Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J1: Ballistic-Diffusive Crossover in Graphene Electron Transport
Sponsoring Units: DCMPChair: Sankar Das Sarma, University of Maryland
Room: Spirit of Pittsburgh Ballroom A
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J1.00001: Electrical transport in suspended graphene Invited Speaker: Many quantum limit transport phenomena in graphene remain yet to be observed due to the omnipresence of carrier scattering. We report a sample preparation method that yields high quality graphene specimens and demonstrates that much of the scattering in traditional graphene-on-silica devices is not intrinsic but rather results from the interaction with the substrate underlying the graphene. We fabricate devices where electrically contacted and electrostatically gated graphene flakes are suspended over a substrate and use current-induced heating to remove the remaining impurities. The measured mobilities are found to exceed 200,000 cm$^2$/Vs in such devices, an order of magnitude improvement over the best values reported in the literature. The very high mobility of our specimens allows us to probe previously inaccessible transport regimes in graphene. At low temperatures transport is near-ballistic in a device of $\sim$2$\mu$m dimension. At large carrier density, we observe linear increase of resistivity with temperature, consistent with scattering off acoustic phonons. At near-room temperature we observe the mobility is $\sim$120,000 cm$^2$/Vs, higher than in any known semiconductor. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J1.00002: A self-consistent theory for graphene transport. Invited Speaker: Arguably, one of the most intriguing properties of graphene transport is the non-vanishing ``minimum conductivity'' at the Dirac point. The carrier density in these single monatomic sheets of carbon can be continuously tuned from electron-like carriers for large positive gate bias to hole-like carriers for negative bias. The physics close to zero carrier density (also called the intrinsic or Dirac region), is now understood to be dominated by the inhomogeneous situation where the local potential fluctuates around zero, breaking the landscape into puddles of electrons and holes. Here, we propose and discuss a particular hierarchy of approximations to understand graphene transport properties that includes a tight binding approximation for the low energy effective Hamiltonian, Random-Phase-Approximation to treat electron-electron interactions, the semi-classical Boltzmann transport theory to treat scattering of electrons by short and long-ranged disorder, and a self-consistent Fermi-Thomas approximation to treat impurity induced density inhomogeneity [1-2]. We find that this self-consistent theory for graphene transport is in remarkable agreement with recent experiments [3-5]. To better understand the range of validity of this theory we relax some of the assumptions and include the effects percolation [6]; calculate transport properties using an effective medium theory [7]; and examine the effects of phase-coherent quantum transport [8]. We believe that while most of the dc transport experiments on bulk graphene samples at zero magnetic field are in the parameter regime correctly captured by the semi-classical diffusive self-consistent transport theory, we demonstrate theoretically that by tuning external parameters, it is possible to access several other transport regimes.\\[4pt] References:\\[0pt] [1] Adam, Hwang, Galitski and Das Sarma, Proc. Nat. Acad. Sci. USA \textbf{104}, 18392 (2007); \\[0pt] [2] Hwang, Adam, and Das Sarma, PRL \textbf{98}, 186806 (2007); \\[0pt] [3] Tan et al. PRL\textbf{ 99}, 246803 (2007); \\[0pt] [4] Chen et al. Nature Physics \textbf{4}, 377 (2008); \\[0pt] [5] Jang et al. PRL \textbf{101},146805 (2008); \\[0pt] [6] Adam et al. PRL \textbf{101}, 046404 (2008); \\[0pt] [7] Rossi, Adam, and Das Sarma, arXiv:0809.1425v1 (2008); \\[0pt] [8] Adam, Brouwer, and Das Sarma, arXiv:0811.0609v1 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J1.00003: Theory of an inhomogeneous electron structure of graphene at its neutrality point Invited Speaker: Graphene is a surprisingly good conductor. Despite its direct exposure to various sources of disorder (charged impurities, non-uniformity of the substrate, etc.), graphene remains conductive even when the nominal concentration of both electron and hole carriers drops to zero - the neutrality point (NP). Theory of the minimal conductivity of graphene is an outstanding challenge because of the non-perturbative nature of disorder at the NP and the still unsettled question of which type of disorder is really dominant. Here, we report on our progress towards analytical solution of the model of graphene subject to the disorder in the form of in-plane charged impurities. Our approach is asymptotically exact for graphene in high dielectric-constant environment where Coulomb interactions of electrons with impurities and electrons with each other become weak. We show that screening of the impurity potential is nonlinear, producing a fractal structure of electron and hole puddles. Statistical properties of this density distribution as well as the charge compressibility of the system are calculated in the leading-log approximation. The minimal conductivity is shown to depend logarithmically on the dielectric constant. We compare our results with other theoretical works and current experiments. Our findings suggest that in real samples charged impurities are either not exactly coplanar with graphene, or are correlated, or are not the only source of disorder. This work is supported by the NSF. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J1.00004: Electronic Transport in Disordered Graphene Sheets and Nanoribbons Invited Speaker: In this talk I will present recent results of our numerical simulations of electronic transport in disordered graphene. Issues related to the scaling of the conductivity and the shot-noise Fano factor of large graphene sheets at zero and finite doping will be discussed. Our calculations are based on an efficient implementation of the recursive Green function method. I will also show how edge and bulk disorder may affect the mesoscopic conductance of graphene nanoribbons under a variety of realistic situations. We find that even for weak edge roughness, conductance steps are suppressed and a transport gap develops near the neutrality point due to strong localization. The gap inferred from our simulations is similar in magnitude to the energy gaps induced by other mechanisms, such as Coulomb blockade, many-body correlations, and lattice distortions. The effects of dephasing will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J1.00005: Electron fractionalization in two-dimensional graphenelike structures Invited Speaker: Electron fractionalization is intimately related to topology. In one-dimensional systems, such as polyacetelene, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. Here we show that fractionally charged topological excitations exist in tight-biding systems where time-reversal symmetry is respected. These systems are described, in the continuum approximation, by the Dirac equation in two space dimensions. The topological zero-modes are mathematically similar to fractional vortices in p-wave superconductors. They correspond to a twist in the phase in the mass of the Dirac fermions, akin to cosmic strings in particle physics. The quasiparticle excitations can carry irrational charge and irrational exchange statistics. These excitations can be deconfined at zero temperature, but when they are, the charge re-rationalizes to the value 1/2. REFS.:Chang-Yu Hou, Claudio Chamon, Christopher Mudry, Phys. Rev. Lett. 98, 186809 (2007); Claudio Chamon, Chang-Yu Hou, Roman Jackiw, Christopher Mudry, So-Young Pi, Andreas P. Schnyder, Phys. Rev. Lett, 100, 110405 (2008); Claudio Chamon, Chang-Yu Hou, Roman Jackiw, Christopher Mudry, So-Young Pi, Gordon Semenoff, Phys. Rev. B 77, 235431 (2008) [Preview Abstract] |
Session J2: Dynamical Layer Decoupling and Stripe Order High Tc Superconductors
Sponsoring Units: DCMPChair: Eduardo Fradkin, University of Illinois at Urbana-Champaign
Room: Spirit of Pittsburgh Ballroom BC
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J2.00001: Towards two-dimensional superconductivity in La$_{2-x}$Sr$_{x}$CuO$_{4}$ in a moderate magnetic field Invited Speaker: We report a novel aspect of the competition and coexistence between antiferromagnetism and superconductivity in the prototypical high-Tc cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214). With a modest magnetic field applied $H \parallel c$-axis, we monitored the infrared signature of pair tunneling between the CuO$_{2}$ planes and discovered the complete suppression of interlayer coupling in a series of La214 single crystals. We find that the in-plane superconducting properties remain largely intact, in spite of increased antiferromagnetism in the planes. Thus, our experiments show that an isolated CuO$_{2}$ plane is capable of maintaining high-Tc superconductivity. The theoretical framework for antiferromagnetic-driven interlayer decoupling is identified in the work of Berg, \emph{et. al.} [E. Berg, \emph{et. al.}, Phys. Rev. Lett. \textbf{99}, 127003 (2007) and Cond-mat arXiv:0810.1564]. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J2.00002: Theory of a striped superconductor Invited Speaker: In the spin and charge striped-ordered cuprate La$_{1.875}$Ba$_ {0.125}$CuO$_4$, the bulk superconducting T$_c$ is strongly suppressed to about 4K. Nevertheless, recent experiments revealed unusual superconducting fluctuations far above T$_c$. In particular, these experiments show evidence for 2D superconductivity in this 3D (albeit anisotropic) system, which is unpreceded. We propose an explanation of these results in terms of a new phase, the ``striped superconductor", in which the superconducting order parameter is modulated periodically in space and averages to zero. Such a state can naturally explain the apparent vanishing of the inter-layer Josephson coupling in La$_{1.875}$Ba$_{0.125}$CuO$_4$. We discuss several phenomenological features of this phase, including a unique sensitivity to disorder which can lead to an xy ``gauge glass" phase that breaks time reversal symmetry spontaneously. Possible microscopic mechanisms leading to striped superconductivity will be briefly discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J2.00003: Electronic liquid-crystal phases, symmetry breaking and Fermi-surface reconstruction in YBa$_2$Cu$_3$O$_{6+x}$ Invited Speaker: The physics of underdoped cuprates is governed by strong correlations and phase competition, and its understanding remains one of the challenges of condensed-matter research. Here we will discuss our systematic doping- and temperature-dependent neutron-scattering investigation of the spin correlations in underdoped YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) in the context of various proposed symmetry-breaking phases. In YBCO6.45 ($T_c=35$~K), we observe the spontaneous onset of a one-dimensional, incommensurate (IC) modulation of the low-energy ($<10$~meV) spin excitations below $T\sim150$~K, Hinkov \emph{et al., Science} \textbf{319}, 597 (2008). We interpret our finding as the occurrence of an electronic liquid-crystal phase breaking $C4$-symmetry, as predicted by Kivelson \emph{et al., Nature} \textbf{393}, 550 (1998). Below $T\sim30$~K, we observe the onset of quasi-static spin-correlations with the same IC geometry as the low-energy spin excitations. Their intensity is doubled by the application of a magnetic field of 15~T. First of all, this resolves a long-standing discrepancy with the La$_{2-x}$(Sr,Ba)$_x$CuO$_4$ family where field-dependent, IC quasi-static spin correlations were reported before (Lake \emph{et al., Nature} \textbf{415}, 299 (2002)). More importantly, our results show how the recently reported quantum oscillations in high magnetic fields in YBa$_2$Cu$_3$O$_{6.5}$ (Doiron-Leyraud \emph{et al., Nature} \textbf{447}, 565 (2007)) can be understood in terms of a Fermi-surface reconstruction induced by IC spin modulations. In the lack of experimental evidence for such IC modulations, this mechanism has not been pursued in the past, although it was discussed as a straightforward explanation for Fermi-surface reconstruction. Finally, we will discuss YBCO with a $T_c$ of 10~K. Compared to the previous example, there is an enhancement of the quasi-static spin-correlations. In addition, there is indication for incipient commensurate AF order in reminiscence of the AF parent compound. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J2.00004: Stripe order, electron pockets, and Fermi arcs Invited Speaker: The recent observation of an electron pocket by quantum oscillation experiments is naturally explained by the presence of magnetic stripe order near 1/8 hole doping in cuprates [1]. A bigger question is how these observations are related to other phenomena in the phase diagram - the pseudogap phase, quantum critical points, and Fermi arcs [2], and the implications this might have for the origin of high temperature superconductivity.\\[4pt] [1] A. J. Millis and M. R. Norman, Antiphase stripe order as the origin of electron pockets observed in 1/8-hole-doped cuprates, Phys. Rev. B 76, 220503 (2007).\\[0pt] [2] M. R. Norman, A. Kanigel, M. Randeria, U. Chatterjee and J. C. Campuzano, Modeling the Fermi arc in underdoped cuprates, Phys. Rev. B 76, 174501 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J2.00005: Magnetic Field and Temperature Dependence of Charge Stripe Order in La2-xBaxCuO4 (x$\sim $1/8). Invited Speaker: We report a comprehensive x-ray scattering study of charge stripe ordering in La2-xBaxCuO4 (x $\sim $ 1/8), for which the bulk superconducting Tc is greatly suppressed. We found that the charge order in this sample is described with one-dimensional charge density waves, which have incommensurate wave-vectors (0.23, 0, 0.5) and (0, 0.23, 0.5) respectively on neighboring CuO2 planes. The structural modulation due to the charge stripe order is simply sinusoidal, and no higher harmonics were observed. Just below the structural transition temperature, short-range charge density wave correlation appears, which then develops into a large scale charge ordering around 40 K, close to the spin density wave ordering temperature. However, this charge ordering fails to grow into a true long range order, and its correlation length saturates at $\sim $23 nm, and slightly decreases below about 15 K. In addition, we report our observation of the unusual magnetic field dependence of the charge order correlation length. Specifically, in the superconducting phase the charge order correlation length increases as the magnetic field greater than $\sim $5 T is applied. [Preview Abstract] |
Session J3: SQUID Amplifiers: at the Quantum Limit
Sponsoring Units: DCMPChair: John Clarke, University of California, Berkeley
Room: 301/302
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J3.00001: Quantum information processing with the Josephson ring modulator Invited Speaker: |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J3.00002: Nearly noiseless amplification of microwave signals with a Josephson parametric amplifier Invited Speaker: A degenerate parametric amplifier transforms an incident coherent state by amplifying one of its quadrature components while deamplifying the other. This transformation, when performed by an ideal parametric amplifier, is completely deterministic and reversible; therefore the amplifier in principle can be noiseless. We attempt to realize a noiseless amplifier of this type at microwave frequencies with a Josephson parametric amplifier (JPA). To this end, we have built a superconducting microwave cavity containing many dc-SQUIDs. This arrangement creates a non-linear medium in a cavity and it is closely analogous to an optical parametric amplifier. In my talk, I will describe the current performance of this circuit, where I show I can amplify signals with less added noise than a quantum-limited amplifier that amplifies both quadratures. In addition, the JPA also squeezes the electromagnetic vacuum fluctuations by 10 dB. Finally, I will discuss our effort to put two such amplifiers in series in order to undo the first stage of squeezing with a second stage of amplification, demonstrating that the amplification process is truly reversible.\\[4pt] M. A. Castellanos-Beltran, K. D. Irwin, G. C. Hilton, L. R. Vale and K. W. Lehnert, Nature Physics, published on line, http://dx.doi.org/10.1038/nphys1090 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J3.00003: Flux-driven Josephson parametric amplifier Invited Speaker: Degenerate parametric amplifiers are phase sensitive amplifiers, which can in principle amplify one of the two quadratures of a signal without introducing extra noise. Parametric amplifiers based on the nonlinear inductance of a Josephson junction have been studied for a long time. Recently, there has been a renewed interest in parametric amplifiers due in part to the increasing need for quantum- limited amplification in the field of quantum information processing using superconducting circuits. In the present work, we design a novel Josephson parametric amplifier, comprising a superconducting transmission-line resonator terminated by a dc SQUID. Contrary to the previous works, the pump is not used to directly modulate a current through the Josephson junction, but is instead used to modulate a flux through the dc SQUID. Because the dc SQUID determines the boundary condition of the resonator, the flux modulation gives the temporal variation of the resonant frequency, which leads to the parametric amplification of the signal coming into the resonator. The practical advantage of the scheme is, first, that the band center of the signal is widely controllable by a dc flux also applied to the SQUID. Second, as the pump and the signal are applied to different ports and their frequencies are twice different, it is straightforward to separate the output signal from the pump. We have operated such a flux-driven Josephson parametric amplifier at around 10 GHz and characterized its basic properties. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J3.00004: Optimizing the Gain and Noise Temperature of Microstrip SQUID Amplifiers Invited Speaker: Micrrostrip SQUID amplifiers (MSA) offer near quantum-limited sensitivity and gains greater than 20 dB at frequencies around 1 GHz. These properties make them desirable for applications ranging from dark-matter axion detection to dispersive readout of superconducting qubits. The input of the MSA is a microstrip transmission line in the shape of a square spiral coil surrounding the hole in the SQUID washer that serves as the ground plane. Near the fundamental resonance, there is strong flux coupling between the input coil and SQUID. To obtain maximum performance it is necessary to know the complete set of complex scattering parameters. We present measurements of the scattering parameters of MSAs cooled to 4.2 K. The input impedance is found by measuring the reverse scattering parameter (S11) and is described well by a low-loss transmission line model. We map the low-loss transmission line model into an equivalent parallel RLC circuit that accurately predicts the observed gain given by the forward scattering parameter (S21). This information is used to optimize the input and output matching circuitry to achieve optimal noise temperature and gain. We will present results for the gain, dynamic range, linearity and noise temperature of these optimized MSAs at 30-500 mK as a function of frequency and SQUID bias. We will compare the results to the prediction of the circuit model and to the theoretical expectation that the lowest noise temperature occurs off-resonance. This work is in collaboration with John Clarke and a portion of this work was supported by DOE. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J3.00005: Lumped-Element DC-SQUID Microwave Amplifier Invited Speaker: We report on the development at NIST of microwave amplifiers in the 6-8 GHz frequency range using DC SQUIDs. Our design approach is to use small SQUIDs which can be modeled as lumped element circuits, thus separating the design process for the SQUID from that of the microwave impedance transformers. We present our model and measurements of the impedance, gain and noise of these SQUID amplifiers. Furthermore, we discuss how our modular hardware design allows for easy deployment in labs around the world where there is a need for lower noise microwave measurement. [Preview Abstract] |
Session J4: Panel Discussion: Around the World in 180 Minutes: Differences and Similarities among Women Physicists
Sponsoring Units: CSWP FIPChair: Eliane Lessner, CSWP, and Cherrill Spencer, FIP
Room: 306/307
Tuesday, March 17, 2009 11:15AM - 11:20AM |
J4.00001: Session Introduction Invited Speaker: A panel discussion session providing a worldwide assessment of the status and experiences of women in physics, paying attention to the different cultures and environments they work in and to how the age of the physicist affects their perspective. We will hear about women physicists in Korea in particular and Asia in general, in Egypt in particular and Africa in general, and in the Caribbean. Six invited speakers will present analyses of the progress being made in promoting women in physics from their personal experiences and as assessed from their participation in the Third International Conference on Women In Physics (ICWIP2008) convened in Seoul, Korea in October 2008. From Albania to Zimbabwe, with representation of all the continents, ICWIP2008 congregated 283 women and men physicists from 57 countries to share the participants' scientific accomplishments and evaluate international progress in improving the status of women in physics. This three-hour session is organized jointly by the Committee on the Status of Women in Physics of the APS (CSWP) and the Forum on International Physics of the APS (FIP). Audience participation in the panel discussion will be strongly encouraged. [Preview Abstract] |
Tuesday, March 17, 2009 11:20AM - 11:45AM |
J4.00002: Looking back, Looking Forward: An International Perspective Invited Speaker: The recent IUPAP International Conference on Woman in Physics (ICWIP) held in Korea brought women physicists together from all over the world. It was a wonderful conference and the third in a series that began in 2002. How did these conferences originate? What was their driving force? What has changed in the intervening 6 years? I will give some of the history and my experiences in the role of secretary general and the first female officer of IUPAP. I will also share some thoughts about the future, and what must be done to make sure that the situation for women in physics continues to improve around the world. [Preview Abstract] |
Tuesday, March 17, 2009 11:45AM - 12:10PM |
J4.00003: The 3$^{rd}$ International Conference on Women in Physics: Global Perspectives, Common Concerns, Worldwide Views Invited Speaker: The 3$^{rd}$ International Conference on Women in Physics (ICWIP), held in Seoul, Korea, in October 2008, brought together 300 participants from 57 countries, including a diverse 22-member U.S. Delegation, for a 3-day summit of stimulating discussions, thought-provoking presentations, inspirational posters, and networking. Held under the auspices of the Working Group on Women in Physics of the International Union of Pure and Applied Physics (IUPAP), this meeting built on the successes of the 1$^{st}$ (Paris, 2002) and 2$^{nd}$ (Rio de Janeiro, 2005) Conferences and further clarified the importance of diversifying the field of physics worldwide. Although considerable progress has been made since 2002, it was clear that the global scientific workforce is still under-utilizing a large percentage of the available female talent pool. If human society is to benefit to its fullest from various contributions that the field of physics can offer in addressing global issues of economic crisis, energy, environment, water, health, poverty, and hunger, women of all races and nationalities need to become fully included and engaged in the national and international physical community. To address these and many other issues, the ICWIP unanimously approved a five-part resolution to IUPAP recommending actions to promote the recruitment, retention, and advancement of women in physics and related fields. [Preview Abstract] |
Tuesday, March 17, 2009 12:10PM - 12:35PM |
J4.00004: Marshak Lectureship Talk: Women in Physics in Egypt and the Arab World Invited Speaker: Until the end of the 19th Century Science was not classified into different disciplines. The first woman named in the history of science was Merit Ptah (2700 BC) in Egypt's Valley of the Kings. In the new Egypt the first girl's school started in Cairo in 1873 and the first University in 1908. Only a few girls attended the University at that time, mainly studying the humanities. The first Egyptian woman physicist graduated in 1940 and received her PhD in nuclear physics in the USA. Nowadays the number of women in physics is increasing in all branches of physics, some of them are senior managers and others have been decorated with various prizes. In this talk some statistics will be given to show the percentage of women in physics in relation to other fields of science in Egypt. In Saudi Arabia the first girls' school started in 1964 and the first college for women, which was a section of King Abdul-Aziz University (where education is not mixed), started in 1975. I was the founder of the Physics Department of this women's section. Egyptians have played significant roles in teaching schoolchildren and university students of both sexes in all the Arab countries: Saudi Arabia, Sudan, Kuwait, Yemen, the Gulf States, Libya, Lebanon, Syria, and Jordan. But with respect to Algeria, Tunisia and Morocco, our role was limited, since classes are taught in French. Arab women living in the countries located east of Egypt still have many difficulties facing them, needing to overcome many technical, academic, and social problems, while women in the countries located west of Egypt have fewer problems. There were many problems in the early days of education in Egypt but the women of Egypt worked hard to gain the same rights as men and were able to pave the way for all Arab women. I myself met many difficulties in my early days. This talk will also describe the impact of the regional conference on Women in Physics in Africa and Middle East, which was held in Cairo in 2007. [Preview Abstract] |
Tuesday, March 17, 2009 12:35PM - 1:11PM |
J4.00005: Women Physicists in Asia Invited Speaker: I will present the history of women scientists in Asian countries and discuss the current status and experiences from women physicists, and ways to create a better future for women in physics. [Preview Abstract] |
Tuesday, March 17, 2009 1:11PM - 1:36PM |
J4.00006: Women in Physics: A Caribbean Perspective Invited Speaker: This paper is concerned with aspects of post-secondary education of women in physics in the Caribbean, focusing more specifically on the main university campuses in Trinidad and Tobago, Jamaica, and Barbados. Within this framework, there are three institutions of tertiary education that provide for undergraduate and post-graduate studies in physics. On average, the bachelor-level graduating class is roughly 40\% female. A great majority of these students go on to seek master's degrees in engineering. Among those enrolled in graduate programs featuring research in astronomy, materials science, environmental physics, medical physics, and quantum physics, 58\% are female. Significant numbers of women from the selected countries and from the Caribbean region are engaged in bachelor and doctoral programs in physics abroad, but no formal survey is available to provide the relevant quantitative information. However, an attempt will be made to quantify this component. Based in part on personal experience, a comparison will be made between domestic and foreign educational pathways, in terms of access to resources, level of research training, and occupational opportunities following graduation. [Preview Abstract] |
Tuesday, March 17, 2009 1:36PM - 2:26PM |
J4.00007: Panel Discussion Invited Speaker: \\ Panelists:\\ Yevgeniya Zastavker, Franklin W. Olin College of Engineering\\ Young-Kee Kim, Fermi National Accelerator Facility and University of Chicago\\ Karimat El-Sayed, AniSharms University, Cairo, Egypt\\ Renee Horton, University of Alabama\\ Kandice Tanner, University of California, Berkeley\\ [Preview Abstract] |
Session J5: Polymer Nanoparticle Interactions
Sponsoring Units: DPOLYChair: Thomas Russell, University of Massachusetts
Room: 401/402
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J5.00001: Simulations of Polymer Grafted Nanoparticles in a Polymer Matrix Invited Speaker: We have performed molecular dynamics (MD) simulations of polymer-grafted nanoparticles in a polymer melt. The model is a coarse-grained representation of spherical nanoparticles with a grafted poly(methyl methacrylate)-like bead-spring polymer in a matrix of the same polymer. Simulations were performed on both a single polymer-grafted nanoparticle as well as for a pair of polymer-grafted nanoparticles. The nanoparticle has a diameter of 5 nm. We have investigated the role of the molecular weight of the grafted and matrix polymer on both brush structure and nanoparticle-nanoparticle interactions. We find that brush density profile is independent of matrix molecular weight. Furthermore, the matrix chains penetrate almost to the particle surface, and there is no extended region with zero or near-zero matrix chain density. Hence, the highly curved brush does not exhibit ``dry brush'' behavior that would be expected at this investigated grafting density. We observe a repulsive interaction between the nanoparticles that sets in at a separation consistent with the polymer brush height. The combined brush-brush plus matrix effect on the nanoparticle-nanoparticle interactions is repulsive at all separations. Our simulations profile reveals no matrix-induced attraction between nanoparticles that is anticipated when the brush are truly ``dry'', i.e., largely non-penetrable by the matrix. Such behavior would be expected for larger particles where the surface curvature effects on brush structure and brush-melt interactions are less important. However, for small nanoparticles, our simulations reveal that surface curvature effects are very important in determining the structure of the grafted polymer as well as nanoparticle-nanoparticle interactions. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J5.00002: Interaction, Structure and Transport of Polymer Grafted Nanoparticles Invited Speaker: Grafting polymers to nanoparticles has proven to be an effective means to disperse isotropic and anisotropic nanoparticles in polymer matrices. Depending on the grafting density, polydispersity and nature of polymer - nanoparticle interaction, such grafted nanoparticles can either be liquid-like, gel-like or crystalline solids. We examine here the nature of interactions between such grafted nanoparticles and correlate those to the structure, dynamics and transport in both solvent and polymer media. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J5.00003: Modification of Block Copolymers Using Surface-Functionalized Hard and Soft Nanoparticles Invited Speaker: Due to their wide range of available nanostructures, ordered block copolymers provide excellent templating media into which nanoparticles can be incorporated with precise spatial modulation for various nanotechnologies. Previous experimental and theoretical studies have demonstrated that the relative size and selectivity of surface-functionalized inorganic nanoparticles can be used to tune the position of the nanoparticles along interfacial regions or within microdomains. Using a combination of experimental and theoretical methods, we furthermore show that these parameters, in addition to nanoparticle concentration, can be used to controllably alter the phase stability of block copolymers. While nanoparticles typically reduce the order-disorder transition (ODT) temperature of ordered block copolymers, a limited window exists wherein the nanoparticles increase the ODT temperature and stabilize the copolymer nanostructure. This nanoparticle-mediated design is extended in this study to include ``soft'' nanoparticles composed of core-shell microgels (CSMG) particles, which can be envisaged as permanent micelles. Addition of CSMG particles to ultrathin films consisting of ordered block copolymers varying in morphology and molecular weight is investigated here by electron microscopy. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J5.00004: Polymer single crystal meets nanoparticle, toward ordered hybrid materials Invited Speaker: Judiciously selected polymer single crystal (PSC) systems can interplay with 1-D and 0-D nanoparticles, forming ordered hybrid structures. In this presentation, I will first focus on patterning PSCs on individual carbon nanotubes (CNT). Using both controlled solution crystallization, thin film crystallization and physical vapor deposition methods, CNTs were periodically decorated with PSCs, resulting in nano hybrid shish-kebab (NHSK) structures. Because the polymer kebabs can be easily removed, these unique NHSKs can serve as templates to fabricate a variety of CNTs-containing hybrid materials with controlled pattering on the CNT surface. Sub-20 nanometer alternating patterning was achieved by using crystalline block copolymers. The mechanism was attributed to the crystallization induced block copolymer phase separation. This pattern was successfully used to template nanoparticles (NP) pattering on CNTs. In the second part of the talk, I will discuss fabricating Janus NPs and patterning these NPs using PSCs. Single crystals of thiol-terminated polyethylene oxide (PEO) were incubated in a gold sol. Au-S bonds were formed between the AuNPs and the PEO single crystal surfaces. The inter-particle spacing was controlled by PEO molecular weights, the incubation time, and the annealing temperatures after incubation. The planar geometry of the PSCs led to Janus NP formation. A series of NP dimers, trimers and tetramers were synthesized. NP nanowires were also fabricated. We anticipate that this observation could lead to controlled synthesis of artificial molecules and NP chains for a variety of optical, electronic, and biomedical applications. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J5.00005: DNA Directed Nanoparticle Assemblies Invited Speaker: While DNA is mostly noted for carrying genetic information, a single strand of DNA is simply a polymer with chemically specific recognition. As a result, DNA is an interesting polymer to consider for the development of new materials. In particular, attaching single strands of DNA to nanoparticles offers the possibility to encode highly specific bonding between nanoparticles to create engineered building blocks, or ``functionalized atoms.'' These core units are an ideal candidate for the development of network-based, nanostructured materials. In this talk, we present results from computer simulations of a coarse-grained model examining several choices and DNA functionalization, and show how these design choices can affect dynamics, phase behavior, and the formation of crystal structures. We first discuss nanoparticles functionalized by four single DNA strands. These units give rise to a material with a hierarchy of interpenetrating networked structure and four thermodynamically distinct amorphous phases, unlike any naturally occurring pure material. On the other hand, the mechanism for the formation of the amorphous phases offers insight into anomalous networked liquids like water and silica. We also consider how varying the number of functionalizing DNA strands alters both the number and shape of these phase transitions. The formation of very low density crystals of nanoparticles tethered by DNA has recently been achieved experimentally, but the factors controlling crystal formation are still not well understood. Therefore, we also discuss the results of nanoparticles uniformly coated with DNA, similar to experimental systems. We show how the DNA strand length and stiffness affects the competition between energy and entropy that controls crystal formation. [Preview Abstract] |
Session J6: Computational Modeling of Crystallization and Nucleation Phenomena
Sponsoring Units: DCOMPChair: Jim Belak, Lawrence Livermore National Laboratory
Room: 406
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J6.00001: Coupled Nucleation Processes in Metallic Liquids and Glasses Invited Speaker: Nucleation processes in condensed systems are often more complicated than expected from classical theory considerations. For example, our recent studies of glasses and deeply supercooled liquids demonstrate that the short- and medium-range order play an important role in the nucleation pathway. High-energy X-ray diffraction data from electrostatically levitated transition metal and alloy liquids demonstrate the frequent development of icosahedral short-range order (ISRO) with supercooling. This ordering has significant consequences for crystallization and vitrification of the liquids. It makes it difficult to nucleate ordered crystal phases, confirming a half-century old hypothesis by Frank. Measurements of the density and surface tension in several supercooled liquids suggest that it may be associated with a liquid/liquid phase transition. Quantitative measurements of the time-dependent nucleation rate in a Zr$_{59}$Ti$_{3}$Cu$_{20}$Ni$_{8}$Al$_{10}$ metallic glass and associated structural studies of the supercooled liquid demonstrate that it increases through the glass transition, providing support for a frustration model of the glass transition. In a Ti-Zr-Ni liquid the ISRO lowers the barrier for a metastable icosahedral quasicrystal, blurring the distinction between homogenous and heterogeneous nucleation. Our studies and those of others suggest that the nucleation of the ordered phase can be coupled with liquid phase transitions, including high order transitions. Coupling between other processes is also common for nucleation. For example, the coupling between the stochastic fluxes of interfacial attachment and long-range diffusion in the nucleation step can be critical when the initial and final phases have different chemical compositions. The implications of coupled nucleation processes on phase formation, stability and nanoscale crystallization are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J6.00002: Out-of-equilibrium processes in suspensions of oppositely charged colloids: liquid-to-crystal nucleation and gel formation Invited Speaker: We study the kinetics of the liquid-to-crystal transformation and of gel formation in colloidal suspensions of oppositely charged particles. We analyse, by means of both computer simulations and experiments, the evolution of a fluid quenched to a state point of the phase diagram where the most stable state is either a homogeneous crystalline solid or a solid phase in contact with a dilute gas. On the one hand, at high temperatures and high packing fractions, close to an ordered-solid/disordered-solid coexistence line, we find that the fluid-to-crystal pathway does not follow the minimum free energy route. On the other hand, a quench to a state point far from the ordered-crystal/disordered-crystal coexistence border is followed by a fluid-to-solid transition through the minimum free energy pathway. At low temperatures and packing fractions we observe that the system undergoes a gas-liquid spinodal decomposition that, at some point, arrests giving rise to a gel-like structure. Both our simulations and experiments suggest that increasing the interaction range favors crystallization over vitrification in gel-like structures. \\[4pt] In collaboration with Chantal Valeriani, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands and SUPA, School of Physics, University of Edinburgh, JCMB King's Buildings, Mayfield Road, Edinburgh EH9 3JZ, UK; Teun Vissers, Andrea Fortini, Mirjam E. Leunissen, and Alfons van Blaaderen, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University; Daan Frenke, FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands and Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, Cambridge, UK; and Marjolein Dijkstra, Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J6.00003: Nucleation of Ice Invited Speaker: The freezing of water into ice is a ubiquitous transformation in nature, yet the microscopic mechanism of homogeneous nucleation of ice has not yet been elucidated. One of the reasons is that nucleation happens in time scales that are too fast for an experimental characterization and two slow for a systematic study with atomistic simulations. In this work we use coarse-grained molecular dynamics simulations with the monatomic model of water mW[1] to shed light into the mechanism of homogeneous nucleation of ice and its relationship to the thermodynamics of supercooled water. Cooling of bulk water produces either crystalline ice or low- density amorphous ice (LDA) depending on the quenching rate. We find that ice crystallization occurs faster at temperatures close to the liquid-liquid transition, defined as the point of maximum inflection of the density with respect to the temperature. At the liquid-liquid transition, the time scale of nucleation becomes comparable to the time scale of relaxation within the liquid phase, determining --effectively- the end of the metastable liquid state. Our results imply that no ultraviscous liquid water can exist at temperatures just above the much disputed glass transition of water. We discuss how the scenario is changed when water is in confinement, and the relationship of the mechanism of ice nucleation to that of other liquids that present the same phase behavior, silicon [2] and germanium [3]. \\[4pt] [1] Molinero, V. {\&} Moore, E. B. Water modeled as an intermediate element between carbon and silicon. Journal of Physical Chemistry B (2008). Online at http://pubs.acs.org/cgi- bin/abstract.cgi/jpcbfk/asap/abs/jp805227c.html \\[0pt] [2] Molinero, V., Sastry, S. {\&} Angell, C. A. Tuning of tetrahedrality in a silicon potential yields a series of monatomic (metal-like) glass formers of very high fragility. Physical Review Letters 97, 075701 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J6.00004: Surface Induced Crystallization In Tetrahedral Liquids Invited Speaker: Freezing is a fundamental physical phenomenon that has been studied over many decades; yet the role played by surfaces in determining nucleation has remained elusive. While common wisdom regards surfaces as unfavorable nucleation sites, both atmospheric data and laboratory measurements on droplets of water support the hypothesis of surface-induced crystallization in some systems. In this talk I will discuss our recent work on employing accelerated molecular dynamics simulations to investigate nucleation in the presence of free surfaces in tetrahedral liquids with a negative slope of their melting line ($dP/dT<0$). Through conducting extensive study on nucleation rates and nucleation pathways in a few systems, {\em e.g.}, Si and Ge, we provide direct computational evidence of surface induced crystallization in supercooled systems with $dP/dT<0$. We show that the possibility of observing preferential nucleation in close proximity of free surfaces is related to the density decrease occurring upon freezing, and surface tension facilitating the initial nucleus formation. Furthermore, in contrast to the common assumption that regards surfaces as heterogeneous center, we identify the {\em homogeneous} nature of surface induced nucleation. This is related to both the local static and dynamical properties of liquid surface. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J6.00005: Growth and optical properties of embedded silicon nanocrystals Invited Speaker: The optoelectronic properties of nanostructured silicon (nc-Si) are governed by the interplay between the local chemical bonding features and the complex overall atomic structure. Interesting enough, a-Si has a larger optical absorption than the c-Si and, therefore, biphasic a-c silicon systems (i.e. nanocrystallites embedded into an amorphous matrix) are currently under investigation for next-generation photovoltaics. Biphasic systems undergo crystallization upon thermal annealing and, therefore, it is quite difficult to predict theoretically their finite-temperature optoelectronic properties. In this talk I will present our ongoing research on the growth and the optoelectronic properties of textured nanocrystalline silicon, here modeled as a distribution of cylindrical grains embedded into an amorphous matrix. As for the growth, I argue that by large-scale atomistic simulations it is possible to infer a continuum model for the crystallinity evolution upon thermal annealing.[1] In particular, at low crystallinity, it is proved that--consistently with the standard Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory--the a-c phase transformation is dominated by the isolated grain evolution; conversely, at later stages deviations from the KJMA theory are observed, mainly due to atomic-scale features. I also prove that such effects can be included by using an improved phenomenological version of the KJMA theory.[2] As for the finite-temperature optoelectronic properties, I present a divide-and-conquer computational procedure, based on a combination of empirical tight-binding and model-potential molecular dynamics. This procedure is applied to investigate local and average optoelectronic properties of very large nanostructured silicon systems and to predict the variation of the optical absorption upon crystallinity.[3] I show that the optical absorption of a nc-Si sample corresponds to a simple linear combination between c-Si and a-Si phases and it is not affected by electron confinement within grains. Strain effects on combined absorption are discussed as well. \\[3pt] [1] A. Mattoni, L. Colombo, Phys. Rev. Lett. 99, 205501 (2007)\\[0pt] [2] A. Mattoni, L. Colombo, Phys. Rev. B 78, 075408 (2008)\\[0pt] [3] A. Mattoni, L. Colombo, submitted (2008) [Preview Abstract] |
Session J7: Complex Cellular Biological Networks
Sponsoring Units: GSNP DBPChair: Takashi Nishikawa, Clarkson University
Room: 407
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J7.00001: Mass-action equilibrium and non-specific interactions in protein binding networks Invited Speaker: Large-scale protein binding networks serve as a paradigm of complex properties of living cells. These networks are naturally weighted with edges characterized by binding strength and protein-nodes -- by their concentrations. However, the state-of-the-art high-throughput experimental techniques generate just a binary (yes or no) information about individual interactions. As a result, most of the previous research concentrated just on topology of these networks. In a series of recent publications [1-4] my collaborators and I went beyond purely topological studies and calculated the mass-action equilibrium of a genome-wide binding network using experimentally determined protein concentrations, localizations, and reliable binding interactions in baker's yeast. We then studied how this equilibrium responds to large perturbations [1-2] and noise [3] in concentrations of proteins. We demonstrated that the change in the equilibrium concentration of a protein exponentially decays (and sign-alternates) with its network distance away from the perturbed node. This explains why, despite a globally connected topology, individual functional modules in such networks are able to operate fairly independently. In a separate study [4] we quantified the interplay between specific and non-specific binding interactions under crowded conditions inside living cells. We show how the need to limit the waste of resources constrains the number of types and concentrations of proteins that are present at the same time and at the same place in yeast cells. \newline [1] S Maslov, I. Ispolatov, PNAS 104:13655 (2007). \newline [2] S. Maslov, K. Sneppen, I. Ispolatov, New J. of Phys. 9: 273 (2007). \newline [3] K-K. Yan, D. Walker, S. Maslov, PRL accepted (2008). \newline [4] J. Zhang, S. Maslov, and E. I. Shakhnovich, Mol Syst Biol 4, 210 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J7.00002: Getting from Genotypes to Phenotypes through Network Reconstruction and Modeling Invited Speaker: sGenome annotations provide a detailed description of the metabolic activities an organism can carry out. A metabolic network can be reconstructed from genomic data and serves as a framework to build computational metabolic models. These models can make phenotypic predictions about the behavior of an organism given different genetic or environmental perturbations. Comparisons between model predictions and experimental data can then be used to identify missing components and interactions in biochemical networks. These comparisons provide a mechanism to improve our understanding of biological networks and genomes and in turn lead to improved models. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J7.00003: Synthetic rescues and spontaneous cascades in metabolic networks Invited Speaker: Using {\it in silico} experiments, I will show that organisms evolving to maximize growth rate, ATP production, or any other linear function of metabolic fluxes tend to significantly reduce the number of active metabolic reactions compared to typical non-optimal states. The reduced number appears to be constant across the microbial species studied and just slightly larger than the minimum number required for the organisms to grow at all. I will show that this massive reaction silencing is triggered by the irreversibility of a large fraction of the metabolic reactions and propagates through the network as a cascade of inactivity. Following these observations, I will introduce a network method to recover the loss of metabolic function due to mutations and other defects, which is based on bypassing rather than correcting the defective pathways. In particular, I will present predictions of {\it synthetic recovery}, in which the knockout of one enzyme-coding gene results in a non-viable phenotype while the concurrent knockout of a second enzyme-coding gene restores viability. In addition to their potential role in metabolic engineering and medical research, these results have puzzling implications for the recently observed temporary activation of latent pathways. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J7.00004: Robustness of metabolic networks Invited Speaker: We investigated the robustness of cellular metabolism by simulating the system-level computational models, and also performed the corresponding experiments to validate our predictions. We address the cellular robustness from the ``metabolite''-framework by using the novel concept of ``flux-sum,'' which is the sum of all incoming or outgoing fluxes (they are the same under the pseudo-steady state assumption). By estimating the changes of the flux-sum under various genetic and environmental perturbations, we were able to clearly decipher the metabolic robustness; the flux-sum around an essential metabolite does not change much under various perturbations. We also identified the list of the metabolites essential to cell survival, and then ``acclimator'' metabolites that can control the cell growth were discovered. Furthermore, this concept of ``metabolite essentiality'' should be useful in developing new metabolic engineering strategies for improved production of various bioproducts and designing new drugs that can fight against multi-antibiotic resistant superbacteria by knocking-down the enzyme activities around an essential metabolite. Finally, we combined a regulatory network with the metabolic network to investigate its effect on dynamic properties of cellular metabolism. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J7.00005: Discrete dynamic modeling of T cell survival signaling networks Invited Speaker: Biochemistry-based frameworks are often not applicable for the modeling of heterogeneous regulatory systems that are sparsely documented in terms of quantitative information. As an alternative, qualitative models assuming a small set of discrete states are gaining acceptance. This talk will present a discrete dynamic model of the signaling network responsible for the survival and long-term competence of cytotoxic T cells in the blood cancer T-LGL leukemia. We integrated the signaling pathways involved in normal T cell activation and the known deregulations of survival signaling in leukemic T-LGL, and formulated the regulation of each network element as a Boolean (logic) rule. Our model suggests that the persistence of two signals is sufficient to reproduce all known deregulations in leukemic T-LGL. It also indicates the nodes whose inactivity is necessary and sufficient for the reversal of the T-LGL state. We have experimentally validated several model predictions, including: ($i)$ Inhibiting PDGF signaling induces apoptosis in leukemic T-LGL. (\textit{ii}) Sphingosine kinase 1 and NF$\kappa $B are essential for the long-term survival of T cells in T-LGL leukemia. (\textit{iii}) T box expressed in T cells (T-bet) is constitutively activated in the T-LGL state. The model has identified potential therapeutic targets for T-LGL leukemia and can be used for generating long-term competent CTL necessary for tumor and cancer vaccine development. The success of this model, and of other discrete dynamic models, suggests that the organization of signaling networks has an determining role in their dynamics. Reference: R. Zhang, M. V. Shah, J. Yang, S. B. Nyland, X. Liu, J. K. Yun, R. Albert, T. P. Loughran, Jr., Network Model of Survival Signaling in LGL Leukemia, PNAS 105, 16308-16313 (2008). [Preview Abstract] |
Session J8: Preparing Physics Students for Careers in Industry
Sponsoring Units: FEd FIAPChair: Lawrence Woolf, General Atomics
Room: 414/415
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J8.00001: The Physics Workforce: The Latest Data on Supply and Demand Invited Speaker: This paper will provide an overview of the trends both in physics education at all degree levels and in the employment of physicists throughout the economy. The paper also includes comparative data on the trends in related fields. There will be a discussion of the common career paths pursued by physicists at different degree levels with particular emphasis on non-academic employment. The paper includes a detailed description of professional master's degree programs in physics departments as well as findings from AIP studies that describe the skills that employers value. The paper concludes with suggestions for how physics departments might assist their students in finding employment during these difficult economic times. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J8.00002: Industrial Physics Careers: A Large Company Perspective Invited Speaker: Statistical data from the American Institute of Physics and the National Science Foundation show that only about a third of physics graduates get permanent jobs in academia. A few work in government labs and hospitals. The majority of physics Ph.D.s, however, find employment in the private sector (industry). This trend has been increasing, i.e., recent Ph.D.s are even more likely to start careers in industry. Industrial physicists work in small, medium or large companies in a broad range of fields, including aerospace, semiconductors, automotive, energy, information technology, contract research, medical, chemical, optics, etc. They are also represented in fields outside of physics, such as finance. Even the ``inventor'' of the Powerball lottery game is a Ph.D. physicist. In my talk, I will describe pathways to success for an industrial physicist, from the perspective of employment in three different large corporations. Based on the NIST Baldridge criteria of Performance Excellence, I will discuss how to achieve and measure organizational success through focus on products and customers. Individual performance is linked to the goals of the organization. Performance has two components: Goals and behaviors. Both are key to success as an individual contributor or manager.\\[4pt] References: \\[0pt] http://www.aip.org/statistics/trends/emptrends.html \\[0pt] http://www.aps.org/about/governance/committees/commemb/index.cfm \\[0pt] http://www.quality.nist.gov/ [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J8.00003: The Rutgers Undergraduate Physics Program: Preparing Students for Varied Careers Invited Speaker: At Rutgers University we offer three main physics major tracks, each tailored towards different kinds of career aspirations. The Professional Option is for students who intend to go on to physics graduate study. The Applied Option is for students who desire technical jobs in industry, but without graduate study. The General Option is for students who have an interest in physics, but do not aspire to a technical career. I will discuss how these Options prepare students for their desired careers, and will give specific examples of jobs obtained. I will especially focus on the Applied Option, explaining how it has evolved based on lessons learned, and what further steps we need to take at Rutgers. I will close by briefly discussing a new, fourth physics major track we have just introduced, our Ocean Physics Option. I will describe this new Option and discuss prospects for its success. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J8.00004: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:39PM - 2:15PM |
J8.00005: Training PhD Physicists for Industrial Careers: The Industrial Leadership in Physics Program at Georgetown University. Invited Speaker: The Physics department at Georgetown University has a unique PhD level graduate program designed to prepare PhD physicists for positions in high-tech business. Launched in 2001, the Industrial Leadership in Physics (ILP) graduate program combines training in technical subjects and business topics with a focus on group learning, communication skills, and practical work experience. Some highlights of the program include a modular curriculum in fundamental physics, centered on solid-state physics, instrumentation, problem solving and computer modeling; a year-long apprenticeship at the site of an industrial partner chosen to match the interests of the student and coursework in the McDonough School of Business at Georgetown. This presentation will give an overview of the program. [Preview Abstract] |
Session J9: Focus Session: Elasticity and Geometry of Thin Objects II
Sponsoring Units: GSNPChair: Pedro Reis, Massachusetts Institute of Technology
Room: 303
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J9.00001: The self assembly of closed surfaces from flat sheet Invited Speaker: We will discuss theoretical and some experimental work to understand the conditions which a flat sheet with embedded permanent magnets can spontaneously fold into a closed surface. The critical question is understanding how to design the system -- both the cut of the sheet and the position of the magnets -- to maximize the yield of assembling into a closed surface without misfolding. This problem also raises novel and interesting questions in thin plate elasticity, which will be highlighted. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J9.00002: Simulating Striped Phases on Curved Surfaces Robin L. B. Selinger, Jonathan V. Selinger Ordered phases on curved surfaces often exhibit geometrical frustration: if the order is incompatible with the curved geometry, the phase must form a complex pattern of defects. Many recent theoretical and experimental studies have explored the relationship between curvature, order, and defects in crystalline, nematic, and striped (smectic or columnar) phases. In earlier work, we developed a new approach for simulating the orientational order of nematic phases on curved surfaces. We now present an analogous approch for simulating striped phases. This approach is based on a lattice gas, in which the Ising spin represents the local density variable. Long-range antiferromagnetic interactions between the spins induce the formation of a striped phase, with a wavelength proportional to the interaction range. This model can be simulated on a random mesh, which can be defined on any arbitrary curved geometry. Through these simulations, we determine how striped phases respond to curvature on spheres, tori, and other geometries. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J9.00003: Simulating Complex Modulated Phases Through Spin Models Jonathan V. Selinger, Lena M. Lopatina, Jun Geng, Robin L. B. Selinger We extend the computational approach for studying striped phases on curved surfaces, presented in the previous talk, to two new problems involving complex modulated phases. First, we simulate a smectic liquid crystal on an arbitrary mesh by mapping the director field onto a vector spin and the density wave onto an Ising spin. We can thereby determine how the smectic phase responds to any geometrical constraints, including hybrid boundary conditions, patterned substrates, and disordered substrates. This method may provide a useful tool for designing ferroelectric liquid crystal cells. Second, we explore a model of vector spins on a flat two-dimensional (2D) lattice with long-range antiferromagnetic interactions. This model generates modulated phases with surprisingly complex structures, including 1D stripes and 2D periodic cells, which are independent of the underlying lattice. We speculate on the physical significance of these structures. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J9.00004: Slow Stress-Relaxation of Thin Sheet Folds Jens Feder, Simon de Villiers, Anders Malthe-Sorenssen We measure the slowly relaxing force required to maintain a fold in thin sheets of aluminum, copper, Mylar, and paper. The relaxation is found to be best described by a Weibull distribution of relaxation times. The exponent $\beta$ of the Weibull distribution characterizes two distinct classes of relaxation observed in metallic ($\beta\simeq 2$) and polymeric materials ($\beta \simeq 1$) respectively. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J9.00005: Nanomechanical properties of few-layer graphene membranes Menno Poot, Herre van der Zant Graphene is a one-atom thick layer of graphite and has only recently been discovered. It combines unique electronic properties with an extremely high Young's modulus of 1 TPa. We have measured the mechanical properties of few-layer graphene and graphite flakes that are suspended over circular holes. The spatial profile of the flake's spring constant is measured with an atomic force microscope. Both the bending rigidity of and the tension in the membranes are extracted by fitting a continuum model to the data. Both parameters show a strong thickness-dependence. Surprisingly, flakes with down to only eight atoms thick can still be described by continuum mechanics. Measurements on single layer graphene could resolve the long standing question whether a one-atom thick membrane can have a finite bending rigidity. Finally we predict that these nanodrums have resonance frequencies in the GHz range. These high frequencies combined with their low mass make them ideal components for quantum electromechanical systems. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J9.00006: Adhesion Transition of Flexible Filaments Arthur Evans, Eric Lauga As forays into fabrication and self-assembly venture to increasingly small length scales, the role of adhesion events between material elements of the system must be closely scrutinized. This area of study is typically dominated by investigations into capillary adhesion, but relatively recent interest in carbon nanotubes and biomimetic devices have spurred interest in intermolecular forces as another source of micro- and nano-scale adhesion. We present here a far-field model for ``dry'' adhesion. We consider a small number N of flexible beams interacting with each other via a typical Lennard-Jones 6-12 potential, and describe the behavior of the system as the ratio of bending rigidity to beam-beam attraction is reduced. Applications ranging from fibrillar systems to the comparatively stiff carbon nanotubes are discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J9.00007: Elastic theory of unconstrained non-Euclidean plates and shells Efi Efrati, Raz Kupferman, Eran Sharon Non Euclidean bodies possess no stress free configuration, thus exhibit residual stress and a rich variety of non-trivial equilibrium configurations in the absence of external constraints. An appropriate hyper-elastic treatment of such bodies is achieved by measuring strain with respect to a reference metric rather than a reference configuration. Applying this formalism to thin sheets, we derive a reduced 2D elastic theory, which enables us to treat thin bodies which are neither plates nor shells in the classical sense. In this reduced theory the elastic energy is given as a function of the mid-surface properties (first and second fundamental forms). We show how prescribing a reference metric for a three-dimensional thin body, corresponds to setting a reference first fundamental form (2D metric) and a reference second fundamental form (curvatures) on the mid-surface. When the prescribed reference curvatures and 2D metric do not comply with one another, the system is frustrated (non-Euclidean). Such systems exhibit various phenomena such as spontaneous buckling and the emergence of a boundary layer. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J9.00008: Experimental Study of Energy Scaling in Non-Euclidean Plates Yael Klein, Eran Sharon We present an experimental study of the three-dimensional (3D) configurations that result from non-uniform lateral growth/shrinking of thin elastic sheets. We construct environmentally responsive hydrogel plates that shrink no uniformly when heated above 33C0. This process prescribes a non-Euclidean ``target'' metric on the sheets. In order to reduce their energy, the plates buckle into 3D configurations associated with their target metric. We study the variations in sheet configurations with decreasing thickness. We observe two types of behaviors: sheets with imposed positive Gaussian curvature have weak thickness dependence, their bending content is bounded and their total bending energy scales like thickness cube. On the other hand, sheets with imposed negative Gaussian curvature undergo a set of bifurcations, as they obtain configurations with increasing number of nodes as thickness decreases. As a result their bending content increases with decreasing thickness, causing the bending energy to scale like the square of the thickness. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J9.00009: The Elastochrone: the descent time of a sphere on a flexible beam Jeffrey Aristoff, Christophe Clanet, John Bush We present the results of a combined experimental and theoretical investigation of the motion of a sphere on an inclined flexible beam. A theoretical model is developed to describe the dynamics, and in the limit where the beam reacts instantaneously to the loading, we obtain exact solutions for the load trajectory and descent time. For the case of an initially horizontal beam, we calculate the period of the resulting oscillations. Theoretical predictions compare favorably with our experimental observations in this quasi-static regime. The time taken for descent along an elastic beam, the elastochrone, is compared to the classical brachistochrone, the shortest time between two points in a gravitational field. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J9.00010: Wrinkles/Folds and the Role of Interfacial Thermodynamics Luka Pocivavsek, Brian Leahy, Enrique Cerda, Binhua Lin, Ka Yee Lee We recently developed a general model for studying instabilities like wrinkling and folding in interfacial membranes on fluid substrates. The dominant length scales describing the instability are set by the elastic response of the membrane (primarily bending) and the ``stiffness'' of the substrate. These length scales, like the wrinkle wavelength and fold amplitude, are independent of the particular interfacial molecular interactions for micron thick membranes where typical system energies like the membrane bending stiffness are thousands of times larger than intermolecular potentials. However, as the membranes become thinner and thinner and eventually approach molecular membranes only a couple of nanometers thin, the chemical interactions between the membrane and the fluid substrate strongly influence the wrinkling and folding length scales. We present data for two such systems (a lipid monolayer and a gold nanoparticle layer) on different hydrogen bonding fluids and discuss possible mechanisms and modifications of our wrinkle-to-fold scaling laws to account for this new degree of freedom. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J9.00011: Universal Shapes of Interacting Mode-I Cracks Melissa Fender, Frederic Lechenault, Pedro Reis, Benoit Roman, Karen Daniels We experimentally investigate the interaction between two parallel cracks propagating towards each other under uniaxial traction in quasi-2D slabs of gelatin. A single crack would propagate perpendicular to the direction of traction. However, after they pass each other, the two cracks rotate and ultimately meet, leaving behind a lens-shaped remnant. We find a universal length-to-width ratio for this remnant, independent of the pulling speed and initial crack separation; the same phenomenon is observed in a variety of elastic materials. Moreover, the overall dimensions of the lens-shaped remnant are set by the initial crack separation. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J9.00012: The Life and Times of a Ruck in a Rug Dominic Vella, Mokhtar Adda-Bedia, Arezki Boudaoud We study the familiar problem of a ruck in a rug. Under lateral compression, a rug bends out of the plane forming a ruck - a localised region in which it is no longer in contact with the floor. We consider the equilibrium of such a ruck. Once the external force that caused the compression is removed, experience tells us that the ruck may either remain or flatten out under its own weight. We quantify the conditions under which each of these two scenarios occurs. We also consider how the propagation of a ruck along the carpet facilitates large-scale sliding. [Preview Abstract] |
Session J10: Focus Session: Optical Properties of Nanostructures II: Quantum-Dot-Coupled Systems
Sponsoring Units: DMPChair: Feng Wang, University of California, Berkeley
Room: 304
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J10.00001: Optical Detection and Control of Single Magnetic Ions in Photonic Microcavities Invited Speaker: As the density of magnetic information storage scales upwards, the number of magnetic moments in each bit decreases. This pathway ends with the need to manipulate a single spin, a requirement that is also important for nascent information processing schemes including quantum computation. Current demonstrations of coherent single spin control include electron spins in semiconductor quantum dots and nitrogen-vacancy centers in diamond \footnote{R. Hanson and D. D. Awschalom, \textit{ Nature } \textbf{453}, 1043 (2008).} . Single magnetic ions in semiconductors have also emerged as an intriguing spin system due to their surprising ability to be manipulated in zero-field. Manganese (Mn) ions in gallium arsenide (GaAs) are strongly exchange coupled to the charge carriers and can be rapidly controlled either optically or electrically in bandgap- engineered heterostructures. Recently we demonstrated optical control and readout of a small ensemble of Mn ion spins in a GaAs quantum well without magnetic materials or applied magnetic fields\footnote{R. C. Myers, M. H. Mikkelsen, J.-M. Tang, A. C. Gossard, M. E. Flatt\'{e}, and D. D. Awschalom, \textit{ Nature Materials} \textbf{7}, 203 (2008).}. In the limit of low doping levels, their spin lifetimes increase with decreasing concentration as the ions become isolated. Here we describe the spatially-resolved observation and manipulation of isolated Mn spins integrated within photonic microcavities. A single magnetically-doped GaAs quantum well is fabricated within both microdisk and vertical Fabry-Perot cavities in which their respective cavity modes are coupled to the neutral Mn acceptor emission. Scanning micro-photoluminescence measurements reveal cavity-coupled emission and a dramatic increase in the measured signal to noise ratio, thereby allowing direct imaging of narrow linewidth luminescence from the Mn moments. These Mn ion spins are optically polarized at zero-field, exhibit long spin lifetimes, and may be manipulated through a variety of techniques. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J10.00002: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J10.00003: Silicon nanocrystal photoluminescence in cylindrical whispering gallery resonators P. Bianucci, J.R. Rodr\'Iguez, C.M. Clements, J.G.C. Veinot, A. Meldrum We present photoluminescence studies of Silicon-nanocrystal (Si-NC) coated cylindrycal microcavities. The coatings were prepared by dip-coating standard optical fibers with a solution-based precursor followed by a high-temperature annealing step. The photoluminescence spectra measured perpendicular to the fiber axis show high Q-factor ($\sim$2500) whispering gallery modes, and allows distinction between TE and TM modes. We also show a proof-of-principle implementation of a far-field refractometric sensor using a nanocrystal-coated fiber. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J10.00004: Optical Response of Strongly Coupled Quantum Dot-Metal Nanoparticle Systems: Double Peaked Fano Structure and Bistability Ryan Artuso, Garnett Bryant We study the optical response of a semiconductor quantum dot (SQD) coupled with a metal nanoparticle (MNP). In particular, we explore the relationship between the size of the constituents and the response of the system. We identify, three distinct regimes of behavior in the strong field limit that each exhibit novel properties. In the first regime, we find that the energy absorption spectrum displays an asymmetrical Fano shape (as previously predicted). It occurs when there is interference between the applied field and the induced field produced by the SQD at the MNP. When the coupling is increased by increasing the size of the SQD, we find a double peaked Fano structure in the response. This second peak occurs when the induced field becomes stronger than the external field. As the coupling is further increased by increasing the sizes of both the SQD and the MNP, we find a regime of bistability. This originates when the self-interaction of the SQD becomes significant. We explore these three regimes in detail and set bounds on each. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J10.00005: Excitons and plasmons in coupled nanoparticles and nanowires Pedro L. Hernandez-Martinez, Alexander O. Govorov In this study, we develop theoretical models of coupled nanoparticles (NPs) and nanowires (NWs). In particular, we focus on exciton energy transfer between NPs and NWs and consider both metal and semiconductor nanocrystals. We obtain analytical equations for the long distance limit and a numerical solution for the general case. For long distances, the energy transfer rate is proportional to 1/d\^{}5, where $d$ is the distance between NP and NW [1]. In a coupled NP-NW structure, excitonic energy can be efficiently extracted and channeled to nanowires/nanotubes by utilizing the F\"{o}rster energy transfer mechanism [1,2]. The calculated energy transfer rates are in good agreement to the experimental values [2]. The NP-NW system has potential for applications in optoelectronic devices and sensors [3]. [1] P. Hernandez-Martinez, A. O. Govorov, Phys. Rev. B, B 78, 035314 (2008). [2] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063-2069 (2005). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials, 6, 291 -- 295 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J10.00006: High-transmission ridge nanoapertures for quantum dot devices Akihiro Kirihara, Junichi Fujikata, Toshihiro Nakaoka, Naoto Kumagai, Katsuyuki Watanabe, Masayuki Shirane, Shunsuke Ohkouchi, Shinichi Yorozu, Yasuhiko Arakawa We report on double-ridge apertures to enhance the coupling between a single quantum dot (QD) and optical field. The double-ridge aperture has two metallic tips protruding inward and facing each other, which work as an effective antenna for a QD just below the tips. We performed FDTD simulation to optimize the apertures, and fabricated them on InAs/GaAs QDs emitting at 960nm. By single-dot PL spectroscopy through the double-ridge aperture, we demonstrated 5-6 times enhancement in PL extraction efficiency, compared to that through a conventional circular aperture. Because our double-ridge aperture works not only as an optical antenna but also as an electrode for a QD, it will be applicable to electrically-driven photon generators or photon detectors. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J10.00007: Polarization and Angle Dependent Spontaneous Emission Rates in Hybrid Metal-Semiconductor Nanostructures Yikuan Wang, Tianyu Yang, Mark Tuominen, Marc Achermann Recently, the coupling of a dipole emitter to surface plasmons (SPs) of metal nanostructures has attracted much attention for its potential applications in light emitting devices. Our time-resolved photoluminescence (PL) study on the emission of CdSe/ZnS core/shell nanocrystals (NCs) deposited on a two-dimensional array of gold nanodiscs demonstrates that the spontaneous emission of dipole emitters is strongly dependent on the detection angles and polarizations. The in-plane, s-polarized PL measurements are independent on detection angles, and can be described by the PL decay dynamics of two NC subsets: the emission from NCs on the dielectric substrate and from NCs on the gold nanodiscs that experience non-radiative quenching by the metal structures. The out-of-plane, p-polarized PL measurements show an additional decay caused by SP-induced enhancement of the spontaneous emission rate. This angular-dependent enhancement is explained by interactions between NC dipole moments and the out-of-plane SP resonance of the gold nanodiscs. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J10.00008: Decay-rate distribution of single quantum dots in nanometer-scale proximity to a metal film Matthew Pelton, Xiaohua Wu Recently, the interaction between fluoresecent colloidal semiconductor quantum dots and plasmonic metal nanostructures has attracted great interest, both for the development of a basic understanding of nanoscale photophysics and for potential applications in improved light-emitting devices and integrated plasmonic circuits. Time-resolved measurements on single dots are required in order to overcome the obscuring effects of ensemble averaging and of time averaging, and thus reveal the physical mechanisms of dot-metal interaction. In this work, we present measurements of photoluminescence decay dynamics from single colloidal CdSe/ZnS core-shell quantum dots in nanometer-scale proximity to a smooth gold film. We extract the decay rate, $k_m$, for each dot when it is in its maximum-intensity state, thereby removing the effects of nonradiative decay-rate fluctuations. We find that, as the separation between the dot and the metal decreases, the $k_m$ distribution becomes broader and its maximum increases. The increase in maximum decay rate is caused by stronger energy tranfer from the quantum dot to the metal film, as expected. The broader distribution of decay rate, on the other hand, reflects inherent variations in the interactions between individual dots and the metal film. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J10.00009: Demonstration of Optical Resonances in a Cylinder-Shell Lattice of Quantum Dots Jared Maxson, Slava Rotkin We present a model for the calculation of the optical response of a cylinder-shell of quantum dots or metallic nanoparticles. We model such a shell cluster as a lattice of non-permanent point dipoles with a known polarizability and a single transition frequency. We then utilize the second quantization formalism to compute the cluster response. The eigenmodes and quantum mechanical response function of the lattice interacting with an external field, polarized respective to the cylinder axis, are calculated numerically. The cylinder radius is treated as a parameter to identify resonator effects due to the cylindrical geometry. Varying the frequency of the external field, regions of response maxima are determined. In these regions resonant interaction between the coupled dipoles results in transferring significant oscillator strength into a few eigenmodes of the cluster, having high spatial and temporal coherence with the external field. Further analysis of the spatial distribution of dipoles in each region of response maxima reveals significant contributions from groups of modes with equal angular momenta, permitting rigorous excitation classification. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J10.00010: Two-Photon Transitions in Molecular Quantum Dot System Michael Scheibner, Ilya Ponomarev, Danny Kim, Allan Bracker, Daniel Gammon Two-photon excitations are at the heart of important nonlinear optical processes and provide a key to the rich physics and fresh opportunities of designer quantum materials. In this study we consider double dot quantum dot molecules (QDMs) that were designed by molecular beam epitaxial growth to exhibit either electron or hole tunnel coupling of the two dots. The electron or hole levels of the two dots can be tuned into resonance with an applied electric field which is created by a Schottky diode structure surrounding the QDMs. Highly sensitive photoluminescence excitation spectroscopy is used to study sequential and simultaneous two-photon transitions in the absorption spectrum of the molecular biexciton in such a QDM. We identify a new two-photon transition which is the first example of a simultaneous, coherent optical excitation of a pair of QDs in a weakly tunnel coupled regime. We further show that a photoluminescence excitation measurement with stereo-chromatic detection can be used to gain access to the spin physics in this regime. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J10.00011: Qubit extraction and manipulation in optically-driven self-assembled quantum dot molecules. J.E. Rolon, S.E. Ulloa Semiconductor quantum dot molecules (QDMs) allow studies of different mechanisms of coherent optoelectronic control of excitonic states in the pursuit of stable and well characterized qubits. In this work, we develop a realistic calculation of the dressed spectrum and exciton dynamics of InAs/GaAs QDMs. The dressed spectrum contains electron and hole tunneling as well as exciton F\"{o}rster resonant energy transfer (FRET) level anticrossing signatures, from which we derive an effective Hamiltonian using a projection operator formalism. The state dynamics is analyzed using a multilevel Lindblad master equation, in which the projected density matrix is obtained by partial tracing of the irrelevant exciton degrees of freedom. We find that the interplay of FRET and carrier tunneling can produce a charge qubit subspace whose indirect exciton character makes it resilient to lifetime limited decoherence. Furthermore, it is shown that a set of universal quantum gates can be constructed and its unitarity assured by the application of an external electric field that prevents the mixture of qubit subspace dynamics with other excitonic degrees of freedom, present upon optical excitation. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J10.00012: Electric field induced manipulation of static and dynamic optical properties of coupled quantum dots Y.K. Verma, S.N. Ghosh, C.G.L Ferri, M. Gallardo, D. Kelley, S. Ghosh A system of coupled quantum dots (QDs) provides pathways for efficient and controlled energy transfer. In such a system, electronic excitations get delocalized over the several QDs and lead to the creation of macroscopic electronic states. We present a novel way to induce structural order in chemically synthesized GaSe QDs by embedding them in a matrix of nematic liquid-crystal (NLC) molecules. Photoluminescence (PL) from the QD-NLC mixture exhibits large red-shift in the emission spectrum ($\sim $200 meV) which implies the formation of strongly coupled QD aggregates. Dynamic Light Scattering measurements on isolated and QD-NLC matrix reveal the aggregates to be composed of several tens of QDs, while PL measurements show that their emission is highly anisotropic, being strongly polarized along the axis of aggregation. These structures can further be spatially re-aligned in situ without destroying the inter-dot coupling by the application of an in-plane electric field. Time-resolved measurements reveal a faster excitonic recombination in the QD-NLC matrix in comparison to that in isolated QDs which is attributed to facile energy transfer processes. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J10.00013: Electronic and optical properties of laterally coupled InGaAs quantum dots Jie Peng, Gabriel Bester We calculate the electronic and optical properties of laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric field using empirical pseudopotentials and configuration interaction. Our model structure is directly taken from recent experiments where an In-poor basin develops below the dots. The coupling of the electron states is significantly enhanced by the presence of the basin, while the holes remain mainly uncoupled. At the proper electric field ---between 0 V/cm and 200 V/cm, depending on the dot molecule--- the electron states can be tuned to be evenly distributed between both dots, forming bonding and antibonding states. The optical absorption is shown to exhibit two bright transitions, mostly independent of the applied field. In emission, we argue that a fast electron-dynamics must be introduced, since the electrons are not subject to a true potential barrier between the dots and consequently only the lowest of the electron states is occupied. Following this approach, we obtain only one bright peak at high electric fields and two peaks (at higher temperature, four peaks) at the tuning point of the electron states. The results are shown to compare very well with recent experiments. A simple 4x4 Hamiltonian is derived to explain the results in the intuitive dot-localized basis. [Preview Abstract] |
Session J11: Quantum Dots and Quantum Point Contacts
Sponsoring Units: DCMPChair: Sergio Ulloa, Ohio University
Room: 305
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J11.00001: Imaging a One Dimensional Quantum Dot in an InAs/InP Nanowire Erin E. Boyd, Halvar J. Trodahl, R.M. Westervelt, Linus E. Froberg, Kristian Nilsson, Lars Samuelson Nanowires are promising contenders for use in novel spintronic and nanoelectronic devices. An InAs/InP nanowire containing a long InAs quantum dot (length$>$2xdiameter), is an ideal system to use a liquid He-4 cooled scanning gate microscope tip to probe electron behavior. This increased understanding would help in the design of quantum devices. For a few electron long dot, no excited states in the transverse direction will be occupied, making the system 1D. The electron density in these systems will change their state, as a function of dot length, from a liquid state to a Wigner-crystal like state[1]. Using a weak tip potential, it should be possible to probe this spatial distribution of the electron probability. By applying a large tip potential, and using the transitions between different partitionings of the dot, information on the relative strength of the electron interaction could be obtained. [1] Jiang Qian et. al arXiv:0809.0834 (September 2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J11.00002: Imaging Double Quantum Dots in InAs/nP Nanowires Halvar J. Trodahl, Erin E. Boyd, R. M. Westervelt, Kristian Nilsson, Linus E. Froberg, Lars Samuelson Coupled quantum dots formed in InAs/InP heterostructure nanowires are attractive candidates for nanoelectronics, spintronics and quantum information processing. The ability to manipulate the charge state of a single quantum dot defined in these nanowire systems using a low temperature scanning probe microscope (SPM) tip has been shown previously [1] and provides a tool to investigate the properties of nanowire systems down to the tens of nanometer scale. In order to realize the above applications, multiple InAs quantum dots can be formed in an InAs/InP nanowire system by using InP barriers. Using a conducting SPM tip as a movable electrostatic gate, the charge can be tuned independently on each dot of a double quantum dot defined in a semiconductor nanowire. [1] A. C. Bleszynski-Jayich, et al., PRB 77, 245327 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J11.00003: Phase Coherence and Mesoscopic Coulomb Blockade in Open Quantum Dots Ileana Rau, Michael Grobis, Sami Amasha, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon The phase coherence of electrons in open systems at low temperatures leads to mesoscopic effects such as universal conductance fluctuations and weak localization of electrons. These effects are encountered in open large quantum dots and are explained by a model of non-interacting electrons. We have investigated the transport properties of a 1.5 $\mu m^2$ and a 3 $\mu m^2$ lateral GaAs/AlGaAs quantum dot in the open regime. The weak localization effect is complicated at low temperatures by the presence of residual interactions (Mesoscopic Coulomb Blockade) that persists even when the dot is coupled by one or two fully transmitting modes to each of the two leads. We present measurements of the electron dephasing rate at low temperatures in the open quantum dots and discuss how they are affected by the suppression of conductance by these Coulomb blockade effects. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J11.00004: Mesoscopic Coulomb Blockade in a Quantum Dot with Two Open QPCs Sami Amasha, Ileana Rau, Michael Grobis, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon A quantum dot consists of a confined droplet of electrons connected to an electron reservoir by two quantum point contacts (QPCs). When the conductance of each of these QPCs is less than $2e^2/h$, the dot is in the closed regime and Coulomb Blockade effects dominate the transport properties. Open quantum dots, in which QPC conductances are $2e^2/h$ or above, are generally thought to be well-described by non-interacting electron theory. While Mesoscopic Coulomb Blockade (MCB) effects can occur in a quantum dot with one open and one closed QPC, these effects are expected to be absent for quantum dots with two open QPCs. We have investigated the transport properties of a 1.5 $\mu m^2$ and a $3 \mu m^2$ lateral GaAs/AlGaAs quantum dot in the open regime and find a clear signature of MCB. We will discuss the dependence of MCB on various controllable parameters, including magnetic field, temperature, and bias. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J11.00005: Quantum Phase Transition to a Zigzag Wigner Crystal A. C. Mehta, C. J. Umrigar, A. D. Guclu, K. A. Matveev, H. U. Baranger We use Quantum Monte Carlo techniques to map out the phase diagram of interacting electrons in a quantum wire. Interacting quasi-one-dimensional systems provide excellent examples of quantum phase transitions that are tractable. Previous work gave a qualitative description of the phase diagram of a quasi-one-dimensional system [Meyer, Matveev, and Larkin, PRL 2007]. At low density, electrons confined to one dimension by a transverse harmonic potential form a linear one dimensional Wigner crystal; as the density increases, symmetry about the axis of the wire is broken and there is a transition to a quasi-one-dimensional zigzag crystal. We use variational and diffusion Monte Carlo to study the phase diagram of this system quantitatively. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J11.00006: Quantum Phase Transition in a single-molecule Quantum Dot N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a manyparticle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, can be observed in several strongly correlated materials such as heavy fermion compounds or possibly high-temperature superconductors, and is believed to govern many of their fascinating, yet still unexplained properties. In contrast to these bulk materials with very complex electronic structure, artificial nanoscale devices could offer a new and simpler vista to the comprehension of quantum phase transitions. This long-sought possibility is demonstrated by our work in a fullerene molecular junction, where gate voltage induces a crossing of singlet and triplet spin states at zero magnetic field. N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer \& F. Balestro, Quantum phase transition in a single-molecule quantum dot, Nature, 2008, 453, 633-637. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J11.00007: Aharonov-Bohm-type quantum interference effects in narrow gap semiconductor heterostructures R.B. Lillianfeld, R.L. Kallaher, J.J. Heremans, Hong Chen, N. Goel, S.J. Chung, M.B. Santos, W. Van Roy, G. Borghs We present experiments on quantum interference phenomena in semiconductors with strong spin-orbit interaction, using mesoscopic parallel ring arrays fabricated on InSb/InAlSb and InAs/AlGaSb heterostructures. Both external electric field effects and temperature dependence of the ring magnetoresistance are examined. Top-gate voltage-dependent oscillations in ring resistance in the absence of an external magnetic field are suggestive of Aharonov-Casher interference. At low magnetic fields the ring magnetoresistance is dominated by oscillations with h/2e periodicity characteristic of Altshuler-Aronov-Spivak (AAS) oscillations, whereas the h/e periodicity characteristic of Aharonov-Bohm (AB) oscillations persists to high magnetic fields. Fourier spectra (FS) reveal AB amplitudes on the same order as AAS amplitudes at low fields, and in some samples reveal a splitting of the AB peaks, which has been interpreted as a signature of Berry's phase. The FS are also used to quantify the temperature dependence of the oscillation amplitudes (NSF DMR-0618235, DOE DE-FG02-08ER46532, NSF DMR-0520550). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J11.00008: Time-resolved detection of single-electron interference Simon Gustavsson, Matthias Studer, Renaud Leturcq, Thomas Ihn, Klaus Ensslin, D. C. Driscoll, A. C. Gossard We demonstrate real-time detection of single electron interference in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity [1]. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage across the quantum point contact a back-action is exerted on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives electronic transitions in the quantum dots [2]. Surprisingly, the efficiency of this process depends strongly on external magnetic field, with variations occurring on a small fraction of the magnetic field scale associated with one flux quantum penetrating the ring. The unexpected features demonstrate the complex interplay between radiation, absorption and coherence in mesoscopic systems. [1] S. Gustavsson et al., Nano Lett. 8, 2547 (2008). [2] S. Gustavsson et al., PRL 99, 206804 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J11.00009: Non-equilibrium Charge Fluctuations as a Source of Inelastic Back-action in Quantum Point Contact Qubit Detectors Carolyn Young, Aashish Clerk Many recent experiments make use of a quantum point contact (QPC) as a qubit readout (e.g., of a double quantum dot (DQD) qubit). It has long been realized that QPC current fluctuations can give rise to inelastic back-action effects on the DQD [1] [2]. In contrast, the role of QPC charge fluctuations in generating inelastic back-action has not been fully studied, despite the fact that this is a more fundamental mechanism. We provide a full theoretical study of charge-noise induced inelastic back-action effects in a QPC plus DQD system, showing that these effects should be appreciable in typical experimental setups. We also discuss a novel contribution to the charge noise associated with the physics of Friedel oscillations. Finally, we discuss how the effects of charge noise back-action can be distinguished from current noise back-action in experiment. \newline [1] S. Gustavsson et al., Phys. Rev. Lett., 99, 206804 (2007).\newline [2] R. Aguado and L.P. Kouwenhoven, Phys. Rev. Lett., 84, 091987 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J11.00010: Hole Spin Filtering by quantum point contacts Taisuke Minagawa, Yuli Lyanda-Geller We calculate the charge carrier spectra in two-dimensional hole systems (2DHS) and in quantum point contacts (QPC) formed in the 2DHS in an in-plane magnetic field B. The origin of the spin splitting for holes differs significantly from that for electrons. For bulk holes, the g-factor is defined not only by the constant of coupling of the angular momentum 3/2 to magnetic field, but also by the Luttinger constants $\gamma_1$, $ \gamma_2$ and $\gamma_3$ defining the heavy and light hole masses. In the high mobility 2DHS, the width of the quantum well (QW) L becomes comparable to the magnetic length $\lambda$ for the in-plane $B > 3$T. We find that the spin splitting for 2D holes and for holes in QPC is strongly affected by the orbital motion in the presence of the in-plane B. We developed the new approach to spectra based on confluent hypergeometric functions. We take into account the anisotropy of the Hamiltonian and calculate the spin splitting for [113] orientation of the 2DHS. For QPC spectra, configurations of in-plane B along and perpendicular to the direction of the current are studied. Our results explain many of the features of spin-resolved QPC conductance observed by Rokhinson group (PRL, ${\bf 100}$, 126401) and by Hamilton group (PRL, ${\bf 97}$, 026403). Our analysis also resolves the puzzling red shift of the Fermi energy discovered in optical spectra for QW in-plane magnetic field by Crooker group (Physica E, ${\bf 22}$:624). [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J11.00011: Ferromagnetic vs. Antiferromagnetic Correlations in a Double Dot System Manas Kulkarni, Robert Konik, Alexei Tsvelik We study a double dot system in a parallel geometry using both a large-N diagrammatic and a SBMFT approach. We consider the role of interdot ferromagnetic correlations upon the conductance. We find at the particle-hole symmetric point that the Friedel sum rule holds and the conductance vanishes. We find that the ground state of the double dot system is a singlet although the correlations between the two dots is primarily ferromagnetic. Hence we observe that the RKKY interaction does not bind the two electrons on the dots into a triplet. We compare our results to a Bethe ansatz analysis of the same system [1]. [1] R.Konik PRL 99, 076602 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J11.00012: Magnetic Splitting of the Zero Bias Peak in a Quantum Point Contact with a Variable Aspect Ratio Tai-Min Liu, Bryan Hemingway, Andrei Kogan, Steven Herbert, Michael Melloch We have measured the nonlinear conductance of a four-gate Quantum Point Contact (QPC) device fabricated in a GaAs/AlGaAs heterostructure containing a 2-dimensional electron gas. By continuously varying the longitudinal potential profile of the QPC, we controllably create and destroy a local bound state. The nonlinear transport data show both a characteristic Coulomb blockade diamand and a zero-bias peak similar to the Kondo effect signature peak in quantum dots. We find that even when the bound state is suppressed the zero-bias peak persists. Applying an in-plane magnetic field perpendicular to the direction of the current produces a splitting of the peak which closely matches the $g$-factor data obtained via the cotunneling spectroscopy method in a separate quantum dot on the same chip. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J11.00013: Topological spin textures in strongly interacting quantum dots Jordan Kyriakidis, Catherine J. Stevenson We present results of configuration-interaction calculations on two-dimensional quantum dots confining charges with long-range Coulomb repulsion. We focus on correlation-induced spin textures formed at zero magnetic field. By looking at chiral structures, at two- and three-point spin-correlation functions, and at explicit symmetry-breaking fields, a consistent picture emerges of incipient topological spin textures formed throughout the dot and particularly at annular regions of increased electron density. In addition to singular vortex-type structures, 2$\pi$-windings are observed in the spin field along these annular regions. These textures are solely due to statistics and repulsion. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J11.00014: Spin interference in quantum rings manipulated with quantum point contacts Francisco Mireles, Leo Diago The Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects are two well known interference phenomena that may appear in semiconductor quantum rings (QR's). Although the AB effect has been long observed, its counterpart, the AC effect has been only recently detected in clever magnetoconductance oscillations experiments on HgTe/HgCdTe based QR's exhibiting strong Rashba SO-interaction [1]. In this work, using the S-matrix formalism we study the role of the contacts between the leads and the QR on the AB and AC conductance oscillations of the device in the presence of Rashba and Dresselhaus type of SO interactions. We describe the backscattering and transparence of the conjunctions lead-to-ring through quantum point contacts (QPCs) modelled with gate-controllable saddle-point potentials. The variable transmitivity of the QPCs, adjusted in the experiment by gate voltages and/or applied magnetic fields, is readily incorporated in our approach. It is shown that manipulating electrostatically the confinement strength at the QPCs, may be of utility to implement a novel way to modulate spin interference effects in semiconductor quantum rings. [1] M. K\"onig {\it et al.}, PRL {\bf 96}, 076804 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J11.00015: NEGF Study of the Spontaneous Spin Polarization in a Quantum Point Contact Junjun Wan, Marc Cahay, Richard Newrock, Philippe Debray A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC) induced by the electric field due to the gradient of the lateral confining potential. A small asymmetry in the confining potential induced by difference of potential between the two side-gates (SGs) leads to an inversion asymmetry in the LSOC which triggers a spontaneous spin polarization in the QPC. In the regime of single-mode transport, the spontaneous spin polarization can reach nearly 100 {\%} when a strong electron-electron (e-e) interaction is taken into account. This leads to the occurrence of a plateau at G $\approx $0.5($e^{2}$/$h)$ in the ballistic conductance without the need of any externally applied magnetic field. Two ingredients are essential for the occurrence of the 0.5 plateau: an asymmetric LSOC and a strong e-e interaction. [Preview Abstract] |
Session J12: Organic Electronic Thin Films
Sponsoring Units: DMP DCMPChair: Renee Diehl, Penn State University
Room: 308
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J12.00001: Valence band structure in crystalline pentacene thin films Richard Hatch, David Huber, Hartmut H\"ochst Organic semiconductors, such as pentacene (Pn), are beginning to show promise as a low-cost substitute for conventional semiconductors for a variety of electronic devices. The overlap of $\pi $-orbitals in the Pn crystal leads to molecular orbital-derived bands. We used angle-resolved photoemission spectroscopy (ARPES) to reveal the Pn in-plane band structure of the two highest occupied molecular orbital-derived bands in crystalline thin film Pn (grown on a Bi substrate) for various temperatures between 75 K and 300 K. We mapped these two bands in several crystallographic directions with special attention given to the region near the top of the valence band and show, within the limits of our experimental resolution, that temperature does not change the dispersions of these bands. We fit the band structure to a tight binding model and compared our results with recent theoretical predictions[1-2]. We also calculated the in-plane reciprocal effective mass for the $\bar {M}$ point and compared it with the measured mobility. [1] H. Yoshida \textit{et. al.} Phys. Rev. B \textbf{77}, 235205 (2008). [2] G. A. de Wijs \textit{et. al.} Synth. Met. \textbf{139}, 109 (2003). [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J12.00002: The growth mechanism of anisotropic organic molecular films A. Al-Mahboob, Y. Fujikawa, J.T. Sadowski, Qikun Xue, T. Sakurai The anisotropy in molecular structure and crystal packing may complicate nucleation and growth processes in organic molecular films. The growth mechanism of pentacene (Pn) films has been studied by real time low-energy electron microscopy. Pn is most promising for FET application as it shows the highest field-effect mobility among organic thin-films. We observed delayed nucleation and formation of large grains (as large as 0.5 mm in diameter) on semiconducting $\alpha \surd $3-Bi-Si(111) and on semi-metallic Bi(0001)/Si(111), with a significant delay in film growth after stopping Pn deposition, indicating long diffusion time. This is in contrast to the growth of Pn on SAMs, oxides or wetting layer on clean silicon surfaces. The long diffusion time could be explained by large barrier for Pn nucleation with standing-up orientation from a lying-down diffusing state due to stronger interaction between lying molecules and Bi-treated substrates. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J12.00003: Morphology and Crystalline Structure of the Epitaxial Growth of Tetracene Thin Films on H/Si(001)-2x1 Substrate Andrew Tersigni, De-Tong Jiang, Xiaorong Qin Epitaxial growth of tetracene films on H/Si(001)-2x1 surface has been studied systematically using AFM, STM and X-ray diffraction. The surface morphology and in-plane lattice structure observed from AFM and STM are compared with that extracted from the diffraction measurements to reach a consistent description on the in-plane and out-plane lattice parameters of different epitaxial domains, respectively. The influence of the substrate roughness and film coverage to the morphology and underlying lattice structures will be discussed in the light of various characteristics revealed by the multiple structural tools. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J12.00004: Structural investigation of pentacene on Ag(111) by density functional theory Fatih Danisman, Ersen Mete, Sinasi Ellialtioglu We have used density functional theory (DFT) calculations based on the projector augmented wave (PAW) method to investigate the initial growth patterns of pentacene (C$_{22}$H$_{14})$ on Ag(111) surface. Here we will report our initial findings and provide a discussion of the results from the point of view of our previous experimental findings. Pentacene prefers to stay planar on Ag(111) surface and aligns perfectly along lattice vector (1,-1,0) without any molecular deformation at a height of 3.74 {\AA}. At 1 ML coverage the separation between the molecular layer and the surface plane extends to 4.14 {\AA} due to intermolecular interactions weakening surface-pentacene attraction. While the first ML remains flat, the molecules on a second full pentacene layer rearrange in a herringbone fashion which is energetically slightly more preferable when compared with a second layer composed of flat lying molecules. With addition of third and fourth layers pentacene molecules continue to maintain the herringbone configuration, with the stability of the herringbone configuration relative to the flat one increasing to 0.13 eV for the 3 ML film, while the first ML always remains flat. Therefore, our calculations indicate bulk-like initial stages for the growth pattern. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J12.00005: Magnetic and structural behaviors of transitional metallo-porphines monolayer and bilayer on Ag(111) X.Q. Tian, J.B. Xu Five monolayers and bilayers of transitional metallo-porphine(MP) (Mn, Fe, Co, Ni, Cu) on Ag(111) substrate are investigated by \textit{ab initio }calculation. The strong coupling of FeP and CoP MP/Ag(111) interface leads to magnetic-nonmagnetic transition. The competition mechanism between intermolecular ferromagnetic exchange interaction and coupling of FeP/Ag(111) prevents the magnetic-nonmagnetic transition of interface. The top layer of all 5 bilayers keeps their intrinsic magnetic properties and planar structures, so the bottom layer of the bilayer serves the role of buffer layer. These theoretical results could be used to explain the recent STM and XPS experiments. A new interfacial magnetic competition mechanism has been proposed and hopefully it will influence the construction and design of future molecular spintronics. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J12.00006: Quantitative Grain Size Distributions of Magnetic Organic Thin Films Thomas Gredig, K. Paul Gentry, Ivan K. Schuller Many electronic, optical, and magnetic properties of organic thin films depend on the precise morphology of grains. Quantitative grain size distributions of an asymmetric organic molecule are presented and correlated with the magnetic characteristics. Iron phthalocyanine (FePc) thin films are grown on sapphire substrates at varied deposition temperatures to study the effect of grain growth and to experimentally quantify the grain size distributions in organic thin films based on atomic force microscopy images. The data of over 3000 grains for each sample show a pronounced asymmetric growth of grains from a spherical to an elongated needle-like shape. The size along the major axes increases from 35nm to 200nm and is distributed in a different way than the minor axes, which grow from 25nm to 90nm. The dissimilar distributions are attributed to an asymmetric growth rate. Low-temperature hysteresis loops and temperature-dependent magnetization curves for these FePc thin films illustrate the effect of the length of quasi one-dimensional Fe chains on the magnetic properties. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J12.00007: The structure of the $\mathrm{C}_{60}$ monolayer on Pb(111) Hsin-I Li, Katharina Franke, Jose Pascual, Renee Diehl Low-energy electron diffraction and scanning tunneling microscopy studies indicate that a monolayer of $\mathrm{C}_{60}$ on Pb(111) forms a modulated structure having an average $\mathrm{C}_{60}$-$\mathrm{C}_{60}$ nearest-neighbor distance of about 10\AA, but a period of about 46\AA. The data were analyzed in the context of the Tkatchenko method$\,^{\dag}$, and the monolayer structure was identified as two coexisting higher-order commensurate structures, namely $(\sqrt{169}\times \sqrt{169})R16.90\,^{\circ}$ and $(\sqrt{169}\times \sqrt{169})R21.31\,^{\circ}$, both with 21 $\mathrm{C}_{60}$ molecules and 169 Pb atoms in their unit cells. This modulated structure has implications for the electronic structure of the $\mathrm{C}_{60}$ film, as measured using scanning tunneling spectroscopy. \newline $\,^{\dag}$A. Tkatchenko, Phys. Rev. B 75, 235411 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J12.00008: The structures of a C$_{60}$ monolayer on Al(111) Heekeun Shin, Hsin-I Li, Katariina Pussi, Renee Diehl The interfaces of C$_{60}$ films with metal surfaces are of particular interest for molecular electronics applications. The electronic properties of these films are known to depend strongly on their structures and the relative molecular orientations of the C$_{60}$ molecules, yet there are few detailed structure determinations for C$_{60}$ films. When grown at room temperature, C$_{60}$ on Al(111) forms a (2$\surd $3x2$\surd $3)R30\r{ } structure with one C$_{60}$ molecule per unit cell, which upon heating converts to a 6x6 structure with 3 C$_{60}$ molecules per unit cell. We present a LEED study of the transition between these structures, a LEED I(E) analysis of the 6x6 structure, and characterization of the adsorption of rare gases onto the surface of the 6x6 C$_{60}$ film. This research is supported by NSF-DMR-0505160 and the Academy of Finland. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J12.00009: The Adsorption of C$_{60}$ fullerene molecules on Nanostructured Au (111) Xin Zhang, Quanmin Guo, Richard Palmer The sub-monolayer growth of C$_{60}$ molecules on the Au (111) surface has been studied using STM in ultra high vacuum. The C$_{60}$ molecules tend to form close-packed layers due to a strong inter-molecular interaction. However, within the close-packed layer, there are finer, secondary structures that are specific to each of all the three C$_{60}$/Au interfacial structures ((2$\surd $3$\times $2$\surd $3)R30\r{ }, in-phase (R0\r{ }) and R14\r{ }) observed [1]. This is a consequence of the molecule-substrate interaction and our findings demonstrate a much more complex structural variation at the molecule-substrate interface than previously assumed. Furthermore, within the R14\r{ } C$_{60}$ layer, slightly darker molecules (30 pm lower) aligned along the \~{a}11-2\~{o} direction with a $\sim $6 nm spacing are observed and these molecules are arranged in a reasonably well-ordered two-dimensional lattice. C$_{60}$ molecules are also found to decorate the elbow sites of the herringbone reconstructed Au(111) even at room, and when fullerenes are deposited to arrays of fabricated monolayer gold stripes (gold-fingers) [2], the molecules show step-specific attachment where the step edges with the (111) micro-facet are preferentially populated.\\[0pt] [1] X. Zhang, F. Yin, R. E. Palmer and Q. Guo, \textit{Surf. Sci.} 602 (2008) 885-892.\\[0pt] [2] Q. Guo, F. Yin and R. E. Palmer, \textit{Small} 1 (2005) 76-79. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J12.00010: Towards 2-D Conjugated Organic Polymers; Br-TTA molecules on Cu(111) J.A. Lipton-Duffin, M.C. Gallagher, J.L. Brusso, D.F. Perepichka, F. Rosei We report on recent experiments using an Ullmann surface polymerization approach to synthesize 2-d polymers on flat metal surfaces in UHV. We previously demonstrated the utility of this technique to perform the surface confined synthesis of individual conjugated polyphenylene chains the Cu(110) surface [1]. In the present work we extend this technique to deposit multidendate Br-TTA monomers onto a Cu(111) surface. The evaporation of Br–TTA monomers at room temperature leads to the formation of flat 2-d islands at submonolayer coverage. We use STM and XPS to investigate the structural and chemical characteristics of these structures. Upon annealing (500K) many of the islands restructure to form highly ordered 2-d nanostructures. These structures are quite robust and are stable up to temperatures of 800K at which point the TTA desorbs.\\[3pt] [1] Lipton-Duffin, Ivasenko, Perepichka and Rosei, in preparation. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J12.00011: Interactions of sulfur with graphite Ko Munakata, Theodore Geballe, Malcolm Beasley Possible evidence for very high temperature superconductivity at the interface of sulfur and graphite in composite materials has been reported in the literature [1]. To examine better this possibility, we are studying the interfacial electronic properties of thin UHV deposited layers of sulfur and other overlayers on graphite and few-layer graphene by means of in-situ UPS and XPS. The results of this study and its implications for the possibility of superconductivity will be presented. [1] S. Moehlecke, Y. Kopelevich, and M. B. Maple, Phys. Rev. B 69, 134519 (2004). [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J12.00012: Surface and trapped charge characterization of epitaxial oxides for applications in graphene electronics$^{1}$ Blake Riddick, Brad Conrad, William Minshew, William Cullen, Ellen Williams, Tassilo Heeg, Darrell Schlom Trapped charges have been shown to play an important role in the transport properties of graphene supported on SiO$_{2}$, and surface roughness may also play a role. Alternative substrate materials, Sc$_{2}$O$_{3}$ ($\varepsilon \sim $ 14, $n\sim $ 1.9) and Gd$_{2}$O$_{3}$ ($\varepsilon \sim $ 22, $n\sim $ 2) were grown epitaxially by molecular beam epitaxy on Si(111) over a range of thicknesses from 2 nm to 100 nm. AFM measurements yield rms roughness, and correlation function analysis reveals the nature of the long range order. For Sc$_{2}$O$_{3}$, the roughness is strongly thickness dependent, with root-mean-square height 0.26 nm$^{2}$ for a 20 nm thick film and 0.55 nm$^{2}$ for a 65 nm thick film; however, the correlation exponent (2$H\sim $ 1) and correlation length ($\xi \sim $ 20 nm) are the same. The roughness characterization for the full range of thicknesses of both oxides will be presented. In addition, frequency-dependent CV measurements are underway to determine the trapped charge densities. [1] supported by a NRI supplement to the UMD-NSF-MRSEC grant {\#} DMR 0520471. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J12.00013: Morphology of hydrocarbon films on decagonal Al-Ni-Co quasicrystalline surfaces Wahyu Setyawan, Renee D. Diehl, Majid Karimi, Stefano Curtarolo Lubricants can affect quasicrystalline coatings by modifying commensurability. We present simulation studies of physically adsorbed hydrocarbons on a decagonal surface of a Al-Ni-Co quasicrystal. We use the Grand Canonical Monte Carlo technique with novel embedded-atom-method potentials fitted to {\it ab initio} calculations and standard hydrocarbons interactions. Methane forms pentagonal structure, benzene monolayer adsorbs in hexagonal domains with 5 orientational degeneracies induced by the substrate, propane forms pentagonal domains with significant disorder, hexane and octane monolayers shows linear ordering. The results support the potential use of quasicrystals as low friction coatings. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J12.00014: First principles study of aromatic molecules on Copper substrates Andrea Ferretti, Arrigo Calzolari, Rosa Di Felice, Alice Ruini, Elisa Molinari Conjugated molecules and oligomers have attracted large attention in the last years due to their interesting electronic and transport properties. The interaction of these molecules with metallic surfaces is attractive both for the properties of the metal-organic interface and for the possibility of tuning the crystal structure of the films using the surface as a template. In the present work we focus on an ab initio investigation based on density functional theory of pentacene adsorbed on Copper surface. We also compare with the case of the DPDI molecule adsorbed on the same substrate. We address structural and electronic properties, and we relate our results to experimental data, STM, XSW, and angle resolved photoemission spectroscopy in particular. Our theoretical findings show a flat adsorption geometry for both pentacene and DPDI molecules. For what concerns the electronic structure, a strong rehybridization of the molecular electron states is found in the range of the occupied $\pi$ states. These results lead to an interpretation of the adsorption mechanism of pentacene in terms of a coupling intermediate between the physi- and the chemi-sorption regimes. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J12.00015: Highly Sensitive Molecular Sensing on Optimally Modified Graphene Monolayers John Russell, Petr Kral We develop a methodology of molecular sensing on optimally modified graphene monolayers. The idea is to modify the monolayers by atomistic (substitution) doping or by covalent binding of short charged ligands at selected positions in such a way that the created local electric fields form selective molecular nests for inorganic, organic and biological molecules [1]. We show by molecular dynamics simulations the nesting of small organic molecules and peptides on modified graphene ribbons. The nested molecules can be detected by electronic, optical or vibrational means [2]. The method is highly sensitive and, at low temperatures, it can even distinguish the configurations of the nested molecules.\\[3pt] [1] B. Wang and P. Kr\'{a}l, Small 3, 580 (2007).\\[0pt] [2] J. Russell and P. Kr\'{a}l, in preparation. [Preview Abstract] |
Session J13: Numerical Methods for Strongly Correlated Systems: Hubbard and Quasi-Particle
Sponsoring Units: DCOMPChair: James Davenport, Brookhaven National Laboratory
Room: 309
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J13.00001: Evaluation of time-resolved photoemission spectra from nonequilibrium, time-domain Green functions B. Moritz, T. P. Devereaux, H. R. Krishnamurthy, J. K. Freericks Recent experiments have shown the power of femtosecond time-resolved, pump-probe photoelectron spectroscopy to probe, directly, the nonequilibrium, real-time dynamics of excitations in a correlated material. We use nonequilibrium dynamical mean-field theory to study the spinless Falicov-Kimball model driven (pumped) out of equilibrium by a constant electric field turned on at $t=0$. We demonstrate the proper evaluation of the time-resolved photoemission intensity as a function of pump-probe delay for both metallic and Mott-insulating phases of the model and the dependence of the intensity profile on the specific details of the probe pulse's lineshape and duration. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J13.00002: Mott-like behavior in the pseudogap region of the Hubbard model Dimitrios Galanakis, Karlis Mikelsons, Ehsan Khatami, Mark Jarrell, Alexandru Macridin, Michael Ma, Juana Moreno We study the phase diagram of the two-dimensional Hubbard model using the Dynamical Cluster Approximation (DCA) in conjunction with the weak-coupling continuous time quantum Monte Carlo (CTQMC) as the cluster solver. We verify the existence of a quantum critical point at a finite electron doping which separates a fermi liquid region at low electron doping from the pseudogap region at high electron doping \footnote{Vidhyadhiraja et. al., arXiv:0809.1477v1}. In the pseudogap region the double occupancy, the two particle correlation functions and spectra reveal a synergism between the development of moment formation and the appearance of short ranged order. We discuss the connection between our results and experiments. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J13.00003: Pairing instabilities and Bose condensation in Hubbard nanoclusters Gayanath Fernando, Kalum Palandage, Armen Kocharian, James Davenport Pairing instabilities found from exact studies of small Hubbard clusters with different topologies appear to provide answers to some long standing puzzles. Electronic charge and spin pairing instabilities in a phase space defined by temperature, magnetic field and chemical potential, lead to properties that are remarkably similar to correlated, inhomogeneous bulk systems such as the high temperature superconductors and colossal magnetoresistance materials. In particular, the role of square- planar geometry is borne out when the vertex coupling in an octahedron is shown to have a detrimental effect on the negative charge and positive spin gaps, which are favorable to forming a Bose condensate in the region of instability. In addition, it is shown that magnetic flux can get trapped in stable minima at half integral units of the flux quantum. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J13.00004: Functional renormalization group beyond the static approximation and its application to two-dimensional Hubbard model. Hirokazu Takashima, Ryotaro Arita, Kazuhiko Kuroki, Hideo Aoki While the functional renormalization group(fRG) is a powerful theoretical method for strongly correlated electron systems which treats diagrams systematically within a framework of quantum field theory, the static approximation is adopted where the Matsubara frequency dependence of the four-point coupling and renormalization for the self-energy are ignored. Here we propose a method to go beyond the static approximation by devising an efficient parameterization for the four-point coupling in the Matsubara frequency space, which is combined with a previous improved algorithm of ours[1] based on a Cartesian box discretization. [1] H. Takashima, R. Arita, K. Kuroki, and H. Aoki, to be published in J. Phys.: Conference Series,\textit{ LT25PROC484.} [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J13.00005: Parquet approximation calculation for the 2D Hubbard model Shuxiang Yang, Herbert Fotso, Jun Liu, Mark Jarrell, Eduardo D'Azevedo, Thomas Maier, Karen Tomko, Richard Scalettar, Thomas Pruschke We present a numerical solution of the parquet approximation on a half-filled 4x4 Hubbard cluster. The parquet formalism is a two-particle self consistent set of equations relating the reducible, irreducible, and fully irreducible vertieces. The simplest approximation from this formalism is the so-called parquet approximation, in which the fully irreducible vertex is approximated by the bare interaction. Our results are compared with results from Self-Consistent 2nd-order approximation, Fluctuation Exchange (FLEX) approximation and the Determinental Quantum Monte Carlo (DQMC) calculation. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J13.00006: Spatially inhomogeneous phase in the two-dimensional repulsive Hubbard model Chia-Chen Chang, Shiwei Zhang Using recent advances in the constrained-path auxiliary-field quantum Monte Carlo method, we study the ground state of the two-dimensional, single-band Hubbard model at intermediate interactions ($2\le U/t \le 8$). In the first part of this study [1], we have determined the equation of state and also calculated the spin-spin correlation functions in square lattices up to size $16\times 16$. Shell effects are eliminated and finite-size effects are greatly reduced by boundary condition integration. It was shown that, upon doping, the system separates into a region with antiferromagnetic (AF) order and a hole-containing region without AF order. In the second part, we study rectangular supercells up to $8\times 64$ to examine the nature of this inhomogeneous phase, in particular to probe phase separation versus stripes and spin-density waves of long wave lengths. [1] Chia-Chen Chang and Shiwei Zhang, Phys. Rev. B 78, 165101 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J13.00007: Inhomogeneous ground state in the Hubbard model: a mean-field study Jie Xu, Chia-Chen Chang, Eric J. Walter, Shiwei Zhang We report unrestricted Hartree-Fock (UHF) results for the ground state of the single-band Hubbard model in two- and three-dimensions with repulsive onsite interaction and nearest-neighbor hopping. At half-filling, the Hartree-Fock (HF) approach is sufficient to capture the basic physics of long-range antiferromagnetic order. Away from half-filling, many earlier HF calculations have been performed in the 2-D Hubbard model, which indicated the formation of domain walls and stripes. We numerically solve the self-consistent UHF equations for a range of densities at weak and intermediate interaction strengths. An annealing scheme coupled with multiple initial configurations is adopted to reach the global minimum. Our goal is to contrast the UHF ground state in the Hubbard model and the HF spin-density wave states in the continuum (jellium) [1]. A second goal is to obtain quantitative information of the UHF ground state for examination by accurate many-body methods such as quantum Monte Carlo. [1] A. W. Overhauser, Phys. Rev. 128, 1437 (1962); Shiwei Zhang and D.M.Ceperley, Phys. Rev. Lett. 100, 236404 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J13.00008: Discontinuous quenching of quasi-particle states in nonequilibrium dynamical mean-field theory Ryan Heary, Jong Han In an effort to model strongly correlated heterojunctions in nonequilibrium we construct a nonequilibrium dynamical mean-field theory for the Hubbard model where each lattice site is a superposition of a left-moving and right-moving state. The left and right movers have the respective chemical potentials, $\mu_L=\frac{\Phi}{2}$ and $\mu_R=-\frac{\Phi}{2}$, where $\Phi$ is the chemical potential bias. The quasi- particle properties are calculated as a function of the Coulomb interaction, $U$, and $\Phi$. As the chemical potential bias is turned on we find that the quasi-particles become strongly renormalized. When $U_d [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J13.00009: Self-consistent solution for the Hubbard model at the two-particle and one-particle level using the parquet formalism. Herbert Fotso, Shuxiang Yang, Jun Liu, Mark Jarrell, Eduardo D'Azevedo, Thomas Maier, Karen Tomko, Richard Scalettar The parquet Formalism is used to solve self-consistently, both at the one-particle and at the two-particle levels, the Hubbard model on a 2-D square lattice. The parquet equation and the Bethe-Salpeter equation are combined into one Newton fixed point problem which is then solved by taking advantage of the existing linear solvers such as GMRES and BiCGStab. Some quantities of interest are calculated and the results are compared to those of Determinental Quantum Monte Carlo (DQMC). We also discuss the importance of this work in the multiscale treatment of the High Tc Cuprates. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J13.00010: Quantum Monte Carlo study of few-electron concentric double quantum rings Leonardo Colletti, Francesc Malet, Marti Pi, Francesco Pederiva We consider few-electron concentric double quantum rings with parabolic confining potential and compare the ground-state energies calculated by exact diagonalization of the Hamiltonian, accurate quantum Monte Carlo and local spin-density functional approaches. Electronic localization in one of the rings and the formation of rotating Wigner molecules is shown respectively from the one-body and the two-body density operators. As the confinement strength is finely increased, the circularly-symmetric electron density exhibits a radial crossover from the outer ring to the inner one without altering the angular character of the system. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J13.00011: Accurate Determination of Tensor Network State of Quantum Lattice Models in Two Dimensions Tao Xiang, H.C. Jiang, Z.Y. Xie, Q.N. Chen, Z.Y. Weng We have proposed a novel numerical method to calculate accurately physical quantities of the ground state using the tensor network wave function in two dimensions. The tensor network wave function is determined by an iterative projection approach which uses the Trotter-Suzuki decomposition formula of quantum operators and the singular value decomposition of matrix. The norm of the wave function and the expectation value of a physical observable are evaluated by a novel second renormalization group method of tensors. Our method allows a tensor network wave function with a high bond degree of freedom to be handled accurately and efficiently in the thermodynamic limit. For the Heisenberg model on a honeycomb or square lattice, our results for the ground state energy and the staggered magnetization agree well with those obtained by the quantum Monte Carlo and other approaches. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J13.00012: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J13.00013: Electronic structure of La$M$O$_3$ (M=Ti$\sim$Cu) by GW approximation Yoshiro Nohara, Takeo Fujiwara We investigate the electronic structure of La$M$O$_3$ ($M=$Ti$\sim$Cu) by GW approximation. The calculated spectra show good agreement with the experimentally observed ones. The on-site Coulomb interaction are affected by strong screening mechanism in trivalent transition metal ion systems, which is qualitatively different from those in mono-oxides $M$O of divalent transition metals. In trivalent transition metal ion systems La$M$O$_3$, 3d electrons are affected by deep atomic potential. Therefore, the 3d orbital locates energetically much nearer to O 2p levels than in $M$O. Moreover, in the cases of $M^{3+}$=Cr$^{3+}$, Mn$^{3+}$ and Fe$^{3+}$ systems, transition metal ions are well spin-polarized, and 3d levels locate very near to O 2p levels. As a result, these systems have large screening effects due to the extended d-electrons. In the cases of $M^{3+}$=Ni$^{3+}$ and Cu$^{3+}$, the systems are metallic and are affected by strong screening effects. In the other cases of $M^{3+}$= Ti$^{3+}$, V$^{3+}$, and Co$^{3+}$, there are small screening effects causing large static screened Coulomb interaction. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J13.00014: GW correlation effects on the quasiparticle energies of Np and Pu Athanasios Chantis, Robert Albers, Axel Svane, Niels Christensen, Mark van Schilfgaarde, Takao Kotani We present results for the electronic structure of plutonium and neptunium by using a recently developed quasiparticle self-consistent $GW$ method (QS$GW$). The self-consistent $GW$ quasiparticle energies are compared to those obtained within the Local Density Approximation (LDA) for several volumes of the unit cell. The goal of the calculations is to understand systematic trends in the effects of electronic correlations on the quasiparticle energy bands as a function of the localization of the $f$ orbitals. We show that correlation effects narrow the $f$ bands in two significantly different ways. Besides the expected narrowing of individual $f$ bands (flatter dispersion), we find that an even more significant effect on the $f$ bands is a decrease in the crystal-field splitting of the different bands. We discuss how these changes affect the topology of the Fermi surface and we demonstrate the importance of the quasiparticle self-consistency scheme in obtaining these results. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J13.00015: Ground state properties of simple solids from GW calculations. Andrey Kutepov, Sergey Savrasov A novel self-consistent implementation of Hedin's GW perturbation theory is presented. This finite-temperature implementation uses Hartree-Fock wave functions to represent Green's function. Hartree-Fock equations are solved with full potential linear augmented plane wave (FLAPW) method at each iteration of a self-consistent cycle. With our approach we are able to calculate total energy and the ground state properties. As an example, the ground state properties of Na, Al, Si, and GaAs obtained with our new code are presented. [Preview Abstract] |
Session J14: Jammed Particles
Sponsoring Units: DFDChair: Corey O'Hern, Yale University
Room: 315
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J14.00001: Porosity of mixed granular media of hard and soft grains Emilie Verneuil, Douglas J. Durian The addition of soft particles to granular materials modifies the packing properties such as the volume fraction and the interconnection of pores as a consequence of the particles squishiness. A macroscopic property that depends on the local arrangement of the grains is the hydraulic conductivity. Hence, hydrogel particles are developed as additives to sandy soils to improve the irrigation efficiency by decreasing the rate of far depth infiltration. However the parameters that control the mixed material porosity have not been explored. Our experimental study of the flow properties of mixtures of glass beads and swollen hydrogels aims at deriving simple arguments to connect the macro-scale measurement of the hydraulic conductivity to the arrangement of the grains around the soft particles, which determines the fraction of blocked pores. Our results show that the porosity decreases with the number of swollen gel per unit volume of the mixture. The conductivity also decreases as the size ratio of gel to glass bead decreases down to 1. A simple description accounting for the elastic contacts between glass beads and gel surface qualitatively accounts for the data. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J14.00002: Experimental characterization of microstate probabilities in mechanically stable packing of frictionless disks. M.D. Shattuck, G.-J. Gao, J. Blawzdziewicz, C.S. O'hern We report on a new experimental technique to produce mechanically stable packings of frictionless disks. The system consists of a quasi-2D vertical cell filled with bi-disperse disks. The disks are vigorously shaken and then allowed to settle under gravity in the presence of high-frequency low-amplitude vibrations to eliminate frictional effects. For a system of 7 particles we find approximately 1000 mechanically stable states. The most probable states occur at least $10^6$ times more often than those that are least probable. This is in direct contradiction to the fundamental postulate in statistical mechanics, that all possible microstates are equally probable and calls into question granular theories based on this assumption. We have measured the frequency distribution of the states in the experiments and in corresponding discrete element simulations, and find excellent agreement. We have also examined how the microstate distribution scales with system size and will connect the microstates to macroscopic quantities such as the density to predict the statistics of macroscopic properties. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J14.00003: Spectral responses in granular compaction Ling-Nan Zou I study the compaction of a granular pack under periodic tapping. The magnitude of acceleration $\Gamma$ at each tap is modulated with frequency $\omega$ and amplitude $\delta\Gamma$: $\Gamma(t) = \Gamma_{\mathrm{DC}} + \delta \Gamma \sin(\omega t)$, where $t$ is time measured by the number of taps. From the temporal modulation $\delta v$ in packing volume $v$, frequency- locked to the modulated tapping input, we can define the real and imaginary volume susceptibilities $\chi_{v}' = (\delta v/\delta \Gamma) \cos \theta$ and $\chi_{v}'' = (\delta v/\delta \Gamma) \sin \theta$; here $\theta$ is the phase lag between $\Gamma(t)$ and $v(t)$. As a function of $\Gamma_{\mathrm{DC}}$, $\chi_{v}'$, $\chi_{v}''$ are peaked at low $\Gamma_{\mathrm{DC}}$, a behavior reminiscent of the temperature-dependent susceptibilities in dielectric and spin glasses. For the packing of small particles ($d = 0.5$ mm) in ambient pressure, $\chi_{v}'$ exhibits memory and rejuvenation effects under $\Gamma_{\mathrm{DC}}$ cycling, similar to that seen in the magnetic susceptibility of spin glasses when subjected to thermal cycling [1]. However this memory effect is suppressed for the packing of larger particles and in vacuum. The measurement of volume susceptibilities shows promise as a new way to study the packing of granular materials, and as an avenue to explore analogies between jammed grains and molecular and spin glasses. \\[3pt] [1] K. Jonason \textit{et al.}, Phys. Rev. Lett. \textbf{81}, 3243 (1998). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J14.00004: Deformed Droplets in Static Two-Dimensional Emulsions Pearl J. Young, Dandan Chen, Eric R. Weeks We confine oil-in-water emulsions between two parallel plates, so that the droplets are essentially squeezed into quasi two-dimensional disks, somewhat analogous to granular photoelastic disks. By varying droplet area fraction, we seek to quantify the jamming transition of this static system. At a critical area fraction, the composition of the system should no longer be characterized primarily by circular disks but by disks deformed to varying degrees. We study a system of toluene droplets in water. As expected, we find that an increase in area fraction corresponds with an increase in average droplet deformation. Further, an increase in average droplet deformity corresponds with an increase in the heterogeneity of deformity within a given sample. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J14.00005: Jamming of Rod-like Granular Materials in Hoppers Summer Saraf, Scott Franklin Long thin rods form solid plugs that are far more rigid than piles of ordinary sand, greatly affecting their ability to flow through small openings. We have built a hopper whose aperture, angle, and width can be independently varied and are studying the frequency with which rods of different length, width, and aspect ratio jam. As the opening aperture becomes larger, the mean number of particles that exit the hopper before a jam occurs naturally increases, but the probability distribution of fluctuations about this mean is unchanged. Unexpectedly, whereas the event distribution function $P(s)$ for spheres decays exponentially, we find the distribution for rods falls off as a power law with exponent $\alpha $=-1.41$\pm $0.08. We are also investigating the growth of the mean event size <$s$> as the aperture increases for possible divergence, which would imply a critical aperture size above which particles would never jam. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J14.00006: Forces and displacements near the granular jamming threshold Mahesh Bandi, Andras Libal, Michael Rivera, Robert Ecke We experimentally study the dynamics of jamming by dragging a probe disk in a two-dimensional bi-dispersed system of randomly packed photo-elastic disks. All measurements are made at packing fractions relative to the critical fraction at which jamming occurs. We measure the local force felt by the probe disk and compare it with the system's global response with sensors placed along the system boundaries. We also visually monitor the disk displacements in the system, which are expected to become increasingly constrained as a function of increasing packing fraction. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J14.00007: The angoricity describes the approach to the jamming Kun Wang, Chaoming Song, Ping Wang, Hernan Makse The application of concepts from equilibrium statistical mechanics to out of equilibrium systems has a long history offering the fascinating possibility to describe a diverse range of systems from glasses to grains. For jammed systems, the key idea was to replace the energy ensemble describing conservative systems by the volume ensemble for dissipative jammed systems. However, this approach is not able to describe the jamming critical point for deformable particles such as emulsions where the volume fraction, coordination number and elastic moduli behaves as power-law of the external stress as the system approaches jamming. The geometrical considerations have to be augmented by the ensemble of stresses described by the angoricity which replaces the role played by the temperature in thermal systems. Here we perform a basic test of the stress ensemble of jammed matter by following two independent approaches: we exhaustively enumerate the available jammed states and numerically follow the dynamics of the system near the jamming point. A direct comparison between both methods supports the idea of thermalization at a given angoricity which is shown to determine the systems state as it approaches the jamming transition. This result opens the possibility to calculate important quantities near J-point. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J14.00008: Equilibration in model granular subsystems: An experimental test for Edwards' compactivity Frederic Lechenault, James Puckett, Karen Daniels We experimentally investigate the statistical features of the stationary states reached by two idealized granular liquids able to exchange volume. The system consists in two binary mixtures of the same number (and area) of soft disks, but with different surface properties. The disks sit on a horizontal air table and are separated by a mobile wall. Energy is injected in the system by means of an array of randomly activated coil bumpers standing as the edges of the cell. Due to the energy injection, the system acts like a slow liquid and eventually jams at high packing fraction. We characterize the macroscopic states by studying the motion of the piston. We find that its average position is different from one half, and is a non monotonic function of the overall packing fraction, which reveals the crucial role played by the surface properties in the corresponding density of states. We then study the bulk statistics of the packing fraction and find confirmation of the macroscopic behavior. However, the local fluctuations of the packing fraction are uniquely determined by its average, and hence independent of the interaction between disks. This result, together with the existence of a point at which the two sub-systems have the same volume, enables us to show that Edwards' compactivity does not have the same value in the two equilibrated subsystems. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J14.00009: Stability of Packings of Soft Elliptical Grains in 2D Mitchell Mailman, Bulbul Chakraborty, Carl Schreck, Corey O'Hern Simulations of hard ellipse packings show that these ellipse packings are generally hypostatic. By using a dynamical matrix approach to analyzing the stability of two-dimensional ellipse packings, we show that the degree of hypostaticity is related to the fraction of zero-frequency modes. The packings are generated using a compression protocol previously employed in disk packings and an energy function based on the overlap model developed by Perram and Wertheim. The density of states exhibits a low frequency peak that approaches zero as the compression is reduced. There is a gap separating this peak from the higher frequency modes. In this talk, we will demonstrate the existence of a scaling relation between the vibrational spectra at different aspect ratios. We will also discuss the origin of the low frequency modes and the origin of the scaling. Analysis of the relationship between contact numbers and vibrational modes will be used to compare and contrast the jamming transition in disks and ellipses. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J14.00010: Shearing dynamics and jamming density Peter Olsson, Daniel V{\aa}gberg, Stephen Teitel We study the effect of a shearing dynamics on the properties of a granular system, by examining how the jamming density depends on the preparation of the starting configurations. Whereas the jamming density at point J was obtained by relaxing \emph{random} configurations [O'Hern et al, Phys.\ Rev.\ E 68, 011306 (2003)], we apply this method to configurations obtained after shearing the system at a certain shear rate. We find that the jamming density increases somewhat and that this effect is more pronounced for configurations produced at smaller shear rates. Different measures of the order of the jammed configurations are also discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J14.00011: Theory of Elasticity and Glassy Dynamics of Suspensions of Soft Particles Jian Yang, Kenneth Schweizer A microscopic theory for the shear modulus and slow dynamics of soft colloidal systems composed of many arm star-polymers and intra-molecularly crosslinked microgels is described. The role of particle volume fraction and softness (arm number for stars and contact modulus for microgel particles) on the ideal mode coupling kinetic arrest transition, elastic modulus, relaxation time in the activated hopping regime, diffusion constant, dynamic fragility, and absolute yield stress and strain have been systematically explored. The low-frequency shear modulus is characterized by two volume fraction regimes: power law scaling at intermediate volume fractions and a linear law beyond the nominal jamming point. Connections between single particle softeness, interparticle packing correlations, and viscoelastic properties have been established. For both microgels and many arm stars, the effective dynamical fragility varies over a wide range as a function of particle softness. Comparisons of the theoretical results with experiments on many arm star and microgel paste systems have been carried out. [Preview Abstract] |
Session J15: Quantum Entanglement
Sponsoring Units: GQIChair: Christopher Fuchs, Perimeter Institute
Room: 316
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J15.00001: Understanding Entanglement as a Resource for Quantum Information Processing Scott M. Cohen Ever since Erwin Schrodinger shocked the physics world by killing (and not killing) his cat, entanglement has played a critical role in attempts to understand quantum mechanics. More recently, entanglement has been shown to be a valuable resource, of central importance for quantum computation and the processing of quantum information. In this talk, I will describe a new diagrammatic approach to understanding why entanglement is so valuable, the key idea being that entanglement between two systems ``creates'' multiple images of the state of a third. By way of example, I will show how to ``visualize'' teleportation of unknown quantum states, and how to use entanglement to determine the (unknown) state of a spatially distributed, multipartite quantum system. Illustrative examples of this entanglement-assisted local state discrimination are sets of orthogonal product states exhibiting what is known as ``non-locality without entanglement'', including unextendible product bases. These ideas have also proven useful in using entanglement to implement a unitary interaction between spatially separated (and therefore non-interacting!) systems. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J15.00002: Genuine tripartite entanglement and nonlocality in 3-qubit GHZ-class states Shohini Ghose, Neil Sinclair, Shantanu Debnath, Rene Stock, Pranaw Rungta Multiqubit entanglement is a crucial ingredient for large-scale quantum information processing and has been the focus of several recent studies. Entanglement between qubits can lead to violations of Bell-type inequalities that are satisfied by local hidden variable models, indicating the nonlocal nature of the correlations between qubits. For 2-qubit pure states, bipartite entanglement is simply related to the Bell-CHSH nonlocality parameter. No such analytical relation between multipartite entanglement and nonlocality has yet been obtained for systems of three or more qubits. We have derived relationships between genuine tripartite entanglement and nonlocality for families of 3-qubit GHZ-class pure states. We quantify tripartite entanglement by the 3-tangle and derive its relationship to the Svetlichny inequality for testing tripartite nonlocality. For the class of generalized GHZ states, although the 3-tangle is always non-zero, we identify some states that do not violate the Svetlichny inequality. Furthermore, we show that states known as the maximal slice states always violate the Svetlichny inequality and analogous to the 2-qubit case, the amount of violation increases with the 3-tangle. The generalized GHZ states and the maximal slice states have unique tripartite entanglement and nonlocality properties in the set of all pure states. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J15.00003: Quantum Noise as an Entanglement Entropy Meter. Israel Klich, Leonid Levitov Entanglement entropy, which is a measure of quantum correlations between separate parts of many-body system, is defined solely in terms of the many-body density matrix, with no relation to any particular observables. Because of that, it has not been clear how to access this quantity experimentally. Here we unveil a universal relation between entanglement entropy of fermions and statistics of current flowing through a quantum point contact. This relation provides a way to experimentally measure entanglement entropy, and test seminal results of conformal field theory such as the prediction of Holzhey, Larsen and Wilczek for entanglement entropy of fermions. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J15.00004: Coherent Light Fields for Entanglement based Quantum Communication Kim Fook Lee Nonlocal polarization correlations of two distant observers based on Stapp's formulation are observed by using coherent light fields. Using a 50/50 beam splitter transformation, a vertically polarized coherent light field is entangled with a horizontally polarized coherent noise field. The superposed light fields at each output port of the beam splitter are sent to two distant observers, where the fields are interfered and manipulated at each observer by using a quarter wave plate and an analyzer. The interference signal contains information of the projection angle of the analyzer, which is hidden by the phase noises. The nonlocal correlations between the projection angles of two distant observers are established by analyzing their data through analog signal multiplication without any post-selection technique. This scheme can be used to implement Ekert's protocol for quantum key distribution. The implementation of two independent coherent states in this scheme is also discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J15.00005: Quantum-entanglement aspects of polaron systems Vladimir Stojanovic, Mihajlo Vanevic We describe quantum entanglement inherent to the polaron ground states of coupled electron-phonon (or, more generally, particle- phonon) systems based on a model comprising both local (Holstein-type) and nonlocal (Peierls-type) coupling. We study this model using a variational method supplemented by the exact numerical diagonalization on a system of finite size. By way of subsequent numerical diagonalization of the reduced density matrix, we determine the particle-phonon entanglement as given by the von Neumann and linear entropies. Our results are strongly indicative of the intimate relationship between the particle localization/delocalization and the particle-phonon entanglement. In particular, we find a compelling evidence for the existence of a non-analyticity in the entanglement entropies with respect to the Peierls-coupling strength. The occurrence of such non-analyticity -- not accompanied by an actual quantum phase transition -- reinforces analogous conclusion drawn in several recent studies of entanglement in the realm of quantum- dissipative systems. In addition, we demonstrate that the entanglement entropies saturate inside the self-trapped region where the small-polaron states are nearly maximally mixed. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J15.00006: Entanglement genesis under continuous parity measurement Andrew Jordan We examine the stochastic dynamics of entanglement for an uncoupled two-qubit system, undergoing continuous parity measurement. Starting with a fully mixed state, the entanglement is zero for a finite amount of time, when it is suddenly created, which we refer to as entanglement genesis. There can be further entanglement sudden death/birth events culminating in the formation of a fully entangled state. We present numerical investigations of this effect together with statistics of the entanglement genesis time in the weak measurement limit as well as the quantum Zeno limit. An analytic treatment of the physics is presented, aided by the derivation of a simple concurrence equation for Bell basis X-states. The probability of entanglement border crossing and mean first passage times are calculated for the case of measurement dynamics alone. We find that states with almost the same probability of entanglement border crossing can have very different average crossing times. Our results provide insights on the optimization of entanglement generation by measurement. Reference: N. S. Williams and A. N. Jordan, arXiv:0809.3248 [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J15.00007: Interferometric Determination of Concurrence of Unknown Two-Qubit Entanglement S.-S. B. Lee, H.-S. Sim We propose a scheme for both distilling and quantifying entanglement, applicable to individual copies of an arbitrary unknown two-qubit state. It is realized in a usual two-qubit interferometry with local filtering. Proper filtering operation for the maximal distillation of the state is achieved, by erasing single-qubit interference, and then the concurrence of the state is determined directly from the visibilities of two-qubit interference. For some representative quantum states, the efficiency is compared between our interferometric scheme and the full state tomography. For some states, our scheme is revealed to be more efficient than the tomography. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J15.00008: Scattering approach to the entanglement entropy area law for fermions Gregory Levine The entanglement entropy (EE) of a critical fermion system coupled to another system by a ``weak link'' is studied perturbatively in the weak link amplitude, $w$. In this model, EE arises from $s$-wave scattering connecting the two subsystems and is computed from perturbative corrections to the subsystem correlation function. The first non-vanishing contribution to the EE, appearing at $O(w^2)$, may be evaluated analytically and is shown to diverge as $w^2 \ln^2{L}$, where $L$ is the linear subsystem size. A generalized version of this model containing many independent weak links is discussed in connection with the entropy area law for fermions. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J15.00009: Mutual information and compactification radius in a c=1 critical phase in one dimension Shunsuke Furukawa, Vincent Pasquier, Jun'ichi Shiraishi We investigate the generic scaling of the mutual information in a class of one-dimensional quantum critical systems described by a bosonic field theory with a central charge $c=1$. A numerical analysis of a spin-chain model reveals that the mutual information is scale-invariant and depends on the compactification radius (or the Tomonaga-Luttinger parameter) of the bosonic field, as opposed to the general prediction of Calabrese and Cardy. Interpretations of the results are given in terms of branch-point twist fields. The present study provides a new way to determine the compactification radius, and furthermore demonstrates the ability of the mutual information to distinguish different conformal field theories with the same central charge. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J15.00010: Universal behavior of the entanglement entropy in 2D conformal quantum critical points and generalized quantum dimer models Benjamin Hsu, Michael Mulligan, Eduardo Fradkin, Eun-Ah Kim We study the scaling behavior of the entanglement entropy of two dimensional conformal quantum critical systems, {\it i.e.\/} systems with scale invariant wave functions. They include two-dimensional generalized quantum dimer models on bipartite lattices and quantum loop models, as well as the quantum Lifshitz model and related gauge theories. We show that, under quite general conditions, the entanglement entropy of a large and simply connected sub-system of an infinite system has a universal contribution which is independent of the size of the region. This universal contribution is computable in terms of the properties of the underlying large-scale conformal structure of the wave function of these quantum critical systems. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J15.00011: Theory of finite-entanglement scaling at one-dimensional quantum critical points Frank Pollmann, Subroto Mukerjee, Ari Turner, Joel Moore We present a quantitative scaling theory of finite-entanglement approximations at one-dimensional quantum critical points. Finite-entanglement scaling is governed not by the scaling dimension of an operator but by the ``central charge'' of the critical point, which counts its universal degrees of freedom. An important ingredient is the universal distribution of density-matrix eigenvalues (the ``entanglement spectrum'') at a critical point recently obtained by Calabrese and Lefevre. The theory is compared to the numerical error scaling of several quantum critical points, obtained by the infinite Time Evolved Block Decimation (iTEBD) method that extends the conventional Density-Matrix Renormalization Group (DMRG) algorithm. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J15.00012: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J15.00013: Entanglement under the renormalization-group transformations on quantum states and in quantum phase transitions Tzu-Chieh Wei We consider the entanglement of states under the renormalization-group (RG) transformations and apply it to the ground states of Hamiltonians that possess quantum phase transitions. We find that near critical points, the ground-state entanglement under RG transformation exhibits singular behavior. The singular behavior under finite steps of RG reveals the correlation length exponent. However, under the infinite steps of RG transformation, the singular behavior is rendered different, and it is not universal unless the critical point can be described by a conformal field theory. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J15.00014: Comparisons of entanglement witnesses for n-qubit systems Richard Bonde, Andrew Schauf, Elizabeth Behrman An overlooked problem in witness design is the possibility of phase offset contamination. For example, the singlet EPS 2-qubit state $\frac{1}{\sqrt 2 }\left( {\left| {\uparrow \downarrow } \right\rangle -\left| {\downarrow \uparrow } \right\rangle } \right)$differs from $\frac{1}{\sqrt 2 }\left( {\left| {\uparrow \downarrow } \right\rangle +\left| {\downarrow \uparrow } \right\rangle } \right)$only by a relative phase factor of $e^{i\pi }$, yet both states are fully entangled. We compare in detail several published witnesses on entangled pure and mixed systems with varying degrees of phase offset. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J15.00015: ABSTRACT WITHDRAWN |
Session J16: Focus Session: Disorder in Ultra-Cold Gases
Sponsoring Units: DAMOPChair: Maxim Olshanyi, University of Massachusetts, Boston
Room: 317
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J16.00001: The Role of Interactions in Disorder Induced Damping of Dipole Oscillations of a Bose-Einstein Condensate Scott Pollack, D. Dries, T.A. Corcovilos, R.G. Hulet We investigate the damping of dipole oscillations of a $^7$Li Bose-Einstein condensate (BEC) in a disordered optical potential. In our highly tunable system we vary the disorder strength $V_D$, the initial velocity of the BEC, and the chemical potential $\mu$ by adjusting the $s$-wave scattering length $a$ via a Feshbach resonance. We observe the breaking of superfluid flow, for values of $V_D$ as small as $0.1\,\mu$, and cessation of motion for $V_D \sim \mu$. Counter-intuitively, at supersonic velocities the flow becomes asymptotically dissipationless regardless of the disorder strength. We test the validity of the scaling $V_D/\mu$ over several decades of $a$, including values of $a$ as small as $0.01\,a_0$, where magnetic dipole effects dominate. We also report on observations of dissipative flow of nearly non-interacting ideal quantum gases and bright matter-wave solitons. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J16.00002: Disorder effects in the evolution from BCS to BEC superfluidity Li Han, Carlos A. R. Sa de Melo We discuss the effects of disorder on the critical temperature of superfluids during the evolution from BCS to BEC. For s-wave superfluids we find that the critical temperature is weakly affected by disorder in the BCS regime as described in Anderson’s theorem, even less affected by disorder at zero chemical potential (near unitarity), but strongly affected by disorder in the BEC regime, where Anderson's theorem does not apply. This suggests that the superfluid is more robust to the effects of disorder at the interaction parameter where the chemical potential vanishes (close to unitarity). We construct a three dimensional phase diagram of critical temperature, disorder and interaction parameter [1], and show that there are regions of localized superfluidity, as well as insulating regions due to Anderson localization of fermions (BCS regime) and molecular bosons (BEC regime). The phase diagram for higher angular momentum (e.g. p-wave and d-wave) is also analyzed, where the effects of disorder are much more dramatic in the BCS regime in comparison to the s-wave case because pair breaking is strong, while the disorder effects in BEC regime are similar to what occurs in the s-wave case. \newline [1] Li Han, C. A. R. Sa de Melo, arXiv:0812.xxxx [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J16.00003: Magnetizm Localization and Hole Localization in Fermionic Atoms Loaded on Optical Lattice Masahiko Okumura, Susumu Yamada, Nobuhiko Taniguchi, Masahiko Machida In order to study an interplay of disorder, correlation, and spin imbalance on antiferromagnetism, we systematically explore the ground state of one-dimensional spin-imbalanced Fermionic atoms loaded on an optical lattice by using the density-matrix renormalization group method [1]. We find that disorders localize the antiferromagnetic spin density wave induced by imbalanced fermions and the increase of the disorder magnitude shrinks the areas of the localized antiferromagnetized regions. Moreover, the antiferromagnetism finally disappears above a large disorder. We also study hole doped cases [2]. Concentrating on the doped-hole density profile, we find in a large $U/t $regime that the clean system exhibits a simple fluid-like behavior whereas finite disorders create locally Mott regions which expand their area with increasing the disorder strength contrary to the conventional sense. References [1] M. Okumura, S. Yamada, N. Taniguchi, and M. Machida, arXiv:0810:3953. [2] M. Okumura, S. Yamada, N. Taniguchi, and M. Machida, Phys. Rev. Lett. \textbf{101} 016407 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J16.00004: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J16.00005: Dynamical Effects of Disorder in Optical Lattices M. Beeler, E.E. Edwards, Tao Hong, S.L. Rolston The precise control available in systems of neutral atoms confined in optical lattices makes them an ideal place to investigate the effects of disorder on crystal structure. We experimentally investigate how disorder affects the dynamical properties of these systems. Using a 1D optical lattice with the addition of one or two weak incommensurate lattices, we investigate the adiabaticity criteria for loading the ground state of the disordered lattice. We find that even a very small amount of disorder greatly increases the timescale needed for adiabatic loading. We attribute this change to the large change in the ground state of the wavefunction with the addition of disorder, as the wavefunction becomes localized. In addition, we will report on experimental efforts to study the effects of disorder in two-dimensional systems and on the timescales of thermalization in one dimension. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J16.00006: Impurity crystal in a Bose-Einstein condensate David Roberts, Sergio Rica We investigate the behavior of impurity fields immersed in a larger condensate field.  The conditions for stability and collapse of this system will be presented.  We discuss the localization of a single impurity field within a condensate and note the effects of surface energy.  We derive the functional form of the  attractive interaction between two impurities due to mediation from the condensate in 1, 2, and 3 dimensions. Generalizing the analysis to $N$ impurity fields, we show that within various parameter regimes a crystal of impurity fields can form spontaneously in the condensate.  Finally, we show the system of condensate and crystallized impurity structure to have nonclassical rotational inertia, which is characteristic of superfluidity, i.e. the system can be seen to exhibit supersolid behavior. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J16.00007: Experiments on Disordered Quantum Gases Invited Speaker: |
Session J17: Focus Session: Superconducting Phase Qubits
Sponsoring Units: GQIChair: Rob McDermott, University of Wisconsin
Room: 318
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J17.00001: High-fidelity gates in Josephson phase qubits Invited Speaker: Complex algorithms for a quantum computer require error correction and robust calibration protocols for extended pulse sequences. We present significant progress towards both of these goals with our detailed measurements of gate fidelity and coupled qubit experiments with multi-pulse sequences. We measure single qubit gate fidelities of 0.98, limited by energy relaxation; and by carefully separating out gate and measurement error we construct a complete error budget. Using the new metrological technique of ``Ramsey filtering'' we show how one important error process can be measured and reduced to a level of 10$^{-4}$, a magnitude believed to be near the fault tolerant threshold. This measurement demonstrates that our quantum system remains in the two-state qubit manifold during our single qubit operations. This precision and accuracy is made possible by custom control electronics that can create arbitrarily shaped microwave pulses. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J17.00002: Universal Quantum Gates in Josephson Junction Phase Qubits Radoslaw Bialczak, M. Ansmann, M. Hofheinz, E. Lucero, M. Neeley, A. O'Connell, D. Sank, M. Steffen, H. Wang, J. Wenner, A. Cleland, J. Martinis Josephson junction phase qubits are at a point where they can be used to create more complex operations such as quantum gates. Here we present work where we have tuned capacitively coupled Josephson junction phase qubits on and off resonance to generate and characterize a SQiSW gate using quantum process tomography (QPT). The SQiSW is the most fundamental universal gate for our system because it arises directly from the Hamiltonian for the physical circuit of our coupled qubits. In order to create more complex gates such as the CNOT, the SQiSW gate must be used to generate the entanglement. We perform QPT and obtain the Chi matrix, from which quantitative measures such as the gate fidelities can be calculated. We also show how to correct for measurement crosstalk and reduced visibilities present in our system and we perform measurements that quantitatively characterize the on/off ratio of our coupling scheme. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J17.00003: Engineering Tripartite Entangled States of Two Phase Qubits Coupled via a Cavity Jae Park, Fabio Altomare, Ray Simmonds We present an experimentally inexpensive scheme for preparing certain tripartite entangled states. We suggest ways to test the degree to which such target states have been successfully prepared. We present a convenient geometrical interpretation of the resonant unitary dynamics which gives a natural interpretation for the characteristic frequencies and provides intuition for the pulse sequence necessary to achieve a desired target state. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J17.00004: Two Qubits and a Cavity: Three's Company Fabio Altomare, Michael Allman, Katarina Cicak, Jae A. Park, Mika A. Sillanpaa, Adam Sirois, Joshua Strong, Jed Whittaker, Raymond W. Simmonds Quantum information theory suggests that there are two inequivalent classes of tripartite entanglement under stochastic local operations and classical communications (PRA, 62, 062314). Representative of these classes are the GHZ state and the W states, respectively. In this talk I will describe our experimental results on two superconducting phase qubits coupled through a cavity: one of the few cases where three is company and not a crowd. This system, effectively three coupled qubits if we restrict the cavity excitation to the single photon manifold, has allowed us to observe the spectroscopic signature and dynamics of Tripartite Entanglement. The rich dynamics of this system has allowed us to also observe a) Bell state between two qubits (with the third one disentangled), and b) W state between the three qubits. Future possibilities include the observation of GHZ state, particularly interesting for its practical applications, and for testing the non-locality of quantum mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J17.00005: Relaxation Dynamics of Fock States in a High Q Microwave Resonator Coupled to a Superconducting Phase Qubit Haohua Wang, Max Hofheinz, Markus Ansmann, Radoslaw Bialczak, Erik Lucero, Matthew Neeley, Aaron O'Connell, Daniel Sank, Jim Wenner, Andrew Cleland, John Martinis We have improved the lifetime of our high $Q$ microwave resonator that is coupled to a superconducting phase qubit. Using high speed electronics, we have successfully generated Fock states with up to 15 photons. We analyze the resonator number state using the qubit to verify the high purity of the Fock states. Finally we monitor the subsequent decay of the Fock states in time, and show that the decay matches that expected from theory, with the $n$-photon lifetime scaling as $T_{1}$/$n$, where $T_{1}$ is the one-photon lifetime. Measurements on the decay of the coherent states, generated in the resonator using classical pulses, are also in agreement with theory. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J17.00006: Controllable Coherent Population Transfers in Superconducting Qubits for Quantum Computing L.F. Wei, J.R. Johansson, L.X. Cen, S. Ashhab, F. Nori We propose an approach to coherently transfer populations between selected quantum states in one- and two-qubit systems by using controllable Stark-chirped rapid adiabatic passages. These evolution-time insensitive transfers, assisted by easily implementable single-qubit phase-shift operations, could serve as elementary logic gates for quantum computing. Specifically, this proposal could be conveniently demonstrated with existing Josephson phase qubits. Our proposal can find an immediate application in the readout of these qubits. Indeed, the broken parity symmetries of the bound states in these artificial atoms provide an efficient approach to design the required adiabatic pulses. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J17.00007: Three and Four Coupled Josephson Junction Phase Qubits Zechariah Thrailkill, Joseph Lambert, Sam Kennerly, Roberto Ramos The Josephson junction phase qubit has been shown to be a viable candidate for quantum computation. The two coupled phase qubit system has been extensively studied theoretically and experimentally. We have analyzed the quantum behavior of systems with more, three and four capacitively-coupled phase qubits, with different possible configurations. We have used anharmonic oscillators to model the systems. We will discuss some of the properties of these simple networks. The focus is on natural state evolution using a time independent, or adiabatically changing Hamiltonian. Analyzing how to transfer quantum information from one qubit to another and performing operations to change the overall state of these systems will give a better understanding of how to utilize the different qubit configurations. We will report on the progress of spectroscopic measurements for the three phase qubit systems. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J17.00008: Tunable Cavity QED with Josephson Phase Qubits Joshua Strong We have designed a tunable Josephson resonator and have coupled it to two phase qubits. The resonator can act as a cavity for QED-type experiments. We discuss results. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J17.00009: Emulation of spin dynamics using a superconducting phase qudit Matthew Neeley, M. Ansmann, R. Bialczak, M. Hofheinz, E. Lucero, A. O'Connell, D. Sank, H. Wang, J. Wenner, John Martinis, Andrew Cleland In superconducting quantum circuits, the nonlinearity of the Josephson junction allows energy-level transitions to be addressed individually by their unique frequencies. Typically this is used to operate the system as an effective two-level system, a qubit. In a recent experiment, we extended our coherent control of a phase qubit to the first five energy levels, allowing us to operate the device as a qu{\it d}it with $d = 3$, $4$, or $5$. We use this system to emulate the dynamics of single spins with spin quantum number $s = 1/2$, $1$ and $3/2$. We show that the phase acquired by a spin under rotation around a closed path follows the theoretical prediction. In particular, we confirm the even (odd) parity of integer (half-integer) spins under $2\pi$ rotation. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J17.00010: Multiplexed Phase qubit readout using SQUID-resonators Jed Whittaker, Michael Allman, Fabio Altomare, Katarina Cicak, Dale Li, Jae Park, Adam Sirois, Joshua Strong, Raymond Simmonds Flux biased phase qubits have traditionally been read out using a critical current switching technique of a coupled DC SQUID. This method has three limitations: it is extremely slow (orders of magnitude longer than typical energy relaxation times), difficult to multiplex, and by exceeding the critical current, it is dissipative and feeds broadband radiation back into the qubit, decohering its state. We are developing a SQUID-resonator readout method that addresses all three of these limitations. By operating the SQUID as a resonator, we can measure the state of the qubit quickly (on the order of its coherence time), we can multiplex resonant readout lines, and we can operate on the SQUID's supercurrent branch eliminating dissipation and decohering radiation. This faster, quieter readout should allow us to use measured results for real-time quantum feedback. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J17.00011: Berry's Phase of a Current-Biased Josephson Junction Anthony Tyler, Roberto Ramos, Zechariah Thrailkill, Sam Kennerly A quantum system, prepared in an eigenstate, can accumulate a geometric phase known as Berry's phase in addition to the expected dynamic phase. This occurs when there are adiabatic changes to the Hamiltonian which trace a closed loop in parameter space. A common example of this phase is an electron in a slowly varying magnetic field which traces a closed path. From this adiabatic variation, the electron's spin state has acquired a Berry's phase in addition to the dynamic phase. Due to the similarities between spin-1/2 particles, such as the electron, and solid state quantum bits (qubits), there should be an analogous process by which these system can gain a Berry's phase. Such processes have been tested in the charge qubit and has been derived for the flux qubit. Here, we will derive the Berry's phase for a phase qubit which can be found experimentally using quantum state tomography. We then utilize this to explore the possibility of creating topological gates with phase qubits. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J17.00012: Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic SQUIDs as candidates of high-T$_c$ phase qubits X.Y. Jin, J. Lisenfeld, Y. Koval, A. Lukashenko, A.V. Ustinov, P. M\"uller An intrinsic SQUID is a superconducting ring made of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal, intercepted by two intrinsic Josephson junction stacks. The inductance parameter $\beta_L$ can be tuned in a wide range by changing the height and the cross-section area of the stacks. When biased with dc current, the device showed typical properties of hysteretic dc-SQUIDs. When a device was coupled with a coil and a Nb readout dc-SQUID, typical rf-SQUID behavior was observed. By choosing a proper reset field, quantum escape from a single minimum has been measured on a sample of $\beta_L\sim10$. The escape rate can be fine-tuned by applying short pulses down to 1 ns, which allows a fast readout technique. With these prerequisites achieved, our experiments have opened the path to directly using these intrinsic SQUIDs as high-T$_c$ phase qubits. The first attempts to measure Rabi oscillations on these devices will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J17.00013: GHZ protocols for superconducting qubits Andrei Galiautdinov, John Martinis Superconducting circuits with Josephson junctions gained considerable attention as promising candidates for scalable solid state quantum computing architectures. While macroscopic quantum behavior of such circuits has already been demonstrated (e.g., Rabi oscillations, high fidelity state preparation and measurement, various logic gate operations, etc.), further progress in developing a workable quantum computer will depend crucially on architecture's ability to implement various multiqubit entangled states. Here we show how Greenberger-Horne-Zeilinger states can be generated in tripartite systems with capacitive and inductive couplings. Generalization to architectures containing arbitrary numbers of qubits is also discussed. [Preview Abstract] |
Session J18: Charge Transport and Optical Properties of Organic Semiconductors
Sponsoring Units: DMP DPOLYChair: Markus Wohlgenannt, University of Iowa
Room: 319
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J18.00001: Light Emitting Transistors of Organic Single Crystals Invited Speaker: Organic light emitting transistors (OLETs) are attracting considerable interest as a novel function of organic field effect transistors (OFETs). Besides a smallest integration of light source and current switching devices, OLETs offer a new opportunity in the fundamental research on organic light emitting devices. The OLET device structure allows us to use organic single crystals, in contrast to the organic light emitting diodes (OLEDs), the research of which have been conducted predominantly on polycrystalline or amorphous thin films. In the case of OFETs, use of single crystals have produced a significant amount of benefits in the studies of pursuit for the highest performance limit of FETs, intrinsic transport mechanism in organic semiconductors, and application of the single crystal transistors. The study on OLETs have been made predominantly on polycrystalline films or multicomponent heterojunctions, and single crystal study is still limited to tetracene [1] and rubrene [2], which are materials with relatively high mobility, but with low photoluminescence efficiency. In this paper, we report fabrication of single crystal OLETs of several kinds of highly luminescent molecules, emitting colorful light, ranging from blue to red. Our strategy is single crystallization of monomeric or oligomeric molecules, which are known to have a very high photoluminescence efficiency. Here we report the result on single crystal LETs of rubrene (red), 4,4'-bis(diphenylvinylenyl)-anthracene (green), 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5) (green), and 1,3,6,8-tetraphenylpyrene (TPPy) (blue), all of which displayed ambipolar transport as well as peculiar movement of voltage controlled movement of recombination zone, not only from the surface of the crystal but also from the edges of the crystals, indicting light confinement inside the crystal. Realization of ambipolar OLET with variety of single crystals indicates that the fabrication method is quite versatile to various light emitting molecular solids, providing novel opportunities to get further insight on the intrinsic optoelectronic processes in organic semiconductors. \\[4pt] [1] T. Takahashi et al., Adv. Funct. Mater. 17, 1623 (2007).\\[0pt] [2] T. Takenobu et al., Phy. Rev. Lett. 100, 066601 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J18.00002: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J18.00003: Modification of the electronic properties of rubrene crystals by extrinsic species Leonidas Tsetseris, Sokrates Pantelides The chemical stability of organic semiconductors is one of the most important factors for the performance of related electronic devices. Here, we report the results of first- principles calculations on the effect of some of the most typical defect culprits in the prototype system of rubrene, the current record-holder organic semiconductor in terms of carrier mobilities. We identify the most stable water and oxygen- related impurity structures, with species in either substitutional or interstitial configurations, and we analyze their complex role in changing the shape and profile of rubrene energy bands. In certain cases the impurities either give rise or help annihilate carrier traps. We discuss the relevance of our findings for the optimization of rubrene-based electronic systems, and, in particular, the possibilities for effective defect engineering. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J18.00004: Charge carrier transport and optical properties of SAM-induced conducting channel in organic semiconductors. Vitaly Podzorov Certain types of self-assembled monolayers (SAM) grown directly at the surface of organic semiconductors can induce a high surface conductivity in these materials [1]. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10 to -5 Siemens per square. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors and provides experimental access to the regime of high carrier density. Here, we will discuss temperature variable measurements of SAM-induced conductivity in several types of organic semiconductors. [1]. M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson and V. Podzorov, ``Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers'', Nature Mat. 7, 84 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J18.00005: Infrared spectroscopy of organic semiconductors modified by self-assembled monolayers O. Khatib, B. Lee, V. Podzorov, J. Yuen, A.J. Heeger, Z.Q. Li, M. Di Ventra, D.N. Basov Recently, self-assembled monolayers (SAMs) were used to modify electronic surface properties of organic single crystals, leading to several orders of magnitude increase in the electrical conductivity$^{1}$.~~Motivated by this discovery, the same technique was applied to polymers.~~Here we present a thorough spectroscopic investigation of organic semiconductors based on poly(3-hexlthiophene) (P3HT) that have been treated with a fluorinated trichlorosilane SAM.~~Infrared spectroscopy offers access to details of charge injection, electrostatic doping, and the electronic structure that are not always available from transport measurements, which can be dominated by defects and contact effects.~~In polymer films, the SAM molecules penetrate into the bulk, leading to a rich spectrum of electronic excitations in the mid-infrared energy range.~ $^{1}$~M. F.~Calhoun,~~J. Sanchez, D. Olaya, M. E. Gershenson, V. Podzorov\textit{,~Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers},~Nature Mater$.$~\textbf{7}, 84--89 (2008)~ [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J18.00006: Gate electric-field tuning of contact barriers between metals and organic semiconductor crystals J. Takeya, M. Yamagishi, K. Nakayama, T. Uemura Metal / organic semiconductor junctions are by all means essential in many organic electronics devices such as organic transistors, reasoning importance of understanding microscopic physics of carrier injection at the boundary. In this presentation, we focus on charge conductance through the metal / organic semiconductor contacts and their gate-electric field effects, fabricating structure of interfacing hole-rich rubrene single crystal crystals with metal electrodes. The hole-rich regions are formed at the top surface of the crystals either by field-effect accumulation using secondary gate electrodes or charge transfer from acceptor films of fluoro-silane molecular layers or F4-TCNQ layers. Application of gate voltage on the bottom surface of the crystals has given rise to a very sharp switching in the conductance through the contacts, because of very short (nanoscale) active length for the conductivity modulation. For the mechanism of the result, energy-level tuning between the metals and the hole-rich rubrene surface is suggested as the result of gradual band-bending in the direction of crystalline thickness. The minimum working length of the device is highly advantageous in high-frequency response and densities of the device integration. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J18.00007: Controlling Grain Size in Solution-Processed Organic Semiconductors for Thin-Film Transistors Stephanie Lee, Chang Su Kim, Enrique Gomez, Cheng Wang, Alexander Hexemer, Michael Toney, John Anthony, Yueh-Lin (Lynn) Loo We present a novel method for controlling the grain size in solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) films through the addition of fractional amounts of fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (FTES-ADT). FTES-ADT can seed the crystallization of TES-ADT during solvent-vapor annealing. The grain size in these films follows an exponential dependence on the concentration of FTES-ADT; varying the FTES-ADT concentration by 2-fold induces a 3-order of magnitude change in the grain size. For channels in which the average grain size is 29 $\mu $m, device mobility of the organic thin-film transistors (OTFTs) is 0.05 cm$^{2}$/V-s. For channels in which the average grain size is 2700 $\mu $m, the device mobility is 0.35 cm$^{2}$/V-s. The relationship between device mobility and grain size is well described by a composite mobility model, which assumes a high intrinsic grain mobility and a low grain boundary mobility. Grazing incidence x-ray diffraction indicates that the crystal lattice of TES-ADT is preserved despite the addition of FTES-ADT. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J18.00008: Charge transport in crystalline organic semiconductors: using polymorphs to explore the effect of crystal packing Oana Jurchescu, Devin Mourey, Sankar Subramanian, Sean Parkin, Brandon Vogel, John Anthony, Thomas Jackson, David Gundlach Organic semiconductors are a fascinating class of materials, with a wealth of properties and diverse technological potential. For small-molecule organic semiconductors, charge transport is closely related to the crystal packing motif. Polymorphism is frequently encountered in these materials, given the weak intermolecular interaction energies. This represents a unique opportunity to explore phenomena related to the fundamental mechanism of charge transport in organic semiconductors, such as the influence of the crystal packing. For example, 5,11-bis(triethylsilylethynyl)anthradithiophene has two polymorphs inter-convertible through phase transition that occurs at $T $= 294 K. We report on their crystal structure, formation, and the effect of the different molecular packing on the electronic properties. We discuss the technological implications that a room-temperature phase transition has on the performance and stability of devices fabricated with this material. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J18.00009: A crystalline organic semiconductor grown from a mesophase: A test of polaron band theory Naresh Shakya, Chandra Pokhrel, Brett Ellman, Shin-Woong Kang, Satyan Kumar, Yulia Getmanenko, Robert Twieg We find that the hole mobility of the crystal smectic phases of the liquid crystal 1,4-di-(5-n- tridecylthien-2-yl)-benzene increases exponentially with decreasing temperature. While qualita- tively consistent with transport via polaron bands, we find that it is quantitatively difficult to explain the data with physically realistic parameters. In particular, the data demand either quite large typical optical phonon frequencies and/or phonon bandwidths. We also find evidence that an unusually highly ordered high temperature smectic-F phase templates the formation of crystalline smectic phases, which may have implications for device development. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J18.00010: First-principles study of charge injection and transport through pentacene multilayers Yong-Hoon Kim Applying a combined density-functional theory and matrix-Green's function approach [1,2], I study the coherent charge transport properties of pentacene nanowires sandwiched between Au(111) electrodes. Junction models based on pentacene trilayers in the ideal $\pi $-stacked and herringbone arrangements with the face-on and edge-on contact configurations at different contact distances are considered. I show that pentacene wires exhibit a robust p-type conductance behavior in agreement with experiments, and analyze the physical origin in terms of charge transfer between molecules and metal electrodes. \\[3pt] [1] Y.-H. Kim, S. S. Jang, Y. H. Jang, W. A. Goddard III, Phys. Rev. Lett. \textbf{94}, 156801 (2005). \\[0pt] [2] Y.-H. Kim, J. Tahir-Kheli, P. A. Schultz, and W. A. Goddard III, Phys. Rev. B \textbf{73}, 235419 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J18.00011: Low temperature, field-dependent mobility in pentacene thin-film transistors. Adrian Southard, Vinod Sangwan, Dan Lenski, Michael Fuhrer, Ellen Williams We measure the field-effect and saturation mobility of Au bottom contact thin-film polycrystalline pentacene field-effect transistors while varying temperature, channel length, and gate voltage. We utilize Au bottom contacts without a wetting layer, and achieve contact resistance as low as 1 k$\Omega $-cm despite disturbance of the pentacene morphology at the drain and source electrodes. By measuring multiple channel lengths, we extract a contact-resistance free mobility. We confirm this value using an alternative technique in which we short the source and drain electrodes and make two terminal measurements of the capacitance and loss between these electrodes and the gate as a function of frequency. We discuss the result of field-dependent mobility in the context of Poole-Frenkel theory to rationalize the non-linear dependence of drain current on drain voltage, and test the predictions of recently developed models for transport in such systems. [Preview Abstract] |
Session J19: Block Copolymer Thin Films II
Sponsoring Units: DPOLYChair: Kevin Cavicchi, University of Akron
Room: 320
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J19.00001: Self-assembled surface patterns from organometallic-containing triblock terpolymers Vivian Chuang, Caroline Ross, Jessica Gwyther, Ian Manners Block copolymers are useful in nanotechnology because they can self-assemble to form periodic nanoscale structures. Here, we demonstrate the formation of hollow ring arrays with a period of 54 nm from a core-shell cylindrical-morphology poly(styrene-b-ferrocenyldimethylsilane-b-2-vinyl pyridine) (PS-b-PFS-b-P2VP) triblock terpolymer thin film. By spin-coating and solvent annealing, thin films of the polymer were self-assembled into arrays of core-shell structures oriented perpendicular to the top surface of the film. Various chemically modified substrates were employed to investigate the effects of interfacial interaction between the substrate and the film, as well as the effects of solvent annealing, on the film morphology. Results will be compared with those obtained from a poly(butadiene-b-styrene-b-methyl methacrylate) triblock terpolymer [1]. The PS core and P2VP matrix blocks were partly removed simultaneously using oxygen plasma, and the remaining PFS ring pattern was successfully transferred into a PS layer by imprinting. \\[3pt] [1] Chuang et al., ACS Nano, 2008, 2, 2007. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J19.00002: Improvement of Extraction Efficiency of LED with Surface Relief Nanotructure Fabricated by Self-Assembled Block Copolymer Pattern Ryota Kitagawa, Akira Fujimoto, Koji Asakawa A surface relief nanostructure was fabricated on the emission surface of light-emitting diodes (LEDs) using a self-assembled diblock copolymer pattern. The pattern of the nanostructure possesses moderate short-range order with slightly deviation in size and spacing, which is different from conventional extraction surface structures, such as photonic crystal and randomly textured surface. The dot pattern of a self-assembled polystyrene -polymethylmethacrylate diblock copolymer (PS-b-PMMA) was used as an etched mask. An average dot spacing was controlled by changing blend ratio of PS-b-PMMA, homo (h-) PS, and h-PMMA in a polymer solution. In the photoluminescence (PL) measurement, the light extraction efficiency of the nanostructure exceeded over twice, compared with a flat surface, by optimizing the average spacing of the nanostructure. It was also revealed that the nanostructure showed more than 10{\%} higher extraction efficiency than the highly ordered nanostructure fabricated by a self-assembled nanosphere pattern. These results can be interpreted as a contribution of structural fluctuation in the nanostructure for enhancement of extraction efficiency. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J19.00003: Self-Assembling Block Copolymer Resist Mixtures towards Lithographic Resists for Sub-10 nm Features Curran Chandler, Vikram Daga, James Watkins Significant improvements in 193 nm photolithography have enabled the extension of device feature sizes beyond the 45 nm and 32 nm nodes, yet uncertainty lies beyond 22 nm features as no single replacement has emerged. Here we show that low molecular weight, nonionic block copolymer surfactant blends are capable of self-assembling into highly ordered domains with feature sizes on the order of 5 nm. These surfactants, most of which lack the required $\chi N$ for microphase separation on their own, exhibit strong segregation and long-range order upon addition of a component capable of multi-point hydrogen bonding that is specific for one of the blocks in the copolymer. This has been demonstrated by our SAXS data for several Pluronic (PEO-$b$-PPO-$b$-PEO) and Brij (PEO-$b$-[CH$_{2}$]$_{n}$CH$_{3})$ surfactants of various molecular weights and PEO volume fractions. Furthermore, we employ these highly-ordered systems as thin film, nanolithographic etch masks for the transfer of sub-10 nm patterns into silicon-based substrates. Small molecule, hydrogen bonding additives containing aromatic or silsesquioxane structure are also used to tune etch contrast between the blocks which is important for reducing line edge roughness (LER) of such small features. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J19.00004: Directed assembly of block copolymers on chemically nanopatterned substrates: enabling science for ultra high resolution lithography Paul Nealey Self-assembling materials based on block copolymers spontaneously form structures with well-defined dimensions and shapes at length scales of interest in nanotechnology. Unfortunately the thermodynamic driving forces for self-assembly are small and low-energy defects can get easily trapped. At issue is the extent of direction or guidance required to meet criteria related to perfection and registration for use of such materials in nanofabrication. Through fundamental understanding of the physics, chemistry, and surface and interfacial phenomena associated with equilibrating block copolymer films in the presence of chemically nanopatterned substrates, we demonstrate how block copolymers may be integrated into and advance the performance of the lithographic process. The technological importance of this approach is discussed with respect to patterned media and the fabrication of integrated circuits. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J19.00005: Conditions for the directed assembly of thick block copolymer films on chemically nano-patterned surfaces Adam M. Welander, Paul F. Nealey The extent to which a lamellae forming block copolymer (bulk period, $L_{0}$ = 48 nm) can be directed to assemble on chemically nano-patterned striped surfaces (period $L_{S})$ with domains registered to and extending vertically away from the underlying pattern with few defects was studied as a function of film thickness, commensurability between $L_{S}$ and $L_{0}$, and temperature. The thickness through which low defect assembly could be achieved increased as $L_{S}$ and $L_{0}$ became more commensurate and as the temperature increased from 190 \r{ }C to 230 \r{ }C. Under certain conditions ($L_{S} \quad \approx \quad L_{0}$, 230 \r{ }C), block copolymer films approaching 750 nm (aspect ratio $\approx $ 30) in thickness still exhibited low levels of defectivity. These results were interpreted in terms of a phenomenological model and minimization of free energy including surface and interfacial energies and chain configuration entropy. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J19.00006: Directed self-assembly of diblock copolymer thin films on chemically-patterned substrates for defect-free nano-patterning Mikihito Takenaka, Yasuhiko Tada, Satoshi Akasaka, Synsuke Aburaya, Hiroshi Yoshida, Hirokazu Hasegawa, Elizabeth Dobisz, Dan Kercher We demonstrate that Polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA) can self-assemble in a well-aligned, long-range ordered nano-pattern over arbitrarily large areas, commensurate with chemically pre-patterned templates prepared by electron beam (EB) lithography. We also demonstrate that the self-assembly process can interpolate points in between the EB generated pattern, thus multiplying the pattern density. Moreover, we show the results of the investigation about the time-evolution of the self-assembled structure during annealing process. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J19.00007: Lamellar and Non-bulk like Morphologies in Thin Films of Block Copolymer on Chemical Nanopatterned Surfaces Guoliang Liu, Francois Detcheverry, Juan J. de Pablo, Paul F. Nealey Thin films of symmetric PS-b-PMMA (bulk lamellae period L$_{o})$ were equilibrated on substrates patterned with periodic stripes such that the adjacent stripes are preferentially wet by the two blocks of the copolymer. The morphology of the films was quantified as a function of the following pattern characteristics: the pattern period, L$_{s}$, where L$_{s}=\delta $ L$_{o}$, 1 $\le \quad \delta \quad \le $ 3, the width of the PMMA wetting strips, W, and the interfacial energies between the blocks and the patterned stripes, $\Lambda _{i, j}$. Under different boundary conditions we can 1) direct the assembly of lamellae perpendicular to the substrate and ordered in linear arrays so as to increase the density of features of the chemical pattern, or 2) obtain a number of stable non-bulk like structures including asymmetric lamellae, mixed orientated lamellae, dots, and check-board structures. The experimental results are compared to a phase-diagram predicted from molecular simulations. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J19.00008: Self-confinement in Block Copolymer Thin Films Induced by Chemical Patterns Made from Electro-oxidation Nanolithorgraphy Ji Xu, Antonio Checco, Benjamin Ocko, Soojin Park, Shiliu Wang, Thomas Russell The effect of confinement from chemical patterns on the self-assembly of block copolymer and related wetting physics has been studied. A variety of geometries designed in a mesh fashion were chemically patterned on OTS modified silicon wafers by electro-oxidation nanolithography. Thin films of a cylinder-forming PS-b-PEO were spin coated onto these patterned substrates. Thermal annealing of these films showed that the films were pinned on the patterned regions, due to the strong interaction between PEO block and carboxylic acid group on patterned surface while, over the non-patterned areas, dewetting was suppressed. The non-favorable interactions of both blocks with the substrate in the non-patterned areas caused the cylindrical microdomains to orient normal to the surface, being confined geometrically by the patterned regions. Defect-free, hexagonally packed cylindrical microdomains that conformed to hexagonal pattern written onto the surface were obtained. Point defects arose in the hexagonal packing of the microdomains when the dimensions or shape of the pattern were not commensurate with the natural packing of the copolymers. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J19.00009: Time-Resolved SAXS Characterization of Block Copolymer Blends on Chemically Nanopatterned Surfaces Karl Stuen, Paul Nealey, Dillip Satapathy, Kim Nygard, Harun Solak The directed assembly of block copolymer/homopolymer ternary blend thin films on chemically nanopatterned substrates was investigated with \textit{in situ} transmission SAXS. A ternary blend was used to match the block copolymer period with the period of a chemical pattern fabricated by x-ray interference lithography. The domain assembly in a 24-nm-thick block copolymer blend film on the chemical nanopattern was monitored with SAXS in real-time as a sample was heated from 100 to 240 \r{ }C at about 20 \r{ }C per minute. The strongest diffraction from the sample was detected after just 4.5 minutes of annealing (maximum temperature $\sim $ 190 \r{ }C). Complementary results were obtained from top-down SEM images of films that were quenched to room temperature after various times during the temperature ramp. The SEM images revealed transient structures in the annealing process that may relate to the non-uniform distribution of homopolymer in the direction perpendicular to the substrate. The results were compared to previously reported Monte Carlo molecular simulations to better understand the three-dimensional structures that form during the annealing process. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J19.00010: Imaging Layer Effect on Density Multiplication in the Directed Assembly of Block Copolymer Thin Films Huiman Kang, Eungnak Han, Padma Gopalan, Paul Nealey Recently, we discovered the assembly block copolymer thin films on chemically nanopatterned surfaces markedly improve both the quality and resolution of the lithographic process. In comparing the assembled block copolymer structures to the lithographically defined chemical pattern, the density of features is increased by a factor of four and the dimensional uniformity is vastly improved, even on the strongly preferential background imaging layer. Here, we investigate the effect of the interactions between the patterned imaging layer and the components of the block copolymer, especially polystyrene-\textit{block}-poly(methyl methacrylate) (PS-$b$-PMMA), by controlling the fraction of styrene in the imaging layer of chemically patterned surfaces from preferential to non-preferential to the polymer. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J19.00011: Molecular Transfer Printing Using Block Copolymers Shengxiang Ji, Chi-Chun Liu, Guoliang Liu, Paul Nealey We report a new parallel patterning technique, molecular transfer printing (MTP), for replicating geometrically complex patterns over macroscopic areas with sub-15 nm feature dimensions, and the ability to replicate the same pattern multiple times. In MTP, inks are mixed with block copolymers (BCPs) and deposited as films on a substrate. The inks are compatible with only one block of the BCP, and sequestered into domains of nanometer scale dimensions after microphase separation. A second substrate is then placed in contact with the surface of the film. By designing the inks to react, adsorb, or otherwise interact with the second substrate, inks are transferred to the second substrate in the exact pattern of domains present at the surface of the ``master'' BCP film. Here we demonstrate high degrees of perfection on both line and dot patterns. We also show that 1) the master template can be regenerated, 2) the resultant replica can be used to direct the assembly of BCPs and as a daughter master for MTP, and 3) the master and daughter templates can be reused tens of times. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J19.00012: Pattern interpolation in thin films of lamellar, symmetric copolymers on nano-patterned substrates Francois Detcheverry, Umang Nagpal, Guoliang Liu, Paul Nealey, Juan de Pablo A molecular model of block copolymer systems is used to conduct a systematic study of the morphologies that arise when thin films of symmetric, lamellar forming block copolymer materials are deposited on nanopatterned surfaces. Over 500 distinct cases are considered. It is found that, in general, three distinct morphologies can arise depending on the strength of the substrate-polymer interactions, the film thickness, and the period of the substrate pattern. The relative stability of those morphologies is determined by direct calculation of the free energy differences. The dynamic propensity of those morphologies to emerge is examined by careful analysis of simulated trajectories. The results of this systematic study are used to interpret recent experimental data for films of polystyrene-PMMA copolymers on chemically nanopatterned surfaces. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J19.00013: Thin Films of Polydimethylsiloxane-Containing Block Copolymers Maurice Wadley, Kevin Cavicchi The self-assembly of block copolymers into ordered nanostructures such as spheres, cylinders, and lamellae in the range of 10-100 nm makes them interesting materials for patterning surfaces. Thin films of poly(dimethylsiloxane) (PDMS) containing block copolymers are attractive for patterning due to their high oxygen etch resistance compared to other polymers. The main disadvantage of these polymers for patterning is the low surface tension of PDMS. This causes the preferential migration of PDMS to the air/film interface driving the formation of domains parallel to the interface and surface wetting layers. In this work a series of AB block copolymers containing PDMS have been prepared via RAFT polymerization where the surface tension of the opposing block was varied. Using a macro chain transfer approach, it is possible to isolate the effect of changing the opposing block while keeping the PDMS the same in each different block copolymer. The effect of changing the surface tension mismatch between the blocks on the thin film morphology will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J19.00014: Interfacial Bending of Lamellar Microdomains of Block Copolymers Sang-Min Park, Meng Dong, Charles Rettner, Qiang Wang, Ho-Cheol Kim We report our investigation on the interfacial bending property of the lamellar microdomains using a symmetric block copolymer of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) deposited on a neutral surface. The degree of interfacial bending of lamellae on surface was controlled by varying the angle of elbow-like topographic guiding patterns prepared by E-beam lithography. The characteristic parameters of lamellae bending including the critical angles of elbow-like patterns which give maximum interfacial bending of lamellae, the lamellae tilting angle at sidewall were determined for both single and paired guiding patterns. The behavior of a block copolymer containing hybrid system, a mixture of poly(styrene-b-ethylene oxide) and organosilicate, was investigated as well. A computational calculation on the lamellae bending which provides more insights on the free energy and interfacial characteristics will be discussed as well. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J19.00015: Observation of Surface Corrugation-Induced Alignment of Lamellar Microdomains in PS-b-PMMA Thin Films. Ho-Cheol Kim, Sang-Min Park, Charles Rettner, Brian Berry, Elizabeth Dobisz Previously we reported the alignment of lamellar microdomains of a block copolymer containing hybrid on a corrugated surface, which provides self-assembled crossbar nanostructures. The alignment of lamellae of the hybrid system is believed due to the anisotropic bending perperty of lamellae. Attempts to similarly align the lamellae of PS-b-PMMA using the same length scales of surface corrugation were not successful. In this study, we investigated the alignment of lamellar microdomains of PS-b-PMMA using even broader ranges of length scales of the surface corrugation. Within specific ranges of roughness scales, we observed that the lamllar microdomains of PS-b-PMMA align perpendicular to the direction of surface corrugation. The effect of relative scales of periodicity and film thickness of PS-b-PMMA to those of surface corrugation on the alignment of lamellae is discussed in this paper. [Preview Abstract] |
Session J20: Frank J. Padden Jr. Award Symposium
Sponsoring Units: DPOLYChair: Glenn Fredrickson, University of California, Santa Barbara
Room: 321
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J20.00001: Pressure Effects on Polymer Coil-Globule Transitions near an LCST David Simmons, Issac Sanchez A model for the pressure - temperature behavior of the coil-globule transition (CGT) of a polymer in dilute solution is developed without adjustable parameters. The predicted pressure-temperature conformational behavior semi-quantitatively correlates with extant experimental data. The model yields a heating induced coil-to-globule transition (HCGT) temperature that increases with pressure until it merges with a cooling induced coil-to-globule transition (CCGT). The point at which the CCGT and HCGT meet is a hypercritical point that also corresponds to a merging of lower critical solution temperature (LCST) and upper critical solution temperatures (UCST). Theoretical results are discussed in terms of a generalized polymer/solvent phase diagram that possesses two hypercritical points. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J20.00002: Structure-property relationships in ABA copolymer gels with A homopolymer additions Michelle Seitz, Rebecca Rottsolk, Kirt Page, Kenneth Shull ABA acrylic triblock copolymers with poly(methyl methacrylate) endblocks and poly(butyl acrylate) midblocks transition from free flowing liquids to elastic solids with decreasing temperature in alcohol solvents. Homopolymer PMMA chains can be solubilized in the micelle cores if they are shorter than the endblocks. Indentation and compression tests were used to determine gel's modulus and large strain behavior. Gels with volume fractions of PMMA less than $\sim $0.2 are highly elastic and have moduli dictated by stretching of bridging midblocks. At higher PMMA contents, gels exhibit greater permanent deformation and moduli over an order of magnitude larger than would be expected from rubber elasticity alone. Small angle X-ray and neutron scattering and mean field simulations were used to correlate changes in gel structure and micelle morphology with the addition of homopolymer. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J20.00003: A Versatile Method for Covalent Layer by Layer Assembly of Thin Organic Films Hernan R. Rengifo, Cristian Grigoras, Jeffrey Koberstein Layer by layer (LbL) assembly techniques construct multilayer thin films by sequential deposition of monomolecular layers of organic molecules. One of the drawbacks associated with their use is that monomolecular layers are usually held together by relatively weak forces such as Van der Waals, electrostatic and hydrogen bonding interactions, and can therefore be lacking in mechanical integrity. We demonstrate herein that heterobifunctional polymers, functionalized with one azide chain terminus and a protected alkyne group as the other chain terminus, constitute a powerful and versatile means for the \underline {covalent} layer-by-layer (CLbL) assembly of thin polymer films. Each monomolecular polymer layer is covalently bound to both the preceding and following layers to produce a robust multilayer structure. Because the coupling chemistry used, ``click'' chemistry, is highly chemoselective, the layering process is virtually independent of the chemical nature of the polymer so that the constitution of each layer can be selected at will. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J20.00004: How do entangled polymeric liquids flow? Sham Sundar Ravindranath, Shi-Qing Wang This work focused on investigating fundamental questions in polymer dynamics such as how entangled polymeric liquids respond to fast external deformation. By developing an effective particle tracking velocimetric (PTV) method, along with conventional rheometric measurements, new insights can be gained into the phenomenology of entangled polymers in presence of startup shear, step strain and large amplitude oscillatory shear (LAOS). During startup shear of well entangled systems, the shear field becomes inhomogeneous after the stress overshoot for a range of applied shear rates beyond the Newtonian region [1]. The emergence of shear banding after stress overshoot helped us to identify the stress overshoot as indicating yielding, whose characteristics obey some scaling laws. In step shear, contrary to the conventional perception that entangled polymers would undergo quiescent relaxation, the PTV observations reveal macroscopic motions after shear cessation [2]. The recoil-like macroscopic motions appears to reflect an elastic breakdown of the entanglement network due to sufficient build-up of retractive forces. LAOS experiments also demonstrate that entangled polymers cannot sustain a high magnitude of fast deformation without undergoing cohesive failure [3]. [1] \textit{Macromolecules }\textbf{2008}, $41$, 2663 [2] \textit{Macromolecules }\textbf{2007}, $40$, 8031 [3] \textit{J. Rheol. }\textbf{2008}, $52$, 341. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J20.00005: Polymer Diffusion in Carbon Nanofiller / Polymer Nanocomposites Minfang Mu, Nigel Clarke, Russell Composto, Karen Winey Polymer tracer diffusion through carbon nanofiller / polymer nanocomposites is measured using elastic recoil detection methods. Tracer diffusion through a single wall carbon nanotube nanocomposite is strongly suppressed at low concentrations ($\le $ 0.4-0.8 vol{\%}) and then increases at higher concentrations. In contrast, the typical Maxwell model predicts only a weak monotonic decrease. We propose a model for the carbon nanotube composite system wherein the SWCNTs function as cylindrical traps. Simulations of this model found that at low concentrations, the isolated traps retard polymer diffusion and at higher concentrations the percolated traps allow polymer diffusion to recover by providing continuous pathways. A comparison of our experimental and simulation results finds that (1) the strength of the trap increases with the molecular weight of the diffusing polymers and (2) the trap diameter increases with the molecular weight of the matrix polymer. Similarly, tracer diffusion through C60 / polymer nanocomposites exhibits a significant decrease at low concentrations and then slowly increases at concentrations larger than 0.7 vol{\%}. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J20.00006: Tunable Wetting of Polymer Nanocomposite Films Marla McConnell, Shu Yang, Russell Composto Surfaces with controlled wettability are of growing technological importance. In this study, nanoparticles (NPs) with tunable spacing were assembled on poly(styrene-\textit{ran}-acrylic acid), S-$r$-AA, films to manipulate the composite films' wetting properties. Amine-modified silica NPs (15-200 nm) were covalently grafted to the AA moieties on the surface of the S-$r$-AA films, in which the S phase imparts mechanical stability and the AA domains swell, increasing the roughness and surface area. By controlling surface roughness and reaction time, NP coverage ranged from 1{\%}-70{\%}. These films displayed NP-coverage-dependent water contact angles between 60$^{\circ}$and 120$^{\circ}$. The enhanced hydrophobicity is attributed to capillary climbing of S-$r$-AA chains to cover the previously hydrophilic NP surface. Upon increasing NP diameter, the contact angle was found to increase at a fixed total coverage. This increase in attributed to the increase in effective surface area with increasing particle size. This system is utilized as a platform to create Janus particles with unique optical properties and templates for investigating molecular motors. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J20.00007: Direct Measurement of Molecular Mobility in Actively Deformed PMMA Glasses Hau-Nan Lee, Keewook Paeng, Stephen Swallen, Mark Ediger To quantitatively understand the response of segmental motions to external stress, we performed optical measurements of dye reorientation in PMMA glasses during tensile creep deformation. Up to 1000-fold increases in mobility are observed during deformation, which supports the view that stress-induced mobility allows plastic flow in polymer glasses. Although the Eyring model describes this mobility enhancement well at low stress, it fails to capture the dramatic mobility enhancement after flow onset. In this regime, in addition to lowering the barriers for molecular motion, external stress apparently forces the shape of distribution of relaxation times to narrow significantly. The effect of stress on physical aging was also investigated. At low stress, physical aging and deformation-induced mobility act as two independent processes. However, after flow onset, the data are consistent with the view that aging has been erased by deformation. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J20.00008: Probing Surface Glass Temperature of Polymer Films via Pentacene Growth Mode, Microstructure, and Thin-Film Transistor Performance Choongik Kim, Antonio Facchetti, Tobin Marks Pentacene-based organic thin-film transistors (OTFTs) have been extensively studied in organic electronics. In this study, we report the fundamental importance of the polymeric gate dielectric glass transition temperature on pentacene film growth mode, and microstructure and corresponding OTFT performance. From the knowledge that nanoscopically-confined thin polymeric films exhibit glass-transition temperatures deviated from the corresponding bulk materials, we show here that pentacene films grown on polymeric gate dielectrics at temperatures well-below their bulk glass transition temperature ($T_{g }$(b)) exhibit morphological/microstructural transitions and dramatic OTFT performance variations at a well-defined temperature [herein defined as the polymer surface glass transition temperature, or $T_{g }$(s)] characteristic of the polymer structure and independent of the film thickness. Our results demonstrate that TFT measurements represent a new methodology to probe polymer surface viscoelastic properties. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J20.00009: Responsive Polymer Surfaces: Crumpling, Folding, and Snapping Films. Douglas Holmes, Alfred Crosby This work focuses on understanding deformation mechanisms and responsiveness associated with folding, crumpling, and snapping of thin polymer films attached to patterned and nonpatterned substrates. By studying folding and crumpling in confined regimes, we gain insight into material properties, while developing new strategies for adhesive, optical, and patterning applications. Using a novel processing technique, microarrays of freestanding polydimethylsiloxane plates are placed in equibiaxial compression and transition through crumpled morphologies that are difficult to attain through traditional patterning techniques. The microstructures also change their curvature through a snap-through instability via environmental stimuli. When triggered via osmotic pressure the snap transition time scales as the square of the plate thickness and the inverse of the plate modulus. Recently, we have transferred this knowledge into the crumpling of ultrathin polymer films. We have fabricated sharply folded films directly on elastomeric and silicon substrates. The fold width scales directly with the film thickness and applied strain. We find that normally brittle, polystyrene films can accommodate excessive compressive strains without fracture by undergoing strain-localizing fold events. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J20.00010: Structure and mobility of PEO/LiClO4 solid polymer electrolytes Susan Fullerton, Janna Maranas Solid polymer electrolytes [SPEs] for use in rechargeable lithium-ion batteries offer many advantages over traditional liquid electrolytes, including mechanical flexibility and environmental friendliness. The practical limitation is that room temperature conductivity remains insufficient to power a portable device. While it is well-established that ion mobility is driven by polymer dynamics, high conductivity values have also been reported through fully crystalline SPEs. PEO-based SPEs have a rich phase behavior, and can form several crystalline complexes depending on the lithium concentration, temperature, and recrystallization time. We investigate the structure, mobility, conductivity, and thermal properties of both semi-crystalline and amorphous PEO/LiClO4 SPEs. Structure is measured with small-angle neutron scattering, and PEO mobility with quasi-elastic neutron scattering. We observe a decoupling of ionic conductivity and PEO mobility in a semi-crystalline sample. We also determine that PEO hydrogen atoms undergo restricted rotation on a circle. The radius of the circle is consistent with a cylindrical, crystalline structure that persists to some extent in the amorphous phase. The results suggest that directed ion transport via ordered structures is perhaps equally important as polymer mobility for increasing conductivity, provided that the structures percolate over large spatial scales. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J20.00011: Architectural effects in strongly hydrogen bonded thermoplastic elastomers Kathleen Feldman, Craig Hawker, Edward Kramer In this work we demonstrate the synthesis of random copolymers of \emph{n}-butyl acrylate with a quadruple hydrogen bonding acrylate monomer based on 2-ureido-4[1\begin{math}H\end{math}]-pyrimidinone (UPy). Despite low T$_g$s and a lack of crystallinity, these materials show thermoplastic elastomer properties through the strong but thermoreversible UPy groups. Through the use of controlled radical polymerization and post-polymerization functionalization we are able to reach high UPy monomer content while maintaining low polydispersity and excellent control over the total molecular weight. It was found that the average distance between UPys along the chain was the major determiner of the overall properties including the plateau modulus, tensile modulus, and relaxation timescale. By using a difunctional initiator it is also possible to synthesize materials containing a homopolymer midblock and random copolymer end blocks, allowing us to address the question of how the MHB group distribution along the chain affects the bulk properties. In concentrating the UPy groups near the chain ends, the plateau modulus remained constant but the crossover frequency decreased dramatically, indicating that the effective lifetime of the hydrogen bonds within the supramolecular network increased, in keeping with prior theoretical predictions. [Preview Abstract] |
Session J21: Focus Sessions: Dopants and Defects in Semiconductors II
Sponsoring Units: DMPChair: Matthew McCluskey, Washington State University
Room: 323
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J21.00001: Defects in Mg doped (Al,In)GaN thin films and nanostructures Invited Speaker: Development of p-type (Al,In)GaN revolutionized the field of solid state lighting in the way that it was hard to imagine, development and introduction to market of light emitters in short period of time and tremendous amount of progress in other areas that was enabled by such development. Although many studies have been done to understand the defects related to Mg incorporation in epitaxially grown AlInGaN films, there are still many open questions. These include the relationship between the defects (type and density) and Mg incorporated and the electrical property of the film. An interesting open question is how optical characteristics of Mg doped (Al, In) GaN can predict its electrical property. In this presentation, we try to address this question. Recent advances in development of nanostructures based on III-nitrides include growth of high quality GaN nanowires. Although large body of work exists in growth and characterization of Si doped GaN nanowires the report work on Mg doped GaN is scarce. In the present work, we will discuss our recent progress in studying optical and electrical characteristics of Mg doped GaN nanowires and defect stabilization in nanostructure and thin films.\\[4pt] In collaboration with M. Reshchikov, Department of Physics, Virginia Commonwealth University, Richmond, VA 23284; N. Tripathi, B. J. Messer, and M. Tungare, College of Nanoscale Science and Engineering, UAlbany-State University of New York, Albany, NY 12203 [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J21.00002: Interaction of hydrogen with defects in GaN Yevgeniy Puzyrev, Matthew Beck, Blair Tuttle, Ron Schrimpf, Dan Fleetwood, Sokrates Pantelides Hydrogen has long been known to have a dual role in Si electronics. It can passivate interfacial dangling bonds and other defects, but it can also be an agent of degradation and aging because it can be released under a variety of conditions. When released, it can depassivate other defects by forming H$_{2}$. In GaN, H has been known to play a key role in p-type doping (it helps Mg enter substitutionally, with an H attached to it; H is then removed to activate the Mg as a dopant). Here we present results of first-principles density-functional calculations exploring the role of H as a degradation agent in GaN devices, e.g., degradation by hot electrons. We find that H binds strongly to N antisite defects and is not removed during the process of dopant activation. Under normal operating conditions, hydrogenated N antisite defects are benign. However, hot electrons can release the H atoms from these defects, making them active electron traps. The results provide an explanation of pertinent data on hot-electron device degradation. This work was supported in part by an AFOSR MURI grant. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J21.00003: Intrinsic spin polarization of cation vacancies in group-III nitrides Yoshihiro Gohda, Atsushi Oshiyama Group-III nitrides are of significant importance not only in optoelectronic devices but also in potential applications to spintronics. Recently, we have clarified by means of total-energy DFT+U calculations that spins of Ga vacancies in Gd-doped GaN interact ferromagnetically [1]. We have also found that the magnetic moment increases monotonically with the increasing number of Ga vacancies, which explains experimental observations of colossal magnetic moments in Gd-doped GaN [2]. In this contribution, we report newly performed first-principles calculations that clarify spin polarization and lattice relaxation of cation vacancies with various charge states in a few group-III nitrides. Spin-polarized electronic configurations obtained in the present study are indicative of intrinsic ferromagnetism due to cation vacancies in nitride semiconductors. \\[0pt] [1] Y. Gohda and A. Oshiyama, Phys. Rev. B 78, 161201 (R) (2008). \\[0pt] [2] S. Dhar et al., Phys. Rev. Lett. 94, 037205 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J21.00004: N-H$_{n}$ complexes in GaAsN:H containing more than two H atoms Lanlin Wen, Michael Stavola, W. Beall Fowler, Antonio Polimeni, Mario Capizzi, Gabriele Bisognin, Marina Berti The III-N-V alloys have attracted much attention because of a large reduction of the band gap that occurs for N concentrations of a few percent. The hydrogenation of these alloys eliminates the effect of N.[1] IR experiments and theory revealed the properties of an NH$_{2}$ complex that can cause these novel effects.[2] Further studies by theory [3] and experiment [4] suggest the formation of defect complexes that contain more than two H atoms per N atom. In the present talk, new IR data provide experimental clues about the structures of NH$_{n}$ defect complexes with n$>$2 that have been found in GaAsN samples that were hydrogenated at reduced temperature. [1] A. Polimeni et al., Phys. Rev. B \textbf{63}, 201204 (2001). [2] S. Kleekajai et al., Phys. Rev. B \textbf{77}, 085213 (2008) and the references contained therein. [3] A.A. Bonapasta et al., Phys. Rev. Lett. \textbf{98}, 206403 (2007). [4] M. Berti et al., Phys. Rev. B \textbf{76}, 205323 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J21.00005: N-H vibrational frequencies in GaAs:N:H$_{n}$ W. Beall Fowler, M. Stavola, Lanlin Wen, A. Polimeni, M. Capizzi While the fundamental H-related defect in GaAs:N and GaP:N appears to have an NH$_{2}$ character,[1] considerable recent evidence suggests that an additional one or two hydrogen atoms may in some cases be involved.[2,3] We have used the CRYSTAL06 \textit{ab} \textit{initio} quantum code[4] with density functional theory to investigate these possibilities, obtaining theoretical equilibrium positions, ground-state energies, and vibrational frequencies with all combinations of H and D. In all cases investigated, the NH$_{2}$ configuration is preserved, but observable shifts in the N-H or N-D vibrational frequencies are predicted. Supported by NSF Grant 541744. [1] S. Kleekajai \textit{et al.,} Phys. Rev. B \textbf{77}, 085213 (2008) and references contained therein. [2] A. A. Bonapasta \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 206403 (2007). [3] M. Berti \textit{et al.}, Phys. Rev. B \textbf{76}, 205323 (2007). [4] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual} (University of Torino, Torino, 2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J21.00006: Effects of Si-N complexes on the electronic properties of GaAsN Yu Jin, Ryan Jock, Hailing Cheng, Cagliyan Kurdak, Rachel Goldman Silicon is the most common n-type dopant in GaAs-based materials and devices; however, in dilute nitride alloys, it has been suggested that Si and N atoms form Si-N complexes which act as deep electron traps. Here, we report the first quantitiative evidence of Si-N complex formation by comparing the properties of GaAsN films doped with Si and Te, with a variety of N-dopant spatial separations. First, we compare bulk-like GaAsN:Si films, where Si and N reside in the same layer, with modulation doped heterostructures, where N and Si atoms are spatially separated. A decrease in free carrier concentration, [n], with increasing N composition is observed in bulk-like films but not heterostructures, suggesting N-Si defect complexes in the bulk GaAsN layers are likely acting as trapping centers. In addition, we compared GaAsN films doped with Si and Te. For GaAsN:Te films, [n] increases substantially with increasing annealing temperature, but little change is observed in GaAsN:Si films. In GaAsN:Si, the annealing-induced increase in [n] is balanced by the formation of additional Si-N complexes. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J21.00007: Influence of N Interstitials on the electronic properties of GaAsN Alloys Ryan Jock, Yu Jin, Hailing Cheng, Cagliyan Kurdak, Rachel Goldman GaAsN alloys contain a significant fraction of interstitial N, which is often reported to act as a scattering and/or trapping center. In some cases, annealing has been reported to reduce the interstitial N fraction, presumably due to N diffusion to As vacancies. However, the influence of interstitial N on the electronic properties of GaAsN alloys remains unknown. In this work, we used annealing to probe the influence of N interstitials on the electronic properties of GaAsN. In as-grown GaAsN films, temperature-dependent Hall measurements reveal a thermally activated increase in free carrier concentration for temperatures above 150 K, suggesting the presence of a defect level below the conduction band edge, presumably due to interstitial N. Upon annealing, the free carrier concentration increases and becomes nearly temperature-independent, indicating a decrease in the concentration of trapping centers, presumably due to a reduced concentration of interstitial N. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J21.00008: Quaternary Ga$_{1-x}$In$_{x}$P$_{1-y}$N$_{y}$ alloys described by clustering of In and N in GaP Koushik Biswas, Alberto Franceschetti, Stephan Lany The interactions between the different atomic constituents in an alloy affect the microstructure, and ultimately, the electronic properties of the alloy. Specifically, in group III-V alloys the energy of formation of single defects and the binding energy of defect complexes play an important role in determining the microstructure.~ We present a model that starts from the dilute defect picture and extends to alloys of low to moderate concentrations. Using a valence-force-field (VFF) method we calculate the energy of formation of isolated N and In defects and that of small defect clusters formed by N and In in a GaP host. Considering a 1:2.12 N to In ratio that conserves lattice matching to GaP, we show that in a N concentration range up to $\sim $ 15{\%}, the formation energy of the random alloy can be described by the random probability to form such defect clusters. This approach allows the thermodynamic modeling of the microstructure of quaternary alloys, such as GaInNP, without intricate lattice-energy expansions and Monte-Carlo simulation techniques. In GaInNP, we find that short range ordering due to large atom to small atom preferential binding (i.e. InN+GaP) strongly reduces the energy compared to the random distribution. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J21.00009: Mechanism of Interaction between Hydrogen and the Two-dimensional Electron Gas in AlGaN/GaN High Electron Mobility Transistors Jason Gu, Mahak Khandelwal, Jacob Melby, Michael Steeves, Yuh-Renn Wu, Robert Lad, Robert F. Davis The large polarization difference between AlGaN and GaN causes a two-dimensional electron gas (2DEG) to form at the interface between the two semiconductors. Capacitance-voltage (CV) measurements revealed a charge density of 4.71x10$^{12}$ electrons/cm$^{2}$ in our 60 nm Al0.2Ga0.8N on 1.5 microns of GaN heterostructure. Exposure to hydrogen in the presence of a catalyst (Pt) resulted in a marked increase in the conductivity through the 2DEG. An interface state passivation mechanism is proposed as the most probable cause of this phenomenon. This mechanism was modeled using a self-consistent Schr\"{o}dinger-Poisson solver, which showed that the passivation of interface states causes the shift of the Fermi level towards the conduction band, thereby increasing the carrier density of the 2DEG by 9{\%}. In-situ CV measurements showed a 16{\%} increase in the carrier density and a non-parallel shift in the CV curve when hydrogen was introduced, indicating in a change in the number of available states. This supports interface state passivation as a cause of the increase in the conductivity through the 2DEG. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J21.00010: Magnetodielectric coupling in Au/GaAs:Si Schottky barriers S. Tongay, A.F. Hebard, Y. Hikita, H. Hwang A surprisingly large ($>$20{\%}) negative magnetocapacitance in non-magnetic Au/GaAs:Si Schottky barriers is attributed to a magnetic field ($H)$ induced increase in the binding energy of the shallow donor Si impurity atoms. Capacitance ($C)$ dispersion is used to identify the impurity ionization and capture processes that give rise to an $H$-dependent density of ionized impurities $N_d (H)$in the depletion region. Internal photoemission experiments confirm that the large $H$-induced shifts in the built-in potential $V_{bi} $, inferred from Mott-Schottky (1/$C^{2}$ versus voltage) measurements, are not due to an $H$-dependent Schottky barrier height (SBH), thus requiring a modification of the abrupt junction approximation which identifies the dependence of $V_{bi} $ on $N_d (H)$rather than the SBH. The linearity of the Mott-Schottky plots is preserved, as experimentally observed. The underlying magnetodielectric coupling not only allows a new opportunity for the tuning of the dopant carrier density by an external means (magnetic field) but should be important for understanding the behavior of related interfacial structures incorporating dilute magnetic semiconductors and/or complex oxides. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J21.00011: Point Defects and Dielectric Loss at MM Wavelengths in Wide-Gap Semiconductors Jyotsna Dutta, Charles Jones, V.V. Parshin, B. Garin, V.I. Polyakov, A. Rukovishnikov Data are presented on wide-gap semiconductors of various grades for their dielectric loss values at millimeter wavelengths to explore their potential for various RF technology related applications. In order to identify the impurities or electrically active defects that give rise to the excess loss, temperature-dependent conductivity and DLTS measurements have been undertaken. Dielectric loss measurements over a wide range of temperatures are in progress to verify the results obtained from electrical methods and help to determine the primary loss mechanisms for these materials in the millimeter wave length range. Experimental results and their implications to loss properties will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J21.00012: Dual-Surfactant effect on enhancing different p-type doping in GaP. Junyi Zhu, Gerald Stringfellow, Feng Liu We report first principles calculations demonstrating a dual-surfactant effect of Sb and H on enhancing Zn, Mg, Be and Cd in vapor phase epitaxially grown GaP thin films. The combined effects of Sb and H lower significantly the doping energy all the p-type dopants in GaP, while neither Sb nor H can work alone as effectively. The role of H is to satisfy the electron counting rule. The role of Sb is to serve as an electron reservoir to help electron redistribution. The enhancement is the lowest for Mg which is probably due to the lowest electronegativity of Mg among these four elements. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J21.00013: InSb epilayers and quantum wells grown on Ge(001) substrates by MBE Mukul Debnath, Tetsuya Mishima, Mike Santos, Khalid Hossain, Wayne Holland For digital logic applications, transistors with both electron and hole channels are required. InSb:Ge heterostructure is an ideal material since the highest carrier mobilities for n and p-type quantum wells (QWs) are observed in InSb and Ge channels, respectively. We report on the MBE growth of InSb-based materials on Ge(001) substrates. A temperature variation two-step growth procedure (TSGP) is more effective than direct growth of InSb on Ge(001). In the TSGP, an initial 100-nm InSb layer was grown at a temperature of 340$^{o}$C before increasing the substrate temperature to 420$^{o}$C for the rest of the growth. The initial growth forms a wetting layer that minimizes defects at the InSb/Ge interface. The X-ray rocking curve width of a 5.0-$\mu $m-thick InSb epilayer is 173 arc sec. Electron mobilities of a 5.0-$\mu $m-thick InSb epilayer and an InSb/Al$_{0.20}$In$_{0.80}$Sb QW at room temperature are 34,500 and 8,600 cm$^{2}$/V-s, respectively. These are the highest mobilities for an InSb epilayer and QW on Ge(001) substrates reported so far. This work was supported by NSF Grant DMR-0520550 and OCAST contract AR071-025. [Preview Abstract] |
Session J22: Focus Session: Optical Control and Electron-nuclear Effects in Quantum Dots
Sponsoring Units: GMAG DMP FIAPChair: Roberto Myers, Ohio State University
Room: 324
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J22.00001: Optical Spin Initialization and Non-Destructive Measurement in a Quantum Dot Molecule Invited Speaker: The spin of an electron is an ideal two level system for realizing a quantum bit. Spatial confinement in a self-assembled InAs quantum dot greatly extends its spin coherence times as well as making them optically addressable. Through the excited trion state the electron can be initialized, coherently manipulated, and read out: the essential operations for quantum information processing. For single quantum dots, initialization of the spin requires a transverse magnetic field in order to turn on the normally forbidden transitions, which breaks the symmetry of the system. A major drawback is that this precludes the use of sensitive 2-level cycling transitions. In a cycling transition measurement the system repeatedly returns to the same spin eigenstate because of selection rules, and in this sense is non-destructive. Cycling transition measurements are the established method of eigenstate readout as in the case of ion qubits. Spin initialization and non-destructive cycling transition read out are incompatible in single quantum dots. In this talk I show how we overcome this fundamental limitation by using a pair of quantum dots that are coupled through coherent tunneling. The electron is isolated in one quantum dot whose spin is initialized or read out by the optical creation of an electron-hole pair in the other quantum dot, forming a molecular trion. The unique energy level structure of a molecular trion eliminates the need for a transverse magnetic field. Instead a longitudinal magnetic field is used to tune two of the trion states into resonance such that an exchange interaction permits a spin-flip Raman process. At the same time other trion states maintain strict selection rules and are used for cycling transition measurements. Overall the singly charged quantum dot molecule forms a ``W'' energy level system which is comprised of a Lambda system and two two-level cycling transitions. Two-laser transmission spectroscopy is used demonstrate initialization and non-destructive measurement, simultaneously. \\[4pt] [1] D. Kim et al. Phys. Rev. Lett, \textbf{101} (2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J22.00002: Dynamical Nuclear Polarization via Triplet States in Self-Assembled Quantum Dot Molecules S. C. Badescu, D. Kim, A. S. Bracker, D. Gammon, T. L. Reinecke Recent experiments on self-assembled quantum dot molecules used molecular trion states to initialize and measure optically the electron spin in one of the dots [1]. The key to this experiment is the anticrossing of two electron-triplet states in a magnetic field in Faraday configuration, which is due to spin-orbit coupling and electron-hole exchange. The experimental spin initialization and readout plots exhibit bifurcation and hysteresis features attributable to nuclear polarization. Here we present results for these effects from a model accounting for the feedback between the dynamical nuclear polarization and the electron spin states in the two dots, determined by the optical pumping and the asymmetric exchange. We explain the correlations between the nuclear polarizations in each of the dots and the asymmetry between the nuclear effects for the two measured electron states. [1] D. Kim et al, Phys. Rev. Lett. \textbf{101} (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J22.00003: Single frequency precession of inhomogeneous ensemble of electron spins. Alex Greilich, Stefan Spatzek, Irina Yugova, Ilja Akimov, Dmitri Yakovlev, Alexander Efros, Dirk Reuter, Andreas Wieck, Manfred Bayer We show that the spins of all electrons, each confined in a quantum dot of an (In,Ga)As/GaAs dot ensemble, can be driven into a single mode of precession about a magnetic field. This regime is achieved by allowing only a single mode within the electron spin precession spectrum of the ensemble to be synchronized with a train of periodic optical excitation pulses. Under this condition a nuclei induced frequency focusing leads to a shift of all spin precession frequencies into the synchronized mode. The macroscopic magnetic moment of the electron spins that is created in this regime precesses without dephasing. This ensemble can be used then as a macroscopic quantum bit. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J22.00004: Control of the direction and rate of nuclear spin flips in InAs quantum dots using detuned optical pulse trains Sam Carter, Sophia Economou, Andrew Shabaev, Thomas Kennedy, Allan Bracker, Thomas Reinecke Using two-color time-resolved Faraday rotation and ellipticity measurements, we show that control of the direction and rate of nuclear spin flips in InAs quantum dots can be achieved through optical manipulation of the electron spin. A circularly polarized pump pulse train excites an ensemble of dots with varying electron spin precession frequencies and pump detunings. Resonant excitation has been described in Ref. [1], in which the electron spin polarization is greatly enhanced when the precession is synchronized to a multiple of the pulse repetition rate. Nuclear spin flips occur rapidly when the electron spin is not synchronized, with equal probability to flip up or down, leading to random walks that eventually lead the system to stable synchronized modes. In detuned dots, rotations of the spin away from the plane of precession lead to asymmetry in the nuclear spin flip rates, giving a clear pathway for nuclear reconfiguration. For dot energies below (above) the pump, the nuclear reconfiguration pushes electron spins towards (away from) synchronization. This effect is observed through a spectral shift in the Faraday rotation/ellipticity amplitudes as a function of probe detuning. [1] A. Greilich, A. Shabaev \textit{et} al., Science \textbf{317}, 1896 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J22.00005: Theory of the effect of detuned optical pulse trains on the electron-nuclear hyperfine interaction in quantum dots Sophia Economou, Sam Carter, Andrew Shabaev, Tom Kennedy, Allan Bracker, Tom Reinecke A train of optical pulses detuned from resonance of the electron spin-trion transition in a quantum dot has the combined effect of generating and of rotating the spin polarization. The rotation is a direct consequence of the detuning and induces an electronic spin component parallel or antiparallel to the magnetic field, depending on the sign of the detuning. This electron spin component directs the nuclear spin to preferably flip in one direction. This pulse-assisted electron-nuclear flip-flop both affects the electron, because it changes the precession frequency and changes whether it is synchronized with the pulses, and it also opens up the opportunity for manipulating the nuclear polarization by using the detuning along with the pulse repetition rate as handles. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J22.00006: Many-body theory of spin bath dynamics for qubit decoherence Ren-Bao Liu, Wen Yang We have developed a cluster correlation expansion (CCE) theory for the many-body dynamics of a finite-size spin bath in a time scale relevant to decoherence of a center spin or a qubit embedded in the bath [1]. In terms of the linked cluster expansion, a cluster correlation term is the infinite summation of all the linked diagrams with all the spins in the cluster flip-flopped. The lowest order of the cluster correlation corresponds to the pair-correlation approximation developed previously [2]. In the thermodynamics limit, the CCE reduces to the standard cluster expansion. The CCE is especially useful for studying multi-spin coherence in small spin baths such as NV centers in diamonds and molecular magnets, where the cluster expansion fails to converge to the exact solution. \\[4pt] [1] W. Yang and R. B. Liu, Phys. Rev. B \textbf{78}, 085315 (2008).\\[0pt] [2] W. Yao, R. B. Liu and L. J. Sham, Phys. Rev. B \textbf{74}, 195301 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J22.00007: The Hyperfine-mediated and Nuclear-Dipolar-Induced Nuclear Spin Eddy Yusuf, Xuedong Hu We study nuclear spin diffusion in semiconductor quantum dots based on the density matrix approach. The nuclear spin interactions that we consider include both hyperfine-mediated and magnetic dipolar interactions. Furthermore, we take into account both the secular and the non-secular terms of the magnetic dipolar nuclear interactions. We discuss how the one- and two-electronic states in the quantum dots lead to quantitatively different nuclear spin polarization relaxation and nuclear spin diffusion time. We explore the behavior of the relaxation time and diffusion constant for various experimentally relevant parameters, and compare our results to the recently measured nuclear spin relaxation in GaAs double quantum dots [1]. \\[3pt] [1] D. J. Reilly, J. M. Taylor, J. R. Petta, C. M. Marcus, M. P. Hanson, and A. C. Gossard, arXiv:0803.3082 [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J22.00008: Theoretical study of the strain-induced nuclear spin depolarization in self-assembled quantum dots Chia-Wei Huang, Xuedong Hu We investigate how strain-induced quadrupole interaction is related to nuclear spin polarization in self-assembled quantum dots. Our calculation shows that the achievable nuclear spin polarization in In$_{x}$Ga$_{1- x}$As quantum dots is sensitively dependent on the strain distribution in the dots. There are two interesting regions of rapid changes in nuclear spin polarization when the Overhauser field is in the opposite direction to the external field. The first one occurs in the low field region (B $<$ 1T) where nuclear spin polarization of individual nuclear species is suppressed due to a degeneracy between different nuclear spin states. The second one is a peak in nuclear spin polarization showing up in the intermediate field region. This peak corresponds to a local maximum of the Overhauser field, which happens when electronic Zeeman energy vanishes. Our results are in qualitatively agreement with the measured nuclear spin polarization in the experimental work of various groups\footnote{B. Eble \textit{et al.}, Phys. Rev. B \textbf{74}, 081306 (2006).}$^,$ \footnote{A. S. Bracker \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 047402 (2005).}$^,$ \footnote{P. Maletinsky \textit{et al.}, Phys. Rev. B \textbf {75}, 035409 (2007).} [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J22.00009: Preparation of Nuclear Spin States in Double Quantum Dots Jacob J. Krich, Michael Gullans, Jacob M. Taylor, Michael Stopa, Bertrand I. Halperin, Mikhail D. Lukin, Amir Yacoby Recent experiments on double quantum dot systems with two electrons have shown rich dynamics associated with the hyperfine coupling to nuclear spins. We examine how the cycles used to produce dynamic nuclear polarization in such double quantum dots can lead to interesting non-equilibrium configurations of the nuclear spins. We develop a master equation for the nuclear spins, which we solve using time-dependent mean field theory. We find a rich set of phenomena in the system, including tendencies of the system to approach two very different configurations, one with equal effective magnetic fields produced by the nuclei in the two dots and the other with a large difference between the magnetic fields produced by the nuclei in the two dots, both of which are seen in experiments. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J22.00010: Zamboni force in pumping of angular momentum from electron to nuclear spins via the Overhauser effect Michael Stopa, Amir Yacoby, Jacob Krich We identify a feedback mechanism between the electron states in a two-electron double quantum dot and the difference between the nuclear spin polarization in the two dots, which we term the ``Zamboni force.'' The Overhauser interaction is known to cause angular momentum transfer, spin flip-flops, between electrons and nuclei in GaAs-AlGaAs heterostructures. In double quantum dots, transport and pumping experiments have been performed to study the evolution of nuclear spin polarization in response to certain electronic transitions. We show that, in flipping from singlet (S) to triplet (T+), ``flopping'' of the nuclear spin can occur in the left dot, the right dot or in the barrier depending on the composition of the singlet state. Assuming a composite nuclear spin for each the left dot, the right dot and the barrier, we numerically integrate the Schr\"{o}dinger equation to study the gate voltage sweep through the S-T+ anti-crossing point. We show that the (nuclear) effective magnetic field gradient tends to produce spin flips in the dot with the weaker field and thereby constitutes a force toward nuclear spin equilibration. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J22.00011: Spin Fluctuations in Magnetic Quantum Dots A.G. Petukhov, R.M. Abolfath, Igor Zutic We present a theoretical description of magnetism in quantum dots (QDs) doped with magnetic ions. It has been recognized that the mean-field theory (MFT) is inadequate for small magnetic systems, such as bound magnetic polarons (BMPs), at finite temperatures [1]. Magnetic QDs are in many respects similar to BMPs, however the latter are one-electron systems while the former may contain many electrons. Our approach requires the minimization of the generalized ``free energy'' functional [2] for QDs, which leads to a set of self-consistent Kohn-Sham-type equations that coincide with MFT-equations [3] in the thermodynamic limit. We reveal that the well-known spurious MFT second order phase transition in magnetization is completely removed by thermodynamic spin fluctuations. \\[4pt] [1] T. Dietl and J. Spalek, Phys. Rev. Lett. {\bf 48}, 355 (1982).\\[0pt] [2] A. G. Petukhov, I. Zutic, and S. C. Erwin, Phys. Rev Lett. {\bf 99}, 257202 (2007)\\[0pt] [3] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. {\bf 101}, 207202 (2008); R. M. Abolfath, P. Hawrylak, and I. Zutic, Phys. Rev. Lett. {\bf 98}, 207203 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J22.00012: Carrier-mediated magnetism and bound magnetopolarons in quantum dots Rafal Oszwaldowski, Andre Petukhov, Igor Zutic While Mn-doped quantum dots (QDs) offer versatile control of magnetic order [1], important challenges remain in understanding of these systems beyond the mean-field approximation. Furthermore, to describe the carrier-mediated magnetism in arrays of magnetic QDs and their non-equilibrium properties, it is important to consider the presence of both electrons and holes in these systems. We develop a formalism that accounts for both equilibrium and light-controlled magnetopolaron effects [2]. We study QDs of different sizes and find that their magnetic and photo-induced properties are extremely size-sensitive. We compare our theory with recent experiments on circularly-polarized photoluminescence in magnetic QDs [3] where both the magnetopolaron energies and power dependence of the circular polarization were measured. We thank I. R. Sellers for valuable discussions. Supported by ONR and NSF-ECCS CAREER. [1] L. Besombes et al., Phys. Rev. Lett. 93, 207403 (2004); R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. 101, 207202 (2008). [2] T. Dietl and J. Spalek, Phys. Rev. Lett. 48, 355 (1982). [3] I. R. Sellers et al., unpublished. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J22.00013: ABSTRACT WITHDRAWN |
Session J23: Spectroscopic Studies of 2D Electron Gas and Other Low Dimensional Semiconductor Structures
Sponsoring Units: DCMPChair: Aron Pinczuk, Columbia University
Room: 325
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J23.00001: Coexistence of quantum phases in the quantum Hall regime of the 2$^{nd}$ Landau Level Trevor Rhone, Jun Yan, Yann Gallais, Aron Pinczuk, Loren Pfeiffer, Ken West We report the experimental study of spin excitation modes in the regime of quantum hall phases of the 2$^{nd}$ Landau Level. In the ferromagnetic state at $\nu $=3 the long wavelength spin wave mode is seen at the bare Zeeman energy. At low temperatures and at filling factors slightly lower ($\nu \sim $2.97), the spin wave attenuates and a broad `overdamped' continuum of low-lying excitations emerges. Under these conditions, sharp and broad modes coexist, suggesting the presence of mixed quantum phases. At slightly elevated temperatures the continuum disappears. Further away from filling factor three, near the odd-denominator state at $\nu \cong $8/3, the continuum dominates at low temperature. However, the sharp spin wave is recovered at T$>$1K. For even lower filling factors, such as $\nu \cong $5/2, low temperature spectra display only the broad continuum of low-lying excitations. At high temperatures (T$\sim $2K) a sharp spin wave is recovered while the broad continuum persists, indicating the emergence of phase coexistence. The interplay between sharp and `overdamped' modes may manifest tendencies towards loss of full spin polarization in the N=1 Landau level and may indicate that spin degrees of freedom have significant impact on the physics of quantum Hall states with 3$\ge \nu \ge $ 2. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J23.00002: The Virtual Scanning Tunneling Microscope: Induced Tunneling in Bilayer Two-Dimensional Electron Systems Adam Sciambi, Matthew Pelliccione, David Goldhaber-Gordon, Seth Bank, Arthur Gossard, Michael Lilly, John Reno We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which will spatially and spectroscopically map two-dimensional electron systems (2DESs) in semiconductor heterostructures. The probe overcomes the typical inaccessibility of a buried 2DES by having a second parallel ``probe'' 2DES grown nearby. A biased tip overhead can then induce local tunneling from the probe 2DES into the original by adjusting the interlayer potential barrier. Prior bilayer studies have shown that a tunneling signal is dominated by the overlap of the layers' Fermi surfaces, hindering VSTM-induced tunneling and obscuring any spectroscopy. We show, however, in widely-space bilayers systems where interlayer inelastic scattering is more prominent that the previous energy-momentum constraints are relaxed. In GaAs/AlGaAs samples grown by two different sources, we show we can increase interlayer tunneling by an order of magnitude with gating, setting the stage for spectroscopy. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J23.00003: Tunneling spectroscopy of a 2D-2D tunnel junction: Towards a local spectroscopic probe of 2D electron systems Matthew Pelliccione, Adam Sciambi, David Goldhaber-Gordon, Seth Bank, Arthur Gossard, John Reno, Michael Lilly We present measurements on GaAs/AlGaAs bilayer two-dimensional electron systems (2DES) that exhibit inelastic tunneling between the 2D electron layers. Due to a relatively large interlayer separation, scattering allows for tunneling events between states of different energy and momentum, which are not observed in similar systems with a small interlayer separation. This behavior can be used to measure spectroscopic information about the 2DES that is obscured when tunneling events conserve energy and momentum exclusively. We study the bulk behavior of this system in the integer quantum Hall regime on samples from different sources, and provide a model to explain the observed tunneling dynamics. We also discuss the prospect of using this system as a virtual scanning tunneling microscope (VSTM), where a scanning probe is used to locally induce tunneling between the 2D electron layers. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J23.00004: Unusual Cyclotron Resonance Line Broadening in Ultra-High Mobility Two-Dimensional Electron Gas System Li-Chun Tung, Changli Yang, L.N. Pfeiffer, K.W. West, R.R. Du, Yong-Jie Wang Microwave induced resistance oscillation in the ultra high mobility two-dimensional electron gas system has attracted an intense interest in recent years. Under the illumination of an intense microwave radiation in the millimeter regime, the system exhibits an unique resistance oscillation with a microwave-frequency-dependent period. We have carried out a FIR magneto-optical study up to 33T on a set of GaAs/AlAs ultra-high mobility heterostructure samples at 4K. At low magnetic field, ultra-sharp electron CR has been observed as expected. At high magnetic field, the halfwidth of CR grows rapidly with increasing magnetic field and this phenomenon is uniquely found in the 2DEG in the heterojunction. At the same time, there is no indication of a reduced mobility at high field in the transport measurement. The sudden broadening of the CR line shape cannot be simply interpreted by either the short- or long-range random potential. The halfwidth changes by at least an order, thus it can hardly be explained by either magnetophonon effect, inhomogeniety nor magnetic oscillation. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J23.00005: Evidence of Auger Satellite Lines in Landau-Level Photoluminescence Spectroscopy in a Two Dimensional Electron Gas S.K. Lyo, W. Pan, J.L. Reno Landau-level spectroscopy has provided a powerful tool for investigating the electronic structure and the scattering dynamics in a two-dimensional electron gas (2DEG) in the past. In this paper, we present theoretical and experimental evidence for Auger satellite lines in the magnetoluminescence of the Landau-level spectroscopy from a 2DEG under a perpendicular magnetic field $B$ at low temperatures. These new anomalous lines with a weak intensity appear below the energy gap in the form of radial ``spokes'' with negative slopes in the so-called fan (energy vs. $B$) diagram, in contrast to the well-known standard spokes of the fan diagrams of the spectral lines which appear above the band gap energy with positive slopes. [S. K. Lyo, E. D. Jones, and J. F. Klem, Phys. Rev. Lett. 61, 2265 (1988)]. Our theoretical predictions yield reasonable agreement with observed low-temperature data from GaAs quantum wells. The satellite lines can be used to determine the conduction (valence) band mass in n-doped (p-doped) systems. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J23.00006: Localization - Delocalization Transition of Indirect Excitons in Lateral Electrostatic Lattices M. Remeika, J. Graves, A.T. Hammack, L.V. Butov, M. Hanson, A.C. Gossard We report on a study of transport of indirect excitons in GaAs/AlGaAs coupled quantum wells in linear lateral lattices created by laterally modulated gate voltage. The localization-delocalization transition for the excitons was observed with increasing density in the directions along and across the lattice. At high lattice amplitudes, the density corresponding to the transition across the lattice was found to linearly depend on the lattice amplitude. Screening of the potential by repulsive exciton-exciton interaction was demonstrated. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J23.00007: Giant upconversion of photoluminescence in semiconductor heterostructures Angelo Mascarenhas, Brian Fluegel, David Snoke, Gamani Karunasiri, Loren Pfeiffer Photoluminescence (PL) upconversion is observed in GaAs/AlGaAs multiple quantum wells at photon energies up to 90 meV above the exciting laser and with an efficiency up to 27\% relative to normal Stokes PL excited from the two-dimensional continuum. The effect is due to electric field-induced bipolar tunneling. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J23.00008: Phase Separation of Bright and Dark Excitons in Coupled Quantum Wells Nicholas Sinclair, Zoltan Voros, Jeff Wuenschell, David Snoke, Kenneth West, Loren Pfeiffer The diversity and complexity of solid-state environments suggests that Bose-Einstein Condensation (BEC) of excitations in a solid might manifest in a variety of interesting ways, in correspondence with the diversity of features among ground states of these systems. The pursuit of excitonic BEC is both enriched and obfuscated by this flexibility of condensate character. Previous research pursuing BEC of interwell excitons in GaAs coupled quantum wells (CQWs) has focused attention on observing unusual luminescence from `bright', dipole-coupled (J=1) excitons to detect a BEC. However, theorists have recently predicted a `dark' (J=2) ground state for interwell excitons in GaAs. Our recent work with interwell excitons confined in stress-induced, in-plane traps shows the critical onset of a dark spot in the exciton-recombination luminescence at trap center, suggestive of a dense population of dark excitons and a phase separation between the dark/bright species. The critical temperature vs. density in the low temperature regime matches well with ideal 2D harmonic trap BEC criteria, and preliminary theoretical work suggests that this degree of species separation cannot be explained by a model based on classical statistical level occupation using the bright/dark state energy separation. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J23.00009: Ultrafast Relaxation Dynamics of a High Density Electron-Hole Plasma in High Magnetic Fields Jinho Lee, Dave H. Reitze, Junichiro Kono, Alexey Belyanin, Glenn Solomon, Steve McGill We study the inter-Landau level relaxation dynamics of a dense electron-hole plasma in high magnetic fields (up to 31 T). Intense 150 fs pump pulses create carrier densities approaching 10$^{13}$/cm$^{2}$ in In$_{0.2}$Ga$_{0.8}$As/GaAs multiple quantum wells. Relaxation dynamics are probed as a function of Landau level (LL) and magnetic field using time-resolved transient absorption (TRTA) and time-resolved photoluminescence (TRPL), which provide complementary information about the relaxation processes. Manifestly non-exponential decays of the TRTA signals are observed at high fields (above 15 T). TRPL emissions measured in the plane of the wells reveal the presence of multiple emission bursts from the LLs at high magnetic fields, suggesting a complicated relaxation process mediated by the field whereby carriers get trapped in a specific LL, emit PL though recombination, and then `reload' as the carriers relax down to the previously occupied LLs. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J23.00010: Spin splitting of the valence band Landau levels in GaAs quantum wells I. Khan, T. Ali, M. Yasar, A. Petrou, A. Hanbicki, G. Kioseoglou, C. Li, B. Jonker We have studied as function of magnetic field the electroluminescence spectra from an n-i-p LED that incorporates three GaAs quantum wells in the intrinsic region. This device has excess n-type doping and as a result, the quantum wells are populated by a two-dimensional electron gas. The broad zero field emission band evolves into a series of discrete features in the presence of a magnetic field. These are identified as interband transitions between the $\ell $ = 0, 1, and 2 Landau levels associated with the e$_{1}$ and h$_{1}$ subbands, with the selection rule $\Delta \ell $ = 0. The EL spectra were analyzed in their $\sigma $+(LCP) and $\sigma $- (RCP) components. An energy splitting between the two polarized components is observed for each Landau level transition, which is equal to the sum of the conduction and valence band spin splittings. Since the electron g-factor value is known ($g$ = - 0.44) we were able to determine the valence band spin splittings. Our experimental values are compared to calculated values (1) and were found to be in reasonable agreement. Work at SUNY was supported by ONR and NSF 1. H.A. Nickel et al., Phys. Rev. B, \underline {62}, 2773, (2000) [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J23.00011: Bose Einstein Condensation in Trapped Polaritons Versus Lasing Effects Ryan Balili, Bryan Nelsen, David Snoke, Loren Pfieffer, Kenneth West Evidence for Bose Einstein condensation (BEC) of exciton-polaritons have been presented recently by several groups in a variety semiconductor microcavity geometries (eg. [1). Objections nevertheless remain as experimental evidence for polariton BEC bear striking similarity to observed behavior in a regular photon laser or microcavity in weak coupling regime [2]. Latest results however show that a both BEC and lasing transitions can occur and are distinguishable in the stress trapped case [3]. \\[3pt] [1] Balili, R., Hartwell, V., Snoke, D., Pfeiffer L., and West, K. Science 316, 1007 (2007). \\[0pt] [2] Bajoni, D., Senellart, P, Lemaitre, A., and Bloch, J. Phys. Rev. B. 76, 201305(R) (2007). \\[0pt] [3] Balili, R., Nelsen, B., Snoke, D., Pfeiffer L., and West, K. arXiv:0808.1861v2 [cond-mat.other](2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J23.00012: Study of Optical Bistability in Coupled Microdisks S.N. Ghosh, Y.K. Verma, B.B. Buckley, X. Li, N. Samarth, D.D. Awschalom, S. Ghosh Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications. We report bi-stable lasing in coupled GaAs microdisks with quantum wells and interface-fluctuation quantum dots in the active region. The inter-disk coupling results in mode-splitting, with the higher energy resonance persistently achieving higher mode Q ($\sim $ 4000). The bi-stability manifests in the form of hysteresis in the intensity of the coupled modes on non-uniform excitation and can be attributed to saturable absorption. This property in the lasing characteristics of coupled cavities gives us a control on the gain modulation and mode-switching and would be useful for applications in optical memories and computing, and in next generation of low-threshold optoelectronic devices. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J23.00013: Stress Induced Splitting of Polariton States in Semiconductor Microcavities Bryan Nelsen, Ryan Balili, David Snoke, Lauren Pfeiffer, Kenneth West Photoluminescence and reflectance measurements of microcavity polaritons indicate that there is an energy splitting of the lower polariton branch on the order of 10 to 100 $\mu $eV. When stress is applied with a pin to the microcavity samples, this energy splitting is found to increase. The states have varying degrees of polarization which depend on the applied stress. Experimental results will be presented, along with theoretical mechanisms that may cause such a splitting. These results may have implications for the spontaneous emission of polarized light associated with the Bose-Einstein condensation transition observed in microcavity polaritons.[1] \newline \newline 1. R. Balili, V. Hartwell, D.W. Snoke,~ L. Pfeiffer and K. West, Science 316, 1007 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J23.00014: Polarization dependence of coherent LO phonon excitation in Si Anca-Monia Constantinescu, Hrvoje Petek The coherent LO phonon mode excitation dependence on the angle $\theta$ between pump and probe polarizations is systematically investigated by transient electro-optic sampling measurement for both $\Gamma_{25}$ and $\Gamma_{12}$ symmetries\footnote{M. Hase, M. Kitajima, A. M. Constantinescu, and H. Petek, \textit{Nature} \textbf{426}, 51 (2003).} We find that for $\Gamma_{25}$ symmetry the phonon exhibits a sin(2$\theta)$ dependence in the amplitude, while for $\Gamma_{12}$ symmetry the signal is considerably weaker and has a cos(2$\theta)$ dependence. The LO phonon maximum amplitude for $\Gamma_{25}$ is larger than the maximum amplitude for $\Gamma_{12}$ by about a factor of 6. We will discuss the results in the context of coherent phonon excitation mechanisms. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J23.00015: ABSTRACT WITHDRAWN |
Session J24: Focus Session: Nanotube Applications
Sponsoring Units: DMPChair: Stefano Curtarolo, Duke University
Room: 326
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J24.00001: Nanotube and Graphene ElectroMechanics Invited Speaker: Carbon nanotubes and graphene have attracted a lot of attentions as high-frequency mechanical resonators. For instance, nanotube resonator devices hold promise for ultralow mass detection or quantum electromechanical experiments. However, the detection of the mechanical vibrations remains very challenging. In this talk, I will present a novel detection method of the vibrations of nanotubes and graphene, which is based on atomic force microscopy. This method enables the detection of the resonances up to 3.1 GHz with subnanometer resolution in vibration amplitude. Importantly, it allows the imaging of the mode-shape for the first eigenmodes. I will also report on a new artificial nanofabricated motor in which one short nanotube moves relative to another coaxial nanotube. The motion is shown to be controlled by how the atoms are arranged within the two nanotubes. The motion is actuated by imposing a thermal gradient along the device, allowing for sub-nanometer displacements. This is, to our knowledge, the first experimental demonstration of displacive actuation at the nanoscale by means of a thermal gradient. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J24.00002: Carbon nanotube based sharp tips and soldering irons Abha Misra, Chiara Daraio High energy electron beam machining has been proven a powerful tool to modify desired nanostructures for technological applications and to form molecular junctions and interconnections between carbon nanotubes. The development of the next generation of miniaturized electronic systems demands the integration of nanoelectronic components creating reliable mechanical and electrical contacts. At the same time, the development of scanning probe techniques and magnetic recording media require an ever decreasing tip size of ultrasharp magnetic read-write heads. We report on the nano-electron beam assisted fabrication of atomically sharp iron-based tips and on the creation of a nano-soldering iron for nano-interconnects using Fe-filled multiwalled carbon nanotubes (MWCNTs). Our technique allows also carving a MWCNT into a nanosoldering iron that was demonstrated capable of joining two separated halves of a tube. This approach could easily be extended to the interconnection of two largely dissimilar CNTs, between a CNT and a nanowire or between two nanowires. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J24.00003: Broken chiral symmetry in nanotube sliding Giuseppe Ernesto Santoro, Xiaohua Zhang, Ugo Tartaglino, Erio Tosatti We discovered, in simulations of the frictional sliding of coaxial nanotubes, an unanticipated example of dynamical symmetry breaking -- in fact a family of examples -- taking place at the nanoscale. While both nanotubes are armchair, thus perfectly left-right symmetric and nonchiral, a nonzero angular momentum appears spontaneously at a series of critical sliding velocities, in correspondence with large peaks of the sliding friction. The angular momentum is not connected with real bodily rotations, but rather to breathing phonon pseudorotations. The nonlinear equations governing this phenomenon turn out to share common elements with another classic problem exhibiting a dynamical chirality breaking, that of forced oscillations of a string or a rope. Several newer elements that are exquisitely ``nano'' appear in the nanotube case making it a richer, more elegant and intricate case, with a variety of different phenomena, and the crucial involvement of umklapp and of sliding nanofriction. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J24.00004: Porphyrin-like defects in CN$_x$ nanotubes Antonio J.R. da Silva, James Almeida, A.R. Rocha, A. Fazzio Carbon nanotubes (CNT) can be used in a variety of nanoscopic electronic devices, and their functionality can be greatly enhanced by the introduction of defects. It has been shown that CNTs doped with nitrogen atoms can act as sensors. Pyridine- like defects, where four N atoms surround a divacancy, have been shown [1] to act as binding sites of molecules, such as ammonia, and to be associated with the behavior of these sensors. In this work we study the adsorption of iron atoms onto these nitrogen defects. We demonstrate that the Fe atoms bind to the four N defect in a configuration similar to a porphyrin molecule. Moreover, this system - CN$_x$ nanotube plus iron atom - has a magnetic moment of 3 $\mu_B$ which is almost entirely localized on the Fe atom. With a combination of density functional theory and recursive Green's functions calculations we study the transport properties of disordered one-dimensional systems [1] composed of such units. In particular, we study the polarization of the current as a function of the average spin orientation of iron atoms randomly dispersed over the nanotubes in a reallistic setup. [1] A. R. Rocha, M. Rossi, A. Fazzio and A. J. R. da Silva, Phys. Rev. Lett. {\bf 100}, 176803 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J24.00005: Dislocation Onset and Glide in Carbon Nanotubes under Torsion Traian Dumitrica, Dong-Bo Zhang, Richard James The torsional plastic response of carbon nanotubes is comprehensively described in the objective molecular dynamics framework~[1-3]. It is shown that an (n,m) tube is prone to slip along a nearly-axial helical path, which introduces a distinct (+1,-1) change in the wrapping index. The low energy realization occurs without loss of mass, via nucleation of a 5-7-7-5 dislocation dipole, followed by a nearly-axial glide of the 5-7 dislocation. The onset of plasticity depends not only on chirality but also on handedness. For a given handedness of the applied twist, chiral tubes of opposed handedness are most susceptible to yield. A right-handed applied twist on an armchair (zig-zag) tube leads to a right- (left-) handed tube. \\[4pt] [1] T. Dumitric\u{a} and R.D. James, {\it Objective Molecular Dynamics}, Journal of the Mechanics and Physics of Solids {\bf 55}, 2206 (2007). \\[0pt] [2] D.-B. Zhang, M. Hua, and T. Dumitric\u{a}, {\it Stability of Polycrystalline and Wurtzite Si Nanowires via Symmetry-Adapted Tight-Binding Objective Molecular Dynamics}, Journal of Chemical Physics {\bf 128}, 084104 (2008). \\[0pt] [3] D.-B. Zhang and T. Dumitric\u{a}, {\it Elasticity of Ideal Single-Walled Carbon Nanotubes via Symmetry-Adapted Tight-Binding Objective Modeling}, Applied Physics Letters {\bf 93}, 031919 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J24.00006: Electrochemical Charging of Carbon Nanotubes for Tunable Electron Field Emission Cathodes Alexander Kuznetsov, Norman Barisci, Anvar Zakhidov, Alexander Zakhidov Carbon nanotubes (CNTs) have very promising applications as electron field emitters. Work function of CNTs greatly affects the performance of such cold electron emitters. It is possible to change emission currents by several orders of magnitude by electrochemical charging. Electrochemical charging changes work function of CNTs by creating so called double layer. It was recently demonstrated that double layer structure remains for several hours after removing the CNTs from an electrolyte [1]. The extensive study of charging single wall carbon nanotube (SWNT) paper in different electrolytes has been performed at different charging potentials Vch. Field emission currents and threshold fields dependence on the charging potential and polarity is studied for various ions, with different valency and size: Na, Mg, Cs. Clear dependence of work function on Vch is demonstrated. AFM micro-imaging with a Kelvin probe allowed to study the micropatterns of work function modulation. Also dissipation of positive charge in air was investigated and its stability was significantly increased. 1. Suh Dong-Seok, Baughman Ray, Zakhidov Anvar, US Patent 20070170071 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J24.00007: Interaction of water and methanol with graphene, C60 and (10,10) nanotube Vijay Kumar, M. Amar, J.F. Maguire, Y. Kawazoe We study interaction of water and methanol molecules with grapheme, C60 and (10,10) carbon nanotube using plane wave pseudopotential method and GGA. The interaction energies, $\Delta $E, of H2O and CH3OH molecules on a (10,10) SWCNT, C60 and a graphene sheet are quite small (a few tens of meV) and are weakly dependent on the orientation of the molecules. The different electronic structures of graphene, nanotubes, and C60 lead to the differences. For (10,10) nanotube $\Delta $E of water (39 meV) is favorable outside the nanotube and it increases for a water dimer. For methanol $\Delta $E outside as well as inside the nanotube is nearly the same (40 meV). A competition between molecules-molecule and molecule-nanotube wall interaction could, however, lead to interesting molecular ordering behavior. $\Delta $E of water on C60 is significantly smaller presumably due to its large HOMO-LUMO gap but for a graphene sheet the band gap vanishes and $\Delta $E has an intermediate value between C60 and (10,10) nanotube. For methanol on graphene sheet $\Delta $E increases to 60 meV due to more significant overlap of the molecular orbitals with those of the graphene sheet. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J24.00008: Hydrogen adsorption on metal coated Multiwalled Carbon nanotubes Xianfeng Zhang, Dinesh Rawat, Toyohisa Furuhashi, Rakesh Shah, Aldo Migone, Saikat Talapatra We present results of volumetric adsorption measurements of hydrogen, on Palladium-Gold (Pd-Au) coated multiwalled carbon nanotubes (MWNT). The nanotubes were prepared using air assisted chemical vapor deposition technique and were subsequently purified (acid treatment) before coating them with Pd-Au. Hydrogen adsorption measurements were performed at 77.3 K on as produced MWNTs as well as purified MWNT and compared with the adsorption isotherm obtained on Pd-Au coated MWNT samples under same experimental conditions. The effect of coating the MWNTs with Pd-Au on the adsorption behavior of hydrogen on these nanotubes will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J24.00009: Enhancement of In Vivo Anticancer Effect of Cisplatin by Incorporation Inside Carbon Nanohorns Masako Yudasaka, Kumiko Ajima, Tatsuya Murakami, Yoshikazu Mizoguchi, Kunihiro Tsuchida, Toshinari Ichihashi, Sumio Iijima We have been studying potential applications of single-wall carbon nanohorns (SWNHs) to drug delivery systems. SWNHs are multiply functionalized with proteins, magnetites, tumor targeting molecules, and others. Various drugs are easily incorporated, and the incorporated drugs are slowly released. Almost no acute toxicity of SWNHs was found through various animal tests. We show in this report that anticancer effect of cisplatin was enhanced by incorporation inside SWNHs (CDDP@SWNH) as evidenced by in vivo tests: CDDP@SWNH was locally injected to tumors subcutaneously transplanted on mice. CDDP@SWNH inhibited the tumor growth more effectively than CDDP. This anticancer enhancement was achieved by large CDDP-quantity incorporated inside SWNH, slow release of CDDP from SWNH, long-term stay of SWNHs at the tumor sites, and an anticancer effect of SWNH itself [1].\\[3pt] [1] K. Ajima et al. ACSNano, 10(2008)2057-2064. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J24.00010: Fabrication of ZnPc/Protein Nanohorns for Double Photodynamic and Hyperthermic Cancer Phototherapy Minfang Zhang, Tatsuya Murakami, Kumiko Ajima, Kunihiro Tsuchida, Atula S. Sandanayaka, Osamu Ito, Sumio Iijima, Masako Yudasaka We developed double photodynamic and hyperthermic phototherapy systems [1] by loading zinc phthalocyanine (ZnPc) on single-wall carbon nanohorns (SWNHs). A protein of bovine serum albumin (BSA) was also attached to the surface of SWNHs. ZnPc performed photodynamic therapy (PDT) effect and SWNH had photohyperthermic (PHT) effect. BSA endowed hydrophilic property to the system. Previous results in vitro showed that the efficiency of phototherapy using ZnPc-SWNH-BSA was higher than that of ZnPc or SWNHs. We show in this report that mouse tests also exhibited the similar tendency. ZnPc-SWNH-BSA was locally injected in tumors subcutaneously transplanted on mice. And the laser (670 nm) was irradiated for 15 minute everyday for 10 days. By this phototherapy, the tumors completely disappeared. The phototherapy using ZnPc or SWNHox-BSA exhibited weaker antitumor effects, and the tumors continued to grow. [1] Zhang et al. PNAS, \textbf{2008}, 105, 14773. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J24.00011: Terahertz Plasmon Oscillations at Room Temperature in Nanotube Transistors Diego Kienle, Fran\c{c}ois L\'{e}onard We present a theoretical study of the high-frequency properties of carbon nanotube transistors. We employ a new theory for AC quantum transport based on a self-consistent Non-Equilibrium Green Function formalism. The theory is applied to calculate the frequency dependent response of a semi-conducting (17,0) nanotube FET device in the ballistic limit applying a time harmonic signal at the gate terminal. We show that in the ON- state the dynamical conductance exhibits divergent resonant peaks at discrete frequencies in the terahertz regime even at room temperature. These conductance peaks can be associated with the excitation of the charge eigenmodes (plasmons) of the quantum cavity formed by the nanotube channel and its surrounding gate, and shows up as pronounced spatial periodic large amplitudes in the AC charge and potential, respectively. The resonant features vanish when the device is operated in the OFF-state in which case the conductance displays smooth oscillations, a signature of single particle quantum interference. Our results indicate that low dimensional devices with nanometer channel length might show potential as novel detectors and emitters of THz radiation operating at room temperature. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J24.00012: Effects of substrate relaxation on adsorption in pores Silvina Gatica, Hye-Young Kim, George Stan, Milton Cole Fluids in porous media are commonly studied with analytical or simulation methods, usually assuming that the host medium is rigid. Large qualitative effects are found for several systems, for which substrate relaxation may not be neglected. One application is a determination of the ground state of 3He in slit and cylindrical pores. With the relaxation, there results a much stronger cohesion than would be found for a rigid host. Similar increased binding effects of relaxation are found for classical fluids confined within slit pores or nanotube bundles. These effects include large changes of the critical temperature (slit pore and nanotube bundle substrates) and condensation of the quasi-one dimensional fluid (carbon nanotube substrate). arXiv:0810.0262v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Session J25: Focus Session: Graphene V: Structure and Raman Spectroscopy
Sponsoring Units: DMPChair: Marcos Pimenta, UFMG, Brasil
Room: 327
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J25.00001: Quantum and transport scattering times in graphene X. Hong, K. Zou, J. Zhu, A. Posadas, J. Hoffman, C. H. Ahn We study the quantum ($\tau_q$) and transport ($\tau_t$)
scattering times of single layer graphene mechanically
exfoliated on SiO$_2$ and polycrystalline Pb(Zr,Ti)O$_3$ (PZT)
substrates. The PZT substrate exhibits a gating efficiency of
$\sim2\times10^{11}$/cm$^2$/V$_g$(V), corresponding to a
dielectric constant of $\sim$12. We extract $\tau_q$ from the
magnetic field dependence of Shubnikov de Haas oscillations and
$\tau_t$ from the zero field mobility, respectively. For the
PZT-gated graphene, in the density range of $2\times10^{12}
$/cm$^2 |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J25.00002: Electrical and structural properties of chemically modified graphene sheets Dmitriy A. Dikin, Inhwa Jung, Rodney S. Ruoff The chemical exfoliation of graphite through oxidation and then dispersion in a solvent is one of the methods of achieving scalable production of single graphene sheets. We use this method for making chemically modified graphene (CMG) sheets with tunable electronic properties, which can be placed flat on any surface or dispersed in various matrices. CMG sheets share some similarities with pristine graphene and with carbon nanotubes, e.g. tunable electron- and hole-type conductivity is observed in single CMG sheets just above the percolation threshold. CMGs may also be considered as a template for a bottom up development of a new class of materials. We have performed electrical measurements of individual CMG sheets and will discuss their electronic properties and the possible mechanisms of the charge transport in relation to their atomic structure and chemical composition. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 12:15PM |
J25.00003: Doping, Strain, Orientation and Disorder of Graphene by Raman Spectroscopy Invited Speaker: Raman spectroscopy is a fast and non-destructive method for the characterization of carbons [1]. These show two features: the G and D peaks, around 1580 and 1350cm$^{-1}$ respectively. The G peak corresponds to the doubly degenerate E$_{2g }$phonon at the Brillouin zone centre. The D peak is due to the breathing modes of sp$^{2}$ atoms and requires a defect for its activation [1-5]. It is common for as-prepared graphene not to have enough structural defects for the D peak to be seen [4,6], so that it can only be detected at the edges [6]. The most prominent feature in graphene is the second order 2D peak [6]. This is always seen, since no defects are required for its activation. Its shape distinguishes single and multi-layers [6]. Raman spectroscopy also monitors doping [7-9]. We report the evolution of the Raman spectra of single [7,8] and bi-layer [9] graphene as a function of doping. A Fermi level shift is induced either by applying a bottom gate [7], or by a polymeric top gate [8,9], or naturally happens as a result of charged impurities [10]. This induces a stiffening of the Raman G peak for both hole and electron doping [7]. This is explained including dynamic corrections to the adiabatic Born-Oppenheimer approximation [7]. The phonon renormalization of bilayer graphene has characteristic features compared to single layer. This allows a direct estimation of the interlayer coupling [7-9]. We then consider the effects strain. Uniaxial strain lifts the E$_{2g}$ degeneracy and splits the G peak in two: G$^{+ }$and G$^{-}$. The peaks downshift as a function of strain allows a direct measurement of the Gruneisen parameter [10]. The polarization dependence of the G$^{+}$/G$^{- }$modes is a probe of the crystallographic orientation of the sample [10]. Finally, we consider the effect of disorder [3,4,11] and show how to discriminate between disorder, strain and doping [11]. We will also discuss how the D peak is a signature of $\pi $ electron localisation, and, thus, of gap opening in chemically modified graphene[12]. \\[4pt] 1. A. C. Ferrari, J. Robertson (eds), \textit{Raman spectroscopy in carbons: from nanotubes to diamond}, Theme Issue, Phil. Trans. Roy. Soc. \textbf{362}, 2267 (2004). 2. F. Tuinstra, J.L. Koening, J. Chem. Phys. \textbf{53,} 1126(1970). 3. A. C. Ferrari, J. Robertson Phys Rev B \textbf{61}, 14095 (2000); \textbf{64}, 075414 (2001) 4. A. C. Ferrari Solid State Comm.\textbf{143}, 47 (2007) 5. S. Piscanec et al. Phys. Rev. Lett. \textbf{93}, 185503 (2004) 6. A. C. Ferrari et al. Phys. Rev. Lett. \textbf{97}, 187401 (2006) 7. S. Pisana et al. Nature Mater. \textbf{6}, 198 (2007) 8. A. Das et al, Nature Nano \textbf{3}, 210 (2008). 9. A. Das et al., arXiv:0807.1631v1 (2008) 10. A. C. Ferrari et al. submitted (2008) 11. C. Casiraghi et al. Appl. Phys Lett. \textbf{91}, 233108 (2007) 12. Elias et al. arXiv:0810.4706 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J25.00004: Controlled Structural Strain in Epitaxial Graphene Layers on 6H-SiC and Effects on Surface Morphology Nicola Ferralis, Jason Kawasaki, Roya Maboudian, Carlo Carraro The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by depolarized Raman spectroscopy and electron channeling contrast imaging. The selection of the depolarized component of the scattered light results in a significant increase in the C=C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, the formation of the buffer layer and a strained graphene layer. The linear strain, measured at room temperature (RT), is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. Whereas film thickness is determined by growth temperature only, the magnitude of the compressive strain and film morphology can be varied by adjusting the growth time at fixed annealing temperature. Annealing times in excess of 8-10 minutes lead to an increase in the mean square roughness of SiC step edges to which graphene films are pinned, resulting in compressively stressed films at RT. Shorter annealing times produce minimal changes in the morphology of the terrace edges and result in nearly stress-free films upon cooling to RT. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J25.00005: Frictional Characteristics of graphene Changgu Lee, Robert Carpick, James Hone The frictional characteristics of graphene were characterized using friction force microscopy (FFM). The frictional force for monolayer graphene is more than twice that of bulk graphite, with 2,3, and 4 layer samples showing a monotonic decrease in friction with increasing sample thickness. Measurements on suspended graphene membranes show identical results, ruling out substrate effects as the cause of the observed variation. Likewise, the adhesion force is identical for all samples. The frictional force is independent of load within experimental uncertainty, consistent with previous measurements on graphite. We consider several possible explanations for the origin of the observed thickness dependence. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J25.00006: Using Defects as Local Electronic Probes of Epitaxial Graphene on SiC Gregory M. Rutter, Kevin D. Kubista, David L. Miller, Ming Ruan, Walter A. de Heer, Phillip N. First, Joseph A. Stroscio Defects play an important role in the transport properties of epitaxial graphene, and understanding this role is essential for realizing potential nanoelectronics based on graphene. In this study, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) performed at 4.2 K are used to measure the local electronic behavior of defects in epitaxial graphene grown on both SiC(0001) and SiC(000-1). Energy-resolved maps of the differential conductance reveal defect-induced standing-wave modulations related to the unique nature of the graphene band structure. In this talk, I will discuss how these defects can be used as a local probe of the graphene electronic properties with the inclusion of an applied magnetic field and the resulting Landau quantization. Supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J25.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J25.00008: Crystallographic Cuts in Single Layer Graphene Leonardo Campos, Vitor Manfrinato, Javier Sanchez, Jing Kong, Pablo Jarillo-Herrero Graphene consists of a single monolayer of carbon atoms in a honeycomb 2D crystal. It is unique because the electrons are described by the Dirac equation, like ultrarelativistic particles with zero rest mass. According to theoretical predictions, it is possible to create field effect transistor just using narrow (d$<$10nm) nanoribbons. With zigzag edges, graphene nanoribbons can have a large magneto-resistance or could be used to produce a spin valve. With armchair edges it is possible to have an energy gap controllable by electric field. In this work we will show how to use Ni nanoparticles to create crystallographic oriented cuts in exfoliated graphene. Using Raman spectroscopy and electronic measurements of Dirac peaks, we have verified that the graphene, after the high temperature nanocut etching process, are still high quality 2D crystals, indicating that this process can be used to produce graphene nanodevices. Using this method we fabricate oriented nanoribbons and equilateral triangles with varying size. We also present a detailed analysis of the fabrication conditions for controlling the etching characteristics. Last, we present our analysis of the chirality of our nanocuts. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J25.00009: Chiral tunnelling of Dirac electrons in strained graphene A. Garcia-Saravia, G. Cordourier-Maruri, M.E. Cifuentes-Quintal, E. Martinez-Guerra, R. De Coss The behavior of the electrons in graphene is like massless Dirac fermions, which is a consequence of the characteristic energy spectrum of this material (E$\sim $k). Perfect chiral tunnelling is expected when Dirac electrons pass through a step barrier (Klein paradox). However, in a two-dimensional system like graphene, the perfect tunneling is obtained only in a small range of incident angles. In the present work, we have studied the uniaxial deformation as a method of tunning the electronic transmittance in graphene. The effect of the armchair and zigzag strain on graphene was studied by means of first principles calculations, using the Density Functional Theory. For the calculations we used the pseudopotential-LCAO method. We found that the uniaxial deformations, induce an ellipsoidal distortion of the Dirac cones and isotropy breaking of the Fermi velocity. Finally, we used the Dirac--like equation to find the electronic transmittance as a function of the incident angle. We obtain that the strain induces a strong changes in the transmittance when the deformation is perpendicular to the incident axis. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J25.00010: Strain and adhesion of graphene sheets in shallow trenches. Constanze Metzger, Sebastian Remi, Silvia Kusminskiy, Antonio Castro Neto, Anna Swan, Bennett Goldberg Detailed high resolution micro Raman mapping of graphene exfoliated over shallow trenches demonstrates that single layer graphene adheres to the bottom of shallow trenches instead of remaining freely suspended. The analysis shows that the strain is surprisingly uniform. The high resolution Raman mapping of G and 2D Raman shift are consistent with uniaxial strain measurements, and the strain map concurs with our theoretical calculations that predict that a graphene sheet reaches minimal free energy sticking to the trench bottom even if that leads to moderate strain of about 0.6{\%} in the sheet. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J25.00011: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J25.00012: Correlation of optical and topographical measurements with electronic transport properties of epitaxial graphene on Si-face SiC Paul M. Campbell, Joshua Robinson, James C. Culbertson, Joseph L. Tedesco, Glenn G. Jernigan, Rachel L. Myers-Ward, Charles R. Eddy, Jr., D. Kurt Gaskill Epitaxial graphene layers grown on the Si face of (0001) SiC substrates by thermal desorption of Si were studied by Raman spectroscopy. Characteristic D, G, and 2D peaks were observed, and the 2D peak was used to extract layer thickness and film strain. These results, along with measured Hall mobility and topography from AFM, were used to establish those factors that influence the transport properties of graphene devices. The combination of uniform strain and nearly uniform thickness usually results in high-mobility graphene with an average room temperature Hall mobility $>$1000 cm$^{2}$/Vs. In contrast, films with nonuniform strain and thickness typically show lower values of mobility. These results will be useful for optimizing growth conditions to produce uniformly high-mobility graphene on full 2-inch and 3-inch SiC wafers, an agenda that we are now pursuing. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J25.00013: Reconstruction of Vacancy Defects in Graphene and Carbon Nanotube Gun-Do Lee, Euijoon Yoon, Nong-Moon Hwang, Cai-Zhuang Wang, Kai-Ming Ho Recently, various structures of vacancy defects in graphene layers and carbon nanotubes have been reported by high resolution transmission electron microscope (HR-TEM) and those arouse an interest of reconstruction processes of vacancy defects. In this talk, we present reconstruction processes of vacancy defects in a graphene and a carbon nanotube by tight- binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. We~found that a structure of a dislocation defect with two pentagon-heptagon (5-7) pairs in grapheme becomes more stable than other structures when the number of vacancy units is ten and over. The simulation study of scanning tunneling microscopy reveals that the pentagon-heptagon pair defects perturb the wavefunction of electrons near Fermi level to produce the $\surd 3 \times \surd$3 superlattice pattern, which is good agreement with experiment. It is also observed in our tight- binding molecular dynamics simulation that 5-7 pair defects play a very important role in vacancy reconstruction in a graphene layer and carbon nanotubes. [Preview Abstract] |
Session J26: Focus Session: Computational Nanoscience IV: Carbon- and Silicon-based Nanostructures
Sponsoring Units: DMP DCOMPChair: Sohrab Ismail-Beigi, Yale University
Room: 328
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J26.00001: New Routes Toward Nanotube Synthesis: Computation and Experiment Invited Speaker: The nanotechnology of the future demands controlled and consistent fabrication of different classes of nanostructures. Computational nanoscience can play an important role in the development of novel nanofabrication techniques. By revealing the fundamental differences in the mechanical bending behavior of nanofilms from that of micro- and macro-films, we have carried out atomistic simulations making significant contributions to advance a novel nanofabrication approach, the so-called ``nanomechanical architecture'' of thin films. This approach allows fabrication of different types of nanostructures, with a high level of control over their size and shape based on a priori theoretical/computational designs. The simulations have revealed a self-bending mechanism of Si (Ge) nanofilms leading to formation of pure Si (Ge) nanotubes, which greatly broadens the repertoire of nanotubes that can be made from multilayer films. Furthermore, applying the principle of nanomechanical architecture to the extreme case of the thinnest film possible, a single atomic layer of patterned graphene sheet, a new method for synthesizing carbon nanotubes with an unprecedented control over their size and chirality was proposed. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J26.00002: Thermal Properties of Vertically Aligned Carbon Nanotube-Nanocomposites Boundary Resistance and Inter- Carbon Nanotube Contact: Experiments and Modeling Hai Duong, N. Yamamoto, M. Panzer, A. Marconnet, K. Goodson, D. Papavassiliou, S. Maruyama, B. Wardle It is very significant to experimentally and numerically examine thermal properties of aligned CNT polymeric nanocomposites (PNCs) with variable volume fraction (vol{\%}) and controlled morphology. MWNTs having 200-1000um length synthesized by CVD are densified mechanically to achieve 1-20vol{\%}~Thermal conductivities of MWNT-epoxy along the MWNT axis with different vol{\%} are measured by the temperature gradient with an infrared microscope. The developed random walk model with taking into account the thermal boundary resistance (TBR) at the CNT-epoxy and/or CNT-CNT interface is validated by experimental results The different vol{\%} and CNT aligned in PNCs allows extracting the TBR values between MWNTs and epoxy or even between MWNTs by combining the developed model and experiment results. Further numerical investigation is conducted to compare systematically the thermal conductivity of both MWNT- and SWNT-epoxy with different CNT orientations under the wide range of the CNT vol{\%} with different TBR between the CNTs and the CNT-matrix and various inter-CNT contact degrees. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J26.00003: Thermal activation of interlayer bonding and its effect on properties of multiwalled carbon nanotubes Chun Tang, Wanlin Guo, Changfeng Chen We report molecular dynamics simulations of multiwalled carbon nanotubes (MWCNTs) at high temperatures. Our results show that thermally activated interlayer bonding have significant influence on structural, mechanical and electronic properties of MWCNTs and lead to new behaviors with implications for their applications. We examine the effect of strain and temperature conditions on the formation of interlayer bonding in MWCNTs and unveil the underlying atomistic mechanisms. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J26.00004: Effective Hamiltonian approach to bright and dark excitons in single-walled carbon nanotubes Sangkook Choi, Jack Deslippe, Steven G. Louie Recently, excitons in single-walled carbon nanotubes (SWCNTs) have generated great research interest due to the large binding energies and unique screening properties associated with one-dimensional (1D) materials. Considerable progress in their theoretical understanding has been achieved by studies employing the ab initio GW-Bethe-Salpeter equation methodology. For example, the presence of bright and dark excitons with binding energies of a large fraction of an eV has been predicted and subsequently verified by experiment. Some of these results have also been quantitatively reproduced by recent model calculations using a spatially dependent screened Coulomb interaction between the excited electron and hole, an approach that would be useful for studying large diameter and chiral nanotubes with many atoms per unit cell. However, this previous model neglects the degeneracy of the band states and hence the dark excitons. We present an extension of this exciton model for the SWCNT, incorporating the screened Coulomb interaction as well as state degeneracy, to understand and compute the characteristics of the bright and dark excitons, such as the bright and dark level splittings. Supported by NSF \#DMR07-05941, DOE \#De-AC02-05CH11231 and computational resources from Teragrid and NERSC. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J26.00005: First-principles studies of the optical properties of carbon nanohoops Joydeep Bhattacharjee, Jeffrey B. Neaton First proposed 70 years ago, cycloparaphenylenes -- cyclic aromatic molecules that are the shortest possible segment of an armchair nanotube -- have been only recently synthesized [1]. Using first-principles density functional theory and a Bethe-Salpter equation approach, we study structural, electronic, and optical properties of this novel class of materials, coined ``carbon nanohoops.'' Remarkably, we find, in agreement with experiments, that smaller hoops have smaller optical absorption gaps. This counterintuitive trend, opposite to that expected from ordinary quantum confinement, reflects a large increase in electron-hole interaction strength with decreasing hoop diameter. The diameter dependence of this interaction is thoroughly explored for several nanohoops, compared with an acyclic series, and discussed in the context of possible applications. [1] R. Jasti, J. Bhattacharjee, J. B. Neaton, and C. R.Bertozzi, submitted (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J26.00006: First-principles study of methane adsorption on defective graphitic nanostructures Brandon Wood, Debosruti Dutta, Ganapathy Ayappa, Shobhana Narasimhan Efficient storage of methane represents a significant challenge to large-scale implementation of natural gas-based consumer transportation. Activated carbons and related carbon-based nanoporous structures have garnered tremendous interest as storage media due to their unusually high absorptive capacities. However, systematic improvement of these materials relies on a fundamental understanding of the physical and chemical processes involved. We present here extensive energetic calculations of methane adsorption in model carbonaceous systems using density-functional techniques. As exact microstructures of activated carbons are difficult to obtain, we have attempted to isolate likely model nanostructures and defects, including surfaces, edges, point defects, and chemical functionalization. For each of these cases, we analyze changes in the structural, magnetic, and electronic properties upon adsorption. The defect structures exhibiting strongest methane adsorption are isolated, and the relevant mechanisms dominant in binding are identified. The impact of our results in terms of increasing methane absorptive capacity in activated carbons is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J26.00007: Effects of hydrogen chemisorption on the structure of carbon nanotubes Andre Muniz, Tejinder Singh, Dimitrios Maroudas We report results of a computational atomic-scale analysis of the effects of atomic hydrogen chemisorption on the structure of single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs). The analysis combines classical molecular-dynamics simulations with first-principles density functional theory calculations. We find that H chemisorption induces structural changes in SWCNTs associated with \textit{sp}$^{2}$-to-\textit{sp}$^{3}$ bonding transitions; increasing the H coverage beyond a critical level leads to axial and radial expansion of the SWCNTs that increases monotonically with H coverage. We also investigated the possibility of H-induced inter-shell \textit{sp}$^{3}$ C--C bond formation in MWCNTs. We find several pathways that lead to stable inter-shell bonded structures, which can act as seeds for nucleation of various crystalline carbon phases embedded into the MWCNTs. Finally, we show how the chiralities and relative alignments of adjacent graphene walls in MWCNTs determine the resulting crystalline structures. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J26.00008: First principles study of NH$_{3}$ adsorption on carbon nanowires Jorge-Alejandro Tapia, Alvaro-Daniel Sanchez, Cesar Acosta, Gabriel Canto Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. Theoretical and experimental studies of the NH3 adsorption in the carbon nanotubes report changes in the electronic properties of the carbon nanotubes. In the present work we have studied the electronic and structure properties of carbon nanowires (chain@SWCNT) when NH3 atoms are adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure and density of states (DOS). We found that the electronic character of the carbon chain of the chain@SWCNT system, can be modulate by NH3 adsorption. This research was supported by SEP under Grant No. PROMEP/103.5/07/2595 and the Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grants No. 82497 and 60534. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J26.00009: First-Principles Studies of Octacyclopropylcubane: A Novel High-Energy Density Material Steven L. Richardson, Reeshemah N. Allen, Daniel Finkenstadt, Michael J. Mehl, Mark R. Pederson The ongoing quest for synthesizing novel high-energy density materials (HEDMs) is clearly motivated by a search for new propellants and explosives. Recently de Meijere {\it et al.} have synthesized a new HEDM, octacyclopropylcubane ($C_{32}H_{40}$), in which the eight hydrogen atoms of cubane were replaced by cyclopropyl groups. In this work we report the results of a first-principles density-functional theory (DFT) calculation using the suite of codes known as NRLMOL (Naval Research Laboratory Molecular Orbital Library) to compute the structural, electronic, and vibrational properties of octacyclopropylcubane. We have calculated the vibrational properties of $C_{32}H_{40}$ and compare our results with experiment. We have also employed a DFT-based tight-binding scheme to compute the vibrational density of states for octacyclopropylcubane and compare our results with our full DFT-based results. Interesting enough, the geometry of the cyclopropyl groups in $C_{32}H_{40}$ does not allow for the quartic- concerted torsional mode (QCTM) that we and other workers have previously studied in octanitrocubane. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J26.00010: Efficient first-principles simulation of non-contact atomic force microscopy for structural analysis James Chelikowsky, Tzu-Liang Chan, Cai-Zhuang Wang, Kai-Ming Ho Non-contact atomic force microscopy (nc-AFM) has made significant advances that have allowed one to image a surface at atomic resolution. However, first-principles simulations of nc-AFM images remain a challenge because they involve calculations of the sample together with an atomic model of the AFM tip. We propose an efficient scheme to simulate nc-AFM images by using a first-principles self-consistent potential from the sample as input and without explicit modeling of the AFM tip. Our method is applied to various types of semiconductor surfaces including Si(111) $(7\times7)$, TiO$_{2}$(110) (1$\times1)$, Ag/Si(111)-$(\sqrt{3}\times\sqrt{3})R30^{\circ}$ and Ge/Si(105) $(1\times2)$ surfaces. Our method takes into account electronic effects of the tip-sample interaction, which are important for predicting the bright spot positions and the contrast change with AFM tip height. In addition, we obtain good agreement with experimental results and previous theoretical studies. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J26.00011: Ab initio calculation of Stokes shifts of hydrogenated silicon clusters Marie Lopez del Puerto, Manish Jain, James R. Chelikowsky There is experimental evidence that hydrogenated silicon clusters may have large Stokes shifts. The absorption and emission processes in these clusters are not symmetric because the clusters may undergo structural changes while in an excited state. Several theoretical methods have been used to study this problem, resulting in an array of predicted Stokes shifts that differ in energy by several eVs, and different predicted minimum-energy structures with either relaxed cores or relaxed outer shells. We calculate Stokes shifts using three different methods: density functional theory within the local density approximation (LDA), density functional theory within the generalized gradient approximation (GGA), and time-dependent density functional theory within the local density approximation (TDLDA). We find that these three different methods give similar results both for magnitude of Stokes shift and excited-state structures. The Stokes shift of hydrogenated silicon clusters of 5 to 35 silicon atoms range from 5.5eV to 0.8eV, decreasing with increasing cluster size. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J26.00012: Calculated Polarizabilities of Diamond and Silicon Nanoclusters Sudha Srinivas, Koblar Jackson, Mingli Yang, Julius Jellinek A scheme for decomposing the electric polarizability of a system into site-specific contributions is applied to hydrogenated nanoclusters of carbon and silicon. Site-specific dipole moments and polarizabilities are obtained from the response of charge densities to external electric fields, and decomposed into local and charge transfer components. We study changes in the polarizabilities of the C and Si atoms as the clusters grow in size. We find that exterior atoms have larger polarizabilities than interior atoms and that the charge transfer contribution to the total cluster polarizability increases with cluster size. We examine the relationship between the atomic polarizabilities in these clusters and bulk polarizability in carbon and silicon. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J26.00013: The Role of Vacancies on the Doping in Silicon Nanocrystals Jae-Hyeon Eom, Tzu-Liang Chan, James Chelikowsky We will present results for the effect of intrinsic defects (vacancies) on the doping of silicon nanocrystals by using first-principles calculations, {\it i.e.}, pseudopotentials in real space. We calculated the total energy of a B doped silicon nanocrystal as a function of the vacancy position and the nanocrystal size. We found that the most stable B site strongly depends not only on the cluster size, but also on the position of the vacancy. We also explored the evolution of the interaction between the vacancy and the B dopant by comparing the total energy for several nanocrystal sizes and configurations. [Preview Abstract] |
Session J27: Focus Session: X-ray and Neutron Instruments and Sciences I
Sponsoring Units: GIMSChair: Albert Macrander, Argonne National Laboratory
Room: 329
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J27.00001: Synchrotron X-ray Ultrafast Phase-Contrast Imaging Study of Fluids Invited Speaker: Visible light imaging has been the traditionally dominant technique for study of fluid mechanical systems. However, it suffers from strong refraction, reflection and scattering effects under various occasions. X-ray phase-contrast imaging technique with its inherent penetrability and edge-enhancement capability can circumvent these difficulties naturally. Here, I report that these advantages when combined with the high flux x-ray photons offered at 3$^{rd}$ generation synchrotron can offer a great tool for studying fluid mechanical systems with microsecond temporal and micron spatial resolutions. Several examples on dynamic multiphase flows and fluid singularities will be demonstrated. Future applications for other soft-condensed matter systems will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J27.00002: Implicit spatial averaging in inversion of inelastic x-ray scattering data P. Abbamonte Inelastic x-ray scattering (IXS) is usually said to measure the imaginary part of the dynamical density response of a material. However this is not rigorously true. The density response $\chi$, which describes the response of the system to a point charge source, is a function of {\it two} spatial coordinates and the time, i.e. $\chi = \chi(x_1,x_2,t)$. Its Fourier transform $\chi(k_1,k_2,\omega)$ is therefore a function of two, rather than just one, momenta. IXS does not probe this full response, but only its longitudinal or ``diagonal" part $Im[\chi(k,k,\omega)]$. In this talk I will show that recently developed IXS inversion algorithms [2], which have shown promise for imaging attosecond dynamics in real space, yield a specific spatial average of the response, i.e. $\chi(x_1,t) = \int dx' \chi(x_1,x_1+x',t)$. This can be thought of as an average over all possible source locations, a real space projection, or a specific type of Fourier space filtering. I will show, within a simple model, that the salient real space dynamics nonetheless survive, and that IXS inversion is still a useful and well-posed technique for imaging attosecond dynamics. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J27.00003: Quasi-zero dimensional CuB$_{2}$O$_{4}$ as an archetype for resonant inelastic X-ray scattering Jason Hancock, Guillaume Chabot-Couture, Yuan Li, Guerman Petrakovskii, Kenji Ishii, Ignace Jarrige, Jun-ichiro Mizuki, Martin Greven, Tom Devereaux We explore the general phenomenology of resonant inelastic scattering (RIXS) using CuB$_{2}$O$_{4}$, a network of CuO$_{4}$ plaquettes electronically isolated by B$^{+3}$ ions. Spectra show a small number of well-separated features, and we exploit the simple electronic structure to explore RIXS phenomenology by developing a calculation allowing for intermediate-state effects ignored in standard approaches. These effects are found to be non-negligible and good correspondence between our model and experiment leads to a simple picture of such phenomenology as the genesis of d$\to $d excitations at the K edge and intermediate-state interference effects. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J27.00004: Inelastic Neutron Scattering Study of $Ce_3Sn$ and $Ce_3In$ C.H. Wang, J.M. Lawrence, A.D. Christianson, E.A. Goremychkin, E.D. Bauer, N.R. de Souza , A.I. Kolesnikov In $Ce_3Sn$ and $Ce_3In$, the linear coefficients of specific heat $\gamma$ are 260 $mJ/mol Ce-K^2$ and 700 $mJ/mol Ce-K^2$, respectively. The Wilson ratio is 7.0 for $Ce_3Sn$ and 11.5 for $Ce_3In$. Such large values suggest the presence of ferromagnetic correlations in the ground state. Hence, this system is a potential candidate for studying the magnetic instability at a quantum critical point (QCP). As an initial measurement, we have measured the magnetic inelastic neutron scattering line shape of polycrystalline samples to determine the crystal field (CF) excitations. The low temperature spectrum of both $Ce_3Sn$ and $Ce_3In$ consist of a quasi- elastic line and two obvious inelastic lines resulting from the two excited crystal field doublets of $Ce^{3+}$ in the tetragonal symmetry. The quasi-elastic linewidth,which is related to the Kondo scale, is 3.2meV for $Ce_3Sn$ and 1.5meV for $Ce_3In$, consistent with the linear coefficients of specific heat. For $Ce_3Sn$ the two CF excitations are at 20meV and 35meV while for $Ce_3In$, the splitting is much larger giving the two excitations at 15meV and 47meV. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J27.00005: Resonant Multi-Wave X-Ray Diffraction Study on Iron Oxides System Shih-Chang Weng, Yen-Ru Lee, Jheng-Gang Chen, Chia-Hung Chu, Shih-Lin Chang The resonant X-ray scattering occurs when the incident photon energy is close to an absorption edge of a constituent atom. Under such circumstances, the corresponding atomic scattering factor will be modified due to anomalous dispersion, which is directly related to unoccupied states, magnetic moment, charge distribution, and the types of near-neighbor atoms. Therefore, the resonant X-ray scattering is widely used to investigate crystal structure, electronic structure, magnetic property and charge distribution, etc. On the other hand, the multi-wave diffraction can provide more information about reflection phase than the normal (two-beam) Bragg diffraction. In this paper, we will show that resonant multi-wave diffraction profiles in the vicinity of iron k-edge give the information about charge-ordering/charge distribution of iron oxide systems, such as Fe3O4. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J27.00006: X-ray Diffuse Scattering from Ultrafast Laser Excited Solids Mariano Trigo, Yu-Miin Sheu, Jian Chen, David Reis, Stephen Fahy, Eamonn Murray, Timothy Graber, Robert Henning Intense, ultrashort laser pulses can be used to excite and detect coherent phonons in solids. However, optical experiments can only probe a reduced fraction of the Brillouin zone and hence most of the decay channels of such coherent phonons become invisible. In contrast, time-resolved x-ray diffuse scattering (TRXDS) has the potential to be the ultimate tool to study these phonon decay processes throughout the Brillouin-zone of the crystal.~ In our work, performed at the BioCARS beamline at the Advanced Photon Source, we use synchrotron time-resolved diffuse x-ray scattering to study Si and Bi under intense laser excitation with 100 ps resolution. We show that reasonable signal levels can be achieved with incident flux of 10$^{12}$ photons comparable to the flux that will be available at future 4th generation sources such as the LCLS in a single pulse.~ These sources will also provide three orders of magnitude shorter pulses; thus, this experiment serves as a test of the feasibility of time-resolved X-ray diffuse scattering as a tool for studying nonequilibrium phonon dynamics in solids. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J27.00007: Antiferromagnetism in a Fe50Pt40Rh10 thin film investigated using neutron diffraction Gary Mankey, Dieter Lott, Jochen Fenske, Andreas Schreyer, Prakash Mani, Frank Klose, Wolfgang Schmidt, Karin Schmalzl, Elena Tartakovskaya The temperature-dependent magnetic structure of a 200 nm thick single-crystalline film of Fe[50]Pt[40]Rh[10] was studied by unpolarized and polarized neutron diffractions. By applying structure factor calculations, a detailed model of the magnetic unit cell was developed. In contrast to former studies on bulk samples, our experimental results show that the film remains in an antiferromagnetic state throughout the temperature range of 10--450 K. Remarkably, it can be demonstrated that the antiferromagnetic structure undergoes a smooth transition from a dominant out-of-plane order with the magnetic moments orientated in-plane to an in-plane order with the magnetic moments orientated perpendicular to the film plane. Theoretically this can be explained by the existence of two competing anisotropy contributions with different temperature dependencies. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J27.00008: Following Transient Phases at the Air/Water Interface Mati Meron, Jeff Gebhardt, Harold Brewer, P. James Viccaro, Binhua Lin A fast pixel array detector, the Pilatus 100K, has been used in studies of organic monolayers at the air-water interface. The area sensitivity and large dynamic range of the detector, in combination with a ``one dimensional pinhole'' geometry, make it possible to observe surface processes which were inaccessible to the previous generation of experimental techniques. Especially, time dependent phenomena acting on time scales ranging from seconds to minutes can be observed and analyzed. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J27.00009: Detecting Hidden Symmetries with Coherent X-Ray Diffraction Thomas Demmer, Alejandro Diaz Ortiz, Peter Wochner, Helmut Dosch An approach to analyze x-ray coherent diffraction patterns of amorphous systems is presented. We have investigated archetype hard-sphere systems where the local environment is simulated using different hundreds of geometric structures (i.e., regular and irregular polyhedra). The effect of positional and orientation randomness on the coherent diffraction pattern is studied numerically for samples containing up to $10^7$ particles. A library of such simulated diffraction data is then used to retrieve the underlying symmetries in amorphous systems. A discussion of the relevant experimental work is also presented. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J27.00010: Measuring phonon dispersion relations with momentum- and energy-resolved x-ray calorimetry. Ruqing Xu, Hawoong Hong, T.-C. Chiang X-ray Thermal Diffuse Scattering (TDS) is a powerful method for determining phonon dispersion relations, as has been demonstrated in a number of experiments. However, most previous studies employed a fitting procedure based on presumed atomic force constant models, and the results could be susceptible to systematic errors. In view of this issue, we have developed a new method based on momentum-resolved x-ray calorimetry: the phonon frequencies at specific wavevectors are determined directly from the temperature dependence of the TDS intensities, with no force constant models needed. A test of this method on Cu has yielded excellent results. However, a limitation exists for this method, as it requires data taken over a wide temperature range, and the minimum temperature must be significantly lower than the Debye temperature. This can be a problem for materials with phonon frequencies that change significantly with temperature. To overcome this problem, we are experimenting with another method: the scattered x-rays are energy analyzed, and the TDS intensity is determined relative to the Compton scattering intensity. With this internal intensity calibration, phonon frequencies can now be determined from TDS data taken over a narrow temperature range. Results of a test study on chromium will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J27.00011: Infrared emission induced by x-rays Richard Rosenberg, Mohammad Abu Haija, Simon Watkins Two of the most powerful methods for studying the properties of matter are Fourier transform infrared (FTIR) spectroscopy and synchrotron radiation (SR) based x-ray techniques. Having the ability to perform both types of research on the same samples at the same time would be a significant synergism. Furthermore, the spatial resolution of conventional FTIR microscopes is limited by diffraction, which in the mid IR is 2-20 $\mu $m, while SR based x-ray microscopes are capable of $<$30 nm diameter resolution. Thus, by utilizing nanometer sized x-ray beams to produce IR emission it should be possible to extend the spatial resolution of IR microscopy by orders of magnitude and simultaneously perform x-ray studies. To test the feasibility of this approach we have incorporated a commercial FTIR instrument into an existing ultra-high vacuum end station on an insertion device beamline at the Advanced Photon Source and measured the bandgap, exciton luminescence (0.4 eV) from InAs thin films. Results using both high intensity, near zero-order and low intensity, monochromatic x-rays will be presented. [Preview Abstract] |
Session J28: Focus Session: Graphene Device and Applications I
Sponsoring Units: FIAPChair: James Hannon, IBM
Room: 330
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J28.00001: Electronic properties of graphene and its operation at GHz frequencies Invited Speaker: Graphene, a two-dimensional carbon crystal, possesses great potential for applications in nanoelectronics because of its high intrinsic carrier mobility and the possibility of being processed using the well-established planar top-down technology in semiconductor industries. The former makes graphene an ideal candidate for electronic devices operating at high frequencies, while the latter allows us to tailor the transport properties of graphene devices by controlling their channel geometry. For example, it is, in principle, possible to create metallic and/or semiconducting graphene nanostructures if a precise edge termination can be achieved. In this talk, I will present our recent experimental studies on transport properties of graphene nanoribbons and high-frequency characteristics of graphene transistors operated at GHz frequencies. High-quality graphene nanoribbons with widths down to 30 nm are fabricated by e-beam lithography. In these graphene nanoribbon devices, clear plateau features are observed in the measured conductance as a function of gate voltage at T < 80K, indicating the formation of subbands due to quantum confinement in nanoribbons. This conductance quantization behavior is observed in both metallic and semiconducting nanoribbons, and provides the direct experimental evidence of quantum size confinement effects and the formation of subbands for 1D graphene nanostructures. To explore the high-frequency transport in graphene, top-gated graphene field-effect transistors are fabricated and S- parameter measurements are performed to obtain their transport properties at microwave frequencies. In these graphene transistors, we found that the measured intrinsic current gain shows the ideal 1/f frequency dependence, indicating an FET- like behavior in these devices. The cutoff frequency $f_T$ at which the current gain becomes unity is proportional to the dc transconductance $g_m$ of the device, and is consistent with the relation $f_T=g_m/(2\pi C_G)$. The peak $f_T$ was found to increase with a reducing gate length, and a cut-off frequency beyond 20GHz was measured in a graphene transistor with a gate length of 150 nm. This work is done in collaboration with Ph. Avouris, D. Farmer, K. Jenkins, A. Valdes-Garcia, V. Perebeinos, J. Small. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J28.00002: Graphene Frequency Multipliers Han Wang, Daniel Nezich, Jing Kong, Tomas Palacios In this paper we demonstrate a new application for graphene: full-wave signal rectification and frequency doubling. Due to its ambipolar transport properties, graphene field-effect transistors (GFET) show a ``V''-shaped transfer characteristic about the minimum conduction point. Frequency doubling can be realized with a single GFET by biasing the gate to the minimum conduction point and superimposing a sinusoidal input signal to the gate. Electrons and holes will conduct in alternative half cycles to produce an output signal at the drain, whose fundamental frequency is twice that of the input. Sub-linear IV characteristics of the GFET near minimum conduction point help improve the spectrum purity of the output signal. In our experiments, for an input frequency of 10 KHz, the output signal showed excellent spectrum purity (94{\%} of RF power at 20 KHz) in the absence of any filtering elements. Given the extremely high electron mobility in graphene ($>$100,000 cm$^{2}$/Vs at room-temperature), such ambipolar devices have the potential to operate at very high frequencies and allow the fabrication of new THz sources and sensors, as well as high speed transmitters and receivers. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J28.00003: Chemically derived graphene nanoribbons: physical properties and electronics Xinran Wang, Yijian Ouyang, Xiaolin Li, Li Zhang, Hailiang Wang, Jing Guo, Hongjie Dai Graphene electronics is a promising field of graphene research due to extremely high carrier mobility and the ability to fabricate true nanometer scale devices. We show that sub-10nm graphene nanoribbons, which are semiconductors with suitable bandgap for nano-electronics, can be synthesized via chemistry. Electrical transport measurements show that GNRs have finite bandgaps which are inversely proportional to widths. Unlike carbon nanotubes, all the sub-10nm GNRs are semiconductors and afford graphene field-effect transistors (FETs) with on-off ratio higher than 10$^{5}$ at room temperature. The performance of individual GNRFET is assessed and compared with CNTFETs. The scattering mean free path of carriers in GNRs is estimated, and the limiting factors are discussed. The performance of chemical GNRs and plasma etched GNRs are also compared and discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:51PM |
J28.00004: Non-volatile memory devices using graphene and ferroelectric thin films Invited Speaker: The unique linear energy band dispersion and its purely 2D crystalline structure have made graphene a rising star not only for fundamental research but also for nanoscale device applications. Here we demonstrate a novel non-volatile memory device using a combination of graphene and a ferroelectric thin film. The binary information, i.e. ``1'' and ``0'', is represented by the high and low resistance states of the graphene working channels and is switched by the polarization directions of the ferroelectric thin film. A highly reproducible resistance change exceeding 300\% is achieved in our graphene-ferroelectric hybrid devices under ambient conditions. The experimental observations are explained by the electrostatic doping of graphene by the remnant electrical field at the ferroelectric/graphene interface. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:27PM |
J28.00005: Tuning Disorder and Interactions in Graphene Invited Speaker: One (of many) unique aspects of graphene is that it is an atomically-thin two-dimensional electron system, open to manipulation and study using surface science techniques. This aspect of graphene has allowed us to tune both the disorder strength the interaction strength, allowing unprecedented control over a condensed matter system. Experiments are performed on atomically-clean graphene on SiO$_{2}$ in ultra-high vacuum. Addition of potassium to graphene is used to study the dependence of the mobility and minimum conductivity point on charged impurity density. Tuning the dielectric environment through addition of an ice overlayer has two effects: charged impurity scattering is reduced, due to reduced Coulomb interaction between impurities and carriers, while short-range scattering is increased, due to reduced screening. In sharp contrast to graphene with charged impurity disorder, which remains metallic at low temperature, even a small amount of irradiation-induced point disorder produces a divergence of the resistivity and insulating behavior at low temperature. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J28.00006: Graphene electronics: joule heat and charge density in active devices Marcus Freitag, Mathias Steiner, Yves Martin, Vasili Perebeinos, Zhihong Chen, James C. Tsang, Phaedon Avouris We use Raman scattering microscopy to measure the shifts of the 2D and G-bands resulting from the electronic power dissipation in the graphene sheet. Extracted images of the temperature distribution show peak temperatures of up to 1000K in the middle of the graphene device. The metallic contacts act as the dominant heat sinks, because the thermal conductivity of graphene is far greater than the gate-oxide thermal conductivity. We model thermal transport and obtain excellent agreement in peak temperature and functional form. Velocity saturation due to phonons with 50meV energy is observed, suggesting that substrate polar phonons limit the high-bias conduction in graphene. Trapped charges are also detected and we find that application of a high current is associated with drain-induced barrier lowering in back-gated graphene devices. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J28.00007: n-Type Behavior of Graphene Supported on Si/SiO2 Substrates Humberto Gutierrez, Hugo Romero, Ning Sheng, Jorge Sofo, Peter Eklund, Parsoon Joshi, Srinivas Tadigadapa Results are presented from an experimental and theoretical study of electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO$_{2}$ substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After $\sim $20 h at 200 \r{ }C in $\sim $5$\times $10$^{-7}$ Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of 4$\times $10$^{12}$ electrons/cm$^{2}$. This value is in excellent agreement with our theoretical calculations on SiO$_{2}$, where we have used molecular dynamics to build the SiO$_{2}$ structure and then density functional theory to compute the electronic structure. The essential theoretical result is that SiO$_{2}$ has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostatic model for the FET is also presented that produces an expression for the gate bias dependence of the carrier density. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J28.00008: Photocurrent imaging and efficient photon detection in a graphene transistor Fengnian Xia, Thomas Mueller, Roksana Golizadeh-mojarad, Marcus Freitag, Yu-ming Lin, James Tsang, Vasili Perebeinos, Phaedon Avouris We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. In this approach, the photon-induced current between the source and drain is measured when the excitation laser beam is scanned across device at various gate biases. Potential profiles are then inferred from photocurrent measurements. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than 1/3 of the total channel length from both source and drain sides, hence most of the channel is affected by the metal. The barrier height between the metal and graphene interface is experimentally determined to be around 95 meV from transport and photocurrent measurements. As the gate bias exceeds the Dirac point voltage, V$_{Dirac}$, the original p-type graphene channel turns into a p-n-p channel. When laser beam from He-Ne laser with a wavelength of 632.8 nm is focused on the p-n junctions, an impressive external responsivity of $\sim $0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection. The possibility of using graphene p-n junctions in high-bandwidth photonic applications is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J28.00009: Quantum capacitance measurement in high performance graphene field-effect devices Zhihong Chen, Joerg Appenzeller We demonstrate first quantum capacitance measurements on single-layer graphene devices and elucidate on their relevance for the extraction of mobility for scaled graphene FETs. We also show experimentally that multi-layer graphene devices can be easily distinguished from single-layer graphene within our capacitance measurement approach, a previously unnoticed fact. [Preview Abstract] |
Session J29: Focus Session: Spin Currents in Metals - New and Miscellaneous Topics
Sponsoring Units: GMAG DMP FIAPChair: Samir Garzon, University of South Carolina
Room: 333
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J29.00001: Electronic transport in ferromagnetic conductors Christian Wickles, Wolfgang Belzig We theoretically study ferromagnetic conductors using the Stoner model to describe the interaction between electron spin and magnetization. The latter can, in general, depend on time and space. We include impurity scattering processes for the electrons that lead to momentum relaxation, spin-flip and spin-dephasing. Utilizing Keldysh theory, we derive transport equations which allow to access interesting quantities such as domain wall resistance, electronic contribution to the magnetization damping coefficients and forces induced by non-equilibrium electron distributions in the presence of current flow. On the other side, a magnetization that shows temporal and spatial variation can induce current flow in the electron system. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J29.00002: Bulk Spin Pumping and Bulk Spin Transfer Torque in Two-band Magnetic Conductors Wayne Saslow For nonmagnetic materials, irreversible thermodynamics shows that thermal conduction and electrical conduction have a new and independent cross-coupling that yields a thermoelectric and an electrothermal effect. All of these terms are dissipative. However, for nonuniform two-band conducting magnets (e.g., within domain walls), electrical conduction and magnetization dynamics are cross-coupled by the up-band and down-band conductivities, without a need for a new cross-coupling. This yields both a bulk spin pumping term driving the current and a bulk spin transfer torque term driving the magnetization. Adiabatic in space, these terms are dissipative. In addition to these spin transfer and spin pumping terms corresponding to existing transport coefficients, for each spin component there are two types of additional transport coefficients. One type modifies the dissipative conductivity-driven terms in spin pumping and spin transfer torque, and itself is dissipative. The other type, non-adiabatic in space, is non-dissipative. We consider the situation where there is a spin current but no net current. Thermal effects are also considered, with temperature gradients having the same symmetry as gradients in the up and down spin electrochemical potentials. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J29.00003: Influence of a Transport Current on Magnetic Anisotropy Ion Garate, Allan MacDonald The microscopic understanding of the spin transfer torque (STT) is an essential ingredient in the quest to develop optimized spintronic devices. It is well-known that STT occurs whenever electric currents travel through non-collinear magnetic systems. In contrast, it is often overlooked that current-induced torques may also arise in uniformly magnetized systems due to the intrinsic spin-orbit coupling in the band structure of the ferromagnet. We relate this effect to the change in magnetic anisotropy in presence of a current, and use simple models to estimate the possible role of transport currents to modify the direction of the ferromagnetic easy axis and assist magnetization reversal. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J29.00004: Quantifying Spin Hall Effects in Gold Hall Bars Goran Mihajlovic, John E. Pearson, Samuel D. Bader, Axel Hoffmann, Miguel Angel Garcia Spin Hall effects manifest the fundamental interdependence between charge and spin transport. We studied these effects experimentally by measuring the non-local resistance,$ R_{nl}$, in mesoscopic Au Hall bars where spin current generation and detection are spatially separated in two side arms, while a bridging arm acts as the spin conduit. The measured $R_{nl}$ decreases monotonically with decreasing temperature, changing sign from positive to negative. This can be understood by modeling $R_{nl}$ as a sum of two components; a positive, ohmic component, arising from the charge current, and a negative component, due to spin Hall effects and spin diffusion. By varying the spacing between the side arms, the components can be separated. We determined the spin diffusion length, the$_{ }$spin Hall angle and the spin Hall conductivity. We found spin Hall angles of order 0.1, with a temperature dependence proportional to the resistivity, while the spin Hall conductivity was almost temperature independent. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J29.00005: Investigation of Spin-Torque Effects on the Exchange Bias of Ferromagnet/Antiferromagnet Bilayers Kiran V. Thadani, R.A. Buhrman, D.C. Ralph Spin-polarized current, generated by one ferromagnetic layer in a magnetic multilayer structure, can deposit spin angular momentum into a second ferromagnetic layer, causing it to either reversibly switch its orientation or oscillate in steady state at microwave frequencies. Recent calculations and experiments have investigated the possibility that spin torque might also alter the structure of an antiferromagnet [1], thereby affecting the exchange-bias field produced by the antiferromagnet on an adjacent ferromagnetic layer [2, 3]. Here we report studies made using nanopillar samples in which the free magnetic layer is exchange-biased to an antiferromagnet, which allow a direct measurement of the magnitude of the exchange bias and its current dependence. We will also investigate the degree to which the exchange bias alters the damping of the free-layer magnet and the extent to which the effective damping can be controlled with current. [1] A. S. Nunez et al., Phys. Rev. B 73, 214426 (2006). [2] Z. Wei et al., Phys. Rev. Lett. 98, 116603 (2007). [3] S. Urazhdin et al., Phys. Rev. Lett. 99, 046602 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:51PM |
J29.00006: Tunnel barrier enhanced voltage signals generated by magnetization precession of a single ferromagnetic layer Invited Speaker: A variety of experimentally observed phenomena involving nonlocal magnetization dynamics in magnetic multilayers are due to two complementary effects: (i) the transfer of spin angular momentum accompanying charge currents driven by the applied bias voltage between ferromagnetic layers results in torques that (for sufficiently high current densities) generate spontaneous magnetization precession and switching; and (ii) the precessing magnetization of a ferromagnet (FM) pumps spins into adjacent normal metal layers (NM) with no applied bias. In particular, the spin pumping effect is a promising candidate for realizing a spin battery device [1] as a source of elusive pure spin currents (not accompanied by any net charge transport) emitted at the FM/NM interface, where steady magnetization precession of the FM layer is sustained by the absorption of external rf radiation under the FMR conditions. We report the electrical detection of magnetization dynamics in an Al/AlO$_{x}$/Ni$_{80}$Fe$_{20}/$Cu tunnel junction, where a Ni$_{80}$Fe$_{20}$ ferromagnetic layer is brought into precession under the ferromagnetic resonance (FMR) conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed in Cu/FeNi/Pt structures [2]. A structure of Cu (100nm)/Al (10nm)/AlO$_{x}$ (2.3nm)/Ni$_{80}$Fe$_{20}$ (20nm)/Cu (70nm)/Au (25nm) was fabricated on a Si substrate with a 1$\mu$m thick thermal oxide layer. The bottom-most 100 nm Cu layer was patterned into a coplanar waveguide (CPW) and the rest of the structure was patterned into a pillar structure on the signal line of the CPW. Dc voltages $\sim\mu$V were observed in the Al/AlO$_{x}$/Ni$_{80}$Fe$_{20}$/Cu tunnel junction when the Ni$_{80}$Fe$_{20}$ is in the ferromagnetic resonance. The dc voltages increase as the precession cone angle and frequency increase. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal. \\[4pt] [1] A. Brataas et al. PRB 66, 060404 (2002)\\[0pt] [2] M. V. Costache et al. PRL 97, 216603 (2006) [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J29.00007: FMR and voltage induced transport in normal metal-ferromagnet-superconductor trilayers Hans Joakim Skadsem, Arne Brataas, Jan Martinek In recent years, hybrid nanoscale circuits containing normal conductors, ferromagnets, and superconductors have been realized. These structures allow observation and understanding of the competition between ferromagnetism and superconductivity. In this talk, we consider charge and spin transport in normal metal-ferromagnet-superconductor trilayers induced by bias voltage and/or magnetization precession. Transport properties are discussed in terms of time-dependent scattering theory. We compute the charge and spin current response to bilinear order in precession frequency and bias voltage and express the results in terms of spin-dependent conductances. Simplified conductance expressions are obtained when the ferromagnet is longer than the transverse spin coherence length. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J29.00008: Spin torque from tunneling through impurities in a magnetic tunnel junction Turan Birol, Piet Brouwer We study impurity-mediated transport in a magnetic tunnel junction (MTJ) in the sequential tunneling regime. We address the conductance of the MTJ as well as the spin transfer torque. We show that the torque from impurity-mediated tunneling can be distinguished from that from direct tunneling through its dependence on the barrier thickness and the angle between the ferromagnetic moments. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J29.00009: Thermoelectric transport and thermal spin currents in ferromagnetic films and nanostructures Azure Avery, Rubina Sultan, Barry Zink For fundamental physics, understanding the mechanisms behind giant magnetoresitance (GMR) and its related properties, magnetoresistance (MR) and magnetothermopower (MTEP), is crucial, especially for nanoscaled structures. Though progress has been made in understanding electron transport through magnetic thin films and multilayers, far less is understood about the mechanisms behind thermal transport in these systems. This is due, in part, to the difficulty of measuring thermal properties of these low-dimensional systems. We present a robust technique for accurately measuring thermal conductivity ($ k_{\parallel} $), thermopower ($ \alpha $), and MTEP in nanoscale magnetic materials using micromachined silicon nitride thermal isolation structures. We outline the fabrication of the structures and present our measurement results for ferromagnetic thin films and nanowires. Finally, we present how this technique is applied to testing the validity of current models explaining the mechanisms of thermal transport, such as thermal spin currents, in ferromagnetic films and nanostructures. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J29.00010: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J29.00011: Magnetoresistive junctions based on epitaxial graphene and h-BN Oleg Yazyev, Alfredo Pasquarello Using a first-principles approach, we investigate the structural, magnetic and transport properties of interfaces based on epitaxially grown monolayer graphene and hexagonal boron nitride ($h$-BN) in combination with ferromagnetic transition metals (Fe, Co and Ni). Such structurally well defined interfaces based on (111) fcc or (0001) hcp transition metals can be produced using simple manufacturing processes. Our calculations predict magnetoresistance ratios over 100\% for certain junction compositions. In addition, such systems feature strong antiparallel (Fe and Co) and parallel (Ni) exchange coupling across the interface combined with low junction resistance. The predicted properties position such magnetoresistive junctions as an interesting alternative to the currently used giant and tunneling magnetoresistance systems and make them suitable for practical applications. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J29.00012: Spin injection and transport in single layer graphene spin valves Wei Han, Wei-Hua Wang, Keyu Pi, Kathy McCreary, Wenzhong Bao, Yan Li, Chun Ning Lau, Roland Kawakami Graphene is an attractive material for spintronics due to its tunable carrier concentration and polarity, weak spin-orbit coupling, and the prediction of novel spin-dependent behavior. We investigate the spin injection and transport in single layer graphene (SLG) spin valves at room temperature. Raman spectroscopy is used to identify SLG. SLG spin valve devices are fabricated by growing cobalt electrodes, defined by electron beam lithography, on top of SLG. Nonlocal resistance measurements are performed on these SLG spin valve devices in order to study the spin injection and transport properties. Our results show that the nonlocal magnetoresistance (MR) is dependent on the gate voltage. Also, the nonlocal MR shows some variation as a function of DC bias current. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J29.00013: Falling Magnets and Electromagnetic Braking Christopher Culbreath, Peter Palffy-Muhoray The slow fall of a rare earth magnet through a copper pipe is a striking example of electromagnetic braking; this remarkable phenomenon has been the subject of a number of scientific paper s [1, 2]. In a pipe having radius R and wall thickness D, the terminal velocity of the falling magnet is proportional to (R\^{}4)/D. It is interesting to ask what happens in the limit as D becomes very large. We report our experimental observations and theoretical predictions of the dependence of the terminal velocity on pipe radius R for large D. [1] Y. Levin, F.L. da Silveira, and F.B. Rizzato, ``Electromagnetic braking: A simple quantitative model''. \textit{American Journal of Physics}, \textbf{74}(9): p. 815-817 (2006). [2] J.A. Pelesko, M. Cesky, and S. Huertas, Lenz's law and dimensional analysis. \textit{American Journal of Physics}, \textbf{3}(1): p. 37-39. 2005. [Preview Abstract] |
Session J30: Focus Session: Multiferroic Manganites
Sponsoring Units: DMP GMAGChair: Despina Louca, University of Virginia
Room: 334
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J30.00001: Order Parameters and Phase Diagram of Multiferroic RMn2O5 Invited Speaker: \def\rhov{{\mbox{\boldmath{$\rho$}}}} \def\tauv{{\mbox{\boldmath{$\tau$}}}} \def\Lambdav{{\mbox{\boldmath{$\Lambda$}}}} \def\sigmav{{\mbox{\boldmath{$\sigma$}}}} \def\xiv{{\mbox{\boldmath{$\xi$}}}} \def\chiv{{\mbox{\boldmath{$\chi$}}}} \def\oh{{\scriptsize 1 \over \scriptsize 2}} \def\ot{{\scriptsize 1 \over \scriptsize 3}} \def\of{{\scriptsize 1 \over \scriptsize 4}} \def\tf{{\scriptsize 3 \over \scriptsize 4}} Recently there has been great interest in systems which display phase transitions at which incommensurate magnetic order and a spontaneous polarization develop simultaneously. Perhaps the most puzzling and seemingly complicated behavior occurs in the series of compounds RMn$_2$O$_5$, where R=Y, Ho, Er, Tb, Tm, and Dy. (For references to experimental data, see [1].) The sequence of magnetoelectric phases of the type I systems R=Tb, Ho, and Dy is slightly different from that of the type II systems R= Y, Tm, and Er. At about 45K both types develop essentially collinear modulated magnetic order into a ``high-temperature ordered" (HTO) phase with a wave vector ${\bf q} = (1/2-\delta , 0, 1/4 + \epsilon)$ where $\delta$ and $|\epsilon|$ are of order 0.01 and the spontaneous polarization is zero. There is a lower-temperature phase transition to a ferroelectric phase in which transverse magnetic order appears and produces a magnetic spiral with $\delta=\epsilon=0$. In type I systems, this transition occurs directly from the HTO phase, whereas for type II systems, there is an intervening ferroelectric phase in which $\epsilon=0$, but $\delta$ remains nonzero. %At low ($<10$K) temperature the classification into types I and II %breaks down and each system requires its own specific description. I will discuss a Landau free energy[1] which allows both type I and type II sequences of phase transitions. This theory is couched in terms of the uniform polarization vector ${\bf P}$ and two complex-valued magnetic order parameters $\sigma_1({\bf q})$ and $\sigma_2 ({\bf q})$ whose symmetry follows from the magnetic structure analyses.[2] The magnetoelectric coupling and the competition between commensurate and incommensurate phases are analyzed. \\[4pt] [1] A. B. Harris, A. Aharony, and O. Entin-Wohlman, Phys. Rev. Lett. {\bf 100}, 217202 (2008) and J. Phys. Condens. Mat. {\bf 20}, 434202 (2008). \\[0pt] [2] A. B. Harris, Phys. Rev. {\bf 76}, 054447 (2007); A. B. Harris, M. Kenzelmann, A. Aharony, and O. Entin-Wohlman, Phys. Rev. B {\bf 78}, 014407 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J30.00002: Local Structure Investigation of ReMn$_{2}$O$_{5}$ A. Masadeh, T. Tyson, S.-W. Cheong The temperature dependent structure of the ReMn$_{2}$O$_{5}$ (Re=rare earth) system has been examined by the x-ray pair distribution function method based on high-q data. Temperature dependent measurements reveal anomalies in the short range structure involving oxygen atoms. Comparison with Rietveld and XAFS analysis will be made. The detailed temperature dependent structure on multiple length scales will be presented with implications for the observed low temperature ferroelectric properties. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J30.00003: Magnetic Field Dependent Changes in the Local Structure of ReMn$_{2}$O$_{5}$ M. Deleon, T.A. Tyson, Z. Chen, S.-W. Cheong The low temperature structure of ReMn$_{2}$O$_{5}$ systems has been studied by x-ray absorption spectroscopy. Temperature dependent measurements indicate that Re-O correlations play an important role in the low temperature properties. While no variation in the Mn-O distribution is observed with magnetic fields, we find evidence for direct coupling of the Re-O distribution to an external magnetic field. The results suggest that polarization of the Re-O bonds may contribute significantly to magnetic field induced electrical polarization. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J30.00004: Studies of Competing Order in Multiferroic RMnO$_{3}$ Prasenjit Guptasarma, Ying Zou, Shishir Ray, Somaditya Sen, Mark Williamsen Unconventional magnetic order, a ferroelectric background and the possibility of coupled ground states, together with competing spin, charge, lattice and orbital degrees of freedom, give systems such as $R$MnO$_{3}$ ($R$=Rare Earth), an ABO$_{3}$-type compound, a rich and fascinating phenomenology. The possibility of using these materials in switching, in spin based electronics, and as materials with negative refractive index, make them important candidates for device applications. Here we present a review of our studies of the detailed phenomenology of a series of single crystals of $R$MnO3 grown from a floating zone, for different Rare Earths $R$, and by inducing structural distortions and charge disproportionation through substitutions at both the A and the B sites. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J30.00005: Enhanced local lattice distortions with the antiferromagnetic transition in the multiferroic LuMnO$_{3}$ Despina Louca, Thomas Proffen, Seung-Hun Lee, Sang-Wook Cheong The ferroelectric hexagonal manganite, LuMnO$_{3}$, has been investigated via neutron scattering and the pair density function analysis to determine the nature of the local atomic distortions with the antiferromagnetic transition, T$_{N}$, of the Mn ions. While in previously reported neutron diffraction data, it was shown that all atomic coordinates changed based on symmetry considerations with T$_{N}$, we hereby show that it is the ferroelectric motion of the Lu ions coupled with O distortions that exhibits a strong temperature dependence below T$_{N}$ as reflected in the Lu-O bonds. This suggests an enhancement of the net electric polarization below T$_{N}$. At the same time, the motion of the apical O1 and O2 ions distorts the MnO$_{5}$ bipyramids, leading to more buckling of the ab-layers. However, the Mn ions do not appear to distort significantly away from their equilibrium position. The oxygen distortions induced with the spin reorientations below T$_{N}$ may be the cause for the Lu ion displacements through electrostatic interactions and this in turn produces coupling to the electric dipole moments. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J30.00006: Pressure Dependence of Structure Stability of Multiferroic Hexagonal-RMnO$_{3}$ Zhiqiang Chen, Peng Gao, Trevor. A. Tyson, Zhenxian Liu, Jinzhu Hu, Chenglin Zhang, Sung-Baek Kim, Sang-Wook Cheong We present high pressure IR and X-ray diffraction measurements of the hexagonal multiferroic systems HoMnO$_{3}$, YMnO$_{3}$ and LuMnO$_{3}$. Measurements were conducted over the pressure range ambient to $\sim $20 GPa. No phase changes were observed over this broad range of hydrostatic pressures. These suggest that the hexagonal structure is stable at higher pressures. The thermal treatment is necessary to overcome the barrier (breaking and reconnection of bonds) to achieve the hexagonal to orthorhombic phase change. A discussion of the effect of hydrostatic pressure on the ferroelectric properties of these systems will be given based on comparisons with density functional calculations. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J30.00007: X-ray absorption spectroscopy studies of YMnO$_{3}$, HoMnO$_{3}$, and Y$_{.4}$Ho$_{.6}$MnO$_{3}$ Relja Vasic, Dario Arena, Joseph Dvorak, Haidong D. Zhou, Chris R. Wiebe, Gerald Lucovsky, Marc Ulrich We have investigated three hexagonal perovskites, YMnO$_{3}$, HoMnO$_{3}$, and Y$_{.4}$Ho$_{.6}$MnO$_{3 }$by O K$_{1}$ and Mn L$_{2,3}$ edge X-ray absorption spectroscopy. In YMnO$_{3}$ and HoMnO$_{3}$ the lowest energy features are predominantly Mn 4p and 3d states with a least five distinct states occurring at approximately the same X-ray energies in both samples. We associate this portion of electronic structure with the trigonal bipyramid bonding symmetry of a five-fold coordinated Mn. Higher energy transitions in the XAS OK$_{1}$ spectra are broader and associated with Ho 5d and Y 4d orbitals. Compared with YMnO$_{3}$ and HoMnO$_{3 }$Mn 3d, and Ho 5d and Y 4d spectral features, the corresponding features in theY$_{.4}$Ho$_{.6}$MnO$_{3}$ O K$_{1}$ spectrum exhibit broader features fewer in number. These are consistent with random alloy bonding in which the Ho and Y are randomly distributed on the A-atom sub-lattice. We will discuss the electronic structure of these empty states in the context of symmetry adapted linear combinations of molecular orbital O 2p*, and Mn 3d*, Ho 5d* and Y 4d* nearest neighbor states. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J30.00008: Competing Magnetic Interactions in Magnetoelectric YbMnO$_{3}$ Shishir Ray, Ying Zou, Mark Williamsen, Somaditya Sen, Larry Buroker, Prasenjit Guptasarma The (\textit{RE)}MnO$_{3}$ (\textit{RE }= \textit{Rare Earth}) series of magnetoelectrics exist as both hexagonal and orthorhombic lattice structures. These have recently attracted much attention due to possible applications in spintronics, in switching, and as media with negative refractive index. YbMnO$_{3}$ is hexagonal with ferroelectricity (Tc$\sim $ 970K) and antiferromagnetism (T$_{N}$: Mn$\sim $80K, Yb$\sim $5K) in the same phase. Here, we report detailed studies of the H-T phase diagram using a high-quality single crystal of YbMnO$_{3}$ grown by floating zone. We examine the magnetically ordered phases of Yb: Yb3+ (2$a)$ via Yb-Yb, and Yb3+ (4$b)$ via Yb-Mn interactions within the hexagonal YbMnO$_{3}$ structure, and report several new features in the magnetic phase diagram. Contrary to recent reports, we observe that the magnetic moment of Yb does not become fully suppressed with external magnetic field, but rather directly transitions from the A$_{1}$ order into the A$_{2 }$order at low temperature. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J30.00009: Resonant Soft X-ray Scattering Study on Multiferroic TbMnO$_{3}$ Hoyoung Jang, J.-S. Lee, K.-T. Ko, J.-Y. Kim, K.-B. Lee, J.-H. Park, C. L. Zhang, S.-W. Cheong TbMnO$_{3}$ has been extensively studied both experimentally and theoretically about its fascinating properties (e.g., magneto-electric coupling, spiral magnetic order). Comprehensive resonant x-ray scattering at absorption edges of Mn and Tb, mainly at Mn $L$-edge, were performed on single crystals of TbMnO$_{3}$ to understand the fascinating properties. Under \textit{Pbnm} space group of TbMnO$_{3}$, we found the forbidden reflections, such as (0 q 0), (0 1-2q 0), and (0 2q 0). Each reflection was also investigated by dependency on temperature, photon energy, photon polarization, and etc, which gives us a clue to unveil hidden properties of TbMnO$_{3}$. Detailed description will be dealt with this presentation. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J30.00010: High Temperature XPS Studies of a Single Crystal of Magnetoelectric TbMnO$_{3}$ Mark Williamsen, Shishir Ray, Ying Zou, Marshall Onellion, Prasenjit Guptasarma Recent interest in magnetoelectric and unconventional magnetic phenomena in materials such as the RMnO$_{3}$ series (R=Rare Earth) reveals competing electronic ground states, together with electronic and crystal structure phase transitions both above and below room temperature[1]. A complete elucidation of these phenomena calls for detailed studies of electronic properties at varying temperatures. Using a new high temperature insert with a resistive heater stage fitted to a Cylindrical Mirror Analyzer at the Synchrotron Radiation Center in Stoughton, WI, we have performed x-ray photoemission studies at variable temperatures (200-1000K) in a large single crystal of magnetoelectric orthorhombic TbMnO$_{3}$ (0.5 x 0.5 x 1cm) grown by us from a floating zone. We observe peak splitting in the Mn 3p and Tb 4d peaks near a structural distortion observed by us. Together with resistivity and detailed Rietveld analysis of powder x-ray diffraction, we ascribe this to possibly an orbital order-disorder transition around 900K. [1]J.S.Zhou,Phys.Rev.Let. 96,247202(2006) [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J30.00011: Emergence of the multiferroic state in $R$MnO$_{3}$ ($R$ = Sm and Nd) crystals G. Balakrishnan, D. O'Flynn, C.V. Tomy, M.R. Lees In order to understand the emergence of multiferroic behaviour in the $R$MnO$_{3}$ compounds, it is educational to study the relationship between ferroelectricity and magnetoelastically induced lattice modulations. Lattice modulations in $R$MnO$_{3}$ are strongly dependent on the Mn-O-Mn bond angle ($\Phi )$, which in turn is determined by the ionic radii ($r_{R})$ of the $R$ atoms. Multiferroic properties have been observed in the orthorhombic RMnO$_{3}$ (R = Tb,Dy) compounds, in which $\Phi $ is close to 145\r{ }. In order to induce multiferroic behaviour in other magnetic members of the orthorhombic $R$MnO$_{3}$, and to tune the structure to be in the same region of the phase diagram as Tb/DyMnO$_{3}$, it is necessary to substitute at the $R$ site with a suitable (smaller) atom. We have achieved this in SmMnO$_{3}$ and NdMnO$_{3}$ by substitutions at the Sm and Nd sites with smaller $R$ ions. In the optimally substituted compounds (40 to 50{\%}), we observe an additional magnetic transition. Investigations of the dielectric properties of the crystals reveal anomalies in the dielectric properties coincident with this magnetic transition, analogous to those exhibited by Tb/DyMnO$_{3}$, indicative of multiferroic behaviour. We present detailed investigations of the magnetic, dielectric and structural properties in single crystals of selected compositions. [Preview Abstract] |
Session J31: Focus Session: Magnetic Multilayers
Sponsoring Units: DMP GMAGChair: Kathryn Krycka, National Institute of Standards and Technology
Room: 335
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J31.00001: 400-Fold Reduction in Saturation Field by Stress Relief in Multilayers William Egelhoff, John Bonevich, Carlos Beauchamp, Gery Stafford, John Unguris, Philip Pong, Robert Mcmichael A common problem in soft magnetic thin films is increased saturation field due to stress buildup with increasing thickness. We have found a solution to the problem using multilayers of a magnetic thin film and a film that is either not lattice matched or has a different crystal structure. Reductions in the saturation field as large as 400 fold are found. The ultrasoft Ni$_{77}$Fe$_{14}$Cu$_{5}$Mo$_{4}$ alloy can have saturation fields as small as 0.005 mT (0.05 Oe) for 10 nm thick films. However, for films 400 nm thick (which are needed for some applications) the saturation field is typically 20 mT. Splitting this magnetic thin film up into segments 100 nm thick separated by a 5 nm Ag film reduces the saturation field to 0.05 mT. Alternatively, using a 2 nm CoFe film yields a saturation field of 0.1 mT. A tensile stresses of 7.35 x 10$^{9}$ dynes/cm$^{2}$ was measured in the 400 nm film and 3.7 x 10$^{7}$ dynes/cm$^{2 }$for the multilayer with Ag. The highly-stressed Ni$_{77}$Fe$_{14}$Cu$_{5}$Mo$_{4}$ develops a magnetostriction coefficient of $\sim $5 ppm, although in the unstressed state its magnetostriction coefficient is near zero. In conclusion, we have found a solution to the stress-induced large saturation fields in an otherwise soft magnetic film. The results should be important for ultra-low magnetic-field tunnel-junction sensors and magnetic flux concentrators. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J31.00002: Structure and magnetic properties of magnetron-sputtered FePt/Au superlattice films Yongsheng Yu, Haibo Li, XingZhong Li, Lanping Yue, Weili Li, Mei Liu, Yumei Zhang, Weidong Fei, David J. Sellmyer FePt/Au multilayer films were prepared with sputtering and the effects of Au thickness and annealing temperature on the structure and magnetic properties were investigated. Superlattice structure was induced by thicker Au layer. The interatomic spacing d(220) in the fcc FePt lattice increases with increasing Au thickness, indicating increasing strain energy in fcc FePt lattice. After annealing at 300$^{\circ}$C, FePt films with Au layer of 3.5nm became ordered and the multilayer structure were retained. The strain energy in fcc FePt lattice appears to be responsible for lowering the ordering temperature of the FePt phase. For films annealed at higher temperatures, thicker Au layer restrained the ordering of FePt phase, which led to a decrease of coercivities. -- This research is supported by DOE and NCMN. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J31.00003: Controlling Interlayer exchange coupling in ultra narrow Fe/Pt multilayered nanowire: an ab initio study Puspamitra Panigrahi, Ranjit Pati Interest in the study of magnetic/non-magnetic multilayered structures took a giant leap since Gr\"unberg and his group established that the value of interlayer exchange coupling (IEC) depends upon the non-magnetic spacer width. The recent increase in demand for device miniaturization compelled researchers to look for novel nanoscale multilayered structures. Towards this effort, we have studied IEC in one dimensional Fe/Pt multilayered nanowires using first principles density functional approach. Our result shows the exchange coupling energy (J) to switch sign as the width of the non-magnetic Pt spacer varies. The competition among short and long range direct exchange and the super exchange is recognized to play an important role for the non-monotonous sign in IEC depending upon the width of the Platinum spacer layer. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J31.00004: Design of Co/Pd multilayer system with antiferromagnetic-to-ferromagnetic phase transition Invited Speaker: Among the known magnetic material systems there are only very few examples of materials that undergo a temperature dependent antiferromagnetic-to-ferromagnetic phase transition, and of these only the chemically ordered alloy FeRh exhibits this transition near room temperature [1, 2]. Here we present a perpendicular anisotropy multilayer structure that mimics FeRh. The basic idea is to use two stacks of Co/Pd multilayers with large perpendicular magnetic anisotropy and high Curie temperature, T$_{C}$, separated by a layer providing antiferromagnetic coupling, and a CoNi/Pd multilayer with perpendicular anisotropy with a lower T$_{C}$, interlayer, in the range of the desired AF-FM transition temperature, T$_{AF-FM}$. At room temperature this system behaves as two antiferromagnetically coupled layers with a low perpendicular remanent magnetic moment. As the temperature is raised to approach T$_{C}$, $_{interlayer}$ the magnetization of the interlayer is gradually reduced to zero, and consequently its coupling strength is reduced. Eventually, the effective coupling between the two high-K$_{U}$, high-T$_{C}$ layers becomes dominated by their dipolar fields, resulting in a parallel alignment of their moments and a net remanent magnetic moment equal to the sum of the moments of the two high-T$_{C}$ layers [2]. \\[4pt] [1] J. S. Kouvel and C. C. Hartelius, J. Appl. Phys. \textbf{33} (1962) p1343 \\[0pt] [2] J.-U. Thiele, E. E. Fullerton, S. Maat, Appl. Phys. Lett. \textbf{82} (2003) p2859 \\[0pt] [3] J.-U. Thiele. T. Hauet. O. Hellwig, Appl. Phys. Lett. \textbf{92} (2008) 242502. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J31.00005: Graded Magnetic Anisotropy in Co/Pd Multilayers B. J. Kirby, J. E. Davies, S. M. Watson, R. D. Shull, J. A. Borchers, G. T. Zimanyi, Kai Liu As the magnetic recording industry looks beyond perpendicular recording, multilayered exchange coupled media have demonstrated potential for increased storage density. Recent work has shown further enhancements when the anisotropy is gradually increased from a soft top to a hard bottom region.[1] However, creating graded anisotropy structures is difficult, and convincingly demonstrating such a gradient is also challenging. Since the coercivity of Co decreases with increasing thickness, we attempted to create graded anisotropy structures by sputtering [Co/Pd] superlattices with progressively varying Co layer thicknesses. We probed the depth dependent anisotropy of the samples using polarized neutron reflectometry (PNR), a technique sensitive to the depth-dependent magnetic composition of thin films. The sample magnetization vector M was bent away from the out-of-plane easy axis direction by an applied magnetic field H, and the depth profile of the in-plane component of M(H) was measured. Our results clearly demonstrate that samples with graded Co thickness also exhibit graded anisotropy. Further, comparisons of samples with different levels of gradient discretization shed light on the nature of the interlayer exchange coupling in a graded anisotropy system. [1] D. Suess, Appl. Phys. Lett. 89, 189901 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J31.00006: Coercivity enhancement in (Co/CoO)$_{n}$ superlattices Srinivas Polisetty, Christian Binek The temperature dependence of the coercivity is studied in (Co/CoO)$_{n}$ periodic multilayer thin film superstructures below and above the exchange bias blocking temperature. The ferromagnetic Co thin films are grown with the help of MBE at a base pressure of 10E-10 m.bar whereas antiferromagnetic CoO thin films are grown from in-situ oxidized Co. The thicknesses of these films are monitored by reflection high energy electron diffraction (RHEED). A mean-field theory$^{1}$ is outlined which provides an analytic and intuitive expression for the enhancement of the coercivity of the ferromagnet which experiences the exchange coupling with a neighboring antiferromagnet. An experimental approach is developed allowing to determine the interface susceptibility of an individual antiferromagnetic pinning layer by systematic change in the thickness of the antiferromagnet thin films in various sets of superlattice samples measured at different temperatures, respectively. The experiment enables us to separate out the intrinsic coercivity from the contribution induced by exchange coupling at the interface. It is the goal of our study to evidence or disprove if it is simply this susceptibility of the reversible interface magnetization creating the spin drag effect and by that the coercivity enhancement. Financial support by NSF through CAREER DMR-0547887, NRI and Nebraska MRSEC. $^{1}$G. Scholten, K. D. Usadel, and U. Nowak, Phys. Rev B. \textbf{71}, 064413 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J31.00007: Antiferromagnetic coupling in Fe/Si/Fe structures with Co ``dusting'' layers Rashid Gareev, Matthias Kiessling, Matthias Buchmeier, Georg Woltersdorf, Christian Back Artificial antiferromagnetic (AF) tunnelling Si-based structures are attracting a special interest due to extremely strong AF coupling, which exceeds 5mJ/m$^{2}$, low resistance-area product and resonant-type tunnelling magnetoresistance (TMR) [1]. A promising way to regulate the spin polarization in TMR structures is to insert Co ``dusting'' layers at interfaces [2]. We present AF coupling in Fe/Co/Si/Co/Fe epitaxial structures with sub-monolayer --thick Co ``dusting'' layers at interfaces and different thicknesses of the Si spacer. We determined the strength of AF coupling from spin-wave frequencies and angular dependence of the resonance field of the ferromagnetic resonance, as well as MOKE hysteresis. We found the AF coupling near 0.1mJ/m$^{2}$ which decays with the spacer thickness and detectable for 2nm-thick Si spacers. The presented results can open an avenue for magnetotransport studies in AF-coupled structures using interface engineering. [1]. R.R. Gareev \textit{et al}, JMMM \textbf{240}, 235 (2002), R.R. Gareev \textit{et al}, APL \textbf{88}, 172105 (2006). [2]. Y. Wang, X.F. Han, and X.-G. Zhang, APL \textbf{93}, 172501 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:39PM |
J31.00008: Probing Magnetic Configurations in Buried Cobalt/Copper Multilayered Nanowires Invited Speaker: Multilayered magnetic nanowires have been a model system for heterostructured junctions that exhibit a host of fascinating perpendicular spin transport phenomena, such as giant and tunneling magnetoresistance (MR), and spin-transfer torque effects. Due to the extremely small physical dimensions the magnetic components in these nanowires or junctions often exhibit complex magnetization reversal behaviors, which are difficult to probe by magnetic imaging since the entities are buried deep inside a matrix. Conventional hysteresis loop measurement alone cannot reliably distinguish the reversal mechanisms either. In this work we have captured magnetic and MR ``fingerprints'' of Co nanodiscs in Co/Cu multilayered nanowires as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method [1]. The nanowires have been electrochemically deposited into nanoporous polycarbonate membranes. In 50 nm diameter [Co(5nm)/Cu(8nm)]$_{400}$ nanowires, a 10{\%} MR effect is observed at 300 K. In 200 nm diameter nanowires, the magnetic configurations can be tuned by adjusting the Co nanodisc aspect ratio. Nanowires with thinnest Co exhibit single domain behavior. Those with thicker Co exhibit vortex states, where the irreversible nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distributions, similar to those observed in arrays of Fe nanodots [2]. They also show a superposition of giant and anisotropic magnetoresistance, which corresponds to the specific magnetic configurations of the Co nanodiscs. \\[4pt] [1] J. E. Davies, \textit{et al}, Phys. Rev. B \textbf{70,} 224434 (2004); Appl. Phys. Lett. \textbf{86,} 262503 (2005); Phys. Rev. B \textbf{77}, 014421 (2008).\\[0pt] [2] R. K. Dumas, \textit{et al, }Phys. Rev. B \textbf{75}, 134405 (2007); Appl. Phys. Lett. \textbf{91}, 202501 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J31.00009: Controlled formation of double-vortex configurations in a shape-engineered F/N/F trilayer stack studied by quantitative off-axis electron holography Lei Huang, Marvin Schofield, Yimei Zhu Vortex domain state, widely existing in submicron size patterned magnetic structures, can be very useful in high density magnetic data storage devices. In this report, we designed a shape engineered ferromagnetic-nonmagnetic-ferromangetic (F/N/F) trilayer stack that would generate four different vortex-based remnant states by applying defined sequences of in-plane magnetic field. These four states are distinguished by different relative chirality orientations of two vortices stabilized in the ferromagnetic layers. Experimentally, we lithographically patterned 400nm sized prototype device, and studied in-situ the switching behavior by off-axis electron holography. Using the integrated approach including single element hysteresis loop, induction contour mapping and quantitative electron phase shift measurement, we revealed the underlying reversal mechanism as separate vortex formation and annihilation in two magnetic layers. We also confirmed the field-control feasibility of such structure by distinguishing unambiguously the presence of all four states after each field recipe was applied. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J31.00010: Magnetic properties of ion-etched magnetic nanodot arrays Ioan Tudosa, Keith Chan, Erik Shipton, Eric Fullerton One pathway for increasing the density in magnetic recording media is to have bits stored as single patterned magnetic islands. While promising, this method has been hindered by the failure to reduce the island to island variation of magnetic switching properties. We have prepared [Co/Pd], [Co/Pd]Co/Ni and CoO/[Co/Pd]/CoO multilayers to tune the anisotropy. The films were subsequently patterned into nanodot arrays by ion etching using self organized di-block copolymers as the etch mask. The resulting patterned islands have a 32-nm diameter and 65-nm pitch. We characterize the time and temperature dependence of the magnetic properties to extract the coercivity, switching field distribution and thermal stability parameters. The distribution of the switching fields, in the range of 10-12{\%} of the coercive field, was separated into intrinsic and dipolar contributions and find small dipolar contribution to the switching field distributions. The room temperature stability parameters are greater than 100 k$_{B}$T for all the samples. However, the magnetic switching volume extracted from the thermal stability is significantly less than the physical volume of the samples suggesting incoherent reversal. We will discuss the physical origin of the incoherent reversal. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J31.00011: Magnetic multilayers and Nanomagnetic Patterns P. Panyajirawut, M.S. Rzchowski We have grown Ni/Co magnetic multilayers by sputtering, finding that the multilayers have in-plane uniaxial magnetic anisotropy. This is induced during growth by the sputtering geometry, and by the interaction between layers. We pattern the multilayers into sub-micron dots and networks using the nanosphere lithography technique, forming well-ordered two dimension arrays of magnetic nanoparticles. We use an oxygen plasma etch to adjust the size of the polystyrene spheres after spin coating. Using self-assembled close-packed monolayer of polystyrene spheres as deposition mask, the magnetic material is deposited through the interstitial areas to form networks. We also form isolated nanoparticles using the polystyrene spheres as a etch mask. We discuss the magnetic behavior of patterned mulitlayers. [Preview Abstract] |
Session J32: Artificial Structured or Self-Assembled Magnetic Materials
Sponsoring Units: GMAGChair: Brian Maranville, National Institute of Standards and Technology
Room: 336
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J32.00001: Surface-state mediated indirect exchange interaction between magnetic nanodots on metallic substrates Di Xiao, Lifeng Yin, Wenguang Zhu, G. Malcolm Stocks, Jian Shen, Zhenyu Zhang We investigate theoretically the ferromagnetic ordering of magnetic nanodots grown on flat or vicinal metal substrates. We first show that, on a flat substrate, the surface state-mediated indirect exchange interaction between the nanodots can be significant enough to account for the high ferromagnetic transition temperature observed in recent experiments. We obtain the quantitative coupling strength and characteristic length scale of the magnetic interaction via detailed Monte Carlo simulations. We then study how the reduced dimensionality of the surface state on vicinal surfaces affects the collective magnetic behavior of the systems, and discuss the findings in connection with latest experimental observations. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J32.00002: Artificial Nanomagnet With Lateral Confinement Lifeng Yin, Zheng Gai, Noppi Widjaja, Di Xiao, Zhenyu Zhang, Ward Plummer, Jian Shen We introduce a novel way---curved Cu(111) substrate---to smoothly modify the surface states by introducing a miscut angle and study the impact of modifying vicinal surface states on the ferromagnetic behavior of Fe dots. With this curved substrate, the same growth parameter can be ensured in the whole miscut angle studied. When the Fe dot assemblies have an in-plane easy axis, two distinct regimes and a critical terrace width, separating these two regimes, can be identified. There are three contributing factors: the vicinal surface state, the competition between the Fe-dots diameter and the terrace width, and the in-plane uniaxial magnetic anisotropy. The couplings between these three factors lead to the interesting behavior observed in the Fe/vicinal Cu(111) nanodot assemblies. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J32.00003: Multiple Quantum Transitions In Magnetic Nanoparticles Natalia Noginova, Adrian Radocea, Vadim A. Atsarkin Absorption at multiple resonance frequencies is observed in magnetic nanoparticles in strong similarity with forbidden multiple quantum transitions known for paramagnetic ions. The detailed studies of these low-field signals in the dependence on temperature, concentration and orientation of the texturized samples will be presented and discussed using a ``quantization'' approach, considering resonance transitions between energy levels of a giant spin corresponding to the total magnetic moment of a nanoparticle. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J32.00004: SEMPA Measurements of Ferromagnetic Nanodisk Phase Diagrams Seok-Hwan Chung, Robert McMichael, Daniel Pierce, John Unguris We use Scanning Electron Microscopy with Polarization Analysis (SEMPA) to image the magnetic domain structures of ferromagnetic nanodisks with different diameters and thicknesses, and thereby determine the phase diagram of the ground state in these technologically important magnetic structures. Depending on the nanodisk dimensions, one of three distinct ground state magnetic configurations is observed: a single domain in-plane, a single domain out-of-plane, or a vortex state. In contrast to previous work, the magnetic states of \textit{individual} nanodisks are determined using simultaneous SEMPA measurements of both the in-plane and out-of-plane magnetization components. By systematically imaging Permalloy nanodisks with diameters that range from 35 nm to 190 nm and with thicknesses that range from 10 nm to 65 nm, we are able to locate phase boundaries and the triple point between the three phases. Near the phase boundaries and triple point we observe a mixture of the different phases. A model magnetic phase diagram generated by using the OOMMF micromagnetic simulator is found to agree well with the phase diagram determined by the SEMPA measurements. This work is supported by the NIST-CNST/UMD-NanoCenter Cooperative Agreement. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J32.00005: Ordering, Texture and Magnetism in Ultrathin FePt Films Tom George, Xingzhong Li, Ralph Skomski, David J. Sellmyer Non-epitaxially grown $L$1$_{0}$ FePt ultrathin films have been fabricated and investigated. All films were magnetron sputtered onto SiO$_{2}$ substrates in the form [Fe/Pt]$_{n}$, with individual layer thicknesses from about 0.1 to 0.4 nm. The films' nominal total thickness ranged from 5 to 20 nm and was controlled by varying the number of bi-layer repetitions. The $L$1$_{0}$ phase and (001) texture were obtained by post-deposition annealing for 300 seconds at 600 \r{ }C. Transmission electron microscopy showed all as-deposited films as continuous; but after annealing, all films except the thickest ones showed agglomeration into a connected island-like morphology. $L$1$_{0}$-ordering and texture were confirmed by x-ray and electron diffraction, and the degree of order tended to decrease with increasing nominal film thickness. The (001) texture was greatest when the nominal film thickness was 12.5 nm, coinciding with an island thickness of the same value. SQUID magnetometry shows a relatively unusual trend of coercivity increasing with island thickness, with the highest value corresponding to the most ordered sample. The process of film agglomeration and the effects of bi-layer thickness and annealing temperature and time are also discussed. -- This research is supported by NSF-MRSEC (RS), INSIC, DOE (DJS), and NCMN. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J32.00006: Numerical Studies of Magnetization Reversal in Thin Annular Nanorings Gabriel Chaves-O'Flynn, Andrew Kent, Daniel Stein, Daniel Bedau The rate of thermally activated magnetization reversal in thin ferromagnetic nanorings has been found analytically in a 1D model in which the demagnetization energy is approximated by a local surface term [1]. Numerical micromagnetic calculations confirm all aspects of the analytic model for narrow thin rings, such as permalloy rings of 200 nm mean radius, 40 nm width and 2 nm thickness [2]. However, the model breaks down in for extremely wide rings, when the ring width approaches its mean diameter. Here we present numerical micromagnetic results for the transition states between the clockwise and counterclockwise state in this limit. We describe how the two transition configurations of narrow rings cease to be saddles of the energy functional. Also, a new low energy metastable state is found to exist for a narrow range of fields. We discuss the results of applying the String Method [3] to determine the transition states and energy barriers between the lowest magnetization configurations of rings. [1] K. Martens, D.L. Stein, and A.D. Kent, PRB 73, 054413 (2006) [2] G. D. Chaves-O'Flynn, D.L. Stein, and A.D. Kent, arXiv:0811.0440 (2008) [3] W. E, W. Ren, E. Vanden-Eijnden, J. Chem. Phys 126, 164103 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J32.00007: Onset of magnetism in supported transition metal encapsulated silicon cages Roberto Robles, Shiv N. Khanna In the past few years, silicon based clusters have attracted a lot of attention as building blocks of nanomaterials. Some of the most promising candidates are the transition metal encapsulated silicon cages, which have been shown to be specially stable, both experimentally and theoretically. However, for the use of these materials in fields like spintronics, it is not only necessary to be semiconductor based, but also that they present a finite magnetic moment. However, it has been shown that the magnetic moment of the transition metal atom encapsulated in silicon cages is quenched due to the hybridization with silicon. By performing density functional calculations in the generalized gradient approximation, we show that the magnetic moment of these clusters can be recovered by depositing then on a surface. Using CrSi$_{12}$ on Si(111) as an example, we have deposited the cluster in different orientations. The studies show that, for most of them, a finite magnetic moment is preserved in the system after a geometrical relaxation. The origin of this behavior is discussed in terms of hybridization, comparing to the unsupported situation. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J32.00008: Metamaterials with tunable refractive index fabricated from amorphous ferromagnetic microwires and optical Magnus effect Andrey Ivanov, Anatoly Vedyayev, Vladimir Galkin, Alexander Shalygin, Valery Ivanov For homogeneous NPVM (negative phase--velocity mediums) [V. G. Veselago, Soviet Physics - Uspekhi \textbf{10} (1968) 509; T. G. Mackay, A. Lakhtakia, Phys. Rev. E \textbf{69} (2004) 026602] anomalous effects such as negative refraction, light pressure, Doppler shift, Cherenkov-Vavilov radiation, $Goos-H\ddot {a}nchen$ effect have been discovered in different frequency ranges. In this presentation the optical circular polarized effect is calculated for inhomogeneous mediums (optical Magnus effect) and it is shown that it is anomalous in NPVM with respect to ``right-handed'' materials. The proposed metamaterials fabricated from glass coated amorphous ferromagnetic Co-Fe-Cr-B-Si microwires are shown to exhibit a negative refractive index for electromagnetic waves over scale of GHz frequencies [A.V. Ivanov, A.N. Shalygin, A.V. Vedyayev, V.A. Ivanov, JETP Letters \textbf{85} (2007) 565]. The magnetostatic interaction between microwires has been taken into account. The phase and group velocities in proposed metamaterial have been calculated. The ratio of thereof depends monotonically on the size of the microwires. Optical properties of such metamaterials are tunable by an external magnetic field and mechanical stress. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J32.00009: Magnetism in 1D Cobalt-Cyclopentadienyl Sandwich Molecular Wire Hannes Allmaier, C. Morari, L. Chioncel, E. Arrigoni, F. Beiuseanu, A. Lichtenstein, M. Katsnelson A challenge for technological applications at the nanometer scale is to find magnetic materials with reduced dimensionality. Recent theoretical studies have predicted ferromagnetic and half-metallic behavior for the 1D-organometallic benzen vanadium wire. Here we discuss a variety of magnetic orderings such as anti-ferromagnetic and ferrimagnetic half-metallicity in the cobaltocen Co2(C5H5)2 nanowire. We performed DFT-calculations to optimize its geometry and used the NMTO downfolding technique to construct the real-space low energy Hamiltonian. To describe electronic correlations beyond the mean-field, we used the developed Variational Cluster Approach. Our preliminary results show that non-quasiparticle states appear in the half-metallic gap, which reduce considerably the spin polarization of such a wire. Ab-initio electron transport calculations are in progress to establish the role of cobaltocen nanowire as part of a future spin filter. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J32.00010: Comparative Density Functional study of Ti on CuN/Cu(100) Jesus Cruz, Pushpa Raghani, Barbara Jones We have performed a Density Functional Theory (DFT) calculation using the Projector Augmented Wave (PAW) technique to study the electronic structure of adatoms of Ti placed on a single layer of copper nitride (CuN) surface grown on top of Cu(100). The insulating CuN surface mediates superexchange interactions between the magnetic adatoms, and also can strongly affect electronic properties. The PAW technique allows us to have elements of the precision of an all-electron (AE) calculation and the performance of an Ultrasoft Pseudopotential (USPPs) calculation. We compare results obtained of the magnetic moment, atomic positions, and charge densities of Ti adatoms with both USPPs and PAW methods, and discuss the similarities and differences between the methods. We also report studies of the effect of the Coulombic repulsion U in the PAW methology, and compare some of these results to an all-electron calculation. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J32.00011: Characteristics of Co islets on Cu(111) from first principles calculations Duy Le, Talat Raman Through first principles electronic calculations, based on the spin-polarized density functional theory using the generalized gradient approximation and the ultrasoft pseudopotential method in the plane wave representation, we have examined the structure and magnetic properties of Co monomer, dimer and several n-mers on Cu(111). We find that the monomer has slight preference for the fcc site as compared to the hcp (about 0.02eV) while there is no such preference in the case of the Co dimer. The dimer bond length is found to be about 2.15?. For the 6 atoms cluster, we find that it prefers to be antiferromagnetic and absolute magnetic moment of each Co atom is about 0.07-0.08$\mu $B. The monomer is non-magnetic while a high magnetic moment of 1.94$\mu $B per Co atom is found in the case of dimer. We discuss our results in the context of recent experimental and theoretical findings [1,2] \\[3pt] [1] S. Borisova \textit{et al}, Phys. Rev. B \textbf{78}, 075428 (2008) \\[0pt] [2] O. Mironets \textit{et al}, Phys. Rev. Lett. \textbf{100}, 096103 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J32.00012: Growth and characterization of MnAs on HOPG Shridhar Hegde, Everett Fraser, Jaesuk Kwon, Hao Zeng, Hong Luo MnAs thin films exhibit room temperature ferromagnetism and have been extensively studied on substrates such as GaAs and Si, for spintronic applications. Film properties, such as vertical transport measurements, are often hindered by the presence of underlying substrates. The non-reactive HOPG surface provides an ideal environment for studies of MnAs with minimal efffect from the substrate. We grew MnAs on HOPG by MBE. AFM measurements indicated that the MnAs particles self-assemble on step edges of the HOPG surface, to form highly ordered wire-like structures. Magnetization measurements showed the MnAs/HOPG sample to be ferromagnetic at room temperature. A temperature dependent AFM/MFM study yielded a Curie temperature of 330K. Individual particles are found to be ferromagnetic and the neighboring particles are antiferromagnetically coupled. Modulation dI/dV measurements showed a clear difference between the electronic states in MnAs and in the HOPG substrate used as a reference. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J32.00013: Aging and {\it in-situ\/} annealing reduction of magnetite (Fe$_3$O$_4$) thin films grown on the polar MgO(111) surface Prasenjit Dey, Michael Weinert, Marija Gajdardziska-Josifovska Previous transmission electron microscopy and diffraction studies of Fe$_3$O$_4$(111)/MgO(111) polar oxide interfaces found \footnote{ V. K. Lazarov, et al., Phys. Rev. Lett. {\bf 90}, 216108 (2003).} the formation of (110)-oriented metallic Fe nano-crystals at the interface and within the magnetite film under oxidizing conditions that result in pure magnetite growth on the neutral MgO(001) surface. The question arises whether these iron nano-crystals oxidize with prolonged aging in air. We find, instead, that they not only persist but grow in average thickness within the magnetite film. We have also explored whether reduction can be achieved by {\it in-situ\/} annealing in vacuum starting from pure phase Fe$_3$O$_4$(111)/MgO(111) samples. We find a phase transformation from Fe$_3$O$_4$ to FeO at 720$^\circ$C and a second phase transformation at 800$^\circ$C from FeO into Fe nanoparticles that tend to nucleate along the surface. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J32.00014: Magnetic and Optical Anisotropy in Epitaxial Cobalt Films Nikolai Yakovlev, Andrew Kaveev, Nikolai Sokolov Cobalt films were grown by molecular beam epitaxy on CaF2 buffer layers on silicon and showed a variety of magnetic and optical effects. When the substrate is Si(111), CaF2 grows as flat layer with (111) surface and Co grows on it in face centered cubic lattice. Magneto-optical Kerr effect (MOKE) from these structures is isotropic in plane. When the substrate is Si(100), CaF2 growth direction is [110] due to unique properties of CaF2/Si(100) interface. Then CaF2 surface has grooves with {\{}111{\}} facets and cobalt grown on it has in-plane magnetic anisotropy with easy axis along the grooves. The dependence of remanence magnetisation and coercivity on azimuthal angle (between the grooves and field) follows single domain model except the range between 80 to 90 degrees - there is a peak related to crystalographic anisotropy. MOKE from these structures can have different sign depending on incidence angle; this is consistent with calculations of multilayer model. We found that the sign of MOKE can change also versus azimuthal angle, which was unexpected. Physics behind this phenomenon will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J32.00015: Structure and magnetism of Mn$_{0.5}$Ni$_{0.5}$/Ni(001) system Bothina Hamad The dependence of the magnetic structure on the geometry of materials is well-established for decades. Most of the theoretical and experimental investigations have focused on transition metals adsorbed on (001) noble metal surfaces. These systems can be considered as two-dimensional magnets with negligible influence of the substrate. Several experimental investigations have reported a c(2$\times $2) ordered alloy structure for MnCu/Cu(001) and MnNi/Ni(001) systems. In this work, I present a theoretical study of the magnetic structure for the c(2$\times $2) ordered Mn$_{0.5}$Ni$_{0.5}$/Ni(001) alloyed system. The calculations were performed using the density functional theory (DFT) and the exchange-correlation potential was treated by the generalized gradient approximation (GGA). In this study, an in-plane ferromagnetic structure was obtained, which is more stable than the antiferromagnetic by 398 meV. The Mn and Ni atoms exhibit local magnetic moments of 3.998$\mu _{B}$ and 0.250 $\mu _{B }$, respectively. I obtained relaxations of +1.5{\%} and -1.05{\%} for Mn and Ni surface atoms, respectively. [Preview Abstract] |
Session J33: Superconductivity: Novel Materials
Sponsoring Units: DCMPChair: Elena Cimpoiasu, United States Naval Academy
Room: 403
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J33.00001: Superconductivity at T$_{C }$=38 K in Pristine and Sulfur Doped Amorphous Carbon Israel Felner, Yakov Kopelevich, R. Ricardo da Silva Detailed dc magnetic studies perormed on pristine amorphous carbon and on sulfur doped amorphous carbon, clearly indicate the existence of inhomogeneous superconductivity with T$_{C }$ranging from 32 to 38 K. The superconducting phase fraction is about 0.5{\%}. Superconductivity is manifested by: (i) the diamagnetic shielding state, (ii) the Meissner effects and (iii) the typical M(H) hysteresis loops. The results indicate that amorphous carbon is a system with non-perculative superconducting phase. Chemical analysis shows that the amorphous carbon contains a small amount of sulfur. It is propossed that superconductivity stems from an unknown C-S phase immersed in amorphous carbon, thus it is concluded that s\'{u}lfur is a effective dopant to induce superconductivity in carbon (graphite) based materials. The various options for the C-S superconducting phase will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J33.00002: Crushing Elemental Calcium into a High Temperature Superconductor. Zhiping Yin, Francois Gygi, Warren Pickett The high temperature superconductivity (up to 25 K) observed in elemental Ca at high pressure extends across several phase boundaries, making understanding the crystal structures of Ca under high pressure of great importance. Above 100 GPa, both experiment and theory indicate three possible structures, having space groups P4$_3$2$_1$2, Cmca and Pnma. The reported room temperature structure, primitive simple cubic in the 32-109 GPa range, is dynamically unstable at T=0 throughout this range, according to linnear response calculations of the phonon spectrum. Structure optimization and constant pressure enthalpy calculations using density functional theory reveal that several structures (the structures mentioned above, an I-43m structure, and simple cubic)are quasi-degenerate, and hence are competing, in the pressure ranges 40-80 GPa and 100-130 GPa. Volume collapse transitions of the Cmca and Pnma structure will also be described. We discuss the implications of these findings for the observed room temperature ``simple cubic'' phase. Linear response calculations give T$_c\sim 20$ K for several of the phases at high pressure. Predictions of electron-phonon coupling in the 120-220 GPa regime will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J33.00003: Competing s and d-wave superconducting order in V$_{3}$Si Shantanu Mukherjee, Daniel Agterberg Competing phases have generated a lot of interest in the study of cuprate and pnictide superconductors. Here we examine the competition between s and d-wave superconducting order in V$_{3}$Si. We provide microscopic arguments as to why both of these phases have a comparable transition temperature in this material. We further argue that many experimental probes provide evidence for this competition. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J33.00004: Theoretical study of FFLO phases and related phases in non-centrosymmetric superconductors Zhichao Zheng, Daniel Agterberg Superconducting order can break translational invariance, leading to a phase in which the Cooper pairs develop a coherent periodic spatially oscillating structure, such as in a FFLO phase. Some such superconductors break inversion symmetry, leading to helical and multiple-q (FFLO-like) phases. We study these related phases with and without vortices. We show that for a FFLO phase, a crisscrossing lattice solution arises from the decay of conventional Abrikosov vortices into pairs of fractional vortices. We further show that the fractional vortex solution can also exist in the multiple-q phase of non-centrosymmetric superconductors. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J33.00005: Giant Magnetoresistance and Unusual Magnetic Behavior in Single Crystals of the Layered Arsenide EuRh$_2$As$_2$ Yogesh Singh, D.C. Johnston Magnetic susceptibility $\chi$, isothermal magnetization $M$, resistivity $\rho$, Hall effect, and heat capacity $C$ measurements on EuRh$_2$As$_2$ reveal complex and unusual magnetic behavior. The $\chi(T)$ data gave a small Weiss temperature $\theta \approx$~12~K indicating predominantly ferromagnetic interactions between the Eu$^{2+}$ moments. Below $T$~=~47~K, however $\chi(T)$ indicates that an antiferromagnetic transition occurs instead. The unusually high $T_{\rm N}$ compared to $\theta$ ($\theta$/$T_{\rm N}\approx$0.26) suggests novel physics. A metamagnetic transition is observed in the $M$ versus $H$ data at $T < T_{\rm N}$ when $H$ is applied in the $ab$ plane. The metamagnetic field shows an unusual $T$ dependence, decreasing slightly between $T$~=~2~K and 30~K and increasing again on approaching $T_{\rm N}$ before vanishing abruptly at $T_{\rm N}$. In zero field the $\rho(T)$ data indicate metallic behavior between 2~K and 300~K. However, at low temperatures $T \leq 30$~K, $\rho(T)$ increases dramatically in an applied field $H$ and we observe a giant positive magnetoresistance of $\approx$90\% at $T$~=~2~K and $H$~=~8~T. For $T < 30$~K $\rho(T)$ increases for $H \geq$~1~T. A monotonic reduction of the electronic specific heat coefficient $\gamma$ with $H$ and a change in sign of the Hall coefficient from negative above $T$~=~15~K to positive for lower $T$ are also observed. \\[0pt] $^*$Supported by DOE-BES under Contract No.\ DE-AC02-07CH11358. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J33.00006: Non-centrosymmetric superconductor La$_3$Bi$_4$Pt$_3$ Gabriel Seyfarth, Cigdem Capan, Andrea Bianchi, Zachary Fisk, Pierre Rodiere, Christine Opagiste Recently, we have discovered that the metallic La$_3$Bi$_4$Pt$_3$ (Y$_3$Au$_3$Sb$_4$ structure) becomes superconducting below a transition temperature $T_c$ of about 1.4K. Our interest in the superconducting phase of La$_3$Bi$_4$Pt$_3$ stems from the fact that it lacks a center of inversion, which may lead to unconventional superconductivity, including nodes in the superconducting gap function, even if the pair wave function exhibits the full spatial symmetry of the crystal. Compared to other non-centrosymmetric \emph{magnetic} compounds in which superconductivity has recently been discovered, like CePt$_3$Si, UIr, CeRhSi$_3$ (under pressure), the nature of the superconducting state in La$_3$Bi$_4$Pt$_3$ is not complicated by strong electron correlations nor the coexistence of magnetism. This makes it a good model system to study superconductivity without inversion symmetry. In our presentation we will focus on the first basic characterization of our La$_3$Bi$_4$Pt$_3$ single crystals (X-ray, specific heat, resistivity, penetration depth, etc.). [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J33.00007: On d-Wave Superconductors with a Zeeman or Exchange Splitting of the Spin-Up and --Down Fermi Surfaces Chia-Ren Hu For a given Zeeman (or exchange) energy $h$, we used the
Fermi-surface
splitting, $\delta $\textit{$\mu $}, as a variational parameter,
and showed: (1) For an
s-wave superconductor, the Sarma state is actually an \underline
{unstable}
equilibrium state, which is known to exist for 0.5$ |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J33.00008: Superconductivity from two dimensional interfaces: CuCl/Si, GaP/Si, ZnS/Si S.H. Rhim, R. Saniz, A. J. Freeman Two-dimensional (2D) interfaces of hetero-bonded semiconductor superlattices are studied using the highly precise FLAPW \footnote{Wimmer, Krakauer, Weinert, and Freeman, Phys.Rev.B, {\bf 24}, 864 (1981)} method. The 2D system, of metal-insulator-metal, is one of the candidate geometries to realize the excitonic mechanism of superconductvity, \footnote{V.L. Ginzburg, Sov. Phys. JETP {\bf 20},1549 (1965)} where $T_C$ can be greatly enhanced over phonon mediation. Epitaxially grown CuCl on Si (111) was reported to exhibit an anomalous diamagnetic susceptibility at 60$\sim$150 K. \footnote{Mattes and Foiles,Physica 135B, 139 (1985)} For all superlattices, 2D metallicity was found at the interfaces due to charge transfer from the polarity mismatch, \footnote{Rhim, Saniz, Yu, Ye, and Freeman, Phys. Rev. B {\bf 76}, 184505 (2007)} as evidenced by their bands, Fermi surfaces, and charge densities. The $T_C$, calculated within the crude RMTA and the McMillan-Hopfield formula, is 0.04$\sim$4.4K for the CuCl/Si case, but vanishes for the other cases. To pursue the excitonic mechanism, we are determining the Kernel function $K(\omega)$, i.e. the average of the effective Coulomb interfaction, with {\bf q} dependent dynamic screening. First results for CuCl/Si show $K(\omega)$ to be attractive for a certain energy range [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J33.00009: Standard Model for Superconductivity in Graphite Intercalation Compounds: Prediction of Optimum $T_c$ Yasutami Takada Based on the model that was successfully applied to the explanation of superconductivity with the transition temperature $T_c$ of about 0.1K or less in the alkali- intercalated graphite compounds such as KC$_8$, RbC$_8$, and CsC$_8$ in 1982 [Y. Takada, {\it J. Phys. Soc. Jpn.} {\bf 51}, 63 (1982) ], we have calculated $T_c$ for the alkaline-earth- intercalated graphite compounds including CaC$_6$, YbC$_6$, and SrC$_6$ with $T_c$ of about 10K or less to find that the same model reproduces the observed $T_c$ in those compounds as well, indicating that it is a standard model for superconductivity in the graphite intercalation compounds with $T_c$ ranging over three orders of magnitude. The difference in $T_c$ by two orders between KC$_8$ and CaC$_6$ can be accounted for by (i) doubling $Z$ the valency of the metal ions, which enhances $T_c$ by one order, and (ii) tripling $m^*$ the effective mass of the superconducting three-dimensional electrons in the interlayer band, which also enhances $T_c$ by one order. Enhancement of $T_c$ well beyond 10 K is also predicted in this model, if intercalant metals are judiciously chosen so that both $Z$ and $m^*$ are increased further. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J33.00010: Crystal structure and physical properties of new layered oxysulfides (Cu$_{2}$S$_{2})$(Sr$_{4}$Sc$_{2}$O$_{6})$, (Cu$_{2}$S$_{2})$(Ba$_{3}$Sc$_{2}$O$_{5})$ and (Cu$_{2}$S$_{2})$(Ba$_{3}$In$_{2}$O$_{5})$ Hiraku Ogino, Koichi Ushiyama, Yukari Katsura, Shigeru Horii, Jun-ichi Shimoyama, Kohji Kishio Recently high-$T_{c}$ superconductors were discovered in layered oxypnictide systems with stacking of fluorite-based oxide layers and anti fluorite-based pnictide layers. Materials having similar stacking structure with perovskite-based oxide layers and anti-fluorite chalcogenide or pnictide layers have been discovered in some oxypnictides and oxychalcogenides, but such systems are relatively less explored. Some materials belongs to these systems show interesting properties such as p-type transparent conductivity in (Cu$_{2}$S$_{2})$(Sr$_{3}$Sc$_{2}$O$_{5})$. This motivated us to explore perovskite-based layered oxysulfide system. New layered oxysulfides (Cu$_{2}$S$_{2})$(Sr$_{4}$Sc$_{2}$O$_{6})$, (Cu$_{2}$S$_{2})$(Ba$_{3}$Sc$_{2}$O$_{5})$ and (Cu$_{2}$S$_{2})$(Ba$_{3}$In$_{2}$O$_{5})$ have been synthesized by conventional solid state reaction. X-ray diffraction patterns show that the structure of these materials consists of stacking of semi-conducting Cu$_{2}$S$_{2}$ layers and perovskite-based oxide layers. Structural features and physical properties of these new materials will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J33.00011: High-T$_{c}$ superconductivity in nanostructured Na$_{x}$WO$_{3-y}$: Sol-gel route Ali Aliev Tungsten trioxide, WO$_{3-y}$ infiltrated into various nanoporous matrix structures such as carbon inverse opal, carbon nanotubes paper, or platinum sponge and then intercalated with alkaline ions (Li$^{+}$, Na$^{+})$ exhibits a pronounced diamagnetic onset in ZFC magnetization in a wide range of temperatures, 125-132 K. Resistivity measurements show non zero jump and intensive fluctuations of electrical resistance below observed transition points. The observed magnetic and electrical anomalies in nanostructured tungsten bronzes (Li$_{x}$WO$_{3-y}$, Na$_{x}$WO$_{3-y})$ suggest the possibility of localized non-percolated superconductivity. The direct evidence of polaron formation from temperature dependence of EPR and photoemission spectra and formation of bipolarons in weakly reduced to WO$_{3-y}$, with 3-y typically in the order of 2.95 suggest bipolarons mechanism of a Bose-Einstein condensation of trapped electron pairs in doped WO$_{3-y}$. On the other hand the strong lattice instabilities in 2D systems like layered cuprates and tungsten bronzes place the upper limit on T$_{c}$. Than, the percolative self-organized mechanism on the metal/insulator interface like Na/WO$_{3}$ and NaWO$_{3}$/nanostructured matrix can facilitate the high T$_{c}$ obtained in sodium bronzes infiltrated into inverted carbon opal or carbon nanotube matricies. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J33.00012: Search for superconductivity in surface-doped WO$_{3}$ films Akio Tsukada, Robert Hammond, Theodore Geballe, Malcolm Beasley We report the search for superconductivity in surface-doped WO$_{3}$ films. Possible evidence for high temperature superconductivity in this system has been reported in the literature. In our work, WO$_{3}$ films were grown by MBE and characterized by \textit{in-situ} XPS, UPS, and \textit{ex-situ} XRD and resistivity measurements. For some films, Na was deposited on the surface at room temperature, and the resultant Na+WO$_{3-x}$ films annealed in vacuum ($\sim $ 10$^{-8}$ Torr) for 1h at various temperatures (300 -- 800C). With increasing thickness of Na, the intensity of the Na1$s$ peak in XPS spectra and area of the W5$d$ state in UPS spectra increased, suggesting some charge transfer to the WO$_{3}$ film. After annealing below 500C, XPS and UPS spectra did not change, while after annealing above 500C, the area of the W5$d$ state drastically increased. The same behavior was observed for pure WO$_{3-x}$ films, however, suggesting that oxygen vacancies are created during the annealing. The films started to decompose above 700C. The resistivity of our films decreased with increasing annealing temperature. However, so far we have not observed any sign of superconductivity down to 2 K in any of our films. This work is supported by Air Force Office of Scientific Research. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J33.00013: Combinatorial Study of the Hall-Effect Sign Change in Overdoped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$ Films Jeffrey Clayhold, Oshri Pelleg, Anthony Bollinger, Gennady Logvenov, Ivan Bozovic We have made a high-resolution study of the sign change of the Hall effect in combinatorially grown samples of over-doped La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4+\delta}$, using our dedicated system for creating and measuring samples with ultrafine stoichiometry resolution. The data are from MBE films grown with a linear stoichiometry gradient and were taken with a characterization system that can measure both the Hall effect and resistivity simultaneously at 31 different locations on the film. Recently improved growth techniques, as well as sample processing, lithography and subsequent handling give both Hall and resistance measurements confirming a linear stoichiometry gradient to better than 1 \%. We report on the variation of the Hall coefficient, T$_{\rm c}$, and the resistivity in the region of the Fermi surface topology change. [Preview Abstract] |
Session J34: Focus Session: Iron Pnictides and Other Novel Superconductors VII: Pressure Effects and Thermal Expansion
Sponsoring Units: DCMPChair: Neil Sullivan, University of Florida
Room: 404
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J34.00001: Effects of pressure on CaFe$_{2}$As$_{2}$ and related materials Invited Speaker: The discovery of CaFe$_{2}$As$_{2 }$[1] and its extreme pressure dependence [2] (even for pressures below 1 GPa) has lead to it being used as a model system for understanding the effects of pressure on the (AE)Fe$_{2}$As$_{2 }$(AE = Ba, Sr, Ca) compounds.[3-5] We have found that the combination of extreme pressure sensitivity with a first order structural phase transition that involves significant changes in the unit cell dimensions makes CaFe$_{2}$As$_{2}$ very sensitive to the pressure medium used. In liquid medium, self clamping cells the higher temperature transitions, while generally detectable and highly reproducible [2,6] are smeared and broadened, especially near the first order phase transition between the low temperature orthorhombic and collapsed tetragonal phases. In He-pressure cells these transitions remain extremely sharp.[3-5] Superconductivity is detected between $\sim $0.3 and $\sim $0.7 GPa in the liquid medium cells and is essentially absent in the He-pressure cell.[2,5,6] This superconducting region can be associated with a coexistence of low temperature phases brought on by non-hydrostatic components associated with the medium's inability to respond to the high-temperature structural phase transitions.[2-5] The origin of the superconductivity in this mixed region remains a topic of keen experimental and theoretical interest. \\[4pt] [1] N. Ni et al., Phys. Rev. B \textbf{78}, 014523 (2008), [2] M. Torikachvili et al., Phys. Rev. Lett. \textbf{101}, 057006 (2008), [3] A. Kreyssig et al., arXiv:0807.3032, [4] A. I. Goldman et al., arXiv:0811.2013, [5] W. Yu et al., arXiv:0811.2554, [6] H. Lee et al., arXiv:0809.3550. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J34.00002: Discovery of a pressure-induced ``collapsed'' phase in CaFe$_{2}$As$_{2}$ A. Kreyssig$^{1,2}$, M.A. Green$^{3,4}$, Y. Lee$^{1,2}$, G.D. Samolyuk$^{1,2}$, P. Zajdel$^{3,5}$, J.W. Lynn$^{3}$, S.L. Bud'ko$^{1,2}$, M.S. Torikachvili$^{6}$, N. Ni$^{1,2}$, S. Nandi$^{1,2}$, J.B. Le\~{a}o$^{3}$, S.J. Poulton$^{3,4}$, D.N. Argyriou$^{7}$, B.N. Harmon$^{1,2}$, R.J. McQueeney$^{1,2}$, P.C. Canfield$^{1,2}$, A.I. Goldman$^{1,2}$ Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe$_{2}$As$_{2}$ undergoes a pressure-induced transition to a non-magnetic, volume ``collapsed'' tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. -- The support by U.S. DOE (DE-AC02-07CH11358) and NSF (DMR-0306165 and DMR-0805335) is acknowledged. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J34.00003: Pressure-induced superconducting state of CaFe$_{2}$As$_{2}$ from an antiferromagnetic spin-density-wave state Hanoh Lee, Eunsung Park, Tuson Park, F. Ronning, E.D. Bauer, J.D. Thompson The spin-density-wave (SDW) antiferromagnet CaFe2As2 has been reported as superconducting under pressure. By measuring electrical resistivity and magnetic susceptibility under pressure in silicon fluid as a pressure medium, we show that bulk superconductivity is present in a narrow pressure range where orthogonal and collapsed tetragonal state coexist. At higher pressures, where the collapsed tetragonal structure is proposed, distinctive behavior appears in resistivity with strong thermal and pressure hysteresis. Magnetic fluctuations combined with structural instability appear to be important for superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J34.00004: Structural changes in pressure induced superconducting BaFe$_{2}$As$_{2}$: Similarities to chemical doping Simon Kimber, Andreas Kreyssig, Fabiano Yokaichiya, Dimitri Argyriou, Jiaqiang Yan, Thomas Hansen, Tapan Chatterji, Robert McQueeney, Paul Canfield, Alan Goldman We have determined the crystal structure of BaFe$_{2}$As$_{2}$ as a function of temperature (4$-$150 K) and pressure (0$-$6 GPa) using neutron powder diffraction. The structural features important to superconductivity, namely suppression of the T$-$O phase transition and reduction in the As$-$Fe$-$As bond angle and Fe$-$Fe distance, show exactly the same behaviour under pressure up to the optimal T$_{C}$ value, as found in chemically doped samples. This result suggests that chemical doping and pressure have similar effects on the electronic degrees of freedom in this family of iron pnictide superconductors. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J34.00005: Dffraction Studies of the P-T Phase Diagram with Single-Crystal Ca122 A.I. Goldman, A. Kreyssig, K. Prokes, D.K. Pratt, D.N. Argyriou, J.W. Lynn, S. Nandi, S.A.J. Kimber, Y. Chen, Y.B. Lee, G. Samolyuk, J.B. Leao, S.J. Poulton, S.L. Budko, N. Ni, P.C. Canfield, B.N. Harmon, R.J. McQueeney Single crystal neutron and high-energy x-ray diffraction have identified the phase lines corresponding to transitions between the ambient-pressure tetragonal (T), the antiferromagnetic orthorhombic (O) and the nonmagnetic collapsed tetragonal (cT) phases of CaFe2As2. We find no evidence of additional structures for pressures up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects and we demonstrate that coexistence of the O and cT phases can occur if a non-hydrostatic component of pressure is present. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J34.00006: Pressure-induced shift of $T_c $ in $K_x Sr_{1-x} Fe_2 As_2 $ (x = 0.2, 0.4, 0.7): Analogy to the high-T$_{c}$ cuprate superconductors Melissa Gooch, Bing Lv, Bernd Lorenz, Arnold Guloy, Ching-Wu Chu Through a systematic study of $K_x Sr_{1-x} Fe_2 As_2 $ (x = 0.2, 0.4, 0.7), by pressure shifts of the $T_c $, similarities between the FeAs and high T$_{c}$ superconductors can be observed. These similarities develop directly from the layered structure seen in both superconductors, which consists of an active superconducting layer and a charge reservoir block. The pressure coefficient of Tc depends on the doping level: dTc/dp$>$0 (underdoped, x=0.2), dTc/dp=0 (optimally doped, x=0.4), and dTc/dp$<$0 (overdoped, x=0.7). This is understood in terms of a pressure-induced charge transfer between the active and charge reservoir layers. In addition to the measured pressure shift in the T$_{c}$, the suppression of the spin density wave can clearly be demonstrated for the x = 0.2 case. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J34.00007: Pressure Effect on the Structural and Magnetic Transition in CaFe$_2$As$_2$ Shiliang Li, Ying Chen, Jeffrey Lynn, Xianhui Chen, Pengcheng Dai We use neutron scattering technique to study both the structural and magnetic phase transitions of CaFe$_2$As$_2$. We confirmed that the nuclear structure changes from orthorhombic to collapsed tetragonal phase with increasing pressure at low temperatures. Strong hysteresis is found in increasing and decreasing temperature processes. The c-axis lattice constant of the orthorhombic phase is found to increase with deceasing temperature under pressure while that of the collapsed tetragonal phase shows almost no change, which suggests a strong magnetic-lattice coupling. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J34.00008: Pressure dependence of the inelastic neutron scattering response of CaFe$_{2}$As$_{2}$. S. Rosenkranz, R. Osborn, E. Goremychkin, I.S. Todorov, D.Y. Chung, H. Claus, J.A. Schlueter, C.D. Malliakas, M.G. Kanatzidis, A.D. Christianson, R.I. Bewley, T. Guidi Application of $\sim $2.5 kbar pressure induces superconductivity in CaFe$_{2}$As$_{2}$ with a T$_{C} \quad \sim $12K that remains constant up to $\sim $ 7kbar, where superconductivity is again suppressed. This modest pressure enables the use of neutron scattering to study in detail changes of the spin and lattice correlations between normal to the superconducting state as a function of pressure. The elastic part of the scattering measured on the MERLIN spectrometer utilizing a He gas pressure cells shows that at 4kbar pressure and 2K only $\sim $ 50{\%} of the sample has transformed to the collapsed tetragonal phase. The inelastic spectra show a suppression of spectral weight at low energies and small momentum transfer on going from ambient pressure to the superconducting state at 4kbar and 2K. The spectral weight is transferred to higher energies and wavevectors, leading to a V-shaped excitation branch in the collapsed tetragonal phase. \newline \newline Work supported by US DOE BES-DMS DE-AC02-06CH11357 [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J34.00009: Thermal expansion and pressure derivatives of $T_c$ in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals S.L. Bud'ko, M.S. Torikachvili, N. Ni, J.-Q. Yan, P.C. Canfield, G.M. Schmiedeshoff We present heat capacity and anisotropic thermal expansion data for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Evolution of structural/magnetic phase transition with Co-concentration is clearly detected by both measurements. Anisotropic pressure derivatives of the superconducting transition temperature are evaluated via the Ehrenfest relations and compared with directly measured hydrostatic $dT_c/dP$. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J34.00010: High-Resolution Thermal Expansion Measurements of CaFe$_2$As$_2$ Ariana de Campos, M.S. da Luz, J.J. Neumeier, E.D. Bauer, F. Ronning, J.D. Thompson, Hanoh Lee, Tuson Park, Eunsung Park The discovery of superconductivity in doped LaFeAsO initiated a surge of interest in layered FeAs systems. The recent discoveries in CaFe$_2$As$_2$ [1,2] of: (i) suppression of the first-order structural phase transition under modest hydrostatic pressure, (ii) superconductivity under pressure and (iii) at higher pressures the suppression of superconductivity with stabilization of a potentially different high temperature phase, establish pressure as a valuable parameter for tuning the behavior of these fascinating compounds. In this work, thermal expansion measurements of a high-quality single crystal of CaFe$_2$As$_2$ are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T$_S$ $\approx$180 K. [1] M.S. Torikachvili, et al. PRL 101, 057006(2008). [2] Park T., et al., J. Phys.Cond.Matter, 20, 322204 (2008). This material is based upon work supported by the Brazilian Agency CNPq (Grant No. 201439/2007-7), the NSF (Grant No. DMR- 0504769) and U.S. DOE Office of Basic Energy Sciences (Grant No. DE-FG-06ER46269). Work at LANL was performed under the auspices of the U.S. DOE Office of Basic Energy Sciences and supported by the LDRD program. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J34.00011: Magnetic Ordering and Negative Thermal Expansion in PrFeAsO D.N. Argyriou, S.A.J. Kimber, F. Yokaichiya, K. Habicht, S. Gerischer, R Klingeler, C. Hess, G. Behr, A. Kondrat, B. B\"uchner, T. Hansen, T. Chatterji We report the structure and magnetism of PrOFeAs, one of the parent phases of the newly discovered Fe-As superconductors, as measured by neutron powder diffraction. In common with other REOFeAs materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with $k =$(1,0,1) is detected below $\sim$ 85 K. Our magnetic order parameter measurements show that the ordered Fe moment along the $a$ axis reaches a maximum at $\sim$ 40 K, below which an anomalous expansion of the $c$ axis sets in, which results in a negative thermal volume expansion of 0.015 \% at 2 K. We propose that this effect, which is suppressed in superconducting samples, is driven by a delicate interplay between Fe and Pr ordered moments. [Preview Abstract] |
Session J35: Focus Session: Iron Pnictides and Other Novel Superconductors VIII: Magnetism (Experiment)
Sponsoring Units: DMPChair: Jeff Lynn, National Institute of Standards and Technology
Room: 405
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J35.00001: Nuclear and Magnetic structures of CeFeAs$_{1-x}$P$_{x}$O Clarina dela Cruz, Pengcheng Dai, Herbert Mook, Q. Huang, M. Green, J. Lynn, N.L. Wang, G.F. Chen, J.L. Lou A new class of superconductors (RO$_{1-x}$F$_{x}$FeAs) has been discovered very recently which has resulted to a flurry of activities in the scientific community. Our initial work on the La based parent compound successfully revealed, for the first time, the AFM order which is preceded by a structural distortion from tetragonal to orthorhombic nuclear structures. A model for the magnetic structure was also proposed. Both magnetic order and structural distortion are suppressed in lieu of the superconducting phase. Our systematic study of the doping dependence of both the nuclear and magnetic structure of CeO$_{1-x}$F$_{x}$FeAs as well as in the La system has established the nature of the competition between the static AFM order of the Fe spins and the superconductivity. This work looks at the effects of the tuning of the structural parameters in the compounds without doping carriers into the system. This is motivated by the belief that structurally driven electronic effects are very important in these highly correlated systems. We study the parent compound CeFeAsO and change the particulars about the structural distortion by substituting P on the As site until it's approach to CeFePO which is a nonmagnetic heavy Fermion. We present interesting details of the phase diagram of CeFeAs$_{1-x}$P$_{x}$O with respect to the As/P concentration. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J35.00002: Lattice and magnetic instabilities in CaFe$_{2}$As$_{2}$ Shibabrata Nandi, Alan Goldman, Dimitri Argyriou, Bachir Ouladdiaf, Tapan Chatterji, Andreas Kreyssig, Ni Ni, Sergey Bud'ko, Paul Canfield, Robert McQueeney Neutron diffraction measurements of a high quality single crystal of CaFe$_{2}$As$_{2}$ are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T$_{S}$=172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural transition we observe a rapid, but apparently continuous, ordering of the Fe moments, in a commensurate antiferromagnetic structure, with a saturated moment of 0.80(5) \textit{$\mu $}$_{B}$/Fe directed along the orthorhombic a-axis. The hysteresis of the structural transition is 1 K between cooling and warming and is consistent with previous thermodynamic, transport and single crystal x-ray studies. The temperature onset of magnetic ordering shifts rigidly with the structural transition providing clearest evidence to date of the coupling between the structural and magnetic transitions in this material and the broader class of arsenides. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J35.00003: High Energy Spin Waves in CaFe2As2 Single Crystals Souleymane Diallo, Vladimir Antropov, Collin Broholm, Toby Perring, Sergey Bud'ko, Ni Ni, Paul Canfield, Andreas Kreyssig, Alan Goldman, Robert McQueeney We present neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic ordered CaFe2As2 (TN = 172 K), the parent compound of the newly discovered iron-arsenide based superconductors. The data reveals steeply dispersive and well-defined spin waves up to an energy of approximately 120 meV. The data below 120 meV can be fit to a Heisenberg model consisting of nearest-neighbor interactions (J1a, J1b and J1c) and next-nearest neighbor interaction (J2), yielding constraining values on the magnetic exchange coupling constants. Above 120 meV, the excitations appear weaker or strongly damped. Ab-initio calculations of the dynamic magnetic susceptibility show that the high energy behavior arises from the damping of itinerant spin waves by particle-hole excitations. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J35.00004: Neutron Scattering Study of the Fe Oxypnictide Superconductors NdFeAsO$_{1-x}$F$_x$ and LaFeAsO$_{0.87}$F$_{0.13}$ Yiming Qiu, Wei Bao, Qingzhen Huang, Taner Yildirim, Jason Simmons, Mark Green, Ying Chen, Jeff Lynn, Maiko Kofu, Seunghun Lee, T. Wu, G. Wu, Xianhui Chen We report the neutron scattering studies of NdFeAsO$_{1-x}$F$_x$ (x=0, 0.2)[1] and LaFeAsO$_{0.87}$F$_{0.13}$[2]. In NdFeAsO, there is a tetragonal to orthorhombic structural transition at Ts$\approx$150 K, where an anomaly in resistivity also occurs. A long range magnetic order with the wave-vector (1/2, 1/2, 0)$_T$ forms below T$_N$=1.96 K. This long range order is dominated by the rare earth Nd ions, however, both the Nd and smaller Fe moments contribute to the antiferromagnetic structure. Neither the magnetic ordering nor the structural distortion occurs in the superconducting samples NdFeAsO$_{0.80}$F$_{0.20}$ and LaFeAsO$_{0.87}$F$_{0.13}$ at temperatures down to 1.6 K. In LaFeAsO$_{0.87}$F$_{0.13}$, no magnetic-resonance peak was observed in the superconducting state at 1.6 K. Two phonon peaks at 12 and 17 meV were observed, consistent with theoretical calculation. Reference: [1] Y. Qiu et al., arXiv:0806.2195 accpeted by PRL(2008) [2] Y. Qiu et al., Phys. Rev. B 78, 052508(2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J35.00005: The crystalline electric field as a probe for long range antiferromagnetic order and superconductivity in CeFeAsO$_{1-x}$F$_x$ Songxue Chi, Devashibhai Adroja, Titiana Guidi, Robert Bewley, Shiliang Li, Jun Zhao, Jeffrey Lynn, Craig Brown, Yiming Qiu, Gen Fu Chen, Jian Lin Luo, Nan Lin Wang, Pengcheng Dai We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of Ce$^{3+}$ in CeFeAsO$_{1-x}$F$_{x}$($x=0,0.16$). For nonsuperconducting CeFeAsO, the Ce CEF levels have three magnetic doublets in the paramagnetic state, but these doublets split into six singlets when Fe ions order antiferromagnetically. For superconducting CeFeAsO$_{0.84}$F$_{0.16}$ ($T_c=41$ K), where the static AF order is suppressed, the Ce CEF levels have three magnetic doublets at $\hbar\omega=0,18.7,58.4$ meV at all temperatures. Careful measurements of the intrinsic linewidth $\Gamma$ and the peak position of the 18.7 meV mode reveal clear anomaly at $T_c$, consistent with a strong enhancement of local magnetic susceptibility $\chi^{\prime\prime}(\hbar\omega)$ below $T_c$. These results suggest that CEF excitations in the rare-earth oxypnictides can be used as a probe of spin dynamics in the nearby FeAs planes. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J35.00006: Inelastic neutron scattering studies on spin excitations of Fe Pnictides Jun Zhao, Dao-Xin Yao, Shiliang Li, Tao Hong, Ying Chen, Sung Chang, William Ratcliff, Jeff Lynn, Herbert Mook, Genfu Chen, Jianlin Luo, Nanlin Wang, Erica Carlson, Jiangping Hu, Pengcheng Dai We used inelastic neutron scattering to study the evolution of spin excitations in the FeAs superconductors and their parent compounds. We show here not only the antiferromagnetic order is suppressed by the doping; the spin excitations also change dramatically with doping. We observed sharp spin-wave excitations in the antiferromagnetically ordered parent compound. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. We also studied the spin excitation spectrum in the superconducting sample and its relationship to the superconductivity. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J35.00007: Resonant spin excitation in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ R. Osborn, S. Rosenkranz, E.A. Goremychkin, D.Y. Chung, I.S. Todorov, H. Claus, A.D. Christianson, M.D. Lumsden, C.D. Malliakas, M.G. Kanatzidis, R.I. Bewley, T. Guidi In the iron arsenides, superconductivity occurs when the antiferromagnetism of a parent compound has been suppressed by chemical doping. We have investigated the evolution of the magnetic response with potassium doping in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. In the parent compound ($x$ = 0), there is evidence of a column of inelastic scattering at the AF wavevector, $Q$ = 1.2{\AA}$^{-1}$, consistent with a steep dispersion of gapped spin waves. A similar inelastic column is seen in the normal phase in the $x$ = 0.4 compound, but it persists to lower energy transfer. However, at the superconducting transition of 38K, there is a transfer of spectral weight into an excitation localized at $Q$ = 1.2{\AA}$^{-1}$ and $\omega $ = 14meV [A. D. Christianson \textit{et al}, Nature, in press]. Such resonant spin excitations, which are a universal feature of the copper oxide superconductors and seen in several heavy fermion superconductors, provide evidence that the energy gap has unconventional symmetry, with opposite sign on portions of the Fermi surface connected by the resonance wavevector. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J35.00008: Antiferromagnetic spin correlations in the tetragonal phase of CaFe$_{2}$As$_{2}$ R.J. McQueeney, S.O. Diallo, J.L. Zarestky, C. Broholm, T.G. Perring, S.L. Bud'ko, N. Ni, A. Kreyssig, P.C. Canfield, A.I. Goldman We present neutron scattering measurements of magnetic excitations in the tetragonal phase of CaFe$_{2}$As$_{2}$. Below $T_{S}$ = 173 K, CaFe$_{2}$As$_{2}$ undergoes a first-order transition to an orthorhombic structure with columnar antiferromagnetic ordering with a wavevector \textbf{Q}$_{AFM}$ = (101). This phase is characterized by strong magnetic interactions giving rise to steep spin waves. In the tetragonal phase above $T_{S}$, broad quasi-elastic excitations are observed near \textbf{Q}$_{AFM}$ that display a weak dependence on $L$, indicating two-dimensional antiferromagnetic correlations. These correlations are observed to persist up excitation energies of $\sim $50 meV and temperatures of at least 300 K. Our results indicate that strong magnetic interactions exist above $T_{S}$, and ordering is likely suppressed by magnetic frustration. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J35.00009: Magnetic order the iron spins in NdOFeAs Ying Chen, J.W. Lynn, J. Li, G. Li, G.F. Chen, J.L. Luo, N.L. Wang, Pengcheng Dai, C. dela Cruz, H.A. Mook Polarized and unpolarized powder neutron-diffraction measurements have been carried out to investigate the iron magnetic order in the parent compound of one of the highest Tc system, NdFeAsO. Antiferromagnetic order is observed below 141 K [1], which is in close proximity to the structural distortion observed in this material [2]. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c axis are antiparallel like LaFeAsO [3]. The ordered moment is 0.25 (7) $\mu$B, which is the smallest ordered moment found so far in these systems. \\[3pt] [1]Ying Chen, J. W. Lynn, J. Li, G. Li, G. F. Chen, J. L. Luo, N. L. Wang, Pengcheng Dai, C. dela Cruz and H. A. Mook, Phys. Rev. B ${\bf{78}}$, 064515 2008. \\[0pt] [2]Y. Qiu, W. Bao, Q. Huang, T. Yildirim, J. M. Simmons, M. A. Green, J.W. Lynn, Y.C. Gasparovic, J. Li, T. Wu, G. Wu, and X.H. Chen, arXiv:0806.2195 (Phys. Rev. Lett. accepted). \\[0pt] [3] C. dela Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P. Dai, Nature ${\bf{453}}$, 899 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J35.00010: Coexistence of the spin-density-wave and superconductivity in the $\rm Ba_{1-x}K_xFe_2As_2$ Yang Ren, H. Chen, Y. Qiu, Wei Bao, R.H. Liu, G. Wu, T. Wu, Y.L. Xie, X.F. Wang, Q. Huang, X.H. Chen The relation between the spin-density-wave (SDW) and superconducting order is a central topic in current research on the FeAs-based high $T_c$ superconductors. Conflicting results exist in the LaFeAs(O,F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the $\rm (Ba,K)Fe_2As_2$ system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high $T_c$ ferropnictide superconductivity. [arXiv:0807.3950 (2008)] [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J35.00011: High magnetic field vortex torque magnetometry in SmFeAsO$_{0.8}$F$_{0.2}$ single crystals Luis Balicas, Alex Gurevich, Younjung Jo, Jan Jaroszynski, David Larbalestier, R.H. Liu, H. Chen, Xianhui H. Chen, N.D. Zhigadlo, S. Katrych, Z. Bukowski, J. Karpinski To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $\tau(\theta)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $\gamma$ extracted from $\tau(\theta)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $\gamma$ depends on both $T$ and $H$, saturating at $\gamma \simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 2:03PM |
J35.00012: Inelastic Neutron Scattering from Fe Pnictide Superconductors Invited Speaker: We present inelastic neutron scattering data from both single crystal and polycrystalline specimens of several of the new Fe-based superconducting materials. The phonon density of states (PDOS) was determined for LaFeAsO$_{1-x}$F$_{x}$. The PDOS for the nonsuperconducting parent compound LaFeAsO was found to be nearly identical to that of superconducting LaFeAsO$_{0.89}$F$_{0.11}$. Good agreement was found between first principal calculations and the experimentally determined PDOS with the exception of a small difference in some of the Fe mode frequencies. The experimental PDOS is not consistent with conventional phonon mediated superconductivity. In the case of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$, a magnetic excitation appears below T$_{c}$ that is not present at any temperature in the parent compound BaFe$_{2}$As$_{2}$. The excitation occurs at an energy of 14 meV and at a wave vector consistent with antiferromagnetic correlations in the FeAs plane. The existence of this excitation is strong evidence for an unconventional superconducting gap symmetry and demonstrates that the superconducting order parameter is strongly coupled to magnetic degrees of freedom in the Fe-based superconductors. [Preview Abstract] |
Session J36: Si and Ge Nanowires: Electrical Transport and Simulation
Sponsoring Units: DCMPChair: Jonathan Pelz, Ohio State University
Room: 408
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J36.00001: Epitaxial growth of Ge-Si$_{x}$Ge$_{1-x}$ core-shell nanowire heterostructures with tunable shell content Kamran Varahramyan, Domingo Ferrer, Emanuel Tutuc, Sanjay Banerjee Core-shell nanowire heterostructures are an interesting testbed for band engineering at the nanoscale. Here we present the growth of germanium (Ge) -- silicon-germanium (Si$_{x}$Ge$_{1-x})$ epitaxial core-shell nanowire (NW) heterostructures, with tunable Si and Ge shell content. The Ge NWs were grown using the Au-catalyzed vapor-liquid-solid (VLS) growth mechanism. Subsequently, the Si$_{x}$Ge$_{1-x}$ shells are grown \textit{in-situ}, conformal onto the Ge NW using ultra-high-vacuum chemical vapor deposition. We use transmission electron microscopy to confirm that both the core and shell are single crystal, and cross-sectional scanning transmission electron microscopy energy dispersive x-ray spectroscopy to determine the shell thickness and content. Our data show that the Si and Ge shell content can be tuned depending on the SiH$_{4}$ and GeH$_{4}$ partial pressures during the shell growth, effectively enabling band engineered core-shell nanowire heterostructures. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J36.00002: The Effects of Strain and Quantum Confinement on the Electronic Properties of Germanium Nanowires Paul Logan, Xihong Peng Germanium nanowires are expected to play an important role as both interconnects and functional components in future nanoscale electronic and optical devices, such as light-emitting diodes, field-effect transistors, chemical and biological sensors. The study of quantum confinement on the band gap of Ge nanowires have been addressed both using theoretical methods and experimental techniques. In the present work, using first principles density-functional theory we studied the uniaxial strain effects on the electronic properties in Ge wires along [110] direction with lateral diameter up to 5 nm. Ge [110] nanowires demonstrate a direct band gap, in contrast to the nature of indirect band gap in bulk. We discovered that the uniaxial strain modulates the band gap of Ge nanowires: compressive strain increases the gap while tensile strain reduces the gap. In addition, the strain also modifies the effective masses of the electron and the hole of Ge wires. Expansion increases the effective mass of the hole, while compression increases the effective mass of the electron. Our results suggest both strain and size can be used to tune the band structure of nanowires, which may help in design of future nanoelectronical devices. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J36.00003: Atomic scale structure of Si nanowire Tao Xu, Jean Philippe Nys, Maxime Berthe, Bruno Grandidier, Didier Stievenard, Wanghua Chen, Rodrigue Larde, Emmanuel Cadel, Philippe Pareige In this work, we have succeeded to observe the atomic structures of Au assisted Vapor-liquid-solid grown Si nanowire facetted sidewalls by scanning tunnelling microscopy (STM) at low temperature. By combining transmision microscopy observations with STM measurements, we were able to identify the differents facets along the growth direction of the nanowires. For nanowires with diameters larger than 150 nm, the facets orientation alternates between the [111] and [113] directions, whereas for smaller diameters, the {\{}113{\}} facets are replaced by facets with an orientations making a larger angle with the [111] direction. Imaging the facets at the atomic resolution clearly revealed that the facet reconstructions are induced by Au atoms. From the spectroscopic measurements, the facets are found to be metallic. In order to obtain the impurity distribution below the surface, 3D atom probe tomography analyses were performed. A uniform distribution of Boron impurities is observed in the core of the nanowire and the impurity concentration agrees well with the ratio of the flow rates between silane and diborane. Finally, such results are compared to the conductivities of single nanowires measured in field effect transistor devices. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J36.00004: Structural and Electronic Properties of Boron Doped Multiply Twinned Silicon Nanowires C.S. Jayanthi, Paul Tandy, M. Yu, S.Y. Wu, Y. Zhao Previous studies of undoped multiply twinned (MT) silicon nanowires (SiNWs) have found these structures to be more stable than the bulk-cut single crystal SiNWs for diameters $<$ 6 nm [1]. The five segments that form the MT-SiNWs result in a strain field, causing the interior region of the MT-SiNW to compress while stretching its exterior. In fact, the distribution of the internal stress field in MT-SiNWs offers a unique opportunity for doping the MT-SiNW, including bi-polar doping, and thus opening doors to novel designs of photovoltaic elements. In this work, we will use highly efficient quantum mechanical simulations based on the semi-empirical Hamiltonian developed in Ref. [2] to investigate the electronic structure of boron doped MT-SiNWs of different diameters. We will first determine the most favorable locations for placing boron atoms by mapping out the stress fields of undoped MT-SiNWs. To understand the doping characteristics, we will compare the local site energies and local electronic density of states of MT-SiNWs of undoped and doped systems, and carry out the calculation for MT-SiNWs of different diameters. 1. Y. Zhao, Phys. Rev. Lett., 91, 035501 (2003). 2. C. Leahy \textit{et al.} Phys. Rev. B74, 155408 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J36.00005: Nanometer-resolution studies of ``end-on'' metal contacts to vertical Si nanowires W. Cai, Y.L. Che, J.P. Pelz, E. Hemesath, L.J. Lauhon There is great interest in semiconducting nanowires (NWs) and carbon nanotubes (NTs) for future electronic devices and fundamental studies of low-dimensional systems. However, the critical \textit{contacts} to NWs and NTs are still poorly understood. For example, it is predicted (but not yet demonstrated) that Fermi level pinning should be much weaker at small ``end-on'' NW or NT Schottky contacts [1]. We have previously used cross-sectional ballistic electron emission microscopy (BEEM) to quantify small-size effects in Schottky contacts to cleaved quantum wells [2]. Here we describe on-going work to study individual end-on contacts to Si NWs. Vertical Si NWs were grown on Si(111) substrates, embedded in spin-on-glass, and planarized with a chemical mechanical polish. A brief HF etch and thin Au film deposition were then used to make end-on NW contacts. Initial studies with AFM, SEM, internal photoemission spectroscopy, and BEEM demonstrate we can make and measure end-on Schottky contacts to 80nm diameter Si NWs. We will discuss on-going work to optimize sample processing (to reduce roughness near the NWs) and then to quantify the dependence of local contact properties on Si NW diameter. Work supported by NSF Grant No. DMR-0805237. [1] F. Leonard\textit{ et al}., Phys. Rev. Lett. \textbf{84}, 4693 (2000). [2] C. Tivarus\textit{ et al}., Phys. Rev. Lett. \textbf{94}, 206803 (2005). [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J36.00006: First-Principles Simulations of Silicon Nanowires with Different Surface Passivations Junwen Li, John W. Mintmire We report first-principles simulation results for the electronic band structure of silicon nanowires along $<$100$>$ and $<$110$>$ directions with different surface passivating groups such as hydrogen, hydroxyl, and methyl within an all-electron, Gaussian type orbital, local density functional approach. We discuss how these different groups affect the band gaps and electron distribution of silicon nanowires. And from the band structures we find that the carrier effective masses of $<$100$>$-oriented silicon nanowires exhibit much more dependence on the diameter and passivation compared to those of $<$110$>$-oriented nanowires. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J36.00007: Study of Electronic Charge Distribution in Silicon Nanowire Transistors : An Atomistic Approach Abhijeet Paul, Saumitra Mehrotra, Gerhard Klimeck Atomistic modeling has been performed to investigate the spatial electronic charge distribution in silicon nanowire cross-sections. The modeling approach involves solution of electronic bandstructure using the 20 band sp3d5s* -SO nearest neighbor Tight-binding (TB) method with spin orbit (SO) interaction (LCAO) solved self-consistently with a two dimensional Poisson equation. Nanowires with rectangular, circular and triangular cross-section shapes have been investigated, with cross-section size of 3.1 nm and 5.1 nm for three different crystal orientations namely [100], [110] and [111]. The observed charge distribution as observed in these wires, is a strong function of cross-section shape, size and crystal orientation. [100] and [110] wires show strong corner effects, however, [111] oriented wires have centralized charge distribution. Charge distribution is sensitive to the structural and crystal symmetry of the nanowire. Structural confinement breaks the symmetry that manifests in the 1D energy dispersion of these wires by lifting up the degeneracy at the gamma valley. Finally, we enable the understanding of atomistic treatment for charge distribution in the capacitance measurements in these ultra-scaled silicon nanowire transistors. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J36.00008: Interface State Disorder Dominated Microwave Conductance in Silicon Nanowires Clark Highstrete, Mark Lee, David H. Dunlap, Aaron L. Vallett, Sarah M. Eichfeld, Joan M. Redwing, Theresa S. Mayer We have developed a technique to measure the microwave conductance spectra of nanomaterials at frequencies from 100 MHz to 50 GHz and at temperatures between 4 K and 300 K. We have used this technique to measure the microwave conductance spectra of doped silicon nanowires (SiNWs) which are found to increase sublinearly with frequency as $f^s$, with \textit{0.25 $<$ s $<$ 0.45}, indicative of disordered conduction. Additionally, the exponents are found to be nearly independent of temperature suggesting that structural disorder in nanomaterial morphology, rather than energetic trapping, dominates the AC transport. A model was developed that explains the SiNW conductance in terms of carrier confinement in a disordered electrostatic potential caused by charged Si/SiOx interface states. These results highlight the importance of topological effects in the microwave conductance of nanomaterials. Results from the measurement of other nanomaterials will also be briefly presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J36.00009: Effect of hydrogen passivation on the structure and energetics of silicon nanowires Abraham Ahmiel, Yongqiang Xue In this work we explore systematically the structure, energetics and electronic properties of silicon nanowires (SiNWs) with different surface structures and growth directions, and the trend of such property variation with increasing nanowire diameters using first principles density functional theory with both local atomic basis and plane waves. Both passivated and unpassivated systems were studied. The unpassivated (100) and (111) wires are found to be metallic with the unpaired electrons on the surface of these wires acting as conducting channels. Hydrogen passivation of these surfaces introduces a direct band gap by confining the electrons to localized bonds. The nature of the electronic states is examined through local density of states and electron density distributions. The relative stability of SiNWs with different growth directions and surface structures are evaluated from the free energy of formation. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J36.00010: Semiconducting nanowire devices in out-of-plane geometry. Pradeep Manandhar, Samuel T. Picraux Semiconducting nanowires are attractive components in the field of nanoelectronics, photonic and sensing applications. Experiments with nanowires have usually been performed in planar geometry. Here, we demonstrate the fabrication of nanowire devices in out-of-plane geometry by taking advantage of inherent growth direction of nanowire using the vapor-liquid-solid (VLS) method. Highly epitaxial semiconducting nanowires are grown on doped Si (111) substrate from Au nanoparticle seeds assembled in e-beam lithography patterns. The directed assembly of Au nanoparticles is achieved by molecular recognition through silanization process, or electrophoretic assembly. The versatility of the VLS method allows the growth of a wide range of semiconducting nanowires with controlled \textit{in-situ} doping. The post-growth processes include CVD of SiO$_{2}$ filler layer, chemical mechanical polishing and light etching of the SiO$_{2}$ layer to expose nanowire tips. Top metal contacts are then deposited for electrical characterization and sensing applications. We will present the results of the vertical nanowire device performance. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J36.00011: Top-gate Ge-Si$_{x}$Ge$_{1-x}$ core-shell nanowire field effect transistors with highly doped source and drain. Junghyo Nah, E.-S. Liu, D. Shahrjerdi, K. M. Varahramyan, S. K. Banerjee, E. Tutuc Semiconductor nanowires (NWs) field effect transistors (FETs) have been considered as candidates for aggressively scaled complementary metal-oxide-semiconductor (CMOS) devices. In particular, germanium (Ge) NW have been of interest thanks to their higher carrier mobility, compared to silicon (Si). Most of the reported semiconductor NW FETs up to date are measured on devices with metal (Schottky) contacts, where the carrier injection efficiency into the channel is significantly limited by the Schottky barrier at the metal/NW interface. Using low (3keV) energy boron ion implantation, we demonstrate here top-gate Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW p-type FETs, with highly doped source (S) and drain (D). The highly doped, up to $\sim $10$^{20}$ cm$^{3}$ levels, S/D areas of the NW FETs allow an efficient carrier injection into the NW and a low contact resistance. Compared to similar top gated NW FETs, but with undoped S/D and with metal-semiconductor contacts, the electrical characteristics of the top-gated NW FETs with doped S/D exhibit up to two orders of magnitude higher current, and an improved ON/OFF current ratio. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J36.00012: Precision transport and assembling of nanowires in suspension by electric fields D.L. Fan, Robert Cammarata, C.L. Chien We describe a method of precision transport of nanowires in suspension using a combination of dielectrophoretic force and electrophoretic force, which, respectively, aligns and transports the nanowires. We revealed the effect of electroosmosis flows on the nanowires and determined the ratio of viscous coefficients for nanowires moving parallel or perpendicular to the orientations. The transport of nanowires can be made to follow any prescribed trajectory with any orientation by the voltages applied to the patterned electrodes. As a demonstration of the high precision of manipulation, we have joined end-to-end two oppositely charged nanowires originally separated by 200 $\mu$m into a microelectromechanical device. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J36.00013: Thermal properties measurements of silicon nanowires at low temperature Heron Jean-Savin, Fournier Thierry, Bourgeois Olivier Phonons transport in nanowires and nanotubes is an effervescent field for theoretician as well as experimentalist. Especially at low temperature, where the dimensions of the sample approximate the dominant phonon wave length, the low dimensionality of these systems has strong impact on the thermal transport. Specific regimes have to be considered: transmission coefficient to the heat bath, quantum regime, transition between diffusive and specular regime etc{\ldots} Firstly, we have performed measurements with the 3$\omega $ method on various suspended silicon nanowires with a section of the order of 100nm$^{2}$ and a length of 10$\mu $m. Above 2 K, the thermal conductance varies like T$^{3}$(Casimir Regime); however at lower temperature, a quadratic regime in temperature appears: the signature of a change in the phonon transport regime. Secondly, we have measured nanowires with various geometries, to deduce the impact of geometrical factors at the mesoscopic scale on the thermal transport. All these results will be discussed in view of the different models describing the heat transfer at the nanoscale. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J36.00014: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J36.00015: Donor-pair defects and doping efficiency in silicon nanowires Byungki Ryu, Chang-Youn Moon, Woo-Jin Lee, Kee Joo Chang We investigate the doping efficiency of dopants in Si nanowires through first-principles density-functional calculations. For hydrogen- passivated Si nanowires doped with group-V elements such as P, As, and Sb, we consider wire diameters in the range of 8 - 30 {\AA} and axis orientations along the [111] and [110] directions. A single substitutional donor prefers to locate on the wire center and acts as a shallow donor. When wire diameters are below a critical value, a donor-pair defect which consists of two dopants at the first-nearest distance can be stabilized, in contrast to bulk Si. The stability of the donor-pair defect is attributed to the confinement effect in nanostructures, which results in the increase of the band gap and thereby the destabilization of the shallow donor level. As the donor- pair defect with a deep level in the band gap is electrically inactive, the doping efficiency is expected to be low in small-diameter nanowires. The formation of the donor-pair defect is found to be more favorable for the P dopants, which have a smaller atomic radius than the As and Sb dopants. [Preview Abstract] |
Session J37: Focus Session: Spectroscopic Probes of Biomolecular Structure and Function II
Sponsoring Units: DCPChair: Angela Gronenborn, University of Pittsburgh
Room: 409
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J37.00001: Structures of Amyloid Fibrils and Protein Folding Intermediates: New Insights from Solid State NMR Invited Speaker: I will present recent results from two projects: (1) We are using a combination of solid state NMR techniques and electron microscopy techniques to develop full molecular models for amyloid fibrils formed by the beta-amyloid peptide of Alzheimer's disease and by other peptides and proteins. Amyloid fibrils are often polymorphic, so that the detailed molecular structure depends on growth conditions or other factors. I will describe two structural models for beta-amyloid fibrils with two distinct morphologies. I will also describe efforts to determine which fibril structure develops in the brains of Alzheimer's disease patients, and solid state NMR methods that contribute to our amyloid studies; (2) Structural properties of unfolded or partially folded states of proteins are not well understood. In principle, solid state NMR measurements on freeze-trapped samples can reveal site-specific, quantitative aspects of protein structures in unfolded states. I will describe experiments on thermodynamically unfolded states (i.e., denatured states) and on transient states that are trapped by freezing on the microsecond time scale. Both types of experiments reveal structural properties that are unanticipated and could not be detected by more conventional protein folding measurements. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J37.00002: Two-Dimensional Infrared Probes of Peptide Conformations: the 3$_{10}$-Helical Secondary Structure Invited Speaker: The 3$_{10}$-helix is a secondary structure that has important biological functions and has been proposed as a picosecond intermediate in the folding of $\alpha $-helices. Two-dimensional infrared (2D IR) spectroscopy with its high structural sensitivity and time resolution is a powerful approach for investigating the structure and dynamics of peptides and proteins. In this talk, we will describe how we are using 2D IR and isotope labeling to study 3$_{10}$-helical oligopeptides that are rich in C$^{\alpha }$-methylated amino acids. These peptides are attractive models for developing and refining experimental and theoretical approaches to peptide conformational analysis. By manipulating networks of vibrational modes using judicious choices of laser polarizations and pulse ordering, we demonstrate that 2D IR can provide diagnostic cross-peak patterns for distinguishing different helical structures and probe the onset of 3$_{10}$-helical secondary structure. Using a series of peptides with $^{13}$C=$^{18}$O and $^{15}$N isotope labels, we observe cross-peak signature that reveals vibrational couplings between amide-I and amide-II modes across a 3$_{10}$-helical hydrogen bond. The results provide a direct evidence for local helical structure formation. Experimental spectra are compared to simulations based on nonlinear response theory, vibrational eigenstates and couplings derived from DFT-optimized structures, and trajectories from molecular dynamics simulations. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J37.00003: Earle K. Plyler Prize Talk: Using High Resolution Electronic Spectroscopy to Probe Reactive Chemical Intermediates Invited Speaker: Gas phase chemical reactions, such as occur in atmospheric chemistry, combustion, plasma processing, etc. are of great importance to our economy and society. These reactions are typically very complex involving up to 1000's of elementary steps with a corresponding number of reactive chemical intermediates. Spectrospic diagnostics, based upon well analyzed and well understood spectra of the intermediates, are crucial for monitoring such reactions and unraveling their mechanisms. These spectral analyses often benefit from the guidance provided by quantum chemical calculations and conversely the molecular parameters, experimentally determined from the spectra, serve as ``gold standards'' for benchmarking such calculations. Such standards are especially valuable for reactive intermediates whose electronic or geometric structure is particularly complex because of electron-spin interactions, Jahn-Teller effects or other vibronic interactions, hindered internal motions, large molecular size and weight, etc. The organic alkoxy, RO$\cdot$, and peroxy, RO$_2\cdot$, (R=alkyl group) free radicals are excellent examples of such species. The talk will focus on our recent characterization of these radicals via their ``high-resolution,'' mostly rotationally resolved, electronic spectra utilizing the techniques of laser induced fluorescence, stimulated emission pumping, and cavity ringdown spectroscopy. Selected spectra, their analysis, and the molecular information resulting therefrom will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J37.00004: Solvent induced fluctuations and the collective librational dynamics of myoglobin, hemoglobin, and lysozyme studied with infrared spectroscopy Kristina Woods We will discuss the use of (THz and Mid-) infrared spectroscopy to investigate the dynamics of several globular proteins under varying hydration and temperature conditions. Analysis of the experimental spectra has revealed that the amount of solvent in the hydration shell has a strong influence on the amplitude and the rate of relaxation associated with the low frequency protein conformational fluctuations and also the arrangement of hydrogen bonds in the protein secondary structure. At at a hydration level $>$ 0.2 we identify modes in the secondary structure of all of the proteins investigated that suggest extra mobility in the protein structure that is not present at low hydration. We will discuss how greater insight into the origin and nature of these detected solvent induced fluctuations may be important for developing a better understanding about energy localization and its relationship with biological function. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J37.00005: Label-Free Determination of Protein Binding in Aqueous Solution using Overlayer Enhanced Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (OE-ATR-FTIR) Travis Ruthenburg, Tolulope Aweda, Simon Park, Claude Meares, Donald Land Protein binding/affinity studies are often performed using Surface Plasmon Resonance techniques that don't produce much spectral information. Measurement of protein binding affinity using FTIR is traditionally performed using high protein concentration or deuterated solvent. By immobilizing a protein near the surface of a gold-coated germanium internal reflection element interactions can be measured between an immobilized protein and free proteins or small molecules in aqueous solution. By monitoring the on and off rates of these interactions, the dissociation constant for the system can be determined. The dissociation constant for the molecule Yttrium-DOTA binding to the antibody 2D12.5 system was determined to be 100nM. Results will also be presented from our measurements of Bovine Serum Albumin (BSA) binding to anti-BSA. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J37.00006: Solute-protein interactions: Variations in correlation times and spin label mobility. Mandy Blackburn, Luis Galiano, Angelo Veloro, Gail Fanucci Using EPR, NMR and fluorescence spectroscopy, the effects of several viscogen monomers (sucrose, glycerol, and ethylene glycol) and macromolecular crowding polymers (Ficoll400 and various size polyethylene glycols (PEG)) on the mobility of spin labels at aqueous exposed sites in the flap of HIV-1 protease, the correlation time of this protein, as well as conformation of the hair pin flaps were investigated. Results show that, as expected, protein correlation time is more strongly altered by the small viscogens compared to the macromolecular crowders. On the other hand, EPR line shapes reveal that the chemistry (ie hydrophobicity) and not the size of the solutes correlates to changes seen in the spectra. The conformations of the $\beta $-hair pin flaps in HIV-1 protease were unchanged by any of solutes as determined by pulsed EPR distance measurements. Thus, indicating that specific solute interactions with the surface of the protein are responsible for the changes observed in the EPR spin label spectra. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J37.00007: On The Electronic Properties of Photoacids In The Gas Phase. Electric Dipole Moments of \textit{CIS}- and \textit{TRANS}-2-Naphthol Adam Fleisher, Philip Morgan, David Pratt The permanent electric dipole moments ($\mu )$ of two conformers of 2-naphthol (2HN) in their ground and electronically excited states have been experimentally determined by Stark-effect measurements in a molecular beam. Upon UV excitation, little change in the magnitudes of $\mu $ is observed, but the orientation of the dipole moment within each conformer shifts significantly, indicating photon-induced rearrangements in electronic distributions. \textit{cis}-2HN has $\Delta \mu $ = +0.17 D and $\Delta $\textit{$\theta $}$_{a}$ = -28\r{ } and \textit{trans}-2HN has $\Delta \mu $ = +0.05 D and $\Delta $\textit{$\theta $}$_{a}$ = +28\r{ } (\textit{trans}-2HN). The $\Delta $\textit{$\theta $}$_{a}$ values for the two conformers differ in sign. The small changes in the magnitudes of the dipole moments suggest that the isolated molecules do not undergo large charge separations upon excitation. Our study, void of solvent perturbations, is of importance to the larger community currently describing aromatic biomolecule and ``super'' photoacid behavior \textit{via} theoretical modeling and condensed phase solvatochromism. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J37.00008: Threading of Binuclear Ruthenium Complex Through DNA Bases Thayaparan Paramanathan, Fredrik Westerlund, Micah McCauley, Per Lincoln, Ioulia Rouzina, Mark Williams Due to steric constraints the dumb-bell shaped binuclear ruthenium complex can only intercalate DNA by threading, which requires local melting of the DNA to occur. By mechanically manipulating a single DNA molecule held with optical tweezers, we lower the barrier to threading compared to bulk experiments. Stretching single DNA molecules with different drug concentrations and holding a constant force allows the binding to reach equilibrium. We can obtain the equilibrium fractional ligand binding and length of DNA at saturation. Fitting these results yields quantitative measurements of the binding thermodynamics and kinetics. In addition, we obtain the minimum binding site size, which may be determined by either electrostatic repulsion or steric constraints. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J37.00009: Raman Spectral Signatures as Conformational Probes of Biomolecules Ilana Bar, Amir Golan, Nitzan Mayorkas, Salman Rosenwaks A first application of ionization-loss stimulated Raman spectroscopy (ILSRS) monitoring the spectral features of four conformers of a gas phase neurotransmitter (2-phenylethylamine) is reported. The Raman spectra of the conformers show bands that uniquely identify the conformational structure of the molecule and are well matched by density functional theory calculations. The measurement of spectral signatures by ILSRS in an extended spectral range, with a relatively convenient laser source, is extremely important, allowing enhanced accessibility to intra- and inter-molecular forces, which are significant in biological structure and activity. [Preview Abstract] |
Session J38: Focus Session: Theory of Electron Transport Through Molecules II
Sponsoring Units: DCPChair: Weitao Yang, Duke University
Room: 410
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J38.00001: Transport in Molecular Junctions: Thoughts Coherent and Incoherent Invited Speaker: Current experimental efforts are clarifying quite beautifully the nature of charge transport in so-called molecular junctions, in which a single molecule provides the channel for current flow between two electrodes. The theoretical modeling of such structures is challenging, because of the uncertainty of geometry, the nonequilibrium nature of the process, and the variety of available mechanisms. The talk will center on the formulation of the problem in terms of non-equilibrium theory, and then on the generalizations needed to make that simple picture relevant to the real experimental situation. These include antiresonances, vibronic coupling and its control, structural disorder and representations for the electronic structure. Comments will be made on the measurements of inelastic spectra, and the information to be gained from them. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J38.00002: Theoretical aspects of modeling the conductance of molecular junctions Invited Speaker: In this talk I will discuss different semi-empirical and ab initio approaches for modeling the coherent electron transport of molecular junctions using the non-equilibrium Greens function formalism [1]. The most important effects for determining the conductance are the energies and coupling of the frontier molecular orbitals to the electrodes. I will discuss the accuracy of different levels of theory for calculating the HOMO-LUMO gap of various molecules, and present a simple correction that improves the accuracy of Density Functional based mean field theories [2]. The physical origin of the correction is illustrated using the Moshinsky atom as test system, and the accuracy is illustrated for a number of small molecules [3]. The coupling of the molecule to the electrodes is controlled by the terminal group on the molecule. We illustrate how a molecule with C60 terminal groups can have a very strong coupling with the electrodes [4]. \\[4pt] [1] Mads Brandbyge, Jose-Luis Mozos, Pablo Ordejon, Jeremy Taylor, and Kurt Stokbro, \textit{Density functional method for nonequilibrium electron transport}, Phys. Rev. B. 65, 165401 (2002).\\[0pt] [2] A. Cehovin, H. Mera, J. H. Jensen, K. Stokbro, and T. B. Pedersen, \textit{Role of the virtual orbitals and HOMO-LUMO gap in mean-field approximations to the conductance of molecular junctions}, Phys. Rev. B 77, 195432 (2008)\\[0pt] [3] H. Mera and K. Stokbro, \textit{Using Kohn-Sham-DFT to describe charged excitations in finite systems, }submitted\\[0pt] [4] C. A. Martin et. Al., Fullerene\textit{-Based Anchoring Groups for Molecular Electronics}, J. Am. Chem. Soc. $130$, 13198 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J38.00003: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J38.00004: Many-body theory of electron transport in single-molecule junctions Charles Stafford, Justin Bergfield Currently, there is no general theory to treat the many-body problem of a single molecule coupled to metallic electrodes. Mean-field approaches such as density-functional theory---the dominant paradigm in quantum chemistry---have serious shortcomings because they do not account for important interaction effects like Coulomb blockade. We develop a systematic theoretical framework for this nonequilibrium many-body problem, starting from an exact diagonalization of the few-body problem of an isolated molecule, and including lead-molecule coupling perturbatively in a novel application of nonequilibrium Green's functions. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J38.00005: Quantum theory of image potential and resonant tunneling in molecular junctions Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner It has recently been realized that the image potential plays an important role in charge transport through single organic molecules. In most cases, the classical image potential -1/4z is used to calculate the modified energy spectrum of the charge carriers in the molecule. In this talk, we will present the theory of resonant tunneling transitions that include the quantum mechanical effects of dynamic image potential due to the polarization interaction of the tunneling charge carrier (electron or hole) with surface plasmons. The application of this theory to organic molecular junctions of experimental interest will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J38.00006: Charge and Spin Memory Effects in Molecular Junctions P. D'Amico, D.A. Ryndyk, G. Cuniberti, K. Richter In the field of molecular electronics, effects like charge-memory, bistability and switching between charged and neutral states have been observed in STM [1] and single-molecule junctions [2] experiments. In this work we use model hamiltonians to describe molecular junctions, including electron-electron and electron-vibron interactions as wel as tunneling coupling to the leads. For a molecular level coupled to a vibron and in the presence of leads, we show that upon applying gate or bias voltage, it is possible to observe charge-bistability and hysteretic behavior. Physical quantities like lifetimes, charge-voltage and current-voltage curves are calculated by the master equation method for weak coupling to the leads [3] and at stronger coupling by the equation-of-motion method for noneq. Green functions, performing a systematic analysis of the bistable behaviour of the system for different internal parameters such as the electron-vibron and the lead-molecule coupling [4]. In the case of a spin-degenerate molecular level in a single and double dot molecule with vibrational coupling and in presence of ferromagnetic leads, we consider the possibility to obtain a spin-memory effect. [1] J.Repp et al, Science 305, 493 (04); [2] E.Lortscher et al, Small 2, 973 (06); [3] D.A.Ryndyk et al, PRB 78, 085409 (08); [4] P.D'Amico et al, NJP 10, 085002(08). [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J38.00007: A simple model for the description of correlation effects in molecular conductors Matthias Ernzerhof, Francois Goyer To model transport through molecular electronic devices (MEDs), we use a non-Hermitian Hamiltonian [1] for the description of open systems that exchange current density with their environment. The infinite contacts are replaced by complex source-sink potentials (SSPs) [1]. Employing a Hubbard interaction term, we include electron-correlation effects in our approach [2]. Electron interaction is considered in the molecule and neglected in the contacts. Among other strongly correlated problems, we discuss the change in conductance upon bond breaking. In the limit where the electron repulsion is strong compared to the binding energy (as it is the case in a stretched bond) a strong suppression of conductance is observed due to the localization of electrons. Other interesting phenomena, which cannot be accounted for with conventional (independent electron) approaches, are discussed as well. [1] F. Goyer, M. Ernzerhof, and M. Zhuang, J. Chem. Phys. 126, 144104 (2007); M. Ernzerhof, J. Chem. Phys. 127, 204709 (2007). [2] A. Goker, F. Goyer, and M. Ernzerhof, J. Chem. Phys. 129, 194901 (2008); M. Ernzerhof, J. Chem. Phys. 125, 124104 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J38.00008: Effects of dephasing on molecular conduction Jesse Maassen, Ferdows Zahid, Hong Guo In this work, we theoretically investigate effects of dephasing on electron transport in molecular wires. The quantum transport analysis is carried out using the density functional theory (DFT) combined with the non-equilibrium Green's function framework (NEGF). The dephasing effect is included at a phenomenological level by introducing fictitious voltage probes to the NEGF-DFT formalism that mimics the randomisation of quantum phase information of the charge carriers. For three systems: (i) a 1,4-benzenedithiol (BDT) molecule connected to Al(001) leads; (ii) an atomic gold chain in contact with Au(001) leads; and (iii) a very narrow Al(001) nanowire, our results indicate that there are two behaviours. When the wires are not conductive as (i,ii), the dephasing effects can increase conduction for a range of system parameters; while for conducting systems (iii), the effect is opposite. These effects can be understood from a quantum interference point of view. We also compare results for two different models on how the phenomenological dephasing effects are introduced into the NEGF-DFT formalism. [Preview Abstract] |
Session J39: Lipid Bilayers: Structure and Function II
Sponsoring Units: DBPChair: Wouter Ellenbroek, University of Pennsylvania
Room: 411
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J39.00001: Membrane curvature sensing by the actin cytoskeleton Ghee Hwee Lai, Abhijit Mishra, Nathan Schmidt, Daniel Kamei, Timothy Deming, Gerard C. L. Wong Biological active molecules such as proteins and oligonucleotides can be transduced across cell membranes with high efficiency by cell penetrating peptides. It has been recently demonstrated using synchrotron x-ray diffraction that such peptides induce saddle-splay (negative Gaussian) membrane curvature, which is the topological requirement for pore formation. Here, we show how the actin cytoskeleton 'senses' and responds to negative Gaussian defects on a membrane, by examining the interaction between cell penetrating peptides and an active polymerizing cytoskeleton encapsulated within giant unilamellar vesicles, and compare the results to cell based studies. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J39.00002: Optically Induced Rotation of Laser-trapped Chiral Lipid Tubules by Linearly Polarized Light Nattaporn Chattham, Thanate Na Wichian, Apichart Pattanaporkratana, Jamras Limtrakul Chiral Phospholipids are found self-assembled into fascinating cylindrical tubules of 500 nm in diameter by helical winding of bilayer stripes under cooling in ethanol and water solution. Theoretical prediction and experimental evidence reported so far confirmed the modulated tilt direction in a helical striped pattern of the tubules. This molecular orientation morphology results in optically birefringent tubules. We investigate them under optical trap of 532 nm linearly polarized optical tweezers. We observed spontaneous rotation of lipid tubules induced by radiation torque. The tubule direction can be controlled by the alignment of polarization direction, and thus the rotation angle can be specified. Other related aspect on optical activity of the lipid tubules is also studied. This work is supported by Kasetsart University Research and Development and National Nanotechnology Center, Thailand. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J39.00003: Diffusion in Tethered Bilayer Lipid Membranes as observed by Z-Scan FCS S. Shenoy, R. Moldovan, S. Rauhala, M. Loesche Tethered bilayer lipid membranes (tBLMs, [1]) are resilient biomimetic systems stabilized by the proximity of an inorganic interface. Synthetic lipids with a hydrophilic oligomer covalently coupled to the substrate serve as membrane anchors while forming a nm-thick aqueous reservoir. This property can be exploited to investigate protein-membrane interactions at the molecular length scale. The anchor is chemisorbed into a self-assembled monolayer, either as a pure compound (densely tethered) or laterally diluted (sparsely tethered) by $\beta$ -mercaptoethanol ($\beta$ME), a small spacer. Phospholipids are then precipitated to complete the bilayer structure. Diffusion measurements were performed using both one-photon and two-photon fluorescence correlation spectroscopy using the Z-Scan approach[2]. While the aqueous reservoir decouples the bilayer from the substrate, we expect the presence of tethers in the inner leaflet to inhibit the free diffusion of lipids. Indeed, we see a drop in the apparent diffusion coefficient by a factor of 2 when comparing a densely tethered membrane to a sparsely-tethered membrane. Importantly, the diffusion coefficients in tBLMs compare favorably with those observed in giant unilamellar vesicles, indicating that tBLM dynamics are similar to those of free bilayers. [1] McGillivray et al., Biointerphases 2007(2): 21-33 [2] Benda et al., Langmuir 2003(19): 4120-4126 [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J39.00004: Cholesterol Perturbs Lipid Bilayers Non-Universally John Nagle, Jianjun Pan, Thalia Mills, Stephanie Tristram-Nagle Cholesterol is well known to modulate the physical properties of biomembranes. Using modern x-ray scattering methods, we have studied the effects of cholesterol on the bending modulus K$_{C}$, the thickness D$_{HH}$, and the orientational order parameter S$_{xray}$ of lipid bilayers. We find that the effects are different for at least three classes of phospholipids characterized by different numbers of saturated hydrocarbon chains. Most strikingly, cholesterol strongly increases K$_{C}$ when both chains of the phospholipid are fully saturated but not at all when there are two mono-unsaturated chains. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J39.00005: Barrier Function of Lipid Membrane in the Interaction with Nanostructures. Sergiy Minko, Yury Roiter Tiny details of the phospholipid (DMPC) membrane morphology in close vicinity to nanostructured silica surfaces have been discovered in the atomic force microscopy experiments. The structural features of the silica surface were varied in the experiments by the deposition of silica nanoparticles of different diameter on plane and smooth silica substrates. It was found that, due to the barrier function of the lipid membrane; only particles larger than 22 nm in diameter, with a smooth surface were completely enveloped by the lipid membrane. However, nanoparticles with bumpy surfaces (curvature diameter of bumps as that of particles $<$22 nm) were only partially enveloped by the lipid bilayer. For the range of nanostructure dimensions between 1.2 nm and 22 nm, the lipid membrane underwent structural rearrangements by forming pores (holes). The nanoparticles were accommodated into the pores but not enveloped by the lipid bilayer. The study also found that the lipid membrane conformed to the substrate with surface structures of dimensions less than 1.2 nm without losing the membrane integrity. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J39.00006: Direct nm-scale observation of lipid membrane fluctuations Sung Chul Bae, Yan Yu, Stephen M. Anthony, Steve Granick Thermal fluctuation of giant unilamellar phospholipids vesicles(GUVs) was observed by a combination of direct imaging using epifluorescence microscopy and forward laser beam scattering of a laser beam from the vesicle edge. The latter technique, owing to the refractive index mismatch, offers nanometer spatial and microsecond temporal resolution. When nanoparticles adsorb, this changes. We will discuss the changes of membrane rigidity caused by nanoparticles, based on membrane fluctuation data. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J39.00007: Effects of mobile membrane proteins on the structure and dynamics of lipid rafts Jun Fan, Maria Sammalkorpi, Mikko Haataja Compositional lipid domains (``lipid rafts''), which reside in the plasma membrane, are thought to facilitate many important cellular processes, including signal transduction and viral entry. Experimentally, raft dynamics have been probed mainly indirectly through observations of raft-associated membrane proteins, and the interpretation of the data relies heavily on assumptions about raft shape and viscosity. Previously, we have shown that strong interactions between rafts and immobile protein clusters may induce the formation of spatially extended raft aggregates [J. Fan et al., PRL 100, 178102 (2008)], thus complicating the interpretation of experimental data. In this work we correlate the dynamics of membrane proteins with the underlying time-dependent raft domain structure via a hybrid continuum-particle simulation scheme, and develop strategies for extracting quantitative information about raft dynamics from observations of the membrane proteins alone. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J39.00008: Multiscale Modeling of supported bilayers Roland Faller, Chenyue Xing, Matthew I. Hoopes Supported Lipid Bilayers are an abundant research platform for understanding the behavior of real cell membranes as they allow for additional mechanical stability. We studied systematically the changes that a support induces on a phospholipid bilayer using coarse-grained molecular modeling on different levels. We characterize the density and pressure profiles as well as the density imbalance inflicted on the membrane by the support. We also determine the diffusion coefficients and characterize the influence of different corrugations of the support. We then determine the free energy of transfer of phospholipids between the proximal and distal leaflet of a supported membrane using the coarse-grained Martini model. It turns out that there is at equilibrium about a 2-3\% higher density in the proximal leaflet. These results are in favorable agreement with recent data obtained by very large scale modeling using a water free model where flip-flop can be observed directly. We compare results of the free energy of transfer obtained by pulling the lipid across the membrane in different ways. There are small quantitative differences but the overall picture is consistent. We are additionally characterizing the intermediate states which determine the barrier height and therefore the rate of translocation. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J39.00009: Thermotropic vibrational spectroscopy of newly developed self-forming PEGylated lipids Rajan Bista, Reinhard Bruch, Aaron Covington Vibrational spectroscopy can provide valuable structural information about lipids, which are important molecular components of biological membranes. In the present study, we have focused on the thermotropic vibrational spectroscopy of two newly developed synthetic PEGylated lipids trademarked as QuSomes{\texttrademark} to investigate the phase behaviors and associated changes in the conformational order. In contrast to conventional phospholipids, this new kind of lipid forms liposomes spontaneously upon hydration, without the supply of external activation energy. Variable-temperature thin-layered Fourier Transform Infrared (FTIR) spectroscopy and Raman spectro-microscopy have been developed and employed in order to plot the transition temperature profiles showing the phase behavior of these new lipids composed of 1,2-dimyristoyl-\textit{rac}-glycerol-3-dodecaethylene glycol (GDM-12) and 1,2-distearoyl-\textit{rac}-glycerol-3-triicosaethylene glycol (GDS-23). Furthermore, several spectral indicators were calculated and correlated which allowed for the deduction of various aspects of molecular structure as well as intramolecular motion and intermolecular interactions occurred during temperature change. To confirm the observations, differential scanning calorimetry (DSC) was applied and revealed a good agreement with the FTIR and Raman spectroscopic results. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J39.00010: Theory of Disk-to-Vesicle Transformation Jianfeng Li, An-Chang Shi Self-assembled membranes from amphiphilic molecules, such as lipids and block copolymers, can assume a variety of morphologies dictated by energy minimization of system. The membrane energy is characterized by a bending modulus (\textit{$\kappa $}), a Gaussian modulus (\textit{$\kappa $}$_{G})$, and the line tension (\textit{$\gamma $}) of the edge. Two basic morphologies of membranes are flat disks that minimize the bending energy at the cost of the edge energy, and enclosed vesicles that minimize the edge energy at the cost of bending energy. In our work, the transition from disk to vesicle is studied theoretically using the string method, which is designed to find the minimum energy path (MEP) or the most probable transition path between two local minima of an energy landscape. Previous studies of disk-to-vesicle transition usually approximate the transitional states by a series of spherical cups, and found that the spherical cups do not correspond to stable or meta-stable states of the system. Our calculation demonstrates that the intermediate shapes along the MEP are very different from spherical cups. Furthermore, some of these transitional states can be meta-stable. The disk-to-vesicle transition pathways are governed by two scaled parameters, \textit{$\kappa $}$_{G/}$\textit{$\kappa $} and \textit{$\gamma $R}$_{0}$\textit{/4$\kappa $}, where $R_{0}$ is the radius of the disk. In particular, a meta-stable intermediate state is predicted, which may correspond to the open morphologies observed in experiments and simulations. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J39.00011: Hybrid Lipid as Biological Surfactants Robert Brewster, Phil Pincus, Sam Safran Systems capable of forming finite-sized, equilibrium domains are of biological interest in the context of membrane rafts where it has been observed that certain cellular functions are mediated by small (nanometric to tens of nanometers) domains with specific lipid composition that differs from the average composition of the membrane. These small domains are composed mainly of lipids with completely saturated hydrocarbon tails that show good orientational order in the membrane. The surrounding phase consists mostly of lipids with at least one unsaturated bond in the hydrocarbon tails which forces a ``kink'' in the chain and inhibits ordering. In vitro, this phase separation can be replicated; however, the finite domains coarsen into macroscopic domains with time. We have extended a model for the interactions of lipids in the membrane, akin to that developed in the group of Schick (Elliott et al., PRL 2006 and Garbes Putzel and Schick, Biophys. J. 2008), which depends entirely on the local ordering of hydrocarbon tails. We generalize this model to an additional species and identify a biologically relevant component, a lipid with one fully saturated hydrocarbon chain and one chain with at least one unsaturated bond, that may serve as a line-active component, capable of reducing the line tension between domains to zero, thus stabilizing finite sized domains in equilibrium. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J39.00012: Distribution of Drug Molecules in Lipid Membranes: Neutron Diffraction and MD Simulations. Mohan Boggara, Ella Mihailescu, Ramanan Krishnamoorti Non-steroidal anti-inflammatory drugs (NSAIDs) e.g. Aspirin and Ibuprofen, with chronic usage cause gastro intestinal (GI) toxicity. It has been shown experimentally that NSAIDs pre-associated with phospholipids reduce the GI toxicity and also increase the therapeutic activity of these drugs compared to the unmodified ones. In this study, using neutron diffraction, the DOPC lipid bilayer structure (with and without drug) as well as the distribution of a model NSAID (Ibuprofen) as a function of its position along the membrane normal was obtained at sub-nanometer resolution. It was found that the bilayer thickness reduces as the drug is added. Further, the results are successfully compared with atomistic Molecular Dynamics simulations. Based on this successful comparison and motivated by atomic details from MD, quasi-molecular modeling of the lipid membrane is being carried out and will be presented. The above study is expected to provide an effective methodology to design drug delivery nanoparticles based on a variety of soft condensed matter such as lipids or polymers. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J39.00013: Calcium-induced domain formation in mixed lipid monolayers Wouter G. Ellenbroek, David A. Christian, Ilya Levental, Andrea J. Liu, Paul A. Janmey Multivalent ions such as calcium play an important role in soft matter and biological systems. This role cannot be captured by a mean field treatment of the electrostatics such as the Poisson-Boltzmann equation, which neglects, for example, the fact that Ca$^{2+}$-ions can mediate attractions between negatively-charged objects. We show, both experimentally and theoretically, that Ca$^{2+}$-mediated attractions lead to phase separation of charged and neutral lipid molecules in mixed lipid monolayers, and discuss the dependence on pH, salt concentration and ion valency. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J39.00014: Membrane Disruption Mechanism of Antimicrobial Peptide Kin Lok H. Lam PG-1, a cationic antimicrobial peptide, kills bacteria by forming pores which increase membrane permeability to ions or larger molecules. It has been proposed that PG-1 selectively induces stable membrane pores in bacterial membranes over mammalian membranes. To study the mechanism of action of PG-1, we directly visualize the topological changes induced by PG-1 in model membranes via atomic force microscopy for the first time. PG-1 induces structural transformations in supported lipid bilayers, progressing from bilayer edge instability, to the formation of pores, and finally to a network of wormlike micelles in a zwitterionic dimyristoylphosphatidylcholine model membrane with increasing PG-1 concentrations. The structural transformation can be understood in the framework of the action of 1d detergent, with PG-1 acts as a line active agent. The results elucidate the mechanism by which PG-1 uses to induce leakage in bacterial cells. [Preview Abstract] |
Session J40: Biological Physics II
Sponsoring Units: DBPChair: Joshua Socolar, Duke University
Room: 412
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J40.00001: Cell Rheology and Embryogenesis Using the Subcellular Element Model Invited Speaker: I will present recent work on grid-free computational modeling of both cell biomechanics and multicellular collective dynamics, the latter in the context of gastrulation in the chick embryo. Mechanics at both subcellular and multicellular scales is modeled seamlessly with the Subcellular Element Model (SEM). The SEM is able to capture basic viscoelastic properties of cells at a semi-quantitative level, and is efficient enough to simulate thousands of cells in three dimensions allowing computational analysis of biological hypotheses regarding collective cell motion during gastrulation. Work done in collaboration with Sebastian Sandersius, Arizona State University. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J40.00002: The effects of viscoelastic polymer substrates on adult stem cell differentiation Chungchueh Chang, Adam Fields, Alex Ramek, Vladimir Jurukovski, Marcia Simon, Miriam Rafailovich Dental Pulp Stem Cells (DPSCs) are known to differentiate in either bone, dentine, or nerve tissue by different environment signals. In this study, we have determined whether differentiation could only through modification of the substrate mechanics. Atomic Force Microscopy (AFM) on Shear Modulation Force Microscopy (SMFM) mode indicated that the spun-cast polybutadiene (PB) thin films could be used to provide different stiffness substrates by changing the thicknesses of thin films. DPSCs were then plated on these substrates and cultured in standard media. After 28 days incubation, Lasar Scanning Confocal Microscopy (LSCM) with mercury lamp indicated that the crystals were observed only on hard surfaces. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX analysis) indicated that the crystals are calcium phosphates. The Glancing Incidence Diffraction (GID) was also used to determine the structure of crystals. These results indicate that DPSCs could be differentiated into osteoblasts by mechanical stimuli from substrate mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J40.00003: Translocation of double strand DNA into a biological nanopore Sunita Chatkaew, Lamia Mlayeh, Marc Leonetti, Fabrice Homble Translocation of double strand DNA across a unique mitochondrial biological nanopore (VDAC) is observed by an electrophysiological method. Characteristics of opened and sub-conductance states of VDAC are studied. When the applied electric potential is beyond $\pm$ 20 mV, VDAC transits to a sub-conductance state. Plasmids (circular double strand DNA) with a diameter greater than that of the channel shows the current reduction into the channel during the interaction but the state with zero-current is not observed. On the contrary, the interaction of linear double strand DNA with the channel shows the current reduction along with the zero-current state. These show the passages of linear double strand DNA across the channel and the electrostatic effect due to the surface charges of double strand DNA and channel for circular and linear double strand DNA. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J40.00004: On-chip growth of polymeric nanowires for electro-mechanical probing of live cells Bret Flanders, Prem Thapa This study characterizes the directed electrochemical nanowire assembly of amorphous polythiophene nanowires on micro-electrode arrays. In this approach, a long range component of an applied voltage signal defines a channel of maximum flux in the laboratory reference frame. Amorphous wires lack a natural growth axis. However, because polymerization is restricted to the channel-region, such materials may be grown with wire-like geometries, and the growth path of these wires may be controlled. The wire-laden electrode arrays are useful substrates for cell physiological studies. To this end, non-invasive methodology for inducing single Dictyostelium cells to approach and attach individual pseudopods to the tips of the polymeric wires will be presented. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 1:03PM |
J40.00005: What do cell rheology experiments really measure? Invited Speaker: It is now widely appreciated that normal tissue morphology and function rely upon cells? ability to sense and generate forces appropriate to their correct tissue context. While the effects of forces on cells have been studied for decades, our understanding of how those forces propagate through and act on different cell substructures remains at an early stage. The last decade has seen a resurgence of interest, with a variety of different micromechanical methods in current use that probe cells dynamic deformation in response to a time varying force. Recently, it has been shown that the seemingly disparate findings from different labs can be fit into a single, workable consensus description. The ability of researchers to carefully measure the mechanical properties of cells subjected to a variety of pharmacological and genetic interventions, however, currently outstrips our ability to quantitatively interpret the data in many cases. Despite these challenges, the stage is now set for the development of detailed models for cell deformability, motility and mechano-sensing that are rooted at the molecular level. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J40.00006: Cargo transport by several molecular motors Stefan Klumpp, Melanie M\"uller, Janina Beeg, Rumiana Dimova, Reinhard Lipowsky In cells, cargoes are often transported by small teams of molecular motors rather than by a single motor. Furthermore, many cargoes perform bidirectional movements, which are based on the presence of two motor species on the cargo. We study the transport by several motors theoretically using a model that describes the stochastic binding and unbinding of motors from filaments and that is based on the properties of individual motors as observed in single molecule experiments. We find that the cooperation of several motors leads to a strongly increased run length, which is confirmed experimentally for beads pulled by several kinesin motors. Furthermore, such cargoes exhibit a non-linear force-velocity relation. For the case of two motor teams pulling into opposite direction we find that a stochastic tug-of-war model, where the motors interact only by pulling their common cargo into opposite directions, leads to surprisingly complex motility. In particular, even for two motor teams with equal strength, we find that a tug-of-war leads to fast bidirectional motion similar to what is observed in cells and usually taken as evidence for some unknown coordination mechanism. This behavior is due to a dynamic instability, which arises from the strong force-dependence of the rate with which motors unbind from filaments. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J40.00007: Adverse Effects of TiO$_{2}$ Nanoparticles on Human Dermal Fibroblasts and How to Protect Cells Zhi Pan, Wilson Lee, Lenny Slutsky, Sowmya Sandaresh, Nicole Elstein, Richard Clark, Nadine Pernodet, Miriam Rafailovich We have studied the effects of exposure of human dermal fibroblasts to rutile and anatase TiO$_{2}$ nanoparticles. We found that these particles can impair cell functions, with the latter being more potent at producing damage. We showed that the exposure to nanoparticles decreases cell area, cell proliferation, mobility, and ability to contract collagen. Individual particles are shown to penetrate easily through the cell membrane, in the absence of endocytosis, while some endocytosis is observed for larger particle clusters. Once inside, the particles are sequestered in vesicles, which continue to fill up with increasing incubation time till they rupture. We also tested particles that were coated with a dense grafted polymer brush and, using flow cytometry, showed that the coating prevented the particles from adhering to the cell membrane and hence penetrating the cell, which effectively decreases reactive oxygen species (ROS) formation and protects cells, even in the absence of light exposure. Considering the broad applications of these nanoparticles in personal health care products, the functionalized polymer coating can potentially play an important role in protecting cells and tissue from damage. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J40.00008: Surface Morphological Studies on Nerve Cells by AFM Goksel Durkaya, Lei Zhong, Vincent Rehder, Nikolaus Dietz Surface morphological properties of fixed and living nerve cells removed from the buccal ganglion of \textit{Helisoma trivolvis} have been studied by using Atomic Force Microscopy (AFM). Identified, individual neurons were removed from the buccal ganglion of \textit{Helisoma trivolvis} and plated into poly-L-lysine coated glass cover-slips. The growth of the nerve cells was stopped and fixed with 0.1{\%} Glutaraldehyde and 4{\%} Formaldehyde solution after extension of growth cones at the tip of the axons. Topography and softness of growth cone filopodia and overlying lamellopodium (veil) were probed by AFM. Information obtained from AFM's amplitude and phase channels have been used for determination of softness of the region probed. The results of structural studies on the cells are linked to their mechanical properties and internal molecular density distribution. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J40.00009: Study of Charge and Spin Distribution Properties in Five-Liganded Helogen-Heme Systems Archana Dubey, Minakhi Pujari, K. Ramani Lata, Alyssa Garcia, A.F. Schulte, S.R. Badu, R.H. Pink, R.H. Scheicher, T.P. Das The current emphasis in biological physics is on the study of the functions of important systems, like for instance hemoglobin and cytochromes at a quantitative level. For these studies an accurate knowledge of the electronic structures of the entire molecules as well as parts of them are very important. In the heme proteins there is great current interest in both electron transport and in attachment and detachment of O$_{2}$, CO, and NO molecules to the iron. For this purpose an in depth understanding at the electronic level of the heme units, the protein chains, and the interactions between the two, is vital. With these aims in mind, we have studied quantitatively at a first principles level the electronic structures of all four halogen five liganded heme compounds, the natures of the charge and spin distributions over them, and the associated hyperfine interactions of the nuclei of the atoms . Results and trends of these properties over the four systems and comparisons with available data will be presented and discussed. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J40.00010: Kalman meets neuron - the intersection of control theory and neuroscience Steven Schiff Since the 1950s, we have developed mature theories of modern control theory and computational neuroscience with almost no interaction between these disciplines. With the advent of computationally efficient nonlinear Kalman filtering techniques, along with improved neuroscience models which provide increasingly accurate reconstruction of dynamics in a variety of important normal and disease states in the brain, the prospects for a synergistic interaction between these fields are now strong. I will show recent examples of the use of nonlinear control theory for the assimilation and control of single neuron dynamics, a novel framework for dynamic clamp, the modulation of oscillatory wave dynamics in brain cortex, a control framework for Parkinsonian dynamics and seizures, and the use of optimized parameter model networks to assimilate complex network data. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J40.00011: Protein Thermodynamics from Maxwell Constraint Counting Donald Jacobs, Dennis Livesay, Oleg Vorov Topological properties of network rigidity explain essential aspects of structural phase transitions and thermodynamic stability in proteins [1]. We present an exact transfer matrix method within a Distance Constraint Model (DCM) that maps interactions into distance constraints having energy and entropy contributions. Conformational entropy is reduced by interactions that rigidify structure, associated with independent constraints. Here, we solve the DCM using a mean-field treatment that assumes distance constraints are well distributed throughout the structure, meaning a distance constraint is independent until the structure is globally rigid. Experimental heat capacity curves are described markedly well with a few adjustable parameters. The universal character of this theory is analogous to the van der Waals model for a liquid-gas transition. This work is supported by NIH R01 GM073082. [Preview Abstract] |
Session J41: Hubbard Model
Sponsoring Units: DCMPChair: Philip Phillips, University of Illinois at Urbana-Champaign
Room: 413
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J41.00001: Origin of the Mott Gap Philip Phillips, Robert G. Leigh We show exactly that the only charged excitations that exist in the strong-coupling limit of the half-filled Hubbard model are gapped composite excitations generated by the dynamics of the charge $2e$ boson that appears upon explicit integration of the high-energy scale. At every momentum, such excitations have non-zero spectral weight at two distinct energy scales separated by the on-site repulsion $U$. The result is a gap in the spectrum for the composite excitations when $U$ exceeds the bandwidth. Consequently, we resolve the long-standing problem of the cause of the charge gap in a half-filled band in the absence of symmetry breaking. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J41.00002: Diagrammatic Monte Carlo Kris Van Houcke, Felix Werner, Evgeny Kozik, Lode Pollet, Nikolay Prokof'ev, Boris Svistunov Diagrammatic Monte Carlo (DiagMC) is an exact technique that allows one to simulate quantities specified in terms of diagrammatic expansions, the latter being a standard tool of many-body quantum statistics. The sign problem, that is typically fatal to Monte Carlo approaches, appears to be manageable with DiagMC. We introduce a general DiagMC scheme for strongly interacting fermions. As an illustrative example, we discuss the application of DiagMC to the Fermi-Hubbard model, and benchmark the technique against state-of-the-art numerical tools for strongly correlated fermions. In addition, we discuss the thermodynamic properties of a Fermi gas at unitarity, obtained through DiagMC simulation. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J41.00003: Unconventional soft gaps in strongly-correlated systems with coexisting short-range interaction and disorder Hiroshi Shinaoka, Masatoshi Imada We report a theoretical study of the Anderson-Hubbard model under coexisting short-range interaction and disorder, which is one of the minimum models of real strongly-correlated materials. We determined the ground-state phase diagram in three dimensions within the unrestricted Hartree-Fock approximation. Although only short-range interaction is present, we found a soft gap in the single-particle density of states of the insulating phases [1]. This unconventional soft gap (soft Hubbard gap) cannot be explained within the conventional theory [2] which ascribes the formation of soft gaps to the long-range part of the Coulomb interaction. We present a phenomenology to clarify the origin of the soft Hubbard gap. We propose a multi-valley energy landscape as their origin. Further support by the exact diagonalization in one dimension beyond the mean-field level is given. Possible experiments to verify the present theory are also proposed. [1] H. Shinaoka and M. Imada, arXiv:0811.2492v1. [2] A. L. Efros and B. I. Shklovskii, J. Phys. C 8, L49 (1975). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J41.00004: Determinant Quantum Monte Carlo method applied to the t-J model Aleksander Zujev, Richard Fye, Richard Scalettar The usual approach to simulating the t-J model with the Determinant Quantum Monte Carlo (DQMC) method starts with the Hubbard model with a finite on-site interaction U which is then increased to ``almost'' infinity. This approach, however, has considerable difficulties with large round-off errors (stability) and variances, and also a very bad fermion sign problem. In this talk, I will describe a different approach which starts with (almost) infinite U by means of a projector operator and further prohibiting double occupancy by using a modified creation operator. The new technique will be shown to solve some of these difficulties. Unfortunately, the sign problem remains significant. I will discuss the different attempts we have made to reduce it. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J41.00005: Ordering and Frustration in a Strongly Correlated Chain system Siddhartha Lal, Mukul Laad We present recent results of our study on the one-dimensional extended Hubbard model with longer-range Coulomb interactions at quarter-filling in the strong coupling limit. With the complex phase diagram of the TMTSF and TMTTF organic charge transfer salts as motivation, we explore the possible charge and spin ordered states that arise from frustrating interactions. We find a quantum critical point in the phase diagram of the single chain and present results for some response functions in the quantum critical regime. RPA studies of coupled chains reveal a phase diagram with the ordered phase extended to finite temperatures and a phase boundary again ending at a quantum critical point. Critical quantum fluctuations at the QCP enhance the transverse dispersion, leading to a dimensional crossover and a low temperature transition from insulating chains to anistropic metallic bulk behaviour. These results have been reported in arxiv:0708.2156 and are in press at Int. J. Mod. Phys. B. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J41.00006: Mott and Wigner-Mott transition away from half-filling Chunhua Li, Ziqiang Wang We study the Mott transition in strongly correlated electron systems away from half-filling in the presence of finite-range Coulomb interaction and/or a superstructure associated with an inhomogeneous electronic state. Using a cluster Gutzwiller approach, we map a minimal single band $t$-$U$-$V$-$\Delta$ model with nearest neighbor Coulomb repulsion $V$ and superlattice potential $\Delta_i$ to a two-orbital Hubbard model with intra and interorbital Coulomb repulsion $U$ and $U^\prime$ and a crystal field splitting $\Delta$. We obtain the Mott transition at quarter-filling from both uniform and $\sqrt{2}\times\sqrt{2}$ charge density wave metals and discuss the physics of the Mott and Wigner-Mott metal-insulator transition. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J41.00007: Effect of Interchain Hopping on Nonequivalent Hubbard Chains Takami Tohyama, Hiroyuki Yoshizumi, Takao Morinari Motivated by recent report on the presence of different orders in each CuO$_2$ plane and their coexistence in multi-layered high-temperature superconductors, we examine a simple model with two nonequivalent Hubbard chains coupled by interchain hopping, employing the density-matrix renormalization group (DMRG) and a mean-field calculation. We consider a coupled non-interacting and spin-density-wave (SDW) chains as an example. We find that the interchain hopping induces a short-rang SDW order from adjacent SDW chain and the induced order decreases with increasing the original SDW order of the adjacent chain. We discuss the finding in connection with the multi-layered cuprates. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J41.00008: High Precision QMC Study of the 2D Hubbard Model Christopher Varney, Simone Chiesa, Richard Scalettar The Hubbard model has provided insight into a wide variety of strongly correlated systems, including the cuprates and manganites. Recent advances in the generation of optical lattices allow for the possibility of experimentally studying the 2D Hubbard model in a new context. To enhance our understanding of the model, we examine the magnetic correlations on a rectangular lattice using Determinant Quantum Monte Carlo. In this talk, we discuss high precision calculations of the anti-ferromagnetic order parameter as a function of interaction strength and the effect of the aspect ratio on finite size scaling. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J41.00009: Quantum phase transition in the one-dimensional half-filled asymmetric Hubbard model Yang Liu, Wen-Ling Chan, Shi-Jian Gu, Hai-Qing Lin We study the quantum phase transition in the one-dimensional asymmetric Hubbard chain at half-filling in terms of spin stiffness by the exact diagonalization method. Since the phase transition is believed to be of Kosterlitz-Thouless type, no local order parameter can describe such a transition. We find instead that spin stiffness might be able to describe the quantum phase transition since the spin degree of freedom is gapless in the Hubbard region. while gapful in the Falicov-Kimball region. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J41.00010: Effect of nonlocal interactions on the disorder-induced zero-bias anomaly in the extended Anderson-Hubbard model Rachel Wortis, Hongyi Chen, W.A. Atkinson Adding disorder to a system of correlated electrons moves single-particle states away from the Fermi surface. In the weakly correlated regime, consensus exists on the evolution of the resulting density of states anomaly between the limits of weak and strong disorder. Recently a number of groups have made progress in understanding the strongly correlated regime, mostly in the context of purely local interactions. We study the extended Anderson-Hubbard model using exact diagonalization on two-dimensional 12-site clusters, exploring the evolution of the zero-bias anomaly with the strength of the nonlocal interaction and with doping. At half filling, an exchange-driven Altshuler-Aronov-like anomaly develops Efros-Shklovskii-like atomic character and moves to a regime of strong charge-density correlations, whereas at quarter filling both the Efros-Shklovskii-like behavior and the charge density correlations are much weaker. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J41.00011: Local Order and the gapped phase of the Hubbard model: a plaquette dynamical mean field investigation Emanuel Gull, Philipp Werner, Xin Wang, Matthias Troyer, Andrew Millis The four-site DCA method of including intersite correlations in the dynamical mean field theory is used to investigate the metal-insulator transition in the Hubbard model. At half filling a gap-opening transition is found to occur as the interaction strength is increased beyond a critical value. The gapped behavior found in the 4-site DCA approximation is shown to be associated with the onset of strong antiferromagnetic and singlet correlations and the transition is found to be potential energy driven. It is thus more accurately described as a Slater phenomenon (induced by strong short ranged order) than as a Mott phenomenon. Doping the gapped phase leads to a non-Fermi-liquid state with a Fermi surface only in the nodal regions and a pseudogap in the antinodal regions at lower dopings $x \alt 0.15$ and to a Fermi liquid phase at higher dopings. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J41.00012: Delocalization effect of the Hubbard repulsion in two dimensions and exact terms Zsolt Gulacsi The physical reasons explaining the delocalization effect of the Hubbard repulsion U leading in 2D to an insulator to metal transition are analyzed. The study is made in exact terms by deducing exact ground states and ground state expectation values of interest based on a positive semidefinite operator technique [1]. First it is shown that always when this effect is observed, U acts on the background of a macroscopic degeneracy present in a multiband type of system. After this step I demonstrate that acting in such conditions, by strongly diminishing the double occupancy, U spreads out the contributions in the ground state wave function, hence strongly increases the one-particle localization length, and consequently extends the one-particle behavior producing conditions for a delocalization effect [2]. \\[3pt] References: [1] Z. Gulacsi, D. Vollhardt, Phys. Rev. Lett. 91,186401(2003); Z. Gulacsi, A. Kampf, D. Vollhardt, Phys. Rev. Lett. 99,026404(2007). [2] Z. Gulacsi, Phys. Rev. B77,245113(2008). [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J41.00013: One-dimensional periodic Anderson model at partial band filling Miklos Gulacsi, Ian McCulloch An effective hamiltonian is derived for the one-dimensional periodic Anderson model via bosonization. The effective hamiltonian is shown to reproduce all the features of the model as identified by DMRG and provides new information on the ferromagnetic to paramagnetic phase transitions and the paramagnetic phase. We are using a non-Abelian DMRG to determine numerically the phase diagram of the one-dimensional periodic Anderson model. We found very good agreement between the bosonization approach and the DMRG results. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J41.00014: The Marginal Fermi Liquid-A Derivation Based on Dirac's Constraints D. Schmeltzer Dirac's method for constraints is used for enforcing the exclusion of double occupancy for Correlated Electrons. The constraintt is given by the pair $Q(\vec{x})=\psi_{\downarrow}(\vec{x})\psi_{\uparrow}(\vec{x})$ which annihilates the ground state $|\Psi>$. Away from half fillings $Q(\vec{x})$ is replaced by a set of $first$ $class$ class Non-Abelian constraints $Q^{(-)}_{\alpha}(\vec{x})$ which are restricted to negative energies. The propagator for the single hole is determined by a measure which is a function of time duration for the hole propagator. a)-The imaginary part of the self energy - is linear in the frequency. b)-In the Superconducting phase the tunneling density of states is asymmetric. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700