Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X3: Interface Phenomena in Oxides
Sponsoring Units: DCMPChair: James Eckstein, University of Illinois Urbana Champaign
Room: Morial Convention Center RO2 - RO3
Friday, March 14, 2008 8:00AM - 8:36AM |
X3.00001: 2D Superconductivity at the LaAlO$_{3}$/SrTiO$_{3}$ interface Invited Speaker: In 2004 Ohtomo and Hwang [1] discovered that the interface between two insulating oxides, LaAlO$_{3}$ and SrTiO$_{3}$ (both band insulators), is metallic with a high mobility. This publication triggered a lot of work around the world (see for instance [2, 3]). We have studied the ground state of this system and discovered superconductivity [4]. The studied LaAlO$_{3}$ samples are grown epitaxialy by pulsed laser deposition on TiO$_{2}$-terminated SrTiO$_{3}$ substrates and are annealed in situ in oxygen. The superconducting properties in the LaAlO$_{3}$/SrTiO$_{3}$ heterostructures display signatures of 2D superconductivity and agree with the Beresinskii-Kosterlitz-Thouless (BKT) predictions. However, for low currents, IV curves show some deviation from the expected behavior of a perfect infinite 2D system. These deviations are attributed to the finite lateral size of the measured path. Experiments to probe this size effect have been done to check this interpretation. \newline \newline [1] A. Ohtomo, H. Y. Hwang, Nature \textbf{427}, 423 (2004) \newline [2] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, J. Mannhart, Science \textbf{313}, 1942 (2006) \newline [3] A. Brinkman \textit{et al}, Nature Materials \textbf{6}, 493 (2007) \newline [4] N. Reyren \textit{et al}, Science \textbf{317}, 1196 (2007) [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X3.00002: Magnetic effects at the interface between nonmagnetic oxides Invited Speaker: The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. In analogy to this remarkable interface-induced conductivity we show how, additionally, magnetic effects can be induced at the interface between the otherwise nonmagnetic insulating perovskites SrTiO$_{3}$ and LaAlO$_{3}$ [1]. A large negative magnetoresistance, up to 50\% at 50 mK, of the interface is found together with a logarithmic temperature dependence of the sheet resistance. These magnetic effects only appear in samples that have been grown under conditions that minimize the incorporation of oxygen vacancies. It is suggested that the interface electronic reconstruction induces localized magnetic moments in the SrTiO$_{3}$ that interact with the conduction electrons. At low temperatures, the sheet resistance reveals magnetic hysteresis with a long relaxation time scale. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise nonmagnetic materials. \\ \\{} [1] A. Brinkman \textit{et al}., Nature Materials \textbf {6}, 493 (2007). [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X3.00003: Transport properties observed at hetero-interfaces of LaAlO$_{3}$ on SrTiO$_{3}$; intrinsic or extrinsic interface effect? Invited Speaker: We have made very thin films of LaAlO$_{3}$ on TiO$_{2}$ terminated SrTiO$_{3}$ and have measured the properties of the resulting interface in various ways. Transport measurements show a sheet carrier density of 10$^{16}$ cm$^{-2}$ and a mobility around 10$^{4}$ cm$^{2}$ V$^{-1}$ s$^{-1}$. \textit{In situ} UPS results indicate that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the hetero-interfaces [1]. Our results explain for the first time the origin of the large sheet carrier densities and high mobility observed previously [2]. XAS and spectroscopic ellipsometry [3] measurements confirm the existence of (oxygen) defects in the SrTiO$_{3}$. Simple model calculations confirm the plausibility of having defects at the origin of charge carriers while still maintaining a high mobility. By means of annealing experiments in atomic oxygen we try to answer the question whether an intrinsically doped interface does indeed exist at lower carrier concentrations [2]. Due to its reactive nature (i.e., binding energy in an oxygen molecule is about 5 eV), atomic oxygen will have much more power to eliminate any oxygen vacancies compared to conventional annealing methods. \newline [1] W. Siemons et al, Phys. Rev. B 76, 155111 (2007) \newline [2] W. Siemons et al., Phys. Rev. Lett. \textbf{98}, 196802 (2007) \newline [3] G. Lucovsky, H. Seo and J. Luning, unpublished [Preview Abstract] |
Session X4: Approaching Quantum Limits in Optomechanical Systems
Sponsoring Units: DCMP GQIChair: Konrad Lehnert, JILA, NIST/University of Colorado
Room: Morial Convention Center 206
Friday, March 14, 2008 8:00AM - 8:36AM |
X4.00001: Sensing nanomechanical motion with a microwave cavity interferometer Invited Speaker: Optomechanical and electromechanical systems utilizing micro and nanomechanical oscillators offer a promising route towards manipulation of macroscopic objects at the quantum level. In this talk I present experiments that use principles of popular optomechanical systems yet employ light at microwave frequencies. Operating at microwave frequencies allows us to also harness technology associated with electromechanical systems, such as very light nanoscale mechanical objects and on- chip circuit elements compatible with a dilution refrigerator environment. Specifically, in our work we embed a nanomechanical flexural resonator inside a superconducting transmission-line microwave cavity, where the mechanical resonator's position couples to the cavity capacitance and thus to the resonant frequency of the cavity. With our device we realize near state-of-the-art force sensitivity (3 aN/$\rm{\sqrt {Hz}}$) and thus add to only a handful of techniques able to measure thermomechanical motion at 10's of milliKelvin temperatures. Our current measurements achieve a promising total displacement uncertainty at 140 times the quantum limit and a measurement imprecision as low as 30 times the quantum limit, as well as elucidate the important steps that will be required to progress towards the full quantum limit of displacement detection with this new system. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X4.00002: Strong dispersive coupling between a micromechanical oscillator and a high finesse optical cavity. Invited Speaker: Very sensitive mechanical detectors spanning roughly seventeen orders of magnitude in size are rapidly approaching a regime in which either the mechanical device itself or its readout should demonstrate quantum behavior. One of the main technical barriers to actually reaching this regime has been the difficulty of integrating ultrasensitive micromechanical devices with high-finesse optical cavities. Recently we have developed a robust means for addressing this issue, and have integrated a 50 nm-thick membrane (with a quality factor $>$ 1,000,000) into an optical cavity with a finesse $\sim $ 20,000. Although the membrane is nearly transparent, it couples to the optical cavity dispersively. This coupling is strong enough to laser-cool the membrane from room temperature to 7 mK. In addition, the dispersive nature of the optomechanical coupling allows us to realize a sensitive ``displacement squared'' readout of the membrane. Such a readout is a crucial requirement for measuring quantum jumps in a mechanical oscillator. We will describe these results, as well as our progress towards observing quantum effects in this system. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X4.00003: Cavity Assisted Sideband Cooling of Mechanical Motion Invited Speaker: This talk will provide a pedagogical introduction to the theoretical ideas that form the basis for cooling a mechanical cantilever using light-induced forces. During recent years, these concepts have been realized in a series of experiments by various groups, that have demonstrated impressive progress in cooling. Several of them will be discussed in the following talks of this session. Ultimately, this line of research may lead to the quantum-mechanical ground state of the center-of-mass motion of objects composed of many billions of atoms. A common ingredient is the use of an optical cavity to resonantly enhance the radiation pressure force affecting the motion of the cantilever. I will start by reviewing the classical description of how a time-retarded force leads to enhanced friction and thus cooling. Then I will present a fully quantum-mechanical description, that takes into account the opposing effect of the photon shot noise [F. Marquardt, J. P. Chen, A. A. Clerk and S. M. Girvin, Phys. Rev. Lett. 99, 093902 (2007); see also I. Wilson-Rae, N. Nooshi, W. Zwerger and T. J. Kippenberg, Phys. Rev. Lett. 99, 093901 (2007)]. This theory yields a quantum-limit for the reachable photon number that can be made arbitrarily small, provided a high-finesse cavity is combined with a high-frequency mirror (the ``resolved sideband limit,'' analogous to ion cooling). Various different ways of experimentally measuring the photon number will be mentioned. Finally, I will briefly give an outlook regarding the opportunities for quantum-coherent experiments that will open up once the ground state has been reached. This talk primarily reports joint work with A. Clerk, J. P. Chen, and S. Girvin. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X4.00004: Laser Cooling of Gram Scale Objects Invited Speaker: Laser cooling of macroscopic mechanical oscillators is a rapidly growing field with applications in high precision measurements, gravitational wave detectors, and exploration of the classical-quantum transition. Here I will describe a series of cooling experiments, which are inspired by gravitational wave detectors, to trap and cool gram scale mirror oscillators. To approach quantum limits of oscillators with such a high mass requires the use of a variety of cooling techniques. The techinques employ non-mechanical forces both to trap the mirror by increasing its effective mechanical resonant frequency, and to cool the mirror by damping its motion within the trap. The non-mechanical forces are created from either radiation pressure in a detuned optical resonator, or from electronic feedback forces in an active servo. As the experiments approach the quantum regime, an assortment of non-classical behavior and effects should become evident, such as quantum radiation pressure noise, and squeezing and entanglement of the light and mirror states. I will discuss the prospects for observation of these effects, in light of current performance and expected upgrades. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X4.00005: Towards experimental optomechanical entanglement between a movable mirror and a cavity field. Invited Speaker: The quantum regime of mechanical systems offers fascinating new possibilities for both applied and fundamental physics. Quantum optics provides a well-developed tool box to help entering and controlling this regime as is evidenced by the recent successes in laser-cooling of micromirrors that promise cooling capabilities to the mechanical quantum ground state. I will discuss the prospects and challenges to generate (opto-mechanical) quantum entanglement, which is an important resource for quantum information processing and is also at the heart of Schr\"{o}dinger's ``cat paradox.`` [Preview Abstract] |
Session X5: Superconducting Sources of THz-Radiation
Sponsoring Units: DCMPChair: Kazuo Kadowaki, University of Tsukuba
Room: Morial Convention Center RO1
Friday, March 14, 2008 8:00AM - 8:36AM |
X5.00001: THz generation using Fluxon dynamics in high temperature superconductor Josephson junctions Invited Speaker: Ever since the Josephson junction was discovered, the possibility of generating high frequency radiation has been considered. Such radiation could have many applications, for example as a local oscillator in an integrated receiver, a spectrometer, an imaging device etc. A brief historical overview of methods and results will be presented. Special emphasis will be given to the BSCCO--type high temperature superconductors that may be considered as a stack of coupled Josephson junctions - potentially having applications as terahertz oscillators. The stack of inductively coupled long Josephson junctions is modeled as a system of coupled sine-Gordon equations. The key point for oscillator applications is to have in-phase coherent motion of fluxons in the different Josephson junctions in the stack. This may be obtained by applying a (large) magnetic field parallel to the layers, or by having the system interacting with a cavity. In both cases the fluxon dynamics is very non-linear, and numerical simulations are necessary. We will also present a few cases where analytical results have provided a new insight in this complicated non-linear system. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X5.00002: Emission of Coherent THz-Radiation from Superconductors. Invited Speaker: Josephson junctions naturally convert dc-voltages into high-frequency electromagnetic radiation, with 1 mV corresponding to 0.483 THz, and many such junctions emitting in phase at the same frequency can produce useful emission power. Stacks of junctions with unsurpassed packing density occur naturally in the layered high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$, in which the superconducting CuO$_{2}$-layers are coupled through the intrinsic Josephson effect. However, achieving synchronization of the high-frequency oscillations of all the junctions in the stack has so far been a major challenge. We demonstrate that coherent continuous-wave THz-radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 $\mu $W at frequencies up to 0.85 THz. The available power is potentially much larger, as there is evidence that 20 $\mu $W of power are pumped into the observed THz cavity resonance. The emission persists up to temperatures of $\sim $50 K. Emission does not require the application of a magnetic field, significantly simplifying the design of superconducting THz-sources. In fact, a single applied D.C. current leads to the efficient excitation of continuous coherent THz-radiation. This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357 -- Basic Energy Sciences, the Japanese Ministry of Education, Culture, Sports, Science and Technology, and the Turkish TUBITAK under Project No. 106T053. In collaboration with L. Ozyuzer, A. E. Koshelev, C. Kurter, N. Gopalsami, Q. Li, M. Tachiki, K. Kadowaki, T. Yamamoto, H. Minami, H. Yamaguchi, T. Tachiki, K. E. Gray, W.-K. Kwok. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X5.00003: Radiation Due to Josephson Oscillations in Layered Superconductors. Invited Speaker: The power of direct tunable radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of highly anisotropic layered superconductors is derived. The super-radiation regime in a current biased crystal is considered when no dc magnetic field is applied. It is assumed that crystal is cut in the form of thin plate parallel to the ac-plane with the thickness of several microns along the b-axis and length of several hundreds microns along the a-axis. At large number of intrinsic junctions oscillations in such BSCCO crystal are synchronized providing high radiation power proportional to squared number of junctions and high efficiency up to 1/3 in the THz frequency range. The radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. When the ac-edge of such a crystal is irradiated by external electromagnetic wave, radiation from both ac-edges of the crystal is enhanced (stimulated radiation) at higher-current part of the Shapiro step. The main part of extra radiation is in the direction opposite to incident wave. This effect of stimulated radiation may be used for amplification of electromagnetic waves. BSCCO crystal with modulated critical current and length several tens microns along the b-axis also provides high THz-radiation power from the ac-edges at discrete Josephson frequencies corresponding to the crystal thickness along the b- axis. Enhancement of radiation power in this case is due to Fiske resonances and super-radiation regime. The powerfuI almost standing electromagnetic wave is excited inside the crystal in the resonance. This wave is homogeneous across the layers meaning that the oscillations are synchronized in all junctions in the stack. In this situation the synchronization of radiation in different junctions is enforced by both super-radiation regime and similar critical current profile in all junctions of the crystal. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X5.00004: THz emission from a slice of high-\textit{Tc} superconducting single crystal Invited Speaker: Copper oxide superconductors possess intrinsically a layered crystalline structure, in which superconducting and non-superconducting layers interleave each other. Therefore the crystal itself consists of a number of superconducting junctions sequentially stacked along the $c$ axis of the crystal, and these junctions are often referred to as intrinsic Josephson junctions (IJJs). In the case of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (BSCCO), each IJJ measures approximately 1.5 nm thick. Many groups have been exploring the possibilities to develop terahertz (THz) detectors and oscillators based on IJJs, due to the high collective plasma frequencies (up to THz region), the uniformity in junction properties, the easiness to make a large junction array, and the low loss at high frequencies. Some years ago, in IJJs singled out from inside a slice of BSCCO single crystal with a double-sided process, THz response was successfully observed as sharp Shapiro steps at frequencies up to 2.5 THz, and harmonic mixings were carried out with harmonic numbers as large as 90. Recently observed have been THz oscillations in various structures of BSCCO IJJs, which can be excited by dc bias, in-plane magnetic fields, or microwave irradiations at several gigahertz. Needless to say, for practical applications, it is necessary to synchronize the emissions from IJJs, couple the THz oscillations into a finite space, guide them in a controllable way, monitor the frequencies and power levels, and preferably do the jobs using an integrated system. We have been making extensive efforts to explore these ideas, and will report our latest results at the meeting. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X5.00005: Mechanism of the terahertz wave emission from intrinsic Josephson junctions of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ Invited Speaker: We propose a mechanism for the strong emission of terahertz wave recently observed in current-biased mesa-shaped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ single crystals at Argonne National Laboratory. When the mesa width is approximately equal to the $c$-axis penetration depth and an external current is applied along the $c$-axis of the crystal, the Josephson current oscillation can cause the resonance excitation of cavity-mode of the transverse Josephson plasma in the voltage state. When the oscillating electric and magnetic fields of the excited plasma wave have antinodes at the surfaces of mesa, the electromagnetic wave with ac Josephson frequency is strongly emitted in the vacuum outside the mesa as terahertz laser wave. For the coherent continuous-wave terahertz-radiation of sizable power, the nonlinear and nonequilibrium superconducting properties of the current biased intrinsic Josephson junction system play an important role. [Preview Abstract] |
Session X6: Tunnel Magnetoresistance: Yesterday, Today, and Tomorrow
Sponsoring Units: GMAGChair: Igor Zutic, State University of New York at Buffalo
Room: Morial Convention Center RO4
Friday, March 14, 2008 8:00AM - 8:36AM |
X6.00001: Tunnel Magnetoresistance, Spin Accumulation, and Spin Hall Effect Invited Speaker: In a tunnel junction consisting of two ferromagnetic (F) electrodes, the tunneling current depends on the relative orientation of the magnetization of F electrodes. This so-called tunnel magnetoresistance (TMR) has been extensively studied for various device applications. When a non-magnetic metal or semiconductor (N) is introduced in such a tunnel junction, the spin-polarized current is injected into N, and the spin current and spin accumulation occur in the region of the order of the spin diffusion length ($\lambda )$, which is in the range of a few 10 $\mu $m $\sim $ a few $\mu $m depending on materials. Therefore, in a device with the size of the order of $\lambda $, the spin current and spin accumulation give size to a variety of novel spin dependent phenomena [1]. The spin current is scattered by the spin-orbit interaction and induces the charge current in the perpendicular direction. It is also possible that the charge current is converted into the spin current in the perpendicular direction by the spin-orbit interaction. These conversions are the Spin Hall effects. If N is superconducting in the tunnel device, the situation is more dramatic. The effects of spin current and spin accumulation are strongly enhanced in the superconducting state. Some of the unique experiments are proposed. \newline [1] ``Concepts in Spin Electronics,'' ed. S. Maekawa (Oxford University Press, 2006). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X6.00002: Spin-Dependent Tunnelling Phenomena in Magnetic Tunnel Junctions with MgO Tunnel Invited Speaker: |
Friday, March 14, 2008 9:12AM - 9:48AM |
X6.00003: Electron tunneling in epitaxial magnetic tunnel junctions Invited Speaker: The remarkable progress in the performance of tunneling magnetoresistance (TMR) junctions using MgO as the barrier layer, has confirmed the theoretical prediction of a very high TMR ratio from first-principles calculations [1,2]. The theoretical prediction was based on the understanding of the band filtering effect by a class of barrier materials, including ZnSe, and even vacuum, in addition to MgO, along particular crystalline orientations. The combination of the preferential filtering for the electrons with the $\Delta_1$ symmetry by the barrier layer, and the presence of the $\Delta_1$ band only in the majority spin channel of Fe, FeCo and Co electrodes, results in the high TMR ratio previously believed only achievable with half-metallic electrodes. Here we show that the remarkable agreement between the experiment and the first-principles theory goes beyond the TMR. The effects of barrier thickness, interface resonance states, and quantum confinement calculated from the first-principles for MgO based magnetic tunnel junctions are compared with experimental data. In addition to excellent agreement for each case, additional insights are obtained from the first-principles calculations that are otherwise not directly available from the experiments. We also discuss other candidate materials for high TMR junctions. \newline \newline [1] W. H. Butler, X.-G. Zhang, T. C. Schulthess et al., Phys. Rev. B 63, 054416 (2001). \newline [2] J. Mathon and A. Umerski, Phys. Rev. B 63, 220403 (2001). [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X6.00004: TMR-related effects in structures involving semiconductors Invited Speaker: The growth of semiconductor heterostructures incorporating ferromagnetic material is a challenge for today's spintronic. We will report on GaMnAs/III-V and MnAs/III-V tunnel junctions that fulfill this condition. In the case of GaMnAs related junctions, the complexity of the transport mechanisms associated with spin-orbit coupled states make this material a powerful means for finding novel effects and provides new challenges for theoretical understandings. This includes tunnel magnetoresistance (TMR) across single and double barriers [1] and tunnel anisotropic magnetoresistance (TAMR). As an illustration, the resonant TAMR on a GaAs quantum well can be used as a probe of the GAMnAs valence band anisotropy [2]. In the case of MnAs, the necessary low temperature growth mode to avoid intermixing at the interfaces favors the insertion of As anti-site in the III-V spacer layer. We will show how the tunneling magnetoresistance is then modified and how the major role of the defects in the conduction can be taken into account. For this we have proposed an analytical model of spin-dependent resonant tunneling through a 3D assembly of localized states (spread out in energy and in space) in a barrier [3]. An inhomogeneous distribution of localized states leads to resonant tunneling magnetoresistance inversion and asymmetric bias dependence as evidenced with a set of experiments with MnAs$/$GaAs(7--10 nm)$/$MnAs tunnel junctions. By extension, we show the possibility of using such spectroscopic measurements to probe the spin asymmetry of ferromagnetic electrodes sandwiching an inhomogeneous tunnel barrier. \newline [1] R. Mattana et al. Phys. Rev. Lett. 90, 166601 (2003) \newline [2] M. Elsen et al. Phys. Rev. Lett. 99, 127203 (2007) \newline [3] V. Garcia et al. Phys. Rev. Lett. 97, 246802 (2006) [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X6.00005: Influence of spin polarized current on superconductivity and spin transport in organic semiconductors Invited Speaker: Spin conserved tunneling from a ferromagnetic (FM) electrode through an insulating barrier with a superconductor as the spin detector, was discovered by Meservey and Tedrow three decades ago. This phenomenon of spin polarized tunneling has been successfully utilized to understand many aspects of magnetism and superconductivity. In recent years enormous success is seen in magnetic tunnel junctions both from the fundamental viewpoint as well as due to their application in information technology. In general, BCS superconductivity with Cooper pairs formed at the Fermi level with electrons of opposite spins is not compatible with ferromagnetism which have parallel spin configuration. The superconducting state can be influenced by injecting spin polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance (MR) of over 1100{\%} for a F/I/S/I/F multilayer system (I - insulator) or even infinite MR can be achieved in epitaxial metallic systems. Due to the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T$_{C})$ in the spin parallel configuration is shifted below that of the spin anti-parallel configuration. This T$_{C}$ shift can much larger than that predicted by theories. Oscillation in Tc is also observed. The field is the spin transport properties in organic semiconductors (OSs), a least explored area. Because OSs are composed of mostly light elements (i.e. C, H, N, O) and thus have a weaker spin-orbit interaction, low hyperfine interactions, spin coherence lengths can be long in these materials. Large spin decay length was observed in amorphous Rubrene. Recent developments in electron spin polarized tunneling through ultrathin layers of the molecular organic semiconductors such as Alq3 and Rubrene will be discussed. [Preview Abstract] |
Session X7: Understanding Strongly Correlated Materials with Dynamical Mean Field Theory Methods
Sponsoring Units: DCOMPChair: Matthias Troyer, Theoretische Physik, ETH Zurich
Room: Morial Convention Center RO5
Friday, March 14, 2008 8:00AM - 8:36AM |
X7.00001: Mott Transition, Antiferromagnetism, and d-wave Superconductivity in Two-Dimensional Organic Conductors and in Cuprates Using Cluster Dynamical Mean Field Theory Invited Speaker: The main features of the phase diagram of high-temperature superconducting cuprates are a Mott insulator at half-filling, a pseudogap at finite doping and, in the ground state, the competition between antiferromagnetism, d-wave superconductivity and possibly other inhomogeneous phases. In the layered organics of the $\kappa$-BEDT-X family, it is pressure instead of doping that is varied but the competing phases and the Mott insulating behavior are similar to the cuprates. Approaches that claim to explain d-wave superconductivity in the cuprates must also explain this phenomenon in all other related classes of compounds described by the Hubbard model. Using Cluster Dynamical Mean-Field theory, we show that the main features of both phase diagrams, cuprates and organics, can be understood from the one-band model with hopping parameters taken from band structure and interaction of the order of the bandwidth. We emphasize the case of the organics, studying the Mott transition, antiferromagnetism and superconductivity on the anisotropic triangular lattice. The Mott transition in the normal phase can be continuous or first order depending on the value of the frustrating hopping $t'/t$. A $d$-wave superconducting phase appears between an antiferromagnetic insulator and a metal for $t^{\prime}/t=0.3-0.7 $, or between a nonmagnetic Mott insulator (spin liquid) and a metal for $t^{\prime }/t\geq 0.8$, in agreement with experiments on layered organic conductors including $\kappa $-(ET)$_{2}$Cu$_{2}$(CN)$_{3}$. These phases are separated by a strong first order transition. The phase diagram gives much insight into the mechanism for d-wave superconductivity and on the question of the glue. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X7.00002: Continuous time quantum Monte Carlo (CTQMC): a fast algorithm to solve the DMFT equations Invited Speaker: Dynamical mean field calculations involve the repeated numerical solution of an impurity problem, which is the time critical step in the self-consistency loop. The performance and flexibility of available impurity solvers therefore defines what type of problems can be treated within dynamical mean field theory. Over the past few years, significant progress has been achieved with the development of so-called continuous- time quantum Monte Carlo methods. These algorithms are based on a diagrammatic expansion of the partition function in either the interactions or hybridizations, and the stochastic sampling of appropriate collections of diagrams. I will explain the key ideas behind this powerful and versatile approach, with a particular emphasis on the expansion in hybridization. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X7.00003: Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator Invited Speaker: Using Cellular Dynamical Mean Field Theory, implemented with exact diagonalization as impurity solver, we study the superconducting state of the hole-doped two-dimensional Hubbard model. We mainly focus on qualitative aspects which characterize the approach to the Mott transition. We will show that our formalism leads to a natural decomposition of the photoemission energy-gap into two components. A first gap, stemming from the anomalous self-energy, dominates near the nodes and decreases with decreasing doping. The second gap has an additional contribution from the normal self-energy, inherited from the normal-state pseudo-gap. It is dominant near the antinodes and increases as the Mott insulating phase is approached. This behavior of the one-particle gap is relevant in the light of recent experimental studies reporting the presence of two different energy scales in the nodal and antinodal regions of high-temperature superconductors. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X7.00004: DCA study of magnetic mediated superconductivity in the Hubbard model Invited Speaker: The Dynamical Cluster Approximation (DCA) with quantum Monte Carlo as a cluster solver is used to study pairing in the two dimensional Hubbard model. The DCA adds non-local corrections to dynamical mean field theory by mapping the lattice onto a self-consistently embedded periodic cluster. The qualitative features of the cuprate phase diagram are captured by the DCA with a 2x2 cluster, which provides a mean field solution of the model. With increasing cluster size, the results are found to converge and display a finite d-wave transition temperature, establishing the presence of superconductivity in the model. A decomposition of the pairing interaction into its cross channels reveals that pairing is mediated by S=1 spin fluctuations. A simple renormalized spin fluctuation model is found to capture many of the properties of pairing and the spectra, including the high-energy kink waterfall structure and the structure of the leading order parameter. However, it fails to capture realistic features including long-ranged hopping, phonons and the pseudogap. Phonons, in particular, are found to enhance the paring interaction by enhancing antiferromagnetism. Despite this, superconductivity is suppressed by local (Holstein, Breathing and Buckling) phonon modes through the formation of polarons which dramatically reduce the particle mobility. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X7.00005: DMFT calculations of materials properties using the continuous time QMC method Invited Speaker: The combination of DFT with DMFT has proven to be an instrumental method for describing realistic strongly correlated electron systems. In essence, DMFT treats the strongly correlated electrons near the Fermi surface while DFT treats the electrons which are less correlated. DMFT effectively maps the intractable lattice many-body problem onto a tractable impurity many-body problem. The DMFT impurity problem must be solved using numerical methods or approximate analytical methods, and this is the bottleneck of the entire procedure. Continuous time QMC has recently emerged as a dominant method to solve the DMFT impurity problem. We present applications of DFT+DMFT(CTQMC-atomic-limit) to the cobaltates and Pu. In Pu, a variety of physical properties are calculated such as the Photoemission spectra, magnetic susceptibility, and the heat capacity. These physical properties are probed as a function of temperature and volume, and compared with experimental measurements. Additionally, we demonstrate the effect of the full on-site exchange interaction on the physical observables. In the cobaltates, the Fermi surface and heat capacity are calculated for Na$_{0.3}$CoO$_2$. We demonstrate that the topology of the Fermi surface depends sensitively upon the bare Hamiltonian. It is shown that consistent agreement with heat capacity measurements and ARPES experiments can only be achieved if the $e_g'$ satellite pockets are not present at the Fermi surface. [Preview Abstract] |
Session X8: Focus Session: Wormlike Micellar Fluids and Vesicles
Sponsoring Units: DFDChair: Andrew Belmonte, Pennsylvania State University
Room: Morial Convention Center RO6
Friday, March 14, 2008 8:00AM - 8:36AM |
X8.00001: Rheo-NMR of shear banded flow in wormlike micelles Invited Speaker: Rheo-NMR gives access to detailed information about the flow field generated by the device used to induce deformational flow. It also provides information about colloidal or molecular organisation and dynamics, under conditions of flow. In particular, NMR offers the possibility of measuring nuclear spin relaxation times and molecular self-diffusion coefficients, sensitive respectively to molecular brownian motions and their restrictions due to local structure. Furthermore, through the use of orientation-dependent terms in the spin interactions, such as the nuclear quadrupole or dipolar interactions, NMR permits the measurement of molecular order parameters. When combined with imaging methods, NMR in principle allows such measurements to be spatially localized, often with resolution down to a few 10s of microns In the study of shear banding phenomena in wormlike micelles, Rheo-NMR has proven of especial value, not only indicating the clear existence of shear bands, but also that they are associated with fluctuations, and sometimes, with molecular alignment. The subtlety of the correspondence (or lack of correspondence) between birefringence effects and shear banded flow has also been revealed. Recent measurements of shear-banded flow under Couette flow of the micellar system 10\% w/v cetylpyridinium chloride and sodium salicylate (CPyCl/NaSal) molar ratio 2:1 in 0.5 M NaCl in $^1$H$_2$O, indicate that shear banding fluctuations are consistent with the shear stress fluctuations observed in rheological measurements. Furthermore we find a coupling between flow fluctuations in the gradient and vorticity directions. Using $^2$H NMR spectroscopy on a deuterated probe molecule (n-decane) in the wormlike micellar interior, direct measurement of the shear-induced nematic phase transition is reported. More recently we have used Rheo-NMR to investigate the flow and alignment properties of worm-like micelles formed by a 5\% w/w solution of the BASF difunctional block copolymer non-ionic surfactant, Pluronic P105 in water along with 4.3\% w/v 1-phenylethanol-d5. A variety of shear-banding and alignment behaviours are observed, along with both stable and fluctuating flows. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X8.00002: Modeling the Inhomogeneous Response of Steady and Transient Flows of Entangled Micellar Solutions Invited Speaker: Surfactant molecules can self-assemble in solution into long flexible structures known as wormlike micelles. These structures entangle, forming a viscoelastic network similar to those in entangled polymer melts and solutions. However, in contrast to `inert' polymeric networks, wormlike micelles continuously break and reform leading to an additional relaxation mechanism and the name `living polymers'. Observations in both classes of entangled fluids have shown that steady and transient shearing flows of these solutions exhibit spatial inhomogeneities such as `shear-bands' at sufficiently large applied strains. In the present work, we investigate the dynamical response of a class of two-species elastic network models which can capture, in a self-consistent manner, the creation and destruction of elastically-active network segments, as well as diffusive coupling between the microstructural conformations and the local state of stress in regions with large spatial gradients of local deformation. These models incorporate a discrete version of the micellar breakage and reforming dynamics originally proposed by Cates and capture, at least qualitatively, non-affine tube deformation and chain disentanglement. The `flow curves' of stress and apparent shear rate resulting from an assumption of homogeneous deformation is non-monotonic and linear stability analysis shows that the region of non-monotonic response is unstable. Calculation of the full inhomogeneous flow field results in localized shear bands that grow linearly in extent across the gap as the apparent shear rate increases. Time-dependent calculations in step strain, large amplitude oscillatory shear (LAOS) and in start up of steady shear flow show that the velocity profile in the gap and the total stress measured at the bounding surfaces are coupled and evolve in a complex non-monotonic manner as the shear bands develop and propagate. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X8.00003: Rheology and Acoustics of Highly Concentrated Wormlike Micellar Fluids Joseph Gladden, Joel Mobley Wormlike micellar fluids have long been studied as a model non-Newtonian fluid. The dynamic microstructure of the fluid gives rise to a rich diversity of hydrodynamic phenomenon. Generally, these fluids are studied in a low concentration regime (0-20 mM of surfactant). In this talk, we will discuss recent rheological and acoustic measurements of highly concentrated wormlike micellar fluids with concentrations of 50 - 400 mM cetyltrimethylammonium bromide (CTAB) and 30 - 240 mM (respectively) sodium salicylate (NaSAL) in the temperature range of 22 - 45 Celsius. In 200 mM CTAB fluids, the activation energy, derived from stress relaxation time measurements, exhibit a discontinuous shift from about 60 kT below 33 celsius to about 40 kT above. Speed of sound and acoustic attenuation measurements were obtained by a broadband Fourier spectroscopic method using ultrasonic waves between 2 and 8 MHz. Each of the concentrations measured exhibits an peak in the acoustic attenuation between 33 and 38 Celsius. The speed of sound increases monotonically over 22 - 40 Celsius, very similar to pure water in both magnitude and temperature dependence. Attenuation measurements as a function of acoustic power using high intensity focused ultrasound will also be discussed. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X8.00004: The flow of wormlike micelles in microchannels: a micro-PIV study of shear-banding, interfacial instabilites and tracers migration Philippe Nghe, Guillaume Degre, Patrick Tabeling, Armand Ajdari We characerize by Particle Image Velocimetry the Poiseuille flow a semi-dilute solution of wormlike micelles (a CTAB and sodium nitrate aqueous solution) in pressure resistant microchannels. Thanks to the high aspect ratio of our channels, we can measure the local rheology of the solution, independantly from the slippage at the wall, according to a method already validated on non-newtonian polymer solutions. As the pressure driving the flow is increased, the velocity profiles reveal first a newtonian phase, then apparition of a dramatically lower viscosity second phase at the walls, which is the so called shear banding regime. First we deduce the local rheology of the solution from these velocity profiles, in agreement with the macroscopic rheology obtained in Couette rheometers. Then we study the development of an instability at the interface between the two phases, with a wavevector in the vorticity direction and a wavelength corresponding to smallest dimension of the channel. Finally we discuss the hypothesis of passive tracers : depending on their size, we observe a tracer depletion in the high-shear phase, which may be to high normal forces. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X8.00005: Reversible and irreversible flow-induced phase transition in micellar solutions Radhakrishna Sureshkumar, Mukund Vasudevan, Eric Buse, Hare Krishna, Ramki Kalyanaraman, Bamin Khomami, Amy Shen It is well known that wormlike micelles form shear-induced structures (SIS). SIS formation is typically accompanied by the appearance of a gel-like phase. While both configurational dynamics of the micelles in flow and electrostatics are recognized as the key factors that influence such phase transitions, there are no universally applicable criteria for the onset strain rate as function of salt concentration. In this work, first, we examine the effect of salt concentration on the critical strain rate for CTAB/NaSal solutions and show that a ``self-similar'' phase transition regime exists. Second, we show that under strong (elongational) flow conditions, the phase transitions are irreversible, leading to the formation of gels that are stable even after the flow is stopped. Results obtained from atomic force microscopy studies of the structure of such gels will be presented. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X8.00006: Investigating the structures and phase behavior of anionic perfluorinated surfactant using SANS Garfield Warren, Dobrin Bossev We have examined the structures formed by mixtures of tetraethylammonium perfluorooctylsulfonate (TEAFOS) and lithium perfluorooctylsulfonate (LiFOS) in water using small angle neutron scattering (SANS). SANS is an ideal method to characterize the morphology of such soft materials because the wavelength of the cold neutrons is comparable to the characteristic length scale of the surfactant structures and the possibility to apply the contrast variation technique. Results were obtained for mixtures at a constant surfactant concentration of 100 mM and different TEA/Li ratios at a temperature of 30\r{ }C. SANS curves were obtained either from the fluorinated micellar core or from the hydrogenated counterion atmosphere surrounding the micelles applying contrast matching. A transitional change in shape from spherical to prolate micelles was observed for TEA fractions greater than 30 mM. For TEA fractions greater than 55mM, threadlike micelle structures are present. From the SANS data we are able to correlate the counterion binding of the two different species to the shape and size of the micellar structure and confirm the role that the counterion environment plays in macroscopic rheological properties. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X8.00007: Fabrication of phospholipid vesicles from double emulsions in microfluidics Insun Yoon, Anderson H. Shum, Daeyeon Lee, David A. Weitz Phospholipids self-assemble into lipid vesicles also known as liposomes. The formation of liposomes via conventional techniques such as electroformation has been studied extensively. However, the liposomes formed through electroformation are polydisperse and have low encapsulation efficiency. We present a new method to fabricate monodisperse phospholipid vesicles with high encapsulation efficiency from water-in-oil-in-water double emulsions. We generate phospholipid stabilized monodisperse double emulsions using a glass microcapillary device. This process allows efficient encapsulation within the inner aqueous drop. The middle oil phase is a volatile organic solvent in which phospholipids are dissolved. As the organic solvent evaporates, phospholipids self-organize into vesicles. This technique is versatile in the choice of phospholipids and we have generated vesicles from different types of phospholipids. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X8.00008: Concentration dependence of dynamics of a droplet microemulsion Michihiro Nagao, Hideki Seto We will present a concentration dependence of dynamics of a spherical droplet microemulsion, consisting of aerosol-OT (AOT), as a surfactant, water, and decane. This mixture forms spherical microemulsion in a wide range of concentration. With keeping water to surfactant ratio constant, concentration of water plus surfactant was changed. The static structure of this system has been determined by small-angle neutron scattering (SANS) using the relative form factor method in the droplet concentration range from 5 to 75 {\%} [1]. Dynamics of droplet microemulsions have been determined using neutron spin echo technique, which is suitable to measure dynamics of systems in nanometer and nanosecond scales. We measured dynamics of the system at 5, 30, and 60 {\%} of droplet concentration. Using analogous data reduction procedure to SANS, contribution of shape fluctuations is decoupled from structure fluctuations. Concentration dependence of shape fluctuations and structure fluctuations will be discussed. [1] M. Nagao et al., Phys. Rev. E 75, 061401 (2007). [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X8.00009: Dielectrophoresis of Functional Phospholipid Vesicles Victoria Froude, Yingxi Elaine Zhu Recently, there has been an emerging interest in using AC-dielectrophoresis (DEP) to transport and assemble phospholipid vesicles (liposomes) and nanoparticles to form functional bio-assemblies where the underlying charge polarization mechanism of colloids in AC fields strongly depends on nano-scaled surface charge. In this work, we study liposomes segregation and aggregation in the presence of nanocolloids and salts in which the biological functionality of liposomes is augmented by the physical functionality of inorganic coating and particles. Liposomes, synthesized by sonication with 1,2-Dioleoyl-sn-Glycero-3-Phosphate (DOPA), are manipulated at varied AC-field frequencies across fabricated micro-electrodes in a quadrapole configuration on glass. We observe the co-assembly of liposome and opposite-charged nanocolloids by confocal microscopy and SEM, where the smaller nanocolloids are captured in between liposome junctions to form stabilized composite vesicles at several distinct frequencies. We observe a strong dependence of the liposome DEP mobility on the number of nanoparticles present in suspension and propose a new mechanism based on charge segregation and charged nanocolloid entrainment in the double layer. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X8.00010: High-throughput Microsphere Encapsulation in Emulsion Droplets by Electrospray Wuen-shiu Chen, Keng-hui Lin Colloidal clusters generated through emulsion encapsulation and evaporation open up the possibilities for assembly of complex crystal structures. Encapsulation in monodisperse emulsion droplets facilitates higher yield of identical clusters as building blocks. We utilize electrospray in an oil-in-water co-flow fluidic device to generate uniform emulsion droplets in micron size and at the rate of ten thousand droplets per second. We investigate the effect of applied voltage, flow rate and the conductivity of liquid on the droplet formation. We further show that incorporation of microspheres into the inner oil fluid enables the encapsulation and formation of clusters. [Preview Abstract] |
Session X9: Focus Session: Spinor Condensates and Dipolar Gases
Sponsoring Units: DAMOPChair: Chris Greene, University of Colorado
Room: Morial Convention Center RO7
Friday, March 14, 2008 8:00AM - 8:12AM |
X9.00001: Amplification of quantum fluctuations across a quantum phase transition in a spinor BEC Sabrina Leslie, Jennie Guzman, Mukund Vengalattore, Dan Stamper-Kurn We study the amplification of quantum fluctuations in a $^{87}$Rb spinor BEC that is rapidly quenched from its paramagnetic phase to its ferromagnetic phase. By characterizing the onset of spontaneous ferromagnetism and the amplification properties of the spinor condensate, we probe the initial quantum fluctuations from which the resulting structures evolve. To characterize the spinor condensate as an amplifier, we temporally and spatially resolve the evolution of the vector magnetization profile as a function of the end point of the quench. In particular, we describe the formation of transversely magnetized domains and vortices as a function of the end point. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X9.00002: Equilibrium phases of a dipolar spinor Bose gas Mukund Vengalattore, S. R. Leslie, J. Guzman, C. Smallwood, D. M. Stamper-Kurn We investigate the effect of magnetic dipole interactions in determining the properties of $F=1$ spinor Bose gases of $^{87}$Rb. Due to the competition between the local ferromagnetic interaction and the long-range, anisotropic dipole interaction, we observe the spontaneous formation of modulated spin domains that exhibit crystalline order. The formation of this modulated spin texture is accompanied by the creation of spin vortices in this dipolar superfluid. We observe this modulated phase both as an equilibrium phenomenon by cooling an incoherent thermal spinor gas, and as a result of a dynamical instability in a pure transversely magnetized spinor condensate. We clarify the crucial role played by dipolar interactions in the creation of the crystalline phase in the spinor condensate and study the finite temperature phase diagram of this dipolar quantum fluid. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X9.00003: Dynamical instability of the XY spiral state of ferromagnetic condensates Robert Cherng, Vladimir Gritsev, Dan Stamper-Kurn, Eugene Demler We calculate the spectrum of collective excitations of the XY spiral state prepared adiabatically or suddenly from a uniform ferromagnetic F = 1 condensate. For spiral wavevectors past a critical value, spin wave excitation energies become imaginary indicating a dynamical instability. We construct phase diagrams as functions of spiral wavevector and quadratic Zeeman energy. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X9.00004: Spin Nematics and Quantum Fluctuation-Controlled Coherent Spin Dynamics of Hyperfine Spin F=2 Cold Atoms Jun Liang Song, Gordon Semenoff, Fei Zhou We show that quantum fluctuations lift the accidental continuous degeneracy that was found in the mean field analysis of spin nematic states of hyperfine spin F=2 $^{87}$Rb. Two distinct spin nematic states with higher symmetries are selected out depending on scattering lengths: a uniaxial spin nematic and a biaxial spin nematic. Recently we also study coherent spin dynamics mainly driven by quantum fluctuations. Unlike the usual mean-field driven dynamics, quantum fluctuation-controlled spin dynamics are sensible to the variation of fluctuations and the potential induced by quantum fluctuations can be tuned by four or five orders of magnitude in optical lattices. These dynamics have unique dependence on quadratic Zeeman fields and potential depth in optical lattices. We find that although these dynamics are difficult to observe in traps, it is possible to observe them in optical lattices; particularly they can survive in F=2 $^{87}$Rb condensates with a relatively short life time. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X9.00005: Nematic order by disorder in spin-2 BECs Ryan Barnett, Ari Turner, Eugene Demler, Ashvin Vishwanath The effect of quantum and thermal fluctuations on the phase diagram of spin-2 BECs is examined. They are found to play an important role in the nematic part of the phase diagram, where a mean-field treatment of two-body interactions is unable to lift the accidental degeneracy between nematic states. Quantum and thermal fluctuations resolve this degeneracy, selecting the uniaxial nematic state, for scattering lengths $a_4$ greater than $a_2$, and the square biaxial nematic state for $a_4$ less than $a_2$. Paradoxically, the fluctuation induced order is stronger at higher temperatures, for a range of temperatures below $T_c$. For the experimentally relevant cases of spin-2 $^{87}$Rb and $^{23}$Na, we argue that such fluctuations could successfully compete against other effects like the quadratic Zeeman field, and stabilize the uniaxial phase for experimentally realistic conditions. A continuous transition of the Ising type from uniaxial to square biaxial order is predicted on raising the magnetic field. These systems present a promising experimental opportunity to realize the `order by disorder' phenomenon. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X9.00006: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 9:12AM - 9:48AM |
X9.00007: Ultracold dipolar gases -- challenge for Experiments and Theory Invited Speaker: Recent experimental progress in trapping and cooling of molecular gases boosts an interest to the interdisciplinary field of quantum gases with dominant dipole-dipole interactions. An unprecedented level of experimental control together with specific physical properties of the dipole-dipole interaction provides a unique possibility to find new physical phenomena and practical applications. In this talk, recent achievements in studies of ultracold dipolar gases, both fermionic and bosonic, are presented. We focus our attention on many-body properties of such systems and discuss how the characteristic features of the dipole-dipole interaction: long range and anisotropy, affect their collective behavior and result in novel macroscopic quantum phenomena. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X9.00008: Spin-orbit interaction effects in cold atomic systems Tudor Stanescu, Victor Galitski We propose a scheme for the realization of spin-orbit interaction in cold atomic systems. We show that, in a system of trapped multi-level atoms moving in the presence of spatially modulated laser fields, the atom-laser interaction generates an emergent pseudo-spin-1/2 degree of freedom that couples to the momentum, leading to an effective spin-orbit interaction. The parameters of the spin-orbit coupling can be modified by controlling the laser fields. Atomic spin-orbit interacting systems open the possibility of studying new effects that are not usually considered, or not accessible in solid state systems. We consider explicitly the problem of strongly non-equilibrium spin dynamics by studying the evolution of an initially spin-polarized Fermi gas in a two-dimensional harmonic trap. We derive the non-equilibrium behavior of the polarization and show that it is characterized by periodic echoes with a frequency equal to the trapping frequency. We also consider a system of multi-level Bose atoms. In the presence of spin-orbit coupling, the single-particle band structure is generally anisotropic and contains two minima at finite momenta. At low temperatures, the bosons condense into these states, leading to a new type of Bose-Einstein condensate. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X9.00009: Dipolar bosons in an array of one-dimensional tubes Julia S. Meyer, Corinna Kollath, Thierry Giamarchi Ultra-cold atomic and molecular gases offer a unique possibility to realize a range of novel interacting many-body systems. While in solid state systems electrons interact via the long-range Coulomb interactions, the interactions in cold gases are essentially local. However, the use of dipolar atoms or molecules allows one to surmount this limitation. We investigate bosonic atoms or molecules interacting via dipolar interactions in a planar array of one-dimensional tubes. In the situation where the dipoles are oriented perpendicular to the tubes by an external field, various quantum phases can be realized by varying the strength of the interactions and the orientation of the dipoles with respect to the plane of the array. We find a `sliding Luttinger liquid' phase in which the tubes remain Luttinger liquids and two-dimensional charge density wave ordered phases with different kinds of order. In particular, a stripe phase in which the bosons in different tubes are aligned as well as a checkerboard phase occur. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X9.00010: Phase space deformation of dipolar Fermi gas Han Pu, Takahiko Miyakawa, Takaaki Sogo, Hong Lu We consider a system of quantum degenerate spin polarized fermions in a harmonic trap at zero temperature, interacting via dipole-dipole forces. Under the semi-classical framework, we introduce a variational Wigner distribution function to characterize the deformation and compression of the Fermi gas in phase space and use it to examine the stability of the system. We emphasize the important roles played by the Fock exchange term of the dipolar interaction which results in a non-spherical Fermi surface. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X9.00011: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 10:36AM - 10:48AM |
X9.00012: Towards Microwave Trapping of Cold Polar Molecules Y.-C. Chen, C.-C. Hsieh, T.-S. Ku, P. Dwivedi, R. Ho We describe our progress on production and trapping of cold SrF molecules. Specifically, we generate the SrF molecules by laser ablation of its solid precursor. We use helium buffer gas cooling as the first cooling stage to cool molecules down to a few Kelvin. To guide the generated cold molecules to ultrahigh vacuum region for further spectroscopic studies and trapping, we are also developing the electrostatic guiding of molecules. The experimental results will be presented. For a second-stage cooling in order to cool molecules down to ultracold regime, we plan to perform the sympathetic cooling of molecules with ultracold cesium atoms in a microwave trap. We have succeeded to build a microwave trap based on a high-power microwave Fabry-Perot resonator. We can couple 1.4 kW power into the cavity with a coupling efficiency more than 80{\%} under locked conditions. The trap depth for the absolute ground state of SrF molecules is 300mK. The design and performance as well as future improvements will be discussed in details. [Preview Abstract] |
Session X10: Superconducting Devices and Applications
Sponsoring Units: DCMPChair: Constantine Vlahacos, University of Maryland
Room: Morial Convention Center RO8
Friday, March 14, 2008 8:00AM - 8:12AM |
X10.00001: A tunable parametric amplifier based on a SQUID array resonator with nearly quantum-limited sensitivity Manuel Castellanos-Beltran, Konrad Lehnert Recently, there has been an increasing number of compelling applications for quantum-limited amplifiers at microwaves frequencies. These include the readout of superconducting qubits and ultrasensitive measurements of the motion of nanomechanical beams. Although quantum-limited microwave amplifiers have already been demonstrated, they have suffered from both limited bandwidth and dynamic range. We create a Josephson parametric amplifier from a transmission line resonator whose inner conductor is made from a series SQUID array. By changing the magnetic flux through the SQUID loops, we are able to adjust the circuit's resonance frequency and, consequently, the center of the amplified band over an octave (4-8 GHz). This tunability circumvents some of the problems related to a limited bandwidth. We will discuss recent results that include demonstration of large gain (30 dB), nearly quantum-limited sensitivity and noise squeezing. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X10.00002: Parametric amplification in a DC SQUID amplifier at 1.7 GHz Jose Aumentado, Lafe Spietz, K.D. Irwin At NIST we have recently developed a DC SQUID-based microwave amplifier employing a resonant input circuit. In conventional operation this amplifier can be operated in a linear, phase-preserving mode. However, it can also be operated as a degenerate parametric amplifier with the SQUID functioning as a first stage amplifier. We will discuss the performance of this hybrid operation and the possibility of generating squeezed states in this system. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X10.00003: Progress at NIST on DC SQUID Microwave Amplifiers Lafe Spietz, K. D. Irwin, J. Aumentado We report on the development at NIST of microwave amplifiers using DC SQUIDs. Our design approach is to use small SQUIDs which can be modeled as lumped element circuits, thus separating the design process for the SQUID from that of the microwave impedance transformers. We present our model of the impedance, gain and noise of such a SQUID and measurements of the characteristics of our amplifiers. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X10.00004: Input Impedance of the Microstrip SQUID Amplifier Darin Kinion, John Clarke We present measurements of the complex scattering parameters of microstrip SQUID amplifiers (MSA) cooled to 4.2 K. The input of the MSA is a microstrip transmission line in the shape of a square spiral coil surrounding the hole in the SQUID washer that serves as the ground plane. The input impedance is found by measuring the reverse scattering parameter (S11) and is described well by a low-loss transmission line model. We map the low-loss transmission line model into an equivalent parallel RLC circuit in which a resistance R, inductance L, and capacitance C are calculated from the resonant frequency, characteristic impedance and attenuation factor. Using this equivalent RLC circuit, we model the MSA and input network with a lumped circuit model that accurately predicts the observed gain given by the forward scattering parameter (S21). We will summarize results for different coil geometries and terminations as well as SQUID bias conditions. A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344 and by Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X10.00005: In vivo T$_{1}$ maps at fields from 0.001 to 150 mT with SQUID based MRI M. Hatridge, S. Busch, T. Wong, M. Moessle, A. Pines, J. Clarke High field magnetic resonance imaging (MRI) uses differences in the longitudinal relaxation times (T$_{1})$ of protons to differentiate tissue types. Using phantoms and \textit{ex vivo} human tissue, we have demonstrated that T$_{1}$ contrast at low fields can be much greater than that at high fields, suggesting that one can, for example, distinguish tumors from healthy tissue without the use of contrast agents. Our MRI system polarizes protons at fields up to 150 mT using a water-cooled electromagnet. Subsequently, we ramp down the polarizing field and measure the proton nuclear magnetic resonance at typically 0.132 mT in the presence of three-dimensional field gradients using a SQUID coupled to an untuned, second-derivative gradiometer. Our system is capable of imaging the human arm \textit{in vivo} with a resolution of 2x2x10 mm$^{3}$. By switching the field to an intermediate field value for a variable time, we can obtain T$_{1}$ maps at fields between 0.001 and 150 mT. We present \textit{in vivo} T$_{1}$ dispersion curves in the same range of magnetic fields for several types of tissue in the human arm. Work supported by USDOE. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X10.00006: Fast, single-photon detection with a superconducting Nb nanowire Anthony Annunziata, Andrew Mack, Joel Chudow, Daniel Santavicca, Aviad Frydman, Michael Rooks, Luigi Frunzio, Daniel Prober We investigate the performance of a superconducting nanowire detector made from an ultra-thin, pure Nb film. Single photon counting performance is shown with good quantum efficiency at 470 nm. We report the reset time, jitter, and dark count rate for single photon detection. We compare these results to reports for NbN detectors. The Nb detector has a faster reset time for the same size active area, with similar quantum efficiency. These detectors have a variety of potential applications ranging from VLSI circuit diagnostics to quantum communication and single molecule spectroscopy. This work is supported by NSF -- EPDT and IBM research. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X10.00007: Local superfluid densities probed via current-induced superconducting phase gradients Alexey Bezryadin, David Hopkins, David Pekker, Tzu-Chieh Wei, Paul Goldbart We have developed a superconducting phase gradiometer consisting of two parallel DNA-templated nanowires connecting two thin-film leads [1,2,3]. We have ramped the cross current flowing perpendicular to the nanowires, and observed oscillations in the lead-to-lead resistance due to cross-current-induced phase differences. By using this gradiometer we have measured the temperature- and magnetic-field dependence of the superfluid density, and observed an amplification of phase gradients caused by elastic vortex displacements. We examine our data in light of Miller-Bardeen theory of dirty superconductors and a microscale version of Campbell's model of field penetration. \newline [1] Hopkins et al., Science {\bf 308}, 1762 (2005). [2] Pekker et al., Phys. Rev. B {\bf 72}, 104517 (2005). [3] Hopkins et al., Phys. Rev. B Rapid Comm. (2007, in press), accepted for publication. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X10.00008: Entanglement of Two Josephson Vortex Quantum Bits in Resonant Cavity Isaac O'Bryant, Ramesh P. Dhungana, Ju H. Kim We investigate the entanglement between two Josephson vortex qubits (JVQ's) in a resonant cavity. A JVQ may be fabricated using two closely spaced microresistor sites in an insulator layer of a long Josephson junction. The phase dynamics of a Josephson vortex (or fluxon) may be described using the perturbed sine-Gordon equation. In a uniform electromagnetic field, it is found that the resonant cavity interacts with fluxons only when they are trapped on a microresistor site. The effect of a resonant cavity on the two JVQ's may be represented as a deformation of the two-qubit potential function. We examine the effects of resonant cavity and magnetic induction on the potential for two non-interacting JVQ's. The deformation of the potential due to the resonant cavity yields a significant increase in the two-fluxon tunneling compared to the single-fluxon tunneling, indicating that entanglement between the two JVQ's is significantly increased. We compute the concurrence to estimate how the entanglement is affected by the magnetic induction effect and the coupling between the fluxons and the resonant cavity. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X10.00009: Death of entanglement of two Josephson vortex qubit due to the dissipation effect Ramesh Dhungana, Isaac O'Bryant, Ju Kim We investigate the effects of the dissipation on the two entangled Josephson vortex qubits (i.e. JVQ) using spin-boson model. It has been suggested that the decoherence time for a JVQ can be long at ultra-low temperature because it couples only weakly to the sources of decoherence. The entanglement of two JVQs due to the magnetic induction effect between two long Josephson junctions and their coupling to a single mode resonant cavity may be destroyed due to the same source of decoherence, which are present in the environment. We consider the decoherence effect on the JVQ system by using a dissipative thermal bath. We estimate its effect on entanglement, which can be measured in terms of concurrence, to show that the entanglement may die down quickly due to this decoherence source. We compare the time scale for entanglement survival of two JVQs in the dissipative environment with the decoherence time for a single JVQ qubit and discuss its effect on the two qubit operation. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X10.00010: Probing Temperature Dependent Noise in Flux Qubits via Macroscopic Resonant Tunneling A.J. Berkley, R. Harris, M.W. Johnson, J. Johansson, P. Bunyk, S. Govorkov, M.C. Thom, S. Uchaikin, C.J.S. Truncik, M.H.S. Amin, S. Han, B. Bumble, A. Fung, A. Kaul, A. Kleinsasser, D.V. Averin Macroscopic resonant tunneling between the two lowest lying states of a bistable RF-SQUID is used to characterize flux noise in a potential qubit. Detailed measurements of incoherent decay rates as a function of flux bias revealed that the Gaussian shaped tunneling rate is not peaked at the resonance point, but is shifted to a flux bias at which the initial well is higher than the target well. This observation indicates that the dominant low frequency (1/f) flux noise in this device is quantum mechanical in nature. The r.m.s. amplitude of the noise, which is proportional to decoherence rate 1/T$^{*}_{2}$, was observed to be weakly dependent on temperature below 70 mK. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X10.00011: Simulation of a YBCO Superconducting Quantum Interference Filter Stephen M. Wu, Shane A. Cybart, John Clarke, R.C. Dynes A Superconducting Quantum Interference Filter (SQIF) consists of a serial or parallel combination of SQUIDs of varying area that at constant current bias produces a sharp voltage peak at zero magnetic field. We simulated a serial array of 300 SQUIDs to calculate the voltage response versus applied magnetic field. We chose representative values of the junction critical current I$_{C}$ and resistance R$_{N}$ for ion damaged YBCO Josephson junctions. We varied the areas to maximize the sharpness and height of the voltage peak. We used the results of the simulation to design a SQIF that we fabricated and tested. The measured voltage response of the device was smaller than the predictions of the model. The agreement was significantly improved by including the effects of the geometric inductances of the SQUID loops and the Fraunhofer diffraction pattern of the individual junctions, both of which reduced the predicted amplitude of the SQIF response. It is likely that the remaining discrepancies are due partly to random variations in I$_{C}$ and R$_{N}$ in the experimental device, which we shall include in future simulations, and partly to the effects of thermal noise. This work was supported by AFOSR. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X10.00012: Measurements of a YBCO superconducting quantum interference filter with planar ion-damaged Josephson junctions. Shane A. Cybart, S. Wu, I. Siddiqi, John Clarke, R.C. Dynes We have fabricated a superconducting quantum interference filter (SQIF) containing 300 SQUIDS connected in series. Loops in a YBCO film were patterned using photolithography and argon ion milling. The Josephson junction barriers were formed with ion bombardment through 30 nm wide slits that were patterned with electron beam lithography and reactive ion etching of a 25 nm germanium / 800 nm photoresist mask. The ion damage lowered the Tc of the 30 nm unmasked region resulting in SS'S junctions, where supercondconductor S has a Tc of 85 K, and S' of 75 K. The IcRn product for individual junctions was determined to be 0.02 mV from current-voltage characteristics measured at 73K. Voltage versus magnetic field curves were measured for different static bias currents. They show a dip at zero field which increases with increasing bias up to a saturation value of 2 mV. The maximum transfer coefficient was 17 mV/mT. Noise properties and linearity measurements will be presented. This work was supported by AFOSR, and by DOE through the LBNL Molecular Foundry. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X10.00013: Fabrication and evaluation of the superconducting d-dot device manufactured with the Y$_{0.9}$La$_{0.1}$Ba$_{1.9}$Cu$_{3}$O$_{y}$ thin film by a DC Sputtering method Masahide Nishiyama, Hiroaki Sato, Masuo Yamamoto, Seiji Adachi, Hironori Wakana, Keiichi Tanabe, Takakazu Ishida The d-dot device is composed of a square shaped d-wave superconductor buried into a s-wave superconductor thin film. The internal phase difference at neighboring square side junction causes a half-quantum-flux at each corner of square of d-wave superconductor. We have developed the method for preparing the d-dot with YBCO thin film by PLD method previously. In the present work, we employed a DC sputtered Y$_{0.9}$La$_{0.1}$Ba$_{1.9}$Cu$_{3}$O$_{y}$ thin film with ramp edge, which is a well-established standard process for fabricating high-T$_c$ single flux quantum (SFQ) device of SRL- ISTEC. Evaluations of this d-dot device are performed by I-V and R-T measurements. The magnetic flux structure has been investigated by a scanning SQUID microscope. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X10.00014: High frequency flux sampling SQUID microscope Constantine Vlahacos, John Matthews, Frederick Wellstood One important application of scanning SQUID microscopes is to locate electrical faults in integrated circuits and multi-chip modules. However, current computer microprocessors operate at over 1 GHz, well above the bandwidth of the present generation of SQUID microscopes. By removing the conventional flux-locked loop electronics we have overcome the bandwidth limitations of traditional scanning SQUID microscopes. Instead we use a pulsed sampling technique with a small Nb/AlO$_{x}$/Nb hysteretic dc SQUID. We present time-varying magnetic field images of room temperature samples obtained with the SQUID mounted on a 4.2 K pulse tube refrigerator in a scanning SQUID microscope, and discuss the advantages and limitations of this method. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X10.00015: Scanning Tunneling Spectroscopy Studies of AlN Tunnel Barriers Y. Li, J. Read, H. Tseng, R. Buhrman Typical Josephson junctions (JJ's) utilize alumina (AlO$_{x})$ tunnel barriers due primarily to the reliable thermal oxidation procedure that yield high quality Nb-Al-AlO$_{x}$-Nb JJs in the low and moderate ($\le $ 10$^{4}$ A/cm$^{2})$ critical current density (J$_{c})$ regime[1]. However, AlN provides the possibility of forming ultra-thin barriers with fewer defects, and hence lower sub-gap leakage currents, and thus could improve device performance in the high J$_{c}$ regime [2-4]. We present results from an X-ray photoelectron spectroscopy (XPS) and scanning tunneling spectroscopy (STS) study of thin AlN layers on Nb formed by reactive radio frequency (rf) sputtering from an AlN target in a mixture of Ar and N gases. The XPS spectra indicates that O is generally incorporated into the nitride layer during growth in high and near-ultra-high vacuum, thus forming AlO$_{x}$N$_{y}$. The STS measurements reveal that these AlO$_{x}$N$_{y}$ layers exhibit an increase in bandgap with increased N content in the process gas. Decreased band-tails and improved surface stability suggest the barrier defect density can be modified through moderate post-growth annealing. We will provide suggestions for optimization of rf sputtered AlO$_{x}$N$_{y}$ layers for use in high J$_{c}$ Nb and NbN based JJs. [1] Miller, APL 63, 1423 (1993) [2] Wang, APL 64, 2034 (1994) [3] Kleinsasser, IEEE TAS 5, 2318 (1995) [4] Kaul, JMRS 20, 3047 (2005) [Preview Abstract] |
Session X11: Focus Session: MgB2-like: Properties of Exotic Superconductors
Sponsoring Units: DMPChair: Peihong Zhang, University of Buffalo
Room: Morial Convention Center RO9
Friday, March 14, 2008 8:00AM - 8:12AM |
X11.00001: Symmetry properties and residual transport in superconducting PrOs$_4$Sb$_{12}$ Tayseer Abu Alrub, Stephanie Curnoe We identify a three-component order parameter in the triplet channel as the most probable candidate for superconductivity in PrOs$_4 $Sb$_{12}$. Two different superconducting phases have been observed in PrOs$_4$Sb$_{12}$; the lower temperature `B phase' occupies the bulk of the phase diagram and breaks time reversal symmetry while the higher temperature `A phase' is found in a narrow region below H$_{c2} $, and possibly does not exist at all. The gap function in the A phase is unitary and has two nodes in the $[001]$ direction. In the B phase, the gap function is nonunitary and the lower branch has four cusp nodes in the $[\pm\alpha,\pm\beta,0]$ directions. The conductivity tensor, due to isotropic impurity scattering, has inequivalent diagonal components due to the off-axis nodal positions of the B phase. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X11.00002: Scanning SQUID imaging of Sr$_{2}$RuO$_{4}$ and PrOs$_{4}$Sb$_{12}$ Clifford Hicks, John Kirtley, Martin Huber, Kathryn Moler We present scanning SQUID magnetometer data on the superconducting materials strontium ruthenate (Sr$_{2}$RuO$_{4})$ and praseodymium-osmium-antimonide (PrOs$_{4}$Sb$_{12})$, both of which are believed to have spin-triplet pairing and to generate spontaneous time-reversal-symmetry-breaking fields below their superconducting transition temperatures. Our images, taken with a SQUID with a resolution of 3$\mu $m and approximately 100$\mu $G, do not show evidence for spontaneous TRSB fields, in contrast with muon spin rotation data which indicates gauss-scale fields in both materials. The fields indicated by $\mu $SR data must therefore have a short length scale and/or a short time scale. Supposing that the TRSB fields are static with the magnitudes indicated by $\mu $SR data we place upper limits on their length scales in both Sr$_{2}$RuO$_{4}$ and PrOs$_{4}$Sb$_{12}$. We also place upper limits on the strength of any distributed fields that might exist at sample edges and order parameter domain walls. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X11.00003: Gauge-invariant electromagnetic response of a chiral $p_x+ip_y$ superconductor Roman Lutchyn, Pavel Nagornykh, Victor Yakovenko We study electromagnetic properties of spin-triplet superconductors with chiral $p_x+ip_y$ symmetry of the pairing order parameter. As a result of spontaneously broken time-reversal symmetry, the electromagnetic response of $p_x\!+ip_y$ superconductor contains additional (anomalous) terms that are not present in conventional superconductors. Using effective action approach, we show that in $p_x\!+ip_y$ superconductors an external electric field may generate transverse Hall-like currents which depend explicitly on the chirality of the pairing order parameter. We also find an analog of the London equation in the anomalous electromagnetic response which implies complete screening of Cooper-pair intrinsic orbital momentum. The implications of our results to the experiments on $\rm Sr_2RuO_4$[1,2] are discussed. \newline [1] J. Xia \emph{et. al.} Phys. Rev. Lett. {\bf 97}, 167002 (2006) \newline [2] J. R. Kirtley \emph{et. al.} Phys. Rev. B {\bf 76}, 014526 (2007) [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X11.00004: Stability of Half-Quantum Vortices in px+ipy Superconductors Suk Bum Chung, Hendrik Bluhm, Eun-Ah Kim We have analyzed the possibility of finding half-quantum magnetic vortices in a quasi-two-dimensional $p_x+ip_y$ superconductor (such as ${\rm Sr_2 RuO_4}$ is believed to be). The predicted exotic properties of these excitations - such as containing Majorana fermion core states with non-Abelian statistics - have recently attracted much attention for their potential application to topological quantum computation. However, these excitations have not been observed yet. In fact, an isolated half-quantum vortex has a divergent energy cost in the bulk due to the associated {\it unscreened} spin current. However, we have shown in our work that tightly bound pairs of half-quantum vortices with a finite separation may be stable or metastable when the ratio of spin superfluid density to superfluid density $\rho_{{\rm sp}}/\rho_{\rm s}$ is small - something we can reasonably expect in ${\rm Sr_2 RuO_4}$. Furthermore, we find that it might be possible to isolate them with present experimental techniques in submicron-sized samples; such samples may selectively allow only single half-quantum vortices to enter. Such an experiment which would be of great fundamental and potential practical interest. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X11.00005: Angle-resolved Photoemission Study of Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$ Madhab Neupane, P. Richard, Z.-H. Pan, Y. Xu, R. Jin, D. Mandrus, X. Dai, Z. Wang, H. Ding We report angle-resolved photoemission spectroscopy results of the Fermi surface of Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$ which is at the boundary between a magnetic metal and an antiferromagnetic insulator in the phase diagram of the Ca-substituted strontium ruthenates. We did observe an orbital-selective Mott transition, which is, however, different with what has been predicted theoretically [1] for this material. Our ARPES results are consistent with both magnetic and transport properties observed in this material. \newline [1] V. I. Anisimov et al., Eur. Phys. J. B \textbf{25}, 191(2002) [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X11.00006: Superconducting pairing symmetries in the 3-K and bulk phases of Sr$_2$RuO$_4$ Zhenyi Long, Benjamin Clouser, Ronald Meyer, Songrui Zhao, Kelly McCarthy, Ying Liu, Hiroshi Yaguchi, Yoshiteru Maeno, Tijiang Liu, Zhiqiang Mao We report recent progress on our single-particle tunneling and phase-sensitive measurements on 3-K and bulk phases of Sr$_2$RuO$_4$. The latter refers to an eutectic phase of Sr$_2$RuO$_4$ that features Ru microdomains embedded in a single crystal of Sr$_2$RuO$_4$. These Ru microdomains are of a mesoscopic size and varying shape. Therefore their pairing symmetries are not subject to the same set of constraints as those in the bulk. We have performed tunneling measurements on the 3-K phase to identify $all$ possible pairing states in this unique superconducting system. The junctions used earlier were prepared by pressing In wire onto a cleaved $ab$ face of a Ru-containing Sr$_2$RuO$_4$ single crystal containing multiple Ru microdomains. More recently we focused on tunnel junctions prepared on pre-selected single Ru microdomains. Possible existence of an intrinsic mixed pairing state in the interior of a Ru microdomain featuring simultaneous presence of both the $s$- and $p$-wave superconducting condensates. We will also discuss briefly our current effort in the phase-sensitive measurements on bulk Sr$_2$RuO$_4$, focusing on detecting possible $/k_z$ dependence of the order parameter and the existence of domains. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X11.00007: Hall Conductivity in a Spin-Triplet Superconductor Wonkee Kim, F. Marsiglio, C. S. Ting We calculate the Hall conductivity for a spin-triplet superconductor, using a generalized pairing symmetry dependent on an arbitrary phase, $\phi$. A promising candidate for such an order parameter is Sr$_{2}$RuO$_{4}$, whose superconducting order parameter symmetry is still subject to investigation. The value of this phase can be determined through Kerr rotation and DC Hall conductivity measurements. Our calculations impose significant constraints on $\phi$. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X11.00008: Upper Critical Field and SdH Quantum Oscillation Studies in Organic Superconductor $\beta''$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ Kyuil Cho, Braunen E. Smith, William A. Coniglio, Laurel Winter, Charles C. Agosta, John A. Schlueter Upper critical fields and SdH quantum oscillations in the organic superconductor $ \beta''$-(BEDT-TTF )$_2$SF$_5$CH$_2$CF$_2$SO$_3$ have been studied by measuring the in-plane rf penetration depth with a tunnel diode oscillator technique. Previous measurements from other groups, with the applied field parallel to the conducting layers, were inconsistent. We report here that for the applied field parallel to the conducting layers the low temperature upper critical fields exceed the Pauli paramagnetic limit calculated by using a semi-empirical method. We will also discuss SdH quantum oscillation with the applied field perpendicular to the conducting layers, which has been found to have a frequency of 181 T and an effective mass of 1.84 $m_e$. The effective mass is consistent with those of other groups, but the oscillation frequency smaller than theirs ($\sim$196 T). [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X11.00009: Absolute penetration depth measurements in deuterated organic superconductor $\kappa $-(ET)$_{2}$Cu[N(CN)$_{2}$]Br Tyson Olheiser, N. Salovich, R.W. Giannetta, J. Schlueter Penetration depth measurements were performed on fully deuterated $\kappa $-(ET)$_{2}$Cu[N(CN)$_{2}$]Br, an organic superconductor. A novel aluminum plating technique was used to determine the absolute penetration depth $\lambda $(T). As the cooling rate is varied from 30 mK/min to 180 K/min we observe systematic changes in T$_{C}$, superconducting fraction and $\lambda $(T). The data is analyzed using a model of superconducting domains embedded in an antiferromagnetic background. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X11.00010: Determination of the Pauli Paramagnetic Limit in Quasi 2D Superconductors Braunen E. Smith, Kyuil Cho, William A. Coniglio, Charles C. Agosta We have calculated the Pauli paramagnetic limit ($H_p$) for different quasi 2D superconductors using a semi-empirical method. We then compared the calculated Pauli paramagnetic limits to penetration depth data obtained using a tunnel diode oscillator technique at low temperatures in a swept applied magnetic field. The organic superconductors examined are layered such that their behavior is dependent on their orientation to the applied magnetic field. In order to eliminate the effect of vortex dynamics, we examined data taken with the conducting layers oriented parallel to the applied magnetic field. For one of these materials, $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$, we find that eliminating vortex effects leaves us with one remaining feature in the data that may correspond to $H_p$. We also find that the material $\beta''$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ exhibits a change in slope for temperature versus upper critical field when the upper critical field exceeds the calculated $H_p$. In addition, many of the examined quasi 2D superconductors, including the above organic superconductors and CeCoIn$_5$, exhibit upper critical fields that exceed their calculated $H_p$ suggesting some type of non-conventional superconductivity. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X11.00011: Second{\_}harmonic thermal conductivity of LSCO in magnetic fields Albert Migliori, Jon Betts, Yoko Suzuki, Izabela Stroe We describe a second harmonic technique to probe the thermal conductivity of LSCO with superconductivity suppressed by high magnetic fields. The technique is suitable for the high noise environment of pulsed magnets. By subtracting the thermal conductivity in field and at zero field, we obtain information about the temperature dependence of the order parameter. Work performed under the auspices of the National High Magnetic Field Laboratory [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X11.00012: Disorder induced weak and strong localization and their influence on superconductivity in underdoped Bi2+xSr2-xCuO6 Huiqian Luo, Peng Cheng, Hai-Hu Wen In-plane resistivity and magnetoresistance(MR) measurements were carried out on high-quality underdoped Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+\delta}$ (0.1$\leq$x$\leq$0.4) single crystals. The results show that the superconductivity and the normal state MR behaviors strongly rely on the doping level of samples. In the slightly doping level of Bi, a metallic normal state and a finite superconducting transition temperature are observed, and there is always a positive MR in the normal state which is explained by the fluctuating superconductivity mixed with the transport of quasiparticles. With further doping, the low temperature resistivity shows an up turn together with a negative MR. Detailed analysis on the negative MR in this region may suggest that weak localization effect is dominant. As the superconductivity is depressed by more doping and thus more disorders, the delocalization gets much harder and the spin-order contribution may be involved in MR. Moreover, in the heavily underdoped doping, the superconductivity is suppressed completely and resistivity evolves into a strong localization behavior with a Coulomb gap opening at the Fermi energy. After summarizing the doping dependence of MR, we construct a new phase diagram to illustrate how does the disorder give the influence both on the superconductivity and magnetoresistance. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X11.00013: Unusual diamagnetic response in p-wave superconductors $\rm Sr_2RuO_4$ Lu Li, Joseph Checkelsky, W. F. Brinkman, M. Kriener, Y. Maeno, Nai Phuan Ong The magnetization $\bf M$ of the $p$-wave superconductor $\rm Sr_2RuO_4$ has not been measured previously in the geometry with magnetic field $\bf H\| c$ because of the small upper critical field ($H_{c2}\sim$ 660 Oe) and low $T_c$ (1.4 K). We have used high-resolution torque magnetometry to study in detail the magnetization curves in this geometry. We find that, in the superconducting state, the $M$-$H$ curves display highly unusual hysteretic behavior. In the critical state, whenever $\rm H$ crosses zero, we observe a break in the slope $\partial M/\partial H$. In a broad field interval $[-H_0, H_0]$ bracketing zero field, $\rm M$ is reversible (to our resolution) under reversal of sweep direction. This anomalous behavior is not encountered in conventional type-II superconductors, where the critical-state behavior is always non-reversible. A possible interpretation of these unusual features is the existence of reversible edge currents. We also discuss the magnetization curves with $\bf H \| ab$, where $\rm M$ jumps sharply at $\rm H = H_{c2}$. Research supported by NSF grant DMR 0213706. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X11.00014: Emergence of dissipative structures in current-carrying stabilized superconducting wires. George Levin, Paul Barnes, Jose Rodriguez, John Bulmer, Jake Connors We discuss the emergence of a dissipative structure in current-carrying superconducting wire. This is a phenomenon similar to the emergence of thermal convection cells, oscillatory chemical reactions, etc. In response to the initial localized temperature perturbation that leads to current exchange between the superconductor and the stabilizer the temperature and critical current density of the wire acquire spontaneous spatial modulation that forces the transport current to crisscross the interface between the superconducting film and metal stabilizer. This generates additional heat that makes such a structure self-sustainable. The central role in this phenomenon is played by the interfacial resistance between the stabilizer and superconducting film. The spatial scale of the modulation is of the order of the thermal diffusion length. This resistance also determines the speed of propagation of the conventional normal zone. We will present the results of numerical and analytical analysis of a model which describes current sharing between the superconducting and normal metal layers - a configuration typical of the state of the art YBCO-coated conductors. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X11.00015: Thermoelectric effects and band-dependent scattering of normal-state quasiparticles in spin-triplet superconductor Sr$_2$RuO$_4$ Zhuan Xu, Xiangfan Xu, Tijiang Liu, David Fobes, Zhiqiang Mao, Ying Liu We present the $first$ measurement on Nernst effects in the normal state of odd-parity, spin-triplet superconductor Sr$_2$RuO$_4$. Below 100 K, the negative Nernst signal was found to be large and nonlinear as a function of magnetic field with its absolute value increasing rapidly as the temperature was lowered. After reaching a maximum around $T$ = $T^*$ = 20 - 25 K, however, the Nernst signal drops linearly with the decreasing temperature. No corresponding feature was found around this temperature in the specific heat. We argue that the large value of the Nernst signal is related to the presence of multibands and the nonlinearity to band-dependent magnetic fluctuation in Sr$_2$RuO$_4$. Furthermore, the quasiparticle scattering from the magnetic fluctuation is suppressed below $T^*$ due to the emergence of coherence among quasiparticles in the $\gamma$ band, an active band for superconductivity in Sr$_2$RuO$_4$. Results on temperature dependence of the thermopower, which was seen to exhibit a sharp kink around $T^*$, provided further support to this picture of band-dependent normal-state properties. Our thermoelectric measurements appear to suggest that the suppression of the magnetic fluctuation makes it possible for the spin-triplet superconductivity to emerge in Sr$_2$RuO$_4$. [Preview Abstract] |
Session X12: Charge Density Wave and Charge Order
Sponsoring Units: DCMPChair: Eduardo Fradkin, University of Illinois at Urbana-Champaign
Room: Morial Convention Center 203
Friday, March 14, 2008 8:00AM - 8:12AM |
X12.00001: Fermi Surface Evolution Across Multiple CDW Transitions in RTe$_{3}$ R.G. Moore, V. Brouet, J. Laverock, S. Dugdale, R. He, N. Ru, I.R. Fisher, Z.-X. Shen The Fermi surface (FS) evolution across multiple charge density wave (CDW) transitions is investigated using angle-resolved photoemission spectroscopy. Low temperature measurements reveal two incommensurate CDW gaps created by perpendicular FS nesting vectors. A larger gap ($\sim $250 meV) arising from a CDW with q$_{CDW}\sim $0.7c* is in good agreement with the expected trend determined from light rare earth members of the bi-layer family of rare earth tritelluride compounds (RTe$_{3})$. A second, smaller gap ($\sim $50 meV) is due to a second CDW with q$_{CDW}\sim $0.7a* never before seen in other RTe$_{3}$ compounds. The temperature dependence of the FS and the two CDW gaps is characterized. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X12.00002: Momentum and time dependent electronic dynamics in the CDW compound TbTe3 Felix Schmitt, Patrick Kirchmann, Laurenz Rettig, Marcel Krenz, Nancy Ru, Jun-Haw Chu, Rob Moore, Uwe Bovensiepen, Martin Wolf, Ian Fisher, Zhi-Xun Shen The rare earth tritellurides RTe3 are charge-density-wave (CDW) systems in which the coupling strength can be tuned by substituting different rare earths. By using ultrashort IR pump and UV probe laser pulses, we have performed time resolved angular resolved electron photoemission spectroscopy (trARPES) on TbTe3 and have observed the time-resolved closing of the CDW gap in the electronic band structure. To our knowledge, this is the first time-resolved study showing momentum dependent charge dynamics. We will also talk about other results on RTe3 compounds. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X12.00003: Phonon Softening and Displacement Pattern in Commensurate Charge Density Wave in 2H-TaSe$_{2}$ Tom Berlijn, Wei-Guo Yin, Wei Ku The formation of the commensurate charge density wave (CCDW) phase of 2H-TaSe$_{2}$ is investigated via a first-principles study of the phonon spectrum. Both the linear response and the frozen phonon approach reveal strong softening of the $\Sigma_{1}$ branch at the CCDW wave vector (2$\Gamma $M/3), as observed by inelastic neutron scattering[1]. Furthermore, to resolve the disagreement between currently proposed patterns by neutron scattering[1], electron diffraction[2] and TDPAC[3], the size and the pattern of the CCDW displacement are evaluated by an $ab initio$ total energy calculation. The role of nesting and electron-phonon coupling in the miscroscopic origin of the instability will be addressed. [1] D.E. Moncton, J.D. Axe and F.J. DiSalvo, Phys. Rev. B \textbf{16}, 801 (1977), [2] D.M. Bird, S. McKernan and J.W. Steeds, J. Phys. C \textbf{18} 499 (1985), [3] T. Butz, S. Saibene and A. Lerf, J. Phys. C \textrm{19} 2675 (1986). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X12.00004: Quantum and classical mode softening near the charge-density-wave/superconductor transition of Cu$_{x}$TiSe$_{2}$: Raman spectroscopic studies Minjung Kim, Harini Barath, S.L. Cooper, P. Abbamonte, E. Fradkin, E. Morosan, R.J. Cava We report temperature- and x-dependent Raman studies of the charge density wave (CDW) amplitude modes in Cu$_{x}$TiSe$_{2}$, which allow us to study the temperature- and x-dependence of the soft mode in this system. Among the key results: we find that the A$_{1g}$ amplitude mode exhibits identical power law scaling with the reduced temperature, p=T/T$_{CDW}$, and the reduced Cu content, p=x/x$_{c}$, i.e., $\omega _{o}\sim $(1-p)$^{0.15}$, suggesting that mode softening is independent of the control parameter used to approach the CDW transition; we provide evidence that x-dependent mode softening originates from the expansion of the lattice, which leads to a x-dependent reduction of the electron-phonon coupling constant; and we infer from our x-dependent mode softening results the presence of a quantum critical point, x$_{c}$(T=0)$\sim $0.07, within the superconducting phase of Cu$_{x}$TiSe$_{2}$. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X12.00005: Insulating charge density wave for a half-filled SU(N) Hubbard model with an attractive on-site interaction in one dimension Jize Zhao, Kazuo Ueda, Xiaoqun Wang We study a one-dimensional SU(N) Hubbard model with an attractive on-site interaction and $N>2$ at half-filling on the bipartite lattice using density-matrix renormalization-group method and a perturbation theory. We find that the ground state of the SU(N) Hubbard model is a charge density wave state with two-fold degeneracy. All the excitations are found to be gapful, resulting in an insulating ground state, in contrast to that in the SU(2) case. Moreover, the charge gap is equal to the Cooperon gap, which behaves as $-2Nt^2/(N-1)U$ in the strong coupling regime. However, the spin gap $\Delta_{s}$ and the quasiparticle gap $\Delta_{1}$ as well open exponentially in the weak coupling region, while in the strong coupling region, they linearly depend on $U$ such that $\Delta_{s}\sim -U(N-1)$ and $\Delta_{1}\sim -U(N-1)/2$. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X12.00006: Spin-soliton excitations from the Bond-Charge-Density Wave spin-Peierls state in the $\frac{1}{4}$-filled band R.T. Clay, R.P. Hardikar, S. Mazumdar The spin-Peierls (SP) transition is commonly discussed in the context of the $\frac{1}{2}$-filled band where the SP state is bond-dimerized. Spin excitations from the SP state generate spin solitons in pairs with opposite-phased bond alternation in between the solitons. As thermal excitations generate additional solitons, oppositely-phased regions overlap and ultimately give way to the uniform phase. Throughout the process the charges on the sites remain uniform. This simple description has to break down in $\frac{1}{4}$-filled band systems where the SP state is a Bond-Charge-Density Wave (BCDW) state with coexisting bond and charge-tetramerization. At $\frac{1}{4}$-filling spin excitations are necessarily accompanied by changes in site charges. We prove the highly interesting result that site charges here can change locally in two different ways, leading to two different kinds of spin solitons. Which kind of soliton dominates is parameter-dependent. The two kinds of solitons promote two different high temperature states, which are either bond-dimerized or charge-dimerized. We discuss experimental consequences of our work. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X12.00007: Measurements of the effect of charge density wave fluctuations on the $^{87}$Rb spin echo decay rate in Rb$_{0.30}$MoO$_{3}$. W.G. Clark, Guoqing Wu, S.E. Brown Measurements and interpretation of the spin echo decay rate (1/$T_{2})$ for the $^{87}$Rb quadrupolar split satellite lines in the charge density wave (CDW) system Rb$_{0.30}$MoO$_{3}$ (blue bronze) are reported over the temperature ($T)$ range 80 K to 250 K at several alignments of the magnetic field (\textbf{\textit{B}} = 9.00 T). In contrast to the central transition, where the contribution of CDW fluctuations to 1/$T_{2}$ is very small, they provide a large contribution to 1/$T_{2}$ for the satellite lines below the CDW transition at $T_{CDW} \quad \approx $ 180 K. This sensitivity to CDW fluctuations shows that 1/$T_{2}$ of the satellite lines should be a much more sensitive probe of both electric field driven and thermal motions of the CDW than the effects of density wave motion on the central transition, which has been used for prior work on this topic. A major goal of our future work is to exploit this property to obtain detailed measurements of electric field driven and thermal motion of the CDW in this material over a wide temperature range. This work has been supported by NSF Grants DMR-0334869 (WGC) and 0520552 (SEB). [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X12.00008: Electro-Optic Search for Critical Divergence of the Charge-Density-Wave Diffusion Constant at the Onset of Depinning in Blue Bronze J.W. Brill, L. Ladino, E.G. Bittle, M. Uddin We have used electro-reflectance measurements to study charge-density-wave (CDW) polarization dynamics at voltages near the CDW depinning onset (V$_{on})$ in the quasi-one-dimensional conductor blue bronze. For low voltages, where the phase-slip rate is low, it is expected that the polarization relaxation time should be inversely proportional to the CDW diffusion constant, which is expected to diverge at V$_{on}$. At T = 78 K, we observe saturation of the relaxation time at low voltages, suggesting that we are in this low phase-slip, ``elastic'' limit and allowing us to estimate the non-critical value of the CDW diffusion constant D($\infty )$ $\sim $ 0.02cm$^{2}$/s, consistent with the measured phason velocity. At other temperatures, the relaxation time increases with decreasing voltage even at the lowest voltages we could measure, indicating we are still dominated by phase-slip. In no case, however, do we observe the expected ``critical speeding up'', setting an upper limit on the critical region of (V/V$_{on}$ -1)$_{critical} <$ 0.06. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X12.00009: Position and Current Dependence of Charge-Density-Wave Polarization Dynamics L. Ladino, J.W. Brill We have studied the frequency and position dependence of charge-density-wave (CDW) polarization by simulating the response to square-waves of variable amplitude and frequency using parameters appropriate for niobium triselenide at T = 90 K, in its upper CDW state. For these simulations, we have numerically solved the phase-slip augmented diffusion model introduced by Adelman \textit{et al} (Phys. Rev. B \underline {53}, 1833 (1996)) for time domain studies. At each position in the sample, the frequency dependence was fit to a modified harmonic oscillator expression and the position and current dependence of the fitting parameters determined. In particular, both the delay time (1/resonant frequency) and relaxation time decrease with increasing current (and phase-slip rate) and increase with distance from the contact, with the delay time vanishing adjacent to the contact, as experimentally observed with electro-optic measurements in blue bronze. No decay of the polarization at long times is observed however, in contrast to electro-optic results. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X12.00010: Thermoelectric studies of charge density wave dynamics. Ross McDonald, Neil Harrison, John Singleton The conventional pyroelectric effect is intimately connected to the symmetry, or rather lack of center of symmetry, of the material. Although the experiments we discuss involve studies of low symmetry materials, the pyroelectric currents observed are of an entirely new origin. Systems with broken-translational-symmetry phases that incorporate orbital quantization can exhibit significant departures from thermodynamic equilibrium due to a change in magnetic induction. For charge density wave systems, this metastable state consists of a balance between the density-wave pinning force and the Lorentz force on the extended currents due to the drift of cyclotron orbits. In this way the density wave pinning potential plays a similar role to the edge potential in a two-dimensional electron gas, leading to a large Hall angle and quantization of the Hall resistance. A thermal perturbation that reduces the pinning potential returns the system towards thermal equilibrium, via a phason avalanche orthogonal to the sample surface. The observation of this new form of pyroelectric effect in the high magnetic field phase (B~$>$~30~T) of the organic charge transfer salt $\alpha $-(BEDT-TTF)$_{2}$KHg(SCN)$_{4}$, thus provides a measure of the phason thermopower. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X12.00011: Super-Crystalline CDW Phase in Organic Conductor (Per)$_2$Pt(mnt)$_2$ Si Wu, Andrei Lebed We suggest a model [1,2], where phase transitions between the Peierls and Super-Crystalline [or soliton wall superlattice (SWS)] charge-density-wave (CDW) phases occur in a magnetic field. The model accounts for peculiarities of an electron spectrum in a quasi-one-dimensional (Q1D) conductor (Per)$_2$Pt (mnt)$_2$ . In particular, we show that the Pauli spin- splitting effects improve the nesting properties of a realistic Q1D electron spectrum, and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In low and very high magnetic fields, a periodic SWS (or Super- Crystalline) phase is found to be a ground state. We discuss [3] possible experimental investigations of the theoretically predicted phase transitions in (Per)$_2$Pt(mnt)$_2$ to discover a unique SWS phase.\newline \newline [1] A.G. Lebed and Si Wu, Phys. Rev. Lett. 99, 026402 (2007). \newline [2] A.G. Lebed and Si Wu, JETP Lett. 86, 135 (2007). \newline [3] Si Wu and A.G. Lebed, Phys. Rev. B, submitted (2008). [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X12.00012: Temperature dependence of charge-ordering in (TMTCF)$_2$X, C = S, Se Sumit Mazumdar, Rahul Hardikar, R. Torsten Clay Quasi one-dimensional 1/4-filled band charge transfer solids (CTS) undergo two distinct phase transitions as temperature decreases. At high temperature ($\approx$ 100K) these materials undergo a 4k$_F$ (period- two) charge or bond ordering transition. At low temperature, these CTS undergo a magnetic transition to either a spin-Peierls (SP) or anti-ferromagnetic (AFM) ground state, both of which coexist with charge-order (CO). Understanding the relationship between the high and low temperature CO states is a key problem here. We show that (i) the critical nearest neighbor Coulomb interaction V that drives the high temperature Wigner crystal CO is spin-dependent; (ii) as a consequence, for intermediate values of V, there occurs a transition from the Wigner cystal CO to a Bond-Charge-Density wave (BCDW) can occur as temperature decreases. This transition is consistent with recent NMR observations of a charge redistribution occurring simultaneously with the SP state. Our theory is able to explain the competition between the Wigner crystal and SP phases, as well as the occurrence of two difference AFM phases. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X12.00013: Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)$_2$Cu(mnt)$_2$ J.L. Musfeldt, S. Brown, S. Mazumdar, R.T. Clay, M. Mas-Torrent, C. Rovira, J.C. Dias, R.T. Henriques, M. Almeida We measured the variable temperature infrared response of the spin ladder candidate (DTTTF)$_2$Cu(mnt)$_2$ in order to distinguish between two competing ladder models, rectangular versus zigzag, proposed for this family of materials. The distortion along the stack direction below 235 K is consistent with a doubling along $b$ through the metal-insulator transition. While this would agree with either of the ladder models, the concomitant transverse distortion rules out the rectangular ladder model and supports the zigzag scenario. A mode analysis provides the microscopic basis for this distortion and an estimate for the degree of charge ordering. Intramolecular distortions within the DTTTF building block molecule also give rise to on-site charge asymmetry. [Preview Abstract] |
Session X13: Quantum Monte Carlo Methods and Strongly Correlated Systems
Sponsoring Units: DCOMPChair: Anders Sandvik, Boston University
Room: Morial Convention Center 204
Friday, March 14, 2008 8:00AM - 8:12AM |
X13.00001: Variational approach for 1D antiferromagnetic Heisenberg chain with matrix-product states Ying-Jer Kao, Ling Wang, Anders Sandvik In order to explore the practical applicability of variational Monte Carlo simulations based on matrix-product states (MPS) [1], we present two implementations for the one-dimensional antiferromagnetic Heisenberg model with periodic boundary conditions [2]. We compare the convergence properties of two different schemes, which use either two sets of matrices corresponding to the two sublattices, or a 2-spin block representation. It is found that the use of symmetries considerably speeds up the convergence with the matrix size D. We also present an efficient ``cooling'' schedule for the stochastic method used to optimize the matrices, which significantly reduces the computational effort. Finally, we will discuss application of the scheme to n-leg ladders with periodic boundary condition. \newline [1] A. W. Sandvik and G. Vidal, arXiv:0708.2232. \newline [2] Y.-J. Kao, L. Wang, and A. W. Sandvik (unpublished) [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X13.00002: Excited states from variational Monte Carlo simulations with matrix-product states Ling Wang, Ying-Jer Kao, Anders Sandvik We report a further development [1] of a recently proposed variational Monte Carlo method for matrix-product states (MPS) [2]. Using the frustrated $J_1-J_2$ Heisenberg chain as a test case, we show how the matrices can be optimized not just for the ground state, but also, simultaneously, for the lowest states in several different lattice and spin symmetry sectors. This is useful in, e.g., studies of quantum phase transitions associated with crossings of excited-state energies. \newline [1] Y.-J. Kao, L. Wang, and A. W. Sandvik (unpublished) \newline [2] A. W. Sandvik and G. Vidal, arXiv:0708.2232. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X13.00003: Scale-renormalized matrix-product states for correlated quantum systems Anders Sandvik A generalization of matrix product states (MPS) is introduced which is suitable for describing interacting quantum systems in two and three dimensions. These {\it scale-renormalized matrix-product states} (SR-MPS) are based on a course-graining of the lattice in which the blocks at each level are associated with matrix products that are further transformed (scale renormalized) with other matrices before they are assembled to form blocks at the next level. Using variational Monte Carlo simulations of the two-dimensional transverse-field Ising model as a test, it is shown that the SR-MPS converge much more rapidly with the matrix size than a standard MPS. It is also shown that the use of lattice-symmetries speeds up the convergence very significantly. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X13.00004: DMRG applied to quantum impurity models Andreas Weichselbaum, Jan von Delft Quantum impurity models are analyzed routinely and reliably at very low energies using the Numerical Renormalization Group (NRG). Its great benefit of energy scale separation, however, comes at the cost of limited resolution at finite energy. By realizing that the NRG shares the same algebraic structure as the density matrix renormalization group (DMRG) given in terms of matrix product states, several strict NRG constraints such as its rigid discretization scheme in energy space can be relaxed due to the variational principle of DMRG. Our recent work in that respect will be discussed. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X13.00005: Ameliorating the sign problem for frustrated magnets using plaquette grouping Kevin Beach Frustrated quantum magnets are not amenable to simulation using conventional quantum Monte Carlo because of the infamous sign problem. In the overcomplete basis of singlet product states, updates have a many-to-one property that allows for grouping of updates around plaquettes in such a way that the negative sampling weights are almost entirely eliminated. Results for the J1-J2 quantun Heisenberg model on the square lattice are discussed. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X13.00006: Bold Diagrammatic Monte Carlo: Generic Technique for Polaron Problems (and More?) Boris Svistunov, Nikolay Prokof'ev We introduce a Monte Carlo scheme for sampling bold-line diagrammatic series specifying an unknown function in terms of itself. The range of convergence of this bold(-line) diagrammatic Monte Carlo (BMC) is significantly broader than that of a simple iterative scheme for solving integral equations. With BMC technique, a moderate ``sign problem" turns out to be an advantage in terms of the convergence of the process. As an illustrative application, we solve the problem of fermipolaron (one spin-down particle interacting with the spin-up fermionic sea). The problem solved is prototypical for all polaron problems, and, probably, for many-particle systems as well. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X13.00007: Interfacing Determinant Quantum Monte Carlo and Density Functional Theory Nikolai Zarkevich, Zhaojun Bai, Sergey Savrasov, Richard Scalettar, Mark Jarrell Over the last decade many body theory and electronic structure calculations have come together within the ``LDA+DMFT" approach in which dynamical mean field theory (DMFT) provides a frequency dependent self-energy $\Sigma(\omega)$ for electronic structure calculation within the local density approximation (LDA). Here we describe initial results with a new approach which uses the determinant Quantum Monte Carlo method to supply the self energy. This technique has the advantage of providing a momentum dependent $\Sigma({\bf k},\omega)$. However, the fermion sign problem can limit the ability to access the ground state value of the self energy. We present tests of the approach on a model of cuprate superconductors. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X13.00008: Spin waves and local magnetizations on the Penrose tiling Attila Szallas, Anuradha Jagannathan The Penrose tiling is a perfectly ordered two dimensional structure with fivefold symmetry and scale invariance under site decimation. Quantum spin models on such a system can be expected to differ significantly from more conventional structures as a result of its special symmetries. We consider a Heisenberg antiferromagnet on the Penrose tiling, a quasiperiodic system having an inhomogeneous Neel-ordered ground state. Spin wave energies and wavefunctions are studied in the linear spin wave approximation. A linear dispersion law is found at low energies, as in other bipartite antiferromagnets, with an effective spin wave velocity lower than in the square lattice. Spatial properties of eigenmodes are characterized in several different ways. At low energies, eigenstates are relatively extended, and show multifractal scaling. At higher energies, states are more localized, and, depending on the energy, confined to sites of a specified coordination number. The ground state energy of this antiferromagnet, and local staggered magnetizations are calculated. Perpendicular space projections are shown, showing the underlying simplicity of this ``complex'' ground state. A simple analytical model, the two-tier Heisenberg star, is presented to explain the staggered magnetization distribution in this antiferromagnetic system. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X13.00009: Loop Algorithm for the SU(N) Heisenberg Model Naoki Kawashima The SU(N) generalization of the Heisenberg model is studied with a new loop algorithms with non-binary loop variables.[1] The split-spin representation is used for high-dimensional representations. While we have confirmed our previous result[3] that the ground state switches from the Neel state to the VBS state around N=5 for the fundamental representation, we also find that there is an apparent U(1) symmetry in the VBS state. For higher representation, we have not observed any VBS state, although the disappearance of the Neel order parameter has been detected as we increase N. \ \\ \ \\ {}[1] N. Kawashima and K. Harada, J. Phys. Soc. Jpn. {\bf 73} 1397 (2004).\\ {}[2] N. Kawashima and Y. Tanabe, Phys. Rev. Lett. {\bf 98} 057202 (2007).\\ {}[3] K. Harada, N. Kawashima and M. Troyer, Phys. Rev. Lett. {\bf 90} 117203 (2003). [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X13.00010: Dynamical properties of SSH and breathing type Hamiltonians Hans Gerd Evertz, Peter Pippan Using a QMC method based on exact phonon integration in Fourier space and on loop updates in particle space, we study fermionic systems coupled to dynamical phonons in one dimension. Within this method it is possible to investigate Su-Schrieffer-Heeger (SSH) as well as Holstein type models, with momentum dependent couplings (e.g. breathing phonons) and arbitrary phonon dispersions. We access the dynamical properties of the systems via momentum dependent phonon spectral functions and electron Greens functions. In case of the standard Holstein model, we present precise data for the phonon spectral function in both the metallic Luttinger liquid and the insulating charge density wave phase, for a wide range of phonon frequencies. [Preview Abstract] |
Session X15: Emerging Nano-based Diagnostics and Therapeutics: Approaches to Cancer Treatment
Sponsoring Units: DBP FIAPChair: Larry Nagahara, National Cancer Institute
Room: Morial Convention Center 207
Friday, March 14, 2008 8:00AM - 8:36AM |
X15.00001: Intracellular Mechanics-Based Drug Screening for Cancer Metastasis Invited Speaker: In 2007 alone, close to 1.5 million new cancer cases and over half of a million deaths from cancer are projected to occur in US. In general, cancer is much easier to be successfully treated before metastasis; the five-year survival rates for most of the cancers in the metastatic stage are lower than 10\%. The origin of cancer is due to genomic instability; however, the genomics or proteomics studies focus on this phenomenon cannot thoroughly elucidate how cancer metastasis proceeds. During this process, cancer cells protrude and conquer their physical barriers, resist shear stress, establish anchorages and finally settle in a new environment. Each development in this process involves mechanical forces. Thus, whether force generation and cancer cells' mechanical properties can be integrated into the current mainstream of cancer research and offer new insight is worthy of being investigated. To measure the change of cell mechanics, specifically intracellular mechanics, a tool that least disrupts the probed cell's behavior and, simultaneously, can obtain real time quantitative measurement is necessary. To satisfy these criteria, we have developed a technique, ballistic intracellular nanorheology (BIN), in which we trace and analyze the trajectories of nanospheres that have been ballistically bombarded into the cytoplasm of individual cells. This technique allows us to probe the effects of chemical or mechanical stimuli on intracellular mechanics in various types of cells, on culture dishes or in a three-dimensional matrix. BIN is, currently, the first and only method available that can be applied to perform such tasks. Using this technique, we have gained detailed information about how the cytoskeletal remodeling pathways control the intracellular mechanics. We have also obtained information on the tempo-correlation between agonists and intracellular mechanics and how cells utilize their intracellular mechanics to react extracellular shear stress. These studies have set the framework for us to understand the mechanical mechanism of cancer cell metastasis on a molecular level. In this talk, I will describe the working principal using this technique to screen cancer drugs that prevent cancer metastasis. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X15.00002: Targeted Multifunctional Nanoparticles cure and image Brain Tumors: Selective MRI Contrast Enhancement and Photodynamic Therapy Invited Speaker: Aimed at targeted therapy and imaging of brain tumors, our approach uses targeted, multi-functional nano-particles (NP). A typical nano-particle contains a biologically inert, non-toxic matrix, biodegradable and bio-eliminable over a long time period. It also contains active components, such as fluorescent chemical indicators, photo-sensitizers, MRI contrast enhancement agents and optical imaging dyes. In addition, its surface contains molecular targeting units, e.g. peptides or antibodies, as well as a cloaking agent, to prevent uptake by the immune system, i.e. enabling control of the plasma residence time. These dynamic nano-platforms (DNP) contain contrast enhancement agents for the imaging (MRI, optical, photo-acoustic) of targeted locations, i.e. tumors. Added to this are targeted therapy agents, such as photosensitizers for photodynamic therapy (PDT). A simple protocol, for rats implanted with human brain cancer, consists of tail injection with DNPs, followed by 5 min red light illumination of the tumor region. It resulted in excellent cure statistics for 9L glioblastoma. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:48AM |
X15.00003: Microcantilever Biosensors Invited Speaker: Micromachined cantilever beams respond to molecular adsorption by with mechanical bending. For small concentrations, the bending signal is directly proportional the surface concentration of adsorbed molecules. Selectivity in detection is accomplished by immobilizing specific receptors on one of the surfaces of the cantilever. We have developed microcantilever arrays for multiplexed, label-free detection of biomolecules. Piezoresistive readout of cantilever bending offers a simple method of signal transduction that is compatible with microfabrication. Although the microcantilever-based biosensing appears to high sensitivity and selectivity, reproducibility of the technique appears to be a challenge. We have developed a novel method of immobilizing receptors that increases the reproducibility. We have demonstrated simultaneous detection of cancer and cardiac markers using cantilever arrays with immobilized receptors. We will also discuss a receptor-free mode of achieving selectivity. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:24AM |
X15.00004: Magnetic sifters and biochips for early diagnosis and therapy monitoring of cancer Invited Speaker: Magnetic nanoparticles conjugated with biomolecules or recognition moieties are finding wide applications in medicine. In this context, we are developing a micromachined magnetic sifter and magnetic nanoparticles aimed for sample preparation applications in early diagnosis of cancer. The microfabricated sifter consisting of arrays of micron sized slits etched through a silicon wafer. A magnetic film is deposited on the wafer, producing high magnetic field gradients, comparable in magnitude to gradients in planar flow devices. As the solution flows through the die, magnetic particles are captured by the magnetic material surrounding the slits. The large number of slits allows for processing of large volumes of liquid, much greater than that of planar microfluidic devices. The sifters can be simply attached to a syringe or tube, resulting in a portable and user-friendly tool for molecular biology. Separation efficiencies of $\sim $ 50{\%} for one pass through the sifter have been achieved. We have also designed and fabricated several types of magnetic biochips consisting of arrays of giant magnetoresistive (GMR) spin valve detectors with appropriate dimensions, surface chemistry, and microfluidics. An advanced electronic test station has been set up as a demonstration vehicle for the integrated evaluation of our magnetic biochips with commercial and custom magnetic nanoparticle labels for DNA or protein biomarkers. The magnetic biochip is capable of detecting down to 1-30 nanotags. Real-time detection of DNA signatures and protein targets in buffer and serum samples has been successfully performed in our laboratories, suggesting that magnetic biochips hold great promises for molecular diagnostics of cancer and other diseases. In collaboration with Chris M. Earhart, Wei Hu, Robert J. Wilson, Sebastian J. Osterfeld, Robert L. White, Nader Pourmand, and Shan X. Wang @ Stanford University. This work was supported by grants from NIH (1U54CA119367-01) and DARPA/Navy (N00014-02-1-0807). [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 11:00AM |
X15.00005: Microdevices for biomolecular detection and single cell analysis Invited Speaker: Recent advances towards developing biomolecular and single cell applications for a mass-based biosensor known as the suspended microchannel resonator (SMR) will be presented. In SMR detection, target molecules or cells flow through a vibrating suspended microchannel and are captured by receptor molecules attached to the interior channel walls. What separates the SMR from the existing resonant mass sensors is that the receptors, targets, and their aqueous environment are confined inside the resonator, while the resonator itself can oscillate at high Q in an external vacuum environment, thus yielding extraordinarily high sensitivity. This approach solves the problem of viscous damping that degrades the sensitivity of cantilever resonators in solution. We have achieved a resolution of approximately 1 femtogram (1 Hz bandwidth) which is represents an improvement of six order of magnitude improvement over a high-end commercial quartz crystal microbalance. This gives access to intriguing applications such as mass based flow cytometry, real-time monitoring of single cell growth, and the direct detection of protein biomarkers. [Preview Abstract] |
Session X18: Focus Session: Dynamics and Structures in Polymer Melts, Gels and Glasses
Sponsoring Units: DPOLYChair: Lynden Archer, Cornell University
Room: Morial Convention Center 210
Friday, March 14, 2008 8:00AM - 8:36AM |
X18.00001: Elastomeric Photopolymers: Shaping Polymer Gels with Light Invited Speaker: Polymer gels that possess a latent ability to change shape, which can be triggered in a spatially resolved manner using light---``elastomeric photopolymers''---have been developed to meet the need for materials that can be reshaped without direct contact, e.g., to non-invasively adjust an implanted lens in the human eye. The physics of diffusion and swelling in elastomers are applied to create a transparent silicone suitable for making a foldable intraocular lens that can be reshaped using near ultraviolet light. A crosslinked silicone matrix dictates the initial shape of the lens, while ``macromers''--short silicone chains with polymerizable end groups—and photoinitiator enable shape adjustment using light: polymerization of the macromer in the irradiated regions, followed by diffusion of free macromer causes local swelling. To predict shape change directly from irradiation profile, a theoretical treatment is presented that captures 1. shape change with no external forces, 2. coupling between diffusion and deformation, and 3. connection between thermodynamics, constitutive equations and equations of motion. Using continuum mechanics complemented with thermodynamics within the auspices of the finite element method, we develop a steady-state model which successfully captures the coupling between diffusion and deformation. Parameter values are drawn from our prior experimental studies of the mechanical properties, equilibrium swelling, penetrant diffusivities and interaction parameters in systematically varied polydimethylsiloxane (PDMS) networks and acrylate endcapped PDMS macromers. Preliminary computational studies show qualitative agreement with experimentally observed phenomena. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X18.00002: Dynamics of Swollen Gel Layers Anchored to Solid Surfaces George Fytas, Maria Gianneli, Robert Roskamp, Ulrich Jonas, Kaloian Koynov, Wolfgang Knoll, Benoit Loppinet Thin responsive hydrogel films are currently under development for biosensor applications. Photocrosslinkable poly(N-isopropylacrylamide) (PNIPAAm) based chains are spin coated as thin films (about 1 micron) and UV irradiated with variable doses to control the crosslink density. The obtained anchored gel layers can swell in ethanol or water up to about 10 microns for low crosslinking densities. Dynamics of the swollen layers and diffusion of different tracers (as analyte mimicks) are studied by dynamic light scattering (PCS) and fluorescence correlation spectroscopy (FCS). PCS resolved fast and slow diffusions, attributed to cooperative diffusion and long range concentration heterogeneities. Higher crosslink densities give rise to faster cooperative diffusion, i.e. short dynamic mesh sizes. FCS revealed the importance of electrostatic interactions between probe and negatively charged network. While a negatively charged dye senses local dynamics with a moderate slow down, a positively charged dye exhibited substantially retarded diffusion. Larger tracers are used to assess the size dependent gel penetrability, whereas large particles, trapped into the network, expectedly follow the network dynamics. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X18.00003: Theory of the effect of deformation on the relaxation and mechanical properties of polymer glasses Kang Chen, Kenneth Schweizer Recently Saltzman and Schweizer have developed a statistical dynamical theory of segmental relaxation in deeply supercooled polymer melts by combining and extending methods of mode coupling, dynamic density functional and activated barrier hopping theories. The approach is built on the concept of a nanometer scale nonequilibrium free energy which quantifies dynamic localization due to interchain forces. We have now generalized this approach to treat quiescent relaxation, physical aging, and nonlinear mechanical properties in the nonequilibrium glass state. Applied stress weakens dynamical constraints in the effective free energy which accelerates alpha relaxation and softens the elastic modulus. A constitutive equation has been constructed which allows the prediction of dynamic yielding and mechanical response under constant strain rate, constant stress (creep), and other modes of deformation. Multiple predictions that fundamentally differ from the phenomenological Eyring model are made for the temperature and strain rate dependence of the dynamic yield stress and the deformation modified segmental relaxation time. Comparison of the theoretical results with experiments on PMMA reveals good agreement. The coupling of stress and aging, the strain softening effect, and large deformation strain hardening, can also be treated within our approach. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X18.00004: Why Temperature Variation of the Chain Relaxation is Universal for Many Polymers? Alexei Sokolov Temperature variations of chain and segmental modes are traditionally described through corresponding friction coefficients. It is usually assumed that the same friction mechanism controls dynamics of both processes. As a result, their temperature dependence is expected to be the same. It is known, however, that segmental relaxation in many polymers varies faster than the chain one when temperature approaches Tg. We present an analysis of temperature variations of segmental and chain modes for different polymers. We demonstrate [1] that the chain relaxation shows universal temperature dependence for many polymers when it is presented vs Tg/T. Even polymers with strongly different temperature behavior of segmental dynamics (fragility) exhibit the same behavior for the chain relaxation. These results indicate complete decoupling in behavior of chain and segmental modes and emphasize our deficiency in understanding the microscopic mechanism of the chain friction coefficient. Possible mechanisms that lead to so universal behavior of chain relaxation are discussed at the end. \newline [1] Ding, Y., Sokolov, A.P., Macromolecules 2006, 39, 3322. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X18.00005: Molecular dynamics simulations of layers of linear and branched alkanes under shear P. Soza, F.Y. Hansen, H. Taub, U.G. Volkmann We have previously studied the equilibrium structure and dynamical excitations in films of the linear alkane tetracosane ($n$-C$_{24}$H$_{50}$) and the branched alkane squalane (C$_{30}$H$_{62})$ in great detail$^{2}$. Here we report the results of nonequilibrium molecular dynamics simulations of these systems in order to compare the rheological properties of alkanes of the same length but with different architecture. The simulations were done in the NVT ensemble using the reverse nonequilibrium algorithm proposed by F. M\"{u}ller-Plathe et al.$^{3}$. The viscosity was calculated for different shear rates and compared with experimental values. Different structural parameters such as the mean end-to-end distance, the radius of gyration, and the angle of alignment of the molecules with the flow were studied as a function of the shear rate. $^{2}$A.D. Enevoldsen et al., J. Chem. Phys. 126, 104703-10 (2007); 126, 104704-17 (2007). $^{3}$F. M\"{u}ller-Plathe et al., Phys. Rev. E, \textbf{59}, 4894 (1998) [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X18.00006: On the influence of excluded volume in polymer melts Hendrik Meyer, J.P. Wittmer, J. Farago, A. Johner, J. Baschnagel Flory's ideality hypothesis states that polymer chains in the melt have random walk like conformations as if there would be no excluded volume. However, it was shown recently that the excluded volume interaction induces corrections to scaling which are long range and which give rise to a power law decay of bond-bond correlation function [1] as well as to corrections to the Kratky plateau of the form factor [2]. In this presentation, we focus on the Rouse mode analysis by switching on the excluded volume potential gradually to study the crossover from phantom chains (representing perfect random walks described by the Rouse model) to real polymer melts [3]. In the melt, significant corrections have to be made to the static Rouse mode spectrum and consequences for the dynamics are briefly discussed. This gives also evidence that subdiffusive behaviour found in contradiction to the Rouse model is also caused by the excluded volume interaction. [1] J. Wittmer, H. Meyer, J. Baschnagel et. al. Phys. Rev. Lett. 93 (2004) 147801. Phys. Rev. E 76 (2007) 011803. [2] J. Wittmer et. al. EPL 77 (2007) 56003. [3] H. Meyer et. al. accepted at Eur. Phys. J. E (2007) [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X18.00007: Observation of Anomalous Viscosity in Entangled Polymer Films near the Glass Transition Zhang Jiang, M. K. Mukhopadhyay, Sunil K. Sinha, Suresh Narayanan, Laurence B. Lurio, Sanghoon Song, Hyunjung Kim We have studied the viscous relaxation of surface waves on molten polystyrene films of various molecular weights ($M_{W})$ using x-ray photon correlation spectroscopy. The relaxation time has been measured as a function of wave vector from high temperatures down to near the bulk glass transition temperature ($T_{g})$. We find a transition from a single exponential regime through a stretched exponential to another single exponential regime as the temperature decreases to $T_{g}$ where the effective viscosity saturates at that of chains with critical molecular weight for entanglement. These results are interpreted in terms of the freezing-out of relaxation modes involving full chains and large segments until only fluctuations of chain segments of critical entanglement length survive. We also find no evidence for a low-viscosity surface layer near $T_{g}$. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X18.00008: Kohlrausch Parameter Determination for Simple Chain Models John Mccoy, Taylor Dotson, Julieanne Heffernan, Keenan Dotson, Joanne Budzien, Douglas Adolf The second Legendre polynomials of the end-to-end vector of freely jointed and freely rotating chains were extracted from molecular dynamics simulations and analyzed in terms of the Kohlrausch-Williams-Watts (KWW) function. Of particular interest is the variation of the stretching exponent, $\beta $, as a function of state point and, consequently, of the detailed compliance with time-temperature superposition. A new analysis methodology is introduced that permits the determination of $\beta $ at the needed level of precision. Detailed time-temperature superposition does not hold for freely-jointed or freely-rotating chain liquids. Indications of a breakdown in time-temperature superposition are also found in violations of Stokes-Einstein and Debye-Stokes-Einstein behavior. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X18.00009: Understanding Fragility in Polymers Kumar Kunal, Christopher Robertson, Alexei Sokolov Glass transition in polymeric liquids is an important phenomenon that still remains poorly understood. It is accompanied by strongly non-Arrhenius temperature variations of segmental relaxation time \textit{$\tau$}$_{\alpha}$. The deviations of \textit{$\tau$}$_{\alpha}$ from Arrhenius behavior is described by the fragility parameter, $m$: $m=\delta \log \tau_\alpha$/$\delta (T_g$/$T$)$\vert_{T=Tg}$ where $T_{g}$ is the glass transition temperature. It appears that polymers are very fragile compared to small molecular weight liquids. However, there are some polymers that have intermediate fragility too. The reason for high fragility of polymers remains a topic of active discussion. We have studied various polymer systems using dielectric spectroscopy and dynamic mechanical analysis, and present an analysis of the results in the framework of chain flexibility, symmetry of the monomer and the packing efficiency of the polymer melt. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X18.00010: Dye Reorientation as a Probe of Stress-induced Mobility in PMMA Glasses Hau-Nan Lee, Keewook Paeng, Stephen Swallen, Mark Ediger To understand the response of molecular motions to deformation, we perform optical measurements of dye reorientation in PMMA glasses under stress. The reorientation of probes can be used to monitor the segmental dynamics of a polymer melt. We utilize this method to quantitatively determine the stress-induced mobility in PMMA glass during tensile creep deformation. At 377 K (Tg-18 K), a stress of 20 MPa accelerates the mobility by a factor of 100, while 11 MPa has almost no effect. After removing the stress, we observe that the enhanced mobility slowly disappears, even though the overall strain is still very large. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X18.00011: On the effect of Molecular weight and Frequency dependence of Tg on the interpretation of Dynamic viscosity data J.P. Ibar In this work, we re-plot dynamic data for a series of monodispersed Polystyrene melts, and their blends, obtained at constant temperature and variable frequency $\omega$, to enable a new analysis at constant (T-Tg) instead, where Tg is made a function of Mn and $\omega$. Tg ($\omega)$ is determined by DMA, from the shift of the max of E'' with $\omega $. Tg increases rapidly with $\omega $ and levels off at higher $\omega$. The Mn dependence of Tg varies in a similar way, quickly plateaus off at approximately M=2 Me, consistent with a free volume interpretation. It is shown that correction for the Tg dependence on Mn and $\omega$ does play an important role in the determination of the scaling parameters exponents, such as the molecular weight dependence exponent for M$<$ 2 Me data, or the low frequency tail representative of the terminal behavior for blends of monodispersed samples. The paper concludes that a true separation of the variables to determine the independent effect of structural (local) and molecular weight (scaling) factors, can only be done at constant (T-Tg). The incidence of Tg($\omega $, ${\rm M})$ on the superposition at low $\omega$, is also reviewed. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X18.00012: Influence of pressure (density) on fast dynamics in polymers Liang Hong, Burak Begen, Alexander Kisliuk, Alexei Sokolov Understanding the microscopic nature of the fast dynamics in disordered materials is still a challenge. In particular, the origin of the collective vibrations, the so-called Boson peak, remains a subject of active discussion. It's known that Boson peak spectra change significantly under pressure. Analysing the role of density in the Boson peak might help to unravel its microscopic nature. In this work, we use light scattering to study influence of pressure (up to 1.5GPa) on fast dynamics in different polymers. In all cases, the observed shift of the Boson peak frequency with pressure is significantly stronger than change of sound velocities. This result clearly indicates that elastic continuum approximation cannot describe the pressure-induced variations. We demonstrate that the main variation of the Boson peak amplitude is due to changes of the Debye level, although detailed quantitative analysis is not possible due to the light-to-vibration coupling coefficient, which also varies with pressure. Analysis also shows there is a correlation between pressure-induced changes in the Boson peak frequency and amplitude. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X18.00013: Visualization and Analysis of the Dynamics of Methanol Transport in Poly(Methyl Methacrylate) Adam Ekenseair, Richard Ketcham, Nicholas Peppas The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to a myriad of transport phenomena, such as Fickian, Case II, and anomalous absorption behavior. Many proposed models account for the majority of anomalous behavior that has been observed. However, there is still a disconnect between theory and experiment, as data must be fit to the model and adjustable parameters determined. We propose that a better understanding of the dynamics of penetrant transport in glassy polymers can be achieved by careful and detailed investigations into the role the polymeric network structure plays in influencing the transport mechanism. We introduce a novel technique to visualize and quantify transport dynamics and mechanisms in- situ. High-resolution X-ray computed tomography, a completely nondestructive technique that can be used to visualize features in the interior of opaque solids, has been successfully adapted to examine the transport dynamics of methanol into glassy poly (methyl methacrylate) discs synthesized by an iniferter- mediated free radical polymerization. In addition to tracking methanol absorption and dimensional swelling, the time- dependent concentration profiles within the polymer disc were determined. [Preview Abstract] |
Session X22: Organic Magnetics and Bio-Electronics
Sponsoring Units: DMP DPOLYChair: Fabio Cicoira, Cornell University
Room: Morial Convention Center 214
Friday, March 14, 2008 8:00AM - 8:12AM |
X22.00001: \textit{Ab initio} simulations of the transport properties of Mn$_{12}$ based spin-devices Chaitanya Das Pemmaraju, Ivan Rungger, Stefano Sanvito Single-molecule magnets (SMMs) represent a unique playground for fundamental quantum physics and exhibit exotic phenomena such as magnetic hysteresis as well as magnetization reversal through quantum tunneling. Recently, transport measurements on Mn$_{12}$ based molecular magnets in single-molecule-transitor devices have been realized. In this work we present \textit{ab initio} transport[1] calculations of Mn$_{12}$ molecules functionalized by thioether groups and sandwiched between gold contacts. We find the transport properties of these SMMs to be dominated by tunneling type behaviour across the organic functional groups and asymmetric coupling to the leads. We observe asymmetric I-V curves under positive and negative bias. In addition we demonstrate that the I-V characteristic changes upon changing the magnetic state of the molecule, suggesting that electrical single-spin detection can be indeed obtained from a detailed knowledge of the I-V. \newline [1] Rocha \textit{et. al}, Spin and Molecular Electronics in an Atomically Generated Orbital Landscape; URL: http://www.smeagol.tcd.ie [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X22.00002: Modeling the organic magnet Fe[TCNE]$_2$ J. Moreno, M.A. Majidi, K.I. Pokhodnya Recent experiments have revealed the crystal structure of Fe[TCNE]$_2$, (TCNE = tetracyanoethylene), an organic-based magnet with a transition temperature around 100 K and a saturation magnetization corresponding to an effective spin of 3/2 per formula unit. Its structure consists of undulating layers of TCNE anion-radicals bound to four Fe(II) ions, where Fe ions between adjacent layers coupled via diamagnetic $\sigma$-dimerized TCNE.$^1$ Since the angular momentum of Fe(II) is almost quenched due to the asymmetric crystal field, we model the system using a Heisenberg Hamiltonian with antiferromagnetic in-plane coupling between the Fe(II) S=2 spins and the near-neighbor (TCNE) S = 1/2 spins and also antiferromagnetic superexchange coupling between the Fe(II) spins at adjacent planes. By comparing our results with magnetization measurements as function of temperature and field, we extract the values of the inter- and intra-plane antiferromagnetic couplings. We discuss how to extend our approach to other TCNE-based magnets, such as the amorphous semiconducting V[TCNE]$_x$, a room temperature ferrimagnet and promising candidate for multifunctional spintronic applications.\\ $^1$ J.-H. Her, P. W. Stephens, K. I. Pokhodnya, M. Bonner and J. S. Miller, An gew. Chem. Int. Ed. 2007, 46, 1521 [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X22.00003: Reversible Photoinduced Magnetism in V-Cr Prussian blue analogues K. Deniz Duman, Jung-Woo Yoo, N.P. Raju, Amber C. McConnell, William W. Shum, Kendric J. Nelson, Joel S. Miller, A.J. Epstein The cyano-bridged bi-metallic compounds, so called ``Prussian blue magnets,'' display a broad range of interesting photoinduced magnetic phenomena. A notable example is Fe-Co Prussian blue magnet, which exhibits light-induced changes in between magnetic states together with glassy behavior [1,2]. Here, we report reversible photoinduced magnetic phenomena in V-Cr Prussian blue analogue (\textbf{K}$_{1.54}$\textbf{V}$_{0.85}$\textbf{[Cr(CN)}$_{6}$\textbf{](SO}$_{4}$\textbf{)}$_{0.16}$\textbf{3.1H}$_{2}$\textbf{O}), one of the few room temperature molecule-based magnets. Illumination with UV light suppresses magnetization, whereas subsequent illumination with green light increases magnetization. This recovery effect of green light is observed only when the sample is previously UV-irradiated. This suggests a hidden metastable magnetic state with a long lifetime at low $T $ ($<$ 100 K). Results of detailed magnetic studies and the likely microscopic mechanisms will be discussed. [1] Hashimoto et al. science \textbf{272}, 5262 (1996); [2] Pejakovic, et al. PRL \textbf{85} 1994 (2000) [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X22.00004: Magnetic properties of organic-based Ni[TCNE](MeCN)$_{2}$][BF$_{4}$] magnet. Konstantin Pokhodnya, Victor Dokukin, Joel S. Miller A new organic-based magnet of Ni[TCNE][BF$_{4}$]( MeCN)$_{2-\delta}$ (\textbf{1}) composition ($\delta $ = 0.15; TCNE = tetracyanoethylene) was synthesized via reaction of NBu$_{4}$(TCNE) and Ni(NCMe)$_{6}$(BF$_{4})_{2}$ in CH$_{2}$Cl$_{2}$. Zero field cooled and field cooled magnetizations, $M(T)_{ZFC}$ and $M(T)_{FC}$, at 0.5 mT rise sharply below 70 K indicative of an onset of a magnetic transition. $M(T)_{ZFC}$ reaches maximum at 25 K followed by a rapid decrease suggesting antiferromagnetic (AF) interaction. In contrast, $M(T)_{FC}$ rises upon further cooling signifying a strong irreversibility in accord with sharp increase of a remanant magnetization below 30 K and hysteretic behavior of $M(H)$. The $M(H)$ at 2 K increases rapidly with field and approaches saturation above $\sim $ 0.5 T. At 9 T $M(H)$ reaches 2.24 $\mu _{B}$ that is significantly higher than 1.30 $\mu _{B}$ expected for AF coupled Ni(II) S = 1 and [TCNE]$^{-}$ (S = 1/2) suggesting a ferromagnetic (FM) interaction. The unpaired Ni$^{II}$ spins and those on the [TCNE]$^{-}$ reside in orthogonal orbitals resulting in FM coupling. Assuming that similarly to Fe[TCNE][FeCl$_{4}$](MeCN)$_{2}$ \textbf{1} consists of Ni$^{II}-\mu _{4}$-[TCNE]$^{-}$ layers we believe that the decrease of $M(T)_{ZFC}$ below 25 K is due to AF coupling \textit{between the layers} while the interaction \textit{within the layer} is FM in contrast to the AF one reported for Fe, V, and Mn analogues. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X22.00005: Magnetoresistance in bulk heterojunction solar cells Ronald Oesterbacka, Sayani Majumdar, Himadri Majumdar, Harri Aarnio, Reino Laiho The magnetoresistance (MR) response of the poly(3-hexyl thiophene) and poly(3-hexyl thiophene):1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]-methanofullerene (PHT:PCBM) based bulk heterojunction solar cells have been studied. Positive MR was always observed at room temperature in both the devices. In both cases the magnitude of the MR signal decreases at lower temperature and shows positive to negative sign inversion at 100K for the solar cells and at 200K for P3HT. The detailed voltage and temperature dependence of MR will be presented which will give important insight of the magnetic field effect on the bulk carrier mobility in the organic solar cells. We have observed tendency of retaining magnetic history in both the devices and it has been studied. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X22.00006: Extending transfer-matrix studies of charge transport in dsDNA: diagonal ladder model Stephen Wells, Rudoph Roemer The $\pi$-stacking of aromatic bases along the axis of the DNA double helix suggests that DNA should be capable of supporting electron transport. This possibility has been investigated by a variety of experimental methods, including charge-transfer between intercalated dye molecules and direct measurement of conductivity in DNA molecules bridging two electrodes. In order to explore either the biological or nanotechnological significance of charge transport in DNA, we need theoretical models capable of predicting the influence of DNA sequence and structure on its charge transport properties. Transfer matrix methods have been used in conjunction with a ladder model of dsDNA (incorporating charge transfer between adjacent bases along a strand, and between hydrogen-bonded base pairs) to predict different transport properties for random, repetitive, or coding DNA sequences. It has been suggested that DNA charge transport may be involved in cellular mechanisms to detect and repair damage to DNA strands. We present extensions to the ladder model to allow for, firstly, charge transfer ``diagonally'' (from a base on a 5' strand to an adjacent base on a 3' strand, for example), and secondly, variations in hopping amplitudes due to bending of the helix (for example, in wrapping round a histone complex). Hence we take into account the extent of the electronic states and the geometry of the DNA strand in our modeling. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X22.00007: Sequence Dependent Charge Transport on Double Stranded DNA Efta Yudiarsah, Sergio E. Ulloa The transport properties of different double-stranded DNA sequences are studied by transfer and scattering matrix methods. The DNA is described by a tight-binding model with realistic sequence-specific hopping integrals. Our results show that, in qualitative agreement with experimental results [1], even a single basis mismatch on the sequence can dramatically change the conductance of short DNA sequences. The change in conductance is larger if the mismatch is on the energetically favorable path of transmission: the path with the most bases with energy close to the Fermi energy of the contacts. This trend is independent on which strand is being connected to the electrodes, although similar sequences have drastically different conductance values. We also study the effect of structural ``nicks'' on the DNA conductance. In accordance with experimental results [2], the conductance is changed by several orders of magnitude in the presence of the nicks, depending on the position of the defect on the strand. As the conductance of a strand is found to be dependent on the sequence of bases, this suggests an electronic approach to sequencing [1]. [1] J. Hihath \emph{et al.}, Proc. Natl. Acad. Sci.U. S. A \textbf{102}, 16979 (2005). [2] B. Hartzel \emph{et al.}, Appl. Phys. Lett. \textbf{82}, 4800 (2003). [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X22.00008: Theory of electron conductance across a DNA basepair Myeong Lee, Otto Sankey In recent years, research on electron tunneling through DNA basepairs has become more important due to its potential application in DNA sequencing technology. The goal is to recognize and identify a specific DNA base by measuring the hydrogen bond mediated tunneling current across a DNA basepair junction. In this talk, we discuss the results of density functional theory on the intrinsic conduction through DNA basepairs (Watson-Crick basepairs, Wobble basepairs, etc), and in particular the role of the hydrogen bond on the tunneling current. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X22.00009: Charge transport in guanine crystals Frank Ortmann, Karsten Hannewald, Friedhelm Bechstedt Charge-transport processes in organic molecular crystals exhibit similarities and differences to those in $\pi$-conjugated polymers. For both types of condensed matter the polaronic effects are of high importance. These effects can cause a transition from bandlike transport to themally activated hopping. While the hopping regime is prevalent for DNA polymers, it is not clear if the same holds also for crystalline guanine or if band transport dominates. Also the influence of the temperature is rarely discussed in literature. In our approach to the problem of charge-carrier transport in these systems [1], we discuss the temperature dependence of the polaron bandwidth and the mobility in guanine crystals [2]. \newline [1] K. Hannewald {\it et al.}, Phys. Rev. B {\bf 69}, 075211 (2004); 075212 (2004). \newline [2] F. Ortmann {\it et al.}, J. Phys. Chem. B (to be published). [Preview Abstract] |
Session X23: Focus Session: Multiferroics III: Other
Sponsoring Units: DMP GMAGChair: Sam Lofland, Rowan University
Room: Morial Convention Center 215
Friday, March 14, 2008 8:00AM - 8:12AM |
X23.00001: Pyroxenes: A novel class of multiferroics D.I. Khomskii, S. Jodlauk, P. Becker, J. Mydosh, Th. Lorenz, S.V. Streltsov, D.C. Hezel, L. Bohaty Pyroxenes with the general formula AMSi$_{2}$O$_{6 }$(A - mono- or divalent metal, M = di- or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi$_{2}$O$_{6}$ becomes ferroelectric in a magnetically ordered state below 6 K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi$_{2}$O$_{6}$ (T$_{C}$ =18 K) and LiCrSi$_{2}$O$_{6}$ (T$_{C}$ =11 K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi$_{2}$O$_{6}$) and synthetic crystals of LiFeSi$_{2}$O$_{6}$ and LiCrSi$_{2}$O$_{6}$ grown from melt solution. For NaFeSi$_{2}$O$_{6}$ a temperature versus magnetic field phase diagram is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X23.00002: Magnetic field induced ferroelectricity in Mn$_{0.9}$Fe$_{0.1}$WO$_{4}$ Rajit Chaudhury, Bernd Lorenz, Yaqi Wang, Yanyi Sun, Ching-Wu Chu We discovered the external magnetic field induce ferroelectric phase in Mn$_{0.9}$Fe$_{0.1}$WO$_{4}$, which is paraelectric at zero magnetic field. The ferroelectricity appears in fields above 4 Tesla applied along the easy axis of magnetization and the spontaneous polarization along the b-axis was measured by the pyroelectric current method as a function of temperature and magnetic field. The temperature and magnetic field dependence of spontaneous polarization shows strong coupling between magnetic and ferroelectric orders. We interpret that the improper ferroelectricity in this compound is driven by non collinear spin structure which breaks the inversion symmetry. We propose high-field neutron scattering experiments to characterize the magnetic structure in the ferroelectric phase. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X23.00003: Quantum theory of multiferroics in quasi-one-dimensional spin-1/2 frustrated magnets Shigeki Onoda, Masahiro Sato, Yasuhiro Saiga, Shunsuke Furukawa A theory is developed to understand recent experimental findings on quasi-one-dimensional spin-$1/2$ multiferroics LiCuVO$_4$ and LiCu$_2$O$_2$. For this purpose, weakly coupled frustrated quantum spin chains with and without the zigzag structure are studied by means of an effective field theory based on the bosonization in one dimension. A chiral ground state with gapless incommensurate spin excitations can be stabilized in the presence of an easy-plane anisotropy. This state is driven by a three-dimensional coupling to the incommensurate helimagnetic state, in accordance with the experimental observations. We also reveal the quantum dynamics of the spin, the chirality and the electromagnon as well as the finite-temperature phase diagram, which reflect the one-dimensional nature of the quantum fluctuations. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X23.00004: Correlation between spin helicity and electric polarization vector in quantum chain magnet LiCu$_2$O$_2$ Shinichiro Seki, Yuichi Yamasaki, Yoshinori Tokura, Minoru Soda, Masato Matsuura, Kazuma Hirota Measurements of polarized neutron scattering were performed on the multiferroic quantum chain magnet LiCu$_2$O$_2$. In the ferroelectric ground phase, the existence of transverse spiral spin component in the $bc$-plane was confirmed. When the direction of electric polarization is reversed, the vector spin chirality as defined as ${\bf C}_{ij} = {\bf S}_i \times {\bf S} _j$ is also reversed. This directly proves that the spin- current model ${\bf P}_{ij} \propto {\bf e}_{ij} \times {\bf C}_ {ij}$ is applicable even to this $e_{\mathrm{g}}$-electron quantum $S$=1/2 system. Differential scattering intensity of polarized neutrons shows a large discrepancy from that expected for the classical $bc$-cycloidal spin structure, implying either the complexity of magnetic structure or the effect of quantum fluctuation. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X23.00005: Multiferroics versus Quantum Fluctuations in Spin-1/2 Frustrated Chains Shunsuke Furukawa, Masahiro Sato, Shigeki Onoda We study interplay of the chiral spin ordering and quantum fluctuations in a spin-1/2 frustrated chain, which is the simplest model for one-dimensional multiferroic cuprates like LiCuVO$_4$ and LiCu$_2$O$_2$. In a Heisenberg chain, it is known that the classical helical magnetic order is suppressed by strong quantum fluctuations and valence-bond solid phases emerge. In fact, weak easy-plane spin anisotropies exist in the above materials, because of the XXZ-type anisotropy and a phonon-induced biquadratic Dzyaloshinskii-Moriya interaction. In particular, when the nearest-neighbor exchange coupling is much weaker than the antiferromagnetic second-neighbor one, our exact-diagonalization calculations combined with the bosonization analyses show that such anisotropies bring about the vector-chiral spin ordering and the associated multiferroic behavior. This chiral state is accompanied by slightly incommensurate algebraic spin correlations, which, with a three-dimensional coupling, explains the magnetic order experimentally observed in LiCuVO$_4$. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X23.00006: Optical spectroscopic study on magnetoelectric MnWO$_{4}$ Woo Seok Choi, Kouji Taniguchi, Soon Jae Moon, Sun Jung Kim, Sung Seok A. Seo, Yoon Sang Lee, Taka-hisa Arima, Tae Won Noh We report optical spectroscopic investigation on a multiferroic oxide compound, MnWO$_{4}$. This compound is known to exhibit ferroelectricity induced by the incommensurate spiral magnetic ordering in a temperature range of 7.6 K and 12.7 K [1]. We grew single crystals of MnWO$_{4}$ by using the floating zone method. To examine the optical anisotropy originating from the monoclinic crystal structure, we measured reflectivity spectra of MnWO$_{4}$ with light polarizations along three crystallographic axes, and calculated the optical conductivity spectra through the Kramers-Kronig transformation for each axis. We discuss the anisotropic phonon structures and electronic structures with temperature and magnetic field dependence in relation to its multiferroic properties. \newline [1] K. Taniguchi \textit{et al}., Phys. Rev. Lett. \textbf{97}, 097203 (2006). [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X23.00007: Ferroelectric domain topology of the multiferroic spin spiral system MnWO$_4$ D. Meier, Th. Lottermoser, G. Yuan, M. Fiebig, P. Becker, L. Bohaty The strong interest in magnetoelectric multiferroics is due to their potential concerning the design of novel multifunctional devices, as well as to their unusual physical properties. Among these, TbMnO$_3$, Ni$_3$V$_2$O$_8$, and MnWO$_4$ form a particularly challenging group: The key factor for ferroelectricity lies in the long-wavelength magnetic order. Many aspects of the precise nature of the ferroelectric state in such a \textit{spiral magnet}, and in particular their coupling to the magnetic order, are still largely unclear. Here we report about the three-dimensional spatial distribution of ferroelectric domains in MnWO$_4$, revealed by optical second harmonic generation (SHG). Although ferroelectricity is induced by cycloidal spiral magnetic order, 180$^{\circ}$ domains as in a conventional ferroelectric are observed. Their coupling to the coexisting magnetic order and modifications of this coupling by external parameters such as temperature variation are discussed using spatially resolved SHG for probing both the magnetic and the ferroelectric order in one experimental run. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X23.00008: Magnetic field control of the ferroelectric polarization in multiferroic MnWO$_{4}$ Kouji Taniguchi, Nobuyuki Abe, Takahisa Arima, Taishi Takenobu, Yoshihiro Iwasa The relationship between magnetic order and ferroelectric properties has been investigated for MnWO$_{4}$. Spontaneous electric polarization is observed in a cycloidal spiral spin phase. The magnetic-field dependence of electric polarization indicates that the noncollinear spin configuration plays a key role for the appearance of ferroelectric phase. Destabilization of the ferroelectric phase and an electric polarization flop from the $b$ direction to the $a$ direction have been observed when a magnetic field is applied along the $b$ axis. On the other hand, the ferroelectric phase is stabilized when a magnetic field is applied along the $a$-, $c$- and the spin easy axes. We have also found that the magnetic field induced ferroelectric polarization disappears in a high magnetic field above 12T along the spin easy axis. Theses phenomena provide us useful information for gigantic magnetoelectric effects because MnWO$_{4}$ is a simple system without rare-earth f-moments. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X23.00009: Ferroelectricity in Mn$_{0.9}$Fe$_{0.1}$WO$_{4}$ induced by magnetic fields: A simple model calculation B. Lorenz, R. P. Chaudhury, C. W. Chu Replacing Mn$^{2+}$ by Fe$^{2+}$ in multiferroic MnWO$_{4}$ results in the complete loss of ferroelectricity at zero magnetic field. However, it was shown that in Mn$_{0.9}$Fe$_{0.1}$WO$_{4}$ an external magnetic field restores the ferroelectric state. We present a simple mean field calculation of the Heisenberg model with ferromagnetic nearest and antiferromagnetic next nearest neighbor interactions and uniaxial anisotropy in an external magnetic field. The various commensurate and incommensurate magnetic phases in Mn$_{1-x}$Fe$_{x}$WO$_{4}$ are well described by the model. The loss of the non collinear helical spin structure (which is associated with the ferroelectric order) with increasing Fe substitution is explained by the enhancement of the anisotropy. We show that the external field does indeed restore the helical spin structure in Mn$_{0.9}$Fe$_{0.1}$WO$_{4}$ and that the observed field-induced ferroelectricity can be explained. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X23.00010: Correlation between magnetic, dielectric properties and strain in a Mn$_{3}$O$_{4}$ single crystal T. Suzuki, T. Katsufuji Mn$_{3}$O$_{4}$ has a tetragonally distorted spinel structure below 1443 K and exhibits a ferrimagnetic ordering at $T_{N}$ = 43 K. This compound exhibits further magnetic phase transitions at 39 K and 33 K, where Mn$^{2+}$ and Mn$^{3+}$ spins are canted from the collinear spin structure. We measured the dielectric constant and strain of a Mn$_{3}$O$_{4}$ single crystal. We found that both dielectric constant and strain have clear anomalies at the magnetic transition temperatures. We also found that dielectric constant is suppressed (enhanced) by 2 \% when magnetic field is applied parallel (perpendicular) to the direction of electric field within the $ab$ plane below $T_ {N}$. In addition, strain along the $ab$ plane also has anisotropic magnetic field dependence. These results can be explained as follows: (1) There is an orthorhombic distortion below $T_{N}$, presumably induced by the orbital ordering of Mn$^{3+} $, (2) anisotropy of dielectric constant and strain within the $ab$ plane appears due to the orthorhombic distortion, and (3) the alignment of crystalline domains with applied magnetic field occurs, resulting in the large magnetic field dependence of dielectric constant and strain. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X23.00011: Polar Behavior in a Magnetic Oxide Via A-Site Size Disorder David J. Singh, Chul Hong Park Density functional calculations are used to test a new mechanism for ferroelectricity in magnetic perovskites based on A-site size disorder. Calculations of the structure and magnetic ordering of (La,Lu)MnNiO$_6$ show that this mechanism is effective for this material, which is predicted to be both polar (ferroelectric or relaxor) and ferromagnetic, depending on the Lu concentration. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X23.00012: Electric field control of magnetic phase transitions in Ni$_{3}$V$_{2}$\O$_{8}$ P. Kharel, C. Sudakar, A.B. Harris, R. Naik, G. Lawes In certain multiferroics, including Ni$_{3}$V$_{2}$O$_{8}$, the ferroelectric order is induced by the magnetic structure, leading to the simultaneous onset of spin and charge ordering. We have prepared thin films of Ni$_{3}$V$_{2}$O$_{8}$ by sputter deposition. Films annealed at 1000$^{0}$C crystallize with closely packed rod-like grains. XRD confirms that the films are single phase Ni$_{3}$V$_{2}$O$_{8}$ and highly oriented along the $b$-axis. We observe a hysteretic magnetic anomaly at 3.6 K, which is consistent with a first order phase transition from a canted magnetic state to incommensurate magnetic order. This transition temperature is suppressed by $\Delta $T=0.2 K in an electric field of 30MV/m. An anomaly in the dielectric constant is observed at 6.3K, corresponding to a transition between two incommensurate magnetic states. Because the electric field acts as a field conjugate to the order parameter, it causes a rounding of the phase transition and an apparent increase in the transition temperature by $\Delta $T=0.2 K when the sample is biased at 25 MV/m. The E-T phase boundary for the 3.6 K transition is linear, while the 6.3 K phase boundary shifts roughly like E vs.T$^{2}$, consistent with estimates from critical scaling. We will discuss the electric field control of magnetic order parameter in these films and some important implications of this result for the multiferoic material thin film research. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X23.00013: Direct Evidence of Magnetoelastic Coupling in Ni$_{3}$V$_{2}$O$_{8}$. Luciana I. Vergara, J. Cao, J. L. Musfeldt, N. Rogado, R. Cava, F. Yen, R. P. Chaudhury, B. Lorenz We investigate the infrared active phonons of the Kagome staircase compound Ni$_{3}$V$_{2}$O$_{8}$ as a function of temperature to elucidate changes in magnetoelastic coupling through the cascade of low-temperature magnetic transitions. A detailed analysis of the $a$- and $c$- polarized vibrational mode trends demonstrates that: i) the approach to the cascade of magnetic transitions is driven by the high frequency stretching modes and the highest frequency bending mode along $a$; ii) the paramagnetic to high-temperature incommensurate phase transition is driven by low frequency $c$-polarized modes; and iii) the high-temperature to low-temperature incommensurate phase transition is driven by all $a$-polarized modes plus the NiO$_{6}$ stretching mode along $c$. Work is in progress to elucidate the trends along $b$. Overall, we find that the phonons are sensitive to the magnetic state, indicating that the lattice is flexible, coupling strongly to the spin system in this multiferroic material. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X23.00014: Magnetic field effect on the magnetic structure of Ba$_{2}$CoGe$_{2}$O$_{7}$ C.R. dela Cruz, S. Li, S.-W. Cheong, Y. Chen, J. Lynn, H. Mook, P. Dai Multiferroic materials have recently attracted much interest fueled by the discovery of the coexistence and mutual interference of long range magnetic and ferroelectric order in them. Further attention to these compounds is gained due to their potential for device applications made possible by the controllability of the spontaneous polarization by a magnetic field or the bulk magnetization by an electric field via the sizable magneto-dielectric coupling in them. The fundamental microscopic mechanism for the phenomena is yet to be fully understood but an essential component has been suggested to be the non-linear coupling of the ferroelectric and magnetic order parameters with a spatially varying magnetization. It is thus the focus of this work to study the static magnetic structure of the compound Ba$_{2}$CoGe$_{2}$O$_{7}$ below its magnetic and ferroelectric ordering temperature of 7K. Neutron diffraction measurements were done on the compound under applied magnetic fields up to 7 T along the crystal's $c$-axis. [Preview Abstract] |
Session X24: Nanotubes: Transport, Synthesis and Growth
Sponsoring Units: DCMPChair: Vincent Crespi, Pennsylvania State University
Room: Morial Convention Center 216
Friday, March 14, 2008 8:00AM - 8:12AM |
X24.00001: Quantum electron transport in toroidal carbon nanotubes Mark Jack, Mario Encinosa Electron transport under bias is treated in tight-binding approximation using a non-equilibrium Green's function approach. Density-of-states D(E), transmissivity T(E), and current I$_{SD}$ are calculated through a (3,3) armchair nanotorus with laterally attached metallic leads and a magnetic field penetrating the toroidal plane. Plateaus in T(E) through the torus are observed as a function of both the relative angle between leads and magnetic flux. Initial computational studies performed with 1800 atoms and attached leads show substantial computational slowdown when increasing the system size by a factor of two. Results are generated by inverting the device Hamiltonian with a standard recursion method extended to account for unit cell toroidal closure. Significant computational speed-up is expected for a parallelized code on a multiprocessor computer cluster. The dependence of electronic features on torus size and torus curvature is tested for three tori with 900, 1800 and 3600 carbon atoms, respectively. References: 1. M. Jack and M. Encinosa, Quantum electron transport in toroidal carbon nanotubes with metallic leads. ArXiv: quant-ph/0709.0760. 2. M. Encinosa and M. Jack, Dipole and solenoidal magnetic moments of electronic surface currents on toroidal nanostructures. J. Comp.-Aided Mat. Design (Springer), 14 (1) (2007) 65 -- 71. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X24.00002: \textit{Ab} initio study of solvent effects on electrical transport of molecular bridge between electrodes Arihiro Tawara, Tomofumi Tada, Satoshi Watanabe The electrical conductance of benzene dithiolate (BDT) between gold electrodes has been actively investigated to realize single molecular devices. However, almost all of previous theoretical studies for the electrical conductance of BDT were done assuming 0K and vacuum in spite that many measurements have been performed at room temperature in solution [1,2]. In this study, we have investigated the electrical transport of BDT molecule between gold electrodes in water solution using \textit{ab initio} nonequilibrium Green's function method and Car-Parrinello molecular dynamics at room temperature. The calculated time-averaged conductance of the BDT in water solution, 0.190 G$_{0}$, is clearly different from the value calculated without water, 0.201 G$_{0}$. Detailed analysis shows that this difference can be attributed to the effect of dipole moments of water molecules on the potential profile of the BDT molecule. [1] X. Xiao \textit{et al}., Nano Lett. \underline {\textbf{4}}, 267 (2004).\textbf{ [}2] M. Kiguchi \textit{et al.}, Appl. Phys. Lett. \underline {\textbf{89}}, 213104 (2006). [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X24.00003: Low-temperature electronic transport in Pt-nanocluster decorated alumina template grown carbon nanotubes Swastik Kar, Caterina Soldano, Li Chen, Saikat Talapatra, Robert Vajtai, Saroj Nayak, Pulickel Ajayan Alumina template grown nanotubes are known to be highly disordered tube when compared to arc-discharge grown tubes. This is due to the particular type of growth process involved. Temperature dependence study reveals a slow power law dependence of the conductance as a function of the temperature. Large value of power law exponents found in pristine tubes, suggest that the transport mechanism takes place through tunneling between adjacent graphene flakes. When platinum-decorated, those devices show a L\"{u}ttinger liquid behavior in the high-T regime and a large suppression of the conductance at low-T due to the interplay of disorder and $e-e$ interactions. Transport properties are studied in light of a recently proposed model for disordered multi-channel quantum wires. Magneto-transport measurements ($\vert $B$\vert <$5T) show the presence of weak localization and a small but distinct Rashba spin-orbit scattering effect in the low-field regime ($\vert $B$\vert <$.5T), the latter attributed to the surface decoration. Coherent transport is found to be recovered with increasing applied electric field. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X24.00004: Electronic properties of doped semiconductor nanowires Mamadou Diarra, Christophe Delerue, Yann-Michel Niquet, Guy Allan Semiconductor nanowires have shown very promising properties, which opens new opportunities for the design of nanoscale devices. The physics of these nanowires is not yet fully understood. In this context, theory and numerical simulation give valuable insights. We present self-consistent tight binding calculations of donor and acceptor impurities in semiconductor nanowires, either in a free standing configuration or surrounded by a metallic gate and an oxide. We show that the dielectric mismatch between the nanowires and their surroundings increases the binding energies of dopant impurities up to a few hundreds of meV [1]. This decreases the doping efficiency and affect the behavior of nanowire devices. The effect of the environment will be discussed. When the nanowires are surrounded by an oxide, polaronic effects largely contribute to the binding energy of the dopants [2]. \newline [1] M. Diarra, Y. M. Niquet, C. Delerue and G. Allan, Phys. Rev. B 75, 045301 (2007). \newline [2] M. Diarra, C. Delerue, Y.M. Niquet, and G. Allan, J. Appl. Phys., accepted. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X24.00005: The effect of the sodium and iodine doping on the electronic band structure of the polyicosahedral Si nanowire: A first principles study. Kengo Nishio, Taisuke Ozaki, Tetsuya Morishita, Wataru Shinoda, Masuhiro Mikami In a previous molecular dynamics study, we predicted a polyicosahedral Si nanowire which has a Si20 fullerene cage per icosahedral Si100 nanodot [1]. The unique cage structure is distinct from the crystalline diamond Si nanowire. Encapsulating a guest atom into the Si20 cage allows us to tune the physical properties of the nanowire. Here, we report on a first-principles study of the effect of the sodium and iodine doping on the electronic band structure of the hydrogen-terminated polyicosahedral Si nanowire [2]. Our calculations reveal that the guest-free polyicosahedral Si nanowire is a semiconductor with a 1.20 eV band gap. We also find that the semiconducting nanowire becomes metallic by the sodium and iodine doping, suggesting that the electronic band structure of polyicosahedral Si nanowires can be tuned by doping appropriate guest atoms. [1] J. Chem. Phys. 125, 074712 (2006). [2] submitted to PRB [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X24.00006: Electronic structures and electron-phonon interactions of boron-doped carbon nanotube Takashi Koretsune, Susumu Saito We study the boron-doped single-walled carbon nanotubes using first-principles method based on the density functional theory. The total energy, band structure and density of states are calculated. From the formation energy of boron-doped nanotubes with different diameter, it is found that the narrower tube needs lower energy to substitute a carbon atom with a boron atom. Using the result of different doping rate in the (10,0) tube, we extrapolate the result to low boron density limit and find that the ionization energy of the acceptor impurity level is approximately 0.2 eV. Furthermore, we discuss the doping rate dependence of the density of states at the Fermi level and the electron-phonon interactions which are important for superconductivity. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X24.00007: Edge states and magnetism in carbon nanotubes with line defects Helio Chacham, Mario S. C. Mazzoni, Simone S. Alexandre Under certain conditions, magnetic ordering has been predicted to occur in carbon nanostructures even in the absence of transition metal impurities. These conditions involve situations in which electronic localization takes place, such as at zig-zag edges or defects in graphene sheets and ribbons or in topological defects in carbon nanostructures. In the present work, we apply first-principles calculations to investigate the interplay between electronic and magnetic properties of carbon nanotubes with line defects. We consider three types of defects: lines of C-O-C epoxy groups, and defects resulting from the substitution of the oxygen atoms by CH$_2$ or C$_2$H$_4$ divalent radicals. We find that the line defects behave as pairs of coupled graphene edge states, and a variety of electronic and magnetic ground states is predicted as a function of defect type, nanotube diameter, and a possibly applied transverse electric field. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X24.00008: Effects of intrinsic spin-orbit coupling in carbon nanotubes Carlos A. B\"usser, Mahdi Zarea, Nancy Sandler The electronic structure and transport properties of carbon nanotubes (NTs) are the subject of intense theoretical and experimental studies. Tight-binding model predict these quasi-1D systems to be metallic or insulating depending on their chiral wrapping. Furthermore, external magnetic fields applied along the NT's axis, are expected to change these behaviorsby opening (metallic) or closing (insulating) gaps. \\ In this work we present preliminary numerical results obtained solving a tight-binding Hamiltonian [1] for band structures of armchair and zigzag NTs in the presence of the intrinsic spin-orbit (I-SO) interaction and magnetic fields. The I-SO interaction has dramatic effects, opening gaps for metallic NTs that are proportional the I-SO coupling constant. When considering an external field, the I-SO generated gaps show a quadratic dependence on the field strength in contrast with the linear dependence predicted in the absence of this interaction. Insulating tubes show increased gaps that do not close even in the presence of external fields. \\ \noindent [1] F.D. Haldane, Phys. Rev Lett. {\bf 61}, 2015 (1988). [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X24.00009: Transport properties of single vacancies in nanotubes Alexandre R. Rocha, J.E. Padilha, Adalberto Fazzio, Antonio J.R. da Silva We present transport, density of states and electronic transport calculations of single vacancies in carbon nanotubes. We confirm that the defect reconstructs into a pentagon and a nonagon following the removal of a single carbon atom. This leads to the formation of a dangling bond. Finally we demonstrate that care must be taken when calculating the density of states of impurities in one dimensional systems in general. We show that obtaining information about the transport properties of such systems with defects solely from the density of states of a periodic DFT calculation can be misleading. The appearance of mini-gaps and oscillations, even in the limit of large unit cells, can be erroneously associated with changes induced by the defect itself instead of a figment of the procedure. In fact, we demonstrate that those mini-gaps vanish if the appropriate approach is taken, namely a Green's function method where the effects of semi-infinite electrodes are considered and a true open system is calculated. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X24.00010: The structural and electronic properties of vacancy clusters in carbon nanotubes Alex Taekyung Lee, Yong-Ju Kang, Kee Joo Chang Carbon vacancies, which can be generated by electron or ion irradiation, significantly modify the structural and electronic properties of carbon nanotubes. Recent experiments showed that vacancy-type defects induce structural changes such as junction, shrinkage, and bending. In this work we study the atomic and electronic structure of vacancy clusters up to six missing atoms in carbon nanotubes through both first-principles and tight-binding calculations. We find that vacancy defects are generally stable when they are aligned along the tube axis, forming a vacancy-chain. Due to the curvature effect, this feature is different from that found for graphene, where vacancies tend to aggregate into a lump. For the even-numbered vacancies in the (5,5) and (9,0) nanotubes, we find that clustering of vacancies leads to the local shrinkage, with a smaller diameter tube sandwiched between two semi-infinite tubes. In this case, the defect levels near the Fermi level are mostly associated with 7- or 8-membered rings, whereas those for odd-numbered vacancies result from the remnant dangling bonds. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X24.00011: Synthesis of Triangle Tungsten Oxide Nanosheets with Potassium Doped Rong Hu, Kunquan Hong, Huasheng Wu We report that large quantity of tungsten oxide nanosheets are synthesized by oxidizing tungsten plate directly. Using potassium hydrate as the catalyst and tungsten plate as the substrate, those triangle tungsten oxide nanosheets with thickness of 50-300 nm and wide up to tens of micrometers are obtained on a large scale. The angles of the triangle nanosheets are around 35$^{\circ}$ and 50$^{\circ}$ statistically. We discussed the potential reason of this peculiar phenomenon, besides the several characteriztions results of the grown nanosheets. The probable growth mechanism is also investigated. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X24.00012: CVD synthesis of graphene Tao Jiang, Joel Therrien We will report on the use of Chemical Vapor Deposition (CVD) to grow sheets of graphene on substrates suitable for the semiconductor industry. Growth starts with the deposition of seed crystals of graphene on the substrate. CVD growth is found to result in growth at the edges of the seeds, rather than on the surfaces. The result is increases in the size of the seed crystal without additional layers of graphene forming on top of the crystal. This technique holds promise for forming large areas of high quality single layer graphene on inexpensive substrates. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X24.00013: New opportunities from controlled growth of carbon nanotubes. Seokwoo Jeon, Robert Caldwell, Yuyao Shan, Hanfei Wang, Jinyao Tang, Sami Rosenblatt, Colin Nuckolls, James Hone Aligned growth of carbon nanotubes (CNT) has been an important issue to researchers involved in CNT, and recent work has proved the possibility of alignment by using special substrates (i.e. quartz, sapphire, etc.) where CNT can grow along crystallographic axes. We present here two applications of aligned tubes produced from quartz substrates. The first is the realization of an integrated platform for using CNTs as electrodes for single molecules toward sensing applications; numerous sensing units can be generated by photolithography without long, tedious e-beam writing steps due to the precise control of location and direction of CNTs over a large area ($\sim $1X1 cm). The second is measurement of the high frequency properties of CNTs, which is difficult due to the high impedance of single-tube devices. By a slight modification of growth parameters, we have achieved growth of 'serpentine' CNTs on on quartz substrates that permit the fabrication of low-impedance devices using multiple identical CNT sections. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X24.00014: Diffusion-limited and pressure-driven electrodeposition in a microfluidic channel Geoff Steeves, Alex Wlasenko, Dennis Zakopcan, David Sinton Self-terminating electrochemical fabrication has previously been devised to create quantum point contacts with single-atom contacts, but the structure of the growth has been poorly controlled. We have introduced a microfluidic channel with which to apply pressure-driven flow during the formation of the junction between two Au electrodes. Without applied flow, dendritic growth and dense branching morphologies were typically observed at the cathode. The addition of applied pressure-driven flow resulted in a densely packed gold structure that filled the channel. Our computer simulation yielded insight into the regimes where the diffusion, flow and electric field between the electrodes individually dominated growth. Proposals for further sophistication in both experiment and simulation will also be presented. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X24.00015: Continuous and Scalable Fabrication of Transparent Conducting Thin Films of Single Walled Carbon Nanotubes Budhadipta Dan, Mainak Majumdar, Matteo Pasquali We report the fabrication of optically transparent and electrically conducting thin films of single-walled carbon nanotubes (SWNT) using the industrially scalable, fast and simple process of Rod-Coating. Rheology was used to study four different surfactants, their capacity to disperse SWNT in water and the viscoelastic properties of the resulting dispersion. A combination of two different surfactants was found ideal to make a uniform dispersion with high concentration of SWNT and the specific viscoelastic properties desired for coating. Rod coating with this coating fluid produced highly uniform, transparent and conducting SWNT thin films. The films were also treated with various strong acids which lead to further significant improvement in properties. Our results show that the choice of surfactant for making the coating dispersion has a strong effect on the electro-optical properties of the SWNT films. Films with sheet resistance of 100 Ohm/sq and 300 Ohm/sq for respective transparency of 70{\%} and 90{\%}, in the visible region, were obtained with this process. The development of this continuous and scalable fabrication process will thus bring the SWNT films closer to commercial application. [Preview Abstract] |
Session X25: Block Copolymer Phase Behavior
Sponsoring Units: DPOLYChair: Nitash Balsara, University of California, Berkeley
Room: Morial Convention Center 217
Friday, March 14, 2008 8:00AM - 8:12AM |
X25.00001: Molecular Simulation of Bicontinuous Phases in Diblock Copolymer Melts Francisco Martinez-Veracoechea, Fernando Escobedo Molecular simulations are used to study the stabilization of different bicontinuous phases in diblock copolymer (DBC) melts. The stabilization approach entails attempting to reduce the packing frustration inside the bicontinuous phases nodes by the addition of a ``filler'' with affinity for the A component. Two different strategies are considered: 1) addition of selective-solvent particles, and 2) addition of homopolymer. Approximate phase boundaries were found via free-energy calculations. A very dissimilar phase behavior is observed upon increasing the amount of the ``additive'' in the two different strategies. While with the first strategy (i.e., addition of selective solvent) we observed the progression Gyroid (G) $\to $ Perforated Lamella $\to $ Lamella $\to $ Reversed-Gyroid. With the second strategy (i.e., addition of homopolymer) we observed the progression of morphologies G $\to $ Cylinder $\to $ Double Diamond (DD) $\to $ Plumber's Nightmare (P). In both the DD and the P phases, the homopolymer concentrates preferentially in the nodes, suggesting the reduction of the nodes' packing frustration. In addition, a novel morphology was observed, wherein cylinders of two different diameters alternate in a tetragonal packing. The contrasting difference in the phase behavior observed for the two strategies is understood as a consequence of the difference in mixing entropy exhibited by the two additives. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X25.00002: Orthorhombic \textit{Fddd} Network in Diblock Copolymer Melts Mikihito Takenaka, Myung Im Kim, Satoshi Akasaka, Tsutomu Wakada, Shotaro Nishitsuji, Hirokazu Hasegawa Poly(styrene-\textit{block}-polyisoprene) (S-I) diblock copolymer melts with asymmetric volume fraction are shown to form an orthorhombic \textit{Fddd} network structure, which Tyler et al. predicted with self-consistent field theory for diblock copolymer melts. The studies with small-angle X-ray scattering and transmission electron microscopy revealed that the phase diagram of the S-I diblock copolymer exhibits the sequence of transition of disorder-gyroid-\textit{ Fddd}-lamellae with decreasing temperature and \textit{Fddd} phase appears within the narrow composition and temperature range where gyroid, lamellae, and hexagonally perforated layer (HPL) phases appear. The ratio of unit cell parameters ($a$:$b$:$c)$ estimated from the peak positions of the scattering function is 1:2.00:3.51, which agrees with the result of the theoretical calculation by Tyler et al. In this orthorhombic structure with the observed unit cell parameters, the higher order reflections 022, and 004 overlaps with the reflection 111 at the first order peak. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X25.00003: Fluctuation effects in block copolymers Erin M. Lennon, Richard Elliott, Glenn H. Fredrickson Using recently developed techniques for locating phase transitions, we study the effects of fluctuations in a field theoretic model on block copolymer behavior. Specifically, we couple the use of complex Langevin dynamics within a field theoretic framework and thermodynamic integration techniques for the calculation of free energies of fluctuating systems to show a revised prediction of the diblock copolymer phase diagram. Further, these methods are extended into blend systems to investigate unbinding transitions and critical micelle concentrations in cylindrical phases. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X25.00004: Scaling of Diblock Copolymer Lamella near the Order Disorder Transition Andrew B. Croll, An-Chang Shi, Kari Dalnoki-Veress Our accumulated knowledge of the physics of diblock copolymer phase transitions is extensive after decades of intense interest. There are, however, several inconsistencies between experiment and current theoretical understanding. Notably, one of the simplest measurable parameters, the length-scale of microphase separation, falls significantly out of agreement with theory near the order disorder transition (ODT). This length scales as $(\chi N) ^{m}$ where $\chi$ is the Flory- Huggins interaction parameter and $N$ is the number of monomers – experiments yield $m=0.8$ while theory predicts $m = 1$. We use optical microscopy to make real space measurements of the thickness of symmetric polystyrene - block - poly (2 vinyl pyridine), which we find to scale linearly - as predicted by theory. Our experiment suggests two simple optical methods for the measurement of $\chi$. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X25.00005: Self-assembly of Asymmetric Architectures: Study of the Phase Behavior of an ABAC Block Copolymer Michael Bluemle, Guillaume Fleury, Timothy Lodge, Frank Bates We have investigated the bulk phase behavior of the asymmetric tetrablock poly(cyclohexylethylene-b-ethylene-b-cyclohexyethylene-b-dimethylsiloxane) (CECD) in order to elucidate the effects of asymmetry created by introducing a third chemically distinct block to the well-studied CEC triblock. These tetrablock polymers are especially attractive due to the potential of degrading the D block, leaving a mechanically robust polyolefin triblock nanoporous material. Starting with CEC triblocks that self-assemble into different morphologies (hexagonally packed cylinders and lamellae), varying amounts of D have been added, creating two series of polymers along distinct isopleths. A combination of small-angle x-ray scattering, transmission electron microscopy and dynamic mechanical spectroscopy have revealed the complex phase behavior of these asymmetric polymers. Addition of as little as nine percent D by volume drastically changes the tetrablock morphological behavior as compared to their precursor CEC triblocks. These promising results exhibit the influence of asymmetry on the self-assembly of complex architectures in block copolymers. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X25.00006: Soft and Strong Thermoplastic Elastomers Through Molecular Design Folusho Oyerokun, Glenn Fredrickson, Dale Handlin Thermoplastic elastomers (TPE) that have a low linear modulus and yet are strong at large extension are of great importance in a variety of technological applications. Current TPE designs based on ABA triblock copolymers are limited in that the maximum volume fraction of the hard A blocks, which correlates with the material strength, is restricted by the constraint that the A domains be discrete while the soft B domains are continuous. In this study, we have investigated new designs of TPEs that utilize polydispersity of the hard blocks in tandem with novel block architectures to control morphology in microphase separated AB block copolymers. Self-consistent field theory calculations confirm that these designs stabilize spherical and cylindrical phases at higher volume fractions of the hard blocks, with the maximum volume fraction of the hard block in some cases approaching twice that of a conventional ABA thermoplastic elastomer. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X25.00007: Influence of Soft Segment Composition on Phase Separated Microstructure of PDMS-Based Multiblock Polyurethane Copolymers. Taeyi Choi, Jadwiga Weksler, Ajay Padsalgikar, James Runt Multiblock polyurethane (PU) copolymers with polydimethylsiloxane (PDMS) based soft segments possess intriguing microphase separation behavior and excellent biocompatibility. In this study we investigate the microphase-separated structure of PDMS-PUs with various well-defined soft segment compositions, which is closely connected to the structural and surface properties of these copolymers. The PDMS-PUs are shown to exhibit a three phase, core-shell like morphology. Intra- and intercomponent hydrogen bonding was explored using FTIR spectroscopy and quantitative analysis of hard/soft segment mixing was determined by small-angle X-ray scattering. The presentation will focus on the latest findings, particularly the role of PDMS in controlling the details of the microphase-separated texture. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X25.00008: Nanoparticle-Regulated Phase Behavior and Morphological Development in an Ordered Block Copolymer Michelle Bowman, Steven Smith, Jon Samseth, Michael Bockstaller, Russell Thompson, Kim Rasmussen, Richard Spontak Although microphase-ordered block copolymer motifs are employed to template inorganic nanoparticles, only recently has the effect of nanoparticles on copolymer self-assembly been explored. In this work, we examine the influence of nanoparticles on the copolymer order-disorder transition (ODT) temperature. Theoretical results from a hybrid self-consistent field/density functional theory -- supported by experimental observations of a model copolymer/nanoparticle system -- confirm that judicious selection of nanoparticle size and selectivity can be used to increase the ODT temperature at constant concentration. For a given nanoparticle size and selectivity, we show that there likewise exists a critical nanoparticle concentration beyond which the ODT temperature decreases. The ability of nanoparticles to increase the ODT temperature is a unique consequence of their size and is not expected for small-molecule additives. At high concentrations, the nanoparticles form percolated colloidal networks that represent highly confined environments for the copolymer molecules. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X25.00009: Effects of Lithium Salts on the Domain Size of Polyethylene Oxide Containing Block Copolymers Nisita Wanakule, Scott Mullin, Nitash Balsara The morphology of block copolymers with and without lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salts are measured with small-angle x-ray scattering (SAXS). 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. Due to the hygroscopic nature of the salts, blend preparation is performed completely in a glovebox and the SAXS samples are sealed off in airtight sample holders. To ensure that moisture contamination does not affect morphology, Karl-Fischer titrations are performed after SAXS measurements. Our data will be compared with literature results that indicate a 300{\%} increase in the domain spacing of PEO-containing block copolymers spacing due to the addition of LiTFSI. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X25.00010: Weak Segregation Theory of Microphase Separation in Block Copolymers: New Results and Perspectives. Igor Erukhimovich The weak segregation theory (WST) of microphase separation in block copolymers (BC) is based on the vision by Landau (1937) and seminal breakthrough by Leibler (1980) into microscopic theory of as well as the Brazovskii-Fredrickson-Helfand (1975, 1987) understanding of the corresponding fluctuation effects. The WST is especially helpful in the situation when one tries to form the well reproducible ordered morphologies, for which purpose they are to be formed as smoothly as possible. Among other new results in this field, I address the following issues: $i)$ non-conventional morphologies and phase transitions in the bulk and confined ternary ABC block copolymers; \textit{ii}) the BC phase diagram control via their chemical modification involving thermoreversible association between the different blocks; \textit{iii}) the WST analysis of the non-centrosymmetric lamellar structures in the blends of the ternary and binary BC; and \textit{iv}) semidulute BC solutions as photonic crystals. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X25.00011: Tunable Microphase Segregation of Gradient Copolymers: Ordering in Materials with Sinusoidal Composition Profiles Michelle Mok, Wesley Burghardt, John Torkelson Gradient copolymers are a class of polymers that exhibit a gradual change in composition along the entire chain from mostly A-monomer to mostly B-monomer. Theoretical work has predicted that gradient copolymers organize into sinusoidal composition profiles rather than the step-like profiles seen for block copolymers. Here, small-angle x-ray scattering and rheological studies were performed to investigate the impact of gradient design and comonomer choice on this unique ordering. Samples showed a variety of non-terminal behaviors consistent with their chain architecture relative to block copolymers, indicating highly tunable microphase segregation. Scattering results also demonstrated that a range of ordering was attained, with higher order peaks visible in more microphase-segregated samples. In addition, it was demonstrated for the first time that application of high amplitude oscillatory shear induced domain shear alignment in a manner similar to block copolymers, even though gradient copolymers do not possess distinct domain boundaries. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X25.00012: Polydispersity effects in block copolymer melts Mark Matsen We examine the effects of polydispersity on the phase behavior of diblock copolymer melts using self-consistent field theory (SCFT). The calculations are performed with an efficient spectral-based algorithm that can handle high degrees of polydispersity with only a modest increase (i.e., a factor of 2 or 3) in computational cost over that of monodisperse melts [Matsen, EPJE, {\bf 21}, 199 (2006)]. We find that even small degrees of polydispersity can have a significant effect on the domain sizes and the position of the phase boundaries. For large polydispersities, fractionation also becomes important and the phase diagram develops large two-phase coexistence regions [Matsen, PRL, {\bf 99}, 148304 (2007)]. As a consequence, the complex gyroid phase becomes unstable with respect to the coexistence of lamellae and cylinders. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X25.00013: Polydispersity-Driven Morphological Transitions in ABC Triblock Terpolymers Adam J. Meuler, Christopher J. Ellison, Christopher M. Evans, Marc A. Hillmyer, Frank S. Bates The use of synthetic polymerization techniques (e.g., controlled radical polymerizations) that often yield polydispersity indices greater than 1.1 is becoming more widespread. Advances in these methodologies have increased the number of monomers amenable to incorporation in block copolymers and will potentially drive commercial costs down. Since many block copolymer properties are governed by the underlying mesostructure, understanding the influence of polydispersity on morphological behavior should prove vital to the success of block copolymer commercialization efforts. This presentation will focus on polydispersity-driven morphological transitions in poly(isoprene-$b$-styrene-$b$-ethylene oxide) (ISO) triblock terpolymers. ISO triblocks with polydisperse polystyrene blocks were prepared by anionic polymerization and their morphological behavior was characterized using small-angle x-ray scattering and dynamic mechanical spectroscopy. Only lamellar microstructures were identified along the f$_{I }$= f$_{S}$ isopleth for polydisperse ISO triblocks, while an orthorhombic network (O$^{70})$ was previously identified in monodisperse ISO triblocks. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X25.00014: Scaling of the ODT of Block Copolymers in Compressed CO$_{2}$ Curran Chandler, Timothy Francis, James Watkins It is well-known that diblock copolymers with sufficient $\chi N$ form periodic microphase-separated domains upon cooling through an order-disorder transition (ODT). We have investigated the scaling behavior of the ODT as a function of polymer volume fraction, \textit{$\phi $}, of several nearly symmetric poly(styrene-$b$-2-vinylpyridine) and poly(styrene-$b$-isoprene) diblock copolymer/diluent systems in relation to the well-known dilution approximation. Using compressed CO$_{2}$ in the place of conventional liquid diluents allowed$_{ }$the determination of the scaling parameter, \textit{$\alpha $}, for highly concentrated systems where \textit{$\phi $ }ranges from 0.85 to 1.0 at high temperatures. The scaling was determined by combining optical birefringence measurements of the ODT ($\chi _{ODT})$ with the ellipsometric swelling measurements (\textit{$\phi $}) of the constituent homopolymers at increasing CO$_{2}$ pressures. We show that sorption of small volume fractions of CO$_{2}$ results in significant reductions in the observed ODTs. Yet, \textit{$\alpha $ }was clearly shown not to be universal even for a specific diblock copolymer. For styrene-b-isoprene copolymers, it appears that \textit{$\alpha $ }is an increasing function of copolymer molecular weight. In contrast, the styrene-b-2-vinylpyridine copolymers studied show no obvious correlation with molecular weight, with \textit{$\alpha $ } taking on both positive and negative values. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X25.00015: Pressure Effect on Phase Behavior of Weakly Interacting Block Copolymers by using FTIR spectroscopy Hye Jeong Kim, Seung Bin Kim, Jin Kon Kim, Young Mee Jung Hydrostatic pressure effect on the phase transition of polystyrene-block-poly(n-pentyl methacrylate) [PS-b-PnPMA] copolymer was investigated by FTIR spectroscopy. The size of closed-loop consisting of both the lower disordered-to-ordered transition and the upper ordered-to-disordered transition became smaller with increasing pressure. The functional groups belonging to the PnPMA block are much more sensitive to pressure compared with those belonging to the PS block. The sensitivity of the functional groups change with pressure is different from that with temperature. [Preview Abstract] |
Session X27: Focus Session: Exchange Bias and Magnetic Interactions
Sponsoring Units: GMAGChair: Christian Binek, University of Nebraska-Lincoln
Room: Morial Convention Center 219
Friday, March 14, 2008 8:00AM - 8:12AM |
X27.00001: Uncompensated moments in antiferromagnets: Origin and role in exchange bias. Igor V. Roshchin, Zhi-Pan Li, Casey W. Miller, Maria Varela, Stephen J. Pennycook, Ivan K. Schuller Exchange bias (EB) is a ferromagnet (FM) -- antiferromagnet (AF) proximity effect. The depth profile of the magnetization across the interface between a FM (Co) and an AF (FeF$_{2})$ in an EB system has been measured. [1] It was found that both uncompensated and compensated magnetic moments are present in FeF$_{2}$. The origin of these moments remains an open question. Our high-resolution STEM study confirms that FeF2 grows epitaxially on MgF$_{2}$, and its structural quality is very high. It also reveals that the substrate surface imperfections do not necessarily affect the quality of the FeF$_{2}$ layer. In magnetization measurements of just a thin film of FeF$_{2}$, without a FM, we find an uncompensated magnetization. This magnetization demonstrates temperature dependence and horizontal hysteresis loop shift, typical for EB. Dependence of this magnetization on the substrate, film parameters and cooling conditions will be discussed. Work supported by US DOE. \\ $[1]$ M. R. Fitzsimmons \textit{et al}., Phys. Rev. B \textbf{75}, 214412 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X27.00002: Strong anisotropy suppression at the CoO Ne\'{e}l temperature in perpendicularly exchange-biased CoO/CoPt multilayers Erik Shipton, Keith Chan, Olav Hellwig, Eric E. Fullerton We have performed high-field torque magnetometry measurements on perpendicularly exchange-biased CoO/CoPt multilayers. From magnetometry measurements we observe perpendicular exchange bias that goes to zero at a blocking temperature of 200 K. Torque measurements were performed from 2 to 400 K in fields up to 9 T. There is significant hysteresis even at 9 T fields suggesting instabilities in the AF layer. This hysteresis persists up to the blocking temperature. Surprisingly, there is a large reduction in the uniaxial anisotropy with increasing temperature with a minimum at 250 K, the estimated Ne\'{e}l temperature of the CoO layers, followed by an increase in the anisotropy with increasing temperature. The anisotropy of Co/Pt multillayers without the CoO layers monotonically decreases with increasing temperature. This suggests that there is an additional planar anisotropy arising from the CoO that counters the anisotropy of the Co/Pt layers, and that the contribution from the CoO is maximized at the Ne\'{e}l temperature as observed by Grimsditch et al. [1]. These results show that the addition of antiferromagnetic layers may be used to tune the temperature dependence anisotropy response of magnetic systems. [1] M. Grimsditch et al., Phys. Rev. Lett. \textbf{90}, 257201 (2003). [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X27.00003: Pinned magnetization in the exchange bias system Permalloy/CoO Sujoy Roy, Elizabeth Blackburn, Cecilia Sanchez-Hanke, Sunil Sinha, Ami Berkowitz Interfacial effects are understood to be crucial in the development of exchange bias. In particular, the role of uncompensated spins is important, although because the interface is buried these uncompensated spins can be difficult to measure. Penetrating radiation such as neutrons or x-rays are one of the few tools available to do this. The problem of exchange bias is further complicated by the myriad differences observed from system to system, indicating that the local environment of the magnetic ions has a strong effect on the type of coupling that dominates across the interface. In turn, understanding this coupling is vital in understanding the microscopic origin of exchange bias in a given system. In this paper, we present soft x-ray reflectivity data that show that in the exchange biased state (i.e. below the Neel temperature 289 K for CoO) there is an interfacial layer between the Py and CoO that possesses a net magnetization at room temperature. In the exchange biased state, this contains the uncompensated spins from the CoO layer, and a significant fraction of the spins in this layer are pinned. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X27.00004: Ultrafast frustration of the magnetization in exchange biased Ni/FeF2 Amit Porat, Ivan K. Schuller, Shimshon Bar-Ad We used the magneto optical Kerr effect to study fast optically-induced magnetization dynamics in a Ni/FeF$_{2}$ exchange bias bilayer. We find that sub-picosecond laser pulses trigger an unexpected out of plane precession of the Ni magnetization, surprisingly in external magnetic fields that overcome the exchange bias (unlike previously reported precessions in bilayers). Even more surprisingly the precession persists for excitation intensities that completely decouple the Ni from the FeF$_{2}$. The experimental results suggest that the FeF$_{2}$ layer at the thermally-excited interface is frustrated by the opposing anisotropy fields created by the external field, the Ni layer, and the underlying thermally unexcited FeF$_{2}$ layer. The frustrated FeF$_{2}$ layer reorients, which in turn triggers the precession of the Ni. This implies that the decoupling at high excitation intensities does not only involve the Ni, but also the interfacial FeF$_{2}$ layer, which decouples from the cold underlying bulk FeF$_{2}$. The decoupling thus leads to a reversal of the exchange bias, as we found experimentally. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X27.00005: Antiferromagnetic Domain Size Measurement in Fe$_{0.70}$Zn$_{0.30}$F$_2$/Co Bilayers David Lederman, Hongtao Shi, Michael Fitzsimmons The size of the antiferromagnetic domains of an epitaxial (110) Fe$_{0.70}$Zn$_{0.30}$F$_2$ dilute Ising antiferromagnetic layer 68 nm thick with a polycrystalline Co overlayer 27 nm thick was studied via neutron diffraction. The sample's exchange bias changed sign from negative to positive as the temperature was increased, with the switching temperature, at which the exchange bias was zero, occurring at $T=20$ K. The width of the (100) antiferromagnetic peak of the Fe$_{0.70}$Zn$_{0.30}$F$_2$ layer was significantly narrower at the switching temperature than at either $T=5.5$ K or $T=30$ K. This result is consistent with models that predict an inverse relationship between the antiferromagnetic domain size and exchange bias. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X27.00006: Scaling behavior of the exchange-bias training effect Srinivas Polisetty, Sarbeswar Sahoo, Christian Binek The dependence of the exchange-bias training effect on temperature and ferromagnetic film thickness is studied in detail and scaling behavior of the data is presented.$^{\dag }$ Thickness-dependent exchange bias and its training are measured using the magneto-optical Kerr effect. A focused laser beam is scanned across a Co wedge probing local hysteresis loops of the Co film which is pinned by an antiferromagnetic CoO layer of uniform thickness. A phenomenological theory is best fitted to the exchange-bias training data resembling the evolution of the exchange-bias field on subsequently cycled hysteresis loops. Best fits are done for various temperatures and Co thicknesses. Data collapse on respective master curves is achieved for the thickness and temperature-dependent fitting parameters as well as the exchange bias and coercive fields of the initial hysteresis loops. The scaling behavior is strong evidence for the validity and the universality of the underlying theoretical approach based on triggered relaxation of the pinning layer towards quasi-equilibrium. $^{\dag }$Srinivas Polisetty, Sarbeswar Sahoo, Christian Binek, Phys. Rev. B \textbf{76}, 184423 (2007). [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X27.00007: Interlayer coupling in Co/NiO/Fe trilayers studied by element-specific XMCD and XMLD effects J. Wu, J. Choi, C. Won, A. Scholl, A. Doran, E. Arenholz, Y.Z. Wu, X.F. Jin, Z.Q. Qiu Co/NiO/Fe trilayers are grown on Ag(001) substrate using Molecular Beam Epitaxy (MBE). Element-specific magnetic domain images on both ferromagnetic (FM) Co and Fe layers and antiferromagnetic NiO layer are obtained using X-ray Magnetic Circular Dichrism (XMCD) and X-ray Linear Dichrism (XMLD), respectively. By comparing these magnetic domain images, we find that the Co-Fe interlayer coupling across the NiO spacer layer exhibits a transition from a 90$^{o}$-coupling to a collinear coupling as the NiO film thickness increases. This observation is shown to be directly related to the NiO magnetic structure and its interaction at the Co/NiO and NiO/Fe interfaces. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X27.00008: Thermodynamics of Co/Cr superlattices T. Mukherjee, S. Sahoo, R. Skomski, D.J. Sellmyer, Ch. Binek Progress in ultra thin film growth has resulted in many novel surface and interface induced properties of artificial heterostuctures. Here, we study magnetic superlattices of ultrathin Co and Cr films grown by Molecular Beam Epitaxy methodology at a base pressure below 1$\times $10$^{-10}$ mbar. Our approach is based on controlling two distinct magnetic degrees of freedom. First, the critical temperature, T$_{c}$, of individual Co films is tailored via geometrical confinement of the correlation length perpendicular to the film. Various thickness dependent values, T$_{c}$(d), between zero and the bulk Curie temperature of 1388 K are realized. Second, the T$_{c}$-tailored Co films are antiferromagnetically coupled through Cr interlayer films. The oscillating coupling strength is tailored via the Cr interlayer thickness. The resulting thermodynamic properties of such Co/Cr superlattices are studied with the help of SQUID magnetometry. Particular emphasis is laid on tailoring magnetic entropy changes in the vicinity of room temperature. X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X27.00009: Cooling Field Dependence of Magnetization Depth Profiles in Exchange-coupled Superlattices Michael Fitzsimmons, Karine Dumesnil, Catherine Dufour In DyFe$_{2}$/YFe$_{2}$ superlattices, competition between ferromagnetic exchange coupling of adjacent Fe spins and antiferromagnetic coupling of Fe spins with rare earth spins leads to an antiparallel arrangement (confirmed with XMCD and neutron scattering) of magnetization across the DyFe$_{2}$/YFe$_{2}$ interfaces in low fields at 300 K. After cooling this simple structure to 12 K, the DyFe$_{2}$ magnetization becomes pinned and the sample exhibits very large exchange bias ($\sim $2 T) and a large (35{\%}) negative shift of the sample magnetization along the magnetization axis. However, when a large magnetic field is applied at room temperature, the magnetization adopts a spin-flop configuration across the DyFe$_{2}$/YFe$_{2}$ interfaces (confirmed with XMCD and neutron scattering). When cooled in a large field, the sample yields neither exchange bias nor a shift of the sample magnetization along the magnetization axis. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X27.00010: In-plane anisotropy of NiCo multilayers P. Panyajirawut, M.S. Rzchowski We have grown NiCo magnetic multilayers on Si substrates by sputter deposition. The Ni$_{0.6}$-Co$_{0.4}$ bilayer that repeats to form the multilayer ranges from 2~nm to 12~nm in thickness. The total multilayer thickness ranges from 50~nm to 75~nm. Room temperature vibrating sample magnetometer (VSM) measurements show that the multilayers have in-plane uniaxial magnetic anisotropy with K$_{u}\sim $1.2x10$^{5}$ erg/cc. This is apparently induced during growth by the sputtering geometry, as we also see uniaxial in-plane anisotropy in individual 30~nm thick Ni (K$_{u}\sim $0.3x10$^{5}$ erg/cc) and Co (K$_{u}\sim $2.9x10$^{5}$~erg/cc) sputtered films. However the multilayer anisotropy is more complex as it arises from an interaction between the Ni and Co layers, with the Ni and Co layer magnetizations to first approximation rotating together. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X27.00011: Determination of inhomogeneous magnetic profiles in an asymmetric Fe/Gd multilayer Evgeny Kravtsov, Daniel Haskel , Suzanne G.E. te Velthuis , Yongseong Choi, J. Samuel Jiang We have studied the dependence of the detailed magnetization depth profile in a [Fe(35 A)/Gd(50 A)]$_{5}$ multilayer on the applied magnetic field and temperature. Utilizing the complementarity of x-ray resonant magnetic reflectivity (element-specificity and high spatial resolution) and polarized neutron reflectivity (large magnetic scattering cross sections), we applied a unified approach by simultaneous refinement and resolved the complex magnetization profiles. It was found that the small number of periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique inhomogeneous magnetic phases, which are characterized by significant twisting of Fe and Gd magnetic moments and non-uniform distribution of magnetization density within the Gd layers. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X27.00012: Magnetic Compensation in Ferrimagnetic Bimetallic Oxalates Peter Reis, Randy Fishman, Fernando Reboredo, Juana Moreno Bimetallic oxalates are layered organic magnets with the chemical formula A[M(II)M'(III)(ox)$_{3}$], where M(II) and M'(III) are transition metal ions, A is an organic cation, and ox = C$_{2}$O$_{4}$ is the oxalate molecule. For some ferrimagnetic bimetallic oxalates, the magnetization changes sign at a compensation temperature below the ferrimagnetic transition temperature. We have initiated a systematic study of these compounds by examining the possibility of magnetic compensation for any possible combination of transition metal ions. Our model includes spin-orbit coupling for both M(II) and M(III) ions, the antiferromagnetic exchange between neighboring metal ions mediated by the oxalate bridges, and the effects of the ligand fields. Using mean-field theory, we predict candidates that may exhibit magnetic compensation for certain choices of the intercalated cation A. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X27.00013: Giant Negative Magnetization in a Class of Layered Molecular-Based Magnets Randy Fishman, Fernando Reboredo Bimetallic oxalates are a class of layered molecular-based magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spin-orbit interactions, that explains the magnetic compensation and giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. By shifting the Fe(II) ions with respect to the oxalate molecules, the organic cation between the magnetic layers alters the C$_{3}$-symmetric crystal field and the orbital angular momentum of the ground-state doublet at the Fe(II) sites. We provide new predictions for the spin-wave gap, the effects of uni-axial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates [1]. [1] R.S. Fishman and F.A. Reboredo, \textit{Physical Review Letters }\textbf{99}, 217203 (2007). [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X27.00014: Effect of thermal fluctuation on the recoil loops of exchange-coupled nanocomposite magnets Chuanbing Rong, Yuzi Liu, J. Ping Liu Exchange-coupled hard/soft nanocomposite magnets have attracted great attention due to the very high potential energy product. One of the most effective ways to characterize the exchange-coupling strength in the nanocrystalline magnets is the measurement of recoil loops. It is often noted that the recoil curves are widely open for the hard/soft nanocomposite magnets but are narrow and even close for the single-phase magnets. In this work, we studied recoil loops of the FePt/Fe$_{3}$Pt nanocomposite magnets. It was interesting to find that the parameter that describes the openness of the loops, $\Delta m_{rc}^m$, is significantly dependent on the sweep rate of applied field, especially for the nanocomposite magnets with high soft-phase content, where $\Delta m_{rc}^m$ is maximum difference between upper and lower magnetization curves of the recoil loops. The quantitative analysis shows a reciprocally linear relation between $\Delta m_{rc}^m$ and the activation volume, which means that the recoil loops are intimately related to the thermal fluctuation. The large open area of the nanocomposite magnets compared to that of single phase magnets is attributed to the more unstable magnetization process in the exchange-hardened soft phase. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X27.00015: Effects of additives on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)12 alloys Jingzhi Han, Changsheng Wang, Honglin Du, Haiying Chen, Yingchang Yang Effects of additives such as Co, Zr, Nb and Ga on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)$_{12}$ alloys are investigated. The results show that Zr or Nb addition increases the temperature of the disproportionation, and Co or Ga addition decreases the temperature of the recombination. This indicates that Zr or Nb addition makes the disproportionation sluggish while Co or Ga addition is effective for enhancing the recombination, which is similar to the effects of the above additives on the hydrogen absorption and desorption characteristics of Nd$_{2}$Fe$_{14}$B alloys. However, according to XRD measurement of the magnetic oriented samples, Anisotropic HDDR NdFe$_{10.5-X}$M$_{X}$Mo$_{1.5}$ (M=Co,Zr,Nb or Ga) powders is not found. In order to analyze the reason leading to isotropic Nd(Fe,Mo)$_{12}$ and anisotropic Nd$_{2}$Fe$_{14}$B powders, the similarities and differences of the Nd(Fe,Mo)$_{12 }$and Nd$_{2}$Fe$_{14}$B systems and their corresponding HDDR phenomena are discussed, and whether leading to anisotropic powders or not is suggested to be related to crystal growth direction of the Nd-Fe-B and Nd(Fe,Mo)$_{12}$ system. At the same time, it is also found that the Nb addition is helpful for improving the magnetic properties of the HDDR Nd(Fe,Mo)$_{12}$N$_{X}$ magnetic materials, and the HDDR NdFe$_{10.3}$Nb$_{0.2}$Mo$_{1.5}$N$_{X}$ powders have a Br of 54 emu/g, a iHc of 3900 Oe. Moreover, the Mr/Ms ratio of the above powders reaches 0.60. [Preview Abstract] |
Session X28: Focus Session: Optical Properties of Nanostructures VII: Surface Plasmons and Periodic Arrays
Sponsoring Units: DMPChair: Ho-Bun Chan, University of Florida
Room: Morial Convention Center 220
Friday, March 14, 2008 8:00AM - 8:12AM |
X28.00001: Plasmonic propagation along Metal/Semiconductor/Metal nanowires Laetitia Bernard, Marleen H. van der Veen, Daniel Turner-Evans, Eric R. Dufresne, Mark A. Reed, Kwan Skinner, Sean Washburn Recent advances demonstrated metallic nanowires as structures allowing selective coupling of photons to fluctuations in the surface density of electrons, and the propagation of these plasmon modes along the wire. We report here the observation of such propagating plasmons in heterogeneous metal/semiconductor/metal nanowires. Specifically, we excite one end of a Au/CdSe/Au nanowire with focused laser light and demonstrate the coupling of photons into the plasmon modes of the wire. These modes propagate along the wire, being emitted as elastically scattered photons, exclusively at the metal/semiconductor interfaces and the distal end. Through control of the excitation wavelength and wire composition, we gain insights about the nature of the plasmon propagation through CdSe, allowing direct comparison with standard metal studies. This contributes to the growing interest in plasmonics within nanoscale devices by extending it to semiconductor materials, and goes towards the integration of optics with nanotechnology. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X28.00002: Plasmonic Interfaces for Electro-Optic Characterization of Molecular Junctions Marleen H. van der veen, Laetitia Bernard, Jason Merrill, Eric R. Dufresne, Mark A. Reed Metallic nanowires have interesting optical applications that make them promising platforms for sensing applications. The optical properties originate from the excitation of surface plasmon polaritons with light. We use this phenomenon to develop new spectroscopic tools to characterize the nature and details of the molecular structures within active regions of nanometer-scale devices. To achieve optical excitation at the nanometer-scale, we use a far field plasmon launching and emission technique based on nanowires coupled to molecular junctions. It is shown that plasmons can couple efficiently across small interfaces and can propagate along crossed nanowires. Furthermore, the light emitted as elastic scattered photons can be used to selectively couple light into the region of molecules for optical excitation. The possibility of doing local optical excitation of molecules simultaneously with their current-voltage measurements could lead to a new sensing platform for molecules based on plasmonic interfaces. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X28.00003: Controllable evanescent field coupling between metallic bilayers of subwavelength apertures Z. Marcet, J. Paster, H. B. Chan, D. W. Carr, J. E. Bower, R. Cirelli, F. P. Klemens, W. M. Mansfield, J. F. Miner, C. S. Pai, J. A. Taylor The optical transmission through a periodical array of subwavelength apertures in a metal film can be extraordinarily high due to resonance of the incident light with surface excitations, accompanied by dramatic enhancement of the local electromagnetic field on the metal surfaces. We have fabricated subwavelength slit arrays in two layers of metal. The two layers are positioned sufficiently close to each other so that the evanescent fields couple strongly at resonance. Depending on the lateral shift between the two layers, the transmission changes from near zero to a value that exceeds single layer transmission. Moreover, the phase delay of the transmitted light depends strongly on the lateral shift. We present both numerical simulations and experimental data to demonstrate that by tuning the lateral shift, an extra phase delay of $\pi $ can be introduced while the transmission remains near maximum. The controllable phase delay in bilayer subwavelength structures opens new capabilities that cannot be achieved with single layers. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X28.00004: Study of the absorption spectra of periodic hole arrays Dimitrios Koukis, Daniel Arenas, Sinan Selcuk, A.F. Hebard, D.B. Tanner, S.V. Shabanov The absorption of light by periodic hole arrays was studied for various open area fractions and wavelengths. We determined the absorption by measurements of the transmittance (T) and reflectance (R) at near normal incidence and setting the absorption as A = 1-R-T. The reflectance and transmittance were both measured, using a microscope photometer in the near infrared region and Bruker IFS 113v in the mid-infrared region. Periodic hole arrays are characterized by ``extraordinary'' transmittance (larger than the predictions of geometrical optics) at frequencies just below the onset of first order diffraction by the periodic array. The absorption maximum occurs at frequencies slightly larger than the transmittance maximum. The absorption results for the various open area fractions will be compared to theoretical predictions. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X28.00005: Steering and multiplexing subwavelength plasmon beams Vitalii Vlasko-Vlasov, Alexandra Imre, John Pearson, Jon Hiller, Ulrich Welp Arc shaped nanoslits in thin silver films are used as sources and lenses for generating sub-wavelength plasmon spots with high optical near-fields. We introduce a continuous phase shift along the nanoslits to achieve steering of the plasmon focus spot. It is experimentally shown that such a phase control allows to move the plasmon focus by micrometers with a nanometer precision and to launch it on separate 250 nm wide silver nanowires placed in the focal plane. Our nanostructures demonstrating scanning and multiplexing functionality show a feasibility of the nanoscale manipulation with optical fields. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X28.00006: Optical mapping of surface plasmon near-field spatial distribution in triangular silver nanoprisms Andrew Jones, Matthias Rang, Benjamin Wiley, Zhiyuan Li, Younan Xia, Markus Raschke Plasmonic metal nanostructures have attracted interest in diverse fields including biomolecular sensing and nano-photonics. To understand the correlation of the optical response with the shape and size of the structure, we optically stimulate localized surface plasmons on chemically prepared Ag triangular nanoprisms and utilize scattering-type Scanning Near-field Optical Microscopy (aSNOM) to map the optical near-field distribution with a spatial resolution down to 10nm. Using homodyne amplification allows for phase selective probing and identification of specific plasmon eigenmodes. For large triangles the superposition of several modes results in a complex distribution of the electric field whereas for smaller particles the distribution takes on a dipolar pattern. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X28.00007: Photonic delocalization and optical propagation in one-dimensional random n-mer dielectric systems Ruwen Peng, Zeng Zhao, Feng Gao, Lushuai Cao, De Li, Zhan Wang, Xiping Hao, Mu Wang We have both theoretically and experimentally investigated the propagation of electromagnetic waves in a one-dimensional random n-mer(RN) dielectric system. Due to the positional correlation in the RN structure, the localization- delocalization transitions of photons happen at expected frequencies of photons. Multiple resonant transmissions are found in the photonic band gap. At each resonant mode, zero- Lyapunov exponent and undecayed field distribution of electromagnetic waves have been found through the whole system. Furthermore, the channel is opened for photonic transport at the resonant frequency, and the density of states of photons increases step by step as frequency increases. The theoretical results are experimentally demonstrated in RN dielectric multilayer films of SiO2/TiO2 for visible and near infrared light. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X28.00008: Spontaneous quantum condensation in an optically-pumped microcavity far from equilibrium Paul Eastham, Richard Phillips We propose and analyze a method for creating highly non-equilibrium quantum condensates from excitons and photons in a semiconductor microcavity on timescales shorter than any thermalization times. In a theoretical study of microcavity dynamics we show that a tailored optical pulse can directly create a supercooled exciton population by an analogue of adiabatic rapid passage. This state involves no macroscopic occupations and is thus not a condensate. At later times, however, it spontaneously develops into a quantum condensate far from thermal equilibrium. This class of condensates encompasses phenomena similar to superradiance and lasing, but also includes states which give access to non-equilibrium Bose condensation in a solid-state system. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X28.00009: Random Laser Emission from ZnO Nanocomposite Hybrids A. Stassinopoulos, S. H. Anastasiadis, D. G. Papazoglou, D. Anglos, D. Tsagarakis, R. N. Das, E. P. Giannelis Highly scattering hybrid structures are produced either by incorporating ZnO nanoparticles in inert polymeric or inorganic sol-gel matrices or by depositing them on flexible substrates. All structures exhibit intense laser-like emission upon optical pumping. The ZnO particles provide both the gain and the strong scattering power, which lead to photon localization due to multiple scattering. The polymer matrix offers ease of material fabrication and processability while the elastic substrate offers flexibility in view of potential applications. Excitation of the hybrids by laser pulses shows threshold behavior demonstrated by a dramatic increase in the emitted light intensity and significant spectral and temporal narrowing. We study the influence of pump pulse duration and sample temperature on the random laser efficiency whereas we measure the coherence length of the emission. Nanocomposite fabrication issues and pumping conditions are varied aiming at performance optimization and, thus, potential use of such materials in future light emission devices. Sponsored by the Greek GSRT, by NATOs Scientific Affairs Division and by the EU. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X28.00010: Exciton energy spectra and optical-transition probabilities in InGaAs/GaAs ring-like nanostructures: Shape effects V.M. Fomin, V.N. Gladilin, J.T. Devreese, N.A.J.M. Kleemans, P.M. Koenraad We analyze the energy spectrum of an exciton and optical-transition probabilities for a model of self-assembled InGaAs/GaAs ring-like nanostructures, which is based on our X-STM data. We calculate the probabilities of optical transitions between the exciton vacuum and one-exciton states in the ring-like nanostructures. Spectral distributions of these probabilities as a function of the applied magnetic field are characterized by rich patterns, which significantly depend on shape, size and composition. Our analysis of the photoluminescence spectrum, observed in magnetic fields up to 30 T, implies a clear anisotropy of the measured nanostructures. For those anisotropic nanostructures, a smooth behavior of the exciton ground state energy as a function of the magnetic field is found in agreement with the experiment. We acknowledge collaboration with H. C. M. van Genuchten and M. Bozkurt. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X28.00011: Collective Plasmon Resonances and Their Influence on Metallic Nanostructures Ping Chu, Douglas L. Mills We present theoretical studies of the influence of collective plasmon resonances and their influence on enhanced fields, laser induced forces and related issues in selected examples of metallic nanostructures. Our new calculations focus on two nearby dissimilar nanospheres, and on nanospheres near a planar substrate. We show that when the response of two dissimilar spheres is compared to that of two isolated spheres, breakdown of a selection rule greatly increases the spectral range over which laser fields may couple to collective plasmons. We shall illustrate this with several selected examples. Collective plasmons also influence zero point fluctuations in the electric field near nanostructures; these produce non radiative transitions and energy level shifts within nearby molecules[1]. We shall present studies of the spatial distribution and frequency spectra of plasmon enhanced zero point fluctuations, with the near vicinity of STM tips in mind. [1] D. L. Mills, J. X. Cao and R. Wu, Phys. Rev B75, 214404, (2007). [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X28.00012: Probing Bright and Dark Surface Plasmon Modes in Au Nanoparticles Using a Fast Electron Beam Ming-Wen Chu, Viktor Myroshnychenko, F. Javier Garc\'Ia de Abajo, Cheng Hsuan Chen We have studied the surface plasmons (SPs) of individual and coupled Au nanoparticles (NPs) with various sizes and shapes by electron energy-loss spectroscopy (EELS) using a 2-nm monochromatized fast electron beam in a scanning transmission electron microscope (STEM). EELS spectra were investigated with the beam in grazing incidence, the optical near-field setup, and \textit{bright} SP modes (visible by light) were identified in both individual, coupled NPs ranging from near-IR to visible regimes. In an individual long rod with an aspect ratio of $\sim $6, a \textit{dark} SP mode invisible by light was surprisingly characterized, supported by calculations of the macroscopic dielectric responses. With the NPs coupling, rich bright, dark SP modes emerge and the suppression of one mode over the other is dictated by the beam position when approaching a geometrically centered site. The electron scattering geometry thus plays the role the SP-mode selection, never documented before. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X28.00013: Quantum confined Stark effect in organic fluorophores. Xihong Peng, John Anderson, Gary Tepper, Supriyo Bandyopadhyay, Saroj Nayak Fluorescent molecules have widely been used to detect and visualize structure and processes in biological samples due to its extraordinary sensitivity. However, the emission spectra of flurophores are usually broad and the accurate identification is difficult. Recently, experiments show that energy shifts by Stark effect can be used to aid the identification of organic molecules [1]. Stark effect originates from the shifting/splitting of energy levels when a molecule is under an external electric field, which shows a shift/splitting of a peak in absorption/emission spectra. The size of the shift depends on the magnitude of the external field and the molecular structure. In this talk we will show our theoretical study of the peak shifts on emission spectra for a series of organic fluorophores such as tyrosine, tryptophan, rhodamine123 and coumarin314 using density functional theory. We find that a particular peak shift is determined by the local dipole moments of molecular orbitals rather than the global dipole moment of the molecule. These molecular-specific shifts in emission spectra may enable to improve molecular identification in biosensors. Our results will be compared with experimental data. [1]Unpublished, S. Sarkar, B. Kanchibotla, S. Bandyopadhyay, G. Tepper, J. Edwards, J. Anderson, and R. Kessick. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X28.00014: Effects of electron heating on femtosecond laser-induced coherent acoustic phonons Jincheng Wang, Chunlei Guo In this work, we employ a surface plasmon technique to resolve the dynamics of femtosecond laser-induced coherent acoustic phonons in noble metals. Clear acoustic oscillations are observed in our experiments. We further find that the initial phase decreases linearly with pump fluence. Our model calculations show that the hot electrons instantaneously excited by femtosecond pulses contributes to the generation of coherent acoustic phonons in metals. [Preview Abstract] |
Session X29: Focus Session: Carbon Nanotubes and Related Materials XV: Electronic Structure and Optical Properties
Sponsoring Units: DMPChair: Eva Andrei, Rutgers University
Room: Morial Convention Center 221
Friday, March 14, 2008 8:00AM - 8:12AM |
X29.00001: Saturation of Photoluminescence from Carbon Nanotubes at High Laser Intensities: Exciton-Exciton Annihilation near the Mott Density Yoichi Murakami, Junichiro Kono We have carried out a nonlinear photoluminescence excitation (PLE) spectroscopy study of carbon nanotube ensembles using intense, femtosecond, and wavelength-tunable optical pulses. For all PL features we examined, their intensities were seen to saturate at high laser fluence, irrespective of whether the excitation was resonant or non-resonant with the $E_{22}$ transition. As the fluence was increased from the linear regime to the saturation regime, excitation resonances at $E_{22}$ energies gradually broadened and eventually became completely flat at the highest fluence, indicating that the PL intensity became independent of the excitation wavelength. However, the energies and lineshape of PL \textit{emission peaks} did not show any changes throughout the entire range of fluence used. Through absorption spectroscopy at high laser intensities, we also demonstrated that $E_{22}$ \textit{absorption peaks} do not show any shift or broadening even at high laser fluence, indicating that state-filling or scattering is not the cause of the observed ``flattening'' of the excitation spectra. We developed a model to explain these observations by carefully taking into account the spatial overlap of excitons when the average inter-exciton distance approaches the Bohr radius in the exciton-exciton annihilation process. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X29.00002: Curvature Effects on the E$_{33}$ and E$_{44}$ Exciton Transitions in Semiconducting Single-Walled Carbon Nanotubes Stephen Doorn, Erik Haroz, Sergei Bachilo, Bruce Weisman We discuss recent measurements of the E$_{33}$ and E$_{44}$ transitions of small diameter (0.7 to 1.2 nm) single-walled carbon nanotubes using deep blue (415 to 465 nm) resonance Raman spectroscopy and photoluminescence excitation spectroscopy in the UV and blue regions (280 to 480 nm). Individual radial breathing mode features, as well as Raman and photoluminescence excitation maxima, are assigned to specific nanotube chiralities. Transition-dependent trends in RBM intensities are discussed. We present a scaling law analysis of transition energies and show that energies for nanotubes with diameter less than 0.9 nm are not explained by previous scaling law descriptions for larger diameter nanotubes. This new behavior at small diameters is interpreted in terms of both a crossing-over of the E$_{33}$ and E$_{44}$ trend lines for a given 2n+m branch, and a chirality dependence in the many-body exciton effects that becomes significant at high curvatures. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X29.00003: Exciton binding energies in metallic single-walled carbon nanotubes are comparable to those in semiconducting ones Zhendong Wang, Sumit Mazumdar Excitons in metallic single-walled carbon nanotubes (M-SWCNTs) have attracted both theoretical \footnote{J. Deslippe {\it et al.}, Nano. Lett. \textbf{7},1626-1630 (2007).} and experimental \footnote{F. Wang {\it et al.}, unpublished} attention recently. It has been claimed that exciton binding energies in M-SWCNTs are an order of magnitude smaller than those in semiconducting single-walled carbon nanotubes (S-SWCNTs). We have investigated M-SWCNTs within a $\pi$-electron Hamiltonian that has previously reproduced quantitatively the absolute energies as well as the binding energies of both longitudinal \footnote{Z. Wang, H. Zhao and S. Mazumdar, Phys. Rev. B {\bf 74}, 195406 (2006).} and transverse \footnote{Z. Wang, H. Zhao and S. Mazumdar, Phys. Rev. B {\bf 76}, 115431 (2007).} excitons in S-SWCNTs. We are able once again to reproduce quantitatively the available absolute exciton energies and the optical absorption spectra of M-SWCNTs with diameters 0.9 - 1.4 nm. While we need a dielectric constant larger than in the S-SWCNTs, our calculated exciton binding energies in this diameter range are 0.2 - 0.3 eV, only slightly smaller than those in S-SWCNTs with similar diameters. \footnote{Supported by NSF-DMR-0705163.} [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 9:12AM |
X29.00004: First-principles calculation of carrier dynamics in CNTs Invited Speaker: |
Friday, March 14, 2008 9:12AM - 9:24AM |
X29.00005: Experimental evidence for the spin triplet exciton in single wall carbon nanotubes A. Mohite, B. Alphenaar, T. Santos, J. Moodera Optical transitions in single wall carbon nanotubes (SWNTs) are dominated by the formation of excitonic states. Most excitonic states predicted for carbon nanotubes are not observable under normal conditions, including the so-called dark excitonic states and the spin triplet states. Here, we describe a photocurrent measurement technique that allows for the first observation of the spin triplet state. A thin (3 nm) semi-transparent film of Europium Sulphide (EuS) is used as a top contact to CVD grown nanotubes. EuS is a spin filtering tunnel barrier. At low temperatures, the conduction band in the EuS splits, so that the spin-up level lies 0.36 eV below the spin-down level. We monitor the nanotube photocurrent at 4.2K for magnetic fields between 0 and 50 mT. With increasing magnetic field, a low energy photocurrent peak splits off from the singlet exciton peak. No splitting is observed in the free carrier peak, but its position shifts by approximately 7 meV. From this we estimate that the EuS layer produces an effective magnetic field of 7 T. This is too small to produce observable Zeeman splitting, or for the dark exciton to be observed. We instead suggest that the EuS provides spin orbit coupling to mix the singlet and triplet states, making the triplet state optically active. Supported by ONR (No. N00014-06-1-0228) and NSF. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X29.00006: Diffusion limited optical effects in Single Walled Carbon Nanotubes (SWNTs) Aruna Rajan, Michael Strano, Tobias Hertel, Klaus Schulten Recently, it was observed that SWNTs smaller than a few hundred~nm have very low quantum yields. We propose that this is due to thermal diffusion of excitons along the nanotube axis and quenching at the ends. By fitting spectroscopic data to a one-dimensional diffusion model, we extract a diffusion coefficient of $6~$cm$^{2}/$sec for excitons in~($7$,$5$) SWNTs. Assuming a mono-exponential decay of exciton photoluminescence, we also predict that the effective length-dependent photoluminescence lifetimes for these excitons lie in the range of $1$~ps to $27$~ps. Experimental observations are shown to be consistent with stochastic rather than wave packet-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs as optical sensors. Calculation of the form of the diffusion coefficient assuming exciton-phonon coupling as the underlying mechanism will be discussed. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X29.00007: $K$-momentum dark exciton energy in carbon nanotubes O. N. Torrens, J. M. Kikkawa, M. Zheng Phonon sideband optical spectroscopy determines the energy of the dark $K$-momentum exciton for (6,5) carbon nanotubes (CNTs). One-phonon sidebands appear in absorption and emission, split by two zone-boundary ($K$-point) phonons. Their average energy locates the $E_{11} \quad K$-momentum exciton 36 meV above the $E_{11}$ bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the $K$-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of CNTs. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X29.00008: Low Magnetic Field Effects in Single-Walled Carbon Nanotubes D. E. Milkie, O. N. Torrens, J. M. Kikkawa We observe room temperature, sub-Tesla magnetic field effects on the photoluminescence of aqueous suspensions of single-walled carbon nanotubes and on the electrical transport of carbon nanotube composites. The nearly identical field scale found in both cases suggests a common origin for the phenomena. In aqueous suspensions, weak magnetic fields decrease the nanotube photoluminescence intensity by $\sim $0.1{\%} - 1{\%}, an effect which saturates by $\sim $1 Tesla. We explore this magnetic darkening as a function of surfactant, magnetic field and direction, temperature, and nanotube chirality. For carbon nanotube epoxy composites and aerogels, low magnetic fields produce a similar decrease in the device resistivity, which is found to be temperature dependent. We discuss these new magnetic field effects in the context of excitonic magneto physics and magnetic field effects observed in other organic semiconductor systems. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X29.00009: Dark excitons in single-walled carbon nanotubes investigated by electroabsorption spectroscopy Hideo Kishida, Y. Nagasawa, S. Imamura, A. Nakamura We report electroabsorption (EA) spectra in micelle-wrapped single walled carbon nanotubes (SWNT). By applying the high electric field (up to 85kV/cm), the absorption spectra of semiconducting SWNTs show field-induced change in the region of $E_{11}$ and $E_{22}$ transitions. The EA spectra are essentially reproduced by the second derivative curves of the absorption spectra. Such spectral features indicate that the bright (one-photon allowed) exciton and dark (two-photon allowed) exciton for each chiral index are nearly degenerate. The closer scrutiny of the EA spectra reveals that the dark excitons for several chiral indices are located on the higher energy side of the bright states. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X29.00010: High-Field Magneto-Photoluminescence Spectroscopy of Highly-Aligned Carbon Nanotubes J. Shaver, J. Kono, S. A. Crooker, J. A. Fagan, E. K. Hobbie We have investigated excitons in semiconducting single-walled carbon nanotubes (SWNTs) through low-temperature magneto-photoluminescence (PL) of highly-aligned SWNT films in magnetic fields ($\textbf{\textit{B}}$) up to 55~T. The magnetic field was generated using the 60~T long pulse magnet powered by a 1.4 GVA motor-generator at the National High Magnetic Field Lab in Los Alamos, NM. Polyacrylic acid films containing DNA suspended CoMoCAT SWNTs were stretch-aligned, and the alignment factor was analyzed by polarized Raman spectroscopy$^1$. Utilizing two well-defined measurement geometries, SWNTs~$\parallel$~$\textbf{\textit{B}}$ and SWNTs~$\perp$~$\textbf{\textit{B}}$, we provide unambiguous evidence that the PL from excitons in SWNTs is sensitive only to the $\textbf{\textit{B}}$-component parallel to the tube axis. We developed a theoretical model of one-dimensional magneto-excitons, based on recently-proposed exchange-split bright and dark exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, which successfully reproduces our observations$^2$. \newline 1. Fagan \textit{et al.} Phys. Rev. Lett. \textbf{98}, 147402 (2007) \newline 2. Shaver \textit{et al.} Nano Lett. \textbf{7}, 1851 (2007) [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X29.00011: Time-Domain Ab Initio Studies of Excitation Dynamics in Carbon Nanotubes Bradley Habenicht, Oleg Prezhdo We have developed state-of-the-art non-adiabatic molecular dynamics techniques and implemented them within time-dependent density functional theory in order to model the ultrafast excitation processes in CNTs at the atomistic level and in real time. Our ab initio studies of CNTs directly mimic the experimental data and reveal many intriguing features of the excitation dynamics, including non-radiative fluorescence quenching, fast intrinsic intraband relaxation, phonon-induced component of fluorescence linewidths, the importance of defects, the dependence of the relaxation rate on the excitation energy and intensity, and a detailed understanding of the role of active phonon modes. \textbf{1}. C. F. Craig, W. R. Duncan, O. V. Prezhdo ``Trajectory surface hopping in the time-dependent Kohn-Sham theory for electron-nuclear dynamics'', \textit{Phys. Rev. Lett}.,\textbf{ 95} 163001 (2005) \textbf{2}. B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Electron and hole relaxation dynamics in a semiconducting carbon nanotube'', \textit{Phys. Rev. Lett.} \textbf{96} 187401 (2006) \textbf{3}. B. F. Habenicht, H. Kamisaka, K. Yamashita and O. V. Prezhdo, ``Ultrafast vibrationally-induced dephasing of electronic excitations in semiconducting carbon nanotubes, \textit{Nano Lett.,} submitted \textbf{4}. B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Quenching of fluorescence in a semiconducting carbon nanotube: time-domain ab initio study'', \textit{Phys. Rev. Lett.}, submitted. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X29.00012: Free Carrier Auger Relaxation of Excitons in Carbon Nanotubes Jesse Kinder, Eugene Mele We study a relaxation pathway in doped carbon nanotubes in which optically pumped excitonic states interact with free electrons or holes through an Auger process. The exciton recombines nonradiatively, transferring its energy and momentum to a free carrier. The calculated decay rate depends on temperature, exciton density, and the amount of doping. For optically excited bright excitons, conservation of energy and momentum forbid this decay below a critical doping density. For the heavier dark excitons, recombination is allowed for any nonzero doping density. By studying the phase space for this Auger process and exciton-exciton annihilation, we find that the free carrier interaction can dominate the relaxation rate at low exciton densities. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X29.00013: Magnetic Brightening of Dark Excitons in Individual Single-Walled Carbon Nanotubes Ajit Srivastava, Junichiro Kono, Han Htoon, Victor I. Klimov We have performed micro-photoluminescence (PL) studies on \textit{individual} single-walled carbon nanotubes (SWNTs) at varying temperatures ($T$ = 4 K -- 100 K) in magnetic fields ($B)$ up to 5 T, which provide direct evidence for the existence of dark excitons in SWNTs. Only when the $B$ was parallel to the tube axis, we observed the appearance of a \textit{secondary peak at a lower energy} with respect to the main emission peak. The secondary peak increased in intensity with increasing $B$ at the expense of the main peak. At the lowest $T$, a complete reversal of emission intensity from the main peak to the side peak was seen as the $B$ was increased. However, the main peak was recovered as the $T$ was increased at a fixed $B$. These behaviors can be explained by assigning the main and secondary peaks to the lowest-energy bright and dark singlet exciton states, respectively. The absence of these behaviors in $B$ perpendicular to the tube axis convincingly suggests that brightening is induced by the Aharonov-Bohm phase. The zero-field dark-bright splitting is found to be $\sim $1-2 meV, which is lower than most theoretical predictions. [Preview Abstract] |
Session X30: Composite and Porous Media
Sponsoring Units: DCMPChair: Stefano Curtarolo, Duke University
Room: Morial Convention Center 222
Friday, March 14, 2008 8:00AM - 8:12AM |
X30.00001: Modeling of multiscale porous media B. Biswal, P.-E. Oren, R.J. Held, S. Bakke, R. Hilfer A continuum geometrical model for reconstructing three dimensional pore scale microstructure of multiscale porous media is presented. Pore scale geometries of different carbonate textures are successfully reconstructed with informations from thin sections. The reconstructed models incorporate correlations with the primordial depositional textures, scale dependent intergranular porosity over many decades, vuggy porosity, a percolating pore space, a fully connected matrix space, strong resolution dependence and wide variability in the permeabilities and other properties. A method to generate synthetic micro-CT images at arbitrary resolutions is also developed. The model can be extended to a wide class of multiscale porous media. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X30.00002: Direct observation of pore blocking and advanced adsorption in nanoporous alumina: cooperative effects at the origin of hysteresis Felix Casanova, Casey E. Chiang, Chang-Peng Li, Ivan K. Schuller We tailor anodized alumina with independent pores with well-defined, simple geometries (inkbottle, funnel), in order to study the effects of pore morphology in hysteretical capillary condensation, independently from other cooperative processes such as network effects. We confirm, by direct observation using optical interferometry, the occurrence of two cooperative phenomena: the classical pore blocking effect in nearly ideal `inkbottle' pores (which has usually been employed to describe hysteresis loops in disordered/interconnected porous materials) and the advanced adsorption in pores with a change in the cross section. Both effects have been predicted in theoretical and simulation works, but not directly observed experimentally before. They are relevant for the development of a theory of the poorly understood hysteresis in complex porous materials. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X30.00003: Acoustic Band Pass, Band Gap and Dispersion in Discrete Media at Micro and Nano Scales Hasson Tavossi Acoustic properties of models of crystals, when measured at macroscopic scale, are found experimentally to have remarkable similarities with the same wave properties observed at atomic and nano-scales. It can be shown that, elastic moduli and other wave properties such as; band-pass, forbidden band, wave tunneling, attenuation, cutoff-frequency, and dispersion, depend on the similar structural factors as for phonons in crystals. Acoustic properties of the macroscopic models of discrete media, in the length scale range; 1.5 mm to 30 micrometers, and the frequency range; audible to ultrasonic are studied. The Band-pass, band-gap, attenuation, and dispersion expressed in wave-number (ka), show similar characteristics as the phonons in solids. These findings can lead to a better understanding of the wave properties of solids at nanoscales. The readily analyzable wave models at large scale are convenient tools to verify experimentally the models for complex binary composites. Experimental findings and numerical results for wave properties of discrete structures at large scale are compared with atomic scale wave behavior of solids, for a wide range of frequencies, from audible to ultrasound, to show the common characteristics with the phonons behavior in solids. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X30.00004: Conducting-tip AFM Studies of Multi-Walled Carbon Nanotube/Polyimide Nanocomposites A. Trionfi, D. Scrymgeour, J.W.P. Hsu, M.J. Arlen, D. Wang, L.-S. Tan, R.A. Vaia Electrical transport studies of multi-walled carbon nanotube (c-MWNT)/polymer nanocomposites have shown metallic behavior with conductivity $\sigma =\sigma _0 \left( {\phi -\phi _c} \right)^t$ above the percolation threshold. The conductivity depends on three aspects of the conducting network (CN): the conductivity of the constituent c-MWNT, the number of c-MWNT making up the CN, and the detailed interconnectivity of the CN. Using conducting-tip atomic force microscopy (C-AFM), we have studied the density and conductivity of the c-MWNT CN as a function of c-MWNT loading between 0.5 - 5.0 wt {\%} in a polyimide matrix. Using the Principle of Delesse, the volume fraction of the c-MWNT CN can be calculated from the conducting areal density measured in the C-AFM scans. The results of the C-AFM tests have shown localized areas of electrical transport associated with c-MWNT as well a clear dependence of conducting areal density and conductivity on the c-MWNT loading. This work was performed in part at the US Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos and Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X30.00005: Systematic Study of Microwave Absorption, Heating, and Microstructure Evolution of Porous Copper Powder Metal Compacts Darin Zimmerman, John Diehl, Earnie Johnson, Kelly Martin, Nicholas Miskovsky, Charles Smith, Gary Weisel, Brock Weiss, JunKun Ma We present a systematic study\footnote{ J. Ma, J. F. Diehl, E. J. Johnson, K. R. Martin, N. M. Miskovsky, C. T. Smith, G. J. Weisel, B. L. Weiss, and D. T. Zimmerman, J. Appl. Phys. \textbf{101}, 074906 (2007)} of the absorption, heating behavior, and microstructure evolution of porous copper powder metal powder compacts subjected to 2.45 GHz microwave radiation and explain our observations using known physical mechanisms. Using a single mode microwave system, we place the compacts in pure electric (E) or magnetic (H) fields and compare the heating trends. The observed trends in the E- and H-field heating reflect the dramatic changes in the conductivity, permittivity, and permeability of the samples caused by the microstructure evolution during heating in the two types of fields. The observed dependence of the initial microwave heating of the samples suggests that the microwave absorption in the sample is dominated by the properties of the individual metal particles composing the sample. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X30.00006: Do pores strengthen materials? -- A pore size effect analysis Catalin Picu, Mohan Nuggehally, Mark Shephard Pores with radius larger than several microns are known to reduce the yield and flow stress of ductile materials and to increase their toughness. In this work we discuss a new mechanism leading to an increase of the strength of a material by nanosized pores. We show that voids grow by the emission of dislocations. As the void radius is reduced, while their volume fraction is kept constant, the mean spacing between voids decreases and their number increases. This makes the concurrent dislocation nucleation from neighboring voids more difficult. The situation is equivalent to increasing the density of dislocation sources in the material. Furthermore, we show that the critical stress for dislocation nucleation form an isolated void also increases as the pore size is reduced. The analysis is performed using a computationally efficient adaptive atomistic-continuum method. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X30.00007: First-Principles Calculations for the Dislocations in Titanium Nitride Ramkumar Gudipati, Y.G. Shen, Wentao Xu, A.S. Rao, H.L. Dang, Sanwu Wang Nanostructured$^{ }$superhard materials have been successfully synthesized in recent years. The hardness of nanosuperlattices$^{ }$and nanocomposites significantly exceeds that of the component materials. While it is believed that the nanodimensions are needed to impede dislocation activity and grain-boundary sliding, relevant calculations are rare. We report first-principles density-functional calculations for the core structures and energetics of various dislocations including the [110]{\{}110{\}}, [110]{\{}111{\}}, [110]{\{}100{\}}, [100]{\{}100{\}} and [100]{\{}110{\}} edge dislocations in bulk TiN. We found that the formation energies of the core dislocations were continuously increased when their sizes decreased. We also found that the most common types of dislocations in TiN are the [110]{\{}110{\}} and [110]{\{}111{\}} edge dislocations. The obtained results are helpful for elucidating the atomic-scale mechanism for the superhardness of nanocomposites. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X30.00008: Interaction of RDX Explosive Molecules with Metal-Organic Khorgolkhuu Odbadrakh, James Lewis We have studied interactions of cyclotrimethylene trinitramine (RDX), a highly energetic explosive's molecule, with metal-organic framework of composition Zn$_4$O (1,4-benzenedicarboxylate)$_3$ (MOF-5), within {\it ab initio} density functional theory method. The structures were optimized with the Fireball atomic orbital basis sets to a good agreement with experimental values. Optimal surface geometries have been obtained for MOF-5-RDX system and first principles estimates of the binding energies, charge transfer, and activation barriers are given. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X30.00009: Control of Microcellular Structure in Polymeric Foams via Nanofiller Size and Surface Chemistry Kerem Goren, Linda Schadler, Rahmi Ozisik Polymeric foams are used in many applications, where thermal insulation, selective sound inhibition or low density materials are needed. They are generally produced by blending polymer with a chemical blowing agent, which releases inert gas at processing temperatures. There are two major drawbacks to this process: chemical residues that form during the decomposition of blowing agent and formation of large (macroscopic) pores that weaken the material. To overcome these drawbacks, supercritical carbon dioxide (scCO2) is used. Use of scCO2 along with nanofillers provide heterogeneous nucleation, and present the opportunity to control pore size and pore distribution. In the current study, the effect of silica size and silica-scCO2 interaction on microcellular structure was investigated. Two different silica fillers were synthesized: 15 and 150 nm. These silica nanofillers were surface modified with tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilanes. Dynamic Light Scattering, FT-IR, TGA, and SEM were used to characterize the samples. Results indicate that there is a strong correlation between surface chemistry, and hence scCO2 and filler interaction on pore size and size distribution. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X30.00010: Diffusion in an array of cavities in two and three dimensions Mykyta V. Chubynsky, Francis Torres, Gary W. Slater We consider diffusion of point-like particles in arrays of cavities separated by infinitely thin walls with holes. We show that in the small-hole limit, the effective diffusion coefficient $D$ is related to the electrical capacitance of a plate of the same shape as the hole. Applying this result to elongated holes in 3D, we find an interesting logarithmic dependence of $D$ on the hole width. A logarithmic dependence is also obtained in 2D. As a consequence, the diffusion rate reaches 10\% of the free diffusion rate when the hole widths are only about $10^{-6}$ of the cavity size. The theoretical predictions are validated using a numerically exact computational method. In the opposite limit, when holes span the cavity boundaries nearly completely and only small pieces of walls remain, analytical calculations are also possible. The result in this case is interesting, since a finite reduction in the diffusion coefficient is obtained even for infinitely thin walls, contrary to predictions of various effective-medium theories that this reduction should be proportional to the excluded volume fraction. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X30.00011: First principles modeling of structure and properties of multi-component amorphous steels Viatcheslav Kazimirov, Despina Louca, Michael Widom Amorphous steels (iron based metallic glasses) exhibit unique physical properties that have opened the venue to new commercial applications as well as renewed the interest in this field. To provide a realistic description of the three dimensional structures and associate the coordinated environment of atoms to physical properties, first principles quantum mechanical molecular dynamics (MD) simulations along with the pair density function (PDF) analysis of neutron and X-ray diffraction data were used by way of considering chemical effects, ionic size ratio and concentration. Direct comparison of the simulated atomic structures obtained from MD with the local atomic structures determined experimentally show a very good agreement between the two, indicating that this theoretical approach can be applied towards simulating multi-component alloys. The atomic coordinates were used to develop the building blocks of cluster-like structures that give rise to the short-range order. The diffusion rates of different atom species were modeled at several temperatures that allowed us to describe the quenching process. In addition, the bulk moduli as a function of chemical composition were simulated and showed a very good agreement with the ones obtained experimentally. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X30.00012: Structural, dynamic and electronic properties of amorphous Al$_2$O$_3$: ab--initio molecular dynamics calculations Gonzalo Gutierrez, Sergio Davis First principles molecular dynamics (MD) calculations of amorphous Al$_2$O$_3$ in a system consisting of a supercell of 80 atoms is reported. A detailed analysis of the interatomic correlations allows us to conclude that the short range order is mainly composed by AlO$_4$ tetrahedra, but in contrast to classical MD results, also an important number of AlO$_5$ units are present. The vibrational density of state, calculated by means of the velocity autocorrelation function, present two main bands, a low frequency one related to the inter-tetrahedron vibration and a high frequency band related to the intra-tetrahedron vibration. By means of a geometry relaxation we obtain a fully relaxed system, and calculated its elastic properties. The reported bulk modulus is 193.4 GPa, the smallest among the several phases of alumina. The electronic properties were characterized by means of both the total and partial electronic density of states as well as by means of the electron localization function. The system present a rather small gap of 2.4 eV. The consequences of these results will be discussed. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X30.00013: Disorder Induced Transition into a One-Dimensional Wigner Glass Shimul Akhanjee, Joseph Rudnick The destruction of quasi-long range crystalline order as a consequence of strong disorder effects is shown to accompany the strict localization of all classical plasma modes of one- dimensional Wigner crystals at $T=0$. We construct a phase diagram that relates the structural phase properties of Wigner crystals to a plasmon delocalization transition recently reported. Deep inside the strictly localized phase of the strong disorder regime, we observe ``glass-like'' behavior. However, well into the critical phase with a plasmon mobility edge, the system retains its crystalline composition. We predict that a transition between the two phases occurs at a critical value of the relative disorder strength. This transition has an experimental signature in the AC conductivity as a local maximum of the largest spectral amplitude as function of the relative disorder strength. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X30.00014: Structural and electronic properties of amorphous silicon carbide: A first principles and experimentally constrained molecular relaxation approach Parthapratim Biswas, Raymond Atta-Fynn We present first-principles modeling of amorphous silicon carbide within a localized basis density functional formalism to study the electronic, vibrational and structural properties for system containing 1000 atoms. Our work for Si$_{.5}$-C$_{.5}$ system shows that the short range chemical order is dominated by heteronuclear Si-C bonds with coordination defect with a degree of chemical disorder. We calculate the electronic density of states that shows a presence of clean optical gap in the spectrum and study the localization nature of the electronic band tail states and vibrational eigenstates. We compare our results with existing models and experimental data available in the literature. Finally, we presents some preliminary results for models obtained by experimentally constrained molecular relaxation technique [1,2] that directly uses experimental data in conjunction with a classical force-field. \newline [1] P. Biswas, D.N. Tafen and D.A. Drabold, Phys Rev B71, 54204 \newline [2] P. Biswas, R. Atta-Fynn, S. Chakraborty, and D.A.Drabold, J.Phys.:Condens. Matter 19 (2007) 455202 [Preview Abstract] |
Session X31: Focus Session: Ruthenates
Sponsoring Units: DMP GMAGChair: Igor Mazin, Naval Research Laboratory
Room: Morial Convention Center 223
Friday, March 14, 2008 8:00AM - 8:36AM |
X31.00001: Metallic Spin Liquid Behavior and Unconventional Anomalous Hall Transport of the Geometrically Frustrated Kondo Lattice Pr$_2$Ir$_2$O$_7$ Invited Speaker: Among metallic magnets on geometrical frustrated lattices, the pyrochlore oxide Pr$_{2}$Ir$_{2}$O$_{7}$ is unique for its metallic spin liquid behavior \footnote{S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. v. Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, \textit{Phys. Rev. Lett.} \textbf{96}, 087204 (2006).}, and unconventional Hall transport phenomena \footnote{Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara, and S. Onoda, \textit{Phys. Rev. Lett.} \textbf{98}, 057203 (2007).}. Despite the Weiss temperature $T^* = 20$ K deu to the RKKY interaction, Pr$_{2}$Ir$_{2}$O$_{7}$ exhibits no magnetic long range order, but spin freezing at a very low temperature $\sim 120$ mK. Instead, the Kondo effect, including ln$T$ dependence in the resistivity, emerges and leads to partial screening of the $4f$-moments below $T^*$. Moreover, the underscreened 4f- moments show spin-liquid behavior below a renormalized energy scale of $\theta_{\rm{w}}\sim 1.7$ K. Interestingly, in this spin-liquidlike paramagnetic regime, the Hall resistivity $\rho_ {xy}$ becomes largely enhanced, and shows behavior far different from anomalous Hall effects (AHE) due to the spin- orbit coupling observed in ordinary magnetic conductors. We discuss the origin of the metallic spin liquid behavior and unconventional AHE in terms of the spin chirality due to the non-coplanar texture of the $<111>$ Ising-like Pr moments. This work is based on the collaboration with Y. Machida, T. Tayama, T. Sakakibara (ISSP, Univ.of Tokyo), Y. Maeno (Kyoto Univ.), S. Onoda (RIKEN, Tokyo), C. Broholm (Johns Hopkins Univ.), C. Stock and J. van Duijn (ISIS), L. Balicas (NHMFL), Jung Young Cho, and Julia Y. Chan (Louisiana State Univ.). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X31.00002: Magnetic phase diagram of (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7 }$ (0$\le $x$\le $1.0) Z.Q. Mao, Z. Qu, L. Spinu, J. Peng, T.J. Liu, D. Fobes, V. Dobrosavljevic, H.Q. Yuan, W. Bao, J.W. Lynn The layered ruthenates (Sr,Ca)$_{n+1}$Ru$_{n}$O$_{3n+1}$ have received widespread attention in recent years since they display a remarkable range of unique superconducting and magnetic properties, such as spin triplet superconductivity and metamagnetic quantum criticality. We have recently studied properties of the double layered (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$ solid solution series and established the magnetic phase diagram of this system using the high-quality single crystals grown by floating-zone method. We have observed rich magnetic ground states in this system: (I) Itinerant metamagnetic state (0.08$>x\ge $0); (II) enhanced PM state accompanied by non-Fermi liquid behaviors near $x\sim $0.08; (III) ferromagnetic cluster glass (CG) phase featuring an extremely large Wilson ratio ($\sim $120) and enhanced electron correlation (0.40$>x>$0.08); (IV) long-range AFM state (1$\ge x\ge $0.40). Furthermore, we found a new phase which shows an unconventional anomalous Hall effect in a low temperature range immediately above the CG phase. The magnetization of this phase is found to follow a new scaling expression derived by phenomenologically extending the quantum Griffiths phase model. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X31.00003: ARPES Investigations of the Electronic Structure Evolution of the Doped Ruthenate (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$ William Dunkel, Felix Baumberger, Milan Allan, Worawat Meevasana, Zhiqiang Mao, Zhi-Xun Shen The bilayer ruthenate system (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$ can be grown in the full range of dopings with the end members being an antiferromagnetic ``bad metal'' (x=1) and a paramagnetic Fermi liquid displaying a metamagnetic transition under applied field (x=0). Recent transport and magnetization measurements have shown that the system displays disorder-induced unconventional quantum critical behavior near the doping level x$\sim $0.3. We present ARPES data for a range of dopings to investigate the associated electronic structure evolution. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X31.00004: Study of the Layered Perovskite $Sr_3Ru_2O_7$ by STS and ARPES. Milan Allan, J. Lee, M. Wang, A. Schmidt, F. Baumberger, A. Tamai, J. Farrell, J.C. Davis, A. Mackenzie The Ruddlesden-Popper series, $Sr_{n+1}Ru_nO_{3n+1}$ exhibits a variety of electronic phases, including triplet superconductivity (n=1) or metamagnetism (n=2). Here we report studies of the electronic structure of the bilayered perovskite $Sr_3Ru_2O_7$ (n=2) by means of scanning tunneling spectroscopy. Special attention is given to the influence of titanium impurity atoms on the local electronic structure. A comparison with angle resolved photoemission experiments on the same set of samples is drawn. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X31.00005: Quasi-3D ordered lattice modulations in a bilayer ruthenate with no long-range order Zahirul Islam, Zhe Qu, Yejun Feng, Jonathan Lang, Jin Peng, Zhiqiang Mao Bulk measurements reveal disorder-induced unconventional quantum critical behaviors in (Sr$_{1-x}$Ca$_x$)$_3$Ru$_2$O$_7$ (SCRO) compounds, in particular, near $x = 0.3$. Here we report X-ray scattering studies on SCRO with $x =0.3$, as well as those for the end members. We find that at $x = 0.3$ robust 2-unit-cell periodic lattice modulations exist that are characterized by $(\frac12,0,0)$ and $(0,\frac12,0)$, respectively, even at room temperature. These modulations are transversely polarized and quasi-3D ordered in that they are fully coherent in the basal plane with {\bf c} axis correlations at least one unit cell in extent. These modulations are due to correlated displacements of the O atoms. The displacement pattern is consistent with $t_{2g}$-modes of distortion of RuO$_6$ octahedra, signifying the presence of lattice and orbital correlations, although no long-range magnetic or orbital order is present. These modulations are absent in the end members. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X31.00006: Doping-Induced Structural and Physical Properties Changes in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ Single Crystals Biao Hu, Manuel Angst, Ovidiu Garlea, V.B. Nascimento, David Mandrus, E.W. Plummer, R. Jin In the homologous strontium ruthenate series Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$, double-layered Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits unique physical properties. However, the partial substitution of Ru by the smaller Mn was found to change its ground state from a paramagnetic metal to an antiferromagnetic insulator with less distorted crystal structure. Interestingly, our Hall effect measurements show no change in the carrier concentration (x $\le $ 0.2), suggesting that Mn is isovalent with Ru in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$. This is further supported by the magnetization data, which yield effective spin S = 3/2 for Mn corresponding to Mn$^{4+}$. In addition to bulk physical properties, the doping dependence of the surface structure and lattice dynamics of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ single crystals will be discussed. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X31.00007: Relation between structure and magnetic properties in (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$ J. Peng, Z. Qu, L. Spinu, T.J. Liu, D. Fobes, Z.Q. Mao The (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$ solid solution series exhibits rich magnetic ground states, ranging from itinerant metamagnetism, ferromagnetic instability, ferromagnetic cluster glass, to antiferromagnet.$\left[1\right]$ In this talk, we report a study on the relation between structure and magnetic properties in (Sr$_{1-x}$Ca$_{x})_{3}$Ru$_{2}$O$_{7}$. Our Rietveld refinement of x-ray scattering data shows that the rotation angle of RuO$_{6}$ octahedra increases with increasing $x$, and approaches saturation for $x>$0.2. The orthorhombicity, which is caused by the tilting of RuO$_{6}$ octahedra, occurs for $x>$0.4 and enhances significantly for $x>$0.7. By comparing these structural characteristics with the magnetic properties, we conclude that the ferromagnetic instability originates from the RuO$_{6}$ octahedra rotation, and the tilting of RuO$_{6}$ octahedra causes a significant magnetic anisotropy.\\ $\left[1\right]$ Z. Qu \textit{et al.} arXiv:0708.1291 (2007). [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X31.00008: QPI in $Sr_3Ru_2O_7$. Jinho Lee, M. Wang, M. Allan, A. Schmidt, F. Baumberger, A. Tamai, J. Farrell, J. C. Davis, A. Mackenzie $Sr_{n+1}Ru_nO_{3n+1}$ family drew substantial attention recently due to its plethora of electronic phases like triplet superconductivity or metamagnetism. $Sr_3Ru_2O_7$ (n=2) is very similar to the triplet superconductor $Sr_2RuO_4$ in 2D conductivity as well as in structure, but shows no superconductivity. Understanding of the underlying band structure is the first step to fathom this strongly correlated oxide. STM has proven to be a high precision tool to measure band dispersions in momentum space along with the atomically resolved real space spectroscopic properties. Here we report the first atomically resolved 2D spectroscopic maps in $Sr_3Ru_{2(1-x)}Ti_{2x}O_7$ where QPI patterns are observed and discuss the band landscape responsible for the QPI in this energy range. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X31.00009: Orbital Hierarchy Inversion and Magnetic Transition in Mn Doped Sr$_3$Ru$_2$O$_7$ Muhammed Hossain, M.W. Haverkort, Z.H. Hu, T. Burnus, C.F. Chang, S. Klein, J.D. Denlinger, H.-J. Lin, C.T. Chen, R. Mathieu, Y. Kaneko, Y. Tokura, S. Satow, Y. Yoshida, H. Takagi, A. Tanaka, I.S. Elfimov, G.A. Sawatzky, L.H. Tjeng, A. Damascelli Ruthenium oxides are a very important class of compounds that are extremely sensitive to impurities. Here we present a study of the Mn doped Sr$_3$Ru$_2$O$_7$ by X-ray Absorption Spectroscopy (XAS) and a combination of density functional theory (LSDA) and cluster calculations. We find that delocalized RuO matrix forces the Mn impurities to behave as a negative charge transfer system and donate a hole. As a result Mn impurities behave as Mn3+ acceptors. Our XAS data clearly shows that the occupied $e_g$ orbitals have an in-plane alignment: a very surprising result in a tetragonally distorted system elongated along the $c$-axis. LSDA calculations reveal very strong anisotropy in oxygen bandwidth leading to a reversal of the $e_g$ crystal field hierarchy. This work further establishes that the Mn atoms are sitting in a bath of a uniaxial exchange field that gradually goes out-of-plane to in-plane as the Mn doping level is increased. Overall, the behavior has some similarities with Mn doped GaAs and may be relevant to the physics of dilute magnetic semiconductors. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X31.00010: Temperature dependence of a zero bias anomaly in scanning tunnelling spectra of Sr(4)Ru(3)O(10) Bernhard Nansseu, Tatjana Novgorodov, Michael Waelsch, J\"urgen Hager, Jiandi Zhang, R. Moore, Ward Plummer, Zhiqiang Mao, Rene Matzdorf We have studied a zero bias anomaly in scanning tunnelling spectra of layered ruthenate Sr(4)Ru(3)O(10). This material shows a dip-like feature in the dI/dV spectra, which has previously observed in the single-layer Sr(2)RuO(4) and double- layer Sr(3)Ru(2)O(7) ruthenates. We have studied in particular the temperature dependence of the zero bias anomaly, which is in all three materials different. The triple-layer material shows intergrowth of single and double layers, which have been identified by their spectroscopic fingerprint. Finally, we discuss different effects as possible explanations for the zero bias anomaly. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X31.00011: Anisotropy of magnetoresistivity in trilayered ruthenate Sr$_{4}$Ru$_{3}$O$_{10}$: Evidence for orbital dependent metamagnetism D. Fobes, M. Zhou, T.J. Liu, Z. Qu, H.Q. Yuan, M. Salamon, Z.Q. Mao Trilayered ruthenate Sr$_{4}$Ru$_{3}$O$_{10}$ is ferromagnetic with T$_{c}$ $\approx $ 100K and moderate in-plane magnetic field (B$_{c }\approx $ 2T) induces an itinerant metamagnetic transition [1,2]. Such metamagnetism within a FM ground state cannot be understood with a single band model of a field-tuned Stoner transition. We have measured azimuthal angular dependence of in-plane and out-of-plane magnetoresistivity ($\rho _{ab}(\phi $,B) and $\rho _{c}(\phi $,B)). We found that for B$<$B$_{c}$, at fixed field $\rho _{ab}(\phi )$ exhibits a two-fold anisotropy attributable to anisotropic behavior of an Ising ferromagnet, while $\rho _{c}(\phi )$ does not show such a behavior. For B$>$B$_{c}$, both $\rho _{ab}(\phi )$ and $\rho _{c}(\phi )$ exhibit a four-fold symmetry anisotropy, but the minimum value in $\rho _{ab}(\phi )$ and $\rho _{c}(\phi )$ occurs along different directions, i.e., [100] and [010] for $\rho _{ab}$, and [110] and [1$\overline{1}$0] for $\rho _{c}$. Such a difference in anisotropic behavior between $\rho _{ab}(\phi )$ and $\rho _{c}(\phi )$ provides strong support that metamagnetism in Sr$_{4}$Ru$_{3}$O$_{10}$ is orbital dependent, i.e., ferromagnetic and metamagnetic bands coexist. \\ $\left[1\right]$ Cao et al., Phys. Rev. B 68, 174409 (2003)\\ $\left[2\right]$ Mao et al., Phys. Rev. Lett. 96, 077205 (2006)\\ Work at Tulane is supported by NSF, DOE and the Research Corporation. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X31.00012: Fermi Surface mapping of Sr$_4$Ru$_3$O$_{10}$ using Angle Resolved Photoemission R.S. Singh, F. Wang, J.W. Allen, J.D. Denlinger, X.N. Lin, Gang Cao Layered strontium ruthenates in the Ruddlesden-Popper series (Sr$_{n+1}$Ru$_n$O$_{3n+1}$) are scientifically interesting and technologically promising materials exhibiting various novel electronic and magnetic properties. We will show the effect of dimensionality on the electronic structure as one goes from two dimensional to three dimensional crystal structures by increasing $n$. In particular, we will present new results and analysis of photon energy dependent angle resolved photoemission spectroscopy measurements on Sr$_4$Ru$_3$O$_{10}$ and make detailed comparison of Fermi surface maps with recently measured Shubnikov-de Haas frequencies and available LDA band structure calculations corresponding to various layered strontium ruthenates. \vspace{.5cm} \\ Work supported by U.S. DOE (DE-AC03-76SF00098 at ALS, DE-FG02-07ER46379 at UM currently) and U.S. NSF (DMR-03-02825 at UM initially, DMR-02-40813 at UK). [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X31.00013: Mid-infrared Hall conductivity in Ca$_x$Sr$_{1-x}$RuO$_3$ films M.-H. Kim, G. Acbas, M.-H. Yang, V. Kurz, M. Eginligil, J. Cerne, P. Khalifah, I. Ohkubo, H. Christen, D. Mandrus, Z. Fang We have studied the complex longitudinal ($\sigma_{xx}$) and transverse (Hall, $\sigma_{xy}$) conductivities in a series of Ca$_x$Sr$_{1-x}$RuO$_3$ films by measuring the magnetic field induced polarization changes of mid-infrared (100 - 1000 meV) radiation in the 10~K-300~K temperature range. The infrared $\sigma_{xy}$ shows strong energy dependence and sign changes. We compare the complex magneto-conductivities $\sigma_{xx}$ and $\sigma_{xy}$ [M.-H. Kim, PRB 2007] with dc measurements and with calculations [Z. Fang, Science 2003] of the intrinsic anomalous Hall effect. To test the accuracy of these measurements and to remove the background magneto-optical response of windows and substrates, the magnetic field induced changes in the polarization of transmitted light also were measured in GaAs, ZnSe, and BaF$_2$. This work was supported by the Research Corp. Cottrell Scholar Award (UB), NSF-CAREER-DMR0449899 (UB), and an instrumentation award from the CAS(UB). Oak Ridge Nat. Lab.is managed by UT- Battelle, LLC, for the U.S. DOE (contract DE-ACO5-00OR22725). [Preview Abstract] |
Session X32: Focus Session: Magnetic Sensors
Sponsoring Units: GMAGChair: Thomas Crawford, University of South Carolina
Room: Morial Convention Center 225
Friday, March 14, 2008 8:00AM - 8:12AM |
X32.00001: Challenges for picoTesla Magnetic-Tunnel-Junction Sensors William Egelhoff, Philip Pong, Robert McMichael, Edmund Nowak, Alan Edelstein, James Burnett, Greg Fisher The extension of small, inexpensive, low-power, low frequency, ultra-sensitive magnetic sensors to fields between 1 nanoTesla and 1 picoTesla, an area currently dominated by fluxgates, optically-pumped magnetometers, and SQUIDS, would be a paradigm shift for the field of magnetic sensors. The necessary elements for picoTesla MTJ sensors have been identified by modeling the noise characteristics. The results help identify the experimental challenges that exist in the integration of the necessary components of the sensor and illustrate the trade-offs that must be considered. For example, values of the TMR above 100{\%} contribute very little, while lowering the saturation field of the free layer below 10 Oe is essential. These and other insights identify the critical issues than need attention and can guide research into productive directions. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X32.00002: X-Ray Diffraction Studies of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions Pinshane Huang, John Read, Robert Buhrman MgO-based magnetic tunnel junctions (MTJs) show high levels of tunneling magnetoresistance (TMR),\footnote{ Y. Lee, et al., Appl. Phys. Lett 90, 212507 (2007).} a very desirable trait for magnetic random access memory and hard drive read heads. Because theory links tunnel junction crystallinity with high TMR, studies of MTJ crystal structure have implications for the development of high-performance MTJs\footnote{ J. Mathon and A. Umerski, Phys. Rev. B. 63, 220403 (2001).}$^{,}$\footnote{ W. H. Butler, et al., Phys. Rev. B. 63, 054416 (2001).}. Varying the anneal temperature, MgO growth methods, and seed layers, we have examined texturing in CoFeB/MgO/CoFeB tunnel junctions using x-ray diffraction. We investigated MTJs with rf-sputtered or electron-beam evaporated MgO barrier layers, and we used TaN, Ta/Ru, and Ta/CuN as alternative seed layers for the MTJ growth. We report on changes in both tunnel barrier and electrode crystallization as a function of anneal temperature, which we find to be dependent on both the MgO deposition method and the seed layer composition. Our complete MTJ structures using rf-sputtered MgO barriers achieve TMR in excess of 200{\%}, and correlation with these XRD results sheds light on the complex dependence of MTJ performance on growth and processing conditions. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X32.00003: Chemical and electronic studies of CoFeB / MgO / CoFeB magnetic tunnel junctions J. Read, J. Cha, P. Huang, W. Egelhoff, D. Muller, R. Buhrman MgO based magnetic tunnel junctions (MTJs), particularly the CoFeB/MgO/CoFeB system, exhibit large tunneling magnetoresistance (TMR) which makes them viable for MRAM [1] and sensor applications. Careful engineering of the MgO tunnel barriers, CoFeB electrodes, and their interfaces is essential for optimizing device performance [2,3], which motivates investigation of the chemical and electronic properties of high quality MTJs. We correlate scanning tunneling (STS), x-ray photoelectron (XPS) [4], and electron energy loss (EELS) [5] spectroscopies with current-in-plane tunneling (CIPT) measurements to gain insight on the electronic structure and chemistry of MgO MTJ structures. The measurements reveal that quite high TMR ($>$200{\%}) can be obtained when there is substantial boron in the tunnel barrier, showing that proper doping of the MgO layer plays a significant role in the performance of such MTJs. We will discuss the impact of materials properties upon transport measurements and provide suggestions for greater control over MTJ device characteristics. [1] Parkin, Nat. Mater. 3, 862 (2004). [2] Nagamine, APL 89, 162507 (2006). [3] Lee, APL 90, 212507 (2007). [4] Read, APL 90, 132503 (2007). [5] Cha, APL 91, 062516 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X32.00004: FMR spectroscopy with very large precession cone angle in magnetic tunnel junctions Takahiro Moriyama, Xin Fan, John Q. Xiao Unlike the small angle precession with low power microwave excitation, which has been extensively studied using conventional ferromagnetic resonance (FMR) techniques, the large angle precession involves nonlinear response of the magnetizations. Large angle ferromagnetic resonance (LA-FMR) measurements can help to understand the magnetization dynamics with high power excitation generally found in current induced spin switching and microwave assisted switching, etc. Recently we have performed magnetoresistance measurements in IrMn/FeCo/AlOx/Permalloy (Py) magnetic tunnel junctions excited by a large power microwave [1]. Using the same structure, we electrically detected the LA-FMR of the Py by measuring the tunneling resistance in a sweeping external dc magnetic field. We found that the resonance frequency depends on the microwave power, i.e. precession cone angle, as well as the dc magnetic field, which can be well explained by using the Landau-Lifshitz-Gilbert equation in non-linear regime. The results also suggest that this new experimental technique to detect FMR is very useful for characterizing the LA-FMR. \newline [1] T. Moriyama et al., Appl. Phys. Lett. 90 (15), 152503 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X32.00005: Technique for Minimizing the Effect of 1/$f$ Noise in Magnetic Sensors Alan Edelstein, Greg Fischer, James Burnette, Shu-Fan Cheng, Edmund Nowak, William Egelhoff, Cathy Nordman Sensors such as magnetic tunnel junctions with MgO barriers offer the possibility of increased sensitivity. The magnetoresistance of these junctions can be as large a 400{\%}. Unfortunately, these magnetoresistance devices suffer from having considerable 1/$f$ noise. We have a device, the MEMS flux concentrator, that modulates the field at the position of the sensor and thus increase the frequency of the field to be detected to kHz frequencies where the 1/$f$ noise is much smaller. It does this by having flux concentrators on MEMS flaps that are driven to move electrostatic comb drives. The flaps on each side of the sensor are connected by springs so that the desired motion is a normal mode. The signal appears as sidebands that can be demodulated using a lock-in amplifier. The device will increase the sensitivity at 1 Hz of many sensors by a factor of 100. Tests indicate that the device does not increase the noise and that it will function at frequencies lower than 1 Hz. Results of initial tests will be reported. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X32.00006: Study of magnetization switching in synthetic antiferromagnets Cosmin Radu, Dorin Cimpoesu, Leonard Spinu, Alexandru Stancu Synthetic Antiferromagnet (SAF) structures are very important in designing modern spintronic devices. The theoretical studies of the toggle writing mode in MRAM$^{1}$ use the concepts of SAF critical curve, which is a generalization of the astroid from the coherent rotation model in the case of uniaxial anisotropy. Although extensively studied theoretically$^{2,3}$ there are no methods proposed to experimentally determine the critical curve of a SAF structure. We propose a way for determining the critical curve of the switching fields using reversible susceptibility experiments (RS) and we prove this to be more sensitive to the switching characteristics of SAF structures than a regular hysteresis loop. For certain coupling strengths the entire critical curve can't be determined using standard RS experiments and a strategy for revealing these hidden parts of the critical curve is proposed. 1. L. Savtchenko, B. N. Engel, N. D. Rizzo, M. F. Deherrera, and J. A. Janesky, US Patent 6,545,906 B1, (2003). 2. S. Y. Wang and H. Fujiwara, J. Magn. Magn. Mater. 286, 27-30 (2005). 3. H. Fujiwara, S. Y. Wang, and M. Sun, J. Appl. Phys. 97, 10P507-10P507-5 (2005). Work supported by DARPA grant HR0011-07-1-0031. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X32.00007: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 9:24AM - 9:36AM |
X32.00008: Exchange bias in a ferrimagnetic/antiferromagnetic system M.R. Hossu, S. Demirtas, M.B. Salamon, A.R. Koymen The effect of antiferromagnetic FeMn on ferrimagnetic (Co4nm/Gd4nm)$_{ 4}$ multilayer was investigated by measuring the exchange bias and coercivity fields. It was observed that magnetic properties depend on whether the multilayers are Co or Gd terminated. The exchange bias increased at low temperatures from 50Oe to 350Oe when FeMn layers are on both surfaces of Co terminated [FeMn10nm/Co4nm/ (Gd4nm/Co4nm)$_{ 4}$/FeMn10nm] multilayer, compared to single FeMn layer. However for the Gd terminated [FeMn10nm/Gd4nm/ (Co4nm/Gd4nm)$_{ 4}$/FeMn10nm] multilayer the Gd/FeMn interface does not induce exchange bias. Moreover exchange spring behavior is observed around compensation temperature due to the uncompensated moment in FeMn layers. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X32.00009: Optical Manipulation of Paramagnetic Particles with On-Chip Detection Using Spin Valve Sensors Lena Wai-Yi Lui, Ke Bin Li, Sean O'Shea, Chorng-Haur Sow In this work, we present a new combinatory approach where an optical tweezers was used to trap and position a single super-paramagnetic particle over a Spin Valve sensor, with the particle then detected by the sensor. This approach is demonstrated using super-paramagnetic particles of 2 $\mu $m (Micromer$^{\mbox{{\textregistered}}}$-M, Micromod) together with a Spin Valve sensor with dimensions of 2 x 4 $\mu $m$^{2}$ whereby a single magnetic particle was positioned over the sensor and a corresponding drop in the voltage across the sensor was detected. The results are explained using a simple model where the particle is treated as a pure dipole. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X32.00010: One dimensional Brownian rotation of magnetic microspheres Miri Shlomi, Brandon McNaughton, Raoul Kopelman, Panos Argyrakis Many experimental observations of 3D random rotations of particles have been interpreted using Einstein's 1D Gaussian solution. However, in contrast to 1D, 3D rotations are non-commutative, and cannot rigorously be described by Einstein's model. To test the potential discrepancy between theory and experiment, we monitor a particle rotating freely around a single fixed axis, subsequently comparing it with the particle's 3D Brownian rotation. To carry out these observations, we use a fluorescent ferromagnetic microsphere that has one hemisphere coated with an opaque metal (aluminum), while the other is left intact. In rotating, the particle undergoes intensity fluctuations which are observed in a fluorescent microscope. Restraining the rotation by aligning the ferromagnetic particles with an external magnetic field, allows us to see, for the first time, Brownian rotation of a sphere around a fixed axis. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X32.00011: Single bacterial cell detection with nonlinear rotational frequency shifts of driven magnetic microspheres Brandon McNaughton, Rodney Agayan, Ron Smith, Raoul Kopelman, Roy Clarke Shifts in the nonlinear rotational frequency of magnetic microspheres, driven by an external magnetic field, offer a dynamic approach for the dynamic detection of single bacterial cells. We demonstrate this capability by measuring such frequency shifts when an \textit{Escherichia coli }attaches to the surface of a 2.0 micron magnetic microsphere, thereby affecting the drag of the system. From this change in drag, the nonlinear rotation rate was reduced, on average, by a factor of 3.8. Sequential bacterial attachments were also monitored using this approach. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X32.00012: Length dependence of the magnetorheological properties of cobalt microwires dispersed in silicone oil. Richard Bell, Joshua Karli, Jeffrey Krug, Darin Zimmerman The rheological responses and dispersion stability of magnetorheological (MR) fluids were experimentally investigated. In order to improve the stability of the MR fluids, spherical particles were replaced with cobalt microwires (260 nm diameter) with varying aspect ratios ranging from 10 to 65 and suspended in silicone oil (0.17 Pa$\cdot $s). Under an external magnetic field ($H_{0})$ and a steady shear flow, the yield stress of the fluids display a $H_{0}^{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$ dependence. The apparent yield stress at magnetic saturation increased linearly with the median length of the particles until a maximum at a wire length of 7 $\mu $m was reached. Further increasing the length of the microwires resulted in a linear decrease in the yield stress of the fluids. Furthermore, the MR fluids containing microwires with lengths greater than 6 $\mu $m exhibited a larger yield stress than fluids containing only 1.6 $\mu $m diameter spherical particles. The microwire-based fluids also display improved stability against rapid sedimentation as compared to the spherical cobalt particles. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X32.00013: Half-metallic ferromagnetism in iron-antimony based Skutterudites with monovalent filler atoms Andreas Leithe-Jasper, Walter Schnelle, Helge Rosner, John Mydosh, Yuri Grin We report the thermodynamic, magnetic and electronic properties of of the filled Skutterudites AFe$_4$Sb$_{12}$ (A=Na,K,Tl) in a joined experimental and theoretical study. Unexpectedly, these compounds show a ferromagnetic transition at $T_c$ $\sim$ 85\,K.[1] According to electronic structure calculations and point-contact Andreev reflection [2] these systems show a rather large spin polarization. On the other hand, these itinerant magnets exhibt strong spin fluctuations. A brief comparison with compounds based on A=Ca,Sr,Ba,Yb,La where spin fluctuations impede long magnetic order will be presented.\newline \newline [1] A. Leithe-Jasper, W. Schnelle, H. Rosner, N. Senthilkumaran, A. Rabis, M. Baenitz, A. Gippius, E. Morozova, J.A. Mydosh, and Yu. Grin, Phys. Rev. Lett. 91, 037208, (2003). \newline [2] G. Sheet, H. Rosner, S. Wirth, A. Leithe-Jasper, W. Schnelle, U. Burkhardt, J. A. Mydosh, P. Raychaudhuri, and Yu. Grin, Phys. Rev. B 72, 180407, (2005). [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X32.00014: Interplay of Magnetic and Structural Anisotropy in Co$\vert$Ni Multilayer Thin Films Joseph Dvorak, Kathryn Krycka, Jean-Marc Beaujour, Wenyu Chen, Andrew Kent, Chi-Chang Kao Interfacial perpendicular magnetic anisotropy (PMA) is important for spin transfer devices and has been predicted to overcome dipolar shape anisotropy for [t Co$\vert$2t Ni] multilayers with thicknesses, t, of 4 $\mathring{A}$ or less [1]. layered between Cu, however, thickness dependent PMA (t=1, 2, 3, 4 and 6 $\mathring{A}$) is not sufficiently strong to produce perpendicular magnetization. Anomalous diffraction reveals that the Co and Ni are expansively strained by the Cu within the sample plane. As calculated in reference 2 this trigonal strain would be sufficient to overcome the PMA. Ferromagnetic resonance measurements [3] indicate that the net Lande g-factors are enhanced above bulk for all thicknesses, and increase further with decreasing layer thickness. By applying element-specific x-ray magnetic circular dichroism (XMCD) we have been able to study the Co and Ni individually. Both elements show increasing spin to orbit ratios with decreasing thickness magnetized either in-plane or along sample normal. In all cases the spin to orbit ratio is enhanced along the sample normal compared with the in-plane direction. \newline [1] Phys. Rev. Lett. 68, 682 (1992) \newline [2] Phys. Rev. B 69, 104426 (2004) \newline [3] Eur. Phys. J. B 59, 475 (2007) [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X32.00015: Classical and Quantum Routes to Linear Magnetoresistance Jingshi Hu, T.F. Rosenbaum We show that either adding a few parts per million of the proper chemical impurities to indium antimonide, a well-known semiconductor, or redesigning the material's structure on the micrometer scale, can transform its response to an applied magnetic field. The former approach is a spectacular manifestation of magnetotransport in the extreme quantum limit, when only one Landau band is partially filled; the latter a classical outgrowth of disorder, turned to advantage. In both cases the magnetoresistive response – at the heart of magnetic sensor technology – can be converted to a simple, large and linear function of field that does not saturate. Harnessing the effects of disorder has the further advantage of extending the useful applications range of such a magnetic sensor to very high temperatures by circumventing the usual limitations imposed by phonon scattering. [Preview Abstract] |
Session X33: Focus Session: Magnetic Resonance in Magnetic Semiconductors
Sponsoring Units: GMAG FIAP DMPChair: Roland Kawakami, University of California, Riverside
Room: Morial Convention Center 224
Friday, March 14, 2008 8:00AM - 8:12AM |
X33.00001: Microscopic mechanism of optically pumped NMR in bulk GaAs Patrick Coles, Jeffrey Reimer In the past decade, optical nuclear polarization in semiconductors has been used as a tool for basic solid-state physics [1], and has been proposed as a means towards sensitivity-enhanced NMR for biological systems [2] and towards information storage in quantum computing architectures [3]. The microscopic mechanism of this process, however, has been debated recently even in the model system of bulk GaAs [4]. We review our modeling and experimental results towards differentiating between recently considered mechanisms for optical nuclear polarization in GaAs based on localized and delocalized electrons. We discuss a simple experiment that quantifies the amount of localization. Results are consistent with localized electrons cross-relaxing with nearby nuclei, and gradual polarization of the bulk by nuclear spin diffusion. [1] A.E. Dementyev et al. Sol. State Comm. 119: 217 (2001) [2] R. Tycko. Sol. State Nuc. Mag. Res. 11:1 (1998) [3] T.D. Ladd et al. Phys. Rev. Lett. 89:017901 (2002) [4] A.K. Paravastu et al. Phys. Rev. B 69:075203 (2004) [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X33.00002: Spin Resolved Cyclotron Resonance and Magneto-absorption in InSb Multiple Quantum Wells Xingyuan Pan, G.D. Sanders, C.J. Stanton, T. Kasturiarachchi, W. Gempel, X.H. Zhang, R.C. Meyer, N. Goel, M. Edirisooriya, T.D. Mishima, R.E. Doezema, M.B. Santos, Y.J. Wang We present calculations of the spin-dependent cyclotron resonance and magneto-absorption spectra in strained, narrow gap AlInSb/InSb multiple quantum wells. Our calculations are based on the 8-band Pidgeon-Brown model generalized to including the effects of the confinement potential and pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with experiments. The large g-factors allow one to spin-resolve the transitions at modest magnetic fields ($<$10T). In the magneto-absorption, we see spin-split transitions and identify both bright and dark transitions in agreement with experiment. For cyclotron resonance, we see not only spin-resolved anticrossings between Landau levels with the same spin, but also anticrossings between levels of opposite spin. These opposite spin anticrossings result from spin-orbit coupling and the resonance-enhanced mixing of the second Landau level of the ground state subband and the lowest Landau level of the first excited state of opposite spin. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X33.00003: High-frequency EPR of impurities on diamond Susumu Takahashi, Mark S. Sherwin, Ronald Hanson, David D. Awschalom The nitrogen-vacancy (N-V) impurity center is a promising solid- state spin system for solid-state quantum information processing. Many desirable quantum properties have been found at room temperature, including long spin-coherence times, demonstration of single N-V spin quantum gate operation, discovery of rapid spin polarization and achievement of readout of single N-V spins. There have been many EPR experiments to investigate electronic structures and dynamics of impurities in diamond. Most of the studies were however performed by low- field EPR. High-frequency EPR generally has a great advantage for spectral and time resolution and absolute sensitivity due to very high spin-polarizations in high magnetic fields. High- frequency EPR for the diamond system therefore enables the investigation of ensembles of low-concentration impurities. In this presentation, we will discuss high-field properties of spin relaxations of impurities on diamond studied with 240 GHz cw and pulsed EPR. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X33.00004: Electron-nuclear interactions in lightly-doped GaAs, studied through optically-detected magnetic resonance (ODMR) John Colton, Lee Wienkes, Andrew Gierke, Susan Allemann, Allan Bracker Electron-nuclear interactions have been studied in lightly-doped GaAs via optically-detected magnetic resonance (ODMR). Thermally-polarized electrons were resonated with $\sim $10 GHz microwaves; the spin states were detected with cw optical Kerr rotation. The electron-nuclear interaction was manifested through an effective magnetic field produced by the hyperfine interaction between nuclei and donor electrons, which caused a shift in the electron ODMR peak position. The effective nuclear field could be eliminated by simultaneous magnetic resonance of the three nuclear species. A measurement of the nuclear spin relaxation time was obtained (5.6 minutes) by tracking the amount of effective nuclear field, in the absence of nuclear magnetic resonance. Finally, optically-detected electron-nuclear double resonance (ODENDOR) was also performed, by monitoring changes in the electron Kerr rotation signal while sweeping through nuclear resonant frequencies one at a time. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X33.00005: Pseudospin Resonance in Semiconductor Bilayers Saeed H. Abedinpour, Marco Polini, Mario P. Tosi, Bilal Tanatar, Allan H. MacDonald, Giovanni Vignale The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic resonance mode of a ferromagnet. We present a theory of the dependence of the energy and the damping of this mode on layer separation $d$. Based on these results, we discuss the possibility of realizing transport-current driven pseudospin-transfer oscillators in semiconductors. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X33.00006: Location and Magnetic Hyperfine Properties of Mn$^{2+}$ in Silicon. R.H. Pink, Archana Dubey, S.R. Badu, R.H. Scheicher, M.B. Huang, Lee Chow, T.P. Das Crystalline Silicon doped with the transition metal ion Mn$^{+2}$ is ferromagnetic at room temperature and thus potentially a useful material for spintronic applications. In attempting to understand from first principles the location of Mn$^{+2}$ and the electronic structure of the ferromagnetic system we have started work first on the dilute system. We have used the Hartree-Fock cluster procedure to determine the binding energies of the three likely locations for Mn$^{2+}$, substitutional (S), tetrahedral interstitial (T$_{i})$ and hexagonal interstitial (H$_{i})$ locations allowing for relaxation of the silicon neighbors. Our calculations show that the H$_{i}$ location is unstable and the S and T$_{i}$ are stable. Our nuclear magnetic hyperfine interactions results for $^{55}$Mn nucleus and $^{29}$Si neighbor will be presented and compared with electron spin resonance [1] experimental data. \newline [1] H.H. Wood bury and G. W. Ludwig Phys. Rev. \underline {117},102(1960) [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X33.00007: The Nuclear Spin Relaxation in the Semiconductor Double Quantum Dots: Study of Spin Diffusion Eddy Yusuf, Xuedong Hu We study the relaxation of the nuclear spins in a semiconductor double quantum dot. We focus on spin diffusion as an important channel for nuclear spin relaxation. The diffusion of the nuclear spin polarization arises from the Fermi contact hyperfine interaction between electrons in the quantum dots and the nuclear spins, as well as the magnetic dipolar interaction between nuclear spins. We calculate the nuclear spin polarization relaxation time and the nuclear diffusion constant within the density matrix framework. We explore the behavior of the relaxation time and diffusion constant for a wide range of conditions, including variations in temperature, the initial degree of nuclear polarization, dot sizes, and the strength of the applied magnetic field. We compare our results to the available experimental data and discuss various experimental schemes to further test the outcome of our calculation. [1] A. Greilich, et al., Science 317, 1896 (2007) [2] S. Das Sarma, et. al., Solid State Commun. 133, 737 (2005) [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X33.00008: Microscopic theory of electron spin relaxation in N@C$_{60}$ Z.G. Yu Endohedral N@C$_{60}$ exhibits an extremely long electron spin relaxation time and offers a great potential in storing and processing quantum information. Here we present a microscopic theory of electron spin relaxation in N@C$_{60}$. The theory combines (1) the spin-orbit interaction of N $2p$ electrons, which mixes the ground state $^4S$ with excited $^2P$ and $^2D$ states, and (2) the coupling between the N $2p$ electrons and C$_{60}$ $H_g$ vibrations, which facilitates transitions between $^2P$ and $^2D$ states. The spin relaxation occurs via a two-phonon (Raman) process by absorbing a $H_g$ phonon and emitting another at the (approximately) same frequency. The theory consistently explains measured spin relaxation time $T_1$ and its temperature dependence, and predicts two distinct spin decoherence $T_2$ constants. In addition, the excellent agreement between theory and experiment suggests a universal importance of the two-phonon Raman process in determining spin relaxation in nanostructures such as quantum dots, where a one-phonon process is ineffectual in flipping electron spins because of the lack of low-energy phonons in nanostructures. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X33.00009: Electronic and transport properties of a lateral triple quantum dot molecule in a magnetic field F. Delgado, Y.-P. Shim, M. Korkusinski, P. Hawrylak We present a theory of spin, electronic and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a Hubbard model and the Linear Combination of Harmonic Orbitals combined with Configuration Interaction method to arbitrary magnetic fields. The one-particle spectra obtained as a function of the magnetic field exhibit Aharonov-Bohm oscillations. As a result, by changing the magnetic field it is possible to engineer the degeneracies of single-particle levels, and thus control the total spin of the many-electron system. For the triple dot with two and four electrons we find oscillations of total spin due to the singlet-triplet transitions occurring periodically in the magnetic field. In the three-electron system we find a transition from a magnetically frustrated to the spin-polarized state. The impact of these phase transitions on the addition spectrum are analyzed and the qualitative behaviour of the current through the quantum molecule under spin blockade conditions is studied as a function of the applied magnetic field [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X33.00010: All optical FMR of ferromagnetic (Ga,Mn)As with various Mn contents Satoi Kobayashi, Yusuke Hashimoto, Hiro Munekata We have investigated all-optical FMR of non-thermal origin [1] on (Ga,Mn)As
with various Mn contents $x$ with time-resolve MO signals obtained by a
single-wavelength pump and probe (P{\&}P) technique, and found clear
dependence of $x$ on the effective magnetic field on ferromagnetically coupled
Mn spins.
(Ga,Mn)As samples $x$ = 0.02-0.11 were grown by molecular beam epitaxy.
Measurements were performed for both as-grown and annealed samples. In-plane
magnetization easy axis was [1-10] for the $x$ = 0.11 sample, whereas it was
$<$100$>$ for the rest. Samples were magnetized prior to P{\&}P
measurements. Polarization of probe pulses was carefully controlled with
respect to the direction of $M$. Pump power was varied between 3.4 - 34 $\mu
$J/cm$^{2}$.
Precession of magnetization was clearly observed at 10-40 K in most of
samples. The period of the oscillation becomes longer with increasing $x$
value, indicating a decrease in an effective magnetic field $H_{eff}$ =
$\hbar \omega $ / \textit{g$\mu $}$_{B}$ on Mn spins from 0.2 to 0.1 Tesla with increasing
$x$. Assuming $H_{eff}=J_{pd} |
Friday, March 14, 2008 10:00AM - 10:12AM |
X33.00011: Nuclear Magnetic Resonance in Semiconductor Nanostructures Ionel Tifrea One measurement for the nuclear spin dynamics in solid state systems is the Knight shift observed in nuclear magnetic resonance experiments. I will present a theoretical investigation of the Knight shift in samples with reduced dimensionality. The nuclear spin dynamics is dominated by the hyperfine interaction between nuclear and electronic spins and depends on the electronic local density of states. As an example, I will discuss the temperature, position, and time dependence of the induced nuclear spin polarization and the resulting Knight shift in semiconductor quantum wells. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X33.00012: Electrically-detected magnetic resonance in accumulation-layer MOSFETs Laurens Willems van Beveren, Dane McCamey, Hans Huebl, Andrew Ferguson, Tim Duty, Robert Clark Spin-dependent transport, originating from neutral-impurity scattering, in silicon accumulation-layer MOSFETs was reported more than a decade ago in an electron-spin resonance (ESR) cavity setup [1]. There, current measurements on the MOSFET showed ESR features with a hyperfine (HF) splitting of 42 G, indicative of electrons whose wavefunctions overlap with phosphorous nuclei in the silicon crystal. Here, we report the observation of electrically-detected magnetic resonance (EDMR) in phosphorous-doped silicon MOSFETs without the constraint of a cavity and down to the mK-regime in a dilution refrigerator with a superconducting magnet. Instead, the ESR-field is generated by an on-chip shorted coplanar stripline (CPS), allowing broadband operation. Continuous-wave EDMR was achieved up to 30 GHz. The EDMR spectra show (i) the two hyperfine-split (42 G) ESR lines and (ii) an EDMR signal that is centered between the hyperfine lines, associated with the `free electron' ESR response. [1] R. Ghosh and W. Silsbee, Phys. Lett. 85, 439 (1992). [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X33.00013: Electrically detected magnetic resonance in Si:P at high magnetic fields (B = 8.5 T) Dane McCamey, Gavin Morley, Louis Claude Brunel, Johan van Tol, Heather Seipel, Christoph Boehme Phosphorus doped silicon (Si:P) is a technologically important material with possible uses in spintronic and quantum information processing devices. A useful way to understand the properties of this material is by investigation of the spin dependence of its transport processes. Whilst numerous studies of this type have been performed on Si:P at low magnetic fields, no systematic investigation has been undertaken at high magnetic fields. We will present an electrically detected magnetic resonance (EDMR) study of Si:P, with a native oxide surface, at $B = 8.5$ T ($f_{resonance} \sim 240$ GHz). The change in the sample photocurrent, $\Delta I/I$, was measured as a function of B using a microwave chopping method. Resonant signals from the P donors, as well as P$_{\textrm{b}}$ defects near the Si-SiO$_2$ interface, were observed. The temperature dependence of the observed signals in the range $T = 3$ K - $10$ K will be presented, and the microscopic processes leading to the signals discussed. Finally, pulsed EDMR (Rabi oscillations, Hahn echos) was performed to investigate spin coherence and manipulation in high fields, and these results will also be discussed. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X33.00014: Pulsed electrically detected magnetic resonance of phosphorus donors near the Si(111)-SiO$_{\mathrm{2}}$ interface Seoyoung Paik, Heather Seipel, Sang-Yun Lee, Thomas Herring, Dane McCamey, Christoph Boehme Recently, there has been a large effort towards the electrically detection of spin coherence of phosphorus donor electrons in silicon. These studies have been undertaken on the Si (100) interface, due to its wide utilization in the semiconductor industry. Here, we present a pulsed electrically detected magnetic resonance study on P donors near the Si(111) interface. We observe the transient current after a short, coherent microwave pulse as a function of both the magnitude and relative orientation (with respect to the [111] direction) of the applied magnetic field. Similar to previous Si(100) studies, we observe three resonant peaks, which we attribute to a) the two well known hyperfine spilt phosphorus resonances, and b) the P$_{\mathrm{b}}$ defect resonance. The P$_{\mathrm{b}}$ resonance exhibited an anisotropy with field direction, in agreement with conventional ESR studies. In addition, we observe a fourth isotropic resonance, with a g-factor of $g=2.0031 \pm 0.0004$. We conclude that, aside from the anticipated and well know P-P$_{\mathrm{b}} $ transition, at least one additional spin dependent recombination pathway exists at the Si(111)-SiO$_{\mathrm{2}}$ interface. [Preview Abstract] |
Session X35: Semiconducting Devices and Applications
Sponsoring Units: FIAPChair: Tom Oder, Youngstown State University
Room: Morial Convention Center 227
Friday, March 14, 2008 8:00AM - 8:12AM |
X35.00001: Improved Ni/SiC and ZrB$_{2}$/SiC Schottky Barrier Diodes by High Temperature Processing Tom Oder, Ta-Lun Sung, Edward Sutphin, Sara Schaefer, Rani Kummari High temperature processing was used to improve the barrier properties of SiC Schottky diodes fabricated with Ni and ZrB$_{2}$ Schottky contacts. The Ni/SiC diodes were annealed in vacuum at 500 $^{\circ}$C in 20-hour intervals for a total of 110 hours. The average Schottky barrier heights determined by I-V measurements, increased with annealing time from 1.48 eV for non-annealed contacts to 1.81 eV for those annealed for 20 hours. The improvement is believed to be due to the formation of nickel silicide. The ZrB$_{2}$ Schottky contacts for the ZrB$_{2}$/SiC diodes were deposited at temperatures between 20 $^{\circ}$C and 800 $^{\circ}$C. The barrier heights increased with the deposition temperature from an average value of 0.87 eV for contacts deposited at 20 $^{\circ}$C to 1.07 eV for those deposited at 600 $^{\circ}$C. The Rutherford backscattering spectra of the ZrB$_{2}$/SiC contacts revealed a decrease of oxygen with increase in the deposition temperature and showed no reaction at the ZrB$_{2}$/SiC interface. We ascribe the improvement of the ZrB$_{2}$/SiC diodes to the removal of oxygen from the contact when deposited at high temperatures. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X35.00002: ``Ultimate'' SOI MOSFETs Thomas J. Walls, Konstantin K. Likharev Silicon-On-Insulator (SOI) field-effect transistors (MOSFETs) are being aggressively scaled toward the 10-nm frontier - see, e.g., Ref. \footnote{M. Vinet {\it et al., IEEE Elec. Dev. Lett.}, vol. 26, pp. 317-319, May 2005.}. In our earlier work \footnote{V. Sverdlov {\it et al., IEEE Trans. on Electron Dev.}, vol. 50, pp. 1926-1933, Sep. 2003.}, we have carried out a detailed analysis of the performance and parameter variation sensitivity of double-gate sub-10-nm MOSFETs using a self-consistent numerical solution of the 2D Poisson equation and 1D Schrodinger equation. However, for very small devices the 1D approximation misses some important details of the device physics. In this work, we have used the momentum-space formalism, developed in 1989 by A. Szafer and A. D. Stone \footnote{A. Szafer {\it et al., Phys. Rev. Lett.} vol. 62, pp. 300-303, Jan. 1989}, to fully account for 2D quantum effects. At the meeting, we will present a comparison of our new results with our previous 1D approximation, as well as calculations of the gate capacitance of the transistors. This work has been supported in part by the ONR. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X35.00003: Novel Ferroelectric CMOS Circuits as a Nonvolatile Logic M. Takahashi, T. Horiuchi, Q.-H. Li, S. Wang, K. Y. Yun, S. Sakai We propose a novel and promising nonvolatile-logic circuit constructed by p channel type (Pch) and n channel type (Nch) ferroelectric gate field effect transistors (FeFETs), which we named a ferroelectric CMOS (FeCMOS) circuit. The circuit works as both logic and memory. We fabricated a NOT logic FeCMOS device which have Pt metal gates and gate oxides of ferroelectric SrBi$_{2}$Ta$_{2}$O$_{9}$ (SBT) and high-$k$ HfAlO on Si. Key technology was adjusting threshold voltages of the FeFETs as well as preparing those of high quality. We demonstrate basic operations of the NOT-logic response, memory writing, holding and non-destructive reading. The memory writing is done by amplifying the input node voltage to a higher level when the node was logically high and to a lower one when it was logically low just before the writing operation. The data retention was also measured. The retained high and low voltages were almost unchanged for 1.2 days. The idea of this FeCMOS will enhance flexibility of circuit designing by merging logic and memory functions. This work was partially supported by NEDO. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X35.00004: Study on threshold voltages of Pt/SrBi$_{2}$Ta$_{2}$O$_{9}$/Hf-Al-O/Si FeFETs Q.-H. Li, M. Takahashi, S. Wang, T. Horiuchi, C.C. Wang, K.Y. Yun, Y. Fuhihata, S. Sakai Complementary ferroelectric-gate field-effect transistors (FeFETs) are attractive for nonvolatile-logic circuit applications after the achievement of long data retention for both $n$- and $p$- channel FeFETs [1, 2]. To demonstrate nonvolatile logic circuits, the threshold voltage should be well controlled. Due to ferroelectricity two threshold voltages $V_{t, left}$ and $V_{t, right}$ can be defined from $I_{d}-V_{g}$ curves as gate voltages at $I_{d}$ =10$^{-6}$A. More than 90 $n$- or $p$-channel Pt/SrBi$_{2}$Ta$_{2}$O$_{9}$/Hf-Al-O/Si FeFETs on a Si chip are studied. The average$ V_{t, left}$ and $V_{t, right}$ are 1.19 and 2.38 V for $n$-channel FeFETs, and -0.76 and 0.40 V for $p$-channel FeFETs, respectively. The standard deviations of $V_{t}$ are 3-5{\%} and 7-8{\%} of the memory window for the $n$- and $p$- channel FeFETs, respectively. $V_{t}$ positions are adjusted by varying the well doping concentrations. Our results indicate possible circuit demonstration. This work was partially supported by NEDO. \newline [1] S. Sakai, et al, \textit{IEEE Electron Devices Lett.}, \textbf{25}, 369(2004). \newline [2] Q.-H. Li, et al , \textit{Appl. Phys. Lett. }\textbf{89}, 222910 (2006). [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X35.00005: The Nanowire iJFET Bart Soree, Wim Magnus The cylindrical geometry of nanowire surrounding gate MOSFETs gives rise to outstanding electrostatic control in comparison to planar devices. On the other hand, we expect that for ultrasmall nanowire diameters, the interaction of electrons with the surface (e.g. surface roughness and high-k) will be detrimental for device performance due to mobility degradation. In order to avoid these surface interactions we consider a surrounding gate nanowire operated not in MOSFET mode, but in ``JFET mode.'' We thus consider a nanowire with silicon body radius $R$ and surrounding oxide of thickness $t_{\rm ox}$ with a surrounding metal gate where both source, drain and silicon body are doped uniformly with a donor density $N_ {\rm D}$. Applying a negative gate voltage pushes the electrons away from the interface between the insulator and metal gate, and as a result a depletion region is induced. For sufficient negative gate voltage the depletion region reaches the center of the silicon body, and pinch-off occurs. For large radii, we construct a compact model, and we show that reasonable pinch- off voltages are realized when the wire radius or the donor density is sufficiently small. Using the gradual channel approximation we are able to obtain current-voltage characteristics that constitute a ``proof of concept'' for this device. In the case of ultrasmall radii, we perform a quantum mechanical analysis of the electronic structure. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X35.00006: Extraordinary Electroconductance in Ti-GaAs hybrid thin film structures Yun Wang, A.K.M. Newaz, Jian Wu, S.A. Solin, V.R. Kavasseri, Niu Jin, I.S. Ahmad, I. Adesida Following the demonstration of extraordinary electroconductance (EEC) in metal-semiconductor hybrids (MSHs), we have developed microscopic circular thin film GaAs-Ti EEC sensors capped by a concentric Ti electrode that acts as both a shunt and Schottky barrier. The geometrical properties of the device are characterized by the parameter $\alpha $ which is the ratio of the shunt radius to that of the GaAs mesa. We investigated samples with $0\le \alpha \le {14} \mathord{\left/ {\vphantom {{14} {16}}} \right. \kern-\nulldelimiterspace} {16}$ in mesa sizes from 80$\mu $m to 200$\mu $m. We define the EEC as the {\%} change of conductance with and without an externally applied electric field. An EEC=20{\%} is obtained with $\alpha $=8/16 at E=0.65kV/cm and a shunt bias current of 50nA. The reverse bias EEC is always larger than the forward bias effect (maximum $\sim $ 5{\%}) due to the asymmetry of the band profile. The {\%} change in sample conductance increases linearly with direct reverse voltage bias across the MS interface and is independent of the radius of the GaAs mesa. This independence is extremely desirable for scaling to the nano regime. Such a static electric field sensor as described above makes high resolution imaging of surface charge density distribution possible. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X35.00007: Optimizing Transport Properties of a Potential Molecular Electronic Device Julio L. Palma, Chao Cao, Hai-Ping Cheng, Jeffrey L. Krause Future generations of electronic devices will have the dimensions of molecular size. The ability to control the transport properties of single molecules will have a major impact on this promising technology. The azobenzene molecule has been proposed recently as a component of a light-driven molecular switch. This molecule has two stable conformations in its ground state: {\it cis} and {\it trans}. The molecule can be converted from one configuration to the other by photo-excitation. Previous calculations showed that the {\it trans} configuration has a considerably higher conductance than the {\it cis} configuration. In this work, we study the effects of chemical substituents on the electron transport properties of azobenzene. The effects of such substituents are crucial in predicting structures that may have optimized properties with slightly different chemical structures. For the azobenzene studies, we include electron donating groups (-NH$_2$) and electron withdrawing groups (-NO$_2$) in meta- and -ortho positions with respect the azo group. The transport properties are calculated using first principles methods that combine non-equilibrium Green's function (NEGF) technique with density functional theory (DFT). [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X35.00008: Ion Sources for Deep and Shallow Ion Implantation Ady Hershcovitch, V. Batalin, A. Bugaev, V. Gushenets, B. Johnson, A. Kolomiets, G. Kropachev, R. Kuibeda, T. Kulevoy, I. Litovko, E. Masunov, E. Oks, V. Pershin, S. Petrenko, S. Polozov, H. Poole, I. Rudskoy, D. Seleznev, P. Storozhenko, A. Svarovski, G. Yushkov Various ions, but mostly B, P, Sb, {\&} As, are implanted, over a wide range of energies into materials used in the construction of semiconductors. These energies range from as low as 100 eV for shallow surface implantations, to as high as multi-MeV for deep implantation into the substrate. State of the art ion sources meet industry needs for the energy range of 10 keV to 300 keV. But at the two extremes (100's of eV and at multi-MeV), there is room for improvement due to space charge limitations at the low energy range and due to inefficiency in acceleration at the higher energy range. A joint R{\&}D effort focusing on meeting industry needs has been in progress for the past four years. This endeavor has resulted in record steady state output currents of higher charge state Antimony and Phosphorous ions as well as Decaborane molecular ions. This talk is a synopsis of an extensive ion source R{\&}D program designed to address industry needs. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X35.00009: Infrared Near-field Microscopy of Semiconductor Devices Fritz Keilmann, Andy J. Huber, Rainer Hillenbrand We report optical imaging at ultrahigh resolution $<$ 30 nm of cross-sectional preparations of state-of-the-art transistors. Our technique employs a scanning optical near-field microscope of scattering type (s-SNOM). It is based on a tapping-mode AFM with a standard, metallized tip, and an interferometric receiver. This detects a pseudo-heterodyne signal detection that is filtered at a low harmonic of the tapping frequency. The illumination wavelength of 10.7 $\mu $m allows to record specific contrasts---which are obtained in amplitude as well as in phase---distinguishing not only all material components in the transistors, but furthermore highlighting the charge carriers. Prospects of quantitative carrier density mapping will be discussed. A.J. Huber et al., Adv. Mat. 19, 2209 (2007) [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X35.00010: Resistance Switching Behavior in Epitaxially Grown NiO S.R. Lee, J.H. Bak, Y.D. Park, K. Char, D.C. Kim, R. Jung, S. Seo, X.S. Li, G -S. Park, I.K. Yoo Reproducible resistance switching behavior has been found in NiO films prepared by a pulsed laser deposition system. The I-V measurements of epitaixally grown NiO on SrRuO$_{3}$ electrode show a bipolar resistive memory switching behavior, in contrast with a unipolar switching behavior of polycrystalline NiO on Pt electrode. In order to understand the resistive memory switching mechanism in NiO, the I-V characteristics and memory switching property of epitaxial NiO prepared under various synthesis conditions and electrodes has been investigated. The IV measurements at room temperature suggest that the interface between NiO and the electrode plays an important role on the resistive switching phenomena. To analyze the role of the interface, our efforts to control the interfaces and to measure the I-V characteristics at low temperature will be presented. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X35.00011: Temperature-Dependence of the Resonant Pump Wavelength in Optical Pumping Injection Cavity Lasers L.J. Olafsen, K.G. Young, T.C. McAlpine, W.W. Bewley, I. Vurgaftman, J.R. Meyer, H. Lee, R.U. Martinelli An optical pumping injection cavity (OPIC) laser contains a type-II W active region enclosed between two GaSb/AlAsSb distributed Bragg reflector mirrors, where the thickness of the etalon cavity surrounded by the mirrors is tuned to the desired pump wavelength. Multiple reflections of the pump photons result in more efficient absorption of the pump beam and consequently higher efficiencies and lower lasing thresholds. An optical parametric oscillator is used to pump the OPIC lasers at resonance, where the threshold pump intensities are minimized and output efficiencies are maximized. The resonant pump wavelength varies linearly with temperature. In addition to presenting light-light results, including efficiencies and thresholds as a function of temperature, the temperature-dependence of the resonant pump wavelength will be discussed, including the relative variations and contributions of lattice constant and refractive index with temperature. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X35.00012: Mid-IR Photonic-Crystal Interband Cascade Lasers Mijin Kim, Chul Soo Kim, William Bewley, Chadwick Canedy, James Lindle, Jill Nolde, Diane Larrabee, Igor Vurgaftman, Jerry Meyer Photonic-crystal distributed-feedback (PCDFB) semiconductor lasers have the potential to maintain optical coherence over very large areas. We report an electrically pumped PCDFB laser operating in a true single mode in the mid-infrared. A two-dimensional grating was formed on top of an interband cascade laser emitting at 3.3 $\mu $m by patterning a high-index Ge layer. The grating and the 400-$\mu $m-wide gain stripe were tilted by 20$^{o}$ with respect to the facet. Current spreading was prevented by ion bombarding the region outside the gain stripe rather than etching of a ridge. The gain region at the back of the cavity was also terminated by ion bombardment, since feedback from the back facet is undesirable. A single mode was emitted with maximum cw output power $>$ 60 mW, resolution-limited spectral linewidth (side-mode suppression ratio 27 dB), and single-lobe spatial far-field with angular full width at half maximum of 0.5$^{o}$. Comparison of the near and far field patterns indicated effective $M^{2} \quad \approx $ 3. The observation of low efficiency is thought to be due primarily to inadequate grating coupling, which can be remedied by thickening the Ge layer. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X35.00013: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 10:36AM - 10:48AM |
X35.00014: Electrical Characterization of Critical Phase Change Conditions in Nanoscale Ge$_{2}$Sb$_{2}$Te$_{5 }$ Pillars Ozhan Ozatay, Barry Stipe, Jordan Katine, Bruce Terris Following the original work of Ovshinsky on disordered semiconductors that exhibit ovonic threshold switching (OTS) there has been substantial interest in the electronic reversible switching properties of chalcogenides$^{1}$. The current induced phase transitions between polycrystalline and amorphous states in these materials offer orders of magnitude changes in the conductance which makes them an ideal candidate for non-volatile data storage applications. In this work we investigate the scaling of critical programming conditions required to observe such transitions between highly resistive (disordered) and highly conductive (ordered) states by constructing a resistance map with various pulse widths and amplitudes under different cooling conditions (as a function of pulse trailing edge). We study the evolution of critical phase change conditions as a function of contact size (50nm-1$\mu $m) and shape (circle-square-rectangle). We compare the resulting switching behaviour with the predictions of a finite-element model of the electro-thermal physics to analyze the nature of the switching dynamics at the nanoscale. $^{1}$ S-H. Lee, Y. Jung, R. Agarwal, \textit{Nature Nanotechnology}; doi:10:1038/nnano.2007.291 [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X35.00015: Charge Transport Phenomena in Detectors of the Cryogenic Dark Matter Search Kyle Sundqvist The Cryogenic Dark Matter Search (CDMS) seeks to detect putative weakly-interacting massive particles (WIMPS), which could explain the dark matter problem in cosmology and particle physics. By simultaneously measuring the number of charge carriers and the energy in athermal phonons created by particle interactions in intrinsic Ge and Si crystals at a temperature of $40 ~ mK$, a signature response for each event is produced. This response, combined with phonon pulse-shape information, allows CDMS to actively discriminate candidate WIMP interactions with nuclei apart from electromagnetic radioactive background which interacts with electrons. The challenges associated with these techniques are unique. Carrier drift-fields are maintained at only a few $V/cm$, else drift-emitted Luke-Neganov phonons would dominate the phonons of the original interaction. Under such conditions, carrier scattering is dominated by zero-point fluctuations of the lattice ions. It has been an open question how well the 8 Kelvin data prominent in the literature depicts this case. We compare the simulated transport properties of electrons and holes in $<100>$ Ge at $40~mK$ and at $8~K$, and apply this understanding to our detectors. [Preview Abstract] |
Session X36: Advances in Instrumentation and Measurements
Sponsoring Units: GIMSChair: Charles Agosta, Clark University
Room: Morial Convention Center 228
Friday, March 14, 2008 8:00AM - 8:12AM |
X36.00001: Newton, Abbe, and the relation between refractive index and dispersion William Karstens, David Y. Smith Chromatic aberration in lenses is corrected by combining glasses with different index/dispersion characteristics. The correction is based on an empirical linear relation between index and dispersion, a rule known qualitatively for many years that was given definitive form by Abbe and Schott in 1886. Despite a long history, its physical basis has remained obscure. Here we show it is a consequence of the spectral shape of the glasses' UV absorption by expanding the Kramers-Kronig relation for the index in a series of absorption-spectrum moments. To first order, the index is determined by the inverse-first moment, the dispersion by the inverse-third moment. For a complex glass, these moments may be rewritten as sums of UV-absorption moments for the glass-former and the glass-modifiers. The total index is then a sum of glass-former index plus a term proportional to the dispersion introduced by the modifier ions. The index \textit{vs}. dispersion plot gives families of straight lines originating at the host-glass index/dispersion point. A line's slope is determined by the inverse-third moment of the modifier absorption, its length by the modifier concentration. Newton's erroneous conclusion that achromatic lenses could not be made was based on the limited range of glasses available to him, all of which fell on a single index-dispersion line. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X36.00002: A Non-Perturbative Series Solution for Diffraction Gratings with Arbitrary Profiles Daniel Nkemzi, Prabasaj Paul In this work, we use a boundary continuation technique to obtain an iterative series expansion for the scattering of a plane wave by a diffraction grating with an arbitrary profile function. The method is efficient and is simple to apply. The results of numerical experiments show excellent agreement with the C-method and the coupled-wave approach. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X36.00003: Surface-enhanced Raman scattering by silver clusters at planar dielectric interface above the critical angle. Denis Pristinski, Siliu Tan, Henry Du Light refraction at the planar interface of dielectric media prevents light propagation in the optically denser medium at angles above the critical value. This limitation is broken when the evanescent wave is excited at the opposite side of the interface. The polarization and angle dependence of the light emitted above the critical angle has been previously studied for both luminescence and elastic scattering. In this work we demonstrate the possibility to quantitatively characterize the effect using surface-enhanced Raman signal from thiocyanate molecules adsorbed on clustered silver nanoparticles at the water-glass interface. Evanescently excited silver nanoparticles demonstrate stronger polarization dependence for both absorption and emission, and wider range of emittance angles, as compared with the model of a radiating dipole at a similar interface. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X36.00004: A novel time resolved resonance Raman technique - applications in correlated systems B. Schulz, I. Mahns, A. Goos, P. Saichu, S. Binder, S.G. Singer, A. Rusydi, M. Ruebhausen, S.-W. Cheong, P. Guptasarma In many modern materials the functionality of a system can be observed by studying the order parameters in the time domain. Here, we present a time resolved pump probe resonance Raman system that allows to study order parameters in an energy range from 5 meV to several eV and in a time domain from 1 ps to several ns. Due to our fully achromatic, all reflective Raman spectrometer, [1] we are able to combine pump and probe beams ranging in energy from the near infrared to the deep ultraviolet. We show exemplary measurements on the melting process in charge order Manganites (La$_{0.5}$Ca$_{0.5}$MnO$_3$) as well as studies on the temporal evolution of the superconducting parameter in high-T$_c$ cuprates.\newline [1] B. Schulz {\it et al}, Rev. Sci. Instrum. {\bf 76}, 73107 (2005). [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X36.00005: The Physics of Coherent Anti-Stokes Raman Spectroscopy Laszlo Ujj Coherent anti-Stokes Raman Spectroscopy (CARS) has been shown to be one of the most powerful experimental methodologies for obtaining vibrational information from both stable and transient molecular species. The general theoretical and experimental principles associated with CARS, together with specific examples of its applications to important molecular systems, are presented. A concise theoretical formalism of CARS, and its electronically resonant variant, is presented with an emphasis on points relevant to the interpretation of experimental spectra. The instrumental components required for obtaining typical CARS data are illustrated in a brief description of a picoseconds and a nanosecond experimental system used to record CARS spectra. The details of the design and operation of an all solid-state broadband nanosecond CARS system will be presented. Specific examples of picosecond CARS data are taken from measurements of stable and transient intermediates comprising the photoreactions of photoactive proteins. Polarization sensitive CARS spectra taken with the nanosecond system are also presented. Finally, an overview of developments in this field to be anticipated is discussed. Reference: Laszlo Ujj and George H. Atkinson, Coherent Anti-Stokes Raman Spectroscopy, in Handbook of Vibrational Spectroscopy, John Wiley and Sons., Ltd., 2002. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X36.00006: High quality optical and mechanical properties of a dispersive optomechanical device Andrew Jayich, Benjamin Zwickl, Cheng Yang, Jack Harris We have characterized the optical and mechanical properties of a high finesse Fabry Perot cavity (F=17,000) dispersively coupled to a micromechanical SiN$_{x}$ membrane. The membrane's fundamental and higher-order vibrational modes show quality factors above 10\^{}6 and a spectrum corresponding to a simple drumhead model. The optical cavity's transverse and longitudinal spectrum is also in agreement with theory. We find that the cavity's full transmission and reflection properties can be modeled quantitatively once the membrane's small optical loss is accounted for. We will discuss the role of avoided crossings between higher-order optical modes in this system, particularly within the context of potential QND measurements of micromechanical devices. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X36.00007: Using Multiple FROG Traces to Generate a Single High Dynamic Range Trace Sebastian W. Winkler, William M. Dennis Frequency resolved optical gating (FROG) is a powerful technique that has been used to characterize the complex electromagnetic field of ultrafast pulses for more than a decade. However, FROG relies on detector with a high dynamic range; at least $10^4$. If the detector's dynamic range is insufficient, the FROG phase retrieval code will not work properly. We describe a method to generate a single FROG trace with high dynamic range from multiple FROG traces of the same pulse but using different integration times. We present successfully retrieved fields from data captured using a spectrometer that would otherwise have insufficient dynamic range for FROG trace acquisition. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X36.00008: Theoretical Considerations for Surface Thermal Lensing Studies Using Polarized Light Marshall Thomsen Surface Thermal Lensing (STL) is a well-established photothermal technique for locally probing the optical, thermal, and mechanical properties of a surface. A periodically chopped laser beam, the pump beam, irradiates a sample. A portion of the pump beam is absorbed by the sample and converted into thermal energy, resulting in a periodic local thermal expansion of the surface. A second, weaker and broader laser beam, the probe beam, is directed off- normal at the surface, covering the entire heated area. The result is a modulated diffraction pattern embedded in the reflected portion of the probe beam. The addition of polarizers both between the probe laser and the sample and between the sample and the detector opens up the possibility of gaining further information about the surface. In particular, the repeated flexing of the surface of a polymer sample may give rise to local realignment of polymer chains. The resulting asymmetry may be visible through a polarization analysis of STL signals. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X36.00009: Intra-valance transitions for uncooled short wave infrared detection A.G. Unil Perera, S.G. Matsik, P.V.V. Jayaweera, H.C. Liu, M. Buchanan An infrared detector based on p-GaAs/AlGaAs heterojunction exhibiting response in the 2-5 $\mu $m range at room temperature is demonstrated. The basic principle of the detector utilizes inter-valance (heavy hole, light hole, and split-off hole) absorption of a highly p-doped GaAs layer (emitter). The dark current is limited by the work function at the interface between the highly doped emitter and the undoped Al$_{x}$Ga$_{1-x}$As barrier. The barrier height can be tailored by varying the Al fraction to obtained the desired operating temperature. The split-off energy of the material determines the split-off threshold and the band offset determines the free carrier threshold for the photo excited carriers. Detector performance can be controlled by varying these two thresholds. A device consisting of 30 periods of 3$\times $10$^{18}$ cm$^{-3}$ p-doped GaAs emitter and Al$_{0.57}$Ga$_{0.43}$As barrier regions between two contact layers shows infrared detection up to 330 K with a peak responsivity of 1.4 A/W and D* of 2.6$\times $10$^{9}$ Jones at 2.5 $\mu $m . Different materials should give rise to different wavelength threshold infrared detectors operating at high temperatures. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X36.00010: Room Temperature Terahertz Detection based on Electron Plasma Resonance in an Antenna-Coupled Gallium Arsenide MESFET Sangwoo Kim, Jeramy Zimmerman, Paolo Focardi, Dong Ho Wu, Arthur C. Gossard, Mark S. Sherwin Terahertz detectors utilizing quantum transitions require cryogens since the thermal energy (kT) needs to be smaller than the transition energy (1 THz $\sim $ 4 meV). A bulk 3-D plasmon is a classical excitation, and hence does not saturate with temperature. Plasma absorptions occur at a density-dependent frequency $1/2\pi \sqrt {n_{3D} e^2/m\varepsilon } $. For 1 THz radiation, the corresponding 3-D free electron density is n$_{3D}$ $\sim $ 10$^{16}$ cm$^{-3}$ in GaAs, a density that can be easily achieved. The density of electrons can be made tunable if a device such as a Field Effect Transistor is employed. We utilize these facts in order to realize a room temperature Terahertz detector. Our device consists of twin-slot antennas, coplanar waveguides, and a GaAs Metal-Semiconductor-Field-Effect-Transistor (MESFET). While the sensitivity of the first set of devices is not competitive, we were able to observe the resonance behavior by sweeping bias voltages. This talk will present design, fabrication, recent measurement, and possible future improvement of our detector. Work supported by NSF-DMR 0703925 and Naval Research Lab. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X36.00011: Signal Restoration from Atmospheric Degradation in Terahertz Spectroscopy Dong Ho Wu, Seong Kong We presents a method of restoring signals in Terahertz (THz) spectroscopy by removing the distortion from the observed THz signals. The distortion is generated by the absorption and scattering of gas molecules and water vapor in the atmosphere, during the transmission of THz beams through the air from the source to the spectrometer. Such atmospheric degradation causes spurious spectral dips and peaks in the THz spectrum, which often obscure the spectral peaks specific to the material of interest. This fact makes it challenging to measure the THz spectroscopic signatures of objects at a distance in a humid air environment. A THz signal restoration filter based on an artificial neural network model can be very effective in removing noisy absorption peaks caused by atmospheric degradation in THz spectra. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X36.00012: Compressed Sensing and its Applications in Imaging Dharmpal Takhar, Ting Sun, Jason Laska, Marco Duarte, Richard Baraniuk, Kevin Kelly Compressed sensing is a new sampling theory which allows reconstructing signals using sub-Nyquist measurements/sampling. This can significantly reduce the computation required for image/video acquisition/encoding, at least at the sensor end. Compressed sensing works on the concept of sparsity of the signal in some known domain, which is incoherent with the measurement domain. We exploit this technique to build a single pixel camera based on an optical modulator and a single photosensor. Random projections of the signal (image) are taken by optical modulator, which has random matrix displayed on it, corresponding to the measurement domain (random noise). This randomly projected signal is collected on the photosensor and later used for reconstructing the signal. In this scheme we are making a tradeoff between the spatial extent of sampling array and a sequential sampling over time with a single detector. In addition to this method, we will also demonstrate a new design which overcomes this shortcoming by parallel collection of many random projections simultaneously. Applications of this technique in hyperspectral and infrared imaging will be discussed. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X36.00013: Test of search methods to extract quasi-periodical signals in noise from gravitational wave detectors. Giovanni Santostasi We have tested several techniques to extract quasi-periodical signals in a simulated advanced gravitational wave detector noise. These methods include chi-square , Zn-square , H-test and Bayesian methods. The efficiency and usefulness, in different contexts, of these methods is discussed. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X36.00014: High temperature resonant ultrasound spectroscopy methods Guangyan Li, Gary Lamberton, Josh Gladden Resonant ultrasound spectroscopy (RUS) is a technique to obtain the full elastic tensor of single crystal materials by measuring the mechanical resonances of a polished sample. Any direct resonance measurement at high temperatures is limited by the fact that most ultrasound transducers have an upper operational limit of 200-300C. High temperature RUS measurements are made possible by separating the sample, placed in a tube furnace, and the transducers with buffer rods made of low acoustic attenuation materials with good thermal stability such as ceramic alumina or fused quartz. Tests on stainless steel demonstrated that the system has the ability of acquiring resonance signals at temperatures up to 800C. Experimental issues such as additional resonance peaks introduced by the buffer rods and sample loading will be addressed. The apparatus has been used to study high temperature elastic properties of p-zintl thermoelectrics, single crystal quartz, a novel piezoelectric material kepertite, and the glass transition around 400C in bulk metallic glass compounds. Good results from these studies and high temperature test runs of aluminum and stainless steel demonstrate the potential for RUS measurements at elevated temperatures. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X36.00015: Development of a low cost, low temperature cryocooler using the Gifford McMahon cycle A. Ramanayaka, R. Mani Although Helium is the second most abundant element, its concentration in the earth's atmosphere is fairly low and constant, as the portion that escapes from the atmosphere is replace by new emission. Historically, Helium was extracted as a byproduct of natural gas production, and stored in gas fields in a National Helium Reserve, in an attempt to conserve this interesting element. National policy has changed and the cost of liquid Helium has increased rapidly in the recent past. These new circumstances have created new interest in alternative eco-friendly methods to realizing and maintaining low temperatures in the laboratory. There have been number of successful attempts at making low temperature closed cycle Helium refrigerators by modifying an existing closed cycle system, and usually the regenerator has been replaced in order to achieve the desired results. Here, we discus our attempt to fabricate a low cost, low temperature closed cycle Helium refrigerator starting from a 15K Gifford McMahon system. We reexamine the barriers to realizing lower temperature here and our attempts at overcoming them. [Preview Abstract] |
Session X37: Semiconductors IV: Electronic and Optical Properties II
Sponsoring Units: FIAPChair: Udo Pernisz, Dow Corning Corp.
Room: Morial Convention Center 229
Friday, March 14, 2008 8:00AM - 8:12AM |
X37.00001: Electronic Structure of Silicon Phases Resulting from Decompression from $\beta$-Sn Brad Malone, Jay Sau, Marvin Cohen We present an \textit{ab initio} study of the electronic structure of the silicon phases that result from decompression from the metallic $\beta$-Sn phase, namely the BC8 (Si III), the hexagonal diamond (Si IV), the R8 (Si XII), and the yet unobserved ST12 phases. To correct for the inadequacies in the DFT-LDA quasiparticle energy spectra we employ quasiparticle corrections with the framework of the GW approximation. In doing so we find that the R8 phase should be semiconducting at lower pressures We also analyze the effect of strain and doping on these materials in an attempt to find novel applications for these phases, from high-mobility semiconductors to superconductivity. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X37.00002: First principles calculations of optical and x-ray spectra from atomic coordinates alone J.J. Kas, M. Prange, F.D. Vila, Y. Takimoto, J.J. Rehr Theoretical calculations of various x-ray and optical spectroscopies often rely on semi-empirical or phenomenological models to account for many-body effects and thermal vibrations. Typically such models include a number of parameters which complicate fitting schemes that extract physical quantities from experimental spectra. Here we present an approach for {\it {ab initio} } calculations of these spectra starting from structure alone. A many-pole model of the dielectric function is introduced to calculate the self-energy and spectral function,\footnote{J. J. Kas et al., Phys. Rev. B {\bf76} 195116 (2007).} while a density functional theory calculation of the dynamical matrix is used to calculate effects of thermal vibrations. \footnote{Fernando D. Vila et al., Phys. Rev. B {\bf 76}, 014301 (2007).} In addition, core-hole effects are incorporated with RPA screening. \footnote{A. L. Ankudinov et al., Phys. Rev. B {\bf 71}, 165110 (2005).} This approach has been incorporated into FEFF9, a new version of the real-space multiple-scattering FEFF code for broad spectrum calculations of various optical and x-ray spectra. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X37.00003: Surface states of the Topological Insulator Bi$_{1-x}$ Sb$_x$ Jeffrey Teo, Liang Fu, Charles Kane The alloy Bi$_{1-x}$Sb$_x$ is a narrow gap semiconductor for
$.07 |
Friday, March 14, 2008 8:36AM - 8:48AM |
X37.00004: Optical, Electronic and Structural Properties of Y$_{x}$Sc$_{1-x}$N Robert Bruce vanDover, John Gregoire, Steve Kirby Semiconducting Y$_{x}$Sc$_{1-x}$N thin films are reactively sputtered onto a variety of surfaces, resulting in films varying from nanocrystalline to epitaxial (single crystalline). As a function of composition, we investigate the variations in the crystalline lattice, Hall mobility, absorptivity and direct and indirect band gaps. We find this material to be a solid solution semiconductor across the entire composition range. The tunable band gap and high absorptivity of this semiconductor make it an interesting photovoltaic material. Details on this application will be presented as well as prospects of this material's use as the host for ferromagnetic transition metals. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X37.00005: Free-electron induced mode mixing and surface-polariton enhanced reflected THz-field Danhong Huang, Godfrey Gumbs, Plau Alsing, Dave Cardimona The main result of this talk is the prediction of large enhancements at the band edges of a coupled Bloch-surface-plasmon-polariton band in the spectrum of the reflected far electromagnetic field due to anti-crossing gaps induced by the strong coupling between the continuous surface-plasmon-polariton mode and the discrete Bloch-like modes. The existence of these Bloch-like modes is a direct consequence of the nonlocal mixing of specular and diffraction modes of the reflected electromagnetic field by free-electron induced optical polarization and the interference of a pair of surface optical-polarization waves with opposite Bragg order numbers in the presence of a grating. The interference of these two counter-propagating surface waves leads to the formation of a Wannier-like state with associated electromagnetic fields localized within the grating-gap regions. The effects of the sheet density, grating period and absorption loss on the optical enhancements of both transmitted and reflected electromagnetic fields are investigated. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X37.00006: Absorption and Photoluminescence Analysis of InAs/GaSb Superlattices Exhibiting Lateral Composition Modulation Julia Wickett, Jianhua Li, Donna Stokes The effects of lateral composition modulation (LCM) in (InAs)$_{13}$/(GaSb)$_{13}$ superlattices on the structure and optical response of the material have been investigated by double crystal x-ray diffraction (XRD), infrared absorption and photoluminescence (PL). Superlattices (SL) were grown by molecular beam epitaxy (MBE) on GaSb (001) substrates with InSb interfacial bonds. Various buffer/substrate combinations were employed to determine if strain manipulation could be used to improve the optical response of the system. Modeling of XRD data has been used to determine the strain state of the SL layers with respect to the growth template. Absorption and PL measurements indicate that strain significantly affects the optical responses of the samples and manipulation and control over the strain state of the system will be key in employing LCM superlattices for optical applications. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X37.00007: Probing ErAs nanoparticle density of states using capacitance-voltage Kasey Russell, Venkatesh Narayanamurti, Joshua Zide, Arthur Gossard Two asymmetric In$_{0.53}$Ga$_{0.47}$As/In$_{0.53}$Al$_{0.47}$As double-barrier samples are fabricated and compared using low-temperature capacitance-voltage measurements. The two samples are identical except for a layer of ErAs nanoparticles embedded within the quantum well layer of one of the samples. A clear difference in the capacitance-voltage profile is observed between the two samples, and the difference is attributed to additional available states associated with the ErAs nanoparticles. These results are compared with a charge-step simulation of the low-frequency capacitance of the device in order to estimate the density of states contributed by the ErAs nanoparticles. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X37.00008: Photon-like plasmon polariton in a nanocoaxial waveguide Yun Peng, Xiwen Wang, Krzysztof Kempa We study propagation of electromagnetic waves in a nano-coaxial waveguide for frequencies below the surface plasmon frequency. We show, that for sufficently low frequencies, the waveguide supports a plasmon polariton mode that resembles, and indeed reduces to the conventional TEM mode of the conventional coax, known in the radiotechnology. We consider also coupling of this mode to the external radiation, and show that it can be made very efficient with appropriate antenna-like arrangements. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X37.00009: X-ray Absorption Study of Amorphous Metal Semiconductor Alloys $M_x Si_{1-x} $ (M: Gd,Y) Near the Metal Insulator Transition Erik Helgren, F. Hellman, Li Zeng, J.W. Freeland, P. Ryan, D. Haskel, R. Winarski, M. van Veenendaal, R. Wu X-ray absorption structure (XAS) at both Si K edges and Gd M edges were measured at compositions close to the metal insulator transition (MIT) for amorphous $Gd_x Si_{1-x} $ (x = 0.11 - 0.21) and $Y_x Si_{1-x} $ (x=0.13) from 10-300K. Spectral lineshape is unchanged as a function of composition, despite the presence of the MIT at x = 0.14. Comparison with calculations indicates that Gd is in the 3+ state for all compositions and temperatures measured. An anomalous temperature dependent absorption is seen below approximately 70K; the energies of the absorption peaks are unaffected, indicating no change in valence, but the absolute magnitude of absorption is temperature dependent for both K and M edges, up to 40 eV from the edges. This temperature dependence is related to changes in the nature of the conduction band states, specifically a transfer of weight from Si p-states to more localized Gd p-states. However similar shifts in the magnitude of the Si K edge are found in the non-magnetic analog system $Y_x Si_{1-x} $. Thus this transfer cannot be solely related to the magnetically-dependent localization phenomena previously observed in $Gd_x Si_{1-x} $, and we argue that it is related to electronic correlation effects present in both systems. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X37.00010: Eu L3-edge Resonant Inelastic X-ray Scattering on EuB6 Jungho Kim, D. Ellis, B.K. Cho, E.J. Choi, A. Said, Y. Shvydko, Young-June Kim We present the frst Eu L$_{3}$-edge resonant inelastic x-ray scattering (RIXS) spectra on the cubic EuB$_{6}$ using vertical and horizontal scattering geometries. EuB$_{6}$ is an undistorted cubic compound being free from complications like lattice distortion. The valence of Eu is 2$^{+}$ (4$f^{7})$. Electronic states near the Fermi edge are formed by the hybridization of Eu 5$d_{x2-y2 }$and B 2$p$. According to optical spectroscopy studies, a number of excitations exist over a wide energy range up to 40 eV. In particular, optical spectrum below 10 eV consists of a number of narrow strong peaks. These peaks were assigned to 2$p$-5$d$ charge transfer or 4$f$-5$d$ on-site excitations. In the current Eu L$_{3}$-edge RIXS experiment, we found that resonant enhancements occur both near and above the Eu L$_{3}$ absorption peak. In both resonance features, we identify a number of narrow energy loss features and compare those features with the optical spectroscopy spectrum. Interestingly, we observed a drastic dependence on measurement geometry in a resonant enhancement above the Eu L3 absorption peak. We discuss the excitation symmetry based on the geometry dependence. This study demonstrates the utility of the rare-earth L$_{3}$-edge RIXS as a probe of excited states in rare-earth systems. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X37.00011: Effect of Dielectric Response on the Magic Angle Mystery in EELS Adam Sorini, John Rehr, Zachary Levine The ``magic'' collection angle in electron energy-loss spectroscopy (EELS) is that angle at which the spectra from an anisotropic sample ``magically'' becomes orientation independent. The ``myster''" is that non-relativistic theory predicts a magic angle typically a factor of two too large in modern EELS experiments. Recently it has been shown that a relativistic treatment largely explains the discrepancy [1]. Here, we suggest that the dielectric response of the sample can lead to still larger magic angle corrections for low energy-loss spectra [2]. These dielectric effects are included in a relativistic, independent particle theory using the generalized Lorentz gauge. The effect is illustrated by a calculation of the magic angle including both relativistic and first principles dielectric corrections for graphite and for boron nitride. \newline [1] P. Schattschneider, C. Hebert, H. Franco, and B. Jouffrey, Phys. Rev. B, 72, 045142 (2005) \newline [2] A. P. Sorini, J. J. Rehr, and Z. H. Levine (UW Preprint, Nov. 2007) [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X37.00012: Wave-front engineering by Huygens-Fresnel principle for nonlinear optical interactions in domain engineered structures Zhu Yongyuan The wave-front engineering for nonlinear optical interactions was discussed. Using Huygens-Fresnel principle we developed a general theory and technique for domain engineering with conventional quasi-phase-matching (QPM) structures being the special cases. We put forward the concept of local QPM, which suggests that the QPM is fulfilled only locally not globally. Experiments agreed well with the theoretical prediction. The proposed scheme integrates three optical functions: generating, focusing and beam splitting of second-harmonic wave, thus making the device more compact. This work was supported by the State Key Program for Basic Research of China (Grant No. 2004CB619003) and also by the NNSF of China (Grant Nos.10523001, 10504013 and 10674065). [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X37.00013: Influence of Phonon dimensionality on Electron Energy Relaxation Ilari Maasilta, Jenni Karvonen We studied experimentally the role of phonon dimensionality on electron-phonon (e-p) interaction in thin copper wires evaporated either on suspended silicon nitride membranes or on bulk substrates, at sub-Kelvin temperatures. The power emitted from electrons to phonons was measured using sensitive normal metal-insulator-superconductor (NIS) tunnel junction thermometers. Membrane thicknesses ranging from 30 nm to 750 nm were used to clearly see the onset of the effects of two-dimensional (2D) phonon system. We observed for the first time that a 2D phonon spectrum clearly changes the temperature dependence and strength of the e-p scattering rate, with the interaction becoming stronger at the lowest temperatures below $\sim$ 0.5 K for the 30 nm membranes\footnote{J. T. Karvonen and I. J. Maasilta, Phys. Rev. Lett. {\bf 99}, 145503 (2007).}. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X37.00014: Hartree-Fock-Cluster Investigation of Nuclear Quadrupole Interactions in Solid Chalcogens, Selenium and Tellurium. M.M. Aryal, N.B. Maharjan, D.D. Paudyal, D.R. Mishra, S.R. Byahut, R.H. Scheicher, S.R. Badu, J. Jeong, Lee Chow, T.P. Das Using the first-principles Hartree-Fock Cluster Procedure, we have studied the electronic structures of pure chain like Selenium and Tellurium, pure ring structured Selenium, Tellurium impurity in chain and ring-structured Selenium and Selenium impurity in chain-structured Tellurium chain. For our investigations in all the systems we have carried out convergence studies with respect to variational basis set sizes,sizes of clusters and electron correlation effects using many-body perturbation theory. Using our calculated electronic field-gradient parameters q in the pure chain systems and employing the experimental quadrupole coupling constants (e$^{2}$qQ), the values Q($^{77}$Se)=(0.50$\pm $0.04) 10$^{-28}$ m$^{2}$ and Q($^{125}$Te)=-(0.2$\pm $0.02) 10$^{-28}$m$^{2}$. Results will also be presented for the asymmetry parameters $\eta $ for the pure chain systems and the e$^{2}$qQ and $\eta $ for $^{77}$Se in selenium ring. Our calculated values for e$^{2}$qQ and $\eta $ for the impurity systems will also be presented and compared with available experimental data and earlier theoretical results. [Preview Abstract] |
Session X39: Miscellaneous Topics in Statistical Physics
Sponsoring Units: GSNPChair: Bill Klein, Boston University
Room: Morial Convention Center 231
Friday, March 14, 2008 8:00AM - 8:12AM |
X39.00001: Enhancement of epidemic extinction by random vaccination Ira Schwartz, Mark Dykman We study the probability of epidemic extinction in large populations. We use the susceptible-infected-susceptible (SIS) model since it forms the foundation of many epidemic processes. Fluctuations in the SIS system have two sources. The major source is the randomness of the ``reactions'' in which the number of susceptibles and/or infected changes. In addition, we assume that vaccination is done at random, leading to the decrease of the number of susceptibles. The vaccination is modeled by a Poisson process. The probability distribution is found from the master equation, which is solved in the eikonal approximation. It is shown that, even in the absence of vaccination, the logarithm of the extinction rate displays scaling dependence on the parameters. It scales as the square of the distance to the parameter value where the average number of infected vanishes. This is very different from the familiar 3/2 scaling law for saddle-node bifurcations. Finally, we show that even weak vaccination can dramatically increase the extinction probability. The correction to the logarithm of the probability becomes exponential in the vaccination rate when this rate is not too small. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X39.00002: Inhomogeneous Coupling in 2-Channel Asymmetric Simple Exclusion Processes Konstantinos Tsekouras, Anatoly Kolomeisky Asymmetric exclusion processes for particles moving on parallel channels with inhomogeneous coupling are investigated theoretically. Particles interact with hard-core exclusion and move in the same direction on both lattices, while transitions between the channels is allowed at one specific location in the bulk of the system. An approximate theoretical approach describing the dynamics in the vertical link and horizontal lattice segments exactly but neglecting the correlation between horizontal and vertical transport is developed. It allows us to calculate stationary phase diagrams, particle currents and densities for symmetric and asymmetric transitions between the channels. It is shown that in the case of the symmetric coupling there are three stationary phases, similarly to the case of single-channel totally asymmetric exclusion processes with local inhomogeneity. However, the asymmetric coupling between the lattices lead to a very complex phase diagram with ten stationary-state regimes. Extensive Monte Carlo computer simulations generally support theoretical predictions, although simulated stationary-state properties slightly deviate from calculated in the mean-field approximation, suggesting the importance of correlations in the system. Dynamic properties and phase diagrams are discussed by analyzing constraints on the particle currents across the channels. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X39.00003: Brownian Gas: a field theory with a Poissonian ground state Andrea Velenich, Claudio Chamon, Dirk Kreimer As a first step towards a satisfying field theory of Brownian particles in interaction, we study exactly the non-interacting case, its combinatorics and its non-linear time-reversal symmetry. The field theory is nevertheless interacting: the vertex is the hallmark of the original particle nature of the gas and enforces the constraint of a strictly positive density field as opposed to a Gaussian free field. We compute exactly all the n-point density correlation functions, determine non-perturbatively the Poissonian nature of the ground state and emphasize the futility of any coarse-graining assumption for the derivation of the field theory. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X39.00004: Wave electrophoretic trapping and chaos Boyd Edwards, Lloyd Carroll, Aaron Timperman, Jarrod Schiffbauer, Jon Mease Synchronized oscillating electric potentials are applied to a periodic array of stationary cylindrical electrodes in a stationary conducting viscous fluid. These potentials produce a longitudinal traveling wave that traps high-mobility ions and partially traps intermediate-mobility ions in periodic and narrowband chaotic attractors with average velocities that are commensurate with the wave speed. Incommensurate broadband chaotic attractors feature ascending and descending geometric series of orbit transitions that converge at the same unstable trapped orbit. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X39.00005: Experimental Study of Energy Localization in a Nonlinear Electrical Lattice Lars English, Ritoban Basu Thakur, Ryan Stearrett Experimental results are presented that reveal the formation of intrinsic localized modes (ILMs) in a nonlinear electrical lattice. The lattice studied is a discrete transmission line consisting of two inductors and a diode (with voltage-dependent capacitance) per unit cell, and it forms a ring. We show that when this ring circuit is driven uniformly at large amplitudes, an instability of the uniform mode leads to the appearance of sharply localized features. Under certain driving conditions, these ILMs can become locked to the driver. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X39.00006: Multiple traveling solitons in one-dimensional monatomic quartic lattices Sanghamitra Neogi, Gerald Mahan We discuss the generation of traveling soliton waves in a one- dimensional monatomic quartic lattice using numerical techniques. We apply an external forcing function or a pulse to the end atom of a free chain of monatomic atoms to generate traveling solitons. When the strength of the forcing function is above a threshold value, multiple traveling solitons are observed to flow down the chain. The number of traveling solitons in the chain increases rapidly with the increase in pulse strength beyond this critical value. The amplitudes and velocities of these multiple solitons increase with the increase in pulse strength for small pulse strengths. For larger values of pulse strengths, the amplitudes and velocities of all the multiple solitons saturate. The frequencies and wave vectors of all the traveling solitons on the quartic lattice are within a very narrow range of values. These values are independent of the pulse strength. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X39.00007: Statistical mechanics rooted in maximum entropy method shows absence of the Gibbs paradox Chih-Yuan Tseng Studying the Gibbs paradox problem indicates Gibbs's statistical mechanics may not be a general theory for thermodynamics. We found that most of conventional resolutions only provide explanations for ``supplementary'' introduction of the Gibbs correction rather than re-develop statistical theory to comprehend corresponding aspects. In this talk, we will show a general theory of statistical mechanics based on generalized maximum entropy method, which is designed for inductive inference. The general theory integrates prior knowledge of the system and measurements of macroscopic properties into a general ensemble. Based on the general theory, the 1/N! is simply a prior distribution that denotes our prior knowledge for indistinguishability of N microstates instead of the correction for the canonical ensemble. There is no Gibbs paradox. It is simply a consequence of incomplete statistical description for classical thermodynamic systems. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X39.00008: High-Precision Thermodynamic and Critical Properties from Tensor Renormalization-Group Flows Michael Hinczewski, A. Nihat Berker The recently developed tensor renormalization-group (TRG) method [1] provides a highly precise technique for deriving thermodynamic and critical properties of lattice Hamiltonians. The TRG is a local coarse-graining transformation, with the elements of the tensor at each lattice site playing the part of the interactions that undergo the renormalization-group flows. These tensor flows are directly related [2] to the phase diagram structure of the infinite system, with each phase flowing to a distinct surface of fixed points. Fixed-point analysis and summation along the flows give the critical exponents, as well as thermodynamic functions along the entire temperature range. Thus, for the ferromagnetic triangular lattice Ising model, the free energy is calculated to better than $10^{-5}$ along the entire temperature range. Unlike previous position-space renormalization-group methods, the truncation (of the tensor index range $D$) in this general method converges under straightforward and systematic improvements. Our best results are easily obtained with $D=24$, corresponding to 4624-dimensional renormalization-group flows. [1] M. Levin and C.P. Nave, Phys. Rev. Lett. 99, 120601 (2007). [2] M. Hinczewski and A.N. Berker, arXiv:0709.2803v1 [cond-mat.stat-mech], Phys. Rev. E, in press. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X39.00009: Nucleation in a long-range repulsive model. Kipton Barros, William Klein We employ a model with long-range repulsive interactions to study nucleation from a fluid to a crystalline phase. The long-range interactions make the metastable fluid phase long lived. From our simulations we find a divergence of the susceptibility at the spinodal, as predicted by mean-field theory. We also observe nucleation events and verify that the nucleating droplets, when they occur, match the numerical saddle point solutions of the free energy functional. In one dimension, where Alexander-McTague type symmetry arguments cannot be satisfied, we find that nucleation does not occur and that the fluid-solid transition is continuous. In two and three dimensions the transition is first order, with nucleating droplets of hexagonal and bcc symmetries. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X39.00010: Phase transitions in the long-range antiferromagnetic Ising model. Rachele Dominguez, Kipton Barros, William Klein We study the long-range antiferromagnetic Ising model to understand microphase separation in block-copolymer systems and dipolar Ising models. When quenched to low temperatures, the system initially obeys a Cahn-Hilliard-Cook-like linear theory and then orders into a ``stripe" or ``clump" phase. The system also exhibits order-to-order transitions between the stable stripe and clump phases. From the free energy density obtained from coarse graining the Ising model, we use Langevin dynamics to investigate these transitions. I will discuss the nature of the transitions into the ordered phases as well as computational and theoretical methods for obtaining the phase diagram of the system. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X39.00011: Emergent Structures in Dissipative Wave-Particle Systems Davit Sivil, Alfred Hubler We study the motion of a particle with mass m on a vibrated string of length L. We assume that there is a friction force between the particle and the string. The string is sinusoidally forced at both ends. We find that the particle has attractors located at x=L/2 - $n\pi c/2 \omega$, where $\omega$ is the frequency of the waves on the string, and $n \in {\boldmath Z}$. We also study the attractors of the same system with multiple driving frequencies. We also compared our results with numerical simulations. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X39.00012: Absence of structural glass transition in a monoatomic model liquid predicted to undergo an ideal glass transition Matthias Troyer, Charlotte Gils, Helmut Katzgraber We study numerically a monodisperse model of interacting classical particles predicted to exhibit a static liquid-glass transition. Using a dynamical Monte Carlo method we show that the model does not freeze into a glassy phase at low temperatures. Instead, depending on the choice of the hard-core radius for the particles, the system either collapses trivially or a polycrystalline hexagonal structure emerges. [Preview Abstract] |
Session X40: Self Assembled Protein Cages
Sponsoring Units: DBPChair: William Klug, University of California, Los Angeles
Room: Morial Convention Center 232
Friday, March 14, 2008 8:00AM - 8:12AM |
X40.00001: A minimal model for protein coat dynamics in intracellular vesicular transport Ranjan Mukhopadhyay, Hui Wang, Greg Huber Within eukaryotic cells, proteins are transported by vesicles formed from coated regions of membranes. The assembly of coat proteins deforms the membrane patch and drives vesicle formation. Once the vesicle has pinched off, the protein coat rapidly disassembles. Motivated by recent experimental results, we propose a minimal model for the dynamics of coat assembly and disassembly and study the spatio-temporal behavior of the system. We will show that for a range of parameters, our model can robustly generate a steady state distribution of protein clusters with characteristic sizes and will obtain the scaling behavior of average cluster size with the parameters of the model. We will also discuss the coupling of coat dynamics to sorting of cargo proteins. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X40.00002: The study of viral assembly with fluorescence fluctuation spectroscopy Joachim Mueller, Bin Wu, Yan Chen Enveloped viruses contain an encapsulating membrane that the virus acquires from the host cell during the budding process. The presence of the enveloping lipid membrane complicates the physical characterization of the proteins assembled within the virus considerably. Here we present a method based on fluorescence fluctuations that quantifies the copy number of proteins within an enveloped viral particles. We choose the viral protein Gag of the human immunodeficiency virus (HIV) type 1 as a model system, because Gag expressed in cells is sufficient to produce viral-like particles (VLPs) of the same size as authentic virions. VLPs harvested from cells that express fluorescently labeled Gag were investigated by two-photon fluorescence fluctuation spectroscopy. The autocorrelation functions of the fluctuations revealed a hydrodynamic size of the fluorescent VLPs consistent with previous results based on electron microscopy. Further analysis of the fluctuations revealed a copy number of Gag per virion that is inconsistent with the prevailing model of HIV assembly. We will discuss the implications of our experimental results for the assembly process of VLPs. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X40.00003: Spherical Proteins and Viral Capsids Studied by Theory of Elasticity Zheng Yang, Ivet Bahar, Michael Widom Coarse-grained elastic network models have been successful in elucidating the fluctuation dynamics of proteins around their native conformations. It is well established that the low-frequency collective motions derived by simplified normal mode analysis depend on the overall 3-dimensional shape of the biomolecule. Given that the large scale collective motions are usually involved in biological function, our objective in this work is to gain more insights into large scale collective motions of spherical proteins and virus capsids by considering a continuous model with perfect spherical symmetry. To this end, we compare the global dynamics of proteins and the analytical solutions from an elastic wave equation with spherical boundary conditions. In addition, an icosahedral discrete model is generated and analyzed for validating our continuous model. Applications to lumazine synthase, satellite tobacco mosaic virus and other viruses shows that the spherical elastic model can efficiently provide insights on collective motions that are otherwise obtained by detailed elastic network models. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X40.00004: Low frequency mechanical modes of viruses with atomic detail Eric Dykeman, Otto Sankey The low frequency mechanical modes of viruses can provide important insights into the large global motions that a virus may exhibit. Recently it has been proposed that these large global motions may be excited using impulsive stimulated Raman scattering producing permanent damage to the virus. In order to understand the coupling of external probes to the capsid, vibrational modes with atomic detail are essential. The standard approach to find the atomic modes of a molecule with $N$ atoms requires the formation and diagonlization of a $3N\times 3N$ matrix. As viruses have $10^5$ or more atoms, the standard approach is difficult. Using ideas from electronic structure theory, we have developed a method to construct the mechanical modes of large molecules such as viruses with atomic detail. Application to viruses such as the cowpea chlorotic mottle virus, satellite tobacco necrosis virus, and M13 bacteriophage show a fairly complicated picture of the mechanical modes. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X40.00005: Diversity of in-vivo assembled HIV-1 capsids Se Il Lee, Toan Nguyen Understanding the capsid assembly process of Human Immunodeficiency Virus (HIV), the causative agent of Acute Immuno Deficiency Syndrom (AIDS), is very important because of recent intense interest in capsid-oriented viral therapy. The unique conical shapes of mature HIV-1 capsid have drawn significant interests in the biological community and started to attract attention from the physics community. Previous studies showed that in a free assembly process, the HIV-1 conical shape is not thermodynamically stable. However, if the volume of the capsid is constrained during assembly and the capsid protein shell has high spontaneous curvature, the conical shape is stable. In this work, we focus on in-vivo HIV-1 capsid assembly. For this case, the viral envelope membrane present during assembly imposes constraint on the length of the capsid. We use an elastic continuum shell theory to approximate the energies of various HIV-1 capsid shapes (spherical, cylindrical and conical). We show that for certain range of viral membrane diameter, the conical and cylindrical shapes are both thermodynamically stable. This result is supported by experimental observation that in-vivo assembled HIV-1 capsids are very heterogeneous in shapes and sizes. Numerical calculation is also performed to improve theoretical approximation. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X40.00006: An elastic model of partial budding of retroviruses Rui Zhang, Toan Nguyen Retroviruses are characterized by their unique infection strategy of reverse transcription, in which the genetic information flows from RNA back to DNA. The most well known representative is the human immunodeficiency virus (HIV). Unlike budding of traditional enveloped viruses, retrovirus budding happens together with the formation of spherical virus capsids at the cell membrane. Led by this unique budding mechanism, we proposed an elastic model of retrovirus budding in this work. We found that if the lipid molecules of the membrane are supplied fast enough from the cell interior, the budding always proceeds to completion. In the opposite limit, there is an optimal size of partially budded virions. The zenith angle of these partially spherical capsids, $\alpha$, is given by $\alpha\simeq(\tau^2/\kappa\sigma)^{1/4}$, where $\kappa$ is the bending modulus of the membrane, $\sigma$ is the surface tension of the membrane, and $\tau$ characterizes the strength of capsid protein interaction. If $\tau$ is large enough such that $\alpha\sim\pi$, the budding is complete. Our model explained many features of retrovirus partial budding observed in experiments. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X40.00007: Calibrating elastic parameters from molecular dynamics simulations of capsid proteins Stephen Hicks, Christopher Henley Virus capsids are modeled with elastic network models in which a handful of parameters determine transitions in assembly [1] and morphology [2]. We introduce an approach to compute these parameters from the microscopic structure of the proteins involved. We consider each protein as one or a few rigid bodies with very general interactions, which we parameterize by fitting the simulated equilibrium fluctuations (relative translations and rotations) of a pair of proteins (or fragments) to a 6-dimensional Gaussian. We can then compose these generalized springs into the global capsid structure to determine the continuum elastic parameters. We demonstrate our approach on HIV capsid protein and compare our results with the observed lattice structure (from cryo-EM [3] and AFM indentation studies).\\{} [1] R. Zandi et al, PNAS 101 (2004) 15556.\\{} [2] J. Lidmar, L. Mirny, and D. R. Nelson, PRE 68 (2003) 051910.\\{} [3] B. K. Ganser-Pornillos et al, Cell 131 (2007) 70. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X40.00008: Coarse-grained mechanics of viral shells William S. Klug, Melissa M. Gibbons We present an approach for creating three-dimensional finite element models of viral capsids from atomic-level structural data (X-ray or cryo-EM). The models capture heterogeneous geometric features and are used in conjunction with three-dimensional nonlinear continuum elasticity to simulate nanoindentation experiments as performed using atomic force microscopy. The method is extremely flexible; able to capture varying levels of detail in the three-dimensional structure. Nanoindentation simulations are presented for several viruses: Hepatitis B, CCMV, HK97, and $\phi$29. In addition to purely continuum elastic models a multiscale technique is developed that combines finite-element kinematics with MD energetics such that large-scale deformations are facilitated by a reduction in degrees of freedom. Simulations of these capsid deformation experiments provide a testing ground for the techniques, as well as insight into the strength-determining mechanisms of capsid deformation. These methods can be extended as a framework for modeling other proteins and macromolecular structures in cell biology. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X40.00009: ABSTRACT HAS BEEN MOVED TO SESSION C1 |
Friday, March 14, 2008 9:48AM - 10:00AM |
X40.00010: Biochemistry in the Nanopores Samir M. Iqbal, Bala Murali Venkatesan, Demir Akin, Rashid Bashir Solid-state technology is fast advancing novel nano-structures for biomolecular detection. The solid-state nanopores have emerged as potential replacement of the Sanger's method for DNA sequencing. While the passage of the DNA molecule through the nanopore has been reported extensively, little has been done to identify the individual base pairs or sequences within the molecule. Learning from the mechanics of ion-channels on the cell surface, we functionalized the solid-state nanopores to recognize and selectively regulate the flow of molecules though the pore. The probe DNA was immobilized by chemical adsorption, and target DNA was passed under electrophoretic bias. The single base mismatch selectivity was achieved by using a hairpin loop in the probe. We could thus identify between the perfect complementary and mismatched target molecules. We will expand on the theoretical framework that governs the interactions of the probe and target molecules, as observed from the pulse behavior. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X40.00011: Poisson pulsed control of particle escape Marie McCrary, Lora Billings, Ira Schwartz, Mark Dykman We consider the problem of escape in a double well potential. With a weak background Gaussian noise, the escape rate is well known and follows an exponential scaling with the noise intensity $D$. Here, we consider adding a small Poisson noise to the Gaussian noise. We compute the change in escape time as we add Poisson distributed pulses of a given duration and amplitude. The escape rate acquires an extra factor which is determined by the characteristic functional of the Poisson noise calculated for a function, which is determined by the system dynamics and is inversely proportional to $D$. As a result, for small $D$ even weak Poisson pulses can lead to a significant change of the escape rate. The Poisson noise induced factor depends sensitively on the interrelation between the noise correlation time and the relaxation time of the system. We compare analytical results with extensive numerical simulations. The numerical computation of escape rates for multiple interacting particles in a well will also be shown. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X40.00012: Quorum sensing and biofilm formation investigated using laser-trapped bacterial arrays Vernita Gordon, John Butler, Ivan Smalyukh, Matthew Parsek, Gerard Wong Studies of individual, free-swimming (planktonic) bacteria have yielded much information about their genetic and phenotypic characteristics and about ``quorum sensing,'' the autoinducing process by which bacteria detect high concentrations of other bacteria. However, in most environments the majority of bacteria are not in the planktonic form but are rather in biofilms, which are highly-structured, dynamic communities of multiple bacteria that adhere to a surface and to each other using an extracellular polysaccharide matrix. Bacteria in biofilms are phenotypically very different from their genetically-identical planktonic counterparts.~ Among other characteristics, they are much more antibiotic-resistant and virulent.~ Such biofilms form persistent infections on medical implants and in the lungs of cystic fibrosis patients, where Pseudomonas aeruginosa biofilms are the leading cause of lung damage and, ultimately, death.~ To understand the importance of different extracellular materials, motility mechanisms, and quorum sensing for biofilm formation and stability, we use single-gene knockout mutants and an infrared laser trap to create a bacterial aggregate that serves as a model biofilm and allows us to measure the importance of these factors as a function of trapping time, surface, and nutritional environment. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X40.00013: Self-Polarization of Cells in Elastic Gels Assaf Zemel, Samuel Safran The shape of a cell as well as the rigidity and geometry of its surroundings play an important role in vital cellular processes. The contractile activity of cells provides a generic means by which cells may sense and respond to mechanical features. The matrix stresses, that depend on the elasticity and geometry of cells, feedback on the cells and influence their activity. This suggests a mechanical mechanism by which cells control their shape and forces. We present a quantitative, mechanical model that predicts that cells in an elastic medium can self-polarize to form well ordered stress fibers. We focus on both single cells in a gel, as well as on an ensemble of cells that is confined to some region within the gel. While the \textit{magnitude} of the cellular forces is found to increase monotonically with the matrix rigidity the \textit{anisotropy} of the forces, and thus the ability of the cells to polarize, is predicted to depend non-monotonically on the medium's rigidity. We discuss these results with experimental findings and with the observation of an optimal medium elasticity for cell function and differentiation. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X40.00014: Active suspensions in shear flow A. Ahmadi, M.C. Marchetti, T.B. Liverpool We report on the structure and rheology of an active suspension of cytoskeletal filaments and motor proteins in shear flow. Hydrodynamics equations for an active suspension were derived earlier by us [arXiv:q-bio.CB/0703029v1] by coarse-graining the Smoluchowski equation for a model of filaments and motors. The model incorporates the coupling of orientational order to flow and accounts for the exchange of momentum between filaments and solvent. In the present study we investigate the role of active crosslinkers on the formation and stability of ordered states (polar and nematic) under external shear flow. We also study the effect of motor activity on the rheological behavior of the ordered states away from boundaries. This work may also be relevant for the understanding of the flow-driven reorientation of endothelial cells under the shear stress imposed by blood flow. [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X40.00015: Selective advantage for sexual replication with random haploid fusion Emmanuel Tannenbaum This talk develops a simplified set of models describing asexual and sexual replication in unicellular diploid organisms. The models assume organisms whose genomes consist of two chromosomes, where each chromosome is assumed to be functional if and only if it is equal to some master sequence. The fitness of an organism is determined by the number of functional chromosomes in its genome. For a population replicating asexually, a cell replicates both of its chromosomes, and then divides and splits its genetic material evenly between the two cells. For a population replicating sexually, a given cell first divides into two haploids, which enter a haploid pool. Within the haploid pool, haploids fuse into diploids, which then divide via the normal mitotic process. When the cost for sex is small, as measured by the ratio of the characteristic haploid fusion time to the characteristic growth time, we find that sexual replication with random haploid fusion leads to a greater mean fitness for the population than a purely asexual strategy. The results of this talk are consistent with previous studies suggesting that sex is favored at intermediate mutation rates, for slowly replicating organisms, and at high population densities. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700