Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S1: Pairing and Symmetry
Sponsoring Units: DCMPChair: John Kirtley, IBM
Room: LACC 152
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S1.00001: Robust d-wave pairing symmetry in hole-doped cuprate superconductors$\dagger$ Invited Speaker: After a debate over many years, it is widely agreed that an order parameter with $d_{x^2-y^2}$ symmetry has been established in optimally doped cuprate superconductors\footnote{C.C. Tsuei and J.R. Kirtley, Rev. Mod Phys. {\bf 72}, 969 (2000)}. The controversy has now shifted to the possibility of changes in pairing symmetry as a result of doping. In this talk, we will first report new results of a precise measurement on the location of nodes in the d-wave gap function at the Fermi surface of a high-T$_c$ superconductor YBa$_2$Cu$_3$O$_7$. We will then present a series of phase-sensitive tricrystal experiments to demonstrate, using the half-flux quantum effect, that the d-wave pair state in several cuprate systems is robust against a wide range of doping variations from under-doped, through optimal doping, to over-doped regimes. Implications of the findings of this work for understanding high-temperature superconductivity will be discussed at the end of the talk. \\ $\dagger$ work done with J.R. Kirtley, A. Ariando, Hans Hilgenkemp, G. Hammerl, J.Mannhart, H. Raffy, and Z.Z. Li. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S1.00002: Phase-sensitive test of odd-parity superconductivity in Sr$_{2}$RuO$_{4}$ Invited Speaker: Ying Liu The possibility of odd-parity, spin-triplet pairing of electrons was first pointed out in 1960 after the publication of the BCS theory for superconductivity. Sr$_{2}$RuO$_{4}$, the only Cu-free layered perovskite superconductor, was predicted to be a material featuring such an unusual pairing state. We have in the past several years pursued the phase-sensitive test of odd-parity pairing in Sr$_{2}$RuO$_{4}$ following the proposal of Geshkenbein, Larkin, and Barone (GLB). The experiment involves the preparation and the measurement of GLB SQUIDs consisting two oppositely faced Josephson junctions between an s-wave superconductor, Au$_{0.5}$In$_{0.5}$, and Sr$_{2}$RuO$_{4}$ as well as control samples. We found that the critical current of GLB SQUIDs was a maximum while for control samples of SQUIDs with two junctions prepared on the same side, the critical current is a minimum at zero magnetic flux. This indicates that the phase of the order parameter changes by \textit{$\pi $} under inversion, showing that Sr$_{2}$RuO$_{4}$ is indeed an odd-parity superconductor [1]. More recent results on corner and the same-side junctions of Au$_{0.5}$In$_{0.5}$-Sr$_{2}$RuO$_{4}$ will be presented. [1] K.D. Nelson, Z.Q. Mao, Y. Maeno, and Y. Liu, Science 306, 1151-1154 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S1.00003: Low temperature scanning SQUID microscopy studies of Sr$_{2}$RuO$_{4}$ Invited Speaker: We present direct imaging of magnetic flux structures in the anisotropic, spin-triplet superconductor Sr$_{2}$RuO$_{4}$ using a scanning SQUID microscope. We detected magnetic flux above the crystallographic ab plane in the superconducting state, at temperatures between 0.35 K and 1.35 Kelvin. Magnetic fields as high as 70 G were applied inplane. Individual quantized vortices were resolved at low perpendicular magnetic fields. At intermediate fields direct imaging revealed coalescing vortices forming flux domains. Based on our observations, we suggest that a pinning mechanism intrinsic to the material stabilizes the flux domains against the repulsive vortex-vortex interaction. Topological defects like domain walls may provide the pinning, implying proof for unconventional chiral superconductivity. We will discuss our observations in view of the predicted vortex domain state in time reversal symmetry breaking superconductors. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S1.00004: The State of Superconductivity in the 1D Organics Invited Speaker: We report on the status of the superconducting state in the quasi-1D molecular organic superconductors (TMTSF)$_{2}$X, on this 25$^{th}$ anniversary of organic superconductivity. A number of experimental results over the last decade, mostly on X=ClO$_{4}$ and PF$_{6}$, address the nature of the Cooper pairing in these materials, which surprisingly is not a settled issue. Critical field (via transport), NMR (including Knight shift) and impurity studies are suggestive of unconventional pairing, converging on spin triplet as a top candidate. Muon spin rotation and thermal conductivity results are less conclusive. More recently, the large $H_{c2}$ (well beyond the Pauli limit) has been confirmed by magnetization, and tunneling on a bicrystal shows a large midgap (zero-bias) state, strengthening the case for triplet superconductivity. The talk will include a discussion of the spin triplet configurations ($p$-- and $f$ --wave) available for (TMTSF)$_{2}$X, and will propose an order parameter \textbf{d}-vector consistent with the experiments. \newline \newline Work done in collaboration with Heon-Ick Ha (present address, Harvard University) and Jeong-Il Oh [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S1.00005: Evolution of superconducting order in Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ Invited Speaker: The filled skutterudite PrOs$_4$Sb$_{12}$ is paradoxical, exhibiting heavy-fermion behavior and unconventional superconductivity absent an ion with a magnetic ground state. Related compounds, PrRu$_4$Sb$_{12}$, LaOs$_4$Sb$_{12}$, and LaRu$_4$Sb$_{12}$ (the last two reported at this conference) are conventional BCS superconductors. To explore the change from unconventional $^3$He like order to fully-gapped conventional order we have measured, with high precision, the penetration depth of the series Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ to low temperatures. We find a persistence of the T$^2$ temperature dependence, found for $x=0$, below a temperature we label $T_{c3} (x)$. The cross over temperature $T_{c3}(x)$ decreases linearly from 0.6 K at $x=0$, appearing to vanish near $x=0.26$. The data above $T_{c3}(x)$, and over the entire range for $x \ge 0.3 $, are well represented by weak-coupling ($0.1\leq x \leq 0.6$) or intermediate-coupling ($x \geq 0.8$) BCS expressions. The results are discussed in terms of proposed mechanisms for unconventional behavior based on the proximity of a triplet excited state to the ground state singlet of the Pr ion.\\ \\Work performed with Elbert E. M. Chia, D. Vandervelde, K. Kikuchi, H. Sugawara and H. Sato and supported by the Department of Energy through the Illinois Materials Research Laboratory [Preview Abstract] |
Session S2a: Granular Impact-Cratering Dynamics and Morphology
Sponsoring Units: DCMPChair: Robert Behringer, Duke University
Room: LACC 151
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S2a.00001: Dynamics of projectile impact in a two-dimensional granular medium Invited Speaker: Our experiments and molecular dynamics simulations on a projectile impacting a two-dimensional granular medium reveal that the average deceleration of the projectile is constant during the penetration and proportional to the impact velocity [M. Pica Ciamarra, A. H. Lara, A. T. Lee, D. I. Goldman, I. Vishik, and H. L. Swinney, Phys. Rev. Lett. 92, 194301 (2004)]. Thus the time taken for a projectile to decelerate to a stop is independent of its impact velocity. The simulations show that the probability distribution function of forces on grains is time independent during a projectile's deceleration in the medium. At all times the force distribution function decreases exponentially for large forces. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S2a.00002: Low-speed impact cratering in loose granular media Invited Speaker: In this talk I shall describe the penetration of projectiles dropped into noncohesive granular media, and how the results vary with the properties of both the projectile and the medium. In contrast to wide assumption, the penetration depth and crater diameter represent two distinct length scales. The diameter scales as the 1/4 power of projectile energy, but curiously the depth is not a simple function of either the projectile energy or momentum at impact. Rather, it scales as the 1/2 power of density, the 2/3 power of projectile diameter, and the 1/3 power of total drop distance. This same result also holds for cylinders with a variety of tips, and so is not an accident of projectile shape. It is crucial to understand the penetration depth because it is directly related to the mechanics of impact, namely the average stopping force acting between projectile and medium. In addition to this discussion, I shall also present new data on the dynamics of impact. All experiments were constructed and carried out at UCLA by undergraduate physics majors: Jun Uehara, Katie Newhall, Chris Santore, and Mike Ambroso.\newline\newline [1] J.S. Uehara, M.A. Ambroso, R.P. Ojha, and D.J. Durian, ``Low-Speed Impact Craters in Loose Granular Media,'' Phys. Rev. Lett. {\bf 90}, 194301 (2003).\newline [2] K.A. Newhall and D.J. Durian, ``Projectile-shape dependence of impact craters in loose granular media,'' Phys. Rev. E {\bf 68}, 06030R (2003).\newline [3] M.A. Ambroso, C.R. Santore, A.R. Abate, and D.J. Durian, ``Penetration depth for shallow impact cratering,'' cond-mat/0411231 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S2a.00003: Simulations of impact cratering in granular media Invited Speaker: We simulate impact cratering in two dimensions using soft particle molecular dynamics simulations. We systematically vary the physical parameters of the grains and the ball (density, size, friction, impact energy). Our results confirm the recently observed scaling of the crater depth with impact energy as observed by Uehara et al and the effect of constant deceleration of the ball during the penetration phase as observed by Pica Ciamarra et al. We focus on the distribution of energy dissipation during the impact among various dissipation mechanisms and conclude that the most significant dissipation occurs due to internal frictional contacts among the grains. [Preview Abstract] |
Session S2b: Supersolidity and Bose-Einstein Condensation in Solid Helium
Sponsoring Units: DCMPChair: Moses Chan, Penn State University
Room: LACC 151
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S2b.00001: Observation of superflow in solid $^4$He Invited Speaker: At temperatures below 2.176 K, liquid $^{4}$He enters into a superfluid state and flows without any friction. There is strong evidence that Bose-Einstein condensates in dilute gases also exhibit superfluidity. Perhaps it is counter to intuition, but superfluid like behavior is thought possible even in solid helium. We employed a torsional oscillator technique and found evidence of superflow in bulk solid $^{4}$He (1) and solid $^{4}$He confined in porous Vycor glass (2). The effect appears as a drop in the resonant oscillation period as the sample cell is cooled below about 0.2 K. A series of control experiments reveals that the effect is due to irrotational superflow as in superfluid helium. The supersolid fraction in the low temperature limit is about 1{\%}. It has a ``universal'' temperature dependence that is different from that of the superfluid transition in liquid and different from that of a weakly interacting Bose gas. The supersolid fraction is strongly attenuated with increasing oscillation speed, indicating that the critical velocity is extremely low. (1) E. Kim and M. H. W. Chan, \textit{Science} \textbf{305}, 1941 (2004) (2) E. Kim and M. H. W. Chan, \textit{Nature} \textbf{425}, 227 (2004) [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S2b.00002: Quantum Phase Transition of $^4$He Confined in a Nanoporous Material Invited Speaker: Confinement of $^4$He in porous medium such as Vycor glass results in suppressions of freezing and superfluidity. The suppressions can be enhanced as the pore size decreases to nanometer scale. From torsional oscillator [1] and pressure studies we have revealed the $P-T$ phase diagram of $^4$He confined in a porous Gelsil glass which has nanopores of 2.5 nm in diameter. We have found that the superfluid transition temperature approaches 0 K at 3.4 MPa, and the freezing pressure shifts from the bulk one to 3.5 MPa. The solid - nonsuperfluid phase boundary is independent of temperature below 1 K, suggesting that the nonsuperfluid phase has low entropy as well as solid. The features indicate that the confined $^4$He undergoes a superfluid - nonsuperfluid - solid quantum phase transition at 0 K. The low - entropy nonsuperfluid phase may be a localized Bose - condensed state, in which global phase coherence is destroyed by strong correlation between $^4$He atoms or by random potential. [1] K. Yamamoto, H. Nakashima, Y. Shibayama, K. Shirahama, Phys. Rev. Lett. 93, 075302 (2004). [Preview Abstract] |
Session S3: Nanoscale Metal-Semiconductor Schottky Contacts
Sponsoring Units: DCMPChair: Jonathan Pelz, Ohio State University
Room: LACC 515B
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S3.00001: Enhanced tunneling across nanometer-scale semiconductor interfaces Invited Speaker: A Schottky diode is one of the simplest semiconductor devices, which makes it a suitable test-case to study the performance of a nanometer- scale semiconductor-device. Experimental systems range from an STM tip pressed onto a semiconductor substrate to semiconductor nanowires in contact with a metal. All show deviations from bulk behavior which is usually attributed to a lowered Schottky barrier. We have investigated systematically the transport properties of nanometer-scale metal-semiconductor contacts, formed by self-assembled epitaxial CoSi2-islands on Si, contacted by an STM. Our measurements indicate that tunneling plays an increasingly important role for smaller devices due to a size effect in the Schottky barrier. For metal-semiconductor contacts with a diameter smaller than the Debye length, the barrier thickness decreases with the contact size. This leads to an increase in tunneling current which explains the measured transport properties without the need to assume a Schottky barrier height deviating from the bulk value. Discreteness of doping atoms can no longer be neglected and its influence on the transport properties has been investigated both experimentally and theoretically. Statistical analysis of the conductance shows that the electronic properties of small diodes are dominated by a single dopant atom close to the metal-semiconductor interface, causing local barrier lowering even at room temperature. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S3.00002: Direct Measurement of Quantum Confinement and Environmental Pinning Effects on Metal/Nanostructure Schottky Contacts Invited Speaker: I will discuss direct nm-resolution measurements of metal/quantum well (QW) Schottky contacts made using Cross- sectional Ballistic Electron Emission Microscopy (XBEEM), in order to quantify the influence of small-size effects on hot- carrier injection into semiconductor nanostructures. Molecular Beam Epitaxy was used to grow a sequence of GaAs QWs with width varying from 1nm to 15 nm, separated by thick Al$_{0.3}$Ga$_ {0.7}$As barrier layers. The samples were cleaved ex-situ and polycrystalline Au contacts were electron-beam evaporated on the cleaved edge using shadow mask or photo-lithography. Samples were studied in ultra-high vacuum using Scanning Tunneling Microscopy and XBEEM. The Schottky barrier height over the QWs was found to systematically increase with decreasing QW width, by up to $\sim$140 meV for the 1 nm QW. This is mostly due to a large quantum-confinement increase ( up to $\sim$200 meV) of the QW conduction band minimum (CBM), as estimated by a simple 1D QW model. We also did finite element electrostatic modeling to estimate the ``environmental" effects of the surrounding metal/Al$_{0.3}$Ga$_{0.7}$As interface on the QW CBM. Excellent quantitative agreement over the full QW width range is obtained when both quantum confinement and electrostatic effects are considered.I will also discuss on-going measurements to use the metal/QW nanocontacts as unique ``nano-apertures" to directly image and quantify the lateral hot-electron spreading profile in the metal film. This profile is surprisingly large, with a FWHM of $\sim$15nm ($\sim$21nm) for a 4nm (7nm) thick Au film. XBEEM images directly show that hot-electron spreading is strongly modified by the grain structure in the metal film. In collaboration with J.P. Pelz, M.K. Hudait, and S.A. Ringel. Work supported by NSF and ONR [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S3.00003: Schottky contacts for quantum dots in SiGe modulation-doped heterostructures Invited Speaker: Silicon has certain unusual properties, including a spin-0 nuclear isotope, that make quantum dots in this material excellent candidates for quantum information processing. Schottky contacts have many advantages for fabrication of such quantum dots, including large dot-gate capacitance and excellent screening of interface states. We discuss recent progress in the fabrication of silicon quantum dots using Schottky contacts to SiGe modulation-doped heterostructures. In addition to understanding the physics of electron confinement in silicon quantum dots, it is also important to understand the Hamiltonian for electrons in silicon modulation-doped quantum wells. We present measurements of the anisotropy of the spin relaxation time in silicon quantum wells, and we show results of a new spectroscopic measurement of the energy gap between the two lowest-lying valleys in silicon quantum wells. Both results have important implications for the application of Schottky-gated silicon quantum dots to quantum computation. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S3.00004: High Performance Schottky Barrier MOSFETs Invited Speaker: Schottky barrier metal-oxide-semiconductor field-effect transistor (SB MOSFETs) has been recognized as one of the potential candidates for nano-scale device applications. Nevertheless, the inherent high off-state leakage and poor on/off current ratio represent major showstoppers for nano-scale device operation. We have recently reported the fabrication of a novel SB SOI MOS transistor featuring electrical source/drain junctions induced by a sub-gate overlying the passivation oxide. Pt or Co salicide process was adopted to form the Schottky barrier source/drain (S/D). The new device is extremely simple in fabrication and requires no implantation or associated annealing steps. We have shown that, a proper sub-gate bias could not only increase the on-state current but also significantly suppress the off-state current. In addition, the device is capable of bi-channel operation, which is unique, interesting, and greatly simplifies CMOS process integration. Ambipolar device characteristics with excellent on/off current ratios ($>$10$^{8})$ have been demonstrated on a same single device with CoSi$_{2}$ S/D. Moreover, near-ideal sub-threshold swing ($\sim $ 60 mV/decade) for both n- and p-channel modes of operation could be realized on a same single device, as the planar channel length is scaled to less than 100 nm. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S3.00005: Carbon Nanotubes as Schottky Barrier Transistors Invited Speaker: Field-effect transistors (FETs) made with carbon nanotubes have many attractive features, and are being widely studied as a potential nanoscale successor to silicon FETs. Remarkably, we found that nanotube FETs generally operate by a completely different principle than ordinary Si FETs. Rather than modulate the conductance of the channel, the gate field acts to modulate the tunneling conductance of a Schottky barrier at the contact [1]. As a result, the device performance is determined by completely different factors than in familiar FETs [2-4]. In particular, the nanoscale electric field distribution near the contacts plays a crucial role. As a result, the geometry and workfunction of the contact become as important as more familiar factors like gate-oxide thickness. In addition, there are fundamental differences in the role of Fermi-level pinning at the metal-nanotube contact, compared to ordinary semiconductor interfaces [5]. \\ 1. S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, and Ph. Avouris, Phys. Rev. Lett. 89, 106801 (2002). \\ 2. S. Heinze, M. Radosavljevic, J. Tersoff, and Ph. Avouris, Phys. Rev. B 68, 235418 (2003). \\ 3. M. Radosavljevic, S. Heinze, J. Tersoff, and Ph. Avouris, Appl. Phys. Lett. 83, 2435 (2003). \\ 4. S. Heinze, J. Tersoff, and Ph. Avouris, Appl. Phys. Lett. 83, 5038 (2003). \\ 5. F. Leonard and J. Tersoff, Phys. Rev. Lett. 84, 4693 (2000). [Preview Abstract] |
Session S4: Surface Structure of Compound Semiconductors
Sponsoring Units: FIAPRoom: LACC 515A
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S4.00001: Scanning Tunneling Microscopy and Spectroscopy of Semiconductor Surfaces Invited Speaker: We discuss recent advances in our knowledge of the structure, electronic spectroscopy, and transport properties of semiconductors, obtained primarily through measurement with the scanning tunneling microscope (STM). For the wide band gap materials GaN and AlGaN, observation of various reconstructions together with first-principles theory have enabled the determination of many of the fundamental surface structures. The occurrence of incommensurate metallic layers on the surfaces, and their relevance for growth and control of thin film properties, will be described. Heterostructures of InGaP/GaAs have been studied in cross-section, permitting determination of the band offset between these materials. Detailed electrostatic simulations of the influence of the STM probe-tip on the measurement are used to provide an error bound on the result. Finally, for both Ge and SiC, spectroscopic measurements as a function of current reveal transport limitations in the conductance through surface states. Extension of the results to the determination of transport properties of other surfaces will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S4.00002: Invited Speaker: |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S4.00003: Atomic Structure of InGaAs Alloys Invited Speaker: The surface structure of a seemingly random alloy layer has a great impact on the compositional homogeneity and subsequent interface formation. For example, it has been suggested that random fluctuations in composition may initiate lateral composition that propagates through the remainder of the film. Our group studies the morphology and surface reconstruction of InxGa1-xAs alloy layers during growth and after annealing. Films of different compositions were grown by Molecular Beam Epitaxy on GaAs and InP to thicknesses less than the critical thickness for 3D islanding or misfit dislocation formation, and examined using in-situ Scanning Tunneling Microscopy and ex-situ Atomic Force Microscopy. The surface reconstruction of these layers is generally more disordered than those of their binary counterparts, and consists of different reconstruction domains. In particular, both surfaces show domains of a mixed-terminated (4x3) reconstruction, which is better ordered for the high In composition. In addition, there are pockets of a2(2x4) in the case of In0.27Ga0.73As/GaAs, and b2(2x4) in the case of In0.81Ga0.19As/InP. The coverage of both (2x4) reconstructions decreases during annealing, concomitant with a decrease in In surface concentration due to In desorption, suggesting that the (2x4) reconstructions are enriched in In compared to the (4x3)/(nx3). The coverage of different reconstructions also changes with film thickness, following changing surface composition and increasing strain energy. In the case of the In0.27Ga0.73As films, the In composition at the surface increases with film thickness and reaches a saturation level, in agreement with previous reports. The coverage of the (4x3) reconstruction reaches a saturation level at the same time, suggesting that a high and stable In concentration at the surface and/or a high strain energy favor a better ordered (4x3). The coverage of the a2(2x4) reconstruction increases initially with film thickness, then it decreases as the strain energy continues to increase, despite the fact that the surface reaches a stable composition. These results point out the importance of considering the effects of strain energy and inhomogeneous composition in the understanding of alloys surface structure. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S4.00004: Unified description of the formation and evolution of self-organized quantum dots in the InAs/GaAs(001) and Ge/Si(001) systems Invited Speaker: Self-organised semiconductor quantum dots, epitaxially grown on lattice-mismatched substrates, are promising candidates for the practical realisation of ``artificial atoms.'' Their peculiar tuneable properties open the way to novel applications in the fields of optoelectronics, single-electron and single-photon devices as well as quantum computation. However, a successful implementation requires a precise control over their shapes and sizes which, at present, is still an open problem. Its solution needs a basic understanding of the actual morphology of the quantum dots and of their further evolution during post-growth treatments. Here, by means of high-resolution scanning tunnelling microscopy, we investigate the model systems of self-organised quantum dots formed from single and binary semiconductor compounds, Ge grown on Si(001) substrates and InAs on GaAs(001), respectively. We demonstrate that for experimental conditions close to the thermodynamic limit (high substrate temperatures and low deposition rates) only two families of faceted and defect-free nanocrystals exist, small pyramids composed of four equivalent shallow facets, and lager multifaceted domes. The analogies between the two material systems extend also to the existence of hut-clusters and embryo islands that act as precursors for pyramid. A shape transition from pyramids to domes is seen to occur in both material systems. The transformation path, essentially consisting of the bunching of incomplete facets at the top of pyramids larger than a critical size, is precisely determined for Ge/Si(001) and explained in terms of surface diffusion processes only. These striking similarities further extend to the capping procedure that is needed in order to transform self-organised islands into true quantum dots. For both material systems we observe a backward dome to-pyramid transition accompanied by a strong height decrease. This complex phenomenology is rationalized in terms of intermixing processes driven by strain release. Our measurements suggest that the unified picture we are presenting for the prototype systems Ge/Si(001) and InAs/GaAs(001), extends, at least qualitatively, to a large number of material combinations that follow the Stranski-Krastanow growth mode. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S4.00005: Nanometer-Scale Structure and Properties of Dilute Semiconductor Alloys Invited Speaker: For many compound semiconductors, the addition of dilute concentrations of impurities leads to dramatic changes in the electronic, optical, and magnetic properties. For example, the introduction of a few percent nitrogen into GaAs leads to a band gap reduction of 100s of meV. Furthermore, the incorporation of a few percent manganese into GaAs enables a combination of semiconducting and ferromagnetic behavior. The resulting dilute semiconductor alloys are promising for several applications ranging from long-wavelength light-emitters and high efficiency solar cells to spin-electronics and spin-optoelectronics. In both cases, the nanometer-scale details of impurity incorporation are critical to understanding and controlling the observed properties. In this talk, I will discuss our recent investigations of the growth, nanometer-scale structure, and properties of dilute GaAsN and GaMnAs alloys, using nuclear reaction analysis and scanning tunneling microscopy, in conjunction with several other measurements. In GaAsN, we examine the role of surface reconstruction on the incorporation of nitrogen into substitutional vs. interstitial lattice sites, as well as the effect of nitrogen incorporation mechanisms on electronic and optical properties [1]. In the case of GaMnAs, we quantify clustering of Mn$_{Ga}$ and As$_{Ga}$ point defects, and its effect on electronic and magnetic properties [2]. [1] M. Reason, H. McKay, W. Ye, S. Hanson, V. Rotberg, and R.S. Goldman, ``Mechanisms of Nitrogen Incorporation in GaAsN Alloys,'' \textit{Appl. Phys. Lett. }\textbf{85}, 1692 (2004). [2] J.N. Gleason, M. Hjelmstad, V.D. Dasika, R.S. Goldman, S. Fathpour, S. Charkrabarti, and P.K. Bhattacharya, ``Nanometer-scale Studies of Point Defect Distributions in GaMnAs Alloys,'' \textit{Appl. Phys. Lett., }in press (January 3, 2005). [Preview Abstract] |
Session S5: Solitons in Physics: 50 Years Since Fermi, Pasta & Ulam
Sponsoring Units: GSNPChair: Charlie Doering, University of Michigan
Room: LACC 502B
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S5.00001: From FPU to Intrinsic Localized Modes Invited Speaker: Intrinsic localized modes (ILMs), also known as ``discrete breathers,'' have been appealing theoretical possibilities for more than a decade. Roughly speaking, they represent the extension of the continuum concept of ``solitons'' to spatially extended discrete (lattice) systems. Importantly, theory suggests that ILMs are far more ubiquitous than solitons, in that they can occur in discrete systems in any number of spatial dimensions and with a wide range of nonlinear interactions. In the past several years, ILMs have been observed experimentally in physical systems as distinct as charge-transfer solids, Josephson junction arrays, photonic structures, and micromechanical oscillator arrays. Beginning with a brief historical overview of the origin of the concept of ILMs, we focus on their current theoretical and experimental and discuss some exciting possible future directions and applications of these novel nonlinear excitations. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S5.00002: The FPU problem and statistical mechanics Invited Speaker: Most of the investigations of the FPU model have led to the discovery of new, very interesting properties of this seminal model such as solitons, breathers, etc., creating the field of non-linear dynamics. However, the original question FPU posed was a statistical mechanical one, namely to verify the approach of a very simple system initially not in equilibrium to equilibrium. To their and most physicists’ consternation their model did not show this, throwing one of the basic tenets of statistical mechanics into doubt. The reasons why FPU did not find an approach to equilibrium will be discussed here as related to stochasticity thresholds in Hamiltonian dynamics and a connection with the phase space geometry of this model will be mentioned. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S5.00003: Solitons and Coherent Structures in Bose-Einstein Condensates Invited Speaker: The Fermi-Pasta-Ulam (FPU) model was formulated fifty years ago in an attempt to explain heat conduction in non-metallic lattices and develop ``experimental'' (computational) methods for research on nonlinear dynamical systems. Further studies of the FPU problem ten years later led to the first analytical description of solitons, which occur ubiquitously in diverse physical situations ranging from water waves to plasmas, optical fibers, and more. More recently, the study of solitons and coherent structures in Bose-Einstein condensates (BECs) has come to the forefront of experimental and theoretical efforts, drawing attention from both atomic and nonlinear physicists. In this talk, I will discuss localized (soliton) and spatially extended solutions of the Gross-Pitaevskii (GP) equation describing the macroscopic behavior of BECs near zero temperature. As a key example, I will discuss BECs loaded into deep, spatially periodic optical potentials, which effectively splits the condensate into a chain of linearly-interacting, nonlinear droplets, the dynamics of which is characterized by nonlinear lattice models. This talk will highlight some of the dynamical structures in BECs reminiscent of the discoveries that originated from the FPU model. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S5.00004: Heat Conduction in Low Dimensions Invited Speaker: The Fourier law describing heat conduction is almost two hundred years old and yet a first-principle derivation of this simple law from statistical mechanics is still lacking. Worse still, the validity of this law in low dimensions and the necessary and sufficient conditions for its validity are also far from clear. In this talk I’ll give a review of work done in this area and I’ll report our recent work on heat conduction in the Frenkel- Kontorova model. Application of knowledge gained in the study of heat conduction in one dimension to the design of a thermal diode will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S5.00005: FPU, Solitons and Nonlinear Science: History, Visiometrics and Art \& Science Invited Speaker: I review how the discipline of ``nonlinear'' physics arose from the '54-`55 Fermi-Pasta-Ulam (FPU) digital computer simulations of the nonlinear one-dimensional alpha-lattice and the discovery of the soliton (1965) and ``n-curve'' states (1967). Recently, applications to accelerated inhomogeneous (Richtmyer-Meshkov) flows have been made and evolving vortex bilayers and ``vortex projectiles'' discovered in the emerging turbulence. Underlying these innovations is the cogent visiometrics approach, whose representations can provide peak insights that lead to pathways for discovery and can stimulate artistic expression. See (http://www.mechanical.rutgers.edu/scart4/ especially ''Links''); also, `` FPU, solitons {\&} the fabric of nonlinear science: History, synergetics and visiometrics'' in CHAOS (to be published in `05).- [Preview Abstract] |
Session S6: Solid State Qubits, Resonators, and Quantum Optics
Sponsoring Units: DAMOPChair: Mikhail Lukin, Harvard University
Room: LACC 502A
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S6.00001: Quantum optics with superconducting qubits in the dispersive limit Invited Speaker: Several recent experiments have demonstrated that superconducting circuits are ideal systems for the study of quantum mechanical effects on large scale and promising candidates for quantum computation. It was recently proposed [1] and experimentally demonstrated [2,3] that superconducting circuits fabricated inside a high quality on-chip transmission line resonator can be used to study solid-state analogs of quantum optics experiments and, in particular, to reach the strong-coupling regime of cavity quantum electrodynamics (CQED). In this talk, this theoretical proposal will be reviewed and experimental results in the dispersive regime will be presented, for both the time and frequency domain. Results in the time domain show our ability to coherently control the state of the qubit, while detailed analysis of the frequency domain results yields insights into the measurement process and its backaction.\par [1] A. Blais, R.-S. Huang, A. Wallraff, S. M. Girvin and R. J. Schoelkopf, Phys. Rev. A {\bf 69}, 062320 (2004).\par [2] A. Wallraff, D. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf, Nature {\bf 431}, 162 (2004).\par [3] D. Schuster, A. Wallraff, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. M. Girvin and R. J. Schoelkopf, cond-mat/0408367. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S6.00002: Mesoscopic cavity quantum electrodynamics with atomic systems and quantum dots Invited Speaker: We discuss electrodynamic techniques for strong, coherent coupling between spatially separated atoms or quantum dots on a microchip. These techniques are based on capacitive interactions between the electron charge and a superconducting transmission line resonator operating in microwave domain, and are analogous to cavity quantum electrodynamics. In the case of isolated neutral atoms, the coupling is achieved by exciting atoms trapped above the surface of a superconducting transmission line into Rydberg states with large electric dipole moments that induce voltage fluctuations in the resonator. In the case of quantum dots, interactions between the resonator and the electron charge may be exploited to couple spatially separated electron-spin states while only virtually populating fast-decaying superpositions of charge states. We discuss potential applications of such electrodynamic coupling for a long-range interaction between a variety of spatially separated quantum systems, for entangling isolated neutral atoms separated by millimeters, or for mapping the quantum states of a solid-state device onto atomic or photonic states. Finally, we discuss prospects for extending these on-chip cavity QED techniques into the optical domain. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S6.00003: Strong coupling between a quantum dot and a photonic crystal nanocavity Invited Speaker: We report the observation of vacuum Rabi splitting (true strong coupling) between a single InAs quantum dot and a photonic crystal nanocavity.$^{1}$ An anti-crossing of the dot and nanocavity resonances occurs as the temperature is increased from 15K to 29K. The zero-detuning splitting is 0.164 meV, slightly larger than the 0.13 meV coupled linewidth, so there is a dip between the peaks of 30-40{\%}. The 2D photonic crystal slab is surrounded by air; three holes in a line are omitted and the end holes are shifted out to form the cavity spacer.$^{2}$ The Q of the cavity mode was 13,300 at high power and about 6000 at the low power used for the anti-crossing. The volume of the cavity mode is computed to be about ($\lambda _{cav}$/n)$^{3}$ = 0.04 $\mu $m$^{3}$, where $\lambda _{cav}$ is the cavity mode wavelength and n is the refractive index. This solid-state system that entangles the quantum-dot-transition qubit with the cavity-mode-photon qubit may find applications in quantum information science as a deterministic single-photon source or quantum phase gate, or for quantum state transfer. 1. T. Yoshie, A. Scherer, J. Hendrickson, G. Khitrova, H. M. Gibbs, G. Rupper, C. Ell, O. B. Shchekin, and D. G. Deppe, Nature \textbf{432}, 200 (2004). 2. Y. Akahane, T. Asano, B.-S. Song, and S. Noda, Nature \textbf{425}, 944 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S6.00004: NMR-like Operation and Analysis of Decoherence of a Superconducting Quantum Bit Invited Speaker: The Quantronium [1], a superconducting circuit with Josephson junctions, can be regarded as a solid state qubit prototype with built-in decoupling from its environment. We demonstrate that arbitrary operators can be applied to it using NMR-like and atomic physics-like techniques that involve quasi-resonant microwave or adiabatic DC pulses [2]. Then, we explain how the symmetry of the circuit limits decoherence of a superposition of states, at an optimal working point. Coherence time measurements, performed both during free and driven evolution of the qubit are presented and analyzed using a simple model involving different noise sources. A complete picture of decoherence in this quantum electrical circuit is thus provided. [1] D. Vion \textit{et al.}, Science \textbf{296} (2002). [2] E. Collin \textit{et al}., Phys. Rev. Lett. \textbf{93}, 15, (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S6.00005: Superconducting qubit storage and entanglement with nanomechanical resonators Invited Speaker: I will discuss work done with Andrew N. Cleland on the design of a quantum computing architecture based on the integration of GHz-frequency mechanical or electromagnetic resonators with Josephson junction (JJ) phase qubits. This system is analogous to one or more few-level atoms (the JJs) in an electromagnetic cavity (the resonator), except that here we can individually tune the energy level spacing of each atom, and can control the electromagnetic interaction strength. We show that the quantum state of a JJ can be passed to the resonator and stored there, and later passed back to the original JJ or transferred to a second JJ. Furthermore, memory devices made from resonators with ultrahigh Q factors can used to effectively extend the coherence of the phase qubits. The resonator can also produce controlled entangled states of the JJs and can mediate quantum logic. We discuss the accuracy of the rotating-wave and adiabatic approximations in this system, and show that these approximations are actually quite poor at predicting the phase of probability amplitudes. Our architecture combines desirable features of both solid-state and cavity-QED approaches, and may make quantum computing possible in a scalable, solid-state environment. [Preview Abstract] |
Session S7: Gene Chips
Sponsoring Units: DBPChair: Ned Wingreen, Princeton University
Room: LACC 408B
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S7.00001: Gene Chips: A New Tool for Biology Invited Speaker: The knowledge of many complete genomic sequences has led to a ``grand unification of biology,'' consisting of direct evidence that most of the basic cellular functions of all organisms are carried out by genes and proteins whose primary sequences are directly related by descent (i.e. orthologs). Further, genome sequences have made it possible to study all the genes of a single organism simultaneously. We have been using DNA microarrays (sometime referred to as ``gene chips'') to study patterns of gene expression and genome rearrangement in yeast and human cells under a variety of conditions and in human tumors and normal tissues. These experiments produce huge volumes of data; new computational and statistical methods are required to analyze them properly. Examples from this work will be presented to illustrate how genome-scale experiments and analysis can result in new biological insights not obtainable by traditional analyses of genes and proteins one by one. For lymphomas, breast tumors, lung tumors, liver tumors, gastric tumors, brain tumors and soft tissue tumors we have been able, by the application of clustering algorithms, to subclassify tumors of similar anatomical origin on the basis of their gene expression patterns. These subclassifications appear to be reproducible and clinically as well as biologically meaningful. By studying synchronized cells growing in culture, we have identified many hundreds of yeast and human genes that are expressed periodically, at characteristically different points in the cell division cycle. In humans, it turns out that most of these genes are the same genes that comprise the ``proliferation cluster,'' i.e. the genes whose expression is specifically associated with the proliferativeness of tumors and tumor cell lines. Finally, we have been applying a variant of our DNA microarray technology (which we call ``array comparative hybridization'') to follow the DNA copy number of genes, both in tumors and in yeast cells undergoing adaptive evolution during hundreds of generations of growth in continuous culture. These studies suggest a basic similarity in mechanism between adaptive evolution in yeast and tumor progression in humans. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S7.00002: Mining gene-chip data Invited Speaker: DNA microarray (``gene chip'') technology has enabled a rapid accumulation of gene-expression data for model organisms such as {\it S. cerevisiae} and {\it C. elegans}, as well as for {\it H. sapiens}, raising the issue of how best to extract information about the gene regulatory networks of these organisms from this data. While basic clustering algorithms have been successful at finding genes that are coregulated for a small, specific set of experimental conditions, these algorithms are less effective when applied to large, varied data sets. One of the major challenges in analyzing the data is the diversity in both size and signal strength of the various transcriptional modules, {\it i.e.} sets of coregulated genes along with the sets of conditions for which the genes are strongly coregulated. One method that has proven successful at identifying large and/or strong modules is the Iterative Signature Algorithm (ISA) [1]. A modified version of the ISA algorithm, the Progressive Iterative Signature Algorithm (PISA), is also able to identify smaller, weaker modules by sequentially eliminating transcriptional modules as they are identified. Applying these algorithms to a large set of yeast gene expression data illustrates the strengths and weaknesses of each approach. [1] Bergmann, S., Ihmels, J., and Barkai, N., Phys. Rev. E {\bf 67}, 031902 (2002). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S7.00003: Combining gene-chip data and bioinformatics to define transcription networks Invited Speaker: The development of DNA microarray technology has made it possible to simultaneously monitor the mRNA abundance of all genes (``transcriptome'') for a variety of cellular conditions. In addition, microarrays have been used to map protein-DNA interactions by measuring occupancy profiles along the chromosome for an increasing number of transcription factors (TFs), especially in the yeast S. cerevisiae. With this data and the complete genome sequence on hand, it is becoming possible to quantitatively model the molecular computation performed near the transcription start site of the gene. This computation has as input the nuclear concentrations of the active form of various regulatory proteins (``regulome'') and as output a transcription rate, which together with the half-life of the transcript determines the mRNA abundance. Our laboratory has pioneered the use of multivariate regression methods to link mRNA expression data with genome sequence data and TF occupancy data. This allows us to: (i) discover cis-regulatory elements in non-coding regulatory regions; (ii) infer the condition-dependent regulatory activities of transcription factors as ``hidden variables''; and (iii) accurately determine which genes are controlled by which transcription factors. Together, our results show that model-based analysis of functional genomics data provides a versatile and extensible conceptual and practical framework for the elucidation of regulatory circuitry, and a powerful alternative to the currently popular methods based on clustering and ``modules''. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S7.00004: Integrating Genetic and Functional Genomic Data to Elucidate Common Disease Tra Invited Speaker: The reconstruction of genetic networks in mammalian systems is one of the primary goals in biological research, especially as such reconstructions relate to elucidating not only common, polygenic human diseases, but living systems more generally. Here I present a statistical procedure for inferring causal relationships between gene expression traits and more classic clinical traits, including complex disease traits. This procedure has been generalized to the gene network reconstruction problem, where naturally occurring genetic variations in segregating mouse populations are used as a source of perturbations to elucidate tissue-specific gene networks. Differences in the extent of genetic control between genders and among four different tissues are highlighted. I also demonstrate that the networks derived from expression data in segregating mouse populations using the novel network reconstruction algorithm are able to capture causal associations between genes that result in increased predictive power, compared to more classically reconstructed networks derived from the same data. This approach to causal inference in large segregating mouse populations over multiple tissues not only elucidates fundamental aspects of transcriptional control, it also allows for the objective identification of key drivers of common human diseases. [Preview Abstract] |
Session S9: Geometrically Frustrated Magnets II
Sponsoring Units: GMAGChair: Oleg Tchernyshov, Johns Hopkins University
Room: LACC 153A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S9.00001: Phase diagram of the S=1/2 checkerboard antiferromagnet Akira Furusaki, Leon Balents, Oleg Starykh We report the phase diagram of the S=1/2 Heisenberg antiferromagnet on the checkerboard lattice, also known as the crossed-chains lattice. It is assumed that the exchange coupling along the crossing chains, $J_2$, is different from that on the inter-chain (square-lattice) links, $J_1$. We show that in the one-dimensional limit $J_2 \gg J_1$, the ground state is spontaneously dimerized in the crossed-dimer pattern. At the isotropic planar pyrochlore point $J_2=J_1$, the ground state is also a valence-bond solid (VBS) but of the plaquette type, as is known from previous studies. We argue that these two VBS states may be connected, as a function of the ratio $J_2/J_1$, by an intermediate magnetically-ordered phase. Two quantum critical points, separating the ordered phase from dimerized VBS ones, are analyzed and argued to belong to the $O(3)$ universality class. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S9.00002: A soluble model for a Spin-1 Kagom\'{e} Antiferromagnet Kirill Shtengel, Gil Refael We propose an exactly soluble spin-1 model on a $2D$ Kagom\'{e} lattice. The Klein-type Hamiltonian involves interactions between nearest and next-nearest spins and, unlike the closely related AKLT Hamiltonians, has extensively degenerate ground states. These ground sates are characterised by an exponential fall-off of correlations between spins which strongly suggests a gap to the excited states. Simple spin-1 and spin-0 excitations can be viewed as bound states of $S=1/2$ spinons. We also show that generic Heisenberg-like perturbations lead to a unique ground state -- a featureless fluctuating valence bond ``solid'' obtained by placing a benzol ring on every hexagon of the lattice. Finally, we consider an additional term of the type $\alpha ({S^z})^2$ which can drive the system into another featureless ground state. We introduce the notion of ``wedge'' excitations that allow to distinguish between these states leading to the conclusion that these sates must be separated by at least one quantum phase transition. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S9.00003: Spectral analysis of higher spins (S$>$1/2) kagome systems Dommange Stephane, Laeuchli Andreas Martin, Mila Frederic, Fouet Jean Baptiste, Normand Bruce We will present some recent numerical results concerning higher spin kagome systems, with emphasis on S=3/2. We focus on pure systems calculating exact spectra for different cluster sizes and conclude that no magnetic long range order is found. We complete the analysis by describing some features of such systems doped with non-magnetic impurities. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S9.00004: Role of Quantum Fluctuations in the triangular antiferromagnet Cs$_2$CuCl$_4$ Martin Y. Veillette, John Chalker We have performed a detailed comparison of static properties of the anisotropic triangular antiferromagnet Cs$_2$CuCl$_4$ with a calculation taking into account the leading order of the zero point fluctuations at zero temperature. The Hamiltonian of the Cs$_2$CuCl$_4$ compound is known to a high degree of accuracy and allows for a parameter-free calculation. A distinction must be made between transverse and longitudinal field due to a weak Dzyaloshinkii-Moriya interaction that introduce an easy-plane anisotropy in the Hamiltonian. The phase diagram is determined in a classical approximation along the two field directions. Building on these results, we calculate the contribution of quantized spin-waves in a large S expansion to themagnetization, ordering wavevector, sublattice magnetization and transverse spin component. The results are shown to depend sensitively on the weak anisotropy. In high field, we find the zero-point fluctuations to be quenched and use a mapping to a dilute Bose gas to determine the exact quantum contribution near the critical field. The results of the linear spin wave analysis are found to be in excellent agreement with the experimental data. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S9.00005: Spin Structure Factor of the Frustrated Quantum Magnet $Cs_2 Cu Cl_4$. Denis Dalidovich, Rastko Sknepnek, Junhua Zhang, Catherine Kallin, John Berlinsky We present the results of a calculation of the spin structure factor for the two-dimensional antiferromagnet on the triangular lattice, with strong directional anisotropy in the nearest-neighbour exchange couplings. The corresponding Heisenberg Hamiltonian describes the physics following from neutron scattering measurements in the frustrated quantum magnet $Cs_2 Cu Cl_4$, [R. Coldea, et. al., Phys. Rev. B, {\bf 68}, 134424, (2003)]. Since the experimental data reveal the presence of a small but finite on-site magnetic moment $S_z$, the calculations are performed using the Holstein-Primakoff representation for spins. The results for the structure factor, computed up to the order in $1/S$ that takes into account interactions between magnons, are compared with experiment. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S9.00006: Superfluid-insulator transition in two-dimensional superfluids on the triangular lattice Anton Burkov, Leon Balents We report on a study of superfluid to Mott insulator transition in two-dimensional superfluids on the triangular lattice at rational fillings, with a particular emphasis on 1/2 and 1/3 fillings. This is a continuation of our earlier study (cond-mat/0408329) of superfluids on the square lattice. At 1/3 filling, not unexpectedly, we find a picture of the transition that is very similar to the 1/2 filling square lattice case. On the other hand, at 1/2 filling on the triangular lattice strong geometric frustration leads to a very different picture, with features that have no analogs in square lattice superfluids. We find that the low-energy action, describing the transition in this case, has an emergent nonabelian symmetry, not present at the microscopic level, and explore the physical consequences of this symmetry. We also identify the essential features of the insulating phases at 1/2 filling, in particular the prevalence of valence bond solid (VBS) phases over simple site-centered charge density wave phases. This has important implications for the search of microscopic models where VBS phases may be realized. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S9.00007: Topological degeneracy in the RVB phase of the Quantum Dimer Model on the triangular lattice Arnaud Ralko, Michel Ferrero, Federico Becca, Dimitri Ivanov, Frederic Mila Using numerical methods such as exact diagonalizations and Green function monte carlo (GFMC), we study ground state properties of the different topological sectors of the Quantum dimer model on the triangular lattice to characterize the phases of the system and the transition points between them. Thanks to the large sizes available with GFMC, we show in particular that in the thermodynamic limit, the four topological sectors are indeed degenerate. Finally, we show that the correlation functions are consistent with the identification of the ordered phases by Moessner and Sondhi. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S9.00008: Two Step Restoration of SU(2) Symmetry in a Frustrated Quantum Magnet Andreas L\"auchli, J.C. Domenge, C. Lhuillier, P. Sindzingre, M. Troyer We demonstrate the existence of a spin-nematic, moment-free phase in a quantum four-spin ring exchange model on the square lattice. This unusual quantum state is created by the interplay of frustration and quantum fluctuations which lead to a partial restoration of $SU(2)$ symmetry when going from a four- sublattice orthogonal biaxial N\'eel order to this exotic uniaxial magnet. A further increase of frustration drives a transition to a fully gapped $SU(2)$ symmetric valence bond crystal. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S9.00009: Spin charge sepration in the doped frustrated J1-J2-J3 Heisenberg antiferromagnet on the square lattice Matthieu Mambrini, Andreas Laeuchli, Didier Poilblanc We study undoped and doped frustrated J1-J2-J3 Heisenberg antiferromagnet on the square lattice using both exact diagonalization techniques and projections onto the short-range RVB subspace. The insulating system shows, in the vicinity of the $(J_3+J_2)/J_1 =0.5$ line, a magnetically disordered ground very well captured by a RVB wave function. The nature, dimer liquid or valence bond crystal, of such a RVB phase is characterized from the computation of dimer-dimer correlations and a singlet spectrum analysis of finite clusters up to 50 sites. We also show that a substancial reduction of the quasi-particle spectral weight of a doped hole can be related to the spin liquid character of the magnetic background. This suggests that spin-charge separation occurs in such a frustrated system. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S9.00010: Order in the resonating singlet valence plaquette model in three dimensions. Sergey Pankov, Roderich Moessner, Shivaji Sondhi We study a generalization of resonating valence bond (RVB) physics to three dimensions. In two dimensions short-range RVB models can exhibit fundamentally interesting phenomena like quantum liquid states with topological order and fractionalization. Whereas in the RVB case, the basic degree of freedom can be thought of as an SU(2) singlet (valence) bond between two spins, in resonating singlet valence plaquette (RSVP) physics, it corresponds to an SU(4) singlet comprising four sites; this might arise for example in a spin-orbital model. Here, we discuss the detailed phase diagram for the simplest case -- the Rokhsar-Kivelson RSVP model on the cubic lattice. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S9.00011: Introducing interactions in quantum dimer, vertex and loop models at the RK point Claudio Castelnovo, Claudio Chamon, Christopher Mudry, Pierre Pujol We present a generalized class of Rokhsar-Kivelson (RK) Hamiltonians that are in one-to-one correspondence with generic stochastic classical systems described by a Master equation in matrix form. We show that the ground state of the quantum system can be computed exactly and the zero-temperature phase diagram is captured by the phase diagram of the corresponding classical system at equilibrium. Moreover, the excitation spectrum over the ground state is given by the relaxation rates of the Master equation for the classical system. We then show how this framework allows to introduce interactions in known systems at the RK point, such as quantum dimer and vertex models, and to study phase transitions along lines of RK points. We also illustrate how one can construct exotic quantum Hamiltonians that are incapable of equilibrating to their ground state when coupled to a local thermal bath, and that exhibit relaxation time scales characteristic of a quantum glass. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S9.00012: Quantum glassiness in clean strongly correlated systems: an example of topological overprotection Claudio Chamon Describing matter at near absolute zero temperature requires understanding a system's quantum ground state and the low energy excitations around it, the quasiparticles, which are thermally populated by the system's contact to a heat bath. However, this paradigm breaks down if thermal equilibration is obstructed. I present solvable examples of quantum many-body Hamiltonians of systems that are unable to reach their ground states as the environment temperature is lowered to absolute zero. These examples, three dimensional generalizations of quantum Hamiltonians proposed for topological quantum computing, 1) have no quenched disorder, 2) have solely local interactions, 3) have an exactly solvable spectrum, 4) have topologically ordered ground states, and 5) have slow dynamical relaxation rates akin to those of strong structural glasses. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S9.00013: Frustrated corner-shared triangles: the B20 structure John Hopkinson, Hae-Young Kee We present a mean-field treatment of the classical Heisenberg model on the B20 lattice which is composed of two intertwined sublattices of corner-shared triangles. When one sublattice \{(x,x,x),(1/2+x,1/2-x,-x),cyclic perm.\} is magnetic we find a non-trivially degenerate ground state over a modified sphere. Addition of next nearest neighbor terms lifts this degeneracy leading to a host of long period magnetic structures--analogues of those shown by helimagnets--along the qqq or qq0 directions. The implications of these results for neutron scattering experiments will be discussed in light of recent surprising experiments under pressure on the itinerant MnSi, and renewed interest in the ``Kondo insulator'' FeSi and its doped semiconducting helimagnetic partner Co$_x$Fe$_{(1-x)}$Si. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S9.00014: Spin frustration controlled by orbital fluctuations Hiroaki Onishi, Takashi Hotta In order to clarify a key role of orbital degree of freedom in geometrically frustrated electron systems, we investigate an $e_{\rm g}$-orbital Hubbard model on a zigzag chain at quarter filling by using numerical techniques. When two orbitals are degenerate, orbital degree of freedom is active, but a $3x^2-r^ 2$ orbital is selectively occupied to suppress the spin frustration. Namely, the occupied orbital shape extends just along the direction of a double chain, and the zigzag chain is reduced to a double-chain spin system due to the spatial anisotropy of orbitals. On the other hand, taking account of the level splitting $\Delta$ between $3z^2-r^2$ and $x^2-y^2$ orbitals, electrons are forced to accommodate in the lower level. When the $3z^2-r^2$ orbital is fully occupied for large positive $\Delta$, the orbital anisotropy disappears in the $xy$ plane and the spin frustration revives. With increasing $\Delta$ from zero to large values, the orbital state gradually changes and the orbital fluctuation is found to be significant in the intermediate region. We will discuss the change in orbital structure and spin incommensurability. [Preview Abstract] |
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S9.00015: Impurity induced frustrations in a non-frustrated antiferromagnet Sasha Chernyshev, Shiu Liu Zn substitution for Cu in La$_2$CuO$_4$ is thought to be an ideal example for a simple site dilution of the antiferromagnetic $S=1/2$, square lattice non-frustrated nearest-neighbor Heisenberg model. We show that starting from the microscopic three-band Hubbard model one obtains quite different, counterintuitive result. Namely, the spinless impurity generates {\it frustrating} interactions around itself. This is because the oxygen orbitals around Zn impurity site can be still engaged in the virtual transitions which produce substantial superexchange interactions between the Cu spins {\it across} the impurity site. This effect can explain noticeable discrepancies between the experimental data and theoretical results for the simple site- diluted Heisenberg model. [Preview Abstract] |
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S9.00016: Random Bonds Effects in the Spin-$\frac{1}{2}$ Heisenberg Antifferomagnet on the Square Lattice Nicolas Laflorencie, Stefan Wessel, Andreas Laeuchli, Heiko Rieger In one dimension, it is well know that the spin-$\frac{1}{2}$ Heisenberg antiferromagnetic (AF) chain, governed by ${\mathcal{H}}_{1d}=\sum_{i} J_i {\vec{S}}_i \cdot {\vec{S}}_{i+1},$ is unstable against any strength of bond randomness [1]. The {\it{quasi}}-long-range-ordered phase is indeed destroyed $\forall \langle J_{i}^{2} \rangle \ne 0$ and then replaced by the so-called {\it{Random Singlet Phase}} [1]. Here, we adress the question of the two-dimensionnal case on the square lattice. When non-frustrating randomness in the AF exchanges is introduced, we show that the situation for the following Hamiltonian ${\mathcal{H}}_{2d}=\sum_{\langle i,j \rangle} J_{i,j} {\vec{S}}_i \cdot {\vec{S}}_{j}$, is completely different from the one dimensionnal case. In fact, the extreme robustness of the $T=0$ AF order parameter as well as the appearance of localized excitations with increasing disorder has been studied with the help of several theoretical tools: Exact Diagonalizations, modified Spin-Waves calculations and Quantum Monte Carlo simulations performed at extremely low temperature over thousands of disordered samples and for systems up to $32\times 32$. Our results also lead to more general consideration about Griffiths singularities in random quantum magnets.\\ $[1]$ D. S. Fisher, Phys. Rev. B {\bf 50}, 3799 (1994). [Preview Abstract] |
Session S10: Focus Session: Magnetic Semiconductor Heterostructures
Sponsoring Units: DMP GMAGChair: Jing Shi, University of Utah
Room: LACC 153B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S10.00001: Magnetomechanical Dissipation in (Ga,Mn)As Nanoelectromechanical Resonators E.B. Myers, S. Masmanidis, H.X. Tang, M. Li, M.L. Roukes, K. De Greve, Geert Vermeulen, W. Van Roy When the magnetization of a ferromagnet is changed (e.g., by applying a magnetic field), a stress develops in the material that can change its shape; this is the well-known magnetostriction effect. The magnetoelastic coupling that governs this effect operates conversely as well, in that an applied stress can alter the magnetization state of the material (the magnetomechanical effect). We have studied this coupling in MHz-range nanoelectromechanical resonators fabricated out of the dilute magnetic semiconductor (Ga,Mn)As. We find that the resonator quality factor and piezoresistive signal amplitude vary strongly with both the magnitude and direction of an applied magnetic field. The data can be interpreted in terms of local AC strain in the resonator driving the Mn moments into heavily damped motion at the resonator frequency; comparisons to magnetoelastic theory will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S10.00002: In-plane uniaxial anistropy in epitaxial InMnAs on GaSb determined using the magneto-optical Kerr effect Philip Chiu, Steven May, Bruce Wessels The in-plane magnetic anisotropy of (001) oriented InMnAs/GaSb layers grown by metal organic vapor phase epitaxy was investigated by the longitudinal magneto-optic Kerr effect. The films exhibited ferromagnetic hysteresis at room temperature. An In$_{0.96}$Mn$_{0.04}$As film with a thickness of 50 nm was tetragonal and coherently strained to the GaSb substrate. The c-axis lattice constant was 1.008 times smaller than that of GaSb, in agreement with elasticity theory. These films exhibit uniaxial in-plane anisotropy with the easy axis of magnetization along the [110] direction. Along the easy axis, a square hysteresis loop was obtained with a coercive field of 990 Gauss. In contrast, an In$_{0.93}$Mn$_{0.07}$As film with a thickness of 320 nm was incoherent. The c-axis lattice constant matched that of bulk In$_{0.93}$Mn$_{0.07}$As. The shape of the hysteresis loops for the fully relaxed film was invariant with respect to in-plane rotation. The coercive field for the unstrained InMnAs film was 280 Gauss. The large difference in coercive fields between the two samples is attributed to the disparity in the uniaxial anisotropy constant ($K_{u})$. In addition, the observation of uniaxial anisotropy for only coherently strained films indicates that the anisotropy is due to the twofold symmetry of a zinc-blende (001) substrate surface. These results are consistent with a previous study of magnetic anisotropy of InMnAs epitaxial films grown on InAs and GaAs (001) substrates.$^{ }$(Chiu APL 2004) [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S10.00003: Studies of the magnetic properties of Amorphous Mn-doped GaAs Thin Films W.A. Iwamoto, R.R. Urbano, C. Rettori, P.G. Pagliuso, J.H.D. da Silva, Andr\'e L.J. Pereira, S.B. Oseroff Recently Mn-doped GaAs films have become compounds of great interest due to their potential technological application. In this work, we report experiments of magnetic susceptibility and Electron Spin Resonance (ESR) of Mn$^{2+}$ ion in amorphous thin films of GaAs doped with different concentrations of Mn (0.5 -- 10{\%}) and hydrogenated films with the same Mn concentration. A single nearly temperature independent g $\sim $ 2 line is observed for the Mn-doped films. The presence of Hydrogen is also verified in the ESR spectra by three narrows g $\sim $ 2 lines presumably due to s=1/2 centers with and without hyperfine splitting. The origin of these s=1/2 centers is unclear. Furthermore, our measurements show the absence of the ferromagnetic ordering for all measured films, in contrast to ferromagnetic ordering observed in crystalline films for Tc$\sim $ 110 K [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S10.00004: Heterojunction Engineering of Semiconductor Ferromagnetism Invited Speaker: While bandgap engineering and wavefunction engineering are established in \textit{nonmagnetic} semiconductor heterostructures, we aim to broaden their field to \textit{magnetic} heterostructures and to extend the degree of freedom in designing spin-related properties in semiconductors. In this study, we introduced delta-doping of magnetic (Mn) impurities in the quasi two-dimensional hole gas at the interface of GaAs/$p$-AlGaAs heterostructures, and successfully maximized the ferromagnetic order among the Mn spins by overlapping the hole wavefunction with the Mn delta-doping profile. Selectively $p$-doped heterostructures (Mn-delta-doped GaAs / Be-doped AlGaAs) were prepared by molecular beam epitaxy, in which holes are supplied from the Be-doped $p$-AlGaAs layer to the Mn-delta-doped channel, and ferromagnetic ordering was clearly observed in magnetotransport measurements [1]. In the heterostructure prepared with proper conditions, its ferromagnetic transition temperature ($T_{c})$ was 172 K, higher than the $T_{c}$ of InAs- or GaAs-based random-alloy magnetic semiconductors [2]. It was also found that in more suitably designed heterostructures with low-temperature annealing, $T_{c}$ can be higher than 200 K [3]. Furthermore, we show the control of ferromagnetism in the heterostructures by using gate electric field and light irradiation at relatively high temperatures ($\sim $100 K) [4]. [1] A. M. Nazmul, S. Sugahara, and M. Tanaka, Appl. Phys. Lett. \textbf{80}, 3120 (2002). [2] A. M. Nazmul, S. Sugahara, and M. Tanaka, Phys. Rev. \textbf{B67}, 241308(R) (2003). [3] A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, submitted. [4] A. M. Nazmul, S. Kobayashi, S. Sugahara, and M. Tanaka, Jpn. J. Appl. Phys. \textbf{43}, L233 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S10.00005: Anomalous Hall Effect in a 2DEG Containing Magnetic Impurities John Cumings, David Goldhaber-Gordon, Keh-Chiang Ku, Gang Xiang, Nitin Samarth The has been great interest recently in the role of electron spin in transport properties, specifically in the phenomena of the spin-Hall and anomalous-Hall effects. We present low- temperature transport measurements on a modulation-doped quantum well containing magnetic (Mn) impurities. The effect of the impurities is to cause a strong (free-spin) paramagnetic Zeeman- splitting of the carriers due to s-d exchange interactions. In recent measurements, we observe a Hall voltage that scales with the magnetic moment of the material, a signature of the anomalous-Hall effect. This is surprising, given that the material is not ferromagnetic and does not have a magnetically- ordered ground state. This has important implications for spin- transport in a large class of two-dimensional electron systems. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S10.00006: Coherent Spin Waves in Thin-Film GaMnAs D. M. Wang, Y. H. Ren, R. Merlin, K. Dziatkowski, X. Liu, J. K. Furdyna, M. Grimsditch We report on the observation of coherent oscillations associated with standing spin waves in thin films of the ferromagnetic semiconductor GaMnAs. The oscillations were observed in differential magnetic Kerr measurements using a standard pump-probe setup. Subpicosecond pump pulses from a Ti: sapphire laser induce a coherent precession of the magnetization which is detected by measuring the rotation of the polarization of the delayed probe pulses. The magnetic anisotropy and spin stiffness constants ($D)$ were determined from the magnetic-field dependence of the spin-wave frequencies. We obtain $D=0.7\pm 0.1\times 10^{-13}T\cdot cm^2$ for two as-grown samples ($T_{C~}$= 65 K) and $D=3.5\pm 0.5\times 10^{-13}T\cdot cm^2$ for an annealed sample with $T_{C~}$= 80 K. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S10.00007: Ultrafast Photoinduced Demagnetization in Ferromagnetic InMnAs Chanjuan Sun, Jigang Wang, Junichiro Kono, Akira Oiwa, Hiroo Munekata Mid-infrared pump-probe magneto-Kerr rotation measurements have been performed on ferromagnetic InMnAs in external magnetic fields. Systematic pump power and temperature dependent studies showed ultrafast demagnetization induced by the pump laser pulses. Complete magnetization quenching was observed at high pump powers, implying a photoinduced phase transition from the ferromagnetic state to the paramagnetic state on a femtosecond time scale. During the demagnetization process, three different time scales were revealed, indicating different mechanisms for magnetic order quenching. Interplay among different energy transfer channels will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S10.00008: Theory of carrier dynamics and coherent phonons in ferromagnetic semiconductor heterostructures G.D. Sanders, C.J. Stanton, J. Wang, C. Sun, J. Kono, A. Oiwa, H. Munekata We model generation and propagation of coherent acoustic phonons in two color time resolved reflectivity experiments on ferromagnetic InMnAs quantum wells with thick GaSb barriers grown on a GaAs substrate. Carriers are created in the InMnAs by ultrafast pumping below the GaSb band gap while differential reflectivity is measured above the GaSb gap. The electronic states in the ferromagnetic InMnAs are obtained in an effective mass model and the generation and subsequent relaxation of photogenerated carriers in the well are treated in a Boltzmann equation formalism. Coherent acoustic phonons are generated in the quantum well via electron-phonon interaction with photogenerated carriers. These propagate into the GaSb bulk layer at the LA sound speed modifying the optical properties of the GaSb layer and giving rise to an oscillatory component in the differential reflectivity with a period of $\sim 23$ ps. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S10.00009: Spin relaxation in InGaN/Ga(Mn)N quantum wells W.M. Chen, I.A. Buyanova, K. Nishibayashi, K. Kayanuma, K. Seo, A. Murayama, Y. Oka, G. Thaler, R. Frazier, C.R. Abernathy, S.J. Pearton, Jihyun Kim, F. Ren, F.V. Kyrychenko, C.J. Stanton, C. -C. Pan, G. -T. Chen, J. -I. Chyi, J. M. Zavada Spin relaxation processes in InGaN/GaN and InGaN/GaMnN multi-quantum wells are studied by transient magneto-optical spectroscopy. Nearly no photoluminescence (PL) polarization was observed immediately after pulsed laser excitation (t=0), regardless of the polarization of the excitation light. Afterwards PL gradually becomes $\sigma ^+$ polarized in an applied magnetic field. This polarization build-up is shown to correspond to an additional decay process (50 ps) of the $\sigma ^-$ PL component. With the aid of the exciton Hamiltonian and rate equations, we show that fast spin relaxation ($<$20 ps) is partly responsible for the vanishing optical polarization at t=0. The fast spin relaxation is attributed to carrier spin relaxation at high K-vectors dominated by the D'yakonov-Perel' (DP) mechanism. When the excitons are at rest (K=0), the DP spin relaxation is suppressed leading to a slower spin relaxation (50 ps). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S10.00010: Digital Ferromagnetic Heterostructure Composed of Mn in Si C. Y. Fong, M. C. Qian We examine the electronic and magnetic properties of digital ferromagnetic heterostructures (DFH) composed of Mn atoms confined in a layer of Si by using an \textit{ab-initio} algorithm. The $\delta $-layer can have the composition of Mn$_{1-x}$Si$_{x}$ with x=0 for 1ML, x=0.5 for 0.5ML, and x=0.75 for 0.25ML coverage of Mn. Each DFH is simulated by a tetragonal supercell formed by stacking 8 cubes of the Si crystal in the [100] direction. For each supercell, the value of x determines the area of the plane perpendicular to the [100] direction. For all cases, the total energy difference between the ferro- and antiferro-magnetic phases, the magnetic moments for the ferromagnetic phase, and the density of states at the Fermi energy as functions of x will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S10.00011: Memory function formalism for the optical response of diluted magnetic semiconductors Fedir V. Kyrychenko, Carsten A. Ullrich Because of the possibility to act as effective spin injectors, diluted magnetic semiconductors (DMSs) play a significant role for spintronics applications. Detailed investigation of the transport and optical properties of these materials is thus important for theory and experiment. Present theoretical calculations of the electrical conductivity and dielectric function in DMSs are based on the relaxation time approximation. Usually, the relaxation time is treated as an adjustable parameter or is calculated within the weak scattering approach. However, real III-V DMS samples are highly disordered, which casts doubt on the validity of the relaxation time approximation in general, and the weak scattering approach in particular. We go beyond the relaxation time approximation and treat disorder in DMSs in terms of the memory function formalism. As an example, we present calculations of the dielectric function in the infrared range and compare with the relaxation time approximation and with experimental data. This work is supported by DOE Grant DE-FG02-04ER46151. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S10.00012: First-principles study of ferromagnetic coupling in Zn$_{1-x}$Cr$_x$Te thin film P. Jena, Qian Wang, Qiang Sun Using gradient corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr doped ZnTe in both bulk and thin film configurations. Calculations with full geometry optimization for Zn$_{1-x}$Cr$_{x}$Te supercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn$_{1-x}$Cr$_{x}$Te in both bulk and thin film is energetically the most preferable state irrespective of the concentration and/or site occupation of Cr atom. The strong hybridization between Cr-3$d$ and Te-5$p$ states is found to be responsible for the ferromagnetic coupling - in agreement with recent experiments. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S10.00013: Magnetic Field Dependence of the Specific Heat of Pb$_{1-x}$Gd$_{x}$Te M. Gorska, J.R. Anderson, Y. Dagan, A. Lusakowski, A. Jedrzejczak, Z. Golacki We have measured the magnetic specific heat, C$_{m}$, of Pb$_{1-x}$Gd$_{x}$Te (x = 0.033 and 0.054) at magnetic fields up to 2 T at temperatures from 0.4 to 9 K. This study is complementary to our magnetization measurements on the same system. The samples were prepared as bulk single crystals by the Bridgman method and were n-type with carrier concentrations of about 1 x 10$^{19}$ cm$^{-3}$. The specific heat measurements were made using a Quantum Design PPMS system. We saw evidence for a maximum in C$_{m}$, which shifted to higher temperatures and became broader with increasing magnetic field. At zero magnetic field this maximum is expected to occur below 0.4 K for both samples and will be several times higher than that predicted by a model of superexchange interactions between nearest neighbors, which was based on previous experiments on Pb$_{1-x}$Eu$_{x}$Te.$^{1}$ The present data will be compared with a model that takes into account the spin splitting of the ground state of the individual Gd ions, possibly due to local lattice distortions. $^{1}$ M. G\'{o}rska, A. Lusakowski, A. Jedrzejczak, Z. Golacki, J. R. Anderson, H. Balci, Acta Phys. Pol. A \textbf{105}, 631 (2004). [Preview Abstract] |
Session S12: Focus Session: Superconductivity: Theory and Computation II
Sponsoring Units: DCOMP DCMPChair: Oleg Starykh, University of Utah
Room: LACC 402A
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S12.00001: Eliashberg Theory of Multiband Superconductors Invited Speaker: The application of the Eliashberg strong-coupling theory to multiband superconductors is discussed. The results of calculations are presented for the electronic densities of states in different bands, the tunneling conductance, the free energy and the specific heat. Effects of impurity scattering are discussed with particular emphasis on the two-band case relevant for new superconductor MgB2. The results are compared with the experimental data for doped MgB2 samples. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S12.00002: Nanostructures in Unconventional Superconductors Dirk Morr, Nikolaos Stavropoulos Nanoscale structures by design provide the unique opportunity to reveal the wave-like nature of electrons in condensed matter systems. This has led to the discovery of many new quantum phenomena, such as quantum imaging using electronic waves. In this talk I argue that a new class of quantum effects can be expected to emerge when nanoscale impurity structures are coupled to host systems with complex electronic correlations, such as superconductors. Using some recently developed theoretical approaches, I show that this coupling leads to new types of quantum imaging which possess geometry based selection rules. As a result, one can create nanoscale copying machines, suppress pair-breaking effects of magnetic impurities, and project quantum images ``around the corner". Moreover, nanostructure consisting of more complex building blocks, such as molecules, can be used to manipulate the electronic structure of unconventional $d_{x^2-y^2}$-superconductors in a controlled way. This in turn allows us to gain insight into the nature of electronic correlations and to study the role played by collective modes in the appearance of unconventional superconductivity. Finally, I discuss the relevance of these results for recent scanning tunneling experiments on the high-temperature superconductors. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S12.00003: Numerical study of transport through a mesoscopic superconducting device Lucian Covaci, Frank Marsiglio Starting from the tight-binding description of a superconductor, with the use of the extended Hubbard Hamiltonian, we rely on real-space methods to describe the properties of a superconducting device. The Bogoliubov de Gennes equation are solved for the superconducting device and the Keldysh Green's functions are calculated. We use a perturbation method, first introduced by Caroli et al., which considers the connection of two semi-infinite leads to the superconducting device as a perturbation. The leads can be either in the normal state or in the superconducting state, leading to interesting effects on the Andreev processes inside the superconductor. Using this approach we calculate microscopic currents through a 2D superconducting device. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S12.00004: Mesoscopic fluctuations in disordered superconductors with broken time-reversal symmetry Shinsei Ryu, Akira Furusaki, Andreas Ludwig, Christopher Mudry The mesoscopic regime in the problem of Anderson localization is a scaling regime in which disorder effects remain weak. It can be realized in quasi-one and higher dimensional systems. Fluctuations in the global density of states, the local density of states, as well as in the conductance were first studied for conventional metals in the pioneering works of Stone and Lee on the one hand, and Altshuler, Kravtsov, and Lerner, on the other hand. Here we extend the analysis of the conductance fluctuations by Altshuler, Kravtsov, and Lerner to dirty superconductors with broken time-reversal symmetry in near two spatial dimensions. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S12.00005: Effect of superconducting fluctuations on ultrasound in unconventional superconductor M. Mar'enko, C. Bourbonnais, A.-M.S. Tremblay We study the renormalization of sound attenuation and sound velocity by fluctuation Cooper pairs in layered superconductors. We consider the influence of $s$- and $d$-wave symmetry of the fluctuating order parameter, on both longitudinal and transverse phonon modes. We show that both unconventional order parameter symmetry and transverse sound polarization suppress the AL and MT terms, while the DOS contribution is the least affected. The combination of these effects can change the sign of the overall fluctuation corrections above $T_c$. We also compare the results obtained using the Ginzburg-Landau formalism with a microscopic derivation of the fluctuation corrections in $s$- wave superconductors with a momentum-independent scalar electron-phonon vertex. These calculations are motivated by ongoing ultrasound measurements in organic superconductors. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S12.00006: LDA+DCA calculations of cuprate superconductors Paul Kent, Alexandru Macridin, Mark Jarrell, Thomas Schulthess, Ole Krogh Andersen, Tanusri Dasgupta, Ove Jepsen We present calculations of the properties of realistic models of single-layer cuprate superconductors. A multi-band Hubbard model is obtained from downfolded material specific local density approximation (LDA) density functional theory (DFT) calculations. The on-site U is obtained from constrained DFT calculations. The resulting model is solved using the dynamic cluster approximation (DCA) and quantum Monte Carlo, for small clusters. Some of us have previously shown that DCA calculations of the single band Hubbard model, with empirical parameters, reproduce key features of the experimental phase diagram, including the d-wave superconducting region and pseudogap. In the multi-band model, we find a superconducting region, and discuss how the computed transition temperature depends on the downfolded band structure. In model calculations, we test the sensitivity of the transition temperature to changes in the individual hopping terms, including the copper-oxygen and oxygen-oxygen hybridization. Work supported by the Division of Materials Science and Engineering, U.S. Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle LLC. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S12.00007: Short-Range Correlation Induced Pseudogap in Strongly Correlated Systems Bumsoo Kyung, Sarma Kancharla, David Senechal, Andre-Marie Tremblay, Marcello Civelli, Gabriel Kotliar We investigate the correlation-driven Mott transition as well as the evolution of the Mott-Hubbard insulator into a correlated metal upon doping in the 2D Hubbard model by means of the Cellular Dynamical Mean Field Theory (CDMFT). By comparing the solutions with and without antiferromagnetic long-range order, it is found that at intermediate to strong coupling the dominant physics is well captured by short range spin correlations in the normal state. These short range spin correlations create two 'additional' bands apart from the familiar lower and upper Hubbard bands in the single particle spectrum. Even a tiny doping into the Mott-Hubbard insulator causes dramatic effects - a jump of the Fermi energy to one of these additional bands and an immediate suppression of the spectral weight in the region that is now at the Fermi energy. This suggests a mechanism for the pseudogap phenomenon observed in the normal state of several cuprates. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S12.00008: Quantum phase fluctuations in a correlated lattice $d$-wave superconductor and Cooper pair density-wave in the underdoped cuprates Ashot Melikyan, Zlatko Tesanovic We introduce and study an XY-type model of thermal and quantum phase fluctuations in a two-dimensional correlated lattice $d$-wave superconductor. We investigate the origin of the charge density-wave of Cooper pairs (CPCDW), which we argue is the state behind the periodic modulation of LDOS discovered in recent STM experiments. We illustrate how Mott-Hubbard correlations near half-filling suppress superfluid density and favor an incompressible state which breaks translational symmetry of the underlying atomic lattice. The formation of CPCDW in such a strongly quantum fluctuating superconductor can naturally be understood as an Abrikosov-Hofstadter problem in a type-II dual superconductor, with the role of the dual magnetic field played by the electron density. The resulting Abrikosov lattice of dual vortices translates into the periodic modulation of the BdG gap function and the electronic density. A 4 $\times$ 4 checkerboard modulation pattern naturally arises as an energetically favored ground state at and near the $x=1/8$ doping and produces LDOS in good agreement with experimental observations. Z. Te\v sanovi\' c, Phys. Rev. Lett. {\bf 93}, 217004 (2004), A. Melikyan and Z. Te\v sanovi\' c, cond-mat/0408344. Supported in part by the NSF grant DMR00-94981. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S12.00009: Gaussian fluctuation corrections to the BCS mean field gap amplitude at zero temperature Simon Kos, Andrew Millis, Anatoly Larkin The leading (Gaussian) fluctuation correction to the weak-coupling zero temperature BCS superconducting gap equation is computed. We find that the dominant contribution comes from the high energies and momenta (compared to the gap) and gives a correction smaller by the weak-coupling factor $gN_0$ than the mean-field terms. This correction is small due to cancellation of singular contributions from the amplitude and phase mode at high energies and momenta. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S12.00010: Quantum Fluctuations and Pairing in Strongly Interacting Fermi Systems: A Tractable Crossing-symmetric Approach Khandker Quader, T.L. Ainsworth We carry out a general study of quantum fluctuations and pairing in 3D Fermi systems using a tractable crossing-symmetric approach. The underlying fermionic interactions are taken to be finite-range, non-local, and of arbitrary strength. Our method allows us to obtain quantum fluctuations, such as density, current, spin, and spin-current fluctuations. Pairing interactions in various pairing channels are obtained self-consistently from the competing quantum fluctuations. Here we explore s, p, and d -wave pairing. The sign of the non-local interaction is found to be important for the types of quantum fluctuations exhibited by a system, and consequently the channel of pairing. Consequences for static spin susceptibility and the effective mass are also discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S12.00011: Metal-superconductor quantum phase transition in ultrathin superconducting cylinders Oskar Vafek, Malcom Beasley, Steven Kivelson We consider a metal-superconductor quantum phase transition induced by a magnetic field in a doubly connected mesoscopic sample i.e. in the geometry of a long cylinder. Since the dynamical critical exponent z=2, the eventual 1D quantum criticality can be accessed controllably by $\epsilon$-expansion starting from the upper critical dimension d=2. We calculate the critical exponents as well as the crossover function for the conductivity and compare it with the existing data of Liu et. al. Science, 294, 2332 (2001). Finally, we discuss the nature of the quantum multi-critical point. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S12.00012: Metal-Insulator Transition from Isotope Effects in the Strongly Correlated Local Jahn-Teller Phonon Systems Jong E. Han Electron-phonon coupling in the strong correlated regime has received a great deal of attention in the past few years, particularly motivated by the suggestion that the phonons in cuprate HTSC systems could play significant role in the $d$-wave pairing. Here we investigate a related model of local Jahn-Teller (JT) phonons interacting with electrons in the strong Coulomb limit, using the dynamical mean-field theory with quantum Monte Carlo technique. It is shown that the isotope effects become important with local JT phonons at low phonon frequency as the local polaron regime is approached. The system eventually goes through a metal-insulator transition driven by the isotope effects in such regime. The JT system is qualitatively different from the the non-JT phonon systems in that the metal-insulator transition in JT coupling is not driven by the strong charge fluctuation, but by internal multiplet fluctuation. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S12.00013: Specific Heat and Resistivity of materials with complex Fermi surface topology David Quesada New families of superconducting materials share a common property; they all have a complex Fermi surface topology. The later is a result of the complex dynamics of carriers in these systems and the layered crystalline structure of most of them. In this paper, the electronic specific heat and resistivity have been computed in the normal and superconducting state for three models of Fermi surface: 1. a tetragonal tight binding dispersion law including second order hopping element, 2. a tetragonal tight binding dispersion law including second order hopping element and interlayer hopping that depends only on plane momentum, and 3. a hexagonal tight binding dispersion law. The effect of the proximity of the Fermi level to the Van Hove singularity is analyzed, as well as its relevance to the phenomenon of superconductivity. The calculations in the superconducting state are done for d-wave symmetry and d+s order parameters. Results are compared with angle resolved specific heat measurements and angle resolved photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2005 5:30PM - 5:42PM |
S12.00014: Chiral symmetry breaking in ${QED}_{3}$ in presence of irrelevant interactions: a renormalization group study Kamran Kaveh, Igor Herbut Motivated by recent theoretical approaches to high temperature superconductivity, we study dynamical mass generation in three dimensional quantum electrodynamics (${QED}_{3}$) in presence of irrelevant four-fermion quartic terms. The problem is reformulated in terms of the renormalization group flows of certain four-fermion couplings and charge, and then studied in the limit of large number of fermion flavors $N$. We find that the critical number of fermions $N_c$ below which the mass becomes dynamically generated depends continuously on a weak chiral-symmetry-breaking interaction. One-loop calculation in our gauge-invariant approach yields $N_{c0} = 6$ in pure ${QED} _3$. We also find that chiral-symmetry-preserving mass cannot become dynamically generated in pure ${QED}_{3}$. [Preview Abstract] |
Session S13: uSR and NMR in Superconductors
Sponsoring Units: DCMPChair: Steve Anlage, University of Maryland
Room: LACC 402B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S13.00001: Electronic phase separation in La$_{2-x}$Sr$_x$CuO$_{4+y}$ H.E. Mohottala, B.O. Wells, J.I. Budnick, H.A. Hines, Ch. Niedermayer, A.R. Moodenbaugh, F.C. Chou We have performed a combination of $\mu $SR and bulk magnetization experiments on oxygen-intercalated single crystals of \textit{La}$_{2-x}$\textit{Sr}$_{x}$\textit{CuO}$_{4+y}$, with various Sr contents x. All of the samples were both superconducting with $T_{C}$\textit{$\sim $40 K} and magnetic with a spin density ordering temperature $T_{M}$\textit{$\sim $40 K}. Interpretation of our results suggests that the superconducting and magnetic states arise from separate regions of the sample and comprise separate phases. The coexistence of these phases implies a phase separation that is purely electronic. In fact, we have seen no evidence for a structural phase separation in x-ray diffraction studies. The magnetic region is not superconducting and appears to be closely related to the anomalous, 1/8 hole doped, weakly superconducting versions of \textit{La}$_{2-x}M_{x}$\textit{CuO}$_{4}$. While the superconducting regions are harder to characterize, there is evidence that these regions are completely non-magnetic. This work reveals the subtleties of a more complex relationship between magnetism and superconductivity in the copper oxides than has previously been assumed. This work was partially supported by the US-DOE through contract DE-FG02-00ER45801 and the Cottrell Scholar Program of the Research Corporation. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S13.00002: Muon Spin Rotation Study of Field Induced Magnetism in Overdoped $\mathrm{La_{2-x}Sr_xCuO_4}$ G.J. MacDougall, J. Rodriguez, C.R. Wiebe, G.M. Luke, A.T. Savici, P.L. Russo, Y.J. Uemura, S. Wakimoto, R.J. Birgeneau The interplay between magnetism and superconductivity in the cuprates has been a topic of extensive research over the last two decades. Muon spin rotation ($\mu$SR) is an ideal probe of these properties because it is a volume sensitive local probe of magnetism and superconductivity. A rich diversity of phenomena has been discovered and explored using this technique in different parts of the electronic phase diagram. Unfortunately, such studies have not been extended to the highly overdoped region, largely due to the lack of high quality single crystals. Our group has recently obtained a series of highly overdoped crystals of $\mathrm{La_{2-x}Sr_xCuO_4}$ and performed transverse muon spin rotation (TF-$\mu$SR) studies as a function of magnetic field. A large, field-induced signal relaxation was seen with unusual temperature and doping dependences, extending well into the non-superconducting regime. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S13.00003: Muon spin rotation measurements of the vortex core size in single-gap and multi-gap type-II superconductors Fergal Callaghan, Mikko Laulajainen, Christina Kaiser, Jeff Sonier, Roger Miller We present muon spin rotation ($\mu$SR) measurements of the vortex core size in the single-gap superconductor V$_3$Si and the two-gap superconductor NbSe$_2$. Both temperature and applied magnetic field strongly affect the electronic structure of the vortex cores in these type-II materials, which in turn affects the spatial extent of the cores themselves. Increasing temperature causes excitations of the localized quasiparticle core states, whereas increasing magnetic field results in a delocalization of the core states. The presence of two gaps at the Fermi surface in NbSe$_2$ leads to an unusually large core size at low magnetic fields, with the ratio of the low-field to high-field core sizes being approximately equal to the ratio of the magnitudes of the two energy gaps. We show that the measured behavior of the vortex core size in these materials is fully explained in terms of these effects, and a comparison to recent theoretical work is made. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S13.00004: Penetration depth, symmetry breaking, and gap nodes in superconducting $\bf PrOs_4Sb_{12}$ Lei Shu, D.E. MacLaughlin, R.H. Heffner, G.D. Morris, O.O. Bernal, F. Callaghan, J.E. Sonier, A. Bosse, J.E. Anderson, N.A. Frederick, W.M. Yuhasz, M.B. Maple The vortex-state field distribution in the filled-skutterudite heavy-fermion superconductor PrOs$_4$Sb$_{12}$, in which there is evidence for time-reversal-symmetry (TRS) breaking, has been studied using transverse-field muon spin relaxation (TF-$\mu$SR)\@. The superconducting-state TF-$\mu$SR relaxation rate $\sigma_s(T)$, a measure of the vortex-lattice field distribution width, is found to be nearly constant below ${\sim}T_c/2$. Our results suggest $\lambda(T) \approx \rm const.$ at low temperatures, consistent with a nonzero gap for quasiparticle excitations. Surface penetration-depth measurements in zero static field yield $\lambda(T) - \lambda(0) \propto T^2$, which suggests point nodes in the gap. A similar discrepancy is found in the TRS-breaking superconductor~Sr$_2$RuO$_4$, but not in a number of non-TRS-breaking superconductors, conventional and unconventional. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S13.00005: NMR study of Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ ($x=0.17$) Guoqing Wu, W.G. Clark, S.E. Brown, F. Zamborszky, H. Balci, R.L. Greene Recent studies of the electron-doped high-temperature superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO) show very unusual physical properties at the doping level $x=0.17$. Transport experiments are consistent with the existence of a quantum critical point in the normal state. Within the superconducting state, specific heat measurements in a magnetic field are interpreted as evidence for a transition in superconducting order parameter symmetry. We report the $^ {63,65} $Cu-NMR spectrum and spin dynamics of PCCO single crystals with $x=0.17$ and contrast their properties with crystals of $x=0.15 $. The anisotropic Knight shifts are dominated by the Pr$^{3+}$ moments and change little between the samples. The spin lattice relaxation rates are affected by the doping, as are the linewidths at low temperatures. \\ $^\ast$This work is supported at UCLA by NSF Grants DMR-0334869 (WGC) and 0203806 (SEB). [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S13.00006: NMR Studies of Plutonium-based Superconductors N.J. Curro, T. Caldwell, E.D. Bauer, L.A. Morales, M.J. Graf, Y. Bang, J.D. Thompson, A.V. Balatsky, J.L. Sarrao The intermetallic plutonium-based compound PuCoGa$_5$ exhibits the highest superconducting transition temperature for a heavy- fermion compound (18.5K), and may bridge the gap between the high temperature superconducting transition metal oxides, and the Ce and U-based heavy fermion superconductors. The unusual magnetic and electronic behavior of the plutonium may play a significant role in the pairing mechanism. We have performed extensive Ga and Co NMR investigations of the electric field gradient (EFG), the Knight shift and the spin lattice relaxation rate in both the normal and superconducting states, and our results suggest an unconventional pairing symmetry. Time dependent studies of the EFG suggest the presence of significant changes to the lattice due to the radioactive decay of the Pu. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S13.00007: Vortex vibrations and NMR T$_2$ relaxation in YBCO Rachel Wortis, Ting Lu The similarity of spin echo decay rates for planar and apical oxygen in the vortex state of YBCO suggest that T$_2$ is dominated by vortex motion. We present a calculation of T$_2$ assuming overdamped harmonic vibrations in an anisotropic 3D superconductor with the field applied parallel to the c axis. The resulting field, temperature and position dependence is compared with available data. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S13.00008: Re-examination of the interpretation of NMR spin-lattice relaxation measurements in cuprates Peter Fritz Meier, Anne-Christine Uldry \newcommand {\ybco}[1]{YBa$_2$Cu$_3$O$_{#1}$} \newcommand {\ybcoE}{YBa$_2$Cu$_4$O$_{8}\:$} \newcommand {\lasco}{La$_{2-x}$Sr$_{x}$CuO$_{4}\:$} We have examined measurements of the nucleus spin-lattice relaxation rates on Cu, planar O and Y reported for optimally doped \ybco{7} in the temperature range between 100 and 300 K. In a representation appropriate to anisotropic materials there is no striking different temperature dependence between the three sites. We have analyzed all the data with the model of fluctuating magnetic fields, factorizing the temperature dependence into a product of two temperature-dependent terms $V(T)$ and $\tau_{\mathit{eff}}(T)$. $V(T)$ contains the static antiferromagnetic spin-spin correlations and their changes in temperature which determine the influence of the hyperfine interaction energies. The effective relaxation time $\tau_{\mathit{eff}}(T)$ reflects the dynamics of the spins. We present a fit of the model parameters to the data and compare the model predictions extrapolated to higher temperatures with experiments. The model predicts two independent contributions to $\tau_{\mathit{eff}}$ which will be discussed in detail. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S13.00009: Comprehensive analysis of NMR spin-lattice relaxation data in underdoped cuprates Anne-Christine Uldry, Peter Fritz Meier \newcommand {\ybco}[1]{YBa$_2$Cu$_3$O$_{#1}$} \newcommand {\ybcoE}{YBa$_2$Cu$_4$O$_{8}\:$} \newcommand {\lasco}{La$_{2-x}$Sr$_{x}$CuO$_{4}\:$} We present an analysis of NMR and NQR spin-lattice relaxation measurements reported for \ybco{6.63}, \ybcoE and \lasco in terms of a model of fluctuating magnetic fields. The model parameters obtained by fitting the data vary among the different compounds and depend on the doping level. We compare these changes and discuss the physics behind them. The model generally fits the data well. In particular, high accuracy NQR measurements on the Cu relaxation in \ybcoE are fitted with excellent precision. The model provides also a simple explanation of the observed temperature and doping dependence of $^{63}T_{1c}^{-1}$ in \lasco, which have been measured up to high temperatures. It is shown that the data provide evidence for a crossover from the relaxation dominated by quasi-particle scattering to one governed by local spin fluctuations. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S13.00010: Temperature dependence of nuclear spin shifts in cuprates Samuel Renold, Anne-Christine Uldry, Peter Fritz Meier There exists by now an extended set of data on NMR spin shifts, $^k\! K_{\alpha,spin}$, measured for various nuclear species $k$ and directions $\alpha$ of the external field in a variety of cuprates. We present a phenomenological model that fits the temperature dependence $^k\! K_{\alpha,spin}(T)$ in the normal state of most of the published data in terms of a few parameters (including the pseudo gap) surprisingly well. The dependence of these parameter values on the compound and the doping level is presented and discussed. In particular, the pseudo gap temperature is compared to corresponding values deduced from the nuclear spin-lattice relaxation rates. Furthermore, we comment on the orbital shifts and their relationship to the measured values of $^k\! K_{\alpha,spin}$ at lowest temperatures. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S13.00011: Properties of the N layer inside of a SINIS sandwich-type Josephson junction Ivan Nevirkovets, Oleksandr Chernyashevskyy, John Ketterson We report characteristics of a multi-terminal SINIS device which has electrical leads connected to the middle N layer; here S, I, and N denote a superconductor (Nb), an insulator (AlO$_{x})$, and a normal metal (Al) respectively. Specifically, we studied properties of the middle (10-20 nm thick) N layer in dependence of Josephson current passing through all layers and on an injection current passing through one of the individual (NIS) junctions. The unperturbed I-V curve of the N film displays a small phase-coherent current near zero voltage, which can be suppressed by a very weak magnetic field (applied in parallel to the layers). In the last case, the state of the N film may be regarded as normal; at the same time, a sufficiently high supercurrent can flow between the S electrodes. An injection current (lower than the Josephson current) from one junction induces additional phase-coherent contribution to the I-V curve. We consider possible explanation of the observed behavior. [Preview Abstract] |
Session S14: Focus Session: Hydrogen Storage II: Measurements
Sponsoring Units: FIAPChair: Frederick Pinkerton, General Motors
Room: LACC 403B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S14.00001: Anelastic Spectroscopic Studies of Point Defect Dynamics and Evolution of Chemical Reactions in Alanates Craig Jensen, Rosario Cantelli, Oriele Palumbo, Annalisa Paolone, Sesha Srinivasan, Martin Sulic As part of our effort to characterize the active species in Ti-doped NaAlH$_{4}$ and elucidate its mechanism of action, we have carried out the first measurements of elastic modulus and energy dissipation in Ti-doped and undoped sodium aluminium hydride. We have found that the dehydrogenation of the hydride can be monitored through its effects on the elastic constants. After a well-defined thermal treatment, a relaxation process appears at 70 K in the kHz range, denoting the existence of a new species, likely involving hydrogen, that has a very high mobility. The species is estimated to ``jump'' at rate of 10$^{3}$ s$^{-1}$ at the peak temperature corresponding to a relaxation rate of about 10$^{11}$ s$^{-1}$ at room temperature. The activation energy of the process is 0.126 eV and the pre-exponential factor 7$\cdot $10$^{-14}$ s, which is typical of point defect relaxation. The peak is very broad with respect to a single Debye process, indicating strong interaction or/and multiple jumping type of the mobile entity. The results of these studies will be presented and discussed in terms of their relationship to the mechanism of reversible elimination of hydrogen from the doped hydride. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S14.00002: Electron Paramagnetic Resonance and X-ray Absorption Studies of Fluctuating Titanium Species During the Reversible Dehydrogenation of Ti-Doped Sodium Alanate Meredith Kuba, Craig Jensen, Sandra Eaton, Job Rijssenbeck, Yan Gao As part of our effort to characterize the active species in Ti-doped NaAlH$_{4}$ and elucidate its mechanism of action, we have carried out tandem electron paramagnetic resonance and X-ray absorption studies. We find that upon mechanical milling NaAlH$_{4}$ with 2 mol {\%} TiF$_{3}$, the majority of the titanium is present as a Ti(III) species. However, following a few cycles of dehydrogenation/re-hydrogenation, the majority of the titanium is converted to a Ti(0) species, \textbf{A} that is subsequently replaced by a different Ti(0) species,\textbf{ B} upon further hydrogen cycling. Hydride milled with TiCl$_{3}$, was found to contain mainly the Ti(0) species \textbf{A} and only a minor amount of a Ti(III) component. However, a parallel is seen with the TiF$_{3}$ doped hydride as after 10 cycles tthe Ti(0) species \textbf{A} is seen to completely convert to Ti(0) species \textbf{B}. These results will be presented and discussed in terms of their relationship to the mechanism of reversible elimination of hydrogen from the doped hydride. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S14.00003: NMR studies of hydrogen storage materials: TiCl3-doped NaAlH4 Sean Barrett, Anatoly Dementyev, Dale Li, Rona Ramos, Yanquan Dong An exciting development in the field of hydrogen storage materials was the 1997 discovery that a small amount of Titanium doping can significantly improve the hydrogen discharging/recharging characteristics of sodium alanate (NaAlH$_{4})$. Understanding the dopant action in this ``model'' compound may translate into the rational design of improved storage materials. We report static NMR measurements of both TiCl$_{3}$-doped and undoped NaAlH$_{4}$, including our detection of the Ti-NMR signal. Future directions will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S14.00004: Lattice dynamics of NaAlH$_4$ from high-temperature single-crystal Raman scattering: Evidence of highly stable AlH$_4^-$ anions Eric Majzoub, Kevin McCarty, Vidvuds Ozolins Polarized Raman scattering on single crystals of NaAlH$_4$ has been used to determine the symmetry properties and frequencies of the Raman-active vibrational modes over the temperature range from 300 to 425~K, i.e., up to the melting point $T_{\mathrm{melt}}$. Significant softening (by up to 6\,\%) is observed in the modes involving rigid translations of Na$^{+}$ cations and translations and librations of AlH$_4^{-}$. Surprisingly, the data indicate mode softening of less than 1.5\,\% for the Al-H stretching and Al-H bending modes of the AlH$_4^{-}$ anion. These results show that the AlH$_4^{-}$ anion remains a stable structural entity even near the melting point. The enhanced kinetics of absorption and desorption in Ti-doped NaAlH$_4$ powders is attributed to the effectiveness of Ti in promoting the break-up of the AlH$_4^{-}$ anions. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S14.00005: NMR studies of the metal-hydrogen system ZrNi(H/D)$_{x}$ Caleb Browning, Timothy Ivancic, Robert Bowman, Jr., Mark Conradi Relaxation studies of the intermetallic ZrNiH$_{x}$ and ZrNiD$_{x}$ were performed using hydrogen and deuterium NMR. Correlation times for atomic diffusion were determined based on the temperature dependence of spin-lattice and spin-spin relaxation times. The motion is shown to be thermally activated over the temperature range 200~-~575~K, and the activation energies for diffusion are determined. The deuterium NMR spectra exhibit comparatively little line narrowing with temperature, indicating that the average electric field gradient is not zero, averaged over the deuterium atom sites of these non-cubic cells. Furthermore, the spectrum of ZrNiD$_{1.87}$ reveals a coexistence of two phases, in agreement with the phase diagram. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S14.00006: Hydrogen clathrate hydrate - novel hydrogen storage material: crystal structure, kinetics, and phase diagram Konstantin Lokshin, Yusheng Zhao The detailed crystal structure information for the hydrogen clathrate hydrate was determined by neutron diffraction as a function of temperature (10-300 K) and pressure (1-2000 bar) for the first time. We found that hydrogen occupancy in the (32+X)H$_{2}$*136H$_{2}$O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between 2 and 4 molecules, but keeping single occupancy of the small (dodecahedral) cage in the sII structure. Above 130-160K the guest hydrogen molecules were found in the delocalized state, rotating around the centres of the cages. Decrease of temperature results in the rotation freezing followed by a complete localization below 50 K. We have discovered an extremely fast method of the clathrate synthesis, which allows the complete hydrogen hydrate formation in minutes. The influence of substitutions of different entities for hydrogen on the clathrate structure and stability was studied. High hydrogen capacity (up to 3.77 mass {\%} at ambient pressure), fast kinetics, and readily accessible $P-T$ range are the features that make hydrogen clathrate an excellent candidate for a hydrogen storage material. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S14.00007: Catalytic role of defective carbon substrates in the dissociation of small molecules Milen Kostov, Aaron George, Erik Santiso, Keith Gubbins, Marco Buongiorno Nardelli A necessary step towards the achievement of a hydrogen economy is the development of a production process that is able to drastically revamp the energetic cost while leaving, at the same time, a smaller environmental footprint than the current industry standards. Chemical reactions are often carried out in nano-structured media, where the reaction mechanism can be dramatically changed due to the interactions of reacting species with the substrate. One point of interest is the recent experimental evidence for stable defects in graphene layers such as vacancies. Our aim is to report that physical and chemical properties of such defects can have an astounding effect on certain chemical reactions. Using state of the art first principles modeling techniques, we have explored the potential of nano-structured carbon materials to lower the activation energy barrier of dissociation reactions for small molecules. Using water as a prototypical example, we will show how the carbonaceous environment and the defects present in it, can aid in lowering the activation energy barrier of adsorption and dissociation reactions. Finally we will discuss this exploration in the context of a complete cycle of energy storage and release through the production of hydrogen in defective carbon substrates. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S14.00008: Doping of AlH$_3$ with alkali metal hydrides for enhanced decomposition kinetics Gary Sandrock, James Reilly, Jason Graetz, Wei-Min Zhou, John Johnson, James Wegrzyn Aluminum hydride, AlH$_{3}$, has inherently high gravimetric and volumetric properties for onboard vehiclular hydrogen storage (10 wt{\%} H$_{2}$ and 0.148 kg H$_{2}$/L). Yet it has been widely neglected because of its kinetic limitations for low-temperature H$_{2}$ desorption and the thermodynamic difficulties associated with recharging. This paper considers a scenario whereby doped AlH$_{3}$ is decomposed onboard and recharged offboard. In particular, we show that particle size control and doping with small levels of alkali metal hydrides (e.g., LiH) results in accelerated H$_{2}$ desorption rates nearly high enough to supply fuel-cell and ICE vehicles. The mechanism of enhanced H$_{2}$ desorption is associated with the formation of alanate windows (e.g., LiAlH$_{4})$ between the AlH$_{3}$ particles and the external gas phase. These alanate windows can be doped with Ti to further enhance transparency, even to the point of accomplishing slow decomposition of AlH$_{3}$ at room temperature. It is highly likely 2010 gravimetric and volumetric vehicular system targets (6 wt{\%} H$_{2}$ and 0.045 kg/L) can be met with AlH$_{3}$. But a new, low-cost method of offboard regeneration of spent Al back to AlH$_{3}$ is yet needed. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S14.00009: Ti-based catalytic effect on hydrogen desorption in crystalline NaBH4: an ab initio investigation C. Moys\'es Ara\'ujo, Rajeev Ahuja, Puru Jena The application of hydrogen fuel cell technology in portable electronic devices and transportation vehicles has led to a great deal of interest in the study of complex alkali hydrides (MXH$_{4}$ with M=Na, Li and X=Al,B) primarily due to their high gravimetric hydrogen density (eg.18.5{\%} in LiBH$_{4})$. In particular, NaBH$_{4}$ slurry has been suggested as the most promising system for applications in fuel cell technology (1) as it provides one of the simplest ways of generating hydrogen. Additionally, the NaBH$_{4}$ itself is also a promising hydrogen storage material since it has one of the highest gravimetric hydrogen density (13.0 wt{\%}) among the alkali metal hydrides. However, its irreversibility with respect to hydrogen absorpton/desorption cycle limits its practical application for hydrogen storage. To overcome this limitation we have explored the role of Ti on the electronic and crystalline structures of NaBH$_{4}$. Using density functional calculations we show that Ti prefers to occupy the Na site in sodium borohydride. In addition, Ti weakens the strength of the covalent bond between B and H atoms and the hydrogen removal energy is reduced from 5.64 eV in pure sodium borohydride to 4.70 eV when doped with Ti. Thus, Ti might work as a catalytic agent allowing hydrogen to desorb at a lower temperature. Calculations are underway to examine if other dopants may be even better candidates for hydrogen desorption from sodium borohydride. 1. Z. P. Li, B. H. Liu, K. Arai, K. Asaba and S. Suda \textit{Journal of Power Sources}\textbf{\textit{ }}\textbf{126}, 28 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S14.00010: A role of Ti dopants in catalyzing NaAlH$_4$ from x-ray absorption studies and first-principal density functional calculations. A.Yu. Ignatov, T.A. Tyson, J. Graetz, J.J. Reilly, J. Johnson We have performed Ti $K$-edge XAFS measurements on 2 and 4 mol$\% $ TiCl$_3$ doped sodium alanates. Ti does not enter substitutially or interstitially into the perfect NaAlH$_4$ lattice. A substance formed as a result of multiple hydrogen cycling is of close resemblance of an amorphous TiAl$_3$ alloy with local structure about the Ti atom given by a cluster expansion of Ti-H$_x$-Al$_{10}$-Ti$_2$-... Interatomic distances and Debye-Waller factors are determined for several structural models. These results are elaborated by Ti $K$-edge XANES measurements which are interpreted in terms of single-electron multiple scattering calculations. Main features of the absorption edge are reproduced reasonably well assuming that either 3-5 hydrogen atoms enter the tetrahedron positions of the bulk $I4/mmm$ phase or a few monolayer thick TiAl$_3$ clusters are formed. Structural properties and phase stability of hydrided Ti-Al alloys, NaAlH$_4$, and Na$_3$AlH$_6$, as well as several products of the decomposition reaction were determined at zero temperature within LDA approximation to DFT using LAPW method. The calculations reveal that partial decomposition of NaAlH$_4$ accompanied by formation of TiAl$_3$ alloy is preferred to Ti substitution for Na, in good agreement with our XAFS finding. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S14.00011: Volumetric Analyses of Sieverts Apparatus Data Anne Dailly, Channing Ahn, John Vajo, Robert Bowman, Jr. Sieverts measurements of volumetric excess physisorption adsorption/desorption data at temperatures other than ambient require assumptions related to temperature gradients within the system. In addition, assumptions related to the sample volume need to be considered if this data is not available {\it a priori}. We consider two approaches to Sieverts analysis for data obtained at 77K. In the first approach, we assume that we have three distinct volumes that consist of the reactor, the manifold, and a transition volume between the manifold and the reactor. In this case, the reactor temperature and the manifold temperature are known and measured with thermocouples. The transition volume temperature is assumed to be at a value half way between that of the reactor and the manifold. In the second approach, we make no assumptions about the transition volume temperature, but use a known blank volume within the reactor to produce an ``instrument response" for an assumed sample volume. This data is normalized for the actual volume and used to generate an isotherm. The results of these analyses show that the two approaches yield almost identical isotherm results for hydrogen sorption in a high surface area activated carbon. We will also present data from metal-organic framework (MOF-5 or IRMOF-1) and alkali metal modified MOF structures. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S14.00012: Theoretical characterization of the NHxBHx compounds Maciej Gutowski The NH$_{x}$BH$_{x}$ (x=1-4) compounds display favorable gravimetric and volumetric properties of hydrogen storage. Molecular species have been characterized using highly correlated electronic structure methods. Extended systems (polymers, solids) have been characterized at the density functional level of theory with a Perdew-Wang exchange correlation functional. The results demonstrate unique cohesive properties resulting from dihydrogen intermolecular bonds between protic and hydridic hydrogens of NH and BH, respectively. The kinetic and thermodynamic parameters for hydrogen release and uptake will be discussed. [Preview Abstract] |
Session S15: Focus Session: Transport in Ensemble of Nanocrystals
Sponsoring Units: FIAPChair: V. Klimov, LANL
Room: LACC 405
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S15.00001: Charge transport properties of nanocrystals studied by electrostatic force microscopy Zonghai Hu, Michael Fischbein, Hugo Romero, Marija Drndic Charge transport in semiconductor and metal nanocrystal multilayers between two electrodes is probed by electrostatic force microscopy. The in-plane charge diffusion coefficients are deduced from the charge distribution imaged in real time. Temperature dependence of the transport properties and effects of photoionization and oxidation are also investigated. Implications of these results on the transport mechanisms will be discussed. This work was supported by the ONR Young Investigator Award N000140410489, the American Chemical Society (ACS) PRF award {\#} 41256-G10, and the startup funds at the University of Pennsylvania. MF acknowledges funding from the NSF IGERT program (Grant {\#}DGE-0221664) and SENS. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S15.00002: Memory Effects in CdSe Nanocrystal Quantum Dots Michael Fischbein, Arif Shiliwala, Marija Drndic Memory effects in the charge transport in arrays of CdSe nanocrystals have been observed and characterized. These semiconducting colloidal quantum dots have previously been shown to demonstrate a non-steady state current transient response to the application of a constant negative source-drain voltage bias. In this study we have shown that CdSe nanocrystals display memory of the voltage pulses applied to them. In particular, for a sequence of two negative voltage pulses, the nanocrystals' response to the second pulse will be dependent on the value and duration of the first pulse. We define the first voltage pulse as the ``write'' step and the second voltage pulse as the ``read'' step. To probe the programmability of the nanocrystals, a range of different write steps were performed and the current transients generated by the read steps were characterized. We have demonstrated the ability to undo the effect of the write steps by either shining band gap light on the nanocrystals or by applying a positive voltage bias; such events are naturally defined as ``erase'' steps. The full write-read-erase cycle demonstrates the potential for the application of CdSe nanocrystals to memory technology and offers new information on the charge transport. * This work is supported by the ONR Young Investigator Award {\#} N000140410489, the American Chemical Society PRF award, and the startup funds at Penn. MF acknowledges funding from the NSF IGERT Program. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S15.00003: Photoconductivity Studies of Treated CdSe Quantum Dot Films Exhibiting Increased Exciton Ionization Efficiency Venda Porter, Mirna Jarosz, Brent Fisher, Marc Kastner, Moungi Bawendi We present a photocurrent study of CdSe quantum dot films exhibiting unity internal quantum efficiency as a result of post-deposition treatments. While the photocurrent of untreated films is highly voltage dependent at all voltages, the treated films depend strongly on voltage at low voltage, linearly with voltage above a voltage threshold, and finally saturate at high voltage. The voltage dependence of the treated films can be reproduced with a model assuming blocking contacts and a field dependent exciton ionization efficiency that saturates to unity. The increase in exciton ionization efficiency is a result of increased surface passivation and decreased QD spacing. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S15.00004: Charging of Colloidal Quantum Dots. Spectroscopy and Transport Invited Speaker: Colloidal inorganic semiconductor nanocrystals promise optimized electronic, optical and magnetic properties using size, shape and composition control. It is also interesting to look at the effect of a few charges in such small structures, particularly when placed in the quantum dot states. We observed that charging the colloidal quantum dots leads to electrochromic response covering the visible and IR spectral ranges via changes in the available interband and intraband transitions. In particular, charging the dots leads to reduced absorption at the band edge and this affords a lowered threshold for stimulated emission. Charging also a-priori saturates traps that otherwise impede transport and charged films of quantum dots are indeed observed to be conducting. Shell to shell ( S or P) transport has been observed and, to date, the films conduct via a variable range hopping mechanism in the Coulomb-gap regime. These findings will possibly impact the use of colloidal quantum dots in their opto-electrical applications and they more generally provide support to the growing effort to generate novel materials based on self-assembled organic/inorganic nanostructures. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S15.00005: Electron transport of n-type semiconductor nanocrystal thin films Dong Yu, Congjun Wang, Brian Wehrenberg, Philippe Guyot-Sionnest The conductivity of thin films of n-type colloidal CdSe nanocrystals increases by up to 12 orders of magnitude as the occupation of the first two electronic shells 1Se and 1Pe increases, either by potassium or electrochemical doping. \textit{[D. Yu, C. Wang, P. Guyot-Sionnest, Science 300, 1277 (2003)}] In the low electrical field regime, the conductivity follows $\sigma \sim \exp (-(T^\ast /T)^{1/2})$ in the temperature range 10K$<$T$<$120K. At higher electrical field, the conductivity becomes strongly field dependent. At 4K, the conductance increases by eight orders of magnitude over one decade of bias. At extremely high field conductivity becomes temperature independent, where $\sigma \sim \exp (-(E^\ast /E)^{1/2})$. The conduction behavior follows Efros {\&} Shklovskii's variable range hopping model with Coulomb gap very well and the parameters determined by experiment agree well with the theoretical prediction. \textit{[D. Yu, C. Wang, B. Wehrenberg, P. Guyot-Sionnest, Phys. Rev. Lett. 92, 216802 (2004)]} Our current interest is to dope magnetic impurities like Mn$^{2+}$ inside the NCs. The doped magnetic spins provide strong local magnetic field and large magnetoresistive effect is expected. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S15.00006: Probing electronic transport in SiO2 films containing Si nanocrystals by conductive atomic force microscopy Tao Feng, Harry Atwater Understanding the mechanisms of charge injection into Si nancrystals embedded in SiO2 is the central issue governing device performance in Si nanocrystal nonvolatile memories. We use conductive atomic force microscopy (c-AFM) to study transport in partially etched SiO2 films containing Si nanocrystals fabricated by Si ion-implantation into a 15 nm SiO2 layer on p-Si (001). The 2$\sim $3 nm size nanocrystals were identified by scanning tunneling microscopy. In the c-AFM experiments, correlation between morphologies and tunneling current images shows dependence of tunneling currents on SiO2 thickness fluctuation. Highly localized tunneling paths were recorded and attributed to localized state-assisted tunneling through nanocrystals and/or defects. The electron tunneling current changes much more rapidly with SiO2 thickness than the hole tunneling current, a result explained by the combination of Fowler-Nordheim tunneling and inversion layer formation in the channel for electron tunneling. Current-voltage spectroscopy data showing resonant tunneling and itinerant single electron storage will be presented and discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S15.00007: Controlling the Assembly of Nanorods Inside Electronic Devices Dong Tran, Hugo Romero, Zonghai Hu, Michael Fischbein, Marija Drndic Semiconductor nanorods are versatile nanostructures with exceptional electrical and optical properties that can be exploited for their applications as functional nanoscale devices. The manipulation and assembly of nanorods inside electronic devices are crucial for the study and fabrication of nanoelectronics. Here we present a simple technique to align colloidal suspensions of CdSe nanorods across lithographically pre-patterned metal electrodes on silicon nitrite substrates by an ac electric field. We synthesized CdSe nanorods with diameters of a few nanometers by a conventional chemical technique and the assembly is characterized by AFM and TEM. We probed the nanorod assembly at different frequencies of the applied ac E-field. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S15.00008: Electrical Transport Properties of CdSe Nanorod Solids Hugo Romero, Marija Drndic Semiconductor nanorods are of great interest for fundamental research because they allow us to study how the electronic and optical properties of semiconductor nanocrystals depend on their shape. Nanorods have also attracted much attention because of their potential applications in light-emitting diodes, in low-cost photovoltaic devices, and their propensity to form liquid crystalline phases. So far, most of the studies have focused on the electrical transport in ``spherical'' nanocrystals, where transport mechanisms from variable-range hopping to Coulomb-glass-like behavior have been reported. Compared with nanocrystals, nanorods are expected to exhibit interesting anisotropic effects. We have chemically synthesized the CdSe nanorods and integrated them into electronic devices. Here, we report on our experimental studies of the charge transport in these CdSe nanorod solids. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S15.00009: DC Electrical Transport Properties Of PbSe Nanocrystal Quantum Dot Solids Hugo Romero, Marija Drndic We have studied temperature-dependent electronic charge transport in three-dimensional, closed-packed arrays of PbSe colloidal nanocrystals in the form of thin disordered films. PbSe nanocrystal quantum dots offer unique access to the regime of extreme quantum confinement because of the large Bohr radii of electrons and holes. These materials are expected to have significantly different physical properties from those of the better-known II-VI (CdSe) nanocrystals. Current-voltage characteristics of PbSe nanocrystal arrays show a variety of phenomena, which can be well described using the framework established in the context of transport measurements in metallic quantum dots. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S15.00010: Integrated colloidal nanocrystal and epitaxical quantum nanostructures Atul Konkar, Siyuan Lu, Yi Zhang, Anupam Madhukar Colloidal semiconductor nanocrystals (NCs) and epitaxically grown semiconductor quantum wells, wires, and dots represent two classes of quantum nanostructures which currently serve complementary purposes, the former being well suited for biological applications the latter for electronic and optoelectronic systems as applied to non-hazardous environments. In this work we report on the integration of InAs NCs with InGaAs/GaAs epitaxy based nanostructures through overgrowth on the NCs. We report on the cleaning conditions needed to remove the chemical contamination arising from the solvent during deposition of the NCs on GaAs, the reduction of the as-deposited NC sizes due to the Kelvin effect during thermal treatment, and the GaAs molecular beam epitaxical overgrowth. High-resolution transmission electron microscopy and photoluminescence spectroscopy examinations reveal high quality overgrowth, thus opening the study of a new class of integrated quantum nanostructures that can provide unprecedented functionalities not to be found in either component. Work supported by DARPA/AFOSR under the DURINT program. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S15.00011: Thermopower Enhancement in PbTe with Pb Precipitates Joseph Heremans, Christopher Thrush, Donald Morelli The thermoelectric power of polycrystalline PbTe samples containing nanometer-sized precipitates of Pb metal is enhanced over that of bulk PbTe. Samples of PbTe containing excess Pb and Ag were prepared using conventional metallurgical heat treatments. These samples are shown, by X-ray diffraction, by microscopy, and by the presence of a superconductive transition, to contain Pb precipitates with sizes on the order of 30 to 40 nm. The thermopower enhancement is related to an increase in the energy-dependence of the relaxation time, as evidenced by a complete set of measurements of thermoelectric and thermomagnetic transport coefficients. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S15.00012: Fully Ordered and Nano-Structured Inorganic-Organic Hybrid Semiconductors Yong Zhang, G. M. Dalpian, B. Fluegel, S.-H. Wei, A. Mascarenhas, X.-Y. Huang, J. Li A family of novel inorganic-organic hybrid nanostructures based on II-VI semiconductors has been synthesized, including the first monolayer inorganic/organic superlattices with all covelent bonds (3D structures) and the smallest quantum wires (1D), the chains being formed of single II-VI atomic bonds [1]. These materials are atomistically reassembled crystals without the structural fluctuation typically found in other nanostrutures, and exhibit a number of remarkable properties (e.g., a giant bandgap tunability of 1-2 eV [1,2]). As a prototype system, a 3D structure $\beta $-ZnTe(en)$_{0.5}$ shows a strongly enhanced free exciton absorption (a few times of that in the II-VI binary), Raman lines as sharp as any binary semiconductor, band edge free exciton emission, and more than 10 times enhancement in the exciton binding energy. First-principles density function band structure calculations have been performed to obtain the band gap shift, dispersion relations (effective masses), and dielectric constants of the hybrid material, and the relevant band offsets. [1] X.-Y. Huang, J. Li, Y. Zhang, and A. Mascarenahs, JACS 125, 7049 (2003). [2] B. Fluegel, Y. Zhang, A. Mascarenahs, X.-Y. Huang, and J. Li, PRB 70, 205308 (2004). [Preview Abstract] |
Session S16: Focus Session: Molecular Materials: Electronic Transport and Growth
Sponsoring Units: DMP DCPChair: Douglas Natelson, Rice University
Room: LACC 404A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S16.00001: Ultrathin epitaxial graphite layers : 2D electron gas properties and a route towards graphene based nanoelectronics Claire Berger, Zhimin Song, Tianbo Li, Xuebin Li, Asmerom Ogbazghi, Rui Feng, Zhenting Dai, Alexei Marchenkov, Edward Conrad, Phillip First, Walt de Heer Nanopatterned ultrathin epitaxial graphite structures have been produced by thermal decomposition on single crystal SiC and conventional lithographic techniques. The films, composed of less than 5 graphene sheets, show remarkable 2D electron gas properties. Large positive magnetoconductances and large magnetoconductance anisotropy, indicate that orbital effects dominate the magnetotransport. Shubnikov de Haas oscillations have been observed as well as a pronounced zero-bias anomaly in low-temperature current versus voltage spectra. The films have been gated by applying potentials to gate electrode structures. These results and the control of the nanographite structure suggest nanoelectronic device applications, and a route towards realizing the device potential of nanographite, predicted to be comparable to carbon nanotubes. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S16.00002: Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite Yuanbo Zhang, Joshua Small, Philip Kim We use a unique micromechanical method to extract extremely thin graphite crystallites from bulk highly oriented pyrolitic graphite samples. Electric field effect devices are subsequently fabricated for galvanomagnetic measurements. Strong modulation of magneto- resistance and Hall resistance as a function of gate voltage is observed as the sample thickness approaches the screening length of graphite. Electric field dependent Landau level formation is detected from Shubnikov de Haas oscillations in magneto-resistance. The effective mass of and hole carriers has been measured from the temperature dependent behavior of these oscillations. Extending similar experimental methods to other layered materials will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S16.00003: Electric Field Effect in Planar Single-Layer Graphene Andre Geim, Kostya Novoselov, Da Jiang, Yaun Zhang, Tim Booth, Irina Grigorieva, Sergey Morozov, Anatoly Firsov, Sergey Dubonos We describe free-standing single-layer crystals of graphene, which are one carbon atom thick but extend over many microns laterally. This two-dimensional fullerene macromolecule is obtained by mechanical exfoliation and allows standard microfabrication procedures, as described in our earlier paper in Science \textbf{306}, 666 (2004). We have found graphene to be stable under ambient conditions, conductive and of remarkably high quality. Using graphene films, we have fabricated transistor-like devices and studied their properties from room to liquid-helium temperatures. Graphene exhibits a strong ambipolar electric-field effect with room-temperature mobilities of electrons and holes up to $\approx $10,000 cm$^{2}$/Vs, which implies ballistic transport over submicron distances. At low temperatures, we have observed pronounced Shubnikov-de Haas oscillations and well-developed plateau-like features, indicating onset of the quantum Hall effect. Graphene is found to be a zero-gap 2D semiconductor. Analysis of the quantum oscillations also indicates the linear, Dirac-like spectrum of its carriers. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S16.00004: Electrical Properties of Metallic and Semiconducting Transition-Metal Dichalcogenide Nanopatches. Enrique Cobas, Anthony Ayari, Ololade Ogundadegbe, Michael Fuhrer Metallic and semiconducting nanopatches of MoS$_2$, TaS$_2$ and WSe$_2$ dichalcogenide crystals on SiO$_2$ substrates were fabricated. The crystals were synthesized by a chemical vapor transport method or obtained in natural form and cleaved by mechanical or chemical exfoliation techniques to thicknesses as small as 2nm. Electrical contact to the nanopatches was established via lithographically defined metal leads to allow measurement of electrical properties, including charge carrier mobility and Hall mobility, in field-effect transistor geometries at various temperatures. These nanopatches represent progress toward studies of electron behavior in self-assembled two-dimensional systems. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S16.00005: X-ray topography and high resolution diffraction of single-crystal rubrene B.D. Chapman, R. Pindak, T. Siegrist, C. Kloc Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is an important step in optimizing organic-based electronic devices. Rubrene is an interesting organic semiconductor material with high charge carrier mobility. Improved characterization of the crystalline quality of rubrene is expected to lead to a better understanding of the role of defects on charge transport. Here, we present x-ray topography and high resolution diffraction measurements of high mobility single-crystal rubrene. The topographs reveal many features commonly found in self-nucleated inorganic crystals and they provide important information about the character and distribution of crystallographic defects. In addition, we find significant differences in the topographs of organic crystals that appear equivalent under optical polarization analysis. We demonstrate that x-ray topography is a suitable tool for the optimization of organic crystal growth. We also present complementary AFM and preliminary surface x-ray diffraction measurements of single-crystal rubrene. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S16.00006: Reciprocal Space Mapping in Organic Electronic Materials Theo Siegrist, Christian Kloc, Charley Chi Semiconductor materials have been traditionally studied using X-ray diffraction to asses stress/strain. However, X-ray rocking curves often do not provide a good separation of the mosaic structure and stress induced shifts in the peak positions. To further study the quality of organic semiconductor materials, the mosaic structure needs to be deconvolved from stress/strain induced peak shifts and peak broadening. Reciprocal space map scans were carried out for pentacene crystals obtained from different growth procedures. Large mosaic spreads were observed, however, individual grains are well crystallized with little strain present. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S16.00007: Charge transport and trap spectroscopy in organic molecular crystals Invited Speaker: Key issues in the field of organic semiconductors are the determination of the intrinsic transport properties and of the distribution and the role of the trap states. We report on the use of temperature-dependent space-charge limited current spectroscopy and of field-effect measurement techniques to determine the bulk and surface transport properties and the distribution of trap states in the bulk of organic molecular crystals (OMCs). Systematic studies of structurally and chemically pure rubrene crystals reveal deep states with a density as low as 10$^{15}$~cm$^{-3}$ and an exponentially increasing density of shallow tail states near the mobility edge. Furthermore, we are able to intentionally generate additional traps, determine their density and energetic position within the band gap, and their influence on the bulk transport. Charge transport at the surface of rubrene, tetracene, pentacene and alpha-sexithiophene crystals measured e.g. by a gated four-terminal ``flip-crystal'' FET technique indicates significant charge trapping with an estimated interface state density of 10$^{11}$ to 10$^{12}$~cm$^{-2}$ in high purity OMCs. Being able to measure the energetic distribution of trap states, to create and characterize defect states, and to study their influence on charge transport in OMCs constitutes significant progress in understanding crystalline organic semiconductors on a microscopic level. In collaboration with C. Krellner, S. Haas, K. P. Pernstich, A. Kloke, D. J. Gundlach, and B. Batlogg. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S16.00008: Study of defects in pentacene single crystals Oana D Jurchescu, Jacob Baas, Thomas T.M. Palstra Our research focuses on the growth of the pentacene single crystals with a high degree of purity, and the investigation of their morphology and physical properties. We are able to reduce the number of traps by two orders of magnitude compared with conventional methods. This is reflected in the value of hole mobility of 35 cm2/Vs at room temperature increasing to 58 cm2/Vs at 225 K. These high mobilities result from a purification of the material, that consists of removal of 6,13-pentacenequinone, as the major impurity, using vacuum sublimation under a temperature gradient. We further study the influence of air exposure on the electronic properties of pentacene single crystals. Our observations show that gases can diffuse reversibly in/out the crystals, and influence the electronic properties. We discern two competing mechanisms that modulate the electronic transport. On the one hand stands the presence of water from ambient air that intercalates into the crystal lattice and forms trapping sites for injected charges. On the other hand, the presence of oxygen increases the hole conduction. The latter effect is enhanced by the presence of visible light. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S16.00009: Thin Bismuth Film as a Template for Growth of Highly Ordered Pentacene: STM and LEEM study Jerzy T. Sadowski, Tadaaki Nagao, Yasunori Fujikawa, Shin Yaginuma, Abdullah Al-Mahboob, Toshio Sakurai, Gayle E. Thayer, Ruud M. Tromp Considering that the atomic bonding in bulk Bi can be described as intermediate between “covalent” and “metallic,” one may expect a unique growth mechanism for thin Bi films, different from that observed in the case of metals or semiconductors. In this talk, the results of the scanning tunneling microscope (STM) and low-energy electron microscope (LEEM) investigations will be used to show that thin Bi film undergoes an unique and unexpected structural transformation of the quasi-cubic, {012} oriented film into a hexagonal Bi(001) film [1]. Subsequently, the possibility of using annealed, well ordered Bi(001)/Si(111) films as the templates for the growth of organic thin films with excellent crystallinity will be demonstrated on the example of pentacene – one of the most promising organic semiconductors. Pn nucleates on Bi(001) into highly ordered, crystalline layer, with pentacene molecules “standing up” on the Bi surface, with (001) plane on the growth front. Pn layer is aligned with the Bi(001) surface, having “point-on-line” commensurate relation with the substrate. Moreover, a bulk-like structure is observed in Pn/Bi(001) film, directly from the first Pn layer. [1] T. Nagao et al., Phys. Rev. Lett. 93 (2004) 105501 [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S16.00010: Submonolayer Pentacene Thin Film Growth on Hydrogen-Passivated Si(111) substrate Weijie Huang, Boquan Li, Jian-min Zuo Exploding interests to implement organic semiconductors in novel electronic and optical applications urge the growth of organic thin film with large single crystalline grain and structural homogeneity. By tailoring growing parameters, such as incident flux,substrate surface chemistry and substrate temperature, we succeeded growing pentacene monolayer on a hydrogen-passivated Si(111) substrate, with average grain diameter of 15 micron, by low flux Organic Molecular Beam Deposition(OMBD). We observe that, using atomic force microscope, the morphology of the islands evolve from independent fractal shapes to correlated compact shapes, as the coverage increases, which can be explained by capture-zone growth model. Dynamic scaling behavior of the 2 dimensional pentacene islands is studied, from which critical island size for nucleation on H-Si(111) surface is extracted. Pentacene's large island size and layer-by-layer growth mode on H-Si(111) allow us to study fundamental processes of metal deposition on pentacene,which include metal-carbon bonding formation,crystal structure of the metal cluster on pentacene layers,and the pentacene-step-mediated growth of metal clusters. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S16.00011: Structures and phase transitions in the growth of tetracene on silicon Jun Shi, Xiaorong Qin We report a study on the evolution of the morphology of tetracene thin films during vapor deposition. The film growth has been carried out at room temperature in vacuum on H-passivated silicon substrates. We investigate the surface structures of the films under different growth conditions with an ex-situ atomic force microscopy. The surface structures have been found sensitive to the kinetic parameters such as the coverage and the deposition rate, and in favor of 3D-island formation. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S16.00012: Morphology of benzene and pentacene self-assembled monolayers and gold-molecule-gold junctions. Leonidas Tsetseris, Sokrates Pantelides The structural properties of self-assembled monolayers of organic molecules or nanostructures on metallic or semiconductor surfaces are critical in determining the electronic and transport properties of such configurations. Here we report results obtained by first-principles density-functional calculations on a number of systems. We first describe the morphology of benzene-dithiolate films on gold surfaces. Special emphasis is given on the behavior of the ultrathin film during the deposition of the second electrode, particularly on the kinetics of cleavage of hydrogen and formation of covalent S-Au bonds. We also discuss the work of Xu and Tao\footnote{B. Xu and N. J. Tao, Science {\bf 301}, 1221 (2003)} on formation of molecular junctions, and we analyze the distinct structural phases that can relate to quantum conductance observed in these experiments. Finally, we present results on the growth of pentacene films on Si and SiO$_2$\ surfaces, examining the possibility of forming chemisorbed structures. [Preview Abstract] |
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S16.00013: Growth of pentacene films for transistor applications George Malliaras, Alex Mayer, Randall Headrick Organic thin film transistors (OTFTs) are being developed in academic and industrial labs for ``disposable'' plastic electronics such as smart identification tags. Among the most promising organic semiconductors is pentacene, which yields transistors with performance similar to that of amorphous silicon. The interfaces of pentacene films with dielectric materials (gate oxide) and conductors (electrodes) play a major role in determining OTFT performance. A combination of synchrotron x-ray diffraction and atomic force microscopy was used to probe these interfaces and help optimize pentacene growth. By varying the growth conditions we were able to obtain polycrystalline films with crystallite sizes of the order of tens of microns. The connection between growth, morphology and OTFT performance will be discussed. [Preview Abstract] |
Session S17: Issues in Physics Education
Sponsoring Units: FEd DCOMPChair: Karen Cummings, Southern CT State Univ
Room: LACC 404B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S17.00001: What is physics? The perceptions of middle and high school students in El Paso Eric Hagedorn, Manuela Ortiz The border region of El Paso, Texas contains many families of low socioeconomic and educational background. The majority of local public high schools are not only failing to achieve in math and science, but few of the students take non-required science courses such as physics and chemistry. This is consistent with the disturbing national trend that only 11{\%} of U.S. Hispanic children take both chemistry and physics in high school. In order to increase the number of students prepared for and willing to take physics in high school, we have been investigating the perceptions of middle and high school aged students about science in general and about physics in particular. Because parental attitudes play such a critical role in students development, this study also solicited the parental perceptions of the middle school students. Fifty-eight middle school students and 18 of their parents as well as 130 high school students responded to the prompt: ``Briefly write what you think physics is about.'' Responses were sorted and coded by hand as well as by using the TextSmart software package. Percentages for the various categories are provided as well as interpretation, theoretical explanations and possible educational implications. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S17.00002: Girls, Cars, and Science Beth Parks For the past two summers, I have run an NSF-funded residential camp for girls ages 14-17. This camp is designed to stimulate girls' interest in science by building on their interest in automobiles. The girls spend half the day in hands-on work with cars at Morrisville State College. The other half of the day is dedicated to laboratory exercises at Colgate University that have been designed to help girls learn the science behind the operation of cars. While it is impossible to assess the long-range impact of this program after only two years, the results seem promising. I will discuss the camp program, with particular emphasis on the laboratory experiments that have been developed, which could easily be incorporated into standard high school or college laboratories. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S17.00003: Internships in Public Science Education program: a model for informal science education Greta Zenner, Olivia Castellini, Arthur Ellis, Wendy Crone The NSF-funded Internships in Public Science Education (IPSE) program provides a unique opportunity for undergraduate and graduate students with varied academic background to experience learning and teaching science--specifically nanotechnology--to the general public and middle-school students. The program is in collaboration with Discovery World Museum of Milwaukee, Wisconsin. IPSE interns have created a number of classroom activities ranging from understanding the scale of a nanometer to experimenting with liquid crystal sensors to critically examining the societal implications of nanotechnology. In a new phase of the program, the interns are developing a museum exhibit on nanotechnology to be housed at the Discovery World Museum. Through this experience, intern teams learn about nanotechnology, brainstorm ideas, present and receive feedback on their ideas, and create an exhibit prototype to explain nanotechnology and related science concepts. The program also focuses on professional development, during which interns learn techniques for presenting to non-technical audiences, strategies for assessing their materials, and work on their skills in teamwork, project design, leadership, and science communication. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S17.00004: The LEAPS GK-12 Program Elisabeth Gwinn, Fiona Goodchild, Wendy Ibsen, Marilyn Garza The NSF-funded GK-12 program at UCSB, ``Let's Explore Applied Physical Science'' (LEAPS), awards full fellowships to competitively selected graduate students in the physical sciences and engineering, to support their engagement in local 8th and 9th grade science classrooms. The Fellows' responsibilities to LEAPS total 15 hours per week during the school year. They join consistently in the same classes to collaborate with teachers on delivery of discovery-oriented science instruction. Fellows work in 3-member, interdisciplinary teams. They benefit from this team approach through interaction with colleagues in other disciplines, validation from peers who share enthusiasm for science and mentoring, increased leadership and teaching skills, and a research safety net provided by teammates who can pick up the slack when one Fellow's research requires undivided attention. For teachers, the disciplinary breadth of the Fellow teams is an enormous asset in covering the broad physical science curriculum in CA. Students benefit from hands-on labs and small-group problem-solving exercises enabled by the Fellows' presence and from mentoring by these young scientists. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S17.00005: Graduate Physics Education – Adding Industrial Culture and Methods to a Traditional Graduate Physics Department Ken Vickers, Greg Salamo, Ron Foster, Ronna Turner The education and training of the workforce needed to assure global competitiveness of American industry in high technology areas, along with the proper role of various disciplines in that educational process, is currently being re-examined. Several academic areas in science and engineering have reported results from such studies that revealed several broad themes of educational need that span and cross the boundaries of science and engineering. They included greater attention to and the development of team-building skills, personal or interactive skills, creative ability, and a business or entrepreneurial where-with-all. We will report in this paper the results of a fall 2000 Department of Education FIPSE grant to implement changes in its graduate physics program to address these issues. The proposal goal was to produce next-generation physics graduate students that are trained to evaluate and overcome complex technical problems by their participation in courses emphasizing the commercialization of technology research. To produce next-generation physics graduates who have learned to work with their student colleagues for their mutual success in an industrial-like group setting. And finally, to produce graduates who can lead interdisciplinary groups in solving complex problems in their career field. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S17.00006: What Computational Approaches Should be Taught for Physics? Rubin Landau The standard \emph{Computational Physics} courses are designed for upper-level physics majors who already have some computational skills. We believe that it is important for first-year physics students to learn modern computing techniques that will be useful throughout their college careers, even before they have learned the math and science required for Computational Physics. To teach such \emph{Introductory Scientific Computing} courses requires that some choices be made as to what subjects and computer languages wil be taught. Our survey of colleagues active in Computational Physics and Physics Education show no predominant choice, with strong positions taken for the compiled languages Java, C, C++ and Fortran90, as well as for problem-solving environments like Maple and Mathematica. Over the last seven years we have developed an Introductory course and have written up those courses as text books for others to use. We will describe our model of using both a problem-solving environment and a compiled language. The developed materials are available in both Maple and Mathaematica, and Java and Fortran90\footnote{Princeton University Press, to be published; www.physics.orst.edu/\~{}rubin/IntroBook/}. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S17.00007: Classroom Physics Applications Using The Open Source Physics (OSP) Library Javier Hasbun During the past few years the OSP library$^{1}$ has gained broad usage due to the various workshops held recently, in addition to the development of a wide variety of applications.$^{2}$ Here, three applications are presented in which such library has been employed and which are suitable for classroom use. The first is a 17 function data curve-fitter. The fitter can do up to 5th order polynomials, and includes power, logarithmic, lorentzian, and gaussian curve fitting capabilities. Such applications can be useful in undergraduate laboratories as well as in research involving undergraduates. The second application involves the simulated solution of a charged particle in crossed electric and magnetic fields, in which the 2D cycloidic behavior of the motion can be easily demonstrated. The third and final application involves the extension of the charged particle interacting with Both E and B fields. The motion that results from solving the equations $md^2\overrightarrow{r}/dt^2=q\overrightarrow{v}\times\overrightarrow{B} +\overrightarrow{E}$ is demonstrated in three dimensions with the ability to experiment with the parameters that affect the particle's trajectory as well as the user's viewpoint. The final versions of the applets will be available at http://www.westga.edu/$^\sim$jhasbun/osp/osp.htm, including the source code. \newline $^{1}$ J. E. Hasbun APS. Bull. \underline{49}, 1361 (2004). \newline $^{2}$ http://www.opensourcephysics.org [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S17.00008: MIT’s 3.091 and NSDL Materials Digital Library: Investigating the Role of Digital Libraries in Freshmen Introductory Science Courses with No Lab Component Donald Sadoway, Laura Bartolo One example of the reality facing science educators is the practical impossibility of providing meaningful laboratory experience in large introductory undergraduate science courses. The Materials Digital Library (MatDL), as part of the National Science Foundation (NSF) National Science Digital Library (NSDL) program, investigates issues associated with the delivery of content in materials science and its cognate areas. The focus of the current work is to investigate the feasibility of using data archived in digital libraries to provide freshmen in large introductory science classes a virtual laboratory experience that meets many of the educational objectives of classical laboratory offerings. Also, the question of scalability and broader applicability, e.g., to freshman physics, is under study. The results of a new pilot project launched this academic year will be reported. The virtual laboratory is associated with ``\textit{Introduction to Solid State Chemistry} 3.091,'' a course that over half the freshman class at MIT choose to satisfy the chemistry requirement but which lacks a laboratory component. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S17.00009: Uncertainty Calculations in the First Introductory Physics Laboratory Shafiqur Rahman Uncertainty in a measured quantity is an integral part of reporting any experimental data. Consequently, Introductory Physics laboratories at many institutions require that students report the values of the quantities being measured as well as their uncertainties. Unfortunately, given that there are three main ways of calculating uncertainty, each suitable for particular situations (which is usually not explained in the lab manual), this is also an area that students feel highly confused about. It frequently generates large number of complaints in the end-of-the semester course evaluations. Students at some institutions are not asked to calculate uncertainty at all, which gives them a fall sense of the nature of experimental data. Taking advantage of the increased sophistication in the use of computers and spreadsheets that students are coming to college with, we have completely restructured our first Introductory Physics Lab to address this problem. Always in the context of a typical lab, we now systematically and sequentially introduce the various ways of calculating uncertainty including a theoretical understanding as opposed to a cookbook approach, all within the context of six three-hour labs. Complaints about the lab in student evaluations have dropped by 80{\%}. * supported by a grant from A. V. Davis Foundation [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S17.00010: Interdisciplinary Materials Science at a Small Liberal Arts College: Integration of Curriculum and Research Sasha Dukan, Scott Sibley Faculty members from the departments of Physics and Chemistry at Goucher College have begun an NSF-funded project to increase the exposure of undergraduates to topics in materials science. The centerpiece of the project was development of a team-taught course offered jointly by two departments. The hands-on projects in the course use investigative methods that model modern research collaborations in physics and chemistry and include study of metals, semiconductors, superconductors and other materials presented from the perspective of both disciplines. In this presentation we will report on the progress in the curriculum development as well as discuss the impact that the project has had on the interdisciplinary student/faculty summer research at a small liberal arts college. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S17.00011: Undergraduate Instruction in Nanoscience: Visualizing Quantum Confinement and Resonance Effects in 2DEG Nanostructures Ronald Cosby, Arkady Satanin, Yong Joe Undergraduate learning about the basic physics of electron transport in nanostructures is enhanced by visuals depicting probe-induced conductance modifications. Historically, studies on electrostatically-defined nanostructures in the two-dimensional electron gas (2DEG) at the AlGaAs/GaAs heterostructure interface have revealed the physics of electron transport at the nanoscale. For structures strongly coupled to leads, a single electron picture satisfactorily explains the effects of quantum confinement, including quantization of conductance and the appearance of transmission resonances. Here, to aid undergraduate instruction, we add the basic concept of charge flow impediment by a local scatterer and probe the flow characteristics. Computationally scanning a short-range potential probe in conducting 2DEG nanostructures produces conductance modifications that illustrate propagating modes and resonance conditions. Structures and patterns selected for their instructional value are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S17.00012: Classical analogy of Fano interference Arkady Satanin, Yong Joe The resonances are main object of theoretical and experimental investigations in modern physics and the concept of resonances is ubiquitous in teaching. Fano resonance is phenomena of the interference between the configurations of discrete level and continuum [1]. The main purpose of present work is to give a simple explanation of the nature of Fano resonances. First, we briefly introduce a general feature and manifestation of resonances. We study basic ideas of resonance manifestation in simple mechanical systems by considering a single oscillator and two coupled oscillators. These models provide a main idea about an analytical zero-pole structure of amplitude and phase behaviors near the resonances. Therefore, we obtain the physical meaning of the zero amplitude in these systems. Second, we examine the Fano interference in quantum systems within the help of similar analogy with oscillators. As an example of quantum interference, we give a detailed examination of wave interference in a waveguide with an embedded attractive potential (quantum dot). In particular, the interference between the narrow group of states (quasi-bound or decaying states) and the continuum will be analyzed. We present a general expression for the scattering amplitude with a complex coupling parameter and an explicit expression for the Fano-profile. [1] U. Fano, Phys. Rev. \textbf{124}, 1866 (1961). [Preview Abstract] |
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S17.00013: Nanotechnology, Society, and Freshman, Oh My! Charles Tahan, Wendy Crone, Karin Ellison, Ricky Leung, Clark Miller, Greta Zenner Nanotechnology has emerged as a broad and exciting, yet ill-defined, field of scientific research and technological innovation. Important questions have arisen about the technology's potential economic, social, and environmental implications by prominent technology leaders, nanotechnology boosters, science fiction authors, policy officials, and environmental organizations. We have developed a freshman-level seminar course that offers an opportunity for students from a wide range of disciplines, including the natural and social sciences, humanities, and engineering, to learn about nanoscience and nanotechnology and to explore these questions and reflect on the broader place of technology in modern societies. The course is built around active learning methods and seeks to develop the students' critical thinking and research skills, written and verbal communication abilities, and general knowledge of nanotech. Continuous assessment is used to gain information about how effective the class discussions are and how well the overall course enhances students' understanding of the interaction between nanotechnology and society. [Preview Abstract] |
Session S18: Focus Session: Wide Band Gap Semiconductors IV
Sponsoring Units: DMPChair: Jennifer Zinck, HRL
Room: LACC 406A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S18.00001: Defect generation and annealing of Al-implanted 6H-SiC Gary Gerardi, Ken Jones, Mark Wood, M.A. Derenge, R.D. Vispute, S.S. Hulllavarad Aluminum implantation and high-temperature annealing of 6H-SiC was carried out to determine the thermal treatment needed to produce the shallow Al acceptor and remove intrinsic defects resulting from the implant. A p-type wafer was box implanted with Al to a depth of 0.4 $\mu $m resulting in a concentration of 6 x10$^{19 }$cm$^{- 3}$ on both sides of a wafer originally doped to 1.3 x 10$^{18}$ cm$^{-3}$. Samples were annealed at temperatures ranging from 1300 to 1600$^{\circ}$C. Implantation resulted in a large concentration of intrinsic defects and charge trapping as evidenced by the loss of EPR signal of the shallow Al acceptor from the substrate. Optical absorption measurements indicate amorphization, which was removed by the 1300 $^{\circ}$C anneal. Intrinsic defects are completely removed after the 1600 $^{\circ}$C anneal. No measurable increase of the Al shallow acceptor was found as a result of the implant and annealing suggesting that the implanted Al does not behave in the same manner as Al incorporated during growth even after the implant has been electrically activated. EPR results suggest that the annealing at different temperatures produces different Al-related defects. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S18.00002: Time-dependent photo-induced electron paramagnetic resonance of Vc+ in semi-insulating 4H SiC: evidence of defect relaxation Haiyan Wang, Mary Zvanut SiC is widely studied because of its superior electronic and physical properties. Many investigations focus on defect levels that act as efficient recombination centers and influence the carrier lifetime. We conducted photo-induced electron paramagnetic resonance (photo-EPR) studies of high purity semi-insulating 4H SiC by illuminating the sample with light of selected energy at 4K. Although several different defects were detected in the samples, the presentation will focus on the defect level of the carbon vacancy, V$_{c}$. Steady state photo-EPR indicates that the intensity of V$_{c}^{+}$ increases at 1.8 eV and reaches a peak at 2.3 eV. Time-dependent photo-EPR results for V$_{c}^{+}$ are consistent with those obtained from steady state measurements. The data suggest that when V$_{c}^{+}$ captures an electron from the valence band becoming V$_{c}^{0}$ the energy required is about 1.8 eV, and when V$_{c} ^{0}$ releases an electron to the conduction band becoming V$_{c}^{+} $ the energy required is 2.3 eV. Initial analysis indicates that lattice relaxation accounts for the energy difference between the plus- to-neutral and neutral-to-plus states. At the talk, we will present the details of these measurements and discuss additional support for the defect relaxation model. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S18.00003: Activation of dopants in SiC: theoretical study Oleg Pankratov, Michel Bockstedte, Alexander Mattausch The electrical activation and solubility of dopants are the limiting factors that hamper the manufacturing of a highly doped SiC. In nitrogen-doped SiC a complete electrical activation was achieved \footnote{M.~Laube {\em et~al\/}, J.~Appl.~Phys.~{\bf 91}, 549 (2002).}\textsuperscript {,} \footnote{D.~Schulz {\em et~al\/}, Mat. Sci. Forum {\bf 338-342}, 87 (2000).} only for impurity concentration below $2-5\times 10^{19}$cm$^{-3}$. Yet for phosphorus a full activation was obtained \footnotemark[1] up to $10^{20}$cm$^{-3}$. We study the different activation behavior of these dopants theoretically by an {\em ab initio\/} DFT approach. We find that phosphorus mainly substitutes for silicon, whereas nitrogen is incorporated exclusively into the carbon sites. In a thermodynamic equilibrium the activation of both donors is not limited by the self-compensation. Phosphorus is found to be fully activated until the onset of precipitation. In contrast, nitrogen preferentially incorporates in the neutral nitrogen-vacancy complexes at concentrations above $2\times10^{19}$cm$^{-3}$. This leads to the nitrogen passivation, in agreement with the experimental findings \textsuperscript {1,2}. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S18.00004: Defect studies of Vanadium doped 4H-SiC using optical admittance spectroscopy Wonwoo Lee, Mary E. Zvanut Semi-insulating SiC is an excellent candidate for a variety of applications, including microwave FET's and other devices for high power and high temperature applications. Vanadium acts as an amphoteric impurity in 4H-SiC with a V$^{3+/4+}$ acceptor level thought to be within 1 eV of the conduction band edge and a V$^{4+/5+}$ donor level known to be 1.6 eV below the conduction band edge. Vanadium is an efficient carrier trap and recombination center. We have studied vanadium doped 4H-SiC with the optical admittance spectroscopy (OAS) at room temperature. After taking into account phonon-assisted optical transitions, the estimated threshold energies can be compared with defect levels measured using thermal techniques. Compared with data reported in the literature, our results show that the defect level E$_{c}$ -1.5 eV is close to the vanadium donor level and the other level E$_{c}$ -0.67 eV is within the range of the value attributed to the vanadium acceptor level. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S18.00005: Electrical and optical properties of Mg doped Al$_{0.7}$Ga$_{0.3}$N Alloys M.L. Nakarmi, N. Nepal, J.Y. Lin, H.X. Jiang Al-rich AlGaN alloys are ideal materials for the development of chip-scale optoelectronic devices such as deep ultraviolet (UV) emitters and detectors operating at wavelengths down to 200 nm. Mg doped Al-rich Al$_{x}$Ga$_{1-x}$N epilayers with Al content as high as 0.7 are required as an electron blocking layers in deep UV LEDs ($\lambda <$ 300 nm). However, little has been reported regarding how to achieve p-type Al$_{x}$Ga$_{1-x}$N epilayers with x $>$ 0.3. Achieving Al-rich AlGaN with high conductivities remains as one of the foremost challenges for the Nitride community. We report on the growth and studies of the electrical and optical properties of Mg doped Al$_{x}$Ga$_{1-x}$N epilayers for x $\sim $ 0.7 grown on AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). We found the epilayers to be semi-insulating at room temperature and confirmed p-type conduction at high temperatures ($>$ 700 K) with a resistivity of about 40 $\Omega $cm at 800 K. From the temperature dependent Hall-effect measurement, the Mg acceptor activation energy was estimated to be about 0.4 eV for Al$_{0.7}$Ga$_{0.3}$N alloys. Deep UV photoluminescence (PL) was employed to probe the impurity transitions. We found that the intensity of the 4.2 eV emission line in Al$_{0.7}$Ga$_{0.3}$N is strongly correlated with the resistivity of the materials. Fundamental limit for achieving p-type Al-rich AlGaN alloys and the effects of the Mg doped Al$_{0.7}$Ga$_{0.3}$N electron blocking layer on the deep UV LED performance will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S18.00006: Electrical and optical activation studies of Si-implanted AlxGa1-xN Mee-Yi Ryu, Timothy Zens, Yung Kee Yeo, Robert Hengehold, Todd Steiner Comprehensive and systematic electrical and optical activation studies of Si implanted Al$_{x}$Ga$_{1-x}$N grown on sapphire substrates by molecular beam epitaxial method have been made as a function of ion dose, anneal temperature, and anneal time. Si ions were implanted at 200 keV with doses ranging from 1x10$^{13}$ to 5x10$^{15}$ cm$^{-2}$. The samples were proximity cap annealed from 1100 to 1250$^{\circ}$C for 5 to 25 min with a 500 {\AA} thick AlN cap in a nitrogen environment. Both the sheet carrier concentration and electrical activation efficiency of Si-implanted Al$_{x}$Ga$_{1-x}$N increase continuously with anneal temperature and/or anneal time. The mobility also increase along with the increase in sheet carrier concentrations as the anneal temperature and/or anneal time increases, indicating successive damage recovery with increased anneal temperature and/or anneal time. The luminescence observations are consistent with the results of electrical activation studies. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S18.00007: The Status of p-type ZnO Invited Speaker: In recent years, ZnO has been proposed as an ideal material for new electronic and optoelectronic devices, such as transparent transistors and UV light-emitting diodes (LEDs). However, the LED application will require both n-type and p-type ZnO and the latter is difficult to produce. Potential acceptors include Group V and Group I elements, substituting for O and Zn, respectively. Unfortunately, the Group I elements (e.g., Li) tend to produce semi-insulating material, because they can, in some cases, enter the lattice either as acceptors or donors. On the other hand, the Group V elements, N, P, and As, have all proven to be viable acceptor dopants. Interestingly, theory predicts that P$_{O}$ and As$_{O}$ should be \textit{deep} acceptors, and not highly soluble in ZnO. Thus, it has been proposed that the As acceptor is not As$_{O}$, but rather As$_{Zn}$-2V$_{Zn}$, which should be more soluble and also should have a lower transition energy. Compensating donors must also be minimized, and the most prominent of these are Al$_{Zn}$, interstitial H, and possibly interstitial-Zn complexes. Some ZnO homojunction p-n UV LEDs have already been produced, but more success has been achieved with heterojunction LEDs, using AlGaN as the hole injector. Future prospects will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S18.00008: New insights in the role of native defects in ZnO Anderson Janotti, Chris G. Van de Walle ZnO is a wide-band-gap semiconductor with unique piezoelectric, optical, and electronic properties suitable for use in optoelectronic devices. The availability of bulk single crystals and a large exciton binding energy of 60 meV make ZnO a serious alternative to GaN. However, as-grown ZnO is nearly always $n$-type and the lack of reliable $p$-type doping still hinder its application for light-emitting devices. The unintentional $n$- type conductivity has been attributed to native defects, but the role of individual defects is still controversial. Here we investigate the electronic and structural properties of native defects in ZnO using Density Functional Theory within the Local Density Approximation. We will discuss methods for correcting the band-gap error inherent in density-functional theory. We explore the local atomic relaxations and their direct effect on the electronic structure of each native defect, diffusion barriers, and defect complexes. We will also report results for previously unexplored configurations. Finally, we discuss the influence of native defects on the control of $p$-type doping. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S18.00009: Light-induced metastability in the wide-gap ZnO and CuGaSe$_{2}$ caused by anion vacancies Stephan Lany, Alex Zunger First-principles electronic structure calculations [1] show that anion vacancies in II-VI and chalcopyrite I-III-VI$_{2}$ semiconductors are a class of intrinsic defects that can produce metastable behavior and persistent photoconductivity (PPC), arising from a pronounced coupling between electronic and structural degrees of freedom. In ZnO, V$_{O}^{0}$ has a deep localized donor state in the gap, while V$_{O}^{2+}$ has a shallow level near the CBM. Illumination excites V$_{O}^{0}$ to V$_{O}^{+}$+e and to V$_{O}^{2+}$+2e, and this transition is accompanied by large lattice relaxation. The latter state is metastable and acts as a shallow donor, leading to persistent \textit{electron} photoconductivity ($n$-type PPC), which persists until it is thermally activated into the deep V$_{O}^{0} $. Comparing the behavior of the anion vacancy in the wide-gap chalcopyrite CuGaSe$_{2}$ to that in ZnO, we find an interesting asymmetry: V$_{Se}$ produces persistent \textit{hole} photoconductivity in $p$-CuGaSe$_{2}$, constituting the unusual case where a donor-like defect creates $p$-type PPC. \par [1] Stephan Lany and Alex Zunger, Phys. Rev. Lett. \textbf{93}, 156404 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S18.00010: Resonant State of Substitutional Oxygen in ZnSe W. Walukiewicz, W. Shan, Y.M. Yu, J.W. Ager III, Y. Nabetani We have studied the effect of hydrostatic pressure on low- temperature photoluminescence (PL) spectra of ZnSe doped with oxygen. MBE-grown samples containing up to 2x10$^{19}$/cm$^{3}$ of oxygen have been studied. A broad PL spectral feature associated with the O-states emerges at the pressures around 30- 40 kbar as the fundamental bandgap of ZnSe increases with pressure. It gradually becomes the predominant emission structure and shifts towards higher energy with increasing pressure but at a much slower rate than the exciton emission in ZnSe. By extrapolating the experimental data to the atmospheric pressure, the energy position of the resonant O-states is found to be $\sim$0.20 eV above the conduction-band edge of ZnSe. The location of the resonant state provides a key parameter for modeling the electronic structure of highly mismatched ZnO$_{x} $Se$_{1-x}$ alloy. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S18.00011: Combined Excitation Emission Spectroscopy of Eu ions in GaN V. Dierolf, Z. Fleischman, C. Sandmann, C. Munasinghe , A.J. Steckl Rare earth doped GaN is a promising candidate for light emitters and electrically pumped laser for various applications, such as displays. The rare earth ions are incorporated in this material in various sites, which exhibit different excitation efficiencies and therefore a precise control of these sites is crucial for optimizing potential devices. To this end, we have performed site-selective combined excitation emission spectroscopy studies on Eu-doped GaN layers that have been prepared using the \textit{Interrupted Growth Epitaxy} (IGE) growth technique. In the spectral regime of the single $^{4}$D$_{0}-^{4}$F$_{0}$ excitation transition of Eu$^{3+}$, a large number of excitation peaks ($>$7) can be observed. At least four of them can unambiguously be assigned to different sites. Their relative number depends on growth condition. Selective excitation allows to produce fingerprints of their crystal field splitting in the $^{4}$F$_{1,2}$ states which allow to identify the sites in electro-luminescence and in PL under above-bandgap excitation. In contrasts to this assignment, other strong excitation peaks exist that yield identical emission spectra and must originate therefore from a \textit{single} defect site. The separation of these peaks coincides with GaN Raman frequencies. We therefore suspect that electron-phonon coupled transitions may account for the discrepancy with the single zero-phonon excitation peak that is expected for a single site. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S18.00012: Change in the Structure and Enhancement of Rare-Earth Emission in amorphous GaN and AlN Thin Films S.B. Aldabergenova, H. Mendel, H.P. Strunk We report strong enhancement of Er$^{3+}$, Ce$^{3+}$ , Tb$^{3+}$, Eu$^{3+ }$and Ho$^{3+}$ emission with annealing in mostly amorphous GaN and AlN thin films prepared by DC magnetron co-sputtering in different laboratories. We observe sharp characteristic emission peaks of intra-4f-shell transitions of Er $^{3+}$, Tb$^{3+}$, Eu$^{3+}$ and Ho$^{3+}$ions and a strong but broad peak of 5d-4f emission from Ce$^{3+}$ ions over the temperature range 2-300K. During annealing small crystallites form in the amorphous matrix. The crystallite diameters are between 4 and 7 nm as analyzed by high resolution transmission electron microscopy. We relate strong enhancement of the rare-earth emission with the occurrence of these small crystallites. Different mechanisms of energy transfer from absorbing states in the nanostructured wide band gap GaN and AlN matrix to the rare-earth ions are discussed [Preview Abstract] |
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S18.00013: Can wide-gap chalcopyrite be doped $n$-type Stephan Lany, Yu-Jun Zhao, Clas Persson, Alex Zunger Wide-gap materials are more difficult to dope than their lower- gap counterparts, as evidenced by $n$-doping of Ge $\rightarrow$ Si $\rightarrow$ C or $p$-doping of GaN $\rightarrow$ AlN. Similarly, whereas CuInSe$_{2}$ (E$_{g}$ = 1.1eV) can be $n$- doped via stoichiometry control, its wider-gap counterpart CuGaSe$_{2}$ (Eg = 1.8eV) so far resisted $n$-type doping. Using the defect formation energies calculated from first-principles supercell calculations, we have studied theoretically doping of CuInSe$_{2}$ and CuGaSe$_{2}$ by Cl, Br, I (on Se-site) and Zn, Cd (on metal sites), as a function of chemical thermodynamic boundary conditions. We find that the bottlenecks are proportional to (a) the ease of forming V$_{Cu}$ (an electron killer) and (b) the ease of doping on the wrong site (e.g. Cd-on-In rather than Cd-on Cu). In CIS, halogen doping does not improve over intrinsic doping by InCu, which yields a net donor concentration of ~10$^{18}$cm$^{-3}$ at T = 800 K. A higher net donor doping can be achieved with Cd and Zn doping, but a high compensation ratio is always present. In CuGaSe$_{2}$, both anion-site and cation-site donor doping is intrinsically hampered by overcompensation due to V$_{Cu}$ formation. [Preview Abstract] |
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S18.00014: Localized states due to oxygen in II-VI semiconductors Paul Kent, Clas Persson, Alex Zunger We study the electronic structure of dilute oxygen impurities in II-VIs (ZnTe, ZnSe, ZnS, CdTe, CdSe, CdS) using fully-relaxed large supercell density functional calculations. We analyze the ensuing localized electronic states, finding that oxygen induces ``cluster states'' (CS) near the conduction band edge, similar to nitrogen localized states in the III-Vs. In contrast to nitrogen in the III-Vs however, the CS in II-VI's exhibit a greater degree of delocalization owing to a greater hybridization with the host band structure. The fundamental character, origin, and energetic alignment of the CS is analyzed using the spectral projection method and hydrostatic pressure calculations. We discuss the ramifications of the CS on the band gap bowing, effective masses, absorption and emission spectra of dilute oxygen alloys. [Preview Abstract] |
Session S19: Bulk Semiconductors: Electronic and Optical Properties
Sponsoring Units: DCMPChair: Stefan Zollner, Freescale Semiconductor
Room: LACC 406B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S19.00001: Reexamination of the Ab-Initio Calculation of the Electronic Structure of ZnSe, Ge, and GaAs G.L. Zhao, L. Franklin, D. Bagayoko We reexamined some of the mathematical and physical properties of the ab-initio LCAO calculations for the electronic structure of ZnSe, Ge, and GaAs. The utilization of non-strongly minimal systems in the self-consistent ab-initio calculations could lead to a non-uniformity in approaching the solution in the Ritz-process. We have proposed that an optimum basis set may be needed so that the calculated electron density is converged and the significant scattering of the Ritz-coefficients may be avoided. We have applied the new method to the calculations of the electronic structure of ZnSe, Ge, and GaAs. Our calculated results of the electronic properties agree well with experimental data. Work was funded in part by US NASA (NASA Award No. NCC 2-1344), ONR (Grant No: N00014-04-1-0587], and NSF and the Louisiana Board of Regents (NSF Award Nos. HRD-0000272 and LEQSF(2002-2003)-ENH-TR-57). [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S19.00002: Local self-energy approach for electronic structure calculations Nikolay Zein, Serguei Savrasov, Gabriel Kotliar We implement method for electronic structure calculations which utilizes GW approximation combined with dynamical mean field theory (DMFT). To study the locality of electronic self-energy operator we compared all the relevant quantities as obtained in both R and k spaces. Convergence of the exchange diagram as well of the correlational part for the self-energy within GW and its first vertex correction are checked as functions of cutoff radius in the real space. Our approach permits calculations beyond GW in a controllable manner. Full self-consistency with respect to Green functions is implemented which erases information on the starting point given either by LDA or Hartree-Fock approximations. Results obtained for a number of covalent and ionic semiconductors will be discussed and compared with various existing calculations and experiments. Work supported by NSF, DOE and CMSN. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S19.00003: Ab-initio calculation of excitons in conventional and anorganic semiconductors Claudia Ambrosch-Draxl, Kerstin Hummer, Stephan Sagmeister, Robert Laskowsky, Niels Christensen The excitonic effects on the optical absorption properties of organic as well as inorganic semiconductors are studied from first-principles. The Coulomb interaction between the electron and the hole is accounted for by solving the two-particle Bethe-Salpeter equation. In the organic semiconductors the exciton binding energies strongly depend on the molecular size, the crystalline packing, as well as the polarization direction of the incoming light. We show that the electron-hole interaction can lead to strongly bound excitons with binding energies of the order of 1eV or to a mere redistribution of oscillator strength. In several cases, the screening is efficient enough such that free charge carriers govern the optical absorption process. In the inorganic counterparts the sensitivity of the exciton binding energy is tested against the structural parameters and the screening of the electron-hole Coulomb interaction. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S19.00004: Linear optical response of bulk GaAs from finite-temperature lattice dynamics A. Shkrebtii, M.J.G. Lee, K. Vynck, T. Teatro, D. Zekveld, W. Richter, T. Zettler, T. Trepk Measurements of the linear optical response of semiconductors are frequently carried out at and above room temperature, and linear optical probes are widely used to monitor various physical properties up to the melting point. We have found that Lorentzian broadening of the linear dielectric function, commonly used to model the effects of lattice vibrations, does not accurately describe the temperature dependence of the linear optical response. Instead, we determine the temperature dependence of the linear optical response of bulk GaAs in the temperature range 100 K to 1100 K from representative structural configurations extracted from finite-temperature molecular dynamics for an 8-atom supercell. The dielectric function corresponding to each structural configuration is calculated within the density functional theory (DFT) by the full-potential linearized augmented plane wave (FP-LAPW) method. The finite-temperature dielectric function is deduced by averaging the dielectric functions that correspond to the various structural configurations (about five structural configurations are sufficient for satisfactory convergence). The resulting finite-temperature dielectric functions are in good agreement with the experimental data over the whole temperature range. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S19.00005: Temperature-enhanced far-infrared absorption in GaAs and GaP Hadley M. Lawler, Eric L. Shirley, Simon G. Kaplan Our first-principles calculations and measurements of the far-infrared absorption in GaAs and GaP are presented and compared. Detailed frequency and temperature dependence are reported. Below the reststrahlen feature, a strong enhancement of the absorption is exhibited as the material is heated from 10 K to 300 K. The appearance of a phonon "gap" between acoustic and optical branches in GaP arises from the large ionic mass mismatch in that material, and causes a narrow infrared transmission band above the reststrahlen. Possible sources of discrepancy between experimental and theoretical spectra are addressed. These include the coexistence of two-phonon infrared channels independent of the anharmonic hybridization of two-phonon states with the infrared-active phonon, which is included in the calculation. Additional oscillator strength may be accounted for by considering direct two-phonon dipoles, or the higher-order charges. These additional channels account for the weak infrared absorption in diamond-type materials. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S19.00006: Deformation Electron-Phonon Coupling in Disordered Semiconductors and Nanostructures Andrei Sergeev, Michael Reizer, Vladimir Mitin We study the effective electron-phonon interaction, which is determined by the interference of electron scattering via the deformation potential and elastic electron scattering from impurities and defects. We have found that in contrast to the destructive interference in metals, which results in the Pippard ineffectiveness condition for the electron-phonon interaction, the interference in semiconducting structures substantially enhances the effective electron-phonon coupling. The interference also significantly changes temperature dependence of the electron-phonon relaxation (dephasing) rate in disordered semiconductors and low-dimensional structures. The obtained results provide an explanation of energy relaxation measurements in Si $\delta$-layers. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S19.00007: Bowing in the Compositional Dependence of Optical Transitions in GeSn alloys Vijay D' Costa, Andrew Chizmeshya, Candi Cook, John Kouvetakis, Jose Menendez Recent ellipsometric measurements provide a detailed picture of the band structure of GeSn alloys. In particular, the compositional dependence of the strongest interband optical transitions was determined in the compositional range x $<$ 0.2. The results show very strong deviations (bowing) from a linear interpolation between the band structures of pure Ge and $\alpha$-Sn. In this presentation we analyze the bowing parameters for GeSn alloys and compare them with similar parameters for the SiGe system. The possibility of a comparative study of bowing between these two systems is exciting due to their isoelectronic nature and the similarities in their structural relaxation parameters. Following the theory of Bernard and Zunger [J.E. Bernard and A. Zunger, PRB 36, 3199 (1987)], the bowing is described as the sum of three terms, involving volume deformation, charge redistribution, and internal relaxation. The first and third mechanisms are expected to scale with the lattice constant mismatch, which is much larger in the Ge-Sn system. The second mechanism is proportional to the electronegativity difference, which is also larger in Ge-Sn. The parameters needed for quantitative predictions are obtained from measured volume dependencies of optical transitions or from calculated band structures for Si, Ge, $\alpha$-Sn, SiGe, and GeSn as a function of volume. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S19.00008: Optical properties of InAsP alloys S.G. Choi, C.J. Palmstrom, T.J. Kim, Y.D. Kim, D.E. Aspnes Optical properties of InAs$_x$P$_{1-x}$ ternary alloys grown by chemical beam epitaxy (CBE) on S-doped InP(100) substrates are presented. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were measured by spectroscopic ellipsometry. The $E_0$, $E_1$, $E_1+\Delta_1$, $E'_0$, $E_2$, and $E_2+\delta$ interband critical point (CP) structures were observed in the spectra. By applying the parabolic-band critical point model to numerically calculated second-energy-derivatives of these spectra, accurate values of the CP parameters and their dependence on alloy composition $x$ were obtained. The experimentally determined variation of the $E_1$ and $E_2$ CP energies and the spin-orbit splitting $\Delta_1$ with alloy composition were in good agreement with the theoretical calculation based on the Van-Vechten-Berolo-Woolley (VV-B-C) model. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S19.00009: Optical Constants Determined by Genetic Algorithms David Y. Smith, William Karstens, Shaheen M. Malghani A recent determination$^{a}$ of the complex refractive index, $n$(\textit{$\lambda $}) + i \textit{$\kappa $}(\textit{$\lambda $}), of porous silicon employed a genetic$^{b}$ algorithm to fit the Fresnel equations to reflectance spectra. The procedure appeared to involve more unknowns than explicit equations available for fitting, an indeterminate problem. However, the index values obtained were reasonable, and predicted the properties of porous-silicon multilayes. We have traced this success to the interpolation formulas used for $n$ and \textit{$\kappa $} in the fitting algorithm. They amount to an implicit optical-constant model with the \textit{de facto }assumption of an analytic complex index that can be approximated by a cubic polynomial. Our analysis suggests the procedure can be improved by explicitly using a more appropriate model, e.g., one that uses wave number as the expansion variable and requires that $n$ and \textit{$\kappa $} be even and odd functions of \textit{$\lambda $}, respectively. $^{a}$ V. Torres-Costa, R. J. Mart\'{\i}n-Palma, and J. M. Mart\'{\i}nez-Duart, J. Appl. Phys. \textbf{96}, 4197 (2004). $^{b}$ D. E. Goldberg, \textit{Genetic Algorithms in Search, Optimization and Machine Learning} (Addison-Wesley, Reading, 1989). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S19.00010: Burstein-Moss effect in n-type MBE-grown ZnSe thin films Brian Karrer, Frank Peiris, Brenda VanMil, Ming Luo, Nancy Giles, Thomas Myers Using ellipsometry and prism coupling, we have measured the dielectric functions of a series of Cl-doped ZnSe epilayers grown on GaAs substrates. The carrier concentrations were determined using Hall measurements for samples between $6.30 \times 10^{16}~cm^{-3}$ and $9.50 \times 10^{18}~cm^{-3}$. Variable angle spectroscopic ellipsometry in the energy range between 0.7 and 6.5~eV was used in conjunction with prism coupling to obtain the complex dielectric functions of the specimens. Upon careful examination of the dielectric functions obtained for these thin films, we find that its fundamental band gap blue shifts with respect to the carrier concentration. As our theoretical calculations indicate, the origin of this blue shift is due to the Burstein-Moss effect. The characteristics of the higher order transitions are also determined for this doped system. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S19.00011: Sulfur isotope shift of the gap of PbS H. J. Lian, A. Yang, M. L. W. Thewalt, R. Lauk, M. Cardona PbS is one of the oldest known semiconductors, occurring naturally as the mineral galena. One of its interesting properties is a strong increase of the band gap energy with increasing temperature, opposite in sign to almost all other semiconductors. We report on the isotope shift of the band gap energy between natural PbS (containing mostly $^{32}$S) and PbS made with enriched $^{34}$S, measured using low temperature photoluminescence spectroscopy. The observed isotope shift is also opposite to the `normal' expectation of larger band gap for the heavier mass. In addition, we report on improved measurements of the temperature dependence of the band gap energy measured using absorption spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S19.00012: Microscopic Origin of Infrared Activity in Graphite Gun Sang Jeon, Gerald Mahan We investigate the phonon dispersion of the graphite within a generalized bond-charge model with emphasis on the microscopic origin of the infrared active mode. The resulting dispersion is in good agreement with those obtained by the experiments which include the full optical spectra obtained by the recent x-ray scattering. The computed strengths of the infrared peak are found to be comparable to the experimental values. We show that the dipole moment responsible for the infrared activity is induced by the interlayer Coulomb interaction between $\pi$- bond electrons. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S19.00013: High Resolution Studies of the Electronic Properties of Graphite and the Effect of Disorder S.Y. Zhou, G.-H. Gweon, C.D. Spataru, J. Graf, S.G. Louie, A. Lanzara We report a high resolution Angle Resolved Photoemission Spectroscopy (ARPES) study on the electronic properties of graphite. Data as a function of momentum, photon energy and temperature are presented. We report evidence of very sharp and well-defined quasiparticle peaks. A detailed analysis of the quasiparticle dispersion and scattering rate is presented. Coupling of quasiparticles to collective excitation as well as the effect of interlayer coupling on the quasiparticle is discussed. In addition we report evidence of orientational and potential disorder, coexisting with these sharp quasiparticle peaks and well-defined dispersions. We will give explanations for this paradox and discuss the implications of these results in line with similar effect observed in other correlated materials. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S19.00014: The electronic structure of boron doped diamond probed with XAS and XES P. -A. Glans, K. E. Smith, J. -H. Guo, M. Mattesini, R. Ahuja, S. Ferro, A. De Battisti Diamond is an interesting candidate for ultraviolet light-emitting devices. Device fabrication properly doped thin films of diamond. The valence and conduction band electronic of boron-doped diamond (BDD) has been measured using soft x-ray emission (XES) and x-ray absorption spectroscopy (XAS). Local density approximation of the electronic structure were also performed, and are in general agreement with XES and XAS data. However, XAS reveals the existence of three doped states in the band , only one of which is predicted by theory. Further improvements in terms of chemical and inertness can be obtained by modifying the surface by insertion of fluorine. The of XES and XAS measurements of BDD and fluorinated BDD will be presented. The Boston University program is supported in part by the NSF under DMR 0311792. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S19.00015: A New Photoluminescence Band in Hafnium-implanted Silicon Ravinder Sachdeeva, Andrei Istratov, P.N.K. Deenapanray, Eicke Weber A photoluminescence band in the energy range of 700 meV to 950 meV associated with hafnium implanted in silicon is reported for the first time. Activation of the Hf-optical centers requires a 1000\r{ }C anneal step. The intensity of the PL lines appear to depend on the cooling conditions. The spectrum consists of five peaks in the rapidly quench sample as opposed to twenty one in the slow cool sample. The peak with the highest intensity, occurred in the rapidly quench sample, is found at 943.8meV with two phonon replicas. Temperature and excitation power dependent PL are performed on this peak. It is also found that oxygen coimplantation enhances the PL intensity. A shift in the position of photoluminescence peaks observed on the samples implanted with two different isotopes of Hf confirms that Hfrelated origin of the observed photoluminescence band. [Preview Abstract] |
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S19.00016: Optical Matrix elements in empirical tight-binding with overlap Titus Sandu Calculations of optical and electronic properties of materials are mostly carried out making two assumptions: an orthogonal, atom-like basis and a diagonal coordinate operator in this basis. Thus the intra-atomic matrix elements of coordinate operator are neglected. We show that including the overlap between the orbitals, intra-atomic contributions between different orbitals on the same atom are considered. [Preview Abstract] |
Session S20: Focus Session: Ferroelectric Thin Films
Sponsoring Units: FIAP DMPChair: Fred Walker, ORNL
Room: LACC 407
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S20.00001: Giant Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films Invited Speaker: Epitaxial thin films often have quite different properties than bulk single crystals due to epitaxial and thermal strains arising from substrate constraints. This offers the opportunity to modify ferroelectric properties by heteroepitaxy and strain engineering. Biaxial compressive strain has been used to dramatically enhance the ferroelectric properties of BaTiO$_{3}$ thin films on (110) GdScO$_{3}$ and (110) DyScO$_{3}$ substrates [1]. This strain, imposed by commensurate epitaxy, can result in a ferroelectric transition temperature ($T_{c})$ nearly 500~\r{ }C higher and a remanent polarization ($P_{r})$ at least 250{\%} higher than bulk BaTiO$_{3}$ single crystals. This is the largest increase in $T_{c}$ ever reported for a ferroelectric, and it is consistent with thermodynamic prediction. This work demonstrates a route to a lead-free ferroelectric for non-volatile memories and electro-optic devices, and is a general means for achieving extraordinary physical properties in thin films through strain engineering. [1] K. J. Choi et al. \textit{SCIENCE}, \textbf{306}, 1005 (2004). This work has been done in collaboration with K. J. Choi, M. Biegalski, Y. L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y. B. Chen, X. Q. Pan, V. Gopalan, L.-Q. Chen, D. G. Schlom. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S20.00002: Polarization Scaling in Ultra-thin Epitaxial Ferroelectric Heterostructures: Experimental Results R. Ramesh, V. Nagarajan, J.Q. He, C. Jia, H. Kohlstedt, R. Waser, S. Prasertchoung, T. Zhao, K. Lee, Y.K. Kim, S. Baik Scaling of the structural order parameter and the polarization was investigated in ultra-thin epitaxial PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ /SrRuO$_{3}$/SrTiO$_{3}$ model ferroelectric heterostructures. High Resolution Electron Microscopy and Synchrotron X-Ray studies show that a high tetragonality (c/a$\sim $1.06) is maintained down to 40 {\AA} thick films, suggesting indirectly that ferroelectricity is fully preserved at such ultrathin thicknesses. However, measurement of the switchable polarization ($\Delta P)$ using a novel pulsed probe setup revealed a systematic drop from $\sim 140\mu C/cm^2$ for a 150 {\AA} thick film to $11\mu C/cm^2$ for a 40 {\AA} thick film. This contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180$^{o}$ polydomain state for the thinnest films. This work was supported by MRSEC Grant {\#} 00-8008, DOE Grant DE-FG02-01ER45937 and NSF-DFG Grant 02-44288. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S20.00003: Polarization scaling in ultrathin epitaxial ferroelectric heterostructures: First-principles results Javier Junquera, Philippe Ghosez, Karin M. Rabe To resolve the apparent inconsistency between the high c/a and the low measured switchable polarization of ultrathin Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ (PZT) films \footnote{V. Nagarajan {\it et al.}, previous abstract}, we have carried out first-principles effective hamiltonian simulations. The epitaxial strain constraints and the thickness dependent residual depolarization field $\mathcal{E}$$_d$, arising from an incomplete screening of the dipole surface density by real metallic electrodes, \footnote{ J. Junquera and Ph. Ghosez, Nature {\bf 422}, 506 (2003)} are properly included. As the thickness decreases below 150\AA, the increase of $\mathcal{E}$$_d$ in the uniformly polarized state drives a phase transition to a state with 180$^o$ stripe domains, similar to that observed for PbTiO$_3$ films on insulating substrates \footnote{D. D. Fong {\it et al.}, Science, {\bf 304} 1650 (2004)}. Although the net polarization is zero, each domain exhibits the bulk strained polarization and tetragonality, 1.25 \% larger than in the unstrained sample, yielding a consistent interpretation of the experimental data. Work supported by DOE Grant DE-FG02-01ER45937 [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S20.00004: Kinetics of polarization switching in epitaxial BaTiO3 David Towner, Bruce Wessels The kinetics of polarization switching in BaTiO$_{3}$ ferroelectric epitaxial thin films were studied using optical second harmonic generation (SHG). Epitaxial films were polydomain having both a and c variants. Upon application of a poling field the SHG signal increased according to the expression 1- exp-At$^{n}$ . The rise is attributed to a time dependent increase in the fraction of aligned domains. The kinetic exponent n was of the order of 0.2 indicating fractal dimensionality. The observed kinetics are consistent with a model developed to describe the electro-optic and dielectric relaxation response of polydomain thin films with a continuous distribution of domain sizes (Hoerman PRB 65 2002). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S20.00005: Ferroelectric properties of epitaxial BiFeO3 thin films grown by MOCVD S. Y. Yang, F. Zavaliche, Y.H. Chu, Y.J. Lee, L. Mohaddes-Ardabili, T. Zhao, Q. Zhan, R. Ramesh Recently, perovskite BiFeO$_{3}$ (BFO) has attracted great attention due to the coexistence of ferroelectric and magnetic properties. Particularly, ferroelectric properties in thin films make the BFO an outstanding candidate as a substitute for Pb-based ferro/piezoelectric material. Epitaxial BFO thin films were grown by Metallorganic chemical vapor deposition (MOCVD) equipped with liquid delivery system on SrRuO$_{3}$/SrTiO$_{3}$ (001) using Bi(thd)$_{3}$ and Fe(thd)$_{3}$ as precursors. A systematic study on deposition process control such as stoichiometric composition and growth temperature was carried out. To investigate the effect of thickness on structure and ferroelectric properties, BFO films with thicknesses in the range from 20 to 200 nm were grown. In addition, results will be discussed by comparison with size effect obtained from Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ thin films. This work has been supported by the ONR under a MURI program and partly under the UMD-MRSEC program. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S20.00006: Finite Temperature Properties of KTaO$_3$ Thin Films from First-Principles Alireza Akbarzadeh, Laurent Bellaiche, Kevin Leung, Jorge \'I\~niguez, David Vanderbilt Thin films made of the incipient ferroelectric KTaO$_3$ are studied using a parameterized effective Hamiltonian, $H_{\rm eff}$. Quantum effects are turned off and on by performing classical Monte Carlo and, path integral quantum Monte Carlo simulations respectively. The films are simulated to be grown along the [001] pseudo-cubic direction. Different electrical and mechanical boundary conditions are investigated. Particular striking predictions are (1) that, unlike in the bulk, quantum effects are unable to suppress ferroelectricity in KTaO$_3$ thin films and, (2) the formation of complex ferroelectric nanodomains, depending on the boundary conditions.\\ This work is supported by ONR grants N 00014-01-1-0365, N 00014-04-1-0413 and N 00014-01-1-0600 and NSF grants DMR-9983678 and DMR-0404335 [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S20.00007: Local Electro-Optic Response of Strained SrTiO$_3$ Films Grown on DyScO$_3$ Hongzhou Ma, Jeremy Levy, Mike D. Biegalski, Darrell G. Schlom, Susan Trolier-McKinstry, R. Uecker, P. Reiche The electro-optical response of SrTiO$_3$ thin films grown on DyScO$_3$ substrates was studied with confocal scanning optical microscopy (CSOM) and apertureless near-field scanning optical microscopy (ANSOM). The polarization dependent electro-optical coefficients reveal that the c axis of the strained SrTiO$_3$ is oriented in-plane along the (110) and (-110)directions. The hysteretic electro-optic response at room temperature results from uniform strain in the SrTiO$_3$ film. Time-resolved ANSOM is used to study the domain dynamics at microwave frequencies. This work was supported by the National Science Foundation through grants DMR-0103354 and DMR-0333192, and the U.S. Department of Energy through contract W-31-109-ENG-38. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S20.00008: Local Piezoresponse of Strained BaTiO$_3$/Si Heterostructures Ajay K. Kochhar, Jeremy Levy, Venu Vaithyanathan, Darrell G. Schlom Ferroelectric-silicon heterostructures can provide new functionality with applications for classical and quantum computing architectures. For quantum computing, it is important to have reversible polarization oriented parallel to the growth direction. Strained BaTiO$_3$/relaxed (Ba,Sr) TiO$_3$/Si heterostructures were grown by oxide-MBE. X-ray diffraction measurements indicate an out-of-plane orientation for the BaTiO$_3$ films. Thin (1.5 nm) Au electrodes are deposited on top of the films, and piezoresponse measurements are performed using an atomic-force microscope in contact mode. Local hysteresis curves are imaged by an interleave method that ensures proper registry of successive scans taken at various applied bias voltages. The results suggest that such thin film structures exhibit local hysteresis with varying coercive fields over sub-micrometer regions. This work was supported by DARPA QuIST DAAD-19-01-1-0650. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S20.00009: Ferroelectric domain structure in BiFeO3 films F. Zavaliche, Y. H. Chu, J. Wang, S.Y. Yang, E. Reilly, T. Zhao, Q. Zhan, L. Mohaddes-Ardabili, R. Ramesh In bulk, BiFeO$_{3}$ (BFO) possesses a rhombohedrally distorted perovskite structure; in thin films, the structure is sensitive to heteroepitaxial constraints, and a large polarization was measured. To investigate the ferroelectric domain structure, films of various thicknesses were grown by both pulsed laser deposition, and metal-organic chemical vapor deposition on (001), (110) and (111) oriented SrTiO$_{3}$ (STO) substrates. The ferroelectric domain structure was studied by piezoelectric force microscopy. Contrary to the case of films grown on STO(111), we found that as-grown BFO films on (100) and (110) oriented STO show a strong in-plane polarization component for thicknesses above $\approx $30 nm. This finding is in agreement with the atomistic model of ferroelectricity in distorted rhombohedral BFO. The stability of switched domains is also investigated. This work has been supported in part by the U. of Maryland NSF-MRSEC under grant {\#}DMR 00-80008, and by the ONR under a MURI program. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S20.00010: Size Effect in Ferroelectric BiFeO$_3$ Films T. Zhao, F. Zavaliche, L. Mohaddes-Ardabili, S.Y. Yang, Y.H. Chu, Q. Zhan, H. Zheng, E. Reilly, D.G. Schlom, R. Ramesh BiFeO$_{3}$ (BFO) has recently attracted attention because it is considered as a promising candidate for the lead-free ferroelectric memory cells and/or piezoelectric sensors and actuators. However the understanding of ferroelectricity of BFO is still limited, especially when the vertical and lateral dimensions decrease in thin films. We are studying the ferroelectric and piezoelectric responses of epitaxial BFO films grown by pulsed laser deposition on single crystal SrTiO$_{3}$(STO) (100,110, 111 orientations) as well as STO/Si. The epitaxy of the films were confirmed by TEM and XRD. The electric properties were characterized by polarization hysteresis, pulsed polarization and piezoelectric force microscopy (PFM) measurements. The polarization in BFO is along the (111) direction of its rhombohedrally distorted perovskite structure which is different from the (001)-polarization in tetragonal PZT. We observe a systematic decrease in piezoresponse as the thickness is decreased from 400nm down to 30nm. We will present the results of this systematic study in this paper. This work is supported by an ONR-MURI. [Preview Abstract] |
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S20.00011: Nanoscale smoothing and the analysis of interfacial charge and dipolar densities Javier Junquera, Morrel H. Cohen, Karin M. Rabe, Philippe Ghosez The transfer of charge that occurs in a metal-dielectric interface is a difficult issue, whose deep understanding requires first-principles calculations. The problem then arises about how to extract from the inmense detail provided by the first-principles results the physical quantities of interest. The difficulty comes from the fact that the interface-related dipole densities are overwhelmed by the much larger variations of the total microscopic charge density. Accordingly, nano-smoothing procedures have been developed \footnote{L. Colombo {\it et al.}, Phys. Rev. B {\bf 44}, 5572 (1991)} in order to localize the physically relevant charge densities to the interface. Here we discuss the criteria for validity that the smoothing procedure should meet to leave the physical quantities unaffected. We have applied the model to compute directly the polarization charge density of a realistic ferroelectric capacitor \footnote{J. Junquera and Ph. Ghosez, Nature {\bf 422}, 506 (2003)} and find it to be much smaller than the bulk ferroelectric polarization. We attribute this reduction to the penetration of the metallic wave functions into the ferroelectric, which leads to screening of the polarization charge within the ferroelectric itself. Work supported by DOE Grant DE-FG02-01ER45937 [Preview Abstract] |
Session S21: Cellular Biomechanics
Sponsoring Units: DBPChair: Alexander Neiman, Ohio University
Room: LACC 409A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S21.00001: Simulation of Actin-Polymerization-Mediated Propulsion Kun-Chun Lee, Andrea Liu An important component of the cellular cytoskeleton is F-actin, a biopolymer whose self-assembly is key to the process of cell crawling. The polymerization and branching of F-actin near the cell membrane is known to drive cell crawling, but the precise mechanism by which these processes lead to the generation of a mechanical force is still controversial. We have constructed a Brownian dynamics simulation of F-actin polymerizing near a surface, which includes all known important processes, including polymerization, depolymerization, branching, crosslinking and capping. Using this model, we study the dynamics of the moving surface in conjunction with the stresses in the system. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S21.00002: Hyaluronan-mediated cellular adhesion Jennifer Curtis, Christian Schmitz, Joachim Spatz Many cells surround themselves with a cushioning halo of polysaccharides that is further strengthened and organized by proteins. In fibroblasts and chrondrocytes, the primary component of this pericellular matrix is hyaluronan, a large linear polyanion. Hyaluronan production is linked to a variety of disease, developmental, and physiological processes. Cells manipulate the concentration of hyaluronan and hyaluronan receptors for numerous activities including modulation of cell adhesion, cell motility, and differentiation. Recent investigations by identify hyaluronan's role in mediating early-stage cell adhesion. An open question is how the cell removes the 0.5-10 micron thick pericellular matrix to allow for further mature adhesion events requiring nanometer scale separations. In this investigation, holographic optical tweezers are used to study the adhesion and viscoelastic properties of chondrocytes' pericellular matrix. Ultimately, we aim to shed further light on the spatial and temporal details of the dramatic transition from micron to nanometer gaps between the cell and its adhesive substrate. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S21.00003: A 3-D Biophysical Model of Mitotic Spindle Formation Stuart Schaffner, Jorge Jose The mitotic spindle is the scaffolding on which plant and animal cell division occurs. It is known that under certain circumstances the spindle self-assembles, using only a few functional elements. We have been developing increasingly realistic biophysical models of spindle self-assembly. Our earlier 2-D model produced spindle patterns under certain conditions. Our more realistic 3-D model is defined by coupled Langevin equations that mimic the mechanical and thermal interactions between microtubules and molecular motors. Microtubules pivot on fixed kinesin motors and are drawn into poles by dynein motors. The distribution of pivot points form boundary conditions whose spherical asymmetry guides spindle formation. Unlike the 2-D model, the 3-D model correctly handles microtubule entanglement. Initial runs of the 3-D model show that spindle self-assembly is indeed possible under certain conditions. We are currently performing calculations to determine how parameter changes affect spindle formation and pattern morphology. In particular, we are varying dynein motor processivity, the degree of spherical asymmetry, and dynein motor concentrations. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S21.00004: Self-organization of cytoskeletal systems: formation and dynamics of bundles, rings, and spindles Alexander Zumdieck, Karsten Kruse, Frank J\"ulicher The cytoskeleton is a complex network of protein filaments. Driven by active processes such as filament polymerization and depolymerization and the action of molecular motors, it represents an active system which by self-organization can form dynamic patterns and exhibit active mechanical properties. Starting from a microscopic picture, we develop a coarse grained description for the dynamics of bundles of filaments and motors in the presence of filament polymerization and depolymerization. We show that filament treadmilling in the presence of passive cross-linkers can, similarly to motor proteins, generate tensile stresses that may result in bundle contraction. Motivated by contractile rings that cleave cells during cell division we extend our description to cylindrical geometries and show that filament rings with contractile properties can form by self-organization phenomena. Furthermore we discuss the stability of bipolar spindles, taking into account the simultaneous action of several types of motor proteins. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S21.00005: MSP dynamics and retraction in nematode sperm Charles Wolgemuth, Long Miao, Orion Vanderlinde, Tom Roberts, George Oster Most eukaryotic cells can crawl over surfaces. In general, this motility requires three distinct actions: polymerization at the leading edge, adhesion to the substrate, and retraction at the rear. Recent \textit{in vitro }experiments with extracts from spermatozoa from the nematode \textit{Ascaris suum} suggest that retraction forces are generated by depolymerization of the Major Sperm Protein (MSP) cytoskeleton. Combining polymer entropy with a simple kinetic model for disassembly I propose a model for disassembly-induced retraction that fit the \textit{in vitro }experimental data. This model explains the mechanism by which deconstruction of the cytoskeleton produces the force necessary to pull the cell body forward and suggest further experiments that can test the validity of the model. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S21.00006: Untwisting the mystery of supercoiling: Mbl configuration in growing bacterial filaments Sulav Mukherjee, Barbara Setlow, Peter Setlow, Charles Wolgemuth \textit{Bacillus subtilis}, a commonly studied prokaryote form long filaments, or chains of cells, when the cells fail to separate upon replication. These mutants undergo supercoiling where the bacterial filament buckles and wraps about itself like an over-twisted phone cord. It has long been supposed that twisting stress is generated in the cell wall during growth and causes this coiling. But, the twisting mechanism has remained an enigma. A recently discovered actin-like protein, Mbl, forms helical structures under the cell wall and controls cell morphogenesis in \textit{B. subtilis}. Based on these findings, a new model suggests how these helical structures could lead to supercoiling. We report here experiments connecting growth, Mbl structure, and supercoiling. We have studied the helical pitch of the Mbl under regular growth conditions, various concentrations of xylose, and under the influence of different concentrations of ammonium and magnesium. These experiments demonstrate how growth effects the configuration of the Mbl cables and suggest that growth induced deformation of the Mbl cables generate twist in the filaments, which eventually leads to supercoiling in bacterial filaments. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S21.00007: Stochastic description of pilus retraction dynamics Martin Lind\'en, Emil Johansson, Mats Wallin, Ann-Beth Jonsson Motility of certain gram-negative bacteria is mediated by retraction of type IV pili surface filaments, which are essential for infectivity. Type IV pili are helical filaments with 4 nm periodicity and 5 subunits per turn. The retraction is powered by a strong molecular motor protein, PilT, producing very high forces in excess of 100 pN[1]. One possible explanation for the high forces are that several ATP are hydrolyzed to retract each subunit.\\ We consider a widely used class of discrete hopping models, which has been used to describe well-known motor proteins such as kinesin[2] and myosin[3]. The model describes recent experimental measurements[1] on \emph{Neisseria gonorrhoeae} well, and makes several interesting predictions for the randomness of the retraction dynamics.\medskip\\ 1. Maier et al, PNAS 101:10961 (2004)\\ 2. M. E. Fisher and A. B. Kolomeisky, PNAS 98:7748 (2001)\\ 3. A. B. Kolomeisky and M. E. Fisher, Biophys. J. 84:1650 (2003) [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S21.00008: Why nozzles are required for bacterial gliding? Junhwan Jeon, Andrey Dobrynin Many microorganisms transduce an energy stored during polymerization reactions into mechanical force propelling them over surfaces. For example, cyanobacteria has nozzles-like organelles secreting a polysaccharide gel. On the other hand, listeria propels itself through the cell by polymerizing a network of actin filaments from its surface. The actin filaments have a persistence length of the order of $2-10\mu m$ leading to the high value of Young's modulus ($10^3-10^4$Pa). The polysaccharide gel has lower shear modulus ($10^2$Pa) than that of the actin gel and does not have sufficient strength to support compression and propel bacteria. In this case nozzles play a role of the compression chambers that improve the elastic properties of a gel resulting in bacteria translocation. To elucidate the effect of chain rigidity on bacterial motility, we performed molecular dynamics simulations of crosslinked semiflexible polymers growing inside a nozzle-like orgenelle and from the surface of a bacteria. We have analyzed the correlation between object's velocity and the chain stiffness. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S21.00009: Force Regulation in Living Tissue Yusuke Toyama, Xomalin G. Peralta, Stephanos Venakides, Daniel P. Kiehart, Glenn S. Edwards Forces within tissue are involved in the shaping of an embryo. Measuring the net forces exerted by groups of cells and tissues in live organisms is a challenging endeavor. Quantitative physical modeling based on experimental results has the potential for increasing our understanding of this question. We use a developmental stage in Drosophila embryo known as dorsal closure, which to a large extent occurs in two dimensions and is a consequence of four biological processes that are synchronized in time and coordinated in space [1]. To reveal the interaction of forces and tensions within the components of these tissues, we use a steerable UV-laser microbeam to cut them and monitor the resulting behavior with confocal microscopy. We present experimental evidence for an increase in the force that could be a consequence of the laser cut supported by a quantitative model and a biological mutant. [1] M. S. Hutson, et al. Science, 300, 145 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S21.00010: The TITS Algorithm: A Simple and Robust Method for Calculating Stable Shapes of Axisymmetric Vesicles Gerald Lim I have implemented a simple and robust numerical technique for calculating axisymmetric equilibrium shapes of one-component lipid bilayer vesicles. This so-called Tethered Infinitesimal Tori and Spheres (TITS) Algorithm gives shapes that are automatically stable with respect to axisymmetric perturbations. The latest version of this algorithm can, but is not restricted to, impose constraints on any of three geometrical quantities: the area, volume and pole-to-pole distance (in the case of tether formation). In this talk, I will introduce the basic principles of the TITS Algorithm and demonstrate its versatility through a few example shape calculations involving the Helfrich and Area Difference Elasticity bending free energies. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S21.00011: Microinterferometric demonstration of actively driven bending excitations of the cell envelope of mouse macrophages. Alexandra Zidovska, Erich Sackmann We observed pronounced undulation excitations of cell envelope of weakly adhering macrophages. This so called flickering of the cell membrane gives rise to strong entropic disjoining pressures which are consequence of freezing in long wavelength undulations of adhering cells. Membrane fluctuations were analyzed by Reflection Interference Contrast Microscopy (RICM) with $\sim $ 0.3 micron lateral and $\sim $ 1 nm vertical resolution. Under physiological conditions we observe amplitudes of 8-10 nm corresponding to apparent bending moduli of the order of $\kappa \quad \sim $ 1000 k$_{B}$T. This anomously small bending stiffness very strongly suggests that the excitations are driven by fluctuating biochemical forces. Latrunculin (an actin polymerization blocker) causes softening of the cell envelope resulting in an increase of the membrane undulations amplitude up to 20 nm. Sequestering of intracellular Ca$^{++}$ by the chelator BAPTA leads to a lowering of the membrane fluctuation amplitude to 5-7 nm. The apoptosis inducing agent camptothecin induced strong reductions of the fluctuating amplitudes to 3-4 nm. The capillary length and elastic moduli were determined using the discrete Fourier Transformation. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S21.00012: Dynamics of Caveolae in Endothelial Cells Meron Mengistu, Linda Lowe-Krentz, H. Daniel Ou-Yang The blood flow subjects endothelial cells to various shear stress conditions, regulating the formation and localization of caveolae for macromolecular transport and potentially mechanosensing. We simulate this condition by exposing cultured bovine endothelial cells to various flow conditions in flow chambers. Using GFP-constructs of caveolar markers such as caveolin-1, dynamin II, and intersectin, we targeted caveolae with optical tweezers laser as probes to measure changes in viscoelastic properties that the cell undergoes in the different flow conditions. We also tracked the transport of fluorescently labeled Bovine serum albumin (BSA) through caveolae using confocal microscopy. This technique allows us to study the transport dynamics of caveolae once they are internalized in endothelial cells. Integrating optical tweezers and confocal fluorescence microscopy will allow us to measure the micro-mechanical properties of caveolae and give us insights into its function as a mechanosensor as well as its role in transcytosis. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S21.00013: Cellular tolerance to pulsed heating Dmitri Simanovskii, Daniel Palanker, Alan Schwettman, Mainak Sarkar, Afraz Irani, Caitlin C. O'Connell-Rodwell, Christopher Contag In many medical applications knowledge about the threshold temperature leading to irreversible cellular damage is critically important. We study the dependence of the threshold temperature on duration of the heat exposure in the range of 0.3 ms to 1 second. Thin layer of cells cultured in a Petri dish was exposed to a pulsed CO$_{2}$ laser radiation. Laser beam was focused onto a surface of Petri dish providing Gaussian intensity distribution in the focal plane with a typical beam diameter (2w) 10 mm. Surface temperature in the central part of the focal spot (1mm in diameter) was measured by thermal IR emission from the sample recorded with a fast (ns) MCT detector. For pulses shorter than 1 s the temperature profile across the focal spot was found to closely correspond to the radial distribution of the laser beam, thus allowing for accurate determination of temperature at any given distance from the center of the spot. Immediate cellular damage was assessed using vital staining with the live/dead fluorescent assay. Threshold temperatures were found to vary from 55 $^{o}$C at 1 s of heating to 160 $^{o}$C at pulses of 0.3 ms in duration. The shorter end of this range was limited by vaporization which occurs during the laser pulse and results in mechanical damage to cells. [Preview Abstract] |
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S21.00014: A physical basis for actin filament lengths in cytoskeletal networks: Dennis Discher, Paul Dalhaimer, Tom Lubensky Actin filaments are typically crosslinked in cells by highly flexible proteins, leading to a wide range of cytoskeletal structures or microphases. Two-dimensional or membrane networks composed of actin and spectrin family proteins have been particularly well-characterized in both the red blood cell (RBC) -- where actin filaments are short -- and the outer hair cell (OHC) of the inner ear, where the filaments are long. General aspects of the phase behavior and anisotropic elasticity of these two systems are addressed here by simulating actin-like rods in two-dimensions crosslinked by a soft overlying network of spectrin-like chains. With short rods (per RBC), networks become glassy with compression whereas longer rods transition to a nematic phase. At zero applied pressure, a locked-in or quenched nematic emerges when actin length equals or exceeds crosslinker length (per OHC). Applying tension to quenched nematic states further reveals a soft response in the direction perpendicular to the director -- the direction of sound propagation through the OHC. Properties such as isotropic surface elasticity of RBC and directional elasticity in OHC would seem respectively useful in flow through blood capillaries and sound propagation in the ear and thus follow from disparate actin filament lengths crosslinked at suitable densities. [Preview Abstract] |
Session S22: Biological Physics
Sponsoring Units: DBPChair: Sonya Bahar, University of Missouri-St. Louis
Room: LACC 409B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S22.00001: Visualizing interactions between Sindbis virus and cells by single particle tracking Mary Williard, Gongbo Wang, Keith Weninger Sindbis virus infects both mammalian and insect cells. Though not pathogenic in humans, Sindbis is a model for many mosquito- borne viruses that cause human disease, such as West Nile virus. We have used real-time single particle fluorescence microscopy to observe individual Sindbis virus particles as they infect living cells. Fluorescent labels were incorporated into both the viral coat proteins and the lipid envelope of the virus. Kinetics characteristic of free diffusion in solution, slower diffusion inside cells, attachment to spots on the cell surface, and motor protein transport inside cells have been observed. Dequenching of the membrane label is used to report membrane fusion events during the infection process. Tracking individual viral particles allows multiple pathways to be determined without the requirement of synchronicity. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S22.00002: Shape transformation of viral capsids and HIV Toan Nguyen, Robijn Bruinsma, William Gelbart We present a continuum description of the shape transformation of viral capsids. The cone-like HIV virus is shown to be an thermodynamic stable shape, intermediate between icosahedral and sphero-cylinder capsid shapes. A generalized Caspar-Klug classification is introduced to describe spherical, conical and cylinderical shapes of virus. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S22.00003: Fracture Toughness of Nacre Phani Nukala, Srdjan Simunovic Nacre exhibits phenomenal fracture strength and toughness properties despite the brittle nature of its constituents. For example, its work of fracture is three orders of magnitude greater than that of a single crystal of its constituent mineral. This is surprising because it is a ceramic composite made up of about 95\% brittle inorganic phase (aragonite mineral) and only a few percent of the soft organic material, and polymer-matrix composites with such high levels of ceramic fillers do not possess these exceptional combinations of stiffness, fracture strength and toughness. This study investigates the fracture properties of nacre using a simple discrete lattice model based on continuous damage random thresholds fuse network. The discrete lattice topology of the proposed model is based on nacre's unique brick and mortar microarchitecture, and the mechanical behavior of each of the bonds in the discrete lattice model is governed by the characteristic modular damage evolution of the organic matrix that includes the mineral bridges between the aragonite platelets. The numerical results obtained using this simple discrete lattice model are in excellent agreement with the previously obtained experimental results, such as nacre's stiffness, tensile strength, and work of fracture. The analysis indicates that nacre's superior toughness is a direct consequence of ductility (maximum shear strain) of the organic matrix in terms of repeated unfolding of protein molecules, and its high fracture strength is a result of its perfectly ordered brick and mortar architecture with significant overlap of the platelets, and shear strength of the organic matrix. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S22.00004: How do protozoa respond to intense magnetic fields? Karine Guevorkian, James M. Valles, Jr. Most microorganisms such as \textit{Paramecium Caudatum}, swim in helical paths in nature. In the absence of any external stimuli (e.g. obstacles, electric field, heat, etc.) the axes of these helical paths, which define the trajectories, are straight lines and are distributed in random directions. Our experiments reveal that these trajectories can be manipulated by applying intense DC magnetic fields of the order of several Tesla. Swimming paramecia, for example, align their trajectories with magnetic fields in excess of about 7 Tesla in fraction of a second. We will describe this phenomenon in fields up to 25 T. We will address whether this effect is an active or passive response to the magnetic torque exerted on the diamagnetically anisotropic structures in \textit{Paramecium}. In addition we will present results for other species as they are obtained. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S22.00005: Geometry and physics of proteins Jayanth R. Banavar, Marek Cieplak, Trinh X. Hoang, Amos Maritan, Flavio Seno, Antonio Trovato We recall some of the key lessons of protein research over the last several decades and show that they strongly suggest a new framework for understanding proteins. The unified framework is useful for understanding protein folding, amyloid formation and protein interactions and has important implications for natural selection. The experimental data and our new approach, supported by computer simulations, reveal an astonishing simplicity underlying the protein problem. \\ \\REFERENCES: Banavar, J. R. and Maritan, A. (2003). Colloquium: Geometrical approach to protein folding: A tube picture. Rev. Mod. Phys. 75, 23. \\Banavar, J. R., Hoang, T. X., Maritan, A., Seno, F. and Trovato, A., (2004). A unified perspective on proteins -- a physics approach. Phys. Rev. E 70, 041905. \\Banavar, J. R., Cieplak, M. and Maritan, A., (2004). Lattice tube model of proteins, Phys. Rev. Lett. (in press). [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S22.00006: Molecular dynamics simulations of peptides interacting with a surface: Do surfaces prevent or promote aggregation? Miriam Friedel, Joan-Emma Shea Understanding the process of protein and peptide aggregation is critical to treating and preventing debilitating diseases such as Alzheimer's. In particular, there is evidence that the aggregation process may be influenced by protein-surface or protein-membrane interactions. Nevertheless, there have been few molecular dynamics (MD) studies of proteins and peptides interacting with surfaces. Here, we present the results of an MD study of an off-lattice peptide model. Simulations of peptides both in a bulk environment and interacting with a hydrophobic surface were performed. With this simple model, we examined the impact of peptide sequence and surface hydrophobicity on the thermodynamics and kinetics of aggregation, and our results indicate that both play a significant role in determining aggregation behavior. Although interaction with a surface allows the peptides to form aggregates not easily achieved in the bulk, the kinetics of assembly is not necessarily enhanced by the presence of a surface. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S22.00007: Motorized adhesive particles in localized phases: A toy model for the controlling of viscoelastic phases of cytoskeletal assembly Tongye Shen, Peter Wolynes The cytoskeleton is not an equilibrium structure. To begin to investigate it, we studied a system of motorized particles that may capture the far-from-equilibrium essence of its dynamics. Variational solutions of the manybody master equation for a set of motorized spherical particles accounts for their Brownian motion as well as for motorized kickings. These approximations yield stability limits for crystalline phases and frozen amorphous structures. The methods allow one to relate the strength of nonequilibrium effects and adhesiveness (effective cross-linking of cytoskeleton) to the mechanical stability of localized phases as a function of density and/or temperature. Interestingly nonequilibrium noise does not necessarily destabilize the structures. The dynamics issues are also touched. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S22.00008: A Comparison Between Adaptive Integration and Path-Sampling Methods for Calculating Free Energy Differences in Molecular Systems F. Marty Ytreberg, Daniel M. Zuckerman, Robert H. Swendsen The calculation of free energy differences ($\Delta F$) is essential for the understanding of protein-ligand binding, the conformational stability of proteins and many molecular processes, yet it remains one of the most challenging tasks in computational biophysics. Here we present a study of two recently developed free energy methods, both applied to molecular systems for the first time. The first approach is an adaptive integration Monte Carlo procedure which continually updates the free energy profile connecting the two states of interest. The second method is a path-sampling implementation of the Jarzynski relation which uses a Monte Carlo procedure to generate an ensemble of non-equilibrium paths connecting the two states of interest. Both techniques are compared to the standard methods of thermodynamic integration and use of the Jarzynski relation, by calculating $\Delta F$ values for growing and for charging a simple ion in explicit SPC water. Adaptive integration is found to have the highest precision and accuracy for long simulation times. However, if very rapid $\Delta F$ estimates are the goal, the path-sampling approach is found to be the most efficient. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S22.00009: Effect of temperature on fd and M13 aggregation Qi Wen, Jay Tang Counterion induced aggregation of like-charged polyelectrolytes(PE) such as DNA, F-actin and fd viruses has been investigated extensively. It has been shown that the like-charge attraction is due to the correlation of counterions. The classic Oosawa model suggests that thermal fluctuations of counterions gives rise to a net attractive interaction between two paralel charged rods by inducing transient dipole moments on them. This attractive force is reminiscent of van de Waals interaction, and the interaction is expected to increase with temperature. Alternatively, positional correlations of counterions has been shown by Brownian Dynamics simulations to induce an attractive force that decreases with increasing temperature. In order to determine the dominant mechanism by which the counterions are correlated to induce attractive force, we measure the temperature dependence of threshold concentrations of divalent counterions that cause the aggregation of fd and M13 viruses. Our priliminary measurements favor the Oosawa type of mechanism, although specific chemical effects may affect the interpretation of the experimental results. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S22.00010: Molecular dynamics study of the Mg - AMP interaction in water Don Brugess, Ioan Kosztin The conformational states of $Mg^{++}$ complexed with adenosine monophosphate (AMP) in water solution is analyzed using classical molecular dynamics (MD) simulations. Relevant portions of the 3D \textit{potential of mean force} (PMF) of $Mg^{++}$ is reconstructed by employing two distinct methods: (1) equilibrium MD simulations using the umbrella sampling and weighted histogram analysis method, and (2) non-equilibrium steered MD simulations using a recently developed method based on the Jarzynski equality. Withing computational errors, both methods yield the same results. Two topologically distinct sets of equilibrium conformations of $Mg^{++}$ in the vicinity of the phosphate moiety are found. The free energy difference between the states within a given set is about $1~kT$, and are separated by potential barriers of $\sim 10~kT$ in height. However, the free energy difference between states from the two distinct sets are found to be unrealistically high ($\sim 10~$kcal/mol). Also, the calculated dissociation energy of Mg++ from AMP exceeds several times the corresponding experimental value. Possible sources of this discrepancy are discussed, and alternative methods to improve the accuracy of the calculations are proposed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S22.00011: Predictive filtering in the phototransduction cascade Ilya Nemenman Animals gather sensory information to guide their actions. But acting takes time, and sense data are useful only to the extent that they carry predictive information, that is, information about the state of the world at the time of the actions. We suggest that efficient maximization, extraction, and transmission of such predictive information, rather than maximization of the overall channel capacity, may be the correct optimization principle responsible for designs of some sensory systems. We support these arguments by analyzing information transmission in the enzymatic amplifier in the phototrandusction cascade, were maximization of predictive information seems to explain various experimentally observed properties, such as time scale and gain adaptation. Further, we emphasize that some standard filters used in signal processing can be viewed as (implicitly) maximizing predictive information as well. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S22.00012: Theoretical Study on the Influence of Solvent on Subtilisin Catalysis Yiming Zhang, Lu Yang, Jonathan Dordick, Shekhar Garde, Saroj Nayak Using a hybrid quantum mechanical and molecular mechanical (QM/MM) approach we have studied subtilisin catalysis in water and tetrahydrofuran (THF). Extensive classical molecular dynamics simulations have been carried out in order to obtain the solvent structure around the protein, while hybrid QM/MM method is used to provide the reaction energy profile for the enzymatic reaction. The reaction energy barrier for the formation of the enzyme's tetrahedral intermediate in water is found to be about 7 kcal/mol lower than formation of the tetrahedral intermediate in THF. This result is in good agreement with experimental data where the reactivity of subtilisin is up to four orders of magnitude lower in THF than in water. The lower reaction barrier in water is related to the enhanced stabilization of the enzyme's transition state (TS) in water through hydrogen bonding between solvent water molecules and the TS complex. In addition, we find that the role of Asp32 in stabilizing the tetrahedral intermediate state abates in THF. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S22.00013: Dynamical and Mechanical Behavior of Associating Protein Hydrogels Darina Danova, James L. Harden, David R. Heine, Gary S. Grest Molecular dynamics simulation is used to study the dynamical and mechanical properties of hydrogels made from synthetic associating proteins. These proteins are triblock architectures composed of a central soluble, unstructured block flanked by associating helical ends. The ends are amphiphilic leucine zippers designed to reversibly assemble into trimeric bundles. Solutions of the triblock proteins reversibly self-assemble into hydrogels with predominantly trifunctional crosslinks. The hydrogel formation may be controlled by changes in temperature and pH, thus providing a switchable material with potential application in biomedical and environmental engineering. A coarse-grained model which mimics the alpha-helical secondary structure and amphiphilic properties of the leucine zipper domains is developed in order to accurately describe the structure and dynamics of the self-assembling hydrogel. Then, mechanical properties of the hydrogel under uniaxial and shear strain are presented and related to changes in the molecular level structure of the hydrogel in an effort to relate the protein structure to the hydrogel material properties. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S22.00014: Encapsidation of Linear Polyelectrolyte in a Viral Nanocontainer Yufang Hu, Roya Zandi, Charles Knobler, William Gelbart We present the results from a combined experimental and theoretical study on the self-assembly of a model icosahedral virus, Cowpea Chlorotic Mottle Virus (CCMV). The formation of native CCMV capsids is believed to be driven primarily by the electrostatic interactions between the viral RNA and the positively charged capsid interior, as well as by the hydrophobic interactions between capsid protein subunits. To probe these molecular interactions, \textit{in vitro} self-assembly reactions are carried out using the CCMV capsid protein and a synthetic linear polyelectrolyte, sodium polystyrene sulfonate (NaPSS), which functions as the analog of viral RNA. Under appropriate solutions conditions, NaPSS is encapsidated by the viral capsid. The molecular weight of NaPSS is systematically varied and the resulting average capsid size, size distribution, and particle morphology are measured by transmission electron microscopy. The correlation between capsid size and packaged cargo size, as well as the upper limit of capsid packaging capacity, are characterized. To elucidate the physical role played by the encapsidated polyelectrolyte in determining the preferred size of spherical viruses, we have used a mean-field approach to calculate the free energy of the virus-like particle as a function of chain length (and of the strength of chain/capsid attractive interaction). We find good agreement with our analytical calculations and experimental results. [Preview Abstract] |
Session S23: Quantum Chaos
Sponsoring Units: GSNPChair: Nicholas Cerruti, Washington State University
Room: LACC 410
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S23.00001: Local Level Velocity Variances and the Fidelity in Integrable Systems Nicholas R. Cerruti, Steven Tomsovic In a chaotic system, a wave packet's Wigner transform will ergodically explore the entire energy surface on an extremely short time scale. However, for integrable systems the Wigner transform will remain on constant action surfaces and can access only a fraction of the available phase space. Thus, the local properties of integrable systems can be more important than the global properties. An example occurs when a system is perturbed and the energy levels are redistributed. Using first order quantum perturbation theory, this redistribution is largely responsible for changes in the evolution of the wave packet. More specifically, it is the local variance of the level velocities, which are defined by the changes in the eigenenergies due to the perturbation, that defines the rate of decoherence between the same initial wave packet evolved through the unperturbed and perturbed systems. We derive a semiclassical expression for these local variances and show an application of the variances to the fidelity. The results are demonstrated in the rectangle billiard which is fully integrable. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S23.00002: Nonlinear $\sigma$ -model for a ballistic quantum dot with random boundary absorption Igor Rozhkov, Ganpathy Murthy The problem of evaluation of a two-point function in an integrable ballistic billiard (circular quantum dot) is formulated in terms of the supersymmetric nonlinear $\sigma$ -model. The dot is assumed to be slightly open. It is modeled by attaching the closed circular billiard to the leads with random coupling constants between the eigenstates of the dot and the outgoing states. In the limit of large number of uniformly distributed leads with an infinitesimal couplings to a single channel in each lead we are able to derive a nonlinear $\sigma$ for interacting angular harmonics of the supersymmetric field. Our procedure is done within saddle point approximation without introduction of a disorder potential or diffusive boundary scattering; no energy averaging was performed. Supported by: NSF DMR 0311761 [1]B. A. Muzikantskii and D. E. Khmelnitskii, JETP Lett. 62, 76 (1995). [2] A. V. Andreev, O. Agam, B. D. Simons, and B. L. Altshuler, Phys. Rev. Lett. 76, 3947 (1996). [3] K. B. Efetov and V. R. Kogan, Phys. Rev. B 67, 245312 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S23.00003: Persistent currents for interacting electrons in ballistic/chaotic billiards Oleksandr Zelyak, Ganpathy Murthy We study persistent currents in a quantum billiard enclosing a magnetic flux $\phi$ by analytical and numerical methods. We concentrate on the family of Robnik-Berry billiards generated by conformal maps of the unit disk. We study the persistent current as a function of magnetic flux and parameters of the billiard in the chaotic regime. We include Fermi-liquid interactions in a mean-field approach, justified by the recent large-$N$ approach[1] for ballistic/chaotic quantum dots. [1] G. Murthy, R. Shankar, D. Herman, and H. Mathur, Phys. Rev. B 69, 075321 (2004); G. Murthy, R. Shankar, and H. Mathur, cond-mat/0411280. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S23.00004: Weak Localization In Periodical Structures Without Disorders Chushun Tian, Anatoly Larkin he dynamics of a moving particle in some periodic structures exhibits (normal) diffusion at the classical level [1]. We find weak localization phenomena in such structures. Remarkably, no random quantum potentials are introduced so that the analytical treatments do not involve calculations with the regularizer. In periodic structures, an additional random quantum potential does not affect the perturbative regime of localization phenomena (loop expansion). In sharp contrast, in the quantum limit, it leads to strong localization (in the $1$D and $2$D cases), but quantum diffractions result in the (extended) Bloch state due to spatial periodicity. At the semiclassical level, we find the one loop (frequency- dependent) quantum correction to the diffusion constant has the same functional form as chaotic systems [2]. However, the Ehrenfest time, which signals the crossover between a classical and quantum picture is found without any random quantum potentials. The predicted classical-to-quantum crossover may be studied experimentally in periodic quantum dot systems. The results may be helpful for understanding the crossover between ray and wave optics in photonic crystals. [1] P. Gaspard, Phys. Rev. E {\bf 53}, 4379 (1995). [2] I. Aleiner and A. Larkin, Phys. Rev. B {\bf 54}, 14423 (1996). [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S23.00005: Spectroscopic Evidence of Discrete Energy Levels in Nanosize Clusters of Metal Atoms using a Low Temperature STM Laura Adams, Brian Lang, Allen M. Goldman A new method of obtaining spectroscopic information about clusters is realized through the interplay between the surface states at a metal-semiconductor interface and the discrete electronic energy levels of nanosize ($\sim $ 30 {\AA} in diameter) Pb clusters. When these surface states come into registry with the energy levels of the cluster, resonant peaks emerge in the I(V) characteristics. The histograms of the peak intensities and spacings are consistent with Porter Thomas and Wigner Distributions, respectively. Metallic clusters were fabricated in situ by a buffer layer assisted growth technique developed by Huang, Chey and J. H. Weaver$^{1}$. [1] L. Huang, S. Jay Chey, and J. H. Weaver, ``Buffer-Layer-Assisted Growth of Nanocrystals: Ag-Xe-Si (111)'', PRL 80, 4095 (1998). [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S23.00006: Geometric Phase of Phase Space Trajectories:Mobius Strip and Nonlinear Oscillators Radha Balakrishnan, Indubala Satija We present a gauge invariant formulation of associating a geometric phase with classical phase space trajectories. This geometric phase which depends upon the integrated torsion of the trajectory, bears a close analogy to the generalized Berry phase associated with the time evolution of the quantum wave functions. This topological quantity serves as an order parameter signalling phase transitions including novel geometrical transitions. One of the interesting aspects seen in Duffing and other nonlinear oscillators is the sudden jumps in the geometric phase which is accompanied by the divergence of the local torsion and the vanishing of the local curvature. Intriguingly, the analogous phenomenon was seen in a mobius strip when the ratio of the width to the length of the strip exceeds beyound a critical value. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S23.00007: Spectrum of Geometric Phases in a Driven Oscillator Indu Satija, Radha Balakrishnan We study geometric phases underlying the time evolution of the quantum wave function of a driven nonlinear oscillator exhibiting periodic, quasiperiodic as well as chaotic dynamics. In the asymptotic limit, irrespective of the classical dynamics, the geometric phases are found to increase linearly with time. However, the fingerprints of classical motion are present in the bounded fluctuations that are superimposed on the monotonically growing phases, as well as in the difference in phases between two neighboring quantum states. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S23.00008: Topological Singularities and Transport in Kicked Harper Model Indubala Satija Quasienergy spectrum of kicked Harper model is found to exhibit a series of diabolic crossings. These conical degeneracies reside mostly on the symmetry line of its two-dimensional parameter space and their location is found to coincide with the location of maxima of the kinetic energy of the kicked system. Additionally, there are also branch point singularities, the exceptional points, that are associated with avoided crossings and are obtained by analytically continuing the kicking parameter in the complex plane. The location of these singularities also appear to be closely correlated with the maxima and minima of the kinetic energy, suggesting a correlation between the transport and the topological characteristics of the system. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S23.00009: Universal Statistics of the Scattering Coefficient of Chaotic Microwave Cavities. Sameer Hemmady, Xing Zheng, Thomas Antonsen, Edward Ott, Steven Anlage We experimentally investigate theoretical statistical predictions [X.Zheng, \textit{et al.} cond-mat/0408327] for the universal scattering coefficient in wave chaotic systems using a microwave analog of a quantum chaotic infinite square well potential [S. Hemmady, \textit{et al. }submitted to Phys.Rev.E]. We consider the statistics of the scattering coefficient $S$ of a two-dimensional chaotic microwave cavity coupled to a single port. The non-universal effects of the coupling in the experimental $S$ data are removed using the radiation impedance of the port, obtained directly from the experiments [S. Hemmady, \textit{et al.} Submitted to Phys. Rev. Lett., cond-mat/0403225]. A normalized scattering coefficient is obtained, and its Probability Density Function (PDF) is predicted to be universal in that it depends only on the loss (quality factor) of the cavity. We compare experimental PDFs of the normalized scattering coefficients for different degrees of quantified loss with those obtained from Random Matrix Theory (RMT), and find excellent agreement. We will discuss how these results apply to scattering measurements on other quantum chaotic systems including those with broken Time Reversal Symmetry. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S23.00010: Spectral Statistics of Pointer States in an Open Stadium Billiard Richard Akis, David Ferry In quantum-measurement theory, a point of discussion has been the manner in which the quantum states of a system evolve into classical states. The interaction of a system with the environment has been suggested to lead to einselection[1], the selection of a discrete set of pointer states that remain robust while their superposition with other states is reduced by decoherence. It has been predicted[1] that pointer states are the basis of the transition to classical behavior, and actually possess classical properties. In the case of open quantum dots, such pointer states yield measurable conductance resonances[2]. In our presentation, we shall discuss the results of a new study of the energy level spacing statistics in a stadium quantum dot cavity perturbed by attached leads. When the leads are \textit{closed,} the eigenstates follow the Wigner distribution associated with chaos. However, when the stadium is sufficiently opened to the external environment so that only the pointer states remain resolved, the distribution becomes Poissonian, indicating that these states are intimately associated with the \textit{regular} classical orbits. [1] W. H. Zurek, Rev. Mod. Phys. \textbf{75}, 715 (2003). [2] D. K. Ferry, R. Akis, J.P. Bird, Phys. Rev. Lett. \textbf{93}, 026803 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S23.00011: Complexity of Quantum Spectra Yuri Dabaghian It has been long recognized that the problem of semiclassical evaluation of quantum spectra is fundamentally more difficult for classically chaotic systems than for the classically integrable ones. It appears now that the quantum spectra of the chaotic systems may also differ among themselves by level of their complexity. This is indicated by the hierarchy of the explicit spectral solutions for 1D quantum networks. [Preview Abstract] |
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S23.00012: Chaos Beyond Linearized Stability Analysis: Folding of the Phase
Space and Distribution of Lyapunov Exponents Peter Silvestrov, I.V. Ponomarev We have found~\cite{Reference} a universal mechanism leading to the enhanced probability, $P(\lambda ,t)$, to find small values of the finite time Lyapunov exponent, $\lambda$. In our investigation of chaotic dynamical systems we go beyond the linearized stability analysis of nearby divergent trajectories and consider folding of the phase space in the course of chaotic evolution. We show that the spectrum of the Lyapunov exponents $F(\lambda)= \lim_{t\rightarrow\infty} t^{-1}\ln P(\lambda ,t)$ at the origin has a finite value $F(0)=-\tilde{\lambda}$ and a slope $F'(0)\le 1$. This means that all negative moments of the distribution $\langle e^{-m\lambda t}\rangle$ are saturated by rare events with $\lambda\rightarrow 0$. Extensive numerical simulations confirm the results. Among the practical applications of our findings are the problem of a gap in spectrum of a semiclassical Andreev billiard, conductance fluctuations in a smooth quantum dot and stability to perturbations in chaotic wave-packet dynamics. \begin{thebibliography}{99} \bibitem{Reference} P. G. Silvestrov, I.V.Ponomarev, preprint nlin.CD/0409053. \end{thebibliography} [Preview Abstract] |
Session S24: Focus Session: Jamming: Effective temperature and Aging
Sponsoring Units: GSNPChair: Daniel Blair, Harvard University
Room: LACC 411
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S24.00001: Measuring effective temperatures in sheared, athermal systems at fixed normal load Ning Xu, Corey O'Hern We perform molecular dynamics simulations of repulsive athermal systems sheared at fixed normal load to study the effective temperature $T_L$ defined from time-dependent fluctuation- dissipation relations for density. We show that these systems possess two distinct regimes as a function of the ratio $T_S/V$ of the granular temperature to the potential energy per particle. At small $T_S/V$, these systems are pressure-controlled and $T_L$ is set by the normal load. In contrast, they behave as quasi-equilibrium systems with $T_L \approx T_S$ that increases with shear rate at large $T_S/V$. The fact that $T_L$ is slaved to the pressure at small $T_S/V$ indicates that the variables $T_L$, pressure, and density are not sufficient to describe dense, slowly-sheared athermal systems. Another important implication for systems at small $T_S/V$ is that $T_L$ for two systems placed in contact will not equilibrate when a pressure gradient is maintained between them. Thus, $T_L$ does not behave as a thermodynamic temperature variable in the pressure-controlled regime and new definitions of effective temperature should be explored. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S24.00002: Off-equilibrium fluctuation dissipation relation in polymer glasses Hassan Oukris, Nathan Israeloff Aging dynamics in thin films of polyvinyl acetate (PVAc) is investigated using a rapid quench process by applying local joule heating to a thin layer capacitor. We study the validity of fluctuation dissipation relation (FDR) during aging in a glassy polymer. The time evolution of noise and dielectric susceptibility is analyzed in the frequency range 0.1 Hz- 10$^{3}$ Hz following quenches to below and near the glass transition temperature (T$_{g})$. It is found that eliminating extraneous noise spikes due to differential thermal contraction is important for accurately determining effective temperature. We find a large violation of FDR which extends to ft$_{w}$=300, much larger than for spin glasses. The results of these studies are important in understanding the relaxation dynamics of structural glasses and in testing the validity and relevance of the effective temperature concepts. Experimental study of FDR in aging systems can give new insight into the interpretation of the aging dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S24.00003: Experimental measurement of an effective temperature for densely packed granular materials Ping Wang, Chaoming Song, Hernan Makse A densely packed granular system is an example of an out-of- equilibrium system in the jammed state. It has been a longstanding problem to determine whether this class of systems can be described by concepts arising from equilibrium statistical mechanics, such as an ``effective temperature'' and ``compactivity.'' The measurement of the effective temperature is realized in the laboratory by slowly shearing a closely-packed ensemble of spherical beads confined by an external pressure in a Couette geometry. All the probe particles, independent of their characteristic features, equilibrate at the same temperature, given by the packing density of the system. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S24.00004: Energy landscape picture of overaging and rejuvenation in a sheared glass Daniel Lacks, Mark Osborne Molecular simulations and an energy landscape analysis are used to investigate the effects of shear on aging in a glass. Shear beyond the yield point is shown to change the state of a glass such that it resembles (but is not identical to) a different stage in the aging process. A cycle of large strain rejuvenates the glass by relocating the system to shallower energy minima, while a cycle of small strain overages the glass by relocating the system to deeper energy minima. The balance between overaging and rejuvenation is controlled by how well the glass was initially annealed. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S24.00005: Overaging and rejuvenation in a polymer glass subjected to shear deformation Matthew L. Wallace, Bela Joos When applying a transient shear on a jammed substance, one can induce both rejuvenation and overaging in the system, as the relaxation times are altered in a non-trivial way. We induce the overaging of a polymer glass by instantaneous, one-time shear deformations, and follow its evolution for long waiting times $t_w$. After each deformation, the characteristic relaxation time of the system $\tau_{1/2}$ increases. We find in our molecular dynamics simulations two distinct regimes, primarily based on how the system evolves after a waiting time $t_w$ following one deformation. In the low-shear regime, both the energy of the inherent structure of the system and $\tau_ {1/2}$ change very little with $t_w$, and there are no apparent structural changes in the system. In the high-shear regime, we see an initial combination of rejuvenation and overaging in the system and $\tau_{1/2}$ has a well-defined logarithmic dependence on $t_w$. Furthermore, it is found that when successive deformations are applied, no memory effect or directional preference arises from the previous shearing. Finally, we investigate the signatures of the overaged state. The polymer glass is obtained by compressing isothermally a melt of short freely jointed chains interacting with van der Waals interactions. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S24.00006: Time evolution of local fluctuations in the aging of a simple glass Horacio Castillo We report results of molecular dynamics simulations of the out of equilibrium dynamics of a simple glass former, a binary Lennard-Jones system, after a quench to low temperatures. We explore in detail the fluctuations in the mean square displacement and local correlator describing small, coarse grained regions of the system. We evaluate the probability distribution funcion (PDF) of those local quantities, as a function of waiting time $t_w$ and final time $t$. We find that both probability distribution functions show very good scaling when the waiting time and final time are chosen to keep the global incoherent scattering function $C_q(t,t_w)$ at a constant value. In fact, the global incoherent scattering function is a better predictor for both probability distribution functions than the ratio of the final and waiting times $t/t_w$ or the value of the global mean square displacement of the particles $\Delta(t,t_w)$. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S24.00007: Shearing glassy materials: insight from computer simulations Invited Speaker: I will discuss some aspects of flow in glassy systems that have been recently investigated using computer simulations. I will first describe some statistical properties of homogeneous flow using the ``effective temperature'' concept. Then I will show that complex phenomena such as yield stress, shear banding, complex stress strain curves, commonly observed in metallic glasses, amorphous polymers or complex fluids, can be observed in a very simple ``computer glass'' model. The existence of shear bands can be understood from the presence of a nonzero yield stress, larger than the small shear limit of homogeneous flow curves. This in turn implies that the flow curve is effectively nonmonotonic, and allows shear band formation in a restricted domain of shear rates. Finally, I will describe some results obtained in athermal systems, in which it is possible to decompose the deformation in terms of elementary plastic events. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S24.00008: Aging effects in shear yielding of glassy solids Joerg Rottler, Mark O. Robbins The shear yield stress of amorphous glassy materials is usually taken to be a function of the loading state, the temperature and the strain rate. Since glasses are out of equilibrium systems, yielding is additionally influenced by the material's own intrinsic aging dynamics. We study the role of aging in the well-known generic Lennard-Jones glasses through molecular dynamics simulations. For temperatures not too far below the glass transition temperature, the (transient peak) yield stress increases logarithmically with the waiting time in the glassy regime. The rate dependence is directly related to the age of the system. If the waiting time is much larger than the time to reach the yield point, the yield stress follows a universal logarithmic rate dependence, but in the opposite limit the system behaves like a system in steady shear that is constantly ``rejuvenated.'' These effects disappear at very low temperatures where the aging dynamics is frozen out. Implications for phenomenological models of plasticity in glassy materials are pointed out. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S24.00009: Local aging effects in glassy polymers probed by 1/f noise Koneswaran Sinnathamby, Nathan Israeloff Local aging effects in glassy system have been investigated by nanometer scale probing of polarization noise fluctuations in a glassy polymer polyvinyl-acetate (PVAc) near to the primary relaxation region. Using ultra high vacuum (UHV) capacitance scanning probe microscopy (SPM) techniques, nanometer scale polarization fluctuations were probed. Time dependent changes in the noise spectrum, and high order statistical variations of the noise time series were studied and analyzed with varying temperature. Local aging was studied by analyzing the variance of the noise spectrum following a temperature quench. The temperature dependent variance is larger than the expectations of Gaussian noise, and this effect decreases with aging. Moreover, the experimental 1/f noise and computer simulations of noise from a simple model were compared in order to better understand the cooperativity and heterogeneity found in glassy polymers. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S24.00010: SPM investigation of local aging effects in glassy polymers Philip Crider, Nathan Israeloff We investigate the cooperative and heterogeneous nature of glassy dynamics by nanometer-scale probing in a glassy polymer, Polyvinyl-Actetate (PVAc), with a Scanning Force Microscope (SFM). Using ultra-high-vacuum (UHV) Scanning Capacitive Force Microscopy techniques, nanometer-scale capacitive responses are probed. Dielectric relaxation near the glass transition is investigated, and scanning capabilities are utilized to analyze spatial response on a nanometer scale. The results of these studies may yield insight into the understanding of temperature-dependent cooperative length scales, local aging properties, and energy landscape properties of evolving dipole clusters on a mesoscopic scale. Results are used to test the validity and relevance of current models of glassy dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S24.00011: Local correlations and local responses in the aging of a sample Allen Dahili, Horacio Castillo We investigate the relationship between local fluctuations in the correlation and the response in a three dimensional strucural glass model. We perform molecular dynamics simulations of a binary Lennard-Jones mixture and measure local correlations and responses in small regions of the system, after a quench into the glass regime. We compare the joint probability distribution of these local quantities with the out-of-equilibrium fluctuation-dissipation relation of the bulk system. We also compare our results with the results of analogous simulations in short range spin glass systems, which support the presence of a soft mode in the aging dynamics. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S24.00012: Structural Details of Aging in Colloidal Glasses Gianguido C. Cianci, Eric R. Weeks Dense colloidal suspensions are good model glass formers. We use fast laser-scanning confocal microscopy, which allows real-time tracking of the trajectories of thousands of colloidal particles in 3-D, to study non-equilibrium phenomena in colloidal glasses. In particular we are interested in aging, the dependence of physical properties on the time elapsed since the creation of the sample. We investigate this non-equilibrium behavior in terms of colloidal packing. Tetrahedra, or triangular based pyramids, represent the ideal packing of 4 spheres in three dimensions, however they do not tile 3-D space. This frustration between local and global packing optimization has been invoked as a possible origin for the glass transition. We therefore study how these tetrahedra evolve as the sample ages. We are particularly interested in how the characteristics of tetrahedra, such as irregularity (the normalized standard deviation of the edge lengths of the tetrahedra), influence their dynamics. Furthermore, we study the correlations between the structure of tetrahedra and the dynamics of the particles that form them. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S24.00013: Growth of spatial correlations during the aging of a simple structural glass Azita Parsaeian, Horacio Castillo We investigate dynamical heterogenities in a binary Lennard-Jones system below the glass transition temperature, by computing spatial correlations of fluctuations in the aging regime. A theoretical framework based on the existence of a soft mode in the nonequilibrium dynamics predicts the presence of a dynamical correlation length which grows with time. In our simulations we find that correlations grow with growing waiting time, as expected from this picture. [Preview Abstract] |
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S24.00014: Aging at criticality in models with absobing states Jose J. Ramasco, Malte Henkel, Maria Augusta Santos, Constantino A. da Silva Santos The long-time dynamics of the critical models with absobing states which are brought suddenly out of an uncorrelated initial situation undergoes ageing in close analogy with quenched magnetic systems. In particular, we show that time-translation invariance is broken and that dynamical scaling holds. We find that the autocorrelation and autoresponse exponents $\lambda_{\Gamma}$ and $\lambda_R$ are equal but, in contrast to systems relaxing to equilibrium, the aging exponents $a$ and $b$ are distinct. A recent proposal to define a non-equilibrium temperature through the short-time limit of the fluctuation-dissipation ratio is therefore not applicable. [Preview Abstract] |
Session S25: Strongly Correlated Oxides, Superlattices and Synthesis
Sponsoring Units: DMP DCMPChair: Zenji Hiroi, University of Tokyo
Room: LACC 501A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S25.00001: Orbital selective gap opening in Ca$_{3}$Ru$_{2}$O$_{7}$ observed by ARPES Felix Baumberger, Naoki Kikugawa, Nicholas Ingle, Worawat Meevasana, Kyle Shen, Donghui Lu, Suman Hussain, Andrea Damasceli, Andrew Mackenzie, Yoshiteru Maeno, Zhi-Xun Shen We present a high--resolution photoemission study of the bilayered ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$, which is right on the verge of a Mott--Hubbard metal-insulator transition. The low carrier density, inferred from transport measurements is manifested in a very small quasiparticle weight, and a characteristic broadening of the valence band states. Coincident with the first order structural phase transition at 48 K, we observe the orbital selective opening of a weak, mean--field like gap. This sheds new light on the mysterious low--temperature metallic state of Ca$_{3}$Ru$_{2}$O$_{7}$, and on the interplay of charge, lattice, and orbital degrees of freedom. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S25.00002: Multi-$q$ crystal and magnetic structure in TbMnO$_{3}$: Evidence for a Soliton-lattice N. Aliouane, D.N. Argyriou, S. Landsgesell, C.J. Milne, J. Strempfer, W. Caliebe In TbMnO$_{3}$, Mn-spins order with a sinusoidal antiferromagnetic (AF) propagation wave vector $Q_{Mn}$=[0,k+/-q,l] ($q\sim0.288b^{\ast}$) at T$_{N}$(Mn)=41K. The propagation vector $Q_{Mn}$ varies with temperature on cooling until T$_{Lock}$(Mn) $\sim$30K, which coincides with a ferroelectric transition. In addition to $Q_{Mn}$ reflections we find magnetic reflections at 3$Q_{Mn}$. Our X-ray measurements show that the magneto-elastic coupling gives rise to a structural modulation at twice the magnetic wavevector (2$Q_{Mn}$). Field cooling the sample under a magnetic field oriented along the a-direction with H$>$9T shows that all magnetic wavevectors for Mn and Tb collapse to a single $q$ structure with $Q$=[0,1/4,0], an up-up, down-down phase, and coincide with anomalies in the polarization. We argue that the temperature and field dependence of the magnetic and superlattice reflections are consistent with a soliton formalism which predicts a stable commensurate single q=1/4 phase [1]. [1] Kimura {\sl et al.}, $PRB$ 68, 60403(2003). [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S25.00003: Incommensurate to commensurate transition under magnetic field in TbMnO$_{3}$ Dimitri N. Argyriou, N. Aliouane, S. Landsgesell, C.J. Milne, J. Strempfer, I. Zegkinoglou, M. von Zimmermann We have studied the evolution of the incommensurate satellite peaks of TbMnO$_{3}$ by neutron and x-ray single crystal diffraction with field H$\| b$ and H$\| c$. For H$\| b$ and a temperature of T=3K, Tb undergoes the first order phase transition characterized by a change of the magnetic wavevector from $q_{Tb}\sim$0.44 to 0.33 r.l.u. at a field of 1.75T. X-ray diffraction measurements show that the structural counterpart for the magnetic Tb reflection follows a similar dependence with 2$q_{Tb}$=0.82 r.l.u. changing to 0.66 r.l.u.. At H=6T and T=3K, the structural superlattice reflections for both the Mn and Tb wavevectors collapse to a single $q$-commensurate structure with propagation vector $Q$=[0,1/2,l]. This transition coincides with anomalies in the polarization P$\| c$. For fields of up to 4T applied along the c-direction no new magnetic phases arise, except for the shift of the antiferromagnetism and locking transitions. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S25.00004: Optical-Pump Mid-Infrared Probe Study of Quasiparticle Dynamics in (La,Pr,Ca)MnO3 Hae Ja Lee, R.P. Prasankumar, R.D. Averitt, D.J. Funk, A.J. Taylor, S.-W. Cheong We have investigated the temperature dependence of the quasiparticle dynamics in single crystal (La,Pr,Ca)MnO3 (LPCMO) using ultrafast optical spectroscopy. As a function of temperature, distinct changes in the dynamics occur upon crossing through the charge ordering and ferromagnetic transition temperatures. We will discuss the origin of the quasiparticle dynamics in light of the phase separation into submicrometer-sized mixtures of ferromagnetic metallic and charge-ordered insulating domains that occur in LPCMO. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S25.00005: Properties of Nonstoichiometric SrRu$_{1-v}$O$_{3}$\\Perovskites B. Dabrowski, S. Kolesnik, O. Chmaissem, J. Mais, M. Avdeev, J.D. Jorgensen Annealing of stoichiometric SrRuO$_{3}$ perovskites in high-pressure oxygen of 600 atm. near 1100$^{\circ}$C produces SrRu$_{1-v}$O$_{3}$ compounds with vacancies on the Ru-sites. The creation of Ru vacancies rapidly suppresses the ferromagnetic ordering temperature, $T_{C}$, from 163 K to 45 K with increase of v $\approx $ 0.09. The resistivity shows a metallic behavior near room temperature with progressively more insulating behavior at low temperatures for increasing v. All samples display clear metallic-like decrease of the resistivity and negative magneto-resistance right below $T_{C}$. Structural changes that accompany creation of Ru-site vacancies indicate reduced charge screening caused by the Ru-vacancies that offsets expected decrease of the average interatomic distance Ru--O. The $b$ and $c$ lattice parameters and the unit cell volume are virtually temperature independent for the stoichiometric material below $T_{c}$. We show that this previously reported invar-effect originates from freezing of the octahedral tilting about the [001] axis that can be observed for both stoichiometric and Ru-deficient samples. Spontaneous magnetostriction has the largest effect on the $b$ axis of the orthorhombic \textit{Pbnm} crystal structure. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S25.00006: Bulk modulus anomaly in RCoO3 (R=La, Pr, and Nd) J.-S. Zhou, J.-Q. Yan, J.B. Goodenough The RCoO$_{3}$ family undergoes a transition from the low-spin state to a higher spin state with increasing temperature. The onset temperature for this thermally driven, progressive transition is 35 K for LaCoO$_{3}$; the onset temperature increases to 200K for PrCoO$_{3}$ and 300K for NdCoO$_{3}$. At room temperature the population of the low-spin state increases as the ionic size of rare earth reduces from La to Nd. High pressure stabilizes the low-spin state by enlarging the crystal field splitting. The structural study with more pressure sampling points in this work has given a more accurate V-P relationship for the RCoO$_{3}$ family. A linear fitting to the V-P relationship instead of the Birch-Murnaghan (B-M) equation with a parameter B'=4 as default has been obtained for LaCoO$_{3}$. The bulk modulus B is even lower than that reported previously. In contrast, NdCoO$_{3}$ shows a regular B by fitting the V-P curve with the B-M equation. The V-P curve of PrCoO$_{3} $ is irregular and has been explained in terms of both a pressure- induced spin-state transition and a structural transition from an orthorhombic to a rhombohedral phase. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S25.00007: Ferroelectricity in one unit-cell period oxide superlattices T.W. Noh, S.S.A. Seo, J.H. Lee, J. Yu, H.N. Lee We present electric properties of one unit-cell period superlattices composed of CaTiO$_{3 }$(CTO), SrTiO$_{3}$ (STO), and BaTiO$_{3}$ (BTO) perovskites, in which the structural symmetry and lattice misfit strain can be systematically varied without changing the chemical valence states. The one unit-cell period CTO/BTO, BTO/STO, and CTO/STO superlattices were grown by high oxygen pressure pulsed laser deposition on atomically flat SrRuO$_{3}$ conducting oxide grown on STO (001) substrates. CTO/BTO and BTO/STO showed ferroelectricity in room temperature, while CTO/STO showed paraelectric behavior. Such spontaneous electric polarization was an unexpected result, because all TiO$_{6}$ octahedron was not in the same structural condition with ferroelectric BTO, but was sandwiched by CaO (SrO) and BaO layers in these superlattices. By performing first principle calculations, ferroelectric ground states can be found in the distorted TiO$_{6}$. Moreover, the ferroelectricity was described as the collective displacement of the titanium-oxygen-titanium ions, which is different from that of bulk ferroelectric material. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S25.00008: Tailoring of ferromagnetic/ferroelectric superlattices for multiferroic properties W. Prellier, P. Murugavel, D. Saurel, Ch. Simon, B. Raveau Superlattices composed of ferromagnetic Pr$_{0.85}$Ca$_{0.15}$MnO$_{3}$ insulating layers and ferroelectric Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ layers were fabricated on (100)-SrTiO$_{3}$ substrates by pulsed-laser deposition. The magnetotransport properties were measured with the current perpendicular to the plane geometry. An increase in magnetoresistance (MR), with no significant low field effect, was observed as the number of ferroelectric layer increases even up to 9 unit cells. This observed large MR cannot be explained by simple interfacial ferromagnetism or by the tunneling magnetoresistance. The capacitance and resistive parts of the samples were also analyzed from the complex impedance measurements, performed on the samples using a special experimental set-up. The superlattice with larger ferroelectric thickness shows unique characteristics which are not present in the parent ferromagnetic thin film and both ferromagnetic and ferroelectric transitions which is an evidence for the coexistence of both the properties. The high magnetoresistance (40{\%} at 80K) shown by the superlattice can be attributed to the coupling between ferromagnetic and ferroelectric layers, i.e, to the magnetoelectric effect. Various examples with different materials will also be presented to confirm these results. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S25.00009: Multi-scale Modeling of Relaxor Ferroelectrics Aravind Asthagiri, Narayani Choudhury, Wu Zhigang, Ronald Cohen The origin of the high piezoelectric response observed in complex solid solution perovskites like PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-PbTiO$_ {3}$ (PMN-PT) is still not well understood. We have taken a multi-scale approach to examine PMN and PMN-PT by developing a shell potential model by fitting to extensive first-principles data that can then be used in molecular dynamics (MD) simulations. For PMN we have performed both total energy calculations with LAPW and linear response calculations using ABINIT to obtain the phonon spectra of ordered 1:2 supercells along the [111] and [001] directions. For both ordered structures, we find small energy differences between the polar ferroelectric and non-polar antiferroelectric structures and large LO-TO splittings. For the [111] structure, we find the ground state to be triclinic. The results above and similar results for PbTiO$_{3} $ have been used to fit a shell model potential for PMN and PT. We will report the temperature and applied field behavior of disordered PMN and the phase diagram of PMN-PT obtained from MD simulations and compare to existing experimental data. Preliminary results for PMN indicate that we obtain qualitatively similar Raman spectra and bulk modulus to experiment. Work supported by the Office of Naval Research (contract number N000149710052) and the Carnegie Institution of Washington. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S25.00010: First-principles study of symmetry lowering in relaxed BaTiO$_3$/SrTiO$_3$ superlattices Karen Johnston, Xiangyang Huang, Jeffrey B. Neaton, Karin M. Rabe The crystal structure and local spontaneous polarization of (BaTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices is calculated using a first-principles density functional theory method. The in-plane lattice constant is 1\% larger than the SrTiO$_3$ substrate to imitate the relaxed superlattice structure and the symmetry is lowered to monoclinic space group $Cm$ which allows polarization to develop along the [110] and [001] directions. The polarization component in the [110] direction is found to develop only in the SrTiO$_3$ layers and falls to zero in the BaTiO$_3$ layers, whereas the polarization in the [001] direction is approximately uniform throughout the superlattice. These findings are consistent with recent experimental data and first-principles results for epitaxially strained BT and ST. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S25.00011: Rare-earth doped chalcogenide thin films for optoelectronic applications by laser ablation Prabhat Dwivedi, T. Allen, T.J. Clement, Y.Y. Tusi, C.J. Haugen, R.G. Decorby, J.N. McMullin, S.O. Kasap Chalcogenide glasses have been extensively studied as host media for rare-earth (RE) doped photonic devices due to potential application as optical amplifiers for optical telecommunication. However, fabrication of RE doped homogeneous thin films of chalcogenide glass systems is a difficult task. Doping high amounts of RE atoms (1 to 2 at{\%}) using conventional preparation methods such as glass quenching or physical vapor deposition techniques often results in physical or chemical clustering of the RE atoms in the glass matrix also. In this paper, we report the deposition and properties of RE doped chalcogenide films fabricated by pulsed laser deposition (PLD), using 15 ns KrF laser pulses at various laser energy densities and substrate temperatures. We examined the effects of changing the substrate temperature during deposition on the optical constants and photoluminescence. The thermal stability was examined using a temperature modulated differential scaning calorimetry (TMDSC) measurements. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S25.00012: Fingering Instability in a Growing Elastic Gel Ariel Balter, Allan Bower, Jay Tang It has been experimentally observed that when an actin gel is made to grow around a small ($\sim$1 $\mu$m) bead, the gel may finger, i.e. grow a small number of stable protrusions. The case of one finger represents the case of ``comet tail'' motility. We present at theory for this process as an instability driven by the interplay of surface energy and elastic energy at the outer edge of the growing gel. In our theory, the number of fingers is selected by the bead size, the growth rate, the elastic modulus and the surface energy, so direct comparisons to experiment can be made. We have also simulated the process with a finite element code. Although we present this as an explanation for the morphology of actin gels around beads, in particular the formation of actin ``comet tails,'' we suggest this as a generic process which may emerge whenever an elastic material is growing away from a curved surface and the elastic material has a stress-dependant chemical stability. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S25.00013: Methylamine Intercalation Rates in Lead Iodide M. Gallegos, B. Magness, W. Tikkanen, H. Goldwhite, T. Berhe, C. C. Coleman The intercalation and deintercalation rates and some optical spectral changes of methylamine guests intercalated into layer structured lead iodide hosts are reported. Vapor diffused purified lead iodide was used to make powder, thick film and thin film samples. Host films of 200 to 500nm were evaporated on quartz crystals. Degassed host samples were exposed to guest gas pressures ranging from 5 to 200 Torr in an isolated glass system. Mass changes were determined by quartz spring and crystal frequency change methods. The final intercalation of methylamine into lead iodide at low guest pressures reaches saturation in 3 hours. This results in an uptake of 1.3:1 mole ratio of guest to host. However, a plateau in uptake occurs at a 0.64:1 guest to host mole ratio within ten seconds of exposure depending upon the pressure of the guest. Deintercalation obtained by pumping takes 15 days suggesting the intercalated state is fairly stable. There are indications of a plateau in the deintercalation data at the 0.64:1 mole ratio as well further suggesting the possibility of a second stage in this system. Optical data were obtained using thin film host samples, which produced a 0.46 eV increase of the optical band edge energy upon intercalation. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S25.00014: Electronic structure of nano-sized iron oxide particles measured by scanning tunneling- and photoelectron spectroscopy Marcus Preisinger, Michael Krispin, Torsten Rudolf, Siegfried Horn We have investigated the electronic structure of nano-sized iron oxide by scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by photoelectron spectroscopy (PES). Nano particles were produced by thermal treatment of Ferritin molecules containing a self-assembled core of iron oxide. Depending on the thermal treatment we were able to prepare different phases of iron oxide nanoparticles resembling $\gamma$-Fe$_{2}$O$_{3}$, $\alpha$-Fe$_{2}$O$_{3}$, and a phase which apparently contains both $\gamma$-Fe$_{2}$O$_{3}$ and $\alpha$-Fe$_{2}$O$_{3}$. Changes to the electronic structure of these materials were studied under reducing conditions. We show that the surface band gap of the electronic excitation spectrum can differ from that of bulk material and is dominated by surface effects. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S25.00015: Structural and Electronic Properties of (HAlO)$_n$ Clusters Yi Dong, Michael Springborg HAlO is a nanostructured material that can be used as a substrate for organized structures of organic molecules. However, except for the fact that HAlO is stoichiometric, very little is known about its structural and electronic properties. Using two different unbiased methods for structure optimization (our own {\it Aufbau} method as well as Genetic Algorithms) together with a parameterized density-functional method in calculating the total energy and the electronic properties for a given structure, we have optimized the structure of (HAlO)$_n$ clusters with $n$ up to 26. We shall briefly outline our theoretical approach and subsequently present the results for isolated (HAlO)$_n$ clusters together with those for interacting (HAlO)$_n$ clusters and for layers of HAlO. [Preview Abstract] |
Session S26: Nanowires I: Theory and Modeling
Sponsoring Units: DCOMP DMPChair: Daniel Stein, University of Arizona
Room: LACC 501B
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S26.00001: Multi-channel impurity scattering effects on the carrier mobility in semiconductor nanowires Kunal Das, Ari Mizel We consider the mobility of charged carriers in a semiconducting nanowire. The suppression of scattering phase space in small radius wires can enhance mobility. This can compete with an increased density of impurities and defects in the interior and surface of small radius wires that can decrease mobility. We study the dependence of these effects on wire radius, providing insight into the transition from bulk transport to effective one dimensional transport. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S26.00002: Nonlinear dynamics of the thinning process of metallic nanocylinders J\'er\^ome B\"urki, Charles A. Stafford A nonlinear partial differential equation for the shape evolution of metallic nanowires is presented and applied to the description of the thinning process of nanocylinders. Using concepts from fluid dynamics, the PDE is derived from a semiclassical energy functional that includes electron-shell effects. The thinning is found to occur through nucleation of kink-like solitons at the boundary of the nanowire, which subsequently move along the wire. We discuss a rich dynamics involving interactions between kinks that substantially alters their motion along the wire, and compare our results with experiments on gold nanowires. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S26.00003: Universal activation barriers for metastable metal nanowires Charles Stafford, J\'er\^ome B\"urki, Daniel Stein We introduce a continuum approach to study the lifetimes of monovalent metal nanowires, which includes the competing effects of surface tension and electron-shell structure on an equal footing. Thermal fluctuations of cylindrical nanowires are modeled through the use of stochastic Ginzburg-Landau field theories. The fluctuation-induced thinning of metal nanowires is predicted to occur via the nucleation of surface kinks at the ends of the wire, consistent with recent electron microscopy studies. The activation barriers of the most stable structures are predicted to be ``universal,'' i.e., independent of the radius of the wire, and proportional to the square root of the surface tension. The reduction of the activation barrier under strain is also determined. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S26.00004: Transport and Magnetic Properties of Metal Nanowire under Finite Voltages Chang-hua Zhang The electronic transport and magnetic properties of metallic nanocylinders have been studied under the finite bias in a generalized mean-field electron model. The electron-electron interaction is treated in the self-consistent Hartree approximation so that the calculated physical quantities are ``gauge invariant.'' The modulation of the cohesion force is a few nano-Newtons for a few Volts and is correlated to the jumps of the differential conductance. The screening of electron-electron interactions is also found to be very sensitive to the magnetic field, which leads to a high sensitivity of the magnetotension and magnetoconductance coefficients on a longitudinal magnetic field. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S26.00005: The charge density wave transition in metallic nanowires Daniel F. Urban, Charles A. Stafford, Hermann Grabert A quantum mechanical stability analysis of metallic nanowires reveals an instability of the Peierls type in the regime where the Rayleigh instability is suppressed by electron shell effects. Therefore the length of stable wires with magic radii is limited. Near the Peierls transition, the singular part of the energy shows finite-size scaling consistent with the hyperscaling ansatz for a quantum phase transition. Based on this critical behavior, we study the dynamics of surface fluctuations and discuss the crossover to the CDW phase as a function of the length of the wire. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S26.00006: Structured Energy Distribution and Coherent AC Transport in Mesoscopic Wires Andrey Shytov Electron energy distribution in a mesoscopic AC-driven diffusive wire generally is not characterized by an effective temperature. At low temperatures, the distribution has a form of a multi-step staircase, with the step width equal to the field energy quantum. Analytic results for the field frequency high and low compared to Thouless energy are presented, while the intermediate frequency regime is analyzed numerically. Manifestations in the tunneling spectroscopy and noise measurements are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S26.00007: Wigner Crystallization in inhomogeneous one dimensional wires Erich Mueller I present a theory of the crossover between weak and strong interactions in a one dimensional electron gas confined by a power law potential [cond-mat/0410773]. Upon increasing the interaction strength, Friedel oscillations in the density of the noninteracting gas smoothly increase in amplitude, eventually resulting in well-separated electrons. I extract the momentum space wavefunction of the electron at the Fermi surface, which can be measured in experiments on tunneling between parallel wires. The onset of localization leads to a dramatic broadening of the momentum space wavefunction together with pronounced sharpening (in energy) of the tunneling spectrum. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S26.00008: Midgap States and Generalized Supersymmetry in Semi-infinite Nanowires Chung-Yu Mou, Bor-Luen Huang, Shin-Tza Wu Edge states of semi-infinite nanowires in tight binding limit are examined. We argue that understanding these edge states provides a pathway to generic comprehension of surface states in many semi-infinite physical systems. It is shown that the edge states occur within the gaps of the corresponding bulk spectrum (thus also called the midgap states). More importantly, we show that the presence of these midgap states reflects an underlying generalized supersymmetry. This supersymmetric structure is a generalized rotational symmetry among sublattices and results in a universal tendency: all midgap states tend to vanish with periods commensurate with the underlying lattice. Based on our formulation, we propose a structure with superlattice in hopping to control the number of localized electronic states occurring at the ends of the nanowires. Other implications are also discussed. In particular, it is shown that the ordinarily recognized impurity states can be viewed as disguised midgap states. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S26.00009: Nanotube in a Wigner crystal regime Dmitry Novikov A narrow--gap nanotube is studied close to half--filling, focussing on the effects of the curvature of the electronic dispersion controlled by the gap. Curvature couples charge and spin--valley SU(4) "flavor" sectors, resulting, in particular, in the effects of commensuration. In the charge sector, the latter are manifest when the tube is subjected to a periodic external potential, in which case the Wigner crystal of electrons becomes locked into incompressible configurations. Commensuration in the flavor sector can be manifest in the presence of external magnetic field. In this case, commensurability between the densities of electrons with opposing spin polarizations results in the flavor ordering that could be revealed in magnetization measurements, when the Zeeman energy is larger than the renormalized gap at half--filling. Work supported by NSF MRSEC grant DMR 02-13706. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S26.00010: Curved Nanowire Structures Jens Gravesen, Morten Willatzen Schroedinger eigenstates and associated eigenvalues are found and discussed in terms of symmetry properties for a quantum- mechanical particle confined to a curved nanowire having arbitrary cross-sectional geometry. The three-dimensional Schroedinger problem is simplified mathematically using differential-geometry arguments so as to obtain three ordinary differential equations which can be solved computationally fast even for complex-curved nanowire structures. This simplification is possible as long as the nanowire radius of curvature is considerably larger than the nanowire cross- sectional dimensions. We consider in details the computational problems of a straight nanowire with two subsequent 90 degree bendings, the sinusoidal-shaped nanowire, the elliptical-shaped nanowire based on the analytical fact that the model presented gives exact (excellent) agreement with the corresponding three- dimensional treatment in the cases of a nanowire with a straight-line shaped (circular-shaped) axis. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S26.00011: Theory of momentum resolved tunneling into a short quantum wire Jiang Qian, Gregory Fiete, Yaroslav Tserkovnyak , Bertrand Halperin Motivated by recent tunneling experiments in the parallel wire geometry[1], we calculate results for momentum resolved tunneling into a short one-dimensional wire containing a small number of electrons. We derive some general theorems about the momentum dependence, and we carry out exact calculations for up to N=4 electrons in the final state, for a system with screened Coulomb interactions that models the situation of the experiments. Both the case of electrons with spin and the case of completely polarized electrons are considered. The electron density and momentum-dependent tunneling matrix elements at various inter-particle spacings are analyzed. At large interactions the spin dynamics of the system can be approximated with a Heisenberg model, and we derive an effective Heisenberg coupling constant J from the gap between ground state and first excited state. We shall discuss implications of the calculations for the experiments on parallel wires. [1] O. Auslaender et. al, unpublished. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S26.00012: Novel Gapped Quantum Wire Trinanjan Datta, Erica W. Carlson High quality state of the art quantum wires (QWRs) can be fabricated by the novel cleaved edge overgrowth technique, proposed by (Pfeiffer {\em et al.}, 1990). Transverse quantization in these QWRs leads to a succession of nested energy bands. With the lowest two successive energy levels occupied, gapped phases are possible including, e.g, an intersubband charge density wave (ICDW) and a Cooper phase with strong superconducting fluctuations (Starykh {\em et al.}, 2000). Due to the possibility of density reorganization, in which it becomes favorable for the two lowest subbands to match their densities, the ICDW is usually the most likely state. Recently, by exploiting the valley degeneracy in AlAs, a single QWR has been fabricated with two degenerate nonoverlapping bands separated in k space by half an Umklapp vector (Moser {\em et al.} 2004). For low densities this structure is able to access a multiple subband regime that is not subject to the density reorganizing ICDW, leaving the Cooper phase to flourish. Using Abelian bosonization, we explore the relevant interaction terms for this system, including Umklapp assisted Cooper scattering, and discuss the phase diagram. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S26.00013: Coulomb correlations in ultrathin wires Michael Fogler We compute the ground-state energy and the density-density correlation function of electron liquid in a thin one- dimensional wire. The calculation is based on an approximate mapping of the problem with a realistic Coulomb interaction law onto exactly solvable models of mathematical physics. This approach becomes asymptotically exact in the limit of small wire radius but remains numerically accurate even for modestly thin wires. Possible experimental realizations of the model include semiconducting carbon nanotubes on high-kappa dielectric substrates and quantum wires near metallic gates. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S26.00014: Quantum thermal conductance of quasi-one-dimensional insulating rods Padraig Murphy, Joel Moore At low temperatures the thermal conductance of mesoscopic quasi-one-dimensional systems is determined by the transmission probability of various propagating modes. We study the effect of disorder on the quantum thermal conductance of thin insulating rods by a dynamical transfer-matrix method valid in the harmonic approximation. In particular we examine the dependence of heat transmission and localization on the width of the system, the degree of disorder, and the temperature. Increasing the wire width changes not only the number of propagating modes, but also the behavior of each mode, in a fashion quite different from electronic localization in one dimension. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S26.00015: Ab initio calculations for the electronic properties of III-V semiconductor nanowires Manuel Alemany, Xiangyang Huang, James Chelikowsky The electronics industry is pushing the size of traditional semiconductor-based devices towards their physical limits, i.e., the miniaturization of devices, which is needed for increasing the efficiency, is restricted by the fundamental limitations of current lithography techniques. In contrast, the use of nanoscale structures as basic units for constructing electronic devices can potentially overcome such limitation. As building blocks for devices, semiconductor nanowires have created great interest in recent years. Here we will present results of an \textit{ab initio} study of the electronic properties of semiconductor nanowires of III-V materials. We have performed calculations using a real-space pseudopotential approach for these systems. We will discuss the role of quantum confinement on the electronic properties of such materials by comparisons to quantum dots and experiment [Preview Abstract] |
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S26.00016: Electronic and Structural Properties of Chains of Metals Michael Springborg, Yi Dong, Abu Assaduzzaman We report results of parameter-free calculations of the structural and electronic properties of isolated metal chains. The calculations have been performed using our own density-functional methods that has been developed for isolated, infinite, periodic, helical chains with a straight chain axis. We study linear, zigzag, double-zigzag, and tetragonal chains of Au, Ag, Al, Pt, Bi, Pb, and Tl, and discuss the relative stability of the different forms, distortion modes, effects of spin-orbit couplings, and band structures. [Preview Abstract] |
Session S27: Focus Session: Carbon Nanotubes: Theory
Sponsoring Units: DMPChair: Mark Pederson, NRL
Room: LACC 501C
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S27.00001: Curvature effects in carbon nanofibers Mina Yoon, Jane Howe, Zhenyu Zhang We apply {\em ab initio} density density functional formalism and also use empirical van der Waals potential to study equilibrium interlayer spacings in multi-wall carbon nanotubes and carbon nanofibers. We find that the generally-accepted empirical potential describing van der Waals interaction cannot explain the experimental observation of the strong curvature effects on interlayer spacings between neighboring graphite sheets. A modified van der Waals potential including curvature effects is constructed based on the experimental observations and theoretical calculations. Furthermore, the possible mechanism on increase or decrease of interlayer spacing in carbon nanofibers will be compared with experimental data. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S27.00002: Bundling up carbon nanotubes through Wigner defects Ant\^onio J. R. da Silva, Adalberto Fazzio, Alex Antonelli We show, using {\it ab initio} total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right besides a vacancy, which are present in irradiated graphite can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizeable amount, clearly indicating its importance for the mechanical properties of nanotube bundles. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S27.00003: Molecular Dynamics Simulation of Nucleation Process of SWNT from a Metal Particle on a Substrate Yasushi Shibuta, Shigeo Maruyama Nucleation process of single-walled carbon nanotubes (SWNTs) from a transition metal cluster on a substrate is studied using classical molecular dynamics (MD) simulations. For describing the effect of the substrate, averaged one-dimensional Lennard-Jones potential is employed between the metal cluster and the bottom boundary of the simulation cell. As the initial condition, a Ni500 cluster is placed on the bottom boundary of the cubic cell of 20 nm. The number of carbon atoms is adjusted to achieve the constant density by adding a new carbon atom to the cell when the metal cluster dissolves a carbon atom. As the metal cluster dissolves carbon atoms, the cluster becomes more wetting to the substrate. This may be due to the different wettability between pure metal and metal-carbide. Graphite structure gradually precipitates from the edge of the cluster. Nucleation mechanism of SWNTs will be discussed by comparing with the simulation using the floated catalyst. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S27.00004: Self-Healing of Divacancies in Carbon Nanotubes. Savas Berber, Atsushi Oshiyama We investigate the occurrence and reconstruction of divacancies in carbon nanotubes, and the electronic structure modification by defects using {\em ab initio} Density Functional calculations. Structure optimization calculations on both arm-chair and zig-zag defective nanotubes with diameters ranging from $4$~{\AA} to $9$~{\AA} reveals self-healing ability of defective nanotubes. Nanotube with ideal divacancy transforms into more stable structure by concerted formation of $2$ new bonds, leaving no unsaturated bonds. Diameter dependence of reconstruction energy and formation energy of divacancies depend on the mechanism of strain distribution for particular orientation of the defect. Divacancy formation is a possible way to stabilize the tubes with high concentration of monovacancies. Band structure of relaxed defective tubes indicate that metallic tubes mostly keep their metallic character while semiconducting tubes may acquire metallic character due to appearance of additional energy levels inside band gap of perfect tube due to new bonds formed during healing process. Five membered rings in relaxed structure can be recognized by a raised profile in simulated Scanning Tunneling Microscope (STM) images, providing a detection tool. In addition, we explore thermal stability of defective nanotubes by elevating the temperature in Nos\'e-Hoover molecular dynamics simulations using a parametrized electronic Hamiltonian. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S27.00005: Effect of intertube bond formation on mechanical and vibrational properties of nanotube ropes Andriy H. Nevidomskyy, G\'abor Cs\'anyi, Mike C. Payne Based on first-principles electronic structure calculations, we predict the formation of chemical bonds between boron nitride (BN) nanotubes, if doped with carbon. Similarly, intertube bonds are predicted to form in ropes of Nitrogen-doped carbon nanotubes. The effect of such bonds on the mechanical properties of carbon and BN nanotube ropes has been analysed, from which we find that the shear modulus of a nanotube rope appears to be greatly enhanced by the presence of intertube bonds. The analysis of phonon vibration spectra of BN nanotube ropes shows the up-shift in the frequency of the radial breathing mode due to the intertube bridging. These findings open prospects both for experimental detection of intertube bonds and possible technological applications. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 4:06PM |
S27.00006: Phonon Scattering and Excitons in Carbon Nanotubes. Invited Speaker: Due to their low dimensionality, carbon nanotubes (CNTs) have striking properties, quite different from these of traditional semiconductors, with important implications for technology. The performance of electronic devices relies on carrier mobility, which is extraordinary high in CNTs at low fields. However, at high fields the mobility is dramatically reduced due to inelastic optical phonon scattering. Optical properties of CNTs, essential for electro-optical devices, are dominated by excitons with binding energies and oscillator strengths orders of magnitude larger than those in conventional semiconductors. We calculate the electron-phonon scattering and binding in CNTs, within a tight binding model [1]. We derive the mobility as a function of temperature, electric field, and nanotube chirality using a multi-band Boltzmann treatment. We find the drift velocity saturates at approximately half the graphene Fermi velocity. Polaronic binding give a band-gap renormalization of 70 meV, an order of magnitude larger than previously suggested. We calculate the properties of excitons in CNTs embedded in a dielectric, for a wide range of tube radii and dielectric environments, by solving the Bethe-Salpeter Equation in a tight binding basis. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength as a function the tube radius, the dielectric constant of the embedding material, and the chirality [2]. In addition we calculate optical absorption including the exciton-phonon interaction. We find a phonon sideband at 200 meV above the zero phonon line, due to the creation of exciton plus one optical phonon [3]. [1] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411021. [2] V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. 92, 257402 (2004). [3] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411618. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S27.00007: Thermal conductivity of single-walled carbon nanotubes with $^{13}$C isotopes Junichiro Shiomi, Shigeo Maruyama Molecular dynamics simulations were performed to investigate the influence of impurities on thermal conductivity (k) of (5,5)-SWNTs. The impurities were represented by mixing $^{13}$C isotopes to a $^ {12}$C-SWNT. Random mixing of $^{13}$C isotopes to $^{12}$C- SWNTs results in decrease of k. The results show not only that k decreases against the fraction of mixed $^{13}$C, but also that k is dependent on the structure of $^{13}$C clusters, seemingly on their sizes. In order to highlight the influence of axial scales of the impurities, we consider SWNTs which consist of $^ {12}$C and $^{13}$C periodically connected with certain interval thickness. The result shows that there is a critical interval thickness which gives the minimum value of k. Spectral analyses reveal the role of phonon modes. Adopting the phantom heat bath model to each end of a SWNT, k can be computed through the Fourier's law. Non-Fourier aspects of the heat transfer in the non-equilibrium SWNTs are also examined by applying a local heat pulse with duration ranging from 40 fs to 4 ps. The results of the simulations exhibit the heat waves of selected phonons traveling from the heated end of the SWNT towards the other. The characteristic properties of the heat flow will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S27.00008: Structural, vibrational and thermodynamical properties of carbon allotropes from first-principles: from graphite to nanotubes Nicolas Mounet, Nicola Marzari We studied the ground state and finite-temperature properties of carbon allotropes using a combination of density-functional theory and density-functional perturbation theory. Equilibrium structures, elastic constants, and phonon dispersions of bulk diamond, graphene, graphite, and zigzag and armchair nanotubes are first obtained at the DFT-PBE level, showing on average an excellent agreement with experiments, with the caveat that for the case of graphite the experimental c/a ratio must be used. Thermal expansion coefficients are then determined from the minimization of the vibrational free energy in the quasi-harmonic approximation. Other thermodynamical properties such as heat capacities and the temperature dependence of the elastic constants are also obtained. Finally, the role of different phonon modes on the graphite, graphene, and nanotube thermal expansion or contraction is discussed, together with a full determination of their Gr\"{u}neisen parameters. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S27.00009: Quantum transport through short semiconducting nanotubes: A complex band structure analysis Pawel Pomorski, Christopher Martin Roland, Hong Guo Within an ab initio nonequilibrium Green's function formalism, we have examined the problem of quantum transport through short, semiconducting nanotube devices contacted with Al electrodes. Metallic behavior is predicted for very short nanotubes, which crosses over to semiconducting behavior as the tube length is increased. This behavior finds its origins in the evanescent modes that are present in these finite-sized systems, which cannot be ignored. A complex band structure analysis makes the contributions of these modes particularly transparent. Our calculation also allowed us to study the Schottky barrier formed between the nanotubes and Al contacts. We were also able to study the configuration where the whole system is in close proximity to a metal gate with some gate voltage, as is usually the case in experiment. Our computational method was able to handle metal gate boundary conditions and also implemented a numerical acceleration based on taking advantage of symmetry. References: Pawel Pomorski {\em et al.}, {\em Phys. Rev. B} {\bf 70}, 115408 (2004), Pawel Pomorski {\em et al.}, {\em Phys. Rev. B} {\bf 69}, 115418 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S27.00010: Structural ordering in nanotube polymer composites Chenyu Wei, Deepak Srivastava, K.J. Cho The structural and mechanical properties of polymeric carbon nanotube (CNT) composites have been studied through molecular dynamics simulations. Polymer molecules (polyethylene in this study) have been found to form layer structures around the nanotube with oscillating features and with orientations aligned with the tube axis. The increase of the structural order parameter of orientations {\$}$<$S{\_}Z$>${\$} is shown to contribute to the enhancement of mechanical modulus of CNT based composites. {\$}$<$S{\_}Z$>${\$} is found to increase with applied strains, and the corresponding structural change of the composite is shown to be inelastic, which is not fully recovered upon removal of the applied strains. [Preview Abstract] |
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S27.00011: Atomistic simulation of the laser fragmentation of single wall carbon nanotubes Harald Jeschke, Aldo Romero, Martin Garcia, Angel Rubio Femtosecond laser induced structural changes in single wall carbon nanotubes (SWNTs) are investigated by extensive molecular dynamics simulations on time dependent potential energy surfaces. The damage threshold of the SWNT is shown to depend on the chirality of the tube, on its diameter as well as on the laser pulse parameters. For the studied laser parameters, zigzag SWNTs are shown to be more stable with respect to laser excitation than armchair SWNTs. The diameter dependence of the thresholds for structural modification turns out to be nonmonotonic, suggesting the possibility of selectively evaporating SWNTs from nonhomogeneous tube bundles. [Preview Abstract] |
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S27.00012: C60 Polymerization in BN and Carbon Nanopeapods Andrea Trave, Filipe J. Ribeiro, Steven G. Louie, Marvin H. Cohen A variety of C$_{60}$ structures, polymerized and non-polymerized, isolated or encapsulated in BN or carbon nanotubes, have been analysed, to gain insight into the atomic and electronic structure of nanopeapods (NPP). Isolated double-bonded C$_{60}$ chains and dimers are found to be more stable than non-polymerized C$_{60}$. NPP geometries and energetics are unaffected by encapsulation, which gives an energy gain larger than the activation energy for C$_{60}$ polymerization, supporting the hypothesis of partial polymerization of C$_{60}$ molecules in NPP's. Upon encapsulation, BN NPP's remain wide-gap semiconductors, while in metallic carbon NPP's the lowest unoccupied C$_{60}$ states lie just above the Fermi level and charge transfert can take place, stabilizing single-bonded C$_{60}$ chains with wider spacing than double-bonded polymers, closely corresponding to the experimental structural observations. This work was supported by NSF (Grant No.DMR-0087088), and by the Office of Energy Research, Office of Basic Energy Sources, Materials Sciences Division of the US Department of Energy (Contract No. DE-AC03-76SF00098). Part of this work was performed under the auspices of the US Department of Energy by the University of California at the LLNL (Contract No.W-7405-Eng-48). Computational resources at NERSC, NCSA, and NPACI are acknowledged. [Preview Abstract] |
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S27.00013: Gate-controlled Tomonaga-Luttinger liquid in peapods Junji Haruyama, Jun Mizubayashi, Toshiya Okazaki, Takeshi Nakanishi, Hisanori Shinohara, Yuji Awano, Naoki Harada We report the charge transport properties of peapods encapsulating a chain of C60 within a field-effect transistor (FET) structure. We find they are very sensitive to the back gate voltage applied and power laws with large power exponents (between 12 and 1), findings not observed before in carbon nanotubes. Based on atomic-like behaviors observed in the single charging effect, we reveal that part of the power laws (between 3 and 1) arises from a Tomonaga-Luttinger liquid with strong electron-electron interaction, which originates from the large number of occupied subbands due to our peapod's unique electronic states. [Preview Abstract] |
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S27.00014: Analysis of band-gap formation in squashed arm-chair CNT H. Mehrez, A. Svizhenko, M.P. Anantram, M. Elstner, T. Frauenheim The electronic properties of squashed arm-chair carbon nanotubes are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into three phases. In the first phase of squashing, the nanotube deforms with negligible force. In the second phase, there is significantly more resistance to squashing with the force being ~40-100nN/per CNT unit cell, and in the last phase the nanotube loses its hexagonal configuration. We compute the change in band- gap as a function of squashing and our main results are: (i) A band-gap initially opens due to interaction between atoms at the top and bottom sides of CNT. The Pi-orbital approximation is successful in modeling the band-gap opening at this stage. (ii) In the second phase of squashing, large Pi-Pi* interaction at the edges becomes important, which can lead to band-gap oscillation. (iii) Contrary to a common perception, nanotubes with broken mirror symmetry can have zero band-gap. (iv) All arm-chair nanotubes become metallic in the third phase of squashing. Finally, we discuss both differences and similarities obtained from the tight binding and density functional approaches. [Preview Abstract] |
Session S28: Excitations, Scattering and Surfaces of Metals
Sponsoring Units: DMPChair: Randy Fishman, ORNL
Room: LACC 506
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S28.00001: Quasi-particle corrections to the LSDA+\emph{U} electronic structure of solid bcc hydrogen Emmanouil Kioupakis, Peihong Zhang, Steven G. Louie Quasi-particle calculations within the GW approximation usually start with the LDA electronic structure as mean field solution, which works well for moderately correlated materials. For strongly correlated systems, such as the transition metal oxides, LSDA can give qualitatively wrong ground states, making any further improvement difficult. By starting with the LSDA+\emph{U} mean field results in the GW approximation calculation of the electron self-energy, we expect to have a better understanding of the quasi-particle properties in these systems. We employ this approach in the study of solid hydrogen, a model system for which previous results exist in the literature. This will test the applicability of the technique to more realistic systems. This work was supported by National Science Foundation Grant No. DMR04-39768 and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Computational resources have been provided by NSF at the National Partnership for Advanced Computational Infrastructure (NPACI) and DOE at the National Energy Research Scientific Computing Center (NERSC) [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S28.00002: {\it Ab initio} calculations of optical spectra M.P. Prange, J.J. Rehr, A.L. Ankudinov We present a real space approach for {\it ab initio} calculations of the optical constants of materials. The approach is based on a generalization of the {\it ab initio} Green's function formalism implemented in the FEFF8 code to include valence and conduction band spectra. This all-electron approach, which is the real-space analog of the KKR method, includes self-consistent potentials, relativistic effects, inelastic losses, self-energy and screened core-hole effects, and multiple-scattering to all orders. Our procedure is based on calculations of the imaginary part of the dielectric function $\epsilon_2$, from which various optical constants can be derived using analytical properties and Kramers-Kronig transforms. Results for several materials are presented and compared with experiment. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S28.00003: 1T-TiTe2: scattering rate from infrared and Raman data Lev Gasparov, G. Guentherodt, H.H. Berger, L. Forro, G.G. Margaritondo, D. Tanner The normal-state properties of high temperature superconductors (HTSC) are usually described as those of a non-Fermi liquid, on account of the unusual temperature and frequency dependence of their optical and dc conductivities. This description can be challenged, however, because one has no good example Fermi-liquid (FL) behavior for comparison, especially considering the quasi two-dimensionality of the HTSC. One could address this issue with the help of a reference compound 1T-TiTe$_{2.}$ Both transport and photoemission data on 1T-TaTe$_{2}$ agree well with the Fermi liquid predictions. Thus 1T-TaTe$_{2}$ is believed to be a ``model Fermi liquid''. However, the key signature of the FL is the frequency and temperature dependence of the carrier scattering rate. In this report we discuss our infrared and Raman measurements of the scattering rate in 1T-TiTe$_{2}$ and compare our date to the available photoemission and transport data. $^{\ast }$Supported by Petroleum Research Fund award {\#} 40926-GB10 and Research Corporation Cottrell College science award {\#} cc6130 [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S28.00004: Combined BSE/TDDFT approach for x-ray absorption calculations A.L. Ankudinov, Y. Takimoto, J.J. Rehr Many-body effects such as local fields and the core-hole interaction can be significant in x-ray absorption spectra, even several hundred eV above an absorption edge. The treatment of these effects requires theories that go beyond the independent particle approximation such as the Bethe-Salpeter equation (BSE) or time-dependent density functional theory (TDDFT). However, neither of these approaches is fully satisfactory; the BSE is usually restricted to low energies while the TDDFT ignores non-locality. Here we present a combined TDDFT/BSE approach which is applicable over a wide spectral range. This approach is implemented within a real-space multiple-scattering formalism, with core-hole interaction and the local screening fields calculated within linear response. The approach is illustrated for a several materials and compared with experiment, including the Mg K-edge of MgO, the Ni L-edges, and W M-edges. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S28.00005: Multiple-scattering approach to inelastic x-ray scattering calculations J.A. Soininen, A.L. Ankudinov, J.J. Rehr We present an implementation of the real-space multiple-scattering (RSMS) approach for calculations of non-resonant inelastic x-ray scattering (NRIXS) from core-electrons, which is also termed x-ray Raman scattering (XRS). The RSMS approach has been used extensively to model x-ray absorption spectra from deep core levels, including both the fine structure and near-edge behavior. This {\it ab initio} method includes final state self-energy and lifetime effects as well as an approximate treatment of the core-hole interaction. Moreover, the method is applicable to aperiodic or periodic systems alike. The momentum transfer dependence of the XRS is related to the dynamic structure factor $S(\vec q, \omega)$. The results for XRS at different momentum transfers yield information concerning local symmetries of the excited states through changes in the spectral weight of mono-pole, dipole, quadrupole, and higher couplings. Our results are compared both with experiment and with other theoretical calculations. Our near edge calculations are in agreement with previous calculations, and we show how the approach can also be used to calculate the fine structure. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S28.00006: Coincidence Measurements of the Auger Cascade Process in Ag R. Sundaramoorthy, A. H. Weiss, S.L. Hulbert, D. Arena, R. A. Bartynski Auger electron spectroscopy provides a probe of many-electron phenomena and the connection between screening, correlation of the final-state holes, and the spectral line shape. Intra-shell and inter-shell hole-hole interactions in the intermediate and final states of the Auger cascade processes shift the measured electron emission spectra of Ag. Here we study the correlation effects of the final state holes created in LMM Auger Cascade process. The final \textit{MM }hole of the \textit{LMM }transition can serve as the initial state for an \textit{MVV }transition, the contribution of which can be separated out from the remaining contributions occurring from other cascades such as LMV and the direct transition using Auger-Auger coincidence spectroscopy. Here we present results for the x-ray excited Auger \textit{MVV }spectra of Ag measured with synchrotron radiation excitation where we study the difference between the MVV -- 2p3/2 Auger photoelectron coincidence spectra and the LMM-MVV Auger-Auger coincidence spectra. The observed differences throw light on the correlation effects in many core-hole excited states. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S28.00007: Interaction between a surface plasmon and surface nano-defects in thin metallic films. Ra\'{u}l Garc\'{i}a-Llamas, Javier Dur\'{a}n-Favela, Jorge Gaspar-Armenta, Jos\'{e} Valenzuela-Benavides The intensity of the electromagnetic near-field produced by the interaction between surface plasmons and surface nano-defects on otherwise planar structure is studied theoretically. The structure is a thin metallic film bounded by glass and vacuum, the Kretchmman configuration used for the excitation of surface plasmons. Exact and perturbation solutions, until fourth order in the surface defect profile, of the reduced Rayleigh equation are found to obtain the intensity. The numerical results are calculated using a single or double Gaussian wells, and triangular shaped one. Rapid oscillations of the near field are found whose period is the inverse of two times the mode propagation constant. A Fourier Transform technique is used to reconstruct the surface defect profile from the near-field intensity obtained at constant height. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S28.00008: Spectral response of the optical second-harmonic scattering from a metallic cylinder Jesus Maytorena, Claudio Valencia We study the scattering of second-harmonic (SH) radiation generated by an infinite cylinder of homogeneous, isotropic, centrosymmetric material and arbitrary radius illuminated perpendicularly to its axis with $p-$ or $s-$ polarized light. We derive analytical expressions for the SH radiated field and illustrate the theory for a simple metal cylinder. The nonlinear source polarization includes both a nonlocal bulk term and a dipole-allowed surface term which corresponds to the interfacial region where the inversion symmetry is broken. We consider the cylinder as locally flat and use the dipolar surface susceptibility resulting from a microscopic calculation based on the density functional formalism for a planar jellium surface. The frequency dependence of this surface contribution manifests itself in the spectral response. The calculated SH scattering cross section shows a separated peak corresponding to a surface-intrinsic collective mode in addition to peaks due to plasmon modes of the cylinder whose frequencies are determined by the linear optical properties at either fundamental or SH frequency. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S28.00009: Observation of growth mode dependence on over-potential values: a combination of in-situ optical monitoring and ex-situ atomic force microscopy characterization. M. K. O'Toole, J. Gray, T. Moffat, C. Orme, W. Schwarzacher, X.D. Zhu We study the over-potential electrodeposition of Pb on Cu(100) using a combination of an \textit{in-situ} oblique-incidence reflectivity difference technique and \textit{ex-situ} atomic force microscopy. We identified two distinctly different over-potential growth modes that depend on the values of the overpotential. We have characterized the morphology corresponding to the two growth modes using AFM. At a high over-potential there is even growth of small clusters across the monolayer thick under-potential deposited (UPD) Pb covered Cu(100) surface. At lower over-potentials (a fraction of a volt from the value for UPD), we observed much larger 3-D clusters, more than 400 nm in height and separated by an averaged distance of one micron. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S28.00010: Determining Film Thickness and Probing Buried Interface Structure with Characteristic Scanning Tunneling Spectroscopy Shin-Ming Lu, W.B. Su, C.L. Jiang, H.T. Shih, C.S. Chang, Tien T. Tsong Structural and electronic properties of atomic-scale flat Ag films grown on Si (111)-7×7 are measured with scanning tunneling microscopy and spectroscopy. Spectroscopy for each film thickness not only reveals the features of transmission resonance and distinct quantized bound states, but can also probes the buried interface structure. First, the energy levels of those states vary with the film thickness and can serve as the fingerprints. The film thickness can thus be determined with the characteristic spectrum, which is especially useful as the film covers the entire substrate. Secondly, the spectra manifest a shift of electronic resonance and quantized bound states due to varied reflection strength at the buried interface. With this effect, the buried interface structure can be probed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S28.00011: Persistent Step-Flow Growth of Strained Overlayers on Vicinal Substrates Wei Hong, Mina Yoon, Zhigang Suo, Ho Nyung Lee, Hans M. Christen, Doug H. Lowndes, Zhenyu Zhang Driven by step-step (SS) attraction, a strained overlayer grown on a vicinal substrate is inherently unstable, as manifested by step bunching. The step-edge (or Ehrlich-Schwoebel) barrier effects may either accelerate, delay, or suppress step bunching, depending on the nature of the ES barriers for a given system. Using linear stability theory and numerical simulations, we analyze the morphological evolution in heteroepitaxial growth with explicit consideration of the competition between the SS and ES effects. We establish the existence of a deposition flux window within which stable and persistent step-flow growth can be achieved. This window for step-flow growth is sandwiched between the island growth mode at too high deposition fluxes, and the step bunching mode at too low deposition fluxes. We express the phase boundaries in terms of intrinsic physical parameters and experimentally controllable growth conditions, and compare the predictions with experimental results from PLD growth studies of metal oxide thin films. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S28.00012: Surface Relaxation of V(100); Experiments and Ab-Initio Calculations: a status report on the Feibelman Issue D. Lacina, A. Ciucivara, B.R. Sahu, L. Kleinman, J.L. Erskine Systematic discrepancies between first-principles calculations and experimental determinations of the surface relaxation of reactive transition metals was first noted by Feibelman.$^{[1]} $ The basic issue is that calculations invariably yield first- layer (inward) relaxations that substantially exceed the results obtained by electron diffraction, and the disagreement lies beyond the accepted accuracy of the experiments and calculations. We report new LEED results and calculations for V (100) that explore this trend and present a status report on the primary issues. The LEED results for several data sets at 300 K and 100 K are robust and consistent with d$_{12}$ = -5.0 \underline{+}1.0 \% and d$_{23}$ = 3.6 \underline{+}1.0 \%with acceptable Pendry and Zanazzi-Jona r-ractors. Preliminary calculations using pseudopotentials without partial core corrections gave d$_{12}$ = -14.0 \% in the GGA (cf. the more accurate FPLAPW d$_{12}$ = -11.1 \%).$^{[2]}$ Meta-GGA$^{[3]}$ calculations resulted in d$_{12}$ = -12.0 \% suggesting meta- GGA FPLAPW calculations might yield d$_{12}$ as small as -9 \%, still an unacceptably large deviation from experiment. \newline Supported by the R.A. Welch Foundation \newline 1. P.J. Feibelman, Surf. Sci. \underline{360}, 297(1996) \newline 2. G. Bihlmayer, T. Asada, and S. Blugel, Phys. Rev. B\underline{62}, 11937(2000) \newline 3. V.N. Starorerov, et al., Phys. Rev. B\underline{69},75102(2004) [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S28.00013: Theory of Current Crowding Effect on Electromigration Lingyun Zhang, King-Ning Tu We investigate the current crowding in the inhomogeneous area and explore the mechanism of gradient force in electromigration. The inhomogeneous field, which is stimulated by the inhomogeneous current density, can provide a force for the vacancies. It is shown that the magnitude of the force along the normal direction of current density is the same order as the electric field force. By using the self-consistent approach, the distribution of electrons and vacancies, and the current density distribution can be obtained. The detailed calculation demonstrates that the current density gradient is a driving force and induces the atomic rearrangement in the inhomogeneous area, which explain why the void can be formed in the low current density regions of an interconnect. Furthermore, the quantum mechanical approach is developed to understand the nature of gradient force in electromigration. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S28.00014: The Ni(111) surface electrons investigated with low-temperature scanning tunneling spectroscopy Kai-Felix Braun, C.F.J. Flipse, K.-H. Rieder The electronic structure of the ferromagnetic Ni(111) surface has been attracting interest for a long time. Despite experimental and theoretical effort, reported values of binding energies, effective masses and number of the surface states and surface resonances differed substantially. Working with a local probe technique reveals relevant contributions from adsorbates and defects. Here we present an extensive scanning tunneling microscopy and spectroscopy investigation at low temperature, employing fourier transform methods for the analysis. The results show a parabolic surface state with an upward dispersion at --165 meV with a surprisingly low effective mass of 0.17 m$_{e}$ and a downward dispersing surface resonance at --230 meV. From the decay of the standing wave pattern at step edges electron and hole lifetimes have been determined with values considerably smaller than found on noble metal surfaces. Details of the surface electronic structure have been revealed including an anti-resonance at the Fermi energy. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S28.00015: Optimal Switching Time Israel Klich, Leonid Levitov We discuss the noise produced in the process of switching mesoscopic conductors between two noiseless states: perfectly connected and disconnected, in the presence of a bias voltage V. We show that there are two main contributions to the noise: a switching noise logarithmic in the time of observation T, and a quantum shot noise accumulated during the process of switching and proportional to V, this leads to a minimization problem for the optimal switching time. Switching noise is expected to be a fundamental parameter in nano-circuits. We also discuss the relation of this result to an estimation of entangelment entropy of a Fermi sea. [Preview Abstract] |
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S28.00016: Surface, Step-Edge, and Facet-Edge Diffusion Barriers in Growth of Pb Nanomesas Yong Han, Feng Liu, Guang-Hong Lu, Byeong-Joo Lee To obtain a better understanding of the growth kinetics of faceted Pb mesas on Si(111) surface, we perform extensive calculations of diffusion barriers for a Pb adatom (1) on a flat Pb(111) surface, (2) crossing a single A- or B-step edge on the Pb(111) surface, and (3) crossing the joint edge of the Pb(111) and (001) facet, using a modified embedded atom method. We have investigated different diffusion mechanisms mediated by vacancies and concerted displacements of multiple atoms. A quantitative comparison will be made between the calculated results and recent experiments. *This work is supported by NSF. [Preview Abstract] |
Session S30: Polymer Blends
Sponsoring Units: DPOLYChair: Alamgir Karim, NIST
Room: LACC 505
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S30.00001: Thermodynamics and Dynamics of Diblock Copolymers at Polymer/Polymer Interfaces Benedict J. Reynolds, Megan L. Ruegg, Nitash P. Balsara, C. J. Radke The efficacy of diblock copolymers for stabilizing interfaces between immiscible polymers depends on both thermodynamic and dynamic factors. We have exploited the slow dynamics of polymer chains to prepare two surfactant-bearing polymeric interfaces that are initially out of equilibrium. The distance between the interfaces was varied from 50 to 600 nm, and the transport of the surfactant molecules from one interface to another was measured by dynamic secondary-ion mass spectroscopy. This transport depends on the traditional diffusion coefficients and the depth of the thermodynamic potential wells that trap the surfactant molecules at the interfaces. The diffusion coefficients of our system were measured in independent experiments and the well depths were obtained from SCFT using a Flory-Huggins interaction parameter and statistical segment lengths measured by small-angle neutron scattering from homogeneous binary blends. This enables a comparison of our experimental interfacial transport measurement and theoretical predictions with no adjustable parameters. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S30.00002: Thermodynamics of Polymer Blends Organized by Balanced Block Copolymer Surfactants Studied by Mean-field Theories and Scattering Megan Ruegg, Benedict Reynolds, Nitash Balsara, Timothy Shaffer, Min Lin, David Lohse The phase behavior of multicomponent blends of two immiscible homopolymers (A,B) and an A-C diblock copolymer was studied by scattering experiments and mean field theories. The interactions between the components were tuned to create organized blends with the copolymer serving as a surfactant. The morphology of A/B/A-C blends changed between lamellar phases, microemulsions, homogeneous phases and macrophase- separated states simply by adjusting the temperature. The experimentally determined phase transition temperatures and domain spacings were compared with calculations based on the Random Phase Approximation (RPA) and Self-Consistent Field Theory (SCFT). The only inputs into the calculations were the binary Flory-Huggins interaction parameters (chi) between the three kinds of monomers in our system, and statistical segment lengths. The domain spacing determined from theory was often within 5\% of the experimental values. In a particular range of molecular weights, we find that a critical A/B blend can be organized into a periodic phase by the addition of only 5\% of the diblock copolymer. To our knowledge, all previous experiments that have led to organized critical mixtures have required a significantly larger copolymer concentration. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S30.00003: Effects of Branch Points and Chain Ends on Interfacial Segregation and Bulk Thermodynamics in Blends of Branched and Linear Polymers Jae S. Lee, Nam-heui Lee, Alexei P. Sokolov, Roderic P. Quirk, Mark D. Foster, Boualem Hammouda, Charles F. Majkrzak The effects of the number of branch points and number of chain ends in a branched chain on interfacial segregation and bulk thermodynamics in binary blends of well-defined, regularly branched polystyrenes with their linear analogues were studied. A novel series of molecules in which the numbers of branch points and numbers of chain ends in the branched molecule were varied in a controlled way was key to the study. The value of the effective interaction parameter determined from small angle neutron scattering data increased both with increasing the number of branch points and the number of chain ends. Neutron reflectometry and Surface Enhanced Raman Spectrometry measurements showed the preferential segregation of the branched molecules to both interfaces of blend films. The strength of the segregation generally increased by increasing the number of branch points while the number of end groups was kept constant at six. The degree of segregation also increased by increasing the number of end groups while keeping the number of branch points constant at four. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S30.00004: Ultra-low interfacial tensions of a polymer/polymer interface with diblock copolymer surfactant Kwanho Chang, David Morse, Christopher Macosko The equilibrium interfacial tension between immiscible homopolymers A and B mixed with an AB copolymer reaches a minimum value $\gamma_{sat}$ beyond a critical concentration of copolymer at which the copolymer begins to self-assemble into micelles or a bicontinuous microemulsion. This saturation value has been measured with a spinning drop tensiometer for systems of poly(isoprene) (PI), poly(dimethyl siloxane) (PDMS), and PI-b-PDMS copolymers, for a sequence of 11 copolymers with PDMS volume fractions $f=0.5 - 0.73$. Ultra-low tensions of $\gamma_{sat} \simeq 10^{-3}$ mN/m have been measured for copolymers with $f = 0.5$. Self-consistent field theory predictions of the tension between a PI phase and a PDMS phase containing swollen micelles agree very well with measured values for copolymers with $f \geq 0.61$. For $f = 0.5 - 0.6$, however, measured values of $\gamma_{sat}(f)$ are much lower than predicted by this theory, and exhibit a discontinuous dependence on $f$ at $f \simeq 0.6$. We suggest that the behavior observed for nearly balanced copolymers could be due to formation a middle-phase bicontinuous microemulsion phase that wets the macroscopic PI-PDMS interface. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S30.00005: Kinetic Hindrance during Diffusion-Controlled Reactions at Polymer-Polymer Interfaces Shane Harton, Frederick Stevie, Harald Ade We have characterized diffusion-controlled (DC) reactions between 75kDa hydroxy-terminated deuterated polystyrene (dPS-OH) and several acyl chloride functionalized poly(methyl methacrylates) (PMMA) using dynamic secondary ion mass spectrometry (DSIMS). When annealed at 393-408K, these systems consistently show a depletion hole at early times, which is indicative of DC reactions, but this depletion hole disappears over time with an apparent reaction extinction. This extinction is characterized by an interfacial excess of block-copolymer (Z*) that is considerably lower than current theories predict. This indicates that reactions at highly immiscible polymer-polymer interfaces, whether inherently DC or reaction- controlled, may actually be controlled primarily by kinetic hindrance at the interface rather than long-range diffusion or reaction mechanisms. To probe this directly, we have measured the extinction excess (Z$_{e})$ as a function of reactive dPS-OH concentration in the PS matrix ($\sim $ 5-20 {\%}) and reaction temperature ($\sim $ 403-433K). The variation of the Z$_{e}$ as a function of initial dPS-OH concentration confirms kinetic hindrance as the underlying cause of the reaction extinction, as opposed to loss of reactivity, because the reactive chains are never fully depleted, and the coupling reactions implemented here are extremely robust and completely irreversible. An effective activation energy is determined from the temperature dependency of the Z$_{e}$. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S30.00006: Phase Behavior of Ternary Block Copolymer – Homopolymer Blends in Thin Films on Chemically Nanopatterned Surfaces Mark P. Stoykovich, Harun H. Solak, Paul F. Nealey Ternary blends of lamellar-forming block copolymers and homopolymers exhibit a complex phase behavior in the bulk with ordered lamellar, two-phase, and disordered morphologies coexisting at a Lifshitz multicritical point. The onset of the unbinding transition and the associated bicontinuous microemulsion phase is predicted to be a function of $\alpha $, the ratio of the degree of polymerization of the hompolymer to the block copolymer. Here we investigate the phase behavior of ternary blends with varying $\alpha $ and lamellar periods, Lb, in thin films on chemically striped surfaces with period Ls. Our findings indicate that: 1) in contrast to model predictions for bulk systems, the unbinding transition is not strongly dependent on $\alpha $, 2) a blend in the microemulsion phase forms lamellar structures on patterned surfaces, 3) chemical surface patterns can induce phase separation of the ternary blend, and 4) the homopolymer components can distribute themselves in such a way as to form adjacent lamellar domains with different Lb. The technological importance of these findings is the ability of ternary blend systems to epitaxially self-assemble into imaging materials for sub-30 nm lithography. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S30.00007: Nucleation in Polymer Blends Timothy Rappl, Nitash Balsara The initial stages of phase separation in an off-critical binary mixture of model polyolefins were studied by time-resolved small angle neutron scattering. We focus on nucleation in the metastable regime which is bounded by the binodal, the location where the nucleation barrier is infinite, and the spinodal, where the nucleation barrier is unimportant. This broad range of barrier heights renders study of the early stage of nucleation a challenging endeavor. We have met this challenge by subjecting our blend to both single- and two-step quenches within the phase separated region of the phase diagram. This enables measurement of the size of the critical nucleus over 80{\%} of the metastable regime. Some aspects of nucleation kinetics follow the linearized theory of Cahn, Hilliard, and Cook, which was originally developed to describe spinodal decomposition. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S30.00008: Discrete combinatorial phase mapping of multicomponent mixtures Joao Cabral, Alamgir Karim We report an experimental investigation of the bulk phase behavior of multicomponent mixtures using a novel discrete combinatorial approach. The technique involves a parallel cloud point detection scheme using discrete composition libraries, which are scanned across a temperature range and optically imaged. Sample substrates are microwell arrays fabricated by contact photolithography on a glass coverslip. Polymer blend libraries are generated using a custom built, programmable liquid dispenser system. The sample arrays are placed in a uniform, but continuously varying, temperature field, scanning the mixture across its phase boundary. Optical turbidity is detected by imaging the entire array and the cloud point curve is determined through automated parallel image analysis. In this demonstration, we investigate mixtures of low molecular mass poly(styrene) and poly(butadiene), exhibiting upper critical solution temperature (UCST) phase behavior. The cloud point curves obtained closely approximate the bulk binodal line, and have a high composition resolution of delta phi = 0.01 (volume fraction), using 10 x 10 sample arrays. We discuss thermodynamic and kinetic effects induced by the addition of copolymers and nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S30.00009: Small Angle Neutron Scattering Studies on Blends of Poly (Styrene-ran-Vinyl Phenol) with Liquid Crystalline Polyurethane Rujul Mehta, Mark Dadmun Molecular composites, composed of uniformly dispersed rigid-rod liquid crystalline polymer (LCP) molecules in a flexible amorphous polymer matrix, have remained hitherto elusive due to a scarcity of miscible systems containing a LCP and an amorphous polymer. The production of such a blend, with an experimentally accessible miscibility window, has become possible by modifying the architecture of the flexible polymer, so as to induce favorable intermolecular hydrogen bonding. Specifically, liquid crystalline polyurethanes (LCPU) are found to be miscible with a copolymer of styrene and vinyl phenol; with optimum hydrogen bonding between the carbonyl groups of the urethane linkages and the hydroxyl groups present in the styrenic matrix. Availability of a truly miscible molecular composite presents a unique opportunity of studying the confirmation of polymer chains containing rigid-rods that are uniformly dispersed in a flexible coil matrix. A system consisting of the LCPU and the deuterated styrenic copolymer containing 20{\%} vinyl phenol is examined by Small Angle Neutron Scattering at the National Center for Neutron Research at Gaithersburg and Technology, and the Institute of Solid State Research (IFF) at J\"{u}lich. Scattering curves for neat dPS-VPh did not fit the Debye-Bueche model; indicating complex structure. A two correlation length Debye-Bueche model was considered to accommodate for this nonlinear behavior. This model utilizes four fitting parameters, including two correlation lengths a$_ {1}$ and a$_{2}$, corresponding to a Debye-Bueche model and Guinier model. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S30.00010: Binary Phase Diagrams of Crystalline Polymers Thein Kyu, Rushikesh Matkar We have extended the phase field free energy for polymer crystallization to binary crystalline polymer blends to generate phase diagrams that are capable of describing a rich variety of phases including eutectic, peritectic, and a host of other intermediate cases. The thermodynamics of liquid-liquid mixing have been modeled based on the Flory-Huggins theory with a $\chi $ interaction parameter. Various coexistence regions have been computed self-consistently by extending the phase field theory of crystallization to binary systems in conjunction with the coupling terms between phase separation and crystallization. The calculated phase diagrams exhibit rich variety of coexistence regions such as liquid + liquid, liquid + crystal, crystal + crystal and neat crystal regions. To describe the spatio-temporal evolution of crystalline morphology, a conserved concentration order parameter and a non-conserved crystal phase order parameter have been utilized in the context of the time-dependent Ginzburg-Landau (TDGL) model C, viz. model A for crystallization and TDGL model B for phase separation. The emerging crystalline morphology is discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S30.00011: Spectroscopic Investigation on Morphology Development of Polymer Blends Tomoko Hashida, Young Gyu Jeong, Ying Hua, Shaw Ling Hsu For the first time, high spatial resolution ($\sim $1 $\mu $m$^{2})$ Raman micro-spectroscopy has been used to measure the composition and crystallite distribution of various crystallizable polymer blends. Crystallization kinetics and the degree of crystallinity were characterized using a combination of optical microscopy, thermal analysis, and time resolved FT-IR spectroscopy. These investigations were conducted for a number of binary blends incorporating crystallizable poly(hexamethylene adipate) (PHMA) or poly(hexamethylene sebacate) (PHMS) mixed with non-crystallizable poly(propylene glycol) (PPG). Although the two polyesters have similar chemical structure, they exhibit different phase behavior. Ternary blends including a high glass transition temperature ($T_{g})$ component were also studied. The local composition of polyester was found to control crystallization kinetics and degree of crystallinity. The compositional distribution in the polyester-rich phase was inhomogeneous. Surprisingly, the degree of crystallinity measured for polyesters did not necessarily correspond to the composition profile. The role of the third relative immobile component significantly changed both chemical and morphological distributions. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S30.00012: Glassy states and microphase separation in crosslinked homopolymer blends Paul Goldbart, Christian Wald, Annette Zippelius We address the physical properties of blends of distinct homopolymers, crosslinked beyond the gelation point, via a Landau approach involving a pair of coupled order-parameter fields: one describing vulcanization, the other describing local phase separation [1]. Thermal concentration fluctuations, present at the time of crosslinking, are \lq\lq frozen in\rq\rq\ to the gel network. The resulting glassy fluctuations are analyzed at the Gaussian level in various regimes, determined by the relative values of certain physical length-scales. We also analyze the enhancement, due to crosslinking, of the stability of the blend with respect to demixing. Beyond the corresponding stability limit, complete phase separation is prevented by gelation and replaced by microphase separation, which occurs up to a length-scale set by the mesh size of the network, as a simple variational scheme reveals. [1] C. Wald, A. Zippelius and P. M. Goldbart, cond-mat/0411056. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S30.00013: Coarse-Grained Description of Polymer Liquids and their Mixtures as Interacting Soft-Colloidal Particles Marina Guenza, Galina Yatsenko, Edward Sambriski, Maria Nemirovskaya We present a novel theoretical approach which maps polymer melts and their mixtures onto fluids of soft-colloidal particles. From liquid-state theory we derive analytical center-of-mass total pair correlation functions, which reproduce those computed from united-atom simulations with no fitting parameters. The coarse-grained description correctly bridges micro- and mesoscopic fluid properties. Molecular dynamics simulations of soft colloidal particles interacting through the calculated effective pair potentials are consistent with data from microscopic-scale simulations and analytical formulas. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S30.00014: Polyolefin blends: Coarse-grained study of melt structures relevant for predicting blend miscibilities Sandeep Jain, Shekhar Garde, Sanat Kumar Miscibility of polyolefins has been the subject of intense research both due to its fundamental importance and industrial applications. Detailed Molecular Dynamics (MD) simulations of long chain polymers and their phase behavior is limited by the enormous computational effort involved. Naturally, emphasis is being placed on development of coarse-graining strategies that allow more efficient sampling of conformational space while retaining the chemical identity of the polymer of interest. We will present results from a novel coarse-graining approach that combines detailed MD simulations of oligomers with inverse (Monte Carlo based) methods to obtain interaction potentials in coarse-grained system. We show that the coarse-grained potentials, thus generated, reproduce a variety of structural properties of the underlying polymer melt systems. The computational efficiency of our coarse-graining apporach allows simulations of truly polymeric (long chain) melts. Extension of these ideas to studies of polyolefin blends with emphasis on miscibility will be presented briefly. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S30.00015: Miscible polyethylene glycol-citric acid gels Justin Barone Polyethylene glycol (PEG) and citric acid (CA) are crystalline solids at room temperature. They are opaque, hard, brittle materials. However, blending the PEG and CA shows that a clear, soft “gel” is formed at certain concentrations. Rheology, differential scanning calorimetry (DSC), FT-IR, and Raman spectroscopy are used to characterize the behavior of the blends as a function of concentration. The solubility parameters for PEG and CA are the same indicating that complete miscibility is possible. It is found that the PEG and citric acid strongly associate through hydrogen bonding and prevent re-crystallization of either phase. [Preview Abstract] |
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S30.00016: Quantitative Predictions of the Enthalpic Component of the Interaction Parameter in Mixtures: An Assessment of the Accuracy and Precision Required From Molecular Simulations David Rigby The use of force field based atomistic simulation methods to calculate the enthalpic part of the interaction parameter in mixtures containing small molecules or polymers is both conceptually simple and appealing in view of the enormous potential savings in the cost of synthesis and experimental measurements (e.g. when one is seeking a new or modified polymer mixture system). In order for such simulations to be ultimately successful however, it is necessary that they be capable of routinely making accurate predictions of excess thermodynamic properties. This in turn requires both that a force field be capable of high accuracy and that the associated simulation protocol be capable of high precision. This presentation will examine the factors that affect precision and accuracy in typical simulations of experimentally well-studied small molecule mixture systems containing aliphatic and aromatic hydrocarbons, and will further discuss additional sources of uncertainty when the simulations are extended to mixtures of oligomers or polymers comprised of similar aliphatic and aromatic moieties. [Preview Abstract] |
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S30.00017: Three-Dimensional Modeling of Holographic Polymer-Dispersed Liquid Crystal Formation via Various Interference Techniques Thein Kyu, Gregory Yandek, Scott Meng Holographic polymer-dispersed crystalline materials (H-PDLC), useful in many optical applications, maintain periodic alignment of liquid crystalline (LC) domains in polymeric hosts. H-PDLC fabrication entails the exposure of a mixture containing monomer, photo-initiator, and inert LC to geometrically arranged light beams where constructive and destructive interference occurs within the sample. Polymerization dominates in regions of high beam intensity such that LC migrates to areas of low intensity resulting in desired periodic structures. It is advantageous to acquire information regarding the physics of the fabrication process through modeling techniques. By coupling reaction-diffusion equations with the Flory-Huggins theory of mixing, Maier-Saupe relations for nematic ordering, and network elasticity terms, modeling has elucidated information without accruing the costs of trial and error. Results predict that a 35{\%} LC volume content and flexible polymer host materials afford optimal structures. Two- and three-dimensional variations in structures have been predicted. [Preview Abstract] |
Session S31: Organic Electronics Materials Characterization
Sponsoring Units: DPOLY FIAPChair: Graciela Blanchet, DuPont
Room: LACC 503
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S31.00001: Optical characterization of single crystals of the organic semiconductor rubrene J.R. Weinberg-Wolf, L.E. McNeil, Shubin Liu, Christian Kloc 5,6,11,12-tetraphenyl tetracene (rubrene) is an organic semiconductor with a reported mobility of up to 20 cm\^{}2/Vs and a near 100{\%} photoluminescence yield. It is receiving much attention for its possible uses in electronic devices. A detailed analysis of the Raman spectrum of rubrene single crystals will be discussed in light of isolated-molecule Raman simulations. The spectra will also be compared to measurements and simulations of single crystal tetracene Raman spectra, as the tetracene molecule is the backbone of the rubrene molecule. The effects of temperature on the Raman spectrum of the crystalline rubrene will also be presented. One important conclusion from the vibrational studies is that there is very small intermolecular coupling between rubrene molecules in the solid state. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S31.00002: Crystal Structures and Band Structures of Acene Chalcogenides: Their Application for OFET. A. Ugawa, T. Kunikiyo, Y. Ohta, M. Murakami, J. Kasahara We have systematically studied acene chalcogenides as active channel materials for Organic Field Effect Transistor (OFET). The molecules have a common structural feature which is of great advantage for carrier channel: chalcogen atoms are located at outside of molecule so that larger orbitals of chalcogen atoms would intensify intermolecular interactions not only for the molecular stacking direction but also for the inter-stacking directions. We then expect that the conduction channels would become more isotropic and the effective mass of the carriers would become lighter compared to the case of aromatic hydrocarbons. The materials we have surveyed are as follows: Hexathiopentacene (HTP), Tetrathiotetracene (TTT), Hexathioanthracene (HTA), Tetrathioanthracene (TTA), Benzo[1,2-c;3,4-c';5,6-c'']tris[1,2]dithiole-1,4,7-trithione (abbreviated as C$_{9}$S$_{9}$, which is its chemical formula), and some selenium analogues. The first-principle band structure calculations based on the crystal structures determined by x-ray analysis reveal that the materials have an anisotropic 2-D HOMO band with an effective mass of 1.1$m_{e}$ for TSeA, 1.3$m_{e}$ for HTA, 1.4$m_{e}$ for TTT, respectively. HTP is proved to have a 1-D HOMO band with $m$*=2.5$m_{e}$ in spite of 2-D structural feature. It is interesting that C$_{9}$S$_{9}$ has an isotropic 3-D HOMO band with $m$*=0.68$m_{e}$, properties which are suitable for OFET channel. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S31.00003: Microscopic evidence for spatially inhomogeneous charge trapping in pentacene. Erik Muller, John Marohn Using high sensitivity electric force microscopy we are investigating the electronic properties at the semiconductor-dielectric interface in pentacene thin film devices. It is believed that the conduction takes place within the first few monolayers of the organic and is adversely affected by the presence of charge traps. We find that charge traps in polycrystalline pentacene are distributed inhomogeneously but do not appear to be associated with grain boundaries as is generally supposed. We will also report on ongoing studies of thin (1-3 monolayers) devices, where the relationship between the topography and the location of the charge traps is more easily interpreted. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S31.00004: Optical properties of pentacene clusters and ultra-thin films (*) Rui He, Nancy G. Tassi, Graciela B. Blanchet, Aron Pinczuk Photoluminescene spectra of pentacene clusters and films of few monolayer in thickness reveal two fundamental excitations that are assigned to the Davydov doublets of the lowest singlet exciton. While the energy splittings of the doublets have minor dependence on cluster thickness, their bandwidths become narrower as the pentacene clusters grow larger and into continuous ultra-thin films. The marked similarity of these Davydov doublets to those in optical absorption spectra of thicker pentacene films and crystals suggests a similarity in molecular arrangements. Luminescence of self-trapped excitons is quenched in the few monolayer clusters and ultra-thin films. Asymmetric 0-0 and 1-0 resonance Raman scattering excitation profiles are observed in these samples. (*) Supported primarily by the Nanoscale Science and Engineering Initiative of the NSF under NSF Award Number CHE- 0117752 and by the NYSTAR, and by a research grant of the W. M. Keck Foundation. We thank I. Dujovne and C. F. Hirjibehedin for their helpful discussions. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S31.00005: Raman spectroscopic studies of polyfluorenes upon thermal cycling M. Arif, S. Guha, B. Tanto, M.J. Winokur Polyfluorenes (PFs) have emerged as attractive alternatives to other pi-conjugated polymers for organic optoelectronics due to their strong blue emission, high charge mobility and excellent chemical and thermal stability. Almost all PFs utilize side- chain substituents that improve solution processing as well as confer new functionality. Thermal cycling of PFs results in distinct backbone and side-chain conformations that lead to improved optical and electronic properties. In this work we present detailed Raman scattering studies of a branched (PF2/6) and non-branched (PF8) PF as a function of temperature to investigate phase transitions and molecular ordering. The low frequency Raman peaks between 100-1000 cm$^{-1}$ are extremely sensitive to the local chain conformation, side chain moiety, and are strongly impacted by thermal cycling. Our results are further compared with X-ray diffraction and gas phase molecular modeling calculations. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S31.00006: Intermolecular bonding in conjugated polymers Jeremy D. Schmit, Alex J. Levine Soluble conjugated polymers may enable the development of fast sensitive biosensors. However, the tendency of these molecules to aggregate even at low concentrations has a profound effect on the fluorescence signal that these sensors rely on. We propose that the aggregation of doped conjugated polymers occurs due to the formation of weak interpolymeric bonds resulting from intermolecular electron tunneling at crossing points of the chains. Although these bonds are essentially covalent in character, they are significantly weaker ($\sim 2 k_B T$) due to poor the intermolecular overlap of the electron wavefunctions as well as the delocalization of the pi-electrons along the polymer backbone. We show that the aggregates resulting from these bonds form either loosely bound braids or tight bundles of parallel chains depending on the strength of the electrostatic repulsion between the polymers. Surprisingly, we find that undoped polymers are unable to form parallel bundles. We also explore the interaction of SSH solitons on the chains with these intermolecular binding sites and demonstrate a roughly a four-fold enhancement of the binding strength when each chain has a soliton at the binding site. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S31.00007: Resonant Soft X-Ray Emission (SXE) and Resonant Inelastic X-Ray Scattering (RIXS) study of the Electronic Structure of Thin Film Vanadium Oxide Phthalocyanine (VO-Pc). Kevin E. Smith, Yufeng Zhang, Lukasz Plucinski, Shancai Wang, Sarah Bernardis, Timothy Learmonth, James Downes We report a synchrotron radiation-excited resonant soft x-ray emission (SXE) spectroscopy study of the electronic structure of thin films of the organic semiconductor vanadium oxide phthalocyanine (VO-Pc). SXE measures directly the element-resolved partial density of states (PDOS) in materials. The VO-Pc films were grown \textit{in-situ} on Si(100) substrates at beamline X1B at the National Synchrotron Light Source. We present measurement of the V, O, N, and C PDOS in VO-Pc, as well as the observation of dipole forbidden V $3d$ -- V $3d$ excitations and O $2p$ - V $3d$ charge transfer excitations. The relationship of these excitations to those observed in bulk vanadium oxide crystals will be explored, and our data will be compared to our earlier study of Cu-Pc ( J.E. Downes, C. McGuinness, P.-A. Glans, T. Learmonth, D. Fu, P. Sheridan, and K.E. Smith, Chem. Phys. Lett. 390, 203 (2004)) [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S31.00008: Two-dimensional dispersion of image electrons on C$_{60}$ thin films on Au(111) and Cu(111) Daniel Quinn, Gregory Dutton, Chad Lindstrom, Xiaoyang Zhu Two-photon photoemission (2PPE) has been used for many years to investigate occupied and unoccupied electronic states in clean and adsorbate-covered metal substrates. In this report, femtosecond 2PPE is employed to investigate charge transfer across a metal/organic-semiconductor interface and electronic structure in the thin film overlayer. Monolayer films of C$_ {60}$ have been grown using organic molecular beam deposition in ultrahigh vacuum on Au(111) and Cu(111) substrates. Such films represent a model system consisting of a metal/organic- semiconductor interface. Due to slightly different interatomic spacing in the two substrates, the epitaxial C$_{60}$ films grow as C$_{60}(4\times 4)$/Cu(111) or C$_{60}(2\sqrt{3}\times 2 \sqrt{3})$R30$^\circ$/Au(111). These distinct overlayers have previously been established by low energy electron diffraction and scanning tunneling microscopy experiments. By studying angle-resolved 2PPE, dispersion of image electrons in the conduction band along the surface plane can be measured directly. The fact that the C$_{60}$ overlayer is rotated by 30 $^\circ$ in the Au(111) case with respect to the Cu(111) case leads to distinct dispersion characteristics which correspond to different cuts in the two-dimensional band structure of the C$_{60}$ thin film. Application of an \emph{s}-band tight binding model leads to a reasonable quantitative fit. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S31.00009: DMRG study of pi-conjugated polymers with additional pi-conjugation in the transverse direction Yongguo Yan, Sumit Mazumdar The excited state ordering in trans-polyacetylene is not conducive to light emission: the 2A$_g$ occurs below theoptical 1B$_u$ in this system, and the excited 1B$_u$ rapidly decays to the 2A$_g$, radiative transition from which to the ground state is forbidden. It has been suggested that systems which have the same backbone pi-conjugation as linear polyenes, but which also have transverse pi-conjugation with finite extent, will simultaneously have small optical gap and excited state ordering that allows light emission. To verify this prediction, we have carried out DMRG calculations within a Hubbard Hamiltonian for a hypothetical simplified polymeric structure. The critical Hubbard U at which the 2Ag occurs below the 1Bu is larger in the subtituted polymer than in the unsubstituted linear polyene of the same length. Our results introduce the possibility of synthesizing conjugated polymers that will emit in the IR. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S31.00010: Optical Coherent Control of Lattice Deformations in Organic Semiconductors M. V. Katkov, C. Piermarocchi We investigate theoretically a semiconducting polymer chain under the effect of an intense off-resonant laser field. The coherent polarization induced by the field couples to the lattice and causes local deformations. Due to the off-resonant nature of the excitation, the deformations are reversible and controllable by the intensity and frequency of the laser. We derive and solve numerically a nonlinear equation describing the distribution of the optical polarization in the chain. Localized solutions exhibit characteristic saturation features. We analyze the light-induced force acting on the lattice in the case of polydiacetylene. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S31.00011: Density functional theory and Molecular Dynamics Studies on Energetics and Kinetics for Electro-Active Polymers: PVDF and P(VDF-TrFE) Haibin Su, A. Strachan, William Goddard III We use first principles methods to study static and dynamical mechanical properties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (TrFE). We find that the energy barrier necessary to nucleate a kink (gauche pairs separated by trans bonds) in an all-T crystal is much lower (14.9 kcal/mol) in P(VDF-TrFE) copolymer than in PVDF (24.8 kcal/mol). This correlates with the observation that the polar phase of the copolymer exhibits a solid-solid a transition to a non-polar phase under heating while PVDF directly melts. We also studied the mobility of an interface between a polar and non-polar phases under uniaxial stress; we find a lower threshold stress and a higher mobility in the copolymer as compared with PVDF. Finally, considering plastic deformation under applied shear, we find that the chains for P(VDF-TrFE) have a very low resistance to sliding, particularly along the chain direction. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S31.00012: Force Field Parameterization and Property Calculation of Aminofluorene-Based Chromophores David Rigby, Rajiv Berry In recent years, a variety of aminofluorene-based chromophores have been synthesized and studied experimentally for potential use in applications ranging from high density storage and imaging to detection of chemical and biological agents. In view of the large number of compounds of this type, it is desirable to be able to perform property prediction on interesting molecules as efficiently and accurately as possible -- a need which may be realized by use of molecular modeling techniques in combination with increasingly accurate force fields. Currently the most significant obstacle to accurate property prediction is the lack of accurate force field parameters for the chromophore functional groups of interest. Accordingly, we have performed ab-initio based valence parameter development combined with a classical approach to nonbond parameter refinement (as employed for the COMPASS force field) for relevant model compounds. The resulting force field has then been applied to investigate prediction of properties, including glass transition temperatures, for several molecules of interest. [Preview Abstract] |
Session S32: Electronic Structure
Sponsoring Units: DCOMPChair: Estela Blaisten-Barojas, GMU
Room: LACC 507
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S32.00001: Nearsightedness of Electronic Matter Emil Prodan, Walter Kohn The concept of ``nearsightedness of electronic matter" (NEM) articulates and quantifies the common, qualitative consensus among chemists and also among many physicists, that static, local electronic properties like the density or energy density at a given point r, of a many-atomic covalent or metallic system depend significantly on the external potential v(r$'$) only for values of r$'$ near r. NEM provides the physical basis for linear scaling [O(N)] electronic structure calculations. We say that a system of electrons is nearsighted at r, if distant potential perturbations w(r$'$) [w(r$'$) vanishing for $\vert $r-r$'\vert <$D] produce insignificant density changes at r. For non-interacting electrons in 1, 2 and 3 dimensions, we have obtained explicit complete asymptotic (D large) functional behaviors and the pre-factors of the density changes generated by distant perturbing potentials. For periodic or non-periodic metals and insulators, these results show that the density changes at r cannot exceed a maximum value regardless of the shape or magnitude of w(r$'$). This allows us to introduce a new concept, the nearsightedness range R, defined as the minimum distance for which the density changes at r, due to any potential w(r$'$) vanishing for $\vert $r-r$'\vert <$R, are less than a desired accuracy. We present here the dependence of the nearsightedness range on the desired accuracy, for gapped and un-gapped electronic systems. We also discuss the implications for O(N) electronic structure calculations. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S32.00002: Static Electricity in Sublimated CO2 Gas -- One in a Class of Phenomena Chung Liao Feng The charge state of CO$_{2}$ gas, sublimated from neutral solid, is observed to be positive. This phenomenon will be demonstrated. This discovery is made by testing a hypothesis which expects a group of molecules to become more positive when moving faster. The same hypothesis has also led to other discoveries of a special class of phenomena which include static electric charges being produced by thermal changes during vaporization, condensation and heating of water, as well as charges being produced by mechanical motion of metal discs. These phenomena were reported at the APS meetings of MAR02 (W30 4), MAR03 (J1 206) and MAR04 (Y38 12 - demonstration using toy gyroscopes). All of these phenomena appear to contradict the idea that the fundamental (or elemental) charge is constant. Each of these phenomena shows readings of positive voltage increasing without a decreasing electron count. Calculations show that the deviation from a static value of the fundamental (or elemental) charge may be the source of magnetism. This appears to answer the question why charges can appear to be constant. One of these calculations will be presented briefly. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S32.00003: First-Principle Perturbative Computation of Phonon Properties of Insulators in Finite Electric Fields Xinjie Wang, David Vanderbilt The methods of density-functional perturbation theory have been shown to provide a powerful tool for realistic calculations of lattice-vibrational, dielectric, elastic, and other response properties of crystals.\footnote{S. Baroni {\it et al.}, Rev. Mod. Phys. {\bf 73}, 515 (2001).} Recently, a total-energy method for insulators in nonzero electric fields was proposed.\footnote{I. Souza, J. \'I\~niguez, and D. Vanderbilt, Phys. Rev. Lett. {\bf 89}, 117602 (2002).} However, the perturbative computation of phonon properties under a dc bias field has not previously been addressed. Here, we start from a variational total-energy functional with a field coupling term that represents the effect of the electric field on the crystal. The linear response of the field-polarized Bloch functions is obtained by minimizing the second-order derivative of the total-energy functional. Due to the presence of the electric field, the field-polarized Bloch functions at each k-point in the Brillouin zone are weakly coupled to those at the neighboring k-points. We implement the method in the {\tt ABINIT} code and perform illustrative calculations of the phonon frequencies for III-V semicondutors. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S32.00004: Relativistic correction to the single-particle kinetic energy term Mark Pederson, Tunna Baruah Evaluation of the relativistic kinetic energy, given by $ \sqrt{(p^2c^2+m^2c^4)}$, is one strategy that may be promising from the standpoint of approximate scalar relativistic treatments. However the square root makes it cumbersome in quantum mechanical operator form. Chandra et al. [P. Chandra et al., Chem. Phys.{\bf 84}, 1 (1984)] have shown that it is possible to derive a simplified method for evaluating the relativistic kinetic energy within a single-particle framework. In this method a finite basis set of the $p^2$ operator is used to evaluate the expectation values of the $p^2$ dependent operators. In our formulation, we use a complete basis set of the $p^2$ operator to determine a variational expression for the above operator that is useful for Gaussian-orbital-based calculations. We compare the results of the corrections to the kinetic energy obtained by our formulation with other methods such as the expansion of the kinetic energy operator method and an incomplete $p^2$ basis method. Comparison to the full Dirac equation is made for simple atoms and the importance of the Darwin term will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S32.00005: Calculation of Raman and infrared spectra by coherent-phonon stimulation Jonathan Yates, Ivo Souza We propose a novel method for the efficient first principles prediction of Infra-Red and non-resonant Raman spectra. The method is inspired by the experimental technique of impulsive- stimulated Raman scattering. We apply initial impulsive forces to the ions in the system. For IR spectroscopy these forces correspond to the first order forces induced by a static electric field; for Raman spectroscopy they are the second order forces. We show how the corresponding vibrational spectrum can be obtained from the ensuing short-time dynamics of the system. The method has better scaling with system size than existing techniques. We present applications of the method to various clusters and molecules. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S32.00006: Exponentially localized quasi-free-particle generalized Wannier functions Bradley A. Foreman A new class of localized basis functions is proposed as a generalization of the Wannier functions for a free particle. The basis is orthonormal and its Fourier transform is given in explicit analytical form. For large values of the coordinate ($x \rightarrow \infty$), the wave functions are localized as $\exp (-C x^{\gamma})$, where $C > 0$ and $\frac12 \le \gamma < 1$ are fixed constants (with the same value for each state in a given basis). In contrast, ordinary free-particle Wannier functions are localized only as $1/x$, while the Wannier functions for a crystal behave as $\exp (-C_n x)$, where $C_n$ vanishes as the band index $n \rightarrow \infty$. Potential applications of the theory are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S32.00007: Ab initio electronic structure calculations of metals by the finite element method John E. Pask, Philip A. Sterne The finite-element (FE) method is a general approach for the solution of partial differential equations. Like the planewave (PW) method, the FE method is a systematically improvable expansion approach. Unlike the PW method, however, its basis functions are strictly local in real space, which allows for variable resolution in real space and facilitates massively parallel implementation. We discuss the application of the FE method to \textit{ab initio} electronic-structure calculations of metals. In particular, we discuss the use of nonlocal pseudopotentials in crystalline calculations, the handling of long-range interactions in the construction of the Kohn-Sham effective potential and total energy, and the synthesis of the metallic charge density. We show that the total energy converges variationally and at the optimal theoretical rate consistent with the cubic completeness of the basis. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S32.00008: Electronic structure of Cu$_2$O within GW approximation Fabien Bruneval, Nathalie Vast, Lucia Reining It is known that density functional theory fails to predict a gap in various insulating oxides like CuO. Cu$_2$O is a good starting point to address the fundamental issue of {\it 3d} electrons of metals in oxides. This semiconductor material has indeed a cubic structure, a closed {\it d} shell, and is non-magnetic. We calculated its band structure within Density Functional Theory and GW approximation [1]. We studied the role of semicore states (3{\it s}$^2$3{\it p}$^6$) and stated that their influence is slight on the Kohn-Sham band structure, but drastic on the GW one. Even a GW calculation including semicore states largely fails with the quasiparticle gap. Further approximations are usually used to perform a ``standard" GW calculation. We extensively discuss the reliability of these technical approximations, and find that they perform well. The most dubious approximation is the equality between GW and LDA wavefunctions. Therefore, we obtained self-consistent quasiparticle wavefunctions within the static COHSEX approximation to the GW scheme. We show that subtle changes in the wavefunctions may have large effects on the different contributions to the band gap. [1] L. Hedin, Phys. Rev. {\bf 139}, A796 (1965). [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S32.00009: Compact representation of the Green function of an infinite periodic system Jonathan E. Moussa, Marvin L. Cohen The single particle Green function of a periodic system is typically constructed by first calculating the band structure of the system and then summing up wavefunctions and energy denominators in the usual way. To construct an accurate Green function, many unoccupied bands must be included and k-points have to be sampled carefully, making use of both symmetries and approximate integration techniques. An alternate form of the Green function is presented, not based on band structures but rather local coordinate transformations. This method makes no use of Bloch's Theorem and instead exploits periodicity using renormalization-like scaling ideas. Calculations are performed with a localized basis set and the cost is demonstrated to be proportional to the log of the number of included unit cells and linearly scaling with unit cell size. \newline \newline This work was supported by National Science Foundation Grant No. DMR04-39768 and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy under Contract No. DE- AC03-76SF00098. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S32.00010: Combining quasiparticle energy calculations with exact-exchange density-functional theory Patrick Rinke, Christoph Freysoldt, Matthias Scheffler, Abdallah Qteish, J\"org Neugebauer We present a systematic {\it ab initio} study of the electronic structure for selected II-VI compounds and group III nitrides in the zinc-blende structure with special emphasis on analysing the role played by the semicore $d$-electrons. We show that applying density-functional theory (DFT) in the exact-exchange (EXX) approach [1] leads to an improved description of the $d$-electron hybridisation compared to the local-density approximation (LDA). Moreover we find that it is essential to use the newly developed EXX pseudopotentials [2] in order to treat core-valence exchange consistenly. In combination with quasiparticle energy calculations in the $GW$ approximation we achieve very good agreement with available photoemission data. Since the DFT energies and wavefunctions serve as input for the $GW$ calculation we conclude that for these materials EXX constitutes the better starting groundstate.\\ $[$1$]$ M. St\"adele {\it et al}, Phys.\ Rev.\ Lett. {\bf 79} 2089 (1997) \\ $[$2$]$ M. Moukara {\it et al}, J.\ Phys.: Condens.\ Matter {\bf 12} 6783 (2000) [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S32.00011: QuasiParticle Self-Consistent, $GW$ Theory Takao Kotani, Mark van Schilfgaarde, Sergey Faleev A formal justification for a new kind self-consistent {\em GW} approximation is developed. In this Landau-Silin picture the $GW$ approximation is based on the ansatz of the existence of bare quasiparticles generated from a noninteracting Hamiltonian $H_0$ and corresponding Green's function $G_0$. In this picture, electrons and holes should have real meaning; $W$ is computed from the time-dependent Hartree approximation; $\Sigma=iG_0W$ means ``exchange effect'' + electrons and holes interacting. A key issue is how to construct the optimum $H_0$. The true Green's function $G$ should have corresponding one-particle excitations, and $H_0$ should approximate the corresponding energies and eigenfunctions as well as possible. We present a prescription for $H_0$ that approximately minimizes the difference between $G^{-1}$ and $G_0^{-1}$. The theory is applied to $sp$ bonded materials, simple and transition metals, transition-metal oxides, some magnetic compounds such as MnAs and some $f$ systems (e.g. CeO$_2$, and Gd). We compare to a variety of experimental data for these different materials classes. The errors are quite small and highly systematic in $sp$ systems, they are somewhat larger but still systematic in transition-metal oxides, and are largest for Gd. Some analysis of the origin of the errors will be presented. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S32.00012: Band structure of wurtzite quantum dots with cylindrical symmetry Lok Lew Yan Voon, Calin Galeriu, Benny Lassen, Morten Willatzen, Roderick Melnik A six-band ${\bf k \cdot p}$ theory for wurtzite semiconductor nanostructures with cylindrical symmetry will be presented. Our work extends the formulation of Vahala and Sercel [{\it Phys. Rev. Lett.} {\bf 65}, 239 (1990)] to the Rashba-Sheka-Pikus Hamiltonian for wurtzite semiconductors, without the need for the axial approximation. Results comparing this new formulation for studying the electronic structure of wurzite GaN and CdS cylindrical quantum dots with the conventional formulation will be shown; our formulation is computationally superior. An application to the search for level crossing in the valence band of cylindrical quantum rods as a function of aspect ratio will be given. \\ \\Supported by NSF CAREER award. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S32.00013: Electronic Properties of Small Virtual Clusters of Ga and In with As and P atoms Liudmila Pozhar, Alan Yeates, Frank Szmulowicz, William Mitchel The electronic energy level structure (ELS) of several virtually (i.e., fundamental theory-based, computationally) pre-designed stable clusters of Ga, In, As and P atoms is investigated by means of the Hartree-Fock method and compared to that of the corresponding virtual clusters grown in vacuum. The results obtained in the course of this study confirm that the ELS and the direct optical transition energy (OTE) of the clusters are sensitive to manipulations with the covalent radii and cluster composition. Thus, a small displacement (in the range of several hundredths of Angstrom) of atoms in stable pre-designed clusters from their respective positions in vacuum clusters leads to a significant decrease in the OTE and formation of the valence and conduction bands. In agreement with previous results, the OTE for such clusters is in the range of several electron Volts and can be manipulated up to 100{\%} by manipulations with cluster parameters. [Preview Abstract] |
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S32.00014: Superbox method for order-N electronic structure calculations for very large systems Kalman Varga, S.T. Pantelides We have developed a superbox method for ab initio electronic structure calculations for very large systems. The method is applicable to density-fucntional or Hartree-Fock-based calculations. In this approach a molecule, a cluster, or a large supercell of a crystal is divided into a number of boxes. Self-consistent calculations are then carried out in parallel for each box, treating it either as a supercell or as a free unit. The wave function or the Green's function of the total system is then constructed by connecting the solutions obtained in the individual boxes by a rigorous and formally exact procedure that yields a self- consistent solution for the entire system. The method has been implemented by using the recently introduced Lagrange-function basis [1]. Examples will be presented using density functional theory to demonstrate the linear scaling of the approach with respect to the number of atoms in the system. [1] K. Varga, Z. Zhang, and S. T. Pantelides, Phys. Rev. Lett. 93, 176403 (2004). [Preview Abstract] |
Session S33: Focus Session: Pathway to Practical Quantum Computing
Sponsoring Units: GQI DAMOP DCOMPChair: Daniel Greenberger, City College of New York
Room: LACC 511C
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S33.00001: Measurement and control in quantum information science Invited Speaker: Quantum information science has a broad interface with control theory. In the region of overlap between these two thriving fields, one finds compelling problems ranging from robust and time-optimal control of quantum dynamics to the analysis and design of concatenated coding schemes. In this talk I will begin with a brief overview of recent work on applications of control theory in quantum information science, and then provide a more detailed review of my own group's research on quantum feedback control, quantum state preparation and quantum metrology. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S33.00002: Exploiting the Quantum Zeno Effect to Beat Photon Loss in Linear Optical Quantum Information Processors Jonathan Dowling We devise a new technique to enhance transmission of quantum information through linear optical quantum information processors. The idea is based on applying the Quantum Zeno effect to the process of photon absorption. By frequently monitoring the presence of the photon through a QND (quantum non-demolition) measurement the absorption is suppressed. Quantum information is encoded in the polarization degrees of freedom and is therefore not affected by the measurement. Some implementations of the QND measurement are proposed. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S33.00003: Control of Anisotropic Spin Exchange in Quantum Dots Dimitrije Stepanenko, Layla Hormozi, Nicholas Bonesteel, Kerwin Foster To first order in spin-orbit coupling, the exchange interaction between spins in coupled quantum dots has the form $J ({\bf S}_1 \cdot {\bf S}_2 + \vec\beta\cdot ({\bf S}_1 \times {\bf S}_2))$. Recently we have shown that the ability to control the Dzyaloshinski-Moriya vector ${\vec\beta}$ is a potentially useful resource for quantum computation.\footnote{D.Stepanenko, N.E.Bonesteel, PRL {\bf 93}, 140501 (2004).} Here we study microscopically the degree of this control for coupled quantum dots in III-V semiconductors. At the level of the Hund-Mulliken (HM) approximation, in which one orbital is kept per dot, spin-orbit coupling enters as a small spin precession during interdot tunneling. $\vec \beta$ is proportional to this precession angle, and its dependence on dot parameters (e.g., interdot distance and dot size) can be strongly enhanced by ferromagnetic direct exchange. We determine the range of effective $\vec \beta$ values in quantum gates produced by pulsing the exchange interaction through numerical integration of the Schr\"odinger equation. Anisotropy in any particular gate is determined by the pulse duration, which is limited by decoherence for slow pulses and adiabaticity for fast pulses. The effects of going beyond the HM approximation, keeping more than one orbital per dot, are also discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S33.00004: Conductance peak splitting in coupled Si/SiGe quantum dots L.J. Klein, Srijit Goswami, K.A. Slinker, S.N. Coppersmith, M.A. Eriksson, J.O. Chu, P.M. Mooney The fabrication and electrical characterization of double quantum dots in modulation doped Si/SiGe heterostructure are presented. Trench line defined quantum dots and in plane gates are fabricated by electron beam lithography and reactive ion etching. Low temperature transport measurements (0.2 K) show split Coulomb blockade peaks over a large range of voltages on the side gates. The split conductance peaks indicates a tunnel coupling between the pair of quantum dots and this coupling can be tuned by varying the voltages applied to the gates. The stability plot diagram of the double dot reveals similar conductance diamonds for the individual dots with well resolved excited states. The possible applications of tunnel coupled quantum dots for quantum information processing are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S33.00005: Thresholds for reliable quantum computation Invited Speaker: |
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S33.00006: Entanglement and quantum computational speed-up Guifre Vidal I will discus recent results on the role of entanglement in quantum computation [G. Vidal, Phys. Rev. Lett. 91, 147902 (2003)] and in the efficient numerical simulation of quantum many-body dynamics [G. Vidal, Phys. Rev. Lett. 93, 040502 (2004)]. A pure-state quantum computation can be efficiently simulated with a classical computer provided that only a restricted amount of entanglement is involved. More generally, an upper bound on the computational speed-up offered by a quantum computation can be given in terms of its amount of entanglement. These results follow from an explicit simulation algorithm that can also be applied to efficiently simulate quantum dynamics in one spatial dimension, including spin chain dynamics. [Preview Abstract] |
Session S35: Molecular Electronics II
Sponsoring Units: DCP FIAP DMPChair: Edwin Chandross, Bell Labs
Room: LACC 511B
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S35.00001: Phonon effects in molecular conduction junctions Invited Speaker: This talk will give an overview of our work on effects of electron-phonon coupling on molecular conduction, including dephasing, dissipation and heating, then describe some recent observations, interpretations and predictions on three phenomena involving phonons in molecular junctions: (a) Heat conduction and its rectification by molecular wires$^{1,2}$; (b) inelastic electron tunneling spectroscopy$^{3-5}$ and (c) phonon-induced multi-stability, hysteresis and negative differential resistance in molecular conduction.$^{6}$ $^{1}$ D. Segal, A. Nitzan and P. H\"{a}nggi, J. Chem. Phys.\textbf{ 119, }6840-6855 (2003) $^{2}$ D. Segal and A. Nitzan, cond-mat/0405472 $^{3 }$M. Galperin, M. Ratner and A. Nitzan, J. Chem. Phys. \textbf{121}, 11965-11979 (2004) $^{4}$ M. Galperin, M. Ratner and A. Nitzan, Nano Lett., \textbf{4}, 1605-1611 (2004) $^{5}$ M. Galperin, A. Nitzan, M. A. Ratner and D. R. Stewart, to be published http://atto.tau.ac.il/$\sim $nitzan/253.pdf $^{6}$ M. Galperin, M. A.Ratner and A. Nitzan, Nano Letters, in press http://atto.tau.ac.il/$\sim $nitzan/254.pdf [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S35.00002: Charge injection and transport in a single organic monolayer island Invited Speaker: We report how electrons and holes, that are locally injected in a single organic monolayer island (where organic monolayers are made from sublimated oligomers (pentacene and other oligoacenes), or made from chemisorption in solution (self-assembled monolayers) of pi-conjugated moieties), stay localized or are able to delocalize over the island as a function of the molecular conformation (order vs. disorder) of this island. Charge carriers were locally injected by the apex of an atomic force microscope tip, and the resulting two-dimensional distribution and concentration of injected charges were measured by electrical force microscopy (EFM) experiments. We show that in crystalline monolayer islands, both electrons and holes can be equally injected, at a similar charge concentration for symmetric injection bias conditions, and that both charge carriers are delocalized over the whole island. On the contrary, charges injected into a more disordered monolayer stay localized at their injection point. These different results are discussed in relation with the electrical performances of molecular devices made from these monolayers (OFET, SAMFET). These results provide insight into the electronic properties, at the nanometer scale, of these molecular devices. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S35.00003: Electrical and Spectroscopic Characterization of Metal-Molecule-Metal Junctions Invited Speaker: Considerable attention has been devoted to developing an understanding of the mechanisms that dominate electrical transport in metal- molecule-metal junctions comprised of single and small ensembles of molecules. In this talk, we will present an overview of recent research on the electrical and spectroscopic characterization of molecular junctions inserted along the length of sub-40-nm diameter Au and Pd metal nanowires (i.e., in-wire junctions) fabricated by template-directed synthesis. In particular, we will show results that investigate the relationship between the temperature dependent (10 -- 300 K) current-voltage (I-V) characteristics and the vibrational spectra measured by inelastic electron tunneling (IET) spectroscopy for candidate molecular wires and bistable switching molecules. The two types of molecular wire junctions that were studied incorporate a self assembled monolayer of dithiolated oligo(phenylene- ethynylene) (OPE) molecules or their -NO$_{2}$ derivatives. The I-V of these junctions are stable and reproducible between +/-1V. Temperature independent I-V are measured for both types of junctions, which is indicative of coherent tunneling transport. Moreover, strong vibrations associated with $\upsilon $(18b) and $\upsilon $(19a) ring modes were observed in both junctions. In contrast, measurements of molecular junctions that incorporate SAMs based on aniline derivatives show reproducible bistable switching with an on-off ratio of $>$10:1 at 1V. Differences are observed in the vibrational spectra that depend on the state of the junction. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S35.00004: Critical Roles of Metal-Molecule Contacts in Electron Transport Through Molecular-wire Junctions Anton Grigoriev, Jonas Skoldberg, Goran Wendin, Zeljko Crljen We use non-equilibrium Green's function DFT methods (TranSIESTA) to study the bonding-site dependence of the transmission through metal-molecule contacts in molecular junctions of type M-S-mol-S-M for a number of different molecular systems, mainly short molecules with DTB as a reference system, and also OPVn, n=3-5. For all systems on Au(111) surfaces the transmission is quite insensitive to the bonding site. However, if S is adsorbed in an Au vacancy, or on- top of a small (3-Au-atom) island, the transmission can drop very substantially due to mismatch and changes of the HOMO structure in the contacts. However, we do not find any examples of several orders-of-magnitude reductions of the conductivity. In several systems with low zero-bias transmission at the Fermi level, we found that buried Au-S contacts (S adsorbed in Au vacancy) are associated with very sharp LUMO levels just above the Fermi level. Such a system will show extremely strong non-linear effects and might work as uni- or bi-directional voltage-controlled 2-terminal switches. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S35.00005: A Physical Interpretation of an Orhtogonal Hiolbert-Space Transformation: Transmision Antiresonances from Long-Range Hopping Seth Rittenhouse, Brad Johnson We provide the physical interpretation for a recently- introduced Hilbert space transformation from a nonorthogonal (overlapping) basis to an orthogonal basis, for the purpose of studying transport through single-molecule systems. The new Hilbert space may be interpreted as an orthogonal basis in the same {\it physical} space, wherein the basis overlap is formally transferred to the hopping matrix elements in the orthogonal system, resulting in a standard tight-binding system in an orthogonal basis with long-range hopping. We utilize the formal procedure to solve for the transmission characteristics of an impurity site (molecule) coupled with semi-infinite leads. We demonstrate that (previously predicted) transmission antiresonances are produced, in the orthogonal space, by the presence of second-nearest neighbor hopping. The parameter range in which transmission antiresonances are possible is formally outlined--a feature of the orthogonal space transformation. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S35.00006: How does thermal motion of atoms influence rates of bridge-mediated electron transfer reactions? 1. Dynamical modulation of the effective tunneling coupling. Ilya Balabin, Spyros Skourtis, Tsutomu Kawatsu, David Beratan Understanding how thermal nuclear motion affects the electron transfer (ET) reaction rates is essential for describing a broad range of vital biological redox reactions as well as designing molecular electronic devices. Theoretical studies of biological ET reaction rates usually assume a) the superexchange ET regime (virtual bridge electronic states), and b) the Franck-Condon approximation (electronic dephasing slower than the time-dependent Franck-Condon factor decay time). We present the first investigation of the electronic dephasing effects and the first quantitative analysis of the modulation of effective tunneling coupling by nuclear dynamics. Molecular dynamics simulations coupled with extended Huckel-level quantum chemical calculations of the effective electronic coupling were performed for the blue copper ET protein azurin. We discuss effects of the donor-to-acceptor distance, tunneling pathway structure, tunneling energy, temperature, and protein motion on the dynamics of the effective tunneling coupling and the ET reaction rate. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S35.00007: Direct Observation of a Molecular Junction using High-Energy X-ray Reflectometry Julian Baumert, Michael Lefenfeld, Eli Sloutskin, Moshe Deutsch, Colin Nuckolls, Ben Ocko Very little is known about the structure of organic molecular thin films at their rest potential. Further, it is not known whether the structure of these films is modified by an applied potential. We present a new x-ray scattering technique, which allows high-resolution structural studies of buried self-assembled monolayers (SAMs) that are sandwiched between silicon and mercury junctions. The high-energy x-ray beams, utilized in the present studies (32 keV), penetrate through the conducting silicon electrode. The x-ray reflectivity interference pattern thus provides information on the thickness and orientation of the molecules in the electronic junction. Our results, for alkane-thiol and alkane-silane layers, show that the SAMs form homogenous densely packed monolayers within the deeply buried interface. The thickness of these layers is compared with the SAMs prepared at the vapor/vacuum interfaces on mercury and silicon. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S35.00008: Programmable logic in the nanocell Jonas Skoldberg, Goran Wendin The investigated nanocell is a two-dimensional network of self- assembled metallic particles connected by molecules that show reprogrammable (i.e. can be switched between high and low conductance states) negative differential resistance (NDR). The nanocell is surrounded by four lithographically defined I/O leads at the edges of the nanocell. By selectively turning molecules on and off, the nanocell can be programmed to implement a set of logic gates. So far, no methods for programming the nanocell by only applying voltages to the access leads have been demonstrated (1). Using the nanocell circuit model described in (1), we will show that there is set of simple target circuit diagrams that implement the logic gates required for the architecture also described in (1). We will then show that the target circuit diagram is simple enough to, under certain assumptions, allow the nanocell to be programmed by applying voltages to the I/O pins. 1. C Husband, S Husband, et al. (2003). ``Logic and memory with nanocell circuits." IEEE Transactions on Electron Devices 50(9): 1865- 1875. [Preview Abstract] |
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S35.00009: Non-volatile multilevel Memory Based on Nanowire / Molecule Heterostructures Chao Li, Bo Lei, Wendy Fan, Chongwu Zhou A multilevel molecular memory device was constructed by coating In$_{2}$O$_{3}$ nanowire FETs with a self-assembled monolayer of Fe$^{2+}$-terpyridine compound. This bottom-up process takes advantage of both the nanowire and the redox-active molecules, as discrete multilevels naturally exist in an ensemble of redox-active molecules, while precise charge sensing can be carried out with a semiconducting nanowire transistor. In the demonstration, charges were precisely placed at up to eight discrete levels by altering the population of reduced / oxidized molecules. Gate voltage pulses and current sensing were used for writing and reading operations, respectively. More importantly, these devices exhibited ultralong retention up to 600 hours and great reliability. This approach solved the long-standing reliability issue by moving molecules outside the conduction path, and multilevel memory represents a conceptual breakthrough for molecular devices. [Preview Abstract] |
Session S36: Trapped Atoms in Optical Lattices
Sponsoring Units: DAMOPChair: James Williams, NIST-Gaithersburg
Room: LACC 510
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S36.00001: Bragg Spectroscopy of Excitations of a Quantum Bose Gas in a Lattice Xu Du, Emek Yesilada, Changhyun Ryu, Shoupu Wan, Daniel Heinzen We have measured the excitation spectrum of a quantum degenerate Bose gas in an optical lattice with Bragg spectroscopy. We begin each cycle of the experiment by producing a magnetically trapped $^{87}$Rb Bose condensate. We then superimpose a three-dimensional optical lattice of cubic symmetry onto the condensate. We turn the lattice potential on adiabatically, so that the gas temperature remains very close to zero. This provides an experimental realization of the Bose-Hubbard model, which exhibits a quantum phase transition between a superfluid and an insulating state. We find that in the superfluid state, the resonant excitation energy in the phonon-like regime decreases with increasing lattice strength. In the insulating regime, we observe the appearance of a sharp increase in the excitation rate at non-zero frequencies, which we interpret as a measurement of the gap in the insulating state of the gas. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S36.00002: Calculation of collective modes of atoms in an optical lattice Sara Bergkvist, Emil Lundh, Patrik Henelius, Anders Rosengren The excitation spectrum of bosons in an optical lattice with a confining potential has been studied using quantum Monte Carlo. Moments of the strength function have been calculated from the ground state expectation values. From these moments information about the collective behavior of the atoms has been extracted. The response of the system to two different perturbations has been examined. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S36.00003: Single-Particle Excitations of a Bose Gas in an Optical Lattice Near the Mott Transition Satoru Konabe, Tetsuro Nikuni, Masaaki Nakamura We study single-particle excitations of a Bose gas in an optical lattice near the Mott transition. We derive the excitation spectra in both Mott insulator phase and superfluid phase. The characteristic feature in the Mott insulator phase is the existence of an energy gap between the particle and hole excitations. We show that this energy gap can be directly probed by an output coupling experiment. Applying the general expression for the output current derived by Luxat and Griffin to the Mott insulator phase, we find that the energy spectrum of the momentum-resolved output current exhibits two characteristic peaks corresponding to the particle and hole excitations. Thus, it can be used to detect the transition point from the Mott insulator to superfluid phase where the energy gap disappears. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S36.00004: Surface Waves at the Mott Insulator-Superfluid interface for confined BEC Eros Mariani, Ady Stern Bose Einstein Condensates in optical lattices under an external confinement are expected to form Superfluid and Mott-Insulating domains [1] reminiscent of the quantum phase transitions for the infinite system [2]. The experimentally observed transition [3] is presently ascribed to relative shrinking/widening of the different domains. The interfaces between the two phases and their excitations therefore play a crucial role in the time evolution of the system under an external perturbation as well as in its thermodynamic properties. In this work we derive the dispersion relation of the surface waves at the interfaces between Mott-Insulating and Superfluid phases. We then calculate their contribution to the heat capacity of the system and show how its low temperature scaling allows a direct experimental test of the existence and properties of Mott-Superfluid domains [4]. [1] D. Jaksch et al. , Phys. Rev. Lett. \textbf{81}, 3108 (1998) [2] M. P. A. Fisher et al. , Phys. Rev. B \textbf{40}, 546 (1989) [3] M. Greiner et al. , Nature \textbf{415}, 39 (2002) [4] E. Mariani and A. Stern, to be submitted to Phys. Rev. Lett. (2004) [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S36.00005: Inhomogeneous Mott phases of bosons in optical lattices Courtney Lannert, Brian DeMarco, Smitha Vishveshwara, Tzu-Chieh Wei The feasibility of creating a spatially-inhomogeneous phase of neutral bosonic atoms (such as Rubidium) in which multiple Mott-insulating states coexist in a shell structure is discussed and analyzed. In the set-up of trapped, tightly confined bosons in an optical lattice, we derive the conditions on experimental parameters, such as number of bosons and the curvature of the trapping potential, for creating an onion-like structure of any prescribed number of Mott shells with different occupation numbers. We discuss the stability of such a structure with an eye toward experimental conditions necessary for measuring the structure with spectroscopic probes. The analyses ought to give a controlled means of achieving the Mott groundstate observed in previous experiments, and give some constraints for how successfully these states can be employed for quantum computing. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S36.00006: Spectroscopic Probe of Inhomogeneous Mott Phases Smitha Vishveshwara, Brian DeMarco, Courtney Lannert, Tzu-Chieh Wei The feasibility of creating a spatially-inhomogeneous phase of neutral bosonic atoms (such as Rubidium) in which multiple Mott-insulating states coexist in a shell structure is discussed and analyzed. In the set-up of trapped, tightly confined bosons in an optical lattice, we derive the conditions on experimental parameters, such as number of bosons and the curvature of the trapping potential, for creating an onion-like structure of any prescribed number of Mott shells with different occupation numbers. We discuss the stability of such a structure with an eye toward experimental conditions necessary for measuring the structure with spectroscopic probes. The analyses ought to give a controlled means of achieving the Mott groundstate observed in previous experiments, and give some constraints for how successfully these states can be employed for quantum computing. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S36.00007: Atomic pair statistics and adiabatic realization of the Mott state in an optical lattice Guido Pupillo, Carl J. Williams, Nikolay V. Prokof'ev In a series of recent experiments, several groups demonstrated the experimental realization of a Mott insulator state, created by loading a trapped atomic Bose-Einstein condensate into an optical lattice. A superfluid-insulator transition is then induced by varying the intensity of lattice laser beams. When the average filling of lattice sites at the trap center is of order one, double occupancy of lattice sites in the Mott state is both consequence of zero-temperature mixing of high energy basis states into the ground state and of finite temperature population of high energy states. Finite temperature may be due to imperfect adiabaticity while increasing the intensity of the lattice laser beams. In this talk we discuss the distribution of atomic pairs in the trapped Mott insulator relevant to current experiments and suggest that statistics of detection of atomic pairs may be used to bound the temperature of the trapped atoms for energies well below the lattice level spacing. We discuss time scales for adiabatic realization of the Mott state in the trap. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S36.00008: Evidence for Phase Variance Oscillations in a Bose-Einstein Condensate and Applications to Precision Interferometry Ari Tuchman, Chad Orzel, Anatoli Polkovnikov, Mark Kasevich We report the dynamic restoration of phase coherence after generating an array of highly squeezed number states loaded from a Bose-Einstein Condensate (BEC) into an optical lattice. We induce oscillations in the phase variance of the array by rapidly reducing the intensity of the lattice. This sequence projects the number squeezed array onto a superfluid groundstate, inducing phase variance oscillations as the quantum state evolves. It is critical to recognize that the experimental signature, of oscillations in the interference contrast, can be nearly reproduced by driving semiclassical excitations. However, by comparing data both with the semiclassical GPE where a coherent state array with identical initial phases is time evolved and with a model which accounts for an initial state with large quantum fluctuations, we find evidence supporting the quantum mechanical nature of these oscillations. We further discuss applications of these phase variance oscillations to precision interferometry. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S36.00009: Role of quantum fluctuations in the dissipative dynamics of a 1D Bose gas in an optical lattice Ana Maria Rey, Julio Gea-Banacloche, Guido Pupillo, Carl J. Williams, Charles W. Clark We will present a theoretical treatment[1] of the surprisingly large damping observed recently in a experiment done at NIST [2] where the transport properties of a harmonically trapped 1D Bose gas in a periodic (optical lattice) potential were studied by observing small amplitude dipole oscillations. In the absence of the lattice these oscillations are expected to be undamped (generalized Kohn's theorem), however, large damping of the dipole mode was observed in the experiment for very weak optical lattices and very small cloud displacements. We will show that the observed damping can be derived from a model whose main ingredients are (a) a large noncondensate fraction that arises as a direct consequence of the enhanced effective on-site interaction due to the tight transverse confinement, (b) the fact that a non-negligible part of it occupies high-momentum states and is therefore affected by dynamical instabilities, and (c) the interaction of the condensate atoms with the random field created by these noncondensate atoms when their equilibrium state is perturbed. We find good agreement between the model and the experimental results. [1] Julio Gea-Banacloche \textit{et al. }cond-mat/0410677. [2] C. D. Fertig, K. \textit{et al.}cond-mat/0410491. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S36.00010: Effect of Quantum Fluctuations on the Dipolar Motion of Bose-Einstein Condensates in Optical Lattices Daw-Wei Wang, A. Polkovnikov We reexamine dipolar motion of condensate atoms in one-dimensional optical lattices and harmonic magnetic traps including quantum fluctuations within the truncated Wigner approximation. In the strong tunneling limit we reproduce the mean field results with a sharp dynamical transition at the critical displacement. When the tunneling is reduced, on the contrary, strong quantum fluctuations lead to finite damping of condensate oscillations even at infinitesimal displacement. We show that there is a smooth crossover between the chaotic classical transition at finite displacement and the superfluid-to-insulator phase transition at zero displacement. We further analyze the time dependence of the density fluctuations and of the coherence of the condensate and find several nontrivial dynamical effects, which can be observed in the present experimental conditions.Many of our prediction has been observed recently by C. D. Fertig et al, in cond-mat/0410491. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S36.00011: Superfluid Insulator transition of two species ultracold bosons in an optical lattice K. Sengupta, M-C Cha, A. Issacson, S.M. Girvin We analyze the superfluid-insulator (SI) transition for a two-species, ultracold bosonic atoms confined in an optical lattice for odd filling at commensurate densities. We find that in contrast to the even-filling case, the SI transition, for most experimentally accessible parameter ranges, occurs either a) with complete depopulation of one species or b) with simultaneous onset of superfluidity for both species or c) with superfluidity of one species and Mott insulator of another species. The analysis consists of a analytical mean-field study of the SI transition using a variational wave function and numerical treatments which includes canonical transformation method and a quantum Monte Carlo study. The numerical studies allow us to study the effect of quantum fluctuations on the SI transition and point out the domain of applicability of the mean-field theory. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S36.00012: $^{87}$Rb in a double well optical lattice Ian Spielman, Chad Fertig, Johnny Huckans, James Porto, William Phillips Bose condensed alkali gasses present an ideal venue for the study of weakly interacting, phase coherent quantum phenomena. The subsequent application of an optical lattice can controllably increase the importance of interactions. Such a strongly interacting, yet tunable, system is interesting of its own right; consider for example the recent direct observation of a Mott insulator-superfluid transition. More practically, however, these strong interactions coupled with long coherence times, and easily tuned parameters suggest this system as a potential quantum information processor. In this talk, we first introduce a scheme for 2 qbit gates between pairs of $^{87}$Rb atoms in a tunable double-well optical lattice. An external phase sets the barrier between wells and can be varied externally; thus, atoms in each well can be controllably interacted. Here we present preliminary experimental results demonstrating loading of the individual lattice sites, and comment on the effects of changing the barrier potential with the loaded atoms. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S36.00013: Quantum walks with ultracold atoms in optical lattices Stuart van der Lee, David Feder The behavior of several ultracold atoms (bosons or fermions) undergoing a quantum walk in a one-dimensional optical lattice is investigated numerically. Both discrete and continuous time quantum walks are implemented, the latter within the context of a tight-binding model. Because the quantum statistics place constraints on the overlap between different many-particle states, the Hamiltonian generates a one-particle quantum walk on a graph with vertices of higher degree. The results will be used to make predictions for experiments with ultracold atoms in optical lattices, as well as to explore fundamental issues related to quantum information, such as graph covering and the role of entanglement. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S36.00014: Dynamics of Bose condensates in an optical lattice with a basis Wen-Chin Wu, Chou-Chun Huang Dynamics of atomic Bose-Einstein condensates in an optical lattice with a basis is investigated. For a 1D optical lattice of two types of potential barrier within a unit cell, similar to the case of a crystal lattice with two-atom basis in a unit cell, acoustic as well as optical phonons can propagate along the lattice of atom clouds. These are in addition to in-phase and out-of-phase collective excitations of the condensates. The dispersions of phonons depend crucially on the relative size of two tunneling amplitudes ($J_1$ and $J_2$) across the two barriers and the ratio of $J_1,J_2$ to the repulsion U between the atoms. Using a variational method, the effect of condensate breathing modes on the phonons is studied in details. The dynamic structure factor of the system is also studied. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S36.00015: Band Effects in Optical Lattices Containing Cold Atoms Vito Scarola, Sankar Das Sarma The possibility of manipulating band structure to engineer novel, many-body ground states of cold bosons in optical lattices is discussed. Effective Hamiltonians are derived using the realistic band structure of one and two dimensional systems. We analyze the mean field phase diagram and stability of these models in experimentally relevant parameter regimes. [Preview Abstract] |
Session S37: Colloids III
Sponsoring Units: DFDChair: Noel Clark, University of Colorado
Room: LACC 512
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S37.00001: Melting of 2D Colloidal Crystals J.R. Savage, A.D. Dinsmore We study the kinetics of melting of colloidal crystals formed by a short-range attractive potential. We use aqueous suspensions of micron-sized latex spheres mixed with surfactant (SDS) micelles, which create a depletion attraction among the spheres. Single-layer crystals appear on the glass surface. Upon uniformly heating the sample to 60 deg., the micelles shrink. The depletion attraction weakens by a factor of approximately 2.25, and the crystals melt. Optical microscopy is used to track the motions of hundreds of colloidal spheres for up to 2 hours, until crystals have melted. We initially observe a steady decrease in the size of the crystallites, limited by diffusion. When the size reaches approximately 15, however, crystallites rapidly shrink. The kinetics of individual bond-breaking events and the evolution of the crystalline order parameter will be presented. This work is supported by the Research Corporation and by NSF-DMR 0305395. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S37.00002: Melting Mechanisms of 3D Colloidal Crystals A.M. Alsayed, M.F. Islam, J. Zhang, A.G. Yodh We study the melting mechanisms of 3D colloidal crystals using aqueous suspensions of thermally responsive NIPA microgel colloidal particles. Below 32 $^{o}$C, the particle radius decreases approximately linearly with increasing temperature. We use this effect to tune the volume fraction of nearly hard-sphere aqueous NIPA colloidal suspensions from 0.74 to 0.54. Using video tracking microscopy, we measured the Lindemann parameter of particles within the crystal as a function of temperature. Interestingly, we find that melting of the 3D colloidal crystals starts at grain boundaries and free surfaces, rather than isolated vacancies or dislocations. Very near the melting temperature, the Lindemann parameter for particles near the grain boundaries and free surfaces was $\sim $0.16; the parameter decreased approximately exponentially with distance into the bulk crystal. These works has been partially supported by NSF through MRSEC DMR-0203378 and DMR-079909 and by NASA grant NAG8- 2172. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S37.00003: Electrokinetic measurements of a model colloidal system in low polar solvents Andrew Hollingsworth, William Russel, Paul Chaikin, Mirjam Leunissen, Alfons van Blaaderen In a low polar environment, sterically stabilized poly(methyl methacrylate) spheres become positively charged and exhibit significant long-range repulsive interactions. Particles were fluorescently labeled for confocal microscopy and suspended in near index and density matching solvents. Calculations show that small differences in the dielectric constant, which ranged from 6 to 8 in our experiments, can dramatically affect electrolyte dissociation. We have computed ionic strengths from conductivity measurements using Fuoss' theory of ionic association, and, in turn, interpreted electrophoretic mobility and dielectric response using the standard electrokinetic model. We hypothesize that the dehydrohalogenation of cyclohexyl bromide (CHB) produces HBr, which weakly dissociates in these organic solvents. To establish the acidic nature of the media, we dissolved CHB in water and measured the conductivity and pH as a function of CHB concentration and time. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S37.00004: Alloys of Suspended Colloidal Donuts and Spheres T.G. Mason, C. Hernandez, C.K. Harrison, P.M. Chaikin We present a study of the structure of mixed dispersions, or ``alloys'', of colloidal donuts and spheres. By contrast to previous studies of colloidal alloys, this study allows for the investigation of how the possible penetration of one species through a hole in another can alter the structures. Three different regimes have been investigated: when the diameter of the donut's hole is much larger than, equal to, and significantly smaller than the sphere's diameter. In the case when the spheres are slightly smaller than the holes, interpenetrating particle configurations are possible, and the spheres can explore a larger positional phase space. In this case, we report the variation of these structures with respect to the two independent volume fractions of spheres and of donuts. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S37.00005: A Microrheological Study of the Time Dependent Gelation of Single Wall Carbon Nanotube Suspensions. L. A. Hough, M. F. Islam, A. G. Yodh Single wall carbon nanotubes (SWNTs) dispersed in water using an anionic surfactant, sodium dodecylbenzene sulfonate (NaDDBS) form reversible gels because of the bonding between the individual nanotubes (L.A. Hough, M.F. Islam, P.A. Janmey and A. G. Yodh Phys. Rev. Lett. \textbf{93}, 168102 (2004)). In this talk, we present a microrheology study of the time dependence of this reversible gelation. We embed fluorescent tracer particles in SWNT suspensions and use optical microscopy tracking techniques to measure the mean-squared displacement during gelation. We then apply a time-cure superposition to obtain a master curve for the viscoelasticity that extends over several decades in frequency. We compare high frequency dynamics of the SWNTs solutions to those expected for semiflexible and rigid rod polymer systems. This work has been partially supported by the NSF through Grants DMR 00-79909 (MRSEC) and DMR-0203378, and by NASA Grant NAG8-2172. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S37.00006: Creaming of Emulsion aggregations and gels Chanjoong Kim, David Weitz Even though creaming of emulsion has been considered as a simple phenomenon due to the hydrodynamics interaction and the density mismatch between dispersed phase and continuous phase, actual creaming behavior is not so simple once emulsion droplets interact with each other. Interaction between droplets was controlled by depletion force, which depends on the concentration of smaller micelles. Creaming behaviors can be categorized to three different groups; 1) The boundary moves up fast at a constant speed with low surfactant concentrations, 2) it moves slowly in the beginning but suddenly collapses up at intermediated concentrations, and 3) it moves up slowly without collapsing at high concentrations. These behaviors are interpreted in terms of poroelatic model. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S37.00007: Internal dynamics of a model concentrated emulsion Vinothan Manoharan, John C. Crocker Foams and concentrated emulsions---glassy systems in which the volume fraction $\phi$ of the bubbles or droplets exceeds the close-packed threshold---display unique relaxation behavior due to intermittent and inhomogeneous structural rearrangements. The rearrangements are caused by diffusion of fluid from small to large droplets, which leads to variations in the local stress. Unfortunately in most of these systems the large refractive index mismatch prevents direct microscopic observation of the internal dynamics of the rearrangements. Furthermore, the concomitantly large density mismatch leads to changes in $\phi$ over time, thus limiting the timescale of dynamical measurements. We report the results of real-space, microscopic experiments on model concentrated emulsions ($\phi > 0.7$) in which the continuous and dispersed phases are both index- and density-matched. We characterize the internal dynamics by measuring the spatial and temporal correlations between the motion of embedded tracer particles. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S37.00008: Phonon spectrum measured in a 1D Yukawa chain J. Goree, B. Liu An experiment is reported for a 1D chain of charged microspheres with a Yukawa pair potential. This work is motivated by chains of colloids confined in laser beams and Xe atoms confined in carbon nanotubes. Polymer microspheres are dispersed in a weakly- ionized rarefied gas. The resulting suspension, termed a dusty plasma, is a colloidal crystal. Depending on the confinement geometry, these suspensions can be 1D, 2D, or 3D. Dusty plasma suspensions resemble charge-stabilized colloidal suspensions: they both allow precise tracking of particles with digital video microscopy and manipulation of particles with lasers. The suspension in a dusty plasma is distinguished, however, by its extreme parameters: it is vastly softer, more weakly damped, and more dilute than colloids in water. In this talk, we describe oscillations of a non-bifurcated 1D chain. Using particle tracking to compute velocities of individual microspheres, we find the phonon spectrum. This is done both with and without time-modulated laser manipulation to excite phonons at a specific frequency. The measured spectrum is compared to a theoretical dispersion relation. Work supported by NASA and DOE. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S37.00009: Shear viscosity measurements in a 2D Yukawa liquid Volodymyr Nosenko, John Goree Shear viscosity was measured for a 2D strongly-coupled Yukawa liquid. First, we formed a dilute monolayer suspension of microspheres in a partially-ionized rarefied gas, i.e., a dusty plasma. In the absence of manipulation, the suspension forms a 2D triangular lattice. We used a new in-situ method of applying a shear stress using the scattering forces applied by counter-propagating laser beams. The lattice melted and a shear flow formed. Using digital video microscopy for direct imaging and particle tracking, the microscopic dynamics of the shear flow are observed. Averaging the velocities of individual microspheres, a velocity flow profile was calculated. Using the Navier-Stokes equation with an additional frictional term to account for gas drag, we fit the velocity profile. The fit yielded the value of the shear viscosity. The kinematic viscosity of our particle suspension is of order 1 mm$^{2}$s$^{-1}$, which is comparable to that for liquid water. We believe this is the first report of a rheological measurement in a 2D dusty plasma. This talk is based on V. Nosenko and J. Goree, PRL 93, 155004 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S37.00010: The Interplay of Osmotic Transport and Coalescence on Stability of Double Emulsions Wang Yafei, Zhang Tao, Hu Gang The long-term stability and controlled release of encapsulated active materials are major concerns of important applications of double emulsions. Only recently can stable monodisperse W/O/W double emulsions be made with a controlled size and internal volume fractions of encapsulated aqueous phase. The size of uniform oil droplets can be varied from sub-micron to tens of microns. Conventional dynamic light scattering encounters significant difficulties to probe the internal microstructure of liquid droplets. The droplets-inside-droplets structure of double emulsions complicates the interpretation of scattering data. To probe the internal microstructure of liquid droplets, diffusing-wave spectroscopy shows a unique advantage to measure the restricted motion of internal aqueous droplets. The destabilization of double emulsions can be initiated by introducing osmotic unbalance between the inner and outer aqueous phases and the process of coalescence transfers a double emulsion into a simple emulsion. Light scattering is a powerful technique to study the kinetic process by probing the structure migration. Diffusing-wave spectroscopy is also used to monitor the aging of double emulsions on a time scale of one year. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S37.00011: Dynamic electrorheological effects of rotating spheres: a self-consistent theory Lei Shen, Kin Wah Yu, Guo Qing Gu, Jun Jun Xiao We have corrected and extended the previous work done by T. K. Wan[1]. By solving self-consistent relaxation equations under coupled dipole approximation, we got the exact analysis solution for two rotating particles. Next, we derived three typical rotating configurations. Several comparisons have been done for interacting forces as well as the angular velocity dependence. Further more, we extended this two-particle model to a lattice model which contains infinite periodically arranged rotating particles. It has been examined by employing Ewald summation method. The rotational motion leads to a shift of the ground state of this electrorheological solid. The original bct(body-centered tetragonal) structure is no longer stable. By noticing that the behavior of our model is extremely similar to the electrorotation case [2], we studied the relations between these two models. The explicit associating expression has been found. \\[4pt] [1] Jones T. K. Wan, K. W. Yu, and G. Q. Gu. Phys. Rev. E, 6846 (2002). [2] C. K. Lo and K. W. Yu. Phys. Rev. E, 031501 (2001). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S37.00012: A Coarse-Grained Simulation of Rheology of Colloidal Suspensions and Polymer Nano-Composites Victor Pryamitsyn, Venkat Ganesan We extend DPD model to address dynamical properties of suspensions of solid particles in complex fluids. In this approach, the solvent particles (polymer segments) are represented as DPD particles. In contrast, the solute particles are represented as spherical hard particles of appropriate size. To provide proper shear friction and grip of the colloids and solvent we utilize Espanol's extensions over standard DPD model by adding rotational degree of freedom and rotational friction and \textbf{non-central} dissipative and random forces. For non-polymeric fluids, our results focus on the equilibrium dynamics and the steady state shear rheological behavior for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. For polymeric fluid, we have studied influence of polymer-particle friction and particle concentration on polymer matrix relaxation dynamics, particle diffusion and rheology of the composite. Our results suggest that the simulation approach can be used as a mesoscale model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S37.00013: Liquid-liquid phase transition and fragile-to-strong transition Limei Xu, Sergey Buldyrev, H. Eugene Stanley Using molecular dynamics simulations we study the dynamic properties for a model of liquid consisting of particles interacting via a spherically symmetric potential with repulsive and attractive ramps, first introduced by Jagla. The model displays anomalies similar to those found in liquid water, namely, expansion upon cooling and an increase of diffusivity upon compression as well as the liquid- liquid phase transition in the region of pase diagram easily accessible by simulations. At lower temperatures the low and high density liquids undergo glass transitions into correspondent glassy phases. We find that similarly to the behavior of water and silicon,the glass transition in the high density phase is fragile while in the low density phase it is strong. The difference is that in the present model, the co-existence line between low-density and high-density liquids as well as its Widom extension above the critical point have positive slopes. Thus, at constant pressure the behavior of diffusion coefficient changes upong cooling from strong to fragile, i.e. in the opposite way than in water and silicon. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S37.00014: Relaxation in a glassy binary mixture: A comparison of a Brownian Dynamics simulation and the mode-coupling theory Elijah Flenner, Grzegorz Szamel We compare results of Brownian Dynamics simulations of a glassy binary mixture with predictions of the mode-coupling theory for the same mixture. The mode-coupling equations for the time evolution of the coherent and the incoherent scattering functions were solved for a number of temperatures using as input the structure factors determined from the simulations. As found in earlier studies, the mode-coupling theory predicts an ergodicity breaking transition at a temperature that is approximately twice higher than the \textit{crossover} temperature inferred from Brownian Dynamics simulation results. However, when compared at the same $T-T_c$, time-dependent quantities predicted by the mode-coupling theory agree reasonably well with those obtained from simulations except at the lowest temperatures. [Preview Abstract] |
Session S39: Quantum Phase Transitions in Strongly Correlated Electron Systems
Sponsoring Units: DCMPChair: Victor Galitski, UCSB
Room: Los Angeles Convention Center 514
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S39.00001: Magnetic properties of Ni$_3$Al and Ni$_3$Ga: Emergent states and the importance of a tri-critical point? Robert P. Smith, Gilbert G. Lonzarich, Siddharth S. Saxena, Mark Ellerby Ni$_{3}$Al and Ni$_{3}$Ga are itinerant d electron systems which lie close to a ferromagnetic quantum critical point. Ni$_{3}$Al is a ferromagnet with a transition temperature at ambient pressure of 41K and a critical pressure of 80kbar while Ni$_{3}$Ga is a paramagnet all the way to zero temperature. These materials are ideal systems in which to test spin fluctuation theory. We present magnetisation data of these two materials and test the results against spin fluctuation theory with no adjustable parameters. While Ni$_{3}$Al (at ambient pressure) is found to fit well with the theory, Ni$_{3}$Ga can be understood by assuming that this system lies close to a quantum tri-critical point. We suggest that such a quantum tri-critical point may be a key feature in the understanding of quantum critical systems more generally. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S39.00002: Quantum oscillations in non-Fermi-liquid metals Stephen Julian, Alix McCollam, Patrick Rourke, Jacques Flouquet, Dai Aoki The temperature dependence of de Haas van Alphen (dHvA) and other quantum oscillations is governed in a Fermi liquid by the Lifshitz-Kosevich (LK) equation\footnote{see e.g. D.\ Shoenberg, {\em Magnetic Oscillations in Metals}, CUP 1984}. Several authors \footnote{e.g. A.\ Wasserman and M.\ Springford, Adv.\ Phys.{\bf 45} (1996) 471, and references therein.} have extended the LK theory to non-Fermi-liquid metals, but these treatments tend to be very technical. We will give a simple interpretation of the non-Fermi-liquid effects that arise in these theories, and will briefly discuss the possible observation of non-Fermi-liquid temperature dependence in dHvA oscillations in CeCoIn$_5$. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S39.00003: Quantum criticality in the Itinerant Ferromagnets Zr$_{1-x}$Nb$_{x}$Zn$_{2}$ D. Sokolov, M.C. Aronson, Z. Fisk We report the results of magnetization measurements performed on the family itinerant ferromagnets Zr$_{1-x}$Nb$_{x}$Zn$_{2}$, (0 $\leq x \leq 0.14)$. Nb doping reduces the moment M$_{0}$ and also the Curie temperature T$_{c}$, which simultaneously disappear at the critical Nb concentration x$_{c}$=0.084. We find that T$_{c}$ $\propto$ (x-x$_{c}$)$^{3/4}$, as predicted for a 3d ferromagnet, while M$_{0}$ $\propto$ T$_{c}$ (x), as expected for a Stoner ferromagnet. For all Nb concentrations and for temperatures which approach 100 K, the extrapolated zero field susceptibility $\chi$ can be expressed with a modified Curie Weiss expression $\chi=C/(T^\gamma +\theta)$. $\theta$ is finite in the paramagnetic state (x$>$x$_{C}$), but vanishes as the system becomes critical at x=x$_{C}$, evidenced by the T=0 divergence of $\chi$ in this system. We find that $\gamma$ is near one in paramagnetic regimes for x$<$x$_{c}$ (T$>$T$_{c}$), and for x $\gg$ x$_{c}$. However, $\gamma$ is substantially enhanced in the vicinity of the quantum critical point (0.08$<$x$<$0.09), indicating the breakdown of the conventional Stoner theory. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S39.00004: Thermoelectrical Transport of Quantum Critical Metals Seiji Yamamoto, Eugene Pivovarov, Qimiao Si Motivated by recent experimental measurements of an anomalously enhanced Nernst coefficient in strongly correlated electron systems, we examine thermal and electrical transport in metals near quantum phase transitions. We consider an electronic topological transition (Lifshitz transition) to mimic exotic quantum critical points with a large Fermi-surface reconstruction, and compare it with the case of spin-density-wave (SDW) transition. In the former, we find a strong enhancement of the Nernst coefficient. Across an SDW transition, on the other hand, the Fermi surface change is too smooth to significantly affect the Nernst coefficient. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S39.00005: Magnetic fluctuations close to an antiferromagnetic quantum critical point as observed in CeNi2Ge2 Almut Schroeder, Bilal ElZoghbi, C.L. Broholm, Y. Qiu, D.F. McMorrow, N. Christensen, J. Mydosh, O. Tegus, G. Aeppli, M. Adams Neutron scattering spectra of the heavy fermion compound CeNi2Ge2 will be presented to characterize the magnetic correlations and dynamics close to an antiferromagnetic quantum critical point. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S39.00006: The strongly paramagnetic-to-spin polarized ferromagnetic transition in Fe$_{1-x}$Co$_x$S$_2$ Song Guo, John DiTusa, David Young, Julia Chan Carrier doping of “fool’s gold”, the paramagnetic insulator FeS$_2$, by way of Co substitution for Fe, results in a insulator-to-metal transition at $x\le 0.001$. Further Co substitution beyond $x~0.035$ produces an itinerant and fully spin polarized ferromagnet by way of either a crossover or quantum phase transition. In order to explore the thermodynamics of this magnetic semiconductor near the paramagnetic-to- ferromagnetic phase transition the specific heat of Fe$_{1-x} $Co$_x$S$_2$ (x=0.045,0.03,0.005) was measured for temperatures down to 0.1 K. Our $x=0.045$ sample displayed a logarithmic like divergence of $C/T =\gamma$ in zero field which saturates to a Fermi liquid like constant below 0.5 K. For $x$=0.03 at zero field, the enhancement of $\gamma$ is extended down to 0.3K while for x=0.005 the logarithmic-like divergence continues down to the lowest temperature measured. For this last sample, $\gamma$ can be enhanced by the application of small magnetic fields ($H<0.15$ T) while higher fields tend to suppress $\gamma$ in all of our samples. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S39.00007: Anisotropic Hall Effect in Single Crystal Heavy Fermion YbAgGe Sergey Bud'ko, Emilia Morosan, Paul Canfield Temperature- and field-dependent Hall effect measurements are reported for YbAgGe, a heavy fermion compound exhibiting a field-induced quantum phase transition. The low temperature, field-dependent measurements reveal well defined, sudden changes with applied field; in specific for $H \perp c$ a clear local maximum that sharpens as temperature is reduced below 2 K and that approaches a value of 45 kOe - a value that has been proposed as the $T = 0$ quantum critical point. Similar behavior was observed for $H \| c$ where a clear minimum in the field-dependent Hall resistivity was observed at low temperatures. Although at our base temperatures it is difficult to distinguish between the field-dependent behavior predicted for (i) diffraction off a critical spin density wave or (ii) breakdown in the composite nature of the heavy electron, for both field directions there is a distinct temperature dependence of a feature that can clearly be associated with a field-induced quantum critical point at $T = 0$ persisting up to at least 2 K. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S39.00008: Numerical renormalization group study of the Bose-Fermi Kondo model Matthew T. Glossop, Kevin Ingersent The Bose-Fermi Kondo model (BFKM) is of current interest in the
context of non-Fermi liquid behaviour in quantum critical heavy
fermion systems [1]. We study the Ising-symmetry BFKM, employing
a novel extension of Wilson's numerical renormalization group to
include coupling of a quantum impurity
to {\it both} a conduction electron band {\it and} a dissipative
bosonic bath described by the spectral function
$\eta(\omega)\propto \omega^s$ ($0<\omega<\omega_c$).
For sub-Ohmic bath exponents $0 |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S39.00009: Metamagnetism and Non Fermi Liquid behavior in CeIrIn5 C. Capan, F. Ronning, E.D. Bauer, R. Movshovich, M.F. Hundley, J.D. Thompson, J.L. Sarrao, L. Balicas, T. Murphy, E. Palm, D. Hall, H. Radovan, S. Tozer, R. Goodrich Investigations of transport and thermodynamic properties near a quantum phase transition have been subject of intense theoretical and experimental efforts in strongly correlated electron systems in recent years. The nature of low energy excitations near a quantum critical point is strikingly different from the Landau quasiparticles, resulting in strong deviations from Fermi Liquid theory in most properties at low temperatures. In this context, recent studies of Sr3Ru2O7 have raised the possibility of a quantum critical point associated with metamagnetism. CeIrIn5, a recently discovered heavy fermion superconductor, offers yet another playground for such investigations. Indeed, CeIrIn5 has peculiar properties at high magnetic fields, with a field induced Non Fermi Liquid behavior in both resistivity and specific heat up to 17T. A metamagnetic transition has also been reported for magnetic fields above 30T. Thus, it is natural to ask whether the Non Fermi Liquid behavior is a result of a quantum critical point associated with metamagnetism in CeIrIn5. We will present new results of resistivity and magnetization up to 33T in an attempt to address this issue. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S39.00010: Antiferromagnetic Quantum Critical Point in CeRh(In,Sn)$_5$ Eric Bauer, D. J. Mixson, F. Ronning, J. D. Thompson, J. L. Sarrao, R. Movshovich, M. F. Hundley, G. R. Stewart CeRhIn$_5$ belongs to the family of CeMIn$_5$ (M=Co, Rh, Ir) heavy fermion superconductors that have attracted attention in recent years due to the rich variety of strongly correlated electron phenomena observed in these materials. The CeRhIn$_5$ compound exhibits antiferromagnetism at $T_N$=3.8 K with a Sommerfeld coefficient $\gamma \sim 300$ mJ/mol K$^2$. The Neel temperature is suppressed at a critical pressure $P_c \sim$25 kbar, while superconductivity is found to coexist with antiferromagnetism above $\sim15$ kbar, reaching a maximum transition temperature $T_c=2.1$ K. de Haas van Alphen measurements reveal a divergence of the effective mass at $P_c$, but signatures of an antiferromagnetic (AFM) quantum critical point (QCP) from other measurements are masked by the occurrence of superconductivity in this pressure range. The substitution of Sn for In in CeRhIn$_5$ offers an alternative way to probe the possible AFM QCP in this system. Preliminary measurements suggest an AFM QCP in CeRhIn$_{5-x}$Sn$_x$ at $x \sim 0.75$ with robust non-Fermi liquid behavior occurring for $x >0.75$. The physical properties of the CeRh(In,Sn)$_5$ system will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S39.00011: Evolution of Superconducting H$_{c2}$ Transition with La-Doping in Ce$_{1-x}$La$_x$CoIn$_5$ Johnpierre Paglione, M.A. Tanatar, E. Boaknin, D.G. Hawthorn, R.W. Hill, M. Sutherland, Louis Taillefer, C. Petrovic Recent measurements of the heavy-fermion superconductor CeCoIn$_5$ have revealed a field-tuned quantum critical point which coincides with a {\it first-order} superconducting transition H$_{c2}$ as $T \to 0$ [1], resulting in a completely unique and intriguing H-T phase diagram with no known magnetic phase. By substituting La for Ce, a gradual destruction of the superconducting phase allows an investigation of the correlation between the order of H$_{c2}$ and the field-tuned quantum critical behaviour. Here we report low temperature heat and charge transport measurements of Ce$_{1-x}$La$_x$CoIn$_5$ as a function of field and doping, revealing an intriguing evolution of the quantum critical behaviour in this system. \newline \newline [1] J. Paglione {\it et al.}, Phys. Rev. Lett. 91, 246405 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S39.00012: Quantum Critical Behavior of the Bose-Fermi Kondo Model with Ising Anisotropy Tae-Ho Park, Stefan Kirchner, Qimiao Si The existence of a continous quantum phase transition of the Bose-Fermi Kondo Model (BFKM) with a self-consistently determined bosonic bath has been demonstrated within the Extended Dynamical Mean Field Approach to the anisotropic Kondo lattice model and $\omega/T$-scaling near the quantum critical point(QCP)was found[1,2]. We study the quantum critical properties of the anisotropic BFKM with specified bath spectral function, where the spectrum of the bosonic bath vanishes in a power-law fashion with exponent $\gamma$ for small frequencies. Motivated by very recent results that the quantum to classical mapping for a related class of models fails[3,4]. We determine the critical local susceptibility using both the classical and quantum Monte Carlo approaches of Ref.5. Our results cover several values of $\gamma$ below and above the upper critical dimension of the classical model for temperatures down to 1\% of the bare Kondo scale. [1]D. Grempel and Q. Si, Phys. Rev. Lett. 91, 026402 (2003). [2]J.Zhu, D. Grempel, and Q. Si, Phys. Rev. Lett. 91, 156404 (2003). [3]L. Zhu, S. Kirchner, Q. Si nad A. Georges, Phys. Rev. Lett. in press (cond-mat/0406293). [4]M. Vojta, N. Tong, and R. Bulla, cond-mat/0410132. [5]D. Grempel and M. Rozenberg, Phys. Rev. B 60, 4702 (1999). [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S39.00013: Two-Impurity Dynamical-Mean-Field-Theory Study of the Periodic Anderson Model Ping Sun, Gabriel Kotliar We solve the periodic Anderson model using a two impurity dynamical mean field theory via QMC. We obtain the temperature v.s. hybridization phase diagram. In the crossover region, we observe logarithmic temperature ($T$) dependence in energy. As the quantum critical point (QCP) is approached, both the Neel and the lattice Kondo temperatures decrease and the two lines do not tend to cross at a finite temperature. We observe strong ferromagnetic spin fluctuations near the QCP on the Kondo side. Our results indicate that the critical spin susceptibility is local in space at the QCP. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S39.00014: Finite Temperature Properties of Quantum Lifshitz Transitions between valence bond phases: An Example of `Local' Quantum Criticality Pouyan Ghaemi, Ashvin Vishwanath, Todadri Senthil We study finite temperature properties near quantum `Lifshitz' transitions between different valence bond solid states of two dimensional quantum magnets. They are the generic versions of phase transitions associated with the solvable Rokshar Kivelson points in quantum dimer models on bipartite lattices. This quantum critical point is described by a free theory that nevertheless has operators with non-trivial scaling dimension. We show that while correlators of such operators exhibit the expected scaling as a function of time, they do NOT show analogous scaling in space. In particular, in the scaling limit, all such correlators are purely LOCAL. In contrast, the zero temperature properties are conventional, and the correlators decay as a power law in both space and time. This provides a valuable microscopic example of how some kind of `local' criticality may arise at finite temperatures (in the scaling limit), although the underlying zero temperature critical point is itself not `local' in any sense. We examine the underlying reasons for this unusual behaviour, present the exact local dynamical correlation functions at finite T for these operators, and the effect of irrelevant operators on the scaling limit results described above. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S39.00015: Extended-DMFT Study of Quantum Phase Transitions in a Kondo Lattice: Dynamical Large-N limit Stefan Kirchner, Lijun Zhu, Qimiao Si In one approach to the quantum critical heavy fermion metals, Kondo lattice systems are studied through self-consistent Bose-Fermi Kondo Model (BFKM) within the extended dynamical mean field theory. In the case with spin-rotational invariance, this model is still difficult to study theoretically or numerically. Very recently, it has been shown[1,2] that a dynamical large-N generalization provides an access to the quantum critical behavior of the spin-rotationally-invariant BFKM with a sub-ohmic boson spectral function. Here, we carry out a self-consistent EDMFT study of the model in this large-N limit. We determine the extent to which a second-order quantum phase transition exists in this limit for two- and three-dimensional spin fluctuations, as well as the critical exponents of the magnetic dynamics. [1] L. Zhu, S. Kirchner, Q. Si, and A. Georges, Phys. Rev. Lett, in press (cond-mat/0406293). [2] S. Kirchner, L. Zhu, and Q. Si, Physica B, in press (cond-mat/0407307). [Preview Abstract] |
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S39.00016: Out-of-Equilibrium Transport near a Quantum Phase Transition Stefan Kirchner, Lijun Zhu, Qimiao Si Non-equilibrium properties are expected to be particularly rich at a second order quantum phase transition, given that statics and (imaginary time) dynamics are already mixed at the equilibrium level. The issue has so far received only limited study, partly because equilbrium methods are not readily generalizable to non-equilibrium situations. Here we consider the quantum critical point of a Bose-Fermi Kondo model, which we have recently shown[1] to model the magnetic quantum dot systems[2]. We extend the saddle-point equations in a dynamical large-N limit[3] to the Keldysh contours, and determine the linear and non-linear conductance in the quantum critical regime. [1] S. Kirchner, L. Zhu, Q. Si, and D. Natelson, to be published (2004). [2] A. N. Pasupathy et al., Science {\bf 306}, 86 (2004). [3] L. Zhu, S. Kirchner, Q. Si, and A. Georges, Phys. Rev. Lett, in press; cond-mat/0406293. [Preview Abstract] |
|
S39.00017: Magnetic field-tuned quantum critical point in CeAuSb$_2$ Luis Balicas, Satoru Nakatsuji, Hannoh Lee, Pedro Schlottmann, Timothy P. Murphy, Zachary Fisk Here we report on the anomalous metallic properties of CeAuSb$_ {2}$. At $H$ = 0, CeAuSb$_{2}$ displays AF ordering at $T_{N}$ = 6.0 K. Above $T_{N}$, the resistivity \textit{$\rho $} displays a $T^{\alpha }$ dependence with $\alpha \quad <$ 1 and $C_{e}$/$T$ has the -\textit{lnT} dependence characteristic of NFL behavior. For $T \quad <$ $T_{N}$, $\rho $ has the \textit{AT}$^{2}$ FL-like dependence and the extrapolation of $C_{e}$/$T$ to $T$ = 0 yields a Sommerfeld coefficient of $\gamma \quad \sim $ 0.1 J/mol.K$^{2}$, so that CeAuSb$_{2}$ is to be considered a heavy- Fermion system. A magnetic field along the inter-plane direction leads to two subsequent metamagnetic transitions and the concomitant \textit {continuous} suppression of $T_{N}$ to $T$ = 0 at $H_{C}$ = 5.3 $\pm $ 0.2 T. As the AF phase boundary is approached from the paramagnetic (PM) phase, $\gamma $ is enhanced and the $A$ coefficient of the resistivity diverges as $H-H_{C}^{-1}$. As $T$ is lowered for $H \quad \sim \quad H_{C}$, the $T$-dependence of \textit {$\rho $} and $C_{e}$/$T$ is sub-linear and -\textit{ln} $T$, respectively. These observations suggest the existence of a field-induced QCP at $H_{C}$. At higher fields an unconventional $T^{3}$- dependence emerges and becomes more prominent as $H$ increases, suggesting that the FL state is \textit{not} recovered for $H \quad >> \quad H_{C}$. [Preview Abstract] |
Session S40: Focus Session: Morphology and Evolution at Surfaces: Wires and Self-Assembly
Sponsoring Units: DMP DCMPChair: Pantelis Kelires, University of Crete
Room: LACC 408A
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S40.00001: Semiconductor Nanowires from Materials Science and Device Physics Perspectives Invited Speaker: Realization of extremely down-scaled devices gives tough challenges related to technology and materials science. One reason for the concern is that top-down fabricated nano-devices tend to have their properties dominated by process-induced damage, rendering ultra-small devices not so useful. Alternatively, bottom-up fabrication methods may allow dimensions on the scale even below 10 nm, still with superb device properties. I will in this talk describe our research on catalytically induced growth of semiconductor nanowires. Our method uses catalytic gold nanoparticles, allowing tight control of diameter as well as position of where the nanowire grows, with our work completely focused on epitaxially nucleated nanowires in which the nanowire structure can be seen as a coherent, monolithic extension of the crystalline substrate material. One of the most important achievements in this field of research is the realization of atomically abrupt heterostructures within nanowires, in which the material composition can be altered within only one or a few monolayers, thus allowing 1D heterostructure devices to be realized. This has allowed a variety of quantum devices to be realized, such as single-electron transistors, resonant tunneling devices as well as memory storage devices. A related recent field of progress has been the realization of ideally nucleated III-V nanowires on Si substrates, cases where we have also reported functioning III-V heterostructure device structures on Si. All of these device related challenges evolve from an improved understanding of the materials science involved in nucleation of nanowires, in altering of composition of the growing nanowire, in control of the growth direction etc. I will give examples of these materials science issues and will especially dwell on the opportunities to form new kinds of materials, e.g. as 3D complex nanowire structures, resembling nanotrees or nanoforests. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S40.00002: STM of Gold-Induced Chains on the Si(775) Surface. Laura Pedri, Laura Toppozini, Mark Gallagher Atomic scale wires on semiconductor surfaces are an attractive system to study the novel physics of one-dimensional (1D) metallic conduction. Recently, quasi-1D chains have been produced by depositing small amounts of Au onto vicinal Si(111) surfaces. The gold-induced chains run parallel to step edges, and photoemission from these surfaces reveals highly 1D metallic bands. The Si(775)-Au surface exhibits 1D chains running along $[1\bar {1}0]$ spaced 2.13~nm apart. It has been previously reported that the (775) surface is prepared by depositing 0.25~$\pm $~0.07~ML of gold onto a silicon surface tilted 8.5$^{o}$ towards $[11\bar {2}]$ [1]. Furthermore, it has been argued that the unit cell contains two Au atoms per-unit cell similar to the on axis Si(111)5x2-Au reconstruction. We have used scanning tunneling microscopy to further investigate the atomic structure and the electronic properties of the (775)-Au surface. In particular, we have used STM with Auger spectroscopy, and LEED to examine the stochiometry of the chain structure. [1] Crain et al., \textit{Phys. Rev. B} \textbf{69}, 125401 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S40.00003: From Multiply-Twinned Particles to Epitaxial Nanocrystals: the crucial role of interface on the structure of Ag nanoparticles Boquan Li, Jian-Min Zuo The effect of substrate on the structure of nanometer-sized metal particles is investigated for Ag on silicon surfaces. The nanometer-sized Ag particles, formed by vapor deposition of Ag on hydrogen-terminated Si (001) surfaces at room temperature, adopt multiply-twined structures, while the Ag particles of similar sizes on H-Si (111) surfaces assume the face-centered cubic (fcc) crystal structure. Upon annealing, the multiply-twined Ag nano-particles on H-Si (001) transform into fcc nanocrystals. The fcc Ag takes up the cube-on-cube epitaxy, with the orientation relationship of Ag (001)//Si (001) and Ag[110]//Si[110]. An energetic model is developed to account for the transition from multiply-twined particles to fcc epitaxial crystals, in which the interface energy change on H-Si(001) due to hydrogen desorption plays a crucial role. A pair-correlation function study will also be presented in an effort to accurately determine the inter-atomic distances and strain in the multiply-twinned Ag nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S40.00004: Nucleation and aggregation in 1D with interactions Hachem Sidi Ammi, Olivier Pierre-Louis, Anna Chame, M'Hammed Touzani, Abdelilah Benyoussef, Chaouqi Misbah We present a study of the aggregation of interacting particles in one dimension. This situation for example applies to atoms trapped along linear defects at the surface of a crystal, such as crystal steps. It is therefore important for the initial stages of the formation of quantum wires. Simulations are performed with two lattice models. In the first model, the borders of atoms and islands interact in a vectorial manner. In the second model, each atom carries a dipole. These two models lead to qualitatively similar but quantitatively different behavior. In both cases, the final average island size $S_f$ does not depend on the interactions in the limits of very low and very high coverages. For intermediate coverages, $S_f$ exhibits an asymmetric behavior as a function of the interaction strength: while it saturates for attractive interactions, it decreases for repulsive interactions. A class of mean field models is designed, which allows one to retrieve the interaction dependence and the coverage dependence of the average island size with a good accuracy. Ref. H. Sidi Ammi et al, Submitted to Phys.Rev.E. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S40.00005: The atomic-scale processes underlying nanoscale pattern formation on solid surfaces Invited Speaker: Gary Kellogg The deposition of Pb atoms on Cu(111) produces two surface phases: a Pb-Cu surface-alloy and a Pb-overlayer. Within a specified range of Pb coverage, the two phases co-exist and spontaneously order into domain patterns with tens-of-nanometer periodicity[1]. The evolution of the domain structures with increasing Pb coverage agrees well with theoretical descriptions based on competing long- and short-range interactions. The system thus provides an ideal model system for probing the interactions underlying 2-D self-assembly. Although a self-consistent picture of the thermodynamic driving forces responsible for pattern formation now exists[2,3], a comprehensive understanding of the kinetic processes underlying self-assembly is only beginning to emerge. In this talk I will discuss recent experiments and calculations that address a key question concerning the kinetics of pattern formation: how do individual domains of each phase, which contain 10s of thousands of atoms, assemble into patterns on timescales of minutes? I will show that the self-assembly occurs by the collective motion of entire domains and present evidence that this motion is facile because (1) Pb atoms move quickly across the surface alloy due to Pb-Cu exchange and (2) Cu atoms move quickly \textit{through} the overlayer phase due to a high concentration of vacancies and Cu atoms embedded within the overlayer. The latter finding is not intuitive and may explain why the remarkable pattern formation seen for Pb/Cu(111) has not been observed for other metal-metal systems. [1] R. Plass, \textit{et al}., Nature \underline {412}, 875 (2001). [2] R. van Gastel, \textit{et al.}, Phys. Rev. Lett. \underline {91}, {\#}55503 (2003). [3] R. van Gastel, \textit{et al}, Phys. Rev. B (in press). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S40.00006: Self-organization and geometry control of Pb islands grown on anisotropic substrates Myron Hupalo, Michael Tringides The growth of uniform nanostructures requires discovering robust and reproducible ways to control their size and geometry. It is found that the growth of Pb on the anisotropic substrate In(4x1)-Si(111) leads to full control of the grown morphology from the interplay of two stabilizing mechanisms which minimize the island energy at preferred geometry. Quantum Size Effects (QSE) i.e. the dependence of the electron energy on the structure dimensions as a result of electron confinement result in uniform height 4-layer Pb islands grown at 180K. More importantly the island shape and width are controllable, with the width an integer multiple of 1.33nm (the period of the initial In(4x1)along [$\mathop 1\limits^- \mathop 1\limits^- 2$]), because the underlying reconstruction generates anisotropic potential energy relief with preferential sticking of the incoming Pb atoms along [$\mathop 1\limits^- 10$]. Growth on the different reconstructed substrate, Si(111)-In$\surd $31x$\surd $31, leads to uniform height Pb islands of hexagonal shapes thus demonstrating island shape control. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S40.00007: Self-assembly of isonicotinic acid molecules into supramolecular films on Ag (111) Bo Xu, Hui Li, Janice Reutt-Robey Self assembly processes of isonicotinic acid (INA) molecules into supramolecular structures on Ag (111) surface are studied with UHV-STM, XPS, and IR spectroscopy. INA molecule contains both a ring nitrogen and carboxyl tail, which lead to a tape-like molecular solid.~At room temperature, INA~molecules organize into 2D islands that exceed~100 nm on Ag (111), demonstrating 2D H-bonding interactions. A series of carboxy O-H\textbullet \textbullet \textbullet N hydrogen bonds assemble INA molecules into linear chains, while~weaker hydrogen bonds between carbonyl O and aromatic~H link the chains sideways into ordered 2D structures. Different orientational domains are observed and the domain walls (carboxyl-carboxyl coupling)~establish the molecular dipole direction. XPS spectroscopy corroborates the H-bonding interactions, while IR spectroscopy was used to assess INA molecular orientation with respect to the surface plane. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S40.00008: Atomically and Time Resolved Pattern Formation: S on Submonolayer Ag/Ru(0001) Bogdan Diaconescu, Katrsten Pohl Strained metallic interfaces can lead to highly ordered arrays of misfit dislocations that can be used as an elegant patterning technique for growing arrays of clusters with specific size and densities. Deposition of S on submonolayer Ag films on Ru(0001) transforms the short herringbone dislocation pattern, with a 60{\AA} x 40{\AA} unit cell, into a large scale ordered triangular array of S filled vacancy islands 50{\AA} apart as shown by STM and LEED. In this process S partially relieves the strain in Ag film as seen by the relaxation of the threading dislocation network. Atomic structure of the S islands and the coverage dependence of the atomic structure of S/Ru(0001) and S islands size is revealed by STM imaging. The low S coverage regime shows a transition from isolated, highly mobile S islands as seen through time resolved STM to an ordered islands array. As the S coverage increases beyond 0.33 ML on the Ru(0001) terrace, the ordering of the 2D S islands array is destroyed, and the compressed S phase pushes Ag onto a second layer. Supported by NSF-CAREER-DMR-0134933 and ACS-PRF-37999-G5 [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S40.00009: MeV Ion Beam Synthesis of Nanopore Arrays in SiO2 Films Andrew Carlson, Anuranjita Tewary, Mark Brongersma, Thomas Felter, Sergei Kucheyev High energy (MeV), heavy ion irradiation can be used as a tool to deform patterned SiO$_{2}$ films in a controlled fashion. We have used this technique to deform micron-sized holes in SiO$_{2}$ films generated by photolithography and focused ion beam milling. The holes were fabricated in 2 $\mu $m thick SiO$_{2}$ films in the size range from 500 nm to 2 $\mu $m and were subsequently irradiated with 4 MeV Xe$^{ }$ions. First, a systematic study will be presented on the deformation of differently shaped holes as a function of the ion fluence. Second, we will present data on the deformation of linear and two-dimensional arrays of holes. Finally, we will test the observed deformations against currently available visco-elastic models that describe this ion irradiation-induced deformation process. This work may find application in nanoscale fashioning of SiO$_{2}$ surface features and the controlled fabrication of nanopore arrays. [Preview Abstract] |
Session S41: Correlated Electrons: Exotic Magnetism
Sponsoring Units: GMAGChair: S.D. Mahanti, Michigan State Univ.
Room: LACC 150A
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S41.00001: Properties and Detection of Spin Nematic Order in Strongly Correlated Electron Systems Daniel Podolsky, Eugene Demler A spin nematic is a state which breaks spin SU(2) symmetry while preserving translational and time reversal symmetries. Spin nematic order can arise naturally from charge fluctuations of a spin stripe state. Focusing on the possible existence of such a state in strongly correlated electron systems, we build a nematic wave function starting from a $t-J$ type model. The nematic is a spin-two operator, and therefore does not couple directly to neutrons. However, we show that neutron scattering and Knight shift experiments can detect the spin anisotropy of electrons moving in a nematic background. We find the mean field phase diagram for the nematic taking into account spin-orbit effects. [cond-mat/0411159]. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S41.00002: Can Frustration Preserve a quasi-two dimensional spin fluid? Marianna Maltseva, Piers Coleman One of the mechanisms recently proposed to account for non-Fermi liquid behavior in heavy fermion materials at a quantum critical point is based on the notion that the spins form a quasi-two-dimensional spin fluid. Using the Heisenberg antiferromagnet as a simple example to explore this line of reasoning, we show with the help of spin-wave theory that in general, geometric frustration fails to preserve a two-dimensional spin fluid. Even though one can eliminate the classical interlayer coupling by frustration, the layers always develop a quantum-mechanical coupling via tunneling. The magnon pair tunneling process responsible for this coupling is analogous to the pair tunneling process responsible for the Josephson effect generating a $\cos 2 \theta$ or biquadratic coupling between layers. To end our discussion we consider a special case of XY model in which decoupled "sliding phases" of spin fluid may exist in certain finely tuned conditions. In general these finely tuned situations are equally susceptible to the strong-coupling effects of quantum tunnelling, forcing us to conclude that in general, geometric frustration can not preserve a two dimensional spin fluid. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S41.00003: Geometric critical line and unconventional ordered phases in quantum kagome antiferromagnets Joel Moore, Cenke Xu We study the phase diagram of the XXZ antiferromagnet for spin$>$=1 on the kagome lattice for comparison with recent neutron scattering results. In addition to the usual geometric classical criticality that appears at zero temperature in the classical limit of the XY model, there is another zero-temperature geometrical critical line that separates two unconventional ordered phases. We also discuss numerical transfer-matrix results and similarities to plaquette phases proposed in the honeycomb lattice quantum dimer model. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S41.00004: Weak Chiral Magnetic Order in the Quantum Pyrochlore Antiferromagnet Valeri Kotov, Maged Elhajal, Michael Zhitomirsky, Frederic Mila Heisenberg (S=1/2) spins on the three-dimensional pyrochlore lattice do not order magnetically due to the strong frustration. However we show that the presence of antisymmetric, Dzyaloshinsky-Moriya (DM) interactions, can lead to weak antiferromagnetic order. This exotic state has chiral symmetry, dictated by the distribution of the DM vectors and is characterized by small magnetic moments induced by the DM interactions. An external magnetic field can also produce complex ordered states, and a quantum transition separates the field-induced and chiral ordered phases. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S41.00005: Pressure induced quantum phase transitions in Ca2RuO4 Patricia Lebre Alireza, Anne-Marie Cumberlidge, Gilbert Lonzarich, Stephen Julian, Fumihiko Nakamura, Yoshiteru Maeno Ca$_{2}$RuO$_{4 }$is a member of the family of ruthenates, which are strongly correlated systems that exhibit a wide range of interesting phenomena including metal-insulator transitions, orbital and magnetic ordering and unconventional superconductivity. Using a novel setup in a miniature anvil cell, we have been able to measure magnetic susceptibility under high hydrostatic pressure and have followed the transitions of Ca$_{2}$RuO$_{4}$ from an antiferromagnetic Mott insulator to a ferromagnetic metal. Additionally, we have investigated the evolution of this ferromagnetically ordered state as the pressure is increased, successfully suppressing this transition towards a quantum critical point. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S41.00006: Magnetic droplets in nearly ferromagnetic metals close to the quantum critical point Yen Lee Loh, Vikram Tripathi, Misha Turlakov Metallic palladium and platinum have anomalously high magnetic susceptibilities because of their proximity to quantum phase transitions, so that giant magnetic moments may form around impurities such as iron atoms, resulting in a magnetic susceptibility which varies sensitively with temperature, but sometimes shows deviations from the Curie law due to quantum fluctuations. We have studied the case of a magnetic droplet with XY anisotropy using perturbation theory and path-integral Monte Carlo simulation. We find that the susceptibility obeys a logarithmic law close to the quantum critical point that is distinct from the logarithms in the Kondo and Larkin-Mel'nikov theories. Our work provides a fuller understanding of the `phase diagram' of magnetic droplet systems and has implications for the design of magnetic thermometers based on giant-moment alloys. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S41.00007: BEC critical exponent near quantum critical point in BaCuSi$_2$O$_6$ Suchitra E. Sebastian, I.R. Fisher, P.A. Sharma, M. Jaime, N. Harrison, C.D. Batista, V. Correa, L. Balicas, N. Kawashima BaCuSi$_2$O$_6$ is a spin gap compound comprising coupled square bilayers of vertical dimers. The magnetic ordering transition in applied magnetic fields above the critical magnetic field (H$_{c1}$ = 23.5T) has been interpreted as a condensation of triplons. Proximity to the Quantum Critical point (QCP) can be tuned in this material by changing the particle density via the applied magnetic field (chemical potential.) We present a series of experiments that measure the critical exponent $\nu$ relating the proximity to the QCP, to the ordering temperature (T$_c$) by the power law $T_{c} \alpha (H-H_{c1})^\nu$. Experimental estimates of $\nu$ in this system agree with the theoretical mean field prediction of 2/3 for Bose Condensation of a dilute Bose gas. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S41.00008: Quantum Oscillations and Competing Ground States in triple layered Sr4Ru3O10 V. Durairaj, X.N. Lin, S. Chikara, E. Elhami, V.A. Bondarenko, J.W. Brill, G. Cao, S. Parkin, L. Balicas, Y. Xin The triple-layered Sr$_{4}$Ru$_{3}$O$_{10}$ is characterized by an unexpectedly strong quasi-two dimensional characteristic, and a sharp metamagnetic transition and ferromagnetic behavior occurring within the basal plane and along the c axis, respectively. The interplane resistivity at magnetic field B, up to 45 T, exhibits low-frequency quantum oscillations associated with the spin polarized state when B is parallel to the c axis, and a large magnetoresistivity accompanied by critical fluctuations governed by the metamagnetic transition when B is perpendicular to the c axis. The complex behavior evidenced in magnetization, specific heat, and resistivity presented is not characteristic of any obvious ground states, and points to an unusual state that shows a delicate balance between fluctuations and order. The results will be presented and discussed along with comparisons with data of impurity doped Sr$_{4}$Ru$_{3}$O$_{10}$. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S41.00009: Non-Fermi-liquid behavior in annealed UCu$_{4}$Pd Ryan Baumbach, Nicholas Butch, M. Brian Maple, Douglas MacLaughlin The effect of disorder in the polycrystalline UCu$_{4}$Pd system is addressed in an annealing study. Samples were annealed at 750$^{\circ}$C for 7, 14 and 56 days and compared to unannealed samples. X-ray diffraction data indicate that all samples are single phase UCu$_{4}$Pd. Magnetic susceptibility vs. temperature data and the Sommerfeld coefficient ($\gamma =C_{e}$/$T $where $C_{e}$ is the electronic specific heat) vs. temperature both show little variation with annealing and can be fit with either a logarithmic or weak power law function for 4.5K$\le T\le $50K and 0.6K$\le T\le $2K, respectively. In contrast, electrical resistivity vs. temperature data develop a Kondo-like minimum near 35K that strengthens with annealing. Additionally, at low temperatures (2K$\le T\le $10K), the resistivity is linearly proportional to temperature for all samples. These data indicate that the non-Fermi liquid behavior of UCu$_{4}$Pd is not suppressed by annealing and may resolve inconsistencies reported in previous studies. Data will be collected and presented for electrical resistivity at temperatures less than 2K. This work was supported by grants from the NSF and DOE. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S41.00010: Theory of Ferromagnetic Transition in One-Dimensional Itinerant Electron Systems Kun Yang Ferromagnetic transitions in itinerant electron systems are among the very first examples of quantum phase transitions studied theoretically. In the Hertz-Millis approach, one decouples the electron-electron interaction using Hubbard-Strotonovish transformation, integrates out the fermionic degrees of freedom, and arrives at a free energy functional that involves the ferromagnetic order parameter only. It has been pointed out recently that the procedure of integrating out gapless fermions may lead to subtle singularities in the bosonic free energy functional, which may complicate the analysis of the theory or even invalidate this approach. In this work we offer an alternative approach of obtaining a bosonic Ginsburg-Landau-Wilson theory that describes ferromagnetic transition in one-dimension. Our approach is based on Abelian bosonization, which allows for a derivation the effective field theory without integrating out fermions. The resultant theory is shown to have dynamical exponent z=2 at tree level and upper critical dimension 2. Thus one dimension is below the upper critical dimension of the theory, and the critical behavior of the transition is controlled by an interacting fixed point, which we study via epsilon expansion. Comparisons will be made with the Hertz-Millis theory, and possible generalizations to high dimensions will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S41.00011: Orbitally driven behavior: Mott transition, quantum oscillations and colossal magnetoresistance in bilayered Ca$_3$Ru$_2$O$_7$ X.N. Lin, V. Durairaj, S. Chikara, E. Elhami, G. Cao, L. Balicas, P. Schlottmann, J.E. Crow We report recent transport and thermodynamic experiments over a wide range of temperatures for the Mott-like system Ca$_{3}$Ru$_{2}$O$_{7}$ at high magnetic fields, $B(\le $ 30 T). This work reveals a rich and highly anisotropic phase diagram, where applying $B$ along the $a$-, $b$-, and $c$-axis leads to vastly different behavior. A fully spin-polarized state via a first-order metamagnetic transition is obtained for $B \quad \ge $ 6 T and $B\vert \vert a$, and colossal magnetoresistance is seen for $B\vert \vert b$, and quantum oscillations in the resistivity are observed for $B\vert \vert c$, respectively. In addition, both magnetic and transport properties of the system are highly sensitive to oxygen content and other impurity doping. The orbital ordering is considered to be the driving force for the rich phase diagram. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S41.00012: Strongly-correlated crystal-field approach to heavy-fermion compounds and to 3d oxides Ryszard Radwanski, Zofia Ropka The description of electronic and magnetic properties of real compounds like LaMnO$_{3}$, LaCoO$_{3}$, Na$_{2}$V$_{3}$O$_{7}$, FeO, NdAl$_{2}$ and ErNi$_{5}$ as well as heavy-fermion superconductor UPd$_{2}$Al$_{3}$ and heavy-fermion metal YbRh$_{2}$Si$_{2}$, both zero-temperature ground state properties and thermodynamics, will be presented pointing out the existence of a discrete atomic-like low-energy, in the meV scale, electronic structure. This low-energy many-electron discrete atomic-like electronic structure is governed by very strong electron correlations, predominantly on-site, by the intra-atomic spin-orbit coupling and by details of the local surrounding (crystal-field interactions), but later is modified by inter-site interactions. Our studies indicate that there is the highest time to ``unquench'' the orbital moment in solid state physics in description of 3d-/4f-/5f-atom containing compounds and that heavy-fermion phenomena are of the relativistic origin. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S41.00013: High-temperature weak ferromagnetism on the verge of a metallic state: Impact of dilute Sr doping on BaIrO3 Esmat Elhami, Gang Cao, Xinu Lin, Shalinee Chikara, Vinobalan Durairaj The 5d-electron-based BaIrO$_{3 }$is a nonmetalic weak ferromagnet with a Curie temperature at T$_{C}$= 175 K. Its greatly extended orbitals generate strong electron-lattice coupling, and the magnetism and electronic structure are thus critically linked to the lattice degree of freedom. Here we report results of our transport and magnetic study on slightly Sr-doped BaIrO$_{3 }$. It is found that dilute Sr doping drastically suppresses T$_{C}$, and instantaneously leads to a nonmetal-metal transition at high temperatures. All results highlight the instability of the ground state and the subtle relation between magnetic ordering and electron mobility. It is clear that BaIrO$_{3}$ along with very few other systems represent a class of materials where the magnetic and transport properties can effectively be tuned by slight alterations in lattice parameters. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S41.00014: Role of Strong Correlations in Disproportionation of Aqueous Actinides Steven E. Horowitz, J.B. Marston We study the role of strong electronic correlations in the disproportionation of aqueous actinide complexes An(aq) and AnO$_2$(aq) where An = U, Np, and Pu. Correlations are expected to be important due to the localized nature of the actinide 5f orbitals. We first confirm that relativisitic DFT\footnote{ADF2004.01, \urllink{SCM}{http://www.scm.com}, Theoretical Chemistry, Vrije Universiteit.}, despite yielding reasonable geometries and bond lengths, fails to reproduce\footnote{P. J. Hay, R. L. Martin, and G. Schreckenbach, J. Phys. Chem. A {\bf 104}, 6259 (2000).} experimentally observed degeneracies of the redox potentials\footnote{D. L. Clark in {\it Los Alamos Science} No. 26 Vol. II (2000).}. By using a continuum model for the water beyond the first solvation sphere we are able to construct and diagonalize reduced Hubbard-like models of the actinide complexes, and incorporate the missing physics of strong intra-atomic Coulomb repulsion\footnote{M. X. LaBute {\it et al.}, J. Chem. Phys. {\bf 116}, 3681 (2002).} \footnote{D. V. Efremov {\it et al.}, \urllink{cond-mat/0303414}{http://arxiv.org/abs/cond-mat/?0303414}; E. Runge {\it et al.}, \urllink{cond-mat/0402124}{http://arxiv.org/abs/cond-mat/?0402124}.}. [Preview Abstract] |
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S41.00015: On the Mott Transition in Ca$_{2-x}$Sr$_x$RuO$_4$ Ansgar Liebsch The Mott transition in multi-band materials like Ca$_{2-x}$Sr$_x$RuO$_4$ involving subbands of different widths is studied within the dynamical mean field theory [1]. Using the multi-orbital Quantum Monte Carlo method and iterated perturbation theory for the quantum impurity problem it is shown that at low temperatures inter-orbital Coulomb interactions give rise to a single first-order transition rather than a sequence of orbital selective transitions. Recent photoemission data [2] seem to confirm these results. Nevertheless, at finite temperatures, the degree of metallic or insulating behavior of the subbands differs greatly. Thus, on the metallic side of the transition, the narrow band can exhibit quasi-insulating features, whereas on the insulating side the wide band exhibits pronounced bad-metal behavior. The transition is therefore partially incomplete for individual subbands. This complexity of the quasi-particle spectra, and the difficulty of clearly identifying metallic and insulating properties at finite temperatures, presumably is the origin of contradictions between several previous works. The role of Hund's rule exchange interactions will also be discussed. \\ {[1]} A. Liebsch, Phys. Rev. B {\bf 70}, 165103 (2004); Phys. Rev. Lett. {\bf 91}, 226401 (2003).\\ {[2]} S.-C. Wang {\it et al.}, Phys. Rev Lett. {\bf 93}, 117007 (2004). [Preview Abstract] |
Session S42: Artificially Structured or Self-Assembled Magnetic Materials - II
Sponsoring Units: GMAGChair: Satish Ogale, University of Maryland
Room: LACC 150B
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S42.00001: GMAG Dissertation Award: Exploring new magnetic properties in coupled magnetic nanostructures Invited Speaker: One of the basic building blocks in constructing magnetic nanostructures is a magnetic sandwich in which two ferromagnetic layers are separated by a nanometer thick nonmagnetic spacer layer. Research on coupled magnetic sandwiches has generated fruitful results such as the oscillatory magnetic interlayer coupling and giant magneto resistance. The basic question on this subject is: how does the interlayer coupling generate new magnetic properties of the sandwich? In order to single out the effect of interlayer coupling, it is necessary to measure the two magnetic layers separately. Such experimental capability becomes available after the development of x-ray magnetic circular dichroism (XMCD) technique which enables element-specific measurement. In particular, Photoemission Electron Microscopy (PEEM) technique allows the element-specific magnetic domain imaging of magnetic nanostructures. In this talk, I will discuss our recent effort in using PEEM to study coupled magnetic sandwiches. First, I will discuss results of Co/Cu/Ni and Co/Fe/Ni in which we studied the effect of interlayer coupling on the magnetic phase transitions of the Co and Ni films. We found that a coupled magnetic sandwich undergoes three types of magnetic phase transitions, depending on the two ferromagnetic films' thickness. The differences and characteristics among these three phase transitions will be discussed with the simple simulation based on an Ising model. Second, spin reorientation of coupled sandwich will be discussed. We reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling. In the last, I will discuss results of FeMn/Co in which the magnetic frustration between the antiferromagnetic FeMn and ferromagnetic Co modifies the Curie and Neel temperatures of the system. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S42.00002: Oscillatory Interlayer Coupling in Co/Pt Multilayers with Perpendicular Anisotropy Jacob Knepper, Fengyuan Yang Oscillatory interlayer coupling in ferromagnet/nonmagnet multilayers has been observed in many materials with the exception of only a few metals, including Pt. Recently, Co/Pt multilayers have attracted great attention because of the perpendicular anisotropy. However, the mechanism of the magnetic coupling in Co/Pt multilayers remains unknown. To our knowledge, oscillatory interlayer coupling has only been observed in multilayers with in-plane anisotropy. We investigated the interlayer coupling in Co(0.4nm)/Pt(0-8nm) multilayers with perpendicular anisotropy and repetition from 5 to 30 made by UHV sputtering. Hysteresis loops were measured between 8 and 293 K. The interlayer coupling is always ferromagnetic, which can be readily detected by coercivity with perpendicular anisotropy. The coercivity shows an oscillatory behavior with the Pt thickness for all multilayers. The period of the oscillation is 3 nm at 80 K. The oscillation of coercivity is a clear indication of the oscillatory interlayer coupling of Co across Pt. This is the first report of oscillatory interlayer coupling in Pt and in multilayers with perpendicular anisotropy. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S42.00003: Antiferromagnetic coupling in soft amorphous ferromagnet/semiconductor multilayers Maria Velez, C. Quiros, J.I. Martin, L. Zarate, J.M. Alameda Antiferromagnetic coupling between ferromagnetic layers separated by nonmagnetic metallic interlayers has been intensively studied due to the fundamental and technological interest in such behaviour. In this work, the presence of antiferromagnetic (AF) coupling has been investigated in multilayers where the nonmagnetic interlayers are not metallic but semiconducting. The analyzed samples are amorphous (Co$_{x}$Si$_ {1-x})_{5 nm}$ /(Si)$_{d}$ multilayers obtained by co-sputtering on Si substrates, and the Si layer thickness has been varied in the range 1 nm $<$ d $<$ 15 nm. X-ray diffraction analysis has shown that the multilayered structure is well defined. The individual (Co$_{x}$Si$_{1-x})_{5 nm}$ ferromagnetic layer presents an uniaxial anisotropy and a soft magnetic behaviour (with coercivity smaller than 1 Oe for fields applied along its easy axis), being suitable to detect the possible AF coupling in the multilayer. Magneto-optical kerr effect and alternating gradient magnetometry measurements have revealed that these multilayers do present AF coupling at room temperature for d $<$ 8 nm. Moreover, the magnetic field required to switch between antiparallel and parallel configurations is as low as 3 Oe and varies slightly with the Si layer thickness [1]. \newpage [1] C. Quiros et al., Phys. Rev. B (in press) [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S42.00004: XMCD studies of antiferromagnetically coupled Co/Pt Multilayers A. Baruth, S. Adenwalla, D.J. Keavney Previous results on multilayered structures of [Pt(5{\AA})/Co(4{\AA})]$_{3}$/NiO(t$_{NiO}${\AA}) /[Co(4{\AA})/Pt(5{\AA})]$_{3}$ show exchange coupling between the two Co/Pt layers as well as exchange bias between the Co and NiO below 200K [1]. The exchange coupling is explained through the canting of AFM NiO spins which were theoretically predicted [2] and seen using X-ray Magnetic Circular Dichroism [3]. Using XMCD we have studied the element specific magnetization of Co and NiO as functions of field and temperature (above and below the blocking temperature, 200K) in two samples with 11{\AA} and 12{\AA} NiO. At these thicknesses of NiO, both sets of Co/Pt multilayers couple antiferromagnetically, but the coupling strength for the 12{\AA} NiO sample is approximately half that of the 11{\AA}. Element specific hysteresis loops showed identical behavior for both Co and Ni implying that the AFM NiO spins at the interface cant in the direction of the Co magnetization. Photoemission electron microscope images on a virgin sample at room temperature revealed the exact correlation between FM domains in the Co and NiO layers in the strongest antiferromagnetically coupled sample. We plan to measure the AFM domain structure of NiO using Magnetic Linear Dichroism. [1] Phys. Rev. Lett. 91, 037207 (2003) [2] Phys. Rev. Lett. 92, 219703 (2004) [3] Z.Y. Liu et. al. Phys Rev B (accepted) Funded by NSF MRSEC [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S42.00005: Spin structure in exchange-biased epitaxial ferromagnetic oxide bilayers Xianglin Ke, Land Belenky, Chang-Beom Eom, Mark Rzchowski We investigate the spin structure of exchange-biased ferromagnetic bilayers for varying thicknesses near the critical thickness of the biasing layer. The epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO)/ SrRuO$_{3}$ (SRO) ferromagnetic oxide bilayers were grown on (001) SrTiO$_{3}$ single-crystal substrates by pulsed laser deposition with atomic-layer control. We find a $\sim $2Ênm critical thickness of the biasing layer for the disappearance of exchange bias. The antiferromagnetic interfacial exchange permits manipulation of the frozen-in spin structure, and the ferromagnetism of each layer enables direct magnetization measurements. From these measurements, we infer the thickness dependence of the spin structure of the biasing layer in terms of domain walls perpendicular and parallel to the bilayer interface. We find that parallel domain walls can be frozen into the biasing layer for thicknesses near the critical thickness. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S42.00006: Growth and characterization of tunable BSTO/BaM multilayers as substrates for magnetic nanoparticles N. A. Frey, R. Heindl, S. Srinath, H. Srikanth, K. R. Coffey, N. J. Dudney Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for multifunctional applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al$_{2}$O$_{3}$ and Si/SiO$_{2}$ substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed distinct interfaces between BSTO and BaM layers. Magnetization measurements taken with a Physical Properties Measurement System (PPMS) showed a large coercivity ($\sim $2000 Oe) consistent with the hard magnetic hexaferrite component. The hysteresis loops also revealed the influence of magnetocrystalline and shape anisotropies at different temperature ranges. The multilayer structures can be made even more versatile with the inclusion of magnetic nanoparticles to help achieve a greater degree of tunability and frequency agility under applied electric and magnetic fields. Fe$_{3}$O$_{4}$ nanoparticles were deposited on the multilayer film surface using the Langmuir-Blodgett technique. Images of the resulting structures will be presented. Work at USF supported by DARPA/ARO through Grant No. DAAD 19-03-1-0277 [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S42.00007: Temperature Dependent Magnetization Reversal of [Co/Pt]/Ru Multilayers Olav Hellwig, Joseph E. Davies, Kai Liu, Eric E. Fullerton Magnetization reversal in antiferromagetically coupled [Co/Pt]/Ru multilayers have been shown to be either laterally or vertically correlated, depending on the layer thicknesses.$^{1}$ Here we report on our investigation of the magnetization reversal as a function of temperature using the First Order Reversal Curve (FORC) method.$^{2}$ At high temperatures the vertically correlated reversal dominates, and we observe the reversal behavior similar to that of just a Co/Pt multilayer.$^{2}$ At low temperatures the effect of antiferromagnetic interlayer coupling, and consequently laterally correlated reversal, becomes more prominent. At intermediate temperatures the two modes coexist and can be tuned by the applied magnetic field, leading to exotic reversal behavior where the FORCs could exist outside of the major hysteresis loop. This is due to the delicate balance among the magnetostatic, domain wall, and interlayer exchange coupling energies. \newline \newline $^{1}$O. Hellwig, et al., Nature Mat. \textbf{2}, 112 (2003). \newline $^{2}$ J. E. Davies, et al., Phys. Rev. B \textbf{70}, (22), Dec. 1$^{st}$ (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S42.00008: Growth and structure-property correlations in perpendicular exchange biased magnetic multilayers Xiaosong Ji, Honglyoul Ju, Kannan Krishnan Si/Pt$_{200\mbox{{\AA}}}$/(Co$_{6\mbox{{\AA}}}$/Pt$_{20\mbox{{\AA}}})_{\times 5}$/FeMn/Pt$_{20\mbox{{\AA}}}$ multilayers with strong perpendicular anisotropy and large exchange bias field were deposited by ion-beam sputtering. It was found that the microstructure and magnetic properties were greatly affected by the growth parameters such as different substrates, buffer layer thickness, multilayer thicknesses, number of bilayers, substrate temperatures, and most critically, by the ion-beam energies which was studied for the first time. The structure-property correlations of the multilayers, as a function of ion beam energy and other sputtering parameters, were investigated with emphasis on the role of interdiffusion at the interfaces. By comparing the structural properties of the multilayers deposited with different ion-beam energies, especially the degree of interdiffusion measured by x-ray reflectivity, with the magnetic properties, we show that lower ion-beam energy deposition has stronger perpendicular anisotropy and exchange bias field due to the better control of interdiffusion. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S42.00009: Diffraction MOKE on multilayer magnetic nanodisks Kristen Buchanan, Marcos Grimsditch, Konstantin Guslienko, Samuel Bader, Valentyn Novosad Diffraction magneto-optical Kerr effect magnetometry (DMOKE) is an excellent tool for investigating magnetization reversal in complex systems as it provides additional information not contained in bulk hysteresis measurements. The hysteresis loops measured on the diffraction beams can be understood in terms of field dependent magnetic form factors. Here the DMOKE technique was employed to investigate the magnetization reversal process in stacked ferromagnetic nano-disks, separated by a non-magnetic layer. The disks interact strongly via magnetostatic interactions and also via interlayer exchange coupling governed by the thickness and composition of the spacer layer. Micromagnetic simulations indicate that the disks will each support vortices of opposite chirality at remanence and reverse through coordinated nucleation, displacement, and annihilation of vortices when interlayer exchange is important. For thin, well separated disks, however, the magnetostatic interactions can be comparable to the vortex nucleation field and the reversal is quite different. Diffraction hysteresis loops for Permalloy (Ni$_{80} $ Fe$_{20}$ ) dots with thicknesses of up to 40 nm and radii of 250-1250 nm separated by a Cu spacer (1-20 nm) will be compared with loops calculated from micromagnetic simulations. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S42.00010: Magnetic stripe melting and metastable bubble domains in Fe/Ni/Cu(001) J. Choi, C. Won, Y.Z. Wu, T. Owens, J. Wu, A. Scholl, A. Doran, W. Kim, X.F. Jin, Z.Q.Qiu Qiu Spin reorientation transition (SRT) of Fe/Ni/Cu(001) was investigated using photoemission electron microscopy (PEEM). Stripe domains were imaged as a function of temperature to monitor the domain melting process. We found that the stripe domains melt in a narrow thickness range of the SRT region. This result indicates that the Curie temperature at the SRT point is lowered by the reduction of the magnetic anisotropy. In addition to the stripe domains, we observed a metastable phase of magnetic bubble domains in the SRT region, which may suggests the importance of higher order magnetic anisotropy in the SRT. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S42.00011: Tunable Thermal Hysteresis in CoNi/Gd Nanolayers Maria R. Hossu, Sezen Demirtas, Ali R. Koymen, Robert E. Camley For the first time we present experimental results proving that artificial ferrimagnetic multilayers show magnetic thermal hysteresis in the total magnetic moment. CoNi/Gd multilayers, a typical ferrimagnetic system, grown by dc magnetron sputtering has \textbf{bow-tie} shaped magnetic hysteresis with temperature over a wide range of layer thicknesses and external magnetic fields. The magnetic phase transition occurs at different temperatures during the heating and cooling cycles. Our results show that we can control the width of the magnetic thermal hysteresis loop over a temperature range of 90K. These results are in good agreement with theoretical calculations. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S42.00012: Field cooling dependence of the anisotropy in exchange biased FeF$_{2}$/Co films A.K. Alsmadi, S.G.E. te Velthuis, Hongtao Shi, David Lederman Using polarized neutron reflectometry we have studied the magnetization reversal in exchange biased single-crystal FeF$_{2} $/Co films grown on MgF$_{2}$. A recent study showed that the anisotropy of the antiferromagnetic FeF$_{2}$ plays an important role in determining the magnitude and effective direction of the exchange bias field H$_{E}$[1]. After field cooling perpendicular to the c-axis (easy axis of FeF$_{2}$), the magnetization curve determined with the applied field parallel to the c-axis, shows a double loop, one with positive bias, and one with negative bias. This behavior suggests that the antiferromagnet is simply split into two types of domains, inducing opposite H$_{E}$ along the c-axis. However, our observation of spin-flip reflectivity in the field regions separating the two loops indicates that some rotation of the Co magnetization also occurs during reversal, implying that H$_{E}$ does not lie exclusively along the c-axis. \\[4pt] [1] Hongtao Shi, David Lederman, Phys. Rev. B 66, 094426 (2002). [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S42.00013: Oscillatory Exchange Bias in Fe/Cr Bilayers J.S. Parker, L. Wang, P.A. Crowell, C. Leighton We have measured the magnetization of Fe{(001),50 \AA}/Cr{(001),t$_{Cr}$(\AA)} bilayers grown on MgO(001) using the magneto-optical Kerr effect (MOKE). Samples were grown by UHV dc magnetron sputtering at 400 C with a wedge of Cr (t$_{Cr}$ = 0-1000 \AA) on top of a 50 \AA Fe layer. Structural characterization of the films was performed by grazing incidence X-ray diffraction (GIXRD), wide angle XRD, and atomic force microscopy (AFM). We observe small ($\sim$1 Oe) periodic oscillations ($\sim$70 K period) in the exchange bias field (H$_E$) as a function of temperature below the blocking temperature (T$_B$ $\sim$ 300K), which is attributed to variations in the wavelength, $\Lambda$, of the incommensurate spin density wave (SDW) spin structure in the antiferromagnetic Cr layer. Using scanning MOKE we have measured and compared the change in oscillation period for different t$_{Cr}$ in a single bilayer. We observe a peak in the coercive field as a function of temperature for thicker Cr layers. This effect may be related to the spin-flip transition between longitudinal and transverse spin density waves in the Cr layer. *Research supported by the NSF MRSEC program under DMR-0212032. [Preview Abstract] |
Session S43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides IV
Sponsoring Units: DMP GMAGChair: Sang-Wook Cheong, Rutgers University
Room: LACC 150C
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S43.00001: Density Functional Theory of Multiferroics Invited Speaker: The electronic and structural properties of complex magnetic oxides are notoriously difficult to calculate accurately because of the strong correlations between their localized, magnetic electrons. Indeed with the traditional local density approximation to density functional theory, ferromagnetic metallic behavior is often predicted for materials which are known experimentally to be antiferromagnetic insulators. This is particularly problematic in the modeling of multiferroic materials, because such spurious metallicity is fundamentally incompatible with the occurrence of ferroelectricity. Fortunately methodological advances such as the LDA+U method, and the use of self-interaction corrections, now allow the reliable calculation of multiferroic phenomena without significant increase in computational cost. Here we review the utility of such modern density functional methods in explaining and predicting multiferroic behavior. We describe the elucidation of ferroelectric mechanisms that are compatible with magnetism, the successful prediction and subsequent synthesis of new multiferroics, and some recent computational explorations of magneto-electric switching. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S43.00002: High magnetic field phase diagram of multiferroic DyMn2O5 up to 45 T K. H. Kim, T. H. Kim, S. Y. Haam, N. Hur, S. Park, S.-W. Cheong, Y. H. Jo, J.-G. Park, A. Migliori Strong magnetoelectric coupling in multiferroic crystals such as ReMn$_{2}$O$_{5}$ (Re=rare earth) has provided unprecedented opportunity to manipulate ferroelectric (FE) polarization using magnetic fields. Most of investigations to dates have yet been limited to a rather low magnetic field region. Herein, we present the first high magnetic field (B) versus temperature phase diagram of DyMn2O5 in magnetic fields up to 45 T and temperatures below 50 K, determined from the dielectric constant, pyroelectric, and magnetoelectric current measurements using various magnets; superconducting magnets up to 17 T, a dc resistive magnet up to 33 T, and a mid-pulse magnet up to 45 T. Our phase diagram reveals that at least 4 different kinds of FE domains, which show strong temperature-and field-history-dependence, develop at low temperatures below 40 K, and exhibit dramatic evolution under B. For example, as B increases at 4 K, FE polarization shows successive flopping at B~2, 7 T and 22 T, producing large dielectric constant changes, $\Delta$$\epsilon$(B)/$\epsilon$(0T) $\sim$5, 70, 20 \%, respectively. We discuss the complex phase diagram in the context of strong spin- lattice coupling that is linked to the exchange interaction between Dy f- and Mn d-spins. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S43.00003: Magnetic Phase Diagram of the Giant Magnetoelectric, DyMn2O5 W. Ratcliff II, V. Kiryukhin, M.A. Kenzelmann, S.-H. Lee, Ross Erwin, N. Hur, S. Park, S.-W. Cheong It has been recently found that DyMn$_{2}$O$_{5}$ develops a spontaneous electric polarization below the Neel ordering temperature. Furthermore, this spontaneous polarization can be switched through the application of a magnetic field. ~Several anomalies were also observed in the dielectric constant. ~We have performed neutron diffraction measurements on a single crystal of this material, and have found that anomalies in the dielectric constant and the polarization are correlated with magnetic transitions induced by field or temperature. ~During this talk, I will present the magnetic phase diagram of this system and show how it corresponds to the spontaneous polarization and anomalies in the dielectric constant. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S43.00004: Evolution of ferroelectric and antiferromagnetic phases of TbMn$_{2}$O$_{5}$ under high magnetic fields up to 45 T S.Y. Haam, T.H. Kim, K.H. Kim, S. Park, N. Hur, S.-W. Cheong, A. Migliori Recent discovery of ferroelectric (FE) polarization reversal/imprint actuated by an external magnetic field in multiferroic TbMn$_{2}$O$_{5}$ has opened up promising device application potentials such as magnetically-recorded ferroelectric memory [1]. For better understanding of the interplay between magnetism and ferroelectricity in the multiferroic, we determined high field vs temperature phase diagram of TbMn$_{2}$O$_{5}$ from dielectric constant, pyroelectric current and magnetoelectric current measurements (along b axis) under static or pulsed magnetic field (B) (along a axis) up to 45 T. Our results reveal that (1) as B increases, negative FE polarization phase coined with the Mn d-spin reorientation transition below T=25 K at B=0 T expands its region in temperature to merge into the main FE phase boundary below T$\sim$38 K and B$\sim$20 T (2) low temperature positive FE polarization phase stabilized with the ferromagnetic order of Tb f-spin survive up to 25 K under B$\sim$4T. [1] N. Hur et al., Nature 429, 392 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S43.00005: Magnetic Order and Spin Dynamics in Multiferroic HoMnO$_{3}$ and Related Systems Invited Speaker: Hexagonal HoMnO$_{3}$ is a c-axis ferroelectric (T$_{C}$=875 K) that couples to the antiferromagnetism at 72 K (1). The S=2 Mn$^{3+}$ ions occupy a frustrated triangular lattice, with the spins forming a non-collinear 120$^{\circ}$ magnetic structure from 72 to 40 K, and then undergoing a spin-flop transition to another 120$^{\circ}$ Mn spin structure below. The spin wave dispersion relations are well described by a two-dimensional nearest-neighbor Heisenberg model with exchange J=2.44 meV, and an anisotropy D that is 0.093 meV above the spin reorientation transition at 40 K, increasing to 0.126 meV below. For H$\Vert$c the phase diagram has been determined, and reveals a re-entrant phase boundary for the structure below 40 K, and additional hysteretic transitions below the magnetic ordering temperature of 8 K for the holmium spins. The effects of an applied electric field in the magnetically ordered phases will be discussed. We also briefly describe the effects of A-site chemical disorder on the ferromagnetic phase transition and spin dynamics of the La$_{1-x}$Ba$_{x}$MnO$_{3}$ perovskite (2). The dramatically reduced Curie temperature of the disordered system primarily originates from the enhanced polaron formation that truncates the ferromagnetic state, rather than a reduction in the exchange. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides. \\ (1) Work in collaboration with O. P Vajk, M. Kenzelmann, S. B. Kim and S.-W. Cheong\\ (2) Work in collaboration with T. Sato and B. Dabrowski\\ Work at UM supported by NSF-MRSEC, DMR 00-80008. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S43.00006: Ferroelectricity and magnetism in the hexagonal manganite YMnO$_3$ from first principles Craig Fennie, Karin Rabe The hexagonal manganites are a class of multiferroic materials that are simultaneously ferroelectric and antiferromagnetic. Here, we describe a first-principles study of the structural energetics and polarization in magnetic YMnO$_3$, with the LSDA+U as implemented in VASP. For selected collinear magnetically ordered structures, the lowest symmetry-allowed terms in the Taylor expansion of the energy as a function of zone-center and zone-boundary distortions are identified and computed. The implications for the phase transitions in YMnO$_3$ will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S43.00007: Large magneto-dielectric coupling in orthorhombic YMnO$_3$ and HoMnO$_3$ Bernd Lorenz, Y. Q. Wang, Y. Y. Sun, C. W. Chu We have found a remarkable increase (up to 60 {\%}) of the dielectric constant with the onset of magnetic order at 42 K in the metastable orthorhombic structures of YMnO$_{3}$ and HoMnO$_{3}$ that proves the existence of a strong magneto-dielectric coupling in the compounds. Magnetic, dielectric, and thermodynamic properties show distinct anomalies at the onset of the incommensurate magnetic order and thermal hysteresis effects are observed around the lock-in transition temperature at which the incommensurate magnetic order locks into a temperature independent wave vector. The orders of Mn$^{3+}$ spins and Ho$^{3+}$ moments both contribute to the magneto-dielectric coupling. A large magneto-dielectric effect was observed in HoMnO$_{3}$ at low temperature where the dielectric constant can be tuned by an external magnetic field resulting in a decrease of up to 8 {\%} at 7 Tesla. By comparing data for YMnO$_{3}$ and HoMnO$_{3}$ the contributions to the coupling between the dielectric response and Mn and Ho magnetic orders is separated. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S43.00008: Giant spin-lattice coupling in magnetic oxides with rare earths Sang-Wook Cheong, N. Hur, S. Park, S. Guha, A. Borissov, V. Kiryukhin Recently, the astonishing interplay between magnetic and lattice properties has been discovered in magnetic oxides with rare earths, including multiferroic Tb(Dy)MnO$_{3}$, Tb(Dy)Mn$_{2}$O$_{5}$, HoMnO$_{3} $. The discovered effects include reversible flipping of polarization (magnetization) by applied magnetic (electric) field, and the giant change of dielectric constant in applied magnetic field. We have investigated other magnetic oxides with various rare earths in order to reveal new phenomena of spin-lattice coupling as well as to find out the exact role of rare earths in the coupling. The results of our extensive investigation will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S43.00009: Spectral origin of the $c$-axis dielectric constant anomalies in hexagonal HoMnO$_3$ Andrei Sushkov, H. Dennis Drew, Sang-Wook Cheong The coupling between ferroelectric (P) and magnetic (M) order parameters in multiferroics is one of the most important problems both for basic understanding and applications. If this coupling in hexa-manganites occurs within a unit cell there should be a phonon, infrared-active in $E||c$ polarization, which modulates simultaneously $P_c$ and Mn--Mn superexchange in the $ab$-plane. The dielectric constant anomalies, observed at magnetic ordering/re-ordering temperatures, indicate existence of a spin-coupled phonon(s). To find the linking phonon we have measured temperature dependence of the $c$-axis infrared phonon spectrum of a hexagonal HoMnO$_3$ single crystal. These optical results and the dielectric anomalies will be discussed in terms of the coupling between the two order parameters in the multiferroic hexa-manganites. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S43.00010: Search for the soft mode phonons in TbMnO$_3$, TbMn$_2$O$_5$, and DyMn$_2$O$_5$ multiferroics using Raman and Far Infrared Transmission Spectroscopy A.A. Sirenko, S. Park, N. Hur, S-W. Cheong, C. Ulrich, L. Machtoub, B. Keimer, G.L. Carr One of the possible explanations of the anomalies in the dielectric properties of TbMnO$_3$ and TbMn$_2$O$_5$ multiferroics [Nature 426, 55 (2003) and Nature 429, 392 (2004)] is the phonon softening. Raman spectra of the optical phonons in TbMnO$_3$, TbMn$_2$O$_5$, and DyMn$_2$O$_5$ single crystals have been measured in the temperature range between 4 and 300 K and in the magnetic field up to 13 T. Different scattering configurations and orientations of magnetic field with respect to the crystallographic directions have been investigated. Structural phase transition in both TbMn$_2$O$_5$ and DyMn$_2$O$_5$ was found at T=150 K. FT-IR transmission spectra in TbMnO$_3$ have been measured in the frequency range between 20 and 250 cm$^{-1}$ and in the temperature range between 10 and 300 K in magnetic field up to 11 T oriented along the c- axis. TbMnO$_3$ single crystal is transparent below 120 cm$^{-1} $. No clear indication for the phonon softening has been found so far. [Preview Abstract] |
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S43.00011: Lamellar phase separation and dynamic competition in La0.23Ca0.77MnO3 Jing Tao, Dario Niebieskikwiat, Myron Salamon, Jian-Min (Jim) Zuo Lamellar charge ordered (CO) and charge disordered (CD) phase separation at low temperatures in La0.23Ca0.77MnO3 was firstly revealed using transmission electron microscopy (TEM). The in-situ TEM observations showed the evolution of the CO-CD phase separation with changed temperatures. Electron diffraction found the different crystal structures of CO and CD phases. Magnetism measurements in La0.23Ca0.77MnO3 indicate two magnetic phase transitions that interestingly coincide with two transition temperatures found in the resistivity relaxation curves. The TEM dark field images observed the dynamic competition between CO and CD phases and the observation is consistent with that in transport properties. The finding here suggests that the CO-CD phase coexistence in this manganite at certain temperature range is driven by magnetoelastic effects rather than being dictated by disorder. [Preview Abstract] |
Session S44: Magnetic and Force Microscopy
Sponsoring Units: GIMSChair: Mark Hersam, Northwestern University
Room: LACC 518
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S44.00001: Detection and Manipulation of the Statistical Fluctuations in Nuclear Spin Ensembles Using Magnetic Resonance Force Microscopy H.J. Mamin, R. Budakian, B.W. Chui, D. Rugar We have detected and manipulated the naturally occurring $\sqrt N $ statistical polarization in nuclear spin ensembles using MRFM. We have studied fluorine nuclei in CaF$_{2}$, as well as protons in the polymer PMMA and the protein collagen. The ensembles studied contained of order 10$^{8}$ nuclear spins, corresponding to volumes of order (200nm)$^{3}$, which resulted in statistical polarizations of order 10$^{4}$ net spins. We have also implemented a scheme similar to one proposed by Weitekamp \textit{et al}, in which we suppressed the effect of the statistical uncertainty so as to extract meaningful information from time-averaged measurements. In this way, we have successfully made nutation and transverse spin relaxation measurements in a nominally unpolarized sample of CaF$_{2}$ at low temperatures. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S44.00002: Force-detected ESR from E$^\prime$ centers Palash Banerjee, Yulu Che, K.C. Fong, Tim Mewes, Vidya Bhallamudi, Denis V. Pelekhov, P. Chris Hammel Magnetic Resonance Force Microscopy (MRFM) is a novel technique that combines magnetic resonance with scanned probe techniques. We report on low temperature force-detected electron spin resonance (ESR) signals from $E^\prime$-centers in fused silica (SiO$_2$). By utilizing the high gradients close to a micron sized SmCo ferromagnetic tip mounted on an AFM cantilever, spin resonance was observed from a sub- micron thick ``sensitive slice'' whose location can be varied with respect to the sample surface. E$^\prime$ centers at low temperatures ($<$ 10 K) are characterized by long spin-lattice relaxation times T$_1$ approaching a few seconds. The spins were adiabatically inverted at the appropriate frequency by means of microwave FM techniques and T$_1$ was studied as a function of field gradient. We also discuss the sensitivity of the microscope and ongoing efforts to improve it. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S44.00003: MRFM and non-contact friction experiments under UHV conditons S. Rast, U. Gysin, P. Ruff, C. Wehrle, H.-R. Hidber, E. Meyer, P. Vettiger, M. Despont, C. Gerber, D.W. Lee In this contribuiton we present magnetic resonance force microscopy (MRFM ) experiments and non-contact friction force experiments made on $\gamma $-irradiated quarz samples under UHV condtions and low temperatures. Ultrasensitve cantilevers with integrated magnetic tips allow us to detect forces in the order of 10$^{-18 }$N/$\surd $Hz [1]. The measurements showed that the sensitivity did not change significantly in a homogenous magnetic field. In quarz samples long-range, non contact friction forces are observerd. A spin-lifetime of 137 ms was measured by cyclic inversion of the spins. The measurement was achieved with an ultrasensitve PLL using the OSCAR protocol. [1] Gysin et al., Phys. Rev. B \textbf{69}, 045403 (2004) [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S44.00004: Tip design and tip-sample interaction in magnetic resonance force microscopy Sean Garner, Seppe Kuehn, John Marohn Magnetic resonance force microscopy (MRFM) is a three-dimensional, subsurface imaging technique which registers the presence of sample spins via the deflection, or change in mechanical resonance frequency, of a magnet-tipped cantilever. At single-nuclear-spin sensitivity, MRFM would have numerous exciting applications, such as imaging of single biomolecules or spin-state readout for solid-state quantum computing. We have previously reported unprecedented sensitivity in nuclear MRFM, and are currently improving our sensitivity by attacking two remaining technical challenges: producing usable nanomagnetic tips, and learning to control excess cantilever energy dissipation to the sample surface. We will discuss our recent results in these areas and our latest MRFM results. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S44.00005: NMR Force Microscopy on an hcp Co single crystal Yuri Obukhov, Denis Pelekhov, P. Chris Hammel We discuss the implementation of NMR Force Microscope setup designed and built for NMR study of ferromagnetic thin films. The details of data collection and analysis as well as the design of RF and Magnetic Field subsystems are presented. As an example of subsurface NMR investigation we present the preliminary data obtained from an hcp Co single crystal followed by the discussion of sensitivity and spatial resolution of the method. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S44.00006: Magnetic Resonance Force Microscopy and Force-Detected NMR of Microcrystals J. T. Markert, A. D. Cambou, J.-H. Choi, S. Guchhait, Y. J. Lee, W. Lu, U. M. Mirsaidov We report our advances in nuclear magentic resonance force microscopy (NMRFM) and NMR of microcrystals using force (mechanical oscillator) detection. For each, we report microfabrication of sensitive single-crystal-silicon multiple-torsional micro-oscillators using both optical and e-beam lithography and a back-etch technique. We characterize mechanical oscillator frequency, quality factor, and spring constant from the noise spectral density of oscillator motion, detected using fiber-optic interferometry. We review past work on scanning-mode detection of the NMR response from volumes as small as 2 $\mu$m$^3$ at room temperature. We primarily discuss progress in two experiments currently underway: 1) the study of $^1$H dynamics in submicron-thick metal hydride films, where the NMRFM technique permits selective response to motion-modulated dipolar interactions with correlation times from microseconds to seconds, and 2) detection of the $^{11}$B resonance in microcrystals over the temperature range $4\ {\rm K} < T < 300{\rm K}$. We also overview measurements made in our $^3$He low-temperature NMRFM system. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S44.00007: Development of a New Generation of Piezoresistive Cantilevers Designed for Torque Magnetometry Stefan Kohout Torque magnetometry is known to be a very sensitive technique to measure the magnetization of anisotropic magnetic materials. Applying the piezoresistive technology pushed forward by atomic force microscopy made extremely small and powerful torquemeters possible. However, commercially available cantilevers, which are optimized for force measurements, are not well suited for torque measurements. Cantilevers specially designed for torque magnetometry greatly improved the performance of such sensors and sensitivities of the order of $10^{-14}$\,Nm are obtained in a dynamic operation mode [1]. Based on this work we present here a new generation of torque sensors with greatly improved performance [2]. Together with our recently improved software for the automatic control of our torque magnetometer this offers new possibilities of performing systematic studies of magnetic phenomena with high resolution. The power of this new device will be demonstrated by magnetization studies in cuprate superconductors. [1] M. Willemin et al., J. Appl. Phys. 83, 1163 (1998) [2] S. Kohout et al., in preparation [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S44.00008: Ultra-Sensitive Micromechanical Cantilevers with Integrated Magnetic Structures Michelle D. Chabot, John M. Moreland, Lan Gao, Sy-Hwang Liou, Casey W. Miller We report on the design, fabrication, and implementation of ultra-sensitive micromechanical oscillators. These novel devices have been developed for use as cantilever magnetometers and as force sensors in nuclear magnetic resonance force microscopy. Our single-crystal silicon cantilevers with integrated magnetic structures are fabricated using a novel process in which magnetic film patterning and deposition are combined in a nondestructive manner with cantilever fabrication. Current magnetic moment sensitivity achieved for the devices, when used as magnetometers, is 10$^{-15}$ J/T at room temperature. Finite element modeling was used for several different cantilever geometries to improve design parameters, ensure that the devices meet experimental demands, and correlate mode shape with observed results. Post-fabrication focused-ion-beam milling was used to further pattern the integrated magnetic structures when nanometer scale dimensions were required. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S44.00009: Zeptogram Scale Nanomechanical Mass Sensing Y.T. Yang, Carlo Callegari, X.L. Feng, K.L. Ekinci, M.L. Roukes We show very high frequency nanoelectromechanical systems (NEMS) that provide a profound sensitivity increase for inertial mass sensing into zeptogram-scale. Measurement and analysis from our still unoptimized experiments already demonstrate mass sensitivity at the level of 7 zg, the mass of an individual 4 kDa molecule or 30 xenon atoms. Implication of the detailed analysis of the ultimate sensitivity of such devices based on experimental results is especially compelling: they indicate NEMS can ultimately provide inertial mass sensing of individual electrically neutral macromolecules with single Dalton sensitivity. The scheme has been employed to study noise arising from adsorption desorption of xenon on the NEMS surface. We also anticipate this will offer an unprecedented opportunity for many interesting applications in surface science, atomic physics and biology. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S44.00010: Toward single-molecule nanomechanical mass spectrometry W.K. Hiebert, X.L. Feng, M.L. Roukes Nanoelectromechanical systems (NEMS) offer immense potential for high-sensitivity applications in sensor technology. In mass sensitivity, recent reports have logged dramatic progress with milestones at the level of, first, single femtogram, then single attogram\footnote{ M. L. Roukes and K. L. Ekinci, U. S. Patent 6,722,200 (filed: 4 May 2001, granted: 20 April 2004); see also Appl.Phys.Lett. \textbf{84}, 4469 (2004).}, and most recently few zeptogram\footnote{ Y. T. Yang, Carlo Callegari, X. L. Feng, Kamil L. Ekinci, and M. L. Roukes, ``Zeptogram Scale Nanomechanical Mass Sensing,'' this meeting.} -- pushing the state of the art to over a billion times the sensitivity of commercially-available mass sensors. It is now conceivable ``to weigh'' single macromolecules of viruses and proteins, simply by accreting them one-by-one onto a NEMS device\footnote{ K. L. Ekinci, Y. T. Yang, and M. L. Roukes, ``Ultimate limits to inertial mass sensing based upon nanoelectromechanical systems,'' J. Appl. Phys. \textbf{95}, 2682 (2004).}. When achieved, the ability to weigh single molecules may provide a tranformationally different core sensing mechanism and a new niche platform for mass spectrometry. The experimental approach underway at California Institute of Technology to achieve this measurement milestone will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S44.00011: On recent developments for high-speed AFM imaging Georg Schitter, Georg E. Fantner, Johannes H. Kindt, Paul K. Hansma This contribution discusses some key challenges for the next generation of high-speed atomic force microscopes (AFM). For high-speed imaging all AFM components have to be optimized in performance, i.e. the scanning unit, the force sensor and the AFM electronics. i) The three dimensional positioner (scanner) needs not only sub-nanometer resolution and a high bandwidth but also must not show any oscillatory behavior in order to achieve the required position accuracy. To this end a new mechanical design that uses stack piezos for the spatial movement is combined with new control schemes [1] that are designed for highest positioning bandwidth combined with high control performance. ii) The force sensor has to be soft and fast in order to minimize the imaging forces and force variations together with a higher sensor bandwidth. This is achieved by using small cantilevers [2]. iii) For high-speed imaging also the feedback and piezo drive electronics as well as the data acquisition system have to fulfill high bandwidth and timing requirements. Combining all these improvements, the next generation of AFMs will enabling imaging speeds two orders of magnitudes faster than current commercial AFM systems. [1] G. Schitter, F. Allgoewer, A. Stemmer, Nanotechnology 15(1), p.108 (2004) [2] T.E Schaeffer, M. Viani, D.A. Walters, B. Drake, E.K. Runge, J.P. Cleveland, M.A. Wendman, P.K. Hansma, SPIE 3009, p.48 (1997) [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S44.00012: Measuring AFM Cantilever Spring Constants Katie Chynoweth, Mahlon Wigton, Kristine Lang Josephson-junctions are prime candidates for the realization of quantum bits. Understanding the properties of Josephson-junction materials is crucial to building a functional qubit. Conducting atomic force microscopy (CAFM), which can simultaneously measure local topography and conductance, is a promising tool for these purposes. Previous results suggest that control of the imaging force in CAFM is vital to achieve reproducible conductance images. In this talk, we discuss how control of the imaging force can be achieved by measuring the CAFM cantilever spring constant, and how this results in reproducible conductance images. The theoretical background and the experimental technique for measuring the spring constant by two distinct methods will be discussed. We present spring constant measurements from these two methods for comparison. Finally, we present our user-friendly program which measures the spring constant in situ on a Digital Instruments atomic force microscope. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S44.00013: Force Microscopy with Light Atom Probes Franz Giessibl, Stefan Hembacher, Jochen Mannhart The charge distribution in atoms with closed electron shells is spherically symmetric, while atoms with partially filled shells can form covalent bonds with pointed lobes of increased charge density. Covalent bonding in the bulk can also affect surface atoms, leading to four tiny humps spaced by less than 100 pm in the charge density of adatoms on a (001) tungsten surface. We image these charge distributions via atomic force microscopy by using a light-atom probe (a graphite atom) to directly measure high-order force derivatives of its interaction with a tungsten tip. Features with a lateral distance of only 77 pm are revealed (Science 305, 380, 2004). [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S44.00014: Coupled ion - nanomechanical systems Lin Tian, Peter Zoller The nanomechanical modes can be manipulated and probed via their coupling with effective quantum two level systems. Here we study a coupled ion - nanomechanical system where the ion is in a nanotrap with the electrodes being nanomechanical resonators. The motion of the ion and that of the nanomechanical modes can be described as coupled harmonical oscillators. The ions play the role of a quantum optical system that acts as a probe and control, and allows entanglement with or between nanomechanical resonators. We show as examples the laser cooling and the entanglement generation between the resonators [1] L. Tian and P. Zoller, quant-ph/0407020 [Preview Abstract] |
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S44.00015: Nanomagnetic Planar Magnetic Resonance Microscopy ``Lens'' Mladen Barbic, Axel Scherer The achievement of three-dimensional atomic resolution magnetic resonance microscopy remains one of the main challenges in visualization of biological molecules. The prospects for single spin microscopy have come tantalizingly close due to the recent developments in sensitive instrumentation. Despite the single spin detection capability in systems of spatially well-isolated spins, the challenge that remains is the creation of conditions in space where only a single spin is resonant and detected in the presence of other spins in its natural dense spin environment. We present a nanomagnetic planar design where a localized Angstrom-scale point in three-dimensional space is created above the nanostructure with a non-zero minimum of the magnetic field magnitude. The design thereby represents a magnetic resonance microscopy ``lens'' where potentially only a single spin located in the ``focus'' spot of structure is resonant. Despite the presence of other spins in the Angstrom-scale vicinity of the resonant spin, high gradient magnetic field of the ``lens'' renders those spins inactive in the detection process. [Preview Abstract] |
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