Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L1: Nobel Prize Session followed by Onsager Prize Session
Sponsoring Units: APS DAMOPChair: Arthur Beinenstock, APS President, Stanford University, followed by Ivan Schuller, University of California, San Diego
Room: Morial Convention Center LaLouisiane AB
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L1.00001: Recent developments and perspective in spintronics Invited Speaker: Recent developments and perspective in spintronics: A. Fert, UMR CNRS/Thales, 91767 Palaiseau and Université Paris-Sud, 91405 Orsay, France After an introduction on the fundamentals of spin transport and the discovery of GMR, I will focus on the most recent developments in spintronics. I will first describe the field of the spin transfer phenomena by reviewing experimental results on magnetic switching and generation of microwave oscillations by spin transfer. The synchronization and phase locking of a collection of STO’s (Spin Transfer Oscillators) is an example of new important problem raised by the experiments of spin transfer. I will present data on the synchronization of electrically connected STO. I will then continue the review with results on spintronics with semiconductors, molecular spintronics and spin Hall effect.\newline \newline Acknowledgements: I thanks all the coworkers of my recent works on spintronics, A. Anane[1], J. Barnas [2], A. Barthélémy [1], A. Bernand-Mantel [1], M. Bibes [1], O. Boulle [1], V.Cros [1], C.Deranlot [1], M.Elsen [1], G. Faini [3], B. Georges [1], JM.George [1], R. Giraud [3], M. Gmitra [2], J.Grollier [1], A.Hamzic [5], L. Hueso [6], H.Jaffrès [1], S. Laribi [1], A. Lemaitre [3], P. M. Levy [7], N. Mathur [6], R. Mattana [1],, F. Petroff [1], P. Seneor [1], F.Van Dau [1], A. Vaurès [1]. \newline \newline [1] Unité Mixte de Physique CNRS/Thales, Palaiseau and Université Paris Sud,Orsay, France\newline [2] Department of Physics, Adam Mickiewicz University, Poznan, Poland\newline [3] CNRS- LPN, Marcoussis, France\newline [4] IEF, Université Paris-Sud, Orsay, France\newline [5] University of Zagreb, Croatia\newline [6] Cambridge University, UK \newline [7] New York University\newline [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L1.00002: From spinwaves to Giant Magnetoresistance (GMR) and beyond Invited Speaker: Standing spinwaves and surface waves in layered magnetic structures can be used for the detection and quantitative evaluation of interlayer exchange coupling (IEC). Using this method antiferromagnetic IEC has been found in Fe/Cr/Fe layered structures. This was applied to switch the alignment of the magnetizations of the Fe films in an external field from antiparallel to parallel whereby the Giant Magnetoresistance (GMR) effect was discovered. Currently the interest focusses on ``Current Induced Magnetic Switching" (CIMS) which can be understood as inverse GMR effect. For all three effects, IEC, GMR and CIMS there are many interesting applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L1.00003: Lars Onsager Prize Talk: Quantum fluids: from liquid helium to cold atoms Invited Speaker: The study of quantum liquids has led to ideas and concepts of broad applicability. I shall illustrate this by examples from the physics of liquid helium-3, heavy-fermion compounds, quark-gluon plasmas and cold atomic gases. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L1.00004: Lars Onsager Prize Talk: Stepping through forty years of quantum fluids Invited Speaker: This talk will trace milestones in quantum fluids from dilute solutions of He-3 in He-4, to superfluids in neutron stars, to cold atoms. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L1.00005: Lars Onsager Prize Talk: A New Challenge for Cold Atom Physics: Achieving the Strongly Correlated Regimes for Cold Atoms in Optical Lattices. Invited Speaker: Cold atoms in optical lattices show great promise to generate a whole host of new strongly correlated states and to emulate many theoretical models for strongly interacting electronic systems. However, to reach these strongly correlated regimes, we need to reach unprecedented low temperatures within current experimental settings. To achieve this, it is necessary to remove considerable amount of entropy from the system. Here, we point out a general principle for removing entropies of quantum gases in optical lattices which will allow one to reach some extraordinarily low temperature scales. [Preview Abstract] |
Session L2: The Physics of Next Generation Photovoltaics
Sponsoring Units: DCMP DMPChair: Arthur Nozik, National Renewable Energy Laboratory
Room: Morial Convention Center LaLouisiane C
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L2.00001: Nano-Structured Silicon Thin Films for Photovoltaic Applications Invited Speaker: The current technology for thin-film silicon photovoltaic panels is based on hydrogenated amorphous silicon and related alloys, such as silicon-germanium and silicon-carbon. Currently there is great interest in using some form of thin-film silicon that includes nano-structured components. This interest is driven in part by the potential for decreased cost, increased efficiency, and increased stability. Also driving this interest is the abundance of silicon as an element and its lack of toxicity. I will review various structures that have been suggested, and discuss recent results on inhomogeneous films of hydrogenated amorphous silicon that contain nanocrystalline inclusions. In particular, I will describe the mechanisms for optical absorption, carrier transport and the role of defects. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L2.00002: Novel ultra high efficiency concepts in solar cells Invited Speaker: The limit efficiency of conventional solar cells is about 45{\%} as obtained in 1960 by Shokley and Queisser. Besides the multijuntion solar cells, other novel concepts have been proposed in the last years with efficiency limit in the range of 85{\%}. They will be reviewed now. The intermediate band (IB) solar cell attempts to increase the photocurrent of a solar cell without reducing the voltage. For it an electronic band is fabricated in the mid of the bandgap, the IB, so that electron hole pairs are created by the absorption of two sub-band-gap photons using the IB as a relay. Furthermore, the preservation of the voltage requires that a new quasi Fermi level appears in the IB, different to those in the valence band (VB) and in the conduction band (CB). So far IB materials have been produced, either using the confined states of quantum dots or by alloys. The basic principles, both of electron hole creation by double photon absorption and the appearance of a third quasi Fermi level have been experimentally proven. The principle of the multiple generation solar cells is based on the creation of several electron-hole pairs by photons whose energy is well above the bandgap. So far up to seven electro-hole pairs have been experimentally proven from a single photon in structures with PbSe quantum dots. Solar cells based on the two preceding concepts have been fabricated although with performances still low. In the hot carrier solar cells what is intended is to recover the energy of gas of electrons excited by the flux of photons before they thermalise with the lattice. Basic requirements of this device are being understood. They require a medium in which electrons and phonons are very decoupled and narrow contacts for extraction the hot electrons. The role of the quantum dots may be important. Experimental research in this field is about to start. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L2.00003: Using Multijunction Solar Cell Designs to Achieve High Efficiency Invited Speaker: Achieving high-efficiency requires minimizing the absorption and carrier-thermalization losses in solar cells. Multijunction solar cells do this by using multiple materials and matching their band gaps with the corresponding portions of the solar spectrum. The p-n junctions formed from each material must be near-perfect so as to avoid non-radiative recombination. The efficiency can be further increased by concentrating the incident light, which increases the generation rate of electron-hole pairs per semiconductor volume. Mirrors or lenses can concentrate the light onto a small area or light trapping can be used to concentrate the light into a thinner layer. The talk will describe the physics of how the different aspects of the design of multijunction cells contribute to achieving high efficiency. This abstract is subject to government rights. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L2.00004: Dye-Sensitized Approaches to Photovoltaics Invited Speaker: Sensitization of wide band-gap semiconductors to photons of energy less than the band-gap is a key step in two technically important processes - panchromatic photography and photoelectrochemical solar cells. In both cases the photosensitive species is not the semiconductor - silver halide or metal oxide - but rather an electrochemically active dye. The gap between the highest occupied molecular level (HOMO) and the lowest unoccupied molecular level (LUMO) is less than the band-gap of the semiconductor with which it is associated. It can therefore absorb light of a wavelength longer than that to which the semiconductor itself is sensitive. The electrochemical process is initiated when the dye molecule relaxes from its photoexcited level by electron injection into the semiconductor, which therefore acts as a photoanode. If the dye is in contact with a redox electrolyte, the negative charge represented by the lost electron can be recovered from the reduced state of the redox system, which in return is regenerated by charge transfer from a cathode. An external load completes the electrical circuit. The system therefore represents a conversion of the energy of absorbed photons into an electrical current by a regenerative device in every functional respect analogous to a solid-state photovoltaic cell. As in any engineering system, choice of materials, their optimization and their synergy are essential to efficient operation. While a semiconductor-electrolyte contact is analogous to a Schottky contact, in that a barrier is established between two materials of different conduction mechanism, with the possibility of optical absorption, charge carrier pair generation and separation, it should be remembered that the photogenerated valence band hole in the semiconductor represents a powerful oxidizing agent. Given that the band-gap is related to the strength and therefore the stability of chemical bonding within the semiconductor, for narrow-gap materials the most likely reaction of such a hole is the photocorrosion of the semiconductor itself. However, only relatively narrow band-gap materials have an effective optical absorption through the visible spectrum, towards and into the infra-red. Materials with an optimal band-gap match to the solar spectrum, of the order of 1.5eV, are therefore electrochemically unstable. A stable photoelectrochemical cell, without some process of optical sensitization, and necessarily using a wide-gap semiconductor is sensitive only to the ultra-violet limit of the visible spectrum. Over recent years a suitable combination of semiconductor and sensitizer has been identified and optimized, so that now a solar spectrum conversion efficiency of over 11{\%} has been verified in a sensitized photoelectrochemical device. One key to such an efficient system is the suppression of recombination losses. When the excited dye relaxes by electron loss, the separated charge carriers find themselves on opposite sides of a phase barrier -- the electron within the solid-state semiconductor, the positive charge externally, in association with the dye molecule. There is no valence---band involvement in the process, so the system represents a majority-carrier device, avoiding one of the major loss mechanisms in conventional photovoltaics. In consequence also a highly-disordered, even porous, semiconductor structure is acceptable, enabling surface adsorption of a sufficient concentration of the dye to permit total optical absorption of incident light of photon energy greater than the HOMO-LUMO gap of the dye molecule. The accepted wide-band semiconductor for photoelectrochemical applications is titanium dioxide in the anatase crystal structure. The size of the nanocrystals making up the semiconductor photoanode can be determined by hydrothermal processing of a precursor sol, and the film can be deposited on a transparent conducting oxide (TCO) substrate by any convenient thin-film process such as screen printing or tape casting. The preferred dye system is inspired by the natural processes involving chlorophyll, the coloring material in plants on which all earthly life depends. Chlorophyll is an organometallic dye, with a metal ion, Mg, within a porphyrin cage of nitrogen atoms. The synthetic chemist of course can select any convenient metal within the periodic table, and experience shows that ruthenium has the optimal properties expected. A ruthenium-pyridyl complex provides the chromophore of the dye, with the HOMO-LUMO gap, and thence the absorption spectrum bring modified by substitution with thiocyanide groups. Chemisorptive attachment of the dye to the metal oxide surface is obtained by carboxyl groups attached to the pyridyl components. The energetics of the dye is such that the LUMO level is just above the conduction band edge of the semiconductor, enabling relaxation by electron injection as required. A satisfactory electroactive dye structure, with good attachment properties and a wide optical absorption spectrum is therefore a sophisticated molecular engineering product. The electrolyte is also an optimized electrochemical system. The basic redox behavior is provided by the iodine/iodide system, with the advantage that the ions, both oxidized and reduced are relatively small, and therefore mobile in the supporting electrolyte. Energy losses due to slow diffusion are minimized. Early experiments used aqueous electrolytes, though with limited cell lifetime due to hydrolysis of the chemisorptive dye---semiconductor bond. A wide range of organic systems were therefore investigated, with the present favored formulation being based on imidazole salts. These have the additional advantage of low vapor pressure, very necessary as the photoactive sites under mid---day sun illumination may reach 80\r{ }C or higher. Low losses at the cathode counterelectrode are also a requirement for cell efficiency. The cathode is not necessarily transparent, and prototype cells on thin metal foils have been produced. However a TCO on glass or polymer counterelectrode is widely used. In either case suitable electrocatalytic behavior is required and frequently a nanodispersed Pt precipitated from haxachloride solution is employed. It is by now evident that the achievement of an industrially-competitive sensitized photoelectrochemical solar cell is the result of the optimization of several components, associated obviously with their effective synergy. Each change of a single component has repercussions on the choice and performance of others. However as already mentioned an efficiency of over 11{\%} has now been certified, and a stability of over 14,000 hours under accelerated testing with continuous simulated AM1.5 illumination was recently reported. In consequence there is increasing confidence on the part of industry. Several licensees of EPFL patents on dye---sensitized photovoltaic systems are now preparing for large-scale production. G24 Innovations PLC in Wales is commissioning a manufacturing plant, and Dyesol PLC in Australia is making available the required materials on an industrial scale. In conclusion, then, it can be stated that the DSC system is much more than a fascinating scientific artifact illustrating charge-transfer mechanisms at electrochemical interfaces; an efficiency and reliability with industrial credibility have been demonstrated and verified, and a significant role in competition with other photosystems can be foreseen. [Preview Abstract] |
Session L3: The Physics of Climate and Climate Change
Sponsoring Units: DFDChair: John Wettlaufer, Yale University
Room: Morial Convention Center RO2 - RO3
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L3.00001: The Disordered Kinetics of Earth's Carbon Cycle Invited Speaker: The carbon cycle describes the transformations of carbon as it cycles through living organisms and the physical environment. In its simplest form, the cycle amounts to a loop between photosynthesis and respiration. Photosynthesis produces organic carbon and molecular oxygen from carbon dioxide and water. Respiration reverses the process by oxidation of organic carbon. The duration of the cycle spans a vast range of time scales: from days or less for fast-growing plankton in the oceans, to hundreds of millions of years or more for the small fraction of organic matter that is buried as rock. The rates at which the cycle is closed set atmospheric carbon dioxide levels at short time scales and oxygen levels at geologic time scales. Respiration rates thereby influence not only climate---by the determination of equilibrium carbon dioxide concentrations---but also biological evolution---because the oxygenation of Earth's atmosphere must have preceded the advent of aerobic metabolism. We review recent advances in the understanding of the rates that control the carbon cycle, with emphasis on the respiratory back-reaction. Given considerable biological, chemical, and environmental variation, it comes as no surprise that measurements of rates vary greatly. Observations suggest, however, some surprising simplicity: for example, the rates of microbial consumption of organic matter in sediments and soils slow down systematically like the inverse of the age of the organic matter. This aging effect can be quantitatively understood as the macroscopic observation of microscopically disordered kinetics. The disorder can arise purely physically as the consequence of a reaction-diffusion process in porous media, but any combination of physical, chemical, and biological parameters that yield a wide range of rates suffices. A predicted practical consequence is a slow, logarithmic decay of organic matter in sediments and soils, which compares well with measurements. Further observations suggest that the effects of such disordered kinetics extends to inorganic processes as well. The carbon cycle thus appears not as a simple reaction network defined by a single set of rates, but rather as complex network in which the rates of specific reactions can be widely dispersed. We conclude by briefly discussing implications for short-term climate and long-term evolution. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L3.00002: The Quantum and Fluid Mechanics of Global Warming Invited Speaker: Quantum physics and fluid mechanics are the foundation of any understanding of the Earth's climate. In this talk I invoke three well-known aspects of quantum mechanics to explore what will happen as the concentrations of greenhouse gases such as carbon dioxide continue to increase. Fluid dynamical models of the Earth's atmosphere, demonstrated here in live simulations, yield further insight into past, present, and future climates. Statistics of geophysical flows can, however, be ascertained directly without recourse to numerical simulation, using concepts borrowed from nonequilibrium statistical mechanics\footnote{J. B. Marston, E. Conover, and Tapio Schneider, ``Statistics of an Unstable Barotropic Jet from a Cumulant Expansion,'' arXiv:0705.0011, J. Atmos. Sci. (in press).}. I discuss several other ways that theoretical physics may be able to contribute to a deeper understanding of climate change\footnote{J. Carlson, J. Harte, G. Falkovich, J. B. Marston, and R. Pierrehumbert, ``Physics of Climate Change'' 2008 Program of the Kavli Institute for Theoretical Physics.}. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L3.00003: Geostrophic Turbulence and the stability of Ocean models Invited Speaker: Despite multiple efforts, predictions of climate change remain uncertain. Where precision is an issue (e.g., in a climate forecast), only ensembles of simulations made across model families which differ for parameterizations, discrete algorithms and parameter choices allow an estimate of the level of imprecision. Is this the best we can do? Or is it at least conceptually possible to reduce these uncertainties? Focusing on ocean models in idealized domains we describe chaotic space-time patterns and equilibrium distributions that mimic nature. Using the Navier-Stokes equations for barotropic flows as a zero-order approximation of analogous flow pattern, we then investigate if is possible, in this overly-simplified set-up, for which smooth-solutions exist, to bound the uncertainty associated with the numerical domain discretization (i.e. with the limitation imposed by the Reynolds number range we can explore). To do so we analyze a series of stationary barotropic turbulence simulations spanning a range of Reynolds number of 10$^{4}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L3.00004: Heat waves, climate change and eggplant harvests - simple models of climate systems Invited Speaker: I discuss a simple box model of soil-vegetation-atmosphere interactions that we recently introduced to study the insurgence of summer droughts at continental midlatitudes (D'Andrea et al, GRL 2006, Baudena et al, AWR 2007). I show that the model possesses multiple equilibria and that, for the same synoptic forcing, soil moisture at the beginning of summer and vegetation cover play a primary role in determining which equilibrium will be reached. We also observe a difference in the drought climatologies associated respectively with the dynamics of natural vegetation, capable of adapting to the prevailing soil moisture conditions, and with cultivated vegetation such as eggplant, that cannot spontaneously modify its areal extent. I conclude with some speculations on a conceptual model of the interaction between vegetation and climate at global scale. The results discussed in this talk are the product of joint work with Fabio D'Andrea (ENS, Paris) and Mara Baudena (ISAC-CNR). [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L3.00005: Physical Problems in Modeling the Global Ocean Invited Speaker: Understanding and modeling the physical ocean circulation is of primary importance for both enhancing the science of the ocean, and for providing rational projections of future climate. This talk aims to outline fundamental physical and numerical aspects of ocean climate modeling. We highlight features associated with representing elements of the continuum ocean fluid using a discrete model lattice. A major challenge of this representation includes the parameterization of scales which are unresolved by the simulation. This subgrid-scale problem is ubiquitous in computational fluid dynamics, and forms a major focus of ongoing research and development with ocean climate models. Another challenge involves developing robust numerical methods whose truncation errors do not adversely corrupt the quasi-ideal nature of much of the ocean circulation outside of boundary layers. Progress has been made on both fronts, with improvements arising from better understanding of the ocean, smarter methods used to simulate the ocean, and enhancements in computational power. [Preview Abstract] |
Session L4: Bosonic Modes in HTSC
Sponsoring Units: DCMPChair: Mike Norman, Argonne National Laboratory
Room: Morial Convention Center 206
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L4.00001: STM Observation of a Bosonic Mode in the Electron-Doped Superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ Invited Speaker: Information on bosonic excitations in high temperature superconductors is part of a critical dataset that is necessary to decipher the puzzle of the pairing mechanism in these materials. In this talk, I will discuss our recent STM investigations of the electron-doped cuprate superconductor Pr$_{0.88}$LaCe$_{0.12 }$CuO$_{4-\delta }$ (PLCCO) ($T_{c}$ = 24 K). Our spectra reveal superconducting gaps with coherence peaks that disappear above $T_{c}$. In addition, multiple step/peak-like features are observed outside the gap. Such features in STM spectra are suggestive of bosonic excitations that couple strongly to the electrons. Analysis of the data indicates that the observed (bosonic) mode energy in PLCCO lies at 10.5$\pm $2.5 meV which is much lower than the bosonic mode observed in hole-doped BSCCO. The energy scale of our mode is the same as the magnetic resonance mode (spin-excitations) in PLCCO$^{ }$measured by inelastic neutron scattering but is also consistent with a low energy acoustic mode. Additionally, I will show that both the local mode energy and the intensity reveal correlations with the local gap energy scale. The sensitivity of the mode intensity to the energy scale of the onset of the continuum of excitations (2$\Delta )$ may indicate an electronic origin rather than phonons. This work was done in collaboration with F. C. Niestemski, S. Kunwar, S. Zhou, Shiliang Li, H. Ding, Ziqiang Wang, and Pengcheng Dai. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L4.00002: Bosonic self energy spectrum of high temperature superconductors from optical spectroscopy Invited Speaker: We address the problem of extracting the bosonic spectral function in high temperature superconductors using optical spectroscopy. Last year, we succeeded in inverting the optical spectra of the cuprates and extract the quantity, analogous to the electron-phonon spectral density $\alpha^2F(\Omega)$ in the conventional superconductors, for YBCO Ortho II system. We used the highly ordered crystals grown by Hardy, Bonn and Liang and compared our results with magnetic neutron spectra on samples from the same source measured by Stock {\it et al}. There was excellent agreement between the results of the two sets of spectroscopies. Since then we have refined our inversion technique and have been able to make a similar comparison for the LSCO system with new neutron scattering data from Vignolle {\it et al}. This magnetic spectrum, together with our Eliashberg inversion, accounts in a straightforward way for the lower $T_c$ of LSCO as compared to other cuprates. We also offer a detailed prediction of the evolution of the magnetic excitation spectrum with temperature and doping for the highly studied system Bi-2212, a material were neutron scattering data are very hard to get. Our data show that the bosonic self energy function evolves continuously from the broad background. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L4.00003: Visualizing pair formation in $Bi_2Sr_2CaCu_2O_{8+x}$ Invited Speaker: Unlike traditional superconductors, the density of states (DOS)
of the high-Tc superconductor $Bi_2Sr_2CaCu_2O_{8+x}$ shows
large nanoscale variations that have been detected using scanning
tunneling microscopy (STM) [1,2]. Such variations are seen in
the low temperature superconducting gap [1] and in features
associated with the coupling of pairs to boson modes [2]. In
order to understand these variations in the spectra, we perform
atomic resolution STM measurements of $Bi_2Sr_2CaCu_2O_{8+x}$ as
a function of temperature [3]. Using newly developed experimental
techniques, we measure the evolution of the DOS from low
temperature $(T< |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L4.00004: Evolution of the gaps through the cuprate phase-diagram Invited Speaker: The actual physical origin of the gap at the antinodes, and a clear identification of the superconducting gap are fundamental open issues in the physics of high-Tc superconductors. Here, we present an electronic Raman scattering study of single layer cuprates, as a function of both doping level and temperature. We examine both the evolution of the gaps close to the nodes and at the antinodes in the normal and superconducting sates. On the deeply over-doped side, we show that the anti-nodal gap is a true superconducting gap. In contrast, on the under-doped side, our results reveal the existence of a break point close to optimal doping below which the anti-nodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal state is distinctly different on each side of this breakpoint and will be discussed. \newline References: M. Le Tacon, A. Sacuto, A. Georges, G. Kotliar, Y. Gallais, D. Colson, A. Forget Two Energy Scales and two Quasiparticle Dynamics in the Superconducting State of Underdoped Cuprates, Nature Physics 2, 537, August 2006; W. Guyard, M. Le Tacon, M. Cazayous, A. Sacuto, A. Georges, D. Colson, A. Forget, Breakpoint in the evolution of the gap through the cuprate phase diagram, Cond Mat 0708.3732 [Preview Abstract] |
Session L5: Phase Transitions in Disordered Magnets
Sponsoring Units: DCMPChair: Raymond Bishop, University of Manchester
Room: Morial Convention Center RO1
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L5.00001: Dilute anisotropic dipolar systems as random field Ising ferromagnets Invited Speaker: We have shown the equivalence, at low energies, of dilute anisotropic dipolar magnets to the Ising model in the presence of an effective random longitudinal field and an effective transverse field, both of which are independently tunable. In the ferromagnetic (FM) regime [1], these systems constitute the first realization of the classical, as well as quantum, random field Ising model in a FM system, allowing, in particular, the application of a longitudinal field conjugate to the FM order parameter. In the spin-glass regime [2,3] we elucidate the role of both the hyperfine interactions, which couple the system to a spin bath and change the low-energy degrees of freedom, and the off-diagonal terms of the dipolar interactions, which lead to the effective random field. This resolves long standing questions regarding quantum spin glasses in general, and the quantum phase transition between the spin glass and paramagnetic phases in particular. \newline [1] $LiHo_xY_{1-x}F_4$ as a random field Ising ferromagnet, M. Schechter, Cond-mat/0611063. \newline [2] Significance of the hyperfine interactions in the phase diagram of $LiHo_xY_{1-x}F_4$, M. Schechter and P. C. E. Stamp, Phys. Rev. Lett. {\bf 95}, 267208 (2005). \newline [3] Quantum spin glass and the dipolar interactions, M. Schechter and N. Laflorencie, Phys. Rev. Lett. {\bf 97}, 137204 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L5.00002: A ferromagnet in a continuously tuneable random field Invited Speaker: The Random-Field Ising Model (RFIM) has been extensively studied as a model system for understanding the effects of disorder in magnets. Since the late 1970s, there has been a particular focus on realizations of the RFIM in site-diluted antiferromagnets. We observe random-field effects in the dilute dipole-coupled ferromagnet $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$. In the presence of a magnetic field transverse to the Ising axis ($H_t$), the behavior of $\mathrm{LiHo}_x\mathrm{Y}_{1-x}\mathrm{F}_4$ becomes increasingly dominated by the influence of random-field terms in the effective Hamiltonian. This is seen experimentally in the shape of the ferromagentic-paramagnetic phase boundary and in changes to the critical exponents near the classical critical point. We find that above the classical critical point the magnetic susceptibility diverges as $H_t\rightarrow0$, and that the susceptibility both above and below the classical critical point can be collapsed onto a single universal curve using a modified Curie law which explicitly incorporates random-field contributions. The discovery of a ferromagnetic realization of the RFIM opens the door to investigation of the random-field problem with the wide variety of techniques available for probing ferromagnets, including the ability to examine both the statics and dynamics of the random-field problem. It also allows studying the effects of controlled amounts of randomness on the dynamics of domain pinning and the energetics of domain reversal. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L5.00003: Activated quantum criticality of complex systems Invited Speaker: Magnetization measurements performed on single crystals of Ho$_{x}$Y$_{1-x}$LiF$_{4}$ with x = 16.5{\%} and 4.5{\%} show the same behavior for both compositions: (i) absence of divergence of the non-linear susceptibility in a transverse field (ii) same absence of divergence in zero field. These results are in sharp contrast with earlier studies of Ho$_{x}$Y$_{1-x}$LiF$_{4}$. In (i) the observed lack divergence results from the presence of random fields induced by the applied transverse field (no spin-glass phase transition, as predicted by Schechter, Laflorencie and Stamp). In (ii) it results from important slowing down of the dynamics due to huge energy barriers. Excellent fits are obtained for the linear and non-linear susceptibilities with ln($M)$ = -$T$f($H,T)$ (f is a functional form), suggesting the possibility of a dynamical phase transition involving thermally activated tunneling states. This model may also be useful for more general quantum dynamics of complex systems at finite temperatures. \newline \newline Scaling of non-linear susceptibility in MnCu and GdAl spin-glasses, B. Barbara, A. P. Malozemoff, and Y. Imry, PRL, 7, 1852 (1981). \newline Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo$_{x}$Y$_{1-x}$F$_{4}$, P. E. J\"{o}nsson, R. Mathieu, W. Wernsdorfer, A. M. Tkachuk, and B. Barbara, PRL, 98, 256403 (2007). \newline Nuclear spin driven quantum relaxation in LiHo$_{0.002}$Y$_{0.998}$F$_{4}$, R. Giraud, W. Wernsdorfer, A. M. Tkachuk, D. Mailly, and B. Barbara, PRL, 87, 057203 (2001). \newline Significance of the hyperfine interactions in the phase diagram of LiHo$_{x}$Y$_{1-x}$F$_{4}$, M. Schechter and P. C. E. Stamp, PRL, 95, 267208 (2005). \newline Quantum spin-glass and the dipolar interactions, M. Schechter and N. Laflorencie, PRL, 97,137204 (2006). \newline Induced Random Fields in the LiHo$_{x}$Y$_{1-x}$F$_{4}$ Quantum Ising Magnet in a Transverse Magnetic Field, S. M. A. Tabei, M. J. P. Gingras, Y.-J. Kao, P. Stasiak, and J.-Y. Fortin, PRL, 97, 237203 (2006). \newline Quantum spin-glass in anisotropic dipolar systems, M. Schechter, P.C.E. Stamp, and N. Laflorencie, J. Phys: Cond. Matt., 19, 145218 (2007). \newline Activated Scaling of Classical and Quantum Spin Glasses, B. Barbara. PRL, 99, 177201 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L5.00004: Glass Phenomenology from the Connection to Spin Glasses Invited Speaker: Using an effective potential replica formalism the properties of supercooled liquids near their glass transition are related to those of an Ising spin glass in a magnetic field. Results from the droplet picture of spin glasses are used to provide an explanation of the main features of fragile glasses such as Vogel-Fulcher-like behavior of the dynamics and the growing size as the temperature is reduced of the dynamically re-arranging regions. Exact solutions of one-dimensional fluids with glass-like features have been obtained and will be used to provide illustrations of the connection between glasses and spin glasses. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L5.00005: Is there an Almeida Thouless line in spin glasses? Invited Speaker: One of the most striking predictions of the mean field of spin glasses is a line of transitions in the magnetic field temperature plane, called the Almeida-Thouless (AT) line, which separates a high temperature, paramagnetic phase, with finite relaxation times, from a low temperature spin glass phase with infinite relaxation times. It is therefore represents an ergodic to non-ergodic transition with no change in symmetry. Whether or not an AT line occurs in real spin glasses has been controversial. Experiments have looked to see if there is a divergent relaxation time at finite field, and Ref. [1], for example, has argued their data indicates no AT line. However, other experimental papers have come to the opposite conclusion. Theoretically, it seems best to investigate a divergent {\em static} quantity, the ``replicon'' susceptibility (which is not accessible experimentally), and the corresponding correlation length. A finite size scaling analysis of the three-dimensional Ising spin glass in a field [2] found no AT line. It is, however, possible that an AT line could occur in higher dimensions, even if it does not occur in d=3. To investigate this question we used an analogous model, a one-dimensional system with long-range interactions which fall off with a power law, in which varying the power is analogous to varying the dimension in the short-range case. We do find an AT line [3] in models corresponding to short-range systems in dimension greater than 6.\\ \\ {[1]} J. Mattsson, T. Jonsson, P. Nordblad, H. A. Katori, and A. Ito, Phys. Rev. Lett. 74, 4305 (1995).\\ {[2]} A.P. Young and Helmut G. Katzgraber, Phys. Rev. Lett. 93, 207203 (2004).\\ {[3]} Helmut G. Katzgraber and A.P. Young, Phys. Rev. E 72, 184416 (2005). [Preview Abstract] |
Session L6: Minorities in Condensed Matter Physics
Sponsoring Units: COMChair: James Dickerson, Vanderbilt University
Room: Morial Convention Center RO4
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L6.00001: Multi-Scale Modeling of Conductive Polymers Invited Speaker: In spite of the tremendous impact of conductive polymers in a number of technological applications, molecular engineering of these materials has not been accomplished yet. One of the main reasons is the lack or limited understanding of the connection between changes occurring at the molecular level and the resulting polymer conductivity. Understanding the influence that local changes to the polymer's structure and chemical composition have on polymer properties, is the key to reach the stage where polymer-based materials and devices can be molecularly engineered with optimum properties. A multiscale model able to predict and accurately describe such a connection is thus a much needed tool to achieve this goal. The main aspect of this project is the bridging between scales in such a way that properties of the polymer at the molecular level are reflected in the observed and measured macroscopic properties. However, to achieve that integration, adapting and improving models at each of the involved scales must be done first. The progress towards improved models at the atomic and at the macroscopic level will be described. The atomic level is dealt with by using quantum mechanics calculations including semiempirical and ab initio methods. A semi-empirical/DFT study of oligomers will be described were extrapolation of electronic properties to an essentially infinite chain show excellent agreement with experimental results. The macroscopic level is addressed with probabilistic models, based on the Monte Carlo Technique, to study the charge transport process. The efforts toward the improvement and implementation of an existing transport algorithm, based on the hopping model, will be described. Existing models consider polymer and polymer devices as a cubic arrangement of sites and incorporate disorder as an ad-hoc parameter, in our model, the use of realistic configurations allows the distinction of intra- vs. inter-molecular conduction and the modeling of polymer devices. In addition, the model is reformulated to incorporate parameters calculated at the atomic level, thus the effect of on macroscopic properties produced by changes at the atomic level can be studies. Plans for the integration across scales, the final step to achieve multiscale modeling, will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L6.00002: Characterization of Hybrid Electronic Materials for Molecular Based Device Electronics Invited Speaker: Nanoscale device development, as an exercise in electronic materials fabrication, comes down to essentially one of two approaches---``top-down'' or ``bottom-up.'' In the former, the drive is to retain the macroscopic properties of the materials involved while shrinking toward ever diminishing device geometries. The latter approach attempts to control individual molecules and/or quantum-level structures, during the course of fabrication, in order to ultimately realize device functionality. This bottom-up design and control of molecular assemblies has united device engineers and scientists in testing new combinations of materials, both organic and inorganic. As a result, an entirely new class of nanoscale electronic structures and devices has emerged to create the field of hybrid electronic materials or HEMs$^{1}$. HEMs have shown promise as the basis for exciting device applications ranging from quantum computing to unique drug delivery methods. Although theoretical proposals of unimolecular transport began as early as the 1970s, it has been within the last decade in particular that research has yielded significant results in terms of nanoscale electronic applications for HEMs$^{2}$. However, many unresolved fundamental issues of electronic materials remain in this field. It is for this reason that in this talk, I will discuss a selection of HEM components (e.g.- self-assembled monolayers of n-alkanethiols, porphyrin molecules), their characterization, and their propensity for future nanoelectronic device development. 1. James R. Heath and Mark A. Ratner, Physics Today, May 2003; 43-49. 2. Mark A. Reed and Takhee Lee (Eds.), ``Molecular Nanoelectronics,'' American Scientific Publishers, Stevenson Ranch, CA, 2003. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L6.00003: Use of X-ray absorption spectroscopy in the search for the best LIGO mirror coatings Invited Speaker: The Laser Interferometer Gravitational-wave Observatory (LIGO) seeks to improve its sensitivity for gravity-wave detection by a factor of ten during its next phase of operation, Advanced LIGO. In order to achieve this goal it is necessary to design and fabricate test mass mirrors that help minimize the noise in the interferometers and in doing so maximize gravity-wave detection capability. In this talk we will present recent results from our program of X-ray absorption spectroscopy measurements to obtain detailed chemical composition and structure of titania (TiO$_{2})$-doped tantala (Ta$_{2}$O$_{5})$ multilayers fabricated via ion beam sputtering on SiO$_{2}$ substrates. Our investigations focus on how the microscopic features of the coatings influence their macroscopic mechanical loss properties. Our goal is to obtain correlations between chemical impurities and/or dopants and the optical absorption and mechanical loss characteristics of these multilayer coatings. ~To examine our samples we use synchrotron-based X-ray absorption Spectroscopy (XAS) techniques including Extended X-ray Absorption Fine Structure (EXAFS), X-ray Absorption Near Edge Structure (XANES) and X-ray Fluorescence (XRF).~ We present chemical and structural data obtained at the titanium K-edge and tantalum L$_{III}$-edge as well as relative elemental distribution information (Ti/Ta, Fe/Ta, and Cr/Ta) obtained via XRF. Following a brief description of the LIGO experiment, our program of research in optical materials for use in advanced versions of the interferometer will be described. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L6.00004: A Mathematical Solution to the Theoretical Band Gap Underestimation: Predictive Calculations of Properties of Semiconductors Invited Speaker: Most density functional theory (DFT) calculations find band gaps that are 30-50 percent smaller than the experimental ones, as illustrated in this presentation that recalls some popular explanations of this band gap problem, i.e., self-interaction effects and derivative discontinuities of the exchange correlation energy. A survey of the increasingly numerous approaches aimed at resolving the theoretical underestimation follows these explanations. These approaches include the Green function and screened Coulomb approximation (GWA), time dependent density functional theory (TDDFT), the exact exchange and screened exchange methods, and the use of local density approximation (LDA) potentials plus additional potentials located at atomic sites. Using the Rayleigh theorem, we describe a basis set and variational effect inherently associated with calculations that employ a linear combination of atomic orbitals (LCAO) in a variational approach of the Rayleigh-Ritz type. This description concomitantly shows a source of large underestimation errors in calculated band gaps, i.e., an often dramatic lowering of some \textit{unoccupied energies} on account of the Rayleigh theorem as opposed to the Hamiltonian. We present the Bagayoko, Zhao, and Williams (BZW) method [Phys. Rev. B 60, 1563 (1999); PRB 74, 245214 (2006); and PRB 76, 037101 (2007)] that follows from the description of the aforementioned effect and that leads (a) to DFT and LDA calculated band gaps of semiconductors in agreement with experiment and (b) theoretical predictions of band gaps that are confirmed by experiment. Unlike most calculations, BZW computations solve, self-consistently, a system of two coupled equations$.$ DFT-BZW calculated effective masses and optical properties (dielectric functions) also agree with measurements. We illustrate ten years of success of the BZW method with its results for GaN, C, Si, 3C-SIC, 4H-SiC, ZnO, AlAs, Ge, ZnSe, w-InN, InAs, and AlN. We conclude with a request to revisit beliefs relative to actual limitations of DFT and of schemes purporting to correct it or to go beyond it. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L6.00005: New Possibilities for Understanding Complex Metal Hydrides via Synchrotron X-ray Studies Invited Speaker: Ultrasmall-angle x-ray scattering (USAXS) and X-ray absorption spectroscopy (XAS)~are used~for the study of chemical and morphological changes in metal hydride powder (e.g. NaAlH4) both before and after transition metal salt catalytic dopant additions by high energy ball milling. The variation in surface fractal dimension and particle size with milling time and dopant content were tracked. These studies show that dopant content level (e.g. 2 mol {\%} and 4 mol {\%}) and dopant type (i.e. TiCl2, TiCl3, VCl3, and ZrCl4) markedly affects NaAlH4 powder particle surface area (determined using USAXS~surface fractal dimension). As well, the chemical reaction between the catalyst and hydride powder was further elucidated using XAS. Ti-metal reacts with the Al desorption product (from NaAlH4) to form TiAlx product phases.~ These studies were able to link powder particle surface area to catalytic doping and were able to link dopant chemical state with dehydrogenation reactant and product phases. [Preview Abstract] |
Session L7: Multiscale Phenomena in Biological Physics
Sponsoring Units: DBP GSNPChair: Jianpeng Ma, Rice University/Baylor College of Medicine
Room: Morial Convention Center RO5
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L7.00001: Self assembly of natural and synthetic membranes using coarse grain models Invited Speaker: The talk will review recent work by the Klein group in deriving force fields for natural and synthetic membrane forming systems. Molecular dynamics studies of the self-assembly will be presented for both types of systems. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L7.00002: Bridging time-scale gaps via reaction path optimization Invited Speaker: In this talk I will present a series of new computational methodologies that can be applied to systematically investigate the mechanism, free energy profiles, and rates of large-scale conformational changes of biomolecules. First, we enhance the efficiency of reaction path optimization methods, which use a series of duplicated systems, or replicas, to represent a discrete path by using holonomic constraints instead of reparametrization or using penalty potential functions that may require force projections to maintain equal distances between replicas. As a result, this formulation allows a straightforward application of super-linear optimization schemes such as the Adopted Basis Newton Raphson method, which uses much fewer energy and force evaluations to optimize a path. Novel objective functions, such as Hamiltonian and action, have also been designed for the search of novel pathways in addition to minimum energy paths. We have also generalized this approach to compute minimum free energy paths of a reaction. Second, constraints for sampling on the hyper-planes along an optimized path have been developed for computing the potential of mean force using the blue- moon approach. For obtaining rate information, we propose to solve the time-dependent Fokker-Planck equation by using the free energy profiles along a path as input. I will present the studies of two important conformational changes using these methods: the cis-to- trans isomerization of an alanine dipeptide and the helix-to-hairpin transition of an amyloid beta peptide. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L7.00003: A Novel Empirical Potential Function and A Monte Carlo Sampling Technique Invited Speaker: In protein folding study, two major issues are effective potential function and powerful sampling technique. In this meeting, recent results in both directions will be presented. In terms of potential function, we have developed an orientation-dependent statistical all-atom potential derived from side-chain packing. Test of the new potential on decoy set recognition indicates that it outperforms all the known statistical potential functions in the literature. Applications of this potential in substantially improving side-chain modeling will also be discussed. In terms of sampling technique, I will discuss some new results of a novel Monte Carlo sampling technique that performs simulation via direct computation of partition functions. The results will be compared with those of the well-known Wang-Landau sampling scheme. Application of this new MC method in studying protein folding will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L7.00004: Energy Landscape of Cellular Networks Invited Speaker: Cellular Networks are in general quite robust and perform their biological functions against the environmental perturbations. Progresses have been made from experimental global screenings, topological and engineering studies. However, there are so far few studies of why the network should be robust and perform biological functions from global physical perspectives. In this work, we will explore the global properties of the network from physical perspectives. The aim of this work is to develop a conceptual framework and quantitative physical methods to study the global nature of the cellular network. The main conclusion of this presentation is that we uncovered the underlying energy landscape for several small cellular networks such as MAPK signal transduction network and gene regulatory networks, from the experimentally measured or inferred inherent chemical reaction rates. The underlying dynamics of these networks can show bi-stable as well as oscillatory behavior. The global shapes of the energy landscapes of the underlying cellular networks we have studied are robust against perturbations of the kinetic rates and environmental disturbances through noise. We derived a quantitative criterion for robustness of the network function from the underlying landscape. It provides a natural explanation of the robustness and stability of the network for performing biological functions. We believe the robust landscape is a global universal property for cellular networks. We believe the robust landscape is a quantitative realization of Darwinian principle of natural selection at the cellular network level. It may provide a novel algorithm for optimizing the network connections, which is crucial for the cellular network design and synthetic biology. Our approach is general and can be applied to other cellular networks. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:30PM |
L7.00005: Finding Transition Pathways Using the String Method with Swarms of Trajectories Invited Speaker: |
Session L9: Colloidal Self Assembly and Interactions
Sponsoring Units: DFDChair: Eric Dufresne
Room: Morial Convention Center RO7
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L9.00001: Tunable Liquid Micromirror Based on Self-Assembly of ``Janus'' Particles Tom Krupenkin, Mike Bucaro, Paul Kolodner, J. Ashley Taylor In optofluidics, control over light propagation is primarily achieved by using the optical properties of liquid-gas and liquid-liquid interfaces. Currently, the vast majority of existing optofluidic systems are refractive optical devices. However, reflective optofluidic devices potentially have a number of important advantages over their refractive counterparts, since they are not constrained by the relatively low refractive index contrast commonly found in liquid-liquid and liquid-gas interfaces. In this work, we propose and experimentally demonstrate a novel approach that makes it possible to create tunable reflective liquid surfaces by combining the flexibility and tunability of liquid-liquid interfaces with the excellent reflective properties of solid metal surfaces. We employ self-assembly of reflective solid ``Janus'' particles at the interfaces between polar and non-polar liquids to create highly flexible, continuous, reflective ``carpets'' capable of acting as spherical micromirrors. We have successfully demonstrated electrowetting-based dynamic tuning of these micromirrors, including electrical control over mirror shape and focal distance. The mirror self-assembly process was studied as a function of the particle functionalization and of the chemical properties of the liquids involved. Potential applications of the proposed mirrors are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L9.00002: Emerging Structures for Colloidal Brushes: from Dispersions and Agglomerates to Spherulites, Wires, and beyond Alberto Striolo A large variety of nanoparticles holds extraordinary promises for practical applications, e.g., in catalysis and materials science. For these and other applications it is necessary to assemble the nanoparticles to yield supra-molecular aggregates of desired morphology. We are interested in the self-assembly of spherical colloids (i.e., nanoparticles) induced by interactions that become anisotropic because of entropic effects. Thus short polymer brushes are grafted on restricted regions of the spherical nanoparticles considered (e.g., the equatorial plane). Monte Carlo simulations were conducted to assess the properties of the self-assembled nanostructures as a function of the length of the brushes and of the strength of the particle-particle attraction. Depending on the specific solution conditions (particle-particle dispersive attractions, as well as length and density of the grafted polymer brushes) it is possible to obtain uniform dispersions, irregular aggregates, spherulites, one-dimensional wires, and two-dimensional colloidal sheets. We will discuss whether or not the effective colloid-colloid pair interactions at infinite-dilute conditions (i.e., the potential of mean force) can be used to predict the emerging behavior of the colloidal nanoparticles at larger concentrations. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L9.00003: Roughness-controlled depletion interactions for controlling colloidal self-assembly Kun Zhao, Thomas G. Mason The surfaces of colloidal particles resulting from many new fabrication methods are not molecularly smooth, so understanding how the surface roughness affects the depletion attraction is very important. We show that the depletion attraction between custom-shaped microscale platelets can be suppressed when the nanoscale surface asperity heights become larger than the depletion agent. In the opposite limit, the attraction re-appears and columnar stacks of platelets are formed. Exploiting this, we selectively increase the site-specific roughness on only one side of the platelets to direct the mass-production of a single desired assembly: a pure dimer phase. Furthermore, we model the interaction between flat plates coated by hemispheres having controlled sizes and densities relative to those of a spherical depletion agent. Overall, these studies provide significant insight into attractive bonds between particles that retain lubrication, and they provide a basis through which more complex assemblies can be made. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L9.00004: Corralled Colloids in Four Dimensions Stephen Anthony, Minsu Kim, Steve Granick Three colloidal particles were placed in small corrals and the strong correlations between their translation and rotation were quantified using the optical anisotropy of MOON (Modulated Optical Nanoprobes) particles to simultaneously measure their translation and rotation in an optical microscope. This system represents the simplest system which can capture one of the relevant components of multi-body interactions, the fact that while two particles can freely rotate together (like gears), once a third particle (or gear) is added there is no universally favorable set of rotations. This simple multi-body system provides a paradigm of how rotation influences translation and vice-versa. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L9.00005: Interactions among microdroplets at the water-air interface Chuan Zeng, Anthony D. Dinsmore There has been a great mystery concerning the origin of measured long-range attraction among microparticles at fluid interfaces. Recent theoretical work$^{1}$ showed that electrostatic interactions should not lead to long-range attraction, but the possibility remains that attraction arises from an irregular contact line on the particles' surfaces. Replacing the solid particles with liquid droplets eliminated surface roughness and thus reduced the complexity of the system. We captured micron-sized oil droplets at water-air interface and measured the interaction between them. The dynamics of droplets at interface were imaged using optical microscopy, from which the droplets' motions were tracked and analyzed. The interaction between two isolated droplets was calculated from their trajectories through the Markovian dynamics extrapolation method developed by J. C. Crocker and D. G. Grier$^{2}$. We acknowledge support from NASA through the Fluid Physics program (NRA 02-OBPR-03-C) and from the NSF-supported MRSEC on Polymers (DME-0213695).\\ $^{1}$ See, for example, M. Oettel, A. Dominguez, and S. Dietrich, \textit{Phys. Rev. E} \textbf{71}, 051401 (2005).\\ $^{2}$ J. C. Crocker and D. G. Grier, \textit{Phys. Rev. Lett.} \textbf{73}, 352 (1994). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L9.00006: Geometrical frustration in colloidal ``antiferromanget'' Yilong Han, Yair Shokef, Ahmed Alsayed, Peter Yunker, Tom Lubensky, Arjun Yodh We report experiments about a self-organized colloidal system that exhibits geometrical frustration similar to that of antiferromagnetic Ising spins on a triangular lattice. Novel thermally sensitive microgel NIPA (N-isopropyl acrylamide) spheres are close packed between two parallel flat walls with a vertical separation of about 1.5-particle diameters. The particles form an approximate in-plane triangular lattice. Neighboring particles tend to push each other toward opposite walls leading to out-of-plane local up and down buckling. We tune the strength of such effective antiferromagnetic interactions by varying temperature-tunable diameter of spheres. ``Spin'' flipping was directly visualized with video microscopy. We investigated the static structures, the dynamics of particles with different degrees of frustration and the degenerated ground state. This experiment is the first dynamic measurement in a geometrical frustrated system at single-particle resolution. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L9.00007: Correlated Motion of Ellipsoids Diffusing in 3D Kenneth Desmond, Eric R. Weeks Currently the hydrodynamic interaction between two ellipsoids in a fluid is not well understood. By observing the Brownian motion of micron sized ellipsoids suspended in a fluid using confocal microscopy, we directly measure these interactions. The ellipsoids exhibit both translational and rotational diffusion. The motion of an ellipsoid induces a flow field, which couples the motion of other ellipsoids with the first one. In our experiments we measure the translational and rotational diffusion of polystyrene ellipsoids suspended in a water glycerol mixture in three dimensions, and examine the spatial correlations between the rotational and translational motion of pairs of ellipsoids. Rotational motions set up a dipolar flow field, and thus the resulting correlations decay quicker than the correlations caused by translations. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L9.00008: Simulating Collective Dynamics of Confined Colloids Jonathan K. Whitmer, Erik Luijten We investigate the dynamical behavior of colloidal particles under confinement, by means of computer simulations that explicitly account for hydrodynamic interactions. Even under dilute conditions, long-range solvent-mediated coupling of the translational and rotational degrees of freedom influences the relative motion of colloidal particles. These effects on the collective dynamics are often ignored in simulations. Our calculations utilize the hybrid Stochastic Rotation Dynamics/Molecular Dynamics method [A. Malevanets and R. Kapral, J. Chem.\ Phys.\ \textbf{112}, 7260 (2000)] to incorporate both hydrodynamic and Brownian forces exerted on colloids by the solvent. The computational results are compared to recent experiments on few-body colloidal systems where the particle number is limited through confinement in a cylindrical trap. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L9.00009: Stripes and their zigzagging in buckled hard spheres Yair Shokef, Yilong Han, Ahmed Alsayed, Peter Yunker, Tom Lubensky, Arjun Yodh We use a hard sphere model to describe recent experiments on buckled colloidal monolayers. Our detailed Monte Carlo simulations exhibit the behavior, observed experimentally, of antiferromagnetic order and the formation of stripes that randomly zigzag around the system. Using free volume calculations, we deduce the strength of the effective antiferromagnetic interactions between neighboring particles. We furthermore explain how the geometrical frustration is partially removed by collective effects arising from sphere packing. We show how lattice distortions enable striped configurations to pack better than disordered ground states of the simple antiferromagnetic Ising model and that zigzagging of these stripes does not affect the free volume of the system. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L9.00010: Breakdown of Pairwise Additivity in Colloidal Electrostatics Sunil Sainis, Eric Dufresne Predictions of the structure and stability of charged colloidal suspensions typically assume pairwise additive forces. We directly measure electrostatic forces in small clusters of two to seven particles in a nonpolar solvent. We find that electrostatic interactions are not pairwise additive when the particle separations are much smaller than the screening length. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L9.00011: Mystery on Charge Asymmetry: Anionic Macroions in Periodic Lattices Held by Hydrated Cations and Not vice versa William Kung, Monica Olvera de la Cruz We propose a mean-field analytical model to account for the observed asymmetry in the ability to form long-range attraction by the negatively charged colloidal particles and not their equivalently charged positive counterpart. We conjecture that this asymmetry is due to solvation effects, and we phenomenologically capture its physics by considering the relative strength of this water-induced short-range repulsion between the different charge species. We then apply our model to the colloidal system of negatively charged disks that are neutralized by a sea of counterions and strongly absorbed to an interface in a compressible binary system. We demonstrate the resulting coexistence between a dilute isotropic ionic phase and a condensed hexagonal lattice phase as a function of density and interaction strength. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L9.00012: Charged colloids in low polar solvents Andrew Hollingsworth, Mirjam Leunissen, William Irvine, Paul Chaikin, Alfons van Blaaderen In a low polar environment, sterically-stabilized poly(methyl methacrylate) spheres become positively charged and exhibit extraordinary long-range repulsive interactions. Confocal microscopy shows that they can form low density, body centered cubic crystals with lattice constants up to 40 microns. We attribute this behavior to the cyclohexyl bromide (CHB) in which the colloidal particles are suspended. CHB is a desirable solvent due to its density matching capability; however, it is difficult to purify. Trace amounts of the hydrogen halide resulting from the hydrolysis of CHB apparently interact with the stabilizer layer, imparting charge to the colloids. Surprisingly, water can also be used to deionize the organic solvent, depending on the relative amounts of the two fluids. The addition of quaternary ammonium salts was used to screen charge (reducing long range particle interaction). Ionic strengths were computed using ionic association theory; in turn, particle charge and surface potentials were estimated from electrokinetic measurements. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L9.00013: Direct measurements of the pair potentials of colloids with light scattering and optical traps Kisun Yoon, Vinothan Manoharan We present a methodology of directly measuring the pair potentials of colloids. We take snapshots of the thermal fluctuation of a pair of colloidal particles in equilibrium. The probability distribution of the separation distance obtained from the snapshots should follow the Boltzmann distribution because the separation distance of the particle pair is the only independent variable necessary to describe the effective free energy of a macrostate of the colloidal particle pair in equilibrium. The measurement of the pair potentials can be achieved by appropriately subtracting the unwanted potentials due to optical traps and optically induced interactions from the effective free energy. Accurate measurement of the separation distance between colloidal particles has critical importance in measuring colloidal interactions. Conventional Video Microscopy used for separation distance measurement is significantly restricted due to the two-dimensional nature of the measurement. Furthermore, the measurement is seriously distorted when the two particles are nearly in contact because of the diffraction of light and multiple scattering effect. We introduce a new technique to accurately measure the separation distances using light scattering. This light scattering technique can measure the separation distance in 3D and appropriately considers the multiple scattering effect. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L9.00014: Concentration Gradients in Mixed Magnetic and Nonmagnetic Colloidal Suspensions Randall Erb, Benjamin Yellen The ability to form concentration gradients in mixed magnetic/nonmagnetic colloidal suspensions using magnetic field gradients has many practical applications in the fields of biosensors and life science diagnostics. Previously, we developed and experimentally confirmed a self-consistent model describing the local distribution of magnetic nanoparticles exposed to a magnetic field gradient. Here, we have derived an analytic expression to describe the local concentration of nonmagnetic colloids which are also affected by field gradients when inside magnetic colloidal suspensions. The model calculates the force on particles as a function of local magnetic particle concentration, and solves for the equilibrium distribution of particles through the drift-diffusion equations. We investigate the ability to concentrate and deplete nonmagnetic particles from specific regions of a substrate, such as nearby patterned micro-magnets on a substrate. Also, we have qualitative experimental results to support our expression. Our results show that nonmagnetic particles which are 5-10 times larger than the magnetic nanoparticles can be effectively concentrated or depleted at specified regions of the substrate. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L9.00015: Interactions and self assembly of two heterogeneously charged surfaces Robert Brewster, Philip Pincus, Samuel Safran Recent experiments$^{1,2}$ have measured attractive interactions between two surfaces that each bear two molecular species with opposite charge. Theoretical considerations predict equilibrium finite-sized domains of each species, consistent with experiment. These domains, whose observed sizes are typically tens of nanometers, are the result of a balance between the line tension, which prefers macroscopic separation, and the electrostatics, which prefers mixing. Additionally, two such surfaces show a long range attraction. We present a theoretical model that predicts the domain size, phase behavior and forces for two such interacting surfaces. \\* \\* (1) E. E. Meyer, Q. Lin, T. Hassenkam, E. Oroudjev, J. N. Israelachvili PNAS {\bf 102}, 6839 (2005). \\* (2) S. Perkin, N. Kampf, J. Klein, Phys. Rev. Lett. {\bf 96}, 038301 (2006). [Preview Abstract] |
Session L10: Focus Session: Electronic and Vortex Mechanisms for Higher Performing Superconductors
Sponsoring Units: DMPChair: John Sarrao, Los Alamos National Laboratory
Room: Morial Convention Center RO8
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L10.00001: Exploring the limits of critical currents in superconductors Invited Speaker: Mechanisms, which determine the ultimate limit of the critical current density $J_c(T,B)$ in superconductors are discussed. The talk is mostly focused on the extreme strong pinning limit of highly deformed vortex segments, the role of anisotropy, current-blocking effects of pinning centers and grain boundaries, thermal fluctuations of vortices in high-$T_c$ superconductors. In particular, the design of optimum pinning nanostructures, which produce the maximum $J_c$ is addressed. The results are applied to YBCO thick-film coated conductors with insulating nanoprecipitates, for which several groups have reported very high $J_c$ values, up to 12-20 $\%$ of the depairing current density. Requirements for a putative room-temperature superconductor to be useful in high-field applications are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L10.00002: Current density in YBCO-based Tapes Studied over 8 Decades of Dissipation J.R. Thompson, Ozgur Polat, D.K. Christen, D. Kumar, P.M. Martin, J.W. Sinclair Many applications of superconductors require conduction of high density electric currents in a magnetic field, with minimal dissipation. We investigated the dependence of current density $J$ on electric field $E$ due to motion of depinned vortices, over a range of $\sim $10$^{8}$ in $E$. The materials are pre-commercial YBa$_{2}$Cu$_{3}$O$_{\sim 7}$ coated conductors (3.5$\mu $m) on buffered Hastelloy substrates prepared by SuperPower, Inc. Experimental methods include conventional 4-probe electrical transport at the highest $E$ fields; inductive measurements of magnetic moment $m\sim J$ using a swept magnetic field d$H$/d$t\sim E$ at lower $E$ fields; and time dependent ``flux creep'' measurement where d$m$/d$t\sim E$. At $T$ = 77 K, a power law variation $E\sim J^{n}$ is found. The resulting $E(J)$ dependencies become steeper, i.e., the characteristic $n$-value increases, as $J$ is reduced, reflecting a diverging activation energy for vortex movement. The inductive studies are easily extended to lower temperatures and a wide range of magnetic fields. Implications for applications will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L10.00003: Isotropic critical currents in anisotropic superconductors: a simple physical model D.K. Christen, Y.L. Zuev, S. Wee, A. Goyal, S.W. Cook Critical current densities, $J_{c}$, that are nearly independent of magnetic field orientation can be observed in intrinsically anisotropic high-temperature superconductors that have specific, very strong flux pinning nanostructure. The phenomenon is observed to occur at specific temperature dependent fields, $H$*($T)$. The possibility of such isotropic behavior can be described by a simple physical model based on the orientation dependence of the irreversibility field $H_{irr}(\theta)$ and the power-law decay exponent $\alpha (\theta )$, where $J_{c}\propto H^{-\alpha }$ in the intermediate field regime. An analysis will be discussed that elucidates necessary conditions for occurrence of the effect, and provides possible predictive tools for tailoring of $H$*($T)$ to practical fields and temperatures by means of defect engineering. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 4:06PM |
L10.00004: Exploring the limits to vortex pinning in superconductors Invited Speaker: Vortices in type II superconductors sit on a potential energy landscape created by material inhomogeneities. In the presence of an electrical current these inhomogeneities produce a restoring force that precludes vortex motion, thus allowing dissipation-free transport, as long as the current density does not exceed the critical current density $J_{c}$. Based on present theoretical understanding, by introducing the appropriate type of pinning centers it should be possible to attain $J_{c}$ values (for low vortex densities) as large as the physical limit determined by the depairing current density $J_{0}$. However, after decades of large efforts and resources dedicated to pinning enhancement (which has obvious technological relevance) we are far below that limit. Presently, the largest $J_{c}$/$J_{0}$ ratios have been obtained for very thin epitaxial YBa$_{2}$Cu$_{3}$O$_{7}$ films and are $\sim $0.3, slightly higher than in the conventional superconductor Nb-Ti ($J_{c}$/$J_{0}\sim $0.25). I will analyze the possible reasons for this limitation and discuss possible ways to circumvent it. I will particularly focus on the influence of thermal fluctuations, which promote some level of vortex motion even below $J_{c}$, resulting in a temporal decay of the supercurrents and consequently lower $J_{c}$ values as determined by standard experimental techniques. Based on general principles, I will discuss what pinning performance we may expect in yet-to-be-discovered superconductors with high $T_{c}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L10.00005: Magnetic field and temperature specific isotropic critical currents in strong-pinning high-temperature superconductors Y.L. Zuev, D.K. Christen, S.H. Wee, A. Goyal, S.W. Cook We report the observation of a unique temperature-dependent magnetic field, $H$*($T)$, at which the critical current of (R)BaCuO (R=rare earth) films with strong $c$-axis pinning can be nearly isotropic. That is, $J_{c}(\theta $, $H$*) $\cong $ constant over nearly the entire interval of sample orientation from \textit{H$\vert \vert $c} to \textit{H$\vert \vert $ab }(in the full Lorentz force configuration). The phenomenon is observed in classes of HTS coatings that contain self-assembled, strongly pinning columnar stacks of second-phase precipitates, BaZrO$_{3}$, oriented near the $c$ axis, and appears to originate from the combination of and offsetting effects of material anisotropies. Systematics of this behavior will be explored and several important control parameters will be identified. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L10.00006: AC losses in multifilamentary YBCO thin films Ran Yang, Andrea Lucarelli, Gunter Luepke, Timothy Haugan, Paul Barnes Striation of superconducting tape allows the reduction of hysteresis losses. We studied the effect of an ac current as a function of the frequency and of a static magnetic field on the flux behavior in ultifilamentary YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin films. The current density, the magnetic and electric field profiles are determined quantitatively during the cycle. The shielding and transport current distribution in the filaments are affected by hysteresis and inductive effects that depend on the number and the distribution of the filaments. Time resolved magneto-optical imaging measurements reveal a cross-talk between adjacent conducting filaments that affects the overall hysteresis losses. This new, quantitative and fast method allows us to determine a of dynamic parameters, such as mapping the transport current density and electric field distribution during the ac cycle, that are important for practical superconducting applications and complementary to conventional transport measurement techniques. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L10.00007: Evaluation of YBCO to 45T over wide temperature range Zhijun Chen, Fumitake Kametani, David Larbalestier The ability to tune the vortex pinning of YBCO in coated conductor form is both an enormous benefit to future superconducting materials applications and a challenge to understanding the properties over a broad range on particular samples. We have been doing such characterizations in support of the design of 30 T magnets. In recent work we have mapped the $J_{c}$ to fields of 33T at temperatures down to 4K and measured the angular dependent $J_{c}$ in similar fields. We see that it is possible to enhance $H_{irr}$ by about 15{\%} as compared to standard YBCO at 55K, when a high density of RE$_{2}$O3 nanoprecipitates is formed in the microstructure These precipitate arrays produce $J_{c}$ 50 MA/cm$^{2}$ almost 20{\%} of the depairing current density. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L10.00008: Vortex phases and dynamics in YBa$_{2}$Cu$_{3}$O$_{7}$+BaZrO$_{3}$ films as a function of angle and field up to 50 Tesla S.A. Baily, B. Maiorov, H. Zhou, S.R. Foltyn, T.G. Holesinger, Q.X. Jia, Leonardo Civale, F.F. Balakirev, M. Jaime Studying the vortex solid-liquid transition (resistivity=0) in high T$_{c}$ superconductors is scientifically and technologically relevant. We have used low current transport measurements to study the melting line of YBa$_{2}$Cu$_{3}$O$_{7}$ films with and without BaZrO$_{3}$ additions in fields up to 50 T. Samples with mostly extended particle defects, mostly columnar defects, or a mixture of both will be compared. Plain YBa$_{2}$Cu$_{3}$O$_{7}$ shows correlated pinning along the crystalline axes and the emergence of a smectic phase when field is aligned with the a-b plane. Inclusion of BaZrO$_{3}$ not only alters the angular dependence of the irreversibility line indicating the stronger influence of c-axis correlated pinning, but also affects dissipation in the vortex-liquid state over the entire angular range. We will discuss the results in terms of vortex pinning, the corresponding types of phase transitions, micro-structural analysis, and information obtained from critical current measurements. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L10.00009: Vortex pinning landscape in MOD-TFA YBCO nanostroctured films J. Gutierrez, T. Puig, A. Pomar, X. Obradors A methodology of general validity to study vortex pinning in YBCO based on J$_{c}$ transport measurements is described. It permits to identify, separate and quantify three basic vortex pinning contributions associated to anisotropic-strong, isotropic-strong and isotropic-weak pinning centers. Thereof, the corresponding vortex pinning phase diagrams are built up. This methodology is applied to the new solution-derived YBCO nanostructured films, including controlled interfacial pinning by the growth of nanostructured templates by means of self-assembled processes [1] and YBCO-BaZrO$_{3}$ nanocomposites prepared by modified solution precursors. The application of the methodology and comparison with a standard solution-derived YBCO film [2], enables us to identify the nature and the effect of the additional pinning centers induced. The nanostructured templates films show c-axis pinning strongly increased, controlling most of the pinning phase diagram. On the other hand, the nanocomposites have achieved so far, the highest pinning properties in HTc-superconductors [3], being the isotropic-strong defects contribution the origin of their unique properties. [1] M. Gibert et al, Adv. Mat. vol 19, p. 3937 (2007) [2] Puig.T et al, SuST EUCAS 2007 (to be published) [3] J. Gutierrez et al, Nat. Mat. vol. 6, p. 367 (2007) * Work supported by HIPERCHEM, NANOARTIS and MAT2005-02047 [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L10.00010: Flux dynamics in a two-band superconductor with delocalized electric fields Milind N. Kunchur, James Knight In conventional flux flow, vortex dissipation is localized to the vicinity of the vortex core leading to a viscous coefficient $\eta$ that is independent of flux density $B$ and a flux-flow resistance $R_f \propto B$. This causes a progressive broadening with $B$ of $I$-$V$ and $R$-$T$ curves, which in turn degrades a superconductor's performance in switching applications. An anomalous behavior arises when a substantial quasiparticle population exists away from the cores and when the electric field and dissipation extend into those regions---a scenario that is realized in a disordered two-band superconductor with slow branch-imbalance relaxation. In this case $\eta$ rises linearly with $B$ and $R_f$ becomes independent of $B$, as observed in disordered magnesium diboride. Such an intrinsically field indifferent mixed-state response makes this system especially suited for magnetic-field induced switching. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L10.00011: In-Field Critical Current by Correlated Anti-Pins in Type-II Superconductors Eric J. Oswald, Jose P. Rodriguez The critical current shown by films of YBa$_2$Cu$_3$O$_y$ that contain naturally occuring linear pinning centers aligned parallel to the c-axis decays with increasing magnetic field that is also aligned in parallel as an inverse-square-root power law. Recent theoretical work based on 2D collective pinning of the vortex lattice by such material line defects recovers this dependence on magnetic field in the weak-pinning limit[1]. It further predicts an in-field critical current for correlated {\it antipins} that decays with magnetic field more slowly, as an inverse power law characterized by an exponent below 1/2. We test these predictions by performing Langevin dynamics simulations of the corresponding 2D vortex lattice driven by the Lorentz force. Long-range logarithmic interactions between vortices are assumed, while (anti)pinning centers are arranged in a ``liquid'' fashion. We find indeed that collective pinning by antipins result in a significant critical current. More detailed comparisons with theory in relation to the power-law decay with magnetic field will be made. \newline [1] J.P. Rodriguez and M.P. Maley, Phys. Rev. B {\bf 73}, 094502 (2006). [Preview Abstract] |
Session L11: Superconductivity: Transport and Fermi Pockets
Sponsoring Units: DCMPChair: Richard Greene, University of Maryland
Room: Morial Convention Center RO9
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L11.00001: Low Temperature Thermal Conductivity in Cuprate Superconductors Amidst Coexisting Charge Order: Part II - Effect of Self-Consistent Disorder and Vertex Corrections Philip Schiff, Adam Durst As in Part I, we consider a d-wave superconductor (dSC) in which the superconductivity coexists with charge density wave (CDW) order of wavevector ($\pi$,0). Here we discuss two significant improvements to the calculation of the low temperature thermal conductivity. First, rather than taking the 4-by-4 self-energy matrix to be a scalar and a constant, independent of the CDW order parameter, we compute it from an impurity density and scattering potential, within the self-consistent Born approximation. We find that self-consistency requires two off-diagonal terms in the self-energy matrix, in addition to the diagonal term. Furthermore, we find that these off-diagonal terms dominate in the clean limit. Second, we include ladder corrections to the bare vertex, as required to satisfy Ward identities. We compute their contribution, which is verified to be small in the low impurity density limit. However, the contribution of the off-diagonal terms in the self-energy matrix is found to be quite important, significantly reducing the critical value of the CDW order parameter beyond which the thermal conductivity vanishes. Results are used to identify signatures of the effect of charge order on low temperature thermal conductivity, with an eye toward understanding the effect of charge order on quasiparticle transport in the underdoped cuprates. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L11.00002: Low Temperature Thermal Conductivity in Cuprate Superconductors Amidst Coexisting Charge Order: Part I - Bare Bubble Calculation Adam Durst, Subir Sachdev We consider a d-wave superconductor (dSC) in which the superconductivity coexists with charge density wave (CDW) order of wavevector ($\pi$,0). While the nodes of the quasiparticle energy spectrum survive the onset of charge order, there exists a critical value of the CDW order parameter beyond which the quasiparticle spectrum becomes fully gapped. We perform a linear response Kubo formula calculation of thermal conductivity in the low temperature (universal) limit, where a simplified model of disorder has been employed and vertex corrections have been neglected. (Effects of self-consistent disorder and vertex corrections are discussed in a separate talk -- Part II.) Results reveal the dependence of thermal transport on increasing CDW order parameter up to the critical value at which the quasiparticle spectrum becomes fully gapped and thermal conductivity vanishes. In addition to numerical results, closed-form expressions are obtained in the clean limit for the special case of isotropic Dirac nodes. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L11.00003: Thermal and electrical transport in a random dissipative quantum system controlled by an infinite randomness critical point Adrian Del Maestro, Bernd Rosenow, Markus Mueller, Subir Sachdev The complicated interplay between quantum fluctuations and disorder in the vicinity of a quantum phase transition can have drastic effects and lead to exotic Griffiths phases and the flow to infinite randomness. We present a study of a dissipative Hertz-like theory in the presence of quenched disorder which may describe the quantum phase transition between a superconductor and metal in ultra narrow metallic wires tuned by an external source of pair-breaking. By finding a numerical solution to the large-N self-consistency equations for real-space chains we are able to directly compute the finite temperature thermal and electrical d.c. conductivities throughout the quantum critical regime. From an analysis of the typical and average correlation functions we find evidence for dynamically activated scaling in accord with recent strong disorder real space renormalization group calculations. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L11.00004: Theory of the Marginal Fermi liquid Arkady Shehter, Chandra Varma Marginal Fermi liquid is a successful phenomenological description of the strange metal phase of cuprates near optimal doping. We cast the theory into the standard microscopic form for interacting fermions, employing particle-hole rescattering and (singular) Landau parameters. We calculate the density-density correlation function and demonstrate its standard relation to the conductivity through the continuity equation. Internal consistency of the theory is ensured by showing that the relevant conservation laws are obeyed. The relation to the kinetic equation is established. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L11.00005: The effect of electron pockets on the quasiparticle interference patterns in cuprates Kangjun Seo, Jiangping Hu We study the hole/electron pockets in the $d$-density wave and the spin density wave states. While the hole/electron pockets can be seen in the presence of the DDW and the SDW orders, the shape, locations, and the number of the Fermi pockets are different, depending on the band structures and the wavevectors of the order parameters. We calculate the local density of states in the presence and the absence of the electron pockets in the DDW and SDW states. We find that the quasiparticle scattering interference patterns by the single impurity are dramatically affected in the presence of the DDW and SDW order in comparison with one without the electron pockets. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L11.00006: Quantum oscillations and Fermi surface in underdoped YBa$_2$Cu$_3$O$_{6.5}$ Nicolas Doiron-Leyraud, Cyril Proust, David LeBoeuf, Julien Levallois, Jean-Baptiste Bonnemaison, Ruixing Liang, Doug Bonn, Walter Hardy, Louis Taillefer We report quantum oscillations in the transport properties of YBa$_2$Cu$_3$O$_{6.5}$ at 10\% doping, showing that a well-defined Fermi surface with closed orbits is a fundamental property of underdoped cuprates. In contrast with the large Fermi surface seen on the overdoped side, we observe a very small orbit whose area is only 1.9\% of the Brillouin zone. Such a small Fermi surface does not come from the band structure of YBa$_2$Cu$_3$O$_{6.5}$ and is most likely the result of a reconstruction. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L11.00007: Shubnikov-de Haas oscillations in YBa$_2$Cu$_4$O$_8$ A. F. Bangura, J. D. Fletcher, A. Carrington, P. J. Heard, N.E. Hussey, J. Levallois, M. Nardonne, B. Vignolle, C. Proust, N. Doiron-Leyraud, D. LeBoeuf, L. Taillefer, S. Adachi The recent report of quantum oscillations in the single-chain underdoped cuprate YBa$_2$Cu$_3$O$_{6.5}$ - ortho II (hole doping p$\sim$0.10) points to the possibility that the underlying electronic structure in the underdoped region of the cuprate phase diagram contains Fermi surface pocket(s), at odds with results from ARPES experiments. In this talk I will describe our observation of quantum oscillations in the Hall resistivity $\rho_{xy}$ of the stoichiometric double-chain cuprate YBa$_2$Cu$_4$O$_8$ (hole doping p$\sim$0.14), in pulsed magnetic fields up to 62T. Our results show that both the area of the quasiparticle orbit and the cyclotron effective mass of YBa$_2$Cu$_4$O$_8$ are larger than those measured for YBa$_2$Cu$_3$O$_{6.5}$ - ortho II. The observed negative Hall coefficient and the failure of LDA bandstructure calculations to account for the oscillations suggests a non-trival origin. However, clear evidence of quantum oscillations in materials with such different levels of doping and the details of the transport properties of the two compounds, allows us to conclude that small Fermi surface pocket(s) are a generic feature of the underdoped side of the Yttrium-based cuprate phase diagram and are associated with the CuO$_2$ planes. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L11.00008: Electron pockets in the Fermi surface of hole-doped high-$T$c superconductors David LeBoeuf, Nicolas Doiron-Leyraud, Julien Levallois, Ramzy Daou, J.-B. Bonnemaison, Nigel Hussey, Luis Balicas, Brad Ramshaw, Ruixing Liang, Doug Bonn, Walter Hardy, S. Adachi, Cyril Proust, Louis Taillefer The Fermi surface of the electronic states in the underdoped `YBCO' materials YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L11.00009: Evidence of Fermi surface reconstruction and the formation of small hole pockets in underdoped La$_{2-x}$Sr$_x$CuO$_4$: Far Infrared Hall measurements D.C. Schmadel, G.S. Jenkins, H.D. Drew, I. Tsukada, T. Ando The Hall Effect in La$_{2-x}$Sr$_x$CuO$_4$ films is measured from 3 to 100 meV as a function of temperature from 5K to 300K and carrier doping ranging from severely underdoped ($x=0.03$) to optimal doped ($x=0.15$). The behavior of the infrared Hall angle with temperature and frequency is found to be consistent with a simple extended Drude model at all dopings. A significant reduction of the Hall mass is observed when the hole doping level is reduced from optimal doping, which is consistent with a drastic reduction of the Fermi surface volume. These results are similar to earlier mid-IR Hall measurements obtained in underdoped YBCO, [1] and related to the recent observations of quantum oscillations reported in YBCO. [2] \newline \newline [1] L. B. Rigal, et al., Phys. Rev. Lett. 93, 137002 (2004). \newline [2] N. Doiron-Leyraud, et al., Nature, 447 565 (2007); A. F. Bangura, et al., Cond-mat/07074461 and E. A. Yelland, et al., Cond mat/07070057 [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L11.00010: Evidence of Fermi surface reconstruction in Pr$_{2-x}$Ce$_x$CuO$_4$: Far IR Hall measurements in electron doped cuprates G.S. Jenkins, D.C. Schmadel, P. Bach, R.L. Greene, H.D. Drew The Hall Effect is measured at far infrared frequencies (24-85 1/cm) in Pr$_{2-x}$Ce$_x$CuO$_4$ films as a function of temperature from 5K to 300K and electron doping levels ranging from severely underdoped ($x=0.10$) to overdoped ($x=0.19$). In underdoped PCCO, the doping and temperature dependence of the complex Hall angle is found to be consistent with a simple Drude model with an associated reduction of the Hall mass in comparison with optimal doping. The mass reduction is consistent with Fermi surface reconstruction and the formation of small electron pockets. In overdoped PCCO, evidence for both electron and hole contributions to $\sigma_{xy}$ is observed even at low temperatures, a contradistinctive result compared with the DC Hall Effect in PCCO as well as the behavior observed in IR Hall measurements in La$_{2-x}$Sr$_x$CuO$_4$. These data suggest interactions through the exchange of incoherent zone corner magnons. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L11.00011: Disorder and the Metal-Insulator Crossover in Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ P. L. Bach, W. Yu, J. S. Higgins, H. Xu, R. L. Greene, B. Weaver One of the outstanding issues in the electron doped cuprates is the role that oxygenation plays in the superconductivity and normal state properties. Oxygen addition can be considered both a doping and a disordering process. To disentangle these two effects, disorder can be introduced by irradiating the samples without altering doping. We report transport studies on optimal and underdoped Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ films subject to proton irradiation and oxygenation. We establish a correlation between the static AFM and the metal-insulator crossover. Our separation of the disorder and doping effects also shed light on oxygen reduction effects in electron-doped cuprates. Supported by NSF grant DMR-0653535. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L11.00012: Doping dependence of the dynamic critical exponent in Pr$_{2-x}$Ce$_x$CuO$_4$ M.C. Sullivan, J. Sousa, M. Salvaggio, R.L. Greene Scaling analysis of voltage vs. current isotherms is a favorite tool to study the normal-superconducting phase transition in cuprate superconductors. This measurement has never been performed on the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$, despite unusual behaviors which may alter this phase transition and yield interesting results (behaviors such as the extended doping range of the anti-ferromagnetic phase and the quantum critical point). This is perhaps due to the lack of consensus regarding the analysis of voltage vs.\ current isotherms, due in part to finite-thickness effects even in thick ($d \approx 3000$\AA) films.\footnote{ Phys. Rev. B \textbf{69}, 214524 (2004)} If finite-thickness effects are taken into consideration, we can find the dynamic critical exponent $z$ in our Pr$_{2-x}$Ce$_x$CuO$_4$ films. We present our results of the dynamic critical scaling exponent $z$ as a function of doping. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L11.00013: Thermal and electrical transport in the cuprate superconductor Nd-LSCO Louis Taillefer, R. Daou, S.Y. Li, David LeBoeuf, Olivier Cyr-Choiniere, J.-Q. Yan, J.-S. Zhou, J.B. Goodenough We present a study of the thermal and electrical transport properties of the cuprate superconductor Nd-LSCO in a wide range of doping from x=0.12 to x=0.25. The behavior of the thermal conductivity in the zero temperature limit allows us to access the quasiparticle transport in both the superconducting and normal state. The impact of stripe order on quasiparticle transport is discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L11.00014: Measurements of the Hall Effect and Resistivity in La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$ with Ultrafine Stoichiometry Resolution, $\delta x$~$\sim$~2.5~$\times$~10$^{\rm -4}$ Jeffrey Clayhold, Bryan Kerns, Michael Schroer, David Rench, Gennady Logvenov, Anthony Bollinger, Ivan Bozovic Recent reports of sharp changes of transport properties with small variations of stoichiometry in cuprate superconductors have motivated us to look for similar behavior in optimally- and over-doped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$, using a recently completed system for creating and measuring samples with ultrafine stoichiometry resolution. The data are from MBE films grown with a linear stoichiometry gradient and were taken with a characterization system that can measure both the Hall effect and resistivity simultaneously at 31 different locations on the film. We will show new data for $x$ ranging from 0.15 to 0.30. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L11.00015: Magnetotransport and noise in lightly doped La$_{2-x}$Sr$_x$CuO$_4$ and La$_{2}$Cu$_{1-x}$Li$_{x}$O$_4$ Ivana Rai\v{c}evi\'{c}, Dragana Popovi\'{c}, Christos Panagopoulos, Takao Sasagawa We report a detailed comparative study of magnetotransport and noise in high quality single crystals of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and La$_{2}$Cu$_{1-x}$Li$_{x}$O$_4$ ($x=0.03$) at temperatures 0.100$\leq T$(K)$\leq 150$ and fields $0\leq B$(T)$\leq 18$ parallel and perpendicular to the c-axis. Our results demonstrate that, in both materials at low $T$, the positive magnetoresistance (MR) exhibits signatures of glassiness, such as hysteretic behavior and memory. At such low $T\ll T_{sg} $ ($T_{sg}$ -- spin glass transition temperature) , the resistance noise data reveal other glassy features, such as slowing down of the charge dynamics and the onset of cooperativity as $T$ is reduced. The crossover to negative MR takes place at higher $T$ and $B$ in all samples and for both $B$ orientations. However, for $B\parallel c$, a steplike decrease in MR has been observed only in La$_{2}$Cu$_{1-x}$Li$_{x}$O$_4$ at high $T$, as the system enters the N\'{e}el state, similar to the result obtained on antiferromagnetic LSCO with $x=0.01$~[1]. *Supported by NSF No. DMR-0403491, NHMFL via NSF No. DMR-0084173, and The Royal Society.\\ \noindent [1] Y. Ando \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 247003 (2003). [Preview Abstract] |
Session L12: Heavy Fermion Theory and Strong Correlations
Sponsoring Units: DCMPChair: Jiangping Hu, Purdue University
Room: Morial Convention Center 203
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L12.00001: Phonon Anomalies in $\alpha$-uranium Xiaodong Yang, Peter Riseborough The temperature-dependence of the phonon spectrum of $\alpha$- uranium has recently been measured by Manley {\it et} al. [1] using inelastic neutron scattering and x-ray scattering techniques. Although there is scant evidence of anharmonic interactions, the phonons were reported to show some softening of the optic modes at the zone boundary. The same group of authors later reported that an extra mode was oibserved to form at a temperature above 450 K [2]. The existence of the proposed new mode is inconsistent with the usual theory of harmonic phonons, as applied to a structure composed of a monoclinic Bravais lattice with a two-atom basis. We investigate the effect that the f electron-phonon interaction has on the phonon spectrum and its role on the possible formation of a breathing mode of mixed electronic and phonon character.\newline [1] Manley {\it et} al. {\it Phys. Rev. Lett.} {\bf 86}, 3076 (2001).\newline [2] Manley {\it et} al. {\it Phys. Rev. Lett.} {\bf 96}, 125501 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L12.00002: Phonon Spectra and Lattice Thermal Conductivity of UO2 and PuO2 Quan Yin, Sergey Savrasov Electronic structure, phonon spectrum and lattice thermal conductivity of UO2 and PuO2 are studied using a combination of Density Functional Theory within Local Density Approximation and Dynamical Mean Field Theory (LDA+DMFT). UO2 and PuO2 are mixed oxides fuel (MOX) used in modern thermal reactors. Both oxides are Mott-insulators with strongly correlated 5f electrons, showing very similar electronic structures and phonon dispersions. The calculated phonon dispersion for UO2 is generally consistent with experiment and we give prediction for PuO2. Thermal conductivity is calculated based on the phonon dispersion. The phonon relaxation times are estimated in form of the Gr\"uneisen constant derived from compressed volume phonon calculations. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L12.00003: Spectral Properties of Plutonium and its Compounds Jian-Xin Zhu, A.K. McMahan, M.D. Jones, T. Durakiewicz, J.J. Joyce, J.M. Wills, R.C. Albers By combining the local density approximation (LDA) with dynamical mean field theory (DMFT), we analyze the spectral properties of plutonium and its compounds. The LDA Hamiltonian is extracted either from a tight-binding fit to full-potential linearized augmented plane-wave calculations, or directly from the full-potential linearized muffin tin orbitals calculations. The DMFT equations are solved by the exact quantum Monte Carlo method complemented with the Hubbard-I approximation. We compare the 5f electron behaviors in Pu elemental solid and compounds. The theoretical results will also be discussed in the context of photoemission spectroscopy data. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L12.00004: Crossover from non-Fermi liquid to Fermi liquid behavior: Amplitude of de Haas-van Alphen oscillations Pedro Schlottmann Deviations from Landau's Fermi liquid behavior in numerous U, Ce and Yb based heavy fermion systems are known as non-Fermi liquid behavior and are frequently attributed to a quantum critical point (QCP). A nested Fermi surface together with the remaining interaction between carriers after the heavy bands are formed may give rise to itinerant antiferromagnetism. We consider an electron pocket and a hole pocket, with Fermi momenta $k_{F1}$ and $k_{F2}$, respectively. The order can be suppressed by increasing the mismatch of the Fermi momenta and a QCP is obtained as $T_N \to 0$. For the tuned QCP the specific heat over $T$ increases as the logarithm of the temperature as $T$ is lowered [1] and the linewidth of the quasi-particles is linear in $T$ and $\omega$. [2] With increasing nesting mismatch and decreasing temperature the specific heat and the linewidth display a crossover from non-Fermi liquid ($\sim T$) to Fermi liquid ($\sim T^2$) behavior. [2] Using the quasi-particle linewidth the temperature dependence of the amplitude of the de Haas-van Alphen oscillations (corresponding to the pocket frequencies) is computed. \par\noindent [1] P. Schlottmann, Phys. Rev. B {\bf 68}, 125105 (2003). \par\noindent [2] P. Schlottmann, Phys. Rev. B {\bf 73}, 085110 (2006). \par\noindent [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L12.00005: Fermi Surface and Symmetry of Magnetic Quantum Phases in Heavy Fermion Metals Seiji Yamamoto, Qimiao Si Magnetic ordering in heavy fermion metals is typically antiferromagnetic, although ferromagnetic ordering does occur in some materials, such as CeRu2Ge2. Extending a previously developed framework [1] for the antiferromagnetic Kondo lattice, we now consider the case where the magnitude of the antiferromagnetic Kondo coupling is small compared to the ferromagnetic RKKY interactions. The coupling between spin waves and conduction electrons is relevant, resulting in a change in the form of the QNLsM in the appropriate energy and momentum range. At the lowest energies, a gap in the continuum of excitations renders the Kondo coupling irrelevant. We conclude that a ferromagnetic state with a ``small'' Fermi surface (where local moments are NOT included in the Fermi volume) is stable. Finally, we discuss how the lack of Kondo screening in magnetic metal phases is reflected in symmetry properties. We show that, in some cases, spin symmetry breaking does not lead to a reduction of spin symmetry, further emphasizing the need to go beyond the conventional order parameter to characterize these magnetic quantum phases. [1] S. J. Yamamoto and Q. Si, Phys. Rev. Lett. 99, 016401 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L12.00006: Anomalous Hall Effect in Heavy Fermion Semiconductors Peter Riseborough, Sukalpa Basu Using the Kubo formula, the off-diagonal components of the conductivity tensor can be written in terms of completely filled states. This is a restatement of the discovery by Luttinger and Karplus that a (dissipationelss) anomalous Hall conductivity can result from the filled Fermi-volume which is occupied according to the equilibrium Fermi-Dirac distribution function, and has been recently interprted in terms of a Berry Curvature. The condition that the net anomalous conductivity is non-zero, is that time-reversal invariance should be broken and that the system should have low spatial symmetry. The heavy fermion semiconductors, such as Ce$_3$Bi$_4$Pt$_3$, are extremely narrow gap semiconductors in which the band-gap is subjected to strong many-body renormalizations at low- temperatures. The electrons in the vicinity of the gap are of mixed f and conduction band character and, therefore are subject to strong spin-orbit scattering, as can also be inferred from the finite low-temperature susceptibility of these compounds. Therefore, when subjected to an external magnetic field, these materials are candidates for showing a finite (dissipationless) intrinsic Hall conductivity of the Luttinger-Karplus type. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L12.00007: Study of the gap formation in Kondo insulators M. A. Majidi, K. Mikelsons, A. Macridin, J. Moreno, M. Jarrell Motivated by recent experimental measurements of the temperature-dependent gap formation in the Kondo insulator SmB$_6$ (Matsunami et al.) we investigate the Periodic Anderson Model at half-filling and in the strong-coupling regime. We solve this model in three dimensions using the Dynamical Cluster Approximation to incorporate non-local correlations and the Maximum Entropy Method to derive the spectral functions. We calculate the temperature dependence of the insulating gap and the screened moment, and discuss how the appearance of the gap is related with the lattice Kondo screening. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L12.00008: Is FeSi a Kondo Insulator? Jan Kunes, Vladimir I. Anisimov Using combination of a simple model of local bandstructure and quantum Monte-Carlo technique we show that 3d 'Kondo insulators' such as FeSi and FeSb$_2$ differ in many respects from classical f-electron based Kondo insulators. In particular we show that hybridization between narrow f-band and a broad conduction band typical for f-electron compounds is not a necessary condition for observation of the typical Kondo insulator behavior of susceptibily and dc as well as ac conductivity. The difference between FeSi and f-electron Kondo insulators becomes apparent when doped, while the former exhibits enhanced susceptibility leading to a Stoner-like ferromagnetic instability the latter are usually characterized by competition between the Kondo screening and local moment magnetism with RKKY inter-site coupling. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L12.00009: A Phenomelogical Theory of Quantum Tricritical Point: Applications to a Heavy-Fermion Compound YbRh$_{2}$Si$_{2}$ Takahiro Misawa, Youhei Yamaji, Masatoshi Imada We construct a phenomelogical theory of quantum tricritical point (QTCP) of itinerant antiferromagnets by extending the self-consistent renormalization theory. We have shown that a novel non-Fermi liquid behavior comes out at the QTCP where a continuous phase transition changes into a first-order one at zero temperature. In contrast to the conventional quantum criticality proposed by Moriya, Hertz and Millis, a remarkable feature of the QTCP is the divergence of the ferromagnetic susceptibility at the antiferromagnetic phase transition under the magnetic field. In a heavy-fermion compound YbRh$_{2}$Si$_{2}$, enhancement of the ferromagnetic susceptibility near the antiferromagnetic quantum critical point has been an unsolved puzzle. We have shown that the quantum tricriticality naturally explains this puzzling enhancement. In particular, singularities of ferromagnetic susceptibility and the magnetization curve near the QTCP are well consistent with the experimental results. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L12.00010: Two Channel Kondo Effect and Superconductivity in Pu and Np compounds Rebecca Flint, Maxim Dzero, Piers Coleman Recently, superconductivity has been observed in two heavy fermion compounds, PuCoGa$_5$ and NpPd$_5$Al$_2$, which transition directly from unquenched spins into the superconducting state, without passing through an intermediate heavy fermi liquid. These two compounds can be modeled with the two channel Kondo model, where the two channels derive from virtual valence fluctuations of different crystal symmetries. The electron-spin scattering develops an Andreev component, creating a composite bound state of a spin-flip and a triplet pair of electrons. This process can be examined in a controlled fashion with symplectic N, with the maximum transition temperature occuring when the two channels have equal strengths. We will also discuss the effect of Pu doping on the superconducting transition temperature of NpPd$_5$Al$_2$. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L12.00011: Fermionic quantum criticality and the fractal nodal surface Frank Kruger, Jan Zaanen Normal metals are characterized by a degeneracy scale imposed by Fermi-Dirac statistics: the Fermi energy. This paradigm breaks down in the high-Tc and heavy-fermion compounds where one encounters metallic states with scale invariant quantum dynamics. A theoretical understanding of these quantum critical states is lacking because the fermionic minus signs render the path integral non-probablistic. We demonstrate that within the constraint-path-integral formalism scale invariance and Fermi-Dirac statistics can be reconciled. The latter is translated into a geometrical constraint structure. We prove that this ``nodal hypersurface'' encodes the scales of the Fermi liquid and turns fractal when the system becomes quantum critical. To illustrate this we calculate nodal surfaces and electron momentum distributions of Feynman backflow wave functions and indeed find that with increasing backflow strength the quasiparticle mass gradually increases, to diverge when the nodal structure becomes fractal. This explains precisely the puzzling behaviors observed in the heavy-fermion intermetallics. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L12.00012: Non-linear Sigma Model of Kondo Lattice in Antiferromagnetic Regime Tzen Ong, B. A. Jones We analyze the antiferromagnetic transition in heavy fermion compounds in two dimensions, which we study using the Kondo-Heisenberg model. The system is assumed to be in the antiferromagnetic regime, with a Heisenberg coupling (J$_{H})$ that is larger than the Kondo coupling (J$_{K})$. The Heisenberg terms are mapped onto a non-linear sigma model, and the fermions are then formally integrated out to obtain an effective theory for the Neel field. We then study the evolution of the Heisenberg and Kondo couplings under perturbative RG, and calculate the critical exponents at the phase transition. Preliminary results indicate the possibility of a quantum phase transition from an AFM to a helical phase with anisotropy in the time-like direction . [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L12.00013: Derivation and study of the Fermi-Majorana bi-resonant level model C.J. Bolech, A. Iucci We review the mapping of the anisotropic two-channel Anderson impurity model to a Fermi-Majorana bi-resonant level model. The correspondence is rigorously proved by using bosonization and explicitly constructing the new fermionic fields and Klein factors in terms of the original ones. We also demonstrate that the fixed points associated to the solvable manifold of the new model are renormalization-group stable and generic, and therefore representative of the physics of the original isotropic model. The simplicity of the mapped model allows for the computation of the full set of thermodynamic quantities as well as the identification of the different physical energy scales. In the absence of external fields the impurity physics is of non-Fermi liquid type. As expected, an arbitrarily small external field breaks some of the symmetries and introduces an extra energy scale below which the system flows to a local Fermi-liquid fixed point. [Preview Abstract] |
Session L13: Ground State Density Functional Theory: Theory Development
Sponsoring Units: DCOMPChair: Andreas Savin, Université Pierre et Marie Curie
Room: Morial Convention Center 204
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L13.00001: Strictly correlated electrons in density-functional theory: A general formulation with applications to spherical densities Michael Seidl, Paola Gori-Giorgi, Andreas Savin We reformulate the strong-interaction limit of electronic density functional theory in terms of a classical problem with a degenerate minimum. This allows us to clarify many aspects of this limit, and to write a general solution, which is explicitly calculated for spherical densities. We then compare our results with previous approximate solutions and discuss the implications for density functional theory. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L13.00002: Spherically and system-averaged pair densities in the strong-interaction limit of density functional theory Paola Gori-Giorgi, Andreas Savin, Michael Seidl The spherically and system-averaged pair density (also known in chemistry as intracule density) plays a central role in the construction and understanding of exchange-correlation energy functionals in density functional theory. We have calculated the intracule density for several atoms in the strong-interaction limit of density functional theory. Comparison of our results with the same quantities calculated in the opposite limit, the non-interacting Kohn-Sham system, provides useful insight on the nature of electronic correlation in density functional theory. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L13.00003: Internally Consistent Local Approximation to Density Functional theory Antonios Gonis, Don M. Nicholson, G. Malcolm Stocks We propose a new non-local functional for the implementation of density functional theory (DFT) within a local approximation. This functional is obtained through the replacement of the conventional form $T_s+\frac{1}{2}\int{\rm d}{\bf r}_1\int{\rm d}{\bf r}_2\frac{n({\bf r}_1)n({\bf r}_2)}{|{\bf r}_1-{\bf r}_2|}$ to represent the kinetic and Coulomb energy of a non-interacting system with the expression $T_s+\frac{1}{2}\int{\rm d}{\bf r}_1\int{\rm d}{\bf r}_2\frac{n_s({\bf r}_1,{\bf r}_2)}{|{\bf r}_1-{\bf r}_2|}$ where $n_s({\bf r}_1,{\bf r}_2)$ is the two-particle density formed from the non-interacting wave function, and $n({\bf r})$ is the single-particle density. Based on this new functional we develop a local approximation and show that it is self-interaction free and also leads to energies that form an upper bound to the exact ground-state energy. We provide a brief comparison with the conventional Kohn-Sham local density approximation and some of the schemes introduced to correct for the presence of self-interaction in the conventional formalism, and comment on our immediate plans for future development. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L13.00004: A new approach to density functional theory Kieron Burke, Peter Elliott, Attila Cangi, Donghyung Lee I will explain a new way to think about density functional theory, based on a simple principle: asymptotic exactness for large particle number. This explains many features of existing functionals [1], and makes the connection between semiclassical and density functional approximations. It underlies the restoration of the gradient expansion in PBEsol[2]. It also provides a path toward highly accurate, orbital-free, non-local functionals of the potential, for both the density itself and the non-interacting kinetic energy.\newline [1] J.P. Perdew, L.A. Constantin, E. Sagvolden, and K. Burke, Phys. Rev. Lett. {\bf 97}, 223002 (2006).\newline [2] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, and K. Burke, arXiv:0707.2088 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L13.00005: The Role of Quantum Stress in Descriptive Chemistry Ilya Tokatly, Giovanni Vignale, Jianmin Tao We show that key concepts of descriptive chemistry, such as atomic shells, bonding electron pairs and lone electron pairs, may be described in terms of {\it quantum stress focusing}, i.e. the spontaneous formation of closed surfaces upon which the electronic pressure has an extremum. This description subsumes previous mathematical constructs, such as the Laplacian of the density and the electron localization function, and provides a new tool for visualizing chemical structure. We also show that the full anisotropic stress tensor can be easily calculated from density functional theory. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L13.00006: Semiclassical Approaches in Density Functional Theory Peter Elliott, Attila Cangi, Donghyung Lee, Kieron Burke We discuss the use of semiclassical methods in understanding, and improving, density functional theory (DFT). It has been found[1] that semiclassical approaches can explain many features of modern exchange-correlation approximations, such as the local density approximation or generalized gradient approximations. In this work, we continue this line of inquiry, showing how semiclassical approximations may be used to construct a form for the non-interacting kinetic energy density. A semiclassical approximation to the Green's function is made[2], which, when integrated appropriately, yields leading corrections to the Thomas-Fermi result. It can be regarded as a functional of the potential. We test this approximation for various 1D systems confined within a box and present the results. This, coupled with the corresponding form of the density[2], could provide an orbital-free DFT which would allow more complex systems to be studied.\newline [1] J. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, and K. Burke, arXiv:0707.2088 (2007).\newline [2] W. Kohn and L.J. Sham, Phys. Rev. {\bf 137}, A1697 (1965). [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L13.00007: Correlation Energy of 3D Spin-Polarized Electron Gas: A Single Interpolation Between High- and Low-Density Limits Jianwei Sun, John Perdew, Michael Seidl We present an analytic model for the correlation energy per electron $e_c (r_s ,\zeta)$ in the three-dimensional (3D) uniform electron gas, covering the full range $0\le r_s <\infty $ and $0\le \zeta \le 1$ of the density parameter $r_s $ and the relative spin polarization $\zeta$. An interpolation is made between the exactly known high-density ($r_s \to 0)$ and low-density ($r_s \to \infty)$ limits, using a formula which (unlike previous ones) has the right analytic structures in both limits. We find that there is almost enough information available from these limits to determine the correlation energy over the full range. By minimal fitting to numerical quantum Monte Carlo data, we predict the value of $b_1 (\zeta)$ at $\zeta $=0 close to the theoretical value [1], where $b_1 (\zeta)$ is the coefficient of the $r_s $term in the high-density expansion. The model finds correlation energies for the unpolarized ($\zeta $=0) and fully polarized ($\zeta $=1) cases in excellent agreement with Monte Carlo data. \newline [1] T. Endo, M. Horiuchi, Y. Takada and H. Yasuhara, Phys. Rev. B 59, 7367 (1999) [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L13.00008: Semiclassical Origin of Density Functionals Attila Cangi, Peter Elliott, Donghyung Lee, Kieron Burke We use methods of semiclassical physics [1] to study the basic ingredients of DFT in terms of an asymptotic expansion in powers of the particle number. As an example we derive an approximate many-particle density [2] of a general potential in a one-dimensional system with hard walls. If the Fermi level lies above the maximum of the potential, we obtain densities very close to the exact answer, even for a small particle number. Density oscillations due to the effect of the boundaries are also present.\newline [1] M. V. Berry and K. E. Mount, Reports of Progress in Physics 35, 315 (1972).\newline [2] W. Kohn and L. J. Sham, Phys. Rev. 137, A1697 (1965). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L13.00009: Construction of an analytic exchange-correlation hole for the Perdew-Burke-Ernzerhof GGA Matthias Ernzerhof, Hilke Bahmann The Perdew-Burke-Ernzerhof (PBE) [1] approximation to the exchange-correlation energy is employed as a starting point for the construction of an approximate, spherically averaged exchange-correlation hole. In a first step, we develop a new model for the PBE exchange hole. This model satisfies the homogeneous electron gas limit; it is normalized and yields the correct small-gradient limit in the system average. A correlation factor [2], i.e., a function multiplying the exchange hole, is proposed that turns the exchange into an exchange-correlation hole. The correlation factor has a simple form and its parameters are determined through a number of known conditions that ought to be satisfied by a PBE exchange-correlation hole. The homogeneous-electron-gas limit of the new hole model is compared to the LSD hole [3]. \newline [1] J.P. Perdew, K. Burke, M. Ernzerhof, PRL 77, 3865 (1996); 78, 1396(E) (1997). \newline [2] P. Gori-Giorgi, J.P. Perdew, PRB 66, 165118 (2002). \newline [3] J.P. Perdew, Y. Wang, PRB 46, 12947 (1992). [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L13.00010: Orbital-free Kinetic Energy Density Functionals of GGA Type with Positive-definite, Finite Pauli Potentials S.B. Trickey, V.V. Karasiev, R.S. Jones, Frank Harris A reliable, orbital-free expression for the Kohn-Sham kinetic energy functional $T_s$ would provide Born-Oppenheimer forces for first-principles molecular dynamics with a computational cost scaling as the relevant system volume rather than some power of the electron count $N_e$. In previous work (J. Computer-Aided Mat. Des. {\bf 13}, 111 (2006)) we obtained improved (compared to published) generalized gradient approximate KE functionals by requiring positive-definiteness of the Pauli potential, $v_\theta = \delta T_\theta / \delta n$, with $T_s = T_w + T_\theta$ and $T_w$ the von Weizs\"acker KE functional. However, such modified conjoint functionals still generate unphysical singularities at the nuclei. Here we discuss a systematic use of gradient expansion truncations to generate constrained enhancement factors for GGA functionals that are guaranteed to yield $v_\theta$ that is both everywhere positive definite and finite at the nuclei. Illustrative results will be reported. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L13.00011: Exact condition on the non-interacting kinetic energy for real matter Donghyung Lee, Kieron Burke From the analysis of the asymptotic expansion [1,2] for the total energies of neutral atoms, we suggest a modified gradient expansion approximation to the kinetic energy which satisfies the exact asymptotic condition as the number of electrons N $\rightarrow$ $\infty$. The resulting functional determines the small gradient limit of any generalized gradient approximation, and conflicts with the standard gradient expansion. We apply this new functional to the atoms up to Z $\sim$ 88 in comparison with the 2nd and 4th gradient expansion approximations. We also give a modern, highly accurate parametrization of the Thomas-Fermi density of neutral atoms.\newline \newline $[1]$ Thomas-Fermi model: The second correction, J. Schwinger, Phys. Rev. A {\bf 24}, 2353 (1981).\newline $[2]$ Semiclassical atom, B.-G. Englert and J. Schwinger, Phys. Rev. A {\bf 32}, 26 (1985). [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L13.00012: Construction of Wave Function Functionals Marlina Slamet, Xiao-Yin Pan, Viraht Sahni We recently proposed [1] expanding the space of variations in calculations of the energy by considering the approximate wave function $\Psi$ to be a functional of functions $\chi$, $\Psi = \Psi[\chi]$, rather than a function. A constrained search is first performed over all functions $\chi$ such that $\Psi[\chi]$ satisfies a physical constraint or leads to a known value of an observable. A rigorous upper bound to the energy is then obtained via the variational principle. In this paper we apply this idea to the ground state of the He atom by constructing $\Psi[\chi]$ that reproduce the exact expectations of the Hermitian single- and two-particle operators $W = \sum_{i} r_{i}^{n}, n = -2, -1, 1, 2$; $W =\sum_{i}\delta ({\bf r}_{i})$; $W=|{\bf r}_{1}-{\bf r}_{2}|^{m}, m=-1,-2,1,2$. The functionals are of the form $\Psi[\chi] = \Phi [1 - f(\chi)]$, where $\Phi$ is a prefactor and $f(\chi)$ a correlation factor. The $\Psi[\chi]$ (\emph{i}) lead to the exact expectation value of $W$; (\emph {ii}) are automatically normalized; and (\emph{iii}) provide a rigorous upper bound to the energy. [1] X.-Y. Pan, \textit {et al}, PRA \textbf{72}, 032505 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L13.00013: LDA+DMFT Charge self-consistency applied to Yb valence transition Erik R. Ylvisaker, W. E. Pickett, A. K. McMahan Ytterbium metal, in a pressure range of 0 to 34 GPa, is known to undergo a gradual transition from a $v^2f^{14}$ state to a $v^3f^{13}$ state where $v$ and $f$ represent valence (spd) and f-orbital occupations, respectively. We present, first, conventional LDA+DMFT studies of this transition using both the Hirsch-Fye QMC and Hubbard I atomic solvers. This application of DMFT to the correlated f-orbitals gives reasonable agreement with the experimental transition. However, the neglect of charge self-consistency is questionable for a valence transition where the concentration of valence electrons changes. Therefore we generalize the procedure and compare and contrast LDA+DMFT results (transition pressure, energy and equation of state) with and without charge self-consistency for Yb using the Hubbard I impurity solver. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L13.00014: Fock exchange in FLAPW method Tatsuya Shishidou, Tamio Oguchi Fock exchange potential has distinct features which cannnot be seen in the LDA exchange potential. (i) It is self-interaction free potential and (ii) nonlocal potential and thus state-dependent potential. With appropriate correlation effects added, these two features may produce significantly improved results over the conventional LDA results, as one can witness in the GW calculations. Massidda et al.\ (1993) proposed a way to calculate Fock exchange potential of extended solids within the FLAPW method. Their idea was to apply Weinert's Poisson solver to infinite lattice summation as is done for the Hartree potential calculation. Due to the long range nature of Coulomb interaction, one encounters singularity problem in this process. They handled it by simply extending Gygi's prescription (1986), which was originally developed for the norm-conserving pseudopotential framework. In this paper, we present our formula in calculating Fock exchange matrix of solids based on the FLAPW method. Following Massidda's idea, we use Weinert's Poisson solver. However, in treating the Coulomb singularity, we have developed more accurate way: the occupied eigenfunctions in Fock operator are expanded upto the second order in terms of $q$ vector based on the $k\cdot p$ perturbation theory, whearas Gygi's way corresponds to the zeroth order expansion. With this higher order expansion, one can achieve faster convergence for the Brillouin zone integration appearing in the Fock operator. [Preview Abstract] |
Session L14: Focus Session: Foundations of Quantum Theory II
Sponsoring Units: GQIChair: Julio Gea-Banacloche, University of Arkansas
Room: Morial Convention Center 205
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L14.00001: General probabilistic theories for quantum foundations and quantum information Invited Speaker: Is there any reason why the universe should obey the laws of quantum theory, as opposed to any other possible theory? Is quantum theory special in any way? The best way to address these questions is to view quantum theory as just one point in an entire space of possible theories, and to compare and contrast quantum theory with its rivals. As the success of quantum information theory makes clear, one means by which different theories may be compared and contrasted is via their information processing capabilities. To this end, following early work of Mackey, Ludwig and others, I show how to write down essentially arbitrary probabilistic models, based on the conditions that state spaces are convex and that separated systems cannot be used for instantaneous signalling. Both the classical and quantum theories are special cases. With a focus on information processing, I then describe (i) some features of quantum theory that one might have expected to be uniquely quantum, but turn out to be highly generic, and (ii) some features that do mark quantum theory as special. Some of this is work done in collaboration with Howard Barnum, Matthew Leifer and Alexander Wilce. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L14.00002: Fully epistemic toy theory Michael Skotiniotis, Aidan Roy, Barry C. Sanders The Spekkens toy model is an interesting example of how to modify classical physics in order to perform several quantum information processing tasks. Spekkens' toy model has four axioms concerning toy states, valid operations, measurements, and composition of single toy systems. Motivated by the empirical indistinguishability of epistemic vs. ontic states in the toy universe, we show that relaxing valid operations to mappings of epistemic rather than ontic states preserves the features of the toy model. Similarly we show that relaxing the axiom regarding the composition of single toy systems also preserves the toy model. Relaxing both axioms simultaneously, however, breaks the correspondence of the toy model with quantum theory because the tensor product composition rule is violated, but these two relaxations together produce a group of operations on epistemic states that is isomorphic to the projected extended Clifford Group. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L14.00003: Tensor products and teleportation protocols for abstract state spaces Alexander Wilce In a well-known generalization of classical probability theory, arbitrary compact convex sets serve as abstract ``state spaces'' for (hypothetical) physical systems, with classical systems corresponding to simplices and quantum systems, to state spaces of C*-algebras. One can define natural tensor products for abstract state spaces, modeling composite systems subject to a no-signaling condition. Remarkably, many basic quantum-information theoretic phenomena, including the no-cloning and no-broadcasting theorems, already appear at this level of generality. However, the existence of a teleportation protocol is a strong constraint, moving us closer to quantum theory. In this talk, after a brief summary of the framework, I will outline what we currently understand about teleportation in this setting. This represents recent and ongoing joint work with Howard Barnum, Jonathan Barrett and Matthew Leifer [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L14.00004: The Density Matrix and the Interpretation of Quantum Theory Owen Maroney Can a density matrix be regarded as a description of the physically real properties of an individual system? If so, it may be possible to attribute the same objective significance to statistical mechanical properties, such as entropy or temperature, as to properties such as mass or energy. Non-linear modifications to unitary evolution can be proposed, based upon this idea, to account for thermodynamic irreversibility. Traditional approaches to interpreting quantum phenomena assume that an individual system is described by a pure state, with density matrices arising only through a statistical mixture or through tracing out entangled degrees of freedom. We discuss how treating the density matrix as fundamental can affect the viability of some of these interpretations, and how the thermodynamically motivated non-linearities do not, in themselves, help in solving the quantum measurement problem. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L14.00005: An Approach to Quantum State Pooling from Quantum Conditional Independence Invited Speaker: In approaches to quantum theory in which the quantum state is taken to represent an agent's belief, knowledge or information about a physical system, it is legitimate for different agents to assign different states to one and the same physical system. The question then arises of what state they should assign if they get together and share their information about the system. This is the problem of quantum state pooling. The classical counterpart of this problem for probability distributions only has a unique solution under additional assumptions about how the data are collected, such as conditional independence constraints. Recently, Spekkens and Wiseman found a quantum pooling rule analogous to the classical one, which is valid if the differing state assignments arise from making indirect measurements on special classes of tripartite quantum state. We show that this pooling rule applies to a much wider class of tripartite states, and that its validity rests on quantum analogs of conditional independence recently studied by one of the authors, as well as a generalization of the notion of a sufficient statistic to the quantum case. Work done in collaboration with Robert Spekkens, University of Cambridge. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L14.00006: New results in the category-theoretic approach to foundations of quantum physics Bob Coecke We report on some recent results in the category theoretic approach quantum physics, which aims to provide an operational foundation, a logical axiomatics as well as a purely diagrammatic language for it. Firstly, we were able to unify several measurement-based quantum computational schemes; in particular, the categorical language is sufficient to provide proofs of universality for each of these. Secondly, we have a manner to abstractly generate arbitrary multi-partite entangled states; hence equipping multi-partite entanglement with a formal interpretation in terms of information-flow. Also, we axiomatised Spekkens' model in purely category-theoretic terms; its quantum-like behaviors are now consequences merely of abstract category-theoretic structure. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L14.00007: Graphical Calculi and Mutually Unbiassed Embeddings of Classical Logic Ross Duncan While arbitrary quantum states may not be freely cloned or deleted [1], we note, following [2], that these distinctively classical operations may be performed on states which lie within a given basis. Each basis therefore provides an embedding of classical logic into a quantum state space. This work provides a categorical axiomatisation (cf [3]) of the interaction of such embeddings when distinct mutually unbiassed bases [4] are used. We provide a graphical language (cf. [5]) for the classical operations that each embedding provides, and demonstrate that this system captures many properties of multi-partite entangled states and can simulate quantum algorithms. [1] W. Wootters and W. Zurek. A single quantum cannot be cloned, 1982. A.K. Pati and S. L. Braunstein. Impossibility of deleting an unknown quantum state, 2000. [2] B. Coecke and D. Pavlovic (2007) Quantum measurements without sums. arXiv:quant-ph/0608035. [3] S. Abramsky and B. Coecke (2004) A categorical semantics of quantum protocols. arXiv:quant-ph/0402130. [4] J. Schwinger (1960) Unitary operator bases. Proceedings of the National Academy of Sciences of the U.S.A. 46 [5] B. Coecke (2005) Kindergarten quantum mechanics. arXiv:quant-ph/0510032 [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L14.00008: Basing quantum theory on information processing Howard Barnum I consider information-based derivations of the quantum formalism, in a framework encompassing quantum and classical theory and a broad spectrum of theories serving as foils to them. The most ambitious hope for such a derivation is a role analogous to Einstein's development of the dynamics and kinetics of macroscopic bodies, and later of their gravitational interactions, on the basis of simple principles with clear operational meanings and experimental consequences. Short of this, it could still provide a principled understanding of the features of quantum mechanics that account for its greater-than-classical information-processing power, helping guide the search for new quantum algorithms and protocols. I summarize the convex operational framework for theories, and discuss information-processing in theories therein. Results include the fact that information that can be obtained without disturbance is inherently classical, generalized no-cloning and no-broadcasting theorems, exponentially secure bit commitment in all non-classical theories without entanglement, properties of theories that allow teleportation, and properties of theories that allow ``remote steering'' of ensembles using entanglement. Joint work with collaborators including Jonathan Barrett, Matthew Leifer, Alexander Wilce, Oscar Dahlsten, and Ben Toner. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L14.00009: Quantum mechanics and the nature of the second law of thermodynamics Ian T. Durham The second law of thermodynamics is inherently a classical law, though quantum analogues have been suggested. What are these quantum analogues and what is their relation to the classical version of the second law? In particular, what can Bell's inequalities tell us about this relation? This continues ongoing work, some of which has been presented at previous APS meetings, and makes a stronger argument on the thermodynamic nature of Bell's inequalities. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L14.00010: Many worlds and the appearance of probability in quantum mechanics Robert A. Van Wesep The theory of measurement has posed a conceptual problem since the beginning of quantum mechanics (QM). One the one hand, the quantum theory of interacting systems says that when a system $\mathcal O$ measures the value of a quantity $A$ associated with a system $\mathcal S$, the state of the compound system $\mathcal S \mathcal O$ following the measurement is a superposition of pure product states, one for each eigenvalue of $A$. On the other hand, one's subjective experience (as the observer $\mathcal O$) is that this statevector ``collapses'' nondeterministically to a pure state with probability given by the Born rule. The \emph{Copenhagen interpretation} (CI) says that this collapse actually occurs. The \emph{many-worlds view} (MW) is that it doesn't. The defects of CI are obvious: there is no way to say which interactions are measurements to which the interpretation applies, and there is no way to describe the process of collapse that it calls for. MW, on the other hand, does not seem able to incorporate the Born rule. If this were true, it would rule out MW as a description of reality. We show that it is not true, in the strongest possible way: the Born rule is actually a derivable consequence of the quantum theory of measurement \emph{as long as we accept the theory as is}, i..e, as long as we accept MW[1]. The proof uses the strong law of large numbers, which is the link between the abstract notion of probability and the concrete properties of sequences of observations.\newline [1] R.A. Van Wesep, Ann. Phys. 321 (10) (2006) 2438--2452. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L14.00011: Quantum catalysis of information Koji Azuma, Masato Koashi, Nobuyuki Imoto In quantum information science, it has been long believed that no one can access quantum information in a system without disturbing it. In fact, the belief has been corroborated by several no-go theorems such as the no-cloning theorem and the no-deleting theorem. Here, however, we show that the belief is incorrect, by exhibiting a novel process, `quantum catalysis of information', in which, without receiving any disturbance, a system certainly exchanges a type of information that cannot be transmitted without quantum communication channel. [Preview Abstract] |
Session L15: Focus Session: Progress toward Scalable Quantum Information Processing
Sponsoring Units: GQIChair: Dana Berkeland, IARPA
Room: Morial Convention Center 207
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L15.00001: Ensemble encoding of quantum registers: it's easy if you can count to one Invited Speaker: We present a new encoding of qubits in multi-bit registers which makes use of the collective population of a set of internal states of an ensemble of identical quantum systems. This establishes a linear rather than exponential relationship between the number of bits and the internal state Hilbert space dimension of our basic physical system. The key requirement of our proposal is that we can count to one and restrict the collective populations to the values zero and unity. We propose physical implementations and recipes for one- and two-bit gates with ground state atoms interacting via Rydberg excited states, offering up to 14 bits in a small cloud of cesium atoms, and with polar molecules interacting via a stripline cavity field and a Cooper pair box, offering even larger register sizes. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L15.00002: Progress toward scalable optical quantum computing Invited Speaker: Quantum computing holds great promise for solving certain problems which would be intractable using classical computing architectures. Compared to other carriers of other quantum information (e.g., ions, spins, or superconductors) photons have the simultaneous advantage and disadvantage of interacting with the environment and each other only weakly. They are thus relatively immune to decoherence, but it is difficult to achieve the required qubit-qubit interactions. Fortunately, in 2001 Knill, Laflamme, and Milburn proposed a scheme that was scalable at least in principle, if not in practice (too many resources per gate were required). Since then, the ideas were merged with those of ``one-way quantum computing'' to realize a scalable approach based on ``cluster states'', with much more modest – though still very challenging - resource requirements. Here I will describe some of the challenges and recent successes, both in implementing the necessary resources (i.e., high-efficiency detectors, single- and entangled-photon sources, and fast logic), and in applying these to realize some basic quantum computing primitives (single- and two-qubit gates and some simple algorithms). [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L15.00003: Experimental demonstration of decoherence-free one-way quantum information processing Robert Prevedel, Mark S. Tame, Andr\'e Stefanov, Mauro Paternostro, Myungshik Kim, Anton Zeilinger In recent years, one-way quantum computing has become an exciting alternative to existing proposals for quantum computers. In this specific model, coherent quantum information processing (QIP) is accomplished via a sequence of single-qubit measurements applied to an entangled resource known as cluster state. However, there has so far been no experimental realization of noise-resilient quantum computation in the one-way model. Here we report the experimental demonstration of a one-way quantum processor reliably operating under the effects of decoherence. Information is protected by a properly designed decoherence-free subspace in which the cluster states reside. We demonstrate our scheme in an all-optical setup by encoding the information into the polarization states of four photons. A one-way information-transfer protocol is performed with the photons exposed to severe symmetric phase damping noise. Remarkable protection of information is accomplished, delivering nearly ideal computational outcomes. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L15.00004: Entangling the optical frequency comb into multiple continuous-variable cluster states Olivier Pfister, Hussain Zaidi, Nicolas Menicucci, Steven Flammia, Russell Bloomer, Matthew Pysher A single multimode optical parametric oscillator (OPO) can be designed so that its nonlinear gain medium (typically a two-photon parametric amplifier) generates a particular network of entangling interactions between the eigenmodes of its optical cavity. We show how this can be formulated using nonstandard graph states and how these are related to the usual graph states, an example of which is the cluster state for one-way quantum computing. We also report on the progress of our very compact experimental implementation, in a single OPO with a single pump field, of a parallel quantum register comprising several independent quadripartite cluster states. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L15.00005: Experimental Teleportation-Based Quantum Gate Kai Chen, Alexander Goebel, Claudia Wagenknecht, Yu-Ao Chen, Jian-Wei Pan For large scale quantum computation, the coupling of physical qubits to the environment imposes a major challenge for a real- life implementation. Teleportation-based scheme offers an alternative way for scalable quantum computing. Most attractively, this architecture allows for realizations of universal quantum gates in a fault-tolerant manner as shown by Gottesman and Chuang, and in fact serves as an important basis for measurement-based quantum computing. We report a proof-in- principle experimental implementation of this architecture by demonstrating a teleportation based two-qubit CNOT (controlled NOT) gate through linear optics with 6-photon scheme. By preparation of high-fidelity four-photon cluster states and applying two Bell state measurements with an arbitrary input of two qubits, the desired quantum gate operations are teleported onto the remaining two qubits of the cluster states. Our novel architecture and experimental demonstration for teleportation- based linear optics quantum computing could serve as an essential basis toward resource-efficient, scalable quantum computation and yielding fault tolerance automatically. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L15.00006: Generation arbitrary permutation symmetric state with projection Fangwen Sun, Chee Wei Wong We proposed a scheme to generate arbitrary permutation symmetric multi-partite state. The system contains N equally single quantum particles (We use atoms for these particles) which may interact with single photon to generate entanglement between them. This entanglement can be obtained by the transition from three-level $\Lambda $ atom's exited state to different low levels and emitting corresponding polarized photon, or by inputting a single-photon to a trapped atom to gain different phase shift . After preparing N photon-atom entangled states, the N photons are coupled into same path mode to erase the \textit{Welcher-Weg} information. By postselection ($\frac{N}{2}+k$) photons in one polarization state and ($% \frac{N}{2}-k$) photons in its orthogonal polarization state with N-fold coincidence counts, we can generate the atom Dicke state $\left\vert \frac{N% }{2},k\right\rangle $. Moreover, the arbitrary superposition of these Dicke states can be generated by constructing corresponding projection measurements, which includes multi-atom GHZ state. Based on the discussion on the entanglement between different degrees of freedom, we will show that the projection measurement can also be constructed in the far- field region without combining all photons in one path mode. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L15.00007: Single photon Mach-Zehnder interferometer for quantum networks based on the Single Photon Faraday Effect: principle and applications Hubert Seigneur, Michael Leuenberger, Winston Schoenfeld Combining the recent progress in semiconductor nanostructures along with the versatility of photonic crystals in confining and manipulating light, quantum networks allow for the prospect of an integrated and low power quantum technology. Within quantum networks, which consist of a system of waveguides and nanocavities with embedded quantum dots, it has been demonstrated in theory that many-qubit states stored in electron spins could be teleported from one quantum dot to another via a single photon using the Single Photon Faraday Effect. However, in addition to being able to transfer quantum information from one location to another, quantum networks need added functionality such as (1) controlling the flow of the quantum information and (2) performing specific operations on qubits that can be easily integrated. In this paper, we show how in principle a single photon Mach-Zehnder interferometer, which uses the concept of the single photon Faraday Effect to manipulate the geometrical phase of a single photon, can be operated both as a switch to control the flow of quantum information inside the quantum network and as various single qubit quantum gates to perform operations on a single photon. Our proposed Mach-Zehnder interferometer can be fully integrated as part of a quantum network on a chip. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L15.00008: High threshold 2D nearest neighbor quantum computation. Austin Fowler, Peter Groszkowski, Robert Raussendorf We describe a quantum computation scheme on a 2D nearest neighbor coupled square lattice of qubits that requires relatively few physical qubits per logical qubit, permits logical operations between arbitrarily distant logical qubits in almost constant time and has a physical gate threshold error rate of almost 1{\%}. To the best of our knowledge, no other quantum computation scheme simultaneously possesses all of these desirable properties. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L15.00009: A Universal Operator Theoretic Framework for Quantum Fault Tolerance. Gerald Gilbert, Robert Calderbank, Vaneet Aggarwal, Michael Hamrick, Yaakov Weinstein We introduce a universal operator theoretic framework for quantum fault tolerance. This incorporates a top-down approach that implements a system-level criterion based on specification of the full system dynamics, applied at every level of error correction concatenation. This leads to more accurate determinations of error thresholds than could previously be obtained. The basis for the approach is the Quantum Computer Condition (QCC), an inequality governing the evolution of a quantum computer. In addition to more accurate determination of error threshold values, we show that the QCC provides a means to systematically determine optimality (or non-optimality) of different choices of error correction coding and error avoidance strategies. This is possible because, as we show, all known coding schemes are actually special cases of the QCC. We demonstrate this by introducing a new, operator theoretic form of entanglement assisted quantum error correction. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L15.00010: Structure and quantum-dynamics relationship in spin networks Luis Cajamarca, Luis Quiroga We report on the relationship of the spin dynamics with the quantum network topology. The network consists of N-1 spins-1/2 arranged along a circle, also referred to as a ring, equidistant to a central spin (Heisenberg star or ring topology). Every spin along the ring interacts with its first neighbors by means of a coupling constant J2, as well as with the central spin by means of a constant coupling J1. Both couplings are of antiferromagnetic nature and the competition among these incorporates the well known magnetic frustration behavior, which is characteristic of this type of systems. A full analysis of the quantum system's dynamics is carried out for the two limiting cases of coupling constants. We analyze the ground state transitions of the system as well as correlations between any pair of spins including the temperature dependence. The time evolution of the central spin is also analyzed for a given preparation state of the whole spin network. Finally, an stochastic element is incorporated into the system by disconnecting the central spin with any spin along the ring in a random manner. Such dynamics is referred to as dilution and allows us to describe how quantum quantities, such as spin coherences, entanglement and general quantum correlations, depend on the different path topologies between the considered spins (classical structural quantities). Extensions to more complex network topologies are also addressed. [Preview Abstract] |
Session L16: Focus Session: Brownian Motors
Sponsoring Units: DBPChair: Martin Bier, East Carolina University
Room: Morial Convention Center 208
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L16.00001: Stochastic path integrals and geometric theory of mesoscopic stochastic pumps and reversible ratchets. Invited Speaker: A variety of stochastic systems, from enzyme kinetics to epidemiology, exhibit pump-like behaviors, where adiabatic changes of parameters result in a nonzero directed current through the system. Using the stochastic path integral technique from mesoscopic physics, we have been able to relate these and similar phenomena to geometric effects in mesoscopic stochastic kinetics and construct their unifying theory. In the talk, this methodology will be demonstrated on three examples: (1) an adiabatic pump effect in the evolution of a Michaelis-Menten enzyme, treated as a classical two-state stochastic system; (2) a reversible ratchet; and (3) a related novel phenomenon in a previously unexplored domain, namely the SIS epidemiological model. In all of these examples, pump-like currents follow from very similar geometric phase contributions to the effective action in the stochastic path integral representation of the moment generating functional, and our construction provides the universal technique for identification, prediction, and calculation of these currents in an arbitrary mesoscopic stochastic framework. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L16.00002: ABSTRACTS WITHDRAWN |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L16.00003: Failure of Overdamped Models for Buttiker-Landauer Heat Engine: Molecular Dynamics Simulation Ronald Benjamin, Ryoichi Kawai A spatially inhomogeneous temperature profile in presence of a periodic potential leads to directed current of Brownian particles, commonly known as B{\"u}ttiker-Landauer ratchet. Under a small external load the system can do work as a heat engine. Overdamped models, neglecting inertial effect ($m=0$), predict that the engine can reach Carnot efficiency. On the other hand, the overdamped limit ($m\rightarrow 0$) predicts the opposite due to the kinetic energy contribution to the heat transfer, suggesting that $m=0$ is mathematically a singular point. A phenomenological argument predicts that the heat from the hot to the cold reservoir diverges as $1/\sqrt{m}$ [1,2]. We confirmed this singular behavior by Molecular Dynamics (MD) simulation and also by numerically solving the corresponding inertial Langevin equation. We obtain good agreement between the MD simulation and the inertial Langevin equation whereas the solution of the overdamped Langevin equation qualitatively disagrees with them. We also confirmed, from the numerical simulation, that the efficiency of the engine does not reach the Carnot limit. \newline [1] I. Derenyi and R. D. Astumian, Phys. Rev.E {\bf 59}, R6219 (1999). \newline [2] T. Hondou and K. Sekimoto, Phys. Rev. E {\bf 62}, 6021 (2000). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L16.00004: Modeling an efficient Brownian heat engine Mesfin Asfaw Taye We investigate the effect of subdividing the ratchet potential on the performance of a tiny Brownian heat engine that modeled as a Brownian particle hopping in a viscous medium in a sawtooth potential (with or without load) assisted by $\it {alternately}$ placed hot and cold heat baths along its path. We obtain analytic expression for the steady state current. The expressions for velocity, efficiency and coefficient of performance of refrigerator are reported for different number of barrier subdivisions. We find that the velocity, the efficiency and the coefficient of performance of the refrigerator maximize as the number of barrier subdivisions increase. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L16.00005: Trajectories of a Brownian Motor Invited Speaker: Many bio-molecular motors are dimers that move by a ``hand-over-hand'' mechanism along polar bio-polymeric tracks. Examples include kinesin, that ``walks" on microtubule and myosin V that ``walks" on actin. These molecular motors share two important symmetries. Typically the motor dimers have approximate mirror symmetry, and their tracks have translational, but not mirror, symmetry. Here we use a trajectory approach to analyze a minimal model for a generic dimeric motor that moves on a polymer track incorporating these two symmetry features. The analysis focuses of the relative probabilities of forward, reverse, backward, backward reverse trajectories and provides an experimentally accessible measure of the relative importance of a ``Brownian motor'' vs. ``Power stroke'' mechanism. Reciprocal relations, similar to those derived for the linear regime by Onsager for the fluxes (generalized velocities), hold for arbitrary magnitude forces (i.e.,far from the linear regime) for the net probabilities for stepping and for chemical reaction. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L16.00006: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L16.00007: Nonequilibrium Fluctuations and Mechanochemical Couplings of a Molecular Motor Andy Lau, David Lacoste, Kirone Mallick We investigate theoretically the non-equilibrium features of a single processive motor operating far from equilibrium using an externsion of the two-state model introduced by Kafri {\em et al} [Biophys.\ J.\ {\bf 86}, 3373 (2004)]. By including an important variable, namely, the number of ATP consumed, we construct a thermodynamic framework, which allows us to characterize the ATP consumption rate of a motor, its run length, and its thermodynamic efficiency. Additionally, with the aid of the Fluctuation Theorem, we analyze the violations of Einstein and Onsager relations as functions of generalized forces. Our main results are (i) one of the Einstein relations holds near stalling, (ii) the degree by which the Onsager symmetry is broken is largely determined by the underlying asymmetry of the substrate, (iii) kinesin's maximum efficiency and its maximum violation of Onsager symmetry occur roughly at the same energy scale, corresponding to that of an ATP hydrolysis ($\sim 20\,k_B T$). [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L16.00008: Influence of non-conservative optical forces on the dynamics of optically trapped colloidal spheres: The fountain of probability Bo Sun, Yohai Roichman, Allan Stolarski, David G. Grier We demonstrate both experimentally and theoretically that a colloidal sphere trapped in an optical tweezer does not come to equilibrium, but rather reaches a steady state in which its probability flux traces out a toroidal vortex. This non-equilibrium behavior can be ascribed to non-conservative optical forces and constitutes a particularly simple thermal ratchet. We briefly discuss ramifications of this effect for previous experiments in which optical tweezers have been treated as conservative potential energy wells. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L16.00009: Anomalous single-particle diffusion in a tilted washboard potential Ke Xiao, Yael Roichman, Sanghyuk Lee, David Grier A corrugated optical vortex acts as a tilted washboard potential for micrometer-scale colloidal particles. A single particle circulating around a corrugated optical vortex undergoes normal diffusion in the limit of strong driving and high temperatures. In the opposite limit, a particle becomes localized. When the effective barrier height is comparable to the thermal energy scale, the particle switches intermittently between stationary and running states. This intermittent switching results in a giant enhancement of the particle's effective self-diffusion coefficient, which has been predicted theoretically and demonstrated experimentally. The observed enhancement is at least one order of magnitude larger than predicted. Simulations of this system reveal that, contrary to predictions, the single particle undergoes anomalous diffusion, and that this explains the unexpectedly large enhancement of the thermally driven fluctuations. In particular, we show that giant diffusivity arises from the competition between sticking and running states, and can be related to the anomalous diffusion characteristics. We show that the system crosses over from superdiffusive behavior to subdiffusion as the driving increases relative to the barrier height, in agreement with experiments. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L16.00010: New Proposed Mechanism for Actin-Polymerization-Mediated Propulsion Kun-Chun Lee, Andrea Liu An important component of the cellular cytoskeleton is F-actin, a biopolymer whose non-equilibrium self-assembly is key to the process of cell crawling. We have reported previously how the polymerization and branching of F-actin near the cell membrane drives cell crawling using a physically-consistent Brownian Dynamics model. Here we show that the creation of new polymerizing filaments by the branching process leads to a steady-state concentration profile of actin away from the moving surface. This non-equilibrium concentration profile is associated with an osmotic pressure profile. The gradient of the osmotic pressure, evaluated at the surface, is the force density on the actin. This force pushes actin backwards, away from the surface. By Newton's third law, this force has a reaction force on the disk; this is the force pushing the disk forwards. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L16.00011: Stochastic Regulation of Actin Bundles Growth Dynamics Pavel Zhuravlev, Yueheng Lan, Garegin Papoian Actin polymerization in living cells exemplifies biological dynamical processes where mechanics is intrinsically coupled to chemistry. Modeling the dynamics of biochemical reaction networks may by itself be challenging, because ordinary chemical kinetics is often inapplicable when a small copy number of individual proteins are involved. Instead, to treat large fluctuations, the reaction dynamics should be computed with stochastic methods. We have developed an extensible mechano-chemical model describing the dynamics of actin bundle growth and retraction, where all reaction and diffusion processes are treated stochastically. We have applied our computational algorithm to study the dynamics of filopodia, where polymerization rate at the tip is coupled to the membrane force and fluctuations. Our approach allows to investigate how a particular regulatory protein, participating in the relevant signaling network, influences the distribution of filaments in the bundle, growth and retraction rates and other dynamical characteristics. Among these proteins, the most interesting are capping proteins (that block polymerization), formins (that promote polymerization), fascins (that connect the filaments in the bundle together) and myosins (molecular motors that have been observed in filopodia and may participate in active transport to the tip). [Preview Abstract] |
Session L17: Solid Helium
Sponsoring Units: DCMPChair: Tony Clark, Pennsylvania State University
Room: Morial Convention Center 209
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L17.00001: Shear Modulus Anomaly in Solid Helium John Beamish, James Day, Alexander Syshchenko The search for supersolidity was given impetus by recent experiments in which solid helium appeared to decouple from a torsional oscillator, but other phenomena which characterize superflow have not yet been observed. Both experiments and theory indicate that defects are involved in supersolidity and these should also affect the solid's mechanical behavior. We have measured the shear modulus of solid helium at extremely low frequencies and strains, using a new method, and observe anomalous stiffening at temperatures below 200 mK. It has the same dependence on temperature, measurement amplitude, $^{3}$He impurity concentration and annealing as the torsional oscillator decoupling. This elastic behavior is explained in terms of a dislocation network which is pinned by $^{3}$He at the lowest temperatures but becomes mobile above 100 mK. Moving dislocations appear eliminate the decoupling and disrupt possible supersolidity. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L17.00002: Effect of Annealing and Stresses on the Shear Modulus of Solid Helium Alexander Syshchenko, James Day, John Beamish Recent measurements suggest that defects are crucial to ``supersolid'' behavior. Dislocations produced during crystal growth or by plastic deformation can have dramatic effects on a solid's mechanical properties. We have made pressure/flow and elastic/acoustic measurements on solid helium and have studied the effects of annealing near melting and of applying large stresses at low temperatures. Pressure gradients are greatly reduced by annealing, but only at temperatures quite close to melting. In our elastic measurements, we observed a large stiffening of the shear modulus in the same temperature range where decoupling was observed in torsional oscillators, behavior which can be understood in terms of the response of mobile dislocations above 100 mK. Annealing usually raises the high temperature shear modulus but leaves the low temperature modulus unchanged, as expected if annealing eliminates some dislocations. Applying large stresses further increases the high temperature modulus, but these changes are reversed by heating above 0.5 K, suggesting that the defects introduced by stressing the crystal are much easier to anneal than the dislocations produced during growth. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L17.00003: Amplitude Dependence of the Shear Modulus Anomaly in Solid Helium James Day, Alexander Syshchenko, John Beamish The shear modulus of solid $^{4}$He increases substantially in the temperature range below 200 mK where torsional oscillator measurements showed mass decoupling apparently associated with supersolidity. The amount of helium which decoupled depended on the oscillator amplitude, which was interpreted in terms of a supersolid critical velocity of order 10 microns/second. We observed a similar amplitude dependence in our shear modulus anomaly - the stiffening at low temperatures decreased above a critical drive level. By varying the measurement frequency (from 20 to 2000 Hz) and by changing the sample's dimensions, we conclude that the amplitude dependence we see is a function of the stress or displacement in the solid helium rather than the velocity. This contrasts with recent torsional oscillator measurements in which the amplitude dependence scaled with sample velocity. However, the amplitude dependence in our modulus measurements begins at stresses comparable to those in torsional oscillators and at low temperatures it shows hysteretic behavior similar to that seen in torsional oscillators. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L17.00004: Finite-Element Modeling of Acoustic Modes in Solid $^4$He Anthony Clark, Jay Maynard, Moses Chan Using a finite-element method we have calculated the low-frequency eigenmodes of various torsional oscillators (TO) in the literature. To elucidate the relationship between the elasticity [1] and apparent nonclassical rotatation inertia (NCRI) of solid $^4$He, we have investigated the dependence of the torsion mode on the shear modulus of $^4$He. If we exclude supersolidity, we find the inferred increase in the shear modulus that is necessary to account for typical frequency shifts in TO studies is significantly larger than that reported in Ref. [1] and nearly unphysical. Experiments are in progress to understand the connection between NCRI and the increased shear modulus. [1] J. Day and J. R. Beamish, arXiv:0709.4666v1 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L17.00005: Decoupling NCRI from shear modulus changes in solid $^4$He Joshua T. West, Moses H.W. Chan Day and Beamish report a significant increase in the shear modulus of solid $^4$He [1] below 250 mK with temperature dependence similar to the non-classical rotational inertia (NCRI) response seen in torsional oscillator measurements [2]. Finite element calculations show that stiffening of the solid $^4$He could mimic very small NCRI signals [3]. We have constructed a one-piece, welded oscillator which is designed to minimize the effect from stiffening of the solid helium. Preliminary data will be presented. \newline [1] J. Day and J. R. Beamish, arXiv:0709.4666v1 (2007). \newline [2] E. Kim and M. H. W. Chan, Science \textbf{305}, 1941 (2004). \newline [3] A. C. Clark and M. H. W. arXiv:0711.3619v1 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L17.00006: Studies of Non-Classical Rotational Inertia of Solid $^4$He Michael C. Keiderling, Yuki Aoki, Harry Kojima We have constructed a compound torsional oscillator which could be operated at two resonant modes (the first at 496 and the second at 1172 Hz). This device allowed us to study the non-classical rotation inertia of the \underline{identical} solid $^4$He at the two oscillator modes driven separately. We present here recent studies of NCRI when the two modes are simultaneously excited. The idea was to drive the first mode at variable high amplitude and to detect its effect on NCRI fraction by the second mode. We expected that when the solid $^4$He was driven at high amplitudes with the first mode to produce significant reduction in NCRI fraction, the same reduction would be measured with the second mode driven simultaneously at very low amplitude. On the contrary, the observed reduction in NCRI fraction by the second mode was much \textit{smaller} than that expected from the first mode. If the driver/detector roles of the first and second modes were reversed, the amount of reduction of NCRI fraction detected and induced by a high drive amplitude of the second mode became \textit{greater} in the first mode driven at a low amplitude. The critical drive amplitude effects of NCRI induced in one mode are not entirely ``seen'' by the other mode in our oscillator. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L17.00007: Torsional oscillator and synchrotron x-ray experiments on solid helium in aerogel J. West, M.H.W. Chan, N. Mulders, C.N. Koddituwakku, C.L. Burns, L.B. Lurio A number of Torsional Oscillator experiments have shown that the Non Classical Rotational Inertia fraction in solid $^{4}$He is strongly dependent on sample preparation. Samples presumed to be the most pristine show the smallest fraction, rapidly quenched samples a much larger one. On the assumption that samples grown in a strongly disordered environment would similarly show a large NCRIf, we have made T.O. measurements on solid samples grown in 95{\%} porous silica aerogel. Contrary to expectation, these samples show a behavior that is very similar to high purity samples grown from the superfluid phase. Subsequent x-ray diffraction experiments show that the solid grown in aerogel is highly polycrystalline, with a hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. X-ray diffraction experiments were performed at the Advanced Photon Source, Argonne national laboratory. This work was supported through NSF DMR-0706339 (MHWC) and DE-FG01-05ER05-02 (CAB). [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L17.00008: Non-classical response of solid helium confined in Vycor glass Duk Young Kim, Hyung Chan Kim, Eunseong Kim The non-classical rotational inertia of solid $^{4}$He was observed by a series of torsional oscillator experiments[1]. Probable heat capacity signature of the supersolid transition which coincides with the non-classical mass decoupling is also observed[2]. Recent torsional oscillator experiments suggest that disorder and the orientation of a helium crystal may play an important role in the appearance of non-classical rotational inertia. Accordingly, the investigation on the role of defects and crystal orientation may provide crucial clues to understand supersolidity. Solid helium confined in porous media cannot possess well-defined crystal orientation and is likely heavily populated with defects. Here we will present the pressure dependence of the non-classical rotational inertia in solid helium confined in Vycor glass by torsional oscillator techniques. [1] E. Kim and M. H. W. Chan, \textit{Science} \textbf{305}, 1941 (2004); \textit{Nature} \textbf{425}, 227 (2004); \textit{J. Low Temp. Phys.} \textbf{138}, 859 (2005); \textit{Phys. Rev. Lett. }\textbf{97}, 115302 (2006). [2] X. Lin, A. C. Clark, M. H. W. Chan, \textit{Nature} \textbf{449,}1025(2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L17.00009: Torsional oscillators and the entropy dilemma of solid $^{4}$He M.J. Graf, A.V. Balatsky, I. Grigorenko, S.A. Trugman, Z. Nussinov Solid $^{4}$He is viewed as a nearly perfect Debye solid. Yet, recent calorimetry measurements by Chan's group (JLTP \textbf{138} (2005) 853 and Nature \textbf{449} (2007) 1025) indicate that at low temperatures the specific heat has both cubic and linear contributions. These features appear in the same temperature range where measurements of the torsional oscillator period suggest a supersolid transition. We analyze (Phys. Rev. B \textbf{75} (2007) 094201) the specific heat and compare the measured with the estimated entropy for a proposed supersolid transition with 1{\%} superfluid fraction and find that the observed entropy is too small. We suggest that the low-temperature linear term in the specific heat is due to a glassy state that develops at low temperatures and is caused by a distribution of tunneling systems in the crystal. We propose that dislocation related defects produce those tunneling systems. Further, we argue (Phys. Rev. B \textbf{76} (2007) 014530) that the reported mass decoupling is consistent with an increase in the oscillator frequency as expected for a glass-like transition. The glass model offers an alternate interpretation of the torsional oscillator experiments in contrast to the supersolid nonclassical rotational inertia (NCRI) scenario. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L17.00010: the excitation spectrum of solid 4He John Goodkind, Elizabeth Blackburn, Sunil Sinha, Collin Broholm, John Copley Speculation about a possible Bose condensation in solid $^{4}$He has existed for decades and has recently been further stimulated by the discovery of an acoustic anomaly and a decrease in the moment of inertia at temperatures below 200 mK. The excitation spectrum played an important role in understanding the properties of superfluid liquid helium so that, if there is such a condensation in solid $^{4}$He,$^{ }$the spectrum might also confirm it and aid in understanding it. We have measured the excitation spectrum of solid $^{4}$He by neutron scattering using the Disc Chopper Spectrometer at NIST. We have identified a sharp line in the spectrum as vacancy wave excitations. The dispersion relation for these excitations has a minimum energy of 1 meV and is quadratic. This vacancy mode intersects the longitudinal phonon mode at 1.1 meV and the two become degenerate at higher energies. The degenerate mode has a linear dispersion law with smaller slope than the longitudinal acoustic mode at lower energies. No change in the spectrum was observed below 200 mK. The spectrum has strong similarities to the spectrum in superfluid liquid helium. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L17.00011: Torsion Oscillator Studies of Solid Helium-4 Ethan Pratt, Benjamin Hunt, Minoru Yamashita, J.C. Seamus Davis We will present results of torsion oscillator experiments on solid helium-4 below 300 mK. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L17.00012: Pairing states for the ring exchange t-J model Ming Lou, Michael Ma, Fu-Chun Zhang Multiple spin interactions, introduced by ring exchange process, are important for many systems, including solid $^{3}$He and High Tc cuprates. In high Tc cuprates, the dominant term is the 2-spin antiferromagnetic interaction, which leads to d-wave singlet pairing upon doping. In solid $^{3}$He, on the other hand, the 4-spin interaction plays an important role, and it's interesting to determine how the Cooper pairing state may differs from that of cuprates. In this work, we apply the renormalized mean-field theory [1] to a modified t-J model, where the J term includes the 4-spin interaction introduced by the ring exchange. Our result shows that a mixed state of singlet and triplet pairing optimizes the energy. At half filling, the pairing state is unphysical, due to the fact that there is no double occupancy. Upon doping or with intrinsic vacancies, the paring state becomes physical. Such a mechanism may introduce supersolidity in bulk solid $^{3}$He and solid $^{3}$He absorbed on a substrate at very low temperature. [1] F.C. Zhang, C. Gros, T.M. Rice and H. Shiba, Supercond. Sci. Technol. 1 36 (1988) [Preview Abstract] |
Session L18: Focus Session: John H. Dillon Medal Symposium
Sponsoring Units: DPOLYChair: Gregory McKenna, Texas Tech University
Room: Morial Convention Center 210
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L18.00001: John H. Dillon Medal Talk: Polymer Droplets Invited Speaker: The simplicity of a liquid droplet, say a dew drop on spider silk, is both esthetically beautiful and scientifically intriguing. The interplay of surface energies, thermal motion, and confinement of the liquid, especially on small length scales can reveal interesting physics. Droplets are an ideal confining geometry because the length scales can be easily controlled and it is possible to arrange the system such that each droplet acts as an independent experiment. The talk will focus on some recent examples where we have used the droplet geometry to learn about material properties. It will become apparent in the presentation that the deviations from the ``expected'' behaviour in confined systems are far from subtle! [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L18.00002: Surface relaxation in glassy polymers James Forrest, Dongping Qi, Zahra Fakhraai We have used nanohole relaxation to measure the viscoelastic
properties of the first 1-4 nm of the surface of glassy polymer
PS and i-PMMA. In both cases we find evidence for complete
relaxation of the surface below the bulk glass transition
temperature $T_g$. For the case of PS, the temperature
dependence of the relaxation was measured in the range
$240K |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L18.00003: Tg and Cure of a Polycyanurate at the Nanoscale Sindee Simon, Qingxiu Li Nanoscale constraint is known to have a significant impact on the thermal properties of materials. Although thermosetting resins have been cured in the presence of nanoparticles and nanotubes, cure of thermosetting resins under the well defined nanoscale constraints imposed by controlled pore glass (CPG) or similar matrices has not been previously documented. In this work, we investigate the isothermal curing under nanoscale constraint of a thermosetting resin, bisphenol M dicyanate ester (BMDC), which trimerizes to form a polycyanurate network material. Differential scanning calorimeter is used to monitor the evolution of the glass transition temperature (Tg) and the conversion during cure as a function of the diameter of the silanized control pore glass matrix which is used for confinement. A Tg depression is observed for both the bisphenol M dicyanate ester monomer and the polycyanurate networks; the depression is only a few degrees for the monomer, whereas a 56 K depression is observed for the ``fully-cured'' network in 11.5 nm pores. The nanoscale constraint is also found to accelerate the cure of the bisphenol M dicyanate ester, but it does not affect the normalized Tg versus conversion relationship. The appearance of a secondary Tg above the primary Tg in the smaller pores and the associated length scale are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L18.00004: Comparison of surface mobility of polymeric and low molecular weight glass-formers. Mark Ediger, Stephen Swallen, Ken Kearns The last ten years has seen considerable effort to understand dynamics at the surface of polymer melts and glasses. For comparison, we present data on two low molecular weight glass formers: trisnaphthylbenzene (TNB) and indomethacin (IMC). Neutron reflectivity provides direct information about mobility in the top several nanometers of TNB glasses. Two other measurements (surface crystal growth rates and the enthalpy of glasses prepared by vapor deposition) offer indirect information on surface mobility for IMC and TNB. These measurements indicate that surface dynamics at Tg are 2-5 orders of magnitude faster than bulk dynamics. The temperature dependence of the surface relaxation process is weak below the bulk Tg, in qualitative agreement with recent measurements on polymer surfaces in this regime. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L18.00005: Modeling Solvent Evaporation from Glass-Forming Polymer Films by MD Simulations Jorg Baschnagel, Simone Peter, Hendrik Meyer By means of molecular-dynamics simulations we study solvent evaporation from glass-forming, free-standing and supported polymer films. Polymers are represented by a commonly employed bead-spring model, solvent molecules are modeled as Lennard-Jones particles, and polymer-solvent interactions are tuned such that good-solvent conditions are realized. We start the simulations from a dense solution with a solvent content of 20\% and explore the evaporation process for temperatures $T$ above and below the glass transition temperature $T_{\mathrm{g}}$ of the pure polymer film. At all $T$ we observe the formation of a polymer-rich crust at the free surface upon solvent evaporation. For $T > T_{\mathrm{g}}$ we can reproduce the simulation results (reduction of film thickness with time, solvent and polymer density profiles, etc.) by a Fickian diffusion model with a constant diffusion coefficient. For $T < T_{\mathrm{g}}$ deviations from Fickian diffusion are observed. We suggest that these deviations may be rationalized by a diffusion coefficient of the solvent, which depends on film composition and distance from the free surface. We attempt to compare our results to recent experiments. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L18.00006: Studies of Glassy Colloidal Systems Under Shear Michael Massa, Chanjoong Kim, David Weitz In analogy with the glass transition of polymer (and other molecular) liquids, colloidal suspensions can undergo dynamic arrest to form a glassy solid, when the system is concentrated beyond a critical volume fraction. However, in contrast to their molecular counterparts, studies of the glass transition in colloidal systems are facilitated by their natural length- and time-scales, which make it possible to directly visualize the behaviour of the individual constituent particles. Using confocal microscopy, we follow the dynamics of colloidal suspensions near the glass transition, and in particular, their reaction to an imposed deformation. We investigate the evolution from a quiescent solid to a shear melted liquid, to elucidate the nature of the structural rearrangements that govern the properties of glassy materials. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L18.00007: Growth and Stability of Polymer Surface Wrinkles Alfred Crosby For certain materials and geometries, the surface of a polymer film will wrinkle upon the application of a critical in-plane stress due to the onset of an elastic instability. The resulting morphology minimizes the in-plane strains and the system energy by locally bending the surface plane. This process and morphology have been studied at an increasing rate over the last decade and demonstrated in applications ranging from materials metrology to adhesion control. In general, the knowledge and use of this phenomenon has been developed for conditions that far exceed the point of initiation, under static equilibrium. In this presentation, we highlight recent experiments that explore the growth and morphological transitions of surface wrinkles. We quantify the growth mechanism under different conditions of mechanical constraint and demonstrate a unique ability to stabilize a wide variety of unique, non-predicted, surface wrinkle morphologies. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L18.00008: Toughening Mechanisms in Polymer Gels Hugh Brown I will describe a simple model that accounts for the very high toughness of double network gels. The model is based on the assumption that the first, stiff network will break up forming multiple cracks when the stress is above a defined value. These cracks are held together by the second network. A multiply cracked damage zone will form round any macroscopic crack in the material causing energy dissipation and shielding the second network. The toughness enhancement by this process is estimated to be about x40. Other techniques of gel toughening will then be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L18.00009: Theory of polymer crystallization M. Muthukumar We will present new conceptual arguments for the spontaneous selection of very small lamellar thicknesses and their melting behavior. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L18.00010: On the effective charge of hydrophobic polyelectrolytes Elie Raphael, Alexei Chepelianskii, Farshid Mohammad-Rafiee In this paper we analyze the behavior of hydrophobic polyelectrolytes. It has been proposed that this system adopts a pearl-necklace structure reminiscent of the Rayleigh instability of a charged droplet. Using a Poisson-Boltzmann approach, we calculate the counterion distribution around a given pearl assuming the latter to be penetrable for the counterions. This allows us to calculate the effective electric charge of the pearl as a function of the chemical charge. Our predictions are in very good agreement with the recent experimental measurements of the effective charge by Essafi et al. (Europhys. Lett. 71, 938 (2005)). Our results allow to understand the large deviation from the Manning law observed in these experiments. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L18.00011: Disentanglement in thin polymer films Hendrik Meyer Molecular dynamics simulations of thin polymer films confined between structureless walls show accelerated in-plane dynamics with decreasing film thickness. Using the primitive path analysis (PPA) introduced by Everaers et al [Science 303 (2004) 823] for chain length up to N=1024, we can show that the entanglement density decreases with decreasing film thickness. However, the effect becomes pronounced only for films thinner than the bulk radius of gyration where also the chain structure becomes modified by the confinement [1]. The PPA algorithm can be modified to estimate the contribution of self-entanglements: The latter become more important for thinner films, however, they do not counterbalance the global decrease of entanglements. [1] H. Meyer et al Eur. Phys. J. Sp.Top. 141 (2007) 167. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L18.00012: Complex Structural Packing of ABC Triblock Copolymers Solvent Annealed at High Humidity Chuanbing Tang, Joona Bang, Gila Stein, Glenn Fredrickson, Craig Hawker, Edward Kramer, Michael Sprung, Jin Wang The use of ABC triblock copolymers to obtain industrially relevant morphologies for block copolymer lithography has been investigated. Nanoporous thin films of poly(ethylene oxide)-poly(methyl methacrylate)-polystyrene triblock copolymer spheres were prepared by solvent annealing under controlled high humidity followed by UV degradation and acid washing. Ordered half spheres at the surface that template ordering of spheres below the surface in thin films were formed as a result of the interaction between the highly hydrophilic PEO segments and water vapor in the chamber. The spherical block copolymer domains exhibit complex packing behavior on the surface and in the interior which is dependent on film thickness. Half sphere ``monolayer'' and half sphere plus whole sphere ``bilayer'' formed in thin films were shown to have hexagonal lattice symmetry. For half sphere plus two whole sphere ``trilayers'', coexistence of regions of hexagonal and square packing was observed by TEM, SFM, SEM and GISAXS. Square packing was consistent with a surface truncated unit cell of a body-centered cubic lattice with the (100) plane parallel to the surface. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L18.00013: High resolution structure of bacterial cell sacculi John Dutcher, Ahmed Touhami, Valerio Matias, Anthony Clarke, Manfred Jericho, Terry Beveridge The major structural component of bacterial cell walls is the peptidoglycan sacculus, which is one of nature's strongest and largest macromolecules that allows the cell to maintain a large internal pressure while allowing the transport of molecules into and out of the cell and cell growth. The three-dimensional structure of this unique biopolymer is controversial, and two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the scaffold model, in which the glycan strands lie perpendicular to the cell surface. In this study we have used atomic force microscopy (AFM) to investigate the high resolution structure of isolated, intact sacculi of both Gram-positive and Gram-negative bacterial cells. We have observed a sponge-like structure for both types of sacculi with pore diameters between 5 to 15 nm. Our data for Gram-positive sacculi provide evidence for the validity of the scaffold model, whereas our data for Gram-negative sacculi indicate an orientation along the short axis of the cell which is consistent with the planar model. To further elucidate the structure, we have exposed sacculi to the tAmiB enzyme which cleaves peptide-peptide bonds. [Preview Abstract] |
Tuesday, March 11, 2008 5:30PM - 5:42PM |
L18.00014: Mechanical Response of Lipid Multibilayers From Micro- and Nano-Particle Embedment Gregory McKenna, Kirthi Deshpande We have used atomic force microscopy (AFM) to image micro- to nano-meter sized particles embedded into lipid multilayer films prepared by a spin coating technique. The lipid investigated was 1, 2-dipalmitoyl-Sn-glycero-3-phosphotidylcholine (DPPC). Gold, silica and polystyrene were used for the embedding particles. Particle diameters ranged from 50 to 300 nm and all tests were performed at atmospheric conditions and ambient temperature. Film thickness was approximately 87 nm based on AFM determination. We used the elastic analysis for contact between a rigid sphere and elastic substrate with the work of adhesion w$_{a}$ acting as the force on the sphere to determine the shear modulus G in terms of w$_{a}$, the particle radius and the height of the sphere that remains above the surface. From the AFM height measurements, we find that the shear modulus for the DPPC falls in the range from 3 to 35 MPa, but seems independent of particle type and particle diameter. The potential for the particle embedment method for mechanical property determination of soft materials will be discussed. [Preview Abstract] |
Session L19: Focus Session: Dopants and Defects in Semiconductors II
Sponsoring Units: DMPChair: Michael Stavola, Lehigh University
Room: Morial Convention Center 211
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L19.00001: Highly Enriched $^{28}$Si -- a New Testbed for Impurity and Defect Structure Invited Speaker: We have recently found that many optical transitions in Si, including those of shallow impurity bound excitons, and the electronic ground state to excited state transitions of shallow donors and acceptors, are remarkably sharper in highly enriched $^{28}$Si than in natural Si, due to the removal of inhomogeneous isotope broadening. This work is now being extended to deeper defects, many of which have been studied for decades in natural Si and were until now thought to be well understood. In $^{28}$Si, due to the narrowness of the individual transitions, changing the isotopic species of the defect constituents results in well-resolved components, rather than the `isotope shift' of the broad, unresolved inhomogeneously broadened lines observed in natural Si. This results in an `isotopic fingerprint' of the defect, revealing not only the participation of a given element in the defect, but also the number of atoms of that element which are involved. We have recently shown [1] that a well known Cu-containing defect with a no-phonon luminescence line at $\sim $1014 meV, which was thought to be a Cu-pair, and for which ab-initio calculations [2] based on a pair-model appeared to agree convincingly with experiment, in fact contains four Cu atoms. A related center at $\sim $944 meV, modelled in the past as a different configuration of a Cu-pair [3], was shown to contain three Cu atoms [4]. We have now found that the 944 meV center also contains one Ag atom, and that another defect exists which contains two Cu and two Ag atoms. We will show that high resolution spectroscopy in highly enriched $^{28}$Si produces many more surprising results regarding the actual constituents of well known deep centers in Si. \newline [1] M.L.W. Thewalt et al., Physica B \underline {401-402}, 587 (2007). \newline [2] S.K. Estreicher et al., Phys. Rev. Lett. \underline {90}, 035504 (2003). \newline [3] S.K. Estreicher, D. West and M. Sanati, Phys. Rev. B \underline {72}, 121201 (2005). \newline [4] A. Yang et al., Physica B \underline {401-402}, 593 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L19.00002: Nitrogen-hydrogen complexes in ZnO: A possible route toward p-type conductivity. M.D. McCluskey, S.J. Jokela Zinc oxide (ZnO) is a wide band gap II-VI semiconductor with optical, electronic, and mechanical applications. The lack of reliable $p$-type doping, however, has prevented it from competing with other semiconductors such as GaN. In this talk, I describe the successful incorporation of nitrogen-hydrogen (N-H) complexes in ZnO during chemical vapor transport (CVT) growth, using ammonia as an ambient. The N-H bond-stretching mode gives rise to an infrared (IR) absorption peak at 3150.6 cm$^{-1}$. Isotopic substitutions for hydrogen and nitrogen result in the expected frequency shifts, thereby providing an unambiguous identification of these complexes. The N-H complexes are stable up to $\sim $700\r{ }C. The introduction of neutral N-H complexes could prove useful in achieving reliable $p$-type conductivity in ZnO. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L19.00003: Vibrational lifetimes of O-H stretch modes in MgO and ZnO Erik Spahr, Gunter Lupke, Norman Tolk, Leonard Feldman Hydrogen is an important and omnipresent impurity in a wide class of oxides. A more complete understanding of the role of hydrogen in wide-bandgap oxides such as MgO and ZnO is crucial for further development of oxide-based optoelectronics. We have measured for the first time the vibrational lifetime of the O-H stretch mode associated with the Mg$^{2+}$ vacancy in MgO for the charge state, V$_{OH}$-, and the neutral state, V$_{OH}$, using picosecond transient bleaching spectroscopy. For the V$_{OH}$ center we find the lifetime ($\sim $11 ps) is longer than for the charged defect state ($\sim $5 ps). These lifetimes are almost an order of magnitude shorter than in covalent semiconductors Si and Ge [1]. Similar measurements will be presented for interstitial hydrogen in ZnO. Our results provide new insight into the coupling of the ionic surroundings to the O-H vibration within the crystal lattice. [1] M. Budde et al., PRL 87, 145501 (2001). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L19.00004: Carrier Dynamics and Photoexcited Emission Efficiency of ZnO:Zn Phosphor Powders John V. Foreman, Henry O. Everitt, Jie Liu Nonstoichiometric ZnO with an excess of Zn atoms (ZnO:Zn) has a long history of use as a green/monochrome phosphor in electron-excited vacuum fluorescent and field emission displays. We previously studied the external quantum efficiency of such ZnO:Zn powders under continuous-wave photoexcitation and found that the efficiency depended sensitively on excitation density [\textit{Appl. Phys. Lett. }\textbf{91}, 011902 (2007)]. Here we study experimentally the time-integrated quantum efficiency and the time-resolved photoluminescence decays of both band edge and defect emission of ZnO:Zn powders under femtosecond pulsed excitation of varying intensity. The results are discussed in terms of a rate equation model which describes energy transfer between band edge and radiative defect levels, as well as nonradiative centers. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L19.00005: Study of the carrier concentration dependent photoluminescence of Ga-doped ZnO thin films grown by molecular-beam epitaxy Zheng Yang, Leelaprasanna Mandalapu, Jianlin Liu The undoped and Ga-doped ZnO thin films were grown on $r$-sapphire using molecular-beam epitaxy (MBE) system. The samples of electron concentrations ranging from 1.4x10$^{18}$ to 3.4x10$^{19}$ cm$^{-3}$ were grown and studied. The RT PL peaks show a monotonic red shift from 3.280 to 3.229 eV with the increase of electron concentration, which is attributed to the band-gap narrowing effect. The red-shifted peak values have been fitted. The evolution of the LT PL spectra were studied and discussed. The free exciton emission at 3.371 eV, the first Ga-level-related peak at 3.313-3.321 eV, and the second Ga-level-related peak at 3.359 eV (assigned as the Ga D$^{0}$X peak) are competing with each other in the LT PL spectra. These three kinds of peaks are dominating in the lightly (or undoped), mediate, and heavily doped ZnO:Ga samples, respectively. From the experiments, we conclude that there are two Ga levels in ZnO. In the lightly doped sample, the Ga atoms contribute to the first Ga level around 3.32eV. When the Ga incorporation reaches some critical amount, Ga atoms form the second Ga level in ZnO at 3.359 eV. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L19.00006: Zinc Vacancy induced magnetism in ZnO thin films and nanowires Qian Wang, Qiang Sun, Puru Jena Extensive theoretical studies based on first-principles have been carried out for the mechanism of magnetism in ZnO thin films and nanowires. It has been identified that the observed magnetism is introduced by Zn vacancy and is affected by its concentration. The main source of the magnetic moment comes from the unpaired 2p- electrons in oxygen sites around the Zn vacancy, instead of Zn 3d electrons. Moreover, Zn vacancy is more energetically favorable to reside on the surface, and its formation energy is found to be less than that of oxygen vacancy that does not introduce any magnetism. These findings suggest that the main vacancy species is Zn vacancy as expected by experiments. The present theoretical study not only provides some deep understandings for the experimentally observed magnetism in un-doped ZnO samples, but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO structure for bio-magnetic applications. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L19.00007: Magnetism of Undoped and Co-Doped TiO2 Clusters X. Wei, R. Skomski, M. Schubert, D. Sellmyer TiO$_{2}$ is a widely used optically active material, and transition-metal doped TiO$_{2}$ has attracted much attention in spin electronics. Recently, it has been argued that ferromagnetism is a universal feature of nanoparticles of nonmagnetic oxides, and our focus is on doped and undoped TiO$_{2}$ nanoclusters. The clusters are examined with TEM, AFM, MFM, and hysteresis loops and zero field cooled magnetization curves were measured by SQUID magnetometry. Both doped and undoped films display hysteresis and magnetic order in the investigated temperature range of 5K to 400~K. The ordering temperature is above 400 K, and~both magnetization and coercivity are enhanced in~the out-of-plane direction. Undoped TiO$_{2}$ particles exhibit a nominal moment of about 0.2 $\mu _{B}$ per surface atom. Small Co concentrations have little effect on the magnetism of the particles. Higher Co doping percentages, about 8{\%}, yield proteretic (clockwise) loops, indicating~the formation of CoO. It has~been suggested that the magnetic moment of 'nonmagnetic' oxide thin films is a surface effect, and the comparison of different particle sizes yields a similar picture for our particles. Our renormalization-group modeling assumes indirect exchange interactions between scarce magnetic moments and yields a logarithmic dependence of the ordering temperature on the particle size - This research is supported by NSF MRSEC and NCMN. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L19.00008: First-principles determination of the electronic structure of native point defects and impurities in rutile TiO$_{2}$ Jun He, Susan Sinnott Density-functional theory calculations are used to determine the electronic structures of native point defects (Ti interstitials and O vacancies) and dopants (Al and Nb) in rutile TiO$_{2}$. The calculated densities of states (DOS) show that in pristine and defective structures that contain charged Ti interstitials or O vacancies, the lower conduction bands of the defective structures are shifted up in energy relative to the perfect structure. This shift leads to a broader lower conduction band that more readily promotes the formation of shallow donor levels. This effect is more pronounced in the case of Ti interstitials. We also find that the charge state of the Ti interstitial influences the extent of orbital overlap. The case of Al dopants is much more complex since Al can either be a donor or an acceptor. In the case of Al and Nb substitutional defects, the calculated DOS is similar to the DOS of the pristine structure. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L19.00009: Group III-A Acceptor-Hydrogen interactions in SnO$_{2}$ Joel Varley, Anderson Janotti, Abhishek Singh, Chris G. Van de Walle Using first-principles calculations we investigate the role of hydrogen in the passivation of p-type dopants in SnO$_{2}$. We focus on group III-A elements, including Al, Ga, and In and investigate the stability of these impurities when substituting Sn under hydrogen-free and hydrogen-rich conditions. Hydrogen effectively passivates the acceptors and removes their electrical activity. Based on calculated binding and migration energies we discuss conditions under which hydrogen can be removed and acceptor activation can take place. We also calculate the stretch-mode vibrational frequencies associated with the hydrogen-impurity complexes, providing a signature for experimental identification in vibrational spectroscopy. We conclude that the group III-A elements studied are suitably shallow acceptors for p-type doping and that the presence of interstitial H will not impede, and potentially enhance, p-type doping of SnO$_{2}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L19.00010: Investigation of Electroluminescent Degradation in doped ZnS phosphors Frank Bridges, Jacob Stanley, Yu Jiang, Laurel Ruhlen, John Willy, Sue Carter We present optical and EXAFS data on a series of ZnS samples doped with Cu, Mn and Cl. These materials (30 micron particles) have a strong electroluminescence (EL) when subjected to a 100V square-wave voltage. At 100 kHz, the luminescence decays significantly in a 20 hr period. We show that this degradation can partially be reversed by annealing the sample and that this can be repeated several times. In addition the EL emission centers reoccur at the same points in the 30 micron particles after the anneal. The optimum annealing temperature is about 180C, but varies slightly for different wavelengths. Surprisingly an anneal at somewhat higher temperatures (240C) dramatically reduces the EL intensity. The EXAFS studies show that the local structure about Cu continues to look like CuS for ``as made", EL degraded, rejuvenated samples (annealing at 180C), and thermally degraded samples (annealed at 240C). This means that most of the Cu is in the relatively inert CuS precipitates, and does not change significantly with EL degradation or annealing. Thus the EL active sites must be dilute. We discuss some possible models. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L19.00011: Theoretical Study of native defects in CdGeAs$_2$ Tula R. Paudel, Walter R.L. Lambrecht First-principles results are presented for various native defects \textit{viz} : V$_{\rm{Cd}}$, V$_{\rm{Ge}}$, V$_{\rm{As}}$, Cd$_{\rm{Ge}}$, Ge$_{\rm{Cd}}$, Ge$_{\rm{As}}$ and As$_{\rm{Ge}}$ in CdGeAs$_2$ under different growth conditions. The defects were calculated by constructing a 64 atom supercell in the full potential linearized muffin-tin orbital implementation of the density functional theory under the local density approximation (LDA). Calculations of the energy of formation show that antisites should be the most abundant type of defect. The LDA band gap is adjusted to experimental band gap by introducing a non-local orbital dependent constant potential shift to the $s$-orbitals of Cd and Ge and $d$-orbitals of Cd within the LSDA+U approach. The defect transition levels for different charge states are calculated. The calculations support the earlier suggestion that Ge$_{\rm{As}}$ is a shallow acceptor. The calculated transition levels are found to be significantly different form corresponding defects levels of ZnGeP$_2$. The defect levels are interpreted in a simple molecular-orbital theory and compared with the available experimental data. [Preview Abstract] |
Session L20: Focus Session: Self-Assembled Organic Overlayers
Sponsoring Units: DMPChair: Philip Cohen, University of Minnesota
Room: Morial Convention Center 212
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L20.00001: Self-assembly at the liquid/solid interface: from patterns to function Invited Speaker: Self-assembly - the spontaneous organization of molecules into stable, structurally well-defined aggregates - has been put forward as a possible paradigm for generating nanoscale templates under ambient conditions. A very convenient method for the formation of extended two-dimensional (2D) networks is physisorption at the liquid-solid interface. The preparation is relatively simple and scanning tunneling microscopy (STM) allows a detailed investigation of the structural aspects of the 2D patterns. A deep understanding and control of the spatial orientation and packing of molecules in self-assembled systems is indispensable for the development of future nanodevices. We have developed hydrophobic - hydrophylic nanopatterns at electrified surfaces via the self-assembly of amphiphilic molecules. For this purpose we selected 5-hexadecyloxy isophthalic acid: this neutral amphiphile forms hydrogen-bonded rows that are commensurate with the Au(111) surface. Based on the successful adsorption of these amphiphiles, multicomponent architectures have been realized at these electrified surfaces as the result of the potential directed assembly of charged and non-charged molecular systems. Following a slightly different approach, nanoporous two-dimensional networks were formed at the interface between an organic liquid and highly oriented pyrolytic graphite. Pore sizes of more than 5 nm in diameter can be realized. As an alternative approach to make nanoporous two-dimensional networks, molecular defined shape-persistent two-dimensional oligomers, such as molecular spoked wheels, are used. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L20.00002: Stress-Induced Striped Domains at the C$_{60}$-Pentacene Interface* Daniel Dougherty, Wei Jin, William Cullen, Gregory Dutton, Janice Reutt-Robey, Steven Robey C$_{60}$:pentacene heterojunctions have been recently employed in functional organic photovoltaic devices [1]. In order to develop a detailed structural understanding of these junctions, we have made STM observations of the growth of pentacene on top of a monolayer film of C$_{60}$ on Ag(111). We observe pattern formation in the first pentacene layer due to the tensile stress arising from interactions with the underlying C$_{60}$. The stress results in a striped pattern of alternating commensurate and incommensurate domains of pentacene with respect to the C$_{60}$ monolayer. The incommensurate domains appear as bright regions 3.6 $\pm $ 0.6 nm in width and are discussed using a 1D Frenkel-Kontorova model. *Supported by the NIST Center of Nanomanufacturing and Metrology and the University of Maryland MRSEC via DMR-05-20471.[1] Yoo et al., Appl. Phys. Lett. 85, 5472 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L20.00003: The growth mechanism of Pentacene-C$_{60}$ heteroepitaxial films A. Al-Mahboob, J.T. Sadowski, Y. Fujikawa, T. Sakurai Pentacene (Pn) and fullerene (C$_{60})$ are of great interest among organic semiconductors as they show highest field-effect hole and electron mobilities respectively. The absorption peak in Pn crystal is located close to maximum of solar visible spectra, making a bipolar Pn-C$_{60}$ diode promising for solar cell application. In order to improve its efficiency to satisfy the requirement for practical application, an in-depth understanding of Pn-C$_{60}$ interface formation is necessary for further optimization. We shall discuss the growth mechanism of Pn-C$_{60}$ heteroepitaxial films on Bi(0001)/Si(111) substrate studied by real time low-energy electron microscopy and complementary scanning tunneling microscopy. A competitive growth between a thin-film phase of Pn having standing-up orientation and a phase with laying-down orientation has been observed. The growth of laying-down phase is suppressed gradually with increasing film thickness. The nucleation of this phase is also suppressed with increasing temperature and the standing up phase without co-presence of laying down phase is achievable at $\sim $75\r{ }C. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L20.00004: Self-Limiting C$_{60}$-Pentacene Network on Ag(111) Wei Jin, Daniel Dougherty, Gregory Dutton, William Cullen, Steven Robey, Janice Reutt-Robey During Scanning Tunneling Microscopy investigations of C$_{60}$:Pentacene (Pc) interfaces on Ag(111), we identified a new network structure. This binary arrangement forms readily by sequential deposition: Pc, of $\sim $0.3 mL coverage, is first evaporated onto the Ag(111), forming a 2-D gas. Subsequent C$_{60}$ deposition produces a network, consisting of chains of close-packed C$_{60}$ molecules, spaced by C$_{60}$ molecules. The characteristic 1 x 2.5 nm$^{2}$ pores are sized to accommodate Pc molecules. Spontaneous formation of this structure from an iniital Pc coverage ranging from 0.3 to 0.6 mL, indicates a self-limiting assembly process. Drawing upon topographic and Z(V) measurements, we propose a structural model and describe molecular mechanisms that could cause this self-limiting behavior. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L20.00005: Guest-host interaction of C$_{60}$ adsorbed on an ordered layer of phthalocyanine derivatives Tomas Samuely, Meike Stoehr, Nikolai Wintjes, Thomas A. Jung, Marco Haas, Shi-Xia Liu, Silvio Decurtins Symmetrically substituted phthalocyanines (Pcs) with eight peripheral di-(tert-butyl)phenoxy (DTPO) groups self-organize on noble metal substrates. The rotational degrees of freedom, specific for the DTPO substituents, allow a bowl-like conformation of the Pc derivatives and thus, hosting of the C$_{60}$ molecules in two clearly distinguishable binding sites. Moreover, controlled manipulation of the C$_{60}$ by the STM tip enables switching from one site to the other. Since Pcs are well-known electron donors and C$_{60}$ molecules are good acceptors, it can be conceived as a system with two morphologically different donor-acceptor complexes, individually addressable by an STM tip. Preliminary STS analysis shows vast differences in the electronic properties. Exploration of such a system is of great interest because of its similarity to fundamental biological processes (photosynthesis, respiration), as well as its potential for application in energy storage, conversion, nanoelectronics, etc. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L20.00006: Scanning Tunneling Microscopy Investigation of Ordered Iron Phthalocyanine Molecules on Ag(111) Ken Park, Kedar Manandhar, S. Ma, Jan Hrbek A well-ordered, molecular thin films (about 0.5 monolayers) of iron phthalocyanine (FePc) on Ag(111) has been investigated using a scanning tunneling microscopy. The room temperature deposition, followed by 30 minutes of annealing up to 475 K results in well-ordered islands of FePc molecules which form a two-dimensional, oblique lattice. The overlayer lattice vectors \textbf{a}$_{1}$ and \textbf{b}$_{1 }$are 16.2 {\AA} each with the angle of 80\r{ } between the lattice vectors. The FePc overlayer lattice is commensurate to the substrate lattice with the relationship of \textbf{a}$_{1}$ = 6\textbf{a} - \textbf{b} and \textbf{b}$_{1}$ = -\textbf{a} + 6\textbf{b}, with \textbf{a} and \textbf{b} are the Ag(111) lattice vectors. Furthermore, FePc molecules at the opposite corners within the unit cell align their isoindole rings at each other, significantly increasing the nearest and the next-nearest neighbor distances in the overlayer. The commensurate overlayer lattice structure and the distinct molecular orientation within the unit cell are attributed to the molecule-surface interaction via the Fe 3d$_{xz,yz}$ and Ag 4d$_{xz,yz}$ orbitals. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L20.00007: Self-assembly of functionalized fullerenes on strained metallic interfaces. Bogdan Diaconescu, Mikael Jazdzyk, Glen Miller, Karsten Pohl The process of growing ordered arrays of molecules or nanometer sized clusters with tailorable properties on the dislocation networks of strained metallic thin films requires a detailed understanding of the nucleation processes and film-molecule and intermolecular interactions. We compare two different self-assembly mechanisms of functionalized fullerenes on strained metallic films of Ag on Ru(0001). We found that by controlling the molecular coverage and/or the strain in the thin film, various interactions can dominate the self-assembly process, thus resulting in a richness of structures with controllable properties. At low molecular coverage on 1 monolayer Ag films on Ru(0001), ordered triangular arrays of clusters, 4.9 nm apart, with a tunable number of molecules can be grown. This process is driven by strain relaxation in the metal film, as confirmed by 2D Frenkel-Kontorova models, and was found to be a general one working for various functionalized molecules. At higher molecular coverage and different Ag film thickness, the intermolecular interaction becomes dominant and the symmetry and unit cell size of the self-assembled monolayer are a consequence of the molecular structure and functionality. Both these processes are generally applicable to many functionalized C60 molecules thus opening avenues towards complex self-assembled structures based on lock and key type approach. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L20.00008: Morphology of thin organic semiconductor layers on vicinal (0001) sapphire surface Gvido Bratina, Primo\v{z} Rebernik Ribic Morphology of pentacene, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), rubrene, and N,N$'$-diphenyl-N,N$'$-bis(3-methylphenyl)-1,1$'$-diphenyl-4,4$'$-diamine (TPD) layers grown on vicinal (0001) sapphire surface was examined by non-contact atomic force microscopy ex situ. The layer thickness ranged from a submonolayer coverage to up to four molecular layers. Pentacene was found to nucleate at the nm-size sapphire steps, and continues to grow on the terraces in a layer-plus-island growth mode. PTCDA nucleates randomly at room temperature, while at 135$^{\circ}$C and low coverage the molecules aggregate at the steps. At increased coverage the island growth proceeds in the directions determined by the intermolecular interactions and not along the steps. Rubrene and TPD nucleate in 3D islands that evolve over time by ripening. TPD nucleates along the steps and also ripening proceeds along the steps. The rate of ripening of rubrene islands is one order of magnitude slower than the rate of ripening of TPD. We associate this difference to the wealth of rotational degrees of freedom in TPD molecules as opposed to only twisting degree of the tetracene backbone in rubrene. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L20.00009: Organic Vapor-Liquid-Solid Deposition for Controlled Growth of Organic Semiconductor Films. David Patrick, Brad Johnson Thin films of low molecular weight organic semiconductors (OS) are drawing much attention for their potential use in a range of different applications. Because the optical and electronic properties of OS films are extremely sensitive to structural imperfections, domain size, and crystallographic orientation, preparation of high quality thin films with controlled microstructural organization under technologically favorable conditions has long been a bottleneck toward practical applications and better controlled fundamental studies. Here we describe an approach for fabricating OS films that comes close to achieving these demanding objectives. The main advance is the combined use of atmospheric pressure vapor-phase deposition into a thermotropic liquid crystal (LC) solvent, which is applied as a thin coating onto a supporting substrate, providing an organized fluid environment in which OS crystals nucleate and grow. The technique produces relatively large crystals, enables control over crystallographic orientation, growth habit, and size, and involves near ambient conditions compatible with a variety of substrates and inexpensive processing conditions. Results will be presented for the model compounds tetracene and pentacene. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L20.00010: Structure and adsorption mechanisms of thiol self-assembled monolayers on GaAs (001) surfaces Oleksandr Voznyy, Jan J. Dubowski Self-assembly of organic molecules on solid substrates attracts a lot of interest from both fundamental and practical perspectives. Particularly, alkanethiol SAMs on GaAs surfaces can be used, e.g., for surface passivation, bio- and chemical sensing, molecular electronics and nanolithography. However, the progress of the investigation of this material system is hindered by problems of reproducibility of SAMs growth due to insufficient understanding of the deposition process on the atomic level and a lack of experimental characterization techniques. In this work, we present the results of ab initio modeling of thiol SAMs on GaAs (001) surface which helps to resolve the SAM structure, chemistry of bonding to substrate, adsorption kinetics and energetics. We also compare the GaAs case to a prototypical system of thiols on Au and discuss differences the semiconductor surface brings to the process. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L20.00011: Self-assembly of long chain alkanes and their derivatives on graphite Teng Yang, Savas Berber, David Tom\'anek, Jun-Fu Liu, Glen P. Miller We combine scanning tunneling microscopy (STM) measurements with {\em ab initio} calculations to study the self-assembly of long chain alkanes and related alcohol and carboxylic acid molecules on graphite. For each system, we identify the optimum adsorption geometry and explain the energetic origin of the domain formation observed in the STM images. Our results for the hierarchy of adsorbate-adsorbate and adsorbate-substrate interactions provide a quantitative basis to understand the ordering of long chain alkanes in self-assembled monolayers and ways to modify it using alcohol and acid functional groups. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L20.00012: Theoretical Modelling of Self-Assembly of Molecular Networks Manuela Mura, Natalia Martsinovich, Lev Kantorovich The phenomenon of self-assembly of atomic and molecular superstructures on crystal surfaces has attracted an increasing interest in nanotechnology. Self-organised nano-templates where the self-assembled monolayer traps other molecules with selected functional properties, can be used as building blocks for larger nanoscale structures. These superstructures can form chiral domains ranging from 1D chains to 2D monolayers. In particular, there have been many scanning tunneling microscopy (STM)studies of self-assembly of melamine, perylene tetra-carboxylic di-imide(PTCDI) or perylene tetra-carboxylic di-anhydride (PTCDA) molecules on the Au(111). STM images of these networks do not reveal the exact details of the intermolecular bonding and process of network growth. It is therefore the task of theory to determine the exact atomic structure of these networks. We present a theoretical study of self-assembly of molecular networks based on different molecules by using a systematic approach to build molecular superstructures. The energies of these structures are calculated using the density-functional theory SIESTA code. The theoretically predicted monolayer structures are in very good agreement with the results of STM measurements. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L20.00013: Polymerization of a perylene derivative on a metal surface Manfred Matena, Jorge Lobo-Checa, Meike Stoehr, Kathrin Mueller, Thomas A. Jung, Till Riehm, Lutz H. Gade By utilizing the concepts of supramolecular chemistry, impressive results for molecular self-assembly on surfaces have been presented. Mostly, non-covalent interactions like metal coordination, hydrogen bonding or dipolar coupling are exploited to create supramolecular patterns. One approach to influence these structures relies mainly on the sophisticated design of the molecular functional groups. Thus it makes use of properties already inherent to the molecules. In our work we have chosen a different concept. A thermally-induced surface-assisted reaction was used to modify the endgroups of a perylene derivative (TAPP) and thereby, the molecular interactions are altered. TAPP was found to form a closed-packed assembly on Cu(111), whereas the molecular interactions are based upon vdW-forces. After annealing at 150\r{ }C, a rectangular network is obtained which is commensurate to the underlying Cu surface. In this case the organic molecules coordinate to Cu atoms through the lone pairs of their nitrogen atoms. A second annealing step at temperatures $>$240\r{ }C modifies the molecule on the surface and leads to covalently linked polyaromatic chains. [Preview Abstract] |
Session L21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials V
Sponsoring Units: DCPChair: Penee Clayborne, Virginia Commonwealth University
Room: Morial Convention Center 213
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L21.00001: Ab Initio Simulations of Nano-Diamond Surface Reconstruction William Mattson, Radhakrishnan Balu We have simulated with in the Density Functional Theory (DFT) spherically cut nano-diamonds from bulk diamond at ambient conditions. The 2.6 nanometer diameter sphere is then allowed to relax at 250K and surface reconstruction is observed. Four hemispherical fullerene like regions form on the surface and while the interior maintains the diamond structure, it undergoes compression equivalent to over 30GPa in the bulk. Results of dynamic shearing will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L21.00002: TiO$_{2}$ nanostructures prepared by ferrocene/cobalt catalyst agents M.E. Gomez, J.C. Caicedo, G. Zambrano, A-M. Lazar, D. Chaumont, Y. Lacroute, M. Sacilotti We present the growth and characterization of TiO$_{2}$ nanocrystals. Nanostructured growth is obtained in a low-pressure CVD system by using an organometallic precursor Ti(OC$_{3}$H$_{7})_{4}$ as both the Ti and O source catalyzed by both ferrocene (an organometallic precursor) and cobalt metallic clusters prepared by the microwave-assisted polyol method. Two kinds of TiO$_{2}$ structures were obtained in the cobalt clusters: a) pine-tree like (with short-leaf structure) and b) long-leaf structures as large as a few micrometers in size and both under 10-nm in thickness. Long-leaf TiO$_{2}$ structures were grown at cobalt grain boundaries. For the growth conditions utilized, the TiO$_{2}$ structures are composed of both anatase and rutile crystallographic phases. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L21.00003: Metal cluster anions produced by attachment of slow electrons: Evaporative cooling, cluster energetics, and restructuring of the abundance spectra Vitaly Kresin, Roman Rabinovitch, Chunlei Xia Metal clusters are able to attach low-energy electrons with very large cross sections by capturing them in a strong long-range polarization potential. But little information has been available about the last stage of the collision process: what happens to the energy deposited by the captured electron, and are the cluster size distributions modified? We have carried out measurements of the mass spectra of negative sodium cluster ions born in the electron-cluster interaction region. Importantly, the arrangement allowed us to monitor the parent and the daughter cluster beams simultaneously. It is found that the electron affinity energy is quickly thermalized and is sufficient to cause rapid evaporative cooling. As a result, the magic numbers shift from Na$_{n}$ to $\mbox{Na}_{\mbox{n-1}}^{\mbox{-}} $, and a host of other significant changes in the abundance structure are observed, compared to the parent cluster beam. These are well reproduced by a detailed analysis based on the evaporation cascade model, and yield new information about cluster binding energies. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L21.00004: Anion Photoelectron Spectroscopy and Density Functional Investigation of Diniobium-Carbon Clusters P.A. Clayborne, K.L. Knappenberger, Jr., J.U. Reveles, M.A. Sobhy, C.E. Jones, Jr., U.U. Gupta, I. Iordanov, J. Sofo, A.W. Castleman, Jr., S.N. Khanna Experimental photoelectron and computational results show diniobium-carbon (Nb$_{2}$C$_{n})$ clusters to coexist in multiple structural isomers: three-dimensional geometries, planar rings and linear chains. Three-dimensional clusters having up to five carbons are formed preferentially with Nb-Nb bonding, whereas only Nb-C bonding is observed experimentally at six carbons. Clusters consisting of an odd number of atoms are also observed with linear geometries. The larger binary clusters (n $\ge$ 7) display properties similar to pure carbon clusters. We provide evidence for niobium substitution of carbon atoms. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L21.00005: Towards Artificial Molecules: Metallodielectric Clusters Dina Aronzon, Vinothan Manoharan, Jonathan Fan Recent advances in the synthesis of metallic colloids have allowed for an explosion in research into their optical properties. It is now possible to synthesize solid metallic colloids, core-shell colloids that mix different metals, and core shell colloids of metallodielectrics. In this talk I propose a mechanism for further exploring the optical properties of such materials by producing clusters of metallodielectric colloids and studying the relationship between the structure and composition of a colloidal cluster and its optical response. To this end, we synthesized a number of solutions, each of clusters consisting of a different average number of colloids. By doing this, we hope to study the effects of different dielectrics and near neighbor interactions on the plasmonic resonances of the metallic shells in the colloid. In the future, we hope to produce and study high yield, pure samples, both in solution and as photonic crystals. All of these options provide new ways of producing specific optical resonances that can be used in sensors, spectroscopy, optical triggers, and many other applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L21.00006: Quantum transport in molecular electronic devices described with complex source and sink potentials Francois Goyer, Ali Goker, Matthias Ernzerhof We present a non-Hermitian model Hamiltonian containing complex potentials [1,2] that is devised to study ballistic transport in molecular electronic devices (MEDs). The complex potentials replace semi-infinite contacts and act as source and sink of probability current density. This approach is rigorous in the sense that the exact wave function is recovered in the interior of the MED. We employ this technique to calculate the conductance through certain prototypical MEDs [3]. We also extend this method [4] such that we can go beyond the one- electron picture by constructing two-electron states explicitly. We present results for simple model system described by Hubbard-type Hamiltonians. The impact of electron correlation effects on the molecular conductance is discussed. [1] F. Goyer, M. Ernzerhof, M. Zhuang, JCP, 126, 144104 (2007). [2] M. Ernzerhof, JCP, to appear nov. 2007. [3] M. Ernzerhof, H. Bahmann, F. Goyer, M. Zhuang, P. Rocheleau, J. Chem. Theory Comput., 2, 1291 (2006); M. Ernzerhof, M. Zhuang, P. Rocheleau, JCP, 123, 134704 (2005). [4] A. Goker, F. Goyer, M. Ernzerhof, work in pogress. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L21.00007: Charge carrier solvation on a polymer chain revealed in \textit{ab initio} computations Michael Mayo, Yuri Gartstein When an excess charge carrier (electron or hole) is added to a semiconducting polymer chain in vacuum, it is well known that the carrier may self-trap into a polaronic state accompanied by a self-consistent localized bond alternation pattern. A different mechanism of self-localization is a solvation of the charge carrier expected to take place when the polymer chain is immersed in a polar medium (such as a common solvents) - in this case a self-consistent pattern of the orientational polarization is formed around a localized charge. The goal of our study is to identify this solvation effect within \textit{ab initio} computations. Using long carbon atom chains (both hydrogen terminated and rings) as model systems, we employ the hybrid B3LYP density functional within the DFT and the Polarizable Continuum Model to find the resulting electronic level structure and atomic charge densities. Our results clearly show trends towards excess charge self-localization due to the solvation. We suggest that this effect may be of importance for various semiconductor nanostructures in polar environments. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L21.00008: Nanotubes in polar environments: Solvated charge carriers and their dynamics Geoffrey Ussery, Yuri Gartstein Excess charge carriers on semiconducting nanotubes immersed in sluggish polar environments (such as common solvents) can undergo self-localization into polaronic states whose properties are profoundly different from the free band states. We explore such solvated states within the adiabatic continuum framework using a simplified picture of an electron or a hole confined to a cylindrical surface in the 3D polar medium. At the static level, the binding energy of a polaron is evaluated and found to be a sizable fraction ($\sim$ 0.3) of the corresponding Wannier- Mott exciton binding energy, which is expected to substantially decrease the thermal activation energy for the exciton dissociation. We discuss the diffusion and mobility of polarons caused by the dielectric fluctuations of the medium and applied electric fields, as well as the local dielectric relaxation modes in the vicinity of the polaron. We also discuss the electronic (optical) transitions between the localized electronic states within a self-consistent potential well due to the orientational polarization pattern. \newline [1] Yu.N. Gartstein, Phys. Lett. A 349, 377 (2006). \newline [2] Yu.N. Gartstein, T.D. Bustamante, S. Ortega Castillo, J. Phys.: Cond. Matter 17, 156210 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L21.00009: Fluorescent silver atom clusters in DNA hairpin loops Patrick O'Neill, Elisabeth Gwinn, Dirk Bouwmeester, Deborah Fygenson We synthesize fluorescent clusters of silver atoms on DNA hairpins, and systematically vary the loop region to probe the effects of DNA sequence and structure on the optical properties and chemical stability of the Ag clusters. We find that these novel fluorophores only form on the single stranded hairpin loop, have Stoke's shifts ranging from 60nm to 120nm and can be tuned to emit at wavelengths ranging from 525nm to 670nm. Furthermore, certain DNA geometries support strong excitation of visible fluorescence by 260-280nm light. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L21.00010: H$_2$O Nucleation Around Noble Metal Cations Patrizia Calaminici, Pavel Oropeza Alfaro, Martin Juarez Flores, Andreas K\"oster, Marcela Beltran, J. Ulises Reveles, Shiv N. Khanna First principle electronic structure calculations have been carried out to investigate the ground state geometry, electronic structure and binding energy of noble metal cations (H$_2$O)$_n^+$ clusters containing up to 10 H$_2$O molecules. The calculations are performed with the density functional theory code deMon2k [1]. Due to the very flat potential energy surface of these systems special care to the numerical stability of energy and gradient calculation must be taken.Comparison of the results obtained with Cu$^+$, Ag$^+$ and Au$^+$ will be shown. This investigation provides insight into the structural arrangement of the water molecules around these metals and a microscopic understanding of the observed incremental binding energy in the case of the gold cation based on collision induced dissociation experiments. \newline [1] A.M. K\"oster, P. Calaminici, M.E. Casida, R. Flores-Moreno, G. Geudtner, A. Goursot, T. Heine, A. Ipatov, F. Janetzko, J. Martin del Campo, S. Patchkovski, J.U. Reveles, A. Vela and D.R. Salahub, deMon2k, The deMon Developers, Cinvestav, 2006 [Preview Abstract] |
Session L22: Confinement-Induced Structures in Block Copolymers
Sponsoring Units: DPOLYChair: Eric Cochran, Iowa State University
Room: Morial Convention Center 214
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L22.00001: Dillon Symposium Break
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Tuesday, March 11, 2008 3:06PM - 3:18PM |
L22.00002: Block copolymers in cylindrical confinement: role of thermal fluctuations and confinement parameters in structure formation Kirill Titievsky Phase segregated block copolymer morphology and dimensions were studied using explicit simulations of interacting bead-spring chains at physically realistic densities. In particular, The distribution of characteristic morphologies and structure dimensions for a given fiber diameter are characterized. Key novel aspects of this work inlude sampling of thermal fluctuations in full three dimension, rather than finding minimum energy surfaces and extensive comparison to experimental data on electrospun block copolymer--polymer fibers. Two methodological innovations are also introduced. First, an unprecedentedly precise estimate of Flory $\chi$ parameters for the model allows a quantitative comparison to field theoretic models. Second, a novel reflective boundary model allows common boundary artifacts in total density to be effectively eliminated, even for fibers with of dimensions comparable to single chains. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L22.00003: Complex Morphologies of Symmetric Diblock Copolymers under Nano-Confinement Dong Meng, Yuhua Yin, Jacqueline Acres, Qiang Wang We have performed parallel self-consistent field (SCF) calculations in continuum to study the self-assembled morphologies of symmetric diblock copolymers under planner and cylindrical confinement by homogeneous surface(s). The SCF equations are solved with high accuracy in real space, without \textit{a priori} knowledge of the possible morphologies. Effects of surface preference and film thickness / pore diameter are investigated in detail. In addition to simple morphologies (i.e., surface parallel and perpendicular lamellae), complex morphologies are found in both cases and their stable regions are determined. Our SCF calculations also reveal the formation mechanism of these complex morphologies. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L22.00004: Nano-structures of block copolymers under confinement Jie Feng, Eli Ruckenstein By employing a bond fluctuation lattice Monte Carlo simulation, block copolymers confined in nano-cylindrical cavities are studied. The effects of preference of the surface for segments and incompatibility between different blocks as well as the symmetry of the chains and the ratio of cavity diameters to the lamellae period of copolymers in the bulk $\left( {D/L_0 } \right)$ are investigated in detail. Numerous novel morphologies such as complicated helical structures, plate morphologies with fins and dendrites etc are presented in this work. Some phase diagrams regarding above parameters are provided in order to understand the transitions between structures. Additionally, the orientation parameters indicating the alignments of the polymer chains were calculated and correlated with the morphologies. The simulation results are compared with experimental results qualitatively. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L22.00005: Morphologies of a diblock copolymer melt confined in a spherical nanopore Bing Miao, Janine Tulkens, Robert Wickham We systematically investigate the microdomain morphologies that self-assemble in a diblock copolymer melt confined in a spherical nanopore, using real-space self-consistent mean-field theory. Near the surface of the nanopore we find that perforated-layer structures form, with four-, five-, and six- fold coordinated pores, for melts that form the cylindrical phase in the bulk. Simultaneously, spherical domains, toroidal domains, or small networks form in the centre of the pore. We vary the diameter of the pore and accurately locate the diameters where phase transitions between these morphologies occur. The effect of confinement on melts that form spherical microdomains in the bulk is also examined. We find that convergence to regular structures is complicated by the formation of defects, and we develop techniques to eliminate these defects. Methods to distinguish the various morphologies will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L22.00006: Spherical nano-shells of block copolymers Marco Pinna, Andrei Zvelindovsky Using cell dynamics simulation we investigate morphologies block copolymers confined in a spherical shell. The shell is formed by coating of a colloidal particle with a block copolymer thin film. We examine the influence of molecular composition, thickness of the film, curvature of the shell on the nano- structure of the shell. Several block copolymer morphologies are investigated: bulk lamellae, cylinders, spheres and bicontinuous. Deviation from the bulk structure develops under influence of confinement between curved surfaces. The curvature influences the defect density in the formed structure. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L22.00007: Water permeable nanotubes from amphiphilic block copolymers Jiun-Tai Chen, Mingfu Zhang, Ling Yang, Margaret Collins, Jim Parks, Armando Avallone, Thomas Russell We used anodic aluminum oxide (AAO) membranes to generate nanotubes of the cylinder-forming polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymer. The PS-b-PEO solution was introduced into the cylindrical nanopores of an AAO membrane by capillary force and polymeric nanotubes formed after solvent evaporation. Owing to the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respsect to the nanotube walls, the nanotubes were permeable to aqueous media. We also prepared PS-b-PEO nanotubes inside amorphous carbon nanotubes (a-CNTs) where the microphase separated morphology could serve as a template for functionalizing the interior of the carbon nanotubes. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L22.00008: Effect of curvature on equilibrium and non-equilibrium properties of a 2D smectic phase. Leopoldo R. Gomez, Enrique M. Valles, Daniel A. Vega We study through the Otha-Kawasaki model for diblock copolymers equilibrium and non-equilibrium features of two dimensional smectic phases on curved sinusoidal substrates. At low curvatures defect free patters are found to be stable. In agreement with theoretical predictions of Vitelli and Nelson [], at high curvatures topological defects are observed in the ground state. In this regime positive (negative) disclinations are located in regions of positive (negative) curvature. The configurations formed by stripes aligned with the lines of longitude are found to be unstable, while the equilibrium state is formed by the stripes aligned with the parallels. General features of smectic phases on curved backgrounds are discussed by a Frank elastic free energy in the one constant approximation. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L22.00009: Phase Transitions in block copolymers under external electric field and in confinements Andrei Zvelindovsky Phase transitions induced by external factors in various block copolymer systems are investigated by means of coarse-grained computer simulations. We develop several meso-scale computational techniques: dynamic self-consistent field theory and cell dynamics simulation. This contribution puts focus on dynamics of systems subjected to two examples of external fields. First, we describe dynamics of phase transitions of various block copolymer morphologies (lamellae, hexagonally packed cylinders, spheres, gyroid, hexagonally perforated lamellae) under an applied electric field. Second example illustrates kinetics of surface phase transitions in confined systems (thin films). [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L22.00010: Fluctuation-Induced Line-Edge Roughness in Nano-Confined Block Copolymer Thin Films August Bosse, Ronald Jones, Alamgir Karim Block copolymer (BCP) thin film systems are currently under intense scrutiny as a potential nano-scale fabrication mask for pattering next-generation semi-conductors and magnetic media on the $5$ to $20$ nm scale. However, there are certain fundamental issues that need to be resolved, or at least well understood, if BCP systems are going to evolve into a feasible fabrication tool, most notable of which is the scale and system-parameter-dependence of microdomain--matrix-interface line-edge roughness (LER). We present a computational study of microdomain--matrix-interface LER for a nano-confined $AB$ diblock copolymer thin film. The BCP system was simulated using a field-theoretic sampling technique based on a ``hybrid'' mean-field--Monte Carlo framework. We present a summary of our simulation technique, and we examine the dependence of LER on the Flory $\chi$ parameter and the copolymer molecular weight. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L22.00011: The Hierarchical Morphology of Dielectric Mirrors Michael Birnkrant, Christopher Li, Lalgudi Natarajan, Vincent Tondiglia, Pamela Lloyd, Richard Sutherland, Timothy Bunning Active hierarchically structured volume reflection gratings can be fabricated by combining top-down and bottom-up nanomanufacturing techniques. In a typical process holographic photopolymerization (HP) formed lamellar structures of $\sim $200 nm in thickness confining a block copolymer (BCP) to $\sim $100 nm domains. The BCP self assembles into a variety of ordered structures with a period of $\sim $20 nm. A lamellar-in-lamellar structure was achieved and by varying the BCP structure more complex cylinder-in-lamellar and sphere-in-lamellar structures are envisioned. The HP and BCP phase separation/crystallization dichotomy during the formation of hierarchical dielectric mirrors will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L22.00012: Crystalline - Crystalline Diblock Copolymers of Linear Polyethylene - Hydrogenated Polynorbornene Richard Register, Sasha Myers, Sheng Li The melt and solid-state morphologies of linear polyethylene (LPE) - hydrogenated polybnorbornene (hPN) diblock copolymers, and their crystallization behavior, were explored over a range of block ratios and diblock molecular weights. LPE and hPN are both highly-crystalline polymers, with similar melting points, so the final structure can potentially be dictated by the melt structure, or by crystallization of either block. For diblocks with molecular weights of 50 kg/mol, symmetric copolymers exhibited lamellar microphase-separated melts, while diblocks with 20 or 80{\%} LPE were homogeneous, indicating a modest LPE-hPN interaction energy density of approximately 1 J/cc. For the relatively slow crystallizations which can be conducted isothermally, hPN appears to always crystallize first; the two crystallization processes can be resolved by time-resolved DSC and x-ray diffraction at small undercoolings. However, the LPE block has a stronger temperature dependence of its crystallization rate, implying that LPE would crystallize first at deep undercoolings. Up to the highest diblock molecular weights explored (100 kg/mol), crystallization ``breaks out'' from the melt mesophase to form spherulites defined by the hPN block; subsequent crystallization of the LPE within these spherulites is apparent from a change in optical texture. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L22.00013: Deformation-induced structure changes in olefin block copolymer Feng Zuo, Yimin Mao, JongKahk Keum, Christian Burger, Benjamin Hsiao, Hongyu Chen, Debbie Chiu, Shih-Yaw Lai Uniaxial tensile deformation was applied on two olefin block copolymer (OBC) samples to investigate the structure, morphology and orientation development using in-situ small-angle X-ray scattering and wide-angle X-ray diffraction techniques. Two samples have similar molecular characteristics but different chain architectures due to the different content of chain shuttling agent in production. The samples behave alike at room temperature, but the difference becomes distinct at high temperatures, such as fracture strain and crystal orientation. As more chain shuttling agents was used, the more frequently the growing chains transferred between catalysts; shorter block segments are produced. The block length has a strong effect on the crystallization kinetics, resulting microstructures (lamellae versus fringe-micelle) and deformation-induced structure changes. [Preview Abstract] |
Session L23: Focus Session: Probing and Modifying Materials with Lasers II
Sponsoring Units: DMPChair: Daniel Bubb, Rutgers University, Jagdish Narayan, North Carolina State University
Room: Morial Convention Center 215
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L23.00001: Femtosecond laser structuring in dielectrics Invited Speaker: Three-dimensional (3D) structuring of glasses, crystals, and polymers by tightly focused femtosecond laser pulses is a promising technique for microfluidic, micro-optical, photonic crystal and micro-mechanical applications [1-4]. The 3D laser micro-structuring of resists is demonstrated by direct laser writing [1] and holographic recording using phase control of interfering pulses [2]. Tightly focused laser pulses can reach dielectric breakdown irradiance without self-focusing when sub-1 ps pulses are used for laser-structuring inside dielectrics. The limiting case of microstructuring, a void recording, can be achieved [3]. The mechanism of void formation has been explained as a result of dielectric breakdown and micro-explosion. The absorption is localized within a skin depth of tens-of-nanometers in the plasma at the focus. This defines an ultimate localization of the energy delivery by a laser pulse. The absorbance reaches 0.6 in a fully ionized solid state density breakdown plasma. The high temperature and pressure buildup is large enough to generate a shock wave (strong micro-explosion). For example, a single 100 nJ laser pulse forms a void under tight focusing conditions even in the high strength sapphire (Young modulus of 400 GPa). It is considered that material fails upon compression rather than tension for which the mechanical failure threshold is by an order of magnitude smaller. This scenario of breakdown by compression is corroborated by numerical modeling of the strong explosion at our experimental conditions. Modification of materials by tightly focused femtosecond pulses opens new material processing routes for inert dielectrics [4] and can possibly be used for creation of new high-temperature and pressure phases inside the volume of irradiated samples. These regions with altered nano-structure have different chemical properties as was found in silica glass, quartz, and sapphire by wet etching of the ``shocked'' regions in aqueous solution of hydrofluoric acid. Current challenges of structural characterization of micrometer-sized volumes of nano-structures materials are discussed. The achievable resolution limits and potential of the fabricated 3D patterns in photonics, micro-fluidics, and sensor applications will be presented. [1] K. K. Seet et al., Adv. Mat. 17, 541, 2005. [2] T. Kondo, et al., New J. Phys. 8, 250, 2006. [3] S. Juodkazis, et al., Phys. Rev. Lett. 96 166101 2006. [4] S. Juodkazis, et al., Adv. Mater. 18 1361 2006. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L23.00002: Mechanisms of nanoparticles size reduction by laser irradiation Alexander Pyatenko, Munehiro Yamaguchi, Masaaki Suzuki Size reduction of nanoparticles after laser irradiation is well known phenomenon. Two different mechanisms of size reduction have been proposed: 1) the photoejection of electrons from a particle into a solution which caused ionization and Coulomb explosion of the ionized particle. 2) a simple heating-melting-evaporation mechanism. In this report we show that the different mechanisms are working under different experimental conditions, and give the criterion for their applicability. The main experimental parameter responsible for such criterion is the laser energy flow density, I$_{0}$ = E/$\tau $s, where E and $\tau $ are the laser pulse energy and duration, and S is the laser beam cross section. We calculated the critical value for this parameter in case of spherical silver and gold particles. When this parameter exceeds the value of about 10$^{10}$ W/cm$^{2}$, the electron ejection can be started. For nanosecond lasers such energy flow density values can be achieved only with beam focusing, but for pico and especially femto lasers this condition can be realized in different experimental arrangements. When I$_{0}$ is smaller than the critical value, the particle heating-melting-evaporation mechanism is responsible for particle size reduction. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L23.00003: Novel Nanostructured Materials and Properties by Pulsed Laser Deposition Jagdish Narayan, Gopinath Trichy Pulsed laser deposition has been used to create novel nanostructured materials either as layered or nanodot structure. By controlling thin-film growth kinetics during island growth, we are able to create three-dimensional self-assembled nanodot structures of Ni and ordered L10 FePt in a given matrix. Epitaxial growth and Integration of Ni and FePt on Si(100) substrate was achieved via domain matching epitaxy which facilitated epitaxial growth across the misfit scale. Magnetic properties can be varied by controlling the orientation and coercivity higher than 1.2 Tesla achieved. These results on ordered L10 FePt will be compared with those Ni with practical implications of information storage (1,2). (1) H. Zhou, D. Kumar, A. Kvit, A. Tiwari, J. Narayan, J. Appl. Phys. 94, 4841 (2003). (2) G.R. Trichy, D. Chakraborti, J. Narayan, J. T. Prater, J. Phys. D: Appl. Phys 40, 7273 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L23.00004: A study of photoemission using CW and pulsed UV light sources to probe surface slip band structure evolution of single crystal aluminium Mingdong Cai, Stephen Langford, J. Thomas Dickinson We report measurements of photoelectron emission from high-purity single crystal aluminum during uniaxial tensile deformation. A 248 nm pulsed excimer laser was used as a light source and the generated photoemission data was compared with that using a filtered mercury lamp. Time-of-flight curves of photoelectrons generated by pulsed excimer laser irradiation were observed showing a two peaked structure. These two peaks correspond to photoelectrons of two energy levels. It was also found that real time total photoelectron charge increases linearly with strain; and the increment is heterogeneous. Photoemission using low-energy photons is sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. The discontinuity in photoelectron intensity and the heterogeneous surface slip band structure prove the production of fresh surface area is not continuous, which is predicted by a recent dislocation dynamics theory based on percolation process. Except for differences in instrumentation and data analysis, the photoemission data from a filtered mercury lamp and from the excimer laser are comparable. Current studies extend the application of the excimer laser into surface dynamics analysis. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L23.00005: Self-Positioning Optically Trapped Microspheres For Nanoscale Laser Direct Write Craig Arnold, Euan McLeod We present a novel method of near-field laser direct-write patterning by incorporating self-positioning optical manipulation of polystyrene microsphere combined with pulsed laser processing. A 532 or 1064 nm CW laser optically traps a water-dispersed microsphere against a polymer substrate using a 2-dimensional Bessel beam trap. The optical scattering force due to the Bessel beam in the propagation direction is balanced by the repulsive interaction near the surface thereby creating an equilibrium spacing between the two, regardless of large scale surface features. A pulsed nanosecond 355 nm laser directed down the same beam path, is then used to ablate or modify the surface below the microsphere. While the pulsed laser has a large spot diameter, the intensity required for material modification is only achieved directly below the sphere due to focusing and near-field enhancement. Using an x-y translation stage, we demonstrate the ability to move the substrate while keeping the bead fixed in the optical trap, but allowing it to maintain its position above the surface. Direct-write nanoscale features are thereby enabled through this process. Characterization of the resulting structures along with advantages and limitations of this technique will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L23.00006: 2D patterned GaN$_{x}$As$_{1-x}$ Quantum structures using Ion Implantation and Pulsed Laser Melting Taeseok Kim, Michael J. Aziz, Venkatesh Narayanamurti We will present two dimensionally patterned GaN$_{x}$As$_{1-x}$ nanostructures fabricated in a GaAs matrix using nitrogen ion implantation followed by pulsed laser melting and rapid thermal annealing (RTA). The arbitrarily patterned GaN$_{x}$As$_{1-x}$ regions are investigated by ballistic electron emission microscopy (BEEM), a three terminal scanning tunneling microscopy technique. BEEM can image both the surface topography and the local hot electron transport. Using ion implantation through a lithographically patterned mask and varying subsequent processing conditions such as nitrogen concentrations and laser fluences, we have made locally confined GaN$_{x}$As$_{1-x}$ dots. By analyzing BEEM images of the quantum dots, we study giant bandgap bowing effects on the Schottky barrier height. We will also discuss the effects of different implanted nitrogen concentrations, laser fluences and RTA conditions on the conduction band structures of GaN$_{x}$As$_{1-x}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L23.00007: Femtosecond laser-induced black metals Anatoliy Vorobyev, Chunlei Guo Metals are one of the most commonly used materials in everyday life. One of the intrinsic properties of nearly all metals is that they are highly reflective for electromagnetic waves. Recently, by treating metal surfaces with high-intensity femtosecond laser pulses, we turned highly reflective metals highly absorptive and created, for the first time, ``black metals''. We also investigated the surface features for metal blackening and characterized the spectral responses of the black metals from UV to IR. The black metals promise potential for a variety of technologically important applications. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L23.00008: Cooling rates and mechanisms of resolidification in short pulse laser processing of metal targets Leonid Zhigilei, Zhibin Lin, William Duff, Dmitriy Ivanov Short-pulse laser irradiation of a metal target can create conditions for generation of non-equilibrium phases and unusual microstructure in the surface region of the irradiated target. The shallow melt depths produced by the short pulse laser irradiation and the high thermal conductivity of metals can result in very high cooling rates, strong undercooling and rapid resolidification. In this work, the melting and resolidification processes occurring under conditions of extreme heating and cooling rates are investigated in large-scale molecular dynamics simulations. The kinetics of the resolidification process and the microstructure of the surface region are found to be defined by a competition between the epitaxial regrowth of the substrate and nucleation of crystallites within the undercooled melted region. The dependence of the final microstructure of the surface region on the irradiation conditions is discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L23.00009: Modeling of early-stage plasma during femtosecond laser ablation of metals Zhaoyang Chen, Samuel Mao We developed a model of early-stage plasma induced by intense femtosecond laser ablation of metals in an ambient gas. We consider a 100 fs FWHM, 800 nm wavelength laser pulse irradiating a copper target in 1 atm nitrogen environment. Electron and lattice temperature of laser-irradiated target were calculated based on a two-temperature model, with surface electron emission due to thermionic and photoelectric effects utilized as the boundary condition for plasma initiation. Plasma development was calculated based on conservation laws for electrons, ions, as well as atoms from ambient gas. Inverse Bremsstrahlung laser absorption by electrons and electron impact ionization were found to be responsible for plasma development, and the simulation results yielded the laser intensity threshold for femtosecond laser-induced plasma formation. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L23.00010: Laser-Ablation Deposited Hafnium-Oxide Films for Triple Point Cathodes Ronald Gilgenbach, Nicholas Jordan, Y.Y. Lau, David French, P. Pengvanich, Brad Hoff, Michael Atzmon The triple-point is defined as the interface between metal, dielectric and vacuum; it provides a copious source of electrons for cold-cathodes. Pulsed-laser-deposition has been utilized to fabricate triple-point cathodes consisting of hafnium-oxide film-islands deposited over metal substrates. A 600 mJ, 20 ns KrF laser ablates a solid target of hafnium metal in a background gas of 20 percent O2 and 80 percent Ar at 100 mTorr at 10-15 pps. Contact lithography is employed to fabricate arrays of Hf-oxide islands on substrates to maximize the area of triple points for electron emission. For materials analysis, the films are deposited on a Si substrate. Plasma plume diagnostics include gated optical emission spectroscopy; neutral and singly-ionized hafnium have been measured. Hafnium-oxide film diagnostics include XEDS, SEM, TEM, profilometry, ellipsometry and x-ray diffraction (XRD). Hafnium-oxide deposition rates are about 0.06 nm/pulse. Cathode experimental current results will be presented at -300 kV. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L23.00011: Growth control of GaAs nanowires using pulsed laser deposition with arsenic over-pressure X.W. Zhao, A.J. Hauser, T. R. Lemberger, F.Y. Yang Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowires (NWs) catalyzed by gold nanoparticles have been systematically investigated. The single-crystal structure and geometry of the NWs have been characterized for various growth conditions. Arsenic over-pressure with As2 molecules was introduced into the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the NWs during growth. GaAs NWs exhibit a variety of geometries under varying arsenic over-pressures. Without As2 over-pressure, branched growth of GaAs with uncontrollable size and geometry was observed due to the decomposition of GaAs NWs, producing metallic Ga which serves as catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimal As2 over-pressure, at substrate temperature of 570 ?C, single-crystal GaAs NWs with uniform diameter of $\sim $50 nm, small diameter distribution, length over 20 micrometers, and thin surface oxide layer of $\sim $0.5 nm were obtained. X-ray diffraction results confirm the zinc-blende crystal structure of the GaAs NWs. A preliminary electrical characterization gives a linear $I$--$V $curve with a reasonable resistance, which leads to more thorough electrical characterization on GaAs NWs and device fabrication. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L23.00012: Phase-locking in Y-coupled Quantum Cascade Lasers Gottfried Strasser, L.K. Hoffmann, S. Schartner, M. Austerer, E. Mujagic, M. Nobile, W. Schrenk, A.M. Andrews, P. Klang A variety of spectroscopic applications call for powerful coherent light sources in the mid and far infrared spectrum [1]. In the past decade this demand has promoted quantum cascade lasers (QCLs) to become crucial light sources for sensing chemical components in the gaseous and liquid phase [2]. Waveguide coupling has pushed forward major developments to fulfill the demands of today's spectroscopers, such as high power output, stable single longitudinal mode operation, narrow spectral linewidth, and frequency tunability. In this work, a monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for GaAs/AlGaAs quantum cascade lasers [3]. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field profile of a double slit. The mode distribution is comprehensively derived by matching the farfield profiles to simulated values and shows a weak temperature and current dependence. The concept enhances the output power of a single facet coherent mid-infrared emitter and opens possibilities for monolithic interferometric sensing devices. [1] F. K. Tittel et al., Top. Appl. Phys. 89, 445(2003). [2] G. Wysocki et al., Appl. Phys. B 81, 769 (2005). [3] L. K. Hoffmann et al., Appl. Phys. Lett. 91, 17 (2007). [Preview Abstract] |
Session L24: Focus Session: Transport in Nanostructures IV: 2DES, Dots, and QPCs
Sponsoring Units: DMPChair: Misha Fogler, University of California, San Diego
Room: Morial Convention Center 216
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L24.00001: Coulomb and Spin Blockade transport through molecules and quantum dots Bhaskaran Muralidharan, Owen Miller, Avik Ghosh, Supriyo Datta In this talk we address some common theoretical grounds between molecular electronics and quantum-dot transport. Here, we focus on how theoretical models based on Coulomb Blockade (CB) theory can be successfully applied in order to theoretically interpret various notable transport experiments in both molecular electronics and quantum dot transport. We first show that, a majority of low-temperature molecular experiments can be explained easily, using a simplified CB theory. In the later part, we focus on how the many-body excitation spectrum of the molecule/quantum dot plays a significant role, in many other experiments. This includes, not-so-commonly observed transport effects such as Negative Differential Resistance (NDR) and bi-stability, resulting from asymmetry within the molecule or within the quantum dot array. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L24.00002: Influence of coherent level mixing on the resonant currents at anti-crossings between two single particle levels of a vertical quantum dot C. Payette, D.G. Austing, G. Yu, J.A. Gupta, S.V. Nair, B. Partoens, S. Amaha, Y. Tokura, S. Tarucha We study single electron resonant tunneling through weakly coupled vertical quantum dot molecules. Using the ground state of one of the constituent dots as an energy filter, we can probe the single particle energy spectrum of the other dot. Overall, the spectra are well modeled by assuming the dots are either circular or elliptical and parabolic, except in the regions where two or more single particle states approach each other. In these regions, we observe pronounced level mixing behavior. Here, we focus on the numerous two level anti-crossings, examining the conditions which lead to either simple transfer of the resonant current strengths between the two branches or concurrent enhancement and suppression of the resonant current in the two branches. We show that both types of behavior can be understood using a simple coherent level mixing model. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L24.00003: Electron-Phonon Kinetics and Transport in 2D Structures of Reduced Electron Concentrations Andrei Sergeev, Michael Reizer, Vladimir Mitin Usually, screening of the electron-phonon (e-ph) interaction is considered in linear approximation. In this case in 2D systems, the Debye screening radius $r_D$ is independent on the electron concentration, $n$. The linear approximation ignores the discreteness of the electron charge and it is not applicable for diluted systems. Here we show that the screening radius for e-ph interaction is in fact $\max (r_D, n^{-1/2}) $. For this reason, e-ph interaction is drastically enhanced in the diluted systems. In particular, a value of the deformation potential is increased by a factor of $n^{1/2}/r_D \approx R_s/a_0 = r_s$. The suggested approach explains puzzling data [1], which demonstrate that the deformation potential between holes and phonons in dilute 2D GaAs is twenty times stronger than expected from the theory. Strong coupling increases all e-ph phenomena. Using the Keldysh diagrammatic technique, we calculate kinetic and transport characteristics for diluted 2D systems. [1] X.P.A. Gao et al, Phys. Rev Lett. 94, 086402 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L24.00004: Novel measurement techniques for probing quantum point contacts Invited Speaker: Conductance measurements of quantum point contacts (QPCs) reveal an anomalous plateau at roughly 0.7 x 2e$^{2}$/h, when the mode occupation is just short of making a fully transmitting 1D channel available. Past experiments have built a consensus that this so-called ``0.7 structure'' is related to electron spin and electron-electron interaction, but the detailed description remains controversial. ~We have performed measurements on two new kinds of devices which give new insight into the interactions of electrons in these clean quasi-one dimensional systems. One device allows us to measure the compressibility of the electrons in a QPC for the first several conduction modes. ~Comparison with density functional calculations give new information about the relative importance of interactions (including exchange) as the density in the QPC is depleted.~ The second device allows us to measure the local density of states (DOS) in the QPC as we tunnel directly into the constriction.~ Deviations from the 1D DOS would help to develop a more complete picture of the transport through a QPC. We acknowledge support from the ONR Young Investigator Program, Award No. N00014-01-1-0569 and a Research Corporation Research Innovation Award, No. RI1260. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L24.00005: Ballistic hole transport and the 0.7 anomaly in p-type GaAs quantum wires A.R. Hamilton, R. Danneau, O. Klochan, W.R. Clarke, L.H. Ho, A.P. Micolich, M.Y. Simmons, M. Pepper, D.A. Ritchie Studying the spin degree of freedom of charge carriers in semiconductors is an area of significant current interest. Although spin-orbit coupling is extremely strong in p- type semiconductors such as GaAs, to date there have been only a limited number of experiments on holes in p-GaAs nanostructures. We have fabricated extremely high quality 1D hole quantum wires that show up to 10 extremely clean and stable quantized conductance plateaus at B=0 [1]. In contrast to 1D electrons, we observe an extreme anisotropy of the Zeeman spin splitting of the 1D energy levels depending on whether the magnetic field is parallel or perpendicular to the quantum wire [2]. We use this anisotropy to show that the 0.7 feature and zero bias anomaly are both spin related in hole quantum wires [3]. [1] O. Klochan \emph{et al}, APL 89, 092105 (2006); R. Danneau \emph{et al}, ibid 88, 012107 (2006). [2] R. Danneau\emph{et al}, PRL 97, 026403 (2006). [3]R. Danneau, et al, PRL (in press). [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L24.00006: Elementary Events of Electron Transfer in a Voltage-Driven Quantum Point Contact Mihajlo Vanevic, Yuli Nazarov, Wolfgang Belzig We find that the statistics of electron transfer in a coherent quantum point contact driven by an arbitrary time-dependent voltage is composed of elementary events of two kinds: unidirectional one-electron transfers determining the average current and bidirectional two-electron processes contributing to the noise only. This result pertains at vanishing temperature while the extended Keldysh-Green's function formalism in use also enables the systematic calculation of the higher-order current correlators at finite temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L24.00007: Zeeman splitting and subband spacing in ballistic Ga$_{0.25}$In$_{0.75}$As/InP quantum point contacts Theodore Martin, A. Szorkovszky, C.A. Marlow, L. Samuelson, H. Linke, R.P. Taylor, A.P. Micolich, A.R. Hamilton Spin-resolved transport in low-dimensional, solid state systems is a leading area of research at the nanoscale, due to potential device applications that combine quantization with the spin degree of freedom. The realization of such devices requires both a well-resolved energy level spectrum and a large splitting of the spin-degeneracy. Here we investigate the transport properties of a ballistic quantum point contact (QPC) etched into a high indium content strained GaInAs/InP heterostructure, a system with strong spin-orbit coupling and large 1D subband spacings. We have measured the 1D subband spacing using two independent methods, and find it to be $\sim $10 meV, with a very steep confining potential. We also present data studying the Zeeman splitting of the 1D subbands for different magnetic field orientations. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L24.00008: Magnetoconductance of interacting electrons in quantum wires in the integer quantum Hall regime. Igor Zozoulenko, Siarhei Ihnatsenka We present systematic quantitative description of the magnetoconductance of the split-gate quantum wires. Accounting for the exchange and correlation interactions within the spin density function theory (DFT) leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of $e^{2}/h$. We show that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps for the spinless Hartree electrons. A detailed analysis of the structure of compressible/incompressible strips and the evolution of the Hartree and the spin-DFT subband structure provides an explanation of this finding. Our spin-DFT calculations reproduce not only qualitatively, but rather quantitatively all the features in the magnetoconductance observed in the experiment [1] including the unexpected effect of the collapse of the odd conductance plateaus at lower fields. \\ $[1]$ I. P. Radu, J. B. Miller, S. Amasha, E. Levenson-Falk, D. M. Zumbuhl, M. A. Kastner, C. M. Marcus, L. N. Pfeiffer, and K. W. West, unpublished. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L24.00009: The Virtual Scanning Tunneling Microscope: A Novel Probe Technique for Imaging Two-Dimensional Electron Systems Adam Sciambi, David Goldhaber-Gordon, Seth Bank, Arthur Gossard We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which will provide both spatial and spectroscopic information about two-dimensional electron systems (2DESs) in semiconductor heterostructures. The VSTM's innovation is the addition of a second 'probe' 2DES separated by a low barrier from the sample 2DES below. Simulations show that a positively-biased tip held above the sample surface can greatly diminish the interlayer barrier and induce tunable tunneling between the two 2DESs. If the tip is scanned, the tunneling region will follow below, acting as a virtual tip while screening the true tip from the sample 2DES. This probe technique is motivated by interesting local 2DES physics that can only be studied indirectly because of the depth of 2DESs; we describe a range of predicted spatially-organized phases of 2D electrons which could be accessed with this new probe. We follow with experimental results showing induced tunneling in a GaAs/AlGaAs bilayer 2DES sample, which we characterize thoroughly and use to tunnel into a quantum Hall liquid. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L24.00010: Virtual Scanning Tunneling Microscope: Modeling Interlayer Tunneling Between Two-Dimensional Electron Systems in the Ballistic Regime Katherine Luna, Eun-Ah Kim, Paul Oreto, Steven Kivelson We study a theoretical model for the virtual scanning tunneling microscope (VSTM), which is a proposal to use interlayer tunneling in a bi-layer system as a way to probe two-dimensional electron systems (2DES) in semiconductor heterostructures. We model the bi-layer in the presence of weak tunneling between the layers using an analog of the spin-boson model. Previously, such a system was modeled in the diffusive regime by Levitov and Shytov [1], and they predicted a zero-bias anomaly, where the tunneling conductance vanishes singularly near zero-bias as a result of Coulomb blocking. Motivated by the availability of high mobility samples and the goal of using VSTM to probe the physics of clean 2DES dominated by interactions, we focus on tunneling in the ballistic regime. We find the absence of a zero-bias anomaly due to extremely efficient screening in the ballistic regime. We discuss the implications of our results on ongoing experimental efforts. [1] S. Levitov and A.V. Shytov. JETP Lett. \textbf{66}, 214 (1997). [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L24.00011: Imaging electron local density of states inside mesoscopic quantum rings B. Hackens, V. Bayot, Marco Pala, X. Wallart, S. Bollaert, A. Cappy, F. Martins, T. Ouisse, H. Sellier, J. Chevrier, S. Huant We combine scanning gate microscopy (SGM) experiments and simulations to demonstrate imaging of the electron local density of states within open quantum rings (QRs). SGM is based on a weak electrostatic perturbation of the electron system by a charged tip, which alters the transmission of electrons through the system. When the QRs are in the ballistic and coherent regime of transport, conductance fringes are observed in SGM images when the tip scans over the QR area as well as in its vicinity. Comparing our results to quantum mechanical simulations of transport in realistic QRs, we demonstrate that the fringes observed over the QR area are directly connected to the local density of states inside the QR [1]. Moreover, the magnetic field dependence of the fringes observed in the vicinity of the QRs indicates that they originate from the electrostatic Aharonov-Bohm effect, and correspond to iso-phase lines for electrons [2]. From these results, one can expect to design new kinds of quantum nanodevices based on a precise spatial control of electron interferences and trajectories. [1]~F. Martins et al., PRL 99,136807 (2007). [2]~ B. Hackens et al., Nat. Phys. 2, 826 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L24.00012: Origin of nonlinear current-voltage characteristics in nanowires Francois Leonard, A. Alec Talin, B. S. Swartzentruber, Xin Wang, Stephen D. Hersee The current-voltage characteristics of nanowires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanowires and demonstrate that the nonlinear behavior originates instead from space-charge limited current. Analysis of published experimental data in several nanowire materials shows that this behavior is common, and should be expected whenever diffusive transport dominates and the effective carrier concentration is low. A theory of space-charge limited current in nanowires is presented, and correctly predicts the scaling of the current with the nanowire aspect ratio. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L24.00013: ABSTRACT WITHDRAWN |
Session L25: Semi-crystalline Polymers
Sponsoring Units: DPOLYChair: Ben Hsiao, Stony Brook University
Room: Morial Convention Center 217
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L25.00001: Dillon Symposium Break
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Tuesday, March 11, 2008 3:06PM - 3:18PM |
L25.00002: Polymer Single Crystals as 2D Templates Stephen Cheng, Ryan Van Horn, Wenbin Zhang Polymer single crystals grown from dilute solution provide a novel approach to studying polymer physics. Because these crystals are lamellar in shape, their fold surfaces provide a platform from which to study two-dimensionally ordered structures. By coupling other polymers (forming multiblock copolymers) or inorganic particles (forming organic/inorganic hybrids) to the crystallizable block, it is possible to analyze the inherent physics of confining this assembly to 2D space and to provide an opportunity to use the crystal as a template. These non-crystallizable components are excluded from the single crystal lattice, tethering them to both surfaces. This control over the 2D assembly of these materials allows us to study materials for various biological, optical, and electrical applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L25.00003: Interfacial and confinement effects to the structure of nylon 6 /clay nanocomposites made by chaotic flow. Dilru Ratnaweera, Dvora Perahia, Chaitra Mahesha, David Zumbrunnen , Mark Kampf The structure of polymers within nanocomposites is strongly affected by the confinement of the polymer to the interface with the nanoparticles and the method of blending. In nanocomposites of Nylon 6 and clay particles are made by chaotic blending, the strength of the chaotic flow affects the internal structure on multiple length scales, where the local structure is determined by the interfacial effects between the polymer and the nano-particle. The structural evolution as a results in alternating layers of nylon and nylon/clay regions forming two distinct interfaces, that with the clay and that of the pure nylon and the composite. The structure has been studied by X-ray, AFM and TEM at different chaotic blending strengths. At the interface with the clay particles, the polymer chains orient perpendicular to the interface. While the alpha crystalline form dominated the structure of the polymer in melts, the confinement to the layers results in an enhanced gamma form where hydrogen bonds form in between parallel nylon 6 chains dominate. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L25.00004: WAXS investigations on Polyethylene -- Carbon Nanofibers Composites Brian Jones, Jianhua Li, Rogelio Benitez, Karen Lozano, Mircea Chipara, Alin Cristian Chipara, Magdalena Dorina Chipara, David J. Sellmyer Nanocomposites have been obtained by high-shear mixing of isotactic polyethylene with various amounts of purified nanofiller (vapor grown carbon nanofibers type PR-24AG from Pyrograf Products, Inc) by utilizing a HAAKE Rheomix at 65 rpm and 180 $^{\circ}$C for 9 min followed by an additional mixing at 90 rpm for 5 min. Composites loaded with various amounts of vapor grown carbon nanofibers have been prepared. Various spectroscopic techniques have been used to assess the interactions between the polymeric matrix and carbon nanofibers. Wide angle X - Ray scattering investigations focused on the effect of carbon nanofibers on the crystalline phases of polypropylene and on the overall crystallinity degree of the polymeric matrix. This research aims at a better understanding of the nature and structure of the polymer -- carbon nanofibers interface. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L25.00005: Crystallization of Propylene-Hexene Random Copolymer Yimin Mao, Feng Zuo, JongKahk Keum, Benjamin Hsiao Time resolved small- and wide-angle x-ray scattering (SAXS/WAXS) were used to study the crystallization behavior of propylene-hexene random copolymer containing non-crystallizable hexene segments and crystallizable propylene segments. It was found that the copolymer would follow two crystallization paths depending on temperature, resulting in two distinct crystalline structures. At high crystallization temperatures (e.g. $100^{\circ}\mathrm{C}$), the combined effects of phase separation and high chain mobility greatly enhance the formation of lamellar structure consisting of the alpha-form of isotactic polypropylene (iPP) crystal. At low temperature (e.g. $40^{\circ}\mathrm{C}$), the lack of phase separation and the low chain mobility mainly result in the formation of fringe-micelle structure also with the alpha-form of iPP. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L25.00006: Probing the crystallisation of polyethylene confined to a system of droplets Jessica Carvalho, Kari Dalnoki-Veress We present results on the crystallisation of polyethylene (PE) confined to a system of dewetted droplets. With the droplet system, we have access to a large ensemble of small, isolated volumes of crystallisable material, allowing for a direct measurement of nucleation rates. In our previous work with dewetted droplets of poly(ethylene oxide), we were able to demonstrate that long chains showed the same nucleation behaviour as chains roughly an order of magnitude shorter[1]. In contrast to this, it has been shown that for systems of n-alkanes molecular weight plays a role[2]. By investigating a wide range in molecular weight, the dependence of nucleation on molecular weight will be addressed in PE. [1] M.V. Massa et al., Phys. Rev. Lett. \textbf{97}, 247802 (2006). [2] H. Kraack et al., Macromolecules, \textbf{33}, 6174 (2000). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L25.00007: Crystalline Morphology of Propylene 1-Octene Random Copolymers Keesu Jeon, Rufina G. Alamo The morphology of isotactic propylene 1-octene random copolymers has been studied by AFM, DSC, WAXS, and FTIR in an octene range of 10-20 mol {\%}. Different morphologies were observed below and above 15 mol {\%}. The morphological components in the higher counit copolymers are not of the lamellae-type, thicker than lamellae observed below 15 mol {\%}, connected and isotropic in their orientation. Their global morphology is developed via nucleation and growth (NG) of spherulitic aggregates. The evolution of heat of fusion with time is also sigmoidal shape, typical of NG-type crystallization mechanism. WAXS diffractograms for the higher counit copolymers are devoid of crystalline reflections, except for small and broad peaks suggesting mesomorphic-like structures, which by FTIR show small contents of the 840 cm$^{-1}$, 12 and higher units regularity bands, and hence formed of short helical sequences. The PO morphology is additionally compared with copolymers with ethylene, 1-butene and 1-hexene counits at matched contents. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L25.00008: Tailoring the Properties of Poly(ethylene terephthalate) without Addition of Fillers via Solid-State Shear Pulverization Cynthia Pierre, Kosmas Kasimatis, John Torkelson We demonstrate the ability to very strongly tune the physical and mechanical properties of poly(ethylene terephthalate) (PET) by changing the processing conditions of neat PET during solid-state shear pulverization without addition of any fillers or nucleating agents. Using differential scanning calorimetry, we observe a roughly factor of 3 increase in crystallinity of PET that has been pulverized and subsequently melted relative to the unprocessed PET. We also observe a dramatic increase in the rate of crystallization of the pulverized samples. Rheological characterization has demonstrated an increase in viscosity of the pulverized material, which can be ascribed to chain branching in the pulverized product. We also observe significant reductions in the oxygen permeability of the PET with pulverization as well as enhancements in mechanical properties that are commensurate with the modified crystallization properties of the pulverized PET. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L25.00009: Crystallization of Bromine Substituted Polyethylenes with Precise Placement or Random Distribution R.G. Alamo, K. Jeon, R.L. Smith, E. Boz, K.B. Wagener The crystalline properties of a series of bromine containing polyethylenes (PEs) with either a random or an exact placement of the Br atom on each and every 21$^{st}$, 19$^{th}$, 15$^{th}$ and 9$^{th}$ backbone carbon have been studied by DSC, NMR, Raman spectroscopy, WAXS and SAXS. Taking into account a larger strain due to the size of the Br atom, the crystallization behavior is analogous to Cl substituted PEs. While all precision Br-PEs crystallize as homopolymers as demonstrated by, 1. DDMAS solid-state $^{13}$C NMR spectra; 2. development of relatively large crystal thickness, and 3. sharp crystallization and melting peaks, the development of the crystalline state in random analogues is led by selection of most crystalline sequences indicated by broad thermal transitions, and lower crystallinities. WAXS patterns are unique for each type of substitution. For precision Br-PEs WAXS contain reflections corresponding to planes containing the Br atoms which are tilted 35$^{\circ}$ in reference to the chain axis. Due to the accommodation of the large Br atom, the crystals of all precisely substituted Br-PEs studied are conformationally disordered as observed by their Raman and NMR spectra. In contrast, crystals from random analogues display negligible conformational disorder. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L25.00010: Molecular and Crystalline Microstructure of Ferroelectric Poly(vinylidene fluoride-\textit{co}-trifluoroethylene) Ultrathin Films on Bare and Self-Assembled Monolayer-Modified Au Substrates Youn Jung Park, Seok Ju Kang, Bernard Lotz, Annette Thierry, Cheolmin Park There has been much interest in polymorphic crystal structures and ferroelectric properties in polymer materials, as one way of an application for organic memory device. We investigate the molecular and microdomain structure of Poly vinylidene fluroride-\textit{co}-trifluoroethylene (P(VDF-TrFE)) thin films spin-coated on bare and self-assembled monolayers (SAMs)-modified Au substrates. The two types of films display similar crystal morphologies with edge-on needlelike crystalline microdomains. They have, however, a different structure depending on the substrate. When the films are deposited on a bare Au surface, the films preferentially have a (110) contact plane with the substrate but a (100) contact plane when deposited on the Au surface modified by SAMs. The polar $b-$axis, along which the ferroelectric polarization is oriented, is therefore tilted to the film (and substrate) surface normal at 30 and 90\r{ }, respectively. In particular, the orientation of the polar $b$-axis tilted at some 90\r{ } to the normal of the polymer films on a CH$_{3}$ terminated SAM modified Au surface explains the smaller remanent polarization at low initial electrical bias. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L25.00011: Chirality Information Transfer in Polylactides: From Main-Chain Chirality to Lamella Curvature Robert Emery Prud'homme, Damien Maillard The behaviour of ultrathin polymer films is very different from that in the bulk phase. In this work, the crystallization of poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA) was followed using in situ atomic force microscopy over a broad range of temperatures and thicknesses. Using a forced nucleation technique, edge-on lamellae were observed, showing a curvature which can be related to the polymer chirality. In the case of PLLA, the lamellae are S-shaped, contrary to the PDLA lamellae which are Z-shaped. This behaviour was also observed on TEM pictures of PLLA and PDLA films crystallized in the same conditions without any external nucleation. For the first time, a relationship has been established between the molecular chirality of poly(lactide)s and their macroscopic behaviour. Moreover, the rotating direction of those lamellae can be directly linked with the sense of twisting of the poly(lactide)s lamellae in banded spherulites. Those observations can lead to a model where the curved crystals in ultrathin films can be considered as half-lamellae, which, when associated together, give twisted complete lamellae. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L25.00012: Effects of Vitamin E on the Oxidative Reaction of Free Radicals in Ultra-High Molecular Weight Polyethylene Benjamin Walters, Muhammad Jahan Free radicals in gamma- or x-irradiated ultra-high molecular weight polyethylene (UHMWPE) are investigated as a function of vitamin E (alpha-tocopherol ($\alpha $-T)). $\alpha $-T is mixed with UHMWPE (GUR 1020) powder (e-PE) before (premix) or after (post-mix) irradiation. Pre-mix powder is also compression-molded (CM) to solid pucks (1'' thick and 2.5'' dia.) at 200$^{o}$C under constant force of 20-40 kN. Free radicals are detected using an X-band electron spin resonance (ESR) spectrometer, and oxidation index (OI) (1720 cm$^{-1})$ by FTIR technique. As expected, no measurable OI is detected by FTIR and thus e-PE suffers no loss in its mechanical properties. ESR data, however, suggest that $\alpha $-T quenches polyethylene radicals during and/or immediately after irradiation, but it does not have any effect on the long-term oxidative reaction. The difference between the pre- and post-mix powder is apparent only at the initial stage, and the terminal oxygen-induced radicals (OIR) are produced in all irradiated samples. Both pre- and post-mix powders are found to have equal amount of residual $\alpha $-T radical (tocopheroxyl). [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L25.00013: Self-Assembly and Chain-Folding in Hybrid Coil-Coil-Cube Triblock Oligomers of Polyethylene-b-Poly(ethylene oxide)-b-Polyhedral Oligomeric Silsesquioxane Lei Zhu, Jianjun Miao, Li Cui Self-assembly and chain-folding in well-defined oligomeric polyethylene-block-poly(ethylene oxide)-block-polyhedral oligomeric silsesquioxane (PE-b-PEO-b-POSS) triblock molecules were studied. The triblock oligomers were synthesized by attaching two kinds of functional POSS molecules to a hydroxyl-terminated PE-b-PEO diblock oligomer. In the crystalline state, PE chains tilted 32 degrees from the lamellar normal, and both Ib-POSS and Cp-POSS molecules stacked into four-layer (ABCA) lamellar crystals, having the same trigonal symmetry as in pure POSS crystals. Because the cross-sectional area for a PE chain in the PE crystals (0.216 nm2/chain) at the interface was much smaller than that for a POSS molecule in POSS crystals (1.136 nm2/molecule), the self-assembly and PE chain-folding were substantially affected by the sequence of PE and POSS crystallization when crystallizing from the melt. This study indicated that confinement effect plays an important role on chain-folding of crystalline block oligomers. [Preview Abstract] |
Session L26: Focus Session: Quantum Control II
Sponsoring Units: DCPChair: Vlasta Bonacic-Koutecky, Humboldt-Universitaet zu Berlin
Room: Morial Convention Center 218
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L26.00001: Hiking Over Quantum Control Landscapes Invited Speaker: Seeking the best control over a posed quantum dynamic objective entails climbing over the associated control landscape, which is defined as the quantum mechanical observable as a function of the controls. The topology and general structure of quantum control landscapes as input output maps dictate the final attainable yield, the efficiency of the search for an effective control, the possible existence of multiple dynamically equivalent controls, and the robustness of any viable control solution. Normal optimization problems in virtually any area of engineering and science typically have landscape topologies that remain a mystery. Quantum mechanics appears out to be quite special in that the topology of quantum control landscapes can be established generically based on minimal physical assumptions. Various features of these landscapes will be discussed and illustrated for circumstances where the controls are either an external field or the time independent portions of the Hamiltonian; the latter circumstance corresponds to subjecting the material or molecules to systematic variation and hence viewed in the context of being controls. Both theoretical and experimental findings on control landscapes and their consequences will be discussed, including issues of robustness to noise, search algorithm efficiency, existence of multiple control solutions, prospects for identifying reduced sets of control variables, simultaneous control of multiple quantum systems (optimal dynamic discrimination (ODD)), and mechanism analysis. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L26.00002: Non-resonant, non-perturbative Dynamic Stark Control of Quantum Dynamics. Invited Speaker: One of the most important non-resonant interactions is the dynamic Stark effect. In the non-perturbative but non-ionizing limit, an effective Hamiltonian can be constructed based upon a hierarchy of approximations (the Born-Oppenheimer Approximation, Slowly Varying Envelope Approximation, the Rotating Wave Approximation). In this situation, the effective Hamiltonian contains first order (dipole) and second order (polarizability) matter-field interactions which can lead to significant yet reversible changes to the molecular Hamiltonian. The first order term leads to a fast evolution which follows each optical cycle. The second order term causes an evolution which follows, by contrast, the envelope of the laser pulse. We discuss the use of the non-resonant second order Dynamic Stark Effect as a tool for controlling quantum systems without any net absorption of light. We illustrate this by examples chosen from problems in: (i) Control of branching ratios during non-adiabatic photodissociation; (ii) Control of 3D field free molecular frame alignment of asymmetric tops. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L26.00003: Controlling and Understanding Laser Filamentation in the Solution and Gas Phase Molecular Systems Invited Speaker: The process of laser filamentation is highly nonlinear, yet amenable to control using laser pulse shaping techniques. Investigations of our ability to control the spatial position of a filament in a water tank and measurements of the forward and back scattered amplified spontaneous emission (resulting from the strong field excitation in the resulting plasma) will be presented. Our time resolved measurements of the dynamics of the filamentation process in various gases will also be reviewed. Finally, a model of the plasma formation will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L26.00004: New Developments in Quantum Control: Phase Space Learning Algorithms and Uncontrollable Quantum Systems Invited Speaker: This talk has two parts. The first deals with a new representation of shaped ultrafast laser pulses based on a von Neumann time-frequency lattice. We show that a pulse defined in terms of an amplitude and a phase at $N$ frequency points can be represented on the von Neumann lattice using $\surd N$ points in frequency and $\surd N$ in time without loss of information. The transformation from the frequency (or time) representation to the von Neumann representation is one-to-one and therefore invertible. We discuss three possible applications of the von Neumann representation of pulses: 1) for cleaning and interpreting complex pulses; 2) for performing systematic scans of the effect of timing and frequency on molecular control; 3) as genes to be used in mutations and crossover in evolutionary algorithms. The second part of the talk deals with the classification of uncontrollable quantum systems. It is well-known that for a quantum system to be controllable the Lie algebra spanned by iterated commutators of H$_{0}$ and H$_{1}$ must span the full space of the dynamical algebra. We pose the following questions: When a system is \textbf{not} completely controllable, can we classify different families of uncontrollable systems? If so, can we associate these different types of mathematical structures with different underlying physics (for example, dark states or generalized entangled states)? We show that uncontrollable quantum systems fall into two categories: reducible and irreducible. The former is associated with dark states and the latter with generalized entangled states. Based on Lie subalgebras we give a complete characterization of irreducible uncontrollable systems for systems up to 9 levels. Finally, we show that an earlier intuitive concept of connectivity only incompletely captures this Lie algebraic structure of uncontrollable systems. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L26.00005: Quantum information processing with a minimal control Peter Pemberton-Ross, Sonia Schirmer, Ivan Pullen Various physical and practical constraints limit the amount and type of control we have in quantum information processing systems, leading to complicated or unreliable implementations. To try and circumvent these problems, we take the most restricted candidate systems where only a single energy transition can be controlled by a piecewise-constant field, and show that even this is sufficient for efficient execution of a range of useful QIP tasks. We show that it is in principle possible to achieve global control with a single, simple, fixed, local actuator, and show how such minimal control could significantly improve information processing in terms of speed, fidelity and transfer efficiency. The scheme presented has a natural application to spin-chain systems, where only one interaction between two spins can be controlled, and the effects of the position of the controller in the 'quantum wire' and the system's symmetries are explored. It may also be relevant for gate-controlled solid-state systems where it is desirable or necessary to limit the number of control electrodes due to the constraints of size, decoherence and cross-talk, and where complex temporal variation of the control voltages is difficult. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L26.00006: A Simulation of Strong-Field Attosecond Electron Dynamics: Effects of Pulse Shape Stanley Smith, Dmitri Romanov, Xiaosong Li, H. Bernhard Schlegel, Robert Levis As the complexity of systems increases from atoms to molecules, the exploration of non-adiabatic electron dynamics in strong fields requires a leap in understanding and in the principles of description. Recently, a time-dependent Hartree-Fock approach (TDHF) was developed to study the dynamics of individual electrons in multielectron systems. We have used this TDHF approach to numerically simulate the non-adiabatic electron dynamics of a few small molecules and polyacenes using basis sets ranging from AUG-cc-pVTZ for smaller molecules to 6-31G(d,p) for larger molecules. The electric field was applied in the direction of the long molecular axis and the attosecond response of the electrons during and after the laser pulse has been obtained. To determine the effects of ionization, electron dynamics for both neutrals and ions was also simulated. As a function of pulse shape, there are significant differences in the excitation spectrum and volume for each molecule. [Preview Abstract] |
Session L27: Focus Session: Low-dimensional Spin Systems
Sponsoring Units: GMAGChair: Christopher Wiebe, Florida State University
Room: Morial Convention Center 219
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L27.00001: Specific heat and magnetocaloric effect of the S=1/2 spin-ladder compound (CH$_{3})_{2}$CHNH$_{3}$CuCl$_{3}$ Younghak Kim, Yasuo Yoshida, Yasumasa Takano, Hiroyuky Tsujii, Keishi Kanada, Takehiro Saito, Akira Oosawa, Takayuki Goto (CH$_{3})_{2}$CHNH$_{3}$CuCl$_{3 }$is the best laboratory model for the S=1/2 spin ladder comprising ferromagnetic rungs and antiferromagnetic legs [1]. We have determined the magnetic phase diagram of this compound in fields up to 18 T by means of specific-heat and magnetocaloric-effect measurements for two crystal orientations, with either the so-called $B$ or $C$ plane perpendicular to the field direction. For both orientations, we find power-law dependences of the critical field of the long-range antiferromagnetic order on temperature, behavior indicative of the Bose-Einstein condensation of spin triplets due to the cancellation of the energy gap by the magnetic field. [1] T. Masuda \textit{et al}., Phy. Rev. Lett. \textbf{96, }047210 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L27.00002: Neutron scattering study of strong- and weakly-coupled, spin-1/2 spin-ladders Bella Lake, S. Notbohm, D.A. Tennant, T.G. Perring, C.D. Frost, R.I. Bewley, P. Manuel, K.P. Schmidt, G.S. Uhrig, P. Ribeiro, C. Sekar, R. Klingeler, C. Hess, G. Krabbes This presentation will discuss two-leg spin-ladders where the magnetic ions have spin-1/2 moments and antiferromagnetic exchange interactions. In the limit of strong rung coupling, the magnetic spectrum is dominated by the gapped magnon mode of an antiferromagnetic dimer, introduction of leg coupling modulates this mode. In the limit of weak rung coupling the excitations are similar to the multi-spinon continuum of the one-dimensional, spin-1/2, Heisenberg antiferromagnet, although the gap remains in the presence of infinitesimal rung coupling. Cyclic exchange interactions are often found in spin-ladders and reduce the gap size. Inelastic neutron scattering measurements will be presented for two ladders. La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ has strong rung coupling and excitations consisting of a gapped one-magnon mode and a two-magnon continuum. In contrast CaCu$_{2}$O$_{3}$ has a weak rung interaction and a substantial cyclic exchange which drives the system gapless and quantum critical. The excitations are similar to a multi-spinon continuum of a spin-1/2 chain, however the presence of rung coupling is revealed by a modulation parallel the rung at low energies. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L27.00003: Incommensurate correlations in a S=1/2 4-leg quantum spin tube Andrey Zheludev, Ovidiu Garlea, Louis-Pierre Regnault, Klaus Habicht Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one- dimensional quantum spin-liquid system Cu2Cl4-D8C4SO2. Contrary to previously conjectured models, the appropriate Heisenberg Hamiltonian is equivalent to that of a S=1/2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation [1]. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations. In high magnetic fields, a Bose-Einstein condensation of magnons induces a quantum phase transition to a incommensurate helimagnetic ordered state. Research at ORNL was funded by the United States Department of Energy, Office of Basic Energy Sciences- Materials Science, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. [1] V. O. Garlea , A. Zheludev, L.-P. Regnault, J.-H. Chung, Y. Qiu, M. Boehm, K. Habicht and M. Meissner, Phys. Rev. Lett., in press (2007); arXiv:0710.0891. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L27.00004: Magnetic and transport properties of the mixed valent vanadium oxides LuV$_4$O$_8$ and YV$_4$O$_8$ S. Das, A. Niazi, D.C. Johnston Mixed valent vanadium oxides have been subjects of wide interest owing to their exotic properties such as heavy fermion behavior below 10 K in spinel LiV$_2$O$_4$ and isosymmetrical phase transition and charge ordering in YbV$_4$O$_8$. \textit{L}V$_4 $O$_8$ (\textit{L} = Y, Lu) crystallize in a structure similiar to that of orthorhombic CaFe$_2$O$_4$, containing four inequivalent V sites arranged in zig-zag chains. The formal oxidation state of V in these compounds is 3.25. In this study, the magnetic and transport properties of these \textit{L}V$_4 $O$_8$ compounds are reported. The magnetic susceptibility indicates two possible structural phase transitions in YV$_4$O$_8 $ in the temperature ($T$) range 70--90 K\@. Anomalies in the heat capacity are also seen in this temperature range. The zero- field-cooled/field-cooled magnetic susceptibilities show possible canted antiferromagnetic ordering in both YV$_4$O$_8$ and LuV$_4$O$_8$ for $T < 50$~K\@. For LuV$_4$O$_8$, the dc electrical resistivity of a sintered pellet measured using a standard four-probe technique shows a strong increase at $T < 100 $~K\@. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L27.00005: Single Crystal Growth and Characterization of Quasi 1-D, Magnon BEC Candidate Pb$_2$V$_3$O$_9$ Benjamin Conner, Haidong Zhou, Christopher Wiebe, Luis Balicas, Youn-jung Jo Recent success in the floating zone growth of single crystal Pb$_2$V$_3$O$_9$ will be highlighted. Pb$_2$V$_3$O$_9$ has been studied as part of the class of antiferromagnetic 1D spin 1/2 dimer compounds that are believed to be driven through a phase transition to a BEC state with applied magnetic field. Magnetic susceptibility and specific heat measurements will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L27.00006: Magnetic structure of frustrated Haldane chain compound CaV$_{2}$O$_{4}$ B. Lake, O. Pieper, A. Daoud-Aladine, M. Reehuis, K. Prokes, M. Enderle, A. Niazi, J.Q. Yan, D.C. Johnston While the Haldane chain, (Heisenberg spin-1 chain with nearest neighbor antiferromagnet interactions) has been much studied and shown to have gapped magnon excitations. The effect of frustrated and single ion-anisotropy has not been investigated experimentally. Theory suggests that frustration can enhance the multi-particle spectrum and the combination of frustration and anisotropy can drive the system into a gapless chiral phase. CaV$_{2}$O$_{4}$ is a candidate for such a system. This compound consists of two inequivalent one-dimensional chains consisting of spin-1 V$^{3+}$ ions which have antiferromagnetic first and second neighour interactions. Interchain coupling gives rise to long-range order below $T_{N}$= 71K. Neutron diffraction measurements will be described. They reveal collinear spin order within the chains and canting between chains, with the average spin direction along the \textbf{b} axis. The ordered spin moment was also measured and found to be 1 $\mu _{B}$ per Vanadium; this reduction of 50{\%} from full spin ordering suggests loss of moment due to quantum fluctuations. Finally, preliminary inelastic neutron scattering reveals a gap due to single-ion anisotropy. In addition a steep dispersion along the \textbf{c} (chain) direction and much weaker dispersions along \textbf{a} and \textbf{b} confirm the one-dimensional nature of CaV$_{2}$O$_{4}$. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L27.00007: The local density of states of a finite quantum wire: New insights from DMRG and bosonization Imke Schneider, Michael Bortz, Alexander Struck, Sebastian Eggert We consider interacting Fermions on a finite one-dimensional lattice. By using an adapted DMRG algorithm we are able to calculate the energy and spatially resolved local density of states (LDOS) for the lattice model directly without using time-dependent correlations. We compare to analytic expressions for individual energy levels in systems with open boundary conditions from Luttinger Liquid theory. In this way, a detailed understanding of the LDOS for each individual energy level can be obtained in both fermionic and bosonic pictures. Certain degeneracies of the Luttinger Liquid spectrum are lifted in the lattice model by band curvature and interaction effects, leading to a large number of states and energy levels in the LDOS. The standing waves in the LDOS reveal the collective bosonic excitations explicitly. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L27.00008: Real-time dynamics of spinons and holons in one-dimensional correlated electron systems H. Onishi, I. Gonzalez, R. Melko, E. Dagotto To clarify the properties of spin-charge separation in correlated electron systems, we study the real time evolution of wavepackets carrying spinons and holons in a one-dimensional Hubbard model, using the time-dependent density-matrix renormalization group method. In the bulk, while spinons and holons move with the same velocity in the non-interacting chain, it is observed that in the interacting Mott insulator they move in opposite directions each with its own individual velocity. Thus, spin-charge separation is clearly observed. On the other hand, when the Mott insulator is connected to a non-interacting chain, the wavepackets created in the Mott insulator moving toward the non-interacting region exhibit interesting reflection and transmission properties at the interface between the two regions, that will be discussed in this presentation. The differences between Mott and band insulators are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L27.00009: Excitation Spectrum Gap and Spin-Wave Stiffness of XXZ Heisenberg Chains: Global Renormalization-Group Calculation Ozan S. Sariyer, A. Nihat Berker, Michael Hinczewski The anisotropic XXZ spin-$\frac{1}{2}$ Heisenberg chain is studied using renormalization-group theory, thoughout the entire temperature and anisotropy ranges in both ferromagnetic and antiferromagnetic regions.[1] We obtain, for all anisotropies, the antiferromagnetic spin-liquid spin-wave stiffness and the Isinglike ferromagnetic excitation spectrum gap, exhibiting the spin-wave to spinon crossover. In the latter case, we also obtain a crossover in the leading algebraic behavior of the specific heat. A number of other purely quantum characteristics are found: The in-plane interaction $s_{i}^{x}s_{j}^{x}+s_{i}^ {y}s_{j}^{y}$ induces an antiferromagnetic correlation in the out-of-plane $s_{i}^{z}$ component. Conversely, an antiferromagnetic $s_{i}^{z}s_{j}^{z}$ interaction induces a correlation in the $s_{i}^{xy}$ component. As another purely quantum effect, (i) in the antiferromagnet, the value of the specific heat peak is insensitive to anisotropy and the temperature of the specific heat peak decreases from the isotropic (Heisenberg) with introduction of either type (Ising or XY) anisotropy; (ii) in complete contrast, in the ferromagnet, the value and temperature of the specific heat peak increase with either type of anisotropy. \newline \noindent [1] O.S. Sariyer, A.N. Berker, and M. Hinczewski, arXiv:0704.1064v1 [cond-mat.stat-mech]. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L27.00010: Dispersive excitations in the $S=1$ antiferromagnet Ba$_3$Mn$_2$O$_8$ Matthew Stone, Mark Lumsden, Eric Samulon, Yiming Qiu, Cristian Batista, Ian Fisher We present powder inelastic neutron scattering measurements on the $S=1$ dimerized antiferromagnet Ba$_3$Mn$_2$O$_8$. The measured $T=1.4$~K magnetic spectrum exhibits a spin-gap of $\Delta = 0.99$~meV and a dispersive spectrum with a bandwidth of approximately $1.67$~meV. Comparison to a coupled dimer model of the dispersion and scattering intensity yields an accurate description of the exchange constants in Ba$_3$Mn$_2 $O$_8$. Interdimer exchange between the stacked triangular lattice dimer bilayers is found to be two orders of magnitude weaker than the intradimer exchange resulting in a quasi-two- dimensional frustrated bilayer triangular lattice. The wave- vector dependent scattering intensity also confirms the proposed $S=1$ dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet- triplet and thermally activated triplet-quintet excitations, with temperature dependent damping and spin-gap consistent with current models for weakly coupled quantum spin-liquids. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L27.00011: Field dependence study on the ordering temperature of [Cu(pz)$_{2}$(NO$_{3})$](PF$_{6})$ Fan Xiao, Christopher Landee, Mark Turnbull [Cu(pz)$_{2}$(NO$_{3})$](PF$_{6})$ is a tetragonal 2D quantum Heisenberg antiferromagnet with an exchange strength of 10.9 K and a zero-field ordering temperature of 3.05 K. Its magnetization has been studied along the two principal directions at various fields and temperatures. The ordering temperature increases with field by as much as 19{\%} in 5 Tesla. This behavior can be explained as the effects of both weak 3D interactions (J'/J $\approx $ 10$^{-3})$ and field-induced Heisenberg-XY crossover. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L27.00012: Raman scattering for the Heisenberg S=1/2 antiferromagnet Natalia Perkins, Wolfram Brenig We study the full Raman intensity for the Heisenberg $S=1/2$ antiferromagnet on the triangular lattice by simultaneously considering the effects of the renormalization of the spectrum by $1/S$ corrections, and the final state magnon-magnon interactions. The analysis of the Raman intensity without final state interactions shows, that it has two peaks, corresponding to two maxima of the bare magnon spectrum $E_k$. Then we calculate Raman intensity with the renormalized spectrum. We obtain that at the energy at which the renormalized dispersion has a plateau, and, therefore, density of states is large, the Raman intensity is strongly enhanced. We also derive explicit expressions for the vertex functions to order $1/S$, and calculate Raman intensity including $1/S$ self-energy and the vertex corrections on equal footing. The vertex corrections is calculated by a summation of ladder diagrams with magnon-magnon interactions. Once interactions are included, the peak smears out and shifts to lower energies. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L27.00013: The central peak problem in the 2d anisotropic heisenberg model Bismarck Costa, Anderson Lima The origin of the central peak in the neutron scattering function, $S(q,\omega)$, for the classical 2d anisotropic Heisenberg model has been a puzzle for several years. Wiesler et al (Z. Phys. B 93, 277-297, 1994) studied the compound $CoCl_2-GIC$ by using neutron scattering. More recently some numerical simulations of the model (H.G. Evertz and D. P. Landau, PRB 54, 12302-12317, 1996) came out. Their results are inconsistent with any analytical study so far. The analytical works suppose that the central peak is due to vortex motion. The theory mimics a successful approach for the one-dimensional version of the model where the central peak is due to kink motion. In this work we show that the central peak in the 2d anisotropic heisenberg model can be explained by a vortex-anti-vortex number fluctuation due to local diffusion and a creation-annihilation process. The phenomenology we propose gives the correct behavior for $S(q,\omega)$ when compared with experimental as well spin dynamics results. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L27.00014: Magnetic susceptibilities of rectangular Heisenberg S=1/2 antiferromagnets Tom Valleau, Rob Butcher, Brian Keith, Christopher Landee, Mark Turnbull, Anders Sandvik Rectangular antiferromagnets are two-dimensional systems with inequivalent exchange strengths (J', J) along the two principle axes with J' $\equiv $ $\alpha $J, $\alpha \quad <$1. They have an intermediate dimensionality that can vary continuously from 1D ($\alpha $ = 0 ) to square 2D ($\alpha $ = 1). There exist a number of physical realizations of rectangular antiferromagnets (CuPzBr$_{2}$, CuPzCl$_{2}$, CuPz(N$_{3})_{2}$ where Pz = pyrazine) but there has been no previous mechanism for interpreting their susceptibilities in terms of two exchange parameters. We have simulated the susceptibility of the rectangular S=1/2 Heisenberg antiferromagnet using the stochastic series expansion quantum Monte Carlo method [1] and used the results to interpret our experimental data. For example, copper pyrazine diazide, CuPz(N$_{3})_{2}$, has a primary exchange of 15.5 K and an anisotropy parameter $\alpha $ = 0.4. The stronger exchange is due to the superexchange pathway through the pyrazine molecule and the weaker corresponds to the azide bridges. [1] A. Sandvik, PRB 59, R14157 (1999). [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L27.00015: Magnetic and Thermal Properties of the Spin $S$~=~1/2 Zig-Zag Spin Chain Compound In$_2$VO$_5$ $^*$ Yogesh Singh, David Johnston The structure of In$_2$VO$_5$ consists of zig-zag V$^{4+}$ (spin $S$~=~1/2) chains along the $b$-axis. Prior to our work, there were two theoretical reports on this material.$^{1,2}$ One report suggested that the nearest-neighbor and next-nearest-neighbor interactions between the V$^{4+}$ moments would be anti-ferromagnetic and frustrated,$^1$ while the second report suggested that both these interactions should be ferromagnetic.$^2$ An experimental study of the physical properties of this material had not been reported. We measured magnetic susceptibility $\chi$, ac susceptibility $\chi_{\rm ac}$ and specific heat $C$ versus temperature $T$ on In$_2$VO$_5$ and $\chi$ and $C$ versus $T$ on the isostructural, nonmagnetic compound In$_2$TiO$_5$. The $\chi(T)$ data for In$_2$VO$_5$ showed that the dominant magnetic exchange between the V$^{4+}$ moments was ferromagnetic above 150~K\@. However, the $\chi(T)$ and the frequency dependence of the $\chi_{\rm ac}(T)$ data indicate that below 3~K the system is in a spin-glass state indicating the presence of disorder and frustrated interactions at these temperatures. Our $C$ and entropy $S$ data suggest that there may be a structural change below 140~K in In$_2$VO$_5$ which could possibly change the interactions between the V$^{4+}$ moments.\\ 1. I. M. Volkova, J. Phys.: Condens. Matter {\bf 19}, 176208 (2007).\\ 2. U. Schwingenschlogl, Phys. Rev. B {\bf 75}, 212408 (2007).\\ $^*$Supported by the USDOE under Contract No.\ DE-AC02-07CH11358. [Preview Abstract] |
Session L28: Material Issues for Superconducting Qubits
Sponsoring Units: DCMPChair: John Martinez, University of California, Santa Barbara
Room: Morial Convention Center 220
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L28.00001: Fabrication and Characterisation of Superconducting Coplanar Waveguide Resonators for Circuit QED Applications Martin G\"oppl, Rob Schoelkopf, Andreas Wallraff Superconducting thin film microwave cavities have gained great interest in recent years for studying qubit-photon or qubit-photon-qubit interactions in circuit quantum electrodynamics (QED) experiments $[1,2]$. Thin film cavities made by using standard optical lithography and microfabrication techniques have the potential for enabling quantum computing applications such as coupling several qubits via a quantum bus $[2]$. In order to specifically design circuit QED systems the cavity parameters such as resonance frequency and quality factor, determined by the input and output coupling strength need to be precisely controlled. We have fabricated niobium and aluminum resonators on Si, Si/SiO$_x$ and Al$_2$O$_3$ substrates. Resonators with quality factors up to several hundred thousands have been characterized. Furthermore, the effect of external parameters such as temperature and drive power have been investigated. The measured frequency dependent transmission of the cavities is found to be in good agreement with analytical circuit models.\\ $[1]$ A. Wallraff et al, Nature \textbf{431}, 162 (2004).\\ $[2]$ J. Majer et al, Nature \textbf{449}, 443 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L28.00002: Materials for superconducting qubits: Measurements of dielectric loss at low temperatures Aaron O'Connell, M. Ansmann, R.C. Bialczak, R. McDermott, M. Hofheinz, N. Katz, E. Lucero, M. Neeley, H. Wang, E.M. Weig, J.M. Martinis, A.N. Cleland The energy relaxation time ($T_{1})$ of the Josephson phase qubit is significantly impacted by the microwave loss in the dielectric materials used in fabrication. This loss mechanism is most likely due to the qubit coupling to a fermionic bath of two-level defect states embedded in the dielectrics. At high temperatures or high excitation voltages, these states can become saturated, so that the low loss tangents reported in the literature do not accurately represent the material performance at low temperatures and very small excitation energies. We have probed the microwave ($\sim $6 GHz) loss tangents of a number of common thin-film dielectrics at $\sim $100mK, using very low excitation voltages, in order to obtain values for the loss tangents relevant to quantum computation. We present these loss tangent data, and illustrate a technique to extract material values from measurements of the quality factors of coplanar waveguide resonators. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L28.00003: Multi-level Spectroscopy of Microstates Coupled to a dc SQUID Phase Qubit Tauno Palomaki, S. K. Dutta, R. M. Lewis, A. J. Przybysz, Hanhee Paik, B. K. Cooper, H. Kwon, E. Tiesinga, J. R. Anderson, C. J. Lobb, F. C. Wellstood We report spectroscopic measurements at 25 mK of discrete two-level systems (TLS) coupled to a 16 $\mu $m$^{2}$ area Al/AlO$_{x}$/Al dc SQUID phase qubit. When the energy level spacing of the qubit equals that of the TLS, the coupling between the two systems lifts the degeneracy. By applying microwaves to excite transitions in the qubit, we map out the ``flat'' splittings in the 0$\to $1 transition spectrum of the qubit. We see 8 splittings, over a 1 GHz range, ranging in size from 14 to 240 MHz. We observe ``tilted'' splittings in the spectrum of 0$\to $2 transitions for the qubit, corresponding to the $\vert $1,e$>$ state interacting with the $\vert $2,g$>$ state, where e and g were the excited and ground state of the TLS, and the first index is the qubit and the second is the microstate. The spectra were compared to predictions from a model with the junction coupled to charged TLS and the agreement was reasonable. Finally, we show that away from TLS the coherence time of the qubit is limited by the bias leads. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L28.00004: Effect of Two Level System Saturation on Charge Noise in Josephson Junction Qubits Magdalena Constantin, Clare Yu, John Martinis It is not widely appreciated that two-level systems in small qubits can easily be strongly saturated when the applied electromagnetic flux $J$ is much larger than the critical flux $J_c$. We show that charge noise $S_Q$ in Josephson qubits can be produced by fluctuating two-level systems with electric dipole moments in the substrate using the standard flat density of states. At high frequencies the frequency and temperature behavior of the charge noise depends on the ratio $J/J_c$. Our results are consistent with experimental conclusions that $S_Q\sim 1/f$ at low frequencies and $S_Q\sim f$ at high frequencies. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L28.00005: Crystalline Josephson phase qubits with improved performance Jeffrey Kline, Seongshik Oh, Haohua Wang, John Martinis, David Pappas One of the greatest challenges in the development of a practical solid-state quantum computer is to overcome decoherence due to coupling between the environment and the qubit. Superconducting quantum computers based on Josephson phase qubits are susceptible to decoherence due to charge noise in both the tunnel barrier and crossover insulators. We have demonstrated that the usage of crystalline tunnel barriers greatly reduce the density of spurious charge fluctuators in the tunnel barrier when compared to the ubiquitous amorphous barrier. However, a performance gain in coherence time was not realized due to decoherence in the crossover insulator. In the latest generation of devices, we have optimized both the tunnel barrier and crossover insulator materials. Low temperature measurements performed on a large area (50 \textit{$\mu $}m$^{2})$ device yielded a coherence time of 500 ns which is tied for the Josephson phase qubit world record. We expect increased performance in future generation devices where small area (13 \textit{$\mu $}m$^{2})$ junctions will be used. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L28.00006: 1/f Flux Noise in Josephson Phase Qubits Robert McDermott, Radek Bialczak, Markus Ansmann, Max Hofheinz, Nadav Katz, Erik Lucero, Matthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis We present the results of a novel measurement in a Josephson phase qubit that uses the resonant response of the qubit to directly measure the spectrum of low-frequency noise. This general method can be applied to any qubit system. By alternating the sense of the qubit bias, we show that the noise is predominantly flux-like, as opposed to a critical-current noise. The magnitude of the noise is compatible with previous measurements of excess low-frequency flux noise in SQUIDs cooled to millikelvin temperatures. We present the results of calculations of flux noise from paramagnetic defects in the native oxides of the superconductors, and show that the measured flux noise cannot be explained by the standard model of two-level state defects. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L28.00007: Measurements of Decoherence in rf SQUID Qubits Douglas Bennett, Luigi Longobardi, Vijay Patel, Dmitri Averin, James Lukens We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID has independent, in situ, controls for the relative positions of levels in different fluxoid wells and the barrier height between the wells. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise. The low frequency flux noise is observed using microwave spectroscopy and resonant tunneling between fluxoid states in addition to the decay of coherent oscillations. These measurements are useful for evaluating the various insulating layers which are believed to be an important source of 1/f noise in many superconducting qubits. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L28.00008: Josephson junction microscope for probing and quantum manipulation of low-frequency fluctuators Lin Tian, Raymond Simmonds The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barrier of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices. Meanwhile, quantum manipulation of the TLSs via the junction oscillator mode can also be achieved. L. Tian and R. W. Simmonds, Phys. Rev. Lett. 99, 137002 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L28.00009: Low-frequency Flux Noise in SQUIDs and Superconducting Qubits Steven Sendelbach, David Hover, Achim Kittel, Michael Mueck, Robert McDermott Superconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a 1/f spectrum. The origin of this excess noise is not understood. We report the results of SQUID measurements that explore the dependence of the excess low-frequency flux noise on SQUID inductance, geometry, materials, and temperature. We discuss contributions to the measured noise from temperature fluctuations, trapped vortices in the superconducting films, and surface magnetic states in the native oxides of the superconductors. We discuss implications of our measurements for qubit dephasing. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L28.00010: Small shadow evaporated junctions for superconducting phase qubits F. Altomare, M.S. Allman, K. Cicak, M.A. Sillanpa\"{a}, J.D. Whittaker, R.W. Simmonds One of the biggest problems facing the fabrication of quantum computers based on superconducting qubits is the short coherence time of the quantum states. This is due to interaction of qubits with both the environment and defects (two level systems-TLS) in the Josephson junction (JJ). Because of the large JJ area, this problem is particularly obvious in phase qubits where it has been shown that TLS greatly affect the coherence time.\footnote{PRL 93, 077003 (2004)} One way to overcome this problem is to reduce the size of the JJ thus reducing the number of TLS.\footnote{PRL 97, 050502 (2006)} We will discuss the results of our approach to this solution, namely using shadow evaporated JJ ($<1\mu m^2$) and low-loss capacitor, and the results of our experiments on coupled qubits. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L28.00011: Investigating two-level systems in the Josephson tunnel barrier Joshua Strong, Fabio Altomare, Raymond Simmonds The presence of two-level system defects within the Josephson tunnel barrier has been made apparent in the spectroscopy of superconducting phase quantum bits. Here, we present a different circuit-- a tunable harmonic resonator based on the Josephson inductance-- which is better suited to the study of these two-level systems. A typical circuit is tunable over a gigahertz range and behaves linearly at sufficiently low drive amplitudes. We use this circuit to investigate the properties of individual defects, extracting parameters such as strength of coupling to the resonator, etc. The sea of two-level systems also has a bulk effect, causing a degradation in the quality factor of the resonator. This means that the Josephson tunnel barrier has a non-zero loss tangent in the microwave regime. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L28.00012: Probing dissipation from vortices with superconducting microwave resonators. C. Song, T.W. Heitmann, M.P. DeFeo, K. Yu, B.L.T. Plourde, R. McDermott One potential source of dissipation in superconducting qubits comes from vortices trapped in the thin films. We present a design for a system of microwave resonators for studying the loss contributed by trapped flux over the frequency range from 2 - 12 GHz. This consists of a multiplexed set of superconducting resonators with a wide range of lengths that are capacitively coupled to a common superconducting feed-line. By cooling the resonators in different magnetic fields, it is possible to probe the loss from vortices as a function of field and frequency, at least at the discrete frequencies of the resonators in our set. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L28.00013: Decoherence in Superconducting Qubits from Surface Magnetic States David Hover, Steven Sendelbach, Achim Kittel, Michael Mueck, Robert McDermott Unpaired spins in amorphous surface oxides can act as a source of decoherence in superconducting and other solid-state qubits. A density of surface spins can give rise to low-frequency magnetic flux noise, which in turn leads to dephasing of the qubit state. In addition, magnetic surface states can couple to high-frequency resonant magnetic fields, and thereby contribute to energy relaxation of the qubit. We present the results of low-frequency measurements of the nonlinear and imaginary spin susceptibility of surface magnetic states in superconducting devices at millikelvin temperatures. In addition, we describe high-frequency magnetic resonance measurements that directly probe the surface spin density of states. We present calculations that connect the measurement results to qubit energy relaxation and dephasing times. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L28.00014: Measurement of low frequency flux noise in superconducting flux qubits Wei Qiu, Bo Mao, Yang Yu, Shaoxiong Li, Siyuan Han The development of superconducting quantum interference devices (SQUIDs) as flux qubits for scalable quantum computation has been impeded significantly in the last several years by excessive low frequency flux noise which has become the dominant decoherence mechanism in several experiments. We measured the low frequency flux noise in SQUIDs with inductance ranging from about 30 pH to 1 nH. We found that for the Nb SQUIDs fabricated with the same process the measured rms low frequency flux noise has a linear dependence on the inductance of the SQUIDs. Implications of the result on material, design, fabrication of flux qubit will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L28.00015: Measurement of Spin Susceptibility of Thin Films and Nano-Scale Structures Julie Bert, Hendrik Bluhm, Nicholas Koshnick, Martin Huber, Kathryn Moler We report measurements of a spin-like paramagnetic susceptibility signal from high purity metallic and insulating thin films.~ The measurements were performed using a Superconducting Quantum Interference Device (SQUID) in a scanning microscope.~ By using the SQUID to scan areas of the sample both near and far from the metallic films, we found a paramagnetic susceptibility associated with both Au and AlOx films that was ten times larger than could be explained by the concentration of impurity spins expected for 6N gold.~ The 1/T temperature dependence and the paramagnetic sign indicate that the susceptibility signal is caused by localized spins that are at most weakly coupled to each other and to the conduction electrons.~ Moreover, the signal exhibits a measurable out of phase response which can be related to 1/f noise due to fluctuating spins [Koch, DiVincenzo, and Clarke, Phys. Rev. Lett. \textbf{98}, 267003 (2007)].~ These results demonstrate the utility of scanning SQUID based susceptibility measurements for characterizing spin related effects.~ Further applications of this technique may include probing 1/f noise origins in superconducting devices as well as imaging magnetic structures such as nanomagnets. [Preview Abstract] |
Session L29: Focus Session: Carbon Nanotubes and Related Materials VIII: Electronic Structure of Graphene
Sponsoring Units: DMPChair: Michael Fuhrer, University of Maryland
Room: Morial Convention Center 221
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L29.00001: Electronic Structure and Morphology of Graphene Layers on SiC Invited Speaker: Recent years have witnessed the discovery and the unique electronic properties of graphene, a sheet of carbon atoms arranged in a honeycomb lattice. The unique linear dispersion relation of charge carriers near the Fermi level (``Dirac Fermions'') lead to exciting transport properties, such as an unusual quantum Hall effect, and have aroused scientific and technological interests. On the way towards graphene-based electronics, a knowledge of the electronic band structure and the morphology of epitaxial graphene films on silicon carbide substrates is imperative. We have studied the evolution of the occupied band structure and the morphology of graphene layers on silicon carbide by systematically increasing the layer thickness. Using angle-resolved photoemission spectroscopy (ARPES), we examine this unique 2D system in its development from single layer to multilayers, by characteristic changes in the $\pi $ band, the highest occupied state, and the dispersion relation in the out-of-plane electron wave vector in particular. The evolution of the film morphology is evaluated by the combination of low-energy electron microscopy and ARPES. By exploiting the sensitivity of graphene's electronic states to the charge carrier concentration, changes in the on-site Coulomb potential leading to a change of $\pi $ and $\pi $* bands can be examined using ARPES. We demonstrate that, in a graphene bilayer, the gap between $\pi $ and $\pi $* bands can be controlled by selectively adjusting relative carrier concentrations, which suggests a possible application of the graphene bilayer for switching functions in electronic devices. This work was done in collaboration with A. Bostwick, J. L. McChesney, and E. Rotenberg at Advanced Light Source, Lawrence Berkeley National Laboratory, K. Horn at Fritz-Haber-Institut, K. V. Emtsev and Th. Seyller at Lehrstuhl f\"{u}r Technische Physik, Universit\"{a}t Erlangen-N\"{u}rnberg, and F. El Gabaly and A. K. Schmid at National Center for Electron Microscopy, Lawrence Berkeley National Laboratory. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L29.00002: Tailoring electronic properties in coated graphene Bruno Uchoa, C.-Y. Lin, N.M.R. Peres, J.M.B. Lopes dos Santos, A.H. Castro Neto Graphene is a single layer carbon material whose unique properties in transport are related to the its peculiar Fermi surface, which is made out of six points at the corners of the Brillouin zone where the conduction and valence bands touch. Due to the vanishing density of states at these points, the low energy excitations are made of massless Dirac fermions, with several anomalous properties in the transport. We propose that some of the unique properties of graphene can be tailored by the chemical adsorption of impurity atoms on its surface. If on one hand alkaline metals are good charge donors and can be used to control the number of charge carriers in graphene, transition metals have a more covalent character and can be used to induce magnetism. We show that despite pure graphene cannot be magnetized, the hybridization of the carbon p orbitals with non-magnetic d orbitals can generate strong itinerant magnetism in graphene coated with transition metal atoms. On the other hand, if an isolated impurity atom is able to form a stable localized level under hybridization with the bath of electrons in graphene, we show that the suppression of the density of states in the bath around the localized level can strongly favor the formation of a local magnetic moment at the impurity. We propose that the local magnetization of the impurity can be controlled by the application of an external gate voltage. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L29.00003: Probing the Band Structure of Mono-, Bi- and Tri-layer Graphene by Infrared Absorption Spectroscopy Kin Fai Mak, Matthew Sfeir, Yang Wu, Chun Hung Lui, Janina Maultzsch, Sami Rosenblatt, Mark Hybertsen, Tony Heinz Absorption spectra in the infrared range (0.3 -- 1 eV) were measured for large-area, single-crystal mono-, bi- and tri-layer graphene samples produced by mechanical exfoliation of graphite. A constant absorption independent of photon energy was observed for monolayer samples. For the bi-layer, a strong absorption peak was seen at 0.37eV. The absorption spectrum of tri-layer graphene was found to be well represented by the sum of those of a mono- and a bi-layer, with the latter spectrum scaled by 2$^{1/2}$ in photon energy. These observations can be explained qualitatively within a tight-binding band structure picture and yield an accurate determination of the nearest-layer hopping constant ($\gamma _1 )$. Explicit calculations of the absorption spectra show that an optimal fit to experiment requires a shift of the Fermi energy of approximately 100 meV from the Dirac point and an empirical broadening of tens of meV. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L29.00004: Electronic structure of epitaxial graphene layers on SiC Laurence Magaud, Francois Varchon, Cecile Naud, Guy Trambly de Laissardiere, Pierre Mallet, Jean-Yves Veuillen, Claire Berger, Didier Mayou Our DFT calculations [1](VASP) demonstrate the existence of a strong interaction between the substrate and the first carbon layer in the epitaxial graphene system. This prevents any graphitic electronic properties for this layer. However, the graphitic nature of the film is recovered by the second and third absorbed layers in agreement with recent STM experiments[2]. We also present evidence of a charge transfer that depends on the interface geometry. It causes the graphene to be doped and may open a gap in agreement with ARPES experiments [3]. The effect of the complex first carbon layer structure on the ontop graphene like layer will also be discussed. Moreover we will show how a rotational disorder between two graphene sheets leads to an effective electronic decoupling of these layers and then to band structures with linear free graphene like dispersions [4]. [1] F. Varchon et al. Phys. Rev. Lett. 99, 126805 (2007) [2] P. Mallet et al., Phys. Rev. B 76, 041403(R) (2007) [3] T.Ohta et al., Science 313, 951 (2006) [4] J.Hass et al., cond-mat/0706.2134, (submitted to PRL) [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L29.00005: Electronic and structural properties of graphene on 6H-SiC M. Weinert, G. Sun, L. Li We present a first-principles investigation of the electronic and structural properties of one and two graphene sheets on both the Si and C-terminations of 6H-SiC. Of particular interest are properties of the interlayer states associated with the graphene sheets, their spatial and energy distribution, how they are modified by the application of external electric fields, and differences with respect to the polarity of the fields. The calculated results will be compared to the electronic and structural properties observed in scanning tunneling microscopy experiments. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L29.00006: Direct Determination of the Absorption of Graphene Mono- and Multi-layers in the Visible and Near-Infrared Yang Wu, Kin Fai Mak, Chun Hung Lui, Janina Maultzsch, Tony Heinz Single-crystal mono- and multi-layer graphene samples were prepared by mechanical exfoliation on quartz substrates. The absorption spectra of samples of 1 -- 8 monolayer thickness were measured in the optical and near-infrared range. The absorption coefficient was found to be largely independent of photon energy and linear in the number of graphene layers. Such absorption measurements can thus be used to determine the thickness of mesoscopic graphite to monolayer accuracy, as already demonstrated in the context of Rayleigh scattering [Casiraghi et al. Nano Letters 2007]. By analysis of the optical transmission problem for a thin film at the air-quartz interface, we deduced an absorption of 2.3{\%} per layer. The magnitude of the monolayer absorption agrees with the value of $\pi \alpha $, where $\alpha $ is the fine-structure constant, and corresponds the result obtained from a tight-binding model of the graphene electronic structure [Gusynin et al. PRL 2006]. The predicted (and measured) optical absorption, we note, is equivalent to a constant optical conductance of$\frac{\pi e^2}{2h}=6.09\times 10^{-5}\Omega ^{-1}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L29.00007: Experimental Measurement of Ultrafast Carrier Dynamics in Mono- and Multi-layer Graphene Samples Daohua Song, Kin Fai Mak, Yang Wu, Chun Hung Lui, Matthew Sfeir, Sami Rosenblatt, Hugen Yan, Janina Maultzsch , Tony Heinz The ultrafast dynamics of charge carriers in mono- and multi-layer graphene was investigated by femtosecond transient reflectivity measurements. The experiments were performed using 100-fs optical pump pulses at a wavelength of 400 nm and probe pulses at a wavelength of 800 nm. We observed a transient response on the time scale of several picoseconds. For bulk graphite, a decay time of $\sim $ 3 ps was found; for thin graphene multilayer samples, a reduced decay time was observed, dropping ultimately to $\sim $ 1 ps for a single graphene layer. The reflectivity transients can be understood in terms of coupling of the photo-generated electronic excitations to optical phonons, and the subsequent loss of energy from this sub-system. The possible role of graphene interactions with the quartz substrate and the effect of the graphene electronic specific heat on the decay rate will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L29.00008: Exposure of Epitaxial Graphene on SiC(0001) to Atomic Hydrogen Nathan Guisinger, Greg Rutter, Jason Crain, Phillip First, Joseph Stroscio Graphene films on SiC exhibit coherent transport properties that suggest the potential for novel carbon-based nanoelectronics applications. Recent studies suggest that the role of the interface between single layer graphene and silicon-terminated SiC can strongly influence the electronic properties of the graphene overlayer. In this study, we have exposed the graphitized SiC to atomic hydrogen in an effort to passivate dangling bonds at the interface. We have used scanning tunneling microscopy to investigate the interface surface structure following exposure to atomic hydrogen for a range of sample temperatures. Initial results indicate that regions of clean SiC were successfully passivated with atomic hydrogen below 400 \r{ }C, while the underlying interface of the graphitized regions appear to be unchanged for all temperatures studied. The threshold temperature for passivating clean SiC suggest that the passivated dangling bonds are primarily from Si atoms that are present within the SiC surface reconstruction. Although the hydrogen does not appear to penetrate below the graphene layer, initial results suggest that it does adsorb to the graphene. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L29.00009: Characterization of Epitaxial Graphene Oxide Fan Ming, Michael Sprinkle, Xuebin Li, Xiaosong Wu, Claire Berger, Walter de Heer Graphite oxide is a layered semiconducting material that is produced from graphite or graphene by chemical oxidation. The material is characterized by various probes such as transport, Raman spectroscopy and optical absorption spectroscopy. Here we present the properties of graphene oxide, which is chemically converted from epitaxial graphene directly on silicon carbide chips. The absorption spectrum indicates a large band gap and the Raman spectrum shows a pronounced D line while the 2D line is absent. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L29.00010: Ballistic Transport of Narrow-Channeled Epitaxial Graphene Tian Shen, Peide Ye, Yang Sui, Yanqing Wu, Mike Capano, James Cooper, Leonid Rokhinson, Lloyd Engel Large-area epitaxial graphene film by thermal decomposition of SiC wafer has provided the missing pathway to a viable electronics technology $^{2}$. Low temperature magneto-transport properties of narrow-channeled epitaxial graphene films with dimension of 100 nm to 500 nm, formed on the carbon face of semi-insulating 4H-SiC substrates in an Epigress VP508 SiC hot-wall chemical vapor deposition reactor, are systematically studied. Typical quasi-ballistic transport features, such as negative magnetoresistance or 1D weak localization, aperiodic magnetoresistance fluctuations, periodic magentoresistance oscillations associated with channel geometry, bend resistance and quench of Hall effect associated with nanoscale junctions, are all observed. Magneto-resistance, being independent on parallel magnetic field up to 18 T, verifies the 2D nature of epitaxial graphene. 2. C. Berger et al., \textit{Science} 312, 1191 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L29.00011: Electrical conductivity of graphene oxide sheets and networks of such sheets Dmitriy Dikin, Inhwa Jung, Geoffrey Dommett, Sasha Stankovich, Rod Ruoff Electrical transport through individual flat graphene oxide sheets and through networks of crumpled graphene oxide sheets has been studied at different temperatures and strengths of the electrical field. Conductivity of individual graphene oxide sheets on silica/silicon substrate were measured with a 4-electrode configuration at room and elevated temperatures (up to 520 K) in steady state conditions and at different states of reduction and oxidation. Crumpled graphene oxide sheets in a powdered form and graphene oxide sheets dispersed inside of a dielectric matrix at different packing densities were also electrically measured in a wide temperature range (between 520 and 20 K). Possible mechanisms of an electrical charge transport through these systems and the electronic properties of graphene oxide sheets will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L29.00012: The epitaxial graphene-graphene oxide junction, a key step towards epitaxial graphene electronics Mike Sprinkle, Xiaosong Wu, Xuebin Li, Fan Ming, Claire Berger, Walt de Heer Epitaxial graphene (EG), grown by thermal decomposition of SiC, was lithographically patterned to form pairs of EG electrodes separated by narrow gaps. Graphene oxide (GO) flakes were deposited by an AC dielectrophoresis method to bridge the gaps and produce all-graphene metal-semiconductor-metal structures. Electrical measurements on these devices indicate the presence of Schottky barriers, due to the band gap in GO, at the junctions. The barrier height is found to be between 0.5 eV and 0.7 eV. It is known that annealing graphite oxide reduces the degree of oxidation; annealing these structures at 180 C reduces the barrier height, implying that the band gap can be tuned by changing the degree of oxidation. A lower limit on the mobility of GO is obtained. Recent efforts towards transistor fabrication by chemically oxidizing selected regions of patterned EG will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L29.00013: Plasmons and The Spectral Function of Graphene Marco Polini, Reza Asgari, Giovanni Borghi, Yafis Barlas, T. Pereg-Barnea, A.H. MacDonald We report on a theoretical study of the influence of electron-electron interactions on the one-particle Green's function of a doped graphene sheet based on the random-phase-approximation and on graphene's massless Dirac equation continuum model. We find that states near the Dirac point interact strongly with plasmons with a characteristic frequency $\omega^\star_{\rm pl}$ that scales with the sheet's Fermi energy and depends on its interaction coupling constant $\alpha_{\rm gr}$, partially explaining prominent features of recent ARPES data. [Preview Abstract] |
Session L30: Metallic and Semiconducting Nanotubes
Sponsoring Units: DCMPChair: Alex Efros, Naval Research Laboratory
Room: Morial Convention Center 222
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L30.00001: Surface scattering in metallic nanowires Xi Chen, Randall Victora We theoretically study the electronic transport with surface scattering in metallic nanowires. The Landauer formula along with a recursive Green's function technique is employed to calculate the transport properties of wires with surface disorder. In the diffusive regime, the resistance increases linearly with the wire length, with a slope determined by the elastic mean free path (MFP). For strong scattering, MFP grows linearly with the wire diameter in a way that violates the semi-classical Fuchs-Sondheimer model. The mechanism for the violation is discussed. In the weak scattering limit, MFP grows with the diameter in an oscillatory manner owing to the quantum size effect. We propose an analytical theory that explains the observed behavior. The MFP is also shown to strongly depend on the surrounding environment of the nanowire. For wires with realistic sizes, we quantitatively show that the surface scattering alone can cause a resistivity that is several times larger than the bulk value. This work is supported by the National Science Foundation under Award Number ECS-0621868 and by the University of Minnesota Supercomputing Institute. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L30.00002: Phonon mode and Breakdown of a Au nanowire between electrodes Liqin Ke, Mark Van Schilfgaarde The deformation behavior of an atomic Au wire placed between Au electrodes, was investigated by using a generalization of the method of linear muffin-tin orbitals (LMTO) within the local-density approximation (LDA). We studied the dynamical motion of the atoms in the wire. Soft phonon modes were studied. It has been found that the lowest phonon eigenstate are associated with motions of atom in the middle of the chain. Atomic motion of this mode was studied by solving the muclear wavefunction of this atom. The quantum and classical effects are compared. At certain stages of the wire breaking process, atomic fluctuations become very large. We also show that the wire conductance is sensitive to the details of the deformation, which implies that the intrinsic noise in the conductance is of the order as the conductance itself. Large displacements which resulted in wire breakage were also studied. The energy barrier associated with this process has been calculated. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L30.00003: Semiconducting chains of gold and silver Ricardo Nunes, Frederico Fioravante The authors introduce a geometry for ultrathin Au and Ag wires that {\it ab initio} calculations indicate to be more stable than previously considered planar zigzag geometries for these systems, by about 0.1 eV per atom. This structure is insulating for both metals and for related Ag$_{0.5}$Au$_{0.5}$ alloys, with gaps of 1.3 eV for Au, 0.8 eV for Ag, and varying between 0.1 eV and 1.9 eV for the alloys. The insulating nature of the geometry is not a result of Peierls instabilities, and is analyzed in terms of an interplay between geometric and electronic structure effects. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L30.00004: Bonding, Conductance and Magnetization of Oxygenated Au Nanowires Chun Zhang, Robert Barnett, Uzi Landman Spin-density-functional calculations of tip-suspended gold chains, with molecular oxygen, or dissociated oxygen atoms, incorporated in them, reveal structural transitions for varying lengths. The nanowires exhibit enhanced strength for both oxygen incorporation modes, and upon stretching tip atoms join the wire. With incorporated molecular oxygen the wire conductance is about 1(2e$^{2}$/h), transforming to an insulating state beyond a critical length. The nanowire conductance with embedded oxygen atoms is low, 0.2 (2e$^{2}$/h), and it develops magnetic moments localized on the oxygens and the neighboring Au atoms. \textbf{Physical Review Letters (in press)} [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L30.00005: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L30.00006: Superconductivity and metal-insulator transition in Bi nanowires. Mingliang Tian, Jian Wang, Nitesh Kumar, Qi Zhang, Thomas Mallouk, Jianendra Jain, Moses H.W. Chan Semi-metallic bismuth has long been a prototype system for quantum transport and finite-size effect studies, due to its long electron mean-free path, low carrier density and small carrier mass. We found Bi nanowires (NWs) of less than 100 nm diameter can be superconducting, metallic and insulating closely depending on the details of their microstructures, morphology and surface condition. For granular Bi NWs with grains showing (001) preferred orientation, the NWs are superconducting with Tc's of 7.2 and 8.3 K. Without (001) preferred orientation, the NWs show superresistive behavior. For single-crystalline Bi NWs, when the wires are embedded inside anodized aluminum oxide (AAO) membrane, insulating behavior is found below 1.0 K with low excitation current but metallic at a higher bias current. However, this metallic state can be tuned into insulating again by an applied magnetic field. We have also made measurements on an individual single-crystalline Bi NWs released from the AAO, an thin oxide layer is found on the wire surface. Interestingly, the wire was found to be superconducting below 1.5 K. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L30.00007: Evolution of Integer Quantized Conductance in Gold Nnowires Yoshihiko Kurui, Yoshifumi Oshima, Masakuni Okamoto, Kunio Takayanagi Conductance of gold junction was measured during many breaking procedures, while simultaneously acquiring transmission electron microscope images. The conductance histogram exhibits the quantized peaks in the vicinity of $G_{0 }$(=2e$^{2}$/h: conductance quantum), 2$G_{0}$, 3$G_{0}$, and 4$G_{0}$. From the TEM images, we found that these values correspond to single, double, triple and quadric atomic strands, respectively. With the exception of the quadric strand, these strands were arranged in a planar sheet with a ladder or zigzag configuration that respectively resembled the (001) and (111) lattice planes of the gold crystal. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L30.00008: Intrinsic Electrical-Transport Properties of Single ZnO Nanowires S.P. Chiu, Y.H. Lin, J.J. Lin, W.B. Jian, Z.Y. Wu, F.R. Chen, J.J. Kai Single-crystalline zinc oxide (ZnO) nanowires (NWs) were synthesized by the thermal evaporation method. The intrinsic electrical-transport properties of ZnO NWs were studied by carrying out four-probe measurements on individual NWs. The electrodes were made by the standard electron-beam lithography technique. The current-voltage characteristics and the zero-bias resistivities, \textit{$\rho $}(T), were measured over a wide range of temperatures between 0.25 and 300 K. We found that, in many cases, the temperature behavior of \textit{$\rho $} could be well-described by the thermal-activation model involving three activation energies ($E_{1}$, $E_{2}$ and $E_{3})$. Our values of $E_{1}$ (approximately, several tens meV) extracted from the \textit{$\rho $}(T) around 300 K are close to the ionization energies of the major known shallow donors in ZnO. Our values of $E_{2}$ (approximately, several meV) extracted from the \textit{$\rho $}(T) in the intermediate temperature regime might originate from electron activation from the lower impurity band to the upper Hubbard subband ($D^{-}$ band). Finally, at very low temperatures ($<$ 5 K), the measured \textit{$\rho $}(T) indicated diminishing values of $E_{3}$ (approximately, thousandths meV), suggesting essentially metallic behavior. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L30.00009: Electrical-transport studies of individual RuO$_2$ nanowires and their nanowire contacts Y.H. Lin, K.J. Lin, F.R. Chen, J.J. Kai, J.J. Lin Single-crystalline RuO$_2$ nanowires (NWs) have been prepared by the thermal evaporation method. With the help of e-beam lithography, individual NWs were contacted by submicron electrodes from above. By employing 4- and 2-probe configurations, not only the intrinsic electrical resistivities of the NWs but also the electronic contact resistances, $R_{\mathrm{c}}(T)$, have been determined. Our measured resistivity behavior of the NWs is found to agree well with the current understanding of this rutile material within the framework of the Boltzmann transport theory. On the other hand, we found that, for high-resistance contacts, $R_{\mathrm{c}}$ increases rapidly with decreasing temperature and finally saturates at liquid-helium temperatures. This behavior of $R_{\mathrm{c}}$ can be satisfactorily explained in terms of the ``thermally fluctuation-induced tunneling'' conduction through a microscopic junction incidentally formed at the interface between the electrode and the NW. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L30.00010: Anomalous Long-Range Proximity Effect in Template-Fabricated Single-Crystal Superconducting Nanowires Wenhao Wu, Haidong Liu, Zuxin Ye, Zhiping Luo, K. D. D. Rathnayaka We report an anomalous proximity effect observed in single-crystal nanowires of Zn, Sn, and Pb of length up to 60 $\mu $m. These nanowires were electrochemically deposited into the pores of anodic aluminum oxide membranes and polycarbonate membranes. Using an \textit{in situ} self-contacting method, single nanowires were electrically contacted on both ends to a pair of macroscopic film electrodes of Au, Sn, and Pb pre-fabricated on both surfaces of the membranes. We observed that superconductivity in the nanowires was strongly suppressed when Au electrodes were used. When superconducting electrodes with higher transition temperatures were used, the nanowires became superconducting at the transition temperatures of the electrodes. We will present measurements of the sample resistance and the $I-V$ characteristics at various temperatures and magnetic fields. Scanning electron microscopy and transmission electron microscopy analyses of the structure and the composition of the nanowires will also be presented. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L30.00011: Electronic Transport of TiO$_{2}$ Nanowire Devices Geetha Dholakia, Steven Kuo, Emily Allen Titanium dioxide (TiO$_{2})$ is a wide band-gap semiconductor with applications in photovoltaics and sensing. Large scale integration of nanowires onto functional devices requires new techniques to manipulate them at the nanoscale. Currently engineering strategies for efficient assembly of nanoscale objects is very limited. Here we report the use of dielectrophoresis to assemble TiO$_{2}$ nanowires onto devices. We use a sol-gel template based synthesis of TiO$_{2}$ nanowires. The nanowires have typical diameters of 100-150 nm and range in length from 3-10 $\mu $m. Devices for two probe and four probe measurements were fabricated by standard lithography. AC dielectrophoresis was used to assemble the TiO$_{2}$ nanowires on devices. A dielectrophoretic translational force and a torque aligns the nanowires onto the devices. FIB assisted platinum deposition on the aligned TiO$_{2}$ nanowires ensures ohmic contacts. Two probe room temperature I-V measurements show a resistivity of 0.22 $\Omega $-cm, which is comparable to 0.26 $\Omega $-cm for a thin film$^{1}$. Temperature dependent transport measurements are being pursued. We have demonstrated an efficient method of assembling and fabricating nanowire device structures. T. Miyata et. al. Thin Solid Films, \textbf{496}, 136 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L30.00012: Conductivity of MgZnO nanoparticles as a function of gas exposure and temperature Chris Berven, Joseph Dick, Leah Bergman, Jesse Huso, John Morrison Changes in the current-voltage (I-V) characteristics of Mg$_{x}$Zn$_{1-x}$O (x = 0.15) nanoparticles as a function gas exposure and temperature are reported. The nanoparticles were prepared using wet chemical techniques on insulating thermally grown SiO$_{x}$ Si substrates. Contact to the nanoparticle film was by gold wires laid across about 2 mm apart. The experiments were performed in a custom-built environmental chamber with the ability to evacuate or introduce various gases. For these experiments, the temperature was tuned over a range of about 300 K to 420 K. Our measurements showed a possible history-dependant behavior in changes of the conductance of the nanoparticle film. When the device was heated to $\sim $120 K in vacuum or in an Ar the current increased by the same amount. When repeated with H$_{2}$, the current increase was less. Initially, the effect was quite pronounced with a relative change by a factor of 20. With repetitions of the experiments, the same effect was observed but to a lesser degree suggesting a saturation phenomena. When the experiment was modified so that the H$_{2}$ gas was introduced at a high temperature to an evacuated chamber the current dropped but not by the same degree as before. A similar response to exposure to H$_{2}$ was found for exposure to O$_{2}$. Possible explanations for the observations will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L30.00013: Noise Characterization of Semiconductor Nanowires C.A. Richter, H.D. Xiong, V.M. Stanford, Wenyong Wang, Xiaoxiao Zhu, Qiliang Li, Woong-Ki Hong, Takhee Lee A thorough understanding of the noise properties of emerging nanoelectronic devices such as those based on semiconductor nanowires is critical because the signal-to-noise ratio is a fundamental factor limiting their performance. We present the systematic characterization of the low frequency noise properties of Si and ZnO semiconductor nanowire field effect transistors. At room temperature, the noise power spectra have a classic 1/f dependence while random telegraph signals (RTS's) are observed in the drain current at 4.2 K leading to a Lorentzian type noise spectra. The RTS's are characterized by estimating a hidden Markov model based on a Gaussian mixture, and quantified using a Viterbi decoder to measure the discrete current switching events. This analysis enables the estimation of parameters such as event lifetime, event amplitudes, and trap cross-section. Under some conditions, three-level switching is observed that can be attributed to two near-interface oxide traps. These data illustrate that the characterization of two- and multi-level RTS's is a valuable tool to determine the energetic and spatial position of individual defects in semiconductor nanoelectronic devices. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L30.00014: Remarkable Effects of Gating on the Photoconductivity of Porphyrin Nanorods C. K. Riley, X. Huang, W. F. Smith, D. E. Johnston, A. T. Johnson Tetrakis(4-sulfonatophenyl) porphine self assembles into well-defined nanorods with intriguing photoelectronic properties.$^{1}$ For example, when light is applied, the conductivity immediately jumps up from zero, then grows further over several hours. This may be due to a light-induced structural change. In recent experiments, we imaged the nanorods with AFM while measuring the photoconductivity; we observed no change in morphology. We also deposited nanorods onto oxidized silicon substrates. We find that the photoconductivity is not sensitive to the \underline {value} of the gate voltage applied to the underlying silicon, but only to \underline {changes} in the gate voltage. The photoconductivity increases when the gate voltage is increased, but then relaxes back to its original level over about one hour. When the gate voltage is decreased, the photoconductivity decreases, and again slowly relaxes back. These results may be associated with structural changes caused by the electric field of the gate, which may affect filling of trap states. $^{1}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L30.00015: Adsorption Kinetics of Alkanes on Purified HiPco Nanotubes Dinesh Rawat, Murat Bulut, Aldo Migone We present results for the adsorption kinetics of methane, ethane and butane on purified HiPco SWNTs. We studied the adsorption kinetics by monitoring the evolution of the gas pressure with time from the instant at which a dose of adsorbate is added to the sample, until the moment at which equilibrium is reached. The waiting times for comparable coverages increase with increasing alkane chain length. For methane and ethane, the equilibration time decreases with increasing fractional coverage. For the butane, on the other hand, the kinetic measurements display a reverse trend: the equilibration times increase with increasing fractional coverage. We speculate that this observed increase in the waiting time is due to a possible reorientation of adsorbed molecules in the film. The observed differences in adsorption kinetics suggest the possibility of using adsorption as a means to achieve the separation of gaseous alkane mixtures. [Preview Abstract] |
Session L31: Graphene: Atomic Structure and Lattice Properties
Sponsoring Units: DCMPChair: Dan Finkenstadt, Naval Research Laboratory
Room: Morial Convention Center 223
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L31.00001: Nano-Engineering Defect Structures on Graphene Mark Lusk, Lincoln Carr We present a new way of nano-engineering graphene using defect domains. These regions have ring structures that depart from the usual honeycomb lattice, though each carbon atom still has three nearest neighbors. A set of stable domain structures is identified using density functional theory (DFT), including blisters, ridges, ribbons, and meta-crystals. All such structures are made solely out of carbon; the smallest encompasses just 16 atoms. Blisters, ridges and meta-crystals rise up out of the sheet, while ribbons remain flat. In the vicinity of vacancies, the reaction barriers to formation are sufficiently low that such defects could be synthesized through the thermally activated restructuring of coalesced adatoms. These defect domains may offer technological applications associated with the confinement and transport of charge. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L31.00002: Solid State Zwitterions realized on carbon graphenic surface ZhaoHui Huang, Vincent Crespi Zwitterions are single molecular species that combine anionic and cationic groups. Here we consider the prospects for introducing the concept of a zwitterion into the solid state, by combining geometrically incompatible anionic and cationic moieties within a single extended structural element whose covalent rigidity frustrates the close approach of the anionic and cationic regions. Specifically, first principles computations for anionic and cationic groups such as NH3 and CO2 covalently attached to a graphenic surface via linker elements demonstrate long-range charge transfer, the hallmark of a zwitterion, while maintaining overall structural integrity. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L31.00003: Atomic structure of corrugated graphene You Lin, Xiang Gu, Ivan Oleynik, Carter White Following its successful isolation, there has been intense interest in a single sheet of graphite, known as graphene, due to its fundamental physical properties as well as its promising applications in nanoelectronic devices. Recent experimental studies of graphene, freely suspended on nanofabricated scaffolding, found appreciable deviations from a perfect two-dimensional crystalline structure [1]. In this presentation, we discuss results of atomistic modeling of graphene with the occurrence of possible corrugated structures examined under the influence of several factors including sample size and temperature. [1] J.C. Meyer, \textit{et al}, Nature \textbf{446}, 60 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L31.00004: \textit{Ab initio} study on atomic and electronic structures of epitaxial graphene Seungchul Kim, Jisoon Ihm, Hyoung Joon Choi, Young-Woo Son Recently, lots of efforts have been devoted to the growth of epitaxial graphene and its geometric and electronic structure measurements. Several models for the interface structure have been suggested but never been successful in explaining the observations from various experiments. Using density functional theory (DFT) calculations, we find an interface atomic structure of graphene on hexagonal silicon carbide which agrees well with the low energy electron diffraction (LEED), scanning tunneling microscopy (STM), as well as angle resolved photo emission spectroscopy (ARPES). Our results clearly resolve the disagreement between $6\times 6$ periodic pattern from STM measurements and the $6\sqrt{3} \times 6\sqrt{3}$ from the LEED measurements. Furthermore, we also investigate the origin of the gap opening at the Dirac point in the present geometry. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L31.00005: Phonon dispersions and vibrational properties of monolayer, bilayer, and trilayer graphene Jia-An Yan, W. Y. Ruan, M. Y. Chou The phonon dispersions of monolayer and few-layer graphene (AB bilayer, ABA and ABC trilayers) are investigated using the density-functional perturbation theory (DFPT). Compared with the monolayer, the optical phonon $E_{2g}$ mode at $\Gamma$ splits into two and three doubly degenerate branches for bilayer and trilayer graphene, respectively, due to the weak interlayer coupling. These modes are of various mode symmetry and exhibit different sensitivity to either Raman or infrared (IR) measurements (or both), and therefore the combination of Raman and IR measurements of the zone-center optical modes should give a clear identification of the layer number as well as the stacking geometry. The splitting is found to be 5 cm$^{-1}$ for bilayer and 2 to 5 cm$^{-1}$ for trilayer graphene. The interlayer coupling is estimated to be about 2 cm$^{-1}$. We found that the highest optical modes at K upshift by about 12 cm$^{-1}$ for bilayer and 18 cm$^{-1}$ for trilayer relative to monolayer graphene. The atomic displacements of these optical eigenmodes are analyzed. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L31.00006: Structural study of the phase evolution of 6H-SiC(0001) by low energy electron microscopy Jiebing Sun, Karsten Pohl, Rudolf M. Tromp, James B. Hannon The surface phase transition of Si-terminated 6H-SiC(0001) upon heat treatment is studied by low energy electron microscopy (LEEM). Bright and dark field imaging demonstrates a direct \textit{in situ} observation of the surface phase evolution, transitions in a sequence from 1$\times $1, 3$\times $3, $\surd $3$\times \surd $3, 6$\surd $3$\times $6$\surd $3 to the graphene phase due to gradually increasing the temperature. Intensity vs. voltage (IV) spectra extracted from single domain diffraction images is used to determine the local surface structure and chemical stoichiometry. Preliminary results from a quantitative dynamical analysis of the LEEM-IV curves show a Si-depleted 1$\times $1 structure and an adatom-trimer-adlayer structure on 3$\times $3 reconstruction. Ongoing work on the structure of the $\surd $3$\times \surd $3 and 6$\surd $3 $\times $ 6$\surd $3 phases is aimed to unraveling the initial growth mechanism of graphene on SiC. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L31.00007: The Initiation of Graphene Growth on SiC(0001)-6H James Hannon, Rudolf Tromp We have studied the evolution of surface morphology on SiC(0001)-6H during annealing at temperatures up to 1250 C using low-energy electron microscopy (LEEM). Surface roughness is dominated by the formation of deep pits or canyons. We show that the canyons form because of the stability of the 6$\surd $3 $\times $ 6$\surd $3 phase, which pins atomic steps during the decomposition of SiC. The density of pits is ultimately determined by how the 6$\surd $3 phase nucleates. Graphene forms preferentially in these pits, where the step density is highest. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L31.00008: Imaging the interface states of epitaxial graphene layers on 6H-SiC G. Sun, Y. Qi, M. Weinert, L. Li Single and bi-layer graphene were epitaxially grown on both the Si- and C-terminations of 6H-SiC. The energy dependence and spatial distribution of their local density of states were investigated using scanning tunneling microscopy and spectroscopy. Of particular interest is the rt3xrt3 reconstructed interface state. Atomically resolved topographs and dI/dV images show clear differences between the single and bi-layer graphene at different length scales. These results will be compared to the electronic and structural properties obtained by first principles calculations. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L31.00009: Interface Studies of Graphene layers on SiC thin films and bulk SiC(0001) Andreas Sandin, J.L. Tedesco, R.J. Nemanich, J.E. (Jack) Rowe A multi-method approach is described based on the well-known high temperature annealing procedure that converts SiC into thin layers of graphite called graphene and has been applied to bulk 6H-SiC(0001) as well as epitaxial thin films of 3C-SiC. Atomic Force Microscopy (AFM) measurements of epitaxial SiC on Si(100) used an UHV Omicron AFM/STM/LEED/Auger multi-probe system. Initial AFM and STM measurements show narrow domains of $\sim $150 nm dimension that have an Auger electron Spectroscopy (AES) signature confirming the formation of graphene on the SiC surface. Our AES measurements show a weak Si KLV transition at $\sim $ 1623 eV which can be used to determine the layer thickness of graphene which is typically $\sim $ 10 {\AA}. Low Energy Electron Diffraction (LEED) also confirms the well-known (6x6) pattern of SiC + graphene reported earlier by several groups.$^{ }$ We find that the conversion temperature appears to be somewhat lower ($\sim $ 1050 -- 1080 \r{ }C) for films of SiC on Si(100) than for bulk SiC (0001) surfaces. This is possibly due to surface dislocations formed during the epitaxial growth process with very large lattice mismatch between SiC and the Si substrate, which provide additional diffusion sites and paths for the surface segregation reaction that forms the graphene layer. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L31.00010: X-ray studies of the rotational fault distributions in multilayer graphene grown on the 4H-SiC$(000\bar{1})$ surface J.E. Mill\'an, J. Hass, F. Varchon, W.A. deHeer, C. Berger, P.N. First, L. Magaud, E.H. Conrad We present x-ray diffraction experiments showing that multilayer graphene grown on 4H-SiC $(000\bar{1})$ (C-face) consists of a high density of rotational stacking faults. The existence of these faults explains why multilayer graphene is electronically similar to an isolated graphene sheet.[1] The faults present themselves as graphite rods rotated through a series of commensurate graphene/SiC angles relative to the SiC rods and as rods shifted due to compressed graphene at the fault boundaries. By analyzing the intensity modulations of these different rods, it is possible to extract information on the distributions of these faults in the film. We will present results from calculations of different models for the fault distribution and compare them against x-ray data for graphene films of different thicknesses. [1] J. Hass, F.Varchon, J. E. Millan-Otoya, M. Sprinkle, W.A. de Heer, C. Berger, P.N. First, L. Magaud, E.H. Conrad (to be published) http://arxiv.org/abs/0706.2134 [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L31.00011: Dispersive Raman Scattering from \textit{n}=1-4 Graphene Layers (\textit{n}GLs) Peter Eklund, Awnish Gupta We present new Raman scattering results from $n$GLs ($n$=1, 2, 3, 4) in the range of 100 - 4500 cm$^{-1}$. Dispersive behavior of Raman peaks was probed at room temperature with 7 laser lines from 1.5-2.7 eV. In addition to the five Raman peaks reported previously, we report on the behavior of five new weaker features that appear in graphene (514.5 nm excitation) at $\sim $1882 cm$^{-1}(\sim $ 125 cm$^{-1}$/eV), $\sim $2035 cm$^{-1 }(\sim $ 177 cm$^{-1}$/eV), $\sim $2218 cm$^{-1 }(\sim $ -43 cm$^{-1}$/eV), $\sim $3174 cm$^{-1 }(\sim $ -40 cm$^{-1}$/eV) and $\sim $4069 cm$^{-1}(\sim $35 cm$^{-1}$/eV). The value in ( ) is the respective band dispersion, or shift with peak position per eV change in the excitation energy. The band dispersion is connected with the ratio of the phonon to Fermi velocity and stems from the double resonance (DR) scattering. New Raman bands that are only observed for $n$GLs ($n>$1) are found at $\sim $1510 cm$^{-1}(\sim $ 16 cm$^{-1}$/eV) and 1737 cm$^{-1}$ ($\sim $ 10 cm$^{-1}$/eV). The identities of the new peaks will be discussed based on DR scattering and the phonon dispersion curve of graphene. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L31.00012: The Thickness Dependence of the Graphene Oxidation Li Liu, Sunmin Ryu, Michelle Tomasik, Elena Stolyarova, Michael Steigerwald, Mark Hybersten, Louis Brus, George Flynn Single-, double-, and triple-layer graphene sheets were heated in an oxygen atmosphere at various temperatures generating nano-sized holes in the sheets. Both AFM topography and Raman spectroscopy indicate that the oxidative reactivity of single-layer sheets is greater than that of thicker sheets. The distribution of hole sizes and STM topography studies suggest that the oxidation reaction is initiated at the pristine carbon surface. Vertical etching of carbon atoms from the graphene surface occurred at a much lower temperature than that from a highly oriented, multi-layer pyrolytic graphite crystal. The mechanism for this thickness dependence of reactivity will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L31.00013: UHV STM/STS and AFM study of oxidized graphene sheets Deepak Pandey, Richard Piner, Ronald Reifenberger Exfoliated oxidized graphene sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of $\sim $5-10 $\mu $m. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a 0.26 nm $\times $ 0.42 nm unit cell over small areas. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduced density of electron states within $\pm $0.1 V around zero bias. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L31.00014: Scanning Tunneling Spectroscopy (STS) studies of graphene films on an insulating substrate Elena Stolyarova, Li Liu, Mark Hybertsen, Philip Kim, Tony Heinz, George Flynn Scanning Tunneling Spectroscopy has been utilized to study the differences between the electronic structure of a three-dimensional graphite crystal and its two-dimensional building block, graphene. Single and few-layer graphene samples were isolated on a non-conductive silicon dioxide substrate and contacted at the edges with a gold electrode. For single layer flakes current-voltage dependent I(V) curves, recorded at 4.6 K under Ultra-high Vacuum (UHV) conditions, show no additional features (for states far from the Fermi energy) that might be considered characteristic of a weak interaction between graphene and the substrate. No significant spatial inhomogeneity of local sample properties was observed. Evolution of spectroscopic curves as a function of graphene layers will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L31.00015: Generation of atomic scale defects in graphene by high-energy positive-ion bombardment Daniil Stolyarov, Elena Stolyarova, George Flynn, Karl Kusche, Igor Pavlishin, Igor Pogorelsky, Peter Shkolnikov , Vitaly Yakimenko We have demonstrated controllable defect generation in graphene (a single layer of a graphite crystal) and in few atomic layer thick graphitic films. Graphene flakes deposited on a silicon dioxide substrate by mechanical exfoliation were bombarded by a collimated high-energy (1 MeV) particle beam consisting of protons and positively charged ions. This beam was produced by Target Normal Sheath Acceleration (TNSA) upon irradiation of a metal foil with a terawatt CO2 laser pulse. Ions and protons with different mass and energy were separated by a permanent magnet installed between the laser target and the sample. After the exposure the graphene flakes were examined by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). Irradiation of graphene films with both protons and positive ions results in the formation of atomic-scale defects without mesoscopic damage to the flake. The density of observed defects depends strongly on the number of atomic layers. This method can be used to modify single chemical bonds in graphene films and to engineer carbon based devices and sensors with tailored properties. [Preview Abstract] |
Session L32: Focus Session: Spin Transfer Torque II
Sponsoring Units: GMAG DMP FIAPChair: Jordan Katine, Hitachi
Room: Morial Convention Center 225
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L32.00001: Time-Resolved X-ray Microscopy for Direct Observation of Spin-Torque and Oersted-Field Driven Vortex Gyration M. Bolte, G. Meier, L. Bocklage, A. Drews, B. Krueger, T. Tyliszczak, A. Vansteenkiste, B. Van Waeyenberge, K.-W. Chou, H. Stoll Due to their symmetry, magnetic vortices are ideal candidates for studying the influence of the spin-transfer torque on the local magnetization. The out-of-plane magnetization of the vortex, the vortex core, can be excited to gyrate around its equilibrium position by in-plane magnetic fields or spin-polarized currents. Here we present results from time-resolved X-ray microscopy on permalloy squares with a vortex in the center. Spin-polarized currents with densities of $4.7~-~12~\cdot $~10$^{10}$~A/m$^2$ are laterally driven through the permalloy sample, and the gyration is imaged for different phases of the ac-excitation. The results are compared to micromagnetic simulations, to good agreement. For vortices having opposite chirality a chirality-dependent phase shift of $40^{\circ}$ is observed that is attributed to Oersted fields from the spin-polarized current. An analytical model estimates corresponding field strengths of $40~\mu$T. This study confirms our assumption that Oersted fields from spin-polarized currents cannot be neglected in spin-torque experiments and shows the sensitivity of the measurement technique. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L32.00002: Time-resolved X-ray Imaging of Spin-Torque-Induced Vortex Oscillation Xiaowei Yu, Vlad Pribiag, Yves Acremann, Venkatesh Chembrolu, Ashwin Tulapurkar, Tolek Tyliszczak, Zhipan Li, Robert Buhrman , Joachim Stohr, Hans Siegmann Recent transport measurements demonstrated a persistent oscillation of a magnetic vortex isolated in a nanoscale spin valve structure driven by a d. c. spin-polarized current [1]. The magnetic information is inferred from the giant magnetoresistance (GMR) signal, which depends on the relative average magnetization of the two magnetic layers. Here, we report spatially resolved measurements of the vortex oscillation driven by spin-transfer torque by using advanced x-ray imaging technique. The microwave-frequency vortex oscillation is synchronized to the fast x-ray pulses. Motion images with 70ps time resolution and 30nm spatial resolution reveal the complicated dynamics underlying the previously observed oscillating GMR signal. [1] V. S. Pribiag, I. N. Krivorotov, G. D. Fuchs, P. M. Braganca, O. Ozatay, J. C. Sankey, D. C. Ralph, and R. A. Buhrman, Nature Physics 3, 498 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L32.00003: Spin-current induced magnetic excitations in single magnetic layer nanopillars Bernd Beschoten, Nicolas M\"usgens, Mark Weidenbach, Eva Maynicke, Coen Smits, Matthias B\"uckins, Joachim Mayer, Gernot G\"untherodt We investigate current-induced spin-wave excitations in Cu/Co/Cu single magnetic layer nanopillar devices with asymmetric Cu leads by means of transport and microwave probes at room temperature. The thin film stack is deposited by MBE in prefabricated nanostencil masks with lateral dimensions below 100 nm. At high current densities, we observe narrow excitations (bandwidth $\sim $ 100MHz) and higher harmonics for magnetic fields perpendicular to the layers. The frequency increases with increasing current and magnetic field, which indicates an out-of plane precessional mode as found in bilayer systems (e.g., Kiselev et al., PRL 93, 036601(2004)). Furthermore, we observe frequency jumps as a function of both current and magnetic field, which might originate from transitions between different localized nonlinear spin-wave modes. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L32.00004: Large angle out of plane steady precession induced by spin-transfer with perpendicular to plane polarizer Invited Speaker: The dynamics of a ferromagnetic system is characterized by a conservative precession torque, as well as a non-conservative damping torque. The damping torque is responsible for the realignment of the magnetisation with the equilibrium direction after excitation. Recently it has been shown that the damping torque can be compensated by a spin transfer torque that is due to the interaction between a spin polarized current and the local magnetization. This additional spin transfer torque can lead to auto-oscillations of the magnetization close to constant energy trajectories. The potential exploitation of such large angle auto-oscillations for tuneable microwave devices is currently driving many research efforts. For an in-plane magnetized thin film with uniaxial anisotropy, two types of constant energy trajectories exist which are commonly called in-plane precession (IPP), where the magnetization oscillates around the in-plane energy minimum, and out of plane precession (OPP) where the magnetization oscillates around the out of plane energy maximum [1]. IPP and OPP oscillations differ substantially in their dependence of frequency and amplitude as a function of current and/or applied bias field. In many experiments so far, IPP precessions have been obtained at the threshold current using in-plane magnetized spin valve structures. However, from an applications point of view it will be of interest to excite OPP oscillations since they will lead to a larger output signal than IPP oscillations. Here, we will present experimental evidence of large angle OPP oscillations using a spin torque oscillator that contains a perpendicularly magnetized polarizing layer and an in-plane magnetized analyzing layer in addition to a planar free layer [2]. We will show that OPP oscillations are induced at the threshold current for moderate current densities of 9x10$^{6}$ A/cm$^{2}$. The experimental current-field state diagram as well as the dependence of the frequency vs. current and applied bias field is in good qualitative agreement with macrospin and micromagnetic simulations. Furthermore, due to the planar analyzer, there exist IPP oscillations which allow a direct comparison of the OPP and IPP precession amplitudes. [1] A. N. Slavin, V. S. Tyberkevich Phys. Rev. B 72, 94428 (2005) [2] Houssameddine et al, Nature Materials 6, 441 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L32.00005: Current-Induced Magnetization Switching with a Spin-Polarized Scanning Tunneling Microscope Matthias Bode, Stefan Krause, Luis Berbil-Bautista, Gabriela Herzog, Roland Wiesendanger The understanding of current-induced magnetization switching is in the focus of many ongoing investigations since switching the magnetization by the injection of a spin-polarized current rather than by magnetic fields may open the gateway for new data storage technologies at much higher bit densities. We show how individual superparamagnetic Fe nanoislands with typical sizes of 100 atoms can be addressed and locally switched using a magnetic scanning probe tip. We demonstrate current-induced magnetization reversal across a vacuum barrier and combine it with the ultimate resolution of spin-polarized scanning tunneling microscopy. This technique allows us to clearly separate and quantify three fundamental contributions that are involved in magnetization switching, i.e.\ current-induced spin torque, heating the island by the tunneling current, and Oersted field effects. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L32.00006: Current-Induced Magnetoresistance in Antiferromagnetic Spin Valves Z. Wei, A. Sharma, J. Bass, M. Tsoi Influence of the magnetic state of a ferromagnet (F) on its electronic transport properties has been found in various phenomena, including giant magnetoresistance (GMR) in magnetic multilayers, where the relative orientation of the magnetic moments of F-layers affects the current flow. MacDonald and co-workers recently predicted that a corresponding effect - antiferromagnetic GMR (AGMR) - should exist in structures where F-layers are replaced by antiferromagnets (AFM). To test this prediction, we measured the (closely) current-perpendicular to plane (CPP) magnetoresistance (MR) of three types of AFM spin-valve multilayers: (I) AFM/N/AFM, (II) F/AFM/N/AFM, and (III) F/AFM/N/AFM/F, with a non-magnetic (N) layer between the two AFM layers. We saw no MR in samples of type I or II at any current density j, or of type III when j was small. But large enough j $\sim $ 10$^{13}$ A/m$^{2}$ applied to type III multilayers gave small positive MRs (largest resistance at high field). As these MRs are inverted from the usual GMR associated with the F-layers, they must be due to the AFM layers, and thus be an AGMR. We will describe how this AGMR varied with applied current j and AFM layer thickness [arXiv:0711.0059]. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L32.00007: RF Assisted Spin Transfer Switching in Nanopillar Spin-Valves S.H. Florez, J.A. Katine, M. Carey, L. Folks, O. Ozatay, B.D. Terris We study at low temperature spin transfer torque (STT) driven free-layer magnetization reversal in current perpendicular to plane (CPP) spin valves with in-plane magnetization. The precessional frequencies of the direct current driven pre-switching modes were measured. Based on this characterization we compare the pre-switching and switching behavior, when driven by direct currents only and in the presence of an additional rf current bias. We find interesting rf induced dynamics such as frequency locking as well as effects on the critical switching boundary. These effects appear for applied frequencies close to the dc-only driven pre-switching resonance frequencies. In particular, we observe a reduction in the critical current for switching when applying rf with frequencies slightly below this range. Macrospin simulations (using Slonczewski STT) reproduce well our experimental data and serve as a basis for the development of a phenomenological model that describes the observed behavior. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L32.00008: Noise Reduction and Other Effects of Rare-Earth Doping in Magnetic Spin-Transfer Systems Eric Ryan, O. Ozatay, P. M. Braganca, N. C. Emley, D. C. Ralph, R. A. Buhrman, J. A. Katine At sufficiently large current densities, spin-torque can excite high frequency dynamics in magnetic trilayers. While this effect can lead to interesting new applications and phenomena, it also introduces detrimental effects, such as creating high frequency noise in next generation GMR read heads. Light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase the damping parameter $\alpha $ by several orders of magnitude [1], which should suppress spin-torque effects. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.004 um$^{2}$ Py/Cu/Py nanopillar spin valves with 0 and 2{\%} Tb in the free layer. We will present data from these devices showing that critical switching current, coercive field, and high frequency noise suppression are all increased in the presences of terbium, and that these effects have a strong and extended temperature dependence. Proper choice of materials can lead to even larger effects at room temperature and beyond, an important regime for technological applications. We will explore the mechanism of the enhanced damping effects as a function of temperature in terbium-doped CoFe and NiFe films and devices. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L32.00009: Spin-torque-driven ferromagnetic resonance of Co/Cu/NiCo spin valves Wenyu Chen, Gregoire de Loubens, Jean-Marc Beaujour, Andrew Kent, Jonathan Sun Spin-torque-driven ferromagnetic resonance is a quantitative tool for studying spin-transfer interactions in nanojunctions that enables tests of microscopic models of spin transport [1]. Using this method we have studied Co/Cu/NiCo spin values, in which the NiCo free layer has perpendicular magnetic anisotropy. Perpendicular field swept resonance lines were measured under low amplitude GHz current excitation. The resonance field and linewidth were measured as a function of rf frequency and dc current bias, from which magnetic anisotropy constants and damping parameters were determined [2]. The magnitude of spin transfer torque, d$\tau $/dI, was estimated from both the zero dc bias resonance amplitude and from the change of the resonance linewidth with dc current. These two sets of results are in agreement with each other, and show a sinusoidal dependence of the torque on the angle between the Co and NiCo layer magnetizations in the range studied, 60$^{o}$ to 80$^{o}$. The resulting torque magnitude will be discussed in the context of theoretical models of spin transfer in metallic structures. [1] J. C. Sankey et al., Nature Physics, doi:10.1038/nphys783 [2] W. Chen et al., arXiv/0711.0405 [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L32.00010: Coherent control of magnetic moment dynamics and switching via spin momentum transfer L. Ye, S. Garzon, T.M. Crawford, R.A. Webb, M. Covington, S. Kaka We have measured the switching probability of CoFe/Cu/CoFe nanopillars driven by shaped current waveforms consisting of two $\sim $30ps FWHM pulses with adjustable amplitudes and delay. We observe oscillations in the switching probability as the delay is varied over the timescale of a free precession cycle, demonstrating large sensitivity to precise pulse timing. We also observe a non-monotonic increase in the switching probability as the amplitudes of the two pulses are simultaneously increased, showing that employing larger current pulses does not necessarily increase switching probabilities. Our data shows that two pulses with precisely adjusted amplitudes and delay can switch a nanopillar device with higher probability than a single pulse with equivalent total power, and that $\sim $100{\%} switching probability can be obtained even with short ($\sim $30ps FWHM) pulses. Our results suggest a new set of techniques for studying coherent time-domain magnetic moment dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L32.00011: Magnetic Dynamics of Single-Domain Planar Spin-Transfer Devices Ya. B. Bazaliy We study spin-transfer devices with dynamic magnets characterized by large easy-plane anisotropy. This situation is standard for planar devices where it arises due to the shape anisotropy. Dominating easy-plane anisotropy keeps the motion of the magnetic moment close to the easy plane, with small out-of-plane deviations. As a result, it is possible to approximately describe magnetization vector by the in-plane angle and derive an effective one dimensional equation for that angle in the absence [1] and in the presence [2] of spin-transfer torques. Effective description maps a spin-transfer device problem onto a problem of an ``effective particle'' moving in external potential with variable friction coefficient. The advantage of such a description is that the motion of the effective particle can be qualitatively understood by applying the usual energy conservation and dissipation arguments. We show how the effective description produces analytic results for current induced precession states [3] and predicts unconventional ``stabilization by repulsion'' of static states [2]. // [1] C. J. Garcia-Cervera, Weinan E, J. Appl. Phys. 90, 370 (2001). [2] Ya. B. Bazaliy, Phys. Rev. B 76, 140402(R) (2007). [3] Ya. B. Bazaliy, arXiv:0705.0508, to be published in Appl. Phys. Lett. (2007). [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L32.00012: Electron transport driven by nonequilibrium magnetic textures Yaroslav Tserkovnyak, Matthew Mecklenburg Spin-polarized electron transport driven by inhomogeneous magnetic dynamics is discussed in the limit of large exchange coupling. Electron spins rigidly following the time-dependent magnetic profile experience spin-dependent fictitious electric and magnetic fields. We show that the electric field acquires important corrections due to spin dephasing, when one relaxes the spin-projection approximation. Furthermore, spin-flip scattering between the spin bands need to be taken into account in order to calculate voltages and spin accumulations induced by the magnetic dynamics. A phenomenological approach based on the Onsager reciprocity principle is developed, which allows us to capture the effect of spin dephasing and make connection to the well studied problem of current-driven magnetic dynamics. In addition, we relate and generalize the results that recently appeared in literature. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L32.00013: Spin-Diffusion Lengths in Ag(4{\%}Sn) and Cu(2{\%}Ge) alloys Amit Sharma, Brandon Richard, Quinton Fowler, Reza Loloee, William Pratt Jr., Jack Bass Alloying Ag with a little Sn, or Cu with a little Ge, greatly increases elastic scattering of electrons---i.e., greatly decreases the electron mean-free-path (mfp), but does not produce much spin-flipping---i.e., leaves the electron spin-diffusion length, $l_{, }$relatively long. Thus, dilute AgSn and CuGe alloys were used to study effects of changing the mfp on current-perpendicular-to-plane (CPP) magnetoresistance [1] and current-induced magnetization switching (CIMS) [2], while leaving spin-flipping weak. Published transport data in dilute AgSn and CuGe alloys give only lower bounds for $l$ [3-5]. We find $l$ = 34 $\pm $ 4 nm for Ag(4{\%}Sn) and $l$ = 125 $\pm $ 10 nm for Cu(2{\%}Ge). [1] K. Eid et al., J. Magn. Magn. Mat. \textbf{224}, L205 (2001). [2] N. Theodoropoulou et al., Phys. Rev. B (rapid comm.) in press. [3] S.-F.Lee et al., J. Magn. Magn. Mat. \textbf{118}, L1 (993). [4] J. Bass et al, Mat. Sci. and Eng. \textbf{B31}, 77 (1995). [5] J. Bass and W.P. Pratt Jr., J. Phys. Cond. Matt. \textbf{19}, 183201 (2007). [Preview Abstract] |
Session L33: Focus Session: Spins in Quantum Dots
Sponsoring Units: GMAG FIAP DMPChair: Alexander Efros, Naval Reseach Laboratory
Room: Morial Convention Center 224
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L33.00001: Ultrafast coherent optical manipulation of a single electron spin in a quantum dot M.H. Mikkelsen, J. Berezovsky, N.G. Stoltz, L.A. Coldren, D.D. Awschalom A key ingredient for spin-based quantum information processing is the coherent rotation of a spin state on timescales much faster than the spin coherence time. By applying off-resonant, picosecond-scale optical pulses, we demonstrate the coherent rotation of a single electron spin in a GaAs quantum dot (QD) through arbitrary angles up to $\pi$ rad.\footnote{ J. Berezovsky, M.H. Mikkelsen, N.G. Stoltz, L.A. Coldren, D.D. Awschalom, {\em Submitted}, (2007)} We directly observe this spin manipulation using time-resolved Kerr rotation spectroscopy\footnote{ M.H. Mikkelsen, J. Berezovsky, N.G. Stoltz, L.A. Coldren, D.D. Awschalom, {\em Nature Physics} \textbf{3}, 770 (2007)} at T=10K. Measurements of the spin rotation as a function of laser detuning and intensity confirm that the optical Stark effect is the operative mechanism and the results are well-predicted by a model including the electron-nuclear spin interaction. Using short tipping pulses and QDs with long spin coherence times, this technique enables one to perform a large number of operations within the coherence time. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L33.00002: Coherent Population Trapping of an Electron Spin in a Singly-Charged Quantum Dot Xiaodong Xu, Bo Sun, Paul R. Berman, Duncan G. Steel, Allan S. Bracker, Dan Gammon, Lu J. Sham When two radiation fields drive coupled transitions in a three-level lambda system, a steady-state coherent superposition of the ground states can be formed in which the system is totally decoupled from the applied fields, a process that is sometimes referred to as coherent population trapping (CPT). Here we report the demonstration of the CPT of an electron spin in a singly-charged quantum dot. By applying a magnetic field in the Voigt geometry, we create a three-level lambda system, formed by two Zeeman sublevels of an electron spin and an intermediate trion state. As we tune the driving and probe fields to the two-photon Raman resonance, we observe a pronounced dip in the probe absorption spectrum, indicating the CPT of the electron spin. An arbitrary superposition of the electron spin states can be prepared by varying the ratio of the Rabi frequencies between the driving and probe fields. This work shows that spin based semiconductor quantum dot systems can exhibit the same interesting quantum behavior that has been found in simple atom-field systems. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L33.00003: Spin polarized current generation from quantum dots without magnetic fields Jacob J. Krich, Bertrand I. Halperin An unpolarized charge current passing through a chaotic quantum dot with strong spin-orbit coupling can produce a spin polarized exit current without magnetic fields or ferromagnets. If there is only one channel in the output lead, no spin polarization can be produced. We use random matrix theory to estimate the typical spin polarization as a function of the number of channels in each lead, finding rms spin polarizations up to 45\% with one input channel and two output channels. Finite temperature and dephasing both suppress the effect, and we include dephasing effects using a new variation of the third lead model. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L33.00004: The role of electron-hole exchange in the spin mixing of excited-shell excitons in quantum dots Stefan C. Badescu, Thomas L. Reinecke In semiconductor quantum dots (QDs) the long-range electron-hole exchange is known to lift the spin degeneracy of ground orbital state excitons for small lateral asymmetries. In the context of solid-state quantum computing recent developments in optical manipulation of spin involve the excited shells of charged excitons in single and coupled QDs. We demonstrate here that this electron-hole exchange is important in the mixing of spin states of excitons in higher shells in QDs, e.g., the p-shell exciton. In contrast to the ground states, no lateral asymmetry is necessary for simultaneous reversal of an electron and a hole spin in these excited shells. For charged excitons, this interaction competes with the asymmetric exchange between electrons or between holes. We discuss the optical selection rules and the role of decoherence from phonons for the dynamics of excited excitons. We conclude that the e-h exchange is an important factor in designing logical gates with excited states. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L33.00005: Theoretical Study of Dynamical Nuclear Spin polarization in Semiconductor Quantum Dots Chia-Wei Huang, Xuedong Hu Dynamical nuclear spin polarization (DNSP) can be achieved optically in a semiconductor quantum dot (QD) via the hyperfine interaction between the optical oriented electron and nuclear spins. Here we explore several mechanisms, combined with the hyperfine interaction, to investigate how efficiently they can transfer the electron spin polarization to nuclei. Specifically, to ensure energy conservation during the spin transfer, we consider both a combination of electron spin-orbit and electron-phonon interactions, and cotunneling processes. Based on these interactions we evaluate the buildup time for DNSP in a semiconductor QD. Our results show that the DNSP buildup time of spin-orbit associated with the hyperfine interaction, accompanied by phonon emission, is of the order of seconds. This is much longer than that of the recent experimental findings. [1] The calculated nuclear spin polarization rate also shows a different Zeeman-energy-dependence from what is observed experimentally. [1] We thus conclude that hyperfine interaction combined with electron level broadening due to cotunneling processes between the QD and the nearby reservoir is more likely to be responsible for the fast buildup of nuclear polarization in experiments. [1] P.Maletinsky, A.Badolato, and A. Imamoglu, Phys. Rev. Lett. 99, 056804 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L33.00006: Emission spectra from InAs quantum dots as a function of carrier spin polarization; experiment and theory. I. Khan, T. Ali, M. Yasar, A. Petrou, G. Kioseoglou, C.H. Li, A.T. Hanbicki, B.T. Jonker, M. Korkusinski, P. Hawrylak We have studied the emission spectra from InAs quantum dots (QDs) under the following conditions: a) recombination of spin-polarized electrons with unpolarized holes; b) recombination of spin-polarized electrons with spin-polarized holes. In the first experiment (a), we recorded the electroluminescence from Fe/GaAs n-i-p spin-LEDs which incorporate a single layer of InAs QDs at the center of the intrinsic region of the device. In the second experiment (b), we studied the photoluminescence spectra from a similar undoped heterostructure using optical pumping. In both experiments, in addition to the typical shell structure of the emission spectra, new polarization maxima are observed on the high and low energy sides of the shell emission features as a result of the imbalance between the two spin populations of the carriers. These results are compared with a calculation for the case of two and six electron-hole occupancy of the QDs. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L33.00007: Determination of spin polarization in InAs/GaAs self-assembled quantum dots F. G. G. Hernandez, T. P. Mayer Alegre, G. Medeiros-Ribeiro The electronic spin in a quantum dot (QD) has been proposed as a candidate two-level system for qubit implementation in a quantum information processing scheme (QIP). For semiconductor quantum dots, the optical transition selection rules provide a natural tool for a direct and quantitative measurement of the electronic spin polarization. In contrast to the optical scheme, electrical readout of the electronic spins orientation do not require any knowledge of hole polarization. Here we perform magneto-capacitance measurements of QDs embedded in a Metal-Insulator-Semiconductor (MIS) capacitor structure. A statistical approach for the population of the spin levels allows one to study the spin orientation in the limit of comparable magnetic and thermal energies. The experimental data are analyzed in terms of the addition energies as measured by magneto-capacitance spectroscopy. The amount of polarization was inferred by measuring the addition energies of electrons sequentially loaded in QDs. In this experiment, we found an electron spin polarization higher than 50\% for $B_{[001]} = 4 $T. Finally, by including the g-tensor anisotropy the angular dependence of spin polarization with the magnetic field B orientation and strength could be explained. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L33.00008: Non-volatile spin bistability based on ferromagnet-semiconductor quantum dot hybrid nanostructure Yuriy Semenov, Hani Enaya, John Zavada, Ki Wook Kim Electrical manipulation of a memory cell based on bistability effect in a nanostructure consisting of a semiconductor quantum dot (QD) adjoining on opposite sides with a dielectric ferromagnetic layer (DFL) and a reservoir of itinerant holes is investigated theoretically. The operating principle is based on the interplay between the exchange field of the holes \textbf{B}$_{h}$ acting on the magnetization vector of the DFL \textbf{M} perpendicular to structure plane and the anisotropy field \textbf{B}$_{a}$ which aligns \textbf{M} along the plane. At low hole population of the QD (B$_{h}<<$B$_{a})$, \textbf{M} is still in plane direction (first stable state ``0''). If an applied bias populates the QD sufficiently (B$_{h}>$B$_{a})$, the subsequent \textbf{M} rotation will decrease the hole energy in the QD; hence the high hole population state is sustained (second stable state ``1'') under a fixed electro-chemical potential set by the reservoir even after bias is removed. The analysis of bit retention time of the proposed memory demonstrates the feasibility of the device with lateral QD size at least 30 nm under room temperature operation. Another advantage is the extremely small dissipative energy for Write/Erase operations. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L33.00009: Method for Full Bloch-Sphere Control of a Localized Spin in a Quantum Dot via a Single Electrical Gate Joseph Pingenot, Craig E. Pryor, Michael E. Flatt\'e Manipulating individual spins in solids requires quickly and coherently reorienting localized spins while leaving neighboring spins unaffected. Difficulties confining oscillating magnetic fields have motivated alternate approaches that use electric fields to change the local magnetic environment, including moving an electron within a hyperfine field gradient or fringe-field gradient. Higher temperatures require spins to be localized in much smaller quantum dots, where these techniques are less effective. In contrast, g-tensor manipulation techniques[1] couple an electric field to the spin via the spin-orbit interaction, and should be scalable to small dots with large confinement. Here we calculate the g-tensor of a single electron in a small quantum dot and show the symmetry of its electric field dependence permits full Bloch sphere control of the spin using an electric field applied in a single direction. We find the spin manipulation frequency of an InAs/GaAs QD in 1 Tesla exceeds 150 MHz. We acknowledge support of an ONR MURI and an NSF NIRT. [1] Kato et al. Nature 299, 1201 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L33.00010: Enhancement of spin lifetime in quantum well lasers using interface fluctuation quantum dots Ryan Woodworth, Ari Mizel, Rusko Ruskov, Gerald Mahan Research on semiconductor heterostructures provides many insights into the next generation of optoelectronic devices. In particular, the gain of a microdisk laser seems to be enhanced by the presence of a long-lived optical cavity mode. Here we analyze a recent experiment using GaAs-AlGaAs microdisks with interface fluctuation quantum dots. A numerical simulation shows enhancement of spin dephasing time in the conduction band due to exchange scattering and D'yakonov-Perel' coupling. Possible applications to quantum computing are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L33.00011: Piezomagnetic quantum dots Ramin Abolfath, A.G. Petukhov, Igor Zutic We study the influence of deformations on magnetic ordering in quantum dots doped with magnetic impurities. The reduction of symmetry and the associated deformation from circular to elliptical quantum confinement lead to the formation of piezomagnetic quantum dots \footnote {R. M. Abolfath, A. G. Petukhov, and I. Zutic, arXiv:0707.2805.}. The strength of elliptical deformation can be controlled by the gate voltage to change the magnitude of magnetization, at a fixed number of carriers and in the absence of applied magnetic field. We reveal a reentrant magnetic ordering with the increase of elliptical deformation and suggest that the piezomagnetic quantum dots can be used as nanoscale magnetic switches. Finally, we discuss thermodynamic stability of piezomagnetism in such quantum dots. [Preview Abstract] |
Session L35: Optoelectronic Devices and Applications
Sponsoring Units: FIAPChair: Fatima Toor, Princeton University
Room: Morial Convention Center 227
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L35.00001: High Efficiency Quantum Cascade Lasers Matthew Escarra, Anthony Hoffman, Scott Howard, Kale Franz, Aishwarya Sridhar, Claire Gmachl Quantum cascade (QC) lasers have proven to be of great interest as powerful and versatile mid-infrared light sources. However, improvement in the wall-plug efficiency of these sources at room temperature and under continuous wave operation is critical to their development across a broad range of sensing applications. The internal, current, voltage, and optical efficiencies all must be maximized. Several different approaches must be taken in conjunction. We will focus primarily on several QC laser designs with low voltage defect. Low voltage defect quantum designs with heterogeneous injector regions have shown efficiencies as high as 13.9{\%} from a single facet in 80K, pulsed operation. Performance at high temperatures can be improved by better confinement of electrons in the upper laser level. The addition of high-reflection and anti-reflection coatings to opposing facets has greatly improved the optical efficiency. Temperature performance can also be improved through InP lateral regrowth, epi-side down mounting, and electroplated gold top contacts. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L35.00002: High-speed wavelength conversion in quantum-dot and quantum-well semiconductor optical amplifiers David Nielsen, S. L. Chuang, N. J. Kim, D. Lee, S. H. Pyun, W. G. Jeong, C. Y. Chen, T. S. Lay All-optical wavelength conversion is an important technology for advanced wavelength division multiplexed networks. The carrier localization available in quantum dots, due to the relatively slow carrier capture and escape times compared to intersubband relaxation in quantum wells, makes it possible to achieve efficient wavelength conversion through the non-linear optical process of four-wave mixing due to enhanced spectral hole burning. To examine the various carrier dynamics we experimentally investigate four-wave mixing in both quantum-dot and quantum-well optical amplifiers. Our results show superior conversion efficiency in a quantum-dot device compared to a quantum well device with identical gain at pump-probe detunings between 100 GHz and 1 THz, and a small-signal modulation bandwidth $>$ 25 GHz. Cross-gain modulation measurements were performed as well and show a much smaller bandwidth of 1 GHz indicating that four-wave mixing is superior for high-speed signals. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L35.00003: Magnetic field controlled sub-THz emission in Quantum Cascade Lasers A. Wade, G. Fedorov, D. Smirnov, S. Kumar, Q. Hu, B.S. Williams We report on the observance of strong multi-wavelength terahertz (THz) radiation in GaAs/AlGaAs based Quantum Cascade Lasers (QCL). The QCL was measured in a strong magnetic field, up to 31 T, applied parallel to the growth axis. The lasing intensity exhibits oscillations due to magnetophonon resonance and Landau Level interaction resulting in a strong increase in the optical power and reduction of the current threshold. By applying the appropriate magnetic field and bias, lasing emission is obtained between 3.27 to 2.61 and 1.53 to 0.68 THz. This demonstrates that a magnetic field offers the unprecedented possibility to control the QCL emission frequency and achieve lasing action as low as 0.68 THz. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L35.00004: Effect of Waveguide Side-Wall Roughness on Quantum Cascade Laser Performance Fatima Toor, Hao Liu, Deborah Sivco, Claire Gmachl Waveguide loss in optical devices can be attributed to two main factors, intrinsic material loss (e.g. free carrier absorption) and scattering loss from imperfections (e.g. fabrication errors). To-date most work for determining the waveguide loss of quantum cascade lasers (QCLs) is concentrated on determining the intrinsic material loss, but there is very little research work done on determining the effect of fabrication errors such as side-wall roughness on QCL performance. Here we report on an experimental and modeling study to determine the effect of side-wall roughness on QC laser performance. The work involved designing and fabricating waveguides with different amounts of side-wall roughness. Measurements were then taken to determine the effect of waveguide side-wall roughness on laser performance parameters like threshold current density and slope efficiency. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L35.00005: Optical coherence imaging using the phase coherent photorefractive effect in ZnSe quantum wells A. Kabir, M. Ajward, S. Tripathy, H.P. Wagner We have performed depth-resolved optical coherence imaging (OCI) of both stationary and moving objects using the exciton resonant phase coherent photorefractive (PCP) effect in ZnSe quantum wells (QWs). PCP QWs operate without electrical contacts thus avoiding elaborate sample processing and avoiding sample destruction due to Joule heating. In addition, the PCP effect exploits the coherence of excitons in OCI experiments thus enabling 3D images of reflecting objects with a depth resolution of $\sim $15 $\mu $m using 90 fs pulses. Due to the high diffraction efficiency of $\eta $ = 5x10$^{-4}$ in our PCP ZnSe QWs we are able to record still images at very low intensities ($\sim $500 $\mu $W/cm$^{-2})$. The OCI movies of moving objects were recorded using a camcorder with frame rates of 60 and 180 Hz. The shortest possible time resolution in these experiments is determined by the decay time of the PCP electron grating being in the $\sim $10 $\mu $s range. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L35.00006: Integrated Plasmonic Terahertz Detector and a Gate Controlled Schottky Barrier$^{1}$ G.C. Dyer, E.A. Shaner, M.C. Wanke, J.L. Reno, G.R. Aizin, J.D. Crossno, S.J. Allen We have successfully fabricated and tested a plasmonic terahertz detector that integrates a gate controlled lateral Schottky diode$^{2}$. As demonstrated in prior work$^{3-4}$, a grating gated two dimensional electron gas can be the basis for a finely tuned terahertz detector. The addition of an independently biased gate adjacent to the drain yields striking Schottky-like behavior and offers increased sensitivity when biased to pinch off. We present measurements and models of the Schottky-like I-V characteristics, resonant plasmonic response ($\sim $50 GHz width), and bias-dependent terahertz rectification. The monolithic Schottky diode plasmonic detector points the way to a plasmonic detector with increased sensitivity. $^{1}$Supported through NSF NIRT Grant No. ECS0609146, and in collaboration with Sandia, 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. $^{2}$V. Ryzhii, et al., Jap. J. Appl. Phys. \textbf{45}, L1118 (2006). $^{3}$E.A. Shaner, et al., Appl. Phys. Lett. \textbf{90}, 181127 (2007). $^{4}$G.R. Aizin, et al., Appl. Phys. Lett. \textbf{91}, 163507 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L35.00007: High precision frequency characterization of THz quantum cascade lasers by heterodyne mixing Mark Lee, Michael Wanke, Maytee Lerttamrab, Erik Young, Albert Grine, John Reno, Robert Dengler, Peter Siegel Terahertz quantum cascade lasers (QCLs) have been used together with a monolithic planar Schottky diode receiver to study the heterodyne mixing between dual internal modes of a QCL and between a single mode of a QCL and a known molecular line from a molecular gas laser. Dual mode mixing using a single QCL shows that the intrinsic linewidth of a free-running QCL is $<$ 30 kHz. Both standard and distributed feedback grating QCLs were mixed against known molecular gas laser lines. Resulting difference frequency spectra gave a high precision measurement of a QCL's absolute frequency against known references. Unusual slow transient turn-on behavior was also observed in a pulsed standard QCL. 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] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L35.00008: Electric Field and Polarization Dependent Spatial Fringe-Patterns in Electro-optic Crystals Randall Hinton, Anthony Garzarella, Dong Ho Wu Electro-optic (EO) crystals, such as LiNbO$_{3}$, are being widely used for high speed optical communication applications, which exploit their fast EO modulation capability. This EO modulation capability is also being used for the detection of an electric field over an extremely broad frequency band, namely DC through THz. It is known that most of EO crystals exhibit the photorefractive, pyroelectric, piezo and photo-elastic effects. While considered as parasitic effects and detrimental for most EO applications, our experimental results seem to suggest that some of these effects can increase the effective EO responsivity. To understand how these effects influence the EO responsivity we have carried out systematic investigations with LiNbO$_{3}$ and Sr$_{0.75}$Ba$_{0.25}$Nb$_{2}$O$_{6}$ crystals. When a linearly polarized laser beam (628 nm) passes through the crystal, to which an external low frequency AC field is applied, we observed a periodic interference pattern, which was dynamically modulated by the AC field. We also found that the interference pattern was produced by the reflected beam from the crystal's front and back surfaces. The patterns and dynamic modulation behaviors of the transmitted and reflected beams were noticeably different from each other. We will discuss the implications of these experimental results to the EO responsivity. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L35.00009: A MHz-Rate High-Power UV Laser Source for High-Speed Planar Laser-Induced Fluorescence Spectroscopy Mikhail N. Slipchenko, Joseph D. Miller, Terrence R. Meyer, Naibo Jiang, Walter R. Lempert, James R. Gord We report on producing MHz-rate pulse burst tunable high energy UV radiation and its application to high-speed temperature measurements of combustion based on planar laser-induced fluorescence (PLIF) spectroscopy. The laser system consists of a narrowband high-speed tunable seeded OPO pumped by harmonics of a MHz-rate pulse-burst pump laser. The pump laser utilizes an AOM-based pulse slicer followed by a 5-stage Nd:YAG amplifier. The pump laser produces 4 bursts per second with a burst duration as long as 1.5 ms and total burst power up to 2 J. Each burst consists of 5 to 100 pulses with each pulse duration variable between 6 to 50 ns. The custom OPO produces up to 5 mJ per pulse in the range from 220 to 315 nm for exciting electronic transitions of species such as nitric oxide and the hydroxyl radical. The laser system performance is tested in well characterized flames and pulse-burst PLIF results are presented. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L35.00010: Theory of THz rectification at a gate controlled Shottky barrier in a FET based plasmonic detector G.R. Aizin, J. Mikalopas, G.C. Dyer, E.A. Shaner, M.C. Wanke, S.J. Allen We present a theory of the resonant THz photoresponse in a grating gated FET with a gate controlled Shottky barrier [1]. We use theoretical modeling to show that the potential barrier induced in the 2D FET channel by an isolated gate finger, biased to pinch off, yields the Shottky-like $I-V $characteristics of the FET. The grating gate couples an external THz radiation to the plasmon excitations in the 2D electron channel. We calculate the photoresponse signal resulting from the THz rectification at the Schottky barrier and demonstrate that it has resonant peaks at plasmon frequencies. The results obtained are consistent with recent measurements [2]. This work is supported by ARO (Grant {\#} W911NF-05-1-0031) and The University at Buffalo NSF NIRT (Grant {\#}ECS0609146 ). 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. 1. E.A. Shaner et al, Appl. Phys. Lett., 90, 181127 (2007). 2. G. Dyer et al, this meeting presentation. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L35.00011: Control of exciton fluxes Alex High, Aaron Hammack, E.E. Novitskaya, Leonid Butov, Micah Hanson, Arthur Gossard We present proof of principle for control of excitonic fluxes by gate in mesoscopic devices. Since excitons are bosonic particles, control of exciton fluxes can extend mesoscopics, the field which electron transport in potential reliefs, to bosons. Also, as are coupled to light, the control of exciton fluxes may lead to development of new optoelectronic devices. The demonstrated devices as a directional switch, star switch, and flux merger. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L35.00012: Experimental Studies of Alignment Tolerance and High Temperature Performance of A Fabry-Perot Interferometric Pressure Sensor Ivan Padron, Anthony Fiory, Nuggehalli Ravindra Fabry-Perot interferometry is one of the most reliable of the several optical techniques that can be utilized to facilitate the fabrication of an optical sensor. Devices based on this technique can provide high degree of sensitivity, versatility and immunity to environmental noise. The Fabry-Perot Interferometric Sensor (FPIS), to be discussed in this presentation, consists of a Fabry-Perot cavity formed between two bonded surfaces: a corrugated diaphragm with a center rigid body (or boss) which deflects under external pressure and keeps a high alignment tolerance and a glass surface with an optical fiber insert. The Fabry-Perot cavity and optical fiber have been used as the sensing element and interconnect, respectively. The Fabry-Perot cavity has been fabricated using the MEMS technology. Micromachining techniques make Fabry-Perot sensors very attractive by reducing the size and cost of the sensing element. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L35.00013: Growth and properties of wide band gap II-VI multi-quantum well structures for mid-infrared quantum cascade lasers William Charles, Kale Franz, Aidong Shen, Maria Tamargo, Claire Gmachl Mid-infrared emission based on intersubband transitions has been recently very actively pursued for the fabrication of quantum cascade lasers (QCLs) operating in that wavelength range. Highly efficient, ultra fast lasing can be achieved from engineered structures in which the emission wavelength is determined by the precisely controlled multi-layered structure rather than the specific materials of choice. However, operation at wavelengths shorter than 3.5 $\mu $m is limited by the conduction band offset of the materials currently available. To address this limitation, we have begun to explore wide band gap II-VI ZnCdMgSe materials grown lattice matched to InP substrates for these applications. Recently, we reported the growth and characterization of multi-quantum well structures that exhibit absorption in the 3-5 $\mu $m region. We have designed a structure consisting of the active/injector regions of a QCL and fabricated electroluminescent devices. Electroluminescence emission at 4.7 $\mu $m has been observed in these structures, suggesting that these materials hold great promise for the fabrication of short wavelength mid-IR QCLs. This work is supported by NSF Grant No. EEC-0540832 (MIRTHE-ERC). [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L35.00014: Behavior of laser diodes in the small particle number quantum limit. Kaushik Roy Choudhury, Anthony F.J. Levi We use master equations to model the steady-state and transient response of a laser diode in the small particle number quantum limit. In scaled laser diodes $n$ electrons and $s$ photons are correlated such that $<$\textit{ns}$>$ may not be factorized and there are significant differences in behavior compared to predictions of continuum mean-field theory. Quantization of photon number is found to supress lasing threshold and create a non-Poisson probability distribution for $n$ discrete electrons and $s$ discrete photons. The same correlation effect damps the transient dynamic response of laser emission. The predictions of conventional mean-field and Langevin theory are recovered in the large particle number limit. [Preview Abstract] |
Session L36: Focus Session: Hydrogen Storage II: Chemical Hydride and Complex Metal Hydride Materials II
Sponsoring Units: DMP FIAPChair: Frederick E. Pinkerton, GM R&D Center
Room: Morial Convention Center 228
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L36.00001: Effects of point defects and impurities on kinetics in hydrogen storage materials Invited Speaker: First-principles calculations are playing an important role in developing a fundamental understanding of the physics and chemistry of hydrogen storage materials. In order to accurately describe the mechanisms of hydrogen uptake and release that are at the core of the hydrogen storage process, it is necessary to consider the addition or removal of individual hydrogen atoms. We have performed supercell calculations to model addition or removal of hydrogen, corresponding to the formation of hydrogen interstitials or vacancies. While the concepts discussed here are general, they will be illustrated with detailed results for sodium alanate, a viable hydrogen storage material. The calculations are based on density functional theory in the generalized gradient approximation, using the projector-augmented-wave approach. We find that hydrogen-related point defects are the dominant defect species involved in (de)hydrogenation of sodium alanate. These defects are positively or negatively charged, and hence their formation energies are Fermi-level dependent - an important feature that has not been recognized in past studies. This dependence enables us to explain why small amounts of transition-metal additives drastically alter the kinetics of dehydrogenation. The rate-limiting step for hydrogen release is the creation of charged hydrogen-related defects, while transition-metal additives (such as Ti) act as electrically active impurities that lower the formation energy of these defects. Comparisons with experimental measurements that confirm the proposed mechanisms will be discussed. The ideas outlined here suggest improved preparation methods for complex hydrides through enhanced control of the addition of small concentrations of impurities. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L36.00002: Density Functional Theory Based Kinetic Monte Carlo Approach for Understanding Atomistic Mechanisms for Reversible Hydrogen Storage in Metal Hydrides: Application to Alane Formation on Ti Doped Al Surfaces A. Karim, J. Muckerman, P. Sutter, E. Muller We describe a density functional kinetic Monte Carlo approach enabling us to study and simulate the steady-state situation of dissociative adsorption of hydrogen along with diffusion and reaction of Al and H atoms leading towards the formation of alane species on Ti-doped Al surfaces. In the first step, density functional theory is used in conjunction with the nudged elastic band/drag method to obtain the energetics of the relevant atomistic processes of Al and H diffusion and their reactions on Al surfaces with different concentration of dopant Ti atoms. Subsequently, the kinetic Monte Carlo method is employed, which accounts for the spatial distribution, fluctuations, and evolution of chemical species at Ti-doped Al surfaces under steady-state conditions. This DFT-based KMC approach provides an insight into the kinetics of alanes at technologically relevant pressure and temperature conditions. Our computed production rates of AlH$_{3}$ on Al surfaces are in agreement with experimental data. We also obtained temperature programmed desorption spectra of different alane species, which is agreeing well with experiments. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L36.00003: Alanes formation on the Al(111) surface Sylvie Rangan, Jean-Francois Veyan, Yves J. Chabal, Santanu Chaudhuri, James T. Muckerman Alane clusters (Al$_{x}$H$_{y})$ are believed to be the ubiquitous intermediates in hydrogen storage reactions for a wide variety of alanates (LiAlH$_{4}$, NaAlH$_{4})$ currently considered for hydrogen storage. The formation and behavior of alanes at surfaces appear to control and limit the efficiency of hydrogen storage. In particular, hydrogen adsorption on the Al(111) surface leads to the coexistence of several adsorbed species, the concentration of which is affected by the step density, the surface coverage and the temperature. We combine density functional theory (DFT) and surface infra-red (IR) absorption spectroscopy to uncover the mechanisms for alane formation on Al(111) surfaces. At low coverage, DFT predicts a two-fold bridge site adsorption for atomic hydrogen, consistent with previous Electron Energy Loss Spectroscopy measurements. At higher coverage, the formation of small chemisorbed AlH$_{3}$ occurs at the step edges. With increasing coverage AlH$_{3}$ is extracted from the step edge and becomes highly mobile on the terraces in a weakly bound state. This mobility is the key factor leading to the growth of larger alanes through AlH$_{3}$ oligomerization. For these large alanes, previous Thermal Programmed Desorption studies are discussed and compared to the thermal stability observed in IR. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L36.00004: Watching the dehydrogenation of alane (AlH3) in a TEM Shane Beattie, Terry Humphries, Louise Weaver, Sean McGrady Alane (AlH3) is a promising candidate for on-board hydrogen storage applications. Its theoretical gravimetric capacity is 10.1 percent and decomposition is achieved with modest heating (60-200 deg C). We studied the dehydrogenation of alane, insitu, in a TEM. Alane powder was loaded into the TEM and heated at 80 deg C. We were able to `watch' the dehydrogenation of the alane to aluminum. Electron diffraction and dark fiend images are used to show how and where the aluminum crystallites grow. Although crystalline aluminum phases were successfully identified, some of the sample remained amorphous. We will discuss the nature of the amorphous material and present images clearly identifying the nature of the aluminum crystallites. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L36.00005: Hydrogen Multicenter Bonds on Small Metal Clusters P. Tarakeshwar, T.J. Dhilip Kumar, N. Balakrishnan We investigate the saturation of hydrogen on metal clusters employing \textit{ab initio} calculations. Our calculations reveal that energetically the most preferred configuration of the hydrogen saturated metal clusters exhibit hydrogen multicenter bonds. The strength of these hydrogen multicenter bonds can be modulated either by changing the extent of hydrogen saturation or using different metal clusters. In the context of hydrogen storage materials, our calculations indicate that early first-row transition metals have the best propensity to form hydrogen multicenter bonds. The relevance of this work in the context of hydrogenation and dehydrogenation kinetics of complex metal hydrides will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L36.00006: The absorption of hydrogen by Nb thin films capped with Pd studied by transmission of visible light J. I. Avila, A. L. Cabrera, G. B. Cabrera, David Lederman Samples of Nb films with thickness between 2.5 to 14 nm were deposited on glass and capped by a continuous 6 nm Pd film in a sputtering system. The light transmission and reflection, in the visible range (400 to 1000 nm), were measured when the sample was exposed to different hydrogen pressure up to 75 Torr. Experiment on continuous pure Pd film and Nb film for were done for comparison. The relative change in transmission for Nb at 74 Torr of hydrogen is about 5 percent but the saturation occurs after 2500 s. A sample of 14 nm Nb capped by Pd shows a 7 percent increased in transmission at the same pressure but the saturation time is reduced to 50 s, same as pure Pd. Change in the kinetics of hydrogen absorption by Nb capped with Pd indicates that the rate limiting step in the absorption process by pure Nb is located on the Nb surface. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L36.00007: Theoretical Analysis on X-ray Absorption Spectra of Ti compounds as Catalysts in Lithium Amide-Imide reactions Takao Tsumuraya, Tatsuya Shishidou, Tamio Oguchi Solid-state storage is conceptually efficient approach for on-board vehicular hydrogen storage. In this context, light-element materials such as lithium amide LiNH$_{2}$ and lithium imide Li$_{2}$NH have been attracted much attention due to their high gravimetric densities of hydrogen. Recently, various transition-metal compounds have been examined with ball-milling technique for exploring catalysts to improve the hydrogen storage and desorption kinetics, and it is found that a small amount (1mol\%) of titanium compounds revealed a superior effect in hydrogen desorption reaction LiNH$_{2}$ + LiH ${\to}$ Li$_{2}$NH + H$_{2}$ [1]. However, these catalysis mechanism and role of Ti in the reaction remain unanswered. Isobe $et$ $al$. have carried out measurements of X-ray absorption spectroscopy(XAS) at Ti $K$-edge to see the electronic states of Ti recently [2]. In this paper, we calculate the electronic structure of Ti metal and its compounds, and obtained theoretical spectra to compare with the measured spectra by using first-principles calculations based on the all-electron FLAPW method. We discuss chemical bonding and local geometry of catalytically active states in the reaction. [1] T. Ichikawa, S. Isobe, N. Hanada and H. Fujii, J. of Alloys and Comp. 365, 271 (2004) . [2] S. Isobe, T. Ichikawa, Y. Kojima and H. Fujii, J. of Alloys and Comp. 446-447, 360 (2007). [Preview Abstract] |
Session L37: Semiconductors II: Structure and Phase Diagrams
Sponsoring Units: DCMPChair: Alex Zunger, National Renewable Energy Laboratory
Room: Morial Convention Center 229
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L37.00001: First-Principles epitaxial phase-diagram, short-range order, microstructure and electronic properties of (In,Ga)N zincblende alloys on GaN Zhe Liu, Paulo Piquini, Alex Zunger A first-principle total energy cluster expansion method is developed to study thermodynamic properties of epitaxial semiconductor alloys coherent to substrate (i.e., $<$ critical thickness $h_c$), in which coherent strain energy as a function of atomic configuration is explicitly described. The search for epitaxial ground state structures of (In, Ga)N alloy grown on GaN (001) substrate concludes that epitaxial strain suppresses phase separation, which is normally observed for bulk (In,Ga)N alloy and relaxed films. Two (102) superlattices: (InN)$_2$/(GaN)$_2$ and (InN)$_4$/(GaN)$_1$ are determined to be the epitaxial ground state structures. Composition-temperature phase diagram calculated by Monte Carlo method shows that homogeneous solid solution phase is thermodynamic stable at typical growth temperature of blue and green LED by MBE and MOCVD ($x$(In)$\sim 0.20-0.30$ and $h_c \sim 10-30$nm). Such calculated phase diagram can be used to understand the controversy regarding atomic microstructures in (In,Ga)N quantum well devices. Short-range-ordering of the solid solution phase and its influence on the electronic properties are also discussed. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L37.00002: First principles and valence force field study of III-V quaternary alloys Koushik Biswas, Alberto Franceschetti, Stephan Lany We report on the elastic properties and formation energies of Ga$_{x}$In$_{1-x}$P$_{y}$N$_{1-y}$ quaternary alloys using first principles and valence force field (VFF) calculations. The elastic constants of the binary compounds (GaP, InP, GaN, and InN) were calculated using the local density approximation (LDA). The resulting VFF parameters, \textit{$\alpha $} (bond stretching) and \textit{$\beta $} (bond angle bending) were used within the Keating model to calculate the formation energies of GaInP, GaInN, InPN, and GaPN ordered structures. We found that the VFF formation energies of phosphide-nitride alloys (e.g. GaPN) were not in very good agreement with the LDA formation energies. Conventionally, the bond bending parameter \textit{$\beta $} for a ternary alloy is chosen as the arithmetic mean of the binary constituents. To improve the accuracy of the VFF model, we lifted such restriction on the \textit{$\beta $}-parameter and we also introduced the parameter \textit{$\sigma $} (bond length-bond angle interaction). The VFF parameters \textit{$\alpha $}, \textit{$\beta $}, and \textit{$\sigma $} were fitted to the LDA-calculated formation energies of a large number of ternary ordered structures and were used to calculate the formation energy of the Ga$_{x}$In$_{1-x}$P$_{y}$N$_{1-y}$ quaternary alloy. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L37.00003: First Principles Phase Diagram Calculation For Al$_{x}$Ga$_{1-x}$N Jeremy Nicklas, John Wilkins First principles phase diagram calculations were performed for the wurtzite and zincblende structures of the quasibinary system AlN-GaN. The cluster expansion method using the code ATAT was performed without and with excess vibrational contributions to the free energy, $F_{vib}$. The ab initio calculations were performed with VASP using the PAW pseudopotentials with PBE for the exchange and correlation energies. Preliminary results show miscibility gaps for both structures with a decrease in the consolute points, ($X_{C}$,$T_ {C}$), when including $F_{vib}$. The wurtzite structure is predicted to be approximately symmetric while the zincblende is predicted to be quite assymetric. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L37.00004: Simulation studies of GST phase change alloys Glenn Martyna In order to help drive post-Moore's Law technology development, switching processes involving novel materials, in particular, GeSbTe (GST) alloys are being investigated for use in memory and eFuse applications. An anneal/quench thermal process crystallizes/amorphosizes a GST alloy which then has a low/high resistance and thereby forms a readable/writeable bit; for example, a ``one'' might be the low resistance, conducting crystalline state and a ``zero'' might be the high resistance, glassy state. There are many open questions about the precise nature of the structural transitions and the coupling to electronic structure changes. Computational and experimental studies of the effect of pressure on the GST materials were initiated in order to probe the physics behind the thermal switching process. A new pathway to reversible phase change involving pressure-induced structural metal insulator transitions was discovered. In a binary GS system, a room-temperature, direct, pressure-induced transformation from the high resistance amorphous phase to the low resistance crystalline phase was observed experimentally while the reverse process under tensile load was demonstrated via ab initio MD simulations performed on IBM's Blue Gene/L enabled by massively parallel software. Pressure induced transformations of the ternary material GST-225 (Ge2Sb2Te5) were, also, examined In the talk, the behavior of the two systems will be compared and insight into the nature of the phase change given. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L37.00005: Local structure and Phase transition of (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ pseudo-binary system for the phase-change memory Jino Im, Jae-Hyeon Eom, Jisoon Ihm A theoretical investigation on the local structure and phase transition between the crystalline and the amorphous phase of (GeTe)$_n$(Sb$_2$Te$_3$)$_m$ pseudo-binary system(GST) for the phase-change memory is presented. Based on the study of the coordination number for the amorphous phase of GST, the local structure of the amorphous phase of the GST is shown to be composed of the stibnite-like building block for the Sb$_2$Te$_3$ and chain-like building block for the GeTe. The phase transition between the crystalline and the amorphous phase of GST is explained by relative repositioning of these building blocks. Density functional total energy minimization calculations show that the crystallization energy and the volume change in transition also agree with experimental data. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L37.00006: Raman scattering studies of Ge-Sb-Te nanoparticles Christine Kim, Hae-Young Shin, Ah Reum Jeong, William Jo, Seokhyun Yoon We have measured Raman scattering spectra of Ge-Sb-Te (GST) nanoparticles which are synthesized by a pulsed laser ablation method. The nanoparticles were grown under different growth conditions such as temperatures and/or pressures. Our measurements could provide information towards the optimal growth conditions for better crystalline quality of the GST nanoparticles. We have also measured nitrogen-doped GST nanoparticles. Comparision between Raman responses of nitrogen- doped- and undoped-GST nanoparticles will be presented. Our results suggest that Raman scattering spectroscopy can be used to study phases and phase changes through local structural information in the GST nanoparticles, which are being developed for low-power non-volatile memory applications. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L37.00007: Theory of unsaturated silicon lattices Feng Zhang, David Stucke, Dragan Stojkovic, Vincent Crespi Several molecules are known to contain stable silicon double or triple bonds that are sterically protected by bulky side groups. Through first-principles computation, we demonstrate that well-defined $\pi$ bonds can also be formed in two prototypical {\it crystalline} Si structures: Schwarzite Si-168 and dilated diamond. The sp$^2$-bonded Si-168 is thermodynamically preferred over diamond silicon at a modest negative pressure of -2.5 GPa. Ab-initio molecular dynamics simulations of Si-168 at 1000 K reveal significant thermal stability. Si-168 is metallic in density functional theory, but with distinct $\pi$-like and $\pi^*$-like valence and conduction band complexes just above and below the Fermi energy. A bandgap buried in the valence band but close to the Fermi level can be accessed via hole doping in semiconducting Si$_{144}$B$_{24}$. A less-stable crystalline system with a silicon-silicon triple bond is also examined: a rare-gas intercalated open framework on a dilated diamond lattice. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L37.00008: Phase diagram of silicon using a DFT-based neural network potential Oliviero Andreussi, Joerg Behler, Michele Parrinello The phase diagram of silicon is computed by means of Classical Molecular Dynamics. A recently developed [Behler and Parrinello, Phys. Rev. Lett. 98 146401 (2007)] neural-network potential based on Density Functional Theory calculations in the Local Density Approximation is used. This potential was shown to be several orders of magnitude faster than corresponding LDA-DFT calculations, while the accuracy is essentially maintained. Results on the liquid-solid coexistence curve are in good agreement with ab-initio calculations and demonstrate the quality of the neural-network potential. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L37.00009: Fine Structures of Ge Nanoclusters on Si(111): STM Observations and First-Principles Theory A.S. Rao, H.F. Ma, M.C. Xu, D.X. Shi, H.J. Gao, R. Gudipati, H.L. Dang, Sanwu Wang Germanium-based nanoclusters grown on silicon substrates have potential applications in optoelectronics and nanotechnology. A variety of Ge nanostructures formed on intact Si(111) have been observed. However, the fine structures of the atomic arrangements in the Ge nanoclusters have remained elusive. We performed scanning tunneling microscopy observations and first-principle calculations for investigating the fine structures of the Ge nanoclusters on the Si(111)-7$\times $7 surface. We obtained atomic structures of the nanoclusters formed with a certain process involving deposition at temperature of 423 K and annealing at 550 K. We found that Ge nanocluster, located predominately in the faulted half unit cells of the Si(111)-7$\times $7 surface, contained approximately six Ge atoms with three bonded center Si adatoms. We also observed that the obtained nanostructures were stable up to 600 K. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L37.00010: Damping of high amplitude phonons in bismuth: classical and quantum mechanical simulations Eamonn Murray, Aaron Hurley, David Prendergast, Tadashi Ogitsu, David Fritz, David Reis, Stephen Fahy Using both classical and quantum mechanical simulations together with first-principles results, we investigate the damping mechanism of high amplitude excitations of the $A_{1g}$ phonon mode in bismuth. Pump-probe experiments using ultrafast lasers can generate and measure large amplitude coherent oscillations of the $A_{1g}$ phonon mode in bismuth. A substantial reduction in the lifetime of the phonon is observed when higher amplitude oscillations are produced. With third-order couplings obtained from first-principles we calculate the rate of energy loss from the $A_{1g}$ mode over several picoseconds. We find that as the highly excited $A_{1g}$ mode decays, it produces highly excited modes in relatively small regions of the Brillouin Zone, leading to an increase in the decay rate into these modes. We show how this can greatly affect the observed lifetime of the high amplitude excitation of the $A_{1g}$ mode. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L37.00011: Structure of Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ supermodulation from ab initio calculations Y. He, S. Graser, P.J. Hirschfeld, H.-P. Cheng We present results of density functional theory (DFT) calculation of the structural supermodulation in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ structure, and show that the supermodulation is indeed a spontaneous symmetry breaking of the nominal crystal symmetry, rather than a phenomenon driven by interstitial O dopants. The structure obtained is in excellent quantitative agreement with recent x-ray studies, and reproduces several qualitative aspects of scanning tunnelling microscopy (STM) experiments as well. The primary structural modulation affecting the CuO2 plane is found to be a buckling wave of tilted CuO$_5$ half-octahedra, with maximum tilt angle near the phase of the supermodulation where recent STM experiments have discovered an enhancement of the superconducting gap. We argue that the tilting of the half-octahedra and concommitant planar buckling are directly modulating the superconducting pair interaction. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L37.00012: Discovery of Dynamics of Jahn-Teller Effect Dan Liu Taking S=1/2 NaSiTi$_{2}$O$_{6}$ as example, we discovered the dynamics of the Jahn-Teller effect of solids by extending the molecular frontier orbital theory from chemistry to solid state physics. At the orbital and spin levels, the dynamics of the Jahn-Teller effect is exposed to involve spin flipping and spontaneous orbital transition that is driven by the spin-orbital coupling to keep the angle momentum reservation. The orbital transition leads to a dramatic structural change, i.e., the JT distortion. In analogue to the singlet-triplet intersystem crossing of photochemistry, the JT effect is a non-adiabatic process, associated with the first-order phase transition. The JT effect agrees well with the low-dimensional, S=1 Haldane gap on the basis of the antimagnetic Heisenberg model, if emphasizing that the S=1 parameter of Haldane gap indicates the parallel spins of neighboring orbitals, rather than the on-site parallel spins. We provide insight to understanding of the unusual structural, magnetic properties of S=1/2 NaTiSi$_{2}$O$_{6}$, as well as the S=1LiVGe$_{2}$O$_{6}$ and S=3/2 LiCrGe$_{2}$O$_{6}$. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L37.00013: Dynamics of A-B transition of the DNA double helices Hao Wang, Thomas Cheatham, Peter Gannett, James Lewis The conformational transitions of DNA and the sensitivity of DNA structure to the surrounding environment are very relevant to its chemical and biological function and potential applications in nano-technology. Different conformations of DNA, even with the same sequence, exhibit different electronic structures, resulting in different conduction properties. We present theoretical work on the dynamical features of electronic states in the A-B transition of a model DNA duplex of d(CGCGCGCGCG)$_2$ (10 base-pairs, 628 atoms) as the molecule undergoes conformational changes and thermal fluctuations at room temperature. We couple state-of-the art empirical force field molecular dynamics (MD) simulations with an {\it ab-initio} electronic structure method based on density-functional theory, called F{\scriptsize IREBALL}. For the A-B transition, we calculated the effects of conformational change on the electronic structure for each snapshot obtained from nanosecond MD simulations. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L37.00014: Mechanism of GeSbTe phase change materials: an ab initio molecular dynamics study Jean-Yves Raty, C\'eline Otjacques, Jean-Pierre Gaspard, Christophe Bichara Among phase change materials, Ge2Sb2Te5 (225) is one of the most successfully used in applications. Accepted models are based on EXAFS spectra and suppose a complete reorganization of bonds during amorphization, with Ge changing from sixfold to tetrahedral coordination. We perform ab initio MD simulations of the (225), (124) and (415) liquid alloys. We show that the crystalline, liquid and amorphous structure of these systems are similar, with very little sp3 hybridization around Ge atoms and a majority of p-sigma bonds. Using a set of quenched liquid configurations we reproduce the EXAFS measurements on the (225) composition and explain how the static Debye Waller factor due to the vacancies in the crystal phase leads to a cancellation of individual neighbors contribution to the EXAFS signal while in the amorphous, a larger coherence occurs, enhancing the EXAFS signal. The computed electrical conductivities of the three phases (cubic solid, liquid and amorphous) prove to be very different, accordingly with the experiment. [Preview Abstract] |
Session L39: Focus Session: Deformation and Fracture
Sponsoring Units: GSNP DMPChair: Christopher Schuh, Massachusetts Institute of Technology
Room: Morial Convention Center 231
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L39.00001: Strain localization as a mechanism for dynamic weakening in amorphous solids M. Lisa Manning, James S. Langer, Jean M. Carlson Solids such as foams, colloids, amorphous metals and granular fault gouge are composed of particles in closely-packed, non-crystalline configurations, and small-scale mechanisms for deformation in these materials are less well-understood than those in liquids or crystals. I will discuss a mesoscopic model for these disordered solids, the theory of Shear Transformation Zones (STZs), and show that it captures macroscopic features seen in experiments as well as interesting internal dynamics such as shear banding. An important component of this model is the effective temperature, which describes the statistical distributions of particle configurations and governs plastic deformation. Shear banding occurs due to a ``frozen''-time instability in the effective temperature field, and one can determine a condition for shear banding based on the initial conditions alone. I will discuss how the STZ formulation can be used as a continuum model for fault gouge and includes a mechanism by which the system can dynamically weaken. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L39.00002: What is the structure of a polymer glass after plastic deformation? Helene Montes, Francois Lequeux, Christiane Alba-Simionesco, Frederic Casas We aim to study the effect of plastic deformation on the structure of a glassy polymer. Using neutrons scattering on a large range of length scales, and comparing samples deformed below and above Tg, we show that: 1) The deformation is extremely homogeneous (or affine) for length scales above the entanglement distance 2) The crossover length scale between affine and non affine deeformation is about half the one of the entanglements, and is independent of temperature below the glass transition 3) The arrangement of the polymer chain is distorded by plastic deformation at the atomic scale We then discuss these results and compare them to the results of the simulation of Hoy and Robbins (J. Polym. Sci., 44 (2006), 3487). As a conclusion we see that the entanglements are respopnsible for the very homogeneous deformations, forcing the individual plastic events to propagate in the sample following the tension of the chains. Thus we conclude that the physics of the plastic deformation of polymer glasses are very different from the one of other glasses. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L39.00003: Depinning transition in failure of disordered brittle materials Laurent Ponson Crack propagation is the fundamental process leading to
material failure.
However, its dynamics is far from being fully understood. In
this work, we investigate both
experimentally and theoretically the growth velocity $v$ of a
crack propagating in brittle materials
in the limit of low velocities compared to the sound speed. The
variations of $v$ with respect to the external
loading $K_I$ are carefully measured on two kinds of brittle
rocks over various orders of magnitude.
The crack dynamics is shown to display two regimes, well
described by a sub-critical creep law
$v \sim e^{-\frac{c}{(K_I-K_0)^{\mu}}}$ with $\mu \simeq 1$ for
$K_I |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L39.00004: Role of microstructural heterogeneities on rupture of polycrystalline materials Invited Speaker: Grain boundary networks are the dominant heterogeneity in many polycrystalline materials, and their performance may be dramatically improved by increasing the fraction of boundaries which have either low grain boundary misorientation or which are special boundaries, such as coincident site lattice boundaries. Significant improvement in properties such as corrosion resistance, critical current in superconductors and mechanical strength and toughness occur, provided special percolating grain or grain boundary structures can be engineered. Nevertheless, grain boundary network correlations constrain the extent to which property improvements can be achieved. A common deleterious effect is that degraded boundaries have a tendency to cluster in linear chains leading to unexpected reductions in performance. After an introduction to the area, scaling laws and the results of large scale simulations of percolation, critical manifolds and rupture in polycrystals will be presented. In particular, deleterious effects due to grain boundary correlations will be elucidated and strategies for their mitigation will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L39.00005: Phase field modeling of liquid metal embrittlement Robert Spatschek, Nan Wang, Alain Karma Liquid metal embrittlement (LME) is a phenomenon whereby a liquid metal in contact with another, higher-melting-point polycrystalline metal, rapidly penetrates from the surface along grain boundaries. This phenomenon is known to be greatly accelerated by the application of tensile stress, resulting in the rapid propagation of intergranular cracks in normally ductile materials. Although this phenomenon has been known for a long time, it still lacks a convincing physical explanation. In particular, the relationship of LME to conventional fracture mechanics remains unclear. We investigate LME using a phenomenological three-order-parameter phase field model that describes both the short scale physics of crystal decohesion and macroscopic linear elasticity. The model reproduces expected macroscopic properties for well separated crack surfaces and additionally introduces short scale modifications for liquid layer thicknesses in the nanometric range, which depend on the interfacial and grain boundary energy as well as elastic effects. The results shed light on the relative importance of capillary phenomena and stress in the kinetics of LME. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L39.00006: Atomistic simulation studies of plastic deformation and dislocation patterning as a function of temperature N. Scott Weingarten, Robin Selinger The mechanical properties of crystalline solids depend sensitively on the mechanisms controlling dislocation nucleation, motion, and patterning. To explore the role of thermal activation in these processes, we carry out atomistic Monte Carlo simulation studies of plastic deformation of 2-d single crystals at a range of temperatures. We find that at intermediate temperature, dislocations readily coalesce to form tilt boundaries, while at high temperature, the defects remain disordered in a gas-like phase, suggesting the possibility of an order-disorder phase transition. Conversely, near $T=0$, dislocation mobility is too low to produce patterning on short time scales, again producing disordered structures. We study also the response of a polycrystalline solid under pure compression and look at the resulting distribution of stresses. We find that the defect-rich grain boundary regions bear higher stresses than those in the bulk, in agreement with Mughrabi's two-component composite theory. Results are compared with recent experiments by L. E. Levine et al. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L39.00007: Novel plastic processes in nanoindented stepped Au surfaces Violeta Navarro, Oscar Rodriguez de la Fuente, Arantzazu Mascaraque, Juan Manuel Rojo While much work has been done recently on defect nucleation during plastic processes, mechanical properties of real surfaces have been seldom studied atomistically. Defect nucleation is well known to be critical in the mechanical behaviour of materials [1]. But the role that surface defects play on the earliest stages of plasticity still needs to be elucidated. We approach realistic surfaces by using vicinal surfaces with a high step density. Nanoindentations with AFM and atomistic simulations have been performed on the Au(788) surface [2]. Force vs penetration curves show a hertzian initial stage and a later incipient plastic regime when dislocations are nucleated. Between these two regimes we report a novel one, in which dislocations nucleate at the steps but no pop-ins are visible. This novel regime is to a large extent reversible in the sense that defects disappear when the tip is retracted [2]. Heterogeneous dislocation nucleation is catalyzed by the presence of the surface steps. \newline [1] J. Li, MRS Bulletin, 32, (2007), 151. \newline [2] V. Navarro et al. Submitted [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L39.00008: Hysteresis in Kinking Nonlinear Elastic Solids and the Preisach-Mayergoyz Model Peter Finkel, Aiguo Zhou, Gary Friedman, Michel Barsoum We show that the stress-induced, dislocation-based, elastic hysteric loops of kinking nonlinear elastic solids -- polycrystalline cobalt, Ti3SiC2 and a 10 vol. {\%} porous Ti2AlC - obey the scalar Preisach-Mayergoyz phenomenological model because they exhibit wipe-out and congruency, two necessary and sufficient tenets of the model. We also present experimental proof of the applicability of the model for the prediction of the response of these materials to complex stress histories. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L39.00009: Strain Relaxation through Structural Phase Transition in Ultrathin Films of FCC Metals Kedarnath Kolluri, M. Rauf Gungor, Dimitrios Maroudas We report a computational analysis of atomistic mechanisms of relaxation of biaxially applied tensile strain over a broad range of strain levels, $\varepsilon $, in freestanding ultra-thin Cu films based on isothermal-isostrain large-scale molecular-dynamics simulations. Our analysis reveals that for $\varepsilon \quad <$ 10{\%}, plastic deformation occurs through ductile void growth and dislocation nucleation and glide from the thin-film surfaces. For $\varepsilon \quad \ge $ 10{\%}, strain relaxation is dominated by the nucleation of a high density of dislocations at the film's surface, leading to a martensitic transformation of the thin film from an fcc to a hcp lattice structure. The hcp phase nucleates at the surface of the thin film and propagates into the film due to the glide of dislocations; in this process, the relative atomic slips have magnitudes identical to those observed in Bain transformations. Furthermore, mechanical analysis according to generalized stability criteria shows that the observed phase transition is consistent with the onset of a shearing instability of the thin film. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L39.00010: Mechanical Properties and Fracture of Electrophoretically Deposited CdSe Nanocrystal Films Shengguo Jia, Sarbajit Banerjee, Dongyun Lee, Joze Bevk, Jeffrey Kysar, Irving Herman The fracture, strain, and stress of electrophoretically deposited (EPD) CdSe nanocrystal films are studied as a function of the film thickness, nanocrystal size, and drying method$. $Fracture results from the film stress that develops with the loss of residual solvent after EPD, when the film exceeds a threshold thickness. Generational crack formation and a preferred direction for film drying are observed in real time. The elastic modulus and hardness of films of 3.2 nm CdSe nanocrystals are $\sim $10 GPa and 450 MPa by nanoindentation. Furthermore, after particle cross-linking and partial ligand removal, the films exhibit compaction of the nanocrystal cores suggesting these films have polymeric features that can be attributed to the organic ligands and granular characteristics due to the inorganic cores. The toughness of the thin films is determined to be $\sim $1000-1400 J/m$^{2}$ for channel cracks in 3.2 nm nanocrystal films; the toughness values would be lower for a (likely) sublinear dependence of stress on strain. This work was supported primarily by the MRSEC Program of the NSF under Award No. DMR-0213574 and by NYSTAR. Nanoindentation studies at the Oak Ridge National Laboratory SHaRE User Center were sponsored under DE-AC05-00OR22725. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L39.00011: Current-induced Stabilization of Surface Morphology in Stressed Solids Vivek Tomar, M. Rauf Gungor, Dimitrios Maroudas We report results on the surface morphological evolution of a conducting crystalline solid under the simultaneous action of an electric field and mechanical stress based on a fully nonlinear model and combining linear stability theory with self-consistent dynamical numerical simulations. Surface diffusional anisotropy is taken into account in the analysis. We address the morphological response of a planar surface for a stress that acts on the solid uniaxially and parallel to the applied electric field, which is directed parallel to the surface plane. For a given stress level, our linear stability analysis predicts three regimes of surface morphological response at weak, moderate, and strong electric fields, respectively. Our key theoretical findings are in agreement with our numerical simulation results. Most importantly, we find that a sufficiently strong electric field, through surface electromigration, can stabilize the surface morphology of the stressed solid against crack-like surface instabilities. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L39.00012: Potts-Percolation Model of Solids Miron Kaufman, H.T. Diep We study a statistical mechanics model of a solid. Neighboring atoms are connected by Hookian ``springs''. If the energy of a ``spring'' is larger than a threshold, the ``spring'' is more likely to fail, while if the energy is lower than the threshold the spring is more likely to be alive. The phase diagram and thermodynamic quantities, free energy, numbers of bonds and clusters etc, are determined using renormalization-group and Monte-Carlo techniques. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L39.00013: Statistical Mechanics with Spatial Resolution: a bottom-up approach to nonuniform deformation Ying Hu The development of theoretical framework connecting pure atomistic simulations to the constitutive macroscopic behavior of a system undergoing nonuniform deformations has been very challenging. Here I report a new formalism that embeds the traditional principles of statistical mechanics with spatial resolution, applied to deformation of crystals. A ``stationary deformation path'' is derived for discrete lattice points. This new atomistic representation of deformation is linked to continuum description of deformed crystalline space through a statistical distribution function whose spatial variation obeys a Liouville-like equation. The formulation is applied to describe local/nonuniform deformations. Dislocations are shown to be represented by an independent-acting nonlocal strain field in nonequilibrium conditions. Continuum equations like Kroner's relation for dislocations are rederived. The formulation can be used in analyzing deformation mechanisms associated with defects involving heterogeneous fields at the nanoscale and macroscale, and in studying nanoscale processes where nonlocality is important. [Preview Abstract] |
Session L40: High Pressure
Sponsoring Units: DCMP DMPChair: Renata Wentzcovitch, University of Minnesota
Room: Morial Convention Center 232
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L40.00001: Complexity in `simple' metals Bruno Rousseau, Neil W. Ashcroft In electronic and structural terms, the light alkalis have long been regarded as `simple systems', at least under ordinary conditions. However, when compressed they exhibit unforeseen complexity; the melting curve of sodium, for example, has a striking maximum, falling to near room temperature melting where a complex structure (CI16) is found, this being in the cubic class but with 16 atoms per unit cell [1,2]. The light alkalis have been extensively studied using ab initio methods with standard assumptions of transferability made for the key pseudopotential input information, largely atomic based. Lacking still, however, is a somewhat more intuitive and physical understanding of the developments in electronic structure with progressive increase in density. In the present work, the problem is treated with non-linear response theory and non-overlapping pseudopotentials, and the structural complexity traced to effective ion-ion interactions with features that both at short range and long display competing state dependence. \newline [1] Gregoryanz et al., Phys. Rev. Lett. 94, 185502 (2005) \newline [2] McMahon et al., Chem. Soc. Rev. 35, 943 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L40.00002: High Pressure and Temperature Behavior of Lithium and Lithium Compounds Amy Lazicki, Alex Goncharov, Maddury Somayazulu, Viktor Struzhkin, Ho-kwang Mao, Russell Hemley Emerging structural complexity and unexpected increase of superconducting transition temperature at high pressure in lithium are some of the recently seen phenomena which indicate that our understanding of the behavior of this element at extreme conditions is incomplete. The heavier alkali metals (as well as alkaline earths and a variety of other elements) exhibit a maximum in the melting curve at high pressure, often attributed to the s-d electronic transition but recently shown most dramatically in light-weight sodium [1] at pressures below the expected s-d transition. In the interest of further exploring the origin of this high pressure-temperature behavior, we will present results of a study of lithium and lithium compounds in a resistively heated diamond anvil cell. [1] E. Gregoryanz, O. Degtyareva, M. Somayazulu, R. J. Hemley, and H. Mao, \textit{Phys. Rev. Lett}. \textbf{94}, 185502 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L40.00003: An improved multiphase equation of state for beryllium Gregory Robert, Arnaud Sollier, Philippe Legrand In our previous articles on beryllium (1)(2), a new theoretical phase diagram with three phases (hcp+bcc+liquid) of beryllium has been proposed : - Melting curve is obtained from quantum molecular dynamics (QMD) calculations along isochors. - Using phonon densities of states and a quasi-harmonic model, solid-solid transition is modeled. Our attempt to construct a three phases equation of state (EOS) failed due to our representation of the liquid phase based on Wallace's approach with the bcc phase, instable at low pressure, as reference. Here, we propose to deal with the instability of bcc phase at low pressure and the discontinuity of physical properties at melting. We also present an improved three phases (hcp+cc+liquid) EOS using simple analytic model constrained by the QMD calculations for the liquid. (1) G. Robert and A. Sollier, J. Phys IV 134, 257 - 2006. (2) G. Robert, A. Sollier and Ph. Legrand, to be published in APS-SCCM, June 2007. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L40.00004: High Pressure-High Temperature Phase Diagram of Beryllium. W.J. Evans, M.J. Lipp, H. Cynn, B.J. Baer, C.S. Yoo, A. Lazicki, Y. Ohishi, N. Sata A detailed understanding of the phase diagram of beryllium impacts fundamental science and technological applications. Despite a simple atomic structure, theoretical modeling of the phase diagram of beryllium has been extremely challenging and remains an area of active investigation [Kadas, ,PRB 07]. Beryllium is important to a range of applications, including structural members, x-ray windows, and nuclear reactors. Extension of the experimental understanding of beryllium will serve to inform and advance theoretical efforts and technological applications. To address these needs, we have extended our previous work [Evans, PRB 05], and performed x-ray diffraction and melt studies of high temperature beryllium. We will describe our measurements of the crystal structure, lattice constants, and melt curve of high-pressure beryllium. We will discuss insights into this simple yet challenging system. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L40.00005: Aluminum Yield Strength on Quasi-isentropes Jeffrey H. Nguyen, J. Reed Patterson, Daniel Orlikowski, L. Peter Martin, Ryan Krone, Roger Minich, Neil C. Holmes Advances in the functionally graded density impactors (FGDI) have made it possible to carry out dynamic experiments at previously inaccessible regions of the phase diagram. We employed these advances in recent tailored dynamic experiments to gain insight into the yield strength of aluminum along ``hot'' quasi-isentropes. The impactor was specifically designed to deliver a triangular compression wave into a sample where the strain rates on the compression and release isentropes were nearly identical. The aluminum samples were initially shocked to a fixed state on the Hugoniot, then quasi-isentropically compressed, and finally allowed to release isentropically. Here, we will discuss the details of the experiments and error analysis in deriving the yield strength of aluminum on a ``hot'' quasi-isentrope. We will also discuss recent advances in the FGDI technology that made these experiments possible with significantly reduced uncertainties. Methods to characterize these advances will be discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L40.00006: Violation of the London law and Onsager--Feynman quantization in multicomponent superconductors Egor Babaev, Neil W. Ashcroft Non-classical response to rotation is a hallmark of quantum ordered states such as superconductors and superfluids. The rotational responses of all currently known single-component 'super' states of matter (superconductors, superfluids and supersolids) are largely described by two fundamental principles and fall into two categories according to whether the systems are composed of charged or neutral particles: the London law relating the angular velocity to a subsequently established magnetic field and the Onsager--Feynman quantization of superfluid velocity. These laws are theoretically shown to be violated in a two-component superconductor such as the projected liquid metallic states of hydrogen and deuterium at high pressures. The rotational responses of liquid metallic hydrogen or deuterium identify them as a new class of dissipationless states; they also directly point to a particular experimental route for verification of their existence. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L40.00007: Ab-initio study on the crystal structure and the superconductivity of calcium in phase IV and V Takahiro Ishikawa, Hitose Nagara, Koichi Kusakabe, Naoshi Suzuki Calcium shows interesting structural phase transitions and the superconductivity under high pressure. Structural transformations from the simple cubic structure (Ca-III) to a complex structure (Ca-IV) at 113GPa and from Ca-IV to another complex one (Ca-V) at 139GPa have been reported, but their structures have not been identified. The pressure-induced superconducting transition has been observed in Ca-III, and the superconducting transition temperature T$_{c}$ dramatically increases at the transition from Ca-III to Ca-IV and reaches to 25K, which is the highest T$_{c}$ in an element, at 161GPa in Ca-V. We explored the structures of Ca-IV and Ca-V via the metadynamics simulation based on the density functional theory and obtained two new structures: A helical structure and a zigzag structure. From comparison of the x-ray diffraction patterns we identified that the helical structure and the zigzag structure are candidate structures of Ca-IV and Ca-V, respectively. For the zigzag structure of Ca-V we calculated T$_{c}$ using the Allen-Dynes formula. We assumed the effective screened Coulomb repulsion constant $\mu $* to be 0.1. We obtained T$_{c}$=18.19K at 140GPa, and the estimated values of T$_{c}$ are high and are close to the experimental values of Ca-V. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L40.00008: Pressure-induced insulator-metal and structural transitions of BaBiO$_3$ from first principles LDA+U Davide Ceresoli, Erio Tosatti At zero pressure and temperature BaBiO$_3$ is an insulator with a structural dimerization, equivalent to a static valence disproportionation of the two Bi ions per cell from 4+ to 3+/5+. Under pressure one would expect an insulator-metal transition and the eventual disappearance of the dimerization. Moreover, the metallic phase should be superconducting, in analogy the metal doped Ba$_{1-x}$K$_x$BiO$_3$ compounds. To date, there are no accurate ab initio predictions under pressure, essentially because LDA or GGA fail to stabilize an insulating phase with the correct distortion and electronic gap. We carried out first principles LDA+U calculations by determining the effective Hubbard U self-consistently at every pressure, and found that the presence of U is mandatory for a correct description of the zero-pressure state. Upon increasing pressure, we found an insulator to metal transition at $\sim$~20~GPa. By further increasing the pressure, we predict the appearance of a superconducting phase, characterized by quantum melting of the weakly dimerized CDW lattice. The dimerization tendency and superconductivity are expected to weaken only at much higher pressures, presently under investigation. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L40.00009: High pressure magnetic phase transitions in the quasi-2D ferromagnet, CeCrSb$_3$ investigated using designer diamond anvils D.D. Jackson, S.K. McCall, S.T. Weir, A.B. Karki, D.P. Young, W. Qiu, Y.K. Vohra Pressure tuning magnetic phase transitions is a powerful method of discovering new physical properties of materials. At ambient pressure, CeCrSb$_3$ undergoes ferromagnetic ordering at 115~K due to the Cr ions, followed by a gradual ferromagnetic alignment of the Ce moments between 48 and 18~K. The evolution of these magnetic transitions was investigated via electrical resistivity and ac magnetic susceptibility to pressures of 20~GPa using designer diamond anvils. The ferromagnetic ordering due to the Cr ions decreases at a rate of $dT_{Cr}/dP=$-1.75~K/GPa, while the onset of the Ce ferromagnetic ordered phase increases at a rate of $dT_{Ce}/dP = $3.6~K/GPa, followed by a sharp drop at $P_c=$11~GPa. In addition, the electrical resistivity reveals that a possible superconducting phase is found between 11~GPa$ |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L40.00010: Zirconium Hydride: Structural Integrity at High-Pressures - A Synchrotron X-Ray Difffraction Study Patricia E. Kalita, A. Cornelius, S. Sinogeikin, A. Martin, T. Hartmann, K.E. Lipinska-Kalita Metal hydrides are of great interest not only form the fundamental research point of view but also because of their many technological applications, including hydrogen storage. Here we present our most recent studies in situ, high-pressure, angle-dispersive, synchrotron x-ray diffraction studies of zirconium hydride. We investigate the effects of hydrostatic and non-hydrostatic conditions. We also show the results of structural refinements as well as the bulk modulus of ZrH2. *Work at UNLV is supported by DOE award No. DEFG36-05GO0850. Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. Use of the HPCAT facility was supported by DOE-BES, DOE-NNSA (CDAC), NSF, DOD --TACOM, and the W.M. Keck Foundation. Use of the APS was supported by DOE-BES, under Contract No. W-31-109-ENG-38. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L40.00011: Structural changes in borohydride hydrogen storage materials Ravhi Kumar, Andrew Cornelius, Malcolm Nicol Angle dispersive powder x-ray diffraction and Raman experiments were performed on ABH$_{4}$ (A = K, Rb) at high pressures up to 27 GPa. We demonstrate that KBH$_{4}$ exhibits structural phase transitions from the ambient $\alpha $-KBH4 phase (cubic Fm-3m) to $\beta $-KBH$_{4}$ (tetragonal-P421c) at 3.8 GPa and to $\gamma $-KBH$_{4}$ phase (orthorhombic-Pnma) at 6.8 GPa which is similar to the phase transition sequence observed for NaBH4 earlier [1]. However, RbBH$_{4}$ undergoes two successive pressure induced structural transitions from the ambient cubic Fm-3m phase to an orthorhombic Pnma phase around 2.9 GPa and then to a monoclinic phase above 8 GPa. The high pressure monoclinic phase is found to be stable up to 27 GPa. The experimental results reveal the phase transition sequence exhibited by RbBH$_{4}$ is different from the pressure induced changes observed in similar cubic compounds NaBH$_{4}$ and KBH$_{4}$. The results further show that both the transition pressure and the structural ordering at high pressures are influenced by the atomic size of the alkali cation in these compounds. [1]. R.S. Kumar and Andrew L. Cornelius, App.Phys.Lett., 2005, 87, 261916. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L40.00012: High-stress phases of SiC, GaN, InN, ZnO, and CdSe Kanoknan Sarasamak, Ambarish J. Kulkarni, Min Zhou, Sukit Limpijumnong Phase transformations of SiC, GaN, InN, ZnO, and CdSe from wurtzite (WZ) to three other different crystalline structures under loading of different stress tensors are studied using first-principle calculations. The first transformation studied is well known and occurs under hydrostatic compression and leads to a six-fold coordinated \textit{rocksalt} (RS) structure. The equilibrium pressures for this transformation of the materials are calculated and found to be proportional to the energy difference between the phases at zero stress and vary monotonically with the materials' ionicity. The second and third transformations studied occur under uniaxial stresses and lead to two new crystal structures previously unknown for these materials. Specifically, uniaxial compression along the [0001] direction or uniaxial tension along the $[01\overline 1 0]$ direction, causes a transformation to a five-fold coordinated \textit{unbuckled wurtzite }structure which we named HX. On the other hand, uniaxial tension along the [0001] direction causes the materials to transform into a body-centered-tetragonal structure which we named BCT-4. The critical equilibrium transformation stresses for these transformations are obtained and their correlation with the ionicity of the materials is analyzed. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L40.00013: Strength Measurements of Shock-Loaded Ta via Heterodyne Velocimetry J. R. Patterson, J. S. St\"olken, J. H. Nguyen, B. W. Reed, H. Hsieh, M. Kumar While knowledge of the constitutive properties of materials at elevated temperatures and pressures is necessary to understand material behavior under these conditions, experimental measurements are generally sparse. In an effort to explore such material behavior, we have performed several dynamic compression experiments on Ta at the 35 mm-bore single-stage gas-gun facility at LLNL. \emph{In-situ} particle velocities on shock-loading and release were measured by heterodyne( or photonic doppler ) velocimetry. We will present the results of a comparative study among different methods of time-frequency analysis, which is required to extract accurate particle velocities in rapidly varying regions. In addition, we have performed hydrocode simulations employing the MTS model to fit our experimental data. We will discuss these results in the context of extracting the flow stress at pressure. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L40.00014: Surface Preparation Methods to Enhance Dynamic Surface Property Measurements of Shocked Metal Surfaces Michael Zellner, Wendy McNeil, George Gray III, David Huerta, Nicholas King, George Neal, Jeremy Payton, Jim Rubin, Gerald Stevens, William Turley, William Buttler This effort investigates surface-preparation methods to enhance dynamic surface-property measurements of shocked metal surfaces. To assess the ability of making reliable and consistent dynamic surface-property measurements, the amount of material ejected from the free-surface upon shock release to vacuum (ejecta) was monitored for shocked Al-1100 and Sn targets. Four surface preparation methods were considered: fly-cut machined finish, diamond-turned machine finish, polished finish, and ball-rolled. The samples were shock loaded by in-contact detonation of HE PBX-9501 on the front-side of the metal coupons. Ejecta production at the back-side or free-side of the metal coupons was monitored using piezoelectric pins, optical shadowgraphy, and x-ray attenuation radiography. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L40.00015: Pressure Correction of Density Functional Theory Calculations Shun Hang Lee, Jones Tsz-Kai Wan The modern implementation of density functional theory algorithms involves approximations in the exchange correlation, which leads to discrepancies between experimental measurements and theoretical predictions. In this talk, we present a comparison of exchange correlation approximations by performing first-principles calculations on bulk structures such as MgO, MgSiO3 perovskite and post-perovskite at pressures up to deep Earth conditions. For a given structure, the calculated results such as the equation of state and bulk modulus corresponding to each exchange correlation are compared. At each volume, the pressure estimated by generalized gradient approximation (GGA) is usually above that by local density approximation (LDA,) resulting in a shift in the equations of states. At ambient conditions, such pressure difference is almost independent of pressure and temperature. However, at extreme pressure and temperature such as planetary interiors, this difference becomes temperature and pressure dependent and could lead to large errors in the predictions of properties for minerals at such conditions. In our study, we quantified such a pressure difference at different volume and temperature. [Preview Abstract] |
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