Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B1: New Experimental Developments in Graphene
Sponsoring Units: DCMPChair: Eva Andrei, Rutgers University
Room: Spirit of Pittsburgh Ballroom A
Monday, March 16, 2009 11:15AM - 11:51AM |
B1.00001: Graphene and its chemical derivatives Invited Speaker: Graphene is a first two-dimensional atomic crystal. In my talk I'll overview our latest results on the electronic properties of graphene, and discuss a possibility of band structure engineering by chemical modification of this material. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B1.00002: The high-field state at the Dirac Point in graphene Invited Speaker: The discovery of the quantum Hall Effect in graphene has generated considerable interest in the state at the Dirac Point in a magnetic field $H$. In intense $H$, the 4-fold degeneracy of the $n=0$ Landau Level (LL) is lifted by the enhanced exchange energy. Among the broken symmetry states proposed are the quantum Hall ferromagnet, the quantum Hall insulator state, excitonic condensation, and charge-density-wave formation. A subset of these theories propose counter-propagating edge states that remain conducting in large $H$. We have performed measurements of the resistance $R_{xx}$ and Hall resistance $R_{xy}$ to fields of 33 T at temperatures $T$ from 0.3 to 50 K in $\sim$6 graphene samples. We find that, as $T$ decreases below 10 K, $R_0$ (= $R_{xx}$ at the Dirac Point) undergoes a steep increase with a divergence consistent with a field-driven transition to an insulating high-field state. The divergence in $R_0$ fits well to the Kosterlitz-Thouless (KT) form $\exp(b/\sqrt{h-1})$ with $h=H/H_c$ and $b\sim 1.4$. The critical field $H_c$ is sample dependent (12 T to 33 T ), and correlates with the disorder as measured by the offset gate voltage $V_0$ and the zero-$H$ mobility. The divergence in $R_0$ is strictly confined to the $n=0$ LL (bracketed by the sublevels $\nu = \pm 1$). The peaks with $n=\pm 1$ remain near the values $h/e^2$. Using an ultralow-power (3 fW), voltage-regulated technique, we show that the KT-fit to $R_0$ is valid over 3 decades (40 k$\Omega$ to 40 M$\Omega$). The steepness of the $R_0$ vs. $T$ curves implies a bulk gap $\Delta$ of magnitude 15-20 K that decreases when $H$ falls below $H_c$. We compare our findings with the various proposed models. We will also report thermopower and Nernst measurements taken to fields of 14 T.\\[4pt] Refs. J. G. Checkelsky, L. Li and N. P. Ong, prl {\bf100}, 206801 (2008); ibid. cond-mat arXiv:0808.0906v1. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B1.00003: Quantum Carrier Collimation in Locally Gated Graphene Heterojunction Devices Invited Speaker: While electron optics such as lensing and focusing have been demonstrated experimentally, building a collimated electron interferometer in two unconfined dimensions has remained a challenge due to the difficulty of creating electrostatic barriers that are sharp on the order of the electron wavelength. Owing to the suppression of backscattering experienced by the chiral quasiparticles, graphene provides an ideal medium to realize the quantum engineering of electron wave functions. In this presentation, we show our progresses in wave function engineering in graphene devices by demonstrating the conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. Analysis of the oscillations confirms that bipolar heterojunctions have a collimating effect on ballistically transmitted carriers. The robustness of the oscillatory conductance to scattering provides a novel probe of the ballistic physics of graphene at the Dirac point and makes graphene heterojunctions a promising tool for the coherent manipulation of carriers in mesoscopic systems. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B1.00004: Robustness of quantum Hall effect in locally gated graphene devices Invited Speaker: Two-terminal conductance of locally gated graphene {\it p-n-p} heterojunctions in the quantum Hall regime is quantized at integer and fractional values, owing to the edge states equilibration. We study the sensitivity of this quantization to finite longitudinal conductivity in the locally gated inner region of the junction. Taking a bulk conductivity approach, we solve spatially non-uniform conduction problem exactly by a conformal mapping method. We find that the robustness of the conductance quantization strongly depends on the geometry of the locally gated region, as well as on the densities in the inner and outer regions. We present a detailed comparison of our predictions with recent experimental data, finding good agreement. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B1.00005: Octet Quantum Hall Effect in Graphene Bilayers Invited Speaker: Interaction driven integer quantum Hall effects are anticipated [1]in graphene bilayers because of the near-degeneracy of eight Landau levels which appear near the neutral system Fermi level at filling factors between $\nu $=-4 and $\nu $=4. The bilayer graphene octet exhibtits a wide variety of broken symmetry states, with Ising, XY and Heisenberg character which can be controlled by an external field which creates an electric potential difference between the two layers. Because of the peculiarities of the bilayer graphene electronic structure states with n=0 and n=1 orbital character are degenerate. I will explain predictions that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate k$^{3/2}$ dispersion. This talk will be based on work performed in collaboration with Yafis Barlas, Rene Cote, Kentaro Nomura, and Jules Lambert. \\[4pt] [1] Y. Barlas \textit{et al. }, Phys. Rev. Lett. \textbf{101}, 097601(2008). [Preview Abstract] |
Session B2: HTSC: Fermi Pockets and Quantum Oscillations
Sponsoring Units: DCMPChair: Robert Laughlin, Stanford University
Room: Spirit of Pittsburgh Ballroom BC
Monday, March 16, 2009 11:15AM - 11:51AM |
B2.00001: Competing order, Fermi surface reconstruction, and quantum oscillations in high temperature superconductors Invited Speaker: Recent quantum oscillation measurements in underdoped high temperature superconductors in high magnetic fields and low temperatures have ushered in a new era. These experiments appear to explore the normal state from which superconductivity arises and provide evidence of a reconstructed Fermi surface consisting of electron and hole pockets in a regime in which such a possibility was previously considered to be remote. Here we explain the observations with the theory that the alleged normal state exhibits a hidden order, the $d$-density wave. The success of our analysis underscores the importance of spontaneous breaking of symmetries, Fermi surface reconstruction, and quasiparticles. We primarily focus on the version of the order that is commensurate with the underlying crystalline lattice, but also touch upon the consequence of incommensuration. In addition, the effect of possible bilayer splitting and the nature of quantum oscillations in the mixed state are addressed. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B2.00002: Quantum Oscillations in underdoped YBCO: the nature of Fermi surface reconstruction and evolution toward the Mott insulating regime Invited Speaker: I will present results of quantum oscillation measurements we have measured in underdoped YBCO$_{6+x}$ that reveal a small Fermi surface. Results of angular dependent measurements enable us to obtain clues as to the nature of order that reconstructs the Fermi surface. I will also present results of doping-dependent measurements that enable us to trace Fermi surface evolution as the insulating regime is approached. \vspace{4 mm} Work was performed in collaboration with N. Harrison, G. G. Lonzarich, C. Mielke, R. Liang, D. Bonn and W. Hardy [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B2.00003: Fermi Arcs or Fermi Pockets Invited Speaker: In the pseudogap phase of the cuprate superconductors, a significant portion of the Fermi surface is still gapped at temperatures above the transition temperature Tc. Instead of a closed Fermi surface, the low-energy electronic excitations appear to form unconnected Fermi arcs separated by gapped regions. It is generally assumed that the spectral function is particle-hole symmetric (at low energies) in both regions - with a peak at the Fermi level on the Fermi arcs centered around the nodes and a local minimum at the Fermi level in the gapped regions away from the arcs. Using high resolution angle-resolved photoemission and new methods of analysis, we show that on a sizable portion of the Fermi surface, including the Fermi arcs, the electronic structure in the immediate vicinity of the Fermi level is not particle-hole symmetric in the pseudogap phase. This is clear evidence that superconducting pairing does not originate from the Fermi arcs. The observations are also consistent with the possibility that the Fermi arcs are in fact the inner surface of the predicted Fermi pockets. This work was carried out in collaboration with Hongbo Yang, Jon Rameau, Tonica Valla, Alexei Tsvelik and Genda Gu and was supported by the Department of Energy. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B2.00004: Field-Induced Quantum Critical Route to a Fermi Liquid in Overdoped Tl$_2$Ba$_2$CuO$_{6+x}$ Invited Speaker: In high temperature superconductivity, charge doping is a natural tuning parameter that takes copper oxides from the antiferromagnet through the superconducting `dome'-shaped region. In the metallic state above $T_{\rm c}$ the standard Landau's Fermi-liquid theory of metals, as typified by the temperature squared ($AT^2$) dependence of resistivity, appears to break down. The expected recovery of the usual Fermi-liquid metal on the high doping side is fundamental but ill understood. Here we uncover a new transformation in an overdoped superconducting copper oxide Tl$_2$Ba$_2$CuO$_{6+x}$ from the non-Fermi to a Fermi-liquid state driven by magnetic field [1]. From the $c$-axis resistivity measured up to 45~T, we show that the Fermi-liquid $AT^2$ features, accompanied by a field-linear magnetoresistance, appear above a field $H_{\rm FL}$. This crossover field $H_{\rm FL}$ decreases linearly with decreasing temperature $T$ and lands at a quantum critical point (QCP) near the upper critical field $H_{\rm c2}(0)$. The Fermi-liquid coefficient $A(H)$ shows a power-law diverging behavior on the approach to the QCP, indicating the second-order quantum phase transition at this field. The connection between the field-induced QCP and the pseudogap observed in the underdoped regime will be discussed. \\ \\ \noindent [1] T. Shibauchi {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 105}, 7120 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B2.00005: Fermi surface reconstruction in high-$T_{c}$ superconductors Invited Speaker: The recent observation of quantum oscillations in underdoped high-$T_{c}$ superconductors (1), combined with their negative Hall coefficient at low temperature (2), reveals that the Fermi surface of hole-doped cuprates includes a small electron pocket. This strongly suggests that the large hole Fermi surface characteristic of the overdoped regime undergoes a reconstruction caused by the onset of some order which breaks translational symmetry. Here we consider the possibility that this order is ``stripe'' order, a form of combined charge / spin modulation observed most clearly in materials like Eu- doped and Nd-doped LSCO. In these materials, the onset of stripe order coincides with major changes in transport properties (3), providing strong evidence that stripe order is indeed the cause of Fermi-surface reconstruction. We identify the critical doping where this reconstruction occurs and show that the temperature dependence of transport coefficients at that doping is typical of metals at a quantum critical point (4). We discuss an interpretation of the pseudogap as a fluctuating precursor of the stripe-ordered phase.\\ \\ This work was performed in collaboration with L. Balicas, D.A. Bonn, J. Chang, O. Cyr-Choini\`{e}re, R. Daou, N. Doiron- Leyraud, W.N. Hardy, N.E. Hussey, F. Lalibert\'{e}, D. LeBoeuf, S.Y. Li, R. Liang, C. Proust, H. Takagi, and J.S. Zhou.\\ \\ (1) N. Doiron-Leyraud \textit{et al}., Nature \textbf{447}, 565 (2007).\\ (2) D. LeBoeuf \textit{et al}., Nature \textbf{450}, 533 (2007).\\ (3) R. Daou \textit{et al}., Nature Physics, in press (DOI 10.1038/nphys1109); http://arXiv.org/abs/0806.2881.\\ (4) R. Daou \textit{et al}., to be published; http://arXiv.org/abs/0810.4280. [Preview Abstract] |
Session B3: 10,000 Undergraduate Physics Majors: Progress on Doubling
Sponsoring Units: FEdChair: Robert Hilborn, University of Texas at Dallas
Room: 301/302
Monday, March 16, 2009 11:15AM - 11:51AM |
B3.00001: Why Do We Need 10,000 Physics Majors? Invited Speaker: The early 1960's saw a huge increase in the number of physics majors, reaching an all time peak of just over 6000 per year. While the number plummeted in the next four decades, it has finally experienced a resurgence to nearly this number. The American Physical Society along with the American Association of Physics Teachers recently endorsed a call to double the number of undergraduate physics majors over the next decade. The main focus of this effort is to increase both the number of high school physics teachers and the fraction of women and under-represented minorities studying physics. In addition, a physics degree prepares an undergraduate with excellent skills that will serve her or him for a variety of occupations both in the sciences and in other fields. This talk will explore some of the data on physics majors and the rationale for taking the bold step of suggesting we try and educate 10,000 majors each year. Sputnik helped catalyze the nation 50 years ago -- What is the Sputnik of today? Bring your thoughts and questions... we hope for a lively discussion. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B3.00002: Successful Minority PhD Producing Programs -- Bell Laboratories and the Meyerhoff Scholarship Program at UMBC Invited Speaker: The Bell Labs Cooperative Research Fellowship Program for Minorities (CRFP), founded in 1972 was one of the first programs of its kind in the US to address the issue of under-representation of minorities in the fields of engineering, mathematics and science. As of 2000, well over 100 PhDs graduated with CRFP sponsorship and a significant fraction joined the research ranks of Bell Labs. In the early days of the program as much as 50{\%} of African American PhDs in Physics in the US were granted to students supported by CRFP. Another unique program initiated by Bell Labs in 1974 that introduced undergraduate students to cutting edge research was the Summer Research Program for Minorities and Women (SRP). The SRP served as a natural feeder to the CRFP. Personally, my career in Optical Physics owes its foundation to these programs and I will give my perspective on participation and impact of the Bell Labs SRP (1974) and CRFP (1975) programs. The Meyerhoff Scholars Program at UMBC was developed in 1988. At that time, UMBC was graduating fewer than 18 African-American STEM majors per year. In 1996 the program was opened to all students with an interest in the advancement of minorities in STEM fields. The program enjoys an overall 18-year retention rate of greater than 95{\%} and has over 500 graduates since 1993. As of May 2006, 75{\%} of these graduates are enrolled in graduate and/or professional programs, with 49 PhDs and 20 MD/PhDs completed as of August 2006. The program challenges notions about minority achievement. Meyerhoff Scholars have changed the perceptions of those around them -- the expectations of faculty who instruct them, the attitudes of students who learn beside them, and the perspectives of scientists who engage them in research. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B3.00003: Best Practices for Recruiting and Retaining Women in Physics Invited Speaker: |
Monday, March 16, 2009 1:03PM - 1:39PM |
B3.00004: Doubling the number of physics majors who teach Invited Speaker: The American Physical Society has adopted a doubling initiative to increase the number of physics majors. One of the main motivations is to increase the number of physics majors certified to teach secondary physics. I will review some of the possible strategies for reaching this goal, and discuss some of the steps we have taken with UTeach, the program for secondary science and mathematics teacher preparation at The University of Texas at Austin.I will discuss the roles of curriculum revision, financial support, and community support in convincing majors to teach. Finally, I will talk about the expansion of UTeach into engineering. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B3.00005: Integrating Research Experiences into the Undergraduate Education Invited Speaker: During the last seven years Michigan State University has been able to increase the number of physics and astrophysics majors by more than a factor of two. Part of this increase can be attributed to introducing special first-year courses on computational physics and on laboratory techniques, designed exclusively for physics majors. Investing into strengthening the Society of Physics Students and Science Theatre and into increased outreach activities also plays a role. But the largest effect is due to integrating a wide variety of research experiences into the Michigan State undergraduate physics and astrophysics experience. An overview of these activities will be given, and ways to upscale these efforts will be discussed. [Preview Abstract] |
Session B4: Polymer Translocation
Sponsoring Units: DPOLYChair: Murugappan Muthukumar, University of Massachusetts
Room: 306/307
Monday, March 16, 2009 11:15AM - 11:51AM |
B4.00001: Polymer Translocation: What Can We Learn From An Exactly Solvable One-Dimensional Model? Invited Speaker: The translocation of a polymer through a narrow hole or channel is generally not a quasi-static process (as we have shown using a detailed Molecular Dynamics simulation with explicit solvent). Nevertheless, numerous analytical models have relied on this approximation/assumption in order to make progress. A simple approach is then to describe the problem in terms of the translocation coordinate (e.g., the number of monomer on the \textit{trans} side of the wall), which effectively makes it a one-dimensional problem with an external driving field and a entropy-related potential landscape. Our group has exploited this simple idea to its fullest using a lattice Monte-Carlo-like model that provides exact numerical results, even for extremely rare events. In this presentation, I will explain how this simplified model is built and how it can be modified to include a variety of additional effects such as polymer stiffness or the differences between the various monomer types in a biopolymer like DNA. I will review the main results obtained to date, focusing on the transitions between the low- and high- field regimes, and between the short- and long- polymer chain limits. Finally, I will examine the role of attractive interactions between the polymer and specific sites inside the channel. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B4.00002: DNA translocation through small channels and pores from molecular models. Hydrodynamic, electrostatic, and hybridization considerations. Invited Speaker: The flow and translocation of long DNA molecules are of considerable applied and fundamental interest. Design of effective genomic devices requires control of molecular shape and positioning at the level of microns and nanometers, and understanding the manner in which DNA is packaged into small channels and cavities is of interest to biology and medicine. This presentation will present an overview of hierarchical models and computational approaches developed by our research group to investigate the effects of confinement, hydrodynamic interactions, and salt concentration, on the structure and properties of DNA, both at equilibrium and beyond equilibrium. The talk will include a discussion of coarse grain descriptions of the flow of DNA in microfluidic and nanofluidic channels over multiple length and time scales, and a discussion of emerging, detailed models that are capable of describing melting and rehybridization at the single nucleotide level, as well as the packaging of DNA into viral capsids and small pores. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B4.00003: Simulation studies of DNA translocation through a nanopore ($^\dagger$) Invited Speaker: The experimental studies of voltage driven translocation of a single stranded DNA through a $\alpha$-hemolysin pore, have stimulated a lot of activities as the phenomenon is rich in fundamental science involved and its prospective technical applications for detecting DNA/RNA sequences. While it is the attributes of heteropolymer translocation that are the key ingredients for prospective new sequencing methods, these experiments have generated stimulating theoretical and numerical studies directed toward a seemingly much simpler problem of homopolymer translocation through a nanopore. The earlier theoretical work of Muthukumar, Sung and Park, and by Kardar and his collaboartors$^2$ have been supplemented by more recent theoretical work by Dubbledam \textit{et. al} and Panja \textit{et. al}$^3$. During this talk I will show results from Langevin dynamics simulation carried out on a coarse-garined bead-spring model of DNA-polymer both for the unbiased and driven translocation$^4$. During the first part of the talk, after a brief review of the current theories of DNA translocation, specifically mentioning the underlying assumptions, I will compare simulation results with those predicted by different theories. Particularly, I will show numerical results for the translocation exponent $\alpha$ defined as $\langle \tau \rangle \sim N^\alpha$ and the exponent for the $s$-coordinate $\beta$ defined as $\langle s^2(\tau) \rangle \sim \tau^\beta$, and discuss how the numerical values differ as one chooses slightly different pore width and geometry. In the second part of my talk I show how a model \textit{attractive nanopore} can distinguish the sequence of a heteropolymer$^4$ and discuss possibility of making a device based on this idea. \\ $^\dagger${work done in collaboration with Kaifu Luo, Tapio Ala-Nissila, See-chen Yin, Andrey Milchev and Kurt Binder}\\ $^1$J. J. Kasianowiczs, E. Brandin, D. Branton and D. W. Deamer, \textit{Proc. Natl. Acad. Sci. U.S.A.} {\bf 93}, 13770 (1996).\\ $^2$W. Sung and P. J. Park, \textit{Phys. Rev. Lett.} {\bf 77}, 783 (1996); M. Muthukumar, \textit{J. Chem. Phys.} {\bf 111}, 10371 (1999); J. Chuang, Y. Kantor and M. Kardar, \textit{Phys. Rev. E} {\bf 65}, 011802 (2001); Y. Kantor and M. Kardar, \textit{Phys. Rev. E} {\bf 69}, 021806 (2004).\\ $^3$J. L. A. Dubbeldam, A. Milchev, V. G. Rostiashvili, and T. A. Vilgis, \textit{Phys. Rev. E} {\bf 76}, 010801(R) (2007); \textit{Europhys. Lett.} {\bf 79}, 18002 (2007); D. Panja, G. T. Barkema, and R. C. Ball, \textit{J. Phys.: Condens. Matter} {\bf 20}, 075101 (2008); H. Vocks, D. Panja, G. T. Barkema, and R. C. Ball, \textit{J. Phys.: Condens. Matter} {\bf 20}, 095224 (2008). \\ $^4$ K. F. Luo, I. Huopaniemi, T. Ala-Nissila, P. Pomorski, M. Karttunen, S. C. Ying, and A. Bhattacharya, \textit{Phys. Rev. E} {\bf }, 050901(R) (2008); A. Bhattacharya, H. Morrison, K. F. Luo, T. Ala-Nissila, S. C. Ying, A. Milchev, and K. Binder, arXiv:0808.1868v3 (2008). \\ $^5$ K. F. Luo, T. Ala-Nissila, S. C. Ying, and A. Bhattacharya, \textit{J. Chem. Phys.} {\bf 126}, 145101 (2007), \textit{Phys. Rev. Lett.} {\bf 99}, 148102 (2007), \textit{Phys. Rev. Lett.} {\bf 99}, 058101 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B4.00004: Anomalous dynamics of polymer translocation Invited Speaker: We study the passage times of a translocating polymer of length $N$ in three dimensions, while it passes through a narrow pore. We show that the behavior of the polymer stems from the polymer dynamics at the immediate vicinity of the pore --- in particular, the memory effects in the polymer chain tension imbalance across the pore. We take as a reaction coordinate the number $s$ of the monomer residing in the pore. in the case of unbiased translocation, these memory effects cause the mobility of $s$ to be anomalous diffusion for times up to the Rouse time $N\sim N^{1+2\nu}$ or Zimm time $N\sim N^{3\nu}$, without or with hydrodynamics, respectively. Here, $\nu$ is the Flory exponent. Beyond this time, the dynamics becomes ordinary diffusion. As a consequence, the pore blockade time scales with length as $\tau_d \sim N^{2+\nu}$. If a force of sufficient strength is pulling on one end, the pore blockade time scales as $\tau_d \sim N^2$ in the absence of hydrodynamics. If a voltage is applied across the pore, which drives the charged polymer, the pore blockade time scales as $\tau_d \sim N^{(1+2\nu)/(1+\nu)}$ without, and $\tau_d \sim N^{3\nu/(1+\nu)}$ with hydrodynamics. In these cases, the pore blockade time decreases inversely with force and field strength, respectively. Our theoretical framework is substantiated with high-precision computer simulations. We will show that memory effects similar to those governing translocation, also play a role in the dynamics of dense polymer solutions and polymer melts. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B4.00005: Polyelectrolyte Translocation Murugappan Muthukumar Theoretical considerations of relative contributions from electrostatic and entropic barriers will be addressed for the phenomenon of polymer translocation through alpha-hemolysin pores. [Preview Abstract] |
Session B5: Heterogeneous Integration on Silicon
Sponsoring Units: FIAPChair: Ganesh Samudra, National University of Singapore
Room: 401/402
Monday, March 16, 2009 11:15AM - 11:51AM |
B5.00001: Scaling Properties of High Performance Ge-Si$_x$Ge$_{1-x}$ Core-Shell Nanowire Field Effect Transistors Invited Speaker: Semiconductor nanowires (NWs), namely highly anisotropic crystals with diameters of the order of a few tens of nanometers, have received increased interest recently as a platform for electronics devices, motivated in part by issues associated with end of the roadmap of complementary metal-oxide-semiconductor (CMOS) device scaling. The performance advantage of such devices stems from superior electrostatic properties compared to planar devices, which in turn help increase their on-state current and on/off-state current ratio. We present recent results on the growth and fabrication of a few key NW device structures, which can potentially outperform conventional CMOS devices. By combing axial Ge NW growth, via the vapor-liquid-solid mechanism, with conformal Si$_x$Ge$_{1-x}$ growth by ultra-high-vacuum chemical vapor deposition, we demonstrate Ge-SixGe1-x core-shell NW heterostructures. Transmission electron microscopy combined with energy dispersive X-ray spectroscopy show that the Si$_x$Ge$_{1-x}$ shell can be grown in-situ, epitaxial onto the Ge NW core, and that the Si/Ge shell content can be tuned depending on the growth conditions, effectively enabling band engineering in these one dimensional nanowire heterostructures. A key component in fabrication high performance nanowire field effect transistors, namely high on-state current and high on/off-state current ratio, is the fabrication of low resistance, unipolar contacts to a semiconductor nanowire. Using low energy ion implantation we demonstrate dual-gated Ge-Si$_x$Ge$_{1-x}$ core-shell nanowire field effect transistors with highly doped source and drain. We discuss the scaling properties as a function of channel length, and intrinsic carrier mobility in these devices. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B5.00002: Integration of CNTs with Silicon Invited Speaker: In this talk, the growth and characterisation of both single and multi wall CNTs is described and a realistic appraisal of the future of CNTs in the electronics field will be provided. Although they are less likely, in the author's opinion, to take over from silicon for use in the active devices such as transistors and diodes etc. in logic circuits their use in vias and interconnects in next generation integrated circuits is considered as being entirely feasible as is their use in transparent conducting contacts. Another major contribution to future electronics could be in complementary applications to CMOS such as their use in sensors, thermal interface materials and solder joints. A novel Liquid Crystal Over Silicon (LCOS) structure will also be discussed. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B5.00003: Heterogeneous Integration of Materials on Si for Nanophotonics Devices Invited Speaker: Optical interconnects are attractive candidates for achieving communication bandwidth well beyond terabit-per-second for high-performance multi-core microprocessors. Silicon has become a desirable material due to its transparency in the infrared wavelength range and the ease for integrating optical devices at the vicinity of CMOS circuitry utilizing standard processes. While state-of-the-art patterning techniques provide precise dimension control as well as pattern placement, standard doping and metallization steps enable utilization of phenomena such as carrier injection and depletion to render the devices tunable. As a result, large progress has been made on Si-based nanophotonic devices such as modulators, switches, and wavelength division multiplexing (WDM) systems [1, 2]. To make photodetectors, however, a heterogeneous integration of other materials that absorb light in the infrared is necessary. Available in standard front-end CMOS processes for gate strain engineering, Germanium is suitable due to its high absorption coefficient at 1.3$\mu $m and 1.5$\mu $m wavelengths. Thus, Ge can be directly integrated into the process to fabricate compact photodetectors simultaneously with amplifier circuits in order to make a receiver for an optical network. Nevertheless, the integration of Ge photodetector into the CMOS process flow is very challenging due to process complexity and severe temperature constraints; as a result, photodetectors fabricated only after completing the front-end processes have been previously demonstrated. This talk will discuss Ge waveguide photodetectors that have been integrated into the front-end before the activation of CMOS well implants. By utilizing a lateral seeded crystallization method wherein the Ge waveguides are melted during high-temperature dopant activation, 20$\mu $m-long single-crystal Ge-on-insulator waveguides were formed. This approach eliminates the need for selective epitaxial growth of Ge, and avoids high-density misfit dislocations formed due to lattice mismatch when growing Ge on Si substrate. The photodetectors operate at low applied bias voltages (0.5-1V) with bandwidth exceeding 40GHz. \\[4pt] [1] W.M.J. Green et al, ``Ultra-compact, low RF power, 10 Gb/s silicon Mach--Zehnder modulator,'' Opt. Express 15, 17106 (2007).\\[0pt] [2] Y. A. Vlasov et al, ``High-throughput Silicon Nanophotonic wavelength-insensitive switch for On-chip Optical Networks,'' Nature Photonics 2, 242 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B5.00004: Beyond Moore's Law: Heterogeneous Integration of III-N Semiconductors and Si CMOS Electronics Invited Speaker: Moore's law has been one of the main drivers behind the unprecedented development of semiconductors in the last forty years. However, this economical and technological paradigm that has helped to create modern Si electronics is now jeopardizing its future. Traditional Si scaling is not only becoming unaffordable, but the performance improvement due to scaling is diminishing. Our group is working on an approach different from Moore's law to increase the performance of electronics: the heterogeneous integration of different semiconductor materials on the same wafer. In this paper, we describe our work on the seamless integration of GaN-based devices and Si electronics. While Si electronics has shown unsurpassed levels of scaling and circuit complexity, nitride semiconductors offer excellent optoelectronics and high frequency/power electronic properties. The ability to combine these two material systems in the same chip and in extremely close proximity would allow unprecedented flexibility for advanced applications. Using wafer bonding technology, virtual Si (001) / GaN / Si (001) substrates have been fabricated for the first time. Due to the high thermal stability of GaN, Si CMOS electronics can be processed in this new substrates without affecting the nitride layers underneath the surface. After the Si devices are fabricated, the Si material is removed from the regions where nitride devices are needed. Then, the nitride devices (transistors, LEDs, lasers or sensors) are processed at room temperature and, finally, an interconnection layer forms the final hybrid circuits. Using this new technology several hybrid circuits are currently being developed, including high power differential amplifiers and normally-off power transistors. These advanced circuits are just a few examples of the potential of heterogeneous integration and how the close integration of Si and other materials enables a vast array of new exciting opportunities for electronics. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B5.00005: Integration of Ferroelectrics, Ferromagnets, and Multiferroics with Silicon Invited Speaker: In this talk I will describe the epitaxial integration of ferroelectrics, ferromagnets, and materials that are both at the same time, with silicon. Until recently, ``oxide'' could only mean one thing to anyone working in the semiconductor industry---SiO$_{2}$. But oxides are an exciting class of electronic materials in their own right. Oxides exhibit the full spectrum of electronic, optical, and magnetic behavior including many functionalities not found in conventional semiconductors. Further, such oxides can be combined epitaxially not only with each other, but epitaxially with the workhorse of semiconductor technology, silicon, enabling the unparalleled variety of physical properties of oxides to be exploited in new ways for electronic applications. The specific oxides that my collaborators* and I have integrated epitaxially with silicon include EuO, ZnO, CaTiO$_{3}$, SrTiO$_{3}$, BaTiO$_{3}$, BiFeO$_{3}$, Pb(Zr,Ti)O$_{3}$, and PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-PbTiO$_{3}$. Highlights from these systems will be presented. * The work reported was performed in collaboration with the groups of Jochen Mannhart (U. Augsburg), Chang-Beom Eom (U. Wisconsin-Madison), Ramamoorthy Ramesh (Berkeley), Jeremy Levy (U. Pittsburgh), David Muller (Cornell), Xiaoqing Pan (U. Michigan), J\"{u}rgen Schubert (J\"{u}lich), Long-Qing Chen (Penn State), Susan Trolier-McKinstry (Penn State), Yves Idzerda (Montana State), Peter B\"{o}ni (TU M\"{u}nchen), Joseph Woicik (NIST), Philip Ryan (Ames), Michael Bedzyk (Northwestern), Yuri Barash (Russian Acad. Sci.), Qing Ma (Intel), and Hao Li (Motorola). [Preview Abstract] |
Session B6: Panel Discussion: Physics in Africa
Sponsoring Units: FIPChair: Abebe Kebede, North Carolina A&T University
Room: 406
Monday, March 16, 2009 11:15AM - 11:51AM |
B6.00001: On the differences between theories of conventional and high temperature superconductors Invited Speaker: After years of successful application of the pseudo and model potential representation of electronphonon interaction to conventional (BardeenCooperSchrieffer) theory of superconductivity, we have developed a generalization (called ``isosuperconductivity'') that not only explains the differences between conventional and highTc in the cuprates and other materials but has enough predictive power to permit the current search for room temperature superconductors. We present a review of more than 30 years of effort and progress report on current research activities in this field. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B6.00002: Physics in Africa: The Case of South Africa Invited Speaker: South Africa plays a special role in building science throughout Africa. The areas of science, particularly those related to physics, which are being developed, will be described together with the involvement of other African countries. Data will be presented that demonstrate the high attrition rate that exists especially in the science track PhD pipeline and highlight the bottleneck at the honors (fourth year) level. Programs designed to overcome this will be presented, and their success discussed. Thoughts on how to go about expanding the interactions between US scientists and South African scientists as well as with African scientists will be presented. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B6.00003: Physics in Africa: The Case of Senegal Invited Speaker: For many years, the research activity in Senegal has been managed through the division of the Ministry of Education. In 2004 the current government established a full-fledged Ministry of Research. This has led to a renewed focus on the organization of the funding of research in Senegal. One important issue to underline is the lack of a budget line devoted to research in most of the local institutions, distinct from support for academic purposes. As a result, the research activity is funded through direct support from the government or thanks to international programs. The main tool for the government to support key research directions is the FIRST program, with a yearly budget of \$700,000 US. For the last call for projects, up to 12 projects have been funded, which translates to about \$58,000 US per project. The other option for research funding lies in different international programs specifically aimed at institutions within the least developed countries. The dominant ones are provided by the French-speaking community, the French-supported AIRE, the European Union framework and ICTP Abdus Salam Centre. In this general context of limited resources, physics is the least supported discipline both in terms of researchers and active laboratories. As a result, particular efforts have to be made to increase the impact of physics and the role of physicists so as to enable them to claim their proper role as the major player in making science and technology the driving forces in the development process of Africa. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B6.00004: Physics in Africa: The Case of Congo Invited Speaker: We will present a review on the state of research and education in physics in the Republic of Congo, one developing country in Central Africa. Special emphasis will be placed on the School of Science of the Marien NGouabi University located in the capital, Brazzaville. We will also discuss the impact of the Physics Department within this University as a whole. One of the main problems in Africa is the serious lack of equipment to provide adequate hands-on trainings for students and for faculty to perform forefront research. To illustrate strategies for the development of Physics in the continent, we will describe some ongoing inter-continental collaborations between our university and some neighboring countries, along with some ways for expanding the framework of the interactions between US and African physicists. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B6.00005: Panel Discussion |
Session B7: Bacterial Growth Laws and Systems Biology
Sponsoring Units: DBPChair: Terence Hwa, University of California, San Diego
Room: 407
Monday, March 16, 2009 11:15AM - 11:51AM |
B7.00001: to be determined by you Invited Speaker: |
Monday, March 16, 2009 11:51AM - 12:27PM |
B7.00002: Growth laws and mechanisms of global control in bacteria Invited Speaker: The growth laws of Schaechter, Maal{\o}e and Kjeldgaard are among the most striking discoveries in bacterial growth physiology: cell composition (mass/cell, RNA/cell, etc.) is a simple function of growth rate alone -- irrespective of how that growth rate is established. I will review the growth laws, and discuss recent experiments that have uncovered new laws. A systems-level mathematical model is developed that suggests the growth laws arise from the partitioning of the protein synthesizing machinery of the cell (the ribosomes), and furthermore indicates a deep connection between growth rate control and central metabolism. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B7.00003: Growth-rate dependent effects on bacterial gene expression Invited Speaker: For fast growing bacteria, which can adapt to wildly different growth conditions, changes in gene expression are often accompanied by changes in growth rates. Because the macroscopic composition of bacteria (e.g., cell size, ribosome concentration, gene copy number) is known to vary greatly for bacteria grown at different rates, significant changes in gene expression may arise 'passively' just due to the growth rate change alone. Towards a quantitative understanding of these passive effects, we analyzed quantitatively available data for the growth rate dependence of various macroscopic parameters affecting gene expression in E. coli, and predicted the growth-rate dependence of gene expression for various simple genetic circuits. For a constitutively expressed gene, the expressed protein concentration is decreased at faster growth, while weak growth-rate dependence is obtained for autorepressing genes and genes under negative control by an autorepressor. We also studied the growth-rate dependence of bistable genetic circuits and determined conditions such that bistability is found over a wide range of growth rates. Our results demonstrate that growth-rate dependent effects play an important role and must be taken into account when analyzing gene expression data under different condition. Buffering against these growth rate dependent effects may be an important requirement underlying the robust operation of endogenous genetic circuits in these bacteria, and should be a prime factor to consider in the design of robust, synthetic circuits. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B7.00004: A Coarse-Grained Biophysical Model of \textit{E. coli} and Its Application to Perturbation of the rRNA Operon Copy Number Invited Speaker: In this work a biophysical model of \textit{Escherichia coli} is presented that predicts growth rate and an effective cellular composition from an effective, coarse-grained representation of its genome. We assume that \textit{E. coli} is in a state of balanced exponential steady-state growth, growing in a temporally and spatially constant environment, rich in resources. We apply this model to a series of past measurements, where the growth rate and rRNA-to-protein ratio have been measured for seven \textit{E. coli} strains with an rRNA operon copy number ranging from one to seven (the wild-type copy number). These experiments show that growth rate markedly decreases for strains with fewer than six copies. Using the model, we were able to reproduce these measurements. We show that the model that best fits these data suggests that the volume fraction of macromolecules inside \textit{E. coli} is not fixed when the rRNA operon copy number is varied. Moreover, the model predicts that increasing the copy number beyond seven results in a cytoplasm densely packed with ribosomes and proteins. Assuming that under such overcrowded conditions prolonged diffusion times tend to weaken binding affinities, the model predicts that growth rate will not increase substantially beyond the wild-type growth rate, as indicated by other experiments. Our model therefore suggests that changing the rRNA operon copy number of wild-type \textit{E. coli} cells growing in a constant rich environment does not substantially increase their growth rate. Other observations regarding strains with an altered rRNA operon copy number, such as nucleoid compaction and the rRNA operon feedback response, appear to be qualitatively consistent with this model. In addition, we discuss possible design principles suggested by the model and propose further experiments to test its validity. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B7.00005: Dynamics of bacterial gene regulation Invited Speaker: The phenomenon of diauxic growth is a classical problem of bacterial gene regulation. The most well studied example of this phenomenon is the glucose-lactose diauxie, which occurs because the expression of the lac operon is strongly repressed in the presence of glucose. This repression is often explained by appealing to molecular mechanisms such as cAMP activation and inducer exclusion. I will begin by analyzing data showing that these molecular mechanisms cannot explain the strong lac repression because they exert a relatively weak effect. I will then present a minimal model accounting only for enzyme induction and dilution, which yields strong repression despite the absence of catabolite repression and inducer exclusion. The model also explains the growth patterns observed in batch and continuous cultures of various bacterial strains and substrate mixtures. The talk will conclude with a discussion of the experimental evidence regarding positive feedback, the key component of the minimal model. [Preview Abstract] |
Session B8: Spectroscopy of Strongly Interacting Fermi Gases
Sponsoring Units: DAMOPChair: Erich Mueller, Cornell University
Room: 414/415
Monday, March 16, 2009 11:15AM - 11:51AM |
B8.00001: Observation of Spin-Polarons in a strongly interacting Fermi liquid Invited Speaker: We have observed spin-polarons in a highly imbalanced mixture of fermionic atoms using tomographic RF spectroscopy. Feshbach resonances allow to freely tune the interactions between the two spin states involved. A single spin down atom immersed in a Fermi sea of spin up atoms can do one of two things: For strong attraction, it can form a molecule with exactly one spin up partner, but for weaker interaction it will spread its attraction and surround itself with a collection of majority atoms. This spin down atom ``dressed'' with a spin up cloud constitutes the spin-polaron. We have observed a striking spectroscopic signature of this quasi-particle for various interaction strengths, a narrow peak in the spin down spectrum that emerges above a broad background. The narrow width signals a long lifetime of the spin-polaron, much longer than the collision rate with spin up atoms, as it must be for a proper quasi-particle. The peak position allows to directly measure the polaron energy. The broad pedestal at high energies reveals physics at short distances and is thus ``molecule-like'': It is exactly matched by the spin up spectra. The comparison with the area under the polaron peak allows to directly obtain the quasi-particle weight $Z$. We observe a smooth transition from polarons to molecules. At a critical interaction strength of $1/k_F a = 0.7$, the polaron peak vanishes and spin up and spin down spectra exactly match, signalling the formation of molecules. This is the same critical interaction strength found earlier to separate a normal Fermi mixture from a superfluid molecular Bose-Einstein condensate. The spin-polarons determine the low-temperature phase diagram of imbalanced Fermi mixtures. In principle, polarons can interact with each other and should, at low enough temperatures, form a superfluid of p-wave pairs. We will present a first indication for interactions between polarons. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B8.00002: Photoemission Spectroscopy for Ultracold Atoms Invited Speaker: We perform momentum-resolved rf spectroscopy on a Fermi gas of potassium-40 atoms in the region of the BCS-BEC crossover. This measurement is analogous to photoemission spectroscopy, which has proven to be a powerful probe of excitation gaps in superconductors. We measure the single-particle spectral function, which is a fundamental property of a strongly interacting system and is directly predicted by many-body theories. For a strongly interacting Fermi gas near the transition temperature for the superfluid state, we find evidence for a large pairing gap. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B8.00003: Competition between final-state and pairing-gap effects in the radio-frequency spectra of ultracold Fermi atoms Invited Speaker: Ultracold Fermi atoms allow the realization of the crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC), by varying with continuity the attraction between fermions of different species. In this context, radio-frequency spectroscopy provides a microscopic probe to infer the nature of fermionic pairing. In the strongly-interacting regime, this pairing affects a wide temperature range comprising the critical temperature Tc, in analogy to the pseudo-gap physics for high-temperature superconductors. By including final-state interactions affecting the excited level of the transition, calculations are here reported of radio-frequency spectra of ultracold Fermi atoms with balanced populations, both below and above Tc, and compared with available experimental data. In the superfluid phase below Tc our calculation rests on the use of the BCS-RPA approximation, while in the normal phase above Tc it includes the Azlamazov-Larkin type contribution which is familiar in the theory of ``paraconductivity'' fluctuations in superconductors, besides the density-of-states contribution. In both cases, the limit of a molecular spectrum is correctly recovered in the BEC regime of the crossover. A competition is revealed between pairing-gap effects which tend to push the oscillator strength toward high frequencies away from threshold and final-state effects which tend instead to pull the oscillator strength toward threshold. In addition, an energy scale associated with pairing is extracted from the spectra and related to a universal quantity recently introduced for Fermi gases. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B8.00004: Theory of RF Spectroscopy in the Normal And Superfluid Phases of Ultracold Fermi Gases Invited Speaker: In this talk we present an overview of radio frequency (RF) spectroscopy in the atomic Fermi superfluids, addressing both momentum integrated and momentum resolved experiments. A general purpose of these RF experiments is to extract the pairing gap size and we present several methodologies for accomplishing this. In addition, we discuss the effects of traps, population imbalance, and final state interactions over the entire range of temperatures. By comparing theory and experiment, we show how a broad range of experimental phenomena can be accomodated within the BCS-Leggett description of BCS-BEC crossover. We also briefly touch on commonalities between photoemission in the cuprate superconductors and RF spectroscopy in the ultracold gases. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B8.00005: Spin polarons, Molecules and Twin Peaks in rf spectra of Fermi gases at unitarity Invited Speaker: We examine pairing, molecule, and spin-polaron formation in strongly-interacting Fermi gases and discuss how radio-frequency (RF) spectroscopy can reveal these phenomena. For an unpolarized gas at unitarity, we show how the double-peak structures observed in recent experiments arise due to the inhomogeneity of the trapped gas. The emergence of stable molecules in the BEC regime results in a two-peak structure in the RF spectrum with clearly visible medium effects on the low-energy part of the molecular wavefunction. For the highly-imbalanced case, we show the existence of a well- defined quasiparticle (a spin polaron) on both sides of the Feshbach resonance, we evaluate its lifetime, and we illustrate how its energy may be measured by RF spectroscopy. The main experimental features observed above the critical temperature in the recent experiments are recovered with no fitting parameters. [Preview Abstract] |
Session B9: GSNP Student Speaker Award Session and Applications of Statistical and Nonlinear Physics in the Life Sciences
Sponsoring Units: GSNPChair: Harvey Gould, Clark University
Room: 303
Monday, March 16, 2009 11:15AM - 11:27AM |
B9.00001: Self-Assembly of Spherical Colloidal Particles at Low N Natalie Arkus, Vinothan Manoharan, Michael Brenner The number of rigid structures that a system of N particles can form grows exponentially with N. Stabilizing any one structure over all others is thus a challenging problem. We consider a system of N spherical colloidal particles that cannot deform or overlap, and which exhibit a short-range attractive force. We present a method, using graph theory and geometry, that solves for all possible rigid packings of N particles - the resultant set of packings is provably complete. We then present a mechanism that is capable of stabilizing any one structure over all others (in the zero temperature limit), and which is experimentally realizable - thereby, potentially allowing us to direct the self-assembly of a desired structure. We compare to preliminary experimental results. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B9.00002: Liquid to solid nucleation through onion-structure droplets Kipton Barros, William Klein We start from a Landau-Ginzburg free energy and develop a theory of crystal nucleation for metastable liquids. Saddle points of the free energy represent nucleating droplets and are obtained analytically and numerically. We find nucleating droplets with hexagonal symmetry in two dimensions and bcc and icosahedral symmetries in three dimensions. Surprisingly, we also find nucleating droplets in three dimensions with a spherically symmetric structure resembling the layers of an onion. These onion-structure objects are the preferred nucleating droplets near the spinodal. We discuss recent experiments and simulations which are consistent with our predictions. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B9.00003: Universal Scaling Relation Near Point J Thomas Haxton, Andrea Liu Recently, several studies (P. Olsson and S. Teitel. \textit{Phys. Rev. Lett.} \textbf{99}, 178001 (2007); T. Hatano. arXiv:0803.2296; L. Berthier and T. A. Witten. arXiv:0810.4405) have indicated the existence of a dynamical phase transition at or near Point J, the point at zero temperature, zero shear stress, and a critical density where repulsive amorphous sphere packings lose rigidity. However, a universal scaling relation connecting the rheology of the jammed solid to that of the viscous liquid has been lacking. We control the temperature, strain rate, and pressure in molecular dynamics simulations to show that the steady-state rheology is described by a universal scaling relation near Point J. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B9.00004: Exact results for currents in nonadiabatic stochastic pumps Jordan Horowitz Biological systems abound with examples of molecular machines: assemblies of molecules that perform specific useful mechanical tasks, such as the motor proteins kinesin and myosin. Remarkably, the first steps in developing useful artificial molecular motors have been taken with the synthesis and manipulation of molecular complexes such as catenanes and rotaxanes. These developments have spurred an interest in developing theoretical frameworks which describe these mesoscopic machines that operate in the presence of thermal noise. In this talk I will analyze a generic model of molecular machines known as stochastic pumps in which useful directed motion (or current) is produced by the variation of external parameters. The main result is an exact expression for the current in the presence of nonadiabatic pumping. This expression connects to a variety of results from the field of brownian ratchets and leads to a surprising ``no-pumping'' theorem: a set of conditions that guarantee no excess or pumped current. These predictions also agree with the observations on catenanes, interlocked ring molecules, made by Leigh et. al. [Nature, 424, 174 (2003)]. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B9.00005: A continuous time random walk description of the hopping dynamics in an aging polymer glass Mya Warren, Joerg Rottler Due to the non-equilibrium nature of the glassy state, structural relaxation becomes increasingly sluggish with the wait time $t_w$ since vitrification. As a result, dynamical correlation functions age, and often obey a simple rescaling with $t_w$: $C(t,t_w) = C_0(t) + C_{age}(t/t_w^{\mu})$. It has recently been shown that, to first order, scaling also applies to the distributions of local correlations and displacements (the van Hove function). In this study, we use molecular dynamics simulations to measure the statistics of the discontinuous hopping events that characterize structural relaxations during aging. This allows us to map the particle dynamics onto a continuous time random walk, which successfully reproduces the entire distribution of displacements. Our results bear a striking resemblance to the popular trap model of aging. We find that the hop time distribution takes the form of a power law which is independent of $t_w$, whereas the time to the first hop shifts to longer times with $t_w$. This two-timescale behavior explains not only the scaling of the distribution functions for times $t\sim t_w$, but also small deviations from perfect scaling that have been observed at longer times. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B9.00006: On the Mass Distribution of Animal Species Sidney Redner, Aaron Clauset, David Schwab We develop a simple diffusion-reaction model to account for the broad and asymmetric distribution of adult body masses for species within related taxonomic groups. The model assumes three basic evolutionary features that control body mass: (i) a fixed lower limit that is set by metabolic constraints, (ii) a species extinction risk that is a weakly increasing function of body mass, and (iii) cladogenetic diffusion, in which daughter species have a slight tendency toward larger mass. The steady-state solution for the distribution of species masses in this model can be expressed in terms of the Airy function. This solution gives mass distributions that are in good agreement with data on 4002 terrestrial mammal species from the late Quaternary and 8617 extant bird species. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B9.00007: Epidemic spread in coupled populations with seasonally varying migration rates Adam Muzyczyn, Leah B. Shaw The H5N1 strain of avian influenza has spread worldwide, and this spread may be due to seasonal migration of birds and mixing of birds from different regions in the wintering grounds. We studied a multipatch model for avian influenza with seasonally varying migration rates. The bird population was divided into two spatially distinct patches, or subpopulations. Within each patch, the disease followed the SIR (susceptible-infected-recovered) model for epidemic spread. Migration rates were varied periodically, with a net flux toward the breeding grounds during the spring and towards the wintering grounds during the fall. The case of two symmetric patches reduced to single-patch SIR dynamics. However, asymmetry in the birth and contact rates in the breeding grounds and wintering grounds led to bifurcations to longer period orbits and chaotic dynamics. We studied the bifurcation structure of the model and the phase relationships between outbreaks in the two patches. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B9.00008: Blasting and Zipping: Sequence Alignment and Mutual Information Orion Penner, Peter Grassberger, Maya Paczuski Alignment of biological sequences such as DNA, RNA or proteins is one of the most widely used tools in computational bioscience. While the accomplishments of sequence alignment algorithms are undeniable the fact remains that these algorithms are based upon heuristic scoring schemes. Therefore, these algorithms do not provide model independent and objective measures for how similar two (or more) sequences actually are. Although information theory provides such a similarity measure - the mutual information (MI) - numerous previous attempts to connect sequence alignment and information have not produced realistic estimates for the MI from a given alignment. We report on a simple and flexible approach to get robust estimates of MI from global alignments. The presented results may help establish MI as a reliable tool for evaluating the quality of global alignments, judging the relative merits of different alignment algorithms, and estimating the significance of specific alignments. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B9.00009: Competition For Resources in a Model for Protein Synthesis Larry Cook, Royce Zia The Totally Asymmetric Simple Exclusion Process (TASEP) is often used to explore translation during protein synthesis. The particles represent ribosomes that move along mRNA, which is represented by the one-dimensional lattice. Unlike ordinary TASEP where the supply of particles is unlimited, there is a finite number of ribosome in a cell. In addition, there are many genes which compete for this pool of ribosomes. Thus, we are motivated to consider the effects of multiple TASEPs (of varying lengths) coupled to a single, finite reservoir of particles. In particular, the total occupation numbers, the density profiles and the particle currents of individual TASEPs are studied, as the overall reservoir of particles is varied. Both Monte Carlo simulation results and analytic considerations will be presented. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B9.00010: Estimating currents in totally asymmetric simple exclusion process with extended particles and inhomogeneous hopping rates. R.K.P. Zia, Jiajia Dong, B. Schmittmann Motivated by translation in protein synthesis, we study the totally asymmetric simple exclusion process with extended particles transported along a 1-D lattice with (quenched) inhomogeneous hopping rates. The particles model ribosomes, the lattice models sequences of codons, and the hopping rates reflect the aa-tRNA concentrations. Taking the latter from data for real \textit{E.Coli} genes, Monte Carlo simulations allow us to find the steady state currents, associated with protein production rates. An application would be to predict the effects of ``silent mutations'' in biological systems. In such mutations, one or more codons are replaced by others which code for the \textit{same} amino-acid, so that the \textit{same} protein (amino-acid chain) is synthesized by a different sequence of codons. However, the rate of production (the overall current), which depends on the details of sequence, will differ. We aim to predict the changes in these currents for all possible silent mutations. Beyond this application, this study of ``quenched distribution of distributions'' is expected to have far reaching implications in other areas of physics. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B9.00011: An information geometric approach to least squares minimization Mark Transtrum, Benjamin Machta, James Sethna Parameter estimation by nonlinear least squares minimization is a ubiquitous problem that has an elegant geometric interpretation: all possible parameter values induce a manifold embedded within the space of data. The minimization problem is then to find the point on the manifold closest to the origin. The standard algorithm for minimizing sums of squares, the Levenberg-Marquardt algorithm, also has geometric meaning. When the standard algorithm fails to efficiently find accurate fits to the data, geometric considerations suggest improvements. Problems involving large numbers of parameters, such as often arise in biological contexts, are notoriously difficult. We suggest an algorithm based on geodesic motion that may offer improvements over the standard algorithm for a certain class of problems. \newline [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B9.00012: Large-Scale Organization of Glycosylation Networks Pan-Jun Kim, Dong-Yup Lee, Hawoong Jeong Glycosylation is a highly complex process to produce a diverse repertoire of cellular glycans that are frequently attached to proteins and lipids. Glycans participate in fundamental biological processes including molecular trafficking and clearance, cell proliferation and apoptosis, developmental biology, immune response, and pathogenesis. N-linked glycans found on proteins are formed by sequential attachments of monosaccharides with the help of a relatively small number of enzymes. Many of these enzymes can accept multiple N-linked glycans as substrates, thus generating a large number of glycan intermediates and their intermingled pathways. Motivated by the quantitative methods developed in complex network research, we investigate the large-scale organization of such N-glycosylation pathways in a mammalian cell. The uncovered results give the experimentally-testable predictions for glycosylation process, and can be applied to the engineering of therapeutic glycoproteins. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B9.00013: Network dynamics mediated by heterogeneous topology as related to hippocampal memory management Jane Wang, Gina Poe, Michal Zochowski Hippocampal-cortical network interactions, including reactivation of recently acquired memories in the hippocampus during sleep, are key to the consolidation of memory traces to long-term storage sites in the neocortex. Network heterogeneities, in the form of regional changes in the connectivity densities of excitatory synapses, support this process in simulated hippocampal-cortical networks by regulating intrinsic network dynamics and thus mediating stimulus familiarity detection as well as selective memory consolidation. We characterize this network model by investigating dynamics due to distributed and overlapping memory structures and examine the ability of regional heterogeneities to both selectively activate in the presence of controlled stimuli and reactivate in the absence of stimuli, the former being indicative of active exploration and the latter of memory replay during sleep. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B9.00014: The topological structure of a network formed during simulations of a reversible polymeric gel M. Wilson, J. Billen, A. Baljon, A. Rabinovitch We investigate the topologies of the ensemble of telechelic polymers for which we previously studied the sol/gel transition [1]. The polymers serve as ``links'' between ``nodes,'' which consist of aggregates of their associating endgroups. The number of associations and hence the topology depends on the employed temperature. Our analysis shows that the degree distribution of the systems is bimodal and consists of two Poisson distributions with different average degrees $<$k$>$. Nodes in the distribution with the higher $<$k$>$ we call ``superpeers,'' those in the other distribution ``peers.'' With decreasing temperature, the fraction of superpeer nodes increases. This increase is steepest at the ``jamming'' transition. The eigenvalue spectra of the networks reveal that in the jammed state peers are only connected to superpeers, a topology known to be very robust. By contrast, at high temperatures peers are connected to each other as well. Due to the finite size of the polymers, our telechelic networks differ from random Erdos-Renyi (ER) bimodal networks. As in many real-world networks, spatial effects play a role. After rewiring the networks obtained in the simulations, we reach the ER limit, that is, the clustering coefficients are equal to those obtained for random ER networks. \\[0pt] [1] JCP 126, 044907(2007) [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B9.00015: Generalized fractional Fokker-Planck equation for anomalous diffusion Alex Veksler, Rony Granek The problem of anomalous diffusion is important for a variety of systems, such as fluids, glasses, polymers, proteins etc. It is characterized by a mean square displacement evolving in time as a power-law $\langle x^2\rangle = 2 D_0 t^{\alpha}$. However, a Fokker-Planck-like equation which could describe a stationary Gaussian process with anomalous-diffusion behavior, such as the one described by the Generalized Langevin equation, is still missing. We propose a generalization for constant force to the fractional Fokker-Planck equation (fFP) [Metzler, R. and Klafter, J., Phys. Rep. \textbf{339} (2000), 1-77], based on a series expansion in spatial and fractional time derivatives and powers of the Fokker-Planck operator. The proposed equation, GfFP, recovers the generalized Einstein relation and leads to Gaussian distribution, in particular, for free particle diffusion. We apply GfFP to 1-D first passage time problem. The long-time asymptote of the probability distribution behaves like $\exp(-t^\alpha)$. This contrasts with the power-law behavior of the corresponding solutions of the fFP. We further propose to generalize GfFP for treating other outstanding problems, such as the anomalous diffusion under an harmonic potential and the Kramers` escape problem. [Preview Abstract] |
Session B10: Focus Session: Ferroelectrics II
Sponsoring Units: DMPChair: Javier Junquera, Universidad de Cantabria
Room: 304
Monday, March 16, 2009 11:15AM - 11:51AM |
B10.00001: Switching behavior and scaling effects in ferroelectric capacitors Invited Speaker: In this presentation, we discuss the results of direct time-space resolved studies of domain switching behavior in micrometer capacitors by means of piezoresponse force microscopy (PFM). The PFM approach allows an insight in the mechanism of polarization reversal and its change as a function of the capacitor size and microstructure. Simultaneous visualization of the instantaneous domain configurations arising during polarization reversal and sub-ms transient current measurements allow us to establish direct relationship between the electrically measured polarization reversal signal and domain switching kinetics and determine the relative contribution of nucleation and wall motion mechanisms into polarization reversal as a function of capacitor size. Effect of microstructure on domain switching kinetics has been studied by comparing the switching behavior of polycrystalline and epitaxial capacitors. It is shown that in epitaxial capacitors the domain kinetics can be described by the classic nucleation model. In the polycrystalline capacitors, interaction of moving domain walls with microstructural defects gives rise to a completely different time dependence of polarization due to a wide distribution of local switching times. In this case the domain kinetics can be fitted by nucleation-limited switching model. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B10.00002: Resolving Deterministic Mesoscopic Mechanisms of Local Bias-Induced Phase Transitions in Ferroelectric Materials S.V. Kalinin, S. Jesse, M.P. Nikiforov, P. Maksymovych, N. Balke, A. Baddorf, H.J. Chang, A.Y. Borisevich, S.J. Pannycook, S. Choudhury, Y. Li, L.-Q. Chen Polarization switching in ferroelectric and multiferroic materials is invariably controlled by defects that act as nucleation and pinning site. Using the synergy of high-resolution spectroscopic Piezoresponse Force Microscopy, materials systems with atomically engineered defects, and phase field modeling, we demonstrate that deterministic mesoscopic mechanisms of polarization switching can be determined. In particular, the artificial bicrystal grain boundary in (100) BiFeO$_{3}$ is found to impede ferroelectric switching, but facilitate ferroelastic switching for one of the constituent crystals. The coupling between ferroelastic domain walls and ferroelectric polarization switching is demonstrated and attributed to the kinetic effects. These studies open the pathway for probing kinetics and thermodynamics of local bias-induced phase transitions and dissipation on a single-defect level using field confinement by an SPM tip. The future potential for atomistic studies is discussed. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B10.00003: Nanometer/Nanosecond Resolved Domain Dynamics Allowing Mapping of Distinct Nucleation and Growth Activation Energies Bryan Huey, Nicholas Polomoff, Vincent Palumbo, James Bosse A high speed variation of AFM is employed to uniquely monitor ferroelectric domain dynamics. Through pump/probe schemes, 20 nanometer resolution and 10 nanosecond temporal resolution is maintained. Consecutive images during switching therefore provide maps of nucleation times, while domain wall growth velocities as high as 25 m/s are observed. By imaging a specific region repeatedly with several pulse amplitudes, activation energies can also be extracted as a function of position, revealing completely independent energies for nucleation and growth that are sample dependent. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B10.00004: Optical cystallography and ferroelectric domain imaging of BaTiO$_3$ nanocrystals with tip-enhanced phonon Raman spectroscopy Samuel Berweger, Catalin C. Neacsu, Yuanbing Mao, Hongjun Zhou, Stanislaus S. Wong, Markus B. Raschke The capability of probing phase transitions, stress, electron-phonon coupling, or doping via their effect on the vibrational structure of crystals has positioned phonon Raman spectroscopy as a powerful tool for the study of semiconductors and dielectrics. In extending the technique to the near-field, the symmetry selectivity of the phonon Raman response allows for optical crystallography on the nanoscale in tip-enhanced Raman spectroscopy taking advantage of the local field enhancement provided by the nanometer size apex of a plasmonic scanning probe tip. The general selection rules that provide the necessary degrees of freedom are derived as a superposition of the crystal Raman tensor, momentum conservation for phonon and light emission, and the symmetry of the near-field tip scattering geometry. The capabilities are demonstrated for the spectrally and spatially resolved identification of intrinsic ferroelectric domains of individual BaTiO$_3$ nanocrystals by probing the A$_1$ TO and E TO phonon modes with nanometer spatial resolution. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B10.00005: Effects of Surface Modification on Photo-Induced Ferroelectric Nanolithography Yang Sun, Chi Xu, Chiyu Zhu, Robert Nemanich This study focuses on the photo-induced deposition of silver on polarity patterned ferroelectric surfaces. The results establish that ferroelectric nanolithography is dependent on the excitation wavelength and that surface modification can affect the process. Depending on the nature of the surface screening, the deposition occurs predominantly on positive domains (internal screening) or at domain boundaries (external screening). In this experimental study periodically poled lithium niobate (PPLN) is used as a template for ``nanolithography'' of metallic nanoparticles and nanowires through a photochemical process. It is shown that the location and rate of Ag nanostructure deposition is dependent on the UV excitation wavelength. Selective deposition is explained by considerations of band-bending, the mechanism of polarization surface charge screening, and the absorption depth of the UV light. In exploring the effects of surface modification, a nm-thick titanium oxide layer is grown on the PPLN surface by molecular beam deposition and the photo-induced Ag deposition process is repeated. It is found that Ag reduction is more selective to the domain boundaries and the positive domains. The results are analyzed in terms of our understanding of the photo-induced process. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 1:15PM |
B10.00006: Time-Resolved Studies of Polarization Switching in Pb(Zr,Ti)O$_{3}$ Capacitors Invited Speaker: The space and time scales of polarization switching in ferroelectric oxides are fundamentally coupled by the speed of elastic deformations that is approximately the speed of sound. Studying this fast polarization dynamics is essential for understanding the relationships between structure and properties of ferroelectric and multiferroic materials. In last several years, new opportunities have been created for synthesizing ultrathin ferroelectric films and for probing fast structural dynamics in these materials at the nanometer scale. Polarization switching dynamics of Pb(Zr,Ti)O$_{3}$ ferroelectric thin films, which have a large magnitude of switchable polarization and relatively large piezoelectric coefficients, have been comprehensively investigated at millisecond and microsecond time scales. In this talk, I will discuss new regimes of polarization dynamics and piezoelectric strain that can be probed at the nanosecond time scale in Pb(Zr,Ti)O$_{3}$ thin film capacitors using time-resolved x-ray microdiffraction. Using this approach, we have visualized the motion of domain walls during polarization switching, tested piezoelectric strain predictions at strains up to nearly 3{\%}, and found an unusual stability of unswitched polarization in ultrathin films at the nanosecond time scale. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B10.00007: Phase diagram in strained epitaxial BaTiO$_3$/SrTiO$_3$ superlattices studied by ultraviolet Raman spectroscopy Dmitri Tenne, J.D. Schmidt, P. Turner, A. Soukiassian, D.G. Schlom, S. Nakhmanson, X.X. Xi, Y.L. Li, L.Q. Chen, M. Bernhagen, P. Reiche, R. Uecker, R. Katiyar Strain effect on phase transitions in nanoscale BaTiO$_3$/SrTiO$_3$ ferroelectric superlattices (SLs) has been studied by ultraviolet (UV) Raman scattering. A series of coherently strained (BaTiO$_3$)$_8$/(SrTiO$_3$)$_4$ SLs have been grown by molecular beam epitaxiy on rare earth scandate (GdScO$_3$, DyScO$_3$, SmScO$_3$, NdScO$_3$) and SrTiO$_3$ substrates. This allowed a systematic strain variation in the SLs. UV Raman data allowed the determination of the ferroelectric phase transition temperature ($T_c$) and indicated the presence of different ferroelectric phases with out-of-plane and in-plane components of polarization in SLs, depending on strain and temperature. Experimental Raman results are supported by first-principles calculations of structural instabilities in BaTiO$_3$/SrTiO$_3$ SLs and thermodynamic phase-field modeling of phase diagrams and ferroelectric polarization as a function of temperature and strain. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B10.00008: Polarization rotation in epitaxially strained perovskite-oxide superlattices Serge Nakhmanson Utilizing first-principles computational techniques, we have mapped out $\Gamma$-point structural instabilities in (BaTiO$_{3}$)$_{8}$/(SrTiO$_{3}$)$_{4}$ superlattices held at varying degrees of epitaxial strain and constrained to P4mm symmetry with fully developed polarization in the out-of-plane direction. We find that at compressive strains larger than -0.5\% (with respect to a fully relaxed P4mm structure) the superlattices exhibit no structural instabilities. However, at a smaller compressive strain, an in-plane ferroelectric instability emerges in the SrTiO$_{3}$ layers. This instability is then complemented by a similar instability in the BaTiO$_{3}$ layers that develops at tensile strain of more than 0.2\%, suggesting nonzero polarization components for both in- and out-of-plane directions throughout the whole superlattice. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B10.00009: Local polarization discontinuities in perovskite superlattices via compensating heterointerfaces Eamonn Murray, David Vanderbilt Using first-principles methods, we investigate an approach to the production of multicomponent perovskite superlattices with discontinuities in the layer polarizations. The recent paper of Wu and Vanderbilt\footnote{X. Wu and D. Vanderbilt, Phys. Rev. B. {\bf 73}, 020103(R) (2006).} demonstrated how 180$^\circ$ domain walls could be formed in a perovskite superlattice through chemical substitution of atoms in the layer at the domain boundary. We examine an alternative approach in which large discontinuities in the local polarization are induced by constructing superlattices out of II-IV and I-V perovskite constituents. The ``polar discontinuities'' at the heterointerfaces provide compositional charges that approximately cancel the polarization bound charges, thereby allowing stable polarized regions to form. We illustrate the concept via first-principles calculations on KNbO$_3$/SrTiO$_3$ superlattices. We also show how the Wannier-based definition of layer polarization described by Wu {\it et al.}\footnote{X. Wu {\it et al.}, Phys. Rev. Lett. {\bf 97}, 107602 (2006).} may appropriately be applied to a system containing non-neutral layers, and use this to quantitatively examine the local variations in polarization in the KNbO$_3$/SrTiO$_3$ system. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B10.00010: Enhanced piezoelectricity in PbTiO$_{3}$/BaTiO$_{3}$ superlattices Valentino R. Cooper, Karin M. Rabe Short period ferroelectric/ferroelectric PbTiO$_{\rm 3}$ (PTO)/BaTiO$_{\rm 3}$ (BTO) superlattices are studied using density functional theory. Contrary to the trends in paraelectric/ferroelectric superlattices the polarization remains nearly constant for PTO concentrations below 50\%. In addition, a significant decrease in the $c/a$ ratio below the PTO values is observed. We predict an enhancement in the $d_ {33}$ piezoelectric coefficient peaking at $\sim$75\% PTO concentration due to the different polarization-strain coupling in PTO and BTO layers. Further analysis with a superlattice effective Hamiltonian reveals that these trends are bulk properties which are a consequence of the reduced $P$ brought about by the polarization saturation in the BTO layers. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B10.00011: ABSTRACT WITHDRAWN |
Session B11: Focus Session: Transport Properties of Nanostructures I: Surface Assemblies and Films
Sponsoring Units: DMPChair: Douglas Natelson, Rice University
Room: 305
Monday, March 16, 2009 11:15AM - 11:51AM |
B11.00001: Following elemental chemical steps by imaging molecular orbitals by STM Invited Speaker: Probing the electronic structure of molecules by STM is complicated by the strong interaction of the molecular orbitals with the metal substrate.[1,2] Adsorbing the molecule on an ultrathin insulating film alleviates this problem and allows direct imaging of molecular orbitals in real space.[1] This approach will be illustrated by two examples: characterizing the operation of a single-molecule switch and controlling and monitoring the formation of an organometallic complex. The inner hydrogens in the central cavity of a naphthalocyanine molecule can be switched between two equivalent positions.[3] This process (hydrogen tautomerization reaction) can be initiated in a controlled fashion by excitation induced by the inelastic tunnelling current. The tautomerization reaction can be followed by resonant tunnelling through the LUMO of the molecule and is expressed as considerable changes in the conductivity. In addition, we demonstrate a coupling of the switching process so that the charge injection in one molecule induces tautomerization in an adjacent molecule. The other example will consider constructing an organometallic complex from individual organic molecules and metal atoms by STM manipulation on the ultrathin insulating film. The manipulation process and the associated changes in the molecular orbitals (energy, spatial extension, symmetry) can be followed STM imaging and spectroscopy. \\[4pt] [1] J. Repp et al. Phys. Rev. Lett. 94, 026803 (2005).\\[0pt] [2] X.H. Qiu, G.V. Nazin, W. Ho Science 299, 542 (2003).\\[0pt] [3] P. Liljeroth, J. Repp, G. Meyer Science 317, 1203 (2007). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B11.00002: Apparent anomaly in electron scattering in Ag nanostructures* Ellen D. Williams, Chenggang Tao, William G. Cullen Electron scattering from diffusing atoms can be visualized via the effects of the corresponding force that biases atomic motion, the ``electromigration force''. Using a combination of scanning tunneling microscopy and scanning electron microscopy, we investigate electron scattering via the biased motion of monatomic islands and C$_{60}$-decorated steps on Ag(111) surfaces in the presence of large current density (j$_{bulk}$ = 6.7x10$^{9}$ A/m$^{2})$. For monatomic adatom islands, the biased motion is opposite to the current direction and thus parallel to the direction of momentum transfer (the ``wind force'' direction), while vacancy islands move oppositely. The measured drift velocity v as a function of the island radius R, vR = 1.9nm$^{2}$/s, yields an anomalously large$^{1}$ effective force per boundary atom $\sim $0.06 meV/nm . An effective scattering force of similar magnitude is also observed via current-induced curvature of C$_{60}$ decorated line-boundaries. Possible mechanisms for this effect, including current crowding, charge transfer and local heating, will be discussed. 1. A. Bondarchuk, et al. Phys. Rev. Lett. \textbf{99, 206801 (2007)} [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B11.00003: Orbital-resolved polaron states in CdSe dots and rods probed by scanning tunnelling spectroscopy Peter Liljeroth, Zhixiang Sun, Ingmar Swart, Christophe Delerue, Dani\"el Vanmaekelbergh Despite the extensive knowledge of phonons in semiconductor crystals, the polaron states formed by the coupling between phonons and single electronic orbitals have not been measured directly due to the negligible spacing between the energy levels in conventional semiconductors. This can be overcome in semiconductor nanocrystals that have discrete energy levels due to quantum confinement. Here, we present scanning tunnelling spectroscopy results on CdSe rods and dots showing the single- electron polaron energy levels with their phonon replica. We measure the spacing and intensity of the replica, and derive the electron-phonon coupling strength for different orbital symmetries. The effect of multiple added electrons on the coupling strength can be assesses under shell-filling conditions. Our results show the formation of polaron eigenstates arising from Fröhlich coupling of an electron to longitudinal phonons with a coupling strength that depends considerably on the size and shape of the nanocrystals. The results are important for understanding electron transport in zero and one-dimensional semiconductors and the intra-band relaxation of hot carriers in quantum dots. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B11.00004: Morphology and transport properties of self-assembled, ligand-exchanged PbSe nanocrystal arrays Ching-Tzu Chen, Weon-kyu Koh, Christopher Murray, Chang C. Tsuei Self-assembled PbSe nanocrystal (NC) arrays have shown strong potential as a viable candidate for producing large-scale quantum dot superlattices. Such superlattices not only have significant technological implications, but they also serve as a model system for simulating strongly correlated transition-metal oxides. At present, highly-ordered PbSe NC arrays can be reproducibly prepared on structured Si-substrates by drop-casting PbSe solution in controlled environments. The as-grown superlattice films are nearly insulating, and post-processing ligand exchange is necessary to induce conduction. However, the lack of understanding of the ligand exchange processes has been a bottleneck to reliably producing highly-ordered conductive arrays. In this talk, we report on in-depth characterizations of the PbSe NC films treated with various ligand molecules in different solvents. The effect of a range of ligands and solvents on the film morphology will be discussed in details. Preliminary temperature-dependent transport and noise studies will be presented. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B11.00005: Tunneling Spectroscopy of Ultrathin Insulating Films: Cu$_2$N on Cu(100) Charles Ruggiero, Taeyoung Choi, Jay Gupta Insulating films of only a few atomic layers offer insight into the evolution of electronic structure at the nanoscale. We report scanning tunneling microscopy (STM) studies of one monolayer Cu$_2$N films grown on Cu(100). Our tunneling spectra indicate that Cu$_2$N acts as an insulator, with a band gap that exceeds 4~eV [1]. We study changes in this electronic structure with size, ranging from few-atom islands to complete films. We find that the conduction band edge first emerges in few-atom islands, and shifts toward lower energy with increasing island size. Images of the local density of states show standing wave patterns consistent with the confinement of electrons to these 2D islands. Measurements of the tunneling barrier height and image potential states indicate that the Cu$_2$N work function is $\sim $0.9~eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory. http://www.physics.ohio-state.edu/$\sim $jgupta \\[3pt] [1] C.D. Ruggiero, T. Choi, J.A. Gupta, Appl. Phys. Lett. \textbf{91}, 253106 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B11.00006: Ambipolar Ballistic Electron Emission Microscopy (BEEM) Studies of Gate-field Modified Schottky Barriers(SBs). Y.L. Che, J.P. Pelz Gate-field modified SBs are important for ``Schottky Barrier FETs'' [1], and could be used to control spin and charge injection into other semiconductor device structures. We have made the first \textit{ambipolar} BEEM measurements on Au/Si SBs that can be changed from effective $n$-type to $p$-type by applying a positive or negative back-gate bias, respectively. Samples were fabricated using SIMOX silicon-on-insulator wafers (35nm Si/150nm SiO$_{2}$/p-Si substrate), with Ti/Au and Pt pads as ohmic contacts for $n$-type and $p$-type operation, respectively. The local SB heights at 80K for electrons and holes were measured \textit{at the same location} to be $\sim $0.785eV and $\sim $0.323eV respectively, which correspond to intrinsic SBHs of $\sim $0.84eV and $\sim $0.36eV after accounting for image force lowering. These sum to 1.20eV, close to the $\sim $1.17eV Si bandgap at 80K. We will discuss on-going measurements of the dependence of the local SBH on temperature, back-gate bias, Si film thickness, and bias between the Schottky and ohmic contacts. Future work will investigate local variations of the conduction and valence bands due to local ``geometry-induced'' electric fields in nanostructured contacts. Work supported by National Science Foundation Grants No. DMR-0505165 and DMR-0805237.[1] S. Heinze, et al., Phys. Rev. Lett. 89, 106801 (2002). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B11.00007: Temperature dependence of lateral hot-electron spreading in Au films using Cross-Sectional Ballistic Electron Emission Microscopy C. Marginean, J.P. Pelz Cross-sectional ballistic electron emission microscopy (XBEEM) was used to investigate the \textit{temperature dependence} of hot-electron lateral spreading in metal films. A sequence of GaAs QWs of 1 to 15 nm width (separated by 200nm Al$_{0.3}$Ga$_{0.7}$As barrier layers) were cleaved \textit{ex situ,} and then 10 nm-thick of Au was thermally deposited on the cleaved edge to form Au Schottky barrier (SB) ``nanoaperture'' contacts [1]. Previous XBEEM results showed an unexpectedly large hot-electron lateral spreading at room temperature consistent with multiple electron scattering inside the metal film [2]. If phonon scattering of hot-electrons is significant in Au films (as previously suggested [3]), then the lateral spreading should increase at lower temperature. However, we found that the lateral spreading at 80K was almost the same as at room temperature, suggesting that electron-phonon scattering is not the dominant scattering mechanism. We will also discuss the temperature dependence of the BEEM current\textit{ amplitude}, as well as Monte-Carlo simulations of the lateral spreading process. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005)~ [2] C. Tivarus \textit{et al}., APL \textbf{87}, 182105 (2005) [3] L. D. Bell, PRL \textbf{77}, 319007 (1996) [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B11.00008: Classical size effect in nanometric Cu films: the dominant role of grain boundary scattering D. Choi, T. Sun, A. Warren, B. Yao, A. Darbal, K. Barmak, M. Toney, R. Peale, K. Coffey Surface and grain boundary electron scattering contribute significantly to resistivity as the dimensions of polycrystalline metallic conductors are reduced to, and below, the electron mean free path. In this work, a methodology is reported to independently evaluate surface and grain boundary scattering in encapsulated polycrystalline Cu thin films, with thicknesses of 28-158 nm, grain sizes of 35-466 nm, and interface roughnesses of 0.2-2 nm. The film resistivity, measured at both room temperature and at 4.2 K, is compared for samples having different grain sizes and film thicknesses. The resistivity contribution from grain boundary scattering is found to be dominant in SiO$_{2}$/Cu/SiO$_{2}$ and Ta/ SiO$_{2}$/Cu/Ta/SiO$_{2}$ films. Resistivity data for a third set of samples, namely SiO$_{2}$/TaSiN$_{x}$/Cu/TaSiN$_{x}$/SiO$_{2}$, will also be presented. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B11.00009: Crystal Orientation Imaging of Nanometric Metal Films in the Transmission Electron Microscope A. Darbal, K. Barmak, N. T. Nuhfer, D. J. Dingley, G. Meaden, J. Michael, T. Sun, B. Yao, K. R. Coffey A reliable method for orientation mapping of nanocrystals is crucial to the study of the impact of grain boundaries on resistivity increase of metal films as thickness is reduced (classical size effect). Here we report on the use of the Automated Crystallography (ACT) system for high-resolution grain and orientation mapping in the TEM. The samples for the study were a 50 nm-thick Pt film annealed at 800\r{ }C and a 40 nm-thick Cu film annealed at 450\r{ }C. In ACT, the diffraction pattern for a given point is constructed by analyzing its intensity variation in a series of dark field images obtained using hollow-cone illumination. The reconstructed diffraction pattern for every point is indexed to obtain the orientation map. The sensitivity of the orientation imaging results to details of sample preparation, data acquisition and choice of indexing parameters is discussed. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B11.00010: Controlling the assembly and electronic properties of solution processed CNT devices: From large area arrays to individual CNT Saiful I. Khondaker, Paul Stokes, Yodchay Jompol, Shashank Shekhar Single walled carbon nanotubes (SWNTs) are considered to be ideal components for nanoelectronic devices because of their exception electronic properties. Integration of these nanostructures into electronic circuits requires the precise positioning of them in different architectures. Here we will summarize our recent progress on the directed assembly of SWNTs using AC dielectrophoresis (DEP). SWNTs are assembled from a surfactant free commercially available aqueous solution using a non uniform electric field. By controlling the electric field strength, frequency, density of solution and novel fabrication techniques, we are able to control the assembly of SWNT from dense arrays mimicking electric flux to single SWNT devices with high yield. Electronic transport properties of field effect transistors and single electron transistors fabricated from such assembly will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B11.00011: Probing the doping state of suspended carbon nanotube films by photo-induced voltage response Benoit St-Antoine, David Menard, Richard Martel A study of the photovoltage (PV) properties of suspended carbon nanotube films was undertaken. Although absorbance exhibits resonance features, changing the photoexcitation using lasers at two different wavelengths did not impact the magnitude of the observed behaviour. Moreover, increasing the thermalization by exposing the films to nitrogen resulted in a steep decrease of the PV with increasing pressures. Interestingly and contrary to previously reported studies, we also found that the PV could be maximized by illuminating the films at a short distance from the metallic contacts. All these findings reveal that the PV response of suspended films in vacuum is mainly driven by thermal mechanisms. Finally, we were able to reverse the sign of the PV by changing the doping state of the carbon nanotube films. Thus, monitoring the magnitude of the PV appears as a powerful tool for evaluating the doping state of suspended films. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B11.00012: Carrier Type and Transport Characteristics of a-B$_{4}$C Marcus Sky Driver, Saad Janjua, Sudarshan Karki, Dae Heum Yeoun, Anthony Caruso Boron carbide has many technological applications, including radiation hard semiconducting applications; the most popular of which is voltaic transduction. Structural defects are known to exist, whose implications in defining the majority carrier type is important, yet unclear. Gaining greater insight into the local physical structure and transport character is crucial toward optimizing the voltaic behavior. Subsequently, gaining information about the majority carrier and carrier concentration from Hall effect and band structure from photoemission spectroscopy gives insight into the electronic structure and transport of boron-rich carbides. Boron carbides are predominately p-type due to their electron deficiency, but it has been suggested that the electron as a majority carrier may also exist. Preliminary studies including Hall effect, photoemission and extended X-ray absorption fine structure will be discussed within the context of amorphous boron carbides with respect to the carrier properties and physical structure for various growth conditions, within the context of local structural defects. [Preview Abstract] |
Session B12: Focus Session: Structure and Dynamics of Metal Thin Films
Sponsoring Units: DMP DCMPChair: Theodore Einstein, University of Maryland
Room: 308
Monday, March 16, 2009 11:15AM - 11:51AM |
B12.00001: Effects of shadowing and steering in oblique incidence epitaxial growth Invited Speaker: Recently the fabrication of novel nanostructures by oblique deposition has drawn much attention due to their potential application in electronic and mechanical devices as well as the interesting morphologies observed in various experiments, such as nanorods, nanocolumns, and nanohelicoids. Unlike self-organization by misfit strain in heteroepitaxial growth, oblique deposition provides a relatively direct way of controlling surface structures of growing films. Recent experiments indicate that oblique incidence deposition can significantly alter materials properties such as surface roughness, magnetic anisotropy, optical transmittance, and porosity. After a review of these experimental results, we first show that a series of morphological transitions observed in oblique incidence Cu/Cu(100) growth near room temperature can be explained primarily by geometrical shadowing effects [1]. We then discuss the modifying effects of steering due to short-range and long-range attraction [2] as well as of substrate rotation on the surface morphology. Finally, we present the results of recent multiscale simulations of Cu/Cu(100) growth at lower temperature (T = 160 - 200 K) [3] as well as parallel accelerated dynamics and molecular dynamics simulations at very low temperature [4]. Based on these simulations we have been able to explain a number of recent intriguing but previously unexplained experimental results including the strong dependence of the surface morphology and roughening behavior on temperature as well as the development of compressive strain in metal thin film growth. \\[4pt] [1] Y. Shim and J. G. Amar, Phys. Rev. Lett. {\bf 98}, 046103 (2007).\\[0pt] [2] Y. Shim, V. Borovikov and J. G. Amar, Phys. Rev. B {\bf 77}, 235423 (2008).\\[0pt] [3] V. Borovikov, Y. Shim and J. G. Amar, Phys. Rev. B {\bf 76}, 241401(R) (2007).\\[0pt] [4] Y. Shim, V. Borovikov, B. P. Uberuaga, A. F. Voter, and J. G. Amar, Phys. Rev. Lett. {\bf 101}, 116101 (2008). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B12.00002: Diffusion of two-dimensional Cu islets on Ag(111) studied with the Molecular Dynamics Method Sadar S. Hayat, Marisol Alcantara Ortigoza, Talat S. Rahman Our molecular dynamics simulations (at 300, 500 and 700 K) of the diffusion of two-dimensional Cu$_{n}$ islets (1$\le $n$\le $9) on Ag(111) using many-body potentials yield an Arrhenius behavior. Concerted motion is seen to dominate the diffusion of smaller islets (2-4 atoms) whereas multiple-atom, shape-adjusting processes control the diffusion of the larger ones. Although the effective energy barrier scales with islet size, the barriers do not change considerably for islands containing 4 to 9 atoms (they are $\sim $ 220 $\pm $ 37 meV). While the diffusion barrier for Cu monomers on Ag(111) is higher than that on Cu(111) (both in experiment and theory), the situation reverses starting from the dimer. Our results for monomer and dimer are in excellent agreement with those extracted from experiments.$^{1}$ On comparing our results with those for Cu islets on Cu(111), we find that the comparatively large Ag-Ag bond-length sets the contrast between Cu monomer diffusion on Cu(111) and on Ag(111). The diffusivity of Cu dimer, however, is boosted on Ag(111) by the competition between optimizing the Cu-Cu and the Cu-Ag bonds. For larger islets (3-9 atoms) our results reveal several novel diffusion processes, including those in which an islet-atom climbs atop. $^{1 }$Morgenstern \textit{et al.} PRL\textbf{93}, 056102 (2005). Work supported by NSF-ITR 0428826. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B12.00003: Atomic structure determination of the (001) surface of the semimetal Bi by STM and LEED J. Sun, J. Wang, J. Wells, Ph. Hofmann, K. Pohl The Bi surfaces differ from the semimetal bulk due to the metallic surface states, induced by the symmetry breaking and strong spin-orbit interaction. All Bi surface states studied are spatially confined to the first layer. Bi(001) is a notable exception with deeply penetrating states, which could have a significant effect on the bulk properties of nanostructures. This work presents surface morphology observation by STM and atomic structure determination by LEED, which are expected to be closely related to the electronic properties. STM shows an unreconstructed surface and wide terraces with double-layer step heights of about 3.76 $\pm$ 0.02 \AA~. We also identify the short termination by obtaining unstable single step heights via special sputtering operations. In the LEED analysis, the termination with an intact bilayer also results in a much better agreement between calculated and measured intensities than the broken bilayer. Strong multilayer oscillatory relaxations (about 10\%) are found to reach deep into the fifth layer, which can be seen as the structural response to the unusually deep surface state penetration at this surface. The measured relaxations agree well with those from first-principles calculations. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B12.00004: Thermal-stability of Pd-Cu surface alloys investigated at the nanometer-scale by LEEM-IV analysis E. Bussmann, J. Sun, K. Pohl, G. L. Kellogg Pd-Cu(100) surface alloys are interesting as model systems for metal/metal epitaxy, as well as for their catalytic properties, and as coatings, e.g. for electromigration resistance. We employ the LEEM-IV technique, with 8.5 nm spatial resolution and submonolayer chemical sensitivity, to investigate Pd interdiffusion into the Cu(100) surface. The LEEM-IV technique is sensitive to the layer-by-layer composition down to the fourth subsurface layer. After annealing a 0.4 ML Pd surface alloy at around 540 K, some regions of the surface develop a Cu$_{3}$Pd structure, a familiar bulk alloy phase. In other regions, the surface Pd concentration becomes dilute due to Pd diffusion into the bulk. We estimate the thermal activation barrier to Pd diffusion from the surface alloy into Cu bulk to be 1.7$\pm $0.15 eV. The LEEM allows real-time, real-space, observation of the interdiffusion process, and the concurrent evolution of the surface structure, at the nanometer scale. Sandia is operated by Sandia Corp., a Lockheed Martin Company, for the U. S. DOE's NNSA under Contract No. DE-AC04-94AL85000. Work at UNH is funded by the NSF under Grant No. DMR-0134933. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B12.00005: Bistability of Nanoscale Ag Islands on Anisotropic Si(111)-(4x1)-In Surface Stress Template Miao Liu, Yaoyi Li, Decai Yu, Jinfeng Jia, QiKun Xue, Feng Liu, Dayan Ma, Xucun Ma We present a combined experimental and theoretical study of stability of Ag nanoislands grown on Si(111)-(4x1)-In surface. Experiments show the existence of two stability regimes: a conventional regime at low temperature where only one island shape is stable, and an unconventional regime at room temperature (RT) where isotropic compact islands coexist with anisotropic elongated ones. First-principles calculations show the unusual bistability at RT arises from the fact that the Ag nanoislands are under anisotropic stress, further supported by a continuum model of island shape evolution as a function of island size. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B12.00006: Impurities in Vacuum Deposition: Effect on Island Nucleation and Surface Morphologies Alberto Pimpinelli, Ajmi BH. Hamouda, T.L. Einstein The effect of impurities on epitaxial growth in the submonolayer regime is studied using kinetic Monte Carlo simulations of a two-species solid-on-solid growth model. Both species are mobile, and attractive interactions among adatoms and between adatoms and impurities are incorporated. Impurities can be codeposited with the growing material or predeposited prior to growth. We discuss the peculiar morphologies observed in copper on copper deposition on vicinal surfaces, and argue that only the presence of impurities can explain all observed features.\footnote{A.BH.Hamouda et al., Phys.\ Rev.\ B 77, 245430 (2008)} We also investigate the effect on island nucleation using a recently developed approach based on capture zone distributions.\footnote{A.\ Pimpinelli \& T.L.\ Einstein, Phys.Rev.\ Lett.\ 99, 226102 (2007)} [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B12.00007: Interface evolution and cluster formation during de-wetting of thin solid films Adi Constantinescu, Leonardo Golubovic, Artem Levandovsky Morphology evolution of solid thin films is investigated within a continuum interface dynamics model that incorporates both the interface relaxation and the long range de-wetting interactions. The model is used to explore the cluster formation phenomena seen on the surfaces of polymeric and metallic thin films. Via numerical simulations and analytic arguments, we obtain the scaling laws governing the coarsening growth of these clusters. These scaling laws are found to be super-universal at long time scales: They do not depend on the dimensionality of the film substrate and the nature of the long range de-wetting interactions, as documented by our numerical simulations on 1-d and 2-d substrates with de-wetting interactions of various forms. However, for the physically interesting 2-d substrates, the long range interactions introduce a distinct early time scaling behavior that persists over many decades of time and may be significant for the understanding of the current experimental phenomenology. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B12.00008: First-passage time approach to kinetic Monte Carlo simulations of metal(100) growth Giridhar Nandipati, Yunsic Shim, Jacques Amar One of the difficulties in carrying out realistic kinetic Monte Carlo simulations is the existence of rapid, repetitive low-barrier processes which can dramatically slow down the simulation. For example, in metal(100) growth the rate for edge-diffusion can be very fast even at moderate temperatures, while the barriers for edge-detachment and corner rounding are relatively high. While one approach to this problem is to artificially reduce the rate of edge-diffusion, such an approach can significantly alter the thin-film evolution. To address this problem while still preserving the relative rates for all processes, we have developed a modified KMC method in which edge-diffusion and corner-rounding are treated using a first-passage time formalism, while the remaining processes are treated as in normal KMC. In simulations of an effective-medium theory (EMT) based model of Cu/Cu(100) growth at $T = 200$ K and above we find that a speed-up of several orders of magnitude is possible, without sacrificing accuracy. Preliminary results for Cu/Cu(100) growth at high temperatures will also be presented. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B12.00009: Growth morphology of ultra-thin Ni films on Pd (100) Patricio H\"{a}berle, Carolina Parra A series of thin Ni films with thicknesses between 0.2 ML to 13 ML were deposited on a Pd(100) substrate at room temperature (RT). Growth morphology was investigated using scanning tunneling microscopy (STM). The STM images indicate the existence of three different growth modes. Up to 6.5 ML, the films grow pseudomorphically, consistent with a face-centered tetragonal (fct) structure. From 6.5 ML to 10.5 ML a new interlayer distance of 1.0 $\pm$ 0.1 ${\AA}$ is established. The new structure is accompanied by the appearance of an arrangement of filaments on the top layer. These filaments are presumably related to a strain relief mechanism of the fct films. Finally above 10.5 ML the Ni films recover their face-centered cubic (fcc) lattice constants. The filaments evolve to form a net-like structure over the whole surface. Preliminary data indicates the magnetic properties of the layers are linked to the evolution of the film's structure. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B12.00010: Dynamical LEED analysis of a quasicrystalline Cu film using periodic approximant structure models Renee Diehl, Katariina Pussi Quasicrystalline surfaces pose a challenge to diffraction techniques since they have no periodicity. They do, however, have good long-range order and produce diffraction patterns with sharp peaks. Copper grows on the 5-fold surface of i-Al-Pd-Mn in a layer-by-layer mode. Although its atomic structure cannot be determined by STM, the diffraction pattern from a 5-layer thick film consists of sharp peaks and streaks, the location of which indicate that this film consists an aperiodic array of rows of periodically-spaced Cu atoms. We have applied the method of periodic approximants in a dynamical LEED analysis of the structure of this quasiperiodic copper film to determine the atomic structure of the Cu film. This analysis indicates that the Cu film has a distorted cubic structure that conforms to the quasiperiodic substrate structure, providing an example of periodic-aperiodic structure matching. This research is supported by NSF-DMR-0505160 and the Academy of Finland. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B12.00011: Theoretical Study of Adsorbates-induced Restructuring of Pb flat-top mesas in the Quantum Regime Wenguang Zhu, Alexander Khajetoorians, Chih-Kang Shih, Zhenyu Zhang Based on first-principles total energy calculations, we study the adsorption and diffusion of a series of metal adsorbates (Fe, Co and Cs) on flat-top Pb mesas, focusing on their influence on the morphology of the mesas. We found that single Fe and Co atoms can easily dive into the subsurface interstitial sites by overcoming a small energy barrier upon deposition onto the mesa top. In contrast, Cs atoms are able to substitute first-layer Pb atoms via a place exchange process resulting in a surface alloy. This induces a dramatic change on the surface morphology of these mesas as observed in recent experiments. This morphological transformation is characterized by the emergence of Cs-decorated monolayer-high Pb islands which are predominately formed on quantum mechanically unstable regions of the Pb mesas. Connections with experimental observations in other related systems will also be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B12.00012: Stress development and relaxation during early stages of oxidation of metals and alloys Guangwen Zhou, Changhong Ke, Judith Yang, Jeffrey Eastman, John Pearson It has been long recognized that oxidation of metals results in the generation of stresses and these stresses play an important role in shaping the microstructure of oxide films. However, the mechanism governing the stress development and relaxation during early stages of oxidation of metals and alloys is still to a significant degree unclear. Using a combination of in-situ ultra-high vacuum (UHV) transmission electron microscopy (TEM) and finite element method, we show how oxidation-induced stresses can be used to tailor the initial oxide formation during early-stage oxidation of Cu(100) and Cu-Au(100). From analysis of the observed morphological evolution of Cu$_{2}$O nanoislands, we establish a close relationship between the stresses generated from the oxide growth and the thermodynamic selection of nanoscale morphology of the oxide film. We expect that our results have implications for controlled production of novel oxide nano structures through controlling the oxidation-induced stresses via oxidation temperature or alloying. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B12.00013: Studies of oxidation of the Cu(100) surface using low energy positrons. W.B. Maddox, N.G. Fazleev, A.H. Weiss Changes in the surface of an oxidized Cu(100) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the positron annihilation induced Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300$^{o}$ C. The intensity then decreases monotonically as the annealing temperature is increased to $\sim $600$^{o}$ C. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. Possible explanation for the observed behavior of the intensity of positron annihilation induced Cu M2,3VV Auger peak with changes of the annealing temperature is proposed. [Preview Abstract] |
Session B13: SPS Undergraduate Research I
Chair: Gary White, American Institute of PhysicsRoom: 309
Monday, March 16, 2009 11:15AM - 11:27AM |
B13.00001: Introductory Physics Experiments Using the Wiimote William Somers, Frank Rooney, Romulo Ochoa The Wii, a video game console, is a very popular device with millions of units sold worldwide over the past two years. Although computationally it is not a powerful machine, to a physics educator its most important components can be its controllers. The Wiimote (or remote) controller contains three accelerometers, an infrared detector, and Bluetooth connectivity at a relatively low price. Thanks to available open source code, any PC with Bluetooth capability can detect the information sent out by the Wiimote. We have designed several experiments for introductory physics courses that make use of the accelerometers and Bluetooth connectivity. We have adapted the Wiimote to measure the: variable acceleration in simple harmonic motion, centripetal and tangential accelerations in circular motion, and the accelerations generated when students lift weights. We present the results of our experiments and compare them with those obtained when using motion and/or force sensors. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B13.00002: Binary Orbital Motion of Electrically Charged Spheres in Weightlessness. Lulu Li, Brad Atkins, Gavin Franks, Joshua Fuchs, Chase Sliger, Jennifer Thompson The similar mathematical forms of Coulombs' Law of Electrostatics and Newton's Law of Gravitation predict that two oppositely charged spheres should be able to move in a binary orbit about their center of mass using only the electric force as the force of attraction. To test this prediction, we conducted an experiment in July 2008 aboard a specialized C-9B aircraft in NASA's Microgravity University Program which simulates the conditions of weightlessness. We successfully achieved multiple binary orbits between the two spheres. The orbital motion was analyzed using VideoPoint software to measure the orbital interaction of the spheres. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B13.00003: Dynamics of a charged Kapitza's pendulum interacting with radiation in thermal equilibrium Jorge Hernandez Gomez, Paulina Pradel Soto The considered system is a Kapitza's pendulum that consists of a disk that spins at constant angular velocity, from which edge is attached a massless rigid rod with an electrically charged bob. The system is studied theoretically and experimentally. The motion equations are settled and solved both, analytically under the small oscillation assumption considering the interaction of the system with black body radiaton as a perturbation, and in the general case using numerical algorithms. The pendulum's dynamics is studied varying both initial conditions and parameters in small steps. In order to identify regions of stable and chaotic motion, Lyapunov's exponents are calculated. Phase and configuration spaces are plotted to notice periodical and erratic behaviors. Poincare sections and fast Fourier transforms are obtained to identify resonance cases. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B13.00004: Electrical energy dissipation in superconducting niobium rings E. M. Dowdell, K. M. Chiola, E. T. Rosauri, J. D. Hettinger Niobium rings were made from films synthesized using sputtering process. Rings and wires were defined in the films using standard photolithographic techniques followed by wet etching. Three rings were fabricated with different diameters and wire widths. One microbridge was created for direct electrical transport measurements. V-I characteristics of the superconducting niobium microbridge were measured at temperatures below 7K and at magnetic fields up to 2T. Dissipation was induced in the superconducting niobium rings by ramping the magnetic field and employing Faraday's Law. The current induced was measured through the magnetic moment and the simple expression $\mu $=IA. We will correlate the magnetically induced measurements with those made directly on the microbridge to investigate this method as a technique for extending transport measurements to lower dissipation levels. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B13.00005: Surface plasmon enhancement of fluorescence on gold nanogratings Stephanie Wiele, Iuri Gagnidze, Jennifer Steele This work focuses on using surface plasmons (SPs) excited on gold wire gratings to enhance the yield from fluorescent molecules. SPs enhance fluorescence by amplifying the electromagnetic near-field that excites the fluorophore as well as providing additional decay channels for the fluorphore. Although it has already been shown that SPs excited on metal nanoparticles can enhance fluorescence, SPs excited on gratings offer several advantages over nanoparticles. Because SPs are excited by the diffraction orders of the grating, changing the angle of incidence of light will change the SP wavelength. The SP excited on the grating can then be tuned on a single plasmonic substrate to overlap the absorption and emission spectrum of many different fluorophores. The ability to tune the SP wavelength through both the absorption and emission wavelength of the fluorophores on a single substrate will give greater insight to how SPs enhance fluorescence and how to maximize the fluorescence enhancement for various biosensing applications. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B13.00006: Diffusive Behavior of Lipid Rafts in Multicomponent Membranes Michael G. Lester, Mohamed Laradji, P.B. Sunil Kumar The diffusion of nanoscale lipid domains (also known as lipid rafts) in multicomponent membranes in the liquid-liquid coexistence region of the phase diagram is investigated via extensive dissipative particle dynamics simulations. In particular we investigated the effect of membrane diameter and shape (curvature) on the diffusivity of the lipid domains. Our results indicate that the domains exhibit Brownian motion, {\em i.e.} the center of mass mean square displacement $(\Delta R)^2=4 Dt$, with the diffusion coefficient decreasing as the domain radius, $r$, is increased. More specifically, we found that $D\sim 1/r$, {\em i.e.} the diffusion of the domains is mainly impeded by viscous drag due to solvent surrounding the membrane. Although the data can also be fitted with the logarithmic expression due to Saffman and Delbr{\"u}ck\footnote{Saffman and Delbr{\"u}ck, Proc. Nat. Acad. Sci. {\bf 72}, 3111 (1975)} $D\sim \ln(1/r)$ describing diffusion where the membrane viscosity plays an important role, the later fit is found to be poorer than that with $D\sim 1/r$. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B13.00007: AC Magnetic Susceptibility Probe for Use in a Commercial SQUID Magnetometer J.D. Cohen, D.M. Pajerowski, M.W. Meisel An AC magnetic susceptibility probe, employing a typical set of mutual inductance coils, has been constructed for operation in a commercial SQUID magnetometer operating down to 1.7 K and up to 7 T. The primary ($\sim$1000 turns) and counterwound secondary (each $\sim$1300 turns) coils were wound with 44 AWG Cu wire on a Kapton tube possessing an ID of 6.4 mm. The ensemble of coils is $\sim$30 mm long and has an OD of 8.7 mm, thereby allowing clearance into the sample region of the SQUID magnetometer. One variation of the probe included optical fibers that passed down the center of the stainless steel support rod. The detection electronics involve a lock-in amplifier and the experiment is controlled by LabView software. Typical AC (1 Hz - 1 kHz) fields of $\sim$10 $\mu$T afford the study of the temperature, frequency, and dc-field bias dependencies of magnetically interesting samples such as the spin ice material Ho$_{2}$Ti$_{2}$O$_{7}$ $[1]$ and nanoparticles of Prussian blue analogs $[2]$.\\ $[1]$ M. Orend\'{a}\v{c} et al., elsewhere in these proceedings.\\ $[2]$ D. M. Pajerowski, F. A. Frye, D. R. Talham, and M. W. Meisel, New J. Phys. 9 (2007) 222. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B13.00008: Recreation of Natural Optical Phenomena Tiffany Paonessa, Peter Sheldon This project was undertaken to study and fully understand optical atmospheric phenomena. Research was done on the structure and formation of colorful atmospheric phenomena including, but not limited to, primary, secondary, and supernumerary rainbows, halos, parhelia, and glories. This study also undertakes an attempt to create some of these phenomena. Using hand-made epoxy crystals for ice, a round bottom flask as a water droplet, and a high-powered halogen lamp for sunlight, primary, secondary, and supernumerary rainbows and halos were created and photographed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B13.00009: Visualization of fracture precursors in vitreous silica: study of under- and over coordinated ions Frank Jones, Romulo Ochoa, Deborah Knox We have conducted classical molecular dynamics fracture studies of vitreous silica. A new visualization program was designed to observe the fracture process of the sample as a whole. The program also allows users to highlight and focus on the under- and over coordinated oxygen and silicon ions. A BKS potential was utilized to model the ionic interactions. DL-POLY* was the program used to perform simulations. Amorphous silica samples were generated at high temperatures; through a series of quenching and equilibration periods vitreous silica was obtained at room temperature. This method resulted in samples having, initially, a number of under- and over coordinated ions (less than one percent of all ions) that were randomly distributed. Radial distributions functions were obtained to verify the amorphous structure of the samples. Stress was applied by uniaxially straining the samples at various rates. As a sample was strained there was an increase in under coordinated ions with seemingly no correlation to the region where the sample would break. In all our simulations the under coordinated ions concentrated in the vicinity of the breakage region a few picoseconds before fracture occurred. *CCLRC Daresbury Laboratory, UK [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B13.00010: Determining the Onset of Amorphization of Crystalline Silicon due to Hypervelocity Impact C. Shane Poletti, Martina E. Bachlechner Atomistic simulations were performed to study a hypervelocity impactor striking a silicon/silicon nitride interface with varying silicon substrate thicknesses. Visualization indicates that the crystalline silicon amorphizes upon impact. The objective of the present study is to determine where the boundary between amorphous and crystalline silicon occurrs. In the analysis, the silicon substrate is separated into sixty layers and for each layer the average z displacement is determined. Our results show that the boundary between amorphous and crystalline silicon occurs between layers 20 and 22 for an impactor traveling at 5 km/s. This corresponds to a depth of approximately 32 Angstroms into the silicon. More detailed analyses reveals that the z displacement is noticeably larger for the layers that do not have a silicon atom bonded beneath them compared to the ones that do. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B13.00011: $^{14}$N (p,p) Scattering with the KN Van de Graff Accelerator Stephanie Lyons, Michael Wiescher The $^{14}$N (p,p) scattering experiment was performed with the 4 MV KN Van de Graff Accelerator at the Nuclear Structure Laboratory at the University of Notre Dame. The KN experienced many problems throughout the experiment requiring several belt changes, a change of the drive motor bearings, and a resistor check. The first run of data was converted to cross-sections, and normalized to 30\r{ }, which was assumed to be completely Rutherford. Resonances were found at 1.06, 1.55, 1.74, 1.80, 2.34, and 2.47 MeV. These values correlated with previous work done. Further experimentation will be required to clarify the resonances and verify that the scattering at 30\r{ } is completely Rutherford. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B13.00012: Orientation of the adiabatic demagnetization refrigerator in the Micro-X sounding rocket Kaitlyn Yoha, Tarek Saab, Tylor Whitmer, Patrick Wikus The Micro-X sounding rocket is a small rocket equipped with an X-ray telescope and will be launched in 2011. For the telescope to function properly, the adiabatic demagnetization refrigerator (ADR) must be aligned with the optics of the X-ray detectors. During the mission, the ADR will move, thus causing errors. A testing prototype was designed and constructed in the lab to simulate the movement the ADR will experience in flight. This method will monitor the orientation of the ADR relative to the detectors, and allow us to counter the resulting measurement errors. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B13.00013: Use of Multivariate Analysis Techniques to Form a Comparison of Mars Odyssey Gamma Ray Elemental Data to Neutron Data Paul Abbazia The Lunar Reconnaissance Orbiter's (LRO) primary mission is exploration. Additional science falls to a secondary focus. LRO does not possess a gamma ray spectrometer, but it has the collimated neutron detector LEND (Lunar Exploration Neutron Detector). It is of interest to determine as much as possible about the moon's elemental composition using LEND. To do so, data from a similar instrument on Mars Odyssey, HEND (High Energy Neutron Detector), was compared to data from Mars Odyssey's gamma ray spectrometer (GRS). Elemental maps were previously derived from the GRS data, and a relation to HEND would allow for LEND to fulfill this role on LRO. Toward this purpose, different multivariate analysis techniques were used to compare GRS and HEND data, including Principal Components Analysis (PCA), K-means clustering, and Pearson product-moment correlation. Results indicate that two elements well known to effect neutron counts, hydrogen and iron, can be identified by these techniques. Further analysis may find additional relations, which would have benefits to the fields of geochemistry and neutron spectroscopy. [Preview Abstract] |
Session B14: Focus Session: Colloids I: Physical Behavior and Mechanisms
Sponsoring Units: DFDChair: Eric Weeks, Emory University
Room: 315
Monday, March 16, 2009 11:15AM - 11:51AM |
B14.00001: Restricted dislocation mobility in crystals of peanut-shaped colloidal particles Invited Speaker: Recent advances in colloidal particle synthesis techniques have enabled the production of a variety of anisotropic yet monodisperse particles, including colloidal ``peanuts,'' which consist of two connected spherical lobes. Since their shape crudely approximates a dimer, colloidal peanut particles constitute a simple but fundamental extension of the classic system of colloidal spheres. Experimental investigations as well as simulations of colloidal peanut monolayers have shown that at high area fractions the particles form a degenerate crystal (DC). In this structure, the peanut particle lobes order into a triangular lattice, much like close-packed spheres, while the connections between lobe pairs are randomly oriented, uniformly populating the three crystalline directions of the underlying lattice. Comparative studies of crystal formation in rapidly compressed monolayers of peanut-shaped versus spherical particles show that DCs harbor many more defects than equivalent crystals of spheres. This suggests that defect annealing may be frustrated by the constraining rigid connections between particle lobes. To elucidate the interactions between these geometric constraints and defect mobility, we directly examine the mechanisms for dislocation nucleation and propagation in DCs. In particular, we show that obstacles formed by certain particle orientations severely limit the range over which dislocations can glide. Furthermore, we observe that transport over longer distances can proceed through dislocation reactions, which switch the direction of propagation and allow dislocations to bypass such obstacles. In this talk I will discuss the impact that these restricted mechanisms have on the macroscopic properties of DCs. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B14.00002: Experimental Study of Brownian Dynamics of Bent-core Colloidal Particles Chun-zhen Fan, Bhuwan Joshi, Ji-ping Huang, Qi-huo Wei Bent-core or banana-shaped molecules exhibit a variety of intriguing liquid crystalline mesophases including nematics and smectic phases. We try to develop suspensions of bent-core shaped colloidal particles to mimic the bent-core liquid crystals. This report will focus on the fabrication of bent-core colloidal particle suspension, and optical microscopic studies of the Brownian dynamics of individual bent-core colloidal particles. The bent-core colloidal particles confined between two glass substrates are observed through dark-field optical microscopy, and their orientation and position are obtained through imaging processing. Results on the translational and rotational Brownian dynamics of these type of particles will be reported. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B14.00003: Pinch-off Dynamics of Non-Newtonian Fluids F. M. Huisman, P. Taborek The pinch-off dynamics of a variety of shear-thinning fluids (foams, concentrated emulsions, and slurries) were studied using high speed videography. The pinch was characterized by the variation of the minimum neck radius rmin as a function of the time to pinch $\tau$. For inviscid fluids, rmin scales as $\tau$ to the 2/3 power. We found that for all the shear-thinning fluids rmin scales with $\tau$ to a power in the range 0.2 to 0.5. To study the transition from conventional inviscid pinch, we systematically varied the concentration of a water-bentonite mixture. As the concentration increased the pinch event transitioned from a needle shape resulting in a satellite drop to a symmetric hyperbolic shape with no satellite drop. These results will be compared with the simulations of Suryo and Basaran (J. Non-Newtonian Fluid Mech. 138 (2006) 134-160). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B14.00004: Melting Dynamics of 3D Hard Sphere Colloidal Crystals Deniz Kaya, N. L. Green, C. E. Maloney, M. Widom, M. F. Islam We use thermally responsive monodisperse micron sized colloidal particles with hard-sphere interactions to study the melting mechanisms in colloidal crystals. As we increase the temperature, these spherical microgel particles decrease in volume, inducing melting in the colloidal crystals. We use video microscopy and image analysis to determine the dispersion relations and the local elasticity near the melting transition. We compare our findings with existing melting and freezing theories. This work has been partially supported by the NSF through Grants DMR-0619424 and DMR-0645596, and by ACS-PRF. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B14.00005: Multiple-Stage Melting and Freezing of Colloidal Crystallites with Short-range Attraction Liquan Pei, J.R. Savage, A.D. Dinsmore We study the dynamics of melting and freezing in a model colloidal system with short-range, temperature tunable attraction. In particular, we mix micron-sized, charge stabilized polystyrene spheres with salt and the surfactant pluronic P103. The pluronic micelles induce depletion attraction whose range is approximately 1.5$\%$ of the sphere diameter and whose magnitude changes strongly with temperature. We use optical microscopy to record the dynamics of freezing and melting following temperature changes. We use particle tracking algorithms to identify the particles with sub-pixel resolution and measure metastable cluster sizes, order parameters, and bond lengths. We have observed that melting and freezing occur in multiple stages, with a metastable liquid phase appearing in both processes. Our results are relevant to protein crystallization where the interactions are also of short range, and to other systems where non-equilibrium states may play a role in phase separation. We thank the NSF for support through grant DMR-0605839. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B14.00006: Melting and Freezing of Colloidal Crystals on Strained Substrates John Savage, Rajesh Ganapathy, Itai Cohen We present results of experiments studying the effect of strain on the dynamics of melting and freezing in single-layer colloidal crystals with a short-range attractive interaction. Our system consists of micron sized colloidal particles and a tunable depletant allowing reversible control of the interaction \textit{via} small temperature changes. We explore the role of strain in the dynamics of melting and freezing by investigating crystallization on a flat patterned substrate. We find that in comparison with previously performed experiments on flat unpatterned substrates, the dynamics of melting and freezing on such surfaces alter dramatically. For example whereas melting of such crystals on a flat substrate was shown to proceed through an intermediary metastable liquid phase, we find that for surfaces templated with a lattice that is commensurate with that of the melting crystal, this intermediary step is suppressed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B14.00007: Transport and Sedimentation of Suspended Particles in Fracture Channels Tak Shing Lo, Joel Koplik Particulate suspensions are ubiquitous in nature and in many artificial situations, and their transport and deposition dynamics are of importance in many chemical, petroleum and environmental processes. While most of the studies in particle transport in confined geometry were done with smooth surfaces in the past, realistic geological fractures usually have irregular rough surfaces that have self-affine structures. We consider the combined effects of sedimentation and inertial transport of particles suspended in a Newtonian fluid in a pressure-driven flow in channels with self-affine surfaces, which is especially relevant to clogging phenomena where sediments may block continuous fluid flows in channels that may occur in geological or industrial processes. We perform a systematic study using the lattice Boltzmann method, which is flexible and particularly suitable for handling irregular geometry. Our results cover a board range in Reynolds and buoyancy numbers, and in particle concentrations. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B14.00008: Particle Organization by Absorbing State Dynamics Laurent Cort\'{e}, David Pine, P.M. Chaikin In a recent study we have found that irreversible collisions can lead to a dynamical phase transition between a constantly evolving state and an absorbing, quiescent state where particles self organize to avoid further collisions. Here we investigate the organization and order in the absorbing state in a model where active, overlapping particles are given random displacements. We contrast the order to what is obtained thermodynamically for hard spheres. We also show that correlated displacements between colliding particles can lead to crystallization and suggest that irreversible flows are a different yet effective tool for ordering particles in desired motifs. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B14.00009: Diffusion through Colloidal Shells under Stress J. Guery, J. Baudry, D.A. Weitz, P.M. Chaikin, J. Bibette The permeability of solids has long been associated with a diffusive process involving activated hopping. Tensile stress can affect the activation energy as originally envisioned by Eyring. Here we use liquid core - solid shell, core-shell, solid colloidal particles that are sensitive to osmotic pressure, to follow the permeation of encapsulated probes at various stresses. We unambiguously show that the tensile stress applied on colloidal shells linearly reduces the local energy barrier for diffusion. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B14.00010: Dispersion relation and density of states of coupled plasmon modes in periodic chains of metallic nanoparticles C.W. Ling, M.J. Zheng, K.W. Yu Energy transmission through one-dimensional chains of equally spaced metallic nanoparticles has been studied via the propagation of coupled surface-plasmon modes. These modes are characterized by well-defined dispersion relation $\omega(k)$ and group velocity $v_g=d\omega/dk$ in a band. The nanoparticles are routinely modelled by Drude metallic spheres and the coupled plasmon modes are calculated in the point-dipole approximation. When the particles approach and finally touch, these bands can differ significantly from those obtained by the point-dipole approximation due to strong multipolar interaction among the particles. In this regard, we have calculated the coupled plasmon modes by a tight-binding approach, taking fully multipolar interactions into account. For approaching particles, the dipolar bands move from the visible down to the infrared region and $\omega(k)$ becomes almost independent of $k$. Concomitantly, the group velocity $v_g$ showed an intriguing non-monotonic behavior versus the particle spacing. When the spacing decreases, $v_g$ increases initially but decreases when the particles approach and touch. For moderate spacing, $v_g$ can be reduced drastically to $0.01 c$, except at $kd=0$ and $kd=\pi$, resulting in a slow propagation. Thus one can tune the propagation of plasmon modes by simply varying the spacing between the particles. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B14.00011: Examining dynamic length scales in a two-dimensional colloidal system Zach Nadler, Cara Hageman, Vikram Prasad, Eric R. Weeks We study polystyrene colloids placed at an oil-water interface as a quasi-two-dimensional colloidal system. As the area fraction of the colloidal particles is increased, we see liquid, hexatic, and crystalline phases. The liquid phase is structurally disordered; the hexatic phase has long range orientational order but poor translational order; and the crystalline phase has long range orientational and translational order. We classify these different phases using structural and dynamic parameters from prior work. Using a laser tweezer we trap and drag a particle along the interface and observe its effect on the surrounding colloids. Our interest is in how the response changes near phase transition boundaries, where the ordering of particles can qualitatively change. We characterize the response by the structural defects induced by the dragged particle, as well as the perturbed motion of the surrounding particles. These responses are localized around the dragged particle, and we study how the localization length scale changes with the area fraction of the colloids. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B14.00012: State Diagram for Optical Tweezers Induced Brownian Motors Bo Sun, David Grier State Diagram for Optical Tweezers Induced Brownian Motors Bo Sun and David G. Grier Center for Soft Matter Research Department of Physics New York University Optical tweezers are extensively used in physics and biology, most study in literatures assume a colloidal particle trapped in optical tweezers relaxes to equilibrium state. To the contrary, we have found experimentally the particle became a Brownian motor. Further more, this Brownian motor showed reversible behavior: given input power, working direction changes when particle size grows; given a particle bigger than wave length of light, changing input power can also change the working direction. Thus we need a state diagram to describe the motor behavior of a colloidal particle in optical tweezers, rather than a potential landscape as most previous study uses. [Preview Abstract] |
Session B15: Current Fluid Mechanics
Sponsoring Units: DFDChair: Shelley Anna, Carnegie Mellon University
Room: 316
Monday, March 16, 2009 11:15AM - 11:27AM |
B15.00001: Walking on water: why your feet get wet Michael Shelley, Jake Fontana, Peter Palffy-Muhoray Walking on wet pavement during or after heavy rain results in wet shoes, and often, wet feet. We describe a peculiar transport process associated with walking on wet surfaces which results in the vamps, and frequently, the insides, of shoes getting wet. We discuss details of this process and compare experimental results with simple model predictions. Strategies for keeping feet dry will be considered. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B15.00002: Breaking beer bottles with cavitation Sunny Jung, Jake Fontana, Peter Palffy-Muhoray, Michael Shelley Hitting the top of a beer bottle, nearly full of water, with an open hand can cause the bottle to break, with the bottom separating from upper section. We have studied this phenomenon using a high-speed camera, and observed the formation, coalescence and collapse of bubbles. The breaking of glass is due to cavitation, typically occurring near the bottom edge. We make numerical estimates of the relevant physical parameters, and compare these with experimental observations. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B15.00003: Optical Tweezer as a Viscometer Daniel Erenso, Samuel Elrod, Taylor Barns, Anthony Farone , Mary Farone An optical tweezer (OT) has been widely used to study the mechanical properties of microscopic living biological systems like red blood cells. These studies are based on measurement of deformations caused by a force exerted directly or indirectly by an optical trap. The trap is usually pre-calibrated using Stokes viscous force of the suspension fluids for the biological system which is directly proportional to the viscosity of the fluids. Therefore, calibration of the trap depends on the viscosity of the fluid which depends on temperature. In this work, we have demonstrated that OT can be used to precisely measure the viscosity of biological fluids affected by temperature. Using a an infrared laser trap which is calibrated using silica sphere suspended in a distilled deionized water and measuring the power as function of escape velocity, we have measured the viscosities of a newborn and unborn bovine serum with a different concentration of antibodies. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B15.00004: Fabrication of a Nanoscale Thermal Anemometry Probe Via Electric Field Assisted Assembly Jason Kawasaki, Sean Bailey, Lex Smits, Craig Arnold A nanoscale thermal anemometry probe (NSTAP) is being developed to measure instantaneous fluid velocity at ultra-small scales using conventional constant temperature anemometry principles. The probe consists of a 50 nm by 10 um platinum nanowire (NW) suspended between two current carrying electrodes. Previous nanoscale anemometry wires had been fabricated via metal deposition on a photolithography-patterned substrate; however, deposited NWs are not free-standing and thus must later be lifted off the substrate resulting in low process yields. In this presentation, we discuss alternative methods of shrinking the probes further and increasing the yield of successful probes, including growing nanowires from solution to bridge the electrodes, and using dielectrophoresis to align pregrown nanowires between the electrodes. In each of these methods, the NWs are directly assembled in the desired structure eliminating the need for additional processing steps. NSTAP probes manufactured using these methods will also exhibit higher spatial resolution and temporal response than previous NSTAP designs. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B15.00005: A Front Tracking Algorithm for Liquid Jet Breakup Wurigen Bo, James Glimm, Xingtao Liu A numerical study of breakup of a high speed jet is presented using the Front Tracking method in 3D. A robust locally grid based method is applied to handle the topological change of the surface mesh in the simulation, the validation of the method is proved mathematically. Numerical results are presented for 3D simulation of the primary breakup of a liquid jet with turbulent inflow. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B15.00006: Coalescence and Pinch-Off in Viscous Liquids Joseph Paulsen, Justin Burton, Sidney Nagel When two fluid drops come into contact, a topological transformation occurs as they rapidly coalesce into a single drop. Because of its speed and geometry, this finite time singularity is difficult to study optically. We therefore use an electrical method to probe viscous coalescence as early as 10 ns after contact. This technique was developed by Burton \textit{et al}.[1] to study mercury drop pinch-off and adapted for salt-water coalescence by Case \textit{et al}.,[2] revealing a breakdown of the expected universal dynamics in early-time inviscid coalescence. For viscous coalescence, we measure a resistance that decreases as t$^{-1}$ at early times and as t$^{-1/2}$ at late times, with a crossover time that increases with viscosity. In the inviscid case, these power laws had been interpreted with a model in which the drops coalesce at a slightly deformed interface.[2] We explore this possibility as well as others, such as an anomalously long viscous regime. This electrical technique is also used to study viscous fluid pinch-off, which we compare with previous optical studies. [1] J. C. Burton, J. E. Rutledge, and P. Taborek, Phys. Rev. Lett. \textbf{92}, 244505 (2004). [2] S. C. Case and S. R. Nagel, Phys. Rev. Lett. \textbf{100}, 084503 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B15.00007: Scaling Law for Driven Spreading and Coalescence of Sessile Droplets Pilgyu Kang, Shahab Shojaei-Zadeh, Christine Appleby, Shelley Anna This study investigates the dynamics of spreading and coalescence of droplets on a surface, a process important in applications such as inkjet printing, spray coating, and flooding of fuel cells. We use a simple microfluidic device to control the spreading and merging processes. Droplet diameter and maximum height are monitored as functions of time. We compare the dynamics with existing scaling models modified to incorporate time dependent volume, and we extend the model to describe the scaling behavior of the liquid bridge growing between merging droplets on a surface. The experiments agree well with the expected scaling. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B15.00008: Temperature profiles near a pinned nucleating bubble Scott Parker, Chang-Ki Min, Sung Chul Bae, David Cahill, Steve Granick We have measured the temperature distribution on solid surfaces in contact with a nucleating vapor bubble by thermal surface plasmon imaging. Vapor bubbles are created by focused laser heating of an underlying metal substrate. Bubbles are pinned in place by suitable surface functionalization and their shape is characterized by interferometry. Varying the wettability of the surface to control the shape and surface lifetimes of bubbles, we have correlated contact angle, lift-off diameter, and local temperature. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B15.00009: Effect of Encapsulated Polymers and Nanoparticles on Deformation of Droplets O. Berk Usta, Dennis Perchak, Andrew Clarke, Julia M. Yeomans, Anna C. Balazs We investigate the effects of polymer chains and nanoparticles on the deformation of a droplet in shear and extensional flow using computational modeling; Our model accounts for both the solid and fluid phases explicitly. We show that under shear flow, both the nanoparticles and the encapsulated polymers reduce the shear-induced deformation of the droplet at intermediate capillary numbers; nevertheless, long polymer chains can induce the breakup of the droplet at high capillary numbers. In contrast, under extensional flow we find that the long polymer chains inhibit the breakup and reduce deformation. We study the chain-length and concentration dependence and also present the effects of various parameters such as the wetting strength. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B15.00010: Electrorheology Leads to Efficient Combustion R. Tao, K. Huang, H. Tang, D. Bell Improving engine efficiency and reducing pollutant emissions are important. Since combustion starts at the interface between fuel and air and most harmful emissions come from incomplete burning, reducing the size of fuel droplets for the fuel injection would increase the total surface area to start burning, leading to a cleaner and more efficient engine. While most efforts are focused on ultra-dilute mixtures at extremely high pressure to produce much finer mist of fuel for combustion, the new technology is still under development and only for next generation vehicles. Here we report our fuel injection technology based on new physics principle that proper application of electrorheology can reduce the viscosity of petroleum fuels. A small device is thus introduced just before the fuel injection for the engine, producing a strong electric field to reduce the fuel viscosity, resulting in much smaller fuel droplets in atomization. Both lab tests and road tests confirm our theory and indicate that such a device improves fuel mileage significantly and reduces emission. The technology is expected to have broad applications, applicable to current internal combustion engines and future engines as well. Supported by STWA and RAND. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B15.00011: Evolution of Electrified Films on a Porous Inclined Plane Uma Balakrishnan, Usha Ranganathan The nonlinear stability of a thin conducting film flow down a porous inclined plane, when an electric field acts normal to the plane is considered. It is assumed that the flow through the porous medium is governed by Darcy's law and the characteristic length of the pore space is much smaller than the depth of the fluid layer above. Integral Boundary Layer method is employed in obtaining a set of exact averaged equations for the film flow system. Linear stability results through normal mode analysis reveal that the destabilizing influence of the electric field is further enhanced by the porosity of the medium. Critical Reynolds number for the onset of instability decreases with the increase in the permeability of the porous plane. Weakly nonlinear stability analysis using method of multiple scales divulges the existence of zones due to supercritical stability and subcritical instability. Permanent finite-amplitude waves in the supercritical stable region are portrayed by solving the nonlinear evolution equation numerically in a periodic domain. The parameter ranges that support complex nonlinear dynamics is obtained through a combination of theoretical analysis and numerical experiments. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B15.00012: Universal cone angle of ac electrosprays due to net charge entrainment Nishant Chetwani, Siddharth Maheshwari, H.C. Chang The slender meniscus that is obtained by the application of high frequency AC field is quite distinct from DC Taylor Cone. This AC cone shows a continuous longitudinal growth and has much smaller half cone of $\sim $ 11$^{o}$. Mass spectrometry on the microjet from the AC cone shows that dissociation reaction occurs at the tip but only the low- mobility anionic species are entrained to produce a charged cone. These free negative charges relax to the interface to produce a non-uniform surface charge density that scales with respect to the azimuthal radius as $\rho ^{-\frac{1}{2}}$ to balance the singular normal capillary pressure. Repulsion of this entrained surface charge and the Maxwell pressure they induce are estimated with an elliptic integral and a variational formulation produces anormal stress balance with capillary pressure that is only satisfied at a universal angle of 12.6$^{o}$ degrees for the liquids with high dielectric constant in good agreement with the measured values for the organic solvents used in experiments [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B15.00013: Impact of Elasticity on Coating Flow near A Moving Contact Line Yuli Wei, Stephen Garoff, Enrique Ram\'e, Lynn Walker The impact of fluid elasticity and shear thinning on the dynamic wetting of polymer solutions is important because many fluids, even those that are normally considered Newtonian, exhibit non-Newtonian behaviors in the high shear environment of the wedge-like geometry near a moving contact line. Even though this behavior is on the microscopic scale, it has significant impact on wetting on the millimeter scale. Shear thinning dramatically modifies the flow field near a moving contact line and results in a reduced curvature of the free surface. In this talk, we will focus on the effects due to fluid elasticity. Both experimental and theoretical results are presented. The fluids we use are the dilute solutions of high molecular weight polyisobutylene (PIB) which exhibit elasticity-dominated rheology with minimal shear thinning. Their wetting behaviors are compared to their oligomer ``solvent,'' which is considered Newtonian based on standard rheometry. We will also discuss a lubrication analysis of the wedge-like flow field using an Oldroyd-B constitutive relation to mimic the stress evolution of the elastic solution. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B15.00014: Field-dependent thermal transfer in magnetic fluids Jun Huang, Zhenyu Zhou, Geoff Huston, Weili Luo The temperature gradient across a quasi one-dimensional magnetic fluid was measured as a function of the magnetic field and field gradient. It was found that when the field gradient, $\nabla B$, is anti-parallel to the temperature gradient, $\nabla T$, the temperature gradient increases with increasing field and field gradient, but decreases for $\nabla B$ parallel to $\nabla T$. For B and $\nabla B$ perpendicular to $\nabla T$ and gravity, the results are complex and depend on the local configuration of the field and field gradient. We will discuss the results in terms of the effect of local magnetic body force that originates from the local field and the local susceptibility on thermal transfer in magnetic fluids. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B15.00015: How does the viscosity of a lubricant effect its tribological behavior? M. Aggleton, P. Taborek The viscosity of many conventional lubricants varies by many orders of magnitude over a small temperature range. We have exploited this variation to explore the effect of large viscosity changes on lubrication. We have used a sliding block tribometer to measure the coefficient of friction of a steel on steel system with a variety of vacuum compatible hydrocarbon lubricants. Each lubricant was thermally cycled in ultrahigh vacuum from room temperature to below the glass transition temperature. This varies the viscosity without changing the chemistry. Several theoretical models for the temperature dependence of the viscosity of hydrocarbons are applied. The theory described in Cameron (1981) is used to relate the change in viscosity to the coefficient of friction. Some lubricants are found to fit these models up to viscosities as high as $10^6$ centiStokes, while for others the model does not even qualitatively describe the data. [Preview Abstract] |
Session B16: Cold Fusion
Sponsoring Units: DCMPChair: Scott Chubb, Naval Research Laboratory
Room: 317
Monday, March 16, 2009 11:15AM - 11:27AM |
B16.00001: Electrodynamic Component of Pd Electrical Conductivity Mitchell Swartz The electrical resistance of Pd$_1$$_-$$_x$D$_x$, [$\rho_P$$_d$] has been used to estimate loading\footnote{Bambakidis, G., et al, Phys. Rev. 177, 1044 - 1048, 1969}$^,$\footnote{McKubre, M. et al, ICCF-1, 1990.}. We discuss with experimental evidence that $\rho_P$$_d$ has electrodynamic components; some may trigger Lattice Assisted Nuclear Reactions (LANR). Type ``B'' (anode plate) Pd/D2O/Pt Phusor$^T$$^M$ LANR devices\footnote{Swartz, M, Fusion Technology, 31, 228-236, 1997.}$^,$\footnote{Swartz, M, Fusion Technology, 31, 228-236, 1997.} (excess heat $\approx$ 175 percent, 1.99 watts) demonstrate two time constants of $\rho_P$$_d$(t). The first ($<$ 5 seconds) is not from deuteron loading. Also, at high loading, Type ``B'' systems produce an instability oscillation. These possible electrodynamic effects, and the supralinear rise of $\rho_P$$_d$, may trigger, or maintain, LANR. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B16.00002: Study Of The Palladium Hydrogen - Deuterium System Jan Marwan Electrochemical deposition of metals from hexagonal lyotropic liquid crystalline phases produces metal films with a unique ordered nanostructure in which the cylindrical pores of 1.7 to 3.5 nm running through the film are arranged in hexagonal array\footnote{Attard, G. S, el al, Mesoporous Science 1997, 278, (31), 838- 840.}$^,$\footnote{Attard, G. S, et al, Langmuir 1998, 14, 7340- 7342.}$^,$\footnote{Attard, G. S.; Goeltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H., Angew. Chem. Int. Ed. Engl. 1997, 36, 1315-1317.}. Nanostructured Pd films were deposited electrochemically from the template mixture of either C$_{16}$EO$_8$ or Brij56. Electrochemical studies showed that the metal films have a high electroactive surface area with the specific surface area of the order of 91 m$^2$/g. These values together with the TEM and X-ray data are consistent with the expected H$_1$ nanostructure. The hydrogen region of nanostructured Pd in the cyclic voltammetry in 1 M H$_2$SO$_4$ was more resolved than that of plain Pd because of the thin walls of the nanostructure and the high surface area. We could distinguish the hydrogen adsorption and absorption processes. The permeation of hydrogen into the Pd metal lattice occurs with fast kinetics when the Pd surface is blocked by either crystal violet or Pt. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B16.00003: Investigation of the Cold Fusion Phenomenon in the Surface Region of Hydrogen Non-occlusive Metal Catalysts; W, Pt, and Au Hideo Kozima, Tadahiko Mizuno There are several experimental data sets showing occurrence of the cold fusion phenomenon (CFP) in such contact metallic catalysts which do not occlude hydrogen isotopes such as tungsten (W), platinum (Pt) and gold (Au). These metals do not occlude hydrogen isotopes and are different from such hydrogen occlusive transition metals usually used in the cold fusion (CF) experiments such as Ti, Ni, and Pd. The non-occlusive isotopes and give us precious information about mechanisms facilitating formation of specific matter for the CFP (CF matter) in the cold fusion material. In the electrolysis experiments with these metals as electrodes and with several electrolytes in light or heavy water, transmuted nuclides were observed in the surface layer of the electrodes. The generated nuclides were localized in areas with a diameter of around a few $\mu$m in the surface layer of thickness less than 103 nm. These specific nuclides observed in the host metals, W, Pt and Au, depend on the composition of the electrolyte. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B16.00004: Sonofusion: Squeezed Deuteron Clusters, With Small Size, High Energy Density but No High Energy Particles Roger Stringham Inertial confined fusion when viewed as a natural process compares with sonofusion's electromagnetically squeezed deuteron cluster. Sonofusion capitalizes on its very small size and its higher energy densities. It is a relatively cool process, with the endothermic removal of heat, 13.6 ev, from a target implanted with clusters of deuterons; the fusion environment. The energy densities approach those of the deuteron sepaaration in muon DD fusion. This helps explain sonofusion's experimental results of heat and helium four. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B16.00005: Time-Dependent Changes in Morphology and Composition of Solid Particles Collected From Heavy Water Electrolyte after Electrolysis with a Palladium Cathode John Dash, Q. Wang Recently, we have observed particles floating on the surfaces of electrolytes after electrolysis, in four cells, each of which contained a heavy water electrolyte and a Pd cathode. Solid particles were unexpected from electrolysis, so it seemed important to characterize these particles. Cu grids were used to collect particles from the electrolyte surface. Then, a scanning electron microscope ( SEM ) and an energy dispersive spectrometer ( EDS ) were used to study the surfaces of these particles and to record time-dependent changes which were occurring. The morphology and composition of the particles were determined . After storage at ambient for 11 days, there were large changes in the morphology and composition of the particles. For example, one portion of the particles contained a large number of microspheres. A typical microsphere contained mostly carbon and palladium, whereas the matrix near the microsphere contained mostly palladium with less carbon and a significant amount of silver. One day later the same microsphere had increased carbon and reduced palladium, but there was no significant change in the composition of the matrix. Results for other particles from other cells will also be presented. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B16.00006: Study of methods to increase cluster/dislocation loop densities in electrodes Xiaoling Yang, George H. Miley Recent research has developed a technique for imbedding ultra-high density deuterium ``clusters'' (50 to 100 atoms per cluster) in various metals such as Palladium (Pd), Beryllium (Be) and Lithium (Li). It was found the thermally dehydrogenated PdHx retained the clusters and exhibited up to 12 percent lower resistance compared to the virginal Pd samples\footnote{A. G. Lipson, et al. Phys. Solid State. 39 (1997) 1891}. SQUID measurements showed that in Pd these condensed matter clusters approach metallic conditions, exhibiting superconducting properties\footnote{A. Lipson, et al. Phys. Rev. B 72, 212507 (2005}\footnote{A. G. Lipson, et al. Phys. Lett. A 339, (2005) 414-423}. If the fabrication methods under study are successful, a large packing fraction of nuclear reactive clusters can be developed in the electrodes by electrolyte or high pressure gas loading. This will provide a much higher low-energy-nuclear- reaction (LENR) rate than achieved with earlier electrode\footnote{Castano, C.H., et al. Proc. ICCF-9, Beijing, China 19-24 May, 2002.}. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B16.00007: Metal Catalyzed Fusion: Nuclear Active Environment vs. Process Talbot Chubb To achieve radiationless dd fusion and/or other LENR reactions via chemistry: some focus on environment of interior or altered near-surface volume of bulk metal; some on environment inside metal nanocrystals or on their surface; some on the interface between nanometal crystals and ionic crystals; some on a momentum shock-stimulation reaction process. Experiment says there is also a spontaneous reaction process. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B16.00008: Potential Role of Energy Band Theory in Finite Solids and in Resonant Phenomena Involving Metal-Induced Fusion and the Fleischmann-Pons Effect Scott Chubb Considerable confusion occurred from a speculative conjecture that Talbot Chubb and I suggested in 1989, concerning the potential role of conventional energy band theory in the ``cold fusion'' claims, suggested by Fleischmann and Pons.\footnote{David Lindley, Nature 344, 375 (1990).}. Two important reasons for this are related to: 1. Misconceptions, about what was taking place in the experiments, and 2. Limitations of conventional energy band theory. In particular, Talbot Chubb and I proposed the idea that deuterium nuclei (deuterons) could occupy energy band states or have overlap with these kinds of states with ``unforeseen'' consequences, including, the possibility of nuclear fusion. Conventional energy band theory has limitations, associated with the underlying quantum mechanics. Talbot Chubb and I have investigated an important problem, relating to extending conventional energy band theory, as it applies to infinitely-repeating ordered crystals, to finite crystal lattices, where energy band theory can be re-expressed more precisely through resonant or nearly-resonant effects and the ``conventional'' Coulomb Barrier problem of fusion can be replaced by a considerably richer problem. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B16.00009: Wave nature of the deuterium flux permeating palladium thin film Xing Z. Li, Bin Liu, Qing M. Wei In the past 20 year research on Condensed matter Nuclear Science, the deuterium flux permeating palladium has been found correlated with the ``excess heat''\footnote{J. Phys. D: Appl. Phys.36 3095(2003)}. An experiment was conducted to reveal the wave nature of the deuterium flux permeating Pd thin film. at the temperature higher than the boiling point of the heavy water. The deuterium flux through Pd thin film was considered as a monotonic function of the thickness of the Pd film because the diffusion theory (Fick's Law) was applied. Indeed the deuteron could not be treated as a granular particle at low energy. It should be treated as a wave, because its de Broglie wave length is comparable with the lattice constant of the Pd crystal (3.84 Anstrons). When the titanium carbide (TiC) thin layer was sputtered on the surface of the Pd subtrate alternatively with Pd layers, we found that the flux might increase with the number of the layers first; then, it decreased after reaching a peak. which is the characteristics of a wave. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B16.00010: Isomorphic Properties of Atoms, Molecules, Water, DNA, Crystals, Earth, SolarSystem and Galaxies F.A. Gareev, G.F. Gareeva, I.E. Zhidkova We discuss the cooperative resonance synchronization enhancement mechanisms of Low Energy Nuclear Reactions (LENR). Some of the low energy external fields can be used as triggers for starting and enhancing exothermic LENR. Any external field shortening distances between protons in nuclei and electrons in atoms should enhance beta-decay (capture) or double-beta decay (capture). We have proposed a new mechanism of LENR: cooperative resonance synchronization processes in the whole system nuclei+atoms+condensed matter+gaseuos+plasma medium, which we suggest can occur at a smaller threshold than the corresponding ones on free constituents. The cooperative processes can be induced and enhanced by low energy external fields. The excess heat is the emission of internal energy, and transmutations at LENR are the result of redistribution inner energy of the whole system. [Preview Abstract] |
Session B17: Focus Session: Progress towards Scalable Quantum Information Processing
Sponsoring Units: GQIChair: Mark Dykman, Michigan State University
Room: 318
Monday, March 16, 2009 11:15AM - 11:51AM |
B17.00001: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 11:51AM - 12:03PM |
B17.00002: Electrons on Helium using circuit quantum electrodynamics[1] David Schuster, Mark Dykman, Stephen Lyon, Robert Schoelkopf It is possible to form a two dimensional electron gas at the interface between superfluid helium and vacuum.~ This unique heterostructure has exceptional bulk properties including electron mobilities exceeding 10$^{7}$ cm$^{2}$/Vs and electron spin coherence times estimated to exceed 100s.~ One of the first proposals [2] for quantum computation employed the \textit{vertical} motional states of electrons on helium but coherent interactions have yet to be realized.~ I will describe a new proposal [3] which uses a high finesse superconducting transmission line cavity to detect and manipulate the \textit{lateral} motional and spin states of a single trapped electron on helium. We estimate that it is possible to attain vacuum Rabi frequencies of g=10 MHz and T$_{1}\sim $T$_{2}\sim $1 ms for the motional state and perhaps even longer coherence times if spin encoding is used. \newline \newline [1] Wallraff, et. al. Nature, 2004, 431, 162 \newline [2] Plattzman and Dykman, Science, 1999, 284, 1967 \newline [3] In preparation [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B17.00003: Simulating electron transport and devices on liquid helium Pascal Bugnion, Stephen Lyon, Forrest Bradbury Manipulation of the spin of electrons in surface states on superfluid helium is a promising method for the implementation of a quantum computer. The electrons can be transported around a substrate along channels in a manner analogous to charge-coupled devices. These devices operate on one or a few electrons, which are sufficiently isolated to be treated as classical point charges. The model must therefore incorporate the discreteness of the charges and their interactions, as well as their response to external potentials. These constraints lead towards considering a ``molecular'' dynamics, multi-electron simulation. The calculation of the electron-electron interactions are complicated by the presence of nearby metallic gates and insulating layers. The concepts necessary for a fast, accurate dynamic simulation of a large collection of individual electrons are elaborated. A computationally cheap approximation of the electrostatic potential due to substrate polarisation for an electron above a channel is proposed. The approximation is compared to an analytic solution. Other substrate geometries which might be used in a quantum computer are also discussed, concentrating on approximations of the potential. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B17.00004: Quantum logic with weakly coupled qubits Michael Geller, Emily Pritchett, Andrei Galiautdinov, John Martinis Effective protocols for performing CNOT quantum logic with qubits coupled by particular high-symmetry (Ising or Heisenberg) interactions are well established. However, many architectures being considered for quantum computation involve qubits or qubits and resonators coupled by more complicated and less symmetric interactions. Here we consider a widely applicable model of weakly but otherwise arbitrarily coupled two-level systems, and use quantum gate design techniques to derive a simple and intuitive CNOT construction. Useful variations and extensions of the solution are given for common special cases. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B17.00005: Quantum gates that correct their own (quantum) errors Lorenza Viola, Kaveh Khodjasteh Scalable quantum computation in realistic devices requires that precise control can be implemented efficiently in the presence of decoherence and operational errors. I will describe a general constructive procedure for designing robust unitary gates on an open quantum system without encoding or measurement overhead. These results allow for a low-level error correction strategy solely based on Hamiltonian engineering using realistic bounded-strength controls, and may prove instrumental to substantially reduce implementation requirements for fault-tolerant quantum computing architectures. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B17.00006: A few-electron triple quantum dot incorporating two fast charge sensors Edward Laird, Charles Marcus, Micah Hanson, Art Gossard A triple quantum dot is defined in a GaAs heterostructure. The occupation of all three dots is monitored using two nearby charge sensing point contacts. Radio frequency multiplexing in a reflectometry setup allows MHz-bandwidth measurements of both charge sensors independently. Configuring the device in the few-electron regime, we achieve coherent spin manipulation using the exchange interaction. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B17.00007: Interqubit coupling mediated by a high-excitation-energy quantum object Sahel Ashhab, Antti Niskanen, Khalil Harrabi, Yasunobu Nakamura, Thomas Picot, Pieter de Groot, Kees Harmans, Hans Mooij, Franco Nori We consider a system composed of two qubits and a high-excitation-energy quantum object used to mediate coupling between the qubits. After reproducing well-known results concerning the leading term in the mediated coupling, we obtain an expression for the residual coupling between the qubits in the off state. We also analyze the entanglement between the three objects, i.e. the two qubits and the coupler, in the eigenstates of the total Hamiltonian. Although we focus on the application of our results to the recently realized parametric-coupling scheme with two qubits, we also discuss extensions of our results to harmonic-oscillator couplers, couplers that are near resonance with the qubits and multi-qubit systems. In particular, we find that certain errors that are absent for a two-qubit system arise when dealing with multi-qubit systems. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B17.00008: Excitons in cavity-embedded quantum dot lattices Michal Grochol, Carlo Piermarocchi We investigate excitons and trions in a two-dimensional quantum dot lattice embedded in a planar optical cavity. The strong exciton (trion)-photon coupling is described in terms of polariton quasiparticles. First, we focus on Bragg polariton modes obtained by tuning the exciton and the cavity modes into resonance at high symmetry points of the Brillouin zone. The effective mass of these polaritons can be extremely small, of the order of $10^8\,m_0$ ($m_0$ is the bare electron mass) and makes them the lightest exciton-like quasiparticle in solids [1]. Second, we consider how disorder affects the properties of Bragg polariton modes. We focus on three kinds of disorder: (i) inhomogeneous exciton energy, (ii) inhomogeneous exciton-photon coupling, and (iii) deviations from an ideal lattice. It is found that in some cases weak disorder increases the light matter coupling and it leads to a larger polariton splitting [2]. Finally, each dot has one electron, and the electron spin determines the polarization of the cavity photon that couples to the dot. Such a ``spin lattice'' can be used for quantum information processing and we show that by using exciton detuning a conditional phase shift gate with high fidelity can be obtained [3]. [1] E. M. Kessler, et al., Phys. Rev. B ${\bf 77}$, 085306 (2008). [2] M. Grochol et al., Phys. Rev. B ${\bf 78}$, 035323 (2008). [3] M. Grochol et al., Phys. Rev. B ${\bf 78}$, 165324 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B17.00009: Low Disorder Si MOSFET Dots for Quantum Computing E.P. Nordberg, L.A. Tracy, G.A. Ten Eyck, K. Eng, H.L. Stalford, K.D. Childs, J. Stevens, R.K. Grubbs, M.P. Lilly, M.A. Eriksson, M.S. Carroll Silicon quantum dot based qubits have emerged as an appealing approach to extending the success of GaAs spin based double quantum dot qubits. Research in this field is motivated by the promise of long spin coherence times, and within a MOS system the potential for variable carrier density, very small dot sizes, and CMOS compatibility. In this work, we will present results on the fabrication and transport properties of quantum dots in novel double gated Si MOS structures. Coulomb blockade is observed from single quantum dots with extracted charging energies up to an including 5meV. Observed dots were formed both from disorder within a quantum point contact, and through disorder free electrostatic confinement. Extracted capacitances, verified with 3D finite element simulations confirm the location of the disorder free dot to be within the designed lithographic structure. Distinctions will be made regarding the effects of feature sizes and sample processing. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B17.00010: Steps toward donor based qubits in Si through integrating single ion Geiger mode avalanche diode detectors J.A. Seamons, E. Bielejec, M.S. Carroll Donor based qubits in Si for solid-state quantum information processing require precise dopant placement into the bulk Si. Placement precision donor is limited by straggle which is strongly dependant upon dopant selection and implantation energy, therefore detection of low energy ions ($<$10 keV) is desired. Great progress has been made using the combination of a $p-i-n$ diode and electron beam lithography patterned surface mask resulting in a signal to noise limited $\sim $10$^{3}$ electron-hole (e-h) pairs detection (D. N. Jamieson \textit{et al.,} Appl. Phys. Lett. \textbf{86}, 202101 (2005)). We present experimental results for a single ion Geiger mode avalanche diode (SIGMA) detector has been shown to be sensitive to a single 250 keV H+ ion with 100{\%} detection efficiency (J. A. Seamons \textit{et al.,} Appl. Phys. Lett. \textbf{93}, 403124 (2008)) as well as advances that have been made with the SIGMA detector in reducing dark (false) counts by three orders of magnitude and placing an upper bound on the e-h pair sensitivity of $\sim $10$^{3}$ produced outside the active region of the SIGMA detector. Future SIGMA designs will enable low energy single ion detection with reduced straggle single donor qubit integration. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B17.00011: Zeno Quantum Gates in Semiconductor Quantum Dots Kaijie Xu, Yuping Huang, Michael Moore, Carlo Piermarocchi Quantum Zeno effect (QZE) is one of the most intriguing quantum phenomena. In the recent literature, there is a series of strongly linked ideas on entanglement generation or computation using the QZE, which have mainly been discussed and explored experimentally in pure quantum optics and superconductors. We propose a scheme for a two-qubit conditional phase gate by QZE with three parallel semiconductor quantum dots [1]. Two of them are charged dots with one additional electron. The spin of these electrons are the logical qubits on which the phase-gate acts. The other dot is an ancillary neutral dot that can perform Rabi oscillations under a resonant laser pulse. With our system setup, we can make use of QZE to gain a $\pi $ phase shift after a 2$\pi $ laser pulse depending on the spin configuration in the logical qubits. This phase shift can realize a conditional phase gate. We solve analytically and numerically the master equation with a realistic set of parameters. The result shows that, despite the widely-held belief that decoherence must always be minimized in quantum information processing, in our scheme decoherence can in principle be harnessed to generate high-fidelity gate operation using the QZE. [1] K.J. Xu, Y.P. Huang, M.G. Moore, and C. Piermarocchi, arXiv: 0810.4489 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B17.00012: Large Scale Quantum Computation in a Linear Ion Trap Guin-Dar Lin, Shi-Liang Zhu, Christopher Monroe, Luming Duan Among the approaches to quantum computation, the trapped ion system remains as one of the leading candidates. The linear Paul trap provides the most convenient architecture for quantum gate operations over a few ions, and the basic requirements for quantum computation have been demonstrated in this setup. However, scaling up this system to a large number of qubits so far remains a formidable challenge because of several obstacles, including the instability of the linear structure and the difficulties of the sideband cooling and addressing for a large ion array. The recent approach to scalable ion trap computation thus has to use a more complicated architecture where the ions are shuttled over different trapping regions. Here, we propose a way to implement large-scale quantum computation in a linear trap by overcoming all the theoretical obstacles. Through excitation of the transverse photon modes in an anharmonic trap, we show that high-fidelity quantum gates can be achieved on ions in a large linear architecture under the Doppler temperature without the requirement of sideband resolving. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B17.00013: Quantifying and Tuning Entanglement for Spin Systems Qing Xu, Sabre Kais, Ahmed Sameh The research carries out a benchmark exact calculation in the field of entanglement in a 19-site two-dimensional spin system. Of particular interest, we study one or more impurities embedded into such systems. We demonstrate that entanglement can be controlled and tuned by varying the ratio of the strength of the magnetic field to the exchange interaction h/J and by introducing impurities. We also discuss the relation of the amount of entanglement, between the impurity spins and the environment, and the decoherence time, which is a quantity measurable in experiments and of relevance in various proposals for traditional and quantum computer hardware. [Preview Abstract] |
Session B18: Focus Session: Templated Assembly of Polymers
Sponsoring Units: DPOLYChair: Alamgir Karim, University of Akron
Room: 319
Monday, March 16, 2009 11:15AM - 11:51AM |
B18.00001: Directed Assembly of Biological Polymers Invited Speaker: The self-assembly of polypeptides into beta-sheet rich nanofibrils has attracted considerable attention in recent years to both understand amyloidgenesis and for their potential biomaterials applications. This self-assembly process is generic to all proteins where fibrillation is typically induced under harsh conditions of low pH and/or high temperature, which are of course not suitable for biomaterials applications. Here we will outline the method developed in our laboratory to create thermo-reversible fibrillar hydrogels from aqueous solutions of a series of proteins by adding a reductant. Proteins studied include beta-lactoglobulin, ovalbimum, lysozyme and bovine serum albimum; all contain an increasing number of disulfide bridges that are disrupted by the reductant. Such disruption destabilises the native state of the protein and this allows us to form transparent, self-supporting hydrogels under physiological conditions. The potential to control and manipulate the gel properties, including mechanical strength and structure (fibre diameter and mesh size of hydrogel) has been explored by varying the protein (consequently the number of disulfide bridges), protein concentration, reductant concentration and ionic strength of the matrix. Our results will be presented here and similarities and differences highlighted. Furthermore we will present both our 2- and 3-dimensional cell culture experiments that show the gel matrix promotes both fibroblast and chondrocyte cell spreading, attachment and proliferation; indicating our hydrogels gels are biocompatible and they can provide a viable support for different cell types. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B18.00002: Hydroxyapatite growth induced by self-assembled noncollageneous extracellular protein: a study on biomineralization \textit{in vitro} Xiaolan Ba, Yizhi Meng, Nadine Pernodet, Sue Wirick, Chris Jacobsen, Helga F\"uredi-Milhofer, Yi-xian Qin, Miriam Rafailovich, Elaine DiMasi Little is known about the role of various ECM proteins in the formation of calcium phosphate during the biomineralization. Here we follow the calcium phosphate mineralization process in vitro using two different ECM proteins, fibronectin and elastin. The mechanical properties of the protein fibers during the early stages were probed by shear modulation force microscopy. The development of the mineral crystals along the protein matrices was investigated by scanning electron microscopy, soft x-ray scanning transmission microspectroscopy, and grazing-incidence synchrotron x-ray diffraction. The elastic modulus of the fibers in the elastin-fibronectin mixture increased to a greater extent than that of the fibers from a single protein. In the presence of fibronectin, longer exposure in the mineral solutions led to the formation of hydroxyapatite crystals templated along the self-assembled fiber structures, while elastin fibers collected calcium without crystallizing. Ca L-edge XANES spectra confirm that Ca in the Ca-elastin complex lacks the mineral anion coordination found in the fibronectin systems and in Ca mineral controls. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B18.00003: Replication of Optical Microstructures of \textit{Papilio palinurus} through Biomimicry Mohan Srinivasarao, Matija Crne, Vivek Sharma, John Blair, Jung Ok Park, Christopher J. Summers The coloration of animals in nature is sometimes based on their structure rather than pigments. Structural coloration based on diffraction, multilayer reflection, cholesteric analogues or photonic crystal-like structures is pervasive especially in the world of insects. The color of \textit{Papilio palinurus} results from microbowl lined with a multilayer of air and chitin. The green color is the result of color mixing of the yellow light reflecting from the bottom of the bowl and the blue light reflecting from the sides of the bowl. We have used breath figure templated assembly as the starting point to mimic the structure of \textit{Papilio palinurus}. We were able to produce microbowls which were then coated with a multilayer of alternating titanium oxide and aluminum oxide. The resulting structure exhibits the same color mixing as the original butterfly structure does. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B18.00004: Polyelectrolyte and nanoparticle adsorption to nanopatterned surfaces Thuy Chastek, Steven Hudson, Vince Hackley The adsorption of polyelectrolytes and nanoparticles onto patterned and curved surfaces is investigated (by fluorescence and electron microscopy) and exploited to produce anisotropic patchy particles. Various anisotropic properties are necessary for the self-assembly of complex structures. In this work, particles were bound temporarily to a substrate, so that part of their surfaces is occluded during subsequent surface modification by the adsorption of polyelectrolyte. The substrate surface charge has a significant effect on the adsorption of particles, which provided several advantages in comparison to bare glass substrates. These include much reduced deposition time, a high degree of coverage, and the ability to accommodate both negatively and positively charged particles. Moreover, patch production yield is consistently 99 $\pm $ 1 {\%}. Rapid coating methods transferable to roll-to-roll processing were tested, and step-by-step characterization methods to evaluate yield were developed. High-yield site-specific binding of complementary spheres to the lithographic region of patchy particles and surfaces was demonstrated, including binding to positive and negative patches. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B18.00005: Vertical Alignment of Single Wall Carbon Nanotubes (SWNTs) in Thin Polymer Films Meagan Mauter, Menachem Elimelech, Chinedum Osuji Thin polymer films (1-10 um) incorporating singly dispersed, vertically aligned carbon nanotubes have a diverse set of potential applications. Desalination membranes that use aligned SWNT as pores, for instance, are predicted to exhibit high flux and salt rejection through size exclusion of hydrated ions. Current fabrication techniques, however, are unable to realize the vertical assembly of narrow diameter SWNTs. Here, we direct the vertical alignment of SWNTs in thin films by using magnetic field aligned lyoptropic surfactant mesophases as structure directing templates. The short alkyl tails of the surfactant impart negative diamagnetic anisotropy to worm-like micelles and lead to parallel alignment of the liquid crystalline (LC) director in an applied magnetic field. The nanotubes orient preferentially with their long axis parallel to the director field of the mesophase, thus promoting their vertical alignment in the system. The LC mesophase incorporates monomers that are polymerized by UV exposure after nanotube alignment to form the polymer matrix. X-ray scattering and optical spectroscopy are used to characterize the field-guided assembly process. The present system may have additional applications for polymer reinforcement using carbon nanotubes. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B18.00006: STM study of stereoselective oligomeric chains on cobalt oxide templates Daejin Eom, Hui Zhou, Kwang T. Rim, Michael Lefenfeld, Colin Nuckolls, George W. Flynn, Tony F. Heinz Stereoselective oligomeric chains of cis-1,4-poly(2,3-dimethyl-1,3-butadiene) have been grown using the cobalt oxide surface as a template. The chains were formed by vacuum deposition of the monomer on an oxidized Co(0001) surface held at room temperature. The geometric structure of the 1-dimensional chains and their relationships to the structure of the surface were probed using scanning tunneling microscopy (STM). The typical chains were more than 10 monomer units in length and were highly linear in structure. When the chains were annealed to a temperature of 480 K, however, their morphology abruptly changed to irregularly shaped curves. We interpret this transformation as the result of scission of the methyl side groups of the oligomers. In addition to comprehensive STM topography data, we have investigated the chains using scanning tunneling spectroscopy (STS). The STS spectra show features with an energy spacing of 0.17 eV. We interpret this response as arising from an inelastic tunneling channeling involving excitation of the symmetric deformation mode of the methyl side groups. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B18.00007: Breath-figure-templated assembly of holey polymer films Vivek Sharma, Mohan Srinivasarao Breath figures formed on evaporating polymer solutions exposed to the blast of humid air involve growth and self-assembly of water drops that are non-coalescent. The hexagonally close packed, nearly monodisperse drops, eventually evaporate away, leaving a polymer film with ordered array of pores. We provide the first quantitative attempt aimed at the elucidation of the mechanism of this breath-figure-templated assembly. The dynamics of drop nucleation, growth, noncoalescence and self-assembly are modeled by accounting for various transport and thermodynamic processes. The theoretical framework developed in this study allows one to rationalize and predict the structure and size of pores formed in different polymer-solvent systems under given airflow conditions. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B18.00008: Gelation in Semiflexible Polymers Venkat Padmanabhan, Sanat K. Kumar Discrete Molecular Dynamics/Collision Dynamics has been employed to study the formation of a physical gel by semi-flexible polymer chains. The formation of a geometrically connected network of bundles of chains is investigated as a function of temperature. As the temperature is lowered, a percolated homogeneous solution phase separates to form a non-percolated nematic fluid and upon further decrease in the temperature, it goes back to a percolated gel state. The gelation, at lower temperatures, is due to the dynamic arrest of chains, preventing them from completing the phase separation process. The cooling rate also plays an important role in deciding the final outcome. Quenching the system, to the final temperature, at a faster rate yields gelation while slower quenches result in phase separation. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B18.00009: Highly Ordered Block Copolymer Templates for the Generation of Nanostructured Materials E. Bhoje Gowd, Bhanu Nandan, Nadja C. Bigall, Alexander Eychmuller, Manfred Stamm Among many different types of self-assembled materials, block copolymers have attracted immense interest for applications in nanotechnology. Block copolymer thin film can be used as a template for patterning of hard inorganic materials such as metal nanoparticles. In the present work, we demonstrate a new approach to fabricate highly ordered arrays of nanoscopic inorganic dots and wires using switchable block copolymer thin films. Various inorganic nanoparticles from a simple aqueous solution were directly deposited on the surface reconstructed block copolymer templates. The preferential interaction of the nanoparticles with one of the blocks is mainly responsible for the lateral distribution of the nanoparticles in addition to the capillary forces. Subsequent stabilization by UV-irradiation followed by pyrolysis in air at 450 $^{\circ}$C removes the polymer to produce highly ordered metallic nanostructures. This method is highly versatile as the procedure used here is simple, eco-friendly and provides a facile approach to fabricate a broad range of nanoscaled architectures with tunable lateral spacing. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B18.00010: Polymeric nanocomposite comprising size-controlled organic nanostructures via copolymer-directed self-assembly Dequan Xiao, Kunhua Lin, Qiang Fu, Qinjian Yin Inspired by inorganic nanomaterials, low-dimensional organic nanostructures have emerged as a new field of nanomaterials with the presence of size-dependent physical properties. Here, we report a polymeric nanocomposite comprising size-controlled organic nanostructures, formed by copolymer-directed self-assembly. By TEM and SEM images, we found the near-spherical shapes of the zero-dimensional organic nanoparticles. A strongly broadened Raman shift band was probed, suggesting the presence of size-dependent quantum confinement effect. By proof-of-principle quantum chemical calculations, we further explain that the strong Raman broadening is caused by the heterogeneous size-distribution of the organic nanoparticles. The present polymeric nanocomposite opens a new route for exploring low-dimensional organic nanostructures with size-dependent physical properties. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B18.00011: Orientational Change of Microphase-Separated Domains of Block Copolymer Thin Films Placed on Ordered Nanoparticle Monolayers Kookheon Char, Taehee Kim Orientation of microphase-separated domains of diblock copolymer (BCP) thin films deposited on ordered nanoparticle (NP) monolayers was investigated. Ordered NP monolayers were prepared on silicon substrates with the Langmuir-Blodgett deposition technique. Parallel orientation of anisotropic microdomains (cylinders and lamellae) of BCP thin films with respect to the substrate is preferred on bare silicon substrates due to the preferential enthalpic interaction with one of BCP blocks, while the perpendicular orientation is preferred on the lattice-like ordered NP monolayers due to the roughness induced from the NP monolayers which can exert elastic deformation on the parallel-oriented microdomains, suppressing the substrate-induced parallel orientation. The effects of NP size as well as BCP film thickness on the orientation of BCP domains were systematically studied with AFM and Grazing Incidence Small-Angle X-ray Scattering (GISAXS). The rectification of perpendicularly oriented BCP microdomains onto underlying NP lattices was analyzed with SEM for thin BCP films (less than 100 nm in thickness). With this experimental technique, we observed the persisted perpendicular orientation of BCP microdomains on NP vacant sites up to the width of NP vacant sites less than 290 nm ($\sim $ 11 L$_{o})$. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B18.00012: Directed self-assembly of block copolymers for resolution enhancement and pattern rectification Joy Cheng, Charles Rettner, Daniel Sanders, Alshakim Nelson, Hoa Truong, Ho-Cheol Kim, William Hinsberg Directed polymer self-assembly which combines lithographically defined substrates and self-assembled polymers has been considered as a potential candidate to extend conventional lithography techniques. Self-assembled block copolymer domains can multiply the spatial frequency and improve pattern quality of the underlying resist prepattern. Lamella-forming PS-b-PMMA is spin-cast on patterned substrate with alternating stripes of resist/ neutral underlayer and annealed to generate well-aligned microdomains. The performance of directed self-assembly depends on the pitch of resist prepattern and resist pattern quality. For linear line-space pattern, defect-free frequency doubling can be achieved within $\pm $5{\%} mismatch between periodicity of block copolymers and pitch of resist patterns. Less mismatch tolerance was observed in concentric circular pattern. The self-healing capability of block copolymers has been explored by introducing dotted resist lines. Pattern rectification and frequency multiplication can be successfully achieved when sufficient local spatial information is given in the resist prepattern. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B18.00013: Tetragonal Ordering in Block Copolymer-Homopolymer Blend Films Laterally Confined in a Square Well Su-Mi Hur, Carlos Garc\'Ia-Cervera, Ed Kramer, Glenn Fredrickson Self-consistent field theory (SCFT) simulations are presented for a melt blend of AB diblock copolymers and A homopolymers in a thin film confined to a square well. The work aims to guide self-assembly towards tetragonal ordering, which is a pattern of technological interest in block copolymer lithography. By using suitable A homopolymer additives, we have succeeded in achieving square lattices of cylinders not observed in the confined or bulk pure diblock system. A phase diagram is presented that shows the region of stability of the tetragonal phase as a function of chain length and volume fraction of the homopolymer additive, in addition to several other interesting phases that result from a competition between surface and bulk contributions to the free energy. Results are also presented on the effect of line edge roughness in the square confinement well on the achievement of robust and defect free tetragonal order. [Preview Abstract] |
Session B19: Focus Session: Polymers and Ionic Liquids
Sponsoring Units: DPOLYChair: Peggy Cebe, Tufts University
Room: 320
Monday, March 16, 2009 11:15AM - 11:51AM |
B19.00001: Block Copolymers and Ionic Liquids: A New Class of Functional Nanocomposites Invited Speaker: Block copolymers provide a remarkably versatile platform for achieving desired nanostructures by self-assembly, with lengthscales varying from a few nanometers up to several hundred nanometers. Ionic liquids are an emerging class of solvents, with an appealing set of physical attributes. These include negligible vapor pressure, high chemical and thermal stability, tunable solvation properties, high ionic conductivity, and wide electrochemical windows. For various applications it will be necessary to solidify the ionic liquid into particular spatial arrangements, such as membranes or gels, or to partition the ionic liquid in coexisting phases, such as microemulsions and micelles. One example includes formation of spherical, cylindrical, and vesicular micelles by poly(butadiene-$b$-ethylene oxide) and poly(styrene-$b$-methylmethacrylate) in the common hydrophobic ionic liquids [BMI][PF$_{6}$] and [EMI][TFSI]. This work has been extended to the formation of reversible micelle shuttles between ionic liquids and water, whereby entire micelles transfer from one phase to the other, reversibly, depending on temperature and solvent quality. Formation of ion gels has been achieved by self-assembly of poly(styrene-$b$-ethylene oxide-$b$-styrene) triblocks in ionic liquids, and by the thermoreversible system poly(N-isopropylacrylamide-$b$-ethylene oxide-$b$-N-isopropylacrylamide), using as little as 4{\%} copolymer. Further, these gels have been shown to be remarkably effective as gate dielectrics in organic thin film transistors. The remarkably high capacitance of the ion gels ($>$ 10 $\mu $F/cm$^{2})$ supports a very high carrier density in an organic semiconductor such as poly(3-hexylthiophene), leading to milliamp currents for low applied voltages. Furthermore, the rapid mobility of the ions enables switching speeds approaching 10 kHz, orders of magnitude higher than achievable with other polymer-based dielectrics such as PEO/LiClO$_{4}$. Finally, we have shown that ordered nanostructures of block copolymers plus ionic liquids show the characteristic self-assembly properties of strongly-segregated systems. Prospects for anisotropic ionic conductivity are also being explored. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B19.00002: Phase Behavior of Block Copolymer Solutions in an Ionic Liquid J.M. Virgili, M.L. Hoarfrost, N.P. Balsara, R.A. Segalman Incorporation of ionic liquids into block copolymers is of interest for applications such as high temperature fuel cell membranes. We investigate the lyotropic and thermotropic phase behavior of solutions of poly(styrene-$b$-2-vinyl pyridine) (S2VP) block copolymers in an ionic liquid consisting of imidazole and bis(trifluoromethane)sulfonamide (HTFSI). Using small angle X-ray scattering (SAXS) and static birefringence, we demonstrate that the ionic liquid behaves as a selective solvent, preferentially solvating the poly(2-vinyl pyridine) segment of the block copolymer. At moderate to high concentrations ($\ge $ 40 wt{\%}) of copolymer, we observe lyotropic phase transitions to lamellar and cylindrical (hcp) nanostructures. At low concentrations of S2VP copolymer ($\le $ 30 wt{\%}), we observe poorly-ordered, microphase-separated structures, which do not resemble the face-centered cubic or body-centered cubic spherical micelles observed in block copolymer solutions in molecular solvents. We observe that the order-disorder transition temperature of the series of SVP copolymers does not depend strongly on the concentration of the block copolymer solution in ionic liquid. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B19.00003: Ordering of Triblock Copolymer Surfactants by Blending with a Room Temperature Ionic Liquid Daniel Miranda, James Watkins, Thomas Russell Well-ordered block copolymer microdomains were obtained by blending Pluronic{\textregistered} PEO-PPO-PEO triblock copolymer surfactants with the room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate. The selective association of the ionic liquid with the PEO blocks increases the segregation strength by increasing the effective interaction parameter between the blocks. The neat copolymer is phase-mixed in the melt whereas the addition of ionic liquid to the copolymer results in phase segregation, forming well-ordered microdomains. The ionic liquid was confirmed to interact with the PEO blocks by a depression in the melting point of the blends with increasing ionic liquid concentration. Further, small angle x-ray scattering experiments show a decrease in the breadth of the first order peak, as well as the appearance of higher order peaks, with increasing ionic liquid concentration. These results confirm the formation of well-ordered microdomains. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B19.00004: Phase separation induced by polymer-ionic molecule complexation Issei Nakamura, An-Chang Shi The miscibility of polymers in ionic solutions has attracted long-standing interest in polymer science. In particular, it has been demonstrated experimentally that phase separation can be driven by complexation of polymers and ionic-molecules. Thermally reversible strong forces such as hydrogen bonding and electrostatic force are often employed to induce the complexation. In this study, we developed a self consistent field theory for polymers which are capable of binding small ionic molecules. Specifically, poly(vinyl alcohol) and borate ion in aqueous solution with sodium chloride are used as a model system. Binding isotherm, phase diagrams, as well as comparisons with experiments, will be presented. The theory provides a closed-loop region for an instability of the homogeneous phase in the phase diagram. Implications of our results to the sol-gel transition arising from the correlation between unoccupied and occupied ion-binding sites of polymers are discussed. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B19.00005: Morphology and Ion Transport in Mixtures of Polymers and Ionic Liquid Jae-Hong Choi, Liang Gwee, Yossef A. Elabd, Karen I. Winey Mixtures of polymers and ionic liquid have been prepared using homopolymers, random copolymers, and block copolymers: poly(methyl methacrylate), poly(methyl methacrylate-\textit{ran}-styrene), and poly(methyl methacrylate-$b$-styrene). The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. These mixtures are investigated using X-ray scattering and electron microscopy. Mixtures of the homopolymer and random copolymer with the ionic liquid are homogeneous and amorphous morphology with excess scattering as content of ionic liquid increases. The block copolymer and ionic liquid mixtures show ordered structures typical of block copolymers that vary with ionic liquid content. The morphologies of the copolymer-ionic liquid mixtures will be correlated with the conductivities. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B19.00006: Dissolving Polymers in Ionic Liquids. David Hoagland, John Harner Dissolution and phase behavior of polymers in ionic liquids have been assessed by solution characterization techniques such as intrinsic viscosity and light scattering (static and dynamic). Elevated viscosity proved the greatest obstacle. As yet, whether principles standard to conventional polymer solutions apply to ionic liquid solutions is uncertain, especially for polymers such as polyelectrolytes and hydrophilic block copolymers that may specifically interact with ionic liquid anions or cations. For flexible polyelectrolytes (polymers releasing counterions into high dielectric solvents), characterization in ionic liquids suggests behaviors more typical of neutral polymer. Coil sizes and conformations are approximately the same as in aqueous buffer. Further, several globular proteins dissolve in a hydrophilic ionic liquid with conformations analogous to those in buffer. General principles of solubility, however, remain unclear, making predictions of which polymer dissolves in which ionic liquid difficult; several otherwise intractable polymers (e.g., cellulose, polyvinyl alcohol) dissolve and can be efficiently functionalized in ionic liquids. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B19.00007: Polyester Spherulite Crystallization in Ionic Liquids Kathy Singfield, Shawna Mitchell A series of polyesters have been crystallized in ionic liquids. Spherulites of the polyesters have been grown isothermally from different ionic liquids after cooling the single phase polymer/ionic liquid system from above the polymer melting point temperature. To the authors' best knowledge this is the first reported account of polyester spherulites grown from these non-traditional solvents. The combination of physical properties of the crystallizing system supports the un-restrained branching/splitting volume-filling growth in all radial directions of the suspended crystallizing entity. The morphology of the collected spherulites at various stages of their formation was examined using scanning electron microscopy (SEM). The SEM results provide a clear visual inspection of the early-stage growth forms and the branching/splitting patterns involved in their evolution to the final spherical form. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B19.00008: Understanding Ion Transport in Polymerized Ionic Liquids using Dielectric Spectroscopy U. Hyeok Choi, Hong Chen, Wenjuan Liu, Yossef A. Elabd, Ralph H. Colby In order to deduce the mechanism of ion conduction in ion-containing polymers, not only the conductivity needs to be measured but also the number density and mobility of conducting ions must be determined using broadband dielectric spectroscopy, covering broad frequency and temperature ranges. To obtain a transference number of unity, one ionic charge is covalently bonded to the polymer so that only the counterions can contribute to ion conduction. In this study, imidazolium-containing monomer was synthesized and polymerized to make a cationic homopolymer with either tetrafluoroborate or bis(trifluoromethanesulfonyl)imide anionic counterions. These ions can associate into pairs and larger aggregates. The degree of ion pairing can be estimated from the temperature dependence of the dielectric constant and knowledge of the dipole moment of the ion pair, using the 1936 Onsager equation. Using the 1953 Macdonald model makes it possible to determine concentration and mobility of mobile counterions from analysis of electrode polarization in dielectric spectroscopy. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B19.00009: Enhanced ionic conductivity of polyurethane ionomers by self-solvating cations Shih-Wa Wang, Ralph Colby We study the effect of different cations on ionic conductivity and dielectric properties of polyurethane ionomeric single-ion conductors with para-phenyl diisocyanate and anionic diols (carboxylate or phosphonate) constituting the hard segments and poly(ethylene glycol) as the soft segment. Bulky cations such as tetra-alkyl ammonium can increase ionic conductivity compared to metallic cations like sodium because bulky cations have lower interaction energy with anions, allowing more dissociation from the anions. In order to increase the conductivity even more, ether oxygens, which are well-known to solvate cations, are incorporated in the alkyl tail of ammonium-type cations. By comparing polyurethane ionomers with sodium, tetramethyl ammonium, and ammonium with ether oxygens in the alkyl tail, we show that the presence of ether oxygen on the ammonium can significantly reduce T$_{g}$ and increase ionic conductivity in our single-ion conductors. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B19.00010: Weakening Ion Interactions in Ionomers using Ionic Liquid Counterions Gregory Tudryn, Ralph H. Colby Anionic poly(ethylene oxide)-based ionomers are candidate materials for electro-active devices due to the ability of ether oxygens to solvate conducting cations. Conventional alkali metal cations in sulfonated PEO-ionomers are exchanged to ionic liquid counterions and electrical and mechanical properties are measured. Electrode polarization in dielectric spectroscopy is used to determine number density and mobility of conducting counterions. Conductivity and mobility increase with counterion size and exhibit Vogel temperature dependences, meaning counterion motion is coupled with polymer segmental motion. Conducting ion concentrations show Arrhenius temperature dependences, with activation energy reduced as counterion size increases. Oscillatory shear and SAXS suggest ions do not microphase separate, presumably due to ether oxygen solvation of cations. Ionomers with small counterions have higher plateau moduli than larger counterions, suggesting small counterions form more stable quadrupoles. Such studies allow fundamental design of ionic conductors for actuators, as ionic liquids provide larger strains and faster response for electro-active devices. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B19.00011: Processing of Natural Polymer-nanocomposites using Ionic Liquids as ``Green Solvents'' Sameer Rahatekar, Asif Rasheed, Rahul Jain, K. Koziol, Alan Windle, Paul Trulove, Satish Kumar, Jeffrey Gilman We report fiber spinning of natural polymers such as cellulose and silk using ionic liquids. Ionic liquids can dissolve cellulose and silk and are less hazardous that the traditional solvents used for dissolving cellulose. We use imidazoluim based ionic liquids as a common solvent to process natural polymers and carbon nanotubes. Cellulose/carbon nanotubes based fibers are spun using wet spinning process. The rheological, mechanical thermal and electrical properties of the fibers are measured. We also characterize the cellulose nanocomposites fibers using ionic liquids by SEM/TEM, X-ray diffraction, TGA and FTIR analysis. Silk and carbon nanotubes fiber processing is also reported using ionic liquids as common solvent. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B19.00012: Biocompatible Ionic Liquid-Derived Conducting Polymers Millicent Firestone, Christopher Burns, Sungwon Lee A significant and frequently encountered challenge when making an electrical connection to a protein is that its electron-transfer sites are buried within the polypeptide matrix and thus, are not readily accessible to bulk metal electrodes. A further complicating factor is that inorganic (i.e., metallic) electrodes are often incompatible with biological samples. These obstacles might be overcome by the use of conducting oligomers and / or polymers, which are flexible, offering a means to access remote redox centers. These oligomers can be readily modified to include chemical moieties that can connect covalently to sites near redox centers. In addition, conducting polymers can be made to be environmentally responsive (dynamic), processable (conformal coating, soluble) and mechanically durable, thus enabling them to function as an electrical conduit (wire or electrode) to biomolecules. In this work, we describe the design, synthesis and electrochemical properties of thiophene-based ionic liquid monomers and their bulk polymerization by chemical oxidation to yield cationic, aqueous-soluble polymers. Preliminary studies evaluating the electropolymerization of these monomers into nanostructured thin films will also be presented. [Preview Abstract] |
Session B20: Focus Session: Polymers and Energy: Photovoltaics, Fuel Cells, Batteries I
Sponsoring Units: DPOLYChair: Bumjoon Kim, Korea Advanced Institute of Science and Technology
Room: 321
Monday, March 16, 2009 11:15AM - 11:51AM |
B20.00001: Polymers for new battery technologies. Invited Speaker: The chemical and electrochemical reactivity of the components comprising today's lithium batteries has severely limited their lifetime and stability, and attempts to push the limits on energy density have exacerbated these stability issues. The weakest link in terms of safety and stability of Li ion systems is the organic liquid electrolyte that facilitates the Li$^{+}$ ion transport between the electrodes. The electrolyte is flammable and electrochemically unstable against the graphitic anode. It is the continuous electrochemical degradation of the electrolyte at the electrodes that leads to poor cycle life of the batteries, and in some cases runaway reactions that lead to explosions. Dry polymer electrolytes alleviate the electrochemical stability problem by offering a stable electrode-electrolyte interface. The absence of flammable liquids prevents runaway reactions. The main hurdle that has prevented dry polymer electrolytes from being commercialized is low ionic conductivity, and challenges in interfacing with the electrode materials. We demonstrate a novel approach towards addressing these challenges that renders batteries with excellent cycle lives, and thermal stability. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B20.00002: Control of Domain Orientation in Block Copolymer Electrolyte Membranes at the Interface with Humid Air Moon Jeong Park, Suhan Kim, Andrew M. Minor, Nitash P. Balsara Access to ion transporting channels in polymer electrolyte membranes depends crucially on the orientation of hydrophobic and hydrophilic domains at the surface. We demonstrate that domain orientation of polymer electrolyte membranes made from poly(styrenesulfonate-b-methylbutylene) (PSS-PMB) copolymers can be tuned by controlling sulfonation level and moisture content of the air. At low sulfonation levels, highly ordered hydrophobic PMB cylinders oriented perpendicular to the film surface are obtained, when the film is contacted with humid air. Increasing the sulfonation level results in a transition from perpendicular to parallel orientation. Our conclusion is based on three-dimensional characterization of membranes using electron microscopy of samples prepared by the shadow focused ion beam technique, grazing incident small angle x-ray scattering, and electron tomography. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B20.00003: Vertical phase-separation due to differences in surface energies in bulk heterojunction polymer solar cells Sarah Cowan, Anshuman Roy, Ji Sun Moon, Sung Heum Park, Alan Heeger The synthesis and testing of new photoactive polymers is steadily improving the light conversion efficiencies of organic bulk heterojunction solar cells. Understanding the physical interactions between the polymer donor material and the electron acceptor is critical in controlling and optimizing the morphology of the blend. While interactions between the donor and acceptor in the blend determine the scale and stability of lateral phase separation, interactions between the constituents of the blend and the neighboring device layers are equally important. In this work, we demonstrate that bulk heterojunction constituents in a polymer solar cell tend to vertically phase-separate due to differences in surface energies leading to surface-directed spinodal decomposition and/or a wetting layer. Using a combination of cross-sectional transmission electron microscopy (TEM), variable angle spectroscopic ellipsometry (VASE), and a contact angle study, we probe the vertical phase separation in poly(3-hexylthiophene) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[N-9$\prime $-heptadecanyl-2,7-carbazole-alt-5,5-(4$\prime $,7$\prime $-di-2-thienyl-2$\prime $,1$\prime $,3$\prime $-benzothiadiazole)] : [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B20.00004: Investigation of the bulk heterojunction structure of organic photovoltaics using neutron reflectivity Jonathan Kiel, Brian Kirby, Michael Mackay Organic photovoltaics have received much attention recently due to their promise of affordable and flexible solar power. A major component of these devices is the bulk heterojunction: an interconnected mixture of an electron donator, a highly conjugated polymer, and an electron acceptor, generally a fullerene derivative. We have performed neutron reflectivity experiments on 200 nm thick films of poly(3-hexylthiophene) and [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) to investigate the structure of this bulk heterojunction. We observe a gradient of PCBM throughout the film that depends on processing conditions, ratio of polymer to PCBM and choice of solvent. These results are compared to working devices to show which bulk heterojunction structures are more suitable to highly efficient solar cells. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B20.00005: Size- and Shape-dependent efficiency of PbSe nanocrystal and nanowire doped organic semiconductor photovoltaics Wenting Li, Christopher Murray, Cherie Kagan Hybrid solar cells based on nanocomposite organic semiconductors and IR sensitive PbSe nanocrystals (NCs) and nanowires (NWs) are fabricated and serve as a model system to test in PV devices. Wet chemical routes are used to synthesize PbSe NCs tunable in size, from 6 to 12nm in diameter, and in shape by tailoring the reaction temperature and selection of surfactants. PbSe NWs are also synthesized through oriented attachment in solution of NC building blocks to form straight, zigzag, helical, and branched NWs. We integrate PbSe NCs and NWs with the organic semiconductors P3HT and pentacene. We are able to fabricate organic-inorganic bulk heterojunctions with pentacene using a solution-processable precursor that is thermally converted to pentacene. We investigate the role of the organic semiconductor pentacene in the solar cell, both as a conductivity booster and as a more stable alternative to P3HT. We find that ligand exchange significantly increases photocurrent by replacing oleic acid ligands used in NC synthesis with shorter pyridine or octylamine ligands. We also report that tailoring the size and shape of the NCs and controlling the deposition and annealing conditions of the nanocomposites enhances the solar cell performance. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B20.00006: Transient photovoltaic behavior of air-stable inverted organic solar cells with solution-processed electron transport layer and high work function top electrode Chang Su Kim, Yueh-Lin (Lynn) Loo In this study, we made air-stable inverted organic solar cells comprising sol-gel derived TiOx as the electron transport layer and Au as the high work function top electrode. The highly transparent TiOx layer, placed between the ITO cathode and the active layer, smooths out ITO and provides better alignment of energy levels for electron transport. The conductivity of TiOx is known to increase with increasing exposure time to light as the excited electrons fill up shallow traps during illumination. The short circuit current of our inverted solar cells thus increases from 1.41mA/cm2 to 8.13mA/cm2 under continuous illumination for 10 minutes. In addition, when our inverted solar cells are stored in air for extended periods of time, the open circuit voltage increases due to oxygen doping of poly(3-hexylthiophene). Exposure to air for 2 days, for example, increases the open circuit voltage from -0.38V to -0.53V. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B20.00007: Controlling Photovoltaic Loss: Recombination of Dissociated Electrons and Holes in Organic Solar Cells Zhihua Xu, Huidong Zang, Bin Hu This presentation reports the studies of charge-transfer complex states formed from the recombination of dissociated electrons and holes at the donor-acceptor interfaces in bulk-heterojunction organic solar cells based on magnetic field effects of photocurrent. Our studies indicate that the formation of charge-transfer complex states is determined by the competition between Coulombic attraction and electrical drifting. Externally, applying electric field can clearly decrease the density of charge-transfer complex states through electrical drifting. Internally, morphology can change the competition between Coulombic attraction and electric drifting through dielectric fields and charge mobilities, and consequently affects the formation of charge-transfer complex states. As a result, changing internal dielectric fields and charge mobilities through internal Coulomb interaction and electrical drifting presents as two mechanisms to control the formation of charge-transfer complex states towards the improvement of photovoltaic efficiencies in organic bulk-heterojunction solar cells. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B20.00008: Photo-induced improvement of Bulk Heterojunction Polymeric Solar Cells Kamil Mielczarek, Alexander Cook, Anvar Zakhidov The effectiveness of BHJ polymeric solar cells depends highly on the formation of continuous three dimensional interconnecting networks of electron donor (typically RR P3HT) and acceptor (typically PCBM) materials. This process is controlled by post-processing heat treatment to induce phase separation of the materials. We demonstrate in this presentation, that in-situ photo-excitation of the BHJ structure during the annealing process controls both the maximal photocurrent and filling factor of the BHJ solar cell. We have found that variations in intensity and spectral composition of the photo excitation affect the resulting morphology of BHJ. The increased diffusivity of constituents and photo-modulation of the carrier recombination upon annealing is discussed as one of the causes of the observed morphology improvement. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B20.00009: High-Vacuum Annealing of Polythiophene:Methanofullerene Bulk Heterojunction Solar Cells Jennifer Segui, Ioana Gearba, Miriam Rafailovich, Charles Black Solar cell device architectures incorporating photoactive layers of immiscible blends of organic semiconductors achieve improved photovoltaic power conversion efficiency compared to planar device geometries. We have fabricated bulk heterojunction solar cells with active layer blends of poly-3 hexylthiophene (P3HT) and the fullerene derivative, [6,6] phenyl C61-butyric acid methyl ester (PCBM). Spin casting the blend from a chlorobenzene solution forms nanometer-scale domains of electron donor and acceptor phases in the device active layer. We solution process the active layers in ambient atmospheric conditions prior to aluminum contact evaporation resulting in inevitable oxygen adsorption in the P3HT bulk and interfaces. We have investigated several device post-fabrication thermal treatments for driving oxygen from the device active layer, including different temperatures, times, and vacuum pressures. We evaluate the efficacy of this technique in improving Al contact quality, film morphology, solar cell efficiency, and reproducibility via analysis of device current-voltage characteristics and tapping mode atomic force microscopy. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B20.00010: Optimizing Ionic Electrolytes for Dye-Sensitized Solar Cells Xiaojuan Fan, Sarah Hall Dye-sensitized solar cells DSSCs provide next generation, low cost, and easy fabrication photovoltaic devices based on organic sensitizing molecules, polymer gel electrolyte, and metal oxide semiconductors. One of the key components is the solvent-free ionic liquid electrolyte that has low volatility and high stability. We report a rapid and low cost method to fabricate ionic polymer electrolyte used in DSSCs. Poly(ethylene oxide) (PEO) is blended with imidazolinium salt without any chemical solvent to form a gel electrolyte. Uniform and crack-free porous TiO$_{2}$ thin films are sensitized by porphrine dye covered by the synthesized gel electrolyte. The fabricated DSSCs are more stable and potentially increase the photo-electricity conversion efficiency. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B20.00011: Improved polythiophene conductivity by thermal crosslinking for solar cell applications I.R. Gearba, C.-Y. Nam, R. Pindak, C.T. Black Organic photovoltaic device power conversion efficiencies are limited in part by low charge mobility within the constituent active layer. For example, the p-type polythiophene polymers used in the highest efficiency organic photovoltaic devices have transverse hole mobilities of only 10$^{-4}$-10$^{-5}$ cm$^{2}$/V-s, despite showing significantly higher values ($\sim $0.1 cm$^{2}$/V-s) in a lateral FET geometry. This mobility anisotropy is caused by poor overlap of $\pi -\pi $ orbitals in the transverse direction, which impedes charge hopping between polymer chains. We have improved the transverse hole conductivity by as much as three times by incorporating the radical initiator di-tert-butyl peroxide into polythiophene thin films. The initiator promotes thermal crosslinking upon annealing at 170C. Crosslinked polythiophene films maintain a similar absorption spectrum to the uncrosslinked material. Grazing incidence X-ray measurements correlate film structural changes to the measured electronic properties, and reveal two possible mechanisms for increased $\pi -\pi $ overlap in crosslinked films. We have increased the power conversion efficiency of planar photovoltaic devices composed of p-type polythiophene and n-type C60 by approx three times (from 0.09{\%} to 0.27{\%}) by crosslinking the polythiophene material. Moreover, crosslinked polythiophene films are rendered insoluble and thus amenable to the further material processing. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B20.00012: Modeling photocurrent transients in organic solar cells Inchan Hwang, Neil Greenham We investigate the transient photocurrents of organic photovoltaic devices by numerical modeling of the drift-diffusion equations. Understanding charge transport in organic solar cells is one of the major interesting issues relevant to improving performance of organic devices. We demonstrate the simulation of the transient photocurrents in a response to a sharp turn-on illumination. Our results show the transient time in photocurrents is determined not only by free charge transport, but also by geminate charge pair dynamics. The dissociation probability of geminate charge pairs is a key parameter in determining the performance of organic devices, controlling the efficiency at low intensity, and also governing the fate of charge pairs formed by bimolecular recombination at high intensity. Bimolecular recombination appears to shorten the typical distance traveled by free charges from where they are generated to the electrode, leading to a reduced turn-on time at high intensity. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B20.00013: Conjugated Polymer Organic Solar Cells made using Low Bandgap Vinylene-linked Benzothiadiazole-thiophene N. C. Heston, J. Mei, S. Vasilyeva, J. R. Reynolds With over 70{\%} of the solar photon flux occurring at wavelengths beyond 700 nm, the broad absorption spectra of low bandgap conjugated polymers offers an additional path towards improving organic photovoltaic efficiencies. Here, we report on polymer solar cells fabricated using a vinylene-linked benzothiadiazole-thiophene polymer and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) blends. We have fabricated cells with various blend film polymer to PCBM ratios as well as film thicknesses and architectures. The performance of these cells was investigated using both AM 1.5 and incident photon to current efficiency measurements. Surface morphologies were characterized using atomic force microscopy. A strong correlation was observed between the percentage of polymer in the blend and the resulting film morphology. We observed photon-generated currents at wavelengths greater than 800nm, though we have not yet obtained high overall power conversion efficiencies. [Preview Abstract] |
Session B21: Focus Session: Dopants and Defects in Semiconductors I
Sponsoring Units: DMPChair: Eugene Haller, University of California, Berkeley
Room: 323
Monday, March 16, 2009 11:15AM - 11:51AM |
B21.00001: Hydrogen multicenter bond in oxide and nitride semiconductors Invited Speaker: Hydrogen is a very reactive atom, occurring in virtually all organic and in many inorganic compounds. It can form a purely covalent bond, in which two hydrogen atoms share a pair of electrons in a two-electron two-center bond, as well as polar covalent bonds, such as in an H$_{2}$O molecule. In solids, hydrogen is usually considered as an interstitial impurity. In elemental semiconductors, such as silicon, hydrogen forms a three-center bond when located at the bond center. In compound semiconductors, hydrogen bonds to the anionic species in p-type material, and to the cationic species in n-type. Thus far, hydrogen in solids has been found to form chemical bonds with one, two, or at most three other atoms. Higher coordination numbers are exceedingly rare and have been reported only for clusters. In this talk we will show that hydrogen is capable of forming multicenter bonds in solids, occupying substitutional sites. As examples, we discuss substitutional hydrogen impurities in oxides (ZnO, MgO, SnO$_{2}$, TiO$_{2})$ [1,2] and nitrides (InN, AlN, GaN) [3]. Based on first-principles calculations we show that hydrogen replaces oxygen (nitrogen) and forms genuine chemical bonds with multiple metal atoms, in truly multicoordinated configurations. These multicenter bonds are surprisingly strong despite the large hydrogen-metal distances when compared to typical values in hydrogen two-center bonds. Hydrogen in the multicenter bond configuration is a shallow donor in a number of materials. In conducting oxides, it provides a consistent explanation for the observed dependence of electrical conductivity on oxygen partial pressure, thus resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity [1,2]. \\[4pt] [1] A. Janotti and C. G Van de Walle, Nature Materials \textbf{6}, 44 (2007). \\[0pt] [2] A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. \textbf{101}, 055502 (2008). \\[0pt] [3] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. \textbf{92}, 032104 (2008). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B21.00002: Hydrogen in anion vacancies of semiconductors Mao-Hua Du, David Singh Hydrogen typically terminates the dangling bonds around vacancies in semiconductors, thereby, partially or completely passivating the vacancies. However, it has been shown recently that hydrogen in anion vacancies of many semiconductors, such as ZnO, MgO, InN, SnO$_{2}$, and GaN, takes multi-coordinated structures and acts as shallow donors, providing $n$-type conductivity to the materials. We study the hydrogen in the anion vacancies of a series of II-VI and III-V semiconductors using density functional calculations. The results on these materials show that, in the anion vacancies of polar II-VI semiconductors, the hydrogen is usually anionic and is coordinated with more than one cation atoms as a result of the relatively high ionicity of the host materials. The hydrogen coordination number depends on the host anion size. On the other hand, in more covalent semiconductors such as some III-V semiconductors, the single cation-H bonding configuration may become most stable. In the anion vacancies of ZnX and CdX where X represents anions, hydrogen is typically amphoteric except for oxides, in which the small anion size prohibits the formation of the cation-cation bond that is required for the acceptor configuration. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B21.00003: Defect Creation and Annihilation in GaN and ZnO Chris Van de Walle, Anderson Janotti ZnO is an extremely attractive material for a number of optoelectronic and electronic applications. Among its advantages is its radiation hardness, which is even greater than that of GaN. Based on our comprehensive investigations of intrinsic point defects [1,2], we have developed a model for defect creation and annihilation during and after irradiation. The calculations, based on pseudopotential-density-functional theory combined with LDA+U [2] produce formation energies, stability of charge states as a function of Fermi level, and migration barriers for each of the point defects. Migration barriers allow us to determine annealing temperatures at which we predict various defects to be mobile. In ZnO, the key factors responsible for radiation hardness are (1) the low migration barriers of point defects and (2) the charge-state matching of dominant defect pairs. Quantitative arguments for both ZnO and GaN will be presented, and the results compared with experimental observations. The insights provided by our modeling can be fruitfully applied to understand irradiation effects in semiconductors and insulators in general. [1] S. Limpijumnong and C. G. Van de Walle, Phys. Rev. B 69, 035207 (2004). [2] A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B21.00004: Asymmetric hole localization and multiple hole binding of acceptors in ZnO Stephan Lany, Alex Zunger Holes bound at cation-site acceptors in oxides, such as Li$_{Zn}$ or the Zn vacancy in ZnO tend to be localized on a single oxygen neighbor rather than to be delocalized over symmetrically equivalent sites. As a consequence of this localization, the acceptor level lies deep in the gap, typically $\sim $1 eV above the VBM. In contrast, conventional local density calculations do not show this symmetry breaking, and predict the acceptor level much too shallow. This failure of approximate functionals has been attributed to the residual self-interaction, which underestimates the energy splitting between occupied and unoccupied states. We identify a criterion for the cancellation of the self-interaction in terms of a generalized Koopmans theorem, and use this criterion to define a self-interaction correction (SIC) potential that does not rely on empirical parameters. After the SIC, the unoccupied hole states are correctly placed in energy with respect to the spectrum of the occupied host states. We use this method to predict the acceptor levels of cation-site acceptors and the Zn vacancy in ZnO, and of acceptors in In$_{2}$O$_{3}$ and SnO$_{2}$. We find that these acceptors have too deep levels to cause $p$-type conductivity, and we further predict that nominal single acceptors can generally bind multiple holes (up to 3). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B21.00005: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B21.00006: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:51PM - 1:03PM |
B21.00007: Unusual uniaxial stress results on the stretch mode of OH related defects in ZnO Kevin Martin, W. Beall Fowler Some uniaxial stress studies of the frequency dependence of O-H-related defects in ZnO have produced surprising results$^{1,2}$. For example, the Li:OH defect in ZnO (H-I*) is oriented along the c-axis, yet the OH stretch mode decreases in frequency when stress is applied along the c direction and increases when stress is perpendicular to the c-direction. Another example is the Cu:OH defect, in which the OH is aligned along one of the three non-c tetrahedral directions. Stress along the c-direction produces a strongly non-linear increase in frequency. These examples and others indicate something unusual is happening in these systems. One possibility is that the piezoelectric effect in ZnO is responsible for the ``backward'' behavior of the frequency shift of these defects. The piezoelectric effect in ZnO is caused by the lack of cancellation between the ``clamped-ion'' term (i.e., electronic contribution) and the term related to the change in the u parameter (``internal strain''), with the latter dominating$^{3}$. When c stress is applied, the value of u increases, thus the two interpenetrating hexagonal lattices (one for Zn, the other for O) increase their overlap. We will attempt to explain the experimental results within this framework. $^{1}$Lavrov and Weber, Phys. Rev. B, \textbf{73}, 035208 (2006), and $^{2}$ Phys.Stat. Sol. (b) \textbf{243}, 2657 (2006) $^{3}$Corso, et al Phys. Rev B. \textbf{50}, 10715 (1994) [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B21.00008: Theoretical study of Si in ZnO John Lyons, Anderson Janotti, Chris Van de Walle Recently, the presence of silicon in relatively high concentrations has been detected in samples of ZnO [1]. The properties of this impurity have not yet been investigated. Here we present a first-principles study of the electronic and structural properties of Si in zinc-blende ZnO using density functional calculations with LDA, GGA, and hybrid functionals. Our calculations show that substitutional Si on a Zn site is lower in energy than either Si on an oxygen site or a Si interstitial. The calculations consistently predict Si to be a shallow donor in ZnO, with the 2+ charge state being most stable across the band gap. The formation energy of substitutional Si is relatively low, supporting experimental evidence which shows a concentration of 10$^{17}$ cm$^{-3}$ Si in ZnO samples. The properties of Ge in ZnO are also studied for comparison and show behavior similar to that of Si. [1] M.D. McCluskey and S.J. Jokela, Physica B \textbf{401-402}, 355 (2007). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B21.00009: Photoinduced EPR study of electron traps in TiO$_{2}$ crystals: Oxygen vacancies and Ti$^{3+}$ ions Shan Yang, Adam Brant, Larry Halliburton Electron paramagnetic resonance (EPR) provides a sensitive method to monitor native defects in wide-band-gap semiconductors. In-situ illumination with laser light at low temperature (photoinduced EPR) forms paramagnetic defects in fully oxidized bulk TiO$_{2}$ crystals. Illumination with 442 nm laser light at 30 K and below produces four electronlike centers and one holelike center. Three of the electronlike centers have S = 1/2 and are assigned, respectively, to a substitutional Ti$^{3+}$ ion in the otherwise perfect lattice, a substitutional Ti$^{3+}$ ion adjacent to a Si$^{4+}$ ion, and a substitutional Ti$^{3+}$ ion adjacent to an oxygen vacancy. The fourth electronlike center has S = 1 and is assigned to two Ti$^{3+}$ ions adjacent to one oxygen vacancy. The holelike center has S = 1/2 and consists of a hole shared equally by two adjacent oxygen ions in the otherwise perfect lattice. Spin-Hamiltonian parameters, obtained from complete sets of angular dependence data, are presented for each of the centers. This work was supported by NSF Grant No. DMR-0804352. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B21.00010: The nature of Group-V acceptor impurities in SnO$_{2}$ Joel Varley, Anderson Janotti, Chris Van de Walle Group-V elements have long been considered leading candidates for achieving p-type doping in semiconducting oxides. Using first-principles calculations, we investigate the feasibility of achieving ambipolar doping in SnO$_{2}$ using the Group-V elements N, P, and As. We address the electronic structure of these impurities by performing systematic density functional calculations using hybrid functionals. This approach overcomes the band-gap problems inherent in calculations using the local density approximation or generalized gradient approximation, thus allowing us to accurately determine energies of defect levels. We discuss the stability of the isolated impurities both as substitutional and interstitial defects, based on calculated formation and migration energies. We also investigate their possible passivation by hydrogen and examine binding energies and activation energies of hydrogen-acceptor complexes. We conclude that the Group-V elements are deep acceptors that will not enable p-type doping of SnO$_{2}$. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B21.00011: Manipulation of Single Oxygen Vacancies on TiO$_{2}$(110) Danda Acharya, Peter Sutter Oxygen vacancies are among the primary chemically active defects on the surface of reducible transition metal oxides, playing a key role in surface chemistry, catalysis, and photocatalysis. We report the controlled manipulation of individual O-vacancies on reduced TiO$_{2}$(110)-1x1 using a low temperature scanning tunneling microscope. Localized voltage pulses trigger the hopping of single vacancies along a bridging oxygen (O$_{br})$ row. We discuss the microscopic manipulation mechanism and demonstrate atomic-scale control by constructing linear and more complex arrangements of vacancies. Single defect manipulation is used to probe the interaction of closely spaced vacancies, and to establish the possibility of forming highly reactive double and a triple O-vacancy clusters. Detailed experimental and theoretical analysis reveals that bridge-bonded O-vacancy pairs are stable and have lower energy than pairs of vacancies separated by two or more lattice spacings. The existence of stable vacancy pairs with exposed low-coordinated Ti atoms has implications on the reactivity of TiO$_{2}$(110) and of similar metal oxide surfaces. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B21.00012: Coordination Defects and Nanoclusters of TiO$_{2}$ Ken Park, Vincent Meunier, Minghu Pan, Nan-Hsin Yu, Ward Plummer Titanium oxide is one of the most investigated photocatalytic systems. It is capable of converting toxic organic and inorganic materials to benign products, as well as turning solar energy into a chemical one. Many believe that the catalytic activation involves charge transfer localized at surface defects with lower stoichiometry and/or coordination. In this study, scanning tunneling microscopy (STM) and density functional theory (DFT) are used to gain insight into such defects on TiO$_{2}$(110). STM reveals defects ranging from a few {\AA}ngstroms to a few nanometers in size, but all of a uniform height of 3 {\AA}. These topographically distinct defects are determined as fully stoichiometric nanoclusters by DFT. Despite the full stoichiometry, they possess undercoordinated atomic sites including 3- and 4-coordinated Ti and 1-coordinated O atoms. Their electronic and chemical properties will be discussed. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B21.00013: Kinetic Monte Carlo study for the thermal stability of hydrogen in ZnO Junhyeok Bang, Kee Joo Chang Zinc oxide (ZnO) has attracted much attention due to a variety of applications to transparent optoelectronic devices. It is known that undoped ZnO exhibits n-type conductivity. Hydrogen, which is unintentionally incorporated, is considered as a promising candidate for shallow donors in ZnO. However, it is still difficult to explain n-type conductivity in annealed ZnO due to the low thermal stability of H. Here we study the diffusion of H in ZnO using first-principles calculations and then perform kinetic Monte Carlo (kMC) simulations for the thermal stability of H. The migration energy of a substitutional H is much higher than that for an interstitial H. Using as input the energy barriers for H diffusion, kMC simulations show that interstitial and substitutional H atoms diffuse out at different annealing temperatures around 125 and 475 $^{o}$C, respectively, in good agreement with experiments. When H atoms are injected from air into ZnO, we find that they are likely to be trapped at O-vacancy sites, leading to the n-type conductivity in annealed samples. [Preview Abstract] |
Session B22: Focus Session: Theory of Spin-based Semiconductor Devices
Sponsoring Units: GMAG DMP FIAPChair: Lukasz Cywinski, University of Maryland
Room: 324
Monday, March 16, 2009 11:15AM - 11:51AM |
B22.00001: Role of motive forces for the spin torque transfer for nano-structures Invited Speaker: Despite an announced imminent commercial realization of spin transfer random access memory (SPRAM) the current theory evolved from that of Slonczewski [1,2] does not conserve energy. Barnes and Maekawa [3] have shown, in order correct this defect, forces which originate from the spin rather than the charge of an electron must be accounted for, this leading to the concept of spin-motive-forces (smf) which must appear in Faraday's law and which significantly modifies the theory for spin-valves and domain wall devices [4]. A multi-channel theory in which these smf's redirect the spin currents will be described. In nano-structures it is now well known that the Kondo effect is reflected by conductance peaks. In essence, the spin degrees of freedom are used to enhance conduction. In a system with nano-magnets and a Coulomb blockade [5] the similar spin channels can be the only means of effective conduction. This results in a smf which lasts for minutes and an enormous magneto-resistance [5]. This implies the possibility of ``single electron memory'' in which the magnetic state is switched by a single electron. \\[4pt] [1] J. C. Slonczewski, {\bf Current-Driven Excitation of Magnetic Multilayers} J. Magn. Magn. Mater. 159, L1 (1996). \\[0pt] [2] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, and B. I. Halperin, {\bf Nonlocal magnetization dynamics in ferromagnetic heterostructures}, Rev. Mod. Phys. 77, 1375 (2005). \\[0pt] [3] S. E. Barnes and S. Maekawa, {\bf Generalization of Faraday's Law to Include Nonconservative Spin Forces} Phys. Rev. Lett. 98, 246601 (2007); S. E. Barnes and S. Maekawa, {\bf Currents induced by domain wall motion in thin ferromagnetic wires.} arXiv:cond-mat/ 0410021v1 (2004). \\[0pt] [4] S. E., Barnes, {\bf Spin motive forces, measurement, and spin-valves.} J. Magn. Magn. Mat. 310, 2035-2037 (2007); S. E. Barnes, J. Ieda. J and S. Maekawa, {\bf Magnetic memory and current amplification devices using moving domain walls.} Appl. Phys. Lett. 89, 122507 (2006). \\[0pt] [5] Pham-Nam Hai, Byung-Ho Yu, Shinobu Ohya, Masaaki Tanaka, Stewart E. Barnes and Sadamichi Maekawa, {\bf Electromotive force and huge magnetoresistance in magnetic tunnel junctions.} Submitted Nature, August, (2008). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B22.00002: Revisiting the ``Spin-Transistor'' Abu Naser Zainuddin, Lutfe Siddiqui, Supriyo Datta A ``spin-transistor'' in principle requires efficient injection (source), efficient detection (drain) and electrical manipulation (gate). For sometime now, electrical manipulation based on the Rashba effect has been well established and in recent years there has been significant progress in the design of injectors and detectors. Lateral spin-valve structures showing $\sim$50\% spin-polarization has been reported. In view of these advances it seems appropriate to evaluate various ``spin- transistor'' concepts. With this in mind, we have developed non- equilibrium Green's function (NEGF) based model and benchmarked against existing experiments. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B22.00003: Dynamical magnetoelectric feedback effects in magnetic resonant tunneling structures Christian Ertler, Jaroslav Fabian Heterostructures made of stacked layers of both magnetic and nonmagnetic semiconductors provide a lot of opportunities for controlling and tuning their spin-dependent transport properties. For instance, highly efficient spin valves,spin switching and spin filtering devices have been demonstrated by using magnetic resonant tunneling structures [1]. Here, we show that in a resonant tunneling double barrier structure, which comprises a ferromagnetic quantum well made of a dilute magnetic semiconductor material, interesting dynamical effects can occur [2]. In such systems the transport and magnetic properties become strongly coupled, since the ferromagnetic order in the quantum well is mediated by the itinerant carriers. Both the Coulomb interaction of the particles and the magnetic exchange field give rise to strong feedback effects on the tunneling current. Interestingly, for a broad voltage range self-sustained high-frequency oscillating currents associated with an oscillating well magnetization appear. The requirements for the occurrence of these dc-driven magnetoelectric oscillations are investigated and possible device setups, which should allow for an experimental observation, are discussed. [1] J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano and I. Zutic, Acta Phys. Slov. 57, 565 (2007). [2] C. Ertler and J. Fabian, Phys. Rev. Lett. 101, 077202 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B22.00004: Theory of Semiconductor Spin Lasers Christian Gothgen, Rafal Oszwaldowski, Igor Zutic Semiconductor lasers with spin-polarized carriers' injection have important advantages as compared to the conventional lasers in which the carriers are unpolarized. While such spin lasers have been successfully realized and shown to provide spin-polarization modulation and threshold current reduction [1-4], there remain important theoretical challenges in understanding their operation. We demonstrate that the maximum threshold reduction is larger than previously thought possible and, surprisingly, can be enhanced by ultrafast spin relaxation of holes [5]. By combining our analytical model [5] and numerical studies of spin lasers we explore the effects of quantum confinement in the gain region and identify different modes of operation. We thank A. Petrou for valuable discussions. This work is supported by US ONR and NSF-ECCS CARRER. [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). [4] D. Basu et al., Appl. Phys. Lett. 92, 09119 (2008). [5] C. Gothgen et al., Appl. Phys. Lett. 93, 042513 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B22.00005: Design Guidelines for Spin-Polarized Lasers Michael Holub, Igor Vurgaftman, Jerry Meyer, Berend Jonker Semiconductor lasers driven by a spin-polarized current are expected to provide a threshold current reduction and optical polarization control. The design of spin-polarized lasers is critical to the realization of these effects. Thus, we have investigated the effect of electron spin injection on semiconductor laser performance using a spin-dependent rate equation model.$^1$ The magnitude of the threshold reduction is shown to depend on intrinsic properties of the active region and laser cavity, and can approach a factor of 3.5 for fully spin-polarized electrons. The threshold reduction is found to be strongest in lasers with undoped active regions, recombination strongly dominated by Auger processes, and low threshold gain. Introduction of a ferromagnetic electrode in the vicinity of the active region for efficient spin injection generally results in higher internal loss and a requirement for greater material gain, which raises the laser's baseline threshold as well as lessens the projected threshold reduction. The placement of a ferromagnetic contact on spin-polarized laser performance will be discussed. \\ \\ $^1$ I. Vurgaftman, M. Holub, B. T. Jonker, and J. R. Meyer {\it Appl. Phys. Lett.} {\bf 93}, 031102 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B22.00006: Spin-orbit coupling effects in Fe/GaAs heterostructures: First principles calculations Martin Gmitra, Alex Matos-Abiague, Claudia Ambrosch-Draxl, Jaroslav Fabian The tunneling anisotropic magnetoresistance (TAMR) effect in semiconductor heterostructures containing a single ferromagnetic layer is potentially useful for spintronics devices. Important, TAMR has recently been observed in a metallic system, namely, in Fe/GaAs/Au junctions. Surprisingly, while all the bulk components of the system are cubic, the observed anisotropy is twofold, of the $C_{2v}$ class. This suggests that rather than coming from the bulk anisotropy of the density of states, the effect arises from the interface that indeed has a reduced symmetry. A phenomenological model reflecting this symmetry in the form of the Bychkov-Rashba and the Dresselhaus spin-orbit coupling was proposed, giving a quantitative fit to the experiment. Here we report on comprehensive first principle calculations of the spin-orbit effects stemming from the interface anisotropy, providing support to the phenomenological theory. In particular, we have performed FPLAPW density functional calculations of an Fe/GaAs slab to extract quantitative information about the proposed model that are Bychkov-Rashba and Dresselhaus parameters. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B22.00007: Magnetic phase transitions driven by non-equilibrium spins and their potential applications to magnetic cooling Lutfe Siddiqui, Abu Naser Zainuddin, Supriyo Datta It is well-known that the Curie temperature in diluted magnetic semiconductors (DMS) like GaMnAs can be controlled by changing the equilibrium density of holes in the material. In this letter we predict,that even with a constant hole density, large changes in the magnetization can be obtained with a relatively small imbalance in the spin population. We show, by coupling mean field theory of diluted magnetic semiconductor ferromagnetism with master equations governing the Mn spin-dynamics, that a splitting of the up-spin and down-spin quasi-Fermi level in the channel by 0.1meV have the same effect as an external magnetic field of 1 T. Experimentally, it has been shown that splitting of the quasi-Fermi level for the two spins by 0.1meV can be conveniently obtained in the channel region of a lateral spin-valve structure with anti-parallel contacts, which can be used to demonstrate the effect we are proposing. Such an effect could also form the basis for a novel class of cooling devices where the electrical input to a spin-valve leads to ferromagnetic ordering of Mn ions which then demagnetize by absorbing energy from the environment. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B22.00008: Magnetic memory and logic based on spin effects in graphene John Zavada, Yuriy Semenov, Ki Wook Kim We report on a novel approach to the problem of low-power-consuming non-volatile magnetic random access memory (MRAM) and logic design that is based on the unique properties of the graphene placed in interface between two magnetic dielectric layers. We find that by combining the electrical effect on the exchange bias field and a giant magneto-resistance effect of the graphene/ferromagnet hybrid structures, a new non-volatile MRAM device is possible. In such a device an electric bias realizes the low energy writing bits instead of an external magnetic field with high energy consumption. In particular, the structure under consideration consists of a three ferromagnetic dielectric layers, which are coupled through monolayer and bilayer graphene films. Interplay of two graphene layers can mediate the exchange bias fields applied to different sides of the free ferromagnets resulting in programmable logic operations. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B22.00009: Metal-insulator transition in a quantum wire with alternating Rashba interaction Henrik Johannesson, George I. Japaridze, Alvaro Ferraz We propose and analyze a device scheme by which an electrical current can be controlled via a gate-operated spin-orbit interaction. The device consists of a quasi-one-dimensional (1D) ballistic channel in a gated semiconductor heterostructure, contacted to a source and a drain and with the gates producing an alternating Rashba spin-orbit interaction. When the period of the Rashba modulation becomes commensurate with the 1D electron density, the spin-orbit interaction opens a charge gap, leading to a suppression of the current. Using bosonization and a perturbative RG approach we explore how electron-electron interactions influence the effect. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B22.00010: Gate control of single-electron spins through Berry Phase in a realistic asymmetric confining potentials in III-V semiconductor Quantum Dots Sanjay Prabhakar, James Raynolds Among recent proposals for next-generation, non-charge-based logic is the notion that a single electron can be trapped and its spin can be manipulated through the application of gate voltages (Rev. Mod. Phys.79, 1217 (2007)). In this talk we present numerical simulations of Berry Phase of electron spins in single electron devices for realistic asymmetric confining potentials in support of experimental work at the University at Albany, State University of New York aimed at the practical development of post-CMOS concepts and devices. We solve the Schr\"{o}dinger equation including spin-orbit effects using a numerical finite-element based technique. We will discuss the calculation of Berry Phase for electrons (Phys. Rev. B 73, 125330 (2006)) in electrostatically defined quantum dots including the Rashba and Dresselhaus spin-orbit interactions computed numerically from realistic asymmetric confining potentials. The new simulation results open the possibility of spin manipulation through the gate induced Berry phase. This work is supported through funding from the DARPA/NRI INDEX center. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B22.00011: Colossal Spincaloritronic Cooling by Adiabatic Spin-Entropy Expansion in Nanospintronics Hiroshi Katayama-Yoshida, Tetsuya Fukushima, Van An Dinh, Kazunori Sato The exchange interactions in DMS are short ranged and can not play an important role for realizing high-T$_C$ because the solubility of magnetic impurity is too low to achieve magnetic percolation [1]. We show that spinodal nano-decomposition under layer-by-layer crystal growth condition (2D) leads to characteristic quasi-one dimensional nano-structures (Konbu- Phase) with highly anisotropic shape and high T$_C$ ($\rangle$ 1000K) even for low concentrations in DMS [2]. We design a spin-currents- controlled 100 Tera bits/icnh$^2$, Tera Hz switching, and non- volatile MRAM without Si-CMOS based on Konbu-Phase [3]. In addition to the conventional Peltier effect, we propose a colossal spincaloritronic cooling based on the adiabatic spin- entropy expansion in a Konbu-Phase (Zn,Cr)Te with very high blocking temperature (T$_B$ $\rangle$ 1000 K) by spinodal nano- decomposition and by nano-column of Half-Heusler NiMnSi (T$_C$ = 1050 K) [4]. [1] K. Sato et al., Phys. Rev. B70, 201202 (2004). [2] H. Katayama-Yoshida et al., Phys. stat. sol. (a) 204 (2007) 15. [3] Japanese Patent: JP3571034, US Patent: US 7,164,180 B2, EU Patent: EP 1548832A1, Taiwan Patent:1262593, Korean Patent: 0557387. [4] H. Katayama-Yoshida et al., Jpn. J. Appl. Phys. 46 (2007) L777. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B22.00012: impurity-impurity interaction in graphene nanoribbons Jian-Ming Tang The high mobility and small spin-orbit interaction makes graphene a promising candidate material for building spin-based quantum devices. Embedded magnetic dopants or magnetized defects due to many-body interaction may act as single spin qubits in these devices. The long decay length for the impurity levels near the Dirac point suggest that the double exchange interaction can compete with the RKKY exchange interaction. The impurity level splitting for two impurities in bulk and in nanoribbons are studied using a tight-binding approach. In the case of nanoribbons, the modification to the interaction due to the presence of edge states will be discussed. [Preview Abstract] |
Session B23: Alloy Theory
Sponsoring Units: DCMPChair: Benjamin Burton, National Institute of Standards and Technology
Room: 325
Monday, March 16, 2009 11:15AM - 11:27AM |
B23.00001: Optimized basis-set representation for electronic-structure methods: Better Energetics Aftab Alam, Duane Johnson We derive an analytic expression for an optimal, and rapidly computed, representation for site-centered basis-set expansion (e.g., spherical harmonic). An optimal site-dependent radius are determined from the local saddle-points derived in terms of overlapping atomic charge densities, typically already used for L\"{o}wden construction of the starting potentials. These ``saddle-point adjusted'' sphere radii separate the ``spherical'' density and potential around an atom from the symmetry-induced, ``non-spherical'' part in the interstitial, and more properly accounts for charge and size of atoms. These radii also properly determine the weighted Voronoi cells (i.e., power diagram or generalized Wigner-Seitz cells) which are mathematically guaranteed to be convex and space filling. For full-potential and forces, exact integrations over Voronoi interstitials is accomplished easily by isoparametric integration. We implement these ideas in a general Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) code within the atomic-sphere-approximation (ASA). For several large-atom/small-atom systems, we show that ASA using saddle-point-adjusted spheres now agrees with formation energies from full-potential calculations and experiments for both chemically ordered and disordered cases, and, hence, predict the stability of the correct phases and its temperature scale. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B23.00002: Applications of the KKR-DCA: A Finite-Temperature Density Functional Theory to Predict Chemical Short-Range Order Effects in Disordered Metallic Alloys D.A. Biava, D.D. Johnson Short-range order (SRO) is ubiquitous in metallic alloys, affecting changes in their electronic, thermodynamic, mechanical, magnetic, and structural properties. For example, SRO is responsible for the yield-strength anomalies observed in Cu-Al at high temperatures, i.e., the materials is more resistant to dislocation motion at high temperature than it is at room temperature. Within the Korringa-Kohn-Rostorker (KKR) electronic-structure method, we present results using the dynamical cluster approximations (DCA) to obtain the temperature-dependent SRO in disordered alloys. We obtain the KKR-DCA SRO energetics versus local neighbor SRO parameters and minimize it at fixed temperature to predict the SRO. We show that the calculated SRO at fixed temperature compares well with available experimental results, and then correlate the results to the electronic structure. We discuss how an accurate analytic estimate can be made for the SRO in most metals due to the dependence of the grand potential on SRO. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B23.00003: Electronic and Magnetic properties of NbFe$_2$: An itinerant magnet near a quantum critical point Alaska Subedi, David J. Singh NMR studies show that pure C14 Laves phase NbFe$_2$ is a weak antiferromagnet below 13K with magnetic moment per Fe of no more than 0.1$\mu_B$. However, the Nb-rich samples do not show antiferromagnetism down to 1.8K, which suggests that they are close to antiferromagnetic QCP. Here we report density functional studies of the magnetic properties, band structure and Fermiology. We elucidate the nature of the ordering between the two distinct Fe sites and discuss the results in relation to the quantum criticality. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B23.00004: First-principles calculations of free energies of unstable phases: The case of fcc W Vidvuds Ozolins Ab initio density-functional theory molecular dynamics simulations are used to solve the long-standing problem of calculating the free energies of harmonically unstable phases, such as fcc W. We find that fcc W is mechanically unstable with respect to long-wavelength shear at all temperatures considered (T$>$2500 K), while the short-wavelength phonon modes are anharmonically stabilized. The calculated fcc/bcc enthalpy and entropy differences at T=3500 K (308 meV and 0.74 kB per atom, respectively) agree well with the recent values derived from analysis of experimental data. The proposed method can be used in first-principles modeling of the thermodynamics of unstable phases and calculations of the thermodynamic driving forces for martensitic transformations in pure elements and alloys. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B23.00005: Anomalies in the bulk properties of single crystalline Niobium Richard K. Bollinger, J. J. Neumeier, B. D. White, Yoko Suzuki, A. Migliori, Jon Betts, H. R. Z. Sandim, C. A. M. dos Santos The thermodynamic properties of single crystal Niobium are presented. Anomalies in thermal expansion, specific heat, elastic constants, and electrical resistivity are observed. The linear coefficient of thermal expansion, $\alpha$, exhibits a large, broad peak in the range 200 K $<$ T $<$ 280 K, with a nearly two-fold increase in $\alpha$. The elastic constants show anomalies over a similar temperature range, while anomalies in heat capacity and resistivity are much narrower. This is surprising since crystalline Nb is a simple system, with only one naturally occurring isotope and a body centered cubic structure. Measurements on a second single crystal and on high purity polycrystalline Nb will also be presented. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B23.00006: Development of an Embedded-Atom Method Potential for Niobium Michael R. Fellinger, John W. Wilkins An embedded-atom method (EAM) potential [1,2] is developed for pure niobium as the first step in the construction of an EAM potential for titanium-niobium alloys. The potential is constructed using the force-matching method [3]: the functions comprising the potential are represented as cubic splines, and the spline knots are chosen such that the potential optimally reproduces a large database of forces, cohesive energies, and stresses computed via density functional theory. The code potfit [4] optimizes the splines using a combination of simulated annealing and conjugate gradient-like minimization algorithms. EAM results are compared to DFT and experimental results for the lattice constant, cohesive energy, single-vacancy formation energy, fcc-bcc and hcp-bcc structural energy differences, elastic constants, and phonon dispersions. \newline [1] M. S. Daw and M. I. Baskes, Phys. Rev. Lett. 50, 1285 (1983). \newline [2] M. S. Daw and M. I. Baskes, Phys. Rev. B 29 6443 (1984). \newline [3] F. Ercolessi and J. B. Adams, Europhys. Lett. 26, 583 (1994). \newline [4] P. Brommer and F. G\"{a}hler, Modelling Simul. Mater. Sci. Eng. 15, 295 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B23.00007: Ab initio up to the melting point: Anharmonicity and vacancies in aluminum B. Grabowski, L. Ismer, T. Hickel, J. Neugebauer At elevated temperatures, the heat capacity of metals strongly deviates from the harmonic prediction. This was pointed out long ago\footnote{M. Born and E. Brody, Zeitschrift f\"ur Physik 6, 132 (1921)} and various explanations have been considered. Ab initio calculations showed\footnote{B. Grabowski, T. Hickel, J. Neugebauer, Phys. Rev. B 76, 24309 (2007)} that a dominant part can be explained by quasiharmonic excitations. However, the {\it detailed} balance of further contributions, such as explicit anharmonicity and vacancies, is not clarified yet even for simple elementary metals. Aluminum is a prototypical example. Even though intensively studied, the ambiguous experimental situation has made a classification of the mechanisms impossible. To resolve the situation, we have calculated the full volume and temperature dependent {\it ab initio} free energy surface employing density-functional theory. In particular, we have included anharmonic and vacancy contributions using numerically highly efficient methods to coarse grain the configuration space. To obtain accurate vacancy energies, we have included the full spectrum of excitations: quasiharmonic, electronic, and explicitly anharmonic. The results are in contradiction to common belief, nevertheless the essential physics can be captured by a simple model. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B23.00008: New Generation Structural Materials: Ab initio Based Modeling of High-Entropy Alloys G. Malcolm Stocks, Xing-Qiu Chen, Easo P. George, Chonglong Fu, Takeshi Egami There is rapidly growing interest in a new generation of structural materials called high entropy alloys. This class of alloys is multi-component ($\sim $ five elements) with approximately equiatomic ratio, and thus have high entropy of mixing by which they are distinguished from conventional alloys. It has been reported experimentally that the single bcc-based AlCoCrFeNi, single fcc-based CoCrCuFeNi and FeCrMnNiCo high-entropy alloys exhibit promising mechanical properties with potential applications. In this work, we introduce ab initio based modeling for understanding structural, magnetic, and elastic properties based on relaxation of randomly generated supercells within the framework of density functional theory. We studied component-dependent phase stabilities, electronic structures, and magnetic properties with all solutes at fixed and relaxed positions. The properties are analysed in terms of the underlying electronic structure and suggestions are made for further experimental studies to further clarify the reasons for the unusual stability of these systems. Research sponsored by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. DOE. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B23.00009: High-throuput formalism and calculation of Ag, Au, Cd, Co, Cr, Ir, W, and Zn solubility in Ti from first-principles Roman Chepulskyy, Stefano Curtarolo Based on statistical-thermodynamic theory of a dilute lattice gas, we developed an approach for calculation of atomic solubility in alloys. The advantage of the approach consists in taking into account all known alloy ground states rather than just pure species. It is shown that the low-solubility obey the simple Arrhenius-type dependence on temperature determined by ``low-solubility formation energy.'' Such quantity is defined as the derivative of the compound formation energy, determined with respect to surrounding ground states, versus composition. ``Low-solubility formation energy'' coincides with the usual ``true'' defect formation energy only in the case of a phase-separating alloy having no intermediate ground states and vacancies. We present a high-throughput formalism where the ``low- solubility formation energy'' can be directly obtained through first-principles calculations. The developed approach is applied to solubility of transition metals in titanium. The obtained values and tendecies are in good qualitative correspondence with experiments. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B23.00010: Alternative alloys for catalysts and platinum jewelry? New structures in Pt-Hf and Pt-Mo Erin Gilmartin, Jacqueline Corbitt, Gus Hart The only known intermetallic structure with an 8:1 stoichiometry is that of Pt$_8$Ti. It is intriguing that an ordered phase would occur at such low concentrations of the minority atom, but this structure occurs in about a dozen binary intermetallic systems. The formation of an ordered phase in an alloy can significantly enhance the performance of the material, particularly the hardness. We have taken a broad look at possible systems where this phase forms. Using first-principles, we calculated the stability of this structure relative to experimentally known phases for more than 80 Pt/Pd binary systems. We find the Pt$_8$Ti structure is a possible ground state in more than 20 cases. Our experimental collaborators have verified our prediction in Pt-Mo and observed order-hardening in Pt-Hf. We discuss the discovery of new ground states that are likely to be verified experimentally and their impact on materials for Pt- and Pd-based catalysts and jewelry. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B23.00011: New structures in Pd-rich ordered alloys Jacqueline Corbitt, Erin Gilmartin, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then a handful of other Pt/Pd/Ni binary systems have been observed to exhibit this curious structure (Pt$_8$Zr, Pd$_8$Mo, Ni$_8$Nb, etc). This ordered structure can significantly increase the hardness of an alloy. For jewelry applications involving Pt and Pd, international hallmarking standards require that the alloys be at least 95\% pure by weight. However, Pt- and Pd-rich alloys are often soft when purity is high if the minority atoms are disordered. Because the 8:1 structure maintains a high weight percentage of Pt/Pd, it can satisfy purity standards while increasing performance. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations and cluster expansion modeling, we have performed a ground state search to find the stable structures in Pd-Nb and Pd-Cu. In collaboration with Candace Lang's group at University of Capetown South Africa, we are working to experimentally validate the predicted ground states. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B23.00012: Verification and refinement of the Al-Mg-Zn $\Phi$ phase crystal structure model Jeff Houze, Bohumir Jelinek, Sungho Kim, Seong-Gon Kim, Mark Horstemeyer Density Functional Theory calculations were performed to validate the crystal structure proposed by L. Bourgeis et al.\ for the $\Phi$ phase of the Al-Mg-Zn system. Their model has ambiguous site occupancies for Zn and Al and definite locations for Mg. The model's simulated electron diffraction patterns agreed very well with experimental patterns. Using DFT calculations we are able to determine optimal Zn and Al aluminum locations. We will also show that the energetically optimal structure's element concentrations are within the experimentally observed range. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B23.00013: An ab initio study of the crystal structure of the Tau-phase in Al-Mg-Zn alloys Laalitha Liyanage, Jeffrey Houze, Sungho Kim, Mark Horstemeyer, Seong-Gon Kim Existing crystal structures for the intermetallic Tau-phase in Al-Mg-Zn alloy are studied by density functional theory calculations using projector augmented wave pseudopotentials. Favorable crystal structures are identified through volume optimization and formation energy calculations. Properties such as elastic constants and bulk modulus of the crystal structure are determined. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B23.00014: Magnesium phase diagrams: Have you seen us? Gus Hart, Stefano Curtarolo Because of it's high strength-to-weight ratio, magnesium is seen as promising material for automotive applications. But magnesium alloys are far less understood that more common alloys such as steel or newer alloys such as aluminum. Even among simple binary magnesium systems, there is a great deal of missing information. There are binary magnesium systems for which no phase diagrams appear in the latest databases (the Pauling File, for example). Using a high-throughput approach, we have undertaken a broad search for ground states in 40 magnesium binary systems using more than 8000 fully-relaxed first-principles calculations. We find new, non-obvious ordering systems and many systems where there are unsuspected ground states. We discuss the results and their potential impact on magnesium alloys. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B23.00015: First-principles thermodynamics of point defects and off-stoichiometry in \textit{$\beta $}-Mg$_{17}$Al$_{12}$ Dongwon Shin, Christopher Wolverton The mechanical strength of Mg-Al alloys may be enhanced by a fine spatial dispersion of \textit{$\beta $}-Mg$_{17}$Al$_{12}$ precipitates. Native point defects, i.e. vacancies and anti-sites, in Mg$_{17}$Al$_{12 }$are important for understanding the phase stability and unusually asymmetric observed off-stoichiometry in this precipitate phase. In an effort to provide a quantitative picture of the phase stability of this system, we have performed a series of first-principles density functional theory calculations of bulk and defect properties of Mg$_{17}$Al$_{12}$. We consider not only the T=0K static energetics, but also key entropic terms such as the configurational and vibrational entropies. The vibrational entropies are calculated from DFT via the direct force-constant approach using the quasiharmonic approximation. We investigate the effect of atomic vibrations on native point defect free energies of Mg$_{17}$Al$_{12 }$and combine the entropic contributions with the point defect formation energies to evaluate the thermodynamics of off-stoichiometry in this phase. We find there is a large vibrational entropy difference between Mg-rich and Mg-deficient defects in Mg$_{17}$Al$_{12}$, consistent with the strong asymmetry in the observed Mg-Al phase diagram. [Preview Abstract] |
Session B24: Focus Session: Nanotube Characterization
Sponsoring Units: DMPChair: Cary Yang, Santa Clara University
Room: 326
Monday, March 16, 2009 11:15AM - 11:51AM |
B24.00001: Raman spectroscopy of individual freestanding single-walled carbon nanotubes of defined chiral structure Invited Speaker: We review the main information that we have obtained from Raman spectroscopy experiments combined with electron diffraction experiments on individual freestanding single-walled carbon nanotubes. This information concerns: the radial breathing mode vs diameter relationship; the dependence of the frequency and lineshape of the G-modes in semiconducting and metallic tubes; the evaluation of the optical transition energies for individual freestanding SWNTs. These experimental Raman results obtained on well-identified individual SWNTs are compared with other experimental data and theoretical predictions. From these data, we can define Raman criteria that allow identifying carbon nanotubes from their Raman features only. We show the efficiency of this approach: (i) to assign the (n,m) indices of individual freestanding single-walled carbon nanotubes, and (ii) to identify the (n,m) tubes organized in a small bundle. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B24.00002: Raman Studies of Exciton and Exciton-Phonon Coupling Behavior in Metallic Single-Walled Carbon Nanotubes Invited Speaker: A scaling law analysis of carbon nanotube transition energies has been found to be valuable in revealing new electronic behaviors for the third and fourth transitions in semiconducting nanotubes. In the work presented here, we discuss resonance Raman data obtained for the E$_{11}^{M}$ and E$_{22}^{M}$ transitions of a broad diameter range (0.7 - 4 nm) of metallic carbon nanotubes. We show that application of the scaling law analysis to transition energies for metallic nanotubes suggests that the transitions are excitonic in nature and that relative scaling of electron self-energies and exciton binding energies in metallic nanotubes closely matches that found in semiconductors. This similarity in behavior can be understood in terms of similar regions of the Brillouin zone being sampled by E$_{11}^{M}$ and E$_{11}^{S}$ and E$_{22}^{S}$ (and by E$_{22}^{M}$ and E$_{33}^{S}$ and E$_{44}^{S})$. Additionally, for large diameter nanotubes ($>$ 1.3 nm) we now observe the previously elusive upper branch signatures for several chiralities for both E$_{11}^{M}$ and E$_{22}^{M}$ excitation. These results are discussed as a consequence of the nodal behavior of exciton-phonon coupling. Also, while theoretical calculations for the ($n,m)$-dependent matrix elements predict the RBM intensity should decrease with increasing diameter; the opposite behavior is observed experimentally. We show this to be a consequence of an increase in the resonance Raman broadening factor $\Gamma $ as diameter decreases. Finally, we present Raman excitation data from surfactant suspensions highly enriched in metallic nanotubes via density gradient ultracentrifugation. Specifically, we will focus on the evolution of G-band behavior over a wide range of chiralities enabled by these new sample types. The variable behavior of the Breit-Wigner-Fano line in these enriched ensemble samples will be discussed. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B24.00003: Resonance Raman Spectroscopy of Armchair Single-Walled Carbon Nanotubes Erik Haroz, William Rice, Benjamin Lu, Robert Hauge, Donny Magana, Stephen Doorn, Pasha Nikolaev, Sivaram Arepalli, Junichiro Kono We performed resonance Raman spectroscopy studies of metallic single-walled carbon nanotubes (SWNTs), including armchair SWNTs from (6,6) through (10,10). The measurements were carried out with excitation of 440-850 nm on aqueous ensemble samples of SWNTs enriched in metallic species. From this, we generated Raman excitation profiles (REPs) of the radial breathing mode and compare the REPs of armchairs and other metallic species. Additionally, we measured REPs of the G-band mode and observed how the Breit-Wigner-Fano line shape of the G$^{-}$ peak evolves in peak position, width and intensity relative to the G$^{+}$ peak as different metallic nanotubes are excited. By combining these studies with absorption and photoluminescence excitation spectroscopy studies, we present a comprehensive examination of the optical signatures of metallic SWNTs. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B24.00004: Direct Measurement of the D-Mode and G-Mode Optical Phonon Lifetimes in Single Wall Carbon Nanotubes Hugen Yan, Daohua Song, Tony Heinz Time-resolved Raman spectroscopy has been applied to determine the population lifetime of both zone-center and zone-edge optical phonons. Non-equilibrium populations of these phonons were produced by the rapid relaxation of charge carriers following photoexcitation of the nanotube sample with a femtosecond laser pulse. The temporal evolution of these phonon populations was recorded using the strength of antiStokes Raman scattering in G-mode (for the zone-center phonons) and D-mode (for the zone-edge phonons) as a function of the time delay of the fs probe pulse. A longer lifetime was found for the D-mode than for the G-mode phonons, a result consistent with recent ab-initio calculations of the anharmonic decay of these phonons [1]. We also report on the transient mode populations for the zone-center and zone-edge phonons that result from carrier cooling. [1] N. Bonini, M. Lazzeri, N. Marzari, and F. Mauri, Phys. Rev. Lett. \textbf{99}, 176802 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B24.00005: Investigation of Nanotube Growth Mechanisms via \textit{In-Situ} Spectroscopy Rahul Rao, David Liptak, Roberto Acosta, Benji Maruyama Analysis of single-walled carbon nanotubes (SWNTs) during growth via Raman spectroscopy offers a unique approach to understand their growth mechanism, which remains unclear due to large variability of parameters in synthesis methods. In our technique the SWNTs are synthesized via chemical vapor deposition inside an environmental cell coupled to an automated stage. Growth occurs from catalyst nanoparticles on thermally isolated islands within substrates. \textit{In-situ }micro-Raman spectroscopy is performed on the radial breathing mode and D/G bands of the growing SWNTs where the excitation laser also serves as a localized heat source for SWNT growth. Computer control over substrate temperature and position, feed gas composition, and chamber pressure enable rapid real-time exploration of SWNT growth parameter space. Comparison of nanotube nucleation and growth kinetics from various metallic catalyst particles will be presented and implications for nanotube catalyst design will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B24.00006: Characterisation of Carbon Nano-Materials with the Confocal Raman AFM Klaus Weishaupt, Thomas Dieing, Matthias Kress, Ute Schmidt Graphene and carbon nanotubes represent perfect model systems for fundamental research. Carbon nanotubes have proven to be unique systems for the study of Raman spectra in one-dimensional systems. Although the diameter of single walled carbon nanotubes (SWCNT) is far below the optical resolution limit, its unique optical and spectroscopic properties due to the one-dimensional confinement of electronic and phonon states leads to resonant enhancement of the corresponding photophysical process. Characteristic for SWCNT only are the radial breathing modes (RBM) providing information about the diameter of the carbon nanotube. The position and width of the G-band is used to distinguish between metallic and semiconducting SWCNT and to probe the charge transfer arising from doping a SWCNT. The G' band, characteristic for interlayer coupling in graphite, arises from phonon resonance in SWCNT. Graphene shows similar unique properties and is a perfect model system for Raman spectroscopy in a two-dimensional system. The combination of two different analytical techniques such as confocal Raman microscopy and atomic force microscopy (AFM) in one instrument, allow the topographical and optical characterization of carbon nano-materials. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B24.00007: Resonance Raman Scattering for Quantification of the Bundling of SWNTs Tao Liu, Zhiwei Xiao The strong attractive van der Waals interaction induces individual SWNTs to form bundles or ropes. It has been demonstrated both experimentally and theoretically that, the various physical properties of SWNTs, e.g., photoluminescence, electrical and electronic, and mechanical, strongly depends upon their bundling states. Upon comparative studies of SWNT dispersions with the preparative ultracentrifuge method, which is a newly developed characterization technique by us for quantifying the structures of SWNTs in a dispersion, and resonance Raman scattering, we demonstrate that the bundling states for a given SWNT dispersion can be quantified with the latter technique. In this presentation, the preparative ultracentrifuge method for studying the processing-structure-property relationships of SWNT dispersion will be introduced. The mechanisms of using resonance Raman scattering to quantify the bundling states of SWNTs will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B24.00008: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 1:39PM - 1:51PM |
B24.00009: Electrical Characterization of Carbon Nanotube Bundles Synthesized from Chemical Vapor Deposition of Ferrocene C. Wolfe, R. Shah, X. Zhang, X. An, S. Kar, S. Talapatra We employed a chemical vapor deposition technique, which used ferrocene both as the catalyst as well as the carbon source, to grow films of carbon nanotubes (CNT). The CNT films obtained using this procedure were characterized using Raman Spectroscopy and Transmission Electron Microscopy which indicated the presence of thin diameter carbon nanotubes as well as single walled CNT ropes. Electrical transport measurements performed on long ropes of CNTs extracted from these bulk films will be presented and will be discussed in the framework of transport theories of quasi-one dimensional systems. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B24.00010: Coherent Phonon Dynamics in Single-Walled Carbon Nanotubes L.G. Booshehri, E.H. Haroz, J. Kono, Y.S. Lim, J.H. Kim, K.J. Yee, G.D. Sanders, C.J. Stanton Understanding how electrons and phonons relax in energy and momentum is one of the current goals in carbon nanotube spectroscopy as well as an important step towards developing novel electronic and optoelectronic devices based on carbon nanotubes. Recent ultrafast pump-probe spectroscopy studies of single-walled carbon nanotubes (SWNTs) have successfully detected coherent phonon dynamics, but the dominant dephasing mechanism that occurs with decoherence of phonon mode oscillations has yet to be understood. Our previous work demonstrating ultrafast coherent phonon spectroscopy of the radial breathing mode (RBM) of semiconducting SWNTs provided a powerful method for determining phonon energies in an ensemble of SWNTs. We now extend our previous studies to provide new insight into the dephasing mechanisms of coherent phonons in SWNTs. Here, we systematically investigated the temperature, polarization, and wavelength dependence of coherent phonon dephasing times, amplitude, and frequency for various types of nanotube film and solution samples. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B24.00011: Microscopic theory of coherent phonon spectroscopy of carbon nanotubes G.D. Sanders, C.J. Stanton, J.K. Kim, K.J. Yee, Y.S. Lim, E.H. Haroz, L.G. Booshehri, J. Kono, R. Saito Using pump-probe spectroscopy with pulse shaping techniques, we study coherent phonons in chirality-specific semiconducting single-walled carbon nanotubes. The signals are resonantly enhanced when the pump photon energy coincides with an exciton resonance, and provides information on the chirality-dependence of light absorption, phonon generation, and phonon-induced band structure modulation. We develop a microscopic theory for generation and detection of coherent phonons in carbon nanotubes. We find that coherent phonon amplitudes satisfy a driven oscillator equation with a driving term depending on photoexcited carrier density. We compare theory with experiment and find that our model predicts correct overall trends in the relative strength of the coherent phonon signal both within and between different mod(n-m,3)=2 families. We predict that phonon intensities are considerably weaker in mod(n-m,3)=1 tubes in comparison with mod(n-m,3)=2 tubes, also in agreement with experiment. [Preview Abstract] |
Session B25: Graphene II: Electronic Properties and QHE
Sponsoring Units: DMPChair: Ziqiang Wang, Boston College
Room: 327
Monday, March 16, 2009 11:15AM - 11:27AM |
B25.00001: Quantum Hall effect of massless Dirac fermions in a vanishing magnetic field Kentaro Nomura, Shinsei Ryu, Mikito Koshino, Christopher Mudry, Akira Furusaki Graphene displays an unconventional quantization of the Hall conductivity when subjected to a magnetic field. The Hall conductivity is measured to be a half-integer in units of 4 times the conductance quantum. We have undertaken a numerical study of the quantum Hall effect of massless Dirac fermions in two-dimensions, and found the following remarkable effect. A negative and a positive quantized Hall plateaus survive even in the limit of vanishing magnetic fields provided inter-valley scattering is negligible. This conclusion is based on our finding that all but one critical states between the different quantized Hall plateaus float away from the charge neutral point as the Landau level mixing becomes stronger. The exception is the state at the neutral point that remains critical whatever the disorder strength is, or equivalently no matter how weak the magnetic field is. K. Nomura, S. Ryu, M. Koshino, C. Mudry, A. Furusaki, Phys. Rev. Lett. 100, 246806 (2008). [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B25.00002: New quantum Hall ferromagnetic states in graphene bilayers Rene Cote, Yafis Barlas, Jules Lambert, Allan H. MacDonald, Kentaro Nomura Quantum Hall ferromagnetic states are expected in graphene bilayers because of the degeneracy of the eight Landau levels which appear near the neutral system Fermi level. Working within the Hartree-Fock approximation, we derive the phase diagram of the two-dimensional electron gas (2DEG) at integer filllings $\nu=-4,-3,...,3,4$ as a function of the magnetic field and an external potential difference between the layers. We show that coulomb interaction leads to broken symmetry ground states that lift the degeneracies associated with spin, valley pseudospin and orbital pseudospin (the $n=0$ and $n=1$ orbital Landau level states are degenerate in graphene bilayers). The phase diagram of the 2DEG in this system is very rich and contains states with interlayer and/or orbital coherence that can be abruptly modified by an interlayer potential difference. We show that some of the broken symmetry states have collective excitations with unusual dispersion relations due to the coupling between valley and orbital pseudospins whose fluctuations give rise to electric dipoles. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 12:15PM |
B25.00003: Novel electronic and transport properties of graphene superlattices Invited Speaker: Charge carriers in graphene show linear and isotropic energy dispersion relation and chiral behavior, like massless neutrinos in particle physics. Because of these novel properties, many interesting and unconventional phenomena occur in graphene. On the other hand, since the 1970's, metallic and semiconducting superlattice structures -- man-made crystals -- have been extensively studied regarding to their fundamental electronic and optical properties as well as many applications. In this talk, I will present calculations on the properties of charge carriers in graphene under an external periodic potential (graphene superlattices) which are found to be greatly different from those of conventional two-dimensional electron gases in similar conditions [1-3]. I will discuss the anisotropies in the group velocity around the Dirac point and in the gap opening at the supercell Brillouin zone boundary [1]. Next, I will focus on the special cases where the group velocity along one direction becomes zero [1,2], emphasizing the phenomena of pseudospin collapse and possible electron beam supercollimation effects in these systems [2]. Finally, I will discuss the properties of a new generation of massless Dirac fermions at the supercell Brillouin zone boundaries and their experimental implications [3]. \\[4pt] [1] C. -H. Park, L. Yang, Y. -W. Son, M. L. Cohen, and S. G. Louie, Nature Phys. 4, 870 (2008). \\[0pt] [2] C. -H. Park, Y. -W. Son, L. Yang, M. L. Cohen, and S. G. Louie, Nano Lett. 8, 2920 (2008). \\[0pt] [3] C. -H. Park, L. Yang, Y. -W. Son, M. L. Cohen, and S. G. Louie, Phys. Rev. Lett. 101, 126804 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B25.00004: Quantum Hall wavefunction for Dirac fermions in high magnetic field Feng Cai, Ziqiang Wang A salient feature of planar Dirac fermion system is the presence of zero modes in high magnetic field with non-trivial topological properties. Based on this fact we develop a field theoretic approach to derive the ground state wavefunction in the $n=0$ Landau level. We discuss possible connections to the unconventional quantum Hall states observed in graphene. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B25.00005: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B25.00006: Edge states in Graphene: from gapped flat band to gapless chiral modes Shengyuan Yang, Wang Yao, Qian Niu We study edge-states in graphene systems where a bulk energy gap is opened by inversion symmetry breaking. We find that the edge-bands dispersion can be controlled by potentials applied on the boundary row. Under certain boundary potentials, gapless edge-states with valley-dependent velocity are found, exactly analogous to the spin-dependent gapless chiral edge-states in quantum spin Hall systems. The connection of the edge-states to bulk topological properties is revealed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B25.00007: Excitonic supersolid in quantum Hall graphene bilayers Yogesh Joglekar, Chang-hua Zhang We study the ground state of two graphene sheets separated by a distance $d$ in the quantum Hall regime where the top layer has electrons and the bottom layer has holes as carriers. We obtain a rich mean-field phase diagram as a function of distance $d$ and the partial filling factor $\nu_e=\nu_h=\nu$ for different Landau levels. We find that the ground state in high Landau levels at large $d$ is a generalized Wigner crystal that includes anisotropic stripe and bubble states, and at small $d$ the ground state is a uniform excitonic condensate. We show that for a wide range of partial filling factors $0\le \nu \leq 1/2$, at intermediate values of $d$, the ground state has interlayer phase coherence as well as a lattice structure, i.e. it is an excitonic supersolid. We discuss the predictions for signatures of such a state in transport and optical experiments. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B25.00008: Transport on quantum antidot made of 4-terminal graphene ribbons Andrea Latge, Carlos Ritter, Pedro Orellana, Monica Pacheco Electronic and transport properties of four-terminal graphene ribbons are discussed taking into account different configurations of quantum antidot potentials, designed at a central conductor. In general, the formation of these antidot potentials promotes a reorganization of the carriers, leading to an electronic localization at the neighboring vacancy sites. Depending upon the position, extension, and symmetries of such antidots, one may find delocalization along the structure due to the formation of new allowed paths. Here we discuss the origin of conductance dips, maximum and complete transport suppressions, within the microscopic scenario of the electronic localization, and using real-space Green function formalism. For such analysis we construct local electronic density of states mapping for different antidot configurations. The results are discussed in comparison with equivalent two-lead devices and perfect structures. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B25.00009: Gate-field effect in multilayer graphenes Mikito Koshino We study the electronic properties of the multilayer graphenes in presence of the external electric field perpendicular to the layers. We calculate the electronic potential of each layer taking account of the screening effect, to obtain the self-consistent band structure within the effective mass approximation. We calculate the conductivity using the self-consistent Born approximation and analyze its external-field dependence for every layer number. We also compute the optical absorption spectra, which is found to be strongly modified by the electric field. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B25.00010: Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices Roksana Golizadeh Mojarad, Damon Farmer, Vasili Perebeinos, Yu-Ming Lin, George S. Tulevski, James C. Tsang, Ali Afzali, Phaedon Avouris We investigate polyethylene imine and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B25.00011: Sub-lithographic Patterning of Extended Arrays of Graphene Nanostructures Ke Li, Wei Han, Sarah C. Parks, Wenzhong Bao, John Ciraldo, Chun Ning Lau, Roland Kawakami, Ezekiel Johnston-Halperin Quasi-one-dimensional graphene nanoribbons (GNRs) with narrow width (w $\le $ 10 nm) and smooth edges have been shown to exhibit bandgaps due to quantum confinement and edge effects. Current fabrication methods of GNRs include electron beam lithography and chemical synthesis. However, the lithographic approach has difficulties in reaching true nanometer-scale widths while the chemical approach lacks fidelity in GNR length and width control. The recent development of sub-lithographic patterning using the superlattice nanowire pattern transfer (SNAP) technique provides a novel approach to fabricating ultra-long ($>$ 1 mm) GNRs with width down to 7 nm. In addition, repeating SNAP at 90\r{ } with respect to the 1st patterning potentially allows graphene rectangular antidot arrays with N$_{dot}$=160,000 and dot density up to 10$^{11}$ /cm$^{2}$ (dot-to-dot spacing 15 nm; dot area 8 nm $\times $ 8 nm). This novel sub-lithographic patterning technique should enable tailored graphene nanostructures and high throughput manufacturing of GNR-based nano-devices for next generation nanoelectronic applications. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B25.00012: Quantum Hall Edge Transport Across Graphene monolayer-bilayer junctions Yue Zhao, Mikito Koshino, Philip Kim We experimentally studied the transport property of a graphene monolayer-bilayer junction in the Quantum Hall (QH) regime. Both the monolayer graphene (MG) and the bilayer graphene (BG) develop their own landau levels under high magnetic field. While the transport measurement shows their distinct QH effect in bulk part of the MG and BG respectively, the transport measurement across their interface exhibits unusual transverse transport behaviors. The transverse resistance across the MG BG interface is asymmetric for opposite sides of the hall bar, and its polarity can be changed by reversing the magnetic field direction. When the QH plateaus of MG and BG overlap, quantized resistance will appear only on one side of the hall bar electrode pairs across the junction. These experimental observations can be ascribed to the QH edge state transport across the MG/BG interface. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B25.00013: Dissipation and Criticality in the Lowest Landau Level of Graphene Pallab Goswami, Xun Jia, Sudip Chakravarty The lowest Landau level of graphene is studied numerically by considering a tight-binding Hamiltonian with disorder. The Hall conductance $\sigma_{xy}$ and the longitudinal conductance $\sigma_{xx}$ are computed. We demonstrate that bond disorder can produce a plateaulike feature centered at $\nu=0$, while the longitudinal conductance is nonzero in the same region, reflecting a band of extended states between $\pm E_c$, whose magnitude depends on the disorder strength. The critical exponent corresponding to the localization length at the edges of this band is found to be $2.47\pm 0.04$. When both bond disorder and a finite mass term exist the localization length exponent varies continuously between $\sim 1.0$ and $\sim 7/3$. [Preview Abstract] |
Session B26: Focus Session: Computational Nanoscience II: Mechanics, Dynamics, and Assembly
Sponsoring Units: DMP DCOMPChair: Dennis Rapaport,, Bar-Ilan University
Room: 328
Monday, March 16, 2009 11:15AM - 11:51AM |
B26.00001: An atomistic approach to viral mechanical oscillations Invited Speaker: Viruses are the simplest ``life'' form. These parasites reproduce by borrowing the machinery of their host cell. Many are pathogenic to plants, animals, and humans. Viruses possess an outer protein coat (capsid) that protects its genomic material that resides inside. We have developed a theoretical technique to model the very low frequency mechanical modes of the viral capsid with atomic resolution. The method uses empirical force fields and a mathematical framework borrowed from electronic structure theory for finding low energy states. The low frequency modes can be ``pinged'' with an ultra-short laser pulse and the aim of the light/vibrational coupling is to interfere with the viral life cycle. The theoretical work here is motivated by the recent work of Tsen et al. [2] who have used ultra-short pulsed laser scattering to inactivate viruses. The methodology can be applied to many systems, and the coupled mechanical oscillations of other floppy biomolecules such as a complete ATP binding cassette (ABC transporter) will also be discussed. Co-authors of this work are Dr. Eric Dykeman, Prof. K.-T. Tsen and Daryn Benson. \\[4pt] [1] E.C. Dykeman et al., Phys. Rev. Lett., 100, 028101 (2008). \\[0pt] [2] K-T. Tsen et al., J. of Physics -- Cond. Mat. 19, 472201 (2007). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B26.00002: A new paradigm for self-assembly: The role of reversibility in viral capsid growth Dennis Rapaport The phenomenon of supramolecular self-assembly, despite its importance, remains an enigma. The formation of virus capsids -- the exquisitely designed protein shells of spherical viruses -- is a well-known example, and there are numerous potential applications for nanotechnology. The capsid assembly process can be modeled using molecular dynamics simulation of simplified particles that are designed to form polyhedral shells. New insights into the mechanism of self-assembly have emerged from simulations carried out using particles immersed in an explicit solvent. Contrary to expectation, self-assembly is found to proceed via a cascade of strongly reversible steps, a feature that helps avoid growth-impeding kinetic traps because partial shells generally tend to lose rather than gain members. This ensures a robust process leading, under suitable conditions, to a high yield of complete shells. Furthermore, despite the large variety of possible intermediate structures, the assembly pathways are found to involve only a small fraction of highly bonded (low energy) forms. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B26.00003: Calculation of the free energy of binding of DNA bases on a single-wall carbon nanotube Robert Johnson, A.T. Charlie Johnson, Michael Klein Biological molecules can be combined with inorganic nanostructures to form multifunctional hybrid materials with unique properties that will drives advances in nanoelectronics, environmental safety, medicine and homeland security. One such material of contemporary interest is the DNA-carbon nanotube hybrid (DNA-CN), which consists of a single-wall carbon nanotube (SWCN) coated with a self-assembled monolayer of single-stranded DNA (ssDNA). Computation and experiment indicate that DNA-CN self-assembles with DNA bases binding to SWCN sidewall. However, the nature, strength and solvation effects of base-SWCN binding have not been studied in detail. To address these issues and expand our understanding of DNA-CN, we have computed the binding free energy of individual DNA bases with SWCN using alchemical free energy methods. Such calculations provide detailed information about the importance of electrostatic, van der Waals and hydrophobic interactions in base-SWCN binding. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B26.00004: Simulations of the self-assembly of CdTe nanoparticles into large pitch helices Aaron Santos, Sudhanshu Srivastava, Sharon Glotzer, Nicholas Kotov Recent experiments have shown that CdTe nanoparticles can self- assemble into wires, sheets, or helical nanoribbons with a large pitch length (300-400 nm) depending on the amount and type of capping group used. While conventional Monte Carlo simulations of electrically charged truncated tetrahedrons successfully predict the formation of wires and sheets, they are inadequate to describe the formation of helical nanoribbons, which require a large number of particles and a long run time to observe their characteristic features. We use a newly developed energy minimization technique, ``binary hierarchical assembly,'' to predict the packing structure of tetrahedral CdTe nanoparticles within the helix. From this packing structure, we construct nanoribbons of various widths and minimize the energy to determine the width of the stable structure. We find the stable width of the ribbon is charge dependent with values that correspond to ribbons observed in experiments. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B26.00005: Sensitivity Limits of Nanomechanical Resonance Spectroscopy P. Alex Greaney The sensitivity limit of the recently proposed chemical sensing method, nanomechanical resonance spectroscopy (NRS) \footnote{P.A. Greaney and J.C. Grossman, \emph{Nano Letters}, {\bf 8}, 2648-2652, (2008).}, is investigated using classical molecular dynamics simulations. The NRS method exploits the preferential transfer of energy between resonant modes, using an array of nanomechanical resonators to interrogate the vibrational spectrum of an analyte directly. We report on the effects of solvent and complex analytes. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B26.00006: Enhancing Molecuar Dynamics to Capture Electronic Effects N.A. Modine, R.E. Jones, D.L. Olmsted, J.A. Templeton, G.J. Wagner, R.M. Hatcher, M.J. Beck In modeling non-equilibrium thermal transport in nanoscale systems, classical molecular dynamics (MD) has the primary strength of explicitly representing phonon modes and scattering mechanisms. On the other hand, electrons and their role in energy transport are missing. Our goal is to couple a MD treatment of the ionic subsystem with a partial differential equation based model of the electronic subsystem in order to accurately capture aggregate behavior at the nanoscale. Along these lines, we have enhanced the LAMMPS MD package by coupling the ionic motions to a finite element based representation of electronic heat transport. The coupling between the subsystems occurs via a local version of the two-temperature model. Key parameters describing the coupling are calculated using Time Dependent Density Functional Theory (TDDFT) calculations with either explicit or implicit energy flow. We will discuss initial demonstrations of our approach focusing on nanowires and carbon nanotubes. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B26.00007: Rotary molecular motion at the nanoscale: motors, propellers, wheels Lela Vukovic, Boyang Wang, Petr Kral We describe by molecular dynamics simulations nanoscale systems that could realize rotary motion. First, we study molecular propellers formed by carbon nanotube rotors with attached aromatic blades [1]. We show that these propellers could pump different types of liquids, and their pumping efficiency strongly depends on the chemistry of the (hydrophobic or hydrophilic) liquid-blade interface. We also investigate nanoscopic wheels with hydrophobic surfaces that show rolling activity on water when driven. Finally, we model efficient molecular motors driven by electron tunneling, which could drive rotary molecular systems [2]. \\[3pt] [1] B. Wang and P. Kr\'{a}l, . Rev. Lett. 98, 266102 (2007).\\[0pt] [2] B. Wang, L. Vukovic and P. Kr\'{a}l, Phys. Rev. Lett. 101, 186808 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B26.00008: Long-Range Hydrodynamic Interactions Implemented into LAMMPS (Parallel MD) Frances Mackay, Colin Denniston We use an explicit solvent method to study the interaction between particles and a fluid. Similar to the Particle-Mesh-Ewald schemes for electrodynamics, the long range hydrodynamic interactions are treated by interpolating the particle density onto a mesh. This is then coupled to the fluid, which we model using a thermal lattice Boltzmann scheme. Mass and momentum conserving noise in the lattice Boltzmann fluid scheme provides a thermostat for both the fluid and the particles. This work has been fully parallelized and implemented into LAMMPS, an open-source molecular dynamics code. We demonstrate the scheme with some examples from colloidal physics and flow over rough surfaces. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B26.00009: Atomistic Simulations of Hydrodynamic and Interaction Forces on Functionalized Silica Nanoparticles J. Matthew D. Lane, Ahmed E. Ismail, Michael Chandross, Christian D. Lorenz, Gary S. Grest It is often desired to prevent the flocculation and phase separation of nanoparticles in solution. This can be accomplished either by manipulating the solvent or by tailoring the surface chemistry of the nanoparticles through functionalization with a monolayer of oligomer chains. Since it is not known how these functionalized coatings affect the interactions between nanoparticles and with the surrounding solvent, we present results from a series of molecular dynamics simulations of polyethylene oxide (PEO) coated silica nanoparticles of varying size (5 to 20 nm diameter) in water. For a single nanoparticle we determined the Stokes drag on the nanoparticle as it moves through the solvent and as it approaches a wall. Due to hydrodynamic interactions there are large finite size effects which we estimate by varying the size of the simulation cell. We also determined both solvent-mediated (velocity-independent) and lubrication (velocity-dependent) forces between two nanoparticles as a function of the coverage and chain length of the PEO chains. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B26.00010: Water Flow in Carbon Nanotubes: Transition from Continuum to Subcontinuum Transport John Thomas, Alan McGaughey Water flow through carbon nanotubes (CNTs) with diameters ranging from 0.83 nm to 4.98 nm is examined using molecular dynamics simulation. A reflecting particle membrane is used to drive the flow and the relationship between the axial pressure gradient, CNT diameter, and volumetric flow rate is examined. The flow enhancement, defined as the ratio of the observed flow rate to that predicted from the no-slip Hagen-Poiseuille relation, is calculated for each CNT. In CNTs with diameters greater than 1.39 nm, flow can be accurately described in terms of continuum mechanics and the enhancement agrees with predictions from the slip-modified Hagen-Poiseuille flow relation. In CNTs with diameters smaller then 1.39 nm, we find that the liquid structure varies with CNT diameter and a continuum-based description of the fluid flow is inappropriate. The flow enhancement for these CNTs do not agree with predictions from the slip-modified Hagen-Poiseuille flow relation. They can, however, be correlated to the diameter-specific liquid structure. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B26.00011: Fluid flow through carbon nanotubes: a new modeling and simulation approach Michael A. Avon, Alper Buldum The flow of fluids through carbon nanotubes was investigated in order to get a better understanding of the unique properties and phenomena of nanofluidics. The previous modeling and simulation efforts were based on diffusion of atoms or molecules that were thrown to the nanotubes with initial velocities. Here, we present molecular dynamics simulations of carbon nanotubes that were embedded in liquid argon. The fluid was pushed through the nanotubes using a moving wall piston of graphene.Single-walled, double-walled, rigid and relaxed nanotubes in different diameters were considered. In order to achieve more continuous flow of fluid through the nanotube, several rounds of pumping were simulated. Pressure difference in different regions was analyzed. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B26.00012: Accelerated Molecular Dynamics Simulation on Friction of Incommensurate Interfaces Woo Kyun Kim, Michael Falk We apply a molecular dynamics (MD) methodology to study the friction of incommensurate interfaces. While the traditional Tomlinson model assumes a single, repeatable transition, the sliding at the real incommensurate interface is comprised of a multitude of transition modes. This may account for recent Atomic Force Microscope (AFM) experimental results that indicate more complex temperature and velocity dependence of friction that deviate from the Tomlinson predictions. Conventional MD simulations are unable to simulate a wide range of sliding rates due to time scale limitations. In this study, we achieve decreases in the simulated sliding velocity by several orders of magnitude compared with conventional MD simulations using Voter's hyperdynamics scheme. This method uses a biased potential to reduce the barrier heights of the original potential to decrease the simulated time between slip events. The decrease in the sliding velocity makes it possible to see the atomic level processes during sliding speeds much closer to the experimental time scale. We carefully analyze the simulation results to elucidate the transition mechanisms. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B26.00013: Molecular dynamics study of the phase transition in the bcc metal nanoparticles Yasushi Shibuta, Toshio Suzuki The phase transition between liquid and solid phases in bcc metal nanoparticles was investigated using a molecular dynamics simulation. The nucleation from an undercooled liquid droplet was observed during cooling in all nanoparticles considered. A nucleus was generated near one side of the particle and solidification spread toward the other side the during nucleation process. On the other hand, the surface melting and subsequent inward melting of the solid core of the nanoparticles were observed during heating. The depression of the melting point was proportional to the inverse of the particle radius due to the Gibbs--Thomson effect [1]. However, the depression of the nucleation temperature during cooling was not monotonic with respect to the particle radius since the nucleation from an undercooled liquid depends on the event probability of an embryo or a nucleus. \\[3pt] [1] Y. Shibuta, T. Suzuki, J. Chem. Phys. 129 (2008) 144102. [Preview Abstract] |
Session B27: Plasmonics and Metamaterials
Sponsoring Units: FIAPChair: Harry Atwater, California Institute of Technology, Chris Van de Walle, University of California, Santa Barbara
Room: 329
Monday, March 16, 2009 11:15AM - 11:27AM |
B27.00001: Controlling electric fields spatially by graded metamaterials Kin-Wah Yu The local electric field of a metal-dielectric composite cylinder, whose complex permittivity is given by a spatially dependent Drude model, has been derived analytically in terms of hypergeometric functions. Our results show that the electric field inside the cylinder can be confined to any desired position. Thus one can achieve the control of electric fields by fabricating graded metamaterials with specific material parameters. The enhanced nonlinear optical response of the composite cylinder has also been calculated [1]. The results suggest that the gradation-controlled electric field distribution may be a consequence of a combination of surface plasmon resonance and the microgeometry in graded metamaterials. Moreover, such a gradation-controlled field distribution serves as a physical mechanism for understanding the enhanced nonlinear optical responses with a broad surface plasmon band [2]. \\[3pt] [1] J. P. Huang and K. W. Yu, Phys. Rep. {\bf 431}, 87 (2006).\\[0pt] [2] J. P. Huang and K. W. Yu, Appl. Phys. Lett. {\bf 85}, 94 (2004). [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B27.00002: Terahertz nanogap antenna detection of nano-bridges and nano-rods H.R. Park, M.A. Seo, J.S. Kyoung, S.M. Koo, O.K. Suwal, S.S. Choi, N.K. Park, D.S. Kim We have measured transmission properties of a composite structure consisting of nano-rods on a long (a$_{y}$=300 micron) nano gap (70 nm) on Au film in broad frequency range of 0.1 THz to 1.0 THz using THz time-domain spectroscopy. The normalized transmittance with no nano-bridge or nanorod structure in the middle shows a half-wavelength resonance: the resonance frequency is $\sim c$/(2\textit{na}$_{y})$ where $n$ is the index of refraction of the substrate. The nano-size bridge at the center of the nano gap gives changes the resonance characteristics profoundly, because in essence, the length a$_{y}$ now halves. Mostly the same resonance-changing behavior is expected with a nano rod structure fabricated by Pt-deposition method using a focused ion beam (FIB). This small rod also acts as a bridge dividing the length of the rectangle. We also positioned nano-rods to lie at one third of each nano gap, dividing the length by the ratio of 1: 2. A resonance peak shift was observed. The structure dependent resonance allows to detect nano-size particles and to tailor resonance characteristics with feature sizes of \textit{$\lambda $}/10,000. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B27.00003: Terahertz Nanogap Plasmonics: Giant Field Enhancement M. A. Seo, H. R. Park, S. M. Koo, O. K. Suwal, S. S. Choi, N. K. Park, D. S. Kim We show that a nanogap dividing two conducting planes can efficiently transmit terahertz electromagnetic waves with wavelengths in the millimeter range. Terahertz time domain spectroscopy is performed to probe transmittance over a frequency range of 0.1 THz to 1.5 THz. It was found that the transmittance continues to increase as the frequency decreases with a dependence of 1/f. The area-normalized transmittance, which is equivalent to the level of field enhancement, reaches the value of 800 at 0.1 THz for a sample with a 70 nm gap. Combined with the 1/f dependence, this indicates that strong local resonance is not a prerequisite for a large field enhancement. It is shown that the accumulation of charges at metal edges via light-induced currents creates a large horizontal electric field, which in effect attracts the incoming light. The enhanced field in the gap fully scatters towards the far-field because there exists no cut-off. With the broad 1/f spectral response, this structure can be an excellent launching pad for inducing terahertz nonlinearity, nano-particle detection, and for surface enhanced Raman scattering. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B27.00004: Active Control of Propagating Surface Plasmons Excited by a Quantum Cascade Laser Daniel Wasserman, Troy Ribaudo, Eric Shaner, Scott Howard, Fow-Sen Choa, Claire Gmachl There has been significant interest, of late, in the optical properties of subwavelength features in metallic films. For instance, resonant transmission through periodic arrays of subwavelength apertures in metallic films is seen at wavelengths determined by the periodicity of the metal film and the relative permittivity of the metal and the surrounding dielectric medium. This phenomenon is referred to as extraordinary optical transmission (EOT) and has been studied for potential applications in display and sensing technologies. Here we demonstrate the ability of an actively tunable EOT grating to control the coupling of incident coherent radiation from a dual wavelength QCL to propagating surface modes on the grating. We use a novel spatially and spectrally resolved Fourier transform infrared spectroscopy technique to image the propagating surface waves on our EOT grating, and are able to extract a plasmon propagation length from the data collected. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B27.00005: Spatial and Spectral Investigation of Extraordinary Optical Transmission T. Ribaudo, B.S. Passmore, E.A. Shaner, D. Wasserman Extraordinary Optical Transmission (EOT), or the enhancement of light transmission through periodic arrays of sub-wavelength holes in metal films, has been investigated for its clear contradiction with conventional aperture theory, as well as for possible applications in chemical sensing and display technologies. In the visible and near-infrared spectral ranges, EOT is argued to be predominantly a result of the excitation of surface plasmon polaritons (SPPs) on metal/dielectric interfaces. Here, we report our investigations of the far-field transmission characteristics of EOT gratings designed for the mid-infrared frequency range. Using a tunable Quantum Cascade Laser, we explore the spatial and spectral dependence of the transmitted far field on the angle of incidence and the exciting frequency of the laser. We show that for frequencies coincident with the EOT maximum, little SPP propagation is observed, while laser frequencies on the high energy falling edge of the EOT peak couple to such propagating modes. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B27.00006: Mid Infrared Beam Steering Using Plasmonic Structures David Adams, Daniel Wasserman The interaction of electromagnetic radiation with periodic features on metal surfaces has received a great deal of attention in recent years. The far-field transmission properties of a sub-wavelength aperture can be controlled when periodic grooves are formed on the exit surface, allowing the shape and directionality of the transmitted beam to be altered. This phenomenon is a result of surface plasmon polaritons propagating along the metal surface and recoupling to photons which are phase mismatched with respect to the transmitted beam. The resulting interference pattern is dependent on the wavelength of the exciting beam, the geometry of the periodic surface structure, and the dielectric permittivity of the materials at the boundary. Here we present simulations using finite element analysis which demonstrate the steering of the beam transmitted through the metal slit as the permittivity of the surrounding material is modified. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B27.00007: Extraordinary Optical Absorption through Plasmonic Subwavelength Slits Justin White, Georgios Veronis, Zongfu Yu, Edward Barnard, Anu Chandran, Shanhui Fan, Mark Brongersma We report on the ability of resonant plasmonic slits to efficiently concentrate electromagnetic energy into a nanoscale volume of absorbing material placed inside or right behind the slit. This gives rise to extraordinary optical absorption (EOA) characterized by an absorption enhancement factor that well-exceeds the enhancements seen for extraordinary optical transmission (EOT) through slits. A semi-analytic Fabry-Perot model for the resonant absorption is developed and shown to quantitatively agree with full-field simulations. We show that absorption enhancements of nearly 1000\% can be realized at 633nm for slits in aluminum films filled with silicon. This effect can be utilized in a wide range of applications, including high speed photodetectors, optical lithography and recording, and biosensors. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B27.00008: Birefringence in Ring Resonator by Free Spectral Range and Wavelength measurement. Chooda Khanal, Magdalena Nawrocka, Xuan Wang, Tao Liu, Roberto Panepucci The spectrum of coupled light through a 10 {\_}m diameter silicon-on-insulator ring resonator from a 1280-1620nm band tunable laser source is investigated. A wavelength dependent group index is calculated by measuring the Free Spectral Range of TM and TE modes. Group index for TM mode decreases with the increase in wavelength while that for TE modes increases slowly in the given wavelength range. Our preliminary data shows that the group index birefringence is almost 0.5 at the lower wavelength band. Moreover, analysis of FWHM, power dependence and dispersion will be presented with wavelength routing applications discussed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B27.00009: Optical Vortexed Transmission through a Nanoslit on the Pyramid Seong Soo Choi, M.J. Park, N.K. Park, D.S. Kim The nanoslit apertures with its gap ranging from 1 nm to $\sim $ 50 nm have been fabricated using microfabrication process such as wet etching, thermal oxidation, circular aperture opening by HF, then followed by Al metal deposition. The light transmission through the single nanoslit apertures has been measured to be dependent upon the gap between two metal edges and independent of length of the slit. With decreasing the gap from 80 nm to 1 nm, the optical transmission rate has been increased upto three order of magnitude. This can be attributed to optical vortexed transmission [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B27.00010: Visible Far-Field Superlens for Two-Dimensional Imaging Below the Diffraction Limit Emily Ray, Rene Lopez Retaining the information carried by evanescent waves scattered from an object could allow for imaging features below the diffraction limit without time consuming scanning procedures. We show experimental results of sub-diffraction limited imaging with visible light using a metal and dielectric multilayer structure with a 2-D diffraction grating. The multilayer structure has an effective negative index of refraction that enhances evanescent waves. Interaction with the diffraction grating converts waves from evanescent into propagating, enabling collection with conventional optics. We are able to tune this far-field superlens (FSL) to our choice of operating wavelengths by modulating the thickness of the metal and dielectric layers. For a wavelength of 532 nm, we use Ag and Al$_{2}$O$_{3}$ layers with 20 nm thickness to image features with 150 nm size. This FSL functions with visible light to amplify evanescent waves and recreate images below the diffraction limit. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B27.00011: Field dependent enhancement of the magneto-optical Kerr effect by surface plasmon resonance C. Clavero, K. Yang, J.R. Skuza, R.A. Lukaszew Surface Plasmon Resonance (SPR) is of interest in a variety of applications ranging from bio-sensing to optical communications. In bio-sensing, the high sensitivity of SPR to changes in the dielectric constant at the metal-dielectric interface is used to detect specific molecular bindings. We have studied the effect of adding ferromagnetic materials to Au films and we have found a remarkable enhancement of the magneto-optical Kerr effect in Au-Co-Au trilayers when SPR is excited$^{1}$. This large enhancement is ascribed to the increase of the electromagnetic field within the Co film due to the excitation of SPR, and has been modeled for ultrathin Co films$^{2}$. At resonance the electromagnetic field within the Au-Co-Au trilayer decays exponentially with the distance from the Au-air interface where the SPPs are propagating. In order to probe the influence of the electromagnetic field within the Co film on its magneto-optical response, Au-Co-Au trilayers with the Co film positioned at different distances from the Au-air interface, while keeping the thickness of all the layers constant where prepared using sputtering deposition in ultra-high vacuum conditions. Our latest results will be presented and differences with proposed model will be discussed. 1. V. I. Safarov et al., Phys. Rev. Lett. \textbf{73}, 3584. 2. C. Hermann \textit{et al}., Phys. Rev. B \textbf{64}, 235422. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B27.00012: Manipulating, Extending, and Re-Routing Optical Near Fields Using Epsilon-Near-Zero Metamaterials in Nano-Optics Nader Engheta, Andrea Alu In recent years, we have developed the notion of lumped optical circuit nanoelements as building blocks for the design of nanoscale systems and components in nano-optics. In this context, we have also introduced the concept of an optical ``nanocircuit board,'' based on epsilon-near-zero (ENZ) metamaterials. Using analytical methods and full-wave numerical simulations, we show here that optical fields in the near field of a source may be efficiently manipulated, ``extended,'' and re-routed in arbitrarily-shaped air grooves carved in ENZ metamaterial substrates, that may effectively act as optical ``wires'' connecting different elements with essentially no phase variation. Such substrates may be obtained by using natural materials with the real part of permittivity near zero, e.g., SiC near 10.3 micron wavelength, or using layered stacks of epsilon-negative (e.g., metal) and epsilon-positive (e.g., dielectric) thin layers in the visible domain, for which the effective bulk substrate may exhibit epsilon-near-zero (ENZ) properties at certain wavelength regimes for transversally polarized electric field. In this talk, we will discuss some of the exciting features of such nano-optical structures for near-field manipulation, re-routing and extension. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B27.00013: Symmetry and degeneracy in metamaterial trimers Chih-Wei Chang, Ming Liu, Sunghyun Nam, Shuang Zhang, Guy Bartal, Xiang Zhang Metamaterial trimers consist of three-coupled split-ring resonators with alternative signs of nearest-neighbor couplings are designed and fabricated. Experimental results from Fourier transform infrared spectroscopy measurements are compared with those of metamaterial dimers. We demonstrate that metamaterial trimers exhibit two-fold degenerate magnetic resonances at infrared frequencies. Remarkably, the degeneracy originates from a new kind of topological symmetry that does not exist in natural materials. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B27.00014: Composite Metal-Semiconductor Metamaterials with Negative Permittivity and No Loss A.M. Bratkovsky, E.V. Ponizovskaya, S-Y. Wang, P. Holmstrom, L. Thylen Close to the resonance in a planar interface between a metal and dielectric, where $\varepsilon _{metal} \quad \approx -\varepsilon _{dielectric}$, very tight electromagnetic field confinement results, but the tighter the confinement, invariably, the higher the propagation losses. For confinement significantly better than that of Si nanowires ($\sim $300 nm) propagation losses become prohibitively high for most interconnect applications. Also, the magnitude of $\varepsilon _{metal}$ needs to be larger than e.g. 2 in order to interface to common dielectrics for close to resonance conditions. The most straightforward way to alleviate this situation is of course to use optical gain. We have analyzed theoretically a metamaterial, which is a mix of quantum dots (QDs) half of them pumped and half absorptive and showed that one could indeed compensate the loss. More efficient way of obtaining the $\varepsilon $'$<$0 is to use metals, and we show that a combination of silver rods, supplying the negative $\varepsilon $ and pumped QDs, providing the gain necessary to compensate the loss in the silver rods [1]. \\[3pt] [1] A. Bratkovsky, E. Ponizovskaya, S-Y. Wang, P. Holmstr\"{o}m, L. Thyl\'{e}n, Y. Fu, and H. {\AA}gren, Appl. Phys. Lett. \textbf{93}, 193106 (2008) [Preview Abstract] |
Session B28: Focus Session: Graphene II: Synthesis and Characterization
Sponsoring Units: FIAPChair: Yu-Ming Lin, IBM
Room: 330
Monday, March 16, 2009 11:15AM - 11:51AM |
B28.00001: Optimizing Graphene Morphology on SiC(0001) Invited Speaker: Many schemes to integrate graphene with microelectronics assume that reliable wafer-scale synthesis processes will be developed. One promising route to wafer-scale synthesis is to form graphene overlayers from the decomposition of SiC at high temperature. We have shown that, even at 1200 C, limited diffusion at the SiC surface leads to pit formation and a non-uniform graphene film thickness [1]. In this talk I will describe our efforts to improve both graphene domain size and thickness uniformity. One way we achieve this is by forming graphene in a background pressure of disilane, which hinders SiC decomposition. Even in rather low Si partial pressures (e.g. 1e-5 Torr), the SiC decomposition temperature can shifted several hundred degrees higher in temperature [2]. Using in situ low-energy electron microscopy (LEEM), we show that this effect can be exploited to form large graphene domains (larger than 10 um) with controlled layer thickness (e.g. 1 ML). Work performed in collaboration with R.M. Tromp. \\[4pt] [1] J.B. Hannon and R.M. Tromp, Phys. Rev. B77, 241404(R) 2008\\[0pt] [2] R.M. Tromp and J.B. Hannon, in press. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B28.00002: Atmospheric pressure growth of graphene on SiC(0001) Invited Speaker: Graphene, a single monolayer of sp$^{2}$-bonded carbon, is a very unique 2-dimensional electron gas system with electronic properties fundamentally different to other 2DEG systems [1]. Several production routes exist for graphene. Among them, the solid-state decomposition of hexagonal silicon carbide (SiC) surfaces [2] is particularly attractive for the development of graphene based electronics [3,4]. The first part of the presentation gives a brief summary of recent studies on the structural and electronic properties of graphene and few-layer graphene grown on SiC(0001) under ultra-high vacuum (UHV) conditions. The second part of the talk is devoted to recent progress in the growth of large domain graphene films on SiC(0001) in Ar atmosphere. It is shown that growth in Ar ambient leads to a significant improvement of the surface morphology and domain size as well as carrier mobility. \\[4pt] [1] A.H. Castro Neto, et al., Reviews of Modern Physics, in print (arXiv:0709.1163v2); and references therein. \\[0pt] [2] A. Charrier, et al., J. Appl. Phys. 92 (2002) 2479. \\[0pt] [3] C. Berger et al., J. Phys. Chem. B 108 (2004) 19912; C. Berger, et al., Science 312 (2006) 1191. \\[0pt] [4] A.K. Geim and K.S. Novoselov, Nature Mat. 6 (2007) 183. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B28.00003: Centimeter scale pattern growth of graphene films for stretchable transparent electrodes Invited Speaker: Large scale pattern growth of graphene is one of the most awaiting problems to be solved in order to bring this material for device application. Recently, macroscopic scale graphene films have been prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films is found to be much larger than theoretically expected values. Here, we report the direct synthesis of centimeter-scale graphene films using chemical vapor deposition (CVD) on thin Ni layers, where the overall structures are connected by lateral electric connections. As a result, the transferred graphene films show very low sheet resistance with excellent optical transparency. At low temperatures, the single layers transferred on SiO$_{2}$ substrates show high electron mobility with the signature of quantum Hall effect, implying that the quality of CVD-grown graphene is as high as mechanically cleaved graphenes. Employing these outstanding mechanical properties of graphenes, we also demonstrate the macroscopic usage of the highly conducting and transparent electrodes for flexible/stretchable/foldable electronics. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B28.00004: Fabrication and measurement of epitaxial graphene nanoribbons Mike Sprinkle, Jeff J. Peterson, Ming Ruan, Yike Hu, XiaoSong Wu, Edward H. Conrad, Claire Berger, Walt A. de Heer Multi-layer graphene grown epitaxially on the C-terminated ($000\overline{1}$) surface of 4H-SiC in a low vacuum ($\sim 10^{-5}$ Torr), high temperature ($\sim$1420 $^{\circ}$C) induction furnace environment has been shown to be of extremely high quality and mobility. Due to its rotational stacking, the material exhibits electronic properties similar to those of isolated graphene sheets. Lithographic techniques, including electron beam lithography, are explored and sub-20 nm ribbon widths are demonstrated. Transport data for gated epitaxial graphene nanoribbons indicates quantum confinement. More than 100 nanoribbons on a single SiC chip are demonstrated, illuminating a technologically viable path towards graphene electronics. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B28.00005: Self Assembly of Graphene Sheets Hailiang Wang, Xinran Wang, Xiaolin Li, Hongjie Dai Chemically derived graphene sheets (GS) were found to self-assemble onto patterned gold structures via electrostatic interactions between noncovalent functional groups on GS and gold. This afforded arrays of single graphene sheets on substrates, characterized by Auger, Raman and scanning electron microscopy (SEM) imaging. Self assembly was used for the first time to produce on-substrate and fully-suspended graphene electrical devices. Molecular coatings on the GS were removed by high current ``electrical annealing,'' which recovered the high electrical conductance and Dirac point of the GS. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B28.00006: Homoepitaxial Diamond Growth on Planar and Non-planar Substrates Using Carbon-13 Precursors Gopi Samudrala, Samuel Weir, Yogesh Vohra The growth of single crystal diamond by microwave plasma Chemical Vapor Deposition has been carried out on [100] oriented Type Ia natural diamond anvils as well as planar Type Ib synthetic diamond substrates. The effects of substrate geometry, concentrations of Carbon-13 gas precursors, nitrogen concentration in the plasma, and substrate temperatures on homoepitaxial diamond growth have been investigated. These results will be presented along with the observed changes in growth rate and surface morphology of the grown films with the variation of each parameter. We have also investigated nitrogen incorporation in diamond lattice by photoluminescence spectroscopy. Results obtained from the study on non-planar substrates have a direct impact on the growth chemistry used in the fabrication of designer diamond anvils for high pressure research. [Preview Abstract] |
Session B29: Spin-Dependent Transport
Sponsoring Units: GMAGChair: Sergei Urazhdin, West Virginia University
Room: 333
Monday, March 16, 2009 11:15AM - 11:27AM |
B29.00001: Spin dependent scattering in all Heusler alloy CPP-GMR nano-structures for magnetic storage applications Oleg Mryasov, Konstantin Nikolaev, Thomas Ambrose At the reduced sensor dimensions necessary for high density magnetic recording, the lower impedance of current-perpendicular- to the plane giant magnetoresistance (CPP-GMR) based read heads likely will take over the currently used tunneling MR based sensors. Main obstacle on the of realizing this transition is relatively low amplitude of the conventional CPP-GMR stacks. In this work, we investigate all-Heusler CPP-GMR spin-valves . The combination of alloys has been chosen in order to match spin states majority channel[1]. We focus on fundamentals of spin dependent interface and bulk scattering as it is affected by substitution disorder in these ternary alloys. Ab-initio electronic structure calculations employed to account for complex band structure of these ternary. We investigate interface scattering contribution within a simple model relying on the electronic band structure calculated for bulk of ferromagnetic and non-magnetic component of all Heusler CPP GMR tri-layer. We use model to investigate how to minimize impact of disorder induced states. Experimentally, these structures have been realized using conventional sputter deposition techniques and found to exhibit significant interface scattering contribution to MR signal. [1] T. Ambrose and O. Mryasov, United States Patent 6,876,522 (April 5, 2005); T. Ambrose, O. Mryasov, ``\textit{Growth and Magnetotransport Properties of Thin Co}$_{2}$\textit{MnGe Layered Structures}'', Springer Verlag Series. 2005. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B29.00002: Sub-gap cotunneling current through a spinfull quantum dot with superconducting leads Jens Paaske, Brian Andersen, Karsten Flensberg A number of recent experiments have measured the voltage driven current through a quantum dot with superconducting leads. By changing the gate voltage of the dot, several unusual properties of the sub-gap current have been revealed. For example, when the dot is occupied by an odd number of electrons, the cotunneling conductance exhibits regions of negative differential conductance as well as significant weight redistributions among the multiple Andreev scattering resonances. We have calculated the sub-gap cotunneling current within a general Hamiltonian approach, allowing us to treat the even occupied (spinless) dot exactly, and the odd occupied (spinfull) dot perturbatively. In the latter case, we present calculations of unusual sub-gap current and relate these finding to the experiments. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B29.00003: Orientation and temperature dependence of the anomalous Hall effect in hcp cobalt Ivo Souza, Eric Roman, Yuriy Mokrousov We calculate from first-principles the evolution of the intrinsic anomalous Hall conductivity vector $\vec\sigma^a$ of hcp Co as the spin magnetization direction $\hat{\bf M}$ is tilted away from the $c$-axis. We find that $\vec\sigma^a$ varies smoothly with the tilt angle $\theta$, and that its magnitude is strongly reduced, by a factor of about four, between $\theta=0$ and $\theta=\pi/2$, in good agreement with the measured anisotropy ratio of about three.\footnote{N. V. Volkenshtein {\it et al.}, Fiz. Metal. Metalloved. {\bf 11}, 152 (1961).} In addition to the anisotropic linear magnetization dependence ($\sigma^a_z/M_z\not=\sigma^a_x/M_x$) expected for any uniaxial crystal, there is a considerable nonlinearity in the dependence of $\sigma_x^a$ on $M_x=M\sin\theta$, while the relation between $\sigma_z^a$ and $M_z=M\cos\theta$ is essentially linear, as in ${\rm Mn}_5{\rm Ge}_3$.\footnote{C. Zeng {\it et al.}, Phys. Rev. Lett. {\bf 96}, 037204 (2006).} The overall angular dependence of $\vec\sigma^a$ is well-described by an expansion in terms of $l=1$ and $l=3$ spherical harmonics. From Zener's model for the influence of thermal fluctuations of $\hat{\bf M}({\bf r})$ on the temperature dependence of magnetic anisotropies,\footnote{C. Zener, Phys. Rev. {\bf 96}, 1335 (1954).} we predict that the $l=3$ terms give rise to an appreciable increase with temperature of the anisotropy ratio. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B29.00004: Determination of spin polarization of amorphous ferromagnet CoFeB by Point-Contact Andreev Reflection Sunxiang Huang, Tingyong Chen, Chia-Ling Chien Amorphous ferromagnet CoFeB plays a key role in spintronic devices. Larger tunneling magnetoresistance (TMR) is resulted when amorphous CoFeB is incorporated into either AlO$_{x}$ or MgO magnetic tunnel junctions (MTJs) than those with CoFe. The critical switching current density in spin transfer torque devices with CoFeB as the free layer is significantly less than that with NiFe. The TMR of MgO-based MTJs are also noticeably different using Co$_{20}$Fe$_{60}$B$_{20}$ and Co$_{40}$Fe$_{40}$B$_{20}$. These phenomena indicate a substantial spin polarization of CoFeB, whose value and compositional dependence should be determined. We have recently determined the spin polarization of amorphous Co$_{x}$Fe$_{80-x}$B$_{20 }(x$=20, 40, 60) using the point-contact Andreev reflection technique [1]. The spin polarization of amorphous CoFeB has been found to be as high as 65{\%}. In contrast to the large enhancement of TMR during crystallization of CoFeB in MgO-based MTJs, the spin polarization of crystallized CoFeB is in fact much reduced. Very recent theoretical studies [2] using density functional theory indicate an enhanced spin polarization in amorphous CoFeB, in good agreement with our measurements. [1] S. X. Huang \textit{et al}., APL, \textbf{92}, 242509 (2008). [2] P.V. Paluskar \textit{et al.,} PRL, \textbf{100}, 057205 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B29.00005: Structural and orientation dependence of the anomalous Hall effect in cobalt crystals Eric Roman, Ivo Souza Co undergoes a structural phase transition at $660$~K from a ferromagnetic hcp phase to a ferromagnetic fcc phase. We present a first-principles study of the anomalous Hall conductivity (AHC) in hcp, fcc, and fct cobalt crystals. We find that the AHC in the fcc phase is about half that of the hcp phase, in good agreement with experiment. \footnote{I.A. Tsoukalas, Phys. Stat.Sol. (a) {\bf 23}, K41 (1974).} By expressing the AHC as the Kramers-Kronig transform of the magnetic circular dichroism (MCD) spectrum, we relate the change in the AHC to differences between the infrared MCD spectra of the two phases. In particular, there is a large, negative spin-flip contribution in fcc, which is absent in hcp. We also describe the effect of tetragonal distortions on the AHC of Co, by studying its evolution along the fcc-bcc Bain path. These distortions alter the dependence of the AHC on the magnetization direction, an effect which is also significant for the orbital moment. \footnote{ O. Hjortstam, K. Baberschke, J. M. Wills, B. Johannson, and O. Eriksson, Phys. Rev. B \textbf{55}, 15026 (1997). } [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B29.00006: Measurement of the transport spin polarization of disordered metals using point-contact Andreev reflection M. Osofsky, G. Woods, J. Sanders, H. Srikanth, S. Kolesnik, T. Maxwell, B. Dabrowski Point contact Andreev reflection (PCAR) studies were done on bulk, polycrystalline SrRu$_{1-x}$(TM)$_{x}$O$_{3}$ (TM = Cr, Mn, Ti) and SrRu$_{0.92}$O$_{3}$ with a high degree of disorder. The curves are typical of many other materials studied, except that the conductance is not constant at large voltages as is commonly observed in most metals. This result is most likely due to the effect of disorder on the density of states (DOS) that produces the well known square-root of V anomaly. After the problem of the V$^{1/2 }$behavior at large V was eliminated by proper normalization, the PCAR spectra for samples were analyzed using the modified BTK model. Pure SrRuO3 undergoes ferromagnetic ordering at a Curie temperature of $T$C $\sim $ 160 K and has a relatively high spin polarization ($\sim $0.6). Our results indicate that, when the lattice is disordered from either the presence of Ru lattice site defects or the substitution of a transition metal for the Ru, the curie temperature, $T_{C}$ changes by a factor of two while the spin polarization is almost unchanged. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B29.00007: Anomalous Hall effect in Y$_2$Fe$_{17-x}$Co$_x$ single crystals Jolanta Stankiewicz, Konstantin Skokov We study experimentally the Hall resistivity of Y$_2$Fe$_{17-x}$Co$_x$ single crystals ($x\leq 8$) for wide temperature and applied magnetic field ranges, and for various magnetic field orientations with respect to the easy-magnetization axis. We find a large anomalous Hall effect (AHE) anisotropy in this system for $x\leq 2$. The AHE resistivity $\rho_{xy}$, measured with an applied magnetic field $H\perp {\it c}$-axis, is nearly one order of magnitude larger than the one for $H$ along the hard magnetization direction ($H\parallel {\it c}$-axis). Furthermore, the former is very large and varies linearly with the longitudinal resistivity $\rho$, whereas the latter follows $\rho^2$ for $T < 150$ K. We tentatively interpret the behavior of $\rho_{xy}$ for $H\parallel \it c$-axis in terms of an intrinsic effect related to the inter-orbital hopping between degenerate {\it d}-orbitals. Such hopping is allowed for high symmetry points at the crystallographic dumb-bell sites in this configuration. On the other hand, there is no inter-orbital hopping for $H\perp \it c$-axis. However, a huge amplitude of the AHE resistivity for this configuration, which follows from skew scattering, is puzzling. Both the AHE anisotropy and the large skew scattering go away as more Fe is substituted by Co. We attribute this to variations in the electronic structure of the Y$_2$Fe$_{17-x}$Co$_x$ system when Co atoms start to occupy the dumb-bell crystallographic sites. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B29.00008: Signal propagation in time-dependent spin transport Yao-Hui Zhu, Hans Christian Schneider Signal propagation in magnetic multilayers is studied using a macroscopic theory of time-dependent spin transport. Our analysis shows that time-dependent spin transport possesses a wave-diffusion duality, i.e., it is wave like for fast signal modulation and reduces to the diffusion equation for slow modulation [1]. The wave-like characteristics allow us to extract a finite spin signal-propagation velocity, which cannot be done using the spin diffusion equation. Applications to different switching scenarios for collinear and noncollinear spin transport through magnetic multilayers will be discussed.\newline [1] Y.-H. Zhu, B. Hillebrands, H. C. Schneider, Phys.\ Rev.\ B \textbf{78}, 054429 (2008) [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B29.00009: The Anomalous Hall Effect in Ultra-Thin Amorphous CNi$_3$ Films Yimin Xiong, Philip Adams We present anomalous Hall effect(AHE) measurements in ultra-thin CNi$_3$ films. Films with sheet resistances in the range $R\ll $$R_Q$ to $R\sim R_Q$ were studied in fields up to 9 T and temperatures down to 2 K. We find that in addition to scattering processes, the AHE in high resistance films is strongly influenced by disorder-enhanced electron-electron interaction effects. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B29.00010: Electronic structure and spin-filter effect of $\gamma$-Fe$_{2}$O$_{3}$ (maghemite) Hiroyoshi Itoh, Syuta Honda, Jun-ichiro Inoue, Hideto Yanagihara, Eiji Kita We theoretically study the electronic structure and spin-dependent transport of spinel-like $\gamma$-Fe$_{2}$O$_{3}$ (maghemite) which is one of candidates for spin-filter devices. By performing first principles calculations (GGA+$U$) for iron vacancy ordered Fe$_{64}$O$_{96}$, the spin-dependent band-gap of the maghemite in the ferrimagnetic insulating ground state is determined. It is also shown that excess of Fe and O atoms significantly affects on the band gap of the minority spin state. In the light of obtained electronic structure, the spin-filter effect and variable range hopping of the maghemite are discussed. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B29.00011: Scanning Probe Microscopy of Quantum Chains Israel Mandel, Gabriel Cwilich, Fredy Zypman Quantum strongly correlated spin systems exhibit energy spectra related to their underlying classical dynamics. For integrable systems, the level-spacing is Poissonian, while for chaotic dynamics, it follows a Wigner-Dyson distribution, corresponding to localized and metallic phases [1,2] Linear spin chains and braids have been considered recently [3] showing that a certain degree of disorder or the presence of impurities along the chain induces a transition from one statistic to the other. We consider a quantum Heisenberg system of two perpendicular linear chains (T-shaped) interacting through the points of closest approach between them. The motivation is to model the tip of a Magnetic Force Microscope, the separation between tip and sample playing the role of an impurity (a different hopping parameter). We study the level-spacing statistics of the whole system as a function of tip-sample separation, observing a transition in level statistics, inducing a change in the total energy, detectable by monitoring the force between tip and sample. We propose a tool to search for signatures of an integrable-to-chaotic transition by looking at the force-separation curves in Scanning Probe Microscopy. [1] Shklovskii B I, Shapiro B, Sears B R, Lambrianides P and Shore H B 1993 \textit{Phys. Rev. }B \textbf{47 }11487 [2] Braun D, Montambaux G and Pascaud M 1998 \textit{Phys. Rev. Lett. }\textbf{81 }1062 [3] Lea F. Santos, Phys Rev E78, 031125 (2008) [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B29.00012: Kondo decoherence: finding the right spin model for iron impurities in gold and silver J. von Delft, T.A. Costi, L. Bergqvist, A. Weichselbaum, T. Micklitz, A. Rosch, P. Mavropoulos, P. Dederichs, F. Mallet, L. Saminadayar, C. B\"auerle We exploit the decoherence of electrons due to magnetic impurities, studied via weak localization, to resolve a longstanding question concerning the classic Kondo systems of Fe impurities in the noble metals gold and silver: which Kondo-type model yields a realistic description of the relevant multiple bands, spin and orbital degrees of freedom? Previous studies suggest a fully screened spin $S$ Kondo model, but the value of $S$ remained ambiguous. We perform density functional theory calculations that suggest $S = 3/2$. We also compare previous and new measurements of both the resistivity and decoherence rate in quasi 1-dimensional wires to numerical renormalization group predictions for $S=1/2,1$ and 3/2, finding excellent agreement for $S=3/2$. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B29.00013: Electronic Interactions between Au Films and the Prussian Blue Analog Co$_{3}$[Os(CN$_{6})$]$_{2}$ T. Wellington, M. Hilfiger, A. Ford, C. Avendano, K. Dunbar, W. Teizer The Prussian blue analog Co$_{3}$[Os(CN$_{6})$]$_{2}$ exhibits photoinduced changes of magnetic behavior as well as charge transfer induced spin transitions at low temperature. Magnetic measurements on the bulk material show an increased magnetic susceptibility after illumination with red light, as the analog exhibits an abrupt spin transition due to enhanced cooperativity. We are exploring electronic interactions between this Prussian blue analog and gold films of varying thickness. Low-temperature measurements of the magnetoresistance of the gold films, with and without a surface layer of the analog, are performed. The study focuses on how the presence of the analog on the surface affects the transport properties within the gold film. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B29.00014: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 2:03PM - 2:15PM |
B29.00015: Enhancement of positive magnetoresistance at the metamagnetic transition in Tb$_{5}$Si$_{3}$ E.V. Sampathkumaran, S. Narayana Jammalamadaka, Niharika Mohapatra It is well-known that, in the antiferromagnetic systems, at the metamagnetic transition field (H$_{t})$, negative MR [defined as {\{}$\rho $(H)-$\rho $(0){\}}/$\rho $(0) where $\rho $ is the electrical resistivity] is observed. Here we present evidence for the opposite behavior of MR in an intermetallic compound, viz., Tb$_{5}$Si$_{3}$, known to form in Mn$_{5}$Si$_{3}$-type hexagonal structure (space group: P6$_{3}$/mcm). We observe a field-induced ferromagnetic transition in the magnetically ordered state (T$_{N}$= 69 K). The key experimental finding is that, below T$_{N}$, $\rho $ is dramatically enhanced at all temperatures (resulting in large ``positive'' MR) at H$_{t}$, in sharp contrast to the expectation based on common knowledge. This finding bears significant relevance to electron scattering phenomena in general, and in particular in metamagnetic systems. [Preview Abstract] |
Session B30: Focus Session: Manganite Superlattices
Sponsoring Units: GMAG DMPChair: Satoshi Okamoto, Oak Ridge National Laboratory
Room: 334
Monday, March 16, 2009 11:15AM - 11:51AM |
B30.00001: Emergent properties of digital superlattices of LaMnO$_{3}$/SrMnO$_{3}$ Invited Speaker: LaMnO$_{3}$ and SrMnO$_{3}$, both antiferromagnetic insulators, are end members of the La$_{1-x}$Sr$_{x}$MnO$_{3}$ phase diagram, which includes a highly spin-polarized ferromagnetic metal and a variety of orbital-ordered antiferromagnets. Interfaces between LaMnO$_{3}$ and SrMnO$_{3}$ provide a unique environment where the spin, charge and orbital degrees of freedom of each of the constituents may `reconstruct', giving rise to collective states at interfaces that are qualitatively distinct from those in either LaMnO$_{3}$ or SrMnO$_{3}$. We have synthesized superlattices of (LaMnO$_{3})_{p}$/(SrMnO$_{3})_{q}$ , where $x=q/(p+q),$ using ozone-assisted molecular beam epitaxy. Here, $p $ and $q $represent integer layers of the constituents. These superlattices can be realized with interfacial roughness/intermixing limited to a region less than one unit-cell in extent. We will explore the properties of these `digital manganites' for a range of $p/q$, including enhanced ordering temperatures compared to randomly alloyed samples, and provide experimental evidence for the interfacial reconstruction that is responsible for their emergent properties. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B30.00002: Enhanced Antiferromagnetic Ordering Temperature in Metallic LaMnO$_{3}$/SrMnO$_{3}$ Superlattices Tiffany Santos, Steven May, Anand Bhattacharya, J. Lee Robertson The perovskite manganite La$_{1-x}$Sr$_{x}$MnO$_{3}$ has a rich magnetic phase diagram, exhibiting ferromagnetism (F) for La-rich compositions and antiferromagnetism (AF) for those that are Sr-rich. Our study focuses on the x=0.5 doping region containing the F-AF phase transition, particularly the role of strain and cation-site disorder in nucleating the F or AF state. Using ozone-assisted molecular beam epitaxy, we have prepared fully-epitaxial superlattices of LaMnO$_{3}$ and SrMnO$_{3}$ on SrTiO$_{3}$ substrates, along with random alloy films of La$_{1-x}$Sr$_{x}$MnO$_{3}$ with equivalent composition. In our digital synthesis method, whereby we interleave single unit-cell layers of undoped LaMnO$_{3}$ and SrMnO$_{3}$, we have eliminated disorder at the La/Sr cation site. Our structural characterization shows atomic layer precision in the synthesis of these superlattices. The structural, magnetic and transport properties of the superlattices are compared with those of the random alloys. A-type AF order (F alignment in-plane, AF alignment of adjacent planes) is verified by neutron diffraction, also revealing an enhanced N\'{e}el temperature with no F phase at higher temperature, in contrast to bulk. These AF thin films display metal-like behavior, opening the possibility of using the discrete layers of opposite spins for coherent spin transport. Supported by DOE, Office of Basic Energy Sciences. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B30.00003: Theory of Manganite Superlattices Andrew Millis, Chungwei Lin A comprehensive theoretical treatment of (001) $(LaMnO_3)_n(SrMnO_3)_m$ manganite superlattices is presented. The charge distribution, conductivity, and propagation through the superlattice of orbital and magnetic order are determined using dynamical mean field calculations in the superlattice geometry. General rules for predicting the behavior of manganite superlattices are outlined. Comparison is made to existing data and inconsistencies between theory and experiment are identified and discussed. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B30.00004: Control of Magnetism via Layer Thickness Modification in the LaMnO$_3$/SrMnO$_3$ Digital Superlattices and the Prediction of a Spin-Polarized 2DEG Birabar Nanda, Sashi Satpathy We study the effect of layer thickness on the magnetic properties in the (LMO)$_{2n}$/(SMO)$_{n}$ superlattices using density-functional calculations. The change in the magnetic properties is shown to be controlled by the leakage of the Mn-e$_g$ electrons from the LMO side to the SMO side. For n = 1 superlattice, the weak potential barrier allows the Mn-e$_g$ electrons to spread across the entire superlattice, so that a uniform ferromagnetic behavior is obtained through carrier mediated Zener double exchange. For larger n, the strong potential barrier restricts the e$_g$ electron transfer to few layers adjacent to the interface, thus leaving the magnetism unchanged and bulk like away from the interface, while modifying the magnetism in the interfacial region. Finally, taking the example of a delta doped superlattice, (SMO)/(LMO)$_1$/(SMO), we predict the formation of a spin-polarized two dimensional electron gas. The 2DEG, generated due to the confinement of the La (d) electrons in the direction normal to the interface, mediates a ferromagnetic alignment of the Mn-t$_{2g}$ spins via double exchange which in turn spin polarizes the 2DEG. \\ $\dagger$ Work supported by the US Department of Energy \\ 1. B. R. K. Nanda and S. Satpathy, arXiv:0810.2126; B. R. K. Nanda and S. Satpathy, Phys. Rev. Lett. {\bf 101}, 127201 (2008) [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B30.00005: The onset of metallic behavior in strained (LaNiO$_3$)$_n$/(SrMnO$_3$)$_2$ superlattices Steven May, Tiffany Santos, Anand Bhattacharya Motivated by predictions of collective ordering phenomena in LaNiO$_3$, we have grown strained (LaNiO$_3$)$_n$/(SrMnO$_3$)$_2$ superlattices on SrTiO$_3$ using ozone-assisted molecular beam epitaxy. The superlattices exhibit excellent crystallinity and interfacial roughness of less than one unit cell. The samples undergo a metal-insulator transition as $n$ is reduced from 4 to 2. Both $n$ = 1 and 2 samples are insulating, however, they exhibit different transport behavior. The $n$ = 1 sample acts as a gapped insulator, while the addition of a second LNO layer ($n$ = 2) leads to hopping transport through non-gapped conduction channels. These results will be compared to (LaMnO$_3$)/(SrMnO$_3$) superlattices to highlight how interfacial charge transfer differs in the nickelate/manganite superlattices from their all-manganite counterparts. Supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences under contract DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B30.00006: Electronic and crystal-field effects in the fine structure of electron energy-loss spectra of La$_x$Ca$_{1-x}$MnO$_3$ Weidong Luo, Maria Varela, Jing Tao, Stephen J. Pennycook, Sokrates T. Pantelides The fine structure of oxygen K-edge electron energy-loss spectra (EELS) of transition-metal oxides is known to correlate with nominal oxidation states (NOS) that are often interpreted as charge states. We report the results of a systematic study of O K-edge EELS fine structures in La$_x$Ca$_{1-x}$MnO$_3$ and their evolution as functions of doping $x$. The calculated spectra, specifically the pre-peak intensities and peak separations, as functions of $x$ are in excellent agreement with experimental data. The calculations show that the variation of the pre-peak's intensity with doping is controlled by the orbital occupancy of the majority-spin Mn 3$d$ states while its width is controlled by crystal-field splitting. The energy separation between the pre-peak and the main peak also has a correlation with the doping parameter $x$ and the NOS. The results confirm that the NOS extracted from EELS correlates with orbital occupancies but does not probe physical charges of the Mn sites. This research was sponsored in part by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and by the McMinn Endowment at Vanderbilt University. Computations were performed at the National Energy Research Scientific Computing Center. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B30.00007: Local Electronic Structure at Oxide-Oxide Interfaces Probed by Atomic Resolution Electron Energy Loss Spectroscopy Amish Shah, Q. M. Ramasse, S.J. May, J.G. Wen, J.N. Eckstein, A. Bhattacharya, J.M. Zuo We report an atomic resolution study of the electronic structure of 12 x 4 LaMnO$_{3}$-SrMnO$_{3}$ and 2 x 2 LaMnO$_{3}$-SrTiO$_{3}$ superlattices and their interfaces grown on SrTiO$_{3}$ by EELS. We correlated the interfacial electronic structure with the interfacial atomic structure using atomic resolution Z-contrast STEM using an electron probes of $<$0.1 nm. The oxide superlattices were synthesized using molecular beam epitaxy. We measured the site-specific unoccupied states of oxygen atoms and transition metals. In the LMO-SMO system we found extra states (holes) near the Fermi level and their dependence on abruptness of interface. In LMO-STO, we will present evidence of site-dependent electronic structure of oxygen and the Mn valence based on the L-edge ratios. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B30.00008: Electronic properties of manganite / titanate superlattices Maria Varela, H. Christen, H.N. Lee, L. Petit, T. Schulthess, S. Pennycook, J. Garcia-Barriocanal, A. Rivera, F.Y. Bruno, Z. Sefrioui, C. Leon, J. Santamaria Here we report on the study of LaMnO$_{3}$/SrTiO$_{3}$ interfaces. While LMO in bulk is an antiferromagnetic Mott insulator and STO is a band insulator, LMO/STO superlattices exhibit ferromagnetism and in some cases metallicity, both of which can be tuned by changing the layer thicknesses. We will compare the structure, chemistry and electronic properties of LMO/STO interfaces in high quality superlattices grown by pulsed laser deposition and high O$_{2}$ pressure sputtering. The distribution of defects and electronic properties will be studied through aberration corrected electron microscopy and electron energy loss spectroscopy. PLD superlattices show two alternating interface terminations, LaO-TiO$_{2}$ and SrO-MnO$_{2}$, which cause an asymmetry in the LMO layer electronic properties. Superlattices grown by sputtering only show one termination, LaO-TiO$_{2}$, giving an overall electron doping to the system. The role of interfacial charge transfer or localization, and any changes in electronic properties due to structural relaxations induced by epitaxial strain will be examined. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B30.00009: Magnetic and structural phase transitions in epitaxial thin films of Manganites Valeria Lauter, Hailemariam Ambaye, Steven Nagler, Hans Christen, Mike Biegalski Understanding the magnetic properties of complex materials near interfaces is important for the development of functional nanostructures and devices. Epitaxial LaMnO$_{3}$ films were grown on SrTiO$_{3}$ substrates. Recent work on such thin-film samples has shown that ``interface doping'' can induce magnetism at interfaces. Our work on LaMnO$_{3}$/SrTiO$_{3}$ interfaces has shown that the nature of the interface determines its magnetic structure - with the MnO$_{2}$-SrO interface showing a different magnetization than the LaO-TiO$_{2}$ interface. To investigate interfacial structures, we used polarized neutron reflectometry with off-specular scattering. Our results give evidence of reversible temperature- and field- dependent structural changes in LaMnO$_{3}$ film which undergo a phase transition. We determined that a structural phase transition in SrTiO$_{3}$ and the misfit strain trigger appearance of twins to reduce stresses and to adjust lattice mismatch between the film and the substrate. We show that a laterally correlated superstructure appear due to interaction of structural modifications with the magnetization the film [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B30.00010: Ferromagnetic Spin State of Manganite/SrTiO$_{3}$ Interfaces in (110) Orientation Xinfei Liu, Jianxing Ma, Tao Lin, G. Y. Gao, W. B. Wu, X. G. Li, Jing Shi The interface spin state of a ferromagnet (FM) can deviate significantly from its bulk spin state and this effect could be strongly orientation-dependent especially in manganites. We have successfully fabricated high-quality (110)-oriented [La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (t) /SrTiO$_{3}$ (3ML)]$_{n}$ superlattices (t ranging from 3 to 15 ML), which are characterized by the atomic force microscopy, high-resolution transmission electron microscopy, x-ray diffraction and magnetometry. Compared to (100)-oriented counterparts, LSMO in (110)-oriented superlattices has a thinner deadlayer at LSMO/STO interface. From the thickness (t) dependence of the superlattice magnetic moment, we extract the interface contribution and find that the interface moment is close to that of the bulk, suggesting that the (110)-oriented interface adopts the FM spin ground state. This differs significantly from the spin canting state at (100)-oriented LSMO/STO interface that was previously reported by other groups. Our results indicate that the magnetism of manganite interface may be manipulated by taking advantage of the orientation-dependent nature of the exchange interactions. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B30.00011: Enhanced Low Field Magnetoresistance in La$_{0.67}$Ca$_{0.33}$MnO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ superlattices P.V. Wadekar, Q.Y. Chen, O. Lozano, P.V. Chinta, W.K. Chu, D. Wijesundera, C.S. Lin, P.H. Tseng, Y.T. Lin, L.W. Tu, C.P. Lin, H. Chou, C.C. Kuo, N.J. Ho, H.W. Seo We have grown [1 nm/ 1nm]$_{n }$ superlattices of manganite (n = 20, 30,40) in which La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.33) serves as the ferromagnetic layer while La$_{1-x}$Ca$_{x}$MnO$_{3}$(x=0.5) serves as the spacer layer on LaAlO$_{3}$ substrates by magnetron sputtering. The samples were characterized by XRD, Magneto-transport measurements, Rutherford backscattering spectroscopy, and atomic force microscopy. Enhanced longitudinal magnetoresistance (MR) under an applied field B, defined as MR (B) = $\rho $(B)/$\rho $(0) -- 1, was as much as -49{\%} at B=0.5 Tesla and T=90 K. The causes for this enhancement not seen at low field in other single-layered films of x=0.33 and the correlation of oxygen annealing with the MR effects for the superlattices will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B30.00012: Strain-controlled electronic properties and magnetorelaxor behaviors in electron-doped CaMnO$_{3}$ thin films and superlattices P.-H. Xiang, H. Yamada, A. Sawa, H. Akoh We present a systematic study on electronic properties of the \textit{electron-doped} manganite Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO,$_{ }$0 $\le x\le $ 0.08) single-layer films and superlattices composed of alternating stacks of non-doped CaMnO$_{3}$ (CMO) and CCMO($x$ = 0.08) layers. The transport properties of the CCMO films are found to be very sensitive to the epitaxial strain. Metallic characteristic observed in the CCMO(0.04 $\le x\le $ 0.06) bulk polycrystal can be realized only in the practically strain-free CCMO epitaxial films on the NdAlO$_{3}$ (NAO) substrate. A large magnetoresistance accompanied with magnetorelaxor-like behavior is observed in the CCMO($x$ = 0.06) film, which can be explained in terms of the phase separation and the irreversible growth of metallic domain in antiferromagnetic insulating matrix. The metallic property is also realized in the superlattices, indicating a charge transfer at the interfaces between CMO and CCMO($x$ = 0.08) layers. When the CCMO($x$ = 0.08) layer in the superlattice is thicker than 8 unit cells, the superlattice exhibits magnetorelaxor-like phenomenon This can be attributed to a phase competition between different antiferromagnetic orderings at the interfaces, resulting in the phase separation [Preview Abstract] |
Session B31: Focus Session: Dimers and Other Frustrated Structures
Sponsoring Units: GMAGChair: Chris Wiebe, Florida State University
Room: 335
Monday, March 16, 2009 11:15AM - 11:51AM |
B31.00001: Multiple magnetic phases in the frustrated S=1 spin-dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$ Invited Speaker: Ba$_{3}$Mn$_{2}$O$_{8}$ is a spin-dimer compound based on pairs of S=1 3d$^{2}$ Mn$^{5+}$ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Interactions between dimers broaden the triplet and quintuplet bands such that application of a magnetic field leads to multiple states marked by long range order above characteristic critical fields. Here we present results of magnetization, heat capacity, magnetocaloric effect and torque magnetometry measurements of single crystal samples which reveal a complex phase diagram containing at least three distinct ordered states across the triplet and quintuplet regimes. Much of the phase diagram can be understood in terms of an effective spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ Hamiltonian containing only the lowest energy states ($\vert $0,0$>$ {\&} $\vert $1,1$>$ and $\vert $1,1$>$ {\&} $\vert $2,2$>$, referred to the dimer states, for the singlet-triplet and triplet-quintuplet regimes respectively). Two distinct ordered states are observed in the singlet-triplet regime, which can be ascribed to the delicate interplay between single ion anisotropy and antiferromagnetic interdimer exchange on the frustrated triangular lattice. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B31.00002: Frustrated magnetism in the diamond-chain like compound Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$ A.V. Mahajan, B. Koteswara Rao, J. Bobroff The structure of Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$, having chains of corner-shared square plaquettes, is reminiscent of the ``diamond chain'' compound Cu$_{3}$(CO$_{3})_{2}$(OH)$_{2}$ which has shown novel magnetic properties. We report preparation of polycrystalline samples of Ba$_{3}$Cu$_{3}$Sc$_{4}$O$_{12}$ followed by temperature dependent magnetic susceptibility \textit{$\chi $(T)} and heat capacity $C_{p}(T)$ measurements in applied magnetic fields up to $H$ = 90 kOe. At high-$T$, \textit{$\chi $(T)} is fitted by the Curie-Weiss law (\textit{$\chi $(T)=C/(T-}$\theta _{CW}))$ and is suggestive of ferromagnetic interactions ($\theta _{CW}$ $\sim $ 70 K). However, in low-fields, the \textit{$\chi $(T)} shows a sharp peak at $T_{N}$ $\sim $ 16 K and the variation at lower temperatures is indicative of antiferromagnetic ordering. Clear evidence of the transition at $T_{N}$ is also seen in heat capacity data. The sharp peak in \textit{$\chi $(T)} and $C_{p}(T)$ moves to lower temperatures with increasing $H$. The $T_{N}$ is found to be strongly lowered by an applied field and $T_{N} \quad \sim $ 0 for H $\sim $ 70 kOe. Further work to understand the relative exchange couplings between various Cu atoms is currently in progress. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B31.00003: NMR Response in Antiferromagnetic Spin-1/2 Chains Jesko Sirker, Nicolas Laflorencie Non-magnetic impurities break a quantum spin chain into finite segments and induce Friedel-like oscillations in the local susceptibility near the edges. The signature of these oscillations has been observed in Knight shift experiments on the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.5}$ and on the spin-chain compound Sr$_2$CuO$_3$. Here we analytically calculate NMR spectra, compare with the experimental data, and give a simple criterion to determine the impurity concentration. Our results are based on a parameter-free formula for the local susceptibility of a finite spin chain obtained by bosonization which is checked by comparing with quantum Monte Carlo calculations. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B31.00004: Controlling Luttinger Liquid Physics in Spin Ladders under Magnetic Field C. Berthier, M. Klanj\v{s}ek, M. Horvati\'{c}, H. Mayaffre, B. Chiari, O. Piovesana, P. Bouillot, T. Giamarchi, C. Kollath, E. Orignac, R. Citro We present a $^{14}$N nuclear magnetic resonance study of a single crystal of CuBr$_4$(C$_5$H$_{12}$N)$_2$ (BPCB) consisting of weakly coupled spin-$1/2$ Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate $T_1^{-1}$ at $250$~mK and for (ii) the phase transition to a 3D ordered phase occuring below $110$~mK due to weak inter-ladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and Luttinger liquid model tested in detail over the whole fermion band.\\ M. Klanj\v{s}ek {\em et al.}, Phys. Rev. Lett. {\bf 101} 137207 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B31.00005: Electronic Excitations in NiCl$_{2}$ Hydrates V.C. Long, N.M. Collins, G.R. Rayner, G.C. DeFotis, A.S. Hampton, J.M. Pothen We measured the temperature-dependent optical absorption spectra of pure polycrystalline NiCl$_{2}\cdot $2H$_{2}$O between 4,000 and 35,000 cm$^{-1}$ and of powdered NiCl$_{2}\cdot $2H$_{2}$O and NiCl$_{2}\cdot $H$_{2}$O (pressed into KBr pellets) in more limited frequency regions. NiCl$_{2}\cdot $2H$_{2}$O has a known complex phase diagram with distinct antiferromagnetic (AF) phases in successively lower temperature regions. It has a rich low temperature spectrum consisting of parity forbidden spin allowed and spin forbidden $d-d$ excitations of the pseudo-octahedrally coordinated Ni$^{2+}$ ion with fine structure due to vibronic sidebands, magnetic dipole allowed transitions, and possible electron-magnon coupling. Although most NiCl$_{2}\cdot $2H$_{2}$O bands show a typical gradually increasing definition with decreasing temperature, a narrow absorption at 12,600 cm$^{-1}$ is strongly suppressed below the lowest AF transition at 6.3 K. The magnetic properties of NiCl$_{2}\cdot $H$_{2}$O are more poorly understood, involving unconventional low dimensional behavior. Although the pressed pellet spectra reveal a loss of detailed spectral information, a downshift of $\sim $ 300 cm$^{-1}$ in the second spin allowed band and associated spin forbidden transition can be discerned for the monohydrate compared to the dihydrate. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B31.00006: Spin order of the classical Kagome antiferromagnet: via effective Hamiltonians Christopher L. Henley The classical Heisenberg Kagom\'e-lattice antiferromagnet (KAF) is only known to have a coplanar ``spin nematic'' (or octupole~\footnote{ M. E. Zhitomirsky, Phys. Rev. B 78, 094423 (2008).}) order, so that low-energy states are labeled by colorings. Contrary to accepted phenomenology,$^1$ I propose that these colorings develop {\it long-range order}.~\footnote{ C. L. Henley, arxiv:0811.0026.} First, from the spin-wave Hamiltonian up to 4th order, most modes are integrated out, leaving an effective quartic Hamiltonian $Q$ for just the ``soft'' (zero at harmonic order) modes. Writing it $Q=Q_0+Q'$, where only $Q'$ depends on the discrete coplanar state, $Q'$ is treated as a perturbation, and its expectation in the $Q_0$ ensemble becomes an effective Hamiltonian $\Phi$ for the colorings. The couplings in $\Phi$ are estimated using ``Coulomb phase'' coarse-grainings.$^2$ Following Huse \& Rutenberg,~\footnote{ D. A. Huse and A. D. Rutenberg, Phys. Rev. B 45, 7536 (1992).} I observe the unweighted coloring model sits at a roughening transition, hence $\Phi$ drives the KAF to long-range order of the $\sqrt{3}\times\sqrt{3}$ type (modulo the inevitable gradual orientation fluctuations of the spin plane). A similar effective Hamiltonian exists for related $d=3$ lattices,~\footnote{ C. L. Henley, arxiv:0809.0079.} but cannot produce order. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B31.00007: Quantum stabilization of 1/3 magnetization plateau in Cs$_2$CuCl$_4$. Oleg Starykh, Jason Alicea, Andrey Chubukov We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrella-like for all fields, but we show that the quantum phase diagram is much richer and contains a 1/3 magnetization plateau, two commensurate planar states, two incommensurate chiral umbrella phases, and, possibly, a spin density wave state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for the spin-1/2 system Cs$_2$CuCl$_4$. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B31.00008: Magnetic properties of bilayer triangular lattice Fei-Ming Hu, Shi-Quan Su, Tian-Xing Ma, Hai-Qing Lin We study magnetic properties of the single-band Hubbard model on a coupled bilayer triangular lattice by using the determinant quantum Monte Carlo method. Simulations are focused in the region near the van Hove singularities. We perform investigations on two kinds of double layer triangular lattices, one is a simple triangular structure which has only one nearest neighbor between two layers for every atom and another one is a graphene-like structure which has three nearest neighbors between two layers for every atom. We compare their magnetic properties in the view of the itinerant electron ferromagnetic theory of attribute their behaviors to the density of states on the Fermi surface. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B31.00009: Dzyaloshinskii-Moriya interactions in valence bond systems I Kirill Shtengel, Kumar Raman, Mayra Tovar We investigate the effect of Dzyaloshinskii-Moriya interactions on the low temperature magnetic susceptibility for a system whose low energy physics is dominated by short-range valence bonds (singlets). Our general perturbative approach is applied to specific models expected to be in this class, including the Shastry-Sutherland model of the spin-dimer compound SrCu$_2 $(BO$_3$)$_2$ and the antiferromagnetic Heisenberg model of the recently discovered $S=1/2$ kagome compound ZnCu$_3$(OH)$_6 $Cl$_2$. The central result is that a short-ranged valence bond phase, when perturbed with Dzyaloshinskii-Moriya interactions, will remain time-reversal symmetric in the absence of a magnetic field but the susceptibility will be nonzero in the zero temperature limit. Applied to ZnCu$_3$(OH)$_6$Cl$_2$, this model provides an avenue for reconciling experimental results, such as the lack of magnetic order and lack of any sign of a spin gap, with known theoretical facts about the kagome Heisenberg antiferromagnet. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B31.00010: Hierarchical mean-field approach to the $J_1$-$J_2$ Heisenberg model on a square lattice Leonid Isaev, Gerardo Ortiz, Jorge Dukelsky We study the quantum phase diagram and excitation spectrum of the frustrated $J_1$-$J_2$ spin-1/2 Heisenberg Hamiltonian. A hierarchical mean-field approach, at the heart of which lies the idea of identifying {\it relevant} degrees of freedom, is developed. Thus, by performing educated, manifestly symmetry preserving mean-field approximations, we unveil fundamental properties of the system. We then compare various coverings of the square lattice with plaquettes, dimers and other degrees of freedom, and show that only the {\it symmetric plaquette} covering, which reproduces the original Bravais lattice, leads to the known phase diagram. The intermediate quantum paramagnetic phase is shown to be a (singlet) {\it plaquette crystal}, connected with the neighbouring N\'eel phase by a continuous phase transition. We also introduce fluctuations around the hierarchical mean-field solutions, and demonstrate that in the paramagnetic phase the ground and first excited states are separated by a finite gap, which closes in the N\'eel and columnar phases. Our results suggest that the quantum phase transition between N\'eel and paramagnetic phases can be properly described within the Ginzburg-Landau-Wilson paradigm. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B31.00011: Mixed exchange antiferromagnetic/ferromagnetic S=1/2 Heisenberg rectangular lattices Brian Keith, Tom Valleau, Fan Xiao, Mark Turnbull, Chris Landee The susceptibilities of mixed antiferromagnetic/ferromagnetic rectangular Heisenberg lattices of $S = 1/2$ have been simulated using Quantum Monte Carlo techniques. These simulations include lattices in which the stronger interaction is ferromagnetic or antiferromagnetic along with the isotropically mixed lattice. The two exchange strengths, $J$ and $J'$, are related by $J'= \alpha J$, where $\alpha$ is the aspect ratio which ranges from $0 \le \alpha \le 1$. These simulations were done for $0 \le \alpha \le 1$ in .05 increments. The results are discussed and the models are used to fit suspected mixed antiferromagnetic/ferromagnetic rectangles such as $ Cu(pyz)(NO_3)(HCO_2) $ and $ Cu(pyz)(N_3)_2 $. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B31.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 2:03PM - 2:15PM |
B31.00013: Ce$_{2}$Pt$_{2}$Pb : Frustrated heavy fermion system with the Shastry-Sutherland lattice Moo Sung Kim, Meigan Aronson We have synthesized single crystals of Ce$_{2}$Pt$_{2}$Pb which has the Shastry-Sutherland lattice network of Ce-ions on the crystallographic c-plane, that can induce magnetic frustration. The specific heat, magnetic susceptibility, and resistivity were measured for as-grown crystals. Above 30 K, Curie-Weiss behavior is found in the magnetic susceptibility, with a Ce moment of 2.33 $\mu _{B}$/Ce and a Weiss temperature of -15 K, indicating antiferromagnetic interactions among the Ce$^{3+}$ ions. The magnetic specific heat rises from a minimum at 15 K to a broad maximum at 2 K, before falling to a heavy fermion value of $\sim $0.8 J/Ce-molK$^{2}$ at the lowest temperature. The entropy reaches only 1/2Rln2 at the maximum in the specific heat, and the full doublet ground state Rln2 is only recovered at 15 K. Ce$_{2}$Pt$_{2}$Pb is an unusual material, in which a heavy fermion liquid with short range antiferromagnetic order emerges from a strongly frustrated and fluctuating paramagnetic state. [Preview Abstract] |
Session B32: Cooperative Phenomena (incl. Spin Structures, Spin Waves, Phase Transitions) I
Sponsoring Units: GMAGChair: Paul Haney, National Institute of Standards and Technology
Room: 336
Monday, March 16, 2009 11:15AM - 11:27AM |
B32.00001: Point Contact Andreev Reflection Spin Spectroscopy in MnSi Single Crystals Pushkal Thapa, Raghava Panguluri, F. Carbone, D. van der Marel, A.A. Nugroho, B. Nadgorny We have carried out a detailed investigation of magnetic and spin transport properties of MnSi, an itinerant helimagnet with the weak Dzyaloshinski-Moria interaction. MnSi single crystals were grown in the form of a rod by the floating zone technique and we used the center of the rod which is clean and free from any impurities for all our measurements. Magnetization measurements suggest the critical temperature, T$_{c}$ of $\sim $ 29 K with the saturation magnetic moment of $\sim $ 0.4$\mu _{B}$/Mn at 2 K. A T$^{2}$ -- dependence of the resistance is observed near T$_{c}$, indicating that the transport is governed by spin fluctuations with a residual resistivity $\sim $ 20 $\mu \Omega $.cm. We used the point contact Andreev reflection (PCAR) technique with electrochemically etched superconducting niobium tips to observe the effects of spin polarization in a variable magnetic field as the magnetic structure of MnSi gradually transformed from helical - to conical - to weakly ferromagnetic. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B32.00002: High Pressure studies of the magnetic phase transition in MnSi Sergei Stishov, Alla Petrova, Vladimir Krasnorusski, Thomas Lograsso New measurements of AC magnetic susceptibility and DC resistivity of a high quality single crystal MnSi were carried out at high pressure making use of helium as a pressure medium. The form of the AC magnetic susceptibility curves at the magnetic phase transition suddenly changes upon helium solidification. This implies strong sensitivity of magnetic properties of MnSi to non hydrostatic stresses and suggests that the early claims on the existence of a tricritical point at the phase transition line are probably a result of misinterpretation of the experimental data. At the same time resistivity behavior at the phase transition does not show such a significant influence of helium solidification. The sharp peak at the temperature derivative of resistivity, signifying the first order nature of the phase transition in MnSi successfully survived helium crystallization and continued the same way to the highest pressure. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B32.00003: Ultrasonic studies of the magnetic phase transition in MnSi Alla Petrova, Sergei Stishov We report results of the ultrasonic studies of a single crystal of MnSi in the temperature range 4-150 K, performed using digital pulse echo techniques. All the elastic constants and their combinations except for $c_{44}$ reveal profound rounded dips at temperature ${\approx}$ 29.6 K with discontinuities at the low temperature sides of the dips at ${\approx}$ 28.9 K, which corresponds to the magnetic phase transition in MnSi. Both of these features are beautifully correlate with the heat capacity, thermal expansion and resistivity data, obtained with the same single crystal of MnSi, but the sharp peaks of the mentioned quantities are replaced by the modest discontinuities in the elastic properties at the phase transition. The sound attenuation displays a double peak structure and looks like an almost exact copy of the corresponding curves, characterizing behavior of the heat capacity, thermal expansion and resistivity in the vicinity of the phase transition in MnSi. All the features observed correspond to a first order phase transition at 28.9 K. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B32.00004: High-Energy Magnon Dispersion and Multimagnon Continuum in La$_2$CuO$_4$ Neil Headings, Stephen Hayden, Radu Coldea, Toby Perring We report high-energy neutron scattering measurements of the magnetic excitations in the S=1/2 antiferromagnet La$_2$CuO$_4$. Measurements were made using the MAPS time-of-flight spectrometer at the ISIS spallation source. Previous measurements found evidence for higher order (cyclic) exchange couplings. We find evidence for significant corrections to linear spin-wave (SW) theory including these higher-order exchange constants. In particular, the intensity of the spin wave pole deviates strongly from that predicted by SW theory near the $\mathbf{Q}=(1/2,0)$ position. We also find evidence for a multi-magnon continuum. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B32.00005: Collective spin-excitations in Cu L and O K edge Resonant Inelastic X-ray Scattering on Sr$_{14}$Cu$_{24}$O$_{41}$. J. Schlappa, T. Schmitt, F. Vernay, V.N. Strocov, B. Thielemann, H.M. Ronnow, J. Mesot, B. Delley, L. Patthey, V. Ilakovac Using Resonant Inelastic X-ray Scattering (RIXS) at the Cu$ L_{3}$ and O $K$ edge we studied the spin-ladder/spin-chain compound Sr$_{14}$Cu$_{24}$O$_{41}$ [1,2]. We observe collective spin-excitations from the ladders, which we assign to two-Triplon modes [3]. At the Cu $L_{3}$ resonance the dispersion of the modes was mapped out, obtaining excellent sensitivity over the full first Brillouin-zone, in contrast to Inelastic Neutron Scattering [4]. At the O $K$ edge site-sensitive hole-excitation studies of the chain- and ladder-subsystems were performed, giving insight into the character of the holes. [1] T. Vuletic et al., Physics Reports 428, 169-258 (2006). [2] A. Kotani and S. Shin, Rev. Mod. Phys. 73, 203 (2001). [3] K. P. Schmidt and G. S Uhrig. Mod. Phys. Rev. Lett. 90, 227204 (2003). [4] S. Notbohm et al., Phys. Rev. Lett. 98, 027403 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B32.00006: Antiferromagnetic instability in Sr$_3$Ru$_2$O$_7$: stabilized and revealed by dilute Mn impurities Muhammed Hossain, B. Bohnenbuck, Y.-D. Chuang, E. Cruz, H.-H. Wu, L.H. Tjeng, I.S. Elfimov, Z. Hussain, B. Keimer, G.A. Sawatzky, A. Damascelli X-ray Absorption Spectroscopy (XAS) and Resonant Elastic Soft X-ray Scattering (RESXS) studies have been performed on Mn-doped Sr$_3$Ru$_2$O$_7$, both on the Ru and Mn L-edges, to investigate the origin of the metal insulator transition. Extensive simulations based on our experimental findings point toward an intrinsic antiferromagnetic instability in the parent Sr$_3$Ru$_2$O$_7$ compound that is stabilized by the dilute Mn impurities. We show that the metal-insulator transition is a direct consequence of the antiferromagnetic order and we propose a phenomenological model that may be applicable also to metal-insulator transitions seen in other oxides. Moreover, a comparison of Ru and Mn L-edge data on 5\% Mn doped system reveals that dilute Mn impurities are generating much more intense signal than Ru which is occupying 95\% of the lattice sites. This suggests the embedding of dilute impurities as a powerful mean to probe weak and, possibly, spatially inhomogeneous order in solid-state systems. In collaboration with: Y. Yoshida (AIST), J. Geck, D.G. Hawthorn (UBC), M.W. Haverkort, Z. Hu, C. Sch\"{u}\ss ler-Langeheine (Cologne), R. Mathieu, Y. Tokura, S. Satow, H. Takagi (Tokyo), J.D. Denlinger (ALS). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B32.00007: Collapse of the Hyperfine Magnetic Field at the Ru site in GdRu$_2$ and HoRu$_2$ D. Coffey, M. DeMarco, P.-C. Ho, T. Sayles, M. B. Maple, J. W. Lynn, Q. Huang The M\"{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $H_{hyper}$, at the ME active site. This is the case in the ruthenates where the temperature dependences of $H_{hyper}$ at $^{99}$Ru sites track the magnetic order. This is not the case in GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, and susceptibility show that there is ferromagnetic order below 93K in GdRu$_2$. Neutron diffraction data reveal that HoRu$_2$ orders ferromagnetically at 15.30(4) K with an ordered moment of 7.98(8) $\mu_B$. However there is no evidence of a correspondingly large $H_{hyper}$ in the $^{99}$Ru ME in either material. {\it Ab initio} calculations shows that spin polarization occurs only on the rare earth sites with $ H_{hyper} < 5T$ on the Ru sites. The results are compared with the corresponding calculations for ferromagnetic SrRuO$_3$. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B32.00008: Neutrons Scattering Study of Magnetic Correlations in Y$_{0.7}$La$_{0.3}$VO$_3$ Sung Chang, Jiaqiang Yan, Robert McQueeney \textit{R}VO$_3$ ($R =$~rare earth) perovskite vanadites exhibit multiple orbital and spin orderings and provide a unique opportunity to study the spin-orbital-lattice coupling of $\pi$-bonding \textit{t} electrons. The nature of the orbital order in these vanadites has been a matter of significant controversy, particularly with respect to whether the different orbital-ordered phases of YVO$_3$ and LaVO$_3$ are best described by a novel orbital-Peierls model or more traditional Jahn-Teller interactions. Here we report on a neutron scattering study of the magnetic correlations in Y$_{0.7}$La$_{0.3}$VO$_3$, which may be expected to depend sensitively on the orbital degrees of freedom. The results are discussed in terms of the interplay between magnetism and orbital order. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B32.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 1:03PM - 1:15PM |
B32.00010: Possible Observation of Electromagnons in BiFeO3 Crystal Pauline Rovillain, Yann Gallais, Alain Sacuto, Ricardo Lobo, Doroth\'ee Colson, Delphine Lebeugle, Rogerio De Sousa We show the first optical observation via inelastic light scattering of sharp low energy modes in multiferroic BiFeO$_{3}$. We have detected two different modes ($\Psi $ and $\Phi $ modes) and interpreted them as the optical fingerprint of the cycloidal magnetic order. The energies of the $\Phi $ modes are equally spaced and exhibit a linear dependence as a function of the mode index (n) down to zero while the energy sequence of the $\Psi $ modes is not equally spaced and exhibit a gap at low energy. Our experimental data are in remarkable agreement with the theory of de Sousa and Moore who pointed out that these modes could possibly be observed due to coupling between spins and lattice degrees of freedom. These experimental findings offer a unique opportunity for optical probing of the electromagnetic excitations of multiferroics. [1] Cazayous et al., Phys. Rev. Lett. 101, 097003 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B32.00011: Magnon Sidebands and Spin-Charge Coupling in Bismuth Ferrite Probed by Nonlinear Optical Spectroscopy Amit Kumar, M. Ramirez, S. Denev, N. Podraza, X. Xu, R. Rai, Y-H. Chu, J. Seidel, L. Martin, J. Ihlefeld, J. Klug, M. Bedzyk, O. Auciello, D. Schlom, R. Ramesh, J. Orenstein, J. Musfeldt, V. Gopalan The interplay between spin waves (magnons) and electronic structure in materials leads to the creation of additional bands associated with electronic energy levels, called magnon sidebands which are difficult to probe due to their smaller energy scales (meV). Linear light absorption and scattering techniques at low temperatures are traditionally used to probe these sidebands. We show that optical second harmonic generation (SHG) can successfully probe the magnon sidebands at room temperature and up to 723K in bismuth ferrite, associated with large wave-vector multi-magnon excitations which linear absorption studies have thus far been unable to resolve. Polarized light studies and the temperature dependence of these sidebands reveal a spin-charge coupling interaction between the spontaneous polarization ($P_{s}$) and antiferromagnetic order parameter, $L$ in bismuth ferrite, that persists with short range correlation well into the paramagnetic phase. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B32.00012: Aging in Co/Cr Superlattices T. Mukherjee, M. Pleimling, Ch. Binek Aging phenomena are observed in various systems brought into non-equilibrium and subsequently showing slow relaxation dynamics. Magnetic specimens with well defined interactions and dimensions can serve as model systems for universal aspects of aging. Magnetic thin films provide access to a wide range of microscopic parameters. Superlattice structures allow tuning the intra and inter-plane exchange and enable geometrical confinement of the spin fluctuations. We use Co/Cr thin film superlattices to study magnetic aging. The static and dynamic properties are affected via the Co and Cr film thicknesses. T$_{C}$ of the Co films is reduced from the bulk value by geometrical confinement. Non-ergodic behavior sets in at a tunable temperature $T^{\ast }$ in a range of some 100K above zero. Cr provides antiferromagnetic coupling between the Co films. Non-equilibrium spin states are set via low field cooling in 5mT in-plane magnetic field to below $T^{\ast }$. Next various in-plane magnetic set fields of some 10-100 mT are applied and the sample is exposed to the latter for various waiting times $t_{w}$, respectively. After removing the field, relaxation of the magnetization is recorded via longitudinal Kerr-magnetometry. The relaxation data are analyzed by scaling plots revealing universal aspects of aging. Financial support by Teledyne-Isco, NRI, and NSF through EPSCoR, Career DMR-0547887, and MRSEC. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B32.00013: Exchange-coupling modified spin wave spectra in the perpendicularly magnetized permalloy nanodot chain arrays Jian Dou, Sarah C. Hernandez, Chengtao Yu, Michael J. Pechan, Liesl Folks, Jordan A. Katine, Matthew J. Carey Spin wave spectra in exchange coupled nanoscale dot chain arrays were studied using ferromagnetic resonance. The dot chain arrays, with dot diameters of 300 nm and thicknesses of 40 nm, coupled via permalloy bridges of width ranging from 0 to 60 nm, were fabricated using e-beam lithography. In the perpendicularly magnetized isolated dots, multiple sharp ferromagnetic resonant peaks were observed\footnote{G.N.Kakazei et al, Appl. Phys. Lett. \textbf{85}, 443 (2004)}, which is associated with the quantized in-plane wave vector due to the finite dot radius. These spectrum lines shift to higher fields for samples with wider bridges due to the increasing effective demagnetizing factor. Additional higher order spin wave modes were observed as satellite peaks near the resonance peaks at both higher and lower fields, with larger separation between adjacent spin wave peaks for wider bridge samples. These extra spin wave modes, associated with the inter-dot exchange coupling,will be described in detail. This work is supported by US Dept. of Energy at MU. [Preview Abstract] |
Session B33: Superconductivity: Mostly Cuprates
Sponsoring Units: DCMPChair: Timir Datta, University of South Carolina
Room: 403
Monday, March 16, 2009 11:15AM - 11:27AM |
B33.00001: Making superconducting transition temperature higher in Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+\delta }$ Xiao-Jia Chen, Viktor V. Struzhkin, Russell J. Hemley, Ho-kwang Mao, Yong Yu, Cheng-Tian Lin We report an experimental finding of \textit{Tc} enhancement in optimally doped Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+\delta }$. We found that the generally observed pressure effect on \textit{Tc}, $i.e.$, \textit{Tc} first increases with pressure and then decreases after passing a maximum at an optimal pressure, is only held below a critical pressure around 24 GPa in this multilayer material. After that \textit{Tc} enhances remarkably upon further compression, considerably surpassing the first maximum. The critical pressure was then considered as the crossover from antiferromagnetism to superconductivity in the inner CuO$_{2}$ plane. The afterwards \textit{Tc} enhancement was suggested through the optimization of two competing energy scales (pairing and phase ordering) of different CuO$_{2}$ planes. The results have important implications for engineering superconductors with much higher \textit{Tc}'s at ambient conditions. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B33.00002: Specific heat of underdoped high $T_c$ superconductors from phenomenological models. J. P. F. LeBlanc, E. J. Nicol, J. P. Carbotte Inspired by phenomenological models for the pseudogap state, for example, the model of Yang, Rice and Zhang[1], we have calculated the specific heat for the underdoped cuprate superconductors. Results will be shown for both the pseudogap and superconducting state as a function of doping. Comparison between models and with experiment will be made. [1] K.Y. Yang, T.M. Rice and F.C. Zhang, Phys. Rev. B 73, 17541 (2006). [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B33.00003: Resistive switching in YBa$_2$Cu$_3$O$_{7-\delta}$ Carlos Acha, Marcelo J. Rozenberg We report on the nonvolatile and polarity dependent resistance switching of metal- YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) ceramic superconductor interfaces that also extends macroscopically to the bulk YBCO. We show that electric pulses mainly modify the connectivity of the ceramic grains of the bulk material, affecting the geometrical conducting factor near the interface and controlling the superconducting percolating path in the bulk. Relaxation processes of the resistivity after applying the pulses, not associated with heating effects, are also observed. We also report on the temperature sensitivity of resistance hysteresis switching loops, where both the difference between high and low resistance states and the voltage needed to produce the switching decrease with increasing temperature. The origin of this switching effect may be related to electric field-induced oxygen ion migration, which modifies the oxygen content at grain boundaries and controls the electric transport of ceramic superconductors. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B33.00004: Calculation of diffusion coefficient in Au diffusion-doped Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ by EDXRF measurements Mustafa Akdogan, Ozgur Ozturk, Ugur Cevik, Ahmet Varilci, Cabir Terzioglu Gold (Au) diffusion in superconducting Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$O$_{y}$ was investigated over the temperature range 500-800 \r{ }C by the EDXRF technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D$_{1}$=6.7 x 10$^{-5}$exp(-1.19eV/k$_{B}$T) and D$_{2}$=9.7 x 10$^{-4}$exp(-1.09eV/k$_{B}$T), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19{\%}. For the Au-diffused samples, transport measurements indicated the T$_{c}$ increased from 100 to 104K and the J$_{c}$ increased from 40 to 125Acm$^{-2}$, in comparison with those of undoped samples. From SEM and XRD measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T$_{c}$ phase to the low-T$_{c}$ phase. The possible reasons for the observed improvement due to Au diffusion are also discussed. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B33.00005: Charge distribution on planar oxygens in an underdoped High-Tc cuprate: La(2-x)Sr(x)CuO(4) via $^{17}$O NMR. Greg Boebinger, Robert Smith, Arneil Reyes, Philip Kuhns, Takashi Imai, P. M. Singer, F.C. Chou, K. Hirota, M. Takaba, T. Kakeshita, H. Eisaki , S. Uchida We use high magnetic fields ($\sim $30T) to suppress the superconducting Tc of La(2-x)Sr(x)CuO(4) (LSCO) in order to investigate the normal state NMR properties at low temperatures. We use $^{17}$O NMR as a local probe of the electron density on the planar oxygens. $^{17}$O Knight shift and linewidths were obtained over a wide temperature range in the normal state for under-doped (x=0.05, 0.115) and optimally doped (x=0.15) LSCO. Our results will be discussed in the context of recent reports of charge segregation on the planar oxygens (from STM experiments) and magnetization and neutron scattering experiments showing glassy behavior and incommensurate spin-waves in LSCO. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B33.00006: Magnetic Breakdown in the Cuprates Jean-Michel Carter, Daniel Podolsky, Hae-Young Kee Following the recent observations of quantum oscillations in YBa$_2$Cu$_3$O$_{6.5}$ materials, we study different Fermi surface topologies in order to make a prediction on the nature of the topology of said Fermi surfaces in the cuprates. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B33.00007: Important issues from uncontrolled cation nonstoichiometry in well-known cuprates Jun-ichi Shimoyama, Hiraku Ogino, Shigeru Horii, Yuhya Yamazaki, Koichi Kaku, Yui Ishii, Kohji Kishio Our recent studies revealed that the post-annealing to control cation composition largely changes superconducting properties of cuprate superconductors, such as Bi-based compounds and the RE123 system. For example, the $T_{c}$ of Bi(Pb)2223 was enhanced up to 118 K by post-annealing at $\sim $950 K in air, while it was a typical value of 110 K before post-annealing. Strong correlation between the $c$-axis length and $T_{c}$ suggested that cation composition plays a crucial role to determine superconducting properties of this compound. On the other hand, substitution of RE for Ba-site in RE123 has been well recognized for light-RE123. However, such RE-rich RE123 compounds were found to form even for heavy-RE123 including Y123, which has been considered as a compound free from cation nonstoichiometry. The Y-rich Y123 exhibited suppressed $T_{c}$ down to $\sim $80 K and apparently short $c$-axis length. These result indicated that intrinsic physical properties of layered cuprates should be reexamined after careful control of cation stoichiometry besides control of oxygen composition, because nonstoichiometric cation composition and its local fluctuation strongly affect electronic and vortex systems and related flux pinning properties. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B33.00008: Evidence for nonmonotonic magnetic field penetration in a Pippard superconductor Vladimir Kozhevnikov, Claudiu Giuraniuc, Margriet Van Bael, Kristiaan Temst, Chris Van Haesendonck, Todor Mishonov, Timothy Charlton, Robert Dalgliesh, Yurii Khaidukov, Yurii Nikitenko, Victor Aksenov, Vladimir Gladilin, Vladimir Fomin, Jozef Devreese, Joseph Indekeu Polarized neutron reflectometry (PNR) provides evidence that \textit{nonlocal }electrodynamics governs the magnetic field penetration in an extreme low-\textit{$\kappa $ }superconductor. The sample is an indium film with a large elastic mean free path (11 \textit{$\mu $}m) deposited on a silicon oxide wafer. It is shown that PNR can resolve the difference between the reflected neutron spin asymmetries predicted by the local and nonlocal theories of superconductivity and therefore can be used for direct measurements of the microscopic intrinsic parameters of superconductors. The experimental data support the nonlocal theory, which predicts a \textit{nonmonotonic decay }of the magnetic field. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B33.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 1:03PM - 1:15PM |
B33.00010: Engineering Thin Film Superconductivity toward Single Quantum Channel Limit Shengyong Qin, Jungdae Kim, Qian Niu, Chih-Kang Shih Traditional studies of two-dimensional superconductors were limited to~the regime where the superconducting order parameter behaves as a~two-dimensional wave function but the underline electrons are still three dimensional.~~Recent advancement of materials synthesis have enabled one to grow epitaxial thin superconductor thin films (e.g. Pb) on semiconductor substrates (e.g. Si or Ge) with unprecedented control in crystallinity, atomic smoothness and the layer thickness, thus opening up new opportunities in investigations of two-dimensional superconductivities. Indeed, quantum oscillations of the superconducting order parameter as a function of film thickness have been observed. Moreover, it was found that superconductivity remains very robustness even for films as thin as 5 ML. An interesting question arises as to what extent the robustness of superconductivity remains in even thinner regime. This work presents the case of thin film superconductivity in extreme confinement limit when only one quantum channel is present (i.e. when k$_{F}$L = $\pi )$. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B33.00011: Competition between pairing and magnetic interactions Raimundo dos Santos, Pedro Bertussi, Andre Malvezzi, Thereza Paiva We discuss the interplay between pairing and magnetism by considering a model system composed of both tight-binding electrons and localized moments; the conduction electrons tend to form Cooper pairs due to a local (on-site) attractive interaction, $U$, while they also have a Kondo-like coupling, $J$, with the local moments. Density matrix renormalization group diagonalization on finite one-dimensional lattices (up to 60 sites) is used to calculate magnetic and pairing correlation functions, as well as structure factors in the ground state, in the case of electron density 1/3. Similarly to what happens in the quaternary borocarbides, we find that superconductivity coexists with a variety of magnetic arrangements of the local moments, ranging from commensurate to incommensurate spin-density waves, up to a critical value $J_c(U)$; the conduction electrons show strong antiferromagnetic fluctuations in this region. Superconductivity is then suppressed by the appearance of a magnetic state with broken rotational symmetry in both the local-moment and itinerant electrons subsystems, so that for sufficiently strong $J$, a spiral-ferromagnetic ground state evolves to a ferromagnetic one. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B33.00012: Alpha particle spectrometry using superconducting microcalorimeters Robert Horansky, Joel Ullom, James Beall, Gene Hilton, Gregory Stiehl, Kent Irwin, Alexander Plionis, Stephen Lamont, Clifford Rudy, Michael Rabin Alpha spectrometry is the preferred technique for analyzing trace samples of radioactive material because the alpha particle flux can be significantly higher than the gamma-ray flux from nuclear materials of interest. Traditionally, alpha spectrometry is performed with Si detectors whose resolution is at best 8 keV FWHM. ~Here, we describe the design and operation of a microcalorimeter alpha detector with an energy resolution of 1.06 keV FWHM at 5 MeV. We demonstrate the ability of the microcalorimeter to clearly resolve the alpha particles from Pu-239 and Pu-240, whose ratio differentiates reactor-grade Pu from weapons-grade. We also show the first direct observation of the decay of Po-209 to the ground state of Pb-205 which has traditionally been obscured by a much stronger alpha line 2 keV away. Finally, the 1.06 keV resolution observed for alpha particles is far worse than the 0.12 keV resolution predicted from thermal fluctuations and measurement of gamma-rays. The cause of the resolution degradation may be ion damage in the tin. Hence, alpha particle microcalorimeters may provide a novel tool for studying ion damage and lattice displacement energies in bulk materials. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B33.00013: High Resolution Gamma-Ray Spectroscopy with Superconducting Microcalorimeters D.A. Bennett, J.N. Ullom, W.B. Doriese, J.A. Beall, G.C. Hilton, R.D. Horansky, K.D. Irwin, N. Jethava, E. Sassi, L.R. Vale, M.K. Bacrania, A.S. Hoover, N. Hoteling, P.J. Karpius, M.W. Rabin, C.R. Rudy, D.T. Vo We are currently developing high resolution gamma-ray microcalorimeters (uCal) for improved analysis of nuclear materials. The uCal consist of a bulk superconducting absorber attached to a transition-edge sensor (TES) biased in its resistive transition and operated at temperatures near 0.1 K. Incoming particles and photons are converted to heat in the absorber and the resulting temperature change is measured by the highly sensitive TES thermometer. The unmatched energy resolution of these devices is useful for nuclear safeguards. A specific application is the determination of Pu isotopics in complex mixtures. Although much of our effort is focused on the construction and multiplexed readout of large arrays of detectors for increased collection area, we are also working on optimizing the performance of individual pixels. To this end, we have developed an analytic uCal model that includes the thermal properties of the attached absorber and the large inductance in the TES circuit bias. We show how this model can be used to maximize the number of sensors that can be multiplexed into a single readout channel and to minimize the response time of individual sensors. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B33.00014: In situ measurement of the permittivity of helium using microwave NbN resonators Loren Swenson, Grigorij Grabovskij, Olivier Buisson, Christian Hoffmann, Alessandro Monfardini, Jean-Claude Villegier Due to their high quality-factors and ease of fabrication, superconducting microwave resonators are increasingly being recognized as ideal sensors in ultra-sensitive, low-temperature measurements. In this talk, we will discuss the design and characterization of superconducting NbN quarter-wave resonators implemented as a high-speed and spatially-sensitive detector for the permittivity of a surrounding helium bath. Measurements of a device with a $\sim$10$^{-3}$ mm$^3$ spatial resolution, a $\sim$6$\times$$10^{-11}$ $\epsilon_0$/Hz$^{1/2}$ sensitivity to changes in the permittivity of helium, and a bandwidth of 300 kHz will be presented in the temperature range from 1.8 to 8.8 K. Potential applications, including operation as a fast, localized helium thermometer and as a transducer in superfluid hydrodynamic experiments, will be discussed. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B33.00015: Instabilities and nonlinearities in the elastic vortex solid Satyajit Banerjee, Shyam Mohan, Jaivardhan Sinha, S. Ramakrishnan, A.K. Grover, Ajay Sood In recent times the influence of microscopic disorder has been viewed to play a crucial role in determining the configuration of vortices inside superconductors. Our recent investigations into the dissipation properties of the driven vortices [1], have found the existence of possible symmetry changes deep within the well ordered elastic vortex solid [1]. The result is significant, as it requires a deeper revision of understanding the properties of the so called `benign' elastic vortex solid. We have also studied the nonlinear properties of the driven elastic vortex solid [2] through a random pinning environment and have found interesting highly nonlinear fluctuation in the time domain. We believe our results indicate the presence of process deep within the elastic driven phase which is a precursor to the plastic transformation in the vortex matter. [1] Shyam Mohan, Jaivardhan Sinha, S. S. Banerjee*, and Yuri Myasoedov, Phys. Rev. Lett. 98,027003 (2007). [2] Shyam Mohan, Jaivardhan Sinha, S. S. Banerjee* A. K. Sood, S. Ramakrishna, A. K. Grover (submitted, 2008) \textit{*satyajit@iitk.ac.in} [Preview Abstract] |
Session B34: Focus Session: Iron Pnictides and Other Novel Superconductors II: Quantum Oscillations, Electronic Structure and Magnetism
Sponsoring Units: DMPChair: Vladimir Antropov, Ames Laboratory
Room: 404
Monday, March 16, 2009 11:15AM - 11:27AM |
B34.00001: Fermi surface of superconducting LaFePO determined from quantum oscillations Amalia Coldea, J. Fletcher, A. Carrington, J. Analytis, C. Andrew, A. Bangura, J.-H. Chu, A. Erickson, I. Fisher, N. Hussey, R. McDonald We report extensive measurements of quantum oscillations in the normal state of LaFePO, using low temperature torque magnetometry and transport in high static magnetic fields (45 T). LaFePO is a bulk Fe-based superconductor (\textit{Tc$\sim $ }6 K) which can be grown in high quality single crystalline form being isostructural to LaFeAsO but without being affected by magnetic or structural transitions at low temperatures. We find that the Fermi surface is that of a compensated metal in broad agreement with the band-structure calculations with frequencies varying between 2.8{\%} to 9{\%} of the basal plane area of the Brillouin zone. The effective masses vary between 1.7-2.1 me and the electronic correlations in LaFePO are moderate corresponding to a mass enhancement of about \textit{$\sim $}2. The observed variation in the electronic scattering between different bands may be related to their different orbital character. The quasi-two dimensional Fermi surface consists of nearly-nested electron and hole pockets, suggesting proximity to a spin/charge density wave instability. The upper critical field anisotropy is a factor $\sim $10 (at 0.325 K) which may linked to its increased two-dimensionality (PRL \textbf{101}, 216402 (2008)). [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B34.00002: Quantum Oscillations in the parent pnictide BaFe2As2: Fermi surface reconstruction of the magnetic ground state James Analytis, Jiun-Haw Chu, Ian Firsher, Ross McDonald, Igor Mazin We have measured quantum oscillations in the magnetically ordered ground state of BaFe2As2, a parent compound of the superconducting ternary pnictides. Measurements were performed in 65T pulsed field at the Los Alamos National High Magentic Field Laboratory, using an atomic force microscope torque cantilever. We also perform detailed band-structure calculations for the spin-density wave ground state and find agreement with our observations of small quasi-two dimensional pockets. These results place significant constraints on our understanding of the magnetism associated with the Fe-As layers and demonstrates that coherent quasiparticles persist in the magnetic ground state, providing an important clue about the nature of superconductivity which emerges when this compound is doped. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B34.00003: Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor Neil Harrison, Suchitra Sebastian, J. Gillet, P. Lau, David Singh, Charles Mielke, Gilbert Lonzarich We report quantum oscillation measurements in SrFe$_2$As$_2$, which is known to become superconducting under doping and pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55~T suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high $T_{\rm c}$ superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B34.00004: Fermi surface changes in LaFeAsO$_{1-x}$F$_x$ using supercells, rigid band shifts, and the virtual crystal approximation Paul Larson, Sashi Satpathy There is currently great interest in the properties of the superconducting material LaFeAsO. While numerous calculations have been performed for this material, questions arise to which approximations describe the changes in the Fermi surface with doping. We have performed {\it ab initio} density functional studies of F-doping in the non-magnetic state using supercell calculations and compared these results to those obtained using rigid band shifts and the virtual crystal approximation (VCA). The Fermi surface consists mainly of Fe d and As p states with La and O states lying far from the Fermi level. Significant differences are found by comparing the supercell results with those of the rigid band shifts, but remarkable agreement is found for the Fermi surface using VCA calculations where the nuclear and electron charge are changed continuously to mimic the addition of electrons. P.Larson and S. Satpathy, condmat:arXiv 0810.4605v1. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B34.00005: Momentum density, Fermi surface and directional Compton profile in the Iron-based superconductor LaOFeAs Y. J. Wang, Hsin Lin, B. Barbiellini , P.E. Mijnarends, S. Kaprzyk, W. Al-sawai, R.S. Markiewicz, A. Bansil We have carried out first principles all-electron calculations of the (001) projected 2D electron momentum density (2D-EMD) and directional Compton profiles (CPs) along the [100], [001], and [110] directions in iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs for various doping concentrations $x$ within the framework of the local density approximation (LDA). We have identified Fermi surface features both in the 2D-EMD and in the CPs. Bonding effects related to the character of wave functions near the Fermi level are revealed by the autocorrelation function B(r) defined as the Fourier transform of the momentum density. Work supported in part by the US Department of Energy. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B34.00006: First Principles studies of 122 Ferropnictide Surface Alexander Kemper, P.J. Hirschfeld, H-P. Cheng We present DFT-GGA calculations for the Ba and Sr-122 ferropnictide materials on the effect of surfaces on the electronic structure. It has been established that there is a strong modulation of the electronic structure by the Fe-As distance, which decreases near a free surface due to surface reconstruction. Indeed, we see significant changes of both Fermi velocities and Fermi surfaces due to these effects for both the paramagnetic and collinear spin density wave states. These changes of the electronic structure, which we exhibit here, will be crucial for the interpretation of surface probes like ARPES and STM. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B34.00007: Structural, spin, and orbital phase transitions in LaOFeAs: I. Total energy calculations Wei Ku, Chi-Cheng Lee, Wei-Guo Yin Recent experimental studies on iron pnictides showed the existence of local Fe magnetic moments even in the superconducting phase, indicating strong fluctuations in a short time scale. We report a investigation of the local electron-lattice and electron-spin couplings via total energy calculations within first-principles density functional theory. Strong coupling in both channels were found to be closely tied to a ferro-orbital order, which drives the structure transition and the stripy magnetic (SDW) transition at high temperature in the undoped system. We suggest that this orbital degree of freedom leads to stronger coupling upon doping and thus possible enhancement of superconductivity. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B34.00008: Structural, spin, and orbital phase transitions in LaOFeAs: II. Wannier function analysis Wei-Guo Yin, Chi-Cheng Lee, Wei Ku A realistic low-energy effective interacting Hamiltonian of doped LaOFeAs is derived quantitatively from a novel first-principles Wannier function analysis. Strong local orbital, spin, and lattice coupling to the doped charge is found associated with the orbital degree of freedom, which suggests a possible explanation of the high transition temperature. The fluctuations in the orbital sector fundamentally distinguish the iron pnictides from the copper oxide superconductors, and ensure the essential role of phonons in the mechanism of superconductivity in this new class of materials, in addition to electronic origin. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B34.00009: Emergence of complex magnetism in three dimensional, yet quasi-layered, iron pnictides: CaFe$_{4}$As$_{3}$ Arthur J. Freeman, Giancarlo Trimarchi, Mercouri Kanatzidis, Iliya Todorov, Duck-Young Chung The class of iron pnictides has been the focus of much attention for the discovery of superconductivity in the layered compounds LaOFeAs, CaFe$_{2}$As$_{2}$, and related ones; the phase diagrams of these pnictides remain still largely unexplored. Here, we report on the electronic and magnetic structure of the recently synthesized CaFe$_{4}$As$_{3}$ compound. This material, as opposed to the layered CaFe$_{2}$As$_{2}$, shows FeAs slabs parallel to the $b$-direction and approximately perpendicular to each other, defining tunnels filled by the Ca atoms. No sign of superconductivity was found in this compound. Instead, the system shows a complex ferromagnetic state at low temperature. DFT calculations performed on the refined crystal structure using the highly precise FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB, {\bf 24}, 864 (1981)} show a pronounced stabilization for the ferromagnetic state which is characterized by four distinct Fe sites with magnetic moments of between 1 $\mu_{B}$ and 2 $\mu_{B}$. The influence of the local topology of the crystal structure on the the electronic and magnetic state is analyzed. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B34.00010: `Unscreening' Effect on Fe-Pnictide Superconductor Masao Ogata, Yuki Fuseya, Toshikaze Kariyado We study a microscopic mechanism of Fe-pnictide superconductor, considering the screening effects of Coulomb interaction in addition to the conventional spin-fluctuation mechanism. It is shown that, by electron doping, the transition temperature of superconductivity increases due to the `unscreening' effect even though the density of states decrease, while that of spin-density wave rapidly decreases due to breaking of nesting conditions. Our results give a clear interpretation to the mystery of interrelation between $T$c and the density of states observed in the Fe-pnictide superconductors. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B34.00011: Quasiparticle scattering interference in superconducting Oxypnictides Yanyang Zhang, Kangjun Seo, Xiaoting Zhou, B. Andrei Bernevig, Jiangping Hu Based on a two-band model of the superconducting iron oxypnictides, we study the effects of single-impurity scattering on the local density of states by exact T-matrix calculations. We compare the quasi-particle interference patterns in different pairing states, as well as different kinds of impurities. The results of this calculation can be used to test and distinguish different sorts of pairing symmetries and impurities in the experiments. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B34.00012: Quasi-particle properties in iron-based superconductors Xiang Hu, Chin-Sen Ting, Jian-Xin Zhu The pairing symmetry is one of the major issues in the study of iron-based superconductors. We adopted a minimum two band model, to introduce the two-band Bogoliubov de Gennes equations. By solving those equations numerically, we checked the possibilities of different pairing symmetry, and made some predictions which can be tested by future experiments in spectrum tunnelling microscopy. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B34.00013: Nature of the SDW state in FeAs-based Compounds Xi Dai, Zhong Fang, Guangtao Wang, Gang Xu We show that the significant underestimation (about 10\%) of Fe-As bond length in FeAs-based compounds by LDA is due to the strong correlation effect. By properly taking into acount the on-site correlation, we are able to reproduce experimental values (to about 1\%) using self-consistent LDA+Gutzwiller method. Also we will show that the strong on-site orbital fluctuation will dramatically reduce the anti-ferromagnetic long range order in the parent compound. All these results are in good agreement with experiments. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B34.00014: High-temperature Superconductivity: Status John D. Dow, Dale R. Harshman, Anthony R. Fiory A theory of high-temperature superconductivity is presented which (i) explains the cuprates, with cuprate-planes; (ii) describes the superconducting ruthenates without cuprate-planes, such as Ba$_2$YRuO$_6$; (iii) treats the rutheno-cuprates, such as GdSr$_2$Cu$_2$RuO$_8$, whose cuprate-planes do not superconduct; (iv) treats the molecule $\kappa$-[BEDT-TTF]$_2$Cu [NCS]$_2$ which superconducts via S, and (v)explains the pnictides. In YBa$_2$Cu$_3$O$_7$, the theory is consistent with the observation that no Cu-containing plane superconducts and the observed superconductivity is $s$-wave, not $d$-wave, once fluxon-de-pinning has been properly accounted for. The superconducting layers are BaO layers, are $p$-type, and are adjacent to the $n$-type cuprate-planes. The theory is consistent with many data which were previously beyond explanation. [Preview Abstract] |
Session B35: Focus Session: Iron Pnictides and Other Novel Superconductors III: General Theory
Sponsoring Units: DMPChair: Alex Koshelev, Argonne National Laboratory
Room: 405
Monday, March 16, 2009 11:15AM - 11:27AM |
B35.00001: Valley density-wave (VDW) and Superconductivity in Iron-Pnictides Vladimir Cvetkovic, Zlatko Tesanovic One of the experimentally observed features of iron-pnictide superconductors is the structural transition and SDW ordering occurring at almost the same temperature. Starting from a tight-binding model [1], we construct an effective theory for iron-pnictides with the distinctive two hole and two electron Fermi surfaces. This theory is then mapped onto a negative-U Hubbard model with additional orbital and spin flavors [2]. We demonstrate that the superconducting instability of the attractive Hubbard model --- valley density-wave (VDW) --- corresponds to the observed structural and SDW orders. The deviations from perfect nesting between the hole and electron Fermi surfaces are mapped onto the Zeeman field which causes portions of Fermi surface to remain ungapped. The origin of pnictide superconductivity in this model, and its ties to the VDW are discussed. [1] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0804.4678. [2] V. Cvetkovic and Z. Tesanovic, http://arxiv.org/abs/0808.3742. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B35.00002: Spin-Density Wave in Iron Pnictides Jian Kang, Valentin Stanev, Zlatko Tesanovic Multi-band Hubbard-like model with appreciable nesting is applied to the study of spin-density wave (SDW) in iron pnictides\footnote{ V. Stanev, J. Kang, and Z. Tesanovic, Phys. Rev. B \textbf{78}, 184509 (2008).}. It is assumed that the SDW particle-hole pairing mechanism arises from the short range interaction between hole bands near $\Gamma $ point and electron bands near M. Within the Hubbard-Stratonovich transformation, an auxiliary field is introduced to obtain the effective action. The mean-field solution is obtained by the stationary phase analysis of this action, and results in an itinerant, antiferromagnetically ordered ground state, with the staggered magnetic moment modulation at wavevector M. We study fluctuations of the spin order around M, both in its direction and amplitude. We present detailed results for the propagation velocity of this mode (spin-wave velocity) as a function of the various parameters of our model and compare them to the available experimental observations of the spin-wave spectrum. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B35.00003: Iron-based superconductors: What can we learn from DFT? Lilia Boeri, Oleg Dolgov, Alexander Golubov, Ole Krogh Andersen The discovery of superconductivity in iron pnicticides has initiated an intense theoretical activity. So far, however, not only the pairing mechanism, but even the basic electronic structure of these materials is not well understood. We use Density Functional Theory to understand the electronic and vibrational properties of LaOFeAs, which can be considered a prototype for iron pnictides. First, we calculate the phonon dispersions and electron-phonon coupling using linear response and show that standard Migdal-Eliashberg theory cannot explain the experimental Tc. Then we derive ab-initio an accurate tight-binding Hamiltonian, using downfolding + N-ization (NMTO), which allows us to elucidate the origin of the complicated band structure of iron pnicticides. As a first application of our model, we study magnetism. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B35.00004: Correlations in Ferropnictides Klaus Koepernik, Helmut Eschrig The strength of correlations in the ferropnictide superconductors is still under debate. While arguments for an electron-electron interaction $U$ of $5$eV have been made, some experimental results support a $U$ of merely $1$eV. Density functional theory in the local spin density approximation (LSDA) seems to describe several aspects of the electronic structure quite reasonably, which would also support a smaller $U$. However, the unusually large error of the calculated lattice structure remains a puzzle. We discuss the influence of correlations on the electronic structure and the properties of the ferropnictides in the framework of LSDA+U calculations. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B35.00005: Superconductivity in SrFe$_{2-x}$Co$_x$As$_2$: Internal Doping of the Iron Arsenide Layers Helge Rosner, Andreas Leithe-Jasper, Walter Schnelle, Christoph Geibel In the strontium iron-cobalt arsenides SrFe$_{2-x}$Co$_x$As$_2$ ($0.2\leq x \leq 0.4$) superconductivity with $T_c$ up to 20\,K is observed in magnetic susceptibility, electrical resistivity, and specific heat data. This first observation of bulk superconductivity induced by electron doping in this family of compounds -- despite strong disorder in the Fe-As layer -- favors an itinerant electronic theory in contrast to the strongly correlated cuprates and renders a $p$- or $d$-wave pairing unlikely. The magnetic ordering present in SrFe$_2$As$_2$ is rapidly suppressed by substitution of Fe by Co. DFT calculations show that this is due to a rigid down-shift of the Fe-3$d_{x^2-y^2}$-related band edge in the density of states. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B35.00006: Linear temperature dependence of the spin susceptibility in Fe-pnictides. Dmitri V. Efremov, Andrey V. Chubukov, Ilya M. Eremin, Maxim M. Korshunov, Dmitri L. Maslov We argue that linear $T$ dependence of the spin susceptibility $\chi >T$ observed in Fe pnictides can be explained within the itinerant Fermi liquid model of hole and electron bands. The spin susceptibility is linear in $T$ in a generic Fermi liquid in 2D. We show that for pnictides, the prefactor for the $T$ term comes chiefly from intra-band scattering and is strongly enhanced compared to an ordinary Fermi liquid as it contains precisely the same interaction that gives rise to spin-density-wave ordering. We compare theoretical slope with the data. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B35.00007: Theory of novel and superconducting properties of Fe-based superconductors Invited Speaker: I will discuss antiferromagnetism and superconductivity in novel $Fe-$based superconductors within the itinerant model of small electron and hole pockets near $(0,0)$ and $(\pi,\pi)$. I will argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, {\it i.e.}, antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended $s$-wave symmetry). I will argue that the $T$ dependencies of the spin susceptibility, NMR relaxation rate, and the penetration depth for such state are exponential only at very low $T$, and can be well fitted by power-laws over a wide $T$ range below $T_c$. I will also discuss the type of a transition between spin-density-wave and superconducting states at $T=0$ and at finite $T$, and the linear $T$ dependence of the spin susceptibility in the normal state. \\ Based on the works done with I. Eremin, D. Efremov, M. Korshunov, D. Maslov, M. Vavilov, and A. Vorontsov. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B35.00008: Nodal Spin Density Wave and band topology of the FeAs based materials Hui Zhai, Ying Ran, Fa Wang, Ashvin Vishwanath, Dung-Hai Lee The recently discovered FeAs-based materials exhibit a $(\pi,0)$ Spin Density Wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two band model and a more realistic 5-band model. The positions of the nodes are different in the two models and can be used to detected the validity of each model. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B35.00009: Normal State Spin Dynamics of Five-band Model for Iron-pnictides Toshikaze Kariyado, Masao Ogata Normal state (assuming absence of SC or AF order) spin dynamics of iron-pnictide superconductors is discussed by calculating spin structure factor $S(q,\omega)$ in an itinerant five-band model within RPA approximation. Due to the characteristic Fermi surface structure of iron-pnictide, column like response is found at $(\pi,0)$ in extended Brillouin zone. This is consistent with recent neutron experiments. Furthermore, we show that the temperature dependence of inelastic neutron scattering intensity is reproduced if we set interaction parameters appropriately. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B35.00010: Symplectic fermion approach to the striped magnetism in the iron arsenides Xun Xue, Jianhui Dai Based on the fact that the near transition temperature of striped SDW and structure distortion in iron pnictides, we propose a symplectic fermion approach to account for this kind of antiferromagnetic properties. The model is expecting for better understanding of the experimental results. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B35.00011: Parquet formalism applied to pnictide superconductors Jun Liu, Karlis Mikelsons, Shuxiang Yang, Herbert Fotso , Mark Jarrell DMFT combined with Parquet approximation is used to study the single particle property of pnictide superconductors (such as FeSe, SrFe2As2,...) in an attemp to understand the enhancement of superconducitivity under pressure. By tracking the evolution of one-particle spectral function, pressure dependence of this type of compound is studied in depth. In the study, inhomogeneous frequency grid is used to high frequency summation. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B35.00012: Jahn-Teller Effect, Structural Phase Transition and Resistivity Anomaly in Iron Pnictides Weicheng Lv, Jiansheng Wu, Philip Phillips We attribute the structural phase transition (SPT) in the parent compounds of iron pnictides to a Jahn-Teller distortion. Due to the anisotropy of the $d_{xz}$ and $d_{yz}$ orbitals in the $xy$ plane, some orbital ordering will make the orthorhombic structure more energetically favorable, thus inducing the SPT. In an orbital ordered system, the sites with orbitals that do not order act as scattering impurities, causing a resistivity anomaly upon the onset of the SPT. Below the SPT, we find that the resistivity displays a $\ln{T}$ divergence. All of these are in agreement with the experiments. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B35.00013: Theory of the Magnetic Moment in Iron Pnictides Jiansheng Wu, Philip Phillips, Antonio Castro-Neto We show that the combined effects of spin-orbit, monoclinic distortion, and p-d hybridization in tetrahedrally coordinated Fe in LaFeAsO invalidates the naive Hund's rule filling of the Fe d-levels. The two highest occupied levels have one electron each but as a result of differing p-d hybridizations, the upper level is more itinerant while electrons in the lower level are more localized. The resulting magnetic moment is highly anisotropic with an in-plane value of $0.25-0.35\mu_B$ per Fe and a z-projection of $0.06\mu_B$, both of which are in agreement with experiment. [Preview Abstract] |
Session B36: Carbon Nanotubes: Devices and Applications
Sponsoring Units: DMPChair: Masa Ishigami, University of Central Florida
Room: 408
Monday, March 16, 2009 11:15AM - 11:27AM |
B36.00001: Density Functional Theory Design of All-metallic Single-wall Carbon Nanotubes Li Chen, Swastik Kar, Saroj Nayak, Pulickel Ajayan We have used density functional theory to investigate the structure and electronic properties of Pt nanocluster decoration of single-wall carbon nanotubes (SWNTs). Energy optimization shows that Pt prefers to form clusters rather than spread out and ``wet'' the SWNTs. Atom-by atom increase in the cluster size is associated with the appearance of a number of new bands in the electronic structure, especially near the Fermi level. These new bands serve to modify the density of states near the Fermi level. While metallic SWNTs remain metallic, semiconducting SWNTs lose their band-gap rapidly with the inclusion of more than 3 atoms per cluster, and continue to remain metallic for all tested cluster sizes (n=0-13 and 19). Room temperature (T=300K) calculations of conductance show that SWNTs of different chiralities (both metallic and semiconducting) remain metallic for beyond n=3, with conductance close to 4e$^{2}$/h. In some cases, the conductance is found to exceed this value. This gives an easy for designing ``all-metallic'' SWNT bundles. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B36.00002: Designing of Single Walled Carbon Nanotubes dispersions for industrial scale processing and roll-to-roll coating applications Budhadipta Dan, Matteo Pasquali Carbon nanotubes (CNTs) combine nanoscale size with high aspect ratio and unique properties, making them ideal candidate materials for high-impact applications. Yet, much as in polymer science and engg, such applications require appropriate fluid based dispersions which can undergo industrial processing that translate the properties of elemental molecules (SWNTs) into macroscopic materials. We report a detailed study on the flow behavior of aqueous SWNT dispersions involving surfactants, its dependence on SWNT {\&} surfactant concentration, and type of surfactant. We also design a SWNT dispersion for use in industrial roll-to-roll (rod based) thin film coating process. Purified, pristine SWNTs were dispersed in water at high concentrations using surfactants and analyzed using rheology and optical microscopy. A SWNT-SDBS-TritonX100 dispersion was found to have the appropriate viscoelastic and shear thinning behavior for rod coating. Rod coating, washing and sulfuric acid treatment resulted in highly uniform thin films of pure SWNT (sheet resistance of 100 and 300 $\Omega $/sq for respective transparency of 70{\%} and 90{\%}). The results presented here, both in terms of scientific understanding of how to control fluid and process, and in terms of a scalable technique, paves the way to the deployment of transparent conductive SWNT films in large scale commercial applications. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B36.00003: Stable Electron Field Emission afrom Opened-Tip Carbon Nanotube Bundles Archana Pandey, Abhishek Prasad, Jason Moscatello, Yoke Khin Yap Effective electron field emission from carbon nanotubes (CNTs) has been known for years but reliable commercial devices are still not available. Most reported works describe low emission threshold field ($E_{th})$ of CNTs and their device architectures. However, fundamental factors that determine stable emission from CNTs are still not clear. We previously reported that graphitic order of CNTs affects their emission stability [1]. Here, we found that both opened tip nanotubes and bundling, when introduced independently, can reduce $E_{th}$ of CNTs and enhance the emission stability. The combined of both factors, i.e., opened tip nanotube bundles are shown to emit electron continuously $>$ ten hours with notable stability. Theoretical simulation was conducted in supporting our explanation on these enhanced emission properties. SEM, TEM and Raman spectroscopy was conducted to characterize the as grown CNTs. Y. K. Yap acknowledges support from the Defense Advanced Research Projects Agency (DAAD17-03-C-0115, through Army Research Laboratory). [1]. Kayastha et al, Nanotechnology 18, 035206 (2007). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B36.00004: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:03PM - 12:15PM |
B36.00005: Fabricating Substrates to Combine Electron Microscopy and Diffraction with Electrical Characterization of Single and Double-Walled Carbon Nanotubes Scott Paulson, Lok-kin Tsuii, Joseph Hardcastle Carbon nanotubes based electronics make model systems for pursuing electronic devices at the nanometer scale. They are chemically robust, and have well defined easily predictable electronic structure. However, device integration requires not just an understanding of the nanotube properties, but also the properties of interfaces between neighboring elements. Ideally the structure-property relationship of the interface between two nanotubes would consist of complete electrical characterization coupled with atomic scale structural information. The former is achieved by lithographic patterning of a nanotube into a circuit, the latter through high resolution TEM imaging and diffraction. Unfortunately, typical TEM preparation of nanotubes is not compatible with lithographic processing and vice-versa. In this talk we will present a fabrication process that integrates carbon nanotubes into devices on a TEM compatible substrate. Sample devices will be shown, and preliminary data will be presented. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B36.00006: Transfer Printed Parallel Carbon Nanotube Devices Andrew Tunnell, Vinod Sangwan, Vincent Ballarotto, Daniel Hines, Michael Fuhrer, Ellen Williams Carbon nanotube (CNT) device properties can be improved by increasing the density and alignment of tubes and avoiding the problems associated with random networks. We are optimizing this approach by preparing devices composed of parallel arrays of CNTs fabricated on quartz and plastic substrates. CNT growth catalysts, ferric nitrate, ferritin and iron, are a point of control of the density and degree of alignment of the grown tubes. Though ferric nitrate produced a denser network, ferritin allows a high degree of alignment, and iron will also be tested. Plastic devices with a 5 $\mu $m channel length and a 22.5 mm width were prepared with approximately 1 channel crossing tube per 4$\mu $m of width. The density is improved by repeatedly printing more CNT's to the same area. The metallic tubes were removed by selective electrical breakdown, marginally increasing the on/off current ratio while decreasing the On current from 800$\mu $A to 450$\mu $A (at Vgs=-20V, Vds=-10V). Results from optimized devices prepared with patterned iron as the growth catalyst will also be presented. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B36.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B36.00008: Textured Network Devices: Overcoming Fundamental Limitations of Nanotube/Nanowire Network-based devices Minbaek Lee, Seunghun Hong, Meg Noah, Young-Kyun Kwon, June Park, Maeng-Je Seong Thin film devices based on single-walled carbon nanotube (swCNT) networks were extensively studied for various practical applications such as transistors, sensors, etc. However, those devices have been suffering various limitations such as poor on-off ratio due to metallic swCNTs in the networks, decreased mobility and conductance for devices with reduced linewidth due to the percolation problem, etc. Herein, we present a simple but efficient strategy to significantly improve the performance of swCNT network device by controlling the network structures. In this strategy, surface molecular patterns were utilized to prepare swCNT network-based devices with desired connectivity. We will discuss systematic study about the effect of swCNT network connectivity, as well as enhancements of on-off ratio, mobility and conductance of textured network-based transistors. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B36.00009: CMOS-analogous wafer-scale nanotube-on-insulator approach for submicron devices and integrated circuits using aligned nanotubes Koungmin Ryu, Alexander Badmaev, Chuan Wang, Chongwu Zhou Massive aligned carbon nanotubes hold great potential but also face significant integration / assembly challenge for future beyond-silicon nanoelectronics. We report our recent advance on full wafer-scale processing of massively aligned carbon nanotube arrays for high performance submicron channel transistors and integrated nanotube circuits, including the following essential components. 1) The massively highly aligned nanotubes were successfully grown on 4 inch quartz and sapphire wafers via meticulous temperature control, and then transferred onto Si/SiO$_{2}$ wafers using our facile transfer printing method. 2) Wafer-scale device fabrication was performed on 4 inch Si/SiO$_{2}$ wafer to yield submicron channel transistors and circuits with high on-current density $\sim $ 20 $\mu $A/$\mu $m and good on/off ratio. 3) Chemical doping methods were successfully demonstrated to get CMOS inverters with a gain $\sim $5. 4) Defect-tolerant circuit design for NAND and NOR was proposed and demonstrated to guarantee the correct operation of logic circuit, regardless of the presence of mis-aligned or mis-positioned nanotubes. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B36.00010: A Novel Fabrication Method for Pristine Suspended Carbon Nanotube Devices Vinod Sangwan, Vincent Ballarotto, Michael Fuhrer, Ellen Williams A simple and scalable method has been developed to fabricate suspended carbon nanotube (CNT) field effect transistors using as-grown CNTs without subsequent chemical processing.\footnote{\textit{V. K. Sangwan} et al, APL \textbf{93}, 113112 (2008)} A printing process is used to transfer CVD-grown CNTs onto specially configured electrode (Au) sets fabricated on SiO$_{2}$. The versatility of the technique is demonstrated by controlling the number of suspended CNTs per device, and by re-using the same electrode set multiple times to produce the desired device characteristics. The quality of suspended CNTs is characterized by electrical transport as well as 1/f noise measurements. Standard resist-processed CNTs on SiO$_{2}$ substrates show p-type behavior and strong hysteresis associated with doping by the SiO$_{2}$ surface and charge trapping in the SiO$_{2}$, respectively. In contrast, suspended CNTs show ambipolar behavior with negligible hysteresis. Low frequency noise measurements on suspended CNT show 1$/f$ behavior with Hooge's constant 2.6 x 10$^{-3}$, around 20 times less than that of CNTs lying on SiO$_{2}$, consistent with reduced effect of the SiO$_{2}$ charge traps, responsible for the bulk of the noise in CNTs on SiO$_{2}$. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B36.00011: As-Grown Single-Walled Carbon Nanotube Diodes Govind Mallick, Shashi Karna, Sarah Lastella, Sangeeta Sahoo, Pulickel Ajayan We present the observation of unidirectional electric current through as-grown single-walled carbon nanotubes (SWNTs) grown by catalytic chemical vapor deposition (CCVD) process. Long strands of as-grown SWNTs were utilized to fabricate multiple arrays of switching devices with the channel length of 3, 5, 7 and 10 $\mu $m on a 15 mm x 15 mm SiO$_{2}$ on Si substrate. Of the fabricated devices, $\sim $ 34{\%} exhibited electrical activity. Of the active devices, about 70{\%} exhibited diode-like unidirectional current, not observed previously in CCVD grown SWNTs. High resolution atomic force microscopic (AFM) analysis of the device structure and surface topology of SWNTs suggests the observed unidirectional current to result from surface irregularities and change in the chirality along the tube axis. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B36.00012: Gate Controlled Negative Differential Resistance and Photoconductivity Enhancement in Carbon Nanotube Intra-connects Seon Woo Lee, Slava Rotkin, Andrei Sirenko, Haim Grebel Field effect transistors were fabricated using carbon nanotubes (CNT). Gate-controlled, N-shaped negative differential resistance (NDR) has been demonstrated. In addition, a large photoconductance effect was associated with the NDR. The intra-connects -- bridges spanning across planar electrodes and contain individual tube or in a small bundle -- were grown using chemical vapor deposition (CVD) precisely between very sharp metal tips on the pre-fabricated electrodes. NDR was observed for intra-connects exhibiting either, ohmic or, non-ohmic contacts. Yet, the enhanced photoconductivity was more pronounced for intra-connects exhibiting ohmic contact at zero gate bias. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B36.00013: Magnetic Carbon Nanotubes: Materials Development and Property Characterization Dereje Seifu, Shashi Karna A versatile chemical method was used to fill multi-wall carbon nanotubes (MWCNTs) with ferromagnetic nanoparticles [1]. For the first time, pulsed laser deposition and magnetron DC sputtering were used to fill vertically aligned MWCNTs. The later approaches gave high-yield nanoparticle filling of MWCNTs. Samples were characterized by Electron Microscopy, Energy Dispersive Spectroscopy, M\"{o}ssbauer Spectroscopy, and magnetization measurements. M\"{o}ssbauer measurements on chemically impregnated MWCNTs clearly show the presence of atomic Fe as well as mixed phases of Fe nano-particles inside the tubes. Magnetization measurements on PLD-filled vertically aligned MWCNTs indicate reasonable coercivity. However, the magnetic anisotropy appears to be randomly oriented, suggesting polycrystalline sample. \newline Acknowledgement: The research at Morgan State University was partially supported by the US ARL-WMRD (W1813LT-5006-7056). \newline [1] D. Seifu, Y. Hijji, G. Hirsch, and S. P. Karna, \textit{J. Magn. Magn. Mat.} \textbf{320}, 312 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B36.00014: Electrical and Thermal Properties of Orientated Multiwall Carbon Nanotube Bulk Materials Keqin Yang, Jian He, Zhe Shu, Apparao Rao Millimeter long vertically oriented multiwall carbon nanotube (MWNT) arrays with typical tube diameter around 30-50 nm were grown on Si substrates using thermal chemical vapor deposition. The arrays were realigned and densified using a spark plasma sintering process to form oriented MWNT bulk samples. Electron microscopy studies on the as-prepared bulk samples corroborate that the MWNTs are fairly well aligned and the pristine tubular morphology of the MWNTs is preserved during the sintering. The temperature dependent electrical, thermopower and thermal conductivity measurements were performed along different directions relative to the preferred orientation of MWNTs. In particular, the longitudinal and transverse thermal conductivity at 300 K are found to be about 35 W/(mK) and 1 W/(mK), respectively. In the temperature regime between 10 -- 300 K, the electrical resistivity is on the order of few m$\Omega $cm and exhibits a thermal excitation type temperature dependence, while the Seebeck coefficient is on the order of few uV/K and exhibits a weak temperature dependence. These results give new insights into the unique electrical and thermal transport mechanisms in and between MWNTs. [Preview Abstract] |
Session B37: Focus Session: Fundamental Developments in Density Functional Theory II
Sponsoring Units: DCPChair: Paula Mori-Sanchez, Duke University
Room: 409
Monday, March 16, 2009 11:15AM - 11:51AM |
B37.00001: Phase-Space Explorations in Time-Dependent Density Functional Theory Invited Speaker: Time-Dependent Density Functional Theory is increasingly popular for calculating excitation and response properties of atoms, molecules, clusters and solids. It has achieved an unprecedented balance between accuracy and efficiency for a wide range of systems, but not all. Although not limited to the linear response regime, there are particular challenges for applications to strongfield processes; for example obtaining momentum distributions, certain electronic quantum control problems, and including memory-dependence necessary in the functional dependence. In this talk we will discuss some of these, and introduce a new extension of the theory where the basic variable is the phase-space density W(r,p,t) (that contains information on both the co-ordinate- and momentum distributions of the electrons), instead of the usual co-ordinate space density n(r,t), to deal with these challenges. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B37.00002: Time-dependent density-functional theory for electronic excitations in materials Invited Speaker: There is currently an intense effort underway to study the optical properties of bulk and nanostructured materials using time-dependent density-functional theory (TDDFT). This talk will discuss challenges and recent advances of TDDFT in this area, and present some new applications to excitonic effects in bulk insulators and to collective charge- and spin-density excitations in doped quantum wells. A TDDFT version of the semiconductor Bloch equations is presented, which describes ultrafast electron dynamics, including excitonic effects, in insulators and semiconductors. From this, an excitonic Wannier equation is derived featuring a nonlocal effective electron-hole interaction determined by long-range exchange-correlation effects. Excitonic binding energies are calculated for several direct-gap insulators. The spin Coulomb drag (SCD), which constitutes an intrinsic source of dissipation for spin currents in metals and semiconductors, originates as a dynamical exchange-correlation effect in time-dependent current-DFT. We develop a linear-response description of collective spin-density excitations in quantum wells including SCD as well as Rashba and Dresselhaus spin-orbit coupling, and show that spin plasmon line widths in quantum wells allow a purely optical, quantitative measurement of the SCD effect. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B37.00003: Many-Pole Self-Energy Model Corrections to Kohn-Sham Calculations of Excited State Spectra J. J. Kas, M. Prange, J. J. Rehr, H. M. Lawler Experimental x-ray spectra are systematically shifted and broadened with respect to conventional density functional theory calculations due to photoelectron self-energy effects. We have recently developed an efficient many-pole model of the GW self-energy based calculations of dielectric responce using a real-space Green's function approach.\footnote{J.J. Kas et al., Phys. Rev. B {\bf 76}, 195116 (2007).} The model is applied \textit{a posteriori} to Kohn-Sham calculations of excited state spectra using a convolution of the spectrum with an energy dependent Lorenzian. The method is found to be widely applicable over a broad range of energies, with little computational cost. Several illustrative examples are presented which show improved agreement between theoretical calculations and experiment for both optical and x-ray spectra. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B37.00004: First principle calculations of long range correlation effects within the random phase approximation Deyu Lu, Yan Li, Hugh Wilson, Giulia Galli The local and semi-local approximations to Density Functional Theory fail to describe correctly certain types of weak interactions (e.g. van der Waals forces) due an incorrect account of long range correlation effects. Such effects may be described by computing correlation energies within the random phase approximation (RPA), using the fluctuation-dissipation theorem and the adiabatic connection. We present an approach to compute RPA correlation energies based on an eigenmode expansion of the dielectric matrix [1,2]. By solving the frequency dependent Sternheimer equation within linear response theory [3], we eliminate the need to compute single particle unoccupied states, which makes our approach more efficient than methods using the direct-summation technique. Furthermore, the use of a dielectric eigenmode representation allows for a physical interpretation of several, different contributions to correlation energies. Results for graphite and the benzene crystal will be discussed. [1] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B, 78:113303, (2008). [2] D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett., 100:147601(2008). [3] S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73:515, (2001). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B37.00005: Tailoring High-Order Harmonics: A Computational Approach Based on Time-Dependent Density-Functional Theory Alberto Castro, Ali Akbari, Angel Rubio, Eberhard Gross Atoms and molecules react in complex manners when they are irradiated with high-intensity electromagnetic pulses: multi-photon, tunnelling and over-the-barrier ionisation, laser driven photo-induced isomerisations or fragmentations, and high harmonic generation are some of the non-linear effects that are observed. The so-called pulse shaping techniques can be used to design pulses that produce a desired effect. A technologically appealing possibility is to tailor the harmonic emission spectrum: enhancement of some given orders, supressions of others, etc. We have undertaken the task of exploring this possibility from a theoretical point of view, by making use of time-dependent density-functional theory to describe the electrons, a real-space numerical representation, and various optimization techniques. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B37.00006: Ab initio calculations of optical spectra by solving the Bethe-Salpeter equation without empty states.Work Dario Rocca, Deyu Lu, Giulia Galli We present a novel first principle approach to solve the Bethe-Salpeter equation (BSE) that builds on recent progress in time-dependent density functional perturbation theory [1], and uses an eigenvalue decomposition representation of the dielectric matrix [2]. This approach does not require the explicit calculation of excited single particle electronic states, making it suitable for calculations involving large basis sets and/or a large number of transitions. The numerical solution of the BSE is obtained through a generalized, non-Hermitian Lanczos iterative algorithm and does not require the use of the Tamm-Dancoff approximation. Furthermore, since Lanczos coefficients are frequency independent, optical spectra may be obtained in a very broad energy range. The efficiency and accuracy of the new approach are demonstrated by calculating the optical properties of silicon nanoclusters with up to 1 nm diameter. [1] D. Rocca, R. Gebauer, Y Saad, and S. Baroni, J. Chem. Phys. 128, 154105 (2008). [2] H.Wilson, F.Gygi and G.Galli, Phys. Rev. B 78, 113303 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B37.00007: Time-dependent transition densities Yonghui Li, Carsten A. Ullrich To visualize and interpret the induced charges and electron-hole coherences of electronic excitations in molecules, a real-space density matrix analysis is a useful computational tool. We extend this technique into the nonlinear, real-time domain and define the time-dependent transition densities in the context of time-dependent density-functional theory. This opens up the possibility of a real-time monitoring of the optical excitation dynamics in molecules, providing a visualization tool for processes such as exciton migration or charge-transfer excitations. The method will be illustrated for simple one-dimensional model systems. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B37.00008: Is the Kohn-Sham Oscillator Strength Exact at the Ionization Threshold? Zenghui Yang, Meta van Faassen, Kieron Burke It is well-established that the highest occupied orbital of the exact Kohn-Sham potential of any system is at -I, where I is the ionization energy. Therefore, in optical response, the non-interacting Kohn-Sham electrons in the ground-state potential have a first ionization threshold that exactly matches that of the real system[1]. We show that corresponding the Kohn-Sham oscillator strength is not exact at the first ionization threshold by explicit demonstration for the helium atom. We use a simple fit of the entire photoabsorption spectrum of both the Kohn-Sham potential for helium and that of real helium. We use oscillator strength sum rules[2] to determine the fit parameters, so this fit should be generally useful. [1] M. A. L. Marques, C. A. Ullrich, F. Nogueira, et al. Time-Dependent Density Functional Theory. Springer-Verlag, Berlin, 2006 [2] U. Fano and J. W. Cooper. Rev. Mod. Phys., 40(3), 441-507, 1968 [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B37.00009: An Exact Condition for the Integrand of Adiabatic Connection Zhenfei Liu, Kieron Burke In density functional theory (DFT), the exchange-correlation functional $E_{\rm XC}$ can be exactly expressed by the adiabatic connection integral [1,2]. The integrand should satisfy several exact conditions [3]. We show that for the low-density limit (as $\lambda \to \infty$), the $\lambda^{-1}$ term in the expansion of the integrand $W(\lambda)$, should vanish. We propose a simple parametric form for $W(\lambda)$, satisfying the new exact condition. We apply this interpolation form to Hooke's atom and helium atom and show that it is accurate for weakly-correlated two-electron systems. \\[3pt] [1] D.C. Langreth and J.P. Perdew, Solid State Commun. {\bf 17}, 1425 (1975). \\[0pt] [2] O. Gunnarsson and B.I. Lundqvist, Phys. Rev. B {\bf 13}, 4274 (1976). \\[0pt] [3] M. Seidl, J.P. Perdew and M. Levy, Phys. Rev. A {\bf 59}, 51 (1999). [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B37.00010: New exact and approximate forms for the Luttinger-Ward correlation energy functional within the GW-RPA approximation Sohrab Ismail-Beigi In principle, many-body Green's function approaches to electronic systems such as the Luttinger-Ward formalism allow one to compute both total energies and quasiparticle excitation spectra (i.e. band structures) simultaneously from first principles. We report on two new results that reformulate the Luttinger-Ward correlation energy functional within the GW-RPA approximation. A first expression is exact and allows for systematic and straightforward evaluation of correlation energies. The second expression is approximate but yields a family of computationally efficient approximations to the correlation energy and the self-energy operator of which the well known Coulomb-hole and screened-exchange (COHSEX) approximation is the lowest order. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B37.00011: Non-empirical hyper-generalized-gradient functionals constructed from the Lieb-Oxford bound Mariana M. Odashima, Klaus Capelle A simple and completely general representation of the exact exchange-correlation functional of density-functional theory is derived from the universal Lieb-Oxford bound for Coulomb-interacting systems. This representation leads to an alternative point of view on popular hybrid functionals. A similar representation of the exact correlation functional allows to construct a family of non-empirical hyper-generalized-gradient approximations (HGGAs), departing from established paradigms of functional construction. Numerical tests and applications of these HGGAs to atoms and molecules demonstrate that even simple Lieb-Oxford based HGGAs are competitive with correlation functionals currently used in solid-state physics and quantum chemistry. [Preview Abstract] |
Session B38: Focus Session: The Chemical Physics of Biological and Biologically-inspired Solar Energy Harvesting II
Sponsoring Units: DCPChair: Bern Kohler, Ohio State University
Room: 410
Monday, March 16, 2009 11:15AM - 11:51AM |
B38.00001: Coherence and Decoherence in the Excited States of Light Harvesting Complexes Invited Speaker: |
Monday, March 16, 2009 11:51AM - 12:27PM |
B38.00002: Electronic coherence in electronic energy transfer despite fast dephasing Invited Speaker: F\"{o}rster resonance energy transfer (FRET) is a common and fundamental photophysical process in life and materials sciences. FRET is an interchromophore relaxation process that transmits the electronic excitation from an initially excited donor to a ground state acceptor chromophore (light-absorbing moleule). FRET is used, for example, to harvest light in photosynthesis, measure distances in proteins, and it accelerates the photodegradation of polymers. In recent years attention has turned to the study of FRET in complex assemblies of molecules. While F\"{o}rster theory has enabled the efficiency of FRET to be predicted and analyzed in numerous and diverse areas of study, recent work has aimed to discover ways beyond the F\"{o}ster mechanism by which electronic energy can be transferred. The talk will compare and contrast theoretical and experimental studies of excitation relaxation in photosynthetic antenna systems with the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylenevinylene] (MEH-PPV). I will report new work where we have used a new anisotropy experiment to examine coherent energy transfer and a complementary technique using two-dimensional electronic spectroscopy expose the role of coherence transfer in the fastest time dynamics. We find that coherent energy transfer occurs for many tens of femtoseconds, even at room temperature. That leads us to examine the nature and implications of the so-called intermediate coupling regime for EET. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B38.00003: Photosynthetic nanoparticle complexes Alexander Govorov We investigate structures composed of a photosynthetic molecule and a semiconductor (metal) nanoparticle [1]. The rate of optical generation of electron--hole pairs inside a photosynthetic system can be greatly increased through conjugation with nanoparticles. In the case of a semiconductor nanoparticle, the enhancement effect comes from the essentially larger optical absorption cross-section of a semiconductor nanoparticle compared to a photosynthetic system. In this hybrid complex, excitons are transferred via the Forster mechanism to the photosynthetic system, where charge separation takes place. For metal nanoparticles conjugated with a photosynthetic system, we predicted a strong enhancement effect due to the plasmon resonance. Such an enhancement effect was recently observed at Munich U. [2]. In summary, we have shown that one can use crystalline nanoparticles to create a 10-fold enhancement of the initial stage of photosynthesis, i.e. the absorption process. Potential applications of nanocrystal complexes are in light-harvesting. [1] A. O. Govorov and I. Carmeli, Nano Lett. \textbf{7}, 620 (2007); A. O. Govorov, Adv. Materils, online, DOI: 10.1002/adma.200702999. [2] S.Mackowski, S. W\"{o}rmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br\"{a}uchle, Nano Lett.~\textbf{8}, 558 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B38.00004: Quantum coherence, decoherence and entanglement in light harvesting complexes Martin Plenio, Filippo Caruso, Alex Chin, Animesh Datta, Susana Huelga Transport phenomena in networks allow for information and energy to be exchanged between individual constituents of communication systems, networks or light-harvesting complexes. Environmental noise is generally expected to hinder transport. Here we show that transport of excitations across dissipative quantum networks can be enhanced by dephasing noise. We identify two key processes that underly this phenomenon and provide instructive examples of quantum networks for each. We argue that Nature may be routinely exploiting this effect by showing that exciton transport in light harvesting complexes and other networks benefits from noise and is remarkably robust against static disorder. These results point towards the possibility for designing optimized structures for transport, for example in artificial nano-structures, assisted by noise. Furthermore, we demonstrate that quantum entanglement may be present for short times in light-harvesting complexes. We describe how the presence of such entanglement may be verified without the need for full state tomography and with minimal model assumptions. This work is based on M.B. Plenio \& S.F. Huelga, New J. Phys. 10, 113019 (2008) and F. Caruso, A. Chin, A. Datta, S.F. Huelga \& M.B. Plenio, in preparation [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B38.00005: Non-radiative decay processes in InAs nanocrystals Marco Califano The mechanisms governing excited state relaxation in semiconductor nanocrystals (NCs) are still not well understood. The validity of the Auger electron cooling and multiexciton recombination hypotheses, which would explain much of the experimental data available to date, has recently been questioned. Moreover the recent observation of sub-picosecond electron relaxation times and biexciton recombination rates of the order of 0.1-1 ps$^{-1}$ in InAs, although qualitatively ascribed to Auger processes, still awaits a quantitative theoretical interpretation. Multiexciton recombination is particularly important as its signatures are used to detect and quantify carrier multiplication efficiency in NCs. Furthermore efficient non-radiative (multi-) exciton decay represents a major obstacle for application of NCs in lasing and photovoltaics. A quantitative theoretical understanding of these processes is therefore critical for any technological implementation of quantum-dot-based devices. The results of a detailed investigation using the pseudopotential method provide an explanation of the observed lifetimes in terms of Auger-like decay mechanisms, supporting the Auger interpretation of excited state relaxation in NCs. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B38.00006: Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers Kwiseon Kim, Muhammet Erkan Kose, Peter Graf, Nikos Kopidakis, Garry Rumbles, Sean E. Shaheen Phenyl cored 3-arm and 4-arm thiophene dendrimers are promising materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules to guide the synthesis of novel dendrimers with improved efficiency. A method is developed to estimate the exciton diffusion lengths for the dendrimers and similar chromophores in amorphous films. The approach exploits Fermi's Golden Rule to estimate the energy transfer rates for an ensemble of bimolecular complexes in random orientations. Using Poisson's equation to evaluate Coulomb integrals led to efficient calculation of excitonic couplings between the transition densities. Monte-Carlo simulations revealed the dynamics of energy transport in the dendrimers. Experimental exciton diffusion lengths of the dendrimers range 10 $\sim $ 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with experiments. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors that determine the exciton diffusion length in amorphous films. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B38.00007: Efficient and Long-lived Charge Separation in a Heteroleptic Ruthenium(II) Polypyridyl Complex Joseph Henrich, Haoyu Zhang, Jeremy White, Prabir Dutta, Bern Kohler The excited-state dynamics of a tris-bidentate mononuclear ruthenium(II) complex, [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ (where bpy = bipyridine, L$_{DQ}$ = 1-[4-(4'-methyl)-2,2'-bipyridyl)]-2-[4-(4'-N,N'-tetramethylene-2,2'-bipyridinium]) was investigated by femtosecond transient absorption spectroscopy in bulk solution and tethered to a zeolite nanocrystal. [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ is a promising photosensitizer molecule for artificial photosynthesis. Broadband transient absorption experiments in bulk acetonitrile solution reveal that excitation of the MLCT absorption band transfers an electron within one picosecond from the metal center to the bipyridinium (DQ) ligand. Back electron transfer then takes place with a time constant of 1.45 ns. Highly efficient charge separation is attributed to the conjugated nature of the bipyridinium-terminated ligand. When [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ is tethered to a zeolite Y particle, charge can be transferred to a methyl viologen molecule encapsulated in the zeolite. Zeolites are promising materials for solar energy conversion because of their ability to slow rates of charge recombination. The effects of the zeolite on the photoprocesses of the ruthenium polypyridyl complex will be presented. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B38.00008: First-Principles Studies of Single-Molecule Photovoltaics Peter Doak, R. A. Segalman, T. D. Tilley, J. B. Neaton Organic photovoltaics consist of electron donor and acceptor polymers or molecules blended together, and are promising inexpensive, lightweight alternatives to conventional silicon solar cells. However, many of the physical processes responsible for their poor efficiencies are not well understood. Here, using first-principles calculations based on density functional theory, including self-energy corrections within the GW approximation and a discussion of excitonic effects, we examine the relationship between molecular structure and electronic level alignment at a covalent donor-acceptor interface. We consider small asymmetric molecules subdivided into discrete covalently linked moieties based on thiophene, tetrafluorobenzene, pyridine, and durene. Excited states of each of these moieties, as well as their covalently-linked combinations, are computed and discussed in the context of their ability to absorb photons and separate charge. Work supported in part by the DOE Helios SERC. Computational resources provided by NERSC. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B38.00009: Intramolecular Interactions in Novel Macrocyclic Materials Theodore Goodson In this presentation I will report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. Strong cooperative enhancement of two-photon absorption cross-section was observed when going from the trimer arrangement to the next generation. Combination of a variety of femtosecond spectroscopy methods such as femtosecond time-resolved fluorescence upconversion, transient absorption, transient grating, three pulse photon echo peak shift experiments complemented with those of steady state spectroscopy allowed us to compare the properties of absorption states with those of fluorescence states, to estimate the reorganization energies, and the extent of inhomogeneous broadening. Our measurements indicated that spectroscopic unit (domain) is different for the trimer system and for the dendrimers of higher generation numbers. This coherent domain extends over the trimer geometry and its size is comparable with the size of the dendrimer G1 comprising nine linear segments. We have also investigated the novel applications of a two-dimensional carbon network structure's building blocks. The material shows very interesting two-photon absorption properties as well as strongly coupled optical excitations. They have also been suggested as good building blocks for molecular electronics applications. [Preview Abstract] |
Session B39: Cellular Biomechanics II
Sponsoring Units: DBPChair: Arpita Upadhyaya, University of Maryland
Room: 411
Monday, March 16, 2009 11:15AM - 11:27AM |
B39.00001: Fibroblast motility on substrates with different rigidities: modeling approach Maria Gracheva, Irina Dokukina We develop a discrete model for cell locomotion on substrates with different rigidities and simulate experiments described in Lo, Wang, Dembo, Wang (2000) ``Cell movement is guided by the rigidity of the substrate'', Biophys. J. 79: 144-152. In these experiments fibroblasts were planted on a substrate with a step rigidity and showed preference for locomotion over stiffer side of the substrate when approaches the boundary between the soft and the stiff sides of the substrate. The model reproduces experimentally observed behavior of fibroblasts. In particular, we are able to show with our model how cell characteristics (such as cell length, shape, area and speed) change during cell crawling through the ``soft-stiff'' substrate boundary. Also, our model suggests the temporary increase of both cell speed and area in that very moment when cell leaves soft side of substrate. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B39.00002: Locomotion of C. elegans through jammed granular media Kevin Lu, Paulo E. Arratia It is quantitatively demonstrated in this experiment on the undulatory swimming of $C.$ (\textit{Caenorhabditis}) elegans that, in a highly-resistive media, the animal only executes beating frequencies and amplitudes in discrete values. This behavior of $C.$ elegans is inferred from the peaks in the particle velocity distributions where the most probable velocities match the transverse velocities of the nematode body. The behavior in the velocity distribution is more pronounced for particles in denser arrangements and for those closer to the thrashing gait of the worm. These results contribute to the existing data on the worm locomotion and further facilitate the identification of the endogenous genes and neural circuitry to the exogenous behavioral responses of $C. $elegans. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B39.00003: Mechanics of an Ultrafast Cellular Contraction Gaurav Misra, Richard B. Dickinson, Tony Ladd \textit{Vorticella Convallaria} is one of a class of fast-moving organisms, traversing its body size in less than a millisecond. It has two main parts, the cell body and a stalk, which attaches the cell body to the substrate. The stalk houses a slender, elastic structure called Spasmoneme, which winds helically inside the stalk and generates a strong tensile force in response to Calcium signaling. We are developing numerical simulations of the collapsing stalk to quantify the magnitude and time scale of the force generation. We have coupled a Kirchhoff model of an elastic rod (representing the stalk) with an embedded helically wound filament (representing the Spasmoneme). Contraction of this assembly is driven by a constant velocity Calcium signal that induces a state of tension in the Spasmoneme. Depending on the speed of the Calcium signal, we observe different mechanical responses from the contracting stalk, which we compare with experimental observations. We follow the interplay of contraction, twist and bend to explain some unexpected features of the retraction process. Two different macroscopic models have been proposed to explain the time-dependent velocity of the cell body; we compare the predictions of these models with the dynamics revealed by our filament model. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B39.00004: Polymer Microlenses for Quantifying Cell Sheet Mechanics Guillaume Miquelard-Garnier, Jessica Zimberlin, Patricia Wadsworth, Alfred Crosby Mechanical interactions between individual cells and their substrate have been studied extensively over the past decade; however, our understanding of how these interactions change as cells interact with neighboring cells in the development of a cell sheet, or early stage tissue, is less developed. We present a recently developed experimental technique for quantifying the mechanics of confluent cell sheets (Zimberlin J.A., et al., Cell Motility and the Cytoskeleton, 65, 9, 762). Living cells are cultured on a thin film of polystyrene [PS], which is attached to a patterned substrate of crosslinked poly(dimethyl siloxane) microwells. As the cell sheet grows, cells apply sufficient force to buckle the PS film over individual microwells to form a microlens array. The curvature for each microlens is measured by confocal microscopy and can be related to the strain and stress applied by the cell sheet. We demonstrate that this technique can be used to decouple mechanical contributions of intercellular junctions and focal adhesions while also providing insight into the important materials properties and length scales that govern cell sheet responses. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B39.00005: Why is Actin Patchy? Anders Carlsson The intracellular protein actin, by reversibly polymerizing into filaments, generates forces for motion and shape changes of many types of biological cells. Fluorescence imaging studies show that actin often occurs in the form of localized patches of size roughly one micrometer at the cell membrane. Patch formation is most prevalent when the free-actin concentration is low. I investigate possible mechanisms for the formation of actin patches by numerically simulating the ``dendritic nucleation'' model of actin network growth. The simulations include filament growth, capping, branching, severing, and debranching. The attachment of membrane-bound activators to actin filaments, and subsequent membrane diffusion of unattached activators, are also included. It is found that as the actin concentration increases from zero, the actin occurs in patches at lower actin concentrations, and the size of the patches increases with increasing actin concentration. At a critical value of the actin concentration, the system undergoes a transition to complete coverage. The results are interpreted within the framework of reaction-diffusion equations in two dimensions. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B39.00006: Direct dynamical measurement of the cytoskeletal contribution to the adhesion and mechanics of living cells Marie-Josee Colbert, Cecile Fradin, Kari Dalnoki-Veress The cytoskeleton is involved in the interaction of the cell with its surroundings through adhesion and the elastic response of the cell. To dynamically probe these properties, we have developed a new tool that takes advantage of an `L' shaped micropipette to micromanipulate a single cell and put it in contact with an adhesive surface mounted on a translation stage. The spring constant of the micropipette is carefully measured and its deflection is used to apply a calibrated force. This technique gives access to real time monitoring of the cell response to an applied deformation, thus exploring the relaxation processes of the cell when subjected to an external load. The polymerization of actin and microtubules is prevented to explore the cytoskeletal contribution to the processes involved in the interaction with the substrate, such as the elastic response and adhesion. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B39.00007: Force generated by polymerization of actin filaments: an entropic role? Jean Baudry, Coraline Brangbour, Olivia du Roure, Emmanu\`ele Helfer, Marc Fermigier, Paul M. Chaikin, Marie-France Carlier, Jerome Bibette Actin polymerization drives protrusions at the cell surface and leads to cell motility. Using magnetic colloids, we measure how the chemical reaction of polymerization generates mechanical forces. Rapid force- distance measurement gives us access to the filaments organisation between colloids, whereas long experiments at constant forces give the force- velocity relation of growing actin filaments. A simple model based on entropic forces seems to explain our observations. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B39.00008: Cytoskeleton mediated spreading dynamics of immune cells King-Lam Hui, Jessica Wayt, Brian Grooman, Arpita Upadhyaya We have studied the spreading of Jurkat T-cells on anti-CD3 antibody-coated substrates as a model of immune synapse formation. Cell adhesion area versus time was measured via interference reflection contrast microscopy. We found that the spread area exhibited a sigmoidal growth as a function of time in contrast to the previously proposed universal power-law growth for spreading cells. We used high-resolution total internal reflection fluorescence microscopy of these cells transfected with GFP-actin to study cytoskeletal dynamics during the spreading process. Actin filaments spontaneously organized into a variety of structures including traveling waves, target patterns, and mobile clusters emanating from an organizing center. We quantify these dynamic structures and relate them to the spreading rates. We propose that the spreading kinetics are determined by active rearrangements of the cytoskeleton initiated by signaling events upon antibody binding by T-cell receptors. Membrane deformations induced by such wavelike organization of the cytoskeleton may be a general phenomenon that underlies cell movement and cell-substrate interactions. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B39.00009: How deep cells feel: Mean-field Computations and Experiments Amnon Buxboim, Shamik Sen, Dennis E. Discher Most cells in solid tissues exert contractile forces that mechanically couple them to elastic surroundings and that significantly influence cell adhesion, cytoskeletal organization and differentiation. However, strains within the depths of matrices are often unclear and are likely relevant to thin matrices, such as basement membranes, relative to cell size as well as to defining how far cells can ``feel.'' We present experimental results for cell spreading on thin, ligand- coated gels and for prestress in stem cells in relation to gel stiffness. Matrix thickness affects cell spread area, focal adhesions and cytoskeleton organization in stem cells, which we will compare to differentiated cells. We introduce a finite element computation to estimate the elastostatic deformations within the matrix on which a cell is placed. Interfacial strains between cell and matrix show large deviations only when soft matrices are a fraction of cell dimensions, proving consistent with experiments. 3-D cell morphologies that model stem cell-derived neurons, myoblasts, and osteoblasts show that a cylinder-shaped myoblast induces the highest strains, consistent with the prominent contractility of muscle. Groups of such cells show a weak crosstalk via matrix strains only when cells are much closer than a cell-width. Cells thus feel on length scales closer to that of adhesions than on cellular scales. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B39.00010: Tether extrusion from biomimetic cells Karine Guevorkian, L\'ea Laetitia Pontani, C\'ecile Sykes, Fran\c{c}oise Brochard-Wyart The plasma membrane of a cell is coupled to its underlying cytoskeleton through membrane binding proteins. By pulling membrane tethers, one can measure the strength of these attachments and also probe the rheology of the membrane. In the past, we have used the hydrodynamic tether extrusion technique to study tether dynamics of Red Blood Cells [1]. To describe the non-linear force-velocity behavior at high extrusion forces, we have developed a theoretical model based on lipid permeation through the network of membrane binding proteins [2]. To test this model, we use a biomimetic system consisting of liposomes encapsulating an actin cortex in which the density of membrane-cytoskeleton linkers can be controlled. Here we will present our recent experimental results and compare them to the theoretical predictions. [1] N. Borghi et al, Biophys. J. 93 (2007) [2] F. Brochard-Wyart, et al, Proc. Natl. Acad. Sci. USA, 103 (2006) [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B39.00011: Mechanics of Nascent Cell Adhesions Cecile O. Mejean, Andrew W. Schaefer, Paul Forscher, Eric R. Dufresne Cells have the ability to sense and respond to mechanical and biochemical cues from their environment. In neurons, the binding and restraint of transmembrane cell adhesion molecules (CAMs) can trigger acute periods of axon growth. Preceding growth, the cell must create a stiff mechanical linkage between the CAM and the cytoskeleton. Using holographic optical tweezers, we manipulate CAM-coated beads on the membrane of the cell. We investigate the dynamics of the mechanical properties of this linkage as a function of time, applied force, and CAM density. We find that CAM-coated beads exhibit stochastic intermittent binding to the cytoskeleton. In time, we observed that the adhesions stiffen and their mechanical properties depend on the applied force. Treatment of cells with small molecules that alter cytoskeletal dynamics are used to probe the roles of actin filament assembly and myosin motor activity in adhesion formation. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B39.00012: Epithelial Mechanics during Germband Retraction in Fruit Fly Embryogenesis Xiaoyan Ma, Holley E. Lynch, M. Shane Hutson During germband retraction in the early embryonic development of fruit fly embryos, the epithelial cells of the amnioserosa (AS) undergo a dramatic change in cell shape. The average cell aspect ratio reduces from $\alpha $ $\sim $10 to $\sim $1 within three hours. We performed laser hole-drilling and confocal microscopy to investigate the mechanics of this process in live fly embryos. We find that the laser-induced recoil dynamics of AS cells during germband retraction (when $\alpha \quad \sim $10) is dramatically different from that during the later dorsal closure stage (when $\alpha $ $\sim $1). First, in the earliest stage of germband retraction, some AS cells actually shrink instead of expand in the first one second after ablation. After this point, the cells do slowly expand. Second, in either phase, the cell speeds were much slower, in the range of $\pm $ 1 $\mu $m/s (compared with speeds in excess of 10 $\mu $m/s during dorsal closure). Theses results suggest a much smaller tensile (and in some cases, compressive) stress in the whole cell sheet in early germband retraction. As retraction proceeds towards dorsal closure, the stresses increase. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B39.00013: Matrix elasticity directs stem cell differentiation in 3D too Allison Zajac, Florian Rehfeldt, Dennis Discher Microenvironments appear important in stem cell lineage specification but can be difficult to adequately characterize or control with soft tissues. Naive mesenchymal stem cells (MSCs) are shown here to specify lineage andcommit to phenotypes with extreme sensitivity to tissue level elasticity. Soft matrices that mimic brain are neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. During the initial week in culture, reprogramming of these lineages is possible with addition of soluble induction factors, but after several weeks in culture, the cells commit to the lineage specified by matrix elasticity, consistent with the elasticity-insensitive commitment of differentiated cell types. Inhibition of nonmuscle myosin II blocks all elasticitydirected lineage specification--without strongly perturbing many other aspects of cell function and shape. The results have significant implications for understanding physical effects of the in vivo microenvironment and also for therapeutic uses of stem cells. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B39.00014: Cell response to long term mechanical interaction with nanopipettes Zulfiya Orynbayeva, Riju Singhal, Elina Vitol, Michael Bouchard, Jane Azizkhan-Clifford, Bradley Layton, Gary Friedman, Yury Gogotsi Traditional microinjection into cells is performed over a relatively short term. Pipettes are typically withdrawn following any kind of injection. On the other hand, there is growing interest in using nanopipettes for cellular and subcellular probing. This interest is partly due to new developments in nanopipette technology which employ carbon nanotubes and provide robustness, flexibility, and biocompatibility. However, as far as we know, no systematic study of physiological, biochemical, and biophysical processes associated with cell response to lengthy mechanical stimulations by nanopipette probing have been performed so far. We present a detailed investigation of a wide range of effects of long term pipette insertion into a cell. Both traditional glass micropipettes and the novel carbon nanotube-tipped probes were involved in this study. The mechanism of Ca2+ response to the mechanical stimuli introduced by the nanopipette, and the role of different organelles in this mechanism were studied. We hypothesize that the calcium response is a function of cytoskeleton integrity and the mode of coupling between the cytoskeleton and the plasma membrane domains. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B39.00015: Gold Nanoparticles effect on Human Dermal Fibroblast Tatsiana Mironava, Nadine Pernodet, Miriam Rafailovich Recently many researchers brought to the light the fact that due to high surface/bulk ratio nanoparticles can penetrate unusually deep human organs and case health problems. Gold nanoparticles are widely used nowadays, however, their effects on cells are still under investigation. Here, we studied the effect of inert citrate/gold nanoparticles as a function of size (13 nm and 45 nm), concentration and time exposure (from 1 to 6 days) on human dermal fibroblasts, since skin is one of the major routs to exposure to nanoparticles. We measured apoptosis rate as a function of nanoparticles size, time exposure and concentration. We found that the presence of 45-nm gold particles had more severe effects on these cells when compared to 13-nm nanoparticles, as the nanoparticles entry use 2 different pathways. In addition the question of cells recovery as a function of time exposure and concentration was investigated. [Preview Abstract] |
Session B40: Proteins: Structure and Function II
Sponsoring Units: DBPChair: Paul Janmey, University of Pennsylvania
Room: 412
Monday, March 16, 2009 11:15AM - 11:27AM |
B40.00001: Amino Acid Free Energy Decomposition Hui Wang, Michael Fairchild, Dennis Livesay, Donald Jacobs The Distance Constraint Model (DCM) describes protein thermodynamics at a coarse-grained level based on a Free Energy Decomposition (FED) that assigns energy and entropy contributions to specific molecular interactions. Application of constraint theory accounts for non-additivity in conformational entropy so that the total free energy of a system can be reconstituted from all its molecular parts. In prior work, a minimal DCM utilized a simple FED involving temperature-independent parameters indiscriminately applied to all residues. Here, we describe a residue-specific FED that depends on local conformational states. The FED of an amino acid is constructed by weighting the energy spectrums associated with local energy minimums in configuration space by absolute entropies estimated using a quasi-harmonic approximation. Interesting temperature-dependent behavior is found. Support is from NIH R01 GM073082 and a CRI postdoctoral Duke research fellowship for H. Wang. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B40.00002: Surface Induced Self-Assembly of Fibrinogen Fibers in the Absence of Thrombin Jaseung Koo, Miriam Rafailovich, Dennis Galanakis Wound healing is a complex process imitated by the formation of fibrin fibers that are involved in clot formation and fibroblast migration. Normally this process is triggered by thrombin cleavage of the E domain on the fibrinogen molecules, which allows them to spontaneously self-assemble into the fibers. Here we demonstrate that this process can also be initiated in the absence of thrombin. We show that by simply placing the proteins in contact with hydrocarbon functionalized clay surfaces, molecular reorientation occurs which allows fibers to form from the intact fibrinogen protein. Furthermore, using monoclonal antibodies, we determined which regions on the $\alpha $C domains are involved in the formation of the new fibrinogen fibers. This allowed us to extend these findings to general hydrophobic surfaces, such as those presented by most hydrocarbon polymers. On the other hand, the carboxyl terminal part of the A$\alpha $ chain, can interact with amine containing polymers, and suppress formation of the fibers. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B40.00003: On the Rate and Mechanism of Proton Transfer Reactions in Proteins Aihua Xie, Yunxing Li, Edward Manda, Beining Nie, Wouter Hoff, Richard Martin One of the fundamental processes in molecular biology is proton transfer reactions in proteins. Proton transfer is essential for the biological functions of proteins responsible in bioenergetics, biological signaling, and enzymatic catalysis. The mechanism of proton transfer is of great interests in order to understand the structural basis of biological functions. Despite of extensive experimental and computational efforts, it remains elusive what causes a proton to move from the proton donor to the proton acceptor. We will report a proof of concept study regarding a general mechanism of internal proton transfer reactions in proteins. Density functional theory, B3LYP/6-311+G(2d,p), is employed in this study. The results of our study provide deep insights into the structural basis to the rate and mechanism of proton transfer reactions in proteins, such as bacteriorhodopsin and green fluorescence protein. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B40.00004: Probing the reversibility of the Dscam Dimer with Light Scattering and Colloids Jesse Collins, Dietmar Schmucker, Vinothan Manoharan Dscam (Down-syndrome cell adhesion molecule) is a fascinating example of the highly specific interactions unique to biomolecules. The extracellular domain is spliced into over 18,000 isoforms. With few exceptions, each isoform, despite conservation of over 95\% of amino acid residues between isoforms, binds to itself and to no other in the set. We investigate the effect of salt and pH on the reversibility of this interaction. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B40.00005: Using Fluorescence Spectroscopy to Evaluate Hill Parameters and Heterogeneity of Ligand Binding to Cytochromes P450 Glenn A. Marsch, Benjamin Carlson, Jennifer Hansen, Elaine Mihelc, Martha V. Martin, F. Peter Guengerich The cytochromes P450 (CYPs) are hemoproteins that oxidize many drugs and carcinogens. Binding interactions of two CYPs with Nile Red, pyrene, and alpha-naphthoflavone were studied using fluorescence quenching. Upon interaction with CYPs, fluorescence from pyrene excited-state dimers was quenched more efficiently than fluorescence from pyrene monomers. Quenching data was fit to the Hill equation to determine binding affinities and the Hill parameter $n$ for the interaction of substrates with CYPs. All ligands showed strong binding to the CYPs, especially alpha-naphthoflavone, but exhibited little or no cooperativity in the interaction. Modified Stern-Volmer plots were used to confirm binding affinities, and suggested heterogeneous populations of amino acid fluorophores. Fluorescence anisotropy experiments suggest that CYP molecules tumble more rapidly when alpha-naphthoflavone is added. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B40.00006: UV Resonance Raman Excitation Profiles and Depolarization Ratios of Peptide Conformations Bhavya Sharma, Sanford Asher UV resonance Raman spectroscopy is a well established technique for probing peptide and protein secondary structure. Excitation between 180 to 215 nm, within the $\pi $ to $\pi $* electronic transitions of the peptide backbone, results in the enhancement of amide vibrations. We use UVRR excitation profiles and depolarization ratios to examine the underlying peptide bond electronic transitions. The present consensus is that three electronic transitions (n to $\pi $* and two $\pi $ to $\pi $*) occur in simple amides between 230 and 130 nm. In $\alpha $-helices a weak n to $\pi $* electronic transition occurs at 220 nm, while a higher frequency $\pi $ to $\pi $* transition occurs at 190 nm. This $\pi $ to $\pi $* transition undergoes exciton splitting, giving rise to two dipole-allowed transitions: one perpendicular to the helical axis (190 nm) and the second parallel to the axis (205 nm). The melted state of alpha-helices resembles left-handed poly-proline II (PPII) helices. The PPII helix electronic transitions have been defined as an n to $\pi $* transition at $\sim $ 220 nm and a $\pi $ to $\pi $* transition at $\sim $ 200 nm. For beta-sheets, the $\pi $ to $\pi $* transition occurs at $\sim $ 194 nm for parallel and $\sim $196 nm for anti-parallel sheets. n to pi* transition occurs at $\sim $217 nm for both. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B40.00007: UV Resonance Raman Spectral Hydrogen Exchange Studies of Poly-L-Lysine's Conformation Lu Ma, Sanford Asher The rate of exchange of peptide backbone NH group with the hydrogen of aqueous solvents is sensitive to the peptide secondary structure. In this work, we use a continuous flow rapid mixing technique and study H/D exchange rates of the model peptide poly-l-lysine (PLL) using UV resonance Raman spectroscopy. Different conformational equilibria of PLL between the helical ($\alpha $, 3$_{10}$, and $\pi $-helix) and extended conformations (PPII and 2.5$_{1}$-helix) are obtained by controlling solvent pH and salt concentration. The AmII' band of the peptide backbone is used as the deuteration marker. The H/D change rate of PLL provides direct information of the stability of different conformations. Additionally, these results provide insight into backbone conformation fluctuations and how various factors affect the conformation. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B40.00008: Polyglycine in Solution, Random or Ordered? Sergei Bykov, Sanford Asher According to the existing theories during folding, the protein backbone undergoes a transition from unordered (random coil) to ordered (native) conformations. Understanding the nature of the unordered state is one of the key problems in protein folding. Some recent investigations indicate that unfolded peptides and proteins in solution form structures close to PPII helices. Glycine based peptides possess greater conformational freedom due to the lack of the side chains. This high flexibility makes polyglycine an important model system for investigating of the conformational preferences of the polypeptides backbone in solution in general and for understanding the nature of the unfolded states in particular. We utilized UV Resonance and Visible Raman spectroscopy to investigate conformational preferences of glycine based peptides of different lengths in water solution at different conditions. We will discuss conformational preferences of the glycine based peptides in solution, and define the major factors which govern these conformational preferences. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B40.00009: Increase in Mechanical Resistance to Force in a Shear-Activated Protein Eric Botello, Nolan Harris, Huiwan Choi, Zhou Zhou, Angela Bergeron, Jing-fei Dong, Ching-Hwa Kiang von Willebrand factor (VWF) is the largest multimeric adhesion ligand found in human blood. Plasma VWF (pVWF) must be exposed to shear stress, like at sites of vascular injury, to be activated to bind platelets to induce blood clotting. In addition, adhesion activity of VWF is related to its polymer size, with the ultra-large form of VWF (ULVWF) being hyper-active, and forming fibers even without exposure to shear stress. We used the AFM to stretch pVWF, sheared VWF (sVWF) and ULVWF, and monitor the forces as a function of molecular extension. We showed a similar increase in force resistance to unfolding for sVWF and ULVWF when compared to pVWF. The increase in force is reduced when other molecules that are known to disrupt their fibril formation are present. Our results provide evidence that the common higher order structure of sVWF and ULVWF may affect the domain structure that causes difference in their adhesion activity compared to pVWF. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B40.00010: Hydrodynamic and Conformational Properties of Unfolded Proteins Guy Berry Published data on the characterization of unfolded proteins in dilute solutions in aqueous guanidine hydrochloride are analyzed to show that the data are not fit by either the random-flight or wormlike chain models for linear chains. The analysis includes data on the intrinsic viscosity, root-mean-square radius of gyration, from small-angle x-ray scattering, and hydrodynamic radius, from the translational diffusion coefficient. It is concluded that residual structure consistent with that deduced from nuclear magnetic resonance on these solutions can explain the dilute solution results in a consistent manner through the presence of ring-structures, which otherwise have an essentially flexible coil conformation. The ring-structures could be in a state of continual flux and rearrangement. Calculation of the radius of gyration for the random-flight model gives a similar reduction of this measure for chains joined at their endpoints, or those containing loop with two dangling ends, each one-fourth the total length of the chain. This relative insensitivity to the details of the ring-structure is taken to support the behavior observed across a range of proteins. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B40.00011: Stability versus flexibility in the dimerization kinetics and thermodynamics of the GCN4 Leucine zipper Dipak Rimal, Yanxin Liu, Prem Chapagain, Bernard Gerstman We present results of computer simulations that show that too much stability of the native state can have the unwanted side-effect of slower and less reliable folding. The time spent in non-native configurations depends on both the depth of the valleys in the energy landscape and the probability for visiting various regions of the landscape. We present computational results for dimerization of the GCN4 Leucine zipper in which both the helical propensities and the ionic interactions are varied in strength. The results show that when interactions are too strong, they not only beneficially stabilize the native state, but also stabilize non-native configurations that act as kinetic traps. In some cases, intermediate structures become so rigid that the peptide does not have the flexibility necessary to fold to the native state. In other situations, such as high temperature, the chain has superfluous flexibility and can form non-native bonds. If these non-native bonds are too strong, the peptide spends significant time in contorted non-native configurations and folding is slowed and less efficient. Therefore, efficient folding must be a compromise between stability and flexibility. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B40.00012: Intermediate states of globular proteins during temperature-induced folding and unfolding studied using small angle x-ray scattering Jose Banuelos, Jacob Urquidi The ability of proteins to change their conformation in response to changes in their environment has consequences in biological processes like metabolism, chemical regulation in cells, and is believed to play a role in the onset of several neurodegenerative diseases. Factors such as concentration, degree of crowding from other entities, and solvent medium affect how a protein folds. As a protein unfolds, the ratio of nonpolar to polar groups exposed to water changes, affecting a protein's thermodynamic properties. Using small angle x-ray scattering (SAXS), we are currently studying the intermediate protein conformations that arise during the folding/unfolding process as a function of temperature for a series of globular proteins. The temporal stability of these ensembles is also under investigation. Trends in the scattering profiles, along with correlations with protein thermodynamics, may help elucidate shared characteristics between all proteins in their folding behavior. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B40.00013: Forced unfolding of proteins within cells -- a proteomic method Brian Chase, Dennis Discher Many cellular activities are mediated by conformational changes in proteins or else involve rearrangement of protein assemblies. These motions are now commonly investigated in vitro as well as at the single-molecule level. But we sought to develop an in-cell method to study these motions and to do so on a proteomic scale. We have been especially interested in studying molecular responses in cells under stress, and we initially developed a labeling technique in the simplest human cell, the red blood cell. The premise is to label cysteines with cell-viable, thiol-reactive fluorophores in both stressed and unstressed cells. Then, differential labeling of proteins would indicate that under stress, previously buried cysteine residues become exposed and thus accessible to the fluorescent probe. Fluorescence imaging and saparations provide initial clues to structures and proteins, but Mass Spectrometry precisely maps the sites that are exposed. Subsequent work on recombinants and in modeling is then used to explain the cell-derived findings, and the method has now been applied to several nucleated cell types. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B40.00014: Energy parameterization of protein coarse-grained models Marcos Betancourt Protein coarse-grained models are simplified representation of proteins that in principle can be used to perform long time scale simulations of protein folding dynamics, thermodynamics, and native structures. The main challenge in realizing these models is to find a physically accurate energy parameterization. Here two approaches are considered for this purpose. The first is the popular knowledge based potential approach, where the energies are extracted from the sequence and structure of known proteins. The advantages and limitations of this approach are examined from the perspective of minimal lattice models. It is concluded that this approach is less accurate in the determination of non-bonded interactions. The other approach involves the straightforward coarse-graining of individual residue pairs by performing molecular dynamics simulations. This approach does not suffer from the approximations involved in knowledge-based potentials and have the advantage that their quality can be controlled. The final energy model is built from a balanced combination of knowledge based potentials and coarse-grained interactions from molecular dynamics. Applications of this model to protein structure prediction are described. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B40.00015: Generalized distance and its application in protein folding Ali Mohazab The concept of Euclidean distance between two points can be generalized to extended objects. The generalized distance $\cal D$ can be used as a reaction coordinate in protein folding process. Here $\cal D$ is compared and contrasted with some well-known reaction coordinates, Q and RMSD and is applied to protein fragments such as alpha helix and beta hairpin. The non-crossing constraint in utilizing $\cal D$ is also discussed. [Preview Abstract] |
Session B41: Heavy Fermion 115 Superconductors
Sponsoring Units: DMP DCMPChair: Carlos Bolech, Rice University
Room: 413
Monday, March 16, 2009 11:15AM - 11:27AM |
B41.00001: Penetration depth study of CeIrIn$_{5}$ Daniel Vandervelde, H.Q. Yuan, Y. Onuki, M.B. Salamon The heavy-Fermion compounds CeTIn$_{5}$, with T a transition metal, provide a fertile ground for studying the interplay between magnetism and superconductivity. The T = Co compound has a transition temperature $T_{c }$ = 2.4 K, and has a d-wave order parameter. As Ir is substituted for Co, the transition temperature decreases sharply to T$_{c}$ = 0.4 K. One report of the thermal conductivity of CeIrIn$_{5}$ supports a d-wave state, another argues in favor of a hybrid gap state with broken time-reversal symmetry. We report penetration depth studies of CeIrIn$_{5}$ to $T_{c}/$5 that supports the d-wave scenario. Converted to superconducting fraction, the data taken with the \textit{rf }measuring field along various crystallographic axes can be scaled to collapse to a single curve that matches a d-wave calculation with a zero-temperature gap of 2.5k$_{B}$T$_{c}$. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B41.00002: Magnetic-field induced quantum critical points of valence transition in Ce- and Yb-based heavy fermions Shinji Watanabe, Atsushi Tsuruta, Kazumasa Miyake, Jacques Flouquet Valence instability and its critical fluctuations have attracted much attention recently in the heavy-electron systems. Valence fluctuations are essentially charge fluctuations, and it is highly non-trivial how the quantum critical point (QCP) as well as the critical end point is controlled by the magnetic field. To clarify this fundamental issue, we have studied the mechanism of how the critical points of the first-order valence transitions are controlled by the magnetic field [1]. We show that the critical temperature is suppressed to be the QCP by the magnetic field and unexpectedly the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be a cooperative phenomenon of Zeeman effect and Kondo effect, which creates a distinct energy scale from the Kondo temperature. This mechanism explains a peculiar magnetic response in CeIrIn$_{5}$ and metamagnetic transition in YbXCu$_{4 }$for X=In as well as a sharp contrast between X=Ag and Cd. We present the novel phenomena under the magnetic field to discuss significance of the proximity of the critical points of the first-order valence transition. [1] S. Watanabe et al. PRL\textbf{100}, (2008) 236401. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B41.00003: Combined Effects of Magnetic Field and Uniaxial Pressure on CeCoIn$_5$ Rena Zieve, Scooter Johnson, Jason Cooley CeCoIn$_5$ exhibits unusual behavior in a magnetic field. The upper critical field is highly anisotropic, more than a factor of two larger for a field applied in the $ab$-plane than for a field along the $c$-axis. In both field orientations the superconducting transition changes from second-order to first-order with increasing field, and possible FFLO phases have been observed at high fields. Here we explore the influence of uniaxial pressure, applied along the crystal $c$-axis, on the temperature-field phase diagram. We find that a magnetic field suppresses the superconductivity far more sharply when the sample is also under pressure. We also examine the slope of the phase boundary near zero field and the nature of the transition at higher fields. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B41.00004: Dependence of Superconducting Transition Temperature on Uniaxial Pressure in CeCoIn$_5$ Scooter Johnson, Rena Zieve, Jason Cooley We apply uniaxial pressure up to 4 kbar along the $c$-axis of single crystal samples of CeCoIn$_5$ and measure how the transition temperature to the superconducting state changes. We mount the sample within an ac susceptibility coil and apply pressure through a helium bellows cell mounted on a dilution refrigerator. We find that pressure shifts the transition to lower temperatures at a rate of 22.5 mK per kbar. Our observation follows the general correlation between transition temperatures and lattice constants among 115 materials, where lower $T_c$'s typically correspond to smaller $c/a$ ratios. Pressure has the additional effect of broadening the transition well beyond what we expect from macroscopic pressure inhomogeneity. We are extending this work to measurements of CeCoIn$_5$ under $a$-axis pressure. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B41.00005: Optical Study of the Heavy Fermion Superconductor CeCoIn$_{5}$ T. Gebre, T. Tokumoto, J. Cherian, T. Murphy, S. Toszer, E. Palm, C. Wiebe, S. McGill We have performed dc magnetization and optical spectroscopy on flux-grown crystals of the heavy fermion superconductor, CeCoIn$_{5}$. We will discuss the growth technique and report results of low-temperature and high magnetic field measurements for different orientations of field and crystal axis. CeCoIn$_{5}$ is a strongly correlated system exhibiting superconductivity below 2.3 K and is believed to show a variety of magnetic field-induced phases near or below the critical temperature. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B41.00006: Coexistence of Superconducting and Magnetic Order in CeCoIn$_5$ Georgios Koutroulakis, Vesna Mitrovic, Mladen Horvatic, Claude Berthier, Gerard Lapertot, Jacques Flouquet The interplay between magnetic and superconducting order near a quantum critical point in heavy fermion materials has attracted intensive research interest in recent years. One of the most intriguing examples is that of CeCoIn$_5$, in which a novel phase within the superconducting phase is observed near the critical field at low temperature. Recent nuclear magnetic resonance [1] and neutron scattering [2] experiments showed that a static magnetic order is stabilized in this phase. The microscopic nature of this magnetic state will be discussed.\\[4pt] [1] B.-L. Young \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 036402 (2007).\\[0pt] [2] M. Kenzelmann \textit{et al.}, Science \textbf{321}, 1652(2008). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B41.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B41.00008: Local Structure and Site Occupancy in Cd- and Hg-doped Ce$T$In$_{5}$ ($T$=Co, Rh, Ir) C.H. Booth, E.D. Bauer, F. Ronning, V. Sidorov, T. Park, J.D. Thompson, J.L. Sarrao, A.D. Bianchi, Z. Fisk Local structure measurements using the extended x-ray absorption fine-structure (EXAFS) technique were performed from the In $K$, Cd $K$, and Hg $L_3$ edges on samples of CeCo(In$_{1-x}$Cd$_x$)$_5$ (0.5\%$\geq x \geq$3.1\%) and Ce$T$(In$_{1-x}$Hg$_x$)$_5$ ($T$=Co, Rh, Ir, 0.7\%$\geq x \geq$3.5\%). Fits indicate no measurable change in the bulk local structure with these substituents. In contrast, the local structure data around the substituent atoms indicates about $f_{Cd}^{Co}=44(3)\%$ of Cd atoms reside on In(1) sites, similar to previous results [$f_{Sn}^{Co}=55(5)\%$] for Sn in CeCo(In$_{1-x}$Sn$_x$)$_5$. Mercury has an even stronger preference for the In(1) site, with $f_{Hg}^{Co}=71(5)\%$, $f_{Hg}^{Rh}=97(3)\%$, and $f_{Hg}^{Ir}=55(5)\%$, although other phases appear to be present in the CeIr(In$_{1-x}$Hg$_x$)$_5$ material. Small distortions from the parent structure are also observed around the substituent atoms. These results will be related to the sharp decrease in the superconducting transition temperature with $x$. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B41.00009: Cd and Sn doping effects on Pauli limiting and HFLT state in CeCoIn$_5$ R. Movshovich, Y. Tokiwa, N. Kurita, F. Ronning, E.D. Bauer, P. Papin, J.D. Thompson, A.D. Bianchi, J.F. Rauscher, S.M. Kauzlarich, Z. Fisk , I. Vekhter We investigated the effect of Cd and Sn doping on the superconducting (SC) transition temperature $T_c$, the superconducting critical field $H_{c2}$, and the High-Field-Low- Temperature (HFLT) phase in heavy fermion superconductor CeCoIn$_5$. $T_c$ decreases monotonically for both dopants, whereas $H_{c2}$ rises initially with Cd substitution, while dropping for Sn doping. Analysis of the magnetization data suggests that these effects are caused by weakening of the Pauli limiting in CeCoIn$_5$ with Cd doping, most likely due to changes of susceptibility of the normal state. Both Cd (leading to AFM ground state at higher concentrations) and Sn impurities, at a very low level of a few hundred ppm, suppress the HFLT phase. We interpret these results as supporting the superconducting origin of the HFLT phase. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B41.00010: Kondo coherence and superconductivity in Yb doped CeCoIn$_5$ Andrea Bianchi, Gabriel Seyfarth, Bobby Prevost, Sjoerd Roorda, Dan Hurt, Cigdem Capan, Zachary Fisk We have studied the effect of Yb doping on superconductivity in the unconventional heavy fermion superconductor CeCoIn$_5$ in a doping series spanning from pure CeCoIn$_5$ to the paramagnetic metal YbCoIn$_5$, which is isostructural with CeCoIn$_5$. By replacing Ce by Yb, we are at the same time removing carriers as well as disrupting the Kondo lattice of CeCoIn$_5$, which is thought to be responsible for superconductivity in this compound. This will give us insight into how the Kondo coherence and Cooper pair breaking in CeCoIn$_5$ are affected by Yb substitution. Our doping series shows that the unit cell volume stays approximately constant up to a Yb concentration of about 40~\%. At higher Yb concentrations the unit cell volume begins to shrink gradually to the value of YbCoIn$_5$. However, is superconductivity in the doped system is remarkably resilient against Yb substitution: At low doping the superconducting transition temperature $T_c$ is only gradually suppressed from the value of 2.24~K of pure CeCoIn$_5$, and only with a Yb concentration of 60~\% $T_c$ is suppressed below 0.3~K. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B41.00011: Conductance asymmetry in point-contact junctions on the heavy-fermion compounds CeMIn$_{5}$ (M=Co, Rh, Ir) L.H. Greene, W.K. Park, E.D. Bauer, J.L. Sarrao, J.D. Thompson The Ce-based 1-1-5 heavy-fermion compounds, CeMIn$_{5}$ (M=Co, Rh, Ir), continue to draw much attention from the community. One of the key questions is how the localized discrete states acquire itinerancy over the conduction electron continuum. As a probe of the evolutionary behavior of the Kondo lattice, we take differential conductance spectra from nanoscale metallic junctions on CeMIn$_{5}$ single crystals over wide temperature ranges. A striking common feature is the systematic development of an asymmetry in the background conductance [1]. Conventional models including the heating model with large Seebeck coefficients of heavy fermions do not account for this behavior. We propose a phenomenological model based on a possible Fano interference effect [2] between two conductance channels, one into the heavy electron liquid (hybridized f-band) and the other into the conduction electrons without hybridization. [1] W. K. Park et al., Phys. Rev. Lett. \textbf{100}, 177001 (2008). [2] U. Fano, Phys. Rev. \textbf{124}, 1866 (1961). [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B41.00012: The Fano effect in the point contact spectroscopy of heavy electron materials Yi-feng Yang Recent experiments on CeCoIn$_5$ reveal similar temperture dependence of the conductance asymmetry in the point contact spectroscopy and the Knight shift anomaly. This suggests a common origin of both anomalies and supports a previously proposed phenomenological two-fluid model that predicts the emergence of a heavy fluid, or Kondo liquid, in heavy electron materials. Here we propose a phenomenological formula for the point contact spectroscopy and describe the spectra by a Fano effect of tunneling electrons due to the Kondo liquid emergence. Our formula explains quantitatively the experimental data of several heavy electron materials. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B41.00013: Evidence for magnetic field-tuned quantum criticality below $H_{c2}$ in CeCoIn$_5$ J. Paglione, J.-P. Reid, M.A. Tanatar, L. Taillefer, C. Petrovic The existence of a magnetic field-tuned quantum critical point coinciding with the upper critical field for superconductivity in the heavy-fermion superconductor CeCoIn$_5$ has remained a puzzling fact, and has proven difficult to study due to the onset of superconductivity. Here we present low temperature thermal conductivity measurements which probe the approach to the quantum critical point in CeCoIn$_5$ as a function of field from within the superconducting state, revealing new evidence for field- tuned quantum criticality. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B41.00014: Insulating Vortex Core near QCP in CeCoIn$_5$ H. Xiao, T. Hu, C. C. Almasan, T. A. Sayles, M. B. Maple We have investigated the vortex core of the superclean unconventional heavy fermion superconductor CeCoIn$_5$ by studying the flux flow dissipation in the mixed state for two magnetic field orientations, i.e., $H \parallel c$ axis and $H \parallel ab $ plane, at temperatures down to 1.8 K. The vortex core in the mixed state of CeCoIn$_5$ is insulator-like, in contrast to the metallic behavior of CeCoIn$_5$ in its normal state, at temperatures $T$ larger than the supercondcting transition temperature $T_c$. Moreover, the abnormal insulating behavior of the vortex core is strongly suppressed when CeCoIn$_5$ is tuned away from the quantum critical point (QCP) by applying pressure. This latter result gives firm evidence that quantum criticality plays an important role in the interaction between superconductivity and magnetism, hence is responsible for the emergence of unconventional superconductivity. A scaling law of the flux flow resistivity has also been found and will be discussed. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B41.00015: The effect of Yb substitution in the heavy-fermion superconductor CeCoIn$_{5}$ Lei Shu, E. Gonzales, K. Huang, T.A. Sayles, J. Paglione, M.B. Maple The discovery of a new family of heavy-fermion superconductors with the formula CeMIn$_{5}$ (M=Co, Rh, Ir ) provides an interesting playground to understand the relation between unconventional superconductivity and quantum criticality. CeCoIn$_{5}$, with the highest ambient pressure superconducting transition temperature of all heavy-fermion compounds, exhibits a magnetic field-tuned quantum critical point. The peculiar magnetic properties of CeCoIn$_{5}$ are determined by the interaction between the magnetic moments of the Ce$^{3+}$ ions and the spins of conduction electrons. Through systematic studies of Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ alloys, where the magnetic Ce ions (4$f^{1}$ configuration) are diluted by Yb (hole analogue; 4$f^{13}$ configuration), it allows us to study the nature of intersite interactions. Lattice constant, magnetic susceptibility, and electrical resistivity data of Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ alloys throughout the range $0\leq x \leq 1$ will be reported. [Preview Abstract] |
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