assemblies involving polyvalent anions As$_{7}^{-3}$ and As$_{11}^{-3}$ and alkali based cations, we will show how the studies can provide novel ways of controlling the bandgap through energy level of the countercation and the degree of charge transfer. The theoretical predictions will be compared with experimental findings. [Preview Abstract] |
Session Y38: Surfaces, Interfaces, and Colloids I
Sponsoring Units: DCPChair: Sunil Sainis, Rowland Institute, Harvard University
Room: 410
Friday, March 20, 2009 8:00AM - 8:12AM |
Y38.00001: Aqueous Solutions on Silica Surfaces: Structure and Dynamics from Simulations Alberto Striolo, Dimitrios Argyris, Naga Rajesh Tummala Our group is interested in understanding the properties of aqueous electrolyte solutions at interfaces. The fundamental questions we seek to answer include: (A) how does a solid structure perturb interfacial water? (B) How far from the solid does this perturbation persist? (C) What is the rate of water reorientation and exchange in the perturbed layer? (D) What happens in the presence of simple electrolytes? To address such topics we implemented atomistic molecular dynamics simulations. Recent results for water and simple electrolytes near silicon dioxide surfaces of various degrees of hydroxylation will be presented. The data suggest the formation of a layered aqueous structure near the interface. The density profile of interfacial water seems to dictate the density profiles of aqueous solutions containing NaCl, CaCl2, CsCl, and SrCl2 near the solid surfaces. These results suggest that ion-ion and ion-water correlations are extremely important factors that should be considered when it is desired to predict the distribution of electrolytes near a charged surface. Our results will benefit a number of practical applications including water desalination, exploitation of the oil shale in the Green River Basin, nuclear waste sites remediation, and design of nanofluidic devices. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y38.00002: Formation of Organic Peroxides and Ethers at Post-Discharge Plasma Plume-Liquid Interfaces Milan Begliarbekov, Steven Kotowich, Vladimir Tarnovsky A direct current (DC) micro-hollow cathode plasma source operating in a mixed glow-streamer regime was used to generate an atmospheric pressure N$_{2}$ discharge. The post-discharge plume / afterglow was interfaced with a target liquid-phase solution, and caused a change in the chemistry of the target solution. In the present work we study the interaction of an N$_{2}$ plume with a mixture of 2-methyl-1-propanol and hexane, which results in the formation of organic peroxides and ethers at the plume-liquid interface. The presence of the peroxide and ether functional groups is established by $^{1}$H-NMR, FTIR, and Raman spectra of the reaction products. Fast Atom Bombardment (FAB) mass spectrometry is also used to further characterize the reaction products. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y38.00003: Deducing 2D Crystal Structure at the Solid/Liquid Interface with Atomic Resolution by Combined STM and SFG Study Arthur McClelland, Seokhoon Ahn, Adam J. Matzger, Zhan Chen Supplemented by computed models, Scanning Tunneling Microscopy (STM) can provide detailed structure of 2D crystals formed at the liquid/solid interface with atomic resolution. However, some structural information such as functional group orientations in such 2D crystals needs to be tested experimentally to ensure the accuracy of the deduced structures. Due to the limited sensitivity, many other experimental techniques such as Raman and infrared spectroscopy have not been allowed to provide such structural information of 2D crystals. Here we showed that Sum Frequency Generation Vibrational Spectroscopy (SFG) can measure average orientation of functional groups in such 2D crystals, or physisorbed monolayers, providing key experimental data to aid in the modeling and interpretation of the STM images. The usefulness of combining these two techniques is demonstrated with a phthalate diesters monolayer formed at the 1-phenyloctane/ highly oriented pyrolytic graphite (HOPG) interface. The spatial orientation of the ester C=O of the monolayer was successfully determined using SFG. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y38.00004: Curved space crystallography at an oil-water interface William Irvine, Stefano Sacanna, Yael Roichman, Andrew Hollingsworth, Mark Elsesser, Mark Bowick, David Grier, Paul Chaikin We study two-dimensional crystallography on a curved oil-water interface. Charged hydrophobic (PMMA) colloids in an oil phase (cyclohexyl bromide) are attracted, without wetting, by image charge effects to an oil-water interface. The micron size spheres form a monolayer on the interface and interact via screened coulomb interactions to form a crystalline lattice. We create a curved oil-water interface by controlling wetting conditions between a water droplet and a substrate or support, to produce interfaces of both constant and varying gaussian curvature with boundary. We simultaneously image and manipulate the full crystal on the curved surface using a setup capable of simultaneous holographic optical tweezing and confocal imaging. We study the resulting dynamics of topological defects. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y38.00005: On the nanometer Gold projectile - surface interaction in SIMS experiments. Francisco A. Fernandez-Lima, Veronica Pinnick, Michael Eller, Stanislav Verkhoturov, Emille Schweikert In an effort to increase the secondary molecular ion signal under ion bombardment, a series of cluster sources have been developed with sputtering yields that deviate from the linear cascade prediction due to the collective cluster beam - surface interaction. In the present talk, the variation of the massive gold Au$_{n}^{q+}$ projectile size (n=1-400) and velocity on the interaction volume and number of desorbed/sputtered particles per impact will be discussed for alkali halide targets. As the projectile size increases, a larger number of cluster ions is observed, where the secondary ion yield can be describe as a decreasing exponential function of the cluster size. Theoretical ab initio calculations show that the relative MS abundances are related to the cluster structure stabilities, defined by a ``fine'' ratio of short and long range interactions between the cluster counterparts. Angular distribution measurements of the secondary ions suggest that under keV bombardment emission normal to the target surface is favored, independent of the cluster ion size. Applications of the massive gold Au$_{n}^{q+}$ projectiles as nanometric imaging probes ($<$ 10$^{4}$ nm$^{2})$ will be presented. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y38.00006: Binding energy of adsorbates on a noble-metal surface: Exchange and correlation effects Michael Rohlfing, Thomas Bredow We discuss the physisorption of atoms (xenon) and molecules (PTCDA) to a noble-metal surface (silver) within a first-principles approach, focusing on the adsorption energy as a function of distance. Instead of density-functional theory (which fails to describe physisorption) we employ a combination of exact exchange and correlation energies, which we evaluate within the adiabatic-connection fluctuation-dissipation theorem. Correlation accounts for non-local dispersion energy, which is crucial in the present cases. At large distance {$Z$} from the surface the correlation causes van der Waals attraction with a characteristic $-C_3/(Z$$-$$Z_0)^3$ asymptotic form. At closer distance the attraction deviates from the asymptotic form and, in combination with the repulsive exact-exchange energy, yields an equilibrium of xenon and of PTCDA on the Ag(111) surface in close agreement with experiment. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y38.00007: Bonding of adenine on Cu(110) Geoffrey Stenuit, Oksana Plekan, Vitaliy Feyer, Kevin Prince, Paolo Umari We present a density functional study of the adsorption of adenine molecules on the Cu(110) surface. In agreement with experimental core level photoemission and x-ray absorption data, our calculations predict the existence of two phases: a parallel one at low coverage and a perpendicular one at high coverage. These findings resolve contradictions between calculated geometries and published vibrational spectra, and illustrate the complexity of the interaction between a relatively simple bio-molecule and a metal. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y38.00008: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y38.00009: Two-photon photoemission spectroscopy of thiophene/Au(111). Jing Zhou, Nicholas Camillone, Michael White The electronic structure of thiophene adsorbed on Au(111) is investigated by two-photon photoemission (2PPE) spectroscopy and density functional theory (DFT) calculation. The adsorption of thiophene lowers the work function from 5.50 eV for clean Au(111) to 4.62 eV for Au(111) exposed to 4.0 L thiophene, due to the electron donation from the thiophene to the substrate. With thiophene adsorbed on Au(111), the surface state of Au(111) attenuates and a localized $\sigma $* starts to form with increasing thiophene exposure on Au(111). This $\sigma $* state is attributed to the $\sigma $* antibonding orbital of a Au-S bond and is evidence of an orientational phase transition of adsorbed thiophene. Preliminary 2PPE results will also be presented for aromatic molecules bound to the Au surface via sulfur or other functional goups (e.g., isocyanide). [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y38.00010: Interfacial Structure imaging of Pentacene/Si(111) by model- independent method Songtao Wo, Hua Zhou, Randall Headrick, Alexander Kazimirov Synchrotron x-ray reflectivity is utilized to study the Pentacene/Si(111) interfacial structure in the direction normal to the surface. Model-independent algorithm is used to analyze the reflectivity data to extract the electron density profile. It indicates two partially ordered layers along the interfacial normal with thickness $\sim $0.6 nm and an interfacial water layer $\sim $0.9 nm as we reported in our previous work. A pentacene monolayer $\sim $1.6 nm can also be revealed. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y38.00011: Optical Trapping of Colloidal Nanoparticles by a Weakly Focused Laser Beam Chun-Yu Lin, Hsia-Yu Lin, Shean-Jen Chen, Steven M.T. Wei, H.D. Ou-Yang We present an analysis of the behavior of an ensemble of colloidal nanoparticles in the focal region of a weakly focused laser beam. Using a mechanical balance of the laser radiation pressure that causes particle migration into the light field and the osmotic pressure of these particles opposing migration, we propose a new method for quantifying the optical trapping potential of individual particles by measuring the increase of the particle concentration as a function of the laser intensity. We find comparable results for the optical trapping potential from this method with values obtained by single particle trapping methods, indicating that radiation-induced particle convection from a weakly focused laser beam does not affect the steady state distribution of the particles in the light field. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y38.00012: Colloidal Crosstalk: Brownian Diffusion of Hydrodynamically Coupled Colloids Stephen Anthony, Minsu Kim, Steve Granick Except at dilute concentrations, the Stokes-Einstein Equation inadequately describes the thermal motion of colloids, due to hydrodynamic interactions between nearby particles. Using single-particle tracking, the rotational and translational motion of hydrodynamically interacting colloids is observed, and deviations from the Stokes-Einstein Equation are readily apparent. The thermal motion of nearby colloids is found to couple in a non-additive fashion, with increasing degrees of non-linearity as particle separation decreases. Similar coupling is observed for colloids near surfaces. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y38.00013: Depletion-Driven Selective Optical Trapping in Nanoparticle Suspensions Yi Hu, Joseph Junio, H.D. Ou-Yang We report results of an optical trapping study that demonstrates the effects of size-asymmetric particles in suspension have on optical trapping efficiency. Using a model colloid system with selective fluorescent dying and particle sizing, we show that the trapping efficiency of nanoparticles can be effectively tuned by adding different sized particles, promoting the use of optical trapping for particle sorting. A variable power IR laser coupled into a high NA objective was used for trapping. For particle detection, we used a 532nm excitation laser aligned to be parfocal with the IR trapping beam through the same objective lens. Fluorescent signals emanating from the focal region common to both beams was band-passed to a pinhole set to be conjugate to the focal region for confocal detection. In a system composed of 160nm and 63nm particles we demonstrated the synergistic effect of size mixing. Experimental results are also shown for fluorescent particles being driven out of the region by size selective trapping of undyed 160nm particles. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y38.00014: Colloid diffusion on phospholipid membranes is anomalous Bo Wang, Stephen Anthony, Sung Chul Bae, Steve Granick We demonstrate experimentally a simple system in which mean-squared displacement is proportional to time yet the distribution function of displacement probability is exponential, not Gaussian as expected for a classical random walk. This is the case of fluorescent submicron-sized beads that diffuse in one dimension in the smooth potential presented by tubes composed of phospholipid bilayer.~ A discussion of possible physical origins suggests that a family of physical systems whose few degrees of freedom couple to slow environmental fluctuations may behave analogously. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y38.00015: Determination of Charge Interactions of Nanoparticles by Optical Trapping Joseph Junio, H.D. Ou-Yang We report an experimental study of interactions in colloidal nanoparticles through optical trapping. Using an optical trap with a size much larger than the trapped particles, we were able to create an optical bottle to confine and concentrate the nanoparticles. We measured the highly focused light-induced particle density fluctuation with confocal fluorescent detection. A theory based on a balance between the optical trapping radiation pressure and the osmotic pressure has been developed to calculate the isothermal osmotic compressibility from the forced density fluctuation. The measured osmotic compressibilities of colloidal crystals are then used to determine the surface charge density of the colloidal particle(1). Comparison of the experimentally determined charge density is compared to t determined by zeta potential measurements. (1)S. Alexander, P. M. Chaikin, P. Grant, G. J. Morales, and P. Pincus, D. Hone, Charge renormalization, osmotic pressure, and bulk modulus of colloidal crystals: Theory, J. Chem. Phys. 80, 5776 (1984) [Preview Abstract] |
Session Y39: Techniques in Biophysics
Sponsoring Units: DBPChair: Andrea Liu, University of Pennsylvania
Room: 411
Friday, March 20, 2009 8:00AM - 8:12AM |
Y39.00001: The Molecular Splash Test: when applied physics help stroke patients. Cedric Hurth, Lena van Nimwegen, Deepthi Jampala, Kris Vijay, Frederic Zenhausern The Molecular Splash Test (MST) stems from recent observations of a solid sphere producing a splash when impacting a liquid (Bocquet et al, 2007, \textit{Nature Physics}, \textbf{3}, 180). We have developed the experimental setup incorporating a high-speed camera operated at 8,000 frames/s to perform biologically-relevant experiments, e.g. rheology studies from the impact of a native glass bead and molecular recognition tests when the impacting bead is functionalized with biomarkers. We present preliminary results with millimeter-sized glass beads impacting water mixtures of increasing glycerol content, i.e. increasing viscosities, as well as biotin-modified glass beads impacting avidin solutions (0.1 -- 1 mg/mL). The viscosity increase of a human blood sample containing heparin, once heparinase II is added to re-induce coagulation, is monitored over time to assess the ability of MST to function on samples of medical interest. The beads can then be derivatized with an antibody for detection of C-reactive protein in blood serum. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y39.00002: Surfaced-Enhanced Raman Scattering of $\lambda $-DNA Diane Alvarez, Jiandi Zhang, Hong Wei, Hongxing Xu The ability to engineer metal particles at the nanoscale in which plasmons can be excited, directed, and manipulated has led to the rapid development of the field of ``plasmonics''. Here we demonstrate that the Raman scattering of $\lambda $-DNA molecules with colloidal silver nanoparticles is drastically enhanced by surface plasmon excitations. Colloidal silver nanoparticles ($\sim $ 90 nm size) were assembled onto DNA molecules using the molecular combing method. Surface-enhanced Raman scattering (SERS) spectra were obtained and compared for different solution concentrations of the DNA/Ag system. It is evident that the SERS peaks were shifted to a lower or higher wavenumber, depending on the concentration of the solution. These different shifts of Raman frequencies may indicate that the different stretching states of DNA molecules in different concentrations probably influence the Raman frequencies. It is speculated that the coiling states of DNA molecules might be different in different concentrations, thus making it a promising method for the study of DNA functionalities and DNA-nanoparticle interactions. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y39.00003: Sub-cellular structure studied by combined atomic force-fluorescence microscopy Andreea Trache A novel experimental technique that integrates atomic force microscopy (AFM) with fluorescence imaging was used to study the role of extracellular matrix proteins in cellular organization. To understand the mechanism by which living cells sense mechanical forces, and how they respond and adapt to their environment, we developed a new technology able to investigate cellular behavior at sub-cellular level that integrates an AFM with total internal reflection fluorescence (TIRF) microscopy and fast-spinning disk (FSD) confocal microscopy. Live smooth muscle cells exhibited differences in focal adhesions and actin pattern depending on the extracellular matrix used for substrate coating. Data obtained by using the AFM-optical imaging integrated technique offer novel quantitative information that allows understanding the fundamental processes of cellular reorganization in response to extracellular matrix modulation. The integrated microscope presented here is broadly applicable across a wide range of molecular dynamic studies in any adherent live cells. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y39.00004: Expanding the applicability of multi-photon fluorescence recovery after photobleaching \textit{in vivo} by incorporating convective flow into the recovery model Kelley Sullivan, William Sipprell, Edward Brown, Jr., Edward Brown, III Multi-photon fluorescence recovery after photobleaching is a well-established microscopy technique used to study diffusion, with expanding applications \textit{in vivo}. We present a new model of fluorescence recovery that explicitly includes the effect of convective flows within a system, thereby improving the efficacy of the technique \textit{in vivo}, where convective flows are omnipresent. We test this ``flow'' model through both simulations and \textit{in vitro} experimentation, and demonstrate the effectiveness of the new model \textit{in vivo}. Our results show that the flow model significantly improves the capabilities of multi-photon fluorescence recovery after photobleaching \textit{in vivo}, by enabling an accurate determination of the diffusion coefficient, even when significant flows are present. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y39.00005: Electrophoretic Migration of Branched DNA in Polymer Solutions Henry Lau, Lynden Archer The electrophoretic migration of large, star-branched DNA molecules has previously been studied in both neutral polymer solutions and gels, and the results have provided insight into the local interactions between the analytes and the sieving matrix during electrophoresis (Electrophoresis, 2006, 27, 3128). This talk focuses on using rigid-rod DNA molecules of complex shapes as model analytes in studying the effects of analyte architecture on mobility in polymer solutions. Electrophoresis of a series of Y-shaped DNA molecules that mimick the shapes of antibodies, was performed in polymer solutions above the overlap concentration and at electric fields up to 300V/cm. The location of the branch point as well as the arm sizes are varied in order to examine their influence on mobility. Our results point to novel, topology-based fractionation strategies for separating biological molecules using capillary electrophoresis with polymer sieving media. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y39.00006: Time-resolved dielectric spectroscopy of protein aggregation performed on model system of hen lysozyme and beta-lactoglobulin Brian Mazzeo, Andrew Flewitt Time-resolved dielectric spectroscopy measurements of solutions containing hen lysozyme and beta-lactoglobulin reveal changes in electrical configuration and hydrodynamic parameters during their interaction. These measurements were performed in a temperature-controlled dielectric cell connected to an HP4194A impedance analyzer. The protein titrations were performed by sequential additions of reacting proteins. Differential spectra reveal the electrical contributions by each species. The computer-controlled measurements and relevant post-processing of the obtained spectra allow quantitative extraction of reaction parameters. This is demonstrated for a model system of proteins consisting of hen lysozyme and beta-lactoglobulin. Reorientation time constants, dielectric increments, and relaxation spread parameters are plotted against time and indicate binding processes. The technique is demonstrated to be a useful analytical tool for monitoring reactions in biological and colloidal systems. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y39.00007: ``Shooting Bead'' Method with Filament Energy Loss Consideration for Finding the Flexural Rigidity of the Rodelike Biological Filaments Abdorreza Samarbakhsh, Jack Tuszynski Flexural rigidity is one of the important characteristics of flexible polymers including biological filaments. For elastic deformation it is analogous to the spring constant in the Hook's law for bending. In this work we propose a new method for experimentally evaluating the cantilever stiffness and flexural rigidity of semiflexible rodlike biological filaments based on the measurement of just two distances. The method is based on applying a force normal to the filament with a microsphere bead trapped in the laser tweezer followed by its sudden release. Through two simple measurements of the initial and final position of the bead, the cantilever stiffness and flexural rigidity of the filament can be found from the formula that has been provided. In the second part, the effect of filament radius has been taken into account and a new formula with filament energy loss consideration, for flexural rigidity and the cantilever stiffness has been found. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y39.00008: Electrostatic Force Microscopy Identification of Different Peptide Structures Casper Hyttel Clausen In this work electrostatic force microscopy (EFM) was used to distinguish between different dipeptides tubes, silver filled peptides, spheres and silver wires, all the samples were placed on pre fabricated SiO$_{2}$ surfaces with a backgate under ambient conditions. The EFM method used for the experiments was force gradient signal, which uses a dual scan approach in order to minimize the atomic force interactions. The investigation shows that it is possible to distinguish between the three types of structures. Further an agreement between the detected signal and the structure of the hollow peptide was demonstrated. These measurements only show qualitative agreement with the mathematical expressing for the peptide tubes. Further during EFM mapping of the silver filled peptide structures showed a changing effect between the tip and the sample. Investigations of this effect were carried out in order to get a better understanding of the physical properties of the peptide structures. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y39.00009: Specific Detection of Vascular Endothelial Growth Factor Using Microcantilever Resonators. Jason Francis, Stephanie Archer, Lisa Holland, David Lederman We demonstrate the specific detection of vascular endothelial growth factor, a protein indicated in tumor angiogenesis, using resonant frequency shifts in microcantilevers due to mass loading, avoiding the need to use tagged antibodies and multiple reagents as is needed with enzyme linked immunosorbent assays (ELISA). Cantilever surfaces were functionalized using F(ab') fragments linked to gold surfaces via their native thiol groups, eliminating the need for complex linking processes. The cantilever surfaces were then passivated with bovine serum albumin to minimize shifts due to nonspecific binding. This technique allows the detection of pg/mL-level concentrations of analyte. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y39.00010: Positioning and guidance of neurons on Au by directed assembly of proteins using Atomic Force Microscopy. Cristian Staii, Chris Viesselmann, Jason Ballweg, Justin Williams, Erik Dent, Susan Coppersmith, Mark Eriksson The specific interactions between neurons and guidance factors as well as the mechanism of axonal navigation toward a target in the developing brain are not well understood. To address this problem we present a new approach for controlling the adhesion, growth and interconnectivity of cortical neurons on Au surfaces. Specifically, we use AFM nanolithography to immobilize extracellular matrix proteins at well-defined locations on Au surfaces, and show that these protein patterns can confine neuronal cells and control their growth and interconnectivity. We will compare this method with other nanofabrication techniques and discuss its main advantages: 1) the procedure is carried out in aqueous solutions, so that the proteins retain their bioactivity, 2) a high degree of control over location and shape of the protein patterns can be achieved, and 3) the minimum protein feature size can be as small as 50nm. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y39.00011: Determination of Frank-Oseen parameters in collagen using polarization modulated second harmonic signal. Clayton Bratton, Karen Reiser, Andre Knoesen, Diego Yankelevich, Mingshi Wang, Israel Rocha - Mendoza A method is presented for determining the Frank-Oseen parameters for the elastic modulus of collagen based on analysis of polarization-modulated second harmonic signal (PM-SHG). The liquid crystal structure of collagen and its associated order parameter, the director field, were characterized in samples of tendon and annulus fibrosus.~ Deformation of the director field caused by controlled stress loading or heating was assessed. Three distinct curvature strains--splay, twist, and bend---were determined, using the PM-SHG data. This optical technique permits highly localized determination of the three major elastic deformations. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y39.00012: Direct measurement of the non-conservative force field generated by optical tweezers Pinyu Wu, Rongxin Huang, Christian Tischer, Alexandr Jonas, Ernst-Ludwig Florin Optical tweezers have been widely used in soft condensed matter physics and biophysics to measure forces in molecular processes on the single molecule level. The usual assumption is that the force applied to a particle confined with the tweezers is directly proportional to the displacement of the particle from the trapping center, which would imply that the force field of the tweezers is conservative. However, the Gaussian beam model indicates that this force field is actually non-conservative, yet no experiments have measured this effect. We developed a new experimental method that can directly measure the force field with femtonewton precision without assuming its conservative character. We successfully obtained the 3-D force field for an optically trapped Rayleigh particle with 10~nm resolution by analyzing its Brownian motion. We found a non-conservative contribution that increases as the trapped particle moves away from the optical axis. In the light of this finding, optical trapping experiments that assumed a conservative force field may need careful reevaluation. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y39.00013: SERS-Active Nanoinjector for Intracellular Spectroscopy Elina Vitol, Zulfiya Orynbayeva, Michael Bouchard, Jane Azizkhan-Clifford, Gary Friedman, Yury Gogotsi We developed a multifunctional nanopipette which allows simultaneous cell injection and intacellular surface-enhanced Raman spectroscopy (SERS) analysis. SERS spectra contain the characteristic frequencies of molecular bond vibrations. This is a unique method for studying cell biochemistry and physiology on a single organelle level. Unlike the fluorescence spectroscopy, it does not require any specific staining. The principle of SERS is based on very large electromagnetic field enhancement localized around a nano-rough metallic surface. Gold colloids are widely used SERS substrates. Previously, the colloidal nanoparticles were introduced into a cell by the mechanism of endocytosis. The disadvantage of this method is the uncontrollable aggregation and distribution of gold nanoparticles inside a cell which causes a significant uncertainty in the origin of the acquired data. At the same time, the nanoparticle uptake is irreversible. We present a SERS-active nanoinjector, coated with gold nanoparticles, which enables selective signal acquisition from any point-of-interest inside a cell. The nanoinjector provides a highly localized SERS signal with sub-nanometer resolution in real time. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y39.00014: Evaluating Epithelial Mechanics with Laser Hole-drilling M. Shane Hutson, David N. Mashburn, Xiaoyan Ma, Holley E. Lynch During the development of an organism, sheets of epithelial cells expand, contract and bend due to intra- and intercellular forces. We have previously developed laser hole-drilling as a technique to probe such epithelial mechanics -- with a focus on the ms-to-s dynamic recoil of single, directly adjacent cell edges. Here we extend the analysis to consider ablation-induced deformations for the entire field of surrounding cells. We treat each epithelium as a homogeneous, linearly elastic, thin sheet. This simplification provides analytical solutions for the expected strain relaxation after hole-drilling (under either plane stress or plane strain). We have developed routines that use these analytic mappings (plus potential rigid body motions) to warp and match pre- and post-drilling images. These mappings account for the majority of the observed deformations and allow one to estimate the epithelium's Poisson ratio and pre-drilling average strain tensor (which yields the anisotropy and direction of principle stress/strain). The unaccounted, residual displacements provide clues to how each epithelium deviates from a homogeneous sheet. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y39.00015: Top-Down Fabricated Silicon Nanochannel Field-Effect Transistors for Biosensing Applications Yu Chen, Xihua Wang, Shyamsunder Erramilli, Pritiraj Mohanty Silicon nanochannel field-effect transistors have great promise for biomolecular sensing. The sensitivity is enhanced at the nanoscale due to the large surface-to-volume ratio. Specificity is achieved by functionalizing the devices with selected antibodies or complimentary target molecules. These devices are important as building blocks for high density bionanoelectronics. Top-down fabrication of these devices is compatible with advanced microfabrication processes. We show top-down fabricated silicon nanochannel devices with 3-dimensional relief can serve as a platform for biosensing applications. Three sides of the silicon nanowire are covered with a thin Al2O3 layer using Atomic Layer Deposition to form an insulating layer. When the surface is modified for binding to specific biomarkers, the device conductance change can be used to detect binding events through a field effect. Applications include building immunosensors to detect the breast cancer antigen 15-3 and other protein biomarkers, and constructing enzyme-based sensors to detect metabolites like glucose and urea. [Preview Abstract] |
Session Y40: Physiological and Medical Physics
Sponsoring Units: DBPChair: Krastan Blagoev, National Science Foundation
Room: 412
Friday, March 20, 2009 8:00AM - 8:12AM |
Y40.00001: Biophysical investigation of the apoptotic force Yusuke Toyama, Xomalin Peralta, Adrienne Wells, Daniel Kiehart, Glenn Edwards Understanding tissue dynamics during development requires knowledge of how cells produce and respond to forces. We have experimentally shown that apoptosis (programmed cell death, which remodels tissue by eliminating cells) also contributes a significant tissue force that promotes cell sheet fusion during dorsal closure in Drosophila development [Science, 321, 1683 (2008)]. By genetically suppressing (enhancing) apoptosis, we slow (increase) the rate of dorsal closure. These changes correlate with the forces produced by the amnioserosa tissue and the rate of seam formation (zipping) for two advancing sheets of lateral epidermis. This apoptotic force is used to drive cell sheet movements during development, a role not classically attributed to apoptosis. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y40.00002: Reorganization of neuronal circuits in growing visual cortex Wolfgang Keil, Siegrid Loewel, Fred Wolf, Matthias Kaschube The dynamics of reorganization of large cortical circuits is rooted in plasticity of individual synapses, but rules governing the collective behavior of large networks of neurons are only poorly understood. The postnatal brain growth partly evoked by extensive formation of new synaptic connections may expose cortical areas to a 'natural perturbation' sufficiently strong to observe signatures of large scale reorganization. Quantifying large sets of imaging data from juvenile cat visual cortex, we observe a novel mode of reorganization of domains that prefer inputs from one eye or the other. Our theoretical analysis shows that this mode can be explained quantitatively by the so called Zigzag instability, a dynamical reorganization, well-known in the field of pattern formation in physics, by which 2D isotropic Turing patterns respond to an increase in their typical spatial scale with a zigzag-like bending of domains. We point out that this instability has in fact been predicted, albeit implicitly, by most models of visual cortical development that have been proposed so far. We conclude that cortical networks can undergo large scale reorganizations during normal postnatal development. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y40.00003: Epithelial oscillations enhance signal detection in a peripheral sensory system Alexander Neiman, Tatiana Engel, David Russell, Brian Helbig, Lutz Schimansky-Geier Rhythmic spontaneous activity was observed in various peripheral sensory systems. Many sensory receptors have a specific structure where detector cells in a sensory epithelium excite primary afferent neurons. We explore how stochastic oscillations of epithelial cells affect the ability of peripheral receptors to detect weak stimuli. We use a simple analytically tractable model to contrast signal detection in two situations: (i) when epithelial oscillations are coherent, and (ii) when the coherence of epithelial oscillations is destroyed. We show that coherent epithelial oscillations decrease the variability of neuronal firing and thus to enhance discriminability of weak signals. Model predictions are supported by the analysis of experimental data from the electroreceptors of paddlefish. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y40.00004: Sound Localization in Lizards: Functioning of a Pressure-Gradient Receiver J. Leo van Hemmen Because of their small interaural distance, lizards as well as some other animals have developed a special hearing mechanism, the ``pressure-gradient receiver''. The lizard peripheral auditory system differs from the mammalian one by a coupling of the two eardrums through the internal mouth cavity. We present a three-dimensional analytical model of the pressure-gradient receiver. The central aspect of the coupling of the membranes through the mouth cavity is realized by means of the boundary conditions. Moreover, the lizard's middle ear, a simple lever construction called \textit{columella}, is asymmetrically attached to the tympanic membrane. This has motivated us to solve the problem of how the middle ear influences the spatial-amplitude profile and the frequency distribution of the tympanic membrane vibration. Finally, we show results from numerical simulations of the eigenfunctions and eigenfrequencies in a lizard's internal mouth cavity bounded by the eardrums. To this end, we have constructed the complex geometry from a cast imprint of the cavity with the help of three-dimensional scans. Our results led to an interesting speculation regarding the neurobiological use of the pressure-gradient system. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y40.00005: Direct Neural Imaging using Ultra-Low Field Magnetic Resonance Karlene Maskaly, Michelle Espy, Mark Flynn, John Gomez, Robert Kraus, Andrei Matlashov, John Mosher, Shaun Newman, Tuba Owens, Mark Peters, J. Sandin, Larry Schultz, Algis Urbaitis, Petr Volegov, Vadim Zotev An enduring challenge in neuroscience is the accurate in vivo mapping of neural activity with high spatial and temporal resolution. A method being developed by our group tries to meet this challenge by using Ultra-Low Field (ULF) MRI. Other groups have attempted direct neural imaging (DNI) using high field MRI. However, the use of ULF presents two advantages. First, the susceptibility artifact at high fields, which masks the DNI signal, is negligible at low fields. Second, the reduced Larmor frequency at ULF may overlap with the frequency spectrum of the neural magnetic field, resonantly enhancing the MRI signal. In this presentation, I will first show our custom-built ULF MRI setups that have successfully produced ULF anatomical images. I will then highlight the numerous studies we have done to investigate the feasibility of DNI with these systems, including both experimental and theoretical studies. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y40.00006: Synaptic vesicle dynamics in Hippocampal Slices Krastan Blagoev, Denis Bragin, Wolfgang Mueller Synaptic vesicle pool dynamics in hippocampal slices have been observed using FM-dye as an activity-dependent contrast agent with two-photon microscopy. Separate vesicle pools for spontaneous and stimulated vesicle release and their dynamics, and vesicle exchange dynamics between the pools was inferred from the signal with and without stimulation. To interpret the experimental results we developed a multi-compartmental kinetic model of the FM-dye dynamics during loading and unloading of vesicles. Using this mathematical model we estimated the exchange rates between the synaptic vesicle pools and the resulting vesicle release dynamics. We will discuss important differences between the vesicle pool dynamics in ex-vivo brain slices and in disassociated neuronal cultures. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y40.00007: The surface properties of a lung mucus model system Markus Weygand, Beautia Dew, Stephen Garoff, Mathias L\"{o}sche, Todd Przybycien, Robert Tilton Adding surfactants to aerosol drug therapies may improve drug delivery by inducing surface tension gradient driven flows along the lung airway surfaces. Understanding the surface structure and properties of the mucus that lines the lung airways is crucial to the proper design of such formulations. In our studies, we use mucin solutions made from porcine gastric mucus as a model system. Surface tension measurements revealed a time and humidity dependence, which led us to investigate the structure of the air/solution interface using X-ray and neutron reflectivity. These studies reveal a compact adsorption layer at the air/liquid interface whose density distribution decays with a long tail extension into the bulk liquid. This structure showed only a minor dependence on the humidity above the mucus surface. To examine possible interaction between lipids present in the lung with the mucus, we deposited DPPC and DMPC onto the mucin solution surface and observed that the lipid layer remained on the solution surface for times long compared to the lifetime of mucus in the lung. The analysis of the reflectivity data impart a microscopic picture of the mucin solution surface and its alteration by lipid deposition. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y40.00008: Structural Measurements from Images of Noble Gas Diffusion Robert V. Cadman, Stephen J. Kadlecek, KiaRash Emami, John MacDuffie Woodburn, Vahid Vahdat, Masaru Ishii, Rahim R. Rizi Magnetic resonance imaging of externally polarized noble gases such as $^3$He has been used for pulmonary imaging for more than a decade. Because gas diffusion is impeded by the alveoli, the diffusion coefficient of gas in the lung, measured on a time scale of milliseconds, is reduced compared to that of the same gas mixture in the absence of restrictions. When the alveolar walls decay, as in emphysema, diffusivity in the lung increases. In this paper, the relationship between diffusion measurements and the size of the restricting structures will be discussed. The simple case of diffusion in an impermeable cylinder, a structure similar to the upper respiratory airways in mammals, has been studied. A procedure will be presented by which airways of order 2 mm in diameter may be accurately measured; demonstration experiments with plastic tubes will also be presented. The additional developments needed before this technique becomes practical will be briefly discussed. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y40.00009: Clusters of decelerations of heart rate appear to be a Hopf bifurcation, and provide early warning of illness in premature infants Abigail Flower, Randall Moorman, Douglas Lake, John Delos The pacemaking system of the heart is complex; a healthy heart constantly integrates and responds to extracardiac signals, resulting in highly complex heart rate patterns with a great deal of variability. In the laboratory and in some pathological or age-related states, however, dynamics can show reduced complexity that is more readily described and modeled. Reduced heart rate complexity has both clinical and dynamical significance -- it may provide warning of impending illness or clues about the dynamics of the heart's pacemaking system. Here we describe uniquely simple and interesting heart rate dynamics observed in premature human infants - reversible transitions to large-amplitude periodic oscillations. We propose a mathematical interpretation based on Hopf bifurcation theory. (Supported by NIGMS, by the National Heart, Lung, and Blood Institute, and by NSF, with computing support provided by William and Mary.) [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y40.00010: New Outlet Conditions for Three-Dimensional Computational Fluid Dynamic Simulations of Blood Flow in Arteries David Johnson, Ulhas Naik, Antony Beris In three-dimensional (3D) simulation of a component of the arterial network, we have the problem of properly specifying outlet conditions due to coupling with the rest of the arterial network. In this work we propose to use an approximate solution, based on a one-dimensional (in space) but time periodic approximation of the flow, in order to obtain these outlet conditions. These are used in fully 3D and time periodic computational fluid dynamic (CFD) simulations of a coronary arterial junction, using the commercial software Fluent. A consistent implementation requires an iterative procedure that has been developed based on a lubrication approximation. The application of these boundary conditions has been applied to both a normal/healthy coronary artery junction and a diseased case, where an occlusion has developed causing impairment of flow. Results will be shown that demonstrate significant changes to the solution. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y40.00011: Millimeter Wave Spectroscopy for Breast Cancer Diagnostics and Detection Konstantin Korolev, Shu Chen, Mohammed Afsar, Stephen Naber Broad-band millimeter wave transmittance measurements of normal and tumorous (cancerous) human breast tissue samples have been acquired \textit{in--vitro} by employing a free-space, quasi-optical spectrometer. Freshly excised breast tissues were prepared and preserved in 10{\%} neutral-buffered formalin solution before testing. Significant differences in the transmittance profiles have been found between the normal and tumorous tissues. It has been found that despite the inhomogeneity and variable structure and composition of each single tissue, the tumorous specimens consistently manifest much higher absorption level of millimeter wave radiation than the normal ones. It has been shown that free space, quasi-optical spectrometer is capable of contributing valuable insights into the dielectric properties of normal and tumorous human breast tissues and aiding in further developments of millimeter wave spectroscopy and mammography for the breast cancer diagnostics and detection. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y40.00012: Diffusion dependence of proton NMR relaxation rates in the presence of ferritin Michael Boss, P. Chris Hammel Ferritin is the predominant iron-storage protein in living organisms. It may serve as an indicator of neurodegenerative diseases such as Alzheimer's. Measuring brain ferritin concentration non-invasively via MRI could enable better diagnoses and treatments of such diseases. Quantitative MRI determination of the ferritin concentration requires an understanding of the NMR relaxation mechanisms of hydrogen protons in the presence of ferritin. In aqueous solutions, ferritin enhances the transverse relaxation rate (R$_{2})$ of the protons. This is thought to occur due to a diffusive mechanism, where protons diffusing near ferritin pass through a region of elevated magnetic field, and a chemical exchange mechanism, where protons bind to the protein for a period of time, experiencing an even higher magnetic field. These two mechanisms exhibit different dependencies on the self-diffusion coefficient of the protons. By adding glycerol to aqueous solutions, we control the self-diffusion of protons. We measure the R$_{2}$ of protons in ferritin-containing binary mixtures of water and glycerol using CPMG sequences, and compare the experimental results to theoretical predictions of diffusion dependence in order to distinguish the relative importance of the mechanisms. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y40.00013: Application of Histogram Analysis in Radiation Therapy (HART) in Intensity Modulation Radiation Therapy (IMRT) Treatments Anil Pyakuryal A carcinoma is a malignant cancer that emerges from epithelial cells in structures through out the body.It invades the critical organs, could metastasize or spread to lymph nodes.IMRT is an advanced mode of radiation therapy treatment for cancer. It delivers more conformal doses to malignant tumors sparing the critical organs by modulating the intensity of radiation beam.An automated software, \textbf{\textit{HART}} (S. Jang et al.,2008,Med Phys 35,p.2812) was used for efficient analysis of dose volume histograms (DVH) for multiple targets and critical organs in four IMRT treatment plans for each patient. IMRT data for ten head and neck cancer patients were exported as AAPM/RTOG format files from a commercial treatment planning system at Northwestern Memorial Hospital (NMH).HART extracted DVH statistics were used to evaluate plan indices and to analyze dose tolerance of critical structures at prescription dose (PD) for each patient. Mean plan indices (n=10) were found to be in good agreement with published results for Linac based plans. The least irradiated volume at tolerance dose (TD50) was observed for brainstem and the highest volume for larynx in SIB treatment techniques. Thus HART, an open source platform, has extensive clinical implications in IMRT treatments. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y40.00014: Role of Physique on Probability of Injury to the Low Back Saami J. Shaibani In a related study of the response of the upper and lower cervical spine[1], there was some correlation between a change in physique and the potential for injury to the neck during automotive events. A similar undertaking in this research on the lumbar spine and sacral spine revealed a much more marked effect, namely an increase in injury potential to the low back when weight is increased. Although there were some exceptions to this, the overall trend was distinct. This is perhaps to be expected when one considers that most additional weight at the same height tends to be located in the center or lower torso. However, it is first time in any comparable analysis of injury causation that there has been a more noticeable pattern for the low back than the neck. The latter was more pronounced with environment geometry, as seen when the height of the seat back was varied. Such changeability again reinforces previous findings that injury outcomes for individual patients cannot always be predicted by what happens in general. 1. http://meetings.aps.org/link/BAPS.2007.MAR.K1.2 (Role of physique on probability of injury to the neck). [Preview Abstract] |
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