Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P1: Magnetism and Localization in f Electron Systems
Sponsoring Units: DCMPChair: Qimiao Si, Rice University
Room: Ballroom A1
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P1.00001: Ce115's and beyond Invited Speaker: Recent studies of members of the Ce115 (CeMIn$_{5}$ (M=Co, Rh)) family of heavy-fermion materials have allowed a new perspective on the relationship between magnetism and unconventional superconductivity in strongly correlated electron systems. The antiferromagnet CeRhIn$_{5}$ under pressure, superconducting CeCoIn$_{5 }$in a magnetic field, and Cd-doped CeCoIn$_{5-x}$Cd$_{x}$ reveal a phase of long-range antiferromagnetic order that coexists microscopically with bulk, nodal superconductivity. Though the detailed relationship between these orders differs in each, evidence suggests that the order parameters are coupled irrespective of these differences and that similar conclusions may hold in structurally related CePt$_{2}$In$_{7}$ and the recently discovered 5f-electron compound PuCoIn$_{5}$. Characteristics of magnetism and superconductivity in theses 4f- and 5f-electron systems bear similarities to those in cuprate and iron-pnictide superconductors. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P1.00002: How spins become pairs: Composite pairing and magnetism in the 115 heavy fermion superconductors Invited Speaker: The highest temperature heavy fermion superconductors are found in the 115 family: CeMIn$_5$ (M=Co,Ir,Rh) and PuMGa$_5$ (M=Co,Rh) [1], where the heavy quasiparticles are only partially formed by the time they pair. The internal structure of the pair is thus just as important as the forces holding it together. We show that the heavy fermion condensate necessarily contains two d-wave components condensed in tandem: pairs of heavy quasiparticles on neighboring sites and composite pairs consisting of two electrons bound to a single local moment. These two components draw upon the antiferromagnetic and two-channel Kondo interactions, respectively, to cooperatively enhance the superconducting transition temperature, as we demonstrate within a symplectic-N solution [2,3] of the two-channel Kondo-Heisenberg model [4]. Additionally, the tandem condensate is electrostatically active, which we predict will result in a superconducting shift in the electronic quadrupolar frequency, as measured in Mossbauer spectroscopy. \\[4pt] [1] J. L. Sarrao and J.D. Thompson, J. Phys. Soc. Jap. 76, 051013(2007). \\[0pt] [2] R. Flint, M. Dzero and P. Coleman, Nat. Phys. 4, 643 (2008). \\[0pt] [3] R. Flint and P. Coleman, Phys. Rev. B 79, 014424(2009). \\[0pt] [4] R. Flint and P. Coleman, arXiv:0912.2339 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P1.00003: Imaging the ``Hidden Order'' Transition in URu$_{2}$Si$_{2}$ Invited Speaker: In URu$_{2}$Si$_{2}$, bulk measurements indicating the formation of heavy bands begin at temperatures around 55 K but are interrupted by an unidentified electronic phase transition, the ``hidden order,'' at $T_{o}$ = 17.5 K. Heavy bands in a Kondo lattice are expected to form due to strong hybridization between electrons localized in real space on magnetic ions and those delocalized in momentum space. Why the ``hidden order'' appears has been an outstanding question in heavy fermion physics. We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to image the electronic structure of URu$_{2}$Si$_{2}$ though $T_{o}$. Above $T_{o}$ we find the Fano spectra expected for Kondo screening of a magnetic lattice, while below $T_{o}$ a partial energy gap opens. Heavy-quasiparticle interference imaging shows that the gap forms due to a light momentum-space band splitting below $T_{o}$ into two heavy fermion bands. Our observations of the ``hidden order'' transition are thus consistent with a sudden alteration in both the hybridization at each U atom and the associated heavy bands. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P1.00004: Scanning Tunneling Microscopy and Spectroscopy of the Heavy Fermion Compounds URu$_{2}$Si$_{2}$ and CeCoIn$_{5}$ Invited Speaker: Heavy electronic states originating from the $f $atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM) to examine the novel electronic states that emerge from the uranium $f $states in URu$_{2}$Si$_{2}$ [1]. We find that as the temperature is lowered, partial screening of the $f $electrons' spins gives rise to a spatially modulated Kondo-Fano resonance that is maximal between the surface U atoms. At T=17.5 K, URu$_{2}$Si$_{2}$ is known to undergo a 2$^{nd}$ order phase transition from the Kondo lattice state into a phase with a hidden order parameter. From tunneling spectroscopy, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that develops in the hidden order state. Spectroscopic imaging further reveals a spatial correlation between the hidden order gap and the Kondo resonance, suggesting that the two phenomena involve the same electronic states. We further study the behavior of the Kondo lattice in a model heavy fermion compound CeCoIn$_{5}$ as a function of temperature and establish a direct comparison between the two heavy fermion compounds. \\[4pt] [1] P. Aynajian, \textit{et al.} \textit{Proc. Natl. Acad. Sci. USA} 107, 10383 (2010). \\[4pt] This work is funded by a DOE-BES grant. Infrastructure at the Princeton Nanoscale Microscopy Laboratory are also supported by grants from NSF-DMR, Keck Foundation, and NSF-MRSEC. PA also acknowledges support of a fellowship through the PCCM funded by NSF MERSEC. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P1.00005: ``Hidden order,'' heavy electron ferromagnetism, and non-Fermi liquid behavior in the pseudoternary system URu$_{2-x}$Re$_x$Si$_2$ Invited Speaker: The identity of the ordered phase that occurs at temperatures below $T_o$ = 17 K in the heavy fermion compound URu$_2$Si$_2$ has eluded researchers for two and a half decades. Features in various physical properties associated with this so-called ``hidden order'' (HO) phase are reminiscent of a charge or spin density wave that forms a gap over about 40\% of the Fermi surface below $T_o$, while the remainder of the Fermi surface is gapped by the superconductivity below $T_c$ = 1.5 K. In order to attain a better understanding of these phenomena, the physical properties of URu$_2$Si$_2$ have been studied as a function of applied pressure, chemical substitution, and magnetic field. Whereas the application of pressure suppresses the superconductivity and induces a phase transition from the HO phase to an antiferromagnetic phase, the substitution of Re for Ru results in the suppression of the superconductivity and the HO transition, the nearby emergence of ferromagnetic (FM) order, and unique critical behavior associated with the FM phase. Magnetization measurements on the URu$_{2-x}$Re$_x$Si$_2$ pseudoternary system as a function of $x$ reveal the onset of ferromagnetism at a concentration $x_{cr}$ $\approx$ 0.15, which apparently represents a FM quantum critical point. Non-Fermi liquid (NFL) behavior in the physical properties such as the electrical resistivity and specific heat at low temperatures is found to extend deep into the FM region of the $T$ - $x$ phase diagram. Experiments conducted on URu$_{2-x}$Re$_x$Si$_2$ single crystals to investigate the superconducting, HO, and FM phases, characterize the NFL behavior, and establish the $T$ - $x$ phase diagram are described. The experimental results are compared to theoretical models for ferromagnetism in a Kondo lattice. Research performed in collaboration with N. P. Butch, J. R. Jeffries, B. T. Yukich, and D. A. Zocco [Preview Abstract] |
Session P2: The Kondo Ground State in Graphene
Sponsoring Units: DCMPChair: Piers Coleman, Rutgers University
Room: Ballroom A2
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P2.00001: Topological Kondo Ground State in Graphene Invited Speaker: Dirac electrons in graphene comprise two-component wavefunctions and quantum symmetries intertwining pseudospin, chirality, and Berry's phase, all ultimately stemming from a node or topological degeneracy in the spectrum known as the Dirac point. Graphene represents one prototype example of a larger class of nodal metals in which a relativistic spectrum causes the density of states to vanish linearly. Based on the unique electronic structure of such systems, a large body of theoretical work has highlighted the propensity for Dirac electrons to condense in strongly correlated ground states when additionally coupled to the real spin degree of freedom. We report the observation of one of these elusive ground states, realized in graphene via unconventional Kondo screening of individual atomic spins by massless Dirac fermions. Low-temperature scanning tunneling microscopy reveals the emergence of a new energy scale and a striking bimodal Kondo resonance localized around magnetic atoms placed on a clean graphene monolayer. Quasiparticle interference maps and concomitant spectroscopy in a high magnetic field demonstrate the spin origin of the associated ground states, and their direct link to local conservation or breaking of effective time-reversal symmetry in the underlying Dirac Hamiltonian. We find these novel spin states to be topologically controlled by Berry phase interference; in the most exotic manifestation, we show experimental evidence for two electron flavors---decoupled in momentum space by a $\pi $ Berry phase shift cancellation---participating in a chiral two-channel Kondo effect. We link these results to a new platform we have developed for the study of topological phases, artificial graphene assembled by atomic manipulation. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P2.00002: Orbitally controlled Kondo effect in graphene Invited Speaker: Graphene differs from usual metals or semiconductors in being a truly two-dimensional material with the charge carriers resembling massless Dirac fermions and the chemical potential being highly tunable by gate voltages. Recently, scanning tunneling spectroscopy experiments opened the exciting possibility to address the interaction of graphene with magnetic adatoms and to investigate the Kondo effect in a material that is simple, of immediate technological importance and offers unprecedented high tunability. Here, we develop a realistic description of the interaction of magnetic adatoms with graphene and explain the role of orbital symmetries: General symmetry arguments show that the Kondo effect in graphene is controlled not only by the spin but also by the orbital degree of freedom and spin-orbit coupling. For the example of Co adatoms, commonly used in experiments, we identify possible scenarios for the Kondo effect based on ab initio calculations. For a Co atom absorbed on top of a carbon atom, the Kondo effect is quenched by spin-orbit coupling below an energy scale of 15K. For Co with spin S=1/2 located in the center of a hexagon, a crossover from SU(4) Kondo physics at higher energies to an SU(2) Kondo effect on the scale of the Co spin-orbit coupling strength is encountered. The interplay of the orbital adatom physics and the peculiar band structure of graphene is directly accessible in Fourier transform tunneling spectroscopy or in the gate-voltage dependence of the Kondo temperature which is predicted to display a very strong, characteristic particle-hole asymmetry. The particular high symmetry situation provided by adatoms on graphene can pave the way for a deeper understanding of Kondo screening in general nanomagnetic structures. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P2.00003: Kondo effect and STM spectroscopy of Dirac electrons in graphene Invited Speaker: We show that graphene, whose low-energy quasiparticles display Dirac like behavior, may exhibit a two-channel Kondo effect in the presence of magnetic impurities. We present a large $N $analysis for a generic spin $S $local moment coupled to Dirac electrons in graphene and demonstrate that the corresponding Kondo temperature can be tuned by an experimentally controllable applied gate voltage. We also study the STM spectra of these Dirac electrons in the presence of such impurities and demonstrate that such spectra depend qualitatively on the position of the impurity atom in the graphene matrix. More specifically, for impurity atoms atop the hexagon center, the zero-bias tunneling conductance, as measured by a STM, shows a peak; for those atop a graphene site, it shows a dip. We provide a qualitative theoretical explanation of this phenomenon and show that this unconventional behavior is a consequence of conservation/breaking of pseudospin symmetry of the Dirac quasiparticles by the impurity. We also predict that tuning the Fermi energy to zero by a gate voltage would not lead to qualitative change in the shape of the conductance spectra when the impurity is atop the hexagon center. A similar tuning of the Fermi energy for the impurity atop a site, however, would lead to a change in the tunneling conductance from a dip to a peak via an antiresonance. We discuss some recent experiments on a doped graphene sample that seem to have qualitative agreement with our theory and suggest further experiments to test our predictions. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P2.00004: Quantum critical Kondo screening in graphene Invited Speaker: Magnetic impurities in neutral graphene provide a realization of the pseudogap Kondo model, which displays a quantum phase transition between phases with screened and unscreened impurity moment. In this talk, I discuss the physics of the pseudogap Kondo model with finite chemical potential $\mu$. While carrier doping restores conventional Kondo screening at lowest energies, properties of the quantum critical fixed point turn out to influence the behavior over a large parameter range. Most importantly, the Kondo temperature $T_K$ shows an extreme asymmetry between electron and hole doping. At criticality, depending on the sign of $\mu$, $T_K$ follows either the scaling prediction $T_K\propto|\mu|$ with a universal prefactor, or $T_K \propto |\mu|^x$ with $x\approx2.6$. This asymmetry between electron and hole doping extends well outside the quantum critical regime and also implies a qualitative difference in the shape of the tunneling spectra for both signs of $\mu$. Finally, the considerations are extended to the two-channel Kondo model where non-Fermi liquid behavior emerges at lowest energies. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P2.00005: Gate-Controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface Invited Speaker: Graphene impurities provide both a source of mobility-limiting disorder as well as a means to alter the graphene electronic structure in a desirable way. While these effects have thus far been primarily studied with spatially averaged techniques, understanding the microscopic physics of such behaviour requires local-probe exploration of the subnanometre-scale electronic and structural properties of impurities on graphene. In this talk I will describe scanning tunnelling microscopy and spectroscopy measurements made on individual Co atoms deposited onto back-gated graphene devices. We observe features in the tunneling local density of states (LDOS) of the Co adatoms related to both atomic resonances and phonon excitations. We also find that the electronic structure of Co adatoms can be tuned by application of the device gate voltage, and that the Co atoms can be reversibly ionized. Large screening clouds are observed to form around Co adatoms ionized in this way, and we observe that some intrinsic graphene defects also show charging behaviour. Our results provide new insight into charged-impurity scattering in graphene, as well as the possibility of using graphene devices as chemical sensors. The relationship between our measurements and recent transport experiments will also be discussed. [Preview Abstract] |
Session P3: FIAP/DCMP/DMP Prize Session: Pake, Adler, IAP
Sponsoring Units: DCMP FIAP DMPChair: Sam Bader, Argonne National Laboratory
Room: Ballroom A3
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P3.00001: George E. Pake Prize Talk: Silicon:Germanium's Road to Market; A Sceptic's Guide From Surface Science to WiFi Invited Speaker: There is an adage that goes, ``Believe none of what you hear and only half of what you see.'' Though this overstates matters just a bit, it is nonetheless a valuable insight, as much of the success in developing and commercializing silicon;germanium(Si:Ge) technology was derived from the suspension of core scientific ``truths'' in favor of rigorous examination of their origins. In this talk I will review a series of foundational discoveries regarding the inception of growth for Si:Ge alloys that ultimately led to the pervasive deployment of this technology. Inextricably linked to the success of this work was a diverse team of extraordinarily talented individuals who each contributed remarkable insights at critical junctures, a valuable reminder of the potential of collaborative innovation to produce seminal discontinuities in science and technology. I will trace the evolution of collaborative innovation from this nucleus to a foundational business model by which much of current silicon technology is derived. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P3.00002: David Adler Lectureship Award in the Field of Materials Physics Talk: Novel Nitride and Oxide Electronics Invited Speaker: Recent progress in development of GaN-based transistors for gas and bio-sensing applications and amorphous IGZO layers for use thin film transistors (TFTs)on flexible substrates, including paper,will be presented. For the detection of gases such as hydrogen, the gateless GaN transistors are typically coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions.The use of enzymes or adsorbed antibody layers on the semiconductor surface leads to highly specific detection of a broad range of antigens of interest in the medical and security fields. We give examples of recent work showing sensitive detection of glucose, lactic acid, prostate cancer and breast cancer markers and the integration of the sensors with wireless data transmission systems to achieve robust, portable sensors. The amorphous transparent conducting oxide InZnGaO4 (IGZO) is attracting attention because of its high electron mobility (10-50 cm2.V-1.sec-1), high transparency in the visible region of the spectrum and its ability to be deposited with a wide range of conductivities.This raises the possibility of making low-cost electronics on a very wide range of arbitrary surfaces, including paper and plastics. N-type oxides such as zinc oxide, zinc tin oxide, indium gallium oxide, and indium gallium zinc tin oxide (IGZO) exhibit surprisingly high carrier mobilities even for amorphous films deposited at 300K. This has been explained by the fact that the conduction in these materials is predominantly through non-directional s orbitals which are less affected by disorder than the directional sp3 orbitals which control electron transport in Si. Examples of progress and discussion of remaining obstacles to use of IGZO TFTs will be presented [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P3.00003: Prize for Industrial Applications of Physics Talk: Low energy spread Ion source for focused ion beam systems-Search for the holy grail Invited Speaker: In this talk I will cover my personal experiences as a serial entrepreneur and founder of a succession of focused ion beam companies (1). Ion Beam Technology, which developed a 200kv (FIB) direct ion implanter (2). Micrion, where the FIB found a market in circuit edit and mask repair, which eventually merged with FEI corporation. and (3). ALIS Corporation which develop the Orion system, the first commercially successful sub-nanometer helium ion microscope, that was ultimately acquired by Carl Zeiss corporation. I will share this adventure beginning with my experiences in the early days of ion beam implantation and e-beam lithography which lead up to the final breakthrough understanding of the mechanisms that govern the successful creation and operation of a single atom ion source. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P3.00004: Prize for Industrial Applications of Physics Talk: Start-up Company (Ad)Ventures -- the Highs {\&} Lows Invited Speaker: Each start-up company is a unique enterprise, with its own strengths, weaknesses, challenges and eventual outcome. However, there are many aspects common to all new technology companies, including the need for: 1. An innovative technology edge. 2. A solid product idea. 3. A team with experienced people in at least some of the key jobs. 4. Adequate initial funding to achieve some significant milestones. 5. Patient investors who can persevere through the inevitable hard times. 6. A liquidity/exit strategy clearly articulated from day one. The case studies of how other companies were started and developed can provide useful insights into what may lie ahead for the founders of a new company. Several examples from my own experience will be discussed. While they are all clearly different, there are common threads running through all of these stories. Some thoughts on what went right or wrong, and what could have been done better will be presented. [Preview Abstract] |
Session P4: Kinetic Pathways to Assembly of Polymers, Particles and Biomolecules
Sponsoring Units: DPOLYChair: Ryan Hayward, University of Massachusetts
Room: Ballroom A4
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P4.00001: Equilibration and metastability in block copolymer micelles Invited Speaker: The strong incompatibility between a solvophobic block and the surrounding medium leads naturally to extremely slow chain exchange kinetics between micelles. The steric barrier between well-solvated coronas similarly inhibits micelle fusion/fission processes. Consequently, equilibration of block copolymer micelles is typically prohibitively slow. As a result, it is possible for one system to adopt quite different micellar shapes and sizes, depending on preparation method. However, by using low volatility solvents such as ionic liquids or paraffinic oils, combined with weakly solvophobic blocks, it is possible to design model systems with accessible critical micelle temperatures. This enables the study of both the mechanisms of chain exchange, by time-resolved small-angle neutron scattering, and the evolution of non-equilibrium structures, by dynamic light scattering. Examples of both approaches will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P4.00002: Uncovering the Biological Identity at the NanoScale; Fundamantal principles governing interactions between nanoparticles and living organisms Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P4.00003: Nanoparticle formation by block copolymer directed rapid precipitations---Flash NanoPrecipitaiton Invited Speaker: With widespread interest in the generic ``nano'' attention has been focused on strategies of making small particles. High-value applications that drive new process innovation include very hydrophobic pharmaceutical actives, dyes and pigments for ink jet printing, or the dispersal of highly toxic insecticides on carriers. While it is relatively easy to make inorganic nano-particles, for example CdS particles, it is much more challenging to make nanoparticles from low surface energy organic solids. Strategies for forming nano particles vary from supercritical spraying, supercritical freezing, milling, solvent exchange precipitation, and imbibing into polymeric micelles. The solute and process combine to give differences in crystalline/amorphous products, individual particles/agglomerates, and uniformity/polydispersity of sizes. We will give an overview of the techniques and the classes of products that each addresses. We have developed a new technology that has two components: (1) rapid and tailored micromxing in an impinging jet, and (2) novel block copolymer stabilizers. The impinging jet process allows the production of nano-particles by: 1) elimination of mass transfer limitations and compositional gradients within 10 ms as determined by independent measurements with competitive-parallel reactions, 2) production of high supersaturations and solute concentrations so that high production rates can be obtained, and 3) control of particle size by stabilization of the particle using block copolymer self-assembly. The process depends critically on control of three time scales: particle nucleation and growth, block copolymer micellization, and polymer adsorption on the particle to produce steric stabilization. We present data on characterization of the mixing times using competitive reactions, data on polymer micellization kinetics, and results on the successful production of $\beta $-carotene and taxol particles with control of the particle size between 40 nm to 600 nm. A range of block copolymers have been used : PS-b{\_}PEO, PBA-b-PAA, and PCL-b-PEO. Homogeneous rapid nucleation and growth produces particle size distributions that are much narrower than those obtained by alternate size-reduction or precipitation routes, and results in a decreased tendency to Ostwalt ripen. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P4.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P4.00005: Soft matter self-assembly driven by specific and nonspecific attractions: dynamic pathways and the Ostwald rule of stages Invited Speaker: Ostwald's rule of stages is one of the few rules-of-thumb we possess that suggests the dynamical pathway a material will take when crystallizing. It states that bulk phases intermediate in free energy between the parent phase and the stable solid will emerge before establishment of the stable solid. Although widely applicable, this rule is frequently seen to break down in experiments and computer simulations, showing it to be without theoretical foundation. A first step in going beyond this rule is to understand why it breaks down. Here we test Ostwald's rule of stages in a statistical mechanical model of crystallization. Our model describes particles that are prototypical of a class of materials (such as proteins and patchy nanoparticles) able to form solid phases stabilized by directional attractions, as well as sparse and dense fluidlike phases. We find that the rule holds in certain regimes of parameter space and breaks down in others. Importantly, its breakdown can be anticipated using simple arguments. We show that the qualitative crystallization pathway of the model depends in general on both the thermodynamic landscape prescribed by inter-particle interactions and on the relative rate of particle rotations and translations. This observation emphasizes that any general rule of crystallization must account for both thermodynamic and dynamic factors. [Preview Abstract] |
Session P5: Broader Impact: Partnerships and Resources to Achieve Successful Public and K-12 Outreach and Engagement
Sponsoring Units: FEd FPSChair: Eric Marshall, Materials Research Society and IBM T.J. Watson Research Center
Room: Ballroom C1
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P5.00001: Science Museum Resources and Partnerships for Public and K-12 Outreach and Engagement Invited Speaker: Science museums engage in a wide range of activities not apparent to exhibit hall visitors. Many of them can support research outreach to public and K-12 teachers and students. In addition to exhibits in science centers, and demonstrations on topics like electricity or cryogenics, science museums offer courses for children and adults, out-of-school programs for students, teacher professional development; some do K-12 curriculum development and some run science magnet schools. In recent years science museums have increased their capacity to communicate with the public about current research. The Museum of Science, for instance, created a Current Science and Technology Center in 2001 dedicated to science in the news and current research developments. Through this Center, the Museum partnered with Harvard University to provide a wide range of public engagement activities as part of Harvard's Nanoscale Science and Engineering Center focused on the Science of Nanoscale Systems and their Device Applications. In the past five years a number of new collaborations among science museums have developed, many in partnership with researchers and research centers. Perhaps the largest or these, the Nanoscale Informal Science Education Network (NISE Net) was launched in 2005 with funding from the National Science Foundation. The NISE Net links informal science education organizations together and to university research centers to raise the capacity of all the participant organizations to increase public awareness, understanding, and engagement with nanoscale science, engineering, and technology. Nearly 300 informal educational organizations in every state nationwide make use of NISE Net's educational materials, professional development, national and regional meetings, and online resources. NISE Net is an open source network with all of its materials freely available to everyone. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P5.00002: Professional Society Resources and Partnerships for Public and K-12 Outreach and Engagement Invited Speaker: Outreach and public engagement lower the barriers that inhibit broader public appreciation of and participation in physics, and are important for inspiring the next generation of scientists and science-literate citizens. The APS and many other professional societies have made significant and sustained investments in public engagement because of the importance of these activities - APS, for example, has an entire department dedicated to outreach. In addition, professional societies have responded to members who desire resources for enabling and enhancing their own outreach efforts. A key question is always, ``What works?'' Professional societies can help provide the answers. In this talk, I will explore the critical interface played by professional societies as a bridge to the public, as a resource to members, and as a broker of partnerships. I will also feature numerous examples of creative and compelling ways to engage the public, including physicscentral.org, LaserFest, NISE Net, Comic Con, SOCKs, citizen science, and many more. A more important question is, ``Is it fun?'' I will show that the answer is an unqualified, ``Yes!'' [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P5.00003: National Laboratory Resources and Partnerships for Public and K-12 Outreach and Engagement Invited Speaker: Nanoscale science and engineering draws upon aspects of chemistry, physics, biology and engineering to address scientific problems in energy, healthcare, security and technology. Scientists in this field often work in a multidisciplinary setting, which suggests a need for educational content unlike that currently offered in single-discipline high school and college science courses. Instructors are faced with the daunting task of accurately describing nanoscience in the context of their discipline, while inspiring students to explore careers in nanoscale science and engineering. The Molecular Foundry, a Department of Energy nanoscience user facility located at Lawrence Berkeley National Laboratory, offers opportunities for high school and college students, along with science and engineering educators interested in learning basic concepts and research developments in nanoscience. Successful partnering with the Nanoscale Informal Science Education Network also provides opportunities for scientists to interact informally with the general public. These interactions convey the role of national laboratories in helping lay audiences understand the breakthroughs, potential issues and societal impact of nanoscience. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P5.00004: Marshalling Corporate Resources for Public and K-12 Technical Education Outreach and Engagement Invited Speaker: In 1988, the Education Task Force of the Business Roundtable recommended that American corporations invest in pre-college education. Prior to that date, corporate investment was targeted at higher education. IBM and other corporations responded by encouraging their employees and their corporate philanthropic organizations to develop programs aimed at enhancing pre-college education. The IBM TJ Watson Research Center initiated a Local Education Outreach program, active for these past 23 years, that marshals the resources of our science-rich institution to enhance STEM education in our local schools. We have broad and deep partnerships between the Research Center and local school districts, including New York City. We have just completed our 19th consecutive year of Family Science Saturdays, which brings 4th and 5th grade children, along with their parents, to our Research Center for hands-on workshops in topics like States of Matter, Polymer Science, Kitchen Chemistry, and Sound and Light. The workshops are staffed by IBM volunteers, assisted by local high school student ``Peer Teachers.'' Since 1990, the IBM Corporation has joined with a coalition of other companies, professional engineering societies, and government agencies to sponsor the annual Engineers Week (EWeek) campaign of technical education outreach, serving as Corporate Chair in 1992, 2001, and 2008. In recent years, we have annually recruited around 5000 IBM volunteers to reach out to more than 200,000 K-12 students in order to increase their awareness and appreciation of technical careers and encourage them to continue their studies of STEM (science, technology, engineering, and mathematics). The speaker, who helped found the APS Forum on Education (FED) and served as FED Councillor for 8 years, will review these and other programs for Public and K-12 Technical Education Outreach and Engagement. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P5.00005: University Research Center Resources and Partnerships for Public and K-12 Outreach and Engagement Invited Speaker: Collaboration and partnerships are essential to successful outreach and engagement in science. Through working together across disciplines and institutions, we can take advantage of a broad range of skills and expertise to strengthen a project and its outcomes. Additionally, building upon outreach and engagement resources developed by others is equally important to efficiently increasing impact and reducing redundancy. Research centers based at universities and other institutions of higher education commonly serve as both partners and resources for those active in education and outreach. This talk will share successful examples of public and K-12 outreach and engagement efforts that partner with a research center and/or use resources developed by a research center. A thorough discussion of strategies and recommendations for fostering such collaborations, as well as a broad survey of partnership models that exist, will help those at all levels of outreach and engagement experience pursue their science education ideas and goals. [Preview Abstract] |
Session P6: Creating and Probing Exotic Optical Lattices
Sponsoring Units: DAMOPChair: Dan Sheehy, Louisiana State University
Room: Ballroom C2
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P6.00001: Quantum Gas Microscope - Simulating the Bose-Hubbard model and beyond Invited Speaker: The Quantum Gas Microscope enables high fidelity detection of single atoms in a Hubbard-regime optical lattice, bringing ultracold atom research to a new, microscopic level. I will report on investigating the Bose-Hubbard model by directly measuring number statistics and correlations across the superfluid - Mott insulator quantum phase transition. I will then give an outlook on how this enables creating novel phases in optical lattices and realizing quantum magnetism. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P6.00002: Probing Mott insulators with single atom resolution Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P6.00003: The Dicke quantum phase transition with a superfluid gas in an optical cavity Invited Speaker: A phase transition describes the sudden change of state in a physical system, such as the transition between fluid and solid. Quantum gases provide the opportunity to establish a direct link between experiment and generic models which capture the underlying physics. A fundamental concept to describe the collective matter-light interaction is the Dicke model which has been predicted to show an intriguing quantum phase transition. We have realized the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and have observed the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-ranged interactions between the condensed atoms, which are induced by two-photon processes involving the cavity mode and a pump field. We have shown that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P6.00004: Strongly Correlated Quantum Gases Trapped in 3D Spin-Dependent Optical Lattices Invited Speaker: Optical lattices have emerged as ideal systems for exploring Hubbard model physics, since the equivalent of material parameters such as the ratio of tunneling to interaction energy are easily and widely tunable. In this talk I will discuss our recent measurements using novel lattice potentials to realize more complex Hubbard models for bosonic $^{87}$Rb atoms. In these experiments, we adjust the polarization of the lattice laser beams to realize fully three-dimensional, spin-dependent cubic optical lattices. We demonstrate that atoms can be trapped in combinations of spin states for which superfluid and Mott-insulator phases exist simultaneously in the lattice. We also co-trap states that experience a strong lattice potential and no lattice potential whatsoever. I will discuss recent measurements revealing a mechanism similar to Kapitza resistance that leads to thermal decoupling in this latter combination. The implications for sympathetic cooling and thermometry using species-dependent lattices will be outlined. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P6.00005: Excitons and Polaritons for Optical Lattice Ultracold Atoms in Cavity QED Invited Speaker: The quantum phase transition from the superfluid to the Mott insulator phase is predicted by the Bose-Hubbard model and realized for optical lattice ultracold atoms. We extend the model to include excited atoms and their coupling to cavity photons. In applying a mean field theory we calculate the phase diagram, where the Mott insulator reappears for deeper optical lattices [1]. In the Mott insulator we consider the system as an artificial crystal similar to molecular crystals with advantages due to the controllability of the system parameters. In such a system electronic excitations are delocalized due to resonance dipole-dipole interactions and in exploiting the lattice symmetry they form collective electronic excitations termed excitons [2]. We show that excitons in low dimensional systems include dark and bright modes, and in free space they can be metastable or superradiant, which deviates from the case of a single atom, the fact that implies the use of resonators [3]. We suggest optical lattice ultracold atoms as new frontiers of matter for cavity QED. In the strong coupling regime excitons and cavity photons are coherently mixed to form new quasiparticles called polaritons [4]. We suggest polariotons as a nondestructive observation tool for the different phases and properties of the system. We present different set-ups that have the potential to realize optical lattice ultracold atoms within a cavity. We emphasize the recent experiment in using tapered nanofibers, which are simultaneously used to trap and optically interface cold atoms through evanescent fields [5]. This system constitutes a hybrid quantum system combining both atomic and solid state devices.\\[4pt] [1] H Zoubi, H Ritsch, \textit{PRA} \textbf{80}, 053608 (2009).\\[0pt] [2] H Zoubi, H Ritsch, \textit{PRA} \textbf{76}, 013817 (2007). [3] H Zoubi, H Ritsch, \textit{EPL} \textbf{90}, 23001 (2010).\\[0pt] [4] H Zoubi, H Ritsch, \textit{EPL} \textbf{87}, 23001 (2009).\\[0pt] [5] H Zoubi, H Ritsch, \textit{NJP} \textbf{12,} 103014 (2010). [Preview Abstract] |
Session P7: System Biology II: The Physics of Morphogenesis
Sponsoring Units: DBP GSNPChair: Shane Hutson, Vanderbilt University
Room: Ballroom C3
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P7.00001: Contractile forces driving embryonic development Invited Speaker: Proper development of an organism requires an orchestrated interplay of large sets of components. Recent developments in live fluorescent imaging methods allow the visualization of many key proteins in cells and tissues. Developing quantitative image analysis methods to measure the dynamics of shape changes in individual cells is central for understanding how a tissue gets sculpted, what molecular machineries are driving this process, and what interactions between cells are regulating it. In this talk, I will present recent advances in our understanding of the dynamical processes during morphogenesis, focusing on the example of tissue folding and invagination at the beginning of gastrulation in Drosophila. I show that this process is driven by a contractile multicellular actomyosin meshwork that dynamically forms within a few minutes at the cell surfaces. In individual cells, contraction is pulsed, with phases of contraction interrupted by pauses in which the cell size is maintained, i.e. a ratchet type dynamics that reduces the surface area of cells incrementally. Measuring the dynamics of whole cell shape changes in 2-photon live imaging data reveals that contraction pulses drive cell lengthening and relocation of cell nuclei, two transformations that are essential for successful invagination of tissue. This analysis further shows that over subsequent stages of invagination, during which cells undergo an elaborate sequence of shape changes, the volume of individual cells is a preserved quantity. These results shed new light on the forces and cellular dynamics driving tissue morphogenesis and are a step towards a quantitative understanding of how an organism's shape and internal structure arises in development. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P7.00002: From Global Stresses to Local Cell Packing During Development Invited Speaker: To perform their functions, cells in epithelial tissues must often adopt highly regular packings. It is still not fully understood how these ordered arrangements of cells arise from disordered, proliferative epithelia during development. I will use experimental and theoretical studies on an attractive model system, the cone cell mosaic in fish retina, to illustrate some ways that mechanical forces and cell signaling can interact to produce this transformation. Experiments examining the response to surgical lesions suggest that the correct mechanical environment at the tissue scale is essential to induce cone cells to rearrange into a rectangular lattice. Starting from this observation, I will argue that large-scale mechanical stresses naturally couple to and orient cell polarization and that this coupling can lead cells to line up in regular rows, as observed in the fish retina. This model predicts that cells in the rows will adopt characteristic trapezoidal shapes and that fragments of rows will persist even in tissue where the mosaic pattern is disrupted by lesions; these predictions are borne out by an analysis of cell packings at the level of the zonula occludens in wildtype and lesioned retinas. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P7.00003: Emergence and Dynamics of Polar Order in Developing Epithelia Invited Speaker: Planar Cell Polarity (PCP) is a conserved process in many vertebrate and invertebrate tissues, and is fundamental for the coordination of cell behavior and patterning. A well-studied example is the orientational pattern of hairs in the wing of the adult fruit fly Drosophila, which is an important model organism in biology. The Drosophila wing is an epithelium, i.e., a two-dimensional sheet of cells, which grows from a few cells to thousands of cells during the course of development. In the wing epithelium, planar polarity is established by an anisotropic distribution of PCP proteins within cells. The distribution of these proteins in a given cell affects the polarity of neighboring cells, such that at the end of wing development a large-scale PCP orientational order emerges. Here we present a theoretical study of planar polarity in developing epithelia based on a vertex model, which takes into account cell mechanics, cell adhesion, and cell division, combined with experimental results obtained from time-lapse imaging of the wing development. We show that in experiment, polarity order does not develop de novo at the end of wing development, but rather cells are initially polarized at an angle with respect to their final polarity axis. During wing development, the polarity axes of cells reorient towards their final direction. We identify a basic mechanism to generate such a large-scale initial polarization, based on the growth of a small number of cells with an initially random PCP distribution. Finally, we study the effect of shear and oriented cell division on dynamics of PCP order, showing that these two processes can robustly reorient the polarity axes of cells. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P7.00004: Simple Physics in Diseases and Embryonic Development of the Eye Invited Speaker: While molecular-level regulation within cells during embryonic development is highly complex, the physical mechanisms which translate this intracellular information into multicellular physical structure at the tissue level are often surprisingly simple. I will discuss an example where regulation of cell-cell contact energies is primarily responsible for robust and evolvable regular patterns, the organization of the ommatidia and supporting cells into the regular tiling characteristic of the Drosophila eye and another example where adhesion failures in the human retina result in choroidal neovascularization leading to blindness. In both cases, simulations based on materials-science techniques can help us understand the patterning mechanisms and the reasons for their robustness and failures. Such simulations are easy to extend to other developmental phenomena and to development-related diseases like cancer. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P7.00005: Morphogenesis of walled cells Invited Speaker: Walled cells have the ability to remodel their shape while sustaining an internal turgor pressure that can reach values up to 10 atmospheres. This requires a tight and simultaneous regulation of cell wall assembly and mechanochemistry, but the underlying mechanisms by which this is achieved remain unclear. In this talk I will discuss the interplay between growth and mechanics in shaping a walled cell, in the particularly simple geometry of tip-growing cells, which elongate via the assembly and expansion of cell wall in the apical region of the cell. Using only conservation laws and describing the observed irreversible expansion of the cell wall during growth as the extension of an inhomogeneous viscous fluid shell under the action of turgor pressure, we determine theoretically the radius of the cell and its growth velocity in terms of the turgor pressure and the secretion rate and rheology of the cell wall material. Moreover, we derive simple scaling laws for the geometry of the cell and find that a single dimensionless parameter, which characterizes the relative roles of cell wall assembly and expansion, is sufficient to explain the observed variation in shapes of tip-growing cells. Our work shows that the physics of cell wall expansion tightly constrains cell shape, providing a unified explanation of the characteristic morphologies of tip-growing cells across species that span several kingdoms, even though their underlying molecular mechanisms of cell morphogenesis are very different. More generally, our description provides a general framework to understand cell growth and remodeling in plants (pollen tubes, root hairs, etc.), fungi (hyphal growth and fission and budding yeast) and some bacteria. [Preview Abstract] |
Session P8: The Physics, Technology and Future of Robotics
Sponsoring Units: FPS FIAPChair: Philip L. Taylor, Case Western Reserve University
Room: Ballroom C4
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P8.00001: Finding Fun and Fame in Physics with Robots Invited Speaker: Physicist are often sought outside their immediate field for difficult solutions. Perhaps this is not surprising. However it might be unexpected how great the needs for physicists in the broadly cross discipline fields as robotics truly are. In this talk the author will describe being lured to Hollywood, by the need to uncover one of those principles, to find the deeper physical issues behind Panavision's needs for motion control. Then later describe using robotics as a tool to introduce students to physical concepts. Robots are excellent tools to focus student interest, both in General Ed courses, and graduate level electronics courses. In this vein, the author will try to highlight, ``Where's the Physics in Robotics?'' [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P8.00002: Recent Advances in Robotics and Career Opportunities for Physicists Invited Speaker: Some of the most significant advances in robotic systems over the last year are shown in this talk, which covers both autonomous and partly autonomous robots. A few robotic employers, both in Texas and elsewhere are profiled, with an emphasis on opportunities of interest to physicists. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P8.00003: Physics and Robotic Sensing -- the good, the bad, and approaches to making it work Invited Speaker: All of the technological advances that have benefited consumer electronics have direct application to robotics. Technological advances have resulted in the dramatic reduction in size, cost, and weight of computing systems, while simultaneously doubling computational speed every eighteen months. The same manufacturing advancements that have enabled this rapid increase in computational power are now being leveraged to produce small, powerful and cost-effective sensing technologies applicable for use in mobile robotics applications. Despite the increase in computing and sensing resources available to today's robotic systems developers, there are sensing problems typically found in unstructured environments that continue to frustrate the widespread use of robotics and unmanned systems. This talk presents how physics has contributed to the creation of the technologies that are making modern robotics possible. The talk discusses theoretical approaches to robotic sensing that appear to suffer when they are deployed in the real world. Finally the author presents methods being used to make robotic sensing more robust. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P8.00004: Robot Competitions Around the World Invited Speaker: This abstract not available. [Preview Abstract] |
Session P9: Liquid Crystals: Smectics, Nano-mixtures
Sponsoring Units: DFDChair: Luz J. Martinez-Miranda, University of Maryland
Room: D220
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P9.00001: Achiral structure of B4 phase in a Bent-Core Liquid Crystal Dong Chen, Michael-Scott Heberling, Joseph Maclennan, Matthew Glaser, Noel Clark, Hideo Takezoe Bent-core smectic layers have a tendency to exhibit spontaneous saddle-splay curvature, driven by the intra-layer structural mismatch. In the chiral B4 phase, the tendency for twist in the orientation of neighboring molecules coupled with the saddle-splay curvature lead to the formation of helical nanofilaments with either clockwise or anticlockwise twist. In addition to the helical nanofilament structure, we observe another microscopic structure in P12OPIMB, which is achiral, with no helical twist. This coffee-bean-like microstructure is dominated by saddle-splay curvature, like the dark conglomerate phase, but appears to have three dimensional order. The origin of these structures will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P9.00002: Two ferroelectric phases in a bent-core liquid crystal C. Zhang, N. Diorio, B.K. Sadashiva, A. J\'akli We report electro-optical, polarization current, dielectric and SAXS studies on novel bent-core materials that contain four ester groups; three in one arm and only one in the other. These materials differ from each other only by the number of carbons (n) in the alkyloxy chain terminating the one ester containing arm: in Ar 35 n=14, while in Ar 39 n=18. The phase sequences of Ar 35 and 39 are very similar to each other. Both have two mesophases, M1 and M2, with M1 in the 115$^{\circ}$C-140$^{\circ}$C temperature range, and M2 in between about 100$^{\circ}$C and 115$^{\circ}$C. Polarization current measurements indicate polarization current and optical switching in the M$_{1}$ phase with spontaneous polarization and switching time in the Ps$\sim $2.5-3mC/m$^{2}$ and $\tau \quad \sim $200$\mu $s range, respectively. While in Ar 35 the M2 phase cannot be switched, in the AR 39 we could detect polarization switching with a polarization value of about 5-6 mC/m$^{2}$ and switching time over a millisecond. Dielectric and X-ray scattering studies were employed to reveal the fine structure of the M$_{1}$ and M$_{2}$ phases. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P9.00003: Surface Induced Reduction of Twisting Power in Liquid Crystal Films LiDong Pan, Cheng-Cher Huang Null transmission ellipsometry was employed to study the temperature evolution of the helical structure in smectic liquid crystal films. Free standing films with thickness ranging from 31 to more than 400 layers were prepared and studied. The experimental results show a reduced twisting power in thin films. A simple model was constructed to explain the results. Surface effect was found to be the reason for this phenomenon. Our findings are consistent with the studies of helically ordered magnetic films. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P9.00004: Evolution of the isotropic to smectic-$A$ phase transition in liquid crystal and acetone binary mixtures Krishna Sigdel, Germano Iannacchione The first-order transition from the isotropic ($I$) to smectic- $A$(Sm$A$) phase in the liquid crystal 4-cyano-4'-decylbiphenyl (10CB) doped with the polar solvent acetone (ace) has been studied as a function of solvent concentration by high- resolution ac-calorimetry. Heating and cooling scans were performed for miscible 10CB+ace samples having acetone mole fractions from $x_{ace}$ = $0.05 (1$~wt.\%) to $0.36 (10\%)$ over a wide temperature range from $310$ to $327 K$. Two distinct first-order phase transition features are observed in the mixture whereas there is only one transition ($I$-Sm$A$) in the pure 10CB for that particular temperature range. Both calorimetric features reproduce on repeated heating and cooling scans and evolve with increasing $x_{ace}$ with the high temperature feature relatively stable in temperature but reduced in size while the low temperature feature shifts dramatically to lower temperature and exhibits increased dispersion. Polarizing optical microscopy supports the identification of a smectic phase below the high-temperature heat capacity signature indicating that the low-temperature feature represents an injected smectic-smectic phase transition. These effects may be the consequence of screening the intermolecular potential of the liquid crystals by the solvent that stabilizes a weak smectic phase intermediate of the isotropic and pure smectic-$A$. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P9.00005: The Power of Poincar\'e: Elucidating the Hidden Symmetries in Focal Conic Domains Elisabetta A. Matsumoto, Gareth P. Alexander, Bryan Gin-ge Chen, Randall D. Kamien Focal conic domains are typically the ``smoking gun'' by which smectic liquid crystalline phases are identified. The geometry of the equally spaced smectic layers is highly generic but, at the same time, difficult to work with. We develop an approach to the study of focal sets in smectics which exploits a hidden Poincar\'e symmetry revealed only by viewing the smectic layers as projections from one-higher dimension. We use this perspective to shed light upon the concentric cyclides of Dupin and several classic focal conic textures which exhibit a more widespread level of geometric organization as in Friedel's law of corresponding cones, the networks and trellises expounded by Bouligand, or Apollonian packings. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P9.00006: Phase behavior of platelets at different aspect ratios Andres Mejia, Ya-wen Chang, Dazhi Sun, Agustin Diaz, Abraham Clearfield, Hung-Jue Sun, Zhengdong Cheng Suspensions of $\alpha$ -ZrP monolayer plates have recently been found to exhibit an isotropic to nematic (I-N) and nematic to smectic (N-S) phase transition. In the past, computer simulations have been developed to study the phase diagrams of platelets. In order to experimentally investigate the phase transitions and rheological behaviors of these particles, it is necessary to be able to manipulate their size, thickness and reduce their size distribution. We demonstrate here the strong dependency of the I-N transition on the aspect ratio (diameter/thickness) via the control of pristine $\alpha$ -ZrP platelets. We confirmed that the I-N transition volume fraction decrease monotonically with the aspect ratio as shown in previous simulations by J.A.C. Veerman and D. Frenkel. Furthermore, we found additional isotropic and gel phases by increasing the polydispersity of platelet sizes. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P9.00007: Liquid Crystal-ZnO Nanoparticle Potential Photovoltaics: Role of LC Order and ZnO Particle Size and Concentration Luz J. Martinez-Miranda, Janelle Branch, Robert Thompson, Jefferson W. Taylor, Lourdes Salamanca-Riba We investigate the role order plays in the transfer of charges in ZnO nanoparticle - 8CB liquid crystal system for photovoltaic applications as well as the role the nominally 5x7nm$^{2}$ ZnO nanoparticles play in improving that order. Our results for the 5nm nanoparticles show an improvement in the alignment of the liquid crystal with increasing weight percentage of ZnO nanoparticles, up to a concentration of 30{\%} wt for the 5nm particles accompanied by an increase by three orders of magnitude in the current generated.\footnote{L. J. Mart\'{\i}nez-Miranda, Kaitlin M. Traister, Iriselies Mel\'{e}ndez-Rodr\'{\i}guez, and Lourdes Salamanca-Riba, Appl. Phys. Letts, \underline {97,} in press (2010).} Our results for the 5 x 7 nm$^{2}$ sample show that the current is larger than the current obtained for the 5 nm samples. The photocurrent can be expressed as the conductivity as a function dependent in the order in the sample times the portion of the electric field that is absorbed and transformed into the current. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P9.00008: Interaction of a Bi-molecular Liquid Crystal Film With Functionalized Nanoparticles Jefferson W. Taylor, Luz J. Martinez-Miranda, Lynn K. Kurihara We investigate the properties of a nominally bi-molecular film of liquid crystal mixed with a magnetic nanoparticle (CoFe) that was functionalized with an organic compound (MHDA or APTS) with the atomic force microscope (AFM). We seek to investigate if the functionalization compound has an effect on the ordering of the liquid crystal in the vicinity of the nanoparticle. Studies in bulk liquid crystals have shown that the functionalization compound influences how the liquid crystal will reorganize.\footnote{L. J. Mart\'{\i}nez-Miranda, and Lynn Kurihara, \textit{J. Appl. Phys, 105, }p. 084305 (2009).} The results of this investigation are compared to the results of work done on phospholipids in close contact with uncovered silica nanoparticles.\footnote{Yuri Roiter, Maryna Ornatska, Aravind R. Rammohan, Jitendra Balakrishnan, David R. Heine, and Sergiy Minko, \textit{Langmuir}, $25$, 6287-6299 (2009).} There seems to be a relation between the way that the two functionalizations behave in the bulk 8CB. The two functionalizations studied behave differently for particles larger than 22 nm, and apparently for the smaller particles. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P9.00009: Directed assembly of CdSe/ZnS quantum dots in cholesteric liquid crystal matrix Andrea Rodarte, Linda S. Hirst, Sayantani Ghosh Controlled self assembly of quantum dots (QDs) over macroscopic scales is important to realizing the potential for new applications such as photovoltaic devices and sensors. Here, we suspend CdSe/ZnS core/shell QDs in a cholesteric liquid crystal (LC) and investigate the dispersion and collective emission of the QDs when loaded into a Grandjean-Cano wedge cell. We use polarized optical microscopy and scanning photoluminescence microscopy to generate spatial and spectral maps of the QD-liquid crystal samples. We find that the LC forms Grandjean steps approximately 200um in width and the spectral effects of the QD emission correlate to the stripe formation. We also find that the cholesteric LC modulates the spectral emission of the QDs, creating a wavelength gradient dependant on the orientation of the collection polarizer with the director axis of the liquid crystal molecules. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P9.00010: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P9.00011: Diffusion-controlled Aggregation of Bucky Balls on Freely Suspended Smectic Liquid Crystal Films Zoom Nguyen, Tatyana Malinina, Cheol Park, Joseph Maclennan, Matthew Glaser, Noel Clark Bucky balls (BB) have the tendency to clump together, making it hard to have them suspended in a solvent. We find that in highly viscous bulk 8CB, a smectic liquid crystal at room temperature, the aggregation happens more slowly. As the result, a freely suspended film made from the 8CB-BB mixture contains mostly small BB clumps. The diffusion coefficients of the clumps in thin films are much bigger than in the bulk, however, accelerating the aggregation process. We measure, via video microscopy, the decrease of the clumps' diffusion coefficients over time, indicating that their sizes increase towards a terminal size determined from the rate of diffusion. The terminal-sized clumps still diffuse around and stick to each other when they meet, forming the classic fractal pattern. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P9.00012: Investigation of the lyotropic liquid crystal phase of Graphene Oxide solution Yue Shi, Rizwan Mahmood, Dong Chen, Noel Clark Graphite Oxide spontaneously exfoliates into single-layer Graphene Oxide flakes in water. As the concentration becomes higher, Graphene Oxide solution shows a phase transition from the isotropic to the lyotropic liquid crystal phase. In the liquid crystal phase, the Graphene Oxide flakes can be ordered spontaneously by flow and shearing forces. We will report the investigation of the liquid crystal phase of the Graphene Oxide solution. In addition, the light scattering studies give dynamic information of the Graphene Oxide solution. Both the translational and rotational diffusion properties are investigated corresponding to different phases formed by Graphene Oxide at different concentrations. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P9.00013: Defect dynamics in monodomain formation of a lyotropic chromonic liquid crystal under confinement Xuxia Yao, Alejandro Rey, Jung Park, Mohan Srinivasarao Lyotropic chromonic liquid crystals are a relatively new class of liquid crystals. We have studied the process of monodomain formation and the associated defect dynmaics of an anionic dye, Sunset Yellow FCF(SSY), under confinement in a flat capillary. SSY solutions were filled into a flat capillary by capillary action in isotropic phase and subsequently cooled to nematic state. Defect coarsening processes due to confinemnet include growth of small uniform domains, splitting of a center disclination line (+1) into two lines (+1/2), merging of uniform domains, and relaxation of defect curvature after pinch-off. Previously we studied the kinematics of a branch point involving a +1 and two +1/2 intersecting lines. Here we report on the collision of two such branch points and the subsequent emergence of two curved +1/2 lines that eventually coarsen into two paralell lines close to the edge of the capillary. A model that includes bending and tension line elasticity describes the branch point post collision and provides the means to asses viscoleatic moduli. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P9.00014: Thermotropic~Gold~Nanorod~Liquid Crystal Phases Paul Luchette, Peter Palffy-Muhoray Large scale, oriented arrays of gold~nanorods~(Au NR) are of interest for a variety of applications, such as negative index materials and hyperbolic dispersion lenses. We report a method for preparing~lyotropic, nematic~LC phases of Au NR by combining polymer coated Au NR with a low molecular weight ($<$ 3200) polymers~solvent. The solvent system was~prepared by reacting hydroxymethyl siloxane and styrene~via a hydrosilylation reaction. At appropriate ratios, these mixtures exhibit liquid crystalline phase behavior. ~Lyotropic LC phases of Au NR were observed for Au NR with aspect ratio above 4, diameter $\sim $15nm, using either linear or cyclic siloxane-styrene polymers as the solvent. Compared with other preparation methods such as lithography or evaporative deposition that produce static films, these self-assembled thermotropic LC phases of Au NR may be re-oriented in response to thermal or electric stimulus. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P9.00015: Organic-Inorganic Liquid Crystalline Composites Petr Shibaev Design of novel liquid crystalline composites consisting of organic liquid crystals, metal oxides (titanium oxide, zinc oxide, etc.) and ``interface'' layer covering inorganic materials is presented and discussed. The composites respond to light irradiation by changing orientation of liquid crystalline molecules, resulting in the changes of transmission and reflection properties of cells made of composite materials. The interaction of composite materials with light results from a complex chain of physico-chemical processes inside both the inorganic component and the ``interface'' layer. The processes that play the major role in the re-orientation of liquid crystalline molecules in the surface layer include: i. light-induced formation of electron-donor pairs inside metal oxides, ii. energy transfer of electron excitations to molecules inside the ``interface'' layer, iii. breaking of hydrogen bonds and conformational changes of molecules inside the ``interface'' layer. The experimental study of the processes resulting in re-orientation of liquid crystals by light is accompanied by theoretical calculations of conformational changes inside the ``interface'' layer and molecular re-orientation on the surface of inorganic materials. [Preview Abstract] |
Session P10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Metals
Sponsoring Units: DMPChair: Jacques Amar, University of Toledo
Room: D221
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P10.00001: Interplay between Quantum Size Effect and Strain Effect on Growth of Nanoscale Metal Thin Film Miao Liu, Yong Han, Feng Liu Quantum Size Effect (QSE) has been shown to be a dominant factor in the growth of metal nanofilms on semiconductor substrates in the so-called electronic growth regime. On the other hand, the strain effect is ubiquitous in heteroepitaxial growth of semiconductor and metal thin films. Most time, however, these two important effects have been studied separately focusing on one while neglecting the other. Here, we develop a theoretical framework to investigate the interplay between QSE and strain effect on the stability of metal nanofilms. The QSE and strain effect are shown to be coupled through the concept of ``quantum stress''. First-principles calculations reveal large quantum oscillations in the surface stress of metal nanofilms as a function of film thickness, which adds extrinsically additional strain-coupled quantum oscillations to surface energy of strained metal nanofilms. Our theory enables a quantitative estimation of the amount of strain in experimental samples from the measured stability patterns, explaining a possible origin for some outstanding discrepancies between the existing theories and experiments. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P10.00002: Contrasting growth modes of Ru thin film nano-structures on Si and Pd Xiangshi Yin, Ao Teng, Mustafa \"Ozer, Hanno Weitering, Paul Snijders We have studied Ruthenium thin film growth on both Si (111) and Pd (111) surfaces. The films were deposited at low (LN2) temperature and at room temperature, and subsequently annealed at elevated temperatures, up to 600C. The surface structure, morphology, and chemical composition were investigated by LEED, STM, AES and XPS. Upon deposition at low temperature, nanoclusters are formed on both Si and Pd. Remarkably, the nanoclusters are approximately 3 nm in diameter and exhibit narrow size distributions on both substrates. In the case of Ru on Si, XPS spectra indicate silicide formation at the interface above 300C, but the nanocluster surface morphology survives up to 600C. On the other hand, nanoclusters on Pd smoothen into atomically flat films above 200C. The striking difference in adatom mobilities on these substrates is surprising in light of the very high melting temperature of Ru (2400C). [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P10.00003: Temperature-dependence of Ni+Al co-deposition on NiAl(110): Atomistic-level modeling of deviations from perfect alloy ordering Yong Han, Dapeng Jing, Baris Unal, P.A. Thiel, J.W. Evans Stoichiometric co-deposition of Ni and Al on NiAl(110) for high enough temperatures (below the order-disorder transition for NiAl) must produce near-perfect alloy islands and overlayers. However, at 300K, island structure is far from perfectly ordered and depends strongly on the deposition protocol (e.g., simultaneous vs. sequential). Realistic atomistic-level modeling of this non-equilibrium behavior must provide an accurate description of not just alloy thermodynamics (through adatom adsorption and interaction energies), but also of diffusion kinetics (for adatom attachment-detachment at and transport along island edges). This is achieved by multi-site lattice gas modeling with DFT input for adsorption and interaction energies for adatoms both at adsorption sites and at transition states for hopping [T. Duguet, Y. Han et al., Proc. Nat. Acad. Sci. 107 (2010) Special Issue on Surface Chemistry; Y. Han et al., submitted (2010)]. Model analysis by KMC simulation shows a transition from poor alloy order at 300K to almost perfect order at 600K. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P10.00004: Electronic origin of kinetic and dynamic processes at atomic steps on vicinal metal substrates Invited Speaker: Single-atomic-layer steps play an important role in the kinetics and dynamics of morphological evolution and structural formation at surfaces. In this talk, we will attempt to elucidate the importance of the electronic nature in determining the bonding of adatoms and the activation barriers for adatom descent at step edges. The insights gained through first-principles case studies have timely and important impacts in understanding the evolution of many nanostructured surfaces and prevention of electrical breakdown in nanodevices. In the first case [1], we establish a clear correlation between the preferred diffusion mechanism and step-edge barrier and the relative degree of electronic shell filling of the adatom and the substrate. We also find an approximate linear relation between the adatom step-edge hopping barrier and the adatom-surface bonding strength with a slope roughly proportional to the number of the nearest neighbors of the adatom in the initial state. These results can serve as simple guiding rules for predicting precise atomic surface morphologies and designing desirable surface nanostructures, such as atom wires [2]. In the second case [3], we discover an optimal surface electromigration inhibitor on the technologically important Cu(111) surface, characterized by energetically favoring and binding strongly at the kink of step edges. Finally, we will briefly discuss how the electronic bonding strengths influence the nucleation and growth behavior of carbon atoms at the step edges of various transition-metal surfaces, a crucial insight in designing optimal kinetic pathways for mass production of quality epitaxial graphene [4]. \\[4pt] [1] Y. Mo, W. G. Zhu, E. Kaxiras, and Z. Y. Zhang, Phys. Rev. Lett. 101, 216101 (2008). \\[0pt] [2] Y. Mo, K. Varga, E. Kaxiras, and Z. Y. Zhang, Phys. Rev. Lett. 94, 155503 (2005). \\[0pt] [3] K. H. Bevan, W. G. Zhu, H. Guo, and Z. Y. Zhang, Phys. Rev. Lett. (submitted). \\[0pt] [4] H. Chen, W. G. Zhu, and Z. Y. Zhang, Phys. Rev. Lett. 104, 186101 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P10.00005: Bilayer islands in heteroepitaxy of transition metals: insights from first principles Talat S. Rahman, Marisol Alcantara Ortigoza, Sergey Stolbov Although not in equilibrium configuration, bilayer islands have been observed in the heteroepitaxy of some transition metals for four decades. Its physical origin, however, was investigated experimentally recently for Ru on Pt(111) [1]. By introducing an energy-gain criterion (upon adlayer formation) and by analyzing the density of electronic states of 1 to 3 Ru adlayers on Pt(111), we show that, even though no bonding stronger than that of atoms in bulk Ru is involved, the energy gain for the formation of the second layer is the largest. We find that the effect of the lattice mismatch is not trivial to elucidate from experiment since the electronic structure of a clean substrate changes in the presence of strain and/or chemical bonding with other species. The lattice mismatch, however, is the key factor for the instability in the formation of a third-layer. We extend the model to explain the well-known case of Co/Cu(111) and to predict other possible bilayer systems. \\[4pt] [1] A. Bergbreiter et al., Vacuum \textbf{84} 13 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P10.00006: Collective Super-Diffusive Motion of the Pb Wetting Layer on Si(111) Michael Altman, K.L. Man, M.M.T. Loy, M.C. Tringides An unusual mass transport behavior has been discovered in the Pb/Si(111) wetting layer. Mass transport is studied by observing non-equilibrium coverage profile evolution with low energy electron microscopy (LEEM). Equilibration of an initial coverage step profile does not exhibit the profile broadening and gradual, x$\sim $t$^{1/2}$, time-dependent evolution that is expected from classical considerations. Instead, the profile edge is displaced linearly in time, x$\sim $t, much faster than expected for thermally activated hopping and without dispersal. LEEM also reveals a wave-like disturbance in the wetting layer that propagates in the direction opposite the step profile motion. The Pb coverage that is left in the wake of this disturbance can be determined accurately and with high lateral resolution using selected-area LEED due to the Devil's Staircase (DS) phases in this system. The expanding wave converts an initial homogeneous DS phase with coverage $\theta >\theta _{c}$ to a final phase with $\theta _{c}$ = 1.25 ML, thereby conserving mass across the initial step profile position. This identifies a collective super-diffusive motion of the Pb layer that can extend rapidly over macroscopic distances. Such motion may facilitate the remarkably efficient self-organization of uniform height, quantum size effect-induced Pb islands on Si(111). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P10.00007: Crossover from concerted motion to periphery diffusion for Cu clusters on Cu(111): Application of Fine Grid On-Lattice SLKMC Syed Islamuddin Shah, Giridhar Nandipati, Altaf Karim, Abdelkader Kara, Talat S. Rahman The ``fine grid on-Lattice'' Self-Learning Kinetic Monte Carlo (SLKMC) technique combines the ideas embedded in the SLKMC [1] method with a new pattern recognition scheme which incorporates both fcc and hcp sites to characterize and store configurations. Application of methods for saddle point searches have revealed several new mechanisms involving multiple atoms which contribute to cluster migration. We present results for the diffusion of 2D Cu islands on Cu(111), using semi-empirical interatomic potentials [2], at three temperatures (300K, 500K and 700K). Long time simulations show a trend in crossover from concerted motion to periphery diffusion for clusters containing more than 14 atoms. The calculated trends in effective energy barriers and diffusion constants are compared with those obtained earlier from the SLKMC Method [1] which allowed only surface fcc site occupancy.\\[4pt] [1] A. Karim et al. Phys. Rev. B 73, 165411 (2006)\\[0pt] [2] S. M. Foiles et al. Phys. Rev. B 33, 7983 (1986) [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P10.00008: Shape transitions in strained islands: kinetics versus energetics Yunsic Shim, Yevgen Kryukov, Jacques Amar Recently, it has been argued that the shape transition from compact to ramified islands observed experimentally in submonolayer Cu/Ni(100) growth is not due to kinetics but can be understood in terms of energetic arguments. In order to determine the responsible mechanisms we have carried out temperature-accelerated dynamics (TAD) simulations as well as energetics calculations. Surprisingly, our results indicate that the strain-energy contribution to the dependence of island-energy on shape is relatively weak. In contrast, our TAD simulations indicate that unexpected concerted motions occurring at step edges may be responsible. The energy barriers for these concerted motions are significantly lower than for Cu/Cu(100) and Ni/Ni(100), decrease with increasing island size, and appear to saturate for islands larger than 300 - 400 atoms. These results suggest that the shape transition is of kinetic origin but is strongly mediated by strain. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P10.00009: Temperature-induced crossovers in the static roughness of a one-dimensional interface Elisabeth Agoritsas, Vivien Lecomte, Thierry Giamarchi At finite temperature and in presence of disorder, a one-dimensional elastic interface displays different scaling regimes at small and large lengthscales. Using a replica approach and a Gaussian variational method (GVM), we explore the consequences of a finite interface width $\xi$ on its small-lengthscale geometrical fluctuations. We compute analytically the static roughness $B(r)$ of the interface as a function of the distance $r$ between two points on the interface, in the specific case of short-range elasticity and random-bond disorder. We find that for a finite $\xi$ two temperature regimes exist, and we determine the corresponding different roughness regimes and their crossover lengthscales. In addition, using a directed polymer description, we study via a second GVM procedure and generic scaling arguments, a modified toy model that provides further insight on those results, which apply to experimental interfaces such as e.g. ferromagnetic domain walls in thin films, subjected to a quenched uncorrelated disorder. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P10.00010: Mean field approach to fluctuations of surface line defects Dionisios Margetis Below the roughening transition temperature, the dynamics of crystal surfaces are driven by the motion of line defects (steps) of atomic size. According to the celebrated Burton Cabrera-Frank (BCF) model, the steps move by mass conservation, as adsorbed atoms (adatoms) diffuse on terraces and attach/detach at step edges. The resulting deterministic equations of motion incorporate nonlinear couplings due to entropic and elastic-dipole step-step interactions. In this talk, I will discuss a formal theory for stochastic aspects of step motion by adding noise to the BCF model in 1+1 dimensions. I will define systematically a ``mean field'' that enables the conversion of the coupled, nonlinear stochastic equations for the distance between neighboring steps (terrace widths) to a single Langevin-type equation for an effective terrace width. In the course of my study, I invoke the Bogoliubov-Born-Green Kirkwood-Yvon (BBGKY) hierarchy for joint terrace-width probability densities and a decorrelation ansatz for terrace widths. By using an example drawn from epitaxial growth (with material deposition from above), I will compare the mean field approach to an exact result from a linearized growth model. [D. Margetis, J. Phys A: Math. Theor. 43, 065003 (2010).] [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P10.00011: Spacing distribution functions for 1D point island model with irreversible attachment Diego Gonzalez, Theodore Einstein, Alberto Pimpinelli We study the configurational structure of the point island model for epitaxial growth in one dimension. In particular, we calculate the island gap and capture zone distributions. Our model is based on an approximate description of nucleation inside the gaps. Nucleation is described by the joint probability density p$^{xy}_{n}$ (x,y), which represents the probability density to have nucleation at position x within a gap of size y. Our proposed functional form for p$^{xy}_{n}$ (x,y) describes excellently the statistical behavior of the system. We compare our analytical model with extensive numerical simulations. Our model retains the most relevant physical properties of the system. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P10.00012: Capture zone area distributions for homogeneous nucleation and growth of islands during deposition Jim Evans, Yong Han, Maozhi Li The size distribution of islands formed by homogeneous nucleation and growth during deposition is known to encode information about the nucleation mechanism. The same was recently proposed for the distribution, g(A), of areas, A, of capture zones (CZ) surrounding islands [1], where most atoms landing within a CZ aggregate with the associated island. We have developed a precise theory for g(A) whose evolution is driven by the nucleation of new islands [2]. g(A) has a complicated form controlled by details of the spatial aspects of nucleation. However, it is reasonably approximated by a Generalized Gamma distribution, g(A) $\sim $ A$^{\beta }$ exp[-cA$^{n}$]. For compact 2D islands, one has n $\sim $ 1.5, and $\beta \quad \sim $ 3(i+2)/2 for critical size i. Here, $\beta $ follows from analysis of the creation of new small capture zones between nearby pairs of islands, and n from analysis of the likelihood that a new CZ overlaps an existing large CZ. \\[4pt] [1] Pimpinelli {\&} Einstein, PRL 99 (2007) 226102; 104 (2010) 149602;\\[0pt] [2] Li, Han {\&} Evans, PRL 104 (2010) 149601. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P10.00013: Modeling Island-Growth Capture Zone Distributions (CZD) with the Generalized Wigner Distribution (GWD): New Developments in Theory and Experiment Alberto Pimpinelli, T.L. Einstein, Diego Luis Gonz\'alez, Rajesh Sathiyanarayanan, Ajmi BH. Hamouda Earlier we showed [PRL 99, 226102 (2007)] that the CZD in growth could be well described by $P(s)=a s^\beta \exp(-bs^2)$, where $s$ is the CZ area divided by its average value. Painstaking simulations by Amar's [PRE 79, 011602 (2009)] and Evans's [PRL 104, 149601 (2010)] groups showed inadequacies in our mean field Fokker-Planck argument relating $\beta$ to the critical nucleus size. We refine our derivation to retrieve their $\beta \approx i + 2$ [PRL 104, 149602 (2010)]. We discuss applications of this formula and methodology to experiments on Ge/Si(001) and on various organics on SiO$_2$, as well as to kinetic Monte Carlo studies homoepitaxial growth on Cu(100) with codeposited impurities of different sorts. In contrast to theory, there can be significant changes to $\beta$ with coverage. Some experiments also show temperature dependence. [Preview Abstract] |
Session P11: Electronic Structure: Theory and Spectra I
Sponsoring Units: FIAPChair: Chris Van Der Walle, University of California, Santa Barbara
Room: D222
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P11.00001: Hybrid functional calculations for defects in TiO$_2$ Chris Van de Walle, Joel Varley, Anderson Janotti Density functional theory (DFT) has proven its value as an immensely powerful tool for assessing structural properties of defects in semiconductors or insulators. Frequently, however, information about electronic structure is required, i.e., the position of defect levels in the band gap. Since DFT in the LDA or GGA severely underestimates the band gap, the position of defect levels is subject to large error bars. Here we show that the use of hybrid functionals allows us to overcome this problem. We illustrate the power of the approach with the example of point defects in TiO$_2$, a material of high interest for electronics, optoelectronics, and photocatalysis. Unintentional n-type conductivity in TiO$_2$ has often been attributed to oxygen vacancies (V$_O$). We find that V$_O$ is indeed a shallow donor [1]. Our calculated formation energies allow us to assess whether vacancy concentrations are consistent with experimental observations of unintentional conductivity. \\[4pt] [1] A. Janotti, J. B. Varley, P. Rinke, N. Umezawa, G. Kresse, and C. G. Van de Walle, Phys. Rev. B 81, 085212 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P11.00002: Bandgap properties of amorphous TiO2 (aTiO2) M. Kylee Underwood, Binay Prasai, Bin Cai, David A. Drabold, James P. Lewis In photocatalytic and photovoltaic applications, TiO$_2$ is a convenient material due to its stability, abundance, and functionality. The natural bandgap of TiO$_2$ is very wide thus limits its usability to the UV region of the solar spectrum. Previous research has indicated that these limitations may be overcome by doping with anionic nonmetal elements such as carbon or nitrogen. However, both experimental and theoretical research suggests that these dopants tend to act more as recombination centers rather than truly enhance the functionality of TiO$_2$. Although naturally occurring in crystalline form, the initial processing of TiO$_2$ for the production of thin films and powders results in mostly amorphous materials and further processing locks the material into a particular crystalline or poly-crystalline form. The properties of anatase and rutile crystalline structures of bulk and nanophase TiO$_2$ have been studied in detail; however, amorphous TiO$_2$ (aTiO$_2$) lacks a similar depth of study. We show, through ab initio density functional theory calculations, that aTiO$_2$ exhibits a bandgap almost identical to crystalline TiO$_2$. We will further discuss our results for anionic nonmetal dopants in aTiO$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P11.00003: Evidences for Ti-N anchoring in organic dyes on TiO2 and its influence on photovoltaic performance Yang Jiao, Sheng Meng New metal-free organic dyes with a novel donor-pi-acceptor design produce efficiencies exceeding 10\% for dye-sensitized solar cells (DSSC) applications since 2010. Based on state-of- the-art electronic structure calculations and real time time- dependent density functional theory (TDDFT) simulations, we present consolidated evidences for novel Ti-N anchoring at the interface for such a broad group of new dyes, inferred from energetics, vibrational recognition, and electronic and optical data. This fact is contrary to what people usually believed and assumed in previous experiments and was largely ignored. We further demonstrate that the presence of interface Ti-N bonds largely benefit the electronic level alignment and photoelectron injection dynamics, greatly contributing to the improved efficiencies of DSSC based on cost-effective, environment-friendly organic dyes. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P11.00004: First Principles Study on Ta$_{2}$O$_{5}$ Polymorphs Yuning Wu, Hai-Ping Cheng, Lan Li Using density functional theory (DFT) with generalized gradient approximations (GGA) and the projector-augmented wave method, we have investigated structure, energetics, elastic tensors and mechanical properties of four crystalline forms of Ta$_{2}$O$_{5}$ with exact stoichiometry and a model amorphous structure. A virtual crystal potential has also been constructed to address partial oxygen occupancy and compared to models of explicit oxygen vacancies and the oxygen-rich system. Calculations show that mechanical properties of these polymorphs are highly anisotropic. By comparison with experimental data, we find that all crystalline phases and the simulated amorphous phase have Young's modulus higher than the amorphous thin film that is measured experimentally, but the variation among crystalline structures is as high a factor of 2. Electronic properties of three Ta$_{2}$O$_{5}$ polymorphs have been calculated using a hybrid DFT and Hartree-Fock functional method that improves gap size obtained by GGA. We suggest that further experimental measurements on tantala crystals are needed to understand physical properties of this import material. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P11.00005: What is the G$^{0}$W$^{0}$ band gap of ZnO? M. Stankovski, G. Antonius, D. Waroquiers, A. Miglio, H. Dixit, P. Rinke, H. Jiang, M. Giantomassi, X. Gonze, M. C\^{o}t\'{e}, G.-M. Rignanese Recently, there has been considerable attention on ZnO as a candidate material for low-cost transparent conducting oxides. Even in its natural wurtzite bulk phase, it is numerically difficult to evaluate $G^{0}W^{0}$ quasiparticle (QP) corrections for ZnO. Therefore we have a wide range of theoretical QP gaps quoted in the literature (from $\sim\!\!1.6$~eV to $\sim\!\!3.6$~eV to be compared with $3.44$~eV experimentally). Typically, many approximations are used \textit{en route}. To find the correct theoretical gap, we have performed calculations of unprecedented accuracy. First, we study the $G^{0}W^{0}$ band gap given different ground-state DFT starting point approximations (LDA and GGA) and the effect of including scalar-relativistic corrections. Second, we present a study of results for norm-conserving pseudopotentials vs. all-electron techniques (both PAW and FP-LAPW). Four different plasmon-pole models are compared with the more accurate contour-deformation approach. Finally, a Hubbard U parameter for the 3d-states of Zn is shown to depend on the exact details of application. This work shows that the band-gap of ZnO is indeed underestimated in the $G^{0}W^{0}$ approach. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P11.00006: Energies of formation and electronic band structure of Zn-IV-N$_{2}$ semiconductors Atchara Punya, Walter R.L. Lambrecht The II-IV-N$_{2}$ semiconductors are expected to have properties closely related to those of the III-N semiconductors. We focus on Zn-IV-N$_{2}$ semiconductors with the group IV-element Si, Ge and Sn. The formation energies of the compounds in this series were calculated by the full-potential linearized muffin-tin orbital method with LDA and GGA. Zero point motion corrections were included. Furthermore, the energies of formation of competing Zn$_{3}$N$_{2}$, Si$_{3}$N$_{4}$, Ge$_{3}$N$_{4}$, Sn$_{3}$N$_{4}$ compounds were also calculated to determine the allowed ranges of the chemical potentials of the elements where the compounds are stable at zero temperature. For comparison, we also calculated the energy of formation of GaN, which is found to be in good agreement with experimental values. All compounds in the series are found to have a large region of stability. The electronic band structures are calculated using the QSGW method. The band gaps span the region from 1.65 - 5.30 eV, increasing from ZnSnN$_{2}$ to ZnSiN$_{2}$, with the bandgap of ZnGeN$_{2}$ close to that of GaN. While ZnGeN$_{2}$ and ZnSnN$_{2}$ are direct band gaps semiconductors, ZnSiN$_{2}$ is found to have an indirect gap slightly smaller than its lowest direct gap. The states near the valence band maximum at Gamma are symmetry labeled and their splittings analyzed in terms of two crystal field parameters. Spin-orbit coupling is found to have negligible effect on these states. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P11.00007: Structural and Electronic properties of $\beta$-In$_{2}$X$_{3}$ (X = O, S, Se, Te) using \emph{ab initio} calculations S.V. Khare, S. Marsillac, N.S. Mangale, V. Gade Several III-VI body-centered tetragonal layered compounds belonging to space group I4$_{1}$/\emph{amd} have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper-indium-gallium-selenide (CIGS) solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds $\beta$-In$_{2}$X$_{3}$ (X = O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants \emph{a} and \emph{c}, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For $\beta$-In$_{2}$S$_{3}$ our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states (LDOS), total density of states (DOS), and band gap E$_{f}$ of these phases have been investigated. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P11.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P11.00009: The surface passivation effects on the optical response of small CdTe quantum dots Osman Baris Malcioglu, Jean-Yves Raty In this work, the optical properties of various small-sized CdTe based quantum dots are investigated using time dependent density functional formalism. \texttt{turboTDDFT}, an implementation of the Lanczos-Liouville approach to linearized time-dependent density-functional theory, designed to simulate the optical spectra of molecular systems made of up to several hundreds atoms and distrubuted as a part of the open source \textsc{Quantum ESPRESSO} project is used. The response of the clusters at ambient temperature is estimated by performing averages of the optical spectra along the molecular dynamics trajectories. Different types of surface passivation schemes are considered in forming the quantum dot structures. Solvent effects on the surfaces that result from different passivation schemes are considered in detail using an explicit solvent approach. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P11.00010: First-principles study of the electronic structure of NiS and NiO Joaquin Noyola, Meng Tao, Qiming Zhang First-principles calculations of the electronic structure of NiS and NiO are performed. The exchange-correlation schemes of GGA, DFT+U and hybrid functional have been applied. The resulting band structures for each scheme are compared and analyzed to assess the reliability of the GGA, DFT+U, and hybrid functional. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P11.00011: Tight-binding based alloy scattering calculations in Si$_{1-x}$Ge$_{x}$ Saumitra Mehrotra, Abhijeet Paul, Gerhard Klimeck Role of alloy scattering in SiGe device performance has been up for debate since long time. The main source of confusion stems from the choice of alloy scattering potential parameter $\Delta $U$_{fit}$. We present a theoretical model within tight-binding representation for treating alloy scattering in SiGe devices. The approach is shown to inherently capture the alloy scattering potential parameter(s) which otherwise are experimentally fitted or determined from first principles calculations for different band edges. It is shown that both onsite (variation in atom type) and off-diagonal (variation in bond type) blocks are important in estimating the potential value. The extracted scattering potential is then used to estimate bulk alloy scattering limited mobility in atomistic SiGe representation. The results show good agreement for both n-type and p-type experimental bulk mobility values. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P11.00012: Predicting the Direct to Indirect Transition in III-V Alloys Jeremy Nicklas, John Wilkins The screened hybrid functional, HSE, used in density functional theory (DFT) has been gaining traction recently for its predictive powers of the band structure in bulk semiconductors. It is natural to assume that these accurate results would carry over to alloy semiconductors, but little work has been done to confirm this. We recently investigated the compositional dependence on the electronic band structure for a range of III-V semiconducting alloys (AlGaAs, InAlAs, AlInP, InGaP, and GaAsP) [1]. These alloys have a critical composition where the band gap crosses over from a direct band gap (having optoelectronic uses) to an indirect band gap (window layers in solar cells). A direct comparison of this critical composition is made between HSE and the standard density functional, PBE, revealing crossover compositions within 12\% atomic composition when compared to experiment while PBE overestimates by as much as 39\% atomic composition. Such results give merit that HSE is a reliable functional for tuning the electronic properties of semiconducting alloys.\\[4pt] [1] Jeremy W. Nicklas and John W. Wilkins, Appl. Phys. Lett. 97, 091902 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P11.00013: Hybrid DFT computes accurate band offsets of semiconductor alloy heterostructures Amita Wadehra, Jeremy Nicklas, John Wilkins Semiconductor alloy heterostructures are the backbone of optoelectronic devices. Among the most important parameters that determine the utility of heterostructure devices are the valence and conduction band offsets. Although DFT with standard functionals such as LDA or PBE does an acceptable job for valence band offsets, it fails to predict accurate conduction band offsets on its own due to the well-known band gap problem. We demonstrate the accuracy of HSE (Heyd-Scuseria-Ernzerhof) hybrid functional for computing the band gaps and band offsets of a broad selection of technologically important semiconductor alloys and their heterostructures, e.g., AlInAs/GaInAs, GaInP/AlGaAs, AlInP/GaInP [1]. The highlight of this study is the computation of conduction band offsets with a reliability that has eluded standard density functional theory. These results demonstrate predictive power of HSE for band engineering of relevant devices. \\[4pt] [1]. A. Wadehra, J. W. Nicklas and J. W. Wilkins, Appl. Phys. Lett. 97, 092119 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P11.00014: Electronic Structure of Random Alloys Chad Waxler, Byounghak Lee, Xavier Cartoix\'{a} We present a theoretical investigation of the evolution of the electronic properties of the random alloys as they undergo a transition from one pure crystal to another. For random substitutional alloys the Bloch wavevector is not a good quantum number due to the lack of translational invariance. In spite of this obvious fact the conventional methods used for random alloys calculations, e.g., Virtual Crystal Approximation and Coherent Potential Approximation, assume a medium that pertains the same symmetries of the parent compounds. The question we ask is how well the band structures from such effective medium theories agree with the real electronic structures. We address this issue using direct simulations of randomly distributed (Al,Ga)As and (In,Ga)P atom structures. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P11.00015: Atomic-scale evolution of interfacial electronic band alignment in epitaxial Gd$_{2}$O$_{3}$ on GaAs (100) B.C. Huang, Y.P. Chiu, M.C. Shih, J.Y. Shen, P. Chang, T.H. Chiang, C.S. Chang, M.L. Huang, M. Hong, J. Kwo Direct imaging of the atomic-scale configuration and interfacial electronic band alignment in epitaxial Gd$_{2}$O$_{3}$ high $\kappa $ oxides grown on GaAs (100) has been demonstrated using cross sectional scanning tunneling microscopy and spectroscopy. Measurements of the local density of states characteristics with atomic precision enabled us to determine the evolution of electronic properties in passivating the Gd$_{2}$O$_{3}$/GaAs hetero-interface. Close examinations suggested excellent electrical passivation at this interface, with low interfacial states and low leakage current density. In addition, from the local electronic states across the gate oxides, the spatial extent of the GaAs wavefunctions extended into the gate dielectric situates a minimum thickness of 0.8 nm for the Gd$_{2}$O$_{3}$ gate capacitance$_{.}$ [Preview Abstract] |
Session P12: Focus Session: Dopants and Defects in Semiconductors: Hyper Doping
Sponsoring Units: DMPChair: Jeff Grossman, Massachusetts Institute of Technology
Room: D223/224
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P12.00001: Limits of doping In$_{0.53}$Ga$_{0.47}$As with Si and Be Sangeetha Vijeyaragunathan, Tetsuya D. Mishima, Michael B. Santos We report on a study of doping efficiency in In$_{0.53}$Ga$_{0.47}$As layers grown on InP (001) substrates by molecular beam epitaxy. Si and Be effusion cells were used to provide n- and p-type dopants, respectively. In epilayers grown at 0.63 monolayers per second with a substrate temperature of 500\r{ }C, doping cell temperatures below T$_{Si}$=1260\r{ }C (T$_{Be}$=907\r{ }C) resulted in electron (hole) concentrations that followed an Arrhenius relation with an activation energy of 5.0 eV (4.0 eV). At higher cell temperatures, the carrier concentration saturated at approximately n=3.1$\times $10$^{19}$cm$^{{\-}3}$ (p=2.6$\times $10$^{19}$cm$^{{\-}3})$. For T$_{Si}$=1300\r{ }C (T$_{Be}$=928\r{ }C), the carrier concentration was increased to n=4.2$\times $10$^{19}$cm$^{{\-}3}$ (p=3.3$\times $10$^{19}$cm$^{{\-}3})$ through use of a lower substrate temperature of 400\r{ }C (470\r{ }C). The maximum carrier concentration achieved through lowering the substrate temperature was n=4.8$\times $10$^{19}$cm$^{{\-}3}$ (p=9.1$\times $10$^{19}$cm$^{{\-}3})$. For Be doping, the maximum hole concentration was increased to 1.3$\times $10$^{20}$cm$^{{\-}3}$ by using a lower growth rate. We will compare our results with the doping of GaSb and InAs, and discuss attempts to increase the maximum concentration through delta-doping and migration enhanced epitaxy. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P12.00002: Se-precipitation in ZnSe under moderate-power laser-irradiation at high-pressure G.P. Lindberg, R.E. Tallman, R. Lauck, M. Cardona, B.A. Weinstein We report evidence for the formation of Se inclusions in ZnSe under laser-irradiation during pressure-Raman experiments. Spectra of high-quality $^{68}$Zn$^{76}$Se crystals are recorded at 300K for pressures of 0-13GPa using 647nm excitation at powers of 10 and 100 mW (focal spot $\sim $ 50$\mu $m.) For runs at the higher power a new Raman peak appears at 1.8 GPa, and shifts to lower energy at the rate --3.5 cm$^{-1}$/GPa with further increase of pressure. Its frequency, 228cm$^{-1}$ at 1.8GPa, is within 7 cm$^{-1}$ of the A1 and E$^{\mbox{'}\mbox{'}}$ Raman peaks in trigonal Se, which both exhibit negative, strongly non-linear, pressure shifts.\footnote{W. Richter, \textit{et. al.}, phys. stat. sol. (b)\underline {56}, 223(1993); K. Aoki, \textit{et. al.}, J. Phys. Soc. Japan \underline {48}, 906 (1980).} In particular, the pressure-shift of the new ZnSe peak gives a reasonable fit to the average dependence of the Se A1 peak over the range 2-8 GPa. No assignment to any of the ZnSe acoustic modes (one- or two- phonon) that also soften with pressure is feasible for the new peak. It is most likely related to the Se A1-mode in Se-inclusions, whose tendency to precipitate appears to increase with pressure. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P12.00003: Band Renormalization in Mn Doped TiS$_{2}$ Timothy Kidd, Paul Shand, Laura Strauss, Jon Rameau, Tonica Valla, Peter Johnson Titanium disulphide is a narrow gap semiconductor with a highly 2D layered structure. Mn dopants can be used to transform the band structure into being truly metallic via a rigid band shift of the electronic states. The system also begins to exhibit a variety of low temperature magnetic phases at Mn concentrations above 5{\%}. We have performed angle resolved photoemission measurements of this system that clearly the transformation of the band structure from semiconducting to metallic. Furthermore, it can be seen that states near the valence band maxima become strongly modified beyond the rigid band shift approximation. The degeneracy of these states is lifted and they show behavior much like the spin splitting classically seen in surface states of gold and more recently in those of topological insulators. This behavior was quite unexpected as the states probed should be essentially bulk bands for this inert material. While no signs of temperature dependence were found to correlate these changes in electronic structure with any magnetic phase transition, it seems likely that this novel behavior arises from magnetic interactions with the Mn dopants. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P12.00004: Persistent Photoconductivity and Magnetotransport in Dilute Nitride Semiconductor Alloys R.L. Field III, Y. Jin, C. Kurdak, R.S. Goldman Nitrogen related defects, such as N interstitials and Si-N complexes, are known to dominate electrical and optical properties of dilute nitride semiconductor alloys [1,2]. We investigate the dependence of these defects on N incorporation for MBE grown Si and Te-doped dilute GaAs$_{1-x}$N$_x$ ($x$ = 0.75-1.9) alloys. Persistent photoconductivity was observed for these heterostructures as high as 160 K, with photo-capture barriers from 216-350 meV. Also, carrier concentrations extracted from Hall measurements reveal a T-independent regime above 150 K and a strong thermally-activated regime below 150 K. These two phenomena are reminiscent of the behavior of n- type AlGaAs, suggesting the presence of similar N-induced DX- center-like states in GaAsN. We will discuss the dependence of these energies on both N composition and annealing temperature. \\[4pt] [1] Y. Jin \emph{et al.}, Appl. Phys. Lett. 95, 092109 (2009).\\[0pt] [2] Y. Jin \emph{et al.}, Appl. Phys. Lett. 95, 062109 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P12.00005: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P12.00006: Magnetic properties of Mn doped zinc selenide clusters: First principles calculations Sachin Nanavati, Sundararajan V., Shailaja Mahamuni, Subhash Ghaisas, Vijay Kumar We report the result of our study on magnetic properties of Mn doped ZnSe clusters within the pseudopotential based density functional theory (DFT). In the present work, we substituted one or two Mn atoms at different cationic sites of small ZnSe clusters and the corresponding stable geometrical configurations are obtained. In general, we find a large magnetic moment of 5 $\mu _{B }$ magnetic moment when one Mn atom is substituted. For the case of doping of two Mn atoms, calculations were performed for both parallel and anti-parallel spin-configurations. The variations in the density of state (DOS), the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the binding energy, and the magnetic moment have been analyzed as a function of the cluster size. This paper will discuss the preferred sites of the dopants, type of magnetization and their bonding characteristics for the above mentioned clusters. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P12.00007: Sub-bandgap absorptance in chalcogen-hyperdoped silicon Invited Speaker: ~It has been shown that optical doping with pulsed lasers~can achieve non-equilibrium concentrations up to one atomic {\%} of heavy chalcogens in silicon.~~Compared to intrinsic silicon, this material exhibits near-unity absorption of sub-bandgap photons and has potential use in silicon infrared photodetectors and high-efficiency photovoltaics.~~Successful application of this material, however, requires better understanding of the exact mechanism responsible for sub-bandgap absorptance.~~Using a variety of techniques, we probe the chemical structure of this material system.~~We find that the short range structure of the dopant atom is correlated to the amount of sub-bandgap absorptance.~~We also compare the structure of different dopant species (S and Se) as well as different hyperdoping mechanisms (fs-laser doping vs. ion implantation followed by pulsed laser melting).~~In conjunction with theoretical modeling of expected chalcogen defect states, we identify dominant structural characteristics related to the observation of sub-bandgap absorptance. ~ Expanding on previous results, we demonstrate control of sub-bandgap absorptance through thermal processing. ~~In addition to suggesting a method to engineer the optical properties of the material, this result provides further insight into the thermodynamics of formation of a possible dopant-related defect state.~~We compare the thermodynamics measurements to the dopant structural measurements and posit a model of sub-bandgap absorptance and defect dynamics. ~These results provide a better understanding of the phenomena of sub-bandgap absorptance in chalcogen-hyperdoped silicon and a pathway to explore other hyperdoped semiconductors. ~~ [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P12.00008: Supersaturated Silicon-Chalcogen Alloys for Thin-film Photodetectors Daniel Recht, Aurore Said, Si Hui Pan, Michael Aziz, Jeffery Warrender, Thomas Cruson, David Hutchinson, Peter Persans, Joseph Sullivan, Mark Winkler, Tonio Buonassisi Supersatured silicon-chalcogen alloys are known to have strong infrared optical absorption and the ability to detect light with energy less than silicon's bandgap. The range of infrared wavelengths these alloys absorb is much broader than the range over which photodiodes made from these alloys respond. We have recently performed several experiments to understand the disconnect between optical absorption and photodetection in thin, monocrystalline films of these alloys fabricated by ion implantation followed by nanosecond laser melting. When subjected to sensitive tests of photoconductivity, these alloys show no optoelectronic response at several absorbed sub-bandgap wavelengths. Furthermore, measurements on photodiodes made from silicon chalcogen alloys suggest that these materials are in fact a potent low-voltage photodetection gain medium. These results, along with temperature dependent transport measurements and sensitive optical spectroscopy, indicate that the mechanism of sub-bandgap response could be substantially more complex than is commonly thought. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P12.00009: Non-radiative Recombination in Intermediate Band Photovoltaics Jacob Krich, Al\'an Aspuru-Guzik Intermediate band photovoltaics (IBPV) promise to absorb low energy photons while maintaining large open circuit voltages, breaking the Shockley-Queisser efficiency limit. Proposals for IBPV include hyperdoping semiconductors with impurities forming mid-gap states, creating a band entirely contained inside the larger semiconductor bandgap. For such devices to function, the electronic states in the middle of the band gap must be extended and thus not contribute to multiphonon recombination. Since the intermediate band is produced by randomly placed impurities, however, there is an inherent disorder in the electronic structure, which produces localized states inside the band gap due to Anderson localization, even at high impurity concentrations. We use a finite size scaling analysis to find the localization properties of a non-interacting intermediate band and its resultant contribution to non-radiative recombination. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P12.00010: A Metal-Insulator Transition in Silicon Hyperdoped with Chalcogens Elif Ertekin, Mark Winkler, Aurore Said, Michael Aziz, Tonio Buonassisi, Jeffrey Grossman Hyperdoped Silicon, the material resulting from the laser doping of Silicon to impurity concentrations orders of magnitude beyond the room temperature solubility limit, can exhibit unique properties. For example, ``Black Silicon'', formed from laser doping with chalcogens S, Se, or Te, exhibits anomalous sub band gap optical absorption at photon energies as low as 0.5 eV and a flat absorption spectrum. While this has piqued interest in the use of Black Silicon for optoelectronics and photovoltaics, there has not yet been a clear explanation for the enhanced optical properties. Focusing on the Se doped systems, we use Density Functional Theory to show that the optical absorption results from an impurity induced insulator to metal transition. Our calculations indicate that an isolated Se impurity introduces a localized electronic state in the band gap. At higher defect concentrations, the transition to the metallic state is demonstrated by an increase in the defect level bandwidth and the eventual merging of the defect state with the conduction bands. The concentration at which this occurs corresponds very well with experimental low temperature Hall effect measurements. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P12.00011: Dopant effects on dislocation width of dislocations in Si Yutaka Ohno, Toshinori Taishi, Yuki Tokumoto, Ichiro Yonenaga Impurities interact with dislocations in semiconductor crystals, resulting in variations of dynamical activities of dislocations such as mobility and immobilization, and also in leading to inhomogeneity of electrical and optical properties of microelectronic and PV devices. Especially in Si in demanded trend of heavily doping for miniaturized transistors, basic knowledge of dislocation-dopant impurity interaction increases the importance. In CZ-Si doped with $n$-type impurities of P, As, and Sb, dislocations freshly induced at 1173 K extended their dissociation width with increasing duration of subsequent annealing at the same temperature. The width increased by annealing when the concentration of $n$-type impurities was high. On the other hand, the dissociation width was unchanged during annealing in Si undoped and doped with $p$-type impurities of B and Ga. These results suggest that the energy of stacking fault bound to partial dislocations is strongly affected by the number of $n$-dopant impurities segregated nearby them via their thermal migration, irrespective of atomic size of the dopant impurities; i.e., $n$-dopant impurities segregate nearby a stacking fault so as to reduce the stacking fault energy. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P12.00012: Entropic Influence on the Aggregation Physics of Interstitial Point Defects in Silicon Talid Sinno, Sumeet Kapur, Alex Nieves The evolution of self-interstitials and their aggregates during the annealing of ion-implanted silicon has received a tremendous amount of attention because of their strong, non-linear effects on the diffusion of dopants. The implantation process leads to extensive lattice damage, which must be healed by thermal annealing. Also generated by the implantation process is a large number of self-interstitials which lead to enhanced dopant diffusion during annealing known as Transient Enhanced Diffusion, or TED. A major obstacle to understanding and quantitatively predicting TED is the formation of a variety of self-interstitial aggregates, which range from small amorphous three-dimensional clusters, to planar stacking-faults with various crystallographic orientations. In the present study, we use large-scale constant-stress MD simulations to dynamically simulate the evolution of an ensemble of highly supersaturated self-interstitials at various temperatures and pressures. We show that the simulated interstitial clustering into various types of planar structures exhibits a complex thermodynamic-kinetic phase diagram that is sensitively controlled by entropic factors. The observations are studied with a recently developed approach that maps out the potential energy landscape in the vicinity of the defect cluster and allows for the total (classical) free energy to be analyzed. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P12.00013: The chemical trends of a new defect cluster: DDX centers Jie Ma, Su-Huai Wei DX center is a major ``killer'' defect limiting n-type doping in group II-VI and III-V semiconductors. It converts a shallow donor to deep one, which is a major reason for the saturation of free-electron carriers in the doping process. Several structure models of isolated DX centers have been proposed in the literatures, such as the broken-bond model (BB-DX), and the $\alpha $ and $\beta $ cation-cation bond model (CCB-DX). All these DX centers can be stabilized with hydrostatic pressure or reduced dimensionality and size. In group III-V and II-VI semiconductors, it has been common believe that cation-site induced DX centers are easier to form than anion-site induced ones. Because DX centers trap an extra electron, therefore, another defect in the system must donate the electron and form a positive charged defect. We show, using GaAs as an example, that in heavily doped semiconductor, the negative charged DX center and positive charged donor can couple strongly through the Coulomb interaction, forming the dominant DDX center. The DDX centers are still deep level defects. However, unlike the DX center, the DDX centers have different chemical trends, i.e., anion-site DDX center is easier to form than cation-site DDX centers. A simple model is proposed to explain the new trends. [Preview Abstract] |
Session P13: Tutorial for Authors and Referees
Sponsoring Units: APSRoom: D225/226
Wednesday, March 23, 2011 8:00AM - 9:30AM |
P13.00001: Tutorial for Authors and Referees Editors from Physical Review Letters and Physical Review will provide information and tips for our less experienced referees and authors. This session is aimed at anyone looking to submit to or review for any of the APS journals, as well as anyone who would like to learn more about the authoring and refereeing processes. Topics for discussion will include advice on how to write good manuscripts, similarities and differences in writing referee reports for PRL and PR, and other ways in which authors, referees, and editors can work together productively. Following a short presentation from the editors, there will be a moderated discussion. Refreshments will be served. [Preview Abstract] |
Session P14: Focus Session: Friction, Fracture and Deformation Across Length Scales II: Plasticity and Rupture
Sponsoring Units: DMP GSNP DCOMPChair: Robin Selinger, Kent State University
Room: D227
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P14.00001: Scaling theory of continuum dislocation dynamics in two and three dimensions Yong S. Chen, Woosong Choi, Stefanos Papanikolaou, James P. Sethna When crystalline materials deform plastically, complex dislocation structures have been observed experimentally.\footnote{P. Hahner et al., Phys. Rev. Lett. 81, 2470, 1998.} We provide a continuum plasticity theory to study the emergent self-similar morphologies.\footnote{Y.S.Chen et al., Phys. Rev. Lett. 105, 105501, 2010.} We analyze the self-similarity in terms of critical exponents for correlation functions of dislocation density, crystalline orientation and plastic distortion, and explore the connection to the power spectrum of the total free energy. In two and three dimensions, we apply anisotropic loadings, and observe little anisotropy in the critical properties. We explore the addition of quenched disorders to our continuum theory, to investigate the relation between dynamics (plasticity avalanches) and static dislocation morphologies. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P14.00002: Saddle node scaling on approach to dislocation nucleation Akanksha Garg, Asad Hasan, Craig Maloney We study the process of dislocation nucleation in a perfect 2D hexagonal crystal under nano-indentation loading in a numerical model using energy minimization techniques and analysis of the energy eigenmodes. The nucleation event takes the form of a saddle-node catastrophe and is governed by associated scaling laws. In particular, on approach to nucleation, a single energy eigenmode descends through the spectrum and its eigenvalue vanishes as the square root of the distance to the nucleation point. The velocity of the system shows the same scaling behavior, and its normal mode decomposition demonstrates that it is dominated by the critical mode responsible for nucleation. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P14.00003: Dislocation dynamics at zero temperature and at finite temperature: analytics and simulations Karin Dahmen, Georgios Tsekenis, Pak Yuen Chan, Thomas Fehm, Jonathan Dantzig, Nigel Goldenfeld, Jonathan Uhl Crystalline materials are known to deform in an intermittent way with avalanches. Power laws govern the statistics of the avalanches. In this work we are studying plasticity as a member of the universality class of depinning phase transition. Results from our Discrete Dislocation Dynamics simulations agree with analytical mean field predictions for distributions of avalanche sizes, durations, power spectra, and avalanche shapes. Results from phase field crystal simulations agree with analytical predictions for the depinning phase transition at finite temperature. Both numerics and analytics indicate that the dynamics of edge dislocations in sheared crystals belong to the mean field universality class of depinning transitions, both at zero temperature and at finite temperature. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P14.00004: Size Matters: size-dependent strength and nucleation-governed deformation mechanisms in nano-scale Cu pillars Invited Speaker: Uniaxial compression and tension tests on single crystalline micro and nanopillars have revealed a strong size effect. For face-centered cubic metals, this size effect is characterized by a power-law: where $n$ is between .5 - .7. The majority of these micro-mechanical tests have been performed on pillars produced by the focused-ion-beam (FIB), a process known to introduce surface damage into the material and to limit the smallest attained pillar diameter to $\sim $150nm while maintaining its shape integrity. In order to overcome these detriments, we developed a new technique combining electroplating and electron beam lithography to create single crystalline Cu nano-pillars with diameters down to 50 nm. We find the mechanical response of these samples to exhibit the same power-law strengthening behavior as other fcc metals down to the diameter of 100nm, as revealed by \textit{in-situ} uniaxial compression and tension tests conducted in a custom-built in-situ mechanical deformation instrument, SEMentor. TEM investigations of the microstructure of pillars produced by the FIB and by electroplating show similar initial dislocation densities of $\sim$10$^{14}$ m$^{-2}$ implying that size-dependent strength at the nano-scale is a strong function of initial microstructure and not of fabrication method. We examine the limits of this power-law trend down to diameters of 50nm, as at these small sizes, deformation behavior has been theoretically predicted to change due to the activation of surface dislocation sources and the increasing influence of the surface stress. Furthermore, we find that these single crystalline Cu nano-pillars show a remarkable strain-rate dependence that increases with decreasing diameter further revealing the thermally activated nature of dislocation sources and corresponding changes in activation volume. HRTEM investigations of post-mortem structures will be presented in the context of dislocation-based phenomenological modeling. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P14.00005: Effect of Inertia and Damping on Avalanche Distributions in Sheared Amorphous Solids K. Michael Salerno, Craig Maloney, Mark O. Robbins Avalanches occur in a variety of contexts from magnets to granular materials. Molecular dynamics simulations of a sheared binary Lennard-Jones glass are used to explore the effect of inertia and damping on avalanche distributions. We find that the energy dissipation rate is one of the key factors in determining the size of an individual avalanche as well as the distribution of avalanche energies. There are three distinct regimes: an overdamped regime where the distribution has an exponential cutoff that varies with dissipation rate, a critical regime where avalanches follow power-law statistics and large events are limited by simulation size, and a run-away regime where inertia leads to a peak at large energies. The same regimes are found for Langevin type viscous damping and Galilean-invariant Kelvin damping. While inertia determines how an avalanche evolves, some properties of the avalanche are predetermined. Weakening of the average shear modulus prior to an avalanche is a good indicator that a large, system-spanning event may occur. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P14.00006: Nonclassical Nucleation and Growth of Cohesive Tensile Cracks Joseph Gran, John Rundle, William Klein We analyze the nucleation and growth of cohesive tensile cracks using a Hamiltonian which is written as a functional of the crack separation (offset field). We simulate the nucleation events on a square lattice using a Metropolis Monte Carlo algorithm. Several modes of crack propagation are seen in the simulations. Our results indicate that for certain materials, crack nucleation and growth proceed through the formation and extension of a diffuse ``halo'' surrounding the classical portion of the crack. This is similar to nonclassical nucleation near the spinodal in magnetic systems. Theoretical considerations and numerical calculations strongly suggest that the diffuse halo can be identified with the fracture ``process zone'' seen in laboratory studies of advancing cracks. We are investigating scaling exponents associated with this apparent phase transition. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P14.00007: Enhanced Strength via crack friction and Pressure Donald Wiegand, Kevin Ellis, Claire Leppard The effect of pressure on the mechanical response of particulate polymer composites is being studied. Between about 0.1 and 7 MPa for one composite the results indicate that slow crack growth is the dominant failure mode. With continuously creasing strain at low pressures the stress initially increases to a maximum, the compressive strength, then decreases indicating work softening and them becomes approximately constant at a plateau value. Both the compressive strength and the plateau stress increases linearly with pressure but the plateau stress increases with a steeper slope such that at higher pressures work softening is not observed. The results are analyzed in terms of shear cracks with friction between the crack surfaces. The model predicts a threshold stress for crack growth which increases linearly with pressure and further predicts that the compressive strength increases linearly with pressure as observed and with the same slope as the threshold stress. These results clearly indicate that the pressure dependence of the compressive strength is due to the pressure dependence of the threshold stress for crack growth. The changes in the plateau region can also be attributed to frictional effects. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P14.00008: Irreversible Damage in Amorphous Silica Cindy Rountree, Damien Vandembroucq, Stephane Roux, Elisabeth Bouchaud Glass touches every aspect of our lives including the glass dishes which we cook with to the storage of nuclear waste. The extensive use of oxide glasses can be attributed to optical transparency, electrical and heat insulation, and large hardness. However, oxide glasses have a major drawback: brittleness. Even small flaws in the structure can lead to the ultimate failure of the material. Recent Atomic Force Microscope experiments and Molecular Dynamics simulations revealed a process zone ahead of the crack tip where damage nucleates, augments, and finally merges with the advancing crack front. Furthermore, when a-SiO$_{2}$ samples are nanoindented, one finds permanent damage under the indenter in the form of densified silica. To shed light on the origin of irreversible deformation in amorphous media, we have expanded our studies to examine what happens to an oxide glass when subjected to shear. MD simulations have been performed in a-SiO$_{2}$ systems which are subject to a shearing force at room temperature. The system was initially isotropic and as long the maximum shear deformation remains under 5{\%} the system remains isotropic upon unloading. However if the system is sheared to a point greater than 5{\%} permanent plastic deformation sets in and the system is no longer isotropic upon unloading. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P14.00009: Molecular dynamics study of the contact strengths between clean metallic surfaces with nanoscale asperities Hojin Kim, Alejandro Strachan A fundamental understanding of the mechanical behavior of contacting surfaces with nanoscale asperities including their adhesion and friction is critical for MEMS and other applications. We characterize the tensile strength of contacts formed between various clean Pt surfaces such as commensurate contacts between (001) and (111) surfaces and incommensurate (001) ones by using MD simulations over wide range of asperity size. In cyclic closing and opening, the first contact shows significant plastic deformation, leading to a considerable reduction in the contact area. After few cycles, steady state is achieved both contact size and the pullout force. The strength of bridges in both commensurate and incommensurate contacts exhibits strong size effects. Their strength increases with decreasing size until a length of approximately 5 nm below which weakening is observed. Commensurate contacts are stronger than incommensurate ones but only during the initial contacts, after steady state is achieved commensurate and incommensurate (001) surfaces lead to similar strengths. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P14.00010: Friction and Sliding of Polystyrene Micro Spheres in the Presence and Absence of Capillary Adhesion Iyam Lynch, Jacqueline Krim Quartz crystal microbalance (QCM) response to varying load geometries, particularly micro particles, is a rapidly growing field of research.\footnote{Dybwad, G.L. J. Appl. Phys. \textbf{1985}, 58, 2789}$^,$\footnote{Dultsev, F.N. et al. Langmuir \textbf{2000}, 16, 5036.} This no doubt is due to its varied applications involving the study of textiles, DNA and viruses$^{3}$, micro adhesion$^{2,3}$, micro sorting$^{3}$, and friction. There are many challenges that must be overcome in this field. One major difficulty is capillary adhesion, which is difficult to quantify. We have created an experiment to greatly reduce the impact of capillary adhesion by employing the shaking motion of a 5MHz QCM to eject micro spheres (15$\mu $m) from its surface, which subsequently land on the surface of a nearby 8 MHz QCM. The experiment is performed in a vacuum chamber to include different environments such as air, vacuum, and dry nitrogen. During the experiment we monitor the behavior of the unloaded QCM by measuring the change in frequency and quality factor as a result of the newly landed spheres. Particle motion and dynamics are observed using a microscope with a camera attached. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P14.00011: Cooperative adhesion and friction of compliant nanohairs Ali Dhinojwala, Liehui Ge, Lijie Ci, Anubha Goyal, Pulickel Ajayan, L. Mahadevan The adhesion and friction behavior of soft materials, including compliant brushes and hairs, depends on the temporal and spatial evolution of the interfaces in contact. For compliant nanofibrous materials, the actual contact area of individual fibers make with surfaces depends on the preload applied upon contact. Using in-situ microscopy observations of preloaded nanotube hairs, we show how nanotubes make cooperative contact with a surface by buckling and conforming to the surface topography. The overall adhesion of compliant nanohairs increases with increasing preload as nanotubes deform and continuously add new side-wall contacts with the surface. Electrical resistance measurements indicate significant hysteresis in the relative contact area. Contact area increases with preload (or stress) and decreases suddenly during unloading, consistent with strong adhesion observed for these complaint nanohairs. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P14.00012: The effect of Coulombic friction on spatial displacement statistics Andreas Menzel, Nigel Goldenfeld We study the effect of Coulombic (dry) friction on the spatial displacement statistics of one-dimensional stochastic motions. In other words, one of the simplest forms of nonlinear friction is added to the Fokker-Planck equation for conventional viscous Brownian motion, and its consequences are investigated. First, we find the eigenfunctions to the problem that includes the velocity component only. This problem can be mapped on the case of a quantum mechanical harmonic oscillator in the presence of a delta potential. Then we show numerically that a crossover from exponential to Gaussian displacement statistics results from the Coulombic frictional contribution. A transient regime of multiscaling is identified for the spatial distribution function. Our results are important for the interpretation of recent experiments in the field of soft matter physics: it turns out that, for practical purposes, higher order moments of the spatial distribution function must be determined to identify the presence of effective Coulombic frictional forces. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P14.00013: Understanding frictional duality and bi-duality: Sb-nanoparticles on HOPG Jan Brndiar, Robert Turansky, Ivan Stich We have simulated [1] the behavior of motion of Sb$_{n}$ nanoparticles on HOPG with the quest to elucidate the experimentally observed frictional bi-duality [2]. The first duality was observed for clean Sb-nanoparticles deposited under UHV conditions. Both frictionless and ``normal'' behavior was observed. Another dual behavior was found for Sb-nanoparticles exposed to ambient conditions, both scaling linearly with contact area. The vanishing friction branch is due to incommensurability of the Sb-HOPG. The non-vanishing friction branch can be accounted for by contaminants due to imperfect UHV, such as water, hydrocarbons, oxygen, etc., including small Sb$_{n}$ clusters. The large friction forces after exposition to ambient conditions result from presence of mobile oxidized multiasperities. The simulations allow for quantitative estimates of impurity concentrations and understanding of the molecular mobility. \\[4pt] [1] J. Brndiar et al. submitted (2010). \\[0pt] [2] D. Dietzel et al. Phys.Rev.Lett. \textbf{101}, 125505 (2008), Phys.Rev. \textbf{B 82}, 035401 (2010). [Preview Abstract] |
Session P15: Focus Session: Spins in Semiconductors - Quantum Computing with Defects
Sponsoring Units: DMP GMAG FIAPChair: Vlatcheslav Dobrivitski, Ames Laboratory/Iowa State University
Room: D171
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P15.00001: Quantum computing with defects Invited Speaker: The development of a quantum computer is contingent upon the identification and design of systems for use as qubits, the basic units of quantum information. One of the most promising candidates consists of a defect in diamond known as the nitrogen-vacancy (NV$^{-1})$ center, since it is an individually-addressable quantum system that can be initialized, manipulated, and measured with high fidelity at room temperature. While the success of the NV$^{-1}$ stems from its nature as a localized ``deep-center'' point defect, no systematic effort has been made to identify other defects that might behave in a similar way. We provide guidelines for identifying other defect centers with similar properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate systems. To elucidate these points, we compare electronic structure calculations of the NV$^{-1}$ center in diamond with those of several deep centers in 4H silicon carbide (SiC). Using hybrid functionals, we report formation energies, configuration-coordinate diagrams, and defect-level diagrams to compare and contrast the properties of these defects. We find that the N$_{C}$V$_{Si}^{-1}$ center in SiC, a structural analog of the NV$^{-1}$ center in diamond, may be a suitable center with very different optical transition energies. We also discuss how the proposed criteria can be translated into guidelines to discover NV analogs in other tetrahedrally coordinated materials.\\[4pt] [1] J. R. Weber, W. F. Koehl, J. B. Varley, A. Janotti, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, Proc. Nat. Acad. Sci. \textbf{107}, 8513 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P15.00002: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P15.00003: Optical Control of Spatial Patterning of Nuclear Polarization in GaAs Jonathan King, Yunpu Li, Le Peng, Maria Tamargo, Carlos Meriles, Jeffrey Reimer We present new results on the optical polarization of nuclear spins in gallium arsenide. Previous work has identified the contact hyperfine interaction at shallow donors as the mechanism for helicity dependent nuclear polarization. We show a new regime, where donors are only partially occupied, where nuclear quadrupolar relaxation at shallow donors is the dominant mechanism. Since quadrupolar relaxation is helicity independent, the incident light polarization may be tuned such that the two relaxation mechanisms drive the nuclear spins to opposite signs of polarization. We show that incident light wavelength and power may be tuned to create spatial patterns of varying donor occupation in a single sample, which in turn creates a pattern of positive and negative nuclear polarization. We have developed an analytical mode which accurately describes the bulk NMR signal in terms of irradiation power and wavelength. We also present stray-field NMR imaging experiments showing direct observation of the patterned nuclear polarization. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P15.00004: Spin Orbit Interaction in Inversion-Symmetric Semiconductors: SrTiO3 and group IV Cuneyt Sahin, Giovanni Vignale, Michael E. Flatt\'e Low-energy effective spin-orbit Hamiltonians have proved effective at describing the effect of spin-orbit interactions on populations of polarized carriers in direct-gap semiconductors such as gallium arsenide. No similar low-energy Hamiltonians are available for materials with inversion symmetry, such as cubic oxides or group-IV semiconductors. In order to construct such low-energy Hamiltonians we have calculated the electronic band structure of strontium titanate, a perovskite material which has recently been used to make high-density two-dimensional electron gases, using a tight-binding electronic structure with atomic spin-orbit interactions. We have also calculated the band structures of several group-IV semiconductors, including germanium, silicon, and diamond. An expression for the effective spin-orbit interaction in the conduction band of these materials has been derived, and calculated for these materials. The symmetry properties of this effective spin-orbit interaction tensor will also be discussed. This work was supported by an ARO MURI. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P15.00005: Excited-State Spin Manipulation and Intrinsic Nuclear Spin Memory using Single Nitrogen-Vacancy Centers in Diamond Invited Speaker: Nitrogen vacancy (NV) center spins in diamond have emerged as a promising solid-state system for quantum information processing and precision metrology at room temperature. Understanding and developing the built-in resources of this defect center for quantum logic and memory is critical to achieving these goals. In the first case, we use nanosecond duration microwave manipulation to study the electronic spin of single NV centers in their orbital excited-state (ES) [1]. We demonstrate ES Rabi oscillations and use multi-pulse resonant control to differentiate between phonon-induced dephasing, orbital relaxation, and coherent electron-nuclear interactions. A second resource, the nuclear spin of the intrinsic nitrogen atom, may be an ideal candidate for a quantum memory due to both the long coherence of nuclear spins and their deterministic presence. We investigate coherent swaps between the NV center electronic spin state and the nuclear spin state of nitrogen using Landau-Zener transitions performed outside the asymptotic regime [2]. The swap gates are generated using lithographically fabricated waveguides that form a high-bandwidth, two-axis vector magnet on the diamond substrate. These experiments provide tools for coherently manipulating and storing quantum information in a scalable solid-state system at room temperature. \\[4pt] [1] G. D. Fuchs, V. V. Dobrovitski, D. M. Toyli, F. J. Heremans, C. D. Weis, T. Schenkel, and D.D. Awschalom, \textit{Nat. Phys.} \textbf{6}, 668 (2010). \\[0pt] [2] G. D. Fuchs, G. Burkard, P. Klimov, and D. D. Awschalom, \textit{in preparation}. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P15.00006: Dynamic Jahn-Teller Effect in Negatively Charged Nitrogen-Vacancy Center in Diamond Tesfaye Abtew, Peihong Zhang The negatively charged nitrogen-vacancy (NV) center in diamond has attracted much research interest recently owing to its desirable optical properties and long spin coherent lifetime. The ground state of NV$^{- }$center has a $^{3}$A$_{2}$ symmetry, which can be optically excited, to a $^{3}$E state. The excited state is orbitally degenerate therefore should experience either static or dynamic Jahn-Teller (JT) effects. We use accurate first-principles methods to study structural and electronic properties of the NV$^{-}$ center in diamond both in the ground and excited states. Our results indicate that the excited state of the NV$^{-}$ center is indeed a dynamic JT system. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P15.00007: Using Adiabatic Pulses for the Control of Nitrogen Impurities in Diamond Zhi-Hui Wang, G. de Lange, R. Hanson, V.V. Dobrovitski High-fidelity quantum control and dynamical decoupling of the NV center in diamond has been recently demonstrated [1]. Efficiently manipulating the spin bath of nitrogen atoms (P1 centers) can add new freedom to the control of NV centers, and can map out the properties of the bath. However, the electron spins of P1 centers have a broad spectrum, and it is difficult to implement accurate rotations uniformly over the whole spectrum. We show that the adiabatic pulses (AP) provide an efficient tool for the bath control. The internal bath dynamics imposes very moderate limitations on the AP parameters so that P1 centers can be controlled with good ($>90$\%) fidelities. The shape of AP can be tailored to the spectral density of the bath for optimized performance. We show how, by manipulating P1 centers, spin echo and dynamical decoupling of the NV center can be achieved in efficient manner. \\[4pt] [1] G. de Lange et al., Science {\bf 330}, 60 (2010); B. Naydenov et al., arXiv:1008.1953 (2010); C. A. Ryan et al., arXiv:1008.2197 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P15.00008: Nanofabrication of single spins and spin arrays in diamond D.M. Toyli, G.D. Fuchs, D.J. Christle, D.D. Awschalom, C.D. Weis, T. Schenkel The properties of isolated nitrogen vacancy (NV) centers in diamond make them a promising solid-state qubit candidate for spin-based quantum information processing. However, scaling this system to multi-qubit NV center devices requires methods to accurately place single NV centers in pure diamond substrates. To address this challenge we have developed a method for fabricating single NV centers on 50 nm length scales based on ion implantation and electron beam lithography.\footnote{D. M. Toyli \emph{et al.}, Nano Lett. 10, 3168 (2010).} Secondary ion mass spectroscopy measurements facilitate depth profiling of the implanted nitrogen to provide three-dimensional characterization of the NV center spatial distribution. Finally, electron spin resonance measurements of single NV centers, including temperature-dependent spin coherence measurements, suggest a pathway for optimizing single spin coherence in future devices. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P15.00009: Nuclear spin diffusion in semiconductor quantum wells Ionel Tifrea, Tom D. Kim We analyze the nuclear spin diffusion effect in semiconductor quantum wells in connection with dynamical nuclear polarization under optical pumping. The natural confinement provided by the particular geometry of quantum well structures is responsible for a position dependent nuclear spin relaxation time and a reduced nuclear spin diffusion. In particular, we consider the case of GaAs quantum wells within GaAlAs barriers and analyze the nuclear spin diffusion for As nuclei. Our results, obtained for different nuclear spin diffusion constants, show that nuclear spin diffusion has a relatively small effect on the overall polarization of As nuclei in these structures. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P15.00010: Single-shot electrical readout of an ensemble nuclear spin memory in silicon Dane R. McCamey, J. van Tol, G. W. Morley, C. Boehme Storing information in spin is widely recognized as a promising technological driver. However, the ability to interact with, and thus control electron spin implies a reasonable coupling to the environment, and thus a limited spin lifetime. This problem can be overcome by using nuclear spins for long term information storage even though mapping nuclear spin information onto device currents has remained challenging. Here, we report on an electrically readable nuclear spin memory implemented using phosphorus donors in silicon [1]. Donor electron spins can be used to encode logical information, which is then transferred to the nuclei. The state can be stored in the nuclear spin and then read out electrically via the hyperfine coupling with the electron. We show that information can be stored in the nuclear spin for longer than 100 seconds, that the information can be read back single shot, and that repetitive measurement does not degrade the stored information. Other nuclei, such as the spin 1/2 $^{29}$Si, can also be used, pointing to the possibility of a nuclear spin memory register. [1] D. R. McCamey, J. van Tol, G. W. Morley and C. Boehme. Science, in press (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P15.00011: Nuclear spin phase transition in the presence of interacting two-dimensional electrons Robert \.{Z}ak, Dmitrii Maslov, Daniel Loss The recent study of the RKKY interaction between localized moments, e.g., nuclear spins of Ga and As atoms in a GaAs heterostructure, mediated by interacting two-dimensional electrons, has shown a possibility of polarizing nuclear spins at currently accessible temperatures [1]. This ferromagnetic phase transition is governed by: (i) anisotropy of the electron spin susceptibility, $\chi$, in the presence of Rashba spin-orbit interaction (RSOI) and (ii) nonanalyticity in momentum dependence of $\chi$. In this talk I will argue that on top of the anisotropy in $\chi$ caused by the RSOI at zero momentum [2], the momentum dependence of $\chi$ is anisotropic itself: while the linear scaling of $\chi_{zz}$ with momentum saturates at the energy scale set by the RSOI, that of the $\chi_{xx}=\chi_{yy}$ continues through this energy scale (in this way it resembles the temperature and magnetic field dependence of $\chi$ in the presence of the RSOI [2]). The effect of the renormalization of the backscattering amplitude in the Cooper channel will be taken into account as well. In the end I will elaborate on possible implications of our results for the stability and nature of the nuclear spin ordered phase. References: [1] P.~Simon and D.~Loss, PRL {\bf 98}, 156401 (2007), P.~Simon, B.~Braunecker, and D.~Loss, PRB {\bf 77}, 045108 (2008); [2] R.~A.~\.Zak, D.~Maslov, and D.~Loss, PRB {\bf 82}, 115415 (2010). [Preview Abstract] |
Session P16: Focus Session: Magnetic Nanostructures, Vortices & Domain Walls
Sponsoring Units: DMP GMAGChair: Axel Hoffmann, Argonne National Laboratory
Room: D173
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P16.00001: Tuneable remote pinning of domain walls in magnetic nanowires L. O'Brien, D.E. Read, J. Sampaio, D. Petit, E.R. Lewis, A.-V. Jausovec, H.T. Zeng, R.P. Cowburn Domain wall (DW) motion in ferromagnetic nanowires has received much attention for its potential technological applications and for probing fundamental physics. The role of DW pinning in nanowires is crucial for these investigations however it is in general a complex process. Distortions of the DW shape make quantitative agreement between modelling and experiment difficult. Here we demonstrate pinning using nanometre scale localised stray fields. This type of interaction gives well characterised, tailorable potential landscapes that do not appreciably distort the DW. Our experimental results are in excellent quantitative agreement with an Arrhenius-N\'{e}el model of depinning - a result only possible when the modelled potential profile agrees fully with that experienced by the DW. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P16.00002: Measurements of nanoscale domain wall flexing in a ferromagnetic thin film A.L. Balk, M.E. Nowakowski, M.J. Wilson, D.S. Rench, P. Schiffer, D.D. Awschalom, N. Samarth We use the anomalous Hall effect to probe the nanoscale behavior of a single magnetic domain wall (DW) in (Ga,Mn)As thin film devices with out-of-plane magnetic anisotropy. Video-rate magneto-optical Kerr microscopy is also used to confirm the variation of the AHE with DW position. Our all-electrical technique allows us to observe a low field flexing regime of DW motion, distinct from the stochastic creep regime that occurs at higher fields. This flexing regime is characterized by a larger DW mobility, linear response to applied field, and non-hysteretic motion which is repeatable within our $\sim 5$ nm experimental resolution. We then analyze the flexing and depinning behavior of the DW to estimate the density and strength of pinning sites. Supported by the ONR MURI program. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P16.00003: Observation of two step magnetization reversal in Fe$_0.25$TaS$_2$ S. Park, S.B. Kim, Y.J. Choi, Y. Horibe, S-W. Cheong, Weida Wu Understanding magnetic coercivity mechanisms in strong ferromagnets is crucial for new technologies. We studied domain wall pinning in a highly anisotropic ferromagnet of single crystalline Fe$_0.25$TaS$_2$ by utilizing variable temperature magnetic force microscopy (VT-MFM). Magnetic domain structure and the magnetization reversal were investigated in magnetic fields up to 8 tesla at several temperature. Our results revealed the existence of two step magnetization reversal in Fe$_0.25$TaS$_2$. The real space images of magnetic domains, showing this intriguing phenomenon, will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P16.00004: Measurement of Annihilation Barriers for Magnetic Vortices Jacob Burgess, David Fortin, Joe Losby, Denys Grombacher, John Davis, Mark Freeman Measurements of the susceptibility of an array of 2 micrometer diameter Permalloy discs are made using the AC magneto-optical Kerr effect. Employing an extended version of the rigid vortex model, saturation magnetization as a function of temperature is extracted from the data. The model also allows extraction of the switching distribution of the array as the discs transition from the vortex state to the quasi single-domain state. Tuning of temperature or sweep rate shows shifts in the distribution peak that confirm vortex annihilation is governed by a thermally activated mechanism. Using the measured saturation magnetization data in conjunction with the measured peak shifts, quantitative extraction of energetic parameters used in semi-empirical models of the annihilation energy barrier is possible. Several models are considered in the context of qualitative observations made in the experiment. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P16.00005: Pinning Mechanisms for Vortices in Ferromagnetic Films Te-Yu Chen, Michael Erickson, Chris Leighton, Paul Crowell In ferromagnetic materials, domain wall motion is generally discontinuous and stochastic in the presence of pinning sites. The pinning energy is typically quantified via a single experimental parameter - the coercivity of the hysteresis loop. We show here that in magnetic structures supporting a vortex, the vortex dynamics provide quantitative information about both the strength and range of the interaction between the vortex and individual pinning sites. Using time-resolved Kerr microscopy, we have measured the defect-induced pinning energy and length scales for magnetic vortices in micron-sized NiFe disks. We find that the pinning length scale matches the size of vortex core, and is insensitive to film thickness and growth conditions. This suggests that the dominant mechanism of vortex pinning is directly associated with the core region. The pinning energy however, is strongly dependent on microstructure. Specifically, we observe large pinning energies in NiFe films that have large roughness on lateral length scales commensurate with the core size (10 nm). The dependence of pinning energy on thickness provides further insight into the relative role of surface roughness versus bulk disorder. The strength as well as the spatial distribution of pinning sites suggest that roughness at this length scale is the dominant source of pinning in these films. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P16.00006: Enhanced current-induced domain wall motion by tuning perpendicular magnetic anisotropy Satoru Emori, Geoffrey Beach The effect of perpendicular magnetic anisotropy (PMA) on current-induced domain wall (DW) motion is investigated by micromagnetic simulations. The critical current density Jc to drive DWs into periodic transformation and continuous motion by adiabatic spin transfer torque decreases with increasing PMA. Also, with optimized PMA that almost exactly compensates the demagnetizing field, the adiabatic displacement of DWs driven by currents less than Jc is strongly enhanced. Since PMA can be controlled easily in multilayer films (e.g. Co/Pt), this technique of enhancing current-induced DW motion may be practical for device applications. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P16.00007: Geometrically Confined Spin Vortices: from Fundamental Physics to Biomedical Applications Invited Speaker: The magnetic ground state of magnetically soft thin film ferromagnets in confined geometries (on the micrometer scale) consists of a curling spin configuration, known as a magnetic vortex state. We have recently demonstrated that the magnetic vortex microdisks can be successfully used as multifunctional magnetic carriers for biomedicine [1]. In particular, we will report on successful interfacing of ferromagnetic nanomaterials with a spin vortex ground state and biomaterials (antibody, whole cell). Namely, the gold-coated lithographically defined microdisks with an Fe-Ni magnetic core were biofunctionalized with anti-human-IL13a2R antibody for specifically targeting human glioblastoma cells. When an alternating magnetic field is applied the vortices shift, leading to the microdisks oscillation that causes a mechanical force to be transmitted to the cell. Cytotoxicity assays, along with optical and atomic force microscopy studies, show that the spin vortex-mediated stimulus creates two dramatic effects: (a) membrane disturbance and compromising, and (b) cellular signal transduction and amplification, leading to robust DNA fragmentation and, finally, programmed cell death [2]. The experiments reveals that by employing biofunctionalized magnetic vortex microdisks the magnetic fields of low frequency of ~a few tens of Hz ~and of small amplitude of $<$ 100 Oe applied during only 10 minutes was sufficient to achieve $\sim $90{\%} cancer cells destruction. \\[4pt] [1] E. A. Rozhkova, et al., J. Appl. Phys. Vol. 105, (2009) 07B306. \\[0pt] [2] D.-H. Kim, et al., Nature Materials, vol. 9, pp. 165 - 171 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P16.00008: Fast transport of superparamagnetic beads by field-driven magnetic domain walls Elizabeth Rapoport, Geoffrey Beach The manipulation of superparamagnetic (SPM) beads with magnetic domain walls (DWs) is of interest for biomedical applications [1, 2]. We present data supporting fast, continuous transport of SPM beads by field-driven DWs along straight magnetic nanowires. If the magnetostatic binding force (F$_{b}$) between a DW and an SPM bead exceeds the Zeeman force (F$_{Z}$) from a driving field, DW velocity is limited by the hydrodynamic drag force on the bead [3], and a wall-bead pair can be propelled at high speeds. We have combined micromagnetic simulations and numerical calculations to determine F$_{b}$, covering the parameter space of bead radius, wire width and thickness, and domain wall type. Comparing F$_{b}$ and F$_{Z}$ for different applied fields, we find that the field, H$_{crit}$, at which the Zeeman force separates the wall from the bead, is maximized by the same wire width, independent of bead size. Optimal conditions for continuous bead transport are achieved with 150 nm wide wires, which can transport 500 nm radius beads in driving fields up to 90 Oe, corresponding to transport velocities of up to 8 mm/s. These results suggest that fast, long-distance transport of SPM beads is possible using simple linear magnetic guide-wire structures. [1] M. Donolato, et al., Nanotechnology 20 (2009) [2] G. Vieira et al., Phys. Rev. Lett. 103, 128101 (2009) [3] M.T. Bryan et al., Appl. Phys. Lett. 96,192503 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P16.00009: Domain wall pinning in magnetic structures with perpendicular magnetic anisotropy Daniel Gopman, Daniel Bedau, S. Park, Dafine Ravelosona, Andrew Kent An experimental technique has been designed to trap domain walls in ferromagnetic nanostructures. Spin valve nanowires and nanopillars with perpendicularly magnetized free and reference layers were engineered with lithographically defined notches of varying depths and lengths. The influence of notch geometry in domain wall pinning has been compared with intrinsic domain wall pinning sites. Thermally activated jumping between metastable states has been observed under rf excitation along with telegraph noise. Coercive fields have been determined to vary linearly with applied direct currents. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P16.00010: Breather states in magnetic domain wall racetrack memory samples John Eves, Remo Hugli, Naoise Grisewood, Beni Braun Proposed magnetic domain wall(DW) racetrack memory [1] exploits controlled motion of magnetic DWs along magnetic nanowires, and the sequence of DWs encodes the bit states. Here we investigate the possibility of the existence of dynamically bound states of pairs of DWs. We show that by the choice of suitable initial conditions for two DWs in a racetrack geometry, such dynamical states can be prepared by a suitable applied field. The breather states correspond to two DWs which have the same chirality and which oscillate around their common center of mass.\\[4pt] [1] S.S.P. Parkin, Science 320, 190 (2008) [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P16.00011: Reversible helicity and higher harmonics in spin textures: stripes and skyrmions Xiuzhen Yu, Yusuke Tokunaga, Yoshio Kaneko, Yoshio Matsui, Yoshinori Tokura The magnetic bubbles viewed as skyrmions have long been attracting attention because of possible application to spintronics. The bubble configuration has been revealed by versatile microscopic techniques such as magnetic-force microscopy, scanning Hall microscopy, and Lorentz transmission electron microscopy (TEM). However, their topological properties, such as topological spin texture and helicity, have not been sufficiently unraveled in spite of possibly important implication in the novel magneto-transport phenomena. In this study, we have scrutinized the spin texture of the thin films of Sc-doped hexagonal barium ferrite with controlled magnetic anisotropy; we have demonstrated the generation of the bubble lattice under external magnetic fields which are applied perpendicular to the film plane. The magnetic component distributions in strips, bubbles and Bloch lines have been successfully achieved by means of high-resolution Lorentz TEM observations and quantitative analyses of the local magnetizations. The results indicate the reversible helicity and higher harmonics in spin textures of stripy and bubble domains. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P16.00012: Domain Growth Behavior in the Compressible Ising Model Meng Meng, David Landau We perform large scale Monte Carlo simulations to study long-time domain growth behavior in a compressible, spin-exchange, two-dimensional triangular-lattice Ising model with continuous particle positions and zero total magnetization. To investigate the effects of compressibility on domain growth behavior, we include an elastic energy term in the Hamiltonian of our model to adjust the rigidity. The system is quenched below the critical temperature from a homogenous disordered state to an ordered phase where multiple domains coexist. Theory expects the domain size $R(t)$ grow as a power law $R(t)=A+Bt^{n}$, where $t$ is the time after quench, and $n$ is the domain growth exponent. Lifshitz and Slyozov have predicted $n$ to be $\frac{1}{3}$ at late-time, but earlier studies\footnote{S. J. Mitchell and D. P. Landau, Phys. Rev. Lett. \textbf{97}, 025701(2006).} suggested that $n$ could be affected by compressibility. We observe the domain growth exponent to be significantly smaller than the Lifshitz-Slyozov value of $n=\frac{1}{3}$. [Preview Abstract] |
Session P17: Focus Session: Bulk Properties of Complex Oxides - 3d Oxides
Sponsoring Units: DMP GMAGChair: Michelle Johannes, Naval Research Laboratory
Room: D174
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P17.00001: Surprises in low dimensional spin 1/2 magnets - from crystal chemistry to microscopic magnetic models of complex oxides Invited Speaker: A microscopic understanding of the structure-properties relation in crystalline materials is a main goal of modern solid state chemistry and physics. Due to their peculiar magnetism, low dimensional spin 1/2 systems are often highly sensitive to structural details. Seemingly unimportant structural details can be crucial for the magnetic ground state of a compound, especially in the case of competing interactions, frustration and near-degeneracy. Here, we present for selected, complex Cu$^{2+}$ systems that a first principles based approach can reliably provide the correct magnetic model, especially in cases where the interpretation of experimental data meets serious difficulties or fails. We demonstrate that the magnetism of low dimensional insulators crucially depends on the magnetically active orbitals which are determined by details of the ligand field of the magnetic cation. Our theoretical results are in very good agreement with thermodynamic and spectroscopic data and provide deep microscopic insight into topical low dimensional magnets. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P17.00002: Spin Waves and Magnetic Interaction in the Multiferroic Antiferromagnet MnWO4 J.A. Fernandez-Baca, Feng Ye, R.S. Fishman, A. Podlesnyak, G. Ehlers, H.A. Mook, Y.Q. Wang, B. Lorenz, C.W. Chu The spin wave excitations of the multiferroic MnWO4 have been measured in the low-temperature collinear commensurate phase using high-resolution inelastic scattering. The spin excitations can be well described by a Heisenberg model with competing exchange interactions up to 11th nearest neighbors. We find the magnetic exchange couplings are highly frustrated within each zigzag spin chain along c-axis and between chains along the a-axis. However, the magnetic interactions are much weaker between chains along the b-axis. Our measurements suggest that the delicate balance of long range magnetic couplings is subject to small perturbations that can lead to a complex magnetic configuration exhibiting magnetoelectric behavior. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P17.00003: Field-induced slow spin relaxation in monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals Hui Xing, Gen Long, Hanjie Guo, Chunmu Feng, Guanghan Cao, Hao Zeng, Zhuan Xu We report the ac susceptibility measurement in the paramagnetic state of the monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals. An unexpected slow spin relaxation is observed in the presence of a nonzero magnetic field. Such behavior is absent in zero field. Distinct features of the relaxation, including the intrinsic frequency on the order of 1 Hz, the field-, temperature- and spin dilution dependence, and its evolution under positive and negative pressures, indicate that the relaxation is associated with an unusual cooperative behavior involving spin correlations. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P17.00004: The d-band manifold in SrTiO$_{3}$: high mobility Shubnikov--de Haas effect in magnetic fields to the quantum limit. S. James Allen, Bharat Jalan, Guru Khalsa, Allan MacDonald, Jan Jaroszynski, Susanne Stemmer The molecular beam epitaxial growth of high mobility ($>$ 30,000 cm$^{2}$/volt.sec), low electron density ( $\sim $ 10$^{17}$ - 10$^{18}$cm$^{-3})$ La doped SrTiO$_{3}$ has provided an opportunity to explore the lowest conduction band states, which are derived from the Ti d-band. Despite the long history of experiments on these d-band states, including magneto transport, we are left without a firm quantitative model of the manifold at the conduction band minimum. But, these states form the basis of quantum confined 2D electron systems at oxide interfaces with SrTiO$_{3}$ and delta doped layers, both the subject of current interest. To remedy this, we have performed magneto transport at temperatures down to $\sim $ 0.4 K, in magnetic fields to 31 Tesla, which is sufficient to reach the quantum limit, in high mobility samples and with carrier densities that tune the Fermi energy through the energy splitting caused by the low temperature tetragonal distortion. In close analogy to hole states in conventional semiconductors, we use 5 ``Luttinger'' parameters and the splitting energy, to describe these results and compare with the current understanding of the SrTiO$_{3}$ d-band structure. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P17.00005: Low Energy Conduction Band Structure of SrTiO3 Guru Khalsa, Bharat Jalan, S. James Allen, Susanne Stemmer, Allan MacDonald The recent observation of a high mobility 2DEG, and truly two-dimensional superconductor, at oxide interfaces with SrTiO3 (STO) and in delta-doped layers of STO have thus far gone without a clear theoretical description. The starting point for any quantitative theory of these systems is a reliable low energy parameterization of the Ti d-band in the bulk parent compound. Here we present a five parameter symmetry constrained model of the t2g band in STO near the conduction band minimum. We use this model to describe a recent high field (up to 31 Tesla), low temperature, angular magneto-transport study of lightly La doped STO and compare our results with other available experimental data. We will also discuss the relation between orbital density and matrix element effects in photoemission experiments of d0 Perovskites. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P17.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P17.00007: Unconventional Electronic Transport in Doped SrTiO$_{3}$ C. Leighton, A. Spinelli, M.A. Torija, C. Liu, C. Jan Resistivity, Hall effect, and magnetoresistance are reported on a large set of semiconducting SrTiO$_{3-\delta }$ single crystals doped n-type (by reduction or Nb substitution) over a broad range of carrier density (10$^{15}$ to $>$10$^{20}$ cm$^{-3})$. Temperature-independent densities, strongly temperature-dependent mobilities (up to 22,000 cm$^{2}$V$^{-1}$s$^{-1}$ at 4.2 K), and a remarkably low critical carrier density for the metal-insulator transition are observed, and interpreted in terms of the quantum paraelectricity of the host. We argue that an unusual, high mobility, low density, metallic state is thus established at carrier densities at least as low as 8.5 x 10$^{15}$ cm$^{-3}$. At low temperatures the temperature dependence of the mobility and resistivity exhibit a non-monotonic carrier density dependence and an abrupt change in character near 2 x 10$^{16}$ cm$^{-3}$, indicating a distinct crossover in conduction mechanism, perhaps associated with a transition from impurity band to conduction band transport. The results provide a simple framework for the understanding of the global transport behavior, and suggest some potential applications. Work supported by NSF. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P17.00008: Pseudogap in metallic layered nickelate $R_{2-x}$Sr$_x$NiO$_4$ Masaki Uchida, K. Ishizaka, Y. Ishida, Y. Onose, R. Arita, S. Shin, Y. Tokura, P. Hansmann, A. Toschi, K. Held, Y. Kaneko, X. Yang, O.K. Andersen, R. Kumai We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates $R_{2-x}$Sr$_x$NiO$_4$. Angle-resolved photoemission spectroscopy (ARPES) on the barely-metallic Eu$_{0.9}$Sr$_{1.1}$NiO$_4$ has revealed a large hole surface of $x^2-y^2$ character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped cuprates, although the antiferromagnetic interactions are one order of magnitude smaller. Our findings strongly indicate that the high-energy momentum-dependent pseudogap (or Fermi arc) is not unique to the high-$T_{\mathrm{c}}$ cuprates but commonly develops in the anomalous quasi-two-dimensional metallic state near the Mott transition reflecting the real-space charge correlation. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P17.00009: Density wave driven metal-insulator transition in nickelates SungBin Lee, Ru Chen, Leon Balents The Mott transition in nickelates, RNiO3, shows unusual magnetic ordering and charge ordering in the insulating phase. For the more itinerant nickelates, one may argue that these unusual density waves are actually driven by Fermi-surface nesting, originated from the large flat regions of Fermi surfaces. Using a tight-binding model of the band derived from doubly degenerate eg orbitals, we obtain the density wave induced metal- insulator transition phase diagram in the presence of on-site Coulomb interaction and Hund's coupling, treated in Hartree-Fock approximation. Furthermore, motivated by recent success in layer by layer growth of nickelates, the thin film effects in nickelates are also studied. Finally we calculate the optical conductivity for the various states in our phase diagram, suggesting experimental measurements to check the theory. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P17.00010: Molecular correlated insulating state in low-valence layered nickelates Victor Pardo, Warren E. Pickett In recent years, there has been an effort on artificially creating Fermi surfaces that resemble those of the superconducting cuprates. A Ni$^{3+}$:d$^7$ (one e$_g$ electron) configuration can be made into the electron-like analog of the d$^9$ (one e$_g$ hole) cuprate electronic structure.[1] Another cleaner alternative would be to grow low-valence Ni$^+$:d$^9$ compounds, which have recently become available through synthesis[2] of members of the series La$_{n+1}$Ni$_{n}$O$_{2n+2}$. We present LDA+U calculations on the layered compounds La$_4$Ni$_3$O$_8$ [3] and La$_3$Ni$_2$O$_6$, with three and two NiO$_2$ layers, respectively. Electron count implies very low Ni formal valencies: 1.33+ and 1.5+, respectively. If charge order is present, Ni$^+$:d$^9$ could occur in a geometry similar to that of the cuprates. However, this is not the case. Both compounds are insulators, which we can attribute to quantum confinement in the NiO$_2$ tri/bi-layers. The only states close to the Fermi level are Ni d$_{3z^2-r^2}$, which couple along the c-axis (Ni trimers or dimers). The insulating behavior must be viewed from a molecular orbital viewpoint, after AFM order within layers has narrowed the bands. Insulating behavior is that of a ``molecular" Mott insulator rather than a charge-ordered insulator. \newline [1] J. Chaloupka and G. Khaliullin, \emph{PRL} \textbf{100}, 016404 (2008). \newline [2] V. V. Poltavets \textsl{et al.}, \emph{Phys. Rev. Lett.} \textbf{102}, 046405 (2009). \newline [3] V. Pardo and W.E. Pickett, arXiv:1008.2707. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P17.00011: Search for structural fluctuations in the disordered stripe state of Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ A.M. Milinda Abeykoon, Emil Bozin, Genda Gu, John Hill, John Tranquada, Simon Billinge We present a temperature series PDF and a Rietveld analysis of Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ system to study the local structural response in the state above the charge-ordered state that has not been characterized in detail to date. We observed NiO$_{6}$ octahedral tilting patterns of different magnitude for short and long-range structure of the system. A sequential Rietveld refinement, and a T-series PDF analysis on the length scale (5-20){\AA} were carried out to characterize the long-range order of the system. A PDF analysis on the length scale (0-4.2) {\AA} revealed a different magnitude local octahedral tilt pattern as a function of temperature. The correlation length of short-range ordered charge stripes existing above T$_{co}$ was estimated using a Box-Car type PDF model. Combining this information with the refined isothermal atomic displacement parameters (ADPs) yields a much more complete picture of the nature of both atomic displacements and how they are correlated with each other in the system. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P17.00012: Lattice normal modes and electronic properties of the correlated metal LaNiO$_3$ Gaoyang Gou, James Rondinelli, Ilya Grinberg, Andrew Rappe We present results from density functional calculations of lattice vibrations and electronic properties of the correlated metal LaNiO$_3$. Using the landau theory of phase transitions and {\it ab initio} derived phenomenological coefficients obtained from local-spin density approximation (LSDA) calculation, we examine the evolution of the Raman-active phonon modes with temperature and find that the LSDA results give excellent agreement with experiments. To study the electronic structure of LaNiO$_3$, we extend to the post-LSDA functional methods, including the local spin density+Hubbard U (LSDA+U) method, and two hybrid exchange-correlation functionals, PBE0 and HSE. By comparing the results obtained from the various functionals with the experimental photoelectron spectroscopy (PES) and X-ray photoelectron spectroscopy (XPS) data, we argue that the screening effect coming from the delocalized O-2p and Ni-t$_{2g}$ electrons will be strong enough to reduce the electron correlation of LaNiO$_3$. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P17.00013: Time-Dependent Recovery of Charge and Spin Order in Stripe-Ordered Nickelates Y.F. Kung, A.F. Kemper, W.-S. Lee, B. Moritz, A.P. Sorini, Z.-X. Shen, T.P. Devereaux Using time-dependent Ginzburg-Landau theory, we study the melting and recovery of charge and spin order in striped nickelates (La$_{2-x}$Sr$_x$NiO$_4$) in response to an ultrashort pump pulse that destroys the order. We find that the critical temperature for onset of spin order varies with increasing coupling between charge and spin order. Solving the Gross-Pitaevskii equations to model the time evolution, we explore the temporal dynamics of charge and spin order parameters, to be compared to experimental observations at LCLS. [Preview Abstract] |
Session P18: Focus Session: Low D/Frustrated Magnetism - Triangular Lattices
Sponsoring Units: GMAG DMPChair: Bruce Gaulin, McMaster University
Room: D172
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P18.00001: Neutron Scattering Studies of the S=1/2 Triangular Lattice Magnets NaNiO$_2$ and LiNiO$_2$ Invited Speaker: NaNiO$_2$ and LiNiO$_2$ are isostructural quantum magnets based on a stacked triangular lattice in which magnetism arises from S=1/2 magnetic moments carried by Ni$^{3+}$ ions. Surprisingly, while these compounds are structurally and electronically very similar, the magnetic properties they exhibit are dramatically different. NaNiO$_2$ undergoes a cooperative Jahn-Teller phase transition at 480K and magnetically orders below T$_N$ $\sim$ 23K, adopting a structure which consists of ferromagnetic sheets of S=1/2 moments stacked in an antiferromagnetic fashion. In contrast, LiNiO$_2$ undergoes a spin glass transition at T$_g$ $\sim$ 9K and remains disordered down to the lowest measured temperatures. Understanding the absence of long-range magnetic order in LiNiO$_2$ is a problem which has attracted considerable interest for more than twenty five years. Among many potential explanations, the answer has most notably been attributed to geometric frustration caused by inherent mixing of the Li and Ni sublattices, or orbital degeneracy resulting from the lack of a coherent Jahn-Teller distortion. In this talk I will describe time-of-flight neutron scattering measurements performed on polycrystalline samples of NaNiO$_2$ and LiNiO$_2$ using the wide Angular-Range Chopper Spectrometer (ARCS) at ORNL and the Disk Chopper Spectrometer (DCS) at NIST. These measurements provide a thorough characterization of the excitation spectra for these two compounds, probing the inelastic scattering over energy scales ranging from $\sim$ 0.1 meV to 1.5 eV. In NaNiO$_2$, our measurements reveal two sets of well-defined spin excitations, which we associate with ferromagnetic spin waves mediated by in-plane interactions and antiferromagnetic spin waves mediated by out-of-plane interactions. In LiNiO$_2$, we observe similar, albeit much broader, excitations consistent with short-range two-dimensional magnetic correlations. In the case of NaNiO$_2$, we have developed a simple linear spin wave theory model to describe these excitations and extract the relevant magnetic exchange couplings for this system. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P18.00002: Magnetic order and glassiness in distorted triangular lattice materials, Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO) Jian Wu, Fletcher Werner, Julia S. Wildeboer, Alexander Seidel, Zohar Nussinov, S.A. Solin We have synthesized two series of distorted triangular lattice materials Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$M, where M = NO$_{3}^{-}$ or C$_{7}$H$_{15}$COO$^{-}$, by hydrothermal reaction. The powder X-ray diffraction measurements show that the substitution of Zn for Cu leads to a series of isostructural doped compounds [1]. The C$_{7}$H$_{15}$COO$^{-}$ long chain intercalated samples display a series of intense (00\emph{l}) reflections, which signals their enhanced 2D structures with an almost doping-independent interlayer distance 24.2\AA. In the DC magnetic susceptibility data for all NO$_{3}^{-}$ samples, we observe clear evidence of transitions from a paramagnetic to antiferromagnet phase. The onset of long-range order is further proven by the prominent features in specific heat data. However, all the long chain intercalated samples were found to display several spin-glass-like behaviors. A clear bifurcation between the ZFC and FC data was observed at T $<$ 15K. The time evolution of isothermal remnant magnetization M$_{ZFC}$(t) has a linear dependence on $\ln(t)$. No peak features or broad maximum have been discovered in the specific heat measurements. Further analysis of the above results suggest that the long chain intercalated samples are cluster spin glasses at low temperature. \newline[1] J.Wu et al, J. Phys. Condens. Matter. 22, 334211(2010). [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P18.00003: Spin glassiness and power law scaling in anisotropic triangular spin-1/2 antiferromagnets Alexander Seidel, Jian Wu, Julia S. Wildeboer, Fletcher Werner, Zohar Nussinov, S.A. Solin We discuss the magnetic properties of a class of spin $S=1/2$ antiferromagnetic quasi-triangular lattice materials, both in the clean limit and in the presence of non-magnetic Zn impurities. These systems are long organic chain intercalated derivatives of copper hydroxy nitrate, with a very large c-axis separation of 24\AA. In these compounds, we find that a spin glass phase is universally preceded by two different power law regimes in the temperature dependence of the DC magnetic susceptibility, separated by a sharp crossover. This is seen both in the presence as well as in the absence of non-magnetic Zn impurities, where the power law exponents are surprisingly unperturbed by the compositional disorder. We argue that these findings may be consistent with a picture based on a self-generated spin glass in the clean undoped compound, where frustration is the driving mechanism of the glassiness rather than disorder. While AC measurements and time dependent magnetization follow traditional spin glass paradigms, the power law structure found in the DC susceptibility is argued to deviate in various ways from scenarios expected based on Griffiths type physics, and may call for new explanations. [1] J. Wu et al., J. Phys. Condens. Matter, 22, 334211 (2010). [2] J. Wu et al., arXiv:1007.0442 [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P18.00004: Competing interactions and continuum excitations in the spin-1 triangular lattice antiferromagnet NiGa$_{2}$S$_{4}$ J. Wen, Y. Nambu, J. Rodriguez, C. Stock, S. Nakatsuji, S. Onoda, Y. Maeno, C. Broholm In some geometrically frustrated magnets, conventional long range order is replaced by static or dynamic short range order. Quasi-2D NiGa$_{2}$S$_{4}$ is the only known S=1 antiferromagnet with an exact triangular lattice. Recent neutron scattering experiment on high quality NiGa$_{2}$S$_{4 }$single crystals revealed short range quasi-2D incommensurate spin correlation with a critical wavevector close to (1/6, 1/6, 0) [1]. Here we report a measurement of the dynamic spin correlation function through a volume of \textbf{Q}-E space for T$<<$J. A gapless spectrum was observed at the incommensurate critical wavevector while a softened but still gapped response was found at (1/3, 1/3, 0). This indicates dominant third nearest neighbor interaction and competing weaker near neighbor interactions. The excitation spectrum takes the form of a bounded continuum throughout the 2D Brillouin zone. The temperature dependence dynamic correlation length shows that short range correlation persist up to $\Theta _{cw}$=-80(2)K. \\[0pt] [1] C.Stock, \textit{et al, }Phys.Rev.Lett.105,037402 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P18.00005: Quantum Antiferromagnet on an Anisotropic Triangular Lattice Sedigh Ghamari, Catherine Kallin, Sung-Sik Lee The effects of quantum fluctuations on the spin 1/2 Heisenberg antiferromagnet on a triangular lattice, with diagonal interchain exchange J' weaker than the intrachain exchange J, are studied. This model is of considerable interest because of its relevance to Cs2CuCl4, where experiments have been interpreted as evidence for a nearby two-dimensional spin liquid and because numerous theoretical studies have proposed that the incommensurate spiral spin density wave order is destroyed by quantum fluctuations well before the one-dimensional limit (J'=0) is reached. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P18.00006: ESR as a probe of spinon excitations of the spin-1/2 antiferromagnet Cs$_2$CuCl$_4$ K. Povarov, A. Smirnov, O. Starykh, S. Petrov, A. Shapiro We report dramatic manifestation of the uniform Dzyaloshinskii-Moriya (DM) interaction in the ESR response of quasi-one-dimensional spin-1/2 antiferromagnet Cs$_2$CuCl$_4$. We find the ESR response in the range 10-90 GHz to be strongly sensitive to the relative orientation of the magnetic field and DM axes. Most importantly, we observe splitting of the ESR line into two lines in the paramagnetic phase, upon lowering the temperature from 10 K to 1.3 K. The latter temperature is about twice the ordering temperature $T_N = 0.62$ K, and yet far below the Curie-Weiss temperature 4 K . The splitting occurs when the static magnetic field H is aligned with one of the DM axes of the material and is absent when H is oriented perpendicular to the axes. This novel phenomenon is a consequence of the critical nature of fractionalized spinon excitations of the individual antiferromagnetic chains in the paramagnetic phase. A uniform (along the spin chain direction) DM interaction provides an effective magnetic field, the sign of which is opposite for the right- and left-moving spinons. In the presence of external magnetic field this difference translates into a pair of ESR frequencies, making the experiment a novel probe of spinon excitations. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P18.00007: Metallic magnetization plateau on triangular lattice Zhihao Hao, Oleg Starykh The $1/3$ magnetization plateau is well established for spin $1/2$ Heisenberg antiferromagnetic model on triangular lattice. The state is stable against a large range of spatial anisotropies and is observed in the triangular compound Cs$_2$CuBr$_4$. A natural question to ask is whether the plateau state remains stable if the on-site repulsion $U$ is lowered for the underlying Hubbard model. In our work, we studied the one-band Hubbard model on triangular lattice. Through mean-field calculations, it is discovered that an up-up-down spin density wave state with $1/3$ of saturation magnetization is stablized for a range of $U$ and magnetic field $h$. For $4.44t\le U\le 4.55t$, the state is a half metal: the spin up bands remain metallic while the spin down bands are insulating. For $U> 4.55t$, the spin up bands become gapped and the system is an insulator. It is speculated the plateau state remains stable for the entire range of $U\ge 4.44t$. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P18.00008: Phase Diagram of a Geometrically-Frustrated Triangular-Lattice Antiferromagnet in a Magnetic Field Randy Fishman The magnetic phase diagram of a geometrically-frustrated triangular-lattice antiferromagnet is evaluated as a function of magnetic field and anisotropy using a trial spin state built from harmonics of a fundamental ordering wavevector. A non-collinear incommensurate state, observed to be chiral and ferroelectric in CuFeO$_2$, appears above a collinear state with 4 sublattices (SLs). A previously unobserved collinear 5-SL phase may be accessible in a magnetic field for a non-stoichiometric compound with excess or deficient oxygen. The apparent absence of multiferroic behavior for predicted chiral, non-collinear 5-SL states poses a challenge to theories of the ferroelectric coupling in CuFeO$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P18.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P18.00010: Phases of spatially anisotropic triangular antiferromagnet in high magnetic field Oleg Starykh, Andrey Chubukov We investigate phases of the Heisenberg spin model on a spatially anisotropic triangular lattice as a function of $J'/J <1$ and a magnetic field $H$ ($J$ is the exchange along the horizontal bonds, and $J'$ is the exchange along the diagonal bonds). The anisotropy of $J$'s competes with quantum fluctuations and this competition leads to a rich phase diagram. Immediately below the saturation field $H_s$ we find three phases: three-sublattice commensurate phase, incommensurate co-planar ``fan'' phase, and incommensurate non-coplanar ``cone'' phase. The former two are supersolids while the latter is a superfluid in the terminology of strongly interacting bosons. At a finite boson density ($H < H_s$) and on approach to the fan-cone phase boundary from within the cone phase with ordering momentum $Q$, we observe softening of the ``roton'' minima at momentum $Q'$ different from $-Q$, which one would expect for a direct cone-fan transition. This points on the existence of the intermediate double-spiral state in which boson density exhibits incommensurate modulations with momenta $Q$ and $Q'$. The extrapolation of our results to $H \sim H_s/3$ predicts that $Q'=Q$, and the intermediate state becomes similar to the ``distorted umbrella'' state that emerges out of up-up-down phase. We discuss the implications of our findings for the global phase diagram of the anisotropic triangular Heisenberg antiferromagnet. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P18.00011: Theory of SU(2) invariant spin liquids on the triangular lattice with spinful Majorana excitations Rudro Biswas, Liang Fu, Chris Laumann, Subir Sachdev We present a theory of SU(2) invariant spin liquids on the 2D spin 1/2 triangular lattice described by a parton representation of the spin in terms of spin-1 Majorana particles. These spin liquids break time reversal symmetry and generically possess a novel Fermi surface consisting of three lines intersecting at $k=0$ as well as an unconventional dynamic critical exponent $z=3$. We also present calculations for observable quantities and discuss possible connections to recent experiments involving spin 1/2 Heisenberg triangular lattices. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P18.00012: Phase Diagram of Classical Heisenberg Antiferromagnets with Four-Spin Interactions on Stacked Triangular Lattice under Magnetic field Shintaro Takayoshi, Masahiro Sato Classical Heisenberg antiferromagnet (HAF) on stacked triangular lattice is a simple and important model of frustrated systems. Although there are some candidate materials for triangular HAF, they are not ideal ones and various kinds of perturbations should be present. While it is well known that the ground state of triangular HAF is 120-degree structure, how perturbations destabilize the structure has not been well studied. In this study, we consider effects of four-spin interactions on magnetic phase diagram of triangular HAF. In fact, some real mechanisms of generating four-spin interactions have been known: higher-order electron hopping processes in Mott insulators, spin-phonon couplings, etc. We complete the magnetic phase diagrams by using Monte Carlo simulation. We will report new phases induced by four-spin interactions. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P18.00013: Magnetic ground state and excitation of SrV_${10}$O$_{15}$ Jooseop Lee, Kazuki Iida, Matthew Stone, Masaaki Matsuda, Tomomasa Kazita, Takuro Katsufuji, Seunghun Lee SrV$_{10}$O$_{15}$ has magnetic bilayers composed of triangular lattices with periodical missing, which can be an interesting playground for the study of frustration. In this new type of magnetic structure, using neutron powder diffraction, we find a magnetic ground state below 45K with ordering vector Q=(0 1/2 1), and study detailed spin configuration. Magnetic excitations have also been investigated using a single crystal at ARCS, a time-of-flight neutron chopper spectrometer at SNS. Our results show quite interesting highly dispersive dispersion relations: a gapless Goldstone mode is strongly dispersive along the a and c axis, and is less-strongly dispersive along the b axis. Another mode is dispersionless along the a and b axis, and is strongly dispersive along the c axis. We determine the spin hamiltonian that sheds light in understanding the interplay between orbital, spin, charge, and lattice degrees of freedom in this compound. [Preview Abstract] |
Session P19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VI
Sponsoring Units: GMAGChair: Stephane Mangin, Nancy-Universite
Room: D170
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P19.00001: Quantum Dynamics of Spin-torqued Nanomagnet Yong Wang, Yaojun Zhang, L.J. Sham Nanomagnet is the key ingredient of many spintronics devices, such as hard disk read head, magnetoresistive random access memory(MRAM),etc. The underlying mechanisms in these structures are due to the interplay between the nanomagnet and spin-polarized electrons. Usually, the nanomagnet is characterized by the classical magnetization vector with its intrinsic quantum fluctuation neglected. By treating the nanomagnet as a huge atom with millions of energy levels, we are able to take account into the magnetization fluctuation, and study the interactions between nanomagnet and spin-polarized electrons on the same footing as quantum objects. We will show that the well-known giant magnetoresistance(GMR) effect and spin transfer torque(STT) effect are the consequences of continuous quantum measurements on the nanomagnet by spin-polarized electrons. We found that the quantum dynamics of nanomagnet is governed by a Fokker-Planck(FP) type equation in the atomic coherent state representation(P- representation). We will also discuss the correlation between the magnetization fluctuation and the electric current fluctuation. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P19.00002: Minimum action paths for single domain ferromagnetic nanostructures under the influence of spin transfer torque Gabriel Chaves-O'Flynn, Daniel Stein, Andrew Kent, Eric Vanden-Eijnden Thermally induced magnetization reversal is an important issue for the design of magnetic storage devices. The problem is usually studied using Kramers' theory of reaction rates, which is applicable when the dynamics can be described as gradient forces. Spin Transfer Torque (STT) is an effect of technological importance which does not fall in this category. For Spin Transfer Torque an action minimization is required to find the most probable paths and transition states between metastable states. We calculate these most probable paths for ferromagnetic nanostructures under the influence of STT in the low noise limit for a variety of current strengths and magnetic fields. Previous action minimization were done in the absence of STT and provide a good basis for comparison [1]. We study thin films with an in-plane easy magnetization axis using the geometrical Minimum Action Method (gMAM) [2]. The action obtained using gMAM is in qualitative agreement with activation energy barriers on previous work by Li-Zhang [3].\\[4pt] [1] R.V. Kohn, M.G. Reznikoff, E. Vanden-Eijnden, J. Nonlinear Sci. 15, 223 (2005)\\[0pt] [2] M. Heymann, E. Vanden-Eijnden, Comm. Pure Appl. Math. LXI, 1052(2008)\\[0pt] [3] Z. Li, S. Zhang, Phys. Rev. B 69, 134416 (2004) [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P19.00003: Giant enhancement of microwave emission in magnetic tunnel junction oscillators by orientating in-plane field Z.M. Zeng, K.H. Cheung, H.W. Jiang, P. Upadhyaya, P. Khalili Amiri, K.L. Wang, J.A. Katine Recently spin-transfer nano-oscillators (STNOs) have attracted considerable attention because they are tunable over a wide frequency rang by varying the applied DC current or magnetic field. One main challenge for practical applications is to boost the relative low emitted power. MgO-based STNOs have exhibited a capability to deliver much larger power. However, they often show multiple emission peaks or broad linewidths. It is necessary to suppress the additional peaks and to reduce the central peak linewidth. In this talk, we present our microwave measurements in MgO-based STNOs as a function of in-plane field orientation. At an optimal orientation, emitted power of a single peak is largely enhanced, together with a significantly narrowed linewidth. The experiment shows that the understanding of intrinsic features of the oscillators as a function of in-plane orientation is important for optimizing the performances of MgO-based nano-oscillators.\\[0pt] [1] see for example, S. I. Kiselev, et.al., Nature 425, 308 (2003). [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P19.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P19.00005: Spin-transfer oscillators in the effective planar approximation Ya. B. Bazaliy Spintronic devices with dominating easy plane anisotropy can be described in an effective planar approximation of the LLG equation. In particular, the effective equation can be used to study the spin-transfer oscillators. We use this approach to study the transitions of the oscillator excited by a combination of an AC and a DC electric currents between the small and large amplitude regimes. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P19.00006: High Speed Spin Torque Memory with Combined Perpendicular and In-Plane Polarizers Graham Rowlands, Tofizur Rahman, Jordan Katine, Juan Alzate, Alexey Kovalev, Yaroslav Tserkovnyak, Kosmas Galatsis, Pedram Khalili Amiri, Kang Wang, Jian-Ping Wang, Ilya Krivorotov The use of a perpendicular polarizing layer in combination with an in-plane free layer has been proposed [1] as a means of reducing the switching time and write energy of spin-torque MRAM cells. While these structures have been realized in nanopillars with metallic spacers [2-4], memory applications demand the use of magnetic tunnel junctions (MTJs) due to their higher magnetoresistance and better impedance matching to a write transistor. We augment standard in-plane CoFeB/ MgO/ CoFeB MTJs to include an additional fixed layer pulled out-of-plane by coupling to the adjacent [Co/Pd] multilayer designed to possess a strong perpendicular magnetic anisotropy. This additional polarizer generates spin torque with an out-of-plane component, resulting in a fast precessional switching with no incubation time or pre-switching oscillations. For a variety of sample sizes we observe switching times approaching 100 ps. References: [1] A. D. Kent et al. Appl. Phys. Lett. 84, 3897 (2004). [2] C. Papusoi et al. Appl. Phys. Lett. 95, 072506 (2009) [3] O. J. Lee et al. 95, 012506 (2009) [4] R. Sbiaa et al. J. Appl. Phys. 105, 013910 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P19.00007: Noise in Spin Torque Oscillators Invited Speaker: In a spin torque oscillator {\_}(STO){\_}, a direct current passing through a reference magnetic layer becomes spin polarized and transfers angular momentum to a second magnetic layer that is excited into steady-state oscillation. The oscillating magnetization causes an oscillating device resistance, through either the giant magnetoresistance effect or the tunneling magnetoresistance effect, which in combination with the bias current generates an oscillating voltage as the output signal. Interest in potential applications of STOs in integrated microwave circuits is driven by their rapid frequency tunability, small size {\_}{\_}($<$100 nm){\_}, and compatibility with standard semiconductor processing techniques. For any oscillator, noise is both an important figure of merit for applications and a useful probe of internal physical processes. I will summarize the theoretical and experimental state of our understanding of frequency and phase noise in a variety of oscillators, considering both time domain and frequency domain measurements. Some aspects can be explained by the effects of thermal fluctuations. Others, such as frequency noise that varies as 1/f at low frequencies, are not yet understood. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P19.00008: Optimal spin current pattern for fast domain wall propagation in nanowires Peng Yan, Zhouzhou Sun, John Schliemann, Xiangrong Wang One of the important issues in nanomagnetism is to lower the current needed for a technologically useful domain wall (DW) propagation speed. Based on the modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski spin-transfer torque and the field-like torque, we derive an optimal temporally and spatially varying spin current pattern for fast DW propagation along nanowires. Under such conditions, the DW velocity in biaxial wires can be enhanced as much as tens of times higher than that achieved in experiments so far. Moreover, the fast variation of spin polarization can efficiently help DW depinning. Possible experimental realizations are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P19.00009: Effects of Spin transfer torques on domain wall nucleation and propagation in perpendicular spinvalve nanopillars Stephane Mangin, Julien Cucchiara, Thomas Hauet, David P. Bernstein, Eric E. Fullerton, Andrew D. Kent, Jordan Katine, Jonathan Z. Sun Controlled manipulation of magnetic domain wall (DW) propagation has spurred intensive research in recent years because of its fundamental interest and the potential impact in spintronic device technology such as racetrack memories. Both magnetic fields and electric currents may be used to control domain walls. Most of the studies have been performed on magnetic nanowires with in plane anisotropy. Here we study domain wall creation and propagation in spinvalve nanopillar composed of magnetic materials with perpendicular anisotropy such Co/Ni multilayers [1]. It is shown that DWs can nucleate and propagate in perpendicularly magnetized nanopillar spin valves as small as 50 $\times $ 100 nm$^{2}$ [2]. The study of the dynamics of DW nucleation and propagation driven by applied fields and injected currents is presented [3]. High domain wall velocities of about 100m/s are found. \\[4pt] [1] S. Mangin, et al , \textit{Nat. Mater.} \textbf{5}, 210 (2006), S. Mangin, et al, \textit{Appl. Phys. Lett.} \textbf{94}, 012502 (2009) \\[0pt] [2] D. Ravelosona, et al (2006) \\[0pt] [3] J. Cucchiara, et al \textit{Appl. Phys. Lett. 94 102503 (2009)} [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P19.00010: Domain Wall Pumping with Spin-Transfer Torque Carl Boone, Ilya Krivorotov We numerically investigate the effects of current-perpendicular-to-the-plane (CPP), angularly asymmetric Slonczewski spin-transfer torque (ST) on transverse domain walls (DW) in nanowires. The CPP ST excites long-range domain wall motion in a direction independent of the current polarity and proportional to the square of the current amplitude. This symmetry with respect to current polarity creates the possibility of DW pumping -- long range DW motion driven by an alternating current. The DW velocity becomes resonantly enhanced near a frequency that depends on the nanowire dimensions, corresponding to the eigenfrequency of a localized, spatially antisymmetric spin-wave mode that exists within the DW. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P19.00011: Current-Perpendicular-to-Plane (CPP) Magnetoresistance at 4.2K of spin-valves with the half-metal Co(2)Fe(Al(0.5)Si(0.5)) and Permalloy Rakhi Acharyya, Reza Loloee, William Pratt, Jack Bass There is interest in current-perpendicular-to-plane (CPP) magnetoresistance (MR) in spin-valves containing half-metallic Heusler alloys such as Co(2)FeAl((0.5)Si(0.5)) (CFAS) [1]. Onto [001] oriented MgO substrates, we sputter epitaxially [001] oriented layers of 150-nm-thick Nb, 10-nm-thick Cu, and chosen thickness CFAS. We complete a spin-valve with 25 nm of Cu, 24 nm of Py = Ni(84)Fe(16), 10 nm of Cu, 25 nm of Nb, and 15 nm of a Au capping layer. With optical lithography and Ar-ion milling, we make 25 micron radius pillars insulated with in-situ deposited SiO. Finally, we lightly ion mill the Au surface and deposit a 150 nm thick Nb cross-strip, then covered by 5 nm of Au. The Nb strips superconduct at our measuring temperature of 4.2K, giving uniform current flow. We will describe how the CPP-MR varies with thicknesses of CFAS ranging from 2 nm to 20 nm. We hope to describe additional studies with Ag instead of Cu and with pinned Py layers.\\[4pt] [1] T.M. Nakatani et al., Appl. Phys. Lett. \textbf{96}, 212501 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P19.00012: Modulation of spin torque from spin transport through two nearby domain walls Elizabeth Golovatski, Michael Flatt\'e The motion of domain walls due to the spin torque generated by coherent coherent carrier transport [1] is of considerable interest for the development of spintronic devices [2]. We model two $\pi$ N\'eel walls [3] separated by a variable distance, and calculate transport characteristics and spin torque through the system [4]. We find that for large separations, the domain walls show the resonant transmission behavior of a spin-dependent double barrier; for small separations, the transmission spectrum resembles that of a $2\pi$ wall. We also find that the spin torque across the system initially increases as the separation between the walls increases from zero, then decreases slightly before reaching a saturation value that is larger than both the spin torque of a $2\pi$ wall and that of two individual $\pi$ walls. This work is supported by an ARO MURI.\\[4pt] [1] M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature 428, 539 (2004).\\[0pt] [2] S. Parkin,M . Hayashi, L. Thomas, Science 320, 190 (2008)\\[0pt] [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89 (2002).\\[0pt] [4] D. Ralph and M. Stiles, J.M.M.M. 320, 1190 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P19.00013: Current-Induced Torques in the Presence of Spin-Orbit Coupling M.D. Stiles, Paul M. Haney In systems without spin-orbit coupling, the simple connection between spin transfer torque and the divergence of the spin current has provided a simple description of current induced torques. Here, we generalize this relationship for systems with strong spin-orbit coupling to a relationship between spin transfer torques, total angular momentum current, and mechanical torques. In such systems, the spin-orbit coupling modifies the behavior of the spin transfer torques. For example, the it can give rise to a persistent spin transfer torque in a spin valve: the spin transfer torque density approaches a constant value rather than decaying away from the interface. This approach also provides a formal expression for the mechanical torque at a single ferromagnetic-nonmagnetic interface. [Preview Abstract] |
Session P20: Focus Session: Physics of Energy Storage Materials IV -- Complex Hydrides and Methane
Sponsoring Units: FIAP/DMP GERA/DCOMPChair: Randall Snurr, Northwestern University
Room: D168
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P20.00001: NaAlH$_{4}$ -- Carbon Aerogel: Kinetic Enhancement of a Complex Hydride by Nanoporous Carbon Invited Speaker: Complex hydrides promise high gravimetric and volumetric hydrogen storage densities, but considerable modification of their thermodynamic and kinetic properties will be required in order to make them feasible for on-vehicle applications. Catalyst additions to achieve fast hydrogen cycling kinetics have been studied for more than a decade. More recently, the concept of nanoconfinement has been explored as a means to improve kinetics, using melt infusion or solvent infusion to embed the hydride into nanoscale pores within a solid. We have achieved enhanced kinetic performance and reversibility of NaAlH$_{4}$ incorporated into nanoporous carbon aerogel by melt infusion, even in the absence of a catalyst. In fact, hydrogen cycling of uncatalyzed NaAlH$_{4}$ in aerogel is almost as good as unconfined NaAlH$_{4}$ catalyzed by addition of TiCl$_{3}$. It remains challenging, however, to obtain NaAlH$_{4}$-carbon aerogel infusions with high hydride loading and/or co-incorporated catalyst. We have therefore investigated combining NaAlH$_{4}$ with carbon aerogel and nanoporous activated carbon by ball milling. The kinetic performance is similar to that of melt-infused NaAlH$_{4}$ at the same loading, and importantly, higher NaAlH$_{4}$ loading can be easily achieved with only modest loss of kinetics. Furthermore, TiCl$_{3}$ catalyst can be easily co-incorporated. In the latter case, a small but significant improvement over TiCl$_{3}$-catalyzed NaAlH$_{4}$ without carbon is observed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P20.00002: Hydrogen release reactions in the {\{}H,Li,B,Na,Al{\}} system Eric Dhall, Vidvuds Ozolins A thermodynamic investigation of the {\{}H,Li,B,Na,Al{\}} system for new solid state hydrogen storage reactions is performed using first-principles DFT calculations and the the grand-canonical linear programming approach (Akbarzadeh, et al. Adv. Mater. 2007, 19, 3233). We report the static, zero-point, and T $>$ 0 K vibrational energies of all known compounds in this system. Enthalpies, entropies, and hydrogen release temperatures are calculated for all thermodynamically reversible dehydrogenation reactions occurring from 0-1000K. Several novel mixtures of reactants with high gravimetric hydrogen storage densities are found using the calculated {\{}H,Li,B,Na,Al{\}} phase diagrams. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P20.00003: First-principles investigations of the quaternary Li-Zn-B-H hydrogen storage system Yongli Wang, Chris Wolverton Mixed metal borohydride hydrogen storage materials are a new class of materials which may possess better thermodynamic and kinetic properties than their separate phases. $LiBH_4$ has an undesirably high $H_2$ desorption temperature, while $Zn(BH_4)_2$ has a lower desorption temperature, but releases $B_2H_6$ upon desorption. We have used density functional theory, as well as Monte Carlo-based crystal structure prediction tools and phase diagram computational methods to explore the stability and decomposition reactions of mixed Li and Zn borohydrides to ascertain whether they possesses an intermediate decomposition temperature. Based on a combination of classical potentials, Monte Carlo optimization, and DFT calculations, we search for low-energy quarternary borohydrides as a function of the Li/Zn context. We find that this system has compounds that are lower in energy than the isolated borohydrides. In agreement with prior work, we confirm the existence of a $LiZn(BH_4)_3$ compound, which as yet has been unobserved. We find that this new mixed compound $LiZn(BH_4)_3$ decomposes via an initial decomposition of $Zn(BH_4)_2$ , and a subsequent decomposition of $LiBH_4$. This sequential decomposition is favored due to the lack of stable intermediate products which involve both Li and Zn. Using this framework, we are searching for stable mixed metal borohydrides in a wide variety of other systems. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P20.00004: First-principles studies of intermediate products in the decomposition of metal amidoboranes Yongsheng Zhang, Tom Autrey, Chris Wolverton Metal amidoboranes [MAB, M=metal cation] form an interesting class of recently-discovered hydrogen storage compounds. However, the decomposition products remain largely unknown. Armed with the combination of the prototype electrostatic ground state search and density-functional theory methodology (PEGS+DFT), we have searched for crystal structures of possible reaction products with [NHBH$_2$]$^-$, [NBH]$^-$, [NBH$_5$]$^-$, polymer-[NHBH$_2$] anion groups in the decomposition of LiAB and CaAB. All these reaction pathways are significantly endothermic, which is in disagreement with the experimentally measured enthalpies in these systems, which are found to be nearly thermoneutral [$-3\sim-5$ kJ/(mol H$_2$) in LiAB and 3.5 kJ/(mol H$_2$) in CaAB]. Using newly developed dianion group [NHBHNHBH$_3$]$^{2-}$, our PEGS+DFT methodology predicts structures and energies of Li/Ca-dianion compounds. Including vibrational thermodynamics and zero-point effects, we successfully obtain a nearly thermoneutral enthalpy of decomposition into these dianion compounds. This agreement lends strong support to the dianion phases as energetically preferred products in the decomposition of metal amidoboranes. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P20.00005: Theoretical study of the vibrational properties of ${\rm NaAlH_4}$ with ${\rm AlH_3}$ vacancies Mei-Yin Chou, Feng Zhang, Yan Wang We investigate from first-principles calculations the vibrational properties in the presence of the ${\rm AlH_3}$ vacancy in both $\alpha$ and $\gamma$ phases of ${\rm NaAlH_4}$. When ${\rm AlH_3}$ is removed from an ${\rm AlH_4^-}$ anion, the remaining H recombines with another neighboring ${\rm AlH_4^-}$ anion and forms an ${\rm AlH_5^{2-}}$ unit with slightly deformed $D_{3h}$ symmetry. For both $\alpha$- and $\gamma$-${\rm NaAlH_4}$, the ${\rm AlH_3}$ vacancy induces several isolated phonon modes that are highly localized on the ${\rm AlH_5^{2-}}$ unit with frequencies within the band gap separating the Al-H stretching modes and Al-H bending modes in pure ${\rm NaAlH_4}$. Similar localized phonon modes also exist in the gap separating the Al-H bending modes and the modes involving the rotation of ${\rm AlH_4^-}$ anions for the $\gamma$ phase. On the other hand, for both $\alpha$ and $\gamma$ phases of ${\rm NaAlH_4}$ with charged ${\rm AlH_4^-}$ vacancies, no isolated phonon modes were found to be localized in the vacancy region with frequencies within the band gap of the pure crystal. These theoretical findings suggest further experimental studies to identify the defects that are involved in the decomposition of ${\rm NaAlH_4}$. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P20.00006: Hexagonal Antiprismatic Metallacarborane Clusters for Hydrogen Storage C\"{u}neyt Berkdemir, Ping Lin, Jorge Sofo We investigated the adsorption properties of molecular hydrogen attached to hexagonal antiprismatic metallacarborane clusters, RuNiC$_2$B$_{10}$H$_{12}$ and Ru$_2$C$_2$B$_{10}$H$_{12}$, using density functional theory. These clusters have been recently synthesized using the reduction-metallation (RedMet) approach [1] and their structures have been resolved. The hydrogen molecules are sequentially attached to these clusters until the H$_2$ binding energies fall below 0.2 eV, which is the minimum value of ideal H$_2$ binding energy in the range of 0.2-0.4 eV/H$_2$ for the practical vehicle applications [2]. We included the van der Waals interactions between metallacarborane clusters and molecular hydrogens. We also evaluated the contribution of zero point vibrational energies to the H$_2$ binding energy. The kinetic stability of these clusters before and after hydrogen adsorption is discussed by analyzing the energy gap. The results show that RuNiC$_2$B$_{10}$H$_{12}$ and Ru$_2$C$_2$B$_{10}$H$_{12}$ clusters can bind up to 8.5 wt \% and 9.8 wt \% molecular hydrogen, respectively. These results suggest that these metallacarborane clusters are potential hydrogen storage materials to meet the targets of DOE for 2015. \\[4pt] [1] D. Ellis et al., Chem. Commu. {\bf 14}, 1917 (2005).\\[0pt] [2] http://www.sc.doe.gov/bes/hydrogen.pdf. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P20.00007: Diffusion-limited Kinetic Pathway for Hydrogen Release from LiNH$_{2}$/LiH Biljana Rolih, Vidvuds Ozolins From experimental work on decomposition of hydrogen storage materials it has been suggested that bulk diffusion of metal species is the bottleneck for hydrogen release. In this work we study the underlying mechanism for diffusion reactions in the dehydrogenation of LiNH$_{2}$. Using first-principle, density functional theory methods we have calculated concentration gradients and diffusivities of neutral and charged defects in LiNH$_{2}$ and Li$_{2}$NH phases. The overall activation energy is obtained from these calculations. The calculated activation energies are found to agree well with experimental work on the kinetics of LiNH$_{2}$ decomposition, suggesting that diffusion of metal species is a possible method for dehydrogenation of Lithium Amide. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P20.00008: Reaction Pathways in the Reactive Composite Mg(NH$_{2})_{2}$ + LiH Deniz Cakir, Gilles A. de Wijs, Geert Brocks Chen \textit{et al} [1] reported reversible hydrogen storage in a mixture of LiH + LiNH$_{2}$ with a storage capacity of 6.5 wt {\%}. However, this system requires an operating temperature in excess of 250 C to achieve a hydrogen pressure of 1 bar. Several efforts including cation substitution have been considered in order to improve the operating conditions, which is necessary for onboard applications. For instance, replacing LiH with MgH$_{2}$ markedly reduces the operating temperature through the reaction MgH$_{2 }$+ 2LiNH$_{2} \quad \to $Li$_{2}$Mg(NH)$_{2 }$+ 2H$_{2} \quad \leftrightarrow $Mg(NH$_{2})_{2}$ + 2LiH. Recent experimental results however indicate that the latter is not a simple one-step reaction and full hydrogenation of Li$_{2}$Mg(NH)$_{2}$ occurs in a two-step sequence via an intermediate Li$_{2}$Mg$_{2}$(NH)$_{3}$ [2]. In this work we examine the stability and structure of possible intermediates compounds, namely Li$_{2-2x}$Mg$_{x}$NH, Li$_{1-2x}$Mg$_{x}$NH$_{2}$, and Li$_{2-x}$Mg(NH)$_{2-x}$(NH$_{2})_{x}$, by means of first-principles DFT calculations. All intermediate compounds are thermodynamically stable with respect to the elements. The hydrogenation reaction of Li$_{2}$Mg(NH)$_{2}$ via the intermediate imides Li$_{2-2x}$Mg$_{x}$NH is energetically favorable compared to other intermediates.\\[0pt] Ref~: [1] Nature \textbf{420}, 302 (2002). [2] J. Phys. Chem. C \textbf{113}, 15772 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P20.00009: First-Principles Study of Native Defects in Li$_4$BN$_3$H$_{10}$ Under Varied Chemical Conditions David Farrell, Christopher Wolverton Hydrogen desorption from many complex hydrides, such as Li$_4$BN$_3$H$_{10}$, is known to be kinetically limited. At temperatures below melting, the motion of point defects is one possible factor affecting chemical reactions. Therefore, an understanding of their formation and migration will yield insight into the kinetic limitation of hydrogen desorption. To explore this, we have determined the 0~K formation energy for a number of neutral and charged point defects in Li$_4$BN$_3$H$_{10}$ under a variety of chemical conditions via density functional theory calculations. We determined chemical potentials based on thermodynamically predicted hydrogen desorption reactions and provide a physical interpretation of the resulting equilibrium conditions. Our results indicate that: 1) The lowest energy defect varies with chemical conditions. 2) neutral defects are always lower energy than analogous pairs of oppositely charged defects. 3) Hydrogen defects are rarely the lowest energy defect. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P20.00010: First-principles Modeling of Diffusion Reactions in the Hydrogenation of NaAlH$_{4}$ Kyle Michel, Vidvuds Ozolins The hydrogenation of NaAlH$_{4}$ has been studied extensively since it was discovered that doping with Ti greatly increases its reversible hydrogen storage capacity. Experimental studies have suggested that diffusion of metal-containing defects may be the rate-limiting step in this reaction. We present a model to study the diffusion of defects during a solid-state reaction and apply it to this hydrogen storage reaction. The flux of defects in simple, model systems is calculated and from these values the activation energy for these processes is determined. We find that the activation energy for the diffusion of metal defects matches well to the experimental activation energy for the reaction when doped with Ti. The model that is presented can easily be applied to other systems in which a reaction takes place in the solid state. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P20.00011: Catalytic effect of carbon nanomaterials on light metal hydride systems Zhao Qian, Rajeev Ahuja, C. Moyses Araujo, Andreas Blomqvist, Biswarup Pathak, Ralph H. Scheicher Carbon nanomaterials are becoming recognized for their use in catalyzing hydrogen desorption from light metal hydride systems, in particular complex borohydrides and alanates. For example, it was shown by us that graphene, carbon nanotubes, and especially fullerenes can improve the hydrogen sorption properties of sodium alanate [Nano Lett. 9, 1501 (2009)]. In parallel to ongoing experimental investigations, we have carried out further theoretical studies in order to better understand the underlying catalyzing mechanism. Our most recent work is concentrated on the interaction of lithium borohydride with fullerene where a complete dehydrogenation process was simulated using the cluster approach. Furthermore, the catalytic effect of graphene nanofibres on sodium alanate has been experimentally demonstrated by our collaborators, and we have studied this system from first principles as well, to better understand the origin of its catalytic effect. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P20.00012: Adsorbed Natural Gas Storage in Optimized High Surface Area Microporous Carbon Jimmy Romanos, Tyler Rash, Erik Nordwald, Joshua Shawn Shocklee, Carlos Wexler, Peter Pfeifer Adsorbed natural gas (ANG) is an attractive alternative technology to compressed natural gas (CNG) or liquefied natural gas (LNG) for the efficient storage of natural gas, in particular for vehicular applications. In adsorbants engineered to have pores of a few molecular diameters, a strong van der Walls force allows reversible physisorption of methane at low pressures and room temperature. Activated carbons were optimized for storage by varying KOH:C ratio and activation temperature. We also consider the effect of mechanical compression of powders to further enhance the volumetric storage capacity. We will present standard porous material characterization (BET surface area and pore-size distribution from subcritical N$_{2}$ adsorption) and methane isotherms up to 250 bar at 293K. At sufficiently high pressure, specific surface area, methane binding energy and film density can be extracted from supercritical methane adsorption isotherms. Research supported by the California Energy Commission (500-08-022). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P20.00013: On the reversibility of the adsorption of methane-mercaptan for natural gas storage M. Golebiowska, L. Firlej, B. Kuchta, M. Roth, C. Wexler Methane is the main constituent of natural gas (NG). As fuel for vehicular applications NG requires sorbents that allow efficient, reversible and safe storage at room temperature and moderate pressure. To enable easy human detection of gas leaks the fuel gas should be added with compounds having low odor threshold, such as thiols (mercaptans). Thus a full understanding of the behavior of methane-mercaptan mixtures is necessary for the development of safe storage systems. In this talk we present results of molecular dynamics simulations in the temperature range 150--350 K and for a large range of methane partial pressures, up to the saturation pressure of methane. We observe the presence of 2D (and to a lesser degree 3D) diffusion of the thiols indicating that though thiols adsorb preferentially relative to methane, the adsorption is still reversible. We estimate that only a small increase in mercaptan concentration is necessary for the desorbed phase to be above the threshold for human detection. [Preview Abstract] |
Session P21: Focus Session: Advances in Scanned Probe Microscopy III - Novel SPM of Spin, Force & Conductance
Sponsoring Units: GIMSChair: Yukio Hasegawa, University of Tokyo
Room: D161
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P21.00001: A New Ultra-Low Temperature, High Magnetic Field STM in an Ultra-Quiet Laboratory Brian B. Zhou, Shashank Misra, Lukas Urban, Jungpil Seo, Andras Gyenis, SeJong Kahng, Ali Yazdani We report progress in the construction of a new UHV STM capable of operating at the extremes of temperature (25 mK) and magnetic field (14 T), allowing atomically resolved studies in previously unexplored areas of phase space. Our novel design is based on a bottom-loading dilution refrigerator in which the entire dilution stage and mounted microscope are moved between measurement and sample transfer positions. Pumping for the dilution fridge and large magnetic fields introduce demanding challenges in vibration isolation, which we have addressed with an ultra-quiet laboratory setting and rigid microscope design. Our system is situated inside both acoustic and RF-shielded enclosures in complement with various stages of isolation for both pump and ambient vibration sources. We will discuss unique aspects of the microscope design, such as a two-in-one double sample holder, and assess preliminary system performance. Supported by the W. M. Keck Foundation. Infrastructure at Princeton Nanoscale Microscopy Laboratory is also supported by grants from DOE, NSF, and ARO. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P21.00002: Low Temperature Scanning Tunneling Microscope for Spin Polarization Measurements Seong Heon Kim, Ryan Jaehne, LeuJen Chen, Alex de Lozanne We describe a new design for a 4K scanning tunneling microscope (STM) with an 8 tesla superconducting magnet to be used for spin polarized measurements. The novel aspects include a compact design for the chamber and the STM, the use of a secondary STM for in-situ tip characterization, and new ideas for vibration isolation. We developed a new STM head unit with 1 inch diameter and 3.2 inch length. This microscope is small enough to be installed within the small space available in the 2 inch diameter bore of our superconducting magnet. To achieve this small size, we modified the typical Pan-type z-approach walker. We also developed new simple and inexpensive electronics to control any stick-slip walker. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P21.00003: Compact probe design for Scanning Hall Probe Microscopy Neliza Leon-Brito, Seongsoo Kweon, Alex de Lozanne In the search for new materials with desirable magnetic properties for applications such as spintronics the study of magnetic properties at the micro and nanoscale is necessary. Magnetic Force Microscopy (MFM) has been the technique of choice for these types of studies, but its invasive nature makes it unsuitable for low coercivity materials like diluted magnetic semiconductors. Scanning Hall Probe Microscopy (SHPM) is an alternative technique which provides a magnetically non-invasive, calibrated measurement of the stray fields above the sample with good resolution ($\sim $1um). We have built a compact cryogenic variable-temperature (4 - 300K) SHPM with unique features such as an inverted tapered seal that also performs as a heat sink for the microscope body and a new coarse approach mechanism. Details of this design will be presented in this talk. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P21.00004: Realizing Spin Logic Atom by Atom Invited Speaker: Scanning tunneling microscopy (STM) has emerged as a leading technique which can address single atom magnetism with high energy and spatial resolution. With the development of sub-Kelvin high-magnetic field STM, two complementary methods, namely spin-polarized scanning tunneling spectroscopy (SP-STS) and inelastic STS (ISTS), can address the fundamental properties of individual magnetic impurities at surfaces [1,2]. We use a map of the distance-dependent RKKY interaction between Fe atoms on Cu(111) obtained by SP-STS to engineer complex magnetic nanostructures with tailored magnetic properties with atomic manipulation. By combining constructed anti-ferromagnetic structures with spin frustration, we realize an atomic-scale logic device which functions solely on the spin-degrees of freedom of its magnetic constituents. This work was done in collaboration with J. Wiebe, S. Lounis, B. Chilian, A. T. Costa, L. Zhou, D. L. Mills, and R. Wiesendanger. \\[4pt] [1] A. A. Khajetoorians, B. Chilian, J. Wiebe, S. Schuwalow, F. Lechermann, and R. Wiesendanger, Nature 467, 1084 (2010). \\[0pt] [2] A. A. Khajetoorians, S. Lounis, B. Chilian, A. T. Costa, L. Zhou, D. Mills, J. Wiebe, and R. Wiesendanger, arXiv:1010.1284v2 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P21.00005: Visualizing spin-dependent scattering in strong spin-orbit systems Anna Strozecka, Asier Eiguren, Jose Ignacio Pascual For surfaces which exhibit spin-orbit coupling, electrons originating from spin polarized surface bands are protected against backscattering by time reversal symmetry. Electron interference patterns observed in STM confirm the chiral spin texture of the surface Fermi contours of such materials and reveal the dominant role of spin in the scattering processes. Using a combined experimental and theoretical approach, we distinguish the role of spin in the electron scattering processes on Bi(110). Utilizing spectroscopic imaging of the local density of states, we studied the energy dependence of the interference patterns formed around single adsorbates. Simulations based on Green`s functions correctly reproduce the interference patterns, unveiling the role of spin in the interference process and allowing identification of the dominant scattering events. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P21.00006: Cotunneling theory for STM spin-flip spectroscopy F. Delgado, J. Fernandez-Rossier Scanning Tunneling Spectroscopy of both magnetic atoms and molecules adsorbed on surfaces is analyzed from the theoretical point of view. We show that cotunneling is the leading mechanism that explains the spin assisted inelastic conductance reported in recent experiments [1-4]. We describe the electronic transport between the scanning tip and the conducting surface through the magnetic system (MS) with a generalized Anderson model. The correlations in the MS are calculated exactly and transport is considered to fourth order in the tip-MS and MS-surface coupling. Our theory accounts for the observed [2,4] asymmetric conductance and provides an explanation of the large inelastic contribution.\\[4pt] [1] A. J. Heinrich et. al, Science 306, 466 (2004)\\[0pt] [2] Xi Chen, et al, Phys. Rev. Lett. 101, 197208 (2008)\\[0pt] [3] A. A. Khajetoorians et al, Nature 467, 1084 (2010)\\[0pt] [4] X. Chen et al, Phys. Rev. Lett. 101, 197208 (2008) [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P21.00007: Analysis of Tunneling Spectra in Constant-Current Distance-Voltage Mode Daniel Dougherty, Alex Pronschinske, Daniel Mardit A technical challenge associated with the use of traditional constant height tunneling spectroscopy in current-voltage mode is that tunneling current increases very rapidly at even modest voltages. This can result in tip-induced damage or motion for soft or delicate materials like organic molecules. One solution to this problem is to measure tunneling spectra in constant current distance-voltage mode where the STM feedback loop maintains a constant small tunneling current. Using the standard integral expression for tunneling current with a WKB transmission function, it is possible to create a first order differential equation connecting distance-voltage spectra with sample density of electronic states. This can be used to experimentally extract density of states or to theoretically predict distance-voltage tunneling spectra from a known density of states. We illustrate the use of this approach with numerical and experimental examples. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P21.00008: Quantitative force measurements with intermodulation atomic force microscopy Daniel Platz, Daniel Forchheimer, Carsten Hutter, Erik Thol\'en, David Haviland Dynamic atomic force microscopy (dynamic AFM) is a key tool for surface characterization on the nanoscale. Operation close to a cantilever resonance increases sensitivity and allows for the measurement of the phase of the cantilever response. This phase is traditionally interpreted as a measure of the energy dissipation due to the tip-sample interaction. However, a full understanding of dissipative processes remains a challenge in dynamic AFM. To address this problem we have developed Intermodulation AFM. With this multi-frequency technique we can tremendously increase the number of information carrying signals close to resonance. Using Fourier analysis and linear algebra we combine the amplitudes and phases of these signals to separately reconstruct the conservative and non-conservative tip-sample interactions. We have tested this method both on simulated and on experimental data. The method works at one tip-surface separation, providing quantitative high resolution maps of surface properties while imaging at normal rates. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P21.00009: Nano-scale Strain Mapping using Near-field Microscopy Antonio Llopis, Arkadii Krokhin, Sergio Pereira, Ian Watson, Arup Neogi Advances in nanophotonics are beginning to allow for the creation of nano-scale light emitting devices. Improving the quality of these next-generation emitters requires similarly advanced methods for characterization. These techniques need to be capable of imaging operational prototypes with nanometric resolution. We demonstrate here a new method for mapping strain capable of meeting the demands of next-generation device characterization. This technique makes use of near-field spectroscopy along with theoretical modelling to achieve non-destructive strain mapping with a resolution on the order of 10-100nm. An InGaN ELOG MQW sample is mapped using a SNOM, producing near-field maps of the intensity and Huang-Rhys parameter. Theoretical calculations are then used to obtain the relation between the Huang-Rhys parameter and the biaxial strain $\varepsilon _{xx}$, thereby allowing the production of a near-field map of the biaxial strain in the sample. Finally, to verify the efficacy of the method, we compare the results with those obtained using high-resolution XRD. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P21.00010: Non-linear optical nano-structured probe for photonic force microscopy Aswini Kanneganti, Harshit Vallabh, Ninad Ingle, Xiao Zhang, Jing Li, Samarendra Mohanty Use of second-harmonic (SH) optical probes for imaging of microscopic samples has distinct advantages over fluorescence, which suffers from photobleaching. Further, SH nanoparticles can be optically trapped for probing interaction forces and even for topographic imaging of nanostructures. Here, we report SH generation in ZnS(pda)$_{1/2}$ (pda = propanediamine), a new class of nanostructured crystals. ZnS(pda)$_{1/2}$ is an isostructure of ZnTe(pda)$_{1/2}$ as confirmed by PXRD pattern. The SHG imaging of the nanocrystals was carried out by an ultrafast ($\sim $100fs) Ti: Sapphire laser beam (wavelength: 960 nm; repetition rate: 80 MHz) focused to a diffraction limited spot by use of a 100X microscope objective leading to very high peak power density. Dependence of SHG intensity as a function of laser power and axial position of the nanoparticle in the focused laser microbeam was quantitated for the purpose of photonic force microscopy. The suspended ZnS(pda)$_{1/2}$ nanocrystals could be trapped using the near-IR Ti: Sapphire laser microbeam. The SHG intensity was found to fall very rapidly as the nanocrystal is displaced from the focused spot, which led to highly sensitive height measurements. Non-linear optical characterization of the ZnS(pda)$_{1/2}$ nanocrystals and its use in photonic force microscopic imaging will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P21.00011: Design of a scanning gate microscope in a cryogen-free dilution refrigerator Matthew Pelliccione, Adam Sciambi, David Goldhaber-Gordon We report on our design of an ultra-low temperature scanning gate microscope housed in a system with no liquid helium bath. The recent increase in efficiency of pulse-tube cryocoolers and pending scarcity of liquid helium have made ``cryogen-free'' dewars popular in recent years. However, this new style of dewar presents challenges for performing scanning measurements, most notably the increased vibrations introduced by the cryocooler. We will highlight the tradeoffs made in choosing such a system to house a scanner, and describe our efforts to achieve a stability suitable for measurements on mesoscopic systems. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P21.00012: Electronic characterization of 1-D defects using scanning gate spectroscopy Steven R. Hunt, Brad L. Corso, Philip G. Collins Scanning gate microscopy (SGM) is a technique particularly useful for characterizing transport in electronic devices. We have extended the SGM technique into a spectroscopy by measuring the entire bias dependence of conductance at every position on a surface. Much as in current imaging tunneling spectroscopy (CITS), the resulting data set is a multidimensional, detailed map of the electronic behavior of a surface. We apply this scanning gate spectroscopy (SGS) technique to scattering in one-dimensional, carbon nanotube circuits. Transport in one-dimensional systems depends critically on inhomogeneities, including isolated point defects. The SGS technique enables straightforward investigation of low-dimensional transport physics at such sites. In our experiments, metallic single-walled carbon nanotubes are investigated before and after the electrochemical introduction of a point defect, in order to clearly establish the contribution of different defect types. SGS directly images the energy dependence of a defect's scattering, providing a way to distinguish between different defect chemistries and quantitatively model its energy levels and transmission. This research is partly supported by the NSF (DMR-0801271). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P21.00013: Scanning gate transconductance microscopy and spectroscopy of a mesoscopic ring Benoit Hackens, Frederico Martins, Sebastien Faniel, Vincent Bayot, Marco Pala, Hermann Sellier, Serge Huant, Ludovic Desplanque, Xavier Wallart In scanning gate microscopy (SGM), a dc voltage is applied to a sharp tip moving in the vicinity of a device. This alters the electrostatic potential seen by electrons inside the device, and consequently changes the device conductance [1]. Here, we superimpose a small ac voltage to the dc bias applied on the tip, and record the change of device conductance at the tip bias modulation frequency, i.e. the local transconductance. We first image the low temperature transconductance of a mesoscopic ring patterned in a two-dimensional electron system (2DES) hosted in an InGaAs/InAlAs heterostructure. The tranconductance images are decorated by concentric features that we associate with charging of electron traps located close to the 2DES. We perform spectroscopy of these traps by positioning the tip close to them, and recording the ring transconductance as a function of the tip dc voltage and the bias accross the quantum ring. We observe Coulomb diamonds in our spectroscopic data, which confirms that Coulomb blockade is at play. [1] B. Hackens et al., Nature Physics 2, 826 (2006). [Preview Abstract] |
Session P22: Metal-Insulator Phase Transitions I
Sponsoring Units: DCMPChair: Rachel Wortis, Trent University
Room: D163
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P22.00001: Revisiting the Anderson Model with Power-Law Correlated Disorder in 1D and 2D Greg Petersen, Nancy Sandler The dimensionality of a disordered system directly affects the critical energy where a localization/delocalization transition occurs. In non-interacting systems with uncorrelated disorder, it is widely known that all states in one-dimension are localized. However, for some correlations there exist transition energies similar to mobility edges or small subsets of extended states that are robust against disorder. In this talk, we will present results on the diffusion of a wavepacket in a power-law correlated random potential of the form $\langle V(r)V(0) \rangle = \frac{1}{(a+r)^\alpha}$. We also report results for the participation ratio $P_r=\frac{1}{N}\frac{\langle |a_i|^2\rangle^2}{\langle |a_i|^4\rangle}$. Preliminary results for 1D chains support the existence of a mobility edge near the band center. Square and graphene lattices will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P22.00002: Higher order correction to the RG $\beta $-function for the 3-d Anderson localization transition at unitary symmetry Tomoyuki Nakayama, Khandker Muttalib, Peter W\"olfle We have recently calculated the $\beta $-function of the conductance for Anderson Metal-Insulator transition including contributions from the ballistic regime. In three dimensional unitary case, the result of two-loop order diagrams is $\beta $(g)=1-a/g, where a is a constant and g is the dimensionless conductance. However, this result is valid only if there is no diagram with extra diffusons which contributes to the order of 1/g. We show that diagrams with extra diffuson propagators only have higher order contributions in the ballistic regime, which confirms our previous result. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P22.00003: Singular Behavior of Electronic Eigenstates in the Anderson Model of Localization Sonika Johri, R.N. Bhatt We report the observation of a singularity in the electronic properties of the Anderson Model of Localization with diagonal disorder\footnote{P. W. Anderson, Physical Review {\bf 109}, 1492 (1958).} which is clearly distinct from the well- established mobility edge (localization-delocalization transition)\footnote{E. Abrahams, P. W. Anderson, D. C. Licciardello and T. V. Ramakrishnan, Physical Review Letters {\bf 42}, 673 (1979).}$^,$\footnote{For a recent review on Anderson Localization, see Ferdinand Evers and Alexander D. Mirlin, Reviews of Modern Physics {\bf 80}, 1355 (2008).} that occurs in dimensions $d>2$. We present results of numerical calculations for various disorder distributions in dimensions $d$ = $1$, $2$ and $3$, of different properties of the electronic wavefunctions to establish this, and to understand its evolution with disorder distribution, dimension and lattice type. Our data suggest that the model is richer than has been originally believed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P22.00004: Temperature dependence of the zero-bias anomaly in the two-site Anderson-Hubbard model R. Wortis, W.A. Atkinson Experiments on disordered strongly correlated electron systems show zero-bias anomalies which are not consistent with either of the two prevailing pictures, by Altshuler and Aronov and by Efros and Shklovskii. Numerical work on the two-dimensional Anderson-Hubbard model shows a zero-bias anomaly with a number of unique features. It has recently been shown that a zero-bias anomaly with many of the same features occurs in an ensemble of two-site Anderson-Hubbard systems. The simplicity of this system allows direct understanding of the mechanism of the anomaly. Here, the temperature dependence of this anomaly is explored. A novel feature is the existence of a temperature driven zero-bias anomaly which appears even in the atomic limit and augments the kinetic energy driven one in the presence of hopping. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P22.00005: Theoretical perspective on nearly frozen coulomb liquids Hanna Terletska, Yohanes Pramudya, Sergey Pankov, Efstratios Manousakis, Vladimir Dobrosavljevic Various studies on systems with charge ordered states, such as Wigner crystal, show their extreme fragility resulting from strong frustrations caused by long-range Colulomb-like interactions. Here, a so-called nearly-frozen Coulomb liquid regime is identified featuring a soft Coulomb pseudo-gap with unconventional insulating-like transport. Despite intensive studies, such pseudo-gap regime is still poorly understood. By employing extended dynamical mean field theory (EDMFT) [1] to study a semi-classical lattice gas model of spinless electrons, we successfully demonstrate the existence of such an intermediate liquid regime, and show that the pseudo-gap is, in fact, a general feature for models with long-range interactions. Our analytical results are well supported by exact Monte Carlo calculations. Moreover, we show that standard theories, like self-consistent Gaussian approximation (``spherical model'') and RPA, are ill-suited to describe this interesting regime. The spherical model approach provides the same as EDMFT freezing temperature Tc, but fails to capture the pseudo-gap feature. RPA, however, not only overestimates Tc, but also completely misses the pseudo-gap regime. \\[0pt] [1] S. Pankov and V. Dobrosavljevic, Phys. Rev. Lett. \textbf{94}, 046402 (2005). [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P22.00006: Bound states in the continuum in a multi-electron system Rahul Hardikar, Gonzalo Ordonez Bound states in a continuum (BIC) occur due to quantum interference of two identical adatoms in a one-dimensional (1D) band. In the past such states have been studied for a one- electron system using several analytical and theoretical methods. We extend the idea of BIC to a multi-electron system. To study this numerically we use the pure Hubbard hamiltoninan and add impurity sites at specific locations. Using this variant of the Hubbard model and an exact diagonalization method we prove that BIC can exist for multi-electron systems. We will also show theoretical proof of such states using the Bethe-Ansatz method [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P22.00007: Spectral functions across the Metal-Insulator transition in the disordered 2D Hubbard model Karim Bouadim, Nganba Meetei, Yen Lee Loh, Nandini Trivedi We study the metal-insulator transition in the repulsive disordered 2D Hubbard model [1,2] using Determinant Quantum Monte Carlo (DQMC). We calculate the spin-spin and current-current correlations to learn about the nature of the conducting and insulating phases. We also obtain local spin-dependent spectroscopic properties, using the maximum entropy method, to understand the role of disorder on the transition in this highly correlated fermion system. We discuss implications of our results for scanning tunneling spectroscopy and dynamical conductivity experiments [3]. \\[4pt] [1]. P.J.H Denteneer, R.T. Scalettar and N. Trivedi, Phys. Rev. Lett.83, 4610 (1999).\\[0pt] [2]. D. Heidarian and N. Trivedi, Phys. Rev. Lett. 93, 126401 (2004).\\[0pt] [3]. M.M. Qazilbash et. al., Science 318, 1750 (2007). [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P22.00008: Time-Correlated Soliton Tunneling in Density Waves John H. Miller, Asanga Iroshan Wijesinghe, Zhongjia Tang, Arnold M. Guloy In the quantum sine-Gordon model of a pinned charge or spin density wave, the electrostatic energy generated by charged soliton domain walls leads to a Coulomb blockade threshold electric field for quantum soliton-antisoliton pair creation. This field can be much smaller than the classical depinning field, since the quantum instability occurs as soon as the formerly lowest energy potential well rises to become a metastable well, or ``false vacuum.'' The analogy to time-correlated single electron tunneling and comparison to recent experimental results, as well as broader implications of the proposed tunneling process, are briefly discussed. This work was supported by the State of Texas though the Texas Center for Superconductivity at the University of Houston and the Norman Hackerman Advanced Research Program, and by NIH R21CA133153 and ARRA supplement 3R21CA133153-03S, and by the Robert A. Welch Foundation, and DoE Basic Energy Sciences. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P22.00009: Level spacing statistics for quantum $k$-core percolation L. Cao, J.M. Schwarz Quantum percolation is the study of hopping transport of a quantum particle on randomly diluted percolation clusters. Quantum $k$-core percolation is the study of quantum transport on $k$- core percolation clusters where each occupied bond must have at least $k$ occupied neighboring bonds. Within the random phase approximation, we found a random first-order phase transition for the $k$-core conduction transition on the Bethe lattice, and $p_q$, the quantum percolation critical probability, is equal to $p_c$, the geometric percolation critical probability [Phys. Rev. B {\bf 82},104211 (2010)]. To further test this result, we numerically compute the level spacing distribution as a function of occupation probability $p$ and system size. The simulation results provide confirmation for the existence of a discontinuous onset of quantum conduction at $p_q=p_c$. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P22.00010: The onset of superfluidity of hardcore bosons in disordered ladders Juan Carrasquilla, Federico Becca, Michele Fabrizio The effect of disorder on the zero-temperature phase diagram of a two-leg ladder of hardcore bosons is investigated using quantum Monte Carlo simulations. We first review some aspects of the clean system which are relevant for the understanding of the disordered case. In the disordered case, an intervening Bose-glass phase between the frozen Mott insulator with zero (or one) bosons per site and the superfluid phase is found. We also investigate the effect of disorder exactly at half filling, where for small values of disorder, there is a commensurate phase with a gap to all excitations, which is eventually destroyed for larger values of disorder. We argue that this phase is always surrounded by the so-called Bose glass and a direct transition from the superfluid is found only in the clean system. Finally, a phase diagram based on our numerical evidence is suggested. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P22.00011: Global phase diagram of the spinless Falicov-Kimball model in $d = 3$: renormalization-group theory Ozan S. Sariyer, Michael Hinczewski, A. Nihat Berker The global phase diagram of the spinless Falicov-Kimball model in $d = 3$ spatial dimensions is obtained by renormalization-group theory [1]. This global phase diagram exhibits five distinct phases. Four of these phases are charge-ordered (CO) phases, in which the system forms two sublattices with different electron densities. The phase boundaries are second order, except for an intermediate interaction regime, where a first-order phase boundary between two CO phases occurs. The first-order phase boundary is delimited by special bicritical points. The cross-sections of the global phase diagram with respect to the chemical potentials of the localized and mobile electrons, at all representative interaction and hopping strengths, are calculated and exhibit three distinct topologies. The phase diagrams with respect to electron densities are also calculated.\\[4pt] [1] O.S. Sar\i yer, M. Hinczewski, and A.N. Berker, arXiv:1002.1821v1 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P22.00012: Improved determination of the self-energy and vertex function in Strong-Coupling Continuous-time Quantum Monte Carlo Hartmut Hafermann, Kelly R. Patton, Philipp Werner The continuous-time quantum Monte Carlo method based on the strong coupling expansion is an efficient and flexible tool for the solution of multiorbital Anderson impurity models. However it is known that it is difficult to accurately compute the intermediate and high-frequency behavior of measured quantities. This leads to large errors, in particular for the self-energy when computed from Dyson's equation. A similar problem occurs for the vertex function when computed directly from the two-particle Green function. We propose an improved way of measuring these quantities, based on higher-order impurity correlation functions. The method yields very accurate estimates for the self-energy and vertex function over the full frequency range. In the segment representation, the improved estimators can be accumulated at essentially no additional computational cost. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P22.00013: Surface effects in doping a Mott insulator Reza Nourafkan, Frank Marsiglio The physics of doping a Mott insulator is investigated in the presence of a solid-vacuum interface. Using the embedding approach for dynamical mean field theory we show that approaching a Mott insulating phase from the metallic side, a dead layer forms at the surface of the solid, where quasiparticle amplitudes are exponentially suppressed. In particular, we have demonstrated that the reduction of the quasiparticle weight detected by surface sensitive photoemission experiments of a doped Mott insulator are caused by both charge transfer and enhanced correlation effects at the surface. The expected modification of the intra-layer hopping at the surface and inter-layer hopping between the surface and the subsurface layer amplifies the surface effects. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P22.00014: The Role of the Van Hove Singularity in the Quantum Criticality of the Hubbard Model Sandeep Pathak, K.-S. Chen, Shuxiang Yang, Mark Jarrell, Juana Moreno A quantum critical point, separating a non-Fermi liquid region from a Fermi liquid, exists in the phase diagram of the Hubbard model [Vidhyadhiraja \emph{et. al}, Phys. Rev. Lett. \textbf{102}, 206407 (2009)]. This quantum critical point is characterized by a vanishing spectral weight and a van Hove singularity (vHS) in the dispersion that crosses the Fermi level. The real part of the critical particle-particle susceptibility exhibits a algebraic decay with temperature, which results in the imaginary part showing scaling at large frequencies. This algebraic decay leads to higher superconducting transition temperatures as compared to the BCS theory, where the pairing susceptibility decays only logarithmically. In this talk, we examine the role of the van Hove singularity in determining this critical behavior. We calculate the bare particle-particle susceptibility of a $d$-wave pair field for the standard two-dimensional tight binding dispersion and for a hypothetical quartic dispersion having ``flatter" or ``extended" singularities. We find that the standard logarithmic vHS cannot correctly describe the critical algebraic behavior and it is essential to have an extended vHS that displays an algebraic singularity. Thus, our results emphasize the possible role of the extended vHS in the unexpectedly higher $T_c$ of cuprates. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P22.00015: Numerical study of real-time quantum dynamics in spin-electron coupled system Wataru Koshibae, Naoto Nagaosa, Nobuo Furukawa The photo-induced metal-insulator transition is studied by the numerical simulation of real-time quantum dynamics of a double- exchange model. The spatial and temporal evolutions of the system during the transition have been revealed including (i) the threshold behavior with respect to the intensity and energy of light, (ii) multiplication of particle-hole (p-h) pairs by a p-h pair of high energy, and (iii) the space-time pattern formation such as (a) the stripe controlled by the polarization of light, (b) coexistence of metallic and insulating domains, and (c) dynamical spontaneous symmetry-breaking associated with the spin spiral formation imposed by the conservation of total spin for small energy-dissipation rates. [Preview Abstract] |
Session P23: Superconductivity: Josephson Effects I
Sponsoring Units: DCMPChair: George Crabtree, Argonne National Laboratory
Room: D165
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P23.00001: Tunable terahertz emission from Bi2Sr2CaCu2O8 mesa devices Timothy Benseman, Ulrich Welp, Wai-Kwong Kwok, Alexei Koshelev, Cihan Kurter, Lutfi Ozyuzer, Kazuo Kadowaki, Takashi Yamamoto The so-called ``terahertz gap,'' covering frequencies from approximately 0.3 to 1.5 THz, is of particular interest for a number of scientific and security applications, although no bright sources of coherent radiation presently exist in this range. However, stacks of high-temperature superconducting intrinsic Josephson junctions are a promising candidate. [1] Here we discuss recent progress in improving the performance of these devices. In particular, we demonstrate that via control of bias voltage and operating temperature, the emission from an 80-$\mu $m wide Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesa can be tuned continuously over a frequency range in excess of 10{\%} in the vicinity of 0.5 THz. We find that as the emission frequency increases from 0.420 to 0.492 THz, the linewidth increases from $<$2.25 GHz (limited by instrument resolution) to $\sim $9 GHz. \\[4pt] [1] L. Ozyuzer et al., Science 318 (2007) 1291-1293. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P23.00002: The relationship between oscillation modes and single crystalline Bi2212 mesa structures T. Kashiwagi, K. Deguchi, M. Tsujimoto, N. Orita, T. Koike, R. Nakayama, K. Delfanazari, H. Minami, T. Yamamoto, K. Kadowaki Continuous electromagnetic (EM) radiation in terahertz region has been observed from a rectangular mesa structure of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(Bi2212) single crystals.\footnote{L. Ozyuzer \textit{et al.}, Science \textbf{318} (2007) 1291.} It has been established that the radiation frequency is determined by both ac Josephson effect and geometrical cavity resonance condition.\footnote{K. Kadowaki \textit{et al.}, J. Phys. Soc. Jpn. \textbf{79} (2010) 023703} The observed radiation frequencies in the many rectangular mesas studied were inversely proportional to the width of the mesa and the fundamental modes equal to twice the mesa width. Recently, several mesas show different radiation characteristics which suggest the existence of the higher excitation modes such as one wave length excitation mode. The observed frequencies from above mesas are almost explained by the geometrical cavity model. In order to clarify the detail of the excitation modes, we also measured the radiation pattern of EM waves. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P23.00003: Coherent THz-wave emission from voltage- and number-controlled intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Manabu Tsujimoto, Ryo Nakayama, Naoki Orita, Takashi Koike, Kota Deguchi, Kaveh Delfanazari, Takashi Yamamoto, Takanari Kashiwagi, Hidetoshi Minami, Masashi Tachiki, Kazuo Kadowaki Intense and coherent terahertz electromagnetic wave (THz-wave) emission from the intrinsic Josephson junctions (IJJs) in single crystalline high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) was reported in 2007 [L. Ozyuzer \textit{et al}., Science \textbf{318}, (2007) 1291.]. In the present work, we demonstrate the relationship between the bias condition and the resonance state by controlling both the applied voltage, $V$, and the number of resistive junctions, $N$. We directly observed that if $N$ junctions are in resistive state, the resonance frequency, $f_{J}$, varies in accordance with the ac-Josephson relation; $f_{J}$ = (2$\vert e\vert $/$h)V$/$N$, although frequency $f_{J}$ has previously been thought to be uniquely determined by the geometrical condition due to the cavity resonance effect [M. Tsujimoto \textit{et al}., Phys. Rev. Lett. \textbf{105}, (2010) 037005.]. We also found that the emission intensity varies as a function of both $f_{J}$ and $N$. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P23.00004: Angular distribution and Josephson plasma modes of THz radiation emitted from Bi2212 mesas with various shapes Kazuo Kadowaki, Takanari Kashiwagi, Manabu Tsujimoto, Takashi Yamamoto, Hidehiro Asai, Hidetoshi Minami So far, we have measured angular dependence of the intensity of the THz emission from mesas of high-$T_c$ superconductor $\mathrm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ with various shapes such as rectangulars, squares, cylinders, triangulars, etc. and various dimensions larger or smaller than the penetration depth $\lambda_c$. The results have been analized by a model developed by Klemm and Kadowaki{\footnote{Richard A. Klemm and K. Kadowaki, J. Phys. Condens. Matter {\bf{22}} (2010) 375701.}}, which assumes coherent generation of the standing of wave cavity modes in a mesa due to the ac-Josephson effect. The higher harmonic modes are observed in some cases as expected in the model. However, it is often observed that the emission frequency does not obey the simple cavity mode, indicating that the cavity resonance may not be a stringent condition for the emission of the THz radiation. The implication is argued in terms of the emission dynamics from the Bi2212 mesa structure. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P23.00005: Superconductivity induced by current injection into non-superconducting Bi$_2$Sr$_2$CaCu$_2$O$_8$ Y. Simsek, Y. Koval, X.Y. Jin, S. Probst, P. M\"uller The carrier-doping induced transition from the antiferromagnetic state to the superconducting phase is still one of the most fascinating properties of high-Tc materials. Usually hole doping is achieved by non-stoichiometry. However, we already have shown that we can change the carrier concentration of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals by current injection along the c-axis. This effect is persistent up to annealing temperatures of approximately 270 K. Now, the interesting question is if ``chemical'' doping by oxygen excess is necessary at all. For this purpose we performed current-injection experiments on fully oxygen depleted Bi$_2$Sr$_2$CaCu$_2$O$_8$ which was not superconducting above 4.2 K. In order to eliminate the contact resistance of the highly resistive depleted material, we realized a ``true'' 4-point geometry by fabricating double cross-bar crystal stacks. C-axis resistivity, critical current, and critical temperature were measured by c-axis transport. We have observed that by carrier injection the conductivity can be increased until superconductivity above 4.2 K is reached. Continuing the doping by current injection, optimum-doped and even overdoped states were obtained. Using current injection at higher bias, we were able to reduce the hole concentration again. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P23.00006: Magnetic-field-driven phase transitions in Josephson arrays Joshua Paramanandam, Matthew Bell, Aleksandr Verevkin, Lev Ioffe, Michael Gershenson We have studied the phase transitions induced by the magnetic field $B$ in arrays of small Josephson junctions. The number of nearest-neighbor junctions connected to a single superconducting island varied between 4 and 11 for different arrays. When frustrated by the magnetic field, the arrays demonstrated several quantum phase transitions at different critical values of the resistance between $R $=3-10~k, which is in line with earlier observations. In particular, with increasing $B$ we observed transitions between three states$: $ a) the superconducting state with zero $R$, b) the ``metallic'' state with a weak $R$ dependence on T in the range 40mK$<$T$<$200mK, and c) the ``insulating'' state with an activation dependent $R(T)$. The activation energy, extracted from the current-voltage characteristics and the Arrhenius fitting of $R(T)$ in the ``insulating'' regime, has been studied in detail as a function of the temperature and the magnetic field. The data indicate the possible development of a strongly inhomogeneous state when approaching the superconducting-to-insulating transition. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P23.00007: Series-parallel two dimensional arrays of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin film ion damage Josephson junctions Jasper Drisko, Shane Cybart, Steven Anton, Stephen Wu, James Parker, Robert Dynes We have fabricated a number of series-parallel two dimensional arrays of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin film ion damage Josephson junctions. The arrays contain 1,000 to 15,000 Superconducting QUantum Interference Devices (SQUIDs) in different dimensional configurations and different SQUID area distributions. We have measured the electrical transport characteristics of each array and compare it to computer simulations based on the resistively shunted junction model to investigate the effect of mutual inductance between the SQUIDS within the array. We find there is good agreement between our simulation model and our experimental device when the parallel dimension of the device is less than 15 SQUIDs. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P23.00008: Properties of linear arrays of Josephson junctions capacitively coupled to a diffusive metal Alejandro Lobos, Thierry Giamarchi Josephson junctions arrays (JJAs) are strongly-correlated quantum systems showing a rich and complex behavior at low-temperatures.\footnote{R. Fazio and H. van der Zant, Physics Reports \textbf{355}, 235 (2001)} Besides their potential uses in applications, JJAs allow to investigate (under controlled conditions) many aspects of low-dimensional superconductivity which remain to be understood. In this work we study the phase diagram and the low-energy properties of a one-dimensional (1D) JJA capacitively coupled to a diffusive two-dimensional electron gas (2DEG) placed at a distance $d$, which provides dissipation.\footnote{A. M. Lobos and T. Giamarchi, Phys. Rev. B \textbf{82}, 104517 (2010)} We derive an effective field-theoretical model for the 1D JJA coupled to the 2DEG, and predict a superconductor-insulator transition (SIT) at $T=0$, in agreement with former theoretical predictions. We discuss implications for transport experiments and for the observed SIT in 1DJJAs. Both in the superconducting and insulating phases, the coupling to the 2DEG produces deviations with respect to the resistivity as a function of $T$ predicted for an isolated array. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P23.00009: Measurement of Aharonov-Casher effect in a Josephson junction chain Ioan Mihai Pop, Florent Lecocq, Bernard Pannetier, Olivier Buisson, Wiebke Guichard We have recently measured the effect of superconducting phase-slips on the ground state of a Josephson junction chain\footnote{I. M. Pop et al. Nature Physics 6, 589\textendash{}592 (2010)} and a rhombi chain.\footnote{I. M. Pop et al. PRB, 78, 104504 (2008)} Here we report clear evidence of Aharonov-Casher effect in a chain of Josephson junctions. This phenomenon is the dual of the well known Aharonov-Bohm interference. Using a capacitively coupled gate to the islands of the chain, we induce oscillations of the supercurrent by tuning the polarization charges on the islands. We observe complex interference patterns for different quantum phase slip amplitudes, that we understand quantitatively as Aharonov-Casher vortex interferences. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P23.00010: Josephson Junction as a Magnetic Switch Liufei Cai, Eugene Chudnovsky We study electromagnetic interaction of a nanomagnet with a weak superconducting link. Equations that govern coupled dynamics of the two systems are derived and investigated numerically. We show that despite very weak magnetic field generated by the weak link, a time-dependent bias voltage applied to the link can initiate a non-linear dynamics of the nanomagnet that leads to the reversal of its magnetic moment. We also consider quantum problem in which a nanomagnet interacting with a weak link is treated as a two-state spin system due to quantum tunneling between spin-up and spin-down states. L. Cai and E. M. Chudnovsky, Phys. Rev B \textbf{82}, 104429 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P23.00011: Switching Experiments on a Current-Biased MgB$_2$ Josephson Junction Roberto Ramos, Jerome Mlack, Joseph Lambert, Steven Carabello As the current through a Josephson junction is increased, the voltage across the junction switches from zero to a finite voltage. This is analogous to the escape of a phase particle originally oscillating with a plasma frequency $\omega$ in a washboard potential well, to the running state. We report results of our switching experiments on current-biased MgB$_2 $/I/Pb thin film junctions through a broad range of sub-Kelvin temperatures. Our results exhibit features in the escape rate $\Gamma$ suggestive of substructure within the pi gap of MgB$_2 $, which is consistent with our recent work demonstrating sub- structure within the pi and sigma superconducting energy gaps of MgB$_2$. Upon irradiation of microwaves with frequencies resonant with the plasma frequency, we observe enhancement of escape rates, which is a clear demonstration of microwave resonant activation in these devices. By manipulating frequency and power, we demonstrate good control over the escape of the phase particle. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P23.00012: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P23.00013: Cavity mode waves during terahertz radiation from rectangular Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesas Richard Klemm, Erica LaBerge, Dustin Morley, Takanari Kashiwagi, Manabu Tsujimoto, Kazuo Kadowaki We re-examined the angular dependence of the radiation from the intrinsic Josephson junctions in rectangular mesas of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, in order to determine if the cavity mode part of the radiation arises from waves across the width $w$ or along the length $\ell$ of the mesas, associated with ``hot spots'' [Wang {\it et al.}, Phys. Rev. Lett. {\bf 105}, 057002 (2010)]. Fits to the data of Kadowaki {\it et al.} [J. Phys. Soc. Jpn. {\bf 79}, 02373 (2010)] using both a uniform $ac$ Josephson current source and a non-uniform cavity mode (or magnetic surface current) source suggest that both scenarios are equally probable. However, when $n\ell/2w$ is integral, where $n$ is the index of the rectangular TM$^z(n,0)$ mode, standing cavity wave modes along the length of the mesa do not radiate in the $xz$ plane perpendicular to the length of the mesa, suggesting experiments on such mesas could help to resolve the question. [Preview Abstract] |
Session P24: Focus Session: What is Computational Physics? II followed by Computational Methods: Numerical Methods for Strongly Correlated Systems II
Sponsoring Units: DCOMPChair: Bernd Berg, Florida State University, and Frank Harris, University of Forida
Room: D167
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P24.00001: US-Japan Workshops on Computational Physics - International Liaison Activities - Yuko Okamoto In this talk, I will report on US-Japan workshops on computational physics. I will summarize what was done in the past workshops that were held in Hawaii and talk about future plans. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P24.00002: Opportunities for Computational Discovery in Basic Energy Sciences Mark Pederson An overview of the broad-ranging support of computational physics and computational science within the Department of Energy Office of Science will be provided. Computation as the third branch of physics is supported by all six offices (Advanced Scientific Computing, Basic Energy, Biological and Environmental, Fusion Energy, High-Energy Physics, and Nuclear Physics). Support focuses on hardware, software and applications. Most opportunities within the fields of~condensed-matter physics, chemical-physics and materials sciences are supported by the Officeof Basic Energy Science (BES) or through partnerships between BES and the Office for Advanced Scientific Computing. Activities include radiation sciences, catalysis, combustion, materials in extreme environments, energy-storage materials, light-harvesting and photovoltaics, solid-state lighting and superconductivity.~ A summary of two recent reports by the computational materials and chemical communities on the role of computation during the next decade will be provided. ~In addition to materials and chemistry challenges specific to energy sciences, issues identified~include a focus on the role of the domain scientist in integrating, expanding and sustaining applications-oriented capabilities on evolving high-performance computing platforms and on the role of computation in accelerating the development of innovative technologies. ~~ [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P24.00003: Discussion of Opportunities in Computational Physics Bernd Berg, Mark Pederson A discussion of the points raised in the previous talk by Mark Pederson (DOE) will be encouraged, including: (1) Identifying models and strategies for effectively organizing and availing complex computational simulation capabilities to a broader scientific and technical community. (2) Identifying cross-discipline communication of capabilities to ensure sharing of algorithms. (3) Opinions on evolution of overlap between the basic energy scientific mission and the fields that are typically represented by the March-Meeting participants. (4) Interactions between the domains of computational physics, computer science, and applied mathematics. (5) The proper balance between individual and group achievement. (6) What role could DCOMP have in this? [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P24.00004: Enabling Computational Discovery and Design Daryl Hess Advanced cyberinfrastructure(CI), increases in computing power, and increasing use of data volumes are revolutionizing how science is done, changing the nature of the questions we ask, and opening new frontiers.~ From discovering new phenomena and states of matter to the challenge of designing new materials and matter, the focus on problems with many complex interacting degrees of freedom through computational investigation often leads to large amounts of data that require analysis, preservation, curation, and sharing across the community. Data from many sources plays an increasingly important role as a driver of discovery. I will discuss opportunities in computational and data enabled science and in building the CI of the 21$^{st}$ century.~ Sustainable, maintained, and reliable shared software is an important component of a National CI framework that will empower computational scientists to engage the scientific frontiers and the pressing problems around us.~ The success of computational data enabled science requires innovation that leads to paradigms in attacking difficult problems. Education will play an important role in realizing the full potential of computation and data enabled science for discovery and design. Participation of the computational science community is an important ingredient to create a CI that will propel science forward; some self-assembly is required. NSF provides funding opportunities to help. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P24.00005: Building Foundations for Future Advances in Computational Physics Barry Schneider, Daryl Hess We continue the discussion on laying the cyberinfrastructure foundations to support future advances in computational science. We will focus on how NSF can help and encourage communication with the community in achieving this goal. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:36AM |
P24.00006: Quo Vadis Computational Physics? Invited Speaker: In this Focus Session, and in the preceding DCOMP Invited Symposium, ``Great Advances of Computational Physics: Past, Present and Future," we have heard about the vision for our field, reviewed (and previewed) cutting-edge numerical work, and considered where the resources might come to support our endeavors. In this talk I will summarize some of the common themes of this discussion, as well as open up the floor for further thoughts on where our APS Division is heading. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P24.00007: Magnetic impurities in real lattices: A DMRG and ECA study Carlos Busser, George Martins, Khaled Al-Hassanieh, Adrian Feiguin Magnetic interactions between strongly correlated impurities coupled to a sea of conduction electrons is a subject of great interest from both, experimental and theoretical studies. When many magnetic impurities are attached to the same conduction band a rich phase diagram can arise. By one hand the magnetic impurities can be strongly coupled to the spin of the electrons of the conduction band forming a Kondo singlet. By the other, through electron of the conduction band, a spin-spin interaction between the impurities can appears as a consequence the RKKY interaction. A competition between this two singles is expected. For these two effects is important to have a good description of the electrons with energy close to the Fermi level. Systems like the square lattice, with a van-Hove singularity at the middle of the band, or Graphene, with Dirac electrons, or Carbon nanotubes with multiple bands and multiples van Hove singularities need a proper description of the electrons in the lattice Hamiltonian. In this work we present, through a canonical transformation, a numerical method to study problems with several magnetic impurities coupled to arbitrary lattices using DMRG or ECA techniques. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P24.00008: Quantum Monte Carlo simulations with tensor-network states Jeong Pil Song, R.T. Clay Matrix-product states, generated by the density-matrix renormalization group method, are among the most powerful methods for simulation of quasi-one dimensional quantum systems. Direct application of a matrix-product state representation fails for two dimensional systems, although a number of tensor-network states have been proposed to generalize the concept for two dimensions. We introduce a useful approximate method replacing a 4-index tensor by two matrices in order to contract tensors in two dimensions. We use this formalism as a basis for variational quantum Monte Carlo, optimizing the matrix elements stochastically. We present results on a two dimensional spinless fermion model including nearest- neighbor Coulomb interactions, and determine the critical Coulomb interaction for the charge density wave state by finite size scaling. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P24.00009: Diagonalization with matrix-product states Chen Liu, Anders Sandvik We consider matrix-product states (MPSs) combined with diagonalization as a method to study correlated quantum many-body systems. The Hamiltonian matrix is constructed in a non-orthogonal basis of MPSs. Diagonalizing this matrix (a generalized eigenvalue problem) gives the ground state as well as excitations. The accuracy is significantly improved compared to individual optimized MPSs. We discuss several ways to generate the MPS basis states in a suitable way and present results for one- and two-dimensional quantum spin systems. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P24.00010: Multi-scale entanglement renormalization for critical systems Bela Bauer, Liza Huijse, Kareljan Schoutens, Guifre Vidal, Matthias Troyer Determining information about the underlying conformal field theory of a critical system in one dimension, such as the central charge and scaling dimensions, is a notoriously difficult problem for numerical methods. Using a suitable tensor network state, the multi-scale entanglement renormalization ansatz [1], this information is directly accessible [2]. We apply this method to several critical systems in one dimension, including a supersymmetric model for lattice fermions and Yang-Lee chains. \\[4pt] [1] G. Vidal, Phys. Rev. Lett. 99, 220405 (2007)\\[0pt] [2] R.N.C. Pfeifer et al, Phys. Rev. A 79(4), 040301(R) (2009) [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P24.00011: Simulation of fermionic and frustrated lattice models in two dimensions with tensor network algorithms Philippe Corboz, Glen Evenbly, Jacob Jordan, Roman Orus, Guifre Vidal, Bela Bauer, Matthias Troyer, Frederic Mila, Frank Verstraete The simulation of strongly correlated fermionic and frustrated systems in two dimensions is one of the biggest challenges in computational physics. Borrowing ideas and tools from quantum information and condensed matter physics, a new generation of simulation techniques for many-body systems, the so-called tensor network algorithms (e.g. PEPS, MERA), have been proposed in the last few years. These algorithms have been generalized to fermionic systems recently. We present a particularly simple formalism to account for the statistics of fermionic degrees of freedom in a tensor network. Benchmark results confirm the validity of this approach, and show that the computational cost of simulations does not depend a priori on the particle statistics, but on the amount of entanglement in the system. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P24.00012: Dynamical simulation of integrable and non-integrable models in the Heisenberg picture Dominik Muth, Razmik Unanyan, Michael Fleischhauer The numerical simulation of quantum many-body dynamics is typically limited by the linear growth of entanglement with time. Recently numerical studies have shown, however, that for 1D Bethe-integrable models the simulation of local operators in the Heisenberg picture can be efficient as the corresponding operator-space entanglement grows only logarithmically. Using the spin-1/2 XX chain as generic example of an integrable model that can be mapped to free particles, we here provide a simple explanation for this. We show furthermore that the same reduction of complexity applies to operators that have a high-temperature auto correlation function which decays slower than exponential, i.e., with a power law. This is amongst others the case for models where the Blombergen-De Gennes conjecture of high-temperature diffusive dynamics holds. Thus efficient simulability may already be implied by a single conservation law (like that of total magnetization), as we will illustrate numerically for the spin-1 XXZ model. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P24.00013: ABSTRACT WITHDRAWN |
Session P25: Superconductivity: Devices and Applications
Sponsoring Units: DCMPChair: Norman Bergren, National Institute of Standards and Technology
Room: D166
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P25.00001: Microwave amplifier based on an inline dc SQUID David Hover, Yung-Fu Chen, Leon Maher, Guilhem Ribeill, Shao-Jiang Zhu, Steve Sendelbach, Robert McDermott The dc SQUID can be used as a sensitive, low-noise microwave amplifier if the signal to be amplified is suitably coupled to the SQUID. We have designed and fabricated microwave amplifiers based on inline dc SQUIDs, where the SQUID loop is formed from a thin ($\sim$ 100 nm) dielectric layer separating the base and counterelectrode wiring traces. The SQUID is embedded in a microstrip transmission line resonator at a current anti-node, and signal current is injected directly into the SQUID loop. With this design we have achieved gain greater than 20 dB at a frequency of 8.5 GHz. We provide a theoretical analysis of amplifier noise temperature, bandwidth, and gain, and describe measurements of amplifier noise temperature. We discuss application of these devices to the readout of superconducting quantum circuits. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P25.00002: Spatial correlations of magnetic fluctuations in DC SQUIDS Steven Sendelbach, Umeshkumar Patel, Robert McDermott Recent experiments indicate that there is a high density of unpaired spins residing on the surfaces of superconducting thin films used to implement SQUIDs and superconducting qubits. Fluctuations of these spins give rise to low frequency flux noise and dephasing of the qubit state. Realization of phase and flux qubits with improved dephasing times will require a deeper understanding of the microscopic physics that governs fluctuations of the surface spins. Here we describe experiments to probe the spatial correlation of magnetic fluctuators in a SQUID circuit. The SQUID loop incorporates multiple current taps, enabling one to locally address magnetic fluctuators. Preliminary data reveal correlated fluctuations on a length scale of order 10 $\mu$m. We discuss implications for qubit dephasing. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P25.00003: Simulations of dependence of low frequency flux noise on SQUID geometry Steven Anton, Keenan Pepper, John Clarke, Ida Sognnaes It is generally accepted that the 1/f magnetic flux noise observed in dc SQUIDs and superconducting qubits originates in the random flipping of a uniform distribution of electron spins localized at the superconductor-insulator interface. Computer simulations and analytical calculations based on this model confirm the experimental result that the noise power at 1 Hz varies only slowly with SQUID dimensions. In particular, analytical calculations for a circular loop with radius R much greater than the loop linewidth W predict that the noise power scales as R/W. We present numerical computations that are valid for arbitrary geometry, including that of the square washer SQUID for which W approaches R. Making use of the reciprocity theorem, we solve the London equations numerically to find the current distribution in the superconductor, evaluate the Biot-Savart integral to find the corresponding magnetic field at any point and integrate over all spins, including those at the edges of the films, to find the total flux noise. We compare our results with our recent experimental measurements. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P25.00004: Dependence of low frequency flux noise on SQUID-washer dimensions J. Birenbaum, S.M. Anton, A.D. Fefferman, S.R. O'Kelley, J. Clarke, H-M Cho, G.C. Hilton, K.D. Irwin, F.C. Wellstood The 1/f spectral density of low frequency magnetic flux noise at 1 Hz in dc SQUIDs and qubits varies slowly with the dimensions of the superconducting loop, in reasonably good agreement with predictions. Previous measurements on SQUIDs fabricated from a variety of superconductors under different conditions and in a variety of geometries, however, showed that the slope of the power spectrum varied considerably. We report flux noise measurements on six resistively-shunted dc SQUIDs fabricated simultaneously on a single Si chip using a Nb-trilayer process. The noise spectra of all six devices were measured using a SQUID in a single cool-down of our dilution refrigerator. The linewidths of the SQUID loops were varied systematically by a factor of more than 30. The variation in noise power at 1 Hz was small compared with the variation in line width, while the slope varied significantly, from approximately -0.5 to -1. Furthermore, for a given SQUID, the slope depended on temperature. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P25.00005: A Novel System for Accurate Cryogenic S-Parameter Measurements Leonardo Ranzani, Lafe Spietz, Jose Aumentado In order to study microwave devices operating at cryogenic temperatures (4K and below), an accurate characterization of their full scattering parameters is needed. Simple response calibration using a single through standard is usually performed at cryogenic temperatures due to its simplicity, but it is inaccurate since it only determines 4 of the 10 unknowns present in a general two port network environment. In this talk we will discuss a fully automated through-reflect-line (TRL) calibration system suitable for accurately characterizing 2-port S parameters for devices such as SQUID amplifiers and other cryogenic microwave circuits. Data for some typical devices up to 8GHz will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P25.00006: Josephson Junction Circuits for Passive Non-Reciprocal Microwave Devices Lafe Spietz, Norm Bergren, Jose Aumentado We propose a method for using Josephson junction circuits to build non-reciprocal passive microwave components. We show that strong analogies can be made between the physics of ferrites used in traditional passive non-reciprocal microwave devices (that of the gyroscope) and certain classes of Josephson junction circuits. We describe a simple circuit which demonstrates these physical principles, and present both theoretical experimental results on this circuit. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P25.00007: Probing the origin of 1/f critical-current noise in nanoscale Al/AlOx/Al Josephson junctions Christopher Nugroho, Vladimir Orlyanchik, Allison Dove, Gustaf Olson, Zachary Yoscovits, James Eckstein, Dale Van Harlingen We present measurements of the low frequency noise in nanoscale Al/AlOx/Al Josephson junctions made by the shadow/angle evaporation technique. We investigate the differences in the nature of the charge trap fluctuations when the junction electrodes are in the normal state vs. in the superconducting state, as a test of some recent theoretical models. To do that, we compare the magnitude, temperature dependence, and magnetic field dependence of junction resistance fluctuations in the normal state above the Al transition temperature to that of the resistance and critical current fluctuations measured in the superconducting state. We also explore whether the observed fluctuators are thermally-activated or tunneling as a function of temperature. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P25.00008: Decoherence and energy shift in phase qubits due to nonequilibrium quasiparticles M. Lenander, R.C. Bialczak, E. Lucero, M. Mariantoni, A. O'Connell, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, T. Yamamoto, Y. Yin, J. Zhao, A.N. Cleland, J.M. Martinis Nonequilibrium quasiparticle excitations are thought to be an important source of decoherence in Josephson qubits. We present a model analogous to the Mattis-Bardeen theory wherein the effects of quasiparticles introduce a complex environmental impedance to the junction. The real part causes energy relaxation in the qubit while the imaginary part causes a frequency shift. We present experimental data comparing these effects while injecting nonequilibrium quasiparticles into the system. The theory is used to qualitatively check the injection process. Then by comparing the decay rate and frequency shift, we quantitatively verify the theory without the need to directly measure the quasiparticle density. Agreement between theory and experiment is observed to within experimental uncertainty, about 10\%. We examine infrared radiation as a source of nonequilibrium quasiparticles. Using these new tools, we hope to develop methods for improving qubit performance and to bound the contribution to energy decay from quasiparticles. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P25.00009: Effect of an Ohmic environment on an optimally controlled flux-biased phase qubit Amrit Poudel, Maxim Vavilov We analyze the effect of environment on the gate operation of flux-biased phase qubits. We employ the master equation for a reduced density matrix of the qubit system coupled to an Ohmic environment, described by the Caldeira-Leggett model. Numerically solving this equation, we evaluate the gate error as a function of gate time, temperature and environmental coupling strength for experimentally determined qubit parameters. Here we present the analysis for single-quadrature microwave (control) pulses as well as for two-quadrature pulses, which lower the gate error significantly for idealized systems in the absence of environment. Our results indicate that two-quadrature pulses with fixed and variable driving frequency have similar performance, which outweighs the performance of single-quadrature pulses, in the presence of environment. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P25.00010: Quasiparticle relaxation of superconducting qubits in the presence of flux Gianluigi Catelani, Jens Koch, Luigi Frunzio, Robert Schoelkopf, Michel Devoret, Leonid Glazman Quasiparticle tunneling across a Josephson junction sets a limit for the lifetime of a superconducting qubit state. We develop a general theory of the corresponding decay rate in a qubit controlled by a magnetic flux. The flux affects quasiparticles tunneling amplitudes, thus making the decay rate flux-dependent. The theory is applicable for an arbitrary quasiparticle distribution. It provides estimates for the rates in practically important quantum circuits and also offers a new way of measuring the phase-dependent admittance of a Josephson junction. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P25.00011: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P25.00012: High count-rate superconducting transition edge sensors for near-IR single photon detection Faustin Carter, Daniel Santavicca, Luigi Frunzio, Anthony Annunziata, Daniel Prober Detection of individual near-IR photons with GHz count rates, good timing resolution, and high quantum efficiency is important in a number of applications. These include quantum key distribution, single-photon classical communication, and CMOS imaging for defect analysis. We propose a nano-scale superconducting niobium transition edge sensor (TES). The extremely small detector volume allows for single-photon sensitivity at 4 K, with a much faster response time (nsec) than conventional TES detectors operating below 0.4 K. Efficient photon coupling is achieved with a resonant near-IR planar antenna. The proposed device is intrinsically photon number resolving, unlike a niobium-nitride nanowire detector or an avalanche photodiode. We present preliminary results for device performance. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P25.00013: Nano-structuring on the surface of high Tc-superconductors by STM\&AFM Kazuto Hirata, Tadashi Machida, Shuuichi Ooi, Minoru Tachiki, Takashi Mochiku We demonstrate local insulation on a cleaved surface of Bi- 2212 single crystals using a local anodic oxidation by a atomic force microscope (AFM) and a scanning tunneling microscope (STM) for the first time. We have investigated the electrical properties of the modified region by using an STM-assisted near- field microwave microscope. From the experimental observations, we conclude that the modified region becomes an insulator with an associated dielectric loss locally. Varying the applied bias- voltage and the scanning speed can control the protrusion height and the line width of the regions. This provides a potential technique for reproducibly fabricating high temperature superconducting devices with stable electronic characteristics. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P25.00014: An Integrated Balanced Superconductor-Insulator-Superconductor Heterodyne Mixer on a Silicon Membrane M.P. Westig, K. Jacobs, M. Schultz, M. Justen, J. Stutzki, C.E. Honingh We have designed and fabricated a 380-520~GHz integrated balanced Nb\textbackslash Al\textbackslash AlOx\textbackslash Nb superconductor-insulator-superconductor (SIS) heterodyne waveguide mixer for submillimeter astrophysics. The response of the mixer measured with a Fourier transform spectrometer shows excellent agreement with the design. The novelty of our device is that we deposit the complete superconducting mixer circuit (tapered slotline antennas, hybrid coupler, MIM capacitors, SIS junctions, tuning circuits and blocking filters) on top of a 9~$\mu$m silicon membrane. The membrane is held suspended in a waveguide by 2.5~$\mu$m thick gold plated beamleads. We will show that silicon membrane technology and a thorough device design render the integration of SIS devices with larger circuits feasible. This is an important step towards large arrays of mixers. When using an appropriate superconductor technology, these devices are scalable to higher frequencies. We will present the design, fabrication results and first results of heterodyne measurements. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P25.00015: c-Axis current flow arises in helically wound wire Ying Jia, U. Welp, G.W. Crabtree, W.K. Kwok, M. Rupich, S. Fleshler, A.P. Malozemoff c-Axis critical current density of second-generation YBCO wires in HTS cables due to the interaction of current flow with the induced magnetic field. However, the importance of c-axis critical current density ($J_{c}^{c})$ on the overall transport critical current is not clearly understood. We measured the temperature and field dependence of $J_{c}^{c}$ using a mesa structure patterned into the YBCO layer of 2$^{nd}$-generation HTS tapes. We found, $J_{c}^{c}$ --values of $\sim $ 4 kA/cm$^{2}$ at 77 K in self-field, corresponding to an unexpectedly high anisotropy of the critical current density $\gamma ={J_c^{ab} } \mathord{\left/ {\vphantom {{J_c^{ab} } {J_c^c }}} \right. \kern-\nulldelimiterspace} {J_c^c }=$ 500$\sim $600. We also investigated the effect of pinning microstructures on $J_{c}^{c}$ and $\gamma $. Our result shows a direct correlation of $J_{c}^{c}$ (77 K, sf) and $\gamma $ to the density of stacking faults. An estimation reveals that the fraction of tape width associated with $c$-axis current flow grows linearly from 5{\%} to 20{\%} with increasing $\gamma $ for a typical geometry and could affect the performance of power transmission in HTS cables. [Preview Abstract] |
Session P26: Focus Session: Iron Based Superconductors -- Spin Dynamics
Sponsoring Units: DMP DCOMPChair: Dmitri Argyriou, European Spallation Source
Room: D162/164
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P26.00001: High Energy Spin Excitations in Optimal doped Superconducting BaFe$_{1.9}$ Ni$_{0.1}$As$_{2}$ Mengshu Liu, Huiqian Luo, Russell Ewings, Tatiana Guidi, Pengcheng Dai The recent discovered iron pnictide superconductor which shows a transition temperature up to 50K has drawn much attention in the community. There are indications that superconductivity in the iron arsenides family may be driven by a magnetic pairing mechanism, the nature of which remains poorly understood. In our recent inelastic neutron scattering experiment on optimal doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ sample, spin excitation data are collected throughout the Brillouin zone and up to energy transfer of 400meV which was not probed before. We found that the scattering persists as high as 300 meV in the superconducting sample, and a spectra transfer similar to the parent compound with a spin nematicity was also observed [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P26.00002: Spin-phonon coupling in iron arsenide superconductors Jennifer Niedziela, Daniel Parshall, Konstantin Lokshin, Athena Sefat, Ahmet Alatas, Takeshi Egami In this work we present the results of an inelastic x-ray scattering experiment measuring the softening of the TA[110] phonon in BaFe$_2$As$_2$ as a function of temperature. Cooling through the structural transition temperature yields a softening of the phonon energy of the transverse acoustic mode nearly 1 meV from the value at room temperature at $\mathbf{q}=0.1$. This phonon controls the structural phase transition, changing the symmetry from tetragonal to orthorhombic at the same temperature as the transition to long range antiferromanetic order. Even though the lattice distortion is minor, the anisotropy in the magnetic exchange constants is very large. We posit that this phonon mode couples to the orbital moment, and softening of this mode is required for the onset of long range antiferromagnetic ordering and the dramatic change in the exchange constants. This observation is suggestive that a mechanism of spin-phonon coupling is present in the pnictide systems, and is a possible contributor to the superconducting mechanism. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P26.00003: Collapse of the spin resonance spectral weight in overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Ray Osborn, Stephan Rosenkranz, John-Paul Castellan, Eugene Goremychkin, Duck-Young Chung, Helmut Claus, Mercouri Kanatzidis, Tatiana Guidi We report inelastic neutron scattering measurements of magnetic excitations in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ over a broad range of electron band filling within the superconducting phase. In an itinerant model, these excitations are resonantly enhanced when the superconducting energy gap changes sign on different parts of the electron Fermi surface. They are therefore sensitive both to the superconducting gap symmetry and to the Fermi surface geometry. Our results show that, in addition to becoming incommensurate because of the growing mismatch in the hole and electron Fermi surface volumes, the resonant spectral weight decreases proportionally to the resonance binding energy, vanishing at $x \sim 0.72$. A tight-binding model including $s_{\pm}$-symmetry pairing is able to reproduce these observations confirming that the resonance arises from the pairing of band electrons. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P26.00004: Spin Correlations in Superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Invited Speaker: Elastic and inelastic neutron scattering are used to study the spin correlations in superconducting Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. Measurements on the antiferromagnetically ordered parents of this system [1] show a strongly anisotropic spin-wave velocity. Here we report [2] measurements of the magnetic excitations in a the superconducting composition, $x$=0.065, up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wavevector- integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed. I will discuss the interpretation of our results in terms of the electronic structure and possible fluctuating nematic order.\\[4pt] [1] S. O. Diallo, \textit{et al.} Phys. Rev. Lett. \textbf{102}, 187206 (2009); J. Zhao , \textit{et al.} Nat. Phys. \textbf{5}, 555 (2009).\\[0pt] [2] Lester \textit{et al.} Phys. Rev. B \textbf{81}, 064505 (2010); unpublished. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P26.00005: Multiband effect on the magnetic resonance spectrum of pnictide superconductors Tanmoy Das, A.V. Balatsky The magnetic resonance behavior which is directly probed by the inelastic neutron scattering (INS) spectroscopy gives valuable information about the pairing mechanism of the unconventional superconductors. In high-$T_c$ cuprate superconductors, INS exhibits a clear signature of a magnetic resonance mode in addition to its characteristic dispersive feature (known as ``hour-glass'' behavior) which are enhanced dramatically below $T_c$ and the mode energy scales universally with the SC gap amplitude. In a multiband unconventional superconductors, the situation is more complex due to the presence of multi-orbital band structure, multiple-SC gaps as well as possibilities of having multiple pairing symmetries. We calculate magnetic susceptibility to show how does the magnetic resonance mode and its dispersion evolve both in energy as well as in momentum as a function of doping in both electron and holed doped pnictide superconductors. The inputs in our calculations are the Fermi surface information from ARPES or LDA and experimental values of superconducting gaps. We find that the magnetic resonance behavior is dramatically different in pnictide than in cuprates. The effects of multiple orbitals, gaps and different pairing symmetry play an important role. We argue that doping dependence of the resonance spectra can be understood from the topological change of the Fermi surface and the gap magnitudes, in good accord with experiments. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P26.00006: Spin-orbit coupling in Fe-based superconductors M.M. Korshunov, I. Eremin, P.J. Hirschfeld The recently discovered iron-based superconductors have attracted considerable attention mainly for their unconventional pairing state. In connection with the determination of pairing symmetry, the resonance peak observed in neutron scattering experiments [1] agrees well with predicted results for the extended s-wave ($s_\pm$) gap symmetry [2]. However, recent neutron measurements shows that there is anisotropy in the spin resonance [3]. In particular, $S^zS^z$ component is different from $S^+S^-$ component of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase without long-range order can occur due to spin-orbit (SO) coupling. We study the role of the SO coupling in the multiorbital model for Fe-pnictides, and discuss how it influences spin resonance feature and the relation to SC pairing. \\[4pt] [1]. A.D. Christianson \textit{et al.}, Nature \textbf{456}, 930 (2008). [2]. M.M. Korshunov and I. Eremin, Phys. Rev. B \textbf{78}, 140509(R) (2008); T.A. Maier and D.J. Scalapino, \textit{ibid}, 020514(R) (2008). [3]. O.J. Lipscombe \textit{et al.}, Phys. Rev. B \textbf{82}, 064515 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P26.00007: A LDA+DMFT+Vertex function study of dynamical magnetic susceptibility in iron based superconductors Hyowon Park, Kristjan Haule, Gabriel Kotliar We developed a method for computing dynamical magnetic susceptibility in complex correlated materials based on LDA+DMFT+Vertex function calculation. The dressed Greens function was obtained from the charge self-consistent LDA+DMFT calculation and the local Vertex function was computed from the quantum impurity model using a CTQMC impurity solver. We applied this method to compute the normal state magnetic susceptibility in iron pnictides and iron chalcogenides. Our result shows good agreement with inelastic neutron scattering data. At a low energy, the dynamical structure factor S(Q,w) is peaked at ($\pi$,0) momentum in BaFe$_{2}$As$_{2}$ and at ($\pi$/2,$\pi$/2) in FeTe, as expected for the distinct ordering of these compounds. At higher energy, the peak positions shifts to the ($\pi$,$\pi$) wave vector, in agreement with recent neutron experiments. We argue that this ($\pi$,$\pi$) magnetic response at high energy and the full spin dispersion above Neel temperature is captured by our realistic band structure method, LDA+DMFT. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P26.00008: Neutron scattering study of spin fluctuations on hole-overdoped KFe$_2$As$_2$ C.H. Lee, K. Kihou, A. Iyo, H. Eisaki, H. K.-Furukawa, H. Usui, K. Kuroki, T. Saito, H. Fukazawa, Y. Kohori, K. Yamada Spin fluctuations in Fe-based superconductors have attracted great attention since they can be a key factor of the formation of superconducting states. The inelastic neutron scattering technique is a powerful method to examine spin fluctuations, whereas measurements using a single crystal were restricted to Fe(Te,Se) or electron-doped AFe$_2$As$_2$ (A=Ba, Ca, or Sr) due to difficulty of growing a large single crystal. To overcome this problem, we have improved growth procedure and succeeded to grow single crystals of heavily hole-overdoped superconducting KFe$_2$As$_2$ ($T_c$ = 3.4 K). It was believed that no spin fluctuation can be observed in KFe$_2$As$_2$, since the nesting of the Fermi surface disappears. To confirm the hypothesis, we have studied spin fluctuations of KFe$_2$As$_2$ by neutron scattering using single crystals at JRR-3 reactor of JAERI in Tokai. As results, a well-defined low-energy incommensurate spin fluctuation has been observed at ($\pi(1\pm2\delta$),0) with $\delta$ = 0.16. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)$_2$As$_2$ at high energies. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped Fe-based superconductors, or a new type of spin fluctuation emerges by heavily hole doping. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P26.00009: Neutron Scattering Experiment on Magnetic Field Effect in Under-doped Superconducting BaFe$_{1.915}$Ni$_{0.085}$As$_{2}$ Miaoyin Wang, Pengcheng Dai, Meng Wang, Huiqian Luo, Jeffrey Lynn, Sung Chang, Songxue Chi, Deepak Singh, Jose Rodriguez In under-doped BaFe$_{2-X}$(Ni,Co)$_{X}$As$_{2}$, superconductivity coexist with the anti-ferromagnetic (AFM) order. By applying a $\sim $10 Tesla magnetic field parallel to a-b plan, we performed a series of elastic and inelastic neutron scattering measurement on BT-7 instrument in NCNR, NIST. We measured how magnetic bragg-peaks and spin excitation in BaFe$_{1.915}$Ni$_{0.085}$As$_{2}$ will change upon the change of the field. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P26.00010: Spin excitations as in hole-doped Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ superconductor Chenglin Zhang, Meng Wang, Miaoying Wang, Jun Zhao, Marty Karol, Mark Lumsden, Songxue Chi, Sung Chang, Jeffrey Lynn, Huiqian Luo, Tao Xiang, Jiangping Hu, Pengcheng Dai We used inelastic neutron scattering to study the optimally doped Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ (Tc=38K). In contrast to electron doped counterpart, we found that resonance is almost none-L dependence as shown in Fig.1 (b), but the spin gaps are. It is gaped along (0.5,0.5,0), however essentially gapless along (0.5,0.5,1). Meanwhile, the spin correlation is strongly temperature dependence which has not been observed in electron-doped 122 materials at all. The above findings clearly suggest that hole-doped region is indeed different from electron-doped region. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P26.00011: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P26.00012: Finite temperature spin dynamics of a square lattice $J_1-J_2$ antiferromagnet and its implications for iron arsenides Elihu Abrahams, Pallab Goswami, Rong Yu, Qimiao Si Motivated by recent inelastic neutron scattering measurements in the paramagnetic phase of iron arsenides, we have studied the finite temperature spin dynamics of a square lattice $J_1- J_2$ antiferromagnet in the parameter regime that gives rise to a collinear $(\pi, 0)$ ground state at zero temperature. We have calculated the dynamical structure factor $S(\mathbf{q}, \omega)$ in the paramagnetic state at finite temperatures using a modified spin wave theory. We have shown that short range antiferromagnetic correlations below the mean-field Ising transition temperature give rise to elliptic features for $S (\mathbf{q}, \omega)$ in momentum space. Employing an effective nonlinear sigma model analysis for the low energy and wave vector limit, we also account for fermion damping and circumvent the shortcoming of modified spin wave theory. Finally, considering a matrix $J_1-J_2$ model, we point out the connection between the Ising transition and the putative orbital ordering in iron arsenides. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P26.00013: Neutron and ARPES constraints on the couplings of the multiorbital Hubbard model for the iron pnictides Qinlong Luo, Adriana Moreo, Elbio Dagotto, George Martins, Dao-Xin Yao, Maria Daghofer, Rong Yu The results of neutron-scattering and angle-resolved photoemission experiments for the Fe-pnictide parent compounds are shown to impose severe constraints on the range of values that can be considered ``realistic'' for the intraorbital Hubbard repulsion U and Hund coupling J in multiorbital Hubbard models treated in the mean-field approximation. Phase diagrams for undoped three- and five-orbital models are discussed, and the physically realistic regime of couplings is highlighted [1]. \\[4pt] [1] Q. Luo {\it et al.}, Phys. Rev. B {\bf 82}, 104508 (2010), and references therein. [Preview Abstract] |
Session P27: Focus Session: Semiconductor Qubits - Spin Readout, Backaction, and Valley Physics in Silicon
Sponsoring Units: GQIChair: Irfan Siddiqi, University of California, Berkeley
Room: C155
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P27.00001: Development of a Si/SiO$_2$–based double quantum dot charge qubit with dispersive microwave readout M.G. House, E. Henry, A. Schmidt, O. Naaman, I. Siddiqi, H. Pan, M. Xiao, H.W. Jiang Coupling of a high-Q microwave resonator to superconducting qubits has been successfully used to prepare, manipulate, and read out the state of a single qubit, and to mediate interactions between qubits. Our work is geared toward implementing this architecture in a semiconductor qubit. We present the design and development of a lateral quantum dot in which a superconducting microwave resonator is capacitively coupled to a double dot charge qubit. The device is a silicon MOSFET structure with a global gate which is used to accumulate electrons at a Si/SiO$_2$ interface. A set of smaller gates are used to deplete these electrons to define a double quantum dot and adjacent conduction channels. Two of these depletion gates connect directly to the conductors of a 6 GHz co-planar stripline resonator. We present measurements of transport and conventional charge sensing used to characterize the double quantum dot, and demonstrate that it is possible to reach the few-electron regime in this system. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P27.00002: Dispersive microwave readout of a double quantum dot charge qubit in silicon Edward Henry, Andrew Schmidt, Mathew House, Ofer Naaman, H. Pan, Ming Xiao, Hong-wen Jiang, Irfan Siddiqi Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in many different architectures in solid state and atomic physics. The electronic states of a semiconductor quantum dot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of semiconductor qubit by coupling to a superconducting resonant circuit. We report on microwave measurements of a lateral quantum dot, realized using a silicon MOSFET structure, where the charge degree of freedom is capacitively coupled to a shorted quarter wave 6 GHz resonator. We characterize the sensitivity of this charge detection scheme and its implications for qubit readout fidelity. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P27.00003: Radio Frequency Single Electron Transistors on Si/SiGe Mingyun Yuan, Zhen Yang, A.J. Rimberg, M.A. Eriksson, D.E. Savage Superconducting single electron transistors (S-SETs) are ideal for charge state readout due to their high sensitivity and low back-action. Upon successful formation of quantum dots(QDs) on Si/SiGe, aluminum S-SETs are added in the vicinity of the QDs. Coupling of the S-SET to the QD is confirmed by using the S-SET to perform sensing of the QD charge state at 0.3 K. We have formed a matching network for an SET with an off-chip inductor. The reflection coefficient of the radio frequency(RF) signal is shown to be modulated by the SET resistance. Efforts to develop an on-chip matching network and perform charge sensing with the RF-SETs are in progress. Recent experimental results will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P27.00004: Two-detector two-qubit correlated continuous measurements and their implications for quantum computing Rusko Ruskov, Charles Tahan We calculate the full counting statistics for a system of two interacting qubits which are simultaneously measured by weakly coupled linear detectors. Two approaches are considered based on rate equations for the full system-detectors density matrix and on quantum filtering equations. Implications for the assessment of quantumness in physical devices based on charge qubits are considered. In addition we consider applications of such systems to practical quantum computing in silicon and/or GaAs quantum dots. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P27.00005: Backaction due to Resonant Phonon Absorption in Quantum Dots Measured by a Quantum Point Contact Carolyn Young, Aashish Clerk Recent experiments have observed unexplained periodic resonances in the charging diagrams of both double [1] and triple [2] quantum dots (DQDs and TQDs). These resonances correspond to the generation of inelastic transitions, driven by energy transfer from a biased quantum point contact (QPC) charge detector used for measurement. In this talk, we present theoretical results describing how quantum backaction due to hot phonons, generated by the out-of-equilibrium QPC, can lead to excited state occupation under certain ``blocking'' conditions that result in slow ground state filling. We propose that recent experiments can be understood in terms of resonant phonon absorption in DQDs and TQDs; a process complementary to resonant phonon emission [3]. Our results shed light on an important contribution to the backaction of the QPC readout scheme widely used for QD-based quantum computation. \\[4pt] [1] D. Harbusch et al., Phys. Rev. Lett., 104, 196801 (2010). \\[0pt] [2] L. Gaudreau et al., App. Phys. Lett., 99, 193101 (2009). \\[0pt] [3] U. Gasser et al., Phys. Rev. B, 79, 035303 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P27.00006: Measurement fidelity in the presence of coherent dynamics or dissipation Jian-Qiang You, S. Ashhab, Franco Nori We analyze the problem of a charge qubit probed by a quantum point contact when the measurement is concurrent with Hamiltonian-induced coherent dynamics or dissipation. This additional dynamics changes the state of the qubit before the measurement is completed. As a result, the measurement fidelity is reduced. We calculate the reduction in measurement fidelity in these cases. References: S. Ashhab, J. Q. You, and F. Nori, New J. Phys. 11, 083017 (2009); Phys. Scr. T137, 014005 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P27.00007: Probing coherent tunneling in semiconductor quantum dots using electromechanical backaction Jamie Gardner, Aashish Clerk Self-assembled quantum dots have been studied intensely because of their possible applications to quantum information processing. While such dots are difficult to characterize using direct electrical transport measurements, it has recently been shown both theoretically [1] and experimentally [2] that a capacitively coupled AFM cantilever can serve as a sensitive probe of dot charge dynamics and electronic level structure. This sensitivity is based on the fact that the dot, which is tunnel-coupled to electrons in a reservoir, acts as a dissipative bath for the cantilever. Here, we extend previous theoretical work to describe an AFM cantilever coupled to a double quantum dot. Unlike a single-dot, the double-dot system exhibits both incoherent tunneling to the leads and coherent tunneling between the dots. We find that the cantilever's motion is affected by both kinds of tunneling and can yield significant information even in regimes where the total double-dot charge does not fluctuate. Cantilever dynamics can also be used to learn about the strength of dephasing processes in the double-dot. After presenting the theoretical approach to this problem, we will discuss the results in the context of current experimental efforts using InAs dots. These effects should also be accessible in a variety of other quantum dot setups. [1] S. D. Bennett, et al., Phys. Rev. Lett. \textbf{104}, 017203 (2010). [2] L. Cockins, et al., Proc. Nat. Acad. Sci. \textbf{107}, 9496 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P27.00008: Few-electron states in SiGe double quantum dot structures with non-planar interfaces A.A. Kiselev, R.S. Ross, M.F. Gyure Valley-orbit effects of planar, non-planar, and imperfect heterointerfaces (both on the intra- and inter-dot scale) are directly captured in numerical simulations and analyzed theoretically for electrostatically defined accumulation mode (001) SiGe multi-dot structures. Our modeling is facilitated by explicitly allowing for an arbitrary and spatially inhomogeneous stacking of heterolayers in the active area of the device. Here we focus on results obtained for a double quantum dot (DQD) system, establishing the detailed structure of few-electron states, and, for two electrons, their spin- and valley-selective dynamics when the system is driven by pulse-modulating dot gate potentials. We identify valley-related avoided crossings and evaluate their strength affecting adiabaticity of applied bias sweeps. We consider a number of experimentally relevant scenarios stemming from (i) macroscopic interface imperfections, e.g., interface steps, and (ii) randomness of the substitutional solid solution in the SiGe barrier layers. Our findings are critically compared with results available for single valley III-V DQDs. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P27.00009: Extended interface states enhance valley splitting in Si/SiO2 Andre Saraiva, Belita Koiller, Mark Friesen Interface disorder and its effect on valley degeneracy in the conduction band present an important theoretical challenge for operating spin qubit in silicon. Here, we demonstrate and investigate a counterintuitive effect occurring at Si/SiO2 interfaces. By applying tight binding methods, we show that intrinsic interface states can hybridize with conventional valley states to produce an anomalously large ground state energy gap. Such hybridization effects have not previously been explored in detail for valley splitting. We find that the splitting can be enhanced by disorder in the chemical bonds at the interface, in agreement with recent experiments. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P27.00010: Interface-mediated intervalley coupling in Si Belita Koiller, A.L. Saraiva, M.J. Calderon, Xuedong Hu, S. Das Sarma The conduction band degeneracy in Si is detrimental to spin qubits, for which a nondegenerate ground orbital state is desirable. The Si valley degeneracy is reduced to 2 near an interface with an insulator, and it may be lifted by the spatially abrupt change in the crystal potential. Basic physical mechanisms for Si/barrier mediated valley coupling in different situations are addressed here. Theoretical studies of the interface-induced valley splitting in Si are presented. Abrupt and smooth interface profiles are considered, and the full plane wave expansions of the Bloch functions at the conduction band minima are included. Simple criteria are suggested for optimal fabrication parameters affecting the valley splitting, emphasizing the relevance of different interface-related properties. Refs: A.L.Saraiva et al, PRB 80, 081305 R (2009); arXiv:1006.3338 [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P27.00011: Atomistic simulations of multi-valley silicon double quantum dots in the presence of disorder in the few electron regime Rajib Rahman, Erik Nielsen, Richard Muller, Malcolm Carroll The singlet-triplet based silicon double quantum dot (DQD) is a promising system for implementing a long-lived and controllable quantum bit. The multiple valleys present in silicon, however, may complicate the operation of such a qubit if the valley splitting is small. The valley splitting is affected by a large number of factors including interface roughness, lattice miscuts, electric and magnetic fields, barrier material, and alloy disorder. We employ an atomistic tight-binding (TB) method and a full configuration interaction (CI) to investigate few electron states of a multi-valley Si DQD. This unprecedented approach involving few million atoms allows us to investigate the role of atomic scale disorder (i.e., random alloy effects or interface roughness) on the energy levels and spin configurations of many electron DQDs. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P27.00012: Engineering anisotropic exchange interactions between quantum dot spin qubits Yun-Pil Shim, Mark Friesen We present a method for engineering anisotropic exchange interactions between quantum dot spin qubits using a Heisenberg antiferromagnetic spin chain as a spin bus. An external magnetic field is applied to create XXZ interactions between spin qubits that are weakly connected to a spin bus whose ground state is non-degenerate. We analyze the dependence of the anisotropy of the effective interaction on the external field and on the length of the spin bus. We show that the tunable XXZ interaction mediated by the spin bus can be used to generate multi-qubit entanglement and to efficiently implement universal gates based on encoded qubits. We also show that the operation of the spin bus is qualitatively different when the spin bus is near one of its magnetic field-induced quantum phase transitions. In this case, the qubits interact with a bus pseudo-spin and the resulting entanglement between pairs of qubits is enhanced. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P27.00013: Asymmetric Quantum Pd Films for Enhanced Hydrogen Separation Guangfen Wu, Wenguang Zhu, Jinlan Wang, Zhenyu Zhang Based on density functional theory calculations and numerical simulations, we have investigated the permeation of H2 through ultra-thin Pd quantum films. The H2 flux can be highly increased by the elevation of the chemisorption-well on the permeate side without significantly blocking the subsurface-surface penetration. We find that Cu-coated asymmetric Pd quantum films (with the Cu monolayer on the permeate side) will enhance the capability for H2 separation: the recombination barrier for H is reduced from 1.36 to 0.79 eV, while the subsurface-surface penetration barrier is only slightly increased from 0.04 to 0.10 eV. Numerical simulations show enhanced H2 flux by 5 orders of magnitude as an upper-limit for asymmetric Pd films over symmetric ones under similar conditions. [Preview Abstract] |
Session P28: Carbon Nanotubes: Optical Properties
Sponsoring Units: DCMPChair: Paola Barbara, Georgetown University
Room: C156
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P28.00001: Empirical Study of the $\pi $ electron Plasmon Energy Dependence on the Bundling/De-bundling Effect in Single Wall Carbon Nanotubes Kiran Lingam, Ramakrishna Podila, Pengyu Chen, Codruta Loebick, Nan Li, Lisa Prefferle, Apparao Rao Many researchers have done detailed studies on optical, thermal and electronic properties of SWNTs. But, very few studies have been done on sub-nanometer SWNTs. Here we studied collective electron excitations in this quasi 1D system. At high excitation energies, broad absorption peak is observed which is attributed to the $\pi $ plasmon (5-7 eV). We used UV-Vis NIR spectroscopy to determine the energy of the $\pi $ plasmons in sub nanometer diameter SWNTs (0.4 nm to 0.9 nm). SWNTs form bundles due to van der Waal forces and this bundling influences their electronic structure. It is known that SWNTs wrapped with a surfactant can be isolated with long centrifugation. The hydrodynamic sizes of the dispersed SWNTs at different centrifugation times were determined by using the Dynamic Light Scattering technique. Systematic studies have been done on the dependence of the $\pi $ plasmon energy on the nanotube bundle diameter. The energy of the $\pi $ plasmon was found to vary with the bundle diameter and the energy to be given by the relation E= (-0.022 eV)*ln(d/1 nm)+5.34 eV. We have done similar studies on HiPCo and Carbolex SWNTs and the empirical relation obtained is consistent with the results above. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P28.00002: Simultaneous measurement of length, concentration and brightness of single-walled carbon nanotubes with fluorescence correlation spectroscopy Denis Pristinski, Constantine Khripin, Xiaomin Tu, Ming Zheng We report on the application of fluorescence correlation spectroscopy (FCS) to simultaneously measure the brightness, concentration, and length of single-walled carbon nanotubes (SWCNTs). The technique relies on the intrinsic bandgap luminescence of (6,5) chirality semiconducting SWCNTs in the near infra-red (NIR) range and does not require sample labeling. The nanotubes used in this study have been dispersed in solution of single stranded DNA and length fractionated via size exclusion chromatography. The SWCNT length measured by FCS was in excellent agreement with more traditional techniques - polarized dynamic light scattering (DLS) and atomic force microscopy (AFM). The apparent nanotube brightness is shown to grow linearly with the mean nanotube length, having a zero intensity cut-off at 110 nm, implying an exciton diffusion length of 55 nm for SWCNTs dispersed in sodium deoxycholate. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P28.00003: Absolute Rayleigh Intensity and Uniform Optical Conductivity in Carbon Nanotubes Lihong Herman, Daniel Joh, Jesse Kinder, Sang-Yong Ju, Michael Segal, Jeffreys Johnson, Garnet Chan, Jiwoong Park We used a novel on-chip Rayleigh imaging technique to measure the absolute intensity of Rayleigh scattering of single-walled carbon nanotubes. The spatial distribution of the radiation scattered by the nanotubes is determined by their shape, but the intensity and spectrum of the scattered radiation are determined by exciton dynamics, quantum-dot-like optical resonances and other intrinsic properties. Moreover, the nanotubes display a uniform peak optical conductivity $\sim $8 e\^{}2/ h, which we derive using an exciton model, suggesting universal behaviour similar to that observed in nanotube conductance. We further demonstrate a radiative coupling between two distant nanotubes, with potential applications in metamaterials and optical antennae. This is in contrast to the optical properties of metal nanostructures and show that nanotubes can form ideal optical wires. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P28.00004: Dielectric screening dependence of excitonic transition energies in single-wall carbon nanotubes Paulo Araujo, Mildred Dresselhaus, Ado Jorio, Kentaro Sato, Ahmad Nugraha, Richiiro Saito The measured optical transition energies E$_{ii}$ of single-wall carbon nanotubes are compared with bright exciton energy calculations. The E$_{ii}$ differences between experiment and theory are minimized by considering first, a diameter/chiral angle-dependent dielectric constant and second, a diameter/exciton size-dependent dielectric constant (k). In our description, k is composed of the screening contributions from the tube, represented by k$_{tube,}$ and from the environment, represented by k$_{env}$. We discuss the main aspects of each approach and show that in the first case, different k dependencies are obtained for (E$^{S}_{11}$, E$^{S}_{22}$, E$^{M}_{11})$ relative to (E$^{S}_{33}$, E$^{S}_{44})$ which is understood as follows: A changing environment changes the k diameter dependence for (E$^{S}_{11}$, E$^{S}_{22}$, E$^{M}_{11})$, but for (E$^{S}_{33}$, E$^{S}_{44})$ the environmental effects are minimal. We show that in order to achieve a single dependence for all E$_{ii}$, the exciton's size should be taken into account, as considered in the second approach. The resulting calculated exciton energies reproduce experimental E$_{ii}$ values within $\vert $50$\vert $ meV for a diameter range (0.7$<$ dt $<$3.8 nm) and 1.2 $<$ Eii $<$2.7 eV, thus providing a theoretical justification for E$_{ii}$ and important insights into the dielectric screening in one-dimensional structures. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P28.00005: Temperature-Dependent Maximum Density of 1D Excitons in Carbon Nanotubes Thomas Searles, Ian Walsh, Takayuki Nosaka, William Rice, Junichiro Kono Previous studies have shown that an upper limit exists on the density of 1D excitons in single-walled carbon nanotubes (SWNTs) due to very efficient exciton-exciton annihilation (EEA). A recent theoretical study based on a dark-bright two-band exciton model predicts that there is a temperature at which the achievable exciton density will be maximized, surpassing the room-temperature upper limit. Therefore, we performed temperature-dependent (300 K to 11 K) photoluminescence (PL) on HiPco SWNTs embedded in an i-carrageenan matrix under high resonant excitation. To achieve high densities, we used pump fluences up to $\sim $ 10$^{14}$ photons/cm$^{2}$, utilizing intense fs pulses from a wavelength-tunable optical parametric amplifier. We found that for each temperature the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Below that critical temperature, the PL intensity decreases with decreasing temperature. Within the framework of diffusion-limited EEA, we successfully estimated the upper limit of the density of 1D excitons in SWNTs as a function of temperature and chirality [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P28.00006: Experimental Kataura Plot in Single-walled Carbon Nanotubes Kaihui Liu, Fajun Xiao, Rodrigo Capaz, Jack Deslippe, Wenlong Wang, Shaul Aloni, Steven Louie, Enge Wang, Feng Wang Single-walled carbon nanotubes (SWNTs) comprise a family of more than 400 structures characterized by different chiral indices n-m, each having a distinct electronic structure that can be either metallic or semiconducting. An outstanding question is how the physical properties, such as optical transitions, vary with the exact nanotube structures. By combining TEM diffraction and Rayleigh scattering spectroscopy on the same individual nanotubes, we determine independently the chiral indices and optical transitions of over 170 single-walled nanotubes. These data permit us to create an experimental Kataura plot for single-walled carbon nanotubes. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P28.00007: Optical Properties of Empty and Water-Filled Single-Wall Carbon Nanotubes J.R. Simpson, J.A. Fagan, J.Y. Huh, A.R. Hight Walker, J.L. Blackburn, B.A. Larsen, J. Holt The necessity for separation of single-wall carbon nanotube (SWCNT) populations to achieve desired properties presents a major technical barrier for the development of SWCNT-based applications, and has been the focus of significant academic and industrial research. Recent advances include the separation of SWCNT populations by diameter through buoyancy differences. Here we report on the optical spectroscopic properties of large diameter SWCNTs synthesized by laser ablation and electric arc methods and then separated by centrifugation to produced isolated bands of empty and water-filled nanotubes. This separation is consistent across multiple nanotube populations dispersed from different source material. Optical absorption, near-infrared fluorescence, and Raman spectroscopic measurements of the resulting empty and filled fractions reveal that water filling leads to systematic changes in the optical properties. Specifically, the peak locations in absorbance and fluorescence display red-shifts with the presence of water in the nanotube cavity and a hardening of the Raman radial breathing modes. The presence of water in the SWCNT interior is found to facilitate the subsequent separation into sub-populations of metallic and semiconducting SWCNTs. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P28.00008: Rapid widefield Raman imaging of individual carbon nanotubes Robin Havener, Sang-Yong Ju, Michael Segal, Lihong Herman, Jiwoong Park Raman spectroscopy is a powerful tool for characterizing carbon nanotubes. Confocal micro-Raman imaging can provide detailed spatial and spectral information about individual nanotubes, but this technique is often time-consuming. We present a widefield Raman microscope capable of rapid and large-area imaging of carbon nanotube samples. Thanks to a widefield excitation geometry, a high-power excitation laser (3W in our experiment) can be used without causing thermal damage to nanotubes, which dramatically shortens image acquisition time ($\sim$20sec for G- band for a 60$\mu$m field of view). With a custom-built tunable bandpass filter, our widefield Raman images let us quantitatively compare the D, G and 2D-band intensities of many nanotubes with different known resonant energies, diameters, and metallicities, while providing the Raman scattering cross-section length for individual nanotubes. This technique allows Raman-based spatially resolved investigation of dynamic processes in nanotubes for the first time, which we demonstrate by real-time imaging of the oxidation of nanotubes at high temperatures. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P28.00009: Absence of Broad G$^{-}$ Feature in Resonant Raman Spectra of Armchair Carbon Nanotubes E.H. Haroz, W.D. Rice, J. Kono, J.G. Duque, C.G. Densmore, S.K. Doorn Unlike the radial breathing mode in carbon nanotubes (CNTs), the G-band Raman feature does not display a strong frequency dependence on nanotube structure. The appearance of a broad G$^ {-}$ peak in CNT Raman spectra has been attributed to numerous phenomena including the presence of metallic nanotubes, although a consensus has yet to be achieved amongst researchers. Here, we present resonant Raman measurements on macroscopic ensembles enriched in armchair CNTs produced by density gradient ultracentrifugation. Our G-band data clearly show that the broad, lower-frequency G$^{-}$ mode is absent for armchair structures, in contrast with recent theoretical and experimental results, and only occurs with resonance of non- armchair metals. This forms a generalized correlation between G-band lineshape and nanotube structure due to the sampling of a large number of nanotubes of several armchair species. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P28.00010: Infrared active vibrations in carbon nanotubes Katalin Kamaras, Aron Pekker The method of choice for the study of vibrational modes of carbon nanotubes has been almost exclusively Raman spectroscopy. Although calculations predict also infrared-active modes in nanotubes, so far only very few experimental results have been published. We conducted a systematic investigation of the infrared transmission of various types of single- and double-walled carbon nanotubes. Experiments were done on self-supporting transparent films in order to avoid perturbation from substrates. We find weak but reproducible vibrational peaks in the infrared spectrum. Their frequency scales with the diameter of the tubes, indicating their intrinsic character. Furthermore, on doping, some of the peaks change from Lorentzian to Fano-like character. This change can be explained by coupling of the tube vibrations to the conduction electrons introduced by doping. Finally, in double-walled nanotubes peaks typical of both outer and inner tubes can be distinguished. The vibrations of the inner tubes occur at the same frequency as those of single-walled tubes with the same diameter. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P28.00011: Far Infrared Optical Studies of Single and Double Walled Carbon Nanotubes Shin Grace Chou, Ahmed Zeeshan, Georgy Samsonidze, Jing Kong, Mildred Dresselhaus, Jeffrey Fagan, David Plusquellic Variable temperature far infrared absorption measurements were carried out for single walled and double walled carbon nanotubes samples (SWCNT and DWCNT) encased in a polymer matrix to investigate the effects of temperature and surface interaction on the low frequency phonons associated with the circumferential vibrations. At a temperature where kBT is significantly lower than the phonon energy, the broad absorption features as observed at room temperature become well resolved phonon transitions. For a DWCNT sample whose inner tubes have a similar diameter distribution as the SWCNT sample studied, a series of sharp features were observed at room temperature at similar positions as for the SWCNT samples studied. The narrow linewidth is attributed to the fact that the inner tubes are isolated from the polymer matrix and from the weak inter-tubule interactions. First principles calculations are carried out to understand the pertinent interactions and the temperature-dependent effects. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P28.00012: Influence of Defects and Doping on Optical Phonon Dynamics in Carbon Nanotubes Daner Abdula, Khoi Nguyen, Kwangu Kang, Scott Fong, Taner Ozel, David Cahill, Moonsub Shim The relaxation of electronic excitations induced by high bias or photoexcitation occurs primarily through optical phonon emission. Optical phonon relaxation may be affected by metallic/semiconducting character of carbon nanotubes, defect concentration, as well as doping. Changes in carbon nanotube G-band optical phonon population and pure dephasing lifetimes with doping and defects are described. Time-resolved incoherent anti-Stokes Raman spectroscopy is used to directly measure phonon decay lifetime, T$_{1}$, while total dephasing rate is inferred from static Raman linewidths. Defect concentration is varied by sample annealing and covalent functionalization showing increasing disorder reduces T$_{1}$ as well as overall dephasing time, T$_{2}$, with an even greater dependence. Samples with different metallic and semiconducting contribution have similar lifetimes, T$_{1}\sim $ 1.2 $\pm $ 0.1 ps in the no defect limit. Doping is shown to increase G-band linewidth, and therefore overall dephasing rate, for semiconducting nanotubes while leaving T$_{1}$ unaffected. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P28.00013: Ultrafast Terahertz Probes of Individualized, Chirality-Enriched Single-Walled Carbon Nanotubes Liang Luo, Ioannis Chatzakis, Jigang Wang Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical-pump, terahertz probe spectroscopy of individualized, (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 1-15 meV at low temperature down to 4K with resonant and off-resonant excitation at the $E_{22}$ transitions of (6,5) and (7,5) nanotubes. The intra-excitonic spectroscopy with THz pulses represents a fundamentally different spectroscopy tools to study dark excitons and shine new lights on the nature of excitonic ground states. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P28.00014: Nonlinear Optical Response of Individual Carbon Nanotubes Tatyana Sheps, Brad L. Corso, Eric O. Potma, Philip G. Collins Single walled carbon nanotubes (SWCNTs) are low dimensional conductors with unique nonlinear electro-optic properties. To investigate these properties we study the third-order, coherent anti-stokes (CAS) response of electrically connected individual SWCNTs on quartz substrates, using a four-wave-mixing (FWM) technique with femtosecond laser pulses. Because the CAS response is primarily electronic in nature [1], the signal from metallic SWCNTs is much stronger than from semiconducting ones. CAS easily distinguishes between the two types, as well as between semiconducting SWCNTs doped to be conductive or insulating. Furthermore, the CAS signal is sensitive to excitation resonances, the same effect that allows SWCNT fingerprinting by photoluminescence and Raman techniques. In addition to the strong electronic signal, we can also resolve a vibrational signal component at the G-band frequency, which suggests a method for studying chemical bond vibrations with this coherent technique. The good spatial resolution and high signal-to-noise achieved with femtosecond laser pulses provides opportunities for time-resolved optical measurements of SWCNT excitation dynamics. Funded by NSF Center for Chemistry at the Space-Time Limit at UCI (CHE-0847097). \\[4pt] [1] H. Kim et al, Nano Lett. 9 2991-2995 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P28.00015: Second Harmonic Generation in Highly Aligned Carbon Nanotubes on GaAs D.T. Morris, G.L. Woods, J. Kono, C.L. Pint, R.H. Hauge Optical properties of carbon nanotubes (CNTs) have been extensively investigated during the last decade, and much basic knowledge has been accumulated on how light emission, scattering, and absorption occur in CNTs. However, their nonlinear optical properties remain largely unexplored, except for theoretical studies predicting highly chirality-selective nonlinear optical processes. In particular, all chiral nanotubes are expected to possess finite second-order nonlinear susceptibilities due to the lack of inversion symmetry. Here, we have observed second harmonic generation (SHG) from samples consisting of highly aligned CNTs on GaAs with linearly-polarized intense mid-infrared femtosecond radiation. SHG is expected from both the CNTs and the substrate, thus the contribution of the CNTs to the overall enhanced SHG signal will be obtained by factoring out contributions from the GaAs substrate, which include any anisotropic effects (absorption, polarization) from the CNTs on the fundamental light incident in the GaAs substrate. We performed detailed angular dependent measurements to separate the two contributions, based on the crystal symmetries of the two. The results will be shown as a function of laser power and wavelength, and discussed in light of CNT band structure. [Preview Abstract] |
Session P29: Focus Session: Superconducting Qubits
Sponsoring Units: GQIChair: Robert McDermott, University of Wisconsin--Madison
Room: C148
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P29.00001: LeRoy Apker Award Talk: Parallel State Transfer and Efficient Quantum Routing on Quantum Networks Invited Speaker: We study the routing of quantum information in parallel on multi-dimensional networks of tunable qubits and oscillators. These theoretical models are inspired by recent experiments in superconducting circuits using Josephson junctions and resonators. We show that \emph{perfect parallel state transfer} is possible for certain networks of harmonic oscillator modes. We further extend our model to analyze the distribution of entanglement between every pair of nodes in the network, and find that the routing efficiency of hypercube networks is both optimal and robust in the presence of dissipation and finite bandwidth. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P29.00002: Towards long coherence superconducting qubits Matthias Steffen, Antonio Corcoles, Jerry Chow, Chad Rigetti, Mark Ketchen, Mary Beth Rothwell, George Keefe, Jim Rozen, Mark Borstelmann, Jack Rohrs, David DiVincenzo The capacitively shunted flux qubit (CSFQ) has recently been shown to have coherence times of 1-2 microseconds repeatedly over many devices at typical qubit operating frequencies. Experiments in our group strongly suggest that losses associated with the shunting capacitor limit the current coherence times. As a result we propose novel approaches towards decreasing capacitive losses by employing geometric and/or materials developments. We show experimental data and compare these with theoretical predictions [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P29.00003: Circuit QED without selection rules: the dispersive regime of the fluxonium qubit Guanyu Zhu, Jens Koch Manipulation and readout of superconducting qubits with microwave photons, as realized in circuit QED, commonly employ the dispersive regime. In this regime, the qubit-photon interaction strength is small compared to the relative detuning $\Delta$, and manifests itself only in the dispersive energy shifts $\chi$, crucial for dispersive readout and spectroscopy of the qubit. For Cooper Pair Box and transmon, these shifts are known to scale like $1/\Delta$ and $1/\Delta^2$, respectively, making readout at very large detuning challenging. We show that the relation between $\chi$ and $\Delta$ is mainly dictated by selection rules, and derive general expressions describing the dispersive regime of a multi-level qubit with arbitrary matrix elements. This generalization turns out essential for describing the dispersive regime of the fluxonium qubit, where no simple selection rules exist. We show that this lack of selection rules explains the suprising magnitude of disperse shifts, at detunings as large as $\sim$8GHz, and also causes peculiarities observed in the fluxonium spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P29.00004: Tunable coupling in circuit quantum electrodynamics with a superconducting V-system Srikanth Srinivasan, Anthony Hoffman, Devin Underwood, Jay Gambetta, Andrew Houck We demonstrate a new superconducting charge qubit that realizes a V-shaped energy level spectrum, enabling tunable coupling between the qubit and a superconducting cavity while retaining all of the advantages, including charge noise insensitivity, common to other charge qubits such as the transmon. Tunable coupling is achieved with quantum interference between the two excited states of the qubit. We report measurements of the vacuum Rabi splitting, showing that the coupling strength can be tuned from greater than 40 MHz to less than 200 kHz using fast flux bias lines. This dynamically tunable coupling is an intrinsic property of the qubit and requires no additional coupling circuit elements. This new qubit design shows great promise for future quantum information processing and quantum optics experiments. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P29.00005: Inductive coupling of superconducting qubits to coplanar waveguide resonators J.D. Strand, M.P. DeFeo, P. Bhupathi, C. Song, M. Ware, B. Xiao, B.L.T. Plourde Superconducting qubits coupled to microwave resonators provide a promising basis for a scalable quantum computing architecture and enable explorations of circuit quantum electrodynamics. One approach for achieving strong coupling between a qubit and resonator involves sharing the kinetic inductance of a narrow superconducting line. We are investigating different designs for inductively coupling qubits, including capacitively shunted flux qubits, to coplanar waveguide resonators. We are working to optimize the coupling while accommodating the space requirements of different qubit types and preserving the performance of the resonator. We present microwave measurements of these structures as well as modeling of the qubit-resonator coupling. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P29.00006: Transmon qubits coupled to compact resonators S. Shankar, K. Geerlings, E. Edwards, L. Frunzio, R.J. Schoelkopf, M.H. Devoret Compact resonators comprising of a meander inductor and an interdigitated capacitor are desirable building blocks for a multi-qubit processor due to their small size. We present an experiment on a superconducting transmon qubit coupled capacitively to such a compact resonator. We have fabricated low-loss Nb based compact resonators with an area within 1 mm$^2$ on a sapphire substrate to operate between 5 and 8 GHz. The resonator geometry was optimized to achieve an intrinsic quality factor above 300,000 at single-photon microwave powers and temperatures below 100 mK. Transmon qubits were made using Al/AlOx/Al Josephson junctions shunted by an Al interdigitated capacitor with an identical width and gap as the resonator. We will present our experimental progress towards measuring relaxation times of these qubits. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P29.00007: Design of a dc SQUID Phase Qubit with Controlled Coupling to the Microwave Signal R.P. Budoyo, A.J. Przybysz, B.K. Cooper, H. Kwon, Z. Kim, B. Cheng, A.J. Dragt, J.R. Anderson, C.J. Lobb, F.C. Wellstood, M. Khalil, S. Gladchenko, M. Stoutimore, B.S. Palmer, K.D. Osborn We have designed an Al/AlO$_{x}$/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area of 0.3 $\mu$m$^2$ to minimize loss associated with two-level systems in the junction oxide barrier. The qubit junction is shunted with a 1.5 pF interdigitated capacitor, and is isolated from the bias leads by an LC filter and an inductive isolation network using a larger Josephson junction. A previous device we built with similar parameters had its relaxation time $T_{1}$ limited by coupling to the microwave line. To reduce this coupling, we adopted a transmission line design and verified the coupling strength using microwave simulations. The new design will also allow us to measure the coupling to the SQUID by throughput measurements. We will discuss our design, the microwave simulations, our estimates for the overall coherence time due to losses and noise from various sources, and our progress towards testing the device. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P29.00008: Two junction effects in dc SQUID phase qubit B.K. Cooper, H. Kwon, A.J. Przybysz, R. Budoyo, J.R. Anderson, C.J. Lobb, F.C. Wellstood The dc SQUID phase qubit was designed to allow one isolation junction to filter bias current noise from a second junction operating as a single junction phase qubit. As junctions shrink to minimize dielectric loss, the Josephson inductances of each junction approach the coupling loop inductance and this single junction picture appears inadequate. We consider a two-junction model of the dc SQUID phase qubit, where the qubit now corresponds to one of the normal oscillatory modes of the full SQUID. We discuss applications of this model to sweet spots in various control parameters and unusual behavior in the tunneling state measurement. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P29.00009: Two-Dimensional quantum dynamic in a dc SQUID Florent Lecocq, I.M. Pop, Z. Peng, I. Matei, C. Naud, F.W. Hekking, W. Guichard, O. Buisson, R. Dolata, A.B. Zorin The dynamics of a dc SQUID presents a large variety of quantum effects at very low temperature such as 2D MQT signature, multilevel and phase qubit dynamics. We have shown that along the zero current bias line, the quantum dynamics is protected from current fluctuations. Along this line, the potential is quadratic-quartic and enhanced phase qubit properties have been demonstrated\footnote{E. Hoskinson et al, Phys. Rev. Lett. 102, 097004 (2009)} When the dc SQUID loop inductance is of the order of the Josephson inductance the dynamic becomes two dimensional. As a consequence, in addition to the oscillation mode producing the phase qubit, a second oscillation mode exists, called transverse mode. Here we report spectroscopic evidence and coherence properties of both oscillators as well as coherent oscillations between the quantum states of these two coupled oscillators. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P29.00010: Stark effect and generalized Bloch-Siegert shift in a strongly driven two-level system Matti Silveri, Jani Tuorila, Mika Sillanp\"a\"a, Yuriy Makhlin, Erkki Thuneberg, Pertti Hakonen A superconducting qubit was driven in an ultrastrong fashion by an oscillatory microwave field, which was created by coupling via the nonlinear Josephson energy. The observed Stark shifts of the ``atomic'' levels are so pronounced that one has to go beyond the rotating wave approximation to properly explain the measurements. The difference between the prediction of the rotating wave approximation and the full calculation including all higher orders constitutes the generalized Bloch-Siegert shift which was verified in the measurement. Based on the Floquet approach for the driven two-level system, we calculate the landscape of the quasienergy splitting and the matrix elements of the probe transition, which were probed by resonant absorption via a cavity. The calculation taking into account both the resonance condition and the magnitude of the probe absorption agrees well with the measurement results. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P29.00011: Time-Reversal Symmetry and Temporal Coherent Back-Scattering in a Driven Two-Level System Simon Gustavsson, Mark Rudner, Jonas Bylander, Leonid Levitov, Will Oliver Coherent backscattering, resulting from quantum interference of the paths related by time-reversal symmetry, is a phenomenon fundamental for quantum-chaotic dynamics. It manifests itself in diverse transport phenomena which were predicted and studied in mesoscopic electron systems in 1980's and 1990's: universal conductance fluctuations (UCF), weak localization and anti-localization, etc. Here we present first experimental realization of the essential physics of coherent backscattering in a driven quantum system, a two-level system repeatedly driven through an avoided level crossing. Our experiment is performed with a superconducting qubit driven through level crossing by a sequence of RF pulses. Each passage through the level crossing serves a Landau-Zener-type ``scattering event,'' with the wave function splitting between the up and down qubit states in a coherent fashion and recombining at a subsequent passage through the level crossing. Time-reversal symmetry can be enforced in our system by the driving protocol, resulting in constructive interference in the up-down transition rates. We observe an enhancement of the speckle-like fringe contrast analogous to UCF, which is suppressed in the absence of time reversal symmetry. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P29.00012: Dark states of cavity-coupled qubits S. Filipp, A.F. van Loo, A. Wallraff In circuit quantum electrodynamics (QED) the cavity-mediated dispersive interaction is the dominant inter-qubit coupling mechanism when the qubits are detuned from the resonator. This mechanism can be used to realize two-qubit gates. Here, we investigate the strength of this interaction explicitly considering the Fabry-Perot like multi-mode structure of the microwave frequency transmission line resonator. We observe the formation of dark states when the qubits are driven jointly by the same resonator microwave field and tuned into resonance with each other [1]. These dark states arise from the symmetry properties of the coupled quantum system at the avoided level crossing. Furthermore, we study the suppression of spontaneous emission of the coupled-qubit system by driving it into its dark state using microwave fields local to the individual qubits.\\[4pt] [1] S. Filipp \emph{et al.}, arXiv:1011.3732 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P29.00013: Entanglement between the charge and phase degrees of freedom in a superconducting qubit Mun Dae Kim The charge and phase are conjugate variables with each other in superconducting qubits which are characterized by either the charge or the phase degree of freedom. In this study we propose a qubit scheme where the charge and phase degrees of freedom are entangled in the qubit. In our qubit the qubit states consists of the phase states of the qubit, while the qubit states can be measured through the charge state detection. The qubit operation can be performed at the optimal point with respect to both the external magnetic flux and gate voltage. We discuss the fidelity of the Rabi oscillation and the possible way of enhancement of fidelity. [Preview Abstract] |
Session P30: Nanowires & Nanotubes: Growth & Absorption Kinetics
Sponsoring Units: DCMPChair: Latika Menon, Northeastern University
Room: C147/154
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P30.00001: Covalent functionalization of ZnO nanowires Andreia Luisa da Rosa, Ney Moreira, Adriel Garcia, Thomas Frauenheim Understanding the interaction of organic species with inorganic nanostructures constitutes a step forward in the development of~semiconductor based biosensors. In this work we have used density functional theory to investigate ZnO-(1010) nanowire surfaces modified with substituted methane molecules (Me-X, with X= OH, NH2, SH, COOH, and CN). We have found three relevant stabilization mechanisms acting on the surface stabilization: passivation of surface oxygen lone-pairs via dissociative chemisorption processes, electrostatic adsorbate-interations involving Zn surface sites and hydrogen bonding interactions involving oxygen surface sites. Covalent adsorbate-substrate interactions were found to play only a marginal role on the surface stabilization. Contradicting the usual chemical intuition, we have found no significant evidence for the formation of classical Lewis acid-base adducts on Zn surface sites. Finally we suggest that the functionalization with Me-COOH is also expected to be stable under ordinary laboratory conditions or in aqueous media. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P30.00002: Growth and Characterization of Serrated GaN Nanowires Zheng Ma, Dillon McDowell, Mohamed Abd Elmoula, Eugen Panaitescu, Dalmau Reig, Latika Menon We describe our results on the growth of single crystalline GaN nanowires on catalyst-patterned substrates by means of chemical vapor deposition. The growth is carried out in a horizontal quartz tube inside a tube furnace wherein gallium oxide powder is used as reactor source and a mixture of ammonia and hydrogen gas is used as precursor. Growth of GaN nanowires are demonstrated on both Au and Ni-catalyst patterned substrates (either sapphire or silicon). The growth temperature is maintained at around 960\r{ }C. We show that by controlling the deposition parameters, specifically the size of the catalyst and amount of gallium oxide GaN nanowires grow in a ``serrated'' pattern. The serrated nanowires maintain a stable, single crystalline state with very regular periodic serrations. The wires have been characterized by means of scanning electron microscopy, transmission electron microscopy and energy dispersive x--ray scattering measurements. Preliminary electrical transport measurements on single serrated GaN nanowires released onto a Si substrate show that the wires exhibit improved electron transport capabilities in comparison with regular GaN nanowires. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P30.00003: GaAs nanowires and GaAs/AlGaAs core/shell nanowires synthesized by MOCVD Brian Peters, Nicholas Minutillo, John Carlin, Fengyuan Yang Nanowires made by the ``bottom-up'' approach can be used in a variety of electrical and optoelectronic devices as well as in the study of low dimensional transport physics. We have grown GaAs nanowires using Au catalysts in a closed couple showerhead MOCVD system. A number of growth parameters, including the substrate temperature, growth rate, and Arsine/TMGa ratio, are explored to identify optimal conditions for growth of GaAs nanowires with large aspect ratio and minimal tapering. Higher substrate temperatures result in larger tapering and lower temperature leads to ``kinks.'' Meanwhile, large V/III source ratio gives large tapering as well. We have found that our optimal conditions are at a substrate temperature of 420\r{ }C and V/III ratio of $\sim $25, which gives a tapering of less than 1 nm increase in diameter per micron in length. In addition, GaAs/AlGaAs core/shell structured nanowires were also grown to minimize the surface states. Characterizations by SEM and photoluminescence will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P30.00004: Fe doped ZnO nanotubes synthesized by low temperature electrochemical process Gopal Sapkota, Karol Gryczynski, Arup Neogi, Usha Philipose We report the synthesis of Fe doped ZnO nanotubes (NTs) with tube diameter of about 60-100 nm and wall thickness of about 20nm. To the best of our knowledge, this is the first report on Fe doped ZnO NTs, that could possibly be ferromagnetic. Fe doping will enable us to tune the electrical, optical and magnetic properties of the NTs which are crucial for practical applications (spintronics and optoelectronics). The morphology of the NTs was found to be very sensitive to concentration and temperature of the electrolyte and growth time. Structural and compositional analysis revealed that Fe was incorporated into the ZnO lattice. High Resolution Transmission Electron Microscopy and X-ray diffraction shows good crystalline quality of the NTs with preferential growth along the wurtzite c-axis. Room temperature photoluminescence (PL) measurement of the NTs exhibit strong UV emission around 370nm whereas low temperature PL of the NTs exhibits the optical signature of Fe doping. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P30.00005: Growth and Characterization of ZnMgO Nanowires by Thermal Chemical Vapor Deposition Gang Shen, Shawn David Wilbert, Nick Harris, Nabil Dawahre, William Baughman, Lee Butler, Joseph Brewer, Seongsin Margaret Kim, Patrick Kung Zinc oxide (ZnO) nanowires are promising structures for nano-optoelectronic devices and applications ranging from solid-state lighting to photovoltaics because of the wide bandgap and large exciton binding energy of ZnO, in addition to serving as template matrices for nanoscale sensors. Alloying ZnO with MgO to achieve ternary ZnMgO compounds represents a potential approach for future optoelectronic heterostructure devices. Well-aligned ZnMgO nanowires (NWs) were grown on a sapphire substrate by thermal Chemical Vapor Deposition (CVD). The alignment of the ZnMgO NWs was confirmed by x-ray diffraction and electron microscopy along with elemental composition information through EDS analysis. The optical and vibrational properties of the ZnMgO NWs were studied by micro-Raman and micro-photoluminescence (PL) measurement. Through the combination of confocal laser scanning microscopy and the micro-Raman and micro-PL, the ZnMgO NWs were imaged at sub-micron resolution. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P30.00006: Coating of Multi-walled Carbon Nanotubes with Inorganic - Organic Silicas Purnatosh Saha, Brian Grady Silica-coated multi-walled carbon nanotubes (MWCNTs) have been prepared by sol-gel techniques. An inorganic silica layer has been formed by hydrolyzation of tetraethoxy silane (TEOS) on surfactant-treated nanotubes. Additionally, a secondary layer has been deposited using organosilane precursors. Coating thicknesses have been measured by transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) has also been used to determine the amount of silicates coating the nanotubes. The thickness of the final coating can be tailored by controlling reaction conditions and the number of layers. It is expected that carefully controlling the inorganic to organic ratio in the coatings will allow for the variation of its stiffness. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P30.00007: Characterization of Co$_{2}$FeAl nanowires Keshab R. Sapkota, I.L. Pegg, J. Philip Heusler alloy, Co$_{2}$FeAl (CFA) is a potentially useful material in the field of spintronics due to its high spin polarization. The CFA nanowires are grown for the first time by the electrospinning method. The diameters of the wires formed are ranging from 80 -- 100 nm. The structural characterization of the nanowires is done using X-Ray diffraction and Raman spectroscopy. The nanowires exhibit cubic structure with a lattice constant, $a$ = 2.44 {\AA}. Parallel arrays of nanowires are grown for magnetic characterization using electric field applied at the collector plate. The nanowires exhibit ferromagnetic behavior with a Curie temperature higher than 400 K. Nanoscale devices are fabricated with single CFA nanowire to understand the magnetotransport properties. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P30.00008: A study of the effect of nitrogen doping in TiO$_2$ Lisa DeBeer-Schmitt, Xiaofeng Qiu, Larry Anovitz, William Heller, Ken Littrell, M. Parans Paranthaman TiO$_2$ nanotube arrays have great potential as photovoltaics due to its unique chemical and physical properties associated with highly ordered tubular geometry. Small-angle neutron scattering (SANS) can characterize the specific nitrogen doping impact to the nanotube array structures. N-doping holds the promise of overcoming the large intrinsic bandgap barrier, which prevent TiO$_2$ from utilizing larger portion of solar energy. Combining with nanotube structures, N-doping could further promote the energy conversion efficiency of TiO$_2$. The SANS data demonstrate that the nitrogen doping during the nanotube growth alters their structure. This stabilized structure is evident in the data via extra peaks in the nitrogen doped sample as compared to the pure. The results demonstrate that the nanotube array morphology can be manipulated by varying the growth conditions, making it possible to tailor the arrays to specific purposes. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P30.00009: Spontaneous Formation of a Nanotube from a Square Ag Nanowire Sondan Durukanoglu, Mine Konuk The recently observed phenomenon of spontaneous formation of a tube from a regular, square Ag nanowire has been investigated through molecular static and dynamic simulations based on the interaction potentials obtained from the embedded atom method. With molecular static calculations, we investigate the effect of strain on this particular type of transformation by focusing specifically on square Ag nanowires. Our results demonstrate that the formation of hollow structures requires a combination of minimum basis size and high gradient stress. Using molecular dynamic simulation, we also discuss the effect of temperature on the evolution of silver nanowire during the elongation. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P30.00010: Effect of catalyst preparation on diameter of single-walled carbon nanotubes synthesized by alcohol CVD Theerapol Thurakitseree, Erik Einarsson, Rong Xiang, Shinya Aikawa, Shohei Chiashi, Junichiro Shiomi, Shigeo Maruyama We investigated the effect of various aspects of the catalyst preparation procedure on the diameters of SWNTs synthesized by the alcohol CVD method. Prior to nanotube growth, a Co/Mo binary catalyst was deposited by dip-coating, and then reduced under flowing Ar/H$_{2}$ at temperatures ranging from 300 to 800\r{ }C. We found that the mean SWNT diameter depends on both reduction time and temperature, with lower reduction temperature or short reduction time resulting in smaller diameter SWNTs. The morphology of SWNTs changed from vertically aligned for reduction temperatures above 500\r{ }C to randomly aligned when reduction occurred below 500\r{ }C. Introducing small amount of water during heating, the mean diameter of the SWNTs was also reduced despite synthesis at 800\r{ }C. Small diameter SWNTs were synthesized with this new cobalt/rhodium (Co/Rh) catalyst. The average diameter SWNT is similar to that from Co/Fe catalysts and slightly smaller than HiPco. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P30.00011: Water monomer interaction with H-passivated Si nanowires from density functional theory Abraham Hmiel, Yongqiang Xue A molecular-level understanding of the interaction between water and nanomaterials is essential for such important phenomena as corrosion, catalysis, electrochemistry and biology. In this talk we present density functional (DFT) study of the structure and energetics of water monomer binding to the surfaces of hydrogen terminated silicon nanowires (SiNWs), which represents the initial phase of interfacial water. We present results on the binding site and orientation of individual water molecules on the surfaces of [112] and [110] oriented SiNWs and analyze the energetics of water adsorption through potential energy surface scan along selected degrees of freedom. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P30.00012: In-situ dynamical study of capillary absorption of molten silver nanodroplets by multiwall carbon nanotubes Yen-Song Chen, Yuan-Chih Chang, Tung Hsu, Chia-Seng Chang Since the discovery of carbon nanotubes (CNTs), they have been widely investigated for their properties. Due to the large aspect ratio and the uniform diameters, the inner cavities of the CNTs are used as nano test tubes, siphons, catalyst carriers, and so on. Based on recent molecular dynamic simulations, a CNT with open end might act as a ``capillary pipette'' which can absorb nonwetting metal nanoparticles. In our study, the in-situ dynamical process of nonwetting Ag nanodroplets drawn into the hollow cores of multiwall carbon nanotubes (MWCNTs) was observed in an ultrahigh-vacuum transmission electron microscope equipped with a scanning tunneling microscopy probe. We discover this capillary absorption of melted Ag nanodroplets can occur only when the ratio of the Ag nanodroplet size to inner diameter of MWNTs is below a critical value, which is dependent on the inner diameter of MWCNTs. With continuous operations of capillary absorption for Ag nanodroplets, the one-dimensional Ag nanowires with a specific length could be fabricated inside the MWNTs for NEMS electronics or other applications. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P30.00013: O$_{2}$ dissociation on nitrogen doped carbon nanotubes (10, 0) from first principles simulation Shizhong Yang, Guang-Lin Zhao, Ebrahim Khosravi Reducing the amount of precious platinum (Pt) loading by identifying non-precious metal catalyst is essential for large-scale applications of fuel cells, which provide a cleaning energy technology. Recent experimental, theoretical, and simulation works accelerate the advance in the research area of doped carbon nanotubes acting as an alternate non-precious metal catalyst for dioxygen reduction in the fuel cells. First principles spin polarized density functional theory(DFT) simulations have been performed to understand O$_{2}$ dissociation on nitrogen doped carbon nanotubes. We have studied nitrogen substitutional doping of carbon nanotubes (CNTs) for dioxygen adsorption, reduction, and dissociation. The calculated results show that nitrogen prefers to stay at the open-edge of short CNTs. Two O$_{2}$ chemisorption sites are found, the carbon-nitrogen complex (Pauling site) and carbon-carbon long bridge (long bridge) sites. The spin polarized DFT calculations using the nudged elastic band (NEB) method show that O$_{2}$ dissociation at the Pauling site has a reaction energy barrier of about 0.55 eV. The unique open-edge structure and charge redistribution are crucial to the novel properties of nitrogen-doped CNTs as a new non-precious metal catalyst for fuel cells. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P30.00014: Adsorption kinetics of polyatomic molecules on a heterogeneous surface Jared T. Burde, M. Mercedes Calbi We study the kinetics of adsorption of diatomic and triatomic molecules on the external surface of a carbon nanotube bundle. The Kinetic Monte Carlo algorithm is employed to track the number of particles adsorbed on the bundle and the orientation of those particles with respect to the surface at any given time. Our model is further complicated by the inclusion of a more complex surface geometry; a two dimensional, heterogeneous lattice better models the reality of groove between adjacent nanotubes on the outside of the bundle. This allows us to see interesting kinetic effects in the adsorption process, as the adsorbates have multiple transitional states through which they can pass as they evolve towards equilibrium. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P30.00015: Simulations of adsorption on a single carbon nanotube Hye-Young Kim, Silvina Gatica, Milton Cole Using the grand canonical Monte Carlo method, we have evaluated the adsorption isotherms of simple gases (Ar, Kr, Xe) on a variety of carbon nanotubes. The adsorption potential is a sum of anisotropic atom-C interactions, dependent on the angle between the outward normal and the atom-C separation vector. For varying gas species and nanotube chirality, different commensurate phases are seen than on the surface of graphite. Comparison is made with recent experiments of Wang, et al, Science 327, 552 (2010). [Preview Abstract] |
Session P31: Focus Session: Materials at High Pressure IV: Geophysical Materials and Magnetic Transitions
Sponsoring Units: DMP GSCCM DCOMPChair: Maddury Somayazulu, Carnegie Institution
Room: C145
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P31.00001: Pressure-induced phase transitions in NaMgF3 post-perovskite Koichiro Umemoto, Renata Wentzcovitch Understanding the behavior of MgSiO$_{3}$ postperovskite(PPV) under extreme pressures is fundamental for modeling the interiors of solar giants and extrasolar planets. In 2006, MgSiO$_{3}$ post-perovskite was predicted to dissociate into MgO and SiO$_{2}$ at 1.1 TPa (Umemoto et al., Science 311, 983 (2006)). However, the predicted dissociation pressure is too high to be easily verified experimentally. Instead, a low-pressure analog, NaMgF$_{3}$ neighborite, has been studied to test for structural predictions in MgSiO$_{3}$. NaMgF$_{3}$ was predicted to dissociate at $\sim$40 GPa (Umemoto et al., Geophys. Res. Lett. 33, L15304 (2006)), but this has not been confirmed experimentally (Martin et al., Geophys. Res. Lett. 33, L11305 (2006); Grocholski et al. Geophys. Res. Lett. 37, L14204 (2010)) and the dissociation MgSiO$_{3}$ PPV is now being questioned. Here, we reexamine in detail the pressure dependence of crystal structures and phonon frequencies in NaMgF$_{3}$ and reveal the apparent reason why dissociation was not observed in this material. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P31.00002: First-principles calculation of thermal conductivity of silicate perovskite at high pressures and high temperatures Jianjun Dong, Xiaoli Tang, Abby Kavner, Moses Ntam The lattice thermal conductivity of silicate perovskite, the most abundant mineral in the Earth's lower mantle, is calculated by combining the first-principles electronic structure theory and Peierls-Boltzmann transport theory. The phonon scattering rate due to lattice anharmonicity and Mg/Fe mass disorder is evaluated for each mode at the extreme P-T conditions of the lower mantle. The predicted thermal conductivity of single crystal MgSiO3 perovskite at ambient condition, about 5.7 W/m/K, is in excellent agreement with experiment. Adding about 6{\%} Fe will lower the thermal conductivity by nearly 40{\%}. Our calculation also reveals an unique pressure dependence for the thermal conductivity of perovskite, and the calculated thermal conductivity of iron bearing perovskite is almost an order of magnitude lower than the previously estimates based on long extrapolation of single crystal data. Including a re-evaluation of radiative contribution, we discuss the implications of our results for the heat flow in deep Earth. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P31.00003: Identification of post-pyrite transition in SiO2 by a genetic algorithm Shunqing Wu, Koichiro Umemoto, Kai-Ming Ho, Min Ji, Cai-Zhuang Wang, Renata Wentzcovitch Here we propose a new phase of SiO$_{2}$ beyond the pyrite-type phase. SiO$_{2}$ is one of the most important minerals in Earth and planetary sciences. So far, the pyrite-type phase has been identified experimentally as the highest-pressure form of SiO$_{2}$. In solar giants and extrasolar planets whose interior pressures are considerably higher than that on Earth, a post-pyrite transition in SiO$_{2}$ may occur at $\sim$ 1 TPa as a result of the dissociation of MgSiO$_{3}$ post-perovskite into MgO and SiO$_{2}$ [Umemtoto et al., Science 311, 983 (2006)]. Several dioxides considered to be low-pressure analogs of SiO$_{2}$ have a phase with cotunnite-type (PbCl$_{2}$-type) structure as the post-pyrite phase. However, a first-principles structural search using a genetic algorithm shows that SiO$_{2}$ should undergo a post-pyrite transition to a hexagonal phase, not to the cotunnite phase. The hexagonal phase is energetically very competitive with the cotunnite-type one. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P31.00004: Spin crossover systems in the deep mantle Invited Speaker: In recent years there has been much interest on spin crossovers found experimentally in the most abundant minerals of Earth's lower mantle ((Mg,Fe)O and (Mg,Fe)(Si,Fe)O$_3$-perovskite) under pressure. Spin crossovers are strongly dependent on thermodynamic conditions and a full understanding of this problem requires its investigation as function of pressure and temperature. There are several controversies, especially in the perovskite systems, and surprises are revealed by electronic structure calculations. The geophysical consequences of these crossovers are yet to be fully understood but could be fascinating. I will review progress we have made in understanding spin crossovers and give an overview of this phenomenon and its potential implications for the Earth.\\[4pt] Research carried out in collaboration with H. Hsu, K. Umemoto, P. Blaha, J. F. Justo, and C. R. S. da Silva. Research supported by the MRSEC Program of NSF under Award Number DMR-0212302 and DMR-0819885, and by NSF/ATM-0428774, EAR-0810212, and EAR-1047629. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P31.00005: Insulator to Metal and Magnetic Transitions in FeO under High Pressure: DFT-DMFT Computations R.E. Cohen, Kristjan Haule, Gabi Kotliar We have applied DFT+Dynamical Mean Field Theory (DMFT) to FeO under varying pressure and strain to understand possible transitions in FeO. We use an LAPW basis set, and the lattice terms are evaluated using the WIEN2K LAPW code. The impurity model is solved using continuous time quantum Monte Carlo (CTQMC). Temperature enters explicitly, so we made special efforts to understand high temperature behavior relevant to geophysics. The computations are fully self-consistent, including the impurity levels and crystal field splitting, and the total energy is evaluated using the full potential and charge density of the lattice plus impurity models. We find with increasing pressure in paramagnetic FeO in a cubic lattice and U=8 eV a high-spin low-spin transition, with a possible intermediate spin state (characterized by intermediate occupancies of the t2g and eg states) between. We find that at 300K cubic FeO remains insulating to a factor of two compression (over 600 GPa). However, high temperatures (e.g. 2000K) and rhombohedral lattice strain promote a metal insulator transition. We are delineating the phase boundaries. This work is supported by NSF. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P31.00006: First-principles study of spin-state crossovers and hyperfine interactions of ferric iron in magnesium silicate perovskite Han Hsu, Peter Blaha, Matteo Cococcioni, Renata Wentzcovitch The spin-state crossover in iron-bearing MgSiO$_3$ perovskite, the most abundant mineral in the Earth, may significantly affect the properties of Earth's lower mantle. However, details of this phenomenon have been very unclear, owing to the complicated nature of this mineral, mainly the coexistence of ferrous and ferric iron. Using the density functional theory plus Hubbard $U$ (DFT+$U$) methods, we investigated the spin states and hyperfine interactions of ferric iron in this mineral. We show that a crossover from high-spin to low-spin state occurs within the lower-mantle pressure range, and it is accompanied by a noticeable volume reduction and an increase in iron nuclear quadrupole splitting (QS). These results are consistent with recent x-ray diffraction and M\"ossbauer spectroscopy measurements [K. Catalli \textit{et al}., Earth Planet. Sci. Lett. \textbf{289}, 68 (2010)]. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P31.00007: Ab initio melting curve of Fe and Fe-S alloys at extreme pressures: implications for Earth's and exoplanets' cores Johann Bouchet, Guillaume Morard, Stephane Mazevet, Francois Guyot Exoplanets with masses similar to that of Earth have recently been discovered in extrasolar systems [1]. A first order question for understanding their dynamics is to know whether they possess Earth like liquid metallic cores. However, the iron melting curve is unknown at conditions corresponding to planets of several times the Earth's mass (over 15 Mbar for planets with 10 times the Earth's mass [2]). In the density-temperature region of the cores of those super-Earths, we calculate the iron melting curve using first principle molecular dynamics simulations based on density functional theory. We also propose an equation of state for iron in this pressure range. Finally we show the melting curve of Fe$_{3}$S and discuss the effects of the addition of sulfur to the melting curve of pure iron. \\[4pt] [1] J. P. Beaulieu, D. P. Bennett, P. Fouque et al., Nature 439 (7075), 437 (2006).\\[0pt] [2] D. Valencia, R. J. O'Connell, and D. Sasselov, Icarus 181, 545 (2006). [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P31.00008: Elasticity of iron-bearing olivine polymorphs investigated by first principles Maribel N\'u\~nez Valdez, Yonggang Yu, Renata Wentzcovitch We calculate by first principles the effect of iron on the high pressure-temperature elasticity of olivine polymorphs: $\alpha$-phase (olivine), $\beta$-phase (wadsleyite) and $\gamma$-phase (ringwoodite), the major constituents of the Earth's upper mantle and transition zone (TZ). We combine the LDA, the quasiharmonic approximation, and a model vibrational density of states for the solid solution to calculate the full elastic tensor $C_{ij}$, bulk ($K$) and shear ($G$) moduli of (Mg$_{0.875}$Fe$_{0.125}$)$_2$SiO$_4$. Comparison with experimental data at ambient conditions validates our results. In the pressure and temperature range of the upper mantle and TZ we study single crystal wave propagation anisotropy and polarization anisotropy in aggregates with preferred orientation.\\[4pt] Research supported by NSF EAR-1019853 and EAR-0810272. Computations were performed at the Minnesota Supercomputing Institute. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P31.00009: Polymerization of methane in the deep Earth Leonardo Spanu, Davide Donadio, Detlef Hohl, Eric Schwegler, Giulia Galli Determining physical and chemical properties of carbon fluids at high pressure and temperature is a key step towards understanding carbon reservoirs and fluxes in the deep Earth. The stability of carbon-hydrogen systems at depth greater than few thousands meters is poorly understood and the abiogenic hypothesis on the synthesis of higher hydrocarbon (HCs) in the Earth mantle remains controversial. We have used ab initio molecular dynamics simulations to investigate the formation of higher HCs from dissociation of pure methane, and of methane in contact with carbon surfaces and transition metals, in a range of pressure of $2-30$ GPa and temperature T=$800-4,000$ K [1]. We present results on the range of stability of pure methane and discuss how the interaction with transition metals or carbon deposits (graphite and diamond) affects the formation of higher HCs.\\[4pt] [1] Leonardo Spanu, Davide Donadio, Detlef Hohl, Eric Schwegler, Giulia Galli (\it{submitted}) [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P31.00010: Solubility and erosion of icy cores in giant planets Hugh Wilson, Burkhard Militzer The core-mantle boundary of a giant planet consists of an interface between dense rock/ice below and fluid hydrogen-helium above. Whether this phase boundary remains stable, however, or whether the core material is dissolved and redistributed throughout the interior, remains unknown, and has major consequences for planetary interior and formation models. In this work we use density functional theory molecular dynamics calculations to compute the free energy of solubility for the icy components of the core into fluid hydrogen, to investigate whether solubility is thermodynamically preferred at the extreme temperature and pressure conditions prevalent at the core-mantle boundaries of Jupiter and Saturn. The consequences for Jupiter and Saturn, as well as for giant exoplanets, will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P31.00011: Post-stishovite transition in AlOOH-incorporated SiO2 Katsuyuki Kawamura, Koichiro Umemoto, Renata Wentzcovitch, Kei Hirose In 2007, Lakshtanov et al. [Proc. Nat. Acad. Sci. 104, 13588 (2007)] showed that the incorporation of AlOOH into SiO$_{2}$ significantly reduces the transition pressure between stishovite and CaCl$_{2}$-type phases. In the present paper, we investigate theoretically the effect of hydrogen and aluminum on this transition. First-principles calculations show that aluminum has no effect on the transition pressure. However, hydrogen bonds play a crucial role, suggesting that a cooperative redistribution of hydrogens aids the post-stishovite transition. Large-scale molecular dynamics simulations using model potentials confirm this effect and reveal the nature of the hydrogen motion. This effect produces a strong temperature dependence on the transition pressure and should make the latter sensitive to hydrogen content in the material. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P31.00012: Thermodynamic properties of MgSiO$_3$ majorite and phase transitions near 660-km depth in MgSiO$_3$ and Mg$_2$SiO$_4$: a first principles study Yonggang Yu, Renata Wentzcovitch, Victor Vinograd, Ross Angel Thermodynamic properties of MgSiO$_3$ tetragonal majorite have been calculated at high $P$-$T$ within the quasiharmonic approximation based on DFT using both LDA and GGA. The LDA results compare exceptionally well with measured thermodynamic properties. A classical Monte Carlo simulation based on a cluster expansion method demonstrates that disorder between Mg and Si in the octahedral sites in majorite does not occur below 3600 K within 30 GPa. The calculated phase boundaries between majorite (mj), perovskite (pv), and ilmenite (il) MgSiO$_3$ agree much better with experiments by using GGA than by LDA. The Clapeyron slopes (CS) predicted by GGA and LDA are close to each other: $0.9$ -- $1.7$ MPa/K for mj-pv, 6.9 -- 7.9 MPa/K for mj-il, and $-7$ -- $-3$ MPa/K for il-pv transition. The triple point predicted by GGA is at $21.8 \pm 1$ GPa and $1840 \pm 200$ K which is $\sim$400 K lower than most experiments. Our calculations also reveal that wadsleyite decomposes to an assemblage of majorite plus periclase above 2280 K with a large negative CS ($-22$ -- $-12$ MPa/K) and that ringwoodite decomposes to ilmenite plus periclase below 1400 K (1.2 MPa/K). The geophysical implications to mantle convection and the composition of the Earth's transition zone will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P31.00013: A first-principles investigation of hydrous defect and IR frequencies in forsterite: The case for Si vacancies Marc Hirschmann, Koichiro Umemoto, Renata Wentzcovitch, David Kohlstedt, Anthony Withers We investigate charge-balanced hydrous magnesium and silicon defects ((2H)$^{X}$$_{Mg}$, (4H)$^{X}$$_{Si}$) by first principles. Here we propose two new lowest-energy hydrogen configurations for (4H)$^{X}$$_{Si}$. With these new configurations, the distribution of OH-stretching phonon frequencies in Group I ($>$ 3450 cm-1) are better reproduced. Substitution of silicon with 4 hydrogens gives rise to significant elongation of distances between oxygen ions at the tetrahedron of the silicon vacancy. Our calculations indicate that the correlation between O-O distances and O-H-stretching phonon frequencies, which has been well established for hydrous minerals, does not apply directly to nominally anhydrous minerals and should not be used to determine the identity of the hydrous defects responsible for infrared absorption peaks. [Preview Abstract] |
Session P32: Optical Properties and Dynamics of Quantum Dots and Quantum Wells
Sponsoring Units: DCMPChair: Sam Carter, Naval Research Laboratory
Room: C144
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P32.00001: Sub micron scale patterning of material optical response through focused ion beam induced InAs/GaAs quantum dot nucleation Timothy Saucer, Jieun Lee, Andrew Martin, Deborah Tien, Joanna Mirecki-Millunchick, Vanessa Sih We report on the technique of using a focused ion beam to produce preferential sites for InAs/GaAs quantum dot nucleation. We mill an array of holes in the GaAs substrate and then deposit a thin layer of InAs below the critical thickness for dot formation in unpatterned areas. The array of holes on the substrate act as preferential nucleation sites and induce quantum dot formation only in the patterned regions. We conduct photoluminescence spectroscopy in a templated multilayer quantum dot sample at temperatures down to 10K and for various patterning conditions. We find that outside of our patterning regions we have no quantum dot luminescence, indicating that the patterning modifies the optical response of the material. We find that we can control this quantum dot formation down to array spacings of 250nm, showing excellent potential for this technique to be used for sub micron spatial control of a material's optical properties. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P32.00002: Photoluminescence imaging of Focused-Ion-Beam induced individual quantum dots Jieun Lee, Timothy Saucer, Andrew Martin, Deborah Tien, Joanna Millunchick, Vanessa Sih Quantum dots are nanostructures that confine electrons in 3 spatial dimensions. Due to their discrete atom-like energy levels, a wide variety of applications related to the optical properties of dots are possible. One such application is to integrate quantum dots in optical nanocavities for the enhanced interaction between electrons and photons. However, self-assembled dots typically nucleate at random locations, hindering the accurate coupling between the dot and cavity. Therefore, spatial control on self-assembled dots at the fabrication level is highly desirable. Here, we report on optical measurements conducted on InAs quantum dots that are prepatterned in a square array by a focused-ion-beam. Using scanning confocal microscopy, we spatially map the photoluminescence of individual quantum dots. Single dot luminescence with 160 $\mu $eV linewidth is observed indicating good optical quality and statistical analysis over 16 array sites show reasonable placement accuracy and emission inhomogeneity. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P32.00003: Strongly confined excitons in self-assembled InGaAs quantum dot clusters Megan Creasey, Xiaoqin Li, Jihoon Lee, Zhiming Wang, Gregory Salamo Quantum dot clusters (QDCs) consisting of regular geometric patterns of six InGaAs quantum dots (QD) are grown on a GaAs substrate using a hybrid growth method that combines droplet homoepitaxy and Stranski-Krastonov growth. These novel structures have potential applications as tunable single photon sources, entangled photon sources, or error corrected qubits - devices critical to the fields of secure optical communications and quantum computing We study the photoluminescence arising from a single cluster using both continuous wave and ultrafast spectroscopic techniques with variations in the sample temperature and excitation power. Our results suggest excitons (bound electron-hole pairs) are strongly confined within the individual QDs rather than loosely confined throughout the entire QDC. The work at Texas is supported financially by NSF, ARO, AFOSR, ONR, the Welch Foundation, and the Alfred Sloan Foundation. The work at Arkansas is supported by the NSF. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P32.00004: Selective control of polarized luminescence from GaN/AlN self-assembled quantum dots Daniel Rich, Ofer Moshe, Benjamin Damilano, Jean Massies GaN/AlN self-assembled quantum dots (QDs) were grown by the Stranski-Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperatures, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing vertically stacked QDs leads to an additional biaxial tensile stress at the Si/III-Nitride interface. We have modified the thermal stress in the QD layers by etching stripes of varying widths using inductively coupled Cl/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width and orientation can create regions of in-plane uniaxial stress ranging from 20-30 kbar which enables a selective and local control of polarized emission from the QDs. Localized cathodoluminescence (CL) spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. We have studied these emissions using time- and polarization-resolved CL for ensembles of QDs. The effects of screening of the polarization field in the QD, state-filling, changes in the polarization anisotropy and lifetime with varying excitation were studied experimentally and modeled with a self-consistent 6x6 k.p calculation method. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P32.00005: Polarization states of charged excitons in coupled InAs/GaAs quantum dot molecules Ramana Thota, Swati Ramanathan, Kushal Wijesundara, Eric Stinaff, Allan Bracker, Dan Gammon The polarization state of charged excitons in coupled InAs/GaAs dots can reveal useful information about the spin state of its charge carriers. In this study, we examine the complete polarization state through Stokes parameter measurements to relate the polarization parameters of the luminescence to the spin configurations of the various charged excitons they originate from. We demonstrate that this method is a useful tool to identify and possibly create spin states for quantum computation applications. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P32.00006: Resonant Fluorescence from Quantum Dot Molecular Excitonic Transitions Mark Kerfoot, Allan Bracker, Daniel Gammon, Michael Scheibner Quantum dot molecules formed by two vertically stacked quantum dots are a rich testing ground for basic concepts regarding the measurement and control of quantum states. The well defined geometry is ideal for studying interaction mechanisms, such as the interaction of two dipoles each located in one of the quantum dots of the quantum dot molecule. A prerequisite for doing so is the ability to detect the interaction mediated changes in the properties of the individual, uncoupled quantum dots. Here we use resonant fluorescence to study exciton transitions in quantum dot molecules. We measure the photoluminescence of the same transition we optically excite with a narrow band laser. With this method, features on the scale of the homogeneous line width of the intradot exciton transition are well resolved. This enables us to study the fine-structure of different charge and spin configurations with high sensitivity. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P32.00007: Exchange-controlled spin dynamics in coupled quantum dots Eric A. Stinaff, Kushal C. Wijesundara, Allan Bracker, Dan Gammon We measure circular polarization memory of neutral exciton states with polarization dependent photoluminescence spectra. As a consequence of anisotropic exchange interaction a low degree of circular polarization memory was observed in the spatially direct and indirect excitons where they anticross. With applied electric field as we tune the excitonic emission from intra-dot to inter-dot the electron-hole wave function overlap reduces and we observe an increase in polarization memory due to reduced exchange interaction. We observe a sudden unexpected dip in circular polarization memory of the spatially indirect exciton state that is coincident with the applied field where the single hole level resonance is observed. Possible mechanisms for this loss of circular polarization memory will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P32.00008: Two-body and three-body interactions in phonon-assisted exciton energy transfer between quantum dots Kaijie Xu, Carlo Piermarocchi We theoretically study the dynamics of exciton energy transfer between semiconductor quantum dots. Phonons play a critical role in the exciton energy transfer process when the energy of the dots involved in the process is different. We find that the phonon-assisted energy transfer cannot be correctly described by two-body exciton-photon and exciton-phonon interactions if each dot is modeled as a single-level exciton system. Higher excited levels of the exciton state have to be included to properly describe the phonon-assisted process. However, excited states can be traced out by introducing a single-level exciton model with an effective three-body exciton-photon-phonon coupling term. The three-body term describes a change in the exciton-photon dipolar coupling due to phonon-induced deformations of the ground exciton wavefunction. The multi-level exciton model with two-body interactions and the single-level exciton model with three-body interaction reproduce the same exciton transfer rates to the leading order contributions of perturbation theory. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P32.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P32.00010: Optical lattices for electrons in semiconductors Carlo Piermarocchi, Michael G. Moore, Martin J. A. Schuetz, Monique Combescot We theoretically investigate the trapping of electrons in a semiconductor using counter-propagating laser beams. We consider two different physical mechanisms that can lead to an efficient electron trapping: (a) Pauli blocking between the electron and a virtual exciton coupled to the laser field, and (b) the virtual excitation of a three-body Coulomb resonance corresponding to a bound charged exciton state (a trion). Both processes induce a momentum transfer between photon and electron, and lead to a sinusoidal trap for electrons with a period determined by the laser beam modulation. The depth of the potential is proportional to the laser intensity and inversely proportional to the exciton-photon detuning. Competing effects such as laser heating, phonons, and disorder are analyzed. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P32.00011: Many-body two-quantum coherences in 2DFT spectra of semiconductors Denis Karaiskaj, A. Bristow, X. Dai, L. Yang, S. Mukamel, R. Mirin, S. Cundiff Investigating the correlations of multiple excitons in semiconductors is a challenging many-body problem that has drawn considerable experimental and theoretical attention over the last two decades. Nonlinear four-wave mixing (FWM) experiments have long been known to provide direct probes for the many-body effects in the ultrafast dynamics of excitons in quantum wells. However, it is very difficult to separate the different contributions such as excitation induced dephasing, excitation induced shift, local field effects, and multiple exciton correlations. With the advent of two-dimentional Fourier-transform (2DFT) spectroscopy, the biexcitonic contributions could be isolated and the many-body contributions could be identified. Phase-resolved 2DFT spectra for the negative delay FWM signal will be presented which show interesting diagonal and off-diagonal peaks. The energy positions, line shapes, and the complexity of the 2D peaks indicate significant many-body coherences and reinforce the ability of 2DFT to disentangle two-quantum transitions (D. Karaiskaj, \textit{et al}., Phys. Rev. Lett. \textbf{104}, 117401 (2010)). [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P32.00012: Electron transfer and relaxation dynamics in heterovalent ZnSe/GaAs quantum well structures Amit Dongol, Hans Peter Wagner We investigate the electron transfer and relaxation dynamics in heterovalent ZnMgSe/ZnSe quantum wells (QW's) grown on GaAs using the nonlinear optical method of three-beam degenerate four-wave-mixing (FWM). We use ultra-short (90 fs) laser pulses with non-collinear wave-vectors \textbf{k}$_{1}$, \textbf{k}$_{2}$ and \textbf{k}$_{3}$ at a center wavelength of 441 nm ($\sim $2.81 eV) which is resonantly tuned to the heavy hole exciton transition energy at 25 K. In the experiment the time coincident strong pump pulses \textbf{k}$_{1}$ and \textbf{k}$_{2}$ creates both an exciton density grating in the QW and an electron-hole pair grating in the GaAs while the delayed weak pulse \textbf{k}$_{3}$ simultaneously probes the exciton lifetime T$_{1}$ as well as the electron grating injection time T$_{t}$ from the substrate into the QW. Intensity dependent experiments reveal that the diffraction efficiency due to the electron grating increases faster with increasing \textbf{k}$_{1}$ and \textbf{k}$_{2}$ pulse intensities than the FWM efficiency due to the generated exciton density grating. This behavior which is attributed to exciton bleaching at high intensities enables the discrimination of times T$_{1}$ and T$_{t}$, both being in the order of a few tens of picoseconds. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P32.00013: Excitons in moving lattices Jason Leonard, Alexander Winbow, Mikas Remeika, Yuliya Kuznetsova, Alexander High, Aaron Hammack, Leonid Butov, Joseph Wilkes, Alrun Guenther, Alexander Ivanov, Micah Hanson, Arthur Gossard We report on the study of indirect excitons in moving lattices--conveyers--created by a set of AC voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage and the speed is controlled by the AC frequency. We probed the conveyer speeds from well below to well above the sound velocity. We observed the dynamical localization-delocalization transitions for excitons in the conveyers and measured its dependence on the exciton density and conveyer speed and amplitude. We also developed the theory of exciton transport via conveyers. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P32.00014: Effect of growth kinetics on intersubband transitions in GaN/AlN multiple quantum wells J. Yang, S.D. Carnevale, T.F. Kent, M.R. Brenner, R.C. Myers The large conduction band offset of nearly 2 eV between GaN and AlN provides very large electron confinement that could be useful for ultrafast intersubband--based photonics operating at telecommunications wavelengths. However, it is difficult to control interface roughness and compositional profiles with monolayer precision, which is crucial for engineering sublevels for quantum cascaded intersubband photonics. Here we examine the effect of Ga-rich and N-rich growth conditions of highly-confined GaN/AlN multiple quantum wells prepared by plasma-assisted molecular beam epitaxy. Structural quality is examined through high-resolution x-ray diffraction and atomic force microscopy. The efficiency of intersubband and interband transitions in these heterostructures is measured using temperature dependent absorption and photoluminescence spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P32.00015: Trions and quatrons in Semiconductor Coupled Quantum Wells Roman Ya. Kezerashvili, Oleg L. Berman The three-body restricted problem for trions, when a spatially separated exciton and electron or hole are located in the parallel quantum wells (QW), is reduced to the 2D two body problem for the exciton and the projection of the electron or hole on the plane of the excitonic QW. In the limit of a large spatial separation of the QWs the eigenfunctions and energy spectrum for the trions are obtained analytically. It is shown that the Schr\"odinger equation for the trion can be reduced to the 2D two-body problem with Coulomb electron-hole interaction for the 2D direct exciton and the Schr\"odinger equation for the 2D harmonic oscillator for the relative motion of the exciton and the image of the projection of the electron or hole on the plane of the quantum well with the exciton. The 2D Wigner crystallization of the trions in the coupled QWs is discussed. The four-body restricted problem for spatially separated exciton and electron and hole, located in the in three parallel QWs, is reduced to the 2D three body problem for the exciton and the projection of the electron and hole on the plane of the excitonic QW. In the limit of a large spatial separation of the QWs the eigenfunctions and energy spectrum for quatron formed by the exciton and electron and a hole are obtained analytically. The 2D superfluidity and Kosterlitz-Thouless phase transition in the dilute Bose gas of quatrons is discussed. [Preview Abstract] |
Session P33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: BiFeO3
Sponsoring Units: DMP DCOMPChair: Nicole Benedek, Cornell University
Room: C143/149
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P33.00001: Manipulation of the domain structure in mixed-phase BiFeO3 epitaxial films Yi-Chun Chen, Hsin-Hua Lee, Feng-Nan Chu, Wen-Chuan Hsieh, Q. He, Wen-I Liang, Ying-Hao Chu Strain-induced phase transformation in epitaxial films is the newly advance in thin-film growth techniques. Under the compressive strain from the substrate, the stable phase of multiferroic BiFeO3 (BFO) films transformed from rhombohedrally- to tetragonally- distorted monoclinic perovskite, which simulated the material system near the morphotropic phase boundary. In this study, we used piezoresponse force microscopy (PFM) to investigate the intrinsic domain structures in the mixed-phase BFO epitaxial films. PFM taken along the principal crystallographic directions revealed the domain polarizations. The IP PFM images indicated the coexistence of at least two monoclinic phases with IP distortions along [100] and [110]. The domains were distributed in the way to minimize the local electrostatic energy, and the mixed phase pattern can be effectively controlled by external fields. The dynamic switching parameters for the domain and phase manipulation, such as switching speed, switching direction, and applying voltages, were systematically investigated. This study provides basic understanding and electrical control of this unique phase boundary. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P33.00002: Oxygen ordering and electrochromism in Ca-doped BiFeO$_3 $ Jan Seidel, Weidong Luo, Phi Nguyen, Suresha Siriyara Jagannatha, Alan Lee, Sang-Yong Kim, Chan-Ho Yang, Stephen Pennycook, Sokrates Pantelides, Ramamoorthy Ramesh We show that calcium-doped bismuth ferrite thin films exhibit an electrochromic effect arising from an intrinsic mechanism due to redistribution of carriers, without the need for additional electrolytes that are needed in common electrochromic devices. The absorption change and coloration efficiency at the band edge are $4.8\times 10^6m^{-1}$and $190cm^2C^{-1}$, respectively, which are among the highest reported values for inorganic electrochromes. These experimental findings are supported by optical absorption calculations from first-principles theory, confirming the strong absorption change at the band edge. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P33.00003: Polarity Control of Ferroelectric BiFeO$_{3}$/Metal Junctions for Switchable Diode and Photovoltaic Devices Tae Won Noh, Daesu Lee, T.H. Kim, S.H. Baek, C.M. Folkman, C.B. Eom, J.-G. Yoon Ferroelectric materials possess spontaneous polarization which can be used to control numerous functionalities of the materials by switching the polarization or modifying domain structure with an electric field. One of emerging phenomena in ferroelectrics is the control of charge transport by switching the polarization. Although charge conduction ($i.e.$, leakage current) in ferroelectrics has been considered as a detrimental factor to practical applications, an interaction between conduction and ferroelectric polarization has recently attracted much attention as a route for novel functionalities. In this presentation, we will report on the ferroelectric control of charge conduction in BiFeO$_{3}$/metal junctions: the BiFeO$_{3}$/metal interfaces can have either blocking or non-blocking contacts according to the polarization direction of the BiFeO$_{3}$ films, allowing non-volatile control of diode-like conduction characteristics. The resulting rectifying and photovoltaic effects can be turned on and off, as well as be flipped in polarity, depending on the ferroelectric domain structures. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P33.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P33.00005: Magnetic Order in single-crystal BiFeO$_{3}$ Mehmet Ramazanoglu, Sang-Wook Cheong, Valery Kiryukhin, William Ratcliff, S. Lee We report neutron scattering studies of the magnetic order in multiferroic Bismuth Ferrite (BiFeO$_{3}$). In ferroelectric monodomain single crystals, there are 3 equivalent magnetic cycloidal domains. The cycloid period slowly grows with increasing temperature, and the antiferromagnetic transition is 2nd order. The equivalent magnetic domain populations do not change with temperature, except in the close vicinity of the Neel temperature. No evidence for the spin-reorientation transitions proposed in previous Raman studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The anharmonicity is much smaller than previously reported in indirect NMR studies. At room temperature, a circular cycloid is observed. The observed anharmonicity provides important clues for understanding electromagnons in BiFeO$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P33.00006: Chemical substitution induced ferroelectric polarization rotation in BiFeO$_{3}$ thin films Ichiro Takeuchi, Daisuke Kan, Anbusathaiah Varatharajan The direction of the polarization vector in ferroelectric materials is an important parameter critical to a number of applications. Polarization orientation in ferroelectric thin films can be controlled by various approaches such as electric- field induced rotation and strain engineering using exotic substrates. We have performed systematic chemical substitution of rare earth cationic dopants, in particular Sm in the BiFeO$_ {3}$thin films, and found that the polarization vector rotates from the (111) to the (001) direction as a continuous function of the dopant concentration. This is accompanied by enhanced dielectric $\epsilon$$_{33}$ as well as piezoelectric coefficient d$_{33}$, and the maximum in d$_{33}$ (110 pm/V) is achieved at 14\% Sm. We will discuss the correlation between the polarization rotation, structural evolution and other properties as a function of chemical substitution. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P33.00007: Strain-stabilized phases of BiFeO3 and the role of first-principles calculations Invited Speaker: After many years of focused attention from the scientific community, the ferroelectric material BiFeO$_3$ (BFO) continues to be one of the most intriguing and technologically promising of the multifunctional ferroelectrics. Here I will discuss some of the recent developments on BFO thin films, including the metastable ``super-tetragonal'' phase achievable in epitaxial thin films. This strain-stabilized phase has been observed to coexist with a bulk-like phase, and reversible switching between the two has been demonstrated by acting on their ferroelectric polarizations with an external electric field. [Zeches {\em et al.}, Science 326, 977 (2009).] Related work finds a phase transition path of rhombohedral-to-monoclinic-to-tetragonal for epitaxially strained BFO, suggesting comparison to a compositional morphotropic phase boundary. I will discuss these results with an emphasis on the contributions from first-principles calculations, and provide context for understanding the calculated behaviors. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P33.00008: Experimental evidence of stress-induced R-M$_{A}$-M$_{C}$-T phase transition in BiFeO$_{3}$ films Hans Christen, Joong-Hee Nam, Charlee Bennett, Hyun-Sik Kim, Michael Biegalski Recent reports on epitaxial BiFeO$_{3}$ films show that the crystal structure changes from nearly rhombohedral (``R-like'') to nearly tetragonal (``T-like'') at strains exceeding $\approx $-4.5{\%}, with the ``T-like'' structure being characterized by a highly-enhanced $c/a$ ratio. While both the ``R-like'' and the ``T-like'' phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M$_{A}$ and M$_{C}$ type, respectively. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the ``T-like'' M$_{C}$ phase is reduced, i.e. the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly-strained films, a phase sequence of rhombohedral(R)-to-monoclinic(``R-like'' M$_{A})$-to-monoclinic(``T-like'' M$_{C})$-to-tetragonal(T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e. near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P33.00009: Strained BiFeO$_{3}$ Films: Rhombohedral-Orthorhombic and Rhombohedral-Tetragonal Phase Transitions. Part I: Phase-Field Simulations Guang Sheng, Jingxian Zhang, Zi-Kui Liu, Long-Qing Chen, Yulan Li In this study, the strain-temperature phase stability diagrams of (001) BiFeO$_{3}$ thin film were constructed using both thermodynamic analysis and phase-field simulations. The predicted diagram reveals a tetragonal to distorted rhombohedral phase boundary around 4.3{\%} compressive strain and rhombohedral to orthorhombic boundary at around 2{\%} tensile strain, both at room temperature. The predicted transition temperatures for rhombohedral-orthorhombic and rhombohedral-tetragonal transitions are in reasonable agreement with experimental observations. We will also discuss domain structure evolutions of BiFeO$_{3}$ thin films during the above two transitions from phase-field simulations. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P33.00010: Strained BiFeO3 Films: Rhombohedral-Orthorhombic and Rhombohedral-Tetragonal Phase Transitions. Part II: Film Growth by Molecular-Beam Epitaxy Carolina Adamo, R. Misra, A. Melville, C. Heikes, Q. He, Y. Chu, J. Lee, R. Haislmaier, S. Denev, V. Gopalan, R. Ramesh, P. Schiffer, D. Schlom Recently, Zeches et al.[1] reported the strain-temperature phase stability diagram of (001) BiFeO3 thin films. Depending on the strain and temperature the stable polymorph of BiFeO3 is predicted to be (monoclinically distorted) rhombohedral, tetragonal, or orthorhombic. To test these predictions commensurate BiFeO3 thin films were grown by adsorption-controlled reactive molecular-beam epitaxy on (110) YAlO3, (110) NdGaO3, (100) LSAT, (001) SrTiO3, (110) DyScO3, (110) TbScO3, (110) GdScO3, (110) SmScO3, (110) NdScO3, and (110) PrScO3 single crystalline substrates. The films span a biaxial strain range from -6.7\% to +1.4\%. Four-circle x-ray diffraction was used to determine the in-plane and out-of-plane lattice parameters. The rocking curve full widths at half maximum in omega of the films were nominally identical to the substrates on which they are grown and ranged from 11 arc sec (0.003 degrees) on (110) PrScO3 to 49 arc sec (0.014degrees) on (110) YAlO3. Magnetic data and second harmonic generation results will be presented. [1] R. J. Zeches, M. D. Rossell, J. X. Zhang, A. J. Hatt, Q. He, C.-H. Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, G. Sheng, Y.-H. Chu, J. F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L. Q. Chen, D. G. Schlom, N. A. Spaldin, L. W. Martin, and R. Ramesh, Science 326, 977 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P33.00011: Strained BiFeO$_3$ films: rhombohedral-orthorhombic and rhombohedral-tetragonal phase transitions. Part IV: ultraviolet Raman spectroscopy study Dmitri A. Tenne, A.K. Farrar, G. Molino, C. Heikes, C. Adamo, J.H. Lee, A. Melville, D.G. Schlom, G. Sheng, L.Q. Chen, Y.-H. Chu, Q. He, R. Ramesh Epitaxial BiFeO$_3$ films grown by molecular-beam epitaxy on substrates inducing different lattice-mismatch strain (YAlO$_3$, SrLaAlO$_4$, PrScO$_3$) have been studied by variable- temperature ultraviolet Raman spectroscopy. Temperature evolution of Raman spectra from BiFeO$_3$ films indicates the phase transitions from rhombohedral to tetragonal phase in compressively strained films on YAlO$_3$ and SrLaAlO$_4$ substrates. The films grown on PrScO3 substrates are subject to $\sim$1.3\% tensile strain, and undergo the transition from rhombohedral to orthorhombic phase at about 550-600 K. The temperature dependence of Raman intensities of certain characteristic peaks indicates the possibility of coexisting rhombohedral and orthorhombic phases in the temperature range 400-550 K. Raman results are consistent with the phase diagram calculated using the phase field model. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P33.00012: Emergence of 90$^\circ$ Domain Walls in Multiferroic BiFeO$_3$ Thin Film Jan-Chi Yang, Ying-Hao Chu, Chun-Yen Peng, Hsiang-Jung Chen, Li Chang, Qing He, Ramamoorthy Ramesh, Chao-Hui Yeh, Heng-Jui Liu, Sheng-Jie Liao, Po-Wen Chiu, Chih-Hung Lai Multiferroics have been a fascinating area for condensed materials research since these materials offer the exciting potential applications that taking advantages of multiple orders. Multiferroic BiFeO$_3$ (BFO) has played a key role in rejuvenating the field after a report of large ferroelectric polarization. Inside this material, domain walls (DWs) of BFO are of great interests. In recent study, room-temperature conductivity at ferroelectric DWs has been observed. In this work, through epitaxial strain, BFO thin films are grown on NdScO$_3$, which provide tensile strain on BFO films, and thus results in orthorhombic-like phase and corresponding periodic 90$^\circ$ DWs. X-ray reciprocal mapping and piezoresponse force microscopy has confirmed the orthorhombic-like and 90$^\circ$ DW structure. The transport behaviors of these natural formed 90$^\circ$ DWs as a function of temperatures and magnetic fields have been probed to understand their fundamental properties. In addition, exchange bias studies and X-ray magnetic dichroism spectromicroscopy have further revealed the magnetic properties in these DWs. Our results show that 90$^\circ$ DW in orthorhombic-like BFO possesses unusual electronic and magnetic behaviors, which are different from that in bulk and might be used for modern electronic devices and nanoelectronic. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P33.00013: ABSTRACT WITHDRAWN |
Session P34: Focus Session: Interfaces in Complex Oxides - Transport and Optics
Sponsoring Units: DMPChair: Jean-Marc Triscone, University of Geneva
Room: C141
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P34.00001: Electrodynamics and electronic structure of LVO/SVO superlattices investigated by optical spectroscopy Da Woon Jeong, Woo Seok Choi, Tae Dong Kang, David Adrian, Yun Sang Lee, Wilfrid Prellier, Tae Won Noh Perovskite vanadium oxide has intriguing coupling between orbital, spin and lattice degrees of freedom that bears novel physical properties. For example, filling controlled insulator to metal transition could be observed in (La$_{1-x}$Sr$_{x})$VO$_{3}$, and orbital ordering was predicted theoretically for the interface valence state (V$^{3.5+})$ between LaVO$_{3}$ and SrVO$_{3}$ [1]. Here, we investigated the charge dynamics and electronic structures of (LaVO$_{3})_{6m}$(SrVO$_{3})_{m}$ (m=1,2, and 4) superlattices using optical spectroscopy. We found a reduction of Drude spectral weight as the superlattice periodicity is decreased, consistent with the transport result [2]. Moreover, interband transition of (LaVO$_{3})_{6}$(SrVO$_{3})_{1}$ was quite different from other larger period superlattices. New peak structure at 3.5eV was developed possibly due to the correlation between the electronic structure and orbital confinement. Electrodynamics and electronic structure reconstruction will be discussed.\\[4pt] [1] G. Jackeli \textit{et al}., PRL, \textbf{101}, 216804 (2008)\\[0pt] [2] W. C. Sheets \textit{et al}., APL \textbf{91}, 192102 (2007) [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P34.00002: Geometrically confined doping in LaVO$_{3}$/SrVO$_{3}$ superlattices U. Lueders, A. David, Ph. Boullay, R. Fr\'esard, W. Prellier, P.-E. Janolin A number of theoretical predictions show that in complex oxides the confinement of t$_{2g}$ electrons to two dimensions can alter strongly the physical properties of these systems compared to their 3D counterpart. To approach experimentally the 2D limit we propose geometrically confined doped superlattices as LaVO$_{3}$/SrVO$_{3}$. Here, a one unit cell thick layer of SrVO$_{3}$ is introduced between insulating LaVO$_{3}$ layers to create conducting zones with a 2D character. We synthesized this kind of superlattices by PLD on SrTiO$_{3}$ (001) substrates. The 2D character of the doped charge carriers influences strongly the physical properties of the superlattices. While the bulk solid solution is an insulating antiferromagnet, in the superlattices, room-temperature magnetism is observed due to the reduction of the bandwidth and a transition from a high temperature weakly localized phase to a low temperature metallic phase is shown to be connected to a structural transition from a metrically tetragonal to monoclinic phase. With the help of theoretical calculations, we will show that these peculiar properties are due to a change of the orbital physics in the vicinity of the SrVO$_{3}$ doping layers. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P34.00003: Theory of LaVO$_3$/SrVO$_3$ superlattices Hung Dang, Andrew Millis We present dynamical mean field theory calculation and compare to experimental data [1] of the magnetic, orbital order and metal-insulator phase diagrams of LaVO$_3$/SrVO$_3$ superlattices. The calculation is based on a three-orbital model system; semiclassical and quantum Monte Carlo impurity solvers are used. \\[4pt] [1] U. Luders, W. C. Sheets, A. David, W. Prellier, and R. Fresard, Phys. Rev. B 80, 241102(R) (2009). [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P34.00004: New Optical Absorption Bands in Atomic Layer Superlattices Invited Speaker: Using atomic layer-by-layer molecular beam epitaxy, atomic layer superlattices can be constructed that exhibit new electronic, optical and lattice effects not present in the individual components. In particular, new optical transitions giving rise to sharp absorption peaks can be created by placing a layer of a material with occupied source states next to a layer of another material with unoccupied destination states. We combine atomic layers of SrTiO$_{3}$ and LaMnO$_{3}$ into superlattice structures with component layers as thin as single monolayer and find a new absorption band due to a transition from manganese- to titanium-derived states. The energy of the new transition depends on how the bands line up at the interface. Furthermore, a substantial shift of spectral weight occurs as well, while retaining a constant sum rule. This work was supported by the Department of Energy Basic Energy Sciences at the Fredrick Seitz Materials Research Laboratory, University of Illinois, Urbana. This work was done in collaboration with Xiaofang Zhai, Mao Zheng, Amish Shah, Chandra Mohapatra, and Jian-Min Zuo. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P34.00005: Colossal Magnetoresistance in thin films of the Mott metal CaVO$_{3 }$ Jiwei Lu Bulk CaVO$_{3}$ (CVO) is a Pauli paramagnetic metal with a singe 3d electron. Some unusual drastic changes in the magneto-resistance, magnetic susceptibility and the Hall effect have been reported in single crystal CVO. We have simultaneously synthesized epitaxial CVO films grown on three differently oriented SrTiO$_{3}$ substrates. The temperature dependent conductivity of these CVO films demonstrated very strong Fermi metal behavior and the resistance ratio, defined as R(300 K)/R (2K) was more than 3000. Colossal magneto-resistance (MR) as well as large crystalline anisotropic was observed at low temperatures. The maximum MR, defined as (R(7 T)-R(0 T))/R(0 T)*100 {\%}, was over 1500 {\%} at 2 K and 7 Telsa on the CVO films deposited on a (110) SrTiO3 single crystal substrate, and didn't show any sign of saturation. An MR of over $\sim $ 500 {\%} and $\sim $ 200 {\%} were observed on (111) and (100) orientation films under the same condition, respectively. The MR ratio was much larger than that of single crystal CVO. We will discuss the peculiar MR in association with the magnetic ordering, oxygen stoichiometry and Fermi surface. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P34.00006: High frequency conductivity of few unit cell thick LaNiO$_{3}$ layers Daniel Ouellette, Junwoo Son, Susanne Stemmer, S. James Allen We have measured the dc and optical conductivity of ultra-thin films of the correlated metal LaNiO$_{3}$ and of superlattices with alternating LaNiO$_{3}$ and insulating SrTiO$_{3}$ layers, all grown epitaxially on LSAT substrates with modest tensile strain in the LaNiO$_{3}$. In the superlattices, the LaNiO$_{3}$ layers are 4 unit cells (1.6 nm) thick and the SrTiO$_{3}$ is either 4 or 10 unit cells thick. Isolated films thinner than 4 nm are insulating; however, both the superlattices and thicker films consistently show a coherent peak in the optical conductivity. Furthermore, the optical conductivity at several hundred GHz is in excellent agreement with the dc electrical conductivity. We consider the possibility of a percolation threshold in ultra-thin LaNiO3 layers and discuss implications for the nature of the LaNiO$_{3}$/SrTiO$_{3}$ interface. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P34.00007: Quasi 2D correlated metals: Unusual transport properties in strained heteroepitaxial ultrathin films E.J. Moon, B.A. Gray, J. Liu, M. Kareev, B. Dabrowski, J.W. Freeland, I-C. Tung, M.J. Bedzyk, L.H. Tjeng, S.G. Altendorf, F. Strigari, V.P. Kunets, G.J. Salamo, J. Chakhalian We explore the electrical transport and magnetoconductance in quasi 2D strongly correlated heteroepitaxial films of LaNiO$_3$ to investigate the effect of quantum confinement and strain on electron-electron and electron-lattice interactions over the whole temperature range (2-300K) including the effect of metal-insulator transition. The quantum corrections to the conductivity indicate that the combination of the weak localization and the electron-electron interaction in the quasi 2D limit gives rise to unusual T-dependent resistivity. Ultrathin films spanning tensile strain up to $\sim$4$\%$ are used to obtain the enhanced driving effects between the two corrections for the observed localization at low temperatures. Intrinsic transport properties of strained LaNiO$_3$ films with the characteristic multi-band structure will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P34.00008: Strain-modified thermopower of ultrathin LaNiO$_3$ films Narayan Prasai, Joshua Cohn, Eun Ju Moon, Jian Liu, Michael Kareev, Benjamin Gray, Jak Chakhalian, James Rondinelli The influence of epitaxial strain on electronic transport in the correlated metal LaNiO$_3$ is investigated through measurements of thermopower (TEP) in the temperature range $5{\rm K}\leq {\rm T}\leq 330 {\rm K}$ on a series of fully-strained, 10-unit-cell-thick films grown by pulsed-laser deposition on (100)-oriented YAlO$_3$, LaAlO$_3$, SrTiO$_3$, and GaScO$_3$ substrates. The TEP exhibits an electron-like, linear-$T$ contribution for $T\geq 150$~K with a slope approximately independent of strain, but a magnitude that varies systematically with strain. A peak in the TEP at $T\approx 25$~K also correlates with strain and is unaffected by a 9-T magnetic field. The implications of these results for strain-modified charge-carrier diffusion and phonon drag contributions to the TEP will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P34.00009: Strain control of the metal-insulator transition of NdNiO3 epitaxial ultrathin films Jian Liu, M. Kareev, B. Gray, P. Ryan, J.W. Kim, J.W. Freeland, J. Chakhalian Metal-insulator transition (MIT) is the hallmark of strongly correlated electron systems. It often couples with the multiple degrees of freedom of $d$ electrons in complex oxides, resulting in diverse and intriguing properties. While MIT has been studied for decades, heteroepitaxy is emerging as a promising way to manipulate correlated electrons and stabilize unusual phases in nanostructures. Understanding its effect on the MIT in ultrathin structures is fundamentally and technologically critical. To this end, we have grown perfectly strained atomic layers of NdNiO3 by laser MBE on a series of substrates with large variation in lattice mismatch. The extensive measurements including electric and thermal transport, synchrotron based XRD and XAS show dramatic modifications of electronic properties with lattice mismatch. Possible microscopic mechanisms are discussed. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P34.00010: Two-dimensional electron gas in SrTiO$_{3}$ Bharat Jalan, S. James Allen, Susanne Stemmer We report on Shubnikov-de Haas (SdH) oscillations in a two-dimensional electron gas (2DEG) in delta-doped SrTiO$_{3}$ thin films. The existence of a 2DEG is confirmed by the angular dependence of the SdH oscillations. The observed SdH oscillation frequency corresponds to a carrier concentration of, which is only 4{\%} of the total Hall carrier density. We show that the only electrons in one of the sub-bands confined by the delta-doping potential have sufficient mobility to exhibit quantum oscillations. Guided by the similarity of the confined $d$-band electron states in SrTiO$_{3}$ to the confined hole systems in conventional semiconductors, quantum oscillations are interpreted in terms of spin and Landau level splitting. Despite the inherent complexity of a sub-band that is derived from four $d$-band states near the conduction band minimum of SrTiO$_{3}$, we show that the quantum oscillations can be modeled quantitatively. Alternative routes to realize high mobility 2DEGs in SrTiO$_{3}$ will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P34.00011: High Temperature Conductance Characteristics of Differently Modified LaAlO$_{3}$/SrTiO$_{3}$-Heterostructures Felix Gunkel, Susanne Hoffmann-Eifert, Jos\'{e}e E. Kleibeuker, Peter Brinks, Mark Huijben, Guus Rijnders, Gertjan Koster, Regina Dittmann, Rainer Waser In order to understand the physical origin of the high charge carrier density at the conducting interface between SrTiO$_{3}$ (STO) and LaAlO$_{3}$ (LAO) the role of defects has to be clarified. In this study, LAO/STO-heterostructures modified in stacking sequence and growth conditions were investigated by means of high temperature conductance (HTC) measurements under changing oxygen ambience. Under measurement conditions the samples are in equilibrium with the surrounding oxygen atmosphere, which rules out the effect of mobile oxygen vacancies on the interface conductivity [Gunkel et al., APL 97(2010)]. The HTC characteristics show a significant dependency on the preparation procedure of STO and LAO close to the interface. Nevertheless, a common conduction and charge compensation mechanism can be identified. The results are discussed with respect to the defect chemistry model of perovskite oxides. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P34.00012: Magnetotransport behavior of LaAlO$_3$/SrTiO$_3$ and Ar-irradiated SrTiO$_3$: implications on Rashba Spin-Orbit coupling and magnetism in quasi-2-dimensional electron gases J.H. Ngai, Y. Segal, F.J. Walker, S. Ismail-Beigi, K. LeHur, C.H. Ahn, D. Su, Y. Zhu Quasi 2-dimensional electron gases (Q2DEG) in complex oxide heterostructures exhibit a wide range of tunable behavior that promises potentially new functionalities. In particular much interest has focused on the Q2DEG at the LaAlO$_3$/SrTiO$_3$ interface, where evidence for Rashba spin-orbit coupling and magnetic behavior has recently been found through magnetotransport measurements. At present it is unclear whether the spin-orbit and magnetic effects are unique to the LaAlO$_3 $/SrTiO$_3$ interface, or if they arise from the properties of doped SrTiO$_3$. In order to shed light on this issue, we compare the magnetotransport properties of LaAlO$_3$/SrTiO$_3$ and Ar$^+$-irradiated SrTiO$_3$. Strikingly similar magnetotransport behavior is observed between LaAlO$_3$/SrTiO$_3 $ and Ar$^+$-irradiated Q2DEGs. However, our analysis indicates changes in the confinement or thickness of the Q2DEG can account for the magnetotransport observed. Implications of our results on Rashba spin-orbit coupling and magnetism at the LaAlO$_3$/SrTiO$_3$ interface will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P34.00013: Can an oxygen vacancy form a Kondo center at the LaAlO$_3$/SrTiO$_3$ interface? Mohammad Sherafati, Birabar Nanda, Sashi Satpathy Recently a Kondo resistance minimum has been observed at the interface between LaAlO$_3$ and SrTiO$_3$[1]. It has been suggested that the effect is due to the scattering of interface electrons from magnetic centers just like in the original Kondo effect; however, the origin of such magnetic centers is not understood. In this work, we evaluate the idea of whether an oxygen vacancy in SrTiO$_3$ might produce a magnetic center. We focus on an isolated vacancy in bulk SrTiO$_3$ from density-functional calculations and provide evidence that of the two electrons released to the system by the oxygen vacancy, one becomes localized near the vacancy site, while the other forms a delocalized state. The results suggest that the localized electron could form a Kondo center resulting in a resistance minimum as observed in the experiments. \\[4pt] [1] A. Brinkman {\it et al.}, Nature Mater. {\bf 6} 493 (2007). [Preview Abstract] |
Session P35: Topological Insulators: Magnetotransport
Sponsoring Units: DCMPChair: Jens Bardarson, University of California, Berkeley
Room: C140
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P35.00001: Transition from n-type to p-type topological insulator thin films of Bi$_{2}$Te$_{3}$ Jin-Feng Jia, Guang Wang, Xie-Gang Zhu, Yao-Yi Li, Tong Zhang, Jing Wen, Xi Chen, Ke He, Li-Li Wang, Xu-Cun Ma, Yi-Yang Sun, Sheng-Bai Zhang, Qi-Kun Xue By using angle-resolved photoemission spectroscopy, we have investigated the electronic structure of the Bi$_{2}$Te$_{3}$ films on Si(111) prepared by molecular beam epitaxy. It is found that the Bi$_{2}$Te$_{3}$ films change from n-type to p-type topological insulator when the growth mode changes layer-by-layer to step-flow, for a given beam flux ratio of Te$_{2(4)}$/Bi. In situ scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal formation of different defects, i. e., Te$_{Bi}$ and Bi$_{Te}$ antisite defects, which are responsible for the n- and p-type conductivity transition. A mechanism for the transition is proposed based on the STM experiment and first-principles calculations. The work suggests a simple way to regulating the chemical potential and Dirac fermion density on the surface of a topological insulator without external doping. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P35.00002: Magneto-Transport in Epitaxial Thin Film $Sb_{2}Te_{3}$ E.C. Gingrich, N.O. Birge, G. Wang, C. Uher We report magneto-transport measurements in epitaxially grown thin films of $Sb_{2}Te_{3}$. $Sb_{2}Te_{3}$ is a topological insulator candidate expected to possess a single Dirac cone on its surface.\footnote{H. Zhang et al., Nat. Phys. 5, 438 (2009).} Both semiconducting and metallic samples were measured, identified by the temperature response of their resistivities. Shubnikov-deHaas (SdH) oscillations were found in measurements with fields up to 9T in metallic samples at liquid helium temperature, but have yet to be observed in semiconducting samples. Measurements will be presented along with plans for further research. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P35.00003: Anomalous galvanomagnetism, cyclotron resonance and microwave spectroscopy of topological insulators Ewelina Hankiewicz, Grigory Tkachov The surface quantum Hall state, magneto-electric phenomena and their connection to axion electrodynamics have been studied intensively for topological insulators. One of the obstacles for observing such effects comes from nonzero conductivity of the bulk. To overcome this obstacle we propose to use an external magnetic field to suppress the conductivity of the bulk carriers. The magnetic field dependence of galvanomagnetic and electromagnetic responses of the whole system shows anomalies due to broken time-reversal symmetry of the surface quantum Hall state, which can be used for its detection. In particular, we find [1] linear bulk dc magnetoresistivity and a quadratic field dependence of the Hall angle, shifted rf cyclotron resonance, nonanalytic microwave transmission coefficient and saturation of the Faraday rotation angle with increasing magnetic field or wave frequency. \\[4pt] [1] G. Tkachov and E. M. Hankiewicz arXiv:1011.2756 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P35.00004: Experimental evidence on a diffusive metallic surface state by the magneto-resistance oscillation in the topological insulating Bi$_{2}$Te$_{3}$ Fengqi Song, Yuyuan Qin, Zhaoguo Li, Qianghua Wang, Jianguo Wan, Guanghou Wang, Zhe Qu The spin helicity seems bound to the generation of the surface state (SS) in a three-dimensional strong topological insulating system. Here we demonstrate a diffusive metallic SS by measuring the magneto-resistance oscillations in the Bi$_{2}$Te$_{3}$ nanoflakes. The products of the oscillating periods and the cross sections of the flakes result in the values of the flux quantum (h/e) and the half quantum (h/2e). The first observation of the h/2e oscillation shows the violence of the spin helicity of the SS The h/e oscillation persists during increasing the magnetic field, while the h/2e oscillation fails, indicating the diffusive origin of the SS and its weak antilocalization. The diffusive SS is found robust against the increases of the circumference till 10 micrometers and the temperatures till 64K, when the h/e oscillation disappears. The diffusive SS can be further activated by the spin-polarized scattering formed by the deposited Co islands. All the evidence agrees to recent simulations on a weak-localized SS upon the presence of strong disorder centers. This research was supported by the National Key Projects for Basic Research of China (Grant numbers: 2009CB930501, 2010CB923401). [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P35.00005: Two-dimensional surface state revealed in the quantum limit of a topological insulator R. McDonad, J. Analytis, J.-H. Chu, S. Riggs, I. Fisher, G. Boebinger Topological insulators possess a metallic surface state of massless particles, known as Dirac fermions whose spin is coupled to their momentum. The realization of this in Bi2Se3 has sparked considerable interest owing both to the potential for spintronic devices and in the investigation of the fundamental nature of topologically non-trivial quantum matter. However, the conductivity of these compounds tends to be dominated by the bulk of the material owing to chemical imperfection, making the transport properties of the surface nearly impossible to measure. We have systematically reduced the number of bulk carriers in the material Bi2Se3 to the point where a magnetic field can collapse them to their lowest Landau level. Beyond this field, known as the three-dimensional (3D)`quantum limi', the signature of the 2D surface state can be seen. At still higher fields, we reach the 2D quantum limit of the surface Dirac fermions. In this limit we observe an altered phase of the oscillations, which is related to the peculiar nature of the Landau quantization of topological insulators at high field. Furthermore, we observe quantum oscillations corresponding to fractions of the Landau integers, suggesting that correlation effects can be observed in this new state of matter. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P35.00006: The surface-state of the topological insulator Bi2Se3 revealed by cyclotron resonance Oscar Ayala-Valenzuela, James G. Analytis, Jiun-Haw Chu, Moaz-M. Altarawneh, Ian R. Fisher, Ross D. McDonald Recently, a large number of surface-sensitive probes have reported the existence of Dirac quasiparticles, similar to those reported in graphene, on the surface of single crystals of Bi2Se3 and related compounds. To date transport measurements of TIs have been dominated by the conductivity of the bulk, leading to substantial difficulties in resolving the properties of the surface. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity of Bi2Se3 at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state. The frequency-magnetic field scaling of this resonance is inconsistent with the bulk effective mass, but more consistent with the dispersion and band filling of a Dirac-like surface state as observed by ARPES, with substantial many-body renormalization. Measurements as a function sample thickness aid in separating bulk and surface contributions and indicate that the band filling of the Dirac cone varies from cleave to cleave. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P35.00007: Surface Hall response of 3D topological insulators Dimitrie Culcer, Sankar Das Sarma We determine the Hall conductivity due to the surface states of 3D topological insulators in the presence of a weak perpendicular magnetic field and/or magnetization. We consider electron doping and calculate all known contributions to the Hall current, including the intrinsic, skew scattering and side jump terms. Skew scattering contributes to in the Born approximation, as well as giving the usual contribution of third order in the scattering potential. We identify a side-jump scattering term together with an intrinsic side-jump term, which give contributions of a similar magnitude. The dominant term by several orders of magnitude is of the order of the conductivity quantum, and includes a topological contribution and a renormalization due to scattering. The result is independent of the Rashba spin-orbit constant, as well as of the impurity concentration. It has different signs depending on whether the principal source of scattering is charged impurities or short-range interface roughness. We expect our results to help disentangle surface transport from bulk transport in these materials [1]. \\[0pt] [1] D. Culcer, E. H. Hwang, T. D. Stanescu, and S. Das Sarma, Phys. Rev. B 82, 155457 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P35.00008: In Search of the Quantum Anomalous Hall Effect in Ferromagnetic Cr-Bi$_{2}$Se$_{3}$ Topological Insulator Thin Films Pascal P.J. Haazen, J.-B. Laloe, D. Heiman, P. Jarillo-Herrero, J.S. Moodera A recent prediction that the topological insulator $Bi_{2}Se_{3}$ can become magnetically ordered upon doping with Cr or Fe opens up the possibility of observing the quantum anomalous Hall effect, in the absence of an external magnetic field [1]. We report on molecular beam epitaxy-grown Cr-Bi$_{2}$Se$_{3}$ thin-films with a Cr content of $0\sim10$ at.\%, and their properties. Our films show highly oriented crystallinity up to a Cr content of 8\%, as required for ferromagnetic ordering. Films with Cr were ferromagnetic; the measured saturation magnetic moment per Cr atom is $1\mu_{B}$ for the crystalline films, with a $T_{C}$ of up to $\sim25 K$. Currently we are investigating the transport characteristics. Varying the electro-chemical potential level in our quantum anomalous Hall insulators by gating should give rise to plateaus of Hall conductance as the Fermi level passes through the energy gap, due to the breaking of time-reversal symmetry caused by the magnetic ions. \\[4pt] [1] R. Yu et al. Science 329, 61 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P35.00009: Quantum Oscillations in a topological insulator Bi$_2$SeTe$_2$ with large bulk resistivity Jun Xiong, DongXia Qu, Robert Cava, N. Phuan Ong To date, transport experiments on Topological Insulators are seriously hampered by bulk conductance $G_b$ arising from impurity bands or band bending. Because of the large $G_b$, the surface currents carried by the massless Dirac surface states have been very difficult to resolve. We report measurements on the new topological insulator Bi$_2$SeTe$_2$ which has an unusually highly bulk resistivity $\rho$ (6 $\Omega$cm at 4 K, or 1,000 times higher than in Bi2Te3). Despite the large $\rho$, Shubnikov-de Haas (SdH) oscillations are clearly resolved in the Hall conductance up to 38 K, which implies a very high surface mobility. In a field $B$ of 14 T, Landau Levels (LLs) n = 4-9 are well resolved. We will describe the value of the Onsager phase $\gamma$ fixed by the index plot of the LLs. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P35.00010: Thermoelectric Properties of Non-Metallic Topological Insulator Bi$_{2}$Te$_{3}$ at High Magnetic Fields Dong-Xia Qu, Yew San Hor, Robert J. Cava, N. Phuan Ong Three-dimensional topological insulators are a new class of electronic systems characterized by a bulk insulating state and conducting surface states with Dirac-like energy-momentum dispersion [1, 2]. One of the interesting aspects of this material is how the surface states affect thermoelectric properties of the whole electronic system, given that the bismuth based topological insulators are also excellent thermoelectric materials. We studied the low-temperature thermoelectric transport properties of high-mobility bulk topological insulator Bi$_{2}$Te$_{3}$ at high magnetic fields up to 35 T. We found remarkably large quantum oscillations in the thermopower of the surface states over a field range of 14 to 35 T. The existence of a non-zero Berry's phase in surface electrons is confirmed from the magneto-oscillations of both thermopower and magnetoresistance.\\[4pt] [1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. \textbf{98}, 106803 (2007)\\[0pt] [2] Y. Xia \textit{et al}., Nat. Phys. \textbf{5}, 398 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P35.00011: Large thermoelectric figure of merit for three-dimensional topological Anderson insulators via line dislocation engineering Oleg Tretiakov, Artem Abanov, Shuichi Murakami, Jairo Sinova We study the thermoelectric properties of three-dimensional topological Anderson insulators with line dislocations. We show that at high densities of dislocations the thermoelectric figure of merit $ZT$ can be dominated by one-dimensional topologically protected conducting states channeled through the lattice screw dislocations in the topological insulator materials with a nonzero time-reversal-invariant momentum such as Bi$_{0.9}$Sb$_{0.1}$. When the chemical potential does not exceed much the mobility edge the $ZT$ at room temperatures can reach large values, much higher than unity for reasonable parameters, hence making this system a strong candidate for applications in heat management of nanodevices. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P35.00012: Quantum Oscillations and Quantum Hall Effect in Topological Insulator Material Bi$_{2}$Se$_{3 }$ Helin Cao, Ireneusz Miotkowski, Tian Shen, Yong Chen Bi$_{2}$Se$_{3}$ has attracted strong attention recently as a prototype topological insulator material. We have measured magneto-transport in metallic Bi$_{2}$Se$_{3}$ crystals. At high magnetic field (B), the longitudinal resistance (R$_{xx})$ displays characteristic Shubnikov--de Haas (SdH) oscillations (periodic in 1/B). The measurements in tilted magnetic field show the SdH oscillations are only controlled by the perpendicular component of B, indicating 2D nature of charge carriers. We also observed quantized plateaus in Hall resistance (R$_{xy})$ concomitant with the minima in R$_{xx}$. From the temperature dependence of the SdH oscillations, we extract a Fermi velocity $\sim $5.9*10$^{5}$m/s, and an effective mass $\sim $0.14m$_{e}$ (m$_{e}$ is the electron mass). We discuss possible relations of our observations to topological surface states, as well as contributions from individual 2D quintuple layers of Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P35.00013: High field magnetotransport in high purity crystals of topological insulator Bi$_2$Se$_3$ Nicholas Butch, Paul Syers, Johnpierre Paglione We have synthesized crystals of high purity undoped Bi$_2$Se$_3$ with carrier density less than 10$^{17}$ cm$^{-3}$ and high bulk mobility. Thorough characterizations of transport and optical properties in fields up to 14 T suggest a high surface scattering rate [1-3]. In a search for quantum oscillations from the surface carriers, we performed measurements of longitudinal and Hall resistance in pulsed magnetic fields of up to 60 T. Due to the as-yet poorly characterized environmental sensitivity of the Bi$_2$Se$_3$ surfaces, we performed on-site cleaving and application of leads under dry flowing nitrogen. The Hall measurements and temperature- and angle-dependence of the longitudinal magnetoresistance will be discussed. \\[4pt] [1] N. P. Butch, et al., Phys. Rev. B 81, 241301 (2010) \\[0pt] [2] A. B. Sushkov, et al., Phys. Rev. B 82, 125110 (2010) \\[0pt] [3] G. S. Jenkins, et al., Phys. Rev. B 82, 125120 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P35.00014: Anomalous Aharonov-Bohm Conductance Oscillations from Topological Insulator Surface States Yi Zhang, Ashvin Vishwanath We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period $\Phi_{0}=hc/e$, twice the conventional period. The conductance maxima are achieved at odd multiples of $\Phi_{0}/2$, implying that a $\pi$ AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on $Bi_{2}Se_{3}$ nanoribbons, and optimal conditions for observing this effect. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P35.00015: First-principles calculation of the full orbital magnetoelectric response Andrei Malashevich, Ivo Souza, Sinisa Coh, David Vanderbilt The possibility of a quantized magnetoelectric (ME) effect in $Z_2$ topological insulators suggests that the orbital part of the ME response can, at least in principle, be comparable in magnitude to the total response of known ME materials.\footnote{S. Coh {\it et al.}, arXiv:1010.6071.} A band theory of the orbital ME response of generic insulators was recently developed,\footnote{A. Malashevich {\it et al.}, New J. Phys. {\bf 12}, 053032 (2010)} paving the way for first-principles calculations. Two types of terms contribute to the response. The Chern-Simons term, which only depends on the unperturbed valence Bloch states, was the subject of a recent Wannier-based calculation.$^1$ The Kubo terms require a knowledge of the Bloch states at first order in the electric field and can be calculated by finite differences from the change in orbital magnetization induced by small electric fields.$^2$ We present preliminary results of such a calculation for Cr$_2$O$_3$, using the Berry-phase approach to calculate the electronic structure under a finite electrical bias. By monitoring the field-induced changes in orbital and spin magnetization and comparing the results obtained with and without structural relaxation, all contributions to the ME coupling can be computed. [Preview Abstract] |
Session P36: Focus Session: Graphene: Growth, Characterization, and Devices: Electronic Structure
Sponsoring Units: DMPChair: Marco Buongiorno-Nardelli, North Carolina State University
Room: C142
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P36.00001: Electronic properties of the Graphene/SiC (000$\overline 1$) interface: a First Principles study Thushari Jayasekera, Shu Xu, K.W. Kim, Marco Buongiorno-Nardelli In this talk, we will discuss the electronic properties of epitaxial graphene on the SiC (000$\overline 1$) surface (C-terminated face) using Density Functional Theory. In our calculations we focus on mono- and bi-layer graphene with AA, AB and turbostratic stacking sequences. Of the three, the turbostratic is the most observed during growth on SiC (000$\overline 1$). However, no theoretical investigations are available to understand the effect of the substrate on this growth sequence. We will investigate the energetics of different stackings and explain their electronic properties. We will also discuss the role of the interfaces in the stabilization of the individual stacking sequences and indicate possible routes for chemical functionallizations at the heterojunction to facilitate the tuning of the electronic and transport properties of these systems. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P36.00002: Local surface potential variations and charge puddling in graphene on SiC(0001) A.E. Curtin, W.G. Cullen, M.S. Fuhrer, R.L. Myers-Ward, L.O. Nyakiti, V.D. Wheeler, D.K. Gaskill We performed Kelvin probe microscopy in ultra-high vacuum on epitaxial graphene grown on SiC(0001). In agreement with previous work, we see discrete surface potentials corresponding to interface layer and monolayer regions separated by steps of $\sim $100 mV. We used the step width to determine the spatial resolution of the probe to be approximately 20 nm. Within a monolayer area we see smaller fluctuations in surface potential of only a few mV. The data set limits on the scale of possible electron/hole puddles in monolayer graphene on SiC(0001). [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P36.00003: Electric Field Effects on Electronic Structures of Epitaxial Graphene on SiC Hyungjun Lee, Seungchul Kim, Jisoon Ihm, Young-Woo Son, Hyoung Joon Choi We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-faced SiC(0001) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications. This work was supported by the NRF of Korea (Grant No. 2009-0081204). Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P36.00004: Plasmarons in Quasi-freestanding Epitaxial Graphene Invited Speaker: Graphene is a remarkable new electronic material with many unique properties. To realize its promise, it is essential to understand how its charge carriers interact. By measuring the spectral function of charge carriers in quasi-free-standing graphene, we show that at finite doping, the well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We find that there also exist composite ``plasmaron'' particles, consisting of holes coupled to density oscillatons of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P36.00005: Effective screening and the plasmaron bands in Graphene A. Walter, K.J. Jeon, A. Bostwick, L. Moreschini, Y.S. Kim, Y.J. Chang, F. Speck, M. Ostler, T. Seyllar, K. Horn, E. Rotenberg In the following we investigate the plasmaron bands in the presence of differing effective screening, by changing the interface layer between graphene and a SiC substrate. ARPES data is presented and the deviation of the band structure from the Dirac cone picture is attributed to electron, hole, plasmon interactions. Comparison to G$_{0}W$ -RPA theory is used to determine the effective dielectric constant of the underlying layer and a range of values ($\varepsilon _{S} \quad \sim $219 to $\sim $11.6) is found. This investigation indicates that, in addition to the long list of unique and interesting properties, graphene is an ideal candidate for investigating the effective screening in the context of electron-hole-plasmon interactions. It is also shown that plasmaronic and electronic properties of graphene can be manipulated semi-independently, a necessity if it is to be employed in future ``plasmaronic'' devices. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P36.00006: Phonon-Induced Gaps in Graphite and Graphene Observed by Angle-Resolved Photoemission Yang Liu, Longxiang Zhang, Matthew Brinkley, Guang Bian, Tom Miller, Tai-chang Chiang Graphene systems, made of sheets of carbon atomic layers, have unusual electronic structures known as Dirac cones. While strong interest in the electronic structure of these graphitic materials has driven extensive ARPES studies, prior work has mostly focused on the quasiparticle band dispersion relations associated with the Dirac cones. Largely unexplored are spectral regions far away from the quasiparticle bands, where direct emission from the quasiparticles is forbidden, but indirect emission through coupling to phonons is allowed. Our ARPES measurements of graphite and graphene layers at low temperatures reveal heretofore unreported gaps at normal emission, one at around 67 meV and another much weaker one at around 150 meV. The major gap features persist to room temperature and beyond, and diminish for increasing emission angles. We show that these gaps arise from electronic coupling to out-of-plane and in-plane vibrational modes at the K point in the surface Brillouin zone, respectively, in accordance with conservation laws and selection rules governed by quantum mechanics. Our study suggests a new approach for characterizing phonons and electron-phonon coupling in solids. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P36.00007: Many-Body Interactions in Quasi-Freestanding Graphene David Siegel, Cheol-Hwan Park, Choongyu Hwang, Jack Deslippe, Alexei Fedorov, Steven Louie, Alessandra Lanzara The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming the basis of any theory of solids. Whether this picture still holds for a semimetal like graphene at the neutrality point, i.e. when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present the first direct measurements of the self-energy in graphene near the neutrality point, by using high-resolution angle-resolved photoemission spectroscopy. These exciting findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P36.00008: Abnormal temperature-dependent self-energy in graphene Choongyu Hwang, Daniel Garcia, David Siegel, Pu Yu, Swanee Shin, Xiaozhu Yu, Alexei Fedorov, Eugene Haller, Ramamoorthy Ramesh, Dung-Hai Lee, Alessandra Lanzara Dynamics of charge carriers are determined by their self-energy associated with many-body interactions. By using angle-resolved photoemission spectroscopy, we study the origin of abnormal temperature-dependent self-energy in graphene, and discuss the results in terms of a quantum phase transition as a function of temperature. Our findings provide another example of novel electronic properties of graphene and deeper understanding of the ground state of charge carriers in graphene. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P36.00009: Graphene/substrate charge transfer characterized by inverse photoelectron spectroscopy Lingmei Kong, Cameron Bjelkevig, Sneha Gaddam, Mi Zhou, YoungHee Lee, GangHee Han, HaeKyung Jeong, Ning Wu, Zhengzheng Zhang, Jie Xiao, Peter Dowben, Jeffry Kelber Wave vector-resolved inverse photoelectron spectroscopy (IPES) measurements demonstrate that there is a large variation of interfacial charge transfer between graphene and various substrates. IPES measurements of CVD single layer graphene on BN(0001)/Ru(0001), Ru, Ni(poly), and Cu(poly) indicate a substrate-to-graphene charge transfer of approximately 0.07, 0.06, 0.03 e- per carbon atom respectively and a charge transfer of 0.02 e- from graphene to the MgO substrate per carbon atom. IPES and photoemission data also indicate that graphene/MgO(111) has a band gap. These data demonstrate that IPES is an effective method for precise measurement of substrate/graphene charge transfer due to the extreme surface sensitivity of IPES. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P36.00010: Characterization of Image States in Graphene on Ir(111) Jerry I Dadap, Marko Kralj, Marin Petrovic, Kevin Knox, Nader Zaki, Rohan Bhandari, Po-Chun Yeh, Richard M. Osgood Jr. Two dimensional electron systems involving graphene and graphene/metallic interfaces are increasingly of interest in condensed matter physics. Here, we demonstrate two-photon photoemission to map the image states of highly perfect and weakly bonded graphene on an Ir(111) substrate to reveal the effects of interaction with the underlying metal substrate. We observe a monotonic decrease in the work function with increasing graphene coverage from 5.6$\pm $0.1 eV for clean Ir to 4.5$\pm $0.1 eV for full graphene ML. We observe $n$=1, 2, 3 image states with nearly free electron dispersion. Despite the minimal coupling between the graphene and Ir, the energy spacing of the image states is consistent with a single Rydberg series description, in contrast to the expected bifurcation of the image states into odd and even states for a pure graphene layer. At large $k_{\vert \vert }$, we observe a weak state deviating from the $n$=1 dipersion. We explain this effect in terms of scattering from the Ir substrate. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P36.00011: Electronic and chemical properties of epitaxial graphene intercalated with FeCl$_{3}$ Kristin Shepperd, Feng Wang, Jeremy Hicks, Holly Tinkey, Edward Conrad Epitaxial graphene has emerged as the platform for large-scale graphene-based electronics. To fully exploit the unique properties of graphene for electronic materials, a number of materials issues need to be resolved. One important challenge is being able to control the doping of graphene without altering its band structure and disrupting the sp2 graphene bonding. One approach to accomplish this is intercalation of atomic or molecular species between individual graphene layers. We report the intercalation of multilayers of epitaxial graphene (EG) with the electron-acceptor FeCl$_{3}$. We will present results on experiments focused on the intercalation of FeCl$_{3}$ into multilayers EG grown on the C-face of SiC(000-1). Intercalation with different staging was achieved by a standard two-zone vapor transport method. The chemical and electronic properties of the EG-FeCl$_{3}$ intercalation compounds were analyzed using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P36.00012: Accessing high energy sub-bands in bilayer graphene - a transport study Dmitri K. Efetov, Patrick Maher, Simas Glinskis, Philip Kim In contrast to single layer graphene sheets with its two distinct valence and conduction bands merging at the Dirac Point, multilayer graphene sheets are known to have additional sub-bands at higher energies. Whereas the low energy sub-bands in these systems are well studied, the higher energy sub-bands could so far not be accessed in a transport measurement of graphene samples sitting on typical SiO$_{2}$/Si back gates. Employing a poly(ethylene)oxide-CsClO$_{4}$ solid polymer electrolyte gate we demonstrate the filling up of the high energy sub-bands in bilayer graphene samples at carrier densities above $\sim $ 2.7x10$^{13}$ cm$^{-2}$ . The onset of these sub-bands is defined by a slight increase of the resistivity and the onset of Shubnikov de Haas (SdH) oscillations. Measurements of the magneto-resistance, the SdH oscillations and the Hall Effect enable us to deduce the carrier densities and mobilities for both, the high and low energy bands simultaneously. In addition, we find that the onset energy of these sub-bands can be tuned by varying the bilayer interlayer asymmetry. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P36.00013: Magnetic-field induced Electron-K-Phonon Interaction in the optical response of multi-layer epitaxial graphene Gerard Martinez, Milan Orlita, Marek Potemski, Mike Sprinkle, Claire Berger, Walter de Heer, Liang Tan, Steven Louie Absolute magneto-optical transmission measurements have been performed in the far-infra-red range under magnetic fields up to 32 T and at a temperature of 1.8 K on a series of multi-layer epitaxial graphene samples. In all samples, transmission data show for the main optical transition involving the n=0 landau level a clear splitting of the transition in the field range 17-18 T corresponding to an energy of about 150 meV which coincides with that of the K zone boundary phonon of graphene. A global analysis of the data using a multi-dielectric model, to fit them with a single transition, reveals in that range of energies an additional increase of the line-width accompanied by a softening of the transition energy. The energy variation of these quantities is characteristic of the emission of phonons. Possible origins of this effect will be discussed but seems to be the consequence of electron-electron interactions between the two valleys K and K' assisted by K-phonons between these two valleys. [Preview Abstract] |
Session P37: Focus Session: Graphene Structure, Dopants, and Defects: Strain Engineering II
Sponsoring Units: DMPChair: Michael Crommie, University of California, Berkeley
Room: C146
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P37.00001: Beller Lectureship Talk: Gauge fields in graphene Invited Speaker: Graphene is a unique material with many special features not found in other systems. Elastic strains, and also topological defects, act on the charge carriers in the same way as effective gauge fields. The emergence of these fields, their dependence on sample parameters, and their relevance to experiments will be reviewed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P37.00002: Limits on electron quality in suspended graphene due to flexural phonons Eduardo V. Castro, H. Ochoa, M.I. Katsnelson, R.V. Gorbachev, D.C. Elias, K.S. Novoselov, A.K. Geim, F. Guinea The temperature dependence of the mobility in suspended graphene samples has been investigated. In clean samples, flexural phonons become the leading scattering mechanism at temperature $T > 10\,\,$K, and the resistivity increases quadratically with $T$. Flexural phonons limit the intrinsic mobility down to a few m$^2$/Vs at room $T$, a value that is routinely achievable for graphene on a substrate. Their effect can be eliminated by applying strain. Similar qualitative behavior, even though with important quantitative differences, has been found for suspended bilayer graphene. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P37.00003: Topological defect clustering and plastic deformation mechanisms in functionalized graphene Ricardo Nunes, Joice Araujo, Helio Chacham We present ab initio results suggesting that strain plays a central role in the clustering of topological defects in strained and functionalized graphene models. We apply strain onto the topological-defect graphene networks from our previous work [1], and obtain topological-defect clustering patterns which are in excellent agreement with recent observations in samples of reduced graphene oxide [2]. In our models, the graphene layer, containing an initial concentration of isolated topological defects, is covered by hydrogen or hydroxyl groups. Our results also suggest a rich variety of plastic deformation mechanism in functionalized graphene systems. \\[4pt] [1] Joice da Silva-Araujo, H. Chacham, and R. W. Nunes, Phys. Rev. B 81, 193405 (2010).\\[0pt] [2] C. Gomez-Navarro et al., Nano Lett. 10, 1144 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P37.00004: Electronic states of graphene grain boundaries Andrej Mesaros, Stefanos Papanikolaou, C.F.J. Flipse, Darius Sadri, Jan Zaanen Recent scanning tunneling spectroscopy measurements on graphite grain boundaries have identified zero energy peaks in the local density of states. These features are tied to intriguing magnetic properties observed in such samples, but are not found in existing theoretical models. We therefore study amorphous grain boundaries in graphene, and find stable structures along the boundary, responsible for local density of states enhancements both at zero and finite energies. We also consider the low energy continuum theory of arrays of dislocations forming a grain boundary in graphene. It predicts the appearance of localized zero energy states, pending the atomic scale dislocation core structure. We discuss possible stable dislocation core structures that actually carry such states. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P37.00005: Spectromicroscopy measurements of surface morphology and band structure of exfoliated graphene Kevin Knox, Andrea Locatelli, Dean Cvetko, Tevfik Mentes, Miguel Nino, Shancai Wang, Mehmet Yilmaz, Philip Kim, Richard Osgood, Alberto Morgante Monolayer-thick crystals, such as graphene, are an area of intense interest in condensed matter research. ~However, crystal deformations in these 2D systems are known to adversely affect conductivity and increase local chemical reactivity. Additionally, surface roughness in graphene complicates band-mapping and limits resolution in techniques such as angle resolved photoemission spectroscopy (ARPES), the theory of which was developed for atomically flat surfaces. Thus, an understanding of the surface morphology of graphene is essential to making high quality devices and important for interpreting ARPES results. In this talk, we will describe a non-invasive approach to examining the corrugation in exfoliated graphene using a combination of low energy electron microscopy (LEEM) and micro-spot low energy electron diffraction (LEED). We will also describe how such knowledge of surface roughness can be used in the analysis of ARPES data to improve resolution and extract useful information about the band-structure. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P37.00006: Snap-through instability of graphene on corrugated substrates Cesar Chialvo, Scott Scharfenberg, Nikhita Mansukhani, Richard Weaver, Nadya Mason We discuss atomic force microscopy measurements of the interplay between interfacial bonding energy and strain energy in few-layer-graphene (FLG) placed on micro-scale corrugated metallic substrates. For fixed corrugation amplitude and wavelength, the theoretical strain energy of conformed FLG scales with the third power of its thickness. We present evidence of a so-called ``snap-through instability,'' where the behavior of the FLG abruptly changes, as a function of thickness, from fully conformed, to completely detached. The large FLG thickness, and by implication strain energy, at the snap-through point implies that the FLG-substrate bonding is larger than expected for van der Waals forces. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P37.00007: A Quantitative Characterization of Thermally Excited Ripples in Graphene Donald Priour, Jr In the framework of an atomistic model, we calculate the amplitude and typical wavelength of undulations in graphene sheets with length scales similar to those encountered in experiment. As part of a quantitative treatment, bond bending and stiffness constants are fixed by appealing to phonon frequency dispersion curves measured experimentally and from \textit{ab initio} electronic structure calculations. Equilibrium thermodynamic quantities, such as mean square atomic deviations and the average length scale (i.e. the typical ``wavelength'') of graphene ripples, are calculated in the context of statistical mechanical Monte Carlo simulations. Thermally induced rippling is examined for suspended graphene, as well as graphene in the presence of a substrate, where the attractive coupling of atomic species to the substrate layer is modeled with a Lennard-Jones potential. The contribution of quenched substrate disorder to undulations in the graphene sheet relative to the component of graphene ripples due purely to thermal fluctuations is studied by examining graphene sheets bound to substrates with various levels of intrinsic positional disorder. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P37.00008: Wrinkling of graphene membranes supported by silica nanoparticles on substrates Mahito Yamamoto, William Cullen, Michael Fuhrer, Theodore Einstein The challenging endeavor of modulating the morphology of graphene via a patterned substrate to produce a controlled deformation has great potential importance for strain engineering the electronic properties of graphene. An essential step in this direction is to understand the response of graphene to substrate features of known geometry. Here we employ silica nanoparticles with a diameter of 10-100 nm to uniformly decorate SiO$_{2}$ and mica substrates before depositing graphene, to promote nanoscale modulation of graphene geometry. The morphology of graphene on this modified substrate is then characterized by atomic force spectroscopy. We find that graphene on the substrate is locally raised by the supporting nanoparticles, and wrinkling propagates radially from the protrusions to form a ridge network which links the protrusions. We discuss the dependence of the wrinkled morphology on nanoparticle diameter and graphene thickness in terms of graphene elasticity and adhesion energy. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P37.00009: Magnetoresistance of Metal-Shunted Graphene Devices Paul Campbell, Adam Friedman, F. Keith Perkins, Jeremy Robinson Graphene, a single atomic layer of hexagonally arranged carbon atoms, presents the optimal platform to study magnetoresistance (MR) effects because of its temperature-independent mobility and linear band structure with zero band gap. Extraordinary magnetoresistance (EMR) can be realized in metal-shunted graphene devices. Here, due to the different magnetic-field-dependent resistances of the metallic shunt, graphene, and shunt-graphene interface, current flows easily through the shunt in zero and low magnetic field, while in high magnetic field, more current flows around the shunt and is redistributed in the graphene. Devices made from chemical vapor deposition (CVD) graphene grown on copper and transferred to a SiO$_{2}$/Si substrate with Ti/Au shunts display gate-tunable longitudinal MR of $\sim $600{\%} at 12 T and also show promise for use as Hall sensors. Graphene magnetoresistance devices have many possible applications including magnetic field sensors and magnetic read-heads. In contrast with the many proposed electronic uses for graphene, which necessitate the creation of a band-gap, graphene magnetoresistance devices that exploit LMR or EMR provide a use for as-grown or deposited graphene. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P37.00010: A Graphene-Based Biosensor Amal Kasry, Ali Afzali, George Tulevski, Bernhard Menges, Satoshi Oida, Matthew Copel, Libor Vyklicky Graphene, a single layer of carbon atoms, has attracted significant interest in several applications including biosensors. In this work graphene was grown by the CVD method. Optical parameters of graphene such as refractive index and extinction coefficient were measure by a mix of techniques including ellipsometer, XPS, Raman Spectroscopy, SPR and MEIS. Determining the optical properties of graphene allowed for study of its ability to sense biomolecular interactions. We also examined graphene modification by electrostatic interaction utilizing a molecule synthesized by IBM Research. Successful modification was proven by XPS, Raman Spectroscopy, and SPR. Studies of the chemical modification, along measurement of electrical and optical properties of graphene are components of our work to develop highly sensitive graphene-based sensors. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P37.00011: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P37.00012: Graphene-Silicon Schottky Diodes Chun Chung Chen, Mehmet Aykol, Chia-Chi Chang, A.F.J. Levi, Stephen B. Cronin By depositing mechanically exfoliated graphene on top of silicon substrates, the graphene-silicon Schottky barriers are observed. The resulting current-voltage characteristics exhibit rectifying diode behavior with a barrier energy of 0.41 eV on n-type silicon and 0.45 eV on p-type silicon at room temperature. The ideality factor is also evaluated for bilayer, three layer, and multiple layer graphene-silicon Schottky diodes at various temperatures. These results indicate that the number of graphene layers and the ambient temperature are major influences for the ideality factor of graphene-silicon Schottky diodes. In this work, photocurrents are observed under 532 nm laser illumination. The transparency of the thin graphene layer allows the underlying silicon substrate to absorb the laser light and generate a photocurrent. The full current-voltage characteristics under illumination are also reported. Spatially resolved photocurrent measurements also reveal the importance of inhomogeneity and series resistance in these devices. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P37.00013: Conductance-Based Temperature Programmed Desorption with Single Defect Resolution Deng Pan, Patrick C. Sims, Brad L. Corso, Philip G. Collins The controlled functionalization of nanotubes and graphene requires methods of chemically attacking these inert surfaces and of removing unwanted oxidation damage. The appeal of reversible chemistries is rarely achieved: the degraded electrical properties of reduced graphene oxide compared to pristine graphene indicates residual damage that remains poorly understood. Using a high temperature, UHV apparatus to perform electrical measurements in situ, we investigate the thermal desorption of adducts that can restore conductivity in oxidized nanographites. The majority of our measurements are accomplished using SWCNTs, due to their enhanced sensitivity to even single point defects. Discrete conductance jumps accompanying the removal of different types of adducts provide a characterization method that directly distinguishes the relative electronic effects of phenolic, epoxide, and carboxylic defects. The electronic measurements complement more traditional, temperature programmed desorption from bulk material, which is insensitive to electronic disorder. [Preview Abstract] |
Session P38: Focus Session: Quantum Coherence in Biology III
Sponsoring Units: DCP DBPChair: Seth Lloyd, Massachusetts Institute of Technology
Room: A130/131
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P38.00001: Barrier compression and tunneling in enzyme catalysed reactions Invited Speaker: Nuclear quantum mechanical tunneling is important in enzyme-catalysed H-transfer reactions. This viewpoint has arisen after a number of experimental studies have described enzymatic reactions with kinetic isotope effects that are significantly larger than the semi-classical limit. Other experimental evidence for tunneling, and the potential role of promoting vibrations that transiently compress the reaction barrier, is more indirect, being derived from the interpretation of e.g. mutational analyses of enzyme systems and temperature perturbation studies of reaction rates/kinetic isotope effects. Computational simulations have, in some cases, determined exalted kinetic isotope effects and tunneling contributions, and identified putative promoting vibrations. In this presentation, we present the available evidence -- both experimental and computational -- for environmentally-coupled H-tunneling in several enzyme systems, from our recent work on redox enzyme systems. We then consider the relative importance of tunneling contributions to these reactions. We find that the tunneling contribution to these reactions confers a rate enhancement of approx. 1000-fold. Without tunneling, a 1000-fold reduction in activity would seriously impair cellular metabolism. We infer that tunneling is crucial to host organism viability thereby emphasising the general importance of tunneling in biology. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P38.00002: The control of electron transfer pathways in biomolecular systems: the role of fluctuations Invited Speaker: Electron transfer reactions are ubiquitous in biology. They are observed in both protein and DNA systems. Biological electron transfer mechanisms range from tunnelling to thermally activated hopping. Due to the floppiness of biomolecules, molecular motion is an important determinant of the electron transfer rate. The electronic couplings that enable electron transfer in biomolecular systems can be understood in terms of competing and interfering electron transfer pathways that are controlled by structure, dynamics, and initial state preparation. We review recent theoretical progress on the effects of conformational distributions, excited-state polarization, and electron-nuclear dynamics on tunneling electron transfer reactions in different biomolecular systems. We discuss how electron-transfer-rate control can be achieved in the presence of a highly fluctuating environment. \\[4pt] [1] S. S. Skourtis, D. H. Waldeck, and D. N. Beratan. Fluctuations in biological and bioinspired electron-transfer reactions. Ann. Rev. Phys. Chem. Vol. 61 461-485 (2010). \\[0pt] [2] I. A. Balabin, D.N. Beratan, and S. S. Skourtis. The persistence of structure over fluctuations in biological electron transfer reactions. Phys. Rev. Lett. 101, 158102 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P38.00003: A molecular breakwater enhances electron transfer between proteins Nathan S. Babcock, Aur\'elien de la Lande, Jan \v{R}ez\'a\v{c}, Barry C. Sanders, Dennis R. Salahub Does natural selection optimize molecular biomachinery at the quantum level? We present statistical characterizations of molecular dynamics at an interprotein electron transfer (ET) interface. In simulations of the wild-type protein complex, we find that the most frequently occurring molecular configurations afford superior electronic coupling due to the consistent presence of a single water molecule hydrogen-bonded between the donor and acceptor sites. We attribute the persistence of this water bridge to a ``molecular breakwater'' composed of several hydrophobic residues surrounding the acceptor site. The breakwater supports the function of solvent-organizing residues by limiting the exchange of water molecules between the sterically constrained ET region and the surrounding bulk. When the breakwater is affected by a mutation, bulk solvent molecules disrupt the water bridge, resulting in reduced electronic coupling. These results suggest that protein surface residues may stabilize interprotein solvent dynamics to enable coherent ET along a single molecular pathway. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P38.00004: Mixed quantum classical simulations of vibrational energy transport in alpha-helices Anne Goj, Eric Bittner We use mixed classical/quantum simulations to study the time dependence of an excitation of a C=O vibration on a 3-10 helix of $\alpha$-Aminoisobutyric acid (AIB), a system which represents a test case for the formation of self-trapped vibrational excitation states on protein helices. Due to the inherent disorder in the system caused by the finite temperature and fluctuations in hydrogen bonding, the excitation tunnels randomly among C=O sites along the helix. Quantum forces are insufficient to establish a coherent relationship between the location of the excitation and the contraction of hydrogen bonds around this site. Our simulations indicate that the excitation frequently becomes localized on the end of the helix due to the defect in helical structure caused by unwinding. Our results generally do not support the existance of Davydov type solitons in biological helix systems under physiological conditions. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 10:12AM |
P38.00005: Quantum processes as a mechanism in olfaction for smell recognition? Invited Speaker: The physics of smell is not well understood. The biological processes that occur following a signalling event are well understood (Buck 1991). However, the reasons how and why a signalling event occurs when a particular smell molecule and receptor combination is made, remains un-established. Luca Turin proposes a signalling mechanism which determines smell molecules by quantum mechanics (Turin 1996). Investigation of this mechanism shows it to be physically robust (Brookes,et al, 2007), and consequences of the theory provides quantitative measurements of smell and interesting potential experiments that may determine whether the recognition of smell is a quantum event. Brookes, J.C, Hartoutsiou, F, Horsfield, A.P and Stoneham, A.M. (2007). Physical Review Letters 98, no. 3 038101 Buck, L. ( 1991) Cell, 65, no.1 (4): 175-187. Turin, L. (1996) Chemical Sences 21, no 6. 773-791 [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P38.00006: Recent developments in the physics of your sense of smell Andrew Horsfield, Luca Turin, Yeong-Ah Soh, Marion Sourribes, Marshall Stoneham, Lianheng Tong, Paul Warburton A radical proposal in 1996 [L. Turin, Chem. Senses 21, 773 (1996)] was that human olfactory receptors use phonon assisted electron tunnelling to probe the vibrational spectrum of odorants in order to determine their chemical identity. A development of this model [J. C. Brookes et al., Phys. Rev. Lett., 98, 038101 (2007)] showed that this Turin mechanism is indeed physically possible, even robust, but left a number of questions open. One such question is: between which sites does the tunnelling electron pass? Our recent calculations support a particular pair of likely sites. Because of the complexity of biological environments, probing the receptor is difficult. Thus we have begun to investigate the properties of a semiconductor nanowire device that mimics the key processes [A. P. Horsfield et al., J. Appl. Phys., 108, 014511 (2010)]. We will present the latest findings of this study. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P38.00007: A Quantum of Solace: molecular electronics of benzodiazepines Luca Turin, Andrew Horsfield, Marshall Stoneham Benzodiazepines and related drugs modulate the activity of GABA-A receptors, the main inhibitory receptor of the central nervous system. The prevailing view is that these drugs bind at the interface between two receptor subunits and allosterically modulate the response to GABA. In this talk I shall present evidence that benzodiazepines work instead by facilitating electron transport from the cytoplasm to a crucial redox-sensitive group in the gamma subunit. If this idea is correct, benzodiazepines should not only be regarded as keys fitting into a lock, but also as one-electron chemical field-effect transistors fitting into an electronic circuit. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P38.00008: Theoretical investigation of coherent exciton flow dynamics in light harvesting complex 2 (LH2) Seogjoo Jang The light harvesting complex 2 (LH2) is a peripheral antenna complex found in photosynthetic unit of purple bacteria. Numerous spectroscopic and computational studies demonstrated that quantum coherence plays an important role in the energetics and the dynamics of excitons created in LH2, but detailed and quantitative understanding is still missing regarding how the quantum coherence influences spectroscopic observables and how it boosts efficient energy transfer despite disorder and soft nature of the system. The present talk reports recent progress in the analysis of the single molecule spectroscopy (SMS) and application of resonance energy transfer theories, which account for multichromophoric and quantum coherence effects. These suggest that spectroscopic modeling allows development of reliable coarse-grained model for LH2 that can capture the major features of the excitons and that LH2 is a highly optimized natural system where the interplay between quantum coherence and disorder/fluctuation is maximally utilized. [Preview Abstract] |
Session P39: Physics of Proteins IV: Folding, Dynamics and Function
Sponsoring Units: DBPChair: Margaret Cheung, University of Houston
Room: A124/127
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P39.00001: Competition between chemical denaturation and macromolecular crowding effects on the folding dynamics of proteins Antonios Samiotakis, Margaret Cheung It is well known that proteins fold and function in the crowded environment of the cell's interior. In the recent years it has been established that the so-called ``macromolecular crowding'' effect can enhance the folding stability of proteins by destabilizing their unfolded states. On the other hand, chemical and thermal denaturation are often used in experiments as tools to destabilize protein structures when probing a protein's folding landscape. However, little is known about the combined effects of these competing phenomena on proteins. In this work, we use coarse-grained molecular simulations to study the thermodynamic and kinetic properties of the small peptide Trp-cage, in the combined presence of macromolecular crowders and chemical denaturant. With the use of an energy function derived by all-atomistic simulations in the presence of urea, we investigate the thermodynamics and kinetics of Trp-cage's folding mechanism at several concentrations of urea. The effects of the competition between stabilization by macromolecular crowding and destabilization by chemical denaturation will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P39.00002: Can understanding the packing of side chains improve the design of protein-protein interactions? Alice Zhou, Corey O'Hern, Lynne Regan With the long-term goal to improve the design of protein-protein interactions, we have begun extensive computational studies to understand how side-chains of key residues of binding partners geometrically fit together at protein-peptide interfaces, e.g. the tetratrico-peptide repeat protein and its cognate peptide). We describe simple atomic-scale models of hydrophobic dipeptides, which include hard-core repulsion, bond length and angle constraints, and Van der Waals attraction. By completely enumerating all minimal energy structures in these systems, we are able to reproduce important features of the probability distributions of side chain dihedral angles of hydrophic residues in the protein data bank. These results are the crucial first step in developing computational models that can predict the side chain conformations of residues at protein-peptide interfaces. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P39.00003: Insight into the mechanics of the selectivity filter of \textit{Escherichia coli} aquaporin Z Guodong Hu, L.Y. Chen Aquaporin Z (AQPZ) is a tetrameric protein that forms water channels in Escherichia coli's cell membrane. The histinine residue in the selectivity filter (SF) region plays an important role in the transport of water across the membrane. In this work, we perform equilibrium molecular dynamics (MD) simulation to illustrate influences of two different protonation states and the gate mechanics of the SF. We calculate the pore radii in the SF region versus the simulation time. We perform steered MD to compute the free energy profile, i.e., the potential of the mean force (PMF) a water molecule through the SF region. We calculate the binding energy of one water molecule with the SF region residues, using Gaussian. The hydrogen bonds formed between the side chains of Hsd 174 and side chains of Arg189 play important roles in the selectivity filter mechanics of AQPZ. The radii of the pores, hydrogen bond analysis, and free energies show that Hsd is favored than Hse. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P39.00004: The Effect of Phosphate Buffered Saline (1x PBS) on Induced Thermal Unfolding and Low Frequency Dielectric Spectra of Lysozyme Klaida Kashuri, Hektor Kashuri, Germano Iannacchione It is well known that the folding / unfolding of proteins is related directly to their structure and functionality. Calorimetry (both AC and MDSC) studies as well as low-frequency (1Hz to 100 kHz) dielectric measurements have been performed on hen egg white lysozyme dissolved in PBS (pH 7.4) from 20 to 100 $^{\circ}$C. From the heat capacity profile, the temperatures and related an enthalpy change of the protein denaturing is probed. The heat capacity peak broadens and new features are reveled as the temperature scan rate is lowered to +0.017 K/min for the AC calorimetric method. Significant differences are observed using the (M)DSC technique at scan rates of from 1 to 5 K/min. The temperature dependence of the permittivity, $\epsilon '$, and the loss factor, $\epsilon ''$, at 100 kHz of the diluted protein show features associated with those seen in the heat capacity (AC and MDSC). All results are interpreted in terms of protein denaturing then subsequent gelation that depend on protein sample concentration, which is supported by the frequency dependence of the permittivity at room temperature after thermally cycling [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P39.00005: The effect of macromolecular crowding, ionic strength and calcium binding on calmodulin dynamics Qian Wang, Kao-Chen Liang, Neal Waxham, Margaret Cheung The flexibility in the structure of calmodulin (CaM) allows its binding to over 300 target proteins in the cell. To investigate the structure-function relationship of CaM in response to the changing intracellular environment, we use a combined method of computer simulation and experiments based on circular dichroism (CD). The conformation, helicity and EF hand orientation of CaM are analyzed computationally to address the effect of macromolecular crowding, ionic strength and calcium binding in the experiments. We applied a unique solution of charges computed from QM/MM to accurately represent the charge distribution in the transition from apo-CaM to holo-CaM. Computationally, we found that a high level of macromolecular crowding, in addition to calcium binding and ionic strength, can impact the conformation, helicity and the EF hand orientation of CaM. Our result may provide unique insight into understanding the promiscuous behavior of calmodulin in target selection inside cells. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P39.00006: $\alpha$-Helical to $\beta$-Helical Conformation Change in the C-Terminal of the Mammalian Prion Protein Jesse Singh, Paul Whitford, Natha Hayre, Daniel Cox, Jos\'e Onuchic We employ all-atom structure-based models with mixed basis contact maps to explore whether there are any significant geometric or energetic constraints limiting conjectured conformational transitions between the alpha-helical ($\alpha$H) and the left handed beta helical (LHBH) conformations for the C-terminal (residues 166-226) of the mammalian prion protein. The LHBH structure has been proposed to describe infectious oligomers and one class of in vitro grown fibrils, as well as possibly self- templating the conversion of normal cellular prion protein to the infectious form. Our results confirm that the kinetics of the conformation change are not strongely limited by large scale geometry modification and there exists an overall preference for the LHBH conformation. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P39.00007: Dynamics of Protein Carbonmonoxyhemoglobin on Multiple Length Scales Jyotsana Lal, Robert Fischetti, Lee Makowski, Peter Fouquet, Marco Maccarini, Nancy Ho, Chien Ho A combination of wide-angle x-ray solution scattering (WAXS) and neutron spin echo spectroscopy (NSE) was used to probe the structure and dynamics of carbonmonoxy hemoglobin (HbCO) in the presence and absence of the allosteric effector inositol hexaphosphate (IHP). IHP shifts the structure of HbCO slightly towards an unliganded, (deoxy)-state conformation. Two potential binding sites for IHP are consistent with the WAXS data, one near each end of the central channel. IHP binding slows the self-correlation times of some protons, most likely those immediately adjacent to the bound IHP, and simultaneously induces an increase in the relaxation rate of correlated motions with length scales comparable to the $\alpha \beta $-dimer. IHP binding increases the spatial extent of these fluctuations by about 20{\%}. This suggests that when hemoglobin binds CO, its conformation is confined to a relatively narrow structural ensemble residing within a functionally well defined energy well. On the other hand, when it binds both CO and IHP, in response to the contradictory stresses applied by these two ligands, it adopts an incommensurate structure with a conformation exploring a broad structural ensemble. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P39.00008: Infrared Spectroscopy Measurements of Protein Dynamics and Mechanism Curtis W. Meuse, Joseph B. Hubbard Infrared spectroscopy has long been used to deduce the concentration and secondary structures of proteins in a variety of static and time resolved applications. Our focus is on developing new infrared methods to compare the structure, dynamics and function of nearly identical protein samples, in different environments, to apply to the problem of identifying bio-similar protein therapeutics. We have developed an order parameter describing protein conformation variations around the average molecular values. By comparing our order parameter and amide hydrogen/deuterium exchange methods, we explore the relationship between protein stability and the dynamics of the protein conformational distribution. Examples include lysozyme and albumin in solution, cytochrome c interacting with lipid membranes of varying net-negative surface charge density, fibrinogen on different polymer surfaces and bacteriorhodopsin during its photocycle. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P39.00009: Comparing potential copper chelation mechanisms in Parkinson's disease protein Frisco Rose, Miroslav Hodak, Jerry Bernholc We have implemented the nudged elastic band (NEB) as a guided dynamics framework for our real-space multigrid method of DFT-based quantum simulations. This highly parallel approach resolves a minimum energy pathway (MEP) on the energy hypersurface by relaxing intermediates in a chain-of-states. As an initial application we present an investigation of chelating agents acting on copper ion bound to $\alpha$-synuclein, whose misfolding is implicated in Parkinson's disease (PD). Copper ions are known to act as highly effective misfolding agents in a-synuclein and are thus an important target in understanding PD. Furthermore, chelation therapy has shown promise in the treatment of Alzheimer's and other neuro-degenerative diseases with similar metal-correlated pathologies. At present, our candidate chelating agents include nicotine, curcumin and clioquinol. We examine their MEP activation barriers in the context of a PD onset mechanism to assess the viability of various chelators for PD remediation. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P39.00010: Investigating hexameric helicases: Single-molecule studies of DnaB and T4 gp41 Omar Saleh, Noah Ribeck, John Berezney Hexameric, ring-shaped motor proteins serve as replicative helicases in many systems. They function by encircling and translocating along ssDNA, denaturing dsDNA in advance of its motion by sterically occluding the complementary strand to the outside of the ring. We investigate the helicase activity of two such motors using single-molecule measurements with magnetic tweezers. First, we measure the activity of the \textit{E. coli} helicase DnaB complexed with the tau subunit of the Pol III holoenzyme. Tau is known from bulk measurements to stimulate DnaB activity (Kim et al., \textit{Cell}, 1996); we investigate the means of this stimulation. Second, we measure helicase activity of the T4 phage helicase gp41 in multiple tethered DNA geometries. Previous work on DnaB showed a dependence of helicase activity on DNA geometry (Ribeck et al., \textit{Biophys. J}., 2010); here, we test gp41 for similar behavior to see whether it is a common characteristic of hexameric helicases. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P39.00011: Copper attachment to prion protein at a non-octarepeat site Miroslav Hodak, Jerry Bernholc Prion protein (PrP) plays a causative role in a group of neurodegenerative diseases, which include ``mad cow disease'' or its human form variant Creutzfeld-Jacob disease. Normal function of PrP remains unknown, but it is now well established that PrP can efficiently bind copper ions and this ability has been linked to its function. The primary binding sites are located in the so-called octarepeat region located between residues 60-91. While these are by now well characterized, the sites located outside these region remain mostly undetermined. In this work, we investigate the properties of Cu binding site located at His 111 using recently developed hybrid Kohn-Sham/orbital-free density functional simulations. Experimental data indicate that copper is coordinated by either four nitrogens or three nitrogens and one oxygen. We investigate both possibilities, comparing their energetics and attachment geometries. Similarities and differences with other binding sites and implications for PrP function will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P39.00012: Pressure and Temperature Effects on Polypeptides and Biomolecules Probed by Micro-Raman Spectroscopy Sanghoon Park, Alfons Schulte We investigate pressure and temperature effects on the secondary structure of Poly-L-glutamic acid (PGA) in D$_{2}$O buffer (pH 5.4) solution. Our setup employs a Raman microscope equipped with a micro-capillary high-pressure cell and a variable temperature stage. Raman spectra are acquired over the pressure range from 0.1 to 300 MPa while the temperature can be varied from 270 K to 330 K. The amide I band of PGA is sensitive to pressure and temperature, and by spectral deconvolution we determine the relative contributions due to $\alpha $-helix and random coil conformations. The amount of $\alpha $-helix increases with increasing pressure. Extensions of these experiments to model proteins and lipids are presented. [Preview Abstract] |
Session P40: Polymer Melts & Solutions
Sponsoring Units: DPOLYChair: Azar Alizadeh, GE Global Research
Room: A122/123
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P40.00001: Radical-cured block copolymer modified thermosets Erica Redline, Lorraine Francis, Frank Bates Poly(ethylene-\textit{alt}-propylene)-$b$-poly(ethylene oxide) (PEP-PEO) diblock copolymers were synthesized and added at 4 wt. {\%} to bisphenol A glycidyl methacrylate (BisGMA). The mixture was thermally cured using free radical chemistry. In separate experiments, 4 wt. {\%} PEP-PEO was added to a combination of poly(ethylene glycol) dimethacrylate (PEGDMA) and BisGMA and cured. Based on small angle X-ray studies of the modified monomers before curing, diblock copolymers self-assembled into well-dispersed spherical micelles with PEP cores and PEO coronas. TEM results showed that these micellar structures were retained during curing. Fracture resistance measurements indicate that the addition of block copolymers does not significantly toughen these thermoset materials. This finding is contrary to the large increase in fracture resistance observed in block copolymer-modified epoxies. We propose that differences in network structure, originating during polymerization, are responsible. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P40.00002: Cavitation in Filled Styrene-butadiene Rubber: A Real Time SAXS Observation Huan Zhang, Arthur K. Scholz, Fabien Vion-Loisel, Edward J. Kramer, Costantino Creton Cavitation of filled and unfilled elastomers under confinement at the macroscopic scale has been experimentally reported and theoretically modeled. However, cavitation occurring at the nanometer length scale has not yet been demonstrated conclusively in rubbers. Real time SAXS with synchrotron radiation was employed to probe the structure changes in carbon black filled styrene-butadiene rubber (SBR) under uniaxial loading. The scattering invariant was calculated and increased sharply at a critical extension depending on both filler content and crosslinking density around q = 0.1 nm-1, which we attributed to the formation of voids. At very large strains, a sharp and wide streak developed perpendicular to the tensile axis in reciprocal space, suggesting the deformation of the voids in elliptical voids along the tensile direction. In step cycle test, we observed that voids only appeared when the current strain exceeded the maximum historical strain (Mullins effect) and attributed the increase of the scattering invariant outside the Mullins region to the creation of new voids rather than to the reopening of old ones. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P40.00003: Why is the tube model inapplicable for entangled polymer dynamics at large deformation Shi-Qing Wang, Yangyang Wang Accumulating experimental revelation of the phenomenology governing dynamics of entangled linear polymers at large deformations has caused us to question the legitimacy of the tube model as an acceptable theoretical description of nonlinear polymer rheology. Upon an explicit investigation of its premise, we have come to realize that the tube model did not overcome the difficulty confronted by other theories and did not contain the basic physics required to explain why and how the entanglement network must break down during large deformations. It considered an unrealistic situation where a load-bearing chain relaxed fast in an affinely deformed tube so that only the chain segment orientation produced the shear stress for applied rates lower than the Rouse rate. A non-monotonic relation between the resulting shear stress and imposed strain for startup shear and step deformations arose from excessive chain orientation not collapse of the entanglement network. In the tube model, the nature of the overshoot is not yielding (transition from elastic deformation to flow), but an elastic instability. Accumulating experimental observations contradict this picture. This presentation will elucidate how the emerging physical picture differs from that of the unrealistic tube model. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P40.00004: Non-Gaussian Stretching Behavior of Entangled Polymers Yangyang Wang, Shi-Qing Wang The behavior of entangled SBR melts and solutions in rapid uniaxial extension has been studied by rheometric and rheo-optical measurements. A yield-to-rupture transition occurs around the same Rouse Weissenberg number of nine for all samples when the failure mechanism changes from chain disentanglement (yielding) to chain scission. Our results show that elastic rupture takes place only when chains between entanglements are near full extension, the strain at rupture grows with increasing entanglement spacing, and the critical stress for rupture is proportional to the polymer concentration. These characteristics validate the well-known idea to represent entangled polymers in terms of a transient network. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P40.00005: Parameters of slip-springs model of polymer entanglement from the maximum likelihood principle Timothy Palmer, Alexei Likhtman, Jorge Ramirez, Mark Matsen The slip-spring model for polymer entanglements proposed by A.E.Likhtman [Macromolecules; 2005; 38(14); 6128] replaces entangling chains with slip-links, which are anchored via springs. The use of such models allows reduction of complex multi-chain problem to a simpler single chain problem. In this work we test the slip-spring model on the simplest possible situation: replacing two entangled chains by one chain with one slip-link. We demonstrate how Maximum Likelihood Estimation (MLE) can be used to generate parameters for the slip-spring model by observing a multi-chain system. The test system being considered consists of two Rouse polymer chains that are anchored by the ends in an entangled state. The effect of the entanglement is enforced by rejecting all steps that lead to topology violation. We show how the results of this MLE indicate that slip-link models with slip-links fixed in space are not satisfactory, and demonstrate analytically the dependence of the plateau modulus upon the strength of the slip-spring. Our results contradict recent calculations of Schieber and Horio [JCP; 2010; 132(7); 074905] who claimed that the plateau modulus must be independent of the slip-spring strength. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P40.00006: Segmental orientation dynamics in bidisperse entangled linear polymer melts Zuowei Wang, Jing Cao, Alexei Likhtman, Ronald Larson Extensive molecular dynamics simulations were performed to investigate the segmental orientation dynamics in mono- and bidisperse entangled linear polymer melts. The binary blends consist of short probe chains diluted in long chain matrices of chain length up to 30 entanglements. With increase of the chain length in monodisperse melts, the bond vector autocorrelation function was found to approach a distinctive time-dependent power law, which is compared with recent NMR experiments. When introduced into long chain matrices, the segmental orientation relaxation and monomer diffusion of short probe chains slowed down strongly due to the suppression of constraint release (CR) effects. The same trend was observed for the end-to-end vector correlation function, reflecting the CR effects on contour length fluctuations. On the other hand, the time-dependent orientation coupling parameter in the entangled systems demonstrates the similar universal behaviour as that discovered in unentangled melts. Considering the stress-optical law was recovered in all simulated systems, our simulations should clarify the connection between rheology and other experimental techniques, which are essential for progress in modeling entangled polymers. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P40.00007: Simulations of polymer melts modeled as chains of interacting soft-colloids Anthony Clark, Marina Guenza The range of time and length scales accessible to dynamical simulations of melts of long polymer chains is strongly limited by the computational demands of calculating large numbers of forces between monomers. Simulations modeling each polymer as a point particle interacting by an analytical soft pair potential have previously been successfully developed to extend this range. For many effects in polymer systems, however, submolecular degrees of freedom remain relevent to molecular-level behavior even at long times and large length scales. To allow for the inclusion of relevant submolecular degrees of freedom, we use analytical effective potentials based on our model of the structure of polymer melts on the level of large chain sub-blocks to simulate homopolymer melts. We demonstrate that structure on the block and center of mass level consistent with the structural model and monomer-level simulation data can be reproduced for large systems and long times at much lower computational cost than monomer-level simulations. Using this model, we also test the effects of additional short-range repulsive interactions between chain subunits on structure and dynamics. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P40.00008: Renormalized dynamics of overdamped driven elastic media Jonathan Landy, Alex J. Levine We present the results of a dynamical renormalization group calculation used to explore the fluctuations of an elastic body steadily driven through a viscous background fluid. Direct applications of this work involve the study of the fluctuation spectrum of semiflexible filament networks driven through a background fluid by e.g. polymerization, but also include the motion of one-dimensional driven elastic objects (e.g. polymers, flux vortices etc.) In that case, a previous linear stability analysis suggests that, when such elastic lines are driven in a direction perpendicular to their axis, they become unstable at any non-zero driving force [1]. We discuss the affect of nonlinearities on these conclusions, showing that such terms can stabilize the system at finite drive velocities. We similarly explore the dynamics of lines driven parallel their axis showing that these systems exhibit ``weak dynamic scaling'' [2]. Turning to the case of driven elastic solids, we report on the effect of molecular motor-induced forces on the long length scale and long time scale dynamics of the driven system. [1] R. Lahiri and S. Ramaswamy, Are steadily moving crystals unstable?, Phys. Rev. Lett. 79, 1150 (1997) [2] D. Das et al., Weak and strong dynamic scaling in a one-dimensional driven coupled-field model: Effects of kinematic waves, PRE 64, 021402 (2001). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P40.00009: Thermodyanmic Scaling of Polymer Dynamics versus Shifting by T-Tg Jiaxi Guo, Sindee Simon A universal scaling law for the relaxation time ($\tau )$ of amorphous liquids as a function of temperature and volume has been proposed by Roland and coworkers: $\tau $(T,V) = F(TV$^{\gamma })$, where $\gamma $ is a material-dependent constant. We test this law for four materials, linear polystyrene, star polystyrene, and two polycyanurate networks using PVT data obtained in our laboratory coupled with the temperature dependent shift factors used to reduce the viscoelastic bulk modulus at different pressures and the dynamic shear properties at ambient pressure. In all cases, $\tau $ can be reduced both by the scaling law and by shifting to account for the changes in T$_{g}$ with pressure, i.e., by plotting versus T - T$_{g}$(P). In the polycyanurate case, time-crosslink density superposition holds and $\tau $ for the two materials can be reduced simply by shifting the temperature with respect to T$_{g}$ to account for the changes in T$_{g }$with crosslink density; however, the thermodynamic scaling for the two materials does not superpose unless the thermodynamic function is normalized by T$_{g}$V$_{g}^{\gamma }$. The validity of the scaling function and its relationship to T - T$_{g}$ will be further examined. In addition, the impact of errors in T, T$_{g}$, and V on the ability to satisfactorily reduce data and obtain universal scaling will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P40.00010: The linear rheological responses of dense branched brush polymers with different side chain lengths and structures Miao Hu, Gregory McKenna, Yan Xia, Chris Daeffler, Andrew Boydston, Robert Grubbs, Julia Kornfield We examined the linear rheological responses of three kinds of dense and regular branched brush polymers. Brush polymers with different degree of polymerization were synthesized from the $\omega $-Norbornenyl macromolecule (as main chain) with linear, three combined short arms, and dendronized brush structures. The master curves for these brush polymers were obtained by time temperature superposition (TTS) of the dynamic moduli from the glassy plateau region to the terminal flow region. The glassy modulus and rubbery modulus for these brush polymers were greatly influenced by the side chain properties. Two different relaxation processes can be observed for those samples with the higher molecular weight, slightly entangled, side chains. The dilution effect of the side chain which is related to the side chain volume fraction doesn't follows theoretical expectations. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P40.00011: Analytical rheology of metallocene-catalyzed polyethylenes Sachin Shanbhag, Arsia Takeh A computational algorithm that seeks to invert the linear viscoelastic spectrum of single-site metallocene-catalyzed polyethylenes is presented. The algorithm uses a general linear rheological model of branched polymers as its underlying engine, and is based on a Bayesian formulation that transforms the inverse problem into a sampling problem. Given experimental rheological data on unknown single-site metallocene- catalyzed polyethylenes, it is able to quantitatively describe the range of values of weight-averaged molecular molecular weight, $M_W$, and average branching density, $b_{m}$, consistent with the data. The algorithm uses a Markov-chain Monte Carlo method to simulate the sampling problem. If, and when information about the molecular weight is available through supplementary experiments, such as chromatography or light scattering, it can easily be incorporated into the algorithm, as demonstrated. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P40.00012: Pressure-Volume-Temperature Behavior of Hyperbranched Polyols: Experiment and Modelling Mukul Kaushik, Sergei Nazarenko, Brian Olson The pressure volume temperature behavior of two generations of hyperbranched polyesters Boltorn$^{TM}$ H40 and H20 was studied by PVT measurements using high pressure dilatometer. Volumetric expansivity, and free volume parameters were determined for both generations in the melt state. The PVT data were fitted to Simha-Somcynsky (SS) equation of state (EOS) and Sanchez--Lacombe (SL) equation of state (EOS) to calculate occupied volume and fractional free volume. The values of occupied volume and fractional free volumes obtained through both the equations of states were similar. Simulated atmospheric pressure V-T data were generated by using Discover module of Accelrys$^{\mbox{{\textregistered}}}$. Quality of equilibrium was confirmed by energy stabilization and closeness of experimental and simulation densities. WAXD and temperature-volume curves obtained by molecular dynamics simulations were comparable to the experimental data. Well relaxed amorphous cell was further utilized to study hydrogen bond network and determination of O-O pair correlation function of terminal hydroxyl groups. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P40.00013: Influence of the Hofmeister Series on Lower Critical Solution Temperature (LCST) Polymers Ryan Toomey, Leena Patra Lower critical solution temperature (LCST) polymers can serve as model systems for probing the effect of ions on the stability of biological macromolecules. In this talk, we show how permutations in the chemical structure of poly(N-isopropylacrylamide), including n-propylacrylamide, cyclopropylacrylamide, and N-vinylisobutyramide influence the action of ions in the Hofmeister series. By using a combination of ellipsometry and FTIR, we show that ions salt out neutral polymers by enhancing the surface tension of the hydrophobic portions of the polymer. Weakly hydrated ions (known as chaotropes) can also lead to salting-in effects through interactions with amide dipoles. This salting-in effect is strongly modulated by the surrounding hydrophobic groups. The larger the hydrophobic group the weaker the salting-in effect, indicating that the specificity of the Hofmeister series results from a combination of ion-dipole interactions and hydrophobicity. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P40.00014: Ionic Liquids for the Imaging of Wet Polymer Morphology David Hoagland, John Harner, Malvika Bihari Unlike convention aqueous and organic solvents, ionic liquids are essentially nonvolatile and thus compatible with the high vacuum environments of electron microscopy. Here is described the room temperature imaging of wet polymer systems such as patterned gels, gel networks, polymeric vesicles, and proteins. Both TEM and SEM images will be offered, along with a discussion of difficulties in applying the two techniques. Via SEM, imprinted surface structures as small as 100-300 nm can be captured for chemically crosslinked gels (polyHEMA), and via TEM, the structure of a physical gel (PEG) is viewed at the 50-to-10-nm scale, revealing network connectivity established by PEG crystallinity. Self-assembled vesicle and micelle structures will be presented for dispersed block copolymers, and the same approach will be applied toward discerning the quality of dispersion for proteins (ferritin) and other nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P40.00015: Biaxial Strain Testing of Extremely Soft Gels Kenji Urayama, Yohsuke Bitoh, Toshikazu Takigawa We present a biaxial tensile tester to characterize the nonlinear stress--strain behavior of extremely soft polymer gels with very low shear moduli, of the order of 100 Pa, under general (equal and unequal) biaxial strain. Stretching of gel sheet specimens in a solvent bath can avoid finite self-weight bending deformations that have precluded biaxial tensile experiments with such extremely soft gels. General biaxial strain covers a wide range of physically accessible deformations in contrast to conventional uniaxial strain that is only a special one among them. The biaxial data for fully swollen chemical gels reveal that the exceptional agreement of uniaxial data with predictions of the simplest rubber elasticity model (ideal gas model), which has been known for over 60 years, is superficial because the model evidently fails to describe the biaxial data. This new biaxial tester will be a powerful tool for the full characterization of the large deformation behavior of extremely soft materials, including biological soft tissues. [Preview Abstract] |
Session P41: Focus Session: Fundamental Issues in Interfacial Charge Transport for Energy Applications I
Sponsoring Units: DCPChair: Lin Chen, Northwestern University
Room: A115/117
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P41.00001: Excitons at Interfaces Invited Speaker: Solar photovoltaics based on molecular and nano materials commonly involve excitons. This results from strong Coulomb attraction between an electron and a hole due to the low dielectric constants of molecules or quantum confinement of nano materials. In this lecture, I will address the question of how excitons dissociate at donor/acceptor interfaces. The first example deals with charge separation in organic photovoltaics. Due to the low dielectric constant of organic materials, an electron-hole pair across an organic donor/acceptor interface is bound by the Coulomb potential. This gives rise to a set of H-atom like states called charge-transfer excitons, as observed experimentally. The lowest energy charge transfer exciton state has a binding energy much higher than kT at room temperature. This leads to the conclusion that hot charge transfer exciton states must be involved in charge separation in organic photovoltaics. The second example deals with hot exciton dissociation due to electron transfer from photo-excited semiconductor nanocrystals (PbSe) to an electron acceptor (TiO2), an issue of particular interest to hot carrier solar cells with theoretical solar conversion efficiency surpassing the Shockley-Queisser limit. We show that, with appropriate chemical treatment of the nanocrystal surface, ultrafast transfer of a hot electron can be competitive with hot exciton relaxation due to phonon scattering. The last example will show recent development on hot carrier scattering and multiple exciton generation (MEG) in semiconductor nanorystals. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P41.00002: The Effect of Photoexcitation and Population Relaxation on Carrier Multiplication Efficiency in Semiconductor Nanocrystals and Bulk Andrei Piryatinski, Kirill Velizhanin The carrier multiplication (CM) is the process of production of two or more electron-hole pairs (excitons) per single absorbed photon. Detailed understanding of the mechanisms of this process is of importance for developing novel cheap and efficient photovoltaic devices. To model the CM dynamics, we have developed an exciton scattering model which accurately treats the contributions of different multi-exciton generation pathways on the same footing. Furthermore, the model allows one to study CM in nanocrystalline and bulk semiconductor materials. Using this model, we performed a numerical study of photogeneration and population relaxation processes contributing to CM in PbSe nanocrystals and bulk. It is found that the photogeneration provides small contribution to the total quantum efficiency compared to the population relaxation process. The resonant incoherent biexciton production is found to be main mechanism of CM in both cases of direct biexciton photogeneration and during the population relaxation. Comparison to the published experimental data shows that the calculations reproduce experimentally observed trends providing insight into the mechanisms of CM. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P41.00003: Non-radiative Energy Transfer in Colloidal Nanocrystals/Silicon Hybrid Structures Hue Minh Nguyen, Oliver Seitz, Damien Aureau, Amandeep Sra, Yves Chabal, Anton Malko The integration of organic and inorganic materials at the nanoscale offers the possibility of developing new photonic devices that could potentially combine the advantages of both classes of materials. Such optoelectronic structures could work both in photovoltaic as well as in light emitting modes depending on the direction of non-radiative \textit{exciton} energy transfer (NRET). In present work, we studied hybrid structures consisting of a monolayer of the colloidal nanocrystal quantum dots (NQDs) grafted on hydrogenated Si surface via amine modified carboxy-alkyl chains linkers. Such approach allowed us to passivate Si surface to suppress non-radiative surface state defects ($N_{s}<<$10$^{11}$ cm$^{2})$ and provided with the controllable spacer lengths between NQDs and Si. We performed systematic measurements of NRET via time-resolved and steady-state photoluminescence (PL) in the range of 10K to 300K and as a function of spacer lengths and quantified NRET rates. Local field effects due to the acceptor surface (Si) are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:36AM |
P41.00004: Quantum Dot Solar Cells. Understanding Charge Transfer at Nanostructure Interface Invited Speaker: Quantum dot solar cells are designed using a chemical approach. Different size CdSe quantum dots are assembled on mesoscopic TiO$_{2}$ films either by direct adsorption or with the aid of molecular linkers. Upon bandgap excitation, CdSe quantum dots inject electrons into TiO$_{2}$ nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The interfacial processes that dictate the photoelectrochemical performance of these solar cells have now been evaluated by comparing photoelectrochemical behavior with charge transfer dynamics between different size CdSe quantum dots and various oxide substrates. The primary photochemical event in these solar cells is the charge injection from excited CdSe quantum dots into nanostructured metal oxide films. This process can be modulated by varying the particle size of CdSe quantum dots or the conduction band of the acceptor oxide. The difference in the conduction band energy of two semiconductors serves as a driving force for the interparticle electron transfer. According to Marcus theory, for a non-adiabatic reaction in the activation limit, the rate of electron transfer depends on the electronic coupling between the donor and acceptor states, the density of states (DOS) per unit volume and the driving force. Because of the quasi continuum of states in the metal oxide conduction band, the total electron transfer rate depends on the sum of all possible electronic transitions. The dependence of electron transfer rate constant on the energy gap and its implication in photoconversion efficiency of quantum dot solar cells will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P41.00005: Ab initio theory of impact ionization applied to silicon nanocrystals Marton Voros, Adam Gali, Dario Rocca, Gergely Zimanyi, Giulia Galli Achieving multi exciton generation (MEG) in semiconducting nanocrystals may lead to overcome the well-known Shockley-Queisser limit when building semiconductor-based solar cells. A thourough, theoretical understanding of the experiments that reported MEG in e.g. Si and PbSe nanocrystals, is still missing and could significantly contribute to clarify the several controversial results in the field. Several theoretical and numerical studies have addressed the origin of the MEG formation, mostly supporting an impact ionization mechanism. However, impact ionization rates have only been evaluated for model nanocrystals by using empirical pseudopotentials fitted to bulk properties, and model dielectric functions to describe the screened Coulomb interaction. We present an ab-initio scheme based on Density Functional Theory in a plane-wave pseudopotential implementation that includes static screening within the random-phase approximation. We will discuss how impact ionization rates are affected by the shape and surface structure of few nm Si nanocrystals. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P41.00006: Optical properties of crystalline and amorphous silicon slabs with adsorbed metal clusters and with dopants: A combined ab-initio electronic structure and density matrix treatment Dimitri Kilin, David Micha, Jessica Ramirez The optical absorbance and surface photovoltage of slabs of Si with varying number of layers have been calculated starting from their atomic structure. Results have been obtained for nanostructured surfaces with adsorbed metal clusters and for group III and V dopants, from ab initio DFT with periodic boundary conditions for extended systems, and from time-dependent DFT for supercells. Density matrix equations of motion (EOM) have been parametrized in a basis set of Kohn-Sham orbitals, for both crystalline and amorphous Si slabs [1]. Results for properties and from electronic charge distributions provide insight on slab confinement effects for electronically excited states and for particle-hole creation. In addition, the integrodifferential EOMs have been solved for an initial femtosecond pulse excitation [2] to analyze the nature of electron transfer at the surfaces, relevant to photovoltaics.\\[4pt] [1] T. W. LaJoie, J. J. Ramirez, D. S. Kilin, and D. A. Micha Intern. J. Quantum Chem. 110, 3005 (2010). \\[0pt][2] A. S. Leathers, D. A. Micha, and D. S. Kilin, J. Chem. Phys. 132, 114702-1(2010)] [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:36AM |
P41.00007: Ultrafast Single and Multiple Exciton Dissociation in CdSe and PbS Quantum Dots Invited Speaker: Charge transfer to and from quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as solar cells and light emitting diodes. Recent reports of multiple exciton generation (MEG) by one absorbed photon in some QDs offer an exciting new approach to improve the efficiency of QD-based solar cells and to design novel multi-electron/hole photocatalysts. However, two main challenges remain. First, the efficiency of MEG process remains controversial and may need to be significantly improved for practical applications. Second, the utilization of the MEG process requires ultrafast exciton dissociation prior to the exciton-exciton annihilation process, which occurs on the 10s to 100s ps time scale. In this presentation we report a series of studies of exciton dissociation dynamics in quantum dots by electron transfer to adsorbed electron acceptors. We show that excitons in CdSe can be dissociated on the a few picosecond timescale to various adsorbates. As a proof of principle, we demonstrated that multiple excitons (generated by multiple photons) per QD can be dissociated by electron transfer to adsorbed acceptors (J. Am. Chem. Soc. 2010, 132, 4858-4864). We will discuss the dependence of these rates on the size and the nature of the quantum dots and possible approaches to optimize the multiple exciton dissociation efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P41.00008: Short time evolution of electronic charge transfer and separation, and quantum coherences, at photoexcited crystalline and amorphous Si surfaces: Adsorbate and dopant effects David Micha, Andrew Leathers, Dmitri Kilin The short time evolution of populations of electronic states and their quantum coherence at nanostructured surfaces of semiconductors provide insight on mechanisms of electronic charge transfer and separation. Starting from atomic structure, density matrix (DM)equations of motion (EOM) have been generated from a general formulation of dissipative quantum dynamics and have been parametrized in a basis set of Kohn-Sham orbitals, for both crystalline and amorphous Si slabs [1] with metal cluster adsorbates and with group III and V dopants. Integrodifferential EOMs have been solved for an initial ground state excited by femtosecond light pulses [2] to provide the time evolution of direct and indirect electron transfer at the surfaces. Results show that one of the transfer mechanisms can lead to long term separation of electronic charge, and what material properties contribute to large charge transfer and separation. \\[4pt] [1] T. W. LaJoie et al., Intern. J. Quantum Chem. 110, 3005 (2010).\\[0pt] [2] A. S. Leathers et al. J. Chem. Phys. 132, 114702-1(2010)] [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P41.00009: Investigation of electron-hole recombination in multi-layered quantum dots using explicitly correlated wavefunction based methods Arindam Chakraborty Electron-hole pairs are generated by photoexcitation of electrons to excited electronic states. Accurate calculations of electron-hole binding energies and recombination probabilities can give important insights into the photovoltaic properties of semiconductor nanocrystals and quantum dots. In the present work, the challenge of accurate treatment of electron-hole correlation is addressed by developing explicitly correlated electron-hole wavefunction that depends on electron-hole interparticle distance. The explicitly correlated ansatz for the electron-hole wavefunction is used to calculate eigenvalues and eigenfunction of the electron-hole Hamiltonian in multi-layered quantum dots using self-consistent field (SCF) and configuration interaction (CI) techniques. These methods are applied to investigate influence of the core/shell structure and chemical composition on electron-hole binding energies and recombination probabilities. The calculations indicate that for a given chemical composition there exists a optimum core/shell structure than minimizes electron-hole recombination. Comparison with experimental studies on similar system show good agreement between the experimental and computed results. [Preview Abstract] |
Session P42: Focus Session: Organic Electronics and Photonics -- Organic Photovoltaic Devices
Sponsoring Units: DMP DPOLYChair: Russell Holmes, University of Minnesota
Room: A302/303
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P42.00001: Increasing Transport Efficiencies of Polymer Based Solar Cells by Electrophoresis Terrence Wong Organic polymer photovoltaic (PV) cells are an active area of Applied Physics research because of four unique characteristics: (1) relatively inexpensive costs, (2) transparent properties, (3) flexibility, and (4) ease of mass production. We are studying the effects of incorporating single-walled carbon nanotubes (SWCNs) into a mixture of poly-(3-hexylthiophene) (P3HT), to test the affects on transport characteristics. The experiment will be segregated into parallel trials, with fixed volume ratios of P3HT:SWCNs to test the effects of (1) random orientation of SWCNs or the control, and (2) an aligned orientation of SWCNs. An electrophoresis-based technique, similar to gel electrophoresis, used to separate DNA fragments of variable masses, is used for partial alignment of the SWCN. Fixed geometry metalized substrates in a four striped copper patternare used for the transport studies and the P3HT:SWCN film's resistivity is monitored in-situ. The oriented films show enhanced conductivity, indicating this plays a major role in the increased efficiencies found in P3HT:SWCN based polymer solar cells. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P42.00002: Transparent Carbon Nanotube layers as cathodes in OLEDs Alexios Papadimitratos, Albert Nasibulin, Esko Kauppinen, Anvar Zakhidov Organic Light Emitting diodes (OLEDs) have attracted high interest in recent years due to their potential use in future lighting and display applications. Reported work on OLEDs traditionally utilizes low work function materials as cathodes that are expensive to fabricate because of the high vacuum processing. Transparent carbon nanotube (CNT) sheets have excellent mechanical and electrical properties. We have already shown earlier that multi-wall (MWCNT) as well as single CNT (SWCNT) sheets can be used as effective anodes in bright OLEDs [1,2]. The true advantage of using the CNT sheets lies in flexible devices and new architectures with CNT sheet as layers in tandem devices [3] with parallel connection. In this work, we are investigating the possibility of using SWCNT as cathodes in OLEDs. SWCNT sheets have been reported to show lower work function compared to MWCNT. Our work attempts to demonstrate transparent OLED devices with CNT anodes and cathodes. In the process, OLEDs with CNT cathodes have been fabricated in normal and inverted configurations using inorganic oxides (MoO3,ZnO) as invertion layers.[1] C.D. Williams et al., Appl.Phys.Lett. 93, 1, 2008.[2] A. Kaskela et al. Nano Lett., 10,11, 4349 ,2010.[3] A. Papadimitratos et al. 8th ICEL,2010. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P42.00003: Nanoscale electro-optical measurements of photovoltaic materials using scanning probe microscopy Nikolai Zhitenev, Behrang Hamadani, Paul Haney, Suyong Jung, Hua Xu The efficiency of photovoltaic devices based on inorganic thin-films or organic polymer blends is often determined by the nanoscale structure and properties of internal and contact interfaces. Measurements of local photo-conductivity, along with other scanning probe based measurements, can link the structural properties to the performance providing the desired feedback for the device optimization. However, the nature of the tip-to-sample contact can be quite different from contact interfaces in devices strongly affecting the injection and collection of charge carriers and complicating the data analysis. Here, we present the characterization of photoconductive channels in a model bulk heterojunction organic solar cell based on a p-type polymer and n-type small molecule. We directly compare the properties of the tip-to-sample interface to the nanocontact interface. We explore the nanoscale photocurrent response on two complementary device architectures using conductive tips suitable for the appropriate charge (i.e., electrons vs. holes) collection. In addition to the measurements at the top surface, we examine the response from the bulk of the film using novel sectioning technique. Our results provide significant insight into the origin of nanoscale variations in photoresponse and nanoscale morphology of such materials. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P42.00004: Organic and hybrid organic-inorganic photovoltaic cells Invited Speaker: The performance and limitations of the world's best organic and dye sensitized solar cells will be presented along with plans to increase the energy conversion efficiency to 15{\%}. Topics of more detailed discussion could include the formation of polymer-fullerene co-crystals and their implications for recombination, the use of energy transfer to improve light harvesting~in~dye sensitized solar cells, solution deposited transparent electrodes or the use of plasmonics to improve light absorption. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P42.00005: Studying recombination in the bulk heterojunction solar cells using lateral solar cell geometries Christopher Lombardo, Eric Danielson, Ananth Dodabalapur Lateral structures are shown to be a very powerful tool to understand transport and recombination phenomena in bulk heterojunction materials and solar cells. Active layers of phase separated P3HT:PCBM were chosen due to their wide use in research devices and potential for commercialization. Studies of current-voltage curves for varying carrier transit lengths have resulted in information about the movement of charge carriers as well as carrier recombination. By examining typical solar cell parameters (open circuit voltage, short circuit current, fill factor, and power conversion efficiency) combined with photocurrent measurements as a function of electrode spacing, carrier density, applied electric field, and temperature under illumination conditions (0.1 -- 100 suns), we have determined how these parameters depend on the carrier concentration, electric field, and temperature. This work provides a clear picture of when bimolecular recombination dominates and also if the recombination is Langevin or non-Langevin. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P42.00006: Scaling behavior and transport in bulk heterojunction materials Eric Danielson, Christopher Lombardo, Ananth Dodabalapur A lateral device geometry has been used to study charge transport in P3HT:C$_{71}$-PCBM bulk heterojunction devices. Analysis of current-voltage curves have previously been used to study charge transport in these materials. We perform ambipolar field effect transistor measurements on these structures to extract carrier mobilities. We are also able to describe the charge transport and recombination properties of these materials. Assymetric electrodes (Al, Au) separated by 100 nm-20$\mu $m enable us to gain considerable insight into transport physics. Photocurrent measruements as a function of channel length, electric field, and illumination intensity (0.1-100 suns) are used to measure the ambipolar mobility-lifetime product and study how this correlates with measured field-effect mobilities at various electric fields. Lateral structures are shown to be a powerful tool to understand transport and the role of carrier mobility on photovoltaic performance. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P42.00007: Charge Carrier Lifetime in Poly(3-hexylthiophe)/ZnO Nanowire Array Based Photovoltaic Devices Lee Butler, William Baughman, Shawn David Wilbert, Nick Harris, Gang Shen, Nabil Dawahre, Joseph Brewer, Patrick Kung, Seongsin Margaret Kim Nanostructured electron donor and acceptor materials have shown potential for greatly improving the efficiency of organic photovoltaic (OPV) devices. Inorganic hybrid OPVs utilizing nanowires, nanorods and nanoparticles have been shown to greatly increase the current through P3HT based devices but have yet to achieve the efficiencies of their corresponding bulk-heterojunction OPVs. Determining the carrier properties and interface structure of these hybrid devices could greatly aid in determining limitation of the device structure on the overall efficiency. Here we report the use of terahertz, micro-Raman and micro-photoluminescence spectroscopy in determining carrier lifetime and optical properties of P3HT/ZnO based OPV devices. We will also discuss the effects of surface functionalization on the available phonon modes, carrier lifetimes and absorption properties. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P42.00008: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P42.00009: Spectral aspects of cavity tuned absorption in organic photovoltaics Brent Valle, Stephen Loser, Jonathan Hennek, Kenneth D. Singer, James Andrews, Tobin Marks In order to increase the power conversion efficiency of organic photovoltaic devices it is necessary to extend absorption to longer wavelengths and to concentrate and capture light in a thin bulk heterojunction (BHJ) layer. In this work, optical transfer matrix formalism is used to model absorption in organic photovoltaic devices as a function of BHJ thickness and incident wavelength in the optical cavity formed by the BHJ layer sandwiched between the aluminum cathode and indium tin oxide (ITO) anode. We have found that absorption can be finely tuned by adjusting the thicknesses of the BHJ and ITO layers within a relatively narrow range. We have also observed distinct spectral effects due to frequency pulling resulting in enhanced long- wavelength absorption. Because the absorption shifts arise purely from optical interference effects, tuning of the absorption spectrum can be achieved by careful cavity design without affecting the open circuit voltage. We have experimentally verified aspects of our modeling and suggest methods to improve device design. Additionally, we consider the effects of BHJ material gradients versus depth on absorption in these devices. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P42.00010: Probing the Thickness Limits of Organic Solar Cells using Monte Carlo Simulation Mike Heiber, Ali Dhinojwala Organic solar cell performance has increased dramatically in recent years, but in order to achieve higher efficiency devices, it is imperative to understand the remaining fundamental challenges. One major shortcoming is that thin film devices cannot absorb all of the targeted incoming light due to the limited optical density of the materials used. To overcome this, thicker devices that can maintain the high quantum efficiency and high fill factor, present in thin state-of-the-art devices, must be developed. We have taken advantage of recent advancements in dynamic Monte Carlo (DMC) simulation methods to study the current-voltage (J-V) behavior of organic solar cells with different thicknesses. This method allows all detailed physical mechanisms of the device to be simulated and as a result, the effects of device morphology and a range of material properties can be captured. Studying device behavior as a function of thickness highlights the importance of the competition between light absorption and charge recombination. The effects of carrier mobility and active layer morphology are also considered. Understanding this tradeoff between absorption and recombination will help direct future experimental efforts to design optimal materials and devices. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P42.00011: Dehydration assisted nanoimprint of PEDOT:PSS nanogratings to improve organic photovoltaics Yi Yang, Koyau Lee, Kamil Mielczarek, Walter Hu, Anvar Zakhidov We demonstrate the fabrication of oly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)nanograting by a dehydration assisted nanoimprint lithographic technique. Dehydration of PEDOT:PSS increases its mechanical strength for high fidelity and fine precision nanoimprinting process, resuling in formation of high quality nanogratings of 60 nmin height, 70 nm in width, and70 nm in spacing. PEDOT:PSS nanograting ar used as hole injection and electron barrier layer in blended poly(3-hexylthiophene-2,5-diyl) (P3HT)[6,6]-penyl-C61-butyric-acid-methyl-ester PCBM) rganic bulk heterojuncton photovoltaic devices (OPV), showing enhancement of photocurrent and increased efficiency in comparison to non-patterned plane PEDOT:PSS film. Improved performance is discussed in terms of increased interface for charge collection and better distribution of internal electric field. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P42.00012: High-performance inverted polymer solar cells with ITO coated with a thin layer of oxide for electron collection Jianyong Ouyang, Hongmei Zhang Solar cells using organic or polymeric materials as the active material have been attracting strong attention due to the low fabrication cost and high mechanical flexibility. The photovoltaic efficiency has been improved to more than 8{\%} under AM1.5 G illumination. However, polymer solar cells are usually not very stable, which severely impedes the practical application. The stability is strongly affected by the electrodes. Both PEDOT:PSS used as the buffer layer on ITO for the hole electron and active metals like Ca for the electron collection are blamed to lower the stability of polymer solar cells. Polymer solar cells with an inverted structure can have much better stability than normal devices because they do not use PEDOT:PSS and active metals. One big challenge in building the inverted polymer solar cells is to lower the work function of ITO for effective electron collection. Here, we report a new method to effectively lower the work function of ITO by depositing a thin layer of oxide and demonstrate high-performance polymer solar cells. The photovoltaic efficiency of the inverted polymer photovoltaic cells is even higher than the normal devices. The mechanism for the oxide effect on the work function of ITO will be presented as well. [Preview Abstract] |
Session P43: Focus Session: Thin Film Block Copolymers III
Sponsoring Units: DPOLYChair: Mark Stoykovich, University of Colorado at Boulder
Room: A306/307
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P43.00001: Effect of Well-defined Roughness on the Microdomain Orientation of Block Copolymer Thin Films Kookheon Char, Youngwoo Choo, Hyo Seon Suh, Taehee Kim It is well known that the perpendicular orientation of block copolymer (BCP) films is preferred over the parallel orientation when the BCP films are placed on rough surfaces due to the unfavorable elastic deformation of BCP chains in the parallel oriented microdomains. More recently, the approach to utilize the rough substrate for BCP domains in long range order has been reported. However, the quantitative analysis to explore the effect of each individual roughness factors, such as lateral or vertical displacement, on the microdomain orientation of BCP films has not been thoroughly studied yet. In order to examine the roughness effect systematically, we prepared substrates with well-defined roughness utilizing either nanoparticle monolayers or line patterns generated by e-beam lithographic technique. Based on the detailed observation of the orientational change of BCP films on the well-defined surface roughness, we analyzed the dependence of BCP domain orientation on each roughness parameter. Furthermore, we found that the BCP film thickness, coated on the substrate, is another important parameter determining the orientation of microphase-separated domains of BCP thin films in addition to the surface roughness. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P43.00002: Multiple Replicas of Block Copolymer Thin Films from a Brushless Organosilicate Substrate Hyo Seon Suh, Hyunsik Yoon, Kookheon Char The chain end-grafted polymer brushes or cross-linked polymer mats have typically been utilized as the surface modification layers to induce the perpendicular orientation of block copolymer (BCP) thin films. Instead of such polymer-based approaches, we have recently introduced a new concept to control the BCP orientation using the brushless organosilicate (OS) substrates, whose surface energy can be finely tuned with thermal treatment. In this brushless case, the BCP chains do not penetrate into the underlying hard OS substrates during thermal annealing of BCP films, therefore, the BCP chains at the interface have no entangled structure with fairly weak adhesion of BCP films against the substrate. Owing to such weak adhesion of BCP films against the OS substrate, the perpendicularly oriented BCP film on a neutral OS substrate could be easily peeled off and transferred to a UV-curable resin applied onto the BCP film. The OS substrate after the peel-off process of a BCP film could regenerate the perpendicularly oriented BCP films since the surface energy of the OS substrate remains intact during the peel-off process. Furthermore, the direct-assembled BCP films on chemically patterned OS substrates could also be peeled off and transferred on to a UV-curable resin, allowing us to produce multiple replicas of direct-assembled BCP thin films from a single chemically patterned OS substrate. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P43.00003: Rational design of block copolymer morphologies via control of the film thickness and substrate patterning: A self consistent field study Xianggui Ye, Brian J. Edwards, Bamin Khomami Chemically patterned substrates can direct the assembly of adsorbed layers or thin films of block copolymers. In this study we have examined the self-assembly of a lamella-forming diblock copolymer on periodically stripe-patterned substrates, and a cylinder-forming diblock copolymer on periodically doted-patterned substrates for various film thicknesses. In general, we have shown that for thin films the morphology of the block copolymer follows the chemical pattern at the substrate; however, with an increase degree of mismatch between the spacing of the pattern and the natural spacing of bulk block copolymer, a host of novel morphologies can been created, which have not to date been experimentally realized. Our studies clearly demonstrate that the film thickness and the pattern of substrate can be judiciously manipulated to rationally design morphologies for various applications such as filtration, conduction, and high-surface area membranes. Overall, these results demonstrate a promising strategy for fabrication of complex interfacial nanostructures from chemically patterned templates. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P43.00004: Utilizing low surface energy moieties to control surface composition of a polystyrene-b-poly(2-vinylpyridine) block copolymer Michael Dimitriou, Daniel Fischer, Craig Hawker, Edward Kramer During processing the interaction of a block copolymer film with a free surface affects its final orientation and surface composition. A strategy to control this interaction and hence tailor the final structure of a polymer film is to introduce low surface energy moieties to the system. A lamellar forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer was synthesized with varying amounts of 1-[(3-butenyloxy)methyl]-4-vinylbenzene copolymerized with the 2VP block. Utilizing thiol-ene chemistry the pendant alkene of 1-[(3-butenyloxy)methyl]-4-vinylbenzene was functionalized with 1H,1H,2H,2H-perfluorooctanethiol to efficiently and selectively incorporate fluorinated hydrocarbons into the 2VP block. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy were used to characterize the polymer surface as a function of 1H,1H,2H,2H-perfluorooctanethiol incorporation. At molar incorporations of the fluorinated monomer in P2VP greater than 4{\%} dynamic secondary ion mass spectrometry and NEXAFS indicate a P2VP rich surface. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P43.00005: Flexible Confinement of Block Copolymer Films Between Tunable Surface Energy Elastomeric Films and Xerogel Substrates Manish Kulkarni, Gurpreet Singh, Sushil Satija, Alamgir Karim Orientation control of block copolymer (BCP) films is important for advanced technological applications such as nanoscale lithography. Here we present a different strategy whereby both interfaces of the poly(styrene)-block-poly(methylmethacrylate) BCP films are tunably controlled. The BCP films were coated on a roughness and surface energy tunable xerogel substrates and the top surface of the polymer film was conformally covered by crosslinked PDMS elastomer. The surface energies of xerogel substrate and crosslinked PDMS is tunable from 28 and 18 mJ/m$^{2}$ to 45 and 55 mJ/m$^{2 }$resp. via UV-Ozone treatment. The confined BCP film was then thermally annealed to induce ordering. Such a unique approach allowed the BCP films to respond in its orientation of cylinders and lamellae from parallel to perpendicular. The morphology of these micro-phase separated BCP films was studied by tapping mode atomic force microscopy and neutron reflectivity. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P43.00006: Directed Assembly of Block Copolymer on Tunable Surface Energy Flexible Substrate Arzu Hayirlioglu, Alamgir Karim We examine the ordering properties of micro-phase separated block copolymer (BCP) films on flexible substrates. In particular, we investigated the wettability characteristics and morphology of the BCP films before and after annealing on tunable surface energy PDMS substrates. The surface energy of PDMS substrates was modified to vary from 20 to 68 mJ/m$^{2}$ by exposing them to UV-ozone (UVO). Two types of block copolymer systems were examined on these UVO exposed PDMS substrates. Our experiments were carried out with surface energy above 42 mJ/m$^{2}$ because the BCP dewets on the PDMS substrate below that surface energy. Atomic Force Microscope (AFM) and Optical Microscope (OM) were used to study of the surface morphology of the BCP films. It was observed that the BCP morphology exhibits perpendicular orientation on PDMS substrate with surface energy in the range of 42 to 66 mJ/m$^{2} $ and parallel orientation above 67 mJ/m$^{2}$. BCP film morphology on other types of soft substrates and different BCP systems as a function of annealing temperatures and film thickness will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P43.00007: The effect of Surface Neutrality on ODT of PS-b-PMMA films Eunhye Kim, Seunghoon Choi, Rui Guo, Du Yeol Ryu, Craig J. Hawker, Thomas P. Russell The film transitions, the order-to-disorder transition (ODT), has been investigated in a symmetric polystyrene- b-poly(methyl methacrylate) (PS-b-PMMA) on a random copolymer (P(S-r-MMA)) grafted substrate where the interfacial interactions are balanced. With decreasing film thickness less than 25L0, the ODT significantly decreases, because the interfacial interactions by a random copolymer grafted to the substrate provide a surface-induced compatibilization toward two block components. However, a plateau of the ODT for films thicker than 25L0 was observed above the bulk value. The elevation of this ODT indicates a suppression of compositional fluctuations normal to the film surface, more than likely because the dominant orientation of the lamellar microdomains was found to be parallel to the film surface. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P43.00008: Surface Affinity Effects On Confined Thin Film Block Copolymers Using Self Consistent Field Theory Modeling Adam Hannon, Alfredo Alexander-Katz, Caroline Ross Self consistent field theory (SCFT) applied to inhomogeneous thin film block copolymer systems allows for the exploration of a wide array of potential equilibrium ordered morphologies at the nanoscale through varying parameters such as $\chi $, the segmental Flory-Huggins parameter, N, the polymer degree of polymerization, and $f$, the volume fraction of the minority polymer component. In addition, boundary conditions of the chemical potential fields in the field theory can be specified to model surface features such as polymer brush layers and topological templating features from lithography that enrich the possible morphologies observed [Macromolecules 2010, 43, 8290--8295]. In this presentation, we show how the orientation and surface morphology of diblock copolymers with large $\chi $N varies with surface affinity, surface shape, and $f $for confined thin film systems. Surfaces neutral to both polymer species are examined, as well as surfaces preferential to both the minority and majority polymer components with fine variances in the magnitude of the surface affinity. Commensurability of the ordered structures is examined as well by varying simulation cell size. The results of the study will be applied to the generation of complex features for nanolithography applications. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P43.00009: Stress induced topographic patterning in thin diblock copolymer films Andrew Croll, Alfred Crosby When a thin rigid polymer film is attached to a soft elastic substrate and placed in a state of compressive stress, the system wrinkles as a critical stress is surpassed. This simple deformation pattern contains information about the mechanical state of both the polymer film and substrate. Although classical mechanics can be used to relate the global deformation of the film/substrate to the local wrinkle geometry as a function of materials properties, relatively little is known about how the thin capping film material accommodates the localized bending (and therefore localized stress). Here we conduct wrinkling experiments using a model diblock copolymer/elastomer composite. Wrinkling a homogeneous, disordered block copolymer film places the film in a well-defined initial stress state. When heated above its glass transition, the wrinkled film flows, microphase separates, and relaxes from the stress imposed by local wrinkle deformations. The periodic stress relaxation leads to the emergence of a new pattern in the microphase separated surface structure, thus providing new insight into how block copolymers react to stress. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P43.00010: Shear-induced sphere-to-cylinder transition in thin films of diblock copolymers and the role of wetting layers Alexandros Chremos, Richard Register, Paul Chaikin, Athanassios Panagiotopoulos The shear-induced sphere-to-cylinder transition in diblock copolymer thin films has been studied using large-scale coarse- grained Langevin dynamics simulations. At zero-shear conditions and below the order-disorder transition temperature the thin film forms a monolayer or bilayer of spheres given the thickness of the film. Mimicking the experimental setup the minority block has an affinity to be adsorbed on the confining surfaces forming brushes which interpenetrate the rest of the film. Once a shear field is applied and above a critical shear rate, the spheres elongate and merge with their neighbors to form cylinders. We find that the mechanism with which the spheres merge is closely related with the stretching of individual diblock chains. In particular, we find that in monolayer thin films it is more difficult to achieve the sphere-to-cylinder transition, which is also an experimental observation, because the brushes restrict the stretching of diblock chains. The simulations were performed with the use of Graphical Processing Units allowing large-scale simulations with long polymer chains to studied. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P43.00011: Shear alignment of standing block copolymer lamellae in thin films Saswati Pujari, Michael Keaton, Paul Chaikin, Richard Register While thin films of cylinder-forming block copolymers (BCPs) can be effective templates for striped patterns (leading to parallel nanowires), a drawback is the modest aspect ratio of the final structures, because of limited etch contrast between the two polymer blocks. Using thin films of lamellar BCPs, with the lamellae standing perpendicular to the substrate, could yield structures with larger aspect ratio. To generate parallel stripes of controlled direction, the in-plane orientation needs to be guided while preserving the out-of-plane perpendicular orientation. In this study, we have produced thin films of standing lamellae of a polystyrene/polymethylmethacrylate BCP of thicknesses up to 1.5 times the domain spacing, by neutralizing the Si substrate with a random terpolymer brush layer. To date, films less than one domain spacing thick have been aligned by shear, while shearing thicker films causes the perpendicular lamellae to switch to parallel orientation. We are currently investigating the alignment as a function of stress, film thickness, terpolymer composition and domain spacing. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P43.00012: Defect generation in thin films of block copolymer cylinders: the effect of cylinder spacing and film thickness Vindhya Mishra, Edward Kramer Understanding the fundamental physics of disordering and defect generation in block copolymer (BCP) films is important for directed assembly based block copolymer lithography. We investigate the defect generation in, and smectic-nematic-isotropic transition temperature T$_{m}$ of, monolayers and bilayers of poly (styrene-b-2vinyl pyridine) diblock copolymer cylinders aligned parallel to the substrate in 2 micron wide channels. Quantitative AFM studies were supplemented with grazing incidence small angle X-ray diffraction line-shape analysis to quantify the decay of translational and orientational correlation functions with increasing temperature. We find that T$_{m}$ decreases, and the dislocation density $n$ below T$_{m}$ increases, if either the number of layers or the cylinder spacing $a$ decreases. These results are expected since $n\sim $ exp(-E$_{d}$/kT) and E$_{d}$, the dislocation formation energy, scales as $a^{2}h$, where $h$ is film thickness. Since only a 10{\%} decrease in $a$ produces a dramatic increase in $n$, these results suggest that using 2D smectic structures such as BCP cylinders aligned parallel to, or BCP lamellae normal to, a substrate to produce more closely spaced features will result in patterns with more and more defects. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P43.00013: Micelle Formation of Diblock Copolymer in a Thin Film Homopolymer: a Comparison with Polymer Brush-Coated Nanoparticles Hengxi Yang, Chelsea Chen, Peter Green We investigated micelle formation of a diblock copolymer polystyrene-$b$-poly(2-vinylpyridine) (PS-$b$-P2VP) of degree of polymerization $N$, in thin films of homopolymer polystyrenes (PS) of different degrees of polymerization $P$, supported on substrates, and compared the results with the phase behavior of PS brush-coated Au nanoparticles in homopolymer PS matrix. PS-$b$-P2VP copolymer chains aggregated to form micelles, composed of an inner P2VP core and an outer PS corona. The size of the micelle cores, $D_{core}$, increased with increasing $P$, and reached a plateau at very large $P$. The transition occurred at a larger $P$/$N$ than expected from brush-melt interaction theories. The organization of micelles at large $P$ regimes suggested attractions between micelles. P2VP block also adsorbed onto the substrate to form a brush layer and the surface adsorption process was affected by micellization of copolymers. We compared the micelle formation of PS-$b$-P2VP in PS with the phase behavior of PS coated Au nanoparticle/PS mixtures: the host chains penetrate into the corona of the micelles more easily than into the PS brush grafted on the particle due to low ``grafting density;'' what's more, micelles can self-adjust their aggregation number as the interaction between host chains and the corona changes. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P43.00014: Bicontinuous nanoporous block copolymer films prepared from a spherical-phase architecture Easan Sivaniah, Paul Zavala, Kevin Channon, Sanna Nataraj, Shaheen Al-Muhtaseb In a recent discovery, we have found a way to make a bicontinuous nanoporous polymer network and subsequently transform this into interconnected mesoporous inorganic oxide sheets. Notably, these structures arise from a spherical block copolymer template. Nanoporous materials of such architecture, both polymeric and inorganic, are rare and also extremely useful.~ Importantly, the process is not restricted to a single block copolymer system or a narrow range of molecular weights or compositions. All of the process steps are scaleable, fast enough to be appropriate to continuous production methods, do not require vacuum technology, and can be achieved by solution processing. We discuss the process and its use to make PLEDs, photovoltaics and filtration membranes. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P43.00015: Arrangement of the Microdomains of Block Copolymers Confined at Hemi-sphere Walls Du Sik Bae, Jin Kon Kim The arrangement of the microdomains of polystyrene-\textit{block}-poly(methyl methacrylate) copolymer (PS-$b$-PMMA) confined at hemi-sphere walls was investigated by scanning and transmission electron microscopy. The hemi-spherical constraint was introduced by using anodic aluminum oxide template.. The wall surface was modified by thin layers grafted by PS, PMMA, and PS-\textit{ran}-PMMA copolymer. We observed interesting morphologies which have not been observed. The observed microdomain arrangement was compared with theoretical predictions. [Preview Abstract] |
Session P44: Focus Session: Assembly, Structure, & Instabilities in Polymer Films, Network Films, & Interfaces I
Sponsoring Units: DPOLYChair: Zhiqun Lin, Iowa State University
Room: A309
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P44.00001: An interplay between self-organization and interfacial modification of diblock copolymers in bi-layered thin film laminates Invited Speaker: We study the interfacial partitioning of poly(styrene-b-methyl methacrylate) (SM) diblock copolymers at interfaces between thin planar films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) homopolymers. SM copolymers with constant PS and varying PMMA block lengths are incorporated into the top PS layer and the resulting dewetting kinetics of the top PS film decrease by reducing the length of the PMMA block and increasing the molecular weight of the host PS homopolymer. Similar behavior is observed when the SM copolymers are added to the bottom PMMA homopolymer. Systems incorporating SM copolymers possessing short PS blocks and long PMMA blocks exhibit dewetting rates that are higher than those of the copolymer-free PS/PMMA bilayer. This behavior is attributed to the segregation of SM aggregates at the PS/PMMA interface, which migrate to and roughen the interface and destabilize the film. The dewetting kinetics of systems with mixtures of asymmetric copolymers (stabilizing and destabilizing) added in the PS homopolymer lie between those of the individual copolymers pure copolymers. Using block copolymer mixtures rather than single copolymers to tune stabilizing/compatibilizing efficacy provides an unexplored route to achieving property control in thin polymer films. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P44.00002: Formation of Nanoparticle Stripe Patterns via Flexible-Blade Flow Coating Dong Yun Lee, Hyun Suk Kim, Cassandra Parkos, Cheol Hee Lee, Todd Emrick, Alfred Crosby We present the controlled formation of nanostripe patterns of nanoparticles on underlying substrates by flexible-blade flow coating. This technique exploits the combination of convective flow of confined nanoparticle solutions and programmed translation of a substrate to fabricate nanoparticle-polymer line assemblies with width below 300 nm, thickness of a single nanoparticle, and lengths exceeding 10 cm. We demonstrate how the incorporation of a flexible blade into this technique allows capillary forces to self-regulate the uniformity of convective flow processes across large lateral lengths. Furthermore, we exploit solvent mixture dynamics to enhance intra-assembly particle packing and dimensional range. This facile technique opens up a new paradigm for integration of nanoscale patterns over large areas for various applications. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P44.00003: Hierarchically Organized Structures Engineered from Controlled Evaporative Self-Assembly Zhiqun Lin, Myunghwan Byun, Wei Han, Ned Bowden By constraining an asymmetric comb block copolymer (CBCP) toluene solution to evaporate in a wedge-on-Si geometry composed of a wedge lens situated on a Si substrate, gradient concentric stripe-like surface patterns of CBCP at the microscopic scale were yielded as a direct consequence of \textit{controlled} evaporative self-assembly of CBCP. The formation of either straight stripes or jagged stripes was dictated by the height of the wedge. Upon subsequent solvent vapor annealing, hierarchically organized structures of CBCP were produced, resulting from the interplay of solvent vapor-assisted, unfavorable interfacial interaction-driven destabilization of CBCP from the Si substrate at the microscopic scale and the solvent vapor-promoted reconstruction of CBCP nanodomains within the stripes at the nanometer scale. This facile approach of combining \textit{controlled} evaporative self-assembly with subsequent solvent vapor annealing offers a new platform to rationally design and engineer self-assembling building blocks into functional materials and devices in a simple, cost-effective manner. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P44.00004: Wrinkling instabilities in compressed networks of polymer supported single-wall carbon nanotubes John Harris, Swathi Iyer, Ji Yeon Huh, Jeffrey A. Fagan, Jun Young Chun, Steven D. Hudson, Jan Obrzut, Christopher M. Stafford, Erik K. Hobbie Strain-induced structural and electronic changes in polymer supported membranes of purified single-wall carbon nanotubes (SWCNTs) are evaluated through the wrinkling instabilities that develop under both uniaxial and isotropic compression. Nanotubes that have been purified by length or electronic type using density-gradient ultracentrifugation are assembled as surfactant-free thin membranes on prestrained polydimethylsiloxane (PDMS) substrates, and the strain response is measured using a broad range of techniques. The small-strain behavior is inferred from kinetic changes in the wrinkling topography of the SWCNT membranes during the slow drying of pre-swelled polymer supports. The measurements suggest a remarkable degree of strain softening that strongly couples to the anisotropic sheet resistance of the films, which we in turn relate to the microscale anisotropy that develops through excluded volume interactions. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P44.00005: Confinement-Induced Molecular Stresses and Wetting Instability in Ultrathin Polymer Films Y. Chein, P.W. Lee, A.C.-M. Yang Chain packing and molecular behavior of polymers confined below unperturbed coil sizes are still poorly understood. To explore the physical state and condensation process, molecular recoiling stresses in polystyrene films (4-100 nm) were measured through wetting instability at above T$_{g}$(100 C). The films demonstrated strikingly different instability mechanisms in regimes divided by entanglement molecular weight (M$_{e})$. Moreover, the recoiling stress decreased with chain length above M$_{e}$, consistent with the condensation process dominated by solvent evaporation, but plunged below M$_{e}$, apparently due to diminishing deformations. A small fraction of MEH-PPV added in films manifested photoluminescence (PL) following the same trend as recoiling stress confirming stress-enhanced PL characteristic of conjugated polymers. As aging temperature lowered but still above T$_{g}$, film stability increased but recoiling stress underwent significant changes, in contrast to that below T$_{g}$ where no changes was observed. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P44.00006: Nucleation and growth of creases on swelled polymer gel surfaces Jinhwan Yoon, Jungwook Kim, Ryan Hayward We have studied the processes by which surface creases form, evolve, and disappear using thin temperature-responsive poly(N-isopropylacrylamide) copolymer hydrogels. For shallow quenches beyond the critical level of compression, creases nucleate and growth, and thus the observed onset and morphology are typically dominated by heterogeneous defects. Measurements of crease growth velocities at different quench depths are used to precisely determine the compression at which the surface first becomes unstable, and this critical strain is found to be slightly elevated as film thickness is reduced. This behavior is captured by a simple model for the energy change upon forming a fold, with a nucleation barrier to fold formation provided by the gel/water surface-energy. While hysteresis between onset and disappearance is often observed in experiments, this reflects predominantly the degree of ``undercooling'' necessary to yield nucleation and growth of creases. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P44.00007: Effect of polymer architecture on the interfacial properties of thin films Emmanouil Glynos, Bradley Frieberg, Peter Green Many physical properties of polymers, such as phase transitions, mechanical properties, dynamics, crystallization and the glass transition, Tg, are influenced by film thickness constraints, and associated with the interactions between the constituent macromolecules and external interfaces. We show that star-shaped molecules, possessing sufficiently high functionality, exhibit significant differences in their average Tg-vs-thickness (H) behavior, both in the magnitude and the thickness dependent trends, from their linear analogs. In this talk, we will discuss the effect of polymer architecture on the interfacial properties of supported thin films, and more specifically how chain segments near the free and the solid interfaces influence the average Tg of the film. These effects are discussed in terms of the role of macromolecular architecture and entropic effects on the structure and dynamics of the polymer chains close to the interfaces, and their influence on the average properties of thin supported films. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P44.00008: Interfacial Effects on Pentablock Ionomer Thin Films Thusitha Etampawala, Dilru Ratnaweera, Naresh Osti, Umesh Shrestha, Dvora Perahia, Jaroslaw Majewski The interfacial behavior of multi block copolymer thin films results from a delicate balance between inherent phase segregation due to incompatibility of the blocks and the interactions of the individual blocks with the interfaces. Here in we report a study of thin films of ABCBA penta block copolymers, anionically synthesized, comprising of centered randomly sulfonated polystyrene block to which rubbery poly-ethylenebutalene is connected, terminated by blocks of poly-t-butylstyrene, kindly provided by Kraton. AFM and neutron reflectometry studies have shown that the surface structure of pristine films depends on film thickness and ranges from trapped micelles to thin layered films. Annealing above Tg for the styrene block results in rearrangements into relatively featureless air interface. Neutron reflectivity studies have shown that annealed films forms layers whose plane are parallel to the solid substrate with the bulky block at the air interface and the ionic block at the solid interface. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P44.00009: Directed Morphology of Nanofilled Polymer Films on Flexible Substrates Diya Bandyopadhyay, Arzu Hayirlioglu, Manish Kulkarni, Alamgir Karim We demonstrate viable meso-patterning techniques that have relevance to electronics and organic photovoltaic applications via tunable control of polymer thin film instabilities. To this end, we examine the influence of fullerene (C$_{60}$) nanoparticles on multicomponent polymer thin films on patterned and flexible polydimethylsiloxane substrates and compare these results to morphologies on hard silica xerogel substrates of variable roughness and surface energy. Controlled incorporation of nanoparticles (NPs) can be used to tune polymer thin film instabilities and morphology. At NP concentrations below a threshold value, we observe directed dewetting of blend thin films consisting of uniformly aligned dewet domains that mimic the periodicity of the confining media, consistent with our previous experiments where it was observed that C$_{60}$ NPs preferentially segregate to a PS/PB blend interface up to a certain saturation concentration. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P44.00010: Fiber Formation From the Melting of Free-standing Polystyrene, Ultra-thin Films: A Technique for the Investigation of Thin Film Dynamics, Rheological Properties and Confinement Effects Jeremy M. Rathfon, Robert W. Cohn, Alfred J. Crosby, Jonathan P. Rothstein, Gregory N. Tew The processes of fiber formation from the melting of ultra-thin films are explored in high detail and produce a new technique for the investigation of rheological properties, confinement effects, and the dynamics of thin films and polymer chains. Ultra-thin films of polystyrene are suspended atop micro-arrays of pillars. Films are then annealed above the $T_{g}$ and studied via optical microscopy. Hole nucleation is quantified with a free energy barrier based on a simple capillary model. Holes then grow exponentially in a shear thinning, high shear strain regime. These holes impinge upon each other to form suspended fibers which thin according to a model for elasto-capillary thinning of fluid filaments. Monitoring fiber thinning allows for the acquisition of rheological properties as well as the apparent extensional viscosity. The breakup of the fiber network indicates the effects of confinement on chain entanglements in ultra-thin films. A transition below a critical film thickness, comparable to the dimensions of a polymer chain, shows reduced interchain entanglements and a remarkably faster breakup of fibers. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P44.00011: Self-Repair of Polymer Films Through Monomer Filled Ni-Zn Microcapsules Marcia Patchan, Lance Baird, Yo-Rhin Rhim, Erin LaBarre, Adam Maisano, Ryan Deacon, Jason Benkoski A novel polymer additive composed of isocyanate resin-filled metal microcapsules has been successfully synthesized through a combination of emulsification, interfacial polymerization, and electroless Ni-Zn deposition. The resulting metallic microcapsules impart self-healing and galvanic protection capabilities to off-the-shelf primers. Once scratched, the microcapsules release their contents into the scratch, where they harden and restore the moisture barrier. If healing is incomplete, the Ni-Zn shell acts as a sacrificial anode to galvanically protect the underlying steel. ASTM adhesion, wear resistance, and moisture resistance tests evaluated the ability of microcapsule-filled primers to heal scratches, provide galvanic protection, and prevent corrosion. We found that self- healing was most effective for broad, shallow scratches (3 mm) and narrow scratches (75 $\mu$m). [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P44.00012: Structural Rearrangement of Semifluorinated Diblock copolymer Thin Films Exposed to Selective Solvents Umesh Shrestha, Dvora Perahia, Stephen Clarson A neutron reflectometry study reveals the response of a semifluorinated diblock co-polymer poly trifluoro propyl methyl siloxane -$b$- polystyrene co-polymer thin films of ca. 50-70nm, to selective solvents. This diblock forms surface induced layered structure at volume fractions of the F segment ranging from 0.03 to 0.50. When exposed to toluene, a selective solvent for PS, the film swelled instantaneously, while retaining its layered structure. The solvent however penetrates into both the PS and the semifluorinated layers. In contact with decane vapor, the solvent penetrates predominantly the top fluorinated layers. The rate of penetration increases with increasing volume fraction of the fluorinated segment. In contrast with toluene, the solvent remains at the top layer only for extended periods again retaining the layered structure. The incompatibility and specific affinity of different blocks drive structural rearrangements at the interface as a response to external stimuli, retaining the layers structure. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P44.00013: Wetting induced instabilities in miscible polymer blends Nigel Clarke, Katherine Thomas, Ullrich Steiner, Rosa Poetes, Mihai Morariu The behaviour of miscible blends of polystyrene (PS)/poly(vinyl methyl ether)(PVME) of varying compositions has been investigated [1] at temperatures where PS and PVME are miscible. The PVME is seen to enrich the polymer-air surface, forming a layer with a width that is comparable to the correlation length. Further heating close to the demixing temperature results in the formation of a capillary instabilities at the polymer surface exhibiting a spinodal-like pattern with a characteristic wavelength that depends on the blend composition. Formation of these instabilities is seen for all blend compositions. We propose that these wetting induced instabilities result from coupled height and composition fluctuations in the PVME enriched surface layer, driving the build-up of long wavelength fluctuations. \\[4pt] [1] Thomas, K.R.; Clarke, N.; Poetes, R.; Morariu, M.; Steiner, U.; Soft Matter, 6, 3517, 2010 [Preview Abstract] |
Session P45: Unitary Fermi Gases and the BCS-BEC Crossover
Sponsoring Units: DAMOPChair: Ken O'Hara, The Pennsylvania State University
Room: A310
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P45.00001: Universal physics in the dilute Fermi gases Soon Yong Chang, Mohit Randeria, Nandini Trivedi Using Quantum Monte Carlo techniques, we investigate universal properties of the repulsive upper branch as well as the superfluid ground state across the Feshbach resonance. We test the Tan relations by computing (a) the equation of state, (b) the short distance behavior of the two-particle density matrix, and (c) the large-$k$ tail of the momentum distribution $n(k)$. We have used twisted boundary conditions to improve the approach to the thermodynamic limit, which allows us to probe $n(k)$ at a dense set of $k$ values. We find consistent estimates of the $1/k_F a$ dependence of the contact $C$ at $T=0$ from all three methods. We show that, just like the superfluid ground state across the BCS-BEC crossover, the repulsive upper branch also obeys these relations, albeit with different values of $C$. This reflects the universal behavior of dilute Fermi gases with a short range potential, in contrast to, for instance, the hard sphere gas in the $k_F a \sim 1$ regime. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P45.00002: Thermal distribution and the contact of Fermi gases at large scattering lengths Joaquin Drut, Timo Lahde, Timour Ten A few years ago, Tan and others derived a set of exact relations valid for strongly interacting non-relativistic Fermi gases in the regime of short interaction range and large scattering length. Recent developments have shown that a central quantity in these identities, the so-called ``contact'' C, actually plays a crucial role in the characterization of these systems, as it determines multiple thermodynamic properties as well as linear-response sum rules. However, computing the ``contact'' presents a challenge as it requires non-perturbative methods such as Quantum Monte Carlo. In this contribution, we present our first results for C as a function of temperature in the limit of infinite scattering length. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P45.00003: Investigating the Effect of Density Inhomogenity on Photoemission Spectroscopy Tara Drake, John Gaebler, Rabin Paudel, Jayson Stewart, Deborah Jin Ultracold atomic gases realize clean and controllable model systems for investigating many-body quantum physics. However, trapped gases are intrinsically spatially inhomogeneous in their density, and in many cases, one would like to compare measurements of these systems with theoretical understanding for a homogeneous gas. In particular, density inhomogeneity can complicate the interpretation of data taken in momentum space, as the original spatial information is lost during time of flight expansion. The effect of density inhomogeneity due to a harmonic trapping potential is studied in a degenerate gas of 40K atoms. Using a method to select only the atoms in the center of the trap, we study how a more homogenous sample changes what can be seen in time of flight experiments, including photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P45.00004: Spectral functions in ultracold Fermi gases William Schneider, Mohit Randeria We study the fermion spectral function in the superfluid state across the BEC-BCS crossover and in the normal Fermi liquid phase in highly imbalanced Fermi gases. We focus on features that can be measured in momentum-resolved radio frequency spectroscopy experiments. We go beyond mean field theory and include the effects of Gaussian order parameter fluctuations in a manner that gives excellent agreement with asymptotically exact results for the $T=0$ equation of state in the BEC and BCS limits, as well as quantum Monte Carlo (QMC) results near unitarity. We show that sharp Bogoliubov quasiparticles, with a substantial coherent spectral weight, exist near unitarity. We argue that this is true generally even beyond the Gaussian approximation. In addition, quasiparticle scattering and interaction with collective modes produces incoherent spectral weight. We show that the dispersion is strongly renormalized at unitarity with its minimum shifted up from its mean field value $\sqrt{2 m \mu}$ and compare our results with existing QMC data. We discuss how the spectral function changes qualitatively compared with its mean field form as $1/(k_Fa)$ increases and the chemical potential changes sign. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P45.00005: Short-range correlations and entropy in ultracold atomic Fermi gases Zhenhua Yu, Georg Bruun, Gordon Baym We relate short-range correlations in ultracold atomic Fermi gases to the entropy of the system over the entire temperature, T, vs. coupling strength, .1/kF a, plane. In the low temperature limit the entropy is dominated by phonon excitations and the correlations increase as T4. In the BEC limit, we calculate a boson model within the Bogoliubov approximation to show explicitly how phonons enhance the fermion correlations. In the high temperature limit, we show from the virial expansion that the correlations decrease as 1/T. By interpolating between the two limits, we predict that the correlations reach a maximum at a finite temperature. We infer the general structure of the isentropes of the Fermi gas in the T,.1/kF a plane, and the temperature dependence of the correlations in the unitary, BEC, and BCS limits. Our results compare well with measurements of the correlations via photoassociation experiments at higher temperatures. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P45.00006: Momentum Resolved Radio Frequency Spectroscopy in Trapped Polarized Gases Kathryn Levin, Chih-Chun Chien, Hao Guo With recent advances in momentum resolved radio frequency (RF) spectroscopy, both experiment and theory, one can consider doing analogous experiments on polarized Fermi gases. In this talk we present predictions for the behavior of the fermionic spectral functions in the majority and minority bands. By truncating the integrated trap contributions at varying radii, the spectral functions will reflect the increase in the local polarization from nearly zero at the center to large values at the edges. We present predictions for these spectral functions and discuss their implications for future experiments.\\[4pt] [1] Stewart, J T et al. Nature 454, 744 (2008)\\[0pt] [2] Chen, Q. and Levin, K. Phys. Rev. Lett. 102, 190402 (2009)\\[0pt] [3] Chen, Q. et al. Rep. Prog. Phys. 72, 122501 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P45.00007: Observation of shock waves in a unitary Fermi gas James Joseph, Manas Kulkarni, Alexander Abanov, John Thomas We study the nonlinear hydrodynamics of a strongly interacting (unitary) Fermi gas comprising a 50-50 mixture of the lowest two hyperfine states of $^6$Li near a broad Feshbach resonance at 834 G. The gas is is cooled via forced evaporation in a cigar-shaped CO$_2$ laser trap with a repulsive optical sheet potential at the center creating two separate clouds. When the repulsive potential is turned off and the two clouds collide we observe exotic nonlinear hydrodynamics distinguished by the formation of a very sharp and stable density peak at the center of the trap and subsequent evolution into a box-like shape with sharp edges. We attribute these characteristics to shock-wave formation in the unitary gas. By solving the hydrodynamic equations numerically we can reproduce the time dependence of the observed density profiles. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P45.00008: Theory of shock waves in a unitary Fermi gas Alexander Abanov, Manas Kulkarni, James Joseph, John Thomas We present here evidence of shock wave formation during the collision of two clouds of a unitary Fermi gas. A unitary Fermi gas is an ideal test ground for extreme quantum hydrodynamics. By its very nature the system exhibits universality, i.e., the properties of the gas, constrained by an underlying scale invariance, depend entirely on the density and temperature. The nonlinear hydrodynamics exhibited in this experiment is understood by using a dimensionally reduced quasi- 1D form of the quantum hydrodynamic equations of motion. We found a near perfect agreement with the experiment. The evidence of shock wave formation is a hallmark of nonlinear physics in a universal quantum hydrodynamic system. The hydrodynamic approach works well deep in the nonlinear regime even at low density and for a system far from equilibrium. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P45.00009: Finite Size Effects in the BCS-BEC Crossover from Functional Renormalization Michael Scherer We apply the functional renormalization group approach to the BCS-BEC crossover for an ultracold gas of fermionic atoms. Formulated in terms of a scale-dependent effective action, the functional RG interpolates continuously between the atomic or molecular microphysics and the macroscopic physics on large length scales. A systematic derivative expansion provides for both a description of the many-body physics and its expected universal features as well as an accurate account of the few-body physics and the associated BEC and BCS limits. Furthermore we put the system into a finite volume and employ periodic boundary conditions to study the effect of a finite size on the few-body scattering physics as well as the critical temperature for the phase transition to superfluidity. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P45.00010: Effects of particle-hole channel on the behavior of BCS-BEC crossover Qijin Chen BCS-BEC crossover is effected by increasing pairing strength between fermions from weak to strong. Such pairing is associated primarily with the particle-particle channel. Effects of the particle-hole channel is often dropped. On the other hand, Gor'kov et al argued that the particle-hole channel can cause a substantial reduction in both Tc and the pairing gap. However, this result has largely been neglected until recent years when BCS-BEC crossover has been realized experimentally in ultracold Fermi gases. In this talk, we study the effects of the particle-hole channel on BCS-BEC crossover in a $G_0G$ scheme. While in the BCS limit, such effects may be approximated by a shift in the pairing strength, the situation becomes more complex as the interaction becomes stronger where the gap is no longer very small. References: Q.J. Chen, I. Kosztin, B. Janko, and K. Levin, Phys. Rev. Lett. 81, 4708 (1998); Q.J. Chen, J. Stajic, S.N. Tan, and K. Levin, Physics Reports 412, 1-88 (2005). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P45.00011: Microscopic Approach to Viscosities in Superfluid Fermi Gases: From BCS to BEC Peter Scherpelz, Hao Guo, Dan Wulin, Chih-Chun Chien, Kathryn Levin We compute the shear viscosity, $\eta$, in a BCS-BEC crossover scheme which is demonstrably consistent, via sum rules, with conservation laws. The onset of a normal state pairing gap and the contribution from bosonic (non-condensed pair) degrees of freedom lead to a considerable reduction in the magnitude of these viscosities at general temperatures $T$. When quantitatively compared with shear viscosity experiments (we independently infer an estimated lifetime from radio frequency data) the agreement is reasonable, as is a comparison of $\eta/s$, where $s$ is the trap entropy density. Our fermionic picture is to be contrasted with that of others in the literature which presume that Goldstone bosons are crucial. As in conventional BCS superconductors, we show these Goldstone bosons do not couple to transverse probes such as the shear viscosity. As a result our calculated viscosity at low $T$ becomes arbitrarily small, rather than exhibiting the upturn predicted by others. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P45.00012: Triply degenerate quantum mixture of $^{41}$K, $^{40}$K and $^{6}$Li Peyman Ahmadi, Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park, Martin Zwierlein We report the observation of a triply quantum degenerate mixture of $^{41}$K, $^{40}$K and $^{6}$Li atoms. It is demonstrated that bosonic $^{41}$K atom is an efficient coolant for sympathetic cooling of fermionic $^{40}$K and $^{6}$Li atoms. The $^{40}$K and $^{6}$Li mixture provides access to a strongly correlated Fermi-Fermi mixture allowing us to study superfluidity and Cooper pairing with imbalanced masses. We also present our investigation of $^{41}$K and $^{40}$K, a Bose-Fermi mixture where a 12 G p-wave resonance and a 40 G s-wave resonance are observed. Negligible differential gravitational sag between $^{41}$K and $^{40}$K makes these resonances excellent candidates for studying unexplored properties of Bose-Fermi mixtures such as Boson mediated Cooper pairing. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P45.00013: The superfluid-insulator transition in disordered Fermi gases near unitarity Sarang Gopalakrishnan Superfluids, whether composed of weakly interacting fermions (i.e., in the BCS limit) or bosons (i.e., in the BEC limit), undergo quantum phase transitions into an insulating phase in the presence of strong disorder. In the BCS limit, such a transition occurs when the disorder is strong enough to overcome the fermions' kinetic energy; in the BEC limit, it occurs when the disorder is strong enough to overcome the bosons' interaction energy. We address the fate of the disorder-driven superfluid-insulator transition in the intermediate ``unitary'' regime, discuss the conditions under which the superfluid-insulator phase boundary is non-monotonic in this regime, and investigate the properties of the insulating phase. Our analysis is quantitatively valid at high densities or for narrow Feshbach resonances, but its qualitative implications are expected to hold beyond these regimes; it can also be adapted to show that the superfluid-insulator transition occurs at infinitesimally weak disorder for a unitary Fermi gas in four dimensions. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P45.00014: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P45.00015: Population imbalance and pairing in the BCS-BEC crossover of three-component ultracold fermions Tomoki Ozawa, Gordon Baym We investigate the phase diagram and the BCS-BEC crossover of a homogeneous three-component ultracold Fermi gas with a U(3) invariant attractive interaction. We show that the system at sufficiently low temperatures exhibits population imbalance, as well as fermionic pairing. We describe the crossover in this system, connecting the weakly interacting BCS regime of the partially population-imbalanced fermion pairing state and the BEC limit with three weakly interacting species of molecules, including pairing fluctuations within a t-matrix calculation of the particle self-energies. [Preview Abstract] |
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