Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S1: Diamond-based Quantum Information Processing
Sponsoring Units: DCMPChair: David Awschalom, University of California, Santa Barbara
Room: Morial Convention Center LaLouisiane AB
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S1.00001: Coherent control of single spins in diamond Invited Speaker: Diamond-based materials have recently emerged as a unique platform for quantum science and engineering\footnote{D.D. Awschalom, R.J. Epstein, and R. Hanson, Scientific American 297, 84 (2007).}. Spins of single Nitrogen-Vacancy (N-V) color centers in diamond can be imaged, initialized and read out optically. These N-V center spins may allow for quantum information processing at room temperature, as measurements have shown long room-temperature electron spin coherence times well into the microsecond regime. We have investigated single N-V center spins that are coupled to electron spins of nearby nitrogen (N) defects, using magneto- optical imaging and coherent single-spin control at room temperature. Some of the N-V centers are strongly coupled to only one single N spin, allowing the controlled polarization and readout of this single `dark' N spin. In contrast, other N- V centers couple to many N spins. We use these latter systems to study the canonical decoherence model of a single central spin coupled to a spin bath. By tuning the internal bath dynamics as well as the spin-bath coupling, we gain access to regimes with strikingly different behaviour. Finally, we have fabricated and characterized photonic crystal microcavities in poly-crystalline diamond and observed quality factors up to 600 \footnote{C.F. Wang, R. Hanson, D.D. Awschalom, E.L. Hu, T. Feygelson, J. Yang, J. E. Butler, Appl. Phys. Lett. 91, 201112 (2007).}. These structures are a first step towards controllable coupling of single N-V spins to single photons in a cavity-QED system in diamond. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S1.00002: Controlling individual electron and nuclear spins in diamond: from quantum registers to applications Invited Speaker: We will discuss our recent work involving the controlled manipulation of individual electron and nuclear spins in a high-purity diamond lattice. Our approach combines ideas from single molecule spectroscopy, quantum optical control techniques and the physics of mesoscopic spin ensembles. It allows us to isolate, polarize and manipulate single nuclear spins and use them to create quantum memory and small quantum registers with exceptional coherence properties, even under ambient room temperature conditions. We will also describe novel applications of these techniques, including new approaches to quantum communication and computation as well as new quantum magnetic sensors with nanoscale resolution. Recent progress towards realization of these ideas will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S1.00003: Fabrication Strategies for Practical Diamond Based Quantum Information Processing Devices Invited Speaker: Optically emitting defect centres in diamond display a range of unique quantum properties that offer exciting possibilities for the construction of quantum devices which employ optical single-spin read-out. Indeed diamond is an ideal material for use in the fabrication of (i) single photon sources for quantum communications, (ii) optical fibre-based single spin read out systems, (iii) photonic platforms for the investigation of quantum entanglement in solid state systems and (iv) optical regenerators and non-linear quantum gates. The toolkit of available fabrication strategies which are used to engineer devices taking advantage of these unique properties will be presented. Our most recent results include demonstrations of (i) optical fibre based single photon sources based on Nickel and Nitrogen optical centres, (ii) waveguiding of light in structures hewn from single crystal diamond, (iii) Electrical Stark shift of the frequency of single optical emitters, (iv) coupling between the spins between single NV and N atoms in devices engineered by ion implantation, and (v) electromagnetically induced transparency in single NV centres. These crucial demonstrations establish the feasibility of a defect tolerant architecture for the fabrication of a few ($\sim $10-50 ) qubit diamond based quantum information processor. We will present one such possible architecture and explain the specific role for ion beam processing in the creation of qubits and the engineering of diamond photonic devices. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S1.00004: Coherent Population Trapping of Single Spins in Diamond under Optical Excitation Invited Speaker: The nitrogen-vacancy (N-V) center in diamond has long-lived electronic and nuclear spin coherence combined with optical addressability, making it an attractive candidate system for building a photonic network for quantum information applications. However, realizing such schemes will require control over the N-V energy level structure and integration into high-quality microphotonic structures operating at visible wavelengths. In this talk I will describe experiments on optical manipulation of N-V centers in low-nitrogen diamond samples. Typically the optical transitions of NV$^{-}$ are spin-conserving, so that if the N-V begins in the m$_{s}$=0 ground state, it can undergo many optical excitation/fluorescence cycles before transitioning to m$_{s}=\pm $1. However, by applying stress to the crystal, or by using strain already present, it is possible to realize a $\Lambda $-type system with one excited state coupled by optical transitions to multiple ground states. By this technique we have observed coherent population trapping both in N-V ensembles and in single N-V centers. These results demonstrate the potential for all-optical spin manipulation in this system. I will also describe initial work on coupling N-V centers to photonic structures with the goal of enhancing emission into the zero-phonon line, as needed for applications such as quantum repeaters. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:30PM |
S1.00005: Controlling single defects: Electric and magnetic fields Invited Speaker: Controlling the optical properties of single defects, e.g. its transition frequency and transition properties is an important prerequisite for their broad application. It is shown that external electric and magnetic fields allow for control over a broad range of parameters. The optical transition frequency can e.g. controlled by an electric field. The same field also determines whether the defect is driven cyclic or lambda transition behaviour dominates. The talk will describe out current knowledge of the defect in this respect and show perspectives toward cavity coupling and entanglement of distant defects. [Preview Abstract] |
Session S2: Can Power Dissipation in a Switch Be Significantly Lowered?
Sponsoring Units: FIAPChair: Jeff Welser, SRC Nanoelectronics Research Initiative
Room: Morial Convention Center LaLouisiane C
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S2.00001: Will a New Milli-Volt Switch Replace the Transistor for Digital Applications? Invited Speaker: In contemplating the headlong rush toward miniaturization represented by Moore’s Law, it is tempting to think only of the progression toward molecular sized components. There is a second aspect of Moore’s Law, that is sometimes overlooked. Because of miniaturization, the energy efficiency of information processing steadily improves. We anticipate that the energy required to process a single bit of information will eventually become as tiny as 1 electron Volt per function, truly indeed a molecular sized energy. Inevitably most logic functions including storage, readout, and other logical manipulations will eventually be that efficient. However there is one information-processing-function that bucks this trend. That is communication, especially over short distances. Our best projections of improvements in the short distance communication function show that it will still require hundreds of thousand of electron Volts, just to move one bit of information the tiny distance of only 10 micro meters. Why this energy per bit discrepancy for communications? It is caused by the difference in voltage scale between the wires and the transistor switches. Transistors are thermally activated, leading to a characteristic voltage >>kT/q. Wires are long and they have a low impedance, allowing them to operate efficiently even at 1milli-Volt. The challenge then is to replace transistors with a new low-voltage switch , that is better matched to the wires. I will present some of the technical options for such a new switch. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S2.00002: Use of negative capacitance to provide voltage amplification for ultra low power nanoscale devices Invited Speaker: It is well known that conventional Field Effect Transistors (FET's) require a change in the channel potential of at least 60 mV at 300K to effect a change in the current by a factor of ten, and this minimum subthreshold slope S puts a \textit{fundamental lower limit} on the operating voltage and hence the power dissipation in standard FET based switches. Here we show that by replacing the standard insulator with a ferroelectric insulator of the right thickness it should be possible to implement a \textbf{\textit{step-up voltage transformer}} that will amplify the gate voltage thus leading to values of S lower than 60 mV/decade and enabling low voltage/low power operation. The voltage transformer action can be understood intuitively as the result of an effective negative capacitance provided by the ferroelectric capacitor which arises from an internal positive feedback that in principle could be obtained from other microscopic mechanisms as well. Unlike other proposals to reduce S this involves no change in the basic physics of the FET and thus does not affect its current drive or impose other restrictions. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S2.00003: Nanowire Impact Ionization FETs Invited Speaker: One limiting factor in the scaling of transistor technology is the room temperature limit of 60 mV/decade of the inverse sub-threshold slope. As supply- and threshold voltages are scaled down leakage currents rise exponentially causing the standby power of highly integrated circuits to suffer. New types of devices based on band-to-band tunneling [1] or impact ionization [2] have recently been demonstrated that can circumvent the 60 mV/decade limit thereby offering lower leakage currents. We have demonstrated vertical integration [3] of a single surround-gated silicon nanowire field-effect transistor (NW FET) having an inverse sub-threshold slope as low as 6 mV/decade at room temperature that spans four orders of magnitude in current [4]. The transistor shows slopes below 60 mV/decade for supply voltages above 2 V. Due to the use of a top Schottky contact and two ungated regions the devices show ambipolar characteristics with impact ionization for both electron and hole branch. The rather small voltages reduce hot carrier injection into the gate dielectric making threshold voltage shifts and degradation of the performance minimal. \newline [1] J. Appenzeller, et al., Phys. Rev. Lett. \textbf{93}, 196805 (2004). \newline [2] K. Gopalakrishnan, et al., IEDM Tech. Dig., 289 (2002). \newline [3] V. Schmidt et al., Small \textbf{2}, 85 (2006). \newline [4] M. T. Bj\"{o}rk \textit{et al.}, Appl. Phys. Lett. \textbf{90}, 142110 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S2.00004: Nanoelectromechanical switches Invited Speaker: Power dissipation is perhaps the most important problem confronting the electronics industry. To address this issue, we investigate vertical nanoelectromechanical (NEM) switches suitable for complementary logic, reconfigurable interconnects, and static power management. NEM switches have the following advantages: (i) Near elimination of source-drain static tunneling losses, (ii) Improved subthreshold characteristics [1]-- allowing lower operating voltage and hence lower dynamic power dissipation, (iii) Ability to run at much higher temperatures than Si-based CMOS. Our approach employs a carbon nanotube-based relay. We have prototyped this approach by inserting a tube into an etched gap between two contacts. Using a nanopositioner to align the tube, the prototype has demonstrated multiple switching at 5V. We will characterize this device and also integrated NEM switches. \newline [1] Ghosh, A. W., Rakshit, T. \& Datta, S. Gating of a molecular transistor: Electrostatic and Conformational. Nano Letters 4, 565-568 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:30PM |
S2.00005: One-dimensional semiconductors for low-power electronic applications Invited Speaker: Power dissipation is rapidly increasing from one to the next generation of silicon CMOS based chips. While following the ideal scaling rules should improve the performance without significantly increasing the power consumption, in particular the supply voltage has not been reduced in the past as required. Since the gate oxide thickness (SiO(N) in common CMOS applications) on the other hand has been decreased substantially, gate leakage currents have become a severe problem when the transistor is turned off. For logic applications, part of the applied voltage is used to switch the device from its off-state into its on-state while the other portion is used to drive the transistor into a regime of high transconductance. When asking the question about how to decrease the supply voltage to reduce power consumption of the device, both states have to be taken into account. Considering that currently more than 3 orders of magnitude current change are required to ensure proper circuit operation, already around 200mV of the supply voltage are used towards driving the transistor from the off-state to the on-state. This is true for charge-based devices that control current transport by means of a gate dependent barrier that can only be overcome by thermal emission. Those types of devices are characterized by an inverse subthreshold slope larger than around 60mV/dec. Altering the logic state by applying smaller voltages is highly desirable. My presentation will elucidate on the possibility of using band- to-band tunneling in carbon nanotubes as a viable approach to address both of the above aspects -- the transistor off- and on- state performance. I will use the example of carbon nanotube based devices to discuss various switching concepts in these low-dimensional geometries and to argue why a certain device structure should be favored over another. [Preview Abstract] |
Session S3: Aging, Shear and Rejuvenation: Mechanics of Glasses, Colloids and Granular Matter
Sponsoring Units: GSNPChair: Criag Maloney, Carnegie Mellon University
Room: Morial Convention Center RO2 - RO3
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S3.00001: Jamming: Relating Shear and Effective Temperature Invited Speaker: In an equilibrium system, temperature not only influences the average properties of a system, such as its pressure or density, but also controls the fluctuations around those averages. In systems driven far from equilibrium, however, temperature is no longer well-defined, and fluctuations can be non-thermal in origin. I will discuss a class of such systems, namely steadily-sheared glasses, for which there is a considerable body of evidence that the idea of an effective temperature is useful, at least in certain regimes. Using nonequilibrium molecular dynamics simulations, we have now calculated seven different definitions that yield a consistent value for the effective temperature, which can be many orders of magnitude higher than the bath temperature. However, if we want to understand the behavior of a material, measuring its temperature is only a start. I will discuss recent results that show that when shear-induced fluctuations dominate over thermal fluctuations, the effective temperature controls materials properties such as the rheology and the extreme slowing down of the dynamics as the system jams. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S3.00002: Simulations of shear banding in metallic glasses Invited Speaker: Metallic glasses represent a promising high strength material, but their use is limited by the onset of a shear banding instability when their material strength is exceeded. Recent simulation studies of the initiation and development of localized deformation in molecular dynamics simulations of a number of amorphous systems reveal the structural changes that accompany plastic deformation and localization involve a decrease in the local short range ordering. We have simulated both two-dimensional and three-dimensional systems in nanoindentation [1,2], uniaxial tension [3] and compression [4] in plane strain. The degree of strain localization depends sensitively on the quench rate during sample preparation, with localization only arising in more gradually quenched samples. A systematic analysis of simulated systems in simple shear geometries [5] reveals that a Boltzmann-like relationship between strain rate and structure holds over large variations in both the applied strain rate and the initial structural state of the glass. Scaling is observed over eight orders of magnitude in strain rate. The consequences of this scaling for constitutive models of glass plasticity will be discussed. \\ $[1]$ Y. Shi and M.L. Falk, ``Structural transformation and localization during simulated nanoindentation of a non-crystalline metal film,'' Applied Physics Letters, Vol. 86, pp. 011914 (2005). \\ $[2]$ Y. Shi and M.L. Falk, ``The structural origin of shear band formation in metallic glass studied via simulated nanoindentation,'' Acta Materialia , Vol. 55, pp. 4317 (2007). \\ $[3]$ Y. Shi and M.L. Falk, ``Strain localization and percolation of stable structure in amorphous solids,'' Physical Review Letters, Vol. 95, pp. 095502 (2005). \\ $[4]$ Y. Shi and M.L. Falk, ``Atomic-scale simulations of strain localization in three-dimensional model amorphous solids,'' Physical Review B, Vol. 73, pp. 214201 (2006). \\ $[5]$ Y. Shi, M.B. Katz, H. Li and M.L. Falk, ``Evaluation of the `disorder temperature' and `free volume' formalisms via simulations of shear banding in amorphous solids,'' Physical Review Letters, Vol. 98, 185505 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S3.00003: Single particle dynamics of aging in colloidal systems Invited Speaker: When a liquid is quenched to form a glass it becomes trapped in a non-equilibrium state, and many of the system's properties depend on the time elapsed since the quench. This phenomenon is known as aging. We study concentrated colloidal suspensions, a model system which has a glass transition when the particle concentration is increased. We use an optical confocal microscope to view the motion of these colloidal particles in three dimensions. Aging is most commonly detected by measuring the evolution of variables (such as the mean squared displacement) averaged over the entire system, but these quantities cannot yield information about the detailed, structural changes that occur during aging. In contrast, confocal microscopy lets us study the the relationship between local structure and the motion of the colloidal particles, in both monodisperse and binary colloidal glasses. We find that particle motion occurs in cooperative groups, and that this motion is facilitated by the relatively poor packing of the particles in these regions. Work done with G. C. Cianci, J. M. Lynch, and R. E. Courtland. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S3.00004: Aging dynamics and the mechanical behavior of glassy solids Invited Speaker: Although aging is observed in most glassy materials, a molecular level understanding of the underlying microscopic processes is incomplete. The intrinsic slow relaxation dynamics in glasses can cause intriguing phenomena: aging changes the mechanical properties of the material, but deformation and yielding also modify aging. This talk will present a series of molecular simulations that provide new insight into the complex, microscopic origins of the elastoplastic behavior of amorphous matter and its relationship to the macroscopic material response. In particular, we investigate the interplay between aging and plastic deformation in coarse-grained models for polymeric and metallic glasses. Molecular dynamics simulations are used to determine the macroscopic shear yield stress as well as the compliance of the model glasses for different loading conditions, temperatures, strain rates and aging times, as well as reveal their relationship to the underlying microscopic distribution of relaxation times. As in experiments on polymer glasses, we find that large stresses can decrease relaxation times and cause mechanical rejuvenation. Furthermore, we find new behavior when the aging glass undergoes more involved thermal protocols such as a temperature step. Phenomenological models will be developed that describe the data over a wide range of temperature, stress and strain rates. We also discuss how continuum models such as energy landscape pictures and the recently formulated shear transformation zone (STZ) theory of amorphous plasticity can account for the aging effects observed in the molecular simulations. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:30PM |
S3.00005: Faults \& Earthquakes as Granular Phenomena: Controls on Stick-Slip Dynamics Invited Speaker: Granular and continuous materials fail in fundamentally different ways, yet inherently discontinuous natural fault materials have often been modeled as continuum processes. Within a sheared or compressed granular material, the internal stresses take the form of a network of force chains. This network of strong connections among the particles is observed to be highly heterogeneous, and the magnitude of the stress varies widely over short distances. I will present the results of laboratory experiments which highlight the granular conrols on earthquake fault behavior. We perform experiments in a quasi-two-dimensional shear zone containing several thousand 5 mm circular and elliptical photoelastic plastic disks, allowing us to monitor the spatiotemporal evolution of both internal stress and strain. While the time, length, and strength scales are vastly different from the natural case, the frictional behavior is found to be in agreement. Therefore, the experiments allow us to isolate the effects of granular interactions and choice of boundary conditions on the fault behavior, through the observation of large populations of stick-slip and creep events. [Preview Abstract] |
Session S4: Keithley Award Symposium
Sponsoring Units: GIMSChair: James Matey, Sarnoff Corporation
Room: Morial Convention Center 206
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S4.00001: Keithly Award Talk: Pushing the limits in ARPES Invited Speaker: A successively higher degree of parallelity in data acquisition has dramatically increased the information rate in angle resolved photoelectron spectroscopy (ARPES). This has been coupled to resolution improvements both in angle and energy. The development leading to the present state of the art is described, and recent results are presented. To extend the field further towards higher or lower energies, as well as higher resolution or larger acceptance angles, novel techniques are needed. Some recent and ongoing developments for this purpose will be described. It is also shown that the application of these techniques can be used to overcome some limitations in energy resolution even when the angular information as such is not important. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S4.00002: Invited Speaker: |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S4.00003: High Resolution Angle-Resolved Photoemission Studies of Correlated Electron Systems Invited Speaker: Bjorn Wannberg's invention of the ``angle mode'' for acquiring 2-dimensional ARPES images with both high energy and angular resolution has revolutionized the way modern ARPES experiments are performed. This advance has lead to both quantitative and qualitative improvements in the resolution as well as collection efficiency. In this talk I will give a few key examples of what has become possible using this new technology for the study of correlated electron systems. Particular focus is on high Tc superconductors, including a 'fingerprinting' of electron-boson couplings using the isotope effect, and recent data on colossal magnetoresistive oxides which shows the critical aspect of the orbital degrees of freedom and k-dependent electron self energies. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S4.00004: Angle-Resolved Photoemission Study of High Temperature Superconductors Invited Speaker: As a hybrid of spectroscopy and scattering experiments, angle-resolved photoemission spectroscopy reveals the direction, the speed, and the scattering mechanism of valence electrons.~ In the past two decades, enormously improved resolution and carefully matched experiments have elevated this technique to an important many-body spectroscopy. Today, ARPES experiments help setting the intellectual agenda by testing new ideas, discovering surprises, and challenging orthodoxies. This technique is at the focal point of the necessary debates leading to new paradigms of physics represented by the high temperature superconductors. In this talk, we will survey the progress of this field over the last two decades and the information we gained about the high temperature superconductors. However, the focus will be the latest ARPES data on the following subjects: i) New temperature and doping dependent data that provide insights on the relationship between pseudogap and superconducting gap; ii) New data on the Fermi surface dependent pairing and many-body interactions in self-doped multilayer cuprates. If time permits, we will also present new time resolved photoemission data from charge density wave systems. [Preview Abstract] |
Session S5: Theory of Orbital Magnetization and Related Properties
Sponsoring Units: DCMPChair: Raffaele Resta, University of Trieste
Room: Morial Convention Center RO1
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S5.00001: Theory of Orbital Magnetization and its Generalization to Interacting Systems Invited Speaker: Recently, a new formula for the orbital magnetization was proposed. In this talk, I will review the original derivation of the formula based on the semi-classical wave-packet dynamics, as well as a general derivation based on the standard perturbation theory of quantum mechanics. The quantum derivation clarifies the origin of the novel aspects of the semi-classical derivation, such as the Berry phase correction to the density of states. It is valid for general systems including insulators with or without a Chern number, metals at zero or finite temperatures. More importantly, we are able to combine the quantum derivation with the exact current and spin density functional theory (SCDFT), proving the validity of the formula for interacting systems. With this development, the new magnetization formula, in combination with the recent advances in the construction of optimized effective potential for SCDFT, will turn out to be a powerful practical tool for the study of systems that have long defied traditional ab-initio methods. \newline \newline [1] J. Shi, G. Vignale, D. Xiao and Q. Niu, Phys. Rev. Lett. 99, 197202 (2007). \newline [2] Z. Wang, P. Zhang and J. Shi, Phys. Rev. B 76, 094406 (2007). \newline [3] D. Xiao, J. Shi and Q. Niu, Phys. Rev. Lett., 95, 137204 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S5.00002: Optical sum rules for the orbital magnetization and anomalous Hall conductivity Invited Speaker: Magnetic circular dichroism (MCD), the differential absorption of left- and right-circularly-polarized light by ferromagnets, results from the interplay between spin polarization and spin-orbit interaction. The same two ingredients are responsible for their spontaneous (``anomalous'') Hall conductivity (AHC) and orbital magnetization. I will discuss how the three phenomena are related by two sum rules for the interband MCD spectrum.\footnote{I. Souza and D. Vanderbilt, {\tt arXiv:0709.2389} (2007).} The sum rules are of the form $\int_0^\infty \omega^{-p}\sigma''_{{\rm A},\alpha\beta}(\omega)d\omega$, where $\sigma''_{\rm A}$ is the absorptive part of the antisymmetric optical conductivity. The sum rule with $p=0$ is the dichroic counterpart of the familiar $f$-sum rule for linearly-polarized light. I will show that it yields a contribution to the ground-state orbital magnetization which in insulators is associated with the circulation of the Wannier orbitals around their centers (more precisely, to the gauge-invariant part thereof). This differs from the net circulation, or total orbital magnetization,\footnote{ D. Xiao, J. Shi, and Q. Niu, Phys. Rev. Lett. {\bf 95}, 137204 (2005).}$^{,}$\footnote{T. Thonhauser, D. Ceresoli, D. Vanderbilt, and R. Resta, Phys. Rev. Lett. {\bf 95}, 137205 (2005).} which has two additional contributions: (i) the remaining Wannier self-rotation, and (ii) the ``itinerant'' circulation arising from the center-of-mass motion of the Wannier orbitals. Contributions (i) and (ii) are not separately meaningful, since their individual values depend on the particular choice of Wannier functions. Their sum is however gauge-invariant, and can be inferred from a combination of gyromagnetic and magneto-optical experiments. The $p=1$ sum rule is the dc limit of the dichroic Kramers-Kronig relation which yields $\sigma'_{\rm A}(0)$, the Karplus-Luttinger AHC. {\it Ab-initio} studies have shown that it is necessary to sample over millions of $k$-points to converge the calculation of this quantity. I will describe an efficient real-space method for computating the AHC\footnote{ X. Wang, J.~R. Yates, I. Souza, and D. Vanderbilt, Phys. Rev. B {\bf 74}, 195118 (2006).} and MCD\footnote{J.~R. Yates, X. Wang, D. Vanderbilt, and I. Souza, Phys. Rev. B {\bf 75}, 195121 (2007).} using Wannier functions, and present some illustrative calculations for ferromagnets as well as field-polarized solid and liquid heavy metals.\footnote{G. Busch and H.--J. G\"untherodt, Solid State Phys. {\bf 29}, 235 (1974).} The possible role of configurational disorder in enhancing the field-induced AHC of liquid metals by introducing low-frequency Drude-related features in the MCD spectrum will be explored. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S5.00003: A converse approach to the calculation of NMR shielding tensors Invited Speaker: We propose an alternative approach for computing the NMR response in periodic solids that is based on a recently developed theory of orbital magnetization [1]. Instead of obtaining the shielding tensor from the response to an external magnetic field, we derive it directly from the orbital magnetization appearing in response to a microscopic magnetic dipole [2]. Our new approach is very general, and it can be applied to either isolated or periodic systems. The converse procedure has an established parallel in the case of electric fields, where Born effective charges are often obtained from the polarization induced by a sublattice displacement instead of the force induced by an electric field. Our novel approach is simple and straightforward to implement since all complexities concerning the choice of the gauge origin are avoided and the need for a linear-response implementation is circumvented. We have demonstrated its correctness and viability by calculating chemical shieldings in simple molecular systems, finding excellent agreement with previous theoretical and experimental results. Applications to more complex systems are currently in progress. \begin{itemize} \item[(1)] T. Thonhauser, D. Ceresoli, D. Vanderbilt, and R. Resta, Phys. Rev. Lett. 95, 137205 (2005). \item[(2)] T. Thonhauser, Arash A. Mostofi, Nicola Marzari, R. Resta, David Vanderbilt, submitted to Phys. Rev. Lett. (2007), arXiv:0709.4429v1. \end{itemize} [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S5.00004: Non-perturbative ab-initio calculation of the g-tensor in periodic boundary conditions Invited Speaker: Electron Paramagnetic Resonance (EPR) spectroscopy is one of the most powerful and versatile analytic tools in materials science to date. The wealth of important information available from EPR spectroscopy, however, cannot be extracted from experiments alone, rather from the combination of experimental date and theoretical calculations. To date, first principle methods for computing the EPR g-tensor rely on the linearization of the effective spin Hamiltonian with respect to spin orbit (SO) coupling [1]. While this approach gives good results for light atoms, it is insufficient when SO coupling is strong, as in transition metal compounds. We have derived a method to calculate the electronic g-tensor of paramagnetic defects from first principles in a non-perturbative way, based on the formula for the orbital magnetization [2]. The main advantage of his method, is that the external magnetic field do not enter the formula explicitly, showing that the g-tensor can be calculated as a ground state quantity by including the spin-orbit term in the SCF hamiltonian. We have found a perfect agreement with linear response calculations for bulk systems and molecular complexes containing light atoms. For heavier atoms, the agreement with experimental data is substantially improved. \newline \newline [1] C. J. Pickard and F. Mauri, Phys. Rev. Lett. 88, 086403 (2002). \newline [2] D. Ceresoli, T. Thonhauser, D. Vanderbilt and R. Resta, Phys. Rev. B 74, 024408 (2006); D. Xiao, G. Vignale, J. Shi and Q. Niu, Phys. Rev. Lett. 99, 197202 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:30PM |
S5.00005: First-principles approach to Non-Collinear Magnetism: towards Spin Dynamics Invited Speaker: Most formulations of spin density functional theory (SDFT) restrict the magnetization vector field to have global collinearity. Nevertheless, there exists a wealth of strong non-collinearity in nature, for example molecular magnets, spin-spirals, spin-glasses and all magnets at finite temperatures. The local spin density approximation (LSDA) can be extended to these non-collinear cases [1] but this extension has the undesirable property of having the exchange-correlation (xc) field parallel to the magnetization density at each point in space. When used in conjunction with the equation of motion for the spin magnetization in the absence of spin currents and external fields [2,3], this local collinearity eliminates the torsional term, resulting in no time evolution. This severe shortcoming of LSDA, where the physical prediction is qualitatively wrong, opens up an important new direction for the development of functionals where this time evolution is correctly described. Towards this goal, I will describe our extension of the Kohn-Sham optimized effective potential (OEP) method to the non-collinear case and derive the corresponding integral equations, applicable to both finite and extended systems [3,4]. Most importantly I'll show that the resulting magnetization and xc field are not locally collinear to each other for real solids, and will therefore produce manifestly different spin-dynamics. \newline \newline [1] J.~Kuebler, K.-H.~Hoeck, J.~Sticht and A.~R.~Williams, J.~Phys.~F{\bf 18}, 469 (1993). \newline [2] K.~Capelle, G.~Vignale and B.~L.~Gyoerffy, Phys.~Rev.~Lett.{\bf 87}, 206403 (2001). \newline [3] S.~Sharma, J.~K.~Dewhurst, C.~Ambrosch-Draxl, S.~Kurth, N.~Helbig, S.~Pittalis, S.~Shallcross, L.~Nordstroem and E.K.U.~Gross Phys.~Rev.~Lett.{\bf 98}, 196405 (2007) \newline S.~Sharma, S.~Pittalis, S.~Kurth, S.~Shallcross, J.~K.~Dewhurst and E.K.U.~Gross Phys.~Rev.~B{\bf 76}, 100401 (Rapid Comm.) (2007) [Preview Abstract] |
Session S6: Superfluid Density in Underdoped Cuprates
Sponsoring Units: DCMPChair: Mohit Randeria, Ohio State University
Room: Morial Convention Center RO4
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S6.00001: Quantum critical behavior in the superfluid density of strongly underdoped ultrathin copper oxide films Invited Speaker: The relationship between transition temperatures T$_{C}$ and superfluid densities n$_{S}$(0) of cuprate superconductors has been a central issue in cuprate superconductivity from the beginning. When mobile holes are removed from optimally doped CuO$_{2}$ planes, T$_{C}$ and n$_{S}$(0) decrease in a surprisingly correlated fashion. Recent measurements of the superfluid density of strongly underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ films and crystals have found a square-root scaling, T$_{C} \propto$ n$_{S}$(0)$^{\alpha}$ where $\alpha \approx$ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$}$, which supplants the approximately linear proportionality that had been deduced long ago from less underdoped samples by Uemura et al. and had been ascribed to the quasi-2D structure of cuprates. This situation leads back to a basic question -- what is the behavior of the fundamental structural unit, namely, a single CuO$_{2}$ layer or bilayer, which is truly two-dimensional by construction? To address this question, we studied 2D samples near the critical doping level where superconductivity disappears. We measured n$_{S}$(T) in films of Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ as thin as two CuO$_{2}$ bilayers. T$_{C}$'s were as low as 3 K. We observed the 2D Kosterlitz--Thouless--Berezinski drop in n$_{S}$ at T$_{C}$, and we recovered the linear scaling T$_{C} \propto $ n$_{S}$(0) expected in 2D due to fluctuations in the phase of the superconducting order parameter. Taken together, results on 3D and 2D samples suggest that the disappearance of superconductivity with underdoping is ultimately due to quantum fluctuations near a quantum critical point. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S6.00002: Superfluid density in the underdoped cuprates Invited Speaker: |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S6.00003: Two energy scales and the nodal-antinodal dichotomy in underdoped superconducting cuprates Invited Speaker: Recent electronic Raman scattering experiments on hole-doped cuprates in the underdoped regime reveal that nodal and antinodal regions behave in very different manners. I will present the conclusions of a theoretical analysis of these experiments, based on a new sum-rule, and on Fermi liquid and phenomenological considerations, which lead to the conclusion that the superconducting state involves a hitherto hidden energy scale, which has the same doping-dependence than the superconducting transition temperature, in contrast to the pseudogap energy scale. The low-frequency Raman response and the temperature-dependence of the superfluid density, both controlled by nodal excitations, are shown to behave in a qualitatively similar manner, which puts strong constraints on microscopic theories of the cuprates. For a reference and list of collaborators on this work, see: M. Le Tacon et al., Nature Physics, 2, 537 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S6.00004: Algebraic charge liquids and the underdoped cuprates Invited Speaker: We describe a possible theoretical route for the evolution with doping of an antiferromagnetic Mott insulator into a gapless d-wave superconductor. Central to this description are ``algebraic charge liquids'' with power law correlations of gapless, spinless, charge e fermions. Based on this understanding a specific theory of the underdoped cuprates that naturally explains a number of puzzling phenomena including the doping and temperature dependence of the superfluid density, the photoemission spectra, and Shubnikov-de Haas oscillations is proposed. Experiments to test the theory are suggested. [Preview Abstract] |
Session S7: Panel Discussion: Non-traditional Careers for Physicists
Sponsoring Units: FGSAChair: Gavi Begtrup, University of California, Berkeley
Room: Morial Convention Center RO5
Wednesday, March 12, 2008 2:30PM - 2:45PM |
S7.00001: Scientific Careers in Public Policy Invited Speaker: Congress is built to respond to the will of its constituents. Representatives and their staffs are awash in information. If scientists do not communicate regularly and effectively with Congress, policies requiring sound scientific underpinnings will be ill-crafted. As a panelist, I will represent scientific careers in public policy, and will also address how civic engagement can be woven into the fabric of any career path. [Preview Abstract] |
Wednesday, March 12, 2008 2:45PM - 3:00PM |
S7.00002: A Road Less Traveled: An Editorial Career Invited Speaker: It has been said that no life is completed the way one had planned for it, and mine is no exception to that rule so far. When I was graduating with a BSc I was convinced I'd be doing physics research for the rest of my life -- and when I was getting my PhD I was sure I'd be teaching high school physics and helping others learn for the foreseeable future. Yet, 9 years later, I am not doing either of these as a full time job, and I've changed my mind a couple more times as to what career path (and broader lifestyle) would work best for me. In the intervening years, I've learnt to embrace change as a tool for carving my own path, and to be wary of the certainties that can tie oneself to a ``safe'' but uninspiring future. I studied at the University of Thessaloniki, Greece, and at Royal Holloway University of London (BSc); also at the University of Illinois at Urbana-Champaign (MSc) and at Imperial College London (PhD). After my PhD (and the national military service in Greece), I taught at high-school level for a couple of years in Crete, Greece. At the same time, I was science editor for Crete University Press, Greece's major university press. From there, I jumped onto the APS editorial boat: First to PRB (2003), then to PRL (2007), where I am now an Assistant Editor. I also have an adjunct research position at Columbia University. [Preview Abstract] |
Wednesday, March 12, 2008 3:00PM - 3:15PM |
S7.00003: Scientific publication: An alternative career in physics Invited Speaker: Scientific research could not go forward without scientific dissemination and publication of research results. As a member of the editorial staff of Physical Review Letters, I hope to be able to shed some light on the nature of the role of APS journal editors in the publication process, what the job entails and field questions on this alternative career in science. [Preview Abstract] |
Wednesday, March 12, 2008 3:15PM - 3:30PM |
S7.00004: Adventures of the Industrial Researcher Invited Speaker: Are you really prepared for the real world? This is a question that is routinely asked - by those that are just entering the work place and those that have been there for years. The current working environment is full of change and is very different that what was imagined even five years ago. This presentation will explore the transition from the academic environment to the corporate research world and to the working world beyond. Tips and tools for survival in the work place will also be presented. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:45PM |
S7.00005: TBD Invited Speaker: |
Wednesday, March 12, 2008 3:45PM - 4:00PM |
S7.00006: TBD Invited Speaker: |
Wednesday, March 12, 2008 4:00PM - 5:30PM |
S7.00007: Panel Discussion on Non-traditional Careers for Physicists |
Session S8: Colloidal Rheology
Sponsoring Units: DFDChair: David Weitz, Harvard University
Room: Morial Convention Center RO6
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S8.00001: Shear Thickening, Gel Elasticity and Internal Stresses in a Colloidal System with Attractive Interactions Chinedum Osuji, David Weitz Dilute dispersions of carbon particles in hydrocarbon fluids flocculate, forming colloidal gels with typical fractal scaling of shear modulus with particle volume fraction. Surprisingly, these systems exhibit shear thickening in two regimes. At low shear rates, shear thickening is concurrent with the formation of vorticity-aligned structures, a general phenomenon in attractively-interacting complex fluids, but not previously implicated in shear thickening. At high P\'{e}clet numbers, thickening involves degradation of particle clusters and an increase in effective volume fraction. This contrasts with the hard-sphere case where thickening is due to pseudo-jamming events that occur with the growth of hydro-clusters with persistent contacts. On cessation of high shear rate flow, these shear thickened gels display a power-law dependence of elasticity on pre-shear stress and the data can be re-scaled simply to provide a universal response for different particle volume fractions. We propose a mechanism and scaling argument that accounts for this behavior in terms of the stress dependence of the cluster number density during pre-shear. We characterize the internal stresses that result from a shear rate quench from the fluid to the gel state and find that the modulus is directly proportional to the internal stress in the system. At short times, t$\approx 10^3$s, the internal stress decays with a weak power law dependence on time. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S8.00002: Microscopic Details of Plastically Sheared Colloidal Gels Gary L. Hunter, Tiffany Soo, Denis Semwogerere, Eric R. Weeks We use fast confocal microscopy to study effects of different shear rates on colloidal depletion gels. Our samples consist of PMMA spheres in a refractive index matched solvent, with polymer added to produce a depletion interaction. We subject these samples to different rates of oscillatory shear with similar strain amplitudes. By tracking the three-dimensional trajectories of several thousand particles, we directly observe how shear modifies the gel's structure at the particle-level and how differences in local structure affect shear-induced dynamics. We find that increasing shear rate significantly increases the rate of plastic bond rearrangement, but that large clusters remain mostly intact, even when the observed deformations are highly non-affine. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S8.00003: Delayed Collapse of Colloidal Gels Juan-Jose Lietor-Santos, Chanjoong Kim, Alberto Fernandez-Nieves, David A. Weitz We study the behavior of colloidal gels under gravitational forces using a system of polystyrene beads and non-adsorbing polymer to induce depletion attraction between particles. As the interaction energy or the volume fraction decreases, a delayed collapse regime is observed, where the sedimentation of the gels starts with a slow initial compression followed, after a delay time, by a rapid collapse characterized by the coarsening of the structure. By means of changing the density mismatch between the network and the surrounding solvent, we are able to explore the dependence of the delay time and coarsening behavior with the gravitational stress. The results clearly show that, even though only the weakest gels undergo delayed collapse, the gravitational stress is not the trigger leading to the coarsening of the structure, although it certainly affects the time it takes the gel to completely sediment. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S8.00004: Effect of Nanoparticle Shape and Size on Shear Rheology Matt K. Petersen, J. Matthew D. Lane, Gary S. Grest The effect of nanoparticle shape and size on the shear rheology of nanoparticle suspensions was explored through non-equilibrium molecular dynamics simulations. Composite nanoparticles consisting of rigid Lennard-Jones particles in a Lennard-Jones explicit solvent were modeled using the M\"{u}ller-Plathe ``reverse'' perturbation method. A series of suspensions were modeled wherein the nanoparticle volume fraction was held constant while the shape and size of the nanoparticles were varied. Specifically, results for the shear viscosity of spherical, plate, and rod-like nanoparticles of size varying from tens to hundreds of interaction sites will be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S8.00005: Shear Thinning in Nanoparticle Suspensions Pieter J. in 't Veld, Matt K. Petersen, Gary S. Grest Results of large scale non-equilibrium molecular dynamics (NEMD) simulations are presented for nanoparticles in an explicit solvent. The nanoparticles are modeled as a uniform distribution of Lennard-Jones particles, while the solvent is represented by standard Lennard-Jones particles. Here we present results for the shear rheology of spherical nanoparticles of size 5 to 20 times that of the solvent for a range of nanoparticle volume fractions and interactions. Results from NEMD simulations suggest that for strongly interacting nanoparticle that form a colloidal gel, the shear rheology of the suspension depends only weakly on the size of the nanoparticle, even for nanoparticles as small as 5 times that of the solvent. However for hard sphere-like colloids the size of the nanoparticles strongly affects the shear rheology. The shear rheology for dumbbell nanoparticles made of two fused spheres is also compared to spherical nanoparticles and found to be similar except at very high volume fractions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04- 94AL85000. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S8.00006: Viscosity of confined suspensions. Philippe Peyla, Yohann Davit, Claude Verdier In this work, we study experimentally and numerically the viscosity of non-brownian confined suspensions of hard spherical particles confined between two walls in a shear flow. By varying the wall-to-wall distance (gap), we show that the viscosity presents a remarkable behavior as a function of the confinement. A transition occurs from a 3D configuration (no confinement) to a quasi2D (Q2D) one when the wall-to-wall distance becomes smaller than twice the spheres diameter. We find, as expected, that the effective viscosity increases when the gap decreases. This is due to dissipation which is enhanced for smaller gaps. But, more precisely, when the wall-to-wall distance decreases, the linear term in volumic fraction (diluted case) increases while the quadratic term decreases to zero when the suspension reaches a Q2D regime and becomes negative for smaller gaps. In a Q2D suspension, it is proven that an anti-drag effect holds between two particles which move perpendicularly to their connecting line. Experimental measurements on diffusion coefficients have been achieved for colloidal Q2D suspensions by Cui et al [1] which shows this behavior without any ambiguity. We suspect that such anti-drag interactions can affect the viscosity of Q2D semi-diluted suspensions. [1] B. Cui, H. Diamant, B. Lin and S. Rice, Phys. Rev. Lett., vol. 92, 258301 (2004) [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S8.00007: Rate dependence, drag balance and role of disorder in linearly sheared foams. Gijs Katgert, Matthias E. M\"{o}bius, Martin van Hecke We linearly shear a bidisperse foam monolayer sandwiched between a glass plate and a fluid surface over 3 orders of magnitude in driving velocity. We find strongly rate-dependent velocity profiles, which become increasingly shear banded with shear rate. We also confirm previous findings that monodisperse foam layers exhibit rate-{\it independent} velocity profiles. Both behaviors are quantitatively captured in a model that balances the viscous drag forces in the foam, provided that we assume the average drag force between bubbles in disordered foams to scale differently than the drag force at the bubble scale. We confirm the scaling of the drag forces in both mono- and bidisperse foams by independent rheological measurements, and confirm the crucial role of disorder on the flow of foams. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S8.00008: Shear Modulus of a Depletion-Induced Colloidal Gel Chanjoong Kim, David A. Weitz Mechanical properties of a colloidal gel are of great interest because they are related to the processability of the colloidal dispersion and its stability. We measure the shear modulus for colloidal gel networks induced by depletion attraction and determine the relationship between the strength of the depletion attraction and the magnitude of the shear modulus. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S8.00009: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S8.00010: Irreversible flow-induced vitrification of nanoemulsions by extreme droplet rupturing James N. Wilking, Thomas G. Mason Some materials weaken through fracturing when subjected to extreme stresses. However, breaking down repulsive bits of condensed matter that are dispersed in a viscous liquid can also potentially cause a dramatic and irreversible increase in the dispersion's elasticity. Here, we demonstrate this principle using dispersions of one liquid in another immiscible liquid. Anionically stabilized microscale emulsions are subjected to a history of extreme high-pressure microfluidic flow, causing the droplets to rupture to nanoscale sizes. As the droplet radius decreases below 100 nm, the nanoemulsion can develop an unusually large elastic modulus, even at droplet volume fractions far below maximal random jamming of uniform hard spheres. Thus, through the history of applied flow, a liquid microscale emulsion can be transformed and vitrified into an elastic nanoemulsion of disordered repulsive droplets without altering the composition. Furthermore, we show that systematic macroscopic shear rheology of the nanoemulsion glass as a function of the droplet volume fraction can be effectively used as a surfaces forces apparatus to deduce the screened Debye interaction potential as a function of separation between the droplet interfaces. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S8.00011: Hindered rising in aggregating polydisperse particle suspensions Shihai Feng, Kim Rasmussen, Alan Graham, Bing Dai We describe a direct simulation method that effectively determines the appropriate hindered rising behavior of polydisperse particle suspensions. Our method allows adequate representation of the hydrodynamic interactions as well as system specific colloidal interactions. Simulation results are in good agreement with experimental data obtained by MRI imaging. Our results demonstrate the importance of particle aggregation in the hindered rising suspensions. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S8.00012: Self-organized criticality of slowly sedimenting sheared suspensions Weining Man, Laurent Corte, Sharon Gerbode, David Pine, Paul Chaikin Suspensions of neutrally buoyant particles driven by slow periodic shear can undergo a dynamical phase transition from an absorbing reversible steady state to a fluctuating irreversible state. For a given strain amplitude {\$}$\backslash $gamma{\$}, this transition occurs at a specific critical volume fraction {\$}$\backslash $Phi{\_}c{\$}. However, if the particles are not neutrally buoyant, they either sink to the bottom or float to the top of the container. New experiments and simulations show that under periodic shear, the particles resuspend, however, and that for a given strain amplitude {\$}$\backslash $gamma{\$}, the particles evolve towards the critical concentration {\$}$\backslash $Phi{\_}c{\$} without any external intervention. In that case, particle collisions nucleated at the bottom of the shear cell propagate through the sample and keep the system suspended close to the critical volume fraction {\$}$\backslash $Phi{\_}c($\backslash $gamma){\$}. Hence, slowly sedimenting particles under oscillatory shear appear as a new class of self-organized critical systems hitherto unreported. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S8.00013: Random organization: A dynamical phase transition Laurent Corte, David J. Pine, Paul M. Chaikin We introduce a simple model motivated by recent experiments in sheared suspensions. We show that completely random displacements of colliding particles are sufficient to generate an organized state where further collisions are suppressed. This organization by self-activated random walkers presents a much more efficient process than when all particles are diffusing. It only occurs provided that the density in particle is lower than a critical value $\rho _{c}$ and is characterized by a dynamical phase transition. A mean-field description captures the existence of this transition. It suggests that the value of $\rho _{c}$ is determined by the ratio p$_{s}$/p$_{c}$, where p$_{s}$ is the probability for a pair of colliding particles to separate and p$_{c}$ is the probability that a ``quiet'' particle be collided. Our results also reveal that the ordering can be enhanced by straining the system periodically. However, these more organized states become less and less accessible as the strain amplitude is increased. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S8.00014: X-ray photon correlation spectroscopy in a shear flow Andrei Fluerasu X-ray photon correlation spectroscopy was used to measure the diffusive dynamics of colloidal suspensions in a shear flow. The results presented here show how the intensity autocorrelation functions measure a coupling between the diffusive dynamics of the particles and their flow-induced, convective motion. However, in the limit of low flow/shear rates, it is possible to obtain the diffusive component of the dynamics. The conditions under which this is possible are easier to achieve at higher values of the scattering wavevector q and this may provide an advantage of X-ray over, for e.g. light, photon correlation spectroscopy. In recent work (A. Fluerasu et al., submitted, 2007) we have shown this result to hold for dillute (particle volume fraction $\Phi \approx$ 10 \%) suspensions when the correlation functions probe, basically, the self-diffusion of individual, non-interacting particles. Here we will also adress the collective motion of concentrated suspensions of hard-sphere systems ($\Phi$ up to 50 \%) and study the coupling between the shear-induced response and the collective diffusion of the suspension. An important benefit of this experimental strategy over more traditional X-ray methods, is the minimization of X-ray induced beam damage, which makes the method suitable for the study of the dynamical properties of a large class of complex soft-matter and biological fluids. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S8.00015: Electrophoretic ``Equilibrium'' Profile of Charged Colloids Romain Planques, Paul Chaikin We perform an electrophoresis experiment of a concentrated colloid against a semipermeable membrane. The electric field forces the charged particles against the membrane and sets up a concentration profile similar to that of a colloid in gravitational sedimentation equilibrium where gravitational forces compete against the osmotic pressure gradient. In the present case there is a current which flows through the electrolyte so the system reaches a steady state profile rather than equilibrium. The electric field, colloid and ionic concentrations adjust self consistently to produce the profile. We use 91 nm polystyrene spheres with sufficient charge that they crystallize and observe their Bragg scattering as a function of height to determine the lattice spacing and particle concentration. We also use 700nm spheres and obtain their concentration profile with X-ray absorption. The fluid flow is zero for a capped system. Connecting a return tube from the supernatant side above the electrophoretic sediment to below the filter yields an electroosmotic flow and circulation. The profile changes substantially and allows us to study the hydrodynamic interactions as a function of concentration for the electrophoresing particles. [Preview Abstract] |
Session S9: Optical Properties of Superconductors
Sponsoring Units: DCMPChair: David Tanner, University of Florida
Room: Morial Convention Center RO7
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S9.00001: Superconducting plasma edge along the \boldmath $c$ axis in La$_{2-x}$Ba$_x$CuO$_4$\unboldmath C. C. Homes, M. H\"{u}cker, Jinsheng Wen, Zhijun Xu, G. D. Gu, J. M. Tranquada The optical properties of La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) have been measured for a number of temperatures over a wide frequency range along the poorly-conducting $c$ axis for the $x=0.095$, $0.125$ and $0.155$ Ba concentrations. In slightly underdoped LBCO ($x=0.095$, $T_c\simeq 32$~K), a sharp plasma edge is observed in the reflectance at low frequency below $T_c$; this plasma edge is associated with Josephson coupling of the copper-oxygen planes and the formation of a bulk three-dimensional superconducting state. A plasma edge is also observed in the more heavily-doped material ($x=0.155$, $T_c\simeq 32$~K) below $T_c$ at higher frequency; however, it is significantly broader in character. Interestingly, for the $x=1/8$ doping, static charge and spin stripe order develop at 54 and 42~K, respectively; the superconducting transition is strongly suppressed ($T_c\simeq 3$~K) and the plasma edge is not observed. The failure to observe a plasma edge for this doping is consistent with the recently proposed view that the layers are decoupled due to the formation of spin stripes, blocking the formation of a coherent three-dimensional superconducting state. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S9.00002: Investigation of the bosonic spectral density in highly under-doped YBa$_{2}$Cu$_{3}$O$_{6.35 }$ Jing Yang, Thomas Timusk, Douglas Bonn, Ruixing Liang, Walter Hardy We studied the doping dependence of the bosonic spectral function in nearly optimally-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$, ortho-II YBa$_{2}$Cu$_{3}$O$_{6.35 }$and highly under-doped YBa$_{2}$Cu$_{3}$O$_{6.35}$ single crystals by optical spectroscopy. With fixed oxygen content, the hole doping of the YBCO system can be fine-tuned by varying the degree of oxygen ordering. After annealing and quenching, we were able to make oxygen less ordered and obtain a highly under-doped YBa$_{2}$Cu$_{3}$O$_{6.35 }$ sample with a very low transition temperature around 18K (about 20{\%} of the optimal T$_{c})$. The a-axis reflectance data of this sample at nine temperatures between 30K and 295K were measured with an infrared spectrometer between 60 and 40 000 cm$^{-1}$ with the aid of three different infrared and optical polarizers. The optical properties of the highly under-doped YBCO sample show dramatic changes compared to the ortho-II YBCO sample. The strong sharp mode in the bosonic spectral function \textit{$\alpha $}$^{2}$\textit{F($\Omega )$ }in the ortho-II YBCO is absent in the highly under-doped sample. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S9.00003: Optical conductivity of electron-doped cuprates, pseudogap and antiferromagnetic fluctuations D. Bergeron, B. Kyung, V. Hankevych, A.-M.S. Tremblay Recent neutron scattering experiments on electron-doped cuprates have shown that the antiferromagnetic correlation length at the pseudogap temperature scales like the thermal de Broglie wavelength. This result, predicted by the Two-Particle Self-Consistent approach (TPSC), suggests that antiferromagnetic fluctuations are at the origin of the pseudogap in electron-doped cuprates. Clearly one needs to verify that other physical properties can also be explained within the same formalism. We thus derive, within TPSC, the formula appropriate for optical conductivity, including the first vertex correction. Our numerical calculations then show that, as observed experimentally, there is a transfer of spectral weight from low to high energy when antiferromagnetic correlations become important near the pseudogap temperature. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S9.00004: Optical Properties of Organic Superconductor $\kappa$-(BETS)$_2$FeBr$_4$ M. Reedyk, N. Hossein Khah, B. Liu, G.V. Sudhakar Rao, H. Fujiwara, H. Kobayashi, M.A. Tanatar, K. Yakushi, T. Nakamura The optical response to far- and mid-infrared radiation has been measured for quasi two-dimensional plate-shaped crystals of $\kappa$-(BETS)$_2$FeBr$_4$ [where BETS = bis(ethylenedithio)-tetraselenafulvalene]. $\kappa$-(BETS)$_2$FeBr$_4$ is the first antiferromagnetic organic superconductor at ambient pressure with N\'{e}el temperature T$_N=2.5$ K and superconducting transition temperature T$_C=1.1$ K. Polarized thermal reflectance measurements were performed to compare the reflectance above and below T$_C$ and T$_N$ using a Martin-Puplett-type polarizing interferometer and $^3$He cryostat. In addition polarized absolute reflectance measurements in the far- and mid-infrared were carried out at temperatures in the normal state between 4 K and 300 K using a Michelson interferometer and cold finger cryostat. Kramers-Kronig analysis was then used to determine the optical conductivity of $\kappa$-(BETS)$_2$FeBr$_4$ at these temperatures. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S9.00005: Optical properties of lattice/spin polarons in underdoped cuprates Simone Fratini, Sergio Ciuchi, Emmanuele Cappelluti In this contribution we investigate the optical spectra of one hole in the Holstein-$t$-$J$ model. We employ a dynamical mean-field theory which becomes exact in the limit of infinite connectivity. This allow us to investigate the local (incoherent) features which are related to the internal structure of the polaron, disregarding coherent motion which should be reflected in the Drude-like peak. We show that magnetic and electron-phonon interactions sustain each other in establishing polaronic regime. Polaron formation is reflected in a peculiar mid-infrared (MIR) band which is however notably different in the case of a lattice or magnetic origin. The dependence of $\sigma(\omega)$ on the electron-phonon coupling constant $\lambda$, on the exchange interaction $J$ and on temperature $T$ is investigated. We compare our results with experimental data in Nd$_{2-x}$Ce$_x$CuO$_4$ showing that the doping and temperature dependences of the optical conductivity in this compounds is naturally reproduced by a spin/lattice polaronic model. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S9.00006: Optical Conductivity and Correlation Strength of the High T$_c$ Cuprate Superconductors Massimo Capone, Armin Comanac, Luca de' Medici, Andrew Millis High temperature copper-oxide-based superconductivity is obtained by adding carriers to insulating ``parent compounds.'' It is widely believed the parent compounds are ``Mott'' insulators, in which the lack of conduction arises from anomalously strong electron-electron repulsion, and that the unusual properties of Mott insulators are responsible for high temperature superconductivity. This paper presents a comparison of optical conductivity measurements and theoretical calculations based on Dynamical Mean-Field Theory which challenges this belief: the analysis indicates that the correlation strength in the cuprates is not as strong as previously believed, that the materials are not properly regarded as pure Mott insulators, that antiferromagnetism is essential to obtain the insulating state and, by implication, that antiferromagnetism is essential to the properties of the doped metallic and superconducting state as well. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S9.00007: Is the Optical Sum Rule Violated in Cuprates? Michael Norman, Andrey Chubukov, Erik van Heumen, Alexey Kuzmenko, Dirk van der Marel Much attention has been given to a possible violation of the optical sum rule in the cuprates, and the connection this might have to kinetic energy lowering. The true optical integral is composed of a cut-off independent term (whose temperature dependence is a measure of the sum rule violation), plus a cut-off dependent term that accounts for the extension of the Drude peak beyond the upper bound of the integral. We find that optical data in the normal state of the cuprates can be accounted for solely by the latter term, implying that the dominant contribution to the observed sum rule `violation' in the normal state is due to the finite cut-off. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S9.00008: Sum rule analysis of YBa$_2$Cu$_3$O$_y$ in magnetic field Andrew LaForge, Willie Padilla, Kenneth Burch, Zhiqiang Li, Alexander Schafgans, Kouji Segawa, Yoichi Ando, Dimitri Basov We present infrared magneto-optical reflectance measurements which characterize the interplane transport of three dopings of YBa$_2$Cu$_3$O$_y$. An optical sum rule analysis reveals the field-evolution of the energy scale from which the superconducting condensate is drawn. We find that fields applied parallel to the $c$ axis totally suppress high-energy contributions to the condensate in underdoped samples while only moderately reducing the superfluid density. For optimally doped crystals the sum rule is satisfied and not modified by field. These results point toward a more conventional, BCS-like condensation mechanism, and will be discussed in relation to the interlayer phase coherence. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S9.00009: Illustration of advantages of optical self-energy for understanding conductivity data on superconductors E.J. Nicol, J.P. Carbotte Over the last 20 years, it has become common to present optical data on exotic superconductors, such as high $T_c$ and heavy fermions, in terms of a generalized optical self-energy. However, this quantity has never been fully examined in the context of conventional superconductivity and tested against experiment. We present a detailed study of the optical scattering rate and mass renormalization with emphasis upon the role of elastic and inelastic scattering, and make comparison with recent high quality data. This illustrates the usefulness of this approach and the new insights that can be obtained. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S9.00010: Optical properties of underdoped high $T_c$ superconductors from a phenomenological model E. Illes, E.J. Nicol, J.P. Carbotte We calculate the optical conductivity predicted by a phenomenological model for the pseudogap state given by Yang, Rice and Zhang [1]. In particular, we present results for both pseudogap and superconducting states as a function of doping. In addition to the conductivity, we examine the optical self-energy (i.e. the optical scattering rate and mass renormalization) and discuss our results in light of experiment. \newline [1] K.Y. Yang, T.M. Rice and F.C. Zhang, Phys. Rev. B 73, 17541 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S9.00011: Signatures of superconducting gap inhomogeneities in the optical conductivity J. LeBlanc, E.J. Nicol, J.P. Carbotte The observation of energy gap inhomogeneities in the BISSCO high $T_c$ cuprates motivates studying such effects on other properties. We have calculated the optical conductivity using an effective medium approximation to mix superconducting regions with different energy gaps. We present our results and comment on possible signatures in the conductivity and optical self-energy. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S9.00012: Time resolved Raman scattering on the pair-breaking peak in Bi-2212 -- direct observation of the dynamics of the superconducting order parameter Pelangi Saichu, Ilka Mahns, Arne Goos, Stephan Binder, Stefan Singer, J. Unterhinninghofen, Benjamin Schulz, Andrivo Rusydi, S.L. Cooper, M.V. Klein, P. Guptasarma, Dirk Manske, Michael Ruebhausen We employ a novel time resolved two-color pump probe Raman technique to study the electronic dynamics in the superconducting state of the high temperature superconductor Bi-2212. By studying the temporal evolution of the gap and the pair-breaking peak in the superconducting state, we reveal two contributions to the superconducting order parameter that respond within 1 ps and 7 ps, respectively. Both effects conserve spectral weight in the sense that the suppression of the pair-breaking peak appears concomitantly with the build up of in-gap states. The recovery times for both contributions to the gap are different by a factor of 4 outlining fundamentally different coupling mechanisms. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S9.00013: Electronic Raman scattering in cuprates William Guyard, Mathieu Le Tacon, Maximilien Cazayous, Alain Sacuto, Antoine Georges, Doroth\'ee Colson, Anne Forget We report electronic Raman response in mercury compound. In cuprates, the superconducting gap reaches its maximum values along the antinodal directions and vanishes along nodal directions corresponding respectively to the principal axes and the diagonal in the Brillouin zone. We will present both the antinodal ($B_{1g}$) and nodal ($B_{2g}$) responses as a function of doping and temperature. We will also report the full symmetric Raman response ($A_{1g}$) as a function of doping. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S9.00014: Self Energy Corrections to Resonant Inelastic X-ray Scattering in the Cuprates Wael Al-Sawai, Robert Markiewicz, Arun Bansil Resonant inelastic x-ray scattering (RIXS) is emerging as a powerful probe of strongly correlated systems by providing direct momentum-resolved information on charge excitations across the Mott gap. We have shown recently that long-range Coulomb interactions and self-energy corrections play an important role in modifying the electronic spectra of the cuprates.[1,2] Here we discuss model calculations to explore how plasmon and magnon corrections to the self-energy influence the RIXS spectra of the cuprates. \newline\noindent [1] R.S. Markiewicz and A. Bansil, Phys. Rev. B{\bf 75}, 020508 (R) (2007). \newline\noindent [2] R.S. Markiewicz, S. Sahrakorpi, and A. Bansil, cond- mat/0701524, to be published, PRB. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S9.00015: Optical conductivity in strongly correlated electron materials Jianmin Tao, Jian-Xin Zhu Ultrafast optical phenomena are of fundamental importance in the investigation of electronic structures of strongly correlated electron materials [1]. Starting from the Hamiltonian of a correlated electron material exposed to a time-dependent laser field, we formulate the particle current density. Within a mean-field approximation, we express the current density in terms of the expectation values of quasiparticle density operators by performing the canonical transformation. Within the Heisenberg picture, we solve a set of equations of motion for these quasiparticle densities. Finally we calculate the optical conductivity in several typical systems. [1] R. D. Averitt and A. J. Taylor, J. Phys: Condensed Matter 14, R1357 (2002). [Preview Abstract] |
Session S10: Superconductors: Synthesis and Physical Properties
Sponsoring Units: DCMPChair: Carmen Almasan, Kent State University
Room: Morial Convention Center RO8
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S10.00001: Growth of MgB$_{2}$ Films by an Impinging Jet HPCVD Reactor Design Daniel Lamborn, R.H.T. Wilke, Qi Li, Xiaoxing Xi, D.W. Snyder, Shufang Wang, Joan Redwing An impinging jet hybrid physical-chemical vapor deposition (HPCVD) reactor design was used for the growth of both thin and thick MgB2 films. This technique was able to independently control the substrate and Mg supply temperatures, and still maintained sufficient Mg overpressure to ensure phase stability. Thin films were predominantly axis oriented with the (0001) sapphire substrate while the thick films were either polycrystalline or showed preferred orientation. Thick films ($\sim $10 $\mu$m) were deposited at a growth rate of $\sim $ 110 $\mu $m/hr and showed a maximum Tc of 39.8 K and residual resistivity ratio of 6.6. The thick films also showed a high J$_{c}$ of 2x10$^{6}$ A/cm$^{2}$ at low applied magnetic fields even at 20 K. The results indicate that the impinging jet HPCVD configuration shows promise for coated conductor processes. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S10.00002: Clean Epitaxial MgB}$_{2}$\textbf{ Films Fabricated by \textit{Ex Situ} Annealing of CVD-Grown B Films in Mg Vapor Mina Hanna, Shufang Wang, Andrew David Eck, Rudeger Wilke, Ke Chen, Arsen Soukiassian, Che-Hui Lee, Wenqing Dai, Qi Li, Joan Redwing, Darrell Schlom, Xiaoxing Xi, Kamel Salama Epitaxial MgB$_{2 }$films have been successfully fabricated by \textit{ex} situ annealing of B films, grown by chemical vapor deposition (CVD), in Mg vapour. The films show a sharp superconducting transition T$_{c}$ of about 40 K, a low residual resistivity of less than 2 $\mu \Omega $cm, and a high residual resistivity ratio RRR of about 10. At self field, the value of critical current density$ J_{c}$ for a 3 $\mu $m thick film is 1.7$\times $10$^{6}$ Acm$^{-2}$ at 5 K and 1.2$\times $10$^{6}$ Acm$^{-2}$ at 20 K. The high $T_{c}$, low residual resistivity, high \textit{RRR} and high $J_{c}$ indicate the cleanness and good connectivity of the films. The results demonstrate that the \textit{ex situ} deposition method can produce clean MgB$_{2}$ films with superior superconducting properties, which is significant for applications such as MgB$_{2}$ superconducting cavities and coated conductor wires and tapes. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S10.00003: High isostatic pressure synthesis of sonochemically modified MgB2 superconductor Brett McCarty, Josua Hugen, Daniel Stoecklein, Ruslan Prozorov A study of the effects of high intensity ultrasound on MgB2 precursors with different additives to improve pinning properties is reported. Additives were either co-sonicated with boron or mixed into sonicated boron afterwards. Hot isostatic pressure (HIP) was used to form fully dense MgB2 samples from precursors. Analysis of magnetization, microstructure and x-ray diffraction will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S10.00004: Preparation and transport measurements of high T$_{c}$ disordered MgB$_{2}$ thin films Li Zhang, Wolter Siemons, Nicholas Breznay, Aharon Kulpitulnik, Malcolm Beasley In this talk we present a method using pulsed laser deposition to fabricate single-layer disordered MgB$_{2}$ thin films. Both Mg and stoichiometric MgB$_{2}$ targets are used during the deposition. The films have been characterized by AFM, XPS, and XRD to establish the quality of the surface and the structural and compositional uniformity of the films. Though the uniformity of the films is still under investigation, transport measurements show that the films exhibit behavior characteristic of the superconductor-insulator transition at relatively high temperatures (2 to 5K). We will also review some preliminary low temperature and high magnetic field transport measurement results. Also of interest is the very low spin-orbit scattering expected in MgB$_{2}$ due to the low Z of Mg and B. This work is funded by DoE and NSF. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S10.00005: Crystal growth of superconducting materials La$_{2-x}$Ba$_{x}$CuO$_{4}$ Genda Gu, J.S. Wen, Z.J. Xu, J.M. Tranquada Since the discovery of the superconductivity in high temperature superconducting oxide La$_{2-x}$Ba$_{x}$CuO$_{4}$ in 1986, a large number of groups have attempted to grow the single crystals. However, no single crystal La$_{2-x}$Ba$_{x}$CuO$_{4}$ with x$>$0.11 has been successfully grown. In this project, the effects of the growth condition and the compositions of a feed rod on the crystal growth of La$_{2-x}$Ba$_{x}$CuO$_{4}$ has been studied by an infrared image floating zone method. The experimental result shows that a planar solid-liquid growing interface tends to break down into a cellular interface when the growth velocity is more than 1 mm/h. When the planar solid-liquid growing interface break down into a cellular interface, the single crystal size decreases abruptly and the as-grown rod is not single phase. The large single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4}$ with x=0 to 0.165 has been successfully grown. The single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4}$ with x=0 to 0.165 up to 6 mm diameter and 150 mm length have been grown. The superconductivity transition temperature T$_{c}$ of as-grown single crystals of La$_{2-x}$Ba$_{x}$CuO$_{4}$ (x=0 to 0.165) have been measured. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S10.00006: Superconductivity in La(Ni$_{1-x}$T$_{x})$C$_{2}$ (T = Cu, Ti, and Pt) H.H. Sung, S.Y. Chou, Y.C. Kung, H.K. Kuo, K.J. Syu, W.H. Lee LaNiC$_{2}$, which crystallizes in an orthorhombic CeNiC$_{2}$ type structure with space group A$_{mm2}$, is the first nickel-based ternary carbide superconductor with T$_{c} \quad \sim $ 2.7 K.$^{1,2}$ Previous report showed that about 50{\%} substitution of La in LaNiC$_{2}$ with the 5f thorium (Th) element could enhance the superconducting critical temperature T$_{c}$ up to 7.9 K.$^{3}$ We present the results of static magnetization and electric resistivity data for fifteen polycrystalline La(Ni$_{1-x}$T$_{x})$C$_{2}$ (T = Cu, Ti and Pt) compounds measured in the temperature range 1.8 $\sim $ 4.4 K and 2.0 $\sim $ 300 K, respectively. Discussion of the improvement, with respect to pure LaNiC$_{2}$, on the superconducting critical temperature T$_{c}$ will be directed toward the changes of valence, lattice parameters as well as the effects of solubility limit in the pseudoternary La(Ni$_{1-x}$T$_{x})$C$_{2}$ alloys. $^{1}$W.H. Lee, H.K. Zeng, Y.D. Yao and Y.Y. Chen, Physica C \textbf{266}, 138 (1996). $^{2}$V.K. Pecharsky, K.A. Gschneidner, Jr., and L.L. Miller, Phys. Rev. B \textbf{58}, 497 (1998). $^{3}$W.H. Lee and H.K. Zeng, Solid State Commun. \textbf{102}, 433 (1997). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S10.00007: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S10.00008: Magnetron sputter deposition of a 48-member cuprate superconductor library: Bi$_2$Sr$_2$Y$_x$Ca$_{1-x}$Cu$_2$O$_{8+\delta}$ (0.5$\le x \le$1) linearly varying in steps of $\Delta x = 0.01$. Kevin Hewitt, Robert Sanderson Using magnetron sputtering, a spatial composition spread approach was applied successfully to obtain 48-member libraries of the Bi$_2$Sr$_2$Y$_x$Ca$_{1-x}$Cu$_2$O$_{8+\delta}$ (0.5$\le$x$\le$1) cuprate superconducting system. The libraries were deposited onto (100) single crystal MgO, mounted on a water cooled rotating table, using two targets: the antiferromagnetic insulator Bi$_2$Sr$_2$YCu$_2$O$_{8+\delta}$ (P=98 W RF) and the hole doped superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (P=44 W DC). A low chamber pressure of 0.81 mTorr argon is used to reduce scattering by the process gas. To minimize oxygen resputtering a substrate bias of -20 V was used as well as a process gas free of oxygen. A rapid thermal processor is used to post-anneal the amorphous deposited films following a step annealing regime - ramp at 5 $^{\circ}$C/s for heating and cooling, with a first plateau at 780 $^{\circ}$C held for 200 s, and a second at 875 $^{\circ}$C held for 480 s. X-ray diffraction reveals that the films develop crystalline order with the c-axis lattice parameter contracting linearly from 30.55 {\AA} (x=0.5) to 30.24 {\AA} (x=1.0) with increasing Y-content, consistent with bulk values. The films are polycrystalline, developing preferred orientation for thinner members of the library. There is a change of 0.01 in doping per library member which will enable further studies to densely map phase space. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S10.00009: Mechanical and superconducting properties of Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$Gd$_{x}$O$_{y}$ system Mustafa Akdogan, Ozgur Ozturk, Huseyin Aydin, Murat Erdem, Cabir Terzioglu The influence of addition of the rare earth ion \textit{Gd} in \textit{Bi}$_{1.8}$\textit{Pb}$_{0.35}$\textit{Sr}$_{1.9}$\textit{Ca}$_{2.1}$\textit{Cu}$_{3}$\textit{Gd}$_{x}O_{y}$ superconductor has been investigated by varying \textit{Gd} addition (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5). The samples were prepared by standard solid-state reaction methods. Phase analyses of the samples by X-ray diffraction (XRD), microstructure examination by scanning electron microscopy (SEM), superconducting properties by dc electrical resistivity and mechanical properties by static Vickers hardness measurements have been carried out to assess the effects of \textit{Gd} addition. These measurements indicated that the superconducting transition temperature, Vickers hardness, Young's modulus, yield strength, fracture toughness values of the samples strongly depend on the \textit{Gd} addition. The values of $T_{c}, H_{v}, E, Y$ and $K_{IC}$ of the samples decreased with the increase in \textit{Gd} addition. Mechanical properties of the samples are also found to be load dependent. In addition, we have calculated the true (load independent) hardness, Young's modulus, yield strength, and fracture toughness of the samples. From XRD and SEM analyses, the addition of the sample by \textit{Gd} degrades formation of the high-$T_{c}$ \textit{Bi-2223} phase, and surface morphology and grain connectivity in comparison with undoped sample, respectively. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S10.00010: Investigation of Some Physical Properties of Gd added Bi-2223 Superconductors Murat Erdem, Ozgur Ozturk, Huseyin Aydin, Mustafa Akdogan, Cabir Terzioglu, Ahmet Varilci, Ibrahim Belenli Static microindentation, XRD, SEM, DC electrical resistivity and critical current density measurements were performed to investigate some physical properties of Bi$_{1.8}$Pb$_{0.35}$Sr$_{1.9}$Ca$_{2.1}$Cu$_{3}$Gd$_{x}$O$_{y}$ superconducting samples with x=0.0, 0.1, 0.3, and 0.5. The indentation load versus diagonal length of the samples under different indentation loads in the range of 0.245-2.940 N were presented. The microindentation measurements showed that, for the Gd added sample, the load dependent (apparent) microhardness value (H$_{v})$ is lower in comparison with that of the pure sample(x=0). The values of H$_{v}$ were found to be load dependent. In addition, we extracted the load independent (true) microhardness using the Kick's law, proportional specimen resistance (PSR), modified proportional specimen resistance (MPSR) model and the Hays-Kendall (HK) approach and compared the true hardness with the apparent hardness. We observed from the transport measurements that addition of Gd decreased T$_{c}$ and J$_{c}$. In addition, surface morphology and grain connectivity of the samples were degraded and the high-T$_{c}$ phase of the samples decreased with increasing Gd addition. The possible reasons for the observed degradation in microstructure, superconducting and mechanical properties due to Gd addition were discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S10.00011: Transport microstructure and mechanical properties of Au diffusion-doped Bi-2223 superconductors O. Ozturk, C. Terzioglu, A. Varilci, I. Belenli We have investigated the effect of the gold-diffusion on the mechanical properties of the \textit{Bi-2223} superconducting samples with different annealing times (10, 20 and 50 hours). The samples are prepared by the conventional solid-state reaction method in the polycrystalline bulk form. Doping of Bi-2223 was carried out by means of gold diffusion during sintering from an evaporated gold film on pellets. The experimental works in this study consist of dc electrical resistivity measurements for electrical and superconducting properties, microhardness measurements for mechanical properties, X-ray diffraction for phase analyses and lattice parameters, and scanning electron microscopy for microstructure examination. These measurements showed that Au-doping, in comparison with the undoped samples, increased the critical transition temperature and enhanced formation of high-T$_{c}$ phase. Additionally, microhardness and grain size were also improved with increasing amount of diffusion. Moreover, the diffusion-annealing time decreased the number and size of voids and increased the transition temperature. The experimental results of hardness measurements were analyzed using the Kick's law, modified proportional specimen resistance (MPSR) model and the Hays-Kendall (HK) approach. Among them HK approach was the most successful. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S10.00012: Critical current density variations with increasing thickness in YBa$_{2}$Cu$_{3}$O$_{7-x}$+ BaSnO$_{3}$ (BSO) films Chakrapani Varanasi, Jack Burke, Lyle Brunke, Haiyan Wang, Paul Barnes To increase the engineering critical current density (J$_{e})$ of YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) films, it is of great importance to grow thicker films with high critical current density (J$_{c})$. However, it has been shown that in pulsed laser ablated (PLD) YBCO films, as the thickness is increased beyond 1 $\mu $m, the J$_{c}$ of the films decreases. Earlier work by this group showed that YBCO+BaSnO$_{3}$ (BSO) films of $\sim $ 300 nm thickness can be grown with more than an order of magnitude increase in the J$_{c}$ in applied magnetic fields using a dual phase sector PLD target approach. In the present work a systematic study of J$_{c}$ dependence on the thickness of YBCO+BSO thick films was undertaken by growing different films with thicknesses ranging from 300 nm to 4 $\mu $m. The J$_{c}$ of these films was measured using a magnetometer indicated that high J$_{c}$ at high fields can be maintained even in thicker films. The cross-sectional TEM analyses of the thick films showed that the BSO nanocolumns grow through out the entire thickness of the samples. Microstructural details and the superconducting properties of thick YBCO+BSO films will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S10.00013: Superconductivity in undoped $T$'-RE$_{2}$CuO$_{4}$ with $T_{C }>$ 30 K Michio Naito, Osamu Matsumoto, Aya Utsuki, Akio Tsukada, Hideki Yamamoto, Takaaki Manabe We report the superconductivity in $T'$-\textit{RE}$_{2}$CuO$_{4}$ (\textit{RE} = Pr, Nd, Sm, Eu, and Gd), which have been for a long time believed as a Mott insulator. The highest $T_{c}$ of undoped $T'$-\textit{RE}$_{2}$CuO$_{4}$ is over 30 K, substantially higher than that of `electron-doped' analogs. Remarkably, Gd$_{2}$CuO$_{4}$, even the derivatives of which have not shown superconductivity so far, gets superconducting with $T_{c}^{onset}$ as high as 20 K. Our discovery was achieved by using metal-organic decomposition (MOD), an inexpensive and easy-to-implement thin-film process. The keys to prepare the superconducting films are rather simple, namely low-$P_{O2}$ firing and subsequent low-temperature reduction. One point to be emphasized is that low-$P_{O2}$ phase field has been almost unexplored in the search for new superconductors because of the belief that high $P_{O2}$ \textit{should} be required in the synthesis of Cu$^{2+}$ compounds. Our discovery contradicts with the past results supporting undoped mother compounds, $T'$-\textit{RE}$_{2}$CuO$_{4}$, to be insulating. The clue to understanding the sharp contrast between the past and our results is impurity oxygen (O$_{ap})$ at the apical site, which has to be cleaned up in order to reach the `generic' electronic phase diagram. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S10.00014: NbN films grown by chemical solution deposition Guifu Zou, Menka Jain, Hongmei Luo, S.A. Baily, T.M. McCleskey, E. Bauer, A.K. Burrell, Q. Jia NbN films were grown on quartz substrate using a chemical solution technique of polymer-assisted deposition for the first time. The precursor films were annealed at different temperatures in ammonia atmosphere. X-ray diffraction and electron microscopy analysis indicated that the films were polycrystalline. Preliminary optical spectroscopy results of these films showed several strong peaks in the visible range that can be attributed to the NbN phase. Wide peaks in the photoluminescence spectrum suggest many defects in these films. The transition temperature (measured from SQUIDs) of these films also will be discussed in this paper. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S10.00015: Superconductivity in YbGa$_x$Si$_{2-x}$ with the AlB$_2$-type structure N. Tsujii, M. Imai, H. Yamaoka, H. Ohashi, D. Nomoto, I. Jarrige, T. Tochio, K. Handa, J. Ide, H. Atsuta, Y. Ito, H. Yoshikawa, H. Kitazawa The discovery of superconductivity in MgB$_2$ has triggered the search for novel superconductors with the AlB$_2$-type structure. Very recently, we discovered superconductivity in the AlB$_2$- type compound YbGa$_{1.15}$Si$_{0.85}$ below $T_{\rm C}$ = 2.5K $^{*}$. We report here on the structural and electronic properties of the YbGa$_x$Si$_{2-x}$ series. XRD and SEM/EDS analysis suggested that the AlB$_2$-type phase can be sustained for $1.0 \leq x \leq 1.4 $. $T_{\rm C}$ is found to decrease from 2.5K for $x$ = 1.0 to 1.9K for $x$ = 1.3 and eventually vanish for $x$ = 1.4. High-resolution x-ray absorption spectra across the Yb-L$_{III} $ edge were measured at SPring-8 on the beamline BL15XU. The valence of Yb was estimated to be 2.3+, suggesting a predominant Yb$^{2+}$ character with minor importance of 4f electrons in the superconductivity.\newline $^{*}$ M. Imai et al., submitted. [Preview Abstract] |
Session S11: Theory of Superconductivity in Cuprates I
Sponsoring Units: DCMPChair: Alexander Balatsky, Los Alamos National Laboratory
Room: Morial Convention Center RO9
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S11.00001: Cuprate higher harmonic gap structure: theory vs. experiment David Parker, Alexander Balatsky We present a detailed comparison to experiment of the generalized gap symmetry predictions of spin-fluctuation mediated superconductivity theory in the hole-doped and electron-doped cuprates, within a weak-coupling BCS framework. We comment on the implications of these results for the ongoing ``one gap vs two gap'' controversy in the cuprates, and discuss the impact of these results on the quasiparticle lifetime model of the cuprate Fermi arcs. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S11.00002: Sublattice model of atomic scale pairing inhomogeneity in a superconductor Vivek Mishra, P.J. Hirschfeld, Yuri S. Barash We study a toy model for a superconductor on a bipartite lattice, where intrinsic microscopic inhomogeneity is produced by two different pairing coupling constants on each sublattice. We consider effects of the inhomogeneity on the transition temperature, the density of states, the specific heat and superfluid density in the framework of the Bogoliubov-de Gennes equations, which may be solved analytically in several interesting cases. The phase diagram in the plane of two pairing coupling constants is found to include a state of gapless superconductivity. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S11.00003: Suppression of $d$-wave superconductivity in the weakly inhomogeneous checkerboard Hubbard Model D.G.S.P. Doluweera, M. Jarrell, Th. Maier, A. Macridin, Th. Pruschke Using a dynamical cluster quantum Monte Carlo approximation we investigate the $d$-wave superconducting transition temperature T$_{c}$ of the doped 2D Hubbard model with a weak inhomogeneity in the form of checkerboard pattern in the hoppings. The hopping within a 2 $\times $ 2 cluster (plaquette) is $t$ and the hopping between the plaquettes is $t'$ ( 0.8$t \le $ \textit{t' $\le $ t }). We find T$_{c}$ decreases monotonically with decreasing $t' $ for both fixed $U/t$ or $U/W$ ($U$ the on site Hubbard interaction and $W$ the bandwidth). The characteristic spin excitation energy scale and the strength of $d$-wave pairing interaction decrease with decreasing T$_{c}$ suggesting a strong correlation between these two quantities. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S11.00004: Electronic properties investigation of YBaCuO using the PAW formalism Simon Pesant, Michel C\^ot\'e Using density-functional theory (DFT) and the projector-augmented wave (PAW), we characterize the electronic properties of the YBa$_{2}$Cu$_{3}$O$_{7-x}$. These systems are metallic or antiferromagnet at room temperature depending on the hole doping induced by oxygen atoms in the basal plane. Also, the impact of an onsite coulomb repulsion term on the DFT know as LDA+U, is investigated in the different structures to take into account the highly correlated character of the electrons of those systems. The addition of an onsite coulomb repulsive term does not strongly alter the electronic properties of the YBa$_{2}$Cu$_{3}$O$_{7}$, and YBa$_{2}$Cu$_{3}$O$_{6.5}$ but gives more accurate details about the electronic structure. In the case of YBa$_{2}$Cu$_{3}$O$_{6}$, the impact of the U term is primordial, the anti-ferromagnetism phase being recovered when LDA+U is used, compared to the standard LDA where it is metallic. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S11.00005: Electronic properties and chain conductivity of underdoped YBa$_2$Cu$_3$O$_{6+x}$ by First-Principles calculations Vincenzo Fiorentini, Alessio Filippetti, Giorgia Lopez, Mauro Mantega Metal-insulating transitions in cuprates represent an historical challenge for first-principles calculations. Here we present results obtained through the pseudo-self-interaction free density functional scheme (PSIC) that is capable to correct the gross failures of LSDA at just a moderate increase of computing effort, and works well in both strong-correlated and metallic limit. Here we describe the properties of the end-point systems YBa$_2$Cu$_3$O$_6$ and YBa$_2$Cu$_3$O$_7$ as well as the chemistry of insulating-metal transition occurring in the CuO chains of underdoped YBa$_2$Cu$_3$O$_{6+x}$ in the region x=[0,0.5]. Coherently with the one-dimensional metallic percolative regime observed at low-doping, we find that the metal-insulating transition occurring at low doping in the non-magnetic Cu(1)Ox chains is induced by chain-like alignment of the doping oxygens within the chains, whereas disorder (i.e. non chain-aligned) distributions are always insulating. In the Cu(2)O2 planes the insulating antiferromagnetic state remains stable up to x=0.25, while at x=0.5 a normal-metal state, similar to that seen for YBa$_2$Cu$_3$O$_7$, take place. The in-plane antiferromagnetic- paramagnetic competition depends on x but is almost unaffected by the intra-chain order-disorder competition. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S11.00006: Holes localization and Fermi Surface morphology of Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ by First-Principles Giorgia Lopez, Alessio Filippetti, Vincenzo Fiorentini The basic chemistry of underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ (and of high-T$_c$ superconductors) is permeated by unexplained features. One, in particular, concerns the nature of the non- superconducting phase and its Fermi Surface (FS) whose character is not univocally described by angle-resolved photoemission and Hall measurements, which detect disconnected arcs and pockets, respectively. But what is really missing for a sound interpretation of these data is a robust link between the observed FS and the corresponding electronic structure. As the description of the underdoped regime is hardly accessible through standard First-Principles calculations (FPC), here we use the innovative pseudo-self-interaction corrected local spin density (PSIC) approach to trace an accurate overview of underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$, with special emphasis on the FS morphology. In particular, the PSIC can predict the formation of Zhang-Rice singlets (ZRS), and we show that electronic states characterized by a mixture of ZRS and antiferromagnetic CuO$_2$ units present, in fact, a variegated series of differently-shaped, disconnected FS that may reconcile the experimental data with a sound interpretation of the underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S11.00007: Nature of high-temperature superconductivity John D. Dow Using muon spectroscopy, the high-temperature superconductivity of YBa(2)Cu(3)O(7) is shown to reside in its BaO layers, not in its cuprate planes. The symmetry of the hole-pairing is s-like, not d-like. The family of superconductors Pb(2)Sr(2)Y(1-x)R(x)Cu(3)O(8) can be doped p-type (with R=Ca) or n-type (with R=Ce or Am). The n-type versions do not superconduct, but the p-type compounds do superconduct. The doped ruthenate Ba(2)YRuO(6) begins superconducting in its BaO layers at 92 K. A successful theory of high-temperature superconductivity must explain the ruthenates, the superconducting organic compounds, and the superconducting cuprates. Presently none do. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S11.00008: Percolation Theory of the Pseudogap Phase Alexei Abrikosov A concept of the pseudogap state in high-$T_c $ layered cuprates on the basis of percolation theory is proposed. Contrary to the self-consistent BCS critical temperature, which defines $T^{\ast}$ - the upper boundary of the psesudogap state, the real critical temperature, $T_c$, is defined, as the percolation threshold, where the infinite cluster appears. This permits to obtain the exact formula for $T_c$, as function of doping and understand its ``dome-like'' shape. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S11.00009: Pairing in Non-Fermi Liquids in Terms of Bethe-Salpeter Equation Yuriy Malozovsky, J.D. Fan The pairing between two fermionic excitations in a non-Fermi liquid is considered in terms of the Bethe-Salpeter equation. We consider the pairing in the Fermi systems with vanishing spectral weight. It is well known that the quasiparticle pole in the single-particle Green's function in non-Fermi liquids is absent or weak. The examples of such systems can be viewed as the ``Marginal'' Fermi liquid and Luttinger liquid. Another example that has also been considered is the Fermi system with pseudogap behavior in the spectral weight. Although the pairing between two excitations in non-Fermi liquids is, in general, absent, yet we show that the Cooper's type pairing can occur and the conditions for such pairing are discussed. The results have also application to the systems with smeared or non-monotonic Fermi distribution. The system that transits from the Fermi type to Bose type behavior can be a showcase of such systems as discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S11.00010: Multiple Pairing in the BCS Model J.D. Fan, Yuriy Malozovsky We study Cooper's paring for more than two particles in terms of the BCS model. We consider the multiple pairing in terms of the BCS Hamiltonian in a quiescent Fermi sea model and in the BCS ground state. Although there is no interaction between Cooper pairs in terms of the BCS Hamiltonian, yet we show that four particles are paired and form a bound state in the singlet state with just twice the bound state energy of a single Cooper's pair. The four-particle bound state only exists as the result of the Pauli principle and the sharp Fermi edge. We have also shown that the smearing of the Fermi edge due to $\Delta \left( k \right)$ as it is in the BCS ground state weakens the pairing of either two or four particles. We show that in the particle-hole channel there exists the multiple particle-hole resonance for four particles and four holes in a quiescent Fermi sea model similar to the case of two particles and two holes resonance. There is no particle-hole resonance in the BCS ground state as shown, which means that the particle-hole resonance is removed by $\Delta \left( k \right)$ due to smearing of the Fermi distribution. The wave function for the multiple-pairing model is discussed as well. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S11.00011: Extraction of the pairing glue spectra of high Tc superconductors Han-Yong Choi, Jaehyun Yoon, Takeshi Kondo, Adam Kaminski, Chandra Varma We report the current progress of extracting the ``pairing glue'' spectra of high Tc superconductors. This is done by inverting the d-wave Eliashberg equation, which is an extension of the McMillan-Rowell analysis of the tunneling conductance for conventional s-wave superconductors. A major difference is that there are two distinct $\alpha ^2F$ functions for the d-wave superconductors. Consequently, we need twice more experimental inputs to perform this analysis; the pairing function $\Delta (\omega )$ and self-energy $\Sigma (\omega )$. This experimental information is currently not available. We therefore first generate $\Delta (\omega )$ and $\Sigma (\omega )$ theoretically using the marginal Fermi liquid like glue spectra. Then, using the generated functions as ``experimental inputs'' we invert the Eliashberg equation to extract the glue spectra. We will compare the input and extracted glue spectra to demonstrate the applicability of the approach. We will next describe how to obtain experimental $\Delta (\omega )$ and $\Sigma (\omega )$ from ARPES data. Then, using the obtained experimental inputs we invert the Eliashberg equation to extract the glue spectra. The results will be reported for several temperatures above and below Tc. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S11.00012: A competing order scenario of two-gap behavior in hole doped cuprates Tanmoy Das, R.S. Markiewicz, A. Bansil Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature $T_c$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations[1] which show that most of the characteristic features observed in the recent angle-resolved photoemission (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order. Work is supported in part by the USDOE. \newline [1] Tanmoy Das, R. S. Markiewicz, and A. Bansil, arXiv/0711.0480. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S11.00013: Local quasiparticle lifetimes in a d-wave superconductor S. Graser, P.J. Hirschfeld, D.J. Scalapino Recently, scanning tunnelling spectroscopy (STS) measurements have exhibited good fits to conductance spectra at the surface of Bi-2212 using a BCS-type model for a d-wave superconductor and assuming a local quasiparticle scattering rate varying linearly with energy. Employing a model of quasiparticle scattering by impurities and spin fluctuations we argue that the broadening of the local density of states is in general given by the self-energy of the system averaged over a small region. The size of this region at low energies is shown to be significantly larger than a gap ``patch'', a region over which the gap is roughly constant in this system; states measured by STS are therefore very homogeneous in this energy range. At energies above a scale determined by disorder, STS averages over states localized within a gap ``patch'', and lifetimes are correspondingly inhomogeneous. We show that the local self-energy in the impurity-plus-spin fluctuation model can explain the data as well as the phenomenological linear scattering rate extracted from experiment. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S11.00014: Disordered superconductors: role of interaction strength Raimundo dos Santos, Felipe Mondaini, Thereza Paiva, Richard Scalettar We have considered the half-filled disordered attractive Hubbard model, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. The configurationally-averaged equal-time pair structure factor has been calculated as a function of temperature, through Quantum Monte Carlo simulations for several $f$ and $U$, and a finite-size scaling {\it ansatz} has been used for the zero-temperature gap. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to the largest values of $U$ we have considered. Also, the normalized zero-temperature gap as a function of $f$, for fixed $U$, shows a maximum near $f_m$, within a range of $U$ values, thus indicating that a small amount of disorder can initially enhance superconductivity. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to ``push'' electrons towards attractive sites. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S11.00015: A singlet-pairing superconductor is always also a super-spin-current-conductor. Chia-Ren Hu A heuristic argument and a simple theory are used to show that, as a fundamental difference between BEC and BCS condensation of fermion pairs, \textit{the later, even for singlet pairing, can carry a sizable dissipation-less spin-current below practically the same T}$_{C}$. The heuristic argument is based on the similarity between a spin-current carried by a singlet-pairing condensate and (coherent) partner changing in a dancing hall. Simple theory: We consider singlet pairing in a normal metal carrying a moderate spin-current, which causes the spin-up- and -down Fermi surfaces (FSs) to be shifted in the momentum space by $\pm $\textbf{q}/2. [(k,$\uparrow )$,(-k,$\downarrow )$]-pairing is clearly still possible over the entire FSs. To favor a spin current in the system, we introduce a vector Lagrange multiplier \textbf{v}$_{sp}$, and add -\textbf{v}$_{sp}$\textbf{$\cdot \Sigma $ }$_{k, \sigma }\sigma $ h\textbf{k c}$_{ k,\sigma }^{\dag }$\textbf{c}$_{ k, \sigma }$to the Hamiltonian. Since time-reversal invariance is not broken, negligible changes to all properties of the singlet-pairing state follow, and the system remains fully gapped. No depairing can be induced even for a sizable spin current. Two experimental tests of this prediction will be discussed. [Preview Abstract] |
Session S12: Strongly Correlated Quantum Phases
Sponsoring Units: DCMPChair: Shailesh Chandrasekharan, Duke University
Room: Morial Convention Center 203
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S12.00001: Fate of the Fermionic Quasiparticles at the Electronic Nematic-Smectic Quantum Critical Point Kai Sun, Benjamin Fregoso, Eduardo Fradkin We use the order-parameter theory of the electronic nematic- smectic transition of the fermionic liquid crystal phases, discussed in the previous talk, to study the effect of the low energy bosonic modes on the fermionic quasiparticles using RPA. Both the continuous model, which has a continuous rotational symmetry, and the lattice model, which has a discrete point group symmetry are studied. We find that at the nematic-smectic critical point, due to the critical smectic fluctuations, the dynamics of the fermionic quasiparticles near several points on the Fermi surface, which eventually become gapped under the development of CDW order, are not governed by a Landau Fermi liquid. Surprisingly, the fermions in the smectic phase also form a non-Fermi liquid. The transition between the quantum liquid crystal phases and the insulating CDW state is also discussed. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S12.00002: Novel Transitions in S=1 Spinor Condensates and XY Ashkin-Teller Universality Shailesh Chandrasekharan, Daniel Poldolsky, Ashvin Vishwanath We study spin-1 polar spinor condensates with magnetic anisotropy, in two spatial dimensions at finite temperatures. The topological binding of vorticity to nematic disclinations leads to a rich phase diagram, which is captured by a U(1) version of the Ashkin-Teller model. In particular, a ``cascaded'' Kosterlitz-Thouless critical point, with two diverging scales, is predicted. Numerical simulations are performed to check our picture. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S12.00003: Theory of the Nematic-Smectic Quantum Phase Transition in Strongly Correlated Electronic Systems Benjamin Fregoso, Kai Sun, Eduardo Fradkin The quantum liquid crystal phases were first proposed in Ref. [1], as one possible way to understand the high $T_c$ superconductors. We discuss the quantum phase transition between a quantum nematic metallic state (a uniform state which breaks spontaneously the point group symmetry) to an electron metallic smectic state (a state with a unidirectional charge density wave order), and construct an order-parameter theory. Its static part has the McMillan-DeGennes form of the classical smectic-nematic phase transition, while its quantum dynamics is dominated by the coupling to the electronic quasiparticles. Both, commensurate and incommensurate cases are studied. The spectrum of the nematic phase has low energy ``fluctuating stripes''. We also provide evidence that, contrary to the classical case, the gauge-type of coupling between the nematic and smectic at the critical point is irrelevant at this QCP. We discuss the relevance of these ideas to the phenomenology of the high $T_c$ superconductors. [1] S. A. Kivelson, E. Fradkin and V. J. Emery, nature 393, 550, 1998. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S12.00004: Universal Scaling in the Fan of an Unconventional Quantum Critical Point Roger Melko, Ribhu Kaul We present the results of stochastic series expansion Quantum Monte Carlo simulations on a 2D S=1/2 Heisenberg model with additional four-spin interaction -- the so-called `JQ' model [1]. Using extensive simulations on lattice sizes containing in excess of 10$^4$ spins, we examine the claim that the observed N\'eel to valence-bond-solid (VBS) quantum phase transition is consistent with the `deconfined' quantum criticality scenario. We discuss finite-temperature properties of the conjectured quantum critical fan [2], including scaling behavior, the calculation of universal critical exponents, and the apparent emergence of a global U(1) symmetry in the VBS order parameter. Finally, we consider several extensions of the model that may help give further insight into the nature of this unconventional quantum phase transition. \newline \newline [1] Sandvik, Phys. Rev. Lett. 98, 227202 (2007). \newline [2] Melko and Kaul, arXiv:0707.2961. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S12.00005: Quantum critical scaling behavior of deconfined spinons Flavio Nogueira, Steinar Kragset, Asle Sudbo The quantum scaling behavior of deconfined spinons for a class of field theoretic models of quantum antiferromagnets is considered. The competition between the hedgehogs and the Berry phases is discussed from a renormalization group perspective. An important result following from our analysis is the computation of the anomalous dimension for the decay of spin correlations. Our results confirm the expectation that the transition from a N\'eel to a valence-bond solid state belongs to a completely new universality class. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S12.00006: Monte-Carlo simulations of su(2) symmetric deconfined criticality action A.B. Kuklov, M. Matsumoto, N.V. Prokof'ev, B.V. Svistunov, M. Troyer We discuss results of Monte Carlo simulations of su(2) symmetric deconfined criticality action in $CP^1$ formulation proposed by T. Senthil,et. al, Science {\bf 303}, 1490 (2004). Using high-temperature expansion we reformulate the partition function in terms of J-currents. The resulting configuration space is explicitly su(2) symmetric. Critical behavior in the region of possible deconfined critical point (DCP) is addressed by the flow method [A.B.Kuklov, et.al., Annals of Physics {\bf 321},1602(2006)] mapping critical properties of a system with small values of the gauge interaction $g$ at large sizes to a system with large $g$ and small sizes. We observe data collapse on a single master curve with the flow toward fluctuation induced I order transition. The unlikely possibility of existence of the lower tricritical point separating I order transitions from the DCP line is assessed in terms of disruption of the flow collapse. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S12.00007: Phase diagram of the anisotropic two-dimensional bilinear biquadratic spin-1 Heisenberg model Christoph Puetter, Michael Lawler, Hae-Young Kee The anisotropic bilinear biquadratic Heisenberg model on a square lattice has been proposed to exhibit deconfined critical phenomena (DCP) based on QMC simulations and effective field theoretical studies [1, 2]. We investigate the phase transitions of the model using slave boson representation. Our mean field approach suggests a first order transition between the nematic and the disordered regime except at the tricritical SU(3) symmetric point. We will also discuss the relevance of our results to the DCP. \newline [1] T. Grover and T. Senthil, Phys. Rev. Lett. 98, 247202 (2007) \newline [2] K. Harada, N. Kawashima and M. Troyer, J. Phys. Soc. Jpn. 76, 013703 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S12.00008: Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models Ralko Arnaud, Poilblanc Didier, Moessner Roderich We revisit the phase diagram of Rokhsar-Kivelson models, which are used in fields such as superconductivity, frustrated magnetism, cold bosons, and the physics of Josephson junction arrays. From an extended height effective theory, two simple generic phase diagrams are obtained. The first one is a first order transition scenario between the columnar and the plaquette phases, common in such models. The second, more exotic, exhibits a second order transition and contains a mixed phase that interpolates continuously between columnar and plaquette states. From exact diagonalization and Green's funtion Monte Carlo techniques, we present evidence that a realization of the latter scenario occurs in the Rokhsar-Kivelson square lattice Quantum Dimer model. This model, originally proposed in the context of high-temperature superconductivity, and its descendants have taken on a central role in the study of quantum systems incorporating a hard local constraint. By combining an analysis of the excitation gaps of different symmetry sectors with information on plaquette structure factors, we show the presence of a phase exhibiting both the plaquette and the columnar properties. This also presents a natural framework for resolving the disagreement between previous studies. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S12.00009: An exact chiral spin liquid with non-Abelian anyons Hong Yao, Steven Kivelson We establish the existence of a ``chiral spin liquid'' (CSL) as the exact ground state of the Kitaev model on a decorated honeycomb lattice, which is obtained by replacing each site in the familiar honeycomb lattice with a triangle [1]. This state spontaneously breaks time reversal symmetry but preserves other symmetries. There are two topologically distinct CSL's separated by a quantum critical point. Interestingly, vortex excitations in the topologically nontrivial CSL (Chern number $\pm 1$) obey non-Abelian statistics. \\ $[1]$ Hong Yao and Steven A. Kivelson, Phys. Rev. Lett. in Press. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S12.00010: Non-abelian topological phases and unconventional criticality in a model of interacting anyons Charlotte Gils, Simon Trebst, Matthias Troyer, Andreas Ludwig, Alexei Kitaev Non-abelian topological phases have recently attracted considerable interest in the context of fault-tolerant quantum computation. However, such phases have only been established in a small set of microscopic models, one of which involves interacting spin-1/2 degrees of freedom on a honeycomb lattice (Levin, Wen 2005). In particular, this model supports quasiparticle excitations that can be described as so-called Fibonacci anyons. We have reformulated this model in terms of anyonic degrees of freedom and consider the case of interacting anyonic quasiparticles by adding a magnetic field term to the Hamiltonian. Our analysis of a quasi-one-dimensional ladder model not only shows the (extended) stability of the topological phase when perturbed by such local terms, but also demonstrates the role of topology in determining the exact nature of these phases. Interestingly, the magnetic field can drive a phase transition between two distinct topological phases. Numerically, we establish that this critical point can be described by a conformal field theory with central charge c=14/15. This observation has lead to an analytical understanding of this critical point which can be mapped to an exactly solvable transfer matrix representation in terms of a restricted-solid-on-solid (RSOS) model. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S12.00011: Topological Mott Insulators Srinivas Raghu, Xiao-Liang Qi, Carsten Honerkamp, Shou-Cheng Zhang We consider extended Hubbard models with repulsive interactions on a Honeycomb lattice, and the transitions from the semi-metal to Mott insulating phases at half-filling. Due to the frustrated nature of the second-neighbor interactions, topological Mott phases displaying the quantum Hall and the quantum spin Hall effects are found for spinless and spinful fermion models, respectively. The mean-field phase diagram is presented fluctuations are treated within the random phase approximation (RPA). Renormalization group analysis shows that these states can be favored over the topologically trivial Mott insulating states. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S12.00012: Critical liquid phases for frustrated bosons in two dimensions Olexei Motrunich, Matthew P.A. Fisher An interesting question in strongly correlated systems is the possibility of a ``metallic'' bosonic liquid -- a quantum liquid phase of bosons that is neither superfluid nor Mott insulator. We present an attempt to construct such states using slave particle technique borrowed from theories of spin liquids; the approach can be also loosely viewed as a flux attachment treatment performed in the absence of time reversal breaking. We describe properties of thus constructed boson liquid states, which support gapless boson excitations residing on ``Bose surfaces'' in the momentum space and exhibit power law correlations in various properties. We also suggest a promising model Hamiltonian of hard-core bosons hopping on a square lattice and with frustrating ring exchanges which may have such a phase. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S12.00013: Pseudogap in strongly interacting and strongly disordered systems Simone Chiesa, Prabuddha Chakraborty, Warren Pickett, Richard Scalettar The interplay of disorder and correlation is known to give rise to anomalies in the density of states at the chemical potential of quantum and classical systems. In particular, in the quantum case, the diagrammatic calculation of Altshuler and Aronov predicts a pseudo-gap for the case of a weakly disordered and weakly interacting metal. Here we report a numerical study suggesting that such anomalies are present also in the case of strongly interacting and strongly disordered systems. We consider the Hubbard model in the presence of diagonal disorder and diagonalize small clusters (up to 12 sites) in the framework of a grand canonical scheme involving twisting the boundary condition. For a given interaction and disorder strength we observe the formation of a pseudo-gap whose shape and depth is largely insensitive to the particle density. For a given particle density the pseudo-gap gets deeper as the interaction and the disorder are increased. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S12.00014: Bose Metal Phase from Inhomogeneous Flow Gergely Zimanyi, Niels Jensen Numerous experiments report a Bose Metal phase between the Superconducting (S) and the Insulating (I) phases at an SI transition. [1,2] However, theoretically the origin of the corresponding dissipation remains unclear. We propose a picture in which inhomogeneous superconducting flow occurs in channels/filaments, defined by islands of localized Bose Glass. The superconducting bosons interact with the localized bosons of the Bose Glass via the Coulomb interaction. This Coulomb drag generates an effective dissipation for the superflow. We developed a new numerical technique to simulate superconductivity by inertial dynamics and a current generator. We found a Bose Metal phase in a finite range of the disorder, bracketed by the superconducting and insulating phases. The noise spectrum was also determined and compared to recent experiments. \newline \newline [1] H.M. Jaeger, D.B. Haviland, B.G. Orr and A.M. Goldman, Phys. Rev. B 40, 182 (1989). \newline [2] A. Yazdani and A. Kapitulnik, Phys. Rev. Lett. 74, 3037 (1995); M. Steiner, N. Breznay and A. Kapitulnik, arxiv: 0710.1822. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S12.00015: Double Occupancy in low-energy theoreis of doped Mott insulators Philip Phillips, Ting Pong Choy, Robert Leigh, George Sawatzky We review how a proper low-energy theory can be constructed for the Hubbard model by explicitly integrating over the degrees of freedom far away from the chemical potential. A surprsing feature of the exact low-energy theory is the emergence of an elementary charge 2e boson which mediates double occupancy much below the Mott scale. We show that within the standard canonical transformation formalismused to derive the $t-J$ model from the Hubbard model, a similar feature (double occupancy below the Mott scale) appears ONLY if the electron creation and annihilation operators are properly transformed as well. By comparing precisely how the electron operators transform in both theories, we are able to show that the charge 2e boson mediates dynamical spectral weight transfer across the Mott gap. At half-filling, the interactions mediated by the charge 2e boson defeat the artificial local SU(2) symmetry found earlier in the projected $t-J$ model. \\ R. G. Leigh, P. Phillips and T. -P. Choy, Phys. Rev. Lett. {\bf 99} 46404 (2007); arxiv:07071554 (PRB, in press). [Preview Abstract] |
Session S13: Focus Session: Frontiers in Electronic Structure Theory I
Sponsoring Units: DCOMP DCPChair: Andrew Rappe, University of Pennsylvania
Room: Morial Convention Center 204
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S13.00001: Total and self-energies beyond LDA and GGA: exact-exchange, {\it GW} and MP2 united by numeric atom-centered orbitals Xinguo Ren, Andrea Sanfilippo, Alexandre Tkatchenko, Patrick Rinke, Volker Blum, Karsten Reuter, Matthias Scheffler Well-known deficiencies of (semi-)local exchange correlation functionals in density functional theory comprise the spurious self-interaction, the absence of long-range correlation, and the absence of the derivative discontinuity with respect to the electron number. Present approaches to overcome these deficiencies (e.g., hybrid functionals, MP2, and {\it GW}) typically involve expensive two-electron Coulomb repulsion integrals. For molecules, the resulting numerical effort usually restricts these methods to Gaussian basis functions. We here show how all these methods can be handled accurately with efficient all-electron numerical atom-centered basis sets [1], by using a second, auxiliary basis for products of basis functions (resolution of the identity). For an extended set of finite systems spanning small molecules (water dimer, benzene), metal clusters (Na$_n$) and biomolecules (polyalanine peptides), we demonstrate that the efficiency of optimized numeric atom-centered basis sets is directly carried over into our new approach. Our approach is then applied to analyze the CO-adsorption problem (CO/Cu(111)). [1] V. Blum {\it et al.}, The FHI-aims project, www.fhi-berlin.mpg.de/aims [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S13.00002: Order N Implementation of Exact Exchange Xifan Wu, Annabella Selloni, Roberto Car Exact (Hartree Fock) exchange is needed to overcome some of the limitations of local and semilocal approximations of density functional theory (DFT). Moreover exact exchange is a basic ingredient in modern approaches to compute excitation properties, like the GW and the OEP schemes. So far, however, computational cost has limited the use of exact exchange in plane wave calculations for extended systems. We show that this difficulty can be overcome by performing a unitary transformation from Bloch to Maximally Localized Wannier functions in combination with an efficient technique to compute real space Coulomb integrals. The resulting scheme scales linearly with system size and, when used in ab-initio molecular dynamics simulations, requires only a modest increase in computational cost compared to standard DFT implementations. We validate the scheme with representative applications. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S13.00003: Accurate and fast DFT calculations with the AM05 functional Ann E. Mattsson The AM05 functional [1] has the same excellent performance for solids as the hybrid density functionals tested in Paier \emph{et.\ al.}\ (J. Chem. Phys {\bf 124}, 154709 (2006); \emph{ibid} {\bf 125}, 249901 (2006)). This confirms the original finding that AM05 performs exceptionally well for solids and surfaces. While hybrid functionals are computationally expensive, preveting them from being used in large systems and/or long molecular dynamics simulations, the AM05 functional is on a regular semi-local GGA form with corresponding computational cost. The performance of AM05 is even superior to an `informed choice' between LDA and PBE. By comparing data from different electronic-structure codes we have determined that the numerical errors in this study are equal to or smaller than corresponding experimental uncertainties. Results for other systems will also be presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. \newline [1] R. Armiento and A. E. Mattsson, Phys. Rev. B {\bf 72}, 085108 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S13.00004: Beller Lectureship Talk: Exploring Exact Exchange for collinear and non-collinear magnetism Invited Speaker: In standard density functional theory, one usually describes exchange and correlation effects by approximations on the same footing, where typical examples are the local density approximation (LDA) and different flavours of the generalized gradient approximation (GGA). Only in recent years the exact exchange (EXX) or optimized effective potential (OEP) method has been investigated in more detail, where most of the work has been dedicated to semiconductors [1]. Here we explore the OEP approach for magnetic materials. We provide a general description of the method which comprises the case of non-collinear magnetism. The equations for the effective Kohn-Sham scalar potential and magnetic field are derived within this framework, where the exact exchange energy functional explicitly depends on two-component spinor orbitals. The example of a magnetically frustrated Cr monolayer shows [2] that the resulting magnetization density exhibits much more non-collinear structure compared to LDA. The finding that a time-dependent generalization of the non-collinear OEP method can be a promising approach for an \textit{ab-initio} description of spin dynamics provides an interesting outlook for future work. In the present study, a series of materials is investigated in view of the reliability of the OEP method in terms of their magnetic properties. \newline [1] M. St\"{a}dele, M. Moukara, J. A. Majewski, P. Vogl, and A. G\"{o}rling, Phys. Rev. B 59, 10031 (1999). \newline [2] S. Sharma, J. K. Dewhurst, C. Ambrosch-Draxl, N. Helbig, S. Kurth, S. Shallcross and L. Nordstr\"{o}m, and E. K. U. Gross, Phys. Rev. Lett. 98, 196405 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S13.00005: A GGA+U approach to realistic modeling of transition-metal complexes Heather Kulik, Nicola Marzari Despite the importance of transition metal centers in a variety of biological and inorganic chemical reactions, density functional theory calculations often fail quantitatively in describing both the stable intermediate electronic structures, splittings, and geometries as well as reaction barriers and geometries of transition states. We have shown$^1$ that augmenting the generalized-gradient approximation (GGA) with a Hubbard U which is obtained from a self-consistent linear response procedure can greatly improve the description of both spin state splittings in the iron dimer as well as reaction barriers in the addition-elimination reaction of hydrogen and methane with FeO$^+$. This fully ab-initio GGA+U approach provides excellent agreement with accurate, correlated-electron quantum chemistry calculations but at a fraction of the cost of these methods. We will further highlight how our method affords substantial improvement in the physical description of hybridization and bonding irrespective of system size. We thus fruitfully employ GGA+U in the study of large-scale complexes which contain hundreds of atoms such as the active site of halogenating enzymes and various porphyrin complexes. \\ \\ $^1$ H. J. Kulik, M. Cococcioni, D. Scherlis and N. Marzari, PRL \textbf{97} 103001, (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S13.00006: Accurate description of the bonding of C$_6$H$_6$ at noble metal surfaces, using a local exchange-correlation correction scheme Erik McNellis, Karsten Reuter, Matthias Scheffler The adsorption of benzene (C$_6$H$_6$) at the Cu(111) surface is a much studied model system for the interaction of larger $\pi$-conjugated molecules with solid surfaces. At first glance a simple system, the suspected predominantly van der Waals type bonding at the extended metal surface poses a severe challenge for accurate first-principles calculations. Density-Functional Theory (DFT) with local and semi-local exchange-correlation (xc) functionals is uncertain to properly account for this type of bonding, while the system sizes required to correctly grasp the metallic band structure are computationally untractable with correlated wave function techniques. We overcome these limitations with a recently introduced ``local xc correction'' scheme [1], correcting the adsorption energetics from present-day DFT xc functionals with hybrid functional and M\o ller-Plesset perturbation theory calculations for small clusters. From the obtained convergence of the xc correction with cluster size we can disentangle short-range and dispersion type contributions to the bonding of the molecule at different heights above the surface. This enables us to qualify the role played by the two contributions in determining the binding energetics and geometry. [1] Q.-M. Hu, K. Reuter, and M. Scheffler, PRL {\bf 98}, 176103 (2007) and {\bf 99}, 169903 (2007); C. Tuma and J. Sauer, CPL {\bf 387}, 388 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S13.00007: Transcorrelated method applied to covalent and ionic solids: total energy and band structure calculation Keitaro Sodeyama, Shinji Tsuneyuki, Rei Sakuma To calculate the electronic structures of solids including
electron correlation effects, we have developed the
transcorrelated (TC) method which was first proposed by Boys and
Handy. In the TC method, the wave function is represented by a
correlated wave function $F \Phi$, where $\Phi$ is a single
Slater determinant and $F$ is a Jastrow function,
$F=\exp[-\sum_{i |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S13.00008: System-averaged exchange and correlation holes in third-row atoms Antonio C. Cancio Recent work is presented on the theoretical calculation of system-averaged exchange and correlation holes (intracules) for a pseudopotential model of the valence shell of third-row atoms. Exchange holes are obtained from numerical fourier transform methods and correlation holes from variational quantum Monte Carlo calculations using the method of correlated estimates. We observe approximate scaling behavior in both exchange and correlation, following the known scaling of the valence density across the row. The holes are compared to density-functional models including LDA, GGA and meta-GGA approaches. Particular attention is paid to self interaction (SI) error; we find that a sizeable error occurs in the same-spin channel of the correlation hole which persists for the LDA and GGA even after standard SI corrections are applied. A simple SI correction that eliminates this error will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S13.00009: RPA Correlation Energy in ACFD Formalism with Thomas-Fermi-von Weizs\"{a}cker Approximation Viet Huy Nguyen, Stefano de Gironcoli It is well known that LDA or GGAs approximations in DFT do not describe correctly systems where long range correlations are important. In the Adiabatic Connection Fluctuation-Dissipation (ACFD) formalism correlation energy can be computed accurately from Kohn-Sham and interacting linear response functions. Although computationally very demanding, this formalism has shown to describe correctly systems where standart DFT fails qualitatively by combining RPA xc-kernel with short-range local-density corrections (RPA+). On the other hand, Thomas-Fermi-von Weizs\"acker approximate kinetic response function can capture reasonably well asymptotic long range interactions via van der Waals coefficients, and has the computationally desirable feature that it only involves a single auxiliary wavefunction regardless of the number of electrons in the system. Here, we show how to use this approach to calculate approximate RPA correlation energies. Numerical results for atoms show that this approach gives approximate RPA correlation energies closer to the experimental values than those obtained by full RPA and, when combined with a short-range local-density correction, it gives results at least as good as those of full RPA+. The possibility is therefore open to address large systems where correlations need to be treated beyond LDA and GGAs. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S13.00010: Generalized density functional theory for effective potentials in many-body electronic structure F. A. Reboredo, P. R. C. Kent We demonstrate the existence of different density functionals that retain selected properties of the many-body ground state in the non-interacting density functional solution. We focus on diffusion Monte Carlo applications that require trial wave functions with Fermion optimal nodes. The theory can be extended and used to understand current practices in several electronic structure methods [GW-BSE,CI,EPM] within a generalized density functional framework. The theory justifies and stimulates the search of optimal empirical density functionals and effective potentials but also cautions on the limits of their applicability. The theoretical concepts are tested against a near-analytic model that can be solved to numerical precision. Research performed at the Materials Science and Technology Division and the Center of Nanophase Material Sciences at Oak Ridge National Laboratory sponsored the Division of Materials Sciences and the Division of Scientific User Facilities U.S. Department of Energy. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S13.00011: Towards QMC benchmarks for large scale dispersive interactions Jonathan L DuBois, Randolph Q. Hood, Sebastien Hamel, Eric R. Schwegler Fixed-node quantum Monte-Carlo (QMC) methods are becoming an increasingly attractive approach for the study of large scale problems in electronic structure. Current challenges lie in efficient application of QMC to large (thousands of electrons) systems and removal or amelioration of the uncontrolled approximations inherent in most practical applications of the method. I will present recent progress and address some of the particular challenges associated with the development of exact potential energy surfaces for weakly interacting closed shell carbon complexes within the fixed-node QMC ansatz. In particular, the efficacy / necessity of backflow corrections and multi-determinant expansions as a method for optimizing the nodal surface in these systems will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S13.00012: Real or artifactual symmetry breaking in BNB: A fixed-node diffusion Monte Carlo study Wissam A. Al-Saidi, Cyrus Umrigar The linear BNB molecule represents one of the most challenging examples of symmetry-breaking effects because of its susceptibility to a second-order Jahn-Teller distortion along the antisymmetric stretching mode. This real symmetry breaking could be confused in calculations with an artifactual one caused by the approximate nature of the theoretical approach. Thus the debate of whether the ground state of BNB is symmetric in the positions of the boron atoms with respect to nitrogen or if this symmetry is broken. Our preliminary investigations with diffusion Monte Carlo shows that the symmetric and the broken symmetry geometries are nearly degenerate, which would suggest a highly floppy quasi-symmetric BNB ground state. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S13.00013: Potential Energy Curves and Excited States of the C$_2$ Molecule by Auxiliary-Field Quantum Monte Carlo (AFQMC) Wirawan Purwanto, Henry Krakauer, Shiwei Zhang, Wissam Al-Saidi The accurate determination of potential energy curves (PECs) and excited states represents two difficult problems in electronic structure calculations. We present AFQMC PECs of the challenging C$_2$ molecule, focusing on the ground state and two singlet low-lying excited states. AFQMC calculates a target many-body wave function (WF) by means of random walks in the space of Slater determinants. We employ truncated complete active space (CAS) trial WFs ($\Psi_T$) to guide the AFQMC projection to obtain the desired state. With the phase-free constraint,\footnote{S. Zhang and H. Krakauer, Phys. Rev. Lett. \textbf{90}, 136401 (2003)} the CAS $\Psi_T$ is effective in controlling the sign/phase problem, and filtering in the desired excited state. The AFQMC results are in very good agreement with exact results. Comparison with experimental spectroscopic constants will also be presented. [Preview Abstract] |
Session S14: Fermions in Optical Lattices
Sponsoring Units: DAMOPChair: Ana Maria Rey, Harvard University
Room: Morial Convention Center 205
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S14.00001: Strongly interacting Fermi gases in an optical lattice Niels Strohmaier, Robert Joerdens, Kenneth Guenter, Yosuke Takasu, Michael Koehl, Henning Moritz, Tilman Esslinger When cold fermionic atoms are placed in the periodic potential of an optical lattice, they behave similarly to electrons in a crystal. However, the properties of this synthetic material can be changed at will. Here, we report on the experimental realization and investigation of strongly interacting Fermi gases with tunable interactions. By changing the interaction strength we are able to control the transport properties: while dipole oscillations are observed for a non-interacting gas, the atomic cloud relaxes very slowly to its equilibrium position for strong attractive interactions. We suggest an interpretation in the framework of the Hubbard model including external confinement: local fermionic pairing occurs, leading to a drastically reduced tunneling rate. Furthermore, experimental results on the behavior of repulsively interacting Fermi gases will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S14.00002: Sqeezing out the entropy of Fermions in an optical lattice. Qi Zhou, Tin-Lun Ho We point out a new scheme for achieving the strongly correlated system in an optical lattice. By turning the bulk of the trapped fermions into a band insulator, the entropy of the system is expelled to the surface and removed by various means. Our scheme also illustrates a general principle of cooling in a many body system. That is, one can use a gapped state to squeeze out the entropy and then turn it into the desired state after the entropy is removed. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S14.00003: Superfluid-insulator transitions of the Fermi gas with near-unitary interactions in a periodic potential Eun-Gook Moon, Predrag Nikolic, Subir Sachdev We consider a gas of spin-1/2 fermions with interactions near the unitary limit. In an applied periodic potential, and with a density of an even integer number of fermions per unit cell, there is a second-order quantum phase transition between superfluid and insulating ground states at a critical amplitude of the lattice potential. We map out the universal phase diagram at $N=\infty$ in a model with Sp($2N$) spin symmetry, and compute the universal ratio between the critical lattice amplitude and molecule recoil energy. As the interactions between fermions are varied, the insulator evolves smoothly between a band insulator of fermions and a Mott insulator of fermion pairs. We discuss implications for recent ultra-cold atom experiments. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S14.00004: Pattern formation in mixtures of different mass ultracold atoms in optical lattices: an inhomogeneous DMFT study James Freericks Dynamical mean-field theory (DMFT) is generalized to include an inhomogeneous trap and applied to the problem of different mass spin-polarized fermionic atoms that have an interspecies interaction $U$. Such a system is described by the spinless Falicov-Kimball model in a harmonic trap (in the limit where the more massive atom is localized on the optical lattice); we examine atoms moving on a 51X51 two-dimensional square lattice. When the temperature is low enough, the system exhibits pattern formation with different types of order, ranging from checkerboard phases and phase separation to an analog of the viscous fingering seen in immiscible liquids. The temperature evolution of these ordered patterns are interesting as well, as the system can form rings of ordered phases surrounding disordered phases, which expand in size as $T$ is lowered. These patterns can be detected with noise-correlation spectroscopy or Bragg scattering, and their evolution with $T$ could by employed for thermometry. The inhomogeneous DMFT algorithm parallelizes well and is quite efficient. The main difference with exact numerical solutions or the local density approximation is that the temperature scales for the ordering are significantly higher in the IDMFT solution. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S14.00005: Fermion mixtures on an optical lattice Shan-Wen Tsai, Tun Wang We investigate mixtures of two species of fermionic ultracold atoms loaded on an optical lattice. We consider fermions with unequal masses and also fermions with unequal spin populations. We discretize the two Fermi surfaces and employ a functional renormalization-group (RG) approach to calculate the flows of effective interaction vertices in order to identify the instabilities of the system. Without any interactions produced via tuning of Feshbach resonances, a question arises as to whether there can be BCS pairing from repulsive interactions (produced by the optical trap) combined with lattice effects. For spin-independent bare contact interaction, the RG flows for unequal spin populations generate effective spin-dependent interactions. We investigate cases with different interactions, both attractive and repulsive, and different shapes of the Fermi surfaces, in particular the case when one of the fermion species has a nested Fermi surface and the other one does not. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S14.00006: Effects of frozen double occupancies on fermions in optical lattices Rajdeep Sensarma, Eugene Demler, Ehud Altman We study the effects of ``frozen'' double occupancies in metastable states of 3D fermionic Hubbard model in the strongly interacting regime. Such long lived states can be created with ultracold fermions in an optical lattice, as the lattice potential is ramped up to access strongly correlated regimes. We shall discuss how the presence of the double occupancies affect anti-ferromagnetism in these systems. We will also discuss possible ``charge'' orders in these systems. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S14.00007: Dynamical instabilities of paired fermion superfluids in optical lattices Ganesh Ramachandran, Anton Burkov, Arun Paramekanti We report on a study of dynamical instabilities in cold atom fermionic superfluids in optical lattices. Of particular interest to us are the properties of such superfluids at fermion densities corresponding to noninteger average number of pairs per lattice site. In this case superfluidity competes with charge-density-wave ordered states, which has a profound effect on how such superfluids respond to flow. In particular, by varying the pairing momentum, we find a complex interplay between dynamical, pair breaking and Landau (phonon excitation) instabilities at different fillings and different magnitudes of the pairing interaction. Using insights from this study, we construct the ``dynamical phase diagrams'' of fermion superfluids, and discuss the experimental observability of the proposed effects. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S14.00008: BCS-BEC Crossover of a Quasi-two-dimensional Fermi Gas: the Significance of Dressed Molecules Wei Zhang, Guin-Dar Lin, Luming Duan We study the crossover of a quasi-two-dimensional Fermi gas trapped in the radial plane from the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime by crossing a Feshbach resonance. Using an effective two-dimensional Hamiltonian with renormalized interaction between atoms and dressed molecules, we calculate the zero temperature cloud size and number density distribution and conclude that the results are consistent with the picture of BCS-BEC crossover. These results are in clear contrast to the predictions of an effective two-dimensional Hamiltonian with renormalized atom-atom interaction, where a constant cloud size and identical density profile are expected for arbitrary detunings. This inconsistence indicates that the inclusion of dressed molecules is essential to describe the two-dimensional Fermi systems, especially on the BEC side of the Feshbach resonance. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S14.00009: Itinerant Ferromagnetism in an Atom Trap Ilya Berdnikov, Piers Coleman, Steve Simon Interest in ferromagnetism has prompted the development of many theoretical techinques to study the phonomenon. However, even the most sophisticated schemes, though well motivated physically, remain intrinsically uncontrolled. This is a clear signal that more experimental input is needed, and the rapidly developing field of ultra-cold atomic gases affords just such an investigation. We propose an experiment to explore the magnetic phase transition in interacting fermionic systems, and establish signatures of ferromagnetic correlations in the observed ground states. We find, that for large trap radii ($R > 4$, in units of coherence length $\xi$), ground states are topological in nature, a ``skyrmion'' in 2D, and a ``hedgehog'' in 3D. Finally, we describe how to obtain the ferromagnetic phase diagram of itinerant electron systems from these experiments. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S14.00010: The expansion of strongly interacting fermions after the release from a trap Fabian Heidrich-Meisner, Marcos Rigol, Alejandro Muramatsu, Adrian Feiguin, Elbio Dagotto Both the recent experimental progress in cold atom gas realizations and developments in computational techniques has fueled interest in nonequilibrium properties of strongly correlated systems. Here we study the expansion of fermions in a one-dimensional lattice after released from a trap. Using the time-dependent density matrix renormalization group method, we analyze properties of the one-particle density matrix as well as the evolution of spin and density correlations. A comparison of particles escaping from a metallic region as compared to a Mott-insulating one shows that some memory on the initial state is preserved during the expansion. We further address the question to what extent the correlations measured during the expansion and thus in a non-equilibrium situation resemble those of appropriately chosen reference systems in equilibrium. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S14.00011: Quantum Antiferromagnetism of Fermion in Optical Lattices with Half-filled p-band Hui Zhai, Kai Wu We study Fermi gases in a three-dimensional optical lattice with five fermions per site, i.e. the s-band is completely filled and the $p$-band with three-fold degeneracy is half filled. We show that, for repulsive interaction between fermions, the system will exhibit spin-$3/2$ antiferromagnetic order at low temperature. This conclusion is obtained in strong interaction regime by strong coupling expansion which yields an isotropic spin-$3/2$ Heisenberg model, and also in weak interaction regime, by Hatree-Fock mean-field theory and analysis of Fermi surface nesting. We show that the critical temperature for this antiferromagnetism of a $p$-band Mott insulator is about two orders of magnitudes higher than that of an $s$-band Mott insulator, which is close to the lowest temperature attainable nowadays. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S14.00012: Stripe Formation and Superfluiditiy Correlation on Two-dimensional Optical Lattice: DMRG Studies for $n$-leg repulsive Hubbard model Masahiko Machida, Masahiko Okumura, Susumu Yamada In order to predict strongly-correlated behaviors on two-dimensional (2-D) optical lattice, we employ the parallel density matrix renormalization group (p-DMRG) method and examine 2-D repulsive (square-lattice) Hubbard model. In the presentation, we firstly suggest that box shape trap enables to observe intrinsic properties of the Hubbard model in a fixed doping in contrast to the harmonic trap bringing about wide spatial variations of atom density profiles. Next, we show atomic density profile on 4-leg repulsive Hubbard model with the open boundary condition under the box trap. The variation parameters in the simulation are the doping rate below the half-filling and the repulsive interaction $U/t$. As a result, we find that stripe formations are universal in a low hole doping range and the stripe sensitively changes its structure with variations of $U/t$ and the doping rate. A striking feature is that a stripe by a hole pair turns to one by a bi-hole pair when entering a limited strong $U/t$ range. Furthermore, a systematic calculation reveals that the Hubbard model shows a change from the stripe to the Friedel oscillation with increasing the doping rate. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S14.00013: Superfluidity of fermions with repulsive on-site interaction in an anisotropic optical lattice near a Feshbach resonance Bin Wang, Luming Duan We present numerical analysis of ground state properties of the one-dimensional general Hubbard model (GHM) with particle assisted tunnelling rates and repulsive on-site interaction (positive-U), which describes fermionic atoms in an anisotropic optical lattice near a wide Feshbach resonance. Our calculation uses the time evolving block decimation algorithm, which is an extension of the density matrix renormalization group and provides a well controlled method for one-dimensional systems. We show that the positive-U GHM, when hole doped from half-filling, shows up a phase with coexistence of quasi-long-range superfluid and charge-density-wave orders. This feature is different from the property of the conventional Hubbard model with positive-U, indicting the particle assisted tunneling in the GHM could bring in qualitatively new physics. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S14.00014: Predicted itinerant ferromagnetism with cold fermions in optical lattices Shizhong Zhang, Congjun Wu Itinerant ferromagnetism is one of the central topics in condensed matter physics. Ferromagnetism is intrinsically strong coupling physics which does not have a weak coupling limit, i.e., spontaneous spin polarization requires strong interactions to overcome the kinetic energy cost. In spite of its importance, ferromagnetism has not received enough attention in the cold atom community because the system is unstable to molecular formation if the interaction is tuned close to resonance from the positive side of the Feshbach Resonance. To overcome this difficulty, we instead propose to realize the ferromagnetic state in the $p$-orbital honeycomb lattice by taking advantage of its flat band structure. Due to the divergent density of states, even weak repulsions can drive the ferromagnetic transition while ensure the stability of the system. This will open up a new opportunity to investigate ferromagnetism with precise controllability and to realize spin transport and even spintronics applications with cold atoms. [Preview Abstract] |
Session S15: Superconducting Qubits III
Sponsoring Units: GQIChair: Franco Nori, University of Michigan & RIKEN
Room: Morial Convention Center 207
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S15.00001: Transmission line cavity as a quantum memory for superconducting phase qubits K. Cicak, F. Altomare, J.I. Park, M.A. Sillanpa\"{a}, R.W. Simmonds A superconducting transmission line cavity coupling two phase qubits has already proven useful as a bus for coherent state transfer$^1$. In this talk we will discuss our efforts in extending the work of Sillanpa\"{a} \textit{et al.}\footnote{NATURE 449, 438-442 (2007)} to use a transmission line as a short term memory element. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S15.00002: Coplanar resonators as computational elements in a superconducting qubit architecture Max Hofheinz, E.M. Weig, R.C. Bialczak, H. Wang, N. Katz, M. Neeley, E. Lucero, A.D. O'Connell, M. Ansmann, J. Wenner, D. Sank, I. Storch, J.M. Martinis, A.N. Cleland We are coupling a superconducting phase qubit, implemented using a current-biased Josephson junction, to a high-Q coplanar waveguide resonator. The interaction between the phase qubit and the resonator can be controlled by tuning the qubit frequency into and out of resonance with the resonator, a tuning that can be achieved dynamically over times short compared to the Rabi time. By combining the quantum control flexibility of the phase qubit with the long coherence time and bosonic nature of the resonator, a number of interesting quantum operations can be explored, including long-term phase coherent quantum memory and two-qubit bus architectures. In this talk we will report on our recent progress with this experiment. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S15.00003: Entangling phase qubits via a common transmission line cavity Jae Park, Ray Simmonds We consider ways of ``distributing'' entanglement to a number of qubits via a common superconducting transmission line cavity. We propose methods of preparing such states and verifying its preparation. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S15.00004: Measuring the quantum states of a superconducting cavity Jae Park, Fabio Altomare, Katerina Cicak, Ray Simmonds We consider the prospects for preparing and measuring some basic quantum states (e.g. number states) of a superconducting transmission line cavity coupled to Josephson-junction qubits. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S15.00005: Cooling of a Resonator with Microwave Induced Charge-Phase Qubit Transitions Jani Tuorila, David Gunnarsson, Antti Paila, Jayanta Sarkar, Erkki Thuneberg, Yuriy Makhlin, Pertti Hakonen We have studied a circuit QED experiment where a superconducting charge-phase qubit is coupled to an electric $rf$-resonator via the phase degree of freedom. The resonator is coupled to a transmission line that allows reflection measurement with narrow band detection. Additionally, the charge degree of freedom is coupled to a $\mu$w-signal. The level spacing $\hbar\Omega$ of the qubit is controlled with constant shifts in both degrees of freedom. Multiphoton absorption from both drives can excite the qubit in the case the level separation is equal to the sum of the photon energies. The results of the measurements show asymmetry between the depths of the side-resonances of the basic resonance $\Omega=\omega_{\mu w}$. Also, a non-monotonic AC-Stark shift is observed in the apparent resonance positions. Solutions of the semiclassical Maxwell-Bloch equations of the whole measurement apparatus show that the measured results can be considered as evidence of cooling/heating of the oscillator. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S15.00006: Fast tuning of a high Q microwave cavity for qubit coupling. M. Sandberg, C.M. Wilson, F. Persson, G. Johansson, V. Shumeiko, T. Duty, P. Delsing In 2004 Wallraff {\it et al.}[1] demonstrated that an artificial atom in form of a superconducting qubit can exhibit coherent interaction with a superconducting high Q microwave transmissionline resonator. In recent experiments [2,3] similar resonators have been used for coupling and reading out qubits. In these experiments the resonance frequency of the resonator is fixed and the qubit frequency is tuned. Here we present measurements on a superconducting transmission line resonator with a tunable resonance frequency that could be used for qubit coupling [4]. With such a device, qubit gates can be performed while the qubits stay at their optimal points. We demonstrate a tunability of 700 MHz for a 4.8 GHz resonator with a linewidth of 500 kHz and that we can tune the resonance frequency by 330 MHz in a few ns. We show that if the resonator is detuned faster than its decay time the photons inside the resonator will shift their frequency with the resonator. \newline [1] A. Wallraff, {\it et al.}, Nature {\bf 431}, 162 (2004). \newline [2] J. Majer, {\it et al.}, Nature {\bf 449}, 443 (2007). \newline [3] M.A. Sillanp\"a, J.I. Park, and R.W. Simmonds, Nature {\bf 449}, 438 (2007). \newline [4] M. Wallquist, V.S. Shumeiko, and G. Wendin, Phys. Rev. B {\bf 74}, 224506 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S15.00007: Vacuum Rabi Mode Splitting at High Drive Powers and Elevated Temperatures J.M. Fink, A. Blais, R.J. Schoelkopf, A. Wallraff The circuit QED architecture $[1,2]$ is ideal to probe the nonlinearity of a strongly coupled cavity QED system at high drive powers populating the cavity with a controllable average photon number in the range from 0.1 $<$ n $<$ 100. While in atomic cavity QED the radiation pressure exerted on the atoms by the drive tends to expel the atoms from the cavity, superconducting qubits remain at a fixed position and maintain constant coupling. This enables us to explore the cross-over from the quantum to the classical regime in the single qubit-field interaction by measuring vacuum Rabi mode splitting spectra. We also investigate the effect of thermal radiation in the cavity leading to a thermal population of excited states in the Jaynes-Cummings ladder, which was theoretically studied in Ref. $[3]$. Simulations have been carried out in order to determine the optimal set of qubit and resonator parameters needed for first experiments. \newline $[1]$ A. Blais \textit{et al.} Phys. Rev. A \textbf{69}, 062320 (2004). \newline $[2]$ A. Wallraff \textit{et al.} Nature \textbf{431}, 162 (2004). \newline $[3]$ I. Rau \textit{et al.} Phys. Rev. B \textbf{70}, 054521 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S15.00008: Observation of Berry's Phase in a Superconducting Qubit P.J. Leek, J.M. Fink, A. Blais, R.J. Schoelkopf, A. Wallraff In quantum information science, the phase of a wavefunction plays an important role in encoding information. While most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance [1]. Here we demonstrate the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit, manipulating the qubit geometrically using microwave radiation, and observing the accumulated phase in an interference experiment [2]. This is achieved utilising the excellent phase coherence and qubit control possible in Circuit QED [3]. We find excellent agreement with Berry's predictions, and also observe a geometry dependent contribution to dephasing. \newline [1] J.A. Jones et al, Nature 403, 869 (2000) \newline [2] P.J. Leek et al, Science, 22 November 2007 (10.1126/science.1149858) \newline [3] A. Wallraff et al, Nature 431, 162 (2004) [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S15.00009: Cooper Pair Box Qubit in the Ultrastrong Coupling Regime Markus Brink, Michael Metcalfe, Luigi Frunzio, Vladimir E. Manucharyan, Jens Koch, Terri M. Yu, Steven M. Girvin, Robert J. Schoelkopf, Michel H. Devoret We propose a new superconducting qubit design, where a small Josephson junction is inserted in the central conductor of a coplanar waveguide resonator. In this distributed element design, the resonator provides a negative reactance for the junction, which modifies the charging energy $E_C$ of the junction and places the qubit far in the ``Transmon regime'', $E_J \gg E_C$, where $E_J$ is the Josephson energy of the junction. We will discuss design details and show preliminary fabrication and measurement results. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S15.00010: Homodyne detection of resonance fluorescence in circuit QED Lev S. Bishop, Jens Koch, Erkki Thuneberg, Jerry M. Chow, Steven M. Girvin, Robert J. Schoelkopf In circuit QED, the transmon qubit[1] allows long coherence times and strong coupling. In this regime, tuning the qubit into resonance with the cavity leads to vacuum Rabi splitting[2] with two transmission peaks very well-resolved in frequency ($\sim$300 linewidths apart). At low probe power, these peaks have Lorentzian shape. As the probe power is increased, each Rabi peak is observed to split into two peaks. Approximating the combined qubit and cavity as a two-level system and applying the theory of resonance fluorescence reproduces the main features of this phenomenon. We explore the effects of including additional levels of the transmon and cavity in the detailed theoretical modeling of the experiment. Additionally, we discuss the possibility to observe the Mollow triplet in the fluorescence spectrum. \newline[1] Jens Koch, TM Yu, JM Gambetta, AA Houck, DI Schuster, J Majer, A Blais, MH Devoret, SM Girvin, and RJ Schoelkopf. Phys. Rev. A \textbf{76}, 042319 (2007) \newline[2] A Wallraff, D Schuster, A Blais, L Frunzio, R-S Huang, J Majer, S Kumar, SM Girvin and RJ Schoelkopf, Nature \textbf{431}, 162 (2004) [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S15.00011: Purcell Effect Limits on the Lifetimes of Transmon Qubits Blake Johnson, Steven Girvin, Robert Schoelkopf Circuit QED couples a superconducting qubit to a transmission line cavity. The presence of the cavity can suppress or enhance the spontaneous emission of the qubit into the cavity, a phenomenon known as the Purcell effect. Consequently, the qubit excited-state lifetime depends on the qubit-cavity detuning. A quantum mechanical calculation of the Purcell effect for a single mode of the cavity does not account for T1s observed in our system. Here we show a semi-classical approximation for the Purcell effect for a multi-mode cavity which we compare with T1 measurements of several transmon [1] qubits. By designing an appropriate cavity we have improved T1 by a factor of 10. \newline \newline[1] Charge-insensitive qubit design derived from the Cooper pair box. Jens Koch et al, Phys. Rev. A 76, 042319 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S15.00012: Suppressing Charge Noise Decoherence in a Transmon Qubit Joseph Schreier, Steven Girvin, Robert Schoelkopf Here we discuss coherence measurements of the transmon qubit, an optimized Cooper Pair Box geometry. We show experimental verification that sensitivity to 1/f charge noise was exponentially suppressed in the transmon qubit. As a result, the effects of gate charge noise and quasiparticle poisoning have been nearly eliminated, and the qubit was seen to be nearly homogeneously broadened. Following an improvement in relaxation times, dephasing times were measured at over a microsecond, nearly twice the relaxation time, without the need of an echo experiment while being tuned over a range of several GHz. The tuning of the qubit excitation energy shows strong agreement with a quantum mechanical treatment of a two level system coupled to our read-out geometry; the spectrum is nearly devoid of unintended avoided crossings. The dephasing times measured are limited by relaxation and further improvements in relaxation time could be matched by dephasing time increases. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S15.00013: Circuit QED with phase-biased qubits Jerome Bourassa, Alexandre Blais, Michel Devoret, Robert Schoelkopf Coupling of a superconducing charge qubit to a transmission line resonator has been shown to lead to the very strong coupling regime of cavity qubit [1]. ~In this talk, we will discuss an alternative approach to circuit QED based on the cavity bifurcation amplifier [2] and where a qubit is directly embedded in the resonator's center line. ~We will show that this type of phase bias leads to very strong coupling and/or non-linearities. ~Readout, decoherence rates and coupling of qubits in this architecture will be discussed. [1] A. Wallraff et al., Nature 431, 162 (2004). [2] M. Metcalfe et al., PRB 76, 174516 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S15.00014: Quantum walk on a circle in phase space via superconducting circuit quantum electrodynamics Barry Sanders, Peng Xue, Alexandre Blais, Kevin Lalumiere We show how a quantum walk, with a single walker and controllable decoherence, can be implemented for the first time in a quantum quincunx created via superconducting circuit quantum electrodynamics (QED). Two resonators are employed to provide simultaneously fast readout and controllable decoherence over a wide range of parameters. The Hadamard coin flip is achieved by directly driving the cavity, with the result that the walker jumps between circles in phase space but still exhibits quantum walk behavior over 15 steps. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S15.00015: Quantum mirror transport of qudits and continuous variables and an implementation in Circuit-QED Jason Twamley, Gerardo Andres Paz Silva, Stojan Rebic We expand on our previous work [J. Fitzsimons and J. Twamley, Phys. Rev. Lett. 97, 090502 (2006)], to derive a globally controlled automata-like protocol for the perfect transmission of quantum information in a chain made up of qudits or a chain made up of harmonic oscillators. The resulting protocol results in perfect spatial reflection of the entire quantum state of the chain about its midpoint. Quantum information can be encoded and then processed in continuous variables if one can engineer highly squeezed states [S. Lloyd and S. L. Braunstein, Phys. Rev. Lett. 82, 1784 (1999)]. We show that appropriately driving a superconducting coplanar microwave cavity coupled to a Cooper-pair box qubit can generate very high squeezings of the cavity mode. We consider a linear array of coplanar cavities nearest-neighbor coupled by Cooper-pair boxes. By controlling the coupling strengths of the cavities to the coupling-CPB qubits and the decoherence rates of the latter with time, we show that we can initialize the cavities to be in highly squeezed CV states, and then execute globally controlled quantum mirroring of the entire chain of CV qubits. We finally show that with extra control on the end cavities of the chain we can further execute universal CV quantum computation. [Preview Abstract] |
Session S16: Focus Session: General Techniques and Radiation Therapies in Biological Physics
Sponsoring Units: DBPChair: Paul Gueye, Hampton University
Room: Morial Convention Center 208
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S16.00001: The Evolution of External Beam Radiation Therapy (EBRT) from a Technological Perspective. Invited Speaker: Since the discovery of x-rays by Roentgen in 1895 ionizing radiations have been used as a treatment for cancer. Such treatments have been based on either implantation of radioactive materials at the site of disease or by aiming external radiation beams at the diseased site. This later method is referred to as \textit{teletherapy} because the beams originate from a location outside of the body distant from the disease site itself. A brief review of the basic radiation biology will be given to illustrate the rationale for therapeutic use of ionizing radiations and the effects of beam energy and beam type- particulate or photon. The remainder of the presentation will focus on the technological \textit{teletherapy} developments supported by the required physical properties of the beams and their associated characteristics that make them suitable for patient treatments. Chronological highlights will include the following sources or devices: superficial x-rays, orthovaltage x-rays, megavoltage x-rays and Cobalt 60 photons, electron beams, neutron beams, negative pi mesons, protons, and heavy ions. The presentation will illustrate how the physical beam properties have been incorporated into modern radiation treatment devices, many of which are equipped with radiation imaging capability. Such devices include: linacs equipped with multileaf collimators for beam shaping and intensity modulation, the Gamma Knife for precise and accurate irradiation of brain tumors or arterial-venous malformations (AVM), the robotic arm based Cyber Knife, and the Helical Tomotherapy unit. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S16.00002: State-of-the-Art External Beam Radiation Therapy: Challenges and Opportunities Invited Speaker: Intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT) allow delivery of highly conformal non-convex dose distributions. However, these treatment modalities require precise knowledge of multimodality imaging, internal organ motion, tumor control probabilities, normal tissue complication probabilities, three-dimensional dose calculation and optimization, dynamic beam delivery of non-uniform beam intensities, and most importantly the knowledge of uncertainties in the radiation therapy planning and delivery process. These uncertainties arise from a variety of sources throughout the whole process that consists of three distinct steps: imaging, planning, and delivery. In the imaging step, 3D patient information is obtained for treatment planning. Any problem in the acquisition, transfer, conversion, registration, or use of imaging data can lead to increased geometrical uncertainties. Dose calculation algorithms have inherent errors, because they are based on approximate solutions to a complex physical situation. The assumptions of a RTP system in modeling a treatment machine significantly impact dose calculation accuracy. The accuracy of the beam geometry depends on the tolerance of each machine parameter and the magnitude of setup errors. Therefore, one must clearly identify and account for all sources of error in imaging, treatment planning, and delivery process to understand the uncertainty in dose delivered to a patient with IMRT/IGRT. It is necessary to properly account for these uncertainties in radiation therapy to improve the accuracy of the dose delivered to patients. This presentation will provide the framework and guidance to safely implement these technologies in the clinic. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S16.00003: Technological Advances in Proton Therapy Invited Speaker: Proton therapy has interested radiation oncologists since the 1946 paper by Robert R. Wilson describing the energy deposition of proton beams and suggesting it would be more suitable for radiation treatments than beams of x-rays. For all its proposed benefits, only 25,000 or so cancer patients worldwide have been treated with high-energy proton beams over the last fifty years. However, during the past decade that number has started to rapidly increase. In the United States alone the number of dedicated facilities has grown from two to five in the last three years and will likely double again by the end of the current decade. We will soon be treating as many patients in one year as was treated during the first fifty years of proton therapy. Surprisingly, the reason is because of what has been happening in x-ray radiotherapy. Conventional radiotherapy underwent a dramatic change during the past decade with the introduction of multiple advances in imaging technology and beam delivery methods. The imaging advances include both imaging for treatment planning (multislice CT systems, high resolution MRI, and increasing use of PET) and imaging of the target location in the treatment room. The treatment delivery advances, dominated by methods that permit intensity modulated beam delivery, were made possible by increased computational power and more computer control of the treatment delivery. These imaging and beam delivery advances should benefit proton therapy treatments even more than x-ray treatments because of the better conformation of dose to the target that one can achieve with proton beams. However, because of the small size of the proton therapy community it has had difficulty implementing some of the advances made in x-ray therapy. The treatment planning imaging is also used by proton therapy but the on-treatment imaging and the intensity modulation often must be specially developed for each proton therapy system. This talk will present the developments in these areas that are expected to be implemented in the next few years. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S16.00004: The active catheter: a novel approach for in-situ dose measurements in brachytherapy Paul Gueye Radiation therapy is the primary mean to combat cancerous tissues. However, although efficient, it still lacks from having a tool that could enable accurate measurement of the dose delivered to the tissue in-situ. The active catheter concept was taken from a common approach used in nuclear/high energy physics where the target is sometimes constructed so as to provide additional information on the scattering process (i.e., active target). By making the catheters used to transport radioactive sources during brachytherapy treatments becoming sensitive to radiation, one is able to extract dose information in-vivo with minimal to no modification during the treatment process. The technique relies on the use of thin (few 1o0s microns) scintillating fibers embedded within the brachytherapy device. We will report on two applications of such active catheters pertaining to breast and prostate cancer. The former was applied to the MammoSite{\textregistered} balloon from Cytyc, the accelerated partial breast irradiation technique that is becoming the preferred mode of radiation in breast brachytherapy. Results from water phantom data will be presented and discussed. For the latter, gel phantom tests were performed simulating prostate brachytherapy treatments. Comparison with a treatment planning to these data will be also presented and discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S16.00005: The proton and carbon therapy experience of the medical physics group at the Italian Southern Laboratories: Monte Carlo simulation and experiment G.A. Pablo Cirrone, C. Agodi, G. Candiano, G. Cuttone, F. Di Rosa, E. Mongelli, P. Lojacono, S. Mazzaglia, G. Russo, F. Romano, L.M. Valastro, S. Lo Nigro, S. Pittera, M.G. Sabini, L. Rafaele, V. Salamone, C. Morone, N. Randazzo, V. Sipala, M. Bucciolini, M. Bruzzi, D. Menichelli At the Italian Southern Laboratories (LNS) of the Italian National Institute for Nuclear Physics the first, and actually unique, Italian proton therapy center is installed and operating. Up to now, 140 patients have been treated. In this environment a big effort is devoted towards Monte Carlo simulation expeciallt with the GEANT4 Toolkit. The authors of this work belong to the Geant4 Collaboration and they use the toolkit in their research programs. They maintain a Monte Carlo application devoted to the complete simulation of a generic hadron-therapy beam line and take active part in the study of fragmentation processes. Moreover they are working in the development of a prototype of a proton Computed tomographic system. In this work we will report our results in the field of proton and carbon therapy either in the simulation as well in the experimental side of our activity. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S16.00006: Real-time High Resolution Plasmonic nanosensors: pH modulated captavidin/biotin binding Jeffrey Anker, Richard Van Duyne The ability to observe real-time molecular binding kinetics is important for understanding the functions and interactions of biological molecules. Localized surface plasmon resonance (LSPR) nanosensors exhibit intense extinction and scattering spectra that redshift when molecules bind to their surface. Herein, we use an array of biotin functionalized silver nanoprisms to detect pH-modulated binding and dissociation of captavidin to biotin as a model system. The captavidin binds at neutral pH and dissociates at high pH. Spectral shifts are monitored in real-time at high resolution during pH-modulated binding and dissociation cycles over hours. After the first cycle, good reversibility is observed. We also observe pH modulated charging and nanoparticle etching effects which are important experimental parameters and also provide a means to control and modulate the nanoparticle spectra. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S16.00007: Electrical noise gives away presence of cancer or toxins in culture David Rabson, Douglas Lovelady, Chun-Min Lo Since 1984, electric cell-substrate impedance sensing (ECIS) has been used to monitor cell behavior in culture and has proven sensitive to morphological changes and cell mobility. Several authors have associated fluctuations in the measured impedance with cellular micromotion; however we are unaware of any previous work applying statistical techniques in order to distinguish two different cell types. We have now demonstrated a method for distinguishing cancerous from non-cancerous cultures of human ovarian surface epithelial cells;\footnote{\frenchspacing D.C. Lovelady {\it et al.}, {\it Phys. Rev. E} {\bf 76}, 041908 (2007).} applying similar ideas, we have also determined the presence and concentration of the toxin cytochalisin B in cultures of 3T3 fibroblasts at levels lower than the detection thresholds of other techniques. Measures indicative of both short-time (autocorrelation) and long-time ($1/f^\alpha$ noise in the power spectrum and Hurst and detrended-fluctuation-analysis exponents) show statistically significant differences between the populations. Our measures confirm that the noise from non-cancerous cultures has a higher degree of temporal order, order which we argue must arise from greater coordination of motion between healthy cells than between cancerous ones. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S16.00008: Delayed Luminescence and Biophotons from Biological Materials Ernst Knoesel, Patrick Hann, Maria Garzon, Erik Pfeiffer, Samuel Lofland There has recently been increased interest in the field of biophotonics, since it is a non-invasive technique.~ Many biological systems, such as yeast, bacteria, leaves, seeds, and algae display the unusual phenomenon of a weak, delayed luminescence on the timescale of seconds to minutes after transient illumination. It is also observed that the time decay of the biophotonic emission is not exponential, even after the delay, and that there can be oscillations in intensity with time, which depend on the duration of the illumination. Results from two types of yeast, i.e. bread yeast, and saccharomyces, as well as those from several types of algae are presented. Possible mechanisms for the source of the ultraweak photon emission are discussed. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S16.00009: Dielectric Spectroscopy: noninvasive and fast method for measuring changes in the membrane potential Corina Bot, Camelia Prodan, Emil Prodan We present a noninvasive and fast method, dielectric spectroscopy, to measure changes in the membrane potential of live cell suspensions, in particular to E. coli. This technique can be applied virtually to any cell suspension, regardless of size or shape and is tested against the traditional one-using voltage sensitive dyes. Precise measurements of the dielectric permittivity $\varepsilon $ and conductivity $\sigma $ of live cells suspensions require prior elimination of the polarization errors. Polarization errors are caused by the ionic content of a buffer, and they affect the total impedance in the low frequency interval. We hereby present our approach of polarization removal in low frequency limit by fitting both real and imaginary experimental curves with an ideal impedance Z=d/i$\omega \varepsilon ^{\ast }$S, where $\varepsilon ^{\ast }=\varepsilon $+1/i$\omega \sigma $. Here, $\varepsilon $ and $\sigma $ represent the fitting parameters; a higher weight is given to each of them for the high frequency domain (3kHz-10kHz), where polarization effects were proven negligible. Measurements were performed in a low electric field (1V/cm) and 40Hz-10kHz frequency domain. Different buffers are measured, such as HEPES, DMEM with different KCl concentrations. Adding different KCl concentration or ionophores triggers changes in the membrane potential of E. coli. Those changes are measured using dielectric spectroscopy and voltage sensitive dyes. [Preview Abstract] |
Session S17: Quantum Fluids and Solids II
Sponsoring Units: DCMPChair: Henry Glyde, University of Delaware
Room: Morial Convention Center 209
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S17.00001: Phonon-roton modes and a Bose glass phase in nanoscale liquid $^4$He Jacques Bossy, Jonathan Pearce, Schober Helmut, Henry Glyde We present neutron scattering measurements of the elementary phonon-roton modes of liquid $^4$He confined in nanoporous media. The aim is to compare phonon-roton (P-R) and superfluid density measurements in helium at nanoscales and in disorder. A specific goal is to determine the region of temperature and pressure in which well defined phonon-roton modes (and therefore BEC) exist and compare this with the superfluid region. In 25 $\AA$ mean pore diameter gelsil Yamamoto et al.[1] find that the superfluid phase extends up to a temperature T$_c$ = 1.4 K at saturated vapor pressure (SVP) (p $\simeq$ 0) and up to a pressure p$_c$ = 34 bar at (T $\simeq$ 0). There is apparently a Quantum Phase Transition at p$_c$ = 34 bar[1]. We find well defined P-R modes (BEC) extend above T$_c$ at SVP (up to T$_\lambda$ = 2.17 K) and to pressures above p$_c$ (up to a pressure p = 36.3-36.8 bars at T $\simeq$ 0 but no modes above this pressure). This suggests that there is a Bose glass phase consisting of local regions of BEC (fragmented BEC) separated by regions with no BEC surrounding the superfluid phase at all p and T. We compare this phase diagram with other dirty Bose systems. [1] Yamamoto et al. Phys. Rev. Lett. 93, 075302 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S17.00002: Elastic transmission of atoms through superfluid $^4$He Yaroslav Lutsyshyn, J. Woods Halley We investigate completely elastic transmission of atoms through a slab of strongly interacting helium superfluid by diffusion Monte Carlo (DMC) method. Both quasiparticle and condensate mediated modes of transmission have been predicted$^{1,2}$ but only quasiparticle mode has been observed$^{3,4}$. We performed numerical calculations of the transmission probability using DMC with a modified fixed-node approach to find the phase shifts of scattering states for elastic transmission process of atoms incident normal to the surface of a free standing helium slab. Transmission coefficients for different energies of the incident atom were computed. Preliminary results for the group velocity of a transmitted wave packet hint at rapid transmission with times characteristic of the virtual condensate mediated process. This work was supported in part by the University of Minnesota Supercomputing Institute. \newline [1] J.~W.~Halley et al., PRL \textbf{71}, 2429 (1993) \newline [2] A.~K.~Setty et al., PRL \textbf{79}, 3930 (1997) \newline [3] K.~A.~Lidke et al., J. Low Temp. Phys. \textbf{140}, 429 (2005) \newline [4] C.~D.~H. Williams et al., PRL \textbf{91}, 085301 (2003) [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S17.00003: Experimental Observation of Quantized Vortex Reconnection and Turbulence in Superfluid Helium Matthew Paoletti, Katepalli Sreenivasan, Daniel Lathrop We present experimental studies of the first direct visualization of reconnecting quantum vortices and the decay of superfluid turbulence in $^{4}$He. Micron-sized solid hydrogen particles are used for particle tracking. The cores of the superfluid vortices trap the hydrogen particles, thereby allowing direct visualization of the dynamics of the line-like defects. We generate superfluid turbulence by driving a thermal counterflow. After pulsing the counterflow, the system relaxes through a cascade of reconnection events. We examine the dynamics of pairs of particles trapped on reconnecting vortices and observe that these particles separate as power laws in time with a scaling exponent distributed about the predicted value of $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. We show that reconnection leads to power-law tails in the velocity probability distribution function, which is in stark contrast to the Gaussian tails that are ubiquitous in classical turbulence and thermal motion. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S17.00004: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S17.00005: Scanning Superfluid-Turbulence Cascade by Its Low-Temperature Cutoff Evgeny Kozik, Boris Svistunov Recent advances in experimental techniques have made it possible to explore highly non-trivial short-wavelength physics of low-temperature superfluid turbulence. We analyze the transformation of the (quasi-)classical Kolmogorov cascade into the Kelvin-wave cascades on individual vortex lines at high enough wavenumbers, revealing a chain of three qualitatively distinct intermediate regimes, supported by local-induction motion of the vortex lines, and distinguished by specific reconnection mechanisms. On the basis of this scenario, we develop a theory of low-temperature cascade cutoff, which predicts a peculiar behavior of the quantized vortex line density, $L$, controlled by the frictional coefficient, $\alpha(T) \ll 1$, responsible for the cutoff. Excellent agreement with a recent experiment by Walmsley {\it et al.} [arXiv:0710.1033]---in which $L(T)$ has been measured down to $T \sim 0.08\,$K---validates our scenario and allows to quantify the Kelvin-wave cascade spectrum. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S17.00006: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S17.00007: Metastable Composite Vortices in Spinor Condensates Ari Turner, Eugene Demler The ground states of condensates of atoms with spin have a variety of symmetries leading to many types of vortices. The quadratic Zeeman effect produces composite metastable vortices, which are configurations of vortices held together by a force resulting from the Zeeman effect as we explain. If the component vortices were to conbine together and react to form a different set of components, then the composite vortex could break up. However, this is prevented by short-range repulsions. Our analysis focuses on the cyclic phase, where the chemistry of the vortices is regulated by the symmetry group of a tetrahedron. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S17.00008: Quantum Monte-Carlo study of a two-band boson Hubbard model Siegfried Guertler, Matthias Troyer, Fu-Chun Zhang We consider a two band boson Hubbard model, in which the on-site interaction is infinity for the intra-band bosons and repulsive for the inter-band bosons. The on-site inter-band boson interaction may facilitate condensation of vacancies of $a$-boson and interstitials of $b$-boson. We report results of large scale quantum Monte Carlo simulations to study possible supersolid phases of the model. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S17.00009: A simple model of thermal conductivity in supersolid Helium Joshua Thibodaux, Ilya Vekhter, Matthias Graf, Alexander Balatsky The recent discovery of the decrease of the torsional oscillator period in solid Helium has led to a renewed interest in a supersolid state. The simplest model for this state is one in which the vacancies undergo Bose-Einstein condensation. Within this model we use the Boltzmann equation to investigate the thermal conductivity of normal $^4$He and super solid by considering phonons interacting with a gas of vacancies. We analyze the temperature dependence of the thermal conductivity and specific heat for different vacancy concentrations. We will discuss the consequences of our calculations for existing and future experiments. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S17.00010: Local melting at surface and isotope impurities in quantum solids Emmanuele Cappelluti, Gianluca Rastelli, Sergio Gaudio, Luciano Pietronero Surface melting is a well known phenomenon in classical solids, and it can be related to a {\em local} instability of the solid phase close to the surface truncation. In this contriibution we employ a self-consistent harmonic approximation to investigate surface melting and local melting close to quantum impurities in quantum solids. We show that surface melting can occur at temperatures much lower than the critical temperature $T_c$ of the solid phase instability in the bulk. Similar effects are driven by the presence of an isotope substitution. In this latter case, we show that stronger local lattice fluctuations, induced by a lighter isotope atom, can induce local melting of the host bulk phase. Experimental consequences and the possible relevance in solid helium are discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S17.00011: A Kosterlitz-Thouless transition in solid $4^$ Helium? Sergio Gaudio, Emmanuele Cappeluti, Gianluca Rastelli, Luciano Pietronero We show that the reproducible ``Non-Classical Rotational Inertia'' signals in solid Helium four are completely ascribable to a Kosterlitz-Thouless transition of liquid $^4$He at the grain boundaries. Despite our toy model, we obtain a surprisingly good agreement with the experimental data when comparing the drop of the momentum of inertia. Within our model, we give an estimate of the average size of the grains, which we argue to be limited by the isotopic impurities and show that the signal is inversely proportional to the size of the average size of the grains. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S17.00012: Disorder-induced Missing Moment of Inertia in Solid $^4$He Jiansheng Wu, Philip Phillips A microscopic model for the experimentally observed missing moment of inertia in the torsional oscillator experiments on partially-annealed solid $^4$He is proposed. We argue that an ordered array of $^4$He atoms is a Mott insulator. Disorder destroys the Mott state producing localized states in the gap which beyond a critical value of the disorder induce a superfluid state. Depending on the magnitude of the disorder, we find that the destruction of the Mott state takes place for $d\le 3$ either through a Bose glass phase (strong disorder and weak disorder) or through a direct transition to a superfluid (intermediate disorder).The critical value of the disorder that separates three region of disorder is shown to be a function of the boson filling, interaction and the momentum cut off. We apply our work to the experimentally observed enhancement $^3 $He impurities has on the onset temperature for the missing moment of inertia. We find quantitative agreement with experimental trends. [Preview Abstract] |
Session S18: Hybrid Organic-Inorganic Nanomaterials II: Assembly and Fabrication
Sponsoring Units: DPOLYChair: Hendrik Heinz, University of Akron
Room: Morial Convention Center 210
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S18.00001: Transparent Organic Field-Effect Transistors with Carbon Nanotube Electrodes Adrian Southard, Vinod K. Sanguan, Tracy L. Moore, Ellen D. Williams, Michael S. Fuhrer, Daniel Hines, Vince Ballaratto Carbon nanotube (CNT) films are promising as electrode material for organic field-effect transistors (OFETs). We have fabricated CNT films over large areas by airbrushing and patterned them either using shadow masks, or via photolithography and reactive ion etching. Pentacene thin film transistors bottom-contacted by CNT source/drain electrodes on Si/SiO$_{2}$ substrates show moderate mobility (up to 0.1 cm$^{2}$/Vs) and contact resistance as low as 20 k$\Omega $*cm. The contact resistance varies linearly with the gate voltage, eventually saturating at high negative gate voltage. OFETs were measured at temperatures from 228 to 353.5 K. After accounting for contact resistance, the intrinsic mobility of the transistors is found to be activated in temperature with activation energy between 0.137 and 0.151 eV in reasonable agreement with values in the literature. An all-carbon transparent OFET with CNT source, drain and gate electrodes and polymethylmethacrylate (PMMA) gate dielectric has been assembled on a polyethylene terephthalate substrate by transfer printing, with a field-effect mobility up to 0.06 cm$^{2}$/(Vs). [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S18.00002: Directed self assembly of macroscopic nanowires from single-wall carbon nanotubes suspended in aqueous bile-salt solutions E. K. Hobbie, J. A. Fagan, M. L. Becker, S. D. Hudson, J. Chun, B. J. Bauer, M. Pasquali Length purified and chirality enriched single-wall carbon nanotubes (SWNTs) suspended in aqueous bile-salt solutions are found to spontaneously self assemble into macroscopically long straight nanowires, both in confined geometries and on patterned substrates. By patterning surfaces with ordered arrays of hydrophobic and hydrophilic regions, we tailor the self assembly of the nanowires for potential applications in the rapid and cheap fabrication of transparent films with strong directional conductivity. We report a phase diagram in the plane of SWNT concentration and bile-salt concentration that delineates regions of stable nanotube dispersion, bulk and surface phase separation, and nanowire self assembly. The predominant mechanisms for this phase behavior are identified as hydrogen-bonding interactions between contacted bile-salt micelles, which lead to the natural formation of macroscopic fibrils, and entropic depletion interactions mediated by free surfactant micelles. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S18.00003: Time and Temperature Dependent Rheological Behavior of Single-Walled Carbon Nanotubes Dispersed in Thermoreversible Acrylic Copolymer {\&} Alcohol Solutions Andrew B. Schoch, Kenneth R. Shull, L. Catherine Brinson SWCNT stabilized by A-B diblock and A-B-A triblock copolymers are excellent model systems for studying the relationship between nanotube dispersion and mechanical response. We have investigated the mechanical properties of these materials with low-amplitude oscillatory shear rheological measurements. The solvent used here, 2-ethyl-1-hexanol, is a poor solvent for PMMA (A) at low temperatures but a good solvent for PnBA (B) over the entire temperature range studied. The solubility of the PMMA blocks in 2-ethyl-1-hexanol drives the formation of an elastic gel in the ABA triblock copolymer at low temperatures. In these SWCNT/copolymer materials the storage and loss moduli have been observed to increase with time at fixed temperature. When triblock copolymer gels are used as the matrix, we find that the aging effect is erased by cycling the temperature through the gel transition. An increase in storage modulus is observed upon cooling before the gel formation. However, the moduli revert back to lower values when the gel dissolves on heating. We believe this is a result of semi-permanent nanotube junctions being pulled apart when the gel forms. This reversibility is not observed when the nanotubes are dispersed in solutions of diblock copolymer, which do not form gels. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S18.00004: Forces between nanorods with end-adsorbed chains in polymer melts Amalie Frischknecht Adsorbed or grafted polymers are often used to provide steric stabilization of colloidal particles. When the particle size approaches the nanoscale, the curvature of the particles becomes relevant. Here I use a classical density functional theory to study the polymer-mediated interactions between two nanorods. The rods are immersed in an athermal, melt polymer blend consisting of: 1) a small fraction of chains of length N=20 with ``sticky'' ends that are attracted to the rods with energy e/kT so that they form a polymer brush on the rods; and 2) a matrix of chains of length P which have no interactions with the rods. The structure of the brushes depends on the nanorod diameter, P, and e/kT. There is an attractive well in the force between the rods near contact, followed by a strong repulsion as the brushes are compressed. The depth of the well increases with increasing P. I will discuss the implications for experimental systems. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S18.00005: Shape and size selection of Au nanorods by reversible flocculation. Kyoungweon Park, Wei Lu, Hilmar Koerner, Richard Vaia For gold nanorods (GNRs) synthesized by wet chemical method, spherical particles as a byproduct is inevitable. Efficient and rapid approaches to separate the NRs are critical to optimize the nanostructure-dependent optical properties and not bias conclusions due to existence of spherical impurities. Relatively monodisperse GNRs can be separated from smaller size spheres through repeated centrifugation steps. The number of centrifugation steps, though, must be minimized to avoid irreversible aggregation of rods due to the loss of their capping surfactant. As an alternative, we demonstrate that size selection of GNRs can be achieved by the formation of controlled flocculates of GNRs, driven by an attractive depletion interaction between the NRs induced by surfactant micelles above a critical concentration. The flocculates sediment after a few hours. Separation yields as high as 90 {\%} in number of particles were obtained without any damage to the surface of NRs. Flocculation is reversible upon varying the concentration of surfactant. The effect of the aspect ratio of rods and surfactant micelle structure on the flocculation is discussed with regards to concentration, type and mixing ratio of binary surfactants. The effect of electrostatic interaction is also considered through the impact of different types of electrolytes. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S18.00006: Industrial viable process of making nanoparticles of various shapes and interior structures Xiaorong Wang Over the past 10 years, we attempted to develop industrial viable processes which were of significance in manufacturing the nanoparticles in good quality and large volume. Our effort relied on the self-assembly concepts of block macromolecules in solutions to prepare particles with a hard core made of crosslinked plastics and a soft shell made of low Tg elastomer. Depending on the type and microstructure of the copolymers, the solvent concentration and other process parameters chosen, a variety of shell-core nano-particles of different shapes (spheres, hollow spheres, ellipsoids, cylinders, linear and branched strings, disks and etc.) and sizes (5-100 nm diameter) were reproducibly synthesized. Scale-up studies led to an optimization of the manufacturing process and the production of nanoparticles in large quantities for various product application efforts. The unique performance of those nanoparticles as performance tuning additives and novel rubber reinforcing elements was explored in rubber compounds. This review describes the synthesis methods used to produce the polymer nanoparticles, the technology to modify the particles through functionalization, the means to optimize their performance for specific applications, and the methods to use those particles in rubber compounds. Collaborators: Victor J. Foltz, Kurasch Jessica, Chenchy J. Lin, Jeff Magestrelli, Sandra Warren, Alberto Scuratti, James E. Hall, Jim Krom, Mindaugas Rackaitis, Michael W. Hayes, Pat Sadhukhan, Georg G. A. Bohm [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S18.00007: Performance of ZnO nanowire-based hybrid solar cells decorated with CdTe quantum dots deposited by a pulsed electron beam technique Roberto Aga, Richard Mu, Kenneth Singer More efficient nanocomposite hybrid solar cells require facile charge transport to the collecting electrodes as well as photon-to-electron conversion over the broad solar spectrum. In this work, we employ ZnO nanowires (ZnO-NW) as direct conduction pathways for electrons to the collecting electrode by growing them directly on ITO-coated glass substrates. Photovoltaic nanocomposites are then formed by spin-coating of poly(3-hexylthiophene). By decorating the ZnO-NW with CdTe quantum dots deposited by a pulsed electron beam technique, we have extended the photon-to-electron conversion sensitivity beyond 600 nm. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S18.00008: Organic and Carbon-based Thin-film Transistors on Flexible Substrates. Daniel R. Hines, A. E. Southard, J.H. Chen, M.S. Fuhrer, E.D. Williams Fabrication of organic {\&} carbon-based thin-film transistors (TFT) was achieved on plastic substrates using transfer printing. Each device component (Au electrodes, polymer dielectric layer and semiconductor layer) was printed using only pressure and temperature, eliminating all chemical processing on the device substrate. Pentacene (Pn), poly(3-hexylthiophene) (P3HT), carbon nanotube mats (CNTM) and graphene TFTs were all fabricated on polyethylene terephthalate (PET) substrates, yielding mobilities of 0.237 cm$^{2}$/Vs for Pn and 0.04 cm$^{2}$/Vs for P3HT. Bottom-gate CNTM TFTs are p-type, with mobilities of 13.7 cm$^{2}$/Vs, on/off ratio of 10$^{3}$ and minimal hysteresis. Top-gate graphene TFTs have mobilities of 1.0x10$^{4}$ cm$^{2}$/Vs for holes and 4x10$^{3}$ cm$^{2}$/Vs for electrons. P3HT TFTs showed little variation in mobility, but strong variation in threshold voltage for different dielectric layers. These TFTs printed onto plastic substrates with a variety of polymer dielectric layers will be presented and discussed.. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S18.00009: Self-assembled contacts to nanoparticles using metallic Ga droplets Kan Du, E. Glogowski, M.T. Tuominen, T. Emrick, T.P. Russell, A.D. Dinsmore We demonstrate a pragmatic approach to forming electronic materials and devices, in which metal droplets serve as electrodes and their spacing is controlled spontaneously, \textit{via} self-assembly, to allow tunneling contact with nanoparticles. We have fashioned devices consisting of droplets of molten metal (Ga). Ga is suspended in acidic solution. Ligand-stabilized Au nanoparticles in solution assemble on the metal surface, as shown by electron microscopy. Coated droplets which are then placed on a substrate and the solvent removed. Electron-transport measurements reveal the Coulomb blockade, in which current is suppressed below a tunable threshold voltage by the energy of charging individual nanoparticles. The threshold voltage for two different sizes of nanoparticles agrees with theory. Our approach provides a straightforward approach to creating nanoscale-precision contacts to nanoparticles and might lead to formation of a large number of microscopic devices from suspension. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S18.00010: ATRP of MMA on Asymmetrically Functionalized Gold Nanoparticles Bingbing Wang, Bing Li, Christopher Li Metal nanoparticles have attracted enormous interest due to their unique optical and electronic properties. After the pioneer work of Brust and Schiffrin, a lot of reports have been focused on the modification of the surface of metal nanoparticles with functional groups. However, it still remains a challenging task to synthesize Janus metal nanoparticles, which could potentially leads to directed assembly of the functionalized nanoparticles, an essential step towards using these nanoparticles in real world applications. Asymmetrical gold nanoparticles (AuNPs) modified with two different kinds of polymers on the opposite sides were synthesized using poly(ethylene oxide) single crystals with thiol end groups as the substrate. After the immobilization of AuNPs, room temperature ATRP was performed using the `grafting from' method to obtain asymmetrical AuNPs, which possess the `Janus' nature: i.e. two types of polymer chains were selectively pattered on the different locations of the AuNPs. The asymmetric nature of these AuNPs was demonstrated by NP decoration. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S18.00011: DNA guided assembly of well-organized nano-architectures. Oleg Gang, Dmytro Nykypanchuk, Mathew Maye, Daniel van der Lelie An incorporation of DNA in nano-object design provides a unique opportunity to establish reversible, chemically weak and highly selective interactions between the components of nanosystems. Assembly approaches based on this addressable interactions promise a possibility for creation of rationally designed multicomponent system. However, understanding interplay of interactions, cooperative phenomena leading to phase formation and experimental realizations of ordered phases has remained elusive. Using in-situ x-ray scattering methods, we have studied an assembly kinetics, structure development, and phase formation of DNA-capped nanoparticles on surfaces and in bulk for various DNA assembly schemes. The observed changes in the 2D DNA/nanoparticle array layer reveal an evolution of particle-surface separations and surface coverages. For 3D systems, formation of 3D assemblies with crystalline long-range order in two-component nanoparticle systems was observed. The DNA design, assembly schemes and thermodynamic pathway leading to this crystallization has been explored. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S18.00012: Schottky nanodiodes based on electrospun polymer nanofibers: Effect of varying fiber diameter Rut Rivera, Nicholas Pinto, Alan Johnson Jr. We report on a simple method to fabricate, under ambient conditions and within seconds, Schottky nanodiodes using electrospun polyaniline nanofibers and an inorganic $n$-doped semiconductor. The objective of the present work is to investigate the role of surface states on the device operation by fabricating Schottky nanodiodes using fibers of varying diameter. The standard thermionic emission model of a Schottky junction was utilized in analyzing the data. As the fiber diameter gets smaller, the diode rectification ratio and the diode turn-on voltage shifts to lower values, while the diode barrier height and the ideality factor increase. The simple construction and high surface to volume ratio of the nanofiber also makes these devices attractive candidates in the potential fabrication of low power, supersensitive and rapid response reusable sensors. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S18.00013: Effects of severe confinement on the structure and dynamics in polymer nanocomposites S.H. Anastasiadis, K. Chrissopoulou, S. Fotiadou, K. Andrikopoulos, G.A. Kourouklis, B. Frick The structure and dynamics of PEO/Na$^{+}$MMT nanocomposites is investigated by XRD, DSC, Raman spectroscopy, and quasi-elastic neutron scattering (QENS). For concentrations up to 20 wt\% PEO, the PEO chains within the galleries form either a single- or a double-layer structure of intercalated chains; at higher PEO content only double-layers of intercalated PEO chains are formed within the 0.9nm galleries. For PEO content below 70 wt\%, the absence of XRD peaks that can be assigned to crystalline PEO and of any DSC melting transition as well as the observation of broad Raman lines reveal that the PEO chains remain liquid-like. It is only for PEO content higher than 70 wt\% that diffraction peaks characteristic of bulk PEO are observed together with sharp Raman lines, proving crystallization of only the excess polymer outside the completely full galleries. QENS investigated the dynamics of PEO in bulk and in confinement. A jump of the bulk PEO dynamics at $T_{m}$ is observed whereas the dynamics of confined PEO shows only weak temperature dependence and goes smoothly through the bulk $T_{m}$. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S18.00014: pH and Protein Sensing with Functionalized Semiconducting Oxide Nanobelt FETs Yi Cheng, C.S. Yun, G.F. Strouse, P. Xiong, R.S. Yang, Z.L. Wang We report solution pH sensing and selective protein detection with high-performance channel-limited field-effect transistors (FETs) based on single semiconducting oxide (ZnO and SnO$_{2})$ nanobelts$^{1}$. The devices were integrated with PDMS microfluidic channels for analyte delivery and the source/drain contacts were passivated for in-solution sensing. pH sensing experiments were performed on FETs with functionalized and unmodified nanobelts. Functionalization of the nanobelts by APTES was found to greatly improve the pH sensitivity. The change in nanobelt conductance as functions of pH values at different gate voltages and ionic strengths showed high sensitivity and consistency. For the protein detection, we achieved highly selective biotinylation of the nanobelt channel with through APTES linkage. The specific binding of fluorescently-tagged streptavidin to the biotinylated nanobelt was verified by fluorescence microscopy; non-specific binding to the substrate was largely eliminated using PEG-silane passivation. The electrical responses of the biotinylated FETs to the streptavidin binding in PBS buffers of different pH values were systematically measured. The results will be presented and discussed. $^{1}$Y. Cheng et al., Appl. Phys. Lett. \textbf{89}, 093114 (2006). *Supported by NSF NIRT Grant ECS-0210332. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S18.00015: On the Miscibility of Polymer / Layered Silicate Nanocomposites K. Chrissopoulou, I. Altintzi, I. Andrianaki, N. Koufaki, S. Fotiadou, S.H. Anastasiadis, E.P. Giannelis In the present work we attempt to control the structure in polymer / layered silicate nanocomposites by understanding and / or altering the interactions between the chains and the surfaces. In this respect, hydrophilic and organophilic systems have been utilized and the final structure of the composites is characterized by X-Ray Diffraction and Transmission Electron Microscopy. The effect of the solvent quality on the final structure, in the case of solution mixing, has been examined and the results are compared with the respective obtained from melt intercalation whereas the role of the chemical structure or of the different glass transition temperature of the polymer has been evaluated. In the case of very immiscible systems like for example polyolefin/silicate composites the effect of a more polar additive has been examined. Phase separated up to exfoliated structures can be obtained in a controlled way by varying the compatibilizer to organoclay ratio. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU. [Preview Abstract] |
Wednesday, March 12, 2008 5:30PM - 5:42PM |
S18.00016: Magnetic fluorescent particles with polypeptide shell Sreelatha S. Balamurugan, Paul S. Russo Magnetic fluorescent particles with a hydrophobic polypeptide shell were synthesized and characterized. The first step was the preparation of an iron oxide magnetic core from ferric chloride and ferrous chloride in presence of ammonium hydroxide. A silica shell was grown on this central nougat by the St\"{o}ber method. In order to introduce fluorescence, a mixture of tetraethoxy silane, a complex of fluorescein isothiocyanate (FITC) with 3-aminopropyl triethoxy silane (APTES), ammonia, and ethanol were added. These particles were further functionalized to place amine groups on the surface. Polypeptide chains were grown from the amine initiators by ring opening polymerization of the N-carboxyanhydride of the glutamate. These particles were characterized by light scattering, transmission electron microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy and fluorescence spectroscopy. [Preview Abstract] |
Session S19: Focus Session: Dopants and Defects in Semiconductors III
Sponsoring Units: DMPChair: Eugene Haller, University of California, Berkeley
Room: Morial Convention Center 211
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S19.00001: Stoichiometry driven impurity configurations in compound semiconductors Invited Speaker: As is well known, crystal growth of defect-free compound semiconductors, in contrast to elemental, is inherently limited by non-stoichiometry. High resolution infrared spectroscopy of localized vibrational modes can display unique signatures which reveal the structure of stoichiometry related defect-impurity complexes. The talk will focus on II-VI semiconductors in which group II cations are replaced with a group IIA or a 3d-transition metal ion as an impurity, on the one hand, and a group VI anion replaced with a group VIA impurity, on the other. Incorporation of O replacing Te with a full complement of nearest neighbor Cd's, i.e. $\mathrm{O_{Te}}$, as well as $\mathrm{O_{Te}}$ in association with a Cd vacancy ($\mathrm{V_{Cd}}$) in the zincblende CdTe result in defect centers with unique i.r. signatures. The occurrence of $\mathrm{O_{Te}}$ with $T_d$ symmetry and ($\mathrm{O_{Te}-V_{Cd}}$) with $C_{3v}$ symmetry can be controlled by favoring or suppressing Cd vacancies. In CdSe, with its wurtzite structure, oxygen incorporation occurs in two ways: in one, it is an \textquotedblleft anti-site\textquotedblright ~defect, $\mathrm{O_{Cd}}$, as revealed in its host isotope related fine structure; in the other, oxygen enters in association with Cd vacancies as ($\mathrm{O_{Se}-V_{Cd}}$). The talk will discuss the number of i.r. signatures specific to each center; their polarization characteristics (in CdSe); the striking temperature behavior of the i.r. signatures of ($\mathrm{O_{Te}-V_{Cd}}$) and ($\mathrm{O_{Se}-V_{Cd}}$); and the occurrence of overtones/combinations of the LVMs in CdTe. These investigations provide a wealth of microscopic insights into orientational degeneracy, host isotope effects and acquisition of the temperature averaged higher symmetries by the switchings of the dangling bond of $\mathrm{V_{Cd}}$. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S19.00002: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S19.00003: Carrier compensation in semi-insulating CdTe Mao-Hua Du, David Singh Carrier compensation in semi-insulating CdTe has been attributed to the compensation of surplus shallow acceptors by deep donors, usually assumed to be Te antisites. However, our first-principles calculations show that intrinsic defects should not have a significant effect on the carrier compensation due either to lack of deep levels near midgap or to low defect concentration. We demonstrate that an extrinsic defect, O$_{Te}$-H complex, may play an important role in the carrier compensation in CdTe because of its amphoteric character and reasonably high concentration. Our findings have important consequences for improving device performance in CdTe-based radiation detectors. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S19.00004: Gadolinium Doping in ZnTe Z.X. Ma, Lei Liu, Kin Man Yu, Wladek Walukiewicz, Dale Perry, Peter Yu, Sam Mao We have investigated, experimentally and theoretically, the effects of Gd doping on the structural and optical properties of ZnTe films grown by pulsed laser deposition. A few {\%} of Gd doping was found to \textit{reduce} the ZnTe lattice constant with \textit{no change} in the fundamental band gap. When the doping level is increased to $>$7{\%}, the lattice constant becomes more or less constant, but the band gap increases abruptly by as much as 50 meV. First principle calculations based on density functional theory using the linearized augmented plane wave method were performed using ZnTe supercells containing either isolated defects or defect complexes. The reduced lattice constant on Gd doping can be attributed to the presence of defect complexes involving substitutional Gd ions and neighboring vacancies. The insensitivity of the band gap at lower Gd concentration can be explained by self-compensation of these defects. The increase in the band gap energy at higher concentration is attributed to band-filling effect. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S19.00005: First Principles Investigation of H in CdTe Hiroyuki Takenaka, David J. Singh, Mao-Hua Du CdTe and alloys particularly (Cd,Zn)Te are of interest for radiation detection and other applications. A key issue is obtaining high mobility compensated material with low concentrations of traps. Hydrogen has been shown to play an important role in various semiconductors with both beneficial and deleterious effects. We investigate the effect of H on vacancies in CdTe using supercells. Our results are obtained from the first principles density functional theory calculations using the full potential linearized augmented planewave method including local orbitals and based on local density approximation. H atoms in Cd vacancies move toward one of Te atoms, which form Td symmetry with Cd in the bulk system. The bond length between H and Te is 1.7 angstrom. This corresponds to a closely bonded Te-H unit similar to OH. We also present H in Te vacancies. We report the electronic structures as well as positron lifetimes. This work was supported by DOE, NA-22. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S19.00006: Origin of doping bottleneck in semiconductor quantum dots Su-Huai Wei, Jingbo Li, Shu-Shen Li, Jian-Bai Xia Doping difficulties in semiconductor nanocrystals have been observed and its origin is currently under debate. It is not clear whether this phenomenon is energetic or depends on the growth kinetics. Using first-principles method, we performed systematic study of defects (donors, acceptors, isovalent defects, etc.) in ZnSe quantum dots. we show that, the transition energies and defect formation energies of the donor and acceptor defects always increase as the quantum dot sizes decrease. However, for isovalent impurities the changes of the defect formation energies are rather small. Our study suggests that for donor and acceptor defects, the doping difficulty is mostly due to energetic effects, whereas for isovalent impurities, the doping difficulty is mostly due to kinetic effects. The origin of the calculated trends is explained using simple band-energy-level models. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S19.00007: Increased binding energy of impurities near a semiconductor vacuum interface. Paul Koenraad, Ineke Wijnheijmer, Jens Garleff, Karin Teichmann, Martin Wenderoth, Sebastian Loth, R. Ulbrich We have recently shown that a STM tip can be used as a tool to manipulate the charge state of a single impurity below the cleavage surface of a semiconductor. This manipulation allowed us to determine the binding energy of single donors and acceptors as a function of their depth (up to 1 nm) below the surface. We found that the binding energy strongly increases near the surface. In the case of a Si-donor in GaAs the binding energy increases continuously from 5.6 meV in the bulk to more than 100 meV close to the surface. Our STM techniques also allowed for the determination of the size and shape of the Coulomb field of single ionized donors. We found that the range of the potential is strongly reduced relative to the bulk value. Both the reduced range of the Coulomb potential and the increased binding energy can be related to a reduced dielectric constant and increased effective mass near the surface. We will discuss the implications of these findings. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S19.00008: First-principles simulations of GaAs defects: The challenge of Ga pseudopotentials Peter A. Schultz, O. Anatole von Lilienfeld Design of norm-conserving gallium pseudopotentials (PP) has been investigated for density functional theory calculations of defects in GaAs. A converged PP construction is described. We examined the performance in cubic zinc-blende structure phases of GaAs, GaP, and GaN. Computed lattice constants, bulk moduli, and, particularly, band gaps vary greatly depending on PP construction and exchange correlation functional. The Kohn Sham band gaps exhibit a distinctive sensitivity on lattice constant, direct with a strong near-linear dependence at larger lattice constants with crossover to indirect near (within 5\%) of the equilibrium lattice constant. Gradient-corrected functionals with a converged PP give a near-zero GaAs gap, a problem for defect calculations. A 3d-core PP ``fixes'' the band gap, but predicts GaAs defect formation energies different from converged 3d-valence PP. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S19.00009: Computational studies of a 2D tight-binding model of randomly dispersed hydrogenic centers Jayson Paulose, Ravindra Bhatt The impurity band arising from s-orbitals of randomly dispersed hydrogenic dopants in semiconductors in two dimensions is studied via exact diagonalization of a tight-binding model. Ensemble averaged density of states (DOS) and inverse participation ratio (IPR) of eigenstates are obtained as a function of energy at low to intermediate doping densities, where small clusters of sites are most significant. A similar calculation is done for p-orbitals for comparison. A strong peak in the DOS is seen about the impurity level. Increasing the density of sites weakens this peak and produces asymmetry in the DOS and the IPR. A nearest-neighbour pair approximation qualitatively explains several features in the DOS at low densities but does not reproduce the singularity. This motivates a comparison to a hierarchically constructed pair model, as well as random bipartite systems, which is pursued further via a renormalization group approach in a concurrent study. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S19.00010: Quenched singularity in the density of states of 2D random hydrogenic systems R.N. Bhatt, Erik Nielsen, Jayson Paulose Delta-doped hydrogenic dopants in semiconductor heterostructures give rise to an impurity band which can be characterized by a two-dimensional tight-binding model with randomly positioned sites. At low densities, the density of states possesses a singularity about the impurity level, which can be understood in terms of the states of a hierarchically constructed set of impurity pairs. As the density is raised, this singularity is quenched due to further neighbors breaking the electron-hole symmetry. The quenching is accompanied by an asymmetry in the density of states, and pair approximations are insufficient to even qualitatively describe the system at higher densities. We motivate and outline a renormalization group technique that captures the quenched singularity and asymmetry in the density of states. This approach motivates the study of random bipartite systems [1], for which it is particularly suited. We compare the results of both types of systems.\newline [1] M. Inui, S. A. Trugman, and Elihu Abrahams, Phys. Rev. B 49, 3190 (1994). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S19.00011: Negative magnetocapacitance and associated Schottky barrier height changes in lightly doped GaAs Sefaattin Tongay, Arthur Hebard We investigate the magnetic field response of Schottky barriers formed on GaAs with Si dopant densities of $\sim $3E16 cm$^{-3}$ and $\sim $9E16 cm$^{-3}$. Negative magnetocapacitance of up to 21{\%} at 20K and increasing Schottky barrier height as determined by various impedance measurement techniques are observed. We attribute these effects to a magnetic-field induced increase in the ionization energies of electrons bound to Si impurity atoms, causing shallow impurity electrons to hop to (ionization process) and from (capture process) the conduction band on longer time scales. The effective field at which these effects are seen is a factor of 10$^{5}$ smaller than it would be in vacuum because of the smaller effective mass and larger dielectric constant of the GaAs host. The dependence of interband hopping on magnetic field decreases the dipole response in the depletion region and gives a corresponding decrease in the measured capacitance. Magnetic field dependent Schottky barrier heights are inferred from linear 1/C$^{2}$ versus bias voltage plots. We note that these magnetic field dependent effects are occurring in the absence of magnetic impurities and thus need to be understood before characterizing the magnetic response of diluted magnetic semiconductors. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S19.00012: Thermally activated persistent photoconductivity {\&} donor binding energy in high mobility AlAs QWs S. Dasgupta, C. Knaak, A. Fontcuberta, M. Bichler, G. Abstreiter, M. Grayson In AlAs, valley index is important quantum number which can help understand interactions. However, important parameters for growth such as donor binding energy and Si $\delta $-doping efficiency were unknown and AlAs quantum wells (QWs) typically did not conduct in dark. We grew series of (001) and (110) oriented double-sided doped n-type AlAs QWs and deduced Si donor binding energy $\Delta $ in Al$_{0.45}$Ga$_{0.55}$ As and doping efficiency $\eta$. They work in dark possibly because dilute charge traps in substrate are screened by backside doping. From dark saturation density for doping series we deduced $\Delta_{dk}$=65.2 meV [1]. Our studies show thermally activated PPC where sample is illuminated at 4 K and returned to dark without appreciable density increase. As temperature is increased to 30 K, density doubles, indicating shallow binding energy $\Delta_{PIA}$=0 meV post-illumination anneal (PIA). Doping efficiency after illumination for (001) facet was found to be $\eta $=n$_{2D}$/n$_{Si}$=35{\%} and for (110) $\eta $=17{\%}. With this understanding, we designed (001) AlAs QW with PIA density n=2.4 x 10$^{11}$ cm$^{-2}$ and mobility $\mu $=4.3 x 10$^{5}$ cm$^{2}$/Vs(330 mK), improvement of almost an order of magnitude over published results. [1] Dasgupta, \textit{et al.} APL (2007) [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S19.00013: The low-temperature 2D mobility for metallic p-type GaAs Quantum Well Theodore Castner At T $<$ 1.2K the mobility $\mu$(T) is determined by charged trap ionized impurity scattering (iis) and T-dependent screening [1]. $\mu$(T) is calculated with $<$$\tau$(E)$>$ given by an empirical expression $\tau$ = $\tau$$_{o}$x/[x + C tanh($\eta$/2)] [x = E/kT, $\eta$ = T$_{F}$/T and a 2D DOS that features a pseudogap. $\mu$(T) exhibits a minimum at T$_{m}$ = T$_{F}$/2.25 and increases slowly for T $>$ T$_{m}$. The physical reason for this unusual increase in $\mu$(T) is explained. The coefficient C is directly related to $\mu$(0)/$\mu$(T$_{m}$ [4.0 $>$ ratio $>$ 3.6 for p-type GaAs data [2]]. The T-dependent screening $\kappa$$_{2}$(T) = s(T) $\kappa$$_{2}$(0) and s(T) is given by [$\mu$(T)-$\mu$$_{m}$]/ [$\mu$(0)-$\mu$$_{m}$]. This s(T) allows the determination of T* [d$\sigma$/dT = 0] where T* is slightly less than T$_{m}$. The data [2] is an example of ideal 2D behavior. The role of interactions for T $<$ T$_{m}$ and T $>$ T$_{m}$ will be discussed. [1] F. Stern, PRL 44, 1469 (1980); [2] X.P.A. Gao et al., PRL 93, 256402 (2004). [Preview Abstract] |
Session S20: Focus Session: Engineering Interfaces for New Materials III: Heterogeneous Interfaces
Sponsoring Units: DMPChair: Eduardo Saiz, Lawrence Berkely National Laboratory
Room: Morial Convention Center 212
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S20.00001: Epitaxial Growth of Iron and Iron Nitrides on Wurtzite Gallium Nitride (0001) Wenzhi Lin, Jeongihm Pak, Kangkang Wang, Abhijit Chinchore, David Ingram, Arthur R. Smith Magnetic transition metal-containing layers on GaN have potential spintronic applications. We explore the epitaxial growth of iron and iron nitride films on wurtzite ($w)$-GaN(0001). First, we investigate the growth of $\sim $ 1:1 iron nitride on $w$-GaN(0001) using rf N$_{2}$-plasma molecular beam epitaxy (MBE) and monitor growth with \textit{in-situ} reflection high energy electron diffraction (RHEED). We find that FeN grows epitaxially with zinc-blende structure and [111]-orientation on $w$-GaN(0001). To achieve high Fe content, such as Fe$_{4}$N, and Fe$_{16}$N$_{2}$, current efforts are aimed at reducing N content in the source gas. In the case of pure Fe deposition, x-ray diffraction and RHEED suggest the epitaxial relationship to be [110]$_{Fe}\vert \vert $ [0001]$_{GaN}$ with Fe in bcc structure. The assignment is based on lattice spacing measurements as well as angular dependence of the RHEED pattern. In this presentation, most recent results for Fe and Fe$_{x}$N$_{y}$ films grown on $w$-GaN(0001) will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S20.00002: Observation of Standing Waves on the GaN(0001) Pseudo (1x1) Surface by Scanning Tunneling Microscopy at Room Temperature L. Li, Y. Qi, G. Sun, M. Weinert The metallic pseudo-1x1 surface, consists of 2 to 2.5 ML Ga on top of the Ga-terminated GaN(0001), provides an ideally confined 2D electron gas (2DEG), which gives rise to complex standing wave patterns. Even at room temperature, these patterns can be observed by scanning tunneling microscopy (STM) and spectroscopy (STS). The analysis of the modulation of the local density of states within various confinement geometries as a function of the bias voltage shows that nearly free-electron like energy dispersive surface states are being probed. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S20.00003: Surface Evolution During Sub-Monolayer Manganese Deposition onto Wurtzite Gallium Nitride (000-1) Surface. Abhijit Chinchore, Kangkang Wang, Wenzhi Lin, Jeohgihm Pak, Arthur R. Smith While transition metal (TM)-doped gallium nitride (GaN) films have been explored as potential spintronic materials, the structural and magnetic effects of various TM adatoms on GaN surfaces are not well understood. In this work, we investigate the deposition of sub-monolayer quantities of Mn onto the N-polar GaN(000-1) 1$\times $1 surface. First, the GaN surface is prepared by molecular beam epitaxy. The smooth surface is then annealed to remove excess Ga adatoms. Next, the surface is exposed to a dose [approximately 0.05-0.1 monolayer (ML)] of Mn at substrate temperature of 200 $^{o}$C. Using \textit{in-situ} reflection high energy electron diffraction (RHEED), we observe the onset of clear 3$\times $ periodicity along [1-100] but only 1$\times $ along [11-20]. Additional 0.05-0.1 ML Mn doses lead to increasing intensity of the 2/3-order RHEED streaks, while 1/3-order and 1$^{st}$-order streaks weaken. For Mn doses up to about 1/3$^{rd}$ ML, the surface appears quite smooth, with the RHEED pattern stable upon heating the surface to 600 $^{\circ}$C. The results suggest a surface evolution process leading to a well-ordered Mn-containing structure at the GaN(000-1) surface. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S20.00004: Van der Pauw and Hall Measurements on Ultra Thin Silicon-on-Insulator Weina Peng, Hongquan Jiang, Sangkeun Ha, Madhu Thalakulam, Donald Savage, Mark Eriksson, Max Lagally Ultra-thin silicon-on-insulator (UTSOI) provides opportunities to study the role of the surface in electrical transport in Si. Because the Si layers can be as thin as 10 nm, surface states, surface induced band bending, and gap states at the oxide-Si interface dominate the carrier density. Transport measurements provide a sensitive probe of the carriers. Previous measurements of thin Si structures have shown that Si/SiO$_{2 }$interface traps deplete Si of mobile carriers, and sheet resistances reach 10$^{11}$ ohm/sq for a 20 nm thick sample [1]. Thus, any perturbation to the surface that induces even modest carrier densities can be detected in transport. We perform van der Pauw and Hall measurements on UTSOI structures with a variety of surface modifications, including hydrogen termination and epichlorohydrin surface attachment. UTSOI that was extremely resistive with oxide on both sides undergoes a drop in resistance of more than 3 orders of magnitude after surface modification. Hall and van der Pauw measurements, reveal the density and the sign of the carriers. We discuss the mechanisms for this increased conductivity. [1] Zhang P. et al. \textit{Nature} \textbf{439} 703 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S20.00005: Segregation of O defects in Si:HfO$_{2}$ heterojunctions: A first principles investigation. R. Ramprasad, C. Tang Driven by a need for device miniaturization in the microelectronic industry, Hf-based high-permittivity materials, such as HfO$_{2}$, have gained interest for their potential application as gate dielectrics. However, undesirable interfacial phases such as Hf silicides and SiO$_{x}$ are known to form and degrade the performance of devices. It has been postulated that these interfacial phases are related to the segregation of O defects (vacancy or interstitial) to the interface. In this work, we examine the thermodynamic and kinetic driving forces for the segregation of isolated and clustered O defects (vacancies and interstitials) to the Si:HfO$_{2}$ interface. Using first principles density functional theory calculations, we have determined the formation and migration energies per O defect within bulk HfO$_{2}$ and at the Si:HfO$_{2}$ interface. Our results indicate that isolated as well as a distribution of point defects display large driving forces for interface segregation, allowing for the formation of silicides and silicates. Thus, while an abrupt Si:HfO$_{2}$ interface may be stable in the absence of O defects, such an interface is unstable to the formation of other phases in the presence of O defects. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S20.00006: Sub-Angstrom Distortions of an Epitaxial Oxide on Silicon (001) Yaron Segal, Fred Walker, J.W. Reiner, C.H. Ahn, Zhan Zhang As metal oxide semiconductor field effect transistor (MOSFET) devices are reduced to the nanometer length scale, atomistic control of the silicon-oxide interface is needed in order to fabricate optimally functioning devices. In this work, we present synchrotron x-ray diffraction measurements of a model system, barium oxide grown epitaxially on Si (001), with an interface phase of submonolayer strontium on silicon. Diffraction results show that the 2x1 surface phase that promotes epitaxy transforms into an interface phase between the oxide and silicon, which also has a 2x1 symmetry on the Si (001) surface. Quantitative analysis of the diffraction is consistent with three classes of models; these involve a 2x1 arrangement of alkaline earth metal in the interface phase, sub-angstrom distortions of the oxide film, or a combination of both. These measurements demonstrate how this reconstruction is a true interface phase that can be used to test our current understanding of silicon-oxide interface physics. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S20.00007: Experiment and theory on metal/ceramic interfaces Invited Speaker: |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S20.00008: Theoretical investigation of the interface structure of $\theta $-Al$_{2}$O$_{3}$/NiAl(001). Jyh-Pin Chou, Ching-Ming Wei The atomic structure of $\theta $c-Al$_{2}$O$_{3}$/NiAl(001) interfaces has been investigated by using \textit{ab initio} pseudopotential method based on the density functional theory. Knowledge on physical origin of adhesion on oxide and metal interface is essential for the development of various industrial applications. However, the atomic structure of the interface has not been fully clarified yet. In this study, surface energies of the Al-terminate and Ni-terminate NiAl(001) are calculated. Geometry configurations and bond adhesion strength between$\theta $c-Al$_{2}$O$_{3}$ and NiAl(001) are determined.. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S20.00009: Calculated atomic arrangement and impurity bonding at a $\kappa $-Alumina -- Al(771) interface. Peter J. Feibelman First principles optimization of a $\kappa $-Alumina -- Al(771) superlattice shows that the incompliant oxide causes substantial disorder in the adjacent, soft metal layers. An H ``probe atom'' is found to bind best in the disrupted metal region, suggesting that this is the locus of initial failure of a protective oxide layer. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S20.00010: Local dielectric constants in metal-oxide and oxide-oxide interfaces: an ab initio approach to address interfacial effects Bora Lee, Choong-Ki Lee, Seungwu Han, Shang-Ho Jeon, Bae-Ho Park Recent experiments indicate that the dielectric constants of thin film oxides are strongly affected by interfaces formed between the oxide and metal or between different oxides. Such interfacial effects will be crucial to high-k dielectric stacks employing oxide materials with nanometer thickness. Therefore, systematic studies on interfacial effects are very important for materials selection and process design of gate stacks. In this presentation, we study on local dielectric constants in metal-oxide and oxide-oxide interfaces within the first-principles framework. Firstly, we introduce an efficient method to calculate local dielectric constants by employing slab models exposed to the vacuum. The static as well as optical dielectric constants are obtained from the change in electrostatic potentials upon the application of external electric fields. Our method can be easily adopted using conventional codes without any modification of the program. We apply this method to investigate interfacial dielectric constants in Au/MgO, Ni/ZrO2, Pt/HfO2, Ni/HfO2, Al/HfO2, SiO2/HfO2, and Al2O3/HfO2 interfaces. Our results show the presence of interfacial region with dielectric constants significantly different from that of the bulk. Microscopic explanations will be provided based on the dynamic charges and hardening/softening of phonons. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S20.00011: The role of metal/transition metal oxide/organic interface Chang-Ting Lin, Guan-Ru Lee, Chih-I Wu, Tun-Wen Pi In this paper, we report a study with UPS and XPS data of metal/transition-metal-oxide/organic interfaces. Transition metal oxides are widely used in organic light- emitting (OLEDs) in recently years, such as Wo$_{3}$, ReO$_{3}$, MoO$_{3}$, and V$_{2}$O$_{5}$. These metal oxides have been proven to be good hole injection layers in OLEDs, interlayers in tandem OLEDs, and nanocomposite electrodes. Although a large number of studies have been made, little is known about the mechanism of metal/transition-metal-oxide/organic interfaces. UPS and XPS data performed by synchrotron radiation research show that these oxides would catch electrons from organic and results in p-type doping in organic material. In addition, there is a significant structure transition from insulating metal oxide to metallic metal oxide. As a result of high work function metallic metal oxides in anode structures and p-type doping organic hole transport layers (HTLs), holes can easily be injected from anode to HTLs. Current-voltage characteristics (I-V) and quantum-efficiency ($\eta $-J) measurements also show the improvement of device performance with insertion of thin transition metal oxides between anodes HTLs. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S20.00012: Stability, structure, and electronic properties of chemisorbed oxygen and thin surface oxides on Ir(111) H. Zhang, A. Soon, B. Delley, C. Stampfl Iridium-based catalysts are widely used in several important chemical reactions. Despite this, very little is known about the surface structure of the catalyst and the atomic and molecular processes involved. As a first step towards a microscopic understanding, we use density-functional theory, coupled with \textit{ab initio} atomistic thermodynamics [1], to investigate chemisorption of oxygen on Ir(111), and the stability of surface oxides. We find for on-surface adsorption, oxygen prefers the fcc-hollow site for all coverages considered, where with increasing coverage, the adsorption energy decreases substantially. Subsurface adsorption is found to be highly unfavourable. The most favourable surface-oxide-like structure has a tri-layer-like (O-Ir-O) configuration, which however, the $(p,T)$ phase diagram predicts is only metastable. For practically all conditions, except ultra-high vacuum, the bulk oxide is thermodynamically the most stable, and the only other stable phase predicted is the on-surface (2x2)-O structure for coverage 0.25 ML [2]. These studies point to the possible importance of oxidized iridium for heterogeneous oxidation reactions. [1] C. Stampfl, Catal. Today 105, 17 (2005). [2] H. Zhang, A. Soon, B. Delley and C. Stampfl, submitted to Phys. Rev. B. [Preview Abstract] |
Session S21: Focus Session: Fundamental Issues in Catalysis III
Sponsoring Units: DCPChair: Steve Overbury, Oak Ridge National Laboratory
Room: Morial Convention Center 213
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S21.00001: Transient FTIR spectroscopy for probing reaction pathways on Au catalysts Invited Speaker: Au is now well known to be an active catalyst if the Au particles are sufficiently small, less than about 5 nm. The causes for this structure sensitivity are now beginning to be better understood. Computational modeling and measurements of size dependence on a single catalyst are consistent with activity at sites with low coordination numbers, due in part to flexibility of adsorbate geometry in these sites. Although small size and low coordinate sites are important in catalyzing, e.g. the CO oxidation reaction, there appear to be other factors which control the observed activity as demonstrated by catalyst deactivation and unusual temperature dependence. We have performed studies of CO oxidation over Au/TiO$_{2}$, Au/SiO$_{2}$, Au/ZnO/TiO$_{2}$ and Au/FePO$_{4}$ catalysts to explore reaction pathways and the causes for activation and deactivation. Three different reactor systems, a fast gas transient FTIR spectrometer, a slower transient DRIFTS cell and a steady state plug flow reactor have been used to correlate activity with surface species. Using this \textit{operando} approach the elementary steps in the CO oxidation reaction have been explored. Striking differences between the supports are found. The effect of various pre-treatments, the evolution of the surface species during ``steady state'' reaction and the role of carbonate, oxygen storage, water, hydroxyl upon catalyst activation and deactivation have been explored and will be described. Reaction pathways and mechanisms will be proposed and compared for the different catalysts. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S21.00002: TBD Invited Speaker: |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S21.00003: Molecular Factors Determining Selectivity in Catalysis. Invited Speaker: Achieving high selectivities is arguably the main challenge in heterogeneous catalysis for the 21st century. In complex reaction with competing parallel pathways, small changes in the relative values of the different activation energies are sufficient to switch the selectivity of those processes from one product to another. We in our laboratory have been carrying out mechanistic studies on model metal surfaces to try to identify the key factors that control such selectivity. In this talk we will present several examples of increasing subtlety from that work, with focus on the conversion of hydrocarbons. Specifically, we will discuss issues of regioselectivity and stereoselectivity in early dehydrogenation steps, and how those affect selectivity in the conversion of olefins. Time permitting, we will also discuss issues related to the bestowing of enantioselectivity on solid surfaces. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S21.00004: The nucleation and growth of ordered Fe and FeO nanoparticles on reconstructed Au(111) surfaces Neetha Khan, Christopher Matranga Iron-based catalysts, including iron oxides, are an important class of materials with relevance to Fischer-Tropsch catalysis and gas-sensing applications. By growing nanostructured particles on single-crystal surfaces, we can create a model system to study size and shape effects on reactivity. We have studied the formation of monolayer thick iron oxide nanoparticles and thin films on the reconstructed Au(111) surface. STM, XPS, ISS, and LEED were used to evaluate the structure and composition of the iron oxide nanoparticles and films as a function of growth conditions. Iron oxide was grown by depositing iron on Au(111), followed by oxidation at room temperature and annealing to 700 K. XPS results indicate that the Fe is oxidized at room temperature, but the STM results indicate that the particles are not ordered until after annealing to 700 K. Atomically-resolved STM images show that at 0.3 ML Fe coverage, iron oxide nanoparticles, pseudo-hexagonal in shape are formed, with large defects occurring in the corners. STM images of FeO particles over 0.5 ML Fe also show evidence of a non-coincidence overlayer lattice with a short periodicity of 0.25-0.3 nm modulated by a larger periodicity of approximately 3.5 nm. The larger periodicity results from a moir\'{e} pattern formed between the iron oxide overlayer and the underlying Au(111) surface. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S21.00005: Dynamic structure in Pt nanoclusters on $\gamma$-alumina F. Vila, J.J. Rehr, J. Kas, R.G. Nuzzo, A.I. Frenkel Pt nanoclusters on $\gamma$-alumina exhibit a number of unusual phenomena including large structural disorder and bond-length contraction with increasing temperature. We investigate this behavior for a prototypical 10-atom Pt cluster using real-time, temperature-dependent simulations combining density functional theory/molecular dynamics and x-ray spectroscopy theory. We find that the cluster structure is dynamically varying in shape and topology on a time scale long compared with internal vibrations. Moreover, the clusters are not rigidly attached to the surface and occasionally pick up or discard a Pt-O bond. This real-time approach suggests that these nanoclusters are comprised of two distinct populations of Pt atoms depending on the charge transfer from the surface, and reproduces many of their unusual properties. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S21.00006: In Situ Characterization of Ethylene Hydrogenation on Pt Powder Using Mass Spectrometry-Sum Frequency Generation Technique Bryan Hsu, Shawn Dougal, Paul Stevens, Mohsen Yeganeh Bridging the pressure gap has been of paramount importance to the field of surface science. Unfortunately, the available techniques used to characterize catalytic surfaces have all been limited in some degree by a combination of factors (e.g. low pressure regimes, ex situ analysis, and low surface area catalysts), which do not fully replicate industrially relevant conditions. Here, we present in situ observation of ethylene hydrogenation of Pt powder in a high pressure regime. Using total internal reflection sum frequency generation (TIR-SFG) we are able to identify ethyl, ethylidyne, di-sigma-bonded ethylene, and pi-bonded ethylene surface intermediates and find that these are all present under reactive conditions as monitored with mass spectrometry (MS). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S21.00007: Dissociation of water and Acetic acid on PbS from first principles Alessandra Satta, Paolo Ruggerone, Giovanni De Giudici The adsorption of complex molecules at mineral surfaces are crucial ingredients for understanding the mechanisms that rule the interaction between minerals and the biosphere and for predicting both the stability and the reactivity of minerals. The present work focuses mainly on the early stages of different adsorption reactions occurring at both the cleavage surface and a high-index vicinal surface of galena (PbS). We have studied the dissociation mechanism of water and acetic acid on the galena surfaces by means of ab initio calculations within the framework of the density functional theory in the generalized gradient approximation and of pseudopotential approach. The calculated adsorption energies of the molecules indicate the stepped surface as the most reactive, as expected. The free energy surface during the reaction process has been explored via metadynamics[1]. The optimized configurations of both reactants and products obtained, were then used to accurately calculate the dissociation energy via the Nudge Elastic Band method[2]. [1] A. Laio and M. Parrinello, PNAS 99, 12562 (2002). [2] G. Mills and H. Jonsson, Phys. Rev. Lett. 72, 1124 (1994). [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S21.00008: Characterization and Reactivity of Mo$_{6}$S$_{8}^{+}$ on Au (111) via Size-Selected Deposition Melissa J. Patterson, James M. Lightstone, Michael G. White Supported MoS$_{2}$ nanoparticles are known for their ability to catalyze a wide range of heterogenous reactions such as hydrodesulfurization (HDS)$^{1}$. However, understanding the role of size, structure, composition and support interactions of the MoS$_{2}$ particles in these heterogenous reactions has not yet been resolved due to the inhomogeneity of commercial catalysts. Work done in our laboratory is geared towards preparing homogenous samples in ultra high vacuum that can serve as model systems for these types of catalytic reactions. We are currently investigating the reactivity of size-selected transition metal clusters generated in the gas-phase and deposited on a Au(111) surface. Using a magnetron cluster source, we are able to produce a wide range of nanocluster stoichiometries including the Mo$_{6}$S$_{8}^{+}$ cluster, which has been observed as the metal core of the well-known Chevrel phase$^{2}$. The work presented focuses on characterization of the Mo$_{6}$S$_{8}^{+}$ cluster deposited on a Au(111) single crystal using techniques such as Auger, photoemission spectroscopy, and thermal desorption. In addition, preliminary reactivity studies will be presented of the supported Mo$_{6}$S$_{8}^{+}$ cluster with small sulfur containing molecules. 1. Topsoe, H.; et.al; \textit{Hydrotreating Catalysis}; Springer: New York, 1996. 2. Umarji, A. M.; et.al.; \textit{J. Phys. Chem. Solids} \textbf{1980}, $41$, 421. [Preview Abstract] |
Session S22: Focus Session: Organic Electronics: Contacts and Interfaces
Sponsoring Units: DMP DPOLYChair: Dave Gundlach, National Institute of Standards and Technology
Room: Morial Convention Center 214
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S22.00001: Energetics of organic semiconductor interfaces: enhancing injection via chemical doping Invited Speaker: Chemical doping of organic molecular films is a powerful way to improve charge injection and transport in organic devices, and to enhance device functionality. The formation of narrow depletion regions at doped organic-conductor interfaces facilitates injection via carrier tunneling through the barrier, and allows the use of moderate work function and non-reactive metals as efficient contacts. P-doping with the electronegative molecule, tetrafluoro-tetracyano-quinodimethane (F$_{4}$-TCNQ), has been used on a number of hole-transport materials. N-type doping is more challenging, often hindered by the energetic requirements of transferring an electron from the dopant HOMO to the host low lying LUMO. We recently demonstrated efficient n-doping of the electron transport material tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP), which has a 4.59 eV electron affinity (EA), with cobaltocene (CoCp, IE = 4.07 eV). We now introduce a stronger n-dopant, i.e. decamethylcobaltocene (CoCp*$_{2}$), and demonstrate n-doping of copper phthalocyanine (CuPc, EA = 3.25 eV). CoCp*$_{2}$ is found to have a remarkably low IE of 3.30 eV. N-doping is evidenced by a large upward swing of the Fermi-level in the gap of CuPc, and confirmed by current-voltage (I-V) measurements. A 10$^{4}$- to 10$^{7}$-fold increase in current density of the interface-doped device a compared to the undoped CuPc device is due to enhanced injection. An additional 10$^{3}$-fold increase in current density is observed for the uniformly doped device and is attributed to enhanced conductivity of the bulk film. The application of p- and n-doping of CuPc to an organic homojunction p-i-n diode with a 1.47 eV built-in potential is demonstrated. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S22.00002: Electronic Structure of Interfaces and Heterojunction Ambipolar Organic Thin Film Transistor. Yongli Gao, Huanjun Ding, Haibo Wang, Donghang Yan There has been a considerable interest in forming ambipolar organic thin film transistors (OTFTs) due to their advantageous for integrated circuits. Recently, Shi \textit{et al.} observed a substantial improvement for both the hole and the electron mobility in ambipolar OTFTs based on the heterojunction formed between copper-hexadecafluoro-phthalocyanine (F$_{16}$CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). We examined the interface formation between F$_{16}$CuPc and BP2T using ultraviolet photoemission (UPS) and inverse photoemission spectroscopy (IPES). It is observed that in F$_{16}$CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F$_{16}$CuPc the band bending is confined in BP2T only. For F$_{16}$CuPc/BP2T, the band bending of BP2T and F$_{16}$CuPc are 0.40 and 0.35 eV, respectively. The band bending region is about 15 nm in both materials, from which the Debye lengths of the materials can be deduced. The combination of the band bending and finite Debye lengths may provide an explanation to the observed ambipolar behavior and improved mobility of the OTFTs based on such heterojunctions. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S22.00003: Sub-100 nm Contact Effects in Poly 3-hexylthiophene (P3HT) Jeff Worne, Douglas Natelson Poly 3-hexylthiophene (P3HT) is a widely studied, versatile material used in organic electronics. Important to understanding the behavior of P3HT lies in its interaction with metal contacts. Contact effects between P3HT and metal electrodes can influence charge injection into P3HT, giving rise to a contact resistance and thereby influencing device performance. The origin of this contact resistance still remains poorly understood, but may result from changes in film morphology near the metal contact, charge transfer and band bending near the contact, or both. Understanding the detailed behavior of the interface between P3HT and metal electrodes will allow for optimization of device behavior. Based on previous work, the voltage drop at the P3HT-metal interface happens over 10-100 nm. We have fabricated devices on the tens of nanometer scale that directly probe this region, and present data on the effect of channel length versus device resistance as well as data on the temperature dependence of device resistance for gold and platinum electrodes. Implications for contact engineering will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S22.00004: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:42PM - 4:18PM |
S22.00005: Infrared study of charge injection in organic field-effect transistors Invited Speaker: We present a systematic infrared (IR) spectroscopic study of charge injection in organic field-effect transistors (FET). These experiments have revealed new unexpected aspects of both polymers and molecular crystals. IR spectromicroscopy was employed to image the charges in poly(3-hexylthiophene) (P3HT) FETs. The charge density profile in the conducting channel uncovers a density-dependent mobility in P3HT due to disorder effects. Our IR studies of single crystal rubrene based FETs show that charge transport in these devices at room temperature is governed by light quasiparticles in molecular orbital bands. This result is at variance with the common beliefs of polaron formation in molecular solids. The above experiments have demonstrated the unique potential of IR spectroscopy for investigating physical phenomena at the nanoscale occurring at the semiconductor-insulator interface in FET devices. This work is in collaboration with G. M. Wang, D. Moses, A. J. Heeger (UCSB), V. Podzorov, M.E. Gershenson (Rutgers), Z. Hao, M. C. Martin (ALS), N. Sai, A. D. Meyertholen, M. M. Fogler, M. Di Ventra and D. N. Basov (UCSD). [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S22.00006: Studies of Au/SAMs/PEDOT-PSS/Au tunnel junctions Nan Sun, Marya Lieberman, Steven Ruggiero We report on tunneling through thin organic films. Junctions of the form: Au/SAMs/Polymer/Au were prepared on electronic-grade Si substrates with Self-Assembled Monolayers (SAMs) including octanedithiol (HS-C$_{8}$H$_{16}$-SH) and mercaptohexadecanoic (HS-C$_{15}$H$_{30}$-COOH). A transitional conducting polymer film PEDOT-PSS was spun on to the SAMs layer, and junctions were completed with a gold film. X-ray photoelectron spectroscopy (XPS) was employed to monitor the quality of the SAMs films. The electron tunneling properties including dI/dV and d$^{2}$I/dV$^{2}$ versus bias for the SAMs are discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S22.00007: Electronic functionalization of organic semiconductors with self-assembled monolayers Vitaly Podzorov Self-assembled monolayers (SAM) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of surface conductivity of organic materials, which can be very large for SAMs with a strong electron withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10\^{}-5 S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors (OFETs). The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity exhibits sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications [1]. [1]. M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson and V. Podzorov, ``Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers'', Nature Materials, Nov. 18, (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S22.00008: Impedance Spectroscopy of Organic Thin Film Transistors and Contacts Daniel Lenski, Adrian Southard, Michael S. Fuhrer We have developed a novel method of characterizing organic thin films using a 2- or 3-contact transmission line configuration, in which an AC voltage is applied to the thin film and the phase and magnitude of the current are measured. This simple method can shed light not only on the bulk properties of the semiconductor film, but also on the contacts, by varying the effective length scale probed in the sample. We present the results of transmission line measurements of pentacene thin films, with several types of contacts including thin films of carbon nanotubes. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S22.00009: Cross-sectional Imaging of Organic Optoelectronic Devices and Molecularly Assembled Nanostructures D.W. Steuerman, A. Garcia, R. Yang, D.S. Seferos, H. Wu, D. Korystov, A. Mikhailovsky, J.P. Lofvander, G.C. Bazan, D.D. Awschalom As the science of organic optoelectronic devices continuously matures, performance often improves at the expense of molecular and architectural complexity. One widespread approach toward optimization is the use of several polymers and hybrid materials, either as blends or in multiple layers. Tools to provide a thorough understanding of interfacial structure are lacking. Therefore, we employed a dual beam scanning electron microscope/focused-ion beam (SEM/FIB) to create device cross-sections that we subsequently investigated by transmission electron microscopy (TEM). High resolution images of an assortment of devices will be presented, including: interfaces of polymer-electrode, polymer-polymer, polymer-nanoparticles, and oligomer-nanoparticles in fully fabricated devices and optical cavities. We directly observed a variety of polymer-polymer interfaces depending upon solvent casting conditions, annealing treatments, and molecular functionality. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S22.00010: Trapping carriers in organic field-effect transistors by metal nanoparticles Yu Chen, Masaya Nishioka, Allen Goldman A thin layer of metallic Au nanoparticles was coated on substrates that were used for organic field-effect transistors, in order to study how the motion of carriers in the organic was affected by metal/organic coupling. A huge reduction of mobility was observed, due to the increase of the characteristic activation energy. We speculate that this follows from the polaronic motion of carriers resulted from the organic/metal coupling, similar to the organic/dielectric coupling. Further experiments demonstrate that the performance of those devices can be adjusted by changing the configurations of nanoparticles. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S22.00011: Charge-retraction time-of-flight technique for mobility measurements in organic materials Jason Wallace, Ralph Young, Ching Tang, Shaw Chen This presentation will explore a recently reported, all-electrical technique, charge-retraction time-of-flight (CR-TOF), for the measurement of charge carrier mobility through an organic layer. Carriers are injected and accumulated at a blocking interface, then retracted. The retraction current transient is nearly indistinguishable from a traditional time-of-flight photocurrent. The CR-TOF technique is validated by measurement of the hole mobility of two well-known compounds using a common hole-blocking layer. An advantage of the technique is the applicability to sample layers less than 300 nm in thickness. This method also offers new opportunities such as catching charges in the middle of the sample layer and an alternate determination of the transition voltage of organic-organic interfaces. [Preview Abstract] |
Session S23: Focus Session: Nanostructured Oxides and Thin Films
Sponsoring Units: DMP GMAGChair: Grace Lu, University of Southern California
Room: Morial Convention Center 215
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S23.00001: Synthesis, structure and properties of nanostructured manganites Invited Speaker: Manganites form a fascinating class of multifunctional perovskite oxides with competing interactions that couple the charge, spin and the lattice. The presence of competing interactions (that often have a comparable energy scale) leads to presence of variety of phases in the manganites that can be tuned by substitution (hole concentration), pressure and magnetic field. Interesting classes of phenomena arise when the size is taken to nanoscales where the ground state can be tuned by the size. The size also tunes the nature of the phase transition and the nature of the electronic transport. Interestingly, the primary change occurs in the lattice structures on size reduction that weakens the orthorhombic distortion. The physical properties of the nanostructured manganites are very distinct and different from that seen in related nanostructured transition metal ferromagnetic oxides like the cobaltates. In this talk we will first describe the methods of synthesis of nanostructured manganites in the form of nanocrystals, nanowires and nanostructured films using soft chemical routes. Arrays of the nanostructured manganites can also be made using such techniques as dip-pen lithography using an Atomic Force Microscope. This will be followed by presentation of results on structures using synchrotron X-rays and neutrons that establish the nature of structural changes on size reduction. The structural changes (as established through the above studies) on size reduction tend to enhance the ferromagnetic interaction in the system. Next we will report a number of physical phenomena that arise as a consequence of the size reduction. This will include destabilization of the charge ordered state, change in the nature of the ferromagnetic transition and non-linear conduction that arises in the nanostructured films due to presence of a large number of grain boundaries. At very low temperatures the transport in the manganites become dominated by such effects as intergrain tunneling and Coulomb blockade. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S23.00002: Epitaxial growth of complex oxide films by a chemical solution method Q.X. Jia, M. Jain, H. Luo, E. Bauer, H. Wang, A.K. Burrell, T.M. McCleskey In the last several years, complex oxides have become the basis for many revolutionary electronic devices because they exhibit a wide range of electronic properties that conventional metallic elements and covalent semiconductors do not possess. Complex oxide films can be grown by physical vapor deposition, chemical vapor deposition, and chemical solution deposition techniques. One of the challenges in solution-based processes of oxide films has been to produce high quality films and at the same time to control the stoichiometry. Here we describe a new chemical solution method called polymer-assisted deposition (PAD) to grow epitaxial oxide films (such as Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3})$. We use a new strategy to control the distribution of metals in solution at a molecular level and a mixture of metal precursor and soluble polymer to form a solution with desired viscosity. By actively binding the metal, the polymer serves to encapsulate the metal to prevent chemical reaction while maintaining a uniform distribution of the metal in solution. This ensures a homogeneous metal distribution and prevents unwanted reactivity that can lead to the formation of undesired phases. The successful growth of epitaxial complex oxide films by PAD suggests that PAD is a feasible alternative approach to the growth of high quality films with desired properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S23.00003: Tailoring The Magnetic Properties of TiO$_{2}$ Nanobelts Shen V. Chong, Kazuhiro Yamaki, Kazuo Kadowaki Over the past decades, titanium dioxide has been discovered to exist in various novel nano-structural forms with high aspect-ratio and good crystallinity. Moreover, the addition of dopants and the self-assembling of foreign molecules on the surface can enrich the physical and chemical properties of this semiconductor, enhancing its versatility and further promoting this metal oxide to be an important nano-based functional material. Herein we report on the doping of TiO$_{2}$ nanobelts with small amount of cobalt, producing a diluted magnetic semiconductor which display a Curie temperature well above room temperature. Co-doped TiO$_{2}$ nanobelts were prepared hydrothermally by powdering a piece of Co$_{x}$Ti$_{1-x}$O$_{2}$ single crystal. The magnetic properties of these nanobelts could be tailored via different heat treatment procedure. Annealing the as-synthesized cobalt titanate in air at 1000 K produced nanobelts which are paramagnetic, while evidence of room temperature ferromagnetism could be observed after the same sample was annealed under vacuum at length. An even more pronounce ferromagnetic behavior was observed when the nanobelts were vacuum annealed direct from the titanate phase. These results again show the importance of oxygen vacancies in unison with the presence of cobalt in inducing room temperature ferromagnetism in this semiconductor. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S23.00004: Surface anisotropy and orbital moment in Fe$_{3}$O$_{4}$ nanoparticles X. Batlle, N. Perez, P. Guardia, O. Iglesias, A. Labarta, A.G. Roca, M.P. Morales, C.J. Serna, L.M. Garcia, F. Bartolome, J. Bartolome, J.C. Cezar Fe$_{3}$O$_{4}$ nanoparticles (NP) in the 5-20 nm range were synthesised in the presence of a variety of surfactants (oleic acid, PVA, {\ldots}). Saturation magnetization M$_{s}$ almost reaches the expected bulk value for those surfactants covalently bonded to the NP, being much larger than in those NP with just a protective coating. Thermo-remanence and ac susceptibility demonstrate that the surface anisotropy constant in covalently bonded NP is similar to the bulk value. XMCD confirms the dependence of the magnetic moment on the surface bond and suggests that the orbital contribution is smaller in covalently bonded NP. The occurrence of bulk M$_{s}$ in Fe$_{3}$O$_{4}$ NP may thus be related to the crystal and magnetic state at the surface. This is of relevance in biomedical applications to reduce the strength of the magnetic field required to obtain a high magnetic response, while the issue of the orbital contribution in Fe$_{3}$O$_{4}$ is under hot debate. Work funded by Spanish NAN2004-08805-CO4-02, NAN2004-08805-CO4-01, MAT2006-03999, MAT2005-02454 and CONSOLIDER CSD2006-12, and Catalan 2005SGR0969. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S23.00005: Novel Oxide Glass-Based Nanocomposites: Development and Structural Characterization Kristina Lipinska-Kalita, Carlo Segre, Patricia Kalita, Oliver Hemmers, Yoshimichi Ohki, J. Cecil, M. Chavarha Glasses can gain new functionalities when specific crystalline phases of nanometer dimensions are nucleated in them. We have developed a series of optically transparent glass-based composites, containing nanometer-sized crystals dispersed within the isotropic host matrix. The composites were characterized using conventional and synchrotron x-ray diffraction as well as synchrotron x-ray absorption spectroscopy. EXAFS studies of Er-doped, gallium-silica glasses and composites will be presented. As the glasses are treated at elevated temperatures, long-range beta-Ga$_{2}$O$_{3}$ is observed to evolve from the initially amorphous local structure. However, the same samples show no significant change in the Er local structure, possibly indicating clustering or preferential of Er in proximity to Ga atoms. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S23.00006: A Memristive Switching Model of Two-Terminal Programmable Nanodevices. Dmitri Strukov, Julien Borghetti, Duncan Stewart, Gregory Snider, R. Stanley Williams The existence of a fourth passive circuit element was proposed by Chua in 1971 from fundamental symmetry arguments. Although he showed that such a device, which he called a `memristor' (acronym for memory and resistor), had many interesting and useful circuit properties, until now no one has presented a physical model or example of such an element. We show here using a simple analytical example that memristance arises naturally in systems for which electronic and atomic transport are coupled under an external bias, and we explore the range of validity of the basic nonlinear circuit model of a more general class of dynamical devices called memristive systems. These results serve as the theoretical foundation for understanding a wide range of hysteretic current-voltage behavior observed in next-generation non-volatile resistive RAM devices, including nanoscale titanium oxide crosspoint switches built in our laboratory. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S23.00007: Nano-crystalline structure and catalytic activity of TiO$_{2}$ supported manganese oxide catalysts Sergey Mamedov, Padmanabha Reddy Ettireddy, Neeraja Ettireddy, Punit Boolchand, Panagiotis G. Smirniotis A series of TiO$_{2}$ supported manganese oxide catalysts were prepared by wet-impregnation method. Raman spectroscopy was used to characterize the structure and chemical environment of these catalysts as well as manganese oxides. Spectra of different Mn-oxides are presented. It was found that there is strong correlation of the position and the width of E$_{2g}$ mode of anatase at 146 cm$^{-1}$ and Mn-oxide(s) concentration. Evolution of Raman spectra described in the frame of the optical confinement model. In this model, the position and width of the Raman band depend on structural disorder and size of nano-crystals. Size of nano-crystal was estimated. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S23.00008: Very Low Frequency ($<$1 mHz) Magnetic Noise in La$_{0.67}$ Ca $_{0.33}$ MnO$_{3}$ Films Sudeshna Samanta, Arup Kumar Raychaudhuri We report an occurrence of very low-frequency ($<$1 mHz) resistance fluctuations (noise) in a rare-earth perovskite manganite film. This fluctuation is distinct from 1/f noise and is larger than that. The fluctuation arises due to coupling to magnetization fluctuation. It reaches a peak close to the ferromagnetic Curie temperature T$_{C}$. The magnetic nature of the transition has been established by sensitivity of the noise to a very low applied magnetic field $<$0.1 Tesla. The magnetization fluctuation has been calculated from the resistance fluctuation using the directly measured magnetoresistance. The magnetization fluctuations show peak at a temperature close to but lower than T$_{C}$ and shows a nontrivial dependence on the applied magnetic field. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S23.00009: Film Growth and Surface Energy of (100) CrO$_{2}$ Hunter Sims, Krishna Chetry, Mairbek Chshiev, Arunava Gupta, William Butler Rutile structure CrO$_{2}$ can be grown epitaxially on a rutile TiO$_{2}$ substrate. Surprisingly, growth in the (100) direction is layer by layer even though surface energies evaluated using the VASP code [1] indicate that the surface energy of TiO$_{2}$ is less than the sum of the surface energy of CrO$_{2}$ and the CrO$_{2}$-TiO$_{2}$ interface energy[2]. It is known however, that CrO$_{2}$ (100) surfaces spontaneously decompose to form an epitaxial Cr$_{2}$O$_{3}$ phase. We have investigated the conjecture that the layer by layer growth of CrO$_{2}$ results from the formation of a surface layer substoichiometric in oxygen. If the conjectured substoichiometric layer forms, it must be converted to CrO$_{2}$ as it is covered. We calculate the energy of a (100) fully stoichiometric surface, a (100) surface with oxygen removed, and a (100) surface with an interior oxygen deficiency using the standard relaxation methods in the VASP code as well as its simulated annealing capabilities. By comparing the energies (taking into account the removed oxygen in the latter two cases), we can predict the behavior of the CrO$_{2}$ films during deposition. \newline [1] G. Kresse and J. Hafner, Phys. Rev. B {bf 47}, 558 (1993); G. Kresse and J. Hafner, Phys. Rev. B {bf 49}, 14 251 (1994). \newline [2] K. Chetry and A. Gupta, unpublished [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S23.00010: Study of Magneto-Transmission Spectra of La$_{0.7}$Pb$_{0.3}$MnO$_{3-\delta }$ Epitaxial Thin Film Sidney Malak, Ryan Clayton-Cox, Jiri Stehlik, Jian-Qing Wang Magneto-transmission (MT) of epitaxially grown La$_{0.7}$Pb$_{0.3}$MnO$_{3}{\rm g}$ was measured. ThermoElectron Nexus 670 FT-IR spectrometer equipped with an electromagnet was used to obtain IR spectra in the range of 350 to 15000 cm$^{-1}$ in various applied magnetic fields up to 1.0 Tesla. For optimal magneto-spectroscopic measurements in transmission, the studied film had a thickness of 190 nm, with a maximum value up to 80{\%} at 320 K in the colossal magnetoresistance (CMR) effect in 5.5 Tesla. It was observed that the MT scale proportionally with the applied magnetic field and was largest at longer wavelengths below 4000 cm$^{-1}$. In this far infrared range, the maximum observed MT value was 4.0 {\%} at 1.0 Tesla. Beyond FIR range the MT curves monotonically decrease with frequency, until the effect vanishes at 12,000 cm$^{-1}$. Such crossover of magneto-spectroscopic responses from IR to optical frequencies is the first evidence of gradual disappearance of the magento-dynamics at higher frequencies. Compared with the CMR effect, the measured MT property resembled that of the CMR closely in the field range and frequency ranges studied. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S23.00011: Ferromagnetism in Co doped anatase TiO$_{2}$ thin films mediated by Co-Ti$^{+3}$-V$_{O}$ complexes Maria Varela, Kelli Griffin-Roberts, Sergey Rashkeev, Sokrates Pantelides, Stephen Pennycook, Kannan Krishnan The correction of spherical aberration in the STEM has enabled sub-Angstrom imaging and spectroscopy, and, in favorable cases, direct imaging of light atoms and interstitials. We identify the origin of ferromagnetism in Co$_{0,03}$:TiO$_{2}$ anatase thin films by combining STEM, EELS and DFT calculations. The films are insulating and ferromagnetic at room temperature. Ferromagnetism is enhanced by a post growth vacuum annealing suggesting a defect-mediated mechanism in these films. DFT finds interstitial Co to be energetically preferred over substitutional Co. STEM imaging reveals the interstitials in the predicted sites, and EELS finds reduced Ti in adjacent columns, also predicted by DFT. The combination of STEM-EELS-DFT therefore identifies the defect responsible for the magnetism: an O vacancy binds to the interstitial Co to form a Co-Ti$^{+ 3}$-V$_{O}$ complex, with a magnetic moment in good agreement with the observed value. Research sponsored by Div. of Materials Sciences and Engineering US DOE, and NSF/ECS 0224138. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S23.00012: Nonmetal-metal transition in anatase Nb-doped TiO$_{2}$ Taro Hitosugi, Hideyuki Kamisaka, Koichi Yamashita, Hiroyuki Nogawa, Tetsukazu Tsuruhama, Shoichiro Nakao, Yutaka Furubayashi, Naoomi Yamada, Yasushi Hirose, Toshihiro Shimada, Tetsuya Hasegawa Anatase TiO$_{2}$ show nonmetal-metal transition on Nb doping. Epitaxial Ti$_{0.94}$Nb$_{0.06}$O$_{2}$ (TNO) thin film exhibits low electrical resistivity, \textit{$\rho $} $\sim $ 1.7 x 10$^{4} \quad \Omega $cm at 300 K, comparable to highly-conducting transition metal oxide, ReO$_{3}$ and Na$_{x}$WO$_{3}$. This TNO is an n-type degenerate semiconductor with carrier density exceeding 10$^{21}$ cm$^{-3}$. We have studied the electronic structure of this TNO system using resonant photoemission spectroscopy and compared with first-principles calculations. The first-principles calculations reveal that there is no impurity state arising from Nb doping, and partial density of states of Nb contribute to both valence band and conduction band. These results imply that Nb is highly-hybridized with Ti and O orbitals, resulting in high activation efficiency of Nb which leads to high carrier density in the TNO system. Resonant photoemission spectra clearly show wide band gap without impurity state with Fermi edge located in the conduction band. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S23.00013: Carrier mediated Ferromagnetism in Cr: In$_{2}$O$_{3}$ Raghava Panguluri, P. Kharel, C. Sudakar, R. Naik, B. Nadgorny, G. Lawes, R. Suryanarayanan, V.M. Naik While a number of Dilute Magnetic Semiconducting Oxides (DMSO), when doped with transition metals, exhibit ferromagnetism well above room temperature, most of them are insulating and hence the relationship between the charge carriers and ferromagnetism remains unclear. Here, we investigate a high Curie temperature DMSO, Cr:In$_{2}$O$_{3}$, which is made conductive down to low temperatures. Films of various thicknesses ranging from 200 nm to 1100 nm were grown on sapphire substrate by rf sputtering. Upon vacuum annealing, the samples became both magnetic (with the saturation magnetization $\sim $0.07$\mu $B/Cr) and highly conductive, in contrast to as-prepared samples which were both non-magnetic and insulating. Point Contact Andreev Reflection (PCAR) spectroscopy with the superconducting Sn tip demonstrated significant (50 {\%} $\pm $ 5{\%}) transport spin polarization, strongly suggesting that ferromagnetism in Cr:In$_{2}$O$_{3}$ is carrier-mediated. We will discuss the implications of these results on our understanding of the nature of ferromagnetic interactions in DMSOs, as well Cr: In$_{2}$O$_{3}$ potential applications as a spin injector. [Preview Abstract] |
Session S24: Fullerenes (not nanotubes)
Sponsoring Units: DCMPChair: Dan Finkenstadt, Naval Research Laboratory
Room: Morial Convention Center 216
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S24.00001: Magnetic molecules made of nitrogen or boron-doped fullerenes Chih-Kai Yang By using density functional theory we investigate the electronic structure of a fullerene C60 molecule doped with nitrogen atoms. We find that as long as the number of the impurity atoms is odd the doped fullerene turns magnetic, with its magnetic moment determined by the way the impurities are bonded with the carbon cage. For even number of impurities the pairing of the electrons exclude the appearance of magnetism. Similar results also apply to fullerenes doped with boron. This simple way of creating single molecular magnets should be useful for the application in magnetic detection, quantum information, and spintronics. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S24.00002: Electron-phonon coupling in C$_{60}$ using exact-exchange functional Jonathan Laflamme Janssen, Michel C\^ot\'e The superconductivity in C$_{60}$ doped crystals is now well understood as a phonon mediated interaction. The strength of the electron-phonon coupling can be deduced by Raman and PES measurements which can then be used to assess the density-functional theory results. Although experimental and computed electron-phonon coupling agree on the total magnitude of the coupling, they do not on the contributions of the individual vibrational modes. Density-functional theory calculations indicate that high frequency modes are responsible for most of the coupling whereas experiments suggest that low frequency modes are the dominating contribution. Up to now, only calculations using the local density approximation (LDA) were performed. In this study, we investigate the effect of exact-exchange functionals, such as B3LYP, on the computed electron-phonon coupling of the different vibrational modes. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S24.00003: Electron Transfer and Localization in Endohederal Metallofullerenes Shenyuan Yang, Mina Yoon, Christian Hicke, Zhenyu Zhang, Enge Wang Endohedral metallofullerenes constitute an appealing class of nanoscale building blocks for fabrication of a wide range of noval materials. One open question of fundamental importance is the precise nature of charge redistribution with the carbon cages (C$_{n})$ upon metal encapsulation. Using ab initio density functional theory, we systematically study the electronic structure of metallofullerenes, focusing on the spatial charge redistribution. For all large metallofullerenes ($n>$32), the valence electrons of the metal atoms are all transferred to the fullerene states. Surprisingly, the transferred charge is found to be highly localized inside the cage near the metal cations, rather than uniformly distributed on the surfaces of the carbon cage as traditionally belied. This counterintuitive charge localization picture is attributed to the strong metal-cage interactions within the systems. These findings may prove to be instrumental in the design of novel fullerene-based functional nanomaterials. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S24.00004: Light Alkaline-Earth-Metal Coated Carbon Fullerenes as Effective Hydrogen Storage Media Mina Yoon, Shenyuan Yang, Christian Hicke, Enge Wang, David Geohegan, Zhenyu Zhang We propose functionalizing carbon nanostructures with light alkaline-earth metals for use as hydrogen storage media. To support this idea, we investigate the feasibility of coating C$_{60}$ fullerenes with light alkaline-earth metals and analyze the hydrogen storage capacities of the resulting compounds. We find a new and unique binding mechanism responsible for the strong binding between Ca or Sr atoms and C$_{60}$. Our theory explains experiments showing that C$_{60}$ can be evenly covered by a monolayer of Ca or Sr atoms. The coating results in a charge redistribution leading to electric dipolar fields around the metal atoms through which the fullerene surface becomes an ideal hydrogen-attractor with a binding strength larger than that of alkali carbon complexes but small enough to prevent hydrogen dissociation as in the case of transition metal decorated fullerenes. With a hydrogen uptake of more than 8.4wt\% and a binding energy of $\approx$ 0.4eV/H2 on C$_{60} $C$_{32}$ Ca is superior to currently used coating elements. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S24.00005: An Investigation of Magnetic, Electronic and Structural Properties of Metallofullerenes S. Vincent Ong, Meichun Qian, Shiv Khanna Gadolinium based endohedral metallofullerenes Gd$_{3}$N@C$_{80}$ functionalized with OH radicals have been found to enhance the relaxivity by orders of magnitude over conventional agents and are being sought as new contrast agents in magnetic resonance imaging (MRI). Using state of the art density functional theory (DFT) in the regime of the local density approximation with the on-site Coulomb interaction (LSDA+U), we have carried out theoretical studies to determine the electronic and magnetic properties of gadolinium-based and lutetium-based nitride fullerenes, namely Lu$_{3-x}$Gd$_{x}$N@C$_{80}$ (x = 1-2). While Gd$_{3}$N@C$_{80}$ has previously shown promising features as a contrast agent, the idea of replacing gadolinium atoms by lutetium has been proposed to result in a mixed-metal species for multi-modal imaging. Our results indicate that Lu$_{2}$GdN@C$_{80}$ is the most stable of all possible configurations with a binding energy 16.57 eV, can be considered for use as both an MRI contrast agent, due to gadolinium's high magnetic moment, and as a potential radioactive therapeutic or diagnostic agent, by neutron activation of a lutetium radioisotope. These results along with details of electronic structure will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S24.00006: First-Principles Investigation of C$_{60}$-Pd Interface Lan Li, Hai-Ping Cheng Conductivity and hybridization of C$_{60}$-Pd nano-system have been investigated using density functional calculations. From analysis of geometry, energetics and electronic structures, the interaction of C$_{60}$ mono-layer and Pd clusters gives rise to electronic charge transfer at the interface and facilitates the dissociation and uptake of hydrogen, which lead to hydrogen storage. The first-principles studies are carried out by self-consistent plane-wave method. The interaction between ions and electrons is described by projector-augmented wave (PAW) approach. In our calculations, the C$_{60}$ monolayer is doped by the Pd$_{n}$ atoms on $h$-BN with $n$ = 1-4 and 15, but it also forms a metal-C$_{60}$ nano-array with the Pd clusters. Charge transfer occurs at the interface, from the Pd atoms towards the C$_{60}$ monolayer. This electronic property strongly depends on the nature and number of metal atoms. A large amount of charge transfer between the Pd atoms and the C$_{60}$ monolayer indicates a strong interaction under the ionic effect, in contrast with the interaction of the C$_{60}$ monolayer and a metallic surface. The $h$-BN surface merely gains 0.1 electrons via C$_{60}$, proving that $h$-BN is an insulating material. We also find that Pd is a good catalyst for dissociation and storage of hydrogen on the C$_{60}$ molecules. Hydrogen is sufficiently dissociated in the presence of the Pd atoms/clusters, which assists in bonding of the individual H atoms to C$_{60}$. Dehydrogenation of C$_{60}$H$_{x}$ is also discussed in energetics. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S24.00007: Investigation of Trapping Positions for Beryllium Atom in C$_{60}$ Fullerene and Electron Densities at$^{ 7}$Be Nucleus. Lee Chow, Archana Dubey, H.P. Saha, Gary S. Collins, R.H. Scheicher, N.B. Maharjan, S.R. Badu, R.H. Pink, M.B. Huang, T.P. Das We are investigating, using first-principles Hartree-Fock Roothaan procedure, the trapping sites for $^{7}$Be atom in C$_{60}$ Fullerene, following broadly the same procedure as in earlier work by our group for trapping of muonium [1]. A number of possible sites, including the center of the C$_{60}$ and various positions near the fullerene surface both outside and inside C$_{60}$ are being studied including the effect of relaxation in the positions of neighboring C atoms. Electron densities at the $^{7}$Be nucleus will be presented for the sites where the binding energy is positive to attempt to understand the observed anomalous electron capture rate compared to other systems where trapped $^{7}$Be atom has been studied [2]. Results of our investigations for$^{ 7}$Be atom in graphite and graphene will also be presented for comparison with $^{7}$Be in C$_{60}$. Possible influence of many-body effects will be discussed. [1] O. Donzelli, T. Briere, T.P. Das, Sol St. Comm. \underline {90 }663(1994), Indian J. Phys. \underline {67 }(Special Issue) 35 (1993) [2] Ohtsuki et al, Phys. Rev. Lett. \underline {93},112501, (2004) [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S24.00008: Transfer of a Single Carbon Fullerene at Small Nano-Gap Yoshifumi Oshima, Yoshihiko Kurui, Kunio Takayanagi A single carbon fullerene shows jump-to-contact behavior in conductance evolution when bringing an electrode close to it. Theoretically, the jump-to-contact behavior has been explained by deformation of the fullerene, but, it has not been proved experimentally. In this study, we investigated the geometry of the fullerene at the moment of jump-to-contact using transmission electron microscope -- scanning tunneling microscope system. A single carbon fullerene was synthesized in-situ [1]. We sometimes observed that the single carbon fullerene was transferred back and forth between both electrodes at the bias voltage of 0.6 V when the gap distance became almost 1nm which was still tunneling regime in conductance. Such a transfer was never observed when the bias voltage was lower than 0.1 V. Since the conductance showed the order of 10$^{-1}$ G0 in pulse at the moment of transfer, the fullerene was suggested to be expanded along the gap to have a contact with the opposite electrode. [1] M. Yoshida et al., Jpn. J. Appl. Phys. 46, L67 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S24.00009: Boron Fullerenes: An Electronic Structure Study Arta Sadrzadeh, Olga Pupysheva, Ihsan Boustani, Boris Yakobson Using \textit{ab initio }calculations, we study electronic structure and frequency modes of B$_{80}$, a member of boron fullerene family made from boron isomorphs of carbon fullerenes with additional atoms in the centers of hexagons. We also investigate geometrical and electronic structural properties of double-rings with various diameters, which are important as building blocks of boron nanotubes, and as the most stable clusters among the studied isomers with no more than 36 atoms. Double-rings also appear as building blocks of B$_{80}$. Furthermore, we investigate the possibility of further stabilizing some of fullerenes by depleting them. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S24.00010: Magic Number of a Spherical Ca Cluster on C$_{60}$ Sungjong Woo, Young-Kyun Kwon Since the discovery of fullerenes, there have been a lot of interest in investigating the metal-fullerene clusters. Mass spectrum on the metal(M) covering on a C$_{60}$ complex showed a peak at M$_{32}$C$_{60}$. This magic number was theoretically explained using the geometry based on the C-Ca binding. However, such theories could not clearly reveal why the peak at M$_{32}$C$_{60}$, especially for calcium clusters, is so prominent compared to smaller number of metal atoms. Using {\it ab initio} MD simulations, we have found that for Ca covering with less than 32 atoms, Ca atoms tend to be retracted to a cluster rather than to be bound on each face of C$_{60}$ even though the Ca atoms are deposited symmetrically. Such a cluster does not have specific number of atoms and it is bound to C$_{60}$ through van der Waals interaction. However, once Ca forms a spherical shell with 32 atoms, the structure is quite rigid so that it will not be retracted to a cluster. We have also found that the interaction between an individual Ca atom and each C$_{60}$ face gets loosen so that C$_{60}$ can rotate within Ca$_{32}$ sphere. The phonon spectrum has been obtained by spectral analysis and electronic orbitals of Ca$_{32}$C$_{60}$ will also be presented. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S24.00011: First principles study of cubane and alkali doped C60 solids Young-Moo Byun, Vincent Crespi Alkali doped fullerene (C60) solids have been studied widely due to their interesting physical properties. Lately, an experimental group succeeded to dope cubane (C8H8) into the octahedral voids of faced-centered-cubic (FCC) C60 solids, demonstrating that not only atoms (and polyatomic cations), but also small neutral molecules can intercalated into C60 solids. We study the electronic properties of cubane-doped C60 solids using first-principles techniques and show that C60 solids doped with both cubane and alkali metals, in which alkali metals such as K and Rb occupy the tetrahedral voids are energetically favorable. Cubane molecules substantially dilate the C60 lattice, resulting in a very large density of states in a single-particle treatment and pronounced tendency towards electronic instability. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S24.00012: First principles electronic structure calculation of interstitial P doped C$_{60}$ solid. Shizhong Yang, Guang-Lin Zhao, Diola Bagayoko C$_{60}$ solid has a very low thermo-conductivity that can be utilized to improve the \textit{figure-of-merit} of thermo-electric devices. The selection of suitable doping elements and doping concentrations in C$_{60}$ bulk semiconductors, for best performance in thermoelectric applications, is of great interest. In this work, we calculated the electronic structure of solid C$_{60}$, interstitially doped with P, at concentrations varying from 1:240 to 1:60. We employed a density functional potential and the plane wave method. Both local density approximation (LDA) and generalized gradient approximation (GGA) potentials were considered. The stability, the electron densities of states, dopant location, carrier type, volume change, and charge transfers of P doped C$_{60}$ were calculated and compared to those of B, N, and Co doped C$_{60}$ solids. In the 1:60 doping case, we found that P doped C$_{60}$ solid is an n-type semiconductor with the dopant energy levels in the band gap, close to the top of conduction band. This work was supported in part by the Department of the Navy, Office of Naval Research (ONR, Grant No. N00014-4-1-0587) and by the National Science Foundation (Award No. HRD0503362 ). [Preview Abstract] |
Session S25: Gels and Elastomers
Sponsoring Units: DPOLYChair: Hiroshi Watanabe, University of Kyoto
Room: Morial Convention Center 217
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S25.00001: Large strain deformation of hydrophobically modified polyelectrolyte hydrogels Guillaume Miquelard-Garnier, Costantino Creton, Dominique Hourdet Hydrogels made from charged polyelectrolytes have been widely studied for their ability to absorb large amounts of water. However this occurs usually at the expense of mechanical properties. Because of recent reports describing very tough charged hydrogels, we have investigated the large strain and fracture resistance of novel chemically and physically crosslinked hydrogels. The backbone was polyacrylic acid modified with hydrophobic side groups and subsequently chemically crosslinked with thiol-ene chemistry. We performed compression/decompression experiments up to large strains and found that at polymer concentrations of 5-8 wt\% these gels depart significantly from Gaussian behaviour at strains above 150\% showing a pronounced strain hardening. We argue that this hardening leading also to a significant hysteresis in the unloading cycle, is due to the formation of clusters of same charge segments of the polyelectrolyte chains. An increase in ionic strength or the substitution of water with an organic solvent reduces or eliminates the effect implying that it is the charges that cause the hardening and the hysteresis. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S25.00002: Creasing instability of surface-attached hydrogels Ryan C. Hayward, Veronica Trujillo, Jungwook Kim, Anesia Burns Surface-attached hydrogels provide a convenient means to tune interfacial material properties such as biocompatibility and tribology. When the gel undergoes hydration, however, the substrate provides a constraint against lateral expansion, thereby generating an in-plane compressive stress within the gel. For sufficiently large degrees of compression a creasing instability takes place, in which the gel surface locally buckles and forms sharp folds. While this instability has been known in practice for well over a century, it remains poorly understood. Using model polyacrylamide hydrogel systems, we have studied the onset of creasing as a function of material properties and gel thickness, and addressed basic questions regarding crease morphologies and growth mechanisms. Using the understanding gained from these studies, we are developing this instability as a route to create active surfaces, where both surface topography and chemical patterns can be controllably modulated. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S25.00003: Swelling-Induced Deformation of Nanopatterned Polymer Lines Vijay Tirumala, Christopher Stafford, Rui Huang, Leonidas Ocola The deformation behavior of isolated poly ($N$-isopropylacrylamide) nanolines due to swelling is studied to quantitatively establish the parameters governing their mechanical stability. The features are patterned using electron-beam lithography and are supported on a rigid substrate. In the range of 50-250 nm linewidth, we show that the swollen lines undergo Euler buckling with stable higher order modes due to the constraint for polymer chain expansion at the substrate interface. The critical wavelength for buckling surprisingly increases with decreasing line length. The linear swelling strain thus becomes a function of the initial length and decreases dramatically as the line length approaches twice the buckling wavelength for infinitely long lines. A critical length, \textit{larger} than the buckling wavelength for infinitely long features, exists below which the lines remain mechanically stable regardless of their crosslink density. For sufficiently long lines with height-to-width aspect-ratio in the range 0.5-1.7, the scaling relationship for buckling wavelength vs. linewidth suggests that swelling is anisotropic and is more dominant through height than width. The results established in this study are more generally applicable to nanopatterned polymer lines since buckling instabilities involve simultaneous bending and compression, which deform the material with equal force but in opposite directions. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S25.00004: Drop spreading and resorbtion on gel surfaces Mehdi Banaha, Adrian Daerr, Laurent Limat We have studied the dynamics of liquid drops on agar gels, using a visualisation method which captures the evolution of the free surface. A first remarquable observation is that drops of water deposited on the surface do not spread, although the gel consists of up to 99.7\% water and as low as 0.3\% agarose. Instead, the drop slowly de-wets and resorbs into the gel which swells locally. If the deposited drop contains surfactants, the dynamics is very different. A sharp circular swelling front develops and progressively invades the whole surface. We study the propagation of this front as a function of surfactant and agarose concentration, and compare its typical properties to similar fronts appearing during mass swarming events of bacterial colonies under the same conditions. The observations reveal the complex nature of gel surface physico-chemistry and its aging, and may be related to recent friction measurements at gel interfaces. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S25.00005: Anomalous Composition-Dependent Swelling Behavior of Photocrosslinked VP/AA Copolymeric Hydrogels J. Hannah Lee, David Bucknall We are investigating the swelling behavior and network structure of vinylpyrrolidone (VP)/acrylic acid (AA) copolymeric hydrogels synthesized using ultraviolet (UV) initiated polymerization. The goal of this work is to develop hydrogels with large expansion volumes but low expansion rates for use in tissue expander applications. In a number of reports, hydrogels composed of VP and AA units have shown that with increasing AA content higher equilibrium swelling ratios ($q_e )$ are produced. This behavior is due to higher osmotic pressure induced by the dissociation of carboxyl groups in the AA units into carboxylate anions and hydrogen ions during the swelling. In contrast, in our UV-cured system we found that the value of $q_e $ showed a distinct maximum at approximately 50 wt{\%} AA. This anomalous swelling behavior has been studied by measuring the modulus and investigating swelling kinetics as a function of AA content, in addition to the composition and thermal analysis of the dried VP/AA gels. These measurements provide detail of the effective crosslink density as well as weight between crosslinks for these hydrogels. From these results we propose a model which describes this anomalous behaviour based on the molecular chain structure of the hydrogels. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S25.00006: Organogels from Polypeptide-based Block Copolymers Daniel Savin, Daniel Bercovici, Sandeep Naik A series of AB diblock and ABA triblock copolymers consisting of poly(Lysine(Z)) (A = P(Lys(Z))) and poly(propylene oxide) (B = PPO) were synthesized and found to form stable, rigid organogels in THF (ca. 1 – 1.5 wt.\% solutions) at room temperature. In these systems, the protecting group on the P(Lys) side-chains remains intact. As such, the secondary structure of the polypeptide chains retains its helicity over a wide range of solution conditions. Gel formation in these systems results from the assembly of the solventphobic P(Lys(Z)) chains, which pack densely in an anti-parallel fashion, minimizing interfacial curvature. These gels all exhibited shear-thinning behavior, and as the temperature was heated to 77$^{\circ}$C exhibited a gel-sol transition. The gels formed over a time scale of about 10 minutes and had a modulus on the order of 55 Pa. The molecular weight dependence of the gel formation and rheological properties was studied in THF, dioxane and toluene. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S25.00007: Soft random solids and their spatial elastic heterogeneity Xiaoming Mao, Paul Goldbart, Xiangjun Xing, Annette Zippelius As a consequence of the disorder that is quenched in at synthesis, random solids are spatially heterogeneous, not only in their structure but also in their response to external conditions. For the case of soft random solids, the heterogeneity in the elasticity is particularly intriguing, owing to its entropic origin and the interplay with incompressibility. We have examined the issue of heterogeneity in the elastic properties of soft random solids by applying replica statistical mechanics to a semi-microscopic model of a random network medium [1]. We have characterized the elastic heterogeneity by random residual stress and Lame coefficient fields, and have determined the statistics of these quantities. We have found that correlations involving the residual stress field are long ranged, and are governed by a universal parameter that also determines the mean shear modulus. Non-affine elastic deformations in soft random solids can also be also studied within this framework. [1] X. Mao, P. M. Goldbart, X. Xing and A. Zippelius, Europhys. Lett. 80, 26004 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S25.00008: Effective removal of entanglement points by network dilution Joshua D. McGraw, Kari Dalnoki-Veress A polymer system in which the chains are much longer than the entanglement molecular weight, $M_{+} \gg M_{e}$ is well entangled. When a thin polymer film is uniaxially strained below the glass transition temperature crazes are formed. Measurements of the volume fraction of the deformed versus the undeformed regions can give information on the entanglement density of the system\footnote{ACM Yang, EJ Kramer, CC Kuo, SL Phoenix, \emph{Macromolecules} \textbf{19} 2020 (1986)}. We present results of such deformation experiments, probed using atomic force microscopy, in which well entangled networks have been diluted with chemically identical species of molecular weight, $M_{-}< \sim M_{e}$ which results in a decrease in the entanglement density. Varying the length of the diluent chains provides molecular information on how the entanglement network is swelled. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S25.00009: Advances in elastomer reinforcement: slow dynamics Paul Sotta, St\'ephane Dupres, Pierre-Antoine Albouy, Didier Long Elastomers reinforced with nanometric solid particles or aggregates exhibit remarkable properties which are still far to be completely understood: reinforcement, non linear effects, irreversibility. Important progress has been achieved recently in modeling these properties, based on glassy layers around filler particles. In some cases, reinforcement as a function of temperature and filler volume fraction was explained quantitatively. We shall present an ensemble of experimental results obtained in various filled elastomers, which give some new insight on the microscopic mechanisms of reinforcement. We shall focus on two aspects: local strain measured by combining various techniques (NMR, mechanics, onset of strain-induced crystallization), indicate the level of stress heterogeneity in the elastomer matrix in the presence of reinforcing fillers; slow relaxation phenomena which occur in these systems: stress relaxation, memory effects and irreversibility effects -such as plasticity- indicate the importance of glassy regions within the matrix. Refs: J Berriot et al., Europhys. Lett. 2003, 64, 50-56; D. Long and P. Sotta, Macromolecules 2006, 39, 6282. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S25.00010: Polydomain-Monodomain Transition of Randomly Disordered Nematic Elastomers with Different Crosslinking Histories Kenji Urayama, Etsuko Kohmon, Ryo Mashita, Toshikazu Takigawa When the mesogenic molecules (polymers) are crosslinked without a special care for global alignment, the resultant networks have a polydomain structure with local orientational order (in the order of micron) but without macroscopic order. It is well known that such polydomain liquid crystal elastomers (PLCE) exhibit a transition to the monodomain state with global orientation under a finite stretching stress. In present study, we examine the polydomain-monodomain (PM) transition of the two types of PLCE (I-PLCE and N-PLCE) that are originally formed in the high-temperature isotropic or low-temperature polydomain nematic states. They show no appreciable difference in the equilibrium properties such as transition temperature and swelling degree. In contrast, their transition behaviors are significantly different: The transition in I-PLCE occurs sharply at a critical stress whereas that in N-PLCE proceeds gradually over a wide range of stress. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S25.00011: New Insights Regarding the Polydomain-to-Monodomain Transition in Smectic Elastomers Ronald Hedden, Harshad Patil, Daniel Lentz Smectic elastomers are rubber-like networks characterized by lamellar mesophases. Crosslinking a smectic polymer in the absence of an aligning field produces a polydomain elastomer containing numerous randomly oriented microdomains. Under uniaxial tension, polydomain smectic elastomers undergo a transition to a globally oriented ``monodomain'' state, which may proceed by rotation and/or transient disordering of microdomains. New studies of smectic main-chain elastomers suggest that disordering of microdomains via unfolding of hairpin structures is the dominant mechanism for elongation at intermediate strains. A ``plateau stress'' is found in plots of the nominal stress vs. strain, which correlates with the average domain size. At very high strains, elastic chains approach the finite extensibility limit, and layer buckling becomes the predominant mechanism for elongation. The elongation mechanism differs significantly from that in mesomorphic poly(diethylsiloxane) elastomers, which also exhibit a ``plateau'' in the nominal stress vs. strain curve due to spontaneous random coil-to-helix transition. Because of chain-folding in main-chain smectics, the P-M transition may exhibit some similarities with the cold drawing of semicrystalline polymers. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S25.00012: Nematic elastomers: From a microscopic model to macroscopic elasticity theory Paul Goldbart, Xiangjun Xing, Stephan Pfahl, Swagatam Mukhopadhyay, Annette Zippelius A Landau theory is constructed for the gelation transition in cross-linked polymer systems possessing spontaneous nematic ordering, based on symmetry principles and the concept of an order parameter for the amorphous solid state. This theory is substantiated with help of a simple microscopic model of cross-linked dimers. Minimization of the Landau free energy in the presence of nematic order yields the neo-classical theory of the elasticity of nematic elastomers and, in the isotropic limit, the classical theory of isotropic elasticity. These phenomenological theories of elasticity are thereby derived from a microscopic model, and it is furthermore demonstrated that they are the universal mean-field descriptions of elasticity for all chemical gels and vulcanized media. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S25.00013: Elasticity of a Chiral elastomer Aparna Baskaran, Xiangjun Xing We study the nonlinear elasticity of an elastomer made by crosslinking a chiral polymer melt in its isotropic state. It is shown that such an elastomer is unstable to uniaxial extension/compression and tends to develop a twist along the axis of deformation. ~Both phonon correlation functions and polarization dependent sound velocity are calculated. ~Furthermore, when such an elastomer is cooled below the isotropic-nematic transition of the underlying polymer melt, it develops inhomogeneous spontaneous deformations to accommodate the emergent chiral ordering of the polymers. We perform variational analysis of these spontaneous deformation in different regimes of system parameters. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S25.00014: Determination of the refractive indices of liquid crystal elastomers Israel Lazo, Peter Palffy-Muhoray Liquid Crystal Elastomers (LCEs) are fascinating materials due to the coupling between orientational order and mechanical strain. We investigate this coupling by studying the optical properties of LCEs. We have measured the ordinary and extraordinary refractive indices of nematic LCEs as function of strain using two different techniques. In both cases, the strain is applied along the nematic director. The first technique is a Brewster's angle measurement which is based on reflection of the incident light and the second is a conoscopic Mach-Zehnder interferometer based on transmission. We present our experimental results and methods of analysis. We compare our observations with theoretical predictions. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S25.00015: Magnetoactive Liquid Crystal Elastomers Moritz Winkler, Andreas Kaiser, Simon Krause, Heino Finkelmann, Annette Schmidt Liquid crystal elastomers (LCEs) offer an interesting spectrum of properties, including temperature induced, fully reversible shape changes connected with considerable development of pulling force, and synthetic diversity. In order to take advantage of LCEs for an extended number of viable devices, it is desirable to trigger such shape changes with electromagnetic fields rather than temperature changes. Magnetoactive LCEs are accessible by the incorporation of superparamagnetic Fe$_{3}$O$_{4}$ nanoparticles into oriented nematic side-chain LCEs and offer a contactless activation pathway to activate the nematic-to-isotrope transition by local magnetic heating in external fields due to relaxational processes. In magnetomechanical measurements at 300 kHz and 43 kA$\cdot $m$^{-1}$, a sample contraction of up to 30 {\%} is observed under field influence, that is fully released when the field is switched off. The load evolved reaches 60 kPa and more. The materials' ability to respond to a contactless electromagnetic stimulus with a well-defined contraction can be of use for various actuator applications. [Preview Abstract] |
Wednesday, March 12, 2008 5:30PM - 5:42PM |
S25.00016: Calamitic liquid crystal elastomers swollen with bent-core liquid crystals M. Chambers, J.T. Gleeson, S. Sprunt, A. Jakli Liquid crystal elastomers are composed of a chemically bonded liquid crystal mesogen and an elastomeric polymer network. They exhibit both the properties of rubber elasticity, liquid crystallinity and their interplay, giving rise to unique systems rich in physics. Additionally, bent-core (``banana'') liquid crystals are also of interest exhibiting rich phase behaviour and non-classical properties such as flexoelectricity. Here we examine the swelling of existing calamitic liquid crystal elastomers with various bent-core mesogens and the intriguing properties of the resulting system. The consequential swollen liquid crystalline elastomer systems, can imbibe many times there weight and volume in bent core mesogens until saturated. The homogenous system displays new properties (transition temperatures and phases) depending strongly on the interaction of the elastomer mesogen and bent-core mesogen. Strangely, for some bent-core mesogens this leads to higher temperature phase behaviour than both compounds originally posses. The authors would like to acknowledge support from ONR (N00014-07-1-0440) and NSF (DMR-0606160). [Preview Abstract] |
Session S26: Focus Session: Advances in Atmospheric Aerosol Science I
Sponsoring Units: DCPChair: Kevin Wilson, Lawrence Berkely National Laboratory
Room: Morial Convention Center 218
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S26.00001: Tropopsheric Aerosol Chemistry via Aerosol Mass Spectrometry Invited Speaker: A broad overview of size resolved aerosol chemistry in urban, rural and remote regions is evolving from deployment of aerosol mass spectrometers (AMS) throughout the northern hemisphere. Using thermal vaporization and electron impact ionization as universal detector of non-refractory inorganic and organic composition, the accumulation of AMS results represent a library of mass spectral signatures of aerosol chemistry. For organics in particular, mass spectral factor analysis provides a procedure for classifying (and simplifying) complex mixtures composed of the hundreds or thousands of individual compounds. Correlations with parallel gas and aerosol measurements (e.g. GC/MS, HNMR, FTIR) supply additional chemical information needed to interpret mass spectra. The challenge is to separate primary and secondary; anthropogenic, biogenic and biomass burning sources - and subsequent - transformations of aerosol chemistry and microphysics. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S26.00002: The Dynamic Interaction of Chemistry and Phase Partitioning in Atmospheric Organic Aerosols Invited Speaker: Atmospheric organic aerosols are a dynamic, chemically evolving mixture in equilibrium between the gas and condensed phases. This applies equally to primary emissions, which span a huge range in volatility, as well as secondary oxidation products generated by chemical reactions in both phases. The degree of volatility of primary emissions has been historically underappreciated, and the role of oxidation reactions has been considered in almost all cases only through their first one or two generations. We have recently developed a `volatility basis set' to address both primary volatility distributions and secondary volatility evolution (sometimes called primary organic aerosol and secondary organic aerosol). Here we shall discuss both facets of this framework as they apply to problems in organic aerosols on all scales, from emissions measurements to global organic aerosol loadings. We shall describe ongoing experimental work to constrain volatility distributions and volatility evolution through chemistry as well as extensions to the basis-set framework to more fully describe evolving aerosol properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S26.00003: Tandem mass spectrometry of single organic aerosol particles: A promising approach for in-situ analysis of mixtures. Pedro Campuzano Jost, Sarah Hanna, Emily Simpson, Damon Robb, Michael Blades, John Hepburn, Allan Bertram We have built a new single particle mass spectrometer for organic aerosol analysis that combines different previously tried approaches into one single instrument. We use soft, wavelength tunable desorption by using a dedicated pulsed CO$_{2}$ laser, (Prather, Baer {\&} coworkers) and soft ionization by tunable VUV radiation (Baer, Wilson {\&} coworkers)) to ensure a minimum of fragmentation. By ionizing the aerosol plume in the center of an ion trap both high sensitivity and the ability to elucidate structure by tandem mass spectrometry (Reilly {\&} coworkers) can be achieved. The analytical performance of the instrument as well as the detection geometry has first been validated by using simpler ionization techniques, 70 eV EI and REMPI, on a suite of aromatic and aliphatic compounds and simple mixtures. The novel tunable VUV laser system has been thoroughly characterized with a host of gaseous organic compounds that has proven both the ability to determine ionization energies with high accuracy and the possibility in many cases to minimize fragmentation by tuning the VUV source close to the ionization threshold. The VUV source has been integrated into the aerosol mass spectrometer and first VUV single aerosol spectra will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S26.00004: A novel flow reactor for studying the hydrolysis of N$_2$O$_5$ on aqueous H$_2$SO$_4$ solutions coated with organic surfactants Daniel Knopf, Lori Cosman, Allan Bertram, Payam Mousavi, Satya Mokamati A flow reactor has been developed which allows the study of heterogeneous kinetics on an aqueous surface coated by organic monolayers. Computational fluid dynamics simulations have been used to determine the flow characteristics for various experimental conditions. A mathematical framework has been developed to derive the true first order wall loss rate coefficient from the experimentally observed wall loss rate. Validation of the flow reactor was performed by measuring reactive uptake coefficients of well studied systems as a function of flow velocity and pressure. We determined the reactive uptake of N$_2$O$_5$ on aqueous H$_2$SO$_4$ solutions coated with a monolayer of 1-octadecanol, 1-hexadecanol, stearic acid, and phytanic acid. The reactive uptake decreased by approximately a factor of 17--61 in the presence of insoluble, straight-chain organic monolayers compared to uncoated solutions. However, the branched monolayer (phytanic acid) did not significantly affect the N$_2$O$_5$ uptake. The reactive uptake coefficients measured on aqueous H$_2$SO$_4$ subphases show a relationship to the surface area occupied by the surfactant molecules. However, data obtained with other subphases do not overlap with this trend. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S26.00005: Spectromicroscopic Studies of the Aging of Carbonaceous Aerosols from Mexico City Ryan Moffet, Yury Desyaterik, Alexi Tivanski, Rebecca Hopkins, Jerome Fast, James Barnard, Alexander Laskin, Mary Gilles Milagro, a multi-national atmospheric field campaign, was conducted in Mexico City during March of 2006. Aerosols were collected at three ground based sites, situated progressively farther from the city center. These aerosol samples are studied with computer controlled scanning electron microscopy with energy dispersive X-ray analysis (CCSEM/EDX) and scanning transmission x-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). This presentation focuses on evidence for carbonaceous aerosol transformation examining aerosol composition, C/O atomic ratios, mixing states, and changes in carbon bonding over time and distance from the source. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:54PM |
S26.00006: A connection between arctic haze and halogen chemistry? Invited Speaker: It has long been known that the Arctic spring time exhibits a phenomenon known as ``Arctic Haze'', characterized by low visibilities, caused by a relatively high concentration of fine aerosol. This aerosol is known to be composed largely of sulfate, with the precursor SO$_{2}$ transported from anthropogenic sources regions in North America and Eurasia. However, since the original studies of Arctic Haze, a complex array of halogen chemistry (involving Cl, Br and I chemistry), initiated by chemistry and photochemistry on frozen saline surfaces and in sea salt aerosol, has been discovered. That halogen chemistry makes the Arctic boundary layer extremely oxidizing, with large concentrations of both halogen radicals and HO$_{x}$. That environment can lead to new particle production via oxidation of organic gas phase precursors and/or enhanced oxidation of SO$_{2}$. Recent observations of a rapidly changing surface in the Arctic raise questions about how this halogen chemistry, which also which results in massive scale depletion of ozone and mercury, will change in the future. In this talk I will discuss what is known about halogen chemistry in polar regions, how that ties to aerosol chemistry, and how these are impacted by climate change and changes in the nature of the frozen surface. [Preview Abstract] |
Session S27: Focus Session: Magnetic Nanowires and Nanodots II
Sponsoring Units: GMAG DMPChair: Daniel Ralph, Cornell University
Room: Morial Convention Center 219
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S27.00001: Probing Magnetic Nanostructures on the Atomic Scale Invited Speaker: Magnetic nanostructures are increasing data storage capacities and are promising candidates for implementations of novel spin-based computation techniques. The relative simplicity and reduced dimensionality of nanoscale magnetic structures also make them attractive model systems for studying fundamental interactions between quantum spins. We used a scanning tunneling microscope to build individual magnetic nanostructures one atom at a time. By measuring their spin-excitation spectra with inelastic electron tunneling spectroscopy, we determined the orientation and strength of the anisotropies of individual Fe and Mn atoms on copper nitride. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network, making them similar to the building blocks of molecular magnets. In linear chains of up to 10 Mn atoms, we observed excitations of the coupled atomic spins that can change both the total spin and its orientation. The large magnetic anisotropy and strong spin-coupling manifested in these structures, which provide atom-by-atom accessibility via local probes, have the potential to produce atomic-scale magnetic structures that have a stable magnetization at low temperatures. \par * This work was done in collaboration with C.-Y. Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones, and A.J. Heinrich at the IBM Almaden Research Center, San Jose, CA 95120 USA. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S27.00002: Initial Metallization and Transition Metal Diffusion in ZnO Single Crystals, CVD-Grown Films, and Nanostructures Senia Katalinic, Sylvie Rangan, Rodney Gateau, Pan Wu, Yicheng Lu, Robert Bartynski Transition metal doped ZnO is a promising candidate room temperature dilute magnetic semiconductor for spintronic applications. In previous studies indicate Fe or Mn dopants exhibit significantly different diffusion properties in ZnO. To explore whether this is an inherent property of ZnO or if it is related to non-ideal aspects of the films or nanostructures, we have studied the initial stages of Mn, Fe, and Cu metallization of the single crystal ZnO(0001)[Zn-terminated] and (11-20) surfaces, as well as MOCVD-grown epitaxial a-plane films using scanning tunneling microscopy and spectroscopy (STM and STS). While deposited Cu forms well defined islands, all three surfaces exhibit substantial disruption upon Fe deposition, including significant change in terrace widths and a markedly smaller fraction of atomic height steps. Upon annealing, Cu islands become mobile and coarsen, but the underlying ZnO structure is not strongly affected. Annealing with Fe on the surface, significant coarsening and roughening of the substrate occurs even at the modest annealing temperature of 200C, and this effect is enhanced upon annealing to 400 C. Initial results suggest that uptake of metals into the epi-film is predominantly determined by the properties of the (11-20) surface that terminates the film. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S27.00003: Unusual size-dependent magnetic anisotropy in Co nanomagnets made from self-organized fast laser processing H. Krishna, C. Miller, Z. Nussinov, A.K. Gangopadhyay, R. Kalyanaraman Unusual size-dependent magnetic anisotropy has been observed in hemispherical polycrystalline Co nanomagnet on SiO$_{2}$ substrates produced by fast pulsed-laser-induced self-organization. The magnetic states of these particles have been characterized by using magnetic force microscopy (MFM) and hysteresis measurements. The results for single domain particles up to a diameter of 180 nm, the magnetization direction of smaller sized particles tends to be in-plane, while the larger particles tend ot have out-of-plane orientation. This finding is not consistent with shape anisotropy which predicts a size-independent in-plane alignment. Microstructural analysis revealed that particles had a granular microstructure with the grain size increasing with particle size. This unusual behavior has been attributed to large residual tensile strain in the hemispherical nanoparticles due to the large heating/cooling rates ($\sim$10$^{10}$ K/s) under ns laser self-organization, the large thermal expansion mismatch and the negative magnetostrictive constant for polycrystalline Co. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S27.00004: Magnetocaloric effect (MCE) in ferrite nanoparticles James Gass, Hariharan Srikanth Enhancement of the magnetocaloric effect (MCE) in nanostructured materials is important for refrigeration applications particularly in potential spot cooling of MEMS and NEMS devices. We have investigated MCE in various classes of polydisperse and monodisperse soft ferrite nanoparticles with different blocking characteristics. Our observations indicate that in some systems, surface properties such as spin disorder and anisotropy lead to considerable enhancement of MCE. This is promising for potentially increasing MCE in nanoparticle systems through systematic engineering of the surfaces via core-shell or other approaches. We report on the magneto caloric effect (MCE) in several ferrite nanoparticle systems and compare them. Characterization of structural and magnetic properties was done using XRD, TEM, DC and AC magnetization, and transverse susceptibility. The change in entropy was calculated using the thermodynamic Maxwell relation from the family of M-H curves taken at different temperatures. The specific role of surface anisotropy and surface structure in ferrite nanoparticles and correlation to the MCE will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S27.00005: A magnetic nanoparticle as an ultimate voltage-controlled nanomagnet Igor Ovchinnikov, Kang Wang We argue that when the conduction band edge of a magnetic nanoparticle is pinched between the majority and minority spins' Fermi energies, the capacitively injected carriers go almost entirely into one of the spin groups, thus varying the magnetization value at nearly 100\% efficiency. Among the IV-group transition metals, Cobalt and Nickel satisfy this requirement. Our proposition designs a source of strong local voltage-controlled nano-scale magnetic fields, which can be an ultimate alternative to the recent quest for DMS devices, in which the efficiency is limited to the magnetic doping concentration $\alt 5$\%. We theoretically support the idea for several transition metals' within the LSDA approach augmented by the kinetic energy functionals, built from the previous ab-initio density of states profiles. Our simulations show that, \emph{e.g.}, for a $N_a=50$ Nickel atom nanoparticle the original spin polarization $2S \sim 0.66 N_a$ gets completely suppressed at the gate voltage of $\approx 15$ Volts. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S27.00006: Anisotropy-Compensated Magnetic Nanostructures Ralph Skomski, Tom A. George, D.J. Sellmyer Nanostructuring can be used to tailor the magnetic anisotropy K$_ {1}$ as function of temperature, which is important in permanent magnetism and magnetic recording. Anisotropy is an atomic quantity, but the ferromagnetic exchange ensures an anisotropy averaging over a few nanometers, in contrast to the absence of nanoscale Curie-temperature averaging [1]. An intriguing and largely overlooked feature is the possibility of temperature- dependent anisotropy zeros, which yields a potential write-field reduction in magnetic recording. On an atomic scale, this effect is well-known but limited to a relatively narrow range of rare- earth transition-metal intermetallics. Nanostructuring greatly extends the range of materials. Explicitly considered structures are thin films, where the determination of the anisotropy zero (s) yields algebraic equations whose roots depend on the thicknesses, Curie temperatures, and zero-temperature anisotropies of the involved phases. On a somewhat larger length scale (> 5 to 10 nm), there is no longer a well-defined anisotropy, but the corresponding micromagnetic corrections are easily incorporated into the theory. - [1] R. Skomski, ``Simple Models of Magnetism,'' University Press, Oxford 2008. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S27.00007: Lattice Induced Configurational Anisotropy in Nanomagnets Wen Zhang, Noah Bray-Ali , Stephan Haas The study of magnetic nanoparticles is evolving into a rich and rapidly growing area, featuring many novel phenomena and potential applications. One of the most important properties of these systems is the magnetic anisotropy, which determines the blocking temperature. Besides the well-known crystalline and shape anisotropies, the competition of exchange and magnetostatic interactions in nanomagnets leads to the formation of a configurational anisotropy, resulting from small deviations of the magnetization from uniformity within the nanostructures. In this talk, I discuss a new type of anisotropy, i.e. the lattice induced configuration anisotropy, which we have studies using Monte Carlo simulations. In particular, a scaling approach has been shown to be effective in obtaining the magnetic properties of nanoparticles. The relationship between anisotropy and blocking temperature will also be discussed. Understanding the influence of anisotropy opens up a new path to designing nanostructured magnetic materials with novel functionalities. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S27.00008: Between ferro- and para- magnetism. Electron magnetic resonance and quantal effects in superparamagnetic nanoparticles Natalia Noginova, Maxim Noginov, Vadim A. Atsarkin Nanometer-scale magnetic objects are at the interface between quantum dynamics of several interacting spins and classical thermodynamics of multi-particle systems. We present results of electron magnetic resonance (EMR) studies in suspensions of two different systems with superparamagnetic iron oxide nanoparticles with average size of 5 nm and 9 nm correspondingly. It is shown that both types of particles demonstrate common EMR behavior, including the broad spectral component, temperature-dependent narrow component with g-factor of $\sim $ 2, and additional low-field signals observed at the fields B$_{0k}$ = B$_{0}$/k, where B$_{0}$ is the resonance field of the main resonance, and k = 2, 3, and 4. These lines correspond to the transitions at the double, triple, etc., resonance frequencies and can be described in terms of the non-secular spin operators arising from the single-particle magnetic anisotropy or/and inter-particle dipole-dipole interactions. These features are common for small quantum systems and not expected in classical case. The relative intensity of the narrow component and low-field signals rapidly decreases with cooling or increase of particle size, marking gradual transition to classical FMR behavior. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S27.00009: Finite size effect in shell nanoparticles Joshua Koch, Renat Sabirianov The magnetic properties of single layer shell particles studied as function of the particle's size using Monte Carlo method with free boundary conditions. We formed truncated octahedron shell-nanoparticles of 4-30 lattice spacings across mimicking particles from 2-12 nm in size. The classical Heisenberg model with nearest neighbor ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions shows the existence of the well defined ground state. FM nanoparticles have susceptibility maximum decreasing with the increase of the nanoparticle size. Finite size scaling analysis predicts small Curie temperature for shells of the large size. The AFM particles built as truncated octahedron with (001) and (111) planes of the cubic lattice show freezing in the noncollinear structure with very low magnetization. The freezing temperature determined as maximum in specific heat in particles with AFM does not change strongly with the size of the shell in the studied particle sizes. We clearly observe the effects of edges and corners on the properties of shell particles resulting in deviation from simple scaling behavior. We fit the size dependence variation of thermodynamic properties maxima using the idea of the continuous dimensionality and the fact that infinite 2D systems do not exhibit long range order at finite temperature in the continuous symmetry. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S27.00010: Low T magnons condensation in FePd and FePt nano-particulate films R. A. Lukaszew, K. Yang, J. R. Skuza, C. Clavero Recently there have been interesting reports on experimental deviations from the Bloch T$^{3/2}$ law at low temperature in magnetic nanostructures. Specifically it has been reported an ``upturn'' of the T$^{3/2}$ law in the magnetization in some nanomagnetic materials. [1] This behavior has been attributed to a Bose-Einstein Condensation of magnons in the nanostructured materials due to more disorder in the spins leading to finite entropy as opposed to the case of crystalline bulk ferromagnets where it can be assumed that all the spins are in the direction of a saturating magnetic field and hence perfectly ordered. In order to further test these ideas, we have carried out systematic low temperature magnetization studies (SQUID) on FePd and FePt nano-particulate thin films. The microstructure and morphology of the thin films have also been extensively characterized with X-Rays, TEM and AFM/MFM. We will show our experimental data and point out significant differences and also similarities in the analyzed samples. [1]. Extension of the Bloch $T^{3/2}$ Law to Magnetic Nanostructures: Bose-Einstein Condensation, E. Della Torre, L. H. Bennett and R. E. Watson, \textit{Phys. Rev. Lett}. \textbf{94}, 147210 (2005) [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S27.00011: Strong-field interactions between a nanomagnet and a cavity mode O. Soykal, M. E. Flatt\'e We analyze the interaction of a nanomagnet with a single mode of a microcavity in a fully quantum-mechanical treatment. We consider a spherical cavity roughly 1 mm$^3$ in volume, and a nanomagnet consisting of $10^9$ spins treated as a macrospin, in the presence of a static magnetic field. For an initial configuration of no photons in the cavity and the macrospin oriented antiparallel to the field, the interaction Hamiltonian contains magnet-microwave mode coupling terms that exceed several GHz. Thus for quality factors in excess of 100, strong-field effects should be observable in the nanomagnet/cavity dynamics. Coherent states of the nanomagnet/photon system are characterized by large oscillations in the photon number (and nanomagnet spin), and are characterized by exceptionally long dephasing times. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S27.00012: Magnetism of Fe double wires deposited on Ir(100) Riccardo Mazzarello, Andrea Dal Corso, Erio Tosatti Bulk bcc Fe is a prototypical ferromagnet (FM), but a single monolayer of Fe on W(001) has been known to be antiferromagnetic (AFM). Very recent spin-polarized STM experiments on Fe double chains deposited on Ir(100) 5x1 [1] showed that these adsorbed nanowires are AFM too [2]. We study the magnetic properties of this system using ab-initio density functional theory and both scalar-relativistic and fully-relativistic ultrasoft pseudo-potentials. In particular, we address the energetics of FM and AFM configurations of several experimentally relevant structures. The AFM configuration is always energetically favored, which is in agreement with experiment but does not yet allow to distinguish between different structures. Investigation of the magnetic anisotropy induced by the spin-orbit interaction is in progress. [1] L. Hammer, W. Meier, A. Schmidt, and K. Heinz, Phys. Rev. 67, 125422 (2003). [2] R. Wiesendanger, private communication. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S27.00013: Tailoring magnetic order in nanowires by alloying 5d Transition Metal elements Javier Guevara, Tristana Sond\'on, Andres Saul The magnetism of pure-element nanowires have been theoretically studied and show, in the case of Au, Pt, and Ir, none or very low magnetic moment values but Os. In this work we study the magnetic properties of A$_{1/2}$B$_{1/2}$ nanowires, being A,B=Os, Ir, or Pt, by using the ab-initio Wien2k code. These alloyed nanowires have large magnetic moment values, and also giant MAE of different signs. We show the evolution of the spin and orbital magnetic moments as the magnetization axis is being varied. [Preview Abstract] |
Session S28: Superlattices and Nanostructures (Wires, Dots, etc.): Optical Properties II
Sponsoring Units: DCMPChair: Michael Scheibner, Naval Research Laboratory
Room: Morial Convention Center 220
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S28.00001: Design of uniaxial metallodielectric metamaterials having large optical nonlinearities Joseph Geddes, Erik Nelson, Paul Braun We describe how the intrinsically large optical nonlinearities of metals could be accessed and increased by fabrication of uniaxial homogenized composites comprising alternating metal and dielectric layers of subwavelength thickness. Such composites are predicted to exhibit effective third-order nonlinear susceptibilities orders of magnitude larger than those intrinsic to the metallic component. The enhancement is due to a resonance effect, and is limited to the direction perpendicular to the layer interfaces. We illustrate our predictions with calculations for several metallodielectric systems, including those consisting of copper and titanium dioxide components. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S28.00002: Spectroscopic and thermal studies of L-arginine doped Potassium Dihydrogen Phosphate crystals Jayesh Govani, Felicia Manciu, Mihir Joshi, Ketan Parikh, Dipak Dave We have used IR transmission and Raman spectroscopy to study the active doping of potassium dihydrogen phosphate (KDP) crystals with L-arginine amino acid. In the present investigation, pure and doped KDP crystals were grown by slow evaporation solution method. Although the dominant bands observed in the infrared absorption spectra correspond to KDP crystals, the existence of vibrational lines at 1401 cm$^{-1 }$(CH$_{2})$, 1637 cm$^{-1}$ (COO$^{-})$, 1716 cm$^{-1}$ (NH$_{3}^{+})$, and 3127 cm$^{-1}$ (NH$_{3}^{+})$ indicate that successful doping was achieved. This affirmation is further corroborated by the FT-Raman data, where strong lines are observed in the 2800 cm$^{-1}$ -- 3100 cm$^{-1}$ region, which is associated with C-H stretching modes of amino acids. The crystal structure and the thermal stability of the samples were also examined by powder X-ray diffraction and thermogravimetric techniques, respectively. Thermogravimetric analysis demonstrates a decrease of the thermal stability with increasing doping amount. An increase of second harmonic generation efficiency was found with more L-Arginine doping. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S28.00003: Optical properties in the visible range of Co clusters capped by Pd under hydrogen A.L. Cabrera, J.I. Avila, C.P. Romero, M.J. Van Bael, P. Lievens Co clusters with mean size of 1.8 nm were deposited to form a 25 nm thick cluster assembled film on glass, capped by a continuous 15 nm Pd film. The light transmission and reflection, in the visible range (400 to 1000 nm), were measured when the sample was exposed to different hydrogen pressures up to 120 Torr. Measurements on 15 nm continuous Pd film were done for comparison. Electrical resistance of the films was also measured as an independent parameter to determine hydrogen absorption by the samples. In both samples the transmission and resistance of the films increase, reaching saturation at 30 Torr hydrogen pressure. Increase of the light transmission and electrical resistance on the pure Pd film indicates absorption of hydrogen in the bulk of the film. Smaller relative change of the resistance and reflection of light on the Co cluster sample capped by Pd indicates that hydrogen absorption is limited to the Pd capping layer only. This work is supported by the Fund for Scientific Research-Flanders (FWO), by the Flemish Concerted Action (GOA), and by the Belgian Interuniversity Poles of Attraction (IAP) programs. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S28.00004: Novel optical signatures of sub-3 nm rare earth sesquioxide nanocrystals. James Dickerson, Sameer Mahajan Europium and terbium based sesquioxide nanomaterials, known for their characteristic red and green luminescence, respectively, have recently garnered much research attention due to their size-dependent optical properties. Here, we present systematic investigation of the size-dependent optical properties Eu$_{2}$O$_{3}$, Tb$_{2}$O$_{3}$, and Gd$_{2}$O$_{3}$:Eu$^{3+}$ / Tb$^{3+ }$nanocrystals (NCs) in the size range of 1-3 nm in diameter. We observe a new luminescence peak at 620 nm in Eu$_{2}$O$_{3}$ and Gd$_{2}$O$_{3}$:Eu$^{3+}$ NCs, which represents modulation of the $^{7}$F$_{2}$ transition in Eu$^{3+}$ ion. Intensity modulation with respect to the 612 nm is observed as a function of nanocrystal size. For the Tb$_{2}$O$_{3}$ NCs, a new luminescence signature at 548 nm characterizes modulation of the $^{7}$F$_{5}$ transition in Tb$^{3+}$ ion. In addition, we probe the effect of NC size on the luminescence efficiencies of the doped and pure sesquioxide NCs. The concentration quenching effect, which leads to low luminescence efficiencies in bulk, pure sesquioxides, is explored in sub-3 nm sesquioxides. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S28.00005: Far-infrared Magneto-Spectroscopic Studies of Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals Jiufeng Tu, Dimitar Dimitrov, Weidong Si, Qiang Li In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials due to their unique combinations of high thermo-coefficient and good metallic transport properties. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out far- infrared magneto-spectroscopic studies of Ca$_{3}$Co$_{4}$O$_{9} $ thin films in Faraday geometry as a function of frequency, magnetic field and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. Far infrared transmission reduces at low frequencies in the presence of magnetic field corresponding to negative magneto-resistance. Below 20K, hysterisis occurs. However, the spectral responses to magnetic field and temperature are different. This indicates that the negative magneto-resistance is due to reduced magnetic scattering when Co spins become aligned. Further infrared studies will be performed with magnetic field parallel to the CoO$_{2}$ layers. A good understanding of our infrared results should shed light on the origin of high thermo-power in these 2D-layered cobaltates. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S28.00006: Ultrafast Time Resolved Transient Absorption and Photoluminescence (PL) Studies of In$_{0.2}$Ga$_{0.8}$As/GaAs Quantum Wells in High Magnetic Fields Jinho Lee, X. Wang, D.H. Reitze, S. McGill, Y.D. Jho, J. Kono, A.A. Belyanin, G. Solomon We investigate the temporal dynamics of dense magneto-plasmas excited by intense femtosecond laser pulses in In$_{0.2}$Ga$_{0.8}$As/GaAs multiple quantum wells in high magnetic fields. To fully fill the Landau levels (LLs), we pump to carrier densities near 10$^{13}$/cm$^{2}$ using 150 fs pulses. Time-resolved transient absorption experiments probe the occupancy of each e1h1 LL, revealing a dramatic decrease in decay times above the zero field e1h1 transition dynamics. Our PL results reveal evidence for multiple short bursts of emission pulses at the highest fields (17.5T) from in-plane PL emission compared with zero field emission. In addition, qualitatively different temporal dynamics from in-plane and out-of-plane collection geometries are observed. Our results are analyzed in the context of ultrafast cooperative emission mechanisms from dense electron-hole plasmas. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S28.00007: Magneto-photoluminescence studies of CdTe/CdSe/ZnS nanoparticles Y.H. Chang, C.C. Huang, L.W. Chang, C.H. Hsu, Chih-Wei Lai, Chien-Liang Liu, Pi-Tai Chou, Y.W. Suen Recently, the effect of the magnetic flux on the excitonic energy has received much attention. Optical Aharonov-Bohm was observed on negatively charged exciton in InGaAs/GaAs quantum ring as well as neutral exciton in type-II InP-GaAs heterostrcture. In this talk we'll present our magneto-photoluminescence studies on the optical properties of CdTe/CdSe/ZnS system. The nanoparticles that were grown by chemical method have size of about 6 nm and the band alignment between the core (CdTe) and the shell (CdSe) is a type--II band alignment. The addition of ZnS layer is to passivate the surface of CdSe and to enhance the light emitting efficiency. Magneto-photoluminescence experiment was performed at T=1.4 K with a 14 T superconducting magnet in conjunction with a green diode laser and a monochromator. Oscillation on the peak energy of the photoluminescence spectra as well as oscillation in the integrated intensity as a function of magnetic field were observed and are attributed to the optical Aharonov-Bohm-like effect. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S28.00008: Absorption and photocurrent of semiconductor quantum wells: a multi-band NEGF study Urs Aeberhard Interband photoexcitation of carriers in semiconductor quantum wells is exploited in various optoelectronic devices such as photodetectors or quantum well solar cells. For a quantitative prediction of the photocurrent, realistic models for the (sub)bandstructure as well as for the transport properties are required. In the present approach, the derivation of optical properties based on a multi-band empirical tight-binding Hamiltonian is combined with the non-equilibrium Green's function formulation of quantum transport. The photocurrent is calculated in presence of elastic and inelastic electron-phonon scattering from the self-consistent Born approximation of the self energy for carrier-light interaction, while the absorption is obtained from the transverse interband polarization function. Since the absorption as experimentally observed is governed by the excitonic contribution, the inclusion of this feature into the calculation of photocurrent and interband polarization via the respective many-body corrections is discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S28.00009: Magneto-infrared study on 2-dimensional electrons and holes in GaSb-InAs-AlSb coupled quantum wells. Li-Chun Tung, P.A. Folkes, Wen Xu, Yong-Jie Wang InAs-AlSb heterostructures have been a subject of interests for their unusual type-II band alignment between InAs and AlSb. The spatially separated 2-dimensional electron and hole gases are confined in different layers and in equilibrium with each other at the InAs/GaSb interface. This unique circumstance has led to several predicted many-body effects, as well as possible applications as infrared detectors and sources. In the past, magneto-infrared studies on InAs-AlSb single quantum wells have revealed a range of phenomena arising from the electron-hole binding. We have carried out an infrared optical study up to 33T on a series of GaSb-InAs-AlSb coupled quantum well structures, in which the electrons and holes are separated by a thin barrier and the Fermi level is tuned by the thickness of the GaSb cap layer. In addition to the electron cyclotron resonance (CR), another transition has been observed at the fields higher than 13 T. The linewidth of the CR shows the oscillatory behavior with the filling factor that is consistent with the electron densities obtained from the transport measurements. The transition energy of this line is close to the energy difference between the lowest Landau level (LL) in the InAs layer and the highest LL in the GaSb layer, which suggests that the it might result from an excitonic transition across the barrier. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S28.00010: Infrared dielectric properties and optical magnetoconductivity of CaRuO$_3$/CaMnO$_3$ superlattices A.V. Boris, P. Yordanov, P. Popovich, B. Keimer, J.W. Freeland, J. Chakhalian, H.N. Lee Spectroscopic ellipsometry and magneto-reflectivity in the far-infared spectral range is used to study the electronic properties of $\rm [(CaRuO_3)_N|(CaMnO_3)_{10}]_6$ superlattices (SLs). The nonlinear regression procedure is employed to extract the dynamical conductivity and dielectric permittivity of bare SLs within the effective medium approximation with a mixture of the ruthenate and manganite layers. We find that the infrared conductivity of the SLs decreases with decreasing individual ruthenate layer thickness, so that the effective number of conducting electrons per Ru atoms remains independent of $N$ and is comparable with the bulk value even for ultrathin $\rm CaRuO_3$ layers (N = 4-10 unit cells). This implies no major charge transfer effects between non-Fermi liquid metal $\rm CaRuO_3$ and antiferromagnetic insulator $\rm CaMnO_3$. While the low-energy electrodynamics of the SLs is governed by $\rm CaRuO_3$ layer behavior, we find a negative magnetoresistivity at temperatures below $\sim$ 150 K, which correlates with the Neel temperature of the AFM state in $\rm CaMnO_3$. The magnetoresistivity effect is discussed to be due to the strong spin dependent scattering from the interface. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S28.00011: Observation of coherent high-wavevector acoustic vibrations in a bulk material using time-resolved X-ray diffraction Mariano Trigo, Yu-Miin Sheu, Emily Peterson, David Reis, Matthew Reason, Rachel Goldman, Roberto Merlin, Eric Landahl, Donald Walko, Dohn Arms We report on the observation of high-wavevector acoustic phonons in bulk InP that originate from folded phonons in a GaInAs/AlInAs superlattice. Synchrotron time-resolved X-ray diffraction is used to probe the evolution of the laser-generated acoustic phonons. Due to the short wavelength, X-ray diffraction gives access high-wavevector components of the acoustic wave-packet in a bulk material. Experiments show a bulk excitation at a wavevector $q=2 \pi/D$, where $D$ is the superlattice period, which propagates into the substrate at the speed of sound. These results are supported by time-resolved dynamical diffraction calculations in which the strain is included as a perturbation from the perfect crystal. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S28.00012: Fr\"{o}hlich phonon modes in PbSe and PbS colloidal quantum dots Jonathan Wrubel, Byung-Ryool Hyun, Nikolay Agladze, Albert Sievers, Frank Wise The measured infrared absorption of colloidal PbSe and PbS quantum dots in hexane is shown to be dominated by absorption near the Fr\"{o}hlich mode frequency position. However, in both sets of quantum dots the mode is at a higher frequency than calculated from the bulk dielectric constant, and shifts to still higher frequency with decreasing diameter of dot. This behavior is shown to be consistent with a decreasing contribution of the near-infrared and visible exciton absorption to the dielectric constant at far infrared frequencies as the particle size decreases and the band gap increases. The unique presence of Fr\"{o}hlich mode absorption in a wide range of dot sizes suggests that the mechanical boundary condition of the quantum dot is a ``soft'' one in which vibrational amplitudes do not go to zero at the boundary. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S28.00013: Coherent defect-assisted multiphonon intraband carrier relaxation in semiconductor quantum dots Alexander Poddubny, Serguei Goupalov A new defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S28.00014: Raman-Brillouin electronic density in GaAs/AlAs superlattices. Nicolas Large, Adnen Mlayah, Javier Aizpurua, Jean-Roch Huntzinger, Bernard Jusserand Raman-Brillouin scattering by acoustic phonons is an accurate experimental method to characterize vibrational states of nanostructures and understand their optical properties. Many nanoscaled systems such as quantum dots, wires, wells and membranes have been studied by the means of this technique. We present here a theoretical and experimental study of the Raman-Brillouin scattering in GaAs/AlAs superlattices. Within third order perturbation theory, we describe the Raman- Brillouin scattering process by introducing a Raman-Brillouin Electronic Density (RBED). The RBED is constructed by combining the superlattice electronic states according to their optical transition rates. This approach is useful when numerous intermediate electronic states are involved in the scattering process. It has been proven to successfully interpret the Raman- Brillouin scattering in quantum dots[1] and membranes[2]. We calculate the RBED for specific GaAs/AlAs superlattices and study the dependence of the Raman-Brillouin spectra on the GaAs/AlAs layer thickness ratio and incident photon energy. Comparison with experiments will be discussed. [1] J.R. Huntzinger et al, Phys. Rev. B 74, 115308 (2006) [2] A. Mlayah, J.R. Huntzinger and N. Large, Phys. Rev. B 75, 245303 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S28.00015: Measured and Calculated Properties of \textit{n}- and \textit{p}-type PbTe-Based Materials for High-Performance Thermoelectrics C. Vineis, T. Harman, S. Calawa, M. Walsh, R. Reeder, W. Goodhue Recent advances in PbTe and other material systems for thermoelectric applications are based on nanostructuring, with a specific goal of substantially reducing lattice thermal conductivity while maintaining good electrical properties. Our work has focused on developing PbTe/PbSe$_{1-x}$Te$_{x}$ ($x\sim $0.02-0.04) nanodot superlattices (NDSLs) for improved thermoelectric performance. In this presentation we will compare the electrical and thermal properties of $n$- and $p$-type NDSLs to baseline homogeneous PbTe, and also compare the electrical data to calculations performed using the Boltzmann transport equation with the relaxation time approximation. Compared to PbTe at the same carrier concentration, NDSL samples generally have reduced mobilities ($\sim $25-35{\%}), the same Seebeck coefficients, and substantially reduced ($\sim $4-6x) lattice thermal conductivities, resulting in a large increase in \textit{ZT}. Specifically, a 300-K in-plane power factor of $\ge $25 $\mu $W/cm-K$^{2}$ has been repeatedly achieved for both $n$- and $p$-NDSLs, while the cross-plane lattice thermal conductivity has been measured as $\sim $0.35-0.4 W/m-K using various techniques. We will also present recent power generation results where 9.9 W/cm$^{2}$ was obtained from a 1 mm$^{2}$, 100-$\mu $m-thick stand-alone $n$-NDSL thermoelement, at a $\Delta $T of 202 K. [Preview Abstract] |
Session S29: Focus Session: Carbon Nanotubes and Related Materials XI: Optical Spectroscopy
Sponsoring Units: DMPChair: James Hone, Columbia University
Room: Morial Convention Center 221
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S29.00001: \textit{In situ} Raman Spectroscopy of Suspended Carbon Nanotubes under High Voltage Bias Invited Speaker: We report recent results of Raman spectroscopy taken of individual suspended single-walled carbon nanotubes exhibiting negative differential conductance (NDC) \textit{in situ} under high voltage biases [1]. The transverse and longitudinal optical phonon modes ($G_{+}$ and $G_{-}$ band) are found to respond differently to the applied voltage bias. We observe preferential downshifting of only one optical phonon mode while the other remains largely unchanged, indicating a non-equilibrium phonon population caused by the preferential electron-phonon coupling of only one optical phonon polarization. This preferential coupling is caused by the differences between the two Kohn anomalies in the TO and LO phonon branches [2]. Surprisingly, in most metallic nanotubes, the narrow $G_{+}$ band (TO band) is more strongly heated by electron-phonon scattering at high biases. The non-equilibrium phonon populations produced under high biases are corroborated by anti-Stokes Raman spectroscopy. We correlate the optically measured phonon population to the electrically measured resistivity using a Landauer model to determine key scattering parameters. The electron-phonon scattering mechanism revealed by these measurements and this analysis show the importance of electron-phonon scattering by phonon absorption from the large non-equilibrium phonon population in explaining the observed negative differential conductance [3]. \newline \newline [1] A. W. Bushmaker, V. V. Deshpande , M. W. Bockrath, S. B. Cronin, Direct observation of mode selective electron-phonon coupling in suspended carbon nanotubes, \textit{Nano Lett.}, in press (2007). \newline [2] S. Piscanec, M. Lazzeri, J. Robertson, A. C. Ferrari, F. Mauri, \textit{Phys. Rev. B} \textbf{75}, 35427 (2007). \newline [3] E. Pop, D. Mann, J Cao, Q. Wang, K. Goodson, H. Dai, \textit{Phys. Rev. Lett.} \textbf{95}, 155505 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S29.00002: Measurement of the absorption cross section of individual single walled carbon nanotubes St\'ephane Berciaud, Laurent Cognet, R. Bruce Weisman, Brahim Lounis We combined time--resolved and cw luminescence studies on highly luminescent individual (6,5) single walled carbon nanotubes in aqueous environments to yield the first experimental determination of the absorption cross section of individual nanotubes. Luminescence decays systematically exhibited a bi-exponential behavior with a short component (mean 45ps) accounting for most of the integrated signal, followed by a weak tail decaying on a 250 ps timescale. We obtained a mean statistical value of 1.0 10$^{-17}$ cm$^{2}$ per carbon atom for nanotubes resonantly excited at their second order optical transition, a value independently obtained by photothermal absorption measurements also performed on individual (6,5) nanotubes. Precise knowledge of the absorption cross section of individual nanotubes is essential for the determination of nanotube quantum yield as well as in quantitative studies of multi-excitonic processes. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S29.00003: Length-dependent optical properties of single-walled carbon nanotubes J. R. Simpson, J. A. Fagan, B. J. Bauer, E. K. Hobbie, A. R. Hight Walker Length proves to be an important physical characteristic of single-walled carbon nanotubes (SWCNTs) strongly affecting optical absorption, photoluminescence, and resonance Raman spectroscopies.\footnote{J. A. Fagan \textit{et al.}, J. Am. Chem. Soc. \textbf{129}, 10607 (2007).} Our samples include HiPco, CoMoCat, and arc-discharge SWCNTs dispersed in aqueous solutions by wrapping with either DNA or surfactant and exhibiting an exceptionally low degree of SWCNT bundling/clustering. Size-exclusion chromatography or density sorting ultracentrifugation collect length fractions ranging in size from approximately 50 nm to 1000 nm. The optical spectral weight specific to the SWCNT electronic states and photoluminescence peak emission intensity, compared to their underlying backgrounds, scale approximately linearly with length. All observed Raman vibrational modes exhibit a monotonic increase of scattering intensity with nanotube length. Resonance Raman of the radial breathing mode reveals a blueshift of the excitation energy for shorter nanotubes. Localization of bound excitons along the nanotube may explain the observed length-dependent optical properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S29.00004: Reflectance spectra of individual single walled carbon nanotubes Hualing Zeng We report back scattering spectroscopic measurements on individual single walled carbon nanotubes (SWNTs). The reflectance spectra show geometry-dependent resonant peaks corresponding to optical transitions between Van Hove singularities in SWNTs' joint density of states. All nanotubes display certain colours as their reflectance spectra demonstrate strong energy dependence. This approach was proved to be an effective tool to probe geometric structures and optical properties of individual SWNTs. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S29.00005: Inverted spectra of SWCNT films John Lehman, Katherine Hurst, Lara Roberson, Kathryn Nield, John Hamlin Diffuse Reflectance for purified single wall carbon nanotube (SWCNT) films and its relation to absorptance in the wavelength range 0.6 $\mu $m to 2 $\mu $m are inverted when compared to absorptivity data in the literature. This surprising behavior has been corroborated by diffuse reflectance measurements and shows that the reflectance is a substantial part of the unique optical behavior. Typically, the absorptance is fairly assumed to be complementary to the transmittance, while the reflectivity is insignificant. Only in certain instances (see for example, Barnes, et. al[1], Wang, et. al[2]), is the small reflectance explicitly accounted for. In the present work, we present diffuse reflectance and specular absorptance at normal incidence of SWCNT films. \newline [1] T. M. Barnes, J. van de Lagemaat, D. Levi,1 G. Rumbles, T. J. Coutts, C. L. Weeks, D. A. Britz, I. Levitsky, J. Peltola, P. Glatkowski, Phys. Rev. B \textbf{75}, 235410 (2007). \newline [2] F. Wang, M. Y. Sfeir, L. Huang, X. M. H. Huang, Wu, J. Kim, J. Hone, S. O'Brien, L. E. Brus, T. F. Heinz, PRL \textbf{96}, 167401 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S29.00006: Broadband Rayleigh Scattering and Photoconductivity Spectra of Individual Semiconducting Single-Walled Carbon Nanotubes Matthew Sfeir, Sami Rosenblatt, Yang Wu, Hugen Yan, Christophe Voisin, Bhupesh Chandra, Robert Caldwell, Yuyao Shan, James Hone, Tony F. Heinz, James A. Misewich Combining a Fourier-transform measurement of photoconductivity with Rayleigh spectroscopy, we have identified the four lowest-lying optical transitions from specific, individual single-walled carbon nanotubes. In these investigations we made use of the previously reported transfer technique [1] to obtain samples with optimized arrangements both for Rayleigh (freely suspended) and photoconductivity (transistor geometry) spectroscopy. The combination of these two optical characterization techniques yields high-resolution spectra of the electronic transitions of individual nanotubes over a spectral range extending from 0.3 -- 2.7 eV. We will discuss the details of the spectra that we have obtained for individual single-walled nanotubes of defined chiral index, including the observation of asymmetric lineshapes for the lowest-lying optical transition. [1] X. M. H. Huang, et al., Nano Lett$.$ \textbf{5}, 1515 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S29.00007: Optical phonon and hot carrier lifetimes in single-walled carbon nanotubes by time-resolved anti-Stokes Raman scattering Kwangu Kang, David Cahill, Taner Ozel, Moonsub Shim The lifetimes of optical phonon and photoexcited carriers in both semiconducting and metallic single-walled carbon nanotubes are determined by time-resolved Raman scattering using a subpicosecond pump-probe method. Non-equilibrium populations of electronic and phonon excitations are observed by incoherent anti-Stokes Raman scattering from a broad continuum and the G mode, respectively. HiPco nanotubes with E$_{22}$ transitions and arc-discharge nanotubes with E$_{11}$ transitions dominate the spectra because of their resonance with the photon energy. To separate Raman scattering created by the probe beam from scattering created by the pump beam, we have developed a two- color pump-probe technique based on the broad bandwidth of the Ti:sapphire laser oscillator and narrow bandpass optical filters. For semiconducting tubes, the optical phonon lifetimes decrease from 1.2 ps to 0.9 ps with increasing laser fluence. The optical phonon lifetimes of metallic tubes, on the other hand, increase from 0.6 ps to 1.1 ps. The hot carrier lifetime is approximately 0.3 ps. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S29.00008: Raman Spectroscopy of isolated Double Wall Carbon Nanotubes (DWNT) Federico Villalpando-Paez, Alfonso Reina Cecco, Daisuke Shimamoto, Antonio G. Souza Filho, Hyungbin Son, Yoong A. Kim, Endo Morinobu , Mauricio Terrones, Mildred Dresselhaus We have developed a method to perform Raman spectroscopy on isolated double wall carbon nanotubes (DWNT). By identifying isolated DWNTs and obtaining their Raman spectra using different laser lines, we are able to find DWNTs whose inner and outer walls are in resonance with the same laser line or with more than one laser lines ranging from 514nm to 785nm. The inner and outer walls of a DWNT can be metallic (M) or semiconducting (S) and each of the four possible configurations (M/M, M/S, S/S, S/M) has different electronic properties. The obtained Raman spectra show simplified radial breathing mode (RBM), G and G' line shapes that allow us to study the inter layer interactions and make comparisons to previous experiments on DWNT bundles and double layer graphene. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S29.00009: Theory of coherent phonon spectroscopy in carbon nanotubes G. D. Sanders, C. J. Stanton, Y. S. Lim, K. J. Yee, J. H. Kim, E. H. Haroz, L. G. Booshehri, J. Kono We develop a theory for the generation and detection of coherent phonons in single wall carbon nanotubes. Coherent phonons are generated in the nanotube by ultrafast laser pulses via the deformation potential electron-phonon interaction with the photogenerated carriers. The electronic states are treated in a tight binding formalism which gives a description of the states over the nanotube Brillouin zone while the nanotube phonon modes are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. Equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations. In coherent phonon spectroscopy the coherent phonons are detected by ultrafast pump probe differential transmission measurements. We find that for uniform illumination with a 5 fs pump pulse only the q = 0 radial breathing mode and a high frequency G mode are strongly excited. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S29.00010: Stability of carbon nanotubes to laser irradiation probed by Raman spectroscopy. Alexander Soldatov, David Olevik, Manuel Dossot, Edward McRae CNTs in a bundled state suffer from overheating effects - exposure to laser irradiation leads to a reversible shift of the RBM resonance window at a moderate laser fluence [1] or even to damaging of certain nanotube types at higher fluencies. Here we report on our systematic study of the influence of laser irradiation on the Raman spectra of HiPCO-produced single wall CNTs. Specifically, we have examined Raman response of bundled CNTs to: i) laser power density; ii) exposure time and iii) photon energy (1.96 and 2.33 eV). Our results show that irreversible destruction of CNTs in the bundles takes place at even a moderate laser power density ($\sim $500 W/cm$^{2})$. Notably, the tubes with smaller diameters are influenced first and the rate of CNT damage increases with photon energy. Finally, we determined that the threshold for the RBM spectrum profile to change at $\sim $200 W/cm$^{2}$, which is apparently below the laser fluencies used typically in Raman experiments on CNT bundles. Based on these results we developed a regime of Raman data collection which was recently used to identify functionalization of different types of CNTs [2] from their RBM Raman spectra. [1] C. Fantini, et al. Phys. Rev. Lett., \textbf{93}, 147406 (2004). [2] J. Liu, et al. Carbon, \textbf{45}, 885, (2007). [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S29.00011: Surface Enhanced Raman Spectroscopy (SERS) and Scanning Electron Microscopy of Individual SERS Hot Spots Rajay Kumar, Stephen Cronin We measure Raman spectroscopy and scanning electron microscopy before and after depositing silver nanoparticles on carbon nanotubes. Individual SERS ``hot spots'' are identified with respect to a lithographically defined grid using micro-Raman spectroscopy. Carbon nanotubes' extremely large aspect ratio enables subsequent imaging of the nanoparticle geometry together with the SERS active molecule. The SERS enhancement factor is determined by comparing the Raman intensity of an individual nanotube before and after nanoparticle deposition. The data, published in R. Kumar et al., Appl. Phys. Lett., 91 (2007), reports SERS enhancement factors up to 134,000 and nanoparticle heating exceeding 600C, as evidenced by the local burnout of nanotubes in SERS hot spot regions. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S29.00012: Electrical Transport Studies of (n,n) Armchair Carbon Nanotubes Robert Caldwell, Bhupesh Chandra, Christophe Voisin, Tony F. Heinz, James Hone By using Rayleigh scattering spectroscopy, a simple mechanical transfer process, and standard E-beam lithography fabrication of metallic leads, we can probe the electrical properties of individual single-walled carbon nanotubes of known chiral indices (n,m) on the substrate of our choosing. Using these techniques, we have discovered that (n,n) `armchair' nanotubes consistently deviate from the predicted metallic behavior, specifically showing a gap in current -- gate voltage curves. We present detailed studies of the transport behavior of these devices, including conductivity as a function of bias, length, and temperature. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S29.00013: Supercurrent in Single Wall Carbon Nanotube Josephson Junctions Gang Liu, Yong Zhang, Chunning Lau We investigate transport in highly transparent single-wall carbon nanotube Josephson Junctions. Gate tunable supercurrent, multiple Andreev reflections and hysteresis current-voltage characteristics are observed, corresponding to on- and off-resonance transmission of charges via the nanotube's quantized energy levels. In the talk we will discuss the dependence of supercurrent on temperature, source-drain separation and gate voltage, and compare with various theoretical models. [Preview Abstract] |
Session S30: Nanotubes, Transport, and Other Properties
Sponsoring Units: DCMPChair: Vincent Crespi, Pennsylvania State University
Room: Morial Convention Center 222
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S30.00001: Direct observation of gate-controlled Josephson inductance in multiwalled carbon nanotube Antti Paila, Lorenz Lechner, Markus Gaass, Christoph Strunk, Mika Sillanp\"a\"a, Pertti Hakonen Electrometers based on Cooper pair tunneling are, in principle, dissipationless, and thus advantageous due to their small back action noise. We have studied the so called L-SET circuit, in which the frequency of Josephson plasma oscillations in a superconducting junction device is tuned below one GHz by a parallel $LC$ circuit. In the present work, we have used a Josephson junction made out of a multiwalled carbon nanotube with Pd/Nb contacts. We observe gate-tunable critical currents up to $I_c \sim 10$ nA, which are deduced from shifts in the LC-circuit resonance frequency caused by the Josephson inductance ($\propto I_c^{-1}$) of the MWNT junction. This work was financially supported by Academy of Finland and by European Union (CARDEQ, FP6-IST-021285-2). [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S30.00002: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S30.00003: Electronic transport in inhomogeneous quantum wires Jerome Rech, K. A. Matveev We study the transport properties of a long non-uniform quantum wire where the electron-electron interactions and the density vary smoothly at large length scales. We show that these inhomogeneities lead to a finite resistivity of the wire, due to a weak violation of momentum conservation in the collisions between electrons. Estimating the rate of change of momentum associated with non-momentum-conserving scattering processes, we derive the expression for the resistivity of the wire in the regime of weakly interacting electrons and find a contribution linear in temperature for a broad range of temperatures below the Fermi energy. By estimating the energy dissipated throughout the wire by low-energy excitations, we then develop a different method for deriving the resistivity of the wire, which can be combined with the bosonization formalism. This allows us to compare our results with previous works relying on an extension of the Tomonaga-Luttinger model to inhomogeneous systems. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S30.00004: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S30.00005: Carbon Nanotubes in Helically Modulated Potentials Paul Michalski, Eugene Mele We study the low energy electronic spectrum of metallic and semi-conducting carbon nanotubes (CNTs) under an applied, helically symmetric potential using the long-wavelength, continuum approximation. We find that the effects of the external potential depend on the strength of the potential and on the dimensionless ratio of the nanotube circumference to the pitch of the helical potential, and we explore the system's response as these two parameters are varied. We find that for semi-conducting CNTs the band gap is always reduced. For metallic CNTs the Fermi velocity is reduced, and in very strong fields two small gaps appear at the Fermi surface in addition to the gapless Dirac point. We develop a simple model to estimate the magnitude of the potential strength and its effect on the spectrum of a DNA-CNT complex in aqueous solution. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S30.00006: Effective Low Energy Lattice Models for Conducting Carbon Nanotubes Sebastian A. Reyes, Alexander Struck, Sebastian Eggert We obtain effective one dimensional lattice theories for the low energy sector of conducting single walled carbon nanotubes. The effective Hamiltonians are otained by keeping only the conducting bands of the tube, but without taking the a continuum limit. For the zigzag and armchair tubes the theory is particularly simple and reduces to two species of well defined orbitals with only nearest neighbor hopping. These models provide a clear picture of the low energy physics of carbon nanotubes on a lattice without linearization. Correlation functions and density oscillations can be calculated. We show how arbitrary bare electron electron interactions can be included. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S30.00007: Heat flow in metallic carbon nanostructures: hot phonons or hot electrons? Marcelo Kuroda, Jean-Pierre Leburton A model for electronic and thermal transport in metallic carbon nanotubes and graphene ribbons under electrical stress is presented. The influence of acoustic and optical phonon scattering is taken into account within a self-consistent electro-thermal scheme. Owing to the linear electronic dispersion relation, two thermalized carrier populations arise as a consequence of inter-carrier scattering. We show that depending on the experimental setup and nanotube parameters, different transport regimes emerge for which non-equilibrium electrons and phonons compete to carry the energy in these carbon nanostructures. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S30.00008: A first principles approach for thermal transport in nanostructures with defects Derek Stewart, Ivana Savic, Natalio Mingo Efficient heat transport is essential for developing viable nanoscale devices. On these small scales, defects, impurities, and even isotopic composition can dramatically affect thermal transport in heat conduits such as nanowires and nanotubes. We present a first principles approach that combines a non-equilibrium Green's function formalism for thermal transport with interatomic force constants derived from density functional calculations. We apply this approach to examine the thermal conductance in several potential heat conduits such as carbon nanotubes, boron nitride nanotubes, and silicon based nanowires. These nanostructures are treated in terms of a central region linked to two semi-infinite leads and interatomic force constants are calculated for the three separate regions. The role of defects on thermal conductance is examined for the different devices and the results are compared to empirical potential calculations and commonly used analytical models. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S30.00009: Thermal and transport properties of a single nickel nanowire Min-Nan Ou, Yang-Yuan Chen, Maw-Kuen Wu, Tzong-Jer Yang, P. C. Lee, S. R. Harutyunyan, C. D. Chen, S. J. Lai Starting with a 100 nm nickel film grown on a Si$_{3}$N$_{4}$/Si substrate by thermal evaporator, a suspended nickel nanowire (Ni-NW) was fabricated through e-beam lithography and etching processes. The Ni-NW was a part of 4-probes circuit which is designed for electrical, thermal and thermopower measurements. The resistivity (\textit{$\rho $}) and thermal conductivity (\textit{$\kappa $}) of a single nickel nanowire have been measured in the temperature range from 4 to 300 K by 4-probes method and the self-heating-3$\omega $ technique. At 300 K the thermal conductivity of nanowire is $\sim $ 20{\%} of the bulk, it diminishes to lower value as temperature decreases, the consequence is opposite to that in the bulk in which it decreases with temperature increase. The result might be explained by the restriction of mean free paths of electron/phonon--phonon interactions due to the grain boundaries. The small relative resistivity ratio (RRR $\sim $ 2) confirms the polycrystalline characteristic of the nanowire. The thermopower (Seedbeck coefficient $S)$ was also investigated by temperature gradient built up between two ends of the nanowire. The figure of merit ZT=S$^{2}\sigma $/$\kappa $ in the one-dimension specimen will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S30.00010: Microwave Kinetic Inductance Measurement of a Carbon Nanotube Y. Yin, J. Chudow, D.F. Santavicca, V. Manucharyan, A.J. Annunziata, L. Frunzio, D.E. Prober, B. Reulet, A. True, C.A. Schmuttenmaer, M. Purewal, Y. Zuev, P. Kim The single-wall metallic carbon nanotube is a model molecular nano-system, and has also been proposed as a candidate for future IC interconnects. For both these reasons, measurement of the kinetic inductance is desirable. This inductance arises from the kinetic energy of electrons in the four quantum channels. It is a fundamental prediction of the Luttinger liquid theory. Direct measurements at room temperature have been reported by Intel. That measurement is very challenging due to the large resistance compared to 50 ohms, and the small inductive impedance. We propose and demonstrate a new approach which uses two on-chip transmission line resonators to transform the nanotube impedance to nearly match the 50 ohm range of the microwave network analyzer. Simulations and cryogenic measurements will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S30.00011: Effects of Confinement Potential and Screening on a Quasi One Dimensional Electron Gas Luke Shulenburger, MIchele Casula, Gaetano Senatore, Richard M. Martin We study a quasi one dimensional system of electrons for different densities and strengths of the transverse confining potential. By means of exact quantum Monte Carlo techniques, we analyze the behavior of electrons interacting via an unscreened potential with a long range $1/r$ tail. We also study the effect of screening due to a metallic gate, which allows a more direct comparison with experiments. \vskip 0.1cm While the high density regime is well described by the RPA, we find that the charge compressibility $\chi_c$ is reduced due to electronic correlation as the density decreases. In the absence of screening, this corresponds to the onset of a quasi Wigner crystal, as shown by the finite size scaling of the static structure factor at 4k$_f$. As the density decreases further, the electrons behave as spinless fermions which is evidenced by the energetics and exponentially small spin velocities. The spinless fermion features persist also in the strongly screened system, and the RPA to Wigner crystal crossover is shifted to higher densities for a thinner wire. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S30.00012: Zone Unfolding and Approximate Bandstructure Calculations in Tight-Binding Timothy Boykin, Neerav Kharche, Mathieu Luisier, Gerhard Klimeck Tight-binding electronic structure calculations for periodic systems are often carried out in non-primitive unit cells, or for imperfect (e.g. random-alloy) nanostructures, such as nanowires. In the first case bands exist but they are difficult to identify due to the choice of unit cell for the calculation, while in the second case bands only exist in an approximate sense. The Brillouin zone unfolding technique applied to tight-binding calculations provides a powerful tool for extracting the true primitive-cell bands from non-primitive cells. Also, it is the starting point for approximate nanowire band calculations. We discuss zone unfolding in tight-binding and its application to both perfect and imperfect systems. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S30.00013: Gate Voltage Dependent Raman Scattering from Semiconducting Carbon Nanotube FETs James Tsang, Marcus Freitag, Vasili Perebeinos, Phaedon Avouris The Raman spectrum of the carbon nanotube in a carbon nanotube FET changes reversibly as a gate voltage is applied, modifying the charge density in the channel. We show that the intensity of the G-line Raman scattering from semiconducting CNTFETs can decrease with applied gate voltage. This is in addition to our previously reported shift of the G-line to higher energies with no change in spectral width as the channel charge density increases. The spectral shift been explained by gate voltage induced changes in the electronic excitations of the carbon nanotube which interact with the G-line. The gate voltage induced G-line shift and intensity changes observed in CNTFETs are similar to the changes observed for the G-line scattering between a suspended carbon nanotube over a trench, and the same tube on the substrate, where the G-line scattering from tube on the substrate shifts to higher energies and is weaker than the G-line scattering from the suspended tube. Gate voltage or substrate induced doping effects can modify the measured intensity of the Raman spectrum of a semiconducting carbon nanotube. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S30.00014: Directed Assembly and Electrical Characterization of Carbon Nanotube-Molecule-Metal Junctions Kansheng Chen, P. Xiong, S.A. McGill Molecular-template directed assembly has been shown to be an effective method for bottom-up assembly of high-performance single-walled carbon nanotube field-effect transistors (SWNT-FETs) $^{1}$. Here, we utilize this platform to carry out a systematic study of the electron transport behavior through SWNT-molecule-metal junctions. The devices were fabricated on doped- Si/SiO$_{2}$ substrates: Au source/drain electrodes were first defined by electron beam lithography. Self-assembled monolayers (SAMs) of thiol molecules with polar ends were then created on both electrodes by immersing the sample in molecule solution or on one of the electrodes by dip-pen nanolithography. Finally, SWNTs were selectively self-assembled onto the electrodes by putting a drop of SWNT solution on the template. The electron transport through the molecular SAM between the SWNT(s) and the Au electrodes were characterized through gated I-V measurements. The same devices were measured before after the desorption of the molecular SAM(by baking) to directly elucidate the role of the molecules on the electron transport. The results will be presented and discussed. \newline $^{1}$S.A. McGill et al., Appl. Phys. Lett. \textbf{89}, 163123 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S30.00015: Thermal Imaging of Electrically-Heated Carbon Nanotubes using Raman Spectroscopy Scott Hsieh, Vikram Deshpande, Adam Bushmaker, Steve Cronin, Marc Bockrath Suspended carbon nanotubes have been known to exhibit striking negative differential conductance under high bias voltages$^{1}$. To better probe the physics underlying this phenomenon, we have recently developed techniques to measure Raman spectra simultaneously with electrical transport, resulting in the direct observation of mode selective electron-phonon coupling$^{2}$. Using similar techniques, we present spatially resolved data taken from long, suspended, and electrically contacted individual carbon nanotubes. Along with electrical transport, Raman spectra are taken at several points along the spatial coordinate, creating a spatial map of the Raman-active phonon populations and the lattice temperature profile. We use a finite element simulation to corroborate our data with a Landauer model and extract numerical values for key scattering and relaxation rate parameters and thermal contact resistances. This use of Raman spectroscopy constitutes a novel non-contact technique for probing local thermal data in nanostructures. \newline $^{1}$ Pop \textit{et al., Phys. Rev. Lett.} (2005). \newline $^{2}$ Bushmaker \textit{et al., Nano Lett.} (2007). [Preview Abstract] |
Session S31: Focus Session: Multiferroics I: 113 and 125
Sponsoring Units: DMP GMAGChair: Valery Kiryukhin, Rutgers University
Room: Morial Convention Center 223
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S31.00001: Dynamics and Phase Transitions in Multiferroic Helimagnets Hosho Katsura, Shigeki Onoda, Jung Hoon Han, Naoto Nagaosa The strong coupling between magnetism and ferroelectricity in multiferroics has recently been attracting much attention due to the fundamental physics involved and promising applications. The representative materials are helical magnets $R$MnO$_3$ ($R=$Gd,Tb,Dy) and they have been extensively studied experimentally. We theoretically study the dynamics and phase transitions in cycloidal helical magnets showing the multiferroic behavior. Our approach reproduces several novel features such as the anomalous dielectric response revealed by recent experiments on $R$MnO3 [1,2]. We also study the nature of the phase transition from collinear to helical spin structure. [1]N. Kida, Y. Ikebe, Y. Takahashi, J. P. He, Y. Kaneko, Y. Yamasaki, R. Shimano, T. Arima, and Y. Tokura, [arXiv:0711.2733]. [2]A. Pimenov, A. Loidl, A. A. Mukhin, V. D. Travkin, V. Yu. Ivanov, and A. M. Balbashov, [arXiv:0707.3614]. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S31.00002: Novel coupling of Tb- and Mn-magnetic orders in multiferroic TbMnO$_{3}$ D.N. Argyriou, O. Prokhnenko, R. Feyerherm, M. Mostovoy, N. Aliouane, E. Dudzik, A.U.B. Wolter, A. Maljuk We report on diffraction measurements on multiferroic TbMnO$_{3}$
which demonstrate that the Tb- and Mn-magnetic orders are coupled
below the ferroelectric transition $T_{FE}=$28 K. For $T |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S31.00003: Electric polarization reversal in multiferroic TbMnO$_{3}$ with rotating magnetic field direction. Nobuyuki Abe, Shintaro Ohtani, Kouji Taniguchi, Taka-hisa Arima, Taishi Takenobu, Yoshihiro Iwasa Recent extensive studies of magneto-electric multiferroics have revealed that the magnitude and direction of electric polarization can be considerably modified by the application of a magnetic field. TbMnO$_{3}$ is a prototypical multiferroic which shows the electric polarization flop from $P$//$c$ to $P$//$a$ by the application of magnetic field along the $b$ or $a$ axis. We have found that the direction of the magnetic-field induced polarization along the $a$-axis ($P_{a})$ is memorized even in the zero field where $P_{a}$ is absent. The polarization direction can be reversed by rotating the magnetic field direction in the \textit{ab} plane. For the memory application of the multiferroic material, such a bistability in zero field and a switching between the bistable states with some noneverlasting stimulus are essential. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S31.00004: Spiral magnetic order in the ferroelectric phase of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ Yuichi Yamasaki, Y. Tokura, K. Sasai, M. Matsuura, K. Hirota, D. Okuyama, H. Sagayama, N. Abe, T. Arima, Y. Noda Perovskite manganite Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ possesses a ferroelectric phase with an electric polarization along the a axis ($P||a$) in zero magnetic field, while $R$MnO$_3$ ($R$=Tb and Dy) undergo ferroelectric transitions with P along the c axis ($P||c$). The $P||a$ phase emerges upon the incommensurate to commensurate transition of the lattice modulation in a similar way of the magnetic field induced $P||a$ phase of TbMnO$_3$. The polarized neutron diffraction and the magnetic structure analysis of the $^{160}$Gd$^{3+}$-enriched single crystal of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ were performed to uncover the coupling between the magnetic order and the ferroelectric polarization $P||a$ on a microscopic level. We found that the ferroelectric transition occurs concomitantly with the collinear to spiral spin transformation and the spin helicity can be controlled by the electric field applied on cooling. Namely, the ferroelectric polarization in the $P||a$ phase can be explained by the spin current model as well as the $P||c$ phases known for TbMnO$_3$. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S31.00005: Reentrant electromagnons in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ in the H-T phase diagram Rolando Valdes Aguilar, A.B. Sushkov, H.D. Drew, Y.J. Choi, C. Zhang, S-W. Cheong The electromagnon spectra of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ has been measured as a function of magnetic field $H\| c$ up to 8 T and temperature between 5 and 300 K. Three magnetic induced electric dipole features reported earlier\footnote{Valdes Aguilar, et al. PRB \textbf{76}, 060404(R) (2007)} are observed to weaken simultaneously but not shift for increasing field. These electromagnon features all show reentrant behavior as a function of temperature for $H >$ 6 T, and track with the anomalies in the static dielectric constant, confirming their electromagnon origin. While the magnetic structure of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ is unknown, it is assumed that it is a cycloidal magnet where the spins lie in the crystallographic \textit{a-b} plane, based on the behavior of the magnetic susceptibility and the direction of static polarization \textbf{P}. Therefore, it appears that the electromagnon selection rule, $e\|$\textit{a}, in all the multiferroic $R$MnO$_3$ manganites is independent of the spin plane and polarization direction. We will compare the phase diagrams of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ and TbMnO$_3$ where similar anomalies are observed. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S31.00006: Ultrafast pump-probe reflectance study of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$ Diyar Talbayev, Antoinette J. Taylor, Richard D. Averitt, Chenglin Zhang, Sang-Wook Cheong Dynamical studies of multiferroic materials help unravel the fundamental interactions between various degrees of freedom and answer technological questions such as achievable switching speeds in multiferroic-based memory elements. We report the results of the ultrafast pump-probe reflectance study of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$. The material undergoes antiferromagnetic ordering and, upon further cooling, ferroelectric ordering that strongly couples to the material's magnetic state. We measured the relaxation time of the pump-probe reflectance in this compound using 800-nm pump and probe pulses. The temperature dependence of the relaxation time follows that of the low-energy spectral weight that includes phonons and electro-active magnons [1]. This suggests a strong coupling between electronic (1.55 eV) and low-energy electro-active excitations in Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$ that can be tuned by magnetic field. The relaxation time increases upon the application of magnetic field along the crystal's $c$-axis in the ferroelectric phase, but exhibits no change in the paraelectric phase. Our results indicate the importance of multiple energy scales (electronic, lattice, and magnetic) for the multiferroicity of Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$. 1. R. Valdes Aguilar et al, Phys. Rev.B \textbf{76}, 060404(R) (2007) [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S31.00007: Theory of terahertz absorption spectra due to two-magnon processes in cycloidal spin magnets Shin Miyahara, Nobuo Furukawa Ferroelectric perovskite magnets $R$MnO$_{3}$ have an attraction both experimentally and theoretically after the discovery of the ferroelectric polarization and its flop by external magnetic field. Recently the measurements by the terahertz time-dominant spectroscopy have been investigated with the several sets of light polarization, and novel magnon states induced by the electric field are observed. Such a spin excitation is called an electromagnon. However, the origin of the electric-dipole active absorption is not clarified yet. We calculated the absorption of light due to the electric dipole transitions associated with two magnon excitations in cycloidal spin magnets. The theory is applied to the ferroelectric magnets $R$MnO$_{3,}$ and absorption peaks in terahertz time domain spectroscopy, which correspond to electromagnons, are interpreted with reasonable parameter sets. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S31.00008: Resonant magnetic scattering of multiferroic HoMnO$_{3}$ in an applied electric field S. Nandi, A. kreyssig, L. Tan, J.W. Kim, J.Q. Yan, J.C. Lang, D. Haskel, R.J. McQueeney, A.I. Goldman The multiferroic material HoMnO$_{3}$ displays electrical polarization $P_{c}$~=~5.6~$\mu $C~cm$^{-2}$ along the hexagonal \textbf{c} axis below the Curie temperature $T_{C}$ = 875 K and antiferromagnetic Mn$^{3+}$ ordering at the N\'{e}el temperature, $T_{N}$~= 75 K. The recently reported ferromagnetic response of Ho$^{3+}$ by an applied electric field opens up the possibility of electric field controlled magnetic data storage. However, both the role of Ho$^{3+}$ in magnetism and details of the magnetic structure of Ho$^{3+}$ have been topics of significant debate. Using element specific x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, we have focused on resolving this controversy. Both quadrupole and dipole Ho $L_{III}$ resonances were observed below 40 K. In zero field, Ho$^{3+}$ orders antiferromagnetically with moments along the c direction below 40 K and undergoes a transition to a different magnetic order below 4.5 K. The role of Ho$^{3+}$ upon the application of an external electric field in the temperature range 1.7-80~K will be discussed. -- The support by U.S. DOE (DE-AC02-07CH11358 and DE-AC02-06 CH11357) is acknowledged. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S31.00009: Enhanced ferroelectric polarization by induced Dy spin-order in multiferroic DyMnO$_{3}$ O. Prokhnenko, R. Feyerherm, E. Dudzik, S. Landsgesell, N. Aliouane, L.C. Chapon, D.N. Argyriou Neutron powder diffraction and single crystal x-ray resonant magnetic scattering measurements suggest that Dy plays an active role in enhancing the ferroelectric polarization in multiferroic DyMnO$_{3}$ above $T_{N}^{\rm Dy}$=~6.5~K. We observe the evolution of an incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering. It closely tracks the evolution of the ferroelectric polarization. Below $T_{N}^{\rm Dy}$, where Dy spins order commensurately, the polarization decreases to values similar for those of TbMnO$_{3}$. The higher $P_{s}$~found just above $T_{N}^{\rm Dy}$~arises from the contribution of Dy-spins so as to effectively increase the amplitude of the Mn spin-spiral. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S31.00010: Electrically driven spin excitation in a ferroelectric magnet DyMnO$_{3}$ N. Kida, Y. Ikebe, Y. Takahashi, J.P. He, Y. Kaneko, Y. Yamasaki, R. Shimano, T. Arima, N. Nagaosa, Y. Tokura In multiferroic manganites, there have been recent experimental and theoretical arguments on the possibility of the presence of the low-lying spin excitation, called $electromagnon$, where the spin excitation electrically becomes active [1]. Here we report on a complete set of low-energy (1--10 meV) electrodynamics of spin excitations for a multiferroic DyMnO$_3 $ with variations of the light polarization in a variety of phases tuned by both temperature (5--250 K) and magnetic field (0--70 kOe) [2]. We identify the pronounced absorption continuum (1--8 meV) with a peak feature around 2 meV, which is electric-dipole active only for the light $E$-vector along the $a$-axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition, but irrespectively of the direction of the $bc$ or $ab$ spiral spin plane. The possible origin of this electric-dipole active band is argued in terms of the gigantic fluctuations of spins and spin-current. [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] N. Kida {\it et al.}, arXiv:0711.2733 [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S31.00011: Spin structures of magnetic phases in YMn2O5 J.-H. Kim, S.-H. Lee, S. Wakimoto, M. Matsuda, H. Kimura, Y. Noda, S. Juerg, M. Kenzelmann, C.F. Majkrzak, S.-I. Park, S. Park, S-W. Cheong A magnetic ferroelectric material, YMn2O5, undergoes several magnetic phase transitions at low temperatures and develops spontaneous electric polarization along the b-axis in a commensurate magnetic phase with a characteristic wave vector of (0.5,0,0.25). We have determined the commensurate spin structure by performing four circle neutron diffraction (FCD) and three-dimensional polarization analysis (CRYOPAD) on a single crystal of YMn2O5. In the spin structure, Mn4+ moments form a transverse (cycloidal) spiral structure along the c-axis that can induce the spontaneous electric polarization along the b-axis. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S31.00012: Magnetic domains in multiferroic YMn$_$2O$_5$ probed by Spherical Neutron Polarimetry under electric field Carlo Vecchini, Laurent Chapon, Paolo Radaelli, Aziz Daoud-Aladine, Jane Brown, Tapan Chatterji, Soonyong Park, Sang-Wook Cheong Precise determination of the magnetic structures in multiferroics RMn$_2$O$_5$ (R: Y, Ho, Bi) have been obtained by single crystal neutron diffraction. The analysis shows the presence of zig-zag antiferromagnetic chains in the ab-plane. An additional weak magnetic component parallel to the c-axis was detected which is modulated in phase quadrature with the a-b components. The nature and population of the coexisting antiferromagnetic domains in YMn$_2$O$_5$ have been determined by Spherical Neutron Polarimetry under an external electric field. We have proved that reversing the electrical polarity results in the inversion of the population of two types of antiferromagnetic domains, with opposite in-plane spin components. This analysis strongly supports theories in which the coupling of the magnetic configuration to the ferroelectric polarisation is due to magnetic exchange striction and likely not related to the small cycloidal modulation in the bc-plane. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S31.00013: Electromagnon H-T phase diagram of multiferroic TbMn$_2$O$_5$ Dennis Drew, Andrei Sushkov, Rolando Valdes Aguilar, Soongyong Park, Sang-Wook Cheong We report the results of infrared (5--250 cm$^{-1}$) transmission study of multiferroic TbMn$_2$O$_5$ as a function of temperature (3--300~K) and H$\|$a for magnetic fields up to 8 T. Our major observation is that the main electromagnon feature softens without splitting with increasing field at T=5 K. This observation is in contrast with the gradual suppression of electromagnons without shifting by magnetic field in RMnO$_3$ compounds. Also, it is in agreement with the theoretical prediction of Fang and Hu \footnote{C. Fang and J. Hu, condmat/0703487} for this system. In this talk, we will also discuss other features of the magnetic field dependence of the low energy electromagnon excitations in multiferroic TbMn$_2$O$_5$. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S31.00014: Far Infrared Anomalies in Orthorhombic Multiferroics (Bi,Pr)Mn$_{2}$O$_{5}$ N.E. Massa, D. de Sousa Meneses, P. Echegut, M.J. Mart\'Inez-Lope, J.A. Alonso We report on far infrared reflectivity between 4 K and 300 K of polycrystalline BiMn$_{2}$O$_{5}$ y PrMn$_{2}$O$_{5}$ known to sustain orbital, lattice, charge, and spin interactions. After conventional temperature mode stiffening band profiles undergo a relative intensity raise with maximum in the interval $\sim $40K to $\sim $30 K (magnetic ordering temperature, T$_{N}$, and the onset of ferroelectricity, T$_{C})$ indicative of a global electric polarization. We do not observe on cooling, in agreement with our high resolution neutron diffraction patterns, new phonon activity that might be associated to structural changes. Below 30 K there is a weak reflectivity attenuation at about the temperature in which the spin glass sets in. In contrast with BiMn$_{2}$O$_{5}$, in PrMn$_{2}$O$_{5}$ we found a Drude shaped overdamped band centered at zero frequency that, active at all temperatures, develops substructure below 15 K. We associate this feature to collective modes responsible for ferroelectricity of magnetic origin ascribed to coupling spins with electronic polarization without atomic displacement In overall, our spectra suggest a qualitative agreement with magnetoferroelectricity originating in spin dislocation and commensuration. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S31.00015: Magnetic X-ray scattering in multiferroic HoMn2O5 Laurent Chapon, Guillaume Beutier, Alessandro Bombardi, Carlo Vecchini, Paolo Radaelli, S. Park, Sang-Wook Cheong The commensurate magnetic phase of multiferroic HoMn2O5 has been studied by x-ray magnetic scattering off resonance and at the L3 edge of Holmium. The magnetic ordering of the Manganese ions below 40 K induces a magnetic order of Ho. Due to the element selectivity of the technique we were able to extract the temperature dependence of the Ho ordering up to temperature very close to TN. Azimuthal scans confirm the recent model of the magnetic structure determined from single crystal neutron diffraction data, for both the Manganese and Holmium ions. The d-f coupling is discussed in the light of these results. [Preview Abstract] |
Session S32: Focus Session: Magnetic Tunneling
Sponsoring Units: GMAG DMP FIAPChair: Jack Bass, Michigan State University
Room: Morial Convention Center 225
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S32.00001: Bound states in electronic transport through Fe/MgO tunnel junctions Ivan Rungger, Nadjib Baadji, Stefano Sanvito Using the ab initio code Smeagol we calculate the electronic transport properties of Fe/MgO/Fe(100) tunnel junctions for applied bias up to 2 Volt. The correct bias-dependent occupation of the interface states (IS) in the Fe/MgO junction is crucial to obtain a physically meaningful potential drop. The coupling of the IS to the Fe electrodes varies strongly for different k-points, and bound states are found along the high symmetry lines and at those k-points where there are no open channels in the Fe leads. A general method for setting the occupation of both weakly-coupled and bound states, based on the notion of a local Fermi energy and a finite relaxation time, is presented. For parallel alignment of the Fe leads the current through the IS is quenched above 20 mV, whereas for the anti-parallel alignment the current flows mainly through IS up to about 0.4 V. In this bias range the TMR shows a pronounced bias dependence, at higher voltages it decays smoothly. If the transmission through the IS is suppressed, for example by adding a finite imaginary part to the energy, the TMR decays monotonically with bias even at low voltage. Finally we show that oxygen vacancies inside the MgO barrier quench the TMR if they are within the first few layers from the interface, and that a similar reduction is achieved by partially oxidizing the Fe interface layer. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S32.00002: Fabrication and characterization of fully epitaxial Fe/MgO/Fe magnetic tunnel junctions on Si(100) and the influence of MgO buffer layer G. X. Miao, M. V. Veenhuizen, M. Munzenberg, J. S. Moodera Spin injection into Si can integrate the spin degree of freedom into future semiconductor technology. Coherent tunneling through epitaxial MgO barrier due to symmetry filtering is expected to yield large spin poarization [1]. Here we report the MBE growth of fully epitaxial Fe/MgO/Fe MTJ stacks on top of Si (100) with epitaxial MgO buffer layer giving rise to a large TMR. Due to the large lattice mismatch between Si and MgO ($\sim $9.6{\%}), the crystalline structure of MgO is critically dependent on its thickness, and the influence is subsequently passed on to the MTJ structure. We observe a systematic change of TMR with the buffer layer thickness, which we attribute to the effect of dislocations. Coherent growth of MTJ on top of Si is a significant step spin injection into Si [2,3]. 1. W. H. Butler et al., Phys. Rev. B, 63, 054416 (2001). 2. Ian Appelbaum et al., Nature 447, 295 (2007). 3. B. T. Jonker et al., Nature Physics 3, 542 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S32.00003: Temperature and Angular Dependences of Dynamic Spin-Polarized Resonant Tunneling Casey W. Miller, Johan \AA kerman, Yan Zhou, Renu Dave, Jon Slaughter, Ivan K. Schuller The bias dependence of tunneling magnetoresistance oscillations due to dynamic resonant tunneling in CoFeB/MgO/NiFe magnetic tunnel junctions was studied as functions of temperature and the relative magnetization angle of the two magnetic layers. The effect of temperature is consistent with thermal smearing, while that of the relative magnetic orientation was typical of a spin valve. A model of tunneling between spin-split free electron bands using the exact solution of the Schr\"{o}dinger equation for a trapezoidal tunnel barrier agrees with experiment.\\ ~\\ C.~W.~Miller \textit{et al.}: J. Appl. Phys., In Press (2008); Phys. Rev. Lett. {\bf 99}, 047206 (2007); Appl. Phys. Lett. {\bf 90}, 043513 (2007); Phys. Rev. B {\bf 74}, 212404 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:42PM |
S32.00004: Transport and spin transfer torques in Fe/MgO/Fe tunnel barriers. Invited Speaker: The prediction of very high tunneling magnetoresistance (TMR) ratios in crystalline Fe/MgO/Fe [1,2] tunnel junctions has been verified by a number of experiments [3,4]. The high TMR can be understood in terms of the electronic structure of the system. In MgO the $\Delta_1$ states at the Brillouin zone center decay the most slowly and dominate the tunnelling current. For coherent interfaces, which are achievable due to the small lattice mismatch between Fe and MgO, these $\Delta_1$ states at the Brillouin zone center are half-metallic in the Fe layers. The dominance of the $\Delta_1$ states and their half-metallicity cause the high tunnelling magnetoresistance measured in Fe/MgO/Fe tunnel junctions [5]. For the spin transfer torque, we calculate the linear response for small currents and voltages. Our calculations show that the half metallicity of the Fe $\Delta_1$ states leads to a strong localization of the spin transfer torque to the interface. As a result, the linear current dependence of the torque in the plane of the two magnetizations is independent of the free layer thickness for more than three monolayers of Fe. For perfect samples we also find a linear current dependence of the out-of-plane component. However, this linear piece oscillates rapidly with thickness and averages to zero in the presence of structural imperfections like thickness fluctuation, in agreement with experiment [6]. In this talk I discuss the bias dependence of the TMR and spin transfer torque effects mentioned above and the influence on them of the following factors: the interface structure Fe/MgO, the barrier thickness, and the structure of the leads [7]. This work has been supported in part by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [1] W. Butler, X.-G. Zhang, T. Schulthess, J. MacLaren, Phys. Rev. B 63 (2001) 054416. [2] J. Mathon, A. Umerski, Phys. Rev. B 63 (2001) 220403. [3] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nature Materials 3 (2004) 868. [4] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yang Nature Materials 3 (2004) 862. [5] C. Heiliger, P. Zahn, I. Mertig, Materials Today 9 (2006) 46. [6] J. C. Sankey, P. M. Braganca, A. G. F. Garcia, I. N. Krivorotov, R. A. Buhrman, and D. C. Ralph, Phys. Rev. Lett. 96 (2006) 227601. [7] C. Heiliger, M.Gradhand, P. Zahn, I. Mertig, Phys. Rev. Lett. 99 (2007) 066804. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S32.00005: Tunneling Magnetoresistance in MgO based double-barrier Magnetic Tunnel Junctions Weigang Wang, Chaoying Ni, Q. Wen, H.W. Zhang, Takahiro Moriyama, John Xiao Double-barrier magnetic tunnel junctions (DMTJs) have attracted much attention due to their fertile physics and promising applications in spintronics devices. Here we report the fabrication and characterization of DMTJs of Si/SiO$_{2}$/Ta 7/Ru 20 /Ta 7/CoFe 2/ IrMn 15/CoFe 2/Ru 1.7 /CoFeB 3/ MgO 2/ CoFeB 3 /MgO 2/CoFeB 3 /Ru 1.7/ CoFe 2/IrMn 15/ Ta 8/Ru 10, where the numbers are layer thickness in nanometers. Single barrier MTJs with similar structure were also fabricated. While the DMTJs exhibit the tunnel magnetoresistance (TMR) of 185 {\%} at room temperature, which is the highest value in DMTJs reported to date, the corresponding single barrier MTJ shows 300{\%} TMR. The reduction of TMR in DMTJs is understood in terms of sequential tunneling through two junctions in serials. The effects of annealing temperature and bias voltage on the TMR of DMTJs will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S32.00006: Influence of spin accumulation on superconducting properties of aluminum layers in magnetic double tunnel junction devices See-Hun Yang, Hyunsoo Yang, Saburo Takahashi, Sadamichi Maekawa, Stuart Parkin We discuss the influence of spin accumulation on the superconducting (SC) properties of thin aluminum layers in crystalline MgO barrier based magnetic double tunnel junction devices composed of ferromagnet-insulator-superconductor-insulator-ferromagnet (FISIF) structures. Below the Al SC transition temperature, when the magnetization directions of the two outer CoFe ferromagnetic layers are antiparallel, the SC energy gap of the Al layer is suppressed, as compared to the case for parallel orientation of these same layers. This is consistent with theoretical models in which spin polarized quasi-particles are accumulated in the SC layer. The accumulated spin depends on the rate at which spin polarized current is injected and leaves and, most importantly, on the spin relaxation rate of the injected quasi-particles. We discuss the dependence of the spin accumulation on the spin-injection rate, which can be varied at a fixed voltage, by varying the MgO barrier thickness. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S32.00007: Enhanced tunneling spin polarization by amorphizing usually crystalline CoFe alloys without any glass forming additives Li Gao, Xin Jiang, See-Hun Yang, Philip M. Rice, Stuart S.P. Parkin Ferromagnetic alloys of Co and Fe are particularly useful electrodes in magnetic tunnel junctions because they exhibit high spin polarization and high Curie temperatures as well as high thermal stability. These alloys are crystalline but they can be made amorphous by adding suitable glass-forming elements such as Boron. Here we show that films of pure CoFe alloys can be made amorphous, without the need of any additives, by sandwiching them between two amorphous layers, a tunnel barrier formed from amorphous Al$_{2}$O$_{3}$ and an amorphous overlayer. The films are amorphous when thinner than $\sim $25 {\AA} but are crystalline for thicker layers. We find that both the tunneling magnetoresistance and the tunneling spin polarization (measured using superconducting tunneling spectroscopy in related junctions) are significantly enhanced when the alloy is amorphous. However, by heating the alloy above its glass crystallization temperature the enhancement is observed to vanish. Possible reasons for this behavior are discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S32.00008: Tunneling Anisotropic Magnetoresistance in Co/AlO$_{x}$/Au Tunnel Junctions Ruisheng Liu, Lukasz Michalak, Carlo Canali, Lars Samuelson, Hakan Pettersson We observe spin-valve-like effects in nano-scaled thermally evaporated Co/AlO$_{x}$/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S32.00009: Transport Studies of reduced RA product MTJs produced by highly charged ion irradiation Joshua Pomeroy, Holger Grube As is commonly known, magnetic tunnel junctions (MTJs) are used for hard drive read heads and are actively being developed for MRAM. In both of these cases, the resistance-area (RA) product is a critical parameter for device speed and bandwidth as well as total power dissipation (particularly for current driven devices). We present transport studies of MTJs whose barrier oxide has been partially reduced by highly charged ions (HCIs). The bias, temperature, and applied field dependence of these devices will be discussed. Beyond magnetic memory applications, HCI modified MTJs provide a compelling new route for preparing order THz spin torque oscillators. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S32.00010: Large voltage from spin pumping in magnetic tunnel junctions Zhifang Lin, Siu Tat Chui We studied the response of a ferromagnet-insulator-normal metal tunnel structure under an external oscillating radio frequency (R.F.) magnetic field. The D. C. voltage across the junction is calculated and is found not to decrease despite the high resistance of the junction; instead, it is of the order of $\mu V$ to $100\mu V$, much larger than the experimentally observed value (100 nano-V) in the ``strong coupled'' ohmic ferromagnet-normal metal bilayers. This is consistent with recent experimental results in tunnel structures, where the voltage is larger than $\mu V$s. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S32.00011: Nature of Voltage Dependence of Spin Torque in Magnetic Tunnel Junctions M. Chshiev, I. Theodonis, A. Kalitsov, N. Kioussis, W. H. Butler Current-induced spin torque [1] is important because it may elucidate fundamental physics and because it may have useful applications. We have recently shown [2] that in magnetic tunnel junctions, both in-plane (Slonczewski) and perpendicular-to-the-plane (field-like) components of the spin torque behave nontrivially as a function of applied bias. Recent experiments [3] are in agreement with these predictions. Here we present a systematic study of voltage-induced spin torque in magnetic tunnel junctions and provide an insight into the nature of its voltage behavior. The explanation is given in terms of the spin and charge current dependence on the interplay between evanescent states in the insulator and the Fermi surfaces of the ferromagnetic electrodes comprising the junction. Calculations are based on the Keldysh formalism with non-equilibrium Green functions technique. [1] J. C. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996) [2] I. Theodonis, N. Kioussis, A. Kalitsov, M. Chshiev, and W. H. Butler, Phys. Rev. Lett. 97, 237205 (2006) [3] J. C. Sankey et al, Nature Physics (2007); H. Kubota et al, Nature Physics, ibid. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S32.00012: Space-Charge Induced Pauli Blockade Effect in Tunnel Junctions with Half-Metallic Electrodes A. P. Li, T.-H. Kim, X.-G. Zhang, J. F. Feng, X. F. Han, Y. Wang, J. Zou, D. B. Yu, H. Yan A space-charge induced Pauli spin blockade effect has been observed in the magnetic tunnel junction consisting of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) electrodes and SrTiO$_{3}$ barrier at temperatures up to 100 K. The blockade voltage under zero magnetic field provides a direct measurement of the energy gap between the Fermi energy and the top of the minority spin valence band of the LSMO. Outside the spin blockade regime, the low temperature conductance oscillates with the bias voltage as the trap centers are charged by electrons. The spin blockade can be lifted when the trap levels are thermally activated or when a large magnetic field lowers the LSMO minority spin mobility edge to below the Fermi energy. A very large magnetoresistance up to 10000{\%} is observed and is correlated to the blockade effect. $^{\ast }$Email: apli@ornl.gov [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S32.00013: First-principles theory of nonequilibrium vertex correction:disordered magnetic tunneling junction Youqi Ke, Ke Xia, Hong Guo The Keldysh Non-equilibrium Green's Function (NEGF) formalism has been developed Within DFT to calculate the electronic structure of disordered system at finite bias based on TB-LMTO+ASA combined with Coherent Potential Approximation. The conditionally averaged NEGF is evaluated by including the vertex correction. The disordered Fe/Va/Fe tunneling junctions are investigated with present first principle method, the vertex correction shows important role in both the electronic structure and transport calculations. The bias dependence of Tunneling Magnetoresistance (TMR) in the disordered Junctions and the disordered dependence of TMR at a finite Bias were illustrated. [Preview Abstract] |
Session S33: Focus Session: Mostly III-V Semiconductors
Sponsoring Units: GMAG FIAP DMPChair: Steve von Molnar, Florida State University
Room: Morial Convention Center 224
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S33.00001: Strain-induced Fermi contour anisotropy of GaAs (311)A 2D holes Javad Shabani, Mansour Shayegan, Roland Winkler There is considerable current interest in electronic properties of two-dimensional (2D) carriers whose energy bands are spin-split at finite values of in-plane wave vector, thanks to the spin-orbit interaction and the lack of inversion symmetry. We report experimental and theoretical results revealing that the spin-subband Fermi contours of the heavy and light heavy-holes (HHh and HHl) can be tuned in high mobility GaAs (311)A 2D hole systems via the application of symmetry-breaking in-plane strain. Our calculations show that the HHl spin-subband Fermi contour is circular but the HHh spin-subband Fermi contour is distorted. Experimentally, we probe the Fermi contour anisotropy by measuring the magneto-resistance commensurability peaks induced by square arrays of antidots. When the spin splitting is sufficiently large, the magneto-resistance trace exhibits two peaks, providing clear evidence for spin-resolved ballistic transport. The experimental results are in good agreement with the calculations, and confirm that the majority spin-subband (HHh) has a severely distorted Fermi contour whose anisotropy can be tuned with strain while Fermi contour of the minority spin-subband (HHl) remains nearly isotropic. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S33.00002: Strain-induced spin splittings in III-V and II-VI semiconductors B.J. Moehlmann, Michael E. Flatt\'e We have calculated the strain-induced spin splittings in III-V and II-VI semiconductors using a fourteen-band basis for a strain-dependent $k \cdot p$ Hamiltonian. Using deformation potentials from pseudopotential calculations we find quantitative agreement with the precessional rates observed in [1]. For GaAs, the contribution of upper-conduction-band deformation potentials to the strain-induced spin splitting is not negligible. The ratio of the strain-induced spin precession frequency to the drift velocity is similar for GaAs and InAs, but is an order of magnitude larger for GaSb and InSb. For ZnSe it is a factor of 2 smaller than GaAs. This work was supported by an ONR MURI. [1] Kato et al., Nature 427, 50 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S33.00003: Electric field dependent $g$ tensor modulation for a hydrogenic Si donor in bulk GaAs Amrit De, Craig E. Pryor, Michael E. Flatt\'e We present calculations demonstrating the feasibility of electrical manipulation of a donor bound electron spin using $g$ tensor modulation resonance. We calculate the electron $g$ factor as a function of applied electric field, and show that $dg/dE$ is largest when the applied magnetic and electric fields are parallel. Unlike quantum dots, the donor's $g$ factor is highly nonlinear as a function of the applied magnetic field, and the anisotropy of the $g$ tensor also has a strong magnetic field dependence. The calculations are for a Si impurity in GaAs, and are done using real-space 8-band k.p theory in the envelope approximation. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S33.00004: Gilbert Damping in (Ga, Mn) As Ion Garate, Allan MacDonald We examine Gilbert damping in (Ga,Mn)As by using a p-d mean-field theory model for the ferromagnetic ground state and a four-band spherical model for the host semiconductor band structure. Within this model it is possible to calculate impurity vertex- corrections to the long-wavelength spin-spin response functions to all orders. Because of spin-orbit coupling in the band structure, beyond leading order vertex corrections make significant contribution to the damping. We comment on the non-monotonic dependence of damping on impurity strength. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S33.00005: Magnetic patterning of GaMnAs layers by hydrogen passivation L. Thevenard, A. Lemaitre, G. Faini, N. Vernier, J. Ferre, S. Fusil In order to study the magnetic switching behavior of diluted magnetic semiconductor (DMS) microstructures, we have patterned thin layers of (Ga,Mn)As by an original method. We have used local hydrogen passivation to locally suppress the carrier-mediated ferromagnetic phase. This purely diffusive process maintains the continuity of the film and smoothes border effects. Two types of structures were examined by Kerr microscopy, and their magnetic behavior compared to that of structures made by dry etching$^{1}$. On hydrogenated arrays of micron-sized magnetic dots, the switching fields were closer to the continuous film coercivity, and with a smaller dispersion. On micron-wide magnetic stripes, current-induced domain wall (DW) propagation was observed with typical currents as low as j = 4.10$^{5}$ A.cm$^{-2}$, a result of the low edge roughness induced by the patterning, and the low density of magnetic atoms. Local hydrogen passivation therefore appears as a viable route towards lower injection currents in DMS spintronics devices based on DW manipulation. $^{1}$L. Thevenard et al., Appl. Phys. Lett. 91, 142511 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S33.00006: Infrared longitudinal and Hall conductivities in Ga$_{1-x}$Mn$_{x}$As films Gheorghe Acbas, M.-H. Kim, J. Cerne, M. Cukr, V. Novak, T. Jungwirth, J. Sinova We determine the complete infrared (0.1-1.2 eV) magneto-conductivity tensor of a series of Ga$_{1-x}$Mn$_{x}$As films from the complex Faraday and Kerr angles as outlined in M.-H. Kim, et al., Phys. Rev. B 75, 214416 (2007). A systematic series of samples with varying Mn and hole concentrations is studied. The samples range from insulating to metallic. The frequency dependence of the real part of the longitudinal conductivity $\sigma _{xx}$ is consistent with the values determined from transmission and reflection measurements. The complex transverse (Hall) conductivity $\sigma _{xy}$ shows resonances associated with the inter-valence band transitions. As the Mn concentration decreases these transitions become broadened due to increased disorder. The temperature dependence shows non-monotonic behavior with sign changes at certain wavelengths. The data is compared with predictions from a disordered valence band model (T. Jungwirth, et al., Phys. Rev. B 76, 125206 (2007)). This work is supported by the Research Corporation Cottrell Scholar Award (Buffalo and Texas A$\backslash ${\&}M), NSF-CAREER-DMR0449899 (Buffalo), an instrumentation award from the CAS, Univ. at Buffalo, ERAS-CT-2003-980409 (Prague)and NSF-CAREER-DMR-0547875 (Texas A$\backslash ${\&}M). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S33.00007: An n-type tunable two-dimensional ferromagnetic semiconductor Angelo Bove, F. Altomare, N. Kundtz, A. Chang, Y.J. Cho, X. Liu, J. Furdyna In the past two decades ferromagnetic semiconductors have been the focus of intense studies because of their potential technological application for spintronics. Particular attention has been dedicated to III-V Diluted Magnetic Semiconductors (DMS), where the ferromagnetism (FM) is hole-mediated and the Curie temperature can therefore be tuned by changing the concentration of free carriers\footnote{T. Dietl \textit{et al.}, Phys. Rev. B \textbf{63}, 195205 (2001)}. In these structures, the Anomalous Hall Effect (AHE) has played a key role in establishing that FM is hole-mediated. We will present data that show the first evidence of electron-mediated FM in GaMnAs. Our heterostructure has a low carrier density ($\sim$ $1.1E12 cm^-2$), a mobility of $\sim$ $600 cm^2/(Vs)$ and excellent gating capabilities. We will also present data that show the first clear bound on the AHE in an electron-mediated DMS and find it much reduced in magnitude when compared to the case of hole-mediated FM. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S33.00008: Electrically-induced Spin Coherence by Ultrafast Electrical Spin Injection. B. Beschoten, L. Schreiber, J. Moritz, C. Schwark, G. Guentherodt, X. Lou, P. Crowell, C. Adelmann, C. Palmstrom Efficient electrical spin injection from a ferromagnet into a semiconductor has been demonstrated for various material systems by steady-state experiments. We introduce a novel time-resolved technique based on electrical pumping and optical probing. As a pump we apply ultrafast current pulses ($\sim $200ps) to electrically inject spin packets from an iron layer through a reverse biased Schottky barrier into a n-GaAs layer. Spin coherence in the semiconductor is probed by subsequent spin precession in a transverse magnetic field using time-resolved Faraday rotation. We observe spin precession for current pulse widths down to 200 ps. The spin polarization of the spin packets is directly measured by Faraday rotation and is found to increase linearly with the current pulse width for pulses shorter than 3 ns at small magnetic fields. This finding together with independent measurements of the samples' high frequency bandwidth indicate that even shorter than 200 ps pulses might be used for generating coherent spin currents in our devices. Work supported by BMBF, DFG and HGF. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S33.00009: Electrical detection of spin polarized current in semiconductors using Andreev reflection Partha Mitra, Meng Zhu, Nitin Samarth Electron transport across the interface between a superconductor and a normal conductor is sensitive to the spin populations of electrons in the conduction band of the latter, leading to the phenomenon of Andreev reflection. A characteristic non-linear behavior in differential conductance is observed in superconductor/ferromagnet bilayers, with a suppression of the conductivity below the superconducting gap. This allows the carrier spin polarization in the normal conductor to be extracted. We attempt to exploit Andreev reflection for measuring hole spin diffusion lengths in p-GaAs by studying a series of hybrid heterostructures of superconducting metal ( In or Nb) and a ferromagnetic semiconductor (GaMnAs), separated by p-GaAs spacer layers of different thickness. Qualitatively, our data show evidence for a finite spin polarization in the current that decreases with spacer thickness. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S33.00010: Reconfigurable spin logic gate in Gallium Arsenide C. Awo-Affouda, O.M.J. van 't Erve, M. Holub, C.H. Li, A.T. Hanbicki, G. Kioseoglou, B.T. Jonker Electrical injection and detection of pure spin currents has recently been shown in semiconductors. Here we concentrate on the realization of spin-based logic circuits in semiconductors. We report on the electrical injection and detection of spin polarized currents using reconfigurable magnetic contacts. Fe/GaAs Schottky contacts are used to create and analyze the spin current in a GaAs transport channel. Non-local detection techniques show that the circuit output voltage can be modulated using current carrying wires that independently switch the magnetization of the contacts. We use this effect to generate a logic function based on pure spin transport in semiconductors. The realization of this integrated spin-based structure may facilitate the development of pure spin-based logic gates. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S33.00011: Inversion of Ferromagnetic Proximity Polarization in GaAs by MgO Interlayers Y. Li, Y. Chye, Y.F. Chiang, K. Pi, W.H. Wang, J.M. Stephens, S. Mack, D.D. Awschalom, R.K. Kawakami Ferromagnet/semiconductor hybrid structures are building blocks for spin transport devices and spin-based logic gates for large- scale circuits. Recent experiments achieved success in making a lateral Fe/GaAs spin transport device, but anomalous bias dependence of the spin injection signal demand an understanding of the role of atomic-scale interfacial structure in determining the spin dependent reflection and transmission coefficients. In our studies, we incorporate a spin-filtering material MgO in the Fe/GaAs structure, and directly study the spin dependent reflection properties of the interface, or the ferromagnetic proximity polarization (FPP) effect, through ultrafast optical measurements. We find that the FPP in Fe/MgO/GaAs can be tuned by controlling MgO thickness, and we observe a sign change by MgO interlayers. Through study of the related nuclear spin polarization, we also observed sign change of FPP with laser intensity when MgO thickness is in the transition range of sign change. By modification of the interface, mainly changing oxygen partial pressure during MgO growth, we find that the Fe-Mg bond is a key factor in the sign change. Supported by CNID, ONR and NSF. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S33.00012: GaMnAs-based hybrid multiferroic memory device . M. Overby, A. Chernyshov, L.P. Rokhinson, J.K. Furdyna, X. Liu In a ferromagnetic semiconductor GaMnAs grown on GaAs there are two equivalent easy axes of magnetization along the [100] and [010] crystallographic directions. These two directions can form a basis for a memory device with the binary state being encoded in the direction of the magnetization. The state can be electrostatically controlled by introducing a compressive (tensile) strain along one of the easy axes. We demonstrate a novel non-volatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs and a piezoelectric material. ~The magnetization direction is monitored via planar Hall effect, which changes sign when magnetization rotates. At zero voltage on the piezoelectric magnetization can be oriented either along [100] or [010], when voltage on piezoelectric is swept between positive and negative values magnetization forms a hysteresis loop with abrupt transitions between the two orientations. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S33.00013: Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: First-principles calculations A.N. Chantis, K.D. Belashchenko, D.L. Smith, E.Y. Tsymbal, M. van Schilfgaarde, R.C. Albers A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin density approximation we calculate spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts [1,2] and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions [3]. \newline [1] S. A. Crooker {\it et al.}, Science {\bf 309}, 2191 (2005) \newline [2] X. Lou {\it et al.}, Nature Phys. 3, {\bf 197} (2007) \newline [3] J. Moser {\it et al.}, Appl. Phys. Lett. {\bf 89}, 162106 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S33.00014: Spin-Polarized Edge-Emitting Lasers M. Holub, A.T. Hanbicki, C.S. Kim, G. Kioseoglou, O.M.J. van 't Erve, C.H. Li, I. Vurgaftman, J.R. Meyer, B.R. Bennett, B.T. Jonker Semiconductor lasers driven by a spin-polarized current are expected to provide a threshold current reduction, optical polarization control, and intensity stabilization. We explore these possibilities in edge-emitting lasers where the low switching fields of in-plane magnetized Fe thin films should enable electronic modulation of the output polarization. Fe/AlGaAs/GaAs heterostructures are designed, grown, and fabricated into surface-emitting light-emitting diodes (LEDs) and double heterostructure lasers. The LED emission is dominated by an H-band feature at 5 K and by the bulk recombination feature at 20 K. An electron spin polarization of 24{\%} is measured in the Faraday geometry. Oxide-stripe lasers are fabricated with and without an Fe capping layer. Lasing is observed at low temperatures with threshold current densities of $\sim $100 A/cm$^{2}$. Magnetic field-dependent studies to examine the effects of spin injection on laser performance will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S33.00015: Spin Injection into Co$_{2}$MnAl by Optical Absorption in GaAs S. Isber, Y.J. Park, J.S. Moodera, D. Heiman Ferromagnet-semiconductor heterostructure devices have been made with half-metallic ferromagnets, which have larger spin polarizations than transition metal based ferromagnets. Spin-polarized electrons generated in the semiconductor by circularly polarized light are injected across a Schottky barrier into the ferromagnet and detected as photocurrent. In this technique, the optically pumped semiconductor is the source of spin polarized electrons, and the ferromagnet is the detector of spin polarized electrons. The heterostructures consist of MBE-grown Co$_{2}$MnAl Heusler ferromagnets which are lattice matched to n$^{+}$GaAs, forming a Schottky barrier. A magnetic field is applied parallel to the light and perpendicular to the ferromagnetic layer, allowing the magnetization to be switched from parallel to antiparallel to the light direction, thus switching the direction of the detected spin polarization. Spin-dependent photocurrent was measured as a function of applied voltage bias across the Schottky barrier. The injection of spin-polarized electrons was determined after subtracting the magnetic circular dichroism (MCD) effect. [Preview Abstract] |
Session S35: Focus Session: Nanotechnology I
Sponsoring Units: FIAPChair: Keith Williams, University of Virginia
Room: Morial Convention Center 227
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S35.00001: Deep UV Pattern Definition in PMMA Brian Burke, Timothy Herlihy, Andrew Spisak, Keith Williams We have patterned polymethyl methacrylate (PMMA) resist by exposing it with the fifth harmonic (213 nm) of an Nd:YAG source through metallized apertures in contact with the resist. Interference patterns with both near- and far-field origins were observed. In order to test the contrast and uniformity of exposure, we deposited germanium onto developed areas to form arrays with feature sizes of approximately 200 nm. We present a straightforward model for interference effects generated in our process, and discuss opportunities for direct-write lithography through single apertures. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S35.00002: Low Temperature Growth of ZnO Nanorods by Chemical Bath Method Matt Baumer, Parameswar Hari, Daryl Spencer ZnO nanorods grown by chemical bath deposition (CBD) methods are of great interest in photonic and electronic device applications because they offer low cost, low temperature techniques compared to conventional vapor deposition and sputtering methods. Our past studies of ZnO nanorods were grown by CBD on indium tin oxide (ITO) coated glass substrates employed heating of an equimolar solution of Zinc (II) nitrate and hexamethylenetramine solution at 95 C. Morphology of ZnO nanorods exhibited both open and closed hexagonal shape under various deposition conditions at or above 95 C. Currently we are studying the effect substrates~ on the morphology of the~ nanorods grown by chemical bath technique. We are using glass coated with tin oxide, aluminum, as well as solid molybdenum as~substrates. We will use scanning electron microscopy and atomic force microscopy to map the changes in morphology of nanorods grown on various substrates. We will present quantitative data on changes in cluster size and shape of nanorods as the growth substrate is varied. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S35.00003: Nano-assembly and Controlled Release Kinetics of Nanoelements from Nanoporous Templates E. Gultepe, D. Nagesha, J. McNulty, S. Sridhar Nanotemplates and nanoparticles have potential for use in the area of nanomanufacturing and biomedical applications. We are using highly ordered nanoporous alumina as a template for drug delivery and to assemble nanoelements such as latex beads and single wall carbon nanotubes (SWNT) by the means of electrophoresis and/or dielectrophoresis. The results of 100{\%} assembly of latex beads and controlled elution of drugs from nanoporous templates will be discussed. Vertically assembled SWNT and with the I-V characteristic as 3D interconnects, will also be presented. We have developed a variety of platforms incorporating superparamagnetic iron oxide nanoparticles for targeted delivery, magnetic hyperthermia and as a contrast agent for magnetic resonance imaging. The results of cell studies on these platforms will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S35.00004: Ionic PN and PNP junctions -- Diodes and Transistors Eric Kalman, Ivan Vlassiouk, Pavel Apel, Zuzanna Siwy There are well-known devices for controlling the transport of electrons, but very few control ions in a solution. We have prepared ionic diodes and transistors that function in a similar manner to their semiconductor analogues. Ionic PN junctions were created by surface patterning single conical nanopores in polymer films, so that the pore walls are split into two sections: one with positive charge, and the other with negative. These diodes can achieve rectification degrees of several hundreds. Ionic PNP junctions were created by surface patterning single double-conical nanopores in polymer films with tip diameter between 2 and 6 nm, so that the pore walls are split into three sections: the two areas near the large pore openings which are positively charged, while the center of the pore, near the pore tip, is negatively charged. This device works in a similar fashion to a semiconducting BJT transistor, and we show that we can control the electric potential chemically in a manner sufficient to gate the ion current through the device. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S35.00005: Metallic glass nanowire Koji Nakayama, Yoshihiko Yokoyama, Guoqiang Xie, Qingsheng Zhang, Mingwei Chen, Toshio Sakurai, Akihisa Inoue Metallic glass nanowires were spontaneously created on the fracture surfaces that were produced by a conventional mechanical test. The presence of the nanowires is directly related to the one-dimensional meniscus configuration with a small viscosity at high temperatures and to the wide supercooled liquid region of the metallic glass. The electron microscopic observations demonstrate the diameters, the lengths, and the amorphous structural states, and the energy dispersive X-ray reveals the chemical components. In addition, we found that round ridges are constructed from nanotubes. The finding of amorphous nanostructures provides not only fundamental understanding of fracture processes but also give a new insight into nano-science and engineering. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S35.00006: CO Gas Sensing with ZnO Nanowire Mat Sirisha Chava, Daqing Zhang, Chris Berven In the past decade, significant advances have been made in the synthesis of ZnO nanostructures. Here, we report the electrical properties of a gas sensor constructed from mats of ZnO nanowires grown on sapphire substrate that showed reversible response to CO exposure. The sensor is a two-terminal design, where the terminals consist of two 25 $\mu $m diameter gold wires laid parallel on the nanowire mat about $\sim $2 mm apart. The nanowires had an average diameter of 50 nm with lengths of about 10 $\mu $m. The mat was about 20 $\mu $m thick and extended over area of about 1 cm$^{2}$. When exposed to Ar, CO$_{2}$, H$_{2}$ no significant changes in the current-voltage (I-V) behavior of the mat were observed. But CO exposure resulted in a dramatic increase in electrical conductivity, with the current increasing by about a factor of four. The response was reversible after evacuation. I-V measurement of the substrate showed near zero current (I $\le $ 100 fA) under vacuum, indicating that all of the current was through the nanowires. The I-V characteristics were acquired with a source-measure unit and the bias voltage was swept over the range of -5 V to +5 V with a typical step size of 50 mV. Typical currents when exposed to CO were in the range of 40 nA. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S35.00007: Mechanical Properties of Nanometric Wire of Water. Manhee Lee, Baekman Sung, Wonho Jhe Water has been one of the perfect newtonian viscous liquids, which are exactly described by navier-stokes equation. Recently, it, however, was found that the effective shear viscosity of water confined between mica crystals at 10nm thickness is very different from the one of 3-dimensional bulk water. While some researchers have measured very high viscoelasticity of the confined liquid. the other researchers reported the fluidic nature of water confined between mica surfaces at $<$3.5 nm interfacial separation like bulk-water viscosity. These conflicting results concerning the mechanical properties of nanometric water have been continually reported for the past several years. None of them clearly clarified the mechanical properties of nanometric water, and the detailed behavior of the viscoelasticity within a tip-sample separation less than 1 nm has not been measured. Here, we present the mechanical properties of nanometric water with a spatial resolution less than 0.1nm such as viscoelasticity, dissipation energy, and phase transitions. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S35.00008: Growth of Periodic Arrays of Vertically Aligned Carbon Nanotube on Glass Trilochan Paudel, Jakub Rybczynski, Zhifeng Ren Periodic arrays of vertically aligned carbon nanotubes on glass have been grown by dc plasma enhanced chemical vapor deposition on patterned Nickel dots prepared by polystyrene nanosphere lithography. A thin buffer layer of Titanium was first coated on cleaned high temperature C1737 Aluminosilicate glass substrates and then a monolayer of self-assembled polystyrene spheres was deposited on the glass. Through the polystyrene spheres, a hexagonal pattern of triangular Nickel dots was obtained after removing the spheres. The sphere size and Nickel thicknesses consequently determine the diameter and the site density of carbon nanotubes. The successful growth of carbon nanotubes on glass substrates with good periodicity and alignment are crucible to bio-sensor and solar cell applications. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S35.00009: Feedback Cooling of a Massive Resonator, Quartz Tuning-fork, in Air. Baekman Sung, Manhee Lee, Wonho Jhe Recently, the cooling of a mechanical resonator through active feedback control has been interested for many researchers and the experiment for a cantilever cooling by using feedback control in vacuum has been done by M.Poggio et al [1]. While the recent cooling experiments have been done by tiny cantilever in vacuum, we performed the feedback cooling experiment in air by using a very massive harmonic oscillator, a tuning fork, which has been used as an useful force sensor due to its high stiffness and dynamic oscillation property in scanning probe microscopy (SPM) such as near field scanning optical microscopy, atomic force microscopy (AFM) [2]. This technique is expected to study the low temperature micro state effect of macrosopic object in air. // [1] M.Poggio, C.L.Degen, H.J.Mamim, and D.Rugar, PRL 99,017201 (2007). [2] F. J. Giessibl, S. Hembacher, M. Herz, Ch. Schiller, and J. Mannhart, Nanotechnology 15, S79 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S35.00010: Molecular quantum-dot cellular automata--from molecular structure to circuit dynamics Yuhui Lu, Craig Lent Quantum-dot cellular automata (QCA) [1] provides a transistor-less paradigm for molecular electronics. In the QCA approach, binary information is stored in the charge configuration of single cells, and transferred via Coulomb coupling between neighboring cells. Single-molecule QCA cells can be realized by using as quantum dots the localized states of mixed-valence complexes. Several candidate QCA molecules have been synthesized and shown to have the required field-induced switching properties [2]. We report progress towards a hierarchic dynamic theory of QCA circuits. We use \textit{ab initio} techniques to calculate the relevant molecular electronic structure, and extract parameters for a simpler Hamiltonian to describe switching behavior. We then apply a coherence vector formalism to model interaction with the thermal environment and generate a circuit-dynamic description. [1] C. S. Lent, P. D. Tougaw, W. Porod, and G. H. Bernstein, Nanotechnology, vol. 4, pp. 49, 1993. [2] H. Qi, S. Sharma, Z. Li, G. L. Snider, A. O. Orlov, C. S. Lent, and T. P. Fehlner, J.Am.Chem.Soc., vol. 125, pp. 15250, 2003. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S35.00011: Theory of Individual Carbon Nanotube Deposition by Nanoscopic Lenses Sheng Liu, Amit Goyal, Zafar Iqbal, Gordon A. Thomas, Reginald C. Farrow, Linus A. Fetter The accurate positioning of an individual vertically aligned carbon nanotube (CNT) is a challenge for nanofabrication. We have successfully deposited individual CNTs into sub-100nm diameter SiN$_{x}$ windows on metal interconnects using electrophoresis in conjunction with the nanoscopic lens effect. The dynamics of the deposition of nanotubes under different CMOS compatible manufacturing conditions was modeled using 2D and 3D finite element analysis. Surface charge accumulation and saturation is the key determinant of the strength of the nanoscopic lens. The modeling predicts that there is an easily obtainable range of conditions where only one nanotube will be deposited in round windows using current generation lithography. Deposition in a slotted window geometry yields a limited number of nanotubes that have an average spacing which is a function of the geometry of the slot and randomly approaching nanotubes. Early integration of vertical carbon based logic with CMOS is feasible. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S35.00012: CMOS based fabrication of single electron devices on a large scale. Vishva Ray, Ramkumar Subramanian, Pradeep Bhadrachalam, Seong Jin Koh Fabrication of single electron devices requires device components (a Coulomb island, source and drain electrodes) to be arranged with nanoscale precision. This has been so far carried out utilizing techniques such as e-beam lithography, shadow evaporation, electromigration, and scanning probe microscopy, which are not suitable for large-scale fabrication for practical use. Here we present new single electron device architecture and its large-scale fabrication within the framework of CMOS fabrication technology. This has been done by employing vertical electrode configuration where the source and drain electrode separation is controlled with nanoscale precision over an entire wafer. Colloidal Au nanoparticles of 10-20 nm diameter were used as Coulomb islands positioned between the source and drain electrodes. We observed clear Coulomb blockade and Coulomb staircase at room temperature for 10 nm Au nanoparticles with charging energies of $\sim $50 meV, in good agreement with self-capacitance values of 10 nm Au particle. The experimental I-V characteristics also agree well with simulated I-Vs carried out using the orthodox theory. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S35.00013: A QCM study of the dynamics and frictional properties of physisorbed polystyrene microspheres on a AU(111) Surface Iyam Lynch, Jacqueline Krim, Jessica McNutt The notion of transporting micro/nano objects has been of interest to the scientific community since the early days of nanotechnology. Progress in this area requires an understanding of the frictional behavior of these objects when they are in motion. In this study we have analyzed the behavior of 5$\mu $m diameter polystyrene spheres physisorbed on the electrode of a quartz crystal microbalance (QCM) in orientations normal and parallel to gravity. By varying the driving voltage of the QCM, the samples exhibit a frequency response with respect to the crystal amplitude (``decoupling curve''). This decoupling curve gives information about the motion of the spheres on the surface and leads to the calculation of the particles sliptime which gives rise to the frictional force between the particles and surface. Optical observations show that the particle motion is dependent on the oscillation direction of the QCM and gravity to a lesser extent. Quantitative comparisons of the friction force and the optically observed sliding motion have been performed and will be reported on. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S35.00014: Dynamic behavior of inter-puls time intervals of neuron-like pulses in a GaAs/InGaAs multi-quantum well structure. Aruna Weerasekara, Steven Matsik, Gennady Cymbalyuk, A. G. Unil Perera Neuron-like triggered pulses in a GaAs/InGaAs multi-quantum well (MQW) structure, which shows an s-type negative differential resistance (SNDR), was investigated. Dynamic behavior of inter-pulse time intervals (IPTIs) was studied by analyzing first return maps, power spectra, and correlation dimension. First return maps of the IPTIs show an interesting grouping pattern at slower pulse rates. The grouping behavior can be empirically modeled using oscillatory behavior of the pulsing threshold level of the MQW structure. This pattern of IPTIs is similar to that of thermoreceptors in fish and mammals. Under different operational conditions, correlation dimension of 8.0, 8.5, and 10.0 were obtained by calculating the correlation integrals. The obtained correlation dimensions suggest that this system possesses a higher dimensional behavior. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S35.00015: Using a quantum point contact as a sensitive detector of cantilever motion M. Poggio, M.P. Jura, C.L. Degen, M.A. Topinka, H.J. Mamin, D. Goldhaber-Gordon, D. Rugar We demonstrate the use of a GaAs quantum point contact (QPC) as a sensitive displacement detector with the ability to resolve the thermal motion of an ultrasoft micromechanical oscillator. The displacement measurement is made by positioning the tip of a metal-coated Si cantilever $\sim $100 nm above a QPC. The application of a small voltage to the lever causes it to gate the conductance through the QPC. As a result, motion of the cantilever's tip modulates the conductance. By measuring this modulation, we achieve a displacement resolution better than 10$^{-12}$ m/Hz$^{1/2}$, which is comparable to the resolution achieved by low power optical interferometry. The flexibility of an on-chip QPC as a sensor of cantilever displacement has a number of potential applications including magnetic resonance force microscopy, the detection of displacement for cantilevers with dimensions smaller than the optical diffraction limit, and the cooling of mechanical resonators through feedback. [Preview Abstract] |
Session S36: Focus Session: Hydrogen Storage III: Novel Porous and Sorbent Materials
Sponsoring Units: DMP FIAPChair: Hui Wu, NIST Center for Neutron Research
Room: Morial Convention Center 228
Wednesday, March 12, 2008 2:30PM - 3:06PM |
S36.00001: Enhancing the interaction strength and capacities of hydrogen storage via surface adsorption Invited Speaker: Storing Hydrogen molecules in porous media based on a physisorption mechanism is one possible approach to reach the US Department of Energy targets for on-board hydrogen storage. Although the storage capacities of metal-organic frameworks (MOFs) have progressed significantly over recent years, some technological obstacles pose challenges for their future improvement. These include the generally low H$_{2}$ adsorption enthalpy limiting room temperature applications and the lack of understanding of surface packing density hindering the efficient improvement of H$_{2}$ adsorption uptake. To improve the H$_{2}$ affinity in MOFs, our previous work has shown that the coordinatively unsaturated metal centers (CUMCs) can greatly enhance the H$_{2}$ binding strength. Our current study of MOF-74 will be presented, showing that its open Zn$^{2+}$ ions bind H$_{2}$ strongly and are identified as being responsible for the large initial H$_{2}$ adsorption enthalpy of 8.4 kJ/mol. In all, there are four H$_{2}$ adsorption sites in MOF-74 identified by neutron powder diffraction. These four hydrogen adsorption sites are closely packed in MOF-74 and form a one dimensional nanoscale tube structure. We also demonstrate an interesting correlation that MOFs with CUMCs generally show larger surface packing densities than that of other MOFs without CUMCs. The implications of this will be addressed. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S36.00002: Nature of the Hydrogen Binding in Metal Organic Frameworks with Exposed Transition Metal Sites Wei Zhou, Taner Yildirim MOFs with exposed transition metal (TM) sites were recently found to exhibit significantly larger experimental heat of H$_{2}$ adsorption than classical MOFs, thus attracted great attention. [1, 2] Understanding the nature of the H$_{2}$ interaction with the exposed metal sites is of critical importance for the further development of these materials. Using Mn$_{4}$Cl-MOF as an example, here we show that the H$_{2}$ binding with the exposed TM site is not of the expected Kubas type, in strong contrast to ``similar'' systems investigated previously (e.g., ref. [3] and [4]). In Mn$_{4}$Cl-MOF, there are a) no charge transfer from TM to H$_{2}$, b) no significant H-H bond elongation, and c) no evidence of any H$_{2}$-\textit{$\sigma $}$^{\ast }$ Mn-$d$ orbital hybridization. We also study the H$_{2}$ binding as a function of Mn$_{4}$-magnetic spin configurations, and find no significant effect of the magnetic state on the binding energy. We further reveal that the major contribution to the overall binding is classical Coulomb interaction arising from the small charge overlap of H$_{2}$-\textit{$\sigma $} and Mn-$d$ orbitals. This coulomb interaction is very anisotropic, and when the quantum nature of H$_{2}$-orientation is taken into account, the actual binding energy is significantly reduced from the calculated classical binding energy. [1] J. Am. Chem. Soc. 128, 16876 (2006). [2] Angew. Chem. Int. Ed. 46, 1419 (2007). [3] Phys. Rev. Lett. 94, 087205 (2005). [4] Phys. Rev. B 76, 085434 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S36.00003: Kinetic and steric differences in adsorption in two porous metal-organic frameworks Kathleen Lask, Vaiva Krungleviciute, Murat Bulut, Aldo Migone, J.-Y. Lee, Jing Li Kinetic and steric differences are two of the three fundamental mechanisms underlying the use of adsorption in applications to gas mixture separations. We present experimental results on kinetics and equilibrium adsorption measurements of tetrafluoromethane and argon on two metal-organic framework (MOF) materials: RPM1-Co or [Co$_{3}$(bpdc)$_{3}$bpy]$\cdot $4DMF$\cdot $H$_{2}$O] (bpdc = biphenyldicarboxylate, bpy = 4,4$\prime $-bipyridine, DMF = N,N- dimethylformamide) and Cu-BTC or Cu$_{3}$(BTC)$_{2}$(H$_{2}$O)$_{3}$ (BTC = benzene-1,3,5-tricarboxylate). The adsorbates display significant differences in their kinetics on RPM1-Co (i.e., there are sizable differences in the time required for each gas to reach equilibrium after it is allowed access to the substrate). Our equilibrium measurements show that CF$_{4}$ is sterically precluded from adsorbing in the small tetrahedral-shaped side pockets present in Cu-BTC. We will compare our experimental results with predictions for how adsorption kinetics depends on the size of the adsorbate and on those of the pores present in the substrate. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S36.00004: A theoretical and experimental study of hydrogen storage in metal organic framework materials. Valentino R. Cooper, Jeong Yong Lee, Jing Li, Yves Chabal, David C. Langreth Metal-organic framework (MOF) materials, assembled by linking metal ions or clusters through molecular bridges, have been shown to be good candidates for H$_2$ storage. We have been successful in fabricating and characterizing MOFs with increased H$_2$ uptake\footnote{J. Y. Lee et al. Adv. Func. Mater., \textbf{17}, 1255 (2007)}, though still too low for commercial applications. Here we present a coordinated theoretical-experimental effort to understand the mechanism of H$_2$ adsorption in true MOF materials. Using the completely \emph{ab initio} van der Waals density functional (vdW-DF)\footnote{M. Dion et al. Phys. Rev. Lett., \textbf{92}, 246401 (2004)}$^,$\footnote{T. Thonhauser et al. Phys. Rev. B, \textbf{76}, 125112 (2007)} we simulate the interactions of H$_2$ within Zn$_2$(bdc)$_2$(ted). We demonstrate that modeling the entire MOF structure can result in different H$_2$ adsorption geometries, binding energies and vibrational frequencies than observed in calculations on fragments of the MOF. Combining these results with experimental IR vibrational frequency studies may provide insights into modifying MOF structure and composition for enhanced H$_2$ uptake. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S36.00005: The flexibility of zeolites for Hydrogen storage Asel Sartbaeva, Stephen Anthony Wells, Peter P. Edwards The flexibility window is a newly discovered theoretical measure which can provide a valuable selection criterion when evaluating hypothetical zeolite framework structures as potential synthetic targets. Today synthetic zeolites are the most important catalysts in petrochemical refineries. There have been considerable efforts to synthesize new zeolites with specific pore geometries, to add to the 167 available at present. Millions of hypothetical structures have been generated on the basis of energy minimization, and there is an ongoing search for criteria capable of predicting new zeolite structures. The flexibility window appears to be a property of existing zeolite frameworks not shared by many hypothetical structures. It provides a valuable selection criterion when evaluating hypothetical zeolite framework structures as potential synthetic targets. We are investigating the use of zeolites as hydrogen storage materials, as small molecules such as molecular hydrogen and ammonia can be easily absorbed into a flexible framework of zeolites. An exiting possibility is the use of framework flexibility to control hydrogen uptake, storage and release. This would allow a safe use of hydrogen for fuel cells. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S36.00006: Inelastic Incoherent Neutron Scattering Studies of the Ti-doped hydrogen-adsorbed SBA-15 Alice I. Acatrinei, Luke L. Daemen, Monika A. Hartl The discovery in 1998 of the hydrothermally stable SBA-15 by Zhao et al. [1] represents a great advance in the synthesis of ordered mesoporous materials. The high porosity, good thermal stability, and low specific weight of these materials makes them good candidates for hydrogen storage research, and metal doping shows an enhancement of their properties and gas storage capabilities. We used inelastic incoherent neutron scattering to look at both Ti-doped and undoped hydrogen adsorbed SBA-15. Powder XRD measurements confirmed the mesoporosity of the material. Adsorbtion isotherm showed that a significant amount of hydrogen was adsorbed in the compound. Our results indicate that hydrogen bonds to the Ti centers in the material. [1]. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S36.00007: Hydrogen Storage in Mesoporous Materials under High Pressure Michelle Weinberger, Maddury Somayazulu, Russell Hemley To date, the materials considered best candidates for hydrogen storage fuel cells include activated carbon and metal organic frameworks. Both very high surface area activated carbon and MOF-5 have been shown to adsorb around 4.5 wt {\%} of hydrogen gas at 78 K. We have investigated the fundamental structural response of these materials to high pressure, as well as their behavior at high pressure when packed with dense hydrogen. Further investigation of these materials at low temperatures while still at elevated pressures may in fact provide a route for recovery of these hydrogen-packed materials to near ambient conditions. Covalent organic frameworks offer the potential for even better hydrogen storage capacity. These materials have significantly lower densities than the MOF materials and offer a significantly larger number of adsorption sites. Diamond anvil cells are uniquely suited for the study of these materials, allowing in situ measurements at high pressure as well as at low temperatures. Using X-ray diffraction and Raman spectroscopy and Infrared Spectroscopy we probe the behavior of the hydrogen confined in these porous materials at high pressure by tracking changes in the in situ high pressure x-ray diffraction patterns and shifts in the hydrogen vibron peaks. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S36.00008: A combined pressure-temperature synthesis approach towards novel hydrogen storage materials Raja Chellappa, Maddury Somayazulu, Viktor Struzhkin, Russell Hemley There is a growing need to explore synthesis of novel hydrogen storage materials with very high hydrogen content ($>$ 30 wt.{\%}) as well as tuning materials in order to facilitate reversible hydrogen desorption and absorption. A combined pressure-temperature ($P-T)$ approach holds considerable promise towards achieving these objectives. In this talk, we will present results from our on-going efforts to synthesize hydrogen clathrates with very high hydrogen content that can be recovered at moderate $P-T$ conditions based on simple molecular systems including clathrates and van der Waals compounds, specifically H$_{2}$O, CH$_{4}$, NH$_{3}$, and boron containing systems. The use of suitable additives to enhance stability will also be explored. Results will also be presented from the direct $P-T$ synthesis of metastable light metal (Li, Mg, B- based) complex hydrides. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S36.00009: Structure of Nanoporous Biocarbon for Hydrogen Storage as Determined by Small Angle X-Ray Scattering Mikael Wood, J. Burress, J. Pobst, S. Carter, P. Pfeifer, C. Wexler, P. Shah, G. Suppes As a member of the Alliance for Collaborative Research in Alternative Fuel Technology (ALL-CRAFT) our research group studies the properties of nanoporous biocarbon, produced from waste corn cob, with the goal of achieving the Department of Energy's gravimetric and volumetric standards for both hydrogen and methane gas storage. Small Angle X-Ray Scattering (SAXS) is a valuable tool in our investigation of the geometry of the pore space in our carbon samples. In this talk, we will compare the experimental SAXS data with theoretical results for various pore geometries to determine which pore models are consistent with experiment. Using data from nitrogen adsorption isotherms, along with SAXS, yields significant structural information about the pore space. This analysis should allow us to fully optimize our production process and to achieve the DOE's target storage capacities. This work supported by: 1. National Science Foundation (PFI-0438469) 2. U.S. Department of Education (P200A040038) 3. U.S. Department of Energy (DE-AC02-06CH11357) 4. University of Missouri (RB-06-040) 5. U.S. Department of Defense (N00164-07-P-1306) 6. U.S. Department of Energy (DE-FG02-07ER46411) [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S36.00010: High Capacity Hydrogen Storage on Nanoporous Biocarbon Jacob Burress, Mikael Wood, Michael Gordon, Phillip Parilla, Michael Benham, Carlos Wexler, Fred Hawthorne, Peter Pfeifer The Alliance for Collaborative Research in Alternative Fuel Technology (http://all-craft.missouri.edu) has been optimizing nanoporous biocarbon for high capacity hydrogen storage. The hydrogen storage was measured gravimetrically and volumetrically (Sievert's apparatus). These measurements have been validated by NREL and Hiden Isochema. Sample S-33/k, our current best performer, stores 73-91 g H$_{2}$/kg carbon at 77 K and 47 bar, and 1.0-1.6 g H$_{2}$/kg carbon at 293 K and 47 bar. Hydrogen isotherms run by Hiden Isochema have given experimental binding energies of 8.8 kJ/mol compared to the binding energy of graphite of 5 kJ/mol. Results from a novel boron doping technique will also be presented. The benefits and validity of using boron-doping on carbon will also be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S36.00011: Preparation and Cryogenic Hydrogen Storage Capacity of Nanoporous Carbon Materials Synthesized using an Aerosol-Assisted Approach Qingyuan Hu, Yunfeng Lu, Gregory P. Meisner Spherical nanoporous carbon particles were synthesized from carbon precursor solutions of sucrose with either silica sols, colloidal silica particles, or both, in a direct one-step aerosol-assisted process, followed by carbonization and then removal of the silica template. The resulting particles show very high porosity with narrow pore size distributions, surface areas up to 2000~m$^{2}$/g, and pore volumes up to 4.0~cm$^{3}$/g. The porosity and pore sizes depend on the type and amount of silica template precursor added to the sucrose precursor solutions. The carbon particles were characterized by transmission electron microscopy, field emission scanning electron microscopy, and nitrogen sorption surface area measurements. Hydrogen adsorption was measured at various temperatures between 77 K and room temperature and at pressures up to 50 bars. The maximum hydrogen uptake of up to 4.0~wt{\%} at 77~K and $>$20~bar was found for nanoporous carbon particles made using the silica sol template. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S36.00012: NMR Characterization of Hydrogen Adsorption and Pore Structures of Carbon-Based Materials B.J. Anderson, Alfred Kleinhammes, Yue Wu Hydrogen adsorption in carbon-based materials such as boron-doped graphite and carbon aero gels were investigated by nuclear magnetic resonance (NMR). $^{1}$H NMR is shown to be a sensitive and quantitative probe for detecting adsorbed gas molecules such as H$_{2}$, methane, and ethane. NMR measurements were carried out in-situ under given H$_{2}$ pressure up to a pressure of over 100 atm, at room temperature and 100 K. From such $^{1}$H NMR measurement, the amount of adsorbed H$_{2}$ molecules was determined versus pressure. In addition to measuring adsorption binding energies via isotherms, the structure and distribution of the nanopores within the material were characterized in order to relate the size of the pores to the rate of diffusion of the H$_{2}$ to the adsorption sites. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S36.00013: Fabrication Procedures and Material Properties of Activated Carbon for Hydrogen and Methane Storage Jeffrey Pobst, Jacob Burress, Mikael Wood, Matthew Beckner, Parag Shah, Michael Gordon, Phillip Parilla, Sarah Barker, Sara Carter, Lauren Aston, Galen Suppes, Peter Pfeifer The Alliance for Collaborative Research in Alternative Fuel Technology (http://all-craft.missouri.edu) has developed nanoporous biocarbons with interesting pore characteristics. These carbons are being optimized for hydrogen and methane vehicular storage. Our current best performer stores 73-91 g H$_{2}$/kg carbon at 77 K and 47 bar, and 1.0-1.6 g H$_{2}$/kg carbon at 293 K and 47 bar. The validity of using methane storage as a predictor for hydrogen storage will be presented. Recent carbons have achieved porosities as high as of 0.8 and BET surface areas of 3,500 m$^{2}$/g. Optimal pore sizes and volumes will be presented for hydrogen storage nanoporous carbon. [Preview Abstract] |
Session S37: Effect of Strain on Ferroelectrics
Sponsoring Units: DCMPChair: Maitri Warusawithana, Pennsylvania State University
Room: Morial Convention Center 229
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S37.00001: First-principles study of novel routes to ferroelectricity using strain Alison Hatt, Nicola Spaldin We use first-principles density functional theory (DFT) to explore the use of strain to induce ferroelectricity in otherwise non-polar materials. First, we investigate the polarization induced in LaAlO$_3$ by changing in-plane bondlengths while constraining the unit cell to tetragonal symmetry. We then relax the symmetry constraint to examine the effect of oxygen octahedra rotations on the polarization. Next we explore whether epitaxial strain in the pseudocubic [100] direction can lift the inversion center in otherwise centrosymmetric BiMnO$_3$ and induce the small polarization reported in thin films. By studying these systems with DFT calculations, we have the ability to finely vary the system constraints, predicting new functional materials and providing insight into the underlying physics. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S37.00002: Single-domain, lattice-tunable, rare-earth scandate templates for strain engineering oxide films Chad Folkman, Chang-Beom Eom, Yanbin Chen, Xiaoqing Pan Epitaxial functional oxides have generated excitement due to the improvement in properties over their amorphous and polydomain counterparts. Generally, high quality epitaxy becomes undesirable with large strain. Widely available substrates with site-terminated and atomically flat surfaces unfortunately have large mismatches ($>$1.0{\%}) with oxides like BiFeO$_{3}$ a$_{p}$ = 3.96{\AA}, BaTiO$_{3}$ a$_{p}$ = 4.00{\AA}, and PZT(MPB) a$_{p}$ = 4.07{\AA}. For this reason, orthorhombic Rare-Earth Scandates (REScO$_{3})$ have been developed in bulk and film where tuning the lattice constant is executed by changing the RE ion. Lattice parameters decrease with increasing Z in the range Z = 51, LaScO$_{3}$ a$_{c}$ = 4.05{\AA} to Z=71, LuScO$_{3}$ a$_{c}$ = 3.89{\AA}. In this work, we demonstrate that single domain REScO$_{3}$ template films of LaScO$_{3}$, PrScO$_{3}$, NdScO$_{3}$, SmScO$_{3}$, GdScO$_{3}$, and DyScO$_{3}$ can be deposited with pulsed laser deposition on highly miscut (001) SrTiO$_{3}$ or orthorhombic (110) NdGaO$_{3}$ substrates with crystalline quality approaching that of the bulk crystal. The rocking curve full width at half maximums were typically below 0.1\r{ } and transmission electron microscopy cross sections exhibit large areas of defect free regions near the surface. The result is a lattice tunable template for growth of strain engineered oxide films. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S37.00003: Effects of Substrate Polarity, Strain, and Chemical Boundary Conditions on Ferroelectricity in PbTiO$_{3}$ on DyScO$_{3}$ M. Highland, D.D. Fong, J.A. Eastman, S.K. Streiffer, T. Fister, P.H. Fuoss, G.B. Stephenson, Carol Thompson Novel substrate materials such as DyScO$_{3}$ have recently been used to control the epitaxial strain imposed on ferroelectric films such as PbTiO$_{3}$, since epitaxial strain effects are predicted to strongly modify the phase transition temperature and domain structure. In addition, the polar nature of these substrates can be expected to impose a different electrical boundary condition than would arise at a traditional non-polar substrate surface such as SrTiO$_{3}$ (001). Chemical conditions at the free surface can also affect polarization in ultrathin films. Here we report synchrotron x-ray investigations of the domain structure and polarization as a function of temperature and oxygen partial pressure for ultrathin films of PbTiO$_{3}$ on DyScO$_{3}$. We observe that the substrate surface charge imposes a strong bias on the net polarization of the film, which can be overcome at low temperatures by the chemical effect of the environment. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S37.00004: In situ LEED-IV characterization of polar distorted ultra-thin BaTiO3 films Von Braun Nascimento, E. Ward Plummer, Jun-Soo Shin, A.Y. Borisevich, Arthur P. Baddorf, Sergei V. Kalinin Ferroelectric phase stability in nanoscale ferroelectrics is governed by the interplay of electrostatic depolarization energy, domain formation, adsorption, and surface band bending. Predictions for the minimum critical film thickness for ferroelectricity in BaTiO3 have continuously decreased with more complex models to a current value of 6 layers. The thinnest experimental value is 12 layers. Using in situ low energy electron diffraction (LEED) I-V, we have characterized the structure of 4 and 10 ML BaTiO3 films, grown using laser molecular beam epitaxy with fully compressive strain on a SrRuO3/SrTiO3 substrate. Analysis of the LEED-IV reveals a a surface dead layer with a single-domain upward (out of surface) polarized state below. Intrinsic asymmetry and the stability to compensation of depolarizing charges by dipoles induced by surface stress can explain the single domain scenario. Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S37.00005: Phase diagram of Ba$_{0.5}$Sr$_{0.5}$TiO$_3$ thin films Kristopher E. Andersen, C. Stephen Hellberg Ba$_{0.5}$Sr$_{0.5}$TiO$_3$ (BST) thin films are a promising material for tunable microwave applications, which require large, tunable dielectric constants and low loss. By controlling the (i) substrate and (ii) oxygen partial pressure, it is possible to tune the tetragonal strain of these films. For example, in-plane strains between -0.48\% (highly compressive) and +0.30\% (highly tensile) have been reported for BST thin films grown on MgO(001) using rf magnetron sputtering. The ability to control the strain makes it possible to optimize the dielectric properties of the film for microwave (and other) applications; however, the strain-temperature phase diagram has not been systematically explored to-date. In this talk, the phase diagram of BST is studied using first-principles calculations, focusing initially on the $T$ = 0 phases. Results for displasive as well as coupled ferroelectric/antiferroelectric phases (recently discussed by Zhang, Cagin, and Goddard in connection to BaTiO$_3$) will be presented. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S37.00006: Influence of strain relaxation and atomic interface configuration on the dielectric response of BST thin film capacitors Regina Dittmann, Rafael Plonka, Patrick Sch\"utzendorf, Pertsev Nikolay, Shaobo Mi, Chunlin Jia, Rainer Waser The collapse of the dielectric response which is commonly observed in Ba$_{x}$Sr$_{1-x}$TiO$_{3}$ (BST) thin films is a topic of general physical interest as well as a key issue in terms of a possible application of this material in future DRAM storage capacitors. We addressed the influence of substrates-imposed strain and electrode interface configuration on the dielectric collapse in epitaxial SrRuO$_{3}$/BST/SrRuO$_{3}$ thin film capacitors. The growth mode of BST thin films was analyzed by RHEED and HRTEM from the thickness range of a few unit cells up to hundreds of nanometers where plastic strain relaxation occurs. The crystalline quality of our ultrathin samples enabled us to resolve the atomic arrangement and to identify the terminating layers at the SrRuO$_{3}$-BST interface by STEM. We obtained bulk-like permittivities in the order of 5000 and its thickness dependence can be well described by an extended Ginzburg-Landau-Devonshire model by taking into account plastic strain relaxation in BST thin films and finite screening of depolarizing fields by the SRO electrodes. We will furthermore present relaxor-type behavior of the BST thin films that becomes visible only in samples with sufficient interface quality and hints on nanoscale structural inhomogeneities. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S37.00007: Strain Effects in Barium Strontium Titanate Films for Tunable Microwave Applications Lisa Alldredge, Wontae Chang, Joseph Woicik, Steven Kirchoefer, Jeffrey Pond Recently, it has been demonstrated that control of lattice structure can improve dielectric tuning in epitaxial ferroelectric (FE) films. Understanding the coupling of strain and lattice structure to the dielectric properties is important for FE-based tunable microwave applications. Ba$_{x}$Sr$_{1-x}$TiO$_{3}$ films on (001) MgO substrates were grown by sputter deposition with c$<$a and c$>$a tetragonal distortions, where a and c are the in-plane and out-of-plane lattice parameters, respectively. The dielectric properties were significantly affected by the type of lattice distortion and by the direction of strain-induced permanent polarization if present. Ti K-edge x-ray absorption fine-structure (XAFS) measurements were taken in several orientations. The anisotropy in the spectra with orientation was used to determine the FE phases of the films. The dependence of the in-plane dielectric constant on strain will be discussed in terms of a theoretical model of the phenomenological thermodynamics of the film strain effect. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S37.00008: An ab initio study of strained PZT films Scott Beckman, Karin Rabe, David Vanderbilt The potential for creating ferroelectric-based devices such as ferroelectric random-access memories (FeRAMs) has led to an intense interest in ferroelectric perovskite thin films, alloys, and superlattices. Engineering of FeRAMs requires the control of both the spontaneous polarization and the barrier for polarization reversal. The two parameters that are simplest to modify are the composition, which is controlled during the film deposition, and the epitaxial strain, which is controlled by substrate selection. Here, using \emph{ab initio} methods, we investigate how the ferroelectric properties of PbZr$_{(x)}$Ti$_{(1-x)}$O$_{3}$ (PZT) depend upon epitaxial strains ranging from $ -0.02 \leq \epsilon \leq +0.02 $. Compositions ranging from $ 0.0 \leq x \leq 0.5 $ are examined by the creation of special quasirandom structures. In agreement with the literature,\footnote{ H.N. Lee, S.M. Nakhmanson, M.F. Chisholm, H.M. Christen, K.M. Rabe, and D. Vanderbilt, Phys. Rev. Lett. 98, 217602 (2007).} the effect of strain on the polarization is found to be small. We also compute the strain dependence of the barrier for polarization reversal, and discuss the results in the context of experimental measurements of coercive fields and polarization retention.$^1$ [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S37.00009: High pressure x-ray diffraction study of single crystal Pb(Sc$_{0.5}$ Nb$_{0.5})$O$_{0.3}$ Maddury Somayazulu, Muhtar Ahart, Ronald Cohen, Russell Hemley We employed high pressure single crystal x-ray diffraction to investigate the pressure-induced phase transition in Pb(Sc0.5Nb0.5)O3 (PSN). At 2 GPa and 300 K, PSN undergoes a phase transition as deduced from earlier dielectric measurements. The pressure dependence of diffuse scattering observed around the (110) Bragg peak indicates that pressure suppresses the local distortion that is coupled to the polar nanoregions. We have monitored the pressure dependence of diffuse scattering at temperatures between 300 - 4 K and used this to understand the P-T phase diagram of PSN. In addition, we also investigated the equation of state at various temperatures. Observed changes in the isothermal compressibility at low temperatures indicate that the first order phase transition changes character at 200 K and 4 GPa. The results can be understood in terms of a pressure-induced decrease in the correlation length among polar nanoregions, which is a unique property of relaxor ferroelectrics. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S37.00010: Strain induced relaxor behavior in PbSc$_{0.50}$Nb$_{0.25}$Ta$_{0.25}$O$_{3}$ thin films: A comparison with the nanoceramics Margarita Correa, Ashok Kumar, Ram Katiyar A comparative studies of the microstructure, micro Raman spectroscopy and dielectric properties of PbSc$_{0.50}$Nb$_{0.25}$Ta$_{0.25}$O$_{3}$ (PSNT) thin films and ceramics were carried out over a wide range of temperature 100-520 K and frequency 100Hz to 1MHz. Microstructure of PSNT films revealed an in plane compressive strain whereas PSNT ceramics showed an average 10-15 nm size nanoordered regions. We observed a shift of 65 K in dielectric maxima temperature towards the lower temperature and frequency dispersion of the dielectric constant in PSNT films compare to bulk that does not exhibit relaxor behavior. We addressed this different dielectric response due to the in plane compressive strain in the films. Temperature dependent micro Raman spectroscopy revealed that the ferroelectric state in PSNT ceramics were accompanied by pronounced changes in both the lowest frequency F$_{2g}$ and highest frequency A$_{1g}$ modes. Micro Raman spectra of the thin films compared to nanoceramics showed shifting of the Raman modes to lower frequencies that confirms the strain state of the films. The in plane compressive strain, dipole arrangement and the size of nanoordered regions change the dielectric response of the PSNT films compare to nanoceramics. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S37.00011: First Principles Theory of Sub-Monolayer Strontium on Silicon (001) Kevin Garrity, James Reiner, Frederick Walker, Charles Ahn, Sohrab Ismail-Beigi Conventional attempts to continue transistor scaling consistent with Moore's law will soon result in unacceptable quantum mechanical leakage currents across the dielectric oxide layer. One promising solution to this problem is to replace the current silicon dioxide layer with a thicker crystalline oxide with a higher dielectric constant, grown epitaxially on silicon. Although there has been progress in growing high quality epitaxial interfaces for some materials, the initial stages of growth, including the deposition of the initial metal layer, are not well understood. Using \textit{ab initio} density functional theory, we study the initial stages of the deposition of strontium titanate on silicon (001), a good model system due its successful epitaxial growth. We present the binding energies of several new low energy structures with sub-monolayer converages of strontium which differ significantly from the conventional view of this surface. Additionally, to include finite temperature effects, we calculate vibrational free energies. We compare our results to experimental samples grown by molecular beam expitaxy. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S37.00012: Sub-monolayer Strontium Phase Diagram on Silicon (100) James Reiner, Kevin Garrity, Fred Walker, Sohrab Ismail-Beigi, C. H. Ahn Crystalline oxides manifest a number of important phenomena, including magnetism, ferroelectricity, superconductivity, and colossal magnetoresistance. Recently, it has become possible to integrate these materials onto a silicon platform in a fully epitaxial structure. These crystalline oxide-silicon heterostructures bring the promise of integrating the rich functionality present in crystalline oxides with modern silicon device technology. The most successful fully epitaxial oxide-silicon (100) heterostructures have been achieved through a deposition recipe that involves manipulating substrate temperature and oxygen pressure on a layer by layer basis during the deposition of an alkaline earth metal. Motivated by a desire to develop a fundamental understanding of this important transition layer between silicon and oxide, we have mapped out the phase diagram of strontium on silicon as a function of temperature and coverage. In particular, recent work on sub-monolayer strontium deposition on the silicon surface suggests the conventional picture of this structure, upon which the entire crystalline oxides on silicon framework is built, is only a low-temperature phase which plays no role in enabling epitaxial oxide growth. Instead, there is strong evidence that a different high temperature phase is the crucial template for epitaxial oxide growth on silicon. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S37.00013: Interfacial effects on ferroelectricity in SrTiO$_3$ thin films supported on Si(001) Alexie Kolpak, Sohrab Ismail-Beigi Experimental and theoretical studies have suggested that the application of a compressive in-plane strain can induce a polar ground state in thin SrTiO$_3$ films. One way to impose such a strain is to grow the film epitaxially on a lattice mis-matched substrate. However, it is unclear whether such films are ferroelectric -- i.e., switchable -- in addition to being polar, and furthermore, what role the substrate plays in determining the switchability. Using density functional theory, we explore the possibility of a ferroelectric ground state in SrTiO$_3$ thin films grown on a Si(001) substrate. In particular, we examine the effects of several proposed interface structures on the interfacial charge compensation and polarization direction, and we discuss the implications of these effects for potential applications in integrated electronics devices. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S37.00014: Polarization of strained SrTiO$_{3}$ films grown on Si (001) F. J. Walker, Yaron Segal, J. W. Reiner, C. H. Ahn, Zhan Zhang Perovskite oxides grown on silicon provide powerful new functionalities for device components built upon the ubiquitous silicon platform. A rich set of applications results from the combination of the perovskite's diverse electrical and physical structures and the semiconducting properties of silicon. Understanding how to develop new functionalities, however, requires detailed knowledge of the real space positions of atoms with sub-angstrom resolution. In this work, we have carried out synchrotron x-ray diffraction studies of crystal truncation rods on 4-5 unit-cell-thick SrTiO$_{3}$ films grown epitaxially on silicon, which have been terminated with different gate metals. We determine the precise atomic structure of these materials, in particular the displacements of the TiO$_{2}$ planes relative to the SrO planes that make up the perovskite structure. We show that the polarization and structure of commensurately strained films depend critically upon the electrical and chemical properties of the terminating metal. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S37.00015: Writing and Reading of Ultrathin Ferroelectric Domains on Commensurate SrTiO$_{3}$ on Silicon Jeremy Levy, Cheng Cen, Charles R. Sleasman, Maitri Warusawithana, Darrell G. Schlom Ferroelectricity in ultrathin epitaxial SrTiO$_{3}$ grown commensurately by oxide-molecular beam epitaxy (MBE) on silicon substrates was investigated using piezoforce microscopy (PFM). A series of samples containing $n$ molecular layers (ML) of SrTiO$_{3}$ ($n$ = 3, 4, 5, 6, 8, 10, 20) was grown on silicon substrates. Room-temperature ferroelectricity was observed for samples containing $n$ = 5, 6, 8, 10 ML. Temperature-dependent measurements indicate that the sample with $n$ = 5 exhibits a ferroelectric phase transition at T$_{c}\sim $317 K. Sample with $n$ = 6 remains ferroelectric up to at least 393K. Polar domains created on the $n$ = 6 was found to be stable at room temperature for more than 72 hours. The implications of these results for fundamental and device-related applications will be discussed briefly. [Preview Abstract] |
Session S39: Systems Far From Equilibrium and Other Topics in Statistical Physics
Sponsoring Units: GSNPChair: Robert Ziff, University of Michigan
Room: Morial Convention Center 231
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S39.00001: Absorbing-state phase transitions: exact solutions of small systems Ronald Dickman I derive precise results for absorbing-state phase transitions using exact (numerically determined) quasistationary (QS) probability distributions for small systems. Analysis of the contact process on rings of 23 or fewer sites yields critical properties (control parameter, order-parameter ratios, and critical exponents $z$ and $\beta/\nu_\perp$) with an accuracy of better than 0.1\%; for the exponent $\nu_\perp$ the accuracy is about 0.5\%. Good results are also obtained for the pair contact process. The QS kurtosis exhibits a sharp minimum at the critical point. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S39.00002: Displacement and velocity correlation functions of magnetic particle chains undergoing Brownian dynamics Yuk Kwan Ho, Mingjie Zheng, Kin Wah Yu Brownian dynamics of physical systems has been studied for a long time since Einstein in 1905. In this work, we report the study of a magnetic particle chain in ferrofluid, with hydrodynamic interactions and harmonic interactions included, by using Rice method [1]. We will focus on the correlation functions and relaxation time of the chain. As qualitative account for the effect of system parameters, analytic solutions of the displacement and velocity correlation functions of a two-body system have been studied. In the case of an over-damped system, very long relaxation time for the displacement correlation function can be obtained from the analytic solutions. Moreover, the size dependence of the relaxation time is also studied numerically for moderate chain sizes. \newline \newline [1] M. C. Wang and G. E. Uhlenbeck, Rev. Mod. Phys. \textbf{17}, 323 (1945). [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S39.00003: Theory of Zener tunneling and breakdown in solids Naoyuki Sugimoto, Shigeki Onoda, Naoto Nagaosa Tunneling and breakdown phenomena are among of the most important problems in condensed matter physics. We study the Zener tunneling and breakdown phenomena in a bulk system taking into account the dissipation due to impurity scatterings in terms of the Keldysh formalism. Three distinct regions are recognized for the current-field characteristics, which are identified as the impurity-conduction, the Zener tunneling, and the Zener breakdown, respectively. The crossovers among them are described in a unified fashion. By examining the local density of states, which can be measured by scanning tunneling spectroscopy, we find that the Zener tunneling and breakdown can be understood as the conduction due to the finite local density of states at the Fermi energy originating from the hybridization between the conduction and valence bands induced by the electric field. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S39.00004: Domain Structure Universality in Coarsening Benjamin Vollmayr-Lee, Andrew Rutenberg, Sohei Yasuda Coarsening systems ubiquitously exhibit power law growth $L\sim t^\alpha$ with self-similiar domain morphology, and much progress has been made in mapping out universality classes of the growth exponent $\alpha$. It has been commonly argued that these universality classes should apply as well to the scaled domain structure, but recent evidence has appeared to the contrary. In particular, surface tension anisotropy has been found numerically and by exact solutions in the dilute limit to modify the domain morphology and structure factor, while leaving growth exponents unchanged. Thus the universality classes of the domain morphology remains an open question. We present a conjecture that the morpology universality is a consequence of the asymptotic trajectories of the topological defects, and then map out the universality classes that follow. Our prediction, in the case of scalar, conserved order parameter coarsening, is that the domain structures depend on surface tension anisotropy and mobility asymmetry, but nothing more. To test this prediction we have conducted extensive simulations of coarsening with mobility asymmetry and have demonstrated its influence on the scaled domain structure. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S39.00005: Exact Solutions for Anisotropic Coarsening in the Dilute Limit William Rosenbaum, Melinda Gildner, Benjamin Vollmayr-Lee We study the influence of anisotropy on coarsening dynamics via two dilute coarsening models: Lifshitz-Slyozov theory for locally conserved order parameter dynamics, and Wagner theory for the globally conserved analog. We adopt a perturbative approach to analyze the effect of surface tension anisotropy on drop shapes and the scaled drop size distribution. In both models we find that coarsening solutions exhibit growth laws that are unchanged from the isotropic theories, $L\sim t^{1/3}$ and $L\sim t^{1/2}$ respectively, and drop shapes that are in general nonspherical and non-Wulffian. We also determine that the drop size distribution varies from the isotropic case. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S39.00006: Domain Morphology and Structure Factor in the Asymmetric Cahn-Hilliard Equation Sohei Yasuda, Benjamin Vollmayr-Lee, Andrew Rutenberg The Cahn-Hilliard equation for conserved order parameter coarsening is modified to allow for unequal mobilities in the two equilibrium phases. This asymmetric Cahn-Hilliard equation is then simulated using an unconditionally stable algorithm, which enables reaching times well into the $L\sim t^{1/3}$ dynamic scaling regime. Our goal is to address questions of the universality of the scaled domain morphology; in particular, to test our conjecture that the morphology should depend on the mobility asymmetry. We study mobility ratios of 1, 2, 4 and 8 and find that the domain size distribution depends strongly on the mobility asymmetry: the higher (lower) mobility phase forms a smaller (larger) backbone cluster with more (fewer) smaller domains, and the magnitude of the effect grows with the mobility ratio. Interestingly, the structure factor shows no discernible dependence on the mobility asymmetry. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S39.00007: Non-trivial statistics crossover in random sequential adsorption due to the presence of a pattern Nuno A.M. Araujo, Antonio Cadilhe, Vladimir Privman The random sequential adsorption (RSA) on a lattice approaches the jammed state exponentially, while in the off-lattice version approaches the jammed state as a power-law. In this presentation, we focus on the kinetics leading to the jammed state. The presence of pattern influences the kinetics of approach to the jammed state. Namely, a pattern consisting of equal size squares embedded on a square matrix lattice provides a rich set of regimes. Measurements of the distribution of areas available for adsorption during the process make possible to predict how the jammed state is attained: power-law or exponential. Finally, we associate these two different kinetic regimes with the breakdown of Pommeau statistics near the jammed state. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S39.00008: Dephasing and the steady state in quantum many-particle systems Thomas Barthel, Ulrich Schollw\"ock We discuss relaxation in many-particle systems. For integrable systems, the time-evolution from an arbitrary initial state can lead, for a given finite subsystem, to a definite steady state. We give an explicit derivation of the steady state ensemble and devise sufficient prerequisites for the dephasing to take place. We also find surprisingly simple scenarios, in which dephasing is ineffective and discuss the dependence on dimensionality and criticality. It also follows that, after a quench of system parameters, entanglement entropy will become extensive. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S39.00009: Assembly and control of self-propelled structures in driven suspensions of magnetic microparticles. Maxim Belkin, Alexey Snezhko, Igor Aranson, Wai-Kwong Kwok Magnetic microparticles suspended on the surface of liquid and subjected to periodic vertical magnetic excitations form a non-trivial dynamic snake-like pattern accompanied by large-scale surface flows. Apparently, controlled suppression of vortices at one end of the snake may lead to a formation of a self-propelled structure. We demonstrate that the suppression of the vortex pair can be implemented by the mechanical disturbance of the one end of the structure by means of the floating particle with characteristic size comparable to the width of the snake's segment. The snake structure with the particle attached to its end becomes a swimmer with parameters effectively controlled by the external driving magnetic field. Experimental studies of such self-propelled structures are presented. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S39.00010: Avalanches of Bose-Einstein-Condensates from open optical lattices Tsampikos Kottos, Gim Seng Ng, Holger Hennig, Ragnar Fleischmann, Theo Geisel We investigate, the outgoing atomic flux of BECs loaded in large OLs using a mean field (Discrete Non-Linear Schroedinger Equation--DNLSE) approach. We show that for some critical values of the rescaled (with respect to the lattice size) interatomic interaction strength, the current decays in avalanches that follow a power-law distribution indicating the existence of a novel phase transition. The origin of this phenomenon is identified to be the collisions between stable and moving breathers which co-exist at the lattice. Using a reduce map we are able to provide bounds for the power law exponent of the avalanche distribution which reflect the complexity of the underlying classical phase-space. Due to the inter-disciplinary nature of the DNLSE, we expect that the same phenomenon will be observed in several other branches of nonlinear physics, ranging from nonlinear optics to polarons and biological molecules. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S39.00011: Study of universality and critical behavior in periodically driven interacting cold atomic system Myoung-Sun Heo, Yonghee Kim, Wonho Jhe, Heung-Ryoul Noh Strongly driven nonlinear oscillators show a variety of interesting phenomena such as period doubling, bifurcation, chaos. Since their resonant behaviors are very sensitive to external perturbation, they have now been widely adopted for the precise determination of physical quantities. The key concept to these researches is the development of bistable or period-2 states, which are, in most cases, energetically degenerate. If particles interact each other, however, this degeneracy of two attractors can be lifted up. For example, in the simple case of all-to-all attractive interaction, particles will be preferably directed into more populated attractors. Being incorporated with random fluctuation which tries to equilibrate population in each attractor, this lifting-up becomes dependent on the system size, or the total number of particles, as recently observed as spontaneous symmetry breaking in cold atomic system which inherently possesses the light-induced attractive interaction. In particular, this dependence seems to show a sort of critical behavior. Here we have elucidated the criticality existing in the strongly driven interacting many-particle system consisted up of cold atoms from static and dynamic perspectives. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S39.00012: Transition of Electromechanical Pendulum into Nonlinear Response Chulki Kim, Hua Qin, Hyun-Seok Kim, Robert Blick We present measurements on a macroscopic electron shuttle in the regime of nonlinear response. The shuttle is formed by a classical mechanical pendulum situated between two capacitor plates. The main advantage of this macroscopic setup is that we can directly measure the onset of nonlinear response of this electromechanical pendulum exhibiting period doubling. The results will find use in application such as bifurcation amplifier in nanoscopic versions of the device. A model nanoelectromechanical device based on the macroscopic experiment has the potential to reveal nonlinear quantum mechanical effects. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S39.00013: Harmonic measure of DLA and percolation clusters Robert Ziff, Leonard Sander, David Adams The problem of efficiently finding the harmonic measure of DLA and percolation clusters is addressed. Deep inside fjords and inlets of irregular surfaces, traditional methods of solving Laplace's equation become very inefficient. A new numerical algorithm that efficiently solves for those measures is developed. The technique uses a step-by-step approach that adds a shell at each time step to the diffusion front, and is essentially linear in the number of sites considered. The method works for lattice systems and discretized versions of continuum systems. With it, we have solved for the multifractal exponents of the fractal surfaces. For DLA, we address the controversy of the behavior of the harmonic measure deep inside the fjord. For percolation, we consider both the relatively smooth accessible hull (D = 4/3), and the very invaginated regular hull (D = 7/4). [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S39.00014: Critical surfaces for general two dimensional bond percolation problems Chris Scullard, Robert Ziff We present a general method for deriving approximate bond percolation critical surfaces for general two dimensional lattices. Our approach is to assume that the critical surface is at most first order in its arguments and then to impose symmetries and known special cases until the function is completely determined. We show also that allowing higher powers of the arguments gives increasingly accurate approximations. For the Archimedean lattices the critical thresholds we find by this method are accurate to at least 4 significant figures. In two other cases, the checkerboard and inhomogeneous bow-tie we find critical surfaces that appear, from our numerical investigation, to be exact but that apparently cannot be rigorously derived by any existing technique. The checkerboard formula that we find is same as that conjectured by Wu (Rev. Mod. Phys. 54, 235 - 268 (1982)) [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S39.00015: Nonuniversal Deviations From Predictions of the Random Matrix Theory of Wave Chaotic Scattering: Theory and Experiment Steven Anlage, James Hart, Elliott Bradshaw, Thomas Antonsen, Edward Ott The eigenfunctions and spectra of chaotic billiards are notoriously sensitive to small perturbations.~ Thus statistical approaches have been developed to model such systems.~ In recent work, we used random matrix theory to develop statistical models for the impedance of a chaotic microwave cavity coupled to a small number of antennas, with the only parameters being the radiation impedance of the antennas, the area of the cavity and a uniform loss parameter Q [S. Hemmady, \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 014102 (2005); X. Zheng, \textit{et al}., Electromagnetics \textbf{26}, 3 (2006)].~ The theory generally agrees well with experiment, but under some circumstances the experimental and numerical results deviate significantly from the Random Matrix Theory predictions.~ We have derived a method of accounting for these deviations and have experimental and numerical results which agree well with our new, non-universal, predictions. [Preview Abstract] |
Session S40: History of Physics
Sponsoring Units: FHPChair: George Zimmerman, Boston University
Room: Morial Convention Center 232
Wednesday, March 12, 2008 2:30PM - 2:54PM |
S40.00001: Development of ultra violet photoemission [UPS]: a history Jerry [ G. J. ] Lapeyre UPS of solids investigations are summarized from the late 1940's through the very early 1960's which measured kinetic energy distributions [ EDC's ] of the emitted electrons. The experiments contributed to the opening of the photoemission spectroscopy fields. This era involved activities at G. E. Research Labs, Univ. of Missouri and W. E. Spicer's lab. The author knew most of the researchers. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:18PM |
S40.00002: Magnetism in matter before the discovery of quantum spin: Bohr's less well-known contribution to the transition from classical to quantum physics. Jean-Francois Van Huele How does one explain magnetic effects in matter when one views matter as a collection of classical charges in motion? The answer is: not at all! This is one of the points that Niels Bohr made in his doctoral dissertation in 1911, two years before addressing the issue of the stability of the hydrogen atom. The result, later rediscovered by H.J.van Leeuwen was amplified and formalized in Van Vleck's 1932 text on electric and magnetic susceptibilities and it is currently known as the Bohr-van Leeuwen theorem. We will review Bohr's two derivations, one statistical and one based on the motion of individual electrons. We will then propose reasons why this result, unlike that on the stability of hydrogen, did not lead to a major development in quantum theory but, instead, had to wait until after the introduction of spin and exchange forces in quantum mechanics to become generally known. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:42PM |
S40.00003: Epistemological Dimensions in Niels Bohr's Conceptualization of Complementarity Gregory Derry Contemporary explications of quantum theory are uniformly ahistorical in their accounts of complementarity. Such accounts typically present complementarity as a physical principle that prohibits simultaneous measurements of certain dynamical quantities or behaviors, attributing this principle to Niels Bohr. This conceptualization of complementarity, however, is virtually devoid of content and is only marginally related to Bohr's actual writing on the topic. Instead, what Bohr presented was a subtle and complex epistemological argument in which complementarity is a shorthand way to refer to an inclusive framework for the logical analysis of ideas. The important point to notice, historically, is that Bohr's work involving complementarity is not intended to be an improvement or addition to a particular physical theory (quantum mechanics), which Bohr regarded as already complete. Bohr's work involving complementarity is actually an argument related to the goals, meaning, and limitations of physical theory itself, grounded in deep epistemological considerations stemming from the fundamental discontinuity of nature on a microscopic scale. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 4:06PM |
S40.00004: Paradox, Natural Mathematics, Relativity and Twentieth-Century Ideas John Ryskamp We are enjoying a renaissance in the historiography of set theory which allows us to pinpoint the effect of Poincare's writing on the development of Einstein as an advocate of natural mathematics (what he called practical geometry). I will briefly describe the importance of Garciadiego's landmark work on Russell (1992), and Grattan-Guinness' epic recounting of the history of set theory (2000). I will present something for which historians of physics have searched but have not previously identified: the precise step at which Einstein incorporated practical geometry into his formulation of the relativitity of simultaneity. This leads to a troubling conclusion, at least for those who look to relativity for internal consistency. However, the trouble is not restricted to physics. We suggest that natural mathematics finds its way into the major twentieth-century ideas. [Preview Abstract] |
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