Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X1: Persistent Currents in Normal Metals
Sponsoring Units: DCMPChair: Karyn LeHur, Yale University
Room: Spirit of Pittsburgh Ballroom A
Thursday, March 19, 2009 2:30PM - 3:06PM |
X1.00001: Capacitance, entanglement energetics and persistent currents of mesoscopic rings Invited Speaker: Small rings are a quintessential mesoscopic system. As a consequence of quantum coherence small normal metal rings support a persistent current. Novel experimental techniques will permit to investigate rings with geometries which highlight the effects of interactions. We discuss rings with in-line and side quantum dots. In the Coulomb blockade regime we derive an effective two-state Hamiltonian and discuss the flux dependence of the Coulomb blockade peaks in the capacitance and the persistent current [1]. Different nearly degenerate charge configurations of the ring-dot system become entangled with an electrical environment. With an environment consisting of an external resistor capacitively coupled to the ring and dot the entire system maps on a spin boson problem. Analysis shows that the visibility of the persistent current decreases with increasing coupling to the environment [2]. The system-bath entanglement which is at the origin of this phenomenon can be detected by projective measurements of the energy of the ring-dot subsystem or the persistent current: even in the ground state of the total system the ring can be found with some probability in the energetically higher lying state [3]. The distribution of energy or of the persistent currents is a direct measure of the system bath entanglement. \\[4pt] [1] M. Buttiker and C. A. Stafford, Phys. Rev. Lett. 76, 495 (1996); \\[0pt] [2] P. Cedraschi, V. V. Ponomarenko, and M. Buttiker, Phys. Rev. Lett. 84, 346 (2000); P. Cedraschi and M. Buttiker, Annals of Physics, 289, 1 (2001). \\[0pt] [3] A. N. Jordan and M. Buttiker, Phys. Rev. Lett. 92, 247901 (2004); M. Buttiker and A. N. Jordan, Physica E 29, 272 (2005); K. Le Hur, Annals of Physics, 323, 2208 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X1.00002: Persistent currents in superconducting rings well above their critical magnetic field Invited Speaker: A striking manifestation of quantum mechanics is the existence of a dissipationless current in a non-superconducting metal ring. The persistent current is analogous to electrons orbiting the nucleus in an atom. The prediction that an atom-like persistent current could be observed in a micron size metal ring generated considerable interest. The persistent current is a signature of electronic phase coherence around the ring and offers insight into many issues in mesoscopic physics such as coherence and electron interactions in metals. The small magnitude of the current and the necessity of measuring it through its associated magnetic moment make persistent current experiments challenging. Few measurements have been reported and inconsistent results have left an unclear picture of the properties of persistent currents. In a novel approach to studying persistent currents we have developed a cantilever based torsional magnetometer capable of detecting a magnetic moment of 1 $\mu _B /Hz^{1/2}$and a force of 1.6 $aN/Hz^{1/2}$ at 300 mK. I present measurements of persistent currents in normal aluminum rings in the presence of large magnetic fields. We have measured single rings and arrays of rings. I discuss the temperature, ring size, and magnetic field dependence of the persistent current and compare it to theory and previous experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X1.00003: Observation of persistent currents in thirty metal rings, one at a time Invited Speaker: It is a central prediction of quantum mechanics that a thermodynamically stable or ``persistent'' current should exist in sufficiently small electronic structures, even if they have a finite resistance as semiconducting or metallic samples do. The magnitude and even the sign of this current should vary greatly from sample to sample, but it should always have a periodic dependence on the applied magnetic field in ring-shaped samples. Due to the extremely high sensitivity required to measure persistent currents in normal metals, only a handful of experiments exist, and most measure ensemble-averaged properties. I will present measurements of the magnetic response of 33 gold rings, measured one ring at a time with a sensitive scanning SQUID technique. We find that the amplitude distribution and temperature dependence of the $h/e$-periodic persistent current is in good agreement with theoretical predictions. This result is in disagreement with the only previous experiment [1] measuring individual metal rings, which found a much larger response than expected in the three measured rings. Our results confirm predictions for the typical, disorder-realization dependent $h/e$ persistent current in diffusive rings at the single ring level, and thus address a major open question in mesoscopic physics. \\[4pt] [1] V. Chandrasekhar et al., PRL \bf{67}, 3578 (1991). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X1.00004: Effect of pair-breaking on superconductivity and on persistent currents well above the transition temperature Invited Speaker: We consider the mesoscopic normal persistent current (PC) in a very low-temperature superconductor with a bare transition temperature much smaller than the Thouless energy. We show that in a rather broad range of pair-breaking strength, in-between the bare transition temperature and the Thouless energy, the transition temperature is renormalized to zero, but the PC is hardly affected. This may provide an explanation for the magnitude of the average PC's in the noble metals, as well as a way to determine their bare transition temperatures. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X1.00005: Paramagnetic or diamagnetic persistent currents? A topological point of view Invited Speaker: A persistent current flows at low temperatures in small conducting rings when they are threaded by a magnetic flux. I will discuss the sign of this persistent current (diamagnetic or paramagnetic response) in the special case of $N$ electrons in a one dimensional ring [1]. One dimension is very special in the sense that the sign of the persistent current is entirely controlled by the topology of the system. I will establish lower bounds for the free energy in the presence of arbitrary electron-electron interactions and external potentials. Those bounds are the counterparts of upper bounds derived by Leggett using another topological argument. Rings with odd (even) numbers of polarized electrons are always diamagnetic (paramagnetic). The situation is more interesting with unpolarized electrons where Leggett upper bound breaks down: rings with $N=4n$ exhibit either paramagnetic behavior or a superconductor-like current-phase relation. The topological argument provides a rigorous justification for the phenomenological Huckel rule which states that cyclic molecules with $4n + 2$ electrons like benzene are aromatic while those with 4n electrons are not. \\[4pt] [1] Xavier Waintal, Genevi\`eve Fleury, Kyryl Kazymyrenko, Manuel Houzet, Peter Schmitteckert, and Dietmar Weinmann {\it Phys. Rev. Lett.}{\bf 101}, 106804 (2008). [Preview Abstract] |
Session X2: Vortex Dynamics and Josephson Lasers in Superconductors
Sponsoring Units: DCMPChair: Alexander Gurevich, Florida State University
Room: Spirit of Pittsburgh Ballroom BC
Thursday, March 19, 2009 2:30PM - 3:06PM |
X2.00001: Thermodynamics and Flow of the Vortex Matter at the Second-Order Glass Transition in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Invited Speaker: We study the low temperature phase diagram of the vortex matter in the high-T$_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$. By employing vortex shaking the vortex system is relaxed towards the equilibrium state. We thus reveal a novel second-order glass transition, manifested by a sharp reversible kink in the measured local magnetization \footnote{HB, N.~Avraham, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, B.~Rosenstein, E.H.~Brandt, T.~Tamegai, \emph{Phys.~Rev.~Lett.}~\textbf{95}, 257004 (2005)}. The glass line bisects the first-order melting line close to its extremum below which disorder is dominant. Consequently, the phase diagram consists of four thermodynamic phases: At high fields, above the melting line, we find amorphous vortex glass and liquid phases; Surprisingly, at low fields the glass transition separates between a low-temperature Bragg glass and a thermally depinned variant of it - possibly a perfect lattice. Studying the oxygen doping dependence of the vortex phase diagram we unexpectedly find that the novel low- temperature glass transition, along which quenched disorder should play a dominant role, has the same anisotropy dependence as that of the high-temperature melting line, where disorder is negligible \footnote{HB, T.~Verdene, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, B.~Rosenstein, D.~Li, T.~Tamegai, \emph{Phys. Rev. Lett.}, \textbf{98}, 167004 (2007)}. Finally, we utilize an indirect measurement technique to reconstruct the low- temperature I-V characteristics in the region which is inaccessible by transport measurements \footnote{HB, Y.~Myasoedov, H.~Shtrikman, E.~Zeldov, E.H.~Brandt, G.P.~Mikitik, T.~Tamegai, T.~Sasagawa, \emph{unpublished}}. At high temperatures the bulk resistance is of a thermally activated flux flow with linear I-V both in the liquid phase above the melting line as well as below it within the ordered phase. At lower temperatures, on approaching the glass transition, the temperature dependence of the bulk resistance becomes much sharper. This deviation from a simple Arrhenius behavior tracks the glass line, and may signify criticality. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X2.00002: Nanomechanics of Individual, Isolated Vortices in a Cuprate Superconductor Invited Speaker: |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X2.00003: Structure and stability of dynamic coherent states in intrinsic Josephson-junction stacks Invited Speaker: Intrinsic Josephson-junction stacks are realized in mesas fabricated out of high-temperature superconductors. Phase oscillations in different junctions can be synchronized via coupling to the intrinsic cavity mode leading to powerful electromagnetic radiation in terahertz frequency range [1,2]. As homogeneous oscillations do not couple directly to the cavity modes, the mechanism of mode excitations is a nontrivial issue. New inhomogeneous dynamic state providing such coupling has been demonstrated recently [3]. In this state, the stack spontaneously splits into two subsystems with different phase-oscillation patterns. The phase shift between the oscillations in the two subsystems is static and varies from 0 to 2$\pi$ in a narrow region near the stack center (phase kink). The oscillating electric and magnetic fields are almost homogeneous in all the junctions. The formation of this state promotes efficient pumping of the energy into the cavity resonance. We will also discuss (i) stability of coherent states (ii) synchronization in inhomogeneous mesas, and (iii) mechanisms of damping of the resonance mode.\newline [1]L. Ozyuzer \textit{et al.} , Science \textbf{318}, 1291 (2007). \newline [2]A. E. Koshelev and L. N. Bulaevskii, Phys. Rev. B \textbf{77}, 014530 (2008). \newline [3]Sh. Lin and X. Hu Phys.Rev.\ Lett., \textbf{100}, 247006 (2008); A. E. Koshelev, Phys. Rev., B \textbf{78}, 174509 (2008).\newline *\underline{\emph{In collaboration with}} L. Bulaevskii (LANL), U. Welp, C. Kurter, K. Gray (MSD, ANL), L. Ozyuzer (Izmir Institute of Technology, Turkey), K. Kadowaki (Tsukuba University, Japan) [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X2.00004: Josephson LASER Working at THz Frequencies in Intrinsic Josephson Junctions Invited Speaker: Strong, continuous and monochromatic THz electromagnetic waves with power of order of $\mu$W have successfully been generated with the mesa fabricated on the single crystal of high-$T_c$ superconductor $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ by either ion milling or FIB (Focused Ion Beam) method\footnote {L. Ozyuzer, \textit{et al.}, Science \textbf{318} (2007) 1291, K. Kadowaki, \textit{et al.}, Physica \textbf{C468} 634.}. The frequency, $f$, of the radiation depends strongly on the shape and the size of the mesa. In the case of rectangular shape it follows the relation, $f$=$c/2nw$, where $c$ is the velocity of lihgt in vaccum, $n$ the refractivity of the superconductor and $w$ the width of the mesa(shorter edge dimension), while it only depends on the radius $a$ in the case of cylindrical mesa. Higher harmonics are also observed. Another stringent requirement for the radiation is the $ac$-Josephson effect, which must be fulfilled in each intrinsic junction with the same frequency determined by the equation: $fh=2eV/N=2ev_n$, where $V$ is the voltage across the whole junction, $N$ the number of junctions involved in the mesa, $v_n$ the voltage appearing betweeen each junctions, $h$ the Planck constant, $e$ the elementary charge. Since the radiation is monochromatic, $v_n$ must be identical and synchronized coherently in all junctions in the mesa. A simple phenomenological interpretation of this synchronization is that it may occur due to the cavity resonance effect inside the mesa. The peculiar temperature dependence and the anisotropic directivity of radiation power observed experimentally may give a hint to understand the mechanism of such synchronized THz radiation from intrinsic Josephson junctions. We think that nonlinearity to be inherent in the Josephson junction as well as thermal nonequilibrium effect plays a crucial role for the synchronized THz oscillation. A more detailed view for the mechanism based on the experimental results will be presented. [Preview Abstract] |
Session X3: Spin Hall and Quantum Spin Hall Effects
Sponsoring Units: DCMPChair: David Awschalom, University of California, Santa Barbara
Room: 301/302
Thursday, March 19, 2009 2:30PM - 3:06PM |
X3.00001: Imaging electrical spin generation and spin Hall dynamics in semiconductors Invited Speaker: The capability to generate and manipulate spin polarization through the spin-orbit interaction drives interest in all-electrical techniques to exploit electron spins for semiconductor spintronics. The spin Hall effect refers to the generation of a pure spin current transverse to a charge current, resulting in a spontaneous spin accumulation near sample boundaries without the need for magnetic fields or materials. Recent experiments toward imaging this electrically generated spin polarization with both spatially and temporally resolved Kerr rotation microscopy in bulk zincblende semiconductors are discussed. Both current-induced in-plane spin polarization and out-of-plane spin accumulation from the spin Hall effect are observed in ZnSe up to room temperature\footnote{N. P. Stern, S. Ghosh, G. Xiang, M. Zhu, N. Samarth, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 126603 (2006)}. In GaAs devices, spatially resolved measurements of steady-state spin Hall accumulation and associated modeling clarify the important role of drift and diffusion in transporting spins generated at sample boundaries to the device interior\footnote{N. P. Stern, D. W. Steuerman, S. Mack, A. C. Gossard, and D. D. Awschalom, \textit{Appl. Phys. Lett.} \textbf{91}, 062109 (2007)}. In these typical optical experiments, electrically-generated spin accumulation is measured using steady-state techniques that do not directly observe dynamics at timescales important for device operation. Here we discuss a time- and spatially-resolved measurement of the spin Hall effect using a pulsed current to drive spin accumulation\footnote{N. P. Stern, D. W. Steuerman, S. Mack, A. C. Gossard, and D. D. Awschalom, \textit{Nat. Physics} \textbf{4}, 843 (2008)}. The dynamical processes of spin accumulation and diffusion reveal spatially-dependent nanosecond timescales comparable to the electric-field dependent spin coherence time. A time-dependent diffusion analysis reconciles the observed spatial and temporal dynamics of spin accumulation from the spin Hall effect in one coherent picture. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X3.00002: The quantum spin Hall effect and the topological magneto-electric effect Invited Speaker: Search for topologically non-trivial states of matter has become a important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction[1] of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation [2]. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators [4]. I shall also present realistic experimental proposals to observe fractional charge [3], spin-charge separation and the deconfinement of the magnetic monopoles in these novel topological states of matter. \\[4pt] [1] A. Bernevig, T. Hughes and S. C. Zhang, Science, 314, 1757, (2006) \\[0pt] [2] M. Koenig et al, Science 318, 766, (2007) \\[0pt] [3] X. Qi, T. Hughes and S. C. Zhang, Nature Physics, 4, 273 (2008) \\[0pt] [4] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, ``Topological Field Theory of Time-Reversal Invariant Insulators", Phys. Rev B. 78, 195424 (2008) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X3.00003: Experimental observation of the quantum spin Hall state in HgTe quantum wells Invited Speaker: Spin-orbit interaction in semiconductors causes many interesting and potentially useful transport effects, such as e.g. the presently very topical spin-Hall effect[1]. So far no direct evidence for a ballistic, intrinsic SHE (i.e. resulting from the band structure) has been obtained by transport experiments. Here, we demonstrate that in specially designed nanostructures[2], which are based on narrow gap HgTe type-III quantum wells, a detection of the spin signal is possible via non-local voltage measurements. Recently, it was pointed out that such HgTe quantum wells, that exhibit an inverted band structure where the ordering of electron- and hole-like states is interchanged, are topologically non-trivial insulators[3], in which the quantum spin Hall insulator state[4] should occur. In this novel quantum state of matter, a pair of spin polarized helical edge channels develops when the bulk of the material is insulating, leading to a quantized conductance. I will present transport data provide very direct evidence for the existence of this third quantum Hall effect: when the bulk of the material is insulating, we observe a quantized electric conductance[5]. Finally, we demonstrate how a combination of the techniques used in the above experiments allows us to verify that the transport in the quantum spin Hall insulator state indeed occurs through spin-polarized helical edge channels. \\[4pt] [1] S. Murakami et al., Science 301 (2003) 1348; J. Sinova et al., Phys. Rev. Lett. 92 (2004) 126603; Y. Kato et al., Science 306 (2004) 1910. \\[0pt] [2] E.M. Hankiewicz, et al., Phys. Rev. B 70 (2004) 241301(R).\\[0pt] [3] B.A. Bernevig et al., Science 314 (2006) 1757. \\[0pt] [4] C.L. Kane and E.J. Mele, Phys. Rev. Lett. 95 (2005) 146802. \\[0pt] [5] M. K\"{o}nig et al., Science 318, 766 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X3.00004: Theory of Topological Insulators Invited Speaker: Topological insulators are materials with a bulk excitation gap generated by the spin orbit interaction, and which are different from conventional insulators. This distinction is characterized by Z$_2$ topological invariants, which characterize the ground state. In two dimensions a single Z$_2$ invariant describes the quantum spin Hall insulator phase. In three dimensions there are four Z$_2$ invariants, distinguishing ``weak'' (WTI) and ``strong'' (STI) topological insulators. The STI phase is characterized by the presence of unique gapless surface states whose Fermi surface encloses an odd number of 2D Dirac points. We will argue theoretically that the semiconducting alloy Bi$_{1-x}$Sb$_x$ is a strong topological insulator -- a prediction that has recently been confirmed experimentally. We will next show that the proximity effect between this unique surface phase and an ordinary superconductor leads to a two dimensional state that resembles a spinless p$_x$+ip$_y$ superconductor, but does not break time reversal symmetry. This state supports zero energy Majorana bound states at vortices, and may provide a new venue to realize proposals for topological quantum computing. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X3.00005: Observation of a New Topological Phase of Quantum Matter : Quantum Hall-like Effect without Magnetic Field Invited Speaker: Most quantum states of condensed-matter are categorized by the spontaneously broken symmetries. The remarkable discovery of charge quantum Hall effects (1980s) revealed that there exists an organizational principle of matter based not on the spontaneously broken symmetry but only on the topological distinctions in the presence of time-reversal symmetry breaking. In the past few years, theoretical developments suggest that new classes of topological states of matter might exist that are purely topological in nature in the sense that they do not break time-reversal symmetry hence can be realized without any applied magnetic field : ``Quantum Hall-like effects without magnetic field.'' In this presentation, I report a series of experimental results documenting and demonstrating the existence of such a topologically ordered time-reversal-invariant state of matter and discuss the exotic electromagnetic and spin properties this novel phase of quantum matter might exhibit and outline their potential use. [Preview Abstract] |
Session X4: Responsive Gels at Surfaces and in the Bulk
Sponsoring Units: DPOLYChair: Ryan Hayward, University of Massachusetts
Room: 306/307
Thursday, March 19, 2009 2:30PM - 3:06PM |
X4.00001: Global Signaling of Localized Impact in Chemo-responsive Gels Invited Speaker: A vital function performed by skin is to send a chemical alarm signal throughout the system in response to irritation or damage. Using our recently developed 3D model for chemo-responsive gels, we design a coating that can perform an analogous, biomimetic function. Our system consists of a polymer gel undergoing the Belousov-Zhabotinsky (BZ) reaction. We show that such coatings respond to a spatially localized mechanical force by exhibiting a range of signaling behavior. For example, an initially stationary gel can emit transient waves in response to a sufficiently weak, localized impact. A stronger localized impact, however, can generate a global signal, which encompasses both chemical waves and surface ripples that propagate across the entire sample. This complex dynamical response persists even after the force is lifted. Furthermore, the spatial patterns formed by these oscillating gels reveal the location and magnitude of the applied force. Our findings open up the possibility of harnessing BZ gels for a range of applications, such as creating sensors that transmit a global signal in response to a local mechanical impact. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X4.00002: Volume-Phase Transitions and Swelling Instabilities in Surface-Tethered Responsive Gels Invited Speaker: Responsive polymer hydrogels have opened exciting opportunities for breathable structures that adopt to environmental cues. Such structures can be designed from a variety of chemical motifs that endow specific response behavior at the material level. Moreover, mechanically pinning a responsive gel to a surface presents further opportunities for designing specific shape- volume transitions due to differential swelling that arises in confined structures. We present a simple technique for fabricating responsive polymer networks based on copolymers comprising the photoactive moiety methacroyloxybenzophenone (MaBP). This approach permits the synthesis of photo-cross- linkable polymers that are easily pattered with thicknesses down to 80 Angstroms. Using a combination of neutron reflection, QCM- D, and ATR-FTIR, we have mapped the volume-phase behavior of ultra-thin layers of responsive networks as a function of chemical functionality, cross-link density, and thickness. Interestingly enough, neutron reflection reveals diffuse interfaces at the periphery of surface-attached networks that grow with the degree of swelling, pointing to surface fluctuations that result from biaxial compression. Finally, confocal microscopy has been used to map three-dimensional swelling in patterned structures revealing swelling instabilities from surface undulations to macroscopic buckling depending on the aspect ratio of the pattern. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X4.00003: Supramolecular Gels in the Bulk and at Surfaces Invited Speaker: Responsive gels might be broadly classified into two groups: those in which the material response is mediated by a phase transition involving the cooperative behavior of many molecules, or molecule moieties, together, and those in which the material response is dictated by the behavior of individual molecular components acting independently of each other. This talk will discuss the properties of supramolecular gels as a representative of the latter class of responsive materials. In the context of this talk, supramolecular gels are solvent-swollen polymer networks in which the connections between polymer chains that define the network are due to specific, directional, and reversible 1:1 interactions between molecular partners. In such cases, the properties of the network are responsive to the same stimuli that influence the reversible interaction between the molecular partners. The magnitude and sensitivity of stimulus-responsiveness in supramolecular gels is greatest in the vicinity of the gel point, and the characteristics of the sol-gel phase transition will be described for a family of coordinatively cross-linked poly(4-vinylpyridine) (PVP) organogels. It will be shown that the cross-linking interaction can have pround effects on the mechanical properties of similar, surface-bound networks prepared from end-grafted PVP. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X4.00004: Photoresponsive Polymers: Converting Light to Mechanical Work Invited Speaker: The ability to remotely control the creation of mechanical work, rapidly, with high spatial precision, and over long distances, offers many intriguing possibilities. Whether driving direct conformational changes, initiating reversible chemical reactions to release stored strain, or converting the photon to a local temperature increase, combinations of photoactive units, nanoparticles, ordered mesophases and polymeric networks are providing an expansive array of photo-responsive polymer options. By combining these material responses with kinematic concepts, mechanical devices can be demonstrated that exhibiting tunable and controllable macroscopic deformation, high-frequency oscillation or directional motion. Specifically, we will discuss recent investigation of liquid crystal polymer networks containing main chain and pendent azobenzene moieties (azo-LCN) with modulus $\sim $1.3GPa that are actuated at the absorptive overlap of the cis and trans confirmers (442 nm). Cantilevers and unconstrained beams of these azo-LCNs can be controlled by polarization angle of the source, as well as by a specific optical exposure history (on-off), with responses ranging from lateral motion to oscillations in excess of 30Hz. These phenomena reflect an underlying interaction between the mesophase order, the network architecture and kinematic constraints of the exposed material. In collaboration with Timothy Bunning, Timothy White, Hilmar Koerner, Air Force Research Laboratory; and Nelson Tabiryan, BEAM Co. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X4.00005: 3D Microfabrication of responsive protein gels Invited Speaker: |
Session X5: Active Soft Matter: From Granular Rods to Flocks to Living Cells
Sponsoring Units: GSNPChair: Cristina Marchetti, Syracuse University
Room: 401/402
Thursday, March 19, 2009 2:30PM - 3:06PM |
X5.00001: Collective dynamics of rigid and deformable self-propelled particles Invited Speaker: We discuss a series of experiments with granular matter with novel shapes which self-organize upon excitation. Previously, we reported experiments with rigid rod shaped particles with asymmetric mass distributions which show directed motion on a vibrated plate [1]. Recognizing that such a system is a simple physical model of self-propelled particles, we discuss the observed collective behavior such as aggregation at the boundaries and swirling motion in the context of various minimal leaderless models of active living systems such as bacterial colonies and hoofed animal herds which show self-organization. We will introduce and discuss the dynamics of deformable shapes consisting of a head and a tail composed of a bead chain which is shown to undergo directed motion because of differential friction associated with the head and the body. [1]: ``Swarming and swirling in self-propelled granular rods," A. Kudrolli, G. Lumay, D. Volfson, and L. Tsimring, Phys. Rev. Lett. 100, 058001 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X5.00002: Three dimensional reconstruction of starling flocks: an empirical investigation of collective animal behavior Invited Speaker: Bird flocking is a striking example of animal collective behaviour: thousands of birds gather above the roosting site, forming sharp-bordered flocks, which wheel and turn with remarkable coherence and synchronization. Despite an increasing theoretical interest, empirical investigations of collective motion have been limited so far by the difficulties of getting data on large systems. By means of stereoscopic photography and using statistical mechanics, optimization theory and computer vision techniques, we have measured for the first time the three-dimensional positions and trajectories of individual birds in groups of up to three thousands elements. This allowed us to analyze global morphological properties of the flocks, as well as structural and dynamical properties. Most notably, we investigated the nature of the inter-individual interaction. We found that the interaction between birds does not depend on their mutual metric distance, as most current models and theories assume, but rather on the topological distance (number of intermediate neighbors). In fact, we discovered that each individual interacts on average with a fixed number of neighbors (six-seven), rather than with all neighbors within a fixed metric distance. We argue that a topological interaction of this kind is indispensable to maintain flock's cohesion against the large density changes caused by external perturbations, typically predation. More recently, we characterized the velocity field, and computed dynamical observables. We showed that flocks exhibit long range correlations, which are a signature of their remarkable collective behavior. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X5.00003: Self Propelled Particles: from microdynamics to hydrodynamics Invited Speaker: In this talk I will illustrate the derivation of a unified continuum description of the large scale collective behavior of active matter from two specific physical microscopic dynamical models: stroke-averaged swimmers moving through a viscous fluid and self-propelled hard rods moving on a substrate. New results at large scales include a lowering of the density of the isotropic-nematic transition, an enhancement of longitudinal diffusion of the self-propelled orientable units, and a strong enhancement of boundary effects in confined self-propelled systems. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X5.00004: Active biopolymer gels: from cells to tissues Invited Speaker: Living cells are active soft materials that are far out of thermodynamic equilibrium. They continuously use up chemical energy in order to generate forces that drive processes such as cell migration and division. Moreover, cells actively remodel their surrounding extracellular matrix (primarily collagen), so whole tissues can also be regarded as active soft materials. The aim of our research is to understand the physical mechanisms underlying the self-organization and mechanics of cells and tissues. To this end we use an experimental approach and study simplified model systems for the cytoskeleton (purified actin, tubulin, and accessory proteins) and for tissues (fibroblast-populated collagen and fibrin gels). We use microscopy and rheology to investigate the structure and mechanics on different length scales, from the single protein up to macroscopic level. I will discuss two examples of active mechanical behavior, namely in purified actin-myosin networks, and in purified fibrin matrices with embedded contractile fibroblasts. In both cases we observe active contraction and active stiffening. We quantify the active forces and examine how the structure and mechanics of the active gels depend on motor/cell density. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X5.00005: Beller Lectureship Talk: Active response of biological cells to mechanical stress Invited Speaker: Forces exerted by and on adherent cells are important for many physiological processes such as wound healing and tissue formation. In addition, recent experiments have shown that stem cell differentiation is controlled, at least in part, by the elasticity of the surrounding matrix. We present a simple and generic theoretical model for the active response of biological cells to mechanical stress. The theory includes cell activity and mechanical forces as well as random forces as factors that determine the polarizability that relates cell orientation to stress. This allows us to explain the puzzling observation of parallel (or sometimes random) alignment of cells for static and quasi-static stresses and of nearly perpendicular alignment for dynamically varying stresses. In addition, we predict the response of the cellular orientation to a sinusoidally varying applied stress as a function of frequency and compare the theory with recent experiments. The dependence of the cell orientation angle on the Poisson ratio of the surrounding material distinguishes cells whose activity is controlled by stress from those controlled by strain. We have extended the theory to generalize the treatment of elastic inclusions in solids to ''living'' inclusions (cells) whose active polarizability, analogous to the polarizability of non-living matter, results in the feedback of cellular forces that develop in response to matrix stresses. We use this to explain recent observations of the non-monotonic dependence of stress-fiber polarization in stem cells on matrix rigidity. These findings provide a mechanical correlate for the existence of an optimal substrate elasticity for cell differentiation and function. \\[3pt] *In collaboration with R. De (Brown University), Y. Biton (Weizmann Institute), and A. Zemel (Hebrew University) and the experimental groups: Max Planck Institute, Stuttgart: S. Jungbauer, R. Kemkemer, J. Spatz; University of Pennsylvania: A. Brown, D. Discher, F. Rehfeldt. [Preview Abstract] |
Session X6: Rahman Prize Lecture and Theory of Multiferroics
Sponsoring Units: DCOMPChair: Craig Fennie, Cornell University
Room: 406
Thursday, March 19, 2009 2:30PM - 3:06PM |
X6.00001: Aneesur Rahman Prize for Computational Physics Talk: Numerical Simulations of Spin Glasses and Related Systems Invited Speaker: Systems with disorder and ``frustration'' occur in many branches of science. There has been considerable effort to understand one such type of system, known as the ``spin glass'', because it can be probed in fine detail experimentally by applying a magnetic field, and because it can be modeled by simple-looking Hamiltonians which are amenable to numerical simulation. Analytical work is very difficult and has been carried out mainly on models with unphysical features such as infinite-range interactions. Hence, much of what we know about spin glasses and related systems comes from numerical simulations on simplified models. In this talk I will describe some of the difficulties in performing \textit{reliable} spin glass simulations. Then I will discuss several questions concerning phase transitions in spin glasses and related systems that have been addressed by simulations in recent years including (i) whether there is universality, (ii) whether there is a ``vortex glass'' transition in a disordered type-II superconductor in a magnetic field, (iii) whether ``chiralities'' play a crucial role in Heisenberg spin glasses, and (iv) whether there is a line of transitions (AT line) in a magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X6.00002: Superexchange-driven Magnetoelectricity in Magnetic Vortices Invited Speaker: We demonstrate that spins in topologically frustrated antiferromagnetic systems can form periodic arrays of magnetic vortices with symmetry allowing for a linear magnetoelectric response. Realization of this magnetic structure can be provided by transition-metal oxides with a layered Kagom\'e lattice. In such systems, an appropriately structured lattice leads to a microscopic coupling between spins and polar lattice distortions via Anderson superexchange, which has the potential to provide a large magnetoelectric response. In order to quantitatively probe the strength of the magnetoelectric coupling, we have performed density functional theory calculations in the presence of an applied electric field (using linear response) for hexagonal manganites. We demonstrate that the coupling is large and summarize the challenges for achieving such a response in real materials. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X6.00003: First-Principles Approach to Lattice-Mediated Magnetoelectric Effects Invited Speaker: I will present a microscopic theory of the magnetoelectric response of an insulator, and derive from it an analytical expression for the lattice-mediated part of the effect. As I will show, such a result provides us with distinct hints at strategies to increase the magnitude of the response, as well as with a convenient method for performing first-principles calculations. I will illustrate the usefulness of the proposed approach with applications to Cr$_{2}$O$_{3}$, a model magnetoelectric crystal, and BiFeO$_{3}$ and related compounds, the best studied, and arguably most technologically promising, family of multiferroics. \newline \newline Ref.: J. I\~{n}iguez, Phys. Rev. Lett. 101, 117201 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X6.00004: First-principles Study of Improper Ferroelectricity in TbMnO$_3$ Invited Speaker: Perovskite TbMnO$_3$ at room temperature forms an orthorhombically distorted lattice with the {\it Pbnm} space group. Below $\sim$27~K the magnetic moments on the Mn atoms adopt an incommensurate cycloidal wave order, and simultaneously a polarization appears along the $c$ direction. We present the results of our first-principles theoretical study of the magnetically induced polarization in TbMnO$_3$ with a {\it commensurate} cycloidal wave of Mn$^{3+}$ moments with a wave-vector close to the experimental value.\footnote{A.~Malashevich and D.~Vanderbilt, Phys.~Rev.~Lett.~\textbf{101}, 037210 (2008).} The calculations are based on density-functional theory in the local-density approximation with the on-site Coulomb correction (LDA+U). The polarization is computed using the Berry-phase technique. We show, in particular, that the spin-orbit interaction is essential for the magnetoelectric coupling. We compute both the electronic and the lattice-mediated contributions to the polarization, and find that the latter is strongly dominant. We analyze the spin-orbit induced forces and lattice displacements from both atomic and mode-decomposition viewpoints, and show that a simple model based on nearest Mn-Mn neighbor Dzyaloshinskii-Moriya interactions is not able to account fully for the results. The direction and magnitude of our computed polarization are in good agreement with experiment. If time permits, calculations on other magnetically induced improper ferroelectrics will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X6.00005: Density functional study of the spin exchange interactions, magnetic structures and ferroelectric polarizations of multiferroics driven by magnetic order Invited Speaker: The electronic structures of magnetic insulators LiCuVO$_{4}$, LiCu$_{2}$O$_{2}$, TbMnO$_{3}$, Ca$_{3}$CoMnO$_{6}$, MnWO$_{4}$, CuFeO$_{2}$, Ba$_{2}$CoGe$_{2}$O$_{7}$ and CuBr were examined on the basis of first principles DFT+U+SOC calculations to evaluate their spin exchange parameters and account for their ordered magnetic structures. We then explored how the electric polarizations of these compounds are related to the magnetic ordering and spin-orbit coupling. In this talk results of our studies will be presented. [Preview Abstract] |
Session X7: DNA Loop Formation, Nucleosome Positioning and Transcriptional Regulation
Sponsoring Units: DBPChair: Phil Nelson, University of Pennsylvania
Room: 407
Thursday, March 19, 2009 2:30PM - 3:06PM |
X7.00001: Statistical mechanics of chromatin: Inferring free energies of nucleosome formation from high-throughput data sets Invited Speaker: Formation of nucleosome core particles is a first step towards packaging genomic DNA into chromosomes in living cells. Nucleosomes are formed by wrapping 147 base pairs of DNA around a spool of eight histone proteins. It is reasonable to assume that formation of single nucleosomes \textit{in vitro} is determined by DNA sequence alone: it costs less elastic energy to wrap a flexible DNA polymer around the histone octamer, and more if the polymer is rigid. However, it is unclear to which extent this effect is important in living cells. Cells have evolved chromatin remodeling enzymes that expend ATP to actively reposition nucleosomes. In addition, nucleosome positioning on long DNA sequences is affected by steric exclusion - many nucleosomes have to form simultaneously without overlap. Currently available bioinformatics methods for predicting nucleosome positions are trained on \textit{in vivo} data sets and are thus unable to distinguish between extrinsic and intrinsic nucleosome positioning signals. In order to see the relative importance of such signals for nucleosome positioning \textit{in vivo}, we have developed a model based on a large collection of DNA sequences from nucleosomes reconstituted \textit{in vitro }by salt dialysis. We have used these data to infer the free energy of nucleosome formation at each position along the genome. The method uses an exact result from the statistical mechanics of classical 1D fluids to infer the free energy landscape from nucleosome occupancy. We will discuss the degree to which \textit{in vitro} nucleosome occupancy profiles are predictive of \textit{in vivo} nucleosome positions, and will estimate how many nucleosomes are sequence-specific and how many are positioned purely by steric exclusion. Our approach to nucleosome energetics should be applicable across multiple organisms and genomic regions. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X7.00002: to be determined by you Invited Speaker: |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X7.00003: Interconvertible Lac Repressor--DNA Loops Revealed by Single-Molecule Experiments Invited Speaker: At many promoters, transcription is regulated by simultaneous binding of a protein to multiple sites on DNA, but the structures and dynamics of such transcription factor-mediated DNA loops are poorly understood. We directly examined in vitro loop formation mediated by E. coli lactose repressor using single-molecule structural and kinetics methods. Small (150 bp) loops form quickly and stably, even with out-of-phase operator spacings. Unexpectedly, repeated spontaneous transitions between two distinct loop structures were observed in individual protein--DNA complexes. The results imply a dynamic equilibrium between a novel loop structure with the repressor in its crystallographic ``V'' conformation and a second structure with a more extended linear repressor conformation that substantially lessens the DNA bending strain. The ability to switch between different loop structures may help to explain how robust transcription regulation is maintained even though the mechanical work required to form a loop may change substantially with metabolic conditions. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X7.00004: The Energy Landscape of Hyperstable LacI-DNA Loops Invited Speaker: The Escherichia coli LacI protein represses transcription of the lac operon by blocking access to the promoter through binding at a promoter-proximal DNA operator. The affinity of tetrameric LacI (and therefore the repression efficiency) is enhanced by simultaneous binding to an auxiliary operator, forming a DNA loop. Hyperstable LacI-DNA loops were previously shown to be formed on DNA constructs that include a sequence-directed bend flanked by operators. Biochemical experiments showed that two such constructs (9C14 and 11C12) with different helical phasing between the operators and the DNA bend form different DNA loop shapes. The geometry and topology of the loops and the relevance of alternative conformations suggested by probable flexible linkers in LacI remain unclear. Bulk and single molecule fluorescence resonance energy transfer (SM-FRET, with D. English) experiments on a dual fluorophore-labeled 9C14-LacI loop demonstrate that it adopts a single, stable, rigid closed-form loop conformation. Here, we characterize the LacI-9C14 loop by SM-FRET as a function of inducer isopropyl-$\beta$,D-thiogalactoside (IPTG) concentration. Energy transfer measurements reveal partial but incomplete destabilization of loop formation by IPTG. Surprisingly, there is no change in the energy transfer efficiency of the remaining looped population. Models for the regulation of the lac operon often assume complete disruption of LacI-operator complexes upon inducer binding to LacI. Our work shows that even at saturating IPTG there is still a significant population of LacI-DNA complexes in a looped state, in accord with previous in vivo experiments that show incomplete induction (with J. Maher). Finally, we will report progress on characterizing the ``energy landscape'' for DNA looping upon systematic variation of the DNA linkers between the operators and the bending locus. Rod mechanics simulations (with N. Perkins) provide testable predictions on loop stability, topology, and FRET. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X7.00005: Mechanics of Protein-Mediated DNA Looping Invited Speaker: The formation of looped DNA-protein complexes in which a protein or protein assembly binds to multiple distant operator sites on the DNA is a common feature for many regulatory schemes on the transcriptional level. In a living cell, a multitude of mechanical forces and constraints act on these complexes, and it is imperative to understand their effects on biological function. For this aim, we study the lactose repressor as a model system for protein-mediated DNA looping in single-molecule experiments. Using a novel axial constant-force optical trapping scheme that allows us to manipulate sub-micron DNA fragments with well-controlled forces down to the 10 fN range, we show that mechanical tension in the substrate DNA of hundred femtonewton is sufficient to disrupt the loop formation process, which suggests that such mechanical tension may provide a mechanical pathway to controlling gene expression in vivo. From the force sensitivity of the loop formation process, we can also infer the topology of the looped complex; in our case an antiparallel conformation. In addition, we will present new tethered-particle microscopy data that shows lifetimes of the looped complexes that are two to three orders of magnitude shorter than those measured in biochemical competition assays and discuss possible interpretations, including the suggestion that operator binding of the lactose repressor tetramer leads to a destabilization of the dimer-dimer interface and that thus the loop breakdown process is mostly a dissociation of the tetramer into two dimers, instead, as widely assumed, an unbinding of the tetramer from the DNA. [Preview Abstract] |
Session X8: 50 Years of Anderson Localization
Sponsoring Units: FHPChair: David Thouless, University of Washington
Room: 414/415
Thursday, March 19, 2009 2:30PM - 3:06PM |
X8.00001: Anderson localization in the seventies and beyond Invited Speaker: Little attention was paid to Anderson's challenging paper on localization for the first ten years, but from 1969 onwards it generated a lot of interest. Around that time a number of challenging questions were raised by the community, on matters such as the existence of a sharp distinction between localized and extended states, or between conductors and insulators. For some of these questions the answers are unambiguous. There certainly are energy ranges in which states are exponentially localized, in the presence of a static disordered potential. In a one-diensional potential all states are localized. There is clear evidence, in three dimensions, for energy ranges in which states are extended and diffusive. Magnetic and spin-dependent interactions play an important part in reducing localization effects. For massive particles like electrons and atoms the lowest energy states are localized, but for massless particles like photons and acoustic phonons the lowest energy states are extended. In a one-dimensional disordered system all states are localized. Uncertainties remain. Scaling theory shows that in two-dimensional systems all states are weakly localized, and that there is no minimum metallic conductivity. The interplay between disorder and mutual interactions is still an area of uncertainty, which is very important for electronic systems. Optical and dilute atomic systems provide experimental tests which allow interaction to be much less important. The quantum Hall effect provided a system where states on the Fermi surface are localized, but non-dissipative currents flow in response to an electric field. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X8.00002: Tests of Localization in Metals and Semiconductors Invited Speaker: The metal-Insulator transition has been a subject of study for decades. It is now well known that entering the critical region of the transition the characteristics of a highly correlated system dominate. The dimensionality of the system is also very important. In this talk I will reminisce about the concepts and experiments to test models, explore systems, and investigate the role of dimensionality. Mott's concept of a minimum metallic conductivity drove my own thinking until the landmark paper of Abrahams, Anderson. Licciardello and Ramankrishnan. A series of careful experiments testing the notions of weak localization followed this paper and provided critical tests of the concept. I will describe some of those experiments and the things we learned from this work. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X8.00003: Anderson Localization of Light Invited Speaker: Photonic lattices are excellent model systems for studying wave localization due to disorder. The recent progress on Anderson Localization of light will be reviewed, including the additional effects of nonlinearity, with an emphasis on the universal features common to all wave systems in nature. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X8.00004: Anderson Localization and Mesoscopics Invited Speaker: I will review certain trends developed within the last thirty years of research on Anderson Localization with emphasis on the description of the Anderson transition in terms of the entire distribution function of the conductance mesoscopic fluctuations, and on the role of electron-electron interactions. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X8.00005: Direct observation of Anderson localization of matter-waves in an optical disorder Invited Speaker: In 1958, P.W. Anderson predicted the localization$^{1}$ of electronic wave functions in disordered crystals, and the resulting absence of diffusion. It has been realized later that Anderson Localization is ubiquitous in wave physics$^{2}$, and this has prompted an intense activity to observe it with light, microwaves, sound waves, and electron gases, but to our knowledge there was no direct observation of exponential spatial localization of matter-waves (electrons or others). We have observed directly$^{3}$ exponential localization of the wave function of ultracold atoms released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. We will present this work, and the prospects of extending that experimental scheme to quantum gases in higher dimensions (2D and 3D), and with controlled interactions. We will also discuss its significance in the rapidly growing field of quantum simulators. \newline 1 Anderson, P.W. \textit{Absence of diffusion in certain random lattices}. Phys. Rev. \textbf{109}, 1492-1505 (1958). \newline 2 Van Tiggelen, B. \textit{Anderson localization of waves}. In \textit{Wave diffusion in complex media 1998}, edited by J.P. Fouque, Les Houches Lectures (Kluwer, Dordrecht, 1999). \newline 3 Juliette Billy, Vincent Josse, Zhanchun Zuo, Alain Bernard, Ben Hambrecht, Pierre Lugan, David Cl\'{e}ment, Laurent Sanchez-Palencia, Philippe Bouyer$^{ }${\&} Alain Aspect. \textit{Direct observation of Anderson localization of matter-waves in a controlled disorder} Nature$,$ 453, 891 (2008). \newline Work published back to back with a related work in the Inguscio's group in Florence: G. Roati et al., Nature$,$ 453, 895 (2008). [Preview Abstract] |
Session X9: Focus Session: Jamming: Theory and Experiment II
Sponsoring Units: GSNPChair: Bulbul Chakraborty, Brandeis University
Room: 303
Thursday, March 19, 2009 2:30PM - 3:06PM |
X9.00001: Jamming in Vibrated Granular Systems Invited Speaker: Granular materials exist all around us, from avalanches in nature to the mixing of pharmaceuticals, yet the behavior of these``fluids'' is poorly understood. Their flow can be characterized by the continuous forming and breaking of a strong force network resisting flow. This jamming/unjamming behavior is typical of a variety of systems, including granular flows, and is influenced by factors such as grain packing fraction, applied shear stress, and the random kinetic energy of the particles. I'll present experiments on quasi-static shear and free-surface granular flows under the influence of external vibrations. By using photoelastic grains, we are able to measure both particle trajectories and the local force network in these 2D flows. We find through particle tracking that dense granular flow is composed of comparable contributions from the mean flow, affine, and non-affine deformations. During shear, sufficient external vibration weakens the strong force network and reduces the amount of flow driven by sidewalls. In a rotating drum geometry, large vibrations induce failure as might be expected, while small vibration leads to strengthening of the pile. The avalanching behavior is also strongly history dependent, as evident when the rotating drum is driven in an oscillatory motion, and we find that sufficient vibration erases the memory of the pile. These results point to the central role of the mobilization of friction in quasi-static granular flow. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X9.00002: Scaling of Foam Flows near Jamming Martin van Hecke, Erik Woldhuis, Brian Tighe, Joris Remmers, Wim van Saarloos We probe the scaling behavior of flows near the jamming transition of soft, viscous discs in a variant of the well known bubble-model for foams, where we assume that the viscous forces between contacting bubbles scale with the relative velocity with an adjustable exponent $\alpha$. This allows us to explore the non-trivial dependence of global flow exponents on the local exponent $\alpha$. Even though we find that elastic stresses dominate the global stresses in the system, the exponent $\alpha$ which governs the sub-dominant viscous interactions still sets the global scaling exponents. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X9.00003: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X9.00004: Rheology of Soft Suspensions near Jamming Kerstin Nordstrom, Emilie Verneuil, Paulo Arratia, Jerry Gollub, Douglas Durian The rheology of a suspension of soft colloidal particles is investigated using a pressure-driven flow in a deep 25 $\mu $m wide microchannel. The system is composed of N-isopropylacrylamide (NIPA) colloidal microgel particles, suspended in aqueous solution. NIPA is temperature-sensitive in that the hydrodynamic radius decreases as temperature increases [1]. Therefore, colloidal suspensions of different packing fraction can be obtained simply by varying the temperature using a temperature-controlled stage. We determine the velocity profile and the local shear rate of the suspension using particle image velocimetry (PIV). We have developed methods to accurately infer the suspension shear viscosity and shear stress as a function of shear rate. The dynamical range of shear rates probed is approximately 5 orders of magnitude, ranging from 10$^{-4}$ to 10$^{1}$ s$^{-1}$. Results show that as the packing fraction is increased towards the jamming point, the velocity profiles are markedly non-Newtonian. Further, above the jamming point, the stress versus shear rate curves show yield stress behavior. [1] Alsayed, A.M.;Islam, M.F.;Zhang, J.;Collings, P.J.;Yodh, A.J., \textit{Science} \textbf{2005}. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X9.00005: Inertia and the Distribution of Avalanches in Sheared Glasses Kenneth Salerno, Mark Robbins, Craig Maloney Many slowly driven condensed matter systems show highly intermittent and spatially organized dynamics where relaxation proceeds via ``avalanches''. This talk describes the role of inertia in determining the distribution of avalanches in sheared glassy systems. Simulations were performed on binary mixtures of Lennard-Jones particles in two dimensions. The distribution of events was evaluated during steady-state quasi-static deformation in pure shear. The temperature was maintained at zero by damping the relative velocity of interacting particles. The damping rate was increased to transition from an inertial to an overdamped limit. The magnitude of avalanches E is defined as the energy dissipated. In the inertial limit the distribution P(E) of events follows power law scaling over more than four decades. As for earthquake fault systems, P(E) $\sim $ 1/E. The largest event and number of events increase roughly linearly with the system width L for L varying from 25 to 400 particle diameters. Increasing the damping or damping the total velocity removes the large events and leads to qualitative changes in the distribution. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X9.00006: Density of states in two-dimensional colloidal system Ke Chen, Zexin Zhang, Peter Yunker, Arjun Yodh The vibrational density of states (VDOS) of particles in a two-dimensional binary colloidal system was investigated using video microscopy. Our ultimate goal is to explore how the VDOS varies near the jamming transition [1].~ Various distributions of NIPA particles, whose diameters can be tuned by small temperature variations, were loaded into parallel-plate microscope cells, and their motions tracked with video microscopy. This approach permits in-situ observation over a wide range of particle packing fractions, from colloidal fluids to colloidal glasses. A search for excess VDOS at low frequencies in colloidal glass is ongoing. 1. N. Xu, M. Wyart, A. J. Liu, and S. R. Nagel, \textit{Phys. Rev. Lett.} \textbf{98}, 175502 (2007) This work is supported by NSF DMR-080488, MRSEC DMR-0520020 [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X9.00007: Applying the model of Soft Glassy Rheology to slowly driven dense granular matter Dapeng Bi, Bulbul Chakraborty In recent work by S. Henkes and B. Chakraborty (PRL 95, 198002 (2005)), a new statistical framework is proposed to describe static granular packings. In this framework, stress replaces energy as the conserved quantity and fluctuations in the stress are controlled by a quantity analogous to the thermodynamic temperature. We adapt this framework in the quasi-static limit and the model of Soft Glassy Rheology (P. Sollich, PRE 78, 2020 (1997)) to describe the rheological behavior of slowly driven dense granular matter. The model explains the experimental observation of R. P. Behringer et al. (Nature 421, 928 (2003)). We will describe ongoing efforts to apply this model to different categories of slowly driven granular media, and to relate the model to threshold critical dynamics in other driven random media. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X9.00008: Theory of random packings Hernan Makse, Chaoming Song, Ping Wang We present a theory of random packings to describe the statistical mechanics of jammed matter with the aim of shedding light to the long-standing problem of characterizing the random close packing (RCP) and random loose packing (RLP) of particles. We describe the jammed system with equations of state relating observables such as entropy, coordination number, volume fraction, and compactivity as well as the probability distributions of volume and contacts. We follow a systematic route to classify packings into a phase diagram of jamming, from frictionless to frictional particles, from hard to deformable particles, from monodisperse to polydisperse systems, from spherical particles to nonspherical convex particles, in an attempt to understand the packing problem from a unifying perspective. The studies of RCP and RLP includes 2d, nd, and the mean field limit of infinite dimension. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X9.00009: Dense packings of hard tetrahedra Amir Haji Akbar, Xiaoyu Zheng, Rolfe Petschek, Peter Palffy-Muhoray, Sharon Glotzer The densest packing of tetrahedra remains an unsolved problem, and there has been much recent debate. We simulate dense packings of mathematically smooth, hard regular tetrahedra using NPT Monte Carlo simulations and determine the density- pressure equation of state. We find disordered packings with densities that significantly exceed the hard-sphere FCC packing density of 0.740480490. Our findings thus demonstrate that tetrahedra obey Ulam's conjecture that spheres pack with a lower maximum packing density than any other hard convex object, despite recent conjecture to the contrary. The dense packings that we have found do not seem to be crystalline but are instead dense random packings. We show that the system is able to achieve such high packing densities by the local ordering of tetrahedra into certain favorable motifs, forming larger structures that pack efficiently but are overall jammed. We speculate that one or several denser crystalline packings exist. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X9.00010: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X9.00011: Continuous Geometric Families of Mechanically Stable Particle Packings Guojie Gao, Jerzy Blawzdziewicz, Corey O'Hern We have performed numerical simulations of quasistatic shear flow of soft disks at zero pressure to generate mechanically stable (MS) packings as a function of applied shear stress/strain in small 2D systems ranging from 4 to 20 disks. In systems composed of frictionless disks, we find that at any given shear strain, there are a finite number of dicrete MS packings characterized by the positions of all particles. In contrast, there are an infinite number of MS packings during continuous shear flow that form a finite geometric families (characterized by the network of interparticle contacts) as a function of shear strain. We count the number of geometric families and measure their length in strain as a function of system size. In particular, we will determine whether the MS packings at finite shear have different structural and mechanical properties from those at zero shear. We also study the effects of friction on MS particle packings. In contrast to frictionless MS packings, frictional packings form continuous geometric families even at a zero shear strain. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X9.00012: Mechanically Stable Packings of Spherocylinders Timothy Green , Scott Franklin Piles of long, thin rods are substantially more stable to perturbations than those of ordinary sand or rice. We generate 3d mechanically stable packings of spherocylinders by alternately enlarging particles (with an elastic repulsive interaction) and using a conjugate gradient minimization of the total elastic energy. The minimum stable packing $\phi_c$ is defined as the least dense packing for which the minimum energy is non-zero, and we investigate the average contact number, the spectrum of vibrational modes in the dynamical matrix, and other properties of this critical packing. We also test whether spherocylinders obey the isostatic conjecture, which states that the average contact number at $\phi_c$ is twice the number of degrees of freedom (for spherocylinders, 5). Spherocylinders' straight edges, compared with the convex sides of ellipsoids, puts the isostatic conjecture in jeopardy, perhaps requiring a greater number of contacts to maintain stability. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X9.00013: On the study of force-balance percolation J. M. Schwarz, M. Jeng We study models of correlated percolation where there are constraints on the occupation of sites that mimic force-balance, i.e. for a site to be stable (remain occupied) requires occupied neighboring sites in all four compass directions in two dimensions. We prove rigorously that $p_c<1$ for the two-dimensional models studied. Numerical data indicate that the force-balance percolation transition is discontinuous with a growing crossover length, with perhaps the same form as the jamming percolation models, suggesting the same underlying mechanism driving the transition in both cases. In other words, force-balance percolation and jamming percolation may indeed belong to the same universality class. We find a lower bound for the correlation length in the connected phase and that the correlation function does not appear to be a power law at the transition. Finally, we study the dynamics of the culling procedure invoked to obtain the force-balance configurations and measure a dynamical exponent similar to that found in sandpile models. [Preview Abstract] |
Session X10: Insulators and Dielectrics: Defects, Structure, and Mechanical and Dynamical Properties
Sponsoring Units: DCMPChair: Tim Fister, Argonne National Laboratory
Room: 304
Thursday, March 19, 2009 2:30PM - 2:42PM |
X10.00001: Dynamics of implant damage as a precursor to nanocrystal nucleation Matthew J. Beck, Sokrates T. Pantelides Ion implantation into \emph{a}-SiO$_{2}$ leads to the self-assembly of metal or semiconductor nanocrystal arrays having applications in optical and non-volatile memory devices. The production of uniform arrays of similarly-sized nanocrystals within the \emph{a}-SiO$_{2}$ matrix has been shown to depend strongly on nucleation conditions. Here we report results of quantum mechanical calculations probing the atomic-scale dynamics in the time immediately following ion-induced low-energy recoils. We show that individual low-energy recoils (with KE$\sim$100 eV) do not produce individual, isolated defects in the \emph{a}-SiO$_{2}$ structure, but rather produce nanoscale defect pockets. These defect pockets are sources for oxygen out-diffusion, and subsequently represent seed regions for nanocrystal nucleation. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X10.00002: Optical Emission from F$_{H}$(CN$^{-}$ ) centers in CsCl Joseph West, Richard Dallinger, Ryan Lidster Strong, previously unreported, emission from at least two different excited electronic states of F$_{H}$(CN$^{-})$ centers in CsCl has been measured following excitation at 532 nm, 514 nm and 568 nm. The temperature dependence of emission at 532 nm was obtained over the temperature range of 16K through 160K. No emission under excitation at 633 nm was detected. The presence of emission following excitation at the higher energies, but absent under 633 nm excitation, may suggest that the well-known energy transfer process from the electronic excited F-center states to the vibrational CN$^{-}$ energy levels in this system occurs from a single common relaxed excited electronic state associated with the absorption band at 633 nm. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X10.00003: Ab-Initio investigation of defects in GaTe Cedric Rocha Leao, Vincenzo Lordi Materials that are good candidates for room-temperature radiation detectors should ideally possess several characteristics that are sometimes contradictory. A high carrier mobility-lifetime product is required to maximize the collection of radiation generated charge, but a relatively large bandgap is desired to minimize thermal noise. High resistivity is usually also desired, to reduce background current which degrades the detection resolution. Furthermore, a high average atomic number increases absorption of high energy radiation. GaTe is attracting recent attention for the potential to satisfy many of these criteria, but its properties are still poorly understood. Like other III-VI compounds, GaTe is a layered material, but its unusual anisotropy in the atomic planes results in rather unique mechanical and electronic properties. In this talk, we discuss ab-initio calculations of the transport properties of GaTe with respect to its application to radiation detectors. Guided by very recent experimental results, we analyze the occurrence of native defects in this materials and possible compensating extrinsic defects, and their effects on the transport properties. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X10.00004: Electronic properties of oxygen vacancy in HfO$_{2}$ within GW calculations Eun-Ae Choi, Kee Joo Chang Hafnia (HfO$_{2})$ has attracted much attention as a high-k dielectric material, which substitutes for silicon gate oxide in nanoscale metal-oxide-semiconductor (MOS) devices. However, there remain several problems to be resolved in hafnia-based devices, such as flat band shift and threshold voltage instability. Oxygen vacancy, as the most common intrinsic defect, is regarded as a major cause of these problems. As previous calculations mostly rely on the local-density-functional approximation (LDA), the defect levels of oxygen vacancy are not accurately determined because of the LDA band gap problem. Here we perform GW calculations for the defect levels of oxygen vacancy in monoclinic HfO$_{2}$. Our calculations show that the Fermi level pinning of p+ poly Si gate electrode is due to the charge transfer from oxygen vacancy to the electrode. In addition, the charge trap of oxygen vacancy can lead to the threshold voltage instability in both nMOS and pMOS devices. Finally, we suggest that oxygen vacancy may be a cause of the gate leakage current by the Poole-Frenkel conduction. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X10.00005: DFT Energetics of Noble Gas Impurities and Schottky Defects in UO2 Alexander Thompson, Chris Wolverton There is a strong need to better understand the mechanisms of nuclear fuel swelling in uranium dioxide (UO2) due to formation of fission product gases. Using density functional theory (DFT+U) calculations, we have explored the energetics of noble gases in UO2, Schottky defects (SD) in UO2, and the interaction between these defects. We find: (i) The noble gas atoms show a strong size dependence of the incorporation energy. (ii) The energetics of the SD in three different geometries shows that the preferred geometry is not simply driven by electrostatic effects. (iii) The energetics of each of the noble gas atoms incorporated in a SD show a strong favorable binding, due to strain relief. (iv) For Ar, Kr, and Xe, the binding energy of a noble gas impurity with the SD is larger than the energy required to form a SD, thereby providing an energetic pathway for the ``spontaneous'' formation of these defects. (v) From our calculations, we have constructed a simple model of the critical number of noble gas atoms required to form a bubble. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X10.00006: Polarization Patterns In GeTe From Bulk To Ferroelectric Nanoclusters Engin Durgun, Riad Shaltaf, Xavier Gonze, Philippe Ghosez, Jean-Yves Raty In this study, we investigated the ferroelectric and structural properties of GeTe crystal and nanoclusters, an alternative type of ferroelectric material, up to 1500 atoms from first-principles calculations based on density functional theory (DFT). Firstly, the dynamical, dielectric and elastic properties of GeTe in ferroelectric phase have been investigated [1]. Next, we demonstrate, for the first time at the DFT level, the existence in the interior of sufficiently large dots of polarization vortices giving rise to a net and reversible toroidal moment of polarization (G) [2]. The amplitude of G decreases with the size of the system and is totally suppressed below a critical diameter of 2.7 nm. The pattern of atomic distortions and the size evolution of the properties are discussed in relationship with the existence of a surface region within which the atoms behave differently. The validity of the results is checked for thicker structures with more layers and also for nanowires which can be considered as an infinite limit. (*)engin.durgun@ulg.ac.be [1] R. Shaltaf, E. Durgun, J.-Y. Raty, Ph. Ghosez, and X. Gonze Phys. Rev. B 78, 205203 (2008) [2] E. Durgun, R. Shaltaf, J.-Y. Raty, X. Gonze, and Ph. Ghosez Nano. Lett. (submitted) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X10.00007: First-principles study of point defects in $\kappa $-Al$_{2}$O$_{3}$ Justin R. Weber, Anderson Janotti, Chris G. Van de Walle The development of gate-stack structures for novel CMOS applications has stimulated interest in point defects that may occur in oxide dielectrics. We consider Al$_{2}$O$_{3}$ as a possible gate-stack material, and study defects in the low density $\kappa $ phase. The choice of the $\kappa $-Al$_{2}$O$_{3}$ phase is based on the similarity of its density to that of amorphous Al$_{2}$O$_{3}$. We analyze native point defects such as vacancies, self-interstitials, and antisites, as well as various relevant impurities. Our first-principles calculations are based on density functional theory (DFT). Hybrid functionals were utilized as a means of overcoming the band-gap problem. This approach allows us to accurately assess the positions of defect levels. We use calculated band offsets to make predictions about the location of these defect levels with respect to the band gap of relevant semiconductors used as channel materials. We will discuss which defects may impede the optimal performance of devices. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X10.00008: First-principles study on the electromigration of oxygen vacancy in metal oxides Sang Ho Jeon, Won-joon Son, Bae Ho Park, Seungwu Han The oxygen vacancy, which is a fundamental defect in oxides, plays a critical role in defining many electrical properties of oxides ranging from ionic conductivities to leakage behaviors. As such, to control the density and spatial distribution of the oxygen vacancy has often been an important goal in many researches on electronic devices, particularly for high-density devices, such as resistance-change random access memories(ReRAM). Despite its importance, the electromigration of oxygen vacancy has not been studied much from the microscopic point of view. In this presentation, we studied on the migration of the oxygen vacancy in metal oxides, such as MgO and TiO2. First, using the nudged elastic band(NEB) method, we estimated the migration barrier of charged oxygen vacancy under an external field. Then, we calculate the zone-center phonon modes of the bulk system to obtain the attempt frequency of the vacancy diffusion. Based on these results, we estimated the migration time of oxygen vacancy in metal oxide by using harmonic transition state theory, and it was in good agreement with the result of molecular dynamics(MD) calculation. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X10.00009: First-principles calculations of Ce activation in RE$_{2}$M$_{2}$O$_{7}$ (RE = La, Y; M = Ti, Zr, Hf). Anurag Chaudhry, Andrew Canning, Rostyslav Boutchko, Stephen Derenzo, Niels Gronbech-Jensen First-principles electronic structure calculations of Ce-doped La and Y compounds with composition RE2M2O7 (RE = La, Y; M = Ti, Zr, Hf) are performed using the pseudopotential method based on the local density approximation in density functional theory. The positions of the 4f states relative to the valence band maximum and the position of the lowest 5d excited state relative to the conduction band minimum of the host material are determined. The prediction of Ce activation is based on the following criteria: (1) The energy difference between the occupied Ce 5d excited state (Ce3+)* and the host material conduction band minimum (CBM) and (2) the degree of localization of the (Ce3+)* excited state on the Ce atom. Our theoretical investigations indicate that Ce activation is not possible in these host materials. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X10.00010: Electric Field Induced Sub-Microsecond Resistive Switching Nilanjan Das, Stephen Tsui, Ya-Qi Wang, Yuyi Xue, Ching-Wu (Paul) Chu Electric field induced resistive switching in metal-oxide interfaces has attracted extensive recent interest. While many agree that lattice defects play a key role, details of the physical processes are far from clear. There is debate, for example, regarding whether the electromigration of pre-existing point defects or the field-created larger lattice-defects dominates the switch. We investigate several Ag-Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO) samples exhibiting either sub-microsecond fast switching or slow quasistatic DC switching. It is found that the carrier trapping potentials are very different for the pre-existing point defects associated with doping (and/or electromigration) and the defects responsible for the sub-$\mu $s fast-switching. Creation/removal of the defects with more severe lattice-distortions and spatial spreading (trapping potential $\ge $ 0.35 eV), therefore, should be the dominating mechanism during sub-$\mu $s switching. On the other hand, the shallow defects (trapping potential $<<$ 0.2eV) associated with doping/annealing are most likely responsible for the resistance hysteresis (slow switch) during quasistatic voltage sweep. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X10.00011: Temperature Dependence of Gigahertz-Range Ultra-High Frequency Micromechanical Resonators Josef-Stefan Wenzler, Tyler Dunn, Diego Guerra, Pritiraj Mohanty We report measurements of bulk mode resonators in the ultrahigh frequency range up to 4 GHz. The devices are fabricated with a stack of materials and actuated using piezeoelectric technique. Typical dimensions of these resonators are 100 $\mu $m in length and width and 10 $\mu $m in thickness. The temperature dependence of mode frequencies and quality factor Q are investigated for temperatures ranging from 0.3 K - 400 K. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X10.00012: Temperature dependence of mechanical stiffness and dissipation in ultrananocrystalline diamond resonators Vivekananda Adiga, Anirudha Sumant, Sampath Suresh, Chris Gudeman, Orlando Auciello, John Carlisle, Robert Carpick We have studied the mechanical softening and dissipation of ultrananocrystalline diamond (UNCD) resonators with temperature. Resonant excitation and ring down measurements were conducted under ultra high vacuum (UHV) conditions in a decoupled UHV atomic force microscope (AFM) to determine the Young's Modulus and quality factor (Q) in UNCD cantilever structures. The temperature dependence of Young's modulus revealed the characteristic Wachtman's empirical relation. From this measurement the Debye temperature was estimated to be $\sim $ 1460\r{ }K, significantly lower than Debye temperature of 1860\r{ }K for single crystal diamond. The quality factors of different resonators increased as the cantilevers were cooled from 300\r{ }K to 30\r{ }K and with the hydrogen termination of the cantilever surface. The results indicate that surface and bulk defects significantly contribute to the observed dissipation as well as the mechanical softening in UNCD resonators. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X10.00013: Evidence of universality in the dynamical response of micromechanical diamond resonators at millikelvin temperatures Matthias Imboden, Pritiraj Mohanty We report kelvin to millikelvin-temperature measurements of dissipation and frequency shift in megahertz-range resonators fabricated from ultra-nanocrystalline diamond. Frequency shift $\delta $f/f$_{0}$ and dissipation Q$^{-1}$ demonstrate temperature dependence in the millikelvin range similar to that predicted by the glass model of tunneling two level systems. The logarithmic temperature dependence $\delta $f/f$_{0}$ is in good agreement with such models, which include phonon relaxation and phonon resonant absorption. Dissipation shows a weak power law, Q$^{-1} \quad \sim $ T$^{1/3}$, followed by saturation at low temperature. A comparison of both the scaled frequency shift and dissipation in equivalent micromechanical structures made of single-crystal silicon and gallium arsenide indicates universality in the dynamical response. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X10.00014: Small to medium atomic size-mismatch leads to alloy phase-separation yet huge mismatch can lead to ordering Xiuwen Zhang, Giancarlo Trimarchi, Mayeul d'Avezac, Alex Zunger Most alkali halide alloys AX-BX (where A and B are alkali elements and X=F, Cl, Br, or I) are expected to have miscibility gaps (phase separation) which increases with the lattice mismatch. Even though LiX-RbX and LiX-CsX with lattice mismatches of $20-33\%$ and $19-40\%$ respectively might be expected to have pronounced miscibility gaps, they were experimentally found to have ordered structures. Here, we investigate the possible stabilization of ordered compounds with respect to random configurations. In the family of LiNaBr$_{2}$, LiKBr$_{2}$, LiRbBr$_{2}$, and LiCsBr$_{2}$, we find that as the lattice mismatch increases, the formation enthalpy of the random structure increases, (mainly due to the volume deformation), but the formation enthalpy of the ordered structure decreases becoming negative for the latter two. The ordered structures consist of distorted LiX4 tetrahedral arranged in layers, with Rb (or Cs) sitting between layers at the center of the resulting triangular prism. We analyze the origin of ordering from the large local distortion induced by the huge lattice mismatch. [Preview Abstract] |
Session X11: Fermions in Optical Lattices II
Sponsoring Units: DAMOPChair: Conjung Wu, University of California, San Diego
Room: 305
Thursday, March 19, 2009 2:30PM - 2:42PM |
X11.00001: Non-Equilibrium Enhancement of Superfluidity in a Trapped Fermi Gas Andrew Robertson, Victor Galitski In 1970, Eliashberg showed that superconductivity could be stimulated by pushing the quasiparticle spectrum out of equilibrium and to higher energies using a periodic perturbation with a frequency of the order of the superconducting gap (Eliashberg, JETP Lett. 11, 114 (1970)). This effect has been observed in thin films (TM Klapwijk et al. JLTP 26, 3-4 (1977)). The theory of this gap enhancement can be mapped onto a cold Fermi gas. We present here the theoretical framework for describing the stimulation of the BCS order parameter in an interacting Fermi gas by means of a periodic perturbation. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X11.00002: Realizing the Strongly Correlated $d$-Wave Mott-Insulator State in a Fermionic Cold-Atom Optical Lattice Michael Peterson, Chuanwei Zhang, Sumanta Tewari, Sankar Das Sarma We show that a new state of matter, the $d$-wave Mott-insulator state ($d$-Mott state) (introduced recently by [H. Yao, W. F. Tsai, and S. A. Kivelson, Phys. Rev. B 76, 161104 (2007)]), which is characterized by a nonzero expectation value of a local plaquette operator embedded in an insulating state, can be engineered using ultracold atomic fermions in two-dimensional double-well optical lattices. We characterize and analyze the parameter regime where the $d$-Mott state is stable. We predict the testable signatures of the state in the time-of-flight measurements. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X11.00003: Spectral function of spinless fermions on a one-dimensional lattice Rodrigo Pereira, Steven White, Ian Affleck We study the spectral function of spinless fermions for an integrable lattice model away from half-filling. The sharp features of the spectral function at arbitrary momentum are argued to be power law singularities analogous to the x-ray edge singularity. Besides the singularity at the energy of the single-particle excitation, we find that at low fillings the spectral function can exhibit a second divergence associated with the formation of a p-wave antibound state. The predictions from the effective field theory are compared with numerical results from the time-dependent density matrix renormalization group. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X11.00004: Superfluidity at the BEC-BCS crossover in two-dimensional Fermi gases with population and mass imbalance Gareth Conduit, Peter Conlon, Ben Simons We explore the zero-temperature phase behavior of a two-dimensional two-component atomic Fermi gas with population and mass imbalance in the regime of the BEC-BCS crossover. Working in the mean-field approximation, we show that the normal and homogeneous balanced superfluid phases are separated by an inhomogeneous superfluid phase of Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type. We obtain an analytical expression for the line of continuous transitions separating the normal and inhomogeneous FFLO phases. We further show that the transition from the FFLO phase to the homogeneous balanced superfluid is discontinuous leading to phase separation. If the species have different masses, the superfluid phase is favored when the lighter species is in excess. We explore the implications of these findings for the properties of the two-component Fermi gas in the atomic trap geometry. Finally, we compare and contrast our findings with the predicted phase behavior of the electron-hole bilayer system. [1] Phys. Rev. A 77, 053617 (2008) [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X11.00005: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X11.00006: Effective theory for weakly coupled one-dimensional imbalanced Fermi gas Erhai Zhao, W. Vincent Liu We present a theory for a lattice array of weakly coupled one-dimensional ultracold attractive Fermi gases (1D ``tubes") with spin imbalance, which are currently under experimental investigation using ultra-cold $^6$Li atoms. We first construct an effective field theory for the 1D Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state based on the exact solution. Special attention is paid to the effect of spin-charge mixing. Then we analyze the instability of the 1D FFLO state against inter-tube tunneling by renormalization group analysis to obtain the phase diagram of the quasi-1D system and further determine the scaling of the superfluid transition temperature with intertube coupling. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X11.00007: Cold Fermionic Atoms in Two Dimensional Traps--Pairing versus Hund's Rule Stephanie Reimann, Massimo Rontani, Jeremy Armstrong, Yongle Yu, Sven {\AA}berg The microscopic properties of a finite many-body system of few interacting cold fermionic atoms confined in a two-dimensional (2D) harmonic trap are studied by numerical diagonalization. For repulsive interactions, a strong shell structure dominates with Hund's Rule acting in its extreme for mid-shell configurations. In the attractive case, odd/even oscillations due to pairing occur simultaneously with deformations in the internal structure of the ground states, as seen from pair correlation functions. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X11.00008: Cold Fermion Pairing in Two-Dimensional Harmonic Traps Jeremy Armstrong, Massimo Rontani, Sven {\AA}berg, Vladimir Zelevinsky, Stephanie Reimann Trapped, ultra-cold atomic gasses have provided a rich testing ground for quantum theories. We apply a pairing model from nuclear physics to a 2D harmonically confined, two-component atomic gas containing 2-9 particles. Our Hamiltonian consists of the oscillator mean field and a contact pairing interaction. We calculate excitation spectra, yrast spectra, the BCS delta, and addition energies for various values of the pairing strength. As expected, when the interaction is weak, the oscillator mean field is dominant, and as the interaction strength is increased, pairing effects become quite clear. Results are compared with \textit{ab initio} calculations. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X11.00009: Ultra-cold fermions with attractive interactions in optical lattices Chihchun Chien, Qijin Chen, Kathryn Levin We address how attractive Hubbard model can be simulated using ultra-cold fermions with attractive interactions loaded into optical lattices. Our study may be relevant to high-temperature superconductivity. For s-wave pairing, smooth crossover behavior similar to the BCS-Bose Einstein condensation crossover in homogeneous Fermi gases can only be observed at low fillings. Near half filling crossover in lattices is interrupted and the BCS wavefunction breaks down. By analyzing the attractive Hubbard model in the strongly attractive regime, we show that states with local pairs instead of Cooper pairs are better ground states. We also study d-wave pairing in optical lattices and find that interruption of crossover occurs at almost all fillings. Our phase diagram for d-wave pairing capture some features of experimental phase diagrams of high-temperature superconductors. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X11.00010: General Hubbard Model for Fermions in an Optical Lattice Jason Kestner, Luming Duan For two-component fermions in an optical lattice, an effective general Hubbard model (GHM) with tunable on-site attraction/repulsion and occupation-dependent hopping rates emerges from very general arguments [1]. This model is quite interesting, containing as special cases both the t-J and the XXZ models. However, the experimental range of applicability and the connection between the model parameters and the actual experimental parameters must be determined explicitly. To this end, we have used a stochastic variational approach with a correlated gaussian wavefunction to numerically find the eigenstates of two atoms interacting in a 3D few-well trap. By matching the few-site spectrum of the GHM to the variational spectrum obtained, the validity of the model and the relationship between experimental and model parameters are determined. [1] L.-M. Duan, Euro. Phys. Lett. 81, 20001 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X11.00011: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X11.00012: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X11.00013: Probing Nagaoka ferromagnetism in optical superlattices Javier von Stecher, Eugene Demler, Mikahil Lukin , Ana Maria Rey In 1966, Nagaoka predicted that interaction-induced ferromagnetism occurs~in lattices with specific geometry when there is one fewer electron than in the half-filled system. Here, we describe a controllable method for observing Nagaoka Ferromagnetism in isolated plaquettes (four lattice sites arranged in a square) created using~optical superlattices. We next discuss the weakly coupled plaquettes and suggest several approaches for creating systems exhibiting itinerant ferromagnetism. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X11.00014: Competition between spin imbalance and mass imbalance in the 1D asymmetric Hubbard model Wen-Long Lu, Zhi-Guo Wang, Shi-Jian Gu, Hai-Qing Lin In this talk, I will discuss the spin imbalance in the 1D asymmetric Hubbard model in the negative U region by the Bosonization method. A ground-state phase diagram has been obtained. We find that, unlike the $N_\downarrow=N_\uparrow$ case, there is no other phase transition in the ground state (always Singlet Superconducting) before it enters into the phase separation region, and the pairing correlation function is founding to oscillate in real space (FFLO state). The maximum mode is only determined by difference of Fermi momenta, and the correlation exponent is determined by both the mass difference and spin polarization. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X11.00015: Spin-coherent and -incoherent Luttinger liquids in trapped ultracold atomic Fermi gases Paata Kakashvili, C. J. Bolech Recent success in manipulating ultracold atomic systems allows to probe different strongly correlated regimes in one dimension. Experimentally, 1D tubes are defined by turning on a 2D optical lattice. Regimes such as the spin-coherent Luttinger liquid and the spin-incoherent Luttinger liquid can be realized by tuning the inter-atomic interaction strength and trap parameters. Due to the trap potential the density decreases near the edges of the tubes and the spin-incoherent regime is inevitably realized. In general, the spin-coherent Luttinger liquid regime in the center of the tube crosses over to its spin-incoherent counterpart at the edges. We identify the noise correlations of density fluctuations as a robust observable (uniquely suited in the context of trapped atomic gases) to discriminate between these two regimes. Finally, we address the concrete prospects of realizing and probing these phenomena experimentally using optical lattices. [Preview Abstract] |
Session X12: Structure and Morphology of Oxide Surfaces and Thin Films
Sponsoring Units: DMP DCMPChair: Shao-Chun Li, Tulane University
Room: 308
Thursday, March 19, 2009 2:30PM - 2:42PM |
X12.00001: Finding stable $\alpha$-quartz (0001) surface structures via simulations Yun-Wen Chen, Chao Cao, Hai-Ping Cheng Reconstruction of $\alpha$-quartz (0001) surfaces is studied using combined classical molecular dynamics and density functional theory. Five reconstruction patterns are identified, including three (2 $\times$ 1) patterns and two (1 $\times$ 1) patterns. The energetically most stable surface structure is found to be a (2 $\times$ 1) reconstruction pattern, and several patterns can co-exist in a large scale surface. A combination of structures can explain the experimentally observed (2 $\times$ 2) diffraction pattern. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X12.00002: Investigation of stability issues of TCO barrier layers for CIGS devices during damp heat and dry heat exposures Rajalakshmi Sundaramoorthy, Ingrid Repins, David Albin, John Pern, Xiaonan Li, Tim Gessert, Thomas Gennett The reliability of In$_{2}$O$_{3}$:SnO$_{2}$ (ITO) and In$_{2}$O$_{3}$:ZnO (IZO) as barrier layers for CuInGaSe$_{2}$ (CIGS) solar cells has been investigated. NREL's high-efficiency CIGS devices are prepared using a three-stage process for the CIGS layer, and insulating ZnO and ZnO:Al as the (bi-layer) transparent conducting oxide (TCO) buffer and conducting layers, respectively. These CIGS devices are processed to explore the effectiveness of barrier layers of ITO and IZO sputtered at room temperature and at various temperatures. Devices are exposed to damp heat at 85$^{o}$C and 85{\%} relative humidity (RH) and dry heat conditions (85$^{o}$C/$\sim $0{\%} RH). Some cells are also tested under 1-sun illumination and open-circuit voltage bias. Optical, electrical, structural, and imaging analyses are used to characterize the samples periodically before and after the exposures. Surface depth profiling and relative concentration of the elements present are analyzed using XPS. Results of these stability studies will be discussed. This abstract is subject to government rights. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X12.00003: Scanning Tunneling Microscopy Study of a Vicinal Anatase TiO$_{2}$ Surface Shao-Chun Li, Olga Dulub, Ulrike Diebold Titanium dioxide finds versatile applications in various technical fields including gas sensing, coatings, pigments, heterogeneous catalysis, photocatalytic degradation of pollutants, and solar cells. TiO$_{2}$ is found in three main crystallographic phases: rutile, anatase and brookite. Rutile is the thermodynamically most stable form and is considered a model system for basic research. However, anatase TiO$_{2}$ is often considered to be catalytically more active than rutile for reasons not yet completely understood. In this work, using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), the structure of the anatase TiO$_{2}(\mbox{5}$ $\overline{14}$) surface, $\sim $10\r{ } vicinal to the -- lowest energy -- (101) plane, has been studied. The surface was found to facet into a structure composed of ridges with a uniform width of 5 lattice units. Based on atomically-resolved STM and electron counting rules, it is proposed that the sides of the ridges are parallel to ($\mbox{1}$ $\overline{10}$) and (112) planes. These sides might be reconstructed to stabilize the microfaceted structure. Vapor-deposited gold shows pronounced clustering between the ridges, indicating a one-dimensional template effect of the vicinal surface, which supports denser and more uniformly sized Au clusters, as compared to the flat (101) surface. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X12.00004: $\beta -$Si$_{3}$N$_{4}$/CeO$_{2-x }$Interface Investigated via Atomic Resolution Z-contrast Imaging, Electron Energy-Loss Spectroscopy and First-Principles Methods Weronika Walkosz, Robert F. Klie, Serdar Ogut, Biljana Mikijelj, Stephen J. Pennycook, Juan C. Idrobo The addition of rare-earth oxides, typically forming intergranular glassy films in ceramics, has long been known to markedly affect toughness and creep resistance of Si$_{3}$N$_{4}$. The present work investigates the bonding characteristics of cerium at the interface between Si$_{3}$N$_{4 }$grains and the secondary ceria phases with aberration-corrected scanning transmission electron microscopy techniques. The obtained Z-contrast images and the electron energy-loss spectra taken at the interface of Si$_{3}$N$_{4}$/CeO$_{2-x}$ suggest that the arrangement of Ce at the interface depends on the thickness of the intergranular film, the electronic structure of the rare earth element, as well as the termination of Si$_{3}$N$_{4}$. Possible reasons for these observed structural and electronic variations at the interface, and their agreement with the theoretical predictions of two stoichiometric terminations of Si$_{3}$N$_{4}(10\mathop 1\limits^- 0)$ surface will be briefly discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X12.00005: Observation of a ($\surd $3x$\surd $3)R30\r{ } Reconstruction on O-Polar ZnO Surfaces Seth King, Somendra Parihar, Kallol Pradhan, H. Trevor Johnson-Steigelman, Paul Lyman Low energy electron diffraction revealed a previously unreported ($\surd $3x$\surd $3)R30\r{ } reconstruction on clean, O-polar ZnO surfaces after\textit{ in-situ} preparation under conditions with an extremely low H background or after \textit{ex-situ }tube-furnace annealing [1]. It has been proposed that unreconstructed, H-free, O-polar ZnO surfaces cannot be produced [2]. As the sample is prepared from the as-received surface, to a clean (1x1), and finally to the clean ($\surd $3x$\surd $3)R30\r{ } reconstruction, x-ray photoelectron spectroscopy shows decreasing intensity of the hydroxyl shoulder (located to the high-binding-energy side of the O1$s$ peak). This reconstruction is stable against H$_{2}$, N$_{2}$, and air, although its formation is suppressed when preparation occurs under an intentional H$_{2}$ background. A structural investigation of the reconstruction with LEED-IV is under way. \underline {\textbf{References:}} [1] S.T. King \textit{et al.}, Surf. Sci. \textbf{604}, L131 (2008) [2] C. W\"{o}ll, Prog. Surf. Sci. \textbf{82}, 55 (2007) [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X12.00006: Density functional study of GaN/ZnO alloy structure and surface properties Li Li, Xiao Shen, Philip B. Allen Recent experiments\footnote{K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito,Y. Inoue, and K. Domen, Nature 440, 295 (2006).} show that GaN/ZnO alloy is a promising host for photo-catalytic water splitting. The band-gap of this alloy is narrowed down to the visible regime, allowing photo-absorption over a large portion of the solar spectrum. Muckerman et\footnote{L. L. Jensen, J. T. Muckerman, and M. D. Newton, J. Phys. Chem. C 112 3439 (2008).} made a first-principles studies of the structural and electronic properties of this alloy. We present further work on this alloy and on the surfaces which may host the water-splitting redox reactions. We look at structural properties such as bond lengths, and the stability of different configurations for various mixing ratios. We examine the nature of the HOMO and LUMO states, aiming for insight about the active sites of the water splitting catalytic chemistry. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X12.00007: Study of oxygen octahedra rotations in Ruddlesden-Popper phases of perovskite materials JeeHye Lee, Tomas Arias We present the most systematic theoretical study of Ruddlesden- Popper(RP) phases in perovskites ($Sr_{n+1}Ti_{n}O_{3n+1}$ and others) to date. Consistent with experimental results, we find that the formation of stacking faults is energetically favored for certain rotational reconstructions of the bulk material. In particular, we enumerate the set of possible rotational configurations of oxygen octahedra next to the stacking fault and then reduce this set to a subset of distinct configurations by considering the symmetries of the RP structure. For this reduced set, we calculate the formation energy of stacking faults, and study the interaction between separated faults. We find that the planar faults attract each other, and that the interaction energy scales as the inverse distance between the faults. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X12.00008: Reconsideration of SrTiO3(100)-$\surd $5\textbf{x}$\surd $5-R26.6 surfaces - existence of O-vacancy and Sr adatom Ichiro Shiraki, Kazushi Miki SrTiO$_{3}$(100)-$\surd $5x$\surd $5-R26.6 surfaces were studied by scanning tunneling microscope (STM) in ultra-high vacuum conditions. The results suggest the existence of O-vacancies on the surfaces, which have not been confirmed experimentally so far. In occupied states observations of -1.2 V sample bias voltage, a pair of dangling bonds of Ti atoms neighboring an O vacancy is clearly seen as an oval shape protrusion that can be resolved apart into two protrusions, which is consistent with previous theoretical study by spin polarized LDA+U calculations [1]. Making a comparison between occupied states and unoccupied states (-1.2 V sample bias voltage) in the same scan area, the oval shape protrusions present dark contrast, which is also consistent with the theoretical study. The present proposed surface structural model requires the existence of Sr adatoms as reported by Kubo et al. [2] [1] Z. Fang and K Terakura, Surf. Sci. 470, L75(2000) [2] T. Kubo and H. Nozoye, Phys. Rev. Lett. 86, 1801(2001) [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X12.00009: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X12.00010: Ab initio modeling of interfacial oxygen defects in ultrathin high-k gate dielectric stacks Blanka Magyari-Kope, Eric Cockayne, Yoshio Nishi As the thickness of the metal-oxide-semiconductor field-effect transistor (MOSFET) is further reduced, and the gate dielectric is under constant voltage, traps due to defects are generated and may form a conductive path. At a critical density of defects this may lead to gate dielectric breakdown. First principles methods based on density functional theory and non-equilibrium Green's function calculations are employed to calculate the tunneling current through ultrathin oxide layers of HfO$_{2}$ and SiO$_{2}$ in a gate stack structure with Pt metal electrode. The microscopic effects of the oxygen defects formation on the electronic transport through the gate oxides are analyzed and discussed. A number of interface models are constructed that include oxygen vacancies at various interfaces and isolated oxygen vacancy defects in the bulk oxide. The band offsets and the tunneling currents are calculated as a function of defects positions in the gate dielectric. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X12.00011: Tunable Metallicity at the Surface of La$_{5/8}$Ca$_{3/8}$MnO$_{3 }$Thin Films Kenji Fuchigami, Zheng Gai, Thomas Z. Ward, Lifeng Yin, Paul Snijders, Ward Plummer, Jian Shen A series of in-situ STM studies of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$(001) thin film reveals that the surface metallicity can be tuned by extrinsic oxygen doping at the surface. By in-situ annealing with or without oxygen, we can convert the surface back and forth between a ({\AA},,2 $\times $ {\AA},,2)R45 reconstructed surface and a (1 $\times $ 1) surface. Electrical properties of the surfaces are investigated by scanning tunneling spectroscopy (STS). I-V curves clearly show that the oxygen doping renders the surface insulating while the (1 $\times $ 1) surface without the oxygen doping is metallic. Structural models and their correlation to the surface metallicity have been proposed. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X12.00012: Surface properties of ultrathin ferroelectric films in external electric field Renat Sabirianov, Minoru Otani, Osamu Sugino The electric polarization of free standing ultrathin films of BaTiO$_{3}$ is analyzed using pseudopotential plane wave calculations within effective screening medium method. The polarization loop in asymmetrically terminated (Pb,Ba)TiO$_{3}$ film is biased, providing the existence of polarization without applied electric voltage across the film. We attribute the origin of bias to a creation of a bias field due to difference in surface work functions of TiO$_{2}$ and BaO terminations. This results in the formation of surface polarizations at each termination and inhomogeneous polarization profile across the thickness of the film. We show that the surface develops in-plane component of polarization in paraelectric state, and also in case of the ferroelectric films when the electric field applied perpendicularly to the plane of the film. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X12.00013: Growth and Characterization of EuTiO$_{3}$ films on SrTiO$_{3}$ (001) H. Q. Wang, J. D. Ferguson, A. R. Woll, D. A. Muller, J. D. Brock Perovskite oxides display a variety of interesting physical properties, and their heteroepitaxial structures are of significant interest in oxide electronics. EuTiO$_{3 }$and SrTiO$_{3}$ are nearly perfectly lattice matched and have the same valence structure, and are therefore well suited as a model system for the study of perovskite interfaces. One outstanding question about such interfaces concerns the factors that determine and limit atomic and electronic abruptness. In this work, several monolayer thick EuTiO$_{3}$ films are grown on single-crystal SrTiO$_{3}$ (001) substrates using Pulsed Laser Deposition (PLD). The growth mechanisms are probed by the combination of synchrotron based \textit{in situ }small angle x-ray scattering (SAXS) and \textit{in situ} Reflection High Energy Electron Diffraction (RHEED). The atomic-scale interfacial properties are investigated by high resolution Scanning Transmission Electron Microscopy (STEM) and spatially resolved Electron Energy Loss Spectroscopy (EELS). [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X12.00014: Density functional calculations of the structure of near-surface oxygen vacancies and electron localization on CeO$_2$(111) M. Veronica Ganduglia-Pirovano, Juarez L.F. Da Silva, Joachim Sauer The use of ceria as a key component in catalysts and as an electrolyte for solid oxide fuel cells relies on its notorious capability of storing and releasing oxygen. This property results from the facility of both formation and healing of oxygen vacancies in ceria. Several studies have been reported in the last years for reduced CeO$_2$(111), however, one of the most topical issues surrounding oxygen vacancies on CeO$_2 $(111), namely, the relative stability of surface and subsurface defects, is still under intense debate. Using density functional theory with the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional as well as the DFT+U approach, we find subsurface vacancies with $(2{\times}2)$ periodicity to be energetically more favorable by 0.45 (HSE06), 0.47 (PBE+U), and 0.22 eV (LDA+U). The excess-electrons localize not on Ce ions which are nearest neighbor to the defect as priorly suggested, but instead on those that are next-nearest neighbors. The excess-electron distribution and the preference for subsurface vacancies are explained in terms of defect-induced lattice relaxation effects. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X12.00015: Interface structure determination of crystalline oxides on silicon using synchrotron x-ray diffraction F.J. Walker, J.W. Reiner, A.M. Kolpak, Y. Segal, Z. Zhang, D. Su, Y. Zhu, M.S. Sawicki, C.C. Broadbridge, S. Ismail-Beigi, C.H. Ahn As electronic devices are reduced in size, well controlled atomically abrupt interfaces become increasingly important. This is especially true for field effect devices where the conducting channel is only a few atoms thick. Here we discuss the experimental determination of the atomic structure of an interface for a model system, crystalline oxides on silicon, which contains the essential elements of field effect devices. For both BaO/Si and SrTiO$_{3}$/Si structures, we use a combination of synchrotron x-ray scattering, Z-contrast transmission electron microscopy and density functional theory to determine the structure. The combination of these approaches has led to a unique detailed model of the interface. We have discovered features of these interfaces that emerge during growth that can be used to understand important elements of the measured electrical properties and microscopically identify sources of fixed charge, interface traps and field-dependent mobility. [Preview Abstract] |
Session X13: Density Functional Methods: Applications
Sponsoring Units: DCOMPChair: Samuel Trickey, University of Florida
Room: 309
Thursday, March 19, 2009 2:30PM - 2:42PM |
X13.00001: System-averaged exchange-correlation holes and self interaction in second-row atoms Antonio C. Cancio Recent work is presented on the theoretical calculation of system-averaged exchange and correlation holes (intracules) for a pseudopotential model of the valence shell for the second row atoms Mg through Ar. Exchange holes are obtained from numerical fourier transform methods and correlation holes from variational quantum Monte Carlo calculations using the method of correlated estimates. We observe scaling behavior in both exchange and correlation, following the known scaling of the valence density across the row, once self-interaction effects are taken into account. The holes are compared to density-functional models including LDA, GGA and related SIC approaches. We note a sizeable error due to self-interaction occurs in the same-spin channel of the correlation hole which persists for the LDA and GGA even after standard self-interaction corrections are applied. The effects of this error and proposed corrections to it on the total exchange-correlation energy will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X13.00002: Constraint-based, Single-point Approximate Kinetic Energy Functionals Frank E. Harris, V. V. Karasiev, R. S. Jones, S. B. Trickey We work toward the development of orbital-free density functionals for the Kohn-Sham kinetic energy $T_s$ of a quality suitable for the computation of quantum-mechanical forces in multi-scale molecular dynamics simulations. The functionals are based on constraints applicable to the Pauli potential $v_\theta=\delta T_\theta/\delta n$, where $T_s=T_w+T_\theta$ and $T_w$ is the von Weizs\"acker kinetic-energy functional. We review our progress to date, and exhibit functionals that do not generate spurious singularities and that produce chemical bonding in semi-quantitative agreement with Kohn-Sham computations and relevant experiments. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X13.00003: Functional minimization scheme for first-principles electronic structure calculations with bi-orthogonal interpolating wavelets William Garber, Wei Ku, James Davenport, Dmitri Volja A new development of first-principles electronic method will be presented based on direct energy functional minimization and bi-orthogonal wavelet basis set. The employment of bi-orthogonal basis allows systematically controlled accuracy while benefiting from the compact support that allows O(N) algorithms. Furthermore, utilization of the interpolating nature of the wavelet, together with the effectiveness of multi-resolution of wavelet, enables very efficient calculation without compromising accuracy. By avoiding solving eigenvalue equation as in standard Kohn-Sham framework, the method is easily extended to parallel algorithms, and allows simple implementation of various non-local functionals. In case of crystals, our method gives directly solution as Wannier functions, further utilizing their sparseness. This new development is ideal for easy implementation and accurate systematic benchmarking of various modern functionals, and holds the potential to attack very large systems such as nano- materials. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X13.00004: Quenched Lieb-Oxford Satisfaction and Improved Performance for PBE-type Functionals S.B. Trickey, V. Medel, A. Vela Success for the orbital-free DFT approach to Born-Oppenheimer forces for first-principles molecular dynamics requires progress on orbital-free exchange-correlation (XC) functionals to go along with newly developed orbital-free kinetic energy functionals [V.V. Karasiev \emph{et al}.\, arXiv 0809.4798, J.\ Comput.-Aided Mat.Des.\ \textbf{13}, 111 (2006)]. We report on development and testing of a non-empirical X functional which generalizes PBE X. It satisfies a reduced Lieb-Oxford bound by quenching to homogeneous electron gas behavior for large values of the inhomogeneity $s \propto |\nabla n|/n^{4/3}$ on the grounds that large $s$ often corresponds (counter-intuitively) to small, smooth density. Used with the PBE C functional, our X functional reduces mean absolute errors for small molecules by 20\% or more with respect to conventional PBE XC. Used with LYP C (a semi-empirical combination), the performance also is improved relative to PBE-LYP. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X13.00005: On the performance of Thomas-Fermi in periodic two-dimensional systems Lazaro Calderin, Malcolm J. Stott The largest missing piece of a completely orbital free Density Functional Theory is the kinetic energy functional $T_s[n]$, and approximations for this are of interest. One of these expands $T_s$ in terms of density gradients with the Thomas-Fermi functional as the first term. But in three-dimensions the expansion appears not to converge, and the sixth and higher order corrections diverge for localized systems. In contrast, a number of authors have shown that the density gradient corrections all vanish in two-dimensions, while numerical test revealed that, even when not exact, TF is a very good approximation. That has been shown for the case of an impurity in a otherwise two-dimensional uniform electron gas. In this work we explore the validity of TF and linear response theory for a periodic two-dimensional system, a system that is likely to be more widely applicable. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X13.00006: Effect of disorder on the electronic properties of strongly correlated systems within the dynamical cluster approximation Unjong Yu, Abdolmajid Nili, Juana Moreno, Mark Jarrell We study the interplay of disorder and strong correlations on the electronic properties of highly correlated systems. We employ the dynamical cluster approximation (DCA) to include the effects of short-range correlations and alloy disorder beyond the coherent potential approximation (CPA). Our study focus on the double exchange model, relevant on the study of dilute magnetic semiconductors, and the periodic Anderson model to study heavy fermion compounds. We present results of several electronic properties as function of disorder strength, alloy concentration, and electron or hole doping. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X13.00007: Basic Variables in Density Functional Theory in the Presence of a Magnetic Field Viraht Sahni, Xiaoyin Pan We have shown$^{\dag}$ via a unitary or equivalently a gauge transformation that for a system of $N$ electrons in an external field ${\vec{\cal{F}}}^{ext} = - {\vec{\nabla}} v({\vec{r}})$, the wave function $\Psi$ is in general a functional of the ground state density $\rho ({\vec{r}})$ and a gauge function $\alpha ({\vec{R}})$; ${\vec{R}} = {\vec{r}}_{1}, \ldots , {\vec{r}}_{N} $, i.e. $\Psi = \Psi [\rho, \alpha]$. The functions $\alpha ({\vec{R}})$ are arbitrary, the choice $\alpha ({\vec{R}}) = 0$ being equally valid. It is the presence of $\alpha ({\vec{R}})$ that ensures the wave function functional is gauge variant. Similarly, in the presence of a magnetic field ${\vec{B}} ({\vec{r}}) = {\vec{\nabla}} \times {\vec{A}} ({\vec{r}})$, we show that in general the wave function is a functional of the density $\rho ({\vec{r}})$, the physical current density ${\vec{j}}_{\vec{A}} ({\vec{r}})$, and a gauge function $\alpha ({\vec{R}}): \Psi = \Psi[\rho, {\vec{j}}_{\vec{A}}, \alpha]$. Again, the $\alpha ({\vec{R}})$ are arbitrary, the choice $\alpha ({\vec{R}}) = 0$ being valid. Hence, it is possible to construct a theory in which the basic variables are $\rho ({\vec{r}})$ and ${\vec{j}}_{\vec {A}} ({\vec{r}})$. The generalized Hohenberg-Kohn theorems, as well as the equations for the noninteracting fermion Kohn-Sham system that reproduces the $\rho ({\vec{r}})$ and ${\vec{j}}_{\vec{A}} ({\vec{r}})$ of the interacting system of electrons, are derived. \\ $^{\dag}$X.-Y. Pan and V. Sahni, Int. J. Quantum Chem. \textbf {108}, 2756 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X13.00008: Projector Augmented Wave database with automatic parameter optimization R.J. Snow, A.F. Wright, C.Y. Fong Projector Augmented Wave (PAW) parameter sets, similar to pseudopotential parameters, can be constructed in many ways. Due to a non-local expansion of projectors, the PAW method can include parameters for each angular momentum channel separately. While this gives the flexibility to optimize projectors individually, it also creates an unfathomable parameter space for searching for good parameter sets. To automatically search for good PAW sets, logarithmic derivatives were analyzed numerically for matching with AE logarithmic derivatives. Logarithmic derivative matching, total energy convergence, and scf convergence were used as scores for automatic optimization of the accuracy and speed of PAW parameter sets using a genetic algorithm within an optimization code. The Dakota [1] program was used for the parameter optimization, while the atompaw program was used for PAW generation. A new database of PAW functions will be introduced and a number of examples discussed. [1] Sand Report Sand 2001-3514, (2002) [2] N.A.W. Holzwrth, A.R. Tackett, and G.E. Matthews, Computer Physics Communications 135, 329 (2001) [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X13.00009: A Projector Augmented Wave Formulation of the Optimized Effective Potential Formalism Xiao Xu, N.A.W. Holzwarth The optimized effective potential (OEP) or exact exchange (EXX) formalism has recently received renewed attention\footnote{ S. K\"{u}mmel and L. Kronik, RMP {\bf{80}}, 3 (2008).} as a method which can improve the accuracy of density functional calculations by representing orbital-dependent functionals and avoiding self-interaction errors found in density functionals. Since the Projector Augmented Wave (PAW) formalism\footnote{ P. Bl\"{o}chl, PRB {\bf{50}}, 17953 (1994); N. A. W. Holzwarth {\em{et al}}, PRB {\bf{55}}, 2005 (1997).} ensures accurate evaluation of interaction integrals by controlling the multipole moments,\footnote{ J. Paier {\em{et al}}, JCP {\bf{122}}, 234102 (2005).} it is a natural choice for implementing OEP within an efficient pseudopotential-like scheme. We developed a frozen core approximation scheme for the atomic all-electron OEP formalism, partitioning the exchange potential into core and valence contributions. The corresponding valence exchange pseudopotential for PAW, $\widetilde{V}_x^{\rm{vale}}({\bf{r}})$, can be derived in a similar way so that for $r > R_c$, $\widetilde{V}_x^{\rm{vale}}({\bf{r}})= {V}_x^{\rm{vale}}({\bf{r}})$. We have investigated the behavior of PAW-OEP basis, projector, and pseudopotential functions for several elements throughout the periodic table. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X13.00010: Density functional study of CO adsorption on d-metal surface using TPSS functional Jianwei Sun, John Perdew Feibelman \textit{et al}$^{[1]}$ presented the puzzle of CO at the Pt(111) surface, showing that the LDA and Perdew-type GGA put the molecule at the wrong, high-coordination site. However, a recent study $^{[2]}$ showed that the BLYP yielded very satisfactory adsorption energies and the correct adsorption sites for CO adsorption on late 4d and 5d transition metal (111) surfaces, although at the price of large errors in the volume of the d metals. Since PBE and BLYP have similar accuracy, it seems the probable reason for the wrong adsorption site is due to the fact that the LDA and Perdew-type GGA's are ``jellium derived'' and hence prefer a more delocalized bonding, rather than that the LDA and GGA inaccurately describe the CO molecule's chemical bond. TPSS meta-GGA is also ``jellium derived'', but improves accuracy for molecules$^{[3]}$. Therefore, as a possible candidate to identify the major reason for the wrong adsorption site, TPSS is used to calculate the adsorption energies and sites of CO on the d-metal surface in the more accurate geometric structure obtained by PBEsol$^{[4]}$. [1] P.J. Feibelman \textit{et al}, J. Phys. Chem. \textbf{105}, 4018(2001). [2] A. Stroppa and G. Kresse, New Journal of Physics \textbf{10}, 063020(2008). [3] V.N. Staroverov\textit{ et al}, J. Chem. Phys., \textbf{119}, 12129(2003). [4] J.P. Perdew\textit{ et al}, Phys. Rev. Lett., \textbf{100}, 136406(2008). [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X13.00011: Fully numerical all-electron solutions of the optimized effective potential equation for diatomic molecules Adi Makmal, Stephan Kummel, Leeor Kronik We present an approach for fully numerical, all-electron solutions of the optimized effective potential equation within Kohn-Sham density functional theory for diatomic molecules. The approach is based on a real-space, prolate-spherical-coordinate grid for solving the all-electron Kohn-Sham equations and an iterative scheme for solving the optimized effective potential equation. The accuracy of this method is demonstrated by comparison with previously reported calculations and new benchmark fully numerical results for selected dimers are provided. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X13.00012: Exchange Energy Density Functionals that reproduce the Linear Response Function of the Free Electron Gas David Garc\'Ia-Aldea, J.E. Alvarellos We present several nonlocal exchange energy density functionals that reproduce the linear response function of the free electron gas. These nonlocal functionals are constructed following a similar procedure used previously for nonlocal kinetic energy density functionals by Chac\'{o}n-Alvarellos-Tarazona, Garc\'ia-Gonz\'{a}lez et al., Wang-Govind-Carter and Garc\'ia-Aldea-Alvarellos. The exchange response function is not known but we have used the approximate response function developed by Utsumi and Ichimaru, even we must remark that the same \emph{ansatz} can be used to reproduce any other response function with the same scaling properties. We have developed two families of new nonlocal functionals: one is constructed with a mathematical structure based on the LDA approximation -- the Dirac functional for the exchange - and for the second one the structure of the second order gradient expansion approximation is took as a model. The functionals are constructed is such a way that they can be used in localized systems (using real space calculations) and in extended systems (using the momentum space, and achieving a quasilinear scaling with the system size if a constant reference electron density is defined). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X13.00013: Non-local exchange-correlation term implemented into the density functional theory Youky Ono, Koichi Kusakabe, Takashi Nakayama The local density approximation (LDA) has serious limitation that this approximation cannot estimate the long-ranged (non-local) exchange-correlation interaction, as typified by the van der Waals (vdW) interaction. In this study we develop a method to calculate the vdW interaction based on the LDA together within the plasmon-pole approximation [1]. The computation code is developed as one of a module program of an existing first principle calculation package. Usefulness and efficiency of the method are confirmed by calculating the interaction energy of simple periodic systems. This method never relies on external parameters and/or on asymptotic model functions, and thus being applicable to any isolated 3 dimensional structures. [1] PRL \textbf{96}, 073201 (2006), PRB \textbf{62}, 6997 (2000), PRL \textbf{92}, 246401 (2004). [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X13.00014: A series expansion of the Coulomb operator for optimization scheme of the multi-reference density functional theory Koichi Kusakabe, Isao Maruyama A quadratic form of the Coulomb operator for the many-electron system is derived.[1] This form of the electron-electron interaction is a sum of quadratic form pairs, which can be redefined in a self-consistent calculation of the multi-reference density functional theory. By virtue of this finding, the extended Kohn-Sham scheme[2] is shown to possess an optimization scheme of the effective electron model, which converges on the exact Coulomb system. For a defined accuracy of computation with given numerical technique, we can provide an algorithm to have an optimized electron model. The present procedure provides also an exact derivation of effective negative interactions in charge fluctuation channels. Relevance to the high-temperature superconductors is discussed. [1] K. Kusakabe, to appear in J. Phys.: Condens. Matter. [2] K. Kusakabe, J. Phys. Soc. Jpn. 70, 2038 (2001). [Preview Abstract] |
Session X14: Micro and Nano Fluid Mechanics
Sponsoring Units: DFDChair: Boyd Edwards, West Virginia University
Room: 315
Thursday, March 19, 2009 2:30PM - 2:42PM |
X14.00001: The dynamical origin of the zeta potential Patrick Tabeling By using evanescent waves, we study equilibrium and dynamical properties of liquid-solid interfaces in the Debye layer for hydrophilic and hydrophobic surfaces. We measure velocity profiles and nanotracer concentration and diffusion profiles between 20 and 300 nm from the walls in pressure-driven and electroosmotic flows. We extract electrostatic and zeta potentials and determine hydrodynamic slip lengths with 10 nm accuracy. The spectacular amplification of the zeta potential resulting from hydrodynamic slippag allows to clarify for the first time the dynamic origin of this potential. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X14.00002: Radiofrequency Nanoelectrolytic Debye-Layer Transistor Jean-Luc Fraikin, Michael Requa, Michael Stanton, Andrew Cleland A voltage-biased metal immersed in an aqueous electrolyte attracts ions from the solution, causing the accumulation of a diffuse layer of charge close to its surface. This layer is called the Debye-Huckel layer, or double layer. For a non-reactive electrode, the electric double-layer (EDL) presents a capacitive electrical impedance, which depends non-linearly on the voltage applied across it. We present a novel transistor whose transduction element is the EDL capacitance, which allows electronic gating of a 50 MHz signal at frequencies close to 1 MHz. The transistor comprises three terminals: a pair of nanofabricated interdigitated electrodes (IDEs) embedded in an electrolyte-filled microfluidic channel, and a third, gating Ag/AgCl electrode in the same fluid volume. We demonstrate direct gating of the transistor with the Debye layer, and make use of the device to measure the voltage dependence of the EDL capacitance over a broad range of electrode bias. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X14.00003: Simulation of Steady-State Non-Equilibrium Ion Distributions Within a Finite-length Nanofluidic Channel William Booth, Jarrod Schiffbauer, Josh Fernandez, Kathleen Kelley, Aaron Timperman, Boyd Edwards Steady-state non-equilibrium distributions of two species of mono-valent ions near and within a charged 2D nanofluidic channel have been examined with and without electroosmotic flow. Large reservoirs are connected by the nanofluidic channel to simulate bulk conditions. Far-from-equilibrium applied voltages create a charge polarization across the nanochannel when the Debye length is comparable to the channel width. Depletion zones of each ion species are observed. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X14.00004: DEP Force Spectroscopy Jingyu Wang, Steven M.T. Wei, Joseph Junio, H.D. Ou-Yang We report measurement of the frequency-dependent dielectrophoretic forces imparted on individual colloid particles in an aqueous suspension. The motion of suspended particles relative to the solvent resulting from polarization forces due to an inhomogeneous electric field is known as the dielectrophoretic force (DEP). In the case of colloidal particles, the Claussius-Mossotti (CM) function containing the frequency dependence of the dielectric behavior of the particle relative to the suspending fluid dictates the direction and magnitude of the resulting DEP force. The magnitude of this force approaches zero as the frequency approaches the point of cross-over to switch the direction of the force. Using optical tweezers as force sensor we have successfully characterized the frequency dependent DEP force with a spatial resolution in the micron range and a force resolution of a fraction of 1pN. To achieve this, we used an AM modulation scheme to administer the oscillating electric field, so that we could monitor the phase and amplitude of the displacement of the particle while it was held by the optical tweezers and acted on by the DEP force. The optical tweezers based DEP force spectroscopy presents a way to understand the fundamental parameters at the microscopic level. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X14.00005: A New Fourier Model of Traveling Wave Electrophoresis Robert Correll, James Eakins, James Vopal, Boyd Edwards Traveling-wave electrophoresis is a new method of separating charged analytes using a series of interlaced electrodes with time-varying electric potentials along a microchannel. It potentially offers several potential advantages over conventional electrophoretic devices, including increased separation efficiency and ease of scalability. A better description of the underlying mathematics is required in order to fully optimize this promising technology. As such, a new Fourier model of the electric potential inside the channel is introduced, along with preliminary computational results. This new representation allows for greatly reduced computation time and greater accuracy. Similarities and differences with other models are highlighted, as well as the dependence of the potential on the electrode and channel geometries. The movement of charged particles in response to the potential is examined, with several critical thresholds highlighted. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X14.00006: Traveling-wave electrophoresis for microfluidic separations Boyd Edwards, Aaron Timperman, Lloyd Carroll, Kyoo Jo, Jon Mease, Jarrod Schiffbauer Models and microfluidic experiments are presented of an electrophoretic separation technique in which charged particles whose mobilities exceed a tunable threshold are trapped between the crests of a longitudinal electric wave traveling through a stationary viscous fluid. The wave is created by applying periodic potentials to electrode arrays above and below a microchannel. Predicted average velocities agree with experiments and feature chaotic attractors for intermediate mobilities. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X14.00007: Trigonometric Model for Traveling Wave Electrophoresis James Vopal, Boyd Edwards The motion of ions in a trigonometric spatio-temporal potential in a fluidic microchannel is investigated. Computer simulations are performed for ions of different mobilities to predict the ionic trajectories and average velocities. In many instances, plotting the average velocity verses mobility results in a Devil's staircase. When Devil's staircases are seen, no chaotic behavior is present. When the average velocity verses mobility does not result in a Devil's staircase, chaotic trajectories can be found. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X14.00008: Electrokinetic interaction between a charged cylindrical particle and a charged planer surface Dolfred Vijay Fernandes, Sangmo Kang, Yong Kweon Suh Electrophoretic motion of a charged particle under an electric field applied parallel to the planer surface has been studied numerically. Effect of electric double layer (EDL) interaction between the particle and the surface on the electrophoretic motion is the main focus of the study. Thick EDL around the particle and on the surface is obtained by solving Poisson-Nernst-Planck (PNP) equations on a hybrid grid system. A Lagrange type cylindrical grid attached to the particle can move freely on Euler type Cartesian grid. Second order accurate bilinear-interpolation scheme is used at the intersection of Lagrange-Euler grid. The linear and rotational motion of particle in the electroosmotically driven fluid is obtain by balancing EDL interaction force, gravitational force, electrostatic force and hydrodynamic force. The fluid flow along the surface and around the particle is computed by solving Stokes equations. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X14.00009: Ac electroosmotic flows above coplanar electrodes Yong Kweon Suh Interactive numerical method has been proposed to calculate the ac electroosmotic flows above a pair of coplanar electrodes. The thin electrical triple layer (ETL) has been modeled by an asymptotic theory developed by the authors. The model corresponds to a simple dynamic equation for the surface charge density representing the integrated charge over the inner layer. Interactive calculation of the dynamic equation and the Laplace equation for several periods of ac frequency then yielded steady-state distribution of potential and the potential drop across the Stern and inner layers. The Smoluchowski's slip velocity was then determined from those two set of data and used as the boundary condition for the calculation of the Stokes' flow above the electrodes. We have shown that our solutions compared well with the experimental data reported in the literature. We investigated the effect of various parameters on the slip velocity distribution, such as the ac frequency, the electrode length, the effective Stern-layer thickness and the adsorption coefficients. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X14.00010: Detection and electrokinetic trapping of single fluorescent molecules in fused silica nanochannels Brian Canfield, Xiaoxuan Li, William Hofmeister, Lloyd M. Davis We describe experimental detection and electrokinetic trapping of single, fluorescently-labeled proteins confined within $\sim $100 nm fluidic channels fabricated in fused silica. Though difficult to fabricate, the fused silica environment yields lower autofluorescence than borosilicate glass, which is especially advantageous given the low light level from single molecules. The molecules are dispersed in a buffer solution at ultralow concentration ($\sim $10$^{-12}$ M) to provide single-molecule occupancy of the sub-femtoliter probe volume within the nanochannel. Fluorescence is excited and collected in a custom-built confocal microscope, using two temporally interleaved beams from a modelocked dye laser focused to adjacent spots along the nanochannel. Detection is accomplished with custom single-photon avalanche diodes for time-resolved single-photon counting, and by using this time stamp information, a field-programmable gate array circuit board controls the electrokinetic trapping by modulating an applied voltage. Fluorescence correlation spectroscopy is also used to monitor the transport of molecules along the nanochannel. Electrokinetic transport can thus be characterized from changes in the autocorrelation function with voltage modulation. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X14.00011: Microfluidic device for the electrokinetic manipulation of single molecules Jason King, Lloyd Davis, Brian Canfield, William Hofmeister, Philip Sampson We are developing a microfluidic device for three-dimensional electrokinetic manipulation of single fluorescent molecules in solution. The device consists of electrode pairs deposited onto glass cover slips via UV microlithography and ionic sputtering. By positioning two such electrode pairs in a tetrahedral configuration separated by 100 microns and applying appropriate digitally-controlled voltages to each, the apparatus generates an electric field of selected directionality in the central bounded region. Proof of concept is demonstrated by controlling the motion of micron-size latex beads, visualized with an EM-CCD camera. By use of a double Mach-Zehnder interferometer configuration, 40 fs Ti:Sapphire laser pulses (repetition rate 76 MHz) are split into four temporally interleaved pulses (effective rate 304 MHz), which are then focused to the vertices of a tetrahedron (approximately one micron per side) within the central electrode region to generate two-photon-excited fluorescence from single molecules. The time stamp data from this four-focus probe, collected with a custom fast-timing single-photon avalanche diode, enables characterization of particle motion through fluorescence cross-correlation spectroscopy. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X14.00012: Microfluidic Channels under Magnetohydrodynamic (MHD) Convection Yogendra M. Panta, Hyun W. Kim, Shizhi Qian Magnetohydrodynamic (MHD) effects have been widely known since many years. MHD effects are used to propel, stir, and pump fluids in various fluid applications especially in the field of microfluidics and Lab On a Chip (LOC) technology. Orthogonally aligned electric flux density and magnetic flux density were applied to straight and torroidal micro-channels both aligned perpendicular to the desired direction of fluid flow. Microfluidic MHD channels in straight and torroidal shapes were fabricated from a thin brass sheet sandwiched between two polycarbonate sheets patterned with two platinum electrodes in the channel walls from inside. When a potential difference of low magnitude ($\sim $ 1 mV) is applied across the electrodes, a current density J transmitted through the electrolyte solution results. In the presence of a magnetic field B, the orthogonal interaction between the resulting current density J and the magnetic field B induces Lorentz forces F (=J$\times $B) which induce and drive fluid motion in the channel. This effect was applied to propel and pump the fluid in presence of a current carrying species both in a straight and torroidal micro-channels. Flow velocities were obtained linearly increasing with the higher magnetic flux densities. A drop of dye was placed into the solution to trace the path of moving fluid under MHD convection. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X14.00013: Cell and Colloidal Substrates for Dielectrophoretic Microfluidic Immunoassays Jill Mazur, Zachary Gagnon, Hsueh-Chia Chang Dielectrophoresis (DEP) is a term commonly used to describe the field induced polarization and translational motion of a polarizable particle in a non-uniform AC field. The frequency at which the induced particle dipole goes to zero, known as the crossover frequency (cof), is highly dependent on the surface conductance of the particle. We have shown previously that DNA hybridization on the surface of a 100 nm functionalized silica particle leads to detectable surface conduction changes which make it possible to detect DNA hybridization reactions by simply measuring changes in particle suspension cof. In this work we present a similar detection scheme using novel colloidal and cell substrates as dielectrophoretic immunosensors. Aminated cell or nanocolloid surfaces are subjected to a polymer coating glutaraldehyde treatment followed by antibody coupling reaction for immunoassay based detection. By varying the polymer coating thickness on the colloid or cell surface we demonstrate the ability to tune, stabilize the cell and colloid cof, and minimized non-specific adsorption of proteins. As such, a library of cof labeled colloids and cells are created and used for multiple antigen analysis. By measuring the colloid and cell specific DEP cof prior to and after antibody-antigen interaction we demonstrate the ability to perform rapid label free protein detection within a microfluidic device. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X14.00014: AC Electrokinetic Cell Separation on a Microfluidic Device Zachary Gagnon, Hsueh-Chia Chang Rapid cell separation and collection is demonstrated through the integration of electrokinetic pumps, dielectrophoretic (DEP) traps and field driven valves into a well designed microfluidic channel loop. We present the ground-up design and analysis of this fully functional microfluidic device for the rapid separation and collection of live and dead yeast cells and malaria red blood cells (RBCs) at low concentrations. DEP cell sorting and concentration schemes are based on the exploitation of cell specific DEP crossover frequencies (cof's). A rigorous DEP study of yeast and RBCs is presented and used to determine optimal conditions for cell separation. By utilizing a glutaraldehyde crosslinking cell fixation reaction that is sensitive to cell membrane protein concentration, we demonstrate the ability to further amplify these differences between healthy and unhealthy cells as well as stabilize their DEP cof's. Pumping is achieved with a new type of electrokinetic flow, AC electrothermal electro-osmosis (ETEO) and is shown to scale inversely with the field induced debye length and drive fluid velocities in excess of 6 mm/sec. The well characterized electrokinetic phenomena are integrated into a microchannel loop with a specifically designed electrode field penetration length for low concentration cell separation and concentration. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X14.00015: Anomalous analyte dispersion at microchannel-nanocapillary membrane interfaces Jarrod Schiffbauer, Kathleen Kelly, Will Booth, Josh Fernandez, Aaron Timperman, Boyd Edwards The dispersion of a plug-like distribution of negatively charged fluorescent dye molecules inside a microchannel is studied by numerical analysis of a time-series of epifluorescence microscope images. The concentration is accomplished using a nanocapillary membrane (NCM) --based concentration device. Dispersion of the analyte after concentration is complete, i.e. after the applied voltage is removed, is of considerable technical interest as a limiting factor in the functionality of lab-on-a-chip concentration devices. Subsequent band-broadening is inconsistent with Taylor dispersion and is shown here to be influenced by the presence of charge-separation between the concentrated analyte and background buffer ions. [Preview Abstract] |
Session X15: Liquid Crystals II
Sponsoring Units: DFDChair: Peter Collings, Swarthmore College
Room: 316
Thursday, March 19, 2009 2:30PM - 2:42PM |
X15.00001: The Taming of the Screw: Nonlinear Interactions in Smectic Liquid Crystals Elisabetta Matsumoto, Gareth Alexander, Randall Kamien From the twist grain boundary phase to the smectic phases of bent core liquid crystals, beautiful and intricate textures composed of screw dislocations appear time and again in a wide variety of smectic systems; yet, little is known about the interactions of screw dislocations. The linear smectic free energy is not sufficient to describe the energetics of single screw dislocations, and superposition cannot shed light on the interaction of many such defects. The full rotationally invariant nonlinear smectic free energy provides insight into systems of multiple screw dislocations. Such nonlinear interactions allow us to begin to understand the stability of the bulk phases observed in both smectic A liquid crystals and their chiral smectic C* counterparts. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X15.00002: Coalescence Dynamics Analysis Of Islands In Smectic A Freely Suspended Films Zoom Nguyen, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark We explore the coalescence dynamics of circular islands in smectic A freely suspended liquid crystal films. The process typically has two distinct stages. First, when the islands make contact initially, the thinner island wraps around the thicker one. These dynamics are fast and determined by the line tensions of the islands and by the film's viscosity. Then the region that used to be the thicker island expands and eventually covers the whole merged island. This process which is dependent on the permeation between layers in addition to the line tension and viscosity, is much slower. The shapes of the islands are extracted from high speed camera images and compared with model calculations. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X15.00003: Orientational fluctuation study in nematic liquid crystals by high speed micrograph Beom-Jin Yoon, Min Sang Park, Jung O. Park, Mohan Srinivasarao The orientational fluctuations in uniaxial and biaxial nematic liquid crystals were investigated with a polarized microscope and a high speed TV camera. Liquid crystals usually have fluctuations with respect to their director, even when the molecular axes tend to be aligned to each other. These fluctuations are sufficiently slow and large, have long wave length and increase with temperature. Herein, we describe our study on fluctuation dynamics by direct observations in real space, while it has been typically done by the photon scattering in reciprocal space. The twinkling of liquid crystals due to orientational fluctuations was observed with a high speed camera up to 500 frames/sec. The time correlation function of the intensity was computed via 2D spatial Fourier transform of each image and then the relaxation frequency was estimated from it. The elastic constant to the viscosity ratio was computed from the relaxation frequency. This approach provides facile route to analyze fluctuation dynamics in liquid crystals. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X15.00004: Modeling twisted distortions in nematic elastomers Vianney Gimenez, Badel Mbanga, Fangfu Ye, Jonathan Selinger, Robin Selinger Experimental studies have reported that nematic elastomers with a twisted nematic director--similar to the configuration in a twisted nematic cell--show a well-controlled deformation under change of temperature. Due to the difference in thermal expansion along and perpendicular to the nematic director, the sample twists and curls dramatically under heating and cooling [1]. We model this shape evolution using both analytical calculations and finite element elastodynamics simulations. In analytical calculations, we determine the optimal shape of an initially flat strip by minimizing a free energy functional that takes into account the coupling between orientational order and mechanical strain. We compare which of two final states--a helical or twisted ribbon shape--is lower in free energy, as a function of the sample's aspect ratio and material properties. We then use finite element simulations to model the dynamics of this spontaneous deformation and examine the resulting equilibrium shapes, which may be intermediate between helical and twisted. Results are compared to relevant experiments. We also use our simulation model to explore a wider variety of director configurations and sample geometries, beyond the ideal cases solvable via analytical methods. [1] G. Mol, K. D. Harris, C. W. M. Bastiaansen, and D. J. Broer, Adv. Funct Mater, 15, 1155 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X15.00005: Behavior of Focal Conic Defects in Shear Flow Sourav Chatterjee, Shelley Anna The rheology of layered liquids is influenced to a large extent by defects present in the system, especially in small gaps. Toroidal focal conic defects are a common type of defect in small molecule layered liquids. We present a study of the influence of flow on focal conic defects in smectic liquid crystals, generated by antagonistic boundary condition at the surfaces. The defects are confined in gaps of the order of tens of microns and are subject to simple shear. The sizes of the focal conic defects vary with the gap size, and hence visual observations are made as to how the gap influences the dynamics of the focal conic defects in a shear flow. We also observe instabilities in initially defect free samples that lead to the creation of defects. The results offer insight into the complex relationship between defects and flow. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X15.00006: Polarization current as evidence of local anticlinic correlations in de Vries smectics Z. V. Smith, P. D. Beale, R.-F. Shao, L. Wang, D. M. Walba, N. A. Clark, M. A. Glaser Previous theoretical work on the electroclinic response of chiral de Vries SmA materials based on the electric field-induced reorientation of independent tilt domains [J. V. Selinger et al., Phys. Rev. E 64, 061705 (2001)] fails to account for the sigmoidal dependence of induced polarization (P) on field (E) seen in some materials. To account for this behavior, we model de Vries smectics as ensembles of small but finite anticlinic tilt domains. Within each domain, interlayer tilt coupling favors anticlinic interfaces, but the finite range of in-layer tilt correlations leads to thermally activated synclinic interfaces and a finite tilt correlation length along the layer normal. This model, equivalent to a generalized one-dimensional XY model in an external field with quadratic and quartic nearest-neighbor interactions, is studied by Monte Carlo simulation and transfer matrix methods. The model successfully reproduces the dependence of P on E for a specific material (W530), and yields physical parameters such as the in-layer correlation length and effective interlayer tilt coupling. The predicted anticlinic tilt correlations should be observable as diffuse superlattice reflections in polarized resonant x-ray scattering experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X15.00007: A one order parameter tensor description of biaxial nematic liquid crystals Xiaoyu Zheng, Peter Palffy-Muhoray We present a simple one order parameter tensor mean field theory of biaxial nematic liquid crystals. We construct the free energy from molecular interactions, identify the components of the order parameter tensors, and obtain self-consistent equations, which are then solved numerically. The phase behavior is described via a 3D phase diagram. We discuss the connection between molecular properties and the coefficients in the Landau expansion. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X15.00008: Elasticities and viscosities of a lyotropic chromonic nematic liquid crystal Krishna Neupane, Yuri Nastishin, Alan Baldwin, Oleg Lavrentovich, Samuel Sprunt We have performed dynamic light scattering studies of the elastic moduli and viscosity coefficients in a uniformly aligned sample of a lyotropic chromonic nematic formed by 14 wt. {\%} water solution of Disodium Cromoglycate [1]. These parameters show a significant anisotropy. In particular, the bend and splay moduli $K_{33}$ and $K_{11}$ are an order of magnitude higher than the twist modulus $K_{22}$, and the ratio $K_{33}/K_{11}$ shows an anomalous increase in temperature, which we attribute to the shortening of the aggregates. The bend viscosity is three orders of magnitude smaller than the splay and twist viscosities; all viscosity coefficients exhibit a strong temperature dependence. \\[4pt] [1] Nastishin \textit{et al.}, \textit{Phys. Rev. E.} \textbf{70}, 051706 (2004). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X15.00009: Reflection and transmission coefficients of a cholesteric liquid crystal film with a negative dielectric coefficient Sabrina Relaix, Wenyi Cao, Peter Palffy-Muhoray A cholesteric liquid crystal (CLC) is a periodic dielectric structure where simple analytic solutions of Maxwell's equations exist: light propagating along the helical axis has been first described by Mauguin in 1911 [1], for wavelengths much smaller than the helical pitch, and was formulated more generally by de Vries in 1951 [2]. The analytical solutions are for bulk CLCs and do not describe the optical properties of a finite thickness CLC film. Recently, analytic expressions for the reflection and transmission coefficients of a CLC slab have been obtained by solving Maxwell's equations and satisfying boundary conditions [3,4], providing results for thick slabs which go beyond the limitation of numerical methods. We discuss how these results are modified when one of the dielectric coefficients is negative. We explore the connection with hyperbolic dispersion and negative index materials. [1] C. Mauguin, Bull. Soc. Fr. Miner. Cristallogr. 34, 6 (1911) [2] H. de Vries, Acta Crystallogr. 4, 219 (1951) [3] W. Cao, Ph.D. dissertation, Chemical Physics, Kent State University (2005) (http://e-LC.org) [4] S. Relaix, W. Cao and P. Palffy-Muhoray, to be published [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X15.00010: Maier-Saupe Theory of Nematics in 4D Peter Palffy-Muhoray, Xiaoyu Zheng We extend the Maier-Saupe theory of nematics to 4 dimensions. We consider the interaction of cylindrically symmetric particles, and derive an effective single particle potential. Using this, we obtain the free energy and the self-consistent equation for the order parameter -- a second rank traceless tensor. In 4D, the order parameter has three independent eigenvalues. We solve the self-consistent equation, and study the solutions as function of temperature. Our results give insight into the relation between orientational order parameters in different dimensions. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X15.00011: Statistical mechanics of the flexoelectric effect in nematic liquid crystals Subas Dhakal, Jonathan V. Selinger Flexoelectricity is the phenomenon in which polarization is induced by imposed deformations of the director field in nematic liquid crystals. Recent experiments [1,2] have found that the flexoelectric effect is three orders of magnitude greater for bent-core liquid crystals than for conventional rod-like liquid crystals. To understand this experimental result, we develop a lattice model for the statistical mechanics of the flexoelectric effect. We perform Monte Carlo simulations and mean-field calculations to find the behavior as a function of interaction parameters, temperature, and applied electric field. The resulting phase diagram has four phases: isotropic, uniaxial nematic, biaxial nematic, and polar. In the uniaxial and biaxial nematic phases, there is a large splay or bend flexoelectric effect, which diverges as the system approaches the nematic-polar transition. This model may explain the large bend flexoelectric coefficient observed in bent-core liquid crystals, which have a tendency toward polar order. [1] J. Harden, B. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J. T. Gleeson, and A. Jakli, Phys. Rev. Lett. 97,157802 (2006). [2] J. Harden, R. Teeling, J. T. Gleeson, S. Sprunt, and A.Jakli, Phys. Rev. E 78, 031702 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X15.00012: Aggregates in Chromonic Liquid Crystal Phases of Aqueous Solutions of Sunset Yellow* Leela Joshi, Shin-Woong Kang, Dena Mae Agra-Kooijman, Satyendra Kumar Molecules of dye Sunset Yellow consist of flat poly-aromatic core and hydrophilic groups at the periphery. In aqueous environments, they self-organize into columnar aggregates mainly via $\pi -\pi $ interactions between aromatic cores. At high concentrations, dye aggregates develop orientational and positional orders to form the nematic (N) and columnar (C) mesophases. Synchrotron x-ray scattering and optical polarizing microscopy were used to better understand the growth of aggregates and mesophase formation. Average column height and their spatial organization strongly depend on concentration, temperature, and pH value of the solution. The aggregate size decreases with temperature exhibiting an Arrhenius behavior with mesophase dependent activation energy. A dramatic decrease in the aggregate size upon addition of \textit{HCl} highlights their sensitivity to electrostatic interactions. *Work supported by grant NSF/DMR-086991. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X15.00013: Aggregation Properties of the Chromonic Liquid Crystal Benzopurpurin 4B Christopher McKitterick, Peter Collings Benzopurpurin 4B (BPP) is a textile dye very similar to the common indicator Congo Red. As is true for all chromonics, the absorption spectrum is concentration dependent at low concentrations. If this dependence is used to estimate a free energy change for aggregation, it is higher than has been determined for other systems. Unlike other recently investigated chromonic liquid crystals, BPP forms a liquid crystal phase at extremely low concentrations, about 0.5 wt\%. Also unlike these other chromonic liquid crystals, the aggregation kinetics are exceedingly slow. X-ray diffraction and light scattering measurements indicate that the aggregates of BPP are much larger than for chromonic systems that form liquid crystals at higher concentrations. BPP aggregates can be imaged using confocal microscopy, revealing a length distribution centered at 3 $\mu$m for a solution forced through a 0.2 $\mu$m filter. Over days the aggregates lengthen to well over 10 $\mu$m. The diameter of the aggregate images is slightly greater than the diffraction limit of the microscope, placing an upper limit on the diameter of 0.14 $\mu$m. These dimensions are consistent with the light scattering results. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X15.00014: Generalized Nematohydrodynamic Boundary Conditions with Application to Bistable Twisted Nematic Liquid Crystal Displays Angbo Fang, Tiezheng Qian, Ping Sheng Parallel to the highly successful Ericksen-Leslie hydrodynamic theory for the bulk behavior of nematic liquid crystals (NLC), we derive a set of coupled hydrodynamic boundary conditions to describe the NLC dynamics near NLC-solid interfaces. In our boundary conditions, translational flux (flow slippage) and rotational flux (surface director relaxation) are coupled according to the Onsager variational principle of least energy dissipation. The application of our boundary conditions to the truly bistable $\pi$-twist NLC cell reveals that the thus far overlooked translation-rotation dissipative coupling at solid surfaces can accelerate surface director relaxation and enhance the flow rate. This can be utilized to improve the performance of electro-optical nematic devices by lowering the required switching voltages and reducing the switching times. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X15.00015: Soft Micro- to Nanolithography Using Highly Periodic Smectic Liquid Crystal Defects Hee-Tae Jung, Yun Ho Kim, Dong Ki Yoon, Hyeon Su Jeong Achieving perfect long-range order with soft building blocks at high speed and high resolution is one of the most exciting interdisciplinary research areas in current materials science and nano-biotechnology. Here, we have developed highly periodic patterns with sub-micrometer features over large-areas using toric focal conic domains (TFCDs) originated from smectic liquid crystal (LC). TFCDs are accomplished by precisely controlling the surface and interfacial properties of smectic LC. In order to apply the smectic liquid crystal defect arrays in lithography, the hexagonal arrays of domain patterns are used as molds for ultraviolet (UV) curable polymers, thereby providing LC defect stamps with high spatial resolution over large areas. Our method was further utilized to transfer patterns with sub-micrometer features from the polymer stamp surface to a secondary surface by microcontact printing ($\mu $-CP). The patterning method based on LC defects has significant advantages over existing lithographic approaches: 1) the masters and stamps are easy to fabricate, 2) the masters and stamps provide long-range surface ordering over large-areas, 3) the periodic arrays are formed quickly in several seconds, and 4) the stamps can generate feature sizes on the micrometer and submicrometer length scales, and 5) the methodology offers the possibility of controlling the array geometry by altering the geometry of the confining channels. [Preview Abstract] |
Session X16: Liquid Helium
Sponsoring Units: DCMPChair: Norbert Mulders, University of Delaware
Room: 317
Thursday, March 19, 2009 2:30PM - 2:42PM |
X16.00001: Measurements of the Thermal Conductivity of Vycor Glass filled with Superfluid $^4$He William Tiernan, Silvia Ionsecu, Michael Ray, Robert Hallock We report on experiments designed to measure the thermal conductivity of Vycor filled with superfluid $^4$He for temperatures in the range 1.2 - 2.0 K and pressures from 2 - 30 atm.. The experimental apparatus, which consists of a rod of Vycor held in a stainless steel tube, will be described. We will report available results for the thermal conductivity as a function of temperature and pressure. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X16.00002: Phonon-roton modes, superfluidity and a Bose glass phase in nanoscale liquid $^4$He Jacques Bossy, Jonathan Pearce, Helmut Schober, Henry Glyde We present neutron scattering measurements of the elementary phonon-roton modes of liquid $^4$He confined in nanoporous media. The aim is to compare phonon-roton (P-R) and superfluid density measurements in helium at nanoscales and in disorder. A specific goal is to determine the region of temperature and pressure in which well defined phonon-roton modes (and therefore BEC) exist and compare this with the superfluid phase region, i.e. with the superfluid-normal phase critical temperature T$_c$ and pressure p$_c$ in porous media. We find well defined P-R modes (BEC) extend to temperatures above T$_c$ (up to T$_\lambda$ = 2.17 K at SVP) and to pressures above p$_c$ (up to a pressure p = 36.3-36.8 bars at T $\simeq$ 0 but no modes above this pressure [1]). This suggests that there is a Bose glass phase consisting of local regions of BEC separated by normal liquid so that there is no phase coherence across the sample lying between the superfluid and normal liquid phase. The Bose glass phase surrounds the superfluid phase at all p and T. We compare this phase diagram with other dirty Bose systems. [1] Bossy et al. Phys. Rev. Lett. 101, 025301 (2008), Phys. Rev. B (in press)(2008) [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X16.00003: Thin $^4$He films on Nano-porous Diblock Copolymer Substrates John Cummings, Robert Hallock In recent years diblock copolymer templates have been a focus of attention due to their potential uses in nano-scale systems. The ability to produce regular arrays of cylindrical pores has applications in areas such as the production of nanowires and magnetic storage. Porous polymer films made by doblock copolymer techniques provide an interesting substrate for helium. Previously studied porous geometries e.g. nuclepore, anopore, aerogel, vycor, and porous alumina have provided interesting insights into capillary condensation, the Kosterlitz-Thouless transition and hysteresis. Here we report on the study of thin superfluid $^4$He films on diblock copolymer substrates by means of quartz crystal microbalance techniques. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X16.00004: Evidence of a Proximity Effect in Liquid Helium Mark O. Kimball, Justin K. Perron, Francis M. Gasparini We report measurements of the specific heat of helium confined to $(2~\mu{\rm m})^3$ boxes connected via a 32~nm thick film. The spacing between the $\sim \!34$ million boxes arranged in a square array is $4~\mu{\rm m}$ edge-to-edge. The specific heat is compared to a similar measurement of helium confined to the same size boxes where the spacing between boxes is $2~ \mu{\rm m}$. Evidence of a coupling between the boxes in the tighter packed array is seen in a temperature region where the filling film is in the normal state. We also report measurments of the superfluid fraction of the film connecting the boxes in the present experiment. The superfluid state persists to higher temperatures than that expected on the basis of finite-size scaling for a 32~nm film. At the temperatures where the measurement of the film occurs, the helium in the boxes is already superfluid indicating, perhaps, the modification in behavior of a thin film in proximity to larger regions of superfluid. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X16.00005: A Novel Method to Create Multielectron Bubbles Jieping Fang, Anatoly Dementyev, Jacques Tempere, Isaac Silvera A multielectron bubble (MEB) in liquid helium is a fascinating object with a spherical two-dimensional electron gas on its surface. Recent theoretical studies of MEBs discuss a plethora of new phenomena$^{1}$. We describe a novel way of creating MEBs and discuss possible ways of trapping them. An electrically heated tungsten filament submerged in superfluid helium is surrounded by a vapor sheath containing electrons due to thermionic emission. We were able to pull MEBs from the sheath by applying electric fields up to 15kV/cm. The motion of MEBs was captured using a high-speed camera (6400 frames/sec). The trajectory of bubbles was clearly influenced by the electric field, which proved that bubbles were charged. In a separate experiment we measured the charge of such an MEB to be as high as 10$^{-9}$ C. We plan to trap MEBs using electromagnetic trap, which will enable extensive experimental studies of these elusive but exciting objects. [1] J. Tempere, I. F. Silvera, and J. T. Devreese, \textit{Surface Science Reports, }\textbf{62}, 159, 2007. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X16.00006: Phase diagram for helium films on lithium substrates. E. Van Cleve, J. C. Burton, P. Taborek We have used an in situ cryogenic pulsed laser deposition system to deposit a lithium film onto a quartz crystal microbalance. Helium 4 adsorption isotherms were measured on a lithium substrate between 2K and 0.6K. Features of these isotherms such as superfluid mass decoupling and variations in the dissipation were used to construct a phase diagram for helium films including the KT line and the 2D liquid-vapor coexistence region. The liquid-vapor critical temperature is approximately 0.8K. The KT transition is anomalous inside the 2D liquid-vapor coexistence region, occurring at constant sub-monolayer coverage independent of temperature. No inert solid-like layers of helium form on lithium substrates, so superfluid films are in direct contact with the substrate. These results will be compared and contrasted with the behavior of helium on other intermediate strength substrates such as exfoliated graphite pre-plated with hydrogen, sodium and magnesium. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X16.00007: Quantized vortices and superflow in arbitrary dimensions: Structure, energetics and dynamics Florin Bora, Paul Goldbart The structure and energetics of superflow around quantized vortices, and the motion inherited by these vortices from this superflow, are explored for the superfluidity of helium-four in arbitrary dimensions. The vortices may be idealized as objects of co-dimension two, such as two-dimensional surfaces in the case of four-dimensional superfluidity. The energy of the superflow is found to take on a simple form for vortices that are smooth and asymptotically large, compared with the vortex core size. The motion of vortices is analyzed in general, as well as for the special cases of hyper-spherical and weakly distorted hyper-planar vortices. In all dimensions, vortex motion reflects vortex geometry. In dimension four and higher, this includes not only extrinsic but also intrinsic aspects of the vortex shape. For the generalizations of the vortex rings of three dimensional superfluidity, the energy-momentum relation is determined. Simple scaling arguments recover the essential features of these results, up to numerical and logarithmic factors. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X16.00008: $^3$He Spin Pump A. Yamaguchi, H. Ishimoto, H. Kojima The superfluid component of $^3$He A$_1$ phase is spin-polarized. The process of forcing the superfluid component through a spin filtering structure, in a manner of mechano-magnetic effect, can be used to increase the spin polarization beyond the equilibrium under a given applied magnetic field. We have constructed a test cell in which a glass capillary array acts as the spin (and entropy) filter and an electrostatically actuated diaphragm forces the superfluid flow through it. Preliminary results show that a maximum \underline{relative} increase of polarization by 50 \% could be achieved. The maximum increase in polarization appears to be limited by the critical superfluid flow through the channels in the glass capillary array. The dependence of the observed effects on temperature, pressure and magnetic field will be presented. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X16.00009: The Spin Diffusion Coefficient of Superfluid $^{3}$He in the A$_{1}$- phase Ayodeju Awobode, Anthony Leggett Using the Boltzmann kinetic approach and perturbation theory, an approximate expression describing the variation with temperature, of the spin diffusion coefficient in the A$_{1}$-phase of $^{3}$He is derived. It is observed that for temperatures close to the transition temperature T$_{c}$, the spin diffusion coefficient D $\sim $ (T$_{c }$-- T)$^{1/2}$ + \textit{const}. Comparison of the theoretical result with related experimental measurements is discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X16.00010: Frequency Dependent Acoustic Properties of Superfluid $^{3}$He in Aerogel. B. H. Moon, N. Masuhara, P. Bhupathi, M. Gonzalez, M. W. Meisel, Y. Lee, N. Mulders Recently, we have reported the absolute sound (9.5 MHz) attenuation in superfluid $^{3}$He impregnated in 98{\%} porosity aerogel for several different pressures in zero magnetic field [1]. It revealed and confirmed many interesting features directly associated with impurity scattering: collisional drag effect, absence of zero sound crossover and order parameter collective modes, and gapless superfluidity. In this work, we report an experimental effort to uncover the detailed gap structure that is expected to be significantly modified by the presence of impurity scattering. We conducted frequency dependent attenuation measurements, which might shed light on this problem as a tunneling experiment does in superconductors. For the B-like superfluid phase of $^{3}$He in 98{\%} aerogel, we report sound attenuation measurements performed between 14 and 33 bar, while using four frequencies between 3.7 and 11.2 MHz. \newline [1] H.C. Choi \textit{et al., }Phys. Rev. Lett. \textbf{98}, 225301 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X16.00011: Superfluid $^3$He in Anistropic Aerogels Johannes Pollanen, John P. Davis, Benjamin Reddy, Kent R. Shirer, Hyoungsoon Choi, William J. Gannon, Charles A. Collett, William P. Halperin Anisotropic quasiparticle scattering has been predicted to modify the properties of superfluid $^3$He in high porosity silica aerogels.\footnote{K. Aoyama and R. Ikeda, Phys. Rev. B {\bf73}, 060504(R) (2006).} For example, anisotropic scattering produced by axial compression (or elongation) of the aerogel has been predicted to stabilize the axial (or polar) state of superfluid $^3$He. We have used a transverse acoustic impedance method to determine the phase diagram of superfluid $^3$He in a 98\% porous silica aerogel subjected to 17\% axial compression. We have found that this uniform axial anisotropy does not increase the stable region of A-like phase but does inhibit the nucleation of the B-phase at low pressure. We have performed optical cross-polarization experiments\footnote{J. Pollanen {\emph{et al.}}, J. of Non-Crystalline Solids {\bf354}, 4668 (2008).} to verify the presence and uniformity of the anisotropy in the aerogel samples. Additionally, we are performing nuclear magnetic resonance experiments on superfluid $^3$He in aerogels with anisotropy introduced with either axial or radial compression. [Preview Abstract] |
Session X17: Semiconducting Qubits II
Sponsoring Units: GQIChair: Xuedong Hu, University of Buffalo
Room: 318
Thursday, March 19, 2009 2:30PM - 2:42PM |
X17.00001: Effect of Substrate Doping in Relaxed SiGe Buffers on Strained Si 2DEG Quantum Devices Kun Yao, Mikhail Gaevski, Alexander Chernyshov, Leonid Rokhinson, Curtin Mike, Ji-Soo Park, James Fiorenza, Anthony Lochtefeld, James Sturm We describe the impact of Si substrate doping on the substrate leakage in strained Si two-dimensional electron gases (2DEG) on SiGe relaxed graded buffers and on quantum devices fabricated from the 2DEG. The best commercially available high quality SiGe relaxed buffers with 30{\%} Ge content, grown at temperature above 1000$^{o}$C, have very low threading dislocation density ($<$1E5cm$^{-2})$. Subsequent strained Si/SiGe heterostructures were grown at 625-700$^{o}$C in a rapid thermal chemical vapor deposition (RTCVD). However, it is shown that the substrate doping (Arsenic) contributes to leakage current origin in relaxed buffers at liquid helium temperatures if the starting Si substrate is heavily doped ($\sim $5E17cm$^{-2})$. The leakage can be attributed to enhanced dopant diffusion along misfit dislocations and high diffusion rate of As in SiGe. The leakage current makes side gating of nanostructures in the 2DEG impossible. With a lightly doped substrate, to avoid leakage, we achieved a high quality 2DEG and successful side gating of a 2DEG quantum dot for a quantum point contact. This work is supported by the NSA under ARO contract number W911NF-05-1-0437. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X17.00002: Robust Fabrication Techniques for Si/SiGe Quantum Dots Mingyun Yuan, Feng Pan, Tim Gilheart, Joel Stettenheim, Mustafa Bal, D. E. Savage, M. A. Eriksson, A. J. Rimberg Si/SiGe quantum dots promise a long spin coherence time due to reduced electron-nuclear spin interaction. Nevertheless, successful device yield has been limited in this novel material system due to difficulties in producing reliable ohmic contacts and Schottky gates. We have successfully developed fabrication processes that produce robust ohmic contacts and non-leaky Schottky gates. The ohmic contacts typically have a two-probe resistance of a few tens of kiloohms and the Schottky gates have no detectable leakage current up to an applied voltage of -5 V. In typical devices we are able to pinch off the quantum point contacts with a voltage range between -1.5 V to -4.5 V. Recent experimental results will be discussed. This work was supported by the NSF under Grant No. DMR-0804488, by the NSA, LPS and ARO under Agreement No. W911NF-04-1-0389, and by the ARO under Agreements No. W911NF-06-1-0312 and No. W911NF-06-01-0361. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X17.00003: Effect of Intervalley Mixing on Qubit Operation in SiGe Quantum Dot Structures A. A. Kiselev, R. S. Ross, B. H. Fong, M. F. Gyure We analyze the effects of valley degeneracy and intervalley mixing on single- and multi-electron states in (001) SiGe heterostructures, including effect of interface steps and variations in interface quality. We focus on the structure of two-electron states in both single and double quantum dot structures in the presence of valley degeneracy in the host material and the oscillatory behavior of exchange coupling in the presence of nonplanar heterointerfaces. We present modeling and simulation results relevant to the design of SiGe based accumulation-mode quantum-dot structures, especially CI calculation in presence of the intervalley mixing. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X17.00004: Transport and charge sensing in Si/SiGe double-quantum dots Christie Simmons, Madhu Thalakulam, E. K. Sackmann, B. J. Van Bael, D. E. Savage, M. G. Lagally, R. Joynt, M. Friesen, S. N. Coppersmith, M. A. Eriksson Gated quantum dots in Si/SiGe are of interest because spins in silicon are weakly coupled to the host material. We demonstrate that Coulomb blockade measurements through a single quantum dot are well correlated with charge sensing in a nearby quantum point contact. Charge sensing enables the determination of the absolute number of electrons in the system, and we present data demonstrating a one-electron single quantum dot. Incorporated with a double quantum dot, charge sensing can be used to probe the inter-dot motion of a single electron at fixed total charge in the double dot. The tunnel coupling between the two dots directly effects the charge localization and thus the sharpness of this inter-dot transition. Here we demonstrate gated electrical control of the exchange coupling -- an important step towards qubit implementation -- showing a smooth transition between two well-isolated dots, two dots so strongly coupled that they act as a single large quantum dot, and the intermediate regime. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X17.00005: Transient and stationary leakage current through a double quantum dot in the Pauli spin blockade regime Farzad Qassemi Maloomeh, William A. Coish, Frank K. Wilhelm We have calculated stationary and transient leakage current through a double quantum dot in the Pauli spin blockade regime. Quite remarkably, even in systems with inhomogeneous hyperfine coupling, we find that the leakage current is often controlled by spin-flip cotunneling processes with the leads. Our calculations show that these processes can be suppressed for one of the spin-triplet states by applying a small magnetic field, allowing for the preparation of a pure spin triplet. We have also found the transient effective charge passing through the double dot between blocking events, which can be strongly modified due to the spin-flip cotunneling processes. These results may explain features observed in several experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X17.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X17.00007: Energy Dependent Tunneling in a Silicon Double Quantum Dot Mark Friesen, C. B. Simmons, Nakul Shaji, R. H. Blick, S. N. Coppersmith, M. A. Eriksson We study transport currents in a few-electron Si/SiGe double quantum dot. A detailed analysis is made of the recently discovered phenomenon of lifetime enhanced transport (LET), in which current may flow in a regime typically considered to be blockaded. To understand this effect, a rate equation model is developed, including both singlet and triplet transport channels. Making use of a simple model of tunneling across a quantum barrier, we map out the energy dependence of the tunneling. This allows us to obtain quantitative estimates for the tunneling rates and transport currents throughout the reverse bias regime. We are then able to identify both resonant and non-resonant phenomena, and provide a physical understanding of the different blockade regimes. In particular, we provide detailed predictions for the conditions under which LET may be observed. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X17.00008: Charge transport in silicon double quantum dots Ted Thorbeck, Neil Zimmerman, Akira Fujiwara, Yukinori Ono, Yasuo Takahashi, Hiroshi Inokawa Double quantum dots are an essential component for many schemes of semiconductor quantum computation. We will present results for transport through a silicon double quantum dot system. Our devices are formed by mesa-etching an SOI wafer to form a nanowire, and then poly-silicon gates are deposited. A global gate is used to invert and local gates form tunnel barriers isolating quantum dots and controlling the potential of the dots. Because the coupling between the two dots is controllable, a transition from a single dot, to two coupled dots, to two uncoupled dots is observed. We will analyze the resulting honeycomb diagram. We also hope to present results in the few electron regime. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X17.00009: Modeling of Accumulation-Mode Quantum Dot Structures for Quantum Information Processing R.S. Ross, A.A. Kiselev, B.H. Fong, M.F. Gyure We present modeling and simulation results relevant to the design of SiGe and III-V based accumulation-mode quantum-dot structures for use as electron-spin-based qubits. We have developed a self-consistent real-space multi-electron simulation tool to efficiently explore and optimize these structures. Specific practical issues we address include the design of double-quantum-well heterostructures, enhancement-gate-electrode design and quantum-dot electronic structure with attention to the effects of electrostatic gate action. We examine the addition and excited-state spectra of single quantum dots (QD), the exchange coupling of nearest-neighbor quantum dots and the vertical tunneling behavior of our accumulation-mode devices. We also present comparisons to recently obtained experimental results on addition spectra in accumulation-mode quantum dots and show that our models correctly capture the relevant behavior. In addition, we address the robustness of device designs with respect to randomly distributed discrete dopants using a semi-analytical model and full numerical simulation based on impurity-induced random potentials. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X17.00010: Si double quantum dot spin qubit in a MOSFET structure Qiuzi Li, Dimitirie Culcer, Lukasz Cywinski, Sankar Das Sarma Motivated by recent experimental developments, we theoretically consider the prospects for creating spin qubits in a lateral double-dot structure fabricated in a Si MOSFET by lithographic patterning. We calculate tunnel coupling, exchange splitting, and other relevant qubit properties as functions of the double-dot structural parameters, i.e. dot separation, central barrier, detuning, etc. Our motivation is to obtain a detailed qualitative comparison between GaAs and Si double-dot systems to see whether a Si MOSFET double-dot structure is feasible as a spin qubit in real quantum computer architectures. We will discuss both regular single electron spin qubit and the successful (in GaAs quantum dots) singlet-triplet spin qubits. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X17.00011: Accumulation-Mode Quantum-Dot Devices Matthew Borselli, Edward Croke, Mark Gyure, Robert Hayes, Ivan Milosavljevic, Adele Schmitz, Jeong-Sun Moon, Andrew Hunter We have developed a quantum-dot device based on a double-well heterostructure in which electrons are localized in the top, mostly empty well by forward biasing a small circular gate. Charge occupancy changes in the dot are monitored by measuring current confined to a narrow channel in the bottom well. In this design, dot occupancy is primarily controlled by a single gate and interacting dots can be straightforwardly fabricated. We have successfully fabricated and characterized single-dot devices of this design in AlGaAs/InGaAs, and are extending the design to SiGe/Si heterostructures. We have measured charging spectra of III-V versions of the device down to zero electron occupancy. Charging spectra show enhanced stability for n=2, 6, 12, and 20 electrons. We have measured the tunneling times as a function of bias to map out excited states of a two-electron dot. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X17.00012: Cross-correlation heterodyne detection part I: Measuring the vacuum fluctuations at microwave frequencies Matteo Mariantoni, Edwin P. Menzel, M. A. Araque Caballero, F. Deppe, E. Hoffmann, T. Niemczyk, A. Marx, R. Gross, E. Solano In order to gain a profound insight into the fundamental properties of quantum electrodynamics (QED), studying the zero-point fluctuations of microwave radiation represents an important task. Here, we present a full experimental characterization of the vacuum fluctuations by measuring the Planck distribution of its noise power at microwave frequencies and very low temperatures. We observe a cross-over from thermal noise to vacuum quantum noise and quantify the level of vacuum fluctuations for a narrow frequency band centered around 5.85 GHz. We demonstrate the change of the vacuum fluctuations level with the center frequency. Finally, we perform a new type of heterodyne detection particularly suitable for circuit QED systems. It is based on microwave beam splitters and cross-correlation measurements and allows for the reconstruction of the entire covariance matrix of the vacuum. We acknowledge support from SFB631, NIM, EuroSQUIP, and the Ikerbasque Foundation. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X17.00013: Cross-correlation heterodyne detection part II: Measuring microwave nontrivial propagating signals Edwin P. Menzel, Matteo Mariantoni, M. A. Araque Caballero, F. Deppe, E. Hoffmann, T. Niemczyk, A. Marx, R. Gross, E. Solano The accurate measurement of the first two moments of Gaussian states (e.g., coherent or squeezed states) allows for their complete characterization. This provides a tool to clarify the quantum nature of microwave radiation, an important issue for example in circuit quantum electrodynamics. We present a full experimental characterization of nontrivial microwave signals with an average photon number of the order of 1, whose variance exhibits an elaborate dependence on external control parameters. We experimentally access the entire covariance matrix by splitting the input signals via microwave beam splitters and performing cross-correlation measurements. In this manner, we are able to precisely resolve the first two moments, a challenging task at microwave frequencies. Furthermore, we succeeded to measure the third central moment of similar nontrivial signals. We acknowledge support from SFB631, NIM, EuroSQUIP, and the Ikerbasque Foundation. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X17.00014: High precision interferometry with a transition edge sensor Christoph F. Wildfeuer, Aaron J. Pearlman, Jun Chen, Jonathan P. Dowling, Jingyun Fan, Alan Migdall In this contribution, we present our studies of Michelson and Fabry-Perot interferometers with a photon-number resolving detector. We show experimentally that with a weak coherent light beam, the use of a photon-number resolving detector improves the interferometric resolution. We also discuss ways the sensitivity of interferometers can be further improved beyond the standard quantum limit by using nonclassical light and photon-number resolving detectors. ~ [Preview Abstract] |
Session X18: Elastomers and Gels I
Sponsoring Units: DPOLYChair: Alfred Crosby, University of Massachusetts
Room: 319
Thursday, March 19, 2009 2:30PM - 2:42PM |
X18.00001: Phase behavior of poly(ethylene oxide) in ethyl alcohol / water mixtures S. H. Shin, R. M. Briber, B. Hammouda, D. L-T Ho PEO in ethanol forms an opaque gel-like mixture with a partial crystalline structure. Addition of a small amount of water disrupts the gel: 5{\%} PEO in ethanol with the addition of 4{\%} water becomes a transparent solution. We have studied the phase behavior of PEO in mixed solvents using small angle neutron scattering (SANS). PEO solutions (5{\%} PEO) which have more than 4-10 {\%} water behave as an athermal polymer solution and the phase behavior changes from UCST to LCST rapidly as the fraction of water is increased. The proposed origin of this unusual phase behavior comes from the formation of a hydration layer around the PEO chain. Two water molecules can hydrate one PEO monomer which is consistent with the suppression in the crystallization and change in the phase behavior observed by SANS. We measured the spinodal temperature and phase behavior of PEO solutions with different concentrations of PEO (2{\%} PEO and 10{\%} PEO) in the mixed water/ethanol solvent system to assess the role of hydration. The observed phase behavior is consistent with a hydration layer forming upon the addition of water and the system shifting from UCST to LCST behavior. The amount of water necessary to form a hydration layer around PEO chains varies in a self-consistent manner as the PEO concentration increases from 2 to10{\%}. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X18.00002: pH-Responsive Swelling of PAMAM Dendrimer-Gels. Ronald Hedden, Burcu Unal End-linked hydrogels containing high mass fractions of amine-terminated poly(amidoamine) (PAMAM) dendrimers are prepared by reaction of dendrimers with monodisperse, epoxide-terminated linear poly(ethylene glycol) chains. PAMAM dendrimers impart pH-dependent swelling characteristics to the gels, which absorb large amounts of water due to protonation of the dendrimers' amine groups under neutral or weakly acidic conditions. The equilibrium swelling of the gels passes through a maximum at pH of approximately 4.5, due to extensive protonation of the amine groups. Interestingly, the equilibrium swelling ratio is markedly lower at both high external pH and low external pH. We model the swelling behavior by invoking the Donnan equilibrium theory, treating the gels as phantom networks that contain a high concentration of Lewis bases having pKb=3.5 The model captures the maximum in swelling near pH=4.5, though equilibrium swelling ratio is overpredicted in some cases. The collapse of the gels at both high and low external pH is explained in terms of the differential between the concentrations of mobile ions inside and outside the gel. We will discuss recent attempts to prepare stimuli-responsive gels based upon the remarkable swelling characteristics of PAMAM dendrimer-gels. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X18.00003: Studies on shear-thinning and recovery properties of beta-hairpin peptide hydrogel Congqi Yan, Radhika Nagarkar, Joel Schneider, Darrin Pochan In solution, freely soluble, unfolded MAX1 peptide ((VK)$_{4}$-V$^{D}$PPT-(VK)$_{4}$-CONH$_{2})$ can undergo a conformation change into a folded$\beta $-hairpin by exposure to a folding stimulus, e.g. pH change, salt addition, or temperature rise. The consequent self-assembly leads to a stiff hydrogel stabilized by physical crosslinks between fibrillar nanostructures. When a proper shear stress is applied, the hydrogel shear-thins and flows. Moreover, as soon as the stress is ceased, the gel immediately reheals into a stiff solid and recovers its original mechanical strength quickly. This shear-thinning and rehealing property makes possible hydrogel delivery via syringe injection. In this work, Rheo-SANS was adopted to monitor the gel network morphology under shear flow. Also, rheological experiments were performed to measure the gel recovery after shear-thinning under various shear treatment conditions. Laser scanning confocal microscopy was used to observe the flow and velocity profile of the hydrogel through a channel. The results explain morphology changes of the gel network during shear-thinning and subsequent rehealing process. The fundamental gel shear-thinning and rehealing mechanisms will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X18.00004: Poly(Z-Lysine)-based Block Copolymer Organogels Sandeep S. Naik, Adam D. Richardson, Daniel A. Savin A series of AB diblock copolymers consisting of poly(Lysine(Z)) (A = P(Lys(Z))) and poly(propylene oxide) or polyhedral oligomeric silsesquioxane (B = PPO, POSS) were synthesized and found to form stable, rigid organogels in THF and chloroform at room temperature. In these systems, the protecting group on the P(Lys) side-chains remains intact. As such, the secondary structure of the polypeptide chains retains helicity over a wide range of solution conditions. Gel formation in these systems results from the assembly of the solventphobic P(Lys(Z)) chains, which pack densely in an anti-parallel fashion, minimizing interfacial curvature. These gels all exhibited shear-thinning behavior, and as the temperature was heated to 350 K exhibited a gel-sol transition. The role of solvent polarity and molecular weight of the P(Lys(Z)) chains on the mechanical strength will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X18.00005: Anisotropic Poly(Vinyl Alcohol) Hydrogel: Connection Between Structure and Bulk Mechanical Properties Stephen Hudson, Jeffrey Hutter, Leonardo Millon, Wankei Wan, Mu-Ping Nieh Poly(vinyl alcohol) (PVA) hydrogels are formed from PVA solution by creation of physical cross-links during freeze/thaw cycling. By choosing a suitable freeze/thaw protocol and applying a strain during thermal processing, gels with permanent, anisotropic bulk mechanical properties matching those of cardiovascular tissues can be made, making them useful for applications ranging from artificial heart valves to vascular grafts. We have performed small- and ultra small-angle neutron scattering (SANS and USANS) measurements covering length scales from 2 nm to 10 $\mu$m, and modeled the structure as interconnected PVA blobs of size 20 to 50 nm arranged in fractal aggregates extending to at least 10 $\mu$m. Here, we discuss the relationship between the microstructure and bulk mechanical properties. Strength increases with the number of thermal cycles due to reinforcement of the small-scale gel phase, while anisotropy is due to elongation of the much larger fractal aggregates. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X18.00006: Kinetics of phase separation of thermoreversible gels Francisco J. Solis, Christine Leon, Brent Vernon We study the kinetics of phase separation and thermoreversible gel formation of LCST-type polymers. A large number of NIPAM-based polymers exhibit transitions near room temperature from a liquid phase to a two-phase state. In the two phase-region of the phase diagram, a polymer dilute phase coexists with a gel. The corresponding shrinking transition for chemically-linked gels has been extensively studied in both its thermodynamic and kinetic aspects. We show that, in the thermoreversible case, the formation of the gel phase proceeds in a similar way. Upon entrance to the two-phase region, the gel volume follows a double exponential decay. The gel undergoes a fast shrinking associated with water ejection, followed by a slower reorganization regime. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X18.00007: Structure and Rheology of Leucine Zipper Protein Hydrogels B.D. Olsen, J.A. Kornfield, D.A. Tirrell Protein hydrogels from telechelic polymers physically crosslinked by the specific association of leucine zipper domains provide fundamental insight into polymer network structures due to the unparalleled control over molecular weight and network junction multiplicity. Two different leucine zippers are used to confer either tetrameric or pentameric end block association. By varying the length of the polyelectrolyte midblock, we show that the structure and rheological properties of the hydrogels depend on both the polymer molecular weight and the aggregation state of the leucine zipper junctions. Cryo-TEM and negative staining are used together to visualize the gels, revealing heterogeneous structures. The gels are strongly shear thinning, and examination of Lissajous figures of stress vs. strain suggest a yielding mechanism. Under many conditions the gels can recover nearly their full strength less than a minute after the cessation of shear. These properties combined with the ease of biofunctionalization and pH and temperature responsive gelation transitions make the materials attractive for tissue engineering. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X18.00008: Morphology of biaxially stretched triblock copolymer gels using SAXS Arjun Krishnan, Tushar Ghosh, Richard Spontak Gels of styrenic triblock copolymers swollen by a low-volatility, midblock-selective oil behave as high-strain, low-field dielectric elastomers in the design of electroactive polymeric actuators. A standard configuration of such devices involves stretching, or ``prestraining,'' the elastomer film biaxially. However, little is known about the effect of biaxial prestrain on copolymer morphology. In this study, small-angle X-ray scattering (SAXS) is used to probe the nanostructure of gels composed of poly[styrene-$b$-(ethylene-\textit{co-}butylene)-$b$-styrene] and mineral oil by systematically changing the concentration of polymer from 5 to 30 wt{\%} and the biaxial prestrain from 0 to 300{\%}. In the azimuthally integrated intensity profiles, the form factor due to scattering from polystyrene microdomains correlates strongly with polymer concentration and does not change with the applied prestrain, indicating that the polystyrene crosslinks remain as polydisperse spheres. The structure factor data correlates with prestrain, and is fitted using the Percus-Yevick approximation for interacting spheres. While a hard sphere interaction model is sufficient for unstrained gels, we resort to a square shoulder hard sphere potential for strained samples. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X18.00009: Computer Simulation of a Switchable Metallo-Supramolecular Gel Shihu Wang, Elena Dormidontova Using Monte Carlo simulation, we studied reversible metallo- supramolecular gel comprised of linear oligomers end- functionalized with ligands and metal ions that can form trans- or cis- ligand-metal complexes with a ratio up to 3:1. We found that cis- isomers exhibit a larger overall degree of association and higher average molecular weight compared to trans- isomers due to a larger fraction of 3:1 or 2:1 ligand- metal complexes. Furthermore the metallo-supramolecular gel formed by cis-isomers occurs within a wider range of metal-to- oligomer ratios at a lower oligomer concentration and exhibits a larger elastic modulus and a smaller mesh-size compared to gel formed by trans-isomers. We found that exchanging cis- to trans- isomers leads to a monotonic change of the materials properties for most cases except for the 2:1 ligand:metal ratio at which the mesh size exhibits a minimum due to the favorable formation of intra-molecular bonds by cis- isomers. These switchable properties suggest interesting application opportunities. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X18.00010: Cavitation Rheology and Fracture Behavior of Polyacrylamide Hydrogels Santanu Kundu, Alfred Crosby Cavitation rheology is a new characterization technique for the measurement of mechanical properties on small length scales, e.g. 10 -1000 $\mu $m, at any arbitrary location within a soft material. The technique involves growing a cavity at the tip of a syringe needle and monitoring the pressure of the cavity at the onset of instability. This critical pressure is directly related to the local modulus of the material. We used this technique to characterize the network mechanics of polyacrylamide hydrogel materials, a common material used in many biological applications. Gels with different moduli, which were obtained by varying initial monomer to water ratio, were investigated. As monomer concentration increased, a transition from stable cavity to fracture was observed. Applying scaling theory for gels, we modify the Lake-Thomas Theory for the fracture of crosslinked networks to relate the transition from cavitation to fracture in terms of molecular parameters. We anticipate this fundamental understanding of cavitation and fracture mechanism will be applicable to biological tissues, as well as the development of advanced soft materials [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X18.00011: Dynamic display of biomolecular patterns through an elastic creasing instability of stimuli-responsive hydrogel surfaces Jungwook Kim, Ryan Hayward Swelling a soft hydrogel film attached to a rigid substrate generates a lateral biaxial compressive stress within the gel. For sufficiently large stresses, the free surface of the gel undergoes a mechanical instability to form sharp creases on its surface. We have taken advantage of this process using temperature-responsive hydrogels to fabricate dynamic scaffolds that reversibly hide and display biomolecular patterns. Desired bioactive ligands are grafted to polyelectrolytes, which are then selectively deposited to pattern the hydrogel surface. The shapes of the patterns are directed by topographic features of the underlying substrate. At room temperature, the functionalized areas of the surface are hidden within creases, but as the temperature is raised, dehydration of the gel leads to unfolding of creases and exposure of the biomolecular patterns. By switching on and off the patterned functionalities, we could engineer dynamic interactions between our scaffolds and target objects such as microscopic beads or cells. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X18.00012: Photo-induced locomotion of chemo-responsive polymer gels Pratyush Dayal, Olga Kuksenok, Anna C. Balazs The need to translate chemical energy into a mechanical response, a characteristic of many biological processes, has motivated the study of stimuli-responsive polymer gels. Recently, it has been shown experimentally that by coupling the mechanical properties of the gel with the Belousov-Zhabotinsky (BZ) reaction it is possible to induce self-sustained oscillations in the gel. One of the means for controlling these chemical oscillations is using light as an external stimulus. To study the effect of light on the mechanical behavior of the gel, we use our recently developed a 3D gel lattice spring model (gLSM) which couples the BZ reaction kinetics to the gel dynamics. In this model, the polymer-solvent interactions were taken into account by adding a coupling term to the Flory-Huggins free energy. By virtue of this coupling term, the swelling---de-swelling behavior of the gel was captured in 3D. In order to include the effect of the polymer on the reaction kinetics, the Oregonator model for the photo-sensitive BZ reaction was also modified. Using gLSM model, we probed the effect of non-uniform light irradiation on the gel dynamics. We were able to manipulate the direction and velocity of locomotion of the gel using light as a control parameter. This ability to control the movement of the gel can be utilized in a variety of applications, ranging from bio-actuators to controlled drug release systems. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X18.00013: Harnessing Labile Bonds between Nanogels Particles to Create Self-Healing Materials German Kolmakov, Krzysztof Matyjaszewski, Anna Balazs Using computational modeling, we demonstrate the self-healing behavior of novel materials composed of nanoscopic gel particles that are interconnected into a macroscopic network by both stable and labile bonds. Under mechanical stress, the labile bonds between the nanogels can break and readily reform with reactive groups on neighboring units. This breaking and reforming allows the units in the network to undergo a structural rearrangement that preserves the mechanical integrity of the sample. The stable bonds between the nanogels play an essential role by forming a backbone that provides a mechanical strength to the material. The simulations show that just a relatively small fraction of such labile bonds (roughly 15\%) are needed to prevent the catastrophic failure of the sample. The findings provide guidelines for creating high-strength, self-healing materials. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X18.00014: Effect of Confinement on the Dynamics of Three-Dimensional Chemo-responsive Gels Olga Kuksenok, Victor V. Yashin, Anna C. Balazs Chemo-responsive gels undergoing the Belousov-Zhabotinsky(BZ) reaction could be ideal candidates for creating materials that can perform sustained mechanical work. We use theory and simulation to investigate the behavior of three-dimensional samples of BZ gels that are spatially confined in various geometric arrangements and show that the spatial confinement has a dramatic effect on the samples' dynamics. We first perform a linear stability analysis in two limiting cases, where a small sample is either completely free or attached at all the boundaries to fixed, hard walls. We find the critical reaction parameters at which the gels undergo a transition from a stationary steady state to an oscillatory regime in each of these cases. We then carry out corresponding computer simulations using our 3D gel lattice spring model and find an excellent agreement between the theory and simulations. Furthermore, we illustrate that the above analysis allows us to predict the behavior of larger samples that are confined in more complex spatial arrangements. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X18.00015: Diffusion of molecular probes and proteins in hydrogels Riccardo Raccis, Robert Roskamp, Annette Brunsen, Bernhard Menges, Ulrich Jonas, Wolfgang Knoll, George Fytas We employ fluorescence correlation spectroscopy to study the diffusion of molecular probes (Cy5) and dye-tagged proteins (Cy5-AntiMouse, hydrodynamic radius 10nm and Alexa488-Streptavidin, 4nm) in surface-attached poly-N-isopropylacrylamide (PNIPAAm) and dextran based hydrogel layers. The diffusion process depends on the crosslinking density and the presence of electrostatic and steric interactions. The protein penetration into the hydrogel layer occurs close to the isoelectric point but the local probe concentration and diffusion rate diminish with increasing penetration depth. Mesh size characterization of the hydrogels is inferred from the diffusivity and the concentration profile of fluorescent probes with different size, with the molecular free dye diffusing deeper into the gel. [Preview Abstract] |
Session X19: Charged and Ion-containing Polymers
Sponsoring Units: DPOLYChair: Jodie Lutkenhaus, Yale University
Room: 320
Thursday, March 19, 2009 2:30PM - 2:42PM |
X19.00001: Molecular Weight and Charge Density Asymmetry in Polyelectrolyte Complexation Debra Audus, Glenn Fredrickson, Dominik Duechs We investigate the phase diagram of oppositely charged polymers in a good solvent using a field-theoretic model. Mean-field solutions fail to predict the experimentally observed macroscopic phase separation into a solvent-rich phase and a dense liquid aggregate of polymers - a ``complex coacervate.'' We therefore study the model within a one-loop approximation, which accounts for Gaussian fluctuations in electrostatic and chemical potentials. Our particular focus is the effect of molecular weight, ionic strength, and charge asymmetry on the phase envelope. A set of dimensionless parameters is identified that dictate the size and shape of the two-phase region. Our results should be helpful in guiding experimental studies of coacervation. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X19.00002: PEG-based Sulfonated Ionomers Microphase Separate with Increasing Temperature Wenqin Wang, Gregory J. Tudryn, Ralph H. Colby, Karen I. Winey A series of Li, Na, and Cs-neutralized sulfonated polyester ionomers with well-defined PEG spacer lengths have been investigated by in situ X-ray scattering over a wide temperature range. At room temperature, no ``ionomer peak'' at q=1-5 nm$^{-1}$ was observed, due to the high dielectric constant of the polymer matrix. As the length of the PEG segment increases, the crystallization of PEG segments is evidenced by multiple crystalline reflection peaks. In addition, crystallization produces periodic low-angle peaks, indicating a layered structure. Scanning transmission electron microscopy will be employed to facilitate the understanding of the nanoscale structures. At high temperatures, the PEG-based ionomers exhibit a new X-ray scattering peak in the angular range of 2-3 nm$^{-1}$, reminiscent of conventional ionomers. The peak intensity increases with temperature while the angular position remains fixed. The appearance of an ``ionomer peak'' at high temperature is attributed to the microphase separation of ionic aggregates as the PEG dielectric constant decreases. A mechanism is proposed to explain the ionic association behavior as a function of temperature. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X19.00003: Hierarchial Structures in PDMS-based Ammonium Ionenes David Salas-de la Cruz, Sudipto Das, Garth Wilkes, Karen Winey Ionenes are polymers with a charged entity in the backbone. Polydimethylsiloxane (PDMS) based ionenes were synthesized from 6-bromohexanoyl chloride and 1,4-diazabicyclo[2.2.2]octane (DABCO). X-ray scattering patterns were recorded at the small, intermediate and wide angle regions for both un-stretched and stretched films. Stretching induces orientation without changing the spacing in the morphology. The hard charged segments, that include DABCO, align in pseudo-cylindrical aggregates in the direction of the stretch and the PDMS soft segments remain amorphous. The orientation relative to the stretching direction suggests that the hard charged segments self assemble into anisotropic aggregates. This self-assembly arises from the hydrogen bonding of the urethane groups and ion sharing between Br- and N+. The characteristic spacing between the pseudo-cylindrical aggregates increases with the molecular weight of the PDMS segments and the size of the hard segments. Ionic conductivity measurements reveal that the through-plane conductivity increases by an order of magnitude upon stretching. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X19.00004: Temperature and pH Responsive Chargeable Copolymers with Tunable LCSTs Kiattikhun Manokruang, Evangelos Manias A series of alternating copolymers, made of $\alpha $,$\omega $-polyethylene glycol oligomers (M$_{w}$ 400 or 900) alternating with 1,6-diamino-hexane-\textit{stat}-lysine, is presented. Specifically their aqueous phase behavior is outlined, exhibiting temperature-controlled solubility (LCST) and a pH-controlled transition (across the point where lysine is charged). The terpolymers are uncharged at low pH while they become charged (containing lysine anions) when the solution pH increases; the phase diagrams in the temperature and pH space are drawn, and the tunability of the critical points in water as it is controlled by the copolymer composition is discussed. These copolymers, due to their hydrophilic polyethylene-oxide comonomers, exhibit a genuine LCST, i.e., a bona fide first order thermodynamic transition, rather than the usual micellization related LCST of copolymers that contain hydrophobic blocks or grafts. This last point is demonstrated by comparisons against micelle-forming copolymers, consisting of hydrophobic and chargeable comonomers. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X19.00005: Ionic Conductivity of Poly(ethylene oxide)-Containing Block Copolymers at Order-Disorder and Order-Order Transitions Nisita Wanakule, Ashoutosh Panday, Scott Mullin, Nitash Balsara The order-disorder transition (ODT) and order-order transition (OOT) of block copolymers with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salts are measured with a combination of small-angle x-ray scattering (SAXS) and birefringence. The block copolymers comprise of polyethylene oxide (PEO), a polymer with a higher dielectric constant that dissolves LiTFSI, and polystyrene (PS), a polymer with a lower dielectric constant that does not dissolve LiTFSI. Ionic conductivity of the block copolymers are measured through the observed ODT and OOT. The effect of morphology on the ionic conductivity will be presented and compared with literature results. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X19.00006: Origin of Lateral Nanoscale Heterogeneities in Weak Polyelectrolyte Brushes You-Yeon Won, Kevin Witte, Jaehyun Hur In this talk, we will first discuss experimental evidence of lateral nanoscale heterogeneities in a single-component weak polyelectrolyte brush system under zero to low salt conditions. Using an amphiphilic diblock copolymer, poly(2-(dimethylamino)ethyl methacrylate-$b-n$-butyl acrylate) (PDMAEMA-PnBA), for Langmuir film compression and Langmuir-Blodgett deposition on a hydrophobic substrate followed by fluid AFM imaging, we show the existence of regions of different brush heights, indicative of the thermodynamic instability (and resultant local clustering) of the PDMAEMA chains in the low-salt limit. Using SCF and scaling theories, we will also show that the lateral heterogeneities occur due to the combined effects of (i) the osmotic instability regulated by charge equilibrium and (ii) the hydrophobicity of the chains. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X19.00007: \textit{In situ} Neutron Reflectivity study of alcohols into ultra-thin ionmer films Thusitha Etampawala, Dvora Perahia, Lilin He, Christopher Cornelius For many applications a polymeric membrane with selective well-controlled transport are desired. A new class of materials that consists of rigid hydrophobic polyphenylene decorated by sulfonic acid functionalized phenylene side chains has shown a potential as controlled transport membranes for energy applications. The nature of the diffusion depends on the polymer structure and the interaction of its different segments with the solvent. In addition, the interfacial layer plays a critical roll in the transport characteristics. The current work introduces an \textit{in situ }neutron reflectivity study of penetration of long-chain alcohols into ultrathin films. The time dependence of the penetration of deuterated n-hexanol into films of 20nm thick ultra-thin films was determined and compared with that of less hydrophobic solvents. The process consists of two stages, a relatively fast stage in which the film thickness increases linearly with time followed by a slow phase in which structural changes take place. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X19.00008: Dynamic behavior of polyelectrolyte multilayer investigated by thin film calorimetry. H. Huth, R. Mueller, A. Fery, C. Schick Polyelectrolyte multilayer can be easily assembled using spraying or dipping of the different polyelectrolytes [1]. The thickness of the produced layers (nanometer range) is well controlled by the preparation conditions. Only a few methods are available for dynamic investigations, as afm for mechanical properties [2]. AC-chip calorimetry is used as a very sensitive tool for calorimetric investigations of such thin films as demonstrated for thin polymeric films in a wide frequency range [3]. To investigate the dynamic behavior of polyelectrolytes the humidity is used as a new parameter in addition to temperature for calorimetry. First measurements with the modified calorimeter for the PSS/PDADMAC polyelectrolyte multilayer system are shown. Further extensions of the calorimeter for better understanding of the phase behavior are discussed. [1] Decher, G. and J.D. Hong, Phys. Chem. Chem. Phys., 1991. 95(11): 1430. [2] Mueller, R., et al. Macromolecules, 2005. 38(23): 9766. [3] Huth, H., Minakov, A. A., Schick, C., J. Polym. Sci. B Polym. Phys. 2006 44: 2996. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X19.00009: Counter-ion fluctuations in the presence of a spherical macromolecule Leandro Boonzaaier, Kristian K. Mueller-Nedebock, Frederik G. Scholtz The effective interactions of charged macromolecules (e.g. polyelectrolytes) are still not fully understood. The role of counter-ion fluctuations, in the presence of these macromolecules, seems to be crucial in understanding these effective interactions. We consider a single charged spherical macromolecule, enclosed in a finite volume, in the presence of point-like counter-ions in an electrically neutral solution. Writing the partition function as a functional integral and only keeping terms up to quadratic order, we calculate the free energy in this approximation exactly. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X19.00010: Counterion Condensation and Collapse of Sodium Polystyrene Sulfonate in Water: A Molecular Dynamics Study Jan-Michael Carrillo, Andrey Dobrynin Hydrophobic polyelectrolytes are known to form necklace-like structures of dense beads connected by strings of monomers. This structure appears as a result of optimization of electrostatic and short-range interactions. To elucidate the effect of counterion condensation on polyelectrolyte conformations we performed two sets of molecular dynamics simulations of sodium polystyrene sulfonate (NaPSS) chains with degree of polymerizations $N$=16, 32 and 64 and fraction of charged monomers $f$=0.25, 0.33, 0.5 and 1.0 in aqueous solutions; (1) water molecules were considered explicitly using TIP3P model and (2) water molecules were modeled as a dielectric continuum with dielectric constant 77.73. Our simulations showed that with increasing $f$ a polyelectrolyte chain adopts an elongated conformation. The transition between collapsed and elongated states does not show any features of abrupt transition due to the fact that only relatively short chains were considered. Furthermore, even for our longest chains ($N$=64) the necklace-like globule was not observed. Effect of the water-ion interactions on counterion condensation was analyzed by comparing the radial distribution function between the sulfonate groups and sodium counterions for chains with different $f$. It was found that in simulations with explicit water ionized groups are located at the globular surface. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X19.00011: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X19.00012: Computational titrations of linear and branched polyethyleneimine Jesse Ziebarth, Yongmei Wang Despite a great deal of interest in polyethyleneimine (PEI) for its potential use as a gene therapy vector, the protonation state of this polycation is currently not well understood. PEI is rare among polyelectrolytes as every third atom along the backbone of the polymer is a protonable nitrogen. The closeness of these potentially charged sites can lead to high electrostatic repulsion and a large shift in the effective pKa of the amine groups as the degree of protonation increases. Previous experimental and theoretical estimates of the degree of protonation of PEI under physiological conditions have ranged from 0.15 to 0.80. Here, we perform computational titrations on coarse-grained models of both branched and linear PEI under various conditions. Chain length, degree of branching, salt concentration, and the solvent dielectric constant are varied to determine how these factors influence the protonation of PEI. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X19.00013: Phase diagrams of effective charge and size of an isolated polyelectrolyte chain and gels Arindam Kundagrami, Murugappan Muthukumar We consider an isolated polyelectrolyte chain and charged gels under poor solvent conditions and calculate the phase diagrams for the effective charge and size of the respective systems as functions of physical parameters such as temperature and salt concentration. Phase boundaries and the critical point for the first order collapse transition - induced cooperatively by counterion adsorption and solvent quality - are calculated self-consistently in terms of both the effective charge and the size of the chain with arbitrary ionizability. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X19.00014: Single chain contraction and re-expansion by counterions of polyelectrolytes Pengxiang Jia, Jiang Zhao We have studied the re-entrant transition of polyelectrolytes at single chain level. Diffusion rate of single chains of polystyrene sulfonate (PSSNa) has been investigated by fluorescence correlation spectroscopy under different counterion condition. Upon the addition of external salts, the hydrodynamic radius of PSSNa was found to decrease with the increase of salt concentration, i.e., the chain contracts due to electrostatic screening and ion bridging effect. When the salt concentration increased beyond a certain value, the chains were observed to increase their dimension for a few mono-, double- and triple-valent counterions. The experimental evidence shows the single chain behavior of re-entrant transition of polyelectrolytes. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X19.00015: Conformation and Dielectrophoresis of Single Weak Polyelectrolyte under AC Electric Field Shengqin Wang, Yingxi Zhu The application of AC-electric fields, when integrated with current microarray and ``lab-on-chip'' platforms, has emerged as a useful tool to manipulate and assemble supramolecular aggregates, such as AC-field induced protein crystallization and DNA hybridization, yet the detailed mechanism remains unclear. To understand the AC-polarization mechanism of biomacromolecules, we examine the conformational and dielectrophoretic behavior of polyelectrolytes under AC-electric field using fluorescence correlation spectroscopy (FCS) at single molecular level. We focus on poly (vinyl pyridine) (PVP) as a model polyelectrolyte whose conformation can be tuned by adjusting the pH and ionic strength. We observe that in the non-uniform AC electric field, PVP in a coil conformation experiences the positive DEP force at applied frequency below 500 kHz; it is also intriguing to observe the elongation of PVP coils at a characteristic AC-frequency of $\sim $ 200 kHz. In sharp contrast, PVP in a globule conformation shows insensitive to imposed AC-fields. We also examine the conformational change in a uniform AC-field where the DEP force is absent; the result suggests a redistribution of counterions with an induced dipole of polyelectrolyte. The behavior is further investigated with varied medium conductivity and microelectrode geometry. [Preview Abstract] |
Session X20: Long Range Order in Polymer Structures and Morphologies
Sponsoring Units: DPOLYChair: Sam Gido, University of Massachusetts
Room: 321
Thursday, March 19, 2009 2:30PM - 3:06PM |
X20.00001: Long range ordering in block copolymer thin films Invited Speaker: Thin films of microphase separated block copolymers, which can form patterns consisting of dense arrays of lines or dots, are attractive materials for self-assembled nanoscale lithography. The long range order of the block copolymer microdomains can be controlled by the use of chemical or topographical patterns. In this work, we discuss how Si-containing block copolymers, polystyrene-b-polyferrocenyldimethylsilane (PS-PFS) and polystyrene-b-polydimethylsiloxane (PS-PDMS), can be templated on substrates patterned with posts or steps. In the case of 40 nm period spherical morphology PS-PDMS, $<$20 nm diameter posts, which are coated with a grafted layer of PDMS homopolymer, define the locations of surrounding PDMS microdomains. The lattice spacing and orientation of the templated PDMS microdomain array can be predicted from the ratio between the post spacing and the equilibrium microdomain spacing. PFS spheres, formed from spherical-morphology PS-PFS, can be aligned within shallow trenches to form a close-packed array with row spacing determined by the trench width. We also show how 32 nm period cylindrical morphology PS-PDMS can be templated using topographical features. Templating using posts or linear substrate features gives arrays of straight parallel cylinders with controllable period and orientation, while templating in circular pits creates sharply curved, concentric toroidal structures. The overall morphology and period of the block copolymer microdomain arrays can be varied by solvent annealing in mixed solvent vapors, for example cylindrical-morphology PS-PDMS can form perforated lamellae by annealing in toluene plus heptane. These results will be discussed in the context of nanolithography, including examples of pattern transfer to form metal, oxide and polymer functional nanostructures. Bita et al, Science 321 939 (2008); Jung et al, Nano Letts. 7 2046 (2007); 8 2975 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X20.00002: Surface Morphology Diagram for Cylinder-Forming Block Copolymer Thin Films Alamgir Karim, Xiaohua Zhang, Jack Douglas, Ronald Jones We investigate the effect of annealing temperature (T) and film thickness (hf) on the surface morphology of flow coated films of a cylinder forming block copolymer, poly (styrene-block-methyl methacrylate) (PS-b-PMMA). A transition from a perpendicular to a parallel cylinder orientation with respect to the substrate is observed upon increasing hf when the substrate interaction is highly selective for one of the blocks (PMMA) and the polymer-air interface has a nearly neutral interaction with both blocks. Surface morphology transitions with increasing hf are observed in these model `frustrated-interaction' films: (a) first, a transition occurs from cylinders oriented parallel to the substrate to a mixed or `hybrid' state where the two orientations coexist (b) this hybrid morphology then transforms to cylinders oriented perpendicularly to the polymer-air interface for larger hf. The characteristic values of hf defining these surface morphological transitions depend on T and we construct a surface morphology diagram as a function of hf and T. The surface morphology diagram is found to depend on the method of film formation (flow coated versus spun cast films) so non-equilibrium effects evidently have a large effect on the surface pattern morphology. In particular, the residual solvent within the film (quantified by neutron reflectivity measurements) can have a large effect on the surface morphology diagram and the physics of glass-formation is also apparently important. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X20.00003: Practical Implementation of Order Parameter Calculation for Directed Assembly of Block Copolymer Thin Films Chi-Chun Liu, Gordon Craig, Paul Nealey, Ricardo Ruiz, Nicola Ferrier The assembled morphologies in thin films of block copolymers are gaining interest for potential applications in advanced lithography and template fabrication due to their low defect density, pattern rectification, and resolution enhancement. For such applications, the ability to quantify the extent of order achieved with different assembly methods and materials is crucial. Previous studies analyzed the translational order parameter ($\Psi $T) based on reciprocal lattice vectors obtained from Fourier Transforms, but discussed neither the accuracy of these vectors nor the effect of domain positioning on the calculation results. In both simulations and real systems we have shown the inaccuracy of these parameters can lead to huge difference in $\Psi $T. Here we present a computational procedure to delineate the importance of these parameters and analyze with a high level of accuracy the translational and orientational order parameters of the guiding pattern, the domains of a block copolymer directed to assemble on this pattern, as well as the underlying structure after lift-off. We observe that order can be preserved over a large area and can be retained in subsequent processing. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X20.00004: Controlling the self-assembly of block copolymer materials in thin-films Eungnak Han, Karl Stuen, Paul Nealey, Padma Gopalan We present a simple and efficient strategy towards surface modification for controlling the self-assembly of P(styrene-b-methylmethacrylate) diblock copolymer (BCP) in thin films. Photo-patternable, substrate-independent neutral surface was created to achieve vertical orientation of block copolymer (BCP) microdomains. A random copolymer of styrene (f= 0.58-0.63), methylmethacrylate (f= 0.41-0.46) and glycidyl methacrylate (f= 0.01-0.02) was synthesized. The copolymer uses photo-crosslinking reaction of epoxy groups by photoacid generator to formulate the neutral surface. Ultra-thin (2-6 nm) crosslinked film was created as a neutral interfacial layer between the block copolymer and the substrate. The composition of the copolymer was fine tuned to tailor the wetting behavior and hence the domain orientation (parallel or perpendicular to the substrate) in the top self-assembled block copolymer film. The effectiveness of the new neutral polymer on a range of substrates such as glass and gold coated silicon wafer and for both symmetric and asymmetric BCPs is demonstrated. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X20.00005: Shear Alignment and Realignment of Cylinder-Forming Block Copolymer Thin Films Andrew Marencic, Richard Register, Paul Chaikin The microdomains in cylinder-forming block copolymer thin films can be oriented by applying a sufficient shear stress to the film; the cylinders lie in-plane and align with the shear direction, as shown by atomic force microscopy (AFM) post-shear. To understand the alignment process, we applied a stress gradient to the film, and focused on the structure in the transition region from unaligned to aligned. To sample a statistically useful number of grains, we used the moir\'{e} patterns formed by interference of the AFM scanning grid with the microdomain lattice to determine the cylinder orientation. The results are compared with a simple melting-recrystallization model, in which grains misaligned with the shear direction are eliminated as the shear stress is increased. In addition, we have applied two shear gradients to the sample in different directions, to directly probe how ordered cylinders can be realigned by a subsequent shear. We again find qualitative agreement with the simple model, but the stress required is a factor of 1.7 larger than required for single shear. We also observed grain boundary generation within the area between alignment with the first shear direction and alignment with the second shear direction. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X20.00006: Structural transition with thickness in films of poly-(styrene-b-2vinylpyridine) (PS-b-P2VP) diblock copolymer/homopolymer blends Vindhya Mishra, Edward Kramer, Su-mi Hur, Glenn Fredrickson , Michael Sprung In multilayer thin films of spherical morphology block copolymers, the surface layers prefer hexagonal symmetry while the inner layers prefer BCC. Thin films with spherical morphology of PS-b-P2VP blends with short homopolymer polystyrene (hPS) chains have an HCP structure up to a thickness n* at which there is a transition to a face centered orthorhombic structure. Using grazing incidence small angle X-ray scattering and transmission electron microscopy we show that that n* increases from 5 to 9 with increase in hPS from 0 to 12 vol{\%}. For thicknesses just below n* the HCP and FCO structures coexist, but on long annealing HCP prevails. We hypothesize that the PS segregates to the interstices in the HCP structure reducing the stretching of the PS blocks and the free energy penalty of HCP versus BCC inner layers. Self consistent field theoretic simulations are being carried out to see if this idea is correct. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X20.00007: A Modular and Hierarchical Supramolecular Block Copolymer Self-assembling Strategy Towards Square Arrays Chuanbing Tang, Erin Lennon, Michael Dimitriou, Glenn Fredrickson, Edward Kramer, Craig Hawker We present a modular and hierarchical self-assembling strategy for the generation of novel nanoscale patterns suitable for block copolymer lithography. Supramolecular block copolymers consisting of poly(ethylene oxide)-b-poly(styrene-r-4-hydroxystyrene) and poly(styrene-r-4-vinylpyridine)-b-poly(methyl methacrylate) diblock copolymer blends with hydrogen-bonding interactions between the polystyrene majority segments were prepared by living free radical polymerization. By combining supramolecular assembly of H-bonding phenolic and pyridyl units with controlled phase separation of diblock copolymers, highly ordered square arrays were obtained. The compositions of H-bonding components were critical for generating both long range order and for controlling the spatial arrangement of ordered arrays. The utilization of these materials as lithographic masks was successful and allowed transfer of the polymeric template with high fidelity to silicon oxide substrates, leading to a highly ordered array of 20 nm cylindrical pores with a spacing of 50 nm. This modular blending approach to block copolymer resists demonstrates a new and powerful strategy for the fabrication of unique patterns for nanolithographic applications. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X20.00008: Macroscopic Addressable Arrays of Block Copolymer Microdomains Soojin Park, Dong Hyun Lee, Bokyung Kim, Sung Woo Hong, Ji Xu, Unyong Jeong, Ting Xu, Thomas P. Russell Generating addressable, macroscopic arrays of nanoscopic elements with perfect lateral order has the potential to revolutionize the microelectronic and storage industries. A novel approach is shown using faceted surfaces of commercially available sapphire wafers to guide the self-assembly of block copolymer (BCP) microdomains into arrays with single crystal textures over the entire wafer surface. Perfectly ordered arrays of BCP microdomains, with areal densities in excess of 10 Terabit/inch$^{2}$, have been produced. The sawtoothed substrate topography provides registered, directional guidance of the BCP self-assembly that is tolerant of surface defects, maintaining the lateral registry and ordering of the microdomains over the entire surface. The approach provides unprecedented areal densities, and opens simple, yet versatile routes to ultrahigh density, addressable systems. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X20.00009: Self-Extinguishing Crystallization: Copolymer Behavior under Flow Diana Smirnova, Meisam Hajimorad, Julia Kornfield It is known that short chain branches in copolymers act as crystal defects, resulting in materials with low crystallinity and poorly-defined morphology. We are interested in the behavior of copolymers under flow in the presence of species that readily form well-defined shish-kebab morphologies. Bimodal blends containing small concentrations of high molecular weight, high density polyethylene (HDPE, Mw = 526 kg/mol, Mw/Mn = 3) in an ethylene-co-hexene matrix (Mw = 50 kg/mol, Mw/Mn = 2, 5 mol {\%} hexene) were studied via rheo-optical and rheo-xray techniques. HDPE concentrations were selected above and below the overlap concentration of 0.6{\%}, but maintained below 1{\%} such that the rheology of the blends was not significantly altered from that of the copolymer matrix. DSC traces were collected to ensure that co-crystallization between the two blend components occurs. Crystallization after shear quickly leveled off revealing a self-extinguishing behavior. The time frame for this extinction is coupled with a loss of anisotropy in scattering patterns indicating random crystallization uncorrelated with existing oriented structures. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X20.00010: Quasicrystalline long-range order in an ABC star block copolymer Tomonari Dotera We report the formation of a dodecagonal quasicrystal (DDQC) in a lattice Monte Carlo simulation of a star-shaped three component polymeric alloy. We have observed a series of Archimedean and quasicrystalline phases $(4.8^2) \rightarrow (3^2.4.3.4) \rightarrow {\rm DDQC} \rightarrow (4.6.12)$ with increase of one component of ABC star polymers. This phase behavior can be regarded as a transition from square tiling to triangle tiling via square-triangle tiling. The simulation is associated with the recent striking experimental manifestation of quasicrystalline order: A mesoscopic tiling pattern with twelvefold symmetry in a three-component star polymer system composed of polyisoprene, polystyrene, and poly (2-vinylpyridine). Since, the same kind of quasicrystalline structures have been found for metal alloys, chalcogenides, and liquid crystals, the present result confirms the universal nature of quasicrystalline long-range order over several hierarchical length scales. \\[4pt] T. Dotera and T. Gemma, Philos. Mag. {\bf 86}, 1085 (2006).\\[0pt] T. Dotera, Phil. Mag. {\bf 88}, 2245 (2008). \\[0pt] K. Hayashida, et al., Phys. Rev. Lett. {\bf 98}, 195502 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X20.00011: Effect of chain extender on the phase behavior and morphology of high hard block content polyurethanes Alberto Saiani, Achilleas Tsiotas Thermoplastic Polyurethanes (TPU) are linear block copolymers typically constructed of statistically alternating soft (SS) and hard (HS) segments. Due to their numerous industrial applications these materials have received considerable attention. We have investigated the phase behavior and morphology of a set of high hard block content polyurethanes and varied the chain extender used. Using mainly calorimetry, scattering and microscopy techniques we were able to elucidate the origins of all the thermal events observed through differential scanning calorimetry. Correlating our thermodynamic work with our structural work we were able to propose a new morphological model of the structure and the phase behavior of high hard block content polyurethanes. We have shown that above 65{\%} hard segment content the melt-quenched samples present a two-phase morphology one pure hard segment phase co-existing with a mixed phase with the same hard segment content of 65{\%} for all samples. When annealed at high temperature the mixed phase undergoes phase separation resulting in the same phase-separated mesophase. Changing the chain extender has a significant impact of the phase behavior and morphology of these systems [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X20.00012: Ligand-induced order in Spotted vesicles and Striped micelles David Christian, Wouter Ellenbroek, Andrea Liu, Dennis Discher Mixtures of amphiphiles are ubiquitous and assemble into various morphologies, including giant vesicles and cylinder micelles that raise the possibility of mesoscopic segregation within the assemblies -- perhaps even in response to binding of small ligands. Here, with anionic and neutral polymer amphiphiles mixed within vesicle and cylinder morphologies, divalent cations are shown to induce meso-scale domains and thus generate `\textit{responsive} \textit{Janus }structures'. Whereas past reports with lipid systems appear conflicted, calcium forms definitive crossbridges between the anionic polymer amphiphiles, rigidifying the charged membranes across a fluid-gel transition and also leading to lateral phase separation without disrupting the assemblies. A systematic phase diagram for these robust assemblies shows that long-lived domains occur in an unexpectedly narrow region near the polyanion's p$K$'s for protonation and cation association. The phase behavior appears well described by a relatively simple model in which -- among electrostatic and entropic contributions -- counterion entropy outcompetes attractive crossbridging to drive remixing of the highly charged polyacid at high pH, contrary to intuition. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X20.00013: Electrospinning of semicrystalline polymer fibers Ying Liu, Shuang Chen, Chunhua Li, Elaine DiMasi, Gad Marom, Miriam Rafailovich Electrospinning of polymeric fibers has been attracted increased interest in recent years. However, the research for ethylene-vinyl acetate (EVA) and linear polyethylene (PE) is still limited, due to their relatively poor solubility in conventional solvent systems at ambient temperature. In this study, EVA and PE fibers were electrospun with different fiber diameter when the electrospinning solution was kept at a temperature greater than that of the solidification temperature of the polymer solutions. The effects of the fiber physical dimension to its crystallization and mechanical properties were thus detected. The morphology of the fibers was measured by scanning electron microscope (SEM) and atomic force microscope (AFM). The shear modulation force microscopy technique (SMFM) was used to measure the melting point, $T_{m}$, which was found to increase with increased fiber diameter and crystallinity. AFM three-point bending test demonstrated that the Young's modulus of the fibers drastically increased as fiber diameter decreased.$_{ }$Grazing-incidence small angle x-ray scattering (GISAX) showed that, compared to the bulk material, the crystallinity of the electrospun fibers had been changed. [Preview Abstract] |
Session X21: Spectroscopic Studies of Semiconductor Structure and Their Growth
Sponsoring Units: FIAP DCMPChair: Yia-Chung Chang, Research Center for Applied Sciences, Academia Sinica
Room: 323
Thursday, March 19, 2009 2:30PM - 2:42PM |
X21.00001: Electronic structure of Na$_{x}$CoO$_{2}$ investigated by X-ray absorption spectroscopy with Ab initio calculation Pao-An Lin, Jiunn-Yuan Lin, Ben Hsu, Horng -Tay Jeng, Chen-Shiung Hsue, Yia-Chung Chang The soft X-ray absorption spectra (XAS) of Na$_{x}$CoO$_{2}$ revealed marked and puzzling polarization dependence. It can not be explained by the degeneracy of e$_{g}$ states generally believed in Na$_{x}$CoO$_{2}$. We fabricated the thin films of x = 0.68 and x = 0.75 to investigate the polarization dependence of XAS. Within the first principles DFT calculations, we have explanations for this phenomenon. After the analysis of the DOS of Na$_{x}$CoO$_{2}$, we presume that the pre-edge peaks at 529 eV and 530 eV of Na$_{x}$CoO$_{2}$ O-$K$ edge may be not solely due to the unoccupied states of Co3+ and Co4+ eg states, but also due to the spacial asymmetry in the occupied Co 3d orbitals. Due to the hybridization between Co 3d {\&} O 2p orbitals, the p$_{x,y}$ and p$_{z}$ states will be non-degenerate. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X21.00002: Investigations of individual quantum dots of InAsP in InP nano-wires Mats-Erik Pistol, Niklas Sk\"old, Kimberley Dick, Craig Pryor, Jacob Wagner, Lars Samuelson We have grown InP quantum wires containing InAsP quantum dots by metal-organic vapor phase epitaxy. These structures were investigated by transmission electron microscop and photoluminescence spectroscopy and were modelled by six-band k.p-theory. We observe sharp emission lines from excitons, bi-excitons and tri-excitons. When we have observe tri-exciton emission we observe lines originating from the s-shell as well as from the p-shell. By changing the size of the dots we observe clear confinement effects. The wires have a wurtzite structure but were modelled (by necessity) using zinc-blende parameters. From the deviaiton between the theory and the experiments we can deduce rough values of the band-gap of the wurtzite InAs as well as the electron effective mass of wurtzite InAs. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X21.00003: Size dependent exciton g-factor in self-assembled InAs/InP quantum dots. Paul Koenraad, Niek Kleemans, Joost van Bree, Murat Bozkurt, Andrei Silov, Richard Notzel, Craig Pryor, Michael Flatte We have studied the size dependence of the exciton g-factor in self-assembled InAs/InP quantum dots. Photoluminescence measurements on a large ensemble of these dots indicate a multimodal height distribution. Cross-sectional Scanning Tunneling Microscopy measurements have been performed and support the interpretation of the macro photoluminescence spectra. More than 160 individual quantum dots have systematically been investigated by analyzing single dot magnetoluminescence between 1200nm and 1600 nm. We demonstrate a strong dependence of the exciton g-factor on the height and diameter of the quantum dots, which eventually gives rise to a sign change of the g-factor. The observed correlation between exciton g-factor and the size of the dots is in good agreement with calculations. The results demonstrate that quantum dots emitting at 1.55 micrometer and showing no Zeeman splitting (g-factor = 0) can be constructed. This makes these does interesting for quantum information processing at optical telecommunication wavelengths. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X21.00004: Optical and Electrical Characterization of Melt-Grown Bulk Ternary In$_{x}$Ga$_{1-x}$As J. Wei, S. Guha, L. Gonzalez, P. Dutta, G. Rajagopllan, Y. K. Yeo, R.L. Hengehold Recent crystal growth technology breakthroughs led to successful growth of good quality bulk melt-grown ternary In$_{x}$Ga$_{1-x}$As single crystals. However, these bulk materials have not been well investigated compared to the epitaxial layers grown on a binary compound semiconductor, GaAs. Therefore, the optical and electrical properties of the bulk grown In$_{x}$Ga$_{1-x}$As have been investigated systematically as a function of temperature and In mole fraction x. The results show that the refractive index increases linearly with temperature from 100 to 300 K and also with In composition x from 0.0 to 0.9 for several IR wavelengths. Typical refractive index values are 3.388 and 3.376 for 4.6 and 10.6 $\mu $m, respectively, at 300 K for x=0.5. The results of Hall-effect measurements show that the electron concentrations increase monotonically with x, while the mobilities decrease as x increases from 0.5 to 1.0. Typical electron concentration and mobility at 300 K are 1.3x10$^{16}$/cm$^{3}$ and 9.1x10$^{3}$cm$^{2}$/V S, respectively, at x=0.75. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X21.00005: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X21.00006: Structural, Optical and Electrical Properties of Sputtered InGaN Alloy Thin Films. Mohammad Ebdah, Daniel Hoy, Joel Vaughn, Martin Kordesch Amorphous and polycrystalline InGaN alloy thin films were successfully fabricated using rf sputtering technique with a sputtering targets of metal In and Ga in pure Nitrogen. Films were deposited on Si and quartz substrates, with the ratio of In to Ga being varied from 0 to 1 in the alloy. Growth under different sputtering conditions has been examined, such as different temperatures, pressures, and substrate-target distances. The corresponding obtained structures have been studied using the x-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The compositions have been verified by means of energy dispersive x-rays (EDX) spectroscopy and Rutherford back scattering (RBS). Multiple crystallographic phases have been investigated upon growth at different temperatures, and the existence of Gallium Nitride (GaN) and Indium Nitride (InN) phases were investigated. Hall effect measurements were made in 0.55 T magnetic field for characterizing the electrical resistivity at room temperature and 77 K, the free carrier concentration, and mobility. The optical bandgap and optical properties were studied by spectrophotometric and spectroscopic ellipsometric (SE) techniques. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X21.00007: Atomic and electronic structure of AlN polar surfaces Maosheng Miao, Anderson Janotti, Chris Van de Walle We studied the stability and electronic structure of AlN (0001) and (000-1) polar surfaces using first-principles DFT methods. A plane-wave basis set and PAW potentials are employed surface calculations. ~In order to correct the band gap of AlN, we applied the hybrid functional in the HSE [1] framework. ~Using this approach, we obtained a band gap of 6.1 eV, and lattice constants in excellent agreement with experimental values. Under Al-rich conditions, the Al adatom at T4 sites on the Al-terminated (0001) surface was found to be the most stable (2x2) reconstruction.~ This reconstruction is characterized by occupied surface states (Al-Al bonding) at 3.0 eV below the conduction-band minimum (CBM) and unoccupied surface states (Al dangling bonds) at 1.1 eV below the CBM. ~Under Al-poor conditions, the N adatom at the H3 site is the most stable reconstruction, with occupied N-Al bonding states at 4.2 eV and an uncoccupied Al dangling-bond state at 1.1 eV below the CBM.~ For the N-terminated (000-1) polar surface, the structure with an Al adlayer is the most stable under Al-rich conditions. ~The impact of the surface states on the properties of materials and devices will be discussed. [1] J. Heyd, G.E.Scuseria, and M.Ernzerhof, J. Chem. Phys. 118, 8207(2003). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X21.00008: Micro-RDS to explore the spatial strain distribution in epitaxial AlN layers Chunhua Wang, Zhiyu Yang, Zhiqiang Yao, Wenjun Zhang A non-destructive method to observe the spatially resolved strain distribution in the sub-micrometer scale has been developed. By using micro-RDS to obtain the RA distribution on AlN films grown on Si and sapphire substrates, combining with the relation of the strain and optical anisotropy, we reveal the local strain distribution in the sub-micrometer scale. Strain domains several micrometers in size have been observed in AlN films grown epitaxially on Si without the amorphous interface layer. Each domain consists of hundreds of AlN grains. In films with many defects or grown on sapphire substrate there are no domains of dominant sizes, and the average strain is about 6 times smaller than the ones without the interface layer. The magnitude of the strain agrees well with the experimental values from the established methods such as XRD, TEM and Raman scattering. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X21.00009: Theoretical Spectra and Optimization of Geometries of GaAs Clusters Ajit Hira, John Auxier, Matilda Fernandez The goal of the current research is to expand our previous work on Ga$_{n}$As$_{n}$ clusters (n = 1 thru 12) and on gallium arsenide nanostructures. Our research group appears to be on the verge of making an original discovery about these clusters, which we want to disseminate through publication in a professional journal. So far in our work on nanotechnology, we used the hybrid ab initio methods of quantum chemistry to derive the optimal geometries of the Ga$_{n}$As$_{n}$ clusters for n going from 1 to 12. We also calculated binding energies, bondlengths, ionization potentials, electron affinities and HOMO-LUMO gaps, and IR spectra for these geometries. Of particular significance is the magic numbers for GaAs cluster stability that we found at n =8, 10, 12 and 16. Also, we recovered 32.7 {\%} of the bulk cohesive energy of 6.67 eV at 2.18 eV for n =10. However, we need to validate the accuracy of our results through more sophisticated computation and through experimental work. This is important; because materials containing controlled GaAs nanostructures provide the capability of preparing new classes of materials with enhanced optical, magnetic, chemical sensor and photo-catalytic properties. The second phase of the investigation will examine the effects of confinement on the optical properties the Ga$_{n}$As$_{n}$ clusters. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X21.00010: Relationship between Strain and Band Structure in Strained-Si Nanomembranes Feng Chen, Chanan Euaruksakul, Ming-Huang Huang, Don Savage, Bing-Jun Ding, Franz Himpsel, Max Lagally The influence of uniaxial and in-plane biaxial strain on the conduction bands of Si is explored using elastically strained single-crystal Si (SiNMs) and high-resolution x-ray absorption measurements (XAS). Strain alters the band structure and hence the mobility of charge carriers, as well as band offsets in heterostructures. In addition to the biaxial lattice-induced tensile strain in SiNMs, their extreme thinness ($<$100 nm) makes them flexible, allowing us to strain the membranes by mechanically bending them (on the host onto which they are transferred). We use UV Raman to determine the amount of strain and XAS with the Si2p-to-conduction band (CB) transition to measure energy shifts and the degeneracy splitting of several CB valleys. The strain-induced splitting of the CB minimum and the energy shifts of two higher CBs near L1 and L3 are clearly resolved. [1] CB shifts and 2p core level shifts for uniaxial strain in different directions and biaxial strain in SiNMs are measured and contrasted, and compared to theory where it exists. Supported by DOE and NSF [1] Euaruksakul, C., Lagally M., et al, P.R.L. 101, 147403(2008) [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X21.00011: Electronic gap-driven amorphization mechanism: a new paradigm in phase-change materials R.A. Nistor, D. Shakhvorostov, L. Krusin-Elbaum, G.J. Martyna, C. Cabral, S. Raoux, D.B. Shrekenhamer, D.N. Basov, M.H. Muser, D.M. Newns Phase change materials are materials that can be thermally interconverted between metallic (crystalline) and semiconducting (amorphous) phases. The interconversion process involves a change in local coordination number in some of the atomic constituents in these typically muticomponent materials. The electronic basis for the interconversion is still controversial. Here we report, in contrast to previous views, that the amorphization process is driven by an electronic reorganization in which lowering of the total energy by opening a Peierls-like gap drives the structural reorganization into the amorphous state, thereby explaining both the formation and semiconducting character of the amorphous phases. Our understanding of the process is based on phase transformation driven both thermally and by pressure, and in particular by analysis of long time {\it ab-initio} simulations of the amorphization process. We demonstrate the equivalence of thermal and pressure-driven interconversions in a system where vacancies are either at very low levels or enitrely absent. These discoveries open a new pressure-driven phase interconversion pathway. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X21.00012: Structural, optical, and thermal stability properties of CdZnO thin films grown by molecular-beam epitaxy (MBE) Zheng Yang, Lin Li, Sheng Chu, Jieying Kong, Jianlin Liu CdZnO thin films with near-band-edge emission from violet (3.07 eV) to orange (2.04 eV) were grown using MBE. The CdZnO thin films evolve from pure wurtzite (wz) structure to mixture of wz and rocksalt (rs) structures, and finally to pure rs structure, with increasing Cd concentration. Wz CdZnO shows a robuster thermal stability than the rs CdZnO. The temperature dependence of the CdZnO bandgap shrinkage was investigated and analyzed based on the empirical Varshni and Bose-Einstein fitting of the variable-temperature photoluminescence (PL) peak positions. The fitting parameters are not only useful for materials study, but also important for future device applications. The temperature-dependence of the integral PL intensity was fitted with and without considering the hopping term. Fitting with hopping term shows significant improvements to the data in the CdZnO thin films, but no evident change in a ZnO reference thin film, indicating the hopping process in the CdZnO thin films. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X21.00013: Electronic properties and stabilities of bulk, nano-cluster, and low-index surfaces of SnO in comparison with SnO$_{2}$: application to high-temperature gas sensor Yuhua Duan High-temperature gas sensors to detect various components of the gas flow in gasification technologies are highly desired. As one kind of the wide band-gap oxide semiconductors, tin oxides (SnO$_{2}$, SnO) are widely used as solid state sensor material, oxidation catalyst and transparent conductor. Due to the electronic structure and possibility of two different oxidation states of Sn$^{4+}$ and Sn$^{2+}$ and high thermal stability, tin oxides are very sensitive to oxidizing and reducing many kinds of gases, and therefore can be used to detect these gases with good sensitivity at high-temperature. In this study, based on density functional theory approach with an empirical correction of van der Waals interactions, the structural and electronic properties of the bulk, nano-cluster, and low-index surfaces of SnO$_{2}$ and SnO are obtained. Our results indicate that the differences between SnO$_{2}$ and SnO are significant and the convertible transition of Sn$^{4+} \quad \leftrightarrow $ Sn$^{2+}$ may have a great application in high-temperature sensor technology. In SnO, the van der Waals interactions play an important rule and may lead to more active sites for interacting with other molecules. By investigating the interactions between gas molecules (such as CO$_{2}$, C$_{2}$H$_{6}$, C$_{2}$H$_{5}$, \textit{etc}.) with the surfaces of SnO$_{2}$ and SnO, the sensing mechanism of tin oxides will be explored. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X21.00014: UV Induced Room Temperature Persistent Photocurrent in In$_{2}$O$_{3}$ Films Raghava Panguluri, A. Dixit, C. Sudakar, P. Kharel, Pushkal Thapa, I.A. Avrutsky, Alexander Efros, R. Naik, G. Lawes, B. Nadgorny We have investigated the effects of UV irradiation on the electrical and optical properties of In$_{2}$O$_{3}$ thin films. These films were grown on sapphire substrate by RF sputtering. Hall effect measurements carried out to measure the carrier concentrations, n$_{c}$, show n-type conduction, with n$_{c} \quad \sim $ 2 $\times $ 10$^{18}$ cm$^{-3}$. We find that UV illumination on In$_{2}$O$_{3}$ films leads to a dramatic increase in the charge carrier concentration, approximately one order of magnitude, and that these elevated carrier densities persist at room temperature on a timescale of days. Consequently, we observe distinct changes in the optical absorption spectra. Also, we studied the decay of the charge carrier density to the pre-illuminated state as a function of temperature. We will discuss possible mechanisms for persistent photoconductivity in these In$_{2}$O$_{3}$ thin films. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X21.00015: Effect of Oxygen Pressure on the Structure and Luminescence of Europium Doped Gadolinium Oxide Thin Films Patrick Wellenius, John F. Muth, Pae C. Wu, Henry O. Everitt, Eric R. Smith Gadolinium oxide has found uses as a dielectric or passivation layer for novel III-V materials and devices, but more recently has been the subject of study as a rare-earth host. It is believed that Gd$_{2}$O$_{3}$ makes a good host for these dopants due to the similarity in ionic radii between the gadolinium ion and the rare-earth dopants. The reported long radiative lifetimes of rare earth dopants in this material make it interesting for optically pumped laser materials. In this study, europium-doped gadolinium oxide (Eu:Gd$_{2}$O$_{3})$ polycrystalline thin films were deposited on sapphire substrates by pulsed laser deposition at 5 and 50 mTorr oxygen pressure. Changes in the crystal structure were observed by x-ray diffraction and photoluminescence. Low-temperature photoluminescence spectra of the $^{5}$D$_{0}-^{7}$F$_{0}$ and $^{7}$F$_{2}$ transitions in the europium ion were recorded with high resolution. Because the $^{5}$D$_{0}-^{7}$F$_{0}$ transition in europium is not subject to fine structure splitting, it provides a useful mechanism for investigation of the local environment. The $^{5}$D$_{0}-^{7}$F$_{2}$ transition is of interest as it results in the most intense emission, making europium doped material useful for red light-emitting phosphors. Radiative lifetimes of the observed transitions are also reported. [Preview Abstract] |
Session X22: Focus Session: Magnetism in II-VI and IV Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Igor Zutic, SUNY Buffalo
Room: 324
Thursday, March 19, 2009 2:30PM - 2:42PM |
X22.00001: Magnetization studies of II-VI semiconductor columnar quantum dots with type-II band alignment M. Eginligil, I.R. Sellers, B.D. McCombe, W-C Chou, I.L. Kuskovsky We report SQUID magnetization measurements of MBE-grown type-II, II-VI semiconductor quantum dot (QD) samples, with and without Mn incorporation. In all samples, the easy axis is out-of-plane, possibly due to columnar QD formation that arises from strain interaction between adjacent \textit{thin} dot-containing layers. In addition, both types of QDs display a non-zero spontaneous magnetic ordering at 300 K. One set of samples consists of five-layers of (Zn,Mn)Te/ZnSe with a nominal (Zn,Mn)Te thickness of 3 nm, and ZnSe spacer thickness of 5 nm and 20 nm. These \textit{magnetic} QD samples show magnetization vs. temperature behavior that can be interpreted in terms of two independent FM phases characterized by transition temperatures T$_{C1}$ $<$ T$_{C2}$. A sample containing no Mn consists of 130 ZnTe/ZnSe layers, which forms Zn(Se,Te) QD layers separated by ZnSe spacers. Evidence of ferromagnetism is also seen in this structure, but the spontaneous magnetization is much weaker. For this sample only \textit{one phase} is seen with T$_{C}$ above 300 K. Results will be discussed in terms of magneto-polaronic effects and defect-level induced ferromagnetism. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X22.00002: Transition metal-doped Sb$_{2}$Te$_{3}$ magnetic semiconductor thin films. Chang-Peng Li, Yi-Jiunn Chien, Lynn Davies Endicott, Ctirad Uher With the doping of vanadium in tetradymite-based Sb$_{2}$Te$_{3}$, magnetic semiconductor thin films Sb$_{2-x}$V$_{x}$Te$_{3}$ have been prepared on (0001) sapphire substrates by low-temperature molecular beam epitaxy. X-ray diffraction measurements and RHEED patterns confirm single crystalline films growing along the c-axis direction. Magnetic and anomalous Hall measurements clearly show stable ferromagnetic ordering with the easy axis along c-axis direction up to Curie temperature, which increases nearly linearly with the content of V incorporated in the lattice. So far, a high Curie temperature of 213 K has been achieved for the composition of Sb$_{1.55}$V$_{0.45}$Te$_{3}$. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X22.00003: MCD investigation on Mn doped CdSe Quantum Ribbons Kritsanu Tivakornsasithorn, Xinyu Liu, Margaret Dobrowolska, Jacek K. Furdyna, Jung H. Yu, Jin Joo, Dong W. Lee, Jae S. Son, Taeghwan Hyeon, Jiwon Park, Young-Woon Kim We have successfully incorporated manganese ions into 1.4 nm thick CdSe nanoribbons, thus generating 1-dimensionally quantum confined diluted magnetic semiconductor nanostructures. A series of CdSe:Mn nanoribbons with Mn concentration ranging from 0.7 to 6.4\% have been investigated using magnetic circular dichroism (MCD) spectroscopy in order to study spin effects in these CdSe:Mn quantum ribbons. In all samples, a strong MCD signal was found at the energy about 2.9 eV (435 nm), which corresponds to the 1S$_h$-1S$_e$ transition, indicating a strong enhancement of the exciton Zeeman splitting due to the $sp-d$ exchange interaction between the CdSe nanoribbon host and the incorporated Mn$^{2+}$ ions. In addition, the magnetic field dependence of the MCD signal obeys a Brillouin function. Finally, we found that the Zeeman splitting energy $\Delta$E calculated from the MCD signal increases with the Mn concentration, which clearly demonstrates that Mn$^{2+}$ ions are indeed present in the CdSe nanoribbons. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X22.00004: Fine Structure of PbSe Colloidal Nanostructures J.G. Tischler, T.A. Kennedy, E.R. Glaser, E.E. Foos, T.J. Zega, R.M. Stroud, Al.L. Efros, S.C. Erwin Although much work has been done on PbSe nanocrystals in order to understand excited states, little is known of the ground state fine structure. Bulk PbSe is an unusual semiconductor material with a direct band gap at the L point. The band structure at this symmetry point is four-fold degenerate for both electrons and light holes, and conduction and valence bands possess similar effective masses and g-factors. In this work, we synthesized high quality PbSe nanocrystals and characterized them using transmission electron microscopy and optical methods. We probed the g-factors and fine structure of excitons in undoped PbSe quantum dots using optically detected magnetic resonance (ODMR) at 24 GHz and polarized photoluminescence in a magnetic field. The results show that the dark states in this system are active even in the absence of external magnetic fields mainly due to thermal population. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X22.00005: Tunable magnetic exchange interactions in manganese-doped inverted core-shell ZnSe-CdSe nanocrystals David Bussian, Scott Crooker, Ming Yin, Marcin Brynda, Alexander Efros, Victor Klimov Magnetic doping of semiconductor nanostructures is pursued for applications in magnetic memory and spin-based electronics. A primary goal is to control interactions between carriers (electrons and holes) and the embedded magnetic atoms. We have demonstrated$^{\ast }$ a tunable magnetic \textit{sp-d} exchange interaction between electron-hole excitations and paramagnetic Mn$^{2+}$ ions using `inverted' core-shell nanocrystals composed of Mn$^{2+}$-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that are tunable in magnitude \textit{and sign}. Effective exciton g-factors are controllably tuned from -200 to +30 at 1.6 K by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystals can be utilized to manipulate carrier-Mn$^{2+}$ wavefunction overlaps \textit{and} the \textit{sp-d }exchange parameters themselves. $^{\ast }$D. Bussian et al., Nature Materials, \textit{in press}. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X22.00006: Observation of Exchange Enhanced Zeeman Splitting in (Zn,Mn)Se Nanowires B. J. Cooley, T. Clark, B. Liu, C. Eichfeld, E. Dickey, S. Mohney, N. Samarth, S. A. Crooker, C. E. Pryor, M. E. Flatte Magnetic semiconductor nanowires (NWs) are of potential interest as model systems for studying the physics of spin polarized 1D Fermi liquids. A high degree of spin polarization is anticipated from the sp-d exchange-enhanced Zeeman splitting of band edge states. Here, we report the vapor-liquid-solid growth of magneto-optically active (Zn,Mn)Se nanowires that show large Zeeman shifts in band edge photoluminescence (PL) at low temperatures. Transmission electron microscopy reveals the formation of single crystal wurtzite NWs oriented along the c-axis and with diameters as narrow as $\sim 10$ nm (approaching the 1D regime). Low temperature magneto-PL measurements of as- grown NW ensembles show Zeeman shifts of $\sim 5$ meV/T. The Zeeman shifts show a Brillouin-like functional dependence on magnetic field and temperature, consistent with mean field expectations for an exchange-enhanced spin splitting. We discuss the magnetic field dependence of the Zeeman shifts and the modified selection rules for PL polarization resulting from quantum confinement and the NW geometry. Supported by NSF MRSEC and the NNIN. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X22.00007: Influence of antiferromagnetic interactions on ferromagnetic properties of p-(Cd,Mn)Te quantum wells T. Dietl, A. Lipi\'nska, C. Simserides, K. N. Trohidou, A. Majhofer Modulation-doped p-type (Cd,Mn)Te/(Cd,Mg,Zn)Te quantum wells (QWs) remain a unique medium allowing to probe carrier-induced Ising-like ferromagnetism in the two-dimensional case, as in this system the mean free path is longer than the QW width [1]. However, a surprising result is the absence of hysteresis loops below the Curie temperature. In order to obtain information on mechanisms controlling spin dynamics, we have extended our previous Monte Carlo (MC) simulations combining Metropolis algorithm with the determination of hole eigenfunctions at each MC sweeps [2], and found that short range spin-spin antiferromagnetic (AFM) interactions play a crucial role in the accelerating magnetization dynamics. Moreover, we reveal that the effect of AFM interactions becomes much reduced if the thickness of the layer containing Mn spins is narrower than the extend of the hole wave function. This implies that magnetic hysteresis should be recovered in quantum wells, in which the thickness of the Mn layer would be smaller than the region visited by the holes. [1] H. Boukari et al., Phys. Rev. Lett. 88, 207204 (2002); [2] D. Kechrakos et al., ibid 94, 127201 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X22.00008: Effects of defects on the half metallicity of a Mn/Si digital ferromagnetic heterostructure Michael Shaughnessy, Ryan Snow, Ching Fong The effects of defects on the half-metallic properties of the Mn/Si digital ferromagnetic heterostructure (DFH) (PRL \textbf{96}, 027211 (2006)) are investigated using a first principles density-functional theory approach. The half metallicity is retained when the $\delta $-layer of the Mn atoms has a vacancy or a Si imperfection. However, the the half metallicity is destroyed if the $\delta $-layer spreads because of the increased d-d bonding of the defect makes the d-p hybridization imperfect. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X22.00009: Magnetism of Mn doped in $a$-Si and $a$-Ge Juexian Cao With experimental studies and density function theory calculations, we report on the properties of Mn-doped amorphous Si and Ge which are designed to understand the fundamentals of cooperative phenomena in highly correlated electronic and magnetic systems. We observed a striking difference in Mn local moment when doped in $a-$Si and $a-$Ge matrices, in great contrast to the previous speculation that these two should behave very similar as the semiconductor host for transition metals. While we observed a large local moment of Mn in $a-$Ge, Mn moment is quenched in $a-$Si. The large difference of local magnetic moment of Mn in $a-$Si and $a-$Ge can be understood by the local atomic environment at the magnetic dopant sites, that is, the bond length and the coordination. Statistical DFT calculations shows that the magnetic dopant Mn with less coordination and large bond length hold large magnetic moment. Otherwise, the magnetic moment would be killed. In $a-$Ge, dopant Mn favours less coordination and large bond while more coordination and small bond length in $a-$Si, which result in the enhancement/quenchement of local magnetic moment Mn in $a-$Si/$ a-$Ge. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X22.00010: Local structure and spin states of Mn-Co and Fe-Co codoped Ge epitaxial films of dilute magnetic semiconductors Brian Collins, Liang He, Frank Tsui, Yong Chu, Daniel Haskel, Evgeny Kravtsov Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe into Ge up to a combined doping level of 15 at. {\%}. Local structure, magnetic moment, and spin-dependant states of these diluted magnetic semiconductors (DMS) have been probed as a function of composition using X-ray and tunneling spectroscopy techniques. Extended X-ray Absorption Fine Structure (EXAFS) measurements reveal an increased level of substitution for each dopant species with respect to traditional single dopant DMS materials. A codoped environment results in 70{\%} of Mn and 90{\%} of Co successfully incorporating substitutionally into the Ge lattice. The remaining dopants are seen to form interstitial dimmers and trimers rather than metallic precipitates. Tunneling spectroscopy measurements on these systems through Al$_{2}$O$_{3}$ barriers using both Nb and Fe as spin detectors will be discussed in their correlation with the structural results. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X22.00011: Ferromagnetic ordering of Mn-As co-doped Ge as diluted magnetic semiconductors studied within a Heisenberg model Hua Chen, Wenguang Zhu, Zhenyu Zhang To achieve room temperature ferromagnetism in group-IV-based dilute magnetic semiconductors, it is required to increase the concentration of the magnetic dopants in the host semiconductors. A recent theoretical study has suggested that the concentration of substitutional Mn in Ge can be greatly enhanced upon co-doping with As, an n-type electronic dopant [1]. Using Monte Carlo simulations based on a classical Heisenberg model with the magnetic coupling parameters calculated from first principles, we study the magnetic property of Mn and As co-doped Ge at low doping concentrations. The estimated Curie temperature increases almost linearly with increasing Mn doping concentration and reaches 264K at 5\% Mn. In contrast, at the same doping level pure Mn doped Ge does not show any finite temperature ferromagnetic ordering. [1] W. G. Zhu, Z. Y. Zhang, and E. Kaxiras, Phys. Rev. Lett. 100, 027205 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X22.00012: Pulsed Laser Melting and Resolidification of GeMn Thin Films Melissa Dolph, Wenjing Yin, Jiwei Lu, Stuart Wolf, Taeseok Kim, Michael Aziz The effect of pulsed laser melting (PLM) and resolidification on the Group-IV diluted magnetic semiconductor (DMS) GeMn is investigated. (100) Ge thin films of thickness 200nm (the top layer of a Germanium on Insulator Wafer) were ion implanted with Mn at concentrations of 0.5 - 4 atomic {\%}. Mn implantation caused amorphization of the near-surface region of the Ge film as well as the formation of Mn nanoclusters. The studies reported here focus on the use of pulsed laser melting to restore crystallinity in the Ge and to redistribute the Mn more uniformly and increase its substitutionality in the Ge lattice. The single crystal Ge at the interface between the Ge device layer and the SiO$_{2}$ acts as a seed layer for single crystal solidification. X-ray diffraction (XRD) data showed a very strong diffraction from (400) Ge at the optimum laser beam fluence. Laser melted films were also found to be ferromagnetic. The effects on the ferromagnetic behavior due to the Mn concentration and post-PLM thermal annealing conditions will also be reported and correlated with transmission electron microscopy (TEM) structure measurements. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X22.00013: Mn-implanted GeC: An Amorphous Ferromagnetic Material Samaresh Guchhait, M. Jamil, D. Ferrer, E. Tutuc, J. Markert, S. Banerjee, A. Li-Fatou, L. Colombo High energy (20 keV) Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains about 1\% carbon. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$C for both samples. A SQUID magnetometer study shows ferromagnetism in both samples. The Curie temperature of the first samples is about 150 K, while that of the second sample is about 170 K. The in-plane saturated magnetic moment for the first sample is about $2.2\times10^{-5} $emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5} $emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon. Further, high-resolution transmission electron microscopy showed that Mn implanted region is amorphous, without any precipitates. It is believed that it is this amorphous phase that is responsible for ferromagnetism. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X22.00014: Magnetic transitions and giant magnetocaloric effect in Eu$_{8}$Ga$_{16}$Ge$_{30 }$clathrates A. Chaturvedi, S. Stefanoski, M.H. Phan, G.T. Woods, G.S. Nolas, H. Srikanth Semiconductors with the clathrate hydrate crystal structure have demonstrated interesting physical properties that are directly related to the fact that ``guest'' atoms reside inside ``host'' polyhedra that are formed by other species. These materials are well known for their excellent thermoelectric properties. One of the interesting ``guests'' in the clathrate structure is europium. Since the magnetic moment of Eu is large and the Eu moments order at low temperatures in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates, these materials are expected to exhibit interesting magnetic and magnetocaloric properties. In this work, we report on the systematic studies of magnetic and magnetocaloric properties of Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates. The magnetic entropy change was numerically calculated from the magnetization isotherms using the Maxwell relation and giant magnetocaloric effect (GMCE) was observed. Experimental results reveal a coherent correlation between the structure, magnetic property and the GMCE in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates. The low-field GMCE, in addition to the absence of thermal hysteresis and field hysteresis, makes this material an attractive candidate for active magnetic refrigeration at low temperatures. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X22.00015: Cr Moment Coupling in Cr-containing Diamond-like Carbon Thin Films Juan Colon, Varshni Singh, Vadim Palshin, Andre Petukhov, Yaroslav Losovyj, Andrei Sokolov, Peter Dowben, Ihor Ketsman Structural and magnetic properties of the Cr-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated diamond-like carbon alloy thin films were investigated. Results showed promising magnetic and spintronics application. Diodes were constructed using the silicon substrate as the n-type material and Cr-DLC film as the p-type material for a wide range of Cr concentrations and temperatures. At low chromium content a high capacitance was observed, limiting the properties of the heterojunction device, however, at high Cr concentration, a large coefficient of negative magneto-resistance was observed even at room temperature, suggesting the possibility for a spintronic application. This negative magneto-resistance effect may be related to the coexistence of various material phases including chromium carbide. [Preview Abstract] |
Session X23: Quantum Hall Effect: Quantum Computation
Sponsoring Units: DCMPChair: Steven Simon, Oxford University
Room: 325
Thursday, March 19, 2009 2:30PM - 2:42PM |
X23.00001: Pfaffian statistics through adiabatic transport in the 1D coherent state representation Alexander Seidel Recent work has shown that certain quantum Hall states and their excitations can be efficiently labeled by simple one-dimensional patterns. One way to achieve this is to observe that the low energy sector of these states is adiabatically connected to simple charge-density-wave patterns that appear when the system is deformed, e.g., into a thin torus. In this talk it will be shown that the patterns emerging in this limit already determine the non-abelian statistics of the $\nu=1$ Moore-Read state. Aside from the knowledge of these patterns, the method essentially relies only on the principle of adiabatic continuity, and topological as well as locality arguments. [References: A. Seidel, Phys. Rev. Lett. 101, 196802 (2008), A. Seidel, D.-H. Lee, Phys. Rev. B 76, 155101 (2007).] [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X23.00002: Formation of novel topological liquids in non-Abelian quantum Hall states with a finite density of interacting anyons Simon Trebst, Charlotte Gils, Eddy Ardonne, Andreas W. W. Ludwig, Matthias Troyer, Zhenghan Wang We discuss the nucleation of novel topological liquids within a `parent' non-Abelian quantum Hall state that arises from a macroscopic occupation of the parent liquid with interacting anyons. The edge states between the `nucleated' and the `parent' liquids are neutral, and correspond precisely to the gapless modes of chains of interacting anyons. Exploring the collective states of anyonic spin chains is thus, at the same time, a tool to systematically study topological phases which can occur inside non-Abelian quantum Hall liquids due to population of such liquids with a finite density of interacting non-Abelian anyons. We discuss several examples including the interface between the Moore-Read and Read-Rezayi quantum Hall states. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X23.00003: Thermopower Measurements in the $\nu = 5/2$ FQHE J.P. Eisenstein, W.E. Chickering, L.N. Pfeiffer, K.W. West The fractional quantized Hall state at $\nu = 5/2$ is currently under intense scrutiny owing to its possible application to topological quantum computation. This connection relies on the possibility that the ground state of the 2D electron gas at $\nu = 5/2$ is the non-abelian Moore-Read paired composite fermion state. A key attribute of this state is its large degeneracy when quasiparticles are present. While most schemes for detecting this degeneracy rely on interferometric braiding operations, it may also have thermodynamic implications. In particular, the entropy of the 2D system at $\nu = 5/2$ should be significantly enhanced relative to conventional abelian quantum Hall states. In this contribution we report measurements of the low temperature thermopower of the 2DES at $\nu = 5/2$. Although thermopower is a transport parameter, it is closely related to the entropy of the system under study and there have been recent suggestions [1] that it may bear signatures of non-abelian quasiparticle statistics. Our measurements will highlight the similarities and differences between the thermopower at $\nu = 5/2$ and other quantum Hall states. This work is supported by Microsoft Project Q and DOE grant DE-FG03-99ER45766. [1] Kun Yang, arXiv:0807.3341 [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X23.00004: Spontaneous Particle-Hole Symmetry Breaking in the 5/2 Fractional Quantum Hall Effect Kwon Park, Michael Peterson, Sankar Das Sarma The essence of the $\nu=5/2$ fractional quantum Hall effect is believed to be well captured by the Moore-Read Pfaffian (or anti-Pfaffian) description. However, an important mystery regarding the formation of the Pfaffian state is the role of the three-body interaction Hamiltonian $H_3$ that produces the Pfaffian as an exact ground state and the concomitant particle-hole symmetry breaking. We show that a two-body interaction Hamiltonian $H_2$ constructed via particle-hole symmetrization of $H_3$ produces a ground state nearly exactly approximating the Pfaffian and anti-Pfaffian states, respectively, in the spherical geometry. More importantly, the ground state energy of $H_2$ is shown to exhibit a ``Mexican-hat'' structure as a function of particle number in the vicinity of half filling for a given flux indicating spontaneous particle-hole symmetry breaking. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X23.00005: Jack Polynomials, W-algebras and application to Fractional Quantum Hall Effect Steven H. Simon, B. Andrei Bernevig, Victor Gurarie We examine Jack symmetric functions and certain W-algebras as schemes for generating fractioanl quantum Hall wavefunctions. We add substantially to the evidence that the Jack functions correspond to certain W-algebras, by calculating the central charge and scaling dimensions of some of the fields in both cases and showing that they match. Except for the Read-Rezayi series all of these Jack symmetric functions turn out to be nonunitary theories. We discuss the (perhaps optimistic) possibility that these approaches may have relevance to various physical quantum Hall systems. Open questions in the field, as well as why this is of importance to those concerned with real experiments, will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X23.00006: Resources needed for peforming Shor's algorithm with Ising Anyons Mara Baraban, Steven Simon, Nick Bonesteel While Ising anyons, the presumed quasiparticles of the $\nu=5/2$ fractional quantum Hall state, do obey non-Abelian statistics, their braid group is not sufficiently rich to support universal quantum computation (UQC). Recently, Bravyi [1] proposed a method for combining topological and non-topological operations that exploits the topological protection of the Ising anyons to allow for UQC even with very low accuracy non-topological operations. Starting from Bravyi's proposal, we calculate the resources required to perform Shor's algorithm. We find that when parallelization is included, the required number of qubits grows as the number of gates ($\sim N^3$, where $N$ is the length of the number to be factored using Shor's algorithm) and that the total time required is nearly independent of $N$. Numerical work has further allowed us to determine how far apart the anyons must be in a realistic sample in order to perform topological operations. We thus estimate how large a coherent sample would be required for Ising anyons to successfully execute modular exponentiation. We compare our results to the requirements for performing Shor's algorithm via fully topological quantum computation with Fibonacci anyons, the presumed quasiparticle excitation of the $\nu=12/5$ fractional quantum Hall state. [1] S. Bravyi, Phys. Rev. A \textbf{73}, 042313 (2006) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X23.00007: Multichannel Kondo Models in non-Abelian Quantum Hall Droplets Gregory A. Fiete, Waheb Bishara, Chetan Nayak We study the coupling between a quantum dot and the edge of a non-Abelian fractional quantum Hall state which is spatially separated from it by an integer quantum Hall state. Near a resonance, the physics at energy scales below the level spacing of the edge states of the dot is governed by a $k$-channel Kondo model when the quantum Hall state is a Read-Rezayi state at filling fraction $\nu=2+k/(k+2)$ or its particle-hole conjugate at $\nu=2+2/(k+2)$. The $k$-channel Kondo model is channel isotropic even without fine tuning in the former state; in the latter, it is generically channel anisotropic. In the special case of $k=2$, our results provide a new venue, realized in a mesoscopic context, to distinguish between the Pfaffian and anti-Pfaffian states at filling fraction $\nu=5/2$. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X23.00008: Topological Phase Transition of the 5/2 Fractional Quantum Hall Effect Hao Wang, D.N. Sheng, F.D.M. Haldane We study the effect of three-body interaction on the fractional quantum Hall effect at filling factor 5/2 using exact diagonization method with torus geometry in a finite-size system. The calculation of the wave function overlap suggests that a repulsive three-body interaction will induce a phase transition to a Pfaffian state. Its particle-hole conjugate (anti-Pfaffian) state is found only in a very narrow region of the attractive three-body interaction while a stronger attraction will push the system into a compressible state. The results from the low energy spectrum and the particle-hole parity evolution further indicate a continuous phase transition between the Pfaffian and anti-Pfaffian states, with the pure Coulomb system sitting at the critical point of the transition. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X23.00009: Topological order in two-dimensional fermion systems with $p_x+ip_y$ pairing Noah Bray-Ali, Letian Ding, Stephan Haas We numerically evaluate the ``entanglement spectrum'' (singular value decomposition of the density matrix) of paired states of fermions in two dimensions that break parity and time-reversal symmetries, focusing on the spinless $p_x+ip_y$ case in which the gap function has orbital angular momentum $\ell=1$. In the weak-pairing phase, the low-lying entanglement spectrum has a gapless structure, which we compare to that of the Moore-Read state, a nonabelian quantum hall fluid. In the strong-pairing phase, we find a different structure, which we compare to the $\ell=0$, $s$-wave case. At the weak-strong transition, we compute the entanglement entropy from the spectrum, and find a logarithmic correction to the generic, ``area'' law behavior. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X23.00010: The fractional quantum Hall effect: The cases of 5/2 and 12/5 Keshav Shrivastava We find that there is a state of zero energy because of a zero value in (1/2)g. When negative sign is used, L=0, S=1/2, g=(2J+1)/(2L+1)=[2(L-S)+1]/[2L+1]=0 so that [n+(1/2)][(1/2)g]=0. For positive sign, L+S, L=0, g=2 so that [n+(1/2)][(1/2)g]=5/2 for n=2. Hence 0 and 5/2 become particle-hole conjugates. In this definition, the sign of the spin for the particle is different from that for the hole as required by the helicity, p.s. For negative sign, L=2, (1/2)g=2/5 and (n-n')[(1/2)g]=12/5 with n-n'=6. For the positive sign, (1/2)g=3/5 for L=2 and for n-n"=4, we get 12/5. Thus 12/5 can arise for up spin as well as for down spin for different Landau levels[1]. On the basis of a product of [n+(1/2)][(1/2)g] we are able to understand all of the fractions given by Pan et al[2]. [1] K. N. Shrivastava, Phys. Lett. A 113,435(1986); A326,469(2004); Mod. Phys. Lett. 13,1087(1999); 14,1009(2000); AIP Conf. Proc. 909, 43-49(2007); 909.50-56(2007);1017, 422-428(2008);1017,326-330(2008); 1017, 47-56(2008), Proc. SPIE(USA)7155,71552F1-8[7155{\_}86](2008). [2] Wei Pan et al, Phys. Rev. B 77, 075307(2008). [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X23.00011: Probing the neutral edge modes in transport across a point contact via thermal effects in the Read-Rezayi non-abelian quantum Hall states Eytan Grosfeld, Sourin Das Non-abelian quantum Hall states are characterized by the existence of neutral gapless edge modes, whose structure is intricately related to the existence of bulk quasi-particle excitations obeying non-abelian statistics. Detecting the presence of these neutral modes is thus an important step towards establishing the non-abelian nature of these quantum Hall states. While it is hard to couple to the neutral modes using an electric field, they will directly couple to a temperature gradient and respond by contributing to the thermal current. By obtaining an expression for the thermal current through a quantum point contact, we demonstrate that a measurement of the thermal current will reveal the presence of the neutral modes. In addition, since thermal measurements are difficult to implement, we propose a setup which uses no external heaters and relies solely on noise measurements to detect thermal effects. The idea is to have two point contacts in series separated by a distance set by the thermal equilibration length of the charge mode. We show that by using the first point contact as a heating device, the excess charge noise measured at the second point contact carries a non-trivial signature of the presence of the neutral mode hence leading to its indirect detection. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X23.00012: Charge-statistics separation and probing non-Abelian states for quantum Hall plateau at $\nu$=5/2 Feifei Li, Dima Feldman Several states were proposed for quantum Hall plateau at $\nu$=5/2. We suggest a transport experiment that can distinguish six of the candidate states. The proposal involves measurements of current and shot noise in a geometry with three quantum Hall edges connected by two quantum point contacts. Unlike interference experiments, this approach can distinguish Pfaffian and anti-Pfaffian states as well as different states with identical Pfaffian or anti-Pfaffian statistics. Moreover, the transport is not sensitive to the fluctuations of the number of quasiparticles trapped in the system.\\[0pt] [1] D. E. Feldman and Feifei Li, Phys. Rev. B {\bf78}, 161304(R) (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X23.00013: Quantum Hall hierarchy revisited Susanne Viefers, Juha Suorsa, Hans Hansson, Maria Hermanns Using techniques from conformal field theory, we construct explicit candidate wave functions for the entire Abelian quantum Hall hierarchy, i.e. quasihole- and quasielectron condensates, as well as mixtures of these. The formalism presented here, generalizes and unifies our previous techniques, which were only able to address quasielectron condensates. In the special cases of the positive and negative Jain sequences $\nu = n/(2np \pm 1)$, our method exactly reproduces Jain's composite fermion wave functions. In general our results are consistent with Wen's topological classification of FQH states. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X23.00014: Non-Abelian quasielectrons. Hans Hansson, Maria Hermanns, Susanne Viefers Using methods from conformal field theory, we construct trial wave functions for quasielectron excitations in both Abelian and non-Abelian quantum Hall states. We briefly explain the underlying theory, and present analytical and numerical results for the case of two and four quasielectrons in the non-Abelian Moore-Read pfaffian state. Our methods generalize to other non-Abelian states, and can possibly also give a description of condensates of non-Abelian quasielectrons.. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X23.00015: Topological Entanglement Entropy of Realistic Quantum Hall States Barry Friedman, Greg Levine The entanglement entropy for the incompressible states of a realistic quantum Hall system was studied by direct diagonalization for square clusters with periodic boundary conditions. The subdominant term to the area law, the topological entanglement entropy, was extracted for filling factors 1/3 and 5/2. The result for filling factor 1/3 is consistent with the topological entanglement entropy for the Laughlin wave function while the 5/2 filling factor exhibits a topological entanglement entropy consistent with the Moore-Read wave function. Preliminary results for the topological entanglement entropy for other incompressible states in the second Landau level will be discussed. [Preview Abstract] |
Session X24: Hydrogen Storage: Complex Hydrides
Sponsoring Units: DMPChair: John Vajo, HRL Laboratories
Room: 326
Thursday, March 19, 2009 2:30PM - 3:06PM |
X24.00001: "Developments in the application of borohydrides for hydrogen storage" Invited Speaker: |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X24.00002: First-Principles Determination of Crystal Structures, Phase Stability, and Reaction Thermodynamics in the Li-Mg-Al-H Hydrogen Storage System Alireza Akbarzadeh, Chris Wolverton, Vidvuds Ozolins First-principles DFT calculations have been used to investigate the crystal structures, thermodynamic stability, and decomposition pathways of Li-Mg-Al-H hydrogen storage compounds. We find that the recently discovered LiMg(AlH$_4)_3$ compound is marginally stable with respect to decomposition into LiAlH$_4$ and Mg(AlH$_4)_2$; however, we also find that LiMg(AlH$_4)_3$ is unstable with respect to H$_2$ release and decomposes exothermically into LiMgAlH$_6$, Al, and H$_2$ in excellent agreement with measurements. Using ICSD crystal structures database, we predict that the hypothetical MgAlH$_5$ compound should assume the orthorhombic BaGaF$_5$ prototype structure. We also discuss that phonon vibrations have sizeable effects on the enthalpies and entropies of hydrogen release reactions of Li-Mg-Al-H compounds. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X24.00003: Hydrogen Storage in Cu-Li-Mg Alloys M. Helena Braga, George Chertkov, Alice Acatrinei, Saurabh Kabra, Luke Daemen CuMg$_{2}$ has an orthorhombic crystal structure (Fddd) and does not form a hydride. However CuLi$_{x}$Mg$_{2-x}$ (x $\sim $ 0.11) has a hexagonal crystal structure (P6$_{2}$22), just like NiMg$_{2}$, a compound known for its hydrogen storage properties. A comparison between the phase diagrams of the systems Cu-Mg and Ni-Mg shows that these binary systems form compounds with similar stoichiometry. NiMg$_{2}$ is formed by peritectic reaction of the elements at 759 $^{\circ}$C (1032 K) and CuMg$_{2}$ at 568 $^{\circ}$C (841 K) by congruent melting. Since the energy of formation of the hydride is related to that of the primary alloy, it was hypothesized that CuLi$_{x}$Mg$_{2-x}$ might also be a hydrogen storage material similar to NiMg$_{2}$. Presumably, its advantage would be that it would release hydrogen at a lower temperature (possibly close to room temperature). In order to determine the properties of the hydrogenated (and deuterated) CuLi$_{x}$Mg$_{2-x}$ material, absorption/desorption experiments were performed at several temperatures and under different pressures of H$_{2}$ (and D$_{2})$. Neutron diffraction patterns and neutron vibrational spectra were collected to elucidate the behavior of hydrogen in the Li-doped CuMg$_{2}$ intermetallic. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X24.00004: Structural Discrimination via DFT: Monoclinic Mg$_{2}$NiH$_{4}$ Jan Herbst, Louis Hector, Jr. Mg$_{2}$NiH$_{4}$ is a semiconductor and forms an ordered low temperature monoclinic phase and a disordered high temperature cubic modification. Two distinct structures for the monoclinic phase from neutron diffraction studies of the deuterated analog, which we designate as LTI and LTII, are available in the published literature. We calculate the enthalpy of formation $\Delta $H with density functional theory (DFT) for both using three different approximations for the exchange-correlation energy functional. Phonon spectra are calculated as well. DFT unequivocally identifies LTII as preferable since $\Delta $H obtained for it is in better agreement with experiment and its phonon spectrum contains no anomalies. Structures approximating LTII derived from analyses of soft modes in LTI and in Mg-substituted CaMgNiH$_{4}$ are also discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X24.00005: Control of thermodynamics and kinetics through anion substitution in metal borohydrides Young-Su Lee, Yoonyoung Kim, Jae-Hyeok Shim, Young Whan Cho High thermal stability of metal borohydrides is one of the bottlenecks in adopting them for practical hydrogen storage materials. For this reason, much effort has been put toward lowering their thermal stability. One of the common routes taken to achieve this aim is to mix with other borohydrides or alantes of less thermal stability hoping to make a compound or an alloy of intermediate stability. Recent studies have proposed a possibility where F$^{-}$ or Cl$^{-}$ anions could incorporate into the lattice of alanates or borohydrides replacing H$^{-}$ or BH$_{4}^{-}$ anions, thus modifying the thermal stability of these materials. We present here a combined experimental and theoretical study on the anion substitution in Ca(BH$_{4})_{2}$ and LiBH$_{4}$. Both thermodynamic and kinetic aspect will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X24.00006: First-Principles Study of the Li-Mg-N-H System: Compound Structures and Hydrogen Storage Properties Kyle Michel, Vidvuds Ozolins The Li-Mg-N-H system is studied with the addition of the Li4Mg(NH)3, MgNH, and Li4NH compounds using first-principles density-functional theory (DFT) calculations. A structure for the mixed imide Li4Mg(NH)3 is proposed, belonging to the Imm2 space group. A new structure for Li2Mg(NH)2 is given that has Pca21 symmetry; this compound has been previously reported as having Iba2 symmetry. The stability of the Li4Mg-imide is studied with respect to its decomposition reactions. The static, zero-point (ZPE), and vibrational energies of all relevant compounds belonging to this system are reported along with their predicted lowest-energy structures. Dehydrogenation reactions are presented that involve these phases and which are found to be spontaneously occurring within 400 K of room temperature. It is predicted that mixing LiH, LiNH2, and Li2Mg(NH)2 at 505 K will form Li4Mg(NH)3 with the release of 2.04 wt. {\%} H2. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X24.00007: De(Re)-hydrogenation Mechanisms of B-N-H Complexes at Elevated Pressure Raja Chellappa, Maddury Somayazulu, Viktor Struzhkin, Russell Hemley, Thomas Autrey The goal of identifying a hydrogen storage material that possesses reversible hydrogen removal/addition characteristics with favorable thermodynamics and kinetics still remains elusive. In case of chemical {\&} complex hydrides, it is increasingly realized that some ``destabilization'' is desired by altering the stiochiometry (dopants) or physical properties via mechanical preprocessing. In this study, a combined pressure-temperature approach (few GPa and 77-400 K) that has been used with great success for synthesizing novel hydrogen clathrates is extended to understand de(re)-hydrogenation mechanisms of B-N-H compounds. For example, Ammonia Borane (NH$_{3}$BH$_{3})$ has very high hydrogen content (19 wt.{\%}) however its thermal polymeric residue does not re-uptake hydrogen. We will present some in situ Raman spectroscopy results from our attempts to create novel hydrogen complexes with this residue as well as with the parent compound. In general, the physics {\&} chemistry of hydrogen interactions at high pressure with light element compounds containing hydrogen has been unexplored and this study is an attempt towards bridging that gap. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X24.00008: First-Principles Prediction of Hydrogen Storage Energetics in the Li-B-N-H System Wenhao Sun, Christopher Wolverton, Vidvuds Ozolins In this talk, we describe recent efforts using first-principles density functional theory (DFT) based methods to elucidate the reaction energetics and phase stability in the Li-B-N-H hydrogen storage system. We have calculated DFT total energies of a large number of phases in this system, including Li$_{4}$BN$_{3}$H$_{10}$, Li$_{2}$BNH$_{6}$, and their decomposition products. We then use these DFT energies in the recently developed ``grand canonical linear programming'' (GCLP) approach to automatically detect the thermodynamically preferred decomposition paths of these compounds as functions of temperature and H$_{2}$ pressure. Using the combined DFT+GCLP approach we calculate thermodynamic phase diagrams in the LiBH$_{4}$ -- LiNH$_{2}$ phase space. Some phases (e.g., Li$_{3}$BN$_{2}$, BN) are found to be very energetically stable, but are often only seen experimentally at very high temperatures, presumably due to hindered kinetics. By removing these phases from the DFT+GCLP calculations and examining the resultant phase diagrams, we can provide insight into the experimental reaction mechanisms. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X24.00009: First-Principles Molecular Dynamics Simulations of Liquid Li$_{4}$BN$_{3}$H$_{10}$:Structural Characterization and Dynamics of Hydrogen Release David Farrell, Christopher Wolverton The recently discovered Li$_{4}$BN$_{3}$H$_{10}$ compound is a promising hydrogen storage material due to its high capacity for hydrogen desorption ($>$10 wt.\%) and favorable thermodynamics for low-temperature H$_2$ release. However, elevated temperatures are necessary for appreciable H$_2$ desorption, pointing to kinetic limitations. Further, Li$_{4}$BN$_{3}$H$_{10}$ is liquid at these H$_2$ release temperatures. In an effort to characterize the liquid structure and uncover the atomistic mechanisms for H$_2$ release, we have performed first-principles molecular dynamics simulations of liquid Li$_{4}$BN$_{3}$H$_{10}$. Our calculations give the temperature-dependent liquid structure, which we compare in detail with that of the crystalline solid. We are also able to ascertain the latent heat of melting, an important contribution to understanding the thermodynamics of H$_2$ release from this material. Finally, we present preliminary work on the atomistic mechanisms of hydrogen desorption from the liquid based on temperature accelerated molecular dynamics. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X24.00010: Hydrogen Desorption Behavior of Nickel-Chloride-Catalyzed Stoichiometric Li$_{4}$BN$_{3}$H$_{10}$ Frederick Pinkerton, Martin Meyer Li-B-N-H quaternary hydrides with the $\alpha $-phase crystal structure form over a range of compositions between Li$_{3}$BN$_{2}$H$_{8}$ and Li$_{4}$BN$_{3}$H$_{10}$ and have up to 11.9 wt{\%} hydrogen capacity. Previous work focused on the non-equilibrium Li$_{3}$BN$_{2}$H$_{8}$ composition created by ball milling because it has maximum hydrogen release and minimum NH$_{3}$ co-generation. Here we report the hydrogen and NH$_{3}$ release characteristics of $\alpha $-phase material having the equilibrium Li$_{4}$BN$_{3}$H$_{10}$ composition. In the absence of a dehydrogenation catalyst, H$_{2}$ and NH$_{3}$ were released simultaneously in roughly equal quantities by weight (or about 10{\%} NH$_{3}$ by volume) at temperatures above 240 \r{ }C. Adding Ni in the form of NiCl$_{2}$ as a dehydrogenation catalyst reduced the H$_{2}$ release temperature by 122 \r{ }C. NH$_{3}$ release, in contrast, still occurred only at the higher temperature. As a result, decomposition occurred in two steps separated in temperature, dominated by H$_{2}$ gas at low temperature and NH$_{3}$ at high temperature. The two gases clearly evolved in two distinct reactions that are coincident in uncatalyzed Li$_{4}$BN$_{3}$H$_{10}$, but can be separated by a dehydrogenation catalyst. We expect that NH$_{3}$ co-generation could be completely eliminated at sufficiently low dehydrogenation temperatures. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X24.00011: Thermochemical Investigations of Nano-phase Ammonia Borane: Effect of Higher Loading Abhi Karkamkar, Ashley Stowe, Tom Autrey Chemical hydrogen storage materials that release H$_{2}$ by thermolysis without generating CO$_{2}$ offer an attractive option. The ammonia borane is an attractive compound containing more than 18 wt{\%} hydrogen. However, the kinetics of hydrogen release in not favorable in bulk materials where H$_{2}$ is released at 114 $^{\circ}$C. We recently reported use of SBA-15 as scaffold material to form a nanophase ammonia borane species which liberated H$_{2}$ at significantly lower temperatures. Hydrogen formation from bulk AB is slightly exothermic (-5 kcal/mol). The reaction enthalpy ($\Delta $H) for release of H$_{2}$ from AB adsorbed into SBA-15 (1:1 w/w) was determined to be nearly thermoneutral---dramatically lower than the bulk material. A near thermoneutral reaction suggests that there would be less restrictive heat management issues, greater thermal stability and potentially a lower energy input requirement for regeneration of AB. One drawback which results for nano-phase AB is that while the hydrogen release properties are enhanced, the gravimetric hydrogen density is reduced by a 50{\%} for the 1 to 1 by mass ratio material. We here report on our efforts to increase the gravimetric hydrogen density of nano-phase AB by developing higher loading conditions of AB adsorbed into mesoporous silica (MCM-41). [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X24.00012: Hydrogen storage in ammonia borane: {\em Ab initio} study of the de- and rehydrogenation mechanisms Kiseok Chang, David Tom\'anek, Eunja Kim, Philippe F. Weck Using {\em ab initio} density functional calculations, we study the microscopic mechanism of hydrogen release from ammonia borane (NH$_3$BH$_3$) and the reverse process leading to its subsequent recharging with hydrogen. Our total energy surfaces indicate the most favorable pathways to thermally convert the NH$_3$BH$_3$ molecular solid to the energetically preferred polymer NH$_2$BH$_2$ and molecular hydrogen. To prevent formation of undesirable side-products such as the cyclic compound borazine (N$_3$B$_3$H$_6$) or other complexes that would prevent subsequent rehydrogenation, we propose to enclose AB in narrow carbon nanotubes. In this constrained space, we investigate possible rehydrogenation pathways using atomic and molecular hydrogen as well as selected protonation agents. [Preview Abstract] |
Session X25: Focus Session: Graphene XVII: p-n Junctions, Nanoribbons, and Quantum Dots
Sponsoring Units: DMPChair: Michael Fogler, University of California, San Diego
Room: 327
Thursday, March 19, 2009 2:30PM - 2:42PM |
X25.00001: Theoretical investigations of deformed graphene nanoribbons Ricardo Kagimura, Mario S. C. Mazzoni, Helio Chacham Graphene nanoribbons (GNRs) have attracted considerable attention in the last two years. Recent experimental work [Science, 319, 1229 (2008)] has reported semiconducting GNRs with width of a few nanometers and suitable band gap widths for electronic applications. One desirable aim in the investigation of GNRs is to control their electronic properties. It has been proposed, for instance, that chemical edge modifications or external strain can modify their electronic properties. In this work, we report density functional calculations for GNRs with structural and topological deformations. In particular, we investigate modifications of the electronic structure as a function of those defornations. Finally, we study the effect of a transverse electric field in those ribbons. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X25.00002: Quantum Dot Behavior in Graphene Nanoconstrictions Kathryn Todd, Hung-Tao Chou, Sami Amasha, Patrick Gallagher, David Goldhaber-Gordon Graphene nanoribbons have been proposed as novel high-frequency transistors due to their high mobility and transport gap that scales inversely with width. In order to understand the origin of the transport gap in long nanoribbons, we measure transport through short side-gated nanoconstrictions. Unlike in long ($\geq$250 nm) nanoribbons, where we observe transport through multiple quantum dots in series, shorter ($\leq$60 nm) constrictions display behavior characteristic of single and double quantum dots. We find that dot size scales with constriction width. In the narrowest short constrictions, high on/off ratios are achievable, while in wider ($\geq$35 nm) constrictions we observe quantum dot behavior overlaid on a highly conducting background. We hypothesize that the metal side gates in close proximity to our short constrictions suppress the importance of edge disorder, and compare constrictions fabricated with and without metal side gates. We propose a model where transport occurs through quantum dots nucleated by disordered background potential in the presence of a confinement gap. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:30PM |
X25.00003: Interaction effects in graphene p-n junctions Invited Speaker: We review our recent analytical and numerical studies of a new class of graphene devices: lateral p-n junctions. Such structures are realized experimentally by modulating the electron density in graphene samples with external gates. Our theory describes the charge density distribution, the electric field profile, and the resistance of such p-n junctions. The proper treatment of the electrostatic screening beyond the linear order is crucial for obtaining correct results for all these quantities. In particular, the electric field at the interface of the electron and hole regions is strongly enhanced due to limited screening capacity of Dirac vacuum. This nonlinear screening effect can significantly reduce the junction resistance. It is necessary to include it in order to obtain a good agreement with the experiments. More subtle interaction effects such as the Bragg reflection of quasiparticles on Friedel oscillations near the p-n interface are also discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X25.00004: Effect of Shape on Electronic and Magnetic Properties of Graphene Nanoribbons (GNRs) Albert K. Dearden, Philip Shemella, Pulickel M. Ajayan, Saroj K. Nayak We have studied electronic and magnetic properties of grapheme nano-structures with different shapes. In particular, we have studied both zigzag and armchair graphene nanoribbons (GNR) of triangular shape using density functional method. We find electronic and magnetic properties of triangular structures are drastically different from their rectangular counterparts and our results suggest that, in addition to size effect, shape of the structure has a large impact on the underlying intrinsic electronic properties of GNRs. We will compare our results with available experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X25.00005: Probing the transport gap in edge disordered graphene nanoribbons Melinda Han, Philip Kim We present experimental studies on the detailed nature of the transport gap observed in etched graphene nanoribbons. Temperature dependent measurements of electronic transport in the ``gapped'' region of suppressed conductance suggest transport via localized states or charge islands, giving rise to separate energy scales for hopping conductance and the transport gap. Distinct temperature regimes with different exponential temperature dependences are observed, with a crossover temperature dependent on ribbon width. A transition to ``bulk'' graphene electronic behavior is observed for wider ribbons, and the size of the transport gap shows a length dependence consistent with conduction due to hopping. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X25.00006: Role of edge states in graphene nano-ribbons: - DFT studies Sumit Saxena, Trevor A. Tyson We present first principle calculations to study the effect of edge states in graphene nano-ribbons. Spin restricted calculations for graphene nano-ribbons were performed using ground state density functional theory. The plot of electron localization function corresponding to the edge dangling bonds has revealed highly reactive edge states in graphene nano-ribbons. The reactivity of the nano-ribbons with respect to the edge structure is discussed. This study has been supplemented by band structure studies in armchair and zigzag edged graphene nano-ribbon systems. This work is supported in part by NSF DMR-0512196. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X25.00007: Electronic properties of Graphene quantum dots Pawel Hawrylak, Pawel Potasz, A. Devrim G\"u\c{c}l\"u We study electronic properties of Graphene quantum dots in magnetic fields. Graphene quantum dots are atomically thick nanometer-scale islands constructed by connecting benzene molecules. Quantum dots with triangular and hexagonal shape have shown to have different edge properties [1,2], and triangular zig-zag structures have recently attracted attention due to their half-filled zero-energy edge states. In this work, we investigate electronic and magnetic properties of triangular and hexagonal shaped islands. We study the effect of first and second nearest neighbour interactions, magnetic field and the number of atoms on the single-particle properties using a tight-binding model. We then use configuration interaction method to study the effect of electron-electron interactions on the ground state properties including magnetization, excitation spectra, and their effect on Coulomb blockade and tunneling through graphene islands. [1] J. Fernandez-Rossier and J.J. Palacios, Phys.Rev.Lett. {\bf 99}, 177204 (2007), [2] M. Ezawa, Phys.Rev.B, {\bf 77}, 155411 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X25.00008: Charge transport in ballistic multiprobe bilayer graphene dots. Igor Romanovsky, Constantine Yannouleas, Uzi Landman We present a numerical analysis of the transport properties of the bilayer graphene quantum dots attached to multiple leads. In the framework of the tight binding model and using the nonequilibrium Green's function technique, we study numerically effects due to: magnetic fields, bias voltage between the layers, geometrical shape, and the arrangement of the attachments of the leads to the device. The results are compared to those obtained for similar quantum dot structures made from a graphene monolayer. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X25.00009: Graphene Josephson Qubit Colin Benjamin, Jiannis K. Pachos We propose to combine the advantages of graphene, such as easy tunability and long coherence times, with Josephson physics to manufacture qubits. These qubits can either be built around a 0 and $\pi$ junction and controlled by external flux or a d- wave Josephson junction can itself be tuned via a gate voltage to create superpositions between macroscopically degenerate states. We show that ferromagnets are not required for realizing $\pi$ junction in graphene, thus considerably simplifying its physical implementation. We demonstrate that one qubit gates, such as arbitrary phase rotations and the exchange gate, can be implemented easily. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X25.00010: Quantum Transport in Graphene pnp Junctions with Contactless Top Gates Jairo Velasco Jr., Gang Liu, Wenzhong Bao, Chun Ning Lau Graphene offers the unique opportunity to explore relativistic physics in a condensed matter system. One such example is the phenomenon of klein tunneling in graphene pnp junctions. By using a contactless top gate, we are able to fabricate very high quality pnp junctions, and perform electrical transport spectroscopy measurements in zero and finite magnetic fields. We observe oscillations in conductance of the pnp junction and changes in magnetoresisitance. Latest experimental progress and comparison with theoretical predictions will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X25.00011: Systematic study of the transport gap and localization in graphene nanoribbons of varying lengths Patrick Gallagher, Kathryn Todd, David Goldhaber-Gordon Recent studies of very short graphene nanoconstrictions\footnote{Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Science \textbf{2008}, 320, 356-358.} have found that short constrictions lack the large transport gap displayed by longer nanoribbons, implying that localization behavior plays a critical role in the transport gap. We present transport measurements on graphene nanoribbons of constant width and varying length and report on gap characteristics and Coulomb blockade behavior. We discuss the relevant theoretical models and compare their predictions to our data. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X25.00012: Signatures of classical chaos in gate-defined graphene quantum dots Jens H. Bardarson, M. Titov, P.W. Brouwer A generic, non-integrable, gate-potential can not confine electrons in graphene. Integrable gate-defined quantum dots, in contrast, do have well defined bound states. This difference between integrable and non-integrable graphene quantum dots is revealed in e.g. the two terminal conductance, whose dependence on the gate potential strength is starkly different for the two cases. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X25.00013: Quantum Hall Effect in Two-Terminal Graphene Devices. James Williams, Dima Abanin, Leonardo DiCarlo, Leonid Levitov, Charles Marcus We report on transport measurements in the quantum Hall regime of two-terminal single and bilayer graphene devices. The mixture of the longitudinal and transverse conductivities in the two-terminal geometry results in departures from the expected conductance values on the Hall plateaus and are found to be device-geometry dependent. The experimental results are compared to theory and discrepancies are discussed. Research supported in part by INDEX, an NRI Center, and by the Harvard NSEC. [Preview Abstract] |
Thursday, March 19, 2009 5:30PM - 5:42PM |
X25.00014: New Computational Approach to Electron Transport in Irregular Graphene Nanostructures Douglas Mason, Eric Heller, David Prendergast, Jeffrey Neaton For novel graphene devices of nanoscale-to-macroscopic scale, many aspects of their transport properties are not easily understood due to difficulties in fabricating devices with regular edges. Here we develop a framework to efficiently calculate and potentially screen electronic transport properties of arbitrary nanoscale graphene device structures. A generalization of the established recursive Green's function method is presented, providing access to arbitrary device and lead geometries with substantial computer-time savings. Using single-orbital nearest-neighbor tight-binding models and the Green's function-Landauer scattering formalism, we will explore the transmission function of irregular two-dimensional graphene-based nanostructures with arbitrary lead orientation. Prepared by LBNL under contract DE-AC02-05CH11231 and supported by the U.S. Dept. of Energy Computer Science Graduate Fellowship under grant DE-FG02-97ER25308. [Preview Abstract] |
Session X26: Nanowire & Nanotube Synthesis and Properties
Sponsoring Units: DCMPChair: Michael Mehl, Naval Research Laboratory
Room: 328
Thursday, March 19, 2009 2:30PM - 2:42PM |
X26.00001: Aligned Gallium Nitride Nanowire Growth by Chemical Beam Epitaxy Method Ryan Munden, Aleksandar Vacic, Erik Castiglione, Weihua Guan, Christine Broadbridge, Mark Reed Gallium Nitride (GaN) Nanowires (NWs) have successfully been grown via a chemical beam epitaxy method. Source gases of Trimethylgallium (TMGa) and Ammonia (NH3) are impinged directly onto a hot growth substrate ($\sim $800 \r{ }C) in high vacuum ($\sim $1x10$^{-8}$ torr, base; $\sim $1x10$^{-5}$ torr, growth). A thin metal film acts as catalyst, but NWs were also grown without catalyst. By this method NWs have been grown on silicon, alumina, sapphire, and GaN-film substrates. NWs grown on GaN films grow aligned to the growth substrate, perpendicular to the c-plane GaN film surface. Wires aligned to the GaN a-planes can also be observed. NWs have been studied by SEM, TEM, and electrical characterization. NW lengths are $\sim $2.5 micron with diameters of $\sim $25 nm. NWs are uniform, straight, and aligned with the substrate over large areas. However closer inspection of the NWs by TEM shows that the NWs are often polycrystalline in nature. There are distinct segments ``stacked'' into a nanowire leading to noticeable diameter variations on the nanoscale. Diameter modulation can be enhanced through choice of growth substrate, temperature, and pulsing of the TMGa source. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X26.00002: Temperature Evolution of Gallium Nitride Nanowire Vapor-solid Growth Matrix K. McElroy, B.W. Jacobs, T.R. Bieler, M.A. Crimp, V.M. Ayres Recent results indicate that vapor-solid growth mechanisms can yield semiconductor nanowires with high crystallinity. In the present experiments, gallium nitride nanowire growth is initiated following formation of a microcrystalline growth matrix. A change in nanowire orientation from wurtzite $<$2-1-10$>$/zinc-blende $<$011$>$ directions at 850\r{ }C and 950\r{ }C to the wurtzite [0001] direction at 1000\r{ }C is observed. The change in nanowire orientation is correlated with changes in the growth matrix. Investigations of the evolution of the growth matrix as a function of temperature using x-ray diffraction with orientation analysis, atomic fore microscopy, high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) are presented. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X26.00003: Interaction between the boron nitride nanotube and biological molecules Chih-Kai Yang By calculating the interaction between boron nitride nanotubes (BNNT) and a variety of biological molecules, including amino acids and nitrogenous bases that are part of a nucleotide, using density functional theory, we conclude that there is no bonding or chemical adsorption between the wide band-gap BNNT and the biological molecules considered. This weak interaction suggests that BNNTs may be used as a safe nanoscale channel for transporting biological molecules. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X26.00004: Formation of Silicon Carbide Y Junctions by the Coalescence of Catalysts Zhenyu Liu, Judith C. Yang, V. Srot, Peter A. van Aken, M. R\"uhle We previously reported the formation of crystalline SiC nanocones by the released iron catalytic procedure, where the initially carbon- encapsulated iron nanoparticles escape from their carbon shells and agglomerate while catalyzing 1D SiC growth. Here we show that the coalescence of the iron nanoparticles can lead to Y junctions. Y junctions where the SiC branches are either parallel or inclined with respect to each other have been observed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The microstructure of the resulting products is analyzed by various techniques, including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) as well as electron energy loss spectroscopy (EELS). The Y junction with two parallel branches of various diameters suggests that the Y junction can be induced by the growth kinetics attributed to the diameter dependence, such as by the Gibbs-Thomson or surface tension effect. The proposed formation mechanism of Y junctions by the coalescence of catalyst droplets is a promising method to the construction of heterostructure nanowire devices. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X26.00005: Quantum Confinement in PbS nanowire Subhasish Mandal, Ranjit Pati One dimensional nanowires have become leading candidates in building nano sensor, nano transistor, optoelectronic devices and logic circuit. In last several years, PbS nanowire has drawn considerable interest for their potential applications in optical switch and solar cell. Controlled synthesis of PbS nanowire with diameter ranging from 1.2 nm to 20 nm have been reported with the photo luminescence study revealing wide band gap behavior for the nanowire. This offers exciting opportunities to study theoretically quantum confinement effect in PbS Nanowire. Here we report first principles density functional calculations of the electronic properties of PbS nanowire as a function of diameter. Our calculation shows, by varying the diameter of the nanowire from $\sim$ 1.17 nm to $\sim$ 3.64 nm, the energy band gap is found to change from 1.55 eV to 0.93 eV, substantially higher than the band gap observed for the PbS bulk - confirming the role of quantum confinement. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X26.00006: Toward complex nanostructures: lead chalcogenide nanoparticles, nanowires, and more Weon-kyu Koh, David K. Kim, Cherie R. Kagan, Christopher B. Murray Lead chalcogenides have been shown to be interesting semiconductor materials due to their small bandgaps and large Bohr radii. Based on lead chalcogenide nanoparticles as building blocks, we studied anisotropic growth of their nanowires and other structures. Growth mechanism of those structures is believed to oriented attachment which is mainly driven by dipole moment; in addition the role of surfactant is also important due to their dynamic binding on the nanoparticle surface. As-synthesized nanoparticles and nanowires were characterized using electron microscopy, X-ray diffraction, optical and electrical measurement. Our initial results open up new opportunities for photovoltaic device, sensor, and other application using those unique structures. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X26.00007: Optimal Annealing Conditions, Mechanism, and Applications for the Crystallization of Titania Nanotubes Powders Obtained by Anodization Eugen Panaitescu, Latika Menon Titanium oxide nanotubes have drawn recent interest due to their possible application in photocatalysis, water splitting and photovoltaics, as they combine the wide gap semiconductor properties of the material with the high surface per unit volume of the nanostructures. Amorphous ordered titania nanotubes arrays can be obtained by anodization of titanium foils, and our group developed a method of ultrafast synthesis of powders containing such nanotubes bundles. Crystallization can be achieved by annealing, and we investigated the influence of annealing parameters using differential scanning calorimetry coupled with spectroscopy and imaging techniques such as SEM, TEM and XRD. Our extensive study revealed the optimal parameters for crystallization without structural damage at the nanoscale, which can occur for temperatures as low as 270 degrees Celsius. Mechanistic explanations and numerical studies offered us a theoretical insight on the phase transition process. Further employing of our crystalline powders in dye sensitized solar cells revealed efficiency results superior to those previously reported for crystallization at higher temperatures and annealing rates. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X26.00008: Controlled Attachment of Gold Nanoparticles on Ordered Titania Nanotube Arrays Mohamed AbdElmoula, Eugen Panaitescu, Christiaan Richter, Laura Lewis, Latika Menon Gold nanoparticles have been deposited on electrochemically synthesized high-aspect ratio Titania nanotubes. Titania nanotubes with very long aspect ratio as long as 50microns are synthesized by means of electrochemical anodization of titanium foils in chlorine containing electrolytes. The tube dimensions (diameter, wall thickness and length) of the tubes can be controlled in our fabrication approach. The gold particles are deposited on the tubes by means of a modified deposition-precipitation method in HAuCl4 solution under controlled concentration of the solution. We show that by adjusting the fabrication conditions, we can obtain a high deposition density of the gold particles over the nanotube surface (over 90{\%} coverage percentage ) and also have good control over the size of the gold nanoparticles ($<$5 nm). We show that there is an increase in particle size upon increasing the deposition period. The samples have been characterized by means of scanning and transmission electron microscopy. The optical and preliminary catalytic properties of such gold-supported Titania nanotubes will also be reported. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X26.00009: Novel Fabrication and Enhanced Photosensitivity of Selenium Filament Arrays by Optical-Fiber Thermal Drawing Daosheng Deng, N. Orf, A. Abouraddy, Y. Fink Structures with high aspect ratio and nanometer cross-sectional dimensions have been the subject of recent studies. These nanometer-scale wire structures are typically processed through a bottom-up approach that yields limited wire lengths lacking global orientation and presenting challenges to handling and electrical contacting. Here, we report a novel physical phenomenon in which a cylindrical shell undergoing a scaling process evolves into an ordered array of filaments upon reaching a characteristic thickness. We propose a fluid front instability mechanism to account for the observed phenomena. The fleeting evolution of fluid breakup from a thin film to a filament array is captured in the frozen state by a thermal drawing process which results in extended lengths of solid sub-100nm filaments encapsulated within a polymer fiber. Furthermore, we demonstrate that the electrical connectivity of centimeter-long filament arrays to external circuitry is readily achieved by contacting the fiber ends, allowing one to study their electrical and optoelectronic properties. Enhanced photosensitivity of filaments is observed compared to a selenium film. This approach offers unique opportunities for fabrication of nanometer scale devices of unprecedented lengths allowing simplified access and connectivity. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X26.00010: Self-assembled Au nanorods - polymer composites Heung-Shik Park, Oleg Lavrentovich, Ashish Agarwal, Nicholas Kotov The unique optoelectronic properties of the anisotropic metallic nanorods (NRs) are of great interest because of their potential applications in biological sensing, solar energy conversion, cloaking devices, etc. In order to utilize NRs, tuning their properties and immobilizing NRs into polymer matrix are essential. We present a simple and universal process for formation of self-assembled nanorods polymer composite. This approach is based on the anisotropic electrostatic interaction between aggregates of chromonic molecules and NRs that lead to either end-to-end or side-by-side assembly of NRs. We discuss encapsulation of these structures with polymer matrices. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X26.00011: Focused Ion Beam Treatment of ZnO Nanowires Gagik Shmavonyan We investigated vapour-liquid-solid-grown ZnO nanowires (NWs) on a Si substrate by SEM. SEM investigations show that there are single NWs and ensembles of NWs, among which we found straight and bend, perfect and non-perfect NWs, as well as NWs with clean surfaces and surfaces with the dark spots and features. After focused ion beam polishing we found that every NW has a clean homogeneous surface, which allow us to conclude that all those dark spots and surface features of the NWs really are just surface features. The focused ion beam milling gives information of the deeper interior of the NWs, i.e. buried structures within the NWs and whether those structures are propagating within the NWs. But also here we found that there are no buried structures inside the NWs and the dark spots and features are not propagating within the NWs, which leads to the result that the NWs are totally homogeneous. The sizes of the NWs were determined: the length is about 2-24 $\mu$m, and the width and height are about 200-500 nm. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X26.00012: Controlled Growth of Zinc Oxide Nanostructures for Applications Abhishek Prasad, Archana Pandey, Yoke Khin Yap Zinc Oxide (ZnO) has proven to be a versatile functional material with promising properties. Here we discuss about the controlled growth and applications of various ZnO nanostructures including novel ZnO nanotubes (NTs) and nanosquids (NSqs). We use a conventional thermal CVD technique for the synthesis of ZnO nanostructures. We found that ZnO nanowires, nanobelts and nanocombs can be readily obtained by applying appropriate gas flow rates and growth temperatures.\footnote{S. L. Mensah et al, J. Phys. Chem. C (Letter) 111, 16092 (2007).} ZnO NTs and NSqs can be formed on the substrates when appropriate cooling rate was applied.\footnote{S. L. Mensah et al, Appl. Phys. Letts. 90, 113108 (2007).} These nanostructures were characterized using XRD, HRTEM, FESEM, Raman spectroscopy, and photoluminescence. Results show that ZnO nanostructures were single crystals in wurtzite structure. Among these ZnO NWs were found to be excellent electron field emitters and field effect transistors. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X26.00013: CO Gas Sensing with ZnO Nanowire Mat Sirisha Chava, Christine Berven, Daquing Zhang We report the electrical properties of a gas sensor constructed from mats of ZnO nanowires grown on sapphire substrate that shows a reversible response which is unique to CO exposure. The sensor is a two terminal design, where the terminals consist of two 25 \textit{$\mu $}m diameter gold wires laid parallel on the nanowire mat. The individual nanowires have an average diameter of 50 nm with lengths of about 10 \textit{$\mu $}m. The nanowire mat is about 20 \textit{$\mu $}m thick and extends over an area of about 1 cm2. When exposed to Ar, CO2 or H2 no significant changes in the current-voltage behavior of the mat are observed. CO exposure results in approximately a three-fold increase in current. The response is reversible after evacuation. Typical currents when exposed to pure CO under room temperature without prior introduction to any other gas are in the range of 40 nA compared to non-exposed 15 nA. Growth technique of nanowires and comparative work will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X26.00014: Transparent Thin Film Transistors based on Pristine and Doped Indium Oxide Nanowires Po-Chiang Chen, Guozhen Shen, Saowalak Sukcharoenchoke, Chongwu Zhou The key to the realization of transparent electronics is the development of transparent thin film transistors (TTFT) with good device performance, in terms of high device mobility, low temperature fabrication, and optical transparency. We present our work on the fabrication of high performance TTFTs using both pristine In$_{2}$O$_{3}$ nanowires and doped In$_{2}$O$_{3}$ nanowires. In$_{2}$O$_{3}$ nanowire TTFTs were made on glass and PET substrates with Al$_{2}$O$_{3}$ as gate insulator and ITO source/drain electrodes. These devices showed a transparency of about 80{\%} and n-type transistor performance. The device characteristics exhibit a subthreshold slope of 0.2 V/dec, a current on/off ratio of 10$^{6}$, and a field-effect mobility of 514 cm$^{2}$V$^{-1}$S$^{-1}$. We also fabricated TTFTs wbuilt on Arsenic-doped In$_{2}$O$_{3}$ nanowires with a field-effect mobility of 1,183.8 cm$^{2}$V$^{-1}$S$^{-1}$ without any post-treatments. In addition, we integrated TTFTs with organic light emitting diode (OLED) to make an active matrix organic light emitting diode (AMOLED) display, and thus made an animation by controlling the OLED light output. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X26.00015: Studies of Surface Exciton Polaritons in Nano-Materials by Electron Energy-Loss Spectroscopy. C.H. Chen, C.T. Wu, M.W. Chu, L.C. Chen, C.W. Chen, K.H. Chen Surface plasmon polaritons (SPPs), which normally occur in the optically \textit{metallic} spectral regime, are collective charge density oscillations of conduction electrons at the surface of metals. In sharp contrast to SPPs, the excitations of surface exciton polaritons (SEPs), which are collective oscillations of delocalized excitons at the surface of semiconductors or insulators, have been shown to be correlated with distinct excitonic onsets (interband transitions) in these materials. Using electron energy-loss spectroscopy (EELS) with a 2-{\AA} electron probe in the near-field geometry and energy-filtered spectral imaging in real space, we have unambiguously demonstrated the existence of the SEP excitations on the surfaces of GaN and ZnO nanorods at energies near the interband transitions. [Preview Abstract] |
Session X27: Optoelectronic Devices and Applications
Sponsoring Units: FIAPChair: Nelson Tansu, Lehigh University
Room: 329
Thursday, March 19, 2009 2:30PM - 2:42PM |
X27.00001: Optical Fiber Tapers for Characterization of Novel Photonic Crystal Devices Jenna Hagemeier, Jan Gudat, Susanna Thon, Dirk Bouwmeester Optical fiber tapers are a useful tool for near-field spectroscopy of solid state devices. There are advantages to using fiber tapers rather than other waveguides because they can be moved spatially with respect to the device being studied, a single taper can be used for both pumping and data collection, and they can be used to probe bare cavities. The experimental procedures and challenges for building these micron-scale tapers will be discussed, as well as their uses for probing new kinds of photonic crystal cavities. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X27.00002: Temporal Wavelength Multiplexing of a Quantum Cascade Laser Fatima Toor, Amirali Shanechi, Jianxin Chen, Claire Gmachl Quantum cascade (QC) laser based sensor systems in the mid-infrared wavelength range (3-30 $\mu $m) have applications in environmental, industrial and medical trace gas sensing. QC laser- based spectroscopic techniques have been developed by several research groups. However, more research work is needed to make these techniques more compact and field deployable. One approach to compactness is to have spectral versatility from a single device. Here we report work on a QC laser based system that is both temporally and wavelength multiplexed, that is, it can emit two different wavelengths at two alternate time slots. A bi-directional and multi-wavelength QC laser source that emits at 10.2 $\mu $m wavelength for positive polarity current and 8.6 $\mu $m for negative polarity current is used. A system is designed so that a single pulsed current source is the input to a pulse-alternator circuit that flips the polarity of every other pulse. The output of the circuit is connected to the bi-directional and multi-wavelength QC laser to emit two different wavelengths for alternate pulses. Contributions by Gary Shu at the beginning of the work are acknowledged. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X27.00003: Whispering-Gallery modes dynamics of GaAs-AlGaAs microdisk lasers Albert Heberle, Botao Zhang Semiconductor microdisk lasers are of great interest because of their low threshold, high Q-factor and potential for quantum optical effects. A microdisk laser consists of a disk with typically 100 nm thickness and several microns diameter freely standing on a pedestal. Total reflection induces high-Q whispering-gallery modes inside the circular outer edge of the disk-shaped cavity. We investigated the picoseconds dynamics of GaAs/AlGaAs microdisk lasers after ultrafast optical excitation at a sample temperature of 10 Kelvin. Surface recombination was prevented by passivation. The emission was measured temporally and spectrally resolved with a streak camera connected to a confocal microscope. The spatial emission patterns of the lasers varied significantly with the position of the exciting laser spot and shifted blue shift with increasing excitation power. The devices emitted in one or two optical modes with an excitation-dependent turn-on delay of the order of 15 picoseconds and a 5 meV red shift with two time constants during the typically 50 picosecond emission time. These effects show the balance between carrier cooling, diffusion and recombination in connection with band gap renormalization and refractive index changes [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X27.00004: Electrically Pumped Quantum Post Vertical Cavity Surface Emitting Lasers Hyochul Kim, Matthew Rakher, Dirk Bouwmeester, Pierre Petroff Quantum dot (QD) lasers based on high quality, small mode volume microcavities have shown low lasing current threshold. A novel MBE grown self assembled nanostructure, the quantum post (QP) is also a very good candidate as a gain medium for low threshold lasing. As opposed to the QD laser, the delta function density of states in the QP is associated with multiple vertically and laterally confined states. In addition, QPs have a large carrier capture cross section. We demonstrate very low threshold electrically pumped lasing in oxide apertured vertical cavity surface emitting lasers (VCSELs) with QPs as the active medium and compare their characteristics with similar structure with QDs as the active gain medium. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X27.00005: Insitu Contact Resistance Evaluation of 2.6-2.9 THz Quantum Cascade Lasers Neelima Chandrayan, Krongtip Tremkoa, Jin Li, Xifeng Qian, Shivashankar Vangala, William Goodhue, Andriy Danylov, Jerry Waldman, Robert Giles, William Nixon The fabrication of THz Quantum Cascade Lasers (QCL) requires a strong understanding of the two electrical contacts of the device. Contact resistance as well as contact/semiconductor interdiffusion properties must be designed to minimize series resistance, free carrier absorption, and e-e scattering. Here, insitu measurement of contact resistance using Transmission Line Measurement (TLM) pads has been implemented in the fabrication of 2.6-2.9 THz quantum cascade lasers. The measurement of contact resistances as part of device processing also verifies the correct etch depth of laser structure, uniformity of the etching, and the ohmic nature of the contacts. For example, the procedure has been used to successfully fabricate 2.93 THz lasers with 5 mW of continuous wave output power. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X27.00006: Magnetic Field Assisted sub-THz Quantum Cascade Lasers A. Wade, Y. Kim, D. Smirnov, S. Kumar, Q. Hu, B.S. Williams, J. Reno In THz QCLs radiative transitions take place between closely-spaced 2D electronic subbands (1THz~$\sim $~4meV) of a multi-QW semiconductor system. THz quantum cascade lasers now cover the frequency range from 1.2 THz to 5 THz, though cryogenic cooling is still required. Further progress towards the realization of devices emitting at longer wavelengths (sub-THz QCLs) and higher temperatures may be realized in a system with additional lateral confinement. Here we use strong magnetic fields to achieve quasi-0D confinement in THz QCL based on the resonance phonon design. We studied two designs: (a) 2-well injector/2 well active region, emitting at 3~THz at B=0; and (b) 1-well injector/3-well active region, emitting at 2~THz at B=0~T. By applying the appropriate electrical bias and strong magnetic fields, we achieved laser emission at 0.8-0.9~THz at B$>$16~T [1], and 0.6~THz at B$\sim $17~T, from devices a and b respectively. The ability to achieve sub-THz lasing is due to magnetic field enhanced population inversion in a quasi-0D QCL. [1] Wade, A et. al., \textit{Magnetic field assisted Terahertz quantum cascade laser operating up to 225K, }\textbf{Accepted for publication Nature Photonics} (2009) [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X27.00007: Conducting a wide-range single-transverse mode operation in a commercial multi-mode VCSEL by beam-profile transferred optical feedback Chuan-Pi Hsu, Tsu-Chiang Yen, Da-Long Cheng, Wang-Chuang Kuo In this research, a beam-profile transferred optical feedback (BTOF) method was employed to conduct a wide range single-transverse mode operation of a commercial multi-mode VCSEL. In BTOF, a spatial modulation optical system was used to reconfigure the spatial distribution of the feedback beam, and to control the laser's transverse mode. Experimental results indicated that, over a range of about 8.7 times of the laser's threshold current, BTOF could conduct the laser to output a single-transverse mode with high spectra purity and low intensity noise. While, without optical feedback, the solitary laser exhibited a multi-mode output with a complicate variation in mode distribution as the laser's current was tuned. More special features of BTOF will be presented in the report. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X27.00008: Single mode operation in ultra-short cavity Quantum Cascade lasers Richard Cendejas, Wendy Sanchez-Vaynshteyn, Dongxia Qu, Claire Gmachl Single-mode continuous wave operation of a $\lambda $=5.3 $\mu $m Quantum Cascade laser (QCL) is achieved through the systematic shortening of the cavity length of ridge waveguide QCLs with lengths ranging from 800 to 100 $\mu $m. Increasing mirror loss was mitigated with highly reflective metallic facet coatings. With smaller cavity lengths, the power consumption of an ultra-short cavity QCL of 110 $\mu $m is $\sim $250 mW at 80K, or about 20 times lower than conventional QCLs. Shortening the cavity length increases the free spectral range of the longitudinal modes placing the side modes at the edge of the gain profile, thus deliberately reducing the number of lasing modes until single-mode operation is achieved. The dominant mode is placed on the gain peak via temperature tuning, increasing the gain margin between the dominant and side modes, further increasing the single-mode current range. Amplified spontaneous emission spectra at various temperatures show that an initial gain margin increase from 1.5:1 to 2:1 doubles the single-mode current range. Ultra-short cavity QCLs with lengths of 110 $\mu $m are shown to operate single mode with the best device having a 10 cm$^{-1 }$single-mode continuous tuning. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X27.00009: Correlated Photon Noise at Threshold of an Interband Cascade Laser Patrick Folkes We report the observation of correlated photon fluctuations at threshold of an interband cascade laser at 30 K and 100 K. Away from threshold, the laser exhibits a frequency-independent photon noise spectral density. The correlated photon noise is manifested by large fluctuations in the low-frequency photon noise spectral density at certain frequencies over a narrow range of current near threshold. Concurrently, we observe the emergence and growth of the lasing mode over the same current range. The data indicates that the correlated photon noise is caused by the onset of laser coherence. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X27.00010: Effect of free carrier absorption on the efficiency of nitride devices. Emmanouil Kioupakis, Andr\'e Schleife, Patrick Rinke, Friedhelm Bechstedt, Chris G. Van de Walle Indium gallium nitride alloys are successfully being used in the fabrication of optoelectronic devices, such as light emitting and laser diodes, in the green to ultraviolet part of the optical spectrum. The wider application of these devices, however, is limited by their reduced efficiency in the green part of the spectrum. Several mechanisms have been suggested as the cause of this efficiency loss, such as Auger recombination and free carrier absorption. We use the band structure and dipole matrix elements from highly accurate many-body perturbation theory calculations in the GW approach (P. Rinke et al., Phys. Rev. B77, 075202 (2008)) to determine the optical absorption coefficient due to free carriers in InGaN. From this we obtain the corresponding photon mean free path and examine the role of free carrier absorption as a possible energy loss mechanism at high carrier concentrations. The computed values indicate that the effect is weak in light emitting diodes but it may become relevant for laser devices that operate at higher current densities. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X27.00011: Light Emission Polarization Properties of a-plane InGaN/GaN Quantum Wells Hung-Hsun Huang, Yuh-Renn Wu This paper discusses the optical characteristics for nonpolar a-plane InGaN/GaN quantum wells (QWs) with different indium compositions, QW well widths, and injection carrier densities. We compared it with the results of the conventional c-plane QWs and analyzed the characteristics of optical anisotropy polarization in (11$\bar{2}$0)-oriented wurtzite a-plane InGaN-based QWs. A self-consistent Poisson, Schrodinger $6 \times 6 $ k$\cdot$p method is used to calculate the electronic band structure including the effect of strain on QWs. We found that different indium compositions, QW well widths, and injection carrier densities have significant influences to polarization ratio of light. We find that the larger indium composition and smaller well width make the energy separation of $|Y\rangle$-like state to $|Z\rangle$-like state larger, and as a result enhance the polarization ratio of light. However, the polarization ratio decreases as the carrier injection increases, which might be a drawback for high power applications. We have studied the optimization condition for designing the a-plane InGaN quantum well LED for applications, such as LCD back light modules and lasers, which would be useful information for device designs. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X27.00012: Auger recombination rates in nitrides from first principles Patrick Rinke, Kris T. Delaney, Chris G. Van de Walle Indium gallium nitride (InGaN) alloys are successfully being used for light emitting and laser diodes in the green to ultraviolet part of the spectrum, but increases in internal quantum efficiency (IQE) are still required to allow broader applications. The IQE of InGaN devices is limited by loss mechanisms that, at high drive currents (i.e., high carrier concentrations) lead to a decrease in IQE, a phenomenon commonly referred to as ``efficiency droop''. We demonstrate by means of rigorous first-principles calculations (density-functional and many-body-perturbation theory), in which individual loss processes can explicitly be isolated, that Auger recombination is a key loss mechanism in wurtzite InGaN. Auger recombination had previously been proposed by Shen {\it et al.} [1] as a loss mechanism in optically pumped InGaN LED devices, but it is difficult to discriminate between different radiationless processes experimentally. We examine two different mechanisms -- inter- and intra-band recombination -- that affect different parts of the spectrum. In the blue to green spectral region and at room temperature the Auger coefficient can be as large as 2$\times$10$^{-30}$cm$^{6}$s$^{-1}$ and in the infrared even larger. Since Auger recombination scales with the cubic power of the free-carrier concentration it becomes an important non-radiative loss mechanism at high current densities. [1] Shen \textit{et al.}, Appl.\ Phys.\ Lett. \textbf{91}, 141101 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X27.00013: Enhancement of terahertz output power in terahertz parametric oscillator with recycled pump beam Dong Ho Wu, Tomofumi Ikari In the terahertz parametric technique the pump beam is used only once, and then dumped, regardless that the dumped pump beam still has substantial laser energy. So the energy efficiency of the technique is low. This paper reports a new parametric technique in which we recycle the pump beam (instead of dumping it) in order to increase the efficiency and enhance the terahertz beam output. Our experiments, in which we used a doped LiNbO$_{3}$ crystal with 5{\%} MgO, indicate that the terahertz beam output increases almost five times magnitude for a terahertz parametric oscillator with recycled pump beam. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X27.00014: Electro-optic sensors for high power microwave measurements Anthony Garzarella, Dong Ho Wu Nonperturbative measurements of high power microwave fields utilizing electro-optic (EO) field sensors are described. Conventional metallic-based field probes (such as dipole antennas) perturb the very fields they measure and typically saturate for field strengths exceeding $\sim $1000 V/m. EO sensors are all-dielectric, have large intrinsic bandwidths (DC to THz), and measure the true waveform of the field noninvasively. Nonlinear EO crystals such as Lithium Niobate or Potassium Dideuterium Phosphate have half-wave retardation fields of the order 10$^{6}$ V/m, making them ideal for high power microwave applications. Initial field tests with our EO sensors revealed several sources of noise and instability which are normally not encountered in laboratory settings. In this presentation, we describe a newly-designed EO sensor and how it is configured to address the noise issues in high power microwave field tests. Using this improved sensor configuration, electric field measurements in the near and far field regions of a radiating microwave horn antenna are presented. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X27.00015: Miniature Wireless BioSensor for Remote Endoscopic Monitoring Alex Nemiroski, Keith Brown, David Issadore, Robert Westervelt, Chris Thompson, Keith Obstein, Michael Laine We have built a miniature wireless biosensor with fluorescence detection capability that explores the miniaturization limit for a self-powered sensor device assembled from the latest off-the-shelf technology. The device is intended as a remote medical sensor to be inserted endoscopically and remain in a patient's gastrointestinal tract for a period of weeks, recording and transmitting data as necessary. A sensing network may be formed by using multiple such devices within the patient, routing information to an external receiver that communicates through existing mobile phone networks to relay data remotely. By using a monolithic IC chip with integrated processor, memory, and 2.4 GHz radio, combined with a photonic sensor and miniature battery, we have developed a fully functional computing device in a form factor compliant with insertion through the narrowest endoscopic channels (less than 3mm x 3mm x 20mm). We envision similar devices with various types of sensors to be used in many different areas of the human body. [Preview Abstract] |
Session X28: Focus Session: Magnetoelectric Coupling in Multiferroic Systems
Sponsoring Units: FIAP DMPChair: Gopalan Srinivasan, Oakland University
Room: 330
Thursday, March 19, 2009 2:30PM - 3:06PM |
X28.00001: Quantitative investigation of magnetoelectric coupling in various forms of multiferroics Invited Speaker: Magnetoelectric susceptibility (MES) is probably the most direct way of estimating the magnitude of magnetoelectric coupling in many forms of magnetoelectric and/or multiferroic materials. Historically, the MES has been measured in numerous existing magnetoelectric materials in broad field, frequency, and temperature ranges and their MES values have been tabulated [1]. With growing interest worldwide toward applications of multiferroics for novel memory and sensor devices, however, there have been ever-increasing demands to measure quantitatively the MES of multiferroic thin films. Yet, the measurements of thin film MES become challenging in spite of its large MES value because the magnetoelectric voltages, proportional to the film thickness, usually get too small to be measured reliably. Herein, we introduce a highly sensitive magnetoelectric susceptometer that can detect the charge variation down to $\sim $10$^{-17}$C in a few gauss oscillating magnetic field. Using this specific setup, we could measure the MES of multiferroic thin films or single crystals with unprecedented accuracy and sensitivity in cryogenic (down to 2 K) and magnetic field (up to 9 T) environments. In this talk, we summarize a number of key results based on this technique; (1) MES of a 300 nm BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanopillar structure as well as those of a 250 nm BiFeO$_{3}$ film and of a BiFeO$_{3}$ single crystal. (2) MES of (Pb,Zr)TiO$_{3}$-NiFe$_{2}$O$_{4}$ nanocomposite films, and (3) temperature- and field-dependent MES in representative multiferroic crystals/films including TbMn$_{2}$O$_{5}$ , GaFeO$_{3}$, and Cr$_{2}$O$_{3}$. In particular, we demonstrate that the MES of the film with the nanopillar structure is enhanced by approximately one order of magnitude reaching 2×10$^{-10}$ s/m at room temperature, compared with those of a pure BiFeO$_{3}$ film and a single crystal. Furthermore, based on detailed field and temperature dependent MES studies, we show that magnetoelectric coupling in TbMn$_{2}$O$_{5}$ has been mediated and amplified by the large magnetoelastic effect. \\[4pt] [1] G. A. Smolenskii and I. E. Chupis, Sov. Phys. Usp. 25, 475 (1982); F. W. Hehl \textit{et al.} Phys. Rev. A 77, 022106 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X28.00002: Magnetoelectric effects induced by domain walls Andrea Scaramucci, Thomas Kaplan, Maxim Mostovoy We explore the possibility to observe high-temperature multiferroic behavior in thin films of ordinary ferrimagnets. In thin films magnetostatic interactions induce periodic stripe domain patterns. We show that stripe domain patterns, stabilized by magneto-dipolar interactions, have ferroelectric properties similar to those of magnetic spirals in bulk materials. We studied behavior of domain patterns and the induced electric polarization in applied magnetic and electric fields using mean field approximation and Monte Carlo simulations. We find a sharp anomaly in dielectric constant close to polarization-flip transition, induced by an external electric field. We also studied the domain walls in conical spiral multiferroics, where magnetization \textbf{M} coexists with the electric polarization \textbf{P} induced by the cyloidal spiral. The structure of these domain walls explains the conservation of \textbf{P}$\times $\textbf{M }recently observed in CoCr$_{2}$O$_{4}$ as well as the magnetic field dependence of the polarization vector \textbf{P} in ZnCr$_{2}$Se$_{4}$. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X28.00003: Large electric polarization in high pressure synthesized orthorhombic manganites $R$MnO$_{3}$ (R=Ho,Tm,Yb and Lu) by using the double-wave PE loop measurements Y.S. Chai, Y.S. Oh, N. Manivannan, Y.S. Yang, Kee Hoon Kim, S.M. Feng, L.J. Wang, C.Q. Jin The magnitude of electric polarization via the conventional pyroelectric current and/or PE loop measurements often is ambiguous due to resistive components of the sample. To avoid this, a new technique called the double-wave method has been recently developed [1], in which only hysteretic PE components can be measured. Using this technique, we have measured the ferroelectric polarization of the orthorhombic $R$MnO$_{3}$ (R=Ho, Tm, Yb, and Lu) synthesized under high pressure. Large remnant polarization P$_{r}$ up to 920 $\mu $C/m$^{2}$ is observed at 10 K for LuMnO$_{3}$. Furthermore, the P$_{r}$ vs. temperature data from the PE loop has shown consistency with that measured through the pyroelectric current measurements, supporting a theoretical prediction of large polarization in the $E$-type spin structure in this system [2]. We also discuss the influence of thermal histories on the ferroelectric domain dynamics and possible internal bias field effects originating from oxygen vacancies in $R$MnO$_{3}$. [1] M. Fukunaga, \textit{et al.} J. Phys. Soc. Jpn. \textbf{77}, 064706 (2008). [2] I. A. Sergienko,\textit{ et al}. Phys. Rev. Lett., \textbf{97, }227204 (2006) [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X28.00004: Chemical engineering of the critical magnetic field for switching ferroelectricity in multiferroic hexaferrites Sae Hwan Chun, Yisheng Chai, So Young Haam, Deepshikha Jaiswal-Nagar, Dong Hak Nam, Yoon Seok Oh, Ingyu Kim, Beom Sung Lee, Kee Hoon Kim, Kyung Tae Ko, Jae Hoon Park, Jae-Ho Chung Multiferroics wherein the magnetic and ferroelectric order parameters coexist with their large cross-coupling effects have promising application potentials for multifunctional devices. To realize various technical exploitations, not only the capability of switching ferroelectricity with low magnetic field but also the tunability of the critical magnetic field ($B_{c})$ for the switching is essential. Herein, we report our discovery of a novel chemical route to engineer $B_{c}$ in a low field regime in the multiferroic hexaferrite system and discuss its mechanism. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X28.00005: Ferroelectric and magnetic properties of multiferroic BaCoF$_4$ thin films Trent Johnson, Felio Perez, David Lederman Thin films of BaCoF$_4$ have been successfully fabricated by molecular beam epitaxy and e beam technique on sapphire substrate (110), with a buffer layer of palladium grown using sputtering dc in argon atmosphere. Here we investigated the structural, morphological and ferroelectric properties were analyzed by means of various characterization techniques. The x- ray patterns showed that the films were oriented, but RHEED showed that the films were polycrystalline in the plane. AFM images showed a relatively granular surface. Measurements of the dielectric polarization showed that the films were ferroelectric at room temperature. The effect of magnetic fields on the ferroelectric properties at cryogenic temperatures will be described. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:30PM |
X28.00006: All thin film magnetoelectric magnetic field sensors. Invited Speaker: We have fabricated prototype ac magnetic field sensors operating at room temperature based on all thin film ME devices showing strong magnetoelectric (ME) coupling. The ME layers consist of a sol-gel derived Pb(Zr$_{0.52}$Ti$_{0.48})$TiO$_{3}$ (PZT) film and a dc magnetron sputter deposited magnetostrictive Fe$_{70}$Ga$_{30}$ (FeGa) film. The bilayer structures are prepared on micromachined Si wafers, and the laser cutting technique is used to release and isolate the cantilevers for optimization of the sensor performance. The PZT layer and the FeGa layer couple via the piezoelectric d$_{31}$ mode and the corresponding ME coupling coefficient is as high as $\approx $ 2 V/(Oe cm) for a lateral dimension of 1 mm$^{2}$ device at the mechanical resonant frequency of 333 Hz of a Si cantilever. The soft magnetic FeGa film requires dc bias magnetic field of around 90 Oe to operate the thin film ME device. The coupling between the PZT and the FeGa films is remarkably improved by depositing a 40 nm thick Pt intermediate layer. The clamping effect on the ME coupling is dramatically reduced by back-etching the Si cantilever down to 35 $\mu $m thick. The present work indicates presence of robust ME coupling in microfabricated multilayer thin film ME devices. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X28.00007: Magnetic anisotropy modulation of magnetite in Fe$_3$O$_4$/BaTiO$_3$(100) epitaxial structures Carlos A.F. Vaz, Jason Hoffman, Agham Posadas, Charles Ahn Temperature dependent magnetometry and transport measurements on epitaxial Fe3O4 films grown on BaTiO$_3$(100) single crystals by molecular beam epitaxy show a series of discontinuities that are attributed to changes in the magnetic anisotropy induced by strain in the different crystal phases of BaTiO$_3$. High resolution x-ray diffraction measurements show that the magnetite film is under tensile strain at room temperature, which is ascribed to the lattice expansion of BaTiO$_3$ at the cubic to tetragonal transition, indicating that the magnetite film is relaxed at the growth temperature. From the magnetization versus temperature curves, the variation in the magnetic anisotropy is determined and compared with numerical estimates for the magnetoelastic anisotropies. In particular, the tensile strain in the Fe$_3$O$_4$ films is shown to give rise to a strong perpendicular magnetic anisotropy, as observed experimentally. These results demonstrate the possibility of using the piezoelectric response of BaTiO$_3$ to modulate the magnetic anisotropy of magnetite films. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X28.00008: Enhanced power output from a magnetically coupled piezoelectric cantilever Ji-Tzuoh Lin, Barclay Lee, Bruce Alphenaar, Walter Jones, Deirdre Alphenaar Piezoelectric cantilevers have been widely studied for energy scavenging applications, but suffer from poor output power outside of a narrow frequency range near the cantilever resonance. Here, we demonstrate how power output can be enhanced by applying a simple passive external force. A symmetrical and repulsive magnetic force is applied to a piezoelectric cantilever beam to compensate the cantilever spring force. The raised and compensated spring potential introduced by the magnetic coupling is found to broaden the frequency response without altering the resonant frequency or introducing damping at resonance. Furthermore, the modified cantilever responds chaotically outside of the resonant frequency, causing increased voltage output across a large spectral region. The total voltage output across the spectrum increases between 31{\%} and 87{\%}. Model calculations support these results. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X28.00009: Quantitative determination of the enhanced magnetoelectric Yoon Seok Oh, Kee Hoon Kim, Steven P. Crane, R. Ramesh, Seongsu Lee, S-W. Cheong With growing interest worldwide toward applications of multiferroic materials for novel memory and magnetic sensor devices, there have been numerous efforts to synthesize multiferroic thin films with large magnetoelectric coupling. Yet, quantitative information on the magnetoelectric susceptibility (MES) of the film is still lacking because it is difficult to measure a reduced magnetoelectric signal due to a tiny thickness. In the present work, we have determined quantitative MES for a 300 nm BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructure, 250 nm BiFeO$_{3}$ film, and BiFeO$_{3}$ single crystal with our highly sensitive magnetoelectric susceptometer operating in cryogenic (down to 2 K) and high magnetic field, $H$, (up to 9 T) environments. We find that the MES of the BiFeO$_{3}$-CoFe$_{2}$O$_{4}$ nanostructure shows a typical anti-symmetric shape with DC magnetic field up to 340 K, as expected in the magnetoelectric coupling mediated by strain between piezoelectric and magnetostrictive materials. At room temperature, the transverse MES of the nanostructure shows a maximum of 2 $\times 10^{-10}$ s/m at low $H$ = 6 kOe. Our results also demonstrate that the MES value of the nanopillar structured film is enhanced by approximately one order of magnitude than that of pure 250 nm BiFeO$_{3}$ film and BiFeO$_{3}$ single crystal. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X28.00010: Epitaxial complex oxide tunnel barriers Junwoo Son, Pouya Moetakef, Jo\"{e}l Cagnon, Susanne Stemmer Tunnel junctions with complex oxide thin film barriers are of interest for studies of the critical thickness of ferroelectricity, of phonon modes in ultrathin films and of traps by inelastic tunneling spectroscopy. We show that high-quality epitaxial SrTiO$_{3}$ and BaTiO$_{3}$ tunnel barriers can be grown on Pt bottom electrodes. Coherent, epitaxial Pt films with roughness of less than a unit cell were grown on (001) SrTiO$_{3}$ to serve as bottom electrodes for epitaxial SrTiO$_{3}$ and BaTiO$_{3}$ tunnel barriers. All interfaces were atomically abrupt as confirmed by atomic resolution Z-contrast imaging. The IV characteristics were non-linear, demonstrating good insulating properties. For the SrTiO$_{3}$ barriers and voltage sweeps up to $\pm$ 0.5 V, the measured tunnel current was independent of the sweep direction. At low biases, dynamic conductance curves showed a symmetrical parabolic shape around the origin in both resistance states. At high bias, deviation from the ideal tunnel behavior was observed. A large increase of the tunnel conductance occurred above a minimum positive bias. A dramatic decrease of tunnel conductance occurred for a large negative bias, indicating bipolar switching. We show the contributions to the resistive switching. Phonon modes and traps are determined using inelastic tunneling spectroscopy with both paraelectric and ferroelectric tunnel barriers. [Preview Abstract] |
Session X29: Focus Session: Current Induced Dynamics in Magnetic Tunnel Junctions and Spin Waves
Sponsoring Units: GMAG DMP FIAPChair: Ioan Tudosa, University of California, San Diego
Room: 333
Thursday, March 19, 2009 2:30PM - 2:42PM |
X29.00001: Influence of electron-magnon scattering on spin transfer torque in magnetic tunnel junctions Aurelien Manchon, Shufeng Zhang Manipulating the magnetization direction using spin transfer torque in magnetic tunnel junctions (MTJs) has been one of the most important challenges in spin electronics for the past five years. Elastic tunneling theories show that the torque possesses two components, one being mainly linear in bias voltage whereas the other shows a quadratic bias dependence. These theoretical results have been recently confirmed by ``spin-diode'' experiments [1]. However, the validity of the elastic tunneling has been questioned by two experimental studies [2]. These studies show that the bias dependence of the out-of-plane torque can be dramatically different from the elastic quadratic dependence. Using the Transfer Hamiltonian formalism, we study the influence of interfacial electron-magnon scattering on the bias dependence of the spin transfer torque. We show that this mechanism can strongly affect the bias dependence of the spin transfer torque, in agreement with the recent experimental studies [2]. [1] J. C. Sankey et al., Nature Physics 4, 67 (2008); H. Kubota et al., Nature Physics 4, 37 (2008). [2] S. Petit, et al., Phys. Rev. Lett. 98, 077203 (2007); Z. Li, et al., Phys. Rev. Lett. 100, 246602 (2008). [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X29.00002: Macrospin model of incubation delay due to field-like spin transfer torque Samir Garzon, Yaroslaw Bazaliy, Richard A. Webb, Mark Covington, Shehzaad Kaka, Thomas M. Crawford While extensive measurements have tested the validity and limitations of the macrospin model with Slonczewski's spin transfer torque in metallic spin valves, recent experiments with magnetic tunnel junctions (MTJ's) have reported an additional ``field-like'' or ``perpendicular'' spin torque. The observed field-like torques generally agree with theoretical predictions, but some controversies remain: frequency domain measurements at low voltages and switching current measurements at large voltages report contradictory signs for the field-like term. Here we show that the absence of pre-switching oscillations (``incubation delay'') reported in magnetic tunnel junctions can be explained within the macrospin model by a sizable field-like component of the spin-transfer torque. Furthermore, we propose that measurements of the voltage dependence of the tunnel junction switching time in the presence of external easy axis magnetic fields can be used to determine the magnitude and voltage dependence of the field-like torque over a broad range of voltages. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X29.00003: Single-Shot Time Domain Studies of Spin-Torque-Driven Switching and Precession in Magnetic Tunnel Junctions Yong-Tao Cui, Robert A. Buhrman, Daniel C. Ralph, Daniele Mauri, Jordan A. Katine We present single-shot time domain resistance measurements of spin-transfer-driven dynamics in CoFeB/MgO/CoFeB tunnel junctions. In the regime of thermally-activated current-driven switching, we have sufficient sensitivity to resolve the pattern of resistance oscillations caused by the magnetic dynamics leading up to switching. When an in-plane hard-axis magnetic field is applied, within a short interval before the switching instant the resistance oscillations show a steadily-increasing amplitude, qualitatively consistent with expectations for large-angle precession in a simple macrospin model, although the oscillation amplitude can vary between individual switching events. Coherent large-angle oscillations are generally absent in the case of an applied field along the easy axis, which can be attributed to the differences in the precession axis and switching barriers as well as effects of thermal fluctuations. We will also report results of single-shot transport measurements in the regime of spin-torque-driven steady-state precession. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X29.00004: Bias and Angular Dependence of Spin-Transfer Torque in Magnetic Tunnel Junctions C. Wang, Y.-T. Cui, R.A. Buhrman, D.C. Ralph, J.Z. Sun We report measurements of the spin-transfer torque vector \textit{$\tau $} in MgO-based magnetic tunnel junctions (MTJs) by means of the spin-transfer-driven ferromagnetic resonance (ST-FMR) technique. We point out that for large applied DC biases, $I$, across the MTJ, for best accuracy one should correct for an artifact that arises because an applied microwave current changes the DC resistance of a MTJ slightly, which results in an extra DC voltage signal in the presence of a current bias. The correction depends strongly on the initial offset angle between the magnetizations of the two electrodes in the MTJ, and it can explain the very different results for the bias dependence of \textit{$\tau $} initially reported by two groups. After the correction is applied, we measure consistent values of \textit{$\tau $} ($I)$ over a wide range of offset angles and achieve sufficiently-improved precision to distinguish among competing theoretical predictions. We determine that the in-plane component of \textit{d$\tau $}/\textit{dV }for the MTJs we study has a weak but non-zero dependence on bias. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X29.00005: Effect of asymmetry on the bias dependence of spin torque in magnetic tunnel junctions N. Kioussis, Y. -H. Tang, Alan Kalitsov The switching of magnetic states in magnetic tunnel junctions (MTJ) by spin-polarized current via the spin torque has been the subject of intensive theoretical and experimental researches. One outstanding question which remains unresolved and controversial is the bias dependence of field-like spin torque, T$_{per}$, perpendicular to the plane of the two magnetizations. In this study, we show that bias behavior of T$_{per}$ can change dramatically with the asymmetry in the ferromagnetic electrodes from purely quadratic with negative curvature in agreement with Kubota's experimental results [1], to linear with sign reverse with bias in agreement of Li's observation [2], and finally to quadratic but with positive curvature in agreement with Sankey's experiments [3]. These results suggest that the asymmetry due to the amorphous alloys may cause the discrepancy in the bias dependence of T$_{per}$ in experimental findings [1-3]. Moreover, our results have important practical applications for MRAM devices, since the magnetic configurations of MTJ can be tuned by external bias and without the application of external magnetic field. This work is supported by NSF PREM Grant No. DMR-00116566. [1] J. Kubota et al., Nature Phys. \textbf{4}, 37 (2008) [2] Z. Li et al., Phys. Rev. Lett. \textbf{100}, 246602 (2008) [3] J. C. Sankey et al., Nature Phys. \textbf{4}, 67 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 4:06PM |
X29.00006: Voltage dependence properties of ballistic spin currents and spin transfer torques in magnetic tunnel junctions Invited Speaker: Interest in spintronics [1] has been strongly accentuated by the discovery of current induced magnetization switching caused by spin transfer torque (STT) [2]. Among the most favorable candidate systems for the realization of STT-based spintronic devices are epitaxial magnetic tunnel junctions (MTJ) [3]. Here we present a systematic study of voltage-induced STT in MTJs and provide an insight into the nature of its voltage behavior by investigating the properties of ballistic spin currents [4,5]. We demonstrate that the band filling has a dramatic impact on voltage dependence properties of both STT components, tunnel magnetoresistance (TMR) as well as on equilibrium interlayer exchange coupling [5]. Both in-plane (Slonczewski) and perpendicular-to-the-plane (field-like) STT components demonstrate a wide range of nontrivial behavior as a function of applied voltage [4,5]. The explanation is given in terms of the spin and charge current dependence on the interplay between evanescent states in the insulator and the Fermi surfaces of the ferromagnetic electrodes comprising the MTJ [5]. In particular we show that in ballistic regime the field-like torque is an even parity function of applied voltage while the parallel torque may exhibit a wide range of behavior [4,5]. Recent experiments [6] are in agreement with these predictions. Calculations are based on the non-equilibrium Green functions technique.\\[4pt] [1] A. Fert et al, \textit{Mat. Sci. Eng. B,} \textbf{84}, 1 (2001); S. A. Wolf, \textit{Science,} \textbf{294}, 1488 (2001)\\[0pt] [2] J. C. Slonczewski, \textit{J. Magn. Magn. Mat.} 159, L1 (1996); L. Berger, \textit{Phys. Rev. B} 54, 9353 (1996\\[0pt] [3] W. H. Butler et al, \textit{Phys. Rev. B,} \textbf{63}, 054416 (2001); J. Mathon and A. Umerski, \textit{Phys. Rev. B,} \textbf{63}, 220403(R) (2001)\\[0pt] [4] I. Theodonis et al, \textit{Phys. Rev. Lett.} \textbf{97}, 237205 (2006)\\[0pt] [5] M. Chshiev et al. \textit{IEEE Trans. Mag. }\textbf{44} (11) (2008);~A. Kalitsov et al., \textit{submitted}\\[0pt] [6] H. Kubota et al, \textit{Nature Physics} \textbf{4}, 37 (2008); J. C. Sankey et al, \textit{ibid.} \textbf{4}, 67 (2008); A. Deac et al, \textit{ibid.} \textbf{4}, 803 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X29.00007: Excitation of magnetization dynamics in patterned thin films using surface acoustic waves A. Baruth, S. Adenwalla The investigation of magnetization dynamics often involves the application of magnetic field or light pulses on very short time scales. Here we outline an alternative method that utilizes the changes in magnetic anisotropy associated with magneto-elastic strain. Surface Acoustic Wave (SAW) transducers are readily available at high frequencies ($>$10GHz), and provide an ideal method for straining thin film magnetic elements. SAWs propagate across a piezoelectric substrate, alternately compressing and expanding the surface with a wavelength and period that depends only on the propagation speed of the piezoelectric and the spacing of the interdigital transducer (IDT) that produces the SAW. Patterning thin film magnetic nanostructures at a spacing identical to the SAW wavelength ensures that all elements will be in phase as the SAW passes through. Passage of the SAW through a magnetic element leads to expansion and compression along the SAW propagation direction dynamically altering the easy axis of magnetization at ultra high frequencies. The subsequent dynamics can be probed using the Kerr effect. Using an IDT of 100 fingers operating at 87.2 MHz with realistic insertion losses, an array of 30nm thick, 10x20$\mu $m rectangular Co bars require voltages of $\sim $3.3V to fully switch the magnetization from the easy to hard axis without the application of an external field. Funded by NSF-MRSEC DMR-0820521. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X29.00008: Spin Wave Quantization by the Stripe Domain Structures in a Continuous Film Chun-Yeol You, Seung-Seok Ha, Jungbum Yoon, Sukmock Lee, Myung-Hwa Jung, Young Keun Kim Abnormal field dependence and dispersion relations of spin wave excitation spectra in the Brillouin light scattering were observed in a continuous CoFeSiB film. The observed spectra are similar to spin wave quantization in laterally-confined magnetic structures such as arrays of magnetic nanowires. In the array of nanowires, the propagating spin wave formed a standing wave due to the reflection from the geometrical confinement, boundaries of the nanowire, and the spin wave exciting modes are quantized. In our observation, the possible reflection source in the continuous CoFeSiB film is the regular domain boundaries, domain walls. As evidence, we observed very regular stripe domain structures by magnetic force microscopy. In the low field region (<1 kOe), the regular stripe domain patterns are formed and additional spin wave excitations are observed, while in the large field region, it behaves as usual continuous film. We believe that the regular domain wall acts as a scattering source of the spin wave, and it causes spin wave mode quantization. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X29.00009: Spin-Wave Resonance and Relaxation in CoFeB/Cr Superlattices Yu Gong, Zehra Cevher, Yuhang Ren, Hassan Imrane , Nian X. Sun We investigated the magnetic anisotropic properties and the spin wave relaxation in ten periods of CoFeB/Cr/CoFeB films grown on seed layers of Cu ($\sim $ 5 nm) with Co : Fe : B composition ratio as 2:2:1. The measurements were taken in samples with 50-angstrom layers of CoFeB using both the time-resolved pump-probe magneto-optical sampling and the ferromagnetic resonance techniques. The thickness of the Cr interlayers ranges from 4-angstrom to 40-angstrom for investigating the mechanisms of interlayer coupling and exchange interactions. Both the acoustic branch and the optical branch in spin wave resonance spectra are identified. We determine the magnetic anisotropic parameters by measuring spin wave frequency as a function of external magnetic field in the time domain and by orthogonally rotating the field aligned axis with respect to the spectral field in the frequency domain. Moreover, we estimate the intrinsic Gilbert damping for the in-plane magnetization orientation. When the interlayer coupling is weaker, the damping increases significantly. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X29.00010: Ferromagnetic resonance study of ion irradiated Co/Ni multilayers with perpendicular magnetic anisotropy J-M Beaujour, A. Kent, D. Ravelosona, E. Fullerton, Y. Samson, C. Beigne Ferromagnetic resonance (FMR) spectroscopy was used to investigate the effect of helium ion-irradiation on the magnetic properties and the magnetization dynamics of Co/Ni multilayer films. The anisotropy in these materials is associated with interfaces, which can be systematically disordered with light ion-irradiation without inducing major structural changes to the films. [Pd/Co]$\times 2|$[8\AA Ni/1.4\AA Co]$\times 3 |$Pd$|$Co$|$Pd$|$ have been exposed to He$^+$ irradiation with fluence up to $10^{15}$ ions/cm$^{2}$ [1]. FMR was conducted with a broad band coplanar waveguide up to 30 GHz. The resonance field and the FMR linewidth were determined as a function of frequency for dc magnetic fields in-plane, out-of-plane and for selected field angles. The perpendicular anisotropy decreases linearly with fluence, and at a particular fluence the direction of easy magnetization switches from perpendicular to in-plane. The Gilbert damping constant of the films irradiated at the higher and lower fluence is about the same: 0.03$\leq \alpha \leq$0.04. However, the linewidth exhibits a non-monotonic dependence on fluence, with a maximum at intermediate fluence. We will discuss this effect as well as possible explanations in terms of the changes in interface structure as a function of fluence. [1] Stanescu $et$ $al.$, J. Appl. Phys. (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X29.00011: A microstrip spin-wave amplifier Mingqiang Bao, Amber Chen, Alex Khitun, Kang Wang, Ajey Jacob To date, the spin-wave logic devices are seen promising for parallel data processing at high speed [Appl. Phys. Lett. 87, 153501 (2005), and Superlattices and Microstructures 38, 184 (2005)]. However, there is no power gain with the current spin-wave logic devices yet, and the spin-wave cannot propagate a long distance because of the spin-wave dumping effect. All those will prevent the spin-wave devices from real applications. Here we report a spin-wave amplifier with a power gain that is controlled by the pumping power level: At the pumping power of 16 dBm at 2.6 GHz, the power gain is 6.4 dB and the frequency is 1.3 GHz. The amplifier is made from a permalloy film with its thickness of 25 nm. The signal input, pump wave input, and signal output are simple microstrip lines, thus the device structure is simple. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X29.00012: Quantative characterization of a thin ferromagnetic film by pi-MFM and FMRFM Y. Obukhov, I.H. Lee, D.V. Pelekhov, E. Nazaretski, P. Banerjee, P.C. Hamme We present a theoretical analysis of two new methods for high resolution magnetic characterization of magnetic materials. These two, probe-induced (pi) Magnetic Force Microscopy and Ferromagnetic Resonance Force Microscopy are related in exploiting the modification of sample properties by the magnetic field of the probe to enable new imaging capabilities. Our analytic theory enables quantitative modeling of signals obtained in these two microscopy methods, and so allows us to extract parameters describing the magnetic properties of ferromagnetic films. We compare our theory with experimental data and find excellent agreement. More detailed experimental data will be presented in an accompanying talk. Our methodology allows detailed local characterization of ferromagnetic films in complementary MFM and MRFM experiments. [Preview Abstract] |
Session X30: Focus Session: Cobaltites
Sponsoring Units: DMP GMAGChair: Igor Zaliznyak, Brookhaven National Laboratory
Room: 334
Thursday, March 19, 2009 2:30PM - 3:06PM |
X30.00001: Doping fluctuation-driven magneto-electronic phase separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals Invited Speaker: The doped perovskite cobaltites, in particular La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), have emerged as productive systems for the study of the magneto-electronic phase separation phenomenon widely observed in complex oxide materials. It is now well established that this system phase separates into hole-rich ferromagnetic clusters embedded in a hole-poor non-ferromagnetic insulating matrix at low doping. These clusters percolate at a critical doping value near $x$ = 0.17 leading to a crossover from short- to long-range ferromagnetic order and a simultaneous insulator-metal transition. In this work we have used multiple complementary experimental probes (e.g. small-angle neutron scattering (SANS), heat capacity, and magnetotransport) to establish that high quality single crystals actually exhibit magnetic phase separation only over a well-defined doping range, 0.04 $< \quad x \quad <$ 0.22. We further show that these limits can be perfectly reproduced by simple statistical simulations where the existence of local ferromagnetism is driven solely by the inevitable local compositional fluctuations that are present at such small length scales. These length scales are defined by the mean cluster size which is determined directly by SANS. The same simulations also reproduce the doping dependence of both the observed magnetic phase fractions and SANS intensity. A remarkable consequence of this analysis is that it suggests that the magnetic phase diagram measured on macroscopic specimens is applicable even at length scales as small as 1 nm. Most importantly, it is clear from this work that models based on true electronic phase separation are not required to explain the physical properties of these cobaltites. Co-authors: C. He, S. El-Khatib, J. Wu, (UMN), J.W. Lynn (NIST), H. Zheng, J.F. Mitchell (Argonne National Lab). Work supported by DoE and NSF. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X30.00002: Heat capacity study of magneto-electronic phase separation in La1-xSrxCoO3 single crystals. C. He, S. Eisenberg, C. Jan, H. Zheng, J.F. Mitchell, J.W. Lynn, C. Leighton We present an investigation of the specific heat (0.35 K $< \quad T \quad <$ 270 K) and ordinary Hall effect (300 K) in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals (0.00 $< \quad x \quad <$ 0.30). The data reveal new information on the nature of the percolation transition, the crystal and electronic structure, and the magneto-electronic phase separation. The observations include a discontinuity in Debye temperature accompanying the percolation-type insulator-metal transition and a large electron mass enhancement, likely due to strong electron correlation effects. The various contributions to the heat capacity provide a detailed picture of the evolution of the phase-separated state with doping. Most importantly, this provides strong evidence for the unanticipated result that the phase separation is restricted to a well-defined doping range, 0.04 $< \quad x \quad <$ 0.22, in close agreement with recent small-angle neutron scattering. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X30.00003: Magnetic Phase Separation and its Relation to the Tolerance Factor in Cobaltites Juan Yu, Despina Louca, Daniel Phelan, Keisuke Tomiyasu, Kazumasa Horigane , Kazuyoshi Yamada The doping of holes into the non-magnetic Mott insulator, LaCoO$_{3}$, induces a magnetic inhomogeneous state resulting from competing magnetic phases. Elastic neutron scattering measurements on Ca, Sr, and Ba doped single crystals showed that phase competition is strongly dependent on the tolerance factor, $t$. When $t$ is small as in La$_{1-x}$Ca$_{x}$CoO$_{3}$, only a ferromagnetic (FM) phase is present. As $t$ gets large as in La$_{1-x}$Ba$_{x}$CoO$_{3}$, an incommensurate (IC) phase coexists with the FM phase. The IC phase becomes commensurate and as strong as the FM phase by x = 0.18, with a very long correlation length. This is in stark contrast from La$_{1-x}$Sr$_{x}$CoO$_{3}$ where the IC phase remains short-range and with its intensity reduced when the system orders ferromagnetically. Our observation shows that increasing $t$ enhances the presence of two phases and favors the growth of nanoscale spin-ordered superstructures. The subtle lattice changes brought about by changing $t$ have a direct effect on the Co-O hybridization that in turn affects the magnetic interactions. Double exchange interactions between Co$^{3+}$ and Co$^{4+}$ result in FM correlations while the superexchange between Co$^{3+}$ ions result in antiferromangetic correlations giving rise to the second phase. This may be mediated by a Jahn-Teller mechanism that sets in at high temperatures. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X30.00004: Incommensurate charge and spin ordering in doped layered Co perovskite oxides: small-polaron charge glass I. Zaliznyak, N. Sakiyama, S.-H. Lee, Y. Mitsui, H. Yoshizawa Using neutron diffraction, we have investigated two families of cobalt-based layered perovskite oxides, Pr$_{2-x}$Ca$_{x}$CoO$_{4}$ (0.39 $<$ x $<$ 0.73) and La$_{2-x}$Sr$_{x}$CoO$_{4}$ (x=0.5, 0.61), which are relatives of high-Tc cuprate superconductors. In the range of heavy doping, 0.5 $<$ x $<$ 0.75, we have discovered the doping-dependent incommensurate short-range ordering of charges and magnetic moments, whose scattering signatures look somewhat similar to those previously found in cuprates and nickelates. The average incommensurability of charge order (CO) propagation vector, \textbf{Q}c = ($\varepsilon _{c}$,0,l), scales roughly linearly with doping and is proportional to the concentration of Co$^{2+}$ ions, $\varepsilon _{c} \sim $ (1-x). CO exists already at room temperature and shows no change on cooling. In cobaltites, this CO can be understood as a glassy state formed by nano-scale patches of commensurate small-polaron superlattices, whose average period is determined by the doping, x, but the long-range coherence is frustrated by the charge neutrality requirement. Static magnetic spin order in cobaltites only develops at low T $<$ 40 K. Its period is roughly twice that of CO, indicating dominant antiferromagnetic correlation between the nearest Co$^{2+}$ spins. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X30.00005: Core level line shape analysis of LaCoO$_3$ E. M. Paisley, J. Stanley, J. Hinton, N. Sundaram, B. S. Mun, A. Bostwick, E. Rotenberg, J. F. Mitchell, D. P. Belanger, G.-H. Gweon The spin state of LaCoO$_3$ is a topic of high interest lately. Here we investigate the electronic structure of LaCoO$_3$ using core level and valence band photoemission spectroscopy. We compare the competing spin models in the literature by using our data obtained as a function of incident photon energy and temperature. Using line shape simulation of the Co 3s core level spectroscopy data and the Co 2p core level spectroscopy data, we address the issue of extracting the spin state information of the ground state from the photoemission data. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X30.00006: Low Temperature Interactions of High SpinMagnetic Excitons in LaCoO$_3$ Sean Giblin, Ian Terry, Andrew Boothroyd, Chris Leighton The low temperature magnetic behavior of LaCoO$_3$, containing oxygen vacancies, is reported. Magnetic susceptibility measurements made in the temperature range 0.5~K to 35~K on a single crystal and a polycrystalline sample provide strong evidence for the existence of magnetic excitons as fundamental entities within in the bulk of the material system. Specifically, two distinct types of excitons form, isolated and interacting excitons, both of which are associated with oxygen vacancies. Isolated magnetic excitons act as high spin paramagnetic particles, whilst the interacting excitons appear to be coupled antiferromagnetically. It is proposed that the interaction arises from the overlap of magnetic excitons as a consequence of the statistical clustering of oxygen vacancies. The striking similarity of these results with those of the lightly doped La$_{0.97}$Sr$_{0.03}$CoO$_3$ suggests that the observed excitons are a precursor to magneto-electronic phase separation and supports the idea that phase separation is initiated by disorder in the material system. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X30.00007: Spin Glass Behavior in the new cobaltite series (BaSr)$_{4-x}$La$_{x}$Co$_{4}$O$_{15}$. Ovidiu Garlea, Rongying Jin, Radu Custelcean, Hao Sha, Jiandi Zhang We report on the structural and magnetic properties of a new class of cobaltites with the chemical formula (BaSr)$_{4-x}$La$_{x}$Co$_{4}$O$_{15}$. These compounds crystallize in a hexagonal structure, where Co ions occupy two different sites with octahedral and tetrahedral oxygen environments. Four Co ions of the unit cell define the vertices of a tetrahedron and their mutual antiferromagnetic superexchange interactions are topologically frustrated. Partial substitution of Sr and Ba atoms for La allows one to adjust the degree of Co valence mixing and finely tune their magnetic interactions. A strong irreversibility between FC and ZFC magnetizations and the absence of magnetic reflections in the neutron diffraction patterns suggest a spin glass-like ground state for all the compositions. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X30.00008: Competition between Jahn-Teller instability and uniaxial magnetism in Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) Yuemei Zhang, Hongjun Xiang, Erjun Kan, A. Villesuzanne, M.-H. Whangbo Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) exhibits a uniaxial magnetism, because the Co$^{n+}$ ions of their CoO$_{6}$ trigonal prisms (n = 2 or 3) possess an electron configuration with unevenly filled degenerate d-states, so the Co$^{n+}$ ions have a nonzero magnetic moment only along the axis of the rotational symmetry causing the degeneracy (i.e., the 3-fold rotational axis along the CoMO$_{6}$ chain). Such ions lead to Jahn-Teller (JT) instability, and the associated distortion removes the rotational symmetry responsible for the uniaxial magnetism. We investigated how these opposing factors compete in Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) on the basis of first principles DFT calculations. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X30.00009: Various magnetic ground states linked to sodium ordering pattern via controlled cooling in Na$_{x}$CoO$_{2}$ (x $\approx $ 0.75-0.85) J. Kanter, V. Wittwer, T. Schulze, P. Haefliger, S. Petitjean, D. Sheptyakov, Ch. Niedermayer, K. Mattenberger, B. Batlogg Detailed characterization is presented of a recently found link between the low temperature magnetic properties of Na$_{x}$CoO$_{2}$ and a sodium rearrangement process at the onset of sodium mobility around 200 K. Switching between different sodium ordering patterns is possible by adjusting the cooling speed, due to the long time constant of the sodium rearrangement. The various magnetic states (with T$_{c}$ of 8, 15 and 22 K) are characterized by transport, magnetization, specific heat and thermal expansion measurements and linked to the sodium ordering process. The magnetic field dependence of the transition temperatures and the magnetic anisotropy were studied in detail. Muon Spin Rotation experiments confirmed the true bulk character of the magnetic transitions and locally probed the different phases. Single crystal diffraction data links the different magnetic ground states to structural changes. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X30.00010: A quantum Monte Carlo study of magnetism in the frustrated Hubbard model Matthew Enjalran Motivated by the observation of complex phases in materials with quasi-two-dimensional triangular lattice structures, Na$_x$CoO$_2$ $\cdot$ yH$_2$O and $\kappa$-(ET)$_2$X, where nearest neighbor interactions are frustrated, we investigate the magnetic correlations in the 2D Hubbard model using constrained path quantum Monte Carlo. In order to develop our understanding of the effect of geometric frustration on the magnetic correlations in an itinerant electron model, we report results for the square and triangular lattice geometries at half-filling. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X30.00011: Does disorder destroy e$_{g'}$ pockets in Na$_{0.3}$CoO$_{2}$? A new ab initio method for disorder Tom Berlijn, Dimitri Volja, Wei Ku Hydrated Na$_{0.3}$CoO$_{2}$ shows interesting superconductivity[1], with evidence of a nodal order parameter[2]. One possible origin of the nodal structure is {\it f}-wave pairing[3] due to the six e$_{g'}$ pockets predicted by the local density approximation[4]. However, ARPES experiments[5] showed no sign of these hole pockets. In this talk, we will investigate a recent proposal[6] of destruction of the e$_{g'}$ pockets due to disorder. An affordable {\it ab initio} Wannier function based method will be presented that takes into account spatial distributions of disorder, beyond existing mean-field approximations (e.g. VCA, CPA). We also use our Wannier functions to analyse the crystal field splitting, the sign of which critically determines the role of correlation in DMFT. \\[3pt] [1] K. Takada et al, Nature {\bf 422}, 53 (2003)\\[0pt] [2] Zheng G. et al, JPCM {\bf 18}, L63 (2006)\\[0pt] [3] Kuroki K. et al, PRL {\bf 93}, 077001-1 (2004)\\[0pt] [4] D. Singh, PRB {\bf 61}, 13397 (2000)\\[0pt] [5] Hasan M.Z. et al, PRL {\bf 92}, 246402 (2004)\\[0pt] [6] D. Singh et al PRL {\bf 97}, 016404 (2006) [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X30.00012: Novel electronic states in the sodium rich phases of cobaltates Na$_x$CoO$_2$ Meng Gao, Ziqiang Wang The cobaltates display many unusual properties in the sodium rich regime. We study the effects of strong local and finite range correlation and the sodium dopant order within the framework of an extended Hubbard model on the triangular lattice. We find that despite the proximity to the band insulating state at $x=1$, the interplay of strong electronic correlation and sodium order leads to the formation of various unconventional inhomogeneous electronic states. We compare these findings to recent experimental observations around $x=0.84$ [Preview Abstract] |
Session X31: Focus Session: Magnetic Nanostructures: Domain Walls, Reversal, Oscillators
Sponsoring Units: DMP GMAGChair: Valnetyn Novosad, Argonne National Laboratory
Room: 335
Thursday, March 19, 2009 2:30PM - 3:06PM |
X31.00001: Reversal mechanisms and defects in perpendicularly magnetized nanostructures Invited Speaker: The problem of switching field distributions (SFDs) is currently plaguing developing technologies which rely on uniform arrays of magnetic nanostructures such as bit patterned media, magnetic random access memory (MRAM) and spin-torque oscillators. Most of these technologies are shifting towards the use of perpendicularly magnetized materials due to the increased device performance and thermal stability that can be achieved. SFDs in such perpendicularly magnetized nanostructures can result from dot-to-dot interactions and size distribution, but is largely dominated by material defects [1- 4]. Such defects can arise from both the material deposition process, and post-deposition processing that occurs during nanofabrication. By comparing nanostructures fabricated by deposition on pre-patterned wafers to those fabricated by ion milling of continuous films, we show that the anisotropy of the edge region can be greatly different in each case. The size, temperature, and angular dependences of the reversal field indicate that the reversal mechanism also differs. In contrast to fabrication induced defects, microstructural variations manifest themselves as a random distribution of local anisotropies. We studied the anisotropy distribution in patterned elements by imaging the localized reversal of low anisotropy regions and mapping these sites as a function of applied field using MFM imaging and TEM. In addition, we used simulations to show the effect a small localized region of lower anisotropy material (such as a grain) has on the reversal field of the entire nanostructure. We find that the reversal field depends on both the relative anisotropy of the defect to the film, as well as, the location of the defect within the structure. \\[4pt] [1] T. Thomson, PRL 96,257204 (2006).\\[0pt] [2] J.M. Shaw, JAP 101, 023909 (2007).\\[0pt] [3] J. Lau, APL 92, 012506 (2008).\\[0pt] [4] J.M. Shaw, PRB 78, 024414 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X31.00002: Controlling Double Vortex States in Low-Dimensional Dipolar Systems Sergey Prosandeev, Laurent Bellaiche The reversal process of the chirality of each opposite vortex belonging to a double vortex state in ferromagnetic hysterons, via the application of in-plane magnetic fields, is reported [1]. Simulations reveal that such a process involves the formation of four intermediate states, including original ones. Hysteresis loops can occur only in a counterclockwise fashion because of one of these intermediate states. Double vortex states can also be controlled by electric fields in ferroelectric nanostructures of different shapes, but with some key differences with respect to the ferromagnetic case. This work is supported by ONR grants~ N00014-04-1-0413 and N00014-08-1-0915,~ DOE grant DE-FG02-05ER46188~ and NSF grants DMR-0701558, DMR-0404335 and DMR-0080054 (C-SPIN). Some computations were made possible thanks to the MRI Grants~0421099 and 0722625 from NSF. [1] S. Prosandeev and L. Bellaiche, Physical Review Letters \textbf{101}, 097203 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X31.00003: Magnetic domain wall shift registers for data storage applications Dan Read, L. O'Brien, H.T. Zeng, E.R. Lewis, D. Petit, J. Sampaio, L. Thevenard, R.P. Cowburn Data storage devices based on magnetic domain walls (DWs) propagating through permalloy (Py) nanowires have been proposed [Allwood et al. Science 309, 1688 (2005), S. S. Parkin, US Patent 6,834,005 (2004)] and have attracted a great deal of attention. We experimentally demonstrate such a device using shift registers constructed from magnetic NOT gates used in combination with a globally applied rotating magnetic field. We have demonstrated data writing, propagation, and readout in individually addressable Py nanowires 90 nm wide and 10 nm thick. Electrical data writing is achieved using the Oersted field due to current pulses in gold stripes (4 $\mu $m wide, 150 nm thick), patterned on top of and perpendicular to the nanowires. The conduit-like properties of the nanowires allow the propagation of data sequences over distances greater than 100 $\mu $m. Using spatially resolved magneto-optical Kerr effect (MOKE) measurements we can directly detect the propagation of single DWs in individual nanostructures without requiring data averaging. Electrical readout was demonstrated by detecting the presence of DWs at deliberately introduced pinning sites in the wire. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X31.00004: Ratchet Effects and Domain Wall Energy Landscapes in Amorphous Magnetic Films with 2D Arrays of Asymmetric Holes J.I. Martin, A. Alija, I. Sobrado, A. Perez-Junquera, G. Rodriguez-Rodriguez, M. Velez, J.M. Alameda, V.I. Marconi, A.B. Kolton, J.M.R. Parrondo The driven motion of domain walls in extended magnetic films patterned with 2D arrays of asymmetric holes has been found to be subject to two different crossed ratchet effects [1] which results in an inversion of the sign of domain wall motion rectification as a function of the applied magnetic field. This effect can be understood in terms of the competition between drive, elasticity and asymmetric pinning as revealed by a simple $\phi^4-$model. In order to optimize the asymmetric hole design, the relevant energy landscapes for domain wall motion across the array of asymmetric holes have been calculated by micromagnetic simulations as a function of array geometrical characteristics. The effects of a transverse magnetic field on these two crossed ratchet effects will also be discussed in terms of the decrease in domain wall energy per unit area and of the modifications in the magnetostatic barriers for domain wall pinning at the asymmetric inclusions. Work supported by Spanish MICINN.[1] A. Perez-Junquera et al, Phys. Rev. Lett. 100 (2008) 037203 [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X31.00005: Magnetic Frustration in Nanowires: Domino Effect Samir Lounis, Peter H. Dederichs, Stefan Bl\"ugel The parity of the number of atoms in finite antiferromagnetic nanowires deposited on ferromagnetic substrates is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear [1]. Using the full-potential Korringa-Kohn- Rostoker method for non-collinear magnetism [2] and a Heisenberg model we show that the magnetic structure of the whole nanowires dramatically changes if a {\em single} adatom is added. Infinite and finite nanochains with even number of adatoms are always magnetically non-collinear while odd numbers of atoms in the wire lead under given conditions to a collinear ferrimagnetic ground state. This unexpected nano-effect, which resembles a domino-effect, occurs only for wires at finite lengths. [1] S. Lounis, P. H. Dederichs, S. Blügel, Phys. Rev. Lett. 101, 107204 (2008). [2] S. Lounis, Ph. Mavropoulos, P. H. Dederichs, S. Blügel, Phys. Rev. B 72 224437 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X31.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X31.00007: Magnetic and mechanical characterizations of ultra-high frequency nanoelectromechanical systems (NEMS) Joe Losby, N. Liu, C. Holt, D. Mitlin, A.E. Fraser, V. Sauer, W.K. Hiebert, M.R. Freeman Recent efforts in our group involve time-domain studies of the motion of silicon NEMS$^{1}$ and spin dynamics in nanometer-scale permalloy elements$^{2}$. Transduction of microwave frequency ($>$ 1 GHz) cantilevers, and time domain coherent control (``unringing'') of nanoscale resonators have been demonstrated. For the next stage of this work, we have fabricated permalloy NEMS cantilevers and doubly clamped beams in order to begin exploration of magnetomechanical dynamics in ferromagnetic nanostructures. The magnetization of these resonators is probed using time-resolved magneto-optical Kerr effect microscopy, while stroboscopic optical interferometry is used for the detection of vibrational modes. \\[0pt] 1. N. Liu, F. Giesen, M. Belov, J. Losby, J. Moroz, A. E. Fraser, G. McKinnon, T. J. Clement, V. Sauer, W. K. Hiebert {\&} M. R. Freeman, Nature Nanotechnology, In Press (2008).2. Z. Liu, R.D. Sydora, and M.R. Freeman, Phys. Rev. B. \textbf{77}. 174410 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X31.00008: Synchronization of spin-torque oscillators via phase-shift control Invited Speaker: The Spin Torque Oscillator (STO) shows great promise as a frequency generating device at microwave frequencies. However its very limited output power has to be significantly improved for any realistic application. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase Q. We have recently demonstrated an intrinsic preferred phase shift between an STO and an injected RF current [1, 2]. This phase shift has direct implications for current-mediated synchronization of serially connected STOs [3]. It is exactly at this phase shift where the multi-STO synchronized state develops the highest robustness and by tuning the total circuit I-V phase shift, synchronization can be enhanced by close to 2 orders of magnitude. Since our initial work, we have now determined both the phase shift and the enhancement factor in all types of STOs (standard, perpendicular [4], wavy torque [5], tilted polarizer [6]). More recently we have also found that the perpendicular torque component present in magnetic tunnel junctions enhances synchronization through a decrease of the intrinsic phase shift. These findings are expected to be critical for future applications and will hopefully accelerate the realization of useful STO-based microwave devices. \\[4pt] References \\[0pt] [1] Yan Zhou, J. Persson, and Johan {\AA}kerman, J. Appl. Phys. 101, 09A510 (2007). \\[0pt] [2] Yan Zhou, J. Persson, S. Bonetti, and Johan {\AA}kerman, Appl. Phys. Lett. 92, 092505 (2008). \\[0pt] [3] J. Persson, Yan Zhou, and Johan {\AA}kerman, J. Appl. Phys. 101, 09A503 (2007). \\[0pt] [4] D. Houssameddine, et al, Nat. Mater. 6, 447 (2007). \\[0pt] [5] O. Boulle, et al., Nat. Phys. 3, 492 (2007). \\[0pt] [6] V. S. Pribiag, et al., Nat. Phys. 3, 498 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X31.00009: Microwave Emitting Nanomagnet Oscillator: Strongly coupled spin-photon modes \"O.O. Soykal, M.E. Flatt\'e We describe a microwave emitting nanomagnet oscillator confined in a high Q-cavity. The precession of the magnetization of a typical Fe nanomagnet of 100 nm in radius, possessing roughly $10^9$ spins and described as a macrospin, is tuned to be in resonance with a microwave cavity of 1 mm$^3$ in volume with an applied magnetic field. The Hamiltonian of the spin-cavity system is quantized in a fully quantum treatment. Due to the very large number of coherently-oriented spins, the interaction Hamiltonian of the spin-cavity system contains magnet-microwave mode coupling terms that exceed several GHz. Coherent coupling of the microwave field with nanomagnet spins is analyzed in terms of the coherent states of the spin-cavity system, which are characterized by large oscillations in the nanomagnet spin and cavity photon number, as well as by exceptionally long dephasing times. Therefore, the nanomagnet-cavity system is predicted to have distinguishable large total spin, long coherence times, and high power output. This may serve as an efficient means of transferring information between a magnetic and a photonic system. [Preview Abstract] |
Session X32: Cooperative Phenomena (incl. Spin Structures, Spin Waves, Phase Transitions) II
Sponsoring Units: GMAGChair: Stephan Rosenkranz, Argonne National Laboratory
Room: 336
Thursday, March 19, 2009 2:30PM - 2:42PM |
X32.00001: A Spin-Orbital Singlet and Quantum Critical Point on the Diamond Lattice: $FeSc_2S_4$ Gang Chen, Leon Balents, Andreas Schnyder We present a theory of spin and orbital physics in the A-site spinel compound $FeSc_2S_4$, which experimentally exhibits a broad ``spin-orbital liquid''(SOS) regime. A spin-orbital Hamiltonian is derived from a combination of microscopic consideration and symmetry analysis. We demonstrate a keen competition between spin-orbit interactions, which favor formation of a local ``Spin-Orbital Singlet'', and exchange, which favors magnetic and orbital ordering. Separating the SOS from the ordered state is a quantum critical point (QCP). We argue that $FeSc_2S_4$ is close to this QCP on the SOS side. The full phase diagram of the model includes a commensurate-incommensurate transition within the ordered phase. A variety of comparison to and suggestion for experiments are discussed. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X32.00002: Quantum Critical Phenomena near Stoner Transition in Two Coupled Quantum Dots with Spin-Orbit Coupling Oleksandr Zelyak, Ganpathy Murthy We consider a system of two coupled quantum dots. Both the dots and connecting region are assumed to be in universal crossover regimes between Gaussian orthogonal and symplectic ensembles. Using a diagrammatic approach appropriate for energy separations much larger than the level spacing, we obtain the ensemble-averaged one- and two-particle Green's functions. The diffusion and Cooperon parts of the two-particle Green's function are described by separate scaling functions. We then use this information to investigate a model of interacting system in which one dot has Stoner exchange interaction, while the other is non-interacting but contains spin-orbit coupling. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X32.00003: Mean-Field Critical Behavior in the Sr$_{1-x}$(Ca$_{0.5}$Ba$_{0.5}$)$_{x}$RuO$_{3}$ (0 $\le $ x $\le$ 0.35) Jinguang Cheng, Jianshi Zhou, John Goodenough Orthorhombic SrRuO$_{3}$ is a metallic ferromagnet with T$_{c} \quad \approx$ 160 K where mean-field (MF) critical behavior has been observed. [1] Recently, we have shown that cubic BaRuO$_{3}$ belongs to the 3D Heisenberg universality class with short-range exchange interactions. [2] The partial chemical substitution by either smaller Ca$^{2+}$ or larger Ba$^{2+}$ in Sr$_{1-x}$A$_{x}$RuO$_{3}$ which changes the bond angle $<$Ru-O-Ru$>$ and introduces an A-cation size variance $\sigma ^{2}$ $\equiv \quad <$r$_{A}^{2}> \quad - \quad <$r$_{A}>^{2 }$, causes a clear deviation from the MF behavior. In order to distinguish effects of $<$Ru-O-Ru$>$ versus $\sigma ^{2}$, we have synthesized Sr$_{1-x}$(Ca$_{0.5}$Ba$_{0.5})_{x}$RuO$_{3 }$(0 $\le $ x $\le $ 0.35) under 1000\r{ }C and 10 GPa in a Walker-type multianvil; these samples have the same average $<$Ru-O-Ru$>$ as that of SrRuO$_{3}$, but a different $\sigma ^{2}$. All samples exhibit perfect MF critical behaviors, which indicates that the peculiar bond angle $<$Ru-O-Ru$> \quad \approx $ 163\r{ } plays an essential role in determining the MF critical behavior of SrRuO$_{3}$.\\[0pt] [1] D. Kim, \textit{et al}., Phys. Rev. B \textbf{67}, 100406 (2003).\\[0pt] [2] J.-S. Zhou, \textit{et al}., Phys. Rev. Lett. \textbf{101}, 077206 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X32.00004: Cross-over of universality class in the Ising chain frustrated by long-range interactions Alessandro Vindigni, Fabio Cinti, Oliver Portmann, Danilo Pescia We investigate a spin chain in which the ferromagnetic nearest-neighbor exchange interaction $J$ competes with a long-range antiferromagnetic interaction of strength $g$ decaying spatially as $\frac{1}{r^{\alpha}}$. For $\alpha$ smaller than a certain threshold $\hat{\alpha}$ (with $\hat{\alpha}\left(\frac{J}{g}\right)>2$), the long-range interaction is able to avoid the global phase separation -- the uniformly magnetized state favored by the exchange interaction -- even at $T=0$. The ground state then consists of an ordered sequence of segments with equal length and alternating magnetization, resulting in a superlattice of magnetic domains. A memory of this periodic spin profile is retained at finite $T$ in the two-point correlation function, which oscillates as well but with a temperature-dependent period. Such an oscillation is then exponentially damped over a spatial scale, the correlation length, which diverges asymptotically, roughly, as the inverse of $T$. This suggests that the long-range interaction drives the Ising chain to acquire a universality class consistent with an underlying continuous symmetry. The $e^{\frac{\Delta}{T}}$-temperature dependence of the correlation length and the uniform ferromagnetic ground state, characteristic of the $g=0$ discrete Ising symmetry, are recovered for $\alpha >\hat{\alpha}$. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X32.00005: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X32.00006: Nature of topological quantum phase transition in chiral spin liquid Suk Bum Chung, Hong Yao, Eun-Ah Kim, Steven Kivelson How to best characterize and detect topological order, which is not associated with any local broken symmetry is one of central questions in the field of topological phases. While the ground state degeneracy that depends on the topology of the manifold the system is defined in has been a successful theoretical indicator of topological order, this concept is applicable only at $T=0$ and not accessible experimentally. Another important indicator has been topological entanglement entropy. However, topological entanglement entropy at $T=0$ can be the same for two distinct topological phases. Here we study an exactly solvable model first introduced in Ref.[1], motivated by the fact that the existence of topological quantum phase transition is known and the full spectrum is available. We examine the nature of Abelian to non-Abelian topological quantum phase transition by studying the expectation value of global flux which shows an abrupt jump at the critical point. We discuss the phase diagram of this quantum phase transition in terms of the global flux and entanglement entropy and discuss to what extent the existence of topologically ordered ground state with non-Abelian excitations is revealed at finite temperature. [1] H. Yao and S.A. Kivelson, PRL ${\bf 99}$ 247203 (2007). [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X32.00007: Scaling in modulated systems and reentrance of order Oliver Portmann, Alessandro Vindigni, Danilo Pescia Ultrathin ferromagnetic iron films exhibit a peculiar reentrance of order.\footnote{O. Portmann, A. Vaterlaus, and D. Pescia, Nature {\bf 422}, 701 (2003).} A less symmetric pattern (stripes) that is present at lower temperatures reoccurs at higher temperatures after a more symmetric intermediate state (labyrinth). We obtain a good qualitative understanding of the system by analytically reducing this problem in two spatial dimensions to an effectively one-dimensional problem that retains important properties of the original system even in the presence of small deviations from mono-dimensionally modulated order. As revealed by a scaling analysis, this system is characterized by a highly anomalous temperature dependence of an elastic constant. This finding is corroborated by mean-field calculations. By means of the scaling analysis, we can relate this experimentally inaccessible elastic constant to experimentally measurable quantities. Comparison with experiment suggests that the driving force for the reentrance of order is indeed the strongly anomalous behavior of this elastic constant. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X32.00008: Generalized Onsager cavity field method for magnets with local spin fluctuations James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko The Onsager cavity field method is extended to magnets described by a classical spin fluctuation model which interpolates between the limits of localized and itinerant magnetism and captures the qualitative features of itinerant thermodynamics. We find that \textit{both} the interatomic exchange coupling and the on-site interaction are renormalized by short-range order. In the localized (Heisenberg) limit Onsager's approximation is recovered, but in itinerant systems it is essential to include both corrections, for in this case a simple subtraction of only the Onsager reaction field leads to poor results. The generalized Onsager method is compared with the results obtained through mean-field and Monte Carlo methods. It is found that for close-packed lattices with nearest-neighbor exchange there is excellent agreement between the generalized Onsager method and Monte Carlo for any degree of itinerancy, and offers a significant improvement over the mean-field approximation in predicting the Curie temperature [1]. [1]A. L. Wysocki, J. K. Glasbrenner, and K. D. Belashchenko, Phys. Rev. B \textbf{78}, 184419 (2008) [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X32.00009: Spin Rotation Technique for Non-Collinear Magnetic Systems: Application to the Generalized Villain Model J.T. Haraldsen, R.S. Fishman This work develops a new generalized technique for determining the static and dynamic properties of any non-collinear magnetic system. By rotating the spin operators in the local spin reference frame, we evaluate the zeroth, first, and second order terms in a Holstein-Primakoff expansion, and through a Green's functions approach, we determine the structure factor intensities for the spin-wave frequencies. To demonstrate this technique, we examine the spin-wave dynamics of the generalized Villain model with a varying interchain interaction. The new interchain coupling expands the overall phase diagram with the realization of two separate spin configurations. The rotational Holstein-Primakoff expansion provides both analytical and numerical results for the spin dynamics and intensities of these phases. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X32.00010: Edge States of Quantum Antiferromagnets Jos\'e Hoyos, S. Chandrasekharan, H. U. Baranger We investigate the edge states of spin systems in the Affleck-Kennedy-Lieb-Tasaki (AKLT) phase. Edges of correlated systems may show novel lower dimensional physics (as in quantum Hall edge states) and have recently garnered increasing experimental attention. Here we study spin-1 systems using the directed-loop quantum Monte Carlo technique. Depending on the configuration and parameters, even though the bulk system has a spin (Haldane) gap, the edge states can be gapless and described by an effective Luttinger liquid. We focus on the behavior of the edge states when the bulk undergoes a quantum phase transition from the AKLT to the N\'eel phase. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X32.00011: Magnetic Properties of a Canted Antiferromagnet Mn(N$_{3}$)$_{ 2}$(4,4'-bpy) Youcef Hamida, Dusan Danilovic, C.L. Lin, Tan Yuen, Kunhao Li, Jing Li Results of magnetic susceptibility \textit{$\chi $}($T)$, isothermal magnetization $M(H)$, and heat capacity $C(T)$ measurements on a manganese complex with mixed ligands Mn(N$_{3})_{2}$(4,4'-bpy) (4,4'-bpy = 4,4'-bipyridine) are reported. The crystal structure of this three-dimensional manganese azide network is tetragonal, with lattice parameters $a=b$ = 8.1 {\AA} and $c$ = 16.7 {\AA}. Fitting the high $T$ data of \textit{$\chi $}($T)$ on powder samples resulted an effective moment \textit{$\mu $}$_{eff}$ = 4.9 \textit{$\mu $}$_{B}$ for the Mn$^{2+}$ moments, and a strong antiferromagnetic interaction of \textit{$\theta $} = - 120 K. An antiferromagnetic transition with rather high transition temperature of 39 K was observed in the $M(T)$/$H$ data of Mn(N$_{3})_{2}$(4,4'-bpy), and large non-compensated component in \textit{$\chi $}($T)$ below T$_{N}$ was seen. The result of $M(H)$ measures showed that Mn(N$_{3})_{2}$(4,4'-bpy) behaves like a ferrimagnet below T$_{N}$, with a small coercive field of $H_{coe}$ = 150 G at 1.8 K. A sizable anomaly was observed in $C(T)$ data, and this confirms the long-range magnetic phase transition and the T$_{N}$. The magnetic behavior of this compound is discussed in terms of a strong Mn-Mn coupling through this unique network with end-to-end azido bridges, and compared with the magnetic behavior of its iron isostructural analogue Fe(N$_{3})_{2}$(4,4'-bpy). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X32.00012: Magnetic switching and phase competition in the multiferroic antiferromagnet Mn$_{1-x}$Fe$_{x}$WO$_{4}$ Feng Ye, Y. Ren, J.A. Fernandez-Baca, H.A. Mook, J.W. Lynn, R.P. Chaudhury, Y.-Q. Wang, B. Lorenz, C.W. Chu Elastic neutron scattering is used to study the spin correlations in the multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$ with $x$=0.035, 0.05, and 0.10. The noncollinear incommensurate (ICM) magnetic structure associated with the ferroelectric (FE) phase in pure MnWO4 is suppressed at $x$=0.035 and completely absent at $x$=0.10. The ICM spin order and FE phase can be restored by applying a magnetic field along the spin easy axis. The low-$T$ commensurate magnetic structure extends in both $H/T$ with increasing Fe concentration. The systematic evolution of the magnetic and electric properties indicates that the noncollinear ICM spin order results from competing magnetic interactions and its stabilization can be tuned by the internal ($x)$ or external magnetic-field perturbations. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X32.00013: The ferromagnetic transition in LiHoF$_4$ Anders Biltmo, Patrik Henelius The rare-earth magnetic compound LiHoF$_4$ is considered to be one of the best realizations of a long-range dipolar Ising model. Several experimental studies have been carried out probing the magnetic properties of the material in the context of classical, as well as quantum, phase transitions. In this work we revisit the effective model for the non-dilute material using numerical simulations. We examine the accuracy of the model in relation to experiments and consider the logarithmic corrections derived in renormalization group theory. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X32.00014: Examination of the magnetic structure of Cs$_{2}$CuCl$_{4}$ by first principles DFT calculations Changhoon Lee, Jinhee Kang, Mike Whangbo The spin-1/2 Cu$^{2+}$ ions of Cs2CuCl4 have a 3D arrangement, but the magnetic properties of Cs2CuCl4 are mainly described by a quasi-2D triangular antiferromagnetic layer model. To understand why the 3D arrangement of (CuCl4)$^{2-}$ ions leads to a 2D magnetic behavior, we evaluated the various spin exchange interactions between adjacent (CuCl4)$^{2-}$ ions by performing DFT calculations. Our results show that the 6p orbitals of Cs$^{+}$ participate in the spin exchange interaction through the Cl?Cs?Cl bridges if the two (CuCl4)$^{2-}$ ions have a symmetric arrangement and if the Cl?Cs?Cl bridges are symmetric, and that the frustrated 2D triangular antiferromagnetism originates from this selective participation of the Cs 6p orbitals in the spin exchange interactions. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X32.00015: Examination of the Coupled Magnetic-Structural Phase Transition in Gadolinium-Silicon-Germanium Magnetocaloric alloys at temperatures well above Tc Ravi Hadimani, Eugene Melikhov, John Snyder, David Jiles The first order phase transition in Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$ from monoclinic phase to orthorhombic phase was investigated from 296-300 K at magnetic fields of up to 9~Tesla. The rate of change of transition temperature with magnetic field was found to be 4.9~K/ Tesla in the field range up to 2.5~Tesla. This linear rate of change of transition temperature with field persisted even at higher magnetic fields of up to 9~Tesla.. Measurements were made on single crystal Gd$_{5}$Si$_{1.95}$Ge$_{2.05}$ and Gd$_{5}$Si$_{2}$Ge$_{2}$ using a high field VSM and a SQUID magnetometer. The single crystal samples were prepared by the Bridgman method at Ames Laboratory US DoE. The first order phase transition temperatures of single crystal samples at nearly zero field were determined to be 264~K and 267~K respectively. The magnetic field required to induce the first order phase transition at 289~K for the single crystal Gd$_{5}$Si$_{1.95}$Ge$_{2.05}$ was 4.8~Tesla and at 300~K it was 8~Tesla. For the single crystal Gd$_{5}$Si$_{2}$Ge$_{2}$ sample, the magnetic field required to induce the first order transition at 289~K was 4.5~Tesla and at 300~K it was 8.4~Tesla. The magnetic field required to induce the first order phase transition increased linearly with the difference T-Tc. [Preview Abstract] |
Session X33: MgB2 and Multigap Superconductivity
Sponsoring Units: DCMPChair: Mike Sumption, Ohio State University
Room: 403
Thursday, March 19, 2009 2:30PM - 2:42PM |
X33.00001: Homogeneity and connectivity of doped MgB2 bulks and strands as probed by heat capacity and current transport Mike Sumption, Mike Susner, Z. Shi, E. Collings Homogeneity and current percolation have been investigated for MgB2 bulks and strands. Sintered bulks were compared to dense bulks produced by HIPping, infiltration, and spark plasma synthesis in terms of their homogeneity as measured by heat capacity and resistivity. Various levels of carbon based dopants were introduced in each case, with control samples for comparison. The influence of higher temperature processing on inhomogeneity was investigated. These results are compared to those of MgB2 based strands made both with and without C-based doping. In addition, comparisons of transport and magnetization measurements at higher magnetic fields showed the onset of a regime where the anisotropy between Jc parallel to the strand and Jc perpendicular to the strand grows rapidly. This leads to large differences between transport and magnetically measured values of not only critical current, but also the irreversibility fields, Birr. Such effects can be described in terms of three different regimes, defining the dimensionality of the system. These regimes are distinct from the various states of vortex matter present in the magnetic phase diagram of MgB2, but in coexisting with them they influence our estimates of these boundaries. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X33.00002: Upper critical field study of CH$_{4}$ HPCVD carbon-doped MgB$_{2}$ F. Hunte, J. Jaroszynski, A. Gurevich, D.C. Larbalestier, Y. Zhu, P.M. Voyles, X.X. Xi, K. Chen, S. Baily, F. Balakirev, C.G. Zhuang, S. Meng , C. Y. Zhang, Q.R. Feng, Z.Z. Gan The $H_{c2}(T)$ of a set of four carbon-doped MgB$_{2}$ films grown on both SiC and Al$_{2}$O$_{3 }$substrates by HPCVD from methane CH$_{4}$ at flow rates from 7 to 10 sccm were measured in fields up to 65T. Compared to early metalorganic C sources which generated high $H_{c2}(0)$, these films have much lower resistivities and higher connectivities. The curvature of $H_{c2}(T)$ derived from low current four point magnetoresistance shows upturn at low temperatures, which is consistent with the dominance of \textit{$\pi $}-band scattering in the theory of dirty two-gap superconductivity. $H_{c2}^{\vert }$(0) $>$ 60T is close to the paramagnetic limit of $\sim $ 66T for the 10 sccm film on SiC, though still a little lower than for the previously used metalorganic (C$_{6}$H$_{7})_{2}$Mg. Differences in the $H_{c2}(T)$ behavior between films grown on the two substrates are attributed to variations in strain fields produced by the substrate coupled to the film at growth. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X33.00003: Absolute Penetration Depth in MgB$_{2}$ Nicholai Salovich, Russell Giannetta, Matt Tillman, Paul Canfield Absolute penetration depth measurements were carried out on single crystals of MgB$_{2}$. $\lambda $(0) was determined by sputtering an Al film onto the sample crystal and measuring the change in Meissner screening as the Al film expels flux. The change in screening was measured with a tunnel diode oscillator [1]. Several samples were sputtered with films of different thicknesses and measured. Thickness dependent changes in Hc and Tc of the thin Al films provided a self-consistency check on properties of the films. Subsequent analysis using FIB/SEM and AFM independently measured the film thickness and roughness. Work at UIUC supported by NSF DMR-05-03882. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [1] R. Prozorov, et al, Appl. Phys. Lett. 77, 4202 (2000) [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X33.00004: High Upper Critical Field and Critical Current Density of Carbon-doped MgB$_{2}$ Films by HPCVD Using TMB Wenqing Dai, Ke Chen, Qi Li, Xiaoxing Xi Carbon-doping is effective to enhance upper critical field $H_{c2}$ and critical current density $J_{c}$ of MgB$_{2}$. Using Trimethylboron (TMB) as the doping source, we have successfully fabricated carbon-doped MgB$_{2}$ thin films by the Hybrid Physical-Chemical Vapor Deposition (HPCVD) method. Large temperature derivative $-dH_{c2}^{//ab} /dT$ values near $T_{c}$, as high as 8.3 T/K, have been achieved for heavily doped samples. These values are much higher than what have been reported before. With $T_{c}$ over 30 K, $H_{c2}^{//ab} (0)$ values over 100 T can be expected for these samples. For lightly doped films, $J_{c}$ values, larger than 10$^{5}$ A/cm$^{2}$ at 5 K under 9 T perpendicular field and 10$^{4}$ A/cm$^{2}$ at 20 K under 5 T perpendicular field, were obtained. The results demonstrate that carbon-doped MgB$_{2}$ films by HPCVD using TMB are promising for high field applications. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X33.00005: Novel phase line in the mixed state of the multi-band MgB$_2$ single crystal Daichi Kubota, Takekazu Ishida High-quality MgB$_2$ single crystals of submillimeter size were synthesized successfully by the vapor transport method for carrying out magnetic torque measurements. The magnetic torque of MgB$_2$ has been analyzed by a single-band model as well as a multi-band model for an anisotropic superconductor. The anisotropy in the coherence length $\gamma_\xi = \xi_a/\xi_c$ and the anisotropy in the penetration depth $\gamma_\lambda = \lambda_c/\lambda_a$ with the multi-band Kogan theory for the magnetic torque were simultaneously determined by means of the least-squares fittings. A systematic variation of two-sorts of effective superconducting anisotropies, $\gamma_\xi$ and $\gamma_\lambda$, in the field-temperature plane was obtained as contours, and can be interpreted as a manifestation of the two-band nature of the MgB$_2$ superconductivity. It is not a gradual crossover but a drastic change in the electronic state that an MgB$_2$ superconductor transits from a $\pi-\sigma$ multiband superconductor in lower fields to a $\sigma$ single band superconductor in higher fields. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X33.00006: Time Dependent Effects in Transport Measurements and Absence of Anisotropic Behavior in Polycrystalline MgB$_{2 }$ Murat Olutas, Atilla Kilic, Kivilcim Kilic, Atilgan Altinkok, Hakan Yetis The flux dynamics in polycrystalline sample of MgB$_{2}$ have been studied by current-voltage (I-V) measurements for different sweep rates (dI/dt) of transport current at zero magnetic field (H=0) and H$\ne $0, and also magnetovoltage measurements (V-H curves) for different sweep rates (dH/dt). It was observed that the time and hysteresis effects in I-V curves which appear upon cycling of transport current are not significant as compared to those of YBaCuO and BiSrCaCuO. The absence of hysteresis effects in I-V curves were attributed mainly to the absence of weak link structure in MgB$_{2}$. The hysteresis effects in V-H curves depend on field orientation of H with respect to I. Furthermore, it was observed that time dependent effects appear in V-H curves as the dH/dt varies and also the counterclockwise behavior which appears in forward region changes its character and becomes clockwise for reverse region. The magnetovoltage measurements were also carried out by varying the angle $\theta $ between H and I (V- $\theta $ curves). The V- $\theta $ curves show that there is no anisotropy in polycrystalline MgB$_{2}$. This behavior was discussed in terms of polycrystalline structure of MgB$_{2}$ and its band structure where the Fermi surface includes four sheets of one electron-like and three hole-like. The observations were interpreted mainly in terms of flux trapping in grains. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X33.00007: Electrical Conductivity Percolation Effects in CrO$_{2}$-MgB$_{2}$ Nanocomposites Xiangdong Liu, Raghava Panguluri, Huang Zhi-Feng, Boris Nadgorny Cold-pressed half-metals/insulators are known to show an enhanced extrinsic powder magnetoresistance (MR) due to intergranular and intergrain spin-dependent tunneling. In this work we use a mixture of metal/superconductor (CrO$_{2})_{x}$(MgB$_{2})_{1-x}$ nanocomposites to study conduction percolation effects. The samples were cold pressed in the form of pellets from the mixture of pure CrO$_{2}$ and MgB$_{2}$ powders. Transport and magneto-transport properties of various composition compacts measured as a function of temperature. Magneto-transport measurements performed over a temperature range of 2-100K show a hysteretic behavior with the peak values of MR coinciding with the CrO$_{2}$ coercive fields, with a maximum MR value for the composition near the percolation threshold of $\sim $ 42{\%} at 2 K. The electrical resistivity displayed a sharp maximum near the percolation threshold, a feature that is likely to be unique for such type of systems. We will discuss the implications of our results for the analysis of conduction percolation within the framework of a simple percolation model and a possible connection to Andreev reflection effect in this system at low temperatures. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X33.00008: Abrikosov flux-lines in a two-band superconductor with mixed dimensionality K. Tanaka, M. Eschrig We study electronic and thermodynamic properties of a two-band superconductor in the vortex state, in which one band is ballistic and quasi-two-dimensional (2D), and the other is diffusive and three-dimensional (3D). A circular cell approximation of the vortex lattice within the quasiclassical theory of superconductivity is applied to a recently developed model appropriate for such a two-band system [1,2]. Motivated by MgB$_2$, superconductivity in the 3D diffusive band is assumed to be ``weak'', i.e., mostly induced. We examine the intriguing effects of induced superconductivity, impurities, and Coulomb interactions in the 3D band on electronic structure in the 2D band. In particular, the Coulomb repulsion and the diffusivity in the ``weak'' band enhance suppression of the order parameter and enlargement of the vortex core by magnetic field in the ``strong'' band, resulting in reduced critical temperature and field. A particularly interesting feature found in our model is the appearance of additional bound states at the gap edge in the ``strong'' band. Furthermore, coupling with the ``weak'' band leads to reduced band gaps and van Hove singularities of energy bands of the vortex lattice in the ``strong'' band. [1] K. Tanaka, D. F. Agterberg, J. Kopu, and M. Eschrig, Phys. Rev. B {\bf 73} 220501(R) (2006). [2] K. Tanaka, M. Eschrig, and D. F. Agterberg, Phys. Rev. B {\bf 75} 214512 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X33.00009: Flux penetration in mesoscale samples of multi-component unconventional superconductors David George Ferguson, Paul Goldbart Multi-component unconventional superconductors bring the possibility of unusual magnetic phenomena. Examples include spontaneous zero-field magnetization, and penetration by magnetic flux not only through one-dimensional vortices but also through two-dimensional domain walls in which superconductivity persists. How flux penetrates, both in and out of equilibrium, depends on the way in which the order parameter distorts---vortex versus domain wall---particularly in view of the fact that flux quantization is required for vortices but not for domain walls that traverse the sample. We study these issues in the setting of mesoscale samples, in which domain walls are more stable and any discreteness of flux penetration should be more readily observable, predicting an unusual variation of the magnetization with the applied magnetic field. The observation of such effects in ${\rm Sr}_{2}{\rm Ru}{\rm O}_{4}$, a proposed unconventional superconductor, via techniques such as cantilever torque magnetometry, should shed light on important issues such as the pairing symmetry and the prevalence of domain walls in bulk samples. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X33.00010: Multi-Gap Superconductivity in Chevrel Phases Alexander Petrovic, Cedric Dubois, Gilles Santi, Christophe Berthod, Oystein Fischer, Rolf Lortz, Albin Demuer, Arlei Antunes, Antoine Pare, Diala Salloum, Patrick Gougeon, Michel Potel Sub-Kelvin scanning tunnelling spectroscopy (STS) in the quasi-3D Chevrel Phases PbMo$_6$S$_8$ and SnMo$_6$S$_8$ reveals two distinct superconducting gaps at $\sim$~3meV and $\sim$~1meV. The relative spectral contribution from each gap changes with the angle between tip and sample, implying a rather anisotropic Fermi surface. Complementary to our local probe studies, specific heat measurements confirm the strong coupling (2$\Delta$/$k_B$$T_c$~$\sim$~4.9) seen by STS in each material and provide further evidence for multi-gap superconductivity. Hexagonal vortex lattices have been imaged by STS for each compound, with vortex core spectroscopy indicating a pseudogap within the cores. No pseudogap is visible in normal-state spectra, suggesting that the gapped vortex cores may be a novel consequence of two-band superconductivity. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X33.00011: Anomalous upper critical field of two-gap superconductor Lu$_2$Fe$_3$Si$_5$ Yasuyuki Nakajima, Hikaru Hidaka, Tsuyoshi Tamegai, Terukazu Nishizaki, Takahiko Sasaki, Norio Kobayashi Ternary-iron silicide superconductor Lu$_2$Fe$_3$Si$_5$ with $T_c$ = 6 K has attracted attention because of the anomalous superconducting properties, such as a large residual linear term in the superconducting specific heat and a reduced normalized specific heat jump at $T_c$ smaller than the BCS value. Our recent specific-heat study has revealed that these anomalies stem from the two distinct superconducting gaps. In order to clarify the details of the two-gap superconductivity in Lu$_2$Fe$_3$Si$_5$, we have prepared the high-quality single crystal and investigated the upper critical field $H_{c2}$ obtained by resistivity measurements. We find that $H_{c2}$ increases linearly with decreasing temperature down to $T_c/3$, and $H_{c2}(T=0)$ exceeds the orbital depairing field described by the simple WHH theory. We also find that the angular dependence of $H_{c2}$ is well described by anisotropic GL model unlike the case of typical two-gap superconductor MgB$_2$. We discuss the origin of these differences based on the nature of two gaps in the two superconductors. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X33.00012: Concomitant enhancement of spin susceptibility and pairing interaction in the reduced carrier-density regime of Li$_x$ZrNCl superconductor Yuichi Kasahara, Tsukasa Kishiume, Takumi Takano, Katsuki Kobayashi, Yoshihiro Iwasa, Eiichi Matsuoka, Hideya Onodera, Yasujiro Taguchi Li-intercalated layered nitrides Li$_x$ZrNCl are novel superconductors, in which superconductivity emerges at relatively high transition temperature $T_c\sim12$ - 15~K with very low carrier density $\sim10^{21}$~cm$^{-3}$. The pristine $\beta$-ZrNCl is a simple band insulator, and electron doping is achieved by Li intercalation. Insulator-to-superconductor (IS) transition takes place at $x\sim0.05$ with maximum $T_c$ value of $\sim15$~K and $T_c$ decreases with further doping, which is opposite trend to the other superconductors in doped band insulators. Here we show the results of magnetic susceptibility measurements on Li$_x$ZrNCl with systematically controlled $x$. Estimated spin susceptibility $\chi_s$ is almost temperature-independent without substantial anisotropy. With decreasing $x$, $\chi_s$ evolves strongly, same as $T_c$. On the other hand, specific heat study revealed that the density of states is reduced but the pairing interaction is enhanced on the verge of IS transition. Therefore, our results may indicate that magnetic fluctuations are enhanced toward a band-insulator and that they are possibly responsible to superconductivity even in the present small carrier-density system. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X33.00013: Absence of Superconductivity in the Hole Doped Li$_{0.38}$BC Engin Ozdas, Bora Kalkan, Ebru Gungor The existence of several borocarbides with crystal structures highly related to MgB$_{2}$, in which one of these, the layered LiBC has been predicted based on the electronic structure calculations that this compound should become superconducting on doping with holes. However, the superconducting features for Li off-stoichiometric borocarbide compounds have not been observed in any experimental studies, because of the difficulties in the sample preparation. In this work, the effects of synthesis conditions on the structure of Li$_{x}$BC samples with the different Li content and the phase stability were investigated. The structural studies showed that the intercalation process has a staging behavior as Li intercalated graphite and a novel Li vacancy ordered structure for off-stoichiometric stage-2 Li$_{0.38}$BC phase. The temperature dependence of the conductivity shows semiconducting behavior over the whole temperature range and the hopping type conduction improved by the hole doping. [Preview Abstract] |
Session X34: Superconductivity: Magnetic Field Effects
Sponsoring Units: DCMPChair: Milind Kunchur, University of South Carolina
Room: 404
Thursday, March 19, 2009 2:30PM - 2:42PM |
X34.00001: Ballistic acceleration phase of a supercurrent Milind N. Kunchur, Gabriel Saracila One of the primitive but elusive current-voltage (I-V) responses of a superconductor is when its supercurrent grows steadily after a voltage is first applied, as per the first London equation. Because this phase lasts for a relatively short duration---until dissipative processes set in---it is difficult to conduct a correlated time-domain I-V measurement of it. The present work employed a measurement system that can simultaneously track and correlate I(t) and V(t) with sub-nanosecond timing accuracy, resulting in a clear time-domain measurement of this transient phase where the quantum system displays a Newtonian like response. The highly controlled technique used here measures the near equilibrium response and should be distinguished from an impulse response measurement, which may probe non-equilibrium processes. The present technique should be of value for the controlled investigation of other types of time-dependent and non-equilibrium phenomena. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X34.00002: Spatial distribution of internal magnetic field in Hight - $T_c$ superconductors with pancake vortices W.P. Halperin, S. Mukhopadhyay, A.M. Mounce, S. Oh, A.P. Reyes, P. Kuhns, H. Takagi, S. Uchida We report here $^{17}$O $T_1$ measurements in single crystals of slightly overdoped ($T_C = 82$ K) Bi2212 at 5 K, in magnetic fields from 15 -- 30 T. In previous work the internal magnetic field distribution in YBCO aligned powders at high magnetic fields has been probed by NMR imaging experiments~[1]. Our results for single crystals of the highly anisotropic superconductor, BSCCO, are remarkably different, and is inconsistent with present theoretical predictions. At 5 K the system is in a 2-D vortex solid phase~[2]. We conclude that the magnetic field distribution for 2-D vortices in the presence of interlayer magnetic and Josephson coupling is radically different from a London vortex lattice.\\[0pt] [1]V.~F.~Mitrovi\ifmmode~\acute{c}\else \'{c}\fi{} \textit {et.al.}, Nature \textbf{413}, 501 (2001). \\[0pt] [2]Bo Chen \textit {et.al.}, Nature Physics \textbf{3}, 239 (2007). [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X34.00003: Intermittent propagation of magnetic flux in superconductors. Vitaliy Yurchenko, Atle Jorstad Qviller, Iouri Galperine, J{\O}rn Bindslev Hansen, Peter Mozhaev, Tom Henning Johansen Regular arrays of planar defects can be introduced in superconducting YBa2Cu3O7 thin films by depositing them on tilted substrates. This results in anisotropy of critical currents flowing in the plane of the film. At optimal tilt angles a substantial increase of the critical currents flowing in both directions, i.e. along and across the planar defects, has been observed. However, the artificially introduced defects also have a dramatic effect on dynamics of the flux propagation: it becomes intermittent. Previously we observed intermittent flux penetration in MgB2 thin films with thermo-magnetic instability, where such flux jumps trigger giant magnetic avalanches. In this report we present results of a real time magneto-optical visualization of the intermittent flux motion in YBCO and discuss possible consequences for its thermo-magnetic stability. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X34.00004: Magnetic field-orientation independence of large basal-plane critical currents in RBCO films with correlated pinning nanostructure David Christen, Y.L. Zuev, S.H. Wee, A. Goyal, C. Cantoni, C. Tarantini, A. Gurevich, D. Larbalestier It has been widely confirmed that self-assembled columnar stacks of second-phase precipitates aligned near to the $c-$axis provide strong flux pinning in RBCO epitaxial films. Such growth-controlled nanostructures can be produced by at least two different deposition techniques and for several species of oxide precipitates. For many of these systems, the usual dependence of in-plane critical current densities, $J_{c}$, on field-orientation nearly vanishes at a specific temperature-dependent field, $B$*($T)$. The phenomenon can be described by a competition between intrinsic electronic anisotropy and orientation-dependent pinning. A simple model parameterizes the effect through the dependencies $H_{irr}(\theta )$ and the power-law decay exponent $\alpha (\theta )$, where $J_{c} \quad \propto $ $H^{-\alpha }$ in the intermediate field regime. Limits to and fundamental aspects of the model with respect to these parameters will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X34.00005: Angularly Dependent, Contact-free Current Density Measurements of YBCO Coated Conductor J.W. Sinclair, J.R. Thompson, D.K. Christen, Y. Zhang Studying the angular dependence of the current density $J$ gives insight into vortex pinning. We investigated a coated conductor of YBa$_{2}$Cu$_{3}$O$_{\sim 7}$ containing $c$-axis correlated defects (stacks of BaZrO$_{3}$ particles), striated into six strips to give a high aspect ratio. The current density was determined inductively from the magnetic moment $m\sim J$, using a SQUID magnetometer. The sample was mounted on a horizontal rotating platform to vary the angle $\theta $ of the sample with respect to the vertical applied magnetic field. The magnetometer allows us to measure both the longitudinal and transverse components of moment \textbf{m}($\theta )$, enabling calculation of the angular dependence of $J$. For a large range of applied magnetic fields at various temperatures, we found a pronounced peak in $J(\theta )$ at an angular value ($\theta _{peak})$ near the $c$-axis. We observed that, for a large range of applied magnetic fields, $\theta _{peak}$ linearly scales with 1/$h$, where $h$ = ($H$/$H_{c2})$ is the reduced field. Research sponsored by DOE, Division of Materials Sciences and Engineering, and Office of Electricity Delivery and Energy Reliability. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X34.00006: Vortex States in Intrinsic Josephson Junctions of B$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ in High Parallel Magnetic Fields Jovan Mirkovic, Saito Takashi, Yuimaru Kubo, Itsuhiro Kakeya, Ahmed Oral, Takashi Yamamoto, Kazuo Kadowaki The $I_{c}$-axis resistivity measurements were performed in the vicinity of the \textit{ab}-plane in order to investigate the interaction between Josephson vortices (JVs) and pancake vortices (PVs) in B$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ mesoscopic single crystals. It was found that the vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region. The magnetic field dependence of the resistance exhibits the non-monotonic behavior probing the different vortex phases in tilted magnetic fields. Sharp dips and steps in the $c$-axis resistance were observed accompanied by penetration of quantized pancake vortices by tilting external fields from the \textit{ab}-plane. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X34.00007: Hole doping-induced evolution of self-organized bulk vortex structure in the high temperature YBa$_2$Cu$_3$O$_{7-\delta}$ superconductor Ahmad Mansour, Rongchao Ma, Mehmet Egilmez, Mohamed Saber, Isaac Fan, Kim Chow, Jan Jung We present systematic studies of the persistent current relaxation of 24 different oxygen contents of YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) as a function of oxygen defect concentration $\delta$. These studies allowed us to map out the ($\delta$,$\mu$) phase diagram of the vortex matter, where the exponent $\mu$ of the vortex structure was obtained using the scaling properties of the energy barrier against vortex motion. The reduction of the hole-doping level (an increase of $\delta$) of the material leads to a transformation of the vortex lattice into a glass and subsequently into a liquid phase. These vortex phases self-organize and produce relaxation plateaus in regions between step-like changes in the dependence of relaxation kinetics on hole doping, revealing the existence of a previously unknown correlation between the vortex structure and the hole-doping level in a cuprate superconductor, such as YBCO. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X34.00008: Temperature Dependence of Flux Pinning Properties for Dilute Impurity Doped Y123 Single Crystals Yui Ishii Most of studies on pinning properties and relating vortex nature of the RE123 system were performed at high temperatures around 77 K, while extensive applications at low temperatures are also expected. However, both pinning properties and vortex states at low temperatures have not been well understood for RE123 crystals having intentionally introduced pinning sites. On the other hand, we have developed the chemical and versatile technique to introduce effective pinning site in the cuprates, that is dilute impurity doping for the target cation sites. In addition, compositional fluctuations of light rare earth (LRE) elements, which induce disorder transition of vortex system, are known to contribute enhanced pinning properties of LRE123. In this paper, we report that the changes of vortex state and $J_{c}$ characteristics of RE123 single crystals in $H$ // $c$ at low temperatures by precise control of cation compositions. Our result suggested that dilute impurity doping to Cu (in CuO-chain) or Ba is more effective to enhance pinning strength of Y123 than introduction of compositional fluctuation for Gd123 single crystals at low temperatures below 70 K. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X34.00009: Hysteresis Effects in Ag-Doped Superconducting Y-Ba-Cu-O Atilgan Altinkok, Kivilcim Kilic, Atilla Kilic, Murat Olutas, Hakan Yetis Time and hysteresis effects have been studied by magneto-voltage ($V-H$ curves) measurements in Ag doped sample of YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO/Ag) as functions of transport current ($I)$, sweep rate of external magnetic field (d$H$/d$t)$ and temperature. Ag was added in the amount of 3{\%} of nominal composition of Cu in YBCO. It was observed that the dissipation in V-H curves does not change as dH/dt increases. This suggests that Ag doping destroys the weak-link structure along inter-grain boundaries and thus the vortices can find enough time to move in the sample irrespective of varying of external H. .The hysteresis effects in V-H curves ride on a background voltage at the temperatures near the $T_{c}$. In one hand, the background voltage of V-H curves decreases by taking low values as the temperature decreases, in the other hand, the hysteresis effects become more significant. It was observed that the evolution of V-H curves depends also on the magnitude of transport current. The increase in $I$ causes a considerable enhancement in background voltage in V-H curves. Similar measurements were repeated for YBCO sample without Ag for a comparison. Experimental observations between YBCO/Ag and YBCO establish that adding of Ag into the superconducting matrix causes the formation of easy metallic flow paths for vortices and thus easy distribution of vortices along grain boundaries. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X34.00010: Effect of pseudo-gap state to the vortices in the high-Tc cuprate superconductors Masaru Kato, Satoshi Tomita The scanning tunneling spectroscopy (STS) experiments for high-Tc cuprate superconductors(SC) show the local density of states (LDOS) around a single vortex is different from that in a purely d-wave superconductor (dSC). Theoretically, in the dSC, there is a quasi-particle bound states peak around the vortex core, which is similar to that for an s-wave superconductor. This discrepancy means the superconductivity in high-Tc superconductors is simple dSC. In the high-Tc SC, dSC always coexists with unknown pseudo-gap state. Recent angle-resolved photoemission spectroscopy experiments show the energy gaps of pseudo-gap states and d-wave superconductivity is different. In this study, we show that the quasi-particle structure is reproducible theoretically, if we take into account the effect of the pseudo-gap state. We consider d-spin density wave ( dSDW) state as the pseudo-gap state. We derived the Bogoliubov-de Gennes equation for the coexistence state with dSC and dSDW. And solving it numerically, we obtained the LDOS, which reproduce the experimental LDOS, qualitatively. This result shows that the pseudo-gap state affect the dSC, especially the bound state in the vortex core, in high-Tc SC's. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X34.00011: Magnetic field dependence of quasiparticle recombination times in superconducting NbTiN at low laser fluence Xiaoxiang Xi, Jungseek Hwang, David Reitze, Christopher Stanton, David Tanner, Lawrence Carr We use a pump-probe technique to study the temperature and field dependent quasiparticle recombination process in superconducting Nb$_{0.5}$Ti$_{0.5}$N in the low laser fluence regime. The sample was excited by a picosecond near-infrared Ti:Sapphire laser pulses and probed with subnanosecond infrared synchrotron pulses at the National Synchrotron Light Source, Brookhaven National Laboratory. Time-resolved photoinduced transmission data were obtained as a function of temperature (2.2-10 K) and magnetic field (0-10 T), from which we calculated the quasiparticle effective lifetime. Both temperature and field dependence were observed, with the effective lifetime increasing as the field became stronger. A model of this phenomenon will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X34.00012: Spin and charge dynamics of photogenerated quasiparticles in superconducting NbTiN Jungseek Hwang, Xiaoxiang Xi, David Reitze, Christopher Stanton, David Tanner, Larry Carr We use time-resolved broadband far-infrared spectroscopy to study the dynamics of photogenerated quasiparticles in superconducting Nb$_{0.5}$Ti$_{0.5}$N. The pump-probe measurements used picosecond pulses from a near-infrared Ti:sapphire laser as a pump and subnanosecond pulses of infrared synchrotron radiation as the probe. Measurements were done as a function of fluence and applied magnetic field. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory.; We report the photoinduced far-infrared transmission at 2 K for two different field orientations: perpendicular and parallel to the Nb$_{0.5} $Ti$_{0.5}$N film. The decay does not follow a simple exponential. We observed a linear scaling behavior between the number of photoexcited quasiparcles and the relaxation rate. In addition, the relaxation rate decreases linearly with magnetic field, attributed to spin polarization of the quasiparticles in the field. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X34.00013: Quantum Oscillations from Fermi Arcs Tamar Pereg-Barnea, Gil Refael, Marcel Franz, Heidi Weber, Babak Seradjeh Recent experiments[1] in a variety of High Tc superconductors revel 1/B oscillations in the vortex-liquid state. The period of oscillations in underdoped samples is short and can be translated, via the Onsager relation to an area in k-space which makes up a few percents of the Brillouin zone. Quantum oscillations are usually thought of as arising from closed orbits in momentum space along the Fermi surface and are used to measure the Fermi vector. Thus, the observation of quantum oscillations in the cuprates seems to be at odds with the observation of Fermi arcs in ARPES experiments[2] due to their fragmented Fermi surface topology. In this talk we show that quantum oscillations can arise from a partially gapped Fermi surface. We adopt a phenomenological model of arcs which terminate at a regime with a superconducting gap of d-wave symmetry to describe the pseudo gap phase. Without invoking any additional order, quantization of energy is found well below the gap maximum. Semiclassically the quantization condition arises from closed orbits in real-space. When translated to momentum space, the area enclosed by the orbits is much smaller than that of the full Fermi surface. [1]N. Doiron-Leyaraud et al. nature 447, 565 (2007) [2]Kanigel et al. Nature Physics 2 447 (2006) [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X34.00014: Giant Thermomagnetic Effects in High-T$_c$ Cuprates: Fermi Liquid vs Vortex Liquid Andrei Sergeev, Michael Reizer, Vladimir Mitin We developed a gauge-invariant formulation of the thermomagnetic effects [1] and prove that the thermomagnetic coefficients in the Fermi liquid with particle and hole excitations are always proportional to the square of the particle-hole asymmetry. Therefore, thermomagnetic effects in the Fermi liquid are always small. Vortices are topological excitations which are completely different from particle-hole excitations. Thermomagnetic transport in the vortex liquid is widely described in terms of the ``transport entropy,'' S$_d$. According to the current views, the main contribution to S$_d$ originates from the electromagnetic energy of superconducting currents circulating around vortex cores. However, this concept strongly contradicts to the London postulate [2]. We revise the theory and show that the transport entropy is, in fact, the entropy of normal electrons in vortex cores determined with respect to the superconducting (or partly superconducting, if cores overlap) background [2]. Only in this form the theory becomes simultaneously consistent with the London postulate and Onsager principle. The theory naturally explains the observed temperature dependence of S$_d$. 1. A. Sergeev et al., Phys. Rev. B 77 064501 (2008). 2. A. Sergeev et al., arXiv:0807.0450. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X34.00015: Absence of superfluid density anomaly at 0.6 K in superconducting PrOs$_4$Sb$_{12}$ D. E. MacLaughlin, Lei Shu, A. D. Hillier, Y. Aoki, D. Kikuchi, H. Sato, Y. Tunashima, H. Sugawara, T. A. Sayles, M. B. Maple The lower critical field $H_{c1}(T)$ in PrOs$_{4}$Sb$_{12}$ exhibits an enhancement below $T\mbox{*} = 0.6$ K [1], suggesting a transition between two superconducting phases. Small anomalies are observed at $T\mbox{*}$ in some other properties but not in the specific heat. We have carried out muon spin rotation experiments in the vortex state for fields just above $H_{c1}$. The muon spin relaxation rate, which is proportional to the rms width~$\delta B_{\rm rms}$ of the vortex-state field distribution, also shows no anomaly at $T\mbox{*}$. In a simple picture both $H_{c1}$ and $\delta B_{\rm rms}$ are proportional to the superfluid density~$\rho_s$, i.e., $\delta B_{\rm rms} \propto H_{c1}$ contrary to observation. Our results suggest that the $H_{c1}$ anomaly is due to flux pinning effects rather than a thermodynamic phase transition.\\ \frenchspacing [1] T. Cichorek et al., Phys. Rev. Lett. {\bf94}, 107002 (2005). [Preview Abstract] |
Session X35: Focus Session: Iron Pnictides and Other Novel Superconductors XV: Electronic Structure and Magnetism
Sponsoring Units: DMPChair: David Parker, Naval Research Laboratory
Room: 405
Thursday, March 19, 2009 2:30PM - 3:06PM |
X35.00001: Electronic Structure of Fe-based Superconductors Invited Speaker: Understanding the electronic structure and electronic interactions in the layered Fe superconductors is prerequisite to understanding their superconductivity and other properties. The purpose of this talk is to overview results obtained within band structure approaches in relation to experiment. So far, many puzzles remain. The materials appear to be much more band-like and show much stronger signatures of metallic (Fermi surface related) physics than cuprates, with correspondinly weaker signatures of on-site Hubbard correlations. However, there remain substantial discrepancies between bare band structure calculations and experiment, and interestingly these discrepancies are in the opposite direction from those found in cuprates. These are discussed in the context of spin-fluctuations. This work was done in collaboration with I.I. Mazin, M.H. Du, Lijun Zhang, Alaska Subedi and Michelle Johannes. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X35.00002: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X35.00003: Theory for lattice collapse and frustrated magnetism in FeAs superconductors Rafael Fernandes, Joerg Schmalian We present a theory for the pressure and temperature dependence of the magnetic and structural phase transitions in FeAs superconductors. Magnetic frustration in the FeAs planes leads to an enhanced coupling between lattice and magnetic degrees of freedom and is responsible for the strength of the first order transition from a paramagnetic tetragonal to an antiferromagnetic, orthorhombic phase. We analyze the phase diagram using a large $N$ expansion for the magnetic degrees of freedom coupled to the lattice. Furthermore, we also address the importance of the lattice collapse in the CaFe$_{2}$As$_{2}$ compound and compare our predictions with experiments. Our results demonstrate that it is crucial to simultaneously include lattice and magnetic degrees of freedom for the FeAs systems. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X35.00004: Formation and suppression of Fe magnetism in ferropnictides Igor Mazin, Michelle Johannes First, I will address the issue of how the Fe magnetic moment in HTC pnictides is formed in the DFT calculation, why is it so large (up to 2 $\mu_B$) and why the moments prefer to order in a stripe-like AFM manner. The role of the onsite Hund rule coupling in forming the momentum and the role of one-electron (band) energy in selecting an AFM pattern will be explained and emphasized. The important distinction between AFM interactions local in real space (superexchange, e.g. in cuprates) and local in momentum space (SDW or AFM in ferropnictides, which is {\it not} an SDW in spin-Peierls sense). This part will be largely based upon our preceeding talk. Next, I will present some speculations about possible solitonic fluctuations (dynamic AFM domain boundaries) in this system, and their relations to experiments. This pictures assumes that in the orthorhombic but nonmagnetic state the system consists of dynamic antiphase AFM domains, in the AFM state the domains are frozen (pinned), and in the nonmagnetic state dynamic twin domains dominate. This picture reconciles experiment, thory and first principle calculations in surprisingly many aspects. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X35.00005: First principles study of magnetic interactions and electronic structure in iron chalcogenide superconductors Myung Joon Han, Sergey Y. Savrasov By using first-principles density functional theory combined with linear response theory we investigate the magnetic interaction of the Fe chalcogenide high T$_C$ superconductors, FeSe$_{1-x}$Te$_x$. The calculated exchange interactions are found to be different from those in pnictides, which suggests possibly different superconducting properties. The nearest neighbor antiferromagnetic coupling ($J_{1a}$) is much stronger than the nearest neighbor ferromagnetic ($J_{1b}$) and the next nearest neighbor coupling ($J_2$). The $J_{1a}$ and $J_{2}$ gradually decreases as $x$ increases while $J_{1b}$ increases and becomes to be stronger than $J_{2}$. Total energy calculation results and the electronic structure will be presented and compared to recent experiments. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X35.00006: Microscopic origin of the structural and magnetic transitions in ferropnictide superconductor parent compounds Michelle Johannes, Igor Mazin, Devina Pillay The parent ferropnictide compounds exhibit two transitions: one is an orthorhombic distortion and the other is a magnetic transition. The transitions are simultaneous in the 122 structural type, but the structural transition precedes the magnetic one in the 1111 type. Although this temperature separation implies that the magnetism depends on the distortion, our computational results show that exactly the opposite is true. The structural distortion is fully dependent on the existence of magnetism and will not occur if a magnetic moment is not present. The particularities of the distortion, namely the expansion along the axis containing aligned spins, occurs as a result of minimizing the one-electron (band) energies. We show that the distortion depends not only on the existence of a magnetic moment, but on the particular ordering pattern chosen by the spins. Imposing a checkerboard ordering results in full x/y symmetry, while a so-called stripe ordering results in near perfect agreement with experimental neutron data below the transition temperature. Our results indicate that, even in the doped (superconducting) compounds, the underlying physics is magnetic. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X35.00007: Magnetic Fluctuation and Anisotropy in High-Tc Iron Pnictides Quan Yin, Myung Joon Han, Warren E. Pickett, Sergey Y. Savrasov Using first-principle density functional theory calculations combined with tight-binding method, dynamical mean field theory, and linear response theory, we extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange constants follow the universal trend of $J_{1}$/$2J_{2}\sim 1$, despite their extreme sensitivity to the z-position of As. This suggests magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast to cuprates. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X35.00008: Role of covalent Fe-As bonding in the magnetic moment formation and exchange mechanisms in iron-pnictide superconductors Kirill Belashchenko, Vladimir Antropov The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong covalent bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X35.00009: The magnetic interactions in iron pnictides Jiji Pulikkotil, Vladimir Antropov, Mark van Schilfgaarde Using static linear response theory we studied the pair wise magnetic interaction parameters in many typical families of iron pnictides. Parameters have been obtained for a wide range of volumes and distance between Fe and As atoms. We demonstrate that two nearest neighbor couplings in plane dominate, with a third and fourth nearest neighbor coupling responsible for the appearance of non-collinear ordering when the magnetic moment is small. We found that the ratio between first and a second neighbor coupling is not universal and greatly varies as a function of pressure or Fe-As distance. A small interplane coupling is found, and it varies by a factor of 10-20 among pnictides. We analyze the Neel temperatures, adiabatic spin wave spectrum and a nature of magneto-structural transitions in different classes of pnictides. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X35.00010: The magnetic phase diagram of iron pnictides German Samolyuk, Jiji Pulikkotil, Vladimir Antropov We study the stability of magnetic structures in iron pnictides as a function of doping, external pressure and the amount of defects. Several collinear and non-collinear magnetic structures are found to be stable in all classes of pnictides. This stability however is a result of a fragile competition between several nearest neighbor exchange couplings and depends greatly on doping. We determined that for a relatively small electron doping the non-magnetic instability is developed, while already for a small hole doping the stripe structure is instable in many pnictides and other magnetic structures are stabilized. For a larger hole doping the local magnetic moment phase with ferromagnetic long range order can be stabilized. A transition to non-collinear state at small moments is explained by a competition between the anisotropy of the nearest neighbors exchange couplings and third or forth neighbor couplings. Using very extensive calculations of magnetic stability parameter we build a generic pressure-concentration phase diagram of iron pnictides. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X35.00011: Magnetic excitations in iron pnictides Vladimir Antropov, Liqin Ke, Takao Kotani, Mark van Schilfgaarde We analyze the dynamical spin susceptibility $\chi({\bf q},\omega)$ in the iron pnictides: FeSe, CaFe2As2 and SrFe2As2 and obtain the spectra of spin excitations. In the longwavelength limit we obtain parameters for the adiabatic Heisenberg model and compare it with parameters generated by a static response method. Antiferromagnons are found for a small q, while for the larger q strong Stoner excitations are developed. These results support the claim that iron pnictides are marginally itinerant systems. We also estimate zero-point fluctuations from $\chi$ and find the following contributing mechanisms: adiabatic spin waves, hole-particle Stoner excitations and longitudinal fluctuations. Taking these effects into account improves the agreement between theory and experiment and indicate the importance of itinerant spin fluctuations. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X35.00012: Collinear (Bi-collinear) antiferromagnetic order in iron-pnictides (chalcogenides) Zhong-Yi Lu, Fengjie Ma, Tao Xiang By the first-principles electronic structure calculations, we find that the ground state of the tetragonal $\alpha$-FeTe is in a bi-collinear antiferromagnetic order, in which the Fe local moments ($\sim2.5\mu_B$) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe-Fe square lattice. This novel bi- collinear order results from the interplay among the nearest, the next nearest, and the next next nearest neighbor superexchange interactions, mediated by Te $5p$-band. In contrast, the ground state of the other iron pnictides or chalogenides is in a conventional collinear antiferromagnetic order, like LaFeAsO, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions, bridged by As atoms. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X35.00013: Effective Hamiltonian for FeAs based superconductors Efstratios Manousakis The Fe-pnictide superconductors exhibit unusual properties attributed to electrons and holes occupying the Fe $d$-orbitals and the outermost occupied $s$ and $p$ pnictide orbitals. Starting from the atomic limit, we carry out a strong coupling expansion for the FeAs layer, where the on-site Coulomb repulsion parameters are assumed to be significantly larger than the hopping between Fe $d$ orbitals and the hybridization parameters between the Fe $d$ and As $4s$ or $4p$ orbitals; we derive an effective Hamiltonian that describes the low energy electron/hole behavior. If this condition for strong coupling expansion is not satisfied, still, we believe that our qualitative results capture important aspects of the physics in these materials. The hopping and the hybridization parameters are obtained by fitting the results of our calculations based on the local density approximation to a tight-binding model. The effective Hamiltonian, in the strong coupling limit, consists of three parts which operate on three sub-spaces coupled through Hund's rule and spanned by the following Fe orbitals: (a) the $d_{x^{2}-y^{2}}$; (b) the degenerate orbitals $d_{xz}$ and $d_{yz}$; and (c) the $d_{xy}$ and $d_{z^{2}}$. Each of these parts is an extended $t-t^{\prime}-J-J^{\prime}$ model and is characterized by different coupling constants and filling factors. For the undoped material the second subspace alone prefers a ground state characterized by a spin-density-wave order similar to that observed in recent experimental studies, while the other two subspaces prefer $(\pi,\pi)$ antiferromagnetic order. The observed spin-density-wave order is imposed by the $d_{xz}/d_{yz}$ subspace as the ground state of the total Hamiltonian of the undoped parent compounds. However, due to the above mentioned frustration the magnetic moment is small in agreement with observation. Our calculation illustrates in a simple manner the reason for the difference in the magnetic ordering between the Fe-pnictides and the cuprates. It also suggests a different evolution of the magnetic order upon electron versus hole doping. [Preview Abstract] |
Session X36: Glassy, Amorphous, and Quasicrystalline Materials
Sponsoring Units: DCMPRoom: 408
Thursday, March 19, 2009 2:30PM - 2:42PM |
X36.00001: Using First-Principles Calculations to Describe Amorphous Metal Films for Hydrogen Purification Shiqiang Hao, Mike Widom, David Sholl The increasing demand for clean and efficient energy has resulted in an increased global willingness to embrace the proposed hydrogen economy. The use of amorphous metal films as membranes to purify hydrogen has potential to overcome at least some of the disadvantages of existing crystalline metal membranes. We introduce a general strategy combining density functional theory and statistical mechanics to quantitatively predict solubility, diffusivity and permeability of interstitial H in amorphous metals. Our methods make it possible for the first time to quantitatively evaluate the performance of amorphous metal films as hydrogen purification membranes. These methods are introduced by examining amorphous Fe3B and a crystalline analogue with the same composition. A membrane made from the amorphous material is predicted to have a hydrogen permeability 1.5-2 orders of magnitude higher than a crystalline membrane. The methods we introduce here will be useful in accelerating the development of amorphous membranes for practical applications. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X36.00002: Atomic structure of PdNiP bulk metallic glass from ab initio simulations Vijay Kumar, T. Fujita, M.W. Chen, A. Inoue, Y. Kawazoe The atomic structure of Pd40Ni40P20 bulk metallic glass (BMG) has been simulated using \textit{ab initio} molecular dynamics plane wave method and PAW pseudopotentials. We use generalized gradient approximation to calculate the exchange-correlation energy and a cubic simulation box whose size and shape have been optimized after the BMG has been formed in simulations. The resulting radial distribution function and density agree remarkably well with the experimental data. The structure is analysed in terms of local clusters centered around Pd, Ni and P atoms and their electronic structures have been used to understand the bonding, stability, and the formation of the PdNiP BMG. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X36.00003: Role of Electronic Structure on Ductility of Iron-Based Bulk Metallic Glasses N. Kvaltine, X.J. Gu, S.J. Poon, G.J. Shiflet, M. Widom Composition effects on the mechanical properties of iron-based amorphous steel alloys have been investigated, with attention to the metalloid content and the relative impact of boron, carbon and phosphorous. Phosphorous-containing amorphous steels exhibited enhanced plastic strains and fracture strengths. Moreover, the plastic strain increased with the decrease in shear modulus. The shear moduli are appreciably lower than those reported for previous amorphous steel compositions that did not contain phosphorus, and the Poisson's ratios obtained are correspondingly high. The ductility of amorphous steels can be improved by chemically tuning the elastic properties which are determined by the amorphous structure and chemical bonding. First-principles electronic structure calculations show that ductility can be improved by partially replacing elements such as boron and carbon that create ionic and covalent bonds with other elements such as phosphorous that favor metallic cohesion. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X36.00004: Bond enthalpy trends of high metalloid Fe-based bulk metallic glasses M. Widom, B. Sauerwine, N. Kvaltine, X.J. Gu, S.J. Poon, G.J. Shiflet Chemical bond types in metal-metalloid glass-forming compounds range from metallic to ionic and covalent. Iron-rich alloys are dominated by metallic bonding, but charge transfer from iron atoms to highly electronegative metalloid elements such as boron, carbon and phosphorous creates ionicity, and the metalloid bonding is intrinsically covalent. For sufficiently large metalloid content their strong bonding character must increase the shear modulus, leading to an associated increase in brittleness of the material. However, for metalloid fractions below 30\%, shear modulus is found to decrease with increasing metalloid content, leading to an associated increase in ductility of the material. We show this unexpected decrease in shear modulus is caused by depletion of the charge density around the iron atoms, weaking the iron-iron bonds. Our calculations are based on Crystal Orbital Hamilton Populations (COHPs) for crystalline structures that locally approximate the amorphous structure. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X36.00005: Effect of excess electrons on hexagonal close-packed Mg and the model clusters for bulk metallic glasses Masae Takahashi, Mikio Fukuhara, Akihisa Inoue, Yoshiyuki Kawazoe Though empirical rules for a large glass forming ability (GFA) were proposed, the formation mechanism of the bulk metallic glasses (BMGs) and the main factors for the GFA have not been clearly elucidated. The advantages of Mg-based BMGs are the lightness and abundance of resources, and a wide supercooled liquid region with the very high thermal stability and extremely large GFA. In 1991, Inoue \textit{et al }developed glassy Mg--Cu--Y alloys with a maximum diameter of 4.0mm. We report here the effect of excess electrons on hexagonal close-packed Mg and the model clusters explained by an inflation process using density functional theory-based calculations, in order to understand the role of conduction electron concentration (CEC) in Mg-based BMGs [M. Takahashi et al,\textit{ J. Phys. D: Appl. Phys.,}\textbf{2008}, $41$, 155424]. The CEC of Mg increase in Mg-based BMGs. In our model calculations, the increased CEC is artificially realized by the injection of electrons into Mg clusters and hcp Mg. We find the volume expansion and distortion to a higher $c/a $ratio in the negative charge state. The increase in the values corresponding to the $c/a $ratio is also observed in the model clusters. In the density of states at the equilibrium cell parameters expanded by charging, the pseudogap near the Fermi level by $\sigma $--$\pi $ mixing becomes small and a spiky structure appears. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X36.00006: Toward Understanding the Giant Frictional Anisotropy on AlNiCo Keith McLaughlin, Heather Harper, David Rabson In a 2005 article in Science [{\bf309}, 1354], Park et al. measured in vacuum the friction between a coated atomic-force-microscope tip and the clean two-fold surface of an AlNiCo quasicrystal. Because the two-fold surface is periodic in one direction and aperiodic (with a quasiperiodicity related to the Fibonacci sequence) in the perpendicular direction, frictional anisotropy is not unexpected; however, the magnitude of that anisotropy in the Park experiment, a factor of 8, is unprecedented. By eliminating chemistry as a variable, the experiment also demonstrated that the low friction of quasicrystals must be tied in some way to their quasiperiodicity. Through molecular-dynamics simulations with pair potentials on quasiperiodic approximants, we investigate generic geometric mechanisms that might give rise to this anisotropy. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X36.00007: Does the morphology of fracture surfaces reveal the structure of quasicrystals? Luc Barbier, Daniel Bonamy, Laurent Ponson The roughness of surfaces obtained by cleavage of i-AlPdMn quasicrystals at room temperature are analyzed using tools of quantitative fractography. From the atomic scale up to 3 nm, they are shown to exhibit scale invariance properties hiding the cluster (0.45 nm) aperiodic structure. These properties are quantitatively similar to those observed on various disordered materials, albeit on other ranges of length scales. These properties are interpreted as the signature of damage mechanisms occurring within a 3 nm wide zone at the crack tip. The size of this process zone finds its origin in the local temperature elevation at the crack tip. This effect is reported to be responsible for a transition from a perfectly brittle behavior to a nanoductile one. It explains also why the cluster structure of quasicrystals is not revealed on the fracture surfaces of i-AlPdMn broken at room temperature. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X36.00008: Quasicrystal Growth and Thermal Expansion Sally June Tracy, Jason Cooley, Heather Voltz, Jason Lashley We have grown Al-Mn-Pd and RE-MG-Zn quasicrystals (RE=Y, Er, Ho, Dy and Tb) from a high temperature metallic solution using a self-flux method with melt compositions presented in previous work by Canfield and Fisher.\footnote{High temperature solution growth of intermetallic single crystals and quasicrystals; Canfield, P.C. ; Fisher, I.R., Journal of Crystal Growth (May 2001) Vol. 255, is 2-4, p. 155-161} The samples showed dodecahedral grains with pentagonal facets. The icosahedral structure was revealed with x-ray powder diffraction. We were able to index the diffraction patterns using Cahn's two index scheme.\footnote{Indexing of icosahedral quasiperiodic crystals; Cahn, J.W. ; Shechtman, D. ; Gratias, D., Journal of Materials Research (Jan.-Feb.) vol. 1, no. 1, p. 13-26} We have measured the thermal expansion of these samples and will present this data. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X36.00009: Jamming is not just isotropic anymore E. Wandersman, Y. Chushkin, A. Robert, E. Dubois, V. Dupuis, R. Perzynski Slow dynamics observed in many disordered systems (colloidal glasses, jammed granular matter{\ldots}) are poorly understood. An approach could consist to discriminate the dynamical properties of such systems by the nature of the interaction potential (attractive/repulsive, isotropic/anisotropic). While the anisotropy of the potential is relevant for the rotational dynamics, its effect on the translational dynamics in glasses is quite absent of current understanding. We investigate here the effect of the interaction potential on the translational dynamics, in a magnetic colloidal glass (charge--stabilized magnetic nanoparticles). By applying a magnetic field H, the potential is tuned, from quasi-isotropic to anisotropic, but remains repulsive on average. The translational dynamics of the nanoparticles is probed (with/without field) using dynamical X-ray scattering [1]. Under field, anisotropic translational dynamics and aging are observed. Moreover, a strong anisotropic cooperativity is reported, almost hundred times larger in the parallel direction. The results are discussed using a phenomenological picture. ~ [1] E. Wandersman et. al., J. Phys. Cond. Mat. 20 (2007) 155104 [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X36.00010: Gradient interactions and the low temperature universality in glasses Moshe Schechter, Philip Stamp Amorphous solids show striking universal characteristics at low temperatures, including unique temperature dependencies of the specific heat, thermal conductivity and internal friction, and a small and rather constant ratio between a phonon wavelength and its mean free path. These universal phenomena are observed in polymers and disordered lattices as well. The standard tunneling model proposed by Anderson, Halperin and Varma, and Philips, accounts well for much of the observed phenomena. However, questions regarding the nature of the two-level systems, the smallness of the above ratio, and the energy scale dictating the temperature range of the phenomena, remain unanswered. We propose a theory that suggests an answer to these questions, along with an explanation of the additional observed phenomena connected with the universal behavior. Our theory is rigorously derived for disordered lattices, and we argue for the plausibility of its applicability to amorphous solids. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X36.00011: Moment based approach to electronic structure calculations: applications to ordered and disordered systems Hiro Shimoyama, Parthapratim Biswas We solve the classical moment problem via maximum entropy optimization to calculate the electronic density of states for ordered and disordered solids. The method employs the Shannon entropy functional and maximize it subject to the moment constraints to construct the spectral distribution of large Hamilton matrix. We illustrate the efficiency and usefulness of the method by applying three candidate systems: a crystalline semiconductor; an amorphous material; and a completely disordered system via tight-binding Anderson model Hamiltonian. The band energy and Fermi level are computed from the reconstructed density of states with a high degree of precision. A possible extension of this method to calculate electronic forces is also discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X36.00012: Reciprocal-Space Approach of Highly Inhomogeneous Systems Volodymyr Bugaev, Alexander Udyansky, Thomas Demmer, Alejandro Diaz Ortiz, Peter Wochner, Helmut Dosch A reciprocal-space approach for the calculation of spatial correlations in highly inhomogeneous systems is presented. Our method is based on the correlation-correction algorithm\footnote{V.N.\ Bugaev, A.\ Udyansky, O.\ Shchyglo, H.Reichert, and H.\ Dosch, Correlation correction algorithm for disordered binary systems, Phys.\ Rev.\ B {\bf 74}, 024202 (2006)} and can also be used in the calculation of thermodynamic potentials of metallic and colloidal glasses. The interaction parameters are calculated in an iterative $k$-space (refinement) procedure that is amenable for both first-principles and semi-empirical calculations. We illustrate our approach on amorphous hard-sphere and Lennard-Jones systems. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X36.00013: What is a dynamical glass transition? Claudio Chamon, Claudio Castelnovo, David Sherrington Using the mapping between the Fokker-Planck description of classical stochastic dynamics into a quantum Hamiltonian, we argue that a dynamical glass transition must have a precise definition in terms of a quantum phase transition. At the static level, the transition affects the ground state wavefunction: while in some cases it could be picked up by the expectation value of a local operator, in others the order may be non-local, and impossible to be determined with any local probe. In general, even in the absence of a local order parameter, the transition can be detected via the quantum fidelity of the groundstate wavefunction, which we show translates directly into a singularity in the heat capacity of the classical system. We illustrate these ideas using exact diagonalizations of the mapped Hamiltonians for the p-spin models and the gonihedric model. [Preview Abstract] |
Session X37: Spectroscopy and Dynamics of Single Molecules and Nanoparticles
Sponsoring Units: DCPChair: Will Castleman, Pennsylvania State University
Room: 409
Thursday, March 19, 2009 2:30PM - 2:42PM |
X37.00001: Reactivity of Aluminum and Doped Aluminum Clusters Arthur Reber, Shiv Khanna, Patrick Roach, William Woodward, A. Welford Castleman Jr. We examine the reactivity of aluminum cluster anions with water, and other nucleophiles. The clusters reveal size dependent reactivity which primarily results in either the chemisorption of one or more water molecules, or no observable reactivity. The reactivity of the clusters is found to be dependent on surface sites which facilitate the splitting of the water on the surface of the cluster. Clusters with two sets of active sites are found to selectively release molecular Hydrogen. We also investigated the reactivity of aluminum clusters doped with another metal and their reactivity with molecular Oxygen. As the reactivity of aluminum clusters with oxygen depends on the electronic shell closing, this serves as a probe of the effects of doping on the electronic shell structure. The results reveal variable effects of doping on the electronic structure depending on the precise geometry and electron count. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X37.00002: Multiple Isomers in the Photoelectron Spectra of NbC$_{n}$ clusters Ivan Iordanov, Jorge Sofo We calculate the photoelectron spectrum of small NbC$_{n}$ clusters (2$<$n$<$7), to identify the atomic structure that best matches experimental photoelectron spectra. We use Density Functional Theory calculations to find all stable isomers. In order to obtain more accurate spectra, we use the Symmetry Adapted Cluster Configuration Interaction method for the smaller clusters where the highly correlated niobium d-orbitals dominate the electronic structure. The most stable isomer configurations are linear and cyclic structures, with the cyclic being the ground state for all but NbC$_{6}$. To fully explain all experimental observations we are required to use the combined spectra of both ring and linear structures. This means that both structures must be present in the cluster beam, even in cases where the higher energy isomer is up to 0.5 eV higher than the ground state. This surprising result is confirmed by both DFT and Configuration Interaction. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X37.00003: Photoelectron Velocity Map Imaging and Density-Functional Investigation of Bismuth and Lead Anions Solvated in Ammonia Kristen Casalenuovo, Mohamed A. Sobhy, J. Ulises Reveles, Ujjwal Gupta, Shiv N. Khanna, A.W. Castleman, Jr. We present the results of photoelectron velocity map imaging experiments for the photodetachment of small negatively charged Bi$_{n}$ and Pb$_{n}$ (n = 1-2) clusters solvated in ammonia using a Nd:YLF 527 nm laser. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters derived from the photoelectron images. Density-functional theory calculations with generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their molecular and electronic structures. Calculated ammonia binding energies and electronic charge transfers are used to rationalize the observed mass spectra distributions. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X37.00004: Dielectrophoresis and Dissociation of Micelles in AC-Electric Fields Victoria Froude, Yingxi Elaine Zhu Dielectrophoresis (DEP) of natural and synthetic colloids has been explored as a new route to rapidly manipulate and assemble colloidal particles in suspensions. Most work has been done with micron to submicron sized particles, yet AC-polarization and dielectrophoretic effects on molecules and nanocolloids remain little understood. In this work, we examine the dynamic responses of micelles to applied AC-electric fields by using fluorescence correlation spectroscopy (FCS) at a single-molecule resolution. We focus on the AC-field induced transport of sodium dodecyl sulfate (SDS) micelles tagged with various fluorescent probes and molecular encapsulates. Micelle concentration and DEP mobility of SDS micelles are examined over a broad range of AC-field frequency (1 KHz -10 MHz) and amplitude (100mV -20V). We observe a strong AC-frequency dependence of micelle concentration, from which the DEP crossover frequency switching between the positive and negative DEP response is determined. Surprisingly, we also observe the AC-field induced dissociation of the micelle structure and the resultant release of fluorescent encapsulates at a characteristic low AC-field frequency of 1-10 kHz; the dissociation frequency can be tuned by encapsulated molecules with a strong dependence of their surface conductivity, which could have a potential application for controlled drug release by AC-electric fields. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X37.00005: Raman Correlation Spectroscopy Maki Nishida, Edward Van Keuren We have developed a simple method for measurement of diffusion coefficients of specific components in complex mixtures of nanoparticles in a suspension. As a variation of Dynamic Light Scattering (DLS), this method analyzes temporal fluctuations of Raman scattered light from particles caused by Brownian motion. Due to the coherent nature of Raman scattering, the time autocorrelation functions of Raman emission lines will yield similar information as that obtained by DLS. Because each Raman emission line arises from a specific type of a molecular bond, only the diffusion coefficient of the particles containing that specific chemical species is measured. We demonstrate that this method can isolate diffusion coefficients from individual components in mutlicomponent nanoparticle dispersions. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X37.00006: Nanosphere Templating Through Controlled Evaporation: A High Throughput Method For Building SERS Substrates Kristen Alexander, Meredith Hampton, Rene Lopez, Joseph DeSimone When a pair of noble metal nanoparticles are brought close together, the plasmonic properties of the pair (known as a ``dimer'') give rise to intense electric field enhancements in the interstitial gap. These fields present a simple yet exquisitely sensitive system for performing single molecule surface-enhanced Raman spectroscopy (SM-SERS). Problems associated with current fabrication methods of SERS-active substrates include reproducibility issues, high cost of production and low throughput. In this study, we present a novel method for the high throughput fabrication of high quality SERS substrates. Using a polymer templating technique followed by the placement of thiolated nanoparticles through meniscus force deposition, we are able to fabricate large arrays of identical, uniformly spaced dimers in a quick, reproducible manner. Subsequent theoretical and experimental studies have confirmed the strong dependence of the SERS enhancement on both substrate geometry (e.g. dimer size, shape and gap size) and the polarization of the excitation source. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X37.00007: Resolving Single Molecule Dynamics with a Point-Functionalized Single-Walled Carbon Nanotube Danny W. Wan, Issa S. Moody, Brett R. Goldsmith, John G. Coroneus, Gregory A. Weiss, Philip G. Collins Outside of fluorescence measurements, there are currently few means of observing characteristic time constants of individual molecules. We describe the development of a single molecule technique utilizing a point-functionalized SWCNT electronic circuit [1]. Time-dependent components of the SWCNT conductance reveal real-time interactions between a covalently attached protein and the immediate electrolytic environment. We will demonstrate electronic transduction of protein-substrate interactions with single molecule resolution. On-line analysis based on normalization of the power spectrum helps to enhance the resulting signals, even to the extent of providing the user with real time feedback regarding the experiment status. [1] B. Goldsmith et al, Science 315 77 (2007) [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X37.00008: Auger Rate Quenching in Nanocrystals George E. Cragg, Xiaoyong Wang, Megan A. Hahn, Sara Maccagnano-Zacher, John Silcox, Todd D. Krauss, Alexander L. Efros Single nanocrystal (NC) photoluminescence (PL) blinking is believed to arise from a photoionization event, resulting in a charged NC state made dark by the dominance of the non-radiative Auger rate. Suppression of the Auger rate has been suggested as the underlying mechanism for the non-blinking PL observed in soft-confinement, single CdZnSe/ZnSe NCs. To probe the interplay between the confinement geometry and the PL, we employ a coupled, two-band NC model which is analyzed with numerical routines based on the propagation matrix formalism. The results obtained will verify whether smooth confining potentials mitigate the Auger process, thereby eliminating blinking by allowing NCs to photoluminesce even in their charged state. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X37.00009: Dynamical charge and structural strain in MoS2 and MnO nanoparticles Qi Sun, Xiaoshan Xu, Janice Musfeldt, Reshef Tenne, Alla Zak, Sheila Baker, Andrew Christianson We measured the far infrared vibrational properties of bulk and nanoscale MoS$_2$ and MnO in order to investigate finite length scale effects and chemical bonding in these materials. From an analysis of frequencies, oscillator strengths, and the high frequency dielectric constants, we extract Born and local effective charges for both materials. In the intralayer direction of MoS$_2$, we find that the Born effective charge of the nanoparticles is decreased significantly compared to the layered bulk, a result that we attribute to structural strain (and resulting change in polarizability). Preliminary results on the 7 nm MnO nanoparticles suggest that structural strain impacts both polarizability and depolarization field. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X37.00010: Ultrafast electronic energy redistribution in hollow gold nanoparticles. Kenneth Knappenberger, Adam Schwartzberg Nanostructured materials offer great potential for novel ways to generate, utilize, store and transport energy. These unique opportunities arise because nanoclusters often portray strikingly different chemical and physical properties than their bulk counterparts, and, perhaps more intriguingly, these vary widely with cluster size and shape. Here we report on the redistribution of electronic energy to thermal phonons in a series of hollow gold nanoparticles using femtosecond transient absorption. Qualitatively, the relaxation processes are similar to those of solid nanoparticles, however distinct differences are observed, likely owing to the unique properties of the hollow structures. In particular, a larger excitation power density is required to observe coherent vibrational oscillations in hollow gold nanoparticles than is needed for solid particles following electronic excitation. This effect is systematically studied over a range of hollow and solid particles, including multiple diameters and wall thicknesses. Models will be presented to account for the different relaxation mechanism observed for hollow and solid gold nanoparticles. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X37.00011: Optical spectra with spin-orbit effects on gold nanostructures Aldo Romero, Alberto Castro, Miguel Marques, Micael Oliveira, Angel Rubio The quest for more efficient optoelectronic devices requires a thorough understanding of the intrinsic properties of the metallic nanostructures such as the optical spectra. Many optoelectronic devices are based upon gold nanostructures but even though, there is a large set of experimental studies, little is known theoretically. Between the concerns, it is important to identify where the spin-orbit effect has influences on the optical spectra on those nanostructural materials. We report here the analysis of the effect of the spin-orbit interaction on the shape of the photoabsorption cross section of small gold clusters (Au$_n$ $n \le 8$ and $n=20$) and small nanowires ($n \le 7$). As it is shown, the spin-orbit coupling has a strong effect on the absortion spectra mainly for nanowires and much less effect on static properties such as the dipole static polarizability. This has strong implications on transport calculations where no spin-orbit effects are considered. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X37.00012: First-principles studies of surface-enhanced Raman scattering: Benzene thiol on Au Alexey Zayak, Jeffrey Neaton First-principles calculations based on density functional theory are used to investigate how chemisorption of organic molecules on metal surfaces affects their Raman spectra. Experiments have long reported Raman intensity enhancements of many orders of magnitude for molecules on rough metal surfaces or near nanofabricated metallic tips. The goal of this work is to explore ``chemical'' effects that may contribute to this enhancement, specifically hybridization and charge transfer between the molecule and its metallic substrate. We consider benzene thiol chemisorbed on extended Au(111) surfaces and finite Au and Ag clusters. Using a finite-difference scheme, we compute the absorption site, molecular orientation, and coverage dependence of Raman-active phonon modes and their intensities. We also examine how the electronic structure of the molecule is modified in each case, and discuss implications for the strength of Raman processes. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X37.00013: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X37.00014: Photoluminescence anti-blinking of single CdZnSe/ZnSe nanocrystals Xiaoyong Wang, Xiaofan Ren, Keith Kahen, Megan Hahn, Manju Rajeswaran, Sara Maccagnano-Zacher, John Silcox, George Cragg, Alexander Efros, Todd Krauss We have synthesized soft-confinement nanocrystals (NCs) of CdZnSe/ZnSe that on the single particle level exhibit complete elimination of photoluminescence (PL) blinking. These continuously emitting NCs have a very short PL decay lifetime of 5 ns. Moreover, single CdZnSe/ZnSe PL spectra are highly unusual with three distinct peaks. These unique and remarkable optical properties are collectively explained by the radiative recombination of a trion due to suppressed Auger processes in a NC. Upon deformation of the soft-confinement potential, the PL intensity of a single CdZnSe/ZnSe NC switches between two bright states, but still never turns off. Possible mechanisms for this PL anti-blinking behavior will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X37.00015: Single-Electron Spectroscopy of Quantum Dots using Vertically Self-aligned Electrode Structure. Ramkumar Subramanian, Pradeep Bhadrachalam, Vishva Ray, Seong Jin Koh We demonstrate single-electron tunneling spectroscopy of individual quantum dots using new vertical electrode structure, where the source and drain electrodes are vertically self-aligned and separated by a thin dielectric spacer. A quantum dot placed on the periphery between the source and the drain electrodes forms a double barrier tunnel junction, allowing for single-electron spectroscopy measurements. CMOS compatible fabrication allows many quantum dot units to be fabricated in parallel processing. This technique not only provides an accurate electronic structure of a ``single'' quantum dot, but such measurement can be made for many of individual quantum dot units fabricated in a single batch process. Thus, this simple procedure provides accurate energy level measurement of ``single'' quantum dots over the entire quantum dot population. The band gap (E$_{g})$, charging energy (E$_{c})$ and energy level spacing ($\Delta $E) were measured directly from the current-voltage and differential conductance spectra for colloidal CdSe quantum dots ($\sim $6.5nm). The band gap was measured to be E$_{g} \quad \sim $1.75-1.85eV, charging energy E$_{c} \quad \sim $60meV and the `s' to `p' level separation ($\Delta $E ) was measured to be $\sim $60-100meV. (Supported by NSF CAREER (ECS-0449958), ONR (N00014-05-1-0030), and THECB ARP (003656-0014-2006)). [Preview Abstract] |
Session X38: Focus Session: Ion Channel Physics and Chemical Physics II
Sponsoring Units: DCPChair: Thomas Beck, University of Cincinnati
Room: 410
Thursday, March 19, 2009 2:30PM - 3:06PM |
X38.00001: Dynamics of Kv2.1 channel cluster formation in mammalian neurons. Invited Speaker: Kv2.1 are potassium channels that play an important role in multiple organs and tissues. In particular, in mammalian neurons Kv2.1 channels have an enormous neuroprotective function attained by their ability to form large clusters on the surface of the neuronal cell body. The regulation of Kv2.1 channel clusters is a key factor in protecting the brain, particularly under sudden ischemic conditions such as those encountered in stroke. It is speculated that this kind of stimulus induces channel declustering in order to suppress neuronal hyperexcitability (i.e. seizures). However, the physical mechanism that forms and maintains Kv2.1 clusters has remained largely unknown. We are investigating the dynamics of channel clusters at the single molecule level using particle tracking with nanometer accuracy in live cells. Here, the cluster structure and individual channels are imaged simultaneously in a total internal reflection microscope. While most Kv2.1 channels in the cell are labeled with green fluorescent proteins (GFP), only a few individual channels are tagged with red quantum dots. This approach allows us to track single molecules and probe their interaction with the cluster perimeter. Different models for the molecular mechanism that localizes Kv2.1 clusters on the cell surface and the implications of our data will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X38.00002: Surfaces and boundaries in the mechanosensitive channel gating Invited Speaker: Mechanosensitive (MS) channels are gated by tension transmitted through the surrounding lipid bilayer. Inorganic ions or amphipathic modifiers that interact with the bilayer surface alter the packing of lipids and perturb the lateral pressure. We describe the effects of lanthanide ions, fluorinated alcohols and esters of parabenzoic acid as potent modifiers of MS channel gating. The other boundary that plays a critical role in channel gating is the water-vapor interface resulting from capillary dewetting of the hydrophobic gate. Molecular simulations predict two alternate positions for this boundary in the pore of the mechanosensitive channel MscS. We approached this problem experimentally by hydrophilizing the outer segment of the pore to resolve if it is `dry' in the closed state. We observed a reduction in activating tension, substantial changes in MscS kinetics and complete removal of gating hysteresis. The kinetic treatment of channel traces recorded in response to steps of tension suggested the sequence of events that leads to the channel opening implying that pore hydration and dewetting are the rate-limiting steps in MscS transitions. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X38.00003: Modeling Conformational Transitions and Energetics of Ligand Binding with the Glutamate Receptor Ligand Binding Domain Invited Speaker: Understanding of protein motion and energetics of conformational transitions is crucial to understanding protein function. The glutamate receptor ligand binding domain (GluR2 S1S2) is a two lobe protein, which binds ligand at the interface of two lobes and undergoes conformational transition. The cleft closure conformational transition of S1S2 has been implicated in gating of the ion channel formed by the transmembrane domain of the receptor. In this study we present a composite multi-faceted theoretical analysis of the detailed mechanism of this conformational transition based on rigid cluster decomposition of the protein structure [1] and identifying hydrogen bonds that are responsible for stabilizing the closed conformation [2]. Free energy of the protein reorganization upon ligand binding was calculated using combined Thermodynamic Integration (TI) and Umbrella Sampling (US) simulations [3]. Ligand -- protein interactions in the binding cleft were analyzed using Molecular Dynamics, continuum electrostatics and QM/MM models [4]. All model calculations compare well with corresponding experimental measurements. \\[4pt] [1] Protein Flexibility using Constraints from Molecular Dynamics Simulations \textit{T. Mamonova, B. Hespenheide, R. Straub, M. F. Thorpe, M. G. Kurnikova} , Phys. Biol., 2, S137 (2005)\\[0pt] [2] Theoretical Study of the Glutamate Receptor Ligand Binding Domain Flexibility and Conformational Reorganization \textit{T. Mamonova, K. Speranskiy, and M. Kurnikova }, Prot.: Struct., Func., Bioinf., 73,656 (2008)\\[0pt] [3] Energetics of the cleft closing transition and glutamate binding in the Glutamate Receptor ligand Binding Domain~\textit{T. Mamonova, M. Yonkunas, and M. Kurnikova~ }Biochemistry~$47$, 11077 (2008)\\[0pt] [4] On the Binding Determinants of the Glutamate Agonist with the Glutamate Receptor Ligand Binding Domain \textit{K. Speranskiy and M. Kurnikova} Biochemistry 44, 11208 (2005) [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X38.00004: ABSTRACT WITHDRAWN |
Session X39: Focus Session: Crystal Growth of and Moderated by Proteins
Sponsoring Units: DBPChair: Raymond Friddle, Lawrence Livermore National Laboratory
Room: 411
Thursday, March 19, 2009 2:30PM - 3:06PM |
X39.00001: Investigating protein controls on crystal growth: how competing timescales and electrostatic interactions lead to bi-stable and catalytic growth Invited Speaker: Structural relationships, chemical interactions, and mechanistic impacts of proteins at the surfaces of growing crystals are poorly understood, despite of their central role in directing formation of mineralized tissues. Here we describe results of in situ AFM investigations into the interactions of aspartic acid-rich peptides and proteins with single crystals of calcium oxalates and carbonates. Using specially designed cantilevers, we have obtained true single molecule resolution and directly imaged protein interactions with atomic steps. We show how the slow adsorption dynamics, strong electrostatic interactions and tendency towards clustering peculiar to macromolecules lead alternately to acceleration and inhibition, as well as switching of growth between two distinct states. We provide a mechanistic model for the observed behavior in terms of altered activation energies and competing timescales for macromolecule adsorption and solute attachment. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X39.00002: Temperature-dependence of global radiation damage to protein crystals Matthew Warkentin, Robert Thorne We have measured the global radiation sensitivity of model protein crystals as a function of temperature in the range 300 K to 100 K. Our data show three regimes, each characterized by a different activation energy for damage. Between 300 and 225 K, the activation energy is considerably less than would be expected if it was limited by the diffusion of radicals or (diffusive) conformational changes in the molecular structure. This suggests that some other mechanism is the bottleneck for radiation damage in this temperature range. From 225 to 160 K, the activation energy is consistent with radiation damage being diffusion limited. Below 160 K, the activation energy is very small, and is consistent with activation energies for damage of small biomolecules. These results shed light on the underlying mechanisms of radiation damage in protein crystallography, and also inform temperature-dependent studies of protein crystals about radiation damage at temperatures other than 100 K. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X39.00003: Directly probing the antifreeze glycoprotein kinetics at the ice/solution interface Salvador Zepeda, Etsuro Yokoyama, Yoshinor Furukawa Antifreeze proteins (AFP) and glycoproteins (AFGP) help fish, plants, insects and bacteria survive sub-freezing environments. It is well known that these proteins function via some surface interaction, but the exact mechanism has eluded scientists. Aside from mutagenesis experiments directed towards examining the functional importance of specific residues, conclusions about the mechanism have been drawn from indirect studies or more precisely from studies that describe the proteins effects on the ice interface. Our work is aimed at directly studying the protein kinetics at the ice/solution interface. Fluorescent microscopy is used to determine interaction planes, surface concentrations as well as adsorption characteristics and the segregation constants, while fourier transform infra-red attenuated total reßectance (FTIR-ATR) is used to determine the protein structure vs. temperature in the liquid and solid states as well as the ice interface characteristics. All data show that AFGP do not function by the characteristic Gibbs-Thomson mechanism. While the surface coverage is similar for the AFPIII, segregation (amount in ice/amount in solution) is non-zero. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X39.00004: Ice-binding protein investigation using microfluidic devices Yeliz Celik, Natalya Pertaya, Christophere P. Garnham, Peter L. Davies, Ido Braslavsky Ice-binding proteins (IBPs) inhibit ice crystal growth and recrystallization. We have developed a novel microfluidic device capable of precise local temperature control in order to grow single ice crystals. This device allows us to expose an ice crystal to an adjustable IBP concentration. We have used this device in conjunction with fluorescence microscopy to examine the affinities hyperactive IBPs have to specific ice planes in comparison with those of a moderately active fish IBP. We also demonstrate that hyperactive IBPs bind irreversibly to ice surfaces. The direct visualization of IBPs on ice using the microfluidic devices reveal the kinetics of attachment of these proteins to ice surfaces, as well as their concentration effects and facet preferences. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X39.00005: Biomineralization of a Self-Assembled Extracellular Matrix for Bone Engineering Yizhi Meng, Yi-Xian Qin, Nadine Pernodet, Xiaolan Ba, Miriam Rafailovich, Elaine DiMasi The mineralization of extracellular matrix (ECM) proteins is an important process in need of new experimental approaches. We present a study of two subclones of MC3T3-E1 osteoblast-like cells, one which mineralizes and one which does not. Using atomic force microscopy we measure the ECM protein fiber morphology and the elastic modulus, which changes as biomineralization proceeds. The non-mineralizing subclone undergoes less remodeling of the ECM over the same development period, compared to the mineralizing subclone. By using synchrotron grazing-incidence x-ray diffraction along with optical and electron microscopy, the development of hydroxyapatite crystals is followed. Cells are shown to mineralize only when an adequately structured ECM is present. Confocal light microscopy indicates that actin restructuring is correlated with mineralization. Correct and complete development of the ECM network, which can be interrupted either by using the non-mineralizing cell line or by culturing cells on an inhospitable substrate, is necessary for osteoblasts to mineralize. We discuss implications for bone biomineralization and for the development of implant materials. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X39.00006: Peptide Probe for Crystalline Hydroxyapatite: In Situ Detection of Biomineralization Marcus Cicerone, Matthew Becker, Carl Simon, Kaushik Chatterjee While cells template mineralization in vitro and in vivo, specific detection strategies that impart chemical and structural information on this process have proven elusive. Recently we have developed an in situ based peptide probe via phage display methods that is specific to crystalline hydroxyapatite (HA). We are using this in fluorescence based assays to characterize mineralization. One application being explored is the screening of tissue engineering scaffolds for their ability to support osteogenesis. Specifically, osteoblasts are being cultured in hydrogel scaffolds possessing property gradients to provide a test bed for the HA peptide probe. Hydrogel properties that support osteogenesis and HA deposition will be identified using the probe to demonstrate its utility in optimizing design of tissue scaffolds. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X39.00007: Superoxide dismutase activity of Cu-bound prion protein Miroslav Hodak, Wenchang Lu, Jerry Bernholc Misfolding of the prion protein, PrP, has been linked to a group of neurodegenerative diseases, including the mad cow disease in cattle and the Creutzfeldt-Jakob disease in humans. The normal function of PrP is still unknown, but it was found that the PrP can efficiently bind Cu(II) ions. Early experiments suggested that Cu-PrP complex possesses significant superoxide dismutase (SOD) activity, but later experiments failed to confirm it and at present this issue remains unresolved. Using a recently developed hybrid DFT/DFT method, which combines Kohn-Sham DFT for the solute and its first solvation shells with orbital-free DFT for the remainder of the solvent, we have investigated SOD activity of PrP. The PrP is capable of incorporating Cu(II) ions in several binding modes and our calculations find that each mode has a different SOD activity. The highest activity found is comparable to those of well-known SOD proteins, suggesting that the conflicting experimental results may be due to different bindings of Cu(II) in those experiments. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X39.00008: Mechanisms of function by AF(G)Ps in ice crystal growth prevention, modification and recrystallization Invited Speaker: Antifreeze activity by proteins and glycoproteins is a form of biomineralization process. Contrasting to covalent or ionic crystals, these hydrogen-bonded molecular crystals (ice) have much weaker crystalline bonding energy profiles. Thus, when interacting with proteins that are equally easy to change in conformation by variations of the hydrophilic/hydrophobic environment, some unique findings are suggestive that local conditions play significant roles in the activity level of these proteins or glycoproteins as they affect ice crystal growth, modification and recrystallization. We review some of these experiments and provide ideas for their functioning mechanisms. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X39.00009: Atomistic Investigation of Cu-Induced Misfolding in the Onset of Parkinson's Disease Francis Rose, Miroslav Hodak, Jerry Bernholc A nucleation mechanism for the misfolding of $\alpha$-synuclein, the protein implicated in Parkinson's Disease (PD), is investigated using computer simulations. Through a combination of ab initio and classical simulation techniques, the conformational evolution of copper-ion-initiated misfolding of $\alpha$-synuclein is determined. Based on these investigations and available experimental evidence, an atomistic model detailing the nucleation-initiated pathogenesis of PD is proposed. Once misfolded, the proteins can assemble into fibrils, the primary structural components of the deleterious PD plaques. Our model identifies a process of structural modifications to an initially unfolded $\alpha$-synuclein that results in a partially folded intermediate with a well defined nucleation site as a precursor to the fully misfolded protein. The identified pathway can enable studies of reversal mechanisms and inhibitory agents, potentially leading to the development of effective therapies. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X39.00010: ESR Spectroscopy Provides Direct Evidence of Cu$^{2+}$ Coordination by Three Histidine Residues in A$\beta _{1-16}$ Byong-kyu Shin, Sunil Saxena We provide direct evidence that all three histidine residues in amyloid-$\beta _{1-16}$ (A$\beta _{1-16})$ coordinate to Cu$^{2+}$. In our approach, we generate three A$\beta _{1-16}$ analogues, in each of which a selected histidine residue is isotopically enriched with $^{15}$N. Pulsed electron spin resonance (ESR) experiments such as electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) clearly show that each of the three histidine imidazole rings at position 6, 13, and 14 in A$\beta _{1-16}$ binds to Cu$^{2+}$ as each of the three Cu$^{2+}$--$^{15}$N-labeled A$\beta _{1-16 }$complexes displays ESEEM and HYSCORE spectra which are distinctively different from those of the Cu$^{2+}$--nonlabeled A$\beta _{1-16}$ complex. The method employed here does not require either chemical side-chain modification or amino acid residue replacement, each of which is traditionally used to determine whether an amino acid residue in a protein binds to a metal ion. We also find that the histidine coordination in A$\beta _{1-16}$ is independent of the Cu$^{2+}$-to-peptide ratio, which is in contrast to the case of A$\beta _{1-40}$. The ESR results suggest tight binding between the histidine residues and the Cu$^{2+}$ ion, which is likely the reason of the high binding affinity of A$\beta $ peptide to Cu$^{2+}$. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X39.00011: Force Spectroscopy of Iron in Nitrosylated Hemes J.T. Sage, A. Barabanschikov, W. Zeng, N.J. Silvernail, W.R. Scheidt Nitric oxide (NO) regulates important physiological processes by interacting with the Fe atom in heme proteins. We investigate the effect of NO binding on the local structure and dynamics of $^{57}$Fe by determining its vibrational density of states (VDOS), both experimentally, using nuclear resonance vibrational spectroscopy (NRVS) and computationally, using density functional theory (DFT). All Fe–-ligand modes contribute to the VDOS, which provides uniquely quantitative information on the frequency, amplitude, and direction of the Fe motion. The VDOS also yields an experimental value for the stiffness, an effective force constant that probes nearest-neighbor interactions by measuring the force required to displace the Fe with the surrounding atoms fixed. Although vibrational mixing between Fe–-NO stretching and FeNO bending character complicates structural interpretations of FeNO vibrations observed near 450 and 560 cm$^{-1}$, we find that the former mode contributes more strongly to the stiffness, indicating its sensitivity to the strength of the Fe-–N bond. Comparison with DFT predictions identifies a feature observed near 130 cm$^{-1}$ in the VDOS of nitrosylated myoglobin as a vibration of the covalent link to the protein. We find that NO binding alters the interaction of the heme Fe with its local environment, and may facilitate NO recognition by heme proteins. [Preview Abstract] |
Session X40: Proteins in Membranes
Sponsoring Units: DBPChair: Sunil Saxena, University of Pittsburgh
Room: 412
Thursday, March 19, 2009 2:30PM - 2:42PM |
X40.00001: Copper coordination in the Glycine receptor by electron spin resonance Sharon Ruthstein, Katherine Stone, Michael Cascio, Sunil Saxena We describe the use of Electron Spin Resonance (ESR) to identify the coordination environment of copper in the extracellular domain of the protein, as well as the number of copper atoms that bind to Glycine receptor (GlyR). The GlyR channel mediates inhibitory neurotransmission in the central nervous system. It belongs to the superfamily of nicotincoid receptors. These receptors are formed by pentameric arrangement of subunits, each sharing a common topology having a large extracellular domain (ECD) and a transmembrane (TM) domain comprised of four membrane-spanning segments (TM1-TM4). For GlyR, four subunits (1-4) and one subunit have been identified to date, although the homomeric expression of just the $\alpha $1 subunit of GlyR is sufficient to reconstitute native-like activity. The results are expected to shed light on the role of metals ion in modulating ion permeation in such receptor. In addition, an identification of copper binding sites will allow the measurement of large range distance constraints in the receptor by pulsed ESR. Such structural information on the GlyR in various allosteric states is essential in order to shed light on the gating mechanism of this protein membrane. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X40.00002: Probing interaction of antimicrobial peptide duramycin with lipid monolayers Izabela I. Rzeznicka, Maria Sovago, Mischa Bonn, Toshihide Kobayashi, Taro Yamada, Maki Kawai Antimicrobial peptides are group of peptides which disrupt the microbial cell membrane through hydrophobic insertion into the outer lipid layer. Duramycin is a small tetracyclic peptide antibiotic, which has recently been shown to bind specifically to phosphatidylethanolamine (PE) lipids. We report the interaction of duramycin with phospholipid monolayers at air-water interface, studied using vibrational sum-frequency generation spectroscopy (VSFG) and fluorescence microscopy (FM). For monolayers containing PE lipids, VSFG reveals binding of duramycin to the monolayer through the appearance of a vibrational peak at 3045 cm$^{-1}$, corresponding to the C-H stretching vibration of phenylalanine amino acid. In addition, the amide I vibrational region shows that peptide has a $\beta $-sheet conformation. Similar experiments performed on phosphatidylcholine (PC) monolayers show the interaction is specific with PE. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X40.00003: Alamethicin Structure in Lipid Bilayers Stephanie Tristram-Nagle, Jian-Jun Pan, John Nagle This investigation uses x-ray diffuse scattering and MD simulations to study alamethicin (Alm) in fully hydrated bilayers of DOPC and diC22:1PC. Comparison of the experimental and simulated form factors supports the standard conclusion that Alm helices are inserted transmembrane along the bilayer normal at high humidity and high concentrations. Little change in membrane thickness with inserted Alm helices occurs for DOPC up to 1/10 Alm/DOPC. By contrast, the x-ray data strongly indicate that the diC22:1PC membrane, which is thicker than DOPC by 7 {\AA}, thins with added Alm. Fitting the data to models of the electron density gives a decrease in thickness of 4 {\AA} at 1/10 Alm/diC22:1PC. Although Alm's helical length is close to the hydrophobic thickness of DOPC (27 {\AA}), it is mismatched with the thicker diC22:1PC. Alm decreases the bending modulus (K$_{C})$ by a factor of $\sim $2 in DOPC and a factor $\sim $10 in diC22:1PC membranes at P/L $\sim $1/10. Determination of the B modulus reveals a large increase in Hamaker parameter when Alm is added to diC22:1PC, but not to DOPC. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X40.00004: Alamethicin Supramolecular Organization in Membranes Jian-Jun Pan, John F. Nagle, Stephanie Tristram-Nagle In this work we investigate the effect of membrane hydration and hydrophobic mismatch on the Alm channel superstructure in an oriented multilayer sample by x-ray scattering. Wide angle x-ray (WAXS) scattering near 14 nm$^{-1}$ indicates that the lipid chain region is not perturbed much by the incorporation of up to 10 mole percent Alm. Low angle x-ray scattering (LAXS) indicates that when the sample is very dry, which promotes interactions between neighboring bilayers, a body centered tetragonal crystal packing of Alm channels is formed. As the hydration level increases closer to biological conditions, the separation between bilayers increases, the interbilayer interactions weaken, and the crystalline order disappears while considerable diffuse scattering remains. The effect of hydrophobic mismatch is examined for two mono-unsaturated lipids, diC18:1PC and diC22:1PC, that differ in bilayer thickness by 0.7nm. There is also in-plane scattering at a medium q of 7nm$^{-1}$ that our analysis suggests may not be from the Alm channel structure. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X40.00005: Geometric cue for protein localization in a bacterium Sigolene Lecuyer, Kumaran Ramamurthi, Howard Stone, Richard Losick Proteins in bacteria deploy to particular places, but the cues for localization are frequently mysterious. We present evidence that the membrane protein SpoVM recognizes a geometric cue in \textit{Bacillus subtilis}. In vivo experiments show that SpoVM localizes to a particular patch of the inner membrane in sporulating \textit{Bacillus subtilis} bacteria, namely the convex surface of the developing spore. Our in vitro experiments support the hypothesis that this localization is driven by geometry rather than biochemical recognition. When purified SpoVM is incubated with polydisperse micrometer-sized DOPC vesicles, we observe that the protein preferentially adsorbs on smaller vesicles, of diameter similar to the size of the bacterial spore ($\sim$ 1 $\mu$m). Using fluorescent GFP-tagged SpoVM, we quantify the amount of adsorbed protein by confocal microscopy. Our results, when interpreted using existing protein adsorption models, suggest the existence of a cooperative adsorption mechanism for high enough membrane curvature, which involves the formation of small clusters of proteins. Membrane curvature could be a general cue for protein localization in bacteria. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X40.00006: The effect of protein on phase separation in giant unilamellar lipid vesicles. J.B. Hutchison, R.M. Weis, A.D. Dinsmore We explore the coarsening and out of plane curvature (budding) of domains in lipid bilayer vesicles composed of DOPC (unsaturated), PSM (saturated), and cholesterol. Green fluorescent protein (GFP) was added to the membrane in controlled amounts by binding to the Ni-chelating lipid, Ni-DOGS. Vesicles with diameters between 10 and 50 microns were prepared via a standard electroformation procedure. As a sample is lowered through temperature Tmix, a previously homogeneous vesicle phase separates into two fluid phases with distinct compositions. Phase-separated domains have a line tension (energy/length) at the boundary with the major phase which competes with bending energy and lateral tension to determine the overall configuration of the vesicle. Domain budding and coarsening were observed and recorded using both bright field and fluorescence microscopy during temperature scans and with varying concentrations of GFP. The addition of a model protein into our system allows for a broader understanding of the effect of protein, which are ubiquitous in cell membranes, on phase separation, budding, and coarsening. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X40.00007: Kinetics and Thermodynamics of Peptide (pHLIP) insertion and folding in a lipid bilayer Oleg Andreev, Alexander Karabadzhak, Dhammika Weerakkody, Vladislav Markin, Donald Engelman, Yana Reshetnyak We study spontaneous insertion and folding across a lipid bilayer of moderately polar membrane peptide pHLIP - pH Low Insertion Peptide. pHLIP has three major states: soluble in water or bound to the surface of a lipid bilayer as an unstructured monomer, and inserted across the bilayer as a monomeric $\alpha $-helix. We used fluorescence spectroscopy and isothermal titration calorimetry to calculate the transition energies between states. The free energy of binding to a surface of lipid bilayer is about -7 kcal/mol and the free energy of insertion and folding across a lipid bilayer at low pH is nearly -2 kcal/mol. We performed stopped-flow fluorescence and CD measurements to elucidate molecular mechanism of pHLIP insertion and folding within a lipid bilayer and to calculate the activation energy of formation of transmembrane helix. pHLIP also has utility as an agent to target diseased tissues and translocate molecules through the membrane into the cytoplasm of cells in environments with elevated levels of extracellular acidity, as in cancer and inflammation. We plan to discuss a number of related kinetics and thermodynamic parameters from our measurements. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X40.00008: Effects of Proteins and Lipids on Each Other in a Simulated Non-equilibrium Biomembrane Model Andrew P. Paradis, Susan R. McKay, Samuel T. Hess Lateral organization in biomembranes plays a major role in membrane topology, and is thus implicated in many basic functions of biomembranes such as endocytosis and signal transduction. In this study, non-equilibrium Monte Carlo simulations are used to investigate two related scenarios: 1. the effect of a rigid distribution of proteins on the lateral organization of lipids in a biomembrane, and 2. the degree to which lipid interactions influence the lateral organization of membrane-associated proteins that are free to translate laterally. Our model includes generic saturated and unsaturated lipids, proteins, and cholesterol, and is driven out of equilibrium through simulated endo- and exo-cytosis events. By varying the temperature, the protein mole fraction, and the interaction strengths, we examine the conditions under which various types of lateral organization occur. Simulation results are analyzed with pair-correlation functions and the Ripley K-test. We compare results from simulations of the two scenarios above and from simulations of biomembranes lacking protein. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X40.00009: Interaction of Defensins with Model Cell Membranes Lori K. Sanders, Nathan W. Schmidt, Lihua Yang, Abhijit Mishra, Vernita D. Gordon, Michael E. Selsted, Gerard C. L. Wong Antimicrobial peptides (AMPs) comprise a key component of innate immunity for a wide range of multicellular organisms. For many AMPs, activity comes from their ability to selectively disrupt and lyse bacterial cell membranes. There are a number of proposed models for this action, but the detailed molecular mechanism of selective membrane permeation remains unclear. Theta defensins are circularized peptides with a high degree of selectivity. We investigate the interaction of model bacterial and eukaryotic cell membranes with theta defensins RTD-1, BTD-7, and compare them to protegrin PG-1, a prototypical AMP, using synchrotron small angle x-ray scattering (SAXS). The relationship between membrane composition and peptide induced changes in membrane curvature and topology is examined. By comparing the membrane phase behavior induced by these different peptides we will discuss the importance of amino acid composition and placement on membrane rearrangement. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X40.00010: Membrane fluidity in the presence of membrane-binding peptides Beatriz Burrola Gabilondo, Wolfgang Losert, Paul Randazzo Arf proteins are GTP-ases that participate in vesicle trafficking inside cells. They are able to interact with membranes through their N-terminus when they are bound to GTP, and they detach from the membrane when GTP is hydrolyzed. The N-terminus of Arf1 (amino acids 2-17) folds into an amphipathic helix that can insert into lipid bilayers. Arf1 is also myristoylated; it has myristic acid, a 14-carbon fatty acid `tail', attached to it. We set out to test the hypothesis that the binding of the myristoylated N-terminus of Arf1 to lipid membranes changes the mechanical properties of the membrane, in ways that myristic acid alone or amphipathic peptides alone do not. We use three reporter molecules embedded in vesicles, whose fluorescence emission spectrum depends on the properties of the environment in which they are found, to measure three distinct aspects of membrane fluidity: Bispyrene is sensitive to lateral motion along the membrane, Prodan's emission gives a measure of the packing of the head groups, and DPH polarization reflects the packing of the hydrophobic tails. We will present effects found for four molecules (myristic acid, myristoylated and non-myristoylated N-terminus of Arf1, and the ALPS domain of KES) in a concentration-dependent manner, and discuss the importance of these results in the vesicle-trafficking picture. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X40.00011: New insights into the bacterial cell wall peptidoglycan architecture John Dutcher, Ahmed Touhami, Manfred Jericho The molecular architecture of the bacterial cell wall peptidoglycan (sacculi) is among the most challenging, yet still unsolved, structural problems in biochemistry. Two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the scaffold model, in which the glycan strands lie perpendicular to the cell surface. We have used atomic force microscopy (AFM) to investigate the molecular structure of this unique biopolymer in the rod-shaped bacterium Bacillus subtilis at high resolution. AFM images recorded in air on single sacculi revealed a porous regular network with 25-50 nm-wide peptidoglycan fibers and a 5-25 nm pore size. Interestingly, the new bacterial pole showed a regular structure with the same fiber sizes but with the fibers running in a direction that is almost perpendicular to that observed away from the pole. This finding combined with our previous data on live hydrated bacteria (1) provides new insights into the three-dimensional architecture of the peptidoglycan of Gram-positive bacteria. 1- A. Touhami, M. H. Jericho, and T. J. Beveridge, J. Bacteriol., 2004 186: 3286-3295. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X40.00012: An Analytic Study of Molecular Motion in Cell Membranes Ziya Kalay, Luca Giuggioli, Paul Parris, Vasudev Kenkre We present a theoretical calculation to describe the confined motion of transmembrane molecules in cell membranes. Understanding the motion of membrane-associated molecules, e.g. various types of receptors, has great modern relevance in cell biology. Our study is divided into two parts. In the first, we consider motion in an ordered system and in the second, we investigate the effects of disorder by employing an effective medium approximation. Both are based on Master equations for the probability of the molecules moving as random walkers, and leads to explicit usable solutions including expressions for the molecular mean square displacement and effective diffusion constants. As a result, the calculations make possible, in principle, the extraction of confinement parameters such as mean compartment sizes and mean intercompartmental transition rates from experimentally reported published observations. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X40.00013: Detergent interaction with tethered bilayer lipid membranes for protein reconstitution Matteo Broccio, Haw Zan Goh, Mathias Loesche Tethered bilayer lipid membranes (tBLMs) are self-assembled biomimetic structures in which the membrane is separated from a solid substrate by a nm-thick hydrated submembrane space. These model systems are being used in binding studies of peripheral proteins and exotoxins. Here we aim at their application for the reconstitution of water-insoluble integral membrane proteins. As an alternative to fusion of preformed proteoliposomes we study the direct reconstitution of such proteins for applications in biosensing and pharmaceutical screening. For reconstitution, highly insulating tBLMs ($R\sim10^{5}-10^{6} ~\Omega$) were temporarily incubated with a detergent to screen for conditions that keep the detergent-saturated membranestable and ready to incorporate detergent-solubilized proteins. We assess the electrical characteristics, i.e. specific resistance and capacitance, by means of electrochemical impedance spectroscopy (EIS) under timed incubation with decylmaltoside and dodecylmaltoside detergents in a regime around their critical micelle concentration, 1.8 mM and 0.17 mM respectively and demonstrate the restoration of the tBLM upon detergent removal. Thereby a range of concentration and incubation times was identified, that represents optimal conditions for the subsequent membrane protein reconstitution. [Preview Abstract] |
Session X41: Strongly Correlated Electrons
Sponsoring Units: DMP DCMPChair: Valery Kiryukhin, Rutgers University
Room: 413
Thursday, March 19, 2009 2:30PM - 2:42PM |
X41.00001: Breakdown of the BCS Ground State at a Quantum Phase Transition Rafael Jaramillo, Yejun Feng, Thomas Rosenbaum, Jonathan Lang, Zahir Islam, George Srajer, Peter Littlewood We use hydrostatic pressure to suppress the magnetism in elemental chromium, a simple cubic metal that demonstrates a subtle form of itinerant antiferromagnetism, formally equivalent to the BCS state in superconductors. By directly measuring the associated charge order with x-ray diffraction in a diamond anvil cell at low temperatures, we reveal a continuous phase transition driven by fluctuations that destroy the BCS-like state while preserving the strong magnetic interaction between itinerant electrons and holes. Cr is unique among stoichiometric magnetic metals studied to date insofar as the quantum phase transition is continuous in nature, allowing experimental access to the naked quantum singularity and a direct probe of the competition between conventional and exotic order in a theoretically tractable material. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X41.00002: Ultra-fast domain formation by visible and high-energy light in low-dimensional photoinduced phase transition systems Kaoru Iwano We propose the detection of domain dynamics in a one-dimensional system by using high energy photons such as synchrotron radiation. The domain here means that of a different phase formed in the background of an original phase. Using high energy photons, it is expected that not only $k$=0 states with $k$ being the momentum of its center-of-gravity motion, but also finite $k$ states are possible to to be detected. We theoretically demonstrate how they are observed, with off-resonant inelastic X-ray scatterings and valence photoemissions as concrete examples. In the latter, we particularly show that, in addition to basic two degrees of freedom, namely, the center-of gravity and the spatial size of the domain, spin excitations inside the domain play essential roles. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X41.00003: Ultrafast photo-induced spin and charge dynamics in correlated electron system Sumio Ishihara, Yu Kanamori, Hiroaki Matsueda Photo-induced phenomena in correlated electron system are one of the attractive themes in recent solid state physics. One of the well known examples is manganites with the perovskite crystal structure. The charge ordered insulating state associated with the antiferromagnetic (AFM) long-range order competes with the ferromagnetic metallic phase. After introduction of the pump photon into the charge-ordered insulating phase, dramatic changes in the optical reflectivity and in the optical Kerr rotation are observed. These results imply that the charge and magnetic structures are changed cooperatively by the photo-irradiation. We present a theoretical study of photo-induced dynamics in a correlated electron system where electronic charge couples with spin and lattice. The generalized double exchange model is analyzed by utilizing the two complementary methods, the exact diagonalization and inhomogeneous Hartree-Fock methods. Time evolutions of the optical absorption spectra, spin correlation, and charge correlation are calculated. There are two time scales in the photo-induced dynamics; the charge and AFM spin orders are collapsed within a short time scale corresponding to 10-100 fs, and the long-range FM spin correlation appears in a long time scale corresponding to a few ps. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X41.00004: Probing orbitons in YTiO3 with Resonant Inelastic X-ray Scattering Lucas Ament, Giniyat Khaliullin, Jeroen van den Brink In YTiO$_3$, a strongly correlated electron system with degenerate orbitals, orbitons are predicted to exist [1]. The hallmark of collective excitations is dispersion. To observe the orbiton dispersion, the rapidly developing technique of Resonant Inelastic X-ray Scattering (RIXS) is especially well suited. We analyze recent experimental RIXS data on YTiO$_3$ in the Ultrashort Core hole Lifetime framework [2]. The Ti ions in this material have a 3$d^1$ configuration, and the electron occupies one of the three degenerate $t_2g$ orbitals. Many of this compound's ground state properties are explained by assuming that the orbitals on these Ti ions talk to each other through a superexchange mechanism [1]. RIXS could couple to the orbital excitations (orbitons) in these kind of materials in two ways: via modulation of the superexchange interactions [3] and via a shakeup process. We compare our theoretical RIXS spectra to experimental ones, giving strong evidence for the existence of orbitons. // [1] G. Khaliullin and S. Okamoto, Phys. Rev. B 68, 205109 (2003) // [2] J. van den Brink and M. van Veenendaal, Europhys. Lett. 73, 121 (2006); L. J. P. Ament, F. Forte and J. van den Brink, Phys. Rev. B 75, 115118 (2007) // [3] compare F. Forte, L. J. P. Ament and J. van den Brink, PRL (2008) [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X41.00005: Polaronic hole-trapping in doped insulating oxide Cesare Franchini, Georg Kresse, Raimund Podloucky In transition-metal oxides, local electronic correlation effects dominate the physics, and lattice degrees of freedom are often only treated as external perturbations. However, in systems dominated by s and p electrons, electronic correlation is expected to be less important, and in agreement with this conjecture, we show that lattice degrees of freedom are crucial to account for the hole doping driven insulator-to-metal/superconducting transition (IMT) in Ba$_{1-x}$K$_x$BiO$_3$. Specifically, by using hybrid-DFT we show that Bi$^{3+}$ sites trap two holes from the valence band accompanied by a large local lattice distortion, commonly referred to as bipolaronic state. We show that in pure $\rm BaBiO_3$ a single peak in the imaginary part of the dielectric function is visible which corresponds to the charge-ordered excitation between Bi$^{3+}$ and Bi$^{5+}$ sub-bands. Upon hole-doping a second peak emerges at $x=0.125$ connected with the bipolaronic excitation. At $x=0.25$ the bipolaronic peak increases in intensity and is shifted towards lower energy, thus indicating the incipient IMT observed at $x\approx0.3$. Our results describe all relevant experimental results. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X41.00006: Systematic enhancement in magnetic susceptibilities and a study of Fermi Liquid behaviour of ReT$_{2}$Al$_{10}$ where Re=Y and La, and T =Fe, Ru, and Os Keeseong Park, Yuri Janssen, Moosung Kim, Carlos Marques, Meigan Aronson DC and AC magnetic susceptibilities, specific heat and resistivity are measured for YbFe$_{2}$Al$_{10}$-structure compounds, ReT$_{2}$Al$_{10}$ where Re =Y, and La, and T = Fe, Ru, and Os. The YT$_{2}$Al$_{10}$ show systematically enhancing paramagnetic behavior in magnetic properties from Os to Ru and to Fe, and Fermi-liquid behaviour below around 100 K. With the linear term of the specific heat ($\gamma _{0})$ and the temperature independent susceptibility ($\chi _{0})$ at low temperature, the Stoner enhancement parameter, Z, is utilized to find how close the compounds are to the ferromagnetic ordering, where Z = 1-(3$\mu _{B}^{2}$/$\pi ^{2}$k$_{B}^{2})(\gamma _{0}$/ $\chi _{0})$ . Specifically, YFe$_{2}$Al$_{10}$ shows a larger Z (0.98) than that (0.83) of Pd, a well known example of nearly ferromagnetic materials. The implied proximity to the quantum criticality is tested by a power law analysis , where 1/($\chi -\chi _{0})$=AT$^{\lambda }$ can describe well a wide range (2K to 100K) of AC magnetic susceptibility for YFe$_{2}$Al$_{10}$ with $\lambda $ = 1.19, which is between the mean-field value ($\lambda $ = 1) and that of the three-dimensional Ferromagnetic Heisenberg model ($\lambda $ = 1.33). [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X41.00007: Soliton Wall Superlattice Charge-Density-Wave Phase in Quasi-One-Dimensional Conductor (Per)2Pt(mnt)2 Si Wu, Andrei Lebed We demonstrate that the Pauli's spin splitting effects in a magnetic field improve nesting properties of a realistic quasi-one-dimensional electron spectrum. As a result, a high resistance Peierls charge-density-wave (CDW) phase is stabilized in high enough magnetic fields in (Per)2Pt(mnt)2 conductor. We show that, in low and very high magnetic field, the Pauli spin-splitting effects lead to a stabilization of a soliton wall superlattice (SWS) CDW phase, which is characterized by periodically arranged soliton and anti-soliton walls. We suggest experimental studies of the predicted first order phase transitions between the Peierls and SWS phases to discover a unique SWS phase. It is important that, in the absence of the magnetic field and in the limit of very high magnetic field, the suggested model is equivalent to the exactly solvable model of Brazovskii, Dzyaloshinskii, and Kirova. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X41.00008: Non Fermi Liquid behavior and disorder in BaVS$_3$ Ana Akrap, Neven Barisic, Florence Rullier-Albenque, Helmuth Berger, Laszlo Forro In strongly correlated BaVS$_3$, the interplay between a wide one-dimensional $d_z^2$ band and the localized $e_g$ electrons leads to a wealth of electronic phases. In this work we investigate the high pressure non-Fermi liquid (NFL) phase of BaVS$_3$ by means of transport measurements, focusing on the influence of disorder, introduced by fast electron irradiation and sulfur deficiency. Our results are interpreted within a novel scenario in which quasi-one dimensional 2$k_F$-CDW fluctuations are responsible for the NFL behavior.\footnote{N. Bari\v{s}i\'c {\it et al.}, arXiv:0712.3393v1} [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X41.00009: The role of charge degrees of freedom in Mott insulators: coupling of dielectric and magnetic properties in Cr-trimer complexes. Ross McDonald, Oscar Ayala Valenzuela, Marcelo Jaime, John Mydosh Materials that are insulating owing to strong electron correlations are pervasive in condensed matter physics-the parent phase of high-Tc cuprates, colossal magnetoresistive manganites and quantum magnets. All are characterized by a large onsite coulomb repulsion relative to the dominant electron hopping. As such, at half-band filling the charge is localized. The properties of these materials are therefore commonly described solely in terms of their spin degrees of freedom, with little attention given to any further role of the charge. Certain classes of Mott insulator are predicted to break this paradigm, providing a direct correlation between the magnetic spin texture and the dielectric properties of a material. We observe such a correlation in Cr- trimer systems, which combined with recent theoretical developments, indicates a purely electronic mechanism for multiferroic behavior. Magnetic field strengths of the order of the exchange interaction strongly perturb the spin texture, which is evident as steps and plateau in the magnetization. The corresponding shifts in dielectric properties reveal the role of the charge degrees of freedom. Electron Spin Resonance (ESR) results and the prospect of novel dipole-active ESR giving rise to the possibility of negative refractive indices will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X41.00010: The Jahn-Teller effect in doped LiCuO$_2$ Chris Marianetti LiCuO$_2$ displays one of the largest known Jahn-Teller distortions, where Cu$^{3+}$ is in a low spin configuration. Previous density functional theory (DFT) calculations verified the fact that the high spin, non-Jahn-Teller distorted LiCuO$_2$ is a metastable phase. In this work, we use DFT calculations to demonstrate that doping this system with ions that are not Jahn-Teller active allows one to tune the energy difference between the high-spin, non-Jahn-Teller phase and the low-spin, Jahn-Teller phase. This occurs due to the elastic penalty of the non-Jahn-Teller ion in the Jahn-Teller phase. The effect of different non-Jahn-Teller dopants is presented, and the electronic nature of the two respective phases is detailed. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X41.00011: Scanning tunneling spectroscopy study of electric-pulse-induced electronic inhomogeneities in GaTa$_{4}$Se$_{8}$ Vincent Dubost, Cristian Vaju, Tristan Cren, Benoit Corraze, Francois Debontridder, Etienne Janod, Dimitri Roditchev, Laurent Cario We have recently discovered a bulk Electric Pulse Induced Insulator-Metal Transition and possible superconductivity in the cluster Mott Insulator GaTa$_{4}$Se$_{8 }$[1]. The transport measurements, conducted on single crystals, are consistent with a two-channel model, which suggests that the electric pulse generates electronic inhomogeneities in the bulk of the samples. Our Scanning Tunneling Microscopy/Spectroscopy experiments indeed confirmed that the observed drop in the electric resistance originates from an electronic phase separation with the coexistence of metallic- like and insulating like domains at the nanometer scale [1]. [1] Vaju \textit{et al.} Advanced Materials, \textbf{20} 2760 (2008), Microelectronics engineering \textit{in press}, (2008) [Preview Abstract] |
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