Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V1: Recent Advances in Soft Complex Materials Using Neutron Scattering
Sponsoring Units: DMPChair: Alan J. Hurd, Los Alamos National Laboratory
Room: Morial Convention Center LaLouisiane AB
Thursday, March 13, 2008 11:15AM - 11:51AM |
V1.00001: Neutron and X-ray Characterization of Nanostructured Polymeric Materials Invited Speaker: Controlling the orientation and lateral ordering of the block copolymer microdomains is key to their use as templates and scaffolds for the fabrication of nanostructured materials. Processes must be robust, rapid and simple to implement and should not introduce disruptive processing steps that would impede their use. Grazing incidence small angle x-ray scattering (GISAXS) and neutron neutron scattering have proven to be critical for the characterization of the static and real time development of structure in thin films of block copolymers. Here, studies on poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) diblock copolymers prepared from mixed solvents will be discussed that show highly oriented, cylindrical microdomains with a high degree of lateral order on a wide range of substrates, including silicon oxide, polystyrene, germanium, polyimide, and poly(butylene terephthalate). The preferential solvation of the P4VP block with an alcohol was used to induce a reconstruction that left a nanoporous film upon drying. The evaporation of gold onto the reconstructed films produced thermally stable films that are resistant to reactive ion etching. GISAXS was used to quantitatively examine the structure of these composite films and the transfer of the patterns to the underlying substrate. (research done in collaboration with Soojin Park, Jia-Yu Wang, Bokyung Kim University of Massachusetts), Benjamin Ocko (Brookhaven National Laboratory) and Jin Wang (Argonne National Laboratory). [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V1.00002: Beyond Wrinkles: Stress and Fold Localization in Thin Elastic Membranes Invited Speaker: Thin elastic membranes supported on fluid or elastic foundations deviate from their flat geometries upon compression. We demonstrate that the periodic and much studied wrinkled state is but one possible solution for such strained membranes. Folds, sharply localized solutions, appear whenever the membrane is compressed beyond a third of its initial wrinkle wavelength. Eventually the surface transforms into a symmetry broken state with flat regions of membrane coexisting with locally folded points, reminiscent of a crumpled unsupported membrane. We study this transition in many systems including lipid monolayers on liquid subphases of differing viscosity and use neutron and x-ray reflectivity to elucidate the role of the subphase in setting the wrinkle and fold size. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V1.00003: Complex Protein Structures by Neutron Scattering Invited Speaker: Neutron scattering by an atom, unlike X-ray scattering, does not depend on the atomic number of that atom. Deuterium atoms scatter neutrons to the same extent as carbon or oxygen atoms and give positive peaks in a nuclear density map, while its isotope, hydrogen, gives a negative peak. Therefore neutron diffraction provides two results that are difficult to obtain from macromolecular X-ray diffraction studies: (1) the locations of hydrogen atoms, including the more mobile ones, and (2) the extent to which a hydrogen atom can be replaced by deuterium. The method shows whether an amino acid side chain (at a given pH value) is ionized or not. For example, one can ascertain whether histidine residues are singly or doubly protonated at the pH of study. Neutron diffraction studies can also be used to determine the absolute configuration of the course of a biochemical reaction by anomalous scattering and enzymatic deuteration of the substrate. Neutron diffraction experiments, however, require large crystals and these are often impossible to obtain for many macromolecules. Examples of reports of the use of neutron diffraction to provide information on enzymatic mechanism will be presented. This includes descriptions of our work on the enzyme D-xylose isomerase for which the orientation of a metal ion-bound water molecule in the active site was found. This water, thought to be involved in the isomerization step, was shown to be water (rather than hydroxyl) at pH 8.0. This analysis also revealed that one lysine has two rather than three attached hydrogen atoms and therefore lacks a positive charge. High-resolution X-ray studies (at 0.94 {\AA}) indicate how some side chains might move during catalysis. This combination of neutron and X-ray diffraction can contribute greatly to the elucidation of enzyme mechanisms. I thank Amy Katz, Xinmin Li, H. L. Carrell, Leighton Coates, Leif Hanson, Joel Harp, Paul Langan, and Benno Schoenborn who were involved in many of the described studies, and particularly Gerard Bunick. We honor his contributions and regret that he is no longer with us. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V1.00004: Observation of a Fragile-to-Strong Dynamic Crossover Phenomenon in Confined Water and Its Relation to the Existence of a Liquid-Liquid Critical Point in Supercooled Water Invited Speaker: We have observed a fragile-to-strong dynamic crossover phenomenon of $\alpha $ relaxation time and self-diffusion constant in deeply supercooled 1-d confined water. The $\alpha $ relaxation time is measured by quasi-elastic neutron scattering and the self-diffusion constant by nuclear magnetic resonance. Water is confined in 1-d geometry in cylindrical pores of porous silica material, MCM-41 and in double-wall carbon nanotubes. The crossover phenomena can also be observed from appearance of a Boson peak in an incoherent inelastic neutron scattering. We observe a pronounced violation of the Stokes-Einstein relation at and below the crossover temperature at ambient pressure. Upon applying pressure to the confined water, the crossover temperature is shown to track closely the Widom line emanating from the existence of a liquid-liquid critical point buried in an unattainable deeply supercooled state of bulk water. Relation of the dynamic crossover phenomena to the existence of a density minimum in supercooled confined water will be discussed. The crossover temperature is shown to be sensitively dependent on the degree of hydrophilicity of the confining substrate. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 2:15PM |
V1.00005: Phase Behavior of Block Copolymer/Inorganic Nanoparticle Composites Invited Speaker: Block copolymers offer as versatile platforms for the fabrication of hybrid nanocomposites with ordered phases useful for various nanotechnology applications. Although the phase behavior of block copolymers is well established the effects of inorganic nanoparticle loading on their phase behavior are not well understood. We carried out a systematic study on the phase behavior of block copolymers with well dispersed nanoparticles. To achieve excellent dispersion of nanoparticles in the polymer phase we used grafted nanoparticles with small polymer chains compatible to a preferred domain of the block copolymer. The nanoparticles sequestered in a preferred domain have profound effects on the thermodynamically induced microphase separation of the block copolymers. To characterize the phase behavior of these systems in a selective solvent we used small angle neutron scattering and that in their bulk and thin film architectures was studied using synchrotron based small angle x-ray scattering and grazing incidence small angle scattering techniques. A number of molecular properties such as the molecular weight of the polymer, segment volume fraction, Flory-Huggins $\chi$ parameter and the nanoparticle concentration influence the state of dispersion of nanoparticles and the nanocomposite morphology in bulk and thin film architectures. The addition of homopolymers provides as yet another variable to alter the interfacial tension and to slow the order-disorder transition. We also probed the nanoscale dynamics at the polymer/nanoparticle interfaces in these systems by using x-ray photon correlation spectroscopy. The dynamics of nanoparticles in the composites is strongly dependent on the dimensionality of the morphology of the block copolymer. Furthermore, the interfacial interaction at the polymer/particle interface plays significant role in the stress relaxation in the composites. \newline \newline $^1$This work was done in collaboration with Chieh-Tsung Lo, Byeongdu Lee, Randall E. Winans, Alec Sandy, XSD Division, Advanced Photon Source and Vilas Pol, David Bohnsack, IPNS, Argonne National Laboratory. \newline $^2$Work benefited from the use of facilities at Argonne National Laboratory funded by DOE, BES under contract DE-AC02-06CH11357 to the UChicago Argonne, LLC. [Preview Abstract] |
Session V2: Facilities for Meeting Grand Challenges
Sponsoring Units: DCMP GIMSChair: Gwyn Williams, Thomas Jefferson Laboratory National Accelerator Facility
Room: Morial Convention Center LaLouisiane C
Thursday, March 13, 2008 11:15AM - 11:51AM |
V2.00001: A Green Field Fourth Generation Light Source Invited Speaker: The success of energy recovering linac technology has opened up new opportunities for the development of light sources to satisfy grand challenges in fundamental physics and materials research. A number of laboratories around the world have proposed extensions or upgrades to existing 2$^{nd}$ or 3$^{rd}$ generation light source facilities to take advantage of the higher brightness and short pulse lengths that 4$^{th}$ generation facilities could offer. Ideas range from multiple synchronized coherent sources in the THz to UV range to Compton X-ray sources, synchrotron emission, and on to achievement of multi-particle coherence in amplifiers or even oscillators at short wavelengths. Such proposals are exciting and begin to show the range of performance that such systems can provide. In this talk I will examine the possibilities for light source development unconstrained by existing physical layouts i.e., on a ``green field'' site. I will then address a set of photonic goals to be achieved and the technological path of development desirable to achieve the full benefits of this next generation of user facilities. In particular, there are specific technical achievements and engineering developments with great leverage on the cost and performance of future machines. I will point toward a development path to set the stage for optimization of technical performance and cost/benefit of this system. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V2.00002: Science Driven the Parameters of 4th Generation Light Sources Invited Speaker: The scientific challenges that can be best addressed with x-rays and the evolution of accelerator based x-ray sources has focused the attention of the x-ray community on new concepts: using the electron beam once rather than storage rings. We will describe this approach and the photon source properties that result with emphasis on peak and average brightness for the study of structure and dynamics in the chemical, materials and biological sciences. Examples from 4th generation sources planned and under construction will be presented. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V2.00003: The Science Motivating the UK's Fourth Generation Light Source Project Invited Speaker: The UK is committed to developing a proposal for a long wavelength (THz - soft X-ray) 4$^{th}$ generation light source that will provide short-pulse synchronised beams to be used in combination. Some of the science motivating the development of this facility is described. The facility will probe ultra-fast dynamics in a wide range of fields. `Pump-probe' experiments will allow the study of chemical reactions and short-lived intermediates on the timescale of bond formation, even for very dilute species. Circularly polarised light sources in a variety of frequency regimes will be used for example to manipulate and monitor carrier charge and spin transport in device structures. The high intensity of the free electron laser (FEL) radiation will allow high resolution in imaging and the opportunity to probe nonlinear regimes and new states of matter. Lower intensity, high repetition rate spontaneous radiation will provide ideal sources for ultra-high energy resolution spectroscopy, especially in the solid state. Around {\pounds}22 M funding has been obtained for the first stages of the project (construction of a prototype energy recovery linac (ERL) source, science demonstrations and design study work, currently underway at STFC Daresbury Laboratory). [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V2.00004: The proposed BigLight fourth-generation light source at the National High Magnetic Field Laboratory Invited Speaker: ``BigLight'' is a compact fourth-generation light source planned for the National High Magnetic Field Laboratory's (NHMFL) quasistatic-field facility at Tallahassee. Designed by George Neil and his team at the Jefferson Laboratory, it is based on an RF superconducting accelerator system driving three free-electron lasers (FELs) that cover the wavelength range from 1 mm to 1.5 microns, plus a broadband THz source. BigLight's specification was derived in a series of five workshops covering potential applications in condensed matter physics, nanoscience, biophysics, chemistry, material processing, microscopy, astrophysics and other disciplines; participants included scientists from current FEL laboratories, potential users and international experts on light sources and high magnetic fields. Consequently, in addition to the robustness and user-friendliness demanded of a user facility, the source has several unique features, including the possibility of running the near- and mid-infrared FELs and the broadband THz source simultaneously; this will allow a wide variety of multi-color, time-resolved, pump-probe and pump-probe-probe experiments with an extremely small timing jitter between pulses. BigLight will also be able to produce either a continuous stream of micropulses ($\sim$1 ps time resolution) or flexible macropulse configurations. Co-location with the world’s highest quasistatic fields presents some unique experimental possibilities; potential users have proposed novel ultra-high-field time-resolved EPR measurements, dynamic nuclear polarization, nonlinear cyclotron resonance, interband magneto-optics, FTICR and many other techniques with very general applicability. In the talk, I will outline the scientific desiderata that have shaped the design of BigLight, and describe some of the proposed experiments. [Preview Abstract] |
Session V3: Industrial Physics History
Sponsoring Units: FHPChair: Gloria B. Lubkin, American Institute of Physics
Room: Morial Convention Center RO2 - RO3
Thursday, March 13, 2008 11:15AM - 11:51AM |
V3.00001: Industrial Research at IBM Invited Speaker: |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V3.00002: Reflections on Three Corporate Research Labs: Bell Labs, HP Labs, Agilent Labs Invited Speaker: This will be a personal reflection on corporate life and physics-based research in three industrial research labs over three decades, Bell Labs during the 1980's, HP Labs during the 1990's, and Agilent Labs during the 2000's. These were times of great change in all three companies. I'll point out some of the similarities and differences in corporate cultures and how this impacted the research and development activities. Along the way I'll mention some of the great products that resulted from physics-based R{\&}D. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V3.00003: Application Oriented R{\&}D: Aphorisms {\&} Anecdotes (The John Bardeen Lecture) Invited Speaker: I have learned many lessons from long experience trying to perform, and lead, application oriented R{\&}D. Some of these lessons have been encapsulated in aphorisms. Some aphorisms will be presented, with explication, and illustrated with anecdotes from experience with the NAVY, ARPA, NASA and General Motors. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V3.00004: The History of Science and Technology at Bell Labs Invited Speaker: Over the last 80 years, Bell Labs has been one of the most scientifically and technologically productive research labs in the world. Inventions such as the transistor, laser, cell phone, solar cell, negative feedback amplifier, communications satellite and many others were made there. Scientific breakthroughs such as discovery of the Big Bang, the wave nature of the electron, electron localization and the fractional quantum hall effect were also made there making Bell Labs almost unique in terms of large impacts in both science and technology. In my talk, I will discuss the history of the lab, talk about the present and give some suggestions for how I see it evolving into the future. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 2:15PM |
V3.00005: 50 Years of ``Scaling'' Jack Kilby's Invention Invited Speaker: This year is the 50th anniversary of Jack Kilby's 1958 invention of the integrated circuit (IC), for which he won the 2000 Nobel Prize in Physics. Since that invention in a laboratory at Texas Instruments, IC components have been continuously miniaturized, which has resulted in exponential improvement trends in their performance, energy efficiency, and cost per function. These improvements have created a semiconductor industry that has grown to over {\$}250B in annual sales. The process of reducing integrated-circuit component size and associated parameters in a coordinated fashion is traditionally called ``feature-size scaling.'' Kilby's original circuit had active (transistor) and passive (resistor, capacitor) components with dimensions of a few millimeters. Today, the minimum feature sizes on integrated circuits are less than 30 nanometers for patterned line widths and down to about one nanometer for film thicknesses. Thus, we have achieved about five orders of magnitude in linear-dimension scaling over the past fifty years, which has resulted in about ten orders of magnitude increase in the density of IC components, a representation of ``Moore's Law.'' As IC features are approaching atomic dimensions, increasing emphasis is now being given to the parallel effort of further diversifying the types of components in integrated circuits. This is called ``functional scaling'' and ``more then Moore.'' Of course, the enablers for both types of scaling have been developed at many laboratories around the world. This talk will review a few of the highlights in scaling and its applications from R{\&}D projects at Texas Instruments. [Preview Abstract] |
Session V4: Electronic Excitations in Organic Molecular Crystals/Fluctuating Fronts: Beyond a Popular Mean-Field Theory
Sponsoring Units: DCMPChair: Vitaly Podzorov, Rutgers University
Room: Morial Convention Center 206
Thursday, March 13, 2008 11:15AM - 11:51AM |
V4.00001: Photoemission study on the charge transport mechanism in pentacene thin film Invited Speaker: Organic molecules are attracting much interest to use for a variety of electronic applications. Pentacene (Pn), which is one of such molecules, has a high application potential due to its high hole mobility. The hole mobility of Pn is almost comparable to that of amorphous silicon. At low temperature, the hole transport in Pn crystal has been reported to have a band-like nature, and the band-like charge transport is expected to play a major part at room temperature as well. One of the origins of the band-like transport is the adequate overlap of the pi-orbitals of adjacent molecules, which produces orbital-derived electronic bands. It is therefore essential to have a proper understanding on the electronic band structures in order to fully comprehend the charge transport mechanism of a Pn crystal. By using angle-resolved photoelectron spectroscopy (ARPES), we have measured the dispersions of the highest occupied molecular orbital (HOMO)-derived bands of single crystal Pn monolayer films grown on substrates. Two HOMO-derived, whose band dispersion widths are larger than the values predicted by theoretical calculations, were clearly observed in the ARPES spectra. Our result indicates that the overlap of the pi-orbitals of adjacent Pn molecules is larger than what was expected from theoretical calculations, and the observed dispersions suggest that the higher binding energy HOMO-derived band mainly contributes to the band-like charge transport mechanism of a Pn crystal. By analyzing the dispersions within a simple tight-binding approximation, the obtained results lead to a hole mobility of $mu_h>$34.1 cm$^2$/Vs at 140 K. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V4.00002: Bandlike transport in organic molecular crystals revealed by subpicosecond transient photoconductivity Invited Speaker: The nature of charge carrier photogeneration and transport in organic molecular crystals is not completely understood. In particular, the mechanism responsible for the observed bandlike behavior of charge carriers in these materials, where the carrier mobility increases as the temperature decreases, remains unresolved and is the focus of much research. Using typical device structures to explore intrinsic properties of charge transport in organic semiconductors is complicated by the presence of defects and the necessity to make contacts to the sample. Recently, however, ultrafast techniques that use terahertz (THz) pulses for assessing the electronic properties of materials have been developed. In particular, time-resolved THz spectroscopy allows transient photoconductivity in materials to be probed with subpicosecond time resolution, providing a sensitive non-contact tool for studying the transport of charge carriers before they are trapped at defect sites. This talk will provide an overview of how THz pulses can be used to probe the nature of conductivity and bandlike behavior in organic molecular crystals and thin films. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V4.00003: Polarons and Coulomb interactions in organic transistors Invited Speaker: In organic Field Effet Transistors (FETs), charge carriers accumulate in a two-dimensional layer at the interface between an organic crystal and a gate dielectric. The possibility of tuning several microscopic parameters such as the carrier density, the electron-electron and electron-phonon interactions makes these devices an interesting playground for fundamental physics. Recent experiments have demonstrated that depending on the gate insulator used, the electric mobility in organic FETs can be tuned from metallic-like to insulating-like. This phenomenon can be explained in terms of the formation of small polarons, due to the remote interaction of the charge carriers with the phonons of the gate material [1]. In the devices with the highest polarizabilities, experiments performed at large gate voltages (corresponding to $\sim 0.1$ carriers/molecule) have revealed a further reduction of the mobility, suggesting the onset of electron-electron interactions [2]. The physics of this novel regime involving both strong electron-phonon and long-range electron-electron interactions will be discussed. If time allows, I shall briefly present how the above picture is modified when the narrow-band organic crystal is replaced by graphene ---a two-dimensional sheet of carbon atoms. Although the effect is less striking in that case, the remote scattering with the substrate phonons still constitutes an important limiting factor of the mobility at room temperature, that should be addressed for the design of future graphene devices [3]. \\ References:\\ $[1]$ I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iosad, G. Rastelli, S. Ciuchi and A. F. Morpurgo, Nature Materials 5, 982-986 (2006)\\ $[2]$ S. Fratini, H. Xie, I. N. Hulea, S. Ciuchi, and A. F. Morpurgo, arXiv:0710.2845 preprint (2007)\\ $[3]$ S. Fratini, F. Guinea, arXiv:0711.1303 preprint (2007) [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V4.00004: Particle versus density models in spark formation: X-rays from pulled fronts? Invited Speaker: Streamer discharges govern the early stages of sparks and lightning, of spark-like phenomena in water, oil, and semiconductors, in industrial corona reactors, or in gigantic sprite discharges above thunderclouds [1,2]. Thunderstorms recently have been found to emit terrestrial gamma-ray flashes or X-rays towards satellites and towards the ground. These emissions might be explained by particle models of ``pulled'' streamer ionization fronts. In general, the growing discharge channel has an inner structure with multiple scales [1-3]. While the largest part of this channel can be treated in a density approximation for the electrons and ions, the dynamics of the ionization front is that of a pulled front; it is determined in the leading edge where the density approach eventually breaks down. We therefore investigate a realistic MC particle model for the motion of single electrons in a discharge in pure nitrogen. The particle model not only incorporates particle fluctuations, but also shows that the electron energies are systematically larger in the leading edge of the front than in the corresponding density model, and that the ionization level behind the front is higher as well, while the front velocity hardly changes [3]. These effects increase with increasing applied electric field and might actually cause the recently observed X-ray emission from lightning through rare very energetic runaway electrons in the tail of the distribution. Comparing the leading edge of the particle front with a linear particle avalanche, the avalanche shows the same mean density gradient and energy overshoot in its leading edge as the nonlinear front; hence the pulled front concept in this sense applies to discrete particle models as well [3]. This gives a key to understanding the above effects through analytical approximations and to develop efficient numerical methods coupling particle and density models in space.\\ {[1]} U. Ebert {\it et al.}, Plasma Sources Sci. Techn. {\bf 15}, S118 (2006) (arXiv:physics/0604023).\\ {[2]} {\it Streamers, sprites, leaders, lightning: From micro- to macroscales}, workshop in Oct. 2007: \\ {\tt http://www.lorentzcenter.nl/lc/web/2007/265/info.php3?wsid=265}; and cluster issue in J. Phys. D in fall 2008; organizers/editors: U. Ebert and D.D. Sentman.\\ {[3]} C. Li {\it et al.}, J. Appl. Phys. {\bf 101}, 123305 (2007) (arXiv:physics/0702129). [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 2:15PM |
V4.00005: Fluctuation Effects on Propagating Waves of Self-Assembly in Organosilane Monolayers. Invited Speaker: Wavefronts associated with reaction--diffusion and self-assembly processes are ubiquitous in the natural world. For example, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes, in polymerization fronts involved in cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. Although it is often claimed that self-sustaining or autocatalytic front propagation is well described by mean-field ``reaction-- diffusion'' or ``phase field'' ordering models, it has recently become appreciated from simulations and theoretical arguments that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean-field theory (MFT) of this type of front propagation. The present work explores these fluctuation effects in a real physical system. In particular, we consider a high-resolution near-edge x-ray absorption fine structure spectroscopy (NEXAFS) study of the spontaneous frontal self-assembly of organosilane (OS) molecules into self-assembled monolayer (SAM) surface-energy gradients on oxidized silicon wafers. We find that these layers organize from the wafer edge as propagating wavefronts having well defined velocities. In accordance with two-dimensional simulations of this type of front propagation that take fluctuation effects into account, we find that the interfacial widths w(t) of these SAM self-assembly fronts exhibit a power-law broadening of in time w(t) $\sim $ t$^{\beta }$, rather than the constant width predicted by MFT. Moreover, the observed exponent values accord rather well with previous simulation and theoretical estimates. These observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials-processing contexts. [Preview Abstract] |
Session V5: Panel Discussion: Lessons Learned from Katrina: How to Prepare a Department for Catastrophic Events
Sponsoring Units: FPS FGSAChair: Andrew Post-Zwicker, Princeton Plasma Physics Laboratory
Room: Morial Convention Center RO1
Thursday, March 13, 2008 11:15AM - 11:30AM |
V5.00001: Lessons Learnt From Hurricane Katrina. Invited Speaker: Hurricane Katrina devastated New Orleans and its suburbs on Monday August 29$^{th}$, 2005. The previous Friday morning, August 26, the National Hurricane Center indicated that Katrina was a Category One Hurricane, which was expected to hit Florida. By Friday afternoon, it had changed its course, and neither the city nor Xavier University was prepared for this unexpected turn in the hurricane's path. The university had 6 to 7 ft of water in every building and Xavier was closed for four months. Students and university personnel that were unable to evacuate were trapped on campus and transportation out of the city became a logistical nightmare. Email and all electronic systems were unavailable for at least a month, and all cell phones with a 504 area code stopped working. For the Department, the most immediate problem was locating faculty and students. Xavier created a list of faculty and their new email addresses and began coordinating with faculty. Xavier created a web page with advice for students, and the chair of the department created a separate blog with contact information for students. The early lack of a clear method of communication made worse the confusion and dismay among the faculty on such issues as when the university would reopen, whether the faculty would be retained, whether they should seek temporary (or permanent) employment elsewhere, etc. With the vision and determination of President Dr. Francis, Xavier was able to reopen the university in January and ran a full academic year from January through August. Since Katrina, the university has asked every department and unit to prepare emergency preparedness plans. Each department has been asked to collect e-mail addresses (non-Xavier), cell phone numbers and out of town contact information. The University also established an emergency website to communicate. All faculty have been asked to prepare to teach classes electronically via Black board or the web. Questions remain about the longer term issues of the size and stability of the faculty. [Preview Abstract] |
Thursday, March 13, 2008 11:30AM - 11:45AM |
V5.00002: TBD Invited Speaker: This abstract has not been submitted. [Preview Abstract] |
Thursday, March 13, 2008 11:45AM - 12:00PM |
V5.00003: Hurricane Katrina at Tulane. Invited Speaker: After hurricane Katrina struck New Orleans on August 29, 2005, Tulane University closed for the fall semester. Buildings on campus were closed and armed guards were hired to protect the campus. Faculty members were not allowed access to their offices and laboratories, except for exceptional cases when a Dean went with them. Many faculty members took their research groups to other universities accepting \textit{much welcomed} invitations from colleagues. Undergraduates went to other colleges and universities, which accepted the without cost and a promise not to recruit them. The university email system went down for months. Collecting information on the welfare of faculty and students was difficult. The university was run from Houston by a small handful of senior administrators. Setting up the schedule of classes for the spring 2006 semester was done without records. Most faculty returned to New Orleans after several weeks. 80{\%} of the city was flooded. Small trailers were provided. Some lived in the FEMA trailers for two years or more. When Tulane reopened, a wide reaching Renewal Plan, worked out by the upper administration, was implemented. A new \textit{emergency preparedness plan} was also developed and put in place. [Preview Abstract] |
Thursday, March 13, 2008 12:00PM - 12:15PM |
V5.00004: Invited Speaker: |
Thursday, March 13, 2008 12:15PM - 12:30PM |
V5.00005: Academic environment and dynamics in response to extreme events: Theory and Practice (Katrina Lessons) Invited Speaker: The possibility of a catastrophic event requires the department as a unit and the university as an organization to devise a comprehensive emergency response plan to minimize the impact and shorten the recovery stage. Does the academic organizational structure and environment possess key features for the possibility of successful response to extreme events? The post Hurricane Katrina experience of Louisiana universities offers data to address this theoretical question. It also emphasizes that the mitigation plan should include two aspects: preparing/protecting a university for/during a catastrophic event and assisting other academic institutions experiencing an extreme event. Short-term and longer-term statistics and other data pertain to the interaction of the University of Louisiana at Lafayette (as an assistance unit) with the universities in New Orleans (units in distress), including the dynamics of student population, faculty influx, course adjustments, and response and recovery actions are presented. An attempt is made to categorize the losses and to assess the recovery quality and time. Faculty and institutional administration interviews are summarized to assist in developing future proactive response plans. UL Lafayette and UNO research capabilities and intellectual resources for developing complex models simulating the multi-variable effects of catastrophic events and providing adaptability in the decision-making process are investigated. [Preview Abstract] |
Thursday, March 13, 2008 12:30PM - 12:45PM |
V5.00006: Hosting a Katrina Evacuee. Invited Speaker: No individual or institution anticipated the impact on the academic research community of hurricane Katrina. When Tulane physicist Wayne Reed asked me to host his research group just a day or two after the disaster, with no authorization or understanding of the commitment, I agreed immediately and then pondered implications. Fortunately, colleagues helped in making the commitment real, only the bureaucracy of my public university posing small hindrances. Industry was remarkably generous in providing Reed with significant ``loaner'' equipment, and amazingly, a suite of custom Reed experiments was running within weeks. At the end, the most productive collaborations for Reed seemed not to have been with my group, with its similar research, but to other groups at my institution, particularly the synthetic chemists, who gained access to methods previously unique to Tulane while offering samples previously unique to UMass. Quickly designed projects exploiting this match turned out remarkably productive. Although begun with trepidation, hosting of Reed had huge positive benefits to me and UMass, and I believe, also to Reed and Tulane. Some key lessons for the future: (i) industry has capacity and willingness to help academic research during disruption (ii) commitment of a host institution must be immediate, without a wait for formal approvals or arrangement of special funding -- delay leads only to discouragement, (iii) continuing academic progress of displaced students must come first, and (iv) intellectual synergy rather than overlap should be the basis for seeking a host. Lastly, NSF or other funding agency should consider a program directly addressing the research needs of unexpectedly disrupted academic scientists, and most particularly, graduate students who face greatly extended studies. [Preview Abstract] |
Thursday, March 13, 2008 12:45PM - 2:15PM |
V5.00007: Panel Discussion
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Session V7: Oscillations and Segmentation: Dynamical Genetic Regulation in Time and Space
Sponsoring Units: DBP GSNPChair: Mogens Jensen, Niels Bohr Institute
Room: Morial Convention Center RO5
Thursday, March 13, 2008 11:15AM - 11:51AM |
V7.00001: Building the Vertebrate Spine Invited Speaker: The vertebrate body can be subdivided along the antero-posterior (AP) axis into repeated structures called segments. This periodic pattern is established during embryogenesis by the somitogenesis process. Somites are generated in a rhythmic fashion from the paraxial mesoderm and subsequently differentiate to give rise to the vertebrae and skeletal muscles of the body. Somite formation involves an oscillator-the segmentation clock-whose periodic signal is converted into the periodic array of somite boundaries. This clock drives the dynamic expression of cyclic genes in the presomitic mesoderm and requires Notch and Wnt signaling. Microarray studies of the mouse presomitic mesoderm transcriptome reveal that the segmentation clock drives the periodic expression of a large network of cyclic genes involved in cell signaling. Mutually exclusive activation of the Notch/FGF and Wnt pathways during each cycle suggests that coordinated regulation of these three pathways underlies the clock oscillator. In humans, mutations in the genes associated to the function of this oscillator such as \textit{Dll3} or \textit{Lunatic Fringe} result in abnormal segmentation of the vertebral column such as those seen in congenital scoliosis. Whereas the segmentation clock is thought to set the pace of vertebrate segmentation, the translation of this pulsation into the reiterated arrangement of segment boundaries along the AP axis involves dynamic gradients of FGF and Wnt signaling. The FGF signaling gradient is established based on an unusual mechanism involving mRNA decay which provides an efficient means to couple the spatio-temporal activation of segmentation to the posterior elongation of the embryo. Another striking aspect of somite production is the strict bilateral symmetry of the process. Retinoic acid was shown to control aspects of this coordination by buffering destabilizing effects from the embryonic left-right machinery. Defects in this embryonic program controlling vertebral symmetry might lead to scoliosis in humans. Finally, the subsequent regional differentiation of the precursors of the vertebrae is controlled by \textit{Hox} genes, whose collinear expression controls both gastrulation of somite precursors and their subsequent patterning into region-specific types of structures. Therefore somite development provides an outstanding paradigm to study patterning and differentiation in vertebrate embryos. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V7.00002: Modelling Ultradian Oscillations and Segmentation Invited Speaker: We model ultradian oscillations in four different eucaryotic systems: Hes1, p53-mdm2, NF-kB and Wnt-Notch. In each of the systems we identify the feed-back loops for the genetic regulations. Oscillations are possible when time delays are present, either by directly introducing a delay, by many steps in the loops or by saturated degradation. The oscillations are important for apoptosis and control of inflammation. The Wnt-Notch system is essential in embryo segmentation and we introduce a model in which the Wnt oscillates by itself but drives the Notch cycle out of phase with the Wnt cycle, in good agreement with experimental observations. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V7.00003: Spatial Patterns of Recurved Sensory Organs in Drosophila Invited Speaker: The fruit fly Drosophila is one of the most intensely studied models of development. A subset of -nominally- identical cells on the anterior wing of Drosophila begins to differentiate at puparium formation, each developing a sensory organ. In wild type flies, every fifth cell becomes such a sensory organ. Recent studies on mutant flies have shown that the transcription factor Senseless and the micro RNA miR-9a play significant roles in the choice of bristle density and the regularity of their arrangement. We propose that this cell differentiation is due to a Turing-type bifurcation whereby periodic concentration gradients emerge spontaneously from a uniform background. A paradigmatic model with intra-cellular networks and lateral activation and inhibition between neighboring cells (for example, through the Notch signaling pathway) is shown to generate the observed arrangements of sensory organs. The theory makes several experimentally verifiable predictions. For example, we propose methods to create mutant flies with systematically increasing numbers of ectopic bristles. In our theory, post-transcriptional regulatory action of the micro RNA occurs through the choice of stable solutions of the network. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V7.00004: Similarities and differences in the p53-mdm2 and NF-kB feedback loops Invited Speaker: Ultradian oscillations in the p53 and NF-kB signalling systems are produced using similar mechanisms: a negative feedback loop combined with an effective time delay. However, seemingly small differences in the molecular implementation of this mechanism mean that the NF-kB system is in equilibrium in the resting state, while the p53 system is far from equilibrium. I will discuss how this affects the dynamical response of the systems. In particular, I will argue that the nonequilibrium driving makes the p53 system respond much faster to external stimuli than the NF-kB system. The interesting question then is whether this makes sense physiologically, and is consistent with the fact that p53 triggers cell-cycle arrest and apoptosis, while NF-kB triggers the immune response. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 2:15PM |
V7.00005: Dynamic Changes in microRNAs may Regulate Robustness of Wnt/Notch Signaling Invited Speaker: The mechanisms by which highly reproducible patterns are formed during embryonic development and organismal evolution despite stochasticity at the single cell level is one of the remaining mysteries in Biology. It has been proposed that a hidden layer of regulation formed through the interaction of microRNAs with protein coding gene networks maybe responsible. Recently developed next generation sequencing technologies afford an unprecedented opportunity to uncover novel aspects of miRNA function and evolution. We find extensive heterogeneity in sequences that correspond to mmu-let-7 (targets Wnt1) and mmu-miR-191 (targets Notch1). Approximately 20{\%} of let-7 and miR-191 have undergone modifications to increase stability and binding to the Wnt1 and Notch1 targets and are likely to be destroyed. In contrast, 80{\%} bind the targets with imperfect complementarity and lower stability and are likely to be sequestered and prevented from forming protein. We propose that these two species together form a highly fluid system that is able to absorb stochastic perturbations in gene expression. A gene that goes on to be translated into functional protein therefore must escape both buffers by significantly high expression. [Preview Abstract] |
Session V8: Liquid Crystals I: Structure and Defects
Sponsoring Units: DFDChair: Oleg Lavrentovich, Kent State University
Room: Morial Convention Center RO6
Thursday, March 13, 2008 11:15AM - 11:27AM |
V8.00001: Effect of Concentration Variations on the Interaction of a Sm-A Liquid Crystal and a Nanoparticle Luz J. Martinez-Miranda, Lynn K. Kurihara, Rahina S. Rabiu We have observed the evolution of the interaction between a Sm-A Liquid Crystal (8CB) and a nanoparticle as the concentration of the nanoparticle is reduced from 30{\%} wt to 0.1{\%}wt. We have observed that a linear structure is observed as the concentration of the particle falls below 15{\%} wt. There is a difference between the functionalization compounds as observed in the study of the 30{\%} wt mixtures. In addition, we have observed that the influence of the nanoparticle in the ordering or disordering of the liquid crystal can be quantified through the integrated intensity of the x-ray signal. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V8.00002: Simulating defect structures in nematic liquid crystal shells Lena Lopatina, Andrew Konya, Jonathan Selinger, Robin Selinger, Alex Travesset Recent theoretical and experimental studies have investigated nematic liquid crystals confined to a shell between two spheres. When the shell is very thin, the structure provides an experimental realization of nematic order and defects in a 2D curved geometry. As the shell becomes thicker, the behavior crosses over to a 3D liquid crystal, with different types of defects. To study this dimensional crossover, we perform simulations of nematic order in a shell. For these simulations, we use a disordered lattice, or mesh, constructed through random sequential adsorption on the inner surface, the outer surface, and within the bulk of the shell, with a nematic director on each site of the mesh. By minimizing the energy, we determine the nematic texture as a function of the radii and thickness of the shell, and as a function of the off-center displacement of the inner sphere. The results show a crossover between half-charged vortex line defects for thin shells and boojum pairs for thicker shells, and demonstrate a new equilibrium state with two vortex lines and one boojum pair. They also show a complex evolution of the structures and energy as the inner sphere moves off-center. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V8.00003: String defects in smectic-C monolayers and hybrid nematic films Fangfu Ye, Zhao Lu, Lena Lopatina, Jonathan Selinger, Alex Travesset Defects are important in determining the structure and statistical mechanics of liquid crystals. In this project, we study the structure of topological defects in smectic-C monolayers and hybrid nematic films. When subjected to boundary conditions requiring a total topological charge of +1, the classical xy model in a flat disk geometry has a ground state of a single vortex of charge +1. By comparison, perfect nematics with directors in the same 2D geometry have two +1/2 vortices repelling each other. We show that slightly tilting directors out of the plane to form a smectic-C monolayer yields two +1/2 vortices bound together by a domain wall, which we call a string defect. We also develop a model easily testable in experiments, in which a thick nematic film is constrained in a cell with weak homeotropic anchoring on one surface and strong planar anchoring on the other surface. For this model, we derive the phase diagram and investigate how the length of string defects changes with the thickness of the cell. We further present numerical simulations of the nematic director in the hybrid film. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V8.00004: Coherent anti-Stokes Raman scattering polarized microscopy of 3D director structures in liquid crystals Ivan Smalyukh, Alexander Kachynski, Andrey Kuzmin, Paras Prasad We demonstrate labeling-free three-dimensional imaging of director structures in liquid crystals using coherent anti-Stokes Raman scattering (CARS) polarized microscopy [1]. Spatial mapping of the structures is based on the strong sensitivity of a polarized CARS signal to the orientation of selected chemical bonds of anisotropic molecules in liquid crystals. As an example, we study director structures in cholesteric, nematic, and smectic materials. We demonstrate that the CARS images of molecular orientation patterns are consistent with the structure models and with the respective computer-simulated CARS textures. \newline [1]. A.V. Kachynski, A.N. Kuzmin, P.N. Prasad, and I.I. Smalyukh, Appl. Phys. Lett. 91, 151905 (2007). [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V8.00005: Light Scattering Study of Biaxiality in Nematic Liquid Crystal Tetrapodes Krishna Neupane, Shinwoong Kang, Sunil Sharma, D. Carney, T. Meyer, George H. Mehl, David W. Allender, Satyendra Kumar, Samuel Sprunt We have performed dynamic light scattering studies on thermotropic liquid crystalline tetrapodes [1], which reportedly exhibit a uniaxial to biaxial nematic phase transition. Our results [2] support the existence of the biaxial nematic phase in tetrapodes. The uni - to biaxial transition is found to be weakly first-order in a 4-ring tetrapode and second-order in a 3-ring tetrapode, while the isotropic to uniaxial nematic transition is weakly first order in both materials. The temperature dependence of the relaxation rates of the biaxial order parameter modes, and of the intensity associated with biaxial director fluctuations, is explained by a Landau-deGennes model of the free energy. \newline [1] R. Elsasser, J. W. Goodby, G. H. Mehl, D. Rodriguez-Martin, R. M. Richardson, D. J. Photinos, and M. Veith, Mol. Cryst. Liq. Cryst. \textbf{402}, 237 (2003) \newline [2] K. Neupane, S. W. Kang, S. Sharma, D. Carney, T. Meyer, G. H. Mehl, D. W. Allender, S. Kumar, and S. Sprunt, \textit{Phys. Rev. Lett.} \textbf{97}, 207802 (2006) [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V8.00006: Effects of dielectric relaxation on the director dynamics of uniaxial nematic liquid crystals Mingxia Gu, Ye Yin, Sergij V. Shiyanovskii, Oleg D. Lavrentovich We derive the reorienting dielectric torque acting on the director, considering the frequency dependence of the dielectric tensor. The model takes account into the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor and predicts the ``dielectric memory effect'' (DME), i.e., dependence of the dielectric torque on both the ``present'' and ``past'' values of the electric field and the director. In a sharply rising electric field, the DME slows down director reorientation for the materials whose dielectric anisotropy is positive at low frequencies, but speeds up the response for the dielectrically negative materials. We also demonstrate, both theoretically and experimentally that an induced ``memory'' polarization leads to a dielectric torque in the switch-off phase which has an opposite sign to that of the LC's dielectric anisotropy, when a specific switching-off profile is used; this reverse torque accelerates the director relaxation back to the equilibrium state. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V8.00007: Condensation of lyotropic chromonic liquid crystals by additives Luana Tortora, H.-S. Park, S.-W. Kang, S. Kumar, K.V. Kaznatcheev, O.D. Lavrentovich Lyotropic chromonic liquid crystals (LCLCs) are formed by molecules with rigid polyaromatic cores and ionic groups at the periphery that aggregate in water. Condensation of LCLCs can be driven by polyamines, organic salt and neutral polymers. At a suitable concentration of additives, a nematic LCLC demixes into a coexisting isotropic phase and a condensed phase with birefringence higher than that in the original N. By employing synchrotron X-ray scattering we demonstrate the formation of a columnar hexagonal (C) phase. Scanning transmission X-ray microscopy, LC PolScope and fluorescent confocal microscopy allow us to map the relative concentration of components in the condensed and isotropic regions. Both electrostatic and entropy effects contribute to the condensation. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V8.00008: Interactions in the NOBOW and 8CB Mixtures Dong Chen, Chenhui Zhu, Noel Clark Mixtures of a bent-core mesogen (NOBOW) and a calamitic mesogen (8CB) are studied using X-ray diffraction (XRD), polarized light microscopy and freeze fracture electron microscopy (FFEM). XRD shows that as the 8CB concentration increases, the transition temperature of Iso-B4 decreases and the correlation length of NOBOW B4 decreases while the correlation length of 8CB SmA increases. Polarized light microscopy reveals that the mixtures have larger chiral domains than pure NOBOW and when the phase of 8CB changes to SmA, they show the same boundary as the chiral domains. FFEM images show more details on the structure of the mixtures. Along with the experiments, we will present theoretical studies on the interactions in the NOBOW and 8CB mixtures. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V8.00009: Effect of Solvent Concentration on the Liquid Crystal Phase Transitions of Octylcyanobiphenyl-Hexane Mixtures Krishna Sigdel, Germano Iannacchione The effect of a non-mesogenic, low-molecular weight, solvent on the phases of a liquid crystal can be profound. High-resolution ac-calorimetry has been carried out on the isotropic to nematic (\emph{I}-\emph{N}) and the nematic to smectic-\emph{A} (\emph{N}-Sm\emph{A}) phase transitions of the liquid crystal octylcyanobiphenyl (8CB) as a function of hexane concentration. Temperature scans were performed above and below these transition temperatures for all samples. Six 8CB+hexane samples were studied having molar concentrations of 0.017 (0.5\% by mass), 0.033 (1\%), 0.063 (2\%), 0.078 (2.5\%), 0.092 (3\%) and 0.119 (4\%) of hexane. Upon increasing dilution of 8CB by the linear form of hexane, the transition temperatures shift lower while the order of both transitions evolves. These effects may be the consequence of the weakening of the liquid crystal molecular interactions due to the presence of the solvent. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V8.00010: Landau potential of polymer stabilized ferroelectric liquid crystals Paul Archer, Ingo Dierking Polymer stabilized liquid crystals (PSLC) [1] consist of a relatively low concentration of a photo-polymerized monomer (typically less than 10{\%}wt) which is phase separated from the continuous liquid crystal medium. For the case of a polymer stabilized ferroelectric liquid crystal, photo-polymerized in the SmA* phase, the network forms parallel to the smectic layer normal. This results in an elastic coupling between the polymer network and the liquid crystal which alters the characteristics of the SmA* to SmC* phase transition. The generalized model of ferroelectric liquid crystals has been modified to encompass this additional interaction through a polymer coupling coefficient. Analysis of experimental tilt angle and polarization data allows the determination of the polymer coupling coefficient and hence the full Landau potential. Results will be shown and discussed for varying polymer concentration. \newline [1] I. Dierking, Adv. Mater. \textbf{12}, 167 (2000) [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V8.00011: Varying the optical properties of cholesteric liquid crystals Sabrina Relaix, Michele Moreira, Peter Palffy-Muhoray, Michel Mitov Cholesteric Liquid Crystals (CLCs) are of particular interest as they form self-assembled photonic band gap (PBG) structures - a macroscopic helical structure, leading to a selective reflection of light - which can be easily tuned by external fields. As PBG materials, CLCs have been used as mirrorless lasers with low lasing thresholds since the density of photon states is suppressed in the reflection band and is enhanced at its edges [1]. The modification of the cholesteric organization -- either by the introduction of a pitch gradient across the cell or by the incorporation of nanoparticles in the medium -- has direct consequences on the PBG and hence the reflected intensity [2,3]. In this presentation, I will describe the variations in the optical properties of CLC caused by these modifications and will discuss possible applications, such as tuning the CLC laser wavelength or adjusting the laser threshold. \newline [1] P. Palffy-Muhoray et al., Phil Trans R Soc A 364, 2747 (2006) \newline [2] S. Relaix et al., Appl. Phys. Lett. 89, 251907 (2006) \newline [3] S. Relaix et al., Liq. Cryst. 34, 1009 (2007) [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V8.00012: Three-dimensional imaging of chemical bond orientation in liquid crystals by coherent anti-Stokes Raman scattering microscopy. Oleg D. Lavrentovich, Heung-Shik Park, Brian G. Saar, X. Sunney Xie Coherent anti-Stokes Raman scattering (CARS) microscopy is used to provide three-dimensional chemical maps of liquid crystalline (LC) samples without the use of external labels. CARS is a polarization-sensitive optical imaging technique that derives contrast from Raman-active molecular vibrations in the sample. Compared to other three-dimensional imaging techniques, CARS offers the most rapid chemical characterization available without the use of external dyes or contrast agents. Examples that illustrate the applicability of CARS microscopy to LCs include textures and defects in nematic and smectic LC, electric Frederiks transition. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V8.00013: Low electric field induced phase transition of the B1 bent-core liquid crystal phase to a switching phase J. Kirchhoff, L.S. Hirst Liquid crystal materials that have ferroelectric and antiferroelectric phases are useful in applications due to their switching properties. The B1 bent-core liquid crystal phase is a columnar phase that does not exhibit switching. A transition from the B1 liquid crystal phase to a switching phase has been seen at an electric field of 10 V/$\mu $m, which is much lower than previously seen fields of greater than 25 V/$\mu $m [1]. This transition is irreversible upon reduction of the applied field and switching continues almost threshold-less down to an applied field of 40 mV/$\mu $m, which has not been previously reported. Any amount of a chiral rod-like dopant increases the field required to transition from the B1 to the switching phase, and the transition becomes reversible with the mixture relaxing back to the B1 phase after a decrease in the electric field. A small concentration of the rod-like dopant also induces a change from the B1 phase to a new liquid crystal phase. These effects were studied using polarized optical microscopy, calorimetry (DSC), and x-ray measurements. \newline [1] J. Ortega et. al., Phys. Rev. E, \textbf{69}, 011703 (2004) [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V8.00014: Evidence of Broken Reciprocity in Chiral Liquid Crystals Michele Moreira, Nithya Venkataraman, Bahman Taheri, Peter Palffy-Muhoray Reciprocity in light scattering is predicated on bounded scattering media with symmetric and linear permittivity, conductivity and permeability. Due to their anisotropy and chirality, cholesteric liquid crystal form periodic dielectric structures. If the periodicity is comparable to the wavelength of light, these phases are self-assembled photonic band gap structures. There appear in the permittivity odd powers of the wave vector resulting from nonlocality and broken inversion symmetry. Evidence of non-reciprocity has been found in optically active crystals by Bennett [1] and in stacks of cholesteric and nematic liquid crystal cells by Takezoe [2]. We present experimental data showing broken reciprocity in transmittance and reflectance in cholesteric cells with different pitches having overlapping but distinct reflection bands. We explain our results in terms of simple analytic descriptions of material properties and propagating modes. \newline [1] P.J. Bennett, S. Dhanjal, Yu. P. Svirko and N. I. Zheludev, \textit{Opt. Lett}. \textbf{21}, 1955 (1996) \newline [2] J. Hwang; M.H. Song; B. Park; S. Nishimura; T. Toyooka; J.W. Wu; Y. Takanishi; K. Ishikawa; H. Takezoe, \textit{Nat. Mat.} \textbf{4}, 383 (2005). [Preview Abstract] |
Session V9: Computational Methods: Multiscale Modeling
Sponsoring Units: DFDChair: Kathie Newman, University of Notre Dame
Room: Morial Convention Center RO7
Thursday, March 13, 2008 11:15AM - 11:27AM |
V9.00001: Step decoration studied with first-principles statistical mechanics Yongsheng Zhang, Karsten Reuter With respect to oxidation catalysis or oxide formation, surface defects like steps, kinks, or vacancies are widely believed to play a decisive role, e.g. in form of active sites or as nucleation centers. Despite this suggested importance, first-principles investigations qualifying this role for gas-phase conditions that are representative of these applications are scarce. This is mostly due to the limitations of electronic-structure calculations in tackling the large system sizes and huge configuration spaces involved. We overcome these limitations with a first-principles statistical mechanics approach coupling density-functional theory (DFT) calculations with grand-canonical Monte Carlo simulations, and apply it to obtain the phase diagram of on-surface O adsorption at a (111) step on a Pd(100) surface. The link between the electronic and mesoscopic techniques is achieved by a lattice-gas Hamiltonian expansion, in which we parameterize the lateral interactions affected by the step from DFT calculations at a Pd(117) vicinal surface, and all remaining lateral interactions from calculations at Pd(100). For a wide range of O gas-phase conditions we find the (111) step to be decorated by a characteristic zig-zag structure. Intriguingly, this structure prevails even up to the elevated temperatures characteristic for catalytic combustion reactions, where only small amounts of disordered oxygen remain at the Pd(100) surface. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V9.00002: Non-Adiabatic Transition Path Sampling: Application to a Model Proton-Transfer Reaction Laura J. Kinnaman, Steven A. Corcelli, Kathie E. Newman A new algorithmic method is discussed, Non-Adiabatic Path Sampling (NAPS), which combines features of transition path sampling (TPS) and molecular dynamics with quantum transitions (MDQT). The goal is to ultimately address problems which involve excited and coupled electronic states, as well as large systems and long timescales ($e.g.$, semiconductor photocatalysis). TPS focuses specifically on trajectories that take a system from reactants to products, which allows the study of chemical processes that are dominated by rare but important events whose timescales are outside the range of direct simulation. In the MDQT algorithm, the nuclear dynamics of the system do not occur on a single Born-Oppenheimer potential energy surface, but rather may involve non-adiabatic transitions between many coupled electronic states. The NAPS algorithm uses the statistical framework of TPS to analyze MDQT trajectories, using the advantages of each method to get results for otherwise inaccessible systems. The algorithm is tested on a simple model of proton transfer: A quantum-mechanical proton in a double-well quartic potential bi-linearly coupled to a thermal bath of classical harmonic oscillators. Results from the model are compared to numerically exact results available in the literature. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V9.00003: First-passage Monte Carlo for materials under irradiation Aleksandar Donev, Vasily Bulatov The key challenge in simulations of irradiated materials is that of time scale. Typically, atomistic simulations extend to less than one nanosecond whereas kinetic Monte Carlo (kMC) simulations struggle to reach hours of simulated irradiation. Based on a time-dependent Green's function formalism, our new kMC algorithm extends the simulated time horizon from minutes to tens and hundreds of years while retaining uncompromising accuracy. This presents an exciting opportunity to extrapolate, through accurate numerical simulations, the material behavior observed under the short and violent irradiation exposures used in the accelerated material tests, to the much longer reactor material lifetimes. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V9.00004: OPAL: A New Multiscale Software Architecture Based on MPI-2 Yun-Wen Chen, Chao Cao, Ming Zhang, Erik Deumens, Hai-Ping Cheng Software integration for multi-scale simulations is a time-consuming process. Common practice is to turn the higher level calculation code ( e.g. DFT ) into a subroutine of the lower level calculation code ( e.g. MD ). This method often requires non-trivial effort. To avoid these difficulties, we have developed a software package OPAL, within which a minimal development effort is required to build a working multi-scale environment. We report our effort of integrating DL\_POLY and SIESTA codes for hybrid quantum-classical simulations. This work is supported the NSF through ITR-medium (NSF/DMR/ITR-0218957) program. The authors want to thank NERSC, CNMS/ORNL and the University of Florida High Performance Computing Center for providing computational resources and support that have contributed to the research results reported within this paper. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V9.00005: MP2 calculations for solid state systems Martijn Marsman, Georg Kresse We present {\it ab initio} total energy calculations at the level of Hartree-Fock + 2nd-order M\o ller-Plesset perturbation theory (HF+MP2) for extended systems using periodic boundary conditions and a plane wave basis set. To characterize the accuracy of this level of theory, HF+MP2 lattice constants, bulk moduli, and atomizations energies for several archetypical semiconducting and insulating solid state systems are compared to those from density functional theory calculations and experiment. The HF+MP2 description of van der Waals interactions is illustrated for several noble gas solids. Important computational aspects of HF+MP2 calculations within the plane wave full potential projector-augmented-wave (PAW) formalism, most notably the basis set extrapolation of the MP2 correlation energy, are addressed as well. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V9.00006: New approaches to the prediction of thermodynamic stability of crystal structures Johannes Voss, Tejs Vegge We present new methods for numerical crystal structure optimization and prediction of structural stability on the basis of density functional theory calculations.[1] Comparison to established approaches to the calculation of lattice free energies differing in numerical complexity and accuracy of the results is provided. We show applications of these methods to complex insulators, semiconductors, and metals, and point out variations of our approaches making them suitable for these different classes of materials. We furthermore briefly outline alternative approaches to the prediction of compound stability avoiding the calculation of free energies. [1] J. Voss and T. Vegge, {\it to be published} (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V9.00007: Solidifying semiconductor nanocrystals from melts: Molecular dynamics simulations Tianshu Li, Davide Donadio, Giulia Galli Understanding the nucleation of semiconductor nanocrystals is of fundamental importance in the field of nanoscience. In this study we employ classical molecular dynamics simulations to explore the crystallization of Si nanocrystals from the melt. We focus on the differences between homogeneous and heterogeneous nucleations, where the heterogeneous case is investigated by simulating a liquid slab. In particular, we use the recently developed forward fluxing method [R.J. Allen, D. Frenkel, and P.R. ten Wolde, JCP 124 024102(2006)] to model the evolution of nucleation processes from melts and to compute nucleation rates. We demonstrate that free surfaces act as catalytic nucleation sites by significantly promoting the formation of solid-like small clusters. The presence of solid-like clusters in proximity of the surfaces is found to occur at temperature higher than those at which solid seed nucleation occurs in bulk liquids, highlighting the important role of heterogeneous nucleation under low under-cooling conditions. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V9.00008: Ab-Initio Density Functional Calculation of Interatomic Potentials for Large-scale Atomistic Material Simulations. G.L. Zhao, S. Yang We propose a new method to calculate interatomic potentials, utilizing an ab-initio density functional formalism. The calculated interatomic potentials can be used for large scale atomistic material simulations and predictions. We benchmark the method for the case of copper. We utilized the ab-initio interatomic potential to calculate various properties of transition metal copper, including the lattice constant, the bulk modulus, thermal expansion coefficient, monovacancy formation energy, and phonon frequencies. The calculated results agree very well with experimental values. We further calculated the properties of BCC Cu, utilizing the interatomic potential derived from the electronic structure calculations of FCC Cu, to demonstrate the predictive capabilities of the interatomic potential. The predicted properties of BCC Cu agree very well with experimental and ab-initio density functional results. Part of the work was performed during the stay of G. L. Zhao at Princeton University. The authors gratefully acknowledge the financial support of the National Science Foundation (Award No. 0508245). [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V9.00009: A New Look at the Evaluation of Embedded Atom Potential Models. James N. Glosli, Kyle J. Caspersen, David F. Richards, Robert E. Rudd, Fred H. Streitz The embedded atom method (EAM) potentials have been used extensively since introduced by Daw and Baskes in the mid 1980's due to their simple incorporation of many-body effects that are missed by simple pair potentials. The computational cost of the inclusion of this additional physics has traditionally been a second pass over the pair data. We will report on an implementation of the EAM model within a molecular dynamics algorithm (MD) that does not require this second pass, substantially reducing the computer time and memory required for evaluation of the potential. The second pass is avoided by using a forward extrapolation in time of the density derivative of the embedding function $dF(\rho(t))/d\rho$. The error in this approximation is controllable and consistent with the error introduced by the finite time step numerical integrators used in the MD. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V9.00010: Electronic structure from Maximum Entropy optimization: Applications to band energy and electronic force computation Hiro Shimoyama, Parthapratim Biswas We apply a new entropy optimization scheme to study the electronic density of states for complex disordered materials from a knowledge of spectral moments. We employ the Shannon entropy functional in our work and maximize it subject to the moment constraints to construct the spectral distribution of large Hamiltonian matrix[1]. We illustrate the efficiency and the usefulness of the method by reconstructing a number of exact functions, which are difficult to reproduce by other function reconstruction techniques. The local and global convergence properties of the resulting distribution is studied and the band energy and Fermi level are computed with a high degree of precision. An extension of this method to calculate electronic forces is presented for the purpose of using in large-scale molecular dynamics simulation of materials. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V9.00011: Lagrangian Time-Reversible Born-Oppenheimer Molecular Dynamics Anders Niklasson A Lagrangian generalization of time-reversible Born-Oppenheimer molecular dynamics [Niklasson et al., Phys. Rev. Lett., vol.97, 123001 (2006)] is proposed. The new formulation enables highly efficient symplectic or geometric integrations of both the nuclear and the electronic degrees of freedom that are stable and energy conserving even under incomplete self-consistency convergence. It is demonstrated how the accuracy is improved by over an order of magnitude compared to previous formulations at the same level of computational cost. The proposed Lagrangian includes extended electronic degrees of freedom as auxiliary dynamical variables in addition to the nuclear coordinates and momenta. While the nuclear degrees of freedom propagate on the Born-Oppenheimer potential energy surface, the extended auxiliary electronic degrees of freedom evolve as a harmonic oscillator centered around the adiabatic propagation of the self-consistent ground state (http://arxiv.org/abs/0711.3466). [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V9.00012: First-principles calculation combined with multicanonical simulation Yoshihide Yoshimoto To tackle statistical complexities in condensed matters such as phase transitions of atomic structures with first-principles calculations, Yoshimoto have studied the combination of first-principles calculations and multicanonical methods. By the multicanonical methods, phase space of atomic coordinates can be explored efficiently. Among phase transitions, Yoshimoto focused crystal$\leftrightarrow$liquid transition because it is a basic procedure for material synthesis and formation of objects (casting). The talk will present his recent results : a direct (not a coexisting) simulation of the crystal$\leftrightarrow$liquid transition by a kind of two-component multicanonical ensemble, a {\em multi-order multi-thermal ensemble}, with an order parameter defined with structure factors that characterize the transition, and optimization of a model interatomic potential in terms of the ensemble from an accurate one called {\em thermodynamic downfolding} of a potential. These provide a principle to project a first-principles approach on a model-based approach conserving thermodynamic properties of multiple phases to a maximum extent. The talk will cover the successful applications of the method to the transition of Si and MgO. Ref: Y. Yoshimoto, J. Chem. Phys. 125, 184103 (2006) [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V9.00013: Topological Properties of Microstructures in Nanocrystalline Materials. Tao Xu, Mo Li Recent experiments show that the topological properties of microstructures in nanocrystalline materials play an important role in the mechanical properties of nanocrystalline materials. However, the fundamental structure-property relationship has not been fully understood due to the difficulties in determining and controlling the microscopic properties of nanocrystalline materials experimentally. In this study, we investigate how different topological properties affect the thermal and mechanical responses of nanocrystalline materials, including grain size distribution, surface area distribution, triple junction length distribution, grain boundary misorientation, etc. Digital microstructures with desired topological properties are generated using Inverse Monte Carlo method and are then relaxed and deformed by large-scale molecular dynamic simulation. In order to characterize the relaxed and deformed digital samples, we use a new grain boundary characterization method to accurately determine the position and thickness of each grain boundary during both relaxation and deformation. Finally, this newly developed algorithm enables us to study the correlation between topological and mechanical properties of nanocrystalline materials. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V9.00014: XML Tools for First-Principles Molecular Dynamics Simulations Francois Gygi We present a set of XML Schema specifications for the representation of electronic structure data and first-principles molecular dynamics (FPMD) simulation data. The schemas (available at http://www.quantum-simulation.org) include the description of FPMD simulation samples and pseudopotentials in an extensible and code-neutral way. Automatic validation of simulation samples can be achieved using publicly available XML parsers such as Apache Xerces-C. We present examples of web-based remote collaboration in which simulation samples and pseudopotentials are accessed using the http protocol. Data analysis using XSLT scripts and a visualization program for remote inspection of simulation samples will also be demonstrated. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V9.00015: Approximating Densities of States with Gaps using Maximally Broken Time-Reversal Symmetry Roger Haydock, C.M.M. Nex When a finite cluster of atoms is used to approximate the electronic structure of a macroscopic system, the appropriate boundary condition for electronic states on the surface of the cluster is maximal flow of probability current through the boundary, or maximal breaking of time-reversal symmetry for the states. For continued fraction representations of electronic Greenians, this boundary condition gives excellent results for both the first and second sheets when there is a single band of states. In this work, the approximation is extended to Greenians for multiple bands separated by gaps, such as arise in semiconductors. [Preview Abstract] |
Session V10: Proximity Effect, Electron-doped Cuprates, STM
Sponsoring Units: DCMPChair: Thomas Lemberger, Ohio State Universtiy
Room: Morial Convention Center RO8
Thursday, March 13, 2008 11:15AM - 11:27AM |
V10.00001: Superconducting proximity effect and Majorana fermions at the surface Liang Fu, Charles Kane A strong topological insulator is an insulating material in which spin-orbit interaction inverts the band gap at an odd number of time reversed pairs of points in the Brillouin zone. These materials have topologically protected gapless spin-split surface states, whose Fermi arc is characterized by a Berry's phase of $\pi$. We study the proximity effect between an s-wave superconductor and the surface states of a strong topological insulator. The resulting two dimensional state resembles a spinless $p_x+ip_y$ superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. Such bound states obey non-Abelian statistics and have been studied in the context of topological quantum computing. We show that linear junctions between superconductors mediated by the topological insulator form a non chiral 1 dimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating and fusing Majorana bound states. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V10.00002: Giant proximity effect in a phase-fluctuating superconductor Dominic Marchand, Lucian Covaci, Mona Berciu, Marcel Franz When a tunneling barrier between two superconductors is formed by a normal material that would be a superconductor in the absence of phase fluctuations, the resulting Josephson effect can undergo an enormous enhancement. We establish this novel proximity effect by a general argument as well as a numerical simulation and argue that it may underlie recent experimental observations of the giant proximity effect between two cuprate superconductors separated by a barrier made of the same material rendered normal by severe underdoping. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V10.00003: Search for Proximity Effect in the Local Pairing Temperature of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Colin Parker, Aakash Pushp, Kenjiro K. Gomes, Abhay Pasupathy, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani The proximity effect is seen when a superconductor is in contact with a metal. The pairing gap in the superconductor is reduced near the interface while superconducting correlations are induced in the metal. Recent results in high-Tc superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ indicate that the pairing gap closes inhomogeneously in space, producing a unique state just above Tc in these compounds where non-superconducting regions are in contact with regions where the pairing gap can still be measured. We will present detailed scanning tunneling measurements that map the local density of states in the same area of the sample from the low temperature fully gapped regime to the high temperature regime where most gaps have closed. We find that the temperature that the gap closes locally is sensitive to the gaps in the surrounding region. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V10.00004: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Soren Flexner, Paul Welander, Kevin Inderhees, Tim McArdle, Jim Eckstein We report results using a novel 3 terminal device structure that provides two independent resistance measurements that we use to examine the influence of the superconducting proximity effect on both the transport properties of the thin film N layer by quantifying its sheet resistance, Rs, as well as independently measuring the junction conductance across the N-S boundary, Gc. When the N layer is a degenerate semiconductor the changes in these quantities are large. Gc increases much more than the factor of 2 that Andreev Reflection or BTK theory predicts, and both Gc and Rs exhibit reentrance as temperature decreases. We interpret these effects as being due to a transition between a phase fluctuating and phase stiff proximity effect in the N layer. This manifests itself by moving the N-S electrical boundary into the semiconductor, increasing Gc, while simultaneously removing volume available to normal transport forcing the measured Rs to increase. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V10.00005: Charge leakage in superconducting $\rm La_{2}CuO_{4}-La_{2-x}Sr_{x}CuO_{4}$ superlattices S. Smadici, P. Abbamonte, J. Lee, S. Wang, C.D. Cavellin, A. Gozar, G. Logvenov, I. Bozovic Ability of grow crystalline high-temperature superconductor (HTS) superlattices using molecular beam epitaxy has opened new avenues of research. Scanning tunneling microscopy measurements have identified a pronounced influence of the dopant atoms on microscopic properties of bulk HTS, therefore raising the question of whether the holes can be spatially separated in an oxide heterostructure from the disordered doped layers in a way analogous to semiconductor modulation doping. We used resonant soft x-ray scattering (RSXS) to answer this question for $\rm La_{2}CuO_{4}-La_{2-x}Sr_{x}CuO_{4}$ superlattices. For a $\rm 15\times [2\times LCO-4\times LSCO]$ superlattice with x=0.36, the measured hole amplitude modulation at the O $K$ edge shows a relatively weak localization of the doped holes to the LSCO layers. By using the interference between ``structural" and resonant scattering at the Cu $L_{3}$ edge, the c-axis stress of the LSCO sublattice was also observed. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V10.00006: Long range proximity effect in High Tc Josephson NanoJunctions: a quantitative study Jerome Lesueur, Nicolas Bergeal, Marco Aprili, Takis Kontos, Martin Sirena, Giancarlo Faini, Jean-Pierre Contour Proximity effect (PE) between a superconductor S and a normal metal N has been a powerful tool to study conventional superconductors. In High Tc (HTc) compounds, low quality interfaces and poor Fermi wave-vector match with most of metals considerably reduce the PE, and make really difficult its study. We have designed Josephson NanoJunctions in which two S reservoirs are coupled through an N layer at a nanoscale, where N is a lightly disordered HTc, whose Tc has been reduced by ion irradiation. In these SNS junctions, Cooper pairs propagate through the N layer by PE. In this situation with no metallurgical interfaces within the same material, we have shown that a long range PE takes place, which can be quantitatively described by the diffusive Usadel equations. The Josephson coupling temperature can be computed. The role of the order parameter symmetry will be also discussed. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V10.00007: The Superconducting State of Pr$_{2-x}$Ce$_x$CuO$_4$ : Tunneling study Yoram Dagan, Roy Beck, Richard Greene We report a tunneling study between the electron-doped high T$_c$ cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ and Lead as a function of doping, temperature and magnetic field. The temperature dependence of the gap follows the BCS prediction. Our data fits a nonmonotonic $d$-wave order parameter for the whole doping range studied. From our data we are able to conclude that the electron-doped cuprate Pr$_{2-x} $Ce$_x$CuO$_4$ is a weak coupling, BCS superconductor in the dirty limit. Phys. Rev. Lett. 99, 147004 (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V10.00008: Local tunneling spectroscopy and infrared spectroscopy of the electron-doped cuprate Sm$_{2-x}$Ce$_{x}$CuO$_4$ A. Zimmers, Y. Noat, T. Cren, W. Sacks, D. Roditchev, B. Liang, R. L. Greene, R. P. S. M. Lobo, N. Bontemps We present infrared and local tunneling spectroscopy of the electron-doped cuprate Sm$_{2-x}$Ce$_{x}$CuO$_4$. In STM, at optimal doping x=0.15, a clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample ($\Delta\sim$~3.5-6meV). Another spectroscopic feature is simultaneously observed at high energy above $\pm$50meV. Its energy scale and temperature evolution is found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors. Using optical spectroscopy, we analyzed the effects of the normal state gap opening (the higher energy gap seen in STM) and phonon structure as a function of temperature and doping from the underdoped to the metallic composition. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V10.00009: Spin correlations and magnetic excitation spectrum of electron-doped Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ Eugene Motoyama, Guichuan Yu, Yuan Li, Inna Vishik, Patrick Mang, Martin Greven, Owen Vajk, Klaudia Hradil, Richard Mole One of the most intriguing issues in the field of high-$T_c$ superconductivity is the electron-hole asymmetry: the hole- or electron-doping of the parent Mott insulators leads to superconductors with differing properties. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping, appearing to overlap with the superconducting phase. Our inelastic neutron scattering measurements have shown the possibility that in the archetypical compound Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$, genuine long-range antiferromagnetism and superconductivity do not co-exist (Motoyama {\it et al.}, Nature {\bf 445}, 186 (2007)). However, some uncertainty remains, due to the inhomogeneity of Ce concentration $x$ in the large single crystals. Here we report new results using improved homogeneity. In addition to the implications for the phase diagram using energy-integrated measurements, we discuss the impact of improved crystals on the (energy-resolved) magnetic excitation spectrum in the superconducting state. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V10.00010: EXAFS study of the role of apical oxygen on superconductivity in Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ S. Rosenkranz, D. Haskel, M. Balasubramanian, S. Heald, S. Li, P. Dai, Y. Ando One of the longstanding puzzles surrounding high-T$_C$ in cuprates concerns the apparent asymmetry between electron and hole doping. Whereas hole doping quickly induces superconductivity, electron doping alone in materials such as R$_{2}$CuO$_{4}$ is insufficient and superconductivity is only achieved after a high temperature, low-oxygen annealing. While it was believed that this annealing process removes small amounts of oxygen in apical positions assumed to induce localization of the doped electrons, this scenario is incompatible with Raman, infrared- transmission, and ultrasound studies. In contrast, based on synchrotron X-ray and neutron diffraction studies combined with chemical and thermo-gravimetric analysis measurements, we recently showed that the annealing process alleviates minor Cu- deficiencies present in the as-grown sample. Here we present EXAFS studies on powder and single-crystal samples of as-grown and annealed Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$. Our results are consistent with no change in the occupation of apical oxygen between superconducting and as-grown samples, providing further evidence that the main effect of the annealing process is to repair defects in the superconducting planes due to Cu- deficiencies present in as grown samples. \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V10.00011: High pressure study on Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta }$ single crystals Costel R. Rotundu, Viktor V. Struzhkin, Alexander Goncharov, Richard L. Greene Transport measurements to 2.5 GPa on electron-doped cuprates revealed modest changes in physical properties, if any [1]. Room temperature synchrotron diffraction on powder samples show that the non-superconducting parent Pr$_{2}$CuO$_{4}$ exhibits a gradual structural transition from T' to T phase when subject to pressures greater than 15 GPa [2]. Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ (PCCO) is superconducting for a Ce doping range 0.12 -- 0.2 (T' structure). Both pressure and Ce doping have the same shrinkage effect on the lattice constants. Here, we report high pressure ac susceptibility, resistivity and Raman shift data on x = 0.15 single crystals to 30 GPa. References: [1] C. Murayama \textit{et al.}, Nature, \textbf{339}, 293 (1989) [2] H. Wilhelm \textit{et al.}, Science and Technology of High Pressure, Proceedings of AIRAPT-17, pp. 740-743, Universities Press, Hyderabad, India (2000) [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V10.00012: Recent ARPES results on electron-doped high-Tc superconductors and comparison to their hole-doped counterparts Pierre Richard, Zihui Pan, Madhab Neupane, Yiming Xu, Patrick Fournier, Shiliang Li, Pengcheng Dai, Ziqiang Wang, Hong Ding Since the CuO$_{2}$ planes of cuprates, where high-temperature superconductivity occurs, can be doped either by holes or electrons, it appears important to establish similarities between these two types of doping in order to have an overview of high-temperature superconductors, especially for the electronic structure. Hence, although much less studied by ARPES than their hole-doped counterpart, the electronic structure of the electron-doped cuprates is believed to provide essential hints towards the understanding of high-Tc superconductivity. We present our recent ARPES results on electron-doped cuprates, focusing on various ranges of energy. We compare the results to those obtained on commonly studied hole-doped cuprates. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V10.00013: Point contact tunnelling as a probe for the superconducting properties of Niobium RF cavities T. Proslier, J. Moore, J. Zasadzinski, M. Pellin, J. Norem Niobium, with its very high H$_{C1}$, has been used in superconducting RF cavities for accelerator systems for 40 years with continuous improvement. The quality of cavities (Q) is governed by the surface impedance \textbf{R}$_{BCS}$, which depends on the quasiparticle gap, \textbf{$\Delta $}, and the superfluid density, \textbf{n}$_{S}$. Both of these parameters are seriously affected by surface imperfections (metallic phases, dissolved oxygen, magnetic impurities). Surface treatments of Nb cavities improved the Q factor, but are not understood from a fundamental point of view. Point contact tunneling spectroscopy is an ideal, surface sensitive probe as the quasiparticle current measures \textbf{$\Delta $}${\rm u}$ We present some measurements on SRF cavity-grade Nb using point contact tunneling demonstrating that the nature of the Nb oxide has a significant effect on $\Delta $. Atomic Layer Deposition was used to grow an alumina oxygen diffusion barrier on Nb providing control of the Nb oxidation layer composition with subsequent annealing. Point contact tunneling on these samples help to unravel the complicated effect of Nb surface oxidation on\textbf{${\rm g}\Delta $}. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V10.00014: Scanning Tunneling Microscopy of Defect-Induced Superstructure in 2H-NbSe$_{2}$ Hui Wang, Jonghee Lee, Michael Dreyer, Barry Barker Scanning tunneling microscopy and spectroscopy measurements were performed on pure 2H-NbSe$_{2}$ with defects introduced by tip-sample interaction at 4K. Domains of a new superstructure with a lattice constant equals $\sqrt {13} $a$_{0}$ instead of 3a$_{0}$ were observed around the defects. Closer to the defects we also observed the disordered phase of this structure. Both of them are stable under the STM measurements at 4K. Atomically resolved microscopy and spectroscopy studies suggest a 2H to 1T phase transition induced by surface atom sliding. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V10.00015: STM Investigation of Bosonic Modes and a Superconducting Gap in the Electron Doped Cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$ Francis Niestemski, Shankar Kunwar, Sen Zhou, Shiliang Li, Hong Ding, Ziqiang Wang, Pengcheng Dai, Vidya Madhavan While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped cuprates less attention has been given to the equally important electron-doped side of the phase diagram. ~We use a variable temperature UHV STM to investigate the electron-doped cuprate superconductor Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$. ~We explore temperatures 2 K and higher and find a superconducting gap that disappears above Tc. ~We also find satellite features anti-correlated to the gap which we associate with bosonic modes. ~We relate our findings to neutron scattering results and discuss how this electron doped superconductor differs from more familiar hole-doped cuprates. [Preview Abstract] |
Session V11: Quasiparticle Dispersion and Coupling in Cuprates
Sponsoring Units: DCMPChair: Dan Dessau, University of Colorado at Boulder
Room: Morial Convention Center RO9
Thursday, March 13, 2008 11:15AM - 11:27AM |
V11.00001: Evidence of Electron Coupling to High Energy Excitations in a High Temperature Superconductor Xingjiang Zhou, Wentao Zhang, Guodong Liu, Lin Zhao, Haiyun Liu, Jianqiao Meng, Xiaoli Dong, Wei Lu, Zhongxian Zhao, Guiling Wang, Hongbo Zhang, Yong Zhou, Zuyan Xu, Yong Zhu, Xiaoyang Wang, Chuangtian Chen, J.S. Wen, Z.J. Xu, Genda Gu, T. Sasagawa We have carried out super-high resolution angle-resolved photoemission measurements on temperature evolution of the electron dynamics along the (0,0)-($\pi$,$\pi$) nodal direction in an optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. The nodal photoemission spectra exhibit dramatic sharpening with decreasing temperature, with an obvious change in the scattering rate across T$_c$. New high energy features are found to develop at $\sim$115meV and $\sim$150meV, besides the prominent $\sim$70 meV one, in the nodal electron self-energy in the superconducting state. These observations provide evidence that, in addition to coupling with low energy excitations like phonons or magnetic resonance mode, there are high-energy excitations involved in the electron coupling. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V11.00002: High energy kink in the single particle spectra of the two-dimensional Hubbard model Alexandru Macridin, Mark Jarrell, Thomas Maier, Douglas Scalapino Employing dynamical cluster quantum Monte Carlo calculations we show that the single particle spectral weight $A(k,\omega)$ of the one-band two-dimensional Hubbard model displays a high energy kink in the quasiparticle dispersion followed by a steep dispersion of a broad peak similar to recent ARPES results reported for the cuprates. Based on the agreement between the Monte Carlo results and a simple calculation which couples the quasiparticle to spin fluctuations, we conclude that the kink and the broad spectral feature in the Hubbard model spectra is due to scattering with damped high energy spin fluctuations. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V11.00003: Theory of quasiparticle excitations in cuprates: universal Fermi velocity and high energy anomalies Qiang-Hua Wang Recent measurements of quasiparticles in hole-doped cuprates reveal highly unusual features: 1) the doping-independent Fermi velocity, 2) two energy scales in the quasiparticle spectral function, and 3) a suppression of the low energy spectral weight near the zone center. The underlying mechanism is under hot debate. We addressed these important issues by a new mean field theory and a novel variational Monte Carlo (VMC) study of the t- J model. We obtained results in both approaches in agreement with the experiments but without invoking extrinsic effects. They reflect the role of strong correlations in the form of local Mottness and antiferromagnetic fluctuations, yielding a strong connection between the low and high energy quasiparticle excitations. Besides, we resolved a long standing issue of the sum rule for quasiparticle spectral weights in traditional VMC studies. The electron doped case was also discussed and we concluded that no high energy anomaly exists in the occupied side, in contrast to the hole doped case. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V11.00004: Dispersion and Spectral weight analysis of ``waterfall'' structure in Pb doped Bi2212. Qiang Wang, Zhe Sun, Eli Rotenberg, Helmut Berger, Daniel Dessau Angle resolved photoemission spectroscopy (ARPES) is used to investigate the property of the ``waterfall'' structure in (Bi, Pb)2Sr2CaCu2O8. The dispersion analysis of the spectrum shows that there is not a ``universal high energy scale'' in this material. And the spectral weight analysis suggests that the matrix element and the inelastic scattering may have an important role in the formation of the waterfall structure. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V11.00005: Magnon Corrections to Cuprate Self Energy Robert Markiewicz, Tanmoy Das, Arun Bansil Recently, high energy kinks (`waterfalls') have been reported above 200 meV in the ARPES spectra of several cuprates. These kinks are a signal of bosonic coupling and may be responsible for the band renormalizations found at lower energies. We have shown that coupling to collective modes in the spin channel can yield waterfall-like effects in the electronic dispersion in the electron as well as hole doped cuprates. [1] Here we further explore the effects of the magnons in the pseudogap regime, including a discussion of how magnons influence optical properties. \newline \newline [1] R.S. Markiewicz, S. Sahrakorpi, and A. Bansil, cond- mat/0701524, to be published, PRB. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V11.00006: Aspects of the electron-phonon interaction in the Cuprates Steve Johnston, Wei-Sheng Lee, Thomas Devereaux, Z.X. Shen The ubiquity of the ``kink''-structures observed in the band-dispersion of the High-Tc cuprates have made this feature the subject of debate for many years now. At present, the community agrees that the feature is due to electron-boson coupling to a collective mode, however, a consensus has yet to be reached on its identity. In this talk we will review the arguments typically made against the phonon interpretation, which are grounded in knowledge gained from metallic systems. We will then show the complications one encounters in extrapolating from these systems to strongly correlated systems such as the cuprates. We will also discuss some of the common methods for extracting information from photoemission spectra that are cited in the modern literature in order to highlight the strengths and weaknesses of each and their reliability for extracting realistic estimates for parameters such as the electron-boson coupling strength $\lambda$. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V11.00007: Phonon anomaly in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$* Jiandong Guo, Huajun Qin, Kehui Wu, R. G. Moore, E. W. Plummer, J. Wen, G. D. Gu, Jiandi Zhang Electron-phonon coupling (EPC) plays in many exotic phenomena displayed by strongly-correlated electron materials, such as the pairing mechanism in high-$T_{C}$ superconductors and metal-insulator transition in ruthenates. Many studies show a strong renormalization of quasi-particle band structure near Fermi energy associated with the coupling to boson modes $\sim $40-50 meV. However, there is no clear picture of the origin of these modes. With angle-resolved electron energy loss spectroscopy, we have studied the lattice dynamics of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ surface. The data indicate a phonon anomaly of modes $\sim $50 meV, identified as the in-plane Cu-O stretching modes, where the energy shifts (softens) while the spectral linewidth and weight vary along (0,0)-($\pi $,0) direction. The other feature $\sim $80 meV, attributed to apical oxygen vibrations, exhibits distinct dispersion toward ($\pi $,$\pi )$. Such behaviors are observed both above and below the superconducting T$_{C}$. These detailed measurements provide new insights into the nature of EPC in such materials. * Supported by China NSF-10704084, NSF DMR-0346826, NSF and DOE (NSF-DMR-0451163 and DMS{\&}E). [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V11.00008: Isotope effect on the nodal kink energy in Bi2212 J.F. Douglas, H. Iwasawa, K. Satou, H. Eisaki, Y. Yoshida, H. Bando, T. Saitoh, A. Ino, M. Taniguchi, M. Arita, K. Shimada, H. Namatame, T. Masui, S. Tajima, S. Uchida, Y. Aiura, D.S. Dessau Using low energy angle resolved photoemission spectroscopy (le-ARPES), we have observed an energy shift of the nodal kink upon substitution of $^{18}$O for $^{16}$O in optimally doped (T$_{c}\sim $92K) Bi2$_{.1}$Sr$_{1.9}$CaCu$_{2}$O$_{8+\delta }$. Studying several samples of each isotope, we find that the kink energy decreases by 3.22 $\pm $ 0.72 meV upon $^{18}$O substitution, in good agreement with the energy shift one would expect from a phononic mode. This strongly supports the view that the nodal dispersion kink arises from electronic coupling to a phonon mode. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V11.00009: Theory of Kink Structure of Quasi-Particle Energy Dispersion in Photoemission Spectra of High Temperature Superconducter Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ via Dipolon Mediated Electron-Electron Pairing Mechanism Ram Sharma We have made use of the four-momenta space diagrams in the dressed particle picture to write the self-energy $\Sigma(p)$ by taking the sum of the exchange diagrams involving dipolon propagator, electron Green's function and electron-electron Coulomb interaction to obtain single quasi-particle energy dispersion in high $T_C$ $\backslash$ Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconductors. The theory contains Mott renormalization and all important and necessary electron correlations. This constitutes an extension of the strong-coupling dipolon theory [1,2] which explains [3] also the peak-dip-hump structure of the line shape of the photoemission spectra of high $T_C$ superconductors. Our calculations of the single quasi-particle energy dispersion for $\backslash$ Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ show a strong kink at the binding energy near 60 meV which has already been identified in the experiments [4] and predicts two additional weak kinks at binding energies close to 100 and 160 meV, yet to be identified experimentally. The Migdal vertex correction does not change our results drastically. [1] R. R. Sharma, Phys. Rev. {\bf B 63}, 054506 (2001). [2] R. R. Sharma, Physica {\bf C 439}, 47 (2006). [3] R. R. Sharma, Physica {\bf C}, in press. [4] P. V. Bogdanov et al., Phys. Rev. Lett. {\bf 85}, 2581, 2000. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V11.00010: Fermi Surface Topology Effects on the Electron-Phonon Coupling in Electron-doped Cuprates S.R. Park, D.J. Song, C.S. Leem, Chul Kim, C. Kim, B.J. Kim, H. Eisaki We have performed high resolution angle resolved photoemission (ARPES) studies on electron doped cuprate superconductors Sm$_{2-x}$Ce$_{x}$CuO$_{4}$ ($x$=0.10, 0.15, 0.18). Imaginary parts of the electron removal self energy by a newly developed method shows kink-like features due to electron-bosonic mode coupling. The kink-like feature is seen along both nodal and anti-nodal directions but at different energies of 50 and 70 meV. Such energy scales can be reconciled by taking the Fermi surface topology and phonon dispersions into account, revealing the kink structures are due to the electron-phonon coupling. Estimated electron-phonon coupling constant $\lambda $ from the self energy is about 0.6 independent of doping and is isotropic. In addition to the low energy feature, we observe a hump structure at 350 meV which is anisotropic and exists only in the anti-nodal spectrum. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V11.00011: Theoretical fits of laser-ARPES energy distribution curves of the high Tc superconductor Bi2212 N.C. Plumb, P.A. Casey, J.D. Koralek, J.F. Douglas, Z. Sun, Y. Aiura, K. Oka, H. Eisaki, P.W. Anderson, D.S. Dessau Laser-ARPES has produced spectral lineshapes in photoemission that are much sharper than any previous data, which is due to increased energy and momentum resolution, increased bulk sensitivity, and decreased final state broadening. The lifetimes of these states, extracted from simple Lorentzian fits to the data, are consistent with bulk-sensitive optical data, implying that we are for the first time measuring the intrinsic spectral function. It therefore is appropriate to study the spectral lineshape in detail. We have looked at standard Lorentzian energy distribution curves, as well as extensions based upon Fermi Liquid theory, Marginal Fermi Liquid theory, and of great interest, a non-Fermi Liquid theory based upon Anderson's treatment of the Gutzwiller projection. The lineshapes based upon the Gutzwiller projection utilize only one free parameter and include no background term, yet they fit the data well over a broad range of temperatures and energies. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V11.00012: Properties of high-T$_C$ superconductors from spin-phonon coupling and band models. Thomas Jarlborg An understanding of the rich doping- and $(\vec{q},\omega)$-dependences of spin excitations in high T$_C$ materials is essential since the mechanism of high-T$_C$ superconductivity might be linked to spin fluctuations. Ab-initio band calculations show important spin-phonon coupling (SPC), i.e. antiferromagnetic fluctations are enhanced when they co-exist with $\vec{q}_x$-phonons involving O, Cu or La distortions. Parameters for these ``1-dimensional'' (1D) electron-phonon and spin-wave couplings are obtained from band calculations for long supercells containing phonon distortions and/or staggered fields. The characteristic 2-D q-dependence of the excitations are calculated for a free-electron like band with the use of the ab-initio parameters. The q-variation depend on the strength of the SPC, which leads to a linear relation between $\vec{q}$ and doping, x, for x$\leq$0.15. The SPC is strongest for in-plane O-modes, weaker for modes involving the heavy atoms, and smallest for apical O, which together with SPC for the phonons at the characteristic frequency lead to a q-dependent excitation spectrum. These and other properties coming from SPC in the band/free-electron model compare favorably with observations. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V11.00013: Universal dispersion anomalies revealed by DQMC simulations of the Hubbard model B. Moritz, S. Johnston, W. Meevasana, C. Kim, T. P. Devereaux, R. T. Scalettar, Z.-X. Shen The recent observation of a ``high energy anomaly" (HEA) in hole-doped high-Tc compounds, as well as the half-filled parent insulators, using angle-resolved photoemission spectroscopy (ARPES) has sparked a great deal of interest and intense theoretical and experimental investigations. Using determinant quantum Monte Carlo (DQMC) and maximum entropy analytic continuation (MEM), we investigate the existence of the HEA in the single-band Hubbard model. The spectral functions obtained from the simulations reveal a universal HEA across the doping spectrum, similar to that seen in experiment. This signals a cross-over from a quasiparticle-like band at low energy to the incoherent lower Hubbard band at higher energy. A comparison of the self-energy obtained from simulation to that extracted from experiment serves as a further llustration of the similarities. In addition, we perform simulations of the single-band Hubbard model on the electron-doped side of the phase diagram and compare and contrast our results to existing experimental evidence and comment on the possibility that future experiments would find a universal HEA in electron-doped compounds. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V11.00014: Local effects of apical oxygen on superconductivity in high-$T_{\rm c}$ cuprates Michiyasu Mori, Takami Tohyama, Sadamichi Maekawa The superconducting critical temperature ($T_{\rm c}$) of high- $T_{\rm c}$ cuprates widely distributes among various series of crystal structures, even if the doping rate is optimized in the CuO$_2$ planes. In addition, the $T_{\rm c}$ is enhanced by applying pressure[1]. These material- and pressure dependences have meaningful correlation with an energy difference of oxygen sites in an apical site and in the CuO$_2$ plane ($V_{\rm A}$)[2]. On the other hand, Slezak et al. has found that locally modulated gap energy has anti-correlation with a distance between a Cu- and an apical O-sites, i.e., the larger distance is related to the smaller gap energy[3]. We study such a local effect of apical oxygen on superconductivity by calculating the Madelung potential. In particular, we focus on a local variation of $V_{\rm A}$, whose value approximately corresponds to stability of the Zhang- Rice singlet state[2]. It is found that, on neighboring sites of apical sites close to Cu sites, $V_{\rm A}$ are locally enhanced compared to other sites. To estimate the gap energy, we propose a toy model like a BCS mean field Hamiltonian with an additional degree of freedom, which describes a role of apical oxygen. We will discuss an anti-correlation between the gap energy and the position of apical oxygen. [1] N. Tanahashi et al: Jpn. J. Appl. Phys. 28, L762 (1989). [2] Y. Ohta, T. Tohyama, and S. Maekawa: Phys. Rev. B 43, 2968 (1991). [3] J. Slezak, PhD thesis. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V11.00015: Variable Temperature Scanning Tunneling Spectroscopy of Inhomogeneous High Temperature Superconductors E.W. Hudson, M. C. Boyer, W.D. Wise, Kamalesh Chatterjee, Yayu Wang, Takeshi Kondo, Tsunehiro Takeuchi, Hiroshi Ikuta Scanning Tunneling Microscopy (STM) of the high temperature superconductor $Bi_2Sr_2CaCu_2O_{8+x}$ (Bi-2212) long ago revealed large gap variations on nanometer length scales. In this talk I will discuss new results from our temperature dependent STM studies of Bi-2201. In particular, I will focus on the effects of these variations on other observables, such as the states generated around single atom impurities, as well as on other measurement techniques, such as angle resolved photoemission (ARPES). [Preview Abstract] |
Session V12: Metal Insulator Transition I: Vanadium Oxides and Others
Sponsoring Units: DCMPChair: M. Brian Maple, University of California, San Diego
Room: Morial Convention Center 203
Thursday, March 13, 2008 11:15AM - 11:27AM |
V12.00001: Electrical measurements at the metal-insulator phase boundary in VO$_{2}$ nanobeams David Cobden, Jiang Wei, Zenghui Wang, Wei Chen We study the electrical properties of vanadium dioxide nanobeams undergoing the metal-insulator transition (MIT), which occurs at a temperature of 67$^{\circ}$C at ambient pressure. The nature of the MIT in bulk VO$_{2}$, although known to involve electron-electron correlations, has remained elusive since its discovery fifty years ago. In nanobeams clamped at both ends there is a coexistence regime which allows electrical measurements along the phase boundary. Remarkably, the resistivity of the insulating phase turns out to be constant along the phase boundary implying that the transition is driven by electron density, consistent with a Mott-type mechanism. The measurements show that the resistance of a domain wall is negligible, and the resistance of a nanowire gives a direct measure of the length of insulating phase present in the wire, allowing one to study the motion of the domain wall electrically with high precision. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V12.00002: Metal-insulator coexistence in VO$_{2}$ nanobeams Jiang Wei, Zenghui Wang, Wei Chen, David Cobden We study the first-order metal-insulator transition (MIT) in vanadium dioxide nanobeams. The MIT occurs sharply at a temperature of $T_{c}$ = 67$^{\circ}$C at ambient pressure. However, in nanobeams clamped at both ends, and hence subjected to a constant length condition, there is a wide coexistence regime between the two phases, which can be visualized in an optical microscope. Above $T_{c}$ the beam is under axial tension and on warming up follows the phase boundary in the tension/temperature plane. Below $T_{c}$ the beam buckles under compressive strain. The metallic phase can be supercooled by up to 50 °C. Usually there is a single metal-insulator domain wall in each beam, but a mobile bubble-like insulating domain can be induced by applying a nonuniform temperature profile.. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V12.00003: Correlation between metal-insulator transition characteristics and electronic structure changes in vanadium oxide thin films D. Ruzmetov, V. Narayanamurti, S. Ramanathan, S.D. Senanayake We correlate electron transport data with energy band structure measurements in vanadium oxide thin films with varying V-O stoichiometry across the VO$_{2}$ metal-insulator transition (MIT). A set of vanadium oxide thin films were prepared by reactive DC sputtering from a V target at various oxygen partial pressures resulting in films with different MIT strength as determined from the electrical resistance measurements. The results of the near edge X-ray absorption fine structure spectroscopy (NEXAFS) of the O K-edge in identical VO films are presented. Redistribution of the spectral weight from $\sigma $* to $\pi $* bands is found in the vanadium oxide films exhibiting stronger VO$_{2}$ MIT. This is taken as evidence of the strengthening of the metal-metal ion interaction with respect to the metal-ligand and indirect V-O-V interaction in vanadium oxide films featuring sharp MIT. We observe also a clear correlation between MIT and the width and the area of the lower $\pi $* band which is likely to be due to the emergence of the d$_{\vert \vert}$ band overlapping with $\pi $*. The strengthening of this d$_{\vert \vert}$ band near the Fermi level only in the vanadium oxide compounds displaying the MIT points out the importance of the role of the d$_{\vert \vert}$ band and electron correlations in the phase transition. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V12.00004: Mott transition in vanadium dioxide (VO$_2$) observed by infrared spectroscopy and nano-imaging M.M. Qazilbash, G.O. Andreev, D.N. Basov, P.-C. Ho, M.B. Maple, M. Brehm, F. Keilmann, A. V. Balatsky, Byung-Gyu Chae, Bong-Jun Kim, Sun Jin Yun, Hyun-Tak Kim The driving mechanism for the temperature-induced insulator-to- metal transition (IMT) in vanadium dioxide (VO$_2$) has been debated for the past five decades. Central to this debate is the relative importance of electron-electron correlations and charge-ordering to the IMT. We report near-field infrared images of VO$_2$ films that directly show the percolative IMT. In combination with far-field infrared spectroscopy, the new data reveal the Mott transition with divergent optical mass in the metallic puddles that emerge at the onset of the IMT. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V12.00005: Avalanches in the Metal-Insulator transition in Vanadium Oxide nano-scaled junctions Amos Sharoni, Gabriel Ram\'Irez, Ivan K. Schuller We present transport measurements on sub-micron devices of VO$_{2}$. Instead of the usual smooth metal insulator transition, we observe for the first time multiple resistive steps. The temperature driven transition between the two phases occurs through a series of avalanches of different amplitude, ranging over 2 decades of resistance. The data is analyzed assuming a generic normalized probability distribution. Results are similar to those obtained in martensitic transitions or Barkhausen noise in ferromagnets, implying universality of first order phase transition. We will discuss the distribution of resistance avalanches as a function of temperature caused by the percolative nature of resistive transition. In particular we will focus on the effect of the VO$_{2}$ junction dimensions and the transport measurement conditions (such as applied current and temperature ramp rate) on the results. Work supported by the US --Department of Energy. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V12.00006: Micro X-Ray Diffraction Study of VO$_{2}$ Films -- Separation between Metal-Insulator Transition and Structural Phase Transition Kim Hyun-tak, Kim Boung-jun, Lee Yong wook, Yun Sun Jin, Lim Jung-wook, Shin Tae-ju, Youn Hwa-sick It has been well-known that VO$_{2}$ undergoes both a structural phase transition (SPT) from monoclinic (insulator phase) to tetragonal (metal phase) and a discontinuous first-order metal-insulator transition (MIT) (Jump) near 68$^{o}$C. When the MIT and the SPT occurs simultaneously, the MIT can be regarded as the Peierls transition. When both take place separately, the MIT can be interpreted as the Mott transition. Peierls transiton and Mott transition in VO$_{2}$ remain controversial. We have investigated a relation of the MIT and the SPT which are simultaneously measured by I-V measurement and synchrotron micro X-ray of the 1B2 line in Pohang accelerator Lab., respectively. A used sample is a VO$_{2}$-based two terminal device. The result shows that the MIT and the SPT does not occur simultaneously. (References on the MIT: New J. Phys. 6 (1994) 52 (http://www.njp.org), Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005) 76, Phy. Rev. Lett. 97 (2006) 266401, Appl. Phys. Lett. 90 (2007) 023515). [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V12.00007: Lateral V/VO$_{x}$/V Tunnel Junctions Formed by Anodic Oxidation David Kirkwood, Kevin West, Jiwei Lu, Stuart Wolf Anodization has been found to be a simple and cost effective technique to produce oxide films of many transition metals. In this work, we have used anodic oxidation as a means of fabricating lateral V/VO$_{x}$/V junctions. Vanadium wires grown by ion beam deposition were patterned by lithography and an active working window was defined on the wire. VO$_{x}$ was then grown under galvanostatic control in a two electrode electrochemical micro-cell. A droplet of oxygen rich saturated Boric acid was used as the electrolyte to electrically connect the Vandium working electrode to a Platinum wire counter electrode. A constant current of approximately 100 $\mu $A/cm$^{2}$ was maintained through the cell for various amounts of time. Electrical measurements of the resulting V/VO$_{x}$/V junctions indicate a metal to insulator transition (MIT) near 340 $^{o}$K that is similar to the structural phase transition and accompanied MIT of VO$_{2}$ which occurs at this temperature. A 4-fold change in resistance is observed in the junctions. Below this transition temperature a typical junction behavior is observed with a dramatic change in resistance state from high to low with increasing applied current. This non-linear IV characteristic on the junction with a size of 5 $\mu $m by 15 $\mu $m suggests that the anodized VO$_{x}$ film behaves like a tunneling barrier. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V12.00008: The metal to insulator transition in correlated materials -- exchange of low and high energy spectral weight Stefan G. Singer, B. Schulz, I. Mahns, M. Bastjan, G. Neuber, A. Goos, P. Saichu, S. Binder, A. Rusydi, K. Horn, S. Eller, M. R\"ubhausen, G. Stryganyuk, L. Cooper, G.A. Sawatzky, O. Sushkov, H. Eisaki, Y. Fujimaki, S. Uchida, K. D\"orr The electronic response of correlated system of manganites and cuprates was investigated by a combination of dc conductivity, ellipsometry, and VUV-reflectance covering an energy range from 0 to 22 eV. By performing a stabilized Kramer-Kronig transformation, we obtain the optical conductivity as a function of temperature around the metal to insulator transition. Our main findings are that changes in the kinetic energy exceed energies of more than 22 eV. Within this range we can establish, that high energy spectral weight gets transferred to low energies into the Drude region. We argue that our findings are general phenomena of correlated materials and outline different scenarios to explain our observations. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V12.00009: Metal-insulating transition and Zhang-Rice singlets in one-dimensional cuprates: the case of Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$ Alessio Filippetti, Vincenzo Fiorentini Chain-like Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$ is the ideal prototype of dopable one- dimensional cuprate with zig-zag Cu-O interactions. The observed abundance of Zhang-Rice singlets in the electronic ground state of the system induces peculiar phase transitions in magnetic and conducting properties upon doping. With the use of unconventional first-principles calculations suited for to the study of strong-correlated materials, we describe the change of electronic and magnetic properties as doping is varied from x=0 to full-doped concentration (i.e. one hole each CuO$_2$ unit). Zhang-Rice singlets are key ingredients to understand the behavior of doped CuO2 units and the rise of high-Tc superconductivity in cuprates. We can visualize ZR singlets in space and energy at varying doping concentration, and their influence on magnetic and dielectric properties. ZR singlets are associated to holes localization on the oxygens, appear for holes concentration above 0.25 per CuO$_2$ and persist up to maximum doping (i.e. one hole per CuO$_2$) that is well above the threshold of the metallic regime. Our findings are indicative of the general behavior of low-dimensional doped cuprates and give evidence that first-principles band-energy approaches can be valuably employed to the study of doped cuprates. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V12.00010: EXAFS and XANES in Fe$_{3-x}$M$_x$O$_4$, M = Zn, Ti, Al with 0 $\leq$ x $<$ 0.065. J. Sabol, D. Owoc, Z. Kakol, C. Kapusta, A. Kozlowski, J. Honig We present an extended EXAFS and XANES study at the Fe K edge of the local structural changes in Fe$_{3-x}$M$_x$O$_4$, M = Zn, Ti, Al, with x within the first and second order Verwey transition regimes. The aim of this work was to investigate how the local Fe symmetry is altered at the transition, in view of the drastic change of the overall crystal structure. The comparison of the interatomic distances and Debye-Waller factors shows that the local Fe-O structure around either the octahedral or tetrahedral Fe atoms in magnetite remains nearly unaltered crossing the Verwey transition. This is also independent of the level of doping and dopant atom, i.e., independent of lattice parameters, as Zn and Ti increase the unit cell volume and Al decreases the unit cell volume. Therefore, the local distortion of Fe sites is not a parameter sensitive enough to elucidate the mechanism of the Verwey transition and the change of its character. An examination of different factors, other than local order, is needed to explain the apparent charge ordering below the Verwey transition temperature \textit{T}$_V$. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V12.00011: Nonequilibrium Relaxations and Aging Effects near a Metal-Insulator Transition in Two Dimensions Dragana Popovi\'c, Jan Jaroszy\'nski The relaxations of conductivity $\sigma (t)$ have been studied in the glassy regime of a strongly disordered two-dimensional (2D) electron system in Si after a temporary change of carrier density $n_s$ during the waiting time $t_w$. Two types of response have been observed~[1]: (a) monotonic, where relaxations exhibit aging, \textit{i.e.} dependence on history, determined by $t_w$ and temperature; (b) nonmonotonic, where a memory of the sample history is lost. The conditions that separate the two regimes have been determined. A detailed study of the aging regime~[2] reveals an abrupt change in the nature of the glassy phase at the metal-insulator transition (MIT) before it vanishes entirely at a higher density $n_g$. Our results provide further support to theories describing the 2D MIT as the melting of a Coulomb glass, and put constraints on the models of glassy freezing.\\ \noindent [1] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 99}, 046405 (2007).\\ \noindent [2] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 99}, 216401 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V12.00012: Photo-induced Metallic States in a Mott Insulator Takashi Oka In the presence of strong AC electric fields, carriers are continuously produced in an insulator and the system may turn metallic. Such photo-induced insulator-to-metal transition was observed in one-dimensional Mott insulators (Iwai et al. $P.R.L.$ (2003)). We have studied the one-dimensional Hubbard model with strong AC electric fields using the time-dependent density matrix renormalization group method. We have changed the strength of the field as well as the frequency. We have observed that metallic states are produced not only when the photon energy, i.e., the frequency of the AC field, is larger than the gap, but also when it is smaller. However, the field must be stronger than a threshold in the latter case. We have also studied the detailed properties of the photo-induced states by calculating the optical and magnetic correlation function. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V12.00013: I3+-Ir4+ Charge Disproportionation of Spinel CuIr2S4 investigated by Synchrotron Radiaton Photoemission Han-Jin Noh, E.-J. Cho, H.-D. Kim, J.-Y. Kim, C.-H. Min, B.-G. Park, S.-W. Cheong We have studied the electronic structure of the spinel CuIr$_2 $S$_4$ using synchrotron-radiation photoemission spectroscopy. The phase transition from a high temperature paramagnetic metal to a low temperature diamagnetic insulator at $\sim$230 K is clearly observed through the significant line shape change of the Ir 4$f$ photoemission spectra and the $\sim$0.09 eV gap opening of the valence band spectra. The photon energy dependence of the Ir 4$f$ photoemission spectra enable us to characterize the satellite peaks in the metallic phase of CuIr$_2$S$_4$, providing compelling experimental evidence for the Ir$^{3+}$-Ir$^{4+}$ charge disproportionation in CuIr$_2$S$_4$. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V12.00014: Local structural aspects of the metal-insulator transition in CuIr$_{2}$S$_{4}$ from total scattering x-ray study E. Bozin, A.S. Masadeh, H.J. Kim, P. Juhas, S.J.L. Billinge, J.F. Mitchell A thiospinel CuIr$\bf _{2}$S$\bf _{4}$ exhibits a metal-insulator (MI) transition at T=230 K, with simultaneous spin-dimerization and charge-ordering [1]. The transition can also be driven by extended exposure to the x-rays at low T [2, 3]. Total x-ray scattering study of CuIr$\bf _{2}$S$\bf _{4}$ was carried out using 100 KeV synchrotron beam and rapid acquisition pair distribution function (RAPDF) approach. The RAPDF results indicate consistency of the local and average structure at high T. At 100 K long x-ray exposure melts the long-range order (LRO) of the dimerization pattern, without affecting the local structure, in agreement with diffuse scattering result [3]. The dependence of the LRO related superlattice peak intensity on the exposure time reveals that the melting occurs within approximately 15 seconds of exposure under experimental conditions used. At 100 K the LRO is recovered without temperature increase quickly after the cessation of the beamtime exposure. Results on Cr doped samples will be addressed as well. [1] P.G. Radaelli {\it et al.}, Nature {\bf 416}, 155 (2002). [2] V. Kiryukhin {\it et al.}, Phys. Rev. Lett. {\bf 97}, 225503 (2006). [3] H. Ishibashi {\it et al.}, Phys. Rev. B {\bf 66}, 144424 (2002). This work is supported by the NSF under grant DMR-0304391. ANL is supported under DOE contract No. DE-AC02-06CH11357. [Preview Abstract] |
Session V13: Focus Session: Frontiers in Electronic Structure Theory III
Sponsoring Units: DCP DCOMPChair: Wei Ku, Brookhaven National Laboratory
Room: Morial Convention Center 204
Thursday, March 13, 2008 11:15AM - 11:27AM |
V13.00001: Dielectric Properties of Ice and Liquid Water from First Principle Calculations Deyu Lu, Francois Gygi, Giulia Galli We present a first-principle study of the dielectric properties of ice and liquid water. The eigenmodes of the dielectric matrix, $\epsilon^{-1}$, are analyzed in terms of maximally localized dielectric functions similar, in their definition, to maximally localized Wannier orbitals obtained from Bloch eigenstates of the electronic Hamiltonian. We show that the lowest eigenmodes of $\epsilon^{-1}$ are localized in real space and can be separated into groups related to the screening of lone-pairs, intra-, and inter-molecular bonds, respectively. The local properties of the dielectric matrix can be conveniently exploited to build approximate dielectric matrices for efficient, yet accurate calculations of quasiparticle energies. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V13.00002: Strong hybridization of Frenkel excitons in Mott insulators: a novel Wannier function perspective* Chi-Cheng Lee, H. C. Hsueh, Wei Ku Linear response scheme of the time-dependent density-functional theory (TDDFT) has been quite successful in the study of the excitations of weakly correlated systems. However, its applicability to strongly correlated systems remains unclear, especially due to the poor quality of the exchange-correlation kernel essential for those systems. On the other hand, the local-density approximation + Hubbard U (LDA+U) approximation has been shown to describe quite successfully the ground-state properties and electronic band structures of Mott insulators. Therefore, it is timely to investigate the linear response of the LDA+U functional in the framework of TDDFT in describing excitations of strongly correlated systems. In this talk, a theoretical (diagrammatic) framework of the linear response of LDA+U (TDLDA+U) functional will be presented and applied to the study of Frenkel excitons in NiO within the first-principles Wannier basis. The advantages and disadvantages of LDA+U functional will be discussed, in comparison with more advanced many-body approaches. [1] B. C. Larson, et al. PRL 99, 026401 (2007)*Work supported by U.S. DOE - CMSN [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V13.00003: Propagation of strongly bound Frenkel excitons in LiF: An effective two-particle kinematic approach of super-atom in ab initio Wannier basis Chen-Lin Yeh, Hung-Chung Hsueh, Wei Ku A general new first-principles Wannier function based method is proposed to better understand the propagation of strongly bound Frenkel excitions. Specifically, long-standing debate of the Frenkel nature of the excitons in LiF is made apparent by the formation of a ``super-atom'' consisting of Wannier orbitals from both Li and F. On this basis, a new approach is proposed by formulating the kinematic contribution to the propagation of the exciton via an effective two-particle hopping kernel. The same kernel contains both the mass enhancement at strong binding and the decay into continuum at weak binding, and is thus exact in both limits. This kinematic effect is compared with found to overwhelm the conventional interaction-based propagations of exciton in LiF. This general theoretical framework can be directly applied to the study of propagation of local excitations of strongly correlated systems. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V13.00004: Ab-initio Total Energy Calculation for Full-potential Multiple Scattering Theory Methods Yang Wang, Aurelian Rusanu, Malcolm Stocks, Don Nicholson, Markus Eisenbach The {\it ab initio} methods (e.g., KKR, KKR-CPA, LSMS) based on multiple scattering theory have the clear advantage of being able to calculate the Green function in a straightforward manner, which has important implications in the application of electronic structure calculations. But these methods have mostly been implemented within muffin-tin approximations. Recent advances in the numerical implementation of full- potential multiple scattering theory and, in particular, the development of an innovative Poisson equation solver have made carrying out the fully self-consistent full-potential calculation possible. In this presentation, we discuss various implementations of the full-potential total energy calculation, and we investigate the convergence of the total energy with respect to the angular momentum expansion cutoff for scattering matrices. Finally, we compare the full-potential total energy with the muffin-tin approximation results. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V13.00005: A quantum chemistry roadmap towards highly accurate adsorption energies at ionic surfaces Bo Li, Angelos Michaelides, Matthias Scheffler A roadmap is established to compute adsorption energies of molecules at ionic surfaces with an accuracy approaching chemical accuracy (a precision of 1 kcal/mol or $\sim$43 meV). The approach relies on established quantum chemistry methodologies and involves a separation of the total adsorption energy into contributions from Hartree-Fock and electron correlation, the use of embedded cluster models of the substrate, and extrapolations to the complete basis set limit. Application of the procedure to the example of water on salt, with electron correlation treated at the CCSD(T) level, yields an adsorption energy for a water monomer on NaCl(001) of 480 $\pm$ 20 meV. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V13.00006: Surface energies of semiconductors by the energy density method Min Yu, Richard M. Martin Energy Density formalism within the first-principles pseudopotential density functional theory has been proposed by Chetty and Martin$^{1}$ in 1990s. Although the energy density function is non-unique, nevertheless integrals over surface regions provide unique results for surface energies, and calculations have been carried out by several groups$^{2,3}$ to study the polar surfaces and interfaces of solid state systems such as GaAs $(111)$ and $(\bar{1}\bar{1}\bar{1})$ polar surfaces. In our work, we apply this method to wurtzite CdSe to determine the energy of of various polar surfaces such as $(0001),(000\bar{1})$, and non-polar surfaces such as $(10\bar{1}0),(11\bar{2}0)$, from which we can estimate the equilibrium crystal shape for large nanoclusters. 1. N. Chetty and Richard M. Martin, Phys. Rev. B 45, 6074 (1992). 2. K. Rapcewicz, B. Chen, B. Yakobson, and J. Bernholc, Phys. Rev. B 57, 7281 (1998). 3. N. Moll, A. Kley, E. Pehlke, and M. Scheffler, Phys. Rev. B 54, 8844 (1996). [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V13.00007: Electron-phonon interaction using Wannier functions: from single-layer graphene to cuprate superconductors Invited Speaker: The interaction between electrons and phonons is central to many phenomena, including electrical and thermal transport and superconductivity. Recently the electron-phonon (e-ph) interaction has been the focus of intense research efforts in the physics of high-temperature superconductivity and nanoscale transport. Despite the continued interest in the e-ph problem, first-principles calculations remain challenging due to the large computational effort required to describe e-ph scattering processes in the proximity of the Fermi surface. In this talk I will present a method based on Wannier functions which greatly reduces the computational cost of e-ph calculations [1,2]. The underlying idea is to exploit the spatial localization of electrons and phonons in the maximally localized Wannier representation. After describing the method I will review recent applications to materials of current interest. I will discuss how the e-ph interaction affects the dynamics of Dirac fermions in graphene [3], the origin of superconductivity in boron-doped diamond [1], and the relation between Fermi surface topology and superconductivity in super-hard carbides. I will conclude this presentation by discussing the role of phonons in the angle-resolved photoemission spectra of cuprates [4]. \newline [1] F. Giustino, J.R. Yates, I. Souza, M.L. Cohen, and S.G. Louie, Phys. Rev. Lett. 98, 047005 (2007). \newline [2] F. Giustino, M.L. Cohen, and S.G. Louie, Phys. Rev. B 76, 165108 (2007). \newline [3] C.-H. Park, F. Giustino, M.L. Cohen, and S.G. Louie, Phys. Rev. Lett. 99, 086804 (2007). \newline [4] F. Giustino, M.L. Cohen, and S.G. Louie, http://arXiv:0710.2146. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V13.00008: Ensemble density functional theory, the atom-in-molecule problem, and reactive charge transfer Susan Atlas, Steven Valone A major challenge in large-scale simulations of complex biomolecular and materials systems is the ability to accurately describe reactive dynamics. We have previously described a new multiscale formalism, based on density functional theory and the embedded-atom method, that enables the rigorous encoding of quantum mechanical excitation effects such as charge polarization and charge transfer within a classical potential. Here we describe a new formulation of a key element of the theory: the deconstruction of molecular densities into subsystem atom-in-molecule components via ensemble constrained-search density functional theory. The method is implemented via the self-consistent solution of coupled sets of Kohn-Sham equations in conjunction with chemical potential equalization across subsystems. This leads to a natural interpretation of dynamical charge transfer and charge polarization in terms of an electronic entropy, thus extending the seminal work of Gross, Oliveira, and Kohn (1988). [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V13.00009: Correction of Finite Size Errors in Many-body Electronic Structure Calculations Hendra Kwee, Shiwei Zhang, Henry Krakauer Finite-size (FS) effects are a major source of error in many-body (MB) electronic structure calculations of extended systems. Reducing FS errors is thus a key to broader applications of MB methods in real materials, and the subject has drawn considerable attention.\footnote{P. R. C. Kent \textit{et. al.}, Phys. Rev. B \textbf{59}, 1917 (1999); S. Chiesa \textit{et. al.} Phys. Rev. Lett. \textbf{97}, 076404 (2006).} We show that MB FS effects can be effectively included in a modified local density approximation calculation. A parametrization for the FS exchange-correlation functional is obtained. The method is simple and gives post-processing corrections that can be applied to any MB results. Conceptually, it gives a consistent framework for relating FS effects in MB and DFT calculations, which is important if the two methods are to be seamlessly interfaced to bridge length scales. Applications to a model insulator (P$_2$ in a supercell), to semiconducting Si, and to metallic Na show that the method delivers greatly improved FS corrections.\footnote{H.\,Kwee, S.\,Zhang and H.\,Krakauer (2007), preprint at http://arxiv.org/abs/0711.0921.} [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V13.00010: Embedding quantum-mechanics in an interatomic potential simulation using local energies Noam Bernstein, Gabor Csanyi Atomistic simulations that use quantum-mechanical total-energy models provide high accuracy and reliability at the price of computational expense. Classical approximations such as interatomic potentials are much faster, but less transferable. We couple the two approaches concurrently, to describe part of the system quantum-mechanically and part with interatomic potentials, using a weighted sum of atomic energies. This enables us to compute a {\em well defined total energy} for the hybrid system with small and controllable errors caused by the boundaries of the QM region. Using tight-binding as a model quantum-mechanical method, we can efficiently evaluate the derivatives of the total energy, including the effects of charge self-consistency, enabling an energy conserving molecular dynamics simulation for a fixed QM region. We show that a localized quantum-mechanical atomic energy can be defined, and that this energy is physically meaningful. We present tests of the method, and discuss convergence with respect to various method parameters, and the effects of moving the QM region during the dynamics. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V13.00011: The search for minimum-energy atomic configurations on a lattice: Lamarckian twist on Darwinian Evolution Mayeul d'Avezac, Alex Zunger We examine how two different mechanisms proposed historically for biological evolution compare for the determination of crystal structures from random initial lattice-configurations. The Darwinian theory of evolution contends that the genetic makeup inherited at birth is the one passed on to offsprings. Lamarck surmised additionally that offspring can inherit acquired traits. In the case of lattice-configurations, such improvements consist in A$\leftrightarrow$B transmutations of atomic sites as guided by ``Virtual Atom'' energy-gradients({\small M. d'Avezac and Alex Zunger, J. Phys.: Cond. Matt. {\bf 19}, 402201 (2007)}). This hybrid evolution is shown to provide an efficient solution to a generalized Ising Hamiltonian, illustrated by finding the ground-states of face-centered cubic Au$_{1-x}$Pd$_x$ using a cluster-expansion functional fitted to first-principles total-energies. For example, finding all minimum-energy structures of a 32-atom supercell with $95\,\%$ confidence requires evaluating $750, 000$ configurations using local improvements only, $150, 000$ using a reciprocal-space genetic algorithm only, and $14,000$ using the hybrid approach. We consider applying the lamarckian search to further functionals. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V13.00012: Using genetic algorithms to find from first-principles the minimum-energy crystal structure starting from random cell vectors and random atomic positions. G. Trimarchi, M. D'Avezac, Alex Zunger We address the global space-group optimization problem in binary metallic A$_{q}$B$_{1-q}$ alloys using an evolutionary algorithm. A set of crystal structures with randomly-selected lattice vectors and atomic positions is evolved, replacing the highest energy structures with new ones generated through mating or mutation, as well as ab-initio structural relaxation to the nearest local minimum. This was applied to a few compounds whose lattice-type is difficult to guess because the constituent solids A and B have different lattice types (e.g., A is fcc and B is bcc): (i) compounds with the crystal lattice of either A or B constituents, i.e., CdPt$_{3}$, AlSc$_{3}$, Al$_{3}$Sc; (ii) compounds with a crystal lattice different than that of either constituents, i.e., AlSc and CuPd; (iii) compounds whose crystal lattice is not even of a Bravais type, e.g., PdTi$_{3}$. The optimization scheme retrieved the lowest energy structures within about 100 total-energy evaluations. Not all independent GA sequences end up giving the same final structure; we select the lowest energy structure from all sequences. Using a model calculation, we will discuss how many independent GA sequences are needed to find the lowest energy structure with given confidence. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V13.00013: From grand-canonical density functional theory towards rational compound design Anatole von Lilienfeld The fundamental challenge of rational compound design, ie the reverse engineering of chemical compounds with predefined specific properties, originates in the high-dimensional combinatorial nature of chemical space. Chemical space is the hyper-space of a given set of molecular observables that is spanned by the grand-canonical variables (particle densities of electrons and nuclei) which define chemical composition. A brief but rigorous description of chemical space within the molecular grand-canonical ensemble multi-component density functional theory framework will be given [1]. Numerical results will be presented for intermolecular energies as a continuous function of alchemical variations within a neutral and isoelectronic 10 proton system, including CH$_4$, NH$_3$, H$_2$O, and HF, interacting with formic acid [2]. Furthermore, engineering the Fermi level through alchemical generation of boron-nitrogen doped mutants of benzene shall be discussed [3].\newline [1] von Lilienfeld and Tuckerman {\em JCP} {\bf 125} 154104 (2006)\newline [2] von Lilienfeld and Tuckerman {\em JCTC} {\bf 3} 1083 (2007)\newline [3] Marcon et al. {\em JCP} {\bf 127} 064305 (2007) [Preview Abstract] |
Session V14: BEC-BCS Crossover
Sponsoring Units: DAMOPChair: Meera Parish, Princeton University
Room: Morial Convention Center 205
Thursday, March 13, 2008 11:15AM - 11:27AM |
V14.00001: Properties of a few-body trapped two-component Fermi gas at unitarity Javier von Stecher, Doerte Blume, Chris H. Greene We consider a trapped two-component Fermi system with even and odd number of fermions N. Unlike fermions interact with a short-range two-body potential which does not support a bound state and is characterized by an infinite scattering length. Using two different numerical techniques, i.e., a correlated Gaussians basis expansion method and a fixed-node diffusion Monte Carlo method, we solve the many-body Schrodinger equation and determine the spectrum and structural properties. Analyzing the excitation spectrum and the wavefunctions, we demonstrate that this system exhibits unique universal properties [1], in agreement with analytical predictions [2]. Furthermore, we determine the excitation gap up to N = 30 and we compare it with recent predictions. [1] D. Blume, J. von Stecher, and Chris H. Greene, e-print arXiv:cond-mat/0708.2734 [2] F. Werner and Y. Castin, Phys. Rev. A 74, 053604 (2006). [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V14.00002: Quantum and thermal fluctuations in the BCS-BEC crossover with unequal mass fermions Roberto Diener, Qi Zhou, Mohit Randeria A lot of progress has been done in the study of the BCS-BEC crossover for equal-mass fermions in recent years by both theory and experimental groups. An extension of this problem which is starting to receive more attention is the study of mixtures of fermions with different masses, such as a mixture of $^{40}$K and $^6$Li. Using our functional integral method, which we have previously used to study the equal-mass case and which includes the effect of collective excitations (see R. B. Diener et al, cond-mat/0709.2653), we have studied the properties of a fermionic gas with unequal masses across the BCS-BEC crossover. We will present results for different thermodynamical quantities as a function of mass ratio and interaction strength: e.g. the beta parameter at unitarity, the ground state energy as a function of $1/(k_Fa_s)$, as well as the dimer scattering in the BEC limit as a function of mass ratio which agrees to within 20\% with the exact four-body calculation of D. Petrov et al., J. Phys. B At. Mol. Opt. Phys. {\bf 38}, S645 (2005). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V14.00003: Critical temperature and thermodynamics of the BEC-BCS crosssover Evgeni Burovski, Evgeny Kozik, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The strongly-correlated regime of the BCS-BEC crossover can be realized by diluting a system of two-component fermions with a contact attractive interaction and an appropriate ultraviolet regularization. We investigate this system via a novel systematic-error-free continuous-space-time diagrammatic determinant Monte Carlo method. The results allow us to predict the universal curve $T_c/E_F$ as a function of the parameter $k_F a$ with the maximum on the BEC side. At unitarity, $T_c/E_F$ = 0.152(7). We also determine the thermodynamic functions and show how the Monte Carlo results can be used for accurate thermometry. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V14.00004: First- and second-sound-like modes at finite temperature in trapped Fermi gases from BCS to BEC Yan He, Chih-chun Chien, Qijin Chen, Kathryn Levin We determine the temperature ($T$) dependence of first- and second-sound-like mode frequencies for trapped Fermi gases undergoing the BCS to Bose-Einstein condensation (BEC) crossover. Our results are based on numerical solution of the two-fluid equations in conjunction with a microscopic calculation of thermodynamical variables. As in experiment and at unitarity, we show that the lowest radial breathing mode is $T$ independent. At finite $T$, higher-order breathing modes strongly mix with second sound. Their complex $T$ dependence should provide an alternative way of measuring the transition temperature $T_c$. We will also discuss collective mode frequancy for polarized Fermi gas. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V14.00005: Transport Properties of a Fermi gas with attractive interactions in the BEC-BCS crossover Chih-Chun Chien, Yan He, Qijin Chen, Kathryn Levin The transport phenomena of a two-component Fermi gas with attractive interactions are studied at finite temperatures focused on the normal state using a t-matrix formalism in the BEC-BCS crossover. We contrast the behavior of both charged and uncharged systems and address such varied coefficients as DC conductivity and shear viscosity. We show how the behavior above the pairing onset temperature $T^*$ appears to depend rather weakly on the scattering length. At lower temperatures by studying the appropriate Maki-Thompson and Aslamazov-Larkin diagrams, we find more pronounced fluctuation effects the closer the system is to the BCS limit. Interestingly, we observe that these fluctuation effects are more apparent in charged than in uncharged systems. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V14.00006: Probing two-fluid hydrodynamics in a trapped Fermi superfluid at unitarity Edward Taylor, Allan Griffin, Hui Hu, Xia-Ji Liu We develop a variational approach to calculate the density response function at finite temperatures of the lowest-lying two-fluid dipole and breathing modes in a trapped two-component Fermi superfluid close to a Feshbach resonance. The out-of-phase oscillations, which are the analogue in trapped gases of second sound in uniform superfluids, have so far not been observed in cold-atom experiments. At unitarity, we show that these modes are observable at finite temperatures via two-photon Bragg scattering, whose spectrum is related to the imaginary part of density response function. This provides direct evidence for superfluidity and a promising way to test microscopic results for thermodynamics at unitarity. (arXiv:0709.0698, 0711.0561). [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V14.00007: Shear Viscosity and the Perfectness of Fluid Gautam Rupak, Thomas Schaefer A recent calculation of the shear viscosity for a unitary gas is presented. Here unitary gas is defined as a non-relativistic Fermi gas with infinite scattering length. The unitary gas is a scale invariant strongly-interacting many-body system, and possesses universal properties that are of interest across subfields in physics. A unitary gas can be realized in cold atomic gas experiments near a Feshbach resonance. Conditions approximating the unitary gas emerge in low energy nuclear physics as well. From general principle, the shear viscosity of a strongly interacting gas should be small, however, quantum mechanics places a lower bound. A strict lower bound indicating how ``perfect'' a fluid can become has been conjectured from calculations in strongly coupled field theories that have a gravity dual. We test this conjecture with an explicit calculation in a unitary gas, the most strongly interacting non-relativistic system experimentally known. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V14.00008: Spectroscopic Signatures of Nonequilibrium Pairing in Atomic Fermi Gases Maxim Dzero, Emil Yuzbashyan, B.L. Altshuler, Piers Coleman We present the results of a theoretical description of the radio-frequency (RF) spectra for non-stationary states of a fermionic condensate. These states can be produced by a rapid switch of the scattering length. We show that the RF spectrum of the nonequilibrium state with constant BCS order parameter has two features in contrast to equilibrium where there is a single peak. The additional feature reflects the presence of excited pairs in the steady state. In the state characterized by periodically oscillating order parameter RF-absorption spectrum contains two sequences of peaks spaced by the frequency of oscillations. Satellite peaks appear due to a process where an RF photon in addition to breaking a pair emits/absorbs oscillation quanta. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V14.00009: Mean Frequency Shift and Finite Width of RF Spectrum of Paired Fermions Zhenhua Yu Using sum rules we derived the mean frequency shift of the rf spectrum of paired fermions in terms of the derivative of free energy, and explained many features of experiments done at the unitarity point between the lowest hyperfine states of $^6$Li.\footnote{G. Baym, C. J. Pethick, Z. Yu, and M. W. Zwierlein, Phys. Rev. Lett. 99, 190407 (2007).} The calculated mean shifts are however some three times larger than the peak shift observed at the center of the trapped atomic cloud, a discrepency due to the long tail of the spectrum. Generating the rf spectrum function self-consistently within BCS-Hartree-Fock, we have determined the characteristic frequency where the behavior of the spectrum transits from $\omega^{-3/2}$ to $\omega^{-5/2}$ at large frequency. We discuss how to subtract out the long tail from the spectrum and give an improved estimate for the peak frequency shift from the sum rules. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V14.00010: Final-state effects in radio-frequency spectrum of ultracold Fermions Sourish Basu, Erich Mueller We model the effects of final-state interaction on the radio-frequency spectrum of a two-component superfluid Fermi gas near resonance. We show how the spectrum evolves as one tunes from weak to strong interactions. Aside from the continuum resulting from the breaking of Cooper pairs, for certain interaction strengths, we predict a sharp peak resulting from converting a pair in one channel to a pair in another channel. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V14.00011: Molecular production at a broad Feshbach resonance in Fermi-gas of cooled atoms Deqiang Sun, Artem Abanov, Valery Pokrovsky The problem of molecular production from a degenerate gas of fermions at a broad Feshbach resonance, in a single-mode approximation, is reduced to the linear Landau-Zener problem for operators. The strong interaction leads to significant renormalization of the gap between adiabatic levels. In contrast to the static problem, the close vicinity of the exact resonance does not play a substantial role. The two our main physical results are: i) The molecular production is sensitive to the initial magnetic field. ii) In the inverse process of molecule dissociation a large BCS condensate distributed over a broad range of momenta is generated. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V14.00012: Bistability in Resonant Fermi Superfluid Lei Jiang, Han Pu, Andrew Robertson, Hong Ling The resonant two-channel Fermi superfluid model can be mapped to a quantum optics model that describes a single-mode laser field, subject to Kerr nonlinearity, interacting with an ensemble of inhomogeneously broadened two-level atoms. Using this analogy, we show that under proper conditions bistability will occur in resonant Fermi superfluids, a matter wave analog of a similar phenomenon encountered in nonlinear optical systems. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V14.00013: Collective spin modes in a Fermionic atomic gas John Feldmann, Hari Dahal, Sergio Gaudio, Kevin Bedell We present our theoretical findings on the structure of the spin mode dispersion of a spin-population-imbalanced Fermionic atomic gas in the highly degenerate regime, but above the superfluid critical temperature, near a Feshbach resonance. We employ standard Fermi liquid theory to describe a gas consisting of two species of spin, up and down, existing in an external magnetic field, which models an atom gas of $^6Li$ atoms in the two lowest Zeeman states. The spin population imbalance creates a net magnetization, and as a result, the transverse magnetization propagates through the system. We find that a diverging scattering length, as occurs near a Feshbach resonance, affects the phenomenological Landau parameters of the system, whose relation to the scattering length is described by the induced interaction model, and thus in turn affects the structure of the collective spin modes, as well. [Preview Abstract] |
Session V15: Quantum Error Correction and Control
Sponsoring Units: GQIChair: Lorenza Viola, Dartmouth College
Room: Morial Convention Center 207
Thursday, March 13, 2008 11:15AM - 11:27AM |
V15.00001: The structure of preserved information in quantum processes Hui Khoon Ng, Robin Blume-Kohout, David Poulin, Lorenza Viola We present a general operational framework for characterizing the types of information that can be preserved by a quantum process. We demonstrate that \emph{information preserving structures} (IPS) -- encompassing noiseless subsystems, decoherence-free subspaces, pointer bases, and error-correcting codes -- are isometric to fixed points of unital quantum processes. This implies that every IPS is a matrix algebra. A structure theorem for fixed points of an arbitrary process further provides a simple and efficient algorithm for finding all noiseless and unitarily noiseless IPS for any quantum process. This framework can be extended to study the structure of approximately preserved information. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V15.00002: Decoherence-free subspaces and incoherently generated coherences Raisa Karasik, Karl-Peter Marzlin, Barry C. Sanders, K. Birgitta Whaley A decoherence-free subspace (DFS) is a collection of states that is immune to the dominant noise effects created by the environment. DFS is usually studied for states involving two or more particles and is considered a prominent candidate for quantum memory and quantum information processing. We present rigorous criteria for the existence of DFS in finite-dimensional systems coupled to the Markovian reservoirs. This allows us to identify a new special class of decoherence free states that relies on rather counterintuitive phenomenon, which we call an ``incoherent generation of coherences.'' We provide examples of physical systems that support such states. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V15.00003: Methods for Producing Decoherence-free States and Noiseless Subsystems Using Photonic Qutrits C. Allen Bishop, Mark Byrd We outline a proposal for a method of preparing a single logically encoded two-state system (qubit) that is immune to collective noise acting on the Hilbert space of the particles supporting it. The logical qubit is comprised of three photonic 3-state systems (qutrits) and is generated by the process of spontaneous parametric down-conversion. The states are constructed using linear optical elements along with three down-conversion sources, and are deemed successful by the simultaneous detection of six events. We also show how to select a maximally entangled state of two qutrits by similar methods. For this maximally entangled state we describe conditions for the state to be decoherence-free which do not correspond to collective errors, but which have a precisely defined relationship between them. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V15.00004: Bounded-strength dynamical control of qubit coherence based on Eulerian cycles Lea F. Santos, Taylor S. Smith, Wenxian Zhang, Lorenza Viola Decoherence and faulty controls are two of the primary obstacles to realize scalable quantum information processing. Here, we investigate dynamical decoupling (DD) techniques for dynamical control and decoherence suppression in the limit of low-power faulty control, using the approach of Eulerian DD introduced in L. Viola and E. Knill, Phys. Rev. Lett. 90, 037901 (2003). By focusing on the illustrative case of single-qubit DD, we identify scenarios where naive transcriptions of bang-bang sequences with finite pulses are outperformed by the Eulerian method -- both in terms of DD fidelity and robustness against systematic errors. Results on Eulerian decoherence control in solid-state qubit devices are presented. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V15.00005: Model-independent dynamical decoupling to combat dephasing decoherence Wayne Witzel, Benjamin Lee, Sankar Das Sarma We present a remarkable finding that a recently [1] discovered series of pulse sequences, designed to optimally restore coherence to a qubit in the spin-boson model of decoherence, is in fact completely model-independent and generically valid for arbitrary dephasing Hamiltonians given sufficiently short delay times between pulses [2]. The series is optimal in that fidelity is maximized for a given number of applied pulses. This is true for sufficiently short delay times because the series, with each pulse, cancels successive orders of a time expansion for the decay of qubit fidelity. Surprisingly, this property is independent of the model of the bath that induces dephasing-type decoherence. For this to be true, a linearly growing set of ``unknowns'' (the delay times) simultaneously satisfy an exponentially growing set of non-linear equations. This is an unexpected and miraculous property of nature and mathematics. [1] G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007). [2] B. Lee, W. M. Witzel, S. Das Sarma, arXiv:0710.1416. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V15.00006: Encoding One Logical Qubit Into Six Physical Qubits Bilal Shaw, Mark Wilde, Ognyan Oreshkov, Isaac Kremsky, Daniel Lidar We discuss several methods to protect one qubit against single- qubit errors by encoding it into six physical qubits. We first present a degenerate six-qubit quantum error-correcting code. We explicitly provide the stabilizer generators, encoding circuit, codewords, logical Pauli operators, and logical CNOT operator for this code. We then prove that a six-qubit code cannot simultaneously possess a Calderbank-Shor-Steane stabilizer and correct arbitrary single-qubit errors. We finally construct a six-qubit non-degenerate entanglement-assisted quantum error- correcting code that uses one bit of entanglement shared between the sender and the receiver. We discuss the advantages and disadvantages for each of our six-qubit quantum error-correcting codes. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V15.00007: Second-order self-refocusing pulse shapes for arbitrary rotation angles Leonid P. Pryadko, Pinaki Sengupta We construct several families of high-precision 1st- and 2nd-order self-refocusing pulse shapes for rotation angles $\alpha=0^\circ$, $10^\circ$, $\ldots$, $360^\circ$. To characterize their performance, we show that for an arbitrarily-coupled qubit driven by a general one-dimensional symmetric pulse shape, in addition to the net rotation angle, the second-order average Hamiltonian is defined by three parameters. Our 1st- and 2nd-order self-refocusing pulses respectively have one or two of these equal to zero, which makes them useful as a drop-in replacement for hard pulses. We illustrate this by analyzing several commonly-used composite pulses in terms of the average Hamiltonian theory. The results are in an excellent agreement with numerical simulations. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V15.00008: Encoded Dynamical Recoupling with Shaped Pulses Yunfan Li, Daniel A. Lidar, Leonid P. Pryadko Encoded Dynamical Recoupling is a passive error correction techique which can be used to enhance the performance of a quantum error correction code (QECC) against low-frequency component of the thermal bath. The elements of the stabilizer group are used in the decoupling cycle which makes the encoded logic operations fault-tolerant. We studied the effectiveness of this techique both analytically and numerically for several three- and five-qubit codes, with decoupling sequences utilizing either Gaussian or self-refocusing pulse shapes. When logic pulses are intercalated between the decoupling cycles, the technique may be very effective in cancelling constant perturbation terms, but its performance is much weaker against a time-dependent perturbation simulated as a classical correlated noise. The decoupling accuracy can be substantially improved if logic is applied slowly and concurrently with the decoupling, so that a certain adiabaticity condition is satisfied. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V15.00009: Generating Novel Spin Echoes Using the Internal Structure of Strong Pi-Pulses Rona Ramos, Yanqun Dong, Dale Li, Sean Barrett Conventionally, strong pulses used to control coherent evolution are approximated as instantaneous, perfect rotations. However, recent experiments have shown a surprising departure from the conventional theory in standard multipulse NMR experiments using strong pi-pulses. Using our understanding of the role that the finite time duration of any real pulse plays in these effects, we design and experimentally demonstrate new classes of spin echoes. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V15.00010: A New Approach to Spin Coherence Control: Extreme Line-narrowing and MRI of Solids Yanqun Dong, Rona Ramos, Dale Li, Sean Barrett The non-zero duration of strong pulses has been shown to have surprisingly large effects in important NMR experiments. The Hamiltonian terms arising from the internal structure of strong pulses provide us with a new technique of spin coherence control. Using this technique, we design and demonstrate new approaches to line-narrowing and magnetic resonance imaging of solids. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V15.00011: Robustness of operator quantum error correction against imperfect initialization Ognyan Oreshkov It is known that perfect unitary evolution inside decoherence-free subspaces and subsystems (DFSs) requires more restrictive conditions if one allows imperfect initialization of the state. It was believed that these conditions are necessary if DFSs are to be able to protect imperfectly encoded states from subsequent errors. By a similar argument, general operator quantum error-correcting (OQEC) codes would also require more restrictive error-correction conditions for the case of imperfect initialization. In this study, we examine this requirement by looking at the errors on the encoded state. In order to quantitatively analyze the errors in an OQEC code, we introduce a measure of the fidelity between the encoded information in two states for the case of subsystem encoding. In contrast to what was previously believed, we obtain that more restrictive conditions are not necessary neither for DFSs nor for general OQEC codes. This is because the effective noise that can arise inside the code as a result of imperfect initialization is such that it can only increase the fidelity of an imperfectly encoded state with a perfectly encoded one. Some implications of this result in the context of fault-tolerant information processing are discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V15.00012: Quantum Convolutional Coding with Entanglement Assistance Mark Wilde, Todd Brun We have recently developed quantum convolutional coding techniques for both entanglement distillation and quantum error correction. These techniques assume that the two parties participating in the communication protocols possess prior shared entanglement. Using these methods, we can import arbitrary classical binary or quaternary convolutional codes for use in quantum coding, with no requirement that these codes be self-orthogonal. Moreover, high-performance classical convolutional codes lead to high-performance quantum convolutional codes. We explicitly show how a convolutional entanglement distillation protocol operates, and how to encode and decode a stream of quantum information in an entanglement-assisted quantum convolutional code. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V15.00013: ``Slow'' and ``Fast'' Gate Limits In Resilient Quantum Computation Eduardo Novais, Eduardo R. Mucciolo, Harold U. Baranger In the study of resilient quantum computation, there are two common approaches to proving the threshold theorem: one either uses a stochastic error model or uses the operator norm to bound the effects of the noise. In many cases, the underlying microscopic Hamiltonian is hidden due to the rapidly growing complexity of the problem. In particular, the microscopic interacting Hamiltonian in the interaction picture depends on the quantum code and its implementation. Nevertheless, there are two possible ways to keep the discussion code independent. The first situation is to imagine very fast gates acting on the system. The second is to derive an upper bound on the effects of correlations by deriving an effective model. In this talk we discuss these two limits, focusing on how to derive the effective model for ``slow gates''. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V15.00014: Optimal Control of Large Spin-Atomic Systems with Coherent Electromagnetic Fields Seth Merkel, Souma Chaudhury, Poul Jessen, Ivan Deutsch Cold atomic systems provide an excellent testing ground for quantum control protocols due to the isolation of these systems from their environment and the availability of high precision fields from the ``quantum optics toolbox''. In this talk, we look at controlling the sixteen dimensional ground state hyperfine manifold of $^{133}$Cs through microwaves and rf-magnetic fields. These controls allow for essentially coherent manipulation of a system that is large enough to exhibit non-trivial dynamics. In particular, we analyze the controllability of this system under different combinations of applied electromagnetic fields. Also, we present a scheme for performing state preparation, which is the mapping a fiducial state to an arbitrary target state, and show simulations that examine the performance of these state preparation protocols.~~~~~ [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V15.00015: Towards a Quantum Predictive Control Mathematical Formulation Yasser A. Davizon-Castillo Quantum Feedback Control presents an interdisciplinary research work between Control Theory and Quantum Physics. A novel mathematical formulation is presented for Quantum Predictive Control (QPC) algorithm, based on Receding Horizon Control philosophy at the quantum level. The application of Heisenberg Uncertainty Principle, to quantify the uncertainty in position and momentum, in a real-time control framework is introduced. An estimate of the Sampling Time at quantum level is demonstrated, applying to a case of study for a double well quantum system using the QPC algorithm. [Preview Abstract] |
Session V16: Focus Session: Medical Imaging and Related Technologies
Sponsoring Units: DBPChair: Christopher Njeh, Tyler Cancer Center
Room: Morial Convention Center 208
Thursday, March 13, 2008 11:15AM - 11:51AM |
V16.00001: Overview of Medical Imaging Invited Speaker: The use of radiation probes to image tissues in the human body has progressed through an extraordinary evolution in the past three decades. Beginning with transmission computed tomography in the 1970s, this evolution has included real-time ultrasound, emission computed tomography, magnetic resonance imaging and digital radiography. These advances have recently yielded major improvements in imaging such as multi-detector transmission computed tomography, functional magnetic resonance imaging, dual imaging modalities built on a common platform, and image-guided intervention. These improvements and others have accelerated the usefulness of imaging methods in the early detection, definitive diagnosis, and effective intervention of a wide spectrum of diseases and disabilities. They also have led to increases in radiation doses to patients and the population, an issue of major concern to physicists and physicians. At this time there are four major frontiers for research in medical imaging: (1) molecular imaging; (2) functional imaging; (3) multi-modality imaging; and (4) information management. These research frontiers, together with the use of sophisticated imaging technologies in clinical practice, offer rich professional opportunities for physicists. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V16.00002: PET Imaging - from Physics to Clinical Molecular Imaging Invited Speaker: From the beginnings many years ago in a few physics laboratories and first applications as a research brain function imager, PET became lately a leading molecular imaging modality used in diagnosis, staging and therapy monitoring of cancer, as well as has increased use in assessment of brain function (early diagnosis of Alzheimer's, etc) and in cardiac function. To assist with anatomic structure map and with absorption correction CT is often used with PET in a duo system. Growing interest in the last 5-10 years in dedicated organ specific PET imagers (breast, prostate, brain, etc) presents again an opportunity to the particle physics instrumentation community to contribute to the important field of medical imaging. In addition to the bulky standard ring structures, compact, economical and high performance mobile imagers are being proposed and build. The latest development in standard PET imaging is introduction of the well known TOF concept enabling clearer tomographic pictures of the patient organs. Development and availability of novel photodetectors such as Silicon PMT immune to magnetic fields offers an exciting opportunity to use PET in conjunction with MRI and fMRI. As before with avalanche photodiodes, particle physics community plays a leading role in developing these devices. The presentation will mostly focus on present and future opportunities for better PET designs based on new technologies and methods: new scintillators, photodetectors, readout, software. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V16.00003: Room temperature STM imaging in air can damage DNA, even at low tunneling biases and currents John Bechhoefer, Philip Grant, Yuekan Jiao STM images of DNA molecules in air or vacuum at room temperature have been plagued by problems of reproducibility. These difficulties have usually been ascribed to substrate artifacts or tip-related effects. However, the recent discovery that low-energy electron beams cause single-strand breaks in DNA suggests that the tunneling electrons used for STM imaging can damage DNA similarly and could be responsible for many imaging problems. Here, we provide experimental support for such a conclusion. Collecting images from an STM that simultaneously detects light-emission from the tip region, we show that the observed DNA structure changes after the first scan. The organic debris from the DNA quenches the light emission from surface plasmons on the gold substrate. Next, we use an atomic force microscope (AFM) whose stiff tip can be used both for tapping-mode AFM and for STM imaging modes. We assess STM-induced damage by re-imaging the same area in tapping-mode AFM. The bias-dependent change in DNA film thickness correlates with the previously observed rate of DNA strand breaks caused by low-energy electron beams of different energies. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V16.00004: Tomography of biological interfaces using defocusing microscopy Oscar Mesquita Transparent objects can render visible in a standard bright-field microscope by slightly defocusing the microscope objective. From contrast fluctuations of images defocused in a controlled way one can measure the fluctuation spectrum of biological membranes and living cells surfaces. We extended our previous defocusing theory, valid for small defocusings, to arbitrarily large defocusings. We discovered that we can measure height fluctuations of transparent interfaces selectively, and obtain elastic properties of layered biological membranes separately. As an example, we measured separately the elastic constants associated with the two opposite surfaces of a red blood cell. The technique is very sensitive and allows us to measure the small increase of surface tension on the surface in contact with the glass-slide, as compared to the one of the free surface. Interface roughness (static and dynamic), down to nanometer amplitudes, can be measured selectively with defocusing microscopy in transparent layered materials. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V16.00005: Terahertz spectroscopy of human skin constituents in suspension Cecil Joseph, Anna Yaroslavsky, Munir Al-Arashi, Andrew Gatesman, Thomas Goyette, Robert Giles Continuous wave terahertz imaging has the potential to offer a non-invasive medical imaging modality for detecting different types of human cancers. The aim of this study was to identify frequencies of interest for continuous wave terahertz imaging of skin cancer. The absorption characteristics of water, collagen, and elastin were studied in the range between 20 and 100cm$^{-1}$. In addition, we have recorded and analyzed the teraherz absorption spectra of several substances that are present in human skin (i.e. tryptophan, tyrosine, melanin, urocanic acid, keratin) and their water suspensions with the goal of using them as biomarkers for skin cancer detection. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V16.00006: Advances in Medical X-Ray Imaging Invited Speaker: |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V16.00007: Inverse Participation Ratio (IPR) Analysis of Transmission Electron Microscopy (TEM) Images: Quantification of Optical Disorder Strength Due to Nanoscale Refractive Index Fluctuations of Tissues/Cells P. Pradhan, V. Turzhitsky, H. Subramanian, A. Heifetz, D. Damania, J. L. Hoogheem, M. J. Jung, H. K. Roy, V. Backman An IPR analysis technique is developed for the first time to analyze and to quantify TEM images of cells/tissues by projecting them to optical lattices and quantifying their short-range nanoscale refractive index fluctuations. The value of IPR of a finite optical lattice provides the measure of the localization of light due to the lattice refractive index fluctuations. The high resolution ($\sim $1nm) of the TEM technique enables imaging of the nanoscale refractive index fluctuations of thin tissue/cell sample slices ($\sim $50-100 nm ). TEM images have been widely used in biology for pathological and visual observation of cells and sub-cellular structures. However, properties of the nanoscale fluctuations in the images have not been fully understood. Results of our IPR study of human tissue/cell TEM images show that average short range nanoscale refractive index fluctuations in tissues/cells increase ( i.e. increase of the IPR value) with the progress of carcinogenesis. Presently available detection techniques are unable to detect these changes. Potential applications of the IPR analysis to probe other nanoscale biological changes are also discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V16.00008: Imaging Stem Cell Aggregation Using Digital Holographic Microscopy Emily J. Gardel, Yonas Yemane, Debra Auguste, Vinothan N. Manoharan Stem cells in solution aggregate and self-assemble into spheres of cells called embryoid bodies (EBs). During this process, cells divide, differentiate, and influenced by interactions with other cells both chemically and mechanically. We use a combination of holographic, interferometric, and spectroscopic techniques to visualize EB formation. Such methods track the cells' positions and allow us to measure the rates and mechanisms of aggregation as well as the overall structure of the EB. The goal is to understand how cell-cell interactions influence the self-assembly process as well as the environmental cues responsible for stem cell differentiation. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V16.00009: Magnetomotive Optical Coherence Elastography for Measuring Biomechanical Properties of Tissue using Magnetic Nanoparticles V. Crecea, A.L. Oldenburg, X. Liang, T.S. Ralston, M.B. Orescanin, M.F. Insana, S.A. Boppart Biomechanical properties of tissue are indicative of health and disease, and the ability to readily measure them is instrumental for the diagnosis of early-stage changes. We present a new method for measuring elastic properties of tissue-like phantoms, which employs Fe$_3$O$_4$ nanoparticles as contrast agents in a technique called magnetomotive optical coherence elastography (MMOCE). PDMS-based samples similar to soft biological tissue (0.5-12 kPa) were prepared, with nanoparticles embedded within their volume. The magnetic nanoparticles are displaced upon probing with an external magnetic field, engaging the sample in axial motion. M-mode MMOCE phase data was acquired concomitantly at a rate of 29 kHz, allowing for the tracking of scatterers in the sample with a displacement sensitivity of 11 nm. The scaterers in the samples underwent underdamped oscillations when the magnetic field was applied step-wise. We extracted the damping constants and the natural frequencies of oscillation (30-200 Hz) from the time-resolved displacement traces. A microindentation apparatus was used to measure the Young's moduli of the samples for validation and calibration with the MMOCE measurements. This novel real-time non-invasive technique affords the potential for \textit{in vivo} studies. [Preview Abstract] |
Session V17: Focus Session: Nonequilibrium Thermodynamics of Small Systems
Sponsoring Units: GSNP DBPChair: Chris Jarzynski, University of Maryland
Room: Morial Convention Center 209
Thursday, March 13, 2008 11:15AM - 11:51AM |
V17.00001: Beyond the second law. Invited Speaker: According to the second law, the work needed to bring a system from one state to another one, when in contact with a heat bath, is at least equal to the difference in free energies of these states. By a refinement of the work theorem, we derive an exact microscopic expression for the extra dissipation. It is expressed in terms of the relative entropy of the phase space density. Furthermore, lower bounds for the extra dissipation are obtained when only limited information is available. The result is illustrated on various Hamiltonian and stochastic models. The connection to the thermodynamics of computation is briefly discussed. Ref R. Kawai, JMR. Parrondo and C. Van den Broeck, Phys Rev Lett 98, 080602 (2007) [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V17.00002: Dynamics of Microtubule Growth and Catastrophe Sidney Redner, Tibor Antal, Paul Krapivsky, Mitch Mailman, Bulbul Chakraborty We investigate a simple dynamical model of a microtubule that evolves by attachment of guanosine triphosphate (GTP) tubulin to its end, irreversible conversion of GTP to guanosine diphosphate (GDP) tubulin by hydrolysis, and detachment of GDP at the end of a microtubule. As a function of rates of these processes, the microtubule can grow steadily or its length can fluctuate wildly. In the regime where detachment can be neglected, we find exact expressions for the tubule and GTP cap length distributions, as well as power-law length distributions of GTP and GDP islands. In the opposite limit of instantaneous detachment, we find the time between catastrophes, where the microtubule shrinks to zero length, scales as $e^\lambda$. We also determine the size distribution of avalanches (sequence of consecutive GDP detachment events). We obtain the phase diagram for general rates and verify our predictions by numerical simulations. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V17.00003: Work fluctuations in modulated systems: what is and what is not universal in the steady state Mark Dykman We study work fluctuations in periodically modulated nonlinear systems. Such systems often have coexisting stable periodic states. We show that the standard relations of the steady-state work fluctuation theorem derived for linear systems do not generally apply to nonlinear systems. Work fluctuations sharply increase near a kinetic phase transition where populations of the coexisting periodic states are close to each other. The work variance in this range is inversely proportional to the rate of fluctuational interstate switching. It exponentially decreases with the increasing distance to the phase transition point. We also show that the work variance in a metastable state displays scaling with the distance to the bifurcation point where this state disappears. The critical exponent in the dependence of the variance on the distance to a saddle-node bifurcation point is -1. The results apply to a broad range of vibrational systems of current interest, from trapped electrons to Josephson junctions and to nano- and micromechanical resonators. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V17.00004: Recognizing the role of microscopic reversibility in stochastic systems with optical trapping experiments as an example Ross Brody With optical tweezer experiments it is possible to confine a particle and observe its Brownian motion in a region of known potential. Recognizing that microscopic reversibility can lead to interesting relationships involving the particle's motion, for example, the ratio of the conditional probability of making a transition between two points with its spatial reverse is equal to the difference between two Boltzmann factors ($e^{-\Delta U/k_BT}$), or that the time to make an up-well transition is identical to the time to make a down-well transition. With these relationships in mind, and the Onsager-Machlup action description of a path, we consider a particle in an optical tweezer of varying strength and investigate relationships involving the conditional probabilities, the actions associated with specific paths, and the external work done by varying the strength of the trap. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V17.00005: What Do We Measure With Single-Molecule Force Spectroscopy? Ching-Hwa Kiang, Nolan Harris, Eric Botello, Wei-Hung Chen Single-molecule force spectroscopy is a powerful technique for studying detailed intra- and inter- molecular interactions by manipulating single biomolecules at the nanometer scale. Force is measured while one pulls on the molecules. However, relating the measured information to equilibrium thermodynamic properties is challenging. Jarzynksi's equality allows one to reconstruct the free energy surface as a function of molecular end-to-end distance$^{1,2}$. Using protein folding as an example, we studied the parameters that influence the unfolding process, such as pulling velocity, tempearture, and chemical denaturant concentration in the solution, to demonatrate that valuable equailibrium thermodynamic information can be obtained using this technique. 1. N.\ C.\ Harris, Y.\ Song, and C.-H.\ Kiang, {\em Phys.\ Rev.\ Lett.}, {\bf 99} 068101 (2007). 2.``Pulling Strings: Stretching Proteins Can Reveal How They Fold,'' {\bf Science News}, {\bf 172} 22 (2007). [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V17.00006: Holographic microrheology of biofilms Fook Chiong Cheong, Simone Duarte, David Grier We present microrheological measurements of polymeric matrices, including the extra-cellular polysaccharide gel synthesized by the dental pathogen S. mutans. As part of this study, we introduce the use of precision three-dimensional particle tracking based on video holographic microscopy. This technique offers nanometer-scale resolution at video rates, thereby providing detailed information on the gels' complex viscoelastic moduli, including insights into their heterogeneity. The particular application to dental biofilms complements previous studies based on macroscopic rheology, and demonstrates the utility of holographic microrheology for soft-matter physics and biomedical research. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V17.00007: Fluctuation relations in a micromechanical oscillator driven far from thermal equilibrium Corey Stambaugh, Ho Bun Chan We explore fluctuation relations in a micromechanical torsional oscillator. In the linear regime when the modulation is weak, we verify that the ratio of the work variance to the mean work is independent of the driving frequency, consistent with standard fluctuation relations for a steady state system near thermal equilibrium. We then apply a strong drive to force the system into nonlinearity. Here the system displays bistability and the relationship between the work and variance is expected to deviate from the linear regime. For a bistable system the total variance has two distinct contributing components. The first results from small fluctuations about a stable state. The work variance is expected to diverge as the system approaches the bifurcation point where the state disappears. The other part of the variance results from the system switching between coexisting states. This part of the variance is expected to show a sharp peak near the kinetic phase transition when the populations of the two states are comparable. We compare our experimental results to theoretical predictions that distinguish nonlinear oscillators from equilibrium systems. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V17.00008: Effect of charge distribution on the translocation of an inhomogeneously charged polymer through a nanopore Aruna Mohan, Anatoly Kolomeisky, Matteo Pasquali We investigate the voltage-driven translocation of an inhomogeneously charged polymer through a nanopore by utilizing discrete and continuous stochastic models. As a simplified illustration of the effect of charge distribution on translocation, we consider the translocation of a polymer with a single charged site in the presence and absence of interactions between the charge and the pore. We find that the position of the charge that minimizes the translocation time in the absence of pore-polymer interactions is determined by the entropic cost of translocation, with the optimum charge position being at the midpoint of the chain for a rodlike polymer and close to the leading chain end for an ideal chain. The presence of attractive or repulsive pore–charge interactions yields a shift in the optimum charge position towards the trailing end and the leading end of the chain, respectively. Moreover, our results show that strong attractive or repulsive interactions between the charge and the pore lengthen the translocation time relative to translocation through an inert pore. We generalize our results to accommodate the presence of multiple charged sites on the polymer. Our results provide insight into the effect of charge inhomogeneity on protein translocation through biological membranes. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V17.00009: Heat Transport in spin chains Jinshan Wu, Mona Berciu The Projection operator technique, usually used to study the relaxation toward thermal equilibrium, is extended to investigate non-equilibrium but stationary processes. Particularly, in this work we apply it to study heat transport in short spin chains with Heisenberg, anisotropic Heisenberg and XY couplings plus a large magnetic field. One long-standing question is under what circumstance the relation between thermal current and local temperature obeys Fourier's Law. Our results suggest that for certain parameters and short chains the heat transport does obey Fourier's Law. The evolution towards a diverging thermal conductivity, expected in the bulk limit, is also elucidated. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V17.00010: Athermal dynamics of strongly coupled stochastic three-state oscillators Lev Tsimring, Bastien Fernandez We study the collective behavior of a globally coupled ensemble of $N$ cyclic stochastic three-state systems with rates of transition from state $i-1$ to state $i$ proportional to the number of systems already in state $i$. While the mean field theory predicts only decaying oscillations in this system, direct numerical simulations indicate that the mean field exhibits stochastic oscillations even in the large $N$ limit. We characterize the regularity of oscillations by the coherence parameter which has a well-defined maximum at the coupling constant of order 1. In contrast, the order parameter characterizing the level of synchrony among oscillators, increases monotonously with the coupling strength. We derive the exact solution of the full master equation for the stationary probability distribution and find the analytical expression for the order parameter. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V17.00011: Entropy-driven hysteresis in a model of DNA overstretching Stephen Whitelam, Sander Pronk, Phillip Geissler When pulled along its axis, double-stranded DNA elongates abruptly at a force of about 65 pN. Two physical pictures have been developed to describe this overstretched state. The first proposes that strong forces induce a phase transition to a molten state consisting of unhybridized single strands. The second picture instead introduces an elongated hybridized phase, called S-DNA, structurally and thermodynamically distinct from standard B-DNA. Little thermodynamic evidence exists to discriminate directly between these competing pictures. Here we show that within a microscopic model of DNA we can distinguish between the dynamics associated with each. In experiment, considerable hysteresis in a cycle of stretching and shortening develops as temperature is increased. Since there are few possible causes of hysteresis in a system whose extent is appreciable in only one dimension, such behavior offers a discriminating test of the two pictures of overstretching. Most experiments are performed upon nicked DNA, permitting the detachment (`unpeeling') of strands. We show that the long-wavelength motion accompanying strand separation generates hysteresis, the character of which agrees with experiment only if we assume the existence of S-DNA. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V17.00012: Replica and extreme-value analysis of the Jarzynski free-energy estimator Matteo Palassini, Felix Ritort We analyze the Jarzynski estimator of free-energy differences from nonequilibrium work measurements. By a simple mapping onto Derrida's Random Energy Model, we obtain a scaling limit for the expectation of the bias of the estimator. We then derive analytical approximations in three different regimes of the scaling parameter x = log(N)/W, where N is the number of measurements and W the mean dissipated work. Our approach is valid for a generic distribution of the dissipated work, and is based on a replica symmetry breaking scheme for x >> 1, the asymptotic theory of extreme value statistics for x << 1, and a direct approach for x near one. The combination of the three analytic approximations describes well Monte Carlo data for the expectation value of the estimator, for a wide range of values of N, from N=1 to large N, and for different work distributions. Based on these results, we introduce improved free-energy estimators and discuss the application to the analysis of experimental data. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V17.00013: Hand-Over-Hand Binding in a Tethered-Particle Method Study of DNA Hybridization Marie T. Ung, John C. Crocker The tethered-particle method is used to probe the complex process of DNA hybridization. This experiment analyzes tethers formed between PEGylated polystyrene microspheres and PEGylated glass surfaces, to which DNA strands have been grafted. Previous studies have looked at single duplex dissociation in isolation. We seek to understand cooperativity effects of short duplexes in multiple tethered states. This experiment is performed near the duplex melting temperature, resulting in dynamic binding and unbinding. Single molecular tethers are analyzed using high performance particle tracking with a high-speed camera. By analyzing 2D trajectories of the particle's centroid versus time, we can distinguish single binding and unbinding events from multiple bridged states. The trajectories' analysis suggests that multiple bridged states are less stable than expected for a zero cooperativity model. We hypothesize that this instability may be due to extreme force sensitivity of individual DNA duplexes. [Preview Abstract] |
Session V18: Focus Session: Properties of Block Copolymers
Sponsoring Units: DPOLYChair: Thomas Epps, University of Delaware
Room: Morial Convention Center 210
Thursday, March 13, 2008 11:15AM - 11:51AM |
V18.00001: Thermodynamics, Structure and Transport in Model Fuel Cell Membranes Invited Speaker: Polymer electrolyte membranes (PEM), used to conduct protons from the anode to the cathode of hydrogen fuel cells, are open systems that exchange water with the surrounding air. Proton conductivity is closely coupled to the presence of contiguous hydrated channels within the membrane. In an attempt to understand the underpinnings of the morphology of these systems, the phase behavior of model PEMs comprising block copolymers in equilibrium with humidified air was studied as a function of the relative humidity of the surrounding air, ion content of the copolymer, and temperature. At low humidity, the copolymers exhibit an order-to-disorder transition as a function of increasing temperature. At high humidity, however, increasing temperature results in a disorder-to-order transition. \textit{In-situ} small angle neutron scattering experiments on the open block copolymer system, when combined with water uptake measurement indicate that the disorder-to-order transition is driven by an increase in the partial molar entropy of the water molecules in the ordered phase relative to that in the disordered phase. This is in contrast to most systems wherein increasing entropy results in stabilization of the disordered phase. The coupling between entropy and proton conductivity will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V18.00002: Water Uptake and Proton Conductivity of Asymmetric Poly(styrenesulfonate-block-methylbutylene) Copolymers Xin Wang, Moon Jeong Park, Nitash Balsara The effect of chain architecture on water uptake and proton conductivity of poly(styrenesulfonate-block-methylbutylene) (PSS-PMB) copolymers at equilibrium with moist air is studied as a function of temperature, and relative humidity of the air. Symmetric and asymmetric PSS-PMB copolymers were synthesized by anionic polymerization of poly(styrene-block- isoprene) copolymers, followed by hydrogenation of the polyisoprene block and sulfonation of the polystyrene block. Previous studies have shown that symmetric PSS-PMB block copolymers in the presence of humid air (relative humidity$>$50\%) are excellent proton conductors at temperatures as high 90 C. Current work is focused on water uptake and conductivity measurements on asymmetric PSS-PMB block copolymers with PSS as the minor component. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V18.00003: Temperature dependent charge transport properties of poly(3-hexylthiophene) block poly(styrene) copolymer field-effect transistor Firoze Haque, Paul Stokes, Lei Zhai, Saiful I. Khondaker Regioregular poly-3-hexythiophene (rr-P3HT) is considered to be one of the most promising and well studied organic semiconductor. Recently attention has been focused in developing di-block copolymers of rr-P3HT by attaching non conjugated blocks which allows one to tune the electrical properties of rr-P3HT. To properly utilize these new types of materials it is necessary to understand the relationship between their molecular structure and electronic transport properties. In this talk we present electronic transport characteristics of poly(3-hexylthiophene) block poly(styrene) copolymer (rr-P3HT-b-PS) field effect transistor at various temperatures. We show that the current voltage characteristic at different temperature follow SCLC type conduction mechanism with $I\propto V^n$, where n varies from 1.5 to 2.2. We also show that the room temperature hole mobility is $\sim $3.5$\times $10$^{-5}$ cm$^{2}$/Vs, and that the mobility decreases with decreasing temperature. The temperature dependent mobility follow activated hoping process. The space charge limited current along with the low mobility of the devices indicates that the charge transport is limited by the insulating polystyrene segment of the di-block polymer. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V18.00004: Morphology and Dynamic Mechanical Properties of Styrene Containing Tri-Block Copolymers for Electromagnetic Wave Interaction Applications S. Peddini, K. Mauritz, D. Nikles, J. Weston Styrene containing triblock copolymers, namely poly(styrene-ethylene/butylene-styrene) (SEBS) and poly(styrene-butadiene-styrene)] (SBS), were selectively modified by attaching polar groups to facilitate the in-growth of an inorganic component. In case of SEBS, the styrene block was sulfonated, and in SBS, the butadiene block was hydroxylated. The extent of modification was determined by analytical and spectroscopic methods. This presentation shows the morphology and dynamical mechanical properties of both block copolymers before and after modification. Nanocomposites of these block copolymers were prepared by inclusion of magnetic metal oxides \textit{via} an \textit{in-situ} precipitation and self assembly processes and their morphology and dynamical mechanical properties were studied. Magnetic properties of these polymers filled with iron oxide nanoparticles were measured using an alternating gradient magnetometer (AGM) at room temperature to observe the magnetic hysteresis. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V18.00005: The Lyotropic Phase Behavior of Diblock Copolymers Swollen with Ionic Liquids Peter Simone, Timothy Lodge The lyotropic phase behavior of poly(1,2-butadiene-$b$-ethylene oxide) diblock copolymers (PB-PEO) has been studied upon addition of two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF$_{6}$]). The copolymer/ionic liquid samples ranged from dilute to concentrated, and were characterized via small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At moderate to high concentrations, SAXS patterns corresponding to the classical copolymer microstructures of body-centered cubic lattices of spheres, hexagonally packed cylinders, and lamellae were observed. Additionally, at several concentrations, coexisting microstructures and what is thought to be a disordered network microstructure were observed. At low concentration, the morphology of the block copolymer micelles ($i.e. $spheres, cylinders, and vesicles) was used as a qualitative gauge of the ionic liquid solvent quality, and it was concluded that for PB-PEO, [BMI][PF$_{6}$] behaves as a more selective solvent than [EMI][TFSI]. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V18.00006: Structure and Thermodynamics of Block Copolymers Doped with Ionic Liquids J.M. Virgili, N.P. Balsara, R.A. Segalman Incorporation of ionic liquids into block copolymers is of interest for applications such as high temperature fuel cell membranes and polymer processing. These applications take advantage of ionic liquids' attractive physiochemical properties, such as low vapor pressure and high thermal stability. We investigate the structure and thermodynamics of poly(styrene-$b$-2-vinylpyridine) (PS-PVP) block copolymers doped with an ionic liquid consisting of imidazole and bis(trifluoromethanesulfonyl)amide (HFTSI). Using small angle X-ray scattering (SAXS), we demonstrate that increased ionic liquid doping leads to swelling of lamellar nanodomains in a symmetric PS-PVP block copolymer. At high ionic liquid loadings, we observe break up of the lamellar domains into hexagonally perforated lamellae. We characterize the effect of ionic liquid loading on the order-disorder transition (ODT) temperature of PS-PVP. We observe depression of the PS-PVP ODT temperature with increasing loading of the ionic liquid. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V18.00007: Controlling the morphology of liquid crystalline block copolymers: interfacial and liquid crystal content effects. Eric Verploegen, Tejia Zhang, Paula Hammond By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase segregated BCP microstructures can be precisely manipulated. A wide range of morphologies can be achieved from a single block copolymer backbone during a one step LC attachment reaction. The anchoring of the smectic LC mesophase to the inter-material dividing surface (IMDS) is a key driving force in determining the morphologies for both the bulk and in thin films. In thin films the orientation of the morphology is determined by the minimization of the surface energy with the substrate and air interfaces and the anchoring of the LC mesophase to the substrate. These competing effects can be utilized to manipulate the orientation of as-cast and thermally annealed thin films. Additionally, by controlling the LC content, the mechanical properties of this system can be tailored over a several orders of magnitude. The tune-ability demonstrated in this system will allow for custom design and tailoring of material properties for specific applications such as electromechanical and mechano-optical devices. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V18.00008: Self-assembly of side chain liquid crystalline block copolymers Manas Shah, Victor Pryamitsyn, Venkat Ganesan We present a new model based on self-consistent field theory (SCFT) approach to characterize the self assembly behavior in side-chain liquid crystalline block copolymers. Our model considers a micromechanical representation of flexible coil-coil diblock copolymers, with rod-like units grafted to one of the blocks. We present results which elucidate self-assembly arising from the interplay between block copolymer microphase separation and the orientational ordering of the rod segments. Our numerical results are in very good agreement with reported experimental observations. Many of the traditional flexible diblock copolymer microphases are also predicted to occur for side chain liquid crystalline polymers, with smectic ordering accompanying within the microphases. The equilibrium phase morphologies are observed to depend on the molecular weight of the copolymer, the length of the rod units, the relative volume fractions of each block, and the energetic and orientational interactions between different components. Moreover, for the parameters considered in this article, microphase separation was observed to be a requisite for developing orientational ordering between mesogenic units [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V18.00009: Structural Formation Process of Microphase Separated Films with Liquid Crystalline Phase Transition Motonori Komura, Tomokazu Iyoda Ordered nanostructures arising from the microphase separation of block copolymers have driven one to fabricate nanofunctional materials as fundamental technology of the coming electronic and photonic materials. Thin films of a series of newly designed amphiphilic block copolymer consisting of hydrophilic polyethylene oxide (PEO) and hydrophobic polymethacrylate with azobenzene-mesogen in side-chain (PMA(Az)) show highly ordered microphase separation with PEO cylinders perpendicularly oriented to the film surface. In the present report, we investigated a structural formation process of the microphase separated films by temperature controlled atomic force microscopy (AFM) and grazing incidence small angle X-ray scattering (GISAXS). These measurements revealed that homeotropic alignments of Az liquid crystalline layers predominated the cylinder orientation, which corresponded to a $<$110$>$ direction of body centered cubic structure under annealing condition, in disagreement with cylinder orientation of order-order transition of traditional block copolymers. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V18.00010: Rod-to-Coil Transition in Polypeptide/$\pi$-Conjugated Polymer/Polypeptide Triblock Copolymers Raffaele Mezzenga, Laurent Rubatat, Xiangxing Kong, Samson Jenekhe, Janne Ruokolainen, Mohamad Hojeij Self-assembly in the bulk of a series of hybrid triblock copolymers formed by a poly(9,9-dihexylfluorene-2,7-diyl) (PHF) internal block and two poly($\gamma $-benzyl-L-glutamate) (PBLG) external blocks has been studied. Since the $\alpha $-helical secondary structure of the PBLG block may be either maintained or suppressed depending on the solvent casting history, the PBLG-PHF-PBLG copolymers exhibit two different conformations: a rod-rod-rod or coil-rod-coil configuration, respectively. To provide insight on the influence of molecular architecture on self-aggregation of these systems, three copolymers with different block ratio were investigated in both conformations using small- and wide-angle scattering techniques and transmission electron microscopy. Time-resolve photoluminescence measurements were performed on the same samples to explore the effect of morphology on photophysical properties. The observed photoluminescence spectra and dominant excited lifetimes of the poly(9,9-dihexylfluorene-2,7-diyl) block were found to differ markedly in rod-rod-rod and coil-rod-coil configurations and were correlated to the morphology of the self-assembled triblock copolymers. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V18.00011: The effect of chain stiffness on the morphology of diblock copolymers G. Leuty, J. Bedard, Mesfin Tsige One of the most interesting and challenging problems in the science of materials concerns the structure and dynamics of the morphology of block copolymers. These materials generally consist of two or more homopolymer chains or blocks that are covalently bonded to each other to form a single polymer chain. Variation in the stiffness of the different block segments can directly affect the morphology of the system and may thus result in a very rich phase behavior. In the present study, the microphase separation of symmetric diblock copolymers with variable block stiffness and chain length is studied using coarse-grained molecular dynamics simulations. In the lamellar phase, the equilibrium lamellar spacing and orientation of the block segments in the system are found to depend on the relative stiffness between the two block segments. As the chain length of the block segments increases, the morphology changes from the expected lamellar appearance to a cylindrical one. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V18.00012: Self-assembled OLEDs from rod-coil block copolymers Y. Tao, R.A. Segalman High efficient OLEDs tend to be made of many stacked layers including layers for hole transport, emission , and electron transport, which are produced via a very tedious sequence of high vacuum steps. Since conjugated rod-coil block copolymers form layer structures due to rod-coil repulsions and rod-rod interactions, they are an alternate route towards multi-layer devices which can be solution processed in one single step. A functional conjugated rod-coil block copolymer, poly(alkoxyphenylene vinylene-b-oxadiazole (PPV-b-OX), incorporates a hole transporting/emissive rod and an electron transporting coil. Grazing Incidence X-ray scattering is used to demonstrate the layered structure of the resulting self-assembled block copolymer film relative to the substrate (electrode). A multi-layer thin film self-assembled from PPV-b-OX shows significant improvement in luminescence and efficiency over pure PPV and PPV/OX blend devices. The correlation between details of thin film structure including lamellar spacing, orientation, and number of layers and device performance will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V18.00013: Self-assembly of linear rod-coil block copolymers Li-jia An, Ji-zhong Chen, Zhao-yan Sun, Cheng-xiang Zhang Rod-coil block copolymer systems have attracted a great deal of attention due to their rich phase behavior. The spontaneous ordering of coil-rod block copolymers is due to the mutual repulsion of the dissimilar blocks and the packing constrains imposed by the connectivity of each block, while the stiff rigid conformation of the rod segment imparts orientation organization. However, few reports investigate rod-coil block copolymers in three-dimension space. In this work, the self-assembly of linear rod-coil block copolymers is studied by applying self-consistent-field lattice techniques in three-dimension space. The stiffness influences on the self-assembly and the possible orientations of the rods in different structures are focused on for rod-coil diblock copolymers; the interfacial grafting density of the separating rod and coil segments is found exerting important influences on the phase behavior of symmetric coil-rod-coil triblock cpolymers; the influences of the intramolecular interactions between the two rods of the symmetric rod-coil-rod triblock copolymer chain on the self-assembly are studied. [Preview Abstract] |
Session V19: New Frontiers for Biological Physics
Sponsoring Units: FIAPChair: Ali Mohazab, University of British Columbia
Room: Morial Convention Center 211
Thursday, March 13, 2008 11:15AM - 11:51AM |
V19.00001: Mechanochemistry of Molecular Motors Invited Speaker: Molecular motors lie at the heart of biological processes from DNA replication to vesicle transport. We seek to understand the physical mechanisms by which these nanoscale machines convert chemical energy into mechanical work. I will overview our ongoing use of single molecule tracking and manipulation techniques to observe and perturb substeps in the mechanochemical cycles of individual motors, before concentrating on our recent efforts to dissect the structural basis of a ``reverse gear'' in myosin VI. The basic actomyosin motor has been embellished, altered, and re-used many times through the evolution of diverse members of the myosin superfamily. Class VI myosins are highly specialized (-) end directed motors involved in a growing list of functions in animal cells, including endocytosis, cell migration, and maintenance of stereociliar membrane tension. How does myosin VI achieve reverse directionality, despite sharing extensive sequence and structural conservation with (+) end directed myosins? We generated a series of truncated myosin VI constructs and characterized the size and direction of the power stroke for each construct using dual-labeled gliding filament assays and optical trapping. Motors truncated near the end of the converter domain generate (+) end directed motion, whereas longer constructs move toward the (-) end, confirming the importance of a class-specific insert that redirects the lever arm. Our quantitative results suggest a surprising model in which the lever arm rotates $\sim$180$^{\circ}$ during the power stroke. We are currently studying the behavior of engineered myosin VI constructs with artificial lever arms, in order to further challenge and refine our power stroke model. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V19.00002: Rare returns on lost effort! Dynamic refolding (after unfolding) of protein domains. Invited Speaker: Dynamic force loading is an established technique for probing the forward kinetics in unfolding of single protein domains. Examined over several orders of magnitude in force rate, the unfolding forces often exhibit a linear dependence on the logarithm of loading rate, revealing the dynamic truncation of a precipitous activation barrier. The slope and force-rate intercept of the linear response characterize the critical molecular length gained in the barrier transition and the force-free rate of barrier passage. On the other hand, if reversed and probed with negative force rates, refolding of a stretched polypeptide chain has been found to yield a linear relation between the squares of the refolding forces and the logarithms of (reverse) force rates. Revealing here the dynamic elevation of a deep harmonic well that confines the unfolded states, the slope and force-rate intercept of the linear response characterize the effective spring constant of the harmonic well and the unstretched refolding rate. Representing a dynamical corollary to predictions of fluctuation theorems for small systems, the most-frequent amount of mechanical work recovered (from the thermal environment) in refolding increases with each decade reduction in the force-unloading rate and approaches the limit set by near-equilibrium transitions over a logarithmic span related to the free energy of transition. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 1:03PM |
V19.00003: Generalization of distance to higher dimensional objects, and its application to protein folding Invited Speaker: After a brief biophysical introduction to motivate the problem, I will show how the notion and calculation of distance between two objects can be generalized to the case where the objects are no longer points, but are one-dimensional. Additional concepts such as nonextensibility, curvature constraints, and noncrossing become central to the notion of distance. I will give some analytical and numerical results for specific examples, and I will discuss applications to biopolymers and protein folding. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:39PM |
V19.00004: Getting into shape: the physics of bacterial morphology Invited Speaker: Bacterial cells come in a wide variety of shapes and sizes, with the peptidoglycan cell wall as the primary stress-bearing structure that dictates cell shape. In recent years, cell shape has been shown to play a critical role in regulating many important biological functions including attachment, dispersal, motility, polar differentiation, predation, and cellular differentiation. Though many molecular details of the composition and assembly of the cell wall components are known, how the peptidoglycan network organizes to give the cell shape during normal growth, and how it reorganizes in response to damage or environmental forces have been relatively unexplored. We introduce a quantitative mechanical model of the bacterial cell wall that predicts the response of cell shape to peptidoglycan damage in the rod-shaped Gram-negative bacterium {\it Escherichia coli}. To test these predictions, we use time-lapse imaging experiments to show that damage often manifests as a bulge on the sidewall, coupled to large-scale bending of the cylindrical cell wall around the bulge. The direction of bending confirms the hypothesis of a longitudinal orientation of peptides and a circumferential orientation of glycan strands in the peptidogylcan layer. Our simulations based on our physical model also suggest a surprising robustness of cell shape to damage, allowing cells to grow and maintain their shape even under conditions that limit crosslinking. Finally, we show that many common bacterial cell shapes can be realized within the model via simple spatial patterning of peptidoglycan defects, suggesting that subtle patterning changes could underlie the great diversity of shapes observed in the bacterial kingdom. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 2:15PM |
V19.00005: Sensing and Selection in Bacteria Invited Speaker: The ability to sense changes in the environment allows bacteria to respond by altering their phenotype, or behavior, to adapt to new conditions. Alternatively, bacteria have the ability to spontaneously change their phenotype, without sensing. Such behavior is known as stochastic switching. By simply observing dividing bacteria, is it possible to tell whether the cells are sensing their environment? This talk presents a theory that can decouple the action of sensing from the action of natural selection using single-cell observation of bacteria. [Preview Abstract] |
Session V20: Phase Transitions: Structural, Electronic, and Magnetic
Sponsoring Units: DCMPChair: Ted Einstein, University of Maryland
Room: Morial Convention Center 212
Thursday, March 13, 2008 11:15AM - 11:27AM |
V20.00001: Inequivalent Down Atom (3x3) structure in Sn/Ge(111). R. Cortes, A. Tejeda, J. Lobo-Checa, C. Didiot, B. Kierren, D. Malterre, E.G. Michel, A. Mascaraque The (3x3) phase of Sn/Ge(111) is formed by three Sn atoms in the unit cell, one of them at a higher level than the other two (1-up, 2-down model). Although this model is mostly accepted, it is still controversial because of apparently contradicting experimental results related to the Sn 4d core level line shape. This work reports on high-resolution photoemission spectroscopy (HR-PES) and scanning tunnelling microscopy (STM) experiments on Sn/Ge(111)-(3x3). Our PES data resolve three main components in the Sn 4d core level [1] instead of two found before [2], which are assigned to the three Sn atoms in the unit cell. This indicates that the two down atoms are at slightly different heights, forming an inequivalent-down-atoms (IDA)-(3x3) structure, also confirmed by STM images. These results conclusively solve the long-standing controversy [2] on the interpretation of the Sn 4d core level line shape, and support a model fully consistent with an initial state picture. [1] A. Tejeda et al. Phys. Rev. Lett. In press [2] R.I.G. Uhrberg et al., Phys. Rev. Lett. 85, 1036 (2000) and references there in. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V20.00002: Formation of $\surd $3$\times \surd $3 structure by depositing Si on Si(111)-(5$\times $2)/Au F.-K. Men, A.-L. Chin, C.-F. Jan, J.-L. Guo By depositing Au on a Si(111) surface at an elevated temperature, 5$\times $2, $\surd $3$\times \surd $3, and 6$\times $6 reconstructions emerge successively as the Au coverage increases. Though great efforts have been made to identify atomic models for each reconstruction, satisfactory result is still lacking. By depositing Si on a 5$\times $2 surface, we have identified the formation of the $\surd $3$\times \surd $3 structure even there was no additional Au being deposited. This observation leads us to speculate (i) the $\surd $3$\times \surd $3 structure has a higher Si density than that of the 5$\times $2 structure, and (ii) the Au density in a single-domain $\surd $3$\times \surd $3 structure, i.e., no domain walls, is roughly equal to that in the 5$\times $2 structure. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V20.00003: Temperature-induced Self-pinning and Nano-layering of AuSi Eutectic Droplets Nicola Ferralis, Roya Maboudian, Carlo Carraro A process for self-pinning of AuSi eutectic alloy droplets to a Si substrate, induced by a controlled temperature annealing in ultra-high vacuum, is presented. Surface pinning of AuSi 3D droplets to the Si substrate is found to be a consequence of the readjustment in the chemical composition of the droplets upon annealing, as required to maintain thermodynamic equilibrium at the solid-liquid interface. Structural and morphological changes leading to the pinning of the droplets to the substrate are analyzed using atomic force microscopy, scanning and transmission electron microscopy. Raman spectroscopy measurements performed on the droplets reveal phase separation upon cooling of the droplets, leading to the formation of amorphous Si-rich channels within the core, and the formation of crystalline Si nanoshells on the outside. The mechanism leading to the pinning and surface layering provide new insight into the role of alloying during growth of silicon nanowires and may be relevant to the engineering of nano-scale Si cavities. We shall also present measurements of the diffusion of Au drops on Si(111) obtained by low-energy electron microscopy. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V20.00004: Adsorbate-Induced Faceting of Ir and Re Surfaces Payam Kaghazchi, Timo Jacob, Matthias Scheffler, Wenhua Chen, Hao Wang, Theodore Madey Since high-index clean metal surfaces typically have lower surface atom densities and higher surface free energies compared to the close-packed surfaces of the same metal they can be used as the basis for surface reconstruction and facet formation studies. In this context experimentally we found recently that on Ir(210) and Re(11$\bar2$1) surfaces strongly interacting adsorbates are able to induce the formation of well defined nanostructures after annealing the system at elevated temperatures. Using density functional theory calculations with the PBE functional and {\it ab initio} atomistic thermodynamics we studied the adsorption of oxygen and nitrogen on the different surface orientations, which are involved in the nanostructures on Ir(210) and Re(11$\bar 2$1). Constructing the corresponding ($p$, $T$)-surface phase diagrams, we find that at experimental pressure conditions ($p_{\mathrm{O}_2}=5\cdot 10^{-10}$\,atm) above 1100\,K for Ir and above 1200\,K for Re the planar surfaces are stable, while lowering the temperature stabilizes the nanofacets found experimentally. While on Ir(210) most nanoscale pyramids consist of smooth and unreconstructed planes, some (110) faces show a stepped double-missing row superstructure, which is only stable on the faceted surface and at higher temperatures. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V20.00005: Oxygen-induced nanoscale pyramidal faceting of Rh(210) surface Govind ., W. Chen, H. Wang, Theodore E. Madey The adsorption of oxygen and nanometer-scale faceting induced by oxygen have been studied on atomically-rough fcc Rh(210) using LEED and AES. The Rh(210) surface remains planar at room temperature after being exposed to oxygen. Upon annealing at temperatures above 600K, Rh(210) covered by $\sim $1ML of oxygen undergoes reconstruction to form 3-sided nanoscale pyramids characterized by two {\{}731{\}} facets and a (2x1)-reconstructed (110) facet. The surface remains faceted for T up to 850K. Oxygen can be completely removed from the faceted surface via CO oxidation at 400K or reaction with H$_{2}$ at room temperature, while preserving the faceted structure. The clean faceted surface remains stable below 600K and irreversibly relaxes to the planar surface at higher temperatures. The clean faceted Rh(210) surface is a potential substrate to study surface reactions whose rates are sensitive to atomic structure and/or nanoscale (facet) size. The results are compared with measurements of oxygen-induced faceting of Ir(210). Work supported by DOE, Office of basic Energy Science [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V20.00006: Order-disorder phase transition of the Cu(001) surface under equilibrium oxygen pressure H. Iddir, D.D. Fong, P. Zapol, P.H. Fuoss, L.A. Curtiss, G.-W. Zhou, J.A. Eastman Understanding atomic processes involved in catalyzed reactions is of great importance and can be achieved by studies of adsorbate-induced surface structures. Copper catalysts are heavily used in methanol and formaldehyde synthesis, two reactions in which oxygen adsorption is an important intermediate step. To better understand catalytic reactions, it is imperative to both identify and characterize the atomic structure of all phases present on metal surfaces at elevated temperatures and pressures relevant to working catalysts. We will report on our discovery of a new high-temperature oxygen-induced surface phase on Cu (001) using a combination of in situ synchrotron x-ray scattering and first-principles theory. This high-temperature phase is characterized by $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ ML of randomly-distributed vacancies in the topmost Cu layer with a c(2x2)-O adlayer. Below 473 K a reversible transition to a $\left( {2\sqrt 2 \times \sqrt 2 } \right)R45^{\circ}$ missing row phase occurs. The results show that this entropy-driven phase transition occurs through the diffusion of Cu vacancies underneath the oxygen superstructure. This work is supported by DOE BES under Contract \# DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V20.00007: Ab initio phase diagram of oxygen adsorption on W(110) Markus St\"{o}hr, Stefan M\"{u}ller, Raimund Podloucky Oxygen adsorption on the tungsten (110) surface has been studied experimentally as well as by semi-empirical theoretical approaches. Up to now, no {\em ab initio} modelling of this adsorption process has been done, for which we present a combined density functional theory (DFT) and cluster expansion (CE) study. For the CE all lateral unit cells with up to 12 atoms were considered. The (2x1) and (2x2) adsorption phases are found to be ground states which is confirmed experimentally [1]. On the basis of effective cluster interactions Monte Carlo (MC) simulations were performed in order to access finite temperature effects. Concerning the atomic structure we find excellent agreement to experimental scanning tunneling microscopy studies [2]. The temperature and coverage dependent short range order parameter is analyzed. From the results of the applied DFT, CE and MC approaches an {\em ab inito} surface phase phase diagram can be derived. [1] Wu et al., Physical Review B {\bf 39}, 7595 (1989). [2] Johnson et al., Phys. Rev. Lett., {\bf 71}, 1055 (1993). [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V20.00008: Lattice gas transition of xenon on a fullerite surface Silvina Gatica, Milton Cole We study a lattice-gas transition of xenon atoms on a honeycomb geometry. It is found from experiments and Monte Carlo simulations that this configuration occurs for xenon adsorbed on a substrate consisting on an array of C60 molecules on Ag(111). At very low coverage the atoms occupy strong-binding 3-fold hollow sites between C60 molecules. In this way, they form a commensurate lattice with nearest neighbor distance 0.58 nm. Using a Lennard Jones model for the Xe-Xe potential, the nearest neighbor interaction strength is U=96.7 K. Using the Ising model we estimate the transition critical temperature to be Tc = 36K. We compare with our results from Monte Carlo simulations based on more realistic interactions. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V20.00009: Stability of the commensurate monolayer solid of xenon/graphite L.W. Bruch, A.D. Novaco A stability analysis based on model calculations of the grand potential finds that the transition from hexagonal incommensurate to commensurate monolayer solid of xenon/graphite is continuous with increasing pressure, in agreement with experimental observations. An atomic-scale interaction model gives an internally consistent account of the thermal expansion of the solid at the 2D sublimation curve and of the chemical potential increase for isothermal compression from monolayer condensation to the commensurate solid. An estimate is given for the corrugation energy of xenon/graphite. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V20.00010: Molecular dynamics simulations of hexane on graphite at various coverages: the difference explicit hydrogens make M.W. Roth, M.J. Connolly, Paul A. Gray, Carlos Wexler Molecular Dynamics simulations of hexane (C$_{6}$H$_{14})$ adlayers on graphite are carried out for coverages of 0.5 $\le $ \textit{$\rho $ }$\le $ 1 monolayers. The hexanes have explicit hydrogens and the graphite is modeled as an all -- atom, six -- layer structure. Above \textit{$\rho $ }$\cong $ 0.9 the herringbone solid loses orientational order at $T_{1}$ = 140 K $\pm $ 3 K. At \textit{$\rho $} = 0.878 the system presents vacancy patches and $T_{1}$ decreases to ca. 100 K. As coverage decreases further, the vacancy patches become larger and by \textit{$\rho $} = 0.614 the solid is a connected network of randomly oriented domains. All cases show a weak nematic mespohase. The melting temperature is $T_{2}$ = 160 K $\pm $ 3 K and falls to ca. 145 K by \textit{$\rho $} = 0.614. The dynamics and energetics observed demonstrate that the explicit-hydrogen model of hexane is substantially more realistic than the UA approximation. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V20.00011: Molecular Dynamics study of tetracosane monolayers adsorbed on graphite L. Firlej, B. Kuchta, M.W. Roth, Paul A. Gray, Carlos Wexler We present the results of Molecular Dynamics (MD) simulations of tetracosane (C$_{24}$H$_{50})$ monolayers physisorbed on graphite. C$_{24}$H$_{50}$ molecules have explicit hydrogens and the graphite is represented by six graphene layers. We focus our analysis on the microscopic mechanism of melting, experimentally observed at $T$ = 340 K. We are looking for the pre-melting transformations with emphasis on the correlation between translational disordering of molecules and their internal degrees of freedom. We analyze several order parameters and their fluctuations along the MD trajectories. We show that the all atom representation of C$_{24}$H$_{50}$ is much more sensitive to the model of intramolecular interactions than united atom model. Footprint reduction during melting involves a simultaneous loss of intramolecular and translational order. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V20.00012: Test of the universality of the scaling energy in alkanes using melting transition of layer adsorbed on graphite B. Kuchta, L. Firlej, M.W. Roth, Paul A. Gray, Carlos Wexler The non-bonded terms together with the torsional energy determine the internal (conformational) degrees of freedom of simulated alkanes. Being able to predict the energy and ordering of conformations correctly is an essential quantity for bringing force field methods into the predictive regime of theoretical methods. To estimate the universality of modern force fields it is helpful to know how different components of classical fields affect the simulated properties. In the case of alkanes, the interplay between torsion potential and the scaling of 1-4 van der Waals and 1-4 charge-charge (QQ) interactions plays a crucial role. It is the purpose of this work to study universality of the scaling factor of 1-4 non-bonded interactions in alkanes. Three alkanes of length from 7 to 30{\AA} (C$_{6}$, C$_{12}$, C$_{24})$, in an all-atom representation have been analyzed. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V20.00013: Heisenberg Spin Systems with Long Ranged Couplings Ute L\"ow Static correlation functions of Heisenberg spin systems with various (non-frustrated) topologies are studied by means of Quantum-Monte Carlo techniques. In particular dimerizations and longer ranged variations of spin couplings and alternations between ferromagnetic and anti-ferromagnetic couplings are considered. The phenomenology of possible ground state phase diagrams is discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V20.00014: Hysteresis in the anomalous Hall effect of MnAs thin films Felix T. Jaeckel, Andreas Stintz, Abdel-Rahman A. El-Emawy, Kevin J. Malloy We report detailed measurements of the Hall effect in MBE-grown MnAs thin films on $(001)$-GaAs as a function of temperature. Hysteresis of the Hall resistivity is observed for temperatures between 300 and 355~K. Non-linear behavior of the Hall resistivity persists up to 390~K. The appearance of hysteresis at low temperatures can be explained by the emergence of stable, out-of-plane domains due to the shape anisotropy of the contracting $\alpha$-phase. However, the persistence of the hysteresis and the anomalous Hall effect at temperatures significantly above 318~K is not consistent with the complete transformation of the $\alpha$-phase and introduces new questions about the magnetic properties of the $\beta$-phase. [Preview Abstract] |
Session V21: Charged and Ion-Containing Polymers I
Sponsoring Units: DPOLYChair: Jodie Lutkenhaus, Yale University
Room: Morial Convention Center 213
Thursday, March 13, 2008 11:15AM - 11:27AM |
V21.00001: Nano-Patterns in Gels of Charged Chains with Self-Attracting Interactions Monica Olvera de la Cruz, Juan J. de Pablo Gels of weakly charged chains have large swelling capabilities that depend on pH and/or salt concentration. In the presence of attractions among elastic units, such as poor-solvent conditions, stable nano-patterns are observed. These systems are ideal actuators, since they undergo large volume changes at the nanophase transition. We find that the nanophases are due to the entropy of the counterions, and are stabilized as the monomer density decreases and as the salt concentration increases by hard-core interactions and network heterogeneities. Our model is constructed with results from a non-linear Poisson Boltzmann approach in the limits when it is applicable (low ionic densities) and a grand canonical Monte Carlo method simulation with Donnan partitioning of counterions and co-ions between the gel and reservoir phases. Our theory and simulations are in close agreement with experiments. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V21.00002: Interactions in Ion-containing Polymers Probed by ab initio Methods Wenjuan Liu, Ralph Colby, Michael Janik We use ab initio methods to estimate dipole moments and interaction energetics in ion-containing polymers. Our calculation quantitatively includes electrostatic interactions (using both permanent and induced dipoles) and effectively estimates solvation energetics for ions interacting with various functional groups. Interactions are reported for various small cations with common functional groups on polymers and carboxylate, sulfonate and phosphonate groups that can be present in anionic ionomers. We demonstrate how these interaction energies can be utilized to design polymer membranes with facile ion transport. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V21.00003: Conformation transition and counterion distribution of single polyelectrolyte chains in aqueous solution Jiang Zhao, Shengqin Wang We adopted single fluorescence techniques to study the conformation transition of poly 2-vinylpyridine in its aqueous solution. The first-order conformation transition from extended coil to collapsed globule was observed as the pH value in the solution was raised. The critical pH value was shifted higher largely upon the addition of salt in the solution. The study shows a difference of proton concentration at the chain to that in the solution (a difference of three orders of magnitude), and the addition of salt in the solution increased the local proton concentration at the chain and therefore shift the transition point. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V21.00004: Unifying Self-Consistent Field Theory for Weak Polyelectrolytes Kevin Witte, You-Yeon Won A self-consistent field (SCF) theory for weak polyelectrolytes has been derived from a grand canonical partition function. The formalism accounts for the location and mixing of the charged and uncharged polymer species, treating the local (spatially dependent) charge fraction as a field variable with which to minimize the total free energy. This method of the derivation gives the resulting equations, especially those governing the local charge fraction, that are identical to the results obtained by Szleifer and coworkers (J. Polym. Sci. B Polym. Phys., 2006) who built upon the mean-field ``annealed'' free energy expression proposed by Raphael and Joanny (Europhys. Lett., 1990). However, we show that these results are further identical to the ``two-state'' model of Borukhov, Andelman and Orland (Eur. Phys. J. B, 1998), namely, the potential field due to the polymer charges with which the chains interact and the local charge fraction are shown to be exactly equal. This annealed model is derived by averaging the partition function with regard to the monomer charges. The charged and uncharged states are weighted by their probabilities which is, in our notation, the bulk charge fraction and one minus the bulk charge fraction, respectively. The utility of this theory is demonstrated by comparing its predictions against various experimental results from bulk potentiometric measurements and also from polyelectrolyte brush compression studies. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V21.00005: Composition and Structure Changes of the Ionic Aggregates with Acid Content and Neutralization Level in Poly (styrene-\textit{co}-methacrylic acid) Ionomers Wenqin Wang, Tsung-Ta Chan, Andrew Perkowski, Shulamith Schlick, Karen I. Winey The morphology of poly(styrene-\textit{co}-methacrylic acid) (SMAA) copolymers neutralized with copper(II), and the corresponding local structures and compositions of ionic aggregates were investigated as a function of acid content and level of neutralization. Using X-ray scattering and scanning transmission electron microscopy, the sizes of ionic aggregates in Cu(II)-neutralized SMAA were found to be independent of acid content and neutralization level. The number density of ionic aggregates increased with acid content and neutralization level, but the increase is significantly less than expected for a fixed ionic aggregate composition. Electron spin resonance spectroscopy indicates \textbf{three} types of cation sites with corresponding relative population changing with acid content, which further indicates a compositional variation of ionic aggregates with neutralization. The correlation between morphology and compositional evolution of the ionic aggregates will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V21.00006: Multiple Nanoscale Morphologies of Poly(Ethylene-\textit{co}-Acrylic Acid) Ionomers Christopher D. Chan, Travis W. Baughman, Kathleen L. Opper, Kenneth B. Wagener, Karen I. Winey We have synthesized linear poly(ethylene-co-acrylic acid) (EAA) copolymers with precisely placed acid groups using ADMET (acyclic diene metathesis). In the acid form, the EAA copolymers with precisely placed acid groups exhibit the typical orthorhombic PE crystal structure along with a new layered structure. The layered structures coexist with the PE crystals and have spacings consistent with the separation between acid groups; at 9.5mol{\%} acid the layer-to-layer spacing is 2.53 nm. When these linear EAA copolymers are neutralized with zinc acetate in solution, high angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the Zn-rich ionic aggregates decrease in size as the level of neutralization increases from 25{\%} to 100{\%}. X-ray scattering indicates that the cations decorate the acid-acid layered structure at low neutralization level, but eventually disrupt the layers at higher neutralization levels. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V21.00007: Conduction, Ion Association and Dynamics in Polyethylene Oxide-based Polyester Ionomers Daniel Fragiadakis, Shichen Dou, Ralph Colby, James Runt A series of single-ion conducting PEO-based polyester copolymers is synthesized, containing different amounts of ionic sulfonate groups covalently attached to the polymer chains. The dynamics of the polymer chains and the mobile lithium cations is investigated using dielectric relaxation spectroscopy. A physical model of electrode polarization is employed to decompose dc conductivity into the contributions of mobile ion concentration and ion mobility, and the physical meaning of these parameters and relation to literature results on similar systems is discussed. The segmental and local dynamics of the polymer chains is studied. We investigate both the modification of the dynamics due to the presence of the ionic groups, as well as the relation of chain motion to ion transport. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V21.00008: Correlation between structure and conductivity in stretched Nafion Elshad Allahyarov, Philip Taylor We have used coarse-grained simulation methods to investigate the effect of stretching-induced structure orientation on the proton conductivity of Nafion-like polyelectrolyte membranes. Recent experimental data on the morphology of ionomers describe Nafion as an aggregation of polymeric backbone chains forming elongated objects embedded in a continuous ionic medium. Uniaxial stretching of a recast Nafion film causes a preferential orientation of these objects in the direction of stretching. Our simulations of humid Nafion show that this has a strong effect on the proton conductivity, which is enhanced along the stretching direction, while the conductivity perpendicular to the stretched polymer backbone is strongly reduced. Stretching also causes the perfluorinated side chains to orient perpendicular to the stretching axis. The sulphonate multiplets shrink in diameter as the stretching is increased and show a spatially periodic ordering in their distribution. This in turn affects the distribution of contained water at low water contents. The water forms a continuous network with narrow bridges between small water clusters absorbed in head-group multiplets. We find the morphological changes in the stretched Nafion to be retained upon removal of the uniaxial stress. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V21.00009: Morphology study in block copolymer electrolytes Scott Mullin, Nisita Wanakule, Nitash Balsara Poly(styrene-b-ethylene oxide)/lithium bis(trifluoromethane)sulfonimide (SEO/LiTFSI) is of interest in battery applications since the doped PEO phase can conduct ions and the glassy PS phase can prevent dendrite growth upon recharging. It is believed that the LiTFSI molecules are localized in the PEO microphases. Previous studies have shown that highly conducting electrolytes can be made from symmetric SEO copolymers. The purpose of this study is to explore the conductivity of asymmetric SEO copolymer systems doped with LiTFSI. Our studies encompass both neat asymmetric SEO copolymers and SEO copolymers blended with PS homopolymers to separate the effects of architecture of the copolymer molecules and morphology adopted by the system in the melt state. Conductivity is measured by AC impedance, morphology is determined by small angle X-ray scattering, and crystallinity of the PEO chains is determined by differential scanning calorimetry. All samples were prepared in hermetically sealed sample cells in an Argon glovebox. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V21.00010: Increased Water Retention in Polymer Electrolyte Membranes Assisted by Capillary Condensation Moon Jeong Park, Nitash P. Balsara We establish a new systematic methodology for controlling the water retention of polymer electrolyte membranes. We show that block copolymer membranes with well-defined hydrophilic channels in the 2 to 5 nm range remain moist in a relatively dry environment (relative humidity = 50 \%) up to temperatures as high as 90 $^{o}$C. This retention of water leads to an increase in the overall conductivity with increasing temperature. Simple calculations suggest that capillary condensation is important at these length scales. The morphology of the hydrated membranes is determined by a combination of in-situ neutron scattering and cryogenic electron microscopy. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V21.00011: Engineering polyelectrolyte multilayer structure at the nanometer length scale by tuning polymer solution conformation. Soheil Boddohi, Christopher Killingsworth, Matt Kipper Chitosan (a weak polycation) and heparin (a strong polyanion) are used to make polyelectrolyte multilayers (PEM). PEM thickness and composition are determined as a function of solution pH (4.6 to 5.8) and ionic strength (0.1 to 0.5 M). Over this range, increasing pH increases the PEM thickness; however, the sensitivity to changes in pH is a strong function of ionic strength. The PEM thickness data are correlated to the polymer conformation in solution. Polyelectrolyte conformation in solution is characterized by gel permeation chromatography (GPC). The highest sensitivity of PEM structure to pH is obtained at intermediate ionic strength. Different interactions govern the conformation and adsorption phenomena at low and high ionic strength, leading to reduced sensitivity to solution pH at extreme ionic strengths. The correspondence between PEM thickness and polymer solution conformation offers opportunities to tune polymer thin film structure at the nanometer length scale by controlling simple, reproducible processing conditions. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V21.00012: Layer Thickness and Charge Compensation of Polyelectrolyte Multilayers Qiang Wang Using a continuum self-consistent field theory, we have modeled the sequential process of layer-by-layer assembly of flexible polyelectrolytes on flat surfaces as a series of kinetically trapped states. Up to 60 depositions of oppositely charged polyelectrolytes (A and B) are performed, each followed by a washing step. Here we focus on the effects of polymer charge fractions, bulk salt concentrations, solvent qualities for A and B, and their incompatibility on the layer thickness and charge compensation of the polyelectrolyte multilayer. We also compare our modeling with available experimental measurements. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V21.00013: pH-Induced Release of Polyanions from Multilayer Films Svetlana Sukhishvili, Eugenia Kharlampieva, John Ankner, Michael Rubinstein Many studies deal with polymer chains irreversibly bound within electrostatically assembled layer-by-layer (LbL) films. Here we present new insights on the desorption of polymer chains assembled within layered polymer films, triggered by pH variation. Specifically, we report that in the case of a polycation assembled with poly(methacrylic acid) (PMAA) at low pH, the films selectively release the polyacid in response to an increase of external pH. By applying \textit{in} \textit{situ} Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), we find that pH-induced film response is controlled by charge imbalance within the film. Studies of the kinetics of PMAA release reveal that the characteristic time of PMAA chain release, $\tau $, scales with molar mass, $M_{w}$, as $M_{w}^{1.1\pm 0.1}$. We present a theoretical model of the ``sticky gel electrophoresis'' of polyacids with excess charge which predicts a release time proportional to the molecular weight of released polyions and to the film thickness, in agreement with experimental results. Finally, neutron reflectivity studies show explicitly that PMAA release results in disordering of the film structure. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V21.00014: Properties of Polyelectrolytes in an Ionic Liquid John Harner, David Hoagland In solvents such as water, polyelectrolyte properties depend strongly on ionic strength, reflecting the ability of free ions to screen electrostatic interactions. At high ionic strength, polyelectrolytes remaining soluble behave similarly to neutral polymers. What happens to polyelectrolyte properties in an ionic liquid? A series of polyelectrolytes were dissolved in [EMIM][EtSO4] (ethyl-methyl-imidazolium ethylsulfate) and studied by viscometry as well as static and dynamic light scattering. Both scattering approaches show that sodium polystyrene sulfonate is more swollen in aqueous 0.1M NaBr than in [EMIM][EtSO4]. Furthermore, classical polyelectrolyte effects (fast and slow mode, increased reduced viscosity with dilution) are absent in the ionic liquid. Lastly, variably quaternized polyvinylpyridine exhibits no evidence of coil expansion as charge density increases. We conclude macroion charges are fully screened in ionic liquids. [Preview Abstract] |
Session V22: Focus Session: Organic Electronics: Molecular Junctions
Sponsoring Units: DPOLY DMPChair: James Kushmerick, National Institute of Standards and Technology
Room: Morial Convention Center 214
Thursday, March 13, 2008 11:15AM - 11:51AM |
V22.00001: Molecular Thermoelectrics Invited Speaker: Thermoelectric materials for energy generation have several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturizability, and CO2 free conversion of heat to electricity. Historically, these materials suffered from low efficiency and have involved very expensive inorganic compounds. We demonstrate that some metal-molecule-metal junctions produce voltage when exposed to heat. This exciting initial demonstration of thermopower from molecular junctions is reinforced by the fact that relatively minor changes to chemical structure result in an unprecedented simultaneous increases in both thermopower and electrical conductance, suggesting that highly efficient devices may be \textit{designed} in a manner impossible in traditional thermoelectric materials. Furthermore, thermopower measurements offer an alternative transport measurement that can characterize the dominant transport orbital and is independent of the number of molecules in the junction. In particular, the effect of substituents and endgroups on the electronic behavior of gold-1,4-benzenedithiol -gold junctions (BDT) is explored. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V22.00002: Chemical Structure and Molecular Switches. Amy Blum, David Long, Martin Moore, James Kushmerick, James Tour, Banahalli Ratna The future of molecular electronics depends on designing molecules to exhibit specific nonlinear properties such as rectification or bistable switching. In pursuit of this goal, two distinct types of switching were observed in matrix isolated and complete monolayers of bipyridyl-dinitro-oligophenylene-ethynylene (BPDN). Several groups have observed conductance state switching in this molecule. However, the mechanism of switching between the two conductance states is still not understood. Using BPDN as a starting point, chemically related structures such as bipyridyl- oligophenylene-ethynylene, dinitro-oligophenylene-ethynylene, and biphenyl- oligophenylene-ethynylene were measured in matrix isolated monolayers. By means of such comparisons to related molecules, we determine the key functional groups leading to switching in BPDN. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V22.00003: Inelastic electron spectroscopy of single alkanedithiol molecules Nicolas Agrait, Carlos R. Arroyo We measure charge charge transport through single molecules between gold electrodes at low temperature using STM. The derivative of the conductance vs voltage (IES spectrum) shows clear peaks corresponding to the vibrational modes of the molecule. The frequency dependence on stretching and comparison with other vibrational spectroscopy studies makes possible to assign these peaks to the longitudinal modes of the molecule. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V22.00004: A Molecular Switch Made of Charge Transfer Complexes on Au (111) U.G.E. Perera, F. J\"ackel, V. Iancu, K.-F. Braun, N. Koch, J.P. Rabe, S.-W. Hla A low temperature scanning tunneling microscope (STM) and spectroscopy study of organic charge transfer complexes is presented. The complexes are formed by self assembly of the electron donor $\alpha $- sexithiophene (6T) and the electron acceptor tetrafluro-tetracyano-quinodimethane (F$_{4}$TCNQ) on Au (111) surface. The formation of new hybrid molecular orbitals in CTCs is evident by STM imaging at different bias voltages as well as by differential conductance spectra. The charge transfer lead to the shift of HOMO and LUMO orbitals of the molecules in complexes with respect to the pure molecular orbitals. Finally, we use a voltage dependent resonance-tunneling scheme to demonstrate a molecular switch made of F4TCNQ/6T charge transfer complexes. This work is financially supported by the US-DOE grant DE-FG02-02ER46012. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V22.00005: Correlating Structure and Conductivity of Pentathiopene Monolayers Bas Hendriksen, Yabing Qi, Florent Martin, Frank Ogletree, Miquel Salmeron Understanding the electrical conduction mechanisms in organic materials is important for the development of plastic and molecular electronics. The charge transport properties of conducting molecular layers are expected to strongly depend on the order of the layer and the conformation of the molecules. We used atomic force microscopy (AFM) to study and correlate the structural, mechanical and electrical properties to molecular monolayers of pentathiophene based molecules on solid substrates prepared by the Langmuir-Blodgett technique. The molecular monolayers consist of two phases: one phase of compact micrometer size flower-shaped islands and a phase with less order and a high density of holes. We found that the perpendicular conductivity, i.e. through the monolayer sandwiched between the conductive AFM probe and the conductive substrate, is more than 5 times higher on the well-ordered island phase. This shows that the molecular lattice order has a significant effect on the electronic properties. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V22.00006: Direct measurement of photomechanical switching cross-sections of single-molecules on a surface Jongweon Cho, Matthew J. Comstock, Niv Levy, Luis Berbil-Bautista, Frank Lauterwasser, Jean M. J. Frechet, Michael F. Crommie The photomechanical switching of photoactive molecules in solution strongly depends on the wavelength of light. This dependence is crucial to reliably control the photomechanical state of target molecules. Recently, reversible photomechanical switching of individual azobenzene molecular derivatives on the Au(111) surface has been reported for one particular wavelength of UV illumination [1]. To further understand this process and its possible applications in future nanotechnologies, we have investigated photomechanical switching rates and saturation behavior for azobenzene molecular derivatives at a surface under optical stimulation at different wavelengths. Using single-molecule-resolved scanning tunneling microscopy, we have determined both the forward and reverse photomechanical molecular switching cross-sections at different wavelengths. In a dramatic departure from solution-based environments, visible light does not efficiently reverse the photoreaction. [1] Matthew J. Comstock, Niv Levy, Armen Kirakosian, Jongweon Cho, Frank Lauterwasser, Jessica H. Harvey, David A. Strubbe, Jean M. J. Fr\'echet, Dirk Trauner, Steven G. Louie, and Michael F. Crommie, Phys. Rev. Lett. 99, 038301 (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V22.00007: Many-body treatment of quantum transport through single molecules Justin Bergfield, Charles Stafford We investigate multi-terminal quantum transport through single molecules including intramolecular correlations exactly by using the nonequilibrium Green function approach, but treating the lead-molecule coupling perturbatively via a Dyson expansion \footnote{Cardamone D. et al. Nano Lett. Vol 6 2422, 2006}, with an extended Pariser--Parr--Pople molecular model. As a validation of the theory we calculate the linear and non-linear transport properties of 1,4-Benzenedithiol(BDT) and compare these results to experiment\footnote{Xiao X. et al. Nano Lett. Vol 4 267, 2004}. We find many transport features which are not accessible via meanfield approaches such as Coulomb Blockade steps and an incipient Hubbard-Mott insulator gap. We also calculated the thermopower exactly and find, in accordance with recent experimental\footnote{Reddy P. et al. Science. Vol 315 1568, 2007} and theoretical reports, that the transport in this junction is dominated by holes (p-type). This result allowed us to then extract the remaining free parameter, the lead-molecule coupling $\Gamma$. The resulting nonlinear I-V curve was found to be in good quantitative agreement with experiment. Finally, we calculated the differential conductance as a function of gating and bias potential to construct a full molecular `Coulomb diamond'. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V22.00008: Single molecule characterization with well-defined contacts Alex Neuhausen, Frank Jaeckel, Jeremy Hiatt, Joseph Sulpizio, David Goldhaber-Gordon, Chris Chidsey, W. E. Moerner, Zhenan Bao We demonstrate a novel method to reliably achieve ohmic contact to single molecules in a geometry that allows for simultaneous transport measurements and Raman spectroscopy. We achieve this by lithographically defining gold contacts to a structure composed of a single molecule bridging a pair of gold nanoparticles. The transport measurements indicate negligible resistance from the contacts as compared to the single molecule behavior, and the Raman spectroscopy benefits from strong field enhancement between the two nanoparticles. We prove the presence of single molecules with both stoichiometric and spectroscopic analyses. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V22.00009: Transport Fluctuations in Metal-Molecule Junctions Jonathan Malen, Kanhayalal Baheti, Peter Doak, Rachel Segalman, Arun Majumdar Thermopower of metal-molecule junctions is an alternative transport characteristic to conductance that can be experimentally measured. A scanning tunneling microscope break junction was used to measure the thermopower of such molecular junctions. Temperature bias applied between gold contacts across the bridging molecules generates a thermoelectric voltage. Hitherto, the statistical analysis of the data from both thermopower and conductance measurements has focused on the histogram peaks rather than the spread of the data. We find that the full width half maximums (FWHM) of the voltage histograms are finite at zero temperature bias and increase in proportion to the temperature bias. Johnson Noise is the most likely cause of the zero bias FWHM, and its magnitude is thereby related to the junction conductance. For 1,4,Benzenedithiol (BDT) the junction conductance associated with the zero bias FWHM is 0.02G$_{0}$, in close agreement with prior conductance measurements of BDT. The dependence of FWHM on temperature bias may provide further insight to the origin of stochastic fluctuations in metal molecule junctions. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V22.00010: Electronic transport through single-molecule- and monolayer-based molecular junctions Luis Agapito, Hai-Ping Cheng We report our development for calculating tunneling electronic transport through molecular junctions, which are composed of two contact leads and the active device in between. The surface Green's function of the contact leads is obtained following a non-iterative, exact procedure using ab initio data computed with the same level of theory and localized basis set than those used for the active device$^{\ast }$. In a 1-dimensional in-wire setting, we describe the electrical switching performance of a single oligo-phenylene-ethynylene molecule connected to graphene-nanoribbons leads. Moreover, in a more realistic 2-dimensional setting, such as the case of self-assembled molecular monolayers, the method is extended to include intermolecular and packing-density effects. * Agapito, L. A.; Cheng, H. P. \textit{Journal of Physical Chemistry C} \textbf{2007}, $111$, 14266. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V22.00011: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V22.00012: Substrate-Dependent Electronic Behavior of Polydiacetylene Nanowires Rajiv Giridharagopal, K. F. Kelly Scanning tunneling microscopy (STM) has been used to study individual polydiacetylene (PDA) nanowires. STM analysis of PDA nanowires on different substrate electrode materials at varying sample bias voltage conditions reveals interesting substrate-dependent effects. PDA nanowires were formed on both graphite and molybdenum disulfide (MoS$_{2})$ substrates. Interestingly, the nanowires on graphite appear with different topographic heights depending on the substrate bias voltage, and the height varies substantially with respect to voltage polarity. A similar effect is observed on MoS$_{2}$ at negative sample bias voltages, except that the nanowires are almost twice as tall on MoS$_{2}$. Even more intriguing is that at positive sample bias voltage conditions, the nanowires on MoS$_{2}$ are invisible in all STM images. A comparison of these voltage-dependent effects points to a strong influence of the substrate electrode material on the electronic behavior of these polymer nanostructures. The results reported here have implications for recently-demonstrated technologies such as monolayer PDA transistors and PDA-based organic solar cell devices as well as potential molecular electronic systems. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V22.00013: Organic memory devices using the negative differential resistance effect R. Osterbacka, J.K. Baral, H.S. Majumdar, F. Jansson, A. Laiho, R.H.A. Ras, J. Ruokolainen, O. Ikkala, H. Jiang, E. Kauppinen Of all the organic memory devices reported so far the ones having the negative differential resistance (NDR) is the best in terms of yield, reproducibility and repeatability. We have observed two different kinds of NDR in nanoparticle based organic memory devices. One is the memory-NDR which follows the observation in SiO2 devices [Simmons et. al. \textit{Proc. R Soc. Lond. Ser. A} \textbf{301}(1967)77]. Here the I-V characteristics trace different paths based on device history. The second type is the tunneling-NDR, where the I-V curves always trace the same path, irrespective of the history. This behavior is similar to the one observed in resonant tunnel diodes and multiple tunneling is the explanation. We will discuss these two phenomena in light of our experimental results in a polystyrene:fullerene nanocomposite system and present a physical model for the same. We have performed a multitude of optical and electrical experiments and clarified the influences of morphology on the complex and interesting device performance observed in this new class of organic electronic devices. [Preview Abstract] |
Session V23: Focus Session: Charge/Orbital Ordering in Complex Oxides
Sponsoring Units: DMP GMAGChair: Jeff Lynn, National Institute of Standards and Technology
Room: Morial Convention Center 215
Thursday, March 13, 2008 11:15AM - 11:27AM |
V23.00001: Evolution of the `Orbital Peierls State' with doping C. Ulrich, G. Khaliullin, B. Keimer, M. Reehuis, K. Schmalzl, A. Ivanov, K. Hradil, J. Fujioka, Y. Tokura Orbital degrees of freedom play an important role in the physics of strongly correlated electron systems. Our extensive investigation of insulating vanadates by neutron scattering has led to the discovery of an unusual magnetic ground state. YVO$_3$ exhibits two magnetic phases, a C-type phase between 116 K and 77 K and a G-type phase below 77 K. While the magnetic properties of the G-type phase are in accordance with standard theories, the C-type phase shows highly unusual static and dynamic spin correlations. Based on the idea of orbital fluctuations we were able to identify this phase as a theoretically predicted 'orbital Peierls state' [1]. Neutron scattering experiments on Y$_{1-x}$Ca$_x$VO$_3$ show that the C-type phase, i.e. the `orbital Peierls phase', is stabilized upon doping, while the orbitally ordered G-type phase is quite unstable and disappears at x = 2 \%. Furthermore, with doping this phase also exhibits a highly unusual spin wave dispersion. These leads us to the conclusions, that the 'orbital Peierls state' becomes more robust with Ca-doping, whereas the formerly well defined G-type phase exhibits a more complex behaviour, probably as a consequence of an increase in orbital fluctuations. [1] C. Ulrich et al., PRL {\bf 91}, 257202 (2003). [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V23.00002: Magnetic interactions and orbital physics in RVO$_{3}$ perovskites J.-Q. Yan, S. Chang, C. Brown, M. Hehlen, F. Trouw, R.J. McQueeney We have performed inelastic neutron scattering study on high quality YVO3 and LaVO3 powders. The magnetic interactions determined from the scattering spectra for YVO3 agree with a previous single crystal study. [1] For LaVO3, a --Jab $>$ Jc is in sharp contrast to the Jc $>$ -Jab in the C-type magnetically (C-AF) ordered state of YVO3. The mechanism that greatly suppresses Jab in C-AF state of YVO3 will be discussed together with thermal conductivity [2] and Raman spectroscopy [3] results. \newline [1] C. Ulrich, et al., Phys. Rev. Lett. \textbf{91}, 257202 (2003). \newline [2] J.-Q. Yan, et al., Phys. Rev. Lett. \textbf{93}, 235901 (2004). \newline [3] S. Miyasaka, et al., Phys. Rev. B \textbf{73}, 224436 (2006). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V23.00003: Pressure-temperature phase diagram for orbital and spin states in $R$VO$_{3}(R$=Y,Tb) Daisuke Bizen, Keisuke Nakatsuka, Tetsuya Murata, Hironori Nakao, Kazuaki Iwasa, Youichi Murakami, Toyotaka Osakabe, Shigeki Miyasaka, Yoshinori Tokura Perovskite-type vanadium oxides $R$VO$_{3}$ ($R$=Y, La-Lu) show various physical properties coupled with the orbital and spin states. Orbitally ordered states of V 3$d^{2}$ in YVO$_{3}$ have been systematically investigated by X-ray scattering technique under high-pressure and low-temperature (HP-LT). The pressure-temperature phase diagram for the orbital state was clearly determined from the crystal parameters, i.e. the lattice constants and the reflection conditions. It indicates that the $C$-type orbital ordering ($C$-OO) is stabilized as compared with the $G$-type orbital ordering ($G$-OO) by applying hydrostatic pressure. Based on the result, we succeeded in controlling the ground state of 3$d$-orbital in TbVO$_{3}$ from $G$-OO to $C$-OO by applying pressure. The spin state coupled with the orbital was also studied by neutron scattering under HP-LT. It elucidated that the magnetic ground state changed from the $C$-type spin ordering to the $G$-type one. This result indicates the strong coupling between orbital and spin states. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V23.00004: Stabilization of Charge Ordering by Magnetic Exchange Invited Speaker: The magnetic exchange energies in charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (LSFO) and its parent compound LaFeO$_{3}$ (LFO) have been determined by inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic exchange between Fe$^{3+}$-Fe$^{5+}$ pairs ($J_{F})$ and antiferromagnetic exchange between Fe$^{3+}$-Fe$^{3+}$ pairs ($J_{AF})$ fulfills the criterion for charge ordering driven by magnetic interactions ($J_{F}/J_{AF} \quad >$ 1). The 30{\%} reduction of $J_{AF}$ as compared to LFO indicates that doped holes are delocalized, and charge ordering occurs without a dominant influence from Coulomb interactions. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V23.00005: Electronic Raman Scattering in Magnetite: Spin vs. Charge gap Lev Gasparov, G. Guntherodt, H. Berger We report Raman scattering data on single crystals of magnetite (Fe$_{3}$O$_{4})$ with the Verwey transition temperature of 123 and 117K. Both single crystals reveal broad electronic background extending up to 900 wavenumbers ($\sim $110 meV). Redistribution of this background is observed when the samples are cooled below the transition temperature. In particular, spectra of the low temperature phase show diminished background below 300 wavenumbers followed by an enhancement of the electronic background between 300 and 400 wavenumbers. $\Downarrow $ To enhance the effect of the background distribution we divide the spectra just below the transition by the spectra just above the transition. The resultant broad peak-like feature is centered at 368 $\pm $5 wavenumbers (45 meV). The peak position of this feature does not scale with the transition temperature. We discuss two alternative assignments of this feature to a spin or charge gap in magnetite. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V23.00006: Electrically-driven phase transition in magnetite nanostructures D. Natelson, S. Lee, A. Fursina, J.T. Mayo, C.T. Yavuz, V.L. Colvin, R.M.S. Sofin, I.V. Shvets In 1939 Verwey found that bulk magnetite undergoes a first-order transition at $T_{V} \approx$120~K from a high temperature conducting phase to a low-temperature insulating phase. High-$T$ conduction occurs via the fluctuating valences of the octahedral iron atoms, and the transition comes from the interplay of charge ordering and structural distortion upon cooling. The Verwey transition mechanism and charge ordering remain highly controversial. We will present data on magnetite nanocrystals and single-crystal thin films demonstrating an electrically driven phase transition below the Verwey temperature. We find sharp conductance switching that is hysteretic in source-drain voltage, and show that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. Scaling of switching voltage with electrode spacing in thin film samples shows that the switching is driven by a critical temperature-dependent electric field. Further studies of this newly observed transition and its low-temperature conducting phase should shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V23.00007: Pressure-Induced Intermetallic Valence Transition in BiNiO$_{3}$ Masaki Azuma, Masahiko Tsujimoto, Shintaro Ishiwata, Seiji Isoda, Yuichi Shimakawa, Mikio Takano, Sandra Carlsson, Jennifer Rodgers, J. Paul Attfield, Matthew G. Tucker The valence state change of BiNiO$_{3}$ perovskite under pressure has been investigated by a powder neutron diffraction study and electronic state calculations. At ambient pressure, BiNiO$_{3}$ has the unusual charge distribution Bi$^{3+}_{0.5}$Bi$^{5+}_{0.5}$Ni$^{2+}$O$_{3}$ with ordering of Bi$^{3+}$ and Bi$^{5+}$ charges on the A sites of a highly distorted perovskite structure. High pressure neutron diffraction measurements and Bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high pressure phase is metallic Bi$^{3+}$Ni$^{3+}$O$_{3}$. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable. \newline [1] M. Azuma \textit{et al}.\textit{, J. Am. Chem. Soc.}, \textbf{129}, (2007) 14433. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V23.00008: Phonon Anomaly across Charge/Orbital Ordering Transition in Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$. Jiandi Zhang, Hao Sha, F. Ye, M.D. Lumsden, P.C. Dai, J. A. Fernandez-Baca, Y. Tomioka, Y. Tokura The lattice dynamics, especially the Jahn-Teller active optical phonon modes across the charge/orbital ordering (CO/OO) transition in the single crystal Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$ has been investigated using inelastic neutron scattering (INS) technique. Three phonon peaks (around 36, 58 and 74 meV near the Brillouin zone center) appear in the scattering spectra, which are associated with the bond-stretching and bond-breathing modes of MnO$_{6}$. Both intensity and phonon energies show changes when the system undergoes the CO/OO transition indicating a strong coupling between the lattice and orbital degrees of freedom. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V23.00009: Charge Ordering in Half-Doped Manganites: Small Charge Disproportion and Leading Mechanisms Dmitri Volja, Wei-Guo Yin, Wei Ku The apparent contradiction between the recently observed weak charge disproportion and the traditional Mn$^{3+}$/Mn$^{4+}$ picture of the charge-orbital orders in half-doped manganites is resolved by a novel Wannier states analysis of the LDA$+U$ electronic structure. Strong electron itinerancy in this charge-transfer system significantly delocalizes the occupied low-energy ``Mn$^{3+}$'' Wannier states such that charge leaks into the Mn$^{4+}$-sites. Moreover, this feature is found to be generic in doped manganites. Based on a realistic effective Hamiltonian derived from first-principles calculations, we further quantify the leading mechanisms of the charge-orbital orders and find that both electron-lattice and electron-electron interactions are essential. \hspace{0.2 cm} {\it Preprint} arXiv:0704.1834. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V23.00010: Orbital ordered phases of Mn$_3$O$_4$ and MnV$_2$O$_4$ investigated by NMR Jeong Hyun Shim, Soonchil Lee, Takuro Katsufuji Compared to spin and charge, investigating orbital state is relatively difficult because it requires higher sensitivity of microscopic experimental tools. In the present study, we have demonstrated that the orbital state of Mn$_3$O$_4$ can be determined by a field-angle resolved NMR technique and the technique also applied to the investigation of MnV$_2$O$_4$ that was reported to have a strong orbital influence to spin and lattice.[1] The orbital ordered phase of MnV$_2$O$_4$ has been controversial, because lattice symmetry alone can not determine the orbital states of V$^{3+}$ ions. From our NMR results, we found that the orbital configuration of MnV$_2$O$_4 $ appears to support the Motome's antiferro-type model with an unexpected tiling of orbital direction within a-b plane, or another explanation is a phase coexistence of antiferro-type and ferro-type orbital ordered domains. \newline [1] T. Suzuki, et al., Phys. Rev Lett. 98, 127203 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V23.00011: Orbital Order and Metal-Insulator Transition in PbRuO$_{3}$ Simon Kimber, Jennifer Rodgers, J. Paul Attfield, Dimitri Argyriou We have prepared the previously uncharacterised perovskite, PbRuO$_{3}$, using a high P/T synthesis technique (10 GPa, 1000 \r{ }C) and performed synchrotron powder x-ray diffraction (ID31, ESRF), powder neutron diffraction (GEM, ISIS) and physical propety measurements. PbRuO$_{3}$ undergoes a metal insulator transition at $\sim $ 90 K at which the resistivity jumps by four orders of magnitude. The susceptibility of PbRuO$_{3}$ shows a paramagnetic-paramagnetic anomaly at 90 K, and at lower temperatures, a broad maximum. At RT, the diffraction profile of PbRuO$_{3}$ is well fitted by a distorted perovskite structure in the space group \textit{Pnma}. On cooling through $\sim $ 90 K, PbRuO$_{3}$ undergoes a structural transition, the low temperature structure is well fitted in the \textit{Imma} space group. The \textit{Imma} phase shows layered Ru$^{4+}$ orbital order, we speculate that this dimensional reduction results in the broad maximum seen in the magnetic susceptibility measurements. The structural transition is first order and phase separation is seen at 75 K. The possible role of Pb -- O covalency in inducing orbital order will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V23.00012: Incommensurate Magnetic Structure of ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$ Fabiano Yokaichiya, Heloisa Nunes Bordallo, Dimitri Argyriou, A Krimmel, A Loidl, V Tsurkan Recent studies of chalcogenide chromium spinels have shown a coupling between ferroelectricity and magnetism. The motivation of this work is to determine the magnetic ground state, (including its symmetry properties), to comprehend the coupling of magnetic and ferroelectric order parameters in the spinels ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$. The incommensurate magnetic structures through the N\'{e}el transition in these systems have been studied by high-resolution powder neutron diffraction. Below T$_{N}$ ($\sim $22K), for both cases, the magnetic structure is described as ferromagnetic layers in the \textbf{\textit{ab}}-plane stacked in a spiral arrangement along the \textbf{c}-axis with a propagation vector \textbf{k} = (0,0,$\sim $0.46). In ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4, }$ the magnetic phase transition is of first order. Therefore to use the irreducible co-representation theory, for symmetry analysis, the magnetic phase is described by a linear combination of irreducible representations. In this talk we present results of Rietveld analysis on the magnetic and crystal structure through the magnetic transition. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V23.00013: ABSTRACT WITHDRAWN |
Session V24: Focus Session: Transport in Nanostructures VII: Si Interfaces and Carbon
Sponsoring Units: DMPChair: Michael Fuhrer, University of Maryland
Room: Morial Convention Center 216
Thursday, March 13, 2008 11:15AM - 11:27AM |
V24.00001: Leakage current in deca-nano MOSFET by surface state hopping Hassan Raza, Edwin Kan Si surface states have been a topic of recent study [Nature 439, 703 (2006), PRB 76, 045308 (2007)]. In this work, we present transport calculations through these surface states, which result in a two-dimensional system. Among the systems being considered are: (1) pi and pi* states on Si(100):(2x1) surface with asymmetric dimer reconstruction, and (2) dangling bond wires along and perpendicular to the dimer row direction. Previously, we have reported the electronic structure of these systems in PRB 76, 045308 (2007). Here, we show that these states can give rise to significant current densities and hence may contribute to subthreshold leakage. Furthermore, the transport depends on the location of Fermi level with respect to the band edge and hence on the Fermi level pinning. We use EHT (extended Huckel theory) for the electronic structure and NEGF (non-equilibrium Green's function) formalism for the mean-field quantum transport. EHT has been applied to Si bulk and surfaces and gives quantitative agreement with experiments, e.g. band gap and band dispersions. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V24.00002: Tunneling properties of ultra-thin SiO$_2$ barriers: a first-principles study Eunjung Ko, Hyoung Joon Choi We performed first-principles simulations of the electron tunneling through ultra-thin SiO$_2$ barriers in Si(100)/SiO$_2 $/Si(100) structures. The atomic structures of the Si/SiO$_2$ interfaces are generated by considering various silicon suboxide states observed in photoemission studies. For comparison, we also consider sharp Si/SiO$_2$ interfaces with dangling bonds. For each atomic structure, the tunneling conductance is calculated by a first-principles scattering-state method based on the {\em ab-initio} pseudopotentials and the density functional theory within the local density approximation. As a result we obtained the dependence of the tunneling probabilities on the oxide thickness and on the interfacial structures. Effects of the dangling bonds on the tunneling probabilities will also be discussed. Computational resource for this work is provided by KISTI under the 8th Strategic Supercomputing Support Program. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V24.00003: C$_{60}$-based devices: large scale simulations and design X. H. Zheng, W. Lu, T. A. Abtew, V. Meunier, J. Bernholc C$_{60}$ is one of the most promising building blocks in the design of molecular devices, due to its spherical symmetry and structural reproducibility. In this work, the electron transport properties of two- and three-terminal devices built of C$_{60}$s are investigated. The C$_{60}$s are connected by alkane chains and then sandwiched between aluminum nanowires. The calculations are carried out using a massively parallel real-space multigrid O(N) implementation of density functional theory. The conductance and nonlinear I-V characteristics are evaluated by a nonequilibrium Green function method in a basis of optimally localized orbitals (W. Lu, V. Meunier, and J. Bernholc, PRL 2005). A conduction mechanism mediated by the LUMOs of the C$_{60}$ molecules is revealed. Due to the bias effect on the LUMO alignment, negative differential resistance (NDR) is observed in both two- and three-terminal devices at a very low bias. Since the LUMOs can be easily modified by molecular adsorption, the NDR position is tunable and can be used in sensor applications without the need for specific molecular receptors. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V24.00004: Quasiparticle Gaps of Nanostructures Weakly-Coupled to their Environments: The Case of C60/Metal Interfaces Jay Sau, Jeffrey Neaton, Hyoung-Joon Choi, Steven G. Louie, Marvin L. Cohen A new approach based on density functional theory is developed to calculate charging energies and quasiparticle energy gaps of molecular systems weakly-coupled to an external environment. The approach is then applied to the case of a C60 molecule adsorbed on the Au(111) and Ag(100) surfaces. For C60/Au(111), the calculated quasiparticle gap is reduced by 2.34 eV relative to the gas-phase, consistent with recent experiments. For the more strongly-coupled C60/Ag(100) system, the predicted gap is also reduced, but differs from experiment by 0.5eV. The discrepancy is identified as being due to screening due to charge transferred from the metal to the molecule and is resolved by solving an effective Anderson Hamiltonian within the GW approximation for the carriers in the HOMO and LUMO states, which results in an extra renormalization of the gap for the Ag(100) case. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V24.00005: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V24.00006: Electron-phonon scattering effects on transport properties of carbon nanotube devices using time-dependent wave-packet approach Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose Single-walled carbon nanotubes have been expected as nanoscale electronic devices, because the nanotubes are very good conductors exhibiting ballistic transport properties. However, the electronic current is saturated by the electron-phonon coupling. To realize the application of carbon nanotube devices, understanding of the scattering mechanism is required. We investigated the electron-phonon coupling effect on the transport properties of the nanotubes with micron order channel length, using the time-dependent wave-packet approach under a tight-binding approximation [1]. The vibrational atomic displacements in real space are introduced through the time-dependent transfer energies. We solved the time-dependent Schr\"odinger equation and obtained the diffusion coefficients of the electronic wave packets. From these data, we can extract the mean free path and conductance. We clarified the difference of the phonon scattering effects on the conductance of the metallic nanotube and the semiconducting one. Furthermore we investigate the channel length dependence of resistance from ballistic to diffusive transport characteristics. [1] S. Roche \textit{et al.}, PRL 95 (2005) 076803 [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V24.00007: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V24.00008: Electron transport through functionalized carbon nanotubes Nicolas Bruque, Rajeev Pandey, Md. Khalid Ashraf, Roger Lake Single molecule functionalization of single walled carbon nanotubes (CNT)s by B. R. Goldsmith, et. al. [1] and single molecule bridges of single wall CNTs by X. Guo, et. al. [2] are elegant examples of CNT contacts to individual molecules for electronic applications. CNTs are of the same physical size as the molecule they contact providing a well-defined covalent bond between CNT electrodes and a molecule. The above two systems are studied to determine how a chemical absorbate and linker influence transport through metallic CNTs. The first system consists of a stand alone metallic CNT with a single oxygen adsorption site, matching a proposed final chemical process for a HNO$_{3}$ oxidation and reduction process. The second system consists of a CNT-Amide-(CH)$_{n}$-Amide-CNT structure in which both CNTs are metallic. Transmission calculations, using our DFT (FIREBALL)-NEGF code show that the amide linker suppresses transmission compared to a direct CNT-polyene linkage studied in [3]. 1. B. R. Goldsmith, et. al., Science, \textbf{315}, 77 (2007). 2. X. Guo, et. al. Science, \textbf{311}, 356 (2006). 3. N. Bruque, et. al. Phys. Rev. B, \textbf{76}, 205322 (2007). [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V24.00009: Multiple Quantum Well Structures of Graphene Haldun Sevincli, Mehmet Topsakal, Salim Ciraci Based on first-principles calculations we predict that periodically repeated junctions of graphene ribbons of different widths form multiple quantum well structures having confined states. These quantum structures are unique, since both constituents of heterostructures are of the same material. The width as well as the bad gap, even the magnetic ground state for specific superlattices are modulated in direct space. Orientation of constituent ribbons, their width and length, the symmetry of the junction and their functionalization by adatoms are structural parameters to engineer electronic and magnetic properties of the quantum structure. Not only the size modulation, but also composition modulation such as the heterojunction of BN in honeycomb structure and graphene gives rise to confined states. Devices made from these graphene quantum structures display negative differential resistance. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V24.00010: Direct measurement of electric-field-screening length in thin graphite film H. Miyazaki, K. Tsukagoshi, S. Odaka, Y. Aoyagi, T. Sato, S. Tanaka, H. Goto, A. Kanda, Y. Ootuka Electric-field-screening length in thin graphite film has been clarified by means of dual gating method. Sandwich type device structure which two gate electrodes are situated over and beneath a graphite film was constructed with Al top electrode. The Al electrode naturally generates thin gate insulator at graphite/Al interface, which enables extremely low voltage operation. Ambipolar charge conduction in a graphite film can be tuned by both top and back gate voltages. A scan of the top gate voltage ($V_{tg})$ generates a resistance peak in the ambiploar response. The back gate voltage ($V_{bg})$ shifts the ambipolar peak depending on the graphite thickness. The shift is larger in thinner film. The thickness-dependent peak shift is clarified in terms of the inter-layer screening length $\lambda $ to the electric field in the dual-gated graphite film. We assume that the gate-induced carriers decay exponentially from both surfaces, and that the conductivity in each layer increases proportionally to the induced carrier density. Then the condition for the ambipolar resistance peak in $V_{tg}$ scan is obtained as a function of $V_{bg}$, $\lambda $, and the graphite film thickness $d$. Applying this model to the thickness-dependence, we obtained a screening length of 1.2 nm experimentally. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V24.00011: Surface Potentials and Layer Charge Distributions in Few-Layer Graphene Sujit Datta, Douglas Strachan, Eugene Mele, A.T. Charlie Johnson Elucidating the electronic interaction between an insulating substrate and few-layer graphene (FLG) films is crucial for graphene device applications. We have performed electrostatic force microscopy (EFM) of FLG films. Our measurements reveal that the FLG surface electrostatic potential increases with film thickness, approaching a `bulk' value for samples with five or more graphene layers - contrasting sharply with behavior expected for conventional conducting or semiconducting films. This is in quantitative agreement with the analytic predictions of a nonlinear Thomas-Fermi theory of the interlayer screening by graphene's relativistic low energy charge carriers. Furthermore, our measurements are able to resolve previously unseen electronic perturbations extended along crystallographic directions of stressed samples, likely resulting from long-range atomic defects. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V24.00012: Valley contrasting physics in graphene: magnetic moment and topological transport Qian Niu, Di Xiao, Wang Yao We investigate physical properties that can be used to distinguish the valley degree of freedom in graphene systems with broken inversion symmetry. We show that the pseudospin associated with the valley index of carriers has an intrinsic magnetic moment, in close analogy with the Bohr magneton for the electron spin. There is also a valley dependent Berry phase effect that can result in a valley contrasting Hall transport, with carriers in different valleys turning into opposite directions transverse to an in-plane electric field. These effects can be used to generate and detect valley polarization by magnetic and electric means, forming the basis for the so-called valleytronics applications. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V24.00013: Quantum transport of 2D Dirac fermions: the 2D symplectic symmetry class of Anderson localization and the Z2 topological term Shinsei Ryu, Christopher Mudry, Hideaki Obuse, Akira Furusaki We discuss the quantum transport of the 2D non-interacting Dirac Hamiltonian, which, underlies theoretical descriptions of graphene and surface states of 3D Z2 topological insulators. For a random scalar potential type disorder, a Z2 topological term is derived in the non-linear sigma model encoding the physics of Anderson localization in the symplectic symmetry class. Unlike the Pruisken term (Chern integer) in the IQHE, the Z2 topological term cannot be expressed, in general, as an integral of a local quantity, but as a sign of the Pfaffian of a family of Dirac operators. The Z2 topological term has a significant effect on the renomalization group flow of the conductance. [Preview Abstract] |
Session V25: Focus Session: Interfaces and Adhesion I
Sponsoring Units: DPOLYChair: Karl Freed, University of Chicago
Room: Morial Convention Center 217
Thursday, March 13, 2008 11:15AM - 11:51AM |
V25.00001: A model for glass transitions in polymer thin films. Invited Speaker: Polymeric materials formulated as thin films can exhibit glass transition temperatures which are significantly shifted relative to bulk values. Depending on whether the film is supported (on a substrate) or freely-standing the temperature shift can go in different directions relative to the bulk. For all films the magnitude of the shift depends on film thickness. For supported films the shift appears to depend on substrate-polymer interactions, while for free-standing films there is a striking dependence on molecular weight. Experimental data published over the last five years have included some elegant and intriguing results which provide a significant challenge for those wishing to understand these phenomena. In this talk a model that predicts glass transitions in both free-standing and supported films will be presented and tested against extant data. Ideas for future experiments will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V25.00002: Creating defect free structures by directed photochemical reaction in a ternary phase separating system Pratyush Dayal, Olga Kuksenok, Anna Balazs We present a technique to create long range ordered structure in photosensitive reaction-diffusion systems. Our approach utilizes a reversible photochemical reaction between species A and B in a ternary immiscible ABC blend to yield a defect-free arrangement of phase separated ABC domains. The process essentially consists of two steps. First, the sample is irradiated by virtue of masks creating an illumination pattern which allows the migration of AB components to the covered regions at the expense of the C component. As a result the C component is pinned onto the irradiated regions which in turn leads to pinning of AB domains thereby creating a long range order in the system. Second, the masks are completely removed and whole sample is irradiated uniformly. In this case, the system evolves into a distinctly different ordered structure. The ordering of the system, either before or after removal of the mask, can be locked in by quenching the system at appropriate time. Simulation studies reveal that the ordering can be controlled by changing the initial concentration of the blend as well as by varying size and arrangement of the holes in the mask. The defect free morphology has been demonstrated for rectangular, hexagonal and parallel arrangement of the masks. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V25.00003: Entropic Effects in the Phase Behavior of Athermal Nanoparticle/Homopolymer Thin Film Mixtures Luciana Meli, Abraham Arceo, Peter Green The phase behavior of an athermal nanoparticle/polymer mixture, composed of polystyrene-coated gold nanoparticles embedded in polystyrene thin film hosts, was examined. It is shown that the spatial distribution of nanoparticles is readily tailored through control of: the grafting density of the brush, the length of both grafted and free chains, and the relative size of the nanoparticles in comparison to the size of the polymer melt chains. The distribution of nanoparticles within the polymer host, including a surface-induced phase separation, may be understood as a balance between the conformational entropy of the polymer chains, which is compromised when the chains have to stretch around the nanoparticles and penetrate their brush, and the translational entropy of the nanoparticles, which favors their homogeneous distribution. This is the first report that systematically studies the entropic effects that lead to surface phase transitions in polymer/nanoparticle thin film mixtures. Our results may also prove helpful in understanding nanofiller dispersion in analogous bulk mixtures. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V25.00004: Mechanism of Interfacial Instability in Thin Polymer Film in Controlled Solvent Atmosphere Parvaneh Mokarian-Tabari, Jonathan. R. Howse, Sasha Y. Heriot, Mark Geoghegan, Richard A.L. Jones Thin films of immiscible polymers made by spin coating have potential for many practical applications like field-effect transistors, LEDs and photovoltaic devices. We have developed a technique based on small angle light scattering and reflectivity to study the process of phase separation in spin cast films \textit{in situ} during formation. Previous experiments\footnote{ \textit{Heriot, S.Y. and R.A.L. Jones, Nature Materials, 2005. }\textbf{\textit{4}}\textit{(10): p. 782-786 }\par } proposed formation of a transient wetting layer which is followed by interfacial instability and leading to lateral phase separation. In our recent work the origin of this instability has been studied. Experiments have been designed to test the Marangoni instability by spin coating the PS/PMMA film in a controlled toluene vapour atmosphere. A fast evaporation rate leads to laterally phase separated structure whereas slow evaporation lowers the solvent gradient inside the film and leads to a self stratified structure. By comparing the data to a model a better understanding of film evolution has been established. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V25.00005: Spanning Trees and the Dynamics of Compact Polymers Armin Rahmani, Andrea Velenich, Claudio Chamon We introduce a lattice model for a compact loop polymer confined to a two-dimensional box. By a mapping to spanning trees on a square lattice we calculate the partition function and the energy of the system as a function of temperature, bending rigidity and elasticity of the polymer. We study the dynamics of the system using a kinetically constrained model whose elementary moves consist of polymer fingering or, in the language of the spanning trees, local bond flips taking place at the leaves of the tree. We study, through Monte-Carlo simulations, the time dependence of the energy and the number of leaves in the system when quenched from infinite temperature to various finite temperatures. We find that for temperatures above a critical value, these observables monotonically decay to their equilibrium values whereas, for lower temperatures, a broad non-equilibrium plateau emerges. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V25.00006: Uper Limit of Superheating in Polymer Crystals Revealed from Linear Heating Covering Seven Orders of Magnitude in Heating Rate Christoph Schick, Alexander Minakov, Andreas Wurm We report about superheating of polymer crystals on linear heating covering the scanning rate range from 0.02 to 1,000,000 K/s. Results obtained by super-fast scanning calorimetry using a thin film sensor [1] are combined with results from DSC. On slow heating semicrystalline polymers tend to recrystallize (reorganize) significantly fast. From previous measurements the onset of melting of isothermally crystallized samples can be attributed to the rising flank of the first melting peak, which is often called the annealing peak, rather than to the peak maximum. The later depends on the counterbalance of melting and recrystallization. Melting kinetics is described by the power law of superheating: $\sim $ ($T$ -- $T_{cr})^{\alpha }$ with \textit{$\alpha $} $<$ 0.2, which does not correspond to the heat transfer but rather to a nucleation process [2]. At high heating rates superheating saturates. The power law behavior and the saturation of superheating will be discussed. 1. A.A. Minakov, C. Schick, Rev. Sci. Instrum. 78 (2007) 073902. 2. A.A. Minakov, A. Wurm, C. Schick, Europ. Phys. J. E, 23 (2007) 43. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V25.00007: Molecular simulation of crystal nucleation of an n-alkane Peng Yi, Gregory Rutledge We report the results of molecular simulations to study crystal nucleation of n-octane from the melt. A realistic united atom force field was employed for n-octane. The melting behavior was first determined by ramping temperature in a set of Monte Carlo simulations. The adiabatic nucleation trajectory was then sampled using the umbrella sampling technique with a set of proposed global and local order parameters, and analyzed for selection of best order parameter. The transition state ensemble has been verified by molecular dynamics simulation. The structure of critical nuclei in the nucleation process is analyzed and the effect of intermediate phases discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V25.00008: Entropically Driven Layering Near a Substrate: A Fluids DFT Study Erin McGarrity, Amalie Frischknecht, Michael Mackay We employ a fluids density functional theory to study the phase behavior of athermal polymer/nanoparticle blends near a hard substrate. These blends exhibit two types of first order, entropically driven layering transitions. In the first type of transition, the nanoparticles order to form a layer which is a fixed distance from the surface. The structure and location of this layer depends on nanoparticle radius. In the second type of transition, which occurs at melt-like densities, the nanoparticles and polymers form laminar structures which resemble colloidal crystals. We examine the effects of packing density, chain length and nanoparticle radius on the system and show that the transitions are first order. In addition we show that the crystalline phase is nucleated by the presence of the surface. 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] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V25.00009: Properties of Ferroelectric Polyvinylidene fluoride-co-trifluoroethylene Nanorods Jodie Lutkenhaus, Thomas Russell Ferroelectric and piezoelectric nanorods of polyvinylidene fluoride (PVdF) are potential candidates for sensing mechanical stimuli on the nano-scale. Left untreated PVdF tends to crystallize in a non-ferroelectric ($\alpha )$ phase. PVdF-co-trifluoroethylene (PVdF-TrFE), on the other hand, readily crystallizes into the ferroelectric ($\beta )$ phase; however, the structure and properties can be significantly influenced by confinement. Here, the creation of PVdF-TrFE nanorods (dia. = 15 to 200 nm) using anodized aluminum oxide membranes was investigated. The crystallization behavior and the Curie transition (ferro- to paraelectric phase) were studied. Changes in structure and phase were investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The effects of electrical poling, which increases dipole-orientation within the ferroelectric phase, are discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V25.00010: Thermodynamics and Kinetics of Crystallization of Flexible Molecules Bernhard Wunderlich The crystallization of a single atom (as in a metal) is a one- step transfer across the liquid-crystal interface. A flexible chain molecule of n segments needs considerably more steps. A simple description gives it (n - 2)$^3$ possible conformations in the liquid, i.e., for pentcontane C50H102 111,000, but only one conformation in the crystal, making the crystallization a multi-step process. At what length are there too many steps to allow the crystallization to be reversible? How can one describe phase separation for flexible molecules containing segments of different chemical nature? How do random segments of different nature and precisely spaced segments of different nature influence the phase separation on crystallization? How is reversible crystallization influenced by chain folding and crystal perfection in the solid state? Some of these old questions can now be answered by temperature-modulated differential scanning calorimetry of precisely made molecules of different length and copolymerized structure. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V25.00011: UV-convergent One-loop Theory of Binary Homopolymer Blends Jian Qin, Frank Bates, David Morse We analyze the effects of long wavelength composition fluctuations in binary homopolymer blends. We use a generalization of Wang's theory [1], in which all dependence upon short-wavelength structure is absorbed into a renormalization of an effective $\chi$ parameter $\chi_e$ and of statistical segment lengths. The theory allows us to calculate the collective correlation function $S(k)$, single chain correlation functions, and the free energy density in homogeneous mixtures. The same formalism can be used to study diblock copolymer melts. The value of $\chi_e N$ at the critical point of a binary blend exceeds that predicted by Flory-Huggins theory by an amount proportional to $1/\sqrt{N}$, though the width of the critical region is proportional to $1/N$. For strongly asymmetric blends, however, the binodal value of $\chi_e N$ is suppressed. The dimensions of individual chains decrease slightly with increasing $\chi$ or with decreasing $N$, even when $\chi_e=0$. [1] Z.-G. Wang, {\it J. Chem. Phys.}, 2002, 117:481. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V25.00012: Effect of intensity gradient profiles on crystal growth subject to holographic free radical photopolymerization Thein Kyu, Pankaj Rathi, Soojeoung Park A theoretical model has been developed to describe a unique phenomenon of photopolymerization-induced crystallization subjected to holographic intensity gradient profiles. A hypothetical phase diagram of a crystalline polymer solution (or blend) is constructed to guide the dynamics of directional crystallization. Calculations of holographic photopolymerization induced crystallization were carried out slightly above the melting temperature of the bends under the conditions of sinusoidal as well as sharp (square) interface. In the case of periodically varying interface, the dynamic calculations revealed that the emerged crystals (spherulites) have out-grown the patterned regions, which is consistent with the crystal growth behavior of the polyethylene oxide/diacrylate system. However, in the case of a sharp holographic the directional growth occurs along the stripes, which is confined within the stratified layers. [Preview Abstract] |
Session V26: Focus Session: Advances in Atmospheric Aerosol Science III
Sponsoring Units: DCPChair: Daniel Knopf, State University of New York
Room: Morial Convention Center 218
Thursday, March 13, 2008 11:15AM - 11:51AM |
V26.00001: Measurements of the Chemical Composition of Atmospheric Nanoparticles Invited Speaker: The Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) is an instrument that is capable of measuring the chemical composition of particles as small as 4 nm. It accomplishes this with a sensitivity that makes it possible to measure the molecular composition of nanoparticles at ambient concentrations in the atmosphere. For the past five years, the TDCIMS has been performing measurements of the smallest particles in the atmosphere in order to determine the chemical species and mechanisms responsible for the growth of aerosols formed by nucleation. In this talk I will summarize what we've learned from these measurements, which took place in urban areas (Atlanta and Mexico City), a remote location (the boreal forests of Finland), and regions that are combinations of both (Boulder). With the exception of one study in urban Atlanta, in which sulfur species were seen to dominate, most measurements indicate a crucial role played by organic species in the growth of atmospheric nanoparticles. Positive ion TDCIMS measurements in a variety of locations show the presence of methyl and dimethyl amines in particles as small as 8 nm. Other oxidized organics detected in positive ion TDCIMS measurements are presumed to be alcohols, aldehydes, or ketones. Negative ion TDCIMS measurements show the presence of multifunctional organics with carboxylic acid moieties. Laboratory studies using pure and multi-component aerosols are assisting us in identifying the many ions that were observed during our campaigns. Our measurements suggest that reactions of organic acids and organic bases on particle surfaces or within particles may form organic ions and/or salts in particles. Based on these measurements, we hypothesize that the organic salt formation mechanism may be the dominant mechanism by which nanoparticles grow in the atmosphere. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V26.00002: Laboratory-Measured Nucleation Rates of Sulfuric Acid and Water from the SO$_{2}$ + OH Reaction David R. Benson, Li-Hao Young, Shan-Hu Lee We present results of the laboratory study of sulfuric acid-water binary nucleation system. H$_{2}$SO$_{4}$ was produced through the reaction of SO$_{2}$ + OH $\to$ HSO$_{3}$ in the presence of SO$_{2}$, OH, O$_{2}$, and H$_{2}$O in a fast flow reactor at 288 K and atmospheric pressure. OH was produced from the photolysis of water vapor. The power dependence of nucleation rate ($J)$ on sulfuric acid concentration ([H$_{2}$SO$_{4}$]) was 2 - 10 in the [H$_{2}$SO$_{4}$] range from 3$\times $10$^{6}$ - 1$\times$ 10$^{9}$ cm$^{-3}$. This power dependence increased with decreasing RH and increasing nucleation time. The power dependence of $J$ on RH was 10 - 15 for the RH values from 10 - 50{\%}. The measured aerosol sizes ranged from 4 - 20 nm. These aerosol sizes were larger for higher [H$_{2}$SO$_{4}$], higher RH, and higher nucleation times. The effects of RH on aerosol growth were also more pronounced at higher [H$_{2}$SO$_{4}$] and with higher nucleation times. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V26.00003: The enhancement of aqueous aerosol formation by ions and radicals Samuel Keasler, Ricky Nellas, Hyunmi Kim, Joseph Francisco, Bin Chen The formation of aqueous aerosols in the atmosphere is of significant importance due the role of these particles in heterogeneous chemistry. One important mechanism for the formation of these aerosols is the multi-component nucleation of water with other compounds present in the atmosphere, such as ions and radicals. We have applied the AVUS-HR approach developed in our group for to examine the nucleation of water in the presence of both single ions and ion pairs, and to the binary nucleation of water with hydroxyl and peroxyl radicals. This method allows us to efficiently calculate the free energy profile for these nucleation processes as a function of the cluster size and composition. This information can give us a clear picture of the role that these ions and radicals may play in forming aqueous aerosols. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V26.00004: Scaled Nucleation in a Lennard-Jones System Barbara Hale, Tom Mahler, Jerry Kiefer Scaling of the vapor-to-liquid nucleation rate, $J$, is examined in a model Lennard-Jones system using Monte Carlo derived rate constant ratios for growth and decay of small clusters. \ The model assumes a dilute vapor system of non-interacting clusters and the steady-state nucleation rate formalism expressed as a summation over products of rate constant ratios. The nucleation rates so obtained are examined in a scaling plot of $\log J$ \ \textit{vs.} $\ \ln S/[T_{c}/T-1]^{3/2}$ [Hale, B. N., \textit {J. Chem. Phys}. 122, 204509 (2005)], the general form of which has been recently used to test the consistency of nucleation rate data [Gharibeh, M., Kim, Y., Dieregsweiler, U., Wyslouzil, B., Ghosh, D. and Strey, R., \textit{J. Chem. Phys}. 122, 094523 (2005); Brus, D., Zdimal, V., and Stratmann, F., \textit{% J. Chem. Phys}. 124, 164306 (2006)]. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V26.00005: The Nucleation Rate and the Gibbs Free Formation Energy of a Cluster H.R. Kobraei In this work, we present an atomistic/molecular model along with the classical approximation for the Gibbs free formation of nuclei. The free formation energy of the critical cluster plays an essential role in the calculation of nucleation rates. Thus, we have constructed a nucleation rate relation which is easy to calculate and its result is relatively simple to compare with experimental data. The energy formation of a cluster has a few more terms than the traditional classical model. Furthermore, the extra terms in this approach have their roots in the molecular treatment of a cluster formation and they are temperature dependent. We have compared the result of this approach with the original classical theory along with some experimental data. Our initial results seem promising and the temperature correction has a correct trend. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V26.00006: Dynamical investigation of water clusters in atmospheric conditions Francesca Baletto, Mal-Soon Lee, Uguette F.T Ndonmgouo, Sandro Scandolo Addressing environmental challenges via first principle calculations is one of the most promising subjects of numerical simulations. Here, we investigate the dynamical evolution of water clusters, namely the dimer and the hexamer, which are abundant in our atmosphere. We use these two clusters as prototypes to clarify long-standing dilemma of greenhouse effects and ozone depletion. To begin with, I will show the behavior of HCl on water hexamers [1]. Our calculations show that at zero temperature the most energetically favorable structure is obtained when the HCl is completely dissociated. At temperatures T $\sim$ 200 K, the vibrational entropic effects stabilize the non-dissociated clusters. This behavior is traced back to the large dynamic effects associated with the flexibility of the planar cluster. Water vapor absorption in the far-infrared region accounts a large portion of the total radiative absorption responsible for the greenhouse effect. We found that at T close to 200K, the dimer dynamics is fully anharmonic and the calculated adsorption strength throughout the far-infrared spectra is smaller than the measured vapor absorption continuum [2]. \newline [1] U.F.T. Ndomgouo et al. JPCA accepted \newline [2] M-S. Lee, et al. submitted [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V26.00007: Modeling the Growth of H$_{2}$O-D$_{2}$O Nanodroplets Somnath Sinha, Barbara E. Wyslouzil, Gerald Wilemski Using experimental data for water condensation in supersonic nozzles, including SAXS measurements of position-resolved nanodroplet size distributions [Wyslouzil, et al., \textit{Phys. Chem. Chem. Phys. }\textbf{9}, 5353 (2007)], we test five different droplet growth models. Three nonisothermal growth models estimate temperature differences between the droplets and the carrier gas; the two isothermal models do not. In general, we found that none of the growth laws agrees well with the experimental data. Although the droplets should be hotter than the carrier gas for our experimental conditions, our results suggest that the nonisothermal models over predict the average droplet temperatures. This leads us to hypothesize that the average temperature is not a good estimate of the most likely temperature of the growing droplets. To accurately predict growth in the nanodroplet regime, droplet growth models will need to account better for the full distribution of temperatures of growing droplets because the main contribution to growth is likely to come from droplets cooler than the average. [Preview Abstract] |
Session V27: Focus Session: Frustrated Theory
Sponsoring Units: GMAGChair: Oleg Tchernyshyov, Johns Hopkins University
Room: Morial Convention Center 219
Thursday, March 13, 2008 11:15AM - 11:27AM |
V27.00001: Path Integral Approach to Geometrically Frustrated Quantum Antiferromagnets Miraculous Bhaseen, John Chalker We develop a path integral approach to geometrically frustrated quantum antiferromagnets. Using Hubbard--Stratonovich transformations to decouple the interactions within clusters of spins, we establish a high temperature expansion of the quantum partition function. This semiclassical approach based on conjugate cluster variables allows us to descend below the Curie--Weiss temperature scale, and to describe the emergent spin liquid regime. We make contact with complementary approaches based on spin wave theory. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V27.00002: Contractor-Renormalization approach to frustrated magnets in a magnetic field Andreas Abendschein, Sylvain Capponi We propose to use the Contractor Renormalization (CORE) technique in order to derive effective models for quantum magnets in a magnetic field. CORE is a powerful non-perturbative technique that can reduce the complexity of a given microscopic model by focusing on the low-energy part. We provide a detailed analysis of frustrated spin ladders which have been widely studied in the past: in particular, we discuss how to choose the building block and emphasize the use of their reduced density matrix. With a good choice of basis, CORE is able to reproduce the existence or not of magnetization plateaux in the whole phase diagram contrary to usual perturbation theory. Furthermore, we present recent results for other, potentially more interesting geometries like the Heisenberg bilayer where we also address the issue of plateau formation and point out the analogy between non-frustrated strongly anisotropic models and frustrated SU(2) ones. Finally, we investigate the magnetization curve of the Shastry-Sutherland model and, however in absence of a magnetic field, we consider the square lattice with four-spin ring-exchange. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V27.00003: Spin-orbital liquid state on the square lattice with emergent Majorana fermions and Z$_2$ topological order Ashvin Vishwanath, Fa Wang Magnetism from d-electrons often retains orbital degeneracy which can enhance quantum fluctuations and lead to exotic liquid-like ground states with no conventional order. Indeed, experimental systems like LiNiO$_2$, FeSc$_2$S$_4$ etc. with orbital degeneracy show a lack of order down to low temperatures. We introduce a Majorana-fermion slave particle theory to study such states in spin-1/2 models with e$_g$ orbital degeneracy. This is first applied to a square lattice model with enhanced SU (4) symmetry. A mean field treatment predicts a spin-orbital liquid state with nodal Majorana fermion excitations and Z$_2$ topological order. A variational Monte-Carlo study of the corresponding wavefunction confirms the absence of magnetic order and bond order, which makes it a candidate state for a spin orbital liquid. Comparing against the exact diagonalization studies in [Bossche et al. Eur. Phys. J. B 17, 367 (2000)], our state is found to have significant overlap with the ground state on small lattices, despite the absence of a variational parameter. More realistic models with lower symmetry and on different lattices are analyzed within our formalism, and applications to S=3/2 atoms confined in optical lattices are pointed out. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V27.00004: Magnetic monopoles in spin ices and spin excitations in other pyrochlores Invited Speaker: Exotic excitations go along with the unusual ground states of frustrated magnets. One of the most striking examples occurs in dipolar spin ice, where the dipole moment of the underlying electronic spin degrees of freedom fractionalises into magnetic monopoles. This constitutes possibly the first instance of fractionalisation in a three-dimensional material. It enables us to account for a mysterious phase transition observed experimentally in spin ice in a magnetic field, which is a liquid-gas transition of the magnetic monopoles. These monopoles can also be detected by other means, e.g., in an experiment modelled after the celebrated Stanford magnetic monopole search. We also discuss other instances of unusual pyrochlore excitations. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V27.00005: Vortex Berry phases and the magnetization of quantum magnets Akihiro Tanaka, Keisuke Totsuka, Xiao Hu We revisit the magnetization process of quantum antiferromagnets subject to an external magnetic field, and show how an interpretation in terms of Berry phases emerges. First we develop a continuum variant of the Lieb-Schulz-Mattis- type approach to the 1d problem, and find that the well-known commensurability condition of Oshikawa et al derives from the Berry-connection theory of the crystal momentum of a magnet first suggested by Haldane. Building on the physical picture which arises from this analysis, we then go on to formulate an effective field theory which can deal with the higher dimensional cases. We show that a topological term associated with vortices of the XY-like slowly varying fields controls the behavior of the system. Finally we utilize this new framework to discuss possible occurrences of fractionalized states. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V27.00006: Kinetic ferromagnetism on frustrated lattices Frank Pollmann, Kirill Shtengel, Peter Fulde Systems with frustrated interactions are generally characterized by a high density of low--lying excitations which can be responsible for interesting physical effects. Much attention has been paid to the effects of frustration in antiferromagnetic systems. In this talk, however, we present a model of ferromagnetism arising in a frustrated system. We study strongly correlated electrons described by an extended Hubbard Hamiltonian on a kagome lattice at 1/6 and 1/3 filling. In the limit $|t|\ll V\ll U$ (where $t$ is a hopping amplitude, $U$ is on-site repulsion and $V$ is nearest-neighbor repulsion), we derive an effective low-energy Hamiltonian for this model. The Perron-Frobenius theorem can then be used to show that the ground state of this system is ferromagnetic. We will also address the robustness of ferromagnetism to finite temperature effects and other interactions and discuss possible extensions to other lattices. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V27.00007: Magnetic Fluctuations in the Hubbard Model on Kagome-based Frustrated Lattices Masafumi Udagawa, Yukitoshi Motome We report our results on the interplay between electron correlation and magnetic fluctuations in the geometrically-frustrated Kagome and hyper-Kagome Hubbard models at half filling. These models have two different geometrical units important in the low-energy physics: the frustrated triangle and the non-frustrated loop with even-number sites. In order to treat both of them on equal footing, we apply cluster dynamical mean-field theory to large-size clusters up to 12 sites. By calculating the spin susceptibility $\chi({\vec q}, \omega)$, we have found in the Kagome system that an anomalous one-dimensional magnetic correlation previously found near the Mott transition [1] is observed even in the non-interacting case at high temperature, and its temperature range gradually suppressed by increasing electron correlation. This behavior is ascribed to the nesting property at the van-Hove singularity preserved under electron correlation. We will also present the results for hyper-Kagome system in relation to the recent experiments on Na$_4$Ir$_3$O$_8$ [2]. \\ \noindent [1]\ T.\ Ohashi {\it et al.}, Phys.\ Rev.\ Lett.\ {\bf 97}, 066401 (2006)\\ \noindent [2]\ Y.\ Okamoto {\it et al.}, Phys.\ Rev.\ Lett.\ {\bf 99}, 137207 (2007) [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V27.00008: Study of the $U(1)-Z_4$ cross-over in 2D valance-bond-solids Jie Lou, Anders Sandvik We consider the 2D Heisenberg model in the overcomplete basis of valence bonds. By tuning matrix elements corresponding closely to the diagonal and off-diagonal terms in Rokhsar-Kivelson quantum dimer model, we show, using a projector quantum Monte Carlo technique, that there is a quantum phase transition into a valence-bond-solid (VBS) state. This system allows us to study the cross-over length-scale [1] associated with emergent U(1) symmetry of the VBS order parameter [2], which has up until now not been possible in other systems [3], where the VBS order is weaker and prohibitively large system sizes are needed to observe the stabilization of a manifestly $Z_4$-symmetric VBS. \break [1] J. Lou, A. W. Sandvik, and L. Balents, Arxiv:0704.1472. [2] T. Senthil, A. Vishwanath, L. Balents, S. Sachdev, and M. P. A. Fisher, Science \textbf{303}, 1490 (2004). [3] A. W. Sandvik, Phys. Rev. Lett {\bf 98}, 227202 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V27.00009: Dimerized Bond-Disordered Quantum Spin Models and Harris Criterion Jonas Gustafsson, Daoxin Yao, Erica Carlson, Anders W. Sandvik We study several different realizations of dimerized bond disorder in the two-dimensional square-lattice S=1/2 Heisenberg model, by introducing strong and weak couplings, $J_s,J_w$, randomly, but in such a way that each spin belongs to one strong bond (a dimer). We study the ground-state phase transition occurring at a critical ratio $g_c=J_s/J_w$ for different ways of distributing the dimers: (a) randomly distributed as in the classical dimer model, whence the dimer-dimer correlation function follows a power law, $c(r) \sim r^{-2}$, and (b) the random plaquette (RP) model, where all dimers are first placed horizontally in columns and thereafter any plaquette with dimers is flipped with probability $p=1/2$ or $1/4$. Our calculations show that the Harris criterion for the relevance of disorder is not applicable to these models. In all cases, the disorder does not appear to change the universality class from that obtaining with a regular dimer arrangement. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V27.00010: Spontaneous parity breaking of Dirac spin liquid in a magnetic field Ying Ran, Wing-Ho Ko, Patrick Lee, Xiao-Gang Wen The Dirac spin liquid was proposed to be the ground state of the spin-1/2 Kagome antiferromagnets. In a magnetic field, we show that the state with Fermi pocket is unstable to the Landau level (LL) state and the XY ordered state. We mainly focus on the fully gapped LL state, which breaks parity symmetry and thus supports a finite temperature phase transition. We discuss experimental signatures which can be used to detect the possible Dirac spin liquid phase in Herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V27.00011: Two-step spin flop transition in quantum spin ladders Toru Sakai, Kiyomi Okamoto It is well known that the antiferromagnet with easy-axis anisotropies exhibits a field-induced first-order phase transition, the so- called spin flop. In one-dimensional quantum spin systems, instead of it, a second-order phase transition occurs because of large quantum fluctuations[1]. Particularly the S=1 antiferromagnetic chain with the easy-axis single-ion anisotropy was revealed to exhibit two successive field-induced second-order transitions by our previous numerical analysis[2]. However, such transitions have not been obseved yet. Recently a two-step spin flop transition was observed in the spin ladder system IPA-CuCl$_3$[3], which has ferromagnetic rung coupling. In order to clarify the mechanism of the two-step field-induced transition, we investigate the anisotropic spin ladder using the numerical diagonalization and the finite-size scaling analysis. As a result, we revealed that two different field-induced second-order quantum phase transitions possibly occur. Several phase diagrams are also presented. In addition we discuss on a possible two-step spin flop in other materials[4] and some frustrated systems. [1] C. N. Yang and C. P. Yang, Phys. Rev. 151 (1966) 258. [2] T. Sakai, Phys. Rev. B 58 (1998) 6268. [3] T. Masuda et al, Phys. Rev. Lett. 96 (2006) 047210. [4] H. Miyasaka et al, Inorg. Chem. 42 (2003) 8203. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V27.00012: Spin and Singlet Dynamics of the S=1/2 Quantum Kagome Antiferromagnet Andreas Lauchli, Claire Lhuillier The kagome Heisenberg antiferromagnet with spin 1/2 has been the topic of many theoretical investigations. Most of these focused on groundstate properties or were aiming at an explanation of the anomalous high density of singlet excitations. In this contribution we report on exact diagonalization studies concentrating on dynamical correlation functions. First the full dynamical spin structure factor $\mathcal{S}(\mathbf{q},\omega)$ on 36 sites has been obtained, showing a broad, rather incoherent spectral response, which furthermore seems to increase significantly at low energies. Then we discuss the time dependent spin autocorrelation function as well as dynamical dimer-dimer correlation functions. All these results combined point towards a highly fluctuating system, both in the singlet and the triplet channel. We conclude by a comparison with recent inelastic neutron scattering measurements on the Herbertsmithite and Volborthite compounds. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V27.00013: Pseudo-Dirac Hamiltonians and Flux Phases of SU(N) magnets F.J. Burnell, S.L. Sondhi, R. Shankar The lattice Dirac Hamiltonian describes a particle hopping on a lattice in a particular background magnetic field. We present a family of hopping Hamiltonians in other background fields that generalize many aspects of the Dirac Hamiltonian to lines and planes of nodes. In our canonical case, hopping on the pyrochlore lattice gives rise to a half filled fermi surface that consists of four intersecting [111] lines. This spectrum is invariant under tetrahedral rotations, rather than all rotations as in the Dirac case, resulting in a more complex matrix anti-commutation structure. This structure arises in a large N treatment of the SU(N) Heisenberg model on the pyrochlore. [Preview Abstract] |
Session V28: Focus Session: Optical Properties of Nanostructures VI: Nanoscale Metamaterials
Sponsoring Units: DMPChair: Vladimir Shalaev, Purdue University
Room: Morial Convention Center 220
Thursday, March 13, 2008 11:15AM - 11:51AM |
V28.00001: Plasmonic Metamaterials and Devices Invited Speaker: |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V28.00002: Backward-wave optical parametric amplification and mirrorless oscillations in negative-index materials Alexander Popov, Sergey Myslivets, Vladimir Shalaev Extraordinary properties of mirrorless backward-wave OPO in nanostructured crystal have been recently demonstrated in [1]. Phase matching of backward waves is inherent to negative-index materials (NIMs). Extraordinary, distributed-feedback, properties of OPA and possibility of OPO in NIMs were predicted in [2,3]. Herewith, we show the feasibilities and explore the features of the mirrorless OPO and of the generation of counterpropagating left-handed signal and right-handed idler photons in NIMs. Two different options are investigated. One is OPO based on intrinsic quadratic nonlinearity of the NIM. Another option is four-wave mixing OPO based on separately engineered strong cubic nonlinearity through resonant nonlinear impurities. It is shown that in the latter case the OPO properties can be tailored by quantum control. [1] C. Canalias and V. Pasiskevicius, \textit{Nat. Photonics} \textbf{1}, 459 (2007) [2] A. K. Popov and V. M. Shalaev, Opt. Lett. \textbf{31}, 2169 (2006). [3] A. K. Popov, S. A. Myslivets, T. F. George and V. M. Shalaev, Opt. Lett. \textbf{32}, 3044 (2007) [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V28.00003: Tunable photonic crystals with nonlinear composite materials Kin Wah Yu, J. P. Huang, G. Wang Photonic crystals (PCs) are periodic dielectric structures that are designed to control the flow of electromagnetic (EM) waves. The main attraction of PCs is the existence of photonic band gaps, for frequencies within which the propagation of EM waves can be forbidden leading to many promising applications in the areas of computing and communication for their advantages over electronics. In this work, we report a novel class of tunable photonic crystals consisting of multilayers of noble-metal nanoparticles in dielectric composites with nonlinear responses [1]. For such PCs, precise tunability of photonic band gaps can be achieved by choosing appropriate pump AC or DC electric fields [2]. Moreover, we study the dynamics of Bloch oscillation in such PCs so as to realize terahertz radiation which is relevant in medical physics. \newline \newline [1] J. P. Huang, K. W. Yu, Phys. Rep. {\bf 431}, 87 (2006). \newline [2] G. Wang, J. P. Huang, K. W. Yu, Appl. Phys. Lett. {\bf 91}, 191117 (2007). [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V28.00004: Enhanced and Resonant Transmission of Light through Dielectric-filled Subwavelength Waveguides Huizhong Xu, Pangshun Zhu, Harold G. Craighead, Watt W. Webb We analyze transmission of light through dielectric-filled subwavelength waveguides in a metal using both analytical and numerical methods. Our analysis revealed that a propagating mode can in principle exist in a waveguide of arbitrarily small size when a certain relationship between the dielectric constants of the metallic cladding and filling material is satisfied. Transmission through a subwavelength aperture of finite length can be further enhanced when the length is such that Fabry-P\'{e}rot-like resonances are excited. Strong, localized near-field intensity at the exit can be achieved with this mechanism for aperture diameters down to $\sim \lambda $/20. We will describe potential applications of this resonantly enhanced transmission phenomenon in near-field scanning optical microscopy and single-molecule spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V28.00005: Transmission of images with subwavelength resolution to distances of several wavelengths in microwave, terahertz and infrared ranges Pavel Belov, Mario Silveirinha, Pekka Ikonen, Constantin Simovski, Sergei Tretyakov, Yan Zhao, Yang Hao, Clive Parini The resolution of conventional imaging systems is restricted by the diffraction limit: the details smaller than half-wavelength of radiation cannot be resolved. Using novel engineered media with extreme optical anisotropy and waveguiding properties it is possible to overcome the classical limit and create devices capable of transmitting images with subwavelength resolution over long distances. We report experimental results that demonstrate transmission of a microwave image by means of an array of parallel metallic rods over a distance 3.5 times greater than the wavelength. The resolution of such imaging device is 15 times less than the wavelength. The magnifying, demagnifying and repeating properties of lenses formed by long metallic rods provide a unique solution for subwavelength imaging at microwave and terahertz ranges. At microwaves, the resolution of such lenses is mainly determined by the characteristic period, which is limited only by the fabrication capability rather than by any physical constraints. At higher frequencies, the resolution is mainly limited by the skin-depth of the rods material. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V28.00006: All Optical Method for Positioning Single Quantum Dots in Photonic Crystal Nanocavities Susanna Thon, Matthew Rakher, Jan Gudat, Hyochul Kim, William Irvine, Dirk Bouwmeester, Nick Stoltz, Pierre Petroff Single self-assembled InAs quantum dots embedded in GaAs photonic crystal (PC) defect cavities are a promising system for cavity quantum electrodynamics experiments. Achieving controllable coupling between the PC cavity mode and quantum dot emission is difficult, however, due to the random nucleation locations and spectral properties of individual quantum dots. We have developed a novel, all optical scheme for locating single dots relative to prefabricated markers on the sample surface with sub-10 nm accuracy which should allow us to custom fabricate PC cavities tuned to the exact position and frequency of the quantum dots. Initial experimental results are presented. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V28.00007: Effect of Shape Parameters on NanoOptronic Circuit Element Optical Response Timothy Corrigan, Dominic Britti, Paul Kolb, Andrie Sushkov, Dennis Drew, Shyhuah Guo, Raymond Phaneuf We examine the effect on the visible-near IR response of a number geometrical parameters of optical circuit elements with potential applications for guiding light for communications and for use in other metamaterial/optoelectronics applications. Rounding of edges produces a shift in the resonance frequency transmission measurements and in numerical simulations. We also examine the effect of shape parameters on the response of u-shaped and single split-ring Ag structures and discuss the LC circuit model in describing their optical response. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V28.00008: Photon-photon correlations from a resonantly driven quantum dot in a microcavity Edward Flagg, Andreas Muller, John Robertson, Thai Tran, Dennis Deppe, Wenquan Ma, Jaiyu Zhang, Gregory Salamo, Chih-Kang Shih We demonstrate strongly driven resonance fluorescence from a single~InGaAs quantum dot in a planar microcavity by measuring the oscillatory second-order correlation function, $g^{(2)}(t)$, of the photoluminescence. Resonance fluorescence is emission from a coherently and resonantly~excited two-level quantum system and under strong CW excitation the system undergoes one or more Rabi oscillations before emitting.~ These oscillations are observed in $g^{(2)}(t)$ rather than the simple anti-bunching dip caused by incoherent excitation. This behavior shows, along with other measurements, that the quantum dot is~well-described by a simple two-level model even at high excitation intensities.~ The dot is resonantly excited with a laser via the waveguide mode of the microcavity and the emission couples into the Fabry-Perot mode where it is collected. The~ability to perform coherent manipulations on a single quantum emitter is a critical step on the~road to many quantum optical devices including high efficiency indistinguishable single photon sources. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V28.00009: Pitfalls of nano-spectroscopy Andrew Walsh, Wolfgang Bacsa, A. Nickolas Vamivakas, Anna Swan We demonstrate the unique and inherent linkage between spatial location and spectral signature caused by an individual nano-scale emitter. We show false spectral shifts and artificial line width broadening which we contrast with results from extended emitters. The findings are general for spectroscopy of any nano-scale 0D or 1D object, such as single fluorescent molecules, quantum dots or nano-rods, but here we demonstrate these effects using Raman spectroscopy of individual carbon nanotubes suspended across gaps tens of microns wide. We explain the origin of these effects and discuss how proper spectrometer alignment and selection of spectrometer parameters is critical in order to avoid misinterpretation of the spectral data. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V28.00010: Wavefront Analysis as a Predictive Tool for Polymer/Liquid Crystal Composites with Nanostructures Jared Coyle, Sameet Shriyan, Adam Fontecchio Nano-scale morphology is a key factor in determining the optical performance of holographically-formed polymer dispersed liquid crystal (HPDLC) thin films. The liquid crystal(LC)/polymer interface and droplet structure in these materials effects optical performance. Established microscopy techniques such as SEM and TEM are conventionally used to examine interfacial properties. In this paper, HPDLC reflective and transmissive wavefront analysis are used to examine gratings created using three different polymer formulations: acrylate, thiol-ene and a combination of acrylate and thiol-ene. Changes in the modulation transfer function, Strehl ratio and wavefront phase of both transmissive and reflective wavefronts are compared to morphological properties shown in SEM images. Wavefront properties were measured using a Shack Hartman wavefront analyzer. Optical performance characteristics were measured using an Ocean Optics spectrometer, halogen light source and oscilloscope. [Preview Abstract] |
Session V29: Focus Session: Carbon Nanotubes and Related Materials XIII: Synthesis
Sponsoring Units: DMPChair: Vasili Perebeinos, IBM
Room: Morial Convention Center 221
Thursday, March 13, 2008 11:15AM - 11:27AM |
V29.00001: High-Resolution Nanofabrication Using a Highly-Focused Electron Beam Thomas Aref, Mikas Remeika, Matthew Brenner, Alexey Bezryadin Carbon nanotubes and metallic nanowires have unusual and potentially useful electrical transport properties. Local control of the parameters of such nano-objects would open even wider possibilities for their applications. We have used the highly focused, high-energy electron beam of a transmission electron microscope to locally modify such nano-objects. In particular, it was possible to drill 2.5 nm diameter nanoholes in multiwall carbon nanotubes (MWNT's). Similarly sized holes were etched through metallic nanowires. We have also fabricated larger nanoholes, as large as 11 nm wide in a 26 nm diameter MWNT, as well as constrictions in MWNT's. Transport measurements of such nanodevices is our future goal. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V29.00002: Optical Patterning of Three-Dimensional Carbon Nanotube Microstructures Wei-Husuan Hung, Rajay Kumar, Adam Bushmaker, Michael J. Bronikowski, Stephen B. Cronin We present an optical, non-contact method for patterning three-dimensional carbon nanotube microstructures. In this method, a 1$\mu $m diameter focused laser spot is used to burn patterns in dense arrays of vertically grown multiwalled carbon nanotubes. The threshold for laser burnout and the depth of burnout are determined by Raman spectroscopy and scanning electron microscopy. Using a high precision translation stage to control the position of the laser spot on the sample, we create several 3D patterns to illustrate this method's potential use for the rapid prototyping of carbon nanotube microstructures [1]. After laser surface treatment, we observe undercut profiles, changes in nanotube density, and nanoparticle formation, which provide insight into the unique evolution of the nanotube microstructures during the burnout process. This non-lithographic method provides new opportunities for chemically sensitive~applications of nanotubes and expands their possible applications into new areas. \newline [1] Hung, Wei Hsuan, Kumar, Rajay, Bushmaker, Adam, Cronin, Stephen B., and Bronikowski, Michael J. Rapid prototyping of three-dimensional microstructures from multiwalled carbon nanotubes. \textit{Applied Physics Letters} \textbf{91}, 093121 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V29.00003: A Real Time Detection System for Dielectrophoretic Deposition of Carbon Nanotubes Sebastian Sorgenfrei, Inanc Meric, Austin Akey, Sarbajit Banerjee, Sami Rosenblatt, Irving P. Herman, Kenneth Shepard Single-walled carbon nanotubes (SWCNTs) have showed considerable potential as building blocks for electronics and sensors but are very difficult to integrate and assemble into larger systems. Dielectrophoretic deposition allows the large-scale positioning and alignment of SWCNTs but requires precise control to reproducibly generate single-tube devices. We investigate dielectrophoretic deposition of SWCNTs using an \textit{in situ} detection system. This apparatus locks into both a small AC signal and the large, mixed-down dielectrophoretic signal, generated by the nonlinearities of the device, making it possible to halt deposition once a nanotube has made electrical contact. This results in a higher yield of single SWCNTs deposited between two electrodes. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V29.00004: Irradiation-induced phenomena in carbon nanomaterials Invited Speaker: The irradiation of solids with energetic particles such as electrons or ions is associated with disorder, normally an undesirable phenomenon. However, recent experiments [for an overview, see A.V Krasheninnikov, F. Banhart, Nature Materials, 6 (2007) 723] on bombardment of carbon nanostructures with energetic particles demonstrate that irradiation can have beneficial effects and that electron or ion beams may serve as tools to change the morphology and tailor mechanical, electronic and even magnetic properties of nanostructured carbon systems. We systematically study irradiation effects in carbon nanotubes and other forms of nano-structured carbon experimentally and theoretically by employing various atomistic models ranging from empirical potentials to time-dependent density functional theory. In my presentation, I will briefly review the recent progress in our understanding of ion-irradiation-induced phenomena in nano-structured carbon and present our recent theoretical [A.V Krasheninnikov, et al., Phys. Rev. Lett., 99 (2007) 016104, A. Tolvanen et al, Appl. Phys. Lett. 91 (2007) 173109.] and experimental [O. Lehtinen et al., to be published] results. I dwell on the ``beneficial'' role of defects and impurities in nanotubes and related systems. Finally, I will present the results of simulations of irradiation-induced pressure build-up inside nanotubes encapsulated with metals [L. Sun, et al., Science 312 (2006) 1199]. Electron irradiation of such composite systems in the transmission electron microscope gives rise to contraction of nanotube shells and thus to high pressure. The irradiation-stimulated pressure can be as high as 40 GPa, which makes it possible to study phase transformations at the nanoscale with high spatial resolution. I will also address the mechanisms of plastic deformation of small metal particles inside carbon shells at high temperatures, which may be important for understanding catalytic growth of carbon nanotubes. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V29.00005: Syntheses and characterization of SWCNT assemblies prepared on silicon substrates with different methods of patterning catalyst particles Irma Kuljanishvili, Venkat Chandrasekhar, Owen Loh, Dmitriy Dikin, Horacio Espinosa, Richard Piner , Rodney S. Ruoff Techniques for controlled way of preparation of single wall carbon nanotubes (SWCTN) on substrates continue to be of interest, including for potential applications in integrated circuits and nanodevices. We report our work, undertaken to elucidate a number of favorable conditions for controlled patterning and growth of high quality SWCNT. Synthesis is carried out under ambient pressure with methane used as feed gas. A key factor for successful SWCNT growth is known to be the catalytic precursor. We compare several methods of controlled deposition of the catalyst precursors on oxidized silicon substrates using methods such as e-beam lithography, photolithography and scanning probe writing methods that allow for maximum flexibility and high efficiency for incorporating SWCNTs into devices or circuit architectures. The advantages and some limitations of these methods of selective patterning will also be addressed. Analysis and characterization of the as-grown SWCNTs was performed by Raman Spectroscopy, AFM and SEM. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V29.00006: \textit{In-situ} calorimetric studies of SWCNT growth Oleg Kuznetsov, Toshio Tokune, Elena Mora, Gugang Chen, Avetik Harutyunyan Single-walled carbon nanotubes (SWCNTs) were grown inside of a differential scanning calorimetry (DSC) apparatus with an attached mass spectrometer (MS), using different hydrocarbons (CH$_{4}$ and C$_{2}$H$_{4})$ and alumina supported (Fe, Fe/Mo, and Ni) catalysts. This set-up allowed to \textit{in situ} follow the evolution of calorimetric, thermogravimetric and MS data during the synthesis. A Raman spectrometer (with laser excitations wavelengths 532 and 785 nm) was used for verification of the growth of SWCNTs. DSC studies at temperatures $\sim $650-900 \r{ }C of the interaction between the hydrocarbons and the preliminary reduced alumina supported catalysts showed a release (C$_{2}$H$_{4})$ or absorption (CH$_{4})$ of heat depending on the type of hydrocarbon used. The effect of this energy on the growth of SWCNTs was studied. We found that the incubation time for nanotube nucleation depends on the hydrocarbon type and flow rate, as well as on the synthesis temperature. The origin of the initial endothermic peak observed during nanotube growth with both hydrocarbon sources will be discussed. Furthermore, the kinetics and thermodynamic of hydrocarbon decomposition, carbon atoms diffusion and solid carbon structure formation dependence on the catalyst and synthesis parameters will also be presented. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V29.00007: The role of carbon solubility in Fe-C nano-clusters on the growth of small single-walled carbon nanotubes Stefano Curtarolo, Neha Awasthy, Wahyu Setyawan, Elena Mora, Toshio Tokune, Kim Bolton, Avetik Harutyunyan Various diameters of alumina-supported Fe catalysts are used to grow single-walled carbon nanotubes (SWCNTs) with chemical vapor decomposition. We find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function particles size, there are three scenarios compatible with steady state-, limited- and no-growth of SWCNTs, corresponding to unaffected, reduced and no solubility of C in the particles. The result raises previously unknown concerns about the growth feasibility of small and very-long SWCNTs within the current Fe CVD technology, and suggests new strategies in the search of better catalysts. Research supported by Honda R.I. and NSF. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V29.00008: Air assisted growth of long aligned carbon nanotube films Xianfeng Zhang, Rakesh Shah, Saikat Talapatra We report on air assisted growth of ultra long aligned bundles of multiwall carbon nanotubes. We found that the growth rate of carbon nanotubes is highly enhanced by introducing a small mount of oxygen during the catalytic decomposition of ferrocene-xylene mixture at 790$^{\circ}$C. Millimeter long aligned carbon nanotube films were easily synthesized on silicon dioxide as well as metal substrates by controlling the air flow. Electron microscopy investigations reveal that the films are composed of dense aligned multi-wall CNTs with the diameters ranging from about 30-100 nm. We will also present our preliminary results on the electrical transport measurement performed on these long nanotube bundles. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V29.00009: Boron-Doped Carbon Nanotube Films Xiao Ming Liu, H.E. Romero, H.R. Gutierrez, P.C. Eklund Here we report room temperature optical and resistivity studies on transparent thin films of bundled single-walled carbon nanotubes exposed to B$_{2}$O$_{3}$ at 1000$^{o}$C. This reaction is proposed to B-dope the films. They are stable in air. At 300K the four-probe sheet resistance and the optical transmission in the NIR-UV range are used to evaluate the effects of this chemical exposure. Our preliminary results show that for films with a visible optical transmittance around 80{\%} (550nm), the sheet resistance in the pristine film is lowered from $\sim $2K$\Omega $ to $\sim $300$\Omega $ via B$_{2}$O$_{3}$ exposure, a factor of five decrease. We find that the magnitude of the decrease in the sheet resistance increases in samples with higher transmission. Our results suggest that boron-doped SWNT may provide a better approach to touch-screen technology, as well as for transparent contacts in solar cells. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V29.00010: \textit{In situ} Transient Growth Kinetics of Vertically-Aligned Carbon Nanotube Arrays Jeremy Jackson, Alex Puretzky, Gyula Eres, Christopher Rouleau, Hui Hu, Bin Zhao, David Geohegan Here, transient growth kinetics are induced during individual VANTA synthesis experiments in order to understand how changes in total pressure and hydrocarbon partial pressure affect subsequent growth kinetics and wall number of nanotubes within the same array under start/stop and pulsed growth delivery. Transient interruptions or changes in hydrocarbon flow are revealed by rapid changes in slope and frequency of the oscillating, exponentially-decaying TRR signal. The associated regions of the nanotube array reveal kinked, band-like patterns along the width of the array as observed in cross-sectional scanning electron microscope (SEM) images. These bands serve as `growth-markers' to measure length intervals and calibrate growth rates before, during, and after transient perturbations to continuous growth. In addition, extended growth interruptions are explored to understand catalyst poisoning mechanisms. Finally, growth of size-selected, multilayered VANTAs was performed to investigate the interfaces between different growth regions by HRTEM [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V29.00011: Chirality-resolved kinetic analysis of single-walled carbon nanotube growth by \textit{in-situ} Raman spectroscopy Takashi Uchida, Masaya Tazawa, Hiroshi Sakai, Akira Yamazaki, Yoshihiro Kobayashi We investigate the chirality-resolved growth kinetics of single-walled carbon nanotubes (SWCNTs) by \textit{in-situ} Raman spectroscopy. The SWCNTs are synthesized by ethanol CVD from Co nanoparticle catalysts with pre-defined size before the CVD process. The chirality-sensitive radial breathing mode (RBM) signals in Raman spectra are observed during the CVD process at 80-120 Pa. We have reasonably assigned the chiral indices of the RBM signals observed at higher temperature during the CVD process by taking into account the temperature dependence of the resonance condition of SWCNTs. The growth kinetics analyzed from the time evolution of each RBM signal in \textit{in-situ} spectra reveals that the nanotube nucleation occurs just after the supply of the carbon source gas and does not significantly depend on the growth pressure and chirality. In addition, we have found that the growth duration depends on the growth pressure and chirality and that the graphitic encapsulation of catalyst particles terminates SWCNT growth. These findings make it possible to clarify the chirality-sensitive growth behavior of SWCNTs. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V29.00012: DC Anhydrous Electrodeposition of Carbon Nanotubes Masahito Sano, Takanori Matsumoto Electrodeposition is a versatile technique to fabricate carbon nanotube films on conducting substrates. Due to different responses of metallic and semiconducting nanotubes against electric field in solution, it can be used to discriminate nanotubes based on these types. Depending on whether the applied field is ac or dc, nanotubes are moved across electrodes by either dielectrophoresis or electrophoresis. In ordinary dc electrodeposition, water has been used as a solvent. We have found that, using organic solvents from which water is carefully removed, low dc fields electrodeposit nanotubes quite efficiently [1]. A millimeter thick film can be obtained within a minute with 15V/cm. Furthermore, the process is highly selective; the films are adhered so strongly that it requires scratching the substrate to remove, all nanotubes form thin straight bundles that lie parallel to the substrate, the films show no metallic Raman peaks and have 4 order of magnitude higher resistivity than the original sample [2]. A recent study shows that the electric field is not responsible for adhesion. Other than the first layer that is directly on the substrate surface, the van der Waals force dominates adhesion of nanotubes. [1] Y. Abe, R. Tomuro, M. Sano, Adv. Mater. 17, 2192 (2005). [2] R. Tomuro, T. Matsumoto, M. Sano, Jpn. J. Appl. Phys. 45, L578 (2006). [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V29.00013: Understanding the Growth of Carbon Nanotubes by Catalyst-Assisted Chemical Vapor Deposition Elif Ertekin, Jeffrey Grossman In catalyst-assisted chemical vapor deposition, carbon nanotubes are formed when a curved graphene island lifts off the surface of the catalyst particle on which it is growing. While this growth technique offers effective control over patterning and alignment, control over nanotube radius and chirality is ultimately tied to understanding the point at which lift-off occurs. We use atomistic approaches to model the lift-off process via the interplay between the excess energy required to grow a curved (and thus, necessarily defected) graphene island and the interaction energy between the growing island and the underlying catalyst. The atomistic approach combines Monte Carlo methods with \textit{ab initio} total energy electronic structure methods to explore island formation, growth, and lift-off on a catalyst surface. Using this approach, we are able to systematically study the effect of incident atomic flux rate, growth temperature, and catalyst curvature. The different defect topologies in the growing graphene cap that result from different growth conditions are a key parameter in determining the chirality of the nanotubes. [Preview Abstract] |
Session V30: Nanotubes and Nanowires II: Other Properties I
Sponsoring Units: DMPChair: Tony Heinz, Columbia University
Room: Morial Convention Center 222
Thursday, March 13, 2008 11:15AM - 11:27AM |
V30.00001: Reversible Charge Induced Separation Between Singlewalled Carbon Nanotubes Sangeeta Sahoo, Ravi Maranganti, Sarah Lastella, Govind Mallick, Shashi Karna, Pradeep Sharma, Pulickel M. Ajayan We report the observation of local separation between single-walled carbon nanotubes in a bundle using low-energy electron-beam (e-beam) irradiation in scanning electron microscope. The effect of the separation is shown to impact the electrical characteristics of small nanotube bundle devices. By semi-analytical calculation we show that the Coulomb repulsive force due to the electrostatic charging can be stronger than the attractive van der Walls force in order to initiate the separation between the nanotubes in a bundle. In addition, the separated nanotubes are observed to return back to their original packed state on removal of electron exposure. We discuss this reversibility of the separation process in the light of thermal fluctuation and discharging of nanotubes at room temperature in the presence of air. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V30.00002: Self-Assembly of Multiwall Carbon Nanotubes from Quench-Condensed CNi$_{3}$ Films Philip Adams, David Young, Amar Karki, Jayne Garno, Johnpeter Ngunjiri Freestanding, vertical, multiwall carbon nanotubes (MWCNT) are formed during the vacuum deposition of thin films of the metastable carbides CT$_{3}$ (T = Ni, Co) onto fire-polished glass substrates. In contrast to widely used chemical and laser vapor deposition techniques, we utilize direct e-beam evaporation of arc-melted CT$_{3}$ targets to produce MWCN's that are self-assembled out of the CT$_{3}$-film matrix. The depositions are made in an ambient vapor pressure that is at least six orders of magnitude lower than the 1-100 Torr typically used in chemical vapor techniques. Furthermore the substrates need not be heated, and, in fact, we observe robust nanotube growth on liquid nitrogen cooled glass and sapphire substrates. High-resolution atomic force microscopy reveals that MWCNT's of heights 1-40 nm are formed in films with nominal thicknesses in the range of 5-60 nm. We show that the growth parameters of the nanotubes are very sensitive to the grain structure of the films. This is consistent with a precipitation mediated root-growth mechanism. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V30.00003: Self-assembly of Carbon Nanotube Based Devices on Programmable DNA Crystals Hareem Maune, Si-Ping Han, Robert Barish, Marc Bockrath, William Goddard, Erik Winfree The greatest challenges of nanoscience today include the ability to interface the new breed of versatile nanomaterial with the conventional electronics and the ability to arrange multiple nanocomponents into arbitrary pre-designed geometries with high density and nanoscale precision. These factors not only limit the feasibility of using the alternative nanostuctures for technological advances, but also hinder the elucidation of the structures' intrinsic properties. We will present a unique process for self-assembling DNA dispersed single-walled carbon nanotubes (SWNTs) into precise geometric arrangements with orientation control and demonstrate its implementation by self-assembling an all SWNT field effect transistor (FET). Not only does the precision, low cost, reproducibility and parallelism of our new process create novel opportunities for the implementation and investigation of complex nanosystems, but it also enables us to gain insight into the dynamics of molecular interactions between nanoscale objects of mega-daltons size. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V30.00004: Diameter-dependent conductance oscillations in carbon nanotubes upon torsion. Nagapriya K.S., Tzahi Cohen-Karni, Lior Segev, Onit Srur-Lavi, Sidney Cohen, Ernesto Joselevich Torsion-induced conductance oscillations have been recently observed in multi-wall carbon nanotubes$^{1,2}$. These oscillations have been interpreted as metal-semiconductor periodic transitions, while an alternative interpretation attributed the phenomenon to changes in registry between the walls. Here we show$^{3}$ that the period of the oscillations is inversely proportional to the squared diameter of the nanotube (\textit{$\delta \phi \sim $1/d}$^{2})$. This dependence is theoretically predicted from the shifting of the corners of the first Brillouin zone of graphene across different subbands allowed in the nanotube, whereas a change in registry should give rise to a simple inverse dependence (\textit{$\delta \phi \sim $1/d}). Hence, the experimental results validate the interpretation of Fermi level shift across subbands \textit{vs}. that of registry change, as a source of torsion-induced conductance oscillations in carbon nanotubes. \newline \newline [1] T. Cohen-Karni \textit{et al}, Nature Nanotech. \textbf{1}, 36 (2006). \newline [2] E. Joselevich, ChemPhysChem \textbf{7}, 1405 (2006). \newline [3] K. S. Nagapriya \textit{et al}, \textit{in preparation}. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V30.00005: Nanomechanical properties of carbon nanotubes determined using a scanning laser vibrometer Laura Biedermann, Ryan Tung, Arvind Raman, Ronald Reifenberger To better understand the nanomechanical properties of nanotubes and nanowires, reliable nondestructive techniques that measure their Young's modulus, E, under ambient conditions are needed. Using a scanning laser vibrometer, the thermally excited eigenfrequencies of plasma-enhanced carbon vapor deposition (PECVD) multiwalled carbon nanotubes (MWNTs) were measured. Due to the small diameters involved, little light is reflected from a bare MWNT. By carefully attaching a small Au-coated glass bead, the intensity of reflected light is sufficiently increased to allow accurate measurements. The length and diameters of the MWNTs are determined using electron microscopy, allowing E to be inferred from an Euler-Bernoulli analysis of a pinned cantilever beam. A unique aspect of our work is that the attached glass bead exerts a torque on the MWNT. A resonance, attributed to a torsional oscillation, appears for the laden MWNTs, allowing an estimate for the torsional modulus G. Values measured for E and G, along with a description of the experimental procedure, will be presented at the talk. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V30.00006: Mechanical Sensing with Flexible Metallic Nanowires Vladimir Dobrokhotov, Mehdi Yazdanpanah, Santosh Pabba, Abdelilah Safir, Robert Cohn A calibrated method of force sensing is demonstrated in which the buckled shape of a long flexible metallic nanowire is interpreted to determine the applied force. Using a nanomanipulator the nanowire is buckled in the chamber of a scanning electron microscope (SEM) and the buckled shapes are recorded in SEM images. Force is determined as a function of deflection for an assumed elastic modulus by fitting the shapes using the generalized elastica model. In this calibration the elastic modulus was determined using an auxiliary AFM measurement, with the needle in the same orientation as in the SEM. Following this calibration the needle was used as a sensor in a different orientation than the AFM coordinates to deflect a suspended PLLA polymer fiber from which the elastic modulus (2.96 GPa) was determined. In this study the same needle remained rigidly secured to the AFM cantilever throughout the entire SEM/AFM calibration procedure and the characterization of the nanofiber. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V30.00007: c-axis GaN nanowires for high-quality-factor mechanical oscillators Jason Gray, Kris Bertness, Norman Sanford, Charles Rogers We report on the electromechanical properties of c-axis GaN nanowires in high-quality-factor mechanical resonators and oscillators. The nanowires are grown by catalyst-free molecular beam epitaxy, are single crystal, hexagonal in cross section, from 50 - 500 nm diameter, and 5 - 15 microns in length depending upon growth time. As-grown nanowires display singly-clamped cantilever mechanical resonances above 1 MHz, with typical resonance full width at half maximum power of less than 100 Hz i.e., a mechanical quality factor, Q, well above 10$^{4}$. We are attempting to obtain similar high-Q for processed nanowires, utilizing dielectrophoresis to position the nanowires within lithographic test structures, and nanowire metallization of two types: First, titanium/aluminum over the ends of the nanowires allows for ohmic contact formation and direct measurement of nanowire resistance versus strain. Second, a layer of aluminum over the entire length of the wires leads to a metallic backbone with a resistance on the order of 50 $\Omega $. This is useful for simple magnetomotive measurements. We will discuss the processing steps and observed behavior. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V30.00008: Self-detecting mechanical resonators made from suspended carbon nanotubes Benoit Witkamp, Menno Poot, Andreas K. H\"uttel, Herre S.J. van der Zant We study the flexural and torsional mechanical properties of suspended carbon nanotubes. We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. Using a continuum model fit to the measurements, we show that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension. We are currently investigating the low-temperature properties of these devices. We are also investigating the torsional mechanical properties of nanotubes. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V30.00009: Characterization and wear resistance of carbon nanotube-based tips for AFM local anodic oxidation nanolithography Lishan Weng, Mark Strus, Arvind Raman, Leonid Rokhinson The AFM local anodic oxidation (LAO) lithography is a powerful scanning-probe-based patterning technique for fabrication and post-fabrication tuning of nanoscale structures and devices. The conventional tips suffer from rapid wear which degrades the quality of imaging and LAO lithography. Much research has been devoted to seek for wear-resistant AFM tips, and carbon nanotubes (CNT) are viable candidates. Apart from featuring small diameter, high aspect ratio, and mechanical flexibility, the CNT tips has been shown to last longer during LAO. We investigate the wear of CNT-based tips during LAO lithography by imaging the tips before and after the lithography using scanning electron microscope. CNTs show small contamination on the tip apex while preserve their original length and diameter after more than 200 micron of the lithography. We also analyze tip-surface interaction in order to optimize the quality of lithography and correlate energy dissipation during tapping mode to the line width and line thickness during LAO. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V30.00010: Ultrahigh Vacuum Scanning Tunneling Microscopy and Spectroscopy of Single-Walled Carbon Nanotubes Interfaced with Silicon Surfaces Peter Albrecht, Joseph Lyding The UHV-STM was used to examine SWNTs directly interfaced with hydrogen-passivated Si(100). Dry contact transfer (DCT) [1] enabled the UHV deposition of SWNTs with minimal disruption of the atomically flat Si(100)-2x1:H surface. Isolated, rather than bundled, SWNTs could be routinely located for atomically resolved imaging, tunneling I-V spectroscopy [2], lateral manipulation [3], and proximal substrate modification. Weakly adsorbed SWNTs initially unstable in the presence of the rastered STM tip could be stabilized by depassivating the underlying H-Si(100) surface via UHV-STM electron-stimulated H desorption [4], which in the case of one chiral semiconducting SWNT also promoted the local alignment of the zigzag symmetry direction on the underside of the tube with the clean Si dimer rows [5]. The growing body of first-principles simulations of the SWNT/Si(100) system [6] was drawn upon in our interpretation of such local perturbations. [1] APL 83, 5029 (2003). [2] Nanotechnology 18, 095204 (2007). [3] Small 3, 146 (2007). [4] Nanotechnology 18, 125302 (2007). [5] Small 3, 1402 (2007). [6] JAP 100, 124304 (2006). [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V30.00011: Cavity-controlled, electrically-induced infrared emission from a single single-wall carbon nanotube Fengnian Xia, Mathias Steiner, Yu-ming Lin, Phaedon Avouris Single-wall carbon nanotube (SWCNT) is attracting significant attention from the photonics community recently due to its unique optical properties and potential applications in optical communications, optical interconnects, and, in particular, nanophotonics. Tunable and truly nanometer-scale (diameters of 0.5 to 2nm) light emitters can possibly be realized using individual semiconducting nanotubes. However, optical cavities are needed to control the emission wavelength, direction, and emission rate of these broadband, nanoscale emitters. Here, we report the monolithic integration of an electrically-excited light emitter based on a single (SWCNT) with a planar photonic $\lambda $/2-cavity. Emission properties were dominated by the cavity characteristics almost regardless of the free-space emission. The broad, free-space emission spectrum of a single SWCNT with a FWHM of 300 to 500nm was transformed to cavity controlled emission with a FWHM of $\sim $40nm. We also modeled the device as a broadband emitter in a planar cavity. Good agreement between the simulated and experimental results was achieved. In our configuration, the maximum enhancement in emission rate is estimated to be 4.4. We consider this result to be an important first step in the development of truly nanometer-scale photonic devices based on SWCNTs. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V30.00012: Fluorination and Defluorination of Double-wall Carbon Nanotubes JungHo Kang, Merlyn Pulikkathara, Valery Khabashesku, Kevin Kelly Due to the unique physical structure double-wall carbon nanotubes (DWNTs), the outer tube can be chemically functionalized while the inner tube is left in pristine condition. Using a fluorinating agent, DWNTs are fluorinated and X-ray photoelectron spectroscopy data indicates that the resulting product composition is equivalent to C3F. The diameter of bare DWNTs is around 2-3 nm as measured by scanning tunneling microscopy, but fluorinated DWNTs possess much larger diameters in a range from 3-10 nm due to a stronger electronic interaction. In addition to imaging the as prepared material, the material was imaged after annealing at temperatures up to 650 K. Due to defluorination, the diameter is decreased down to that of the initial bare DWNTs and atomic resolution of the lattice was recovered. In addition, it was possible to observe that the initial and final structures on the same nanotubes and the evolution of their associated defect structures. Lastly, Raman spectroscopy was employed to confirm the defluoration by revealing the recovery of the RBM peak which disappeared upon fluorination. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V30.00013: Magnetic studies of multi-walled carbon nanotube mats: Ultra-high temperature ferromagnetism or superconductivity? Pieder Beeli, Guo-meng Zhao We report magnetic measurements up to 1200 K on multi-walled carbon nanotube mats using a Quantum Design vibrating sample magnetometer. Extensive magnetic data consistently show two ferrromagnetic-like transitions at about 1000 K and 1275 K, respectively. The lower transition at about 1000 K is associated with an Fe impurity and its saturation magnetization is in quantitative agreement with the Fe concentration measured from an inductively coupled plasma mass spectrometer. On the other hand, the saturation magnetization for the higher transition phase corresponds to about 0.6$\%$ Co impurity concentration, which is about four orders of magnitude larger than that measured from the mass spectrometer. We show that this transition at about 1275 K is not consistent with ferromagnetism of any carbon-based phases or magnetic impurities but with the paramagnetic Meissner effect due to the existence of $\pi$ Josephson junctions in a granular superconductor. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V30.00014: A quadruply twinned core for the growth of nanotetrapods and related structures. S.K. Hark, Z. Liu Nanotetrapods and related complex nanoarchitectures, such as multi-armed and tricrystal structures, are key functional elements and interconnections in future ``bottom-up'' approach to nanotechnology. The growth of these special nanostructures is believed to proceed from a core, . several models of have been suggested. However, the occurrence of some tetrapod related nanostructures, observed by us and also reported by others, can not all be explained by these models. We have obtained aligned ZnCdSe tetrapod related nanostructures using metalorganic chemical vapor deposition on GaAs substrates. Based on high resolution transmission electron microscopy and crystallographic analyses, we propose a new quadruply-twinned model for their core, from which nanotetrapods, nanoswords and related nanoarchitectures can grow. In this model, the core contains four wurtzite structured heptahedrons connected by six {\{}0-113{\}} twins. Different from the other existing models, no polarity of surface is needed to explain the growth of the branching arms of the nanotetrapods [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V30.00015: Individual carbon nanotubes on trench vs. substrate: A Raman study Mathias Steiner, Marcus Freitag, James Tsang, Ageeth Bol, Phaedon Avouris Raman excitation spectroscopy is a powerful tool for characterizing the electronic and phononic structure of single-wall carbon nanotubes (SWNTs). The transitions associated with Raman-active phonon modes in SWNTs, e.g. the radial breathing mode (RBM), the defect-induced mode (D) and the tangential modes (G), carry unique information regarding both electron-phonon interactions in SWNTs and phonon-specific non-radiative coupling to their nano-environment. We study individual, spatially isolated SWNTs grown by chemical vapor deposition across micron-wide trenches etched in a silicon dioxide substrate. Using an optical microscope, we spatially address parts of the same SWNT that are either supported by the substrate or freely suspended across a trench. In a first step, we probe a specific electronic level of a SWNT by monitoring the intensities of different Raman bands, i. e. RBM, D and G, as a function of the laser excitation wavelength. In a second step, taking advantage of the resonance Raman condition, we demonstrate how the positions, widths and shapes of the spectral bands associated with individual Raman transitions of the same SWNT are modified if brought into contact with the substrate. We discuss the results in the context of performance limits of optoelectronic devices based on SWNT. [Preview Abstract] |
Session V31: Focus Session: Oxide Interfaces II
Sponsoring Units: DMP GMAGChair: Arunava Gupta, University of Alabama
Room: Morial Convention Center 223
Thursday, March 13, 2008 11:15AM - 11:51AM |
V31.00001: Field effect tuning of superconductivity at the LaAlO$_{3}$/SrTiO$_{3}$ interface Invited Speaker: At interfaces between complex oxides, electronic systems with unusual properties can be generated [1]. As reported first by Ohtomo and Hwang [2], a highly mobile electron gas is formed at the interface between LaAlO$_{3}$ and SrTiO$_{3}$, two insulating dielectric perovskite oxides. It will be shown that the ground state of this system is superconducting [3]. The superconducting critical temperature is about 200mK. The field effect allows the normal state and superconducting state properties to be spectacularly tuned. The characteristics of the observed superconducting transitions are consistent with a superconducting sheet as thin as a few nm. \newline [1] ``When oxides meet face to face''. E. Dagotto, Science \textbf{318}, 1076 (2007) \newline [2] ``A high mobility electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface''. A. Ohtomo, H. Y. Hwang, Nature \textbf{427}, 423 (2004). \newline [3] ``Superconducting interfaces between insulating oxides''. N. Reyren, \textit{et al.}, Science\textbf{ 317}, 1196 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V31.00002: Stabilization mechanisms at LaXO$_3$/SrTiO$_3$ (X=Ti, Al, V) Heterointerfaces R. Pentcheva, W. Pickett Multivalent transition metal ions that are being incorporated into oxide heterointerfaces offer more degrees of freedom to compensate the charge imbalance at the interface than is the case for conventional semiconductor interfaces. Density-functional theory calculations including a Hubbard U (LDA+U) have shown that a charge and orbitally ordered layer of Ti$^{3+}$ and Ti$^{4+}$ is formed at the interface (IF) between the Mott and the band insulator LaTiO$_3$ ($d^1$) and SrTiO$_3$ ($d^0$) [Phys. Rev. Lett. 99, 016802 (2007)] as well as between the two band insulators LaAlO$_3$ and SrTiO$_3$ [Phys. Rev. B. 74, 035112 (2006)]. Additional complexity relating to charge state and orbital occupation arises at the LaVO$_3$/SrTiO$_3$ interface between these compounds that are $d^2$ and $d^0$ respectively in the bulk: the electron-doped ...-TiO$_2$-LaO-... IF may promote a Ti$^{3+}$ charge state, the hole-doped ...-SrO-VO$_2$-... IF may encourage a V$^{4+}$ ion, or a metallic IF may result. We will present LDA+U predictions of charge states, orbital and spin order, and conducting behavior at these heterointerfaces, and contrast the results with the $d^1-d^0$ and $d^0-d^0$ interfaces mentioned above. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V31.00003: Charge accumulation in nonpolar perovskite quantum well sandwiched by polar Mott-insulating perovskites Cheng-Ching Wang, Sahu Bhagawan , Hongki Min, Wei-Cheng Lee, MacDonald Allan. H. We theoretically examine the possibility of having charge accumulation in the (LaTiO3)$_{n}$/Ba2VO4/(LaTiO3)$_{n}$ layered oxide quantum well system with polar barrier material and non-polar quantum well material using a LDA+U approach. The charge accumulation we find reflects electronic reconstruction which tends to occur near polar/nonpolar heterojunctions. We find enormous orbital reconstruction and both antiferromagnetic and ferromagnetic local in different planes. Lattice relaxation in the structure was allowed as a partial test of the robustness of LDA+U predictions for the properties of this system. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V31.00004: A First Principle Study of the LaAlO$_3$/SrTiO$_3$ Heterointerface Hanghui Chen, Sohrab Ismail-Beigi In order to understand the origin of the intriguing high mobility quasi two dimensional electron gas formed at the LaAlO$_3$/SrTiO$_3$(001) heterointerface, we carry out first principle calculations on the electronic structures and properties of complementary interface. The intrinsic polar properties are investigated and the average electronic potential increase by each LaAlO$_3$ layer is calculated, which can account for the recent observed experiment fact that the heterointerface is not metallic until the number of LaAlO$_3$ layers reaches a critical value. We also study the effects of different surface terminations of SrTiO$_3$ which surprisingly turn out to influence the electronic structure of the interface and so far have not been focused on in experiments. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V31.00005: Electrostatic doping in oxide heterostructures Jaekwang Lee, Alexander A. Demkov Recent experiments on perovskite heterostructures grown by molecular beam epitaxy or pulsed laser deposition suggest the possibility of creating high mobility two dimensional electron gas at the oxide/oxide interface. However, the origin of charge in these insulating materials is still not clear and deemed controversial. We report a first-principles study of SrTiO$_{3}$/LaAlO$_{3}$ heterostructures using density functional theory at the LDA+U level. We consider the energetics and electronic structure of the junction, while focusing on the role of electrostatics. Our results suggest that a complex balance of the crystal filed, Jahn-Teller effect, lattice dynamics and internal electric field result in the robust electrostatic doping for carefully chosen thickness of the polar oxide. We explore the possible uses of this effect in other oxide-based heterostructures. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V31.00006: Electronic and magnetic properties of two-dimensional electron gas formed at LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3}$ interfaces Karolina Janicka, Julian Velev, Evgeny Tsymbal We perform first-principles electronic structure calculations to elucidate the electronic and magnetic properties of LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3 }$superlattices. We find that TiO$_{2}$-terminated interfaces are $n$-type conducting which is consistent with experimental observations. In both heterostructures we find that charge resides in Ti conduction band and is localized within a few nanometers from the interface. This charge distribution is consistent with metal induced gap states in the gap of SrTiO$_{3}$ produced by the interfacial charge. In order to understand the magnetism of these oxide heterostructures, we performed spin-polarized calculations which reveal that this interface in a (LaAlO$_{3})_{3}$/(SrTiO$_{3})_{3}$ superlattice is magnetic with magnetic moment on the Ti$^{3+}$ atom of 0.2$\mu _{B}$. For thicker SrTiO$_{3}$ layers the magnetism decreases and eventually disappears because the electron gas spreads over more than one unit cell making the electron delocalized and violating the Stoner criterion for magnetism. The inclusion of electron correlations via the LDA+U approximation with U=5eV on Ti atoms makes the two-dimensional electron gas more localized and half-metallic and strengthens the interface magnetization. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V31.00007: Fabrication and characterization of complex oxide \textit{RE}NiO3/LaAlO3 superlattices M. Kareev, J.W. Freeland, J. Liu, B. Kirby, B. Keimer, J. Chakhalian Nowadays there has been growing interest to synthesis of atomically thin complex oxide superlattices which can result in novel electronic and magnetic properties at the interface. Here we report on digital synthesis of single unit cell nickel based heterostructures of \textit{RE}NiO3/LaAlO3 (\textit{RE} = La, Nd and Pr) superlattices on SrTiO3 and LaAlO3 by laser MBE. RHEED analysis, grazing angle XRD and AFM imaging have confirmed the high quality of the epitaxially grown superlattices. The magnetic and electronic properties of the superlattices have been elucidated by polarized X-ray spectroscopies, which show a non-trivial evolution of magnetism and charge of the LNO layer with increasing LNO layer thickness. The work has been supported by U.S. DOD-ARO under Contract No. 0402-17291. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V31.00008: Band gap engineering in digital manganite-titanate superlattices Xiaofang Zhai, Chandra Mohapatra, Amish Shah, Jian-Min Zuo, Zoran Popovic, Sashi Satpathy, Anand Bhattacharya, James Eckstein The electronic structure of short period digital superlattices of two different phases depends strongly on the superlattice architecture. We have fabricated digital superlattices of super cells consisting of N layers of LaMnO$_{3}$ and N layers of SrTiO$_{3}$, N=1, 2, 3, 8, and measured their optical conductivities to probe the way in which the electronic structure depends on N. We found the valence band structure systematically changes, as N decreases, from bulk like band gaps to interface modified band gaps. The temperature dependence of the derived Mn Jahn-Teller/Mott gap in large period superlattices is completely different from the small period, suggesting different spin ordering states reside in them. The electronic structure of two parent materials are coherently blended at the interface, and the spin ordering is strongly modified. This method of producing meta materials is a promising new way of material engineering. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V31.00009: Charge leakage and effective doping of atomically flat (LaMnO$_{3}$)$_{2n}$ / (SrMnO$_{3}$)$_{n}$ superlattice interfaces J.J. Kavich, J.W. Freeland, A. Bhattacharya, M.P. Warusawithana, J.N. Eckstein Atomically flat molecular beam epitaxy (MBE) grown interfaces in high quality digital superlattices provide a unique investigation of the role of substitutional disorder in complex oxide systems. Circularly polarized x-rays are used to measure the electronic and magnetic properties of the random alloy La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ as it is digitally ordered into discrete layers of insulating A-type antiferromagnetic LaMnO$_{3}$ (LMO) and G-type antiferromagnetic SrMnO$_{3}$ (SMO) with superlattice configurations of (LMO)$_{2n}$/(SMO)$_{n}$. The diffusion of carriers across the interface effectively dopes the interface region. The electronic and magnetic structure of the n=1 digital structure is nearly identical to the solid solution of the same doping showing that, on average, the charge distributes uniformly over the entire sample. The ferromagnetic insulating nature of the n = 5 superlattice suggests a finite lengthscale of the diffusion of carriers at the interface. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V31.00010: Structural Basis of Conduction at LaAlO$_{3}$-SrTiO$_{3}$ Interfaces Roy Clarke, D. Kumah, C. Cionca, P.R. Willmott, R. Herger, C.M. Schlepuetz, S.A. Pauli, D. Martoccia, B.D. Patterson, B. Delley, Y. Yacoby We present new results on the application of the x-ray phase retrieval method, Coherent Bragg Rod Analysis (COBRA), to heteroepitaxial interfaces in LaAlO$_{3}$ thin films grown on SrTiO$_{3}$ substrates, a system known to form an interfacial quasi-2D electron gas. We observe a dilated, mixed-valence interface which modifies the electronic band structure, lowering the minimum of the conduction band below the Fermi level and thereby rendering the dilated interface conducting. In particular the COBRA measurements reveal the formation of an interfacial La,SrTiO$_{3}$ layer with an accumulation of trivalent Ti at the interface which is responsible for the lattice dilation and minimizes the electrostatic energy at the TiO$_{2}$-terminated SrTiO$_{3}$ substrate surface. The work presented here establishes a structural basis for the formation of the conducting interface. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V31.00011: Signatures of Enhanced Ordering Temperatures in Digital Superlattices of (LaMnO3)m/(SrMnO3)2m A. Bhattacharya, S. J. May, L. Robertson, P. Ryan, X. Zhai, J. N. Eckstein, S. D. Bader Digital superlattices of (LaMnO3)m/(SrMnO3)2m , for integers m = 1 - 4 have been synthesized with ozone assisted oxide MBE. These superlattices are ordered analogues of La1/3Sr2/3MnO3, an antiferromagnetic insulator. Signatures of enhanced ordering temperatures have been observed in transport and magnetic measurements. We interpret this in terms of the effects of enhanced A-site order on dx2-y2 orbital occupancy. We shall also present evidence in support of our findings with x-ray and neutron scattering. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V31.00012: Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature Cheng Cen, Jeremy Levy, Stefan Thiel, German Hammerl, Christof W. Schneider, Jochen Mannhart, Kristopher E. Anderson, C. Stephen Hellberg We report the creation and erasure of nanoscale conducting regions at the interface between two insulating oxides, LaAlO$_{3}$ and SrTiO$_{3}$. Using voltages applied by a conducting atomic force microscope (AFM) probe, the buried LaAlO$_{3}$/SrTiO$_{3}$ interface is locally and reversibly switched between insulating and conducting states. Persistent field effects are observed using the AFM probe as a gate. Patterning of conducting lines with widths $\sim $3 nm, as well as arrays of conducting islands with densities $>$10$^{14}$/in$^{2}$, are demonstrated. The patterned structures are stable for $>$24 hours at room temperature. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V31.00013: Theory of the Switchable Interfacial Metal-Insulator Transition in LaAlO$_3$ thin films on SrTiO$_3$ C. Stephen Hellberg, Kristopher Andersen, Cheng Cen, Jeremy Levy, Stefan Thiel, German Hammerl, Christof Schneider, Jochen Mannhart Although bulk LaAlO$_3$ and SrTiO$_3$ are both insulating, an electron gas can form at the interface between these compounds. For up to 2 unit cells (uc) of LaAlO$_3$, the interface remains insulating, while for 4 uc and beyond, it is metallic. For 3 uc films, the interface can be reversibly switched from metallic to insulating using voltages applied by an atomic force microscope (AFM). We present first- principles density functional calculations of LaAlO$_3$ films on SrTiO$_3$. We show the AFM induced interfacial metal- insulator transition can be explained by the formation and annihilation of oxygen vacancies on the LaAlO$_3$ surface. [Preview Abstract] |
Session V32: Focus Session: Magnetic Semiconductors and Novel Magnetic Materials
Sponsoring Units: GMAG DMPChair: Kenneth Burch, Los Alamos National Laboratory
Room: Morial Convention Center 225
Thursday, March 13, 2008 11:15AM - 11:27AM |
V32.00001: Structural and chemical ordering and spin polarization in epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$ B.A. Collins, L. He, F. Tsui, Y.S. Chu, Y. Zhong The Heusler alloys of Co$_{2}$MnGe and Co$_{2}$MnSi have been predicted to be half-metallic. However, half-metallicity has not been realized owing to its sensitivity to disorders associated with off-stoichiometry and to epitaxial constraints. Combinatorial epitaxial films of Co$_{x}$Mn$_{y}$Ge$_{1-x-y}$ and Co$_{x}$Mn$_{y}$Si$_{1-x-y}$ have been grown on Ge (111) substrates in and around the Heusler stoichiometry using molecular beam epitaxy techniques. The structural and chemical ordering of the films has been examined using x-ray microbeam techniques, including energy dependent anomalous diffraction and diffraction anomalous fine structure. These experiments are shown to be sensitive to various disorders, including stacking faults, and chemical dependent vacancies and site swapping. They reveal that the ordering is very sensitive to the atomic ratio between Co and Mn. Magnetic and spin dependent properties have been examined by magnetoopitc effects and in-situ point contact Andreev reflection spectroscopy, and they exhibit strong correlation with the structural and chemical ordering. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V32.00002: Control of selforganized magnetic nanocrystals aggregation in (Ga,Fe)N by co-doping with shallow donors and acceptors A. Bonanni, A. Navarro-Quezada, T. Li, M. Kiecana, M. Sawicki, T. Dietl A number of possible room temperature functionalities has recently been proposed for magnetically doped semiconductors, in which spinodal decomposition leads to the self-organized formation of coherent ferromagnetic nanodots or nanocolumns [1]. It has also been suggested that the decomposition can be controlled in a wide range by growth conditions and co-doping [2]. We have extended our previous structural and magnetic studies of (Ga,Fe)N [3] by examining the effects of Si and Mg co-doping. As before, we have found the magnetic response to consist of a paramagnetic signal from substitutional Fe and of a ferromagnetic component due to Fe1-xN nanocrystals. Our results demonstrate that the co-doping reduces the fractional concentration of Fe contributing to the nanocrystals. This shows that tuning of the Fermi energy by changing the charge state of the transition metal ions affects their aggregation, as proposed recently [2].\\ 1. H.Katayama-Yosida et al., phys.stat. sol. (a) 204, 15 (2007); T.Dietl, arXiv:0711.0343. 2. S.Kuroda et al., Nature Mat. 6, 440 (2007). 3. A.Bonanni et al., Phys. Rev. B 75, 125210 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V32.00003: Giant anisotropic magnetoresistance in ultra thin (Ga,Mn)As Rashid Gareev, Markus Schlapps, Janusz Sadowski, Werner Wegscheider, Dieter Weiss We describe the effect of giant anisotropic magnetoresistance (GAMR) for epitaxial ultra thin (Ga,Mn)As below the metal-insulator transition (MIT). The GAMR is observed for 5 nm-thick Ga$_{0.95}$Mn$_{0.05}$As films after annealing in optimized conditions in planar geometry for patterned Hall bars at T$<$10K, where longitudinal resistance R$_{o}\sim $h/e$^{2}$. The GAMR manifests itself in magnetization-dependent high-resistance (HR) and low-resistance (LR) states along different crystallographic directions. We demonstrate that holes are strongly localized in HR states and localization depends on orientation of magnetization, amplitude of current and magnetic field. The decrease of current amplitude is accompanied by an enhancement of the GAMR for both in-plane and orthogonal-to-plane orientations of magnetic field. Changes of R$_{xx}$ between HR and LR states reach$\sim $100{\%} at T=1.7K. In the Hall geometry the changes of transverse component R$_{xy}$ in magnetic field exceed 1000{\%}. The behaviour of GAMR we ascribe to anisotropic spin-orbit scattering, strong localization below MIT and localization-delocalization effects in magnetic and electric fields. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V32.00004: Atom-byatom substitution of Mn in GaAs and visualization of their hole-mediated interactions Invited Speaker: |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V32.00005: Ferromagnetism in CuO-ZnO multilayers Sudakar Chandran, B.J. Kirby, K. Padmanabhan, G. Lawes, R. Naik, Sanjiv Kumar, V.M. Naik The magnetic properties of CuO-ZnO heterostructures are examined to elucidate the origin of the ferromagnetic signature in Cu doped ZnO. The CuO and ZnO layer thickness varied from 15 nm to 350 nm, and we observed no significant diffusion of either Cu$^{2+}$ in the ZnO layers or of Zn$^{2+}$ in the CuO layers using Rutherford backscattering spectrometry. Bulk magnetization measurements established that the multilayers exhibit a ferromagnetic moment at room temperature, with a saturation magnetization ($\sim $2-5 emu/cc of CuO) that depends on the CuO size, but not the CuO-ZnO interfacial area. Polarized neutron reflection studies suggest that the ferromagnetism arises from the CuO layers, and not from the interdiffusion of CuO and ZnO. These results indicate that the ferromagnetism in these multicomponent structures arises from the uncompensated surface spins of CuO nanoparticles in the layer rather than from regions of interdiffusing ZnO and CuO. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V32.00006: Role of Donor defects in stabilizing room temperature ferromagnetism in Co doped ZnO Lubna Shah, Yuan-Qiang Song, Huai-Wu Zhang, Weigang Wang, Zhu Hao, John Xiao Extensive experimental and theoretical work has been done on transition metal doped ZnO diluted magnetic semiconductor, in which defects play a vital role in promoting carrier-mediated ferromagnetism. We explored the influence of interstitial Zinc on physical properties of Co doped ZnO by exposing Co0.05Zn0.95O in a Zn vapor. Both X-ray diffraction and X-ray photoelectron spectroscopy indicate the substitution of Co2+ in the ZnO lattice. Observed room temperature ferromagnetism in bulk samples shows a decreasing trend with decreasing temperature. This can be explained by taking into the reduction of the carrier density, induced by the interstitial Zn ions, at low temperature. Carrier density at room temperature is about 8.5 x 10$^{19}$/cc, which is consistent with predicted value for polaron percolation threshold to induce long range ferromagnetism. The carrier density fell below this threshold at low temperature, results in the disappearance of the ferromagnetism. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V32.00007: Ferromagnetism in Rutile Structure Cr doped VO$_{2}$ Thin Films prepared by Reactive Bias Target Ion Beam Deposition Kevin G. West, Jiwei Lu, Li He, David M. Kirkwood, Wei Chen, T. Paul Adl, Michael S. Osofsky, Syed B. Qadri, Robert Hull, Stuart A. Wolf First generation spintronics has entered the mainstream of information technology through its utilization of the Magnetic Tunnel Junction (MTJ) in applicable devices such as read head sensors for hard disk drives and Magnetic Random Access Memory (MRAM). The future of spintronic devices requires next generation spintronic materials. Here we report on the novel structural, transport, and magnetic characteristics of V$_{1-x}$Cr$_{x}$O$_{2}$ (0.1 $\le $ x $\le $ 0.2) thin films deposited on (001) Al$_{2}$O$_{3}$ substrates. We show that the metal-insulator transition (MIT) of VO$_{2}$ is suppressed and the rutile structure is stable down to 100 K. The films are remarkably smooth having a root-mean squared (RMS) surface roughness of 0.3 nm. Films are conductive at room temperature and appear to follow a variable-range-hopping conduction mechanism below that. Ferromagnetism is observed at room temperature and is dependent on Cr concentration. The combination of these characteristics makes V$_{1-x}$Cr$_{x}$O$_{2}$ a viable candidate material for next generation spintronic multilayer devices. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V32.00008: Phase transition temperatures and magnetic entropy change in Ni-Mn-In-B based Heusler alloys Arjun Pathak, Bhoj Gautam, Igor Dubenko, Naushad Ali One of the aspects of great attention of Heusler alloys is the large value of magnetic entropy change ($\Delta $S$_{M})$ and their possible application as a working material in magnetocaloric effect based magnetic refrigerators. It was reported earlier that Ni$_{50}$Mn$_{34.8}$In$_{15.2}$ has first order martensitic transition temperature T$_{M}\approx $ 212K, Curie temperature of austenitic phase T$_{C}\approx $ 328K and $\Delta $S$_{M}$ value associated with T$_{M}$ and T$_{C}$ are respectively 13 and -7 J/kg K [1]. In the present study, we are reporting the effect of partial substitution of In by B in Ni$_{50}$Mn$_{34.8}$In$_{15.2}$ by AC susceptibility, thermal expansion, and magnetization measurements. We observed that substitution of boron sharply increase T$_{M,}$ and significantly enhance the $\Delta $S$_{M}$ peak value higher than 30 J/kg K at T$_{M}\approx $ 296K; however the $\Delta $S$_{M}$ value remains almost same at T$_{C}$. Therefore, the Ni-Mn-In-B based Heusler alloys will be potential material for the study of room temperature magnetic refrigerator materials. Reference: [1] A. K. Pathak, M. Khan, I. Dubenko, S. Stadler, and N. Ali, Appl. Phys. Lett. \textbf{90}, 262504 (2007). [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V32.00009: Magnetic excitations in a quantum spin dimer system Ba$_{3}$Cr$_{2}$O$_{8}$ Maiko Kofu, Jung-Hwa Kim, Seung-Hun Lee, Bella Lake, Yiming Qiu, Hiroaki Ueda, Yutaka Ueda We report our neutron scattering measurements on a powder sample of a new quantum spin system Ba3Cr2O8 in which Cr5+ (3d1, S = 1/2) ions form a double-layered triangular lattice. The system does not undergo any magnetic ordering down to 50 mK. Instead, bulk susceptibility data exhibit a broad peak around 16K. Our inelastic neutron scattering data at temperatures lower than 30 K show a prominent excitation at hw = 2.2 meV. Q- and temperature dependences of the integrated intensity of the excitation can be well accounted for by a model of weakly coupled quantum dimers. When an external magnetic field is applied, the excitation splits into three energies, which confirms the dimer model. We have investigated how the singlet-to-triplet excitations evolve with increasing the magnetic field up to 14.5 Tesla. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V32.00010: Understanding Magnetic Frustration on the Diamond Lattice of Transition Metal Oxide Spinels Brent Melot, Katharine Page, Ram Seshadri, Doron Bergman, Thomas Proffen We present structural and magnetic measurements on the solid solution CoAl$_{2-x}$Ga$_{x}$O$_4$ and demonstrate how frustration on the diamond sublattice can be modified through chemical substitution. The effect of substitution is two-fold. Increasing values of $x$ are accompanied by an increase in the lattice parameter resulting in an elongated and consequentially weakened exchange pathway. Our data indicates that the amount of site mixing also increases across the substitution series weakening the A-O-B-O-A superexchange pathway. Monte Carlo simulations were used to compute the exchange coupling constants between nearest and next-nearest neighbor Co atoms. Density functional calculations were also performed to approximate the nearest neighbor coupling constant. For most values of $x$, we find that the frustration parameter, $\Theta_{CW}/T_N$, decreases which we attribute to the weakening competition between nearest and next-nearest-neighbor Co exchange interactions. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V32.00011: Half-,metallic Silicon Nanowires Engin Durgun, Deniz Cakir, Nurten Akman, Salim Ciraci This study investigates the atomic structure, mechanical, electronic and magnetic properties of silicon nanowires using first-principles plane wave calculations within density functional theory. We considered bare, hydrogen terminated and transition-metal adsorbed silicon nanowires oriented along [001] direction. Nanowires of different sizes are initially cut from the bulk Si crystal in rod-like forms and subsequently their atomic structures are relaxed. We first presented an extensive analysis of the atomic structure, stability, elastic and electronic properties of bare and hydrogen terminated Si nanowires. The energetics of adsorption and resulting electronic and magnetic properties are examined for different level of transition metal atom coverage. Adsorption of transition metal atoms resulted in magnetic ground state. The net magnetic moment increases with increasing coverage. While specific Si nanowires acquire half-metallic behavior at low coverage, at high coverage ferromagnetic nanowires become metallic for both spin-direction and some of them have very high spin polarization at the Fermi level. Present results are not only of scientific interest, but can also initiate new research on spintronic applications of silicon nanowires. [1] \newline \newline [1] E.Durgun, D. Cakir, N. Akman and S. Ciraci, Phys. Rev. Lett (2007) (in press). [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V32.00012: Spin dynamics in single-molecule magnets combining surface acoustic waves and high frequency electron paramagnetic resonance Stephen Hill, Jonathan Lawrence, Ferran Macia, Joan Manel Hernandez, Javier Tejada, Paulo Santos, Christos Lampropoulos, George Christou We report a new experimental technique that integrates high frequency surface acoustic waves (SAWs) with high frequency electron paramagnetic resonance (HFEPR) spectroscopy in order to measure spin dynamics on fast time scales in single-molecule magnets. After driving the system out of equilibrium by triggering magnetic avalanches, or simply by heating with short SAW pulses, the evolution of the spin populations within fixed energy levels is measured using HFEPR spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V32.00013: Crystal Growth and High Field Magnetization of the Spin Dimer Compound Ba$_{3}$Cr$_{2}$O$_{8}$ Adam Aczel, Hanna Dabkowska, Graeme Luke, Eun-Sang Choi Spin dimer compounds provide means to access exotic magnetically ordered states. The ground state of these systems is a product of singlets as a result of antiferromagnetic intradimer exchange. However, an applied magnetic field can be used to close the spin gap to excited triplet states, resulting in a state characterized by long range magnetic order at low temperatures. Ba$_{3}$Cr$_{2}$O$_{8}$ is an example of a system that shows the behaviour discussed above. To investigate the field-tuned phase transition in this system, we grew single crystals of this material for the first time by the traveling solvent floating zone method. We then proceeded to measure the magnetization of the resulting crystals by the torque magnetometry technique at the National High Magnetic Field Lab. The magnetization is flat until $\sim $ 12.5 T, then shows a rapid, nearly linear increase with field and saturates at $\sim $ 23 T.$_{ }$ This behaviour is reminiscent of that seen in the spin dimer compounds BaCuSi$_{2}$O$_{6}$ and Ba$_{3}$Mn$_{2}$O$_{8}$ and has been attributed to Bose-Einstein condensation of triplet excitations. [Preview Abstract] |
Session V33: Focus Session: Theory of Spin Phenomena in Semiconductors
Sponsoring Units: GMAG FIAP DMPChair: Michael Flatte, University of Iowa
Room: Morial Convention Center 224
Thursday, March 13, 2008 11:15AM - 11:51AM |
V33.00001: Design of dilute magnetic semiconductors with room temperature ferromagnetism by controlling spinodal decompostion Invited Speaker: Owing to the recent development of the first-principles method for calculating magnetic properties of dilute magnetic semiconductors (DMS), it has been recognized that the magnetic percolation effect is disastrous to the high temperature ferromagnetism in DMS in particular for low concentrations [1]. The exchange interactions calculated from first-principles are strong for nearest neighbors, but those interactions are short ranged and can not play an important role for realizing high- $T_{\rm C}$ because the solubility of magnetic impurities into DMS is too low to achieve magnetic percolation. To overcome this difficulty and realize room temperature ferromagnetism, we focus on the spinodal decomposition in DMS, and suggest that by controlling the spinodal decomposition high blocking temperature can be realized leading to ferromagnetic behaviour at high temperature [2]. We calculate electronic structure of DMS from first-principles by using the Korringa- Kohn-Rostoker coherent potential approximation (KKR-CPA) method. Then, chemical pair interactions and magnetic exchange interactions between magnetic are calculated. We use the Monte Carlo techniques to simulate spinodal decomposition of DMS and to estimate the magnetic properties of them [3]. The computer simulations for the magnetization process of the decomposition phases indicate that we can control super-paramagnetic blocking temperature by optimizing the size of the clusters by changing the crystal growth condition. This simulation suggests the material design of high blocking temperature DMS by controlling the spinodal decomposition [2].As another approach for realizing high-Tc DMS we propose co-doping method to increase solubility limit of transition metal impurities in DMS [4]. This work is based on the collaboration with H. Katayama-Yoshida and T. Fukushima. \newline [1] L. Bergqvist et al, Phys. Rev. Lett. 93, 137202 (2004), K. Sato et al., Phys. Rev. B 70, 201202 (2004) \newline [2] K. Sato et al., Jpn. J. Appl. Phys. 46, L682 (2007) \newline [3] K. Sato et al., Jpn. J. Appl. Phys., 44, L948 (2005), T. Fukushima et al., Jpn. J. Appl. Phys., 45, L416 (2006) \newline [4] K. Sato et al., Jpn. J. Appl. Phys. 46 L1120 (2007) [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V33.00002: Defect-induced intrinsic magnetism in wide-gap III-nitrides Peihong Zhang, Pratibha Dev, Yu Xue Cation vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3 $\mu_B$ with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wavefunctions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect induced or mediated collective magnetism in these otherwise nonmagnetic $sp$ systems. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V33.00003: Design of Colossal Solubility of Magnetic Impurities for Semiconductor Spintronics by the Co-doping Method Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Based on first-principles calculations, we propose co-doping method for increasing solubility of magnetic impurities in dilute magnetic semiconductors (DMSs). The concentration dependences of the mixing energy of DMS, such as (Ga,Mn)N, (Ga,Cr)N, (Ga,Mn)As, and (Zn,Cr)Te, show large convexity and these systems have a tendency toward spinodal decomposition. By introducing compensating impurities into these DMS, the mixing energy shows gradual transition from convex to concave concentration dependence resulting in negative mixing energy of magnetic impurities. This observation suggests that the co-doping method dramatically increases the solubility of magnetic impurities in DMS, thus high concentration doping of magnetic impurities into DMS becomes possible. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V33.00004: Suppresion of ferromagnetism in Zn$_x$Cr$_{1-x}$Te: A DFT study Juarez L.F. Da Silva, Gustavo M. Dalpian, Su-Huai Wei The possibility to control the charge and spin degrees of freedom independenlty in diluted magnetic semiconductos (DMS) provides an opportunity in the realization of spintronic devices. Therefore, there is a great desire to understand the physical parameters that control $T_{\rm C}$ in DMS. In this talk, we will report a density functional theory study of Zn$_x$Cr$_{1-x}$Te and Ga$_x$Cr$_{1-x}$As, which show interesting carrier dependence of the $T_{\rm C}$. We found that the stability of ferromagnetism (FM), which is calculated with respect the anti-ferromagnetic (AFM) phase, can be controlled by $p$- and $n$-type co-doping. For randomly distributed Cr, the maximum FM stability is found to be at $\approx1.25$ holes per magnetic ion. Furthermore, we studied the formation of magnetic Cr precipitates by analysing the binding energy of two Cr atoms in the host semiconductors. Our results indicate that a simple Coulomb binding picture cannot provide a consistent explanation for the formation of Cr-rich aggregates. Instead, we show that the variation of Cr-Cr binding energy can be discribed quite well using the band coupling model [G. M. Dalpian, S.-H. Wei, X. G. Gong, A. J. R. da Silva, and A. Fazzio, Solid State Commun. {\bf 138}, 353 (2006)]. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V33.00005: Eight-band model of the dilute magnetic semiconductor GaMnAs A. Nili, M. A. Majidi, J. Moreno, P. Kent, M. Jarrell, R. S. Fishman The ordered state of GaAs doped with Mn is studied using a self-consistent Dynamical Mean Field treatment parameterized with the LDA band structure of the parent material. The resulting model is an eight-band k$\cdot$p model including the heavy and light valence bands, the split-off valence band and the conduction band. The interaction between randomly distributed magnetic ions and itinerant charge carriers is modeled with a modified double-exchange coupling, which includes the attractive potential between holes and charged magnetic ions. Since our method is not perturbative we can treat both strong and weak coupling regimes. We calculate the transition temperature, the temperature dependence of the local magnetization, the density of states and the spin polarization of the holes. We compare our results with more simplified models. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V33.00006: Spin relaxation of two-dimensional excitons in a strong magnetic field O. Olendski, T. V. Shahbazyan We study spin relaxation of quantum well excitons in a perpendicular magnetic field. In a strong field, the dominant relaxation mechanism for an optically-excited magnetoexciton is phonon-assisted spin-orbit transition of the constituent electron or hole. We show that in a quantum well the exciton spin relaxation rate is quadratic in magnetic field in contrast to the fifth power dependence in a quantum dot. We calculate relaxation rates for excitons in GaAs quantum wells excited by both left and right-polarized light and find that they are much slower than those in quantum dots due to the dominant contribution of exciton kinetic energy in the final state. We also find that the relaxation rate is very sensitive to the applied in-plane electric field that causes exciton drift in the ground state. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V33.00007: Theory of xchange splittings of bands in diluted magnetic semiconductors Tomasz Dietl, Cezary Sliwa A series of recent photoemission and magnetooptical experiments, particularly on magnetically doped nitrides and oxides, but also on (Ga,Mn)As, points to the limiting understanding of the physics governing the sp-d exchange interaction in these systems. We have developed theory of band splittings in diluted magnetic semiconductors taking into consideration a possibility that the magnetic ion can trap a hole. We have found that the exchange coupling between the bound and delocalized carriers has actually a similar magnitude to that characterizing the sp-d exchange interaction [1]. Furthermore, our results demonstrate that the presence of the corresponding bound state itself renormalizes extended states in a spin-dependent fashion [2]. We show that these two effects can explain the unexpected sign and magnitude of the apparent s- d and p-d exchange integrals determined by magnetooptical studies carried out for (Ga,Mn)As [3] as well as for (Zn,Co)O, (Ga,Mn)N, and (Ga,Fe)N [4].\newline [1] C. Sliwa, T. Dietl, cond-mat/arXiv:0707.3542. [2] T. Dietl, cond-mat/0703278. [3] J. Szczytko et al., Solid State Commun. 99, 927 (1996); M. Poggio et al., Phys. Rev. B 72, 235313 (2005). [4] W. Pacuski et al., ibid. 73, 035214 (2006); ibid. 76, 165304 (2007); arXiv:0708.3296. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V33.00008: Computational Nano-materials Design for Spin-Currents Control in Semiconductor Nano-spintronics Hiroshi Katayama-Yoshida, Tetsuya Fukushima, Van An Dinh, Kazunori Sato We design the different exchange mechanism like Zener's double exchange, Zener's p-d exchange and super-exchange in dilute magnetic semiconductors (DMS) by ab initio calculations. We obtain a universal trend for the exchange interactions [1]. We show that self-organized spinodal nano-decomposition (Dairiseki- Phase) offers the functionality to have high Curie temperatures[2]. We show that spinodal nano-decomposition under layer-by-layer crystal growth condition (2D) leads to quasi-one dimensional nano-structures (Konbu-Phase) with highly anisotropic shape and high $T_{\rm C}$[2]. We design a spin-currents- controlled 100 Tera bits/icnh$^2$, Tera Hz switching, and non- volatile MRAM without Si-CMOS based on Konbu-Phase [2]. In addition to the conventional Peltier effect, we propose a colossal thermoelectric-cooling power based on the adiabatic spin-entropy expansion in a Konbu-Phase [3]. [1] B. Belhadaji et al., J. Phys.-Condens. Matter, 19 (2007) 436227. [2] H. Katayama-Yoshida et al., Phys. stat. sol. (a) 204 (2007) 15. [3] H. Katayama-Yoshida et al., Jpn. J. Appl. Phys. 46 (2007) L777. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V33.00009: Effect of Fermi level on Microstructure and Magnetism in (Ga,Mn)N Alloys Jennifer Chan, Zhe Liu, Stephan Lany, Alex Zunger GaN doped with Mn has been shown experimentally to exhibit either ferro- or antiferro- magnetic behavior, the results varying considerably depending on the microstructure of the sample. Indeed, the electronic structure and magnetic properties appear to be heavily dependent upon growth conditions and ordering of the material. We used {\it ab-initio} calculations to investigate the magnetism of various ordered structures of (Ga,Mn)N with respect to Mn composition. The results show that high-spin states with the spins on the Mn aligned in parallel (HS-FM) are stable at low Mn composition ($<$0.5\%) but not at high Mn composition ($>$0.5\%). Instead, for high Mn composition, low-spin states (LS-FM) or states where the spins on the Mn are anti-parallel (FI) are found. Interestingly, upon raising the Fermi level, the HS-FM states are stable for all Mn compositions and their formation enthalpies lower with respect to the neutral Fermi level case indicating that electron doping leads to enhanced Mn solubility and hence charged microstructure. The short and long range ordering, tendency for clustering and magnetic properties will be investigated. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V33.00010: First-Principles Study on Electronic Structure of TiO$_2$-Based Dilute Magnetic Semiconductors Hidetoshi Kizaki, Masayuki Toyoda, Kazunori Sato, Hiroshi Katayama-Yoshida We investigate the electronic structure in rutile-TiO$_2$-based dilute magnetic semiconductors (DMS) within self-interaction- corrected local density approximation (SIC-LDA). These results are compared with those calculated within standard LDA. Although the calculated band-gap energy and energetic position of Ti 3{\it d} bands are different in the LDA and the SIC-LDA, half-metallic density of states is predicted in transition- metal-doped TiO$_2$ for both methods. While the LDA calculations predict high-spin state only for Fe-doped one, in the SIC-LDA calculations high-spin state is realized in V-, Cr- and Mn-doped one and low-spin state is realized in Fe- and Co- doped one. However, the absorption and soft x-ray magnetic circular dichroism measurements in (Ti$_{0.97}$, Co$_{0.03}$)O$_ {2-\delta}$ indicate the Co$^{2+}$ high-spin state in the ${\it D}_{2h}$-symmetry crystal field at the Ti site. These experimental results do not agree with our calculated results. We will discuss the origin of the discrepancy between the theoretical predictions and the experimental observations. In addition, we will discuss the ferromagnetism in TiO$_2$-based DMS. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V33.00011: Ferromagnetism in GaN:Gd: A density functional theory study Lei Liu, Peter Yu, Z.X. Ma, Sam Mao First principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the Generalized Gradient Approximation (GGA) of the density functional theory (DFT) with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic $p-d$ coupling is found to be over two orders of magnitude larger than the$ s-d$ exchange coupling. The experimental colossal magnetic moments and room temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4$f$ spins via $p-d$ coupling involving holes introduced by intrinsic defects such as Ga vacancies. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V33.00012: Thermodynamics of carrier-mediated magnetism in semiconductors A.G. Petukhov, L. Makinistian, S.C. Erwin, R. Abolfath, I. Zutic We propose a model of carrier-mediated ferromagnetism in semiconductors that accounts for the temperature dependence of the carriers\footnote{A. G. Petukhov, I. Zutic, and S. C. Erwin, Phys. Rev. Lett. {\bf 99}, 257202 (2007)}. The model permits analysis of the thermodynamic stability of competing magnetic states, opening the door to the construction of magnetic phase diagrams. As an example we analyze the stability of a possible reentrant ferromagnetic semiconductor, in which increasing temperature leads to an increased carrier density, such that the enhanced exchange coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised. We apply this approach to studying thermodynamic fluctuations of magnetization in small systems such as bound magnetic polarons and magnetic nanoislands. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V33.00013: Electronic structure and magnetism of Fe$_{4-x}$Mn$_{x}$N compounds Li Chen The electronic structure and magnetism of Fe$_{4-x}$Mn$_{x}$N compounds have been studied by a periodic quantum-mechanical calculation based on density functional theory. The results show that a ferrimagnetic ordered phase is stable when Fe is substituted by Mn on cube corner sites, whereas the antiferromagnetic phase is energetically favored when Mn substitutes for Fe on face-centered sites. Mn atom concentration and the substitutional sites have significant influence on the exchange coupling. We predict covalent bonds between face-centered Fe or Mn 3d and N 2p orbitals. In contrast, bonding between the atoms at cube corners and face-centered sites is mainly ionic or metallic. [Preview Abstract] |
Session V34: Panel Discussion: International Gender Issues in Physics
Sponsoring Units: FIP CSWPChair: Beverly Hartline
Room: Morial Convention Center 226
Thursday, March 13, 2008 11:15AM - 11:25AM |
V34.00001: Session Introduction Beverly Hartlline This abstract not available. [Preview Abstract] |
Thursday, March 13, 2008 11:25AM - 11:44AM |
V34.00002: The APS 2007 Meeting on Gender Equity in Physics Invited Speaker: This abstract not available. [Preview Abstract] |
Thursday, March 13, 2008 11:44AM - 12:03PM |
V34.00003: Promoting Positive Images of Women in Physics Invited Speaker: Why are so few women among the students who study physics? Worldwide statistics show that there have been very few female students in the physics in any country, in the past and in current times. The situation in Germany is outstandingly precarious. The fraction of women to study physics is lower than 15\% of all students who study physics. In the last years we have had a lot of changes in structure and also activities to improve the situation for women. Here I shall present some of the activities in Germany and in other countries. The aims are to bring more girls in to physics, modify the image of scientists and find ways to stop women to drop out of physics. The experiences and the expectations will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:22PM |
V34.00004: She's a Physicist!? Invited Speaker: Any minority often feels either left out or at the center of attention. The difficulty of blending in creates both challenging obstacles and unique opportunities. Such negative and positive distractions, whether major or minor, can have a profound impact on accomplishing one's goals. Success requires sustained focus, a constant eye on the ``big picture'', and the ever-important sense of humor. In other words, don't be distracted by the ups and downs, keep your passion focused on the science, and enjoy the ironies. I will discuss some of my personal experiences as a woman and how I have learned to navigate the physics world since before 1970. Through losses and wins, I will share how I have managed to, at least most of the time, keep on keeping on. [Preview Abstract] |
Thursday, March 13, 2008 12:22PM - 12:41PM |
V34.00005: Women in Physics in a Rapidly Changing China Invited Speaker: Despite the upheavals of the 20th century, physics managed to survive quite well in China, where the first woman president of the American Physical Society was born and bred. During the 1950s as a result of policies that emphasized science and engineering, declared equal rights and equal pay for men and women, and assigned jobs to college graduates irrespective of sex, the number of women in physics increased rapidly, many of whom made notable achievements. Since China's opening up over the last thirty years tremendous changes have taken place, and women now face new opportunities as well as challenges in all aspects of society. Whereas physics used to be regarded as the most elite of the sciences, new fields such as computer science, biotechnology and business are now competing for the best students. Compared with other countries the statistics are not bad; in schools and many physics departments the ratio of women teachers may be 30{\%} or higher, but the numbers drop drastically with rank. Moreover, in some research institutions the ratio of female physicists is actually declining, due to retirement of the older generation and fewer successors. Compulsory retirement for women at an earlier age than for men is also a new factor. Conversely, in recent years the ratio of female graduate students enrolling in physics has increased, even reaching 40{\%} in some universities. However, the reasons for this do not bode well: men are not performing so well as women in entrance exams, while the latter are facing increasing discrimination in employment so they have to seek higher degree qualifications. With the further development of China's economy there will be abundant demand for qualified personnel including women with a physics background. It is imperative to actively support the upcoming generation of women physicists and not lose them in the leaky pipeline. The Chinese Physical Society has taken certain positive steps, such as the recent establishment of the Xie Xi-De Prize for Women in Physics. However, there is complacency and a general lack of awareness of the special difficulties and discrimination that women in physics face, and this needs to be brought to the attention of all sectors at all levels. [Preview Abstract] |
Thursday, March 13, 2008 12:41PM - 1:15PM |
V34.00006: Marshak Lectureship Talk: Women in Physics in the Baltic States Region: Problems and Solutions Invited Speaker: In this contribution the gender equality problem in physics will be discussed on the basis of the results obtained implementing the project ``Baltic States Network: Women in Sciences and High Technology'' (BASNET) initiated by Lithuanian women physicists and financed by European Commission. The main goal of BASNET project was creation of the regional Strategy how to deal with women in sciences problem in the Baltic States. It has some stages and the contribution follows them. The first one was in depth sociological study aiming to find out disincentives and barriers women scientists face in their career and work at science and higher education institutions. Analysis of results revealed wide range of problems concerned with science organization, management and financing common for both counterparts. However it also proved the existence of women discrimination in sciences. As main factors influencing women under-representation in Physics was found: the stereotypes existing in the society where physics is assigned to the masculine area of activity; failings of the science management system, where highest positions are distributed not using the institutionalized objective criteria but by voting, where the correctness of majority solutions is anticipated implicitly. In physics where male scientists are the majority (they also usually compose executive boards, committees etc.) results of such a procedures often are unfavorable for women. The same reasons also influence women ``visibility'' in physicist's community and as the consequence possibility to receive needed recourses for their research as well as appropriate presentation of results obtained. The study revealed also the conservatism of scientific community- reluctance to face existing in the scientific society problems and to start solving them. On the basis of the results obtained as well practice of other countries the common strategy of solving women in physics (sciences) in the Baltic States region was formulated. As changing the stereotypes is long lasting process it was decided firstly to concentrate strategy implementation plans on changes in science management policy tackling the problem from the top and allowing receive the most quick results. For this we created the regional Baltic States Network among the corresponding international women working groups, professional organizations (Scientific societies) and corresponding departments of the governmental institutions. BASNET also became a full member of European Platform of Women Scientists (EPWS)-prestige women organization signally influencing the European Community science policy. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 2:15PM |
V34.00007: Panel Discussion
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Session V35: Focus Session: Thermoelectric Phenomena in Nanostructured Materials
Sponsoring Units: DMP FIAPChair: Chris Vineis, MIT Lincoln Laboratory
Room: Morial Convention Center 227
Thursday, March 13, 2008 11:15AM - 11:51AM |
V35.00001: Thermoelectric Properties of Complex Zintl Phases Invited Speaker: Complex Zintl phases make ideal thermoelectric materials because they can exhibit the necessary ``electron-crystal, phonon-glass'' properties required for high thermoelectric efficiency. Complex crystal structures can lead to high thermoelectric figure of merit (\textit{zT}) by having extraordinarily low lattice thermal conductivity. A recent example is the discovery that Yb$_{14}$MnSb$_{11}$, a complex Zintl compound, has twice the \textit{zT} as the SiGe based material currently in use at NASA. The high temperature (300K - 1300K) electronic properties of Yb$_{14}$MnSb$_{11}$ can be understood using models for heavily doped semiconductors. The free hole concentration, confirmed by Hall effect measurements, is set by the electron counting rules of Zintl and the valence of the transition metal (Mn$^{+2})$. Substitution of nonmagnetic Zn$^{+2}$ for the magnetic Mn$^{+2}$ reduces the spin-disorder scattering and leads to increased \textit{zT} (10{\%}). The reduction of spin-disorder scattering is consistent with the picture of Yb$_{14}$MnSb$_{11}$ as an underscreened Kondo lattice as derived from low temperature measurements. The hole concentration can be reduced by the substitution of Al$^{+3}$ for Mn$^{+2}$, which leads to an increase in the Seebeck coefficient and electrical resistivity consistent with models for degenerate semiconductors. This leads to further improvements (about 25{\%}) in \textit{zT} and a reduction in the temperature where the \textit{zT} peaks. The peak in \textit{zT} is due to the onset of minority carrier conduction and can be correlated with reduction in Seebeck coefficient, increase in electrical conductivity and increase in thermal conductivity due to bipolar thermal conduction. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V35.00002: A theoretical study of thermoelectric metamaterial Jianwei Cai, Gerald Mahan An artificial crystalline material with nanoparticles connected by conducting linkers is proposed for thermoelectric application. Such materials can be synthesized with chemical approaches. The advantage of such materials for thermoelectric application will be discussed. A simple theoretical model for such materials is proposed. Basic theoretical results about the band and properties of the materials based on perturbation theory will be presented. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V35.00003: High Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. B. Poudel, Q. Hao, Y. Ma, A. Minnich, A. Muto, Y.C. Lan, B. Yu, X. Yan, D.Z. Wang, D. Vashaee, X.Y. Chen, M.S. Dresselhaus, G. Chen, Z.F. Ren Bismuth Telluride and its alloys are best thermoelectric materials for near room temperature applications like refrigeration and waste heat recovery. We have been pursuing an approach of random nanostructures in bulk to improve ZT of these materials. Here we report that ZT values of these random nanostructured materials were improved significantly over the state-of-the-art values. Experimental data coupled with microstructure studies and modeling shows that the ZT improvement mainly comes from a lower thermal conductivity because of the increased phonon scattering by defects and grain boundaries. Significantly improved power generation and cooling data produced from these samples confirmed the high ZT values. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V35.00004: The Effect of Bi/Sb ratio on Maximum ZT in Nanostructured p-type Bi$_{x}$Sb$_{2-x}$Te$_{3}$ Alloys Yi Ma, Bo Yu, Bed Poudel, Yucheng Lan, Jian Yang, Zhifeng Ren, Qing Hao, Gang Chen Alloy nanoparticles Bi$_{x}$Sb$_{2-x}$Te$_{3}$ (x = 0.2, 0.3, 0.4, 0.5 and 0.6) were synthesized from bismuth, antimony, and tellurium and compacted into nanostructured bulk samples with full density. The thermoelectric properties were measured in the temperature range of 25 to 250$^{\circ}$C. We showed that a maximum dimensionless figure-of-merit (ZT) reached 1.27 at about 100$^{\circ}$C for Bi$_{0.4}$Sb$_{1.6}$Te$_{3}$(x = 0.4), a significant improvement over the state-of-the-art p-type Bi$_{2}$Te$_{3}$ alloys. It is demonstrated that the enhanced ZT mainly comes from a reduced thermal conductivity due to the increased phonon scattering due to the nano size of the grains. Also, the ZT peaks shift from low temperature to high temperature with the decreasing of x, which indicates the less Bi content would benefit the potential power generation application while the more Bi content materials could be used as a potential cooling device. This new route of enhancing ZT value can be applied to other materials. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V35.00005: XAS study of Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ Superlattices and Sb$_{1.5}$Bi$_{0.5}$Te$_{3}$ Alloy Film Azzam Mansour, Rama Venkatasubramanian Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices are being developed for high performance thermoelectric devices. We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Sb in 20/40 {\AA} and 10/50 {\AA} Bi$_{2}$Te$_{3}$/Sb$_{2}$Te$_{3}$ superlattices, a Sb$_{1.5}$Bi$_{0.5}$Te$_{3}$ alloy film, and a Sb$_{2}$Te$_{3}$ reference film. The films were grown on GaAs using a novel low temperature metal-organic chemical vapor deposition method. Initially, we evaluated the local structure parameters of Sb in the superlattices and the alloy relative to those of the reference Sb$_{2}$Te$_{3}$ film. Using temperature dependent Sb K-edge XAS measurements, we were able to separate structural disorder from thermal disorder for the Sb-Te pair. The temperature dependence of thermal disorder was analyzed using the Einstein and Debye models for lattice vibrations. A comparison of the results for the superlattices, the alloy and the reference film will be made. The goal is to identify key structural factors that may contribute to the enhanced value of the figure of merit ``ZT'' and, hence, the improved conversion efficiency for the superlattices relative to the alloys. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V35.00006: Thermoelectric properties of PbTe/PbSe mesomaterials Feng Chen, Yaqi Wang, Yuyi Xue, C. W. Chu, Jun Zhang, Jiye Fang, Chunhu Tan, Zhigang Lin, Bob Liu Ball milled PbTe mixed with PbSe nano-wires (PTSW) or with PbSe nano-crystals (PTSC) are sintered under high pressure. Different sintering conditions are tested to preserve the mesostructures. Thermoelectric properties (resistivity, Seebeck coefficient and thermal conductivity) are measured at various temperatures. Pure ball milled PbTe are also sintered and measured for comparison. In this talk, we will present these data and compare with various PbTe data from the literature. Our results show that this mesostructure approach is promising and the sintering condition is the key factor for further improvement. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V35.00007: Transport Properties of Lead Chalcogenide Nanocomposites George Nolas, Joshua Martin, Stevce Stefanoski, Li Wang, Lidong Chen Lead chalcogenide dimensional nanocomposites were prepared by densifying nanocrystals, synthesized in high purity - high product yield employing an alkaline aqueous solution-phase reaction, by Spark Plasma Sintering. Transport properties were evaluated through temperature dependent resistivity, Hall, thermopower, and thermal conductivity measurements. The physical properties of these materials are strongly sensitivity to stoichiometry, doping, and porosity. The transport properties of the nanocomposites were compared to that of bulk polycrystalline materials with similar compositions and carrier concentrations. An enhancement of the room temperature thermoelectric properties of up to 30{\%} over that of the bulk was realized. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V35.00008: Nanostructured thermoelectric skutterudite Co$_{1-x}$Ni$_{x}$Sb$_{3}$ alloys Qinyu He, Xiaowei Wang, Jian Yang, Yucheng Lan, Xiao Yan, Bo Yu, Yi Ma, Bed Poudel, Giri Joshi, Dezhi Wang, Zhifeng Ren, Qing hao, Gang Chen Nanostructured Ni-doped skutterudites Co$_{1-x}$Ni$_{x}$Sb$_{3}$ (with x ranging from 0.01 to 0.09) have been prepared by ball-milling and direct-current induced hot press. It was found that the thermal conductivity was much reduced due to strong electrons-phonons scaterring from Ni-doping and grain boundary phonons scattering from nano-structures in the samples. A maximum dimensionless figure-of-merit of 0.71 has been obtained in Co$_{0.91}$Ni$_{0.09}$Sb$_{3}$ at 525 \r{ }C. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V35.00009: Thermal conductivity reduction by interface roughness in AlN$_{x}$-GaN$_{y}$ superlattices. Yee Kan Koh, Yu Cao, David Cahill, Debdeep Jena A reduction of cross-plane thermal conductivity $\Lambda $ by a factor of two is achieved in AlN$_{4 nm}$-GaN$_{52 nm}$ superlattices by varying the plasma power during growth. This reduction is attributed to interface roughness, introducing a new parameter to be considered in the design and fabrication of superlattices for thermoelectric applications. Thermal conductivity of AlN$_{x}$-GaN$_{y}$ superlattices, x $\sim $ 4 nm and 2 $<$ y $<$ 1000 nm, with rough interfaces is then measured by time-domain thermoreflectance. $\Lambda $ decreases monotonically as the GaN thickness y decreases, $\Lambda $ = 6.35 W m$^{-1}$ K$^{-1}$ at y = 2.2 nm. We observe no minimum thermal conductivity as a function of period for these rough superlattices. A continuum model incorporating the effects of interface roughness indicates that diffuse scattering is dominant when y $>$ 20 nm, and significant coherent transmission occurs when y $<$ 20 nm. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V35.00010: High dimensionless figure-of-merit in nanostructured p-type bulk SiGe alloys Giri Joshi, Hohyun Lee, Xiaowei Wang, Goahua Zhu, Yucheng Lan, Dezhi Wang, Bed Poudel, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon Germanium (SiGe) alloys have been used for high temperature power generation in thermoelectric generators that provided the onboard electrical power to several US space vehicles. Since their performance is related to dimensionless figure-of-merit (ZT), material scientists have focused their attention on possible improvements in ZT of SiGe alloys through an increase in power factor and decrease in thermal conductivity. We have been pursuing an approach of random nanostructures to reduce the thermal conductivity based on the understanding that the reduction of thermal conductivity is primarily responsible for ZT enhancement in superlatices. We have observed 100{\%} improvement in ZT, compared to the state-of-the-art values, in p-type SiGe nanostructured bulk materials, which comes mainly from decrease in thermal conductivity due to the increase in phonon scattering by defects and grain boundaries in random nanostructures. These bulk materials also possess superior mechanical properties making them more suitable for fabrication and integration into systems which were made by hot pressing of nanopowders prepared by using high energy ball milling. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V35.00011: Nanotructured high thermoelectric figure-of-merit in n-type bulk SiGe alloys Xiaowei Wang, Hohyun Lee, Gaohua Zhu, Yucheng Lan, Dezhi Wang, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Silicon germanium alloys (SiGe) have been the exclusice choice for radiosotope thermoelectric generators (RTGs) because of its reliability and high operating temperatures. In this paper, by using mechanical alloying method, nano-sized phosphorous-doped (n-type) SiGe alloy powders were produced, followed by direct current induced hot press technique, bulk samples were obtained with $\sim $100{\%} density. The thermoelectric properties, including electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured in the temperature range from 25 to 900 $^{\circ}$C. A maximum ZT of around 1.3 at 900$^{\circ}$C was obtained under certain doping concentration and hot press conditions, mainly due to thermal conductivity reduction from nano-structured SiGe grains of 20 nm. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V35.00012: Thermoelectric properties of n-type nano bulk Si Gaohua Zhu, Hohyun Lee, Xiaowei Wang, Giri Joshi, Yucheng Lan, Jian Yang, Dezhi Wang, Mildred Dresselhaus, Gang Chen, Zhifeng Ren Nano Si has been noticed as a promising substitute material for SiGe. We have observed a 150 to 350 {\%} increase in ZT values in the heavily doped n-type nano Si over the single crystal Si, which mainly comes from the significantly lower thermal conductivity due to the nano size of grains achieved by mechanical alloying and hot press. The key to get higher ZT value is to optimize the doping concentration to achieve high power factor and avoid grain growth during hot-pressing to achieve nano particle size in the final bulk form. So far our research on nano bulk Si has shown promising ZT values close to 1, comparable to that of the traditional SiGe alloys. Silicon is lighter, more refractory, and has better thermal stability than Si$_{0.8}$Ge$_{0.2}$, and also it is much cheaper than Ge. Pure Si may have advantage over SiGe alloy in mass application of power generation systems. [Preview Abstract] |
Session V36: Advances in Scanned Probe Microscopy IV: Optical and High Frequency Methods
Sponsoring Units: GIMSChair: Phillip First, Georgia Institute of Technology
Room: Morial Convention Center 228
Thursday, March 13, 2008 11:15AM - 11:27AM |
V36.00001: High-resolution element-selective microscopy using X-ray enhanced Scanning Tunneling Microscopy Volker Rose, John Freeland, Kenneth Gray, Stephen Streiffer, Matthias Bode Nanoscale structures are at the forefront of fundamental research as well as the keystone for whole new classes of potential applications. Proper understanding of these systems requires tools with both the ability to resolve the nanometer scale as well as provide detailed information about chemical, electronic and magnetic structure. Scanning probe microscopies achieve the requisite high spatial resolution; however, direct elemental determination is not easily accomplished. X-ray microscopies, on the other hand, provide elemental selectivity, but currently have spatial resolution only of tens of nanometers. We present a radically different concept that employs detection of local x-ray interactions utilizing a scanning probe that provides spatial resolution, and x-ray absorption directly yields chemical, electronic, and magnetic sensitivity. The achievement of nanometer spatial resolution with direct elemental selectivity will have a tremendous impact on our ability to probe and understand complex phenomena occurring in nanostructures. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V36.00002: Scanning optical homodyne detection of high-frequency picoscale resonances in cantilever and tuning fork sensors J. C. Randel, G. Zeltzer, A. K. Gupta, R. Bashir, S.-H. Song, H. C. Manoharan Hybrid high-frequency sensors represent the next generation of scanned probe technology. In this work, higher harmonic modes in nanoscale silicon cantilevers and microscale quartz tuning forks are detected and characterized using a custom scanning optical homodyne interferometer. Capable of both mass and force sensing, these resonators exhibit high-frequency harmonic motion content with picometer-scale amplitudes detected in a 2.5 MHz bandwidth, driven by ambient thermal radiation. Quartz tuning forks additionally display both in-plane and out-of-plane harmonics. The first six electronically detected resonances are matched to optically detected and mapped fork eigenmodes. Mass sensing experiments utilizing higher tuning fork modes indicate greater than six times sensitivity enhancement over fundamental mode operation. (This work supported by NSF and ONR). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V36.00003: Radio Frequency Scanning Tunneling Microscopy: Instrumentation and Applications Utku Kemiktarak, Tchefor Ndukum, Keith C. Schwab, Kamil L. Ekinci A severe limitation of the scanning tunneling microscope (STM) is its low temporal resolution, originating from the diminished high-frequency response of the tunnel current readout circuitry. In order to overcome this limitation, we have built a radio-frequency STM (RF-STM). Using this instrument, we can attain electronic bandwidths as high as 10 MHz by measuring the reflection from or transmission through the tunnel junction, which is embedded in a resonant inductor-capacitor (LC) circuit. This $\sim $100-fold bandwidth improvement upon the state-of-the-art translates into fast surface topography as well as delicate measurements in mesoscopic electronics and mechanics. Broadband noise measurements across the tunnel junction using this radio-frequency-STM (RF-STM) have allowed us to perform nanoscale thermometry. Furthermore, we have detected high-frequency mechanical motion with a sensitivity approaching 15 fmHz$^{-1/2}$. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V36.00004: Thermomechanical Noise Measurements of Very High Frequency (VHF) Nanomechanical Resonators Carl Hart IV, Kamil Ekinci We have designed and built a near infrared (NIR) optical interferometer for ultra-sensitive displacement measurements on nanomechanical resonators. At moderate optical power levels, we are able to resolve the thermomechanical displacement fluctuations of stiff doubly-clamped beams with fundamental mode frequencies in the 100 MHz range. In a first set of noise measurements, we have determined the quality factors and resonant frequencies of the nanomechanical resonators under ambient atmospheric pressure at room temperature. We will compare these values to those extracted from driven resonance measurements and discuss sources of disagreement. Furthermore, we will discuss extraction of the local temperature of the nanomechanical resonators in order to assess the heating due to the optical probe. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V36.00005: Development of an Evanescent Microwave Probe / Scanning Tunneling Microscope to study Localized Electron Spin Resonance Christian Long, Naoyuki Taketoshi, Ichiro Takeuchi, Haitao Yang, Xiao-Dong Xiang We have constructed a microwave microscope with an integrated scanning tunneling microscope. In addition to the measurement of complex dielectric constant and conductivity we also perform atomic resolution scanning tunneling microscopy (STM). In this work the probe is operated in a magnetic field, which causes unpaired spins in the sample to precess at the Larmor frequency. When the magnetic field is such that the Larmor frequency of the spins in the sample matches the resonant frequency of the resonator, the transmission coefficient of the resonator (S12) is changed. We measure local ESR by measuring the variation in S12 as a function of magnetic field. In this report, we give an outline of the experimental setup and preliminary spin detection data for various spin radical molecules (one unpaired spin each) on an HPOG substrate (no unpaired spins). Supported by the W. M. KECK Foundation and NSF MRSEC (DMR 0520471). [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V36.00006: High Frequency Piezoresponse Force Microscopy in the 1-10 MHz Regime Katyayani Seal, Stephen Jesse, Brian Rodriguez, Arthur Baddorf, Sergei Kalinin Imaging mechanisms in Piezoresponse Force Microscopy (PFM) in the high frequency regime above the first contact resonance are analyzed. High operation frequencies are expected to provide several advantages including (a) higher signal to noise ratios due to a larger number of oscillations per pixel time and increased separation from the 1/$f$ noise corner (b) imaging at cantilever resonances with an associated increase in mechanical signal amplification (c) inertial stiffening of the cantilever that minimizes the non-local electrostatic force contribution to the signal and improves tip-surface contact. Furthermore, high frequency operation is an essential component of the PFM-based ferroelectric data storage systems, currently limited by the bandwidth of electromechanical detection (1-10 kHz). At the same time, operation at a high mode number can give rise to several problems, including the (a) response averaging due to the finite size of the cantilever beam (b) loss of sensitivity if the tip-surface spring constant becomes smaller than the effective spring constant of the cantilever and (c) signal loss due to the bandwidth of the photodetector. Analytical expressions for these limits are considered. We analyze the operation mechanisms in PFM at high frequencies, and demonstrate high quality PFM imaging at 1-10 MHz. Prospects for imaging in the 10-100 MHz range are explored. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V36.00007: Contrast sensitive imaging with a cantilever-based near-field microwave probe Keji Lai, Worasom Kundhikanjana, Michael Kelly, Zhi-xun Shen We have developed a procedure to systematically study the contrast mechanism of our cantilever-based near-field scanning microwave probes with separated excitation and sensing electrodes. Finite-element analysis was employed to model the small impedance change due to the tip-sample interaction. The near-field signal can then be calculated from the S-parameters of the matching network that routes the tip impedance to 50 Ohm feed lines. Using a common-mode cancellation scheme, the microwave electronics is sensitive down to 1aF capacitance change at our working frequencies near 1GHz. Experimental characterization of the microwave probes was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals, in qualitative agreement with simulation, can be obtained using different reflection modes of the probe. Our microscope also shows very high contrast due to conductivity variations of the sample, which can lead to potential applications of the technique. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V36.00008: STM and SNOM Type of Scanning Probe Microscopes in the Same Unit: Towards Electrical Modification and Optical Characterization at Nanoscale Ilya Sychugov, Hiroo Omi, Tooru Murashita, Yoshihiro Kobayashi Optical and electrical properties of nanostructures can be addressed using radiation or electrical current as a probe. In general, a near-field type of electromagnetic interaction is necessary for an optical probe to enter nanoscale regime. However, a typical scanning near-field microscope utilizes a dielectric fiber tip as an aperture, which makes it unsuitable for electrical measurements. Here, in order to realize both electrical and optical probing at nanoscale, we have combined it with a scanning tunneling microscope (STM). An STM-luminescence (STML) instrument with a conductive and transparent tip, featuring about 40 nm spatial resolution, was reported previously. We have complemented it with a beamsplitter unit in a configuration typical for the fluorescent microscopy. The excitation light is guided through a beamsplitter unit to the indium tin oxide (ITO) tip and the signal is collected via the same fiber transmission line in a spectroscopy mode or in a photon mapping regime. The influence of tip geometry on collection efficiency and spatial resolution as well as limitations of such an instrument are discussed. This approach may find its niche not only for combined electrical and optical measurements, but also for electrical modification with subsequent \textit{in situ} optical probing. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V36.00009: A Unique Probe for Tip Enhanced Raman Scattering and Shadow NSOM Aaron Lewis, Hesham Taha, Rimma Dekhter, Galia Zinoviev, Galina Fish We present a unique atomic force microscope [AFM] probe for tip enhanced Raman scattering [TERS] and a new form of near-field microscopy, ``Shadow Near-field Scanning Optical Microscopy''. The probe consists of a single gold nanoparticle grown at the tip of a cantilevered nanopipette, exposed to the optical axis of an upright or inverted optical microscope. When these probes are used in combination with a Nanonics MV 2000 AFM/NSOM system, we show that a protocol for independent motion of the probe and the sample can produce enhancement or a shadow effect. Both of these effects are enhanced by the ability to affect different Raman spectra with the tip in {\&} out of contact while independently scanning the sample. We analyzed Raman signals of a thin nanometric strained Si layer deposited on bulk Si and developed an understanding of optical mechanisms of enhancement, scattering and shadowing. Our results show different optical mechanisms occur as a result of tip {\&} sample interactions, including TERS effect obtained by near-field interaction of the probe with the top layer of strained Si. Large enhancements of at least 4 orders of magnitude are seen and analyses of relative intensities of bulk and strained Si Raman peaks show an increase in light scattered by bulk or effective shadowing of the surface. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V36.00010: Chemical Imaging of the Surface of Polymeric Nanostructures Using Apertureless Near-Field IR Microscopy Zahra Fakhraai, Kerstin Mueller, Melissa Paulite, Xiujuan Yang, Gilbert C. Walker The chemical composition of the surfaces of thin films of polystyrene-poly (methyl methacrylate) (PS-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. In this technique a tunable IR beam is scattered from an oscillating atomic force microscopy tip. The scattered light is enhanced using a reference signal with the same optical frequency (homodyne) or slightly shifted IR frequency (heterodyne) and detected after demodulation in order to eliminate the background scattering. Using this technique a lateral chemical imaging resolution of $<$20 nm is achievable. It is demonstrated that this technique can be successfully used to image the surface of PS-PMMA diblock copolymers. It is shown that an increase in the IR absorption is observed in the PMMA rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a topography induced artifact can be observed, when homodyne detection technique is used, the chemical signature of the sample can be detected clearly. This technique can be further used in a variety of different systems to detect the surface structure of polymers or proteins. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V36.00011: Scanning thermal microscopy with a fluorescent nanoprobe Benjamin Samson, Elika Saidi, Lionel Aigouy, Peter Low, Beomjoon Kim, Christian Bergaud, Michel Mortier We have developed a scanning thermal microscope that uses a fluorescent particle settled at the extremity of an atomic force microscope tip as a nanoscale temperature sensor. When a temperature change occurs, a modification of fluorescence is detectable, enabling to perform thermal images and to determine the local temperature. We will describe the technique in details and present some thermal images on submicron sized nickel nanoheaters heated by an electrical current. We will show that this apparatus works in both DC and AC regimes, in a low frequency range whose upper limit is around the kilohertz. By performing tip approach/retraction curves on a heated wire, we will describe the different thermal transfer mechanisms between the surface and the fluorescent probe. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V36.00012: A versatile technique for fabrication of SiC SPM probes Joel Therrien, Daniel Schmidt, Sheetal Barrot, Bhavin Patel To date SPM probes have largely been fabricated via methods borrowed from the semiconductor industry for fabricating Micro Electro Mechanical Systems. Although these techniques have enabled SPM to see widespread use, the processes put significant limitations on what structures can be made. We report our progress on fabricating SPM cantilevers composed of Silicon Carbide using polymer molding techniques. A pre-ceramic polymer is molded into the desired probe shape and then converted to SiC via pyrolisys. We will also report on progress in using photo-sterolithography for fabrication of even more complex geometries. In addition to opening up a much larger set of probe structures, the use of SiC leads to improved wear resistance of the resulting probes. Among the potential applications, this method enables the fabrication of low spring constant, high resonant frequency cantilevers via cross sectional geometries not accessible to standard fabrication techniques. Such probes are required for high speed tapping and non-contact imaging. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V36.00013: \textit{In-situ} broadband microwave calibrations and measurements using cryogenic probe stations Jeffrey Lindemuth, Scott Yano Until recently, calibration of microwave measurements in cryogenic environments required custom fixtures [1,2]. These fixtures were necessary to accommodate the limited space in typical ``down bore'' cryostat designs. The typical three measurements (through, short, load, for example) required removing the fixtures from the cryostat three times before loading the sample of interest. We will show that with a 4-probe cryogenic probe station it is possible to simultaneously load a commercially available calibration test strip and the measurement sample. We will demonstrate calibration of the complete S matrix of the microwave network analyzer at each temperature followed by measurement of the sample. In addition, the temperature-dependent effects of the calibration can be determined. We will show short term and long term calibration stability. \newline [1] Broadband calibration of long lossy microwave transmission lines at cryogenic temperatures using nichrome films, M. L. Stutzman, Mark Lee, and R. F. Bradley, Rev. Sci. Instrum. 71, 4596 (2000) \newline [2] Broadband microwave spectroscopy in Corbino geometry for temperatures down to 1.7 K [Preview Abstract] |
Session V37: QHE and FQHE
Sponsoring Units: FIAPChair: Wei Pan, Sandia National Laboratories
Room: Morial Convention Center 229
Thursday, March 13, 2008 11:15AM - 11:27AM |
V37.00001: Optical Studies of Quantum Phases with Filling Factors 2$\le \nu \le $3 in the Second Landau Level Trevor David Rhone, Jun Yan, Yann Gallais, Aron Pinczuk, Loren Pfeiffer, Ken West We report low temperature inelastic light scattering and optical recombination measurements of quantum phases in the second Landau level of 2D electron systems. We focus on states with filling factors 2$\le \nu \le $3. An ultra high mobility, high density GaAs quantum well (240nm) is probed at low temperature (42mK -- 1.2K). Low energy spin excitations are studied by resonant inelastic light scattering. We confirm the existence of a ferromagnetic state at $\nu $ = 3 by the observation of a well-defined long wavelength spin wave mode at the Zeeman energy. Surprisingly, the ferromagnetic spin wave collapses at filling factors slightly away from $\nu $ = 3. While this behavior may be a signal of the disappearance of ferromagnetic order in the second Landau level, experiments in progress may offer deeper insights on fundamental interactions and quantum phases in the second Landau level. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V37.00002: Investigations of possible Landau level anticrossings in p-type (110) GaAs square wells Nebile Isik, S.F. Roth, M. Bichler, A. Fontcuberta i Morral, G. Abstreiter, M. Grayson In p-type (110) quantum wells, a single anomalous magnetoresistance peak is observed within the lowest Landau level ($\nu= 1$) of a two-dimensional hole system when the confinement is a triangular potential well [1]. This peak is interpreted as the transport signature of a $\pm$ 3/2 spin-reversal level-anticrossing in the lowest Landau level. In recently fabricated 400 \AA wide square quantum wells, we observe peaks within several Landau levels, for example, at both $\nu=1$ and $\nu=2$. The peak positions in magnetic field $B$ are observed shift with top gate bias and the peak magnitudes are observed to increase with increasing temperature. At a critical top gate bias, however, both original peaks disappear, just as new features appear at higher fields within both the $\nu=1$ and $\nu=2$ minima. Dependence of feature position on front and back gate bias will be shown, and temperature dependences will be studied in terms of activated energy gaps. The cause of these peaks will be discussed in terms of possible multiple Landau level anticrossings. \newline [1] F. Fischer, R. Winkler, D. Schuh, M. Bichler, G. Abstreiter and M. Grayson Phys. Rev. B 75, 073303 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V37.00003: Current Instability Induced by Negative Differential Conductivity in the Quantum Hall Regime Kuan Ting Lin, Yuling Tsai, Yiping Lin, J. C. Chen, T. Ueda, S. Komiyama We report a new kind of nonlinear effect at $\nu=2$ Hall plateau in a two-dimensional electron gas system formed on a GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. Within the temperature 16 K $\leq$ T $\leq$ 28 K, the current shows discernible zig-zag feature as the applied dc electric field increases beyond a critical value. Such an unstable feature is not only reproducible but also exhibits hysteresis as changing the sweeping direction of the electric field. We identify this current instability as the occurrence of the negative differential conductivity induced by the electric field. Physical origin of the nonlinearities will be presented and discussed. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V37.00004: New Observations in the $N\ge1$ Landau Levels of Wide GaAs Quantum Wells D.R. Luhman, W. Pan, D.C. Tsui, L.N. Pfeiffer, K.W. Baldwin, K.W. West We have studied a series of high-quality wide GaAs/AlGaAs quantum wells at various tilt angles ($\theta$) with respect to magnetic field. Several interesting observations have been made when the Landau level index is $N\ge1$. The quantum Hall states corresponding to total odd filling factors are seen to be reentrant with increasing $\theta$, i.e. they disappear and reemerge as $\theta$ is increased. This observation can be explained by considering oscillations in the tunnelling amplitude with increasing $\theta$. As $\theta$ is increased further, anisotropy emerges at values of half filling. This is particularly visible in the $N=1$ Landau level at total filling factors $\nu=9/2$ and $11/2$. For even larger values of $\theta$, anisotropy develops at $\nu=5$ followed by $\nu=7$. We will discuss how each of these observations are effected by quantum well width and also in the context of previous theoretical and experimental results. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V37.00005: Edge state tunneling in a point contact at filling fraction $\nu$=5/2 Iuliana P. Radu, J.B. Miller, C.R. Dillard, C.M. Marcus, M.A. Kastner, L.N. Pfeiffer, K.W. West We investigate low temperature transport properties of quantum point contacts (QPCs) fabricated in a GaAs/AlGaAs 2-dimensional electron gas (2-DEG) with mobility 2000 m$^{2}$/Vs in a perpendicular magnetic field. The 2-DEG exhibits fractional quantum Hall effect, including a well-quantized plateau at $\nu$=5/2. We study the temperature and DC current bias dependence of the transport through the QPC at $\nu$=5/2 while preserving the same filling number in both the QPC and the bulk of the sample. We compare our results to theoretical predictions for quasi-particle tunneling in the weak coupling regime, and extract the quasi-particle charge and the strength of the Coulomb interaction, as reflected by the Luttinger liquid parameter $g$. This work was partially supported by ARO (W911NF-05-1-0062), by the NSEC program of NSF (PHY-0117795), by NSF (DMR-0353209) at MIT and by Project Q of Microsoft Corporation at Harvard University. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V37.00006: Stripes and Bubbles in the N=2 Landau Level: A DMRG Study Barry Friedman, Lauren Rod, Candice Withrow The phase diagram of the N=2 Landau level has been reexamined using the density matrix renormalization group (dmrg). Very good agreement at filling factor 18/42 has been attained with previous dmrg calculations of Shibata and Yoshioka by using 200 states in the blocks. Near 1/2 filling, we have studied whether the ground state is an anisotropic crystal, suggested by some mean field approaches, or a stripe state, as suggested by other mean field approaches and previous dmrg results. Previous dmrg calculations, i.e. Shibata and Yoshioka, have placed the phase diagram between stripes and bubbles at a filling factor slightly less then .4 by looking at the projected pair correlation function at the special lines x=0 and y=0. We reexamine this boundary by studying the Fourier transform of the projected pair correlation. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V37.00007: Interactions and Landau level mixing at large half odd-integer filling Peter Smith, Malcolm Kennett Recent transport experiments on two-dimensional hole systems (2DHSs) in a strong perpendicular magnetic field show variations in the anisotropy of resistivity at half odd-integer filling fractions $\nu$=7/2, 9/2, and 11/2 that differ from those seen in two-dimensional electron systems \footnote[1]{M. J. Manfra et al., Phys. Rev. Lett. {\bf 98}, 206804 (2007).}. It has been suggested that spin-orbit coupling in 2DHSs leads to these unusual transport properties, as it gives rise to Landau level mixing. We consider the general problem of interacting fermions subject to a perpendicular magnetic field with Landau level mixing in the limit of large Landau levels, and discuss the effect of Landau level mixing on charge-density wave formation in comparison to the single Landau level case \footnote[2]{R. Moessner and J. T. Chalker, Phys. Rev. B. {\bf 54}, 5006 (1996).}. We study the case of spin-orbit induced mixing in detail, and discuss implications for experiments. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V37.00008: Interaction effects in transport through an electronic Mach-Zehnder interferometer Vitaly Golovach, Florian Marquardt We study theoretically transport through an electronic Mach-Zehnder interferometer in the presence of Coulomb interaction inside the interferometer, using a discrete wave-packet model. We find that the mutual capacitance between the arms of the interferometer leads to a suppression of the visibility of the Aharonov-Bohm oscillations at a large source-drain bias $\Delta\mu\gg\hbar v_F/L$, where $L$ is the length of the arms and $v_F$ is the electron drift speed. Our numerical simulations indicate that the visibility of the Aharonov-Bohm oscillations is a non-analytic function of the mutual capacitance strength, in the limit $\Delta\mu\to\infty$. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V37.00009: Universal Periods in Quantum Hall Droplets Gregory A. Fiete, Gil Refael, Matthew P. A. Fisher Using the hierarchy picture of the fractional quantum Hall effect, we study the the ground state periodicity of a finite size quantum Hall droplet in a quantum Hall fluid of a different filling factor. The droplet edge charge is periodically modulated with flux through the droplet and will lead to a periodic variation in the conductance of a nearby point contact, such as occurs in some quantum Hall interferometers. Our model is consistent with experiment and predicts that superperiods can be observed in geometries where no interfering trajectories occur. The model may also provide an experimentally feasible method of detecting elusive neutral modes and otherwise obtaining information about the microscopic edge structure in fractional quantum Hall states. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V37.00010: Variational Monte Carlo study of the nematic state at half-filled Landau level of the two dimensional electron gas Quoc Doan, Efstratios Manousakis The nematic state of 2DEG at half-filled Landau level (LL) is
investigated using the variational Monte Carlo method. The
wavefunction used has Jastrow correlations of the form
$\prod_{i |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V37.00011: Anisotropy in two-dimensional electronic quantum Hall systems at half filling of valence Landau levels Orion Ciftja, Carlos Wexler A recent experimental discovery in the quantum Hall regime has been the observation of very strong magneto-transport anisotropies at certain low values of magnetic field below a critical temperature of about 100 mK. While the origin of such anisotropy is yet unknown, we explain the emergence of such anisotropies in terms of electronic liquid crystalline states with broken rotational symmetry. We investigate the stability of liquid crystalline phases with nematic order at half filling of the valence Landau level. Quantum Monte Carlo simulations indicate that while isotropic states are stable in the lowest and first excited Landau level, there are regions of instability towards liquid crystalline states in higher Landau levels. A possible connection of the recently discovered magneto-transport anisotropy in low magnetic fields and these liquid crystalline states is discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V37.00012: Quantum Hall hierarchy wave functions from conformal field theory Susanne Viefers, Emil Bergholtz, Hans Hansson, Maria Hermanns, Anders Karlhede It has long been known that Laughlin's wave functions, describing the fractional quantum Hall effect at filling fractions $\nu=1/(2k+1)$, can be obtained as correlation functions in conformal field theory. We show how to generalize this approach to construct explicit trial wave functions for all states in the quantum Hall hierarchy corresponding to quasiparticle (as opposed to quasihole) condensates, including the recently observed state at $\nu = 4/11$. At the filling fractions $\nu = n/(2np+1)$ this construction exactly reproduces Jain's composite fermion wave functions. An explicit connection is made to Wen's topological classification of FQH states. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V37.00013: FQHE-the solvable limit and beyond Maria Hermanns, Emil J. Bergholtz, Thors Hans Hansson, Anders Karlhede, Juha Suorsa We consider the quantum Hall system in the torus geometry. In the limit where the torus becomes thin, the problem is exactly solvable and the hierarchy of quantum Hall states is manifest. Explicit wave functions for a large set of them are constructed with help of conformal field theory. This construction provides a continuation from the exactly solvable limit to the experimental regime. Numerical results on 4/11 supports this picture. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V37.00014: The Quantum Hall Effect in Spin Quartets in Graphene Keshav Shrivastava Using the non-relativistic Schroedinger equation, we find that for (1/2)g=(1/2)$\pm $s gives zero charge for negative sign and one charge for positive sign. This explains the conductivity at i = 0 and 1. For s=3/2, (1/2)g=2 for positive sign and hence g=4. The substitution in the series, -(5/2)(g$\mu _{B}$H), -(3/2)(g$\mu _{B}$H), -(1/2)( g$\mu _{B}$H), +(1/2)( g$\mu _{B}$H),+(3/2)( g$\mu _{B}$H), +(5/2)( g$\mu _{B}$H), {\ldots}, etc., g=4 gives, -10, -6, -2, +2, +6, +10, etc. This series is the same as observed in the experimental data of quantum Hall effect in graphene. When we take n=2 in the flux quantization, i.e., 2(hc/e), we generate the plateaus at $\pm $4. Thus the plateaus can occur at 0, 1, 4 and at 2, 6, 10, 14, {\ldots}, etc. Thus the quantum Hall effect in graphene is understood by means of non-relativistic theory. The fractions such as 1/3 or integers such as 0,1,4,{\ldots}, 2,6,10,14, {\ldots} multiply the charge and hence describe the ``effective charge'' of the quasiparticles. This means that there is ``spin-charge locking''. \begin{enumerate} \item K. N. Shrivastava, Phys. Lett. A 113, 435(1986); 115, 436(E)(1986); Phys. Lett. A, 326, 469(2004); AIP Conf. Proc. 909, 43(2007);909,50(2007. \item Z.Jiang, et al, Phys. Rev. Lett. 98,197403(2007);Y.Zhang et al, Phys. Rev. Lett. 96, 136806(2006). \end{enumerate} [Preview Abstract] |
Session V38: Focus Session: Multiferroics and Multiferroic Composites
Sponsoring Units: DCMPChair: Claude Ederer, Trinity College, Ireland
Room: Morial Convention Center 230
Thursday, March 13, 2008 11:15AM - 11:51AM |
V38.00001: In pursuit of strongly coupled multiferroic oxides Invited Speaker: The rational design of new materials with emergent properties is a riveting challenge today in materials physics. It begins with understanding a mechanism to control the interplay between diverse microscopic degrees of freedom in order to create targeted macroscopic phenomena and ends with the discovery or design of new material realizations. When combined with first-principles density-functional theory, this approach provides an efficient strategy to survey the vast space of possible materials to target for synthesis. In this talk I will discuss our recently proposed strategies to identify new multiferroics oxides in which magnetism not only coexists with but also is strongly coupled to ferroelectricity. In one case the interplay of spins, optical phonons, and strain leads to a competition between different ordered states producing a colossal magnetoelectric effect. In a second case, a ferroelectric distortion can be designed to induce weak-ferromagnetism facilitating the electric-field control of a switchable magnetization. We also present first-principles density-functional calculations for several potential realizations. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V38.00002: Growth and characterization of YMnO$_{3}$ nanocrystalline prisms E. Ramos-Moore, Cameron Keenan, Sandeep Chandril, A. L. Cabrera, Thomas H. Myers, David Lederman YMnO$_{3}$ epitaxial self-assembled nanoprisms were grown on GaN by molecular beam epitaxy. The prisms formed suddenly after 6 unit cells during Stranski-Krastanow epitaxial growth at 850 $^{o}$C, with sizes ranging from 20 nm to 60 nm in height and 50 nm to 500 nm in lateral dimensions. They were characterized via energy dispersive analysis of x-rays (EDAX) mapping and x-ray diffraction and reflectivity. EDAX line scans across the prisms showed that the prisms were composed of YMnO$_{3}$. X-ray diffraction showed a 2.2{\%} strain along the growth direction and a 1.6{\%} compression in the plane when compared to bulk lattice parameters. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V38.00003: Properties of YMnO3 Self-assembled Nanocrystalline Prisms on GaN Cameron Keenan, Sandeep Chandril, Thomas H. Myers, David Lederman, Esteban Ramos-Moore, Alejandro L. Cabrera The high-temperature (850 $^{\circ}$C) molecular beam epitaxy deposition of YMnO$_{3}$ on HVPE GaN(0001) resulted in the spontaneous formation of YMnO$_{3}$ nanoprism structures due to Stranski-Krastanow growth mode. Their dimensions ranged from 20 nm to 60 nm in thickness and 50 nm to 500 nm in lateral size. The local dielectric properties of the sample were investigated using scanning surface potential microscopy (SSPM). Remanent hysteresis loops were used to compare the switching properties of the nanoprisms and underlying film. A larger remanence was observed for the prisms, most probably due to the lack of surrounding film areas that would clamp the ferroelectric response. As a result, the remanent polarization increased roughly linearly with increasing prism surface area. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V38.00004: Epitaxial growth and electrical properties of {\it perovskite} HoMnO$_{3}$ thin films Daisuke Kan, M. Murakami, W. Yu, R.L. Greene, S.W. Cheong, I. Takeuchi Recently, it was predicted that orthorhombic HoMnO$_{3}$ would exhibit relatively large polarization (of the order of $\mu$C/cm$^ {2}$) due to the existence of a ferroelectric order in the {\it E}- type magnetic structure [1]. We have fabricated perovskite HoMnO$_{3}$ thin films on SrTiO$_{3}$ (001) substrates by pulsed laser deposition. X-ray 2$\theta$-$\theta$ scan shows (00 {\it l}) peaks only indicating that there are no secondary phases. The reciprocal space mapping around the SrTiO$_{3}$ (103) Bragg reflection and the $\phi$ scan of the HoMnO$_{3}$ (103) peak reveals the tetragonal symmetry of the films and the epitaxial relationship, [100]HoMnO$_{3}$ // [100]SrTiO$_{3}$ and [001] HoMnO$_{3}$ // [001]SrTiO$_{3}$, confirming that the film has the perovskite structure and not the hexagonal one. The films show good insulating properties at low temperatures. We will present electrical properties of the films including results of PUND measurements in applied magnetic field below the Neel temperature. This work is supported by NSF MRSEC, ARO, and the W. M. Keck Foundation. \newline [1] Sergienko et al., PRL 97, 227204 (2006). [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V38.00005: Magnetic ground state of PbVO$_{3}$ and origin of the large tetragonal distortion Kengo Oka, Ikuya Yamada, Masaki Azuma, Mikio Takano, Yuichi Shimakawa, Soshi Takeshita, Kohki H. Satoh, Akihiro Koda, Ryosuke Kadono, Hiroshi Kojitani, Masaki Akaogi Magnetic property of PbTiO$_{3}$-type perovskite PbVO$_{3}$ with a large tetragonal distortion ($c/a$ = 1.23)$^{1}$ was investigated. Long-range antiferromagnetic ordering with $T_{N}$ = 45 K was found by a $\mu $SR measurement. Magnetic susceptibility of multi domain crystal showed a broad maximum centered at 180 K indicated the 2-dimensional nature in magnetism. This is consistent with the $d_{xy}$ orbital ordering predicted by the band calculation.$^{2}$ We propose a mechanism that the pyramidal coordination with a short vanadium-apical oxygen bond is stabilized to lift the orbital degeneracy in this $d^{1}$ system. Accordingly, PbMnO$_{3}$ (Mn$^{4+}$, $d^{3})$ newly stabilized at 15 GPa has a pseudo cubic structure like PbCrO$_{3}$. This scenario explains the pressure induced tetragonal to cubic, insulator to metal transition$^{1}$ of PbVO$_{3}$ and similar large tetragonal distortion of BiCoO$_{3}$. [1]A. A. Belik, \textit{et al., Chem. Mater.}, \textbf{18} (2006) 798. [2]Y. Uratani, \textit{et al.}, \textit{Jpn. J. Appl. Phys.}, \textbf{44} (2005) 7130. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V38.00006: A one-cent room-temperature magnetoelectric sensor Casey Israel, Neil Mathur, James Scott The replacement of silver-palladium with nickel as an electrode material is reducing the cost of industrially manufactured barium titanate based multilayer capacitors, but nickel is magnetic. We discuss direct magnetoelectric coupling at room temperature in these one-cent multilayer capacitors, the influence of the multilayer geometry on this strain-mediated coupling, and why these capacitors might be adopted as cheap magnetic-field sensors that do not require an electrical power source. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V38.00007: Inversion symmetry controlled multiferroic response in an Ising Chain Magnet YounJung Jo, SeongSu Lee, HeeTaek Yi, SangWook Cheong, Luis Balicas Recently, Choi et al. [1] discovered that ferroelectricity in the Ising chain magnet results from inversion symmetry breaking due to the formation of an alternating ionic order of two different ions leading to an up-up-down-down spin configuration. Here we report the observation of drastic changes in the multiferroic character as a function of magnetic field in the Ising chain magnet. Application of a magnetic field up to 33 T, leads to two abrupt changes in the dielectric constant, respectively at $H_1$ = 5 T and at $H_2$=20T, with the first one leading to a fast suppression of the dielectric polarization. The saturation values of the magnetic moment observed by using a vibrating sample magnetometer and a cantilever beam magnetometer suggest that the one ionic moments align along the field for fields above 5 T followed by the subsequent alignment of the other ionic moments above 20 T. [1] Y.J. Choi $et al.$ Physical Review Lett. submitted [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V38.00008: Inter-relationship among magnetization, polarization and magnetic wave vector in multiferroic spinel CoCr$_{2}$O$_{4}$ Young Jai Choi, Jun Okamoto, Di-Jing Huang, Kuo Sheng Chao, Hong-Ji Lin, Chien-Te Chen, Michel van Veenendaal, Thomas A. kaplan, Sang-Wook Cheong It has been known that the spinel CoCr$_{2}$O$_{4}$ is a multiferroic with conical spiral magnetic order characterized by three physical parameters: magnetization (\textbf{M}), polarization (\textbf{P}) and magnetic modulation vector (\textbf{Q}). We have found that these observables have a close inter-relationship in the multiferroic state and their inter-relationship changes in a methodical manner upon temperature variation and through applied magnetic fields. In order to understand the origin of this inter-relationship, we have performed comprehensive characterization experiments on the multiferroic compound, including measurements of dielectric constant, polarization, specific heat, magnetic susceptibility, and resonant x-ray scattering as functions of temperature and applied magnetic field. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V38.00009: Structural and Dielectric Properties of CoFe$_{2}$O$_{4}$-Ba$_{0.90}$La$_{0.067}$Ti$_{0.91}$Zr$_{0.09}$O$_{3}$ composite thin films. Eduardo Delgado, Carlos Ostos, Maria Martinez, Lourdes Mestres, David Lederman, Pedro Prieto CoFe$_{2}$O$_{4}$-Ba$_{0.90}$La$_{0.067}$Ti$_{0.91}$Zr$_{0.09}$O$_{3 }$(CFO-BLZT) composite thin films were grown via RF oxygen magnetron sputtering from a CFO-BLZT mixed target on electrically-conducting single-crystal Nb-doped SrTiO$_{3}$ (100) substrates at 1033 K. From scanning electron microscopy coupled with energy dispersive analysis of x-rays we determined that the CFO and BLZT were phase separated. X-ray photoemission spectroscopy showed that the TiO$_{6}$ octahedron in the perovskite structure was modified by the lanthanide incorporation. The dielectric characterization showed that the samples were ferroelectric at room temperature. The ferroelectric hysteresis loops measured as a function of magnetic field showed that these samples are multiferroic. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V38.00010: Studies of Magnetoelectric properties in (1-x)Pb(Fe$_{2/3}$W$_{1/3})$O$_{3}$-xPbTiO$_{3}$ solid solutions thin films Ashok Kumar, Ram Katiyar We have fabricated (1-x)Pb(Fe$_{2/3}$W$_{1/3})$O$_{3}$-xPbTiO$_{3}$ (PFWT) (x=0.50) thin films on Pt/Si substrates using sol-gel technique. The XRD patterns revealed a single-phase nature of the compound. The microstructure and surface morphology were investigated using SEM and AFM techniques that indicated good homogeneity and had surface roughness of 10-15 nm with particle size of $\sim $ 30-50 nm. The dielectric relaxation studies in these films were carried out measured in the temperature range of 100K-650K and the frequency range of 100Hz-1MHz. Raman and dielectric data indicate that the crystal structure changes from tetragonal to cubic i.e. a ferroelectric phase transition at 575K. The dielectric properties of PFWT thin films were studied in the temperature range 80-600 K over a wide range of frequencies. The slope of the reciprocal of the dielectric constant is 2:1, matched well with the simplest Landau free energy model and it indicates continuous second order displacive ferroelectric phase transition. The polarization hysteresis curve at room temperature illustrated a ferroelectric nature. The detailed of magnetic and magneto electric properties with of PFWT will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V38.00011: Magnetoelectric Effects in hexagonal ferrite-PZT bilayers V.M. Mathe, G. Srinivasan Magnetoelectric (ME) bilayers consisting of magnetostrictive and a piezoelectric layer are of interest for studies on the nature of ME interactions and useful technologies. Co$_{2}$Z and Zn$_{2}$Y are well known hexagonal ferrites with easy plane or uniaxial anisotropy. PZT has high piezoelectric coefficient. This study is on samples with Co$_{2}$Z or Zn$_{2}$Y as a magnetostrictive layer and PZT as a piezoelectric layer to form magnetoelectric bilayers. Low frequency (100 Hz) ME coefficient was measured over 0-17 kOe for various orientations of bilayers in a plane parallel to ac and bias magnetic fields. We measured a strong dependence of the ME voltage coefficients on magneitude and orientation of the bias field. The data are compared with theory. VLM gratefully acknowledge the award of a BOYSCAST fellowship and a FAST TRACK fellowship by DST, India. The research was supported by NSF grants. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V38.00012: Magnetic properties in multiferroic Fe/BaTiO$_{3}$/SrTiO$_{3}$(001) M. Pechan, C. Yu, S. Srivastava, C. Palmstr{\O}m, M. Biegaslski, C. Brooks, D. Schlom Fe (001) films (30 nm) have been epitaxially grown on ferroelectric BaTiO$_{3}$/SrTiO$_{3}$(001) to study the interaction between the multiferroic components. We report the magnetic properties as a precursor to a full investigation of the multiferroic interactions. Ferromagnetic resonance measurements were carried out at 36 GHz cavity and variable frequency microstrip resonators. Four-fold anisotropy is present in these Fe (001) films with energy density $\sim $1.3x10$^{5}$ erg/cm$^{3}$, consistent with bulk Fe values and indicating high quality Fe grown on the ferroelectric. Also present is a small, additional resonance peak, with large out-of-plane uniaxial anisotropy [$\sim $0.75*(2$\pi $M$^{2})$] attributed to strained Fe at the interface, although structural data shows the majority of the film is relaxed. This interfacial Fe will likely provide the desired coupling between the ferromagnetic and ferroelectric components in the system. Frequency/field in-plane dispersion curves further confirm the high quality of the Fe films in these samples. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V38.00013: Dynamic observation of magnetoelectric coupling effect using magnetic force microscopy and Lorentz TEM Sung Hwan Lim, Todd Brintlinger, Daisuke Kan, Yi Qi, John Cumings, Ichiro Takeuchi, Lourdes Salamanca-Riba Dynamic observation of the strain mediated magnetoelectric (ME) coupling effect was pursued by applying an external electric (E) field to a Fe$_{0.7}$Ga$_{0.3}$/BaTiO$_{3 }$(FeGa/BTO) TEM sample in the Lorentz imaging mode in a TEM. An epitaxial piezoelectric BTO layer followed by a polycrystalline magnetostrictive FeGa layer were synthesized by pulsed laser deposition and magnetron sputtering, respectively. The BTO film had Ps = 17 $\mu $C/cm$^{2 }$measured using SrRuO$_{3}$ (bottom) and FeGa (top) electrodes. The FeGa film showed strong magnetic anisotropy with in-plane easy axis. To apply an E field in-plane, FeGa was patterned on BTO using e-beam lithography. The change of magnetic domain structure was observed under an applied magnetic field in TEM. The ME effect was also investigated using MFM. After applying an E field (100 kV/cm) on the BTO layer, a reorientation of the magnetic moments in the FeGa layer along the E field direction was observed. [Preview Abstract] |
Session V39: Focus Session: Jamming I: Theory
Sponsoring Units: GSNPChair: Wouter Ellenbroek, University of Pennsylvania
Room: Morial Convention Center 231
Thursday, March 13, 2008 11:15AM - 11:27AM |
V39.00001: Anisotropic power-laws in sheared amorphous solids Craig Maloney, Mark Robbins The local deformation of two-dimensional Lennard-Jones glasses under imposed shear strain is studied via computer simulations. Spatial correlations in the strain field are highly anisotropic and show apparent power-law behavior with a dramatic angular dependence of the effective scaling exponent. The strongest correlations are for wavevectors roughly perpendicular to the line of maximum resolved shear stress with systematic deviations from this which can be understood in terms of a Mohr-Coulomb effect. These results shed light on the nature of the so-called Jamming transition, supporting the notion that the dense steady flowing state is effectively critical in the slow-driving limit, and provide important, testable predictions for experiments on sheared amorphous materials such as bubble rafts, foams, emulsions, granular packings, etc., which can directly access the particle displacements. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V39.00002: Apparent critical scaling for a steady-state sheared glass Thomas Haxton, Andrea Liu We conducted simulations of a two-dimensional model glass at nonzero temperature under steady-state shear, and calculated the shear stress and shear viscosity as a function of temperature, shear rate and density. Over a dynamic range of two to three decades, we find excellent collapse of the data using critical scaling of the shear viscosity as a function of the distance from the jamming surface in the parameter space spanned by temperature, shear stress, and density. The shear viscosity can be rescaled to collapse onto a master function of the rescaled stress, where the scale factors are powers of the displacement in parameter space from the critical jamming surface. The master function separates into two branches, a high-temperature or low-density branch that approaches a finite rescaled viscosity at low stress, and a low-temperature or high-density branch that appears to diverge at a finite value of the rescaled stress. These results are consistent with those of Olsson and Teitel[1], who found scaling collapse near the zero-temperature, zero shear-stress jamming transition. We compare our results to mode-coupling calculations of sheared systems. [1] P. Olsson and S. Teitel, Phys. Rev. Lett. 99, 178001 (2007). [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V39.00003: Predicting the Viscosity of a Supercooled Liquid Xi Lin We present an atomistic description of the viscosity of a supercooled liquid capturing the highly non-Arrhenius temperature variation for which no previous calculation has been given. A temperature dependent activation energy for structural relaxation is derived by mapping the potential energy surface and extracting saddle-point configurations and associated atomic coordinates. This essential information is combined with the temperature variation of an effective local energy minimum (inherent structure) to describe shear relaxation by thermal activation. For a binary Lennard-Jones model the calculated viscosity shows a characteristic crossover from strong (Arrhenius) to fragile (highly non-Arrhenius) behavior upon appreciable undercooling, followed by a second crossover from fragile back to strong behavior on approaching the glass transition temperature, both features we believe to be generic. Analysis of atomic displacements associated with barrier crossing in the fragile regime suggests a scenario of correlated motions along a chain of particles as the underlying mechanism for slow viscous relaxation in glassy states. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:27PM |
V39.00004: Jamming in systems with attraction Invited Speaker: Many materials jam. As density increases or temperature decreases, structural relaxation becomes sluggish and the system approaches mechanical equilibrium without spatial ordering. The concept of a universal jamming transition and the conjecture that the mechanical response at zero temperature is linked to slow dynamics at non-zero temperature has inspired research in a variety of glassy materials such as colloidal suspensions, emulsions, granular media and foams. While most recent theoretical and simulation studies of the jamming transition have focused on systems with purely repulsive interactions, many materials also possess attractive forces. I will present our recent numerical results on the jamming transition in particulate systems with attractive interactions. At zero temperature, instead of the single discontinuous jamming transition observed in purely repulsive systems, attractive systems exhibit two second-order transitions---connectivity and rigidity percolation---which belong to different universality classes than their lattice counterparts. This observation also holds for low temperature before diffusion and activation/bond-breaking become relevant. At higher temperatures, the universality class of the jamming transition can depend on the age of the system. Finally, I will discuss a proposed phase diagram for gelation and rigidification in the temperature- density plane. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V39.00005: Is there a connection between structure and heterogeneous dynamics in supercooled liquids? William Krekelberg, Venkat Ganesan, Thomas Truskett Structurally arrested (jammed or glassy) states are often prepared from supercooled fluids, though there a number of open questions regarding how that process occurs. One involves explaining why modest increases in structural order accompany the pronounced slowing of liquid-state dynamics near structural arrest. Another involves understanding why self-diffusion in deeply supercooled fluids occurs much faster than would be predicted from knowledge of the viscosity and the Stokes-Einstein relation. Single-particle displacements become heterogeneous near the glass transition, but corresponding structural heterogeneities have been difficult to identify. Collectively, these observations call into question the general prospect of understanding and predicting dynamical behavior of liquids based on structural information. In this talk, we present simulation data on several model systems that show that the dynamics of supercooled liquids can be quantitatively correlated to structure in a simple way. Specifically, we show that the breakdown of the Stokes-Einstein relationship reflects simple and distinct couplings between structure, viscosity, and diffusivity in the supercooled fluid. We also demonstrate how heterogeneous dynamics correlate with dynamically heterogeneous structure. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V39.00006: An effective field theory for soft granular matter Silke Henkes, Corey O'Hern, Bulbul Chakraborty Work on packings of soft spheres (PRE \textbf{68}, 011306 (2003)) has demonstrated the existence of a jamming transition and has highlighted the need for a general statistical framework to describe granular packings. We have shown that a statistical ensemble, based on conservation properties of the global stress tensor, is consistent with simulated packings of frictionless disks (PRL \textbf{99}, 038002 (2007)). We construct an effective field theory based on this ensemble, in the spirit of an earlier attempt (PRL \textbf{95}, 198002 (2005)). The field theory is constructed by synthesizing results from simulations into one functional form for the effective free energy. We will describe ongoing efforts to derive this form by combining scaling ideas with microscopic properties of the packings. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V39.00007: Jamming and correlated percolation on energetically-evolved graphs Shiliyang Xu, Jennifer Schwarz Numerical simulations suggest that the zero-temperature jamming transition in repulsive soft spheres has an unusual mixed second-order/first-order character whose exponents appear to be in the same universality class as mean-field $k$-core percolation. In $k$-core percolation model, every occupied site must have at least $k$ occupied neighbors. The $k$-core analogy of jamming is similar to the kinetically constrained analogy of the glass transition where the geometric constraint of $k=d+1$ contacts needed for local mechanical stability drives the transition. We now introduce energetics explicitly into the analogy by investigating $k$-core percolation on a graph where edges are dynamically evolved with the goal of minimizing an xy-model-type interaction between a node and each of its neighbors. The xy-model-type interaction captures the angular arrangements of jammed configurations at the onset of jamming. Moreover, the graph dynamics captures nonequilibrium aspects of jamming that cannot be captured by static approaches such as a version of rigidity percolation with repulsive forces only. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V39.00008: A ``Hamiltonian'' for Jammed Granular Matter Chaoming Song, Ping Wang, Hernan A. Makse We introduce a ``Hamiltonian''-like function, called the volume function, to describe the microstates of jammed matter such as granular materials and emulsions from a geometrical point of view. We present a theory of volume fluctuations and derive the volume function defined in terms of the available free volume of the particles in the jammed systems. At the microscopic level the volume function provides an analytical formula for the calculation of the Voronoi volume associated with a single particle in terms of field variables. We then coarse-grain the volume function over a scale of a few particle diameters and provide a mesoscopic volume function which is now solely a function of the coordination number. We predict an exponential tail in the distribution of volumes in general agreement with experiments. Our analysis allows the calculation of macroscopic obervables using the statistical mechanics of jammed states when it is supplemented by the condition of mechanical equilibrium of jamming. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V39.00009: Vacancy localization in the square dimer model Mark Bowick, Jeremie Bouttier, Emmanuel Guitter, Monwhea Jeng We study the classical dimer model on a square lattice with a single vacancy by developing a graph-theoretic classification of the set of all configurations which extends the spanning tree formulation of close-packed dimers. The motion of a vacancy induced by dimer slidings is analyzed including the size distribution of the domain accessible to the vacancy and the probability for a vacancy to be strictly jammed in an infinite system. More generally, the size distribution of the domain accessible to the vacancy is characterized by a power law decay with exponent 9/8. In a finite system, the probability that a vacancy in the bulk can reach the boundary falls off as a power law of the system size with exponent $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $. The resultant weak localization of vacancies still allows for unbounded diffusion with a diffusion exponent related to that of diffusion on spanning trees. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V39.00010: Velocity fluctuations in dense granular flows John Drozd, Colin Denniston We use simulations to investigate velocity fluctuations in dry granular flow. Our system is comprised of mono- and poly-disperse sets of spherical grains falling down a vertical chute under the influence of gravity. We find three different classes of velocity distributions depending on factors such as the local density. The class of the velocity distribution depends on whether the grains are in a free-fall, fluid or glassy state. The analytic form of the distributions match those that have been found by other authors in fairly diverse systems. Here, we have all three present in a single system in steady-state. Power-law tails that match recent experiments are also found but in a transition area suggesting they may be an artifact of crossover from one class of velocity distribution to another. By studying both fast and slow flowing systems, we find that the velocity fluctuations are related to collision times by a scaling with the glass transition temperature. We measure collision time distributions along the height of the chute and find that the collision time distributions evolve from exponential tails into power-laws. This suggests that the particles may be forming clusters as they approach the glass state which may correspond to a second order dynamical phase transition. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V39.00011: Energy Transport of Jammed Systems Ning Xu, Vincenzo Vitelli, Matthieu Wyart, Andrea Liu, Sidney Nagel We performed computer simulations to calculate the thermal diffusivity of vibrational modes in jammed sphere packings near the jamming transition (Point J). The diffusivity $d(\omega)$ is low for all modes, including those at low frequency $\omega$, and appears to be finite in the zero frequency limit. In ordinary solids, by contrast, $d(\omega)$ diverges at low frequencies due to long wavelength plane waves. The low- frequency modes near Point J are very different from plane waves: they are quasi-localized with large anharmonic corrections. Thus, these modes, which can be viewed as harmonic precursors to two-level systems, are poor conductors of energy. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V39.00012: Testing ergodicity in dense granular systems Guo-Jie Gao, Jerzy Blawzdziewicz, Corey O'Hern The Edwards' entropy formalism provides a statistical mechanical framework for describing dense granular systems. Experiments on vibrated granular columns and numerical simulations of quasi- static shear flow of dense granular systems have provided indirect evidence that the Edwards' theory may accurately describe certain aspects of these systems. However, a fundamental assumption of the Edwards' description---that all mechanically stable (MS) granular packings at a given packing fraction and externally imposed stress are equally accessible---has not been explicitly tested. We investigate this assumption by generating all mechanically stable hard disk packings in small bidisperse systems using a protocol where we successively compress or decompress the system followed by energy minimization. We then apply quasi-static shear flow at zero pressure to these MS packings and record the MS packings that occur during the shear flow. We generate a complete library of the allowed MS packings at each value of shear strain and determine the frequency with which each MS packing occurs. We find that the MS packings do not occur with equal probability at any value of shear strain. In fact, in small systems we find that the evolution becomes periodic with a period that grows with system-size. Our studies show that ergodicity can be improved by either adding random fluctuations to the system or increasing the system size. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V39.00013: Anomalously Slow Dynamics in the Manhattan Model Prasanta Pal, Corey O'Hern We study the Brownian dynamics of hard rods in a Manhattan-like traffic grid, in which a series of narrow horizontal and vertical channels intersect at right angles and particles are forbidden from turning at the intersections. We measure the mean-square displacement (msd) as a function of packing fraction $\phi$ and determine the $\phi_g$ at which dynamical arrest occurs as a function of system size, number of intersections, and topology of the grid. We observe that structural relaxation occurs via a complex out-of-equilibrium process in which particles occupy locally dense regions of the grid and then undergo a first passage process. We compare our results for the msd and $\phi_g$ to that found in model glass-forming liquids in two and three dimensions. [Preview Abstract] |
Session V40: Nucleic Acids: Structure and Function
Sponsoring Units: DBPChair: Noel Clark, University of Colorado
Room: Morial Convention Center 232
Thursday, March 13, 2008 11:15AM - 11:27AM |
V40.00001: Toward multiscale modeling of the chromatin fiber: a coarse grain model for DNA Alexey Savelyev, Garegin Papoian In eukaryotic cells DNA is compacted a million-fold into a chromatin. Understanding the mechanism of chromatin folding is of great biological importance. All-atom Molecular Dynamics (MD) simulations could provide crucial insights into the electrostatic and structural mechanisms of chromatin folding. However, because of the enormous size of even short chromatin fiber segment and long folding time-scales, atomistic simulations are computationally impractical. Our long-term aim is to build an accurate coarse-grain (CG) model of the chromatin, derived systematically from all-atom simulations of its smaller parts. Here we report the development of the CG model for a linear DNA chain, playing the role of a linker DNA segment in the chromatin. We derived CG inter-DNA electrostatic potential from atomistic simulations with explicit solvent and mobile ions, instead of relying on the standard models of continuum electrostatics, which are inadequate at small intermolecular distances. In addition, we used the ideas of renormalization group theory to construct an optimization scheme for parameterizing the CG force field. This novel approach is designed to accurately reproduce correlations among various CG degrees of freedom. The implementation of these correlations was left as an open question in the prior studies of CG polymer models. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V40.00002: Sequence and Temperature Dependence of DNA Bending Fluctuations Robert A. Forties, Ralf Bundschuh, Michael G. Poirier Recent DNA cyclization experiments measured J factors indicating that DNA may form sharp bends more easily than predicted by the worm-like chain model. One proposed explanation is that local melting of a few base pairs (bp) introduces flexible hinges [Yan, J. et al., \textit{Phys. Rev. E} \textbf{71}, 061905 (2005)]. We incorporate sequence dependence of the local melting into this model and obtain specific predictions for the dependence of J factors on temperature and sequence. We then measure J factors for a 200 bp fragment of lambda DNA and two synthetic 116 bp sequences with different proclivities for melting. The measured temperature and sequence dependence of J factors is found to be in agreement with the sequence dependent model predictions using previously measured free energy costs for melting and reasonable estimates for the flexibility of melted segments of DNA. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V40.00003: Mechanically unzipping dsDNA with built-in sequence inhomogeneities and bound proteins Abhijit Sarkar We theoretically analyze the force signal from unzipping dsDNA with bound proteins and sequence inhomogeneties. Two different force traces are obtained determined by binding and sequence parameters. Sawtooth force curves, as observed in experiments, are found for short enough designed sequences and binding sites. Longer inhomogeneities lead to force plateaus which correspond to gradual, piece-meal, unzipping of the variant stretch of DNA. We generalize our model to allow comparisons to recent experiments on unzipping decorated DNAs. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V40.00004: Base-pair elasticity of free and complexed DNA Nils B. Becker, Ralf Everaers The elastic properties of the DNA molecule are important for its function. On a base-pair scale, they modulate protein binding strength, while over hundreds of base--pairs, they govern the statistics of DNA loops. We have used the rigid base--pair (RBP) model to link experiments on DNA elasticity across these scales. In a study of the indirect readout mechanism in protein--DNA binding, we compare calculated DNA elastic free energy differences to experimental affinities. While quantitative predictions are beyond the precision of current parameter sets, qualitative predictions are meaningful; we propose a statistical marker for indirect readout sub-sites in a given co-crystal complex structure. Furthermore, we relate the RBP model to the worm--like chain (WLC) by a systematic coarse-graining procedure, reducing a total of 270 parameters to 6, which agree remarkably well with direct experimental results. Introducing sequence randomness adds fluctuations and renormalizes WLC stiffness. On short scales, sequence variability and bending anisotropy have a large effect, exhibiting the limits of applicability of the WLC model. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V40.00005: Hartree-Fock Cluster Study of Electronic Structures and Nuclear Quadrupole Interactions in Solid Nucleobases. R.H. Scheicher, Archana Dubey, S.R. Badu, H.P. Saha, R.H. Pink, K. Nagamine, E. Torikai, Lee Chow, T.P. Das In recent work [1] we have studied nucleobases attached to a CH$_{3}$ group to simulate the influence of their binding to the sugar rings and the phosphate groups in DNA and RNA and the effect of this binding on the nuclear quadrupole interactions of $^{14}$N, $^{17}$O and $^{2}$H nuclei. Our results from this work have indicated that for $^{17}$O, the binding to the CH$_{3}$ group moves our results from the free nucleobases closer to the experimentally observed data [2] in the solid nucleobases. We are now investigating the solid nucleobases by the first --principles Hartree-Fock cluster procedure that we have employed earlier for the halogen molecular solids [3]. Our results for the binding energy of an imidazole molecule in the molecular solid system and the $^{14}$N, $^{17}$O and $^{2}$H nuclear quadrupole interaction parameters will be presented. \newline [1] T.P. Das et al (at this APS meeting), [2] Gang Wu et al, J. Am.Chem. Soc. 124, 1768(2002). [3] M.M. Aryal et al Hyperfine Interactions (to be published). [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V40.00006: Nuclear Quadrupole Interaction Study as a Probe of Interaction between Nucleobases and Suger Rings and Phosphate Groups in DNA. T.P. Das, Archana Dubey, R.H. Scheicher, S.R. Badu, R.H. Pink, K. Nagamine, E. Torikai, H.P. Saha, Lee Chow, M.B. Huang We have been investigating the influence of the interaction between the nucleobases and sugar rings and phosphate groups in DNA using Nuclear Quadrupole interactions (NQI) of $^{14}$N and $^{17}$O and $^{2}$H nuclei as probes. We have first simulated the influence of the interaction between a nucleobase and a sugar ring using a CH$_{3}$ group attached to the former. For our electronic structure investigations, we have employed the Hartree-Fock-Roothaan procedure using the Gaussian set of programs. Our preliminary investigations have shown that there are comparable indirect and direct effects on the NQI parameters, the former effect referring to the influence of changes in molecular geometries produced by the CH$_{3}$ group and the direct effect is due to the electronic interaction between the CH$_{3}$ group and the nucleobase. More quantitative results from our current investigations using the actual sugar rings and phosphate groups will be presented as in earlier work by our group[1] for hyperfine interactions of trapped muonium atoms in DNA.[1] R.H. Scheicher et al Physica B \underline {374-375}, 448(2006) [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V40.00007: Extraction of complementary from non-complementary DNA sequences through phase separation and centrifugation Taiquitha Robins, Dacia McPherson, Chenhui Zhu, Mark Moran, Dave Walba, Giuliano Zanchetta, Tommaso Bellini, Noel Clark Double stranded deoxyribonucleic acid (DNA) is known to form lyotropic liquid crystal (LC) phases, nematic and then columnar with increasing DNA concentration in water. Single stranded (DNA) does not form liquid crystal phases. We study the phase separation of both long (900bp) and short (6-20bp) DNA. In the mixture solution of a self complementary sequences (scDNA) and non complementary sequences (nscDNA), the scDNA forms DNA double helices and hence forms LC phases while the nscDNA stays in the isotropic phase, the LC appearing in the form of phase separated droplets. We report results of the use of centrifugation to produce complete spatial segregation of complementary and noncomplementary DNA, based on their different LC-formation tendencies. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V40.00008: Phase Separation and Liquid Crystallization of Complementary Sequences in Mixtures of Random Oligonucleotides Giuliano Zanchetta, Michi Nakata, Marco Buscaglia, Tommaso Bellini, Noel Clark We have investigated the phase behavior of mixtures of DNA oligomers, 8-22 bp in length. When only a fraction of the sample is composed of mutually complementary sequences, and hence the solution is effectively a mixture of single strands (ss) and double stranded helices (ds), the system is found to phase separate via the nucleation of ds-rich liquid crystalline domains from an isotropic background rich in ss. This spontaneous partitioning is the combined result of the free energy gain from the end-to-end stacking and LC ordering of duplexes, and of depletion-type interactions favoring the segregation of the more rigid duplexes from the flexible ss. Phase separation and liquid crystallization are also found in mixtures of oligos with various degrees of randomness in the sequence, enabling to establish the phase behavior in an extended phase space including a radomness axis. The observed phenomena offer a new route to the purification of duplex oligomers and, if in the presence of ligation, could provide a mode of positive feedback for the preferential synthesis of extended complementary oligomers, a mechanism of possible relevance in prebiotic environments. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V40.00009: Scanning Tunneling Microscopy study of ssDNA-CNT on Au(111) surface Xinghua Lu, Eric Brandin, Jene Golovchenko, Daniel Brandon The single-strand deoxyribonucleic acid (ssDNA) - carbon nanotube (CNT) complex on Au(111) surfaces has been studied via scanning tunneling microscopy (STM). The interaction between ssDNA and CNT not only disperses the nanotubes, but also makes the ssDNA more accessible for the STM study. Sputtering on the ssDNA-CNT complex helps to reveal the internal structure. Scanning tunneling spectroscopy (STS) has been carried out to study the electronic structure of the ssDNA-CNT complex. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V40.00010: Liquid Crystal Alignment on Sheared DNA Films Michi Nakata, Giuliano Zanchetta, Marco Buscaglia, Tommaso Bellini, Noel Clark We have studied the alignment of commercial nematic and smectic A liquid crystals (8CB, 6CB, CCN47, MBBA) on the chiral surface obtained by shearing double stranded DNA on a glass surface. Simple characterization of hybrid cells (DNA-homeotropic) and partially sliding cells (DNA-GLYMO) reveal that the nematic director at the DNA surface is oriented at an angle from 50$^{\circ}$ to 100$^{\circ}$ with respect to the shearing direction, indicating that the liquid crystal molecules align preferentially perpendicularly to the DNA grooves. These observations present clear evidence for a large chiral orientational effect in the anchoring of a typical nematic/SmA LC on dehydrated sheared DNA films. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V40.00011: Cholesterol improves the transfection efficiency of lipoplexes by increasing the effective membrane charge density Cyrus R. Safinya, Alexandra Zidovska, Heather M. Evans, Kai K. Ewert Motivated by its important role in lipid-mediated gene delivery, we have studied the effect of cholesterol on the transfection efficiency (TE) of lamellar, cationic lipid-DNA (CL-DNA) complexes. A successful \textit{in vivo }liposome mixture seems to require cholesterol. Recent work in our group has identified the membrane charge density ($\sigma )$ as a universal parameter for TE of lamellar, DOPC containing CL-DNA complexes (A.J. Lin et al, \textit{Biophys. J.}, 2003, K. Ewert et al, \textit{J. Med. Chem.}, 2002, A. Ahmad et al., \textit{J. Gene Med., }2005), with TE following a universal bell-shaped curve as a function of $\sigma $. Theoretical calculations considering the headgroup area of cholesterol and thus necessarily counting for an increase in $\sigma $, when DOPC is replaced by cholesterol, show that TE strongly deviates from the TE universal curve. However, experimental determination of $\sigma $ via X-ray diffraction shows full agreement with the TE universal curve demonstrating that the real $\sigma $ is higher as predicted, therefore the effective headgroup area of cholesterol is lower as expected by theory, suggesting that cholesterol is inserted deep into lipid bilayer partially hidden by the neighboring lipids. Funding provided by NIH GM-59288 and NSF DMR-0503347. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V40.00012: Measuring the 3D Size of Large RNA Molecules Ajaykumar Gopal, Defne Egecioglu, Li Tai Fang, Charles M. Knobler, William M. Gelbart, Marc Niebuhr, A. L. N. Rao Large single-stranded (ss) RNAs are ubiquitous in cells and constitute the genomic content of many viral species. Besides being the primary means of intra-cellular information transfer, some of their functions require them to form stable structural motifs. ssRNA molecules possess intrinsic self-complementarity leading to a partially double-stranded, branched, secondary structure. We measure, in solution, the physical dimensions of several sequences of ssRNA ranging from a few hundred to a few thousand nucleotides in length. Sizes are reported as radii of gyration ($R_g$) and hydrodynamic radii ($R_h$), respectively determined by small-angle x-ray scattering (SAXS) and fluorescence correlation spectroscopy (FCS). For RNAs of fixed nucleotide length ($\sim$2000) and composition, we find that $R_g$s and $R_h$s can vary by over 30$\%$. By changing solvent conditions, we demonstrate that these size discrepancies are a generic property of the secondary structure arising from sequence-dependent base-pairing. Some viral RNAs that self-assemble into spherical protein capsids have highly evolved sequences that code for unusually compact size and shape. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V40.00013: Predicting the Size of Large RNA Molecules Aron Yoffe, Peter Prinsen, Ajaykumar Gopal, Charles Knobler, William Gelbart, Avinoam Ben-Shaul We present a qualitative theory of how the 3D sizes of large single-stranded (ss) RNA molecules depend on their sequences. The work is motivated by the fact that the genomes of many viruses are large ssRNA molecules and that these RNAs are spontaneously packaged into small rigid protein shells. We argue there has been evolutionary pressure for the genome to have large-scale spatial properties -- including an appropriate radius of gyration, $R_{g }$-- that facilitate and optimize this assembly process. We introduce the average maximum ladder distance (\textit{AMLD}) as a measure of the `extendedness' of the RNA secondary structure. We find that the \textit{AMLDs} of viral ssRNAs are smaller than those of equal-length randomly permuted sequences. By mapping these secondary structures onto simple linear or star polymer models, and using \textit{AMLD} as a measure of effective contour length, we predict that the $R_{g}s$ of viral RNAs are smaller than those of random sequences. More generally, we derive results for how the \textit{AMLDs} of large ssRNAs, and their $R_{g}s$, scale with the number of nucleotides. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V40.00014: Observations of simple RNA suboptimal structures including pseudoknots suggests that the folding landscape is often funnel shaped Wayne Dawson, Gota Kawai Many RNA structure are known to fold up into complex function structures such as ribosomal RNA, transfer RNA (tRNA), riboswitches, etc. We are currently developing a novel theoretical approach for predicting the base pairing topology of folded RNA structures [1,2], a term known as RNA secondary structure. A good prediction of this base pairing can significantly speed up computation of the full 3D structure of these complex molecules. In recent work, we reported a pseudoknot prediction application using this model [3]. We have now upgraded this application to also predict suboptimal structures. The results of this model suggest that structures like tRNA often have a folding landscape of suboptimal structures that is essentially funnel shaped; similar to what is known to be the case for many simple proteins. This model has also been applied to simple protein structure topology prediction in a similar fashion. [1] Dawson, et al. (2001). \textit{J Theor Biol. }213, 359-386 and 387-412. [2] Dawson, et al. (2006). \textit{Nucleosides, Nucleotides, and Nucleic Acids} 25, 171-189. [3]Dawson, et al. (2007). \textit{PLoS One}, 2, 905. [Preview Abstract] |
Thursday, March 13, 2008 2:03PM - 2:15PM |
V40.00015: Simplified Hamiltonians for coarse-grained properties of large single-stranded RNA molecules Peter Prinsen, Aron Yoffe, William Gelbart Large single-stranded RNA (ssRNA) molecules with a length of a few thousand to a few tens of thousands of nucleotides are quite common in nature. These RNAs generally have a highly branched secondary structure with many short, double-stranded sections. The secondary structure is important for function. However, the prediction of the thermally accessible secondary structures of large ssRNAs is complicated.There are several computer programs available that predict secondary structures of ssRNA. They produce good results for small molecules but are not very reliable for large ones. We are not interested in ``high-resolution`` structures, however, but in more coarse-grained properties, for example the average three-dimensional size of the molecule. We expect that the available computer programs are useful for determination of these coarse-grained properties but the complicated Hamiltonians they use limit the usefulness of these models for further theoretical investigations. We show that one can simplify these Hamiltonians considerably and still retain important predictive power. The inclusion of stacking energies is crucial but many of the detailed energy rules are not. We define several measures for the size of a secondary structure and we show how these measures are related to each other. [Preview Abstract] |
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