Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L1: Recent Advances in Ultrafast Studies of Condensed Matter
Sponsoring Units: DCMP DCPChair: Ee Min Elbert Chia, Nanyang Technological University
Room: Ballroom A1
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L1.00001: Ultrafast and Nonlinear Optical Spectroscopy of Carbon Nanotubes Invited Speaker: Single-walled carbon nanotubes (SWNTs) provide a variety of unique opportunities for studying the dynamics and interactions of one-dimensional (1-D) electrons and phonons. We have carried out a series of ultrafast and nonlinear optical experiments on SWNTs, revealing novel properties of high- density 1-D excitons as well as coherent lattice vibrations.\footnote{Y.~Murakami and J.~Kono, Phys.~Rev.~Lett.~ {\bf 102}, 037401 (2009); Phys.~Rev.~B {\bf 80}, 035432 (2009); A.~Srivastava and J.~Kono, Phys.~Rev.~B {\bf 79}, 205407 (2009); J.-H.~Kim {\it et al}., Phys.~Rev.~Lett.~{\bf 102}, 037402 (2009); G.~D.~Sanders {\it et al}., Phys.~Rev.~B~{\bf 79}, 205434 (2009); Y.-S.~Lim {\it et al}., ACS Nano {\bf 4}, 3222 (2010); L.~G.~Booshehri {\it et al}., arXiv:1007.3144v1.} We have shown that there exists an upper limit on the density of 1-D excitons in SWNTs, which results in photoluminescence saturation. Using a model based on diffusion-limited exciton- exciton annihilation, we provided realistic estimates for the exciton densities in the saturation regime. We also predicted and demonstrated that there is an optimum temperature at which the exciton density can be maximized, due to the existence of a dark exciton state. Using ultrashort pulses, we have also investigated the dynamics of coherent phonons (CPs) in SWNTs, including both the low frequency radial breathing mode and high frequency G-mode phonons. Pulse shaping techniques allowed us to generate and detect CPs in SWNTs in a chirality-selective manner, which provided insight into the chirality dependence of light absorption, phonon generation, and phonon-induced band- structure modulations. Finally, we observed novel large- amplitude CPs through near-band-edge excitations as well as strongly polarization-dependent CP signals in highly-aligned SWNTs. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L1.00002: Femtosecond magnetism and spin manipulation on a time-scale of the exchange interaction Invited Speaker: The dynamics of phase transformations on the time-scale pertinent to atomic, orbital and spin motion is a rather unexplored field in physics. This is also a particularly interesting problem of modern magnetism, a study of which may have tremendous consequences for future development of magnetic recording technology. However, generation of magnetic field pulses much shorter than 100 ps and strong enough to reverse magnetization (more than 1T) is an extremely challenging technical problem.As a result the dynamics of the magnetic phase transitions at the sub-100 ps time-scale remains to be one of the most intriguing areas of modern magnetism [1,2]. Recently it has been observed that a 40 fs laser pulse influences spins in a magnet as an equally short pulse of effective magnetic field with a strength up to 20 T [3,4]. In my talk I will discuss how these opto-magnetic pulses can be used to excite a magnet on a time-scale of the exchange interaction between the spins [5-7]. Novel insights into the physics of non-equilibrium magnetism will be provided, showing that two exchange-coupled magnetic sublattices of a ferrimagnet may have totally different spin dynamics [8]. As a result, ultrafast spin reversal of two antiferromagnetically coupled magnetic sub-lattices appears to proceed via a novel ferromagnet-like transient state. \\[4pt] [1] J. St\"ohr and H. C. Siegmann, \textit{Magnetism: from fundamentals to nanoscale dynamics} (Springer, Berlin, 2006). \\[0pt] [2] A. Kirilyuk, A. V. Kimel, Th. Rasing, \textit{Rev. Mod. Phys.} \textbf{82} 2731 (2010). \\[0pt] [3] A. V. Kimel et al,\textit{ Nature }\textbf{435 }655 (2005) \\[0pt] [4] C. D. Stanciu et al,\textit{ Phys. Rev. Lett. }\textbf{99}, 047601 (2007). \\[0pt] [5] K. Vahaplar et al \textit{Phys. Rev. Lett.} \textbf{103}, 117201 (2009). \\[0pt] [6] A.V. Kimel et al, \textit{Nature-Physics} \textbf{5} 727 (2009). \\[0pt] [7] A. H. M. Reid et al \textit{Phys. Rev. Lett.}\textbf{105} 107402 (2010). \\[0pt] [8] I. Radu et al (submitted). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L1.00003: Theory of ultrafast pump-probe phenomena in high-temperature superconductors Invited Speaker: The physics underlying the pairing mechanism for high-temperature superconductors remains a topic of current interest. The complexity lies with the existence of competing interactions in these strongly correlated electronic materials. The ultra-fast pump-probe technique can make a stride to untangle the competing degrees of freedom (DOF). In this talk, the theoretical underpinning for this technique will be reviewed. In particular, we have developed a three-temperature model [1] to simulate the real time dependence of the electron and phonon temperatures in high-temperature superconductors. The model considers anisotropic electron-phonon coupling [2]. Based on this model, we have calculated the time-resolved spectral function, which exhibits interesting features with time delay. It has been found that the excitation of phononic DOF can provide a defining signature for the evidence of electron-vibration mode coupling [1]. In addition, the time-resolved optical conductivity and Raman spectra will also be discussed within the same model [3]\\[4pt] [1] Jianmin Tao and Jian-Xin Zhu, Phys. Rev. B \textbf{81}, 224506 (2010);\\[0pt] [2] T. P. Devereaux \textit{et al}., Phys. Rev. Lett. \textbf{93}, 117004 (2004); Jian-Xin Zhu \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 177001 (2006);\\[0pt] [3] Jianmin Tao and Jian-Xin Zhu \textit{et al}., unpublished (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L1.00004: Ultrafast Optical Excitation in YBa$_2$Cu$_3$O$_{7-\delta}$: Tracing the Optical Phonons Invited Speaker: The time-resolved spectroscopy of nonequilibrium states proved to be a powerful tool for observation of the electron-phonon scattering dynamics and the recombination of photoexcited quasiparticles (QP), particularly in high-temperature cuprate superconductors. However, most of the reported experiments monitor only the electronic subsystem [1-5]. Thus, a detailed dynamics of the various phonon modes during an initial non-thermal regime has been beyond reach. Here we utilize the field-resolved ultrabroadband THz spectroscopy to resonantly trace ultrafast phonon and QP dynamics of optimally doped single crystals of YBa$_2$Cu$_3$O$_{7-\delta}$ [6]. The superconducting state is perturbed by 12-fs optical pump pulses, and the induced changes in the mid-infrared optical conductivity are probed by THz transients. Thus, we simultaneously observe the dynamics of nonequilibrium QPs and two specific phonon modes with a time resolution of 40 fs. A quantitative line shape analysis of the apex oxygen vibration allows us to separately follow its transient occupation and coupling to the Josephson plasma resonance. A strong phonon population and the maximum QP density are reached within the same time scale of 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the QPs are thermalized. Our results indicate substantial electron-phonon scattering in YBa$_2$Cu$_3$O$_{7-\delta}$ and introduce a powerful approach for characterizing transient phonon dynamics in a broad variety of solids.\\ [4pt] [1] S. G. Han et al., Phys. Rev. Lett. \textbf{65}, 2708 (1990);\\ [0pt] [2] R. A. Kaindl et al., Science \textbf{287}, 470 (2000);\\ [0pt] [3] R. D. Averitt et al., Phys. Rev. B \textbf{63}, 140502 (2001);\\ [0pt] [4] L. Perfetti et al., Phys. Rev. Lett. \textbf{99}, 197001 (2007);\\ [0pt] [5] R. P. Saichu et al., Phys. Rev. Lett. \textbf{102}, 177004 (2009);\\ [0pt] [6] A. Pashkin et al., Phys. Rev. Lett. \textbf{105}, 067001 (2010). [Preview Abstract] |
Session L2: Single Molecule Transistors and Graphene Quantum Dots
Sponsoring Units: DCMPChair: Daniel Ralph, Cornell University
Room: Ballroom A2
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L2.00001: Spectroscopy and read-out of STM-patterned donor based qubits Invited Speaker: We report low temperature transport measurements of few-to single P donor based quantum dots in silicon. Dots with a high donor number (approx. 7) show a surprisingly dense spectrum of excited states with an average energy spacing of 100 micro eV. The energy spacing of these features is much too low to be accounted for by the nm-scale lateral confinement of either the dot or the leads and can be explained by lifting of valley degeneracy of the dot orbital states [1]. The use of all epitaxial in plane P:Si gates allow us to tune both the electron number in the dot and modulate the transparency of the tunnel barriers [2]. We also present transport through a deterministic single donor device, where we observe both the signature of a single donor directly through STM imaging and demonstrate that the charging energy and excited state spectrum is consistent with the orbital states of a single P-donor. Finally we present our latest results of spin read-out in STM-patterned donor based devices. \\[4pt] [1] M. Fuchsle et al, Spectroscopy of few electron single crystal silicon quantum dots, Nature Nanotechnology 5, 502 (2010). \\[0pt] [2] A. Fuhrer et al, Atomic-Scale, All Epitaxial In-Plane Gated Donor Quantum Dot in Silicon, Nano Letters 9, 707 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L2.00002: Single-donor transport in silicon: Atomic physics in restricted momentum space Invited Speaker: Technology reached a level of miniaturization where we can realize transport through a single dopant atom in a transistor. Such transport spectroscopy can probe the atomic orbitals and the interaction of the atom with the environment. This interaction with the environment in a nano-device leads alters the dopants properties, such as the level spectrum and the charging energy, from those of the bulk. The system discussed here is a gated arsenic donor in a silicon field effect transistor. Electronic control over the wavefunction of dopants is one of the key elements of quantum electronics. This talk focuses on the role of the restricted momentum space which has a severe impact on the charge and spin configuration of a donor atom in a nano-device. The combined experimental and theoretical study of the gated two-electron state of the donor led to the realization of the pseudo spin nature of the valleys. We observe a blocked electronic relaxation due to combined spin and valley selection rules. Time averaged transport measurements put a lower bound of 50 ns on the rate of the blocked transition, 1000 times slower than a bulk transition. For the low lying excited states Hund's rule is violated due to vanishing exchange in orthogonal valleys. Furthermore, we observe reduced charging energies and bound singlet and triplet excited states for this negatively charged donor that can be explained in the self consistent tight binding model. Finally, experiments demonstrating coherent coupling between two donors and between a donor and the leads will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L2.00003: Orbital Gating of Single Molecule Transistors Invited Speaker: Electron devices containing molecules as the active region have been an active area of research over the last few years. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device; e.g., one that operates by modifying the internal energy structure of the molecule, analogous to conventional FETs. Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. We have realized a molecular transistor made from the prototype molecular junction, benzene dithiol, and have used a combination of spectroscopies to determine the internal energetic structure of the molecular junction. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunneling spectroscopy, demonstrating for the first time direct molecular orbital gating in a molecular electronic device. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L2.00004: A single-molecule optical transistor Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L2.00005: Optical, magnetic and electronic properties of graphene quantum dots Invited Speaker: We present a theory of optical, magnetic and electronic properties of graphene quantum dots. We demonstrate that there exists a class of triangular graphene quantum dots with zigzag edges [1-8] which combines magnetic, optical and transport properties in a single-material structure. These dots exhibit robust magnetic moment and optical transitions simultaneously in the THz, visible and UV spectral ranges due to the existence of a band of degenerate states lying at the Fermi level in the middle of the energy gap [1-6]. The magnetic and optical properties[5,7] are determined by strong electron-electron and excitonic interactions in the degenerate band, treated exactly using numerical techniques combining tight-binding, DFT, Hartree-Fock and configuration interactions methods. We show that the spin polarized degenerate band leads to quenching of the absorption spectrum at half-filling, while addition of a single electron fully depolarizes all electron spins and turns the absorption on. It is thus possible to design gate and size tunable graphene quantum dots with desired optical and magnetic properties for optoelectronic and photo-voltaic applications. Collaborators: P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak.\\[4pt] [1] J. Fernandez-Rossier, and J. J. Palacios, Phys. Rev. Lett. 99, 177204 (2007).\\[0pt] [2] W. L. Wang, S. Meng, E. Kaxiras, Nano Lett. 8, 241 (2008).\\[0pt] [3] M. Ezawa, Phys. Rev. B 76, 245415 (2007).\\[0pt] [4] J. Akola, H. P. Heiskanen, and M. Manninen, Phys. Rev. B 77, 193410 (2008).\\[0pt] [5] A. D. G\"u\c{c}l\"u, P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak, Phys. Rev. Lett. 103, 246805 (2009).\\[0pt] [6] P. Potasz, A. D. G\"u\c{c}l\"u, P. Hawrylak, Phys. Rev. B 81, 033403 (2010).\\[0pt] [7] A. D. G\"u\c{c}l\"u, P. Potasz, and P. Hawrylak, Phys. Rev. B 82, 155445 (2010).\\[0pt] [8] O.Voznyy, A. D. G\"u\c{c}l\"u, P. Potasz and P. Hawrylak, arXiv:1011.0369. [Preview Abstract] |
Session L3: Gap Structure of the Ba-122 Iron Based Superconductors
Sponsoring Units: DCMPChair: Peter Hirschfeld, University of Florida
Room: Ballroom A3
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L3.00001: Electronic structure studies of Ba/EuFe$_2$As$_2$ based superconductors by angle and time-resolved photoemission spectroscopy Invited Speaker: We report high-resolution ARPES studies on the evolution of the electronic structure of Ba/EuFe$_2$As$_2$ compounds upon n-type doping by replacing Fe by Co and applying chemical pressure by substituting As by P. In particular, we have investigated the nesting conditions between the hole pockets in the centre and the electron pocket at the corner of the Brillouin zone (BZ) for various wave vectors perpendicular to the FeAs layers. In the case of chemically doped systems we observe a shift of the Fermi level in an almost rigid band system. These changes of the electronic structure upon doping cause a reduction of the nesting conditions, possibly yielding a microscopic explanation of the phase diagrams in which antiferromagnetic (AF) order is destroyed, followed by the appearance and disappearance of superconductivity at higher doping concentration. On the basis of the almost equivalent phase diagram obtained upon chemically pressurizing the compound, one expects a similar change of the electronic structure. However, in this case, with increasing P concentration, we observe a non-rigid-band-like change of the electronic structure in the centre of the BZ. In spite of this difference, also here the nesting conditions decrease with increasing P substitution, possibly providing a microscopic explanation for the phase diagram. Finally, we have performed femtosecond time-resolved ARPES studies on undoped and doped Ba/EuFe$_2$As$_2$ after optical pumping. Regarding the relaxation processes we obtain information on the complex dynamics of the excited electronic state in these semi metallic systems. Furthermore, we derive a small electron-phonon coupling constant making electron-phonon coupling an unlikely candidate for the mechanism of high-$T_c$ superconductivity in these compounds. This work is performed in collaboration with S. Thirupathaiah, E. Rienks, H. A. D\"urr, S. de Jong, E. van Heumen, E. Slooten, Y. Huang, R. Huisman, M. S. Golden, L. Rettig, R. Cortes, U. Bovensiepen, M. Wolf, A. Erb, T. Wolf, H.S. Jeevan, P. Gegenwart. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L3.00002: Symmetry of spin excitation spectra in 122-ferropnictides Invited Speaker: We have studied the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on Ni- and Co-doped BaFe$_2$As$_2$ samples close to the optimal doping level, which exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum does not follow the $I4/mmm$ space group of the crystal, but instead inherits its symmetry from the unfolded Brillouin zone of the Fe- sublattice. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode and the SC spin gap. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional theory calculations without invoking a symmetry-broken (``nematic'') ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode, as well as its intensity and the SC spin gap, inherit the normal-state intensity modulation along the out-of-plane direction. Apparently, it can be traced back to the three-dimensional band structure and the superconducting gap, both of which were reported to disperse along the out-of- plane direction. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L3.00003: London penetration depth as a sensitive tool for determining the superconducting gap structure in iron-pnictide superconductors Invited Speaker: In the high$-T_c$ cuprates, experiments and theories have relied on a single-band picture that is essentially two- dimensional with a single superconducting gap, which provided a simple way to understand the angular dependence of the superconducting order parameter. In iron-based superconductors, the experimental mapping of the superconducting gap structure is complicated by the doping- dependent, multi-band electronic structure with three- dimensional character and the existence of at least two distinct superconducting gaps. Focusing on precision measurements of the London penetration depth, $\lambda(T)$, in ``122'' Ba(Fe$_{1-x}$T$_{x}$)$_{2}$As$_{2}$ (T=Co,Ni,Ru,Pt,Pd,Co+Cu) single crystals, I will discuss the systematics of the ubiquitous power law temperature variation of the in-plane penetration depth, $\lambda_{ab}(T)=\lambda_{ab} (0)+\beta T^n$, and of the absolute value, $\lambda_{ab}(0)$, with the doping level, $x$. To understand the role of disorder and pairbreaking scattering, the effect of heavy ion irradiation has been systematically studied and the results are compared with other systems, most notably stoichiometric LiFeAs. Together with the doping dependence of the out-of- plane London penetration depth, $\lambda_c(T)$, and comparisons to thermal conductivity and specific heat data, these results strongly suggest the development of a significant in-plane anisotropy of the superconducting gap(s) and are also consistent with the appearance of accidental c-axis nodes (not imposed by symmetry) for concentrations moving away from optimal doping. By taking pairbreaking scattering into account, the data for the optimally doped compounds are well described by weak-coupling superconductivity with two nodeless superconducting gaps having amplitudes that differ by about a factor of two. I conclude by emphasizing the significant role of three-dimensionality and scattering in determining the electrodynamics of iron-based superconductors. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L3.00004: Phase diagram and superconducting gap structure of the iron-pnictide superconductor (Ba,K)Fe$_{2}$As$_{2}$ Invited Speaker: Measurements of the Nernst and Seebeck coefficients were used to delineate the T-x phase diagram of the iron-pnictide superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. The sensitivity of these two coefficients to the reconstruction of the Fermi surface caused by the onset of antiferromagnetic order below a temperature T$_{N}$ allowed us to track T$_{N}$ precisely as a function of concentration x, even when the electrical resistivity, for example, shows no anomaly at the magnetic transition. In the region of concentrations where superconductivity appears out of an antiferromagnetic normal state (T$_{c} \quad <$ T$_{N})$, we investigate the evolution of the superconducting gap structure of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ by measuring the thermal conductivity in the T=0 limit. This is a sensitive and directional probe of nodal quasiparticles. As the concentration x is reduced, we find a sudden change in the gap structure from a full gap without nodes to a gap with nodes. We ascribe this change to the onset of antiferromagnetism below a critical doping x$_{N}$ inside the superconducting phase, whose effect is most likely to alter both the Fermi surface and the angular dependence of the gap. We compare these results with our earlier study on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ [1,2]. \\[4pt] [1] M. Tanatar {\it et al.}, Physical Review Letters {\bf 104}, 067002 (2010).\\[0pt] [2] J.-Ph. Reid {\it et al.}, Physical Review B {\bf 82}, 064501 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L3.00005: Superconductivity in a 3D tight-binding model for Ba-122 Invited Speaker: Theoretical investigations of the superconducting state in the iron pnictides have shown that weak-coupling approaches based on a tight-binding parametrization of the LDA band structure can be successfully applied to describe both magnetism and superconductivity in these materials. FLEX, RPA, and fRG studies find in most cases a superconducting state with $s$-wave symmetry that exhibits a $\pi$-phase shift between the gap on the electron and the hole Fermi surfaces, often called a sign-changing $s$-wave state. Besides this general agreement about the symmetry of the superconducting state these studies have also revealed that the momentum dependence of the gap, including the possibility of gap nodes, is highly sensitive to details of the electronic structure, in particular to the orbital composition of the Fermi surface. Since superconductivity in these materials is restricted to the FeAs layers that, at least for the 1111 compounds, are well separated and only weakly coupled most tight-binding models used to study the superconducting state have been limited to two dimensions. On the other hand the 122 compounds as well as the binary compounds show a very pronounced 3D electronic structure with changing orbital weights on the Fermi surfaces along the $k_z$ direction. An RPA based calculation of the spin susceptibility for the Ba-122 material demonstrates that the necessary averaging over the full three dimensional Brillouin zone leads to a broader and more commensurate spin response compared to a corresponding two dimensional calculation in agreement with experimental observations. In addition changes of the orbital character of the Fermi surface lead to a complicated three dimensional gap structure exhibiting V-shaped or near horizontal nodes on the hole sheets near the zone boundary that can in part explain the puzzling transport measurements. [Preview Abstract] |
Session L4: Quantum Information: Featured Experiments
Sponsoring Units: GQIChair: Richart Slusher, Georgia Tech Research Institute
Room: Ballroom A4
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L4.00001: Entanglement of spin waves among four quantum memories Invited Speaker: Quantum networks are composed of quantum nodes that interact coherently by way of quantum channels and open a broad frontier of scientific opportunities [1]. For example, a quantum network can serve as a `web' for connecting quantum processors for computation and communication as well as a ``simulator'' for enabling investigations of quantum critical phenomena arising from interactions among the nodes mediated by the channels. The physical realization of quantum networks generically requires dynamical systems capable of generating and storing entangled states among multiple quantum memories, and of efficiently transferring stored entanglement into quantum channels for distribution across the network. While such capabilities have been demonstrated for diverse bipartite systems, entangled states have so far not been achieved for interconnects capable of ``mapping'' multipartite entanglement stored in quantum memories to quantum channels. In my presentation, I will describe an experiment [2] that demonstrates measurement-induced entanglement stored in four atomic memories; user-controlled, coherent transfer of the atomic entanglement to four photonic channels; and characterization of the full quadripartite entanglement by way of quantum uncertainty relations [3]. Our work thereby provides an important advance for the distribution of multipartite entanglement across quantum networks. Moreover, our entanglement verification method can be applied for the study of entanglement order for condensed matter systems in thermal equilibrium. With regard to quantum measurement, our multipartite entangled state can be applied for sensing an atomic phase shift beyond the limit for any unentangled state. \\[4pt] [1] ``The Quantum Internet,'' H. J. Kimble, Nature \textbf{453}, 1023 (2008). \\[0pt] [2] K. S. Choi, A. Goban, S. Papp, S. J. van Enk and H. J. Kimble, Nature \textbf{468}, 412 (2010). \\[0pt] [3] S. B. Papp \textit{et al.}, Science \textbf{324}, 764 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L4.00002: Quantum Networks with Atoms and Photons Invited Speaker: Trapped atomic ions are among the most promising candidates for quantum information processing. All of the fundamental quantum operations have been demonstrated in this system, and the central challenge now is how to scale the system to larger numbers of qubits. By entangling atomic qubits through both deterministic phonon and probabilistic photon interfaces, the trapped ion system can be scaled in various ways for applications in quantum communication, quantum computing, and quantum simulations. I will discuss several options and issues for such atomic quantum networks, along with state-of-the-art experimental progress. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L4.00003: Quantum-logic clocks for fundamental physics and geodesy Invited Speaker: We have compared the rates of two quantum-logic clocks based on the optical $^1$S$_0$-$^3$P$_0$ transition in Al$^+$. The performance of the newer clock is unmatched, and despite many differences, their rates agree to $1.8 \pm 0.7 \times 10^{-17}$, within the accuracy limit of the older clock. The newer clock has an accuracy of $8.6 \times 10^{-18}$ and stability near $10^{-15} (\tau/s)^{-1/2}$. Quantum-correlation spectroscopy yields an improved measurement stability of $3.7\times10^{-16} (\tau/s)^{-1/2}$. This technique also allows Q-factors beyond $6\times10^{15}$ to be seen. This is the highest observed Q-factor in physics. The talk will discuss the basic operation of quantum-logic clocks based on Al$^+$, together with recent results that include a first geo-potential difference measurement, and constraints on the temporal variation of the fine-structure constant. Potential uses of entangled states in such clocks are also explored. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L4.00004: Towards Quantum Information Processing with Superconducting Circuits Invited Speaker: In the dozen years since the initial demonstrations that superconducting circuits based on Josephson junctions could be considered as qubits, there has been remarkable progress in the field. Several different ``species'' of these artificial atoms have been designed and tested, and coherence times have increased by more than 1,000, or a factor of 10 every three years. While real devices are still far from satisfying all the DiVincenzo criteria with fidelities that would meet the error correction threshold, one can nonetheless perform preparation, control, quantum logic, and measurement on multiple superconducting qubits, all with surprisingly high purity and precision given that these are man-made, solid-state systems. In recent years we have seen the preparation of highly-entangled multi-qubit states that violate the Bell and Mermin inequalities, as well as the demonstration of single quantum algorithms, which all benefit from the strong coupling, addressability, and all-electronic control that is possible with these systems. Many experiments employ the concept of a ``quantum bus,'' where qubits couple via superconducting transmission lines that form high-quality resonant cavities. A spinoff of this work is the advent of quantum optics on a chip: microwaves are photons too! The combination of qubits coupled to cavities has allowed the preparation and detection of single gigahertz photons, as well as other highly non-classical states of microwave light. Great progress has also been made in quantum measurement, and other Josephson circuits are now delivering amplifiers that operate at or beyond the Heisenberg limit. In this talk I will attempt to give an overview of some of the key concepts, some experimental highlights from recent years, and point out some possible directions for the future in this field. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L4.00005: Quantum Information and the Foundations of Quantum Mechanics: a story of mutual benefit Invited Speaker: Fundamental tests, particularly of quantum nonlocality, in the 1970s were crucial for the development of the new field of quantum information science. The consequent development of new technologies has led to novel possibilities to do fundamental tests. A most simple and clear test of noncontextuality, i.e. the existence of joint probability distributions, is due to a proposal by Klyachko et al. [PRL 101, 020403 (2008)]. There, the experimental tests became possible because of technology developed for quantum communication. It turns out that a very simple and intuitive picture of the contradiction with realism can be given. In parallel, an experiment closing the freedom of choice loophole in quantum entanglement [T. Scheidl et al., Proc Natl Acad Sci USA (2010) 19709], together with earlier experiments testing the Leggett-type inequality and objectivity, i.e. the existence of observables without the context of observation, might be challenged. Current micro-optics technology and the exploitation of external states of light like Hermite-Gauss and Laguerre-Gauss allows to extend this kind of experiments into higher-dimensional Hilbert Spaces. There, interesting connections between entanglement and mutually unbiased bases have been found. [Preview Abstract] |
Session L5: Topics in Alternative Energy
Sponsoring Units: COMChair: Lou Strolger, Western Kentucky University
Room: Ballroom C1
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L5.00001: Efficient High Surface Area Vertically Aligned Metal Oxide Nanostructures for Dye-Sensitized Photoanodes by Pulsed Laser Deposition Invited Speaker: Dye Sensitized Solar Cells (DSSCs) differ from conventional semiconductor devices in that they separate the function of light absorption from charge carrier transport. At the heart of a DSSC is a metal oxide nanoparticle film, which provides a large effective surface area for adsorption of light harvesting molecules. The films need to be thick enough to absorb a significant fraction of the incident light but increased thickness results in diminished efficiencies due to augmented recombination. Losses in efficiency are due to the slow trap-limited diffusion process responsible for electron transport. This process limits the effective electron diffusion length to about $\sim $ 10 $\mu $m and results in an efficiency-limiting trade-off between light absorption and carrier extraction. Here we introduce a new structural motif for the photoanode in which the traditional random nanoparticle oxide network is replaced by vertically aligned bundles of oxide nanocrystals. This structure improves absorbed photon to current efficiencies (APCE) to values above 90{\%} over most of the dye absorption range. The bundled anode is fabricated by a simple laser deposition process and features a surface area $\sim $ 2 times larger than that of traditional anodes. The direct pathways provided by the vertical structures also appear to provide for an enhanced collection efficiency for carriers generated throughout the device. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L5.00002: Predictive Defect Science for Cost-Effective Photovoltaics Invited Speaker: Low-cost photovoltaic materials are typically defect-rich, and defects impede electronic transport and photoconversion efficiency. Since efficiency and cost are inversely related, defect-rich materials have until recently resulted in poor-quality, economically uncompetitive solar cells. In this presentation, we review defect physics in low-cost photovoltaic absorbers. Accurate identification of performance-limiting defects requires multiscale characterization, evaluating cm-size devices while probing down to the nanometer scale for defect recognition. We will review recent advances in macroscopic CCD-based PV characterization tools, and elucidate how these can be coupled to synchrotron nanoprobe techniques. Once the nature and underlying physical behavior of these defects are known, we demonstrate how manipulation of defect distribution and state, aided by predictive modeling, can enhance solar cell performance. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L5.00003: Nanoscale heat transfer and thermoelectrics for alternative energy Invited Speaker: In the area of alternative energy, thermoelectrics have experienced an unprecedented growth in popularity because of their ability to convert waste heat into electricity. Wired in reverse, thermoelectrics can act as refrigeration devices, where they are promising because they are small in size and lightweight, have no moving parts, and have rapid on/off cycles. However, due to their low efficiencies bulk thermoelectrics have historically been a niche market. Only in the last decade has thermoelectric efficiency exceeded $\sim $20{\%} due to fabrication of nanostructured materials. Nanoscale materials have this advantage because electronic and acoustic confinement effects can greatly increase thermoelectric efficiency beyond bulk values. In this talk, I will introduce our work in the area of nanoscale heat transfer with the goal of more efficient thermoelectrics. I will discuss our experiments and methods to study acoustic confinement in nanostructures and present some of our new nanostructured thermoelectric materials. To study acoustic confinement we are building a nanoscale phonon spectrometer. The instrument can excite phonon modes in nanostructures in the $\sim $100s of GHz. Ballistic phonons from the generator are used to probe acoustic confinement and surface scattering effects. Transmission studies using this device will help optimize materials and morphologies for more efficient nanomaterial-based thermoelectrics. For materials, our group has synthesized nano-layer superlattices of Na$_{x}$CoO$_{2}$. Sodium cobaltate was recently discovered to have a high Seebeck coeficent and is being studied as an oxide thermoelectric material. The thickness of our nano-layers ranges from 5 nm to 300 nm while the lengths can be varied between 10 $\mu $m and 4 mm. Typical aspect ratios are 40 nm: 4 mm, or 1:100,000. Thermoelectric characterization of samples with tilted multiple-grains along the measurement axis indicate a thermoelectric efficiency on par with current polycrystalline samples. Due to phonon confinement in nano-structures, it is expected that the thermoelectric efficiency of these sheets will be much higher than that of single crystalline Na$_{0.7}$CoO$_{2}$, when the nanosheets have single grains along the heat transport path. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L5.00004: Alternative Energy: A New Frontier for Microfluidics Invited Speaker: Microfuidics is classified as the physics of fluid manipulation at sub-mm length scales. Typically, microfluidic techniques benefit from small sample volumes, low power consumption, and increased surface-to-volume ratio. Because of their high surface to volume ratio, microfluidic systems often utilize surface phenomena such as wettability (i.e. droplet microfluidics) and surface charge (i.e. electrokinetics) for actuation. To date, most applications of microfluidics are in medicine or biology with the purpose of creating ``lab on a chip'' devices. However, the scale of microfluidics is favorable for other engineering problems as well. In this talk we will discuss how phenomena typically applied to lab on a chip devices can be used to enhance energy systems. Specifically, we explore electric field driven fluid and particle flows such as electrophoresis, electroosmosis, and dielectrophoresis. We will show how these phenomena can solve a diverse array of problems, from water management in fuel cells to the selection of microorganisms for bio-energy applications. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L5.00005: Fusion related physics: Understanding the basic physics of High Energy Density Plasmas (HEDP) using ultra-short pulse laser-matter interactions Invited Speaker: Nuclear fusion is one nature's most fundamental methods of generating energy. In stars, the fusion reactions that occur deep within stellar interiors generate radiation and particles that fill the Universe. For many years, a goal of scientists has been to utilize these processes on earth to generate energy. However, understanding the basic physics of the interacting particles is required to exploit this energy source. We present data and analysis from one technique (ultra-short pulse laser matter interactions) currently being used to understand this physics. High power, short pulse lasers offer the ability of studying matter heated to extremely high temperatures ( as high as 700 eV) and near solid density (10$^{22}$ part/cm$^{3})$. Two aspects of the basic physics will be presented, namely radiation absorption and particle energy exchange currently under investigation using these lasers. [Preview Abstract] |
Session L6: Complexity in Invention: The Strongly Coupled Systems that Contribute to Innovation Success
Sponsoring Units: FIAPChair: Steven Rosenblum, Corning, Inc., Science and Technology
Room: Ballroom C2
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L6.00001: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L6.00002: Value analysis for advanced technology products Invited Speaker: Technology by itself can be wondrous, but buyers of technology factor in the price they have to pay along with performance in their decisions. As a result, the ``best'' technology may not always win in the marketplace when ``good enough'' can be had at a lower price. Technology vendors often set pricing by ``cost plus margin,'' or by competitors' offerings. What if the product is new (or has yet to be invented)? Value pricing is a methodology to price products based on the value generated (e.g. money saved) by using one product vs. the next best technical alternative. Value analysis can often clarify what product attributes generate the most value. It can also assist in identifying market forces outside of the control of the technology vendor that also influence pricing. These principles are illustrated with examples. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L6.00003: Innovation, Novel Solutions and New Devices: The Engines that Drive the Magnetic Storage Industry; Choosing the Right Combination Invited Speaker: Magnetic storage technology aims to achieve recording densities $>$ 10$^{12}$ bits/in2 in the foreseeable future. The dimensions of the magnetic domains and sensor minimum feature sizes at this density will be $\sim $ 15 -- 25 nm. These nanoscale dimensions present major challenges for both the materials utilized for magnetic recording, and to the sensors employed to reliably detect the minute magnetic fluxes emanating from such nanoscale domains. These include fundamental physical limits of material properties on account of the reduced dimensionality, as well as nanofabrication challenges to attain the required nanometer feature sizes with the stringent dimensional tolerances required. Since its invention in 1954, the storage density in magnetic recording has incremented by 10$^{9}$ and the cost of storage, measured in {\$}/MB, has undergone a price reduction of the same order. Impressive as these accomplishments are, is the fact that the fundamental engineering principles of the technology today are essentially the same as when it was invented. This is in spite of numerous efforts to replace it with new alternative technologies or by dire predictions by its own practitioners of its impending death based on perceived limitations. In this talk the state-of-the art and challenges facing the HDD industry in its efforts to continue incrementing the storage density will be discussed. I will illustrate how advances in materials engineering, new physical phenomena and breakthroughs in nanofabrication have facilitated such an impressive technology evolution. Moreover, the key ingredients for said innovations to be implemented as technology solutions will be discussed.. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L6.00004: Manufacturing physics: using large(r) data sets and physical insight to develop great products Invited Speaker: Early stage research does a fantastic job providing knowledge and proof-of-feasibility for new product concepts. However, the handful of data points required to validate a concept is typically insufficient to provide insight on the whole range of effects relevant to manufacturing the product. Moving to manufacturing brings larger data sets and variability; opportunistic analysis of these larger sets can yield better product design rules. In the early 2000s Corning developed an optical transmission fiber optimized to suppress stimulated Brillouin scattering (SBS). Analyzing the larger data set provided by the manufacturing environment using the same theoretical framework developed by the original researchers refined our understanding of how to improve SBS in optical fibers beyond what was known from our early efforts. This greater understanding allowed us to design better performing products. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L6.00005: Foundational Forces {\&} Hidden Variables in Technology Commercialization Invited Speaker: The science of physics seems vastly different from the process of technology commercialization. Physics strives to understand our world through the experimental deduction of immutable laws and dependent variables and the resulting macro-scale phenomenon. In comparison, the~goal of business is to make a profit by addressing the needs, preferences, and whims of individuals in a market. It may seem that this environment is too dynamic to identify all the hidden variables and deduct the foundational forces that impact a business's ability to commercialize innovative technologies. One example of a business ``force'' is found in the semiconductor industry. In 1965, Intel co-founder Gordon Moore predicted that the number of transistors incorporated in a chip will approximately double every 24 months. Known as Moore's Law, this prediction has become the guiding principle for the semiconductor industry for the last 40 years. Of course, Moore's Law is not really a law of nature; rather it is the result of efforts by Intel and the entire semiconductor industry. A closer examination suggests that there are foundational principles of business that underlie the macro-scale phenomenon of Moore's Law. Principles of profitability, incentive, and strategic alignment have resulted in a coordinated influx of resources that has driven technologies to market, increasing the profitability of the semiconductor industry and optimizing the fitness of its participants. New innovations in technology are subject to these same principles. So, in addition to traditional market forces, these often unrecognized forces and variables create challenges for new technology commercialization. In this talk, I will draw from ethnographic research, complex adaptive theory, and industry data to suggest a framework with which to think about new technology commercialization. Intel's bio-silicon initiative provides a case study. [Preview Abstract] |
Session L7: System Biology I: The Physics of Development
Sponsoring Units: DBPChair: David Lubensky, University of Michigan
Room: Ballroom C3
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L7.00001: Robustness in multicellular systems Invited Speaker: Cells and organisms cope with the task of maintaining their phenotypes in the face of numerous challenges. Much attention has recently been paid to questions of how cells control molecular processes to ensure robustness. However, many biological functions are multicellular and depend on interactions, both physical and chemical, between cells. We use a combination of mathematical modeling and molecular biology experiments to investigate the features that convey robustness to multicellular systems. Cell populations must react to external perturbations by sensing environmental cues and acting coordinately in response. At the same time, they face a major challenge: the emergence of conflict from within. Multicellular traits are prone to cells with exploitative phenotypes that do not contribute to shared resources yet benefit from them. This is true in populations of single-cell organisms that have social lifestyles, where conflict can lead to the emergence of social ``cheaters,'' as well as in multicellular organisms, where conflict can lead to the evolution of cancer. I will describe features that diverse multicellular systems can have to eliminate potential conflicts as well as external perturbations. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L7.00002: Axis Specification in Hydra Invited Speaker: Hydra is an about cm sized polyp of roughly 10$^5$ cells exhibiting surprising robustness: it can regenerate even from a random cell aggregate made from its own cells. During such a reorganization, hydra first forms a hollow cell sphere. We show that even a weak temperature gradient directs the axis of the regenerating animal -- but only if it is applied during the symmetry-breaking moment. We observe that the spatial distribution across the cell sphere of the early expressed, head-specific gene ks1 has become scale-free and fractal at that point. We suggest that in order to break the symmetry and define an axis during the regeneration process, the cell network organizes towards a state, that is characterized by an unusually high sensitivity to external perturbation as well as spatially self-similar gene expression patterns. The observed behavior arises naturally from next-neighbor cell communication, when long-range signaling as required for axis definition is achieved through increased synchronization of expression profiles. Numerical results in progress show that our observations can be robustly reproduced with avalanches of gene expression patterns generated from gene switching above a stimulation threshold. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L7.00003: Surface cell differentiation controls tissue surface tension and tissue positioning during zebrafish gastrulation Invited Speaker: Differences in tissue surface tension (TST) between different tissue types are thought to guide tissue organization and cell sorting in development. Measurements of TST have been useful to predict the outcome of in vitro cell sorting and envelopment experiments. However, the outcome of cell sorting experiments in vitro often substantially differs from tissue positioning in vivo, raising questions as to the actual contribution of TST to tissue positioning within the developing embryo. Here, we show that surface tension of germ layer tissues during zebrafish gastrulation critically relies on the differentiation of their surface cells. We also show that surface differentiation of the different germ layer tissues varies and is considerably different between the situation in vitro and in vivo, explaining the apparent dissimilar outcome of cell segregation between these two situations. To analyze germ layer TST as a function of surface cell differentiation, we interfere with surface cell properties of germ layer aggregates by misexpressing genes involved in surface cell differentiation specifically within surface cells using the GAL4-UAS system, and measure tissue surface tension using both parallel plate compression and micropipette aspiration techniques. Our data provides evidence in favor of a critical function of surface cell differentiation in modulating TST and subsequently tissue positioning within the developing embryo. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L7.00004: Self-organized cytoskeletal dynamics during fruit fly epithelial morphogenesis Invited Speaker: Epithelial morphogenesis plays a major role in embryonic development. During this process cells within epithelial sheets undergo complex spatial reorganization to form organs with specific shapes and functions. The dynamics of epithelial cell reorganization is driven by forces generated through the cytoskeleton, an active network of protein filaments and motor proteins. In this talk, I will present a novel mesoscopic-scale physical description of force generation by the cytoskeleton, and show that this minimal description can account for a wide range of phenomena associated with fruit fly epithelial morphogenesis. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L7.00005: Collective Chemotactic Cell Movement; a Key Mechanism of Development and Morphogenesis Invited Speaker: We investigate the molecular mechanisms by which cells produce and detect chemotactic signals and translate this information in directed movement up or down chemical gradients in the social amoebae \textit{Dictyostelium discoideum,} and during gastrulation in the chick embryo. Investigation of Dictyostelium mutants with changes in cAMP cell-cell signalling dynamics and chemotaxis, show how cellular heterogeneity in signalling dynamics and polarised activation of the actin-myosin cytoskeleton drive aggregation, cell sorting, slug formation and migration. Chemotactic cell movement also plays a critical role during gastrulation in the chick embryo a model for amniote development. We suggest that epiblast cell movement during the formation of the primitive streak as well as the movement of the mesoderm cells after their ingression through the streak is controlled by a combination of attractive and repulsive guidance cues. We use computer models explore signalling and cell movement interact to give rise to emergent phenomena at the tissue and organism level such as pattern formation and morphogenesis. [Preview Abstract] |
Session L8: J. H. Van Vleck: Quantum Theory and Magnetism
Sponsoring Units: FHPChair: Chun Lin, University of Wisconsin
Room: Ballroom C4
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L8.00001: Van Vleck from Spectroscopy to Susceptibilities: Kuhn Losses Regained Invited Speaker: As a young assistant professor in Minneapolis, John H. Van Vleck spent much of his time between 1923 and 1926 writing a book-length Bulletin for the National Research Council. As its title, Quantum Principles and Line Spectra, suggests, the book focuses almost exclusively on spectroscopy, the core pursuit of the old quantum theory. By the time it finally appeared in 1926, the old quantum theory had given way to the new quantum mechanics. Van Vleck soon realized that matrix mechanics reinstated some well-confirmed results of the classical theory of susceptibilities that had been lost in the old quantum theory. In the history and philosophy of science literature, such losses are called 'Kuhn losses'. Using mathematical techniques similar to those presented in his NRC Bulletin, Van Vleck started to work on the theory of susceptibilities. In 1929, now a full professor in Madison, he began writing another book, which appeared in 1932 and has become a classic: The Theory of Electric and Magnetic Susceptibilities. In this talk I follow Van Vleck's trajectory from spectroscopy to susceptibilities and examine how his two books reflect and helped shape research traditions.The talk is based on joint work with Charles Midwinter. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L8.00002: J. H. Van Vleck and Magnetism at the University of Wisconsin: 1928 -1934 Invited Speaker: In 1928, John Van Vleck returned to his alma mater to take a position in the Physics Department. Six years later he left to join the faculty of Harvard University. While Van Vleck was at Wisconsin, he began a series of theoretical studies that helped lay the foundation for the modern theory of magnetism in solids. In 1932 Van Vleck published his celebrated monograph, \textit{The Theory of Electric and Magnetic Susceptibilities, }in which he made use of the new theory to explain the results of experimental studies in a variety of magnetic materials. In my talk, I will review the accomplishments of Van Vleck and his students during this period and also comment briefly on his notes for a second edition of the book. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L8.00003: My interactions with J.H. Van Vleck as a student and colleague at Harvard Invited Speaker: In the summer of 1947 I participated as a graduate student in discussions with professors J.H. Van Vleck, C.J. Gorter and E.M. Purcell on exchange narrowing of magnetic resonance lines. Subsequent work on exchange broadening and narrowing in nuclear spin systems will be reviewed and interspersed with personal reminiscences. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L8.00004: Remembering Van: Three Madison families, and other tales Invited Speaker: The talk will present some history, in part personal, in part scientific, of Van's influence on other scientists and on magnetic resonance. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:30PM |
L8.00005: Van Vleck and the magnetic susceptibilities of gaseous molecules Invited Speaker: In his 1927 Physical Review article and in his 1932 book, ~\textit{The Theory of Electric and Magnetic Susceptibilities,~}Van Vleck used the new quantum theory to derive the magnetic susceptibilities of O$_{2}$ and NO in their gaseous form ~and compared them with experiments. ~He was therefore very interested in low temperature susceptibility experiments on O$_{2}$ at Oxford University in 1954 where individual O$_{2}$ molecules were trapped in small, almost spherical cages in organic clathrates. Correspondence between him and this speaker, then at Oxford, led to further measurements of O$_{2}$ and also of NO in such clathrates, to theory and to subsequent publications and correspondence. Later communication with Van Vleck on the magnetism in rare earth iron garnets, a subject of long-term interest to him, will be described ~in connection with experiments carried out at Duke University. Some fond personal ~recollections of this speaker of his interaction with Van Vleck - both while at Harvard, during visits and through correspondence which extended into the seventies - will be presented. [Preview Abstract] |
Session L9: Micelles and Vesicles I
Sponsoring Units: DFDChair: Robin Selinger, Kent State University
Room: D220
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L9.00001: Faceting of multicomponent charged elastic shells Rastko Sknepnek, Cheuk Leung, Liam C. Palmer, Graziano Vernizzi, Samuel I. Stupp, Michael J. Bedzyk, Monica Olvera de la Cruz Combining coarse-grained molecular dynamics simulations with continuum elastic theory, we show that electrostatic interactions between charged lipid head groups can lead to the crystallization of the bilayer. Regions with different molecular charge ratios have distinct elastic properties and naturally tend to segregate inducing an effective line tension between neighboring patches. The line tension and local patch-dependent elastic properties, i.e., bending rigidity and Young modulus, have a drastic effect on the shell shape. We explore a wide region of parameter space and find a gallery of faceted structures, closely resembling shapes of shells recently identified experimentally. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L9.00002: Modeling co-evolution of defects and curvature in lipid vesicles: coarse-grained simulation studies Robin Selinger, Jun Geng, Jonathan Selinger To explore interaction between topological defects and curvature in lipid vesicles, we present a coarse-grained simulation approach in which defects and vesicle shape both evolve in time. First we model a vesicle cooled into the tilted gel phase. To represent the tilt field at the mesoscale, we superimpose an XY model onto a coarse-grained liquid membrane [1] where each particle represents a patch of lipid bilayer. The presence of two +1 defects drives the vesicle to a prolate equilibrium state as previously predicted; but extra +1/-1 defect pairs may induce a highly disordered shape which is deeply metastable. We discuss comparison with relevant experiments. Next we consider a lipid vesicle with nematic order, e.g. composed of lipids with a rod-shaped head group. With weak coupling between defects and curvature, the vesicle is spherical with four +1/2 defects. With stronger coupling, the vesicle becomes prolate with two defects clustered at each end. As coupling is further increased, pores nucleate at the defects and coalesce, producing a hollow cylinder. We compare simulation results with theoretical predictions and consider further applications e.g. to study tilt and defects in gel phase lipid rafts.\\[4pt] [1] H. Yuan et al,~\textit{Phys Rev E}~82~(2010) 011905 [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L9.00003: Coarse-grained model for lipid bilayer membranes and vesicles Jun Geng, Jonathan Selinger, Robin Selinger We present a coarse-grained model for simulation studies of lipid bilayer membranes and vesicles. We separately track the behavior of the leaflets in each bilayer, allowing us to model the mechanics of vesicles and a rich array of other phases, topologies, and defect structures. Each particle in the coarse-grain model represents a patch of lipid molecules and carries a vector degree of freedom, representing the local average lipid chain orientation. Particles interact via a pair potential depending on separation distance and relative chain orientation. Solvent is treated as implicit, and membrane fluctuations are modeled via a Langevin thermostat. Resulting bilayer structures show liquid-like diffusion within each leaflet. We show that bilayer vesicles coalesce spontaneously from a random initial state, even though no spontaneous curvature is imposed by the model. We also explore the transition from vesicles to lamellar phases as a function of increasing density. We discuss potential application to the study of vesicle fission and fusion. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L9.00004: Molecular dynamics study of shape transitions in aqueous micelle solutions A. Sangwai, R. Sureshkumar It is well known that surfactant molecules self-assemble in aqueous solutions to form various micellar structures such as spheres, rods or sheets. Although this phenomenon is widely studied experimentally, the molecular mechanisms of shape transitions are not well understood. Atomistic simulations of self-assembled micellar systems are computationally prohibitive to sample several hundred nanoseconds necessary to capture shape transitions. We demonstrate that MARTINI coarse-grained (CG) force field for CTAC is capable of accurately representing micellar assemblies by comparing the CG system to fully atomistic ones. Microsecond molecular dynamic simulations using MARTINI CG models in explicit water are used to predict sphere to rod transitions in micelles. Inter-micelle association free energies are estimated to distinguish between the chemical environments in which the micelle assumes a spherical versus rod-like shape. Presence of hydrophobic salt e.g. Sodium Salicylate, is shown to greatly promote the formation of rodlike structures. CG MARTINI molecular dynamics is benchmarked as a practical approach to study nano-scale micellar structures. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L9.00005: Ordered bulk aggregates of lipid vesicles Ana Hocevar, Primoz Ziherl We study the structure of bulk assemblies of identical lipid vesicles. In our model, each vesicle is represented as a convex polyhedron with flat faces, rounded edges, and rounded vertices. Each vesicle carries an elastic and an adhesion energy and it turns out that in the limit of strong adhesion, the minimal-energy shape of cells minimizes the weighted total edge length. We calculate the shape of the rounded edge exactly and show that it can be well described by a cylindrical surface. We compare several candidate space-filling polyhedra and we find that the oblate shapes are preferred over prolate shapes for all volume-to-surface ratios. We also study aggregates of vesicles whose adhesion strength on lateral faces is different from that on basal/apical faces. We determine the anisotropy needed to stabilize prolate shapes and we show that at any volume-to-surface ratio, the transition between the oblate and the prolate shapes is very sharp. We compare the geometry of the model vesicle aggregates with the shapes of cells in certain simple animal tissues. Predictions of our model are consistent with available experimental data. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L9.00006: Understanding crumpling lipid vesicles at the gel phase transition Linda Hirst, Adam Ossowski, Matthew Fraser Wrinkling and crumpling transitions in different membrane types have been studied extensively in recent years both theoretically and computationally. There has also been very interesting recent work on defects in liquid crystalline shells. Lipid bilayer vesicles, widely used in biophysical research can be considered as a single layer smectic shell in the liquid crystalline phase. On cooling the lipid vesicle a transition to the gel phase may take place in which the lipid chains tilt and assume a more ordered packing arrangement. We observe large scale morphological changes in vesicles close to this transition point using fluorescence microscopy and investigate the possible mechanisms for this transition. Confocal microscopy is used to map 3D vesicle shape and crumpling length-scales. We also employ the molecular tilt sensitive dye, Laurdan to investigate the role of tilt domain formation on macroscopic structure. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L9.00007: Thin shell vesicles composed of hydrophilic plate-like nanoparticles Anand Subramaniam, Jiandi Wan, Arvind Gopinath, Howard Stone Nanopowders of graphene oxide, montmorillonite and laponite spontaneously delaminate into ultrathin nanoscopic plates when dispersed in water. These plates, which are typically $\sim $ 1 nm thick and microns in lateral dimension, have found many uses as precursors to graphene, ceramics, layer-by-layer structures, and as structural modifiers of nanocomposites. Here we show that mechanical forces due to shear in a narrow gap can assemble hydrophilic plate-like particles on air bubbles, forming stable nanoplated armored bubbles. Translucent inorganic vesicles (vesicles defined here as closed thin-shelled structures with the same liquid inside and outside) of these particles are produced when the nanoplated armored bubbles are exposed to common water-miscible organic liquids and surfactants. These inorganic vesicles are mechanically robust, have walls that are about six nanometres thick, and are perforated with pores of submicron dimensions. We characterize the phenomenon and find that a wetting transition at the scale of the nanoparticles is the primary mechanism of formation. The discovery of these novel inorganic structures raises a wealth of questions of fundamental interest in materials and surface science. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L9.00008: The effect of interlayer distance of thickness fluctuations in a swollen lamellar phase: A neutron spin echo study Michihiro Nagao Thickness fluctuations in surfactant membranes have been measured using small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques as a function of the membrane thickness in a swollen lamellar structure composed of nonionic surfactant, water and oil. An excess dynamics from the bending motion was observed around the length scales of the membrane thickness, which originates from thickness fluctuations of the membranes. The amount of oil in the bilayers controls the interlayer distance (membrane thickness) and the bending motion of the membranes. An enhancement of the thickness fluctuations suppresses the bending motion, which introduces the increase in the bending modulus at low swelling condition. The decrease in the bending modulus with further increase in the thickness indicates the decrease of the synchronization between monolayers. In the high swelling conditions, the monolayer movement dominates the dynamics of the membranes in the measured dynamic range. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L9.00009: The effect of interlayer distance on thickness fluctuations in a swollen lamellar phase: A molecular dynamics study Sukhum Chawang, Takumi Hawa Molecular dynamics simulations have been conducted to characterize thickness fluctuations in a swollen lamellar structure, composed of a non-ionic surfactant, water, and oil, to verify the results of the neutron scattering experiments by Nagao. The thickness fluctuations are measured as an excess dynamics from the bending motion around the length scales of the membrane thickness and as a function of the interlayer distance (membrane thickness). The enhancement of the thickness fluctuations is observed in all ranges of thickness we simulated; however, it decays with increase of the membrane thickness. Dependence of directions of sampling wave vectors q on the thickness fluctuation is also investigated. At more normal direction (perpendicular to the membrane surfaces) the excess dynamics is clearly observed, while at more lateral direction (parallel to the membranes) the bending motion is more clearly observed. The present results show the existence of the enhancement of the thickness fluctuations and the importance of the sampling directions. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L9.00010: Studies of lipid vesicle mechanics using an optical fiber dual-beam trap Tessa M. Pinon, Linda S. Hirst, Jay E. Sharping Fiber-based optical traps can be used for manipulating micron-sized dielectric particles such as microspheres and biological cells. Here we study the mechanics of giant unilamellar vesicles (GUVs) which are held and stretched by light forces in a fiber-based dual-beam optical trap. Our GUVs are suspended in a buffer solution and encapsulate various concentrations and molecular weights of poly(ethylene glycol) (PEG) polymer yielding a range of refractive index contrasts and trapping conditions. We find that we can trap GUVs in solution with index contrasts of less than 0.01. We explore the mechanical response of the GUV membrane to a range of forces which are proportional to laser power and refractive index contrast. Our trapping system is a compact and inexpensive platform and trapping is viewed in real time under a microscope. We hypothesize that forces within the high-tension regime will induce a linear response in vesicle surface area. This project sets the stage for membrane mechanics and lipid phase change studies. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L9.00011: Thermal Stress of Supported Lipid Bilayer Induces Formation and Collapse of Uniform Radius Tubules Kimberly Weirich, Deborah Fygenson Supported lipid bilayer (SLB) provides a model system in which to quantitatively investigate fluid bilayer transitions from planar to tubular and tubular to spherical morphologies. Following a small increase in temperature, flexible filaments extrude from a fluid SLB. Individual filaments can reach hundreds of microns in length before spontaneously collapsing into discs. We demonstrate that the filaments are tubular and report the effects of lipid composition and flow-induced tension on their properties. At high ionic strength, the sub-resolution tubules are adsorbed to the SLB, enabling the measurement of their radius to within $\pm $5 nm using fluorescence microscopy. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L9.00012: A Time-Resolved Study on Nanodisc-to-Vesicle Transformation Mu-Ping Nieh, Suanne Mahabir, Wan Kei Wan, John Kastaras Structural phase diagram of a phospholipid mixture composed of dimyristoyl phosphatidylcholine (DMPC), dihexanoyl phosphatidylcholine (DHPC) and dimyristoyl phosphatidylglycerol (DMPG) contains many rich morphologies, e.g., nanodiscs also known as ``bicelles'', bilayered ribbons, unilamellar vesicles (ULVs), multi-lamellar vesicles (MLVs) and perforated lamellae. In this report, we will present time-resolved small angle neutron scattering and dynamic light scattering measurements of the structural transformation from nanodiscs to ULVs as a function of temperature, lipid concentration and charge density. The result will reveal the growth rate of nanodiscs and all the intermediate structures along the transformation process. Through the understanding of the kinetic pathway, the size and polydispersity of the self-assembled nano-size ULVs can be well-controlled. These ULVs can be used as a carrier for therapeutics or imaging probes. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L9.00013: Crystallization Induced by Electrostatic Correlations in Vesicles of Mixed-Valence Ionic Amphiphiles Cheuk Yui Leung, Rastko Sknepnek, Liam Palmer, Graziano Vernizzi, Megan Greenfield, Samuel Stupp, Michael Bedzyk, Monica Olvera de la Cruz Charged amphiphilic molecules, including molecules with biological motifs, have been predicted to organize into elastic membrane or crystalline shells with non-spherical shapes. We demonstrate that pure electrostatic interaction allow (-1) anionic water insoluble amphiphiles and (+3) cationic amphiphiles, which form only micelles in water, to co-assemble into buckled vesicles. The strong interaction between the +3 and -1 head groups increases the cohesive energy of the amphiphiles and favors the formation of crystallized membranes or shells that facet spontaneously into buckled shapes predicted by simulations of vesicles with heterogeneous elastic properties. In situ small-angle and wide-angle X-ray scattering (SAXS-WAXS) experiments conducted at the Advanced Photon Source DND-CAT confirm the presence of crystalline bilayers. Our simulations verify that ionic lateral correlations among the oppositely charged head groups of the co-assembled amphiphiles are responsible for the observed tail crystallization. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L9.00014: Endocytic internalization of nanoparticles into polymeric vesicles. Anja Kroeger, Karmena Jaskiewicz, Antje Larsen, George Fytas The monitoring of transport through cell membranes is essential for proper functioning of all living organisms. Poorly understood mechanisms of endocytosis have become the focus of intense investigations. Here we present a photon correlation spectroscopy study of the uptake of polystyrene nanoparticles (hydrodynamic radius, R$_{h}$=16nm) by poly(dimethylsiloxane)-b-poly(2-methyloxazoline) polymersomes (R$_{h}$=150nm) in aqueous solution. The relaxation function C(q,t) for a particle/polymersome mixture with a molar ratio 100:1 at different scattering wave vectors (q) reveal the presence of free and bound particles. Both the experimental form factor P(q) and the effective diffusion coefficient D(q) of the polymersome in the q-range of 0.005-0.033nm$^{-1 }$are consistently described by modeling these q-patterns by a filled polymersome with about 30 particles under the examined conditions. The emerged picture is supported by cryo-TEM imaging. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L9.00015: AC-Electrokinetic Characterization and Induced Encapsulation Release of Micelles in Aqueous Suspensions Victoria Froude, Yingxi Elaine Zhu Micelles and polymers vesicles have been of increasing interest as drug delivery systems for controlled release, specific cell targeting, and medical diagnostics. In addition, AC-electrokinetic techniques have emerged as a viable option for colloidal and biocolloidal manipulation. In this work, we examine the dielectrophoresis (DEP) characteristics of complex micellar nanoparticles under non-uniform AC-electric field of varied ac-field frequencies (5 kHz-20 MHz) and amplitudes (0.1-10 Vpp) by fluorescence correlation spectroscopy (FCS) at a single-molecule resolution. We focus on the AC-field induced transport of sodium tetradecyl sulfate (STS) and sodium dodecyl sulfate (SDS) micelles tagged with various fluorescent and drug encapsulates in aqueous media. We observe a strong AC-frequency dependence of micelle concentration between two microelectrodes, from which the DEP crossover frequency is determined. Surprisingly, we also observe an AC-field induced dissociation of the micelle structure and a resulting release of fluorescent encapsulates at a characteristic low AC-field frequency of approximately 1-10 kHz, where the dissociation has been found to be dependent on the surface charge of the interior encapsulate. [Preview Abstract] |
Session L10: Nanoclusters and Nanowires on Surfaces
Sponsoring Units: DCMPChair: Phillip Sprunger, Louisiana State University
Room: D221
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L10.00001: Structure and dynamics of PtSn/$\gamma$Al$_2$O$_3$ F.D. Vila, J.J. Rehr, S.D. Kelly, S.R. Bare Supported metal clusters have many industrial applications, especially in heterogeneous catalysis. Their activity and durability is determined by their internal atomic and electronic structure, as well as by their interaction with the support. We have previously shown\footnote{F. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).} that unusual phenomena such as large structural disorder and negative thermal expansion in supported Pt clusters can be understood by using a combination of MD and x-ray absorption spectroscopy simulations. Here we present results for prototypical Pt$_{10}$Sn$_{10}$ alloy clusters on $\gamma$Al$_2$O$_3$. Our simulations show that the internal structure and surface location of the clusters varies dynamically on a time scale of a few ps. While the Sn atoms are especially mobile, the clusters have well defined Pt-Pt and Pt-Sn coordination shells at $\sim$2.75\AA. Moreover, at any instant there are between 2 and 5 bonds between the Pt/Sn and the O atoms in the surface. Finally, we present simulations of the XANES spectra and their relation to charge transfers between atoms in the cluster and between the cluster and the surface. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L10.00002: Polarization dependent Pd deposition structure on LiNbO$_{3}$ \{0001\} surface Seungchul Kim, Andrew M. Rappe We investigate effects of polarization orientation on atomic structure of palladium deposited on lithium niobate (LiNbO$_{3}$) \{0001\} surface, using density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations. Adsorption, diffusion, aggregation and clustering process -- include geometries, paths and energies -- of Pd clusters were calculated from DFT simulations. It has been observed that energy barriers of Pd motions on the negatively poled ($c^{-}$) surface are much larger than those on the positively poled surface ($c^{+}$), which indicates the Pd motions on the $c^{-}$ surface are much slower than that of $c^{+}$ surface. We demonstrate, using kMC with kinetic parameters from DFT, very slow motion of Pd on $c^-$ surface leads dispersed small clusters or atoms while fast motion on $c^+$ surface leads large clusters, indicating larger Pd-covered area on $c^-$ surface than $c^+$ after Pd deposition. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L10.00003: Cu/CuOx Nanoclusters on ZnO(1010): Electronic, Catalytic, Morphological Structure Ziyu Zhang, Fei Wang, Maomin Ren, Frank Womack, Minh Le, Yaroslav Losovyi, Richard Kurtz, Phil Sprunger, John Flake To develop a high performance catalyst for CO2 reduction, we tried bi-layers based on CuOx (x=0, 1, 2) and ZnO. The highest yield rate is found for Cu(I) on ZnO. The repeatability of the experiment illustrates that the Cu(I) catalytic clusters are stable in the air, due to the interface of the bilayer.STM and ARPUS results reveal that the preparation process are highly dependent on the annealing temperature and cluster size. EELS and UPS data show that CO adsorption is distinctly different between Cu and CuOx clusters on ZnO, which explains the different yield rate. Based on TDS and EELS of adsorption such as CO2, H2O, combined with DFT calculation, the mechanism of methanol synthesis is given by introducing intermediate products. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L10.00004: Role of nitrogen dopants in the stabilization of nancrystalline cubic zirconia Renat Sabirianov, G. Wang, Y.L. Soo, G. Luo, H.J. Lin, W.N. Mei, F. Namavar, C.L. Cheung The role of the nitrogen doping in the stabilizing the nanocrystalline cubic phase of zirconia films fabricated by nitrogen beam assisted deposition (IBAD) is investigated. The IBAD zirconia films have diameters three times larger than those previously reported in a sol-gel method. Confocal Raman spectroscopy study indicates that the atomic structure of these IBAD zirconia films evolve from cubic to tetragonal and then to monoclinic phase upon annealing at elevated temperatures. The presence of nitrogen in the films is confirmed by secondary ion mass spectroscopy. X-ray absorption near edge structure study of these films infers that the nitrogen atoms are incorporated at the substitutional sites of these films. Ab-initio density functional calculations suggests that the substitutional nitrogen atoms could effectively immobilize native defect including oxygen vacancies and interstitial ions in nanocrystalline cubic zirconia. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L10.00005: Preparation dependent neutralization efficiency of Pt/TiO2 nanoparticles Alex Arjad, Yarmoff Jory Metal nanoclusters resident on an oxide surface can be produced by sputtering thin films as well as by direct deposition. We previously used the neutralization of scattered low energy alkali ions to demonstrate that Au nanoclusters formed by sputtering a thin gold film on TiO$_2$ have similar electronic properties as those formed by deposition [1]. In this work, we compare Pt nanoclusters grown on TiO$_2$ by both sputtering and deposition. It is shown that Pt nanoclusters formed by deposition are more efficient at neutralizing scattered low energy Na+ ions than those formed by sputtering a thin platinum film. We attribute this difference to the strong-metal-support-interaction (SMSI) present in the Pt/TiO$_2$ system, but not with Au/TiO$_2$. \\[4pt] [1] P. Karmakar, G.F. Liu, Z. Sroubek and J.A. Yarmoff, Phys. Rev. Lett. 98, 215502 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L10.00006: Nanoparticle shape instability by Coulomb interactions Natalya Zimbovskaya Metal atoms adsorbed on few-layer graphenes condense to form nanometer-size droplets whose growth in size is limited by a competition between the surface tension and repulsive electrostatic interactions from charge transfer between the metal droplet and the graphene. Under certain conditions a growing droplet can be unstable to a family of shape instabilities. This phenomenon was observed for Yb deposited and annealed on few-layer graphenes. A theoretical model to describe it is developed. The model describes the onset of shape instabilities for nanoparticles where their growth is limited by a generic repulsive potential and provides a good account of the experimentally observed structures for Yb on graphene [1]. \\[4pt] [1] L. A. Somers, N. A. Zimbovskaya, A. T. Johnson, and E. J. Mele, PhPhys. Rev B 82, 115430 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L10.00007: Structure and Energy Stability of Metal Nanoparticles Hector Barron, Juan Pedro Palomares-Baez, Jesus Velazquez-Salazar, Jose Luis Rodriguez-Lopez, Miguel Jose-Yacaman In this work we present a theoretical model for the structural evolution and energy stability for metal nanoparticles from the small (1-2 nm) to the big ($\sim $50 nm) size ranges. We have found that the appearances of structural lattice defects as well as surface reconstructions are important factors that highly influence the growth process. A simple assembly model for a path transformation for metal nanoparticles is presented and compare with experimental evidence. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L10.00008: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L10.00009: Attachment of Quantum Dots on Zinc Oxide Nanorods Jared Seay, Huan Liang, Parameswar Harikumar ZnO nanorods grown by hydrothermal technique are of great interest for potential applications in photovoltaic and optoelectronic devices. In this study we investigate the optimization of the optical absorption properties by a low temperature, chemical bath deposition technique. Our group fabricated nanorods on indium tin oxide (ITO) substrate with precursor solution of zinc nitrate hexahydrate and hexamethylenetramine (1:1 molar ratio) at 95C for 9 hours. In order to optimize the light absorption characteristics of ZnO nanorods, CdSe/ZnS core-shell quantum dots (QDs) of various diameters were attached to the surface of ZnO nanostructures grown on ITO and gold-coated silicon substrates. Density of quantum dots was varied by controlling the number drops on the surface of the ZnO nanorods. For a 0.1 M concentration of QDs of 10 nm diameter, the PL intensity at 385 nm increased as the density of the quantum dots on ZnO nanostructures was increased. For quantum dots at 1 M concentration, the PL intensity at 385 nm increased at the beginning and then decreased at higher density. We will discuss the observed changes in PL intensity with QD concentration with ZnO-QD band structure and recombination-diffusion processes taking place at the interface. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L10.00010: Controlled Deposition of Nanocrystal Quantum Dots on Silicon Surfaces Oliver Seitz, Hue M. Nguyen, Damien Aureau, Amandeep Sra, Anton V. Malko, Yves J. Chabal Studying Forster resonant energy transfer (FRET) has constantly been a challenge because of the poor control in transferring nanocrystal quantum dots (NQDs) onto various substrates. This lack of control often resulted in formation of aggregates (3D growth), inhomogeneity, and poor adhesion. In this study, using self assembled monolayers (SAMs), dense monolayer of NQDs have been attached onto silicon substrate, with and without the presence of oxide interlayer, allowing investigating FRET effects via photoluminescence measurements. Such SAMs, directly attached to the silicon, via Si-C bonds, display an interface quality with low interface states. Moreover, the ability to be prepared with tunable thicknesses renders them ideal for FRET investigation. Such hybrid colloidal NQD/Silicon optoelectronic structures could potentially be attractive for both photovoltaic as well as light emitting applications. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L10.00011: Synthesis of a long gold atomic strand incorporated with carbon atoms Yoshifumi Oshima, Tomoya Ono, Kunio Takayanagi, Nguyen Duy Huy Single metal or carbon atomic strands have attracted much interest because of their unique properties. They have been usually fabricated by stretching or thinning the junction between both electrodes, but limited to be several atoms in length at maximum. We show that a long atomic strand can be synthesized by pulling one-dimensional reconstructed structure formed on the electrode surface. In the transmission electron microscope combined with a scanning tunneling microscope, gold atoms were observed to be pulled out one-by-one from carbon contaminated gold (111) surface layer each 0.5 nm elongation till diminishing the layer. This observation was explained by the first-principle calculation, showing that two carbon atoms are incorporated at each interval (0.5 nm) between two neighboring gold atoms aligned along the [112] direction to form the one-dimensional reconstructed structure, when the gold (111) surface is contaminated with carbon atoms. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L10.00012: Scanning Tunneling Microscopy Study of Quantum Cobalt Chains Nader Zaki, Danda Acharya, Denis Potapenko, Peter Johnson, Percy Zahl, Peter Sutter, Richard Osgood We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model. LT-STM measurements, taken on self-assembled Co chains, reveal a charge-density modulation that is dependent on tip bias. These charge-modulations are observed for tip-bias relatively far away from the Fermi level, both at negative and positive bias. We present arguments for the identification of these modulations as due to either charge-density-like waves (CDW) or excited states of this 1-D system. \\[4pt] [1] N. Zaki et al, Phys. Rev. B 80, 155419 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L10.00013: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L10.00014: Characterization of the surface environment of PbSe nanoparticles by correlating calculated and measured x-ray spectra Keith Gilmore, Aaron Hammack, April Sawvel, Evelyn Rosen, D. Frank Ogletree, Jeffrey Urban, Delia Milliron, Brett Helms, Bruce Cohen, David Prendergast Given that defining characteristics of nanoparticles are often dictated by their surfaces, it is desirable to be able to control the surface environments. We seek this control through ligand exchange chemistry and investigate PbSe as a model system. We correlate calculated and measured x-ray spectra to quantify the extent of ligand exchange, validate our structural models, and characterize the optical and electronic properties induced by the new surface environment. Chemical shifts in x-ray photoelectron spectra indicate changes in atomic bonding at the surface, whereas x-ray absorption spectra reveal ligand conformation and binding coordination at the surface. The colloidal synthesis of PbSe particles is highly reliable and the resulting particles are technologically useful size-tunable IR absorbers. Such particles have Pb rich surfaces and native oleic acid coats. We replace the oleic acid with alternate ligands of choice, which may change the Pb:Se ratio at the particle surface. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L10.00015: Anchoring platinum on graphene using metallic adatoms F.G. Sen, Y. Qi, A.T. Alpas To anchor Pt on the graphene surface 25 different metallic adatoms were individually inserted into Pt(111)/graphene interface and the work of separation required to break the interface between Pt-adatom and C-adatom bonds were computed using first principles calculations. With the exception of Al, Zn and Au, all metals increased the strength of the Pt/graphene interface, while many transition metals with unfilled d orbitals, such as Sc, Ti, V, Cr, Co, Ni, Zr, Nb, Mo, Ru, Rh, Ta, W, Re, Os, Ir, could increase the Pt/graphene interface strength from 0.009 J/m$^{2}$ to above 0.5 J/m$^{2}$. The Pt-adatom bond had metallic character and its strength was proportional to the amount of charge transferred from the adatom to the Pt. The strength of carbon-adatom bond was proportional to the ratio of charge transferred to the carbon over charge transferred to the platinum from the adatom. As this ratio was $>$1.0 for Ir, Os, Ru, Rh and Re and these emerged as the most promising adatoms for anchoring Pt on graphene. [Preview Abstract] |
Session L12: APS Editorial Q&A: APS and Open Access
Sponsoring Units: APSRoom: D223/224
Tuesday, March 22, 2011 3:00PM - 4:00PM |
L12.00001: APS and Open Access The movement toward Open Access continues to gain momentum. A brief review of APS efforts in this area will be presented by APS Editor in Chief, Gene Sprouse. Editors from Physical Review A, B, E, Focus, Letters, and X, Reviews of Modern Physics, and Physics will address your questions about publishing in this evolving environment. [Preview Abstract] |
Session L13: Focus Session: Jamming Theory and Experiment III
Sponsoring Units: GSNPChair: Robert Behringer, Duke University
Room: D225/226
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L13.00001: Relaxation of stresses and dynamical heterogeneities close to jamming in a granular experiment Corentin Coulais, Olivier Dauchot, Robert Behringer Dynamical Heterogeneities have been found to exhibit maximal size and scale invariance at Jamming. We address here the question of the link with stresses in the materials. To that end, we use a confined, vibrated layer of 8000 bidisperse grains under uniaxial compression. The vibration is horizontal, transverse to the direction of compaction. First, an intruder is pulled at constant velocity through the assembly and force measurements reveals maximal time correlations at Jamming. Then, the experimental setup is slightly modified to accept photoelastic grains, made of soft or hard materials. By measuring positions and stresses, decorrelation of forces as well as dynamics in structure, spontaneous fluctuations are probed. Both quenches and intruder pulling protocols are performed, and novel behavior at Jamming is seen. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L13.00002: Nonlinear elasticity near jamming probed in bidisperse foams Alexander Siemens, Martin van Hecke An unusual characteristic of the jamming transition is the difference in scaling of the bulk and shear modulus of frictionless soft particles near jamming. We probe this scaling by compressing a bidisperse foam monolayer sandwiched between a glass plate and a fluid surface. We also determine the weakly nonlinear effective bubble-bubble interactions in a 1D chain of bubbles under compression. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L13.00003: Reversible plasticity near Jamming in foams Gijs Katgert, Wilson C.K. Poon We study the response of a disordered foam monolayer, confined between a soapy solution and a glass plate to an oscillatory compressive strain brought about by inflating a central bubble. We show that, when driven quasistatically slowly, the foam as a whole can exhibit kinematically reversible plasticity or {\it anelasticity}: the bubble packing alternates between two reproducible configurations, which are separated by multiple plastic events and global displacements. After establishing that the timescale beyond which the foam behaves quasistatically is set by the scaling of the foam compressive modulus with packing fraction $\phi$, we map out the boundary between reversible and irreversible plasticity in the space spanned by $\phi$ and the compressive strain $\varepsilon$ and tentatively find the strain to scale as $\varepsilon \sim (\phi-\phi_c)^{1/4}$, with $\phi_c$ the jamming point. We finally extract a plasticity lengthscale from our experiment and show it to grow on approach to $\phi_c$. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L13.00004: Local origins of volume fluctuations in granular materials James Puckett, Frederic Lechenault, Karen Daniels Recent experiments and simulations have observed that the fluctuations in the local volume fraction, $\phi$, decrease as the granular material approaches jamming. We investigate the role of boundary condition and inter-particle friction, $\mu$, on these fluctuations for a dense bidisperse granular monolayer driven at the perimeter. Using a radical Voronoi tessellation, we find a universal linear relationship between the mean variance of $\phi$ independent of boundary condition and $\mu$. We examine the universality and origins of this trend using the recent granocentric model modified to draw neighbors from an arbitrary distribution $P(s)$, the edge-to-edge distance between neighbors. The mean and variance of the observed particle separation $s$ are described by a single length scale controlled by mean $\phi$. We tested diverse functional forms of $P(s)$ and found that each produces the trend of decreasing fluctuations, but only the experimentally-observed $P(s)$ provides quantitative agreement with the measured $\phi$ fluctuations. In conclusion, we find $P(\phi)$ and $P(s)$ encode similar information about the distribution of free volume in a driven granular system under different boundary conditions and inter-particle friction. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L13.00005: Intermittent Jamming in Quasi-2D Microfunnels Carlos Ortiz, Karen Daniels, Robert Riehn Both athermal granular jamming and thermal glass transitions have recently received extensive attention. We experimentally investigate the jamming transition in a quasi-2D system of nearly hard-sphere, micron-sized PMMA-PHSA particle suspension in a density and index-matched medium flowing through a microfunnel. We observe a packing fraction driven transition from a gas-like to a liquid-like to a solid-like phase. At sufficiently high packing fractions we observe intermittent jamming under constant pressure. Further increase in the packing fraction forms a stable solid-like jammed phase which is disordered on long-ranges, and susceptible to re-melting by reverse flow, agitation, and diffusion. By displaying properties of both athermal granular jamming and thermal glass transitions, our experiment provides a useful testing ground for understanding the jamming transition as a unifying framework. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L13.00006: Controllable jamming of amorphous granular materials applied to robotics Eric Brown, Rodenberg Rodenberg, John Amend, Hod Lipson, Annan Mozeika, Erik Steltz, Mitchell Zakin, Heinrich Jaeger We demonstrate the practicality of using a controlled jamming transition in an amorphous mass of granular material for applications to robotic gripping, and how the gripping capabilities depend on the properties of the jammed state. A mass of granular material contained in a flexible membrane in an unjammed state flows and conforms to almost any object it is pressed against. Upon application of a vacuum, the external pressure on the membrane jams the granular mass with a volumetric contraction $< 1\%$, allowing it to pinch the object. By measuring the holding force on a test sphere at different levels of envelopment, we show that three mechanisms contribute to the holding force: friction, suction, and interlocking. We use a solid mechanics model to relate the holding force from each mechanism to the measured stress response of jammed granular materials to compressional, extensional, and bending strains. This opens up new possibilities for the design of simple systems that excel at gripping objects of arbitrary shape. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L13.00007: Jamming in Vertical Channels G. William Baxter, Fiona Steel We study jamming of low aspect-ratio cylindrical Delrin grains in a vertical channel. Grain heights are less than their diameter so the grains resemble antacid tablets, coins, or poker chips. These grains are allowed to fall through a vertical channel with a square cross section where the channel width is greater than the diameter of a grain and constant throughout the length of the channel with no obstructions or constrictions. Grains are sometimes observed to form jams, stable structures supported by the channel walls with no support beneath them. The probability of jam occurrence and the strength or robustness of a jam is effected by grain and channel sizes. We will present experimental measurements of the jamming probability and jam strength in this system and discuss the relationship of these results to other experiments and theories. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L13.00008: Geometry Dependence of the Clogging Transition in a Tilted Hopper Charles Thomas, Douglas Durian We report the effect of system geometry on the clogging of granular material flowing out of a flat-bottomed hopper. We vary the hopper tilt angle, aperture shape, and granular media shape, investigating smooth spheres (glass beads), compact angular grains (beach sand), and rod-like grains (rice). We measure the average number of grains discharged before a clog halts the flow. This value grows with hole size as a power law, diverging above a critical hole size. We determine the critical value by performing a least-squares fit to the data. Beyond that critical hole size, the flow does not clog for any given tilt angle. This critical hole size grows with increasing tilt, diverging at $\pi - \theta_{r}$, where $\theta_{r}$ is the angle of repose. The value of the critical hole size as a function of tilt angle describes a well-defined transition on a clogging phase diagram. For circular apertures, the shape of this transition is similar for all grain types. However, this is not the case for the narrow slit apertures, where the rate of growth of the critical hole size with tilt angle depends on the material. The growth rate is the fastest for angular grains, then smooth spheres, with rod-like grains showing the slowest growth. This suggests a profound link between the aperture geometry and the particle shape. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L13.00009: Jamming of Granular Flow in a Two-Dimensional Hopper Junyao Tang, Sepehr Sadighpour, Robert Behringer We seek an understanding of the physics of jamming for hopper flow using high speed spatio-temporal video data for photoelastic disks flowing through a two-dimensional hopper. We have found experimental support for the hypothesis that jamming events of granular flow in a hopper is approximately a Poisson process. The mean flow time between two consecutive jams increases rapidly with the hopper opening size, but it is insensitive to changes of the hopper wall angle.Through particle tracking and photoleastic measurements, we measure stress fields, velocity fields and density fields, as well as their fluctuations during the flow. Current work is focusing on understanding how to combine these results to give us further insights of the relation between mean flow properties and jamming and their dependence on hopper configuration.These data are part of an IFPRI-NSF Collaboratory for comparing physical data and simulations. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L13.00010: Structural Stability and Jamming of Self-Organized Cluster Conformations in Granular Materials A. Tordesillas, B. Behringer, Q. Lin, J. Shi, J. Zhang We probe emergent self-organized particle cluster conformations in slowly deforming dense granular materials. We invoke structural mechanics to devise a new stability measure for clusters, and use this measure to explore stability of jammed states of cluster conformations consisting of particles in force chains and minimal contact cycles. Knowledge of the spatio-temporal evolution of the (relative) stability of jammed conformations offers valuable clues to granular rheology and self-assembly. We use data from assemblies of bi-/poly-disperse disks subject to 2D deformation in two biaxial strain tests: one computational and one experimental. Self-assembly occurs on multiple length scales with jammed force chains and minimal cycles forming the basic building blocks. Three-cycles are stabilizing agents acting as granular trusses to load-bearing force chain columns. The co-evolution of minimal cycles and force chains form a generic feature of these materials and loading paths. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L13.00011: Decoupling of Rotational and Translational Diffusion in Supercooled Colloidal Fluids Kazem V. Edmond, Gary L. Hunter, Mark T. Elsesser, HyunJoo Park, David J. Pine, Eric R. Weeks Using high-speed confocal microscopy, we directly observe the three-dimensional rotational dynamics of rigid clusters of microspheres suspended in dense colloidal suspensions. The clusters are highly ordered packings of fluorescently-labeled PMMA particles, fabricated using a recently developed emulsification technique. Our colloidal suspensions serve as good approximations to hard-sphere fluids, while the clusters probe the system's local rotational and translational dynamics. Far from the colloidal liquid's glass transition, both rotational and translational motion of the clusters are purely Brownian. However, in the liquid's supercooled regime, we observe a decoupling between the two types of motion: as the glass transition is approached, rotational diffusion slows down even more than translational diffusion. Our observation supports the notion that supercooled liquids are not merely liquids with large viscosities but that diffusion takes place by fundamentally changed mechanisms. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L13.00012: Confinement of Colloidal Suspensions in a Cylindrical Geometry Nabiha Saklayen, Gary L. Hunter, Kazem V. Edmond, Eric R. Weeks We study binary colloidal suspensions confined within a glass microcapillary to model the glass transition in confined cylindrical geometries. We use high speed three-dimensional confocal microscopy to observe particle dynamics. The use of a slightly tapered microcapillary enables us to probe a range of local volumes for a single colloidal sample. We observe that confinement of the sample slows down particles. In addition, the particles form layers against the capillary walls; these layers also influence particle mobility. We see that even though confinement is primarily responsible for slowing down particles, particles within a layer are seen to move even slower. Within each region of the microcapillary, the mobility perpendicular to the confining boundaries is influenced by distance from the confinement boundary, while the parallel component of mobility is not. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L13.00013: Shear-induced dynamics of polydisperse jammed emulsions Eric R. Weeks, Joaquim Clara Rahola We study dense and highly polydisperse emulsions at droplet volume fractions ranging from phi = 0.65 to 0.85. We apply oscillatory shear and observe the subsequent droplet motions using confocal microscopy. Both affine and nonaffine droplet motions are observed, with the large droplets typically moving affinely and pushing the smaller droplets around in non-affine ways. Despite the polydispersity of the sample and the complex droplet trajectories, we observe dynamic correlation length scales. These length scales grow from one to four times the mean droplet diameter, with larger length scales corresponding to higher strain amplitudes (up to strains of about 6\%). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L13.00014: A Jamming Phase Diagram for Pressing Polymers Chao Teng, Zexin Zhang, Xiaoliang Wang, Gi Xue Molecular glasses begin to flow when they are heated. Other glassy systems, such as dense foams, emulsions, colloidal suspensions and granular materials, begin to flow when subjected to sufficiently large stresses. The equivalence of these two routes to flow is a basic tenet of jamming, a conceptual means of unifying glassy behavior in a swath of disordered, dynamical arrested systems. However, a full understanding of jamming transition for polymers remains elusive. By controlling the packing densities of polymer glasses, we found that polymer glasses could once flow under cold-pressing at temperatures well below its calorimetric glass transition temperature (Tg). The thermomechanical analysis (TMA) results confirmed that Tg changed with density as well as the applied stress, which is exactly what to be expected within the jamming picture. We propose a jamming phase diagram for polymers based on our laboratory experiments. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L13.00015: Visualization of force networks in a three-dimensional granular system Chantal Carpentier, Kinga Lorincz, Peter Schall, Daniel Bonn, Fred Brouwer Force networks form the skeleton of granular matter. The understanding of the rigidity to flow transition of granular materials requires the study of the three-dimensional distribution of forces between the particles. Here we propose a new method to visualize and measure contact forces in three-dimensional suspensions. We use a rigidochromic dye which we attach chemically to the surfaces of the particles to measure local forces at the contact point. This dye exhibits non-fluorescent transitions, when it is free to relax mechanically, but shows strong fluorescence when confinement blocks mechanical relaxation. Preliminary experiments suggest that the fluorescent intensity is a direct measure of the local contact force. We use confocal microscopy to create spatial intensity maps to reconstruct the entire contact force distribution. [Preview Abstract] |
Session L14: Physics Education Research
Sponsoring Units: FEdChair: Kendra Redmond, American Institute of Physics
Room: D227
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L14.00001: A Proactive Approach for Improving the Mathematical Foundation of Students Taking College Physics Leigh Smith, James Sullivan, Howard Jackson We report on preliminary results using the mathematics teaching program ALEKS (see aleks.com) along with the use of Just-in- Time-Teaching (JiTT) and Peer Instruction (PI) to improve the performance of students in College Physics, an algebra-based course. ALEKS, an adaptive program based on artificial intelligence and long-used in the mathematics community, was made available to students 5 weeks ahead of the first class session with participation encouraged by the award of a small class credit. Student participation and engagement was remarkable with many students making significant gains in their mathematics performance. Preliminary data suggests that performance on the first midterm was strongly correlated with performance within ALEKS. The use of JiTT and PI in two out of the four classes suggested overall a modest increase over standard lecture sections, but with women performing significantly better in these classes. We acknowledge the financial support of McGraw-Hill and ALEKS and the National Science Foundation through CCLI grant DUE-1022563. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L14.00002: Improving Students' Interest and Motivation in Introductory Physics Laboratory: A Comparative Study Yevgeniya V. Zastavker, Jennifer A. Simonovich, Emily Towers Project-based learning (PjBL) has shown to be an effective method to enhance student learning in many disciplines, including science and engineering fields. Due to the complex nature of PjBL, however, the effectiveness of this learning environment has been linked, to a large degree, to the specifics of its implementation. This talk will present a comparative study of two technical PjBL courses required for engineering majors at a small technical school, \textit{Introductory Mechanics Laboratory} and \textit{Introductory Engineering Design}. Twelve semi-structured in-depth interviews are analyzed using grounded theory approach. The results indicate that despite similarity in the course goals of these PjBL environments, students' interest and motivation varies dramatically based on the relative levels of student autonomy and scaffolding provided in each course. We propose a framework for creating appropriate PjBL environments in \textit{Introductory Physics Laboratories} with an emphasis on improving engineering and physics students' interest and motivation in the relevant coursework and improving student retention. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L14.00003: Integrating pre-, in- and post-lecture activities to improve students' learning in a large introductory physics course Kwan Cheng, Mehmet Caglar, Amy Pietan, Hani Dulli Monitoring and assessing the students' learning activities before, during and after lecture teaching in a large (more than 150 students) introductory physics class setting are important to evaluate the efficacies of various teaching pedagogies and methods. At Texas Tech, an online and integrative computer-based approach of using an interactive pre-lecture tutorial, an in-class concept test using a wireless student response system and a homework/tutorial system has been implemented in Fall 2010. The strategy of implementation of this integrative approach and the assessment results from various in-house and standard Mechanics tests will be presented. In addition, how this approach may create synergism of lab and lecture teaching efforts will also be addressed. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L14.00004: Design and development of physics simulations in the field of oscillations and waves suitable for k-12 and undergraduate instruction using video game technology Trevor Tomesh, Colin Price Using the scripting language for the Unreal Tournament 2004 Engine, Unreal Script, demonstrations in the field of oscillations and waves were designed and developed. Variations on Euler's method and the Runge-Kutta method were used to numerically solve the equations of motion for seven different physical systems which were visually represented in the immersive environment of Unreal Tournament 2004. Data from each system was written to an output file, plotted and analyzed. The over-arching goal of this research is to successfully design and develop useful teaching tools for the k-12 and undergraduate classroom which, presented in the form of a video game, is immersive, engaging and educational. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L14.00005: A Physics of Semiconductors Concept Inventory Emanuela Ene Following the trend in science and engineering education generated by the visible impact that the Force Concept Inventory (FCI) has created, a Physics of Semiconductors Concept Inventory (PSCI) has been developed. Whereas most classroom tests measure \textit{how many} facts students can remember, or if they \textit{can manipulate} equations, PSCI measures \textit{how well} students interpret concepts and \textit{how well} they can infer new knowledge from already learned knowledge. Operationalized in accordance with the revised Bloom's taxonomy, the multiple--choice items of the PSCI address the ``understand'', ``apply'', ``analyze'' and ``evaluate'' levels of cognition. Once standardized, PSCI may be used as a predictor for students' academic performance in the field of semiconductors and as an assessment instrument for instructional strategies. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L14.00006: A semantic space mapping of introductory physics concepts Nic Rady We know that physics students and their professors organize knowledge differently, but exactly how do they organize knowledge. The Physics Perception Assessment (PPA) has been designed to create a map of the semantic space of 15 commonly encountered concepts in a first-semester physics course. Preliminary data taken with the PPA will be presented and from this data, an ``expert'' semantic space configuration will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L14.00007: A student's guide to searching the literature using online databases Casey W. Miller, Dustin Belyea, Michelle Chabot, Troy Messina A method is described to empower students to efficiently perform general and specific literature searches using online resources [Miller et al., Am. J. Phys. \textbf{77}, 1112 (2009)]. The method was tested on undergraduate and graduate students with varying backgrounds in scientific literature. Students involved in this study showed marked improvement in their awareness of how and where to find scientific information. Repeated exposure to literature searching methods appears worthwhile, starting early in the undergraduate career, and even in graduate school orientation. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L14.00008: Mutual Mentoring Makes Better Mentors Cindy Blaha, Amy Bug, Anne Cox, Linda Fritz, Barbara Whitten In this talk we discuss one of the impacts of an NSF ADVANCE sponsored horizontal, mutual mentoring alliance. Our cohort of five women physicists at liberal arts colleges has found that mutual mentoring has had a profound impact on many aspects of our professional lives. In this talk we will describe how our peer-to-peer mentoring has enabled us to become better mentors for our undergraduate students, for recent graduates beginning their careers and for colleagues at local and neighboring institutions. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L14.00009: Impact of Mutual Mentoring on Research Barbara Whitten, Cynthia Blaha, Amy Bug, Anne Cox, Linda Fritz In this talk we discuss one of the impacts of an NSF ADVANCE sponsored horizontal, mutual mentoring alliance. Our cohort of five women physicists at liberal arts colleges has found that mutual mentoring has had a profound impact on many aspects of our professional lives. In this talk we will give some specific ways that we have supported and helped to expand each other's research. For some new areas of research were opened, for others new focus was brought to existing areas, and still others found acceptance for where they were. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L14.00010: Developing Effective Undergraduate Research Experience Michael Evans, Carolina C. Ilie Undergraduate research is a valuable educational tool for students pursuing a degree in physics, but these experiences can become problematic and ineffective if not handled properly. Undergraduate research should be planned as an immersive learning experience in which the student has the opportunity to develop his/her skills in accordance with their interests. Effective undergraduate research experiences are marked by clear, measurable objectives and frequent student-professor collaboration. These objectives should reflect the long and short-term goals of the individual undergraduates, with a heightened focus on developing research skills for future use. 1. Seymour, E., Hunter, A.-B., Laursen, S. L. and DeAntoni, T. (2004), ``Establishing the benefits of research experiences for undergraduates in the sciences: First findings from a three-year study''. \textit{Science Education}, 88: 493--534. 2. Behar-Horenstein, Linda S., Johnson, Melissa L. ``Enticing Students to Enter Into Undergraduate Research: The Instrumentality of an Undergraduate Course.'' \textit{Journal of College Science Teaching} 39.3 (2010): 62-70. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L14.00011: Seriously? Freshmen In A Physics Research Lab? Rosa Elia C\'{a}rdenas, Isaac Manzanera Esteve, John T. Markert, Sarah Simmons We report on the University of Texas College of Natural Sciences Freshman Research Initiative (FRI) program as a whole and more specifically, its physics stream. The FRI program was developed in an effort to improve retention in the College of Natural Sciences (CNS). The general goal of the program is to bring students at the freshman level into a research laboratory. The reasoning is that as students become part of a research laboratory he or she will feel more involved with science, both academically and socially, and will be more likely to continue on a research science route. We will present the college wide statistical tracking data which shows that the FRI program has indeed improved retention in the CNS, has improved GPA and has improved graduate school matriculation. We will also discuss the tracking of three generations of physics stream participants. We describe the curriculum, training, precautions and techniques used as we bring freshmen into a physics research laboratory. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L14.00012: Mentoring undergraduates for experimental research in physics Jeremy Levy Undergraduate research experiences are pivotal in shaping careers in physics. I will support this thesis$-$the implicit focus of this session$-$with anecdotes highlighting research performed by undergraduates in my group at the University of Pittsburgh, as well as my own recollections as an undergraduate researcher. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L14.00013: Modular Curriculum Approach (MCA) for teaching of introductory physics: Tablet PCs and flexible instructional space to stimulate active learning Tikhon Bykov, Yelena Kosheleva Modular Curriculum Approach is an innovative model designed at McMurry University and adopted for teaching of introductory physics courses. In MCA, traditional lab/lecture structure is converted into a system of flexible instructional modules, with lecture, lab, and discussion being merged into one technologically and collaboratively rich experience. Different elements are integrated with Tablet PCs as a single unifying platform to improve continuity among module components. A technology suite incorporated with tablets includes: Physlets, tablet-adapted personal response systems, data acquisition systems, and tablet-based note-taking tools. The MCA has been further reinforced by creating a new instructional space with movable partitions, allowing for easy transformation between lecture and lab modes. The space is supportive of small peer- group activities with easy-to-reconfigure table clusters, multiple white and black board surfaces, multiple TVs and projection screens. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L14.00014: Impact E-Learning Platform Moodle on the Physic's Learning Process in the High School's Students Jonas Torres-Montealban, Gregorio Ruiz-Chavarria, Enrique Armando Gomez-Lozoya As a didactic proposal, moodle e-learning platform was implemented in one of two Physics High School's group at UACH, in order to show how the use of new technologies can improve the learning progress linked to physics concepts. As a result, the first group worked at the same time with inside class activities as well as outside resources from the moodle e-platform. The second group only worked with inside class activities. This teaching application was developed in six sections. Section I defines the educational framework. Section II identifies the key physic's concepts to be studied in each proposed activity. Section III describes the didactic model. Section IV displays the compared results between similarities and differences in both groups. Section VI shows the gathered information in order to be discussed as a topic related on how new technologies improve the Physic's learning process in the high school' students. [Preview Abstract] |
Session L15: Focus Session: Spins in Semiconductors - Spin Torque and Spin Injection
Sponsoring Units: DMP GMAG FIAPChair: Roberto Myers, Ohio State University
Room: D171
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L15.00001: Spin Torque Switching in GaMnAs Magnetic Tunnel Junctions Lin Xue, R. A. Buhrman, D.C. Ralph, D.W. Rench, M.J. Wilson, P. Schiffer, N. Samarth We have fabricated and measured submicron magnetic tunnel junctions made from GaMnAs multilayers: GaMnAs/GaAs/GaMnAs/MnAs, where GaMnAs is a ferromagnetic semiconductor, GaAs serves as the tunneling barrier, and MnAs is a ferromagnet that provides an exchange bias to the upper GaMnAs (reference) layer. The devices have magnetoresistances of order 50{\%} at 4.2 K and exhibit clear spin-torque switching of the lower GaMnAs layer between parallel and antiparallel orientations relative to the reference layer. We report the switching phase diagram as a function of current and magnetic field. We also describe efforts to probe the high-speed magnetic dynamics in GaMnAs driven by spin torque from ns-scale current pulses and microwave-frequency currents that can drive ferromagnetic resonance. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L15.00002: Theory of current-induced torque in uniform ferromagnets Karel Vyborny, Liviu Zarbo, T. Jungwirth In a model ferromagnetic semiconductor (Ga,Mn)As with strong spin-orbit interaction, it has been experimentally shown that magnetization can be manipulated by injection of unpolarized currents [Chernyshov et al., Nat. Phys. 5, 656 (2009)]. We critically review the existing theoretical approaches to this phenomenon and present a model whose results are compared to more recent measurements of the current-induced torque driven by ferromagnetic resonance in (Ga,Mn)As. The results entail the dependence of the effect on carrier concentration, various types of strain, and temperature. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L15.00003: Intrinsic spin-orbit coupling effects on spin and charge pumping in magnetic tunnel junctions with microwave-driven precessing magnetization Farzad Mahfouzi, Branislav Nikolic, Naoto Nagaosa We develop a microscopic quantum transport approach to the problem of spin pumping by precessing magnetization in one of the ferromagnetic layers within FIF or FIN (F-ferromagnet; N-normal metal; I-insulating barrier) magnetic tunnel junctions (MTJs) in the presence of intrinsic spin-orbit couplings (SOC) at the FI interface. Our approach evaluates the nonequilibrium Green functions (NEGFs) by starting from the time-dependent Hamiltonian of these junctions. To express the time-averaged charge current, or the corresponding dc pumping voltage in open circuits that was measured in recent experiments on MTJs [T. Moriyama {\em et al.}, Phys. Rev. Lett. {\bf 100}, 067602 (2008)], we construct a novel solution for the double-time-Fourier-transformed NEGFs where their two energy arguments are connected by the Floquet theorem describing emission and absorption of finite number of photons. Within this fully quantum-mechanical treatment of the conduction electrons, we find that only in the presence of the interfacial Rashba SOC non-zero dc pumping voltage in F$|$I$|$N junctions emerges at the adiabatic level (i.e., proportional to microwave frequency). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L15.00004: Manipulation of ferromagnetic state by means of spin-orbit interactions Invited Speaker: The current state of information technology accentuates the dichotomy between processing and storage of information, with logical operations performed by charge-based devices and non-volatile memory based on magnetic materials. The major obstacle for a wider use of magnetic materials for information processing is the lack of efficient control of magnetization. Reorientation of magnetic domains is conventionally performed by non-local external magnetic fields or by externally polarized currents. Efficiency of the latter approach is greatly enhanced in materials where ferromagnetism is carrier-mediated. In such materials control of carriers' polarization provides an alternative mean to manipulate orientation of magnetic domains. In some crystalline conductors the charge current couples to spin via intrinsic spin-orbit (SO) interactions and generates electron spin polarization. Unlike the Oersted field, the SO-induced polarization is spatially localized and can be controlled by local electric fields. This non-equilibrium electron spin polarization couples to magnetic moments of magnetic ions and is capable of controlling magnetization of the ferromagnet. We show that magnetization can be reversibly manipulated by SO-induced polarization of carrier spins generated by the injection of unpolarized currents. We demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor GaMnAs. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L15.00005: Ferromagnetic resonance driven by current-induced torque in uniform ferromagnetic micro-structures H. Kurebayashi, D. Fang, J. Wunderlich, K. Vyborny, L.P. Zarbo, R.P. Campion, A. Casiraghi, B.L. Gallagher, T. Jungwirth, A.J. Ferguson We show that the recently demonstrated current induced torque (CIT) [A. Chernyshov, et al. Nat. Phys. 5, 656 (2009)] can excite magnetisation dynamics in micro-bars of uniform ferromagnetic semiconductors. Due to the combined effects of the spin-orbit and exchange interactions, a microwave current injected into (Ga,Mn)As (or (Ga,Mn)(As,P)) micro-bars generates an oscillating effective magnetic field. By using a sensitive electrical detection technique, we accurately measure the magnitude and direction of driving fields for samples under different strain. We confirm the observation of a field with the symmetry of the Dresselhaus spin-orbit interaction and observe an additional field with the symmetry of the Rashba spin-orbit interaction. Our work demonstrates a new scalable FMR technique which provides a sensitive method to study the nature of current-induced torques and to perform magnetic characterisation of uniform ferromagnetic micro-structures. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L15.00006: Non-local spin transport devices with a tunable channel J. Misuraca, J.-I. Kim, P. Xiong, S. von Molnar, K.K. Meng, J. Lu, J.H. Zhao The spin lifetime in GaAs is known to vary strongly with carrier density near the metal to insulator transition [1]. However, a detailed study to optimize this lifetime is complicated because many replica samples need to be made and measured. This difficulty can be circumvented by employing Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor, as the spin transport medium. This material, which is structurally similar to GaAs, has been characterized and shown to have an effective carrier density which can be tuned \textit{in situ} via photo-excitation from 10$^{14}$ to 10$^{18}$ cm$^{-3 }$[2]. Heterostructures (2-$\mu $m Si:Al$_{0.3}$Ga$_{0.7}$As, a thin epitxial Fe layer, and a GaAs graded junction to create linear contacts between them) have been grown by MBE and non-local spin devices have been patterned by photolithography and wet etching. Magnetic measurements on Fe micro-patterns demonstrated the possibility of controlling the coercivity of the Fe electrodes [3]. Electrical characterization of the devices will be presented. [1] J. Kikkawa et al., Phys. Rev. Lett. 80, 4313 (1998) [2] J. Misuraca et al., Phys. Rev. B. 82, 125202 (2010) [3] K. K. Meng et al., Appl. Phys. Lett. 97, 072503 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L15.00007: Electron Charge and Spin Transport in Ferromagnet/Semiconductor Schottky Barrier Heterostructures Qi Hu, Chris Palmstrom, Eric Garlid, Chad Geppert, Paul Crowell Previous studies of Fe/$n^+$-GaAs/$n$-GaAs heterostructures in the lateral non-local geometry have shown that spin accumulation is observed only when tunneling current across ferromagnet/semiconductor Schottky barrier is sufficiently large. The tunneling mechanism is modeled by band diagram simulations and transport calculation using the WKB approximation for the barrier. These suggest that tunneling from localized states in the quantum well (QW) which forms just inside the Schottky interface dominates over tunneling directly from the semiconductor bulk. Electron tunneling spectroscopy is utilized to probe the predicted localized states in the QW. Valleys in the derivative of tunneling conductance spectra were observed at discrete forward bias voltages which are attributed to localized 2-dimensional energy states. The spin escape time of the 2D energy states in the QW is calculated using a spin-dependent WKB approximation. The results are compared with the spin lifetime and escape time extracted from three-terminal spin Hanle measurements. This work was supported by the NSF MRSEC and ONR MURI programs, and NSF DMR-0804244. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L15.00008: Dependence of spin-injection spectra of CoFe/GaAs contacts on temperature and annealing conditions Gian Salis, Andreas Fuhrer, Santos F. Alvarado Spin injection from CoFe contacts into bulk GaAs epilayers is studied experimentally. Close to the metal/semiconductor interface the GaAs epilayer is highly n-doped, allowing efficient spin injection through the Schottky tunnel barrier. Spin polarization in the GaAs channel is measured as a non-local voltage at CoFe detection contacts. Similar to spin injection from Fe contacts, an inversion of the sign of injected spin polarization is found at a finite forward bias $U_c$ applied to the injection contact. We investigate the dependence of the nonlocal signal on $U_c$. From the data, the spin polarization of the differential interface conductance is obtained, providing spectral information on the spin-polarized density of states. The dependence of these spectra on measurement temperature as well as on annealing and growth conditions is discussed and compared to samples with Fe injection contacts. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L15.00009: Electrical Measurements of the Extrinsic Spin Hall Effect in Fe/In$_{x}$Ga$_{1-x}$As Heterostructures Chad Geppert, Eric Garlid, Mun Chan, Paul Crowell, Qi Hu, Chris Palmstr{\O}m We report on all-electrical measurements of the extrinsic spin Hall effect in Fe/In$_{x}$Ga$_{1-x}$As heterostructures with $n$-type channel doping (Si) and highly doped Schottky tunnel barriers. The spin Hall effect refers to the transverse spin current generated by application of a longitudinal unpolarized charge current. Complementary spin accumulation at opposing edges of the channel is detected via a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. The spin Hall conductivity is extracted by fitting the data to a drift-diffusion model incorporating spin precession and relaxation. Tuning the channel conductivity with applied bias allows the skew and side-jump contributions to be determined independently. The resulting magnitude is in agreement with models based on ionized impurity scattering. Further quantitative comparison to theoretical models is achieved by increasing the In concentration beyond previously reported values. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L15.00010: Ab initio Investigation of the Failure of Efficient Spin-Injection in Fe/GaAs Superlattices Sinead Griffin, Nicola Spaldin Magnetic metal-semiconductor systems have been widely studied for use as spintronic devices. The injection of elemental ferromagnetic Fe into GaAs shows great device potential because of the relativity high Curie temperature of Fe compared to other possible injection materials. However, spin-injection in Fe/GaAs has not been successful with several phases such as FeAs and Fe$_{2}$As forming at the interface. We perform Density Functional Theory calculations on bulk FeAs and Fe$_{2}$As to elucidate the structural and magnetic ground states. We then incorporate these Fe-As layers into GaAs/FeAs superlattices and investigate the resulting structures. Both the effects of Fe content and the number of layers in the heterostructure on the magnetic and electronic properties are considered. Our results show that the magnetic ground state of the FeAs compounds helps to explains the failure of spin-injection in these superlattices. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L15.00011: Lateral spin injection and detection through electrodeposited Fe/GaAs interfaces Sarmita Majumder, Anthony Arrott, Karen Kavanagh, Anthony Spring Thorpe We report results on spin injection and detection through epitaxial, electrodeposited Fe/GaAs tunnel barriers formed ex-situ on epitaxially grown GaAs (001). The BCC-Fe ?lms are predominantly single crystalline with large mosaic spread [1]. Tunnel junctions, fabricated with bulk GaAs wafers, and epitaxially grown MBE or MOCVD GaAs (001), showed the expected increase in tunneling current with increasing surface Si dopant concentration. Spin transport through in situ coherently strain, MBE Fe/GaAs interfaces have been reported at spin polarization levels as high as 42{\%} at 50K.[3]. In our experiments the design of the epitaxially grown GaAs substrates followed those used successfully for in situ MBE Fe spin contacts. [2]. A spin voltage (4 mV) has been detected for 2x10$^{18}$/cm3 doped tunnel junctions at liquid nitrogen temperatures (77 K) using an injection current of 20 A/cm2 while varying the applied in-plane magnetic ?eld ($\pm $ 300 Gauss) along a $<$100$>$ easyaxis of the Fe contacts. 1. Z. L. Bao, S. Majumder, A. A. Talin, A. S. Arrott, K. L. Kavanagh, JES 155 (2008) H841. 2. X. Lou, C. Adelmann, A. S. Crooker, E. S. Garlidi, J. Zhang, K. S. Reddy, S. D. Flexner, C. J. Palmstr{\o}m, and P.A. Crowell, Nature Phys. 3 (2007) 197. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L15.00012: All-electrical spin injection and detection in an AlGaN/GaN two-dimensional electron gas D.R. Hoy, Y. Pu, S.D. Carnevale, E. Johnston-Halperin, R.C. Myers Materials with low spin-orbit coupling, including wide band gap semiconductors, may allow practical semiconductor spintronics. Here we investigate all-electronic spin injection and detection using ferromagnetic Fe electrodes on a polarization doped AlGaN/GaN two-dimensional electron gas (2DEG) grown by molecular beam epitaxy. The ultrathin AlGaN cap provides polarization doped electrons and serves as a thin tunnel barrier for spins. The surface morphology is characterized by atomic force microscopy and the electron density, resistivity, and mobility are characterized by Hall measurements. Through the Hanle effect, we investigate the dependence of the spin injection efficiency and spin lifetime with temperature and bias. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L15.00013: Studies of spin injection into thin film InSb from CoFe Yong-Jae Kim, R.L. Kallaher, J.J. Heremans Spin-based electronics requires manipulation of spin-polarized carriers in materials. The narrow gap semiconductor InSb is a promising material for spin-based devices due to its strong spin-orbit interaction, allowing spin manipulation using electric fields. Yet, spin injection from spin-polarized electrodes into InSb has not yet been demonstrated. In order to electrically characterize spin injection and detection in InSb, we use InSb/CoFe lateral spin valve geometries studied at low temperatures and in tilted magnetic fields. The geometries are fabricated by depositing two non-epitaxial CoFe electrodes at mesoscopic separations on high-mobility InSb thin films through an insulator window. The anisotropy of the ferromagnetic electrodes provides parallel and anti-parallel configurations. We have observed two-state non-local output voltages, which are consistent with a spin injection signal. Interestingly, the switching signal is very sensitive to temperature in the range studied. The switching signal appears at low external fields due to CoFe magnetic anisotropy effects. The results are discussed in the light of the CoFe anisotropy and expected spin-coherence properties of InSb (partial support from DOE DE-FG02-08ER46532). [Preview Abstract] |
Session L16: Focus Session: Magnetic Nanostructures: Probing Using Advanced Methods
Sponsoring Units: DMP GMAGChair: Olav Hellwig, Hitachi Global Storage Technologies
Room: D173
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L16.00001: Soft X-ray Spectroscopy of Magnetic Nanostructures: New Phenomena and Applications Invited Speaker: The delicate balance between charge, spin, orbital, and lattice degrees of freedom in transition metal oxides leads to unique phenomena such as colossal magnetoresistance, high temperature superconductivity, as well as a remarkable diversity of charge, spin, and orbital ordered phases. The rich phase diagrams are determined by the strong local interaction of electrons in transition metal $d$ orbitals. Subtle changes in $d$ occupancy and overlap---and thereby phase transitions---can be induced by variations in temperature, by external fields, through doping and lattice distortions. In particular, interfaces can hold surprising electronic and magnetic properties that differ remarkably from the adjacent layers. Soft x-ray based techniques are ideal tools to study these systems as they are inherently element-specific, allow characterizing the valence state and the symmetry of lattice sites and provide detailed information about the electronic and magnetic structure with nanometer spatial resolution and on ultrafast time scales. Here we show that the to-date little explored angular dependence of the x-ray magnetic dichroism provides unique insights in the correlation between atomic, magnetic and electronic structure in these systems [1-4]. Taking advantage of this approach will prove invaluable for the engineering of novel nanoarchitectures to be used in low cost and energy efficient devices with improved performance and multiple functionalities. \\[4pt] [1] G. van der Laan \textit{et al}., Phys. Rev. Lett. \textbf{105}, 067405 (2010).\\[0pt] [2] E. Arenholz \textit{et al}., Phys. Rev. B \textbf{82}, 140103(R) (2010).\\[0pt] [3] G. van der Laan \textit{et al}., Phys. Rev. Lett. \textbf{100}, 067403 (2008).\\[0pt] [4] E. Arenholz \textit{et al}., Phys. Rev. Lett. \textbf{98}, 197201 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L16.00002: Investigation of ferromagnetic/antiferromagnetic nanostructures using X-ray magnetic dichroism J. Wu, J.S. Park, W. Kim, E. Arenholz, M. Liberati, A. Scholl, Chanyong Hwang, Z.Q. Qiu The spin structure of epitaxially grown antiferromagnetic/ferromagnetic bilayer was investigated using X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD) techniques. The XMLD measurement on the antiferomagnetic film (NiO or CoO) gives a direct probe of the spin orientation of the compensated antiferromagnetic spins. This capability enables us to give a clear clue to the mechanism of the exchange bias in the bilayer and to give a direct observation of the antiferromagnetic vortex. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L16.00003: XPCS Study of Antiferromagnetic Domain Fluctuation Keoki Seu, Sujoy Roy, San-Wen Chen, Xiangshun Lu, Hongyu Guo, Sunil Sinha, Karine Dumesnil We have studied magnetic domain fluctuations in Yttrium-Dysprosium-Yttrium trilayer films using X-ray Photon Correlation Spectroscopy (XPCS) in conjunction with resonant soft X-ray magnetic scattering. Dysprosium thin film possesses a helical antiferromagnetic phase below Tn = 180 K and a ferromagnetic phase below Tc = 64 K. With coherent soft x-rays we observed magnetic speckle in the scattering from the antiferromagnetic domains. We determined critical points and found a shift of Curie temperature from the bulk value. Hysteresis associated with a first order phase transition was also observed. We observed magnetic speckle around the magnetic satellite peak at (0,0,q$_{m})$ due to static disorder as well as magnetic domains. At temperatures above Tn the system showed static behavior on times scales up to $\sim $ 300 sec which is indicative of non-fluctuating static disorder that persists above Tn. Close to Tc, there appears to be time-dependent fluctuations due to slow domain wall motion and these will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L16.00004: Deposition-order dependent magnetization reversal of graded anisotropy Co/Pd films Peter Greene, Brian J. Kirby, Julie A. Borchers, June W. Lau, Kai Liu We report deposition-order-dependent, and depth-dependent, magnetization reversal in Co/Pd with graded anisotropy, which are technologically important as they address both writability and thermal stability challenges. Multilayers of [Co(0.4nm)/Pd(0.6nm)]$_{60}$ have been deposited by sputtering, where the Ar pressure has been varied from 5 to 12 and 20 mtorr in type A samples and in the reverse order in type B samples. An extensive set of structural and magnetization reversal studies with depth-resolution has been performed using XRD, cross-sectional TEM, magnetometry, PNR and XMCD. In type A samples, due to the larger grain size, lower interfacial roughness and less disorder in the magnetically softer layer, magnetization reversal proceeds via domain nucleation, propagation, and annihilation. Type B samples show a more localized reversal. Layers grown at higher pressure contain more disorder and rougher interfaces, which is carried into the magnetically softer layers deposited on top, thus impeding domain movement. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L16.00005: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L16.00006: Magnetization reversal and magnetic imprinting in a giant exchange bias system M.R. Fitzsimmons, J.A. Borchers, M. Laver, K.L. Krycka, W.C. Chen, S. Watson, C. Dufour, K. Dumesnil We present compelling experimental evidence, obtained with small angle neutron scattering (SANS) that magnetization reversal of an exchange biased DyFe$_{2}$/YFe$_{2}$ superlattice occurs via reversal of magnetic domains with at least two different length scales. Our SANS studies used both unpolarized neutron beams (with a high field magnet) and polarized neutron beams using $^{3}$He filter polarization analysis. Magnetic length scales ranging from tens of nanometers to greater than several hundreds of nanometers were observed. The magnetization contained within nanometer large domains constituted a significant fraction of the total magnetization at the exchange bias field. During magnetization reversal some of the domains were arranged in a quasi-periodic manner. Because the ferromagnetic domains are so small, they exchange couple to relatively small portions of the pinned magnetic layer (i.e., DyFe$_{2})$, which at the nm length scale may appear relatively perfect. This reasoning may explain why the DyFe$_{2}$/YFe$_{2}$ system is a system that exhibits among the largest exchange bias observed to date. This work was supported by the Office of Basic Energy Science, U.S. Department of Energy and the National Science Foundation. ML acknowledges support from DanScatt. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L16.00007: Polarized Neutron Reflectivity and Electron Microscopy Analysis of the Magnetic Microstructure in Antiparallel-Coupled Co Multilayers John Unguris, Benjamin McMorran, Julie Borchers, Brian Maranville, Brian Kirby, Theresa Ginley Antiparallel exchange-coupled thin films are a convenient way to provide a ferromagnetic surface in situations where zero net magnetization is required, for example, when studying superconducting-ferromagnetic proximity effects with spin-triplet superconducting correlations.\footnote{T. Khaire, etal. Phys. Rev. Lett. 104, 137002 (2010)} We use the complementary techniques of polarized neutron reflectivity (PNR) and scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic structure of such an antiferromagnetically coupled Co/Ru/Co multilayer. We find that, although the average macroscopic magnetization follows the simple antiparallel coupling picture, at the nanoscale the 3-dimensional magnetic structure is much more complex. The films are mostly antiparallel, but the magnetization directions fluctuate by as much as $\pm $ 40\r{ } over lengths as small as 100 nm. This structure has significant implications when trying to understand the local spin-dependent transport properties at the ferromagnetic interface. Applying magnetic fields further complicates the structures, leading to spin-flop related magnetic arrangements. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L16.00008: Distinguishing the ultrafast dynamics of orbital and spin magnetic moments in solids Invited Speaker: Ultrafast magnetization dynamics is an important issue for both fundamental science and for applications in order to optimize spin manipulation on a microscopic level. Since the first observation of laser induced spin dynamics,\footnote{Beaurepaire, E., Merle, J.C., Daunois, A., and Bigot, J.-Y. \textit{Phys. Rev. Lett.} \textbf{76}, 4250 (1996).} the mechanisms of angular momentum dissipation at picosecond timescales have been widely debated. In order to progress in the understanding of such microscopic ultrafast mechanisms, it is now possible to probe absolute values of magnetization with a high temporal resolution (100 fs). In this context, we have used ultrashort optical laser pulses (60 fs duration) to induce changes of the magnetization in a ferromagnetic CoPd alloy film with perpendicular anisotropy. The dynamics was probed with ultrashort circularly polarized femtosecond X-ray pulses, measuring the X-ray magnetic circular dichroism (XMCD) at Co L$_{2,3}$ edges.\footnote{Stamm, C. Kachel, T., Pontius, N., Mitnzer, R., Quast, T., Holldack, Khan, S., Lupulescu, C., Aziz, E. F., Wietstruk, M., D\"{u}rr, H. A., and Eberhardt, W. \textit{Nature Mater.} \textbf{6}, 740-743 (2007).}$^,$\footnote{C. Boeglin, E. Beaurepaire, V. Halt\'{e}, V. Lopez-Flores, C. Stamm, N. Pontius, H. D\"{u}rr, J. -Y. Bigot ``Observing how fast the spin-orbit interaction branches spin and orbital moments in solids'' Nature 465, 458 (2010).} We observe that the two components of the magnetic moments (L and S) show different ultrafast dynamics and that the spin-orbit coupling related to the magneto-crystalline anisotropy in solids is strongly affected by fs laser pulses in the ultrashort time scales. These dynamics can be compared to the purely electronic effect at the CoL$_{3}$ edge. Electronic excitations and their response to the laser pump pulse will be discussed and related to the modifications in the spin-orbit coupling. We will compare our results with time resolved MOKE experiments recently performed on CoPd alloys. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L16.00009: Neutron Reflectivity Study in Py/CoO Exchange Bias System San-Wen Chen, Xiangshun Lu, Sunil Sinha, Ami Berkowitz, Eric Fullerton, Keith Chan, Valaria Lauter, Hailemariam Ambaye, Elizabeth Blackburn We have studied the permalloy-cobalt monoxide exchange bias system using polarized neutron reflectivity. Both polycrystalline and epitaxial single crystalline (with the (111) and (100) CoO planes at the interface) CoO films were studied. By fitting the reflectivity for both directions of the applied field relative to the cooling field, we are able to obtain both the nuclear and spin depth profiles, as well as locating the pinned spins which are responsible for the exchange bias effect. The pinned spins at the interface can be resolved in the polycrystalline sample, which is consistent with our previous study with resonant soft x-ray reflectivity. One could reasonably have expected a stronger exchange bias effect in the (111) single crystal CoO film, because it has more uncompensated spins at the interface. The neutron reflectivity, however, shows lesser pinned spins. In the presentation, we will show the difference between the magnetic density profiles of the samples involving polycrystalline, (111) and (100) single crystalline CoO films respectively. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L16.00010: Effects of hydrogen/deuterium absorption on the magnetic properties of Co/Pd multilayers Kineshma Munbodh, Felio Perez, Cameron Keenan, David Lederman, Mikhail Zhernenkov, Michael Fitzsimmons The effects of hydrogen and deuterium absorption were studied in two Co/Pd multilayers with perpendicular magnetic anisotropy using Polarized Neutron Reflectivity (PNR). PNR measurements were performed with the field in the plane of the sample with the magnetization M saturated at H = 6.0 T and unsaturated at 0.65 T. The nominal thicknesses of the Co/Pd layers were 2.5 {\AA}/21 {\AA}. Therefore, the actual layer chemical composition, thickness, and interface width parameters were defined from the nuclear scattering length density (SLD) profile obtained from both x-ray reflectivity (XRR) and PNR and their derivatives. The nuclear PNR SLD profile showed that although deuterium absorption occurred throughout the sample, the multilayer stack did not expand. The magnetic SLD showed that M in both the Pd and Co layers was affected. At saturation, M decreased, while at H = 0.65 T M increased upon deuterium exposure. Magnetization measurements confirmed hydrogen absorption decreased the total M at saturation and increased the component of M parallel to the field when not at saturation. These results indicate that hydrogen or deuterium absorption decreases both the perpendicular anisotropy and total magnetization of the samples. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L16.00011: Revisit of Magnetism of Fe overlayers on Cu(001) Chanyong Hwang, Yong Rok Oh, Y.S. Park, Wondong Kim, Leela Poornima, Z.Q. Qiu Fe films on Cu(001) have drawn a great attention due to the complicated structural and magnetic properties. It is known from the previous investigations that depending on the Fe thickness, Fe/Cu system has three distinguishable regions in terms of the structural and magnetic properties. Especially the spin structure in region II, where the film thickness ranges from 5 ML to 11 ML, has been controversial for the last two decades. We have studied the spin structure of Fe films on Cu(001) in region II via x-ray magnetic circular dichroism (XMCD) and surface magneto-optic Kerr effect (SMOKE) measurements. Wedge-shaped Fe films, which ranged from 6 ML to 10 ML, have been grown on Cu(001) at room temperature by an e-beam evaporator. Our results suggest a new model, which is totally different from the previous models, based on the incommensurate spin density wave. [Preview Abstract] |
Session L17: Focus Session: Magnetic Oxide Thin Films - Ferroic and Oxide Tunnel Junctions
Sponsoring Units: GMAG DMPChair: James Eckstein, University of Illinois at Urbana-Champaign
Room: D174
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L17.00001: All-Manganite Tunnel Junctions with Interface-Induced Barrier Magnetism Invited Speaker: The recent discovery of several unexpected phases at complex oxide interfaces is providing new insights into the physics of strongly correlated electron systems. The possibility of tailoring the electronic structure of such interfaces has triggered a great technological drive to functionalize them into devices. In this communication, we describe an alternative strategy to produce spin filtering by inducing a ferromagnetic insulating state in an ultrathin antiferromagnetic layer in contact with a ferromagnetic layer. This artificially induced spin filtering persists up to relatively high temperatures and operates at high applied bias voltages. The results suggest that after playing a key role in exchange-bias for spin-valves, uncompensated moments at engineered antiferromagnetic interfaces represent a novel route for generating highly spin-polarized currents with antiferromagnets.\\[4pt] Work done in collaboration with M. Bibes, C. Carr\'{e}t\'{e}ro, A. Barth\'{e}l\'{e}my (Unit\'{e} Mixte de Physique CNRS/Thales, Campus de Polytechnique, 1, Avenue A. Fresnel, 91767 Palaiseau (France) and Universit\'{e} Paris-Sud, 91045 Orsay (France)), F.A. Cuellar, C. Visani, A. Rivera-Calzada, , C. Le\'{o}n, J. Santamaria (Grupo de F\'{\i}sica de Materiales Complejos, Universidad Complutense de Madrid, 28040 Madrid (Spain)), M.J. Calder\'{o}n, L. Brey (Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid (Spain)), K. March, M. Walls, D. Imhoff (Laboratoire de Physique des Solides, CNRS, Universit\'{e} Paris-Sud, 91405 Orsay (France)), R. Lopez Anton, T.R. Charlton (ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX (United Kingdom)), E. Iborra (Universidad Polit\'{e}cnica de Madrid, Escuela T\'{e}cnica Superior de Ingenieros de Telecomunicaciones, 28040 Madrid ( Spain)), F. Ott (L\'{e}on Brillouin, CEA/CNRS, UMR 12, 91191 Gif-sur-Yvette (France)). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L17.00002: Nanostructured CoFe$_{2}$O$_{4}$ Films for Magnetic Logic Applications Ryan Comes, Man Gu, Mikhail Khokhlov, Jiwei Lu, Stuart Wolf CoFe$_{2}$O$_{4}$ (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response when strained. Previous work has shown that when CFO is co-deposited with BiFeO$_{3}$ (BFO) nanostructured phase segregation occurs and electrical control of the magnetic anisotropy is possible [1]. Such a system offers unique possibilities for an electrically-controlled spintronic logic scheme. To that end, CFO films were grown on MgO and SrTiO$_{3}$ (STO) substrates using pulsed electron deposition. Films grown on MgO substrates exhibit large perpendicular anisotropy due to epitaxial strain, while films on STO exhibit mixed anisotropy. Using electron-beam lithography, nanopillars were etched into the film in dense arrays and characterized using magnetic force microscopy. Pillar arrays were produced with diameters between 30 and 75 nm with pitch ranging from 90 to 200 nm. The dipole interactions in these pillars were examined and their potential applications for spintronic logic were evaluated. Thinner CoFe$_{2}$O$_{4}$ islands were also patterned on STO with EBL and then used as a template for a co-deposited BFO/CFO film. Results of this work will also be presented. \\[4pt] [1] Zavaliche, F., et al. \textit{Nano Lett.}, 2007, 7 (6), pp 1586--1590 [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L17.00003: Spin-polarized tunnelling across single or double ferrite-based spin-filters Sylvia Matzen, Jean-Baptiste Moussy, Richard Mattana, Karim Bouzehouane, Cyrile Deranlot, Frederic Petroff, Jagadeesh Moodera, Guoxing Miao The generation of highly spin-polarized electron currents at room temperature is the basis of most spin-based device technologies. One approach known as spin filtering, has the potential of generating 100{\%} spin-polarized currents by the spin selective transport of electrons across a ferromagnetic tunnel barrier. In this work, we investigate the spin-polarized tunnelling characteristics of ferrites (CoFe2O4, NiFe2O4 and MnFe2O4), which are exciting candidates for spin filtering at room temperature. Tunnel junctions containing epitaxial ferrite tunnel barriers have been grown by oxygen plasma-assisted molecular beam epitaxy. Their structural, chemical and magnetic properties having previously been optimized by a number of in situ and ex situ methods, we focus on the spin-polarized tunnelling in the ferrite-based systems using different measurement techniques and we propose an unconventional device combining two ferrite magnetic tunnel barriers in order to get large tunnelling magnetoresistance without the necessity of magnetic electrodes. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L17.00004: Effect of Strain on Electronic and Magnetic Structure of Fe-doped CoFe$_{2}$O$_{4}$ Jarrett Moyer, Carlos Vaz, Ezana Negusse, Dario Arena, Victor Henrich The development of new materials with large room temperature spin polarizations and small conductivity mismatches with semiconductors is key for more complex spintronics devices. CoFe$_{2}$O$_{4}$ has a high Curie temperature (T$_{C}$ = 793 K), a large predicted spin polarization, and, when doped with iron, a conductivity similar to semiconductors; however, the magnetic properties of thin films are different from the bulk. To investigate the effect of strain, Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films (0 $\le x\le $ 0.65) are grown epitaxially on MgO (001) and SrTiO$_{3}$ (001) by MBE. UPS probes filled valence band states, while X-ray Linear Dichroism (XLD) determines d-orbital occupations. SQUID magnetometry and XMCD are used to determine bulk and site-specific magnetic moments, respectively. These measurements allow us to understand how strain affects the electronic and magnetic structure of Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L17.00005: Fabrication and properties of LuFe2O4 thin film Wenbin Wang, Xiaoshan Xu, Zheng Gai, Paul C. Snijders, Thomas Z. Ward, Jian Shen We have succeed in growing the LuFe2O4 polycrystalline thin film on the MgO(111) substrate with the Pulsed laser deposition(PLD) method. The surface structures, crystallographic and magnetic properties of the sample were characterized by XRD, AFM, SEM and SQUID. XRD pattern shows the sample crystallized in both (001) and (110) directions, which is also reflected in their morphological appearance in both AFM and SEM images. SQUID measurements reveal strong ferromagnetic signal in the thin film. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L17.00006: Magnetic Force Microscopy of Magnetite Thin Films with Transition Metal Buffer Layers Alfred KH. Lee, Mark C. Monti, John T. Markert, Alex de Lozanne, Priyanga B. Jayathilaka, Chris A. Bauer, Casey W. Miller Magnetite (Fe$_{3}$O$_{4}$) has been the subject of interest as a material for use in spin devices. Its ideal properties for this application break down in thin film morphologies due to the occurrence of antiphase boundaries (APBs). The density of APBs can be adjusted to some degree via film strain. This is accomplished in this work by including a variety of transition metal buffer layers between Fe$_{3}$O$_{4}$ and its MgO substrate. We investigate the microscale magnetic domain structure via magnetic force microscopy of Fe$_{3}$O$_{4}$ films on MgO with no, a Mo, or an Fe buffer layer across a temperature range surrounding the Verwey temperature (T$_V \sim$ 120K) and compare to bulk measurements from a SQUID magnetometer. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L17.00007: A structural, electronic and magnetic study of ultrathin iron oxides M. Monti, B. Santos, J. Marco, J. de la Figuera, M.A. Ni\~no, T.O. Mente\c{s}, A. Locatelli, K.F. McCarty, A. Mascaraque, O. Rodr\'Iguez de la Fuente Iron oxides continue to fascinate us after nearly a century of ``modern'' science devoted to their growth, properties and structure. Recently, a revival of research has been spurred by the multiferroic character of magnetite, and by its predicted half-metal character, both interesting for spintronic applications. Maghemite is, on the other hand, an interesting counterpart to magnetite. They both present the same inverse spinel structure but maghemite is a ferrimagnetic insulator. In this work we individually characterize flat triangular islands, less than 10 atomic layers thick, of magnetite and maghemite on Ru(0001) by means of selected-area X-ray photoemission and absorption, X-ray circular dichroism and low-energy electron diffraction and reflectivity. We grow magnetite islands in-situ, with well-defined magnetic domains inside, surrounded by a w\"{u}stite wetting layer by depositing iron in a molecular oxygen background pressure. Further exposure to NO$_{2}$ transforms the magnetite islands into maghemite, while changing the w\"{u}stite wetting layer into hematite. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L17.00008: Giant tunnel electroresistance and electrical control of spin polarization with ferroelectric tunnel barriers Invited Speaker: At room temperature, we use piezoresponse force microscopy to show robust ferroelectricity in BaTiO$_{3}$ ultrathin films, and conductive-tip atomic force microscopy to demonstrate the resistive readout of the polarization state via its influence on the tunnel current [1]. This giant electroresistance nondestructive readout paves the way for ferroelectric memories with simplified architectures, higher densities and faster operation. Additionally, ferroelectric tunnel junctions with ferromagnetic electrodes were engineered to demonstrate local, large and non-volatile control of carrier spin polarization by switching ferroelectric polarization [2]. Our results represent a giant interfacial type of magnetoelectric coupling and suggest a new low-power approach for spin-based information control. \\[4pt] [1] V. Garcia \textit{et al.}, Nature 460, 81 (2009) \\[0pt] [2] V. Garcia \textit{et al.}, Science 327, 1106 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L17.00009: Electrically Controlled Spin Valve at a Complex Oxide Interface Evgeny Tsymbal, J.D. Burton Since the discovery of giant magnetoresistance exploration of spin-dependent electronic transport has proved promising for applications. To avoid the costly generation of magnetic fields in these devices there have been recent efforts toward manipulating magnetization by \textit{electric} fields. Such magnetoelectric effects can be induced at the surfaces and interfaces of many ferromagnetic metals. Ferroelectric materials are especially helpful in this because their spontaneous electrical polarization can induce a large response at the interface with a magnetic metal. One example is the ferroelectric control of magnetic order at the interface between La$_{1-x}A_{x}$MnO$_{3}$ (where $A$ is a divalent cation), and the ferroelectric BaTiO$_{3}$ [1]. Importantly, ferroelectric films can now be made thin enough (less than a few nm) to allow measurable electron tunneling while still maintaining a stable and switchable polarization [2]. Here we show that those few atomic layers near the interface sensitive to the ferroelectric polarization can act as an atomic scale spin-valve in series with the ferroelectric tunnel barrier. Switching the ferroelectric barrier induces more than an order of magnitude change in the conductance due to the interfacial spin-valve, constituting a substantial spin-dependent transport phenomenon controlled by an electric field alone.\\[0pt] [1] J. D. Burton and E. Y. Tsymbal, Phys. Rev. B \textbf{80}, 174406 (2009).\\[0pt] [2] A. Gruverman\textit{ et al.}, Nano Lett. \textbf{9}, 3539 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L17.00010: Magnetic state switching controlled by a voltage in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ tunneling junctions Weijin Hu, Ke Chen, Xiaoxing Xi, Qi Li, Zhidong Zhang We report the switching of the two magnetic states (parallel and antiparallel states) in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ magnetic tunneling junctions by measuring the tunneling magnetoresistance after applying a voltage pulse. The junction size ranges between 5x5 to 20x20 $\mu $m$^{2}$ with the barrier thickness in the range of 1-3 nm. We have found that magnetic state of the junction can be switched both from the antiparallel to parallel state and from the parallel to antiparallel state in certain and different field ranges, respectively. The switching does not depend on the polarity of the electrical field direction and the magnetic field direction. The critical voltage for the switching depends on the magnetic field with higher voltage needed for lower magnetic field. The critical voltage depends almost linearly on the bias magnetic field when the switch occurs. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L17.00011: Four resistance states in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ multiferroic tunnel junction at room temperature Peng Xu, Yuewei Yin, Weijin Hu, Muralikrishna Raju, Qi Li, Xiaoguang Li Multiferroic tunnel junction (MFTJ), composed of two ferromagnetic electrodes separated by a thin ferroelectric barrier, has been predicted to serve as a four-state device as a result of the coexistence of tunneling magnetoresistance and tunneling electroresistance effects. Our previous results have demonstrated such devices, but only at low temperatures. Here, we report a MFTJ composed of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ fabricated by pulsed-laser deposition. A typical R-H loop with a sharp-switched resistance states (magnetic parallel and antiparallel) similar to that of magnetic tunnel junctions has been observed up to room temperature. Upon polarization reversal of the barrier, both the parallel and antiparallel resistances will switch to a different value. Clear tunneling magnetoresistance and tunneling electroresistance, hence the four-resistance states, have been observed at room temperature. The resistance states can be switched between them by electric and magnetic fields and the manipulation of the states will be discussed. [Preview Abstract] |
Session L18: Focus Session: Low D/Frustrated Magnetism - 2D Lattices
Sponsoring Units: GMAG DMPChair: Marcos Rigol, Georgetown University
Room: D172
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L18.00001: Exotic quantum phases in a frustrated quantum spin model on a honeycomb lattice Christopher Varney, Kai Sun, Victor Galitski, Marcos Rigol A quantum spin liquid is a phase that defies the usual conventions, i.e. quantum fluctuations prevent long range order even at $T = 0$. The search for models that exhibit this type of behavior has intensified in recent years. In this work, we utilize the Lanczos algorithm to study hard-core bosons on a frustrated honeycomb lattice with nearest-neighbor ($t$) and next-nearest-neighbor hoppings ($t^\prime$). The two limits of this model, $t^\prime / t = 0$ and $t^\prime / t = \infty$, favor two different superfluid phases. In between, we find that an anomalous phase is stabilized by the strong frustration in this system and compare its properties with a quantum spin liquid and a fragmented Bose-Einstein condensate. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L18.00002: Protecting the Kitaev honeycomb model from external fields Haitan Xu, Jacob Taylor We propose an approach to generate many-body interactions from two-body interactions with stable cat states. Applied to the celebrated Kitaev honeycomb model, our approach opens a spectral gap in the gapless phase of the model without any external magnetic field. We confirm the non-Abelian topological properties of a generalized Kitaev model and demonstrate our approach's robustness to sources of error. Our work provides a complete framework for experimentally realizing and manipulating non-Abelian anyons, with direct application in topological quantum computation. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L18.00003: Paramagnetic ground states and field-driven N\'eel order in S=3/2 Heisenberg antiferromagnets on a honeycomb lattice Ganesh Ramachandran, D.N. Sheng, Y.J. Kim, A. Paramekanti We study the spin-3/2 Heisenberg antiferromagnet on a honeycomb lattice with exchange interactions which frustrate N\'eel order. Our motivation stems from the recent synthesis of $Bi_3 Mn_4 O_{12} (NO_3 )$, a spin-3/2 bilayer honeycomb lattice antiferromagnet which remains paramagnetic to the lowest temperature, but shows a field-induced N\'eel transition. We use a combination of spin wave theory, exact diagonalization, and bond operator theory to study the effects of quantum and thermal fluctuations, second-neighbor exchange, biquadratic exchange and bilayer coupling. Biquadratic terms give rise an AKLT valence bond solid ground state, and bilayer coupling leads to an interlayer dimer solid. Upon applying a magnetic field, both these states undergo a phase transition into a N\'eel long range ordered state. We comment on experimental consequences and disorder effects. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L18.00004: Gapped $Z_2$ spin liquid and chiral antiferromagnetic phase in Hubbard model on the honeycomb lattice Yuan-Ming Lu, Ying Ran In Schwinger-fermion representation we identify a $Z_2$ spin liquid called the sublattice-pairing state (SPS) as the gapped spin liquid phase discovered in recent Quantum Monte study of Hubbard model on a honeycomb lattice. We show that SPS is identical to the zero-flux $Z_2$ spin liquid state in Schwinger-boson representation by an explicit duality transformation. SPS is connected to an \emph{unusual} antiferromagnetic ordered phase, which we term as chiral-antiferromagnetic (CAF) phase, through an $O(4)$ critical point. CAF phase breaks $SU(2)$ spin rotation symmetry completely and has three Goldstone modes. Our results indicate that there is likely a hidden phase transition between CAF phase and the usual antiferromagnetic (Neel) phase at large $U/t$. We also propose numerical measurements to reveal the CAF phase and the hidden phase transition. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L18.00005: Exotic phases in Mott insulating Iridates with strong spin-orbit coupling: Phase diagram of the Kitaev-Heisenberg model in a magnetic field Hong-Chen Jiang, Zheng-Cheng Gu, Xiao-Liang Qi, Simon Trebst Motivated by the recent proposal of a Mott insulating state with strong spin-orbit coupling for the Iridate Na$_{2}$IrO$_{3}$[1], we discuss the collective ground states of the effective Iridium moments in the presence of Heisenberg-Kitaev exchange interactions and a time-reversal symmetry breaking magnetic field. For a field pointing in the (111) direction we find a rich phase diagram with both symmetry breaking magnetically ordered phases as well as an unconventional topological phase which is stable over a small range of coupling parameters. Our numerical simulations further indicate two exotic multicritical points at the boundaries between these ordered phases, which we will discuss.\\[4pt] [1] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L18.00006: Schwinger boson spin liquid states on honeycomb lattice: projective symmetry group analysis and critical field theory Fa Wang Motivated by the numerical evidence of a gapped spin liquid in the honeycomb Hubbard model [Meng et al. Nature 464, 847 (2010)], we analyse possible Z$_2$ spin liquids with gapped bosonic spinons coupled to Z$_2$ gauge field on honeycomb lattice within the Schwinger boson formalism. By the projective symmetry group method we find that there are only two relevant Z$_2$ spin liquids on honeycomb lattice with different (zero or $\pi$) gauge flux in the elemental hexagon. The zero-flux state seems to be a good candidate for the numerically observed spin liquid. It can acquire collinear AFM Neel order via a continuous O(4) transition. In the critical field theory of this transition the coupling of bosonic spinons to the Higgs field contains cubic powers of spatial derivatives, therefore does not break honeycomb lattice symmetry and allows for a continuous transition to a commensurate collinear Neel order. We will also discuss several observable features of this spin liquid. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L18.00007: Nature of the spin liquid state of the Hubbard model on the honeycomb lattice Bryan Clark, Dmitry Abanin, Shivaji Sondhi Recent numerical work (Nature 464, 847 (2010)) indicates the existence of a spin liquid phase (SL) that intervenes between the antiferromagnetic and semimetallic phases of the half filled Hubbard model on a honeycomb lattice. To better understand the nature of this exotic phase, we study the quantum $J_1-J_2$ spin model on the honeycomb lattice, which provides an effective description of the Mott insulating region of the Hubbard model. Employing the variational Monte Carlo approach, we analyze the phase diagram of the model, finding a phase transition between antiferromagnet and an unusual $Z_2$ SL state which we identify as the SL phase of the Hubbard model. At higher $J_2/J_1 > 0.3$ we find a transition to a dimerized state with spontaneously broken rotational symmetry. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L18.00008: Finite-temperature phase transition to $m=1/2$ plateau phase in a S=1/2 XXZ model on Shastry-Sutherland Lattices Takafumi Suzuki We study the finite-temperature transition to the $m=1/2$ magnetization plateau in a model of interacting $S=1/2$ spins with longer range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. This model was shown to capture the qualitative features of the field-induced magnetization plateaus in the rare-earth tetraboride, ${\rm TmB_4}$. Our results show that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, whereas a single phase transition takes place in the purely Ising limit. To better understand these behaviors, we perform Monte Carlo simulations of the classical generalized four-state chiral clock model and compare the phase diagrams of the two models. The magnetic properties and critical behavior of the finite-temperature transition to the $m=1/2$ plateau state are also discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L18.00009: Study of spin-lattice coupling in the Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ Ramzy Daou, Marcelo Jaime, Scott Crooker, Franziska Weickert, Michael Nicklas, Frank Steglich, Hannah Dabkowska, Bruce Gaulin SrCu$_2$(BO$_3$)$_2$ supports a network of orthogonally coupled spin-dimers whose ground state consists of localized spin-singlets which can be described by the exactly solvable Shastry-Sutherland Hamiltonian. On applying strong magnetic fields ($>$20T), however, the spin gap in SrCu$_2$(BO$_3$)$_2$ is closed and triplet excitations are generated. As a consequence of the strong geometric frustration, the triplet band is nearly dispersionless and a sequence of steps and plateaux in the magnetisation at integer fractions of the saturation magnetisation are observed, corresponding to static magnetic textures that are commensurate with the lattice. Here we present high resolution measurements in pulsed magnetic fields up to 65T of the magnetostriction and magnetocaloric effect which 1) shed light on the coupling between spin and lattice degrees of freedom and 2) are aimed to address discrepancies between existing data and theoretical predictions for the sequence of field-induced plateaus. Experiments were carried out at the Dresden High Magnetic Field Laboratory and the pulsed field facility of the National High Magnetic Field Laboratory. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L18.00010: Phase Transitions in the $J_1-J_2$ Ising Model on the Square Lattice Songbo Jin, Arnab Sen, Anders Sandvik The $J_1-J_2$ Ising model on the square lattice is one of the simplest classical models
to study the effects of competing interactions and the resulting phase transitions. In
spite of previous studies, there remains a controversy regarding the nature of the
transition into the ``stripe`` phase in this model for $J_2/J_1>0.5$. In this study, we
use the Binder cumulant of the order parameter to address this question. We use the
Wang-Landau and Metropolis algorithms to simulate the model in the relevant
parameter space. From our numerics, we determine that the transition is first-order for
$0.5 |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L18.00011: Low Temperature $^{31}$P-NMR Study of the Frustrated Square-Lattice Compound BaCdVO(PO$_{4}$)$_{2}$ B. Roy, R. Nath, D.C. Johnston, Y. Furukawa, C. Geibel BaCdVO(PO$_{4})_{2}$ is known to be a S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ frustrated square-lattice (FSL) system with a ferromagnetic nearest neighbor exchange coupling J$_{1}\sim $ -3.36 K and an antiferromagnetic next nearest neighbor exchange coupling J$_{2}\sim $ 3.53 K. We have carried out $^{31}$P-NMR measurements at low temperatures down to 0.1 K to investigate magnetic properties of this compound from a microscopic point of view. $^{31}$P spin-lattice relaxation rates (1/T$_{1})$ measured at H = 0.8 T are almost independent of temperature above 2 K, show a peak at 1.05 K and become constant below 0.4 K. The temperature dependence of 1/T$_{1}$ indicates the existence of antiferromagnetic ordering at T$_{N}\sim $ 1.05 K which is also evidenced by the broadening of the NMR spectrum below that temperature. We will compare our NMR results with those of a similar FSL system, Pb$_{2}$VO(PO$_{4})_{2}$ and discuss the similarities and differences in the magnetic properties of these two systems. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L18.00012: Magnetic phase diagram of spatially anisotropic, frustrated spin-1/2 Heisenberg antiferromagnet on square and stacked square lattices Kingshuk Majumdar Magnetic phase diagram of a spatially anisotropic, frustrated spin-1/2 Heisenberg antiferromagnet on a square and a stacked square lattice is investigated using second-order spin-wave expansion. It is shown that with increase in next nearest neighbor frustration the second-order corrections play a significant role in stabilizing the magnetization. We obtain two ordered magnetic phases (Ne\'{e}l and stripe) separated by a paramagnetic disordered phase. Within second-order spin-wave expansion we find that the width of the disordered phase diminishes with increase in the interlayer coupling (for the 3D case) or with decrease in spatial anisotropy but it does not disappear. Our obtained phase diagram differs significantly from the phase diagram obtained using linear spin-wave theory. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L18.00013: Study of Orbital Degenerate System in Frustrated Checkerboard Lattice Joji Nasu, Sumio Ishihara Orbital degree of freedom is one of the recent attractive themes in transition-metal oxides. In contrast to the spin degree of freedom, the orbital interaction explicitly depends on the bond direction, and a certain kind of frustration exists. In the geometrical frustrated lattice, cooperating and competing effects between the orbital frustration and the geometrical frustration provide new features in the static and dynamical properties of orbital. The present purpose is to study the intrinsic orbital frustration effect in a geometrical frustrated lattice. We introduce the spin-less Hubbard-type model with the doubly degenerate $d_{yz}$ and $d_{zx}$ orbitals in the checkerboard lattice. The effective Hamiltonian for the strong correlation limit is derived. We have the $J_{z} S_{i}^{z} S_{j}^{z}$ type Ising interaction for the nearest-neighbor bonds and the $J_{x} S_{i}^{x} S_{j}^{x}$ type Ising one for the next nearest-neighbor bonds. Here \textbf{\textit{S}} is the orbital pseudo-spin operator. In the mean-field approximation, there is a macroscopic number of degeneracy at the frustration point $J_{x}$/$J_{z}$=2. In the classical Monte-Carlo simulation, we have a staggered orbital order and the reentrant phase-boundary. In the analyses by the spin-wave approximation and the exact diagonalization method, a large damping of the high-energy orbital dynamics due to the frustration is observed. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L18.00014: Thermodynamics of the AF Heisenberg Model on the Checkerboard Lattice; a Numerical Linked-Cluster Expansion Study Ehsan Khatami, Marcos Rigol Employing numerical linked-cluster expansions (NLCEs) along with exact diagonalizations of finite clusters with periodic boundary condition, we study the energy, specific heat, entropy, and various susceptibilities of the antiferromagnetic (AF) Heisenberg model on the checkerboard lattice. NLCEs, combined with extrapolation techniques, allow us to access temperatures much lower than those accessible to exact diagonalization and other series expansions. We find that the high-temperature peak in specific heat decreases as the frustration increases, consistent with the large amount of unquenched entropy in the region around maximum classical frustration, where the nearest-neighbor and next-nearest-neighbor exchange interactions ($J$ and $J'$, respectively) have the same strength, and with the formation of a second peak at lower temperatures. The staggered susceptibility shows a change of character when $J'$ increases beyond $0.75J$, implying the disappearance of the long-range AF order at zero temperature. For $J'=4J$, in the limit of weakly-coupled crossed chains, we find large susceptibilities for stripe and N\'{e}el order with ${\bf Q}=(\pi/2,\pi/2)$ at low temperatures with AF correlations along the chains. Other magnetic and bond orderings, such as a plaquette valence-bond solid and a crossed-dimer order suggested by previous studies, have also been investigated. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L18.00015: Quantum Phases of the Cairo Pentagonal Lattice Ioannis Rousochatzakis, Andreas M. Laeuchli, Roderich Moessner We present an analytical and numerical study of the spin S=1/2 antiferromagnetic Heisenberg model on the Cairo pentagonal lattice. This is the dual of the Shastry-Sutherland lattice and has been discussed as a possible new candidate for having a spin liquid ground state [1]. More recently a S=5/2 version of this model has been realized in the Bi2Fe4O9 system [2]. Here we use a model with two different types of exchange couplings and investigate the nature of the ground state as a function of their ratio. This strategy allows us to understand the nature of a number of phases and derive effective models for their description with and without a magnetic field. Of particular interest is a surprising interplay between a collinear and a four-sublattice orthogonal phase due to an underlying order-by-disorder mechanism. Furthermore we address the issue of possible nonmagnetic ground states such as singlet and spin nematic phases. \\[4pt] [1] K. S. Raman, R. Moessner, and S. L. Sondhi, PRB 72, 064413 (2005)\\[0pt] [2] E. Ressouche, V. Simonet, B. Canals, M. Gospodinov, and V. Skumryev, PRL 103, 267204 (2009) [Preview Abstract] |
Session L19: Focus Session: Spin Transport & Magnetization Dynamics in Metals V
Sponsoring Units: GMAG DMPChair: John D. Burton, University of Nebraska–Lincoln
Room: D170
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L19.00001: Thermal spin fluctuations in itinerant ferromagnets: Aspects of magnetic thermodynamics and transport properties Invited Speaker: The character of thermal spin fluctuations in itinerant ferromagnets is a long-standing problem. Our recent theoretical results offer new insights in this issue. First, I will discuss a classical effective spin model with both rotational and longitudinal spin fluctuations, which allows for a variable degree of itinerancy. Magnetic thermodynamics in this model was analyzed for fcc and bcc lattices using Monte Carlo simulations compared (favorably) with mean-field and generalized Onsager approximations. It was found that magnetic short-range order is relatively weak and almost independent on the degree of itinerancy. The ambiguity of the phase space measure will be emphasized. Next, I will discuss our first-principles calculations of spin-disorder resistivity (SDR) of Fe, Ni, and heavy rare-earth metals (Gd-Tm series), in which the Landauer conductance is explicitly averaged over spin disorder configurations. For Fe the SDR agrees very well with experiment. For Ni, comparison with experiment suggests that the average local moment in the paramagnetic state is reduced to $\sim $0.35 $\mu _{B}$. The effect of magnetic short-range order on SDR is found to be weak in both Fe and Ni. Overall, the results suggest that thermal spin fluctuations in Fe and Ni have an effectively classical character. While the crystallographically averaged paramagnetic SDR for rare earth metals agrees quite well with experiments, its anisotropy systematically and significantly exceeds the available measurements. This discrepancy is critically evaluated, suggesting the need for additional experiments. \\[4pt] [1] A. L. Wysocki, J. K. Glasbrenner, and K. D. Belashchenko, Phys. Rev. B \textbf{78}, 184419 (2008). \\[0pt] [2] A. L. Wysocki, R. F. Sabirianov, M. van Schilfgaarde, and K. D. Belashchenko, Phys. Rev. B \textbf{80}, 224423 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L19.00002: Theory of the Kondo Temperature in Multilevel Quantum Dots Ion Garate, Ian Affleck We develop a simple but general method to evaluate the Kondo temperature in a multilevel quantum dot that is weakly coupled to conducting leads. Our analysis reveals that the Kondo temperature is strongly enhanced when the intradot energy-level spacing is smaller than the charging energy. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L19.00003: Role of initial quantum correlation in transient linear response Chikako Uchiyama, Masaki Aihara We study the transient linear response of a two-level system coupled with an environmental system for correlated and factorized initial conditions. We find the significant differences between the transient linear response in these cases, especially for strong system-environment interaction at intermediate temperatures. This means that we need to pay attention to the initial conditions when we analyze experiments on transient linear response. This is because the conventional factorized initial condition, in which the system-environment correlation is disregarded, results in an incorrect response.\\[0pt] Reference: C. Uchiyama and M. Aihara, Phys. Rev. A, {\bf 82}, 044104(2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L19.00004: Even-odd parity effects of a spin-1 Heisenberg chain on long-range interaction and entanglement Sangchul Oh, Mark Friesen, Xuedong Hu A strongly coupled spin chain can be used as a quantum data bus, to mediate long-range interactions and entanglement between remote qubits.By obtaining numerical solutions for finite size systems, we investigate even-odd parity effects in a spin-1 chain and their consequences for long-range interactions and entanglement. We observes some similarities with a spin-1/2 chain~$[1]$. For example, we find that an even-size chain mediates an RKKY (Ruderman-Kittel-Kasuya-Yosida)-like interaction between weakly coupled attached spins, while an odd-size chain acts as a central spin to the attached spins. On the other hand, we find that the RKKY-like interactions have a different character. For a spin-1/2 chain, the interactions decay with qubit separation as they oscillate between ferro and antiferromagnetic couplings. For a spin-1 chain, the interactions also oscillate, but they decay very little as a function of qubit separation. We believe this behavior is a manifestation of the finite Haldane gap of an integer spin chain in the thermodynamic limit.\\[4pt] $[1]$ S. Oh, M. Friesen, X. Hu, Phys. Rev. B {\bf 82}, 140403(R) (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L19.00005: Theory of Magnetization of Interacting Bloch Electrons Prasanta Misra, Gouri Tripathi We derive a theory of magnetization of interacting Bloch electrons in the paramagnetic limits. We start with a thermodynamic potential, which includes both the quasi-particle and correlation contributions. The startling result obtained by us is that the modifications brought about by the electron-electron interactions for the magnetization in the quasi-particle interaction is precisely cancelled by the contributions due to electron correlations and thus the magnetization is devoid of explicit many-body corrections. In contrast, it is well known that both the spin susceptibility and the spin Knight-shift are exchange enhanced by electron-electron interactions. This is due to second order effects in the sense that while both the spin vertices are renormalized, the renormalization of only one of the vertices is cancelled by the contribution due to electron correlations. However, there is only one spin-vertex in the expression for magnetization which is renormalized in the quasi-particle approximation. We discuss the importance of self-energy corrections on the single-particle spectrum and we have shown as how to predict that the interacting electron system is magnetic or not by considering a variant of the Hubbard Hamiltonian in the momentum space. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L19.00006: Exact solution for permeability of a ferromagnet under parametric bichromatic irradiation Adil-Gerai Kussow, Alkim Akyurtlu If two parametrically coupled electromagnetic fields are applied to a ferromagnet, a non-linearity of the equation of precession of magnetic moment strongly affects the permeability. If the coupling constant $\vert \beta \vert <1$ between the probe (p) wave and the support (s) wave is small, the permeability $\hat {\mu }(\omega)$, at the frequency of the probe wave $\omega =\omega _p =2\omega _s$, is still monochromatic-like, with the re-normalized resonance spin waves frequency $\Omega _r \to \Omega (1+\beta )$. If a coupling is strong, $\beta \;\,\le -\,\,1$, unusual response effects are possible (the magnetic transparency and profoundly non-monochromatic permeability $\hat{\mu}(\omega)$. Possible optical applications to the homogeneous negative refractive index materials are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L19.00007: Element specific analysis of magnetic anisotropy in practical Mn-based antiferromagnetic alloys from first principles Khmelevskyi Sergii, Alexandr B. Shick, Peter Mohn Magnetic Anisotropy Energy (MAE) and element specific contribution to MAE has been studied for practical Mn-based antiferromagnetic alloys with layered L1$_{0}$ structure in the framework of the Local Spin Density Approximation and fully relativistic torque method. It is found that the contribution to the total MAE from non-magnetic 3d and 4d-elements in MnNi and MnPd alloys is comparable to the contribution of the magnetic Mn atoms. In the 3d-5d MnIr alloy the Ir contribution is found to be dominating. The origin of this contribution from the elements with total zero atomic spin moment is linked to the calculated non-trivial spin density distributions on the corresponding atom, which gives a zero moment only on average. We have also found and discuss a strong dependence of the total and element specific contribution to MAE on the state of the magnetic order. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L19.00008: Theoretical search for new permanent magnets with no rare earth atoms Liqin Ke, Vladimir Antropov We performed the extensive computational search for better permanent magnets containing no rare earth atoms. Our initial studies are concentrated on the intrinsic properties of magnetic materials such as magnetization, the Curie temperature and magnetic anisotropy. A computational tool based on the electronic structure methods has been developed to describe these physical properties as a function of electronic concentration. The application of this technique allowed us to provide several possible directions to perform a search for new materials. We discuss some physical limitations of required properties in iron based materials using the analysis of their electronic structure and simplified magnetic models. The issues of chemical substitutions, modification of geometry and changing the dimensionality of systems will be discussed as well. The specific results will be shown for Fe and Co based systems with the additions of N, C, W, Al and other atoms. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L19.00009: Ab initio study of d$^{\bigcirc }$ magnetism in CaC Hadi Akbarzadeh, Zahra Nourbakhsh, S. Javad Hashemifar The half-metallic ferromagnetism in d$^{0}$ ionic compounds has attracted considerable attention in the spintronics community [Phys. Rev. B 73 024404, 2006]. In this work we employ density functional theory to study electronic, magnetic, and mechanical properties of the high ionic CaC compound in the Zinc Blend (ZB), Rock Salt (RS), B$_{2}$, Wurtzite (WZ), NiAs, and tetragonal structures.The observed ferromagnetic equilibrium state in the RS, NiAs, WZ, tetragonal and ZB structures is attributed to the partially filled sharp p band of carbon around the Fermi level. Half-metallicity has been found in the equilibrium ZB and expanded WZ structures with a magnetic moment of 2 $\mu _{B}$/formula units. Comparing the Gibbs free energy of various structures indicates favorability of the half metallic phases in negative pressures. Topological analysis of the electronic charge density reveals topological character of the paramagnetic-ferromagnetic phase transition of the revised NiAs and B$_{2}$ structures as well as the geometrical character of the magnetic phase transitions of the RS and ZB structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L19.00010: Greigite and Spinorbitronics Baomin Zhang, Gilles de Wijs, Rob de Groot Greigite(Fe$_{3}$S$_{4})$ and magnetite(Fe$_{3}$O$_{4})$ are isoelectronic and isostructural ferrimagnets. In biology, the motility of magnetotactic bacteria is based on any or both of them. Not much work is known on greigite. Unlike half-metallic magnetite, greigite is a normal metal. Although the constituent elements are light, the complex Fermi-surface of greigite is remarkably sensitive to relativistic effects. The existence of several Fermi-surface sheets is dependent on the direction of the magnetization. This implies spintronics based on a homogeneous material rather than a device. Since this effect is intrinsically relativistic, spin-contamination is irrelevant here. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L19.00011: Experimental and theoretical investigations into the twinning energy of an FSMA system P.K. Mukhopadhyay, Madhuparna Karmakar, Rajini Kanth B., S.N. Kaul Ferromagnetic shape memory alloys (FSMA) are smart materials with largest magnetic field induced strain below austenite - martensite transformations. The lower temperature martensitic state is characterized by the presence of structural twins that have this exceptional magnetoelastic coupling. To understand this behavior, we carried out sound velocity and attenuation measurements on a typical FSMA material, NiFeAl system, and determined the Young's moduli under various stresses and associated strains. We found that the effect of stress is to alter the martensite temperature. We also studied a theoretical thermodynamic constitutive model and Clausius-Duhem inequality, to determine the stress resulting from an applied strain for an isothermal system. In the absence of an applied magnetic field the free energy of the system consists of only the mechanical energy contribution which in turn is dependent upon the elastic moduli pertaining to the elastic and twinning strains. The paper describes the details of measurements and the model chosen, along with the discussions on the correspondence between the experimental observations and various theoretically determined quantities. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L19.00012: Influence of Goldstone modes on the electronic properties of helical magnets Yan Sang, Dietrich Belitz, Kwan-yuet Ho, Ted Kirkpatrick We have investigated the influence of Goldstone modes on the electronic properties of helical magnets, such as MnSi, in which a Dzyaloshinsky-Moriya term in the action leads to helical order with a pitch wave number $q$ [1]. In the presence of a homogeneous external magnetic field $H$ the helix is superimposed by a homogeneous magnetization, which leads to a conical phase [2]. The Goldstone mode in this conical phase has the form $\Omega^2 \propto c_z k_z^2 + c_{\perp} k_{\perp}^4/q^2 + c_H k_{\perp}^2$, where $k_z$ and $k_{\perp}$ denote the components of the wave vector parallel and perpendicular to $H$, respectively. The elastic constants $c_z$ and $c_{\perp}$ are independent of $H$ for small $H$, whereas $c_H \propto H^2$. This Goldstone mode couples to the conduction electrons and leads to nonanalytic temperature dependences of various observables. In the conical phase, the strongest effect is on the thermal conductivity and the single-particle relaxation rate, which both are proportional to $T^{3/2}$. We also report results for the specific heat and the electrical resisitivity, both in the conical phase and in other phases of MnSi [3]. [1] P. Bak and M.H. Jensen, J. Phys. C 13, L881 (1980). [2] Y. Ishikawa, G. Shirane, J.A. Tarvin, and M. Kohgi, Phys. Rev. B 16, 4956 (1977). [3] Kwan-yuet Ho, T.R. Kirkpatrick, Yan Sang, D. Belitz, Phys. Rev. B 82, 134427 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L19.00013: Is CrO$_2$ Fully Spin Polarized? Analysis of Andreev Spectra and Excess Current Tomas Lofwander, Roland Grein, Matthias Eschrig We report an extensive theoretical analysis of point-contact Andreev reflection data available in the literature on ferromagnetic CrO$_2$. We find that the spectra can be well understood within a model of fully spin-polarized bands in CrO$_2$ together with spin active scattering at the contact. This is in contrast to analyses of the data within extended Blonder-Tinkham-Klapwijk models, which lead to a spin polarization varying between 50\% and 100\% depending on the transparency of the interface. We propose to utilize both the temperature dependence of the spectra and the excess current at voltages above the gap to resolve the spin-polarization in CrO$_2$ in a new generation of experiments.\\[4pt] T. L\"ofwander, R. Grein, and M. Eschrig, Phys. Rev. Lett. {\bf 105}, 207001 (2010) [Preview Abstract] |
Session L20: Focus Session: Thermoelectric Materials: Skutterudites, Novel and Nanostructured Materials
Sponsoring Units: DMP GERA FIAPChair: George Nolas, University of South Florida
Room: D168
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L20.00001: Synthesis and thermoelectric property of Ca-doped n-type Bi$_{85}$Sb$_{15}$ alloy Kamal Kadel, Wenzhi Li, Giri Joshi, Zhifeng Ren Bi$_{1-x}$Sb$_{x}$ (0.09$<$x$<$0.20) alloys are n-type semiconducting materials that exhibit a good thermoelectric property at low temperature, around 80 K. In the present work we investigated the thermoelectric properties of undoped Bi$_{85}$Sb$_{15}$ and different Ca-doped Bi$_{85}$Sb$_{15}$Ca$_{x}$ alloys (x=0.5, 2, and 5) synthesized via arc-melting first and followed by ball milling and hot pressing. Effect of different Ca doping levels on transport properties of Bi$_{85}$Sb$_{15}$ alloys has been investigated. It is found that thermal conductivity decreases with increasing Ca. Electrical transport measurements show that power factor increases with doping level of Ca up to Bi$_{85}$Sb$_{15}$Ca$_{2}$ and then decreases, yielding the maximum value of power factor of 3.8$\times $10$^{-3}$ Wm$^{-1}$K$^{-2}$ and ZT of 0.39 at room temperature for Bi$_{85}$Sb$_{15}$Ca$_{2}$. Properties at below room temperature will also be presented. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L20.00002: Exploration of Electron Poor Materials and their thermoelectric properties Daryn Benson, Ulrich Haussermann, Otto Sankey The Electron Poor Materials (EPM); ZnSb, ZnAs which have an average 3.5 valence electrons are explored via ab initio calculations. These materials are of interest for thermoelectric research. The EPM are then compared to valence balanced zinc-blende materials; InSb, GaSb, ZnSe, and ZnTe. Analysis of binding to assess the interesting electronic properties such as the role of nonclassical four-center bonds and the thermoelectric Seebeck coefficient are discussed. Bandstructure comparisons to a simple tight-binding model (Linear Combination of Atomic Orbitals (LCAO)) are preformed in order to test the effects of the atomic orbitals on the electronic structure. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L20.00003: Chemical Doping Effect on the Thermoelectric Properties of $T$Ga$_{3 }(T$ = Fe, Ru, Os) Neel Haldolaarachchige, Amar Karki, Adam Phelan, Yimin Xiong, Rongying Jin, Julia Chan, Shane Stadler, David Young Thermoelectric properties of chemically-doped intermetallic narrow-band semiconductors: $T$Ga$_{3 }(T$ = Fe, Ru, Os) are reported. The parent compounds show semiconductor-like behavior ($E_{g} \quad \sim $ 0.2 eV, $n_{290K} \quad \sim $ 10$^{18 }$cm$^{3})$ with large $n-$type Seebeck coefficients at room temperature ($S_{290K }\sim $ -300 $\mu $V/K). The semiconductor-like FeGa$_{3}$ becomes metallic upon chemical doping (adding electron carriers), but RuGa$_{3}$ and OsGa$_{3}$ remain semiconducting. While the electrical resistivity and the Seebeck coefficients of all the compounds decrease with electron doping, the Seebeck coefficients remain fairly large and $n-$type, which leads to larger power factors than those of the pure samples. The thermal conductivity (\textit{$\kappa $}$_{290K }$= 1.6 W/m K) of electron-doped FeGa$_{3}$ decreases, which increases the room temperature power factor by a large percentage ($S^{2}$\textit{/$\rho $}$_{290K}$ = 60 $\mu $W/m K$^{2})$ over that of pure FeGa$_{3}$. This improvement in the power factor leads to a corresponding enhancement in the thermoelectric figure of merit (\textit{ZT}) -- a factor of 5 increases above undoped polycrystalline FeGa$_{3}$ and two orders of magnitude improvement over that of pure single crystalline FeGa$_{3}$. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L20.00004: Doping dependence of thermoelectric performance in Mo$_3$Sb$_7$: first principles calculations David Parker, Mao-Hua Du, David Singh Experimental studies have indicated the substantial thermoelectric promise of doped Mo$_3$Sb$_7$, with a figure-of-merit ZT of 0.9 (H. Xu {\it et al}, J. Appl. Phys. {\bf 105}, 053703 (2009)) already achieved at high temperature. However, optimal doping levels have not yet been achieved. We study doping of Mo$_3$Sb$_7$ with transition metals (Ni,Fe,Co,Ru) via first principles calculations, including electronic structure, lattice dynamics and Boltzmann transport. We discuss the selection of dopant and the potential thermoelectric performance of optimally doped Mo$_{3}$Sb$_{7}$. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L20.00005: Giant Seebeck Coefficient in V-TCNE thin films Audrey Chamoire, Christopher Jaworski, Chi-Yueh Kao, Joseph Heremans, Arthur Epstein The disordered structure of organic conductors results in a naturally low thermal conductivity ($\kappa )$ but their ZT is known to be low because of their low thermopower (S) and electrical conductivity ($\sigma )$. Here we report an exception, with results obtained from 220 to 320K for the thermopower of V-TCNE$_{x}$ (V-(C$_{2}$(CN)$_{4})_{x})$ thin films deposited on a Si wafer (111). At room temperature S=+21.8 mV/K and increases with decreasing temperature. Those values are matched only by very pure semiconductors such as Si at low temperature, Bi nanowires, or strongly correlated electron systems like FeSb$_{2}$. The valence band of V-TCNE has a very high density of states over a very narrow energy range, ascribed mostly to vanadium 3d(t$_{2g})$ orbitals,\footnote{Y-J Yoo et al., Nat. Mat. \textbf{9} 638 2010} which is consistent with the exceptionally large value of S. The dependence of S and $\sigma $ upon illumination will also be shown, alongside preliminary estimates for the ZT. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L20.00006: Einstein Modes in the Phonon Density of States of the Single-Filled Skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$ Ivo K. Dimitrov, Michael E. Manley, Steven M. Shapiro, Jiong Yang, Wenqing Zhang, Lidong Chen, Qing Jie, Georg Ehlers, Andrey Podlesnyak, Jorge Camacho, Qiang Li Measurements of the phonon density of states by inelastic neutron scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator (\emph{rattler}) at ${\omega}_{E1} \approx$ 5.0 meV in the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at intermediate transfer energies (14 meV $\leq$ \emph{$\omega$} $\leq$ 20 meV) are exhibited in both the experimental and theoretical \emph{density of states} of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at ${\omega}_{E2} \approx$ 12.8 meV obtained from heat-capacity data. The emergence of local modes at intermediate transfer energies is attributed to altered properties of the host CoSb$_{3}$ cage as a result of Yb filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at ${\omega}_{E1} \approx$ 5.0 meV. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L20.00007: On the role of nanostructure on the thermal conductivity of skutterudite thermoelectrics Marco Fornari, Dmitri Volja, Jivtesh Garg, Daehyun Wee, Boris Kozinsky, Nicola Marzari One of the most effective strategies to improve the thermoelectric figure of merit in skutterudites is to reduce thermal conductivity via alloying, filling, or nanostructuring. The latter is most effective when the dimension of the domains is comparable in size to the mean free path of the dominant heat-conducting phonons. In bulk, pristine semiconductors and insulators thermal conductivity and phonons' mean-free paths can nowadays be calculated fully from first-principles from the anharmonic terms in the ionic displacements. We show here our results for the lattice thermal conductivity of several compounds with the skutterudite structure, obtained from the Boltzmann transport equation using phonon lifetimes determined from density functional calculations. We will also discuss the effect of fourth-order terms, albeit as obtained using phenomenological approaches. Last, we comment on the interplay between the different length scales for the nanostructured domains and the relevant heat-carrying phonons. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L20.00008: Thermoelectric Properties of p-type Yb Filled Skutterudite Yb$_{y}$Fe$_{x}$Co$_{4-x}$Sb$_{12}$ Chen Zhou, Donald Morelli, Xiaoyuan Zhou, Guoyu Wang, Ctirad Uher Since the discovery in 1995 of high thermoelectric figure of merit in skutterudite compounds, much work has been done to optimize the thermoelectric properties of these materials. As a result of this effort, n-type skutterudites are available today with figure of merit \textit{ZT} approaching 1.8. By contrast, p-type skutterudites have lagged behind, with the best materials having figure of merit less than unity. In this study, we report the thermoelectric and magnetic properties of p-type Yb-filled skutterudites of nominal composition Yb$_{y}$Fe$_{x}$Co$_{4-x}$Sb$_{12}$ with the aims of extending our knowledge of the filled skutterudite family and enhancing the thermoelectric properties of these p-type materials. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L20.00009: First principles calculations of the interactions of a filling atom with its neighboring atoms in a skutterudite (LaFe4Sb12) Joseph Feldman, David Singh, Noam Bernstein The room temperature lattice thermal conductivity of filled skutterudites is about a factor of 5 smaller than that of unfilled skutterudites which has caused a great deal of attention to be focused on these materials from a scientific standpoint, as well as a technological one, i.e., thermoelectric applications. In an effort to gain a microscopic understanding of this we have previously used a central force model and Green-Kubo techniques with force parameters heavily based on first principles results [Bernstein et al., Phys Rev. B {\bf 81}, 134301 (2010)]. However, as we had no first principles information on the La-Fe cubic anharmonic parameters in LaFe4Sb12 we have performed new direct method calculations for a larger supercell than the Bravais cell used previously to compute not only the six independent La-Fe cubic anharmonic parameters but numerous other parameters. Atomic forces were computed in various structural configurations differing only by the coordinates of one of the two La positions in the simple cubic supercell. DFT results are compared for LAPW, PAW, and plane-wave pseudopotential methods. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L20.00010: Thermoelectric Technology for Automotive Waste Heat Recovery Gregory Meisner Essential to the long term success of advanced thermoelectric (TE) technology for practical waste heat recovery is fundamental physics and materials research aimed at discovering and understanding new high performance TE materials. Applications of such new materials require their development into efficient and robust TE modules for incorporation into real devices such as a TE generator (TEG) for automotive exhaust gas waste heat recovery. Our work at GM Global R{\&}D includes a continuing investigation of Skutterudite-based material systems and new classes of compounds that have potential for TE applications. To assess and demonstrate the viability of a TEG using state-of-the-art materials and modules, we have designed, fabricated, installed, and integrated a working prototype TEG to recover exhaust gas waste heat from a production test vehicle. Preliminary results provide important data for the operation and validation of the mechanical, thermal, and electrical systems of the TEG in combination with the various vehicle systems (e.g., exhaust bypass valve and controls, thermocouples, gas and coolant flow and pressure sensors, TE voltage and output power). Recent results from our materials research work and our functioning automotive TEG will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 5:06PM |
L20.00011: Bottom-Up Strategy for Thermoelectric Nanocomposites Invited Speaker: Thermoelectric (TE) materials that incorporate nano-scale domains offer potential control over electrical and thermal properties simultaneously. A bottom-up strategy may provide cost-effective, scalable, and reproducible processing of TE materials~with improved~TE properties above existing materials. The strategy involves composition and size controlled syntheses of TE materials as nanocrystals by employing facile solution based processes followed by densification into bulk nanocomposite pellets using Spark Plasma Sintering. In this talk an overview of the various solution phase synthesis processes for preparing nanocrystals of different TE materials will be presented. In addition the TE properties after SPS densification will be discussed in relation to composition and grain size within the nanocomposites. Experimental results will be assessed together with theoretical modeling in describing the effect of the nano-scale domains on the TE properties. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L20.00012: Formation Mechanisms of Embedded Zincblende and Wurtzite Nitride Nanocrystals Adam Wood, X. Weng, Y.Q. Wang, R.S. Goldman Semiconductor nanocomposites have been proposed for high figure of merit thermoelectrics. A promising approach to nanocomposite synthesis is matrix-seeded growth, which involves ion-beam-amorphization of a semiconductor film, followed by nanoscale re-crystallization via rapid thermal annealing (RTA) [1]. In this work, we are studying the formation and evolution of N ion-implanted InAs and GaAs. Low temperature (77K) N ion implantation into InAs leads to the formation of an amorphous layer with crystalline InAs remnants. RTA at up to 550\r{ }C leads to the nucleation of zincblende (ZB) InN nanocrystals (NC). RTA at 600\r{ }C leads to nucleation of both ZB and wurtzite (WZ) InN, with an increase in average NC size. These results are consistent with the predictions of a thermodynamic model for the nanoscale-size-dependence for nucleation of ZB and WZ InN. We are also developing a novel approach to \textit{direct} the seeding of nanostructure arrays, using a combination of focused-ion-beam (FIB) implantation and conventional ion implantation. To date, we have demonstrated the selective positioning of WZ and ZB GaN NCs using 75keV and 100keV N implantation, followed by FIB patterning and 800\r{ }C RTA. [1] X. Weng, et al, \textit{J. Appl. Phys}. \textbf{97,} 64301 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L20.00013: Thin film thermocouples for thermoelectric characterization of nanostructured materials Matthew Grayson, Chuanle Zhou, Andrew Varrenti, Seung Hye Chyung, Jieyi Long, Seda Memik The increased use of nanostructured materials as thermoelectrics requires reliable and accurate characterization of the anisotropic thermal coefficients of small structures, such as superlattices and quantum wire networks. Thin evaporated metal films can be used to create thermocouples with a very small thermal mass and low thermal conductivity, in order to measure thermal gradients on nanostructures and thereby measure the thermal conductivity and the Seebeck coefficient of the nanostructure. In this work we confirm the known result that thin metal films have lower Seebeck coefficients than bulk metals, and we also calibrate the Seebeck coefficient of a thin-film Ni/Cr thermocouple with 50 nm thickness, showing it to have about 1/4 the bulk value. We demonstrate reproducibility of this thin-filmSeebeck coefficient on multiple substrates, and we show that this coefficient does, in fact, change as a function of film thickness. We will discuss prototype measurement designs and preliminary work as to how these thin films can be used to study both Seebeck coefficients and thermal conductivities of superlattices in various geometries. The same technology can in principle be used on integrated circuits for thermal mapping, under the name ``Integrated On-Chip Thermocouple Array'' (IOTA). [Preview Abstract] |
Session L21: Focus Session: Imaging and Modifying Materials at the Limits of Space and Time Resolution II
Sponsoring Units: DMP GIMS DCPChair: Richard Haglund, Vanderbilt University
Room: D161
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L21.00001: Nanoscale phase transitions within single ion tracks. William Weber, Ram Devanathan, Pedro Moreira The dynamics of track development due to the passage of energetic ions through solids is a long-standing issue relevant to nuclear materials, age-dating of minerals, space exploration, and nanoscale fabrication of novel devices. We have integrated computer simulation and experimental approaches to investigate nanoscale phase transitions under the extreme conditions created within single tracks of energetic ions in the Gd$_{2}$Zr$_{2-x}$Ti$_{x}$O$_{7}$ system and ZrSiO$_{4}$. Based on the inelastic thermal spike model, we have used molecular dynamics simulations to follow the time evolution of the structure of individual tracks and to reveal the phase transition pathways to experimentally observed concentric track structures. The molecular dynamics simulations clearly demonstrate the dependence of track evolution on composition, deposited energy density, and the complex competition among melting, disordering and recrystallization processes. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L21.00002: Exploring electron beam induced heat and mass transport at the atomic scale Christian Kisielowski In recent years the performance of mid-voltage electron microscopes was significantly boosted to reach deep sub-{\AA}ngstrom resolution around 0.5 {\AA} at 300 kV in broad beam (TEM) and focused probe (STEM) modes. Atomic resolution microscopy at voltages as low as 50 kV (and possibly below) was fostered. As a result the detection of single atoms across the Periodic Table of Elements is now possible even if light atoms are considered. After decades of striving for resolution enhancement, electron microscopy has now reached a limit that is given at a fundamental level by the Coulomb scattering process itself and by beam-sample interactions, which set a maximum dose limit that can be easily reached for soft and hard materials with the developed high-brightness electron guns. Consequently, new frontiers for electron microscopy emerge and this contribution addresses dynamic processes at the single atom level that can now be captured in time series of images at frequencies below 1 Hz reaching towards kHz. In this frequency range much of the observed atom dynamics is electron beam induced and the control of beam-sample interaction imposes constraints as well as opportunities. In this contribution it is shown that it seems feasible to exploit beam sample interactions to gain better insight into heat and mass transport in soft and hard matter at atomic resolution. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L21.00003: Gold nanoislands for sensitivity enhancement in organic and imaging mass spectrometries (LDIMS, keV- and MeV-SIMS) Arnaud Delcorte, Oscar Restrepo, Aneesh Prabhakaran Gold nanoparticles condensed on the surface of organic materials induce large ion yield enhancements in secondary ion mass spectrometry, using atomic projectiles. Here, we first show that the interest of surface metallization extends to MeV-SIMS and to UV laser desorption/ionization, in which the energy of the primary beam is deposited through the electronic subsystems (but not to keV-cluster-SIMS). For the three methods, gold nanoislands induce at least a ten-fold increase of the characteristic fragment and molecular ion yields, making surface metallization an interesting approach for imaging MS of organic surfaces. In the second part of this report, we discuss the underlying physics. For instance, using molecular dynamics simulations, we explain why 10 keV atomic projectiles interacting with metallized organic surfaces desorb more molecules, and why it is not the case with cluster projectiles such as C$_{60}$ and Au$_{400}$. For the other regimes of irradiation, arguments involving photon absorption and electronic effects are proposed. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L21.00004: Surface characterization at the spatial resolution limit with Individual Cluster Impacts Francisco Fernandez-Lima, Michael J. Eller, John D. DeBord, Stanislav V. Verkhoturov, Serge Della-Negra, Emile A. Schweikert The use of cluster bombardment (e.g. C$_{60}$ and Au$_{400})$ for surface analysis and characterization has shown significant advantages due to enhanced emission of molecular ions, low damage cross section, and reduced molecular fragmentation. At temporally and spatially discrete cluster impacts, the small impacted volume (10$^{3}$ nm$^{3})$ and ionized ejecta are ideal candidates for surface molecule interrogation. In the present talk, recent measurements of co-emitted photons, electrons and secondary ions from individual cluster impacts for several projectile-target combinations will be presented. Inspection of the photon and electron emissions show that the emission profiles are correlated with the target structure/composition at the nanometer level, with the particularity that co-emitted photons, electrons and secondary ion pairs can be used as indicators of the surface content and homogeneity. Examples of surface mapping of intact molecules via electron emission microscopy combined with secondary ion detection will be shown. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L21.00005: Electronic response of dielectric covered metal surfaces to highly charged ions R.E. Lake, J.M. Pomeroy, C.E. Sosolik The strong Coulombic perturbation on a solid target from a highly charged ion (HCI) initiates a complex many-body response from target electrons that can produce novel effects such as potential energy sputtering, nanofeature formation and huge secondary electron yields. Far above the surface, HCIs reach a critical electron capture distance and neutralization proceeds via resonant charge transfer over the vacuum barrier [1]. Motivated by recent experiments [2], we detail the onset of charge transfer between a HCI and a metal covered with a dielectric thin film (Co with 1.5 nm Al$_{2}$O$_{3}$) to determine the film's effect on the critical distance. Surprisingly, we find that the first captured electrons are pulled through the exposed dielectric and come from the underlying metal. Additionally, the Al$_{2}$O$_{3}$ film lowers the effective work function of the target and extends the critical distance compared to a clean metal. I will discuss how the experimental parameters (thin film material/thickness and ion charge state/velocity) can be tuned to allow the ion to interact with electrons in either the metal or thin film.\\[4pt] [1] Phys. Rev. A \textbf{44}, 5674 (1991).\\[0pt] [2] J. Phys.: Condens. Matter \textbf{22}, 084008 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L21.00006: Extraordinary sensitivity of nanoscale infrared spectroscopy demonstrated on Graphene and thin SiO$_2$ Greg Andreev, Z. Fei, W. Bao, Z. Zhao, C.N. Lau, L.M. Zhang, M. Fogler, G. Dominguez, M. Thiemens, F. Keilmann, D. Basov Infrared Spectroscopy is a powerful tool for characterizing materials by their vibrational mode fingerprint and/or electron conductivity. Its application to nanoscale resolved studies is highly desirable but remained challenging mainly for two reasons: a suitable source of intense, broadband infrared illumination was not widely available and the spatial resolution of conventional microscopes was limited by diffraction. We have resolved both issues by utilizing tunable External Cavity Quantum Cascade Lasers (EC-QCLs) as an intense illumination source for a scattering Scanning Near Field Optical Microscope (s-SNOM), capable of $<$10nm spatial resolution. With this combination of EC-QCLs + s-SNOM we demonstrate $<$10nm resolution imaging and spectroscopy of extremely thin materials: Silicon oxide layers (SiO$_2$) as thin as 2nm and even single atomic layers of Carbon (Graphene). The spectra register contrasts for volumes as small as 20x20x1nm$^3$ = 400 yoktoliters of SiO$_2$, and about 70 yl of Graphene over a broad spectral range: 1065-2250cm$^{-1}$. We explain the origins of this extraordinary sensitivity with an improved theoretical framework for calculating the near field response of a multilayer system. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L21.00007: Mesoscopic metal-insulator transitions at twin domain walls in improper ferroelastic VO$_2$ Invited Speaker: Appearance of unusual phenomena at interfaces of different materials due to symmetry breaking and atomic, electronic, or spin reconstructions is well established area of intensive research. Domain walls in ferroic materials also can show unusual behavior due to symmetry discontinuities. VO$_2$ is a strongly-correlated-electron material, which exhibits a metal-insulator phase transition with a structural, lattice symmetry-lowering transformation making this material an improper ferroelastic. We observe mesoscopic metal-insulator transitions at the ferroelastic domain walls in the lower-symmetry phase of VO$_2$ that occur at temperatures as much as 10-12 $^{\circ}$C below the bulk transition, resulting in the formation of metallic channels in the semiconducting material. The experiments are made using AFM-based scanning near-field microwave microscopy, which allows simultaneous accurate imaging of topography and the low-frequency dielectric function with a special resolution as high as 50 nm. The latter is possible due to a relatively high frequency used (in a few GHz range), when the sample-probe capacitive coupling becomes sufficiently strong and the electric current path is complete by displacement currents between the sample, probe tip, and the probe shield electrode. Density functional calculations indicate that ferroelastic domain walls of this type possess metallic character at low temperatures, which can be ascribed to elevated structural symmetry at the domain walls. The observed behavior, linked as well to the strain inhomogeneity inherent to ferroelastic materials, is generally relevant to symmetry-lowering phase transitions in other material systems. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L21.00008: Simulation of Non-contact Atomic Force Microscopy for Structural Analysis James Chelikowsky, Tzu-Liang Chan, C.Z. Wang, Kai-Ming Ho A powerful probe of materials centers on the use of atomic force microscopy (AFM). However, an analysis of AFM images can be complex and problematic. We will present an efficient scheme to simulate non-contact AFM images by employing a first-principles self-consistent potential from the sample as the essential input. This scheme does not require an explicit modeling of the AFM tip. Our method will be illustrated by applying it to various types of semiconductor surfaces including Si(111) (7x7), TiO2 (110) (1x1), Ag/Si(111)- ($\sqrt{3} \times \sqrt{3}$) R30$^o$ and Ge/Si(105) (1x2) surfaces. We obtain good agreement with experimental results and previous theoretical studies by using this scheme. The method can quickly and efficiently aid in identifying different models for surface structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L21.00009: Synchrotron X-ray Enhanced Scanning Tunneling Microscopy Volker Rose, John Freeland Proper understanding of complex phenomena occurring in nanostructures requires tools with both the ability to resolve the nanometer scale as well as provide detailed information about chemical, electronic, and magnetic structure. Scanning tunneling microscopy (STM) achieves the requisite high spatial resolution; however, direct elemental determination is not easily accomplished. X-ray microscopies, on the other hand, provide elemental selectivity, but currently have spatial resolution only of tens of nanometers. We present a novel and radically different concept that employs detection of local synchrotron x-ray interactions utilizing a STM that provides spatial resolution, and x-ray absorption directly yields chemical, electronic, and magnetic sensitivity. If during tunneling the sample is simultaneously illuminated with monochromatic x-rays, characteristic absorption will arise. Electrons that are excited into unoccupied levels close to the Fermi level modulate the tunneling current giving rise to elemental contrast. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L21.00010: Frequency comb generation in a tunneling junction by intermode mixing of ultrafast laser pulses Mark Hagmann, Dzmitry Yarotski, Anatoly Efimov, Antoinette Taylor Nonlinear interaction of electromagnetic radiation with tunneling electrons results in a number of peculiar physical phenomena, such as frequency mixing and imaging of insulating surfaces with scanning tunneling microscopy (STM). Arguably, the most promising among them is coupling of femtosecond laser pulses to the STM for material dynamics observation at nm/ps scales. However, the underlying physics is still poorly understood and the majority of existing studies of nonlinear mixing have been restricted to the use of CW lasers in a narrow range. Here, we present a new method for the hyper-spectral characterization of the nonlinear effects in tunneling junction. We use a 10-fs laser pulses at a nominal repetition rate of 74.25 MHz to generate a frequency comb in the tunneling current with frequencies up to 1 GHz. The typical output power at the fundamental (repetition) frequency is -120 dBm, and decreases for higher harmonics. The observed magnitude and square-law dependence of the signal power on the tunneling current and incident laser power are in good agreement with theoretical predictions. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L21.00011: First principles computation of dynamical structure factor in real and momentum space in cuprates Yung Jui Wang, B. Barbiellini, Hsin Lin, Tanmoy Das, Susmita Basak, P. E. Mijnarends, S. Kaprzyk, R. S. Markiewicz, A. Bansil We present a method for efficient, accurate first-principles calculations of the dynamical structure factor $S(\textbf{q},\omega)$ in periodic systems, using products of real space Green functions and fast Fourier transforms (FFT). We further invert $S(\textbf{q},\omega)$ via Fourier transformation [1] to reconstruct the propagator of electron density $X(\textbf{x},t)$ in real space and time domain, thereby visualising spatially the dynamics of an electron doped cuprate system in real time. The present method is useful for many-body perturbation theories of excitations based on Density Function Theory (DFT) and modeling of various highly resolved spectroscopies going beyond the standard LDA [2-5]. Some illustrative examples will be presented. Work supported by the US DOE.\\[4pt] [1] P. Abbamonte \textit{et al.}, Phys. Rev. Lett. {\bf 92}, 237401 (2004).\\[0pt] [2] Susmita Basak \textit{et al.}, Phys. Rev. B {\bf 80}, 214520 (2009).\\[0pt] [3] J. Nieminen \textit{et al.}, Phys. Rev. B {\bf 80}, 134509 (2009). \\[0pt] [4] R. S. Markiewicz \textit{et al.}, Phys. Rev. B {\bf 77}, 094518 (2008).\\[0pt] [5] G. Stutz \textit{et al.}, Phys. Rev. B {\bf 60}, 7099 (1999). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L21.00012: Beam self-focusing in the near field emission scanning electron microscopy Fuxiang Li, Artem Abanov Recent experiment on the near field emission scanning electron microscopy shows an unexpectedly high lateral and vertical resolution. We show that these effects can be explained by the beams self-focusing. We derive the equations for the beam propagation and solve them numerically. Our results are in a very good agreement with the experiment. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L21.00013: Enhancing the spatial resolution in PEEM beyond 30nm using diamondoid surface coating Hitoshi Ishiwata, Hendrik Ohldag, Zhi-Xun Shen, Nick Melosh, Andreas Scholl The spatial resolution in Photoemission Electron Microscopy typically does not allow imaging features smaller than 30nm. PEEM resolution is limited by chromatic and spherical aberrations of the electrostatic lenses in the microscope column in combination with a wide angular and energy distribution of the secondary electrons to make these aberrations significant. Diamondoids have recently been shown to act as a monochromator for secondary electrons, thus reducing chromatic aberration in PEEM. In addition to improving the resolution of the microscope the diamondoid coating will also enhance the image intensity since now more secondary electrons will be accepted by the aperture. At 10kV the spatial resolution of PEEM3 is of the order of 150-200nm so that the magnetic domains can hardly be recognized anymore without the diamondoid coating. However, they become visible on the sample that was coated with diamondoids, indicating that the coating improved the spatial resolution by monochromatizing the secondary electrons. We also find that the image intensity is enhanced by a factor of 2-3 with the diamondoid coating. These initial findings on samples with relatively large domains of 150nm are very encouraging and we are therefore convinced that we can push the resolution limit below 30nm studying samples with smaller domains at higher acceleration voltages of 20kV. [Preview Abstract] |
Session L22: URu2Si2
Sponsoring Units: DCMP GMAGChair: Nicholas Butch, University of Maryland
Room: D163
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L22.00001: Quantification of U f-valence in URu$_{2}$Si$_{2}$ from 3D Bulk Fermi Surface Topology Jonathan Denlinger, O. Krupin, J.W. Allen, B.J. Kim, K. Haule, Kyoo Kim, G. Kotliar, J.L. Sarrao, N.P. Butch, M.B. Maple The three-dimensional bulk Fermi surface (FS) topology of paramagnetic-phase URu$_{2}$Si$_{2}$, as measured by photon-dependent angle-resolved photoemission spectroscopy of UHV-cleaved surfaces, is presented and discussed. Complete characterization of silicon-terminated surface states using spatial dependence, surface adsorption and theoretical surface slab calculations, allows identification of the bulk electronic band structure. The bulk FS topology is shown to be distinctly different from those of both localized ThRu$_{2}$Si$_{2}$-like f-core LDA calculations (U$^{4+})$ and fully itinerant LDA calculations. Key experimental FS topologies can be matched to itinerant LDA contours with suitable Fermi energy shifts that may mimic strong correlation effects not well treated in LDA. The intermediate-sized FS band topologies point to a mixed valent f-occupation between those of the two LDA calculations, n$_{f}\approx $2.2 (f-core) and n$_{f}\approx $2.6 (f-itinerant). [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L22.00002: Band renormalization at the hidden order transition in URu$_2$Si$_2$ T. Durakiewicz, G. Dakovski, Y. Li , G. Rodriguez, J.J. Joyce, E.D. Bauer, P.H. Tobash, P.M. Oppeneer, P.S. Riseborough The temperature-dependent evolution of the band structure in the hidden order (HO) system URu$_2$Si$_2$ was investigated by angle-resolved photoemission(ARPES) and time-resolved photoemission (trARPES) methods. The band structure changes with temperature, and the two dominant effects set the scale for the observed variations near the Fermi level. A hybridization gap is opening at around 70K, and the smaller gap forms below the HO transition temperature. The quasiparticle dynamics across the transition is measured with trARPES. The 3D nature of the electronic structure results in differences obtained by ARPES performed at different photon energies. We show how three different experiments, performed at 7eV, 21.2eV and 34eV can be reconciled with one model when the 3D electronic structure is taken into account. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L22.00003: Complete mapping of dynamic spin correlations in the Hidden Order phase of URu$_2$Si$_2$ H. Barath, Z. Yamani, W.J.L. Buyers, T. Williams, G. Luke, J. Rodriguez, J. Leao, D. Garrett, K.J. McClellan, E.D. Bauer, J.L. Sarrao, C. Broholm URu$_2$Si$_2$ is a heavy fermion compound which undergoes a phase transition at 17.5 K to a ``Hidden Order (HO) phase''. We use inelastic neutron scattering to investigate spin correlations in the HO and paramagnetic phases and find them to be qualitatively different. While the normal state response has sharp features in Q but not in energy, consistent with an itinerant magnet, a distinct spectral gap develops in the HO phase. These results are consistent with previous measurements [1]. Here we present a full measurement of the scattering function, S(\textbf{Q},$\omega$), for energy transfers, $\hbar$$\omega$ $<$ 11 meV throughout the [H 0 L] plane. The data shows qualitatively different dispersion within, and perpendicular to, the tetragonal basal plane and provides information about the Fermi surface and its reconstruction upon entering the HO phase.\\[4pt] [1] C.R. Wiebe \textit{et. al.}, Nature Physics, \textbf{3}, 96-100 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L22.00004: Broken Rotational Symmetry in the Hidden Order Phase of URu$_2$Si$_2$ T. Shibauchi, R. Okazaki, H.J. Shi, Y. Haga, T.D. Matsuda, E. Yamamoto, Y. Onuki, H. Ikeda, Y. Matsuda The nature of the so-called `hidden order' phase transition at $T_h = 17.5$ K in the heavy fermion compound URu$_2$Si$_2$ has posed a long-standing mystery, because despite 25 years of study it remains unidentified what symmetry is broken in this ordered phase. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below $T_h$, which breaks four-fold rotational symmetry in tetragonal URu$_2$Si$_2$. Two-fold oscillations in the magnetic torque, which is measured in magnetic ?elds rotating precisely within the ab plane, are sensitively detected in small pure crystals for the first time. The amplitude of the two-fold oscillations onsets precisely at $T_h$, indicating its close link to an order parameter of the hidden order phase. Our findings uncover that the hidden order phase is an electronic `nematic' phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L22.00005: Optical evidence of Fermi liquid scattering in URu$_2$Si$_2$ Thomas Timusk, Jesse Hall, Sarah Purdy, Travis Williams, Graeme Luke, Toomas R\~o\~om, Taaniel Uleksin, Urmas Nagel, Ricardo Lobo, P. Lejay, Christopher Homes We present new high resolution, low noise, data that demonstrates that in the coherent heavy Fermion state of URu$_2$Si$_2$ the conductivity is due to heavy ($m \approx 50 m_e$) quasiparticles with a self energy that is dominated by Fermi liquid scattering according to $1/\tau(T,\omega) = A(\omega^2 + (\pi T)^2)$ where the coefficient $A=0.23$ $\mu\Omega$cmK$^{-2}$. We use this property to develop a new method of reducing the noise in the low frequency reflectance spectra of this material. In the hidden order state the spectra show evidence of anisotropy of the hidden order gap parameter with $2\Delta_{max} = 6.0$ meV and $2\Delta_{min} = 4.6$ meV. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L22.00006: Signature of Hidden Order in URu$_2$Si$_2$ in the c-axis Optical Conductivity Jesse Hall, Sarah Purdy, Travis Williams, Graeme Luke, Thomas Timusk, Toomas R\~o\~om, Taaniel Uleksin, Urmas Nagel, Ricardo Lobo We present high quality c-axis far infrared optical data for the heavy fermion compound URu$_2$Si$_2$. In particular, we compare the signature of the as yet poorly understood 'hidden order' state along the a- and c-axes of the tetragonal structure. The results presented here demonstrate the presence of the hidden order in the ac plane along the c- direction, although there is a very pronounced difference from the absorption at 5 meV seen along the a-axis. We present an assessment of the nature and significance of the a-c anisotropy of the hidden order signature. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L22.00007: Cyclotron resonance of ultra-clean URu$_{2}$Si$_{2}$ single crystals in the hidden order and superconducting states Sho Tonegawa, Ken-ichiro Hashimoto, Yao-Han Lin, Ryo Katsumata, Kousuke Ikada, Yoshinori Haga, Tatsuma Matsuda, Etsuji Yamamoto, Yoshichika Onuki, Takasada Shibauchi, Yuji Matsuda The cyclotron resonance is a powerful probe to detect the effective mass and scattering time of the electron, but there is few example of the report in the heavy fermion compounds. We succeeded in observing cyclotron resonance in the heavy fermion superconductor URu$_{2}$Si$_{2}$ not only in the hidden ordered state, but also in the superconducting state. In the hidden ordered state, we observe the missing heavy band which has not been detected by de Haas van Alphen (dHvA) measurements. In the superconducting state, the resonance lines exhibit an unexpected sharpening below the transition temperature, suggesting the realization of quasiparticle Bloch state in the vortex lattice state. We will compare our data to the dHvA measuments and discuss the possible electronic structure of the hidden order state. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L22.00008: Neutron Scattering Study of URu$_{1.9}$Re$_{0.1}$Si$_{2}$: Driving Hidden Order Towards Quantum Criticality Travis Williams, N.P. Butch, G.M. Luke, M.B. Maple, Z. Yamani, W.J.L. Buyers We report inelastic neutron scattering measurements in the hidden order state of URu$_{1.9}$Re$_{0.1}$Si$_{2}$. We have fit the data to a resolution convolved simple harmonic oscillator model, plus a continuum extending to 10meV. We observe that towards the Quantum Critical Point (QCP) induced by Re-doping, the gapped incommensurate fluctuations are fairly robust, being nearly identical to the parent material. The gap at the commensurate point (1 0 0) is driven down as the doped system approaches the QCP. The response of this commensurate spin fluctuation associated with the hidden order acquires substantial damping. The particle-hole spectrum of nested fermions [1] can be fitted to the energy and damping of the excitations, but there is no evidence for the static charge density wave that the model implies [2], in agreement with STM [3]. We conclude that Re-doping weakens, but does not destroy, the hidden order on approaching the QCP transition to ferromagnetism. \newline [1] Balatsky et al. Phys. Rev. B 79 (2009) 214413 \newline [2] Su et al. arXiv/cond-mat:1010.0767 (2010) \newline [3] Schmidt et al. Nature 465 (2010) 570 \newline Research at UCSD supported by U.S. DOE Grant \#DE-FG02-04ER46105. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L22.00009: Fano resonance and the hidden order in URu$_{2}$Si$_{2}$ probed by quasiparticle scattering spectroscopy$^{\ast }$ W. K. Park, L. H. Greene, E. D. Bauer, P. H. Tobash, F. Ronning, X. Lu, J. L. Sarrao, J. D. Thompson The nature of the hidden order transition occurring at 17.5 K in URu$_{2}$Si$_{2 }$remains puzzling despite intensive investigations over the past two and half decades. Recent experimental and theoretical developments render it a timely subject to probe the hidden order state using quasiparticle tunneling and scattering techniques. We report on the Fano resonance in pure and Rh-doped URu$_{2}$Si$_{2}$ single crystals using point-contact spectroscopy. The conductance spectra reproducibly reveal asymmetric double peak structures slightly off-centered around zero bias with the two peaks merging well above the hidden order transition temperature. An analysis using the Fano resonance model in a Kondo lattice [1] shows that the conductance peaks arise from the hybridization gap opening. Our estimated gap size agrees well with those reported from other measurements. We will present experimental results over a wide parameter space including temperature and doping dependences and discuss their underlying physics. [1] M. Maltseva, M. Dzero, and P. Coleman, Phys. Rev. Lett. \textbf{103}, 206402 (2009). $^{\ast }$The work at UIUC is supported by the U.S. DOE under Award Nos. DE-FG02-07ER46453 and DE-AC02-98CH10886, and the work at LANL is carried out under the auspices of the U.S. DOE, Office of Science. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L22.00010: Tunneling into clean Heavy Fermion Compounds: Origin of the Fano Lineshape Peter Wolfle, Yonatan Dubi, Alexander Balatsky Recently observed tunneling spectra on clean heavy fermion compounds show a lattice periodic Fano lineshape similar to what is observed in the case of tunneling to a Kondo ion adsorbed at the surface. We show that the translation symmetry of a clean surface in the case of \emph {weakly correlated} metals leads to a tunneling spectrum given by the superposition of the local weighted density of states of all energy bands involved, which does not have a Fano lineshape. In particular the spectrum will show any hybridization gap present in the band structure. By contrast, in a \emph{strongly correlated} heavy fermion metal the heavy quasiparticle states will be broadened by interaction effects. The broadening grows as one moves away from the Fermi surface, up to a value of the order of $T_K$ , the Kondo scale. We show that the hybridization gap is completely filled in this way, and an ideal Fano lineshape of width $T_K$ results, similar to the impurity case. We also discuss the possible influence of the tunneling tip on the surface, in (i) leading to additional broadening of the Fano line, and (ii) enhancing the hybridization locally, hence adding to the impurity type behavior. The latter effects depend on the tip-surface distance. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L22.00011: A $\Gamma_5$ composite density wave model for the hidden order of URu$_2$Si$_2$ Piers Coleman, Premala Chandra, Rebecca Flint Motivated by recent experiments on URu$_2$Si$_2$[1-4], we propose a theory for the hidden order in this material in which the hidden order parameter is a composite density wave formed formed between conduction electrons and a $\Gamma_5$ 5f$^2$ doublet. In this theory, two-channel quadrupole fluctuations in the $Gamma_5$ proceed via virtual fluctuations into a 5$f^1$ Kramer's doublet with $\Gamma_7$ symmetry. Hybridization in these two channels is described by a Schwinger boson which condenses in both the hidden order and the magnetic phase. In the magnetic phase, the hybridization develops in the up or down channels, whereas in the hidden order phase, it develops with equal amplitude in both channels. Our theory can account for the development of an anomalous $\Delta \chi_{xy}$ as a consequence of the composite order. It also predicts the formation of a tiny orbital moment aligned along the xy axis in the basal plane of the crystal that should be observable in neutron scattering experiments. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L22.00012: Hybridization wave as the ``Hidden Order'' in URu$_{2}$Si$_{2}$ Jonatan Dubi, Alexander Balatsky A phenomenological model for the hidden order transition in the heavy Fermion material URu$_{2}$Si$_{2}$ is introduced. The model assumes an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band, appearing after a Fano hybridization takes place. The hybridization wave is identified as the ``Hidden Order'' order parameter. The model, simplified to one dimension, qualitatively reproduces numerous experimental results obtained from e.g. neutron scattering and scanning tunneling microscopy, and mainly the gap-like features in the density of states and the appearance of features at an incommensurate vector $Q^*\sim 0.6 \pi/a_0$. Finally, the model allows us to make various predictions which are amenable to current experiments. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L22.00013: Hexadecapolar Kondo effect in URu$_2$Si$_2$? Anna Toth, Gabriel Kotliar Motivated by recent findings on the electronic structure of URu$_2$Si$_2\,$, we derive the coupling of a localized hexadecapolar mode to itinerant fermionic quasiparticles in tetragonal crystal field, and show how it maps onto the two-channel Kondo (2CK) model. Channel symmetry is a consequence of time-reversal symmetry, and a 2CK regime can be observed if the crystal field splitting is less than the Kondo temperature. Corollary to the derivation, for an $f^2$-configuration in tetragonal environment, a relevant crystal field splitting is always present in addition to the 2CK interaction--even if the local degrees of freedom are a $\Gamma_5$ doublet. Solving the coupling by the numerical renormalization group, we are able to fit the susceptibility and the specific heat of the dilute system, Th$_{1-x}$U$_x$Ru$_2$Si$_2$, in magnetic field and place the measurements on the verge of the local moment and the 2CK regimes. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L22.00014: Electronic structure and multipolar fluctuations in URu2Si2 Hiroaki Ikeda, Ryotaro Arita, Tetsuya Takimoto The intriguing phase transition at 17.5K in URu$_2$Si$_2$ was discovered by Palstra in 1985. In spite of intensive research studies over a quarter century, the order parameter remains still unknown; so-called ``hidden order'' phase. Many recent experimental data indicate that the magnetic and electronic properties can be easy to understand from the viewpoint of the itinerant picture rather than the localized picture. Thus, to elucidate the complicated electronic structure will be our important first step to comprehend the nature. Recently, by using a state-of-the-art {\it ab initio} downfolding, we have succeeded to construct the Wannier orbitals and to obtain the tight-binding Hamiltonian in terms of these basis set. Adding on-site Coulomb interactions between f orbitals, we obtain a multi-band Anderson lattice model, including full f orbitals. We here analyze the model Hamiltonian within the random phase approximation, and investigate magnetic fluctuations and multipolar fluctuations in URu$_2$Si$_2$. From these results, we discuss possible order parameters in the ``hidden order'' phase. [Preview Abstract] |
Session L23: Focus Session: Search for New Superconductors II: Towards Theoretical Design
Sponsoring Units: DMPChair: Philip Phillips, University of Illinois
Room: D165
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L23.00001: Spectral Weight Transfer in a Multi-Orbital Mott System Wei-Cheng Lee, Philip Phillips One of the unique properties in a single band Hubbard model is the spectral weight transfer upon doping. Unlike in a Fermi liquid the redistribution of the spectral weights occurs predominantly near the chemical potential, a significant amount of spectral weights can be transferred from the high energy scales (upper Hubbard band) down to the chemical potential as a Mott insulator is doped. In this talk, we analyze the spectral weight transfer in a multi-orbital Mott system. We find that the spectral weights transferred from the high energy scales are greatly increased due to the multi-orbital structure, leading to a reduction of the critical doping level exhibiting zero thermopower. We argue that this indicates a suppression of the pseudogap phase and also predict the existence of new branches of charge 2e bosons carrying spin 1 at low energy in a multi-orbital Mott system. Relevant experimental consequences will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L23.00002: Electron-electron interaction in superconducting Lithium under pressure Amandeep Kaur, Erik Ylvisaker, Deyu Lu, Yan Li, Giulia Galli, Warren Pickett Lithium is known to exist in different phases and to uperconduct under pressure (P). We investigate ~the screened electron-electron interaction in Li as a function of P by analyzing the dielectric band structures of several phases. These band structures are obtained by iterative diagonalization\footnote{H.Wilson, F.Gygi and G.Galli, Phys. Rev. B, 78,113303 (2008); Hugh F. Wilson, Deyu Lu, Francois Gygi Phys. Rev. B. 79, 245106 (2009).} of the dielectric matrix as a function of wave vector and frequency. Even though the superconductivity in lithium is electron-phonon mediated, lithium is a good test system to study the screened e-e interaction more generally, which might be a primary mechanism for the superconductivity in high T$_{c}$ nitrides of the form MNCl (M=Ti,Zr,Hf). [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L23.00003: An investigation of non-superconducting PuPt2In7 Hahnbidt Rhee, Warren Pickett, Filip Ronning, Jian-Xin Zhu, Eric Bauer PuPt$_2$In$_7$, like the heavy-fermion 115s, is member of a family of systems that are made up of RM$_3$ (R=Ce, Pu, ...; M=In, Ga, ...) building blocks. Superconductivity is observed in many of these materials, and it is understood to arise from an unconventional pairing mechanism due to antiferromagnetic spin fluctuations. Experiments discover that PuPt$_2$In$_7$, however, is an enhanced Pauli paramagnet with a Sommerfeld coefficient of $\sim$250 mJ/mol K$^2$. Here we present a DFT (Density Functional Theory) study of its electronic structure, with direct comparisons made to superconducting PuCoGa$_5$ and PuCoIn$_5$. Fermi surfaces, orbital decomposition of density of states, and band structures reveal many similarities between the two compounds. Our goal is to understand why we observe superconductivity in one but not the other. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L23.00004: Why positive hole carriers and negatively charged planes are conducive to high temperature superconductivity Invited Speaker: The vast majority of superconducting materials have positive Hall coefficient in the normal state, indicating that hole carriers dominate the normal state transport. This was noticed even before BCS theory, and has been amply confirmed by materials found since then: the sign of the Hall coefficient is the strongest normal state predictor of superconductivity. In the superconducting state instead, superfluid carriers are always electron-like, i.e. negative, as indicated by the fact that the magnetic field generated by rotating superconductors is always parallel, never antiparallel, to the body's angular momentum (``London moment''). BCS theory ignores these facts. In contrast, the theory of hole superconductivity, developed over the past 20 years (papers listed in http://physics.ucsd.edu/$\sim $jorge/hole.html) makes charge asymmetry the centerpiece of the action. The Coulomb repulsion between holes is shown to be smaller than that between electrons, thus favoring pairing of holes, and this fundamental electron-hole asymmetry is largest in materials where the conducting structures have \textit{excess negative charge}, as is the case in the cuprates, arsenides and MgB$_{2}$. Charge asymmetry implies that superconductivity is driven by lowering of kinetic energy, associated with expansion of the carrier wavefunction and with \textit{expulsion of negative charge} from the interior to the surface of the material, where it carries the Meissner current. This results in a macroscopic electric field (pointing outward) in the interior of superconductors, and a macroscopic spin current flowing near the surface in the absence of external fields, a kind of macroscopic zero point motion of the superfluid (spin Meissner effect). London's electrodynamic equations are modified in a natural way to describe this physics. It is pointed out that a dynamical explanation of the Meissner effect \textit{requires} radial outflow of charge in the transition to superconductivity, as predicted by this theory and not predicted by BCS. The theory provides clear guidelines regarding where new higher T$_{c}$ superconductors will and will not be found. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L23.00005: The Possibility of Phonon-Mediated Superconductivity in an Iron-Based Material Sheena Shah, Elena Roxana Margine, Aleksey Kolmogorov We have identified a synthesizable candidate FeB4 material with a potential for conventional superconductivity at 15-20 K [1,2]. The strong electron-phonon coupling in the proposed material is unexpected as the recently discovered iron-based superconductors are considered to display an unconventional pairing mechanism. The new nonmagnetic ground state crystal structure has been predicted with an ab initio evolutionary search [3] and shown to be marginally stable at ambient pressures. \\[4pt] [1] A. N. Kolmogorov, S. Shah, E. R. Margine, A. F. Bialon, T. Hammerschmidt, R. Drautz, Phys. Rev. Lett. 105, 217003 (2010). \\[0pt] [2] A. F. Bialon, T. Hammerschmidt, R. Drautz, S. Shah, E. R. Margine, A. N. Kolmogorov (submitted) \\[0pt] [3] A. N. Kolmogorov, MAISE (http://maise-guide.org) [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L23.00006: A DFT (LDA+U) study of the electronic properties of layered, square-planar coordinated, copper monoxide structures Paul M. Grant It is now 25 years and two months since Georg Bednorz observed the onset of high temperature superconductivity in copper oxide perovskites, and yet today its origin remains still largely unresolved. However, it quickly became evident the phenomenon was restricted to those structures possessing a common feature -- square planar coordinated ``sheets,'' or ``layers'' of copper monoxide, and thus now thought to be essential to effect superconductivity in this family of materials. We examine the structural stability and electronic properties of these 2D approximations to the layered CuO compounds as a function of Hubbard U within the DFT (LDA+U) framework, especially for those particular values yielding metallic band formation, and their subsequent fermiology and electron/hole-phonon coupling properties. Although such particular 2D embodiments do not, as yet, exist, we consider their study via DFT as valuable \underline {proxies}\footnote{P. M. Grant, Journal of Physics: Conference Series \textbf{129} (2008) 012042.} to aid eventual understanding of that flavor of superconductivity revealed by the Bednorz-Mueller breakthrough. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L23.00007: Design Algorithms for Novel High Temperature Superconductors O. Paul Isikaku-Ironkwe, Emeka Oguzie, Uko Ofe A grand challenge in superconductivity is to develop a ``materials specific'' theory that enables us to design superconductors from the Periodic Table. Using the Periodic Table properties of electronegativity, valence electrons, formula weight and atomic number, we have been able to quantitatively describe all superconductors in terms of those parameters. We have observed specific correlations with various families of superconductors that enable us to reverse engineer those superconductors. We have developed simple equations, maps and algorithms that facilitate the design of superconductors and predict their approximate transition temperatures. Our design method does not employ density functional theory, even though DFT can be used to verify it. In this paper, we provide many examples of predicted ``materials specific'' novel high temperature superconductors that should test the authenticity of our design algorithms. We also propose a design for possible room temperature superconductivity. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L23.00008: Ionic Plasma Screening and Long-Range Electron Correlations in Quasi-One-Dimensional Conductors Yuri Gartstein, Anvar Zakhidov In quasi-one-dimensional systems with the intercalation-type doping, the dynamical response of dopant ions can substantially affect the interplay of density-wave and superconducting instabilities. We study a generic model of the system of Coulombically coupled Luttinger-liquid chains modified by the Coulomb interaction with displacements of dopant ions. Our interest is in the macroscopic, long wave-length, effects of the ionic response. This three-dimensional electron-ion model system is exactly solvable in the forward-scattering channel allowing us to find the resulting system excitations and electron correlations. For a jellium-like ion response, the effect of the bare electron-electron repulsion on the long- range correlations is essentially canceled by the ions with the effective electron-electron interactions now exhibiting regions of shorter-range repulsion and longer-range attraction. This picture is clarified and reproduced within the macroscopic dielectric function framework. If the system also features a non-polarizational interaction with another optical phonon mode, superconducting correlations are developed already due to the forward-scattering only. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L23.00009: Phase transitions in a three dimensional $U(1) \times U(1)$ lattice London superconductor: Metallic superfluid and charge-4e superconducting states Egil Herland, Egor Babaev, Asle Sudbo We consider a three-dimensional lattice $U(1) \times U(1)$ and $[U(1)]^N$ superconductors in the London limit, with individually conserved condensates. The $U(1) \times U(1)$ problem, generically, has two types of intercomponent interactions of different characters. First, the condensates are interacting via a minimal coupling to the same fluctuating gauge field. A second type of coupling is the direct dissipationless drag represented by a local intercomponent current-current coupling. We study phase transitions and two types of competing paired phases which occur: (i) a metallic superfluid phase, (ii) a composite superconducting phase where there is order in the phase sum of the order parameters which has many properties of a single-component superconductor but with a doubled value of electric charge. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L23.00010: Exact calculations of phase separation instabilities and pairing in two-dimensional Betts nanoclusters Armen Kocharian, Gayanath Fernando, Kun Fang The energy eigenvalues and eigenstates of the Hubbard model with nearest and next nearest neighbor hoppings are calculated by exact diagonalization and Lanczos (algorithm) techniques in isotropic Betts nanoclusters with the square symmetry and periodic boundary conditions. The electron pairing instabilities and quantum critical points for one hole off half filling are evaluated by monitoring the charge and spin pairing gaps and level crossings instabilities in the ground state and at finite temperatures. The calculated spin and charge energy gaps and quantum critical points in optimized 8 and 10 site Betts clusters of square symmetry pertain universal critical behavior and are fully consistent with the exact results obtained for an ``elementary'' bipartite square geometry [Kocharian et al., Phys. Rev. B 78, 075431 (2008)]. We found the strong particle-hole assymetry effect in the electron pairing instability due to the presence of the next nearest neighbor hopping term. Correlated lectrons in various contrasting bipartite and non-bipartite two- and three- dimensional cluster topologies display a number of inhomogeneous, coherent and non-coherent nanoscale phases seen by scanning tunneling microscopy in high Tc cuprates, iron pnictides, manganites, etc. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L23.00011: Electromagnetic Interactions between Electrons moving in the Layered Conductors with a Dielectric Interlayer Kenji Tanahashi Electromagnetic interactions between two electrons moving in the two layered conductors separated with a dielectric interlayer have been estimated. We assume a simple situation in which the two electrons in the layered conductors move with the constant velocity in the same direction. The electric and magnetic fields of a moving electron are derived from the scalar and the vector potentials in the non-relativistic frame. The total electromagnetic force exerted between two electrons is obtained by the Lorentz expression, and the force depends on the velocity of the moving electrons. With increasing the velocity of the electrons, the magnetic force increases and the magnetic attractive force exceeds the electric repulsive force, when $v/c\;\ge 1 \mathord{\left/ {\vphantom {1 {\sqrt {\varepsilon _r \,\mu _r } }}} \right. \kern-\nulldelimiterspace} {\sqrt {\varepsilon _r \,\mu _r } }$, where v is the velocity of the two electrons moving parallel in the same direction, c is the speed of light, and$\varepsilon _r \;$is the relative dielectric constant in the direction of the perpendicular to the plane of the layers, and$\mu _r $ is the in-plane permittivity of the conduction layers. In vacuum the magnetic interaction between moving electrons never surpasses the electric interaction. However, in the highly anisotropic structures in conductivity, the magnetic interaction between moving electrons should be taken into consideration to investigate the behavior of the electrons. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L23.00012: Electrodynamics of Nearly Ferroelectric Superconductors in the non-local Pippard limit Upali Aparajita, Joseph Birman We report the structure of the magnetic field and secular current in a Nearly Ferroelectric Superconducting (NFE-SC) thin film. It was shown that unlike in conventional superconducting films, the external radiation causes alternating pattern of current strips. The strength of the innermost current torrents is governed by the laser field intensity as well as resonance with the ferroelectric component. The latter is modeled by secular reflection and random scattering in the Pippard non-local limit. Our calculations suggest that corresponding magnetic field pattern affects vortex formation in such material. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L23.00013: Planar-coordinated nickelates, isoelectronic to overdoped cuprates: an LDA+DMFT comparison Chuck-Hou Yee, Gabriel Kotliar, Kristjan Haule We show the Ni-O planes in the bilayer and trilayer T'-type nickelates, recently synthesized by Poltavets, {\it et al.} [1], are electronically analogous to the Cu-O planes in overdoped superconducting cuprates. The density of states, Fermi surface, nickel valence and mass renormalization, computed using LDA+DMFT, are in good agreement with available experiment, and indicate that the compounds are well-described by multilayer Hubbard physics. Significant interlayer coupling generates bonding-antibonding Fermi surfaces, similar to those seen in the cuprates. We investigate the possibility that interlayer coupling can explain the presence of a phase transition with $R \log 2$ entropy in the trilayer, and the absence of such a transition in the bilayer. \\[4pt] [1] Poltavets, {\it et al.}, Phys. Rev. Lett. {\bf 104}, 206403 (2010). [Preview Abstract] |
Session L24: Focus Session: Quantum Transport Simulations and Computational Electronics -- GNRs and QDs
Sponsoring Units: DCOMPChair: Zlatan Aksamija, University of Wisconsin-Madison
Room: D167
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L24.00001: Numerical simulation of time-dependent transport in graphene Dharmendar Reddy, Priyamvada Jadaun, Leonard F. Register, Sanjay K. Banerjee We present a numerical method for modeling time-dependent quantum transport in graphene. The time-dependent Schrodinger equation is solved with a pi-orbital-based atomistic tight-binding Hamiltonian. A novel variation of an alternating-direction semi-implicit scheme is employed on the hexagonal tight-binding lattice to maintain stability and conserve probability while achieving computational efficiency. Open boundaries including source terms to allow time-dependent non-equilibrium Green's function (NEGF) calculation of graphene devices will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L24.00002: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L24.00003: Current-Voltage Characteristics of Graphane Nanoribbon Transistors Jun-Qiang Lu, Daniel Valencia Using first-principles transport calculations, we investigate current-voltage characteristics of transistors made by graphane nanoribbons (or hydrogenated graphene nanoribbons). Our results show that transistors made by graphane nanoribbons can achieve better performance than those made by graphene nanoribbons because of the intrinsic large band gap presented in graphane. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L24.00004: Switching Behavior of Carbon Chains Bridging Graphene Nanoribbons: Effects of Uniaxial Strain Brahim Akdim, Ruth Pachter Recently, several experiments [1,2] demonstrated the stability of chain-like carbon nanowires bridged between graphene nanoribbons, paving the way for potential applications in nano-devices. On the basis of density functional tight-binding calculations, we demonstrated switching for chains terminated with a five-membered ring under an applied strain, serving as a model for morphological changes in realistic materials. Electron transport calculations showed an increase of up to 100{\%} in the output current, achieved at a reverse bias-voltage of 2V and an applied strain of just 1.5{\%}. Structural analysis suggested that the switching is driven by conformational changes, in our case triggered by the formation and annihilation of a five-membered ring at the interface of the chain-graphene edge. In addition, we showed that a five-membered ring can easily be formed at the interface under a source-drain bias or through a gate voltage. This mechanism can serve as an explanation of experimentally observed conductance for the materials. \\[4pt] [1] Jin, C.; Lan, H.; Peng, L; Suenaga, K.; Iijima S. \textit{Phys. Rev. Lett.} \textbf{2009}, 102, 205501. \\[0pt] [2] Chuvilin, A.; Meyer, J. C.; Algara-Siller, G.; Kaiser, U. \textit{New J. Phys.} \textbf{2009}, 11, 083019 [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L24.00005: Giant Mechanoelectrical Switching in Ferromagnetic Graphene Nanoribbons Hong Li, Rui Qin, Jing Zhou, Qihang Liu, Zhengxiang Gao, Jing Lu, Wai-Ning Mei, R.F. Sabirianov Giant mechanoelectrical effect is observed when twisting a ferromagnetic zigzag-edged graphene nanoribbon (ZGNR) with collinear spin configuration from ab initio quantum transport calculations. The resulting switch ratio is up to 10$^{10}${\%} when the ZGNRs are overturned once and can be even enhanced to over 10$^{14}${\%} via multiply overturnings. We find such a switch equivalent to a spin valve without resort to an external magnetic field. Furthermore, consideration under Noncollinear situation is aslo in progress. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L24.00006: Transport in Carbon Nanotubes: 2LSU(2) regime reveals subtle competition between Kondo and Intermediate Valence states G. Martins, C. Busser, E. Vernek, P. Orellana, G. Lara, E. Kim, A. Feiguin, E. Anda Three different numerical techniques are used to study the two-level SU(2) regime, obtained from an SU(4) Hamiltonian by orbital mixing via coupling to the leads. SU(4) Kondo physics has been experimentally observed, and studied in detail, in Carbon Nanotube Quantum Dots. Adopting a two molecular orbital basis, the Hamiltonian is rewritten, such that one of the molecular orbitals decouples from the charge reservoir, although still interacting capacitively with the other molecular orbital. This basis transformation explains in a clear way how the charge transport in this system turns from double- to single-channel when it transitions from the SU(4) to the 2LSU2 regime. The charge occupancy of these molecular orbitals displays gate-potential-dependent occupancy oscillations that arise from a competition between the Kondo and Intermediate Valence (IV) states. The determination of whether the Kondo or the IV state is more favorable, for a specific value of gate potential, is assessed by the definition of an energy scale $T_0$, which is calculated through DMRG. We speculate that the calculation of $T_0$ may provide experimentalists with a useful tool to analyze correlated charge transport in many other systems. For that, a current work is underway to improve the numerical accuracy of its DRMG calculation and explore different definitions. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L24.00007: Scattering matrix approaches for dissipative quantum transport Invited Speaker: The Usuki method, which is closely related to both the scattering matrix approach and recursive Green's functions provides a stable numberical method for the simulation of quantum transport in semiconductors. It has major advantage over the Green's function method for self-consistent simlations in that the electron density involves integrals in the contacts where the distribution is near equilibrium, rather than throughout the active area. Various applications of this approach have been studied, and we discuss primarily ballistic transport in quantum dots and dissipative transport in gated quantum wire transistors. Dissipation is introduced via a proper self-energy determined in the mode representation, which is then transformed to the site representation used in the recursive approach. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L24.00008: Electron hopping between Wurtzite CdSe Quantum Dots Linked by Molecules Iek-Heng Chu, Marina Radulaski, Nenad Vukmirovic, Hai-Ping Cheng, Lin-Wang Wang Recent experimental results show that the transport properties of quantum dot (QD) arrays will be tremendously improved after attached by cross-linking molecules. Here, we present an \textit{ab initio} study on the electron hopping rates between wurtzite CdSe QDs connected by Sn$_{2}$S$_{6}$ molecules. The conduction band minima (CBM) transports among connected QDs are calculated. The charge patching method (CPM) is used to construct the charge density of the QDs and the connected systems. The folded spectrum method (FSM) was applied to find the band edge states and the electronic coupling between the neighboring QDs. Electron-phonon couplings are calculated to yield the reorganization energy. The electron hopping rate is then calculated by Marcus theory and its corresponding quantum treatments. Hopping rates for three different sizes of QDs, and two different types of molecular attachments are also presented here for comparison. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L24.00009: Coulomb Drag in Open Quantum Dots Canran Xu, Maxim Vavilov We investigate the Coulomb drag effect in a system of two open quantum dots, in the presence of interdot and intradot Coulomb interactions. We present an analytical expression for the drag current at the low temperature limit obtained in the random-phase approximation. We show that the non-zero current arises from the asymmetry of electronic states with respect to the Fermi level. This asymmetry originates due to fluctuations of the transmission amplitudes in the chaotic quantum dots described by a random-matrix theory, and therefore the drag current exhibits interesting sample-to-sample mesoscopic fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L24.00010: Nonequilibrium quantum many-body transport in multiple lead quantum dot devices Jong Han Recently proposed imaginary-time formalism of steady-state nonequilibrium is extended to three reservoir systems and discuss their interference effects. We first consider a quantum dot coupled to three non-interacting leads in the context of the Anderson impurity model driven by source-drain bias. We discuss the difference between the two and three reservoir systems. We then consider the system of interacting leads, used as a prototype for two-channel Kondo model in quantum dot device.\footnote{R. M. Potok et al, Nature {\bf 446},167 (2007)} We rewrite the charging interaction on the large dot via a gauge transformation to a correlated tunneling and perform quantum Monte Carlo simulation for equilibrium and nonequilibrium using the Matsubara-voltage formalism. We discuss the cross-over from local Fermi liquid to non-Fermi liquid as a function of the Coulomb parameter in the large dot in the electron self-energy and the magnetic susceptibility. We discuss the nonequilibrium spectral evolution of local Fermi liquid. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L24.00011: Continuous measurements of electron tunneling through a quantum dot by a quantum point contact Hsi-Sheng Goan The time-resolved charge detection through a quantum dot (QD) by a nearby quantum point contact (QPC) detector, each coupled to its own independent electrodes and gates, has been demonstrated. The conditional counting statistics of electron transport in this QD-OPC system has also been measured [1]. The conditional counting statistics that is the statistical current fluctuations of one system given the observation of a particular current in the other system could be substantially different from their unconditional counterparts. We provide a thorough analysis on the QD-QPC system. We use the stochastic master equation (or quantum trajectory) approach to describe the conditional dynamics of the QD under continuous measurements by a QPC. We simulate in each single experimental realization the observed QPC current which reveals the real time information of single-electron tunneling events through the QD. We then use the $n$-resolved master equation approach to calculate the conditional counting statistics through the QD (QPC) conditioned on the observed current in QPC (QD). Our investigation goes beyond the analysis presented in Ref.[1] in which they neglected, in the noise power(second cumulant) of the QPC, the QPC shot noise as compared to the telegraph noise contribution induced by the single-electron tunneling events through the QD. \\[0pt] [1] E.V.Sukhorukov et. al, Nature Physics, {\bf 3}, 243 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L24.00012: Designer switches: Effect of contact geometry on the transient current of a strongly correlated quantum dot Ali Ihsan Goker, Zhiyong Zhu, Udo Schwingenschlogl, Aurelien Manchon The time-dependent non-crossing approximation is utilized to investigate the influence of the geometry of contacts made of gold on time dependent current through a quantum dot suddenly shifted into the Kondo regime via a gate voltage. For an asymmetrically coupled system, instantaneous conductance exhibits complex fluctuations. We identify the frequencies participating in these fluctuations and they turn out to be proportional to the separation between the sharp features in the density of states and the Fermi level. Increasing ambient temperature or bias quenches the amplitude of these fluctuations. This suggests that the interference between the emerging Kondo resonance and the van Hove singularities in the density of states is the underlying microscopic mechanism for these fluctuations. Based on these observations, we predict that using different electrode geometries would give rise to drastically different transient currents which can be accessed with state-of-the-art ultrafast pump-probe techniques. [Preview Abstract] |
Session L25: Superconductivity: Vortex Phenomena II
Sponsoring Units: DCMPChair: Milind Kunchur, University of South Carolina
Room: D166
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L25.00001: Vortex Pinning in MoGe Thin Films Containing Periodic Hole Arrays Michael Latimer, Zhili Xiao, Wai-Kwong Kwok, Alexandra Joshi-Imre, Castro Abughayada Resistivity measurements on MoGe thin films containing periodic hole arrays were carried out to study the effects of the lattice symmetry and the size of the pinning centers. Thin films of MoGe were prepared with holes drilled using focused-ion-beam milling to create pinning sites for the vortex lattice. We investigate periodic arrays with hexagonal, square and triangular geometry to determine the change in transport properties with varying magnetic fields. Hole sizes from 50nm to 100nm were tested determine the effects of single and multiple vortices in a single pinning site. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L25.00002: Flux dynamics across MoGe bridges in the parallel field orientation Milind Kunchur, Manlai Liang, Alexander Gurevich We have investigated flux motion in amorphous molybdenum-germanium (MoGe) film bridges with the magnetic field B parallel to the film plane but perpendicular to the current direction. In a temperature range close to the transition temperature Tc we observe an exponential dependence of the Ohmic resistance R on B at low values of the current I, and a nonlinear R(I) at higher currents. In this regime, the diameter of the vortex is approximately equal to the film thickness and the applied magnetic field is comparable to the lower critical field for the parallel field orientation. Dissipation presumably occurs by thermally activated flux jumps over the thickness of the film involving nucleation and expansion of vortex kinks. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L25.00003: Vortex instability in molybdenum-germanium superconducting film Manlai Liang, Milind Kunchur We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium (MoGe) superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov picture with a quasiparticle energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L25.00004: Small Angle Neutron Scattering Studies of the Vortex Lattice in CeCoIn$_5$ with $H \perp c$ M.R. Eskildsen, P. Das, A.T. Holmes, E.M. Forgan, A.D. Bianchi, J.S. White, S. Gerber, M. Kenzelmann, J.L. Gavilano, M. Zolliker, C. Wang, E.D. Bauer, J.L. Sarrao, C. Petrovic We report on small-angle neutron scattering measurements on the vortex lattice (VL) in the mixed state of CeCoIn$_5$ with the magnetic field ($H$) along $[100]$ and $[110]$. For both field orientations a distorted hexagonal VL is observed, reflecting the penetration depth anisotropy of the screening current plane. With $H \parallel [100]$ the VL is oriented with Bragg reflections along the [001]-axis at all fields. For $H \parallel [110]$ the same VL orientation is observed at low fields, followed by a $90^{\circ}$ first-order reorientation transition as $H$ is increased. We attribute this behavior to Fermi surface anisotropy coupled with non-local effects. For $H \parallel [100]$ we obtain the field dependence of the form factor ($|F|^2$) both within (50~mK) and outside (350~mK) the magnetic $Q$-phase. At both temperatures $|F|^2$ varies with $H$ in a manner similar to $H \parallel [001]$ [J.S. White {\em et al.}. New J. Phys. {\bf 12}, 023026 (2010)], due to the competition between Pauli paramagnetism and the antiparallel spin alignment of $d$-wave pairing giving rise to ``magnetized" VL cores. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L25.00005: Low temperature enhancement of the remanent magnetization in CeCoIn$_{5}$ C.F. Miclea, M. Nicklas, A.C. Mota, F. Steglich, M.M. Altarawneh, N. Harrison, I. Vekhter, J.D. Thompson, R. Movshovich We investigated the vortex dynamics together with RF penetration depth measurements in the heavy fermions compound CeCoIn$_{5}$ down to~50~mK. No strong pining is observed and the relaxation curves are logarithmic as expected from Kim-Anderson theory. The temperature dependence of the relaxation rate, $S$, with a small but finite residual value indicate that quantum tunneling plays a role in the vortex creep only at very low temperatures. Remarkably, a new phase transition marked by a strong increase in the remnant magnetization, M$_{rem}$ is observed around $T$~=~0.3~K in very low magnetic fields. $M_{rem}$ increases roughly by a factor of two at 50 mK and we discuss if this can be caused solely by the change in the vortex lattice symmetry or underling magnetism has to play a role. Moreover, this anomaly is corroborated by the RF measurements at very low fields. We extended the vortex dynamics investigation to Pb irradiated CeCoIn$_{5}$. While the defects created by irradiation have a clear effect on the relaxation rates the enhancement of $M_{rem}$ still takes place at the same temperature. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L25.00006: Vortex Core Size Measurements in YNi$_2$B$_2$C and TmNi$_2$B$_2$C P. Das, C. Rastovski, K. Schlesinger, M.R. Eskildsen, J.M. Densmore, S.L. Bud'ko, P.C. Canfield The vortex core size in type-II superconductors is typically determined from measurements of a related quantity combined with a theoretical model, with the best known example being the upper critical field and the GL-result: $\xi = \sqrt{\phi_0/2\pi H_{c2}}$. However, for many non-conventional superconductors such an approach is problematic, as for example in the case of TmNi$_2$B$_2$C and CeCoIn$_5$ where $H_{c2}$ is suppressed by coexistence with magnetism. In such instances a direct, model independent determination of the vortex core is desirable, and can be obtained by small-angle neutron scattering (SANS) measurements of the vortex lattice (VL) if a sufficient number of reflections are recorded [J. M. Densmore {\em et al.}, Phys. Rev. B {\bf 79}, 174522 (2009)]. Here we report on VL SANS studies on two members of the borocarbide superconductors, YNi$_2$B$_2$C and TmNi$_2$B$_2$C. Non-magnetic Y1221 measurements at $0.2$ and $0.5$~T show clear evidence of a vortex squeezing effect. In magnetic Tm1221 the vortex core size was found to be $\xi = 10.8$~nm, roughly a factor of two smaller than the value estimated by the measured $H_{c2}$ ($21$~nm). Supported by NSF award no. DMR-0804887 (Notre Dame) and DOE BES contract No. DE-AC02-07CH11358 (Ames). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L25.00007: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L25.00008: Revealing the effect of edge contamination on vortex matter structure in a Nb single crystal with neutron diffraction techniques Helen Hanson, Xi Wang, Michael Luk, Jing Shi, Xinsheng Ling, Brian Maranville, Charles Majkrzak The vortex matter of type II superconductors provides a model system to study the effect of quenched random disorder on an elastic lattice, particularly in the framework of Bragg glass theory. Neutron scattering techniques are used to examine the structure of the vortex matter and to quantify the phase diagram. After measuring various thermal-magnetic histories, our data provided evidence for the edge contamination model in a Nb single crystal. Since surface oxidation is known to suppress the Bean-Livingston Surface barrier and the inhomogeneous distribution of surface impurities in Nb, we oxidize our sample surface and repeat our measurements. By comparing the data, we are able isolate the dynamic impact of the edge disorder from the static influence of bulk pinning. We discuss the various experimental obstacles in measuring the predicted Bragg glass state. We also report on Reverse Monte Carlo Refinement simulations modeling possible structures of our vortex matter. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L25.00009: Exploring the vertical variation of the flux line lattice angular orientation using a novel neutron diffraction technique Xi Wang, Helen Hanson, Jing Shi, Xinsheng Ling, Brian Maranville, Charles Majkrzak, Mark Laver, Uwe Keiderling, Margarita Russina We use a slicing neutron diffraction technique, employing neutron reflectometry collimation, to study the orientational order of the flux line lattice in a Nb single crystal. We are able to reveal the spatial variation of the different orientation distributions along the length of the flux lines. The results are strongly dependent on the magnetic history of the vortex matter, suggesting various interactions with the disorder in the system. After thermally annealing the different initial states, memory of the growth procedure is removed from the data and a possible ground state is reached. In this final state, the novel vertical slicing reveals the persistence of a domain splitting. We suggest that this domain splitting is due to the quenched disorder in the underlying Nb atomic lattice. We believe that this new insight will be instrumental in growing a true Bragg glass, the theoretically predicted ground state with topological order. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L25.00010: Vortex activation energy in high-Tc superconductors from transport measurements: new systematics Yuri L. Zuev, John Sinclair, James R. Thompson, Sung Hun Wee, Claudia Cantoni, David K. Christen From electrical transport measurements on thin films of YBCO with and without nano-precipitate ``columnar defects,'' we extract information about the activation energy for vortex creep. We also obtain new scaling between temperature, critical current density $J_C$, and the power law index $n$ describing the voltage-current relation $V\propto I^n$. This scaling occurs in the range of field and temperature where $J_C$ decays as a power-law of magnetic field. In this regime the $n$-value can unexpectedly increase as applied field increases. We will discuss implications of these observations and compare systems with and without such scaling. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L25.00011: Comparison of free flux flow in two single crystals of V$_{3}$Si with slightly different pinning strengths Ozarfar Gafarov, Albert A. Gapud, Sunhee Moraes, James R. Thompson, David K. Christen, Arneil P. Reyes Results of recent measurements on two very clean, single-crystal samples of the A15 superconductor V$_{3}$Si are presented. Magnetization and transport data confirm the ``clean'' quality of both samples, as manifested by: (i) high residual resistivity ratio, (ii) low critical current densities, and (iii) a ``peak'' effect in the field dependence of critical current. The (H,T) phase line for this peak effect is shifted in the slightly ``dirtier'' sample, which also has higher critical current density J$_{c}$(H). High-current Lorentz forces are applied on mixed-state vortices in order to induce the highly ordered free flux flow (FFF) phase, using the same methods as in previous work. A traditional model by Bardeen and Stephen (BS) predicts a simple field dependence of flux flow resistivity $\rho _{f}$(H), presuming a field-independent flux core size. A model by Kogan and Zelezhina (KZ) takes core size into account, and predicts a deviation from BS. In this study, $\rho _{f}$(H) is confirmed to be consistent with predictions of KZ, as will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L25.00012: Magnetic-field-induced stripe order and a 2D vortex glass phase in La$_{1.905}$Ba$_{0.095}$CuO$_4$ John Tranquada, Jinsheng Wen, Qing Jie, Su Jung Han, Qiang Li, Markus Huecker, Zhijun Xu, Liyuan Zhang, Genda Gu, M.V. Zimmermann, D.K. Singh We have measured the resistivity parallel and perpendicular to the CuO$_2$ planes in La$_{1.905}$Ba$_{0.095}$CuO$_4$ ($T_c = 32$ K) as a function of perpendicular magnetic field. We have discovered a significant regime of field and temperature where the perpendicular resistivity is finite (and large) but the parallel resistivity is zero. This regime appears to correspond to a quasi-two-dimensional vortex glass phase, a state that theory predicts cannot exist at finite temperature. It seems to be stabilized by field-induced charge and spin stripe order, which we have detected with x-ray and neutron diffraction, respectively. [Preview Abstract] |
Session L26: Focus Session: Iron Based Superconductors -- ARPES
Sponsoring Units: DMP DCOMPChair: Gabriel Kotliar, Rutgers University
Room: D162/164
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L26.00001: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L26.00002: Angle-resolved photoemission spectroscopy study of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ Tian Qian, Nan Xu, Pierre Richard, Yingbo Shi, Guanghan Cao, Zu'an Xu, Hong Ding Ru-doped BaFe$_{2}$As$_{2}$ compounds were discovered recently to show superconductivity at a relatively wide doping range. We will present angle-resolved photoemission spectroscopy results of electronic structure and Fermi surface of Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$, and discuss implications to its superconductivity. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L26.00003: Chiral orbital angular momentum and warping effect in topological insulator Bi2Te3 Won Sig Jung, Y.K. Kwan, B.Y. Kim, J.Y. Kim, B.K. Cho, C. Kim The spin of a topologically protected metallic surface state on topological insulators has a chiral state. The Spin chiral state is aligned with orbital angular momentum of the electron in the surface states. We observe orbital angular momentum direction by using angle resolved photoemission (ARPES) with circularly polarized lights. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L26.00004: Pseudogap in non-superconducting pnictides, CaFe$_2$As$_2$ and EuFe$_2$As$_2$ K. Maiti, G. Adhikary, N. Sahadev, D.N. Biswas, R. Bindu, N. Kumar, A. Thamizhavel, S.K. Dhar Superconductivity in Fe-pnictides are studied extensively recently as they provide a non-cuprate domain to study unconventional superconductivity via doping induced suppression of magnetism. In order to study the role of magnetic interactions in the electronic structure and its implication in superconductivity, we probed the electronic structure of the parent compounds CaFe$_2$As$_2$ and EuFe$_2$As$_2$ using high resolution photoemission spectroscopy. Single crystalline samples were prepared by flux method. Photoemission measurements were carried out using a Gammadata Scienta analyzer R4000 and monochromatic photon sources. The high resolution spectra exhibit signature of a pseudogap above the spin density wave (SDW) transition temperature in both CaFe$_2$As$_2$ and EuFe$_2$As$_2$. The intensity at the Fermi level show a sudden decrease across the SDW transition indicating more prominent pseudogap. An additional gap opens up in EuFe$_2$As$_2$ across the antiferromagnetic transition temperature as expected. Interestingly, CaFe$_2$As$_2$ also exhibit signature of another gap opening at low temperatures although no phase transitions observed in this temperature range. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L26.00005: Unconventional superconducting gap in NaFe$_{0.95}$Co$_{0.05}$As observed by ARPES Pierre Richard, Z.-H. Liu, K. Nakayama, G.-F. Chen, S. Dong, J.-B. He, D.-M. Wang, T.-L. Xia, K. Umezawa, T. Kawahara, S. Souma, T. Sato, T. Takahashi, T. Qian, Y. Huang, N. Xu, Y. Shi, H. Ding, S.-C. Wang The size, the symmetry and the temperature evolution of the superconducting (SC) gap in a given material are directly related to the SC pairing mechanism. The momentum-resolution capability of angle-resolved photoemission spectroscopy (ARPES) allows precise determination of these key parameters, even for complex multi-band systems such as the iron-based superconductors. We performed an ARPES study of NaFe$_{0.95}$Co$_{0.05}$As. The fermiology of this electron-doped 111-pnictide is similar to that of other pnictides. Similarly, the measured SC gaps are nearly isotropic and their size indicates that the system is in the strong coupling regime. Surprisingly, the SC gaps show little change upon increasing temperature towards $T_{c}$, while coherence vanishes. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L26.00006: Studies on the orbital characters and quasi-particle dynamics of LiFeAs Yeongkwan Kim, B.Y. Kim, Chul Kim, D.J. Song, W.S. Kyung, C. Kim, B.S. Lee, K.H. Kim Since the discovery, iron-based superconductors have been intensively and extensively studied by using various techniques including angle resolved photoelectron spectroscopy (ARPES). So far, most of ARPES studies have been performed on 122-compounds and 1111-compounds. However, 122-phase materials do not have neutral cleavage surfaces and have 3-dimensional band structures. These traits of 122- and 1111-phase make the spectral shape generally broad and do not allow investigation of the intrinsic electronic structures in detail. In that respect, LiFeAs is an ideal material with neutral cleavage surfaces and quasi-2 dimensional band structures. In this presentation, our recent ARPES work on the electronic structure of LiFeAs will be presented. We investigated the orbital character of each band by ARPES with various polarizations of the photon. Since the main valence band of LiFeAs comes from iron d-orbitals, pin-pointing the characters of bands should be an important starting point. In addition, we analyzed details of the spectral function in regard to the quasi-particle dynamics.. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L26.00007: ARPES studies on the pairing mechanism of iron-based superconductors Invited Speaker: Angle-resolved photoemission spectroscopy (ARPES) has been used extensively in studying electronic structure and superconducting gap of the iron-based high- temperature superconductors (pnictides). In this talk, I will present our recent ARPES results on these pnictide materials, mainly focus on high-resolution measurements of the superconducting gap function of many different pnictide superconductors. Our results strongly suggest that the pairing mechanism of the pnictides is likely to be driven by short-range antiferromagnetic fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L26.00008: Nodeless superconductivity in the stoichiometric superconductor LiFeAs Hyunsoo Kim, Makariy A. Tanatar, Ruslan Prozorov, Yoo Jang Song, Yong Seung Kwon The in- and out-of-plane London penetration depths were measured in single crystals of the intrinsic LiFeAs superconductor using a tunnel diode resonator (TDR) down to $0.03 T_c$. This compound appears to be in the clean limit with a residual resistivity of $\rho_0 \approx 5~\mu\Omega\cdot$cm and $RRR = 65$; it can be placed at a slightly overdoped value when compared to the charge-doped pnictides. The low-temperature region of the penetration depth, which is sensitive to the superconducting gap symmetry, is exponentially flat implying a nodeless gap. The superfluid density is well described by the self-consistent two-gap $\gamma$-model, where the larger gap is $\Delta_1/T_c\sim2$ and the smaller gap is $\Delta_2/T_c\sim1$. Together with the previous data, our results support the $s_\pm$ symmetry that evolves from nodeless to a nodal gap structure upon departure from optimal doping in Fe-based superconductors. We also conclude that pairbreaking scattering plays an important role in the deviations of the low-temperature behavior from exponential in $\lambda(T)$ of Fe-based compounds. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L26.00009: Superfluid Density in the 111 Fe Pnictide Superconductors C. J. Arguello, T. Goko, J.P. Carlo, Y.J. Uemura, A.A. Aczel, T.J. Williams, G.M. Luke, C.Q. Jin We performed muon spin relaxation studies in two kinds of '111' iron pnictides, Li$_{1.1}$FeAs and Li$_{0.9}$FeP. The zero field spectra of the latter show a fast relaxation in a small volume fraction (approximately $13\%$) due probably to magnetism. In the case of the arsenide, the zero field spectra show a completely paramagnetic state. Below Tc, an applied transverse field allowed us to measure the superfluid density (via relaxation rate $\sigma$) for both compounds. We found that both of them have very high superfluid density and low Tc: $\sigma(T=2K)\simeq1.8 \mu s^{-1}$ with Tc$\simeq$18K for Li$_ {1.1}$FeAs, and $\sigma(T=2K)\simeq2.0 \mu s^{-1}$ with Tc$\simeq$4K for Li$_{0.9}$FeP. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L26.00010: Spin lattice relaxation rate measurements in Ba$_{0.69}$K$_{0.31}$Fe$_{2}$As$_{2}$ by nuclear magnetic resonance Sangwon Oh, Andrew Mounce, William Halperin, Chenglin Zhang, Pengcheng Dai, Arneil Reyes, Philil Kuhns Magnetic impurities have been a problem with NMR measurements of single crystals in the K doped Ba-122 system because of extremely wide linewidth that can be more than 1MHz at low temperature [1]. We have a high quality single crystal of Ba$_{0.69}$K$_{0.31}$Fe$_2$As$_2$ ($T_c$ = 34K) for which the NMR linewidth does not significantly increase at low temperatures and at very large external magnetic fields. In this sample we measure the spin-lattice relaxation rate, $1/T_1$, from 300 K to 4 K at various magnetic fields 6.4 T, 13 T, and 16 T. The rapid increase of $1/T_1T$ down to $T_c$ on cooling can be attributed to spin fluctuations above $T_c$. In the superconducting state, $1/T_1$ has a kink around 20 K, and below this temperature in a field of 13 T it exhibits a power law dependence, $\backslash$varpropto T$^3$. This behavior can be explained by an impurity effect in a superconductor with extended s-wave symmetry [2]. \\[4pt] [1] S. Mukhopadhyay {\it et al.} New J. Phys. {\bf 11}, 055002 (2009)\\[0pt] [2] Y. Bang {\it et al.} Phys. Rev. B {\bf 79}, 054529 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L26.00011: Frustrated proximity effects between s and s$_{\pm }$ superconductors Valentin Stanev, Alexei E. Koshelev The nature of the superconducting order parameter (OP) in iron pnictides and chalcogenides is a hotly debated issue. It was theoretically proposed that the OP has opposite signs on the hole and the electron bands, i.e., it belongs to the unconventional class of s$_{\pm }$ (or extended s)-wave. There are, however, very few experiments that can directly distinguish this state from the ordinary s-wave OP. One way to address this problem is to study the proximity effects in a sandwich composed of conventional and iron pnictide superconductors (SC). If the pnictides indeed have the s$_{\pm }$ OP this system is intrinsically frustrated. In the case of strong frustration, a time-reversal symmetry-breaking (TRSB) SC state emerges, in which the OP phases in different bands are tilted at an angle, different from $\pi $, and controlled by the coupling strength. Observation of such state in the iron-based SC materials would give definite evidence for the s$_{\pm }$ OP. We present a microscopic, fully self-consistent approach to this problem, based on Usadel equations. We have studied the conditions for existence of the TRSB state and its experimental signatures. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L26.00012: Doping dependence of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin films by the THz conductivity measurement D. Nakamura, F. Nabeshima, Y. Imai, A. Maeda, T. Katase, H. Hiramatsu, H. Hosono We investigated the THz conductivity for thin films of Fe-based superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_{2}$ with different Co concentrations. For the optimally doped sample, we found a structure corresponding to superconductivity gap, 2 $\Delta$, whose magnitude is 2.8 meV at lowtemperatures, leading to 2 $\Delta/k_BT_c=$ 4.1[1]. This value is in good agreement with the smaller gap found in an ARPES measurement[2]. For the underdoped sample in which the coexistence of antiferromagnetic ordering with superconductivity was observed, we found the strong suppression of the carrier lifetime around the antiferromagnetic phase transition temperature ($T \sim$ 40 K). However, the real part of the complex conductivity did not be clearly suppressed in this temparature regigon. This behavior may be related to the responce of carriers at the Dirac cone, which observed in BaFe$_2$As$_2$[3]. Details will be discussed in the presentation.\\[4pt] [1] D. Nakamura {\it et al.}, arXiv: 0912.4351.\\[0pt] [2] K. Terashima {\it et al.}, PNAS 106 (2009) 7330.\\[0pt] [3] P. Richard {\it et al.}, Phys. Rev. Lett. 104 (2010) 137001. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L26.00013: Pair breaking in iron-based superconductors Kevin Kirshenbaum, Shanta Saha, Tyler Drye, Steven Ziemak, Johnpierre Paglione The relative ease of crystal growth combined with the range of elements available for chemical substitution, especially on the transition metal site, has allowed for numerous studies of different iron-based superconductors. There are, however, remaining questions about the pairing symmetry in this system. We present transport scattering rate data for optimally-doped single crystals from several superconducting 122 materials and discuss the relationship between superconducting transition temperature and transport scattering rate in the context of pair breaking. [Preview Abstract] |
Session L28: Focus Session: Computational Materials Design - Property Optimization
Sponsoring Units: DCOMP DMPChair: Stefano Curtarolo, Duke University
Room: C156
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L28.00001: Computational Design of Microstructures Invited Speaker: Many important engineering materials are designed by controlling their phase transformations and microstructure evolution. Examples include the improvement of mechanical properties through solid state precipitation reactions in alloys such as Ni-based superalloys and age-hardened Al-alloys, the useful dielectric properties and electro-mechanical coupling effects by manipulating the phase transitions in ferroelectric crystals, the memory effect of shape-memory alloys by utilizing martensitic transformations.~ In this presentation, recent effort on integrating the phase-field approach with other computational methods such as first-principles calculations and CALPHAD will be discussed. A number of examples of coupling phase-field simulations and experimental measurements will be presented. It will be demonstrated that one can use the phase-field method to not only help interpreting experimental observations but also provide guidance to achieve desirable transition temperatures and specific domain/microstructure structures. The possibility to directly obtain the effective responses of a microstructure under an applied field from phase-field simulations, and thus the evolution of effective mechanical and transport properties will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L28.00002: Phase-field model and its numerical solution for coring and microstructure evolution studies in alloys Patrice E. A. Turchi, Jean-Luc Fattebert, Milo R. Dorr, Michael E. Wickett, James F. Belak We describe an algorithm for the numerical solution of a phase-field model (PFM) of microstructure evolution in alloys using physical parameters from thermodynamic (CALPHAD) and kinetic databases. The coupled system of PFM equations includes a local order parameter, a quaternion representation of local crystal orientation and a species composition parameter. Time evolution of microstructures and alloy composition is obtained using an implicit time integration of the system. Physical parameters in databases can be obtained either through experiment or first-principles calculations. Application to coring studies and microstructure evolution of Au-Ni will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L28.00003: High Temperature Thermal Conductivity from First Principles Christian Carbogno, Ramamurthy Ramprasad, Matthias Scheffler In spite of significant research efforts, little is yet known about the atomistic details and mechanisms that underlie peculiarly low (or high) thermal conductivities, especially at elevated pressures and temperatures. Under such extreme conditions, systematic experimental measurements are hard to perform; conventional theoretical approaches typically fail to capture significant physical aspects of the problem, since these methods are either inherently limited to (a) low temperatures and/or (b) to perfect crystals. A recently developed {\it ab initio} simulation strategy~[1] allows to overcome the latter limitation, but the assessment of the high temperature regime remains an unsolved challenge. Within this work, we present efficient strategies to overcome this serious restriction and show their applicability for zirconia based ceramics - a material typically used in high temperature applications, for instance in thermal barrier coatings~[2].\\[0pt] [1] T. M. Gibbons, and S. K. Estreicher, {\it Phys. Rev. Lett.} {\bf 102}, 255502 (2009).\\[0pt] [2] D. R. Clarke, and C. G. Levi, {\it Annu. Rev. Mat. Res.} {\bf 33}, 383 (2003). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L28.00004: First Principles Studies of the Thermoelectric Figure of Merit of Zintl Compounds Ca$_{14}$AlSb$_{11-x}$As$_{x}$ Trinh Vo, Paul von Allmen, Jean-Pierre Fleurial We present predictions for the thermoelectric Figure of merit (ZT) of zintl compounds Ca$_{14}$AlSb$_{1-x}$As$_{x}$ obtained from Density Functional Theory calculations. The Seebeck coefficient, S, was obtained using the Boltzmann transport equation in the relaxation time approximation and first principles electronic structure calculations. We found that the Seebeck coefficient changes dramatically when one or more Sb atoms in the zintl compound Ca$_{14}$AlSb$_{11}$ are replaced with one or more As atoms, and that the difference in S between the original Ca$_{14}$AlSb$_{11}$ and the substituted one, Ca$_{14}$AlSb$_{11-x}$As$_{x}$, depends strongly on the positions of substituting As atoms. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L28.00005: First-principles model of absolute band shifts induced by (001) biaxial strain in group IIIA-VA semiconductors Eugene Kadantsev, Pawel Hawrylak A new model for the evolution of conduction and valence bands of IIIA-VA (InAs, GaAs, InP) semiconductors under (001) biaxial strain is developed. The model is based on ab initio calculations which take into account finite strain dependent relaxation of the reference levels. It is shown that in type I heterostructures subjected to (001) compressive biaxial strain, the confinement of holes can be reduced as compared to some existing models of biaxial strain. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L28.00006: Possible calcium centers for hydrogen storage applications: An accurate many-body study by AFQMC calculations with large basis sets Wirawan Purwanto, Henry Krakauer, Shiwei Zhang, Yudistira Virgus Weak H$_2$ physisorption energies present a significant challenge to first-principle theoretical modeling and prediction of materials for H storage. There has been controversy regarding the accuracy of DFT on systems involving Ca cations. We use the auxiliary-field quantum Monte Carlo (AFQMC) method\footnote{ S.~Zhang and H.~Krakauer, Phys.~Rev.~Lett.~\textbf{90}, 136401 (2003); W.~A.~Al-Saidi, S.~Zhang and H.~Krakauer, J.~Chem.~Phys.~\textbf{124}, 224101 (2006). } to accurately predict the binding energy of Ca$^+$\,-\,4{H}$_2$. AFQMC scales as $N_{\textrm{\small{basis}}}^3$ and has demonstrated accuracy similar to or better than the gold-standard coupled cluster CCSD(T) method. We apply a modified Cholesky decomposition to achieve efficient Hubbard-Stratonovich transformation in AFQMC at large basis sizes. We employ the largest correlation consistent basis sets available, up to Ca/cc-pCV5Z, to extrapolate to the complete basis limit. The calculated potential energy curve exhibits binding with a double-well structure. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L28.00007: Theoretical modification of $WO_3$ for water splitting Predrag Lazic, Maria K. Chan, Rickard Armiento, Yabi Wu, Gerbrand Ceder Using the sun's energy to produce hydrogen from water through photocatalytic process has been a dream since its first demonstration by Fujishima and Honda 40 years ago. Since then significant effort has been made to find a suitable material for this purpose but so far efficiency of the available materials is too low to be commercially interesting. However there are some promising candidates that have some very desirable properties for solar water splitting and their other properties are believed to be improvable by some changes in the material. One of such candidates is $WO_3$ which shows a very good light absorption and very high stability in aqueous environment. Unfortunately it also has a position of conduction band minimum slightly too low to support $H^{+}/H_2$ reaction of hydrogen evolution and also has a relatively large gap which prevents it from using a large part of solar spectrum and thus yielding a low efficiency for water splitting. We have tried to remedy those two problems by substitutions and codoping in the pure $WO_3$ material within the density functional theory. For some of the modifications we see improved material properties. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L28.00008: Ab initio investigations of complex oxides Altynbek Murat, Julia E. Medvedeva We employ {\it ab-initio} density functional approach to investigate the structural, optical and electronic properties of twelve complex oxides with layered structure RAMO$_4$, R=In or Sc, A=Al, Ga, M=Ca, Cd, Mg, and/or Zn. We find that presence of the light metal (e.g., Al, Ca, Mg and Sc) oxides significantly affects the optical band gap which varies from 0.64 eV (InGaCdO$_4$) to 4.35 eV (ScAlMgO$_4$). At the same time, the electron effective mass remains nearly isotropic in all oxides, and both structurally and chemically distinct layers are expected to participate in charge transport once the materials are degenerately doped. Further, for a comparative systematic investigation of carrier generation mechanisms in complex oxides, we calculated the electronic properties of fluorine doped (F$_O$) and oxygen-reduced RAMO$_4$ materials as well as their single-cation constituents in various phases. We determine most preferable spatial distribution of the F impurity and the oxygen defect in the layered structure of each material and find that the dopant/vacancy site locations correlate with the formation energy of the single-cation oxides. The results allow us to draw conclusions on the role played by each constituent oxide and to predict how the properties of multicomponent materials can be controlled via chemical composition, crystal structure and carrier generation. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L28.00009: Tuning of Metal-Metal Bonding by Counterion Size in Hypothetical AETiO2 Compounds Xiaodong Wen, Roald Hoffmann The structures and electronic properties of a number of real and hypothetical ABX2 compounds sharing (or evolving from) a single P4/mmm structural type are examined utilizing first principle calculations. These include the known CaCuO2 and SrFeO2 phases. A number of variations of this P4/mmm ABX2 framework, some obvious, some exotic, all with a chemical motivation, were investigated: A=alkali metal, alkaline earth metal or La, B= Ti, Fe, Cu or Pt, and X=C, O, S, C2, H2 or F. Careful attention was given to the d-orbital splitting patterns and magnetic states (ferromagnetic or antiferromagnetic) of these compounds, as well as their stability gauged by phonon dispersions and energetics. The most interesting as yet unmade compounds that emerged was an AETiO2 (AE = alkaline earth metal, Be, Mg, Ca, Sr and Ba) series, with Ti-Ti bonding, part $\sigma $, part $\pi $, tuned by the AE2+ cation size. The Ti-Ti bonding in 3D AETiO2 structures has a unique electronic feature of 1D metal chain. These AETiO2 (M=Ca, Sr and Ba) structures are calculated to be thermodynamically and dynamically stable. Experimentally, the high temperature method fails so far. Perhaps a low temperature method offer a better pathway to synthesize the AETiO2 (M=Ca, Sr and Ba) structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L28.00010: Rational Band Structure Engineering of TiO$_{2}$ for Photoelectrochemical Water Splitting Su-Huai Wei, Wan-Jian Yin, Yanfa Yan The search for new semiconducting materials or the engineering of existing semiconductors for commercially viable photoelectrochemical (PEC) water splitting has been extremely challenging. Meeting that challenge requires the discovery of a semiconductor with several tightly coupled material property criteria such as appropriate band gap (1.6 -- 2.2 eV), efficient visible light absorption, high carrier mobility, and correct band edge positions that straddle the water redox potentials. However, previous searches/modifications of semiconducting materials for PEC water splitting application have often focused on a particular individual criterion such as band gap, neglecting the possible detrimental consequence to other important criteria. In this talk, general strategies for the rational design of semiconductors such as TiO$_{2}$ to simultaneously meet all of the requirements for a high efficiency solar-driven PEC water splitting device are discussed. Density-functional theory calculations reveal that with appropriate donor-acceptor co-incorporation, heavily doped anatase TiO$_{2}$ hold great potential to satisfy all of the criteria for a viable PEC device. Other approaches to modify the band structure of TiO$_{2}$, such as the application of strain, will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L28.00011: Structural and Electronic Properties of LaTiO$_2$N with O/N Disorder Wei Kang, Mark S. Hybertsen LaTiO$_2$N is an attractive candidate photo-catalyst for water-splitting, showing strong absorption in the visible range with an optical gap about 2.1 eV and catalytic activity for hydrogen and oxygen evolution in the presence of auxiliary co-catalysts. It is also a good prototype suitable for theoretical study. It has a small unit cell while exhibiting several key characteristics found in the more complex oxides and oxynitrides synthesized in the search for improved photo-catalysts. This includes the reduced band gap and the disorder in one of the components, the O/N anion sublattice. We study the structural properties using a first-principles cluster expansion method. Our results reveal that at the temperatures characteristic of synthesis and annealing conditions, the occupation of O/N in LaTiO$_2$N is intrinsically disordered. However the structure retains residual long-range order, in agreement with anion site occupancies measured in powder neutron diffraction. Short-range order in the O/N occupation is also observed. We use many-body perturbation theory to study the electronic and optical properties for some low-energy structures. The fundamental gap is found about 0.5 eV lower than the apparent absorption edge observed in experiments. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L28.00012: The Se effect on the oxygen reduction reaction on the Se/Ru electro-catalysts. Insight from first principles. Sergey Stolbov Rational search for new efficient low-cost electrocatalysts for oxygen reduction reaction (ORR) on the hydrogen fuel cell cathodes focuses on varying the material composition to modify the local densities of electronic states (LDOS) of the surface atoms, in order to tune the surface-adsorbate electronic state hybridization and hence binding energies of the ORR intermediates. My calculation results for the Se/Ru electrocatalysts suggest an alternative way of tuning the binding energies. The Se atoms deposited on the Ru surface are found not to change Ru LDOS noticeably, however, Se atoms are negatively charged due to ionic Se-Ru bonding. As a result, they repeal electrostatically the adsorbed negatively charged O and OH intermediates, and this way reduce their binding energies. Since for the Ru case, reduction of the O and OH binding energies makes ORR energetically favorable, Se deposition dramatically improve the ORR rate on Ru. The ORR rate can thus be enhanced by changing coverage of the deposited halchogen atoms or by tuning the charge transfer to those by modifying the substrate composition. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L28.00013: First principles studies of the oxygen reduction reaction on Se-Ru nanostructures Sebastian Zuluaga, Sergey Stolbov Experiments show an enhanced rate of the oxygen reduction reaction (ORR) on Se-Ru nanostructures (NS) in hydrogen fuel cell cathodes. We use first principles methods to study Ru and Se-Ru NS of approximate 1.2 nm size and shine some light on how the Se affects the O and OH adsorption, which is the bottle neck of the power delivered by the fuel cell. Experiments shows that the Se-Ru NS have a Ru core but is not clear how the Se is distributed on the surface. Our calculation shows that the Se atom adsorbs on the Ru surface with a binding energies in the range 5.7 to 7.1 eV with electronic charge transfer from the Ru atoms. Due to repulsion between negatively charged Se atoms, they tend to spread uniformly over the the Ru NS rather than form islands on its surface. We have also found that, in contrast to the flat Ru surface, the Se bond to the low coordinated Ru atoms have significant covalent component. Our calculation shows how the presence of Se atoms affects the adsorption of the ORR intermediates on the NS. In particular, we show that the electrostatic repulsion between charged Se and O or OH reduces the binding energy of the latters. [Preview Abstract] |
Session L29: Quantum Entanglement
Sponsoring Units: GQIChair: Lana Sheridan, National University of Singapore
Room: C148
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L29.00001: Entanglement entropy between two coupled Tomonaga-Luttinger liquids Shunsuke Furukawa, Yong Baek Kim We consider a system of two coupled Tomonaga-Luttinger liquids (TLL) on parallel chains and study the R\'enyi entanglement entropy $S_n$ between the two chains. The limit $n\to1$ corresponds to the von Neumann entanglement entropy. The system is effectively described by two-component bosonic field theory with different TLL parameters in the symmetric/antisymmetric channels. We argue that in this system, $S_n$ is a linear function of the length of the chains followed by a universal subleading constant $\gamma_n$ determined by the ratio of the two TLL parameters. We derive the formulae of $\gamma_n$ for integer $n\ge 2$ using (a) ground-state wave functionals of TLLs and (b) conformal boundary states, which lead to the same result. These predictions are checked in a numerical diagonalization analysis of a hard-core bosonic model on a ladder. Although our formulae of $\gamma_n$ are not analytic in the limit $n\to 1$, our numerical result suggests that the subleading constant in the von Neumann entropy is also universal. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L29.00002: Entanglement from Charge Statistics: Exact Relations for Many-Body Systems Francis Song, Christian Flindt, Stephan Rachel, Israel Klich, Karyn Le Hur We present exact formulas for the entanglement and R\'{e}nyi entropies generated at a quantum point contact (QPC) in terms of the statistics of charge fluctuations, which we illustrate with examples from both equilibrium and non-equilibrium transport. The formulas are also applicable to groundstate entanglement in systems described by non-interacting fermions in any dimension, which in one dimension includes the critical spin-1/2 XX and Ising models where conformal field theory predictions for the entanglement and R\'{e}nyi entropies are reproduced from the full counting statistics. These results may play a crucial role in the experimental detection of many-body entanglement in mesoscopic structures and cold atoms in optical lattices. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L29.00003: Quantum Monte Carlo Calculation of the Topological Entanglement Entropy in a Kagome Spin Liquid Roger Melko, Sergei Isakov, Ann Kallin, Matthew Hastings We develop a quantum Monte Carlo procedure to compute the Renyi entanglement entropy of interacting quantum many-body systems at nonzero temperature. We illustrate the method by calculating the topological entanglement entropy in a featureless Mott Insulating phase of a Bose-Hubbard model on the kagome lattice. The topological entanglement entropy displays a characteristic finite-temperature crossover behavior discussed previously in the context of the toric code. At zero-temperature it becomes the log of the quantum dimension of the topological order, confirming the existence of a Z2 spin liquid phase in the groundstate of this model. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L29.00004: Entanglement entropy and boundary operators in quantum impurity systems Erik Eriksson, Henrik Johannesson Entanglement in quantum impurity systems can be studied analytically using boundary conformal field theory (BCFT). In particular, the effect from an impurity on the entanglement entropy of a surrounding region is governed by the boundary operator content of the model. We present general results for the corrections to scaling of the R{\'e}nyi entanglement entropies when perturbing the BCFT with boundary operators [arXiv:1011.0448]. These results are then used to predict the asymptotic large-block behavior of the impurity contribution to the entanglement entropy in various Kondo systems. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L29.00005: Global quantum correlations in the spin-1 bilinear-biquadratic chain Roman Orus, Tzu-Chieh Wei We investigate global properties of the ground state of the spin-1 bilinear-biquadratic quantum spin chain in the thermodynamic limit, focusing on the geometric entanglement and fidelity diagram. The two quantities are computed via iTEBD and they appear to be capable of detecting the various well-known phase transitions in the system, including a Kosterlitz-Thouless one. The two quantities also behave distinctively at other points in the phase diagram. In particular, this is the case for the fidelity diagram at $\theta \approx 1.34 \pi$ (around a possible transition to a spin nematic phase), and also for the geometric entanglement at the integrable gapped point $\theta = 3 \pi /2$, where we conjecture an infinite entanglement length in the system. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L29.00006: Definitions of entanglement entropy of spin systems in the valence-bond basis Yu-Cheng Lin, Anders Sandvik The valence-bond structure of spin-1/2 Heisenberg antiferromagnets is closely related to quantum entanglement. We investigate definitions of entanglement entropy based on individual valence bonds connecting two subsystems, as well as shared loops of the transposition graph (overlap) of two valence-bond states [1]. We reformulate a previously used definition based on valance bonds in the wave function as a true ground state expectation value, and find that its scaling for the Heisenberg chain agrees with an exact result. The loop-based entanglement entropy of the two-dimensional Heisenberg model is shown to satisfy the area law (with an additive logarithmic correction), unlike single-bond definitions (which exhibit multiplicative logarithmic corrections). \\[4pt] [1] Y.-C. Lin and A.W. Sandvik, arXiv:1005.0821. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L29.00007: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L29.00008: General relation between energy spectrum and entanglement spectrum Xiaoliang Qi, Hosho Katsura, Andreas Ludwig We demonstrate that the bipartite density matrix, arising from a spatial bipartitioning of a gapped topological state which possesses gapless edge modes in the form of a conformal field theory (CFT ) (when terminated against a topologically trivial state/vacuum), such as e.g. a general quantum Hall state, is the density matrix of a the chiral edge state CFT at a finite temperature. We obtain this result by applying a physical instantaneous cut of the gapped system, and by viewing the cutting process as a sudden ``quantum quench'' into a CFT, using the tools of boundary conformal field theory. In particular, we obtain a general relation between the Hamiltonian spectrum of gapless theories and the entanglement spectrum of the gapped theory obtained from coupling two gapless theories. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L29.00009: The entanglement spectrum of perturbed Chern-Simons theories Thomas Jackson, Israel Klich Topological field theories --- theories insensitive to the metric of the space they live on --- have been shown to be applicable to a remarkable variety of condensed matter systems. A natural and important question is how perturbations relevant for real systems (interactions, etc.) deform these topological structures. In this work, we consider perturbations of Chern-Simons theory by a small Yang-Mills term, which breaks topological symmetry by introducing local bulk degrees of freedom in the form of massive gluons. We consider the behavior of the entanglement spectrum (the eigenvalues of the reduced density matrix) of this theory under this perturbation. We argue that the act of taking the partial trace may be viewed as adding a chemical potential gradient for the gluons near the boundary of the space, with a length scale determined by the gluon mass --- or, colloquially, a ``hot edge.'' [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L29.00010: Entanglement, Dissipation and the Casimir effect Israel Klich The role of dissipation in the Casimir force between metals or dielectric has a been discussed in many works and is an important part of the Casimir theory, where puzzles about the finite temperature corrections to the effect are still being worked out. Here, we study the contribution of dissipation in creating distance dependent entanglement between materials, and on the meaning of the corresponding entropy. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L29.00011: Entanglement Entropy Scaling of 2D Critical Wave Functions Michael Zaletel, Jens Bardarson, Joel Moore While CFT calculations have revealed a variety of universal predictions for the entanglement spectrum of critical 1+1D field theories, much less is known about higher-dimensional systems. CFT methods can be extended to a class of 2+1D theories characterized by a $z = 2$ critical point, the so-called Rokhsar-Kivelson wave functions. The entanglement entropy of RK-type critical wave functions contains a universal logarithmic contribution $\gamma \log( L )$ for some geometries arising from a trace anomaly in the corresponding CFT. We first re-examine the free boson, where the existence of order-unity contributions that depend on the boson compactification radius has been discussed in several recent papers (Hsu et al., St\'ephan et al., Oshikawa). We find analytically and numerically that the logarithmic contribution exists with the coefficient predicted by Fradkin and Moore and is independent of the compactification. However, it appears that their conjecture that general CFTs show the same dependence of $\gamma$ on central charge as the free boson is incorrect. We present arguments and numerical evidence for this conclusion in $c = 1/2$ and $c=1$ lattice models. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L29.00012: Entanglement spectra of Hofstadter and related models Zhoushen Huang, Daniel Arovas We compute the bipartite entanglement spectra for the Hofstadter model on various two-dimensional lattices. The behavior of the entanglement eigenstates in the vicinity of a partition boundary is investigated in detail. We also investigate the formation of entanglement edge states as one tunes through a topological phase transition in Haldane's honeycomb lattice model and other related systems. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L29.00013: Entanglement Spectrum In Condensed Matter B. Andrei Bernevig I will review the information that entanglement spectra give for a wide range of systems in condensed matter physics, such as fractional quantum hall effect, quantum spin chains, topological insulators, and disordered systems. (the results are based on a series of works performed in collaboration with N. Regnault, R. Thomale, A. Chandran, A Sterdyniak, M. Hermanns, Z. Papic, T.L. Hughes, E. Prodan, D.P. Arovas) [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L29.00014: Simultaneous generation of multiple quadripartite continuous-variable cluster states in the optical frequency comb of a single optical parametric oscillator Matthew Pysher, Yoshichika Miwa, Reihaneh Shahrokhshahi, Russell Bloomer, Olivier Pfister We report the experimental generation of multiple, four-mode, continuous-variable cluster states from a single optical parametric oscillator (OPO) operating below threshold. We use a PPKTP crystal phasematching two concurrent nonlinear interactions to entangle the optical frequency comb formed by the OPO cavity. Four independent entanglement witnesses (a.k.a. infinitesimal operators of stabilizers, or ``nullifiers'') display squeezing in each cluster state, and we utilize the large phase-matching bandwidth of the nonlinear interactions to display the simultaneous creation of several such cluster states using only a single pump frequency. A slightly more sophisticated version of this experimental method, using a crystal with three nonlinear interactions and 15 pump frequencies, has theoretically shown the ability to produce arbitrarily large square-grid cluster states suitable for universal one-way quantum computing. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L29.00015: Multipartite entanglement in the optical frequency comb of a depleted-pump optical parametric oscillator Reihaneh Shahrokhshahi, Olivier Pfister The optical frequency comb (OFC) of a single optical parametric oscillator (OPO) has been shown to be a very interesting candidate for scaling the size of quantum entangled states. In sophisticated OPOs below threshold, square-grid cluster states of very large size can in principle be generated. Here, we study a very simple OPO well above threshold, in the linearized fluctuation approximation, and investigate the effect of pump depletion on multiple, simultaneously resonant, signal-mode pairs. We find that the depleted quantum pump mediates quantum correlations between the signal fields. These correlations lead in turn to inseparability of these fields, as evidenced by the well-known van Look-Furusawa entanglement criteria. Due to its simplicity and its scalability, this fully inseparable multipartite entangled state could used as a resource in quantum information protocols. [Preview Abstract] |
Session L30: Graphene: Thermal Conduction and Phonons
Sponsoring Units: DCMPChair: Vitor Pereira, National University of Singapore
Room: C147/154
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L30.00001: Thermal transport in suspended and supported monolayer graphene grown by chemical vapor deposition Weiwei Cai, Arden Moore, Shanshan Chen, Yanwu Zho, Li Shi, Rodney S. Ruoff Although electron transport in graphene has been studied extensively and graphene is predicted to have very high thermal conductivity near room temperature, there is only limited experimental data in the literature on phonon transport in graphene because of experimental challenges. We report results based on micro-Raman spectroscopy for the measurement of the thermal conductivity of large-area, monolayer graphene grown by CVD on copper and subsequently suspended over a circular hole. The obtained optical absorption is measured directly by measuring the transmission through the graphene covered hole. Based on the thermal interface conductance of (28+2.8/-3.8) MW/m$^{2}$ K, the contact thermal resistance is determined to be considerably smaller than the measured thermal resistance of the suspended graphene. The obtained thermal conductivity of the supported graphene is (370 +490/-300) W/m K, which is considerably smaller than that of suspended graphene in agreement with recent measurements of mechanically exfoliated graphene supported on SiO$_{2}$. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L30.00002: Surprising Effects of Substrate on Thermal Transport in Supported Graphene Zhun-Yong Ong, Eric Pop We study thermal transport in graphene ``supported'' on SiO2 using molecular dynamics (MD) simulations. We find that coupling to the substrate leads to an order of magnitude decrease in the apparent thermal conductivity (TC), explaining recent experiments [1]. This reduction is due to the substrate damping of flexural acoustic (ZA) phonons, which implies that the high TC of isolated graphene is due to the large mean free path of long-wavelength ZA modes [2]. However, we find that by increasing the strength of the interfacial interaction, the apparent TC is enhanced by up to a factor of four. Using a continuum model [3], we relate the apparent TC enhancement to the ZA modes coupling with the substrate Rayleigh waves. In the weak coupling limit, the ZA modes have a quadratic dispersion and small group velocities at long wavelengths; in the strong coupling limit, the hybridized interfacial modes have a linear dispersion and larger group velocities. This finding suggests that the TC of supported graphene may be tunable through interfacial interaction. \\[4pt] [1] J. H. Seol et al., Science 328, 213 (2010)\\[0pt] [2] L. Lindsay et al., PRB 82, 115427 (2010).\\[0pt] [3] B. N. J. Persson et al., EuroPhys. Lett. 91, 56001 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L30.00003: Two Dimensional Phonon Transport in Graphene Insun Jo, Jae Hun Seol, Arden L. Moore, Michael T. Pettes, Lucas Lindsay, Natalio Mingo, David Broido, Zhen Yao, Li Shi We present thermal conductivity measurements of monolayer graphene exfoliated on a silicon dioxide substrate at different temperatures. A nanofabricated resistance thermometer device is developed to measure the thermal conductance of graphene and supporting 300nm thick SiO$_{2}$ layer, which allows us to extract the thermal conductivity of graphene while supported on this layer. The measured value is as high as 600 W/mK near room temperature, which is lower than that of suspended graphene, 1500-5800 W/mK, but still higher than those of metal interconnects. Theoretical calculations show that the strong interface-scattering of flexural modes across the graphene-oxide interface is responsible for the decreased value. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L30.00004: Influence of Polymeric Residue on the Thermal Conductivity of Suspended Bi-Layer Graphene Michael Pettes, Insun Jo, Zhen Yao, Li Shi The thermal conductivity (\textit{$\kappa $}) of two bi-layer graphene samples suspended between two micro-resistance thermometers was measured to be close to 600 W m$^{-1}$ K$^{-1}$ at room-temperature and exhibits a \textit{$\kappa $} $\propto \quad T^{1.5}$ behavior at temperature ($T)$ between 50 -- 125 K. The lower thermal conductivity than the basal plane values of graphite and the temperature dependence are attributed to scattering of phonons in the bi-layer graphene by a residual polymeric layer that was clearly observed by transmission electron microscopy. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L30.00005: Phonon thermal conductivities of multi-layered graphene Lucas Lindsay, David Broido Using an exact numerical solution of the phonon Boltzmann equation, we show that the intrinsic lattice thermal conductivities, $\kappa $, of $N$-layer graphene ($N$=1-5) are dominated by contributions from out-of-plane, flexural (ZA) phonon modes contrary to previous theories based on the relaxation time approximation, which assumed this contribution to be negligible [1, 2]. We find a reduction of $\kappa $ with increasing $N$ due to interlayer coupling, which: 1) lifts the degeneracy of the flexural acoustic mode frequencies, 2) makes the ZA phonon branch become linear near the zone-center, and 3) breaks a selection rule for anharmonic phonon-phonon scattering in two-dimensional systems. \\[4pt] [1] P. G. Klemens and D. F. Pedraza, Carbon vol. 32, pp. 735-741 (1994). \\[0pt] [2] B. D. Kong, S. Paul, M. B. Nardelli and K. W. Kim, Phys. Rev. B 80, 033406 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L30.00006: Thermal properties of novel 2D hybrid graphene-BN nanostructures Nikhil Medhekar, Jun Song Graphene, a 2D honeycomb carbon crystal of one-atom thickness, has been widely recognized as a very promising material for next generation optoelectronic and NEMS applications. Recent developments have shown that it is possible to obtain hybrid 2D structures by combining sp2-graphene lattice with sp2-lattice of non-carbon materials such as hexagonal Boron Nitrides. The atomically thin sheets containing both hexagonal-Boron Nitride and graphene can result in new materials with properties complementary to their individual properties and further enrich the potential applications. Here, using molecular dynamics simulations, we elucidate the characteristics of thermal transport in 2D hybride h-BN and graphene materials. We find the thermal conductivity of the hybrid material is a strong function of the relative domain widths, interface type (e.g., zigzag and armchair) as well as the interface quality. Our results provide crucial insights on the role of the interfaces and defects in phonon scattering in the hybrid material and can potentially provide means to tailor its thermal properties. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L30.00007: Thermal Expansion in Graphene and Graphane: Role of Anharmonic and Harmonic Effects Arunima Singh, Richard G. Hennig As the practical application of graphene nears realization, knowledge of effects of temperature on mechanical properties of graphene becomes important. In this study we use empirical potentials and density-functional perturbation theory (DFPT) to determine the thermal expansion of free-standing graphene, graphene on substrates, and its hydrogenated derivative graphane. Comparisons of MD simulations with calculations using the quasi- harmonic approximation using an empirical potential show that anharmonic effects are negligible at temperatures below 2200K. In contrast to the DFPT calculations using the quasi-harmonic approximation, MD results show that free-standing graphene has a positive thermal expansion coefficient above 600K. For graphene on a substrate our DFPT results agree with those of Jiang et. al [1] and show that the substrate suppresses the negative thermal expansion coefficient with increasing strength of the substrate- graphene interaction. We also investigate the thermal expansion of the thermodynamically stable conformers of graphane using DFPT.\\[4pt] [1] J. W. Jiang, J. S. Wang, B. Li, Phys. Rev. B 80, 205429 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L30.00008: Observation of coherent G-mode phonon oscillations in graphene films J.-H. Kim, M.H. Jung, B.H. Hong, E.H. Haroz, J. Kono, K.J. Yee We have observed coherent G-mode lattice vibrations in three stacked-mono layer and multi-layer graphene films by using ultrashort pulses from a Ti:Sapphire laser. The degenerated $E_{2g} $modes were excited through the impulsive stimulated Raman scattering process, and detected through induced reflectivity modulations. The G-mode frequency from the stacked-mono layer graphene is shifted toward higher energy compared with that of the multi-layer graphene. A dephasing time of abound 0.6 ps for the stacked-mono layer graphene was found to be shorter than that of semiconducting single-walled carbon nanotubes (1.48 ps) and slower than that of metallic single-walled carbon nanotubes, due to stronger electron-phonon interactions, where the phonon energy can be dissipated by exciting electrons between a linear bands of graphene. Through the strong polarization dependence of coherent G-mode lattice vibrations, we confirmed that the $E_{2g}^{(2)} $ symmetry is dominant. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L30.00009: Theory of coherent phonons in graphene G.D. Sanders, C.J. Stanton, J.-H. Kim, K.-J. Yee, M.H. Jung, B.H. Hong, E.H. Haroz, J. Kono We develop a theory for the generation and detection of coherent phonons in graphene. Coherent phonons are generated via the deformation potential electron-phonon interaction with photogenerated carriers. In our theory the electronic states are treated in a third nearest neighbor extended tight binding formalism which gives a good description of the states over the entire graphene Brillouin zone while the phonon states are treated in a valence force field model. The equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations for each value of the phonon wavevector. Comparison is made with recent experimental measurements. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L30.00010: Coherent phonon spectroscopy of the shearing mode in bilayer and few-layer graphene Davide Boschetto, Lenadro Malard, Chun Hung Lui, Kin Fai Mak, Hugen Yan, Zhiqiang Li, Tony F. Heinz The interlayer shearing vibration in graphite, a low-energy optical phonon, is known to consist of adjacent atomic planes moving laterally in opposite directions with respect to one another. We have applied coherent phonon spectroscopy, based on a sensitive femtosecond pump-probe measurement, to investigate the corresponding mode in few-layer graphene samples down to bilayer thickness. Here we report on the evolution of the frequency and lifetime of this mode with thickness. To model the expected behavior, we have analyzed a model of identical nearest-neighbour couplings. We find that this model predicts most of the observed reduction in frequency with decreasing layer thickness. We consider to the remaining deviations between the model and our experimental data in terms of a slight increase in the interlayer spacing, leading to a reduced restoring force, with decreasing graphene layer thickness. This decrease in lattice spacing with thickness is expected for layered materials governed by van der Waals forces. We also show experimentally that the shearing mode frequency is robust against external perturbations, such as different substrates and the presence of adlayers. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L30.00011: Tuning the Kohn Anomaly in the Phonon Dispersion of Graphene by Interaction with the Substrate and by Doping Ludger Wirtz, Adrien Allard, Claudio Attaccalite, Michele Lazzeri, Francesco Mauri, Angel Rubio The phonon dispersion of graphene displays two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points G and K. The slope of the HOB around K is a measure of the electron-phonon coupling (EPC) and determines the dispersion of the Raman D and 2D lines as a function of the laser energy. We show that the EPC can be strongly modified both due to interaction with a metallic substrate and due to doping. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around K becomes completely flat. This is due to the strong hybridization of the graphene p-bands with the Nickel d-bands which lifts the linear crossing of the p-bands at K. From experimental phonon dispersions one can therefore draw conclusions about the interaction strength between graphene and its different substrates. Furthermore, we present a new way to tune the EPC in graphene through electron/hole doping. We show that for the highest optical branch at K, the EPC is strongly dependent on the doping level. This dependency influences the dispersion of the Raman D and 2D lines and makes it possible to measure the charge state of graphene via resonant Raman spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L30.00012: Thermoelectric effect in high mobility single layer epitaxial graphene Xiaosong Wu, Yike Hu, Ming Ruan, Nerasoa K. Madiomanana, Claire Berger, Walt A. de Heer The thermoelectric response of high mobility single layer epitaxial graphene on silicon carbide substrates as a function of temperature and magnetic field have been investigated. For the thermopower, a strong deviation from the Mott relation, i.e. a quardratic correction to the linear temperature dependence, has been observed even when the carrier density is high. In the quantum Hall regime, the amplitude of the TEP peaks is lower than a quantum value predicted by theories, despite the high mobility of the sample. A systematic reduction of the amplitude with decreasing temperature suggests that the suppression of the TEP is intrinsic to Dirac electrons in graphene. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L30.00013: Thermoelectric transport in graphene with tunable mobility Xinfei Liu, Deqi Wang, Jing Shi Thermoelectric transport properties of single layer graphene have recently been studied both experimentally and theoretically. The unique band structure of graphene leads to unusual thermoelectric properties which are very sensitive to the carrier mobility. However, all previous experiments were carried out in graphene devices with different mobility values and comparisons were drawn among different devices. Recently, we have shown that by controlling the charge state of the ligand-bound nanoparticles on graphene it is possible to tune the mobility of the same graphene device over a wide range, e.g. 5000-19000cm$^2$/Vs. In this work, we adopted this method and successfully tuned the mobility of graphene while systematically studied the Seebeck and Nernst effects in a magnetic field up to 14 Tesla for each fixed mobility value. Our results show that at zero magnetic field, the width of the transition region near the Dirac point decreases sharply and the diverging behavior in the Seebeck coefficient becomes more pronounced as the mobility is tuned from low to high. At high magnetic fields, the Seebeck coefficient in the high mobility state clearly reveals additional features that are related to the splitting of the zeroth Landau level near the Dirac point. Moreover, we demonstrate that the Nernst peak height at the Dirac point depends linearly on the carrier mobility in graphene. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L30.00014: Thermoelectric Properties of Graphene Ribbons Enrique Munoz Several theoretical and experimental studies have been recently concerned with electric and thermal transport in graphene layers and ribbons, where propagation of electrons [1] and phonons [2] seems to be dominated by a ballistic mechanism. Of particular interest in this context is the identification and characterization of thermoelectric effects [3], which represent a promising alternative for energy recovery in technological applications. In the present work, the effect of the electron- phonon interaction over a predominantly ballistic transport mechanism in graphene ribbons is studied in the context of thermoelectricity. Theoretical estimations of the thermopower S, and the corresponding figure of merit ZT, are presented for this system as a function of temperature. \\[4pt] [1] K. Saito, J. Nakamura, and A. Natori, ``Ballistic thermal conductance of a graphene sheet,'' Phys. Rev. B 76, 115409 (2007).\\[0pt] [2] E. Munoz, J. Lu, and B. I. Yakobson, ``Ballistic thermal conductance of graphene ribbons,'' Nano Lett. 10, 1652 (2010).\\[0pt] [3] Y. Ouyang and J. Guo, ``A theoretical study on thermoelectric properties of graphene nanoribbons,'' Appl. Phys. Lett. 94, 263107 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L30.00015: Theromelectricity in Graphene: Effects of a gap and magnetic fields Subroto Mukerjee, Aavishkar Patel We calculate the thermopower of monolayer graphene in various circumstances. First we show that experiments on the thermopower of graphene can be understood quantitatively with a very simple model of screening in the semiclassical limit. We can calculate the energy dependent scattering time for this model exactly. We then consider acoustic phonon scattering which might be the operative scattering mechanism in free standing films, and predict that the thermopower will be linear in any induced gap in the system. Further, the thermopower peaks at the same value of chemical potential (tunable by gate voltage) independent of the gap. Finally, we show that in the semiclassical approximation, the thermopower in a magnetic field saturates at high field to a value which can be calculated exactly and is independent of the details of the scattering. This effect might be observable experimentally. [Preview Abstract] |
Session L31: Focus Session: Materials at High Pressure III: Electronic Transitions
Sponsoring Units: DMP GSCCM DCOMPChair: Koichiro Umemoto, University of Minnesota
Room: C145
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L31.00001: Pressure Induced Metal Insulator Phase Transition in Eu$_2$Ir$_2$O$_7$ Fazel Fallah Tafti, Jun Ishikawa, Yo Machida, Alix McCollam, Satoru Nakatsuji, Stephen Julian The metal to insulator phase transition of the pyrochlore iridate Eu$_2$Ir$_2$O$_7$ has been studied by means of resistivity measurements under pressure in the range 2 to 12 GPa. At ambient pressure, the system is a ``metal'' at high temperatures with a non-metallic rise of resistivity with decreasing temperature followed by a metal-insulator phase transition at T$_{MI}$ below which it becomes insulating. With increasing pressure, a cross-over from non-metallic to metallic appears in the resistivity curves at a temperature $T^*>T_{MI}$. As the pressure is further increased T$^*$ rises, T$_{MI}$ drops and the low temperature insulating phase melts into a metallic phase through a continuous transition at P $\sim$ 7.8 GPa. The high pressure metallic phase is rather curious and exhibits two characteristic features of Kondo metals: a minimum resistivity and a logarithmic rise of resistivity at low temperatures. We will show that there is a remarkable correspondence between the resistivity curves measured at various pressures and those obtained by successively replacing the R site of the R$_2$Ir$_2$O$_7$ family by larger rare earth atoms. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L31.00002: Pressure-induced Metallization of Carbon Disulfide Ranganath Dias, Mathew Debessai , Choong-Shik Yoo We will report high pressure electrical resistivity measurements on solid CS$_{2}$ in diamond anvil cell to 60GPa. The result shows a steady decrease in resistivity to that of metal at around 55GPa. Its visual appearance of CS$_{2}$ also supports its insulator-metal transition: the initially transparent CS$_{2}$ becomes opaque and eventually reflective with increasing pressure. We will also present a plausible mechanism for the observed metallization. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L31.00003: Electrical resistance measurement of optimal doped YBCO under pressure Takaki Muramatsu High pressure effect on nearly optimal doped high $T_{C}$ cuprate superconductor YBa$_{2}$Cu$_{3}$O$_{7-x}$ was studied by the electrical resistance measurements up to about 30 GPa. Superconducting phase of YBa$_{2}$Cu$_{3}$O$_{7-x}$ in pressure-temperature phase diagram was confirmed. $T_{C}$ has the broad maximum at about 8 GPa and then decreases with pressure and disappears at the pressure between 23 GPa and 25 GPa. In higher pressure region, the resistance shows upturn below about 60 K, indicating the possibility of crossover on YBa$_{2}$Cu$_{3}$O$_{7-x}$ from superconductor to semiconductor at about 24 GPa [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L31.00004: Pressure induced phase transition in FeGa alloys Christopher DeVreugd, Muhtar Ahart, Peter Gehring, Dwight Viehland, Russell Hemley Giant magnetostriction in Fe-- x Ga alloys ( 15 -- x - 27 ) offers potential for future generations of sensors and actuators. A maximum in the magnetostrictive strain is found at Ga content of about 19 percent, which is ten times higher than that of pure alpha-Fe. To investigate the behavior of FeGa alloys under pressure, we chose a slow cooled alloy of FeGa-19 as our sample and performed x-ray diffraction experiments in a diamond anvil cell up to 45 GPa. Diffraction pattern shows powder rings associated with (110), (200), and (211) Bragg reflections from expected bcc structure of iron below 24 GPa. We also observed the intensity increases along the powder rings associated with the crystal structure of Galfenol. Considering the (110) Bragg peak splits into three peaks above 24 GPa, our results indicate that FeGa alloy undergoes a bcc cubic to a hexagonal transition around 24 GPa. When the pressure is decreased, the hcp phase transforms back to the bcc phase. The transition mechanism can be understood by using the analogy to the bcc-hcp phase transition in pure iron under pressure. The transition in iron is a martensitic or displacive one. The hcp structure can be derived from the bcc structure through a relatively minor distortion of the bcc structure. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L31.00005: High Pressure Studies of UO$_{3}$ Zsolt Jenei, Magnus Lipp, Jae-Hyun Klepeis, Bruce Baer, Hyunchae Cynn, William Evans, Changyong Park, Dimitri Popov It has been reported that upon compression t ambient temperature $\delta $-UO3 becomes amorphous at 2.2 GPa. (Journal of Alloys and Compounds 315 p59--61). We studied the properties of $\gamma $-UO3 in diamond anvil cell up to 75 GPa. Powder diffraction experiments performed at HPCAT/Advanced Photon Source show the crystalline uranium trioxide transforms to an amorphous solid between 12 and 14 GPa and remains amorphous up to 75 GPa. The transition has been confirmed by Raman spectroscopy as well. In this paper we'll present our findings on the amorphous transition together with the equation of state of both the crystalline phase and the amorphous phase. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L31.00006: High-pressure equation of state of U$_{3}$O$_{8}$ Jae-Hyun Klepeis, Zsolt Jenei, Magnus Lipp, William Evans, Dmitry Popov, Changyong Park We will present experimental studies at high pressures of the equation of state of U$_{3}$O$_{8}$. Isothermal pressure-volume measurements of U$_{3}$O$_{8}$ were made at ambient/elevated (600 K) temperatures in the pressure range of 1 atm $\sim$ 80 GPa (10 $\sim$ 70 GPa). Angle dispersive X-ray diffraction patterns at ambient temperature indicate that the A-centered orthorhombic structure of U$_{3}$O$_{8}$ transforms to the face centered cubic (fcc) structure above 9 GPa. Both the orthorhombic and cubic phases co-exist between 9 GPa and 30 GPa. As the temperature is increased at 10 GPa, we find that U$_{3}$O$_{8}$ also transforms to the fcc structure. As the pressure is increased at 600 K, the fcc structure undergoes a phase transition to the body centered tetragonal structure. Since the uranium in U$_{3}$O$_{8}$ is the dominant x-ray scatterer, the behavior of the oxygen at the phase transitions was measured using Raman spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L31.00007: High pressure x-ray diffraction of uranium oxide formed by natural oxidation of uranium Hyunchae Cynn, William J. Evans, Bruce J. Baer, Simon MacLeod, Magnus J. Lipp, Zsolt Jenei, J.H. Park Klepeis, Yue Meng, Stanislav Sinogeikin Naturally oxidized uranium has been compressed using a diamond anvil cell. Although X-ray diffraction shows the anisotropic nature in the pressure dependent changes to the lattice parameters of pure uranium as previously recorded, uranium oxide appears stable at high pressure in the fluorite structure with no clear evidence of a phase transition observed above the transition pressure previously measured for bulk uranium oxide. The lattice parameters of uranium oxide formed by natural surface oxidation have been determined along with those of the underlying pure uranium employing Rietveld refinement. We will discuss the seemingly unexpected findings about uranium oxide. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L31.00008: Uranium hydride (UH$_{3}$ ) and deuteride (UD$_{3}$) under conditions of high pressure and temperature Magnus Lipp, Zsolt Jenei, Jae Hyun Park Klepeis, Bruce Baer, Hyunchae Cynn, William Evans, Don Fujino, Blake Nolan, Joe Wermer, Changyong Park, Dmitry Popov Uranium hydrides are currently being evaluated as fuels in new reactor designs. They also serve as sources for very clean hydrogen by decomposing when heated at ambient pressure. We have examined their behavior over a large pressure and temperature range by placing small quantities and a pressure marker in a diamond anvil cell for angle dispersive x-ray diffraction. Neon was chosen as pressure transmitting medium to ensure the best possible hydrostatic conditions. We'll discuss crystal structures, the equation of state, the bulk modulus and the phase diagram. Work performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Diffraction studies were performed at HPCAT (Sector 16), APS/ANL. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from~DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L31.00009: Structural Stabilities and Electronic Properties of Cobalt Hydrides Yasuyuki Matsuura, Tatsuya Shishidou, Tamio Oguchi Cobalt forms ferromagnetic hydrides CoH$_x$ at high pressures of hydrogen [1]. As the hydrogen pressure increases at temperatures 250-350${}^{\circ}$C, the concentration of hydrogen in the hcp phase monotonically increases, and reaches $x\sim 0.6$ at 7 GPa. At higher pressures, an fcc-based hydride with $x\sim 1.0$ is formed. At ambient pressure and 120 K, hydrogen atoms in the solution with $x\le 0.26$ are randomly distributed over octahedral interstitial sites [2]. In the solution with $x=0.34$ ($x\ge 0.38$), hydrogen atoms occupy every third (second) layer. The magnetic moments of the hcp-based hydrides are oriented to the $c$-axis, and are decreased with increasing hydrogen concentration at a rate of about 0.36 $\mu_{\mathrm{B}} $ per hydrogen atom. In this study, we optimize the structural parameters for several structures, and investigate the structural stabilities and related electronic properties by using first-principles calculations. The full-potential linearized augmented plane wave method with the generalized gradient approximation is adopted.\\[4pt] [1] V. E. Antonov, J. Alloys Compd. \textbf{330}-\textbf{332}, 110 (2002).\\[0pt] [2] V. K. Fedotov, V. E. Antonov, T. E. Antonova, E. L. Bokhenkov, B. Dorner, G. Grosse, and F. E. Wagner, J. Alloys Compd. \textbf{291}, 1 (1999). [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L31.00010: Formation of collapsed tetragonal phase in EuCo$_{2}$As$_{2}$ under high pressure Matthew Bishop, Walter Uhoya, Georgiy Tsoi, Yogesh Vohra, Athena Sefat, Brian Sales The structural properties of EuCo$_{2}$As$_{2}$ have been studied up to 35 GPa, through the use of x-ray diffraction in a diamond anvil cell at a synchrotron source. At ambient conditions, EuCo$_{2}$As$_{2}$ ($I$4/\textit{mmm}) has a tetragonal lattice structure with a bulk modulus of 48 $\pm $ 4 GPa. With the application of pressure, the $a $axis exhibits negative compressibility with a concurrent sharp decrease in $c$-axis length. The anomalous compressibility of the $a $axis continues until 4.7 GPa, at which point the structure undergoes a second-order phase transition to a collapsed tetragonal (CT) state with a bulk modulus of 111 $\pm $ 2 GPa. We found a strong correlation between the ambient pressure volume of 122 parents of superconductors and the corresponding tetragonal to collapsed tetragonal phase transition pressures. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L31.00011: The behavior of semi-metal Bi$_{4}$Te$_{3}$ under pressure Jason Jeffries, A.L. Lima Sharma, P.A. Sharma, C.D. Spataru, S.K. McCall, J.D. Sugar, S.T. Weir, Y.K. Vohra As a member of the (Bi$_{2})_{m}$(Bi$_{2}$Te$_{3})_{n}$ adaptive series, Bi$_{4}$Te$_{3}$ exhibits identical crystallographic symmetry and similar electronic properties to the archetypal thermoelectric material Bi$_{2}$Te$_{3}$. The extra Bi atoms in Bi$_{4}$Te$_{3}$ serve to increase the electronic density of states, making Bi$_{4}$Te$_{3}$ a semi-metal, as opposed to semiconducting Bi$_{2}$Te$_{3}$, at ambient pressure. We will report the results of high-pressure structural and magnetotransport characterization of Bi$_{4}$Te$_{3}$, focusing on the interplay between structural parameters and the underlying electronic properties. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L31.00012: Theoretical X-ray Spectroscopy for Strongly Correlated Materials at High Pressure Adam P. Sorini, Cheng-Chien Chen, Shibing Wang, Wendy L. Mao, Thomas P. Devereaux, Chi-Chang Kao We present theoretical x-ray spectra for correlated d- and f-electron materials under extreme conditions. We use exact-diagonalization to study small clusters of atoms including ligand charge-transfer and atomic-multiplet effects. These techniques allow us to extract information from spectroscopic measurements regarding phase transitions in strongly correlated materials as a function of pressure. We show recent results for hematite (Fe2O3) which undergoes a variety of phase transitions (structural, spin, metal/insulator) near 50 GPa, which have been observed using hard x-ray quadrupolar absorption. We also apply our models to the correlated f-electron ``volume collapse" systems which show complex behavior under pressure. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L31.00013: Fermi surface of SnO under pressure Niels Christensen, Axel Svane Tin monoxide undergoes a pressure induced insulator-metal transition around 5 GPa. The pressure effects on the electronic band structure, the Fermi surface (FS) and its nesting properties of SnO in the metallic phase have been derived from ab initio calculations within the local density (LDA) and quasiparticle selfconsistent GW (QSGW) approximations. It is found that the topologies of the FS determined by the two approaches are very similar. Nesting occurs between two different sheets of the FS, most pronounced via (1,1,0) Q- vectors connecting the outer electron surface with the hole surface The present study was motivated by recent observation of superconductivity in SnO under pressure [1]. \\[4pt] [1] M.K. Forthaus el al., Phys. Rev. Lett. {\bf 105}, 157001 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L31.00014: Dynamic response of Cu46Zr54 metallic glass to high-strain-rate shock loading: Plasticity, spall, and atomic-level structures Bedri Arman, Sheng-Nian Luo, Timothy Germann, Tahir Cagin Dynamic response of Cu46Zr54 metallic glass under adiabatic planar shock wave loading with molecular dynamics simulations was investigated. We analyzed the Hugoniot (shock) states up to 60 GPa, shock-induced plasticity and dynamic spall strengths. Especially, the spall strengths likely represent the limiting values achievable in experiments such as laser ablation. To characterize local deformation and structure at various stages of shock, release, tension and spallation, the local von Mises shear strain and Voronoi tessellation analyses were used. Modeled glass showed plasticity as localized shear transformation zones rather than thermal origin. Nucleation of voids occurred preferentially at the highly shear-deformed regions. Our simulations through the Voronoi and shear strain analyses suggest that the atoms having different local structures are of different shear resistances that lead to shear localization. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L31.00015: Single Crystal X-ray Diffraction at Megabar Pressures and Temperatures of Thousands Degrees Leonid Dubrovinsky, Natalia Dubrovinskaia, Marco Merlini, Michael Hanfland The most reliable information about crystal structures and their response to changes in pressure and temperature is obtained from single crystal diffraction experiments. We have developed a methodology to perform single crystal X-ray diffraction experiments in laser-heated diamond anvil cells and demonstrate that structural refinements and accurate measurements of the thermal equation of state of metals, oxides, silicates from single crystal intensity data are possible in a pressures ranging up to megabars and temperatures of thousands degrees. New methodology was applied to solve \textit{in situ} high-pressure high-temperature structure of iron oxide and study structural variations of iron and aluminum bearing silicate perovskite at conditions of the Earth lower mantle. [Preview Abstract] |
Session L32: Focus Session: Nano-Optics, Semiconductor and Metal Nanostructures
Sponsoring Units: DMPChair: Henry O. Everitt, Department of the Army, Redstone Arsenal
Room: C144
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L32.00001: Linear Optical and SERS Study on Metallic Membranes with Subwavelength Complementary Patterns Qingzhen Hao, Yong Zeng, Lasse Jensen, Douglas Werner, Vincent Crespi, Tony Jun Huang An efficient technique is developed to fabricate optically thin metallic films with subwavelength patterns and their complements simultaneously. By comparing the spectra of the complementary films, we show that Babinet's principle nearly holds in the optical domain. A discrete-dipole approximation can qualitatively describe their spectral dependence on the geometry of the constituent particles and the illuminating polarization. Using pyridine as probe molecules, we studied surface-enhanced Raman spectroscopy (SERS) from the complementary structure. Although the complementary structure posses closely related linear spectra, they have quite different near-field behaviors. For hole arrays, their averaged local field gains as well as the SERS enhancements are strongly correlated to their transmission spectra. We therefore can use cos$^{4}\theta $ to approximately describe the dependence of the Raman intensity on the excitation polarization angle $\theta $, while the complementary particle arrays present maximal local field gains at wavelengths generally much bigger than their localized surface plasmonic resonant wavelengths. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L32.00002: Optically controlled patchy modification of metal nanoparticles Stefan Stoianov, Jason Ridley, Brandon Thorpe, Webster Santos, Hans Robinson It is well known that that metal nanostructures strongly concentrate light intensity at hot spots located at sharp corners or in narrow gaps, either due to plasmonic resonances or the lightning-rod effect. This is exploited a several important applications, such as surface enhanced Raman spectroscopy (SERS) and apertureless NSOM. We propose using this phenomenon to induce photochemical reaction on the surface of metal nanoparticles, leading to differential, or patchy, functionalization of the particles. We have functionalized gold and silver nanoparticles fabricated using nanosphere lithography with ligands that contain o-nitrobenzyl functional groups. Upon absorption of a photon, these compounds cleave off, leaving behind a modified surface. Differential functionalization will be demonstrated by comparing the rate of photoreactions at the hotspots (measured with SERS) to the average cleavage rate (measured with FTIR). [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L32.00003: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L32.00004: TDDFT studies of plasmonic excitations in small transition metal-doped gold chains Neha Nayyar, Alamgir Kabir, Volodymyr Turkowski, Talat S. Rahman We apply a TDDFT approach to study the absorption spectra of pure Au chains and those doped with transition metal (TM) atoms (Ni, Rh, Fe) up to 24 atoms. We find that for gold chains with more than 10 atoms a collective plasmon mode is formed whose intensity grows with the number of atoms. The plasmon energy approaches asymptotically a value of ~0.6eV when the number of atoms is about 20. However, in the chains with odd number of atoms, an additional low-energy excited state close to the plasmonic peak is found which can be related to an excitation at the chain edge. Doping with TM atoms also leads to the formation of additional plasmon peaks close in energy to the main one, especially pronounced in the case of Ni- doped chains. We compare the results for the optical absorption spectrum of the system in the case of doping by different TM atoms and the role of the d-electron states of these atoms in formation of the additional plasmon peaks. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L32.00005: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L32.00006: Ultrafast photoconductive response of LaAlO$_{3}$/SrTiO$_{3}$ nanoscale photodetectors Yanjun Ma, Cheng Cen, Chung Wung Bark, Chad M. Folkman, Chang-Beom Eom, Jeremy Levy Conducting AFM lithography can be used to create nanoscale field effect transistors at the LaAlO$_{3}$/SrTiO$_{3}$ interface.\footnote{C. Cen et al., Nature Material, 7, 2136(2008)}$^,$\footnote{C.Cen et al., Science, 323, 1026 (2009)} Such devices exhibit gatable photoconductive response, which spans from visible to near-infrared regime.\footnote{P.Irvin et al., Nature Photonics advanced online publication, 14 Nov.2010 (DOI 10.1038/nphoton.2010.238)} By implementing the pump-probe measurement with a home-built femtosecond laser, we observe an ultrafast nonlinear optical response of these nanoscale photodetectors. We explore the feasibility of these devices for molecular-scale THz spectroscopy applications. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L32.00007: Infrared and Terahertz Nanoscopy Invited Speaker: During the last years, near-field microscopy based on elastic light scattering from atomic force microscope tips (scattering-type scanning near-field optical microscopy, s-SNOM [1]) has become a powerful tool for nanoimaging of local dielectric material properties [2-5] and optical near fields of photonic nanostructures [6-8]. After an introduction of s-SNOM, I will discuss recently developed applications in materials sciences and nanophotonics. I will focus particularly on IR and THz imaging at wavelengths $\lambda $ around 10 and 118 $\mu $m, where we typically achieve a wavelength-independent resolution better than 40 nm, corresponding to $\lambda $/250 and $\lambda $/3000, respectively [3]. Using metal-coated tips, the strong field enhancement at the tip apex probes the local dielectric properties of a sample, allowing for the simultaneous recognition of materials and free-carrier concentration in semiconductor nanodevices [3] and nanowires [5]. Quantitative free-carrier mapping is enabled by near-field plasmon-polariton spectroscopy, which can be also applied to study strain-induced changes of carrier concentration and mobility [4]. Nanoscale imaging of strain and nanocracks in ceramics can be achieved by near-field infrared phonon-polariton spectroscopy [4]. I will also discuss the capability of s-SNOM to image the vectorial near-field distribution of photonic nanostructures. In this application, a dielectric tip scatters the near fields at the sample surface. I will discuss how the amplitude and phase-resolved measurement of different near-field components allows for mapping of the polarization state in nanoscale antenna gaps [8], of near-field modes in loaded infrared gap antennas [7] and of mid-infrared energy transport in nanoscale transmission lines. \\[4pt] [1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004). \\[0pt] [2] R. Hillenbrand et al., Nature 418, 159 (2002). \\[0pt] [3] A. Huber et al., Nano Lett. 8, 3766 (2008). \\[0pt] [4] A. Huber et al., Nature Nanotech. 4, 153 (2009). \\[0pt] [5] J.M. Stiegler et al., Nano Lett. 10, 1387 (2010). \\[0pt] [6] T. Taubner et al. Science 313, 1595 (2006). \\[0pt] [7] M. Schnell et al., Nature Photon., 3, 287 (2009). \\[0pt] [8] M. Schnell et al., Nano Lett., 10, 3524 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L32.00008: Single GaN nanowire polariton luminescence Ayan Das, Marc Jankowski, Wei Guo, Pallab Bhattacharya Polariton emission from a single GaN nanowire in the strong coupling regime has been investigated in the temperature range of 200-300 K. GaN nanowires grow in the wurtzite structure with the c-axis along the growth direction. The polariton dispersion characteristics are determined from angle-resolved reflectivity measurements. The light emission characteristics measured as a function of incident optical power density reveal a distinct non-linear behavior and threshold, accompanied by a sharp decrease in linewidth over an order of magnitude and a small blue-shift that is ascribed to polariton-polariton interactions. Angle-resolved photoluminescence measurements above threshold indicate polariton cooling to the bottom of the lower polariton branch, triggered by the onset of stimulated scattering which is characterized by a fast relaxation time as obtained from time resolved photoluminescence measurements. Emission above threshold is linearly polarized. Second order correlation measurements and interferomtery indicate significant bunching below threshold and a coherent emission above threshold. These measurements indicate a coherent emission. Photon lasing due to carrier population inversion is observed at higher pump power densities. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L32.00009: Photoreflectance spectroscopy of single GaAs/GaP Core-shell Nanowires A. Wade, M. Montazeri, M.A. Fickenscher, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish We present a direct observation of the light hole (lh) and heavy hole (hh) valence band splitting in highly strained GaAs/GaP core/shell nanowires obtained by photoreflectance (PR) from a single nanowire. The NWs were prepared by Au nanoparticle (100 nm) catalyst-assisted MOCVD growth with two different shell thicknesses, where the induced strain is controlled varying the core/shell ratio. They were then dispersed on silicon for the PR measurement. The spectra show a $\sim $140eV splitting of the lh and hh bands for two different wires. Raman spectroscopy was carried out on the same growths in order to measure the hydrostatic and shear strain [1]. From the measured strain we calculate the hh and lh splitting and find them to be in reasonable agreement with PR. \\[4pt] [1] M. Montazeri, et. al., Nano Letters 10, 880-886 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L32.00010: Photoreflectance measurements of single wurtzite InP nanowires M. Montazeri, A. Wade, S. Perera, K. Pemasiri, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish We have carried out photoreflectance measurements from a single semiconductor nanowire for the first time to our knowledge. We show that photoreflectance is an easy, quick and nondestructive technique which could be used to study the electronic band structure of a single semiconductor nanowire at both room and low temperatures. We have used photoreflectance to study electronic band structure of single wurtzite InP nanowires at room and low temperatures. Nanowires were grown by MOCVD using 100nm Au-nanoparticle catalysts. Derivative like features in the photoreflectance spectrum around the fundamental gaps allow us to extract energies of 1.50eV, 1.53eV and 1.70eV for A, B and C excitons of wurtzite an InP nanowire at low temperature. These values are compared to values obtained by photoluminescence-excitation and photocurrent measurements. Supported by the NSF ({\#}0701703, {\#}0806700 and {\#}0806572) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L32.00011: Investigation of Electronic Structure in Wurtzite InP Nanowires Saranga Perera, K. Pemasiri, M. Fickenscher, A. Wade, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H. Tan, C. Jagadish We use photoluminescence excitation (PLE), time-resolved photoluminescence (PL), and CW photoluminescence to investigate the electronic structure of wurtzite InP nanowires (NWs) as a function of diameter (30, 50, 100~nm) at 10 K. The NWs were prepared by Au catalyst-assisted MOCVD growth with a 420\r{ }C growth temperature and a V/III ratio of 700. A tunable Titanium-Sapphire laser was used to excite the nanowire sample. PL from the NWs show a dominant defect line near 840nm (1.475eV) that obstructs the view of the free exciton line which should be around 824nm (1.504eV). PLE was performed by measuring the intensity of the defect emission as a function of the excitation laser. The laser was polarized parallel and perpendicular to the nanowire and the PL was collected with circular polarization. PLE spectra show three peaks for the A, B and C hole bands (\textit{APL} \textbf{97}, 023106-2010). Polarization measurements may probe optical selection rules. Support for this work was provided by the NSF (0701703, 0806700 and 0806572) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L32.00012: Photoluminescence in Strain-Engineered Si/SiGe Three Dimensional Nanostructures Nikhil Modi, Leonid Tsybeskov, David Lockwood, Xiao Wu, Jean-Marc Baribeau The effect of strain on the degeneracy of energy band minima in composition-controlled Si/SiGe nanostructures with high germanium content ($\sim $ 50{\%}) is studied by low temperature photoluminescence (PL) spectroscopy, ultra-high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy measurements. PL spectra obtained from selective excitation of the multilayered nanostructures show a reduction in the strained-silicon fundamental energy bandgap and a splitting of energy levels presumably associated with partial removal of two-fold degeneracy of the SiGe valence band. PL kinetics recorded using different excitation wavelengths show dramatically different PL lifetimes, ranging from $\sim $ 2 $\mu $s to $<$ 10 ns. We show that it is possible to obtain high quantum efficiency luminescence at 1.3-1.6 $\mu $m. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L32.00013: State filling dependent tunneling in hybrid InAs/GaAs-InGaAs/GaAs dot-well structures Vitaliy Dorogan, Yuriy Mazur, Euclydes Marega Jr., Mourad Benamara, Georgiy Tarasov, Christoph Lienau, Gregory Salamo A strong dependence of quantum dot (QD) - quantum well (QW) tunnel coupling on the energy band alignment is established in hybrid InAs/GaAs - In$_{x}$Ga$_{1-x}$As/GaAs dot-well structures by changing the QW composition to shift the QW energy through the QD wetting layer (WL) energy. Due to this coupling a rapid carrier transport from the QW to the QD excited states takes place. As a result, the QW photoluminescence (PL) completely quenches at low excitation intensities. The threshold intensities for the appearance of the QW PL strongly depend on the relative position of the QW excitonic energy with respect to the WL ground state and the QD ground state energies. These intensities increase by orders of magnitude as the energy of the QW increases to approach that of the WL due to the increased efficiency for carrier tunneling into the WL states as compared to the less dense QD states below the QW energy. [Preview Abstract] |
Session L33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Lattice Dynamics, Polarons, and Structure
Sponsoring Units: DMP DCOMPChair: Matt Dawber, Stony Brook University
Room: C143/149
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L33.00001: Ultrafast Polaron Dynamics in Multiferroic LuFe$_{2}$O$_{4}$* R.P. Prasankumar, J. Lee, D. Talbayev, A.J. Taylor, C.L. Zhang, S.-W. Cheong, X.S. Xu The multiferroic material LuFe$_{2}$O$_{4}$ has attracted much recent attention due to its strong magnetoelectric coupling. We used ultrafast optical spectroscopy to examine LuFe$_{2}$O$_{4}$ by employing a 1.55 eV probe pulse to examine the Fe$^{2+}->$Fe$^{3+}$ polaronic excitation as a function of temperature and pump photon energy. After 1.55 eV excitation, the photoinduced reflectivity change $\Delta $R/R decreases within $\sim $1 picosecond (ps), after which a $\sim $30 ps acoustic phonon oscillation is observed. The initial fast drop in $\Delta $R/R can be explained by Fe$^{2+}->$Fe$^{3+}$ polaron hopping, and the subsequent rapid recovery is due to polaron redressing; this is observed at all temperatures. Pumping the Fe$^{2+}$ on-site excitation at 3.1 eV revealed different dynamics. Notably, coupling between the on-site and charge transfer excitations was strongly suppressed above the antiferromagnetic ordering temperature, demonstrating the strong influence of charge and spin order on polaron dynamics. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L33.00002: Amplitude and phase gratings based on spatially modulated densities of optically generated polarons in thermally reduced LiNbO$_3$ Hauke Bruening, Mirco Imlau In thermally reduced, nominally pure LiNbO$_3$ a variety of small polarons can be observed, being responsible for the distinct photochromic properties of this material. In this contribution we use a spatially modulated excitation of polarons for the recording of holographic gratings. These gratings inherit some of the pronounced features of the polarons like a stretched-exponential relaxation behavior with a lifetime in the ms-range. Beside amplitude gratings we also find phase gratings leading to a high diffraction efficiency in some recording and readout geometries. The origin of these phase gratings can't be explained by the classic photorefractive effect due to Fe$_{\mathrm{Li}}$ or other photorefractive dopants. In contrast, our findings are discussed in the frame of a model taking into account a local change of the refractive index by the polaronic distortion of the crystal lattice. Measurements of activation energies also indicate that these gratings can be attributed to the small bound ($\mathrm{Nb}^{4+}_{\mathrm{Li}}$)-polaron. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L33.00003: Hamiltonian approach to Feynman's path-integral polaron treatment J.T. Devreese, S.N. Klimin The Feynman path-integral based all-coupling variational approach for the Fr\"{o}hlich-polaron is re-formulated and extended using the Hamiltonian formalism with time-ordered operator calculus. Special attention is devoted to the excited polaron states. The energy levels and the inverse lifetimes of the excited polaron states are, for the first time, explicitly calculated within this all-coupling approach. The resulting transition energies are compared with the peak positions of the polaron optical conductivity, as recently calculated numerically using diagrammatic quantum Monte Carlo. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L33.00004: Comparison of the Raman low frequency spectra of NBT and KLT Daniel Jackson, Radha Pattnaik, Haosu Luo, Dwight Viehland, Jean Toulouse We present the results of a detailed comparative study of the low frequency central peak in sodium bismuth titanate (Na$_{0.5}$Bi$_{0.5}$TiO$_{3}$ or NBT) and potassium lithium tantalate (K$_{1-x}$ Li$_{x}$TaO$_{3}$ or KLT) from 90 degree angle Raman scattering with a resolution of 1 cm$^{-1}$. The Raman spectra of NBT were obtained over a wide temperature range from 78 to 950 K, spanning the two transitions, from cubic to tetragonal at $\sim $820 K and tetragonal to rhombohedral in the range 480-600 K. In an effort to better understand the nature of these phase transitions in NBT, we performed a detailed analysis of the central peak and soft mode combined, using different models. In particular, we compare the model in which these two features are uncoupled with the model in which they are coupled with a strength parameter, $\delta ^{2}$. These models are also discussed in the more general context of A-site substituted ABO$_{3}$ perovskites. The effects of an external electric field and mechanical pressure on the transitions will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L33.00005: Phonon structures and IR intensities in strained BaTiO3 Aldo Raeliarijaona, Huxiang Fu While soft modes and structural instability in cubic ferroelectrics have been well studied, the vibration properties in structurally stable phases are relatively less understood, however. Here we have carried out first-principle calculations, using density-functional perturbation theory, to determine the phonon structures and IR intensities at gamma point for tetragonal Barium titanate under different in-plane strains, in which lattice parameter ranges from 3.93 {\AA} to 3.80 {\AA}. We find that some modes shift strongly with the inplane strain, while other modes show surprisingly little change. The response of IR intensity is also revealed to be mode-dependent. The microscopic insight for these behaviors is examined. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L33.00006: Neutron Diffuse Scattering Measurements of PZT D. Phelan, P.M. Gehring, J. Rodriguez, C. Stock, X. Long, Y. Xie, Z.-G. Ye Neutron diffuse scattering measurements were performed on a single crystal of PbZr$_{0.675}$Ti$_{0.325}$O$_3$. The data is compared and contrasted with other Pb-based piezoelectrics, such as the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_3$, and is discussed within the context of random fields. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L33.00007: A Neutron Study of the Structure and Lattice Dynamics of Single Crystal PZT Invited Speaker: The outstanding piezoelectric properties of PbZr$_{1-x}$Ti$_{x}$O$_{3}$ (PZT) perovskite ceramics have long been exploited in numerous device applications, making PZT arguably the most technologically important ferroelectric material in use today. Efforts to understand the piezoelectric mechanism have inspired a plethora of structural studies spanning decades, but solving the PZT phase diagram has proven to be famously problematic because single crystals have not been available save for Zr- and Ti-rich compositions that lie very near the end members PbZrO$_{3}$ and PbTiO$_{3}$, where the piezoelectricity is weakest. Thus, whereas PZT has been the subject of thousands of powder and ceramic investigations, no consensus regarding the crystal structures of PZT exists. We report the first neutron diffraction study of single-crystal PZT with compositions x = 0.325 and 0.460 [1]. Our data refute the thesis that the ferroelectric phases of PZT within this composition range, all of which are highly piezoelectric, are purely monoclinic (Cc or Cm). The broadening of certain Bragg peaks can be interpreted in terms of coexisting rhombohedral and monoclinic domains, whereby monoclinic order is enhanced by Ti-doping. This is consistent with the theoretical proposal that the tendency to form macroscopic monoclinic phases facilitates the mechanism of polarization rotation by reducing the energy required to reorient the electric polarization. Dispersions of the lowest energy TO and TA phonon modes were measured on a single crystal of PZT with x = 0.325 in the paraelectric phase at 650 K [2]. The TO mode energy drops at small wave-vectors suggesting that it is a soft mode associated with the ferroelectric phase transition at 590 K. Evidence of a second soft-mode, corresponding to a phase transition at 370 K at the R-point, is provided based on the redistribution of spectral weight as a function of temperature. \\[4pt] [1] D. Phelan \textit{et al}., Phys. Rev. Lett. \textbf{105}, 207601 (2010). \\[0pt] [2] D. Phelan \textit{et al}., submitted. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L33.00008: Correlations between tetragonality, polarization, and ionic displacement in lead titanate-derived ferroelectric perovskite solid solutions Andrew Rappe, Tingting Qi, Ilya Grinberg We use first-principles density functional theory calculations to investigate the dependence of tetragonality on local structure in a variety of ferroelectric solid solutions. We demonstrate that tetragonality is strongly coupled to the $B$-cation displacement and weakly coupled to the $A$-cation displacement. Examination of various Bi$M^{3+}$O$_3$ additives to PbTiO$_3$ for different $M^{3+}$ ionic sizes reveals that substitution of either small $B$-cations or low doping of large $B$-cations gives rise to large spontaneous polarization and tetragonality. Understanding how the phase transition temperature ($T_c$) and tetragonality are affected by Pb- and Bi-based perovskite additives provides a rational path for designing new high-temperature piezoelectric materials. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L33.00009: First principles prediction of a morphotropic phase boundary in the Bi(Zn$_{1/2}$Ti$_{1/2}$)O$_3$-(Bi$_{1/2}$Sr$_{1/2}$)(Zn$_{1/2}$Nb$_{1/2}$)O$_3$ alloy Valentino R. Cooper, Asegun S. Henry, Shigeyuki Takagi, David J. Singh We present a density functional theory study on alloys of the tetragonally distorted Bi(Zn$_{1/2}$Ti$_{1/2}$)O$_3$ (BZT) and the rhombohedrally oriented (Bi$_{1/2}$Sr$_{1/2})($Zn$_{1/2} $Nb$_{1/2})$O$_3$ (BSZN). We find that compositions with $\ge$ 50\% BZT are tetragonally distorted with the polarization pointing mainly along the [001] direction. Conversely, for low concentrations of BZT the polarization is rhombohedrally oriented. Based on these results we propose a phase diagram with a possible monoclinc phase between 25\% and 50\% BZT where this material may have a useful piezoelectric response. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L33.00010: Probing Ferroelectricity in Thin-film Perovskite SnTiO$_3$ with First-principles Structural Instability Analysis William Parker, James Rondinelli, Serge Nakhmanson Perovskite SnTiO$_3$ has been recently identified in a number of computational studies as an attractive, environmentally-friendly material with potential to replace ferroelectric PbTiO$_3$. However, additional computational evidence suggests that bulk perovskite SnTiO$_3$ may be metastable. Using density-functional theory and phonon-band instability analysis, we investigate avenues for epitaxial stabilization of ferroelectric perovskite SnTiO$_3$ thin films with applied strains of up to $\pm$ 2\%. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L33.00011: Room temperature metastable monoclinic phase in BaTiO$_{3}$ crystals Tom Lummen, Jianjun Wang, Martin Holt, Amit Kumar, Eftihia Vlahos, Sava Denev, Long-Qing Chen, Venkatraman Gopalan Low-symmetry monoclinic phases in ferroelectric materials are of considerable interest, due to their associated enhanced electromechanical coupling. Such phases have been found in Pb-based perovskite solid solutions such as lead zirconate titanate (PZT), where they form structural bridges between the rhombohedral and tetragonal ground states in compositional space. In this work, we directly image such a monoclinic phase in BaTiO$_{3}$ crystals at room-temperature, using optical second harmonic generation, Raman, and X-ray microscopic imaging techniques. Phase-field modeling indicates that ferroelectric domain microstructures in BaTiO$_{3}$ induce local inhomogeneous stresses in the crystals, which can effectively trap the transient intermediate monoclinic structure that occurs across the thermal orthorhombic-tetragonal phase boundary. The induced metastable monoclinic domains are ferroelectrically soft, being easily moved by electric fields as low as 0.5 kV cm$^{-1}$. Stabilizing such intermediate low-symmetry phases could very well lead to Pb-free materials with enhanced piezoelectric properties. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L33.00012: Infrared spectroscopy of KDP under high pressure Ana Akrap, Christopher C. Homes, Ricardo P.S.M. Lobo, Patrick Simon We have determined infrared reflectivity of potassium dihydrogen
phosphate (KDP) in
the paraelectric ($T>T\rm_c=$135 K) and ferroelectric phase
($T |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L33.00013: Elastic Anizotropy and Domain Stability in Ferroelectric Thin Films and Problem of Critical Thickness for Memory Alexander Bratkovsky, A.P. Levanyuk The most important effect of the depolarizing field in thin ferroelectric (FE) capacitors is the emergence of domains in place of monodomain state desirable for memory applications, and it depends on the parameters of system Ferroelectric- Electrode. We have studied FE films of BaTiO3, PbTiO3, and Pb (Zr)0.5(Ti)0.5O3 with SrRuO3 electrodes on SrTiO3 (100) substrate. Due to lattice misfit, the FE film becomes tetragonal with the polar axis perpendicular to the film. We have studied rarely addressed topic of relation between the equilibrium domain structure and limits of absolute stability of the monodomain state. We have found that in films with thickness close to the minimal one compatible with FE the stripe domains form with domain walls along the cubic axes in BTO and PTO films, while in PZT it is at 45 degrees to the axes. The orientational dependence of their energy is actually very weak, less than 1\% is all above systems. The threshold of absolute instability of the monodomain state is shifted by electrostriction most significantly in BTO and PTO, where it gets close to the (formal) ``critical thickness'' for FE in monodomain films. In PZT, on the other hand, electrostriction hardly affects it [1]. \\[0pt] [1] A.M. Bratkovsky and A.P. Levanyuk, Phil. Mag. 90, 113 (2009); arXiv: 0801.1669; Phys. Rev. Lett. 100, 149701 (2008). [Preview Abstract] |
Session L34: Focus Session: Interfaces in Complex Oxides - Heterointerfaces
Sponsoring Units: DMPChair: Jak Tchakhalian, Univeristy of Arkansas
Room: C141
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L34.00001: Electronic instabilities at paraelectric and superconducting interface: A mean field approach J.T. Haraldsen, A.V. Balatsky We examine the modified electronic states at the interface between superconducting and ferro(para)-electric films. We find that the coupling of a classical fluctuating paraelectric $P$ and superconducting $\psi$ order parameters can significantly modify these orders at the interface. Using a Ginzburg-Landau formalism, we show that linear and quadratic terms of the electric polarization produce instabilities in $\psi$ at the interface, where the linear interaction produces a modulation of the order parameters and create an interface-induced ferroelectric polarization within the paraelectric bulk state. We will discuss implications of this work for the experiments on the epitaxial oxide films. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L34.00002: Effect of Cu magnetism on superconductivity at YBa$_{2}$Cu$_{3}$O$_{7}$ / La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$ interfaces J. Tornos, C. Visani, J. Garcia-Barriocanal, C. Leon, N.M. Nemes, J. Santamaria, M. Garcia-Hernandez, Yaohua Liu, A. Hoffmann, S.G.E. te Velthuis, J. Freeland, M. Varela, S.J. Pennycook The induced magnetism at the Cu edge of cuprate manganites interfaces has been proposed to depend on interface termination. We have prepared YBa$_{2}$Cu$_{3}$O$_{7}$ / La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$trilayers showing Cu magnetism at both cuprate interfaces as evidenced from an XMCD experiment. This result results from the same termination occurring at both interfaces. The effect of Cu magnetism on superconductivity depression proposed by J. Salafrance and S. Okamoto is discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L34.00003: Quantum Phase Transitions in Ultrathin YBCO/LCMO Superlattices Benjamin Gray, M. Kareev, E.J. Moon, J. Liu, I-C. Tung, M.J. Bedzyk, M. Veenendaal, J.W. Freeland, J. Chakhalian The rational design of complex oxide heterostructures enables the investigation of novel materials with antagonistic order parameters. Our previous work has provided insight into the role of orbital reconstruction and covalent bonding at the interface of such heterostructures. In this talk, we will further address the intriguing interfacial properties and possible coupling between layers in superlattices composed of alternating superconductive YBa2Cu3O7-x and ferromagnetic La2/3Ca1/3MnO3 layers upon approaching the ultra-thin limit. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L34.00004: Unconventional proximity effect and inverse spin-switch behavior in a model manganite/cuprate/manganite trilayer system Juan Salafranca, Satoshi Okamoto The proximity effect in a model manganite/cuprate system is investigated theoretically. We consider a situation in which spin-polarized electrons in manganite layers antiferromagnetically couple with electrons in cuprate layers as observed experimentally. The effect of the interfacial magnetic coupling is found to be much stronger than the injection of spin-polarized electrons into the cuprate region. As a result, the superconducting transition temperature depends on the thickness of cuprate layer significantly. Since the magnetic coupling creates {\em anti}-spin-polarization, an applied magnetic field and the {\em anti}-polarization compete resulting in the inverse spin-switch behavior where superconducting transition temperature is increased by applying a magnetic field. This work was supported by the NSF Grant DMR-0706020 (J.S.) and by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, the US DOE (S.O.). [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L34.00005: Magnetoelectric Coupling in P(VDF-TRFE)/LCMO Heterojunctions Anil Kumar, Evgeny Kirianov, Vasily Moshnyaga, Pankaj Sharma, Alexei Gruverman, Andrei Sokolov Engineered magnetoelectric heterojunctions have recently attracted significant interest due to the possibility to control magnetic properties by external electric fields. Doped lanthanum manganites are attractive candidates to use as a part of such junctions because of their strong coupling between charge, spin and lattice effects. On the other side the use of a ferroelectric (FE) as a gate electrode has dual benefits: it offers the possibility to design a non-volatile data storage device and provide large charge density change at the interface. The ferroelectric polymer, polyvinylidene fluoride (PVDF), is an interesting candidate due to its outstanding electromechanical, dielectric, and mechanical properties. Here we present results of our transport studies of La.$_{67}$Ca.$_{33}$MnO$_{3}$ /P(VDF-TrFE) heterojunction. Manganite thin films were grown by MAD technique, followed by Langmuir-Blodgett deposition of ferroelectric polymer. Results are explained by electron accumulation induced metal-insulator transition in the LCMO layer. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L34.00006: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L34.00007: Emergent phenomena at the heterointerface of multiferroic BiFeO$_{3}$ and ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Invited Speaker: Novel phenomena and functionalities at the heteroepitaxial complex oxide heterostructures have been attracting much scientific attention from the fundamental physics as well as the technological applications. Essentially, the charge and spin reconstruction at the interface could lead to exotic, totally unexpected state of matters at the interface, such as conductive interface between insulating materials and interfacial ferromagnetism at the proximity of antiferromagnet. In this talk, I will present a systematic study of the electronic (charge) and magnetic (spin) interactions in an all-oxide model heterostructure system consisting of the ferromagnet (FM) La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and the multiferroic (ferroelectric (FE) and antiferromagnetic (AFM)) BiFeO$_{3}$(BFO). The study has demonstrated the existence of magnetic coupling at this interface, manifested in the form of an enhanced coercive field as well as an exchange bias. Using x-ray magnetic circular dichroism, the origin of the significant exchange bias has been attributed to a novel ferromagnetic state in the antiferromagnetic BFO sublattice at the interface with LSMO. Based on this finding, the electrical control of magnetic coupling has been explored in the field effect geometry. The magneto-transport measurement clearly demonstrates a reversible switch/control between two distinct exchange bias states by isothermally switching the FE polarization of BFO. This is an important step towards controlling magnetization with electric fields, which may enable a new class of electrically controllable spintronic devices and provide a new basis for producing electrically controllable spin polarized currents. Finally, at the end of the talk, a generic interpretation will be proposed for the understanding of magnetoelectric coupling in the current model system. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L34.00008: Modified Magnetotransport in Digital Manganite Superlattices Brittany Nelson-Cheeseman, Tiffany Santos, Sam Bader, Anand Bhattacharya We investigate how the series of manganite superlattices, (LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$, responds to an out-of-plane magnetic field in order to better understand how the magnetism and magnetotransport are modified in a short period superlattice. The n=1 superlattice shows magnetotransport and magnetic hysteresis similar to the random alloy with positive low field magnetoresistance (MR) due to anisotropic magnetoresistance (AMR) and negative MR at high fields due to colossal magnetoresistance (CMR). However, the n=2 superlattice behaves differently with large negative MR at high fields, and no positive low field MR for T$<$100K. The lack of positive low field MR has also been seen in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ films with perpendicular magnetic anisotropy, suggesting that the n=2 sample moments contain an out-of-plane, canted or frustrated component at low fields. The reemergence of positive low field MR for the n=2 sample above 100K indicates that the driving force for the different moment orientation is strongly temperature dependent. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L34.00009: La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Epitaxial Films on SrTiO$_{3}$(001): Interface Effects \& Electronic Distribution J.-S. Lee, D.A. Arena, C.-C. Kao, P. Yu, R. Ramesh La$_{1-x}$Sr$_{x}$MnO$_{3}$ is an attractive material for incorporation into spin-dependent electronic devices and optimally doped La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) is among the most widely studied colossal magneto-resistance materials. Using a combination of soft x-ray absorption spectroscopy and hard x-ray reflectivity, we found that epitaxial films of LSMO grown on STO(001) substrates exhibit an inhomogeneous 3d electron-distribution along surface normal direction, divided between an intermediate layer (enriched in Mn$^{3+}$) and a nominal mixed-valence layer (Mn$^{3+}$ \& Mn$^{4+}$) of LSMO. This electronic redistribution near the interface is in turn correlated with an unusual remanent magnetic state. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L34.00010: La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Epitaxial Films on SrTiO$_{3}$(001): Interface Effects \& Magnetic Configuration D.A. Arena, J.-S. Lee, C.S. Nelson, C.-C. Kao, P. Yu, R. Ramesh, R. Fan, C.J. Kinane, S. Langridge Mixed valence manganites, in which a delicate interaction between electronic, orbital, magnetic and structural degrees of freedom produces rich phase diagrams reflecting the competing, nearly-degenerate ground states, have been under intense investigation for decades. We present evidence for an unusual magnetic configuration in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) epitaxial films grown on SrTiO$_{3}$(001) substrates. At low temperatures, the remanent state of the near-surface region in thick LSMO films is aligned anti-parallel to the the applied magnetic field. This unusual magnetic configuration is also correlated with an in-plane structural fluctuation, as measured by x-ray diffraction. We suggest that the unexpected magnetic ordering in these films may also be associated with an orbital reconstruction of the Mn $e_g$ orbitals. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L34.00011: Magnetoelectric coupling at the interface of BiFeO$_3$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ multilayers Maria J. Calderon, R. Yu, S. Liang, J. Salafranca, S. Dong, S. Yunoki, A. Moreo, E. Dagotto, L. Brey Magnetoelectric coupling has recently been demonstrated in a system composed of the ferromagnetic manganite La$_{0.7}$Sr$_ {0.3}$MnO$_3$ (LSMO) and the ferroelectric antiferromagnetic BiFeO$_3$ (BFO) [1,2]. Using a realistic microscopic model we study the effects of the charge redistribution and orbital reconstruction on the LSMO/BFO interface ground state. We find that the BFO interface (ferro)magnetism is affected by the charge density at the interface which, in turn, can be modified by the ferroelectric polarization on BFO. This interface induced magnetoelectric coupling leads to the recently observed electric field controlled exchange bias.\\[4pt] [1] S. M. Wu et al, Nature Materials 9, 756 (2010).\\[0pt] [2] P. Yu et al, Phys. Rev. Lett. 105, 027201 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L34.00012: Modulation doping of double-exchange ferromagnetism in an antiferromagnetic manganite: Theory and Synthesis Anand Bhattacharya, T.S. Santos, B.J. Kirby, Sanjeev Kumar, S.J. May, J.A. Borchers, B.B. Maranville, J. Zarestky, S.G.E. te Velthuis, Jeroen van den Brink In this talk we shall discuss the concepts that underlie modulation doping in the context of manganites, particularly the high bandwidth La$_{1-x}$Sr$_{x}$MnO$_{3}$, and how modulation doped structures are realized using oxide-MBE based techniques. The transport and magnetic properties of modulation doped antiferromagnetic digital superlattices of (LaMnO$_{3})_{1}$/(SrMnO$_{3})_{1}$ will be discussed in the context of theoretical ideas about exchange interactions in these materials going back to the seminal work of de Gennes, and compared to similar structures in other parts of the La$_{1-x}$Sr$_{x}$MnO$_{3}$ phase diagram. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L34.00013: Modulation doping of double-exchange ferromagnetism in an antiferromagnetic manganite: Magnetic Structure T.S. Santos, A. Bhattacharya, S.G.E. te Velthuis, B.J. Kirby, J.A. Borchers, B.B. Maranville, S.J. May, S. Kumar, J. van den Brink, J. Zarestky In his pioneering work, de Gennes described a canted antiferromagnetic (AF) state that arises when mobile carriers are added to an insulating AF manganite. However, attempts to realize this canted AF state have been impeded by phase segregation into mixed F and AF phases for x=0.1-0.2. Using a digital synthesis technique to carry out modulation doping of charge carriers into an AF host near x=0.5, we exploit the competing double-exchange and superexchange interactions to realize the canted AF state predicted by de Gennes. We observed the canted AF state using polarized neutron reflectometry and neutron diffraction using polarized neutrons and polarization analysis. Theoretical consideration using the two-orbital model shows that these additional carriers cause a local enhancement of F double-exchange with respect to AF superexchange, resulting in local canting of the AF spins, where the canting angle depends on the doping level. We observe that the canting angle varies with the spreading of charge near the delta-doped layer. Funded by DOE-BES: Scientific User Facilities Div. \& Div. of Materials Science \& Engineering, and US Dept. of Commerce [Preview Abstract] |
Session L35: Metals: Alloys and Impurities
Sponsoring Units: DCMPChair: Michael Mehl, Naval Research Laboratory
Room: C140
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L35.00001: First Principles Stability and Coherency Strain in Mg3RE (RE=rare earth) D019 Metastable Precipitates in Mg A. Issa, J. Saal, C. Wolverton As the need for strong yet lightweight materials intensifies, magnesium alloys have become increasingly important. Although lightweight, these alloys exhibit low strength, particularly in comparison to aluminum alloys. The potential to greatly strengthen magnesium alloys has driven current research, with a recent focus on strengthening precipitates, particularly involving rare earth (RE) dopants. The morphology of these precipitates dictates their effect on the strength of the alloy, and quantifying the coherency strain between the precipitates and the Mg matrix is key to determining the morphology of the precipitate. The large size of the potential composition space makes a systematic experimental study costly and time consuming. Therefore, we apply density functional theory (DFT) to systematically predict the formation energies and coherency strains of D019 precipitates in Mg-RE systems along several crystallographic directions. In particular, we look for D019 precipitates that favorably form plate-shaped morphologies along non-basal planes, as this morphology should be effective obstacles to plastic deformation. These Mg-RE systems also provide an interesting testing ground for the accuracy of DFT methods for intermetallic compounds containing f- electrons. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L35.00002: Chemical Correlations in Atomic Size-Mismatch Disordered Alloys Predicted from KKR-DCA D.A. Biava, D.D. Johnson The dynamical cluster approximation (DCA) has been implemented in a Korringa-Kohn-Rostoker (KKR) electronic-structure method to predict electronic and structural properties of disordered alloys, in particular, chemical short-range order (SRO). We adapted an optimal-basis method\footnote{A. Alam and D.D. Johnson, Phys. Rev. B 80, 125123 (2009)} to the KKR-DCA to account for variations in atomic size due to different configurations present in size-mismatch alloys. In comparison to experiment, we find excellent agreement for predicted lattice constants and SRO, with origins identified in the electronic structure and affecting mechanical properties at finite temperatures. We also show how coarse-grained symmetry of the DCA can be exploited to reduce memory and computation time, allowing us to perform for the first time self-consistent KKR-DCA calculations with $2^{16}$ or more configurations (and atoms) on a single compute node. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L35.00003: Modified Embedded Atom Method potential for Fe-C system Laalitha Liyanage, Jeff Houze, Seong-Gon Kim, Mark Tschopp, Sungho Kim, Mike Baskes, Mark Horstemeyer A Modified Embedded Atom Method potential for the Fe-C alloy system was developed. Pair parameters were constructed based on the structural and elastic properties of element pairs in the L12 reference structure from ab-initio simulations and then adjusted to reproduce heat of formation and elastic constants of cementite, and the interstitial energies for iron. The single element potential of carbon correctly predicts graphite and diamond as the two minimum energy structures.The potential parameters were optimized using an optimization method combining Latin hypercube sampling of the N-dimensional parameter space and multi-objective optimization. The potential was tested for stability of cementite by molecular dynamic simulation at room temperature. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L35.00004: The unified mechanism of aging effects in both martensite and parent phase for shape-memory alloys: atomic-level simulations J. Deng, X. Ding, T. Suzuki, K. Otsuka, T. Lookman, A. Saxena, J. Sun, X. Ren Most shape-memory alloys (SMAs) subject to the aging effects not only in the martensite phase but also in the parent phase. These aging effects have been attracted much attention as they strongly affect the practical applications of SMAs. So far, the intrinsic mechanism of them has remained controversial due to the difficulty in visualization of what happens in atomic scale. In the present study, by using a combination of molecular dynamics method and Monte-Carlo method [1], we investigate the aging effects in both martensite and parent phase. We successfully reproduced the thermal behaviors of aging effects for SMAs, i.e., the $A_{f}$ temperature increase with aging time in martensite and the $M_{s}$ temperature decrease with aging time in parent phase, which keep good agreement with the experimental observations [2]. In addition, quantitative analysis of the atomic configurations during aging reveals that the aging effects are not associated with a change in the average structure. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L35.00005: Enhanced High Temperature Mechanical Behavior of FeCo-Based Alloys Robert Cammarata, Dezhi Zhang, Chia-Ling Chien FeCo alloys have been used for a variety of soft magnetic material applications, including for use in high temperature engine applications. However, inferior mechanical properties, in particular relatively low creep resistance, can limit their use at elevated temperatures. We have investigated a variety of microstructural engineering approaches to improve the creep resistance without significantly degrading the magnetic properties. Two such approached will be discussed: oxide dispersion strengthening and annealing treatments leading to grain growth and precipitation hardening. We have shown that both of these methods allow for sensitive control of the resulting microstructural evolution. This control in turn allows for substantial improvement in both the room temperature yield strength as well as the high temperature creep resistance. Detailed microstructural characterization as well as tensile and testing results will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L35.00006: Fermi surface of an important nano-sized metastable phase: Al$_{3}$Li Stephen Dugdale, Jude Laverock, Ashraf Alam, Mina Roussenova, Joanne Wensley, Jadwiga Kwiatkowska, Nobu Shiotani Nanoscale particles embedded in a metallic matrix are of considerable interest as a route towards identifying and tailoring material properties. In particular, Al-Li alloys, which form ordered nanoscale precipitates of $L1_2$ Al$_3$Li for a range of Li concentrations, have been deployed successfully in the aerospace industry owing principally to their superior strength-to-weight ratio. These precipitates, however, are metastable and only form within the surrounding Al matrix, meaning their electronic structure, thought to be important in contributing to the enhanced material properties through its Young's modulus, has so far been inaccessible through conventional techniques. Here, we take advantage of the strong positron affinity of Li to directly probe the Fermi surface of metastable Al$_3$Li nanoscale precipitates of Al-Li. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L35.00007: Metastable states along the Bain path in AgZr with AFLOW Michael Mehl, Gus Hart, Michal Jahnatek, Stefano Curtarolo AgZr crystallizes in the B11 structure, which is bcc-like with stacking AABB along [001]. Using AFLOW\footnote{S. Curtarolo {\em et al.}, http://materials.duke.edu/aflow.html} we find another low energy structure, ``Z2'' ,\footnote{Z.W. Lu {\em et al.}, {\em Phys. Rev. B} {\bf 44}, 512 (1991)} an fcc-like variant of B11. The B11 to Z2 transition follows the Bain path, with c/a changing from 1.9 (B11) to 2.6 (Z2). This seems similar to results for elemental bcc solids,\footnote{M. J. Mehl {\em et al.}, {\em Phys. Rev. B} {\bf 70}, 014105 (2004)} where we find a secondary Bain path minimum which is elastically unstable. Here there is no simple path from the Z2 structure back to the B11 structure, and the Z2 structure is metastable. Using first- principles DFT we demonstrate the possibility of a pressure induced phase transition from B11 to Z2 at 35~GPa. We also examine the L1$_0$ structure, which is higher in energy than Z2 at zero pressure. We find that a transition from B11 to L1$_0$ at 32~GPa, so that L1$_0$ is the true high-pressure phase of AgZr. We discuss the stability of all three of these phases at both zero and high pressure, and the possibility of similar transitions in more useful materials. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L35.00008: Localized Rattling of Al atoms in VAl$_{10+\delta}$ Douglas Safarik, Tomasz Klimczuk, Anna Llobet, Darrin Byler, Ekhard Salje We have studied the localized rattling mode in the \lq{Einstein solid}\rq~VAl$_{10+\delta}$ using a suite of thermodynamic, transport, and neutron diffraction measurements. The rattling originates from Al atoms that occupy the large void within Z$_{16}$ Friaf polyhedra, of which there are eight per unit cell in the VAl$_{10+\delta}$ structure. Our heat capacity, thermal expansion, and electrical resistivity data are all qualitatively consistent with a low-frequency harmonic vibration of the atom. However, our neutron diffraction data show that the rattling atom potential is better described as sixth-order, rather than harmonic. Using a single-site, sixth-order effective potential for the rattling atom, we can explain our thermodynamic, transport, and structural data, including the unusual temperature dependence of the elastic constants. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L35.00009: High-energy diffraction measurements of deeply undercooled Co-Pd liquids using electrostatic levitation G.E. Rustan, N.A. Mauro, J.C. Bendert, K.F. Kelton, A. Kreyssig, A.I. Goldman Co-Pd liquids in their deeply undercooled state have attracted a great deal of interest because of the potential for magnetically triggered nucleation of the solid phase. We report on the results of high-energy x-ray diffraction measurements, using 129 keV x-rays at the Advanced Photon source, on a series of liquid Co-Pd alloys in a containerless environment employing electrostatic levitation. Diffraction data were collected using a flat-plate two-dimensional detector during free cooling from temperatures well above the liquidus, to as much as 200 degrees C of undercooling for compositions ranging from 50:50 to 80:20 (Co:Pd). The composition dependence and temperature dependence of differences in structure will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L35.00010: Ab initio studies of the effect of nanoclusters on magnetostriction of Fe1-xGax alloys Hui Wang, Yanning Zhang, Teng Yang, Z.D. Zhang, Lizhi Sun, Ruqian Wu We investigated the effects of B2-like and D0-like nanoclusters on the magnetostriction of Fe-xGax alloys, through systematic density functional calculations. While the B2-like Fe-Ga clusters still undergo slightly tetragonal distortion, D03-like Fe-Ga clusters remain to be cubic in the Fe matrix. Moreover, we found that B2-like nanostructures produce negative magnetostriction whereas D03- like nanostructures give small positive magnetostriction in Fe-xGax alloys. The formation of nanoscale precipitates cannot be the reason for the extraordinary enhancement of magnetostriction of Fe1-xGax alloys. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L35.00011: The lost heat capacity and entropy in the helical magnet MnSi Sergei Stishov, Alla Petrova, Anatoly Shikov, Thomas Lograsso, Eyvaz Isaev, Borie Johansson, Luke Daemen We report results of measurements and analysis of the heat capacity of MnSi. The measurements included data collection at a magnetic field of 4T, which suppresses strongly the longitudinal spin fluctuations and the phase transition. To analyze the experimental data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. Inelastic neutron scattering with a polycrystalline sample of MnSi was used to validate the computational results. The combination of the experimental and theoretical data turned out to be decisive in revealing some hidden features of the thermal excitations in MnSi. In particular, the analysis of the available data led conclusively to the existence of a negative contribution to the heat capacity and entropy in MnSi at $T>T_c$, implying that a specific spin ordering process did occur in the paramagnet phase of MnSi. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L35.00012: A common magnetic origin for the Invar effects in fcc iron-based ferromagnets Chris Hooley, Francois Liot Using first-principles calculations, in conjunction with Ising magnetism, we undertake a theoretical study to elucidate the origin of the experimentally observed Invar effects in disordered fcc iron-based ferromagnets. First, we show that our theory can account for the Invar effects in iron-nickel alloys, the anomalies being driven by the magnetic contributions to the average free energies. Second, we present evidence indicating that the relationship between thermal expansion and magnetism is essentially the same in all the studied alloys, including those which display the Invar effect and those which do not. Hence we propose that magnetism plays a crucial role in determining whether a system exhibits normal thermal expansion, the Invar effect, or something else. The crucial determining factor is the rate at which the relative orientation of the local magnetic moments of nearest-neighbor iron atoms fluctuates as the system is heated. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L35.00013: Electronic Origin of Fast Sulfur Diffusion in 3d Transition Metals Dmitri Novikov, Alan Cetel, Michael Maloney, Kevin Schlichting, Brad Cowles, Sergey Okatov, Iliya Lomayev, Yuri Gornostyrev, Segei Burlatsky The microscopic origins of abnormally fast diffusion of sulfur in nickel have been investigated. Transition state theory of vacancy mediated diffusion of substitutional impurities with parameters calculated from first-principle density-functional theory (DFT) was used to determinate of the diffusion coefficients of S and Al impurities in fcc Ni. Sulfur diffusion coefficient was found to be two orders of magnitude higher than for aluminum in good agreement with experimental data. We found that sulfur has a very low barrier for jump toward vacancy and also significantly decreases migration barriers for neighboring nickel atoms. We discuss the microscopic factors contributing to the dramatic difference in S and Al diffusion coefficients and show that electronic structure and chemical bonding play crucial role in enhanced diffusion of S. We also found that S considerably increases Ni self-diffusion rate. The implications of S effect on the stability of thermally grown oxides in superalloys are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L35.00014: Nitrogen Adsorption, Solubility and Transport within Group V Metals Panithita Rochana, Ekin Ozdogan, Jennifer Wilcox It is well known that Group V metals have strong-binding characteristics to diatomic molecule, e.g. N$_{2}$, O$_{2}$, H$_{2}$, and CO. Within this study, N$_{2}$ has been investigated to determine the mechanism of surface adsorption, dissociation and subsequent atomic diffusion into the bulk crystal structure of vanadium(V). Alloys of ruthenium(Ru)-V have been examined indicating that Ru can be used to tune the electronic structure of the bulk to enhance atomic diffusion. Electronic structure calculations based on density functional theory have been studied on the investigation of N$_{2}$ adsorption on 3 low-index surfaces, (110), (100) and (111). Preliminary investigations indicate that the V(111) surface binds N$_{2}$ the strongest at fcc site (E$_{ads}$ = 1.4eV). To determine bulk solubility, binding energy calculations are carried out as a function of N concentration. N was found to be stable primarily at O-sites within the bulk V lattice. Bader charge and density of states analyses are analyzed to investigate the mechanism of bulk absorption and solubility phenomena. Results will be presented on the adsorption, bulk solubility, and transport of N in V and V-based alloys. The application to this study is toward the design of an N$_{2}$-selective dense membrane in which atomic N may be produced on the permeate side with hydrogen as a sweep gas for the ammonia synthesis process. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L35.00015: The wake of H in V, Nb and Ta at elevated temperatures: Irreversibility and non-central forces revisited Franz Reidinger At elevated temperatures U and Do of the Arrhenius equation for diffusion describe the amplitude and relaxation rate, respectively, of the stern wave wake of H in V, Nb and Ta. The key evidence for this hypothesis is the close correlation between the isotope dependence of U derived from the Gorsky measurements$^{1}$ and the shear distortion of the orthorhombic phases of NbH(D) and TaH(D). The isotope dependence of U can be expressed in closed form: U=a$\sqrt{M}$ + b$\sqrt{m}$ where M and m are the atomic numbers of the host metal and H isotope and a and b are 7.4 and 37 for Nb and Ta, and 0 and 55 for V, respectively, in units of meV. I explain this correlation in two steps: a) the cubic symmetry of the nearest neighbor strain field$^{2}$ of the interstitial H is the result of a dynamic superposition, possibly caused by a JT resonance$^{3}$, of the two orthorhombic variants of $\beta $-NbH0.75 and b) the successful characterization of the diffusion process as jump diffusion$^{4}$ eliminates the transition state from consideration. Instead it is the relaxation of the just emptied site from its residual orthorhombic distortion towards the cubic symmetry of the bcc metal which is being measured. 1)Z Qi, J Voelkl, R Laesser and H Wenzl: J. Phys. F 13, 2053 (1983) 2)G Bauer, E Seitz, W Schmatz and H Horner: Sol. State Comm. 17, 161 (1975) 3)G C Abell: J. Phys. F 12, 1143 (1982) 4) V Lottner, A Heim and T Springer: Z. Physik B 32, 157 (1979). [Preview Abstract] |
Session L36: Focus Session: Graphene Structure, Dopants and Defects: Adsorbates
Sponsoring Units: DMPChair: Dmitry Abanin, Princeton University
Room: C142
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L36.00001: First-principles calculations of gated adatoms on graphene Kevin T. Chan, Hoonkyung Lee, Marvin L. Cohen The two-dimensional surface of graphene is well-suited for adsorption of adatoms or molecules. The application of a gate voltage can be used to precisely control the electron concentration of the adsorbate-graphene system. Such control over electronic properties of adsorbates on graphene might have useful applications in areas such as catalysis and hydrogen storage. In this work, the gating of a variety of adatoms adsorbed on graphene is studied using first-principles calculations. We compute the projected density of states, local electrostatic potential, and charge density of the adatom-graphene system as a function of gate voltage. We demonstrate that adatoms on graphene can be ionized by gating, and that the ionization causes a sharp change in the electrostatic potential. Additional interesting features of our results are also discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L36.00002: High-resolution measurement of SiO2 surface potential using scanning Kelvin-probe microscopy William Cullen, Kristen Burson, Mahito Yamamoto, Michael Fuhrer It is now widely recognized that the dominant contribution to disorder in SiO$_2$-supported graphene is due to scattering from charged impurities. These charged impurities give rise to a conductivity which is linear in carrier density, and create electron-hole puddles in graphene. The screened potential variation produced in graphene has been imaged using scanning tunneling microscopy/spectroscopy (STM/STS) by spatially mapping the variation in the Dirac point, revealing a length scale of ~20 nm for the charge puddles. However, there is a substantial gap in resolution between the STM measurements and previous measurements with much greater potential sensitivity but limited spatial resolution. Here we attempt to bridge this gap using scanning Kelvin-probe microscopy (SKPM) of SiO$_2$ in ultrahigh vacuum. Our measurement takes advantage of the high spatial resolution allowed by UHV non-contact AFM while maintaining UHV control of the sample environment.\\[4pt] [1] Y. Zhang et al., Nature Physics 5, 722 (2009)\\[0pt] [2] J. Martin et al., Nature Physics 4, 144 (2008) [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:30PM |
L36.00003: Revealing the dominant scatterer in Graphene on SiO$_{2}$ Invited Speaker: Freely suspended graphene sheets display high-field effect mobility, reaching 2$\times $10$^{5}$ cm$^{2}$ /V s. Yet, suspended graphene sheets are fragile and impractical for most experiments and applications. Graphene sheets on SiO$_{2}$ are easier to handle but possess low-carrier mobilities, which can even vary by an order of magnitude from sample to sample. Poor and unpredictable transport properties reduce the utility of SiO$_{2}$-bound graphene sheets for both fundamental and applied sciences. Therefore, understanding the impact of substrates is crucial for graphene science and technology. We [1] have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. The saturation coverages of atomic hydrogen for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach 1.5$\times $10$^{4}$ cm$^{2}$ /V s in the absence of the hydrogen-adsorbing sites. The affinity to atomic hydrogen is the signature of the most dominant type of native scatterers in graphene-based field-effect transistors on SiO$_{2}$. The dominant scatterer is identified by comparing the reactivity of charge puddles, ripples and resonant scatterers to atomic hydrogen. \\[4pt] [1] J. Katoch, J.H. Chen, R. Tsuchikawa, C. W. Smith, E.R. Mucciolo, and M. Ishigami, Uncovering the dominant scatter in graphene sheets on SiO2, Physical review B. Rapid Communications, 82, 081417 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L36.00004: Magnetic field effects on the local electronic structure near a single impurity in Graphene Ling Yang, Jian-Xin Zhu, Shan-Wen Tsai Impurities in graphene can have a significant effect on the local electronic structure of graphene when the Fermi level is near the Dirac point. We study the problem of an isolated impurity in a single layer graphene in the presence of a perpendicular magnetic field. We use a linearization approximation for the energy dispersion and employ a T-matrix formalism to calculate the Green's function. We investigate the effect of an external magnetic field on the Friedel oscillations and impurity-induced resonant states. Different types of impurities, such as vacancies, substitutional impurities, and adatoms, are also considered. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L36.00005: Tunable magnetoresistance behavior in suspended graphitic multilayers through ion implantation Carlos Diaz-Pinto, Xuemei Wang, Sungbae Lee, Viktor Hadjiev, Debtanu De, Wei-Kan Chu, Haibing Peng A linear positive magnetoresistance (MR) is often observed in graphitic multilayers, yet its origin remains inconclusive. Recently, a non-Markovian transport theory predicts a strong positive MR in two dimensional systems under the influence of both short- and long-range disorders, while a negative MR is expected with only one type of disorder. Here, we address the role of disorders on the MR behavior of suspended graphitic multilayers through ion implantation. Boron implantation is found to drastically change the MR behavior: the linear positive MR transforms into a negative MR after the introduction of short-range disorders (boron), in consistence with the non-Markovian theory. This suggests that the origin of the unexplained linear positive MR in graphitic structures is the non-Markovian transport under the interplay between long-range disorders (charged surface adsorbents) and short-range disorders (defects inside the lattice). After ion implantation, short-range disorders dominate, leading to a distinct negative MR behavior. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L36.00006: Raman spectroscopic study of chemically-doped few layer graphenes PingHeng Tan, WeiJie Zhao, Jun Zhang, Jian Liu Graphene, the latest carbon allotrope discovered at 2004, has attracted intensively scientific interest owing to its distinctive properties. Chemical doping is expected to substantially increase the density of free charge carriers by charge transfer and to modify the Fermi level of doped materials. Here, we investigated charge transfer and optical phonon mixing in few layer graphenes in detail by utilizing sulfuric acid as an electron-acceptor dopant. Sulfuric acid molecules are found to be only physically adsorbed on the surface layers of graphenes and no intercalation happens. The top and bottom layers of bilayer graphenes can be intentionally doped differently by concentrated sulfuric acid. The difference of hole doping between the top and bottom layers results in phonon mixing of symmetric and antisymmetric modes in bilayer graphenes. The Raman frequency evolution with the doping level qualitatively agrees with recent ab initio theoretical calculations. Sulfuric acid molecules can be expected as a stable electron-acceptor dopant for graphenes to study the physical properties of few layer graphenes at different doping levels. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L36.00007: The doping mechanism in graphene Razvan A. Nistor, Dennis M. Newns, Glenn J. Martyna Doping graphene by adsorbing chemical species on its surface is one way to control the carrier concentration of this novel material. Using large-scale {\it ab initio} simulations and electronic structure calculations, we show the carbon layer acts as a metal catalyst facilitating the disproportionation reaction of adsorbed chemical species on its surface. This reaction leads to the formation of charge transfer complexes which thereby dope the graphene. We also investigate the charge transfer in graphene-silicon and defected graphene-silicon-oxide interfaces. Our microscopic understanding of the doping mechanism in graphene, which brings to light the catalytic power of the material, is important in the development of carbon-based electronics. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L36.00008: Understanding Graphene Coatings: Characterization of Solvent Exfoliated Few-Layer Graphene by Raman Scattering Jorge Camacho, Lester Lampert, Willson Arifin, Robby Flaig, Timothy Rue, Tyler Krisko, James Hamilton Graphene has unique properties like its ballistic transport at room temperature combined with chemical and mechanical stability and these properties can be extended to few-layer of graphene. Potential large-area applications that include transparent conductive coatings and fuel cell electrodes require dispersing graphene in a fluid phase. Graphene nano-platelets can be synthesized by dispersion and exfoliation of graphite in organic solvents such N-methyl-pyrrolidine (NMP) and cyclohexylpyrrolidone (CHP). However, liquid-phase exfoliation produces graphene with defects that can disrupt the electronic properties. One of the remaining questions is whether the defects created during synthesis can be minimized. We report a Raman spectroscopic study showing that defects in few-layer graphene produced by liquid-phase exfoliation of graphite can be controlled by the type or mixture of solvents used. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L36.00009: First Principles Study of Interactions between Dopant Atoms in Graphene Nabil Al-Aqtash, Igor Vasiliev We study the interactions between the boron (B) and nitrogen (N) dopant atoms in graphene. Our calculations are carried out using density functional theory combined with the generalized gradient approximation for the exchange-correlation functional. The total energies, equilibrium geometries, electronic charge distributions, and densities of states of doped graphene sheets are examined in cases of B-B, N-N, and B-N co-doped graphene. The interaction energy between the two dopant atoms is found to be inversely proportional to the square of the separation distance. We find the B-B and N-N interactions to be repulsive and the B-N interaction to be attractive. The changes in the density of states observed in B- and N-doped graphene are explained in terms of electronic charge transfer. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L36.00010: Bonding and charge transfer induced by metal adatom adsorption on graphene Xiaojie Liu, C.Z. Wang, M. Hupalo, Y.X. Yao, M.C. Tringides, Wen-Cai Lu, K.M. Ho Structures and adsorption energies of alkali, simple, transition as well as rare earth metal adatoms on graphene were studied systematically by first-principles calculations. Bonding character and charge transfer between the metal adatoms and the graphene were also analyzed using the quasi-atomic minimal basis set orbitals (QUAMBOs) approach. We showed that the interaction between the alkali metal adatoms and graphene can be characterized as ionic with minimal effects on the lattice and electronic states of the graphene layer. On the other hand, transition metal adsorption exhibits strong covalent bonding and induces large distortion in the lattice and electronic states of the graphene. For trivalent simple metal adatom adsorption, mixed covalent and ionic bonding is observed. Interaction of rare earth adatoms with graphene can be either ionic or covalent depending on the specific elements. Charge redistributions upon the metal adsorptions also induce significant electric dipole moments and changes in the magnetic moments of the adatoms. These results are confirmed by STM studies of the nucleated island density in epitaxial growth experiments. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L36.00011: Transport properties of metalo-organic functionalized graphene Stefan C. Badescu, Victor M. Bermudez, Thomas L. Reinecke Transition metal atoms can act as strong covalent anchors for organic molecules on graphene. The hybridization between the metallic d orbitals and the p orbitals of graphene provides a doping method without breaking C-C bonds. Using first-principle calculations for a range of adsorbed transition metals we identify the induced impurity levels and we reveal a dependence of the spin states on adsorbate coverage. We construct sets of maximally localized Wannier functions that interpolate accurately the calculated bandstructures. These sets are used then to describe the electronic transport from the dilute regime to finite coverages. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L36.00012: The electronic structure and chemical bonding of graphene doped with Group IA and Group VIIA elements: A density functional theory study Yiming Mi, Shuichi Iwata The ground state geometry and the electronic structure of graphene doped with group IA and VIIA elements were calculated with the first principles plane wave pseudopotential approaches within the density functional theory formalism in this paper. In terms of the generalized gradient approximation(GGA), GW approximation and optimizing atomic positions, a reliable geometry of the structure was acquired. The calculated formation energies for different configurations under ambient temperature implied that a new kind of material will produce. The results acquired here allow one to suggest new material with semiconductor or semimetallic behavior by adjusting the relative concentration of the doped atoms carefully. [Preview Abstract] |
Session L37: Focus Session: Graphene Structure, Dopants, and Defects: Magnetism
Sponsoring Units: DMPChair: Roland Kawakami, University of California, Riverside
Room: C146
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L37.00001: Missing atom as a source of carbon magnetism Invited Speaker: Introducing vacancies in graphene-like systems by irradiation has been shown to be an efficient method to vary its mechanical behavior, tune its electronic properties and even to induce magnetism in otherwise non-magnetic samples [1-2]. While the role played by these vacancies as single entities has been extensively addressed by theory [3-6], experimental data available refer to statistical properties of the whole heterogeneous collection of vacancies generated in the irradiation process. Here, by artificially generating isolated vacancies on a graphite surface and measuring their local density of states on the atomic scale, we have shown how single vacancies modify the electronic properties of this graphene-like system [7]. Our scanning tunneling microscopy experiments, complemented by tight binding calculations, reveal the presence of a sharp electronic resonance at the Fermi energy around each single graphite vacancy, which implies a dramatic reduction of the charge carriers' mobility and can be associated with the formation of local magnetic moments. Finally, we have extended our investigations to other graphene systems. \\[4pt] [1] P. Esquinazi, D. Spemann, R. H\"{o}hne, A. Setzer, K.-H. Han and T. Butz, \textit{Phys. Rev. Lett.} \textbf{91}, 227201 (2003).\\[0pt] [2] A. V. Krasheninnikov and F. Banhart, \textit{Nature Materials} \textbf{6}, 723 (2007)\\[0pt] [3] V. M. Pereira, F. Guinea, J. M. Lopes dos Santos, N. M. R. Peres and A. H. Castro Neto, \textit{Phys. Rev. Lett.} \textbf{96}, 036801 (2006)\\[0pt] [4] P. O. Lehtinen, A. S. Foster, Y. C. Ma, A. V. Krasheninnikov and R. M. Nieminen , \textit{Phys. Rev. Lett.} \textbf{93}, 187202 (2004).\\[0pt] [5] J. J. Palacios, J. Fern\'{a}ndez-Rossier and L. Brey, \textit{Phys Rev. B} \textbf{77}, 195428 (2008)\\[0pt] [6]. O. V.Yazyev, \textit{Phys}.\textit{ Rev. Lett.} \textbf{101}, 037203 (2008).\\[0pt] [7] M. M. Ugeda I. Brihuega, F. Guinea and J. M. G\'{o}mez Rodr\'{\i}guez, \textit{Phys. Rev. Lett }\textbf{104}, 096804 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L37.00002: Kondo quantum criticality in graphene Bruno Uchoa, T.G. Rappoport, A.H. Castro Neto Graphene fits in a large class of ``pseudogap'' materials which are allowed to exhibit quantum criticality as a result of the interplay of strong correlations and a vanishing density of states near the Fermi points. In the presence of magnetic impurities, we show there is a symmetry class of localized orbitals which, in combination with quantum interference effects inbuilt in the honeycomb lattice, can lead to a novel class of Kondo quantum criticality in graphene [1]. In this class, graphene effectively screens the local spin as a super-ohmic dissipative environment and the RKKY interaction decays spatially with a fast power-law $\sim 1/R^7$, rather than the standard $1/R^3$ decay expected for Dirac fermions. We also show that unlike metals, the exchange coupling between the localized and itinerant spins can be controlled across the quantum critical region with the application of an external gate voltage. This effect may permit the first experimental observation of quantum criticality in graphene at zero magnetic field, directly with scanning tunneling probes and gating. \\[4pt] [1] B. Uchoa et al., arXiv:1006.2512 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L37.00003: Kondo effect in graphene in the presence of Rashba spin-orbit interaction Mahdi Zarea, Nancy Sandler, Sergio Ulloa We present an exact solution for the Anderson model of a single-orbital magnetic impurity on graphene in the Kondo regime. Different positions for the impurity are considered: on top of a carbon atom, substitutional or interstitial (middle of the hexagon cell). We show that regardless of the impurity position, the effective exchange Hamiltonian always describes a single- channel Kondo problem. The inclusion of the Rashba spin-orbit interaction changes the linear energy dispersion to a quadratic one near the Dirac points with the corresponding change in the density of states. This in turn, modifies the value of the critical Kondo coupling as compared to the case where the spin-orbit is absent. Moreover, spin-orbit interactions, introduce a Dzyaloshinsky-Moriya (DM) term in the Kondo Hamiltonian away from particle-hole symmetry. Although still in the single channel region, the effective exchange coupling is augmented by the DM term and the Kondo temperature shows an exponential increase. Supported by NSF-PIRE and MWN/CIAM [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L37.00004: Transport in irradiated graphene: Kondo and charge fluctuation effects Vivek Aji, Sung-Po Chao Observation of an upturn in resistance at low temperatures in irradiated graphene has renewed the interest in the nature of the Kondo effect in systems with linear density of states. The vanishing density of states near the Dirac point leads to a much wider local moment regime but a cross over to the charge fluctuation at very low carrier densities indeed occurs. In this talk I will show how the Kondo scale and the resistance versus temperatures evolves from one regime to the other, and compare our results with experimental data. Our chief conclusion is that a good agreement with data can be achieved only if one posits that the energy of the impurity level varies linearly with the chemical potential. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L37.00005: Spin and Transport Properties of Doped Graphene Kathleen McCreary, Wei Han, Roland Kawakami Graphene is an ideal system to investigate the interplay of magnetic moments and conduction electrons. Electrostatic gates are able to tune the electron and holes concentrations substantially, and localized magnetic moments can form, in principle, through a variety of methods including vacancies, edges, and adsorbed impurities. Theory predicts a coupling of the localized moments and the conduction electrons, leading to gate tunable indirect coupling between moments which can be ferromagnetic or antiferromagnetic. In this study, we perform magnetotransport measurements on graphene devices where the graphene surface is modified inside an ultrahigh vacuum chamber through a variety of methods including hydrogen adsorption, Ar sputtering, and molecular beam deposition of transition metals [1]. Both /in situ/ and /ex situ/ magnetotransport measurements are performed, where the latter involves the air-free transfer to a low temperature (1.6 - 300 K), high field (7 T) cryostat. We will report results on the temperature-dependent, high-field magnetotransport characteristics of doped graphene. \\[4pt] [1] K. Pi, K. M. McCreary, W. Bao, W. Han, Y. F. Chiang, Y. Li, S.-W. Tsai, C. N. Lau, and R. K. Kawakami, Phys. Rev. B 80, 075406 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L37.00006: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L37.00007: Can Carbon Be Ferromagnetic? Hendrik Ohldag, Pablo Esquinazi, Elke Arenoholz, Daniel Spemann, Martin Rothermel, Annette Setzer, Tilman Butz The existence of long range magnetic order at room temperature in carbon based structures without magnetic elements is very unexpected. Theoretical results from different groups suggest that the existence of long range magnetic order in a graphite structure is possible, if one takes the effects of defects and/or the incorporation of hydrogen atoms into account. SQUID results provided first systematic hints for the existence of magnetic order at room temperature in virgin as well as irradiated highly oriented pyrolytic graphite (HOPG) samples. We present a x-ray dichroism study of graphite surfaces [1] that addresses the origin and magnitude of ferromagnetism in metal-free carbon. Using element specific x-ray microscopy we can show that metallic impurities do not play a role in the ferromagnetism of carbon and that carbon can be ferromagnetic without ferromagnetic impurities. A detailed spectroscopic study shows that in addition to carbon \textit{pi-}states, also hydrogen-mediated electronic states exhibit a net magnetization with magnetic remanence at room temperature. The observed magnetism is restricted to the top \textit{$\sim $}10 nm of the sample where the actual magnetization reaches a value similar to classic ferromagnetic materials like e.g. Nickel. [1] H. Ohldag et al., Phys. Rev. Lett. \textbf{98, }187204 (2007) and submitted to NJP (2010) [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L37.00008: Extinction of ferromagnetism in HOPG by thermal annealing Xiaochang Miao, Arthur Hebard, Sefaattin Tongay, Bill Appleton Observations of ferromagnetism (FM) in highly ordered pyrolytic graphite (HOPG) have generated vigorous research activity to clarify its origin, especially when transition metals are known to be absent. We report that the ferromagnetism of pristine HOPG samples as measured by hysteretic magnetization loops can be diminished and eventually extinguished with sufficiently long high vacuum anneals at temperatures greater than 2000\r{ }C. Concomitant with the extinction of ferromagnetism, we observe an anneal-induced increase in grain size (accompanied by possible edge reconstruction) confirmed by XRD measurement and improved transport properties, including lower in-plane and out-of-plane resistance, higher electron and hole mobility and improved charge compensation. The implied anneal-induced reduction of defects and vacancies suggests that the FM of pristine HOPG is correlated with localized states located at zigzag edges, vacancies and related defects. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L37.00009: Local Moment Formation of an Anderson Impurity on Graphene Chunhua Li, Jian-Xin Zhu, C. S. Ting We study the effect of a magnetic impurity on a single layer of graphene within an Anderson impurity model. Due to the vanishing local density of state at the Fermi level in graphene, the impurity spin cannot be effectively screened out. Treating the problem within the Gutzwiller approximation, we found a region in the parameter space of $U$-$E^f$ where the impurity electron is in the local moment state, which is characterized by a zero effective hybridization between the bath electron and magnetic impurity. Here $U$ is the onsite Coulomb repulsion of the impurity and $E^f$ is its energy level with respect to the Fermi energy. The competition between $U$ and $E^f$ is also discussed. While larger $U$ reduces double occupation and favors local moment formation, a deeper impurity level prefers double occupation and a nonzero hybridization and thus a Kondo screened state. For a fixed $U$, by continuously lowering the impurity level, the impurity first enters from a Kondo screened state to a local moment state and then departs from this state and re-enters into the Kondo screened state. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L37.00010: Spin-dependent scattering from gated potential obstacles in graphene systems Mahmoud Asmar, Sergio Ulloa We study the scattering of Dirac fermions in a sheet of graphene from potential obstacles created by external gates in the presence of both intrinsic and extrinsic Spin-Orbit(SO) interactions [1]. Obtaining an analytical solution in real-space representation for the eigenvectors allows us to calculate the phase shifts generated by a finite-size obstacle in the presence of SO interactions [2]. These states take into account the total angular momentum of the Hamiltonian, which includes spin, pseudo-spin and orbital angular momentum. We find an interesting interplay of both SO interactions, which results in oscillations of the spin-flip cross sections with energy; this also generates a difference between both cross sections for different interaction ranges. These results may open a possibility of obtaining spin-polarized currents that are of importance in the field of spintronics.\\[4pt] [1] C. L. Kane and E. J. Mele, PRL 95, 226801 (2005).\\[0pt] [2] A. H. Castro Neto and F. Guinea, PRL 103, 026804 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L37.00011: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L37.00012: Ferromagnetically coupled local moments along an extended line defect in graphene Carter T. White, Smitha Vasudevan, Daniel Gunlycke Recently an extended line defect was observed composed of octagonal and pentagonal carbon rings embedded in a graphene sheet [Nat. Nanotech. 5, 326 (2010)]. We report results of studies we have made of this defect using both first-principles and semi-empirical methods. Two types of boundary-localized states arising from the defect are identified. The first (second) type has eigenstates with wavefunctions that are anti- symmetric (symmetric) with respect to a mirror plane that is perpendicular to the graphene sheet and passes through the line defect center line. The boundary-localized anti-symmetric states are shown to be intimately connected to the zigzag edge states of semi-infinite graphene. They exhibit little dispersion along the defect line and lie close to the Fermi level giving rise to a spontaneous spin polarization along the defect once electron-electron interactions are included at the level of a mean field approximation to a Hubbard Model. Within this approach, symmetry requires that the principal moments couple ferromagnetically both along and across the line defect leading to approximately 2/3 more up than down spin electrons per defect repeat unit. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L37.00013: Correlating Magnetotransport and Diamagnetism of sp2-Bonded Carbon Networks Through the Metal-Insulator Transition P.M. Vora, J.M. Kikkawa, P. Gopu, M. Rosario-Canales, J.J. Santiago-Aviles, C.R. Perez, Y. Gogotsi Titanium carbide-derived carbons (TiC-CDCs) are porous sp2-bonded networks synthesized by exposing TiC to chlorine gas at an elevated temperature. The latter ``chlorination temperature'' adjusts the size of graphitic domains within this material. We perform magnetoresistance, temperature dependent resistance, and SQUID magnetization measurements on TiC-CDC samples prepared at different chlorination temperatures. Transport reveals a metal-insulator transition where high (low) chlorination temperature samples are on the metallic (insulating) side of the transition. Magnetoresistance measurements are consistent with electronic transport in the weak and strong localization regimes for metallic and insulating samples, respectively. The diamagnetic contribution to the total magnetization increases with chlorination temperature, suggesting that the metal-insulator transition is associated with the expansion of graphitic domains. We also discuss a magnetoresistance anomaly observed in insulating samples. This work supported by NSF DMR-0907266 and NSF MRSEC DMR-05-20020. [Preview Abstract] |
Session L38: Focus Session: Quantum Coherence in Biology II
Sponsoring Units: DCP DBPChair: Andreas Buchleitner, University of Freiburg
Room: A130/131
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L38.00001: The role of quantum coherence in excitonic energy transfer: quantum process tomography, molecular dynamics and efficiency measures Invited Speaker: Long-lived electronic coherences in various photosynthetic complexes at cryogenic and room temperature have generated vigorous efforts both in theory and experiment to understand their origins and explore their potential role to biological function. The ultrafast signals resulting from the experiments that show evidence for these coherences result from many contributions to the molecular polarization. Quantum process tomography (QPT) is a technique whose goal is that of obtaining the time-evolution of all the density matrix elements based on a designed set of experiments with different preparation and measurements. The QPT procedure was conceived in the context of quantum information processing to characterize and understand general quantum evolution of controllable quantum systems, for example while carrying out quantum computational tasks. We introduce our QPT method for ultrafast experiments, and as an illustrative example, apply it to a simulation of a two-chromophore subsystem of the FMO photosynthetic complex, which was recently shown to have long-lived quantum coherences. Our FMO model is constructed using an atomistic approach to extract relevant parameters for the simulation of photosynthetic complexes that consists of a quantum mechanics/molecular mechanics approach combined with molecular dynamics and the use of state-of-the-art quantum master equation approaches. We provide a set of methods that allow for quantifying the role of quantum coherence, dephasing, relaxation and other elementary processes in energy transfer efficiency in photosynthetic complexes, based on the information obtained from the atomistic simulations, or, using QPT, directly from the experiment. The possible presence or absence of effects due to correlated protein motion is discussed. The role of non-Markovianity will be discussed. The ultimate goal of the combination of this diverse set of methodologies is to provide a reliable way of quantifying the role of long-lived quantum coherences and obtain atomistic insight of their causes. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L38.00002: Multidimensional electronic spectroscopy of phycobiliproteins from cryptophyte algae Invited Speaker: We describe new spectroscopic measurements which reveal additional information regarding the observed quantum coherences in proteins extracted from photosynthetic algae. The proteins we investigate are the phycobiliproteins phycoerythrin 545 and phycocyanin 645. Two new avenues have been explored. We describe how changes to the chemical and biological environment impact the quantum coherence present in the 2D electronic correlation spectrum. We also use new multidimensional spectroscopic techniques to reveal insights into the nature of the quantum coherence and the nature of the participating states. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L38.00003: Simulation study of 2D spectrum of molecular aggregates coupled to correlated vibrations Darius Abramavicius, Vytautas Butkus, Leonas Valkunas, Shaul Mukamel Oscillatory dynamics of two-dimensional (2D) spectra of photosynthetic pigment-protein complexes raise the questions of how to disentangle various origins of these oscillations, which may include quantum beats, quantum transport, or molecular vibrations. We study the effects of correlated overdamped fluctuations and under-damped vibrations on the 2D spectra of Fenna-Matthews-Olson (FMO) aggregate, which has well-resolved exciton resonances, and a circular porphyrin aggregate (P6), whose absorption shows vibrational progression. We use a generic exciton Hamiltonian coupled to a bath, characterized by a spectral density. Fluctuations have smooth, while vibtations have $\delta$-type spectral densities. We show how various scenarios of correlated molecular fluctuations lead to some highly oscillatory crosspeaks. Molecular vibrations cause progression of diagonal peaks in the 2D spectrum and make their corresponding cross-peaks highly oscillatory. We, thus, demonstrate that bath fluctuations and molecular vibrations of realistic molecular aggregates are highly entangled in 2D spectroscopy. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L38.00004: Coherent Control of Single Molecules at Room Temp Niek van Hulst, Daan Brinks, Richard Hildner Electronic coherence plays a key role in natural processes like ultrafast energy transfer and charge separation. Coherent control has proven powerful, however in complex biosystems with different conformations and environments, the intrinsic inhomogeneity of the synchronized subset severely limits the achievable degree of control. The ultimate solution to overcome intrinsic inhomogeneities is the investigation of the behavior of one molecule at a time. Here we report the observation and manipulation of vibrational wave-packet interference and electronic coherence in \textit{individual molecules} at ambient conditions. Adapting time and phase distribution of the optical excitation field to the dynamics of each molecule we achieve a superior degree of control. The time-phase maps show distinct diversity between different, yet chemically identical, molecules. We induce Rabi-oscillations and control the coherent superposition state in a single molecule. Broadly distributed coherence decay times are found for different individual molecules giving direct insight into the structural heterogeneity of the local surroundings. Our approach allows single-molecule coherent control in a variety of complex inhomogeneous systems and thus to study the role of coherence in energy transfer of single biocomplexes under natural conditions. D.Brinks\textit{ et al. Nature} \textbf{465}, 905 (2010); R.Hildner\textit{ et al. Nat.Physics} doi:10.1038/nphys1858 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L38.00005: Shaped ultrafast pulses for coherent control of energy flow in light harvesting complexes Mohan Sarovar, K. Birgitta Whaley We report on preliminary investigations of the use of evolutionary algorithms for the design of shaped femtosecond laser pulses to control energy flow in the Fenna-Matthews-Olson (FMO) light harvesting complex. We shape the experimentally accessible phase degrees of freedom of pulses of various duration and assess the ability to control (i) the exciton population on distinct chromophores, and (ii) the purity of the FMO complex state at short times. We assess the experimental feasibility of the designed pulses and sketch directions for future improvement of the pulse design technique. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L38.00006: Towards experimental verifications of the transport mechanisms in light-harvesting dynamics F. Caruso, S. Montangero, T. Calarco, S.F. Huelga, M.B. Plenio Recently, we identified the key mechanisms explaining the very- high efficiency and robustness of excitation energy transfer in bacterial photosynthesis, finding that dephasing noise may remarkably enhance the capability of transmitting energy (classical/quantum information) in light-harvesting systems (in communication complex networks [Caruso et al., PRL 2010]), by opening up additional transport pathways and suppressing the ineffective ones. To verify the relevance of such mechanisms in the actual bio-molecular systems, we propose how to gain control over the light-harvesting dynamics by using quantum optimal control tools. In this way, by means of optimally shaped and `robust' laser pulses, we can: i) faithfully prepare the photosystem in some specific initial state (local site or coherent superposition, e.g. quasi-dark and -bright states), and ii) probe efficiently the dynamics, under realistic experimental conditions, i.e. sample of randomly oriented light-harvesting complexes and extra laser constraints related to an experiment in progress. These results could allow us to more easily discriminate the different transport pathways, to characterize the environmental properties, and so enhance our comprehension of coherent processes in biological complexes. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L38.00007: Photobiomodulation (PBM) Applications in Ophthalmology Robert Dotson In a very real sense, we are all creatures of light. This fact is just now beginning to impact medicine, as quantum theory begins to spread outside the confines of physics and into the life sciences. No longer can living organisms simply be viewed as retorts for biochemical reactions. They also demonstrate an energy component that will prove to be the unifying force of life in all its varied forms. With the advent of this shift in the life sciences, light is becoming an increasingly important diagnostic and therapeutic tool within medicine. Ophthalmologists have long been concerned with light and its application and, consequently, have an interest in the coming scientific revolution, photomedicine. A brief history of the use of low energy light for healing, a review of known mechanisms by which photons interact with living cells, and a review of some of the established cellular effects will be presented. Finally, brief clinical studies will be presented illustrating the benefits of PBM - specifically regarding: corneal healing, glaucoma, and dry age-related macular degeneration. The purpose of this talk is to introduce the emerging field of PBM to the physics community at large. [Preview Abstract] |
Session L39: Focus Session: Single Molecule Biophysics III: Novel Single Molecule Approaches to Biology
Sponsoring Units: DBP DPOLY DCPChair: Ching Hwa Kiang, Rice University
Room: A124/127
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L39.00001: High-resolution laser-based detection for magnetic tweezers Invited Speaker: Magnetic tweezers are a versatile and powerful single-molecule manipulation technique capable of applying force and torque on single bio-molecules. They afford several unique advantages over other single-molecule manipulation techniques such as optical tweezers or atomic force microscopy. The hallmark of magnetic tweezers is the ability to twist bio-molecules without the need for complex optical instrumentation. Perhaps less known but of equal significance, magnetic tweezers rely on a slowly decaying magnetic field gradient (1 mm) to impose force so they are intrinsically configured in a passive force clamp mode. These features make magnetic tweezers particularly well suited for the study of nucleic acid structure, DNA topology, and protein-nucleic acid interactions. The one downside to most magnetic tweezers to date is that they rely on video tracking methods to determine the position of the particle. Despite recent progress, the spatial and temporal resolution and accuracy are fundamentally limited by image tracking techniques. I will describe recent improvements utilizing laser-based detection to overcome these limitations. We implemented back-scattered laser-based detection combined with video image tracking to achieve high-resolution, high-bandwidth, three-dimensional position tracking. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L39.00002: Massively Parallel Single-Molecule Manipulation Using Centrifugal Force Wesley Wong, Ken Halvorsen Precise manipulation of single molecules has led to remarkable insights in physics, chemistry, biology, and medicine. However, two issues that have impeded the widespread adoption of these techniques are equipment cost and the laborious nature of making measurements one molecule at a time. To meet these challenges, we have developed an approach that enables massively parallel single- molecule force measurements using centrifugal force [1]. This approach is realized in the centrifuge force microscope, an instrument in which objects in an orbiting sample are subjected to a calibration-free, macroscopically uniform force- field while their micro-to-nanoscopic motions are observed. We demonstrate high- throughput single-molecule force spectroscopy with this technique by performing thousands of rupture experiments in parallel, characterizing force-dependent unbinding kinetics of an antibody-antigen pair in minutes rather than days. Currently, we are taking steps to integrate high-resolution detection, fluorescence, temperature control and a greater dynamic range in force. With significant benefits in efficiency, cost, simplicity, and versatility, single-molecule centrifugation has the potential to expand single-molecule experimentation to a wider range of researchers and experimental systems.\\[4pt] [1] K. Halvorsen, W.P. Wong, Biophysical Journal - Letters 98 (11), (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L39.00003: Horizontal Magnetic Tweezers for Micromanipulation of Single DNA-Protein Complexes C. McAndrew, A. Sarkar, P. Mehl We report on the development of a new magnetic force transducer or ``tweezer'' that can apply pico-Newton forces on single DNA molecules in the focus plane. Since the changes in DNA's end-to-end extension are coplanar with the pulling force, there is no need to continually refocus. The DNA constructs ($\lambda $-DNA end labeled with a 3$\mu $m polystyrene bead and a 2.8$\mu $m paramagnetic sphere) and appropriate buffer are introduced to a custom built 400$\mu $L to 650$\mu $L closed cell. This closed cell isolates our sample and produces low-noise force and extension measurements. This chamber rests on a stage fixed to a three axis micromanipulator. Entering the flat chamber are two micropipettes, a 2.5$\mu $m id pipette for aspirating the polystyrene bead and a 20$\mu $m id pipette for injecting proteins of interest. The suction and the injection pipettes are rigidly mounted to a hydraulic, three-axis micromanipulator. DNA-bead constructs, once introduced to the chamber, can be located by moving the stage over the objective. We have shown that we can easily and reputably find, capture, and manipulate single molecules of DNA within a force range of 0.1pN to 100pN. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L39.00004: Modeling the effects of internal and external fluctuations on lifetimes of proteins measured by an AFM Eric Corwin, Maxime Clusel Measurements of the distribution of the time to unfold a single- molecule of a given protein under an externally applied force have emerged as an important tool with which to study the mechanical stability and energy landscape of a protein. In such an experiment the protein is potentially subject both to internal fluctuations in structure as well as external fluctuations in temperature and applied force. We report on a theoretical exploration of the effects that each kind of fluctuation may have on the measured lifetime distribution. We show that it is extremely difficult to distinguish internal fluctuations from external fluctuations in the lifetime distribution. We find that the rate distribution has higher sensitivity to the origins of fluctuations. Therefore, we propose an experimental protocol to estimate the approximate magnitude of internal fluctuations by intentionally adding increasing amounts of external fluctuations and measuring the skewness of the resulting rate distribution. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L39.00005: Onset of excluded volume in poly(ethylene oxide) elasticity measurements Andrew Dittmore, Dustin B. McIntosh, Omar A. Saleh We use magnetic tweezers to control tension in an 80 kDa poly(ethylene oxide) (PEG) chain. In good solvent, force effectively transforms the swollen coil into a series of smaller polymers ("tension blobs") and progressively diminishes self-avoidance interactions between distant parts of the chain. Excluded volume effects dominate the low-strain elasticity, where the extension follows a 2/3 power law in force in accordance with scaling predictions. These effects disappear as the polymer first enters a linear power-law regime, and then a high-force asymptotic regime well described by the Marko-Siggia wormlike chain model. All told, we observe two transitions between three elastic regimes. We show that the transition forces can be used to determine the polymer's Kuhn length, excluded volume, and thermal blob size, and find that PEG requires roughly 30 Kuhn lengths before self-avoidance becomes significant. Thus, we show that single-molecule elasticity can quantify the onset of a polymer's excluded volume, a problem that has eluded bulk measurement techniques. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L39.00006: Peptide Nucleic Acids as Tools for Single-Molecule Sequence Detection and Manipulation Hagar Zohar, Craig Hetherington, Carlos Bustamante, Susan Muller The ability to strongly and sequence-specifically attach modifications such as fluorophores and haptens to individual double-stranded (ds) DNA molecules is critical to a variety of single-molecule experiments. We propose using modified peptide nucleic acids (PNAs) for this purpose and implement them in two model single-molecule experiments where individual DNA molecules are manipulated via microfluidic flow and optical tweezers, respectively. We demonstrate that PNAs are versatile and robust sequence-specific tethers. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L39.00007: Resolving Single Molecule Lysozyme Dynamics with a Carbon Nanotube Electronic Circuit Yongki Choi, Issa S. Moody, Israel Perez, Tatyana Sheps, Gregory A. Weiss, Philip G. Collins High resolution, real-time monitoring of a single lysozyme molecule is demonstrated by fabricating nanoscale electronic devices based on single-walled carbon nanotubes (SWCNT). In this sensor platform, a biomolecule of interest is attached to a single SWCNT device. The electrical conductance transduces chemical events with single molecule sensitivity and 10 microsecond resolution. In this work, enzymatic turnover by lysozyme is investigated, because the mechanistic details for its processivity and dynamics remain incompletely understood. Stochastically distributed binding events between a lysozyme and its binding substrate, peptidoglycan, are monitored via the sensor conductance. Furthermore, the magnitude and repetition rate of these events varies with pH and the presence of inhibitors or denaturation agents. Changes in the conductance signal are analyzed in terms of lysozyme's internal hinge motion, binding events, and enzymatic processing. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L39.00008: Mechanical properties of NRR domain from human Notch 1 studied by single molecule AFM force spectroscopy Robert Szoszkiewicz, Ashim Dey For proteins in living cells, forces are present from macroscopic to single molecule levels. Single molecule atomic force microscopy in force extension (FX-AFM) mode measures forces at which proteins undergo major conformational transitions with $\sim$10 pN force sensitivity (FX-AFM). Here, we present the results of the FX-AFM experiments on a construct comprising the NRR domain from human Notch 1. It is believed that understanding the mechanical properties of Notch at the single molecule level can help to understand its role in triggering some breast cancers. The experimental results on the Notch construct and further analysis revealed several conformational transitions of this molecule under force. These results opened a path for further investigations of Notch constructs at various physiologically relevant conditions. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L39.00009: Non-Perturbative Tracking of Processive DNA Synthesis with Single-Molecule Fluorescence Everett Lipman, Charles Wickersham We have demonstrated recently that double-stranded DNA labeled with a periodic series of fluorescent dyes can be used to track a single helicase. Here we describe how this technique can be modified to follow DNA synthesis. By means of a stepwise loss of fluorescence during strand displacement, we monitor processive motion of a single $\phi 29$ DNA polymerase without labeling or altering the enzyme or the template strand, and without applying any force. We observe a wide range of speeds, with the highest exceeding by several times that observed in other single-molecule experiments. Because this method enables repeated observations of the same polymerase traversing identical segments of DNA, it should prove useful for studying sequence-specific effects in DNA replication and transcription. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L39.00010: Single molecule studies reveal new mechanisms for microtubule severing Jennifer Ross, Juan Daniel Diaz-Valencia, Margaret Morelli, Dong Zhang, David Sharp Microtubule-severing enzymes are hexameric complexes made from monomeric enzyme subunits that remove tubulin dimers from the microtubule lattice. Severing proteins are known to remodel the cytoskeleton during interphase and mitosis, and are required in proper axon morphology and mammalian bone and cartilage development. We have performed the first single molecule imaging to determine where and how severing enzymes act to cut microtubules. We have focused on the original member of the group, katanin, and the newest member, fidgetin to compare their biophysical activities in vitro. We find that, as expected, severing proteins localize to areas of activity. Interestingly, the association is very brief: they do not stay bound nor do they bind cooperatively at active sites. The association duration changes with the nucleotide content, implying that the state in the catalytic cycle dictates binding affinity with the microtubule. We also discovered that, at lower concentrations, both katanin and fidgetin can depolymerize taxol-stabilized microtubules by removing terminal dimers. These studies reveal the physical regulation schemes to control severing activity in cells, and ultimately regulate cytoskeletal architecture. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L39.00011: Polymer Nanocomposites as a Facile Method for Engineering Acto-Myosin Networks at the Interface Matthew Caporizzo, Yujie Sun, Yale Goldman, Russell Composto Filamentous actin acts as the rails for the molecular motor myosin in muscle contraction and intercellular mass transport. Consequently, understanding the process by which actin organizes, polymerizes, and binds is fundamental for the design of myosin based actuators capable of responding to external stimuli. Starting with atomically smooth, freshly cleaved mica optically coupled to glass slides, a random copolymer nanoparticle composite is engineered for \textit{in situ} single molecule TIRF/AFM studies with controlled roughness, electrostatic binding strength, and binding site density. Four distinct regimes of actin binding are observed; no attachment, end-on attachment, weak side-on attachment, and side-on immobilization. Transitions between regimes are likely to mark competition between the affinity to charged nanoparticles and the inherent resistance of the semi-rigid filaments to bending. Surface conditions optimal for actin immobilization are identified, and Myosin V stepping kinetics are studied on the artificially immobilized filaments, confirming filament support of motility. Supported by NSF grant DMR-0425780. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L39.00012: Electrostatic Effects on the Elasticity of Single ssDNA Molecules Dustin B. McIntosh, Omar A. Saleh Nucleic acids are highly-charged polyelectrolytes whose structure and function strongly depend on the concentration and type of salt ions in solution. We have created a simple experimental system for studying nucleic acid/ ion interactions, based on magnetic-tweezer measurements of the elasticity of single denatured ssDNA molecules in solutions with a known salt concentration. Using this system, we were able to reconcile single-molecule force-extension data with scaling theories of self-avoiding polymers, and we found that the Kuhn length of ssDNA scales with the Debye length in NaCl solutions. (Saleh et al., PRL 102, 068301 (2009)). Here, we use the system to investigate interactions of ssDNA with multivalent salts. We find that, in divalent salt, ssDNA elasticity is qualitatively similar to that in monovalent salt, but with significant quantitative differences. Notably, at low ionic strength, ssDNA in divalent salt maintains the same low-force scaling behavior (``Pincus blob'' regime) as seen in monovalent salts. However, there are differences in the elastic behavior at high forces ($>$ a few pN). In addition, analysis of the low-force scaling behavior indicates it requires $\sim$100 fold smaller concentrations of divalent salt to condense ssDNA. We discuss the data in the context of electrostatic theories, including Debye-Huckel, as well as bulk experiments on similar systems. [Preview Abstract] |
Session L40: Focus Session: Noisy Dynamics as Survival Strategies and Nanopores
Sponsoring Units: DBPChair: Gabor Balazsi and Gurol Suel, MD Anderson Cancer Center
Room: A122/123
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L40.00001: TBA Invited Speaker: This abstract not available. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L40.00002: Evolution and Biophysics of the \textit{Escherichia coli lac} Operon J. Christian Ray, Oleg Igoshin, Selwyn Quan, Russell Monds, Tim Cooper, G\'{a}bor Bal\'{a}zsi To understand, predict, and control the evolution of living organisms, we consider biophysical effects and molecular network architectures. The lactose utilization system of \textit{E. coli} is among the most well-studied molecular networks in biology, making it an ideal candidate for such studies. Simulations show how the genetic architecture of the wild-type operon attenuates large metabolic intermediate fluctuations that are predicted to occur in an equivalent system with the component genes on separate operons. Quantification of gene expression in the \textit{lac} operon evolved in growth conditions containing constant lactose, alternating with glucose, or constant glucose, shows characteristic gene expression patterns depending on conditions. We are simulating these conditions to show context-dependent biophysical sources and costs of different lac operon architectures. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L40.00003: The effects of nongenetic memory on population level sensitivity to stress Rhys Adams, Dmitry Nevozhay, Elizabeth Van Itallie, Matthew Bennett, Gabor Balazsi While gene expression is often thought of as a unidirectional determinant of cellular fitness, recent studies have shown how growth retardation due to protein expression can affect gene expression levels in single cells. We developed two yeast strains carrying a drug resistance protein under the control of different synthetic gene constructs, one of which was monostable, while the other was bistable. The gene expression of these cell populations was tuned using a molecular inducer so that their respective means and noises were identical, while their nongenetic memory properties were different. We tested the sensitivity of these two cell population distributions to the antibiotic zeocin. We found that the gene expression distributions of bistable cell populations were sensitive to stressful environments, while the gene expression distribution of monostable cells were nearly unchanged by stress. We conclude that cell populations with high nongenetic memory are more adaptable to their environment. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 4:06PM |
L40.00004: Interplay of Noisy Gene Expression and Dynamics Explains Patterns of Bacterial Operon Organization Invited Speaker: Bacterial chromosomes are organized into operons -- sets of genes co-transcribed into polycistronic messenger RNA. Hypotheses explaining the emergence and maintenance of operons include proportional co-regulation, horizontal transfer of intact ``selfish'' operons, emergence via gene duplication, and co-production of physically interacting proteins to speed their association. We hypothesized an alternative: operons can reduce or increase intrinsic gene expression noise in a manner dependent on the post-translational interactions, thereby resulting in selection for or against operons in depending on the network architecture. We devised five classes of two-gene network modules and show that the effects of operons on intrinsic noise depend on class membership. Two classes exhibit decreased noise with co-transcription, two others reveal increased noise, and the remaining one does not show a significant difference. To test our modeling predictions we employed bioinformatic analysis to determine the relationship gene expression noise and operon organization. The results confirm the overrepresentation of noise-minimizing operon architectures and provide evidence against other hypotheses. Our results thereby suggest a central role for gene expression noise in selecting for or maintaining operons in bacterial chromosomes. This demonstrates how post-translational network dynamics may provide selective pressure for organizing bacterial chromosomes, and has practical consequences for designing synthetic gene networks. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L40.00005: Population-level control of gene expression Dmitry Nevozhay, Rhys Adams, Elizabeth Van Itallie, Matthew Bennett, Gabor Balazsi Gene expression is the process that translates genetic information into proteins, that determine the way cells live, function and even die. It was demonstrated that cells with identical genomes exposed to the same environment can differ in their protein composition and therefore phenotypes. Protein levels can vary between cells due to the stochastic nature of intracellular biochemical events, indicating that the genotype-phenotype connection is not deterministic at the cellular level. We asked whether genomes could encode isogenic cell populations more reliably than single cells. To address this question, we built two gene circuits to control three cell population-level characteristics: gene expression mean, coefficient of variation and non-genetic memory of previous expression states. Indeed, we found that these population-level characteristics were more predictable than the gene expression of single cells in a well-controlled environment. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L40.00006: Functional Differentiation in EVS modeling Irina Trofimova, William Sulis Ensembles with Variable Structures (EVS) were introduced in mid-1990s as stochastic milti-agent models in which agents possessed either formal diversity (described in a multi-dimensional vector space of abstract characteristics) or resource-oriented diversity (Trofimova, 2000). The process of functional differentiation (i.e. appearance of functional roles) is modelled as constraints on the flow of resources which pass through agents of the model. These constraints are: 1) the maximum amount of resource that an individual can accept from outside, 2) the maximum amount of resource that an individual can give back to the population or other environment, 3) distribution of the exchange of the resource over time (frequency and amount of the resource per step), and 4) the maximum amount of contacts that an individual can hold with such environment (sociability). Sociability appears to have a major impact on clustering dynamics within the population and to be an order parameter in phase transition in clustering behaviour, therefore it interfered with functional differentiation. Two patterns of functional differentiation were observed, before and after the phase transition in clustering, corresponding to sociability values below and after the critical points. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L40.00007: Fitness in fluctuating environments Sorin Tanase Nicola, Ilya Nemenman Often environments change faster than the time needed to evolve optimal phenotypes through cycles of mutation and selection. We focus on this case, but assume that environmental oscillations are slower than an individual's lifetime. This is relevant, for example, for bacterial populations confronted with daily environmental changes. We analyze a resource-limited competition between a mutant phenotype and the ancestor. Environmental dynamics is represented by periodically varying, off-phase parameters of the corresponding Lotka-Volterra model. For the very slow dynamics (but still faster than the fixation time scale) the strength and the sign of selection are functions of the birth/death rates averaged over all of the environmental states and independent of the period of the fluctuations. For faster fluctuations, selection depends on the particular sequence of the successive environmental states. In particular, a time reversal of the environmental dynamics can change the sign of the selection. We conclude that the fittest phenotype in a changing environment can be very different from both the optimal phenotype in the average environment, and the phenotype with the largest average fitness. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L40.00008: Development of an electrical nanopore device towards the control of the translocation of DNA with single base resolution Hongbo Peng, Binquan Luan, Stanislav Polonsky, Stephen Rossnagel, Gustavo Stolovitzky Recently, application of nanopores to low-cost DNA sequencing has attracted great interest as there is great need to reduce the cost of sequencing a whole human genome to \$1000. A key issue in the field of nanopore DNA sequencing is to control the DNA translocation. Here we will report the development of what we call a ``DNA transistor'': a nanopore-based electrical device for controlling the translocation of DNA with single base resolution. The key part of this device is a free standing membrane, within which multiple layers of electrically addressable metal electrodes separated by dielectric layers are embedded. A 1-5 nanometer size pore is made through the membrane. We demonstrated that such a device is electrically viable for the electrode layer or the spacing dielectric layer as thin as 3 nm in 1 mM KCl solution. Induced electrical signals on the nano-electrodes by the translocating DNA, as well as the modulation of DNA translocation speed by the voltage bias applied on the nanoelectrodes are also observed. Our ongoing experiments test if the modulated electrical field can trap or translocate DNA at a single base resolution. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L40.00009: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L40.00010: Polymerization of nanopores for controlled surface charges Waseem Asghar, Azhar Ilyas, Richard Timmons, Samir Iqbal The solid-state nanopores have emerged as a novel candidate for DNA sequencing and protein analysis. Traditional approaches for nanopore diameter shrinking use electron microscopy induced shrinking and deposition processes. These approaches are limited due to less control on surface composition of the deposited film, slow deposition rate and initial membrane thickness dependant shrinking processes. We report a novel approach of pulsed plasma polymer deposition which addresses all of the above described issues. The surface chemical composition and geometry of solid-state nanopores are controlled by plasma deposition of highly conformal thin polymeric films. Surface energy and pore-wall surface charges are controlled using appropriate monomer during plasma deposition process. [Preview Abstract] |
Session L41: Condensed Phase Dynamics and Structure
Sponsoring Units: DCPChair: David Nesbitt, University of Colorado at Boulder
Room: A115/117
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L41.00001: Temperature and Lengthscale Dependence of Solvophobic Solvation in a Water-like Liquid John Dowdle, Peter Rossky Temperature and lengthscale dependence of the solvation of cavity solutes is investigated along the saturation curve of the Jagla liquid, a simple liquid consisting of particles that interact via a spherically symmetric potential combining hard and soft core interactions. The results are compared with an identical calculation for a model of a typical atomic liquid, the Lennard- Jones potential, and with predictions for cavity solubilities in water made by the recently developed cavity equation of state. We find that the Jagla liquid captures the qualitative thermodynamic behavior of hydrophobic hydration as a function of temperature for both small and large lengthscale solutes. The results suggest that a competition between two lengthscales that favors low- density, open structures as temperature is decreased is an essential interaction of a liquid that models hydrophobic hydration. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L41.00002: Effects of Physical Confinement on the Hysteresis between Melting and Freezing Temperatures of Decanol Samuel Amanuel, Jargalsaikhan Dulmaa, Amer Khraisat There is substantial evidence that physical confinement alters melting and freezing temperatures of materials. These have been qualitatively explained using free energy considerations. However, it is not clear how physical confinement influences melting and freezing when the bulk material itself exhibits substantial supercooling. Bulk 2-decanol, for instance, exhibits substantial hysteresis between its melting (approximately -23$^{\circ}$C) and freezing (-3$^{\circ}$C) temperatures. Evidently, both its melting and freezing temperatures are influenced by physical size. However, the hysteresis between the freezing and melting temperatures seems less sensitive to physical size. This may be the result of differences in homogeneous versus heterogeneous nucleation in physically confined 2-decanol. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L41.00003: Evaluating the Liquid Liquid Phase Transition Hypothesis of Supercoooled Water David Limmer, David Chandler To explain the anomalous behavior of supercooled water it has been conjectured that buried within an experimentally inaccessible region of liquid water's phase diagram there exists a second critical point, which is the terminus of a first order transition line between two distinct liquid phases. The so-called liquid-liquid phase transition (LLPT) has since generated much study, though to date there is no consensus on its existence. In this talk, we will discuss our efforts to systematically study the metastable phase diagram of supercooled water through computer simulation. By employing importance-sampling techniques, we have calculated free energies as a function of the density and long-range order to determine unambiguously if two distinct liquid phases exist. We will argue that, contrary to the LLPT hypothesis, the observed phenomenology can be understood as a consequence of the limit of stability of the liquid far away from coexistence. Our results suggest that homogeneous nucleation is the cause of the increased fluctuations present upon supercooling. Further we will show how this understanding can be extended to explain experimental observations of hysteresis in confined supercooled water systems. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L41.00004: Anomalous lattice parameter isotope-shift in hexagonal ice Ih from first principle calculations Bet\"{u}l Pamuk, Marivi Fernandez-Serra, Philip Allen The lattice parameters of light (H$_2$O) and heavy (D$_2$O) Ih ice differ by 0.09\% [1]. The larger lattice constant is that of the heavier isotope, contrary to normal expectations. This isotope shift of the lattice constant is linked to the zero point energy of phonons in ice. In particular, it can be linked to the anti-correlation of the O-H stretch frequency and the O-O distance in H-bonded materials. In order to determine which phonons give the anomaly, we calculate Gr\"{u}neisen parameters of H$_2$O and D$_2$O ice using first principles density functional theory, within the frozen phonon approximation. Our results show a strong dependence on the density functional chosen. We analyze these differences and make connections to experiment. These results indicate that not only H-bond effects but also van der Waals interactions are necessary to reproduce the correct lattice constant zero-point shifts in ice. \\[4pt] [1] B. K. R\"{o}ttger et. al., Acta Cryst. B {\bf 50}, 644-648 (1994). [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L41.00005: Local effects in the X-ray absorption spectrum of salt water Eric Schwegler, Heather Kulik, Nicola Marzari, Alfredo Correa, David Prendergast, Giulia Galli We have used first principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS) to investigate the aqueous solvation of cations in MgCl$_2$, CaCl$_2$, and NaCl solutions. We focus our discussion on the species-specific effects that Mg$^{2+}$, Ca$^{2+}$, and Na$^{+}$ have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the water molecules that form a rigid first solvation shell around Mg$^{2+}$ and a more flexible solvation shell around Ca$^{2+}$ also exhibit differing hydrogen bonding characteristics. Acceptor hydrogen bonds present in the water surrounding Ca$^{2+}$ enhance a post-edge peak near 540 eV in the XAS spectrum, while the absence of such hydrogen bonding features for the first shell surrounding Mg$^{2+}$ corresponds to a diminished intensity at the post-edge peak. For Na$^+$, we find that a broad tilt angle distribution results in broadened post-edge features, despite donor-and-acceptor populations comparable to Ca$^{2+}$. We present re-averaged spectra of the MgCl$_2$, CaCl$_2$, and NaCl solutions that provide an explanation of concentration-dependent features that have been found in corresponding experimental measurements. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L41.00006: Nuclear momentum distribution and potential energy surface in hexagonal ice Lin Lin, Joseph Morrone, Roberto Car, Michele Parrinello The proton momentum distribution in ice Ih has been recently measured by deep inelastic neutron scattering and calculated from open path integral Car-Parrinello simulation. Here we report a detailed investigation of the relation between momentum distribution and potential energy surface based on both experiment and simulation results. The potential experienced by the proton is largely harmonic and characterized by 3 principal frequencies, which can be associated to weighted averages of phonon frequencies via lattice dynamics calculations. This approach also allows us to examine the importance of quantum effects on the dynamics of the oxygen nuclei close to the melting temperature. Finally we quantify the anharmonicity that is present in the potential acting on the protons. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L41.00007: The quantum nature of the hydrogen bond: insight from path-integral molecular dynamics Brent Walker, Xin-Zheng Li, Angelos Michaelides Hydrogen (H) bonds are weak, generally intermolecular bonds, that hold together much of soft matter, the condensed phases of water, network liquids, and many ferroelectric crystals. The small mass of H means H-bonds are inherently quantum mechanical; effects such as zero point motion and tunneling should be considered, although often are not. In particular, a consistent picture of quantum nuclear effects on the strength of H-bonds and consequently the structure of H-bonded systems is still absent. Here, we report \textit{ab initio} path-integral molecular dynamics studies on the quantum nature of the H-bond. Systematic examination of a range of H-bonded systems shows that quantum nuclei weaken weak H-bonds but strengthen relatively strong ones. This correlation arises from a competition between anharmonic intermolecular bond bending and intramolecular bond stretching. A simple rule of thumb enables predictions to be made for H-bonded bonded materials in general with merely classical knowledge (e.g.\ H-bond strength or H-bond length). Our work rationalizes the contrasting influence of quantum nuclear dynamics on a wide variety of materials, including liquid water and HF, and highlights the need for flexible molecules in force-field based studies of quantum nuclear dynamics. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L41.00008: Hydration phase diagram for BaO terminated BaTiO$_3$ John Mark Martirez, Wissam Al-Saidi, Andrew Rappe This study reveals geometries H{$_2$}O adopts upon adsorption on BaO terminated BaTiO$_3$(BTO) at low to high saturation. A hydration phase diagram for the aforementioned termination is presented, for moderate temperatures, and moderate to ultra high vacuum H$_2$O pressures. Calculations suggest a very stable H$_2$O adsorption for wide range of pressures, including high vacuum conditions (p$_{H_2O}$ 10$^{-12}$ bar). This opens venues for mechanistic studies and hopefully will serve as a guide to condition that might suppress H$_2$O adsorption on BTO for applications where it is undesired. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L41.00009: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L41.00010: Topological reaction coordinates to explore the structure of atomic clusters and organic molecule isomers from first principles Fabio Pietrucci, Wanda Andreoni We introduce a simple reaction coordinate based on spectral graph theory which describes the topology of the network of chemical bonds around a given atom. We employ the reaction coordinate in combination with DFT-based first-principles metadynamics to systematically explore the possible structures of silicon and carbon clusters (including fullerene-like cages) for sizes of tens of atoms. From our extensive exploration we are able to estimate the fractal dimension of the configuration space, which both for silicon and carbon clusters turns out to be quite low. Using the same approach we simulate the interconversion among a large number of chemically relevant organic molecules which are isomers of the C$_4$H$_5$N formula unit, and we demonstrate the possibility of automatically exploring isomerisation, association, and decomposition reactions without prior knowledge of the products involved. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L41.00011: Transport through a quantum dot with excitonic dot-lead coupling Florian Elste, David R. Reichman, Andrew J. Millis We study the effect of a Coulombic dot-lead interaction on transport through a quantum dot hybridized to two Luttinger-liquid leads.\footnote{F. Elste, D. R. Reichman, and A. J. Millis, arXiv:1010.2251} A bosonization approach is applied to treat the interaction between charge fluctuations on the dot and the dynamically generated image charge in the leads.\footnote{F. Elste, D. R. Reichman, and A. J. Millis, Phys.\ Rev.\ B \textbf{81}, 205413 (2010)} The nonequilibrium distribution function of the dot and the tunneling current are computed within a master-equation approach. Particular attention is paid to two situations: (i) a quantum dot placed between two leads such that it cuts the Luttinger liquid into two semi-infinite quantum wires; (ii) a quantum dot side-hybridized to two parallel infinite quantum wires. The presence of the excitonic dot-lead coupling is found to enhance transport in the vicinity of the Coulomb-blockade threshold. This behavior is in contrast to the usual power-law suppression of electronic tunneling which is found if this interaction is ignored. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L41.00012: Spectroscopic and Theoretical Investigations of the Potential Energy Surfaces of Molecules with Intramolecular $\pi $-type Hydrogen Bonding Esther Ocola, Hee-Won Shin, Abdulaziz Al-Saadi, Jaan Laane Spectroscopic methods and theoretical calculations have been utilized to investigate the conformations of several cyclic organic molecules. The laser induced fluorescence (LIF) spectra of 2-indanol show the presence of four conformations. The one with intramolecular hydrogen bonding between the --OH group and the benzene ring is of lowest energy. The potential energy surface (PES) in terms of the ring puckering and internal rotational vibrations, which govern the conformational changes, was determined. 3-Cyclopenten-1-ol possesses a similar PES as established from its infrared and Raman spectra and theoretical calculations. This PES also shows the presence of four conformations. The $\pi $-bonding conformer lies at lowest energy. LIF has been used to study the conformational energies of 2-hydroxytetralin, and 2-cyclohexenol has been investigated by infrared and Raman techniques. The analyses of the hydrogen bonding in these molecules as well as in a dozen others were supported by both \textit{ab initio} and DFT calculations. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L41.00013: Raman Spectra and the Potential Energy Function for the Internal Rotation of 1,3-Butadiene and its Isotopomers Jaan Laane, Praveenkumar Boopalachandran, Norman Craig The gas-phase Raman spectra of 1,3-butadiene-d$_{0}$, 2,3-d$_{2}$, 1,1,4,4-d$_{4}$, and --d$_{6}$ have been collected with CCD detection with numerous scans of ten hours or more. For each isotopomer eight Raman transitions in the 240-330 cm$^{-1}$ region corresponding to double quantum jumps of the A$_{u}$ internal rotation ($\nu _{13})$ were observed for the \textit{trans} conformer. Weaker bands in the 170-260 cm$^{-1}$ region were assigned to the \textit{gauche} conformation, which lies at higher electronic energy. A periodic potential function for the internal rotation, which fits the data for all the isotopomers, was determined. This function shows the \textit{gauche} form to be 966 cm$^{-1}$ higher in energy and the barrier between the \textit{trans} and \textit{gauche} structures to be 2055 cm$^{-1}$. The \textit{cis} structure has an energy 408 cm$^{-1}$ higher than the \textit{gauche}. Fourteen combination band or hot band series involving $\nu _{13}$ for the \textit{trans} conformer were also observed, and these allow the internal rotation levels in various excited vibrational states to be determined. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L41.00014: Transition and Excited States of 1,1'-azo-bis-1,2,3-triazole Vladimir Goncharov, Olga Goncharova, Kalman Varga A novel photochromic molecule has been recently synthesized\footnote{Yu-Chuan Li et al. \emph{J. Am. Chem. Soc.}, 2010, 132, 12172}. The photo-isomerization of this nitrogen-rich small molecule is efficiently controlled by a xenon flash lamp suggesting a potential in photonic and molecular mechanics applications. We perform a synergistic quantum molecular dynamics (QMD), real-time time dependent density functional theory (TDDFT) and TDDFT- perturbation theory study to capture and elucidate the transition state, excitation energies and optical properties of the molecule. We also use it to test performance of recently developed real-time TDDFT method\footnote{V. Goncharov, K. Varga, \emph{Phys. Rev. B} 2010, submitted.} to calculate hyperpolarizabilities and compare results with the Sternheimer method. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L41.00015: Importance of Electronic Relaxation for Inter-Coulombic Decay in Aqueous Systems David Prendergast, Craig P. Schwartz, Richard J. Saykally, Shervin Fatehi, Keith V. Lawler, C. William McCurdy Inspired by recent photoelectron spectroscopy experiments on hydroxide solutions, we have examined the conditions necessary for enhanced (and, in the case of solutions, detectable) intermolecular Coulombic decay (ICD) -- Auger emission from an atomic site other than that originally excited. We present general guidelines, based on energetic and spatial overlap of molecular orbitals, for this enhancement of ICD-based energy transfer in solutions. These guidelines indicate that this decay process should be exhibited by broad classes of biomolecules and suggest a design criterion for targeted radiooncology protocols. Our findings indicate that ICD processes in hydroxide solutions are not dependent on hydroxide hydrogen bond donation. [Preview Abstract] |
Session L42: Dillon Medal Symposium
Sponsoring Units: DPOLYChair: Spiros Anastasiadis, IESL- FORTH
Room: A302/303
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L42.00001: John H. Dillon Medal Talk: Protein Fibrils, Polymer Physics: Encounter at the Nanoscale Invited Speaker: Aggregation of proteins is central to many aspects of daily life, ranging from blood coagulation, to eye cataract formation disease, food processing, or neurodegenerative infections. In particular, the physical mechanisms responsible for amyloidosis, the irreversible fibril formation of various proteins implicated in protein misfolding disorders such as Alzheimer, Creutzfeldt-Jakob or Huntington's diseases, have not yet been fully elucidated. In this talk I will discuss how polymer physics and colloidal science concepts can be used to reveal very useful information on the formation, structure and properties of amyloid protein fibrils. I will discuss their physical properties at various length scales, from their collective liquid crystalline behavior in solution to their structural features at the single molecule length scale and show how polymer science notions can shed a new light on these interesting systems. 1) ``Understanding amyloid aggregation by statistical analysis of atomic force microscopy images'' J. Adamcik, J.-M. Jung, J. Flakowski, P. De Los Rios, G. Dietler and R. Mezzenga, Nature nanotechnology, 5, 423 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L42.00002: Interdiffusion in bilayers of PCBM fullerene and poly(3-hexyl thiophene) P3HT Edward J. Kramer, Neal D. Treat, Michael A. Brady, Michael F. Toney, Michael L. Chabinyc, Craig J. Hawker Bulk heterojunction (BHJ) photovoltaic materials, typified by blends of PCBM and P3HT, are often regarded as immiscible in the absence of casting solvent. We use dynamic secondary ion mass spectrometry and grazing incidence wide angle X-ray scattering to probe the interdiffusion of bilayers of P3HT and deuterated dPCBM. We find that the as cast P3HT film is semicrystalline, while the dPCBM film is amorphous, and that there is complete interdiffusion between dPCBM and P3HT after annealing for 300 s at T = 150$^{\circ}$C, a typical treatment to improve device efficiency. This interdiffusion occurs without disrupting the ordered lamellar stacking in the P3HT crystallites, showing that PCBM is miscible with amorphous P3HT at this temperature (the size and/or perfection of the P3HT crystallites actually increases during the annealing). At T $<$ 150$^{\circ}$C rapid diffusion of dPCBM into P3HT still occurs but the dPCBM concentration reaches an apparent solubility limit after long anneals, a limit that decreases with decreasing temperature. This result suggests that dPCBM will phase separate from amorphous P3HT in the BHJ on cooling from 150$^{\circ}$C to room temperature. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L42.00003: Geroges Hadziioannou |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L42.00004: Coarse grained polystyrene simulations: Static and dynamic properties Kurt Kremer By combining input from short simulation runs of rather small systems with all atomistic details together with properly adapted coarse grained models we are able quantitatively predict static and especially dynamical properties of both pure polymer melts of long entangled chains but also of systems with low molecular weight additives. Comparisons to rather different experiments such as diffusion constant measurements, NMR relaxation experiments and dielectric spectroscopy show a remarkable quantitative agreement without any adjustable parameter. The model is also able to distinguish different tacticities and to study the consequences for static and dynamic properties. Reintroduction of chemical details into the coarse grained trajectories allows the study of long time trajectories in all atomistic detail providing the opportunity for rather different means of data analysis. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L42.00005: Effects of Side Chains on the Self-Assembly and Photovoltaic Properties of Conjugated Polymer Semiconductors Samson Jenekhe Conjugated polymer semiconductors are of growing interest in electronics and optoelectronics. Although it is now well established that the electronic band structure, charge transport, and electronic properties of conjugated polymers can be varied over a wide range through manipulation of the molecular backbone structure, little is known about the effects of alkyl side chains on the solid state morphology and properties of these materials. We have investigated homologous series of conjugated homopolymers, block copolymers, and random copolymers with controlled variation of their alkyl sides towards understanding the effects the size and typology of the side chains on self-assembly, morphology, and photovoltaic properties. We found that diblock copoly(3-alkylthiophenes) exhibit highly crystalline and phase-separated nanostructures in blend films with fullerene derivatives, resulting in superior photovoltaic properties compared to the corresponding homopolymers. The solid state morphology and photovoltaic efficiency of a series of donor-acceptor copolymer semiconductors that have the same optical band gap but different alkyl side chains were found to vary dramatically. Self-assembled block copolymer nanowires with widths of 10-30 nm and aspect ratios of up to 900 have been found to be promising building blocks for constructing efficient bulk heterojunction solar cells. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L42.00006: Nacre-Mimetic Composites via Single-Step Self-Assemblies of Polymer-Coated Colloids Olli Ikkala We demonstrate a scalable single-step self-assembly of polymer-modified plate-like colloidal platelets for nacre-mimetic materials, which overcomes the problem of sequential deposition to prepare hard and soft nacre-mimetic assemblies, which is inherently slow and not scalable. The materials have low density and show good mechanical properties, i.e., modulus of 45 GPa and strength 250 MPa, i.e. partly surpassing those of nacre (Walther et al, Nano Letters 2010, Angew Chem 2010). We expect that the concepts open a route for biomimetic materials from the lab to technology. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L42.00007: On the Crystallinity and Chain Conformation in PEO / Layered Silicate Nanocomposites Spiros H. Anastasiadis, K. Chrissopoulou, S. Bollas, K. Andrikopoulos, S. Fotiadou, D. Christofilos, G.A. Voyiatzis The structure of nanohybrid materials as well as the chain conformation under confinement is investigated in hydrophilic polymer / layered silicate nanocomposites. A series of PEO / sodium montmorilonite hybrids was synthesized utilizing melt intercalation with compositions covering the whole range from pure polymer to pure clay. Intercalated nanocomposites with mono- and bi-layers of PEO chains are obtained in all cases. The intercalated chains as well as the ones adsorbed on the outer surface of the clay particles remain purely amorphous; nevertheless, their conformations exhibit different characteristics from those of the amorphous bulk material with the intercalated PEO chains adopting preferably gauche conformations. It is only for compositions where a large amount of excess polymer exists outside the completely full galleries that the polymer crystallinity is recovered. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L42.00008: Formation and application of functional coatings on synthetic fibers Kiran Goli, Ali Ozcam, Kristen Roskov, Richard Spontak, Orlando Rojas, Jan Genzer We present two simple methods for modifying synthetic fibers made of polypropylene (PP) and poly(ethylene terephthalate) (PET). Specifically, we alter the inert PP fiber mats by physisorbing denatured proteins, and cross-linking the protein layers using glutaraldehyde. The amino- and hydroxyl-functionalities present in the protein coatings serve as attachment points for polymerization initiators. In addition, PET fibers are modified chemically by amidation with 3-aminopropyltriethoxysilane (APTES), followed by hydrolysis, which yields silanol groups that permit surface attachment of the initiator molecules. ``Grafting from'' polymerization from such modified PP and PET surfaces is employed following the atom transfer radical polymerization protocol to form functional and responsive polymer coatings. These include arrays of poly(2-hydroxyethyl methacrylate) (PHEMA) as well as chemically-modified PHEMA layers. Selected applications of these functional fibers will be outlined briefly, including, capture of metals or other contaminants from waters, prevention of protein adsorption, and attachment of metallic nanoparticles. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L42.00009: Peptide assembly for nanoscale control of materials Darrin Pochan Self-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic, charged synthetic molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. Design strategies for materials self-assembly based on small (less than 24 amino acids) beta-hairpin peptides will be discussed. Self-assembly of the peptides is predicated on an intramolecular folding event caused by desired solution properties. Importantly, kinetics of self-assembly can be tuned in order to control gelation time. The final gel behaves as a shear thinning, but immediately rehealing, solid that is potentially useful for cell injection therapies. The morphological, and viscoelastic properties of these peptide hydrogels will be discussed. In addition, slight changes in peptide primary sequence can have drastic effects on the self-assembled morphology. Additional sequences will be discussed that do not form hydrogels but rather form nanoscale templates for inorganic material assembly. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L42.00010: Food Materials - a natural playground for soft matter physics Adam Burbidge Traditional food science has its origins in chemistry, and has therefore tended to focus on trying to link molecular formulation and functional performance. Nevertheless, foods are almost always complex hierarchically structured materials of biological origin, far from thermodynamic equilibrium. These kinds of systems provide a challenge of relating structure to function, which is a natural playground for many ideas and concepts of soft matter physics. In this talk I will briefly outline the incredibly rich structural complexity of food products and highlight some areas which are appear to be amenable to physically based reasoning. Despite some notable contributions, food materials physics is a field very much in it's infancy, and I will highlight some outstanding (in both senses of the word) problems! [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L42.00011: Assembly of conjugated-polymer-based nanostructures driven by solution-state crystallization Ryan Hayward, Felicia Bokel, Eunji Lee, Brent Hammer, P.K. Sudeep, Emily Pentzer, Todd Emrick Conjugated polymers such as regioregular poly(3-alkyl thiophenes), are well known to crystallize into extended one-dimensional nanowires or fibrils. This behavior is not only important for the efficiency of charge transport in device layers, but can also provide a driving force to assemble different optoelectronic components into well- defined nanostructures. We have investigated the assembly of two systems that rely on solution-state crystallization of poly(3-hexyl thiophene) (P3HT). In the first case, co- crystallization of freely dissolved and particle-bound P3HT provides hybrid fibrils of polymers flanked with n-type inorganic nanoparticles. In the second case, crystallization of P3HT-poly(3-triethylene glycol thiophene) diblock copolymers yields fibrils that can form supramolecular helical assemblies in the presence of salt. We seek to elucidate the mechanisms of self-assembly and the optoelectronic properties of the resulting nanostructures. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L42.00012: Temperature responsive hydrogel nanofibers and nanoparticles Janne Ruokolainen Poly(N-isopropylacrylamide) (PNIPAM) is one of the most extensively investigated synthetic temperature-responsive polymers. In this work temperature-responsive PNIPAM based triblock copolymer hydrogels, their self-assembly and phase behavior in bulk, are described. Additionally, recent results from responsive hydrogel nanofibers and hydrogel nanoparticles are shown. It is known that block copolymers form well-organized nano structures in bulk or thin films when annealed thermally or in solvent vapours. However, in the case of nanofibers or nanoparticles, the annealing leads in most cases to aggregation and particle sintering. This work utilizes aerosol-based gas phase method where the preparation and annealing of hydrogel nanoparticles with well-organized, hierarchical inner structures are performed without any particle coagulation or sintering. In the method, the block copolymers assemble within aerosol nanoparticles to form, for instance, lamellar onion-like or gyroid inner structures. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L42.00013: Orientation and Order in High-Efficiency Polymer Solar Cell Active Layers M.R. Hammond, R.J. Kline, L.J. Richter, C.L. Soles, D.M. DeLongchamp, T. Xu, L. Yu, M.F. Toney Using a combination of scattering and spectroscopy techniques, we have characterized molecular orientation and order in bulk heterojunction (BHJ) organic photovoltaic cells based upon thieno[3,4-\emph{b}]thiophene-\emph{alt}-benzodithiophene copolymers (PTB) and fullerene derivatives. The various techniques probe complimentary aspects of the full distribution of (e.g.) polymer chain orientations, so analysis of the data in tandem allows us to evaluate the relative likelihood of several model distributions. Specifically, X-ray diffraction data yielded a full orientation distribution for the polymer pi-stacking direction within well-ordered regions, with the nominal result of a strong preference for pi-stacking in the vertical direction. This structural characteristic may enhance vertical charge mobility and thus be one factor leading to the outstanding performance of PTB based devices. However, the most plausible model distribution would suggest that those ordered (diffracting) regions of the PTB BHJ films comprise only a small percentage of the total polymer volume within the film. [Preview Abstract] |
Session L43: Focus Session: Polymers for Energy Storage and Conversion -- Emerging Applications
Sponsoring Units: DMP DPOLY GERAChair: Lou Madsen, Virginia Polytechnic Institute and State University
Room: A306/307
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L43.00001: Dillon Medal Break |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L43.00002: Block copolymers exhibiting simultaneous electronic and ionic conduction for use in lithium battery electrodes Anna Javier, Shrayesh Patel, Daniel Hallinan, Nitash Balsara A block copolymer system that can demonstrate both electronic and ionic conductivity is analyzed for its performance in rechargeable lithium batteries. Here, the electrically active polymer is poly(3-hexylthiophene), while poly(ethylene oxide) is used as the lithium ion conductor. This block copolymer is then mixed with LiFePO$_{4}$ and used as the cathode material. Other components in the battery include a lithium metal anode and poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) as the solid electrolyte. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) is utilized to facilitate ionic conductivity in both the electrolyte and the cathode. The synthesis of the block copolymer and its device performance in rechargeable lithium metal batteries will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L43.00003: All Solid State Rechargeable Lithium Batteries using Block Copolymers Daniel Hallinan, Nitash Balsara The growing need for alternative energy and increased demand for mobile technology require higher density energy storage. Existing battery technologies, such as lithium ion, are limited by theoretical energy density as well as safety issues. Other battery chemistries are promising options for dramatically increasing energy density. Safety can be improved by replacing the flammable, reactive liquids used in existing lithium-ion battery electrolytes with polymer electrolytes. Block copolymers are uniquely suited for this task because ionic conductivity and mechanical strength, both important properties in battery formulation, can be independently controlled. In this study, lithium batteries were assembled using lithium metal as negative electrode, polystyrene-b-poly(ethylene oxide) copolymer with lithium salt as electrolyte, and a positive electrode. The positive electrode consisted of polymer electrolyte for ion conduction, carbon for electron conduction, and an active material. Batteries were charged and discharged over many cycles. The battery cycling results were compared to a conventional battery chemistry. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L43.00004: Solubility of Lithium Polysulfides in a Block Copolymer Electrolyte for Lithium/Sulfur Batteries Alexander Teran, Nitash Balsara The primary challenges to commercialization of the high-energy-density lithium sulfur battery are dendrite growth of the lithium metal at the anode and capacity fade due to loss of active mass through dissolution at the cathode. Nanostructured solid polymer electrolytes offer one potential solution to reduce the amount of capacity fade seen in lithium metal/sulfur batteries by keeping the active material localized at the cathode and to prevent the growth of dendrites at the anode due to their high shear moduli. The block copolymer electrolyte poly(styrene)-\textit{block}-poly(ethylene oxide) (SEO) has shown acceptable ionic conductivity and sufficient shear modulus to retard lithium dendrite growth. The solubility of the lithium polysulfide reaction intermediates Li$_{2}$S$_{x}$, where 1 $\le $ x $\le $ 8, was studied in SEO copolymers with a range of molecular weights and salt concentrations using small angle X-ray scattering, X-ray diffraction, and differential scanning calorimetery. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L43.00005: Solution Processable Hybrid Polymer-Inorganic Thermoelectric Materials Shannon Yee, Nelson Coates, Kevin See, Jeffrey Urban, Rachel Segalman In the last decade thermoelectric material improvements have largely been attributed to a reduction in thermal conductivity due to nanostructuring. An alternative approach is to decouple and optimize the power factor using the unique properties of organic-inorganic interfaces. One method to do this could rely on the electrical properties of a conducting polymer in combination with the thermoelectrical proprieties of an inorganic semiconductors. It is expected that the thermal conductivity of this hybrid material would be low due to the inherent phonon mismatch between polymers and inorganics. Recently we have developed a method for producing a solution processable bulk thermoelectric material (ZT$>$0.1) using a hybrid polymer-inorganic systems consisting of crystalline tellurium nanowires coated in a thin layer of PEDOT:PSS. The interface properties of these materials scale and bulk films demonstrate enhanced transport properties beyond those of either component. Here, we present our methodology, theoretical explanation, and experimental transport properties of this new class of materials where the thermal conductivity, electrical conductivity, and thermopower predictably vary as a function of polymer loading in the hybrid composite. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L43.00006: Microscopic mechanism of energy storage in PVDF-CTFE from ab-initio calculations V. Ranjan, Marco Buongiorno Nardelli, J. Bernholc Polypropelene is most used capacitor dielectric for high energy density storage. However, exotic materials such as copolymerized Polyvinylidene fluoride (PVDF) could potentially lead to an order of magnitude increase in the stored energy density [1,2]. In contrast to linear dielectric properties of polypropelene, several polymers in the PVDF family display nonlinear dielectric properties under electric field. The nonlinearity was postulated to be due to a phase transition from non-polar to a polar structure, whose energy is lowered by an electric field [2]. Our calculations map out the atomistic details of phase transformations for both pure PVDF and PVDF-CTFE. Interestingly, admixture of a small amount of copolymer lowers both the polarization and the energy barriers for the transformation. The barrier lowering facilitates the transformation and may result in reduced loss in the charge-discharge cycle, enabling tuning of material properties for energy storage applications. \\[4pt] [1] B. Chun et al, Science \textbf{313}, 334 (2006).\\[0pt] [2] V. Ranjan et al, Phys. Rev. Lett. \textbf{99}, 047801 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L43.00007: Nanotube Forests for Electrochemical Energy Storage from Electrostatic Assembly Lin Shao, Woo-Sik Jang, Jodie Lutkenhaus With increasing global energy consumption, efficient energy storage sytems are urgently needed. Currently, lithium-ion batteries are prevalent in many of these applications because of their established reliability and superior performance relative to older technologies; however, Li-ion batteries can be limited by mass transfer and safety concerns. Here, we present nanostructured polymer-based electrodes that potentially address these limitations. We apply layer-by-layer (LbL) assembly and nanotemplating to realize LbL-nanotube cathode arrays containing vanadium pentoxide and polyaniline. Both polyaniline and V2O5 store charge via doping/undoping and intercalation/deintercalation, respectively. The aim is to create high surface area electrodes that minimize the diffusion resistance of reactants, which could boost power density. The (LbL) growth profile was monitored using UV-Vis spectroscopy and profilometery. Electrochemical properties were characterized using cyclic voltammetery. Scanning electron microscopy images confirm that large areas of LbL nanotubes can be made. Future work will assess how nanostructured cathodes will behave electrochemically as nanotube aspect ratio is varied. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:54PM |
L43.00008: Block-Copolymer Lithium Battery Electrolytes Invited Speaker: With high energy density at low cost, Li ion has become the most prevalent portable rechargeable battery chemistry in the world. As demand for smaller and lighter batteries grows, the energy density limitation of Li ion batteries presents a significant hurdle. Pushing the existing Li ion platform to higher energy densities compromises lifetime and safety, and these have emerged as the most pressing challenges in today's industry. The weakest link in terms of safety and stability of Li ion batteries is the organic liquid electrolyte that facilitates ionic transport between the electrodes. The continuous electrochemical degradation of the electrolyte at the electrodes causes poor cycle life of the batteries, and in some cases, runaway reactions that lead to explosions. Dry polymer electrolytes coupled to Li metal anodes had been considered a high energy alternative to liquid-based systems, as the solid-solid interface promised to alleviate the stability problems of the liquid electrolyte. However, repeated cycling of Li metal anodes leads to dendrite formation, reducing battery life and compromising safety. Recent theoretical work indicates that dendrite growth can be stopped if the shear modulus of current polymer electrolytes can be increased by three orders of magnitude without a significant decrease in ionic conductivity. Thus, the mechanical properties of polymer electrolytes are particularly important in rechargeable solid-state lithium batteries. Because ion transport in polymers is coupled to the motion of the molecules that are solvating the ions, the presence of mobile molecules is essential to allow for a conductive medium. However, the same mobility of molecules is detrimental to the polymer's structural integrity. There is, thus, a clear need to develop methodologies for decoupling the conductive and mechanical properties of polymer electrolytes. Electrolytes comprised of self-assembled block-copolymer nanostructures overcome this principal constraint. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L43.00009: Effect of Nanoscale Morphology on Selective Ethanol Transport through Block Copolymer Membranes Ashish Jha, Nitash Balsara We have examined the possibility of using A-B block copolymers for selective separation of alcohols from aqueous mixtures. The A block is not soluble in the liquids of interest and serves as the structural block while B serves as the transporting block. The size of the transporting channels has been controlled by varying the molecular weight, and the geometry has been controlled by varying the composition of the copolymer. Experimental results that reveal the dependence of membrane transport on the size and geometry of the transporting domains will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L43.00010: Simulation study of charge distribution near an ionomer-electrode interface Elshad Allahyarov, Philip Taylor Molecular dynamics simulations have been used to investigate the nature of the electrostatic field and of the proton density distribution in a Nafion-like ionomer in contact with an electrode. We compare our results for a heterogeneous ionomer, in which a partial phase separation has resulted in separate nanoscopic regions of hydrophobic and hydrophilic material, with those predicted by one-dimensional theoretical models in which Poisson-Boltzmann techniques are used to derive self-consistent potentials and concentration distributions. We further examine the effects of the strong inhomogeneous electrostatic fields in changing the morphology of the ionomer in the vicinity of the electrode from its original form in the bulk material. [Preview Abstract] |
Session L44: Surfaces, Interfaces, and Polymer Thin Films II
Sponsoring Units: DPOLYChair: Russell Thompson, University of Waterloo
Room: A309
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L44.00001: Dillon Medal Break |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L44.00002: Confinement and elastic modulus in polymer nanofibers Eyal Zussman, Michael Burman, Arkadii Arinstein Size-dependant behavior is considered in electrospun polymer nanofibers. Experimental results unambiguously show that the abrupt increase in the elastic modulus of polymer nanofibers, below a cross-over diameter, relative to the bulk could not be attributed to surface energy effect. Polyamide (\textit{Nylon-6.6}) nanofibers were tested by using either bending or tensile deformation modes (the surface energy affects the effective modulus only in the case of bending, and has no effect in the case of tensile deformation). It turns out that the obtained experimental data cannot be explained by the influence of surface energy upon the elastic modulus either qualitatively or quantitatively. This fact supports the explanation which is based on the geometrical confinement of the supermolecular structures of nano-objects. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L44.00003: Size-dependent behavior of electrospun polymer nanofibers under small deformation Arkadii Arinstein, Eyal Zussman A model describing a mechanism resulting in size-dependent behavior of electrospun polymer nanofibers under small deformation is proposed. According this model, the polymer matrix of the nanofibers consists of correlated groups of chains/subchains, partially orientated along the fiber. These supermolecular structures which were formed during electrospinning are confined by the fiber boundary. Thus, when the fiber elongates under external force the relative rotations of these correlated regions are hindered. As a result the elastic modulus depends on the diameter of the deformed fiber. In case of small fiber diameters this restriction is dominant while this effect decreases with increase of fiber diameter, and tends to zero for large fiber diameters according to square-law which was verified by experimental observations. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L44.00004: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L44.00005: Ionic conduction at liquid-liquid interfaces Francisco Solis, Monica Olvera de la Cruz In coexisting liquid phases with different dielectric constants, ionic species do not have, in general, uniform concentrations. Salt ions accumulate predominately in the liquid with higher dielectric constant. Furthermore, excess accumulation and depletion of ions appears at the interface between the liquids, In this presentation we explore the effects of these non-uniform ionic distributions in the AC conductivity of the liquid mixture. We describe in detail the frequency dependence of of the conductivity for each of the ionic components. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L44.00006: Vapor-Phase Free Radical Polymerization in the Presence of Ionic Liquids Malancha Gupta Ionic liquids (ILs) have recently attracted significant interest as an environmentally-friendly alternative to traditional volatile organic solvents because ILs are non-volatile, non-flammable, and can be easily recycled. ILs can be exploited in many ways to improve the selectivity and kinetics of chemical reactions, including polymer synthesis. Ionic liquids have negligible vapor pressure and are therefore stable under vacuum. A few studies have investigated ILs as substrates in inorganic vacuum deposition processes, but to our knowledge ILs have not been used in vapor phase polymerization systems. We have recently introduced ionic liquids into the initiated chemical vapor deposition (iCVD) process for the first time. The iCVD polymerization process occurs via a free-radical mechanism, and the deposited polymeric films are compositionally analogous to solution-phase polymers. Despite the wide range of polymers that have been synthesized using iCVD, it has proven difficult to polymerize monomers with low surface concentrations such as styrene and low propagation rates such as methyl methacrylate and it is difficult to produce block copolymers. In this talk, we will show that our novel ILiCVD system can address some of these shortcomings. We will explain the effects of deposition time, temperature, and monomer solubility on the morphology of the polymer and the molecular weight of the polymer chains. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L44.00007: Molecular orientation and photo-degradation of PTCDA films on TiO$_{2}$(110) Orhan Kizilkaya, Eizi Morikawa, Phillip Sprunger The molecular orientation and photo-degradation process of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on TiO$_{2}$(110) were investigated by near edge X-ray absorption spectroscopy (NEXAFS). As the incidence angle of p-polarized synchrotron light with respect to the substrate surface normal increases, the intensity of the $\sigma $* resonances diminishes and the $\pi $* resonances is greatly enhanced. This finding indicates that the molecular orientation of the PTCDA film is flat on the TiO$_{2}$(110) surface. NEXAFS results of pristine and photo-degraded PTCDA films exposed to synchrotron white light at the VLSPGM beamline of CAMD revealed the photo-degradation mechanism. We found that the intensity of $\sigma $* states diminishes and the intensity of $\pi $* states of increases upon the PTCDA film exposed to white light for 30 minutes. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L44.00008: Functionalization dependence of the electron beam sensitivity of bridged calix[6]arenes Gregory Spencer, Daniel Ralls, Anup Bandyopadhyay, Michael Blanda Calixarenes have long been studied as a class of high resolution, negative electron beam resists. Previous work has shown the sensitivity can be improved by adding functional groups to the monomer's molecular rim to allow for a more efficient cross-linking mechanism. However, all previous studies dealt with either unfunctionalized resists or monomers that were fully functionalized. In this study, the number of attached functional groups was deliberately varied to directly observe its effect on sensitivity. A bridged calix[6]arene monomer was used as the basic structure. The number of these attached allyl groups ranged from 0 to 8 in steps of 2 per separate synthesis. The bridging units were xylenyl groups which produced both a cone conformer and a 1-2-3-alternate conformer. Resists were formed using all nine different calix[6]arenes and each was subjected to testing. Contrast curves for the cone and alternate conformers were measured by AFM. Resist sensitivities were found as a function of the number of attached groups. The sensitivity was found to be a strong function of the number of attached groups. These results will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L44.00009: Surface Segregation of Small Macrocyclic Shih-Fan Wang, Xiaopeng Li, Renfeng Hu, Bulent Akgun, Rebecca Agapov, Chrys Wesdemiotis, David T. Wu, Mark D. Foster Surface segregation of the thin film blends containing 20wt{\%} 2k macrocyclic polystyrene were studied using surface MALDI-ToF mass spectrometry (MS), time of flight secondary ion mass spectrometry (ToF-SIMS), and neutron reflectometry (NR). To provide contrast between the species for these techniques, the linear polymer in the blend was deuterated. MALDI-ToF MS results show that the 2k macrocyclic chains are depleted from the surface after a film of an isotopic macrocylic/linear blend (h-CPS2K/ d-LPS2K) is annealed at 125$^{\circ}$C for 12hrs. The surface concentration of CPS is less than 1wt{\%}, while the surface concentration of the hydrogenous component in an analogous h-LPS2K/ d-LPS2K film is 20wt{\%} after annealing. The isotopic effect is not significant for the 2k blends and the architecture effect determines the surface segregation. ToF-SIMS and NR results corroborate this view. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L44.00010: Structure Formation and Transition Mechanism in Two-Dimensional Molecular Chiral Phases Ye-liang Wang, Bing Yang, Nan Jiang, Huan-yao Cun, Shi-xuan Du, Yue Wang, Karl-Heinz Enrst, Hong-jun Gao The self-assemble behavior of prochiral species, QA16C molecules, on a Au(111) surface and the induced chirality by 2D confinement on solid surfaces as well as its chiral transferring process will be presented in this presentation. Initial stages of a chiral phase transition in the monolayer of QA16C molecules on the Au(111) surface were investigated by scanning tunneling microscopy (STM) at submolecular resolution. The prochiral molecules form a homochiral lamella phase at low coverages upon adsorption. A transition to a racemate lattice is observed with increasing coverage. Enantiomers of a homochiral lamella line become specifically substituted by opposite enantiomers such that a heterochiral structure evolves. To explain this phenomenon, we propose a ``chiral replacement'' model: enantiomers replace QA molecules in enantiopure phase, leading to racemic one. Our findings are significant for the understanding and control of chiral phase transitions in related molecular systems like liquid crystals. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L44.00011: Formation and Collapse of Biodegradable Polymer Monolayers at the Air-Water Interface Hae-Woong Park, Kimberly Ohn, You-Yeon Won Poly(lactide-\textit{ran}-glycolide) (PLGA) is widely used as an excipient in formulations of aerosol drugs. It has recently been reported that the surface pressure-area isotherm of PLGA at the air-water interface shows a plateau at intermediate compression levels and a sharp rise in pressure upon further compression. In order to investigate the molecular origin of this behavior, we have conducted an extensive set of surface pressure and AFM imaging measurements with PLGA materials having a range of different molecular weights. The results suggest that (1) the plateau occurs due to the formation (and collapse) of a continuous water-free monolayer of the polymer under continuous compression, and (2) the monolayer becomes significantly resistant to compression at high compression because at that condition the collapsed domains become large enough to become glassy. We will also demonstrate that this property of PLGA allows the polymer to be used as an anchoring block to form a smooth biodegradable monolayer of block copolymers at the air-water interface. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L44.00012: 8CB-Langmuir Layer at air/water interface: Line Tension vs. Dipolar Repulsion Pritam Mandal, Andrew Bernoff, Adin Mann, James Alexander, Elizabeth Mann Langmuir \={ }lms of 8CB, a smectic liquid crystal at room temperature, exhibits coexistence of phases with di{\textregistered}erent thicknesses. With decompression of a 8CB-liquid-monolayer gaseous holes appear in liquid monolayer. Molecular interactions controlling the phase separation include short-range van der Waals attraction and long range dipolar repulsion. At small distances where attraction dominates gaseous domains return to energy-minimizing circular shapes. But with size of the holes increasing beyond a critical value, dipolar repulsion becomes strong enough to deform the domains; forming even labyrinth patterns. We use Brewster angle microscopy to study the \={ }lm. Our objective is to obtain a critical diameter of the domains beyond which they are non-circular. Experimental value will be compared with that from theory. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L44.00013: Origins of the Failure of Classical Nucleation Theory for Nanocellular Polymer Foams Russell Thompson, Yeongyoon Kim The behavior of nanocellular polymer foams, in which nanometer-sized bubbles of fluid are dispersed in a polymer matrix, is dominated by its internal surfaces. In particular, nucleation of a nanocellular foam can involve fundamentally different physics from microcellular or regular foams due to properties of the surfaces. Nucleation rates for nano-bubbles in polymer have been calculated using both classical nucleation theory and self-consistent field theory. An identical model is used for both calculations showing that classical nucleation theory predictions are off by many orders of magnitude. The cause of the failure of classical nucleation theory can be traced primarily to its representation of a bubble surface as an infinite planar interface. For nanoscopic bubbles, the curvature of the bubble surface is comparable to the size of the polymer molecules. Polymers on the outside of a curved bubble surface can explore more conformations than can polymers next to a flat interface. This results in a lower free energy for the curved interface system with respect to a flat interface system, which gives a significantly smaller barrier energy to nucleation and thus a much higher nucleation rate. [Preview Abstract] |
Session L45: Strongly Correlated Physics with Atoms and Molecules
Sponsoring Units: DAMOPChair: Joseph Thywissen, University of Toronto
Room: A310
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L45.00001: Quantum phase transitions in a polarized gas of dipolar molecules forming flexible chain Barbara Capogrosso-Sansone, Anatoly Kuklov We numerically demonstrate the formation of quantum flexible chains in a gas of polar molecules confined into a stack of N 1d or 2d optical lattice layers, and with dipole moment aligned perpendicularly to the layers. Molecules interact via dipole-dipole interaction. Ab initio simulations of a single chain pinned at one end reveal quantum roughening transition. Multi-chain ensemble is studied in the J-current model approximation and chain superfluidity (CSF) is found. Increasing density of the chains leads to quantum phase transition from CSF to N-layered molecular superfluid (N-SF). We discuss the nature of this transition and its dependence on density, and the conditions for experimental realization and detection of the chain soup. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L45.00002: The Hyperfine Molecular Hubbard Hamiltonian Lincoln D. Carr, Michael L. Wall An ultracold gas of heteronuclear alkali-metal dimer molecules with hyperfine structure loaded into a one-dimensional optical lattice is investigated. The hyperfine molecular Hubbard Hamiltonian (HMHH), an effective low-energy lattice Hamiltonian, is derived from first principles [1]. The large permanent electric dipole moment of these molecules gives rise to long-range dipole-dipole forces in a dc electric field and allows for transitions between rotational states in an ac microwave field. Additionally, a strong magnetic field can be used to control the hyperfine degrees of freedom independently of the rotational degrees of freedom. By tuning the angle between the dc electric and magnetic fields and the strength of the ac field, it is possible to control the number of internal states involved in the dynamics as well as the degree of correlation between the spatial and internal degrees of freedom. The HMHH's unique features have direct experimental consequences such as quantum dephasing, tunable complexity, and the dependence of the phase diagram on the molecular state.\\[4pt] [1] M. L. Wall and L. D. Carr, Phys. Rev. A \textbf{82}, 013611 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L45.00003: The Prediction of a Gapless Topological ``Haldane Liquid" Phase in a One-Dimensional Cold Polar Molecular Lattice Jason Kestner, Bin Wang, Jay Sau, Sankar Das Sarma We show that ultracold two-component fermionic dipolar gases in an optical lattice with strong two-body on-site loss can be used to realize a tunable effective spin-one model. Fermion number conservation provides an unusual constraint that $\sum_i \left(S^z_i\right)^2$ is conserved, leading to a novel topological liquid phase in one dimension which can be thought of as the gapless analog of the Haldane gapped phase of a spin- one Heisenberg chain. The properties of this phase are calculated numerically via the infinite time-evolving block decimation method and analytically via a mapping to a one-mode Luttinger liquid with hidden spin information. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L45.00004: Density wave patterns for fermionic dipolar molecules on a square optical lattice: mean field theory analysis Karlis Mikelsons, Jim Freericks We model a system of ultra cold fermionic dipolar molecules on a two dimensional square lattice. Assuming that the molecules are in their nondegenerate hyperfine ground state, and that the dipole moment is polarized perpendicular to the planes, we approximate these molecules as spinless fermions with long range repulsive dipolar interactions. We use mean field theory to obtain the phase diagram as a function of the filling, the strength of interaction and the temperature. We find a number of ordered density wave phases in the system, as well as phase separation between these phases. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L45.00005: Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms Nicolas Lopez Nicolas Lopez (University of California, Riverside, USA) Chi-Yong Lin (National Dong Hwa University, Taiwan) Shan-Wen Tsai (University of California, Riverside, USA) Cold atom experiments have realized a variety of multicomponent quantum mixtures, including Bose-Fermi atomic mixtures. Mixtures of fermionic atoms and diatomic molecules, which are boson, have also been obtained by tuning of the interactions with external fields [1]. We study many-body correlations in such a system where the molecules are weakly bound and therefore pairs of fermionic atoms easily convert into and dissociate from the bound molecule state and this exchange mediates a long-range interaction between the fermions. We consider a simple many-body Hamiltonian that includes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa [2]. We employ a functional renomalization-group approach and calculate the renormalized frequency-dependent interaction vertices and fermion self-energies. We find an instability from the disordered quantum liquid phase to a BCS phase and calculate the energy scale for the transition. The unusual frequency-dependence of this mediated interaction leads to strong renormalization of the self-energy, and also affects the couplings in the BCS channel. [1] M. Greiner, C. A. Regal, J. T. Stewart, and D. S. Jin, Phys. Rev. Lett. {\bf 94}, 110401 (2005) [2] E. Timmermans, K. Furuya, P. W. Milonni, and A. K. Kerman, Phys. Lett. A {\bf 285}, 228 (2001) [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L45.00006: Stability and Properties of the Polaron Condensate in a Strongly Interacting Boson-Fermion Mixture Zeng-Qiang Yu, Shizhong Zhang, Hui Zhai In this work we study dilute bosons embedded in a single component Fermi sea across a boson-fermion wide Feshbach resonance using a single channel model. The ground state is a condensation of bosonic polarons, and its stability requires that the interaction strength between bosons exceeds a critical value, which is a universal number at boson-fermion resonance and exhibits a maximum in unitary regime. We calculate the condensate fraction and sound velocity across resonance. The transition from polaron condensate to molecular Fermi gas is also discussed. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L45.00007: Colliding clouds of strongly interacting fermions and out-of-phase spin modes Edward Taylor, William Schneider, Shizhong Zhang, Mohit Randeria Motivated by recent experiments at MIT, we consider the problem of what happens when two Fermi clouds prepared in different hyperfine states collide with each other at low velocities close to a Feshbach scattering resonance. Upon coming into contact with each other, we show that the two clouds evolve preferentially into a metastable upper branch (with amplitude given by the coherent quasiparticle residue Z) where interactions are repulsive, and not the ground state lower branch. As a result, even though the underlying interaction between the fermions is attractive, for sufficiently strong interactions in the unitary region, the clouds will ``bounce" off each other. Using Boltzmann, sum rule and hydrodynamic approaches, we make predictions for the frequency of the bounce mode on the BEC side of resonance, including unitarity, where the scattering length is positive. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L45.00008: Ferromagnetic ordering in two-component Fermi gas: four particle problem ShengQuan Zhou, David Ceperley, Shiwei Zhang To interpret the experiment of Jo et al. on implementing the Stoner model of itinerant ferromagnetism, we investigate the energy spectrum of a system of four interacting spin-half fermions using exact diagonalization on a finite grid. The formation of molecular bound states and the ferromagnetic transition of the excited scattering states are examined systematically as a function of the interaction coupling constant. If the interaction is modeled by an effective positive scattering length, the transition density to ferromagnetism changes significantly. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L45.00009: Atom-dimer and dimer-dimer scattering in fermionic mixtures near a narrow Feshbach resonance Jesper Levinsen, Dmitry Petrov We develop a diagrammatic approach for solving few-body problems in heteronuclear fermionic mixtures near a narrow interspecies Feshbach resonance. We calculate s-, p-, and d-wave phaseshifts for the scattering of an atom by a weakly-bound dimer. The fermionic statistics of atoms and the composite nature of the dimer lead to a strong angular momentum dependence of the atom-dimer interaction, which manifests itself in a peculiar interference of the scattered s- and p-waves. This effect strengthens with the mass ratio and is remarkably pronounced in 40K-(40K-6Li) atom-dimer collisions. We discuss the collisional relaxation of the dimers to deeply bound states and evaluate the corresponding rate constant as a function of the detuning and collision energy. Finally, we calculate the scattering length for two dimers formed near a narrow interspecies resonance. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L45.00010: Phases of the attractive Hubbard model in a trap Elias Assmann, George Batrouni, Simone Chiesa, Hans Gerd Evertz, Richard Scalettar We present a quantum Monte Carlo study of the fermion attractive Hubbard model in a quadratic trap. A rather dramatic failure of the local density approximation occurs in the half-filled region where coupling to nearby superfluid domains induces a strong suppression of charge fluctuations. By monitoring the behavior of the equal-time pairing correlations, we show the existence of a low temperature phase consistent with quasi-long-range order. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L45.00011: Thermodynamics of the 3D Hubbard model on approach to the Neel transition Lode Pollet, Sebastian Fuchs, Emanuel Gull, Evgeny Burovksi, Evgeny Kozik, Thomas Pruschke, Matthias Troyer We study the thermodynamic properties of the 3D Hubbard model for temperatures down to the N{\'e}el temperature using cluster dynamical mean-field theory. In particular we calculate the energy, entropy, density, double occupancy and nearest-neighbor spin correlations as a function of chemical potential, temperature and repulsion strength. To make contact with cold-gas experiments, we also compute properties of the system subject to an external trap in the local density approximation. We find that an entropy per particle $S/N \approx 0.65(6)$ at $U/t = 8$ is sufficient to achieve a N{\'e}el state in the center of the trap, substantially higher than the entropy required in a homogeneous system. Precursors to antiferromagnetism can clearly be observed in nearest-neighbor spin correlators. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L45.00012: Superconductivity in strongly repulsive fermions: the role of kinetic-energy frustration Leonid Isaev, Gerardo Ortiz, Cristian Batista We discuss a physical mechanism of a non-BCS nature which can stabilize a superconducting state in a {\it strongly repulsive} electronic system. By considering the 2D Hubbard model with spatially modulated electron hoppings, we demonstrate how kinetic-energy frustration can lead to robust d-wave superconductivity at {\it arbitrarily} large on-site repulsion. This phenomenon should be observable in experiments using fermionic atoms, e.g. ${}^{40}K$, in specially prepared optical lattices. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L45.00013: Non-perturbative predictions for cold atom Bose gases with tunable interactions Bogdan Mihaila, Fred Cooper, Chih-Chun Chien, John F. Dawson, Eddy Timmermans We discuss a theoretical description for dilute Bose gases as a loop expansion in terms of composite-field propagators by rewriting the Lagrangian in terms of auxiliary fields related to the normal and anomalous densities. We demonstrate that already in leading order this non-perturbative approach describes a large interval of coupling-constant values, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second-order, and is consistent with the critical temperature predicted in the weak-coupling limit by the next-to-leading order large-N expansion. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L45.00014: Density functional theory for fermionic atom gases Matthias Troyer, Ping Nang Ma, Sebastiano Pilati, Xi Dai We will show how Kohn-Sham density-functional theory (DFT), which forms the basis of most electronic structure calculations in material science, can be applied to ultracold atomic gases in optical lattices. We present the derivation of an exchange correlation functional for atomic gases and show first applications within a local spin density approximation. In particular we will show that the local density approximation in DFT is much more accurate than what is commonly referred to as ``local density approximation'' in the atomic gases community. As an outlook we will discuss how the development of DFT for ultracold atomic gases can form a strong link between materials science and atomic physics. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L45.00015: Using off-diagonal confinement as a cooling method Valery Rousseau, Kalani Hettiarachchilage, Juana Moreno, Mark Jarrell, Dan Sheehy We show that the recently proposed ``off-diagonal confining" (ODC) method (Phys. Rev. Lett. 104, 167201 (2010)) can lead to temperatures that are smaller than with the conventional ``diagonal confining" (DC) method, depending on the control parameters of the system. We determine these parameters using exact diagonalizations for the hard-core case, then we extend our results to the soft-core case by performing quantum Monte Carlo simulations for both DC and ODC systems at fixed temperatures, and analysing the corresponding entropies. [Preview Abstract] |
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