Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J1: Pake Prize, McGroddy Prizes, Buckley Prize Session
Sponsoring Units: DCMP FIAP DMPChair: Allen Goldman, University of Minnesota
Room: Morial Convention Center LaLouisiane AB
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J1.00001: Oliver E. Buckley Prize Talk: Why are we so excited about carbon nanostructures? Invited Speaker: There is much current excitement about the interesting new physics and unusual physical properties of carbon nanostructures, particularly carbon nanotubes and graphene. A brief review will be given of the physical underpinnings of carbon nanostructures that were developed over the past 60 years, starting with the electronic structure and physical properties of graphene and graphite, and then moving to graphite intercalation compounds which contained the first carbon nanostructures to be studied experimentally. Liquid carbon studies were precursors to the fullerene family of nanostructures and vapor grown carbon fibers were precursors to carbon nanotubes. Particular emphasis is given to the recent developments in our understanding of the photophysics of carbon nanotubes and graphene, with perspectives on future research directions for these fields. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J1.00002: George E. Pake Prize Talk: Science and the Energy Security Challenge: The Example of Solid State Lighting Invited Speaker: Securing a viable, carbon neutral energy future for humankind will require an effort of gargantuan proportions. As outlined clearly in a series of workshops sponsored by the DOE Office of Basic Energy Sciences (http://www.sc.doe.gov/bes/reports/list.html), fundamental advances in scientific understanding are needed to broadly implement many of the technologies that are held out as promising options to meet future energy needs. Technologies of interest range from solar energy, to nuclear energy, to approaches to clean combustion. Using solid state lighting based on inorganic materials as an example, I will discuss some recent results and new directions, emphasizing the multidisciplinary, team nature of the endeavor. I will also offer some thoughts about how to encourage translation of the science into attractive, widely available products -- a significant challenge that cannot be ignored. This case study offers insight into approaches that are likely to be beneficial for addressing other aspects of the energy security challenge. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J1.00003: James C. McGroddy Prize Talk: Superconductivity in alkali-metal doped Carbon-60 Invited Speaker: Carbon sixty (C$_{60})$, which was first identified in 1985 in laser desorption experiments, is unquestionably an arrestingly beautiful molecule. The high symmetry of the 12 pentagonal and 20 hexagonal faces symmetrically arrayed in a soccer-ball like structure invites special attention and continues to stimulate animated speculation. The availability in 1990 of macroscopic amounts of purified C$_{60}$ derived from carbon-arc produced soot allowed the growth and characterization of both bulk and thin-film samples. Crystalline C$_{60}$ is a molecular solid held together by weak van der Waals forces. The fcc structure has a 74{\%} packing fraction thus allowing ample opportunity (26{\%} available volume) for the intercalation of foreign atoms into the interstitial spaces of the three dimensional host. This opportunity catalyzed much of the collaborative work amongst chemists, physicists and materials scientists at Bell Laboratories, and resulted in the discovery of superconductivity in alkali-metal doped C$_{60 }$with transition temperatures (T$_{c})$ in the mid-30-kelvin range. In this talk I will review how the successes of this initial team effort stimulated a worldwide collaboration between experimentalists and theorists to understand the promise and potential of an entirely new class of superconductors containing only two elements, carbon and an intercalated alkali metal. Although the cuprates still hold the record for the highest T$_{c}$, there are still open scientific questions about the mechanism that gives rise to such unexpectedly high T$_{c}$'s in the non-oxide carbon-based superconductors. The doped fullerenes have unusual attributes (e.g., narrow electronic bands, high disorder, anomalous energy scales, and a tantalizing proximity to a metal-insulator Mott transition), which challenge conventional thinking and at the same time provide useful insights into new directions for finding even higher T$_{c}$ materials. The final chapter of the `soot to superconductivity' story has yet to be written. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J1.00004: James C. McGroddy Prize Talk: What Was New About C60 Invited Speaker: C60 was named molecule of the year by Science in 1991, and in this talk I will discuss what I consider to be the most novel features of the molecule. In some ways C60 is truly unique and the discovery of the molecule in 1985 and its subsequent synthesis in 1990 blazed a trail of new chemical and physical properties that is unlikely to be surpassed by any other molecule. I will discuss the electronic structure of C60, its magnetism, and the conductivity and superconductivity shown by the alkali metal-doped phases. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J1.00005: The p-and d-electron superconductors -Struggle to find higher-$T_{c}$ superconductors Invited Speaker: After the discovery of MgB$_{2}$, 7 years have already passed, and a new higher-$T_{c}$ superconductor has now been desired. In this invited session, we review our present status (struggle?) to find higher-$T_{c}$ superconductors along the following lines. 1) 2-dimensional Cu-oxides having different crystal structures with CuO$_{2}$ planes, such as ladders, Lieb model Cu-oxide etc. 2) Metal superconductors including light elements (boron, carbon etc.), being suggested with MgB$_{2}$, diamond etc. Recently, we found a new superconductor boron doped SiC which belongs to the same category with boron doped diamond and Si etc. 3) We also present the superconducting properties of the clathrate-type silver oxides Ag$_{6}$O$_{8}$AgNO$_{3}$ ($T_{c}$=1.04K) and Ag$_{6}$O$_{8}$AgHF$_{2}$ ($T_{c}$=1.36K). [Preview Abstract] |
Session J2: 50th Anniversary of Physical Review Letters
Sponsoring Units: FHPChair: Reinhardt Schuhmann
Room: Morial Convention Center LaLouisiane C
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J2.00001: PRL at 50: A history of moving physics forward Invited Speaker: Fifty years ago, Editor Sam Goudsmit announced the introduction of a new journal, Physical Review Letters, which would collect the pre-existing ``Letters to the Editor'' in The Physical Review into a separate Review. According to his July 1958 editorial, the new journal would consider only ``Letters which really deserve rapid publication'' in order to ``maintain the high speed and high standards.'' Fifty years after its creation, Physical Review Letters has grown into a journal of choice for publishing important work, which includes many Nobel-Prize-winning discoveries, in all fields of physics. Today, the journal continues to attract a steady growth of worldwide submissions that have reached the level of over 10,000 submitted manuscripts per year. To gain insight into the evolution of the new journal from its beginning as an ``experiment'' to its current state as an established world leader among physics journals, I will present a brief historical perspective of key developments starting in 1893 when three physicists founded the parent physics journal, The Physical Review, at the physics department of Cornell University, Ithaca, New York. Other major events before the birth of Physical Review Letters in 1958 include the immediate introduction of ``Minor Contributions'' in 1893, the foundation of The American Physical Society in 1899 and its takeover of The Physical Review in 1913, and the publication of the first ``Letter to the Editor'' in 1929. Since 1958, Physical Review Letters experienced a steady growth of submissions as well as a few major format and procedural changes, which include the increase in Letter length from one printed page to four printed pages in the 1960s and the establishment of the editorial board for handling appeals in the 1970s. Despite early technical difficulties, the ``experiment'' was very successful at carrying physics into the twenty-first century. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J2.00002: Phase Transitions and Critical Phenomena Invited Speaker: |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J2.00003: Condensed Matter Theory: From Models to First Principles Invited Speaker: It can be argued that modern condensed matter theory (CMP) started 100 years ago. Models of materials explained many solid state phenomena and properties. However, only in the past 50 years---during the ``PRL Era''---can it be argued that a significant number of ab initio calculations for real materials have been done. After some historical comments, the primary conceptual models will be described. This discussion will be followed by examples of current theoretical work on explaining and predicting properties and phenomena associated with ``real materials.'' [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J2.00004: NMR and the BCS Theory Invited Speaker: The talk will review the status of superconductivity research in the early 1950s, Bardeen's thoughts about the role of an energy gap in producing superconductivity, our ideas that NMR experiments might test his ideas, and about the experimental challenge my student Chuck Hebel and I had to overcome: how can one do NMR in a perfect diamagnet (which therefore excludes magnetic fields!), the surprising results we found, then the arrival of the theory of Bardeen, Cooper, and Schrieffer, and how applying their theory to relate NMR to ultrasonic absorption verifies the essential idea of the theory (their wave function of electron pairs). [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J2.00005: The Future of Scientific Publishing Invited Speaker: |
Session J3: Kondo Screening and Quantum Criticality from the Spatial Limit: From Single Spins to Droplets to Lattices
Sponsoring Units: DCMPChair: James Davis, Cornell University
Room: Morial Convention Center RO2 - RO3
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J3.00001: How does a Kondo impurity respond to its local environment? Invited Speaker: The interplay between localized electrons on a magnetic atom and the conducting electrons in a metal can lead to intriguing many-body ground states such as the Kondo effect. When a spin is Kondo screened by conduction electrons the entire spin system performs a complicated dance that results in the formation of a spin singlet at sufficiently low temperature. For simplicity, most theoretical considerations of Kondo screening focus on magnetic impurities with the lowest possible spin S~=~1/2. Such systems can be studied experimentally in exquisite detail and with great control using quantum dots in semiconductor heterostructures or carbon nanotubes. However, in Kondo systems consisting of localized magnetic atoms, the spin is often larger, making the Kondo effect richer and more complex. Here we use the imaging and spectroscopy capabilities of a scanning tunnelling microscope to study how the Kondo screening of a known high-spin atom is determined by its local environment. Co and Ti atoms were deposited on a thin insulating layer (Cu$_{2}$N) on a copper substrate. We study the influence of external magnetic fields, crystalline magnetic anisotropy, as well as spin-coupling to surrounding atomic spins on the Kondo effect that forms on the Co or Ti atoms. We find that the anisotropy of the crystalline field quenches the high-spin system of Co (S~=~3/2) into an effective S~=~1/2 Kramers doublet. Surprisingly, much of the impact of these environmental factors on the complex many-body ground state can be understood simply through their effects on the energy levels of the unscreened spin. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J3.00002: Geometric Manipulation of Quantum Phase and Correlations in Nanoassembled Spin Systems Invited Speaker: The single-impurity Kondo problem, in which an isolated magnetic impurity in a non-magnetic metallic host has its spin screened by spins of conduction electrons, has been extensively studied both theoretically and through bulk experiments. Recently new methods have allowed detailed experimental probing of the prototype single-impurity Kondo effect of individual magnetic atoms. In addition, spin interactions with confined electron states were used to materialize ``quantum mirages'' consisting of a nonlocal single-impurity Kondo effect. When many spin and spatial states are present in a bulk conductor or on its surface, the interactions between them may engender novel collective effects. Using a scanning tunneling microscope we assembled and studied atomically precise arrangements of (magnetic) Co atoms and (non-magnetic) CO molecules on the Cu(111) surface. The spin degeneracy of single magnetic atoms and the conduction electrons alone provide necessary ingredients for Kondo physics. When combined with quantum resonators, Kondo phase shifts can be measured by using the nanostructures as quantum interferometers. We study the effects of adding to the degeneracy of the system in two controlled ways: engineering degeneracies in the spatial states of confined electrons coupled to the spins, and engineering lattices of many spins coherently coupled through electrons. The first type of experiment has enabled a novel method to read out and geometrically manipulate the quantum phase associated with state superpositions. The second class of experiments has enabled investigation of the finite size spin physics of Kondo droplets. In these periodic structures we observe signs of quantum interference and spin correlation effects when the geometries are suitable tuned relative to the Fermi wavelength of the host electron systems. These new quantum materials act as model systems for understanding complementary physics in complex matter. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J3.00003: Kondo Physics at the Nanoscale Invited Speaker: Recent experimental advances have made it possible to study the Kondo effect in nanoscale structures. These achievements are of great importance because they do not only allow us to manipulate the Kondo screening of single magnetic impurities, but also provide us with the unique opportunity to study how Kondo screening and coherence evolve on different lengthscales from a single Kondo impurity to the Kondo lattice. In this talk, I present two examples of novel Kondo physics emerging in nanostructures. First, I demonstrate that the presence of electronic eigenmodes in a nanostructure gives rise to unconventional properties of a Kondo screened magnetic impurity [1]. In particular, the Kondo temperature, $T_K$, of a magnetic impurity located inside the nanostructure varies with the impurity's location and is determined by the eigenmodes' spatial structure. Moreover, the modes' frequency dependence leads to a linear relation between $T_K$ and the local density of states, in stark contrast to the conventional Kondo effect. Second, I discuss Kondo screening and the onset of coherence in finite size Kondo lattices, so-called {\it Kondo droplets}. I show that in such Kondo nanostructures, the hybridization and the coherent coupling of the Kondo resonances can be resonantly enhanced or suppressed via changes in the droplet's geometry and lattice constant. Moreover, I demonstrate how these properties of the Kondo droplet evolve with increasing droplet size. Finally, I discuss how the ability to manipulate the properties of Kondo droplets might provide novel insight into the origin of quantum criticality, which is a central point in understanding the unconventional non-Fermi liquid properties of Kondo lattice systems, such as the heavy-fermion materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J3.00004: Bi-layer $^{3}$He: a simple two dimensional heavy fermion system with quantum criticality Invited Speaker: Two dimensional helium films provide simple model systems for the investigation of quantum phase transitions in two dimensions. Monolayer $^{3}$He absorbed on graphite, with various pre-platings, behaves as a two dimensional Mott-Hubbard system, complete with a density driven ``metal-insulator'' transition [1, 2] into what appears to be a gapless spin-liquid. In two dimensions the corrections to the temperature dependence of the fluid heat capacity, beyond the term linear in $T$, are anomalous and attributed to quasi-1D scattering [3]. On the other hand, bi-layer $^{3}$He films adsorbed on the surface of graphite show evidence of two-band heavy-fermion behavior and quantum criticality [4, 5]. The relevant control parameter is the total density of the $^{3}$He film. The $^{3}$He bilayer system can be driven toward a quantum critical point (QCP) at which the effective mass appears to diverge, the effective inter-band hybridization vanishes, and a local moment state appears. A theoretical model in terms of a ``Kondo breakdown selective Mott transition'' has recently been suggested [6]. * In collaboration with: A Casey, M Neumann, J Nyeki, B Cowan. [1] Evidence for a Mott-Hubbard Transition in a Two-Dimensional $^{3}$He Fluid Monolayer, A. Casey, H. Patel, J. Ny\'{e}ki, B. P. Cowan, and J. Saunders Phys. Rev. Lett. \textbf{90}, 115301 (2003) [2] D Tsuji et al. J. Low Temp. Phys. 134, 31 (2004) [3] A V Chubukov et al. Phys. Rev. \textbf{B71}, 205112 (2005) [4] Bilayer $^{3}$He; a simple two dimensional heavy fermion system with quantum criticality, Michael Neumann, Jan Nyeki, Brian Cowan, John Saunders. Science \textbf{317}, 1356 (2007) [5] Heavy fermions in the original Fermi liquid. Christopher A Hooley and Andrew P Mackenzie. Science \textbf{317}, 1332 (2007) [6] C Pepin, Phys. Rev. Lett. \textbf{98}, 206401 (2007) and A Benlagra and C Pepin, arXiv: 0709.0354 [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J3.00005: Quantum Criticality in 3He bi-layers Invited Speaker: |
Session J4: Panel Discussion: Making the Invisible Scientist Visible: Science in Emergent Countries
Sponsoring Units: FIPChair: Anita Mehta, SN Bose National Centre
Room: Morial Convention Center 206
Tuesday, March 11, 2008 11:15AM - 11:45AM |
J4.00001: Scientists in an alternative vision of a globalized world Invited Speaker: Why should ``increasing the visibility of scientists in emergent countries'' be of interest? Can increasing the relevance and connectedness of scientific output, both to technological applications at home and cutting edge basic research abroad contribute to the general welfare in such countries? For this to happen, governments, inter-governmental and non-governmental organizations must provide incentives for the local industry to help fund and actively engage in the creation of new technologies, rather than settling for the solution of well understood engineering problems under the rubric of collaboration between scientists and industry. However, the trajectory of the highly industrialized countries cannot be retraced. Globalization facilitates closer interaction and collaboration between scientists but also deepens the contrasts between the center and the periphery, both world wide and within national borders; as it is understood today, it can lead to the redundancy of local technology oriented research, as the idea of a ``local industry'' is rapidly made obsolete. Scientists from all over the world are sucked into the vortex as both the economic and the cultural world increasingly revolve around a single axis. The challenge is to redefine our terms of reference under these rapidly changing boundary conditions and help bring human needs, human security and human happiness to the fore in elaborating and forging alternative visions of a globalized world. Both natural scientists and social scientists will be indispensable in such an endeavor. [Preview Abstract] |
Tuesday, March 11, 2008 11:45AM - 12:15PM |
J4.00002: Globalizing Science and Engineering Invited Speaker: In this talk I will review recent trends in Science and Engineering Research and Education in an increasingly interconnected, ``flat'' world. The enormous economic transformation being fueled by technology will, over a period of time, lead to new models of interaction among universities, industry and governments. Scientists and Engineers in ``emergent'' countries may be expected to play a key role in this ``new'' world order. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:45PM |
J4.00003: The Mutual Benefit of International Research Interactions Invited Speaker: Emergent economies provide a fruitful source of scientific knowledge. We have the responsibility to nurture interactions to increase scientific knowledge in the world. Moreover, we have benefited greatly from discoveries and education provided to scientists and engineers in emergent economies. I will give examples of successful modes of interactions between the US and emergent economies, and in particular with Latin American countries. I will review the impact of our interactions in their countries and in ours, and ways to increase the impact. [Preview Abstract] |
Tuesday, March 11, 2008 12:45PM - 1:15PM |
J4.00004: The Invisible Scientist in India -- a case study for emergent countries Invited Speaker: Emergent countries such as India, China, and Brazil, face unique problems in the realisation of their science and technology potential. I will discuss the situation in India, which may be relevant at least in part to the others. The basic focus of the talk will be the possible ways in which the invisible scientist, the person on the ground who has no part in the pyramid of the science establishment, can be rendered more visible—both for her or his own sake, and to prevent the erosion of an enormous intellectual potential by a still continuing brain drain. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 2:15PM |
J4.00005: Panel Discussion |
Session J5: Where is the Center of Mass for Family, Career, and Self?
Sponsoring Units: CSWP FGSAChair: Peter Sheldon, Randolph College
Room: Morial Convention Center RO1
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J5.00001: Career Planning in Harmony with Family Values and Needs Invited Speaker: Balancing career and family! Balancing what you love and who you love!! It is such an attention getting topic. And yet, if you really think about it, people have been doing it for ages. What makes it challenging in today's world is the dual income families that throw off-balance of traditional style of balancing family and profession. Balancing family and career is not as difficult. The question is more meaningful when you ask how do you find the right balance, and in fact, what is the right balance? How do you know you are there? Happiness at home and self esteem due to work is genderless issue however, it is essentially talked more in the context of women. Some of the things that could be helpful in achieving the right balance, are time management, proper prioritization, asking for help, a caring family, friends, and most importantly colleagues. In the portfolio of professional passions, it is important to identify the areas that are conducive to possibilities of changing family needs, international families, spouse's career and job relocation, etc. So, the bottom line question is whether it is possible to find a right balance between family and career? I would submit to you that with passion, courage, open- mindedness, and proper career planning, it is definitely possible. We just need to utilize the same techniques in choosing and sustaining the right balance that we use in identifying research topics and executing it. This discussion will look into further details of the challenges of balancing family and career from the perspective of also an immigrant, and possible ways of overcoming them. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J5.00002: Faculty Work-Family Issues: Finding the Balance at a Liberal Arts College Invited Speaker: The demands and expectations on science faculty at liberal arts colleges are in many ways distinct from those at research universities. While these differences can work in favor of easing work-family conflicts, there are also unique problems that faculty can confront in a setting of smaller departments and undergraduate-only institutions. I will discuss how these issues play out for junior and senior faculty, with an emphasis on how concrete policy changes can make the workplace a more family-friendly and supportive environment for all faculty, as well as making liberal arts colleges more attractive options for those seeking physics faculty jobs. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J5.00003: The eye of the storm: Balancing my storm of family, career and self Invited Speaker: In knowing that the path I travel is not the usual path traveled by most; this has turned out to be the best path for me and my family. It is very important to prioritize what is important to you and then define the best path for you versus choosing a path and the path chooses your prioritizes. Coming from a loving and supportive middle class upbringing created a deep sense of family and the importance of family. Early in my life I was determined to have children and a career. Over the last ten years there have been several obstacles to overcome in my storm, but with careful planning, due diligence, and a support system to help maintain calm at the center of my storm I have been able to achieve my goals of pursuing my Doctorate. A complete research plan was put into place into choosing the institution that I would further my academic endeavors in the same manner in which my dissertation research topic has been defined. Just as any successful business, all persons involved in my future success were consulted with equal input into the new endeavor with the full understanding of what this new plan entailed. We decided on the University of Alabama for several reasons: location, weather, flexibility, policies, research and my ability to make a change in the face of science. According to my advisor, I will do that in about two and half years at my graduation ceremony when I become the first African American to receive a PhD in Material Science from the University of Alabama. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J5.00004: Balancing academic career and children: a personal perspective Invited Speaker: For women in academia, the tenure review and the desire to have children often happen around the same time. How does one cope with the challenges of an academic career while raising small children? From a personal perspective of an assistant professor and a mother, I will discuss the great challenge of efficient time management and the practical strategies to deal with it. [Preview Abstract] |
Session J6: Effective Potentials and Force Fields for Simulating Biological Macromolecules
Sponsoring Units: DCOMPChair: Ronald Levy, Rutgers University
Room: Morial Convention Center RO4
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J6.00001: Artifact or reality? Force field issues in the simulation proteins and nucleic acids Invited Speaker: Access to ever-increasing computational power is providing the means to critically evaluate the performance of atomistic force fields of biomolecules. With greater sampling, and more detailed comparisons to experiment, limitations and artifacts in the applied simulation protocols and force fields can be discovered and ultimately overcome. Additionally, we are able to more carefully validate and assess the performance of the simulations in comparison with experiment. In this talk, we will outline our experiences in large-scale simulations of protein and nucleic acid systems in the context of the AMBER biomolecular simulation program. Issues related to salt and dihedral parameters will be highlighted in applications ranging from ligand-induced remodeling of dihydrofolate reductase and cytochrome P450 2B4 protein structures to large-scale decoy sets and NMR comparisons of various RNA structures. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J6.00002: Development of a polarizable force field based on the classical Drude oscillator Invited Speaker: Empirical force field development requires a systematic approach allowing for the development of a practical potential energy function and optimization of physically realistic parameters that reproduce a range of target data. Ongoing efforts in our laboratory include the development of a polarizable force field based on the classical Drude oscillator for a range of molecules representative of biological systems. A central theme in these efforts is the accurate treatment of both atomic interactions as well as condensed phase properties. To achieve this goal extensions of the energy function have been implemented and parameter optimization has been performed targeting a variety of quantum mechanical results and experimental condensed phase properties. An overview of these studies will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J6.00003: The AGBNP implicit solvent model: recent advances and applications to biological macromolecules Invited Speaker: The Analytical Generalized Born plus Non-Polar (AGBNP) model is an analytical implicit water model suitable for molecular dynamics simulations of small molecules and macromolecules. It is based on an analytical pairwise descreening implementation of the continuum dielectric Generalized Born (GB) model and a non-polar hydration free energy model. AGBNP computes the descreening scaling factors that account for atomic overlaps from the geometry of the solute rather than treating them as geometry-independent parameters fit to numerical or experimental data. The non-polar hydration free energy model is decomposed into a cavity component based on the solute surface area and a solute-solvent van der Waals dispersion energy estimator. The aim of the model is to achieve atomic-resolution accuracy for modelling the many biological systems in which global conformational features are regulated by small and localized control elements. Since its introduction AGBNP has been employed to study a variety of biological problems ranging from peptide conformational propensity and folding, protein allostery, conformational equilibria of protein-ligand complexes, binding affinity prediction, and, more recently, to intrinsically disordered proteins, protein aggregation, the design of virus vaccine carriers, and macromolecular X-ray structure refinement. Recent development work has focused on computational performance enhancements and on improving the accuracy of the model with respect to explicit solvent simulation results. By comparing the details of the solvent potentials of mean force of several peptides calculated with explicit and implicit solvation, we have identified some aspects of the AGBNP model in need of improvement. We are exploring several strategies to address them including the adoption of a molecular surface description of the solute volume, the modelling of high-occupancy hydration sites, and the optimization of the non-polar free energy model. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J6.00004: Atomic-level simulations of biomolecular systems with a modified Amber force field Invited Speaker: Experimental methods have been highly successful in determining 3-dimensional biomolecular structures. However, most approaches provide only time- or ensemble-averaged data, making it much more difficult to study the dynamic and energetic aspects of biological systems. Atomic-resolution simulations are highly complementary to experiments, and can provide data with unparalleled resolution in time and space. Due to the long timescales of biologically relevant events, as well as the complexity of the energy function, accurate and precise simulations remain highly computationally challenging. This seminar will highlight recent progress in both areas, illustrating how energy functions that have been trained on simple peptide models can be successfully used for the study of much more complex systems. We demonstrate that our newly trained energy parameters significantly reduce the secondary structure bias reported for previous Amber parameter sets. Applications of the parameters include studies of folding behavior of peptides and small proteins, and the dynamic behavior of larger biomolecular systems such as conformational changes during drug binding in HIV-1 protease. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J6.00005: Beyond force fields. QM/MM conformational searches in biomolecules using Replica Exchange molecular dynamics. Invited Speaker: Force fields have been extremely successful for our understanding of structure, energetics and dynamics of biomolecules. However, they neglect, for the most part, quantum effects such as charge transfer and polarization. In this talk I will present our work using our newly developed QM/MM interface in the program Amber, which can very efficiently treat small peptides in explicit solvent using a number of different semiempirical methods. We use Replica Exchange molecular dynamics to sample the surface properly and ensure convergence. A comparison of the conformational space sampled by different semiempirical methods in explicit water classical models will be presented. The computational results will be compared against NMR experiments. [Preview Abstract] |
Session J7: Undergraduate Nanotechnology and Materials Physics Education I
Sponsoring Units: FEdChair: Lawrence Woolf, General Atomics
Room: Morial Convention Center RO5
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J7.00001: NCLT Contributions to Nanoscience Education at the Undergraduate Level Invited Speaker: The National Center for Learning and Teaching in Nanoscale Science and Engineering (NCLT) has a mission to build national capacity in Nanoscale Science and Engineering Education (NSEE) by reaching to millions of learners. This mission calls for the development of a globally competitive national nano workforce and national cadre of leaders in NSEE. Part of the NCLT's integrated program focuses on higher education initiatives and the development of undergraduate resources in NSE. The Center has developed an online educational resource repository for the NSEE community, the NanoEd Resource Portal at http://www.nclt.us. This talk involves a description of the applications and context for integrating NSE into undergraduate courses. It will provide research and development examples on new degree programs and concentrations in NSE. The following are a few highlights of NCLT's contributions in undergraduate education: \begin{itemize} \item Example of several short introductory units on Scanning Tunneling Microscopy, Scanning Electron Microscopy and Nanopatterning Techniques \item Simulations that can be incorporated into undergrad courses on Information Storage Technology (i.e. Nanomagnetism simulations and accompanying introductory material) \item Archive of seminars on various topics on NSE concepts \item Working prototype of Nanoconcentration in Physics \item Database of Degree Programs highlighted on the NCLT NanoEd Resource Portal \item Rubric for course development criteria \item Potential venue for professors to post their courses, degree programs, etc. for national and global dissemination \end{itemize} [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J7.00002: A Cutting-Edge Education: Incorporating Nano into the Undergraduate Curricula. Invited Speaker: The Interdisciplinary Education Group (IEG) of the Materials Research Science and Engineering Center (MRSEC) on Nanostructured Interfaces at the University of Wisconsin-Madison (UW) develops and uses hands-on, interactive education and outreach materials to engage a variety of audiences in learning about nanotechnology and advanced materials. Many of the education products created are inspired by UW MRSEC research; and faculty, staff, and students regularly contribute to the IEG's work to share nanotechnology with a broader audience. The UW MRSEC has developed numerous teaching modules, labs, and education resources devoted to nanotechnology concepts, and many of these materials have been integrated into key introductory and advanced undergraduate courses at UW and other institutions, including small liberal arts colleges and community colleges. This effort has taken place through both the creation of new courses and the modification of existing courses to include cutting-edge content based on current research and emerging applications in nanotechnology. In this talk, I will present some of the new instructional materials we have developed based on advances in nanoscale science and technology, the implementation and integration of these materials into undergraduate curricula, and an overview of the UW MRSEC education efforts. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J7.00003: Integrating Condensed Matter Physics into a Liberal Arts Physics Curriculum Invited Speaker: The emergence of nanoscale science into the popular consciousness presents an opportunity to attract and retain future condensed matter scientists. We inject nanoscale physics into recruiting activities and into the introductory and the core portions of the curriculum. Laboratory involvement and research opportunity play important roles in maintaining student engagement. We use inexpensive scanning tunneling (STM) and atomic force (AFM) microscopes to introduce students to nanoscale structure early in their college careers. Although the physics of tip-surface interactions is sophisticated, the resulting images can be interpreted intuitively. We use the STM in introductory modern physics to explore quantum tunneling and the properties of electrons at surfaces. An interdisciplinary course in nanoscience and nanotechnology course team-taught with chemists looks at nanoscale phenomena in physics, chemistry, and biology. Core quantum and statistical physics courses look at effects of quantum mechanics and quantum statistics in degenerate systems. An upper level solid-state physics course takes up traditional condensed matter topics from a structural perspective by beginning with a study of both elastic and inelastic scattering of x-rays from crystalline solids and liquid crystals. Students encounter reciprocal space concepts through the analysis of laboratory scattering data and by the development of the scattering theory. The course then examines the importance of scattering processes in band structure and in electrical and thermal conduction. A segment of the course is devoted to surface physics and nanostructures where we explore the effects of restricting particles to two-dimensional surfaces, one-dimensional wires, and zero-dimensional quantum dots. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J7.00004: Engaging undergradate students in interdisciplinary courses in nanotechnology Invited Speaker: Two new courses at UCSB engage both undergraduate and graduate students in situated learning so that they can acquire the knowledge and skills they will need for future academic courses and career development. These courses are designed and taught by research faculty and education staff at the California Nanosystems Institute (CNSI) at UC Santa Barbara. The speaker, Dr. Goodchild, Education Director at CNSI, collaborated in the course design and is advisor on assessment and pedagogy for both courses. The first course, entitled INSCITES, is aimed at first and second year students who are interested in the impacts of science and technology in society. This general education course is team taught by three Graduate Teaching Scholars from across engineering, science and social sciences. They collaborate with lead faculty from Materials Science and History to design both the curriculum and instructional format for the 10 week course that is supported by the National Science Foundation. INSCITES was taught for the first time in Spring 2007 and feedback indicated that the course had convinced the undergraduate students that they would like to take further courses outside their majors. The second course, entitled the \textit{Practice of Science} is open to all majors in science and engineering, especially those in second and third year who are interested in scientific research and related career opportunities. The course has been taught for the past 4 years as a two quarter course by two research faculty who focus on the nature of scientific discovery, the role of graduate researchers and faculty, the challenges of collaboration across disciplines and the mechanisms for funding research in academia and industry. In the first quarter each students is expected to identify a mentor and a research group in which they can pursue an individual research project, to be completed during the second quarter when the classes are designed to operate like research group meetings. Evaluation indicates that both courses attract students from underrepresented groups in science who value gaining a broader perspective about nanotechnology and the career opportunities that it offers to undergraduate students. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J7.00005: Educating the workforce for the nantoechnoogy industry at CNSE Invited Speaker: The College of Nanoscale Science and Engineering of the University at Albany is the first college in the world~dedicated to research, development, education, and deployment in the emerging disciplines of nanoscience, nanoengineering, nanobioscience, and nanoeconomics. CNSE's Albany NanoTech complex, a {\$}4.2 billion, 450,000-square-foot facility has attracted over 250 global corporate partners, is the most advanced research complex of its kind at any university in the world. CNSE has evolved into an internationally recognized education and research center due to it's eminently successful model for collaboration between industry, government and academia. CNSE's curriculum comprises a cutting-edge, inherently interdisciplinary academic program centered on intellectual rigor, educational diversity, and technical and pedagogical innovation to create a unique experience for faculty and students. The proposed undergraduate curriculum constitutes a four-part educational program comprised of a `\textit{Foundational Principles'} component, a `\textit{Core Competency'} component, a `\textit{Concentration'} component and a \textit{`Capstone Research/Design'} component. These four elements of the baccalaureate curricula in Nanoscale Science and Nanoscale Engineering inherently exploit the unparalleled academic, professional, and infrastructural resources of the College of Nanoscale Science and Engineering and its NanoFab Complex. [Preview Abstract] |
Session J8: Granular Flows
Sponsoring Units: DFD GSNPChair: Wolfgang Losert, University of Maryland
Room: Morial Convention Center RO6
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J8.00001: Washboard Road: The dynamics of granular ripples formed by rolling wheels Nicolas Taberlet, Anne-Florence Bitbol, Stephen Morris, Jim McElwaine We report laboratory experiments on rippled granular surfaces formed under rolling wheels. Ripples appear above a critical speed and drift slowly in the driving direction. Ripples coarsen as they saturate, and exhibit ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft particle simulations in which a disk rolls over smaller disks in a periodic box. The simulations show that compaction and segregation are inessential to the ripple phenomenon. We describe a simplified scaling model which gives some insight into the mechanism of the instability. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J8.00002: Evolution of sand ripples in pulsed flow Jos\'e Eduardo Wesfreid, Joachim Kruithof We present high-resolution experiments showing the temporal evolution of sand ripples formed by oscillatory flow. We discuss the decompaction process observed during the formation of the ripples pattern. We have also studied the evolution of different parameters during the transition of rolling grain ripples to vortex ripples, as the slope of these ripples and we tested the validity of the Sleath criterion to discriminate the transition. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J8.00003: Granular Erosion of Pebbles Adam Roth, Douglas Durian Flowing grains are strongly abrasive, and cause erosion both of themselves and their surroundings. ~In a geophysical setting, the erosion of pebbles has traditionally been quantified by global measures such as aspect ratio. ~Recently we have focused on curvature, and its distribution around the contour, as a local measure more directly related to the microscopic action of erosion. ~Here we apply this method to linoleum shapes, eroded by rotation in an abrasive grit. Several shape parameters are measured at different stages in the erosion process, including the curvature distribution. ~A simple model of erosion is developed, and its predictions are compared to the data. ~The results are in reasonable agreement, and could be useful for understanding natural erosion processes. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J8.00004: Impact and Penetration of Granular Materials by Discrete Element Simulations Justin W. Garvin, Jeremy B. Lechman, J. Matthew D. Lane Granular material response to impact is important in a range of fields, from munitions delivery, to meteorite collision and crater formation. Recently a model for the force experienced on a penetrator has been proposed [L.S. Tsimring and D. Volfson, Powders and Grains 2005, 1215-1223] and shown to fit experimental data well [H. Katsuragi and D.J. Durian, Nature Physics, Vol. 3, June 2007]. This model describes two components of the force: i) a velocity dependent, depth independent term related to the inertial force required to mobilize a volume of grains in front of the penetrator; and ii) a velocity independent, depth dependent, Coulomb friction-like term. In the current study, massively parallel, discrete element simulations have been performed to study the penetration of a large spherical impactor into a multi-million particle bed of granular material. Results agree with previous work for slow impact speeds ($<$ 400cm/s). In addition, the current work extends the comparison with the proposed model to higher speeds ($\sim $1000cm/s). The physics of the phenomenon is discussed along with the challenges for modeling and simulation in the even higher velocity regime. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J8.00005: Gas-Mediated Impact Dynamics in Fine-Grained Granular Materials John Royer, Eric I. Corwin, Bryan Conyers, Mark L. Rivers, Peter J. Eng, Heinrich M. Jaeger Non-cohesive granular media exhibit complex responses to sudden impact that often differ from those of ordinary solids and liquids. We investigate how this response is mediated by the presence of interstitial gas between the grains. Using high-speed x-ray radiography we simultaneously track the motion of a steel sphere through the interior of a bed of fine-grained granular material and measure local changes in the bed packing density below the sphere. In an initially loosely packed bed, interstitial gas allows for nearly incompressible, fluid-like flow of the bed and aids the penetration of the sphere. In an initially densely packed bed the interstitial gas plays the opposite role, strengthening the bed and inhibiting the penetration of the sphere. These two seemingly incongruous effects are both due to the low permeability of the fine grained-bed, which traps the interstitial gas in the bed. This trapped gas resists changes in the bed packing density, inhibiting compaction in the loose bed and inhibiting dilation in the dense bed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J8.00006: Impact cratering in fluidized granular matter Patrick Mayor, Hiroaki Katsuragi, Douglas J. Durian Impacts by projectiles dropped into granular media are an important example of how particulate materials respond to externally applied forces. Beyond the obvious geophysical case of planetary craters, understanding the details of impact mechanisms can provide valuable information on these systems, and the phenomenon has been actively investigated. In particular, recent experiments have studied the penetration depth of projectiles impacting granular materials at relatively low speeds, and measured the dynamics of the impact process, yielding force laws accounting for the observations. We have studied how the impact phenomenon is affected when the granular medium is submitted to a vertical upward (or downward) gas flow, in a range of flow rates below the bubbling regime. These fluidized granular systems yield, logically, deeper impacts, and dynamics measurements reveal that the stopping time is also longer, contrary to what is observed when deeper craters are obtained by increasing the impact velocity. We observe that the parameters involved in previously obtained force laws are modified in a simple way as a function of the flow rate and find a velocity-dependent inertial term and a depth-dependent friction force that vanishes as the flow rate approaches the fluidization threshold. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J8.00007: Drag Force in a Gas Fluidized Granular Bed T.A. Brzinski, D.J. Durian We use a rheometer to measure the torque acting on a rotating bar in a bed of gas-fluidized glass beads. We vary rotation rate from .001-10rps, vary depth from 1-10 cm, and increase the fluidizing gas flow from no flow well into the fluidized regime. We observe that at high rotation rates the drag is roughly proportional to velocity squared. At low rates we can rescale the measured torque by depth, and observe a collapse of the data. These results agree with the predictions of a granular drag force model which has proven effective in predicting granular impact dynamics. The model consists of an inertial drag term, which is depth-independent and scales as velocity squared, and a frictional drag term, which is independent of rate and varies linearly with depth. We find, as expected, that while the frictional term is airflow-dependent the inertial term is uncoupled from the fluidization. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J8.00008: Exploring penetration through granular media Daniel J. Costantino, Thomas J. Scheidemantel, Matthew B. Stone, Julia Cole, Casey Conger, Kit Klein, Matthew Lohr, William McConville, Zachary Modig, Krysten Scheidler, Peter Schiffer The motion of objects through granular media is an important physical problem involving local jamming of the grains. We report on an experiment dealing with the force needed to initiate upward motion through a granular pile, $F_{ini}$. As expected, this force scales monotonically with the depth of the intruder as well as its size, $D_{plate}$. However, unlike previous experiments this force also depends on the size of the particles making up the pile, $d_{grain}$. The force can be represented by the function $F_{ini}=A D_{plate} \quad d_{grain}+B D_{plate}^{2}$; which can be qualitatively explained within a simple model. Finally, preliminary results from a new experiment dealing with horizontal motion through a granular pile will be discussed. In this study, the effect of interstitial fluids on a granular material's resistance to an intruder will be investigated. Research supported by NASA grant NAG3-2384 and the NSF REU program. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J8.00009: Fluctuations in an agitated granular fluid Kiri Nichol, Martin van Hecke Granular media can be fluidized by a flow that occurs far away. Intruders placed in such a 'stationary granular fluid' sink until they reach a depth given by a granular analogue of Archimedes law. Once they float at this depth, these intruders effectively probe the microscopic agitations in the material that cause the fluidization. The spectrum of these fluctuations is anomalous. We present its dependence on experimental parameters such as driving rate, floating depth and probe size, and discuss the possibility of applying a non-equilibrium Fluctuation Dissipation relation to this system. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J8.00010: Jamming in Hopper Flow of Large Aspect Ratio Granular Materials Scott Franklin The clogging of granular materials at the exit of a silo or hopper is a matter of tremendous practical importance, as well as a canonical example of jamming. We investigate the effect of particle aspect ratio (length:width) on the jamming probability through experiments and discrete element simulations. Preliminary experimental results on particles with aspect ratios of 16 and 32 show that the probability $P(m)$ for $m$ grains to exit the hopper has an exponentially decaying tail that, when scaled by the mean number that exit, is independent of exit aperture size. This scaling of $P(m/\langle m \rangle)$ is also observed in hopper flow of ordinary round materials, but the proposed phenomenological explanation of uncorrelated behaviors seems unlikely in long, thin rods. Furthermore, while the mean exit number obviously increases with aperture size, it is not clear which length scale is most relevant: particle length, width, or some combination of the two. We are also writing new discrete element simulations that can be compared with the experiments, and I will discuss some of the computational nuances introduced by particle asymmetry and present initial results. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J8.00011: Granular Flow of Fluid-Submerged Particles: Effects of Fluid Viscosity H. King, D. Ertas, A. Kushnick, F. Zhou, P. Chaikin Gravity-driven flows of granular materials are often influenced by interstitial fluids. Using the rotating half-filled drum geometry, we investigated particle and fluid velocities for granular flows of nearly monodisperse spherical glass particles with interstitial fluids of varying dynamic viscosity (air to 4 cP). We utilize direct particle imaging and PIV methods. For dry flows the fundamental time scale is set by the gravitational constant and particle size. We observe two primary influences of the interstitial fluid on the granular rheology. First, density of the fluid changes both the driving force (due to buoyancy) and the inertial response (due to added mass), increasing the characteristic time scale. Second, the intrinsic time scale is influenced by the dynamic viscosity of the fluid. As a result, the changes associated with the 1 cP viscosity increase in going from air-to-water are considerably larger than those for subsequent viscosity increments. We also see that the surface drag associated with the fluid boundary layer progressively affects the grain velocity profile near the surface as the viscosity increases, giving a several-particle-deep zone of constant velocity. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J8.00012: Rheology and structure of granular flows in split-bottom geometries. Joshua Dijksman, Martin van Hecke Combining rheological methods with surface flow imaging, we probe the flow of slow dry granular media as function of driving rate and geometry. The flow rate affects the spatial structure of the flow much stronger than the stresses, while details of the boundary conditions significantly modify both stresses and flow. We discuss our results in the context of recent numerics on rapid flows in these split-bottom geometries, and various theories developed for slow flows. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J8.00013: Self-diffusion in bulk sheared granular materials Andreea Panaitescu, Ashish Orpe, Arshad Kudrolli We will discuss the diffusion and structural properties of granular particles in the bulk of a cyclically sheared three dimensional rectangular cell. The particles are visualized away from the side walls using a fluorescent refractive index matched interstitial fluid. Previous studies have shown that the diffusion is anisotropic with respect to the vorticity plane, but these results have been confined to either two dimensional systems or small three dimensional systems where the boundary effects could not be decoupled. In a cyclic shear cell, the packing fraction the particles and their orientational order vary smoothly over time. The particle positions are identified and tracked over long durations to obtain particle diffusivity, mean-squared displacements and probability distributions of particle displacements. An analysis of the effect of structural order on the motion of the particles will be presented. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J8.00014: Studies in a 2D granular pure shear experiment Jie Zhang, Peidong Yu, Trush Majmudar, Robert P. Behringer We have performed two dimensional granular experiments under pure shear using bidisperse photo-elastic disks. Starting from a stress free state, a squre box filled with granular particles is subject to shear. The forward shear involved thirty steps, leading to maximum strain of 0.1. The network of force chains gradually built up as the strain increased, leading to increased pressure and shear stress. Backward shear was then applied to return the system to zero strain in the next thirty steps. Following each change of the system, contact forces of individual disks were measured by applying an inverse algorithm. We also kept track of the displacement and angle of rotation of every particle from frame to frame. We present the results for the contact forces, particle displacement, particle rotations, fabric, etc. Work supported by NSF grant DMR0555431. [Preview Abstract] |
Session J9: Fluid Structure and Properties
Sponsoring Units: DFDChair: Nikolai Priezjev, Michigan State University
Room: Morial Convention Center RO7
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J9.00001: The Interplay of Short- and Long-Ranged Forces in Simulations of Confined Water using Local Molecular Field Theory Jocelyn Rodgers, John Weeks A molecular model of water confined between walls is studied using local molecular field (LMF) theory. LMF theory splits the long-ranged Coulomb $1/r$ potential between charge sites into a short-ranged core potential and a long-ranged, slowly-varying potential ideal for mean-field averaging. The core potential may be treated explicitly by simulations using the minimum image convention with a renormalized external field defined by mean field averaging of the longer-ranged potentials. Here we apply local molecular field theory to molecular dynamics simulations of molecular water confined between walls, with and without an electric field. This is a geometry where short-ranged spherical truncations of Coulomb interactions can fail spectacularly, but in tandem with the effective external field defined by LMF theory such truncations correctly predict structural and electrostatic properties of water. Further the concepts behind LMF theory elucidate the varying contributions of hydrogen-bonding and dipolar interactions in determining the structure of water at surfaces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J9.00002: Effects of a solute on a simple model solvent Paolo De Gregorio, Jonathan C. Toledo, B. Widom We studied the effect of the addition of a solute on a one-dimensional model solvent (high density, low compressibility, low coefficient of thermal expansion), at infinite dilution. The solute has a solubility which is low and decreases with increasing temperature. The effect of the addition of solutes on the chemical potential of the solvent at constant volume differs from that at constant pressure in a way similar to that of non-polar solutes in water. The solvent-solvent pair distribution function determines fully the modes of decay of the solute-solute counterpart. At the largest distances, the ultimate decay is strictly monotonic (exponential) for both. But while for the solvent-solvent correlations the amplitude associated with that mode is negligible, it is huge for the solute-solute case. Formally, the correlations vanish in identical fashion at infinite distances, but they differ substantially over an extended range of physical interest. The osmotic second virial coefficient is very large and negative, not only as an effect of the proximity `attraction' between the solutes, but also of the very long tail in the correlations. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J9.00003: Ultrafast Phase-Contrast Imaging Study of Finite-time Hydrodynamic Singularities Yujie Wang Most of the nonlinearity induced hydrodynamic singularities are transient and requires high-speed imaging to be studied. There exist some intrinsic problems of visible-light imaging on fluid mechanical research.. The key advantage of x-ray phase-contrast-imaging is that it is interface-based technique and the boundaries are highlighted naturally. It is a highly penetrative technique in which all complex structures along the path will be picked up. Additionally, it is naturally immune of the complexity of multiple scattering and strong optical reflection or refraction. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J9.00004: Liquid State Properties from \emph{ab initio} Density Functional Theory Calculations Nicolas Bock, Travis Peery, Eric Chisolm, Giulia De Lorenzi-Venneri, Duane Wallace, Erik Holmstr\"{o}m, Raquel Lizarraga For the solid state, density functional theory (DFT) has been successfully applied to calculate material properties in a large range of materials. In the liquid state however, thermodynamic properties are calculated by molecular dynamics (MD) simulations in which the forces are calculated with DFT. These simulations are computationally significantly more expensive than comparable solid state calculations. We present a novel approach which does not rely on MD simulations, but instead uses Vibration-Transit (V-T) theory to make predictions of the thermodynamic properties of the liquid phase. This approach is computationally significantly less expensive than an MD simulation. The accuracy of this approach is demonstrated by a comparison to experiment. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J9.00005: Reconstructing the structure and dynamics of density fluctuations in water near a moving proton Robert Coridan, Ghee Hwee Lai, Nathan Schmidt, Peter Abbamonte, Gerard C. L. Wong ~~~~~~The structure and dynamics of water on femtosecond timescales is relevant to many topics in physical chemistry such as ion solvation. We computationally reconstruct the angstrom-scale spatial and femtosecond-scale temporal evolution of density fluctuations in water using high-resolution inelastic x-ray scattering (IXS). The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations.~ The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the real-space density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse.~ We use this density propagator from IXS data and linear-response theory to reconstruct the hydration behavior of a proton moving at different speeds through water. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J9.00006: Classical Density Functional Theory of Inhomogeneous Polar Molecular Liquids Johannes Lischner, T.A. Arias We show how free energy functionals for classical assemblies of interacting rigid molecules, composed of an arbitrary number of atoms, can be constructed, such that the entropy of the noninteracting assembly, the thermodynamic properties and the microscopic order of the uniform phase and the dielectric properties in both weak and strong electrostatic fields are reproduced. We use our approach to predict density profiles of liquid hydrogen choride in a parallel plate capacitor with different wall potentials and varying external fields. We show that our theory can easily be coupled to electronic structure calculations within the Joint Density Functional approach and will comment on potential application to water. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J9.00007: Localized Voronoi analysis of quenched liquid configurations Travis Peery, Nicolas Bock, Giulia De Lorenzi-Venneri, Duane Wallace, Erik Holmstrom We developed a set of {\it localized\,} statistical tools to explore and characterize condensed matter particle configurations, particularly amorphous distributions associated with the liquid state. Typically global measures of atomic packing are used to characterize atomic configurations, such as pair distribution functions. For large systems, such calculations can be computationally expensive and tend not to be sensitive to localized symmetries. Our localized tools are based upon the geometric or topological analysis of (static) atomic arrangements using Voronoi polyhedra. As each atom in the configuration has a unique Voronoi polyhedron defined by its near neighbors, our tools can describe the geometry and symmetry of local neighborhoods. We have defined, for example, a local, Shannon-type entropy for the Voronoi coordination number for each atom in a 500-atom, monatomic system. This {\it localized} entropy tool was able to find small (9--40 atom) crystallites or regions of high symmetry in an otherwise random 500-atom configuration quenched from a liquid MD state. These tools will help to define and characterize not only random liquid state configurations and the minimum structures associated with liquid potential energy surfaces, but also the symmetry properties of the quenching process itself. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J9.00008: Phase Separation in the Dipolar Hard-Sphere System Revisited Wonki Roh, Erik Luijten We investigate the liquid-vapor transition in the dipolar hard-sphere system. Since the suggestion of de Gennes and Pincus [Phys.Kondens. Mater.\ \textbf {11}, 189 (1970)] this phase transition has proven both elusive and controversial, with conflicting numerical results regarding its existence and its nature. Employing extensive and efficient grand-canonical Monte Carlo simulations, we revisit this issue. High-precision results on the low-temperature heat capacity are presented along isotherms as well as isochores. In addition, we study the density distribution function and its moments for a wide range of chemical potentials, and identify anomalous finite-size effects that can give rise to incorrect conclusions. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J9.00009: X-ray tracer study of Rheology and Hydrodynamics of Fatty acids Mengning Liang, Ross Harder, Ian Robinson In wormlike micelles, the breaking and reforming of the micelle rods and the shearing of the rods and between the carbon chains themselves result in a complex diffusive behavior with more than one characteristic time constant. This is one of the characteristics of a Maxwellian fluid. We have studied the rotational Brownian motion of an alumina crystal suspended in a fatty acid liquid. Synchrotron generated hard x-rays are used to do single particle tracking of the rotational orientation by tracking the Bragg intensity of alumina crystals in diffraction geometry. This technique allows the tracking of particles to sub-milliradian precision. We have observed multiple time scales of relaxation which is evidence of subdiffusive behavior. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J9.00010: Statics and dynamics of a cylindrical droplet under an external body force James Servantie, Marcus M\"uller We study the rolling and sliding motion of droplets on a corrugated substrate by Molecular Dynamics simulations. Droplets are driven by an external body force (gravity) and we investigate the velocity profile and dissipation mechanisms in the steady state. The cylindrical geometry allows us to consider a large range of droplet sizes. The velocity of small droplets with a large contact angle is dominated by the friction at the substrate and the velocity of the center of mass scales like the square root of the droplet size. For large droplets or small contact angles, however, viscous dissipation of the flow inside the volume of the droplet dictates the center of mass velocity that scales linearly with the size. We derive a simple analytical description predicting the dependence of the center of mass velocity on droplet size and the slip length at the substrate. In the limit of vanishing droplet velocity we quantitatively compare our simulation results to the predictions and good agreement without adjustable parameters is found. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J9.00011: Velocity-dependent friction coefficient at the interface between a polymer melt and a solid substrate Nikolai Priezjev, Anoosheh Niavarani Molecular dynamics simulations are carried out to investigate the dynamic behavior of the slip length in thin polymer films confined between atomically smooth thermal surfaces. For weak wall-fluid interactions, the shear rate dependence of the slip length acquires a distinct local minimum followed by a rapid growth at higher shear rates. With increasing the fluid density, the position of the local minimum is shifted to lower shear rates. We found that the ratio of the shear viscosity to the slip length, which defines the friction coefficient at the liquid/solid interface, undergoes a transition from a nearly constant value to the power law decay as a function of the slip velocity. In a wide range of shear rates and fluid densities, the friction coefficient is determined by the product of the value of surface induced peak in the structure factor and the contact density of the first fluid layer near the solid wall. A relation to recent slip flow experiments is discussed. Reference: A. Niavarani and N.V. Priezjev, Phys. Rev. E (2008) (cond-mat/0711.0178). [Preview Abstract] |
Session J10: Superconductivity: Vortex I
Sponsoring Units: DCMPChair: Wai Kwok, Argonne National Laboratory
Room: Morial Convention Center RO8
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J10.00001: Progression of the vortex-solid to vortex-liquid phase boundary with oxygen doping in Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{x}$ Films Benjamin Taylor, Ryan Baumbach, M. Brian Maple By extending magneto-transport measurements to magnetic fields of 35 tesla we have been able to examine the vortex-solid to vortex-liquid transition of thin film Y$_{0.8}$Ca$_{0.2}$Ba$_{2}$Cu$_{3}$O$_{x}$ samples (6.45~$\le $~x~$\le $ 7.0) over a field-temperature range larger than heretofore reported. It is found in this work that the shape of the phase boundary, H$_{g}$(T), evolves from a very shallow low-field temperature dependence to an extremely rapid high field temperature dependence in the highly underdoped regime (x~$\approx $ 6.45). However, in the lightly overdoped regime (x $\approx $ 6.9 - 7.0), H$_{g}$(T) displays an increasingly steep low-field temperature dependence followed by a lessening of the steepness of the high-field region as oxygen content increases. This trend suggests that the boundary of the dissipation-less superconducting region of this unconventional high-T$_{c}$ cuprate based compound is evolving in the overdoped state towards a form that is consistent with what is observed in conventional superconductors. This research was supported by U.S. DOE Grant No. DE-FG02-04ER46105. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J10.00002: Measurements of the anisotropic irreversibility field in the electron-doped high-T$_{c}$ superconductor Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ Guoqing Wu, W.G. Clark, S.E. Brown, R.L. Greene, H. Balci, A.P. Reyes, P. Kuhns, W.G. Moulton We report measurements of the irreversibility field $(H_{\rm {irf}})$ in single crystals of the electron-doped high-T$_{c}$ superconductor (HTSC) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ ($x$ = 0.15 and 0.17) with an applied magnetic field ($B_{0}$) up to 28 T, using the method of the shift in a nuclear magnetic resonance (NMR) probe circuit resonance frequency (f) caused by the susceptibility of the sample. It is observed that $H_{\rm{irf}}$ is highly anisotropic, and that as the temperature $T$ $\rightarrow$ 0 the upper critical field [$H_{c2, \parallel c}$($T$ $\rightarrow$ 0)] at $B_{0}$ $\parallel$ $c$ is far less than the Pauli limit and very different from that at $B_{0}$ $\perp$ $c$. A phase diagram that involves the vortex solid and/or vortex liquid states depending on the alignment of $B_{0}$ relative to the lattice $c$-axis is proposed, and the obtained anisotropic $H_{c2}$ character along with the evaluated zero $T$ coherence length [$\xi_{ab(c)}$($T$ $\rightarrow$ 0)] and penetration depth [$\lambda_{ab(c)}$( $T$ $\rightarrow$ 0)] at $B_{0}$ $\parallel$ $ab(c)$ is compared with that of hole-doped HTSCs. This work is supported at UCLA by NSF Grants DMR-0334869 (WGC) and 0520552 (SEB), at U. Maryland by 0352735 (RLG), and NHMFL by 0084173 and the State of Florida. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J10.00003: Evolution of Vortex Phase diagram in heavy ion irradiated YBCO R. Xie, A. Rydh, U. Welp, W.-K. Kwok, M.R. Eskildsen, Lisa Paulius We present a systematic study of the effect of columnar defects induced by heavy ion irradiation on the vortex phase diagram of single-crystal YB2Cu3O7 using ac-specific heat measurements obtained with a micro-calorimeter. The first order vortex melting line where the vortex lattice transforms into a vortex liquid at intermediate magnetic fields is tracked by the peak in the specific heat. In our pristine untwinned YBCO crystal, the vortex melting line extends from a lower critical point Hlcp=0.2T to an upper critical point Hucp $>$ 6T. The crystal was cleaved into several pieces and then irradiated along the c-axis with 1.4GeV Pb ions with different dose matching fields, B$_{\Phi}$ ranging from 100G to 3000G. We explored the behavior of Hucp and Hlcp in the presence of increasing columnar defects to determine whether the transformation of the first order melting line to higher order occurs abruptly at a defect threshold value or continuously with increasing amount of defects. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J10.00004: Evidence for Hexatic Bose Glass in the Mixed Phase of Type-II Superconductors with Material Line Defects Jose P. Rodriguez, Charles E. Creffield Dislocation lines and nano-rod inclusions in thin films of YBa$_2$Cu$_3$O$_y$ aligned parallel to the c-axis are known to significantly enhance the critical current in external magnetic field that is also aligned in parallel. In contrast to correlated pinning centers created by irradiation, the former material line defects notably arrange themselves in a ``liquid'' fashion that shows no clusters or voids. Theoretical calculations predict the existence of a hexatic Bose glass at low temperature in such case[1]. We test that prediction by performing Monte-Carlo simulations of the corresponding two-dimensional Coulomb gas ensemble with close to 3000 vortices. In the regime of weak (``liquid'') pinning centers, we find a 2D hexatic vortex liquid at non-zero temperature characterized by isolated edge dislocations. It freezes into a phase-coherent hexatic vortex glass in the zero-temperature limit in accordance with theory [1]. \newline [1] J.P. Rodriguez, Phys. Rev. B {\bf 72}, 214503 (2005); Phys. Rev. B {\bf 70}, 224507 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J10.00005: Competition of Point and Correlated Vortex Pinning in Irradiated YBCO Wai-Kwong Kwok, Ulrich Welp, John Schlueter, Ruobing Xie, Jiong Hua, Zhili Xiao, Lisa M. Paulius, Morten R. Eskildsen We present a systematic study of vortex pinning on an optimal-doped untwinned YB$_{2}$Cu$_{3}$O$_{7-\delta }$ single-crystal irradiated with 1.4 GeV Pb ions and subsequently irradiated with protons. Irradiation to a dose matching field of B$_{\Phi }$=2T completely transforms the first order vortex melting transition to a higher order Bose glass transition. The transformation is also marked by a pronounced increase in vortex pinning at all temperatures, determined from SQUID measurements. We compare the irreversibility line and the remanent moment of the irradiated sample after subsequent irradiation with protons to determine the contribution to vortex pinning from point and correlated defects on the same sample. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J10.00006: Dynamics of driven vortices in type-II superconductors in the presence of strong point or columnar pinning centers Thananart Klongcheongsan, Thomas J. Bullard, Uwe C. Tauber We investigate the nonequilibrium steady state of driven magnetic flux lines in type-II superconductors subject to strong point or columnar pinning centers. We employ a three-dimensional elastic line model and Metropolis Monte Carlo simulations. We characterize the system by means of the force-velocity / current-voltage curve, static structure factor, mean vortex gyration radius, number of double-kink and half-loop excitations, and velocity / voltage noise features. We use different annealing methods to minimize numerical artifacts originating from long-lived metastable states. We compare the results for the above quantities for randomly distributed point and columnar defects. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J10.00007: Vortex dynamics and critical current in superconductors with unidirectional twin boundaries Hidehiro Asai, Satoshi Watanabe The pinning of superconducting vortices is important in device applications of superconductors, because immobilization of vortices at pinning sites is essential for lossless transport. Twin boundary (TB) is one of possible candidates for effective pinning centers, in particular in YBa$_{2}$Cu$_{3}$O$_{7-x}$. Thus, their pinning properties have been actively studied both experimentally and theoretically. However, the pinning characteristics of high-density TBs, which have recently been fabricated successfully, are still unclear. We have studied the dynamics of vortices interacting with unidirectional twin boundaries in a superconductor using molecular dynamics simulation. Current-voltage curves and critical currents have been calculated as a function of vortex density. We found that the critical current as a function of vortex density reveals a staircase pattern and this pattern depends on the pinning strength. This behavior corresponds to discontinuous change of vortex configurations, which reflects vortex pinning characteristics of superconductors with TBs. We also discuss the matching effect of vortex lattice with TBs, and reveal its behavior is different from the one in superconductors with columnar pinning. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J10.00008: Non Arrhenius creep motion of interacting vortices in high-$T_c$ cuprate superconductors A. Maeda, D. Nakamura Dynamics of driven vortices attracted much attention in many different fields of physics. In addition to the understanding of the dynamic phase diagram, the understanding of the elementary process of the vortex motion is crucially important. For a rather large driving force, the existence of the so-called washboard motion has been well established (Y. Togawa et al. : PRL 85 (2000) 3716.). On the other hand, very close to the critical driving force (critical current density), the Anderson-Kim creep picture has been believed for a long time, which is characterized as the Arrhenius process with the linear decrease of the potential barrier as a function of the external driving force. Quite recently, Luo and Hu proposed the possibility of the non-Arrhenius creep process for the vortex motion in Bragg glass phase, based on a numerical simulation (M. B. Luo and X. Hu, PRL98(2007) 267002.). Stimulated by this theoretical work, we performed a detailed $I- V$ study in a high-$T_c$ cuprate, LSCO, and found (1) The creep process is non-Arrhenius like in the Bragg glass phase, (2) With increasing magnetic field, the creep process changes into the Arrhenius type one suddenly. (3) The field where the process changes roughly agrees with the Bragg-glass-Vortex glass transition, (4) Aging effect of the critical driving force also changes close to the Bragg glass - Vortex liquid boundary. These features agree well with predictions by Lou and Hu. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J10.00009: Anisotropy of the Vortex Magnetic Field Distribution in LuNi$_2$B$_2$ M. R. Eskildsen, L. DeBeer-Schmitt, K. Rovira, N. Jenkins, C. D. Dewhurst, S. L. Bud'ko, P. C. Canfield It is well known that the vortex lattice (VL) symmetry and orientation in type-II superconductors is very sensitive to any anisotropy within the screening current plane. A classic example is the sequence of transitions from hexagonal to rhombic to square symmetry, which was first observed in the borocarbide superconductors and explained by a Fermi surface anisotropy coupled with the non-local electrodynamics responsible for vortex-vortex interactions. Recently, however, this is mounting experimental evidence for a strong gap anisotropy and possible point nodes in the basal plane of these materials. Here we report on small-angle neutron scattering studies of the VL in a carefully annealed, high quality LuNi$_2$B$_2$C single crystal, which permitted us to measure the VL form factor for a large number of reflections. These measurements allow a reconstruction of the real space profile of the magnetic field around the vortices, reflecting the basal plane anisotropy of the screening currents in LuNi$_2$B$_2$C. The results will be compared to predictions for both Fermi surface and gap anisotropies, and will serve as a valuable reference for more complicated compounds as e.g. Sr$_2$RuO$_4$, heavy fermions and high-$T_c$'s. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J10.00010: An infrared study of the pancake vortex state in La$_{2-x}$Sr$_{x}$CuO$_{4}$ across the phase diagram Alexander Schafgans, Andrew LaForge, Sasa Dordevic, Muhammad Qazilbash, Seiki Komiya, Yoichi Ando, Dimitri Basov We report on a doping dependent study of the far-infrared interlayer response in the high-temperature superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214). A magnetic field up to 8 Tesla applied perpendicular to the CuO$_{2}$ planes is found to increasingly suppress the Josephson plasma resonance (JPR) with decreased doping. By 6 Tesla at a temperature of 8 Kelvin, the c-axis reflectivity is identical to that of the normal state in the most underdoped samples, suggesting the sample is in a two dimensional superconducting state. This behavior is in contrast to fields parallel to the CuO$_{2}$ planes, where only a small suppression of the JPR is seen up to 17 Tesla. Vortex wandering and static spin density waves are considered as possible mechanisms for plane decoupling. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J10.00011: Torque magnetometry on the electron-doped high-temperature superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta }$ J.I. Oh, P. Dhakal, S. Li, P. Dai, M.J. Naughton We have used cantilever and extraction magnetometry to measure magnetization in optimally doped $n$-type high-temperature superconductors Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta }$ ($T_{c}$ = 24K) for magnetic field aligned close to the $c$-axis, over the temperature range (4K to 300K). We observed a distinct irreversibility line below which the torque magnetization is irreversible. Also, we observed a complex torque behavior where the sign of normal state torque response with field (d$\tau $/dH) is the same as that of the superconducting counterpart. From dc magnetization experiments, we conclude that superconducting torque signal arises primarily from out-of-plane diamagnetism, whereas in-plane paramagnetism dominates for the normal state. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J10.00012: Vortex pinning and local density of states in an inhomogeneous d-wave superconductor Daniel Valdez-Balderas, David Stroud We study a model for vortex pinning in a two-dimensional, inhomogeneous, type-II superconductor at low temperatures. The model is based on the Ginzburg-Landau free energy functional with position dependent coefficients, which we chose in such a way that regions with large gap also have large penetration depth. This choice of parameters (suggested by scanning tunneling spectroscopy experiments) results in vortices being pinned by superconducting regions where the gap is large, in contrast to the usual pinning picture. We also compute the density of states of a model BCS Hamiltonian with $d$-wave symmetry, in which the pairing field is given by the superconducting order parameter appearing in the free energy functional described above. We find that the type of inhomogeneity that we introduce is an indispensable ingredient for our model to reproduce some of the most salient experimental features of the local density of states spectra of cuprates. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J10.00013: Superconducting Froth Ruslan Prozorov, Andrew F. Fidler, Jacob Hoberg, Paul C. Canfield Studying the structure and dynamics of froths helps to understand the behaviour of complex systems where topological intricacy prohibits exact evaluation of the ground state. Though exact solutions are difficult, general laws that take into account both, the topological constrains and physics and chemistry of the froth matter have been developed. We used low temperature magneto-optical imaging in superconducting lead to add a new member to the froths family, - superconducting quantum froth, in which the boundaries are the superconducting and the interior is the normal phase. Despite very different microscopic origin, the topological analysis of the structure has shown that von Neumann's and Lewis' laws apply. Furthermore, for the first time in the froths analysis there is an external global parameter of known behaviour - the total magnetic moment. We show that the statistical laws are in a good agreement with the predicted macroscopic response. We assert that superconducting froth is the new playground for the analysis of complex physics of froths with magnetic field and temperature as tuneable control parameters. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J10.00014: Vortex relaxation of the noncentrosymmetric CePt$_{3}$Si A.C. Mota, C.F. Miclea, F. Steglich, M. Sigrist, E. Bauer The discovery of superconductivity (SC) in CePt$_{3}$Si by Bauer et al. (PRL. 92, 027003 (2004)), has attracted much interest since the compound lacks an inversion center and it has an unusually high upper critical field $H_{c2}$. Theoretical studies have pointed out that spin-orbit coupling could lead to a pairing state of mixed parity. For CePt$_{3}$Si it has been proposed that the combination of spin triplet $p$-wave and spin-singlet $s$-wave symmetries could explain most of the experimental facts consistently. Here we report on flux dynamics on a single crystal of CePt$_{3}$Si. The SC probed by means of magnetic susceptibility and specific heat shows a sharp transition at $T_{c}$= 0.45 K with a width of 0.1 K. Decays of the remnant magnetization display a clean logarithmic time dependence with rates that follow the temperature dependence expected from the Kim -- Anderson theory. However, the creep rates are extremely low, lower than observed in any other superconductor. The low rates are not caused by high critical currents. On the contrary, the critical current in CePt$_{3}$Si is considerably lower than in other superconductors with higher vortex relaxation rates. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J10.00015: Model for nodal quasiparticle scattering in a disordered vortex lattice Marianna Maltseva, Piers Coleman Recent experiments by T. Hanaguri et al. on underdoped Ca2-xNaxCuO2Cl2 [1] have observed quasiparticle interference effects [2], which are sensitive to the sign of the d-wave order parameter. In a magnetic field, they observe a sizable transfer of scattering spectral weight from scattering events between anti-nodes of opposite sign to scattering events between anti-nodes of the same sign. We interpret high momentum phase-coherent scattering in terms of the quasiparticle scattering off the vortex walls. The reduction of scattering at even-odd scattering points indicates that the vortices ``screen'' some of the underlying impurity scattering, as the impurities get trapped inside the vortex cores. [1] T. Hanaguri, Y. Kohsaka, J. C. Davis, C. Lupien, I. Yamada, M. Azuma, M. Takano, K. Ohishi, M. Ono, H. Takagi, cond-mat/07083728. [2] Y. Kohsaka, C. Taylor, K. Fujita, A. Schmidt, C. Lupien, T. Hanaguri, M. Azuma, M. Takano, H. Eisaki, H. Takagi, S. Uchida, J. C. Davis, Science 315, 1380-1385 (2007). [Preview Abstract] |
Session J11: Focus Session: MgB2-like: Novel Non-Boride Superconductors
Sponsoring Units: DMPChair: Vladimir Butko, Brookhaven National Laboratory
Room: Morial Convention Center RO9
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J11.00001: Muon spin rotation study of perturbation of the crystalline-electric-field induced by interstitial muon in PrOs$_4$Sb$_{12}$ and PrRu$_{4}$Sb$_{12}$ Lei Shu, D.E. MacLaughlin, R.H. Heffner, O.O. Bernal, W.P. Beyermann, N.A. Frederick, W.M. Yuhasz, T.A. Sayles, T. Yanagisawa, M.B. Maple Muon spin rotation measurements of the temperature dependence of the positive muon Knight shift in single crystals of PrOs$\rm_{4}$Sb$\rm_{12}$ and PrRu$\rm_{4}$Sb$\rm_{12}$ reveal a linear scaling of the Knight shift with the bulk magnetic susceptibility at high temperatures. A small deviation from the linear relation appears in PrOs$\rm_{4}$Sb$\rm_{12}$ below $6.3$ K. However, a large magnitude of deviation is observed in PrRu$\rm_{4}$Sb$\rm_{12}$ below $32$ K. The deviation can be explained by the positive muon induced modification of the susceptibility of neighboring Pr$^{3+}$ ions due to a change of the crystalline-electric-field (CEF) splitting. The data indicate that this modification is much smaller in PrOs$\rm_{4}$Sb$\rm_{12}$ than in PrRu$\rm_{4}$Sb$\rm_{12}$. A model calculation based on CEF theory is in progress. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J11.00002: Interplay of superconductivity and rattling phenomena in $\beta $-pyrochlore KOs$_{2}$O$_{6}$ studied by photoemission spectroscopy Takahiro Shimojima, Yuki Shibata, Kyoko Ishizaka, Takayuki Kiss, Ashishi Chainani, Takayoshi Yokoya, Tadashi Togashi, Xiaoyang Wang, Chuangtian Chen, Shuntaro Watanabe, Jyunichi Yamaura, Shigeki Yonezawa, Yuji Muraoka, Zenji Hiroi, Tomohiko Saitoh, Shik Shin The electronic structure near Fermi level of KOs$_{2}$O$_{6}$ is studied by a laser-excited photoemission spectroscopy. The superconducting(SC) gap clearly opens across the SC transition at 9.6 K, with the strong electron-phonon coupling value of 2$\Delta $(0)/k$_{B}$T$_{c} \quad \ge $ 4.56. Fitting analysis identifies clear anomalies at 7.5 K in the temperature dependences of the SC gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons, suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs$_{2}$O$_{6}$. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 12:15PM |
J11.00003: Controlling physical parameters of layer-structured nitride-halide superconductors Invited Speaker: Metal-intercalation into band insulators sometimes affords superconductors, well-known examples of which are carbon-based materials, such as fullerides and graphite. Layer-structured nitride-halide Li$_x$ZrNCl and Li$_xM_y$HfNCl ($M$ denotes molecule) belong to another class of intercalation-induced superconductors with relatively high $T_c$, in which doping level and interlayer distance (and hence interlayer hopping interaction) can independently be controlled by changing Li concentration and the size of the co- intercalated molecule. The controllability provides a unique and interesting opportunity to investigate the effect of the two important physical parameters on $T_c$ in a single system. Recent progress in the synthesis technique enabled us to obtain for the first time a series of single-phase samples of Li$_x$ZrNCl with finely controlled doping-levels which were notoriously difficult to prepare. Using these samples, we have established[1] an electronic phase diagram to find anomalous doping evolution of $T_c$, which takes a maximum value on the verge of superconductor- insulator transition. Based on this phase diagram and the results of systematic Raman scattering and transport measurements, we will discuss possible roles in producing relatively high $T_c$ played by charge fluctuation and reduced disorder scattering in the layered structure reminiscent of modulation-doped semiconductors. We will also briefly refer to our very recent results on the Hf-based materials in which both of the doping level and interlayer distance were varied. \newline [1] Y. Taguchi {\it et al.}, Phys. Rev. Lett. {\bf 97}, 107001 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J11.00004: Independent control of carrier concentration and interlayer spacing in Li$_x$HfNCl layered superconductors Takumi Takano, Atsushi Kitora, Tsukasa Kishiume, Yasujiro Taguchi, Yoshihiro Iwasa Alkali-metal and organic molecule co-intercalated HfNCl is a new class of layered superconductors with relatively high transition temperature ($T_c$) of 25.5 K. Recently, we have succeeded in synthesis of single phase samples of Li$_x$(molecule)$_y$HfNCl with a wide range of doping concentration of 0.10 $\leq$ $x$ $\leq$ 0.50, where we are able to, continuously and independently, control the carrier density and interlayer distance \textit{d} between the conducting Hf-N layers by means of co-intercalation of Li and organic molecule. Without any molecule, superconductivity appears at $x$ $\sim$ 0.15 and $T_c$ is almost constant against $x$ above this critical value. Furthermore, we found that $T_c$ is enhanced ($\sim$30$\%$) from 20 K to 25.5 K with increasing of \textit{d}. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J11.00005: Unconventional superconductivity in single crystal Lu$_{2}$Fe$_{3}$Si$_{5}$ R. Gordon, M.D. Vannette, C. Martin, T. Tamegai, Y. Nakajima, R. Prozorov Dynamic magnetic susceptibility for a single crystal of the ternary superconductor Lu$_{2}$Fe$_{3}$Si$_{5}$ has been measured using a tunnel diode resonator (TDR) technique. The London penetration depth exhibits non-exponential temperature dependence. We analyze the obtained superfluid density by comparing models of two-gap superconductivity, a gap with nodes or a highly anisotropic gap. The upper critical field is highly anisotropic and is unusually large. Furthermore, hysteresis in the susceptibility implies unusually strong temperature dependence of the critical current. The results are discussed in terms of possible unconventional behavior of this low-T$_{c}$ superconductor. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J11.00006: Two-Gap Superconductivity in Lu$_{2}$Fe$_{3}$Si$_{5}$ Tsuyoshi Tamegai, Yasuyuki Nakajima, Guoji Li Lu$_{2}$Fe$_{3}$Si$_{5}$ is a superconductor with $T_{c}\sim $ 6 K containing nonmagnetic irons. Anomalous temperature dependence of specific heat in the superconducting state has been reported in polycrystalline samples; reduced specific heat jump at $T_{c}$ and apparent residual $T$-linear term in the limit of $T$=0 K. We have successfully grown high-quality single crystals of Lu$_{2}$Fe$_{3}$Si$_{5}$ using the floating-zone technique, and characterized its superconducting and normal state properties. The anomalies of the specific heat reported in polycrystalline samples are reproduced in the single crystals. In addition, we find a second drop of the specific heat below 1 K. We can fit the temperature dependence of the specific heat by assuming two superconducting gaps as in the case of MgB$_{2}$. Temperature dependence of Hall coefficient is nonmonotonic, and also suggests the presence of multiple bands in this compound. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J11.00007: Point-contact Andreev reflection tunneling spectroscopy (PCARTS) of the superconducting gap structure in LuNi$_{2}$B$_{2}$C Xin Lu, W.K. Park, L.H. Greene, Sunmog Yeo, Kyu-Hwan Oh, Sung-Ik Lee, Sergey L. Bud'ko, Paul C. Canfield The PCARTS technique is employed to investigate the gap anisotropy and proposed existence of point-nodes in LuNi$_{2}$B$_{2}$C (T$_{C}$ $\sim$ 16 K). Differential conductance spectra are taken from two different sets of single crystal samples along three major orientations: [001], [110], and [100]. Analyzing using the single-gap Blonder-Tinkham-Klapwijk (BTK) model reproducibly shows the gaps along these directions are 2.4, 2.6, and 2.3 meV, respectively, for one set of samples and 2.4, 2.8, and 2.7 meV, respectively, for the other set. This is smaller than the gap anisotropy reported by other groups[1]. At low temperatures, the single-gap BTK model does not satisfactorily fit our data. Models employing an anisotropic gap are being investigated, as are experiments parameterizing the tunneling cone effect.\newline [1] Y.G. Naidyuk, et al,condmat/0609769(2006); N.L.Bobrov, et al,PRB 71, 014512 (2005); S. Mukhopadhyay, et al,PRB 72, 014545 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J11.00008: Anisotropic properties of aligned weak-ferromagnetic superconductor RuSr$_2$GdCu$_{2}$O$_{8}$ H.C. Ku, B.C. Chang, C.H. Hsu, Y.F. Chen, M.F. Tai The $\it{ab}$-plane aligned powder in epoxy matrix for the tetragonal RuSr$_{2}$GdCu$_{2}$O$_{8}$ weak-ferromagnetic superconductor was achieved using a field powder alignment method with $\it{ab}$-plane parallel to the applied magnetic field. The $\it{c}$-axis aligned powder can also be obtained using the field-rotation method where $\it{c}$-axis is perpendicular to the applied magnetic field and along the rotation axis. The temperature dependence of magnetic moment m (T) for the aligned powder provides the desired anisotropic properties where larger magnetic moment along the $\it{ab}$- plane was observed. The field-cooled (FC) and zero-field-cooled (ZFC) data in low applied field (1 G) for both directions indicate a weak-ferromagnetic (canted-antiferromagnetic) transition of Ru moment at T$_N$(Ru) = 131 K and a superconducting transition in the CuO$_{2}$ plane at T$_{c}$ = 39 K. The low temperature antiferromagnetic ordering of the rare earth Gd moment is observed at T$_{N}$(Gd) = 2.5 K. Diamagnetic superconducting shielding signal is much weaker than bulk sample due to small powder diameter (1-10 $\mu$m), long penetration depth $\lambda$ and the two-dimensional (2D) character of CuO$_{2}$ plane. Low temperature, low field magnetization data m(B$_{a}$,T) will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J11.00009: Order parameter suppression and structure of the surface states in non-centrosymmetric superconductors Anton Vorontsov, Ilya Vekhter, Matthias Eschrig We consider the structure of the surface states at the pairbreaking boundaries of non-centrosymmetric superconductors. In the region of the order parameter suppression multiple Andreev reflections significantly modify the energy and the intragap density of states due to bound states. We elucidate the physics behind this modification by considering a simple model of gap suppression, and comparing it with a fully self- consistent microscopic calculation. We emphasize the experimentally relevant consequences of the lack of inversion symmetry for the surface states. As the discontinuity in the spin-orbit coupling at the boundary makes the interface spin-active, we analyse the resulting spin structure of the bound states. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J11.00010: Superconductivity in Sn(1-x-d)In(x)Te and Pb(1-y)Tl(y)Te Ann Erickson, Theodore Geballe, Ian Fisher Recent evidence for a charge-Kondo effect in superconducting samples of Pb$_{1-y}$Tl$_y$Te [PRL \textbf{94}, 157002 (2005)] raises the possibility that systems of degenerate semiconductors doped with valence skipping elements may be an ideal realm in which to study suggested negative U electronic pairing mechanisms in superconductors [PRL \textbf{61}, 2713 (1988)]. However, questions of exactly how the proposed charge-Kondo behavior relates to superconductivity in Pb$_{1-y}$Tl$_y$Te remain. In this work, we study the relationship between the DOS at the Fermi level and Tc in the related superconductor Sn$_{1-x-d}$In$_x$Te, where enhancement of Tc is found above a critical concentration $x_c > d/2$, where the Fermi level is pinned in the impurity band and the DOS is enhanced [Sov. Phys. Solid State \textbf{28}, 612 (1986)]. We find that the elevated DOS is insufficient to explain the enhanced Tc for these samples, suggesting an additional pairing mechanism is involved, such as the negative U mechanism mentioned above. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J11.00011: Novel superconductivity in a new noncentrosymmetric superconductor Lei Fang, Xiyu Zhu, Gang Mu, Hai-hu Wen Low temperature specific, resistivity and magnetization are measured in a newly fabricated superconductor. It is found that this material has no central inversion symmetry. Specific heat measurement show that the major part of the system has a s-wave symmetry and the superconducting gap is thus derived. However, when the superconductivity is suppressed by the magnetic field, a further drop of specific heat coefficient is observed just at T$_{c}$. This unexpected behavior remains to a very high magnetic field and without any obvious shift of the transition temperature. It is tempting to argue that this drop of specific heat coefficient may be induced by the spin triplet pairs. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J11.00012: Superconductivity in the new Platinum Germanides APt$_4$Ge$_{12}$ (A=Sr,Ba,La,Pr) Helge Rosner, Roman Gumeniuk, Walter Schnelle, Michael Nicklas, Andreas Leithe-Jasper, Yuri Grin New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. Magnetic susceptibility, specific heat, and electrical resistivity measurements find superconductivity in LaPt$_4$Ge$_{12}$ and PrPt$_4$Ge$_{12}$ below ca.\ 8\,K. The parameters of the normal and superconducting states were established. Strong electron-phonon coupling and a crystal electric field singlet groundstate is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level, predominantly due to Ge $4p$ orbitals. Similar behavior, albeit with lower $T_c$, was observed for SrPt$_4$Ge$_{12}$ and BaPt$_4$Ge$_{12}$. [Preview Abstract] |
Session J12: Experimental Studies of the Heavy Fermion 115 Compounds
Sponsoring Units: DCMPChair: N. Peter Armitage, The Johns Hopkins University
Room: Morial Convention Center 203
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J12.00001: 3D Fermi Surface Mapping of Ce$_2$RhIn$_8$ J.D. Denlinger, F. Wang, J.W. Allen, M.B. Maple, S. Elgazzar, P.M. Oppeneer Photon-dependent multi-Brillouin zone angle-resolved photoemission (ARPES) measurements are presented for the antiferromagnet heavy fermion bilayer compound Ce$_2$RhIn$_8$. Highly automated Fermi Surface (FS) mapping for excitation energies of 90-120 eV allow visualization of k$_z$-variations of the electronic structure and permit a k$_z$-tomographic determination of the three-dimensional (3D) FS topology with sufficient detail for quantitative comparison to dHvA orbit areas and LDA-predicted topological shapes. In addition to confirming the quasi-2D circular and square topologies centered on the Brilluoin zone corner, highly 3D k$_z$-variations are observed along the (100) directions $\Gamma$-X (Z-R). Results are compared to a similar ARPES determination of the 3D electronic structure and FS of the single layer compound CeCoIn$_5$. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J12.00002: Anisotropic quantum criticality in heavy-fermion metal CeCoIn5 Ramzy Daou, Makariy Tanatar, Cedomir Petrovic, Johnpierre Paglione, Louis Taillefer We previously reported a violation of the Wiedemann-Franz law in the heavy-fermion metal CeCoIn5 when tuned to its quantum critical point, depending on the direction of electron motion relative to the crystal lattice, which points to an anisotropic destruction of the Fermi surface [1]. Here we present new measurements of electric, thermal and thermo-electric transport coefficients which reveal different anisotropic responses. [1] M.A. Tanatar et al., Science 316, 1320 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J12.00003: Nature of the superconducting state of CeCoIn$_5$ as revealed by NMR. Georgios Koutroulakis, Vesna Mitrovic, Marc-Andr\'e Vachon, Mladen Horvatic, Claude Berthier, Georg Knebel, Gerard Lapertot, Jacques Flouquet We report low temperature nuclear magnetic resonance (NMR) measurements of the heavy-fermion superconductor CeCoIn$_5$ in high magnetic fields. The effect of the RF penetration on the NMR spectrum for the different parts of the phase diagram is studied. The implications of this study for the nature of a possible inhomogeneous superconducting state, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J12.00004: Thermal transport in the vortex state of heavy-fermion superconductor CeCoIn$_5$ M.A. Tanatar, J.P. Reid, J. Paglione , C. Petrovic , Louis Taillefer The thermal conductivity of heavy-fermion superconductor CeCoIn$_5$ was measured as a function of temperature and magnetic field throughout the vortex state. An anomalous decrease is found in the field dependence at low fields and low temperature. We discuss the origin of this behavior and its possible relation to the presence of uncondensed electrons, found in a previous doping study of this material [1]. \newline \newline [1] M.A. Tanatar et al., Phys. Rev. Lett. 95, 067002 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J12.00005: NMR investigation of a hole doped CeCoIn5 R. R. Urbano, N. J. Curro, V. A. Sidorov, J. D. Thompson, L. D. Pham, Z. Fisk We have investigated the local environment of In and Co sites of the heavy fermion compound CeCo(In$_{1-x}$Cd$_{x}$)$_{5}$ ($x$ = 0.0, 0.10 and 0.15) using Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements. Recently, it was found that Cd-doping acts as an electronic tuning agent in CeCoIn$_{5}$ and that superconductivity (SC) and antiferromagnetism (AFM) coexist at ambient-pressure for $0.05 < x < 0.15$. It has also been observed on Cd doped compounds that pressure $P$ recovers the SC ground-state observed for the undoped compound suggesting it as a reversible tuning parameter. In this work we report data indicating that these phases indeed coexist microscopically. The NMR/NQR spectra of In and Co indicate the presence of electronic inhomogeneity, and the spin-lattice relaxation measurements $T_{1}^{-1}$ suggest that Cd doping induces changes to the low frequency spin dynamics only below $T \simeq 5 K$. Furthermore, $T_{1}^{-1}$ measurements for $x = 0.10$ under pressure show a different spin dynamics response in the paramagnetic state, in contrast with the effect of the chemical pressure. We show that chemical doping and pressure are not equivalent. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J12.00006: Thermal transport at a field-tuned quantum critical point in CeCoIn5 Jean Philippe Reid, Makariy Tanatar, Johnpierre Paglione, C. Petrovic, L. Taillefer The heavy-fermion metal CeCoIn5 exhibits a field-tuned quantum critical point which coincides with the upper critical field for superconductivity for directions of magnetic field both parallel [1] and perpendicular to the tetragonal c-axis of the material [2]. Here we report a study of this field-tuned critical point using electrical resistivity and thermal conductivity measurements performed in magnetic fields parallel to the conducting plane. [1] J. Paglione et al., Phys. Rev. Lett. 91, 246405 (2003). [2] F. Ronning et al., Phys. Rev. B. 71, 104528 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J12.00007: Evolution of the superconducting properties of CeCoIn$_5$ with Yb substitution D. Hurt, C. Capan, Z. Fisk, A. D. Bianchi We report on the evolution of the physical properties of the Yb substitution series starting from the unconventional superconductor (SC) CeCoIn$_5$ to the isostructural normal metal YbCoIn$_5$. This study was motivated by the recent results of Cd or Hg substitution at the percentage level on the In site in CeCoIn$_5$ which for low concentrations first was shown to lead to the coexistence of antiferromagnetism with SC and to a complete suppression of SC at higher concentrations. At the same time, the lattice constant of YbCoIn$_5$ indicates that Yb enters this compound in a partially divalent configuration suggesting that Yb could also be suitable for doping holes into CeCoIn$_5$. In our substitution series we find that that the unit cell volume stays roughly constant up to an Yb concentration of about 40~\%, after which the cell volume begins to decrease gradually to the value of YbCoIn$_5$. At the same time we observe a gradual suppression of the transition temperature $T_c$ to zero at an Yb concentration of 60~\%. Interestingly, the shape of $H$-$T$-phase diagram remains the same when the axis is scaled with the respective $T_c$ and upper critical field $H_{c2}$, suggesting that the ratio between Pauli $H_p$ and the orbital critical field $H_{c20}$ remains constant. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J12.00008: Quantum oscillations in heavy fermion CeRhIn5 Huiqiu Yuan, Tuson Park, Eric Bauer, Joe Thompson, John Singleton Construction of Fermi surface is the key to understand the physical properties of variant materials. In correlated electron systems, the heavy carrier mass strongly attenuates the amplitude of quantum oscillations, and, therefore, the Fermi surface sheets from the heavy electrons are usually not observable at low fields. High magnet fields are an indispensable tool to uncover these important features of electronic structure. Furthermore, strong magnetic field above the Neel critical field may lead to the reconstruction or a volume change of Fermi surface in f-electron antiferromagnets (N. Harrison et al, PRL 99, 056401, 2007). Using the unique facilities in the national high magnetic field lab at LANL, in this presentation we will study the quantum oscillations in the heavy fermion compound CeRhIn$_5$ with a field up to 65T. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J12.00009: Pressure effect of single ion Kondo temperature in Ce$_{.02}$La$_{.98}$RhIn$_{5}$ H. Lee, V.A. Sidorov, L.M. Ferreira, T. Park, F. Ronning, E.D. Bauer, J.D. Thompson Near a critical pressure P$_{c} \quad \sim $ 25 kbar, CeRhIn$_{5}$ assumes characteristics of CeCoIn$_{5}$ at atmospheric pressure: they have comparable T$_{C}$, similar dHvA frequencies, and display quantum-critical behaviors. Many properties of CeCoIn$_{5}$ can be interpreted within a two-fluid phenomenology$^{1}$ in which there are interpenetrating fluids, a localized f-electron Kondo gas (energy scale T$_{K})$ and an interacting Kondo liquid (energy scale T*). We have measured transport properties of Ce$_{.02}$La$_{.98}$RhIn$_{5}$ under pressures to 50 kbar to determine T$_{K}$(P), which at P=0 is estimated to be $\sim $ 0.03K from specific heat measurements. T$_{K}$(P) increases rapidly, reaching $\sim $1.35K at 25 kbar, where it becomes comparable to T$_{K}$ ($\sim $1.8K) of CeCoIn$_{5}$ at P=0. A comparison of T$_{K}$(P) with T*(P), determined from the pressure studies of CeRhIn$_{5}$, reveals the same correlation between T$_{K}$ and T* inferred from a two-fluid analysis of CeCoIn$_{5}$, further supporting the similarity of these two compounds and the two-fluid phenomenology. [1] S. Nakatsuji et al., Phys. Rev. Lett. 92, 016401 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J12.00010: Uniaxial Pressure and the Superconducting Transition of CeIrIn$_5$ Owen Dix, Adrian Swartz, Rena Zieve, Todd Sayles, Brian Maple We measure the superconducting transition temperature of CeIrIn$_5$ with applied uniaxial pressure. Heat capacity shows almost no shift in T$_c$ as pressure is applied along the {\em c}-axis, contrary to the large decrease expected from earlier thermal expansion measurements. With {\em a}-axis pressure, however, T$_c$ increases about 20 mK per kbar. These results indicate that another factor besides the {\em c}/{\em a} ratio has a strong effect on T$_c$. Furthermore, applied pressure along either crystal axis strongly reduces the size of the heat capacity transition. We will also discuss the effect of uniaxial pressure on the resistive transtition, which at ambient pressure occurs at a temperature well above the heat capacity transition. Finally, we present x-ray diffraction measurements correlating our applied pressures with changes in the crystal lattice constants. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J12.00011: Evolution of the superconducting state through quantum criticality in CeRh$_{1-x}$Co$_x$In$_5$ Johnpierre Paglione, M.A. Tanatar, J.P. Reid, Louis Taillefer, M.B. Maple The Ce-based 115 materials exhibit a host of novel ground states separated by experimentally tunable quantum instabilities. In the single-crystal alloy series CeRh$_{1-x}$Co$_x$In$_5$, long range antiferromagnetic order is gradually suppressed upon chemical substitution of Co for Rh and followed by a robust superconducting state extending to the 2.3~K transition of the infamous heavy-fermion superconductor CeCoIn$_5$. Here we present a thorough study of heat transport measurements of high- quality single crystals of CeRh$_{1-x}$Co$_x$In$_5$ for several different superconducting samples spanning both the coexistent magnetic and non-magnetic regions of the $x$-$T$ phase diagram. By extracting the residual ($T\to 0$ limit) electronic thermal conductivity of samples at several $x$ values, we analyze the nature of the superconducting state on either side of the incipient quantum critical point near $x\simeq 0.65$ and study the influence of coexistent magnetism on the pairing state of these materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J12.00012: Electronic Duality in the Pressure-tuned Quantum Critical Metal CeRhIn$_{5}$ Tuson Park, M.J. Graf, Lev Boulaevskii, J.L. Sarrao, J.D. Thompson The heavy fermion compound CeRhIn$_{5}$ is a prototypical strongly correlated antiferromagnet where the localized 4f electron of Ce hybridizes weakly with ligand electrons. Applying pressure to this material increases hybridization and induces bulk unconventional superconductivity that arises from pressure-enhanced itinerancy of 4f electrons and that simultaneously coexists with large-moment antiferromagnetic order among localized 4f electrons. This microscopic coexistence of local-moment magnetic order and superconductivity in CeRhIn$_{5}$ is distinctly different from conventional models that attribute coexisting spin-density wave magnetism and superconductivity to a Fermi-surface instability. Electronic duality, which is unambiguously revealed in the single 4f electron of cerium in CeRhIn$_{5}$, is a new framework emerging from strongly correlated electron matter, ranging from the high-T$_{c}$ cuprates and heavy fermion superconductors to plutonium. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J12.00013: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J12.00014: Interplay of Magnetism and Superconductivity in CeM(In$_{1-x}$Hg$_x$)$_5$ E.D. Bauer, F. Ronning, Y. Tokiwa, J.D. Thompson, R. Movshovich, Z. Fisk The CeMIn$_5$ (M=Co, Rh, Ir) heavy fermion superconductors have attracted interest in recent years due to their high superconducting transition temperatures (e.g., $T_c$=2.3 K in CeCoIn$_5$), unconventional superconductivity, and magnetic- field induced exotic ground states. In particular, field- induced quantum criticality at the upper critical field H$_{c2} $=5 T in CeCoIn$_5$ and a possible field-induced magnetic state within the superconducting state suggests close proximity to antiferromagnetism.$^1$ The exciting discovery of slight changes in the electronic structure of CeMIn$_5$ with Cd or Hg substitution$^2$ at the percent level in CeCoIn$_5$ appears to have ``uncovered'' the hidden magnetism in this material. Therefore, substitution of Hg in CeMIn$_5$ offers yet another way to probe the proximity to magnetism in CeCoIn$_5$, the field-induced magnetic state under pressure in CeRhIn$_5$, and the coexistence of magnetism and superconductivity. The physical properties of CeM(In$_{1-x}$Hg$_x$)$_5$ system will be discussed and contrasted with those found with isoelectronic substitutions. $^1$ {J. Paglione {\textit{et al.}} Phys. Rev. Lett. {\textbf {91}} 246405 (2003); A. Binachi {\textit{et al.}} Phys. Rev. Lett. {\textbf{91}} 257001 (2003)} $^2$ {L.D. Pham {\textit{et al.}} Phys. Rev. Lett. {\textbf {97}} 056404 (2006)} [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J12.00015: $d_{x^{2}-y^{2}}$ paring symmetry of heavy fermion CeIrIn$_{5}$ remote from antiferromagnetic quantum critical point Yuichi Kasahara, T. Iwasawa, Y. Shimizu, H. Shishido, T. Shibauchi, I. Vekhter, Y. Matsuda Quasi-two dimensional heavy Fermion CeIrIn$_{5}$ involves two distinct superconducting domes in the phase diagram, which appear as a function of pressure or Rh substitution of Ir. In the analogy to CeCu$_{2}$Si$_{2}$, two distinct superconducting domes with different symmetry has been invoked. We report on the results of low-temperature thermal transport of CeIrIn$_{5}$ in the second dome, which locates away from an antiferromagnetic quantum critical point. The thermal conductivity is measured under a magnetic field rotated with respect to the crystal axes, which give direct evidence for superconducting gap structure. Clear fourfold oscillation with minima at [110] and [1-10] directions is observed as rotating magnetic field within the basal $ab$-plane, while no oscillation is observed within the $bc$-plane. In sharp contrast to previous reports that suggested $E_{g}$ symmetry with horizontal line node within the $ab$-plane [1], our results are most consistent with $d_{x^{2}- y^{2}}$ symmetry with vertical line nodes along the $c$-axis. These results imply that two superconducting domes have the same gap symmetry which appears to be mediated by antiferromagnetic spin fluctuations. \newline [1] H.~Shakeripour et al., Phys. Rev. Lett. 99, 187004 (2007). [Preview Abstract] |
Session J13: Focus Session: Simulations of Matter at Extreme Conditions IV: Crystalline Solids, Liquids, and Methods
Sponsoring Units: DCOMP GSCCMChair: Ann Mattsson, Sandia National Laboratories
Room: Morial Convention Center 204
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J13.00001: Structure and dynamics of supercooled liquid silicon under pressure: A first-principles molecular-dynamics study. Tetsuya Morishita Recent investigations have suggested that silicon (Si) may exhibit liquid-liquid transitions under pressure and/or supercooling [1]. Here, we report first-principles molecular-dynamics simulations of supercooled liquid Si focusing on the pressure dependence of structure and dynamics [2]. The pair correlation function of deeply supercooled liquid Si (1100 K) for pressures 0 - 18 GPa shows considerable structural changes resulting from the collapse of tetrahedral configurations. The power spectrum of the velocity auto-correlation function also confirms the reduction of the tetrahedral order by pressurization. The self-diffusion coefficient as a function of pressure shows a broad maximum around 10 GPa. However, at a higher temperature (1500 K), the diffusion coefficient simply decreases with increasing pressure, indicating conspicuous dependence of the dynamics and relevant structure upon temperature. [1] T. Morishita, Phys. Rev. Lett. 93, 055503 (2004); ibid. 97, 165502 (2006). [2] T. Morishita, Phys. Rev. E 72, 021201 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J13.00002: Molecular dynamics simulation of the shock-induced wurtzite-to-rocksalt transition in CdSe and CdS Aidan Thompson, Marcus Knudson The shock-induced wurtzite-to-rocksalt structural transformation is studied using large-scale molecular dynamics simulation. The primary goal is to understand the atomistic mechanisms underlying the interesting transformation kinetics observed in the case of cadmium sulfide [M. D. Knudson and Y. M. Gupta, J. Appl. Phys, v. 91, p. 9561, 2002]. Since the mechanical and structural properties of CdS are very similar to those of CdSe, we have performed multi-million atom MD simulations of the shock-induced phase transformation in CdSe single-crystals using the well-established interatomic potential of Rabani, which has been shown to correctly describe the wurtzite and rocksalt phases and the transformation pressure. In MD simulations of shock along the wurtzite $c$-axis, the elastically-compressed wurtzite transforms directly to grains of rocksalt. Along the $a$-axis, a three-wave structure is observed; the wurtzite first transforms to a tetragonal crystal phase, which in turn transforms to rocksalt grains. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J13.00003: Large-scale molecular dynamics modeling of shock wave propagation in silicon Xiang Gu, Mikalai Budzevich, Ivan Oleynik, Sergey Zybin, Carter White We performed molecular dynamics simulations of shock wave propagation in silicon. The different regimes of materials response were studied as a function of shock wave intensity and crystalline orientation of shock wave propagation. The shock Hugoniots are predicted in a wide range of piston velocities (0-12 km/s), and for several crystallographic orientations $<$100$>$, $<$110$>$, and $<$111$>$. Shock Hugoniots were used for a detailed analysis of a material's response to complex, split shock-wave structures. The special regime of an anomalous response of the material which is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities was investigated. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J13.00004: Shock front broadening in polycrystalline materials John Barber, Kai Kadau We analyze a model for the evolution of weak shock fronts (or elastic precursor waves) in polycrystalline materials. This model is based on the idea of Meyers and Carvalho [Mater. Sci. Eng. \textbf{24}, 131 (1976)] that the shock velocity anisotropy within the polycrystal is the most important factor in shock front broadening. Our analysis predicts that the shock front width increases as the 1/2 power of the front penetration distance into the crystal. Our theoretical prediction is in plausible agreement with previous experimental results for the elastic precursor rise time, and it should therefore provide a useful shock width estimate. Furthermore, our theoretical framework is also applicable to other problems involving front propagation in heterogeneous media. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J13.00005: Large Scale Molecular Dynamics Simulations of Dense Plasmas. Jerome Daligault, Guy Dimonte Plasmas are generally created and probed by depositing energy into matter, driving it far from equilibrium. Knowledge over a wide range of physical conditions of the rate at which the electronic and ionic subsystems come into thermal equilibrium is important for explicit practical purposes. The microscopic mechanisms vary with the strength of the coupling among particles and the degree of degeneracy of the electrons. Though a variety of models for the electron-ion energy equilibration rate were proposed, these models apply to specific regimes, their range of validity and the transition from one regime to another remains unclear. Molecular dynamics (MD) simulations provide a powerful tool to investigate the validity of the various models. In order to study the temperature relaxation rates over a wide range of plasma coupling, from very weakly coupled to strongly coupled, multi-million particles simulations are necessary. To this end, we have developed a parallel MD code that employs the particle-particle particle-mesh algorithm and allow the simulation of very large, complex Coulomb systems and over long time scales. We have performed detailed, multi-million particle MD simulations to investigate and shed some new light on the electron-ion energy relaxation in hot, dense plasmas. In this talk, we will describe the MD code and discuss the original results obtained for the temperature relaxation rates. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J13.00006: Effect of Crystalline Anisotropy on Shock Propagation in Sapphire. W.J. Nellis, G.I. Kanel, S.V. Razorenov, A.S. Savinykh, A.M. Rajendran The major impediment to measuring reshock temperatures is opacity induced in anvil/windows by shock. We report measured shock profiles of c-, d-, and r-cut single crystals and comparison of these mechanical responses with optical snapshots measured by Hare et al. Profiles were measured at three peak stresses and two sample thicknesses. Particle velocity histories were recorded for sapphire/LiF interfaces. VISAR waveforms are noisy as a result of heterogeneous inelastic deformation and noise depends on crystal orientation and stress amplitude. Heterogeneity is least for r-cut and most for c-cut, which correlates with observed optical heterogeneity. At 2.4 mm thickness r-cut has a three-wave structure that might indicate several elastic-wave speeds off an axis of symmetry. The small signal of the third wave might also indicate a phase transition in the small volume of the sample at higher temperatures. The Hugoniot elastic limit of c-sapphire scatters from shot to shot; scatter in the HEL of r- and d-cut are smaller. Radial pre-stressing of c-sapphire resulted in some increase of the rise time of the second wave; no significant effect of pre-stressing was observed for d- and r- samples. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J13.00007: Shock Pulse Effects in PTFE Shocked Through the Crystalline Phase II--III Transition Eric N. Brown, George T. Gray III, Philip J. Rae, Neil K. Bourne We present an experimental study of crystalline structure evolution of polytetrafluoroethylene (PTFE) due to pressure-induced phase transitions in a semi-crystalline polymer using soft-recovery, shock-loading techniques coupled with mechanical and chemical post-shock analysis. Gas-launched, plate impact experiments have been performed on pedigreed PTFE 7C, mounted in momentum-trapped, shock assemblies, with impact pressures above and below the phase II to phase III crystalline transition. Below the phase transition only subtle changes were observed in the crystallinity, microstructure, and mechanical response of PTFE. Shock loading of PTFE~7C above the phase II--III transition was seen to cause both an increase in crystallinity from 38{\%} to $\sim $53{\%} (by Differential Scanning Calorimetry, DSC) and a finer crystalline microstructure, and changed the yield and flow stress behavior. We particularly focus on the effect of pulse duration on the microstructure evolution. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J13.00008: Electrical Breakdown and Lock-On in Photoconductive Semiconductor Switch (PCSS) Devices Harold Hjalmarson, Kenneth Kambour, Fred Zutavern, Charley Myles Optically-triggered, high-power photoconductive semiconductor switches (PCSS's) using semi-insulating GaAs are being developed at Sandia Labs. These switches carry current in high carrier-density filaments. The properties of these filaments can be explained by redistribution of carrier energy caused by carrier-carrier scattering within the filament. This process enhances the impact ionization rate thus allowing these filaments to be sustained by relatively low fields, a process called lock-on. For GaAs, the sustaining field is approximately 4.5 kV/cm. For this talk, the physics mechanisms for lock-on and high-field electrical breakdown are described. Also, a continuum implementation of these physics mechanisms is used to compute the properties of these filaments. These continuum calculations are based on previous calculations in which the filament properties are computed using a Monte Carlo method to solve the steady-state Boltzmann equation. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J13.00009: Ideal Strength of Hexagonal Diamond and Related B-C-N Compounds Changfeng Chen, Yi Zhang, Zicheng Pan, Hong Sun We have performed first principles calculations on the ideal strength of h-diamond, w-BN, and $\Phi$- BC$_2$N. We have considered structural deformation under pure tensile, pure shear or biaxial stress fields. The calculated results reveal new atomistic fracture mechanism for these materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J13.00010: Equation of state of crystalline FeO from diffusion Monte Carlo simulations Jindrich Kolorenc, Lubos Mitas We investigate equation of state of stoichiometric FeO (at $T=0$ K) by means of the diffusion quantum Monte Carlo method (DMC). We find a pressure induced transition from the B1 (rocksalt) structure, which represents the ambient pressure ground state, to the inverse B8 (NiAs) lattice. Experimental evidence for such a transition is still rather controversial, being detected in some measurements and not seen in others. Our DMC estimate for transition pressure, $P=65\pm 5$ GPa, is compared to outcome of other computational approaches, such as the density functional theory combined with hybrid exchange-correlation functionals. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J13.00011: An efficient method for calculating high PT elastic constants Zhongqing Wu First-principles quasi-harmonic calculations play a very important role in mineral physics because they can accurately predict the structure and thermodynamic properties of materials at pressure and temperature conditions that are still challenging for experiments. It also enables calculations of thermoelastic properties by obtaining the second-order derivatives of the free energies with respect to strain. However, these are exceedingly demanding computations requiring thousands of large jobs running on 10$^{1}$ processors each. Here we introduce a simpler approach that requires only calculations of static elastic constants and phonon density of states for unstrained configurations. This approach decreases the computational time by more than one order of magnitude. We show results on MgO and forsterite that are in very good agreement with previous first-principles results and experimental data. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J13.00012: Radiation in Particle Simulations of Hot Dense Matter Frank Graziani, Richard More The variety of complex processes that take place in hot dense radiative plasmas where temperatures are in excess of several keV and densities are higher than metals, has forced computational physicists in ICF and astrophysics to make a number of assumptions regarding how to model non-equilibrium plasmas undergoing thermal relaxation.~ In order to make the simulations feasible, variations on the Landau-Spitzer model are frequently invoked. There has been recent work on the theoretical properties of thermal relaxation in such plasmas, but there is controversy due to the various approximations needed to make the calculations tractable. Experimental validations in the regimes of interest are prohibitive. Direct Numerical Simulation (DNS) of the many-body interactions of plasmas is a promising approach to model validation but unfortunately, previous work either relies on the collisionless approximation or radiation is entirely absent. We present a new numerical simulation capability that will address a currently unsolved problem: the extension of molecular dynamics to collisional plasmas where Brehmstrahlung and Compton scattering are present. This new tool provides a method for assessing the accuracy of energy and momentum exchange models in hot dense plasmas. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J13.00013: Trajectories and escape rates from a collapsing basin: dependence on the rate of collapse of the basin Antonio Cadilhe, A.F. Voter It has been shown that trajectories with different initial conditions synchronize in harmonic basins during Langevin-thermostatted molecular dynamics under the same time-noise sequence. Synchronization leads to exponential trajectory coalescence onto a single master trajectory [1]. We present our preliminary results extending previous research to include time dependent harmonic potentials, of the form $V (x, t)= A(1-e^{\alpha t})V_o$, where $A$, $\alpha$, and $V_o$ are constants, which are relevant for understanding the dynamics of driven systems. We present analytical results for the synchronization behavior in the various regimes arising in this context. We also complement the study with numerical estimates of the escape rates for basins with escape paths and compare them to static, conventional rates of escape. [1] Blas P. Uberuaga, Marian Anghel, and Arthur F. Voter, J. Chem. Phys. 14, 6363 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J13.00014: Momentum Transfer in Soft X-ray - Induced Shock Loading of Meteorite and Planetary Materials John Remo, Michael Furnish The response of meteorite and planetary materials to high- intensity $<$1 keV x-rays from Z-pinch sources is described. These materials include iron and stony meteorites, magnesium rich olivine (dunite), and Al and Fe calibration samples. Input stresses varied from 6.1 to 12.4 GPa, attenuating to $\sim$1.4 to 2.5 GPa for the iron meteorites, $\sim$0.3 to 1.9 GPa for the stony meteorites, and 1.64 to 1.91 GPa for dunite. The calibration (pure) metals showed less attenuation than the highly inhomogeneous natural materials: 9.5 to $\sim$5 GPa for Fe and 12.4 to 10.6 GP for Al. Methods for deducing momentum and energy coupling into these materials from the radiation are discussed. These data are useful for planetary and astrophysical modeling and for near-Earth object mitigation studies requiring momentum coupling, and momentum enhancement coefficients. [Preview Abstract] |
Session J14: Focus Session: Berezinskii-Kosterlitz-Thouless Regime and Rotating Quantum Gases
Sponsoring Units: DAMOPChair: Hui Zhai, University of California, Berkeley
Room: Morial Convention Center 205
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J14.00001: Kosterlitz-Thouless Transition in Finite-Size BEC Systems Gary Williams The Kosterlitz-Thouless transition in two-dimensional BEC condensates is calculated taking into account the fact that these are finite-size systems. The outer boundaries of the condensate effectively act as hard walls, and this has a polarizing effect on the vortex pairs. As a consequence the superfluid fraction becomes strongly anisotropic, with the tensor component perpendicular to a wall falling to zero there, while the parallel component remains finite. The decreased pair energy near the walls results in an enhanced vortex density near the boundaries. Possible experiments are proposed to probe the anisotropic properties of the superfluid density, including an unusual sharp dip in the superfluid density that is predicted to occur down the middle of a long superfluid strip. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J14.00002: Interference of a stack of coupled two-dimensional BEC pancakes David Pekker, Vladimir Gritsev, Eugene Demler We study the superfluid-normal transition in a stack of Josephson coupled two-dimensional BEC pancakes. Using a combination of the Renormalization Group and the self-consistent harmonic approximation we look at the transition from Kosterlitz-Thouless type behavior to 3D XY type behavior in this finite sized system. We compute the form of the interference patterns that can be observed experimentally if the gas is allowed to expand. In particular, we concentrating on the amplitude modulations in the direction normal to the two-dimensional pancakes, i.e. the direction of fastest expansion. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J14.00003: Evolution from BCS to BKT superfluidity in one-dimensional optical lattices Menderes Iskin, Carlos A.R. Sa de Melo We analyze the finite temperature phase diagram of Fermi-Fermi mixtures in one-dimensional optical lattices as a function of fermion-fermion interaction strength. At low temperatures, the Fermi-Fermi mixture evolves from a three-dimensional (3D) Bardeen-Cooper-Schrieffer (BCS) to a two-dimensional (2D) Berezinskii-Kosterlitz-Thouless (BKT) superfluid as the interaction strength increases. We show that the Ginzburg- Landau-Wilson action near the critical temperature is of the Lawrence-Doniach type for all interaction strengths, and explore the phase space of interaction strength versus hopping (or lattice depth) to determine the characteristic line where the behavior of mixture changes from 3D to 2D. Furthermore, we discuss the existence of vortex loop excitations, and how they evolve as a function of interaction strength. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J14.00004: Berezinskii-Kosterlitz-Thouless Crossover in a Trapped Atomic Gas Invited Speaker: Physics of a Bose gas in 2D is quite different from the usual 3D situation. In a homogeneous 2D fluid of identical bosons long-range order is always destroyed by long wavelength thermal fluctuations and this system cannot undergo conventional Bose-Einstein condensation. Nevertheless, it can become superfluid at a finite critical temperature. This phase transition does not involve any symmetry breaking and in the Berezinskii-Kosterlitz-Thouless (BKT) paradigm it is explained in terms of binding and unbinding of pairs of vortices with opposite circulations. Above the critical temperature, proliferation of unbound vortices is expected. Using optical lattice potentials we can create two parallel, independent 2D atomic clouds with similar temperatures and chemical potentials. When the clouds are suddenly released from the trapping potential and allowed to freely expand, they overlap and interfere. This realizes a matter wave heterodyning experiment which gives direct access to several features of the phase distributions in the two planes. Long wavelength phase fluctuations create a smooth and random variation of the interference fringes and free vortices appear as sharp dislocations in the interference pattern. Both the temperature study of these effects and the measurements of the critical point support the BKT picture of the development of quasi-long-range coherence in these systems. \newline \newline [1] Zoran Hadzibabic, Peter Kruger, Marc Cheneau, Baptiste Battelier, and Jean Dalibard, Nature 441, 1118 (2006). \newline [2] Peter Kruger, Zoran Hadzibabic, and Jean Dalibard, Phys. Rev. Lett. 99, 040402 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J14.00005: Density Matrix Renormalization Group study of rapidly-rotating two-dimensional bosons Dmitry Kovrizhin We study a system of two-dimensional bosons with contact interactions in a rapidly-rotating anisotropic trap. The ground state phase diagram and the excitation spectra are investigated using the Density Matrix Renormalization group (DMRG) and the exact diagonalization methods. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J14.00006: A vortex dipole in a trapped 2D Bose condensate Weibin Li, Masud Haque, Stavros Komineas We study the dynamics and the stationary states of a vortex-antivortex pair in a two-dimensional Bose condensate in a circular trap. The dynamics of this system turns out to be surprisingly complicated, reflecting the nonlinearity of the underlying Gross-Pitaevskii equation. We use a combination of methods --- a time-dependent variational calculation, explicit numerical solutions of the time-dependent and time-independent Gross-Pitaevskii equations, and an exact solution of the non-interacting case --- to uncover the physics of the vortex dipole system. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J14.00007: Vortex lattices of bosons in deep rotating lattices Daniel Goldbaum, Erich Mueller We study vortex-lattice phases for a Bose gas trapped in a rotating optical lattice near the Mott-Hubbard transition. Unlike the case of shallow lattices, the physics in this regime is dominated by the strong on-site interaction between bosons. We find a series of first-order structural transitions between square lattices where vortices are pinned with their cores on plaquettes/sites. We discuss connections between these vortex structures and the Hofstadter butterfly spectrum of free particles on a rotating lattice. We also investigate vortex configurations in a harmonic trap, where superfluid and Mott phases can coexist in a shell structure. \newline \newline [1] D. Goldbaum and E. Mueller, \emph{Vortex lattices of bosons in deep rotating lattices}, arXiv.org:0710.1090 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J14.00008: Trapped Fermi Gases in Rotating Optical Lattices: Realization and Detection of the Topological Hofstadter Insulator Mehmet Oktel, Hui Zhai, Onur Umucalilar We consider a gas of non-interacting spinless fermions in a rotating optical lattice and calculate the density profile of the gas in an external confinement potential. The density profile exhibits distinct plateaus, which correspond to gaps in the single particle spectrum known as the Hofstadter butterfly. The plateaus result from insulating behavior whenever the Fermi energy lies within a gap. We discuss the necessary conditions to realize the Hofstadter insulator in a cold atom setup and show how the quantized Hall conductance can be measured from density profiles using the Streda formula. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J14.00009: Phase transitions in rotating Bose-Einstein condensates Mahir Hussein, Piet Van Isacker, Klaus Bartschat, Oleg Vorov The transition to the Abrikosov state has been observed in cold atomic gases. Such critical behavior is very sensitive [1] to the interaction between the particles in the condensate [2,3]. We give an analytic description [1] of the phase-transition point and classify the types of the corresponding instabilities that depend on the interaction. This toy model of a continuous phase transition predicts the same behavior patterns for all systems governed by a similar energy functional. [1] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, Phys. Rev. Lett. 95, 230406 (2005). [2] O. K. Vorov, M. S. Hussein and P. Van Isacker, Phys. Rev. Lett. 90, 200402 (2003). [3] O. K. Vorov, P. Van Isacker, M. S. Hussein and K. Bartschat, to be submitted to Nature (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J14.00010: Schrodinger Leopards in Bose-Einstein Condensates Lincoln D. Carr, Dimitri R. Dounas-Frazer We present the complex quantum dynamics of vortices in Bose-Einstein condensates in a double well via exact diagonalization of a discretized Hamiltonian. When the barrier is high, vortices evolve into macroscopic superposition (NOON) states of a vortex in either well -- a Schrodinger cat with spots. Such {\it Schrodinger leopard} states are more robust than previously proposed NOON states, which only use two single particle modes of the double well potential. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J14.00011: Vortex lattice transitions in cyclic spinor condensates Subroto Mukerjee, Ryan Barnett, Joel Moore We study the energetics of vortices and vortex lattices produced by rotation in the cyclic phase of $F=2$ spinor Bose condensates. In addition to the familiar triangular lattice predicted by Tkachenko for $^4$He, many more complex lattices appear in this system as a result of the spin degree of freedom. In particular, we predict a magnetic-field-driven transition from a triangular lattice to a honeycomb lattice. Other transitions and lattice geometries are driven at constant field by changes in the temperature-dependent ratio of charge and spin stiffnesses, including a transition through an aperiodic vortex structure. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J14.00012: Condition for the existence of complex modes in a trapped Bose-Einstein condensate with a highly quantized vortex Makoto Mine, Eriko Fukuyama, Masahiko Okumura, Tomoka Sunaga, Yoshiya Yamanaka We consider a trapped Bose-Einstein condensate (BEC) with a highly quantized vortex. For the BEC with a doubly, triply, or quadruply quantized vortex, the numerical calculations have shown that the Bogoliubov-de Gennes equations, which describe the fluctuation of the condensate, have complex eigenvalues. In this talk, we show the analytic expression of the condition for the existence of complex modes, using the method developed by Rossignoli and Kowalski$^1$ for the small coupling constant. To derive it, we make the two-mode approximation. With the derived analytic formula, we can identify the quantum numbers of the complex modes for each winding number of the vortex$^2$. Our result is consistent with those obtained by the numerical calculation in the case that the winding number is two, three, or four. We prove that the complex modes always exist when the condensate has a highly quantized vortex$^2$. \\ $^1$ R. Rossignoli and A. M. Kowalski, Phys. Rev. A {\bf 72}, 032101 (2005). \\ $^2$ E. Fukuyama, M. Mine, M. Okumura, T. Sunaga and Y. Yamanaka, Phys. Rev. A {\bf 76}, 043608 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J14.00013: Control of atomic currents using a quantum stirring device Moritz Hiller, Tsampikos Kottos, Doron Cohen We propose a BEC stirring device which can be regarded as the incorporation of a quantum pump into a closed circuit. It produces a DC circulating atomic current in response to a cyclic adiabatic change of the on-site potentials and the tunneling rates between adjunct sites of an optical trap. We show that the nature of the transport process depends crucially on the sign and on the strength of the interatomic interactions, ranging from a one-by-one transport of atoms (for strong repulsive interaction) to a regime where the particles are are glued together and behave like a huge classical ball that rolls from site to site (for strong attractive interaction). We demonstrate the feasibility of this concept and point out that such device can be utilized in order to probe the interatomic interactions. [Preview Abstract] |
Session J15: Quantum Computation
Sponsoring Units: GQIChair: Gerardo Ortiz, Indiana University
Room: Morial Convention Center 207
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J15.00001: Quantum simulated annealing Sergio Boixo, Rolando Somma, Howard Barnum We develop a quantum algorithm to solve combinatorial optimization problems through quantum simulation of a classical annealing process. Our algorithm combines techniques from quantum walks and quantum phase estimation, and can be viewed as the quantum analogue of the discrete-time Markov Chain Monte Carlo implementation of classical simulated annealing. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J15.00002: Domain Nucleation as a Failure Mode of Adiabatic Quantum Computation William Kaminsky, Seth Lloyd The bottleneck limiting the runtime of an adiabatic quantum algorithm is generically a quantum phase transition point for the computer's qubits. We show that if the qubits undergo this phase transition simultaneously as in a continuous quantum phase transition in a homogeneous system, then the runtime is only polynomial in the number of qubits. However, we next show that if the qubits have finite range interactions in 3 or fewer dimensions, then it is much more likely that they undergo this phase transition in piecemeal fashion by nucleating domains. The runtime then grows faster than any polynomial, though it often remains subexponential. We show this via a scaling argument based on the Suzuki-Trotter mapping. Our argument extends previous similar ones in that it explicitly shows how domain excitations typically must lead to computational errors as inter-domain couplings typically are insufficient to allow the excitations to reconcile with one another and lead back to a valid solution. We close by remarking on ways to minimize domain nucleation, identifying algorithms with continuous symmetries and/or nearly fully-connected topologies as promising. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J15.00003: A quantum algorithm for finding the modal value Mark Coffey, Zachary Prezkuta We present a quantum algorithm for finding the most often occurring (or modal) value of a data set. We thereby supplement other algorithms that can determine the mean value or similar quantities. Our algorithm [1] requires the combined use of quantum counting and extended quantum search, and gives a quadratic speed up over the classical situation. For a data list of N elements, each entry an integer in the range [1,d], our method requires O(d N$^{1/2})$ oracle calls, and further complexity results are described. [1] to appear in Quantum Information Processing. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J15.00004: A Many Body Eigenvalue Problem for Quantum Computation Selman Hershfield A one dimensional many body Hamiltonian is presented whose eigenvalues are related to the order of $G_N$. This is the same order of $G_N$ used to decode the RSA algorithm. For some values of $N$ the Hamiltonian is a noninteracting fermion problem. For other values of $N$ the Hamiltonian is a quantum impurity problem with fermions interacting with a spin-like object. However, the generic case has fermions or spins interacting with higher order interactions beyond two body interactions. Because this is a mapping between two different classes of problems, one of interest in quantum computing and the other a more traditional condensed matter physics Hamiltonian, we will show (i) how knowledge of the order of $G_N$ can be used to solve some novel one dimensional strongly correlated problems and (ii) how numerical techniques, particularly for quantum impurity limit, can be used to find the order of $G_N$. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J15.00005: Generating Pseudo-Random Quantum States with Cluster Computation Winton G. Brown, Yaakov S. Weinstein, Lorenza Viola We revisit existing algorithms for generating pseudo-random pure quantum states in the light of cluster-state quantum computation. Reformulation of previous network-based algorithms in terms of appropriate measurement patterns suggests a (nearly) optimal distribution of local single-qubit gates, which results in a significant improvement in the asymptotic rate of purity decay. Surprisingly, this distribution is not the one corresponding to arbitrary random single qubit rotations. We find that the rate at which the expected purity approaches the prediction based on the Haar measure is asymptotically constant with respect to the number of logical qubits. Connectivity of the underlying qubit coupling topology as well as the occurrence of saturation and cut-off effects are analyzed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J15.00006: Ground States as Resources for Universal Measurement-Based Quantum Computing Adam G. D'Souza, David L. Feder Measurement-based quantum computation (MBQC) requires a massively entangled resource state (such as a cluster state) as input. Experimental efforts towards generating such states have typically focused on performing global entangling operations on uncorrelated qubits. As the states that result from this type of procedure are not generally ground states, they are very sensitive to decoherence effects. A more robust resource would be one that is in fact a ground state of some Hamiltonian that exhibits a reasonably large energy gap between the ground state and the various excited states. We will discuss the possibility of finding simple two-body spin Hamiltonians whose ground states are equivalent to resource states for MBQC under stochastic protocols comprised solely of local operations and classical communication. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J15.00007: Quantum Non-demolition measurements of single spins in semiconductors Mohan Sarovar, Kevin Young, Thomas Schenkel, K. Birgitta Whaley For the development of large-scale quantum computers, electron spin-encoded qubits in solid-state are appealing because of their favorable decoherence time scales, high potential for scalability, and many handles for precision control. However, an additional requirement that is traditionally challenging in the solid-state is a capacity for high-fidelity qubit readout. We propose a scheme for measuring the state of a single donor electron spin using a field-effect transistor induced two-dimensional electron gas and electrically detected magnetic resonance techniques. The scheme is facilitated by hyperfine coupling to the donor nucleus. We analyze the potential sensitivity and outline experimental requirements. Our measurement provides a single-shot, projective, and effectively quantum non-demolition measurement of an electron spin-encoded qubit state. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J15.00008: Adiabatic optical two-qubit operation with electron spins in separate quantum dots Semion Saikin, Clive Emary, Duncan Steel, Lu Sham We develop an adiabatic scheme to control the entanglement of two electron spins localized in separate InAs/GaAs quantum dots via the Coulomb interaction between two negative trions optically excited in the different dots. The scheme gives a unitary operation in the spin subspace and can be used as a two- qubit gate for quantum information processing. The slowly- varying adiabatic pulses drive the system in a such way that effects of pulse imperfections and relaxation of the trion states are minimized. For spin dynamics we provide an exact numerical solution that accounts for dissipation and analyze the essential processes within a ``dressed state'' model. Our calculations for vertically-stacked quantum dots show that for a broad range of dot parameters a two-spin state with the concurrence $C>0.85$ can be prepared coherently from an initially polarized state by four optical fields with the pulse duration $\Delta t\sim 1$~ns. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J15.00009: Universal sets of quantum gates based on geometric phases Yu Shi, Qian Niu We rigorously study adiabatic and nonadiabatic geometric phases of two Heisenberg-coupled identical spins in a rotating magnetic field. The geometric phase of the total system is still some solid angle, and is independent of Heisenberg coupling constant. The adiabatic geometric phase is also independent of the magnetic field rotating speed. Using this result, for both adiabatic and nonadiabatic cases, we explicitly and exactly construct novel robust dynamic-geometric-hybrid two-qubit square root of swap and controlled-NOT gates, as well as purely geometric single qubit gates, including $\pi/8$ and Hadamard gates, thus presenting a complete scheme of robust universal quantum computing. This scheme can be implemented in NMR, quantum dots and cold atoms. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J15.00010: Quantum Logic with Composite Pulse Sequences on $Sr^+$ Ruth Shewmon, Jaroslaw Labaziewicz, Yufei Ge, Shannon Wang, Isaac L. Chuang The optical $5S_{1/2} \rightarrow 4D_{1/2}$ transition in $Sr^+$ is an attractive qubit because it can be addressed by diode lasers, which are relatively inexpensive and easy to operate. We characterize single-qubit rotations as well as a CNOT gate on a $Sr^+$ ion in a surface electrode Paul trap. To improve these operations, the frequency of the clock laser is stabilized to a high-finesse optical cavity. The resulting linewidth of the laser is approximately 300Hz. Composite pulse sequences, a technique adapted from NMR, have been shown to reduce the effects of systematic errors in a variety of quantum systems. We demonstrate several composite sequences that improve the fidelity of quantum logic operations on $Sr^+$. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J15.00011: Static and Dynamic Properties of Flying Spin Qubits Vanita Srinivasa, Jeremy Levy Domain walls in dimerized spin-1/2 chains may be used to transport spin qubits rapidly and with high fidelity (PRB \textbf{76}, 094411 (2007)). Three-spin rings constitute the simplest system in which these ``flying spin qubits'' may be realized. We explore some general properties of three-spin rings with modulated Heisenberg exchange coupling by calculating the corresponding static and dynamic spin states. We also discuss how the motion of domain walls in individual and coupled pairs of rings may be used to carry out single and two-qubit unitary operations within these systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J15.00012: Topological Quantum Computing with Read-Rezayi States Layla Hormozi, Nick Bonesteel, Steven H. Simon In topological quantum computation quantum operations are carried out by moving quasiparticle excitations of certain quantum systems around each other in two space dimensions or equivalently by braiding their world-lines in three-dimensional space-time. Fractional quantum Hall states are among the prime candidates for realizing such quasiparticles. In particular, it has been shown that quasiparticles associated with the so-called Read-Rezayi (RR) states at $k>2, k\neq 4$ can be used for universal quantum computation. In previous work we have shown how to construct two-qubit gates by braiding the so-called Fibonacci anyons --- a class of non-Abelian anyons that are closely related to the quasiparticles of the $k = 3$ RR state. These two-qubit gates then together with single-qubit gates form a universal set of quantum gates. In this talk we point to certain properties of the quasiparticles of the $k = 3$ RR state which are unique to this state and which allow us to construct two-qubit gates in a simple and efficient way. We then present a method that can be used to efficiently construct two-qubit gates for any quasiparticle in the RR sequence at $k>2, k\neq 4$. This work is supported by US DOE. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J15.00013: Polyoxometalates as spin qubits A. Gaita-Ari\~no, M. AlDamen, J.-M. Clemente-Juan, E. Coronado, J. Lehmann, D. Loss, P. Stamp Polyoxometalates (POMs) are discrete fragments of metal oxides, clusters of regular MO$_n$ polyhedra. POMs show a remarkable flexibility in composition, structure and charge state, and thus can be designed according to specific electric and magnetic needs. The two localized spins with S = 1/2 on the V atoms in [PMo$_{12}$O$_{40}$(VO)$_2$]$^{q-}$ can be coupled through the delocalized electrons of the central core. This system was recently used for a theoretical scheme involving two-qubit gates and readout: the electrical manipulation of the molecular redox potential changes the charge of the core and thus the effective magnetic exchange between the qubits. Polyoxometalates can encapsulate magnetic ions, protecting them by a diamagnetic shell of controlled geometry. A great potential of POMs as spin qubits is that they can be constructed using only even elements, such as O, W, Mo and/or Si. Thus, there is a high abundance of polyoxometalate molecules without any nuclear spin, which could result in unusually low decoherence rates. There is currently an effort involving highly anisotropic, high magnetic moment, lanthanide@polyoxometalate molecules acting as spin qubits. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J15.00014: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J15.00015: Hitting time for the continuous quantum walk Martin Varbanov A new definition of hitting (absorbing) time is formulated for the case of continuous quantum walks. The walk is measured randomly according to a Poisson process with measurement rate $\lambda$. We derive an explicit formula for the hitting time and explore its dependence on the measurement rate. We show that in the two limits of the measurement rate going to 0 or infinity the hitting time diverges, where the second limit is representative of the quantum Zeno effect. Several different conditions for existence of infinite hitting time are proven by analyzing the analytical structure of the formula for the infinite hitting time. A different condition for existence of infinite hitting times based on the disconnectedness of the complementary graph is proven as well. [Preview Abstract] |
Session J16: Focus Session: Biochip Physics II
Sponsoring Units: DBP DFDChair: David Nolte, Purdue University
Room: Morial Convention Center 208
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J16.00001: Direct-Print Organic Photonics for Biodetection Chips Invited Speaker: The development of commercial portable Biochip applications based on optical detection is hindered by the lack of imaging systems that can be directly integrated into the chip itself. Currently, fluorescence/luminescence signals are read out with power-hungry, bulky and expensive off-chip imaging systems, like CCD cameras or photomultiplier tubes. Here we present an enabling technology that for the first time allows cheap and easy integration of imaging systems directly into disposable Biochip systems. Our technology is based on organic semiconductor materials that can be processed in liquid form by inkjet and screen printing, in a process much faster and cheaper than the complicated fabrication of silicon-based imaging sensors. Organic photosensors can be printed on various substrate materials like plastic foil or glass or directly onto Biochip systems. The ultrathin photodiodes with an overall thickness of only 300 to 500 nm show quantum efficiencies better than 0.5 and linear light-response over 6 orders of magnitude. The pixel size can range from 50 to over 1000 $\mu $m and inkjet fabrication allows tailoring the sensor layout to the needs of the specific application. Single photodiodes, photodiode line-arrays or 2D arrays of photodiodes can be printed onto diverse materials. Besides the dramatically reduced production costs for printed photodiodes, the presented readout architecture allows detection of e.g. chemiluminescence signals with highest sensitivities and minimum crosstalk due to the close proximity of sample and printed photodiode. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J16.00002: On-the-flow differentiation between cells based on native fluorescence spectroscopy on a chip Markus Beck, Michael Bassler, Peter Kiesel, Noble M. Johnson, Oliver Schmidt Native fluorescence spectroscopy is a promising approach for the detection of pathogens without specific binding or tagging of the analyte. The distinction between different species is possible with (multi-color) UV excitation together with the detection of several spectral bands. We have developed a compact platform that combines a microfluidic quartz channel with chip-size wavelength-selective detection of the fluorescence from particles traversing the channel. The interaction between the UV excitation light and the analyte is enhanced by anti-resonantly guiding the light within fluid. We have recorded the intrinsic fluorescence of single cells (e.g. yeast, e-coli, and BT) passing the detection area. Knowing the particle speed and the physical dimensions of the observation window, we are able to determine particle positions with microscopic ($\sim $10 microns) resolution. A special modulation technique allows us to achieve a high signal to noise ratio even for high particle speeds. Combining our technique with a cell sorting mechanism would allow for on-the-chip characterization and sorting of untagged cells. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J16.00003: Design of a Molecular Diode: Nanoratchets Robert Austin, Jason Puchalla, Peter Galajda, Keith Morton We use the concepts hydrodynamic flow in asymmetric structures and apply them to our own asymmetric bump array/diffusion array technology at the nanoscale. Our basic premise is that asymmetrically designed metamaterials at the nanoscale can act, under the influence of externally applied forces, as molecular ratchets which will sort molecules based on their size. At some nano length scale, we believe that the classical concepts of stick boundary conditions break down and a new regime of transport begins. We present computer simulations and experiments which show that at the nanoscale level we can efficiently separate objects the size of proteins. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J16.00004: Toward on-chip directed evolution of unicellular organisms for efficient hydrogen production David Liao, Caleb Howe, Cecilia Muldoon, Peter Galajda, Juan Keymer, Robert Austin To provide an energy resource alternative to fossil fuels, photosynthetic organisms must increase their energy conversion efficiency. The green algae \textit{C. reinhardtii} stores light energy in hydrogen gas at 0.1\% efficiency, less than the 10\% required to compete with established fuels. This work combines hydrogen sensing in liquid culture with micro habitat patch (MHP) chips for directing hydrogen-producing organisms to evolve improved energy conversion efficiency. A MHP chip contains 87 1 mm $\times$ 1 mm $\times$ 100~$\mu\mathrm{m}$ interconnected chambers. By measuring hydrogen output from different chambers, we will select less productive patches to annihilate. We microfabricated chips from poly(dimethylsiloxane). Color changes in fluorescence micrographs confirm that 254~nm radiation kills algae in MHPs, liberating nutrients and space for exploitation by adjacent populations. We demonstrated colorimetric detection of hydrogen gas production at a rate of $10^{-8}~\mathrm{mol~H_2~mL^{-1}~s^{-1}}$ using tungsten film on sub-mL liquid cultures of \textit{C. reinhardtii} during 2-hrs. of fermentation in darkness. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J16.00005: Rapid Detection of Microorganisms--State of Art and Future Directions Invited Speaker: For the last several decades, nutrient-based culture growth methods have been accepted as the standard for microorganism detection and identification. However, since the discovery of nucleic acids and molecular breakthrough technologies such as restriction enzymes and polymerase chain reactions, the detection and identification of microorganisms have advanced to culture-independent methods that fall under the category of rapid microbial detections. Here, we present an overview of major rapid microbial detection technologies. These technologies will include both amplification and non-amplification based methods for the detection and identification of target microorganisms. The technologies described can be applied to detecting a wide variety of microorganisms, including bacteria, viruses, mycoplasma, and fungi and have the potential sensitivity to detect a single microorganism. Also in this presentation, we will present examples of real-life applications as well as future challenges for the advancement of the field of rapid microbiology. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J16.00006: Guidance and detection of neuronal cells using Si nanomembranes Cristian Staii, Weina Peng, Hyuk Ju Ryu, Don E. Savage, Yu Huang, Sookin Nam, Justin Williams, Erik Dent, Max G. Lagally, Susan N. Coppersmith, Mark A. Eriksson "Lab-on-a-chip" microfluidic technology [1] has emerged as a powerful tool for studying biological systems. Unlike standard macro-scale systems used for decades, microfluidics allows the micro-environment of a neuronal cell culture to be finely regulated. The reduction in feature sizes gives control over fluid phenomena such as laminar flow, shear stresses, and velocity profiles. Here we present a new approach to ``lab-on-a-chip'' design for studying neuronal cells, integrating microfluidic systems with silicon nanomembrane-based microelectronics. We show that this technology permits rapid production of microchannels with a large variety of shapes/sizes, thereby allowing the exposure of neuronal cell cultures to multiple environments, both mechanical and chemical, simultaneously. In addition, these microfluidic channels can be easily integrated with silicon nanomembrane based electronics. [1] A.J.Blake, T.M.Pearce, N.S.Rao, S.M.Johnson and J. C. Williams, Lab Chip, 2007, 7, 842. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J16.00007: Well-Oriented NanoWell Array Metrics for Digital NanoBioChip HeaYeon Lee, BongKuk Lee, Tomoji Kawai Recently many researchers have sought new paradigm for nanobiochip that can be miniaturized and integrated to produce intelligent analysis systems in numerous biotechnology. We have been tried to develop biocompatible materials based nanopatterning, self-assembly array to address challenging problem in nanobioscience. In this time, we describe the nanometrics geometry of a well-oriented nanowell (ONW) array derived from nanofabrication technology which can easily be employed for digital detection with a high S/N ratio, miniaturization, integrated assays and single molecule analysis. We fabricated the self-organized nanopatterning of copolymer as a platform of biomolecular nanoarry using nanolithography. We also present a strong specific antibody-antigen interaction on lipid-membrane modified gold surface using ONW. We believe these findings can be related to various nanobiochip applications. References 1. H.Y. Lee, T. Kawai, K.Y.Suh et al, Advanced Materials, In press (2007). 2. H.Y. Lee, T. Kawai et al, Appl. Phys. Lett. 89, (2006) 113901. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J16.00008: Magnetically Directed Cell Co-Localization for Cell-Cell Interaction Studies Edward Felton, Daniel Reich, Christopher Chen The ability to create ordered patterns of cells has enabled new approaches to various areas of biological interest, such as tissue engineering, biosensing, and the study of interactions between cells. In this work, we apply forces to cells through binding with magnetic nanowires. The nanowires feature high remanent magnetization, allowing for effective manipulation in low-strength magnetic fields, and when used in conjunction with lithographically patterned magnetic microstructures can precisely position cells into predetermined locations. Chemical functionalization then confines the cells to these substrate areas. We have used this technique to create large numbers of isolated pairs of cells by magnetically guiding them to sites on cobalt and permalloy arrays. Further, the use of two different cell types leads to arrays with heterotypic cell pairs in numbers that exceed those attainable with random cell seeding. Initial experiments applying this magnetic cell trapping technique to perform biological studies of cell-cell interactions will also be described. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J16.00009: Electromagnetic Sensors of Biological Motors Jie Fang, K. Rajapakshe, D. Padmaraj, H. Infante, V. Vajrala, G. Mercier, W. Widger, W. Wosik, J. Miller Biological motors operate on time scales that readily couple to oscillatory electric fields. Modest ac fields applied to cells in an aqueous medium lead to greatly enhanced fields across the plasma membrane or (at kHz frequencies) internal membranes. Membrane complexes thus contribute to both linear and nonlinear responses to sinusoidal fields. For example, activity of motors in mitochondrial and (for chloroplasts) photosynthetic electron transport chains correlate with frequency-dependent second and third harmonics. Our electrode-based biosensors are scalable for micro- and nano-fluidic biochips. At low frequencies (less than 100Hz) we find it advantageous to use SQUIDs, which reduce contact effects and could lead to clinical applications. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J16.00010: Single-molecule stochastic sensors for proteins using engineered nanopores Liviu Movileanu We were able to design an unusual temperature-responsive pore-based nanostructure with a single movable elastin-like-polypeptide (ELP) loop. If a voltage bias was applied, the engineered pore exhibited transient current blockades, the nature of which depended on the length and sequence of the inserted ELP. These blockades are associated with the excursions of the ELP loop into the nanopore. At low temperatures, the ELP is fully expanded and blocks the pore completely, but reversibly. At high temperatures, the ELP is dehydrated and structurally collapsed, thus enabling a substantial ionic flow. Acidic binding sites comprised of negatively-charged aspartic acid residues, engineered within the pore lumen, produced dramatic changes in the functional properties of the nanopore, catalyzing the translocation of cationic polypeptides from one side of the membrane to the other. For example, when two electrostatic binding sites were introduced, at the entry and exit of the nanopore, both the rate constants of association and dissociation increased substantially, diminishing the free energy barrier for translocation. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J16.00011: Imaging Protein-Functionalized Quantum Dot Diffusion and Binding at Surfaces Jack Rife, James Long, Lloyd Whitman Understanding single biomolecule and nanoparticle interactions with surfaces at fluid-solid interfaces is a key to improving molecular transport and binding in many biotechnology applications. Biosensor sensitivity, for example, is typically limited by diffusion [2] and non-specific binding to analytical surfaces. We have assembled a Total Internal Reflectance Fluorescence (TIRF) microscopy system with single-photon-sensitive cameras to image diffusion and binding of fluorescently-labeled biomolecules on surfaces under both static and laminar flow conditions. We have acquired movies (57 frames/s) of streptavidin-functionalized CdSe quantum dots (QDs) diffusing, transiently attaching, and permanently immobilizing on repulsive, hydrophilic silica surfaces. From the single-particle trajectories we have extracted diffusion coefficients and transient attachment lifetimes. The binding of protein-functionalized QDs to our nominally repulsive surfaces can be attributed to surface defects, adsorbates, and protein conformational changes. In flow, the QD elevation above the no-slip surface can be approximated, giving a picture of elevated transport between transient attachments and QD departures to and from the surface. [2] Sheehan and Whitman, Nano Lett. 5, 803 (2005). [Preview Abstract] |
Session J17: Focus Session: General Biological Patterns
Sponsoring Units: DBP DMPChair: Shane Hudson, Vanderbilt University
Room: Morial Convention Center 209
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J17.00001: Relating biophysical properties across scales: implications for early development and applications for tissue engineering Invited Speaker: A distinguishing feature of a multicellular system is that it operates at various scales and levels of organization. Genes set up the conditions for physical mechanisms to act, in particular to shape the developing organism and establish its material characteristics. As development continues the changes brought about by the physical processes lead to changes in gene expression. It is through this interplay that the organism acquires its final structure and composition. It is natural to assume that in this multi-scale process the smaller defines the larger. In case of biophysical properties, in particular, those at the subcellular and cellular level are expected to give rise to those at the tissue level and beyond. Indeed, the physical characteristics of tissues vary greatly in physical properties: blood is liquid, bone is solid. In between these extremes lie most of the organs and tissues with intermediate viscoelastic properties. However, a blood cell is not the same as a liquid drop and a single bone-forming cell itself is not a solid. Little is known on how tissue and organ level properties are related to cell and subcellular properties. We introduce a novel combined theoretical-computational-experimental framework to address this question. The basis of our approach is a representation of a cell by a network of interacting `organelles' (i.e. modules) with cell-specific properties. Cells form tissues and eventually organs through interactions either directly with each other or through secreted substances. The experimental and theoretical inputs of the formalism are inseparable: it cannot even be set up without one or the either. The method can serve as the basis for ``computational tissue engineering''. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J17.00002: Dynamics and Mechanics of Zebrafish Embryonic Tissues. Eva-Maria Schoetz, R.D. Burdine, M.S. Steinberg, C.-P. Heisenberg, R.A. Foty, F. Julicher In early zebrafish embryonic development, complex flows of cell populations occur, which ultimately lead to the spatial organization of the three germ layers: Ectoderm, mesoderm and endoderm. Here, we study the material properties of these germ layer tissues which are important for their dynamics and spatial organization in the embryo. In general, tissues can be classified as inherently active complex fluids. However, here we present examples of observed tissue behavior, which can be described satisfactorily in terms of passive visco-elastic fluids. We determined the material properties of the germ layer tissues quantitatively and found that differences in their properties influence tissue interaction. Specifically, quantitative differences in tissue surface tension result in tissue immiscibility and cell sorting behavior analogous to that of ordinary immiscible liquids. Surface tensions were measured with a tissue surface tensiometer. Furthermore, by tracking individual cells in the developing zebrafish embryo, we found differences in the migratory behavior of the different tissue types, which are, to some extent, governed by their mechanical properties. Finally, we generated a 3D velocity flow profile describing the tissue movements during zebrafish embryonic organizer development. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J17.00003: Laser Hole-Drilling as a Probe of Morphogenetic Stresses in Embryonic Epithelia: Experimental Observations Xiaoyan Ma, M. Shane Hutson During the development of an organism, sheets of epithelial cells expand, contract and bend due to forces generated within the cell sheets. These forces can be probed by laser hole-drilling -- a method borrowed from the analysis of residual stress in manufactured widgets -- in which a laser microbeam ablates a single cell surface or the edge shared by adjacent cells. We have applied this method to the embryonic epithelia of GFP-labeled fruit fly \textit{(Drosophila)} embryos. After ablation of one shared edge, we follow the recoil dynamics (strain relaxation) of adjacent cell edges (with time resolution down to 2 ms). The recoils show two distinct phases; and the initial recoil velocity can be consistently retrieved through a double-exponential fit. We observe a strong correlation between the initial recoil velocity and the orientation of the ablated cell edge. This correlation is particularly pronounced in embryos during late dorsal closure. Measuring orientation with respect to the long (anterior-posterior) axis of the embryo, both the recoil velocities and the distribution of cell edge orientations have sharp peaks near 30\r{ } and 150\r{ }. In early dorsal closure, the distribution of cell edge orientation has three much weaker peaks and the recoil velocities only show a weak maximum near 90\r{ }. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J17.00004: Laser Hole-Drilling as a Probe of Morphogenetic Stresses in Embryonic Epithelia: Finite Element Models M. Shane Hutson, Xiaoyan Ma, Jim Veldhuis, G. Wayne Brodland During the development of an organism, sheets of epithelial cells expand, contract and bend due to forces generated within the cell sheets. These forces can be probed by laser hole-drilling; however, the observed recoil dynamics (or strain relaxations) depend strongly on the local cellular geometry. To better understand this dependence and help interpret our experimental observations, we have conducted a series of laser hole-drilling simulations using cell-level finite element models. Even the simplest of these simulations (i.e. homotypic cell sheets with constant cell boundary tensions) produce a wide range of initial recoil velocities. The velocities are correlated with particular aspects of the local geometry -- most notably the aspect ratio and orientation of cells adjacent to the ablated cell edge. These simulations also produce biphasic recoils; however, the two phases are not as distinct as those observed experimentally. To more closely reproduce the experimental recoils, the cell edges must include an elastic component. We will discuss using such finite element models to inversely determine local stresses in an epithelial sheet from the observed strain relaxations after laser ablation. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J17.00005: Forces driven by morphogenesis modulate Twist Expression to determine Anterior Mid-gut Differentiation in \textit{ Drosophila} embryos Invited Speaker: By combining magnetic tweezers to \textit{in vivo} laser ablation, we locally manipulate \textit{Drosophila} embryonic tissues with physiologically relevant forces. We demonstrate that high level of \textit{Twist} expression in the stomodeal primordium is mechanically induced in response to compression by the 60$\pm $20 nN force developed during germ-band extension (GBE). We find that this force triggers the junctional release and nuclear translocation of Armadillo involved in Twist mechanical induction in the stomodeum in a Src42A dependent way. Finally, stomodeal-specific RNAi-mediated silencing of Twist during compression impairs the differentiation of midgut cells, as revealed by strong defects in Dve expression and abnormal larval lethality. Thus, mechanical induction of Twist overexpression in stomodeal cells is necessary for subsequent midgut differentiation. \newline \newline In collaboration with Nicolas Desprat, Willy Supatto, and Philippe-Alexandre Pouille, MGDET, UMR168 CNRS, Institut Curie11 rue Pierre et Marie Curie, F-75005, Paris, France; and Emmanuel Beaurepaire, LOB, Ecole Polytechnique, CNRS and INSERM U 696, 91128 Palaiseau, France. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J17.00006: Mechanical forces in the development of leaf venation networks Francis Corson, Arezki Boudaoud, Mokhtar Adda-Bedia Leaf venation patterns, like leaf shapes, are extremely diverse, yet their local structure has been shown to satisfy a simple, universal property: the angles veins form at junctions are related to their diameters by a vectorial equation analogous to a force balance. This structure is the signature of a reorganization of vein networks during the development of leaves, a process we investigated numerically using a cell proliferation model. Provided that vein cells are given different mechanical properties, tensile forces develop along the veins during growth, causing the network to deform progressively. The statistics of the patterns obtained in these simulations are in good quantitative agreement with observations on leaves, supporting the notion that the local structure of leaf venation networks reflects a balance of mechanical forces. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J17.00007: Pattern Formation in a Synthetic Multicellular System Ting Lu, David Karig, Ron Weiss Pattern formation has been studied for a long history since the Turing's proposal for a reaction-diffusion system and been found in numerous physical, chemical and biological examples. However, experimental study about pattern formation advances slowly. Here we present an artificial pattern formation system. By engineering cellular communication in bacteria \textit{E. Coli} and plating these engineered cells onto a solid-phase agarose plate, we are able to program the pattern formation of this multicellular system. The pattern changes dramatically with different levels of an external inducer IPTG. A simple model is developed to explain the experimental results. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J17.00008: A Model of R8 Cell Specification in the \textit{Drosophila} Eye Matthew Pennington, David Lubensky R8 photoreceptors are specified in a precise hexagonal pattern behind an advancing front as it traverses the eye imaginal disc during \textit{Drosophila} development. In an attempt to better understand this patterning event, we have developed a mathematical model consisting of coupled differential equations on a lattice incorporating auto-activation, long-range activation, and short-range inhibition. The model is based on known elements of the regulatory gene network involved in patterning, and an analogy with discrete Nagumo systems is helpful in understanding its dynamics. We have developed analytic and numeric results for its behavior on a 1D lattice. Significantly, this model can reproduce patterns similar to those seen both in wild-type eye discs and in several mutant phenotypes. We argue that much of the model's behavior is a consequence of the fact that self-activation is cell-autonomous; this behavior represents a novel mode of pattern formation distinct from classical ideas such as Turing patterns or morphogen-dependent positional information. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J17.00009: Emergence of hyper-hexagonal patterns in orientation map models of reduced rotation symmetry Wolfgang Keil, Michael Schnabel, Fred Wolf Neurons in the primary visual cortex preferentially respond to visual stimuli of a particular orientation. These orientation preferences are arranged in aperiodic 2-D patterns, known as orientation preference maps (OPMs). Symmetry assumptions have been used successfully to derive a class of theoretical model which accounts for the emergence of aperiodic pinwheel-rich OPMs. Measurements revealed anisotropic coupling statistics in the underlying neural tissue, suggesting that the symmetry of models for the formation of orientation maps is reduced from the previously assumed E(2)xO(2) to E(2). In dynamical models for OPMs with E(2)xO(2) symmetry interactions represented by quadratic terms cannot occur but may be present in models of reduced E(2) symmetry. Here, we present a general analysis of the impact of such interactions on the formation of OPMs. We demonstrate that near the onset of pattern formation only two basic types of quadratic interaction terms exist, introduce a general parametric representation of permissible quadratic interactions near pattern formation onset, and derive the most general amplitude equations describing pattern selection in models incorporating quadratic interactions. We study the impact of such interactions on the spatial structure of OPMs, by incorporating them into a Swift-Hohenberg-model of OPM formation. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J17.00010: Dynamics of Gas Exchange through the Fractal Architecture of the Human Lung, Modeled as an Exactly Solvable Hierarchical Tree Michael Mayo, Peter Pfeifer, Stefan Gheorghiu The acinar airways lie at the periphery of the human lung and are responsible for the transfer of oxygen from air to the blood during respiration. This transfer occurs by the diffusion-reaction of oxygen over the irregular surface of the alveolar membranes lining the acinar airways. We present an exactly solvable diffusion-reaction model on a hierarchically branched tree, allowing a quantitative prediction of the oxygen current over the entire system of acinar airways responsible for the gas exchange. We discuss the effect of diffusional screening, which is strongly coupled to oxygen transport in the human lung. We show that the oxygen current is insensitive to a loss of permeability of the alveolar membranes over a wide range of permeabilities, similar to a ``constant-current source'' in an electric network. Such fault tolerance has been observed in other treatments of the gas exchange in the lung and is obtained here as a fully analytical result. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J17.00011: Evolution of Optimum Foraging Distributions in Two Dimensions Nathan Dees, Sonya Bahar, Frank Moss In the pursuit of optimally efficient foraging, preferred distributions of movement characteristics have been shown to exist for many types of animals and environments.~ Specifically, planktonic organisms such as \textit{Daphnia} use exponential distributions of turning angles, $\alpha $, in a \textit{``hop, pause, turn by angle $\alpha $, hop{\ldots}'' }random walk-type sequence of movement when traversing experimentally prepared feeding solutions consisting of freeze dried \textit{Spirolina} and water. We investigate the evolution of such random walks in a two-dimensional foraging model. In this model, agents traverse a feeding patch of finite size and for a finite amount of time using hop lengths and turning angles chosen randomly from inherited distributions. Distributions evolve as the choices made by the most efficient forager of one generation influence the distributions available to the succeeding generation. Preliminary results show that initially uniform turning angle distributions evolve to explicit exponential distributions after thousands of generations, consistent with the experimental observations described above. [Preview Abstract] |
Session J18: Frank J. Padden Award Symposium
Sponsoring Units: DPOLYChair: Russell Composto, University of Pennsylvania
Room: Morial Convention Center 210
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J18.00001: Use DNA solutions to model polymer entanglement in flow: simultaneous rheometric and particle-tracking velocimetric measurements Pouyan Boukany, Shi-Qing Wang Entangled aqueous DNA solutions are ideal as a model system to examine nonlinear flow features including stress overshoot in startup shear and shear thinning phenomenon. These soft systems can be strongly entangled with 60 entanglement points per chain and a terminal relaxation time as long as 1000~s at 1 {\%} concentration [1-2]. They allow a comparison between the steady state attained with a startup shear and that attained through an ``infinitely'' slow ramping up of the applied shear rate. Indeed, startup shear in the nonlinear (stress plateau) region causes the DNA solutions to yield inhomogeneously, resulting in permanent shear banding. However, the slowly ramped-up shear into the same final rate as applied in startup shear allowed the solutions to avoid shear inhomogeneity. Thus, we demonstrated that it is possible for the final steady states to be different depending on how an entangled system is brought into the same final experimental condition. This result implies that it is ill-defined to pursue conventional constitutive relationship in flow of entangled polymers. \newline [1] Boukany, P. E.; Hu, T. H.; Wang, S. Q. textit{Macromolecules} 2007, under review. [2] Boukany, P. E.; Wang, S. Q. \textit{J. Rheol}. 2007, under review. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J18.00002: Membrane-Enhanced Surface Acoustic Wave Analysis of Polymer Brushes David A. Brass, Kenneth R. Shull We show that detailed structural information about polymer brushes can be obtained from a simple membrane inflation technique in conjunction with a quartz crystal resonator. The sensitivity of these quartz crystal resonators is determined by the propagation of acoustic shear waves through the materials that are placed in contact with the oscillating crystal. Coupling of the these 'surface acoustic waves' into the membrane is strongly affected by the thickness of the brush layer, which is in turn affected by specific interactions between the polymer brush and the surface with which it is brought into contact. The mechanical resonance of the quartz crystal is affected by these brush parameters, and we refer to the accompanying analysis of this resonance as 'membrane-enhanced surface acoustic wave analysis'. The analysis combines self consistent mean field theory of the polymer brush with the relevant theories of acoustic wave propagation. The model has been tested experimentally with grafted poly(ethylene glycol) brushes in contact with thermoplastic elastomer membranes. We also show how the technique can be used to quantify the strength of specific interactions between the electrode surface of the quartz resonator and functional groups placed at the ends of the PEG brushes. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J18.00003: Advances in Organic Single-Crystal Transistors Alejandro L. Briseno, Zhenan Bao, Younan Xia, Samson A. Jenekhe Organic semiconductors, including conjugated small molecules and polymers, constitute next-generation materials for displays, circuits, and a vast array of other electronic applications. The performances of organic single-crystal transistors have recently surpassed the performance levels of amorphous silicon devices. Despite the high mobilities of single-crystal devices, there are many factors limiting their applications. Currently, single crystals are handpicked and made into an individual device. Another challenge is to achieve control of crystallinity in polymer nanostructures. There is a need to explore nanowires as solution-processable materials because of the cost-effective aspect in fabricating devices. Therefore, in order to meet the requirements for fabricating practical devices, we have resolved the aforementioned issues by patterning organic single-crystal transistors and polymers. We have developed solution-phase methods for preparing organic single-crystal nanowires from p- and n-type semiconductors and highly oriented nanowires from polymer semiconductors. Furthermore, we have realized high-performance transistors and demonstrated the first all-polymer complementary circuit. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J18.00004: Effect of lithium ion distribution on conductivity of block copolymer electrolytes Enrique Gomez, Nitash Balsara Energy-filtered transmission electron microscopy (EFTEM) was used to determine the distribution of lithium ions in mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene-\textit{block}-ethylene oxide) copolymers (PS-PEO). EFTEM results show that the salt is increasingly localized to the middle of the PEO lamellae as the molecular weight of the copolymers is increased. Computer simulations by Borodin and Smith (Macromolecules, 1998, 31, 8396) demonstrate that coordination between lithium ions and PEO chains is diminished for chains that are stretched. Local stretching in block copolymers is modeled using self-consistent field theory (SCFT). Good agreement between EFTEM and SCFT is obtained by postulating a linear relationship between local chain stretching and lithium ion concentration. AC impedance spectroscopy experiments show an increase in the conductivity of PS-PEO/salt mixtures with increasing molecular weight of PS-PEO. The EFTEM/SCFT results suggest that the increase in ionic conductivity with increasing molecular weight is due to segregation of the lithium salt away from PS/PEO interfaces where segmental motion is retarded due to connectivity to slow-moving PS chains. These results may aid in developing all-solid state rechargeable lithium batteries with PS-PEO serving as a dry electrolyte. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J18.00005: Self-assembly of metal--polymer analogues of amphiphilic triblock copolymers Zhihong Nie, Daniele Fava, Eugenia Kumacheva, Shan Zou, Gilbert Walker, Michael Rubinstein We proposed a block copolymer approach to the self-assembly of inorganic nanrods terminated with polymer molecules at both ends. We organized metal nanorods in structures with varying geometries by using a striking analogy between amphiphilic ABA triblock copolymers and the hydrophilic nanorods tetheredwith hydrophobic polymer chains at both ends. The self-assembly was tunable and reversible and it was achieved solely by changing the solvent quality for the constituent blocks. The distance between adjacent nanorods along chains can be tuned by varying the composition of mixture solvents or the molecular weight of polymer blocks, which allows us precisely control the plasmonic band of self-assembled structures. A systematic study of the self-assembly as a function of solvent composition and the molecular weight of the polymer blocks allowed us to construct a diagram that maps the assembled structures. This approach provides a new route to the organization of anisotropic nanoparticles by using the strategies that are established for the self-assembly of block copolymers. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J18.00006: Rod-Coil Block Copolymer Self-Assembly in Thin Films B.D. Olsen, V. Ganesan, R.A. Segalman The phase behavior of rod-coil block copolymers differs from that of traditional block copolymers due to the interplay between liquid crystallinity of the rod blocks and microphase separation of the rods and coils. A universal phase diagram for rod-coil diblock copolymers is prepared using experimental measurements of both the rod aligning interaction and the rod-coil repulsive interaction to transform the temperature-dependent phase transitions of a model-rod coil system into dimensionless parameter space. The rod aligning interaction, parameterized by the Maier-Saupe parameter, may be estimated from the dependence of the nematic-isotropic transition temperature on the molecular weight of the rod homopolymer. The rod-coil interaction, parameterized by the Flory-Huggins parameter, is calculated from the temperature-dependent interfacial segregation of block copolymer to a rod/coil homopolymer interface. The Flory-Huggins parameter is extracted by using it as a fitting parameter in self-consistent field theory to match simulated block copolymer surface excesses to experimental values. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J18.00007: Why nanoconfinement may lead to the development of polymer glasses that do not physically age Rodney Priestley, Linda Broadbelt, John Torkelson With the advent of nanotechnology, polymers will be used at increasingly smaller length scales, $i.e$., the nanoscale. Recently, it has been shown that nanoconfined polymers can exhibit astounding changes in glassy-state properties relative to bulk. Physical aging, $i.e$., the change in properties as a function of annealing time below the glass transition temperature, determines the end-use properties of polymer glasses. How nanoconfinement impacts aging has emerged as a key technological and scientific question. Conventional techniques for monitoring aging of bulk polymers are incapable of doing so for nanoconfined polymers. Here, we present work in which we have developed fluorescence methods to monitor aging in thin polymer films. More importantly, our technique allows for the monitoring of aging at specific locations in films near interfaces. Our work indicates that nanoconfinement and interfacial effects strongly alter aging and that the development of polymer glasses that do not physically age may be possible. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J18.00008: Ionic Complexation Enhanced Block Copolymer Alignment with an Electric Field Jia-Yu Wang, Thomas P. Russell Alignment of microdomains in block copolymer (BCP) films by an electric field offers the possibility of fabricating ordered nanostructures that are use as templates, scaffolds and masks. In polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA) copolymer films, the formation of lithium-PMMA complexes, as a result of the added lithium salts, markedly enhanced the alignment of BCP microdomains under an electric field, due to the increased dielectric contrast and the weakened surface interactions which reduced the critical field strength. The formation of lithium-PMMA complexes also increases the segmental interaction, $\chi _{eff,}$ between PS and PMMA blocks with lithium-PMMA complexes, evidenced by a disorder-to-order transition (DOT) and an order-to-order transition (OOT) from spheres to cylinders. The increased $\chi _{eff}$ drove the system into a stronger phase separation, leading to a transition in the reorientation mechanism of lamellar microdomains from a disruption and re-formation to a grain rotation mediated by movement of defects. The formation of large grains amplified the ability of the electric field to overcome the interfacial interactions and eliminate defects. Consequently, the complete alignment of BCP microdomains can be achieved. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J18.00009: Polymer Surface Diffusion as a Function of Molecular Weight Janet Wong, Steve Granick This talk describes the first measurements to elucidate polymer surface diffusion over the full range of surface concentration and as a function of molecular weight. The model system, polystyrene adsorbed onto mica and quartz from chloroform, was selected to allow molecular weight (M) to vary by more than 2 orders of magnitude and the surface coverage to vary by more than 3 orders of magnitude. Spatially-resolved measurements of surface translational diffusion (D$_{s})$ were made using fluorescence correlation spectroscopy (FCS), which is a single-molecule technique. The value of D$_{s}$ was found to scale as a power-law in M. Remarkably, the absolute value of the power-law was -1 for mica and the most homogeneous quartz surfaces, -3/2 for less homogeneous quartz surfaces, and never took intermediate values. Explanations remain speculative but appear to involve the dominance of Rouse and reptation diffusion mechanisms, respectively. In the latter case, curvilinar motion is guided not by entanglement with other chains but instead by patchiness (topographical and chemical) of the surface adsorption sites. [Preview Abstract] |
Session J19: Focus Session: How to Develop an Education Component for an NSF Proposal
Sponsoring Units: FEdChair: Greg Topasna, Virginia Military Institute
Room: Morial Convention Center 211
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J19.00001: How to develop an education component for an NSF proposal Invited Speaker: NSF has two merit criteria that must be satisfied for a proposal to be successfully funded: Intellectual Merit and Broader Impact. The development and presentation of a quality broader impact program represents a challenge for many researchers. One option is an education component and there are many possible approaches: in formal or informal education; in engagement of traditional, non-traditional or underrepresented participants; in coverage over a variety of age groups; and in scope. This presentation explores such issues and is intended to elicit discussion. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J19.00002: Preparing Scientists for Scientific Careers: Broader Impacts from an NSF CAREER Award Alfred Crosby The scientific focus of my NSF CAREER Award is the impact of patterns, topographical and surface chemical in design, on the adhesion of soft polymer interfaces. Although this topic has provided a strong foundation for the mentoring and training of graduate students, the primary broader impacts of my award have focused on the development of ``soft'' skills in graduate and post-doctoral researchers in STEM disciplines. I have developed a course on ``Scientific and Engineering Management,'' which provides an open forum for students to explore the skills that, in many ways, define successful careers for many scientists. Topics include: leadership, proposal writing, group management, communication in diverse environments, and ethics. In this presentation, I highlight the primary phases of this program, how it meshes with scientific goals, and general statements about the mission of education outreach within STEM disciplines. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J19.00003: Educational Component of an NSF-CAREER Award on State Variables in Dense Granular Materials Karen Daniels I will discuss the design and goals of the educational component of an NSF-CAREER award whose research mission centers around experiments on granular materials. Two of the activities primarily involve undergraduates: providing laboratory research opportunities in support of the scientific goals of the project, and developing and disseminating an introductory seminar course on nonlinear/nonequilibrium systems. In addition, the students from both the course and the laboratory will develop, in collaboration with NC State's Science House, hands-on outreach activities for use with local school/youth groups. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J19.00004: Education component of NSF awards -- what can be done and where can we make a difference Alessandra Lanzara The education component of the NSF awards is often neglected or considered a minor component of the research. In this talk I will share my personal view of the importance of this component. In particular I will provide few examples of situation I have encountered in the past few years as NSF career awardees and will discuss what in my opinion are the aspects one should focus on to make a difference. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J19.00005: Introducing Change in Undergraduate Education (Easy Steps for Junior Faculty) Eric Hudson Bringing change to the undergraduate curriculum -- for example, as new faculty might consider proposing for the education component of their NSF Career proposals -- can be a daunting task. At many institutes classes have been ``taught this way forever'' and even the mention of changing them can induce complaints from students and faculty alike. In this talk I will describe TEAL (technology enabled active learning), a major reform to the introductory physics sequence for non-majors at MIT. I will then focus on a few aspects of the course, such as the use of in-class feedback and real world problems. These relatively small changes (in terms of expense and effort) have been very beneficial, and point to a variety of improvements which faculty (including junior faculty) could make to well established courses while avoiding the difficulties often associated with change. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J19.00006: Integration of Education and Research: proposed and completed activities Vera Smolyaninova In my presentation I will share my ideas of integrating education and research in a setting of an undergraduate institution. The proposed educational component of my CAREER proposal will be compared to what has been completed taking into account evolving needs and goals of our department, college and university. Organization of undergraduate research, curriculum development, and outreach activities will be discussed. Future plans will be introduced. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J19.00007: Caltech Classroom Connection: An Outreach Partnership Program Between Caltech Scientists and K-12 Teachers James Maloney, Jennifer Franck, Tara Gomez, Christina Smolke, John Keith The Caltech Classroom Connection (CCC) is a volunteer outreach program whose goal is to supplement science, math and engineering education in local K-12 classrooms through individual scientist-teacher partnerships. Caltech graduate students, postdocs, staff and faculty volunteers are paired with teachers to develop a mutually beneficial and sustainable partnership. Targeted schools include the Pasadena Unified School District in which 76{\%} of the student demographic consists of Hispanic and African American students, historically underrepresented in science, math and engineering careers. Student surveys are being developed to follow trends in science attitudes and science appreciation after interaction with a Caltech volunteer throughout the school year. The students are also affected by the increase in science awareness and confidence of the teacher, especially at the elementary level. We will present the program's results over the past five years as well as future plans for improvement and expansion. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J19.00008: Squishy Physics Field Trips Eric R. Weeks, Gianguido Cianci, Piotr Habdas Our laboratory studies soft condensed matter, which means we investigate squishy materials such as foams, emulsions, and colloidal suspensions. These materials include common things such as peanut butter, toothpaste, mayonnaise, shampoo, and shaving cream. We have conducted several field trips for grade school students, where they come to our laboratory and play with squishy materials. They do both hands-on table-top projects and also look at samples with a microscope. We have also developed some of these activities into labs appropriate for first-year college students. Our first goal for these activities is to show students that science is fun, and the second goal is to get them intrigued by the idea that there are more phases than just solids, liquids, and gases. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J19.00009: Moving Research into the Classroom with the Electron Microscopy Database Paul Voyles Due to the strongly interdisciplinary nature of research in nanotechnology and materials, a course on transmission electron microscopy (TEM) must often serve student from a very broad range academic disciplines, level of background, and research interests. Someone in the class will want to learn about all the possible capabilities of the TEM, which span diffraction, spectroscopy, and imaging. Research students learn best from real-world examples, which are usually drawn from the research of the instructor, but very few instructors have the breadth of research and instrumentation needed to obtain high-quality examples of all the possible combinations of techniques and materials. I have therefore developed the Electron Microscopy Database (EMdb, http://tem.msae.wisc.edu/emdb/) as part of the education plan of my NSF CAREER project. The goal of the EMdb it to enable TEM teachers to easily exchange high-quality TEM example data and associated homework problems. This serves the NSF education goals of promoting excellence in research training and of bringing cutting-edge research into the classroom, and has significantly improved my own teaching. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J19.00010: Research Experiences for Teachers: How professional development through directed research can revitalize your classroom teaching Shelly Hynes Research Experiences for Teachers (RET) is an NSF-funded program that provides high school teachers with the opportunity to do research at select institutions across the country in a wide variety of fields. I performed research at two institutions under this program; The National Radio Astronomy Observatory in Green Bank, WV in 2006 and Baylor University in Waco, TX in 2007. My work at NRAO utilized IDL programming to analyze the nonlinearities in the signal processing components of the GBT (Green Bank Telescope). My research at Baylor University required me to write a program in Mathematica to analyze the frequency of pulsation of variable white dwarfs, data that was taken at the Paul and Jane Meyer Observatory in Clifton, TX. I will explain how I have incorporated both research experiences into my courses and how each of these experiences has refocused my teaching. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J19.00011: Having Fun with High School Teachers and Students Making Atomic Force Microscopy Chang Y. Ryu, Paul Fedoroff, Tom Pittman As a type of scanning probe microscopy, atomic force microscopy (AFM) is a powerful tool that allows us scientists and engineers to visualize, characterize and manipulate nanostructures in both hard and soft materials. In particular, AFM has served as an invaluable tool for researchers working on nanotechnology, which is a subject of uprising interests to many high school students and teachers. As a part of high school educational outreach program called ``Bringing Nanotechnology to the Classroom'' in the NSF Nanoscale Science and Engineering Center (NSEC) at Rensselaer Polytechnic Institute, we were fortunate to interact with high school teachers and learn more about the opportunities to infuse principles used in AFM into the physics laboratory sessions in high schools. After showing AFM at NSEC to high school teachers, using wood blocks, hex saw and speaker coils, we have constructed AFM model and used it to demonstrate the resonance frequency of vibration and magnetic properties of materials in high school physics classroom. In addition, using LEGO Mindstorm Robotics, conceptual AFM has been made by high school students as a part of high school engineering course. Principles of controlling the motion of objects using gears with an emphasis of team work have been covered in the LEGO AFM project for high school students. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J19.00012: Conceptual Learning Approach to Waves John Cerne, Frank Nappo, Michael Gerfin Waves represent one of the most important concepts in physics, playing a crucial role in topics ranging from acoustical phenomena, electricity and magnetism, optics, Fourier analysis, and even quantum mechanics. However, since waves have both a temporal and spatial dependence (often in more than one dimension) that may be difficult to visualize, many undergraduate and graduate students have a poor understanding of even basic wave concepts. We are creating a web site (electron.physics.buffalo.edu/claw/) that explains many basic wave concepts using dynamic and interactive graphical simulations. Our goal is to create simulations that enable students to visualize how waves behave and better connect this behavior to the equations and concepts that describe the use of waves in applications. There are many excellent web sites using similar graphical interactive tools, but they tend to focus on mechanics, electrostatics, and magnetism. I am actively using this site for my introductory physics courses, as well as a magneto-polarimetry teaching lab that I have created (www.physics.buffalo.edu/cerne/education/moke\_manual.pdf). [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J19.00013: Inciting High-School interest in physics. Jiandi Zhang We report on our outreach effort on material-physics education program as one part of my NSF Career award project. This is a program incorporated with the NSF funded Physics Learning Center at FIU, focusing on the material physics enrichment both high school students and teachers. We particularly pay attention to minority students by taking the advantage of FIU's composition and location. The program offers a special/session-style workshop, demonstrations, research lab touring, as well as summer research activities. The goal is to enrich teacher's ability of instruction to their students and inspire students to pursue scientific careers. The detailed outreach activities will be discussed. [Preview Abstract] |
Session J20: Focus Session: Electronic and Lattice Properties of Surfaces and Thin Films
Sponsoring Units: DCMPChair: Saw-Wai Hla, Ohio University
Room: Morial Convention Center 212
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J20.00001: Atomic and Electronic structure of Fe$_2$O$_3$ films on MgO(111) K. Pande, A. Celik-Aktas, M. Gajdardziska-Josifovska, M. Weinert Polar oxide surfaces have interesting properties due to the strong ionic character of the metal-oxygen bonds and the layering of metal and oxygen planes parallel to the surface. In this first-principles study we investigate the effect of MgO(111) surface polarity on the structure of iron oxide films. The calculated energetics of different surface terminations on the unreconstructed MgO(111) surface suggest that the interface has a Mg-O-Fe stacking with no intermixing of O and Fe atoms at the interface. The magnetic ordering of Fe atoms both within the layers and between neighboring (bi)layers will be described. The calculated structural and electronic properties will be compared to the results of high-resolution transmission electron microscopy (HRTEM) and selective area diffraction (SAD) experiments of Fe$_2$O$_3$ films on MgO(111) surface. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J20.00002: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J20.00003: The semiconducting surface of In$_{4}$Se$_{3 }$ M. Klinke, E. Cai, I. Rodriguez, Jiandi Zhang, R. Matzdorf, Y. Losovyi, J. Lui, P.A. Dowben, L. Makinistian, E.A. Albanesi, A. Petukov, Ya. Fiyala, P. Galiy The layered crystal In$_{4}$Se$_{3}$ is of growing interest because of its natural two dimensional structure and anomalies concerning transport properties. We have studied both the lattice and electronic band structures of cleaved (001) surface of In$_{4}$Se$_{3}$. Both LEED and STM reveal a $p$(1$\times $1) surface structure with quasi-one dimensional atomic chains. ARPES data also confirm such quasi-one dimensional character with a very anisotropic band structure. The surface electronic density of states measured by tunneling spectroscopy is compared with theoretical calculations. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J20.00004: Theoretical Study of Tip-Induced Band Bending and Local Tunneling Barrier Height on H-Terminated Si(100) Surface Hideomi Totsuka, Satoshi Watanabe In scanning tunneling microscopy (STM) measurements on semiconductor surfaces, tip-induced band bending (TIBB) occurs due to the applied high bias voltage, and influences the local electronic structures of the surface detected by STM and scanning tunneling spectroscopy (STS). Recently, Yoshida et al. have reported the effect of the TIBB on local tunneling barrier height (LBH) on Si(100) surfaces by light-modulated STS [1]. However, theoretical studies on the TIBB and LBH have not been performed yet on semiconductor surfaces. In this study, we have analyzed the TIBB and LBH on an H-terminated Si(100) surface theoretically using the boundary-matching scattering-state density functional method [2], which can calculate the electron states under applied bias voltages self-consistently. In particular, we focus on the bias voltage dependences of the TIBB and LBH, and show that measured LBHs can be basically understood as the sum of TIBB and intrinsic barrier height. [1] S. Yoshida, et al., e-J. Surf. Sci. Nanotech. 4, 192 (2006). [2] Y. Gohda et al., Phys. Rev. Lett. 85, 1750 (2000). [3] M. McEllisterm, et al., Phys. Rev. Lett. 70, 2471 (1993). [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J20.00005: Studies of the oxidized Cu(100) surface using positron annihilation induced Auger electron spectroscopy W. Maddox, N. G. Fazleev, M. P. Nadesalingam, A. H. Weiss We discuss recent progress in studies of an oxidized Cu(100) single crystal subjected to vacuum annealing over a temperature range from 293K to 1073K using positron annihilation induced Auger electron spectroscopy (PAES). The PAES measurements show a large monotonic increase in the intensity of the positron annihilation induced Cu M2,3 VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 573 K. The intensity then decreases monotonically as the annealing temperature is increased to 873 K. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption and surface reconstruction on localization of positron surface state wave functions and annihilation characteristics are analyzed. Possible explanations are provided for the observed behavior of the intensity of positron annihilation induced Cu M2,3VV Auger peak with changes of the annealing temperature. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J20.00006: Visualization of 2D subband states formed in Si(111)-$\beta -\surd $3x$\surd $3-Bi surface by STM Katsumi Nagaoka, Shin Yaginuma, Tadaaki Nagao, Tomonobu Nakayama We have investigated electronic subbands formed underneath of Si(111)-$\beta -\surd $3x$\surd $3-Bi ($\beta -\surd $3-Bi) surface by using STM. The $\beta -\surd $3-Bi surface is semiconducting, and the band gap is larger than that of bulk Si. In the dI/dV images, standing waves are found around defects, and the obtained energy dispersion is in good agreement with the free-electron-like model. Apparently, formation of a 2D electronic state is suggested. However, this standing wave is observed only on p-type, but not on n-type Si substrates, and consequently the 2D state is attributed to the subband formed in the depletion layer just below the $\beta -\surd $3-Bi surface. The subband formation is also consistent with the calculation of electronic states inside a potential-well caused by interfacial band bending. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J20.00007: One-Dimensional Electronic Bands of Monatomic Cu Chains Pingheng Zhou, Paolo Moras, Luisia Ferrari, Gustav Bihlmayer, Stefan Bl\"ugel, Carlo Carbone The electronic structure of an array of monatomic Cu chains grown on the Pt(997) surface has been examined by angle-resolved photoemission. The monatomic wires exhibit properties associated to 3d electron confinement in one-dimension. Along the wire direction the 3d bands states display a dispersive character, with periodicity in reciprocal space defined by the wire array geometry. These observations are compared and analyzed with ab initio calculations based the full potential linearized augmented plane wave method. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J20.00008: Calculating work functions with density functional theory: the effect of finite temperature, surface alloying, and oxidation Thomas R. Mattsson, Dwight R. Jennison The work functions for W, Cu, and Al are calculated using density functional theory (DFT). We go beyond the perfect lattice at zero Kelvin by employing molecular dynamics techniques. Effects of surface alloying and -oxidation are also investigated. The effect of alloying and oxidation is, as expected, found to be significant, while the temperature dependence, although clearly seen, is in comparison weak. In addition, the exchange-correlation density functional AM05 is compared to the results of LDA and PBE. The calculated work functions compare well to available experimental results. This work was supported by the LDRD office and the simulations were performed at the High Performance Computing facilities at Sandia National Laboratories, NM. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J20.00009: Longitudinal Phonon Resonance of the Cu(111) Surface Vasse Chis, Bo Hellsing, Marco Bernasconi, Giorgio Benedek, J. Peter Toennies A density functional perturbation theory investigation of Cu(111) surface dynamics proves that the surface longitudinal acoustic resonance is not an artifact of the inelastic He atom scattering mechanism but a genuine dynamical feature of metal surfaces, and suggests the ability of He atoms to probe the atomic displacements in the second layer through the surface charge modulation they produce. The present results solve the long-standing Bortolani-Mills paradox and reconcile many early apparently divergent interpretations, based on semi-empirical as well as ab-initio methods, of the ubiquitous longitudinal acoustic resonance and clarifies its intrinsic nature. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J20.00010: Rashba-type spin-orbit splitting in ultrathin Bi films G. Bihlmayer, Yu. M. Koroteev, E.V. Chulkov, S. Bl\"ugel Due to their electronic similarity with graphene sheets, surfaces and thin films of the semimetal bismuth have recently received considerable interest. A systematic study of thin ($1-6$ bilayers) films in (111) and (110) orientation is presented, employing density functional theory calculations. Due to the different coordination of the surface atoms in these two cases, a large variation of the conducting properties of the films is found. The evolution of surface states is studied as a function of the film thickness and by comparison to thicker films and simulations of the semiinfinite crystals. Interesting features arise from the strong spin-orbit effects in Bi and the resulting Rashba-type spin-splitting of the surface states. The spin-polarization of these states changes as these states transform into quantum well states at the Brillouin zone boundary. The results are compared with recent experimental results on Bi films on Si substrates. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J20.00011: Lattice Dynamics of RuO$_{2}$: Bulk and (110) Surface Klaus-Peter Bohnen, Rolf Heid, Omar de la Pena Seaman Although RuO$_2$ has been studied as a prototype catalyst for CO oxidation no careful study of the lattice dynamics for this material has been presented so far. Using modern ab-initio methods we obtain the phonon dispersion and the generalized density of states (GDOS). Inelastic neutron-scattering experiments allow for an experimental determination of the GDOS. In contrast to what is known from structural studies, we find that the local-density approximation gives a much better description of the phonon spectrum than the generalized gradient corrected form. This is also consistent with Raman measurements. Besides the bulk we have also studied the lattice dynamics for the (110) surface. Our calculations indicate an instability over a large range of the BZ. Unfortunately, no complete experimental phonon study of this surface has been carried out so far. Consequences for the structure of the (110) surface will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J20.00012: Simulation of four-probe measurement based on density-functional tight-binding method Asako Terasawa, Tomofumi Tada, Satoshi Watanabe Four-probe measurements are powerful tools to investigate electric properties of materials precisely. Recently, the minimum probe spacing has reached to the order of 10 nm using nanotube probe tips [1]. However, it is not clear if the procedure used in macroscopic measurements can eliminate the effects of contact resistance even in such microscopic measurements. Keeping this in minds, we have developed a multi- probe transport simulator on the basis of Green's function method combined with the density-functional tight-binding method [2]. So far, we have succeeded in self-consistent calculations at the limit of zero bias voltage for four-probe models consist of more than 1000 atoms, such as an infinite graphite ribbon with two or four semi-infinite nanotube tips. The calculation results indicate that the effects of the contact are not fully eliminated by the usual procedure. \newline [1] S. Yoshimoto, et al., Nano Lett. 7, 956 (2007) \newline [2] T. Frauenheim, et al., Phys. Stat. Sol. 217, 41 (2000) [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J20.00013: Quantum Blockades and Loop Currents in Graphene with Topological Defects Yan-Yang Zhang, Jiang-Ping Hu, B.A. Bernevig, Xiangrong Wang, Xin-Cheng Xie, Wu-Ming Liu We investigate the effect of topological defects on the transport properties of a narrow ballistic ribbon of graphene with zigzag edges. Our results show that the longitudinal conductance vanishes at several discrete Fermi energies where the system develops loop orbital electric currents with certain chirality. The chirality depends on the direction of the applied bias voltage and the sign of the local curvature created by the topological defects. This novel quantum blockade phenomenon provides a new way to generate a magnetic moment by an external electric field, which can prove useful in carbon electronics. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J20.00014: Orbital Order in (LaMnO$_3$)$_m$(SrMnO$_3$)$_n$ Superlattice Chungwei Lin, Claude Ederer, Andrew Millis A realistic model for (LaMnO$_3$)$_m$ (SrMnO$_3$)$_n$ is constructed and solved by the semiclassical approximation. The model includes electron-electron, electron-lattice, and lattice-lattice interactions. Technically, we generalize the semiclassical approximation to the multi-orbital system and include the cooperative Jahn-Teller effect in the impurity problem. Within this framework, we present the orbital order in the superlattice. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J20.00015: Direct measurement of core-level relaxation dynamics on a surface- adsorbate system Jing Yin, Luis Miaja-Avila, Guido Saathoff, Chan La-o-vorakiat, Margaret Murnane, Henry Kapteyn, Stefan Mathias, Martin Aeschlimann, Michael Bauer Electronic coupling between an adsorbate and the surface on which it resides is fundamental to the understanding of many surface interactions. However, the interaction of \textit{highly-excited} adsorbate states is an area that has been explored only indirectly to-date. In this work, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By implementing laser-assisted Auger decay on an adsorbate/surface system, we directly measure the lifetime of the 4d$^{-1}$ core level of Xenon on Pt(111) to be 7.1 $\pm$ 1.1 fs. This result opens up time domain measurements of highly-excited state dynamics in materials systems where, because of complex interactions, energy-resolved measurements provide incomplete information. [Preview Abstract] |
Session J21: Focus Session: Clusters, Cluster Assemblies, Nanoscale Materials IV
Sponsoring Units: DCPChair: Meichun Qian, Virginia Commonwealth University
Room: Morial Convention Center 213
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J21.00001: Quantum Mechanics and Electrodynamics Studies of the Optical Properties of Metal Clusters/Nanoparticles Invited Speaker: This talk will describe the use of electrodynamics and quantum mechanics methods to describe the optical properties of silver and gold nanoparticles and other nanostructures. This work has been done in collaboration with several experimental colleagues, including Chad Mirkin, Rick Van Duyne and Teri Odom. Our recent work has focused on the optical properties of metal nanoparticles that are coated with molecules that are detected either through their influence plasmon resonance excitation, or via surface enhanced Raman spectroscopy (SERS). Electrodynamics calculations using either the DDA or FDTD methods provide a quantitative tool for characterizing far field properties, and at a more primitive level estimates of SERS intensities. Quantum mechanics, as developed using time dependent density functional theory, is restricted to small metal clusters, but the same methods of far field spectroscopy and SERS can still be studied. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J21.00002: Low Temperature Static Dipole Polarizability of Free Sodium Clusters with from 2 to 250 atoms Anthony Liang, John Bowlan, Xiao- Shan Xu, Shuange-Ye Yin, Walt A. de Heer The electric dipole polarizabilities of all sodium clusters Na$_{n}$ were measured from the atom up to n =250 using the molecular beam deflection method. Clusters were formed in cryogenic laser vaporization source operating at a temperature of 20 K. This complete sequence of high-resolution polarizablities measurements greatly enhances previous measurements. Electronic shell effects are observed as well as several features that are not readily understood in the shell model. The asymptotic limit of the measurements appears not to converge to the bulk sodium polarizability value. The data are compared with theoretical predictions. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J21.00003: Novel Properties of Diamondoid Molecules William Clay, Zhi Liu The recent isolation of a number of diamond-like hydrocarbons molecules (diamondoids) has sparked renewed interest in these unusual molecular systems. Several unique properties of these molecules are investigated. Diamondoid monolayers have been found to profoundly alter the electron emission tail of metal substrates in recent photoemission experiments, producing a sharp, nearly monochromatic peak. It is postulated that the cause of this effect is negative electron affinity combined with a strong electron-phonon interaction. New data and simulation results are presented to support this theory. Additionally, photoluminescence spectra for a number of diamondoid crystals are presented, taken with a 229 nm laser. To our knowledge, this is the first observation of UV photoluminescence in a saturated hydrocarbon molecule. Possible explanations for this phenomenon are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J21.00004: Room temperature stability of mass selected Ag clusters on C$_{60}$ functionalized surfaces Stefanie Duffe, Lukas Patryarcha, Torsten Richter, Benedikt Sieben, Heinz H\"{o}vel, Chunrong Yin, Bernd von Issendorff, Michael Moseler Mass selected clusters from Ag$^+_{55}$ to Ag$^+_{561\pm5}$ were soft landed on HOPG and Au(111) functionalized with 1 and 2 monolayers (ML) of C$_{60}$ molecules [1]. Depositions at 165 \,K gave extremely narrow cluster height distributions in STM images measured at 77\,K. Using C$_{60}$/HOPG or 2\,ML C$_{60} $/Au(111) the cluster heights are stable for more than 12\,h at room temperature (RT). For 1\,ML C$_{60}$/Au(111) the cluster height decreases and finally all clusters disappear at RT. Molecular dynamics simulations reveal a process by which the clusters decay atom by atom through 1\,ML C$_{60}$/Au(111) at RT. A sharp maximum at 1.7\,nm cluster height forms during the cluster decay, indicating that there exists some metastable 'supported magic number'. \par \noindent [1] S. Duffe et al., Eur. Phys. J. D (2007), published online [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J21.00005: Modification on the melting of aluminum nanoclusters by a copper atom: heat capacities of CuAl$_{n-1}^{-}$ nanoalloys Baopeng Cao, Colleen M. Neal, Anne M. Starace, Martin F. Jarrold The melting of alloyclusters is currently of great interest and emerging as an important research area. In this talk, we report the synthesis and melting transition of CuAl$_{n-1}^{-}$ nanoalloy clusters (n = 49 -- 62). Heat capacities and melting behaviors have been determined for CuAl$_{n-1}^{-}$ nanoalloy clusters using a novel collision induced dissociation method and are compared with those of pure aluminum cluster Al$_{n}^{+}$. All these nanoalloys present a first order melting transition at temperatures well-below the melting temperature of the bulk aluminum and the eutectic temperature of their bulk alloys. No eutectic characteristic is detected for these nanoalloyclusters. Upon substitution of Al with a single copper atom, the melting of pure aluminum clusters has been altered considerably. Size and charge effects of the doping atom on the melting of host nanoclusters are discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J21.00006: Forces between Functionalized Silica Nanoparticles J. Matthew D. Lane, Ahmed E. Ismail, Michael Chandross, Gary S. Grest Polymer-coated nanoparticles have a wide variety of applications, including drug delivery, adhesives, coatings, and magnetics. Although, the complexity of these nanoparticles precludes atomistic simulations of large numbers of nanoparticles in solution, it is possible to study the interaction between pairs of nanoparticles in an explicit solvent using molecular dynamics. From these simulations, we can compute the potential of mean force (PMF) between nanoparticles, which can be used in coarse-grained simulations at larger length and time scales. In particular, we present results for PMFs between polymer-grafted silica nanoparticles as a function of chain length, core size, and approach velocity. We report results for explicit-atom models of poly(ethylene oxide)-coated nanoparticles in water and alkylsilane-coated nanoparticles in decane. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J21.00007: Photo-fragmentation of the closo-carboranes Jing Liu, Danqin Feng, P.A. Dowben, A.P. Hitchcock, A.L.D. Kilcoyne, T. Tyliszczak, J.D. Bozek, E. R\"uhl Single and multi-cation ionic fragmentation of three isomeric carborane cage compounds [closo-1,2-orthocarborane, closo-1,7-metacarborane, closo-1,12-paracarborane (C$_{2}$B$_{10}$H$_{12})$] following B 1s and C 1s excitation were studied by time of flight mass analysis. The energetic cycles were constructed to gain some insights into some of decomposition processes. CH$^{+}$ or BH$_{2}^{+}$ fragment dominates the single ion fragmentation of the closo-carboranes. Double ion fragmentation yields and charge separation mass spectra of all three isomers are generally quite similar in that H$^{+}$ and BH$_{2}^{+}$/CH$^{+}$ ion pairs, BH$_{2}^{+}$/CH$^{+}$ and Y$_{11}^{+ }$ion pairs (where Y = (BH) or (CH)), Y$_{3}^{+}$ and Y$_{9}^{+ }$ion pairs (where Y = (BH) or (CH)) yields dominate. The H$^{+}$ and BH$_{2}^{+}$/CH$^{+}$ ion pairs dominate at the B 1s threshold for ortho- and metacarborane, while Y$_{1}^{+}$ and Y$_{11}^{+ }$ion pairs (where Y = (BH) or (CH)) dominates the multi-photofragment ion yield of paracarborane at the B1s threshold. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J21.00008: Coupled quantum dot / quantum shell systems: optical gain, ultrafast charge transport, and single particle blinking. Patanjali Kambhampati, D.M. Sagar, Eva Dias, Samuel Sewall, Ryan Cooney, Amy Grimes, Douglas English The CdSe/ZnS/CdSe core/barrier/shell nanostructure forms an electronically coupled quantum system that is a spherical analog to the quantum well superlattice. The core's brightness is enhanced via light harvesting by the shell. This material offers an opportunity to study charge transport in spherical nanoscale materials. Here, we present new results on the femtosecond dynamics of radial charge transport in these materials. With a combination of excitonic state selectivity and femtosecond time resolution, we monitor the ultrafast relaxation dynamics of either the core or the shell, having optically excited either phase. The femtosecond experiments reveal strong optical gain as well as evidence of spatially separated biexcitons, and coupling between phases. Finally, we present single dot data on the two-color blinking kinetics of these coupled quantum dot quantum shell systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J21.00009: Sampling of stable and metastable cluster structures by a first-principles Monte Carlo approach Ralf Gehrke, Karsten Reuter Size-selected nano-scale atomic clusters are now systematically becoming accessible in experiment, but characterizing their ground-state and metastable isomer ensemble averages from first principles requires a global and local exploration of vast configuration spaces. We here explore a first-principles Monte Carlo scheme to efficiently sample the minima of the corresponding total energy landscapes. The energetics is obtained at the density-functional theory level, using an all-electron local orbital based first principles code,$^1$ which allows to switch seamlessly from minimal size effective tight-binding like to meV-level chemically accurate basis sets within a single fundamental framework. The sampling strategies rely on basin hopping, using different schemes to create new trial structures. We demonstrate the reliability and performance of the approach for Cu and Si clusters, discussing in particular the scaling behaviour with the system size.\newline $^1$M. Scheffler and V. Blum; R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, P. Rinke, A. Sanfilippo, A. Tkatchenko, The FHI - Ab Initio Molecular Simulations (aims) project, www.fhi-berlin.mpg.de/aims [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J21.00010: The Effects of pH and Acid Type on Porous Alumina Formation Adam Friedman, Derrick Brittain, Latika Menon Porous aluminum oxide prepared by anodization has an enormous variety of uses in nanomanufacturing, as it can be used as scaffolding to grow nanowires and tubes of exacting size specifications. However, there is no complete physical model for its growth. Three models in particular have been suggested in the past. We experimentally study the stability of porous alumina formation and the effects of changing anodization voltage, acid pH, and acid type. Using this information, we show that the models err in their primary assumptions, we pinpoint the location of these errors, and we suggest a method to correct them. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J21.00011: Probing the Coulomb Barrier Towards Ionic Fragmentation Shaun Ard, Nasrin Mirsaleh-Kohan, Robert Compton Quintessential to understanding the stability of multiply charged anions (MCAs) is the characterization of the so-called ``Coulomb Barrier'' (CB) toward the loss of an excess electron and/or dissociation into charged fragments. The CB arises due to the superposition of the long-range Coulomb repulsion of the excess electron (or anion), and the short-range attractive polarization binding energy of the anion (or fragment). The CB adds to the stability of MCAs, often rendering thermodynamically unstable species to be metastable toward autodetachment or dissociation. The magnitude and shape of the CB is expected to depend heavily on the decay pathway. Whereas dissociation into charged fragments is often the lowest energy pathway for many MCAs, the vast majority of previous research has focused on electron loss. In this work, collision-induced dissociation (CID) is employed to study fragmentation of disulfonic dianions of increasing ``length''. Energy threshold for the production of SO$_{3}^{-}$ plus its conjugate anion are used to estimate the magnitude of the Coulomb barrier to dissociation. These measured thresholds are compared with \textit{ab initio }calculations of the dissociation energy. The relationship between these magnitudes and the distance between the excess charges will then be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J21.00012: Vibrational modes in metal core-shell nanoparticles A.S. Kirakosyan, T.V. Shahbazyan We study the spectrum of lowest vibrational modes of bimetallic nanoparticles in dielectric surrounding. For solid spherical particles, the mode's period and decay time are determined by the ratio of particle radius to longitudinal sound velocities in metal and outside medium, respectively. In contrast, in bimetallic nanoparticles, the dependence of both period and damping time on aspect ratio is highly non-monotonic. In particular, for Au/Ag core-shell nanoparticles, in a wide range of aspect ratios, the fundamental mode period is lower than in both Au and Ag solid particles of the same overall size, while the damping time exhibits a minimum at aspect ratios around 0.5. The unique acoustical signature of complex nanostructures allows unambiguous determination of their composition from ultrafast pump-probe and Raman spectroscopy measurements. [Preview Abstract] |
Session J22: Organic Electronics, Photonics and Magnetics: Theory
Sponsoring Units: DMP DPOLYChair: Zhiqiang Li, University of California, San Diego
Room: Morial Convention Center 214
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J22.00001: Dependence of Mobility on Density of Gap States in Organics by GAMEaS - Gate Modulated Activation Energy Spectroscopy Woo-young So, David Lang, Arthur Ramirez We develop a spectroscopic method for determining the density of states (DOS) in the energy gap - GAte Modulated activation Energy Spectroscopy (GAMEaS), We also report the relationship of these gap states to the mobility of organic field-effect-transistors (FETs). We find that the field-effect mobility is parameterized by two factors: (1) the free-carrier mobility and (2) the ratio of the free carrier density to the total carrier density induced by the gate bias. We show that the highest mobility FETs have shallow exponential band tails of localized states with characteristic slope of 1/kT at 300K. Most remarkably, state-of-the-art crystalline FETs fabricated from rubrene, pentacene, and tetracene all have a very high free-carrier mobility, up to 200cm2/Vsec at 300K, with the somewhat lower effective mobilities dominated by localized gap states. This strongly suggests that further improvements in device performance could be possible with enhanced material quality. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J22.00002: Charge mobility of discotic mesophases of polyaromatic hydrocarbons: a multiscale quantum/classical study Denis Andrienko, Valentina Marcon, Kurt Kremer, James Kirkpatrick, Jenny Nelson Discotic liquid crystals form columnar phases, where the molecules stack on top of each other and the columns arrange in a regular lattice. The self-organization into stacks results in the one-dimensional charge transport along the columns. Using atomistic molecular dynamics (MD) simulations we study columnar discotic phases formed by various polyaromatic hydrocarbons. Combining Kinetic Monte Carlo and MD trajectories a correlation between the material morphology and charge mobility is then established. We are able to reproduce the trends and magnitudes of mobilities as measured by pulse-radiolysis time-resolved microwave conductivity technique. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J22.00003: Optical spectra and exchange-correlation effects in molecular crystals Na Sai, Murilo L. Tiago, James R. Chelikowsky, Fernando A. Reboredo We report first-principles GW-Bethe Salpeter Equation and Quantum Monte Carlo calculations of the optical and electronic properties of molecular and crystalline rubrene (C$_{42}$H$_{28}$). We predict the formation of intermolecular, charge-transfer spin-singlet excitons with energies in close agreement with the observed yellow-green photoluminescence in rubrene microcrystals. In contrast, spin-triplet excitons are localized and intramolecular with a predicted phosphorescence at the red end of the optical spectrum. We find that the exchange energy plays a fundamental role in raising the energy of intramolecular spin-singlet excitons above the intermolecular ones. Exciton binding energies are predicted to bearound 0.5~eV (spin singlet) to 1~eV (spin triplet). The calculated electronic gap is 2.8~eV. The theoretical absorption spectrum agrees very well with recent ellipsometry data. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J22.00004: Monte Carlo Simulation of Carrier Dynamics in an Organic Field Effect Transistor Dharmendar Reddy Palle, Leonard Register, Ananth Dodabalapur We demonstrate here for the first time a self consistent Monte Carlo Simulation [MCS] of carrier dynamics under high charge densities in an Organic Field Effect Transistor (OFET). Given the stochastic nature of carrier transport in organic semiconductors, MCS is ideally suited for simulating the carrier dynamics in these systems. Previous work on MCS of carrier transport in organic devices has been limited to two terminal device configurations with low carrier densities. Also, a modified Miller Abrahams hopping rate is introduced to account for the anisotropy of hopping events between molecules with different spatial orientation. The semiconductor is modeled as a polycrystalline region with a Gaussian density of states. Injection and transport have been modeled as hopping events with different hopping rates. Comparison of measured and simulated transient and steady state current voltage [I-V] characteristics of Pentacene FETs will be presented. The effect of various parameters such as interface barriers, grain boundaries, and temperature on simulated transient and steady state I-V characteristics of Pentacene FETs with channel lengths varying from a few hundred nano-meters to a micron based on Kinetic MCS will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J22.00005: Dynamical Barrier to Impurity Trapping in Organic Semiconductors David H. Dunlap, Paul E. Parris, Stephan De Bievre We consider the trapping of a moving electron by a polar impurity in an uncompensated organic semiconductor when the multipole moment of the impurity is coupled to an intramolecular vibration, a dynamical generalization of the Vannikov-Novikov dipole trap model.[1] Due to the slow power-law dependence of the multipolar interaction, the vibrational coordinate shifts adiabatically with the approach of the charge carrier. The fast molecular motion can be decoupled from the electron's translational motion to first approximation by transforming to the polaron basis, leading to a polaron binding energy which serves to enhance the propensity for the moving charge to be captured. For an isolated impurity, however, the transformed Hamiltonian contains a repulsive ponderomotive term not described in conventional polaron theory. The repulsion can outweigh the attractive force at long range, presenting a barrier to trap formation. [1] S. V. Novikov and A. V. Vannikov, Chem. Phys. 169 (1993) 21-33. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J22.00006: Inverse Molecular Design in a Tight-Binding Framework Dequan Xiao, Weitao Yang, David Beratan The number of chemical species of modest molecular weight that can be accessed with known synthetic methods is astronomical. An open challenge is to explore this space in a manner that will enable the discovery of molecular species and materials that exhibit optimized properties. Recently, a strategy was developed to perform continuous optimization of molecular properties, the linear combination of atomic potentials (LCAP) approach.$^{1}$ Here, using a simple tight-binding (TB) implementation, we show that the LCAP strategy can successfully explore vast chemical libraries that are based on planar $\pi $-electron motifs. We show that LCAP property optimization of $\pi $-electron polarizabilities and hyperpolarizabilities is effective for libraries with 10$^{4}$ to 10$^{16}$ members. This approach finds optimal structures among 10$^{4}$ candidates with about 40 individual molecular property calculations. As such, for molecular candidates with strong structural similarity, the TB-LCAP approach may provide an effective means of identifying structures with optimal properties. \newline $^{1}$M. Wang, X. Hu, D. N. Beratan, and W. Yang, J. Am. Chem. Soc. \textbf{128}, 3228 (2006); S. Keinan, X. Hu, D. N. Beratan, and W. Yang, J. Phys. Chem. A \textbf{111}, 176 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J22.00007: Ab initio Evaluation of the Charge-Transfer Integrals and Band Structures of Phenanthroline-based Molecular Crystals H. Li, J.-L. Bredas, C. Lennartz Ab initio calculations are carried out to study the charge-transport properties of phenanthroline-based molecular crystals, BCP and Bphen. The charge-transfer integrals in the two crystalline structures and in a quasi-1D chain model for BCP are evaluated based on: (i) a tight-binding approximation for dimers; (ii) the band structures of the periodic systems. In these compounds, the LUMO/LUMO+1energies of the isolated molecules are very close, which results in the LUMO and LUMO+1orbitals both having significant contributions to the LUMO level in the dimer. In this case, the usual definition based on the electronic coupling between the two LUMO orbitals from each molecule cannot be applied to describe the charge-transfer characteristics in the dimer. A new definition of ``effective transfer integrals'' based on ``mixed states'' was proposed (H. Li, J.L. Bredas, and C. Lennartz, J. Chem. Phys. 126 (2007) 164704) to address this problem. Within the tight-binding approximation, the ``effective transfer integrals'' for both hole and electron transfers are found to be in very good agreement with the valence and conduction band dispersions obtained from plane-wave DFT calculations. We acknowledge many stimulating discussions with Veaceslav Coropceanu and Demetrio da Silva Filho. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J22.00008: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J22.00009: Electron-phonon coupling in naphthalene crystal Roel S\'anchez-Carrera, Pavel Paramonov, Veaceslav Coropceanu, Jean-Luc Br\'edas We investigate the electron interactions with optical phonons in crystalline naphthalene. The lattice phonon modes were computed at both DFT and empirical force field levels. The electron-phonon couplings were evaluated by means of numerical differentiation; in this approach, the crystal geometry is distorted along normal modes and transfer integrals for several selected molecular pairs are then computed at each distorted geometry. We find that the DFT and force field results for phonon frequencies and electron-phonon couplings compare very well. Interestingly, several phonon modes are calculated to display significant quadratic electron-phonon coupling. In addition, we have also performed electronic band-structure calculations and derived the effective masses for both electrons and holes. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J22.00010: Solid state effects on the photophysics of $\pi$-conjugated polymer thin films Alok Shukla, Zhendong Wang, Sumit Mazumdar The photophysics of thin films of $\pi$-conjuated polymers (PCPs) are remarkably different from that of dilute solutions. The difference is generally ascribed to interchain interactions and disorder in films. Microscopic understanding of the consequences of interchain interactions has remained incomplete in spite of intensive investigations. We present a theory of the complete energy spectrum of interacting PCP chains that leads to correct qualitative, and perhaps even semiquantitative description of the photophysics of PCP films. Within our theory branching of photoexcitations occurs to excimers that occur both below and above the optical exciton. Emission, as well as ultrafast photoinduced absorption (PA) in films are from the lower excimer. The lowest energy PA at 0.35 - 0.4 eV corresponds to the allowed transition from the lower excimer to the lowest polaron-pair. We explain why the energies of the PA at $\sim$ 1.0 eV and higher are the same in films and solutions, even though the origins of these PAs are different. Finally, we give consistent explanations of the peculiarities associated with emission in films, including delayed emission, its quenching by electric field and the reappearance of the delated emission upon removal of the field.\footnote{Supported by NSF-DMR-0705163} [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J22.00011: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J22.00012: Ab Initio Generated UPS of Electron Donors Reeshemah Allen, Tunna Baruah, Mark R. Pederson We have calculated the ultraviolet photoelectron spectra (UPS) data for a variety of molecules that have been suggested for donors for organic photovoltaic materials. The method employed here uses NRLMOL and the PBE-GGA density-functional based method for approximating energies of molecules within a constrained occupation approach. The UPS data is then simulated by calculating the energy difference between the N-electron ground state and a large set of self-consistently generated (N-1)-electron states. Incorporating the relevant absorption probabilities is also discussed. Comparison of calculated UPS spectra to experiment allows us to determine the conditions under which self-interaction corrections$^{1}$ to the energy functional are necessary for determining accurate excitation energies will also be discussed. 1. M. R. Pederson, R. A. Heaton, and C.C. Lin J. Chem. Phys. 82, 2688 (1985). [Preview Abstract] |
Session J23: Focus Session: Cuprates and Nickelates
Sponsoring Units: DMP GMAGChair: John Tranquada, Brookhaven National Laboratory
Room: Morial Convention Center 215
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J23.00001: Unidirectional charge modulations in underdoped cuprates observed with STM Invited Speaker: |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J23.00002: Phase Separation and Magnetism in High Tc Superconductors Samuel Emery, Barrett Wells, Hashini Mohottala, Joseph Budnick, William Hines, Linda Udby, Kim Lefmann, Neils Hessel Andersen, Christof Niedermayer, Niels Christensen, Jeffrey Lynn, Fangcheng Chou Previous work by our group has determined that the low temperature phase diagram of super-oxygenated, La$_{2}$CuO$_{4}$ consists of only a few line phases that are either superconducting (SC) or magnetic. Samples with doping levels between the stable phases will segregate into separate domains; this raises the question as to the nature of the interaction between SC and magnetic domains. We have begun a neutron scattering study of the magnetic behavior of two such crystals. The oxidation states vary such that in one sample we have a phase separation between a low Tc (30K) SC phase and a striped magnetic phase, while the other features a high Tc (40K) SC phase and striped magnetic phase. Elastic neutron scattering reveals little field dependence of the magnetic peaks in the former, but in the latter we see an enhancement of the magnetic intensity. We also are deriving a method for separating contributions to the inelastic magnetic scattering by the SC and magnetic phases. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J23.00003: Spin order and dynamics in a low doped nickelate Andrei Savici, Igor Zaliznyak, Genda Gu, Vasile Garlea Due to their similarities with high-Tc cuprates, doped 214 layered nickelates attract significant interest. We have recently performed elastic and inelastic neutron scattering experiments studying the low-Sr-doped material La$_1.85$Sr$_0.15$NiO$_{4+\delta}$. This sample has nominal hole concentration similar to that in optimally doped cuprate supeconductors. We observe static spin and charge ordering patterns and dynamic spin correlations, which we will compare to those arising from one dimensional physics expected in the stripe picture. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J23.00004: Detection of the Spin Reorientation and Glassy Dynamics in La$_{1.55}$Sr$_{0.45}$NiO$_4$ Sean Giblin, Paul Freeman, Dharmalingham Prabhakaran, Andrew Boothroyd The magnetism of charge stripe ordered La$_{1.55}$Sr$_{0.45}$NiO$_4$ was studied by the combined techniques of neutron diffraction, muon spin relaxation and mass susceptibility. Magnetic ordering was observed at a lower temperature by muon relaxation than neutron diffraction, indicating a glassy transition to the ordered phase, in agreement with the susceptibility. Neutron diffraction determined a second magnetic transition that is observed by all techniques, to be a spin reorientation. On cooling below $T{\rm_{SR}} = 42$\, K the spins re-orientate from lying 33.7$\pm 0.6 ^{\circ}$ away from the stripe direction at 70\,K to 57.4 $\pm 0.4^ {\circ}$ at 10\, K. The magnetic order was observed by neutron diffraction to be of both anisotropic 3D and 2D (without any correlation on the $c$ axis) character. Muon relaxation confirmed this to be consistent with a single magnetically ordered spin stripe phase. The muon site associated with the spin stripe indicated critical behavior expected from a two dimensional magnetically ordered state, whereas the site associated with the charge stripe showed three dimensional critical behavior indicating spin frustration at the charge stripe. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J23.00005: Anomalous diffusion on a hypersphere and time structure of two-point spin correlations in short-range-ordered doped oxides. Igor Zaliznyak, Georg Ehlers, Genda Gu Recently, much attention was paid to exploring charge and spin- ordered phases in strongly correlated transition metal oxides, such as superconducting cuprates and related nickelates, manganites and cobaltites. Using elastic neutron scattering, we have investigated the nano-scale structure of short-range charge and spin ordering in a half-doped cobablite La1.5Sr0.5CoO4, and found that they can be understood in the framework of an effective anisotropic 3D random field Ising model. Here we report on the measurement of the time-dependence of the spin- spin correlation function in this material by means of Neutron Spin Echo (NSE) spectroscopy. Our high-precision NSE data exclude simple Arrhenius and stretched-exponential relaxations and can be best described by the model of anomalous diffusion on the appropriate hypersphere. We argue that such time dependence is generic for short-range-ordered spin systems. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J23.00006: Transverse displacement modulation of the 1D metallic chains in optimally doped yttrium barium copper oxide superconductor Xuerong Liu, Zahirul Islam, Sunil Sinha, Simon Moss, Jonathan Lang, Ulrich Welp We report on comprehensive quantitative analyses of x-ray diffuse scattering studies of nanoscale inhomogeneities in the optimally doped YBCO superconductor. In addition to previously studied ${\bf q}_0=\left(\frac{1}{4},0,0\right)$ superstructure due to oxygen vacancy ordered ORTHO-IV phase and Huang diffuse scattering due to coherent long-range strain, we present a clear x-ray scattering observation of a transverse displacement modulation of the 1D CuO metallic chains. This modulation co-exists within the well-formed ORTHO-IV patches and persists at temperatures well below $T_c$. Interestingly, the periodicity of this modulation is close to that of $\frac{1}{2k_F}$ according to electronic band calculations. The significance of these modulation and their role in the formation of the electronic inhomogeneities on a nano-meter length scales will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J23.00007: Investigation of a Model for the Magnetic Properties of RuSr$_2$GdCu$_2$O$_8$ based on the Temperature Dependence of M\"{o}ssbauer Spectra D. Coffey, M. DeMarco, B. Dabrowski, S. Kolesnik, M. Maxwell, S. Toorongian, M. Haka M\"{o}ssbauer spectra were measured from 4.2K to 145K on a $^{99}$Ru enriched sample of RuSr$_2$GdCu$_2$O$_8$ which magnetically orders at 138K and has a full transition to superconductivity at 8.7K with an onset at $\sim$13K. The superconducting transition has no effect on the spectrum which is determined by the hyperfine magnetic field. At low temperatures there is a rapid decrease of this hyperfine magnetic field with increasing temperature indicating a gapless magnon spectrum. We use a local moment model which includes coupling between nearest neighbor in-plane Ru moments and between the Ru moments and Gd moments to calculate the magnon spectrum and use this to estimate the strength of the exchange interactions based on the hyperfine field temperature dependence. The coupling strength is $\sim$250K for Ru-Ru coupling and $\sim$ 15K for Ru-Gd coupling. We discuss the possible microscopic origin of these coupling strengths. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J23.00008: Layer Dependence of Charge Distribution and Electronic Structure of HgBa$_2$Ca$_4$Cu$_5$O$_{12}$ Wei-Guo Yin, Dmitri Volja, Wei Ku, Warren Pickett, Deepa Kasinathan Recent experimental observation of layer-dependent properties of the five-layer superconducting cuprate HgBa$_2$Ca$_4$Cu$_5$O$_{12+\delta}$ has inspired intensive interest. In general, multi-layer cuprates are of great interest because the influence of the apical oxygen $p$ states, the main source of the material dependence of cuprate structural and electronic properties [1], could be significantly layer-dependent. In this talk, the layer dependence of the charge distribution and electronic structure of HgBa$_2$Ca$_4$Cu$_5$O$_{12}$ will be investigated via the recently developed many-body Wannier-state method [1]. Possible implications on modulation of local pairing gaps, hole mobility, ``super-repulsion'' [1], and electron-phonon interaction among these distinct CuO$_2$ layers will be discussed, in connection with the intriguing experimental findings of coexisting superconducting and antiferromagnetic orders as well as strong interlayer charge inhomogeneity. Work partially supported by DOE-CMSN. \hspace {0.2cm} [1] W.-G. Yin and W. Ku, cond-mat/0702469. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J23.00009: Electron doping of cuprates via interfaces with manganites Elbio Dagotto, Satoshi Okamoto, Seiji Yunoki, Adriana Moreo, Srivenkateswara Kancharla , Atsushi Fujimori The possible electron doping of some undoped high-$T_c$ cuprates via the transfer of charge from undoped manganites (or other oxides) using heterostructure geometries is discussed theoretically [1]. An analysis of photoemission and diffusion voltage experiments locate the Fermi level of some manganites above the bottom of the upper Hubard band of some cuprate parent compounds. The addition of electrons to antiferromagnetic Cu oxides may lead to a superconducting state at the interface with minimal quenched disorder. Model calculations support this view. [1] S. Yunoki et al., Phys. Rev. B {\bf 76}, 064532 (2007) and references therein. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J23.00010: Tuning Hole Mobility, Distribution and Repulsion in High-$T_c$ Cuprates via Apical Atoms Wei Ku, Wei-Guo Yin Using a newly developed first-principles Wannier-states approach that takes into account large on-site Coulomb repulsion, we derive the low-energy effective one-band interacting Hamiltonians for several prototypical cuprate superconductors. The material dependence is found to originate primarily from the different energy of the apical atom $p_z$ state. Specifically, the general properties of the low-energy hole state, namely the Zhang-Rice singlet, are significantly modified, via additional intra-sublattice hoppings, nearest-neighbor ``super-repulsion,'' and other microscopic many-body processes. Possible implications on modulation of local pairing gaps, charge distribution, hole mobility, electron-phonon interaction, and multilayer effects will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J23.00011: Mid-IR band in cuprates : A consequence of strong electron correlations Shiladitya Chakraborty, Dimitrios Galanakis, Philip Phillips Optical conductivity data in lightly doped cuprates show an anomalous peak - like feature in the mid-IR regime ,not naturally expected of doped Mott insulators. Investigating this phenomenon in the light of strong electron correlations, we employ Cluster Dynamical Mean Field Theory (CDMFT) on a four site square plaquette to compute the optical conductivity in the 2-d Hubbard model as a function of hole doping and temperature. The computed optical conductivity shows a peak in the mid-IR regime, consistent with experimental data. Using Non - Crossing Approximation (NCA) as our impurity solver for CDMFT, we have identified the plaquette eigenstates that give rise to the mid-IR feature. The relevant eigenstate has 4 electrons on a plaquette with zero total spin and spatial properties consistent with $d_{x^2 - y^2}$ symmetry. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J23.00012: Fluctuation corrections to the conductivity and the Hall conductivity near a spin--density--wave quantum critical point Jie Lin, Andrew Millis On the mean field level, the presence of the spin density wave order gives rise to distinct features in the transport properties of the electron--doped cuprates, which agree qualitatively with experimental data. Here, we determine how fluctuations modify the mean field behavior of the conductivity and the Hall conductivity, with particular attention to the vicinity of the quantum critical point. We developed a theory that respects the spin rotation symmetry. We determine the electron--spin fluctuation vertex and thus the transport and spectral properties of the electrons. Our formulation leads to a correctly gauge-invariant description of the transport properties of the pseudogap state occurring for a 2D Heisenberg system at $T>0$. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J23.00013: Inhomogeneous phases of itinerant antiferromagnets Leonid Isaev, Gerardo Ortiz, Cristian D. Batista Although magnetic properties of high-Tc cuprates and heavy fermion compounds has received great attention, only little investigation was performed in the purely antiferromagnetic (AF) phases of these materials. For instance, the mechanism of suppression of the magnetic order was never addressed. In the present work we use a 3D repulsive Hubbard model in an external magnetic field with anisotropic hopping parameters to show that this suppression occurs through a sequence of inhomogeneous states, which are coexisting charge/spin density waves and can be mapped onto the usual Larkin-Ovchinnikov states of type-II superconductors. At the mean-field level we compute the phase diagram of this model as a function of doping and lattice anisotropy. It is shown that morphology of the inhomogeneous phases is determined by the topology of the Fermi surface, which is controlled by the anisotropy (Lifshitz transitions). Insight into the properties of collective modes, such as damping, is gained by computing the magnetic response function in the random phase approximation. Our results are directly applicable to the striped phase of the nickelates and may be useful for understanding the interplay between AF and superconducting orders in the underdoped phase of high-Tc and heavy fermion materials. [Preview Abstract] |
Session J24: Focus Session: Optical Properties of Nanostructures IV: Quantum Dots
Sponsoring Units: DMPChair: Ado Jorio, Universidade Federal de Minas Gerais
Room: Morial Convention Center 216
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J24.00001: Assessment of carrier multiplication in semiconductor nanocrystals by transient photoluminescence spectroscopy Gautham Nair, Scott Geyer, Moungi Bawendi The enhancement of carrier multiplication (CM) is an important aim that could increase solar cell performance and widen the range of materials suitable for future solar technologies. Pump-probe measurements have shown evidence of strongly enhanced CM in lead chalcogenide, InAs, and CdSe nanocrystals (NCs). However, the nature of the enhancement mechanism is not well understood. We have carried out an experimental assessment of CM yields in semiconductor NCs by carefully studying exciton and biexciton signatures in transient photoluminescence decays. In the case of CdSe NCs, though the technique is particularly sensitive due to the biexciton's relatively fast radiative rate, we have found no evidence for CM up to photon energies as high as 3.1 Eg. This result is strongly in disagreement with previous reports on CM in CdSe NCs. The implications of our findings on the efficiency and material dependance of CM are discussed within a general physical framework. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J24.00002: The shape and symmetry dependence of excitonic radiative decay in CdSe nanocrystals Qingzhong Zhao, Alberto Franceschetti, Peter A. Graf, Wesley B. Jones, Kwinseon Kim, Lin-Wang Wang For the last decade, the exciton recombination dynamics of nanocrytal quantum dots (NQDs) through radiative decay have been extensively studied by experimental and theoretical methods, because some NQDs, like CdSe and CdS/ZnS, show near unity quantum yield. Using atomistic, semiempirical pseudopotential calculations, we investigate the radiative decay of band-edge excitons in CdSe nanocrystals with perfect and imperfect shapes. While the lifetimes of bright excitons are in the nanosecond range and not sensitive to size and shape, we find that the radiative lifetimes of the ground state dark excitons are highly dependent on the surface shape and symmetry. The introduction of one [100] or [101] facet can drastically reduce the dark exciton lifetime from milliseconds to microseconds, and such faceting is observed by STEM. This provides an alternative to the explanation by spin-flip assisted or surface-assisted recombination mechanisms of the observed microsecond dark exciton lifetime. Our results highlight the importance of QD surface shape and broken symmetry in exciton dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J24.00003: Peculiar many-body effects revealed in the spectroscopy of highly charged quantum dots M. Ediger, Gabriel Bester, A. Badolato, P. Petroff, K. Karrai, A. Zunger, R. Warburton We have discovered new consequences of Coulomb interactions in self-assembled quantum dots by interpreting experimental spectra with results of atomistic pseudopotential calculations. The Coulomb effects are evident in the photon emission process and we can tune them in situ by controlling the quantum dot charge in the range from $+6e$ to $-6e$. We find two regimes in the same dot: $J \le \Delta E$ for electron charging yet $J \simeq \Delta E$ for hole charging. We discover a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole-hole interactions; promotion-demotion processes in the final state of the emission process; and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J24.00004: Anomalous Polarization Behavior of a Zeeman Doublet in CdSe Nanocrystal Quantum Dots H. Htoon, M. Furis, S. A. Crooker, Al. L. Efros, S. Jeong, V. I. Klimov It is well known that a Zeeman doublet observed in emission spectra of a degenerate quantum state in the case of detection along an applied magnetic field (B field) is characterized by left and right circular polarizations. However, our single nanocrystal quantum dot (NQD) studies conducted in B-fields up to 5 T indicate that the Zeeman doublet of some of the CdSe NQDs exhibits a completely different polarization behavior. Specifically, we observe that the lower-energy state of the doublet becomes increasingly circularly polarized with increasing B field, while the higher-energy state shows a zero degree of circular polarization (i.e remains linearly polarized).~ We explain this anomalous polarization behavior in terms of mixing between the Zeeman split levels derived from the low- and high-energy bright exciton states. This mixing relies on strong long-range electron-hole exchange interactions that are unique to ultrasmall nanocrystals. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J24.00005: Influence of the electronic structure and the multi-exciton spectral density on the multiple exciton generation in semiconductor nanocrystals Christophe Delerue, Guy Allan Several experimental works have reported that a single high-energy photon could generate multiple excitons in semiconductor nanocrystals and several theories are proposed to explain these results. We calculate the electronic structure of InAs, Si and PbSe nanocrystals and we investigate two models of the multiple exciton generation (MEG). We show that the impact ionization process is efficient at high energy, with lifetimes as small as 10 fs. We present simulations of the MEG showing that, in PbSe and Si nanocrystals, the impact ionization alone cannot explain the observed efficiencies, even without relaxation by electron-phonon scattering. We calculate the spectral densities of multi-exciton states and we evaluate the possibility of direct photo-generation of multi-excitons. We confirm the importance of the multi-exciton spectral densities because of their rapid variation over several orders of magnitude. The high MEG efficiencies in PbSe and Si nanocrystals imply a very efficient relaxation in multi-exciton states characterized by a negligible density. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J24.00006: State-resolved optical pumping and single exciton gain in CdSe quantum dots. Ryan Cooney, Patanjali Kambhampati, Samuel Sewall, D.M. Sagar Optical gain in semiconductor quantum dots has been under intense investigation. Optical gain has been difficult to produce unless special geometries were employed such as thin films of CdSe quantum dots, or more recently, type II CdS/ZnSe core /shell structures. The prototypical CdSe quantum dots in dispersion typically show small gain even at extremely high carrier concentrations. The key problem is induced absorptions due to multi-exciton interactions which result in losses that cancel the expected gain. Our recent state-selective approach can be used to prepare initial excitonic states, which has yielded much insight on exciton relaxation dynamics. This approach was used here to generate some of the largest gains ever measured, at the lowest thresholds, for all sizes of CdSe quantum dots in dispersion. These results show that gain in quantum dots is general, if the system is driven correctly. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J24.00007: Optical conditional gates in laterally coupled quantum dots: the role of electron-hole exchange interaction Sophia Economou, Thomas Reinecke We propose a fast, optically induced two-qubit C-PHASE gate in laterally coupled quantum dots. We use a model potential with two asymmetric local minima to account for the difference in size and composition of the two dots. By making use of the excited bound states of the total potential, which extend over both dots and which gives rise to an effective coupling between the two resident electron spins, we avoid the need for an external bias, such as that typically used in vertically coupled dots. The electron-hole exchange interaction is shown to play an important role in our proposal. By lowering the symmetry of the eigenstates, it allows for a simple design of a fast (about 50 ps) C-PHASE gate. The dissipative dynamics of the excited states have been taken into account in our numerical simulation of the fidelity. The calculated fidelity depends on the values of the decay rates. Our proposal is consistent with the single qubit rotations we proposed [Phys. Rev. Lett. \textbf{99}, 217401 (2007)], and the combination of the two allows for universal quantum gates. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J24.00008: Fine Structure of Charged Excitons and Multiexcitons in Self-Assembled InGaAs/GaAs Quantum Dots Vladan Mlinar, Jun-Wei Luo, Gabriel Bester, Alex Zunger As a quantum-dot is loaded with Ne electrons and Nh holes, complex charged excitons and multiexcitons are formed. Their fingerprint is the splitting of each exciton line into a set of multiplets separated by ``fine structure'' splitting, whose calculation [1] poses a serious test to many-body theories. Previously, such fine-structure splittings were calculated and measured for charged excitons with (Ne,Nh) of (2,1), (1,2), (3,1), and (1,3), demonstrating good agreement with experiment. Here, we extend the calculation to both charged and neutral multi-excitons with (Ne,Nh) of (2,2), (3,2), (2,3), (3,3), (4,3), (3,4), and (4,4). The energy splittings, oscillator strength, and polarizations of the optical emission are obtained from many body pseudopotential calculations. We present here predictions for the optical emission from negatively and positively charged biexcitons which reveal fine structure splittings in the order of 100 micro-eV, within the experimental accuracy of single dot micro photoluminescence. We will discuss the evolution of the patterns of multiplet lines, their spacings, and regularities vs. the number of particles (Ne,Nh). [1] M. Ediger, G. Bester, et al., Phys. Rev. Lett 98, 36808 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J24.00009: Absorption spectroscopy of quantum dot molecules Danny Kim, M.F. Doty, M. Bashkansky, M. Scheibner, A.S. Bracker, D. Gammon The energy levels of a vertically-coupled self-assembled InAs/GaAs quantum dot pair are probed using differential transmission spectroscopy. This technique offers very fine spectral resolution ($<0.1~\mu eV$) allowing us to resolve the linewidths and fine structure for the various energy levels found in the rich spectrum of coupled quantum dots. For example, we observe increased broadening of the neutral exciton as it approaches the anticrossing point, as a result of a non-zero tunneling term well away from resonance. Excitons particular to coupled dots ---i.e. a positive trion/biexciton, where the additional hole is on the spectator dot--- exhibit polarization and power-dependent behavior that is in marked contrast to their single dot counterparts. Finally, the occupation of these exciton states are manipulated by using a second laser that is resonant on a related energy level. These experiments are a crucial step in using these molecules for coherent nonlinear optical processes. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J24.00010: F\"{o}rster optical signatures in quantum dot molecule photoluminescence Juan E. Rolon, Sergio E. Ulloa We formulate a realistic model that predicts the optical signatures of the F\"{o}rster resonant energy transfer processes (FRET) in InAs/GaAs self-assembled quantum dot molecules (QDMs) in presence of laser illumination and electric fields. We study the time evolution of a multiexcitonic Hamiltonian and construct a map of its dressed spectrum, resulting in effective coupling of the different states under laser illumination. In addition to interdot hole- and electron-tunneling, FRET is found to be an important quantum coupling mechanism in QDMs. We find FRET optical signatures to be highly dependent on structural parameters and severely constrained by the narrow spectral overlap between excitonic transitions in the donor-acceptor pair. However, detailed analysis of the orbital character of the localized hole reveals that it is possible to obtain strong spectral overlap between the first excited single exciton level in the donor dot and the lowest energy single exciton level in the acceptor dot. Remarkably, although FRET occurs via a single pair of exciton levels, its effects are evident throughout the calculated dressed spectrum. We observe a redistribution of spectral weights of direct and Stark shifted exciton lines, and a set of anticrossings among exciton states not directly coupled by FRET. Our results suggest experimental schemes to quantify FRET in photoluminescence experiments. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J24.00011: Excited-state absorption and quantum confined Stark effect in embedded silicon and germanium nanocrystals Ceyhun Bulutay Realistic-sized Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices are studied theoretically using an atomistic pseudopotential Hamiltonian. Based on this electronic structure, first the interband absorption is studied which shows the importance of surface polarization effects that significantly reduce the absorption when included. This reduction is found to increase with decreasing NC size or with increasing permittivity mismatch between the NC core and the host matrix. The intraconduction and intravalence band absorption coefficients are also obtained in the wavelength range from far-infrared to visible region. Next, excited-state absorption at three different optical pump wavelengths, 532 nm, 355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong absorption windows in the case of holes and a broad spectrum in the case of electrons which can especially be relevant for the discussions on achieving gain in these structures. Finally, the interband absorption of NCs is studied under high DC electric field causing the quantum-confined Stark effect. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J24.00012: Level Anti-Crossing Spectroscopy - Optically Mapping the Electronic Structure of Coupled Quantum Dots M. Scheibner, M. Yakes, A.S. Bracker, I.V. Ponomarev, M.F. Doty, C.S. Hellberg, L.J. Whitman, T.L. Reinecke, D. Gammon We introduce an all optical level anti-crossing spectroscopy (LACS) with which the ground \textit{and} the excited state energy levels of quantum dots (QDs) can be measured for a hole(electron) by itself and in the presence of other charges.$^{1}$ Analogies are drawn to the atomic shell-model and connections are made in an average way to structural STM measurements. An applied bias provides an electric field between two InAs/GaAs QDs which ``tilts'' the energy levels of both QDs relative to each other. Molecular resonances between energy levels of the two QDs are measured purely optical as sequences of anti-crossing patterns in the electric field dependent PL spectrum. These sequences provide in situ characterization of the level structure. We anticipate that such measurements will precede more sophisticated quantum control demonstrations, allow precise reverse engineering and boost detailed theoretical modeling of QD structures. This work is supported by NSA/ARO. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J24.00013: Charged Quantum Dots in High Quality Micropillar Cavities Matthew Rakher, Nick Stoltz, Larry Coldren, Pierre Petroff, Dirk Bouwmeester We report on nanodevices that for the first time allow for charge tuning of single InAs quantum dots located near the field maximum of high quality micropillar cavities. Through the innovation of a novel trench style cavity design, we are able to embed doped layers for electrical gating within a microcavity and obtain Q values greater than 50,000. Using these devices, we demonstrate record high single photon count rates with a capture efficiency of 38{\%} and a Purcell effect up to 8. We also show high frequency polarization modulation of single photons enabled by Stark shift tuning a charged quantum dot between two polarization modes of a slightly elliptical micropillar with frequencies up to 100 KHz. Furthermore, we demonstrate a charge tunable quantum dot coupled to a micropillar cavity mode, which is an important step in quantum communication protocols involving trapped single electrons or holes. This type of device enables a quick, non-destructive measurement of the spin state of the trapped charge. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J24.00014: Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays Suresh Sharma, Jay Murphree, Tonmoy Chakraborty, Ajani Ross, Cecil Shive We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under $\sim $ 100 W/cm$^{2}$ laser radiation, the PL intensity (I$_{PL})$ increases with irradiation time upto about 500s and thereafter declines linearly. The wavelength of the PL emission ($\lambda _{peak} )$ exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm$^{2}$ 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I$_{PL}$ and $\lambda _{peak} $ are significantly different; I$_{PL}$ increases to a saturation level, and the magnitude of the blue-shift in $\lambda _{peak} $ is reduced. We also present data on the effects of the density of the quantum dots on the temporal behavior of the PL spectra, as well as additional data on samples synthesized with CdSe/ZnS quantum dots embedded in conducting polymer films. We discuss possible mechanisms underlying our observations. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J24.00015: Control of sp-d exchange interactions in pseudo-type II Mn:ZnSe/CdSe core-shell nanocrystal quantum dots David Bussian, Ming Yin, Scott Crooker, Victor Klimov Dilute magnetic semiconductors (DMSs) have been the focus of considerable research due to their potential usability in spin-based electronic devices. Unpaired electrons of dopant atoms, such as Mn$^{2+}$, can couple strongly to electrons of the semiconductor (sp-d exchange interaction), which should allow for the manipulation of the spin degree of freedom using traditional microelectonic circuitry. We have developed a novel approach for manipulating sp-d interactions between the dopant and the semiconductor wherein Mn ions are incorporated into cores of ZnSe/CdSe core-shell semiconductor nanocrystal quantum dots (NQDs). These NCs represent quasi-type II hetero-structures that allow one to tune both the band edge transition energy and dopant-carrier wavefunction overlap by changing the size of the core and/or shell thickness. We will report our recent results from a set of doped heterostructures for which we demonstrate tunability of both the magnitude and the sign of the sp-d exchange interaction energy as a function of hetero-NQD geometry. [Preview Abstract] |
Session J25: Focus Session: Biopolymers: Molecules, Solutions and Networks I
Sponsoring Units: DPOLY DBPChair: Paula Hammond, Massachusetts Institute of Technology
Room: Morial Convention Center 217
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J25.00001: Conformation and Trapping of DNA at a Convergent Stagnation Point Jennifer Kreft, Yeng-Long Chen, Hsueh-Chia Chang We use a lattice-Boltzmann based Brownian dynamics simulation to investigate the elongation of DNA at a convergent stagnation point trapped by a uniform attractive potential. Surprisingly, we find that the coiled state is favored over the stretched state at high Peclet number, $Pe$. The final elongation is determined by conformation changes during transport to the stagnation point, rather than hydrodynamic stretching at that point. The trapping rate of the DNA is consistent with the classical mean-field convection-diffusion theory and scales as $Pe^{1/3}$. This scaling is insensitive to the attractive potential used. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J25.00002: DNA Surface Hybridization Regimes Rastislav Levicky, Ping Gong Surface hybridization reactions, in which sequence-specific recognition occurs between immobilized and solution nucleic acids, are routinely carried out to quantify and to interpret genomic information. At a surface, molecular interactions are amplified by the two-dimensional nature of the immobilized layer which focuses the nucleic acid charge and concentration to levels not encountered in solution, and which impacts the hybridization behavior in unique ways. We find that, at low ionic strengths, an electrostatic balance between the concentration of immobilized oligonucleotide charge and solution ionic strength governs the onset of hybridization. As ionic strength increases, the importance of electrostatics diminishes and the hybridization behavior becomes more complex. Suppression of hybridization affinity constants relative to solution values, and their weakened dependence on the concentration of DNA counterions, indicate that the immobilized strands form complexes. Moreover, an unusual regime is observed in which the surface coverage of immobilized oligonucleotides does not significantly influence the hybridization behavior, despite physical closeness and hence compulsory interactions between sites. These results are interpreted and summarized in a diagram of hybridization regimes. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J25.00003: Hybridization Pathways and Mechanisms of Model DNA Oligonucleotides in Solution. Juan Araque, Athanassios Panagiotopoulos, Marc Robert We propose a coarse-grained lattice model of short DNA strands to investigate the microscopic pathways and mechanisms of oligonucleoides hybridization in solution. The extent to which hybridization in solution conforms to two-state thermodynamics is also analyzed. Monte Carlo simulations with parallel tempering are performed to estimate the equilibrium population of single- and double-stranded states and the associated free-energy landscapes. Sequence complexity is shown to largely dominate the nucleation and helix propagation pathways. The two-state nature of the transition is found to exhibit strong sequence dependence. Although the model predictions are consistent with classical cooperativity signatures, stable intermediates appear, in violation of the two-state assumption. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J25.00004: Structure and applications of a temperature responsive recombinant protein hydrogel based on silk- and elastin-like amino acid motifs Lawrence Drummy, Melanie Tomczak, Joseph MacAuliffe, Richard Vaia, Rajesh Naik Proteins form the main components of many natural materials, and they can be designed to offer tailored functionality and material properties. Silk elastin-like proteins (SELP)s come from a family of repeat sequence protein polymers based on \textit{Bombyx mori} silk and mammalian elastin that are recombinantly expressed in \textit{E. coli}. SELP gels are formed by heating the protein solutions in order to induce physical crosslinking of the silk $\beta $-sheet regions, they contain approximately 80-90{\%} water by weight and they can be used for encapsulation of enzymes or nanoparticles. For example, horseradish peroxidase demonstrates added resistance to drying and heat treatment when encapsulated in the gel matrix. During gel formation, small angle X-ray scattering shows intensity increases in two distinct regions of reciprocal space, one reversible with temperature and one irreversible. By fitting the scattering data to a unified power-law/Gunier model, morphological parameters are extracted. The thermally reversible intensity changes are attributed to a hydrophilic/hydrophobic transition in the elastin segments, while the irreversible intensity change is due to the crystalline regions formed by the silk blocks. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J25.00005: Fractal Nature of Semiflexible Networks in beta-Hairpin Peptide Hydrogels Rohan Hule, Darrin Pochan De novo designed beta hairpin peptides with asymmetric beta strands, capable of self-assembly and hydrogel formation, were investigated. The stimuli responsive self-assembly occurs via a strand interdigitation mechanism, resulting in physically crosslinked fibrillar networks. Fibrils with distinct nanostructures varying from non-twisted, twisted to laminated morphologies were rationally designed by modulating the peptide strand registry. The fractal dimension and correlation lengths of these networks, both, at the network as well as individual fibril length scales varies significantly with concentration and is directly related to the fibril morphology, as evidenced by SANS and cryogenic TEM. In case of the laminated fibrils, an increase in the peptide concentration induces a change from surface to mass fractal behavior at high q due to the disruption of fibril lamination as a result of faster assembly kinetics from higher peptide concentration. Non-twisting peptide fibril morphologies exhibit an increase the network density with higher peptide concentration and, therefore, an increase in mass fractal dimension. Oscillatory rheology of hydrogels reveals enhanced moduli for laminating networks over non-twisting or twisting networks. These interdigitating peptides constitute a model system to study structure-property relations in other semiflexible networks. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J25.00006: A molecular model for toughening in double-network hydrogels Wen-li Wu, Vijay Tirumala, Taiki Tominaga, Sanghun Lee, Paul Butler, Eric Lin, Jian Ping Gong, Hidemitsu Furukawa A molecular mechanism is proposed for the toughness enhancement in double network (DN) hydrogels prepared from poly (2{\-}acrylamido, 2-methyl,1-propanesulfonicacid) (PAMPS) polyelectrolyte network and polyacrylamide (PAAm) linear polymer. It is an extension of the phenomenological model set forth recently by Gong \textit{et al}. This mechanism takes into consideration all the observed changes in molecular structure of the constituents via \textit{in-situ} small angle neutron scattering (SANS) measurements, the composition dependence of the solution viscosity and the thermodynamic interaction parameters of PAMPS and PAAm molecules from our previous neutron scattering studies. More specifically, this proposed mechanism provides an explanation of the observed periodic compositional fluctuation in the micrometer range induced by large strain deformation. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J25.00007: De novo designed peptide and peptide-polymer conjugate for biomolecular materials Invited Speaker: Peptides, nature's ``own'' building blocks, provide control of functional groups over nanometer distances with sub-Angstrom resolution and can be de novo designed to self-assemble into multidimensional molecular constructs that mimic natural proteins or perform functions not found in nature. Conjugating synthetic polymers to peptides, forming peptide-polymer conjugates, takes advantages of both the stability and processibility of synthetic polymers and the built-in peptide functions. Helical bundles, a ubiquitous folding motif, underpin many structural and catalytic functions of natural proteins. By attaching a polymer chain to a helical bundle-forming peptide, the polymer chain will mediate the interactions between the helical bundle and its external environment, enable the macroscopic self-assembly and, potentially, allow the helical bundle to function in non-biological environments. A new design of peptide-polymer conjugates will be presented where the polymer chain is covalently linked to the side chain of the peptide. Upon attaching poly(ethylene glycol) (PEG) to the exterior of the helix bundle, the peptide secondary structure and also the tertiary structure, i.e. coiled-coil helix bundle formation, are stabilized. More importantly, using a photoactive heme-binding 4-helix bundle peptide as an example, this new design preserves the built-in functionalities in the interior of the helix bundle. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J25.00008: Single polymer stretching in elastic turbulence of polymer solution Yonggang Liu, Victor Steinberg Coil-stretch transition of single T4 DNA molecule in an elastic turbulence is studied in a polymer solution with the same molecules. Two mechanisms of saturation of polymer stretching in elastic turbulence, the nonlinearity of polymer stretching and the back reaction of stretched polymer chains to the flow, are demonstrated based on experiments of single polymer dynamics at different polymer concentrations. The elastic stress calculated from single polymer stretching agrees with the PIV measurement of the flow properties, indicating that polymer stretching can be used as an elastic stress probe of the flow properties. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J25.00009: Shape and conformation of confined biopolymers. Ya Liu, Bulbul Chakraborty Biological macromolecules living in cells are confined on length scales comparable to their intrinsic persistence length. In these environments, the bending rigidity plays a decisive role in determining shape and conformations. We have used numerical simulations to investigate the statistical properties of a semiflexible polymer confined in a square box. Simulations exhibit a shape transition when the bare persistence length becomes comparable to the box size. An order parameter is introduced to quantify and analyze the nature of this transition. The shape change is accompanied by a qualitative change in the effective persistence length, which starts differing significantly from the intrinsic persistence length. A mean-field model, including Gaussian fluctuations around the saddle point solution, provides a quantitative description of the evolution of the tangent-tangent correlation function with increasing confinement. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J25.00010: Raft Formation of Rod-like Polyelectrolytes Daniel W. Sinkovits, Erik Luijten We investigate the formation of raft-like aggregates by charged rod-like polyelectrolytes, as reported from experiments employing F-actin as well as from theoretical analyses. Through extensive molecular-dynamics simulations of pairs of rods at different salt concentrations we construct free-energy landscapes, which in turn elucidate the most likely kinetic pathways to aggregation. Additional simulations of layers of rods at varying skew orientations and lateral spacings demonstrate to what extent the lessons learned from pair simulations apply to large aggregates. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J25.00011: Vapor-liquid coexistence of patchy attractive fluids: Wertheim theory study Hongjun Liu, Sanat Kumar, Glenn Evans Our system consists of spherical particles whose pair potential contains hard core repulsion, short-ranged square well attraction and several distributed attractive patches on its surface. The simplicity of the model makes it possible to compare simulations and theoretical predictions based on Wertheim's thermodynamic perturbation theory (TPT). Wertheim's TPT is in good agreement with simulation data. In the broader parameter ranges, we show the patchy hard sphere fluids obey a generalized law of corresponding states (GLCS) and GLCS seems to hold for all patchy square well fluids with four to six interaction sites. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J25.00012: Strong Keratin-like Nanofibers Made of Globular Protein Yael Dror, Vadim Makarov, Arie Admon, Eyal Zussman Protein fibers as elementary structural and functional elements in nature inspire the engineering of protein-based products for versatile bio-medical applications. We have recently used the electrospinning process to fabricate strong sub-micron fibers made solely of serum albumin (SA). This raises the challenges of turning a globular non-viscous protein solution into a polymer--like spinnable solution and producing keratin-like fibers enriched in inter S-S bridges. A stable spinning process was achieved by using SA solution in a rich trifluoroethanol-water mixture with $\beta $-mercaptoethanol. The breakage of the intra disulfide bridges, as identified by mass spectrometry, together with the denaturing alcohol, enabled a pronounced expansion of the protein. This in turn, affects the rheological properties of the solution. X-ray diffraction pattern of the fibers revealed equatorial orientation, indicating the alignment of structures along the fiber axis. The mechanical properties reached remarkable average values (Young's modulus of 1.6GPa, and max stress of 36MPa) as compared to other fibrous protein nanofibers. These significant results are attributed to both the alignment and inter disulfide bonds (cross linking) that were formed by spontaneous post-spinning oxidation. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J25.00013: Electrospinning of Natural Polymers Aihua He, Shanshan Xu, Huarong Nie, Junxing Li, Charles C. Han Electrospinning is an effective and simple method to fabricate polymer fibers in the range of nano scale. However, electrospinning of natural polymers is a challenge. The key reason for this problem is that natural polymers have very different chain conformation and hydrodynamic responses in solution, especially in aqueous solution, when compared with synthesized polymers. The objective of our study is to find the key parameters in order to have a good control in the electrospinning process. We studied the electrospinnings of gelatin from its aqueous solution, of hyluronic acid without airblowing, and of pure alginate. It was found that electrospinning of those natural-polymer solutions could be successfully carried out when key parameters were properly adjusted, such as viscosity, elasticity (chain entanglement) and sufrace tension. [Preview Abstract] |
Session J26: Focus Session: Quantum Control I
Sponsoring Units: DCPChair: Vlasta Bonacic-Koutecky, Humboldt-Universitaet zu Berlin
Room: Morial Convention Center 218
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J26.00001: Quantum Control of Femtochemistry in the Gas Phase, Liquid Phase and on Surfaces Invited Speaker: By using coherent control techniques we control the behavior of quantum systems on their natural fs-time scale by applying ultrashort coherent light fields in the wavelength range from the IR to the UV. These laser pulses can be variably shaped in space and time using a laser pulse shaper consisting of a liquid-crystal display [1]. Laser-optimized femtochemistry in the gas phase and liquid phase is one field in which this new technique is successfully employed. Automated optimization of branching ratios and total product yields of gas phase photodissociation reactions as well as chemically selective molecular excitation in the liquid phase is performed~[2][3]. Structural changes of a molecule in the liquid phase have been controlled by laser-optimized photoisomerization of a cyanine dye molecule [4] and of retinal in bacteriorhodopsin [5]. So far, optimal control techniques have been restricted to gas phase and condensed phase optimization experiments. Recently we have demonstrated femtosecond laser-assisted catalytic reactions on a Pd(100) single crystal surface. By applying a closed-loop optimal control scheme, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage. The results represent a first step towards selective photocatalysis of molecules. \newline [1] T. Baumert et al, Appl. Phys. B 65, 779 (1997) \newline [2] A. Assion et al, Science 282, 919(1998); T. Brixner et al, J. Mod. Opt. 50, 539 (2003) \newline [3] T. Brixner et al, Nature, Vol. 414, 57 (2001) and J.~Chem. Phys. 118, 3692 (2003) \newline [4] G. Krampert et al, Phys. Rev. Lett. 94, 068305 (2005) \newline [5] G. Vogt et al, Chem. Phys. Lett. 433, 211 (2006) P. Nuernberger et al, Phys. Chem. Chem. Phys. 9, 2470 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J26.00002: Quantum control spectroscopy with multipulses Invited Speaker: The manipulation of molecular vibrations by laser light has been always considered as a very promising means to control chemical reaction. The coherently controlled time-dependent superposition of vibrational states may represent motion along a reaction coordinate and therefore allows for a high degree of selectivity. Pulse shapes for manipulating vibrations can be predicted to be trains of pulses with temporal spacing between the sub-pulses equal to an integer of the vibrational phase. If the manipulation of molecular vibrations with pulse trains is expected to be one of the important mechanisms on the long standing aim of mode selective chemistry, it is necessary to understand its application limits. In this work, the interaction of pulse trains with matter is discussed under the light of time-resolved nonlinear experiments and density matrix simulations. Emphasis is given to the role of electronic coherence between excited and ground-state, to the excited state population relaxation time and to the electronic resonance. In particular the lifetime of the excited state poses a challenge for the coherent control with multipulses and, thus, for the mode filtering capability in the excited state. This is investigated by applying a shaped femtosecond excitation pulse to different molecules in solution and probing the response by transient absorption, nonlinear Raman and DFWM spectroscopy. Finally, the effect of the phase of sinusoidal modulation on the envelope of the multipulse sequence and its consequences on pump-probe spectroscopy is discussed, particularly near zero delay between pump and probe pulses. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J26.00003: Strategies for optimal control in complex systems Invited Speaker: We present strategies for the optimal control of the ground and excited state dynamics in complex systems, based on the combination of the quantum chemical molecular dynamics ``on the fly'' with the semiclassical Wigner distribution approach [1]. We first demonstrate our strategy for the optimal control of the ground state dynamics based on the MD ``on the fly'' with explicit treatment of the interaction with the laser field which is optimized using a genetic algorithm [2]. This approach will be illustrated on two prototype systems representing rigid symmetrical molecules and floppy biomolecules with low frequency modes. Our results show that the ground state isomerization process can be selectively driven by ultrashort laser pulses with different shapes which are characteristic for the prototype systems. Furthermore, for the optimal pump-dump control involving ground and excited electronic states we have developed a new ``field induced surface hopping'' method in which the nuclear dynamics is treated classically while the laser induced electronic transitions are treated fully quantum mechanically. We illustrate our approach on the optimal control of cis-trans isomerization in prototype Schiff base molecular switches. Our theoretical approach allows us to explore the controllability of dynamics in complex systems and to unravel the mechanisms underlying the control of molecular processes. Furthermore, the outlook for laser selective photochemistry of nanoparticles and nanoparticle-biomolecule hybrid systems will be given. \newline \newline [1] V. Bona\v{c}i\'c-Kouteck\'y, R. Mitri\'c , Chem. Rev. 105, 11 (2005). \newline [2] R. Mitri\'c, V. Bona\v{c}i\'c-Kouteck\'y, Phys. Rev. A, 76, 031405 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J26.00004: Phase control of molecular fragmentation with a pair of femtosecond-laser pulses Karl-Michael Weitzel, Georg Breunig, Gunter Urbasch We demonstrate the control of molecular fragmentation on a femtosecond-time scale in two-pulse measurements with a pair of femtosecond-laser pulses. The measurements were performed with o-xylene (C$_{8}$H$_{10})$. Parent and fragment-ion yields were recorded as a function of inter-pulse delays, i.e. different relative phases of the excitation pulses. The experiments revealed different fragmentation mechanisms in the temporal region of direct optical overlap and for separated pulses. For overlapping pulses all ion yields followed the excitation intensity which oscillated as a function of inter-pulse delay due to the change of constructive and destructive interference. For larger delays, in particular the oscillations of the C$^{+}$ and CH$_{3}^{+}$ fragment-ion yield showed a significant deviation from each other. This deviation vanished in measurements with chirped femtosecond-laser pulses where both parent and fragment-ion yields oscillated in phase for all investigated delays. The results are interpreted as a manifestation of optical phase-dependent electronic excitations mapped onto the nuclear fragmentation dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J26.00005: Coherent control on cold alkali systems Albrecht Lindinger Optimal control of photo-induced molecular processes has attained considerable success in recent years. An important issue in this regard is the information coded in the optimized laser pulse shape which supplies insight about the underlying processes. Small alkali systems are suitable since they exhibit bound states available for resonant transitions with weak fields which aids the theoretical description and thus the interpretation. New control methods are presented to extract the most relevant information from the optimized laser field. Moreover, novel pulse shaper schemes for simultaneous phase, amplitude, and polarization pulse control were designed and applied to alkali dimers, even in a parametric encoding. The results demonstrate the perspectives of adding the polarization and hence all properties of the light field in the pulse modulation. Currently, coherent control was applied to ultracold ensembles motivated by the perspective to perform photoassociation and photostabilization of alkali systems. First results are received regarding optimized multi-photonic excitation to molecular ions and pump-probe experiments exposing signal oscillations. They provide indications for photoassociation and open the perspective for transitions to lower vibrational levels in the electronic ground state, which would be a first step to an internally cold molecular Bose Einstein condensate. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J26.00006: Direct frequency comb measurement and control of vibrational dynamics in ultracold molecular samples Avi Pe'er, Evgeny Shapiro, Moshe Shapiro, Jun Ye We propose a new class of control schemes for robust transfer of population between quantum states via a wave packet that utilize trains of coherent pulses (optical frequency comb). Our approach draws from analogy to adiabatic passage techniques in three-level systems, but is more general. We show that breaking a slow adiabatic passage into a train of short, perturbative pulses, enables highly efficient population transfer between single states through an arbitrary wave packet. Alternatively, it is possible to directly deduce the intermediate multi-state structure by a simple scan of the pulse train parameters (repetition rate and envelope phase), in a method similar to two-dimensional Fourier spectroscopy. Viewed in the spectral domain, these techniques rely on quantum pathway interference in an adiabatic passage. The scheme is most suitable for applications in cold and ultracold molecular samples. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J26.00007: Formation of a gas of ultracold LiCs molecules J. Deiglmayr, J. Lange, S.D. Kraft, A. Grochola, R. Wester, M. Weidem\"uller, M. Aymar, O. Dulieu Ultracold polar molecules offer intriguing perspectives for the study of many-body effects in strongly interacting gases and the manipulation by external fields. A promising approach to the creation of a large ensemble of ultracold polar molecules in the absolute translational and electronic ground state is the direct formation of ultracold molecules through photoassociation of ultracold atoms. We recently observed the spontaneous formation of ultracold LiCs molecules in a double species magneto optical trap. After spontaneous decay into the electronic ground state, the molecules were ionized by one-color two-photon ionization and detected with a high-resolution time-of-flight mass spectrometer~[1]. Here we present the active photoassociation of ultracold LiCs molecules and discuss the state distribution of the produced ground state molecules. Precise knowledge of the molecular structure is required to find the most efficient route for the creation of molecules. We present ab-initio calculations of excited molecular states of LiCs including spin-orbit coupling and study the alignment and orientation of bialkali molecules in combinations of static electric fields and strong laser fields. The perspectives for the production of molecules in the absolute ground state are evaluated. [1] S. D. Kraft \textit{et al.}, J. Phys. B {\bf 39}, S993 [Preview Abstract] |
Session J27: Focus Session: Triangular Lattice
Sponsoring Units: GMAGChair: Andrew Cornelius, University of Nevada, Las Vegas
Room: Morial Convention Center 219
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J27.00001: Mean filed study of disordered spin-$1\over 2$ antiferromagnetic systems Vladimir Dobrosavljevic, Sen Zhou, Eduardo Miranda We present a mean filed theory picture of disordered spin-$1\over 2$ antiferromagnetic system as a function of the degree of disorder, in connection to the insulating doped semiconductors. The system is a resonant valence bond (RVB) liquid state at zero disorder, and a possible RVB glass state when the disorder is finite but weak. For a highly disordered system, we show that the essential physics is the formation and decimation of strongly coupled bonds, and the thermodynamics shows an effective power-law singularity, in qualitative agreement with renormalization group result of Bhatt and Lee. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J27.00002: Dynamical properties of spatially anisotropic frustrated Heisenberg models in a magnetic field Masanori Kohno, Leon Balents, Oleg. A. Starykh We investigate spectral features of spatially anisotropic spin-1/2 frustrated antiferromagnets in a magnetic field in terms of fractional excitations. Restricting the Hilbert space to that spanned by the eigenstates of the Heisenberg chain [1], we calculate dynamical structure factor $S$(\textbf{\textit{k}},$\omega )$ in the two-dimensional momentum space. We obtain sharp peaks and broad continuum depending on the momentum. We interpret the sharp peaks as signatures of bound states of psinons and antipsinons, fractional excitations in a magnetic field. We compare these spectral features with available experimental results on Cs$_{2}$CuCl$_{4}$ [2], and make detailed predictions on momentum- and field-dependence of peak structures and line shapes of $S$(\textbf{\textit{k}},$\omega )$ for Cs$_{2}$CuCl$_{4}$ in a magnetic field. [1] M. Kohno, O. A. Starykh, and L. Balents, Nature Phys. \textbf{3}, 790 (2007). [2] R. Coldea, \textit{et al}., Phys. Rev. Lett. \textbf{79}, 151 (1997). [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J27.00003: Quantum Order by Disorder in Frustrated Diamond Spinel Antiferromagnets Jean-Sebastien Bernier, Michael J. Lawler, Yong Baek Kim We study the effect of quantum fluctuations on the frustrated diamond lattice antiferromagnet where frustration arises from the presence of second neighbor interactions. Such an antiferromagnet describes the magnetic properties of spinel AB$_2$X$_4$ where magnetic ions are located on A-sites. We compare the resulting phase diagram of the quantum model and that of its classical counterpart, and discuss the difference/similarity between the quantum and thermal order by disorder phenomena. Implications for experiments on CoRh$_2$O$_4$, Co$_3$O$_4$ and MnSc$_2$S$_4$ will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J27.00004: Random Fields and the Partially Paramagnetic State of CsCo$_{0.83}$Mg$_{0.17}$Br$_{3}$ John-Paul Castellan, B.D. Gaulin, W.J.L. Buyers Partially paramagnetic Neel states are among the exotic magnet states known to exist in nature as a consequence of geometrical frustration. This unusual magnetic structure occurs in the stacked triangular lattice antiferromagnets such as CsCoBr$_{3}$ and CsCoCl$_{3}$. CsCoBr$_{3}$ displays at least 2 magnetic phase transitions. The first, T$_{n1}\sim $28K where the system enters a 3-sublattice state in which on of the sublattices remains disordered and the second, T$_{n2}\sim $13K where the remaining disordered sublattice orders[1]. Critical neutron scattering measurements were performed on the doped system CsCo$_{(1-x)}$Mg$_{(x)}$Br$_{3}$ with x =0.17. We will discuss the evolution of the observed two component scattering below T$_{n1}$ in terms of a Random Field Ising model in both zero applied magnetic field and an applied magnetic field of 2.6T along the c-axis. [1] M.Mao et al. Phys. Rev. B 66, 184432 (2002). [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J27.00005: New ordered phases of the spin-1/2 triangular-lattice antiferromagnet Cs$_{2}$CuBr$_{4}$ Nathanael Fortune, Scott Hannahs, Yasuo Yoshida, Yasu Takano, Toshio Ono, Hidekazu Tanaka Quantum fluctuations and geometric frustration are theoretically expected to produce a gapped, collinear 'up-up-down' phase in spin-1/2 Heisenberg and XY antiferromagnets on a triangular lattice. Experimentally, this phase should manifest itself as a magnetization plateau at 1/3 of the saturation value. Despite being a fundamental theoretical property of such systems, this behavior has to date only been observed in one triangular lattice antiferromagnet: Cs$_{2}$CuBr$_{4}$. We have investigated the magnetic phase diagram of this compound by means of specific-heat, magnetocaloric-effect, and magnetic-torque measurements in magnetic fields up to the saturation field of about 30 T, finding a cascade of new ordered phases adjacent to the up-up-down phase. The evolution of these phases as a function of the field orientation with respect to the crystallographic bc plane suggests that they arise from the competition between the scalar exchange interaction and the symmetry-breaking Dzyaloshinskii-Moriya interaction. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J27.00006: Dynamics of the Spin Liquid Phase of Cs$_2$CuCl$_4$ Ookie Ma, Marc-Andre Vachon, Vesna F. Mitrovi{\'c}, Brad Marston The dynamics of a spin-liquid phase of an antiferromagnet on the anisotropic triangular lattice and in a magnetic field are studied with a combination of Gutzwiller-projected wavefunctions and mean-field theory. Candidate ground states that support fermionic gapless spinon excitations include four different U(1) spin liquids\footnote{Y. Zhou, X. G. Wen, cond-mat/0210662 (2003).}. The lattice and the states interpolate between limiting cases of 1D decoupled chains ($J/J^{\prime} = 0$) and the isotropic 2D square lattice ($J/J^{\prime}= \infty$). Parameters of the mean field theory are chosen to minimize the ground state energy of the corresponding Gutzwiller-projected wavefunction. The spin-lattice relaxation rate $1/T_1$, calculated within the mean-field approximation, is compared to NMR measurements\footnote{M. A. Vachon, O. Ma, J. B. Marston, V. F. Mitrovi{\'c}, unpublished (2007).} in the spin liquid phase of Cs$_2$CuCl$_4$\footnote{Y. Tokiwa, T. Radu, R. Coldea, H. Wilhelm, Z. Tylczynski, F. Steglich, PRB 73, 134414 (2006).}. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J27.00007: Photoemission study of triangular lattices in NiGa$_2$S$_4$, FeGa$_2$S$_4$, and Fe$_2$Ga$_2$S$_5$ Kou Takubo, Takashi Mizokawa, Yusuke Nambu, Keisuke Onuma, Hiroshi Tonomura, Osamu Sakai, Satoru Nakatsuji, Yoshiteru Maeno The newly-discovered NiGa$_2$S$_4$ (Ni$^{2+}$, $S$=1) and FeGa$_2$S$_4$ (Fe$^{2+}$, $S$=2) form frozen spin-disordered states within the triangular lattice [1,2]. The spins of both compounds have no long range order even at lowest temperature. We have performed photoemission spectroscopy of NiGa$_2$S$_4$, FeGa$_2$S$_4$, and Fe$_2$Ga$_2$S$_5$. The photoemission results and subsequent model calculations indicate that the ground state of NiGa$_2$S$_4$ has the $d^9L$ character ($L$ is a S 3$p$ hole) and that the strong S 3$p$ hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites. In contrast, the ground state of FeGa$_2$S$_4$ is dominated by the $d^6$ configuration and the superexchange interactions between the second and third neighbor sites are less important. [1] S. Nakatsuji, {\it et al}., Science 309, 1697 (2005). [2] S. Nakatsuji, {\it et al}., Phys. Rev. Lett. 99, 157203 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J27.00008: The origin of anomalous 3rd neighbor exchange in 2D triangular magnets (NiGa2S4 and others) Igor Mazin 2D magnetic materials with triangular lattices have been attracting much interest. Among them one finds the parent compound of an exotic superconductor, Na$_x$CoO$_2\cdot y$H$_2$O, A-type antiferromagnets like NaNiO$_2$, in-plane antiferromagnetism (LiCrO$2$), spin-liquid type materials (NiGa$_2$S$_4$), charge-order (AgNiO$_2$). The main structural motif in all of them is the AB$_2$ plane, where A is a transition metal and B is oxygen or sulfur. Experiments and calculations inevitably find anomalously strong 3rd neighbor exchange coupling in all these triangular planes, despite different band fillings and different magnetic ground states. I will explain why this happens, why this effect is so universal, and why it can be understood entirely on a one-electron level. I will use as an example NiGa$_2$S$_4$, with a reference to Na$_x$CoO$_2$ as well. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J27.00009: Elementary excitations in the spin liquid phase of Cs$_2$CuCl$_4$ as revealed by $^{133}$Cs NMR spin-lattice relaxation rate measurements. Marc-Andre Vachon, Georgios Koutroulakis, Ookie Ma, Brad Marston, Vesna F. Mitrovic, Arneil P. Reyes, Philip L. Kuhns, Radu Coldea, Z. Tylczynski We present $^{133}$Cs NMR spin-lattice relaxation rate measurements in the spin liquid phase of Cs$_2$CuCl$_4$ as a function of temperature and external magnetic field. At fixed temperature, we found that the rate increases with increasing field, reaching a maximum at the phase boundary of the spin liquid state. The results are compared with the calculated NMR rates in different proposed spin liquid states. The implications of the results on determining the fermionic or bosonic nature of the elementary excitations in the spin liquid phase will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J27.00010: Spin-1/2 Heisenberg Antiferromagnet on the Spatially Anisotropic Kagome Lattice Andreas Schnyder, Oleg Starykh, Leon Balents We study the quasi-one-dimensional limit of the Spin-1/2 quantum antiferromagnet on the Kagome lattice, a model Hamiltonian that might be of relevance for the mineral volborthite [1,2]. The lattice is divided into antiferromagnetic spin-chains (exchange $J$) that are weakly coupled via intermediate ``dangling'' spins (exchange $J'$). Using bosonization, renormalization group methods, and current algebra techniques we determine the ground state as a function of $J'/J$. The case of a strictly one-dimensional Kagome strip is also discussed. \newline \newline [1] Z.~Hiroi, M.~Hanawa, N.~Kobayashi, M.~Nohara, Hidenori~Takagi, Y.~Kato, and M.~Takigawa, J. Phys. Soc. Japan \textbf{70}, 3377 (2001). \newline [2] F.~Bert, D.~Bono, P.~Mendels, F.~Ladieu, F.~Duc, J.-C.~Trumbe, and P.~Millet, Phys.~Rev.~Lett.~\textbf{95}, 087203 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J27.00011: Magnetic Order and Spin Fluctuations in the Spin-1/2 Three-Dimensional Frustrated Magnet Clinoatacamite, Cu$_{2}$(OH)$_{3}$Cl Joel Helton, Kittiwit Matan, Jae-Ho Chung, Matthew Shores, Bart Bartlett, Emily Nytko, Ying Chen, Qingzhen Huang, Jeffrey Lynn, Daniel Nocera, Young Lee We have performed thermodynamic and neutron scattering measurements on the S=1/2 three-dimensional antiferromagnet clinoatacamite, Cu$_{2}$(OH)$_{3}$Cl. The crystal lattice feature Cu$^{2+}$ ions arranged on a distorted kagom\'{e} lattice with weak magnetic coupling between adjacent planes. Long range magnetic order with a weak ferromagnetic moment emerges at the N\'{e}el ordering temperature, T$_{N}$ = 6.2 K. The value of $\Theta_{CW}$ is roughly 30 times larger than T$_{N}$, demonstrating that the material is highly frustrated. Magnetic Bragg peaks are not observed above background for temperatures between 6.2 and 18 K, even though previous $\mu$SR measurements observed muon oscillations in this temperature range. We present a possible model of the magnetic transitions and analyze the inelastic spectrum of the ordered state, taking into account anisotropic terms in the spin Hamiltonian. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J27.00012: Magnetic Properties of Yb$_{2}$Pt$_{2}$Pb with the Shastry-Sutherland Lattice Moo Sung Kim, Marcus Bennett, Meigan Aronson We have synthesized single crystals of Yb$_{2}$Pt$_{2}$Pb, which crystallize in the tetragonal U$_{2}$Pt$_{2}$Sn-type structure. Comparison of the crystal structure with that of other compounds reveals a structure dependent Yb valence state of Yb$_{2}$T$_{2}$M (T=transition metal; M=Cd, In, Sn, and Pb). The magnetic susceptibility $\chi$ of Yb$_{2}$Pt$_{2}$Pb is highly anisotropic. The $\chi_{\lbrack100\rbrack}$ for $B$$\parallel$$\lbrack100\rbrack$ is thirty times larger than $\chi_{\lbrack001\rbrack}$ for $B$$\parallel$$\lbrack001\rbrack$ at the lowest temperatures. A broad maximum in $\chi_{\lbrack100 \rbrack}$ is found around 3 K just above magnetic transition temperature 2.07 K. In agreement, the electrical resistivity shows a broad maximum around 5 K and the specific heat shows a long tail up to 8 K, due to the magnetic frustration originating from antiferromagnetic exchange interaction between Yb ions arranged in the network of the Shastry-Sutherland lattice. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J27.00013: Dynamic Frustration in PrAu$_2$(Si$_{1-x}$Ge$_x$)$_2$ Raymond Osborn, Eugene Goremychkin, Brian Rainford, Robin Macaluso, D.T. Adroja, Marek Koza We have recently proposed that frustration is dynamically induced in the spin glass, PrAu$_2$Si$_2$, a singlet ground state system, in which the interionic exchange is just sufficient to induce a magnetic moment. The degree of static disorder is far too small to produce spin glass freezing but dynamic fluctuations can destabilize the induced moments and frustrate the long-range magnetic correlations. We have studied the crystal field excitations in PrAu$_2$(Si$_{1-x}$Ge$_x$)$_2$, for which long-range order is stabilized at $x\approx12$\%. The excitation energy is independent of $x$ but inhomogeneous broadening increases linearly for $x<0.2$, \textit{i.e.}, long-range order is stabilized even though static disorder is increasing. Tuning the dynamic frustration with concentration or pressure could provide a test of the concept of ``avoided criticality'' in glassy systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J27.00014: KTi(SO$_{4}$)$_{2}$ .H$_{2}$O - a possible candidate for a new spin-Pierles system Deepa Kasinathan, Goran Nilsen, Henrik Ronnow, Stefan-Ludwig Drechsler, Helge Rosner Recently a large number of compounds belonging to the family of J$_{1}$-J$_{2}$ chain models with competing ferromagnetic (FM) and antiferromag- netic (AFM) interactions have been discovered. In most cases, FM-J$_{1}$ and AFM-J$_{2}$ is observed, leading to helical order with no spin gap (for frustration ratio $\alpha$ = $\frac{J_{1}}{J_{2}}$ $\geq$ -0.25). Systems with both J$_{1}$ and J$_{2}$ being AFM causing a spin gap are rather rare. The thermodynamic data of the recently prepared KTi(SO$_{4}$)$_{2}$.H$_{2}$O reveal that this system is a quasi 1D spin 1/2 chain compound with both J$_{1}$ and J$_{2}$ being AFM, and a frustration ratio $\alpha$ $\approx$ 0.29. Here we report the results of electronic structure calculations within the LSDA+$U$ method along with tight-binding models. Our calculations confirm that both J$_{1}$ and J$_{2}$ are AFM. In contrast to the experiments we obtain a larger $\alpha$, slightly depending on the choice of the Coulomb repulsion $U$ . Therefore KTi(SO$_{4}$)$_{2}$.H$_{2}$O might be a new candidate for a spin-Pierles ground state. A brief comparison with other systems belonging to the class of frustrated chain materials is given with respect to their position in the general phase diagram of the 1D J$_{1}$ - J$_{2}$ model. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J27.00015: Valence bond solid phases in the spin-1 XXZ model on the kagome lattice Sergei Isakov, Yong Baek Kim We study the spin-1 XXZ model on the kagome lattice using quantum Monte Carlo simulations. We find a rich phase diagram including two different valence bond solids and other quantum paramagnetic phases. We also characterize the nature of quantum phase transitions between those phases. These results are compared with various effective field theory approaches. [Preview Abstract] |
Session J28: Focus Session: Thermoelectricity in Semiconductor Nanostructures
Sponsoring Units: DMP FIAPChair: Li Shi, University of Texas, Austin
Room: Morial Convention Center 220
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J28.00001: Theory of enhancement of thermoelectric properties of materials with nanoinclusions. Sergey Faleev, Francois Leonard Based on the idea of Schottky potential as an energy filter for the electrons, we developed a theory that explains the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. The Boltzmann transport equation with relaxation time approximation is used for description of both electron and phonon scattering. The theory has been applied to optimize the ZT factor for n-doped PbTe with metallic nanoinclusions. We found that the contribution of electron scattering to optimized ZT is important for high electron concentration (n 3x10 cm$^{3}$), while at low concentrations (n 10 cm$^{3}$) enhancement of the ZT factor is primarily due to decrease of the phonon thermal conductivity. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J28.00002: Measurement of Cross-plane Thermoelectric Properties of Thin Film Structures with UHV Scanning Thermoelectric Microscopy Yong Lee, Anastassios Mavrokefalos, Michael Pettes, Li Shi In recent years various thin film structures have been under intense research in the hope for achieving increased thermoelectric figure of merit (ZT) compared to bulk materials. Accurate measurements of three quantities ($S$\ :\ Seebeck coefficient, $\sigma$\ :\ electrical conductivity, and $\kappa$:thermal conductivity) used to calculate ZT have been a challenge especially for thin film structures which may have very different values between in-plane and cross-plane directions due to the anisotropy. Here, we report our progress toward accurately measuring these thermoelectric properties of thin films along the cross-plane direction with a ultrahigh vacuum (UHV) scanning probe microscope. In particular, cross-plane Seebeck coefficient and electrical conductivity measurement of a thin film with a conductive AFM or STM probe will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J28.00003: Micro- and nanomachined tools for measuring thermopower and in-plane thermal conductivity of thermoelectric thin films Azure Avery, Rubina Sultan, Greg Stiehl, Barry Zink Many of the potential next-generation thermoelectric materials being studied are either thin films or nanostructures that are expected to have anisotropic properties. Techniques such as the $3\omega$ method and picosecond thermoreflectance allow accurate measurements of $k_{\perp}$ at temperatures relevant to thermoelectrics, but measuring $k_{\parallel}$ is often difficult. In this talk we discuss our efforts to design and demonstrate accurate measurements of $k_{\parallel}$ of thin films from $77-475$ K using micro- and nanomachined thermal isolation platforms. Using thin-film structures to support the thin-film sample reduces background contributions, and careful control of the geometry keeps radiation errors small. We will discuss the optimization and micromachining of the measurement platforms and their application for studying the growth and characteristics of our first doped amorphous thin films. We will present our first tests of the devices on materials with established thermal properties. Finally, we will discuss the use of these measurement platforms to determine $k$ and $ZT$ for doped amorphous silicon thin films. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J28.00004: Thermopower and Electrical Conductivity of PbSe Nanocrystal Thin Films Robert Wang, Joseph Feser, Jong-Soo Lee, Dmitri Talapin, Rachel Segalman, Arun Majumdar Thin films assembled of solution-processed PbSe nanocrystals have a thermopower 2 -- 3 times greater than bulk PbSe. In addition, the thermopower and electrical conductivity both exhibit a size-dependence on nanocrystal size. As the nanocrystal diameter changes from 4 to 9 nm, the thermopower and electrical conductivity change from 850 to 650 $\mu $V/K and 10$^{-4}$ to 10$^{-2}$ S/cm, respectively. If electrical conductivity can be improved, these materials represent a new class of inexpensive and scalable thermoelectric materials. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J28.00005: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J28.00006: First Principles Studies of Phonon Dispersion and Lattice Thermal Conductivity of Silicon Nanowires Trinh Vo, Tadashi Ogitsu, Eric Schwegler, Giulia Galli We present phonon dispersions of Si nanowires using \textit{ab initio} and linear response theory. The effects of nanowire surface structures, growth directions, and quantum confinement on phonon dispersions and phonon confinement are also discussed. The thermal conductivity of Si nanowires using the obtained full dispersion curves are also evaluated, using Boltzmann Transport Equation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J28.00007: Enhanced Thermoelectric Performance in Rough Silicon Nanowires Renkun Chen, Allon I. Hochbaum, Raul Diaz Delgado, Wenjie Liang, Erik C. Garnett, Mark Najarian, Arun Majumdar, Peidong Yang Due to the disparity between electron ($<$10 nm) and phonon ( $\sim $100 nm) mean free paths in silicon, nanostructured Si could effectively block phonon transport by boundary scattering while maintaining electron transport, thereby enhancing thermoelectric figure of merit, ZT. Here we report the wafer-scale electrochemical synthesis and thermoelectric characterization of rough Si nanowires with enhanced ZT, relative to the bulk material. Single nanowire measurements show that their electrical resistivity and Seebeck coefficient are similar to those of bulk Si with similar dopant concentration. Thin nanowires, however, exhibit a 100-fold reduction in thermal conductivity (k), yielding a large ZT = 0.6 at room temperature. Although bulk Si is a poor thermoelectric material, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J28.00008: Enhanced thermoelectric properties in silicon nanowires Slobodan Mitrovic, Jen-Kan Yu, Akram Boukai, Jamil Tahir-Kheli, William A. Goddard III, James R. Heath Recently, we demonstrated that silicon nanowires can be designed and fabricated to achieve an approximately 100-fold enhancement in thermoelectric efficiency compared to bulk silicon. Independent measurements of thermoelectric power, and thermal and electrical conductivities suggest that this improvement is due to phonon effects rather than quantum confinement. Here, we present the study of the scaling laws (i.e. nanowire length/width dependence) for the phonon dynamics and transport. We investigate the influence of the phonon drag, carrier mobility and doping on the thermoelectric properties, and the universality of these findings. This work is supported by the Office of Naval Research, the Department of Energy, the National Science Foundation, and the Defense Advanced Research Projects Agency. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J28.00009: Atomistic Modeling and Optimization of thermoelectric properties of SiGe nanowires Maria Chan, Ying Shirley Meng, Tim Mueller, Gerbrand Ceder, John Reed, Trinh Vo, Andrew Williamson, Giulia Galli Nano-structured thermoelectric materials have been shown experimentally to have superior figure of merit compared to bulk materials. To understand the origin of this superiority, it is of interest to develop physically accurate methods to compute the thermoelectric transport coefficients of nanowires. In addition, computationally inexpensive parameterization of these physical models are needed in order for efficient sampling, e.g. in atomic configuration space, so as to design systems with optimal thermoelectric properties. We consider aspects of electron and phonon transport in SiGe nanowires. For electronic transport, we work in the diffusive regime with Boltzmann transport, combining ab initio density functional theory (DFT) calculations with a perturbative treatment of electronic scattering to obtain electronic relaxation times, conductivity and thermopower. The phonon contribution to thermal conductivity is obtained from classical equilibrium molecular dynamics simulations using the Green-Kubo formalism. Cluster expansion and effective potential techniques are used to parameterize the transport coefficients for efficient sampling and optimization. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J28.00010: Strain-superlattice nanowires via SiGe epitaxy on ultrathin Si ribbons C. Ritz, Yu Zhang, Decai Yu, D. Savage, C.-H. Lee, Feng Liu, M. Lagally We demonstrate a method to create a system of precisely positioned strain superlattice nanowires. These are similar to superlattice nanowires, which are traditionally created by VLS growth techniques and are of great interest for thermoelectric applications. We pattern the top layer of thin Si-on-insulator (SOI) into nanoribbons and undercut them, leaving freestanding bridges that are directly integrated into microfabricated devices. The thin freestanding Si is used as a substrate for the Stranski-Krastanov growth of coherent 3D Ge islands, where the thinness of the Si allows for island-island interactions through the ribbon thickness. This elastic interaction causes lateral order in island positions, forming a strain superlattice. A periodic bandgap modulation can result from the periodic island-induced strain. The combination of strain and bandgap modulation should act to improve the thermoelectric figure of merit of these structures. Thermal conductivity measurements of such structures will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J28.00011: Thermoelectric Properties of Semiconducting Silicide Nanowires Song Jin, Jeannine Sczech, Jeremy Higgins, Feng Zhou, Li Shi Semiconducting silicides are promising thermoelectric materials. In addition to their respectable thermoelectric figure-of-merit (\textit{ZT} up to 0.8), silicides have the advantages of low cost, excellent thermal stability and mechanical strength, and outstanding oxidation resistance, making them suitable for high temperature applications. We have developed general synthetic approaches to single crystal nanowires of silicides to investigate the enhancement of thermoelectric properties due to the reduced nanoscale dimension and to explore their applications in thermoelectrics. We will discuss the synthesis and structural characterization of nanowires of chromium disilicide (CrSi$_{2})$ prepared via a chemical vapor transport (CVT) method and chemical vapor deposition (CVD) of organometallic precursors to synthesize the Novontony Chimney ladder phase MnSi$_{1.75}$. The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual CrSi$_{2}$ nanowires were characterized using a suspended microdevice and correlated with the structural information obtained by microscopy on the same nanowires. This combined Seebeck coefficient and electrical conductivity measurements also provide an effective approach to probing the Fermi level, carrier concentration and mobility in nanowires. We will also discuss our progress in using individual nanostructures combined well-defined structural characterization to conclusively investigate the complex thermoelectric behaviors of silicide materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J28.00012: Investigation of Thermoelectric Transport in Individual Bismuth Nanowires Arden Moore, Michael Pettes, Anastassios Mavrokefalos, Li Shi Bismuth is a material of special interest for studying nanoscale transport behavior. Its extremely small effective mass and long electron mean free path suggest that quantum and classical confinement effects might be observed at realistic dimensions and higher temperatures than in other material systems. In addition, the predicted enhancement of the thermoelectric figure of merit \textit{ZT} due to quantum confinement effects has only served to increase the desire to measure the transport properties of individual bismuth nanowires. However, efforts to measure the thermoelectric properties of bismuth nanowires have been hindered thus far by the presence of a highly stable surface oxide layer, making reliable ohmic contact to individual nanowires problematic. In this work, we present the synthesis and measurement methods used by our group to make electrical contact with individual nanowires in order to measure the thermal conductivity, thermopower, and electrical conductivity of individual bismuth nanowires of varying diameter. The obtained data is presented with comparison to bulk values and analysis of the transport behavior. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J28.00013: Thermopower Measurements of Pure and Sn (Te) Doped Bismuth Nanowires Tito Huber, A. Adeyeye, A. Nikolaeva, L. Konopko, R. Johnson, M.J. Graf Theoretical work based on one-dimensional (1D) models indicates that Bi wires with diameter smaller than 50 nm can exhibit superior thermoelectric properties since the density of states at the Fermi level of a 1D system can be tuned to very high values. Also, recently, angle-resolved photoemission spectroscopy (ARPES) studies of Bi thin films have shown that Bi nanowires support Rashba spin-orbit surface states, with high carrier densities of around$5\times 10^{12}cm^{-2}$, that are hybridized with ``bulklike'' electrons and holes, a phenomenon that has not been considered in current models of Bi nanowires. We carried out an experimental study of the transport properties and thermopower of bismuth nanowire arrays (NWA) with wire diameters ranging between 60 nm and 13 nm at temperatures ranging between 4 K and 300 K, for magnetic fields of up to 1 T. Both pure bismuth and doped Bi were studied. The results are interpreted in a multicarrier diffusion thermopower model. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J28.00014: Tuning the Thermoelectric Properties of Metal-Molecule-Metal Junctions K. Baheti, J.A. Malen, P. Doak, T.D. Tilley, A. Majumdar, R. Segalman Thermoelectric materials have application in power generation and refrigeration, with several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturization, and CO2 free conversion of heat to electricity. Nonetheless, low thermodynamic efficiency has limited their applicability. Here we examine a new class of inexpensive thermoelectric materials composed of organic-inorganic heterostructures. Thermopower measurements of 1,4-Benzenedithiol (BDT) molecule between Au electrodes, using a modified scanning tunneling microscope (STM), have been previously reported. This method is used to interrogate junctions where, the BDT molecule has been doped by the addition of substituent groups on the benzene ring. Our measurements show that we can tune the thermoelectric properties of such junctions in a controllable way by the addition of substituents. This in conjunction with a calculated transmission function imply a simultaneous increase in the thermopower and conductance, which has hitherto been impossible to attain in simple materials. We observe an increase of $\sim $50{\%} in the power factor, defined as S$^{2}\sigma$, of the junction upon substitution of electron donating groups in benzenedithiol. Hence, a ground up approach to building thermoelectric materials, from an endless array of possible organic-inorganic heterostructures, evokes hope for efficient thermoelectric energy conversion. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J28.00015: Transport properties and the thermoelectric figure-of-merit of single molecule systems. Padraig Murphy, Subroto Mukerjee, Joel Moore The thermoelectric properties of molecules are both of fundamental interest and of interest for the construction of energy conversion devices. These transport properties are sensitive to interactions within the molecule, the hybridization energy between the molecular energy levels and the leads, and to the temperature. We present numerical and theoretical results on the conductance and thermopower, and discuss the parameter values for which the figure-of-merit, which parametrizes the efficiency of energy conversion devices, is optimal. The numerical results for the thermopower can be obtained at fixed particle number for finite systems using an appropriate generalization of the approach of Gogolin and Prokof'ev to electrical conductance. [Preview Abstract] |
Session J29: Focus Session: Carbon Nanotubes and Related Materials VII: Electronic Properties
Sponsoring Units: DMPChair: Philip Kim, Columbia University
Room: Morial Convention Center 221
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J29.00001: Biased bilayer graphene: Hall effect and zero-energy Edge States Invited Speaker: We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magneto-transport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to mid-infrared energies by using fields of $\alt 1$ V/nm, below the electric breakdown of SiO$_2$. The opening of a gap is clearly seen in the quantum Hall regime. We further report the existence of zero energy surface states localized at zigzag edges of bilayer graphene. It is shown that zero energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene; (ii) states with finite amplitude over the two layers, with an enhanced penetration into the bulk. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J29.00002: Cyclotron resonance in bilayer graphene Erik Henriksen, Zhigang Jiang, Li-Chun Tung, Mollie Schwartz, Maika Takita, Yong-Jie Wang, Philip Kim, Horst Stormer The hyperbolic dispersion of bilayer graphene leads to a Landau level (LL) spectrum that is linear in the magnetic field, B, at low energies but shifts to a $\sqrt{\textrm{B}}$ dependence with increasing energy. Here we present the first infrared transmission measurements of the unique B-field dependence of LL transitions in bilayer graphene, in a gated $400 \mu$m$^2$ sample in fields up to $\textrm{B}=18$ T. Eight intraband transitions are observed among LL indices $|n| \leq 4$, including the unusual zero-energy $n = 0$ level, and are found to follow a selection rule of $\Delta n = +1$. We find the change in field dependence is plainly visible between the behavior of the transition energies for $n = -1 \to 0$ and $n = 0 \to +1$, which are close to linear in B, as compared with all other transitions which display a clear $\sqrt{\textrm{B}}$ behavior. However, the shift in field dependence occurs at energies well below where it is expected based on nearest-neighbor tight-binding calculations, and a single set of fitting parameters within this theory fails to describe our results. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J29.00003: Graphene as an electronic membrane Eun-Ah Kim, Antonio Castro Neto Experiments are finally revealing intricate facts about graphene which go beyond the ideal picture of relativistic Dirac fermions in pristine two dimensional (2D) space, two years after its first isolation. While observations of rippling [1, 2, 3] added another dimension to the richness of the physics of graphene, scanning single electron transistor images displayed prevalent charge inhomogeneity [4]. The importance of understanding these non-ideal aspects cannot be overstated both from the fundamental research interest since graphene is a unique arena for their interplay, and from the device applications interest since the quality control is a key to applications. We investigate the membrane aspect of graphene and its impact on the electronic properties. We show that curvature generates spatially varying electrochemical potential. Further we show that the charge inhomogeneity in turn stabilizes ripple formation.[5]\newline [1] Meyer, J.C., et al., Nature 446, 60 (2007).\newline [2] Stolyarova E. et al., PNAS, 104, 9209 (2007).\newline [3] Ishigami, M. et al., Nano Letters 7, 1643 (2007).\newline [4] Martin, J. et al., unpublished, cond-mat/0705.2180 (2007).\newline [5] E.-A, Kim and A. Castro Neto, cond-mat/0702562 [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J29.00004: Demonstration of one-parameter scaling at the Dirac point in graphene Jens H. Bardarson, J. Tworzydlo, P. Brouwer, C.W.J. Beenakker We numerically calculate the conductivity $\sigma$ of an undoped graphene sheet (size $L$) in the limit of vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function $\beta(\sigma)=d\ln\sigma/d\ln L$. Contrary to a recent prediction, the scaling flow has no fixed point ($\beta>0$) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data supports an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering --- without reaching a scale-invariant limit. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J29.00005: Electron localization in gapped bilayer graphenes with disorder Mikito Koshino The bilayer graphene is a zero-gap semiconductor, and it is known that the width of energy gap can be controlled by the electric field perpendicular to the layer [1,2]. Even in zero magnetic field, the electronic states carry the non-zero Hall conductivity in presence of the gap, while the Hall currents cancel out in summation over two valleys (K and K' points). Here we numerically calculate the electronic states in gapped bilayer graphenes with the smooth disorder potential, and estimate the localization length as a function of the gap width $\Delta$. We find that the conductivity at zero Fermi energy does not simply goes down as $\Delta$ increases, but has a maximum at a certain finite $\Delta$, and the localization length diverges there. We show that this can be interpreted as a ``Hall plateau transition'' in each decoupled valley, even though the total Hall conductivity remains zero. [1] E. McCann Phys. Rev. B 74, 161403(R) (2006) [2] Eduardo V. Castro, et al, Phys. Rev. Lett. 99, 216802 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J29.00006: Effect of electron interactions on the infrared conductivity of a monolayer graphene M. M. Fogler, L. M. Zhang Recent experiments on the infrared spectroscopy of a monolayer graphene has revealed an unexpected non-Lorentzian form of the Drude peak and an anomalously large broadening of the interband absorption edge in this material. We present a theoretical investigation that attributes these features to Coulomb scattering between Dirac quasiparticles. This scattering shows up in the dynamical conductivity because relativistic collisions lead to the current relaxation despite the conservation of the total momentum. This is in contrast to the conventional case where electron-electron scattering along does not cause a finite resistivity. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J29.00007: Excitonic Effects in the Optical Spectra of Graphene Nanoribbons Li Yang, Marvin Cohen, Steven Louie We present a first-principles calculation of the optical properties of graphene nanoribbons (GNRs) with many-electron effects included, employing the GW-BSE approach. The reduced dimensionality of GNRs gives rise to an enhanced electron-hole binding energy for both bright and dark exciton states and changes the optical spectra significantly. The characteristics of the excitons of different types of GNRs are compared and discussed. The enhanced excitonic effects found here are expected to be of importance in considering possible applications (such as optoelectronics) of graphene-based nanostructures. This work was supported by National Science Foundation Grant No. DMR07-05941, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by Datastar at the San Diego Supercomputer Center. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J29.00008: Localized states at zigzag edges of graphene multilayers and graphite steps Eduardo V. Castro, J.M.B. Lopes dos Santos, N.M.R. Peres, F. Guinea, A. H. Castro Neto Among the uncommon features of graphene monolayer we find the presence of zero energy states localized at zigzag edges, leading to the self-doping phenomenon and inducing edge magnetization. Here we report the existence of zero energy surface states localized at zigzag edges of bilayer graphene and stacks with any number of layers. Working within the tight-binding approximation we derive an analytic solution for the wavefunctions of these peculiar surface states. It is shown that zero energy edge states in bilayer graphene can be divided into two families: (\emph{i})~states living only on a single plane, equivalent to surface states in monolayer graphene; (\emph{ii})~states with finite amplitude over the two layers, with an enhanced penetration into the bulk. The effect of edge states on the electronic structure and magnetic order of bilayer graphene nanoribbons is also studied. We show that edge states measured through scanning tunneling microscopy and spectroscopy of graphite step edges belong to family~(\emph{i}) or~(\emph{ii}) mentioned above, depending on the way the top layer is cut. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J29.00009: Minimal conductivity of graphene: role of the Coulomb interaction Vladimir Juricic, Igor F. Herbut, Oskar Vafek The effect of the Coulomb interaction on the zero-temperature low-frequency conductivity in undoped graphene is studied. We will show that the Coulomb interaction introduces a universal and positive leading logarithmic correction to the gaussian value of the dc conductivity [1]. This finding suggests that the origin of the unusually large minimal conductivity observed in graphene may be intrinsic, and arises from the Coulomb correction effectively cut off by finite temeperature/disorder/size effects. A mechanism of such a cutoff based on the non-trivial interplay between the Coulomb interaction and the ripples, both unavoidably present in the graphene sheet, will be briefly discussed. References: [1] I. F. Herbut, V. Juricic, and O. Vafek, arXiv.0707.4171. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J29.00010: Low density ferromagnetism in a biased bilayer graphene Tobias Stauber, Eduardo Castro, Nuno Peres, Nuno Silva We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect both to carrier density and temperature. We find that the ferromagnetic transition is first order, lowering the value of $U$ relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is hole like in one plane and electron like in the other. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J29.00011: Minimal conductivity of rippled graphene with topological disorder. Alberto Cortijo, Maria Vozmediano We study the transport properties of a neutral graphene sheet with curved regions induced or stabilized by topological defects. The proposed model gives rise to Dirac fermions in a random magnetic field and a random space dependent Fermi velocity induced by the curvature. This last term leads to singular long range correlated disorder with special characteristics. The Drude minimal conductivity at zero energy is found to be inversely proportional to the density of topological disorder, a signature of diffusive behavior. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J29.00012: Weak Localization of Dirac Fermions in Graphene Xin-Zhong Yan, C. S. Ting In the presence of the charged impurities with screened Coulomb potentials, we study the weak localization (WL) effect by evaluating the quantum interference correction (QIC) to the conductivity of Dirac fermions in graphene. With the inelastic scattering rate due to electron-electron interactions obtained from our previous calculation, we investigate the dependence of QIC on the carrier concentration, the temperature and the size of the sample. It is found that WL is present in large size samples at finite carrier doping where the strength of the intervalley scatterings due to the charged impurities is not weak. In addition, we argue that the system is delocalized at very low doping. We also analyze the absence of WL in experiment. It is found that WL is quenched at low temperature for small size samples as studied in the experiments. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J29.00013: Electric Transport Theory of Dirac Fermions Yousef Romiah, Xin-Zhong Yan, Chin-Sen Ting The self-consistent Born approximation is employed to calculate transport properties in graphene with finite-range impurity potentials. The current-current correlation function is determined by a system of four coupled integral equations, unlike the case of short-range impurity scattering, and yet the results for the latter can exactly be reproduced in our formalism. As a test, we numerically calculate the dc electric conductivity of graphene for charged impurities with screened Coulomb potential, the linear dependence of the dc conductivity on the carrier concentration and the extrapolated value for zero-doping is shown to be finite, in a qualitative agreement with the experimental observations. [Preview Abstract] |
Session J30: Optical Properties and Experimental Characterization of Graphene and Related Structures
Sponsoring Units: DMPChair: Marco Fornari, Central Michigan University
Room: Morial Convention Center 222
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J30.00001: Photo-induced structural dynamics of graphitic carbon studied by ultrafast electron nanocrystallography. Ramani K. Raman, Yoshie Murooka, Ryan A. Murdick, Chong-Yu Ruan We report the studies of photo-induced structural dynamics of graphite and multi-wall carbon nanotubes (MWCNT) using ultrafast electron nanocrystallography. Graphite, upon excitation, contracts along its c-axis causing a reduction of the interlayer distance, which is the first step towards diamondization. MWCNT on the other hand, display an energy-dependent electron-phonon coupling mechanism. Upon excitation at 400nm, the promoted carriers can transfer their excess energy to the lattice rapidly within 5-10 ps whereas at 800nm it takes around 20-30ps for the same. This indicates a more efficient electron-phonon coupling at 400nm where the excited carriers are more strongly coupled to the lattice. Both graphite and MWCNT also exhibit a transient photovoltaic effect where an accumulation of excited charge carriers at the sample interface causes a collective shift of the Bragg peaks. We found that the charge dynamics and atom dynamics are intimately correlated at interfaces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J30.00002: Non-destructive optical characterization of DNA-wrapping of single-walled carbon nanotubes S.E. Snyder, S.V. Rotkin Single-stranded DNA can form a stable hybrid structure with a single-walled carbon nanotube, allowing dispersion of individual nanotubes in aqueous solution and facilitating the development of methods to separate nanotubes by type. Optical and electronic properties of specific DNA-nanotube structures are the focus of our study due to potential optoelectronic device applications. Within a semi-empirical tight-binding approach, we have studied changes in optical absorption of a single-walled carbon nanotube resulting from a helical wrap of ionized single-stranded DNA. The one-electron absorption spectrum for light polarized across the tube is sensitive to bandstructure modulation due to the wrapping. For a non-chiral tube, the helical perturbation generates ``natural'' optical activity in the DNA-nanotube complex, yielding circular dichroism. Symmetry breaking due to the Coulomb potential of the wrap lifts optical selection rules and allows new optical transitions. These optical effects are predicted to serve as qualitative tools to directly identify DNA wrapping. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J30.00003: Incommensurately Stacked Graphene Bi-Layer: A Raman Scattering Study Awnish Gupta, Y. Tang, T. Russin, V. Crespi, P. Eklund We report results of Raman scattering studies of two novel graphene systems based on incommensurate stacking of sp$^{2}$ carbon: (1) an incommensurate bi-layer (IBL) formed by folding a graphene sheet onto itself; (2) a graphene scroll formed by rolling up a graphene sheet via a shearing motion between scotch tape and substrate during the micro-mechanical cleaving process . In (1), we have a flat bi-layer system; in (2) it is a gently curved multilayer system -- both should be incommensurate. Interestingly, although no significant D-band is observed in the parent graphene system, the incommensurate contact of the graphene sheet in (1) and (2) leads to strong D-band scattering near 1350 cm$^{-1}$ using 514.5 nm excitation. The dispersion of the D-bands in (1) and (2) is significantly different: scroll ($\sim $38 cm$^{-1}$/eV) and IBL ($\sim $50 cm$^{-1}$/eV). A second Raman band is observed nearby at $\sim $1384 cm$^{-1}$ in both the IBL and the scroll. However, the $\sim $1384 cm$^{-1}$ band is non-dispersive in both cases and is much sharper in the IBL than in the scroll. Our data will be compared to theoretical calculations based on double-resonant (DR) scattering and the electronic states of an IBL. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J30.00004: Anharmonic Effects in Raman Scattering Few-Layer Graphene System Timothy Russin, Awnish Gupta, Peter Eklund Result of Raman scattering studies from supported and suspended n-layer graphene films (nGLs) are presented for the temperature range of -190 to 500 $^{\circ}$C. The samples were either supported directly on a Si/SiO$_{2}$ substrate or suspended over lithographically produced trenches; the measurements were performed in a N$_{2}$ atmosphere. For both supported and suspended films, the magnitude of the negative temperature coefficient of the G-band frequency (cm$^{-1}$/K) is found to decrease with number of layers n in the nGL films (i.e., supported 1GL, 2GL and 5GL show -0.037, -0.031, and -0.028 cm$^{-1}$/K, respectively.) The anharmonic coefficients are significantly higher than for highly oriented pyrolytic graphite (HOPG). Surprisingly, the G-band linewidth increases with temperature for the supported films and decreases with temperature for the suspended films. Furthermore, we see evidence for a permanent morphological change at T $\sim $ 200 $^{\circ}$C for supported $n$GLs via new D-band and D'-band scattering. Unsupported films exhibit these changes at higher temperatures. The mechanism and details of the irreversible morphological change(s) is not yet known. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J30.00005: Tunneling spectroscopy of single- and double-layer graphene planar tunnel junctions Conor Puls, Neal Staley, Ying Liu It is of fundamental as well as technological concern if there exists an energy gap in single- and double-layer graphene devices. Single-layer graphene is thought to be gapless while double-layer graphene features an energy gap tunable by controlling the charge difference between the two layers. Previously, scanning tunneling microscopy and spectroscopy studies have been employed to examine energy spectra of graphene films. This approach produces local charge inhomogeneity at the probe tip that could significantly alter the local density of states (DOS) in graphene. Planar tunnel junctions provide a probe of the DOS that should not induce such an inhomogeneity in the charge carrier density. We fabricated planar tunnel junctions on single- and double-layer graphene using an ultrathin quartz filament as a shadow mask over mechanically exfoliated graphene as an alternative to lithographic procedures so as to avoid possible contamination in a wet lithography process. We have measured tunneling spectra for both weakly and strongly disordered samples. For single-layer graphene, we observed an unexpected gap. For double-layer graphene, we found a gap and other features in the tunnel spectra by changing the back gate and tunnel junction bias voltages independently - thereby tuning the charge difference between the top and bottom layer - as well as varying magnetic field and temperature. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J30.00006: The G* (2450 cm$^{-1})$ Double Resonance Raman Peak in single-, few-layer graphene and DWNTs Alfonso Reina, Hyungbin Son, Federico Villalpando-Paez, Hootan Farhat, Jing Kong, Mildred Dresselhaus The dispersion and skewness of the 2450 cm$^{-1}$ peak in the raman spectra of carbon structures was analyzed. The dispersion of this peak for graphene is smaller in magnitude and of opposite sign than that for the G' ($\sim $2700cm$^{-1})$. This dispersion is independent on number of layers. The peak shows asymmetry (skewness) which increases with E$_{laser}$. The observations can be explained by viewing this double resonance process arising by the scattering with both an iTO and a iLA phonon. The peak becomes more symmetric in DWNTs and it shows a stronger curvature dependence than the G' peak. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J30.00007: Probing Non-equilibrium Phonon Dynamics in Graphite by Time-Resolved Raman Spectroscopy Hugen Yan, Daohua Song, Kin Fai Mak, Ioannis Chatzakis, Janina Maultzsch, Tony Heinz Time-resolved Raman spectroscopy has been applied to obtain direct information about phonon lifetimes in graphite. A non-equilibrium population of zone-center optical phonons was produced by the rapid relaxation of charge carriers following photoexcitation of the sample with a femtosecond laser pulse. The subsequent evolution of the phonon population was recorded using the strength of G-mode anti-Stokes Raman scattering from a time-delayed femtosecond probe pulse. A population lifetime for the G-mode phonons of approximately 2 ps was found. Analogous measurements of optical-phonon lifetimes were also conducted in few-layer graphene samples produced by mechanical exfoliation of bulk graphite. Results obtained for graphite and few-layer graphene will be compared with one another, as well as with earlier data on the lifetime of G-mode phonons in single-walled carbon nanotubes [1]. \newline [1] D. Song, F. Wang, G. Dukovic, M. Zheng, E. D. Semke, L. E. Brus, and T. F. Heinz, submitted. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J30.00008: Transfer of Graphene to Alternative Substrates Tracy Moore, J.H. Chen, D.R. Hines, E.D. Williams, J. Simspon, A.R.H. Walker Graphene transport properties are limited by charge defects in SiO$_{2 }$, and by large charge density due strong interaction with SiC. We have investigated the transfer of graphene from one substrate to another using high pressures and temperatures to achieve control of the substrate interactions and thus their effects on graphene. The direct transfer from HOPG to alternative substrates PET and PMMA yields mostly multilayer, opaque graphite flakes. Raman signatures of the thinner, translucent flakes on PMMA can be clearly distinguished from the PMMA spectra and show a downshift in the G' peak that occurs around 2700 cm$^{-1}$ and a relative intensity of G to G' peak of approximately one; characteristics of graphene spectra. In addition the transfer from SiO$_{2}$ to alternative substrates occurs readily for PET substrates, and infrequently for PMMA substrates with thicker flakes transferring more readily than thin flakes. Graphene transfer from 1) direct HOPG, 2) flakes on SiO$_{2}$, and 3) the possibility of direct transfer from epitaxial graphene on SiC will be presented, along with the resulting device characteristics. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J30.00009: Gap and Impurity induced states on graphene layers Chanyong Hwang, Dohyun Lee, Wondong Kim, Junghwa Yang, Jisang Hong One of the interesting phenomena in graphene is the linear Fermi level crossing at the Dirac point. For the measurement of electronic structure, few layers of graphenes are formed on top of SiC substrate by thermal treatment. As the thickness of graphene layers increases, the formation of the gap near Dirac point is somewhat controversial. Recently this gap has been demonstrated to be tunable by the electric field. We have used angle-resolved photoemission spectroscopy and STM to characterize this gap state and actual morphology of the graphene layers to clarify this controversial issue. In addition, we have shown that the adatom carbon can play an important role in gap state. First principles calculation on this carbon adatom state will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J30.00010: Single Layer Graphene formation on Silicon Oxide surface(001) HeeSung Choi, Young-Kyun Kwon Recently graphene is one of most interesting topics in physics and other research fields. For future nanoelectronics applications, graphene formation becomes an important issue. Here we present our theoretical study of how to make a graphene layer on silicon oxide surfaces. In this work, density functional theory calculations are used to determine atomic structures and energies for graphene formation from various carbon sources, such anthracene, on silicon oxide. We will also preresent optimal graphene formation conditions obtained from our ab inito molecular dynamics simulations. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J30.00011: Soldering to a single atomic layer Caglar Girit, Alex Zettl The standard technique to make electrical contact to nanostructures is electron beam lithography. This method has several drawbacks including complexity, cost, and sample contamination. We present a simple technique to cleanly solder submicron sized, Ohmic contacts to nanostructures. To demonstrate, we contact graphene, a single atomic layer of carbon, and investigate low- and high-bias electronic transport. We set lower bounds on the current carrying capacity of graphene. A simple model allows us to obtain device characteristics such as mobility, minimum conductance, and contact resistance. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J30.00012: Synthesis of Carbon Nanotubes by Rolling Up Patterned Graphene Nanoribbons Using Selective Atomic Adsorption Decai Yu, Feng Liu We demonstrate a new method for synthesizing Carbon Nanotubes (CNTs), using first principles and classical molecular dynamics simulations. The single-walled nanotubes (SWNTs) are formed by rolling up graphene nanoribbons patterned on graphite films, through adsorption of atoms of varying coverage, which introduces an external stress to drive the folding process. The diameter and chirality of SWNTs can be \textit{a priori} controlled by patterning graphene nanoribbons with predefined width and direction, so that the post-synthesis sorting process is eliminated. Our method allows potentially mass production of identical tubes and easy integration into device structures on a substrate. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J30.00013: $^{13}$C NMR studies on the organic zero-gap system, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under pressure Kazuya Miyagawa, Motoaki Hirayama, Masafumi Tamura, Kazushi Kanoda We present NMR data for the organic material, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under pressure, which is a candidate for zero-gap conductor with cone-like dispersion. The quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$ is known to show peculiar behaviors under high pressure. The resistivity is insensitive to temperature, while the Hall coefficient is strongly dependent on temperature. The band calculation suggests that this system is in the zero gap state with a linear dispersion around the Fermi energy. While the $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ is metallic under ambient pressure, above 5 kbar temperature dependences of resistivity and Hall coefficient are similar to those of $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$. It is remarkable that the graphine like zero-gap state is likely realized in a bulk system. We have performed preliminary investigation into the magnetism of $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under 8 kbar by $^{13}$C NMR. The external filed is applied to parallel to the conducting layer. In constant to the simple metallic behavior observed under ambient pressure (Korriga's relation holds), the Knight shift vanishes in proportion with temperature and 1/$T_{1}$ shows a steep decrease, which is roughly $T^{3}\sim T^{4}$ down to 4 K . These behaviors are consistent with the zero (or narrow) gap state picture. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J30.00014: UHV Growth of Graphene on SiC Paul Campbell, Glenn Jernigan, Keith Perkins, Brenda VanMil, Rachel Myers-Ward, Kurt Gaskill, James Culbertson, Jeremy Robinson, Eric Snow We report graphene growth on Si- and C-face semi-insulating 6H SiC in UHV by thermal Si desorption /reconstruction of the remaining C. The SiC was etched in H$_{2}$ up to 1580 \r{ }C to smooth the surface. XPS shows the H$_{2}$-etched surfaces are covered by an oxide which desorbs at 1000 \r{ }C, resulting in a surface containing excess Si. At 1300 \r{ }C, the surface becomes stoichiometric in Si and C and a $\surd $3 x $\surd $3 R30 LEED pattern is observed. At 1350 \r{ }C, we observe a 6$\surd $3 x 6$\surd $3 R30 LEED pattern develop when graphene has formed, and a 1x1 LEED pattern for graphite films formed at temperatures greater than 1400 \r{ }C. Graphene layers were grown under a variety of temperatures and conditions and characterized using XPS, LEED, AFM, Raman spectroscopy, and Hall effect. Top-gated FETs were fabricated with a wide range of gate lengths (1-25 microns) and gate widths (2-130 microns), and transistor operation was obtained for both single and multiple graphene layers. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J30.00015: UHV electron-probing of micro-mechanically cleaved Graphene on SiO$_{2}$ K.R. Knox, S. Wang, P. Kim, R.M. Osgood, T.O. Mentes, M.A.N Orti, A. Locatelli, D. Cvetko, A. Morgante While graphene's distinctive Dirac-cone electronic structure and simple 2D atomic structure have attracted major interest in the physics community, the inherent limitations of isolated graphene samples mounted on an insulating substrates have made it difficult to study such systems with typical UHV probes such as photoemission and low energy electron diffraction (LEED).~ While most single layer graphene transport measurements are done on micro-mechanically extracted samples on SiO$_{2}$, all photoemission and LEED measurements of graphene performed so far have used films grown on SiC substrates.~ In this talk, we will discuss the first results of UHV probes carried out exfoliated graphene bonded to SiO$_{2}$. Using the high spatial resolution of the nanospectroscopy beamline at the ELETTRA synchrotron light source, we have been able to overcome the size limitations, which have prevented previous UHV study of this system. We will discuss the results of our X-ray photoemission (XPS), UV photoemission (UPS) and LEED measurements on single and multilayer graphene samples. [Preview Abstract] |
Session J31: Focus Session: Computational Nanoscience IV: Nanocrystals
Sponsoring Units: DMP DCOMPChair: Igor Vasiliev, New Mexico State University
Room: Morial Convention Center 223
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J31.00001: Atomistic design of semiconductor nanostructures with optimal thermoelectric properties Invited Speaker: The search for novel materials with optimal thermoelectric properties (for either thermoelectric power generation or heat dissipation) is an active field of research. We present atomistic and \textit{ab-initio} simulations of selected nanomaterials, aimed at predicting thermal conductivities and electronic transport properties, and ultimately at designing materials with optimal thermoelectric figure of merit. In particular we focus on carbon nanotubes [1], silicon wires [2] and nanoporous silicon [3] and we discuss both strategies and algorithms to optimize thermoelectric properties at the nanoscale. \newline [1] D. Donadio and G.Galli, Phys. Rev. Lett. 2007 (in press). \newline [2] T.Vo, A.Williamson, V.Lordi and G.Galli (submitted) and J.Reed, A.Williamson, E.Schwegler and G.Galli (submitted). \newline [3] J.-H. Lee, J.C.Grossman, J.Reed and G.Galli, Appl. Phys. Lett. 2007 (in press). [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J31.00002: Ab initio method for the electron-phonon scattering times in semiconducting nanostructures Nathalie Vast, Jelena Sjakste, Valeriy Tyuterev The interaction of excited electrons with phonons plays a central role for electronic and transport properties at the nanoscale. It is the dominant process limiting the excitation lifetime at medium excitation energies. Despite its importance, a reliable approach within ab initio methods for phonon interaction with excited carriers was still lacking. We present in this work our fully ab initio approach to calculate the electron-phonon scattering times for collisions of carriers in the conduction band with short-wavelength phonons. We apply it to the deexcitation of hot electrons in GaAs [1,2], and to the lifetime of the direct exciton in GaP and GaAs [2,3], all in excellent agreement with experiments. Finally, we discuss the effect of nanostructuring on the electron-phonon coupling constants in GaAs/AlAs superlattices. \newline [1] J. Sjakste, N. Vast, V. Tyuterev, 2007, accepted in Phys. Rev. Lett. \newline [2] J. Sjakste, V. Tyuterev, N. Vast, Appl. Phys. A 86 (2007) 301. \newline [3] J. Sjakste, V. Tyuterev, N. Vast, Phys. Rev. B 74, 235216 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J31.00003: Quantum-size-induced phase transitions in quantum dots: Indirect-band gap GaAs nanostructures Alex Zunger, Jun-Wei Luo, Alberto Franceschetti Quantum nanostructures are often advertised as having stronger absorption than the bulk material from which they are made, to the potential benefit of nanotechnology. However, nanostructures made of direct gap materials such as GaAs can convert to indirect-gap, weakly-aborbing systems when the quantum size becomes small. This is the case for spherical GaAs dots of radius 15 {\AA} or less (about 1000 atoms) embedded in a wide-gap matrix. The nature of the transition: $\Gamma$-to-X or $\Gamma$-to-L is however, controversial. The distinction can not be made on the basis of electronic structure techniques that misrepresent the magnitude of the various competing effective mass tensors (e.g, LDA or GGA) or wavefunction coupling (e.g, tight-binding). Using a carefully fit screened pseudopotential method we show that the transition occurs from $\Gamma$ to X, and, more importantly, that the transition involves a finite V ($\Gamma$-X) interband coupling, manifested as an ``anti-crossing'' between the confined electron states of GaAs as the dot size crosses 15 {\AA}. The physics of this reciprocal-space $\Gamma$-X transition, as well as the real-space (type II) transition in GaAs/AlGaAs will be briefly discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J31.00004: Co-doping of Boron and Phosphorus in Silicon Nanoclusters Jae-Hyeon Eom, Tzu-Liang Chan, James R. Chelikowsky The effect of cluster size on the interaction between impurity atoms is studied using the first-principles calculations, i.e. pseudopotentials in real space. We calculate the stable configurations of B and P co-doped silicon nanoclusters as a function of size. We evaluate the evolution of interactions between impurity atoms by comparing the stable configurations. The evolution of photoluminescence is discussed. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J31.00005: Quantum Confinement and Non-Magnetic-Doped Dilute Magnetic Semiconductors Hyunwook Kwak, Tzu-Liang Chan, James R. Chelikowsky Dilute magnetic semiconductors are of interest for their unique magnetic properties and their promising role in development of ``spintronic'' semiconductor devices. Recently, a new dimension has been brought to this class of material by observing room temperature ferromagnetism in non-magnetic doped semiconductors and insulators. Using real-space pseudopotential applied to nitrogen-doped ZnO nanowires and nanocrystals, we report the theoretical evidence of magnetism in spatially confined non-magnetic doped semiconductor nanocrystals. Detailed electronic structures and magnetic properties are examined by comparing the total energy of different spin orderings and defect configurations. Besides the prediction of high Curie temperature, our results show that the ferromagnetic order becomes more stable when the nitrogen defects experience strong quantum confinement. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J31.00006: Impurity Doping in PbSe Nanocrystals Steven Erwin We recently proposed that impurity doping in colloidally grown nanocrystals is controlled primarily by kinetics, rather than by thermodynamics.\footnote{S.C. Erwin, L. Zu, M.I. Haftel, Al.L. Efros, T.A. Kennedy, and D.J. Norris. Doping semiconductor nanocrystals. Nature 436, 91 (2005).} In this ``trapped dopant'' model, the diffusion of an impurity through a nanocrystal is negligible at colloidal growth temperatures. Therefore, an impurity can only be incorporated into a growing nanocrystal if it first adsorbs on the surface and is then overgrown. But this simple surface adsorption process is complicated by a competing process: the binding of the impurity by surfactant molecules, which are used in the growth solution to passivate the nanocrystal and control its growth. Here we use density-functional theory to study the interplay and outcome of these two processes for the doping of PbSe nanocrystals by a variety of candidate impurities (Mn, Cl, In, Cd, Tl) in the presence of several widely used growth surfactants (oleic acid, trioctylphosphine, hexadecylamine). [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J31.00007: Size Limits on Doping Phosphorous into Silicon Nanocrystals Tzu-Liang Chan, Murilo L. Tiago, Efthimios Kaxiras, James R. Chelikowsky The evolution of the semiconductor industry requires continued miniaturization. As this trend continues, devices will ultimately approach the nanometer-scale and it is expected that device construction based on macroscopic laws will start to fail. Using a real-space first-principles pseudopotential method, we study doping in the nano-regime using phosphorus-doped Si nanocrystals as the prototypical system. We simulate phosphorus-doped Si nanocrystals with diameter up to 6 nm and study the evolution of the defect state with the size of the nanocrystal. Our calculated size dependence of hyperfine splitting is in excellent agreement with experimental data. The effect of quantum confinement is also manifested in the higher binding energy of the dopant electron, we estimate that phosphorus in Si nanocrystals of less than 20 nm in diameter will not be a shallow donor. We also find that for Si nanocrystals smaller than 2 nm in diameter, the phosphorus atom will be energetically expelled to the surface, leading to a self-purification mechanism that hinders the incorporation of impurity atoms into nanocrystals. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J31.00008: Interface chemistry of silicon nanocrystals embedded in silica Dundar Yilmaz, Ceyhun Bulutay, Tahir Cagin Molecular dynamics simulations of realistic-sized silicon nanocrystals (NCs) with the diameters in the range from 1~nm to 3~nm embedded in amorphous oxide are carried out till steady state conditions with the chemical environment described by the reactive force field model. We identify different types of three-coordinated oxygen (3cO) complexes, previously not noted, on the oxide interface. No double bonds were observed. We reveal that the interface bond topology evolves among different oxygen bridges through these 3cO complexes. The abundance and the charge distribution of each oxygen complex is determined as a function of the NC size as well as the transitions among them. The number of bridge bonds is observed to scale with surface area, thus the curvature has a small effect on the number of bridges. Among the three bonds of 3cO, the weaker bond is more susceptible to bond breaking which is also likely to take part in an optical activity through bond breaking and reformation. Our results indicate that the Si NC-oxide interface is more complicated than the previously proposed schemes which were based on solely double and bridge bonds. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J31.00009: Structure and electronic properties of gold-tipped CdSe nanorods Robert N. Barnett, Uzi Landman We investigate CdSe nanorods capped by gold contacts and passivated by phospho-organic molecules of varying chain length. The geometry is optimized and the electronic structure obtained using first-principles quantum mechanical methods. We discuss the formation of Schottky barriers, the development of interfacial dipoles, the presence and extent of gap states induced by the metallic contact and of states in the semiconductor energy gap associated with the passivant carbon chains. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J31.00010: F{\"{o}}rster resonant energy transfer between CdSe nanocrystals: An empirical pseudopotential/transition density cube approach Joshua Schrier, Lin-Wang Wang We study the energy transfer between semiconductor nanocrystal dots and rods of CdSe using a semiempirical pseudopotential method (SEPM) description of the electronic structure of the nanocrystals, followed by evaluation of the Coulombic contribution to the energy transfer evaluated using the transition density cube (TDC) method. Our results are compared to the dipole-dipole theory of F{\"{o}}rster to characterize the effects of nanocrystal shape, distance, and orientation. In agreement with previous effective-mass and tight-binding studies, we find that the coupling between spherical nanocrystals is well described by the F{\"{o}}rster model. In contrast, we find that rod-shaped nanocrystals display more complicated behavior, which may be relevant to exciton migration in all-inorganic nanorod-based photovoltaic devices. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J31.00011: Polar properties of ZnO nanostructures. Giancarlo Cicero, Andrea Ferretti, Alessandra Catellani The advent of nanostructured devices critically enhances the role of surface and interface effects on bulk properties and determines the physical characteristics of the material: in particular, the understanding of the electronic properties of nanosized structures requires a proper accurate treatment. Here we report on first principles density functional calculations of the structural and electronic properties of the (1-100) ``non-polar'' surface of hexagonal zinc oxide (ZnO) and compare the results with those of ZnO nanowires grown along the [0001] direction and analogous exposed cuts, with a diameter range of about 9-23 {\AA}. We discuss the changes in the nanostructures polarity in terms of two contributions, one related to changes in equilibrium lattice parameters at the nanoscale and the other related to surface effects. Variations of other relevant observables such as the piezoelectric response of the nanowire are addressed. We compare our results with those obtained for nanostructures based on materials with different polarity (e.g. SiC and InN). [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J31.00012: Softening of ultra-nanocrystalline diamond at small grain sizes Georgios Kopidakis, Ioannis Remediakis, Pantelis Kelires Ultra-nanocrystalline diamond is a polycrystalline material, having crystalline diamond grains of sizes in the nanometer regime. We study the structure and mechanical properties of this material as a function of the average grain size, employing atomistic simulations. Using the bulk and Young's moduli as probes of stiffness, we observe softening of the material as the size of its grains decreases, similar to the reverse Hall-Petch effect observed for nanocrystalline metals. This softening is attributed to the enhanced fraction of interfacial atoms. The calculated scaling of the cohesive energy and bulk modulus with respect to average grain size agrees very well with this picture. Our results suggest that softening at very small grain sizes might be a generic property of nanocrystalline materials. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J31.00013: A Molecular Dynamics Study of the Melting and Nucleation of Iron Nanoparticles Yasushi Shibuta, Toshio Suzuki The melting and nucleation of iron nanoparticles were investigated by molecular dynamics simulation using a Finnis-Sinclair potential. The nanoparticle of the bcc single-crystal was uniformly melted from the surface at a melting point during heating, whereas a nucleus was generated near one side of an undercooled liquid droplet and the solidification spread toward another side at a lower temperature during cooling. The melting point and nucleation temperature decreased with particle radius. Moreover, the solid-liquid interfacial energy was estimated to be 0.101 J/m$^{2}$ using a Gibbs-Thomson equation, which is of the same order as the experimental value based on Turnbull-Fusher's classical nucleation theory. [Preview Abstract] |
Session J32: Focus Session: Damping and Spin Relaxation
Sponsoring Units: GMAG DMP FIAPChair: Casey Miller, University of South Florida
Room: Morial Convention Center 225
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J32.00001: Spin-orbit damping in transition metals Invited Speaker: Magnetization dynamics are routinely described with the Landau-Lifshitz-Gilbert (LLG) equation. However, it is expected that the LLG equation fails to properly describe the large amplitude dynamics that occur during magnetization reversal. Improving switching speeds in nanoscale devices by tailoring materials requires both a qualitative understanding of the relaxation processes that contribute to damping and the ability to quantitatively calculate the resulting damping rates. We consider small amplitude LLG damping in transition metals as a prelude to approaching the more complicated mechanisms expected in complete reversal events. LLG damping rates in pure transition metal systems have non-monotonic temperature dependencies that have been empirically shown by Heinrich et al. [1] to have one part proportional to the conductivity and one part proportional to the resistivity. Kambersky [2] postulated that both contributions result from a torque between the spin and orbital moments. We have conducted first-principles calculations that validate this claim for single element systems [3]. Our calculations for Fe, Co, and Ni both qualitatively match the two trends observed in measurements and quantitatively agree with the observed damping rates. We will discuss how the spin-orbit interaction produces two contributions to damping with nearly opposite temperature dependencies and compare calculations of the damping rate versus resistivity with experimental results. \newline [1] B. Heinrich, D.J. Meredith, and J.F. Cochran, J. Appl. Phys., 50(11), 7726 (1979). \newline [2] V. Kambersky, Czech. J. Phys. B, 26, 1366 (1976). \newline [3] K. Gilmore, Y.U. Idzerda, and M.D. Stiles, Phys. Rev. Lett., 99, 027204 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J32.00002: Non-local damping effects in Py-Cu-FeMn trilayers Eric R.J. Edwards, Lei Wen, Zeenath Reddy Tadisina, Claudia K.A. Mewes, Subhadra Gupta, Tim Mewes We report a non-local enhancement of the Gilbert damping in Py-Cu-FeMn trilayers for in-plane ferromagnetic resonance measurements. We observe a 1/t dependence, t being the thickness of the permalloy, of the slope of the linewidth vs. frequency measurements indicating non-local contributions to the damping. In view of experimental verification of the adiabatic spin pump theory [1] in ferromagnetic heterostructures [2], these results suggest parallels in exchange biased systems. With the thickness of the non-magnetic layer, Cu, fixed below its spin-diffusion length, we understand this effect to be non-local resulting from the injection of spins by the precessing magnetization at the Py-Cu interface and subsequent scattering at the Cu-FeMn interface. \newline [1] Tserkovnyak et al., Rev. Mod. Phys. \textbf{77}, 1375 (2005). \newline [2] Heinrich et al. Phys. Rev. Lett. \textbf{90}, 187601 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J32.00003: Gilbert Damping in Single-Crystalline Ni/MgO(001) Kevin J. Smith, R. Ale Lukaszew, Anne Reilly, Gunter L\"{u}pke The dynamical properties of Nickel and Nickel alloys are of great interest in spintronic applications, as these materials exhibit low coercivity and significant magnetoresistance, however, the interplay of the various damping mechanisms, such as two magnon scattering and local resonance, is not well understood. The frequency, $\omega$, and Gilbert damping, $\alpha$, behavior of uniform spin precession on Ni/MgO(001) are studied over a wide range of external field angles and magnitudes using the Time-Resolved Magneto-Optical Kerr effect (TR-MOKE). The damping parameter shows a strong dependence on the magnetocrystalline anisotropy, suggesting a tuneability of $\alpha$ over the range of 0.09 to 0.5. Two separate trends in $\alpha$ vs. $\omega$ are observed depending on the magnitude and direction of the applied field when applied in-plane, indicating the presence of competing damping mechanisms. We further investigate these properties in measurements in which the external field is applied normal to the sample plane, thereby minimizing the role of two magnon scattering. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J32.00004: Inhomogeneous Gilbert damping from disorder and electron-electron interactions Ewelina Hankiewicz, Giovanni Vignale, Yaroslav Tserkovnyak We present a unified theory of Gilbert damping in itinerant ferromagnets at order $q^2$ ($q$ being the wave vector of the spin modulation) including electron-electron interactions and electron-impurity scattering -- the idea being that these interactions are much stronger than the spin-orbit interaction, which controls the damping in the homogeneous case ($q=0$). We show that Gilbert damping can be expressed in terms of the spin conductivity, which can be calculated straightforwardly (no vertex corrections involved) leading to a Drude-like formula in which the inverses of the disorder and interaction scattering times enter in a simple additive form. We also make contact with earlier theories of the transverse spin susceptibilities and with the spin-pumping picture of the Gilbert damping. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J32.00005: FMR Investigations on Ni-Co thin films Justin Baize, Steven Michalski, Roger D. Kirby, Mircea Chipara, David J. Sellmyer Magnetic interactions between thin films of Co and Ni spaced by a conducting, non-magnetic film (Pt) are analyzed by utilizing a Bruker ELEXSYS - EPR spectrometer operating in the X-band (9 GHz). Ferromagnetic resonance spectra have been recorded at room temperature in the out-of-plane configuration, for different orientation of the external applied film relative to the plane of the sample. The effect of the metallic film (Pt) thickness on the interaction between Ni and Co is investigated. For most orientations, the ferromagnetic line of such multilayers is the result of a convolution between the lines of Ni and Co films and has been fitted by a superposition of two Lorentzian lines. The outcome of the interaction between the two magnetic layers on the position and the width of ferromagnetic resonance lines is analyzed in detail. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J32.00006: Determination of ferromagnetic resonance of thin films with coplanar waveguide Xin Fan, Takahiro Moriyama, Rong Cao, John Xiao Determination of ferromagnetic resonance of thin films with coplanar waveguide X. Fan, T. Moriyama, R. Cao, John Q. Xiao Department of Physics and Astronomy, University of Delaware, Newark Delaware 19716 USA Recently, Coplanar Waveguide (CPW) has been applied to determine Ferromagnetic Resonance (FMR) of magnetic thin film$^{1}$ due to its simple geometry and broadband nature. Compared to the conventional method using reflection in a resonant cavity, CPW should be interpreted by the transmission line theory, taking into account of both transmission and reflection. It has been shown that FMR linewidth extracted from S-matrix after four port calibration $^{2}$ differs from that extracted from transmission only, which neglects the impedance mismatch effect. However, the four port calibration is rather complex and tedious. In this presentation, we introduce a new method to extract FMR spectrum without performing four port calibration. We use both transmission and reflection signals, and consider the impedance mismatch, We will demonstrate the difference between these two methods are negligibly small. 1. Y. Ding, T. J. Klemmer, T. M. Crawford, J. Appl. Phys. 96, 2969, 2004 2. C. Bilzer, T. Devolder, P. Crozat, J. Appl. Phys. 101, 074505, 2007 [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J32.00007: Magnetostatic micro-resonators. Alexander Kozhanov, Zach Griffith, Mark Rodwell, Jim Allen, Dok Won Lee, Shan Wang, Ajey Jacob Small scale magnetostatic wave devices are potentially important for on-chip filters for communication systems and more exotic gated spin wave devices. We describe experimental results that measure transmission and reflection resonances in micron size resonators coupled to coplanar waveguides. Ferromagnetic CoZrTa films were sputtered onto Si wafers covered by SiO$_{2}$ and lithographically patterned into stripes and crosses of varying length and width. Magnetostatic waves were excited and detected by overlaying coupling loops patterned as shorted coplanar waveguides. Transmission and reflection S-parameters of fabricated structures were measured in the frequency range (0-50)GHz. Transmission and reflection resonances strongly dependent on the geometry of the ferromagnetic device and applied magnetic field are observed. The results are modeled as standing magneto static waves in micro-resonators. We discuss effect of biasing magnetic fields, approaches to enhanced coupling to the magnetostatic resonators, magnetostatic wave interferometers and magnetization controlled magnetostatic wave switching in junctions. This work is supported by the Nanoelectronics Research Initiative (NRI) - Western Institute of Nanoelectronics (WIN) [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J32.00008: Measurements of spin diffusion length in gold with point contact Andreev reflection spectroscopy Muhammad Faiz, Raghava Panguluri, Benjamin Balke, Sabine Wurmehl, Claudia Felser, Andre Petukhov, Boris Nadgorny Spin diffusion length, $L_{s}$, is of fundamental importance for spin dependent transport and spintronic devices. So far, most of the measurements of $L_{s}$ in non-magnetic metals have been done in the lateral non-local geometry, with the chemical potential difference characterizing the spin imbalance. In our approach the spin diffusion length was measured directly with point contact Andreev reflection spectroscopy. A spin polarized current was injected from a ferromagnetic electrode, Co$_{2}$Mn$_{0.5}$Fe$_{0.5}$Si Heulser alloy, into Au films of variable thickness. The spin current, which gradually decays with the increased thickness of the film, was measured with a superconducting Nb tip. We developed a phenomenological theory which allowed us to determine the values of $L_{s}$ in such a system. We found $L_{s}$ to be on the order of 250 nm at 4 K, comparable to the results obtained by other techniques. Similar results were obtained with a Gd single crystal. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J32.00009: A single minority spin-flip detector Y. Hancock A family of inhomogeneous Hubbard models is shown to have multi-level spin-switching properties. The generic structure of the device is NNNMMMNNN, which defines a linear (quasi-0D) cluster, having open boundary conditions. Within the context of the Hubbard model, N refers to a nonmagnetic (U=0) atom and M is a magnetic atom having finite value of the Hubbard U. The model is solved by numerically exact diagonalization. Localized spin-switching is obtained as a function of the electron filling and is activated by application of an external magnetic field. Arbitrary numbers of nonmagnetic atoms at the edges of the chain, relative to the number of magnetic atoms in the system, lead to a range of spin-switching signals. One potential application to be demonstrated is that of a single minority spin-flip detector. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J32.00010: Anisotropic spin exchange between electrons mediated by spin-orbit interaction Suhas Gangadharaiah, Jianmin Sun, Oleg Starykh It is well known that the exchange coupling between electrons in the presence of Rashba spin-orbit interaction (SOI) is anisotropic. In particular, due to the SOI, electron spins are not conserved during a tunneling process as a result anisotropic exchange terms of the Dzyaloshinkii-Moriya (DM) type are generated. In this talk, we re-visit and clarify the role of spin-orbit interaction in lowering the symmetry of the exchange coupling between spins. We point out that the exchange Hamiltonian, despite of its anisotropic appearance, retains spin-rotational ($SU(2)$) invariance to the second order in the spin-orbital coupling. We argue that spin- rotational symmetry is broken only in the fourth order in SOI coupling. To capture this, we calculate the exchange problem along the lines of the Heitler-London approach [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J32.00011: Noise Spectrum Signature of Large Jumps in the Magnetization of a Finite System Near a Phase Transition Zhi Chen, Clare Yu It is well known that a finite-size spin system can undergo thermally driven flips of the magnetization of the system as a whole. But what is an experimentally measurable signal of this? We show that the low frequency noise spectrum of the magnetization has a distinct signature of these total magnetization flips which as particularly evident just below the phase transition temperature. To see this, we studied the magnetization and energy spectra of the 2D Ising model by using Monte Carlo simulations. We find that at $T_C$ the noise power is a power law in the frequency where the power is given by the critical exponents. As the frequency decreases for a finite system just below $T_C$, the magnetization noise spectrum crosses over to $f^{-2}$. We show that this is due to large jumps in the entire magnetization. Finally at the lowest frequencies, the noise spectrum saturates at a frequency that depends on the system size. The method we used can be applied without much modification to quantify the contribution of jumps to the dynamics of other systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J32.00012: Alkanethiol capping-induced changes in the magnetoresistance of Co Brad Knaus, Samir Garzon, Thomas M. Crawford We demonstrate that chemisorption of a $\sim $ 1 nm thick dodecanethiol (C$_{12}$H$_{25}$SH) self-assembled monolayer on the surface of an Au film alters the magnetotransport of an underlying Co film. Giant paramagnetism has been previously detected in Au thin films and nanoparticles capped with alkanethiols via SQUID magnetometry. By observing the impact of Au-thiol magnetism on a ferromagnetic thin film we avoid background subtraction and the influence of impurities. After thiolation, significant changes are observed in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) from Co/Au bilayers (30 nm/5-60 nm) patterned into 1 x 5 mm$^{2}$ bars. Driven with nearly perpendicular external fields, we observed domain nucleation shifts of $\sim $0.65 T and changes in the FWHM of the AMR. We have also measured differences in the MR as a function of ambient exposure time presumably due to oxidation effects known to occur in alkanethiols. Effects of surface scattering were eliminated with the introduction of a 12nm Al$_{2}$O$_{3}$ insulation barrier between Co and Au. We have calculated that the effects of magnetostriction are too small by more than two orders of magnitude to explain our observations. We therefore hypothesize that Au-thiol magnetization acts as a source of magnetic field which biases the underlying Co film. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J32.00013: The role of uncompensated spins in exchange biasing Hans J. Hug, Iris Schmid, Peter Kappenberger, Sara Romer The origin of the exchange bias (EB) effect has been traced back to the existence of pinned uncompensated spins (UCS) in the antiferromagnet (AFM) or at its interface. However, the understanding of the underlying mechanism is still clouded by contradictory reports: For example, both a parallel as well as an antiparallel orientation of the UCS relative to the magnetization direction of the ferromagnet (FM) were reported for similar FM/AFM systems. Here different magnetization histories in magnetometry and high resolution magnetic force microscopy measurements are used advantageously to demonstrate the co-existence of pinned UCS that are parallel and antiparallel to the cooling field in metallic (IrMn) and oxidic (CoO) EB systems. We further conclude that the EB effect is mainly a result of pinned interfacial UCS, which are antiparallel to the FM spins [1]. In further experiments, the distribution of density of the UCS were imaged on the length scales of single grains. A surprisingly strong fluctuation of the local UCS density (UCSD) was observed. A correlation between the UCSD and the local exchange field was performed. Clearly, a high UCSD results in a high local exchange bias field. Regions with an anti-biasing effect were found. Using grain-boundary engineering, exchange-biased materials without such regions could be fabricated that showed a substantially increased exchange bias effect. \newline [1] Schmid et al. EPL, 81 (2008) 17001 [Preview Abstract] |
Session J33: Focus Session: Mostly Spins in Group IV Semiconductors and Organics
Sponsoring Units: GMAG FIAP DMPChair: Jiwei Lu, University of Virginia
Room: Morial Convention Center 224
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J33.00001: Ferromagnetism in Mn-implanted Ge and epitaxial GeC Samaresh Guchhait, John Markert, Mustafa Jamil, Sanjay Banerjee 20 keV energy Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains less than 1\% carbon. X-ray diffraction data shows a single crystal phase for the GeC film, and the surface rms roughness is about 0.3 nm, measured with AFM. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$C for both samples. For this relatively low energy Mn ion implant, the range is about 17 nm and the straggle is about 9 nm. A SQUID magnetometer study shows ferromagnetism in both samples. While the Curie temperature for both samples is about 180 K, the in-plane saturated magnetic moment per unit area for the first sample is about $2.2\times10^{-5}$emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5}$emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J33.00002: Influence of carrier type on the ferromagnetism in Ge$_{1-x}$Mn$_{x}$ thin films Wenjing Yin, Jiwei Lu, Li He, Jiani Yu, Wenbin Fan, Robert Hull, Stuart Wolf We have been studying the magnetic and transport properties of Mn doped Group IV semiconductors. Mn ions have been implanted into both boron doped P type and phosphorus doped N type Ge thin films respectively. A ferromagnetic hysteresis loop has been observed in P type samples at low temperatures and supermagnetism remains strong at 300 K. The P type samples show much stronger ferromagnetism than N type samples. At 5K, the ferromagnetic saturation moment (Ms) of 5{\%} Mn doped p-Ge sample is $\sim $0.65 Bohr magneton per Mn, which is almost twice as much as that of the 5{\%} Mn doped n-Ge. Rapid thermal annealing has been used to reduce the ion implantation damage as well as to help Mn ions to incorporate into Ge lattice. In this talk we will present magnetic, transport and electron microscopy characterization of these samples. We are in the process of trying to understand the nature of the ferromagnetism in these films and its correlation to carrier type. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J33.00003: Epitaxial Growth and Properties of Multilayers Containing (CoMn)$_{0.1}$Ge$_{0.9 }$and Ge (001) Liang He, Charles Malmberg, Brian Collins, Frank Tsui Epitaxial growth and magnetic and magnetotransport properties of superlattices containing Co and Mn codoped Ge magnetic semiconductor layers with Ge (001) interlayers have been studied. Layer-by-layer epitaxial growth has been observed during the deposition of the doped magnetic semiconductor layers, i.e. (CoMn)$_{0.1}$Ge$_{0.9}$, and that of the undoped Ge interlayers, as indicated by persistent oscillations in the intensity and width of reflection high energy electron diffraction. The superlattices exhibit a ferromagnetic transition near 100 K and superparamagnetism at higher temperatures, as determined by temperature and field dependent magnetic measurements using SQUID magnetometry. Magnetotransport properties indicate that the superlattices are p-type semiconductors with very large positive magnetoresistance (MR) and anomalous Hall effect (AHE). Conduction at low temperatures ($<$10 K) is dominated by variable range hopping in the impurity band. Below 100 K, the MR is found to scale with square of the magnetization, whereas the AHE exhibits a linear dependence on magnetization. Above 100 K, in the superparamagnetic regime the magnetotransport parameters scale with magnetic field. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J33.00004: Different magnetic moment in Mn-doped amorphous group-IV semiconductors: a comparison study between Si and Ge matrices. Li Zeng, Erik Helgren, Cinthia Piamonteze, Elke Arenholz, Addison Huegel, Frances Hellman Mn-doped amorphous Si ($a-$Si) and Ge ($a-$Ge) are prepared by $e$-beam co-evaporation for a wide range of concentrations (0.5-18 at.{\%}) to explore the Mn local moment in group-IV semiconductors. We find that Mn behaves quite differently in these two matrices: in $a$-Si, the Mn local moment is quenched, even for the lowest doping (0.5 at.{\%}), while in $a-$Ge, a large Mn moment is observed, with a spin-glass ground state. X-ray absorption spectra (XAS) of $a-$Mn$_{x}$Si$_{1-x}$ have very broad $L$-edge absorption peaks which correlate with the quenched magnetic state. The quenched Mn moment in $a$-Si is unexpected and can be understood by the formation of Anderson-localized itinerant states even on the insulating side of the metal-insulator transition. By contrast, XAS of $a-$Mn$_{x}$Ge$_{1-x}$ show atomic multiplets. $a$-Mn$_{x}$Si$_{1-x}$ has positive magnetoresistance (MR) like typical non-magnetic disordered electronic systems, while $a-$Mn$_{x}$Ge$_{1-x}$ has negative MR, consistent with magnetization data. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J33.00005: Magnetic Properties of Ge$_{1-x}$Mn$_{x}$Te Thin Films James R. Anderson, W. Knoff, Malgorzata Gorska, T. Story, Costel R. Rotundu We have measured the magnetization of Ge$_{1-x}$Mn$_{x}$Te thin films with 0.08 $<$ x $<$ 0.19 at magnetic fields up to 7 T at temperatures from 2 to 385 K. The monocrystalline epitaxial layers of Ge$_{1-x}$Mn$_{x}$Te were grown on (111)-oriented BaF$_{2}$ crystalline substrates in a home-built MBE system. The layer structure was rhombohedral, thickness in the range 0.5 -- 1 micron, and hole carrier concentration of the order 10$^{21}$ cm$^{-3}$. Magnetization measurements were made using a Quantum Design MPMS system. At low temperatures the samples were ferromagnetic. The ferromagnetic -- paramagnetic transition was observed in various samples in a broad temperature range from 20 -- 100 K. In some samples we have seen two peaks in the temperature dependence of the low-field magnetization. These peaks may be evidence of two chemical phases or of an electronic phase separation. The origin of this effect is under investigation at the present time. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J33.00006: Spin Engineering with Ion Implantation of Diamond Nitrogen-Vacancy Centers G.D. Fuchs, F.J. Heremans, D.D. Awschalom, R. Hanson, A. Batra, T. Schenkel, S. Shirvastava, T. Mugato, E. Sideras-Haddad Nitrogen-vacancy (NV) defect centers in diamond exhibit long coherence times of spin states at room temperature. Individual NV centers can be optically initialized and read-out, making them attractive candidates for quantum information. By exploiting the interactions with naturally occurring, nearby spins, NV centers have been incorporated into two quantum bit (qubit) systems. Despite these successes, scaling NV qubit systems with naturally occurring spins is a challenge. Here we present an alternative approach where we deliberately place NV centers using spatial and energy selective ion-implantation of nitrogen into synthetic diamond samples with low nitrogen content. Since we use isotopically pure N-15 for implantation, we can distinguish the implanted NV centers from the naturally occurring N-14 centers by measuring the hyperfine splitting of the electron spin resonance peaks of individual NV centers. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J33.00007: Electronic transport in nitrogen-rich diamond F.J. Heremans, G.D. Fuchs, C.F. Wang, D.D. Awschalom, R. Hanson Electronic transport in carbon-based materials, including carbon nanotubes, graphene, and diamond, have been receiving significant attention as potential alternatives to silicon-based electronics. In particular, diamond`s excellent thermal properties provide a promising alternative in power-sensitive applications. Here we present studies of the photo-excited electronic transport in nitrogen-rich type IB diamonds. In addition to the study of the carrier dynamics within this system, we discuss a charge storage effect that may find potential application in charging-based memories.~ We find that the discharge curves follow a ``stretched-exponential'' form [1] with a fixed exponent, which does not depend on electrode spacing, voltage, and illumination intensity.~ These findings are discussed in the context of a transport mechanism in this nitrogen-rich diamond substrate. \newline \newline [1] C.G. Van de Walle Phys. Rev. B, \textbf{53}, 11292 (1996) [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J33.00008: Polarization-selective excitation of nitrogen vacancy centers in diamond T. P. Mayer Alegre, C. Santori, G. Medeiros-Ribeiro, R. G. Beausoleil The nitrogen-vacancy (N-V) center in diamond is promising as an electron spin qubit due to its long-lived coherence and optical addressability. Nevertheless, some work remains in determining the detailed energy level structure. In particular, little emphasis has been put on the microwave polarization selection rules. In typical optically-detected magnetic resonance (ODMR) experiments, the microwave transitions are driven by linearly polarized fields. Thus the individual transitions can be selected only by applying a constant magnetic field to lift the degeneracy. Typically the magnetic field is applied along the quantization axis of the NV center to avoid mixing of the spin states. This applied magnetic field is in principle unnecessary with circularly polarized microwave excitation. Using a resonator designed to produce circularly polarized microwaves, we have investigated the polarization selection rules of the N- V center. We first apply this technique to N-V ensembles in [100] and [111]-oriented samples. Next, we demonstrate an imaging technique, based on optical polarization dependence, that allows rapid identification of the orientations of many single N-V centers. Finally, we test the microwave polarization selection rules of individual N-V centers of known orientation. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J33.00009: Spin transport studies in high mobility organic semiconductor, Pentacene (C$_{22}$H$_{14}$) J.H. Shim, V. Karthik, J.S. Moodera Spin transport in organic semiconductors (OSs) is currently an attractive research area because these materials in general can have long spin coherence length due to weak spin-orbit scattering and hyperfine interaction. Charge and spin transport in OSs strongly depend on the intermolecular overlap of electronic wave functions and defects in OS which can be influenced by the thin film growth conditions. Here we chose OS, pentacene (C$_{22}$H$_{14})$, which is among the most promising materials due to its high mobility. From the measured temperature dependence of the conductance in Al/Pentacene/Co thin film tunnel junctions, different transport mechanisms were observed for pentacene barriers of various thicknesses. Thin, amorphous pentacene films were deposited with and without Al$_{2}$O$_{3}$ seed layer at room temperature. Significant tunnel magnetoresistance (TMR) was observed at room temperature in junctions with Co and Fe electrodes and pentacene tunnel barrier. These results will be presented and discussed. Research supported by ONR and KIST-MIT programs. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J33.00010: Is there really spin transport in Alq$_{3}$ spin-valves? J. Samuel Jiang, J.E. Pearson, S.D. Bader There have been reports of GMR and extremely long spin relaxation in Alq$_{3}$- based spin valves.\footnote{Z. H. Xiong et al. Nature, 427, 821 (2004); S. Pramanik et al. Nature Nanotech. 2, 216 (2007).} However, it has also been suggested that direct tunneling through locally-thin regions of the Alq$_{3}$ layer could be the magnetoresistance (MR) mechanism, i.e. the reported MR may be due to artifacts rather than spin transport via the molecular levels in Alq$_{3}$.\footnote{W. Xu, et al. Appl. Phys. Lett. 90, 072506 (2007).} We present transport measurements on Alq$_{3} $-based spin valves and unipolar devices where the Alq$_{3}$ thickness is beyond the tunneling limit. The I-V characteristic is highly asymmetric and strongly temperature- dependent, different from the behaviors of devices where GMR has been reported. The charge transport in the Co/Alq$_{3}$/Fe spin valves is by holes only and is injection- limited. More importantly, we observe no measurable MR in our non-tunneling Co/Alq $_{3}$/Fe spin valves, or in Co/AlO$_{x}$/Alq$_{3}$/Fe structures where spins can be injected via the AlO$_{x}$ barrier. These results indicate that spin transport in Alq$_ {3}$ is unlikely. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J33.00011: Inelastic tunneling spectroscopy study on organic semiconductor tunnel barriers with magnetic electrodes K.V. Raman, J.H. Shim, J.S. Moodera Spin injection and transport through organic semiconductor (OS) is recently being researched extensively. Exploring the interfacial structural and chemical modifications in FM/OS/FM tunnel junctions can lead to a better understanding of spin injection and transport in OS. Inelastic tunneling spectroscopy (IETS), a high sensitivity technique, measures the vibrational and excitational modes of the molecules within a tunnel barrier, which are greatly influenced by any distortions in the molecules. These measurements are performed on thin films of OS, rubrene and pentacene, using Co/seed/OS/Py and Al/seed/OS/Al junctions, all grown in-situ, for two different seed layers viz. Al$_{2}$O$_{3}$ and LiF. The IETS spectra matches well with the reported Raman and IR spectroscopy measurements performed for powder and bulk single crystal samples. In addition, the IETS spectra show weak signatures of the molecular distortions through modifications to certain phonon peaks. Due to the amorphous nature of the films certain electronic states are also observed at higher bias voltages. The effect of vibrational modes on the spin conserved tunneling and the effect of different electrodes on the IETS spectra will also be presented and discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J33.00012: Spin transport studies through spin filter and organic semiconductor hybrid tunnel barriers DeungJang Choi, Tiffany Santos, Tae Hee Kim, Jagadeesh Moodera Spin polarized tunneling through a hybrid tunnel barrier of spin filter EuO magnetic semiconductor and an organic semiconductor Rubrene was investigated. With magnetic tunnel junction structures such as Co/Rubrene/EuO/Al or Cu we observed a magnetoresistance (MR) of up to 8.5{\%}, whereas from the junction resistance versus temperature data, we deduced the capability to produce spin polarization (P) of up to 99{\%}. Thus the observed low MR has been attributed to spin scattering by defects at various EuO and Rubrene interfaces and possible nonstoichiometry in EuO. With further optimization it should be possible to reach towards the expected large MR in such systems. Moreover, Rubrene which has a low barrier height also serves to magnetically decouple the ferromagnetic electrode from EuO, necessary for independent magnetic switching and reaching high MR. This study demonstrates the possibility of combining organic and spin filter materials as tunnel barriers. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J33.00013: Quantitative analysis of molecular spintronics Zhanyu Ning, Yu Zhu, Jian Wang, Hong Guo We report quantitative analysis of nonequilibrium spin injection from Ni contacts to octanethiol molecular spintronic system. Our calculation is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. The first principles results allow us to establish a clear physical picture on how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magneto-resistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent {\it ab initio} transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of Ni surface is crucial for accurate results. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J33.00014: Hall conductivity and colossal magnetoresistance in FeSb2 Rongwei Hu, Vesna Mitrovic, Cedomir Petrovic We report the anisotropic Hall conductivity and discuss mechanism of colossal magnetoresistance in Co doped FeSb2. The Hall conductivity is described in a mutil-carrier picture, in which large carrier mobility was found in Co doped FeSb2. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J33.00015: Electric field induced metal-insulator transition and colossal magnetoresistance in CdCr$_{2}$S$_{4}$ C.P. Sun, C.C. Lin, J.L. Her, S. Taran, C.C. Chou, C.L. Chan, C.L. Huang, H. Berger, H.D. Yang Multiferroic ordering existing in a single material is a recent hot topic in the field of condensed matter physics due to its potential application in device control. The chromium chalcogenide spinel CdCr$_{2}$S$_{4}$ is one of the attractive materials investigated by Hemberger \textit{et al.} recently.[1] Based on the electrical measurement, there is no discontinuity through the ferromagnetic ordering at $T_{C} \quad \sim $ 85K.[2] We measure the temperature dependent resistance under various electric fields to investigate the electrical properties of the present material. To our knowledge, we first observe the electric field induced metal-insulator transition in this material around $T_{C}$. Moreover, a colossal magnetoresistance (CMR), which is comparable to that of manganese-based CMR material, is also observed near $T_{C}$. The origin for these properties is discussed. [1] J. Hemberger, P. Lunkenheimer, R. Fichtl, H.-A. Krug von Nidda, V. Tsurkan, A. Loidl, Nature 434, 364 (2006). [2] P. K. Baltzer, H. W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965). [Preview Abstract] |
Session J35: Focus Session: Emerging Materials and Devices II
Sponsoring Units: FIAP DMPChair: Leonid Tsybeskov, New Jersey Institute of Technology
Room: Morial Convention Center 227
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J35.00001: Silicon Photonics: An Affordable Solution for Communications at Terabit Data Rates Invited Speaker: Over the past 20 years, laser sources, filters, modulators and detectors have followed the example of lithographic miniaturization provided by the silicon microelectronics industry. Optical devices with sizes smaller than the wavelength of light have evolved, and can be integrated into compact photonic systems. More recently, with the introduction of silicon on insulator (SOI) wafers and with the capability to pattern silicon with 100nm resolution, it has become possible to build high quality optical devices within silicon fabrication lines. Using the very same procedures as silicon electronics inexpensive optical and electronic components can thereby be constructed side by side on 8 inch wafers. This capability enables tuning and switching of complex optical systems with on-chip electronics, as well as data communications at Terabit/second bit-rates, all available at the less than 10 dollars per square centimeter cost traditional in the silicon fabrication industry. By integration of nonlinear optical materials with silicon and the fabrication of high index contrast structures that concentrate light to approximately 1 GW per square centimeter fields, it is also possible to fabricate very fast optical modulators and switches that can operate at THz frequencies. In this presentation, we review the present capabilities of ``silicon photonics'' and examine the prospect for inexpensive silicon photonic devices to switch, generate and detect THz frequencies. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J35.00002: Coherence and polarization properties of individual antenna-like metal nanowires Levente Klein, Hendrik Hamann, Yat-Yin Au, Snorri Ingvarsson We investigate the coherence and polarization properties of thermal radiation from resistively heated individual metal nanowires. The high aspect ratio nanowires are fabricated by e-beam lithography with widths from 60 nm up to 2 um, dimensions well below the wavelength of the emitted thermal radiation. The thermal radiation from the metal nanowires has a spatial orientation perpendicular to the substrate with a radiation patterns similar to an electric dipole radiation. Furthermore, the emitted thermal radiation becomes highly polarized as the width of the nanowires decreases. For very narrow nanowires the polarization is oriented along the long axis of the nanowires while its orientation becomes perpendicular to the long axis for widths above 1 um. While in the far field the thermal radiation is incoherent, in the near field the emitted thermal radiation becomes coherent and interference fringes are observed as a mirror approaches the nanowire at gaps smaller than 20 um. The interference fringes are generated by the thermal radiation from the nanowire and its image in the mirror with an increased fringe visibility for narrower nanowires. A lower bound for the coherence length for thermal radiation is 30 um for very narrow metal nanowires. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J35.00003: Fullerene microcrystals under pressure Murilo L. Tiago, Fernando A. Reboredo Solid buckminsterfullerene (C$_{60}$) is known to be very soft, with a large number of crystalline phases that can be accessed by temperature or pressure. External pressure reduces the intermolecular distance, which affects the electronic structure in three ways: by increasing the overlap between molecular orbitals on neighbor molecules, by inducing additional chemical bonds between molecules, and by deforming the molecular structure. Having an exciton gap that is sensitive to pressure suggests that C$_{60}$ can be used as active element in an optical pressure gauge: a device that can detect pressure dynamically on the material from the red-shift of its optical spectrum. Using first-principles many-body theories, we calculate the optical gap of solid C$_{60}$ and its pressure dependence. We also analyze the dependence of optical gap with deformations in the molecule. Our calculations are based on solving the Bethe-Salpeter for electron-hole excitations. The electron self-energy is calculated within the GW approximation. We use pseudopotential density-functional theory to determine the electronic structure of C$_{60}$ in its ground state. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J35.00004: Nanowire waveguide made of extremely anisotropic metamaterials Y.J. Huang, W.T. Lu, S. Sridhar We consider wave propagation along a cylindrical fiber with anisotropic optical property. Exact solutions are obtained for all the modes. For extremely anisotropic cylinder where the transverse permittivity is negative while the longitudinal permittivity is positive, only transverse magnetic and hybrid modes will propagate on the waveguide. At a given frequency the waveguide support infinite number of eigenmodes. For the TM modes, there exists at most one forward wave. The rest of them are backward waves. These waveguides can be used as filter and phase shifter in integrated optical circuits. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J35.00005: Doping Poly(p-phenylene vinylene) with Phosphomolybdate through Layer-by-Layer Fabrication for Optoelectronic Applications Chris Nelson, Bin Wang Poly(p-phenylene vinylene) (PPV) mulilayers have been prepared from its cationic precursor via the layer-by-layer deposition. The photoluminescence (PL) and film thickness of the multilayers have been examined via fluorimetry and atomic force microscopy. The PL of the multilayers has been observed that is consistent with the literature results. When phosphomolybdate PMo12 is incorporated into the multilayer structure, PL quenching is detected that is proportional to the amount of PMo12 used. The quenching is interpreted as exciton diffusion through the polymer multilayers, followed by exciton dissociation at the polymer/PMo12 interface. We show that the modeling used for calculating the PL intensities derived from inorganic semiconductors is also applicable to conjugated polymers. According to the model, an exciton diffusion length is found to be 11.5$\pm$0.4 nm. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J35.00006: Scattering studies of stacked polymer/liquid crystal composites Sameet Shriyan, Kashma Rai, Adam Fontecchio In this work we investigate light scattering from stacked polymer/liquid crystal composite films on several different substrates. Effect of different substrates such as glass, PET and PMMA coated with conductive layers such as Indium-Tin-Oxide (ITO) and PEDOT:PSS conducting polymer are analyzed using scattering and wavefront measurements. Scattering of light in both transmission and reflection mode is measured as a function of angle and its effect using different substrate layers coated with different conducting layers is analyzed. Shack Hartmann wavefront sensor is used to characterize the effect of various substrate and conducting layers on the transmitted wavefront quality in both transmission and reflection mode. Optimal stack length at which the scattering and wavefront degradations are at acceptable levels is derived form the scattering and wavefront measurements. Spectrometry results show notch formations at different wavelength and the effect on transmitted baseline reduction due to scattering using different substrates. SEM imaging shows the effect of different substrates and conducting layers on the formation and quality of gratings along with the LC droplet size which contributes to scattering. Optical path and stack length reduction of up to 45{\%} for a stack of 10 layers is confirmed. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J35.00007: Broadband Wavelength Spanning Holographic Polymer Dispersed Liquid Crystals Kashma Rai, Sameet Shriyan, Adam Fontecchio Broadened interaction wavelength of holographic polymer dispersed liquid crystals (HPDLCs) have extensive applications in beam steering for instrument clusters, hyperspectral imaging, wavelength filtering and construction of lightweight optics. A novel simultaneous time and spatial multiplexing formation configuration is proposed here, to increase narrow wavelength reflecting notch to broad range wavelength spanning device. HPDLC films have electro-optic controllability by applying field. No moving parts, light weight, small footprint compared to prisms and lenses, high color purity make the broadband wavelength HPDLCs desirable for the above applications. Varying the incident laser beam exposure angles using motorized rotating stage, during formation is the key step here for their formation in a single medium. The fabricated broadband wavelength sensitive HPDLCs are characterized for the uniformity of the reflected peak and electro optic response. Their output wavefront is analyzed using wavefront analysis technique. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J35.00008: 1.54 $\mu$m emitters based on monolithic integration of Er doped GaN with nitride emitters Rajendra Dahal, Cris Ugolini, Ashok Sedhain, Jingyu Lin, Hongxing Jiang, John Zavada Er doped III-nitride semiconductors have emerged as very promising materials for applications in photonic devices due to their novel optical and physical properties. Optoelectronic and photonic devices based on Er doped GaN are expected to meet the demand for next generation telecommunication devices due to efficient and temperature stable 1.54 $\mu $m emission from Er doped GaN. We report here on the successful fabrication of a chip size current injected 1.54 $\mu $m emitters by monolithic integration of Er doped GaN epilayers with 365 nm nitride light-emitting diodes (LEDs). Er doped GaN and In$_{0.06}$Ga$_{0.94}$N epilayers were grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD). The photoluminescence excitation (PLE) and absorption spectra of these epilayrs were investigated to understand the 1.54 $\mu $m emission mechanism. A strong correlation between PLE and absorption spectra near the energy bandgap of host nitride epilayers suggest that band to band absorption and subsequent energy transfer to Er ion for 1.54 $\mu $m emission is a much more effective excitation mechanism compared to the direct absorption by Er ion. The success opens the possibility for next generation IR integrated photonic devices such as emitters, detectors, and optical amplifiers. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J35.00009: Synthesis of Metal Silicides by Low Energy Ion Implantation Prakash Poudel, Lee Mitchell, Jianyou Li, Brian Gorman, Arup Neogi, Bibhudutta Rout, Jerome Duggan, Floyd McDaniel A 55KeV Osmium beam was used to implant (5x10$^{16}$ atoms/cm$^{2 })$ into p-type-Si (100).The implantation was performed with the ion source of a National Electrostatic Corp. 3 MV Tandem accelerator. The implanted sample was annealed at 650 \r{ }C in a gas mixture that was 4{\%} H$_{2}$ + 96{\%} Ar. Measurements showed that the samples contained a mixture of continuous polycrystalline osmium disilicide and a silicide layer. Rutherford Backscattering Analysis with 1.5 MeV Alpha particles was used to monitor the precipitate formation. Photoluminescence measurements were also performed to study possible applications of silicides in light emission. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J35.00010: Sb-doped p-type ZnO and its application on light emitting devices Sheng Chu, Leelaprasanna J. Leela, Zheng Yang, Jae Hong Lim, Lin Li, Jianlin Liu Reliable Sb-doped p-type ZnO on silicon substrate was grown by molecular beam epitaxy. The hole concentrations up to 10$^{19}$/cm$^{3}$ were achieved by tuning the Sb cell temperature. The results from XPS and photoluminescence spectrum confirmed the theoretical prediction that the Sb doping mechanism in ZnO is the formation of complex shallow acceptor Sb$_{Zn}$+2V$_{Zn}$, with a low ionization energy of about 150meV. Then ZnO p-n homojunction light emitting diodes (LED) were fabricated based on the p-type Sb-doped layer, and the Ga-doped n-type ZnO layer. Low specific resistivity Au/NiO and Au/Ti contacts were deposited on top of the p-type and n-type layers, respectively, and the contacts were annealed to obtain ohmic conduction. Electroluminescence measurements were performed on the devices under different temperatures and injection currents. Strong near-band edge emissions were clearly observed at room and low temperatures. The device exhibited dominant UV peak at 3.31eV over the deep level emissions at 9K, which is the result from the large build in potential in the junction and the good film quality. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J35.00011: 2.4 $\mu$m GaInAsSb Mesa Photodiode Detectors: Leakage Currents and Ultimate Performance John Prineas, Jeff Yager, Jon Olesberg, Shahram Seydmohamadi Short-wave infrared photodiodes play an important role in areas such as molecular sensing, thermophotovoltaics, and astronomical study of galaxy, star, and planetary formation. Here we present results and analysis of uncoated, unpassivated, GaInAsSb mesa photodiodes. We have currently achieved room temperature peak specific detectivity D*=6x10$^{10}$ Jones, dynamic resistance of 25 $\Omega $-cm$^{2}$, and quantum efficiency of 50{\%}. Devices are limited primarily by sidewall leakage currents, initially due to generation-recombination, and over time due to Ohmic leakage from buildup of sidewall oxides. Based on material parameters obtained in this as well as other studies, ultimate diode performance is predicted, and compared to extended-wave InGaAs/InP and HgCdTe detectors. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J35.00012: Optically Active Erbium with Co-dopants in Silicon S. Abedrabbo, A. Haddad, K. Albath, Q. Younis, A.T. Fiory, N.M. Ravindra Erbium impurity centers in silicon with strong optical emission properties in the near-Infrared are being sought for efficient silicon-based light sources because of the inherent advantages of integrating silicon photonics with VLSI technology. This work reports investigations of adding proper co-dopants to erbium in silicon through a cost-effective combination of techniques, comprising physical vapor co-deposition, implantation doping, ion beam mixing, and thermal annealing. Processed samples are characterized optically by photoluminescence and structurally by Rutherford backscattering. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J35.00013: Characteristics of a Cr$^{4+}$-doped glass-ceramic; a new material for photonic devices L.L. Isaacs, A.B. Bykov, V. Petricevic, I. Popov, M. Yu. Sharonov, J. Steiner The compound 1.0(Cr-doped Ca$_{2}$GeO$_{4})$-1.0(Li$_{2}$O)-0.2(A$l_{2}$O$_{3})$-0.5(B$_{2}$O$_{3})$, on quenching from the melt and subsequent heat treatment, yields a transparent glass-ceramic. The nanocrystallites formed by the ceramming procedure are distributed homogeneously in the bulk. Differential scanning calorimetry was used to determine the glass to crystal transformation temperature, T$_{g}$, and its dependence on heating rate. The activation energy for the glass to crystallite nucleation is 62kJ/mol. The calculated Avrami exponent is 1, in agreement with scanning electron microscopy observations. X-ray diffraction data indicates that the structure of the nanocrystallites is that of distorted Cunyite (Ca$_{2}$GeO$_{4})$. Electron microscopy indicates that the crystallite sizes are less than 1$\mu $m. The growth mode of the crystallites is ``needle'' like. The material exhibits broadband emission between 1050 to 1600 nm, with a maximum at $\sim$1260 nm. The spectroscopic and optical properties indicate, that this material is a promising candidate for use in microelectronics, micro lasers and as fiber optic transmission lines. [Preview Abstract] |
Session J36: Focus Session: Materials for Photovoltaics and Photocatalysis I
Sponsoring Units: DMPChair: Matthias Batzill, University of South Florida
Room: Morial Convention Center 228
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J36.00001: Polycrystalline TiO$_{2}$ films with silver nanoclusters for photocatalysis Frank Womack, Fei Wang, Asoka Sekharan, Phillip Sprunger, Richard Kurtz Metallic Ag nanoclusters nucleate on TiO$_{2}$(110) due to weak substrate interactions and the need to minimize their surface free energy. EELS results on single crystal TiO$_{2}$ have shown that those nanoclusters have a distinct plasmon resonance at 3.7 eV which can be red-shifted when incorporated within a dielectric of titania. Polycrystalline oxides consist of mostly low surface free-energy faces and nanoclusters are expected to nucleate when dosed with Ag. We will present a combined synchrotron-based photoemission, Auger, and optical spectroscopy study of polycrystalline TiO$_{2}$ films functionalized with Ag. We have made thin films of TiO$_{2}$ by thermal evaporation of titanium followed by oxidation at elevated temperatures. Ag was then deposited via thermal evaporation of silver, and covered with additional layers of Ti that were subsequently oxidized and characterized with photoelectron and optical spectroscopies. We will discuss these data in the context of nanocluster formation \textit{vs} bandgap doping. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J36.00002: Buried silver nanoclusters on TiO$_{2}$(110) for photocatalysis Fei Wang, Frank Womack, Asoka Sekharan, Brendan Waston, Phillip Sprunger, Richard Kurtz Ag nanoclusters grown on TiO$_{2}$(110) can promote photochemistry by enhancing photoabsorption via their plasmon resonances. Overcoating the Ag clusters with a thin layer of titania red-shifts the plasmon to better match the solar spectrum and protects the nanoclusters from the environment. Our STM studies show that Ag clusters $\sim $5nm across and 2nm high nucleate on the TiO$_{2}$(110) surface at room temperature. Photoemission performed at the LSU CAMD synchrotron shows that the clusters interact weakly with the substrate, although there is charge transfer from surface defects to the first nanoclusters that nucleate. EELS shows that the bare clusters exhibit a plasmon resonance located at 3.8 eV. Ti overgrowth and subsequent oxidation gives rise to new losses at about 1.5eV as observed in EELS. We will discuss our work at incorporating the nanoclusters within the titania matrix in light of their potential for producing hot electron-hole pairs for surface chemistry. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J36.00003: Epitaxial In$_{2}$O$_{3}$ and Sn-doped In$_{2}$O$_{3}$ thin films with (100) and (111) orientation Erie Morales, Matthias Batzill, Ulrike Diebold In$_{2}$O$_{3}$ and Sn-doped In$_{2}$O$_{3}$ (Indium-Tin Oxide, ITO) have optical transparency and low electrical resistivity. Relatively little is known about their atomic-scale surface properties because of challenges in preparing single crystal samples. We have grown epitaxial In$_{2}$O$_{3}$ and ITO films on Yttrium Stabilized Zirconia. The (100) surface has polar character and the (111) orientation is non-polar. Films were prepared using oxygen-plasma assisted e-beam epitaxy under UHV conditions and the growth was monitored by RHEED. \textit{In-situ} characterization with XPS, ARXPS, LEED and synchrotron-based UPS was used. In$_{2}$O$_{3}$ (100) facets while ITO(100) stays with a 1x1 termination and Sn segregates to surface. In$_{2}$O$_{3}$ and ITO (111) exhibit a 1x1 termination. On both orientations valence band maximum is 2.7 eV below the Fermi level. For the ITO films resonant photoemission measurements indicate a Sn-derived band gap state. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J36.00004: Epitaxial growth of In2O3(100) on Y-stabilised ZrO2(100) by O-plasma assisted molecular beam epitaxy: a study by HRTEM and XPS. Anne Bourlange, David Payne, Russell Egdell, John Foord, Peter Dobson, John Hutchison Thin films of In2O3 have been grown on Y-stabilised ZrO2(100) by radiofrequency oxygen plasma assisted molecular beam epitaxy with a substrate temperature of 650\textsc{\r{ }C}. Ordered epitaxial growth was confirmed by HRTEM and selected area electron diffraction taken across the interface between the substrate and the epilayer. Excellent crystalline order was preserved up to the surface of the films. The valence band onset in the X-ray photoemission spectra of the epitaxial films was found at 2.90 eV relative to the Fermi energy. The discrepancy between this value and the widely quoted value of 3.75 eV for the bandgap will be discussed in relation to recent theoretical work [1]. \newline [1] A. Walsh \textit{et al.}, Phys. Rev. Lett. submitted. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J36.00005: Radiation-induced defect formation and reactivity of model TiO2 capping layers with MMA: a comparison with Ru Theodore E. Madey, Boris V. Yakshinskiy, M. Nejib Hedhili, Manish Chandhok Our goal is to provide insights into surface processes that affect the reflectivity of TiO2- and Ru-capped multilayer mirrors used in EUV lithography by 13.5 nm (92 eV) photons. EUV-generated secondary electrons from the substrates cause surface reactions that lead to mirror contamination in background vacuum. In our experiments, low-energy electron beams mimic excitations initiated by EUV radiation. Oxygen vacancies are produced at energies above 25 eV. Carbon accumulation is measured on both Ru and TiO2 surfaces during 20 eV and 100 eV electron bombardment in methyl methacrylate vapor (MMA). The initial rates on the clean surfaces are very different: a C film grows more rapidly on TiO2 than on Ru. However, the limiting growth rates are the same for C thicknesses greater than $\sim$1 to 1.5 nm, when MMA interacts with a C film. Irradiation of the C films in O2 gas has a mitigating effect. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J36.00006: In-situ vacuum studies of photocatalytic oxidation of isopropanol on nanometer thick TiO$_{2}$ films grown on silicon D. Kazazis, S. Guha, N.A. Bojarczuk, H.-C. Kim, A. Zaslavsky We report on measurements of the photocatalytic activity of ultra-thin TiO$_{2}$ films grown on $n$ and $p$ type Si wafers.~Using the oxidation of isopropanol to acetone as a model system, photocatalytic studies were carried out in-situ, in a high vacuum chamber equipped with leak valves for injecting isopropanol, oxygen and water vapor onto the TiO$_{2}$ sample.~The sample was irradiated through a quartz widow with a UV strobe light source.~The reaction was monitored with a line-of-sight mass spectrometer coupled to a lock-in amplifier tuned to the strobe frequency.~We find that the photocatalytic efficiency is enhanced as the TiO$_{2}$ thickness is reduced from 50\textit{nm} to 2\textit{nm}.~We also find that the efficiency is enhanced by lowering the substrate Fermi level in going from $n$ type to $p$ type Si.~The results strongly point to the hypothesis that only near surface electron-hole pair generation is relevant to the photocatalytic process; and that the reaction rate can be controlled by varying the substrate Fermi level which in turn changes the electrostatic potential variation within the heterostructure. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J36.00007: Generation of Organic Radicals During Photocatalysis on TiO$_{2}$ Invited Speaker: It is well-known that water-related radicals (such as OH$\cdot $ species) are produced by charge transfer events at UV-irradiated TiO$_{2}$ surfaces. In contrast, organic radicals are generally viewed as being formed by reactions with OH$\cdot $ groups and not by direct charge transfer events. Using rutile TiO$_{2}$(110) as a model photocatalyst, we show that organic radicals are generated in single-step charge transfer events during photodecomposition of adsorbed carboxylates and ketones. Some organic radicals (e.g., methyl) are ejected from the surface and, in high surface area catalysts, experience reactions away from the surface of origin. Other radicals (e.g., ethyl and t-butyl) have limited ability to escape the surface of origin without capture and subsequent thermal reactions. Understanding the chemistry associated with organic radical formation on TiO$_{2}$ opens the door for more detailed examinations of charge transfer dynamics and energy redistribution during photon-initiated reactions important to heterogeneous photocatalysis. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J36.00008: Photocatalytic reaction of catechol on rutile titanium oxide Peter Jacobson, Chundao Wang, Ulrike Diebold In an attempt to understand the fundamental aspects of photocatalysis we have studied the substituted benzene catechol on TiO$_{2}$(110). Previous studies have given detailed information about the catechol bonding configuration letting our group focus on molecular level interactions with scanning tunneling microscopy and X ray photoelectron spectroscopy. Under UV exposure (248 nm) in an oxygen background, catechol is observed to degrade via oxidation. This oxidation process results in removal of roughly 10{\%} of the initial monolayer. The removal of carbon from the TiO$_{2}$ surface is shown to depend upon the background gas. Formation of a residual carbon layer is achieved by annealing the catechol monolayer to 600C. This carbon layer is more difficult to remove by photocatalytic oxidation than a pristine catechol monolayer. Work supported by Intel Corporation [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J36.00009: Water on anatase TiO$_{2}$(101): a scanning tunneling microscopy study Y.B. He, O. Dulub, L.H. Ying, U. Diebold, C. Di Valentin, A. Tilocca, A. Selloni The discovery of photochemical water splitting on TiO$_{2}$ has motivated numerous studies of water on the surfaces of this important photocatalytic material. Previous temperature-programmed desorption and X-ray photoelectron spectroscopy studies of water on anatase TiO$_{2}$(101), the most stable surface of the photocatalytically efficient anatase form, have revealed that water adsorbs molecularly on the surface in accordance with theoretical predictions.$^{ }$In the present study, we have employed low-temperature scanning tunneling microscopy to study water adsorption on anatase TiO$_{2}$(101). We dose various amounts of water (0.2-1.8 Langmuir) at sample temperature T $\sim $ 130 K. Besides confirming that water favors molecular adsorption, atomically resolved STM images further reveal that water molecules adsorb at Ti$_{5c}$ sites forming preferentially one-dimensional chains with local doubling of the periodicity along the [010] direction. Near room temperature, the water molecules become mobile and hop between the Ti$_{5c}$ sites. Density Functional Theory calculations are under way to clarify the origin of the observed doubling of periodicity. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J36.00010: Chemical Deposition and Photoactivity of Anatase and Rutile TiO$_{2}$ Films on Si(111). John F. Anderson, Erie Morales, Ulrikie Diebold Dilute Aqueous Chemical Bath Deposition (CBD) from highly acidic (pH $<$ 1) TiCl3 HCl solutions at room temperature and slightly higher (23$^{\circ}$C - 40$^{\circ}$C) produced thin titanium dioxide films on clean Si(111).~ We report initial results of X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) measurements.~ The films thicknesses varied from 300 nm to $\sim $ 1$\mu $m.~ It was found that the films required annealing to ensure adherence to the Si(111) substrate.~ XRD showed that the anatase and rutile structures were present in the TiO2 as a function of post-deposition annealing temperature.~ Additionally, photo decomposition results of methyl orange and methyl blue on TiO2/Si(111) system under UV light is observed and reported. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J36.00011: Photoinduced Desorption of O$_{2}$ and Photooxidation of Organics from TiO$_{2}$(110) Surfaces David Sporleder, Daniel Wilson, Michael White We present here, a study of photoinduced O$_{2}$ desorption from, and reaction with coadsorbates on, a single crystal rutile TiO$_{2}$(110) surface. Translational energy distributions of O$_{2}$ photodesorbed with a photon excitation energy between 3.5 and 4.2~eV were measured using a pump-probe, time-of-flight (TOF) method. This method utilized a one-photon VUV ionization scheme for product detection that was developed in our lab. The translational energy distribution was found to be trimodal, indicating that different O$_{2}$ species (i.e. O$_{2}^{-}$ or O$_{2}^{2-})$ or binding sites may play a role. It was found that the O$_{2}$ translational energy distributions did not depend on the excitation energy over the range studied, which is consistent with a substrate mediated excitation mechanism involving hole capture. More recent experiments are exploring the mechanism for photooxidation of organic molecules. Specifically, we are determining translational energy distributions of small radicals originating from photoinduced fragmentation of simple ketones on a TiO$_{2}$(110) surface. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J36.00012: Influence of the Sensitizer Protonation and Adsorption Mode on the Efficiency of Dye-sensitized solar cells Annabella Selloni, Filippo De Angelis, Simona Fantacci, Moammhed Nazeruddin, Michael Graetzel Dye sensitized solar cells (DSSCs) represent a promising approach to the direct conversion of light into electrical energy at low cost and with high efficiency. In these devices, a dye sensitizer absorbs the solar radiation and transfers the photoexcited electron to a nanostructured TiO2 electrode. We have studied the electronic structure of different Ru(II)-polypyridyl dyes adsorbed onto a model TiO2 nanoparticle by means of first principles Density Functional Theory calculations. Our results suggest that two different electron injection mechanisms (adiabatic and non-adiabatic) may be present in DSSCs employing dyes carrying a different number of protons. We also found that sensitizers with inequivalent bipyridine ligands exert strong dipolar fields at the TiO2 surface, causing a conduction band down-shift and a reduction of the cell open circuit potential, thus resulting in a reduced DSSC efficiency. [Preview Abstract] |
Session J37: QHE: Bilayers and Graphene
Sponsoring Units: FIAPChair: Herb Fertig, Indiana University
Room: Morial Convention Center 229
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J37.00001: Thin cylinder limit of Halperin bilayer quantum Hall states Alexander Seidel, Kun Yang The traditional framework to study fractional quantum Hall states is based on Laughlin type wavefunctions and Chern-Simons field theories. Recently, a new framework has been proposed that puts stronger emphasis on the one-dimensional (1d) Hilbert space structure of Landau levels. One way to obtain this framework is by observing that states describing fractional quantum Hall liquids may be adiabatically evolved into simple one-dimensional charge-density-wave (CDW) patterns when the system is deformed, e.g., into a thin torus or cylinder. Many general properties of fractional quantum Hall systems are rooted in these CDW states, such as degeneracies and fractional quantum numbers. In this talk, the thin cylinder limit of Halperin $(m,m',n)$ bilayer quantum Hall states will be discussed. The corresponding CDW patterns are quite complicated for general $(m,m',n)$, and can be worked out from a discrete version of the plasma analogy in a ''squeezed space''. The simpler cases map onto well-known spin-1/2 physics. This has some implications for a possible phase transition of the (331) state into the Moore-Read state. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J37.00002: Metamorphosis of quantum Hall bilayer state into a composite fermion metal B. Karmakar, V. Pellegrini, A. Pinczuk, L.N. Pfeiffer, K.W. West In the regime of strong interlayer correlation and tunneling gap $\Delta _{SAS}>$0, the quantum Hall (QH) ground state of bilayers at filling fraction $\nu _{T}$=1 can be viewed as an excitonic insulator [1]. Here it will be shown that a phase transition occurs between this excitonic state and a composite-fermion CF metal as $\Delta _{SAS}$ decreases [2,3]. The observations are based on inelastic light scattering of spin-wave (SW) mode at the Zeeman energy and spin-flip (SF$_{SAS})$ mode across $\Delta _{SAS}$. These experiments show that the SF$_{SAS}$ excitation collapses to the SW and disappears at a critical value of $\Delta _{SAS}$ while a low energy continuum of spin transitions below the SW mode appear. These transitions are interpreted as spin-flip SF$_{CF}$ excitations of the CF metal in which orientation of spin and CF Landau level index change simultaneously. Measurements of SW excitations at $\nu _{T}$=1 in the regime of $\Delta _{SAS}\approx $0 will be also shown [4]. The behavior of the SW thermal activation gap as a function of the Zeeman energy suggests a subtle competition between interlayer correlation and spin effects in the broken-symmetry QH state at $\Delta _{SAS}\approx $0. [1] S. Luin, et al., PRL. \textbf{94}, 146804 (2005); [2] S. Luin, et al., PRL. \textbf{97}, 216802 (2006); [3] B. Karmakar, et al., Solid State Comm. \textbf{143}, 504 (2007); [4] B. Karmakar, et al., work in progress. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J37.00003: Evidence for a finite temperature phase transition in a bilayer quantum Hall system A.R. Champagne, J.P. Eisenstein, L.N. Pfeiffer, K.W. West We study the Joshepson-like interlayer tunneling signature of the quantum Hall bilayer excitonic state at total filling factor $\nu_T = 1$ as a function of the layer separation, interlayer charge imbalance and temperature. The tunneling amplitude collapses to zero as either the temperature or interlayer spacing is increased. The interlayer tunneling amplitude dependences on the layer spacing at various temperatures are very similar, but the layer separations where the tunneling disappears scale linearly with temperature. Our results offer evidence [1] that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The phase boundary is found to be re-entrant as a function of charge imbalance thus suggesting an intricate competition between the interlayer coherent phase and various independent layer states. This work was supported by the NSF and the DOE. \newline [1] A.R. Champagne, J.P. Eisenstein, L.N. Pfeiffer, K.W. West, Cond-mat/0709.0718 [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J37.00004: Area and perimeter dependence of tunneling in a bilayer 2D electron system in the \boldmath{$\nu_T=1$} quantum Hall state A.D.K. Finck, A.R. Champagne, J.P. Eisenstein, L.N. Pfeiffer, K.W. West When interlayer interactions between electrons in two parallel 2D electron systems become comparable to intralayer interactions, a quantum Hall state can occur at total filling factor $\nu_T=1$. In this state, the 2D-2D tunneling conductance at zero bias is greatly enhanced. In some theoretical models, tunneling is expected to occur primarily along the edge of the system. In large regions the tunneling conductance would therefore be proportional to the perimeter of the sample. To test this idea, we use electrostatic top gates of various sizes and shapes to locally define a $\nu_T = 1$ state. By subtracting out background tunneling originating from ungated regions of the sample, we can measure the tunneling conductance of individual gated regions. Our data show that the tunneling conductance at $\nu_T =1$ is approximately proportional to area. This implies that tunneling at $\nu_T=1$ is a bulk phenomenon in our samples. How this result is connected with the inevitable disorder in the sample will be discussed. This work is supported by the NSF under grant DMR-0552270. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J37.00005: Theory of Activated Transport in Bilayer Quantum Hall Systems Bahman Roostaei, Herbert Fertig, Kieran Mullen, Steven Simon We analyze the transport properties of bilayer quantum Hall systems at total filling factor $\nu = 1$ in drag geometries as a function of interlayer bias, in the limit where the disorder is sufficiently strong to unbind meron-antimeron pairs, the charged topological defects of the system. We compute the typical energy barrier for these objects to cross incompressible regions within the disordered system using a Hartree-Fock approach, and show how this leads to multiple activation energies when the system is biased. We then demonstrate using a bosonic Chern- Simons theory that in drag geometries, current in a single layer directly leads to forces on only two of the four types of merons, inducing dissipation only in the drive layer. Dissipation in the drag layer results from interactions among the merons, resulting in very different temperature dependences for the drag and drive layers, in qualitative agreement with experiment. We conclude with predictions for future experiments. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J37.00006: Non-perturbative approach to the quantum Hall bilayer Milica Milovanovic, Zlatko Papic We study the disordering of the superfluid phase in the quantum Hall bilayer at the filling factor one with increasing distance between the layers. We find that the possibilities for ground state wave functions that describe the superfluid at an arbitrary distance fall into two universality classes. They correspond to (1) Berezinskii-Kosterlitz-Thouless (BKT) (2D XY) model of superfluid disordering in the presence of charged impurities and (2) $\lambda$ transition (3D XY) model in a translatory invariant quantum Hall bilayer system. The BKT type of disordering likely ends with unbinding of created pairs of neutral fermionic vortices (in a transition reminiscent of the one reported in Champagne et al.). In the translatory invariant system the ensuing quantum phase transition proceeds via condensation of loops of elementary charged vortices - merons into a topological phase associated with the toric code model. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J37.00007: Phonon mediated resonances in bilayer magnetodrag Gokul Gopalakrishnan, Sanghun An, Dongkyun Ko, Yuko Shiroyanagi, Thomas Gramila, Loren Pfeiffer, Ken West The properties of the 2-dimensional electron gas (2DEG) have been studied extensively in the integer and fractional quantum Hall regimes. Much less is understood about the 2DEG at intermediate fields, where thermal fluctuations are comparable to the size of the Landau-level spacing. We have explored this regime by measuring frictional drag, which probes electron-electron interactions, in a bilayer system at temperatures from 1.5K to 8.5K in fields smaller than 1T. In addition to an unusual overall field dependence, we have discovered a series of oscillations in the drag resistivity which are periodic in 1/B, but are distinct from variations in the density of states, as seen in Shubnikov-de Haas oscillations. These novel magnetodrag oscillations are consistent with a phonon mediated interlayer momentum transfer mechanism. Resonances are observed when the frequency of $2k_F$ phonons matches an integer multiple of the cyclotron frequency, and they are suppressed as the densities of the two layers are mismatched. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J37.00008: Unusual quantum Hall Ferromagnetism in Bilayer Graphene Barlas Yafis, Rene Cote, Allan MacDonald Bilayer graphene has eight nearly degenerate Landau levels at the Dirac point compared to the four nearly degenerate levels which occur in an isolated layer. The additional degeneracy is associated with the degeneracy of $n=0$ and $n=1$ orbital Landau level states in the bilayer case and adds a Landau level pseudospin degree of freedom to the spin and valley pseudospins present in the single layer case. We predict broken symmetry states which lift the associated degeneracies with a Hunds rule which orders spin first, then valley, and finally the Landau-level pseudospin. It follows that the Landau level pseudospin orders at all odd total filling factors. We find unusual collective modes which are not gapped even though the system has uniaxial anisotropy, and a $q^{3/2}$ dispersion at small $q$ because the divergence of the pseudospin magnetization produces charges with long-range Coulomb interactions. Because of the charge carried by these collective modes, they are dipole active. We predict unusual intra-Landau-level contributions to the cyclotron resonance signal. We will also discuss unusual properties of the Skyrmions spin-textures of these quantum Hall ferromagnets. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J37.00009: Symmetry Broken States of Dirac Fermions in Graphene with A Partially Filled High Landau Level Hao Wang, D.N. Sheng, F.D.M. Haldane We report on numerical study of symmetry broken states of the Dirac electrons in partially filled N=3 Landau level (LL) in graphene. At half-filling, the static density-density correlation function displays sharp peaks at nonzero wavevectors. Finite-size scaling shows that the peak value grows with electron number and diverges in the thermodynamic limit, indicating an instability toward a charge density wave. A weak disorder potential plays the role of selecting a symmetry broken stripe phase as the ground state from the nearly degenerated low-energy manifold. Such a quantum phase is experimentally observable through transport measurements. Associated with the special wavefunctions of the Dirac LL, both stripe and bubble phases become possible candidates for the ground state at lower filling numbers in the N=3 LL. We have also studied the ground state evolution and quantum phase transitions of the 2D electron gas at half-filled N=1 LL under an additional repulsive three-body interaction. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J37.00010: Midgap states and the fractional quantum Hall regime in graphene quantum dots Igor Romanovsky, Constantine Yannouleas, Uzi Landman Graphene quantum dots (QDs) with zigzag edges exhibit midgap single-paticle states associated with such edges. At zero magnetic field ($B$), these states form a manifold of degenerate states similar to the lowest Landau level that forms in semiconductor QDs at high $B$. It has been recently suggested\footnote{B. Wunsch {\it et al.\/}, arXiv:0707.2948v2} that the midgap-state manifold in graphene dots can support correlated many-body states similar to the rotating-electron-molecule (REM) states (also referred to as rotating Wigner crystallites) that are well known in semiconductor QDs at high $B$.\footnote{C. Yannouleas and U. Landman, Rep. Prog. Phys. {\bf 70}, 2067 (2007)} Here, we will report systematic exact-diagonalization calculations (for $N=4-10$ QD electrons) describing the REM states in graphene QDs. We anticipate that the graphene REM states exhibit for all $N$ a single polygonal ring of localized electrons, in contrast to the multiple polygonal-ring configurations known from semiconductor QDs. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J37.00011: SO(3) theory of the integer quantum Hall effect in graphene Igor Herbut I will discuss the Hubbard model of graphene in an external magnetic field. In the continuum limit and in the Hartree-Fock approximation, the ground state energy at half filling becomes nearly symmetric under rotations of the three-component vector (N1, N2, m), with the first two components representing the Neel order parameter orthogonal to and the third component the magnetization parallel with the external magnetic field. When the symmetry breaking effects arising from the lattice, Zeeman coupling, and higher Landau levels are included the system develops a quantum critical point at which the antiferromagnetic order disappears and the magnetization has a kink. The observed incompressible states at filling factor one are argued to arise due to a finite third component of the Neel order parameter at these electron densities. Recent experiments appear consistent with N1=N2=0, and N3 finite, at the filling factors zero and one, respectively. I. F. Herbut, Phys Rev. B vol. 76, 085432 (2007); ibid. vol. 75, 165411 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J37.00012: Phase Diagram for Quantum Hall States in Graphene Jianhui Wang, Andrew Iyengar, Herb Fertig, Luis Brey We investigate integral and half-integral fillings (uniform and unidimensional stripe states respectively) for graphene using the Hartree-Fock approximation in the continuum limit. For fixed filling factor, the ratio between the scales of the Coulomb interaction and Landau level spacing $g=(e^{2}/ \epsilon\ell)/(\hbar v_{F}/\ell)$ is a field independent constant. However, when B decreases, the number of filled negative Landau levels increases, which surprisingly turns out to ${\it decrease}$ the amount of Landau level mixing. The resulting states at fixed filling factor $\nu$ (for $\nu$ not too big) has very little Landau level mixing even at arbitrary weak magnetic fields. This means many different phases should emanate from the origin of the phase diagram when plotted in the B v.s. density plane, in contrast to regular 2 dimensional electron gas which has a Wigner crystal state in the vicinity of the same point. The stripe amplitudes scale roughly as B, so that the density waves ``evaporate'' continuously as $B\rightarrow0$. These results will be compared to those of tight binding calculations. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J37.00013: Chirality Sum Rule in Graphene Multilayers Hongki Min, Allan H. MacDonald We show that the low energy electronic structure of arbitrarily stacked graphene multilayers with nearest-neighbor interlayer tunneling consists solely of chiral pseudospin doublets. Although the number of doublets in an $N$-layer system depends on the stacking sequence, the pseudospin chirality sum is always $N$. It follows that $N$-layer stacks always have $N$ distinct Landau levels at $E=0$ for each spin and valley, and that the quantized Hall conductivity $\sigma_{xy} = \pm(4 e^2/h)(N/2+n)$ where $n$ is a non-negative integer. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J37.00014: Valley-skyrmion and Wigner crystals in graphene Rene Cote, Jean-Francois Jobidon, Herbert A. Fertig At low-energy, the band structure of graphene can be approximated by two degenerate valleys $(K,K^{\prime })$ about which the electronic spectra of the valence and conduction bands have a linear dispersion relation. An electronic state in this band spectrum is a linear superposition of states from the $A$ and $B$ sublattices of the honeycomb lattice of graphene. In a quantizing magnetic field, the band spectrum is split into Landau levels with level $N=0$ having zero weight on the $B(A)$ sublattice for the $K(K^{\prime })$ valley. Treating the valley index as a pseudospin and assuming the real spins to be fully polarized, we compute the energy of skyrmion crystals in the Hartree-Fock approximation. We show that skyrmion crystals have lower energy than Wigner crystals i.e. crystals with no pseudospin texture. The collective mode spectrum of the valley- skyrmion crystal has three linearly-dispersing Goldstone modes in addition to the usual phonon mode while a Wigner crystal has only one extra Goldstone mode with a quadratic dispersion. We comment on how these modes should be affected by disorder and how, in principle, a microwave absorption experiment could distinguish between Wigner and skyrmion crystals. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J37.00015: Lattice-Induced Double-Valley Degeneracy Lifting in Graphene by a Magnetic Field I.A. Luk`yanchuk, A.M. Bratkovsky We show that the recently discovered double-valley splitting of the low-lying Landau level(s) in the Quantum Hall Effect in graphene can be explained as a {\em perturbative} orbital interaction of intra- and inter-valley microscopic orbital currents with a magnetic field. This effect is provided by the translationally-non-invariant terms corresponding to graphene's crystallographic honeycomb symmetry but do not exist in the relativistic theory of massless Dirac Fermions in Quantum Electrodynamics. We discuss recent data in view of these results [1]. [1] I.A. Luk'yanchuk and A.M. Bratkovsky, arXiv:0707.0466 (2007) [Preview Abstract] |
Session J39: Focus Session: Friction and Contact
Sponsoring Units: DMP GSNPChair: Mark Robbins, Johns Hopkins University
Room: Morial Convention Center 231
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J39.00001: A rigorous approach to the contact mechanics of rough, elastic solids Martin Muser The basic ideas of a statistical field theory is presented, which allows one to calculate the displacement field and the pressure distribution $\Pr(p)$ in a contact formed by an elastic body and a rigid counter face of arbitrary topography. The theory is a cumulant expansion, which contains Persson's contact mechanics theory as the leading-order term. The cumulant approach provides a framework with which corrections can now be systematically derived. Comparison is made to numerical data for surfaces that interact via exponentially repulsive forces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J39.00002: Molecular dynamics study of contact mechanics: contact area and interfacial separation from small to full contact Chunyan Yang, Bo Persson We report a molecular dynamics study of the contact between a rigid solid with a randomly rough surface and an elastic block with a flat surface. We study the contact area and the interfacial separation from small contact (low load) to full contact (high load). For small load the contact area varies linearly with the load and the interfacial separation depends logarithmically on the load [1-4]. For high load the contact area approaches to the nominal contact area (i.e., complete contact), and the interfacial separation approaches to zero. The present results may be very important for soft solids, e.g., rubber, or for very smooth surfaces, where complete contact can be reached at moderate high loads without plastic deformation of the solids. \newline References: \newline [1] C. Yang and B.N.J. Persson, arXiv:0710.0276, (to appear in Phys. Rev. Lett.) \newline [2] B.N.J. Persson, Phys. Rev. Lett. 99, 125502 (2007) \newline [3] L. Pei, S. Hyun, J.F. Molinari and M.O. Robbins, J. Mech. Phys. Sol. 53, 2385 (2005) \newline [4] M. Benz, K.J. Rosenberg, E.J. Kramer and J.N. Israelachvili, J. Phy. Chem. B.110, 11884 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J39.00003: Atomistic effects on friction and contact area in single and multi asperity contacts Binquan Luan, Mark Robbins Contact and friction are universal phenomena in our daily life. Theoretical studies of macroscopic contact and friction are usually based on continuum theories such as Hertz theory and Amontons's laws. Recent advances in nanotechnology have stimulated research into friction at the nanometer scale where new phenomena emerge. Contact and friction in single- and multi-asperity contacts with nanometer dimensions were studied using molecular dynamics simulations (MD) and a hybrid method. The hybrid method retains a full atomistic treatment near contacts and replaces more distant regions with a more efficient finite element description. Our results demonstrate that atomic-scale changes in surface structure produce huge changes in friction and contact area and substantial deviations from the predictions of continuum theories. Unanticipated surface plasticity is observed near peaks on crystalline surfaces. In the case of multiasperity amorphous systems, the rate of local plastic deformation near the surface is directly related to the frictional dissipation of energy. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J39.00004: Local contact stress measurements at a rough interface Julien Scheibert, Alexis Prevost, Eytan Katzav, Mokhtar Adda-Bedia, Georges Debr\'egeas, Joel Frelat An original MEMS-based force sensing device has been designed. It allows for spatially resolved measurements of both normal and tangential stress fields at the base of an elastomeric film in contact with a rigid substrate [1]. Model contact geometries involving a rough, nominally flat film pressed against smooth spherical and cylindrical glass substrates have been studied, in two different regimes, normal indentation and steady sliding. The measured stress profiles have been compared to calculations which assume a smooth contact obeying Amontons-Coulomb's friction law. For the normal indentation a Finite Elements method was used, whereas for the sliding regime a semi-analytical model was developed. These direct comparisons showed that our device was accurate enough to discriminate between dry and lubricated contact conditions and evidenced load-dependent deviations from Amontons-Coulomb's profiles. These deviations are qualitatively interpreted by taking into account the finite compliance of the contacting micro-asperities population. [1] J. Scheibert\textit{ et al}., arXiv:0711.1117v1 [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J39.00005: Friction induced displacement and stress fields within contacts with elastomers Antoine Chateauminois, Christian Fretigny Friction is known to be associated with strongly heterogeneous stress and displacement fields within the contact zone. However, experimental approaches are often based on the measurement of friction forces (or mean shear stress), which makes difficult a detailed analysis of interface dynamics within sliding contacts. We have developed a new methodology for the determination of the interface shear stress distribution within macroscopic sliding contacts. It is based on an in situ measurement of the displacement field induced at the surface of highly deformable solids such as elastomers. An inversion of this field using contact mechanics models then provides the interface shear stress distribution. The experiments were carried out using two different contact configurations. The first one involves the linear sliding of a glass sphere on the elastomer substrate. The second one corresponds to an original torsional contact configuration which minimizes bulk viscoelastic dissipation during steady state sliding. Experimental distributions of frictional shear stress will be discussed in the light of theoretical models assuming either a constant interface shear stress (Tabor's model) or a local Coulomb's friction law. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J39.00006: Optical measurements of pressure and displacement fields at a rough interface Alexis Prevost, Julien Scheibert, Georges Debr\'egeas We report on optical measurements of both pressure and displacement fields at the interface between a rough, nominally flat transparent elastomeric film and a smooth spherical glass lens. The multi-contact interface is imaged by transmission and the pressure field is deduced from the spatial distribution of the transmitted light. The displacement field is obtained using Digital Image Correlation, allowing for a submicron resolution. For normal loading, the radial pressure profiles deviate from Hertz theory, as expected for a rough interface. A good quantitative agreement has been obtained within the statistical description of a rough sphere-on-plane contact by Greenwood and Tripp. When the interface is tangentially loaded below the macroscopic sliding threshold, analysis of the displacement field has shown a coexistence between an inner stuck region and an outer slipping annulus, as suggested by Catteneo and Mindlin. Quantitative comparison with this model yields a good overall agreement. However, small deviations are observed and can be related to the tangential compliance of the rough layer. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J39.00007: Beller Lectureship Talk: Crack-like processes govern the onset of frictional motion Invited Speaker: The dynamics of frictional slip have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of slip. First described by Coulomb and Amontons as the transition from static to dynamic friction, the onset of frictional slip is central to fields as diverse as physics, tribology, the mechanics of earthquakes and fracture. We study the dynamics of how this transition takes place by performing real-time visualization of the \textit{true} contact area which forms the interface separating two blocks of like material. The results show that the onset of frictional motion is driven by the interplay of three different types of coherent crack-like fronts, which propagate along the interface, reducing the contact area as they progress. Two of these, whose propagation speeds are, respectively, slightly below and significantly above the shear wave velocity, appear to be related to known propagation modes of shear cracks. The third type of front does not correspond to known fracture modes. It propagates over an order of magnitude more slowly, and is the most efficient of the three modes in reducing contact area along the interface. We first show that, at applied stresses that are well below the (Coulomb-Amontons) threshold for the onset of frictional motion, significant precursor activity occurs along the interface. This activity is comprised of propagating (subsonic) shear cracks which arrest before traversing the entire interface. In their wake, these ``precursor'' cracks systematically transform the intial spatially uniform contact area along the interface to a highly nonuniform one. \textit{Only} at the transition to overall motion will these precursor cracks simultaneously excite, at their point of arrest, both the slow propagation modes and the intersonic ones. Until to this point, no overall frictional motion occurs. Frictional sliding only takes place when either the slow modes or additional shear cracks excited by the slow modes traverse the entire interface. These results suggest that to understand the transition to frictional motion, the dynamics of this entire chain of events must be taken into account. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J39.00008: Precursor events and the onset of frictional sliding Andras Libal, Mark Robbins The precursor events leading to steady state sliding friction are investigated using a simple two-dimensional model of a rectangular block on a flat surface. As in experiments[1], a succession of cracks nucleates from the rear of the block. Each propagates rapidly and then arrests after a distance that scales with the height at which the lateral force is applied to the block. The propagation distance grows with each successive crack until a steady sliding state is attained. The distribution of local shear stress at the interface can be obtained directly in our simulations. The relation between this stress distribution, the static friction, the normal load, and the nucleation and propagation of successive cracks will be discussed. [1] S.M. Rubinstein, G. Cohen and J. Fineberg, PRL 98, 226103 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J39.00009: The Duality of Nanoscale Friction: Amontons' Law vs. Superlubricity U. D. Schwarz, D. Dietzel, C. Ritter, T. Monninghoff, H. Fuchs, A. Schirmeisen One of the most fundamental questions in nanotribology is the contact area dependence of frictional forces on the nanometer scale. Unfortunately, conventional friction force microscopy techniques are limited for analyzing this problem due to the unknown and ill-defined tip-sample contact. This limitation can be circumvented by measuring the lateral force signal during the manipulation of nanoscale particles with a well-defined, clean contact to the substrate. In our study, the samples under investigation were metallic islands with diameters between 50-500 nm grown by thermal evaporation of antimony on highly oriented pyrolytic graphite (HOPG). Experiments that included the controlled manipulation of a large number of nanoparticles in ultrahigh vacuum show two distinct frictional states during particle sliding: While some particles show finite friction increasing linearly with interface area, thus reinforcing Amontons' law at the nanoscale, other particles assume a state of frictionless or `superlubric' sliding. Additional experiments revealed a similar result even in air, which can be explained by contamination effects of the interface that alter the frictional properties. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J39.00010: Dragging shadows causes real friction: sliding Moire' patterns A. Vanossi, C. Negri, N. Manini, G.E. Santoro, E. Tosatti Surface Moire' patterns are shadow-like modulations (kinks) which form at crystalline overlayers that are out of registry with their substrates. They were hardly considered in the context of friction so far; we here argue that they can be relevant. 1D model calculations suggest in fact that under the action of an external slider, the kinks are the real objects being rigidly dragged, as opposed to the real particles, which are not [1]. For a crystalline periodic slider, we predict peculiar phenomena on the fly caused by the pinning/depinning of the kink lattice to the slider, in full analogy with the well known real lattice static counterpart [2]. The frictional dissipation by a vibrating and/or sliding AFM probe should moreover be enhanced at the kinks, where atoms take poorly stable positions. Thus, AFM frictional maps [3] should reveal with much more contrast the Moire' patterns than topographic maps of the same patterns. This concept is demonstrated by means of a simple model, which also provides a guide to the key parameters determining the enhancement. [1] A. Vanossi et al., PRL 97, 056101 (2006). [2] A. Vanossi et al., PRL 99, 206101 (2007). [3] C. Loppacher et al., PRB 62, 13674 (2000). [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J39.00011: Contribution of Plowing to Nanotribology of Self-Assembled Monolayers Michael Chandross, Christian Lorenz, Gary Grest, Erin Flater, Robert Carpick Atomic force microscope experiments and molecular dynamics (MD) simulations on self-assembled monolayer (SAM) systems have demonstrated that the nanotribology of these systems may be dominated by a microscopic plowing mechanism. Due to relatively weak chain-to-chain interactions, compression only affects molecules directly under the probe tip, and not those outside the contact area. Under shear, the tip must plow into the molecules in front leading to frictional energy dissipation. We will present the results of coupled experiments and MD simulations of alkylsilane SAMs studying the plowing mechanism in detail. In particular, combinations of uncoated and SAM-coated substrates and tips are studied to probe the relationships between friction force and both contact area and applied load. As a SAM coating on the substrate (tip) is (is not) expected to result in plowing during shear, the contrast in these results, combined with detailed calculations using the MD results, will shed light on the complicated response of these systems. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J39.00012: Accelerated Molecular Dynamics Simulation of AFM Experiments Using the Bond-Boost Method Woo Kyun Kim, Michael Falk We apply an accelerated molecular dynamics (MD) methodology to simulate the friction between a silicon tip and a silicon surface under perfect vacuum conditions. These simulations model recent Atomic Force Microscope (AFM) experiments which observed stick-slip motion and a lateral force showing dependences on temperature and sliding velocity. Our AFM models consist of crystalline silicon with an oxidized layer. We achieved the decrease in the simulated sliding velocity by several orders of magnitude compared with conventional MD simulations using the bond-boost method. This method is based on Voter's hyper molecular dynamics scheme accelerating the process between slip events. The decrease in the sliding velocity makes it possible to simulate systems closer to the regime of the actual experiments. We compare the simulation results with the experimental data to elucidate the atomic level processes during sliding. We studied the effects of atomic mass transfer between the tip and the substrate on friction. Moreover, the dependence of friction on temperature and sliding velocity has been quantified, and compared with the modified Tomlinson model. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J39.00013: Measurement of lateral tip-sample forces in the attractive regime with picometer resolution in three dimensions B.J. Albers, T.C. Schwendemann, M.Z. Baykara, N. Pilet, U.D. Schwarz Three-dimensional (3D) dynamic force spectroscopy, i.e., the acquisition of frequency shift vs. distance curves in a dense raster over a surface in order to recover the true tip-sample interaction forces with high local resolution, has so far suffered from relatively low resolution, as long-term drift stability has been a problem. Nevertheless, its promise to deliver not only the normal forces with atomic resolution, but also the lateral forces as well as the energy dissipated during an individual oscillation cycle makes it interesting for high-resolution nanotribology. Using our recently completed home-built low temperature, ultrahigh vacuum NC-AFM, we were able to map the full 3D force field over highly oriented pyrolytic graphite, which was chosen due to its qualities as a solid lubricant. Lateral forces have been measured quantitatively in a grid with spacing better than 6 pm in all three directions and pN resolution. We will discuss the distance-dependence of the static lateral forces, their local distributions with regard to the underlying lattice, as well as influences of the tip shape. [Preview Abstract] |
Session J40: Focus Session: Earth and Planetary Materials II
Sponsoring Units: DMP DCOMPChair: Boris Kiefer, New Mexico State University
Room: Morial Convention Center 232
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J40.00001: The melting curve of MgSiO3 perovskite from ab initio molecular dynamics using the coexistience method Invited Speaker: Despite its importance in understanding such things as the crystallisation of the Earth's mantle from a magma ocean or the existence of melt in the current mantle, the melting temperature of the lower mantle phase MgSiO3 perovskite is poorly know. Estimates of its melting temperature at the core-mantle-boundary range from 5400 K to over 8000 K. We have used, therefore, ab initio molecular dynamics simulations to predict its melting temperature throughout the Earth's mantle using the coexistence method. We used 900 atoms (a 3x3x5 super-cell) with atoms in one half of the super-cell melted and the other half solid. Both halves are thermalised to the desired temperature individually. We then turned off the thermalisation and allowed the system to evolve in an NVE simulation, using DFT forces calculated within the GGA. Those systems which were too hot melted within 10 ps. Those which didn't remained with both solid and melt coexisting in the super-cell for over 25 ps. These where assumed to be either on the melting curve of just below it. Our results agree well with the higher temperature melting curves found experimentally, and we predict a melting temperature of about 6500 K at the core-mantle boundary. We will also present results on simulating the melting temperature of the MgO-MgSiO3 binary. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J40.00002: First-Principles Molecular Dynamics of Melts in the MgO-SiO$_{2}$ System Bijaya Karki, Nico de Koker, D. Bhattarai, Lars Stixrude We have recently completed simulations of five melt compositions in the MgO-SiO$_{2}$ system within density functional theory. These results allow us to investigate the structural and thermodynamical, transport properties of melts along the MgO-SiO$_{2}$ join as a function of pressure. In particular, we have found that the mixing in MgO-SiO$_{2}$ system is significantly non-ideal at low pressures with negative excess volume and enthalpy of mixing. With increasing pressure, the volume of mixing decreases rapidly to a value close to zero at pressures above 50 GPa whereas the enthalpy of mixing remains negative. The radial distribution functions and coordination environments are found to show interesting changes with varying composition. Also, the effects of composition on diffusivity are shown to be substantial at low pressures whereas the effects are increasingly suppressed with increasing pressure. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J40.00003: A New look into the spin transition in Fe$_2$O$_3$ Dipta Bhanu Ghosh, Stefano de Gironcoli The wide range of intriguing characteristics exhibited by Fe$_2$O$_3$ with pressure and temperature has renewed the attention of the scientific community in the last decade. Experimental and theoretical efforts are on to address and unravel the complexity of the system. The ambient pressure phase, hematite ($\alpha$-Fe$_2$O$_3$) transforms to a new structural phase (HP1). That the HP1 phase is orthorhombic perovskite (Pbnm) or Rh$_2$O$_3$-II type (Pbcn) is still a debate and yet to be explored theoretically. On top of this ambiguous assignment of HP1, there has been a long-standing issue of an isostructural high spin (HS) to low spin (LS) transition. Experimental data till date are divided into two horizons--one assigning the spin transition in the hematite phase and the other in the HP1 phase. In this work, motivated by these exotic unresolved controversies of the system, we have tried to gain an insight of the system from first principles density functional calculations. Our results favor the Rh$_2$O$_3$-II type as the HP1 phase, in agreement with recent experiments. Also a (new) mechanism governing the HS to LS transition is proposed. This mechanism, we believe, might help in removing the boundary between the two horizons as mentioned above. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J40.00004: Prediction of an ultrahigh-pressure form of Al$_2$O$_3$ Koichiro Umemoto, Renata Wentzcovitch We predict by first principles a pressure induced phase transition in alumina at $\sim$3.7 Mbar, relevant for interiors of the giant planets and terrestrial exoplanets, at room temperature from the CaIrO$_3$-type polymorph to another with the U$_2$S$_3$-type structure. This transformation should be important for the analysis of shock data in this pressure range, since alumina is used as window material. Our calculated compression curves agree with shock data excellently, indicating that the presence of two phase transitions (corundum--Rh$_2$O$_3$(II)-type and Rh$_2$O$_3$(II)-type--CaIrO$_3$-type) had gone unnoticed in shock data. Our prediction suggests that the multi-Mbar crystal chemistry of planet-forming minerals might be related to that of the rare-earth sulfides. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J40.00005: Models of Giant Planet Interiors Derived from First-Principles Simulation Burkhard Militzer, Jan Vorberger, William Hubbard Our understanding of the interior of giant planets is based on the accurate characterization of hydrogen and helium at megabar pressures and temperatures of several thousands of Kelvin. Theoretical method including first-principles computer simulations have been the preferred tool to study these dense fluids because laboratory experiments cannot yet probe deep into Jupiter's interior despite great progress in shock wave measurements with precompressed samples. Results from an extensive set of density-functional molecular dynamics simulations will be presented [J. Vorberger \textit{et al.,} ``Hydrogen-Helium Mixtures in the Interiors of Giant Planets,'' \textit{Phys. Rev. B} \textbf{75} (2006) 024206]. A new and more accurate equation of state (EOS) will be derived that spans the interior of giant planets. Differences from the widely used Saumon-Chabrier-Van Horn (SCVH) EOS will be analyzed. An updated model for the interior of Jupiter will be introduced. Estimates for the heavy element enrichment as well as for the size of Jupiter's core will be discussed and compared with previous models based on the SCVH EOS. This work is supported by NASA grants PGG04-0000-0116 and NAG5-13775 as well as NSF grant 0507321. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J40.00006: Titan's Interior Chemical Composition: Possible Important Phase Transitions Michael Howard, Joseph Zaug, Bishun Khare, Christopher McKay We study the interior composition of Titan using thermal chemical equilibrium calculations that are valid to high pressures and temperatures. The equations of state are based on exponential-6 fluid theory and have been validated against experimental data up to a few Mbars in pressure and approximately 20000K in temperature. In addition to CHNO molecules, we account for multi-phases of carbon, water and a variety of metals such as Al and Fe, and their oxides. With these fluid equations of state, chemical equilibrium is calculated for a set of product species. As the temperature and pressure evolves for increasing depth in the interior, the chemical equilibrium shifts. We assume that Titan is initially composed of comet material, which we assume to be solar, except for hydrogen, which we take to be depleted by a factor 1/1000. We find that a significant amount of nitrogen is in the form of N$_{2}$, rather than NH$_{3}$. Moreover, above 12 kbars, as is the interior pressure of Titan, a significant amount of the carbon is in the form of graphite, rather than CO$_{2}$ and CH$_{4}$. We discuss the implications of these results for understanding the atmospheric and surface composition of Titan. . [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J40.00007: Extended-Solid Phases of Carbon Dioxide at High Pressures Valentin Iota, Zsolt Jenei, Jae-Hyun Klepeis, Choong-Shik Yoo, William Evans At high pressures and temperatures, CO$_{2}$ transforms to a series of solid polymorphs with differing crystal structures, intermolecular interactions and chemical bonding. Among them are a number of covalent (extended) solid phases, with crystal structures analogous to SiO$_{2}$ polymorphs. Above 40GPa and 1500K CO$_{2}$ transforms to phase V, a network of corner sharing CO$_{4}$ tetrahedra -- structurally similar to SiO$_{2}$ tridymite. At room temperatures, CO$_{2}$ forms \textit{a-carbonia, }an amorphous extended-solid phase similar to silica glass. Recently, we reported another phase, with a structure resembling that of SiO$_{2}$ stishovite, formed by compressing associated phase II above 50GPa. Here, we present a systematic picture of the structural and bonding diagram of carbon dioxide, focusing on the relationship between its molecular and extended phases at high pressures and temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J40.00008: High-pressure Neutron Powder Diffraction and Inelastic Neutron Scattering Studies on the Mineral Jarosite KFe$_{3}$(SO$_{4})_{2}$(OH)$_{6}$ Monika Hartl, Alice Acatrinei, Luke Daemen, Hongwu Xu, Kim Tait, Yuejian Wang, Sven Vogel, Jianzhong Zhang, Yusheng Zhao The mineral jarosite KFe$_{3}$(SO$_{4})_{2}$(OH)$_{6}$ has been detected in rocks at the Meridiani Planum region of Mars [1 and cited therein]. Jarosite is typically formed in aqueous environments at acidic pH. It decomposes to ferric oxohyroxides in humid climate. This gives rise to the question under which conditions jarosite was formed on Mars and what it can tell us about the climatic cycles and the former presence of water on Mars. We are looking at the phases of jarosite at elevated temperature and pressure and were able to show the stability of jarosite up to 6 GPa at room temperature and up to 3 GPa at 300 $^{o}$C using neutron powder diffraction. Furthermore, we used inelastic incoherent neutron scattering to look at the vibrational modes of the hydroxyl groups in jarosite at various temperatures between 10K and 200K. [1] A. Banin, Science 309 (2005) 888 [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J40.00009: Lattice Dynamics and Thermal Equation of State of Platinum Tao Sun, Koichiro Umemoto, Zhongqing Wu, Jincheng Zheng, Renata Wentzcovitch Platinum is widely used as a pressure calibration standard. However, the established thermal EOS has uncertainties, especially in the high $P$-$T$ range. We use density functional theory to calculate the thermal equation of state of platinum, up to $550$~GPa and $5000$~K. The static lattice energy is computed by using the LAPW method, with LDA, PBE, and the recently proposed WC functional. The electronic thermal free energy is evaluated using the Mermin functional. The vibrational part is computed within the quasi-harmonic approximation using density functional perturbation theory and pseudopotentials. Special attention is paid to the influence of the electronic temperature to the phonon frequencies. We find that in overall LDA results agree best with the experimental ones. To provide accurate thermal EOS for pressure calibration, we combine the computed temperature dependence of the Gibbs energy with the room temperature Gibbs free energy corrected by experiments. The resulting thermal EOS seems reasonably accurate and can be used as a reference for pressure calibration. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J40.00010: Ultrafast shock wave propagation at high ambient pressure in a diamond anvil cell Michael Armstrong, Jonathan Crowhurst, Joseph Zaug, William Howard The measurement and characterization of acoustic phenomena at high pressure is critical to the modeling of planetary dynamics, seismic events, and chemistry in extreme environments. Here we present the results of experiments using ultrafast laser excitation and detection of shock waves starting from high precompression (10s GPa) in a standard diamond anvil cell (DAC) with transient single shot shock pressures $>$ 10 GPa. Using ultrafast interferometry, we directly detect surface motion with $\sim $nm spatial resolution and $\sim $ps time resolution. Such experiments enable examination of shock waves with significant strain starting at high ambient pressure using a convenient and relatively inexpensive apparatus. Ultrafast time resolution enables the observation of shock-induced chemistry on the scale of a picosecond shock rise. Furthermore, standard DACs can reach 100s GPa precompression, enabling the examination of phase transitions and chemical reactions starting from a wide range of thermodynamic initial conditions. [Preview Abstract] |
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